The molecular mechanisms influencing healthspan are unclear but mitochondrial function, resistance to oxidative stress and proteostasis are recurring themes. Tumor necrosis factor Receptor Associated Protein 1 (TRAP1), the mitochondrial analog of Hsp75, regulates levels of reactive oxygen species in vitro and is found expressed at higher levels in tumor cells where it is thought to play a pro-survival role. While TRAP1-directed compartmentalized protein folding is a promising target for cancer therapy, its role at the organismal level is unclear. This study reports that overexpression of TRAP1 in Drosophila extends healthspan by enhancing stress resistance, locomotor activity and fertility while depletion of TRAP1 has the opposite effect, with little effect on lifespan under both conditions. In addition, modulating TRAP1 expression promotes the nuclear translocation of homeobox protein Dve and increases expression of genes associated with the mitochondrial unfolded protein response (UPRmt), indicating an activation of this proteostasis pathway. Notably, independent genetic knockdown of components of the UPRmt pathway dampen the enhanced stress resistance observed in TRAP1 overexpression flies. Together these studies suggest that TRAP1 regulates healthspan, potentially through activation of the UPRmt (Baqri, 2014).

Age related impairment of locomotor, reproductive, and physiological functions is a universal phenomenon in animals. Intriguingly, several of the lifespan extension animal models are not healthy through their life. In worms, mitochondrial mutants such as isp-1 and nuo-2 live long, but are slow growing and have a reduced brood size. Similarly, in long lived dwarf mice with deficiencies in growth hormone, prolactin and thyroid stimulating hormone, reproductive fitness is compromised. Finally, Drosophila treated with the anti-convulsive agent Lamotrigine display significantly enhanced lifespan but poor locomotor performance at older ages. An emerging goal in aging research is developing interventions that not only increase longevity, but also increase the healthspan (Baqri, 2014).

The mechanisms that modulate healthy aging are unclear, but mitochondria are thought to play a central role. Mitochondria generate high levels of reactive oxygen species (ROS) that damage cellular macromolecules and gradually impair cellular function. Some evidence suggests that accumulation of ROS contributes to the physiological decline associated with age, though this theory is not universally accepted. Mice that overexpress catalase in their mitochondria have an enhanced lifespan, delayed age-associated pathologies, reduced mtDNA deletions with age and reduced levels of ROS. Knockdown of components of the electron transport chain (ETC) not only decrease ATP production but surprisingly also increase lifespan. Furthermore, overexpressing mitochondrial LON protease in the fungal aging model Podospora anserina results in enhanced healthspan, suggesting that mitochondrial proteostasis is an important regulator of organismal health (Baqri, 2014).

Mitochondrial function is tightly associated with energy sensing and stress response pathways that contribute to aging, in particular the mTOR and AMPK pathways. These pathways are also associated with oxidative stress resistance and regulation of cellular proteostasis. Compartments such as the endoplasmic reticulum and mitochondria are especially vulnerable to diminished proteostasis because of their high intrinsic protein folding requirements. To regulate protein quality control, these organelles have dedicated stress response mechanisms called the unfolded protein response (UPR) (Ron, 2007 and Haynes, 2010). Accordingly, it has been hypothesized (Powers, 2009) that enhanced proteome maintenance would push the limits of the minimal proteostasis boundary (folding energetics necessary for folding of a protein at a defined proteostasis network capacity) towards health (Baqri, 2014 and references therein).

Mitochondrial chaperone Tumor necrosis factor Receptor Associated Protein 1 (TRAP1) is a member of the Hsp90 family and is the mitochondrial analog of Hsp75 (Felts, 2000). TRAP1, originally identified as a binding partner of the intracellular domain of TNFR1, also binds retinoblastoma protein during mitosis, and the tumor suppressors EXT1 and EXT2 (Altieri, 2012). Despite the high homology with Hsp90, TRAP1 does not interact with its traditional client proteins (Kang, 2007), indicating a distinct functional role for this mitochondrial chaperone. In vitro evidence suggests that TRAP1 regulates ROS levels and protects cells against oxidative damage (Hua, 2007). Furthermore, TRAP1 was found expressed at higher levels in tumor cells where it is thought to play a pro-survival role by inhibiting apoptosis (Kang, 2007; Costantino, 2009). Recent evidence also implicates TRAP1 in Parkinson's disease. PINK1 mediated phosphorylation of TRAP1 is considered important in suppressing cytochrome c release from mitochondria to regulate apoptosis (Pridgeon, 2007). TRAP1 also ameliorates cellular toxicity induced by α-synuclein in dopaminergic neurons (Butler, 2012) and expression of TRAP1 was able to rescue mitochondrial dysfunction in Pink1 deficient flies (Zhang, 2013; Costa, 2013). In summary, TRAP1 lies at an exciting intersection between ROS, aging, mitochondrial proteostasis and human disease (Baqri, 2014).

To determine whether modulating mitochondrial proteostasis would influence longevity and healthspan in vivo, the effect was studied of modulating TRAP1 expression levels in Drosophila. This study demonstrates that TRAP1 regulates ROS levels, oxidative stress resistance and heat stress resistance in Drosophila, without significantly altering lifespan. Intriguingly, there is robust sex-specificity in TRAP1-mediated stress resistance. In addition, ubiquitous overexpression of TRAP1 enhances fertility and remarkably improves locomotor ability in aging flies. In contrast, loss of TRAP1 results in decreased fertility and locomotor ability, indicating strong modulation of healthspan by TRAP1. Finally, it was shown that dosage modulation of TRAP1 activates the mitochondrial UPR (UPRmt) in males but not the females, indicative of similar sex-specificity as observed in stress resistance. Finally, impairment of the UPRmt inhibits TRAP1 mediated resistance to stress. These findings suggest that TRAP1 mediated alterations in UPRmt can modulate stress resistance and healthspan (Baqri, 2014).

This study finds that dosage modulation of the mitochondrial chaperone TRAP1 regulates oxidative stress and heat stress resistance in males and modulates healthspan of both sexes, yet has only a negligible effect on lifespan. To gain a better understanding of the mechanism underlying these changes, the effect was examined of TRAP1 dosage on the UPRmt. Modulation of TRAP1 expression induces the nuclear translocation of transcription factor Dve and drives expression of Hsp60, mtHsp70 and a putative protease, CG5045. These results provide evidence for the presence of the UPRmt in Drosophila, confirming the conservation of this stress response pathway across phylogeny. Furthermore, dampening of TRAP1 mediated stress response upon impairment of the UPRmt indicates that alterations in mitochondrial proteostasis can influence stress resistance and healthspan (Baqri, 2014).

Recently published studies have also indicated a similar role of TRAP1 in regulating stress resistance in flies (Costa, 2013; Zhang, 2013). However, this study further demonstrates that dosage modulation of TRAP1 has remarkable sex-specific influences and is correlated with the sex-specific activation the UPRmt. Sex specificity has been reported previously in stress survival studies involving mitochondrial proteins (Magwere, 2006; Mourikis, 2006). One possible explanation is that mitochondria in female Drosophila have dissimilar bioenergetics from males, with higher oxygen consumption, higher hydrogen peroxide production and lower levels of catalase, along with higher mtDNA copy number (Yin, 2004; Ballard, 2007). In oxidative stress and heat stress survival assays, TRAP1 mutant and overexpression animals displayed considerable sex-specificity that correlated directly with the expression of mtHsp70 and CG5045, but not Hsp60 in males and females. These findings suggest that sex-specific differences in activation of proteostasis pathways underlie sex dependent differences in resistance to stress (Baqri, 2014).

The finding that both loss and overexpression of TRAP1 activate the UPRmt is indicative of the complex regulation of this response system. Because TRAP1 is a chaperone, induction of the UPRmt in TRAP1 mutants is likely a compensatory response to protect against excess unfolded proteins. While this response pathway may allow the TRAP1 mutants to have a near normal lifespan, the absence of TRAP1 function has obvious negative consequences on fitness. In contrast, the induction of the UPRmt in TRAP1 overexpressing flies could occur by two non-exclusive mechanisms. One possibility is that the mild stress of over-abundant TRAP1 in the mitochondrial matrix induces the UPRmt. Alternatively, given the potential role of TRAP1 in multiple signaling pathways involving Rb, myc, TNF, cyclophilin D (Kang, 2007; Altieri, 2012) overexpression of TRAP1 may lead to efficient protein folding, direct suppression of apoptosis, and the coordinated induction of other UPR genes through a TRAP1 mediated stress response. In the context of a minimal proteostasis boundary (Powers, 2009), either an increase in TRAP1 activity or the indirect induction of UPRmt would result in enhanced protein quality control in TRAP1 overexpression flies and would shift the proteostasis boundary towards health. Altogether, the results suggest that because of the critical roles played by TRAP1 in regulating several cellular processes, it is important that TRAP1 expression itself is finely regulated (Baqri, 2014).

The role of TRAP1 in activation of the UPRmt has broad clinical relevance. TRAP1 is reportedly expressed at higher levels in cancer cells where it has been suggested to play a pro-survival role by inhibiting apoptosis (Kang, 2007, Costantino, 2009; Leav, 2010). Hsp90 molecules, including TRAP1, antagonize the cyclophilin D-dependent mitochondrial permeability transition, and this cytoprotective pathway is a potential target in cancer therapy (Kang, 2007). The results demonstrate that complete loss of function of TRAP1 has little effect on organismal lifespan and there is no discernible delay in development. While caution must be used when speculating how results in model organisms translate to therapeutics in humans, these results suggest that aggressive disruption of TRAP1 in the context of cancer treatment may provide a means to sensitize tumors to chemotherapy without significantly reducing the lifespan of patients (Baqri, 2014).

In addition, this study has shown that overexpression of TRAP1, ubiquitously or in the nervous system, extends healthspan in both sexes. These results suggest that augmentation of TRAP1 expression may help offset normal age-related decline of physiological and motor capacity, as well as the pathological decline associated with neurological disorders (Baqri, 2014).

TRAP1 (tumor necrosis factor receptor-associated protein 1), a mitochondrial Hsp90 family chaperone, has been identified as a critical regulator of cell survival and bioenergetics in tumor cells. To discover novel signaling networks regulated by TRAP1, Drosophila TRAP1 mutants were generated. The mutants successfully developed into adults and produced fertile progeny, showing that TRAP1 is dispensable in development and reproduction. Surprisingly, mutation or knockdown of TRAP1 markedly enhanced Drosophila survival under oxidative stress. Moreover, TRAP1 mutation ameliorated mitochondrial dysfunction and dopaminergic (DA) neuron loss induced by deletion of a familial Parkinson disease gene PINK1 (Pten-induced kinase 1) in Drosophila. Gamitrinib-triphenylphosphonium, a mitochondria-targeted Hsp90 inhibitor that increases cell death in HeLa and MCF7 cells, consistently inhibited cell death induced by oxidative stress and mitochondrial dysfunction induced by PINK1 mutation in mouse embryonic fibroblast cells and DA cell models such as SH-SY5Y and SN4741 cells. Additionally, gamitrinib-triphenylphosphonium also suppressed the defective locomotive activity and DA neuron loss in Drosophila PINK1 null mutants. In further genetic analyses, enhanced expression of Thor, a downstream target gene of transcription factor FOXO, was shown in TRAP1 mutants. Furthermore, deletion of FOXO almost nullified the protective roles of TRAP1 mutation against oxidative stress and PINK1 mutation. These results strongly suggest that inhibition of the mitochondrial chaperone TRAP1 generates a retrograde cell protective signal from mitochondria to the nucleus in a FOXO-dependent manner (Kim, 2016).

This study generated and characterized Drosophila TRAP1 mutants. TRAP1 mutants successfully developed into adults and showed no significant defects in their life span. Although TRAP1 has been regarded as a mitochondrial protective protein, no meaningful defects were observed in mitochondrial morphology, ATP level, and mtDNA content in 3- or 30-day-old TRAP1 mutants. However, under treatment of free radical inducers, such as rotenone or paraquat, loss of TRAP1 significantly increased survival rate. Moreover, TRAP1 mutation ameliorated oxidative stress sensitivity, mitochondrial dysfunction, and DA neuronal loss in Drosophila PINK1 null mutants (Figs. 3 and 4). Consistent with these fruit fly data, TRAP1 KO mice showed reduced age-associated tissue degeneration with activated oxidative chain complex activities (Kim, 2016).

These genetic data were fully supported by the following pharmacological analyses. G-TPP inhibited cell death and restored the decreased mitochondrial membrane potential in various paraquat-treated mammalian cells, such as MEFs and DA neuron model cell lines. Moreover, G-TPP treatment ameliorated decreased motor activity and DA neuron degeneration in Drosophila PINK1 null mutants and rescued mitochondrial dysfunction in PINK1 null MEFs. Overall, genetic and pharmacological data clearly demonstrated that TRAP1 inhibition can induce resistance against oxidative stress and rescue PINK1 null defects in both Drosophila and mammalian systems (Kim, 2016).

These results raised important questions: How does TRAP1 suppression induce oxidative stress resistance although it increases ROS levels? What is the molecular mechanism underlying the cell protection induced by TRAP1 inhibition? ROS has been regarded as detrimental to many biological processes. However, recent reports showed that ROS can activate beneficial signals especially from mitochondria. When Schulz (2007) restricted glucose availability in Caenorhabditis elegans, life span extension and oxidative stress resistance accompanied by increased ROS production. Pretreatment of anti-oxidants, such as NAC, inhibited elevated expression of cell protective enzymes in glucose-restricted worms and subsequently blocked the extension of life span and the resistance against oxidative stress. Consistently, in TRAP mutants, NAC treatment suppressed the enhancement in survival on paraquat-containing media, suggesting that ROS generated by TRAP1 mutation is not detrimental but beneficial as shown in previous studies. Then what makes ROS beneficial? Types of ROS from mitochondria are observed in long-lived C. elegans mutants. Mitochondrial superoxide, which was also detected in dihydroethidium staining of TRAP1 mutants, was critical to the life span extension induced by several mitochondrial protein mutations. In other analyses, low doses of paraquat, which generate various types of ROS from mitochondria, successfully prolonged life span, whereas higher concentrations shortened it. These results suggest that certain types or amounts of ROS are critical to its beneficial roles and indicate that TRAP1 down-regulation potentially induces appropriate types or amounts of ROS for cell protection (Kim, 2016).

In genetic analyses to find a molecular link between TRAP1 inhibition and cell protection, FOXO loss of function nullified the oxidative stress resistance induced by TRAP1 mutation or TRAP1 inhibition. Consistently, loss of function mutations of FOXO reaggravated the rescued phenotypes of PINK1 null mutants by TRAP1 mutation or G-TPP treatment and also suppressed TRAP1 mutation-induced gene expression of Thor, a FOXO target gene that has a critical role in mitochondrial protection and oxidative stress resistance. It was also observed that TRAP1 inhibition requires FOXO transcription factors to induce cell protection against oxidative stress in mammalian cells. These data consistently demonstrated that FOXO transcription factors mediate cell protection and survival signal induced by TRAP1 inhibition. Moreover, NAC suppressed the enhanced Thor expression and the resistance against oxidative stress in TRAP1 mutants, suggesting that ROS generated by TRAP1-inhibited mitochondria induces FOXO-mediated gene expression to protect cells and animals from oxidative stress and PINK1 mutation (Kim, 2016).

It was observed that G-TPP successfully protects various mammalian cells, such as NIH 3T3, MEF, SH-SY5Y, and SN4741, from oxidative stress. Contrarily, it also potentiated oxidative stress-induced cell toxicity in HeLa and MCF7 cells that are very sensitive to TRAP1 inhibitors. In biochemical analyses, G-TPP caused toxicity in cells with elevated expression of TRAP1, whereas G-TPP protects cells expressing TRAP1 at relatively low levels. These results raise a possibility that TRAP1 expression level reflects different cellular contexts such as amounts of stress on mitochondria. In cells under heavy mitochondrial stress, TRAP1 is overexpressed to protect mitochondria. In this case, TRAP1 inhibition by G-TPP treatment abruptly stops mitochondrial protection mechanisms and subsequently induces cell death. However, it is possible that, in cells not mainly dependent on TRAP1-mediated protection, TRAP1 is weakly expressed, and its inhibition can generate weak and beneficial mitochondrial stress to induce cell protective signals. Testing this hypothesis and finding the molecular mechanism underlying the correlation between TRAP1 expression levels and the sensitivity to G-TPP will be future topics (Kim, 2016).

This report has shown that genetic and pharmacological inhibition of TRAP1 protects cells from oxidative stress and mitochondrial dysfunction. Furthermore, they can generate a compensatory retrograde signal from mitochondria, also known as mitohormesis, to up-regulate cell protective gene expression. These unexpected results raise the possibility that TRAP1 inhibitors developed for anti-cancer therapy might be used to treat human pathology induced by mitochondrial disorders, including PD (Kim, 2016).

Mitochondrial dysfunction caused by protein aggregation has been shown to have an important role in neurological diseases, such as Parkinson's disease (PD). Mitochondria have evolved at least two levels of defence mechanisms that ensure their integrity and the viability of their host cell. First, molecular quality control, through the upregulation of mitochondrial chaperones and proteases, guarantees the clearance of damaged proteins. Second, organellar quality control ensures the clearance of defective mitochondria through their selective autophagy. Studies in Drosophila have highlighted mitochondrial dysfunction linked with the loss of the PTEN-induced putative kinase 1 (PINK1) as a mechanism of PD pathogenesis. The mitochondrial chaperone TNF receptor-associated protein 1 (TRAP1) was recently reported to be a cellular substrate for the PINK1 kinase. This study characterised Drosophila Trap1 null mutants and describes the genetic analysis of Trap1 function with Pink1 and parkin. Loss of Trap1 results in a decrease in mitochondrial function and increased sensitivity to stress, and its upregulation in neurons of Pink1 mutant rescues mitochondrial impairment. Additionally, the expression of Trap1 was able to partially rescue mitochondrial impairment in parkin mutant flies; and conversely, expression of parkin rescued mitochondrial impairment in Trap1 mutants. It is concluded that Trap1 works downstream of Pink1 and in parallel with parkin in Drosophila, and that enhancing its function may ameliorate mitochondrial dysfunction and rescue neurodegeneration in PD (Costa, 2013).

Overexpression or mutation of alpha-Synuclein is associated with protein aggregation and interferes with a number of cellular processes, including mitochondrial integrity and function. This study used a whole-genome screen in the fruit fly Drosophila melanogaster to search for novel genetic modifiers of human [A53T]alpha-Synuclein-induced neurotoxicity. Decreased expression of the mitochondrial chaperone protein tumor necrosis factor receptor associated protein-1 (TRAP1) was found to enhance age-dependent loss of fly head dopamine (DA) and DA neuron number resulting from [A53T]alpha-Synuclein expression. In addition, decreased TRAP1 expression in [A53T]alpha-Synuclein-expressing flies resulted in enhanced loss of climbing ability and sensitivity to oxidative stress. Overexpression of human TRAP1 was able to rescue these phenotypes. Similarly, human TRAP1 overexpression in rat primary cortical neurons rescued [A53T]alpha-Synuclein-induced sensitivity to rotenone treatment. In human (non)neuronal cell lines, small interfering RNA directed against TRAP1 enhanced [A53T]alpha-Synuclein-induced sensitivity to oxidative stress treatment. [A53T]alpha-Synuclein directly interfered with mitochondrial function, as its expression reduced Complex I activity in HEK293 cells. These effects were blocked by TRAP1 overexpression. Moreover, TRAP1 was able to prevent alteration in mitochondrial morphology caused by [A53T]alpha-Synuclein overexpression in human SH-SY5Y cells. These results indicate that [A53T]alpha-Synuclein toxicity is intimately connected to mitochondrial dysfunction and that toxicity reduction in fly and rat primary neurons and human cell lines can be achieved using overexpression of the mitochondrial chaperone TRAP1. Interestingly, TRAP1 has previously been shown to be phosphorylated by the serine/threonine kinase PINK1, thus providing a potential link of PINK1 via TRAP1 to alpha-Synuclein (Butler, 2012).

TRAP1 is a component of a pro-survival mitochondrial pathway up-regulated in tumor cells. The evaluation of TRAP1 expression in 26 human colorectal carcinomas showed up-regulation in 17/26 tumors. Accordingly, TRAP1 levels were increased in HT-29 colorectal carcinoma cells resistant to 5-fluorouracil, oxaliplatin and irinotecan. Thus, this study investigated the role of TRAP1 in multi-drug resistance in human colorectal cancer. Interestingly, TRAP1 overexpression leads to 5-fluorouracil-, oxaliplatin- and irinotecan-resistant phenotypes in different neoplastic cells. Conversely, the inhibition of TRAP1 activity by TRAP1 ATPase antagonist, shepherdin, increased the sensitivity to oxaliplatin and irinotecan in colorectal carcinoma cells resistant to the single agents. These results suggest that the increased expression of TRAP1 could be part of a pro-survival pathway responsible for multi-drug resistance (Costantino, 2009).

PTEN-induced kinase 1 (PINK1) is a serine/threonine kinase that is localized to mitochondria. It protects cells from oxidative stress by suppressing mitochondrial cytochrome c release, thereby preventing cell death. Mutations in Pink1 cause early-onset Parkinson's disease (PD). Consistently, mitochondrial function is impaired in Pink1-linked PD patients and model systems. Previously, in vitro analysis implied that the protective effects of PINK1 depend on phosphorylation of the downstream factor, TNF receptor-associated protein 1 (TRAP1). Furthermore, TRAP1 has been shown to mitigate alpha-Synuclein-induced toxicity, linking alpha-Synuclein directly to mitochondrial dysfunction. These data suggest that TRAP1 seems to mediate protective effects on mitochondrial function in pathways that are affected in PD. This study investigated the potential of TRAP1 to rescue dysfunction induced by either PINK1 or Parkin deficiency in vivo and in vitro. Overexpression of human TRAP1 is able to mitigate Pink1 but not parkin loss-of-function phenotypes in Drosophila. In addition, detrimental effects observed after RNAi-mediated silencing of complex I subunits were rescued by TRAP1 in Drosophila. Moreover, TRAP1 was able to rescue mitochondrial fragmentation and dysfunction upon siRNA-induced silencing of Pink1 but not parkin in human neuronal SH-SY5Y cells. Thus, these data suggest a functional role of TRAP1 in maintaining mitochondrial integrity downstream of PINK1 and complex I deficits but parallel to or upstream of Parkin (Zhang, 2013).

Molecular chaperones of the heat shock protein-90 (Hsp90) family promote cell survival, but the molecular requirements of this pathway in tumor progression are not understood. This study shows that a mitochondria-localized Hsp90 chaperone, tumor necrosis factor receptor-associated protein-1 (TRAP-1), is abundantly and ubiquitously expressed in human high-grade prostatic intraepithelial neoplasia, Gleason grades 3 through 5 prostatic adenocarcinomas, and metastatic prostate cancer, but largely undetectable in normal prostate or benign prostatic hyperplasia in vivo. Prostate lesions formed in genetic models of the disease, including the transgenic adenocarcinoma of the mouse prostate and mice carrying prostate-specific deletion of the phosphatase tensin homolog tumor suppressor [Pten(pc-/-)]), also exhibit high levels of TRAP-1. Expression of TRAP-1 in nontransformed prostatic epithelial BPH-1 cells inhibited cell death, whereas silencing of TRAP-1 in androgen-independent PC3 or DU145 prostate cancer cells by small interfering RNA enhanced apoptosis. Targeting TRAP-1 with a novel class of mitochondria-directed Hsp90 inhibitors (i.e., Gamitrinibs) caused rapid and complete killing of androgen-dependent or -independent prostate cancer, but not BPH-1 cells, whereas reintroduction of TRAP-1 in BPH-1 cells conferred sensitivity to Gamitrinib-induced cell death. These data identify TRAP-1 as a novel mitochondrial survival factor differentially expressed in localized and metastatic prostate cancer compared with normal prostate. Targeting this pathway with Gamitrinibs could be explored as novel molecular therapy in patients with advanced prostate cancer (Leav, 2010).

Mutations in the PTEN induced putative kinase 1 (PINK1) gene cause an autosomal recessive form of Parkinson disease (PD). So far, no substrates of PINK1 have been reported, and the mechanism by which PINK1 mutations lead to neurodegeneration is unknown. This study reports the identification of TNF receptor-associated protein 1 (TRAP1), a mitochondrial molecular chaperone also known as heat shock protein 75 (Hsp75), as a cellular substrate for PINK1 kinase. PINK1 binds and colocalizes with TRAP1 in the mitochondria and phosphorylates TRAP1 both in vitro and in vivo. PINK1 protects against oxidative-stress-induced cell death by suppressing cytochrome c release from mitochondria, and this protective action of PINK1 depends on its kinase activity to phosphorylate TRAP1. Moreover, it was found that the ability of PINK1 to promote TRAP1 phosphorylation and cell survival is impaired by PD-linked PINK1 G309D, L347P, and W437X mutations. These findings suggest a novel pathway by which PINK1 phosphorylates downstream effector TRAP1 to prevent oxidative-stress-induced apoptosis and implicate the dysregulation of this mitochondrial pathway in PD pathogenesis (Pridgeon, 2007).

Natural killer (NK) cells play an important role in innate immunity against virally infected or transformed cells as the first defense line. Granzyme M (GzmM) is an orphan granzyme that is constitutively highly expressed in NK cells and is consistent with NK cell-mediated cytolysis. It was recently demonstrated that GzmM induces caspase-dependent apoptosis with DNA fragmentation through direct cleavage of inhibitor of caspase-activated DNase (ICAD). However, the molecular mechanisms for GzmM-induced apoptosis are unclear. GzmM was found to cause mitochondrial swelling and loss of mitochondrial transmembrane potential. Moreover, GzmM initiates reactive oxygen species (ROS) generation and cytochrome c release. Heat shock protein 75 (HSP75, also known as TRAP1) acts as an antagonist of ROS and protects cells from GzmM-mediated apoptosis. GzmM cleaves TRAP1 and abolishes its antagonistic function to ROS, resulting in ROS accumulation. Silencing TRAP1 through RNA interference increases ROS accumulation, whereas TRAP1 overexpression attenuates ROS production. ROS accumulation is in accordance with the release of cytochrome c from mitochondria and enhances GzmM-mediated apoptosis (Hua, 2007).

Molecular chaperones, especially members of the heat shock protein 90 (Hsp90) family, are thought to promote tumor cell survival, but this function is not well understood. This study shows that mitochondria of tumor cells, but not most normal tissues, contain Hsp90 and its related molecule, TRAP-1. These chaperones interact with Cyclophilin D, an immunophilin that induces mitochondrial cell death, and antagonize its function via protein folding/refolding mechanisms. Disabling this pathway using novel Hsp90 ATPase antagonists directed to mitochondria causes sudden collapse of mitochondrial function and selective tumor cell death. Therefore, Hsp90-directed chaperones are regulators of mitochondrial integrity, and their organelle-specific antagonists may provide a previously undescribed class of potent anticancer agents (Kang, 2007).