The long-term objective of this project is to develop a novel targeted therapy for human cancers by reactivating the tumor suppression pathways. Although p53 is frequently mutated in almost 50% of human cancers, many human tumors retain wild-type p53 but its activities are downregulated through multiple mechanisms. Inactivation of Mdm2 is a validated approach for the treatment of human cancer retaining wild-type p53 by reactivating the p53 tumor suppressor function. Through nearly two decades of intense efforts, a number of highly potent small-molecule inhibitors and peptides that inhibit the MDM2?p53 interaction (also called Mdm2 inhibitors) have been successfully developed and validated in vitro, and several of these Mdm2 inhibitors have been moved to human clinical trials for in vivo validation. Nevertheless, some serious challenges remain to be addressed. The major issue is the dose-limiting toxicity because of low efficacy and severe toxicity to normal tissues with increasing dosage of these Mdm2 inhibitors. Moreover, emergence of p53 mutations in cancer patients developed highly resistance after the initial treatment with Mdm2 inhibitors. Thus, additional cancer targets aiming at this pathway are clearly needed for more effective therapeutic purpose. In this application, we plan to characterize novel small molecule USP7 inhibitors in activating p53 and cancer therapy. The deubiquitinase USP7 (also called HAUSP) was one of the first deubiquitinases (DUBs) that exhibit a specific role in regulating protein stability in vivo. Previous studies from our lab and others demonstrated that inhibition of USP7 leads to p53 activation by destabilizing both Mdm2 and Mdmx. Notably, USP7 inhibitors are also able to induce p53-independent tumor suppression functions in vivo. For example, we identified N-Myc, a major driver in neuroblastoma tumorigenesis as a critical target for USP7. Recently, we discovered PD-L1 as another important target of USP7. Taken together, these studies reveal that USP7 inhibitors have better efficacy because USP7 inhibition activates p53-mediated tumor suppression by downregulating both Mdm2 and Mdmx and also induces p53-independent tumor growth suppression by destabilizing N-Myc and PD-L1. Moreover, the tumors with high levels of N-Myc or PD-L1 may not develop drug resistance even when the p53 gene is mutated. The major hypothesis to be tested here is whether USP7 inhibitors are more effective and better therapeutic agents for the treatment of human cancers.
In Aim 1, we will further characterize novel USP7 inhibitors obtained from our high-through-put screening assays in suppressing tumor growth through both p53 activation and N-Myc destabilization in human neuroblastomas.
In Aim 2, we will examine whether the USP7 inhibitor is able to promote immunotherapy by downregulating PD-L1 in human cancer cells.
Our studies indicate that USP7 inhibition activates p53-mediated tumor suppression by downregulating both Mdm2 and Mdmx and induces p53-independent tumor growth suppression by destabilizing N-Myc and PD-L1. The major hypothesis to be tested here is whether USP7 inhibitors are more effective and better therapeutic agents for the treatment of human cancers.