Induction of cell cycle progression and acceleration of apoptosis are two separable functions of c-Myc: Transrepression correlates with acceleration of apoptosis

Analysis of amino-terminus mutants of c-Myc has allowed a systematic study of the interrelationship between Myc's ability to regulate transcription an d its apoptotic, proliferative, and transforming functions. First, me have found that c-Myc-accelerated apoptosis does not directly correlate with its ability to transactivate transcription using the endogenous ornithine deca rboxylase (ODC) gene as readout for transactivation, Furthermore, deletion of the conserved c-Myc box I domain implicated in transactivation does not inhibit apoptosis. Second, the ability of c-Myc to repress transcription, u sing the gadd45 gene as a readout, correlates with its ability to accelerat e apoptosis, A conserved region of c-Myc implicated in mediating transrepre ssion is absolutely required for c-Myc-accelerated apoptosis, Third, a lymp homa-derived Thr58Ala mutation diminishes c-Myc-accelerated apoptosis throu gh a decreased ability to induce the release of cytochrome c from mitochond ria, This mutation in a potential phosphorylation site does not affect cell cycle progression, providing genetic evidence that induction of cell cycle progression and acceleration of apoptosis are fire separable functions of c-Myc. Finally, me show that the increased ability of Thr58Ala mutant to el icit cellular transformation correlates with its diminished ability to acce lerate apoptosis. Bcl-2 overexpression blocked and the lymphoma-associated Thr58Ala mutation decreased c-Myc-accelerated apoptosis, and both led to a significant increase in the ability of Rat1a cells to form colonies in soft agar, This enhanced transformation mas greater in soft agar containing a l ow concentration of serum, suggesting that protection from apoptosis is a m echanism contributing to the increased ability of these cells to proliferat e in suspension, Thus, me show here for the first time that, in addition to mutations in complementary antiapoptotic genes, c-Myc itself can acquire m utations that potentiate neoplastic transformation by affecting apoptosis i ndependently of cell cycle progression.