Activation of human B-MYB by cyclins

Abstract

B-MYB expression is associated with cell proliferation and recent studies have suggested that it promotes the S phase of mammalian cells. Based on its homology to the transcription factors c-MYB and A-MYB, B-MYB is thought to be involved in transcriptional regulation; however, its activity is not detectable in several cell lines. It was postulated that B-MYB function may depend on the presence of a cofactor, and recent studies suggested that B-MYB is phosphorylated specifically during S phase in murine fibroblasts. In this report we provide evidence that the product of the human B-myb gene can be activated in vivo by coexpression with cyclin A or cyclin E. Transfection studies showed that B-MYB was a weak transcriptional activator in SAOS-2 cells and was unable to promote their proliferation. In contrast, overexpression of both B-MYB and cyclin A or cyclin E caused a drastic increase in the number of SAOS-2 cells in S phase. Also, overexpression of cyclin A and cyclin E in SAOS-2 cells enhanced the ability of B-MYB, but not c-MYB, to transactivate various promoters, including the cdc2 promoter, the HIV-1-LTR, and the simian virus 40 minimal promoter. A direct role for cyclin-dependent activation of B-MYB was demonstrated using an in vitro transcription assay. These observations suggest that one mechanism by which cyclin A and E may promote the S phase is through modification and activation of B-MYB.B-MYB is a transcriptional regulator whose activity appears to be associated with cellular proliferation and differentiation (1-5). B-MYB is highly homologous to c-MYB within the DNA-binding domain region, and this correlates with a similar affinity displayed by both proteins for the myb-binding sequence (C͞T)AAC(G͞T)G (reviewed in ref. 6). A large body of evidence implicates B-MYB as a player in cell-cycle progression. First, although B-myb expression is ubiquitous, it is strictly regulated in cycling cells: B-myb transcription is downregulated in quiescent cells and expression is detected, upon reentry into the cell cycle, in late G 1 (7,8). Repression of B-myb transcription in G 0 ͞early G 1 is dependent upon an E2F binding site within the promoter and appears to involve negative regulation by the retinoblastoma-related proteins p107 and p130 (9, 10). B-MYB is a downstream target of growth suppressors such as p107 and p53 (5, 11) and its transcription is induced by E2F-1 (12), whose activity is associated with cell-cycle progression and, possibly, transformation. Overall, these data support the hypothesis that B-MYB may play a role in G 1 ͞S transition or during S phase itself. B-MYB can affect the growth rate of certain, but not all, cell lines, and it can activate or repress transcription depending on the promoter, cell type, and species. (13-15). The parameters regulating the specificity of B-MYB transcriptional activity have not been elucidated; however, there is evidence to suggest that the conserved region of B-MYB protein binds to a set of cellular factors that may be involved in cell-typedependent t...

Myb binding proteins: regulators and cohorts in transformation

“…Also, by focusing on c-Myb and v-Myb, this review has neglected the interactions known for Myb related proteins, which will certainly turn out to be just as varied and complicated as the ones a ecting c-Myb. For example, a lymphoid-speci®c protein has been identi®ed which interacts with A-Myb (Ying et al, 1997), and interactions between B-Myb, cyclin A and cyclin-dependent kinases have been well documented (Lane et al, 1997;Sala et al, 1997;Ziebold et al, 1997).…”

“…For example, the Myb-related protein B-Myb has S phase speci®c transactivation activity, and is regulated by cyclindependent kinases (Lane et al, 1997;Sala et al, 1997;Ziebold et al, 1997). Ironically, B-Myb has not been shown to be oncogenic, and no evidence linking c-Myb activity to cell cycle regulation has been reported.…”

Physical interaction between CDK9 and B-Myb results in suppression of B-Myb gene autoregulation

“…Mybs are molecules negatively regulated by their carboxyl-termini, and protein truncation results in activation of the transcriptional activity of both c-Myb and B-Myb (Kalkbrenner et al, 1990;Ziebold et al, 1997;Lane et al, 1997). Several studies have demonstrated that B-Myb is a substrate for the cdk2/cyclinA kinase and that phosphorylated B-Myb is more active, in terms of its transactivating activity, than the unphosphorylated form (Ziebold et al, 1997;Lane et al, 1997;Sala et al, 1997). c-Myb expression is high in the hematopoietic lineages, especially in immature cell types (Gonda and Metcalf, 1984).…”

Targeting an E2F site in the mouse genome prevents promoter silencing in quiescent and post-mitotic cells

“…Although we have yet to examine whether the failure to silence B-myb expression in the brains of B-myb mE2F/mE2F mice (Figure 3f) led to increased apoptosis, neuronal defects were not apparent. B-myb activity is regulated not only at the transcriptional level, but also positively by post-translational modification by cyclin-dependent kinases, in particular cyclin A/cdk2 (Robinson et al, 1996;Lane et al, 1997;Sala et al, 1997;Ziebold et al, 1997). It is likely, therefore, that the B-Myb protein made inappropriately in post-mitotic cells of B-myb mE2F/mE2F mice is not activated, as the cyclin A gene is also an E2F target and would be silenced.…”