Lipid Transport in the Intestine Fatty acids (FA) and monoacylglycerol (MG) are the primary hydrolytic products of dietary triacylglycerol (TG). Substantial gaps remain in our understanding of the basic mechanisms of MG and FA assimilation by the intestine. These include a molecular level understanding of the individual functions of two highly expressed enterocyte fatty acid-binding proteins (FABP), intestinal FABP and liver FABP (IFABP and LFABP). In previous studies using in vitro lipid transport kinetics, we suggested that IFABP and LFABP are likely to have at least some unique functions in the enterocyte. Recent studies using mice null for each of these FABPs strongly support this hypothesis. Another important gap in our understanding of intestinal lipid assimilation concerns the metabolic fate of FA and MG in the enterocyte. The absorptive epithelial cell exhibits marked differences in the metabolism of FA added at the dietary or apical (AP) surface of the cell, compared to the basolateral (BL) surface of the cell, and we have recently demonstrated striking metabolic polarity for MG as well. However, the mechanisms that underlie this compartmentation remain largely unknown. In this proposal, we will use an integrated approach of cellular and animal studies to address the functions of IFABP and LFABP, and the fundamental mechanisms that underlie the polarity of intestinal FA and MG metabolism.
The specific aims are 1) to determine the functions and structure/function relationships for IFABP and LFABP in enterocyte transport and metabolism of FA and MG using cell-based approaches. Our previous work using model membrane systems has defined the different kinetic mechanisms for these proteins, and the structural domains and amino acid residues responsible for their distinct properties. In these studies, direct protein transfer and DNA transfection approaches will be used to introduce wild-type and specific mutant forms of the FABPs into cultured cell models that recapitulate the enterocyte phenotype, and effects on lipid uptake and localization, lipid metabolism, and lipoprotein secretion will be determined. 2) To determine the specific functions of IFABP and LFABP in intestinal lipid assimilation using null mouse models. Mice null for IFABP and LFABP have been established in our laboratory, and several interesting intestinal phenotypic changes have been observed, including increased TG synthesis in the IFABP-/- mouse and decreased FA 2-oxidation in the LFABP knockout. Further, we have recently identified LFABP as a critical component in chylomicron biogenesis. Using the two FABP null mouse models, the mechanisms of FABP action will be explored using a combination of animal studies, proteomic, lipidomic, and structure-function approaches. 3) To determine the mechanisms underlying the metabolic compartmentation of FA and MG in the enterocyte. Studies in mouse models supplemented by cell culture approaches will be used to address the role of plasma membrane transporters and specific intracellular enzymes. The overall goal of this research program is to provide a molecular level picture of intestinal lipid traffic in order to enable the control of both the rate and extent of dietary lipid assimilation and postprandial lipid levels, by modulating specific transport and metabolic processes.
Lipid Transport in the Intestine Westerns diets often contain large quantities of dietary lipid, all of which must be digested, absorbed, and transported by the small intestine. Elevated serum lipid levels following meals are thought to be important in the development of atherosclerosis and diabetes. In the proposed research, we will investigate two incompletely understood areas of intestinal lipid assimilation. We will elucidate the specific functions of two intracellular fatty acid binding proteins in fatty acid transport, and we will determine how the products of lipid digestion, fatty acids and monoacylglycerols, are trafficked within the intestinal cell, leading to alterations in lipid metabolism. These studies will enable us to understand how to regulate the rate and extent of dietary lipid assimilation.
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