A Stapled p53 Helix Targets HDMX to Overcome Nutlin-3 Resistance and Reactivate the p53 Tumor Suppressor Pathway in Cancer

p53 is a transcription factor that induces cell cycle arrest or apoptosis in response to DNA damage and cellular stress, and thereby plays a critical role in protecting cells from malignant transformation. The E3 ubiquitin ligase HDM2 controls p53 levels through a direct binding interaction that neutralizes the transactivation activity of p53 and targets it for degradation via the ubiquitylation-proteasomal pathway. Whereas the HDM2-homologue HDMX lacks ubiquitin ligase function, it participates in regulating the p53 axis by heterodimerizing with HDM2 and sequestering p53 through protein interaction. Loss of p53 activity, either by deletion, mutation, or HDM2/HDMX overexpression, is the most common defect in human cancer. Tumors expressing wild type p53 are rendered vulnerable by pharmacologic approaches that stabilize and upregulate p53. In this context, HDM2 and HDMX have emerged as independent therapeutic targets for restoring p53 activity and resensitizing cancer cells to apoptosis in vitro and in vivo. The small molecule nutlin-3 is an effective antagonist of the p53-HDM2 interaction. However, several studies have demonstrated the inability of nutlin-3 to disrupt the p53-HDMX complex, rendering tumor cells that overexpress HDMX nutlin-3-resistant. We have previously described the synthesis and characterization of a hydrocarbon-stapled alpha-helical p53 peptide (SAH-p53-8) that binds HDM2 with low nanomolar affinity, targets HDM2 in situ, and reactivates the p53 tumor suppressor pathway in HDM2-overexpressing osteosarcoma cells. We now report that SAH-p53-8 binds HDMX with even higher affinity, co-immunoprecipitates with endogenous HDMX, and induces apoptosis and cell cycle arrest in nutlin-3-resistant cancer cells that overexpress HDMX. Thus, by inserting a chemical staple into a peptide fragment of the p53 transactivation domain, we have generated the first bifunctional inhibitor of HDM2 and HDMX, enabling the investigation and pharmacologic modulation of both targets in human cancer.