c-myb and growth control

The available evidence indicates that c-myb plays an important role in the proliferation of hematopoietic cells and in those nonhematopoietic cell types in which c-myb is expressed. A critical aspect in the regulation of c-myb expression rests in the positive autoregulatory mechanism, which is dependent on the interaction of myb protein with the 5' flanking region of the human c-myb gene. The positive autoregulation of c-myb, in conjunction with tissue-specific mechanisms that most likely involve efficient transcription beyond the site of "transcriptional pause" in the c-myb first intron, might allow the generation of c-myb transcripts at levels sufficiently high for optimal biological activity (e.g., at the G1/S transition of the cell cycle). Other transactivating factors, such as the Jun family members, also appear to be involved in regulating c-myb expression. Such factors might act to increase basal levels of c-myb expression to allow activation of the autoregulatory mechanism, or might cooperate with myb in transcriptional regulation of c-myb expression. The function of c-myb is ultimately dependent on the genes that are regulated by the myb product. Preliminary evidence suggests that DNA polymerase-alpha and cdc2, two genes that are critical for DNA synthesis, contain myb binding sites in their promoter region that appear to be required for myb transactivation of their expression. The paradox of the generality of the mechanisms by which c-myb affects cell proliferation and the apparent tissue-specific expression of this gene might be resolved by the growing evidence that the tissue distribution of c-myb is more general than previously appreciated, and that many cell types with no detectable c-myb expression contain a functional equivalent of this gene. For example, B-myb a gene that is homologous to c-myb in the DNA binding and transactivating domains and appears to be ubiquitously expressed, is also required for cell proliferation and, like c-myb, appears to regulate the expression of cdc2, a gene required for cell cycle progression. Together, these findings indicate a general role of members of the myb family in regulation of cell proliferation.