MYC-MEDIATED APOPTOSIS REQUIRES WILD-TYPE P53 IN A MANNER INDEPENDENTOF CELL-CYCLE ARREST AND THE ABILITY OF P53 TO INDUCE P21(WAF1 CIP1)/

Deregulated expression of the c-myc proto-oncogene can lead to apoptos is under certain physiological conditions. By introducing a conditiona lly active Myc allele into primary embryo fibroblasts null for p53, an d into fibroblasts without endogenous p53 expression but ectopically e xpressing a temperature-sensitive p53 allele, we show that expression of wild-type p53 is required for susceptibility to Myc-mediated apopto sis. Although ectopic expression of wild-type p53 blocked cells in the G(1) phase of the cell cycle, G(1) arrest by isoleucine starvation, i n a manner independent of p53, did not confer susceptibility to apopto sis. Thus, growth arrest per se is not sufficient to induce Myc-mediat ed apoptosis; instead, a property intrinsic to p53 is specifically req uired. Moreover, apoptosis did not require induction of p53 target pro teins, including the cyclin-dependent kinase inhibitor p21(waf1/cip1). Therefore, the role of p53 in apoptosis may be distinct from its role in cell cycle arrest.