c-myb encodes a transcriptional activator that is essential for the development of the hematopoietic system but appears to lack any major role in non-hematopoietic cells. The identification of two conserved myb-related genes, designated A-myb and B-myb, has raised the possibility that these genes are functional equivalents of c-myb in non-hematopoietic cells. The mouse A-myb gene maps to the proximal region of chromosome 1 and encodes a transcriptional activator with properties similar to those of the c-myb and v-myb proteins. During embryogenesis, A-myb is predominantly expressed in several regions of the developing central nervous system (CNS) and the urogenital ridge. Expression in the CNS is confined to the neural tube, the hindbrain, the neural retina and the olfactory epithelium, and coincides with the presence of proliferating immature neuronal precursor cells. In the adult mouse, A-myb is expressed during the early stages of sperm cell differentiation and in B lymphocytes located in germinal centers of the spleen. Taken together, these results suggest a role for A-myb in the proliferation and/or differentiation of neurogenic, spermatogenic and B-lymphoid cells (Trauth, 1994).

To further explore the relationship between the different myb family members, the expression of A-myb and c-myb has been compared during mouse embryogenesis. In accordance with the important role of c-myb in the hematopoietic system, high levels of c-myb expression are detected in hematopoietic organs, such as the fetal liver and the thymus. Surprisingly, high levels of c-myb expression are not restricted to hematopoietic cells. c-myb is strongly expressed in the neural retina and in epithelia of the respiratory tract. The side-by side analysis of c-myb and A-myb expression clearly shows that both genes are expressed in different, but overlapping sets of tissues. These results suggest that the function of c-myb may not be restricted to hematopoietic cells (Sitzmann, 1995).

The MafB transcriptional activator, a member of a family of genes encoding bZIP transcription factors, plays a pivotal role in regulating lineage-specific gene expression during hematopoiesis by repressing Ets-1-mediated transcription of key erythroid-specific genes in myeloid cells. To determine the effects of Maf family proteins on the transactivation of myeloid-specific genes in myeloid cells, the ability of c-Maf to influence Ets-1- and c-Myb-dependent CD13/APN transcription was tested. Expression of c-Maf in human immature myeloblastic cells inhibits CD13/APN-driven reporter gene activity (85 to 95% reduction) and requires the binding of both c-Myb and Ets, but not Maf, to the promoter fragment. c-Maf's inhibition of CD13/APN expression correlates with its ability to physically associate with c-Myb. While c-Maf mRNA and protein levels remain constant during myeloid differentiation, formation of inhibitory Myb-Maf complexes is developmentally regulated, their levels being highest in immature myeloid cell lines and markedly decreased in cell lines representing later developmental stages. This pattern matches that of CD13/APN reporter gene expression, indicating that Maf modulation of c-Myb activity may be an important mechanism for the control of gene transcription during hematopoietic cell development (Hedge, 1998).

B-myb is a member of the myb family of nuclear sequence-specific DNA-binding proteins, which have been highly conserved among vertebrates. B-myb has been implicated in the control of cell proliferation, particularly at the G1/S transition of the cell cycle. So far, most of the work on B-myb has been performed in immortalized cell lines. Since these cells might show aberrant behavior of genes involved in proliferation control, the role of B-myb in normal cells is being investigated. As a first step, the expression of B-myb has been studied during mouse development. B-myb is expressed at similar levels during all stages of embryogenesis. In situ hybridization reveals a tight linkage between B-myb expression and proliferative activity (as assessed by the expression of the S-phase specific histone H4 gene) in most tissues and throughout embryonic development. However, B-myb and histone H4 expression are uncoupled during spermatogenesis in the adult mouse. Histone H4 is expressed at high levels in the early spermatogenic progenitor cells but not in successive stages of sperm cell development. By contrast, the highest levels of B-myb expression are found during the intermediate stages of spermatogenesis. B-myb mRNA isolated from the testis differs in size from that of other tissues. The data presented here strongly support the notion that B-myb plays a general role during the proliferation of most cells. These results raise the possibility that the function of B-myb in cells undergoing meiosis may be different from its role in cells dividing mitotically (Sitzmann, 1996).

In adult male mice, A-myb is expressed predominantly in male germ cells. In female mice, A-myb is expressed in breast ductal epithelium, mainly during pregnancy-induced ductal branching and alveolar development. Mice homozygous for a germline mutation in A-myb develop to term but show defects in growth after birth and male infertility due to a block in spermatogenesis. Morphological examination of the testes of A-myb-/- males reveals that the germ cells enter meiotic prophase and arrest at pachytene. In adult homozygous null A-myb female mice, the breast epithelial compartment shows underdevelopment of breast tissue following pregnancy and the female mice are unable to nurse their newborn pups. These results demonstrate that A-myb plays a critical role in spermatogenesis and mammary gland development (Toscani, 1997).

The transcription activator c-Myb is expressed at high levels in immature thymocytes and during T-cell activation and may be a regulator of T-cell differentiation. To investigate the role of c-Myb in T-cell development, two dominant interfering Myb alleles were expressed from early developmental times onward in the T cells of transgenic mice; one allele was a competitive inhibitor of DNA binding, and the other, an active repressor comprising the Myb DNA-binding domain linked to the Drosophila Engrailed transcription repressor domain. Both alleles partially block thymopoiesis and inhibit proliferation of mature T cells. The Myb-En chimera is the more efficient repressor and might serve as an archetype for the manufacture of other dominant interfering transcription factor alleles (Badiani, 1994).

The c-Myb transcription factor is important for fetal hematopoiesis and has been proposed to mediate later stages of lymphocyte development. Using homozygous null c-Myb/Rag1 chimeric mice, it has been determined that c-Myb plays an important role in the differentiation of macrophages and lymphocytes from precursor stem cells. Deletion of c-Myb leads to a complete block in early T cell development just before the oligopotent thymocyte matures into the definitive T cell precursor. These data indicate that c-Myb plays an important role at multiple stages of hematopoiesis and is required at an early stage of T cell development (Allen, 1999).

c-Myb and B-Myb produce a differentiation block and a cell cycle shift in cultured cells. The human myeloid leukemia cell line GFD8 is a useful model for comparing the biological function of the structurally related c-Myb and B-Myb proto-oncogenes and to investigate the c-myb domains required for this function. GFD8 cells are dependent for growth on granulocyte-macrophage colony-stimulating factor and differentiate in response to phorbol myristate acetate (PMA). This cell line has been stably transfected with constructs constitutively expressing c-Myb or B-Myb. Deregulated expression of both c-Myb and B-Myb inhibit the differentiation observed in response to PMA and, in particular, the induction of the CD11b and CD11c antigens on the cell surface, and the induction of adherence. c-Myb and B-Myb enhance expression of CD13 upon PMA treatment. Although deregulated Myb expression does not alter the growth factor dependence of the cells, it leads to an increase in G2 relative to G1 arrest in cells induced to differentiate in response to PMA, whereas control vector-transfected cells were blocked mostly in G1. This decrease in G1 block takes place despite normal induction of the cyclin-dependent kinase inhibitor protein p21 (CIP1/WAF1). Thus, GFD8 cells stably expressing the human B-Myb protein behave in a manner indistinguishable from those stably expressing C-Myb, for both differentiation and cell cycle parameters. In agreement with these findings, and in contrast to most previous reports, transactivation assays show that B-myb can indeed act as a strong activator of transcription. Although the DNA-binding domain of c-myb is required for both the differentiation block and the shift in cell cycle after PMA treatment, phosphorylation by casein kinase II and mitogen-activated protein kinase at positions 11 and 12 or 532 of c-myb, respectively, are not. It is concluded that c-Myb and B-Myb may activate a common cellular program in the GFD8 cell line involved in both differentiation and cell cycle control (Golay, 1997).

The myb gene family consists of three members, named A-, B-, and c-myb. All three members of this family encode nuclear proteins that bind DNA in a sequence-specific manner and function as regulators of transcription. The biochemical and biological activities of murine B-myb have been examined and these properties have been compared with those of murine c-myb. In transient transactivation assays, murine B-myb exhibits transactivation potential comparable to that of c-myb. An analysis of B-myb and c-myb deletion mutants shows that while the C-terminal domain of c-Myb acts as a negative regulator of transcriptional transactivation, the C-terminal domain of B-Myb functions as a positive enhancer of transactivation. To compare the biological activities of c-myb and B-myb, the two genes were overexpressed in 32Dcl3 cells, which are known to undergo terminal differentiation into granulocytes in the presence of granulocyte colony-stimulating factor (G-CSF). c-myb blocks the G-CSF-induced terminal differentiation of 32Dcl3 cells, resulting in their continued proliferation in the presence of G-CSF. In contrast, ectopic overexpression of B-myb blocks the ability of 32D cells to proliferate in the presence of G-CSF and accelerates the G-CSF-induced granulocytic differentiation of these cells. Similar studies with B-myb/c-myb chimeras show that only chimeras that contain the C-terminal domain of B-Myb are able to accelerate the G-CSF-induced terminal differentiation of 32Dcl3 cells. These studies show that c-myb and B-myb do not exhibit identical biological activities and that the carboxyl-terminal regulatory domain of B-Myb plays a critical role in its biological function (Oh, 1998).

The three members of the myb gene family (A-, B-, and c-myb) encode nuclear proteins that bind to DNA and function as regulators of transcription. Murine A-myb is a poor transactivator of transcription compared with murine c-myb. Deletion of the COOH-terminal domain of A-Myb, or co-expression with Ets-2 results in increased transactivation potential. While ectopic overexpression of c-myb in 32Dcl3 cells results in a block of terminal differentiation, resulting in indefinite growth in granulocyte-colony-stimulating factor (G-CSF), a similar overexpression of A-myb results in growth arrest and the concomitant terminal differentiation of 32D cells into granulocytes. Co-expression of A-myb and ets-2 in these cells results in the restoration of the proliferative activity of the cells in G-CSF, but fails to induce a block to G-CSF-induced terminal differentiation. However, overexpression of the COOH-terminal deletion mutant of A-myb results in a block to G-CSF-induced differentiation of 32D cells, suggesting that the distinctive biological phenotypes produced by A-myb and c-myb genes are mediated by their COOH-terminal domains (Oh, 1997).