Activation of human B-MYB by cyclins.

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.