HDM2 antagonist Nutlin-3 disrupts p73-HDM2 binding and enhances p73 function

Nutlin-3, a small molecule inhibitor, activates p53 by disrupting p53-HDM2 association. In this study, we found that Nutlin-3 suppressed cell growth and induced apoptosis in the absence of wild-type p53, suggesting a p53-independent mechanism for Nutlin-3-induced cell death. Like p53, its homolog p73 transactivates proapoptotic genes and induces cell death. Since HDM2, a key negative regulator of p53, also binds to and inhibits p73, we asked whether p73 could mediate Nutlin-3-induced apoptosis. We demonstrate that Nutlin-3 inhibits endogenous binding between the proapoptotic p73 isoform TAp73 α and HDM2 in p53-null cells. Dissociation of p73 and HDM2 leads to increased p73 transcriptional activity with upregulation of p73 target genes noxa , puma and p21 , as well as enhanced apoptosis. p73 knockdown by siRNA results in rescue of Nutlin-3-treated cells, indicating that Nutlin-3-induced apoptosis is, at least in part, p73 dependent. In addition, Nutlin-3 treatment increases TAp73 α protein levels with prolongation of p73 half-life. These results provide the first evidence that Nutlin-3 disrupts endogenous p73–HDM2 interaction and enhances the stability and proapoptotic activities of p73 and thus, provides a rationale for the use of Nutlin-3 in the large number of human tumors in which p53 is inactivated.