Dissociation Between p93B-myband p75c-mybExpression During the Proliferation and Differentiation of Human Myeloid Cell Lines

Direct and indirect evidence strongly indicates that the proto-oncogene c-mybplays an important role in the regulation of both the proliferation and differentiation of hematopoietic cells. In addition, recent data suggest that the structurally related B-mybgene is also necessary for the proliferation of these cells. To help understand the relationship between these two related gene products during proliferation and differentiation of myeloid cells, we have studied in parallel the regulated expression of c-myband B-myb RNAs and proteins in human myeloid cells that were either growth-arrested or induced to differentiate along different pathways. For this purpose, we have produced a polyclonal antibody directed against a fragment of the recombinant B-mybprotein. We have thus been able to detect the B-mybprotein in human cell lines and have found it to be a 93-kD protein localized in the nucleus. We have chosen two models to study the expression of both c-myband B-mybmRIMAs and proteins during myeloid proliferation and differentiation. One of the models was the HL-60 cell line, which can be induced to differentiate towards the monocytic pathway with either phorbol ester (phorbol myristate acetate) or vitamin D3 and towards the granulocytic pathway with either dimethyl sulfoxide or retinoic acid. In addition, we have studied another recently established human leukemic cell line, called GF-D8, which is strictly dependent on granulocyte-macrophage colony-stimulating factor (GM-CSF) for proliferation. The results show that the expression of B-mybRNA and protein closely correlates with proliferation in all experimental setups studied, whereas the c-mybprotein levels do not always do so. We observed that the c-mybprotein levels decreased well before the decrease of B-mybprotein and of proliferation itself during differentiation toward monocytes. Such a difference was not present during granulocytic differentiation, in which c-myblevels decreased, if anything, later than those of B-myb and proliferation. Most striking was the finding that high levels of c-mybRNA and protein, but not of B-myb, were present in the GF-D8 cell line, even after growth arrest by GM-CSF deprivation. These data suggest that B-mybmay function solely in the regulation of cellular proliferation, whereas c-mybhas additional functions, for example, in the maintenance of an undifferentiated state.