Regulation of hepatic Δ-6 desaturase expression and its role in the polyunsaturated fatty acid inhibition of fatty acid synthase gene expression in mice

Dietary polyunsaturated fatty acids (PUFA) of the (n-6) and (n-3) families uniquely suppress the expression of lipogenic genes while concomitantly inducing the expression of genes encoding proteins of fatty acid oxidation. Although considerable progress has been made toward understanding the nuclear events affected by PUFA, the intracellular mediator responsible for the regulation of hepatic lipogenic gene expression remains unclear. On the basis of earlier fatty acid composition studies, we hypothesized that the Δ-6 desaturase pathway was essential for the production of the fatty acid regulator of gene expression. To address this hypothesis, male BALB/c mice (n = 8/group) were fed for 5 d a high glucose, fat-free diet (FF) or the FF plus 50 g/kg 18:2(n-6) with and without eicosa- 5,8,11,14-tetraynoic acid (ETYA) (200 mg/kg diet), a putative inhibitor of the Δ-6 desaturase pathway. ETYA had no effect on food intake or weight gain, but it completely prevented 18:2(n-6) from suppressing the hepatic abundance of fatty acid synthase mRNA. ETYA ingestion was associated with a decrease in the hepatic content of 20:4(n-6) and an increase in the amount of 18:2(n-6). The fatty acid composition changes elicited by ETYA were accompanied by a decrease in the enzymatic activity of Δ-6 desaturase. Interestingly, the hepatic abundance of Δ-6 desaturase mRNA was actually induced by ETYA one- to twofold. When the product of Δ-6 desaturase, i.e., 18:3(n-6), was added to the ETYA plus 18:2(n-6) diet, the hepatic content of 20:4(n-6) was normalized. In addition, 18:3(n-6) consumption reduced the level of hepatic Δ-6 desaturase mRNA by 50% and completely prevented the increase in fatty acid synthase mRNA that was associated with ETYA ingestion. Apparently, Δ-6 desaturation is an essential step for the PUFA regulation of the fatty acid synthase gene transcription. Finally, the suppression of Δ-6 desaturase by PUFA and its induction by ETYA suggest that the Δ-6 desaturase gene may be regulated by two different lipid-dependent mechanisms.