The OBJECTIVES of this application are to advance understanding of genetic pathways in hepatic fatty acid (FA) metabolism that influence the development and regression of hepatic steatosis. The BACKGROUND to this proposal is our elucidation of critical gene-environment interactions that underlie the metabolic pathogenesis of hepatic steatosis, a requisite metabolic precursor to nonalcoholic fatty liver disease. In this application we will focus on pathways modulated by tissue-specific deletion of liver fatty acid binding protein (L-Fabp) and microsomal triglyceride transfer protein (Mttp), each of which play a dominant role in hepatic lipid metabolism. Our OVERARCHING HYPOTHESIS is that L- Fabp regulates metabolic trafficking of FA, cholesterol and bile acids and, in concert with Mttp, modulates substrate utilization for lipoprotein secretion versus storage. Our PRELIMINARY DATA demonstrate that L-Fabp-/- mice are protected against hepatic steatosis following a prolonged fast, and against obesity and hepatic steatosis when fed high saturated fat (SF) diets. We further identified kinetic defects in both hepatic and intestinal FA uptake, re-esterification and secretion in L-Fabp-/- mice. Based on these findings, studies in AIM 1 will ask, "How does L-Fabp deletion protect against high SF diet induced obesity and hepatic steatosis?" Other work demonstrated that FAs promote coordinated transcriptional regulation of hepatic L-Fabp and Mttp. L-Fabp-/- mice demonstrate attenuated hepatic steatosis with pharmacologic inhibition of Mttp, suggesting that FA trafficking via L-Fabp may be a requisite step in their metabolic channeling for storage as well as for utilization in VLDL assembly and secretion. Based on these findings, studies in AIM 2 will ask, "How does L-Fabp interact with Mttp to modulate FA trafficking for VLDL production versus storage and is this mediated in a tissue-specific manner?" Quantitative trait mapping identified a chromosomal locus colocalizing with L-Fabp, as a positional candidate for gallstone susceptibility in mice. We demonstrate that L-Fabp-/- mice are dramatically more susceptible to lithogenic diet (LD)-induced gallstones compared to C57BL/6 congenic controls with a phenotype including increased serum and hepatic free cholesterol, increased bile acid pool size and decreased fecal bile acid excretion. These findings suggest that LD-fed L-Fabp-/- mice manifest alterations in both hepatic cholesterol metabolism and biliary lipid secretion as well as changes in intestinal bile acid (BA) metabolism. Based on these findings, studies in AIM 3 will ask, "How does L- Fabp deletion predispose to gallstone susceptibility and alter enterohepatic BA and cholesterol flux?" Taken together, these studies will provide continued insight into the tissue-specific regulation and pathways of FA utilization relevant to both the pathogenesis and reversal of hepatic steatosis, as well as to cholesterol gallstone formation.
While much is known about the clinical features of NAFLD, relatively little is known about the genetic pathways that predict individual susceptibility to high fat diet-induced hepatic steatosis and specifically the metabolic origins and functional compartmentalization/biological significance of the FA species that accumulate. There is also a paucity of information concerning the integrated roles of intestinal and hepatic FA flux, the factors that regulate the metabolic dialog between de novo lipogenesis and utilization of plasma derived FA, and how these different sources of FA substrate influence the balance between storage versus VLDL production and biliary lipid secretion. Our experiments will use novel and unique mutant mouse genetic models to probe distinct metabolic and tissue-specific pathways of FA utilization and elucidate the adaptive mechanisms that influence development and regression of hepatic steatosis.
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