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 ligase MDM2. However, we found in this study that blockade of the interaction of p53 and MDM2 by the MDM2 antagonist nutlin-3 in melanoma 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. Unexpectedly, nutlin-3 upregulated Mcl-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 the Mcl-1 protein to a much greater extent than the Mcl-1 mRNA, 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 fresh melanoma 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.
Molecular Cancer Therapeutics Vol. 9, Issue 12, p. 3363-3374