Differential apoptotic behaviors of c-myc, N-myc, and L-myc oncoproteins

c-, N-, and L-myc are related nuclear oncoproteins that bind similar DNA sites and cooperate with activated ras oncogenes to transform primary fibroblasts. Although c-myc can also promote apoptosis in some cells after growth factor withdrawal or exposure to cytotoxic agents, roles for N- and L- myc in apoptosis remain undetermined. To address this, c-, N-, or L-myc were stably expressed in the interleukin 3 (IL-3)-dependent 32D hematopoietic cell line. The apoptotic response of each cell line was assessed after IL-3 withdrawal or treatment with four structurally unrelated cytotoxic agents. All three oncoproteins accelerated apoptosis after IL-3 withdrawal. In contrast, whereas c-myc overexpression generally sensitized cells to cytotoxic drugs, N-myc and L-myc overexpression produced resistance. myc expression tended to be associated with a more robust G₂-M arrest after drug exposure, but this did not correlate with drug sensitivity or resistance. Bcl-2 and Bcl-X(L) protected control cells against apoptosis after either IL- 3 withdrawal or drug exposure, although in some cases this effect could be overridden by myc oncoproteins, particularly N-myc and L-myc. Our results suggest that the apoptotic pathways activated upon IL-3 withdrawal and cytotoxic drug treatment are distinct and differentially affected by members of the myc and Bcl-2 families.