Hepatokine Control of Metabolic Crosstalk and Insulin Resistance

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Project Summary The major goal of this project is to understand how the NAD+-dependent deacetylase SIRT1 regulates hepatokines that are secreted by the liver and how this process slows the progression of non-alcoholic fatty liver disease (NAFLD) and obesity. Obesity or being overweight affects approximately 70% of U.S. adults and increases the prevalence of developing NAFLD and Type 2 diabetes. Type 2 diabetes is a life-threatening disease characterized by peripheral insulin resistance, which dysregulates inter-tissue communication to promote hyperglycemia and dyslipidemia. However, whether the liver controls systemic metabolism by functioning as an endocrine organ to engage other metabolic tissues through secreted factors is a novel and under-explored area. Our preliminary studies reveal a series of novel and exciting observations that support our hypothesis. The loss of SIRT1 in the liver leads to an obese phenotype manifested by increased fat mass and decreased energy expenditure. Strikingly, gene expression profiling analyses identify that fibroblast growth factor 21 (FGF21)—a “lean factor” secreted by the liver (called hepatokine)—is the most markedly downregulated gene in the liver of liver-specific SIRT1 knockout (SIRT1 LKO) mice. Thus, our Central Hypothesis is that hepatic SIRT1-regulated hepatokines have therapeutic implications for NAFLD and obesity through the autocrine regulation of hepatic lipid metabolism and endocrine control of adipose tissue function. Because hepatic and circulating levels of FGF21 are remarkably decreased in SIRT1 LKO mice, we choose to study the role of hepatokines such as FGF21 in SIRT1 action. Using gain- and loss-of-function mouse models, this central hypothesis will be tested in three Specific Aims: 1) To determine whether hepatic SIRT1, via stimulating the hepatokine FGF21, protects against whole-body insulin resistance and metabolic abnormalities in obesity; 2) To elucidate the molecular mechanisms by which the hepatocyte-derived SIRT1- FGF21 signaling exerts an autocrine effect to ameliorate hepatic steatosis; and 3) To investigate whether hepatic SIRT1-induced FGF21 hormone has an endocrine effect on beige adipocytes and insulin resistance in white adipose tissue. This project is an innovative departure from the study of a single tissue or pathway and thus is likely to reveal the mechanisms by which hepatic SIRT1 defects alone give rise to many features of obesity. Innovative aspects of the application also include: the novel concept that SIRT1-mediated regulation of hepatokines represents the molecular basis for liver and adipose tissue communication, the new mechanistic insight into that SIRT1 regulates FGF21 transcription via a mechanism involving deacetylation, and the technical innovation of RNA-sequencing and metabolomics analyses. Overall, accomplishing this proposal will not only provide fundamental insight into a previously unrecognized endocrine role of the liver in controlling systemic and adipose tissue metabolism but will also identify new targets for treating NAFLD and obesity.