The Role of PHLPP in Colon Cancer

Colorectal cancer (CRC) is the second leading cause of cancer-related deaths in the United States. Approximately 142,000 new cases and 51,000 deaths are predicted for the year 2013. A better understanding of the molecular events leading to cancer progression is needed in order to improve the treatment and prognosis of CRC patients. Our 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 tumorigenesis and metastasis of CRC. We have made substantial progress in understanding the functional importance of PHLPP as a tumor suppressor in CRC as well as the molecular mechanism underlying PHLPP regulation. In this proposed study, we will determine the effect of PHLPP-loss on changing lipid metabolism in CRC. Although recent advances in cancer metabolism research have begun to provide a better understanding of how metabolic changes support cancer cell growth and survival, the regulation and functional importance of altered lipid metabolism in CRC remain elusive. This proposal centers on a novel hypothesis that PHLPP-loss plays a pivotal role in driving CRC progression by controlling lipogenesis. In exciting recent findings, we demonstrate that the expression of nuclear SREBP1, a key activator of lipid biosynthesis, is increased in PHLPP knockdown cells suggesting a potential upregulation of lipogenesis. Consistently, MEF cells isolated from PHLPP knockout mice have enhanced lipogenesis during differentiation into adipocytes. In addition, silencing PHLPP expression leads to increased glucose uptake, lactate production, and Krebs cycle activity in CRC cells indicating altered metabolism. The central hypothesis driving this proposed study is that PHLPP plays an essential role in inhibiting lipogenesis by negatively regulating the PI3K/Akt/mTOR pathway, and loss of PHLPP expression promotes CRC progression as the result of metabolic reprogramming. The following specific aims are proposed: 1) to delineate the molecular mechanism by which PHLPP regulates lipogenesis in CRC cells. We will perform the Stable Isotope-Resolved Metabolomics (SIRM) analysis to determine how PHLPP-loss affects lipid metabolism in CRC cells; 2) to determine the functional importance of PHLPP-mediated regulation of lipogenesis. We will test if PHLPP-loss renders CRC cells more resistant to energy stress and chemotherapy drugs as the result of alterations in lipogenesis; and 3) to define the role of PHLPP in suppressing lipogenesis in vivo. The effect of PHLPP-loss on modulating lipid metabolism will be determined using both genetically modified mouse models and patient-derived xenograft models. Results from our studies will fill an important knowledge gap on how altered lipogenesis affects the prognosis and clinical outcome in CRC patients. Ultimately, by providing insight into the mechanisms by which PHLPP-mediated lipogenesis, our findings will help to develop new treatment strategies in CRC patients.