The Phlpp2 phosphatase protects Myc and is a target for the prevention of prostate cancer metastasis

Prostate cancer (PC) is one of the most prevalent cancers among men, causing almost 30,000 deaths per year in the US alone. Death caused by PC is mainly due to metastasis, and the 5-year survival rate of metastatic PC is only 28%. In contrast, the 5-year survival rate of PC confined to the prostate is nearly 99%. In contrast to primary PC tumors, metastatic tumors commonly present with targeted, bi-allelic mutations of Pten and Tp53 tumor suppressor genes. Whereas 49% of metastatic PC tumor samples have Pten/Tp53 co-deletions, these same co-deletions arise in only 10% of primary tumors. Because of the scarcity of human samples from metastatic disease, our lab has developed a genetically engineered mouse model (GEMM), in which Pten/Trp53 co-deletion using lentiviral plasmids gives rise to metastatic PC. As a result of the propensity of these mice to quickly develop metastatic PC, the model is termed RapidCaP. In the setting of our model for metastatic PC, we sought to investigate other genes which could be regulating the switch from a relatively benign primary PC to a highly lethal metastatic PC. Interestingly, most candidate tumor suppressor genes in PC are part of large recurrent hemizygous deletions, such as the common chromosome 16q-deletion, which involves the AKT-suppressing phosphatase, Phlpp2. Using our RapidCaP model, we find evidence that complete loss of Phlpp2 in the setting of Pten/Tp53 co-deletion paradoxically blocks prostate tumor growth and metastasis burden in RapidCaP mice. In fact, when Phlpp2 is deleted using a lentiviral vector, only 7% of the mice have PC tumors with metastasis compared to 60% in Pten/Tp53 co-deletion alone. Furthermore, we show that the Phlpp2 phosphatase activates Myc, a key driver of PC metastasis, by regulating Myc stability. Phlpp2 deletion in embryonic fibroblasts (MEFs) derived from RapidCaP mice does not affect Myc transcript levels by qPCR, but decreases Myc protein expression by over 75% as measured by Western blot. Using the same MEFs, we further show that the half-life of the Myc protein decreases from 19.3 minutes to 12.7 minutes upon deletion of Phlpp2 using lentiviral vectors. We propose that Phlpp2 dephosphorylates the threonine 58 (T58) site of Myc, thus directly increasing its stability. Importantly, we show that small molecule inhibitors of the Phlpp2 phosphatase can suppress Myc and cause cell death. Using the MEFs derived from the RapidCaP mice, Myc concentration decreases to an undetectable level in a dose-dependent manner following treatment with a Phlpp2 inhibitor between the concentrations of 0-200 υM, which coincides with a dose-dependent increase in cell death as measured by PI staining. A similar trend is seen in the PC3 cell line derived from human PC. In sum, our findings reveal that the frequent hemizygous deletions on chromosome 16q present a possible druggable vulnerability for targeting the Myc protein in metastatic PC with Phlpp2 phosphatase inhibitors. Citation Format: Vincent D9Andrea, Dawid Nowak, Kaitlin Watrud, Alexandra Ambrico, Irene Casanova-Salas, Lloyd Trotman. The Phlpp2 phosphatase protects Myc and is a target for the prevention of prostate cancer metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1097.