The transport of long-chain fatty acids through the cell cytoplasm and their subsequent efficient utilization for energy production and glycerolipid synthesis may be facilitated by a diverse family of 14-15 kDa cytoplasmic fatty acid binding proteins (FABP). Differences in the structure, tissue expression and ligand-binding properties of the several FABP gene products also suggest that they are functionally specialized, but the exact role of these proteins in fatty acid transport and metabolism remains poorly understood. Through their binding of fatty acids and promoting their cytoplasmic diffusion and metabolic utilization, the FABP may also protect the cell from high concentrations of unbound fatty acids. In keeping with such a role, gene transcription of liver FABP (L- FABP) is induced in concert with enzymes of the extramitochondrial fatty acid oxidation pathways by xenobiotics (including hypolipidemic peroxisome proliferators) that inhibit mitochondrial beta-oxidation, and by fatty acyl metabolites that accumulate under conditions of cellular fatty acid overload. The mechanism of induction of enzymes of the extramitochondrial fatty acid oxidation pathways in peroxisomes and endoplasmic reticulum involves the activation of a nuclear receptor, the peroxisome proliferator activated receptor (PPAR) by ligands which are largely bound by L-FABP. L- FABP may thus modulate the activation of PPAR by fatty acyl metabolites and thereby play an important role in the maintenance of cellular homeostasis in disease states characterized by either excessive delivery or impaired cellular utilization of long-chain fatty acids, e.g., starvation, diabetes, alcoholic liver disease, Reye's Syndrome and inborn errors of mitochondrial beta-oxidation.
The aims of this proposal have, as their broad goals: 1) the elucidation of the function of the FABP in fatty acid transport and metabolism and 2) the understanding of the relationship of L-FABP function, expression and regulation to the function of PPAR- mediate gene regulation, and are to: (a) determine, through the selective overexpression of FABP in Xenopus laevis oocytes, the function of L-FABP and the comparative function of its structural congeners in the cellular transport, oxidation, esterification and synthesis of long-chain fatty acids; (b) determine the effect of L-FABP (and L-FABP antisense) expression in mammalian cell lines on PPAR activation by L-FABP ligands in expression-reporter gene transfection assays; (b)determine the regional patterns of L-FABP, peroxisomal enzyme and PPAR expression in the liver lobule by in situ hybridization, and (c) establish the molecular basis for L-FABP induction by localizing PPAR response elements in the L-FABP gene 5' promoter region using promoter-reporter gene constructs transfected into primary hepatocytes. Through the use of these approaches, the proposed studies will provide important new information regarding the poorly understood function of the cytosolic FABP, and the molecular mechanisms that underlie the cellular adaptive response to disease states characterized by increased fatty acid flux and accumulation.
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