The overall objective of this renewal is to define the cellular mechanisms linking hepatocyte cytotoxicity to hepatic injury, inflammation and fibrogenesis in the syndrome of nonalcoholic steatohepatitis (NASH), a common human liver disease which can progress to cirrhosis and its lethal sequelae. NASH is characterized by elevated levels of circulating free fatty acids (FFAs), hepatocyte lipoapoptosis and macrophage (M?)- associated liver inflammation, termed hepatic lipotoxicity. Recent evidence from our laboratory supported by this grant indicates that: a) FFA metabolism to lysophosphatidyl choline (LPC) contributes to hepatocyte lipoapoptosis; b) LPC stimulates hepatocyte release of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-bearing microvesicles by a caspase-dependent mechanism; and c) these microvesicles activate M?s via ligand-dependent, TRAIL receptor (TR)-mediated signaling events. Furthermore, we have developed a mouse model of diet induced obesity, which phenocopies human NASH, by feeding mice a diet high in fructose, saturated fats and cholesterol (FFC diet). Liver injury and hepatic M? numbers are reduced in TR knockout (TR-/-) mice fed the FFC diet compared to the wild-type mice. Based on these extensive preliminary data, we propose the novel CENTRAL HYPOTHESIS that toxic lipids, via TR-dependent signaling pathways, induce extracellular release of TRAIL-bearing microvesicles from hepatocytes which, in turn, activate resident and recruited M?s promoting steatohepatitis. We will now employ current and complementary, molecular, biochemical and cell biological approaches to ascertain how TR signaling mediates hepatic lipotoxicity. Our integrated SPECIFIC AIMS will test three hypotheses. FIRST, we will directly test the hypothesis that hepatic lipotoxicity induces release of hepatocyte microvesicles: a) by a caspase-dependent, proteolytic activation of Rho-associated kinase1 (ROCK1); and b) by a ligand-independent, TR-dependent proapoptotic signaling pathway. SECOND, we will test the hypothesis that extracellular microvesicles activate M?s: a) by ligand-dependent, TR initiated receptor interacting protein 1 (RIP1) protein complex; and b) by a c- Jun-N-terminal Kinase (JNK)-dependent signaling pathway. FINALLY, in an animal model of NASH, we will test the hypothesis that: a) liver injury and fibrogenesis are attenuated in TR-/- mice; and b) TR signaling in M? is critical for their activation and inflammation. This proposal is technically and conceptually innovative as it exploits state-of-the-art understanding in nanomedicine (i.e., microvesicle generation) with a three compartment systems biology approach (i.e., hepatocyte injury -> microvesicles -> M? activation) to solve the conundrum, how does lipoapoptosis promote liver inflammation? This work will not only provide new mechanistic insight into the pathogenesis of liver injury in NASH, but also should identify therapeutic strategies (e.g., ROCK1 inhibition of microvesicle generation) that can be utilized to treat this disorder.
The grant examines the cellular mechanisms by which the liver is injured by conditions of nutrient excess, obesity and diabetes. We propose that toxic lipids such as free fatty acids and their metabolites stimulate activation of prodeath proteins causing liver cells to release microvesicles which induce inflammation by white blood cells. These processes culminate in progressive liver injury with scarring and fibrosis of the liver, conditions resulting in cirrhosis and its sequelae of chronic liver failure. The results of these studies are germane to mechanisms of liver injury in the common syndrome of nonalcoholic steatohepatitis, and should identify new therapeutic strategies for this human liver disease.
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