Preclinical Development Contrasting Effects of Nutlin-3 on TRAIL- and

Wild-type p53 is commonly expressed in melanoma but does not appear to be effective in the induction of apoptosis. One explanation is that p53 is targeted for degradation by the E3 ligaseMDM2. However, we found in this study that blockade of the interaction of p53 andMDM2 by theMDM2 antagonist nutlin-3 inmelanoma cells did not induce apoptosis, even though it upregulated p53 and its proapoptotic targets. Nevertheless, nutlin-3 enhanced TRAIL-induced apoptosis as a result of p53-mediated upregulation of TRAIL-R2. Unex-pectedly, nutlin-3 upregulatedMcl-1, which attenuated apoptotic signaling triggered by TRAIL, and inhibited apoptosis induced by the microtubule-targeting drug docetaxel. The increase in Mcl-1 was related to a p53-independent transcriptional mechanism, but stabilization of the Mcl-1 protein played a dominant role, as nutlin-3 upregulated theMcl-1 protein to amuch greater extent than theMcl-1mRNA, and this was associated with prolonged half-life time and reduced ubiquitination of the protein. Knockdown of p53 blocked the upregulation of the Mcl-1 protein, indicating that p53 plays a critical role in the stabilization of Mcl-1. The contrasting effects of nutlin-3 on TRAIL- and docetaxel-induced apoptosis were confirmed in freshmelanoma isolates. Collectively, these results show that nutlin-3 may be a useful agent in combination with TRAIL and, importantly, uncover a novel regulatory effect of p53 on the expression of Mcl-1 in melanoma cells on treatment with nutlin-3, which may antagonize the therapeutic efficacy of other chemotherapeutic drugs in addition to docetaxel in melanoma. Mol Cancer Ther; 9(12); 3363–74. 2010 AACR