The role ofc-myc and E2F-1 as negative regulators of terminal myeloid differentiation

Abnormalities in both growth and differentiation appear to be required for leukemogenesis. Hematopoiesis provides an excellent system to dissect the molecular genetics of differentiation and to study genetic lesions which block terminal differentiation. Negative regulators of terminal differentiation are likely to be encoded by genes whose expression is rapidly down-regulated upon induction of differentiation. The role of two such genes, c-myc and E2F-1, during terminal differentiation was examined in murine M1 myeloid leukemic cells. Derivative cell lines of M1 that expressed either c-myc or E2F-1 in a deregulated mode were tested for their response to the differentiation inducer, interleukin-6. Deregulated c-myc blocked terminal differentiation at an intermediate stage and also induced a program of p53-independent apoptosis. Interestingly, this is the first study to attribute a role for c-myc in mediating apoptosis during the physiologically relevant process of differentiation. In order to characterize the molecular machinery underlying the c-myc-mediated apoptosis, advantage was taken of the fact that unlike M1myc cells, M1myb cells, which express deregulated c-myb and are also blocked for terminal differentiation, do not undergo apoptosis when treated with IL-6. A comparision of the differential expression patterns of key gene products in M1myc and M1myb cells revealed that in IL-6-treated M1myc cells, but not in M1myb cells, bcl-2 is down-regulated, p15 is induced and Rb is concomitantly hypophosphorylated. However, further analyses showed that overexpressing bcl-2 in M1myc cells only delayed cell death and preventing Rb hypophosphorylation also did not abrogate apoptosis. The results of this study indicate, therefore, that none of these molecular changes are causal events in the myc-mediated apoptosis during differentiation. The deregulated expression of E2F-1 also blocked differentiation of M1 cells, but at an earlier stage than the c-myc block. M1E2F cells did not undergo apoptosis and continued to proliferate normally even in the presence of induced p15, p16, and Rb hypophosphorylation. Hence, deregulated E2F-1 prevents the terminal differentiation of M1 cells and uncouples the expression of p15, p16, and Rb hypophosphorylation from growth arrest