Dietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor ␣ (PPAR␣) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPAR␣-deficient mouse to examine the role of PPAR␣ in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNA AOX (2.3-fold) and mRNA CYP4A2 (8-fold) and suppressed mRNA FAS and mRNA S14 by >80% in wild type mice. In PPAR␣-deficient mice, PUFA did not induce mRNA AOX or mRNA CYP4A2 , indicating a requirement for PPAR␣ in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNA FAS and mRNA S14 in the PPAR␣-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPAR␣ and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPAR␣ and is involved in the PUFA-mediated suppression of lipogenic gene expression.

Polyunsaturated fatty acids (PUFA) suppress hepatic lipogenic gene transcription through a peroxisome proliferator activated receptor ␣ (PPAR␣)-and cyclooxygenase-independent mechanism. Recently, the sterol response element-binding protein 1 (SREBP1) was implicated in the nutrient control of lipogenic gene expression. In this report, we have assessed the role SREBP1 plays in the PUFA control of three hepatic genes, fatty acid synthase, L-pyruvate kinase (LPK), and the S14 protein (S14). PUFA suppressed both the hepatic mRNA SREBP1 through a PPAR␣-independent mechanism as well as SREBP1c nuclear content (nSREBP1c, 65 kDa). Co-transfection of primary hepatocytes revealed a differential sensitivity of the fatty acid synthase, S14, and LPK promoters to nSREBP1c overexpression. Of the three promoters examined, LPK was the least sensitive to overexpressed nSREBP1c. Promoter deletion and gel shift analyses of the S14 promoter localized a functional SREBP1c cis-regulatory element to an E-box-like sequence ( ؊139 TCGCCTGAT ؊131 ) within the S14 PUFA response region. Although overexpression of nSREBP1c significantly reduced PUFA inhibition of S14CAT, overexpression of other factors that induced S14CAT activity, such as steroid receptor co-activator 1 or retinoid X receptor ␣, had no effect on S14CAT PUFA sensitivity. These results suggest that PUFA regulates hepatic nSREBP1c, a factor that functionally interacts with the S14 PUFA response region. PUFA regulation of nSREBP1c may account for the PUFA-mediated suppression of hepatic S14 gene transcription.

Polyunsaturated fatty acids (PUFAs) have been shown to have significant effects on hepatic lipogenic gene expression. The S14 gene has been used as a model to examine the effects of PUFAs on hepatic lipogenic gene expression. In vivo studies showed that feeding rats a high carbohydrate diet containing menhaden oil rapidly (within hours) and signfficantly (250%) attenuates hepatic S14 gene transcription and S14 mRNA abundance. The suppressive effect of menhaden oil was both gene and tissue specific. The effect of PUFAs on expression of the S14 mRNA and a transected S14 fusion gene (i.e., S14CAT4.3) was examined in cultured hepatocytes in the presence of trilodothyronine (T3), insulin, dexamethasone, and albumin under serum-free conditions. Whereas T3 stimulated both S14 mRNA (>40-fold) and S14CAT4.3 (>100-fold), eicosapentaenoic acid (C20:5a3) significantly attenuated (>80%) both S14 mRNA and S14CAT activity in a dosedependent fashion. The effects of C20:5 on hepatocyte gene expression were both gene and fatty acid specflc. Deletion analysis of transfected S14CAT fusion genes indicated that the S14 thyroid hormone response element (at -2.5 to -2.9 kb) was not sensitive to C20:5 control. The cis-linked PUFA response elements were localized to a region within the S14 proxinal promoter (at -80 to -220 bp). This region also contains cis-acting elements that potentiate T3 actVation of S14 gene ranscription. These studies suggest that C20:5 (or its metabolites) regulates factors within the S14 proximal promoter region that are important for T3 activation of S14 gene tanscription.Dietary polyunsaturated fatty acids (PUFAs), particularly those rich in 20-and 22-carbon (w3 and w6) fatty acids, have several unique metabolic effects including suppression of very low density lipoprotein production, reduction of cholesterol synthesis, diminution of blood pressure, and modulation of the growth of certain carcinomas (1-3). A particularly intriguing feature of PUFAs is their ability to regulate the expression of several genes involved in lipid metabolism, such as the genes encoding apolipoprotein AI (4), low density lipoprotein receptors (5), glucose-6-phosphate dehydrogenase (6), and fatty acid synthase and the S14 protein (7)(8)(9).PUFAs suppress the hepatic mRNAs coding for the S14 protein (pI 4.9; 17 kDa) in both adult and weaning rats by inhibiting S14 gene transcription (7-9). S14 gene transcription is induced by 3,5,3'-triiodothyronine (T3) (10,11) Rats (one rat per cage) were meal-fed a high carbohydrate (Hi-CHO; 58% glucose; ICN, Cleveland, OH)/fat-free diet from 9 a.m. until 12 noon daily (9). Diets were supplemented (at 10% wt/wt) with either triolein [1,2,3-tri(cis-9-octadecenoyl)-glycerol; >95%; Sigma] or menhaden oil (MaxEPA, Scherer, FL). All diets contained butylated hydroxytoluene at 0.1% wt/wt (9,21). Gas chromatographic analysis showed that menhaden oil contained 433.5 mg ofpolyenes perg as w03 fatty acids (347.1 mg/g) and w6 fatty acids (26.6 mg/g). EPA and DHA were present at 160.9 mg/g and 112.4...

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