The role of Myb proteins in normal and neoplastic cell proliferation.

The c-myb protooncogene is the prototype of a gene family that contains two other recently described members, A-myb and B-myb. The c-myb gene encodes a transcription regulatory protein, c-Myb, that has distinct DNA-binding domain structure and binding specificity compared with unrelated transcription factors. All three members of the myb protein family, however, display a high degree of homology within their DNA-binding domain, suggesting that they may regulate transcription of a similar set of target genes. We examine here whether, by implication, the individual members of the myb gene family play analogous roles within the cell, in particular focusing on their potential functions in the control of cell proliferation. Expression of both c-myb and B-myb is subject to regulation in the cell cycle, transcripts of these genes being induced within the G1 phase of the cell cycle and persisting at maximal levels through S phase. Consistent with this timing of expression, inhibition of c-Myb and B-Myb synthesis by treatment of cells with anti-sense oligonucleotides indicates that both proteins are required for transition from the G1 to S phase of the cell cycle. The c-Myb and B-Myb proteins cannot be considered as analogs, however, as they show certain differences in trans-activation activity of their target genes. Moreover, while superficially similar, expression of c-myb and B-myb is not coordinated, and it is conceivable that their products have quite distinct functions in the regulation of cell proliferation. The ubiquitous expression of B-myb in cycling cells and its strict regulation during late G1 suggest a universal function for B-Myb in G1 to S phase transition. Consistent with this conclusion, B-myb transcription is controlled by the transcription factor E2F, which has been implicated in cell cycle regulation of a number of genes involved in DNA synthesis. The restricted tissue tropism of c-myb expression and its regulation by certain growth factors such as IL-2 suggest a specific function in transduction of extrinsic proliferation (and differentiation) signals. The role of A-myb in the cell remains to be determined; however, its expression shows no correlation with cell cycling, implying that it has no direct role in cell proliferation. Recent data show that the necessity for c-myb function in proliferation of a number of cell types provides a promising opportunity for intervention in the treatment of certain tumors.