THE P53 TARGETS MDM2 AND FAS ARE NOT REQUIRED AS MEDIATORS OF APOPTOSIS IN-VIVO

The tumor suppressor p53 can exert its anti-oncogenic activity in part by inducing apoptosis in cells that have sustained damage to their DN A. It is likely that p53 activates the transcription of target genes t hat mediate this response. Known p53 targets with potential roles in c ell cycle control and apoptosis induction include: p21(WAF1/CIP1), mdm 2, cyclin G, bax and Fas. We examined the p53 pathway in the thymus of the mouse after irradiation, FACS analysis demonstrated that the thym ocytes of mice with wild-type p53, but not those lacking p53, underwen t apoptosis after irradiation. Expression analysis of the target genes revealed that all tested genes underwent p53-dependent induction, alt hough the extent and timing varied. The target genes implicated in cel l cycle (p21, mdm2 and cyclin G) were induced 2 h after irradiation, i n contrast to targets with a possible role in apoptosis (bax and Fas), which were induced at 4 h. This analysis is the first demonstration t hat Fas is a p53-responsive gene in vivo. Since p21 and bax expression are not required for p53-dependent apoptosis, we tested whether other target genes affected apoptosis in vivo. We discovered that mdm2 has no role in preventing apoptosis independently of p53 inactivation, and that Fas, like p21 and bax, is not necessary for p53-mediated inducti on of apoptosis. Therefore, no p53 target identified and tested to dat e is singly responsible for p53-dependent apoptosis in response to DNA damage in vivo.