Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are rapidly becoming the commonest reason patients in the United States seek advice from a gastroenterologist. Insulin resistance is the primary pathophysiologic problem that results in a net accumulation of triglycerides in hepatocytes. At present there is limited therapy for fatty liver disease. Glucagon-like peptide (GLP-1) is a naturally occurring gut peptide secreted by the L cells of the small intestine. It is cleaved by dipeptidyl peptidase IV (DPPIV) and thus it biological half-life is short. Exendin-4 is a homologous peptide resistant to DPPIV cleavage. Recent data reveals that GLP-1 and its homologue improve insulin resistance. These peptides have pleotropic effects including stimulating growth of b cells of the pancreas and action on adipocytes. To date there is limited data about a direct role of GLP-1 in hepatocytes. Our long-term goal in this application is to address the fundamental biological actions of GLP-1 on fat metabolism in the hepatocyte and prove that the beneficial effects of GLP-1 are not only related to insulin sensitizing effects but also result from direct biological actions on hepatocytes. We will provide a molecular basis for a potentially novel and safe treatment modality for NAFLD and NASH since GLP-1 proteins are also anorexigenic and consequently such therapy would have multiple benefits for this patient population. Preliminary data in long-term administration of Exendin-4 to ob/ob mice indicate marked improvement in key parameters associated with enhanced insulin sensitivity including weight loss, improved hepatic histology and loss of hepatic steatosis, and reduced thiobarbituric acid reactive substances (TEARS), a surrogate marker of oxidative stress. In this proposal we will test the hypothesis that GLP-1, or its synthetic analogue, acts directly on the hepatocyte to reduce net hepatic triglyceride stores via activation of its G-protein coupled receptor (GPCR).
Three aims have been developed to test this hypothesis.
Specific Aim 1 :To determine the cellular basis for GLP-1- hepatocyte interactions in the liver by studying (i) GLP-1-hepatocyte binding characteristics, (ii) measurement of cAMP production, and (iii) elucidation of other hepatocyte signal transduction proteins phosphorylated by GLP-1.
Specific Aim 2 : To employ RNA interference for GLP-1 receptor and determine whether the gene profile associated with GLP-1, which favors hepatocyte fatty acid depletion, is reversed;and, to measure the effect of GLP-1 on key enzymes involved with hepatic fatty acid metabolism.
Specific Aim 3 : To quantify the biochemical reduction of de novo synthesis of triglycerides and enhanced oxidation of fatty acids and VLDL secretion by GLP-1. The completion of this work will provide potential treatment for fatty liver disease, the most common cause of liver function abnormalities in the United States. This proposal meets the goals of the US Public Health Service in treating chronic liver diseases.
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