The Role of PHLPP in Colon Cancer

Colorectal cancer (CRC) is the second leading cause of cancer-related deaths in the United States. Approximately 136,000 new cases and 50,000 deaths are predicted for the year 2017; and this mortality is predominantly due to poor responses to available treatment options. A better understanding of the molecular events leading to cancer progression and chemoresistance is needed in order to improve the overall survival of CRC patients. My lab has been intensively focused on elucidating the role of a novel family of protein phosphatases, PHLPP (PH domain Leucine-rich-repeats Protein Phosphatase), in inhibiting CRC initiation and progression. We have made substantial progress in understanding the functional importance of PHLPP as a tumor suppressor as well as the molecular mechanism underlying PHLPP regulation. The overall objective of this study is to further develop a mechanistic understanding of PHLPP-mediated regulation of cellular stress response in supporting cell survival and tumorigenesis. In exciting recent findings, we demonstrated that chemotherapy-induced ER stress promotes PHLPP degradation and PHLPP-loss provides a survival advantage by upregulating eIF2á/ATF4-mediated signaling. In addition, we found that that PHLPP-loss induced by inflammatory stimulation potentiates phosphorylation of Drp1, a key regulator of mitochondrial fission, suggesting a novel role of PHLPP in regulating mitochondrial dynamics. Collectively, the central hypothesis driving this proposed study is that PHLPP serves an essential stress sensor in CRC, in which cellular stress signals trigger PHLPP degradation to promote cell survival and tumorigenesis. The following specific aims are proposed: 1) to delineate the molecular mechanism underlying PHLPP-mediated regulation of eIF2á/ATF4 signaling. We will determine if downregulation of PHLPP renders colon cancer cells resistant to chemotherapy drugs as a result of autophagy activation; 2) to determine the functional importance of PHLPPmediated regulation of mitochondrial dynamics. We will test the hypothesis that PHLPP plays an important role in regulating mitochondrial dynamics by enhancing Drp1 activity in order to cope with proinflammatory stress signals; and 3) to define the role of mitochondrial dynamics in cooperating with PHLPP-loss to promote tumorigenesis in vivo. We will utilize Drp1 knockout mice to determine the effect of blocking mitochondrial dynamics on colon cancer tumorigenesis in vivo. Our proposed study centers on a novel hypothesis that that PHLPP-loss plays a pivotal role in orchestrating multiple pro-survival responses downstream of cellular stress signals to promote tumorigenesis. Our study will fill an important knowledge gap on how altered mitochondrial dynamics contributes to tumor initiation and progression in colon cancer. Ultimately, by providing mechanistic insights into PHLPP-dependent regulation of cellular stress responses, our findings will help identify new treatment options and better predict the effectiveness of chemotherapy agents based on PHLPP status in CRC.