Non-alcoholic steatohepatitis or non-alcoholic fatty liver (NAFL) is a growing clinical problem that may account for a newly recognized etiology for cryptogenic cirrhosis. Leptin, a 16-kilodalton protein resulting from the transcription of the obese gene, is a hormone associated with weight and satiety control. NAFL, along with obesity, type II diabetes mellitus, and hyperlipidemia, are conditions characterized by high circulating concentrations of the hormone leptin. The overall goal of this proposal is to demonstrate the biological consequences of leptin as a novel mediator of liver fibrosis, the precursor to cirrhosis. Preliminary data indicate leptin has profibrogenic activity in the principal collagen producing cells of the liver, hepatic stellate cells (HSCs). In vivo data indicate that leptin is also required for liver fibrosis induced by carbon tetrachloride (CCl4) in lean, but not obese, mice. The preliminary data and the current proposal meet the long-term objectives of this laboratory: to understand basic mechanisms underlying chronic liver fibrosis. In this proposal, it is hypothesized that leptin is a profibrogenic cytokine in activated hepatic stellate cells and increases (2(I) collagen expression by phosphorylated signal transduction and activator of transcription (pSTAT) enhancing AP-1 binding to the alpha2(I) collagen promoter.
Three aims are outlined to test this hypothesis. First to (a) further characterize leptin as a novel profibrogenic cytokine by examining genes responsible for increased extracellular matrix (ECM) production in fibrotic liver; and (b) to elucidate the specific signal transduction pathway(s) responsible for the effect of leptin on (2(I) collagen expression in HSCs. Second to (a) characterize leptin-associated alpha2(I) collagen mRNA stabilization; to (b) identify specific cis-acting elements along the human alpha2(I) collagen promoter affected by leptin signaling that are responsible for increased collagen gene expression by employing deletion mutant and site-directed mutagenesis; and (c) to identify, by DNase I protection analysis and electrophoretic mobility shift assay, specific transcription factors associated with leptin altered collagen gene expression. Third, to exploit rodent animal models of obesity, and their lean littermates, to determine the mechanisms by which leptin may be required for liver fibrosis in wild-type mice exposed to CC14 but not in identically treated db/db or ob/ob mice; and whether leptin is required in a common bile duct ligation (CBDL) injury model using fa/fa, or Zucker Diabetic Fatty (ZDF) rats and their lean littermates.
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