InteractiveFly: GeneBrief

Fatty acid synthase 1: Biological Overview | References

De novo lipogenesis is a highly regulated metabolic process, which is known to be activated through transcriptional regulation of lipogenic genes, including fatty acid synthase (FASN). Unexpectedly, this study found that the expression of FASN protein remains unchanged during Drosophila larval development from the second to the third instar larval stages (L2 to L3) when lipogenesis is hyperactive. Instead, acetylation of FASN is significantly upregulated in fast-growing larvae. This study further showed that lysine K813 residue is highly acetylated in developing larvae, and its acetylation is required for elevated FASN activity, body fat accumulation, and normal development. Intriguingly, K813 is autoacetylated by acetyl-CoA (AcCoA) in a dosage-dependent manner independent of acetyltransferases. Mechanistically, the autoacetylation of K813 is mediated by a novel P-loop-like motif (N-xx-G-x-A). Lastly, this study found that K813 is deacetylated by Sirt1, which brings FASN activity to baseline level. In summary, this work uncovers a previously unappreciated role of FASN acetylation in developmental lipogenesis and a novel mechanism for protein autoacetylation, through which Drosophila larvae control metabolic homeostasis by linking AcCoA, lysine acetylation, and de novo lipogenesis (Miao, 2022).

De novo lipogenesis (DNL) is a complex yet highly regulated metabolic process which converts excess carbohydrates into fatty acids that are then esterified to storage triglyceride (TAG). Abnormal upregulation of DNL is a vital contributor to increased fat mass in the pathogenesis of various metabolic disorders involving non-alcoholic fatty liver disease and diabetes and in the progression of tumors. DNL is known to be transcriptionally regulated via sterol regulatory element-binding protein 1 (SREBP1) and carbohydrate-responsive element-binding protein (ChREBP) in response to metabolic and hormonal cues. However, it has been recently proposed that allosteric regulation and post-translational modifications (PTMs) are indispensable to control metabolic flux since the timescale of the gene expression is too long to balance the quick turnover of metabolites. Lipid anabolism is active during Drosophila larval development, and the excessive TAG storage in larvae is an essential reservoir for surviving from starvation in the post-feeding pupa stage of metamorphosis. The fatty acids stockpiled in TAG are either derived from the diet or DNL. Regulated by hormonal and transcriptional programs, the mRNA expressions of multiple enzymes in DNL pathways, including acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN), correlate with the dynamic changes in TAG levels in fly embryonic and larval development. However, whether PTMs are involved in the regulation of developmental DNL is poorly studied (Miao, 2022).

Lysine acetylation has recently risen as a novel player that links metabolites [e.g., acetyl coenzyme A (acetyl-CoA)], cell signaling, and gene regulation. Previous acetylome studies found that almost all metabolic enzymes are acetylated, including FASN . FASN, an essential cytosolic enzyme in DNL pathway, catalyzes the biosynthesis of saturated fatty acids from acetyl-CoA (AcCoA) and malonyl coenzyme A (malonyl-CoA). Recently, FASN has emerged as a novel therapeutic target for the treatment of obesity, diabetes, fatty liver diseases, and cancers. Although FASN is known to be regulated through SREBP1-mediated transcriptional activation, several conflicting results show little correlation between FASN expression and its enzymatic activity. These findings suggest a possible involvement of PTMs in the regulation of FASN function. Phosphorylation and acetylation have been proposed as alternative mechanisms of FASN regulation. Nevertheless, how PTMs regulate FASN activity and lipogenesis remains largely unknown (Miao, 2022).

Although protein acetylation is mainly catalyzed by lysine acetyltransferases (KATs), it has been recently reported that acetylation also arises from a nonenzymatic reaction with AcCoA in eukaryotes. AcCoA is the acetyl donor for protein acetylation and is a reactive metabolic intermediate involved in various metabolic pathways. The levels of AcCoA fluctuate in response to both intracellular and extracellular cues (e.g., growth signals and nutrient conditions), which consequently impacts chromatin modifications and transcriptional reprogramming. Previously, it was thought that nonenzymatic acetylation only occurs to mitochondria proteins, as high concentrations of AcCoA and alkaline environment inside mitochondrial matrix favor lysine nucleophilic attack on the carbonyl carbon of AcCoA. In recent years, the capability of enzyme-independent acetylation of cytosolic proteins was also determined. Yet, the mechanism of nonenzymatic acetylation, especially of cytosolic proteins, remains elusive (Miao, 2022).

This study shows that the expression of Drosophila FASN (dFASN or FASN1) protein remains unchanged during larval development, the stages when lipogenesis is hyperactive. In contrast, acetylation of dFASN at K813 is significantly induced in response to increased cellular AcCoA levels, which elevates dFASN enzymatic activity and lipogenesis in fast-growing larvae. Strikingly, acetylation of K813 is controlled through a unique KAT-independent mechanism that involves a novel motif "N-xx-G-x-A." In summary, these findings uncover a novel AcCoA-mediated self-regulatory module that regulates developmental lipogenesis via autoacetylation of dFASN (Miao, 2022).

Metabolic homeostasis plays an important role in animal development and growth. One novel mechanism underlying the coordination of metabolic homeostasis and growth is the interplay between metabolic intermediates and PTMs. This study uncover a novel role of AcCoA-mediated autoacetylation of dFASN in lipogenesis during Drosophila larval development. On the one hand, AcCoA fuels dFASN as the carbon donor for the growing fatty acid chain. On the other hand, AcCoA, as the acetyl-group donor, directly modulates dFASN enzymatic activity through acetylation of the critical lysine residue K813. Surprisingly it was found that acetylation of dFASN does not require KATs; instead, it is mediated by a conserved P-loop-like motif N-xx-G-x-A neighboring K813. Lastly, Sirt1 was identied as the primary deacetylase for dFASN, which acts as a negative regulatory mechanism (Miao, 2022).

TAG levels are tightly controlled during Drosophila development, and the massive buildup of TAG storage is a feature of larval growth. TAG synthesis is catalyzed by isoenzymes and competes with pathways that consume fatty acids, such as fatty acid oxidation and membrane lipid synthesis. During the entire embryonic and larval development, the mRNA expressions of multiple lipogenic enzymes, including FASN, correlate with TAG levels. Moreover, flies with mutations in FASN1 and FASN2 store less TAG in both larval and adult stages of Drosophila, suggesting that DNL is a vital contributor to TAG storage throughout development (Miao, 2022).

Under conditions like excess nutrition, growth factor stimulation, obesity, diabetes, fatty liver diseases, or cancer, DNL is significantly elevated, and the mRNA expression of FASN positively correlates with elevated lipogenesis. However, the protein levels of FASN are rarely characterized. Interestingly, several conflicting results show little correlation between FASN protein expression and its enzymatic activity. Consistently, it was found that the protein levels of dFASN remain unchanged from L2 to L3 and do not match the pattern of its enzymatic activity. In contrast, acetylation of dFASN at lysine K813 is positively associated with dFASN activity, developmental lipogenesis, and TAG accumulation. However, although dFASN level is not upregulated with elevated lipogenesis and TAG accumulation at L3 larvae, it correlates with TAG levels when the entire embryonic and larval development stages are considered. These findings suggest that despite the well-established transcriptional program controlling the expression of dFASN at different stages of development, lysine acetylation of dFASN plays a crucial role in accelerating dFASN activity and fine-tuning dFASN-mediated lipogenesis in fast-growing L3 animals (Miao, 2022).

Indeed, genetic and biochemical analysis further demonstrates that K813R substitution reduces dFASN enzymatic activity and lipogenesis, while AcCoA-mediated acetylation of recombinant dFASN proteins increases its enzyme activity. Although acetylation of FASN has been reported in several previous global acetylome studies, the functional roles of FASN acetylation remain largely unknown. A recent study investigated the role of FASN acetylation in DNL in human cell culture. The study shows that treatment of KDAC inhibitors induces the acetylation of hFASN, promotes FASN degradation, and reduces lipogenesis. However, it remains to be determined whether the regulation of lipogenesis by KDAC inhibition is due to global acetylation, or if it is directly through FASN acetylation. In addition, the functional lysine residues of hFASN that are responsible for altered lipogenesis are not identified. Because of the high conservation between K813 of dFASN and K673 of hFASN, it is possible that K673 is the key lysine residue mediating DNL in human (Miao, 2022).

Apart from K813, other three lysine residues of dFASN (K926, K1800, and K2466) are highly conserved among animal species, and their homologs are also found to be acetylated in other animal species. Since these lysine residues are located on different domains, it is not hard to imagine that acetylation of each lysine may play distinct roles related to their associated domains. The present study shows that acetylation of K813, but not K926, modulates dFASN activity, body fat accumulation, and Drosophila developmental timing. It is likely that acetylation of K926 affects other aspects of enzyme properties that are less important for larval development. K813 is at the substrate docking pocket of the MAT domain. This unique localization suggests that acetylation of K813 might introduce conformational changes to the docking site and modulate dFASN catalytic activity in response to substrate availability during larval development (Miao, 2022).

Another surprising finding from this study is that acetylation of dFASN at K813 does not require a KAT; rather, it is autoacetylated by AcCoA in a dosage-dependent manner. The cytosolic pool of AcCoA increases under feeding or excess nutrient conditions. Consistently, these studies reveal that the amount of AcCoA elevates in fast-growing larvae, which could modulate dFASN activity by promoting both the biosynthesis of MalCoA and autoacetylation of K813 for the conformational changes of MalCoA docking pocket (Miao, 2022).

It was previously thought that only mitochondria proteins were nonenzymatically acetylated since no KATs have been identified in mitochondria. Besides, the high AcCoA concentration and relatively high pH of the mitochondrial matrix facilitate the lysine nucleophilic attack on the carbonyl carbon of AcCoA. Recently, KAT-independent acetylation of cytosolic proteins has been reported. Yet, the underlying mechanism for nonenzymatic acetylation remains largely unknown. When investigating how dFASN is autoacetylated by AcCoA, this study uncovered a novel motif N-xx-G-x-A near acetylated K813. Substituting any of the three key amino acids largely blocks AcCoA-mediated dFASN autoacetylation. The N-xx-G-x-A motif resembles the signature P-loop sequence (Q/R-xx-G-x-A/G) of KATs, which is required for AcCoA recognition and binding. It is predicted that the N-xx-G-x-A motif of dFASN performs a similar function as the P-loop of KATs for AcCoA binding. Moreover, the N-xx-G-x-A motif is highly conserved, pointing out a conserved mechanism for autoacetylation of FASN (Miao, 2022).

In addition to the well-established KATs of the MYST, p300/CBP, and GCN5 families, there are over 15 proteins that have been reported to possess KATs activity, such as CLOCK and Eco1. Since FASN may contain an AcCoA binding motif of KATs, it is possible that FASN, particularly MAT domain, possesses KATs activity and acetylates other proteins, especially those in DNL pathways. This possibility may be further explored through acetylome analysis in the future. In summary, this study uncovered a previously unappreciated role of FASN acetylation in developmental lipogenesis and a novel mechanism for lysine autoacetylation. These findings provide new insights into AcCoA-mediated metabolic homeostasis during animal development. In addition, rhwaw studies underscore a promising therapeutic strategy to combat metabolic disorders by targeting autoacetylation of FASN (Miao, 2022).

Drosophila STING protein has a role in lipid metabolism

Stimulator of interferon genes (STING) plays an important role in innate immunity by controlling type I interferon response against invaded pathogens. This work describes a previously unknown role of STING in lipid metabolism in Drosophila. Flies with STING deletion are sensitive to starvation and oxidative stress, have reduced lipid storage and downregulated expression of lipid metabolism genes. Drosophila STING was found to interact with lipid synthesizing enzymes acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN). ACC and FASN also interact with each other, indicating that all three proteins may be components of a large multi-enzyme complex. The deletion of Drosophila STING leads to disturbed ACC localization and decreased FASN enzyme activity. Together, these results demonstrate a previously undescribed role of STING in lipid metabolism in Drosophila (Akhmetova, 2021).

STimulator of INterferon Genes (STING) is an endoplasmic reticulum (ER)-associated transmembrane protein that plays an important role in innate immune response by controlling the transcription of many host defense genes. The presence of foreign DNA in the cytoplasm signals a danger for the cell. This DNA is recognized by specialized enzyme, the cyclic GMP-AMP synthase (cGAS), which generates cyclic dinucleotide (CDN) signaling molecules. CDNs bind to STING activating it, and the following signaling cascade results in NF-κB- and IRF3-dependent expression of immune response molecules such as type I interferons (IFNs) and pro-inflammatory cytokines. Bacteria that invade the cell are also known to produce CDNs that directly activate STING pathway. Additionally, DNA that has leaked from the damaged nuclei or mitochondria can also activate STING signaling and inflammatory response, which, if excessive or unchecked, might lead to the development of autoimmune diseases such as systemic lupus erythematosus or rheumatoid arthritis (Akhmetova, 2021).

STING homologs are present in almost all animal phyla. This protein has been extensively studied in mammalian immune response; however, the role of STING in the innate immunity of insects has been just recently identified. Fruit fly D. melanogaster STING homolog is encoded by the CG1667 gene, which is refered to as dSTING. dSTING displays anti-viral and anti-bacterial effects that however are not all encompassing. Particularly, it has been shown that dSTING-deficient flies are more susceptible to Listeria infection due to the decreased expression of antimicrobial peptides (AMPs) - small positively charged proteins that possess antimicrobial properties against a variety of microorganisms. dSTING has been shown to attenuate Zika virus infection in fly brains and participate in the control of infection by two picorna-like viruses (DCV and CrPV) but not two other RNA viruses FHV and SINV or dsDNA virus IIV6. All these effects are linked to the activation of NF-κB transcription factor Relish (Akhmetova, 2021).

The immune system is tightly linked with metabolic regulation in all animals, and proper re-distribution of the energy is crucial during immune challenges. In both flies and humans, excessive immune response can lead to a dysregulation of metabolic stores. Conversely, the loss of metabolic homeostasis can result in weakening of the immune system. The mechanistic links between these two important systems are integrated in Drosophila fat body. Similarly to mammalian liver and adipose tissue, insect fat body stores excess nutrients and mobilizes them during metabolic shifts. The fat body also serves as a major immune organ by producing AMPs during infection. There is an evidence that the fat body is able to switch its transcriptional status from 'anabolic' to 'immune' program. The main fat body components are lipids, with triacylglycerols (TAGs) constituting approximately 90% of the stored lipids. Even though most of the TAGs stored in fat body comes from the dietary fat originating from the gut during feeding, de novo lipid synthesis in the fat body also significantly contributes to the pool of storage lipids (Akhmetova, 2021).

Maintaining lipid homeostasis is crucial for all organisms. Dysregulation of lipid metabolism leads to a variety of metabolic disorders such as obesity, insulin resistance and diabetes. Despite the difference in physiology, most of the enzymes involved in metabolism, including lipid metabolism, are evolutionarily and functionally conserved between Drosophila and mammals. Major signaling pathways involved in metabolic control, such as insulin system, TOR, steroid hormones, FOXO, and many others, are present in fruit flies. Therefore, it is not surprising that Drosophila has become a popular model system for studying metabolism and metabolic diseases. With the availability of powerful genetic tools, Drosophila has all the advantages to identify new players and fill in the gaps in understanding of the intricacies of metabolic networks (Akhmetova, 2021).

This work describes a novel function of dSTING in lipid metabolism. Flies with a deletion of dSTING were found to be sensitive to the starvation and oxidative stress. Detailed analysis reveals that dSTING deletion results in a significant decrease in the main storage metabolites, such as TAG, trehalose, and glycogen. Two fatty-acid biosynthesis enzymes were identified - acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN) - as the interacting partners for dSTING. Moreover, this study also found that FASN and ACC interacted with each other, indicating that all three proteins might be components of a large complex. Importantly, dSTING deletion leads to the decreased FASN activity and defects in ACC cellular localization suggesting a direct role of dSTING in lipid metabolism of fruit flies (Akhmetova, 2021).

STING plays an important role in innate immunity of mammals, where activation of STING induces type I interferons (IFNs) production following the infection with intracellular pathogens. However, recent studies showed that the core components of STING pathway evolved more than 600 million years ago, before the evolution of type I IFNs. This raises the question regarding the ancestral functions of STING. In this study it was found that STING protein is involved in lipid metabolism in Drosophila. The deletion of Drosophila STING (dSTING) gene rendered flies sensitive to the starvation and oxidative stress. These flies have reduced lipid storage and downregulated expression of lipid metabolism genes. It was further shown that dSTING interacted with the lipid synthesizing enzymes ACC and FASN suggesting a possible regulatory role in the lipid biosynthesis. In the fat body, main lipogenic organ in Drosophila, dSTING co-localized with both ACC and FASN in a cortical region of the ER. dSTING deletion resulted in the disturbed ACC localization in fat body cells and greatly reduced the activity of FASN in the in vitro assay (Akhmetova, 2021).

Importantly, it was also observed that ACC and FASN interacted with each other. Malonyl-CoA, the product of ACC, serves as a substrate for the FASN reaction of fatty acid synthesis. Enzymes that are involved in sequential reactions often physically interact with each other and form larger multi-enzyme complexes, which facilitates the substrate channeling and efficient regulation of the pathway flux. There are several evidences of the existence of the multi-enzyme complex involved in fatty acid biosynthesis. ACC, ACL (ATP citrate lyase), and FASN physically associated in the microsomal fraction of rat liver. Moreover, in the recent work, a lipogenic protein complex including ACC, FASN, and four more enzymes was isolated from the oleaginous fungus Cunninghamella bainieri. It is possible that a similar multi-enzyme complex exists in Drosophila and other metazoan species, and it would be of great interest to identify its other potential members (Akhmetova, 2021).

How does STING exerts its effect on lipid synthesis? Recently, the evidence has emerged for the control of the de novo fatty acid synthesis by two small effector proteins - MIG12 and Spot14. MIG12 overexpression in livers of mice increased total fatty acid synthesis and hepatic triglyceride content. It has been shown that MIG12 protein binds to ACC and facilitates its polymerization thus enhancing the activity of ACC. For Spot14, both the activation and inhibition of de novo lipogenesis have been reported, depending upon the tissue type and the cellular context. Importantly, there is an evidence that all four proteins - ACC, FASN, MIG12, and Spot14 - exist as a part of a multimeric complex. It is plausible to suggest that Drosophila STING plays a role similar to MIG12 and/or Spot14 in regulating fatty acid synthesis. It is proposed that dSTING might ‘anchor' ACC and FASN possibly together with other enzymes at the ER membrane. The resulting complex facilitates fatty acid synthesis by allowing for a quicker transfer of malonyl-CoA product of ACC to the active site of FASN. In dSTINGΔ mutants, ACC loses its association with some regions of the ER resulting in the weakened interaction between ACC and FASN. Less FASN immunoprecipitated with ACC in dSTINGΔ mutants compared to control flies, and the opposite effect was found in flies expressing GFP-tagged dSTING (Akhmetova, 2021).

It has been shown that de novo synthesis of fatty acids continuously contributes to the total fat body TAG storage in Drosophila. It is hypothesized that the reduced fatty acid synthesis due to the lowered FASN enzyme activity in dSTINGΔ deletion mutants might be responsible for the decreased TAG lipid storage and starvation sensitivity phenotypes. Sensitivity to oxidative stress might also be explained by the reduced TAG level. Evidences exist that the lipid droplets (consisting mainly of TAGs) provide protection against reactive oxygen species. Furthermore, flies with ACC RNAi are found to be sensitive to the oxidative stress (Akhmetova, 2021).

In addition to its direct role in ACC/FASN complex activity, STING might also affect a phosphorylation status of ACC and/or FASN. Both proteins are known to be regulated by phosphorylation/dephosphorylation. In mammals, STING is an adaptor protein that transmits an upstream signal by interacting with kinase TBK1 (TANK-binding kinase 1). When in a complex with STING, TBK1 activates and phosphorylates IRF3 allowing its nuclear translocation and transcriptional response. It is possible that in Drosophila, STING recruits a yet unidentified kinase that phosphorylates ACC and/or FASN thereby changing their enzymatic activity (Akhmetova, 2021).

Drosophila STING itself could also be regulated by the lipid- synthesizing complex. STING palmitoylation was recently identified as a posttranslational modification necessary for STING signaling in mice. In this way, palmitic acid synthesized by FASN might participate in the regulation of dSTING possibly providing a feedback loop (Akhmetova, 2021).

The product of ACC - malonyl-CoA - is a key regulator of the energy metabolism. During lipogenic conditions, ACC is active and produces malonyl-CoA, which provides the carbon source for the synthesis of fatty acids by FASN. In dSTING knockout, FASN activity is decreased and malonyl-CoA is not utilized and builds up in the cells. Malonyl-CoA is also a potent inhibitor of carnitine palmitoyltransferase CPT1, the enzyme that controls the rate of fatty acid entry into the mitochondria, and hence is a key determinant of the rate of fatty acid oxidation. Thus, a high level of malonyl-CoA results in a decreased fatty acid utilization for the energy. This might explain the down-regulation of lipid catabolism genes that was observed in dSTINGΔ mutants. A reduced fatty acid oxidation in turn shifts cells to the increased reliance on glucose as a source of energy. Consistent with this notion, an increased glucose level was observed in fed dSTINGΔ mutant flies, as well as increased levels of phosphoenolpyruvate (PEP). PEP is produced during glycolysis, and its level was shown to correlate with the level of glucose. A reliance on glucose for the energy also has a consequence of reduced incorporation of glucose into trehalose and glycogen for storage, and therefore, lower levels of these storage metabolites, which was observed. To summarize, based on the current findings, a model is presented in (see Model of dSTING deletion effect on Drosophila metabolism), which suggests a direct involvement of dSTING in the regulation of lipid metabolism (Akhmetova, 2021).

Based on the data, dSTING interacts with lipid synthesizing enzymes acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN). In the absence of dSTING, the activity of FASN is reduced which results in decreased de novo fatty acid synthesis and triglyceride (TAG) synthesis. Low TAG level in turn lead to sensitivity to starvation and oxidative stress. Reduced FASN activity in dSTING mutants also results in ACC product malonyl-CoA build-up in the cells leading to the inhibition of the fatty acid oxidation in mitochondria. Reduced fatty acid oxidation shifts cells to the increased reliance on glucose as a source of energy resulting in reduced glycogen and trehalose levels in dSTING mutants. Palmitic acid synthesized by FASN might participate in the regulation of dSTING via palmitoylation possibly providing a feedback loop (Akhmetova, 2021).

Recent studies show that in mammals, the STING pathway is involved in metabolic regulation under the obesity conditions. The expression level and activity of STING were upregulated in livers of mice with high-fat diet-induced obesity. STING expression was increased in livers from nonalcoholic fatty liver disease (NAFLD) patients compared to control group. In nonalcoholic steatohepatitis mouse livers, STING mRNA level was also elevated. Importantly, STING deficiency ameliorated metabolic phenotypes and decreased lipid accumulation, inflammation, and apoptosis in fatty liver hepatocytes (Akhmetova, 2021).

Despite the accumulating evidences, the exact mechanism of STING functions in metabolism is not completely understood. The prevailing hypothesis is that the obesity leads to a mitochondrial stress and a subsequent mtDNA release into the cytoplasm, which activates cGAS-STING pathway. The resulting chronic sterile inflammation is responsible for the development of NAFLD, insulin resistance, and type 2 diabetes. In this case, the effect of STING on metabolism is indirect and mediated by inflammation effectors. The data presented in the current study strongly suggest that in Drosophila, STING protein is directly involved in lipid metabolism by interacting with the enzymes involved in a lipid biosynthesis. This raises the question if the observed interaction is unique for Drosophila or it is also the case for mammals. Future work is needed to elucidate the evolutionary aspect of STING role in metabolism. Understanding the relationships between STING and lipid metabolism may provide insights into the mechanisms of the obesity-induced metabolism dysregulation and thereby suggest novel therapeutic strategies for metabolic diseases (Akhmetova, 2021).

NAD kinase sustains lipogenesis and mitochondrial metabolism through fatty acid synthesis

Lipid storage in fat tissue is important for energy homeostasis and cellular functions. Through RNAi screening in Drosophila fat body, this study found that knockdown of a Drosophila NAD kinase (NADK), which phosphorylates NAD to synthesize NADP de novo, causes lipid storage defects. NADK sustains lipogenesis by maintaining the pool of NADPH. Promoting NADPH production rescues the lipid storage defect in the fat body of NADK RNAi animals. Furthermore, NADK and fatty acid synthase 1 (FASN1) regulate mitochondrial mass and function by altering the levels of acetyl-CoA and fatty acids. Reducing the level of acetyl-CoA or increasing the synthesis of cardiolipin (CL), a mitochondrion-specific phospholipid, partially rescues the mitochondrial defects of NADK RNAi. Therefore, NADK- and FASN1-mediated fatty acid synthesis coordinates lipid storage and mitochondrial function (Xu, 2021).

Lipid homeostasis is important for human health, and its dysregulation is tightly associated with many metabolic diseases, such as type 2 diabetes, hepatic steatosis, cardiovascular disease, and cancer. Cellular lipid homeostasis is regulated by the opposing actions of lipid accumulation, including lipid uptake, de novo lipogenesis and lipid storage, and lipid mobilization, such as lipolysis, lipid oxidation, and lipid efflux. Excess lipid storage or insufficient lipid storage causes obesity or lipodystrophy, respectively (Xu, 2021).

Acetyl-CoA carboxylase (ACC) and FASN mediate fatty acid synthesis from acetyl-CoA during de novo lipogenesis. The fatty acids are then esterified for storage as neutral lipids such as triglycerides (TAGs). The lipid droplet, an organelle with a neutral lipid core and a phospholipid monolayer, is the hub for lipid storage. Understanding of the regulation of lipid storage and lipid droplet dynamics has significantly advanced in recent years. Many processes, including neutral lipid synthesis and degradation, composition of phospholipids, lipid droplet biogenesis and fusion, calcium homeostasis, and lipophagy, together determine lipid storage. Nevertheless, the mechanisms regulating lipid storage and lipid droplet dynamics in vivo are not completely clear (Xu, 2021).

To reduce lipid storage, TAG is mobilized through cytosolic lipolysis to release fatty acids, which are subsequently broken down, mainly in mitochondria, into acetyl-CoA units by lipid oxidation. Therefore, defective mitochondria often lead to lipid accumulation. For example, inhibition of β-oxidation in mitochondria causes lipid accumulation in Drosophila brain. Interestingly, besides conducting fatty acid oxidation, mitochondria also provide substrates and energy for de novo fatty acid synthesis. Both the acetyl-CoA and ATP required by fatty acid synthesis are derived from mitochondria. Impairment of mitochondrial function affects lipogenesis and lipid droplet accumulation. Therefore, impairment of mitochondrial function probably has a context-dependent effect on lipid storage (Xu, 2021).

Conversely, dysregulation of lipid storage also affects mitochondrial function. In the heart, cytoplasmic adipose TAG lipase (ATGL), which hydrolyzes TAG from lipid droplets, affects lipid storage and mitochondrial biogenesis and oxidative metabolism. Similarly, in islet β cells, ATGL knockdown impairs mitochondrial respiration and ATP production, and a PPARδ agonist rescues these mitochondrial defects (Xu, 2021).

Mechanistically, ATGL-mediated lipid droplet lipolysis induces the expression of genes involved in mitochondrial oxidation and respiration by activating the master regulators PPARα/PPARγ and PGC-1α. These studies pinpoint a close relationship between the mitochondrion and the lipid droplet, despite the compartmentalized features of lipid storage and lipid breakdown. Several metabolites, including acetyl-CoA and fatty acids, appear to mediate the two-way communication between these two organelles. De novo lipogenesis is tightly associated with acetyl-CoA and fatty acids. However, despite a few reports showing that lipogenesis inhibitors cause various mitochondrial dysfunctions in cancer, the question of whether and how de novo lipogenesis affects mitochondrial function has not been properly addressed (Xu, 2021).

Through an RNAi screen in Drosophila, this study found that CG6145, a cytosolic NAD kinase (NADK), affects lipid storage in fat body by providing NADPH, an essential reductant in lipogenesis. NADK RNAi causes similar de novo lipogenesis defects as FASN1 RNAi. More importantly, both NADK RNAi and FASN1 RNAi larvae exhibit reduced mitochondrial content. Finally, it was revealed that de novo fatty acid synthesis regulates mitochondrial mass, at least partially, by controlling PGC-1α acetylation and cardiolipin (CL) synthesis (Xu, 2021).

This study shows that NADK affects lipid storage and mitochondrial metabolism in Drosophila. NADK is essential for generating NADP and NADPH, the latter of which is important for de novo fatty acid synthesis. Besides lipid storage, NADK-mediated fatty acid synthesis also contributes to mitochondrial function, possibly through two different mechanisms: one is through acetyl-CoA and PGC-1α acetylation, and the other is through synthesis of the mitochondrion-specific phospholipid CL (Xu, 2021).

Despite the obvious requirement for NADPH in de novo fatty acid synthesis and other metabolic reactions, knowledge about the physiological function and impact of NADK on metabolic homeostasis in different organisms and tissues is limited. This study demonstrated the importance of NADK in animal lipid storage in vivo. NADK determines the level of NADP(H). Increasing NADPH availability rescues the defects in NADK RNAi, which confirms that NADPH is a key determinant of lipid storage. In support of this idea, NADPH-producing enzymes, such as G6PD and ME, promote lipid production in oleaginous microbes. The expression and activities of these enzymes are also correlated with lipid storage in mammals. These observations suggest that NADK and the level of NADPH are previously unappreciated regulators of organismal lipid storage. Interestingly, insulin, which promotes the synthesis and storage of lipids, activates NADK by Akt-mediated phosphorylation, which suggests that NADK may respond to physiological conditions to regulate lipid storage (Xu, 2021).

Besides lipogenesis, this stufy found that NADK also influences mitochondrial metabolism. The amounts of mitochondria and lipid droplets are decreased in both NADK RNAi and FASN1 RNAi, raising the possibility that these two closely linked organelles are co-regulated. Mitochondria regulate lipid metabolism by providing energy and substrates for lipogenesis and a site for fatty acid degradation. Lipid droplets, acting as an important organelle of lipid metabolism, also regulate mitochondrial function. Interestingly, elevating lipolysis by ATGL overexpression reduces the amount of lipid droplets, but it increases mitochondrial content, which suggests that reduced lipid storage per se is not the cause of the reduced mitochondrial mass in both NADK RNAi and FASN1 RNAi. Previous studies have shown that ATGL-mediated lipolysis promotes mitochondrial metabolism and biogenesis through activation of PPARs or Sirt1/PGC-1α. NADK RNAi and FASN1 RNAi exert a stronger effect on mitochondrial function than on lipolysis, which might be attributed to the severe decline in the level of fatty acids. Interestingly, PGC-1α acetylation mediates the regulation of mitochondrial function by both lipolysis and lipogenesis. Therefore, de novo fatty acid synthesis regulates the dynamics of both lipid droplets and mitochondria (Xu, 2021).

Fatty-acid-dependent activation of PPARs and Sirt1 is rather specific. The ligands of PPARs are primarily unsaturated and long-chain fatty acids, while Sirt1 is activated by monounsaturated fatty acids within a restricted range of concentrations. This study found that RNAi of the fat-body-specific PGC-1α homolog srl only moderately reduced mitochondrial mass, in contrast to the strong effect of NADK and FASN1 RNAi. In addition, knockdown of PPAR homologs in fat body caused no obvious mitochondrial phenotype. Therefore, it is likely that fatty acids also regulate mitochondrial function through other mechanism(s). In addition, the rescue of NADK RNAi and FASN1 RNAi by different exogenously supplied fatty acids (including saturated, monounsaturated, and odd-chain fatty acids) and by BMM overexpression suggests a general mechanism with limited or low fatty acid selectivity (Xu, 2021).

Phospholipid synthesis, which affects mitochondrial function in many ways, also requires fatty acids. CL is a mitochondrion-specific phospholipid and is important for almost every aspect of mitochondrial integrity, including crista organization, mitochondrial protein import, and assembly. It is a rather unique phospholipid, harboring four fatty acyl chains, and it undergoes remodeling, which makes it sensitive to the availability and composition of fatty acids. Importantly, the rescue of mitochondrial defects in NADK RNAi and FASN1 RNAi by several genetic manipulations to increase CL production suggests that decreased CL synthesis contributes to the mitochondrial phenotype in NADK RNAi and FASN1 RNAi. The mitochondrial morphology in NADK RNAi and FASN1 RNAi is not completely identical with CLS RNAi. In addition, the rescue effect of CLS overexpression is not comparable with fatty acid supplementation. These observations suggest that fatty acids might also regulate mitochondria through other mechanisms (Xu, 2021).

De novo fatty acid synthesis is important for many biological processes. For example, the activity of fatty acid synthesis is stimulated in some cancer cells or proliferating stem cells. Its inhibition suppresses cell proliferation and survival. It is generally thought that fatty acid synthesis mainly affects these processes by providing structural and signaling lipids\, and limited attention has been paid to the causative role of mitochondrial dysfunction, which is also important for cancer progression and stem cell homeostasis. For example, inhibition of PGC-1α or OXPHOS suppresses cancer cell survival and metastasis under oxidative or bioenergetic stress conditions. In addition, mitochondrial mass is associated with prostate cancer progression. Inhibition of mitochondrial biogenesis was identified as a therapeutic strategy for acute myeloid leukemia. Although OXPHOS activity is restricted in many cancer cells, mitochondrial content, dynamics, and metabolic activity are important for tumorigenesis and stem cell homeostasis (Xu, 2021).

Considering the findings of this study, it is possible that fatty acid synthesis-regulated mitochondrial function may be critical for cancer cell growth and stem cell differentiation. For example, fatty acid and lipid synthesis promote hepatocellular carcinoma development, accompanied by increased CL levels and OXPHOS activity. Inhibition of FASN or ACC reduces mitochondrial oxygen consumption, changes mitochondrial morphology, and affects the levels of mitochondrial proteins and metabolites in cancer and stem cells (Xu, 2021).

Both NADK and FASN are considered as potential targets for cancer therapy because of their lipogenic and other functions. NADK and FASN act as important regulators of lipid storage by restricting the capacity of fatty acid synthesis. This study showed that NADK- and FASN1-mediated fatty acid synthesis regulates mitochondrial function, probably by altering the levels of acetyl-CoA and CL. More physiological functions and molecular mechanisms of NADK and fatty acid synthesis may be revealed through the fatty-acid-mitochondrion link (Xu, 2021).

This study has demonstrated that increased PGC-1 acetylation and reduced CL synthesis are responsible for mitochondrial phenotype in NADK RNAi and FASN1 RNAi. However, reduced acetyl-CoA level and CLS overexpression only partially rescued mitochondrial phenotype. Exogenous fatty acid supplement completely restored mitochondrial mass in NADK RNAi and FASN1 RNAi, suggesting that fatty acid synthesis might regulate mitochondrial mass via other mechanisms as well. In addition, these studies were conducted in fat cells, which are specialized for lipid storage. It remains to be determined whether these findings apply to other cell types (Xu, 2021).

IGFBP-3 promotes cachexia-associated lipid loss by suppressing insulin-like growth factor/insulin signaling

Progressive lipid loss of adipose tissue is a major feature of cancer-associated cachexia. In addition to systemic immune/inflammatory effects in response to tumor progression, tumor-secreted cachectic ligands also play essential roles in tumor-induced lipid loss. However, the mechanisms of tumor-adipose tissue interaction in lipid homeostasis are not fully understood. The yki-gut tumors were induced in fruit flies. Lipid metabolic assays were performed to investigate the lipolysis level of different types of insulin-like growth factor binding protein-3 (IGFBP-3) treated cells. Immunoblotting was used to display phenotypes of tumor cells and adipocytes. Quantitative polymerase chain reaction (qPCR) analysis was carried out to examine the gene expression levels such as Acc1, Acly, and Fasn. This study revealed that tumor-derived IGFBP-3 was an important ligand directly causing lipid loss in matured adipocytes. IGFBP-3, which is highly expressed in cachectic tumor cells, antagonized insulin/IGF-like signaling (IIS) and impaired the balance between lipolysis and lipogenesis in 3T3-L1 adipocytes. Conditioned medium from cachectic tumor cells, such as Capan-1 and C26 cells, contained excessive IGFBP-3 that potently induced lipolysis in adipocyted. Notably, neutralization of IGFBP-3 by neutralizing antibody in the conditioned medium of cachectic tumor cells significantly alleviated the lipolytic effect and restored lipid storage in adipocytes. Furthermore, cachectic tumor cells were resistant to IGFBP-3 inhibition of IIS, ensuring their escape from IGFBP-3-associated growth suppression. Finally, cachectic tumor-derived ImpL2, the IGFBP-3 homolog, also impaired lipid homeostasis of host cells in an established cancer-cachexia model in Drosophila. Most importantly, IGFBP-3 was highly expressed in cancer tissues in pancreatic and colorectal cancer patients, especially higher in the sera of cachectic cancer patients than non-cachexia cancer patients. This study demonstrates that tumor-derived IGFBP-3 plays a critical role in cachexia-associated lipid loss and could be a biomarker for diagnosis of cachexia in cancer patients (Wang, 2023).

Differential metabolic sensitivity of insulin-like-response- and TORC1-dependent overgrowth in Drosophila fat cells

Glycolysis and fatty acid (FA) synthesis directs the production of energy-carrying molecules and building blocks necessary to support cell growth, although the absolute requirement of these metabolic pathways must be deeply investigated. This study used Drosophila genetics and focused on the TOR (Target of Rapamycin) signaling network that controls cell growth and homeostasis. In mammals, mTOR (mechanistic-TOR) is present in two distinct complexes, mTORC1 and mTORC2; the former directly responds to amino acids and energy levels, whereas the latter sustains insulin-like-peptide (Ilp) response. The TORC1 and Ilp signaling branches can be independently modulated in most Drosophila tissues. This study shows that TORC1 and Ilp-dependent overgrowth can operate independently in fat cells and that ubiquitous over-activation of TORC1 or Ilp signaling affects basal metabolism, supporting the use of Drosophila as a powerful model to study the link between growth and metabolism. Cell-autonomous restriction of glycolysis or FA synthesis in fat cells was shown to retrain overgrowth dependent on Ilp signaling but not TORC1 signaling. Additionally, the mutation of FASN (Fatty acid synthase) results in a drop in TORC1 but not Ilp signaling, whereas, at the cell-autonomous level, this mutation affects none of these signals in fat cells. These findings thus reveal differential metabolic sensitivity of TORC1- and Ilp-dependent growth and suggest that cell-autonomous metabolic defects might elicit local compensatory pathways. Conversely, enzyme knockdown in the whole organism results in animal death. Importantly, this study weakens the use of single inhibitors to fight mTOR-related diseases and strengthens the use of drug combination and selective tissue-targeting (Devilliers, 2021).

Drosophila PDGF/VEGF signaling from muscles to hepatocyte-like cells protects against obesity

PDGF/VEGF ligands regulate a plethora of biological processes in multicellular organisms via autocrine, paracrine and endocrine mechanisms. This study investigated organ-specific metabolic roles of Drosophila PDGF/VEGF-like factors (Pvfs). Genetic approaches and single-nuclei sequencing were combined to demonstrate that muscle-derived Pvf1 signals to the Drosophila hepatocyte-like cells/oenocytes to suppress lipid synthesis by activating the Pi3K/Akt1/TOR signaling cascade in the oenocytes. Functionally, this signaling axis regulates expansion of adipose tissue lipid stores in newly eclosed flies. Flies emerge after pupation with limited adipose tissue lipid stores and lipid level is progressively accumulated via lipid synthesis. This study found that adult muscle-specific expression of pvf1 increases rapidly during this stage and that muscle-to-oenocyte Pvf1 signaling inhibits expansion of adipose tissue lipid stores as the process reaches completion. These findings provide the first evidence in a metazoan of a PDGF/VEGF ligand acting as a myokine that regulates systemic lipid homeostasis by activating TOR in hepatocyte-like cells (Ghosh, 2020).

The presence in vertebrates of multiple PDGF/VEGF signaling ligands and cognate receptors makes it difficult to assess their roles in inter-organ communication. Additionally, understanding the tissue-specific roles of these molecules, while circumventing the critical role they play in regulating tissue vascularization, is equally challenging in vertebrate models. This study investigated the tissue-specific roles of the ancestral PDGF/VEGF-like factors and the single PDGF/VEGF-receptor in Drosophila in lipid homeostasis. The results demonstrate that in adult flies the PDGF/VEGF like factor, Pvf1, is a muscle-derived signaling molecule (myokine) that suppresses systemic lipid synthesis by signaling to the Drosophila hepatocyte-like cells/oenocytes (Ghosh, 2020).

The Drosophila larval and adult adipose tissues have distinct developmental origins. The larval adipose tissue undergo drastic morphological changes during metamorphosis and dissociate into individual large spherical cells. These free-floating adipose cells persist to the young adult stage where they play a crucial role in protecting the animal from starvation and desiccation. These larval adipose tissue cells are ultimately removed via cell death. Adult-specific adipose tissue cells develop during the pupal stage from yet unknown progenitor cells and have very little lipid stores in newly eclosed flies. Over the period of next 3-5 days the adult adipose tissue builds up its lipid reserves through feeding and de-novo lipid synthesis. However, at the end of the lipid build-up phase, the rate of lipid synthesis must be suppressed to avoid over-loading of the adipose tissue and prevent lipid toxicity. The data suggest that muscle Pvf1 signaling suppresses lipid synthesis at the end of the adult adipose tissue lipid build-up phase. Pvf1 production in the adult muscles peaks around the time when adult adipose tissue lipid stores reach steady state capacity. In turn, muscle-derived Pvf1 suppresses lipid synthesis and lipid incorporation by activating TOR signaling in the oenocytes (Ghosh, 2020).

This study reveals that Pvf1 is abundant in the tubular muscles of the Drosophila leg and abdomen. In these striated muscles, the protein localizes between individual myofibrils and is particularly enriched at the M and Z bands. Drosophila musculature can be broadly categorized into two groups, the fibrillar muscles and the tubular muscles, with distinct morphological and physiological characteristics. Drosophila IFMs of the thorax belong to the fibrillar muscle group and are stretch-activated, oxidative, slow twitch muscles that are similar to vertebrate cardiac muscles. By contrast, Drosophila leg muscles and abdominal muscles belong to the tubular muscle group. These muscles are striated, Ca2+ activated, and glycolytic in nature. The tubular muscles are structurally and functionally closer to vertebrate skeletal muscles. Expression of Pvf1 in the tubular muscles of the Drosophila leg may reflect a potentially conserved role of this molecule as a skeletal-muscle-derived myokine. The fact that most of the myokines in vertebrates were identified in striated skeletal muscles supports this possibility . Moreover, vertebrate VEGF ligands, VEGF-A and VEGF-B, have also been shown to be stored and released from skeletal muscles (Ghosh, 2020).

Interestingly, in vertebrates, the expression and release of VEGF ligands are regulated by muscle activity. In mice, expression of VEGF-B in the skeletal muscles is regulated by PGC1-α, one of the key downstream effectors of muscle activity. Additionally, expression of VEGF-B is upregulated in both mouse and human skeletal muscles in response to muscle activity. Similarly, expression of VEGF-A is induced by muscle contraction. No effect of muscle activity on the expression levels of pvf1 was observed in the Drosophila muscles. Whether muscle activity regulates release of Pvf1 primarily could not be demonstrated due to the difficulty in collecting adequate amounts of hemolymph from the adult males. However, the localization of Pvf1 to the M/Z bands suggests a potential role for muscle activity in Pvf1 release. The M and Z bands of skeletal muscles are important centers for sensing muscle stress and strain. These protein-dense regions of the muscle house a number of proteins that can act as mechano-sensors and mediate signaling events including translocation of selected transcription factors to the nucleus. Pvf1, therefore, is ideally located to be able to sense muscle contraction and be released in response to muscle activity. Further work, contingent on the development of new tools and techniques, will be necessary to measure Pvf1 release into the hemolymph and study the regulation of this release by exercise (Ghosh, 2020).

Previous work has shown that Pvf1 released from gut tumors generated by activation of the oncogene yorkie leads to wasting of Drosophila muscle and adipose tissue (Song, 2019). Adipose tissue wasting in these animals is characterized by increased lipolysis and release of free fatty acids (FFAs) in circulation. However, no role was observed of Pvf signaling in regulating lipolysis in the adipose tissue of healthy well-fed flies without tumors. Loss of PvR signaling in the adipose tissue did not have any effect on lipid content. Additionally, over-expressing Pvf1 in the muscle did not lead to the bloating phenotype commonly seen in cachectic animals with gut tumors. It is concluded that Pvf1 affects wasting of the adipose tissue only in the context of gut tumors and that the effect could involve the following mechanisms: (1) the gut tumor releases pathologically high levels of Pvf1 into circulation leading to ectopic activation of PvR signaling in the adipose tissue, and, that such high levels of Pvf1 are not released by the muscle (even when pvf1 is over-expressed in the muscle); (2) Pvf1 causes adipose tissue wasting in the context of other signals that emanate from the gut tumor that are not available in flies that do not have tumors (Ghosh, 2020).

Only oenocyte-specific loss of PvR signaling phenocopies the obesity phenotype caused by muscle-specific loss of Pvf1, indicating that muscle-Pvf1 primarily signals to the oenocytes to regulate systemic lipid content. Additionally, muscle-specific loss of Pvf1, as well as oenocyte-specific loss of PvR and its downstream effector TOR, leads to an increase in the rate of lipid synthesis. These observations indicate a role for the Drosophila oenocytes in lipid synthesis and lipid accumulation in the adipose tissue. Oenocytes have been implicated in lipid metabolism previously and these cells are known to express a diverse set of lipid metabolizing genes including but not limited to fatty acid synthases, fatty acid desaturases, fatty acid elongases, fatty acid β-oxydation enzymes and lipophorin receptors. Functionally, the oenocytes are proposed to mediate a number of lipid metabolism processes. Oenocytes tend to accumulate lipids during starvation (presumably for the purpose of processing lipids for transport to other organs and generation of ketone bodies) and are necessary for starvation induced mobilization of lipids from the adipose tissue. This role is similar to the function of the liver in clearing FFAs from circulation during starvation for the purpose of ketone body generation, and redistribution of FFAs to other organs by converting them to TAG and packaging into very-low density lipoproteins. However, a [1-14C]-oleate chase assay did not show any effect of oenocyte-specific loss of PvR/TOR signaling on the rate of lipid utilization, indicating that this pathway does not affect oenocyte-dependent lipid mobilization (Ghosh, 2020).

Oenocytes also play a crucial role in the production of very-long-chain fatty acids (VLCFAs) needed for waterproofing of the cuticle (Storelli, 2019). Results of a starvation resistance assay indicate that loss of the muscle-to-oenocyte Pvf1 signaling axis does not affect waterproofing of the adult cuticle. Storelli (2019) recently showed that the lethality observed in traditionally used starvation assays is largely caused by desiccation unless the assay is performed under saturated humidity conditions. Since a starvation assay was performed under 60% relative humidity (i.e. non-saturated levels), it is likely that desiccation played a partial role in causing starvation-induced lethality. Any defects in waterproofing of the adult cuticle would have led to reduced starvation resistance. However, both muscle-specific loss of Pvf1 and oenocyte-specific loss of PvR led to increased starvation resistance suggesting normal waterproofing in these animals. The increased starvation resistance in these animals is likely the result of these animals having higher stored lipid content that helps them to survive longer without food (Ghosh, 2020).

Insect oenocytes were originally believed to be lipid synthesizing cells because they contain wax-like granules. These cells express a large number of lipid-synthesizing genes and the abundance of smooth endoplasmic reticulum further suggest a role for this organ in lipid synthesis and transport. However, evidence for potential involvement of the oenocytes in regulating lipid synthesis and lipid deposition in the adipose tissue has been lacking. The fact that two of the three fatty acid synthases (fasn2 and fasn3) encoded by the Drosophila genome are expressed specifically in adult oenocytes suggests a potential role for these cells in lipid synthesis. The observation that oenocyte-specific loss of PvR and its downstream effector TOR leads to increased lipid synthesis and increased lipid content of the adipose tissue strongly supports this possibility. The data further suggests that involvement of the oenocytes in mediating lipid synthesis is more pronounced in newly eclosed adults when the adipose tissue needs to actively build up its lipid stores. In later stages of life, the lipid synthetic role of the oenocytes is repressed by the muscle-to-oenocyte Pvf1 signaling axis. This observation also raises the question of whether FFAs made in the oenocytes can be transported to the adipose tissue for storage. This possibility was tested by over-expressing the lipogenic genes fasn1 and fasn3, which regulate the rate limiting steps of de-novo lipid synthesis, in the oenocytes. It was found that excess lipids made in the oenocytes do end up in the adipose tissue of the animal leading to increased lipid droplet size in the adipose tissue. Taken together, these results provide evidence for the role of Drosophila oenocytes in lipid synthesis and storage of neutral lipids in the adipose tissue of the animal. Interestingly, the vertebrate liver is also one of the primary sites for de-novo lipid synthesis and lipids synthesized in the liver can be transported to the adipose tissue for the purpose of storage. Hence, the fundamental role of the oenocytes and the mammalian liver converge with respect to their involvement in lipid synthesis (Ghosh, 2020).

Oenocyte-specific loss of the components of the Pi3K/Akt1/TOR signaling pathway was observed to lead to increased lipid synthesis. The increased rate of lipid synthesis in flies lacking TOR signaling in the oenocytes is paradoxical to current knowledge of how TOR signaling affects expression of lipid synthesis genes. In both vertebrates and flies, TOR signaling is known to facilitate lipid synthesis by inducing the expression of key lipid synthesis genes such as acetyl CoA-carboxylase and fatty acid synthase via activation of SREBP-1 proteins. Therefore this study checked how oenocyte-specific loss of TOR signaling affects expression of oenocyte-specific fatty acid synthases (fasn2 and fasn3) and oenocyte non-specific fatty acid synthesis genes (fasn1 and acc). Oenocyte-specific loss of TOR strongly down-regulated only fasn2 and fasn3, while the expression of adipose tissue specific fasn1 and acc did not change, indicating that TOR signaling is required for the expression of lipogenic genes in the oenocytes. An increase in lipid synthesis in response to loss of TOR in the oenocytes is quite intriguing and the mechanism remains to be addressed. The increase in lipid synthesis is thought not to happen in the oenocytes since loss of TOR signaling rather reduces expression of lipogenic genes in the oenocytes. The increase in lipid synthesis could happen either as a result of compensatory upregulation of lipid synthesis in the adipose tissue or due to disruption of an as yet unknown role of the oenocytes in lipid synthesis that hinges on TOR signaling. The fact that the expression levels of fasn1 and acc does not change significantly in animals lacking TOR signaling in the oenocytes indicates that compensatory upregulation of lipid synthesis, if present, does not happen through transcriptional upregulation of lipid synthesis genes in the adipose tissue. It is still possible, however, that the increase in lipid synthesis is caused by post-translational modifications of the enzymes. Alternatively, loss of TOR in the oenocyte could affect tissue distribution of lipids or impair clearing of dietary lipids via formation of cuticular hydrocarbons. Understanding the tissue specific alterations in gene expression and changes in the phosphorylation states of key lipogenic proteins in the adipose tissue of animals lacking TOR signaling in oenocytes could shed more light on the mechanisms involved (Ghosh, 2020).

Interestingly, the data suggests that the Drosophila InR does not play a role in activating TOR signaling in the oenocytes. While loss of TOR signaling in the oenocytes leads to obesity, loss of InR signaling does not. Additionally, loss of oenocyte specific InR signaling did not have any effect on p4EBP levels in oenocytes. Moreover, InR signaling and TOR signaling also diverge in their roles in regulating the size of oenocytes. While loss of InR signaling leads to a significant reduction in the size of oenocytes, loss of TOR does not. Further suggesting that TOR does not act downstream of InR in oenocytes. Rather, the data suggests that in wildtype well-fed flies TOR signaling in oenocytes is activated by the Pvf receptor. Interestingly, insulin dependent activation of TOR is not universal. For instance, in the specialized cells of non-obese mouse liver, InR does not play any role in activation of TOR and downstream activation of SREBP-1c (Ghosh, 2020).

Drosophila larval oenocytes are known to accumulate lipids in response to starvation. It has also been showed that starving adult females for 36 hr is capable of inducing lipid accumulation in the oenocytes and that this response is dependent of InR signaling. Since TOR signaling is a known metabolic regulator, one alternate hypothesis that could explain some of the data is that loss of PvR/TOR signaling leads to a starvation like response specifically in the oenocyte leading to InR-dependent accumulation of lipid droplets. To address this possibility, single nuclei sequencing of the adult male abdominal cuticle (and the tissues residing within) derived from oenots>tsc1,tsc2 flies. The animals were raised under identical experimental conditions as control animals. Then the two snRNA-seq data sets were re-analyzed after correcting for batch effects using harmony. The resulting UMAP plots for both genotypes look similar to the original UMAP plot for the control flies and identifies all the clusters reported (see Differential snRNA-seq of the abdominal cuticle upon oenocyte-specific loss of TOR). The percentage of nuclei that constitute each of the major clusters remained similar in both genotypes and the top marker genes for each of the clusters did not change. The oenocyte-specific gene expression profiles from both data sets were subsequently converted to pseudobulk expression for the genes that were detected. This allowed comparison of the expression profiles of the oenocytes from control animals and animals lacking TOR signaling in oenocytes. The effect of losing TOR on the expression of the 47 genes that had been reported to be up-regulated in oenocytes in response to starvation was specifically looked at. Thirty-six of these genes were detected by single nuclei sequencing analysis, however, none of them changed significantly. Based on this observation, it is concluded that loss of TOR signaling most likely does not mount a starvation like response in the oenocytes (Ghosh, 2020).

Serum levels of VEGF-A is high in obese individuals and drops rapidly in response to bariatric surgery, suggesting a role for VEGF-A in obesity. However, evidence on whether VEGF-A or other VEGFs are deleterious vs beneficial in the context of the pathophysiology of obesity is unclear. Adipose tissue-specific over-expression of both VEGF-B and VEGF-A has been shown to improve adipose tissue vascularization, reduce hypoxia, induce browning of fat, increase thermogenesis, and protect against obesity. At the same time, blocking VEGF-A signaling in the adipose tissue of genetically obese mice leads to reduction of body weight gain, improvement in insulin sensitivity, and a decrease in adipose tissue inflammation. Moreover, systemic inhibition of VEGF-A or VEGF-B signaling by injecting neutralizing monoclonal antibodies have also shown remarkable effects in improving insulin sensitivity in the muscle, adipose tissue, and the liver of high-fat diet-induced mouse models of obesity and diabetes. Although the evidence on the roles of VEGF/PDGF signaling ligands in obesity and insulin resistance is well established, the mechanisms clearly are quite complex and are often context dependent. Consequently, a wider look at various tissue specific roles of PDGF/VEGF signaling will be necessary to comprehensively understand the roles of PDGF/VEGF signaling in lipid metabolism. The current work demonstrates an evolutionarily conserved role for PDGF/VEGF signaling in lipid metabolism and a non-endothelial cell dependent role of the signaling pathway in lipid synthesis. Additionally, these findings suggest an atypical tissue-specific role of TOR signaling in suppressing lipid synthesis at the level of the whole organism. Further studies will be required to determine whether vertebrate VEGF/PDGF and TOR signaling exerts similar roles either in the vertebrate liver or in other specialized organ (Ghosh, 2020).

This study made use of snRNA-Seq technology to identify expression of Pvr precisely in certain tissues in the complex abdominal region, which harbors several metabolically active tissues including adipose tissues, oenocytes, and muscle in Drosophila. As yet, there is no systematic study of a complete transcriptomics resource of each of these tissues considering the difficulty in dissecting and segregating these tissues for downstream sequencing. Thus, this study also provides a rich resource of gene expression profiles, paving way for a systems-level understanding of each of these tissues in Drosophila (Ghosh, 2020).

Glial Hedgehog signalling and lipid metabolism regulate neural stem cell proliferation in Drosophila

The final size and function of the adult central nervous system (CNS) are determined by neuronal lineages generated by neural stem cells (NSCs) in the developing brain. In Drosophila, NSCs called neuroblasts (NBs) reside within a specialised microenvironment called the glial niche. This study explored non-autonomous glial regulation of NB proliferation. Lipid droplets (LDs) which reside within the glial niche were shown to be closely associated with the signalling molecule Hedgehog (Hh). Under physiological conditions, cortex glial Hh is autonomously required to sustain niche chamber formation. Upon FGF-mediated cortex glial overgrowth, glial Hh non-autonomously activates Hh signalling in the NBs, which in turn disrupts NB cell cycle progression and its ability to produce neurons. Glial Hh's ability to signal to NB is further modulated by lipid storage regulator lipid storage droplet-2 (Lsd-2) and de novo lipogenesis gene fatty acid synthase 1 (Fasn1). Together, these data suggest that glial-derived Hh modified by lipid metabolism mechanisms can affect the neighbouring NB's ability to proliferate and produce neurons (Dong, 2021).

Drosophila Snazarus regulates a lipid droplet population at plasma membrane-droplet contacts in adipocytes

Adipocytes store nutrients as lipid droplets (LDs), but how they organize their LD stores to balance lipid uptake, storage, and mobilization remains poorly understood. Using Drosophila fat body (FB) adipocytes, this study characterized spatially distinct LD populations that are maintained by different lipid pools. Peripheral LDs (pLDs) were identified that make close contact with the plasma membrane (PM) and are maintained by lipophorin-dependent lipid trafficking. pLDs are distinct from larger cytoplasmic medial LDs (mLDs), which are maintained by FASN1-dependent de novo lipogenesis. Sorting nexin CG or Snazarus (Snz) associates with pLDs and regulates LD homeostasis at ER-PM contact sites. Loss of Snz perturbs pLD organization, whereas Snz over-expression drives LD expansion, triacylglyceride production, starvation resistance, and lifespan extension through a Desaturase 1-dependent pathway. It is proposed that Drosophila adipocytes maintain spatially distinct LD populations, and Snz is identified as a regulator of LD organization and inter-organelle crosstalk (Ugrankar, 2019).

Life presents energetic and metabolic challenges, and metazoans have developed specialized nutrient-storing organs to maintain energy homeostasis and buffer against the ever-changing availability of dietary nutrients. Drosophila melanogaster is a key model organism to study energy homeostasis as many aspects of mammalian metabolism are conserved in the fly. The major energy-storage organ of insects is the fat body (FB), a central metabolic tissue that exhibits physiological functions analogous to the mammalian adipose tissue and liver including nutrient storage, endocrine secretion, and immune response. Consequently, the FB makes intimate contact with both the gut where dietary nutrients re-absorbed and circulating hemolymph that transports lipids between organs. Drosophila larvae feed continuously to promote an increase in animal mass, and absorb dietary nutrients into the FB to store these as glycogen or triacylglyceride (TAG) that is incorporated into cytoplasmic lipid droplets (LDs). TAG storage ultimately requires LD biogenesis on the surface of the endoplasmic reticulum (ER), the primary site of TAG synthesis (Wilfling, 2014). During development or when nutrients are scarce, FB cells adapt their metabolism to mobilize LDs via cytoplasmic lipases. These mobilized lipids are delivered to other organs in the hemolymph via protein shuttles called lipophorin (Lpp) particles that are analogous to mammalian VLDL particles, but how LD mobilization is related to Lpp particle lipid loading remains poorly understood (Ugrankar, 2019).

The mechanisms that govern lipid flux across the FB cell plasma membrane (PM) also remain poorly characterized, but are essential for lipid export as well as lipid uptake and storage in LDs. In insects, the internalization of hemolymph lipids into both the FB and imaginal discs is unaffected when endocytosis is blocked, suggesting a non-vesicular uptake mechanism. In line with this, Lpp proteins are not degraded via endolysosomal trafficking within the FB, consistent with a model where Lpp particles can donate and receive lipids directly at the FB cell surface. Furthermore, Lpp particles primarily transport diacylglyceride (DAG), suggesting Lpp-derived lipids are processed during their uptake and delivery to the ER by ER-resident acyl CoA:diacylglycerol acyltransferase (DGAT) enzymes, which convert DAG to TAG. In addition to storing extracellular Lpp-derived lipids, FB cells also generate their own lipids via fatty acid de novo lipogenesis. FB cells deficient in fatty acid synthesis (FAS) enzymes exhibit severe lipodystrophy, indicating FB cells somehow balance the storage of Lpp-derived and de novo synthesized lipids to maintain fat homeostasis (Ugrankar, 2019).

Due to their specialized function in lipid uptake and storage, many fat-storing cells exhibit a unique surface architecture: their PM is densely pitted with invaginations that increase the surface area exposed to the extracellular space. In mammals, up to half the surface of white adipocytes is decorated with caveolae, invaginations that organize surface receptors as well as promote lipid and nutrient absorption. Surprisingly, Drosophila do not encode caveolin genes that are required to form caveolae. Nevertheless, Drosophila FB adipocytes exhibit their own intricate networks of surface invaginations that are stabilized by the cortical actin network. Perturbing this cortical actin network disrupts FB lipid homeostasis, suggesting a functional connection between FB surface architecture and lipid storage (Ugrankar, 2019).

Although LDs serve as organelle-scale lipid reservoirs, how cells organize their LD stores to balance storage with efficient mobilization is largely unresolved. An intuitive mechanism to organize LDs is to attach them to other organelles, as this allows them to exchange lipids with these organelles as well as potentially compartmentalize them in distinct regions of the cell interior. Recent work using Saccharomyces cerevisiae reveal that even simple yeast contain functionally distinct LD sub-populations that are spatially compartmentalized. This compartmentalization is achieved by LD-organizing proteins that bind to LDs and cluster them adjacent to the yeast vacuole/lysosome. One such organizing protein is Mdm1, an ER-anchored protein that binds to LDs and attaches them to the vacuole/lysosome surface. Mdm is highly conserved in Drosophila as CG1514/Snazarus (Snz), originally characterized as a longevity-associated gene of unknown function that is highly expressed in the Drosophila FB (Suh, 2008). Both yeast and human Snz homologs bind to LDs and regulate LD homeostasis, but the function of Snz in Drosophila remains unclear (Hariri, 2019; Datta, 2019). This study investigated how FB cells functionally and spatially organize their LD stores. FB cells contain functionally distinct LD populations that are spatially segregated into regions of the cell interior. These LD populations require distinct lipid pools for their maintenance, with LDs in the cell periphery (peripheral LDs, pLDs) requiring Lpp-dependent trafficking, whereas LDs further in the cell interior (medial LDs, mLDs) are maintained by FASN1-dependent de novo lipogenesis within the FB. Snz was also characterized as a novel regulator of pLD homeostasis that localizes to ER-PM contacts and promotes LD growth and TAG storage (Ugrankar, 2019).

Professional fat-storing cells must organize their fat reserves to balance long- term storage with the ability to efficiently mobilize lipids during energetic crises like starvation or metamorphosis. How this organization is achieved is unknown but presents significant spatial and metabolic challenges for the cell. This study reports that Drosophila FB adipocytes contain functionally distinct LD populations that are spatially segregated in the cell cytoplasm. A pLD population is maintained adjacent to the cell surface and makes intimate contact with the PM. pLD size and abundance are altered in response to fasting, suggesting pLDs are mobilized to provide circulating lipids for other larval tissues. Consistent with this, loss of lipophorin (Lpp) particles by ApoLppRNAi impacts pLD abundance and morphology, suggesting pLD maintenance requires some aspect of Lpp lipid trafficking. FB cells also contain a larger mLD population in the cell mid-plane region that is unaffected by loss of Lpp, but is drastically affected by loss of FASN1-mediated de novo lipogenesis in the FB. Remarkably, pLDs were still observed docked on the inner surface of the PM in FASN1RNAi FB tissue, further suggesting that pLDs contain lipids derived from extracellular sources that may be delivered into the FB via Lpp-dependent trafficking. This study also found that pLDs and mLDs are differentially dependent on perilipins, with mLDs relying on LSD for their morphology whereas pLDs require LSD2. Finally, this study has identified Snz as a LD-associated protein that is required for proper LD homeostasis in the FB. Snz localizes to ER-PM contacts in the FB cell periphery, and its over-expression increases TAG storage, consistent with a model whether Snz regulates ER-PM inter-organelle crosstalk that promotes lipid storage in LDs. In line with this, Snz functionally interacts with the ER- resident FA desaturase DESAT1, which is required for Snz-driven TAG accumulation (Ugrankar, 2019).

LDs have long been observed to be tethered to other organelles such as mitochondria and peroxisomes, and this impacts their sub-cellular distribution as well as their ability to exchange lipids with these organelles. Recent studies have identified specific proteins that bind to the surfaces of LDs, and mediate their attachment to other cellular organelles. Among these, yeast Mdm directly binds to nascent LDs, and promotes their attachment to the yeast vacuole. This Mdm1- vacuole interaction is critical for defining this LD positioning, as replacement of Mdm1's vacuole-binding PX domain with a PM-binding domain re-localizes Mdm to ER-PM contact sites and causes LDs to bud instead from the cortical ER adjacent to the PM (Hanaa Hariri, 2019). In addition to its role as an organelle tether, Mdm also positively regulates LD biogenesis by recruiting the fatty acyl-CoA ligase Faa to the ER surface, where it induces the incorporation of FAs into TAG in the LD (Hanaa Hariri, 2019). This study finds that Snz may function similarly to Mdm in both the spatial positioning of LDs, as well as promoting LD biogenesis. Snz localizes to the FB cell periphery and co-localizes with the ER-PM contact site biomarker dMAPPER, suggesting it enriches at regions of close contact between the ER and PM and potentially functions as an inter-organelle tether. Consistent with this, the Snz PX domain binds to liposomes containing phospholipids normally enriched on the PM, and exhibits a non-canonical lipid binding surface that mediates electrostatic interactions with phospholipids that would be present on the PM. This suggests a model where Snz functions in some aspect of functional coupling between the ER and PM, and potentially assists in lipid uptake from the hemolymph. Snz may also help to localize ER-resident lipid processing enzymes such as DESAT to the cell periphery, creating a localized pool of FA processing enzymes in the peripheral ER network which can efficiently process lipids prior to their incorporation into TAG. Consistent with this, Snz over-expression in the FB promotes TAG storage, and Snz can directly associate with LDs (Ugrankar, 2019).

Once thought to be pathogenic, adipocyte fat stores have more recently been proposed to act as metabolic buffers that protect against caloric overload and serve as sinks to reduce circulating lipids and sugars. As such, factors that enhance adipocyte fat storage may be protective against insulin insensitivity, organismal lipotoxicity, and other T2D-like pathologies. The data are consistent with this model, and indicate that Snz up-regulation promotes TAG storage in the FB through a DESAT1-dependent pathway. These elevated TAG stores not only prolong organismal survival during sustained fasting, but also promote organismal homeostasis that extends Drosophila lifespan, as well as buffers the pathological effects of chronic HSD (Ugrankar, 2019).

Collectively, these data support a model where Snz-associated pLDs provide a spatially-compartmentalized sink for lipids derived from the extracellular hemolymph. This pLD population may serve multiple functions. It could allow FB cells to quickly and efficiently process and store incoming Lpp-derived lipids from the hemolymph, thus avoiding their potentially lipotoxic accumulation in the cytoplasm. This would promote general cellular homeostasis and could minimize FA lipotoxicity during elevated lipid uptake. In addition, the high surface-to-volume size ratio of small pLDs may promote their efficient mobilization by cytoplasmic lipases during fasting. Since they are near the surface, pLD mobilization could also allow liberated FAs to be efficiently transferred to Lpp particles docked on the FB surface, where they can be subsequently trafficked to other organs. Snz has clear mammalian homologs including SNX which also bind PM-associated phospholipids. Whether SNX or other Snz homologs are also able to interact with LDs in distinct regions of the mammalian cell interior is unclear, but will be the focus of future studies (Ugrankar, 2019).

The regulation of triglyceride storage by ornithine decarboxylase (Odc1) in Drosophila

Polyamines are low molecular weight, organic cations that play a critical role in many major cellular processes including cell cycle regulation and apoptosis, cellular division, tissue proliferation, and cellular differentiation; however, the functions of polyamines in regulating the storage of metabolic fuels such as triglycerides and glycogen is poorly understood. To address this question, focus was placed on the Drosophila homolog of ornithine decarboxylase (Odc1), the first rate-limiting enzyme in the synthesis of polyamines. Mutants in Odc1 are lethal, but heterozygotes were viable to adulthood. Odc1 heterozygotes appeared larger than their genetic background control flies and consistent with this observation, weighed more than the controls. However, the increased weight was not due to increased food consumption as heterozygotes ate less than the controls. Interestingly, Odc1 heterozygous flies had augmented triglyceride storage, and this lipid phenotype was due to increased triglyceride storage per cell and an increase in the number of fat cells produced. Odc1 heterozygous flies also displayed increased expression of the lipid synthesis genes fatty acid synthase (FASN) and acetyl-CoA carboxylase (ACC), suggesting increased lipid synthesis was the cause of the augmented triglyceride phenotype. These results provide a link between the expression of Odc1 and triglyceride storage suggesting that the polyamine pathway plays a role in regulating lipid metabolism (Leon, 2019).

Ceramide-protein interactions modulate ceramide-associated lipotoxic cardiomyopathy

Lipotoxic cardiomyopathy (LCM) is characterized by abnormal myocardial accumulation of lipids, including ceramide; however, the contribution of ceramide to the etiology of LCM is unclear. This study investigated the association of ceramide metabolism and ceramide-interacting proteins (CIPs) in LCM in the Drosophila heart model. Ceramide feeding or ceramide-elevating genetic manipulations are strongly associated with cardiac dilation and defects in contractility. High ceramide-associated LCM is prevented by inhibiting ceramide synthesis, establishing a robust model of direct ceramide-associated LCM, corroborating previous indirect evidence in mammals. Several CIPs were identified from mouse heart and Drosophila extracts, including caspase activator Annexin-X, myosin chaperone Unc-45, and lipogenic enzyme FASN1, and remarkably, their cardiac-specific manipulation can prevent LCM. Collectively, these data suggest that high ceramide-associated lipotoxicity is mediated, in part, through altering caspase activation, sarcomeric maintenance, and lipogenesis, thus providing evidence for conserved mechanisms in LCM pathogenesis in mammals (Walls, 2018).

Fatty acid synthase cooperates with Glyoxalase 1 to protect against sugar toxicity

Fatty acid (FA) metabolism is deregulated in several human diseases including metabolic syndrome, type 2 diabetes and cancers. Therefore, FA-metabolic enzymes are potential targets for drug therapy, although the consequence of these treatments must be precisely evaluated at the organismal and cellular levels. In healthy organism, synthesis of triacylglycerols (TAGs)-composed of three FA units esterified to a glycerol backbone-is increased in response to dietary sugar. Saturation in the storage and synthesis capacity of TAGs is associated with type 2 diabetes progression. Sugar toxicity likely depends on advanced-glycation-end-products (AGEs) that form through covalent bounding between amine groups and carbonyl groups of sugar or their derivatives alpha-oxoaldehydes. Methylglyoxal (MG) is a highly reactive alpha-oxoaldehyde that is derived from glycolysis through a non-enzymatic reaction. Glyoxalase 1 (Glo1; see Drosophila Glo1 works to neutralize MG, reducing its deleterious effects. This study used the power of Drosophila genetics to generate Fatty acid synthase (FASN) mutants, allowing an investigation of the consequence of this deficiency upon sugar-supplemented diets. FASN mutants were found to be lethal but can be rescued by an appropriate lipid diet. Rescued animals do not exhibit insulin resistance, are dramatically sensitive to dietary sugar and accumulate AGEs. FASN and Glo1 cooperate at systemic and cell-autonomous levels to protect against sugar toxicity. It was observed that the size of FASN mutant cells decreases as dietary sucrose increases. Genetic interactions at the cell-autonomous level, where glycolytic enzymes or Glo1 were manipulated in FASN mutant cells, revealed that this sugar-dependent size reduction is a direct consequence of MG-derived-AGE accumulation. In summary, these findings indicate that FASN is dispensable for cell growth if extracellular lipids are available. In contrast, FA-synthesis appears to be required to limit a cell-autonomous accumulation of MG-derived-AGEs, supporting the notion that MG is the most deleterious alpha-oxoaldehyde at the intracellular level (Garrido, 2015).

Proteolytic activation of fatty acid synthase signals pan-stress resolution

Chronic stress and inflammation are both outcomes and major drivers of many human diseases. Sustained responsiveness despite mitigation suggests a failure to sense resolution of the stressor. This study shows that a proteolytic cleavage event of fatty acid synthase (FASN) activates a global cue for stress resolution in Caenorhabditis elegans. FASN is well established for biosynthesis of the fatty acid palmitate. The results demonstrate FASN promoting an anti-inflammatory profile apart from palmitate synthesis. Redox-dependent proteolysis of limited amounts of FASN by caspase activates a C-terminal fragment sufficient to downregulate multiple aspects of stress responsiveness, including gene expression, metabolic programs and lipid droplets. The FASN C-terminal fragment signals stress resolution in a cell non-autonomous manner. Consistent with these findings, FASN processing is also seen in well-fed but not fasted male mouse liver. As downregulation of stress responses is critical to health, these findings provide a potential pathway to control diverse aspects of stress responses (Wei, 2024).

Reduced food intake and body weight in mice treated with fatty acid synthase inhibitors

With the escalation of obesity-related disease, there is great interest in defining the mechanisms that control appetite and body weight. This study has identified a link between anabolic energy metabolism and appetite control. Both systemic and intracerebroventricular treatment of mice with fatty acid synthase (FAS) inhibitors (cerulenin and a synthetic compound C75) led to inhibition of feeding and dramatic weight loss. C75 inhibited expression of the prophagic signal neuropeptide Y in the hypothalamus and acted in a leptin-independent manner that appears to be mediated by malonyl-coenzyme A. Thus, FAS may represent an important link in feeding regulation and may be a potential therapeutic target (Loftus, 2020).

Fatty acid synthesis configures the plasma membrane for inflammation in diabetes

Dietary fat promotes pathological insulin resistance through chronic inflammation. The inactivation of inflammatory proteins produced by macrophages improves diet-induced diabetes, but how nutrient-dense diets induce diabetes is unknown. Membrane lipids affect the innate immune response, which requires domains that influence high-fat-diet-induced chronic inflammation and alter cell function based on phospholipid composition. Endogenous fatty acid synthesis, mediated by fatty acid synthase (FAS), affects membrane composition. This study shows that macrophage FAS is indispensable for diet-induced inflammation. Deleting Fasn in macrophages prevents diet-induced insulin resistance, recruitment of macrophages to adipose tissue and chronic inflammation in mice. FAS deficiency was found to alters membrane order and composition, impairing the retention of plasma membrane cholesterol and disrupting Rho GTPase trafficking-a process required for cell adhesion, migration and activation. Expression of a constitutively active Rho GTPase, however, restored inflammatory signalling. Exogenous palmitate was partitioned to different pools from endogenous lipids and did not rescue inflammatory signalling. However, exogenous cholesterol, as well as other planar sterols, did rescue signalling, with cholesterol restoring FAS-induced perturbations in membrane order. The results show that the production of endogenous fat in macrophages is necessary for the development of exogenous-fat-induced insulin resistance through the creation of a receptive environment at the plasma membrane for the assembly of cholesterol-dependent signalling networks (Wei, 2016).

Search PubMed for articles about Drosophila Fatty acid synthase

Akhmetova, K., Balasov, M. and Chesnokov, I. (2021). Drosophila STING protein has a role in lipid metabolism. Elife 10. PubMed ID: 34467853

Devilliers, M., Garrido, D., Poidevin, M., Rubin, T., Le Rouzic, A. and Montagne, J. (2021). Differential metabolic sensitivity of insulin-like-response- and TORC1-dependent overgrowth in Drosophila fat cells. Genetics 217(1): 1-12. PubMed ID: 33683355

Dong, Q., Zavortink, M., Froldi, F., Golenkina, S., Lam, T. and Cheng, L. Y. (2021). Glial Hedgehog signalling and lipid metabolism regulate neural stem cell proliferation in Drosophila. EMBO Rep 22(5): e52130. PubMed ID: 33751817

Garrido, D., Rubin, T., Poidevin, M., Maroni, B., Le Rouzic, A., Parvy, J. P., Montagne, J. (2015). Fatty acid synthase cooperates with glyoxalase 1 to protect against sugar toxicity. PLoS Genet, 11(2):e1004995 PubMed ID: 25692475

Ghosh, A. C., Tattikota, S. G., Liu, Y., Comjean, A., Hu, Y., Barrera, V., Ho Sui, S. J. and Perrimon, N. (2020). Drosophila PDGF/VEGF signaling from muscles to hepatocyte-like cells protects against obesity. Elife 9. PubMed ID: 33107824

Leon, K. E., Fruin, A. M., Nowotarski, S. L. and DiAngelo, J. R. (2019). The regulation of triglyceride storage by ornithine decarboxylase (Odc1) in Drosophila. Biochem Biophys Res Commun. PubMed ID: 31870547

Loftus, T. M., Jaworsky, D. E., Frehywot, G. L., Townsend, C. A., Ronnett, G. V., Lane, M. D., Kuhajda, F. P. (2000). Reduced food intake and body weight in mice treated with fatty acid synthase inhibitors. Science, 288(5475):2379-2381 PubMed ID: 10875926

Miao, T., Kim, J., Kang, P., Fujiwara, H., Hsu, F. F. and Bai, H. (2022). Acetyl-CoA-mediated autoacetylation of fatty acid synthase as a metabolic switch of de novo lipogenesis in Drosophila. Proc Natl Acad Sci U S A 119(49): e2212220119. PubMed ID: 36459649

Ugrankar, R., Bowerman, J., Hariri, H., Chandra, M., Chen, K., Bossanyi, M. F., Datta, S., Rogers, S., Eckert, K. M., Vale, G., Victoria, A., Fresquez, J., McDonald, J. G., Jean, S., Collins, B. M. and Henne, W. M. (2019). Drosophila Snazarus regulates a lipid droplet population at plasma membrane-droplet contacts in adipocytes. Dev Cell 50(5): 557-572 e555. PubMed ID: 31422916

Walls, S. M., Cammarato, A., Chatfield, D. A., Ocorr, K., Harris, G. L. and Bodmer, R. (2018). Ceramide-protein interactions modulate ceramide-associated lipotoxic cardiomyopathy. Cell Rep 22(10): 2702-2715. PubMed ID: 29514098

Wang, X., Li, J., Zhang, W., Wang, F., Wu, Y., Guo, Y., Wang, D., Yu, X., Li, A., Li, F. and Xie, Y. (2023). IGFBP-3 promotes cachexia-associated lipid loss by suppressing insulin-like growth factor/insulin signaling. Chin Med J (Engl). PubMed ID: 37014770

Walls, S. M., Cammarato, A., Chatfield, D. A., Ocorr, K., Harris, G. L. and Bodmer, R. (2018). Ceramide-protein interactions modulate ceramide-associated lipotoxic cardiomyopathy. Cell Rep 22(10): 2702-2715. PubMed ID: 29514098

Wei, H., Weaver, Y. M., Yang, C., Zhang, Y., Hu, G., Karner, C. M., Sieber, M., DeBerardinis, R. J., Weaver, B. P. (2024). Proteolytic activation of fatty acid synthase signals pan-stress resolution. Nature metabolism, 6(1):113-126 PubMed ID: 38167727

Wei, X., Song, H., Yin, L., Rizzo, M. G., Sidhu, R., Covey, D. F., Ory, D. S., Semenkovich, C. F. (2016). Fatty acid synthesis configures the plasma membrane for inflammation in diabetes. Nature, 539(7628):294-298 PubMed ID: 27806377

Xu, M., Ding, L., Liang, J., Yang, X., Liu, Y., Wang, Y., Ding, M. and Huang, X. (2021). NAD kinase sustains lipogenesis and mitochondrial metabolism through fatty acid synthesis. Cell Rep 37(13): 110157. PubMed ID: 34965438

date revised: 8 May 2024

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