mTORC1 activation and alcoholic liver injury

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Summary mTORC1 activation and alcoholic liver injury Key words: alcohol fatty liver, mTORC1, DEPTOR, ER stress, and SREBP-1 Alcoholism is a leading cause of liver disease in Western societies. Hepatic steatosis (fatty liver) is an early and reversible stage of alcoholic liver disease. However, unchecked hepatic steatosis can develop into irreversible steatohepatitis, fibrosis, cirrhosis, and ultimately hepatocellular carcinoma. Alcoholic fatty liver disease (AFLD) is attributed to the activation of endoplasmic reticulum (ER) stress signaling. However, the molecular mechanisms underlying hepatic ER stress in AFLD are not fully understood. Although great progress has been made in the identification of mammalian target of rapamycin complex 1 (mTORC1) pathway components, relatively little is known about the in vivo role of the mTORC1 pathway in alcoholic liver pathophysiology. Our recent studies demonstrate that hepatic inhibition of mTORC1 by the NAD-dependent deacetylase SIRT1 suppresses hepatic ER stress, downregulates lipogenesis, and thereby ameliorates hepatic steatosis in diabetic mice. Exciting preliminary data show that hepatic mTORC1 is activated in chronic binge alcohol-fed mice, which is accompanied by induction of ER stress, activation of lipogenesis, and hepatic steatosis. Importantly, new studies suggest that consistent with mTORC1 activation, hepatic levels of DEPTOR, a newly identified endogenous inhibitor of mTORC1, are reduced in alcohol-fed mice. To better understand the pathogenesis of alcoholic liver disease and develop alternative therapeutic strategies for the disease, our CENTRAL HYPOTHESIS is that mTORC1 plays a key role in alcoholic fatty liver disease by promoting hepatic ER stress and stimulating lipogenesis. Two specific aims are proposed: 1) To characterize the functional and mechanistic role of mTORC1 in alcohol-induced ER stress and lipogenesis in hepatocytes; 2) To determine whether mTORC1 inhibition ameliorates hepatic steatosis and ER stress in mice with alcoholic fatty liver. In response to the NIAAA program (PA-10-094) entitled “stress pathways in alcohol induced organ injury and protection”, the proposed studies will determine whether chronic alcohol exposure results in mTORC1 activation via mTORC1 components such as DEPTOR, TSC1/2 or Raptor and thereby accelerates the development of hepatic steatosis and ER stress. In vitro cell-based mechanistic studies as well as in vivo pharmacologic and genetic approaches of manipulating hepatic mTORC1 activity will be utilized. Innovative aspects of these proposed studies include: (1) new insight into mTORC1 as a novel regulator of alcohol-induced ER stress pathways and fatty liver; (2) the novel concept that DEPTOR-dependent inhibition of mTORC1 ameliorates alcoholic fatty liver and liver injury by relieving ER stress and inhibiting lipogenesis; (3) DEPTOR/mTORC1 as new targets for the therapeutic interventions of AFLD and ER stress-related liver diseases such as cancer. Our long-term objective is to elucidate the pathological mechanisms of AFLD, to identify novel molecular targets for intervention at this early and reversible stage of alcoholic liver disease, and to develop a potential marker of AFLD to aid early diagnosis and prognosis.