DIETARY FAT REGULATION OF HEPATIC GENE EXPRESSION

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Dietary polyunsaturated fatty acids (PUFA), particularly those rich in 20- and 22-carbon (n-3) fatty acids, have several unique metabolic effects including suppression of VLDL, inhibition of cholesterol synthesis, and modulation of growth of certain carcinomas. A particularly intriguing feature of PUFA is their ability to regulate expression of several genes involved in lipid metabolism including fatty acid synthase, glucose-6-phosphate dehydrogenase and apo-lipoprotein. We recently reported that PUFA suppressed the expression of fatty acid synthase (FAS) and the S14 protein, a putative lipogenesis-related protein. PUFA inhibited FAS and S14 gene expression by reducing hepatic cellular mRNA levels through inhibition of gene transcription. These studies suggest dietary fats may represent important mediators of hepatic gene expression at the nuclear level. We propose that PUFA inhibit FAS and S14 gene transcription by interfering with the normal endocrine signalling mechanisms which are operative for these two genes (i.e. T3 or insulin) or that PUFA act through unique trans-acting factors which control the transcription of these genes. In order to test this hypothesis, the following studies are proposed: 1) To characterize the in vivo (in rats) and in vitro (in hepatocytes) regulation of FAS and S14 gene transcription by dietary saturated, mono-, di-saturated and polyenic fatty acids; 2) To identify specific PUFA-regulated cis-acting elements which control S14 gene transcription using the extensively characterized rat liver S14 chromatin structure model. To corroborate the in vivo studies, S14-CAT (Sl4-chloramphenicol acetyl transferase) and FAS-CAT fusion genes will be used to transfect cultured hepatocytes. 3) To identify specific PUFA-regulated trans-acting factors, hepatic nuclear extracts from control and PUFA fed rats will be examined for specific effects on DNA-protein interactions within the regulatory regions of the S14 and FAS genes using established in vitro transcription initiation and gel shift analysis. The information gained from These studies will be of significant biomedical importance because elucidation of the molecular mechanisms by which PUFA control the expression of genes coding for proteins involved in lipid synthesis may allow for the development of potential hypolipemic agents, as well as provide insight into a novel approach for the development of inhibitors which may have utility as anti-obesity agents. Finally, it is not unreasonable to expect that the PUFA mechanism of gene control may extend beyond lipogenesis and could explain other metabolic actions of PUFA.