p53-independent apoptosis associated with c-Myc-mediated block in myeloid cell differentiation

Previously we have shown that deregulated expression of c-myc in M1 myeloid leukemic cells blocked IL-6-induced differentiation and its associated gro wth arrest; however, the cells proliferated at a significantly reduced rate compared to untreated cells. The basis for the increased doubling time of IL-6-treated M1myc cells was found to be due to the induction of a p53-inde pendent apoptotic pathway. The apoptotic response was not completely penetr ant; in the same population of cells both proliferation and apoptosis were continuously ongoing, Down-regulation of Bcl-2 was insufficient to account for the apoptotic response, since deregulated expression of Bcl-2 delayed, but did not block, the onset of apoptosis, Furthermore, our results indicat ed that the IL-6-induced partial hypophosphorylation of the retinoblastoma gene product (Rb), observed in M1myc cells, was not responsible for the apo ptotic response. Finally, the findings in M1 cells were extended to myeloid cells derived from the bone marrow of wild type and p53-deficient mice, wh ere the deregulated expression of c-myc was also shown to block terminal di fferentiation and induce apoptosis independent of p53, These findings provi de new insights into how myc participates in the neoplastic process, and ho w additional mutations can promote more aggressive tumors.