Co-regulation of p53 and PD-L1 by the VPRBP-USP2 axis in transcription and ubiquitylation - 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. For example, Mdm2 is a key repressor of p53 by acting as a major E3 ligase for ubiquitylation-mediated degradation. Inhibition of Mdm2 is well accepted as a validated approach for the treatment of human cancers retaining wild-type p53 by reactivating the p53 tumor suppressor function. Nevertheless, despite of intense efforts in developing highly potent Mdm2 inhibitors, none of them has been approved therapeutically very effective because of the toxicity of these inhibitors in vivo. To overcoming this major issue, it is necessary to search for a candidate by thinking about a ‘out of box’ strategy. In this application, we identify the VPRBP-USP2 axis as such a candidate. Like Mdm2, VPRBP can suppress p53 function by both transcriptional repression and ubiquitylation-mediated degradation. Interestingly, unlike Mdm2, VPRBP acts as a master regulator of PD-L1.Moreover, the stability of VPRBP is tightly control by USP2. Blocking the PD-L1/PD-1 pathway has been shown remarkable anti-tumor effects in cancer patients. Interestingly, clinical studies demonstrated that the success of PD1–PD-L1 blockade by either anti-PD1 or anti-PD-L1 antibody has a positive correlation with PD-L1 expression levels in tumor cells. Thus, although high levels of PD-L1 prevents cytotoxic T cells from effectively targeting tumor cells, it apparently also serves as a potential selective marker for patient stratification for PD1–PD-L1 blockade therapy. Strikingly, our preliminary studies showed that combining VPRBP knockdown/or USP2 inhibitors with an anti-PD-1 antibody treatment dramatically enhanced tumor growth suppression in the immune-proficient mice bearing the tumors expressing wild type p53. Notably, unlike Mdm2, knockout of the USP2 gene has no obvious effect on normal development or cell viability in normal tissues. These data suggest that USP2 is a promising therapeutic target for activating p53-mediated tumor suppressive effects in human cancers without causing severe toxicity to normal tissues. Thus, it is very important to dissect the precise mechanism of the VPRBP-USP2 axis in regulating both p53 and PD-L1 in human cancers and provide critical insights into potential cancer treatment. In Aim1, we will elucidate the molecular mechanism of the VPRBP/USP2 axis in regulating p53 and PD-L1 through transcription repression and the ubiquitylation pathway. In Aim 2, will examine whether the combination of USP2 inhibitors (or VPRBP knockdown) with the anti-PD-1 treatment is able to significantly improve the efficacy in p53-mediated tumor growth suppression in vivo by triggering anti-tumor immunity without cause severe toxicity.
