Mechanisms of glucagon-like peptide 1 (GLP-1) in fatty liver disease

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DESCRIPTION (provided by applicant): Non-alcoholic fatty liver disease (NAFLD) is rapidly becoming a major health concern among US veterans. Liver fat is usually innocuous; however a growing body of literature suggests that a significant number of Americans will develop liver injury from hepatic fat stores including hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). We have found that the naturally occurring hormone glucagon-like peptide 1 (GLP-1) has the capacity to significantly reduce hepatocyte steatosis in human cells. CLINICAL RELEVANCE: With the advent of long-acting synthetic isoforms of GLP-1 agonists such agents may be attractive therapy for treating NAFLD. The goal of this proposal is to focus on how GLP-1 proteins reduce hepatocyte storage of fat, and prevent hepatocyte injury. OBJECTIVE: GLP-1 agonists enhance hepatocyte capacity to handle endoplasmic reticular (ER) stress reducing lipotoxicity, hepatocyte apoptosis, and progression of NAFLD. RESEARCH PLAN AND METHODS - SPECIFIC AIM 1: To demonstrate the specificity and therapeutic efficacy of GLP-1 agonists to prevent progression of NAFLD in a model of diet-induced fatty liver in wild-type mice, and by proof-of-concept studies in mice with a deletion mutant for the hepatocyte GLP-1R (GLP- 1RCre+/flox/flox). Preliminary work includes establishing 1) a dietary model capable of inducing NAFLD and hepatocyte insulin resistance; and 2) successful breeding of a tissue-specific knockout mouse for the GLP-1 receptor (GLP-1R) in the liver, GLP-1RCre+/flox/flox confirmed by qPCR of liver mRNA. SPECIFIC AIM 2: To further characterize the human hepatocyte GLP-1 receptor and to clarify the signaling mechanisms associated with potential hepatocyte insulin-sensitizing effects of GLP-1 agonists which reduce free fatty acid hepatocyte stores. Preliminary data regarding fat-loaded hepatocytes that GLP-1 receptors are G- protein coupled receptors (GPCRs), and can reduce fatty acid stores and modulate key genes associated with fatty acid ¿-oxidation with respect to enhanced mRNA and protein production as well as enzyme activity. These data have implications for decreased lipotoxicity. SPECIFIC AIM 3: To identify how GLP-1 proteins regulate the integrated stress response including activation of the three ER transmembrane proteins associated with stress transcriptional activation, and post-transcriptional modification o c-¿ homologous protein (CHOP) as well as expression of key proteins associated with cell survival and apoptosis. Preliminary in vitro data indicate that free fatty acids are lipotoxic to human hepatocytes and the GLP-1 agonist, exendin-4, suppresses human hepatocyte apoptosis including reducing cleaved caspase-3 production compared to untreated controls. The exendin-4 competitive antagonist fails to demonstrate such protection. Additional data indicate that exendin-4 suppresses CHOP expression both in vitro and in mice fed a high-fat diet, but in CHOP knockout mice fed an identical diet, fat-laden hepatocytes are protected from apoptotic death.