Arachidonic acid and PGE

N–6 polyunsaturated fatty acids (PUFA) suppress hepatic and adipocyte de novo lipogenesis by inhibiting the transcription of genes encoding key lipogenic proteins. In cultured 3T3-L1 adipocytes, arachidonic acid (20:4,n–6) suppression of lipogenic gene expression requires cyclooxygenase (COX) activity. In this study, we found no evidence to support a role for COX-1 or -2 in the 20:4,n–6 inhibition of hepatocyte lipogenic gene expression. In contrast to L1 preadipocytes, adipocytes and rat liver, RT-PCR and Western analyses did not detect COX-1 or COX-2 expression in cultured primary hepatocytes. Moreover, the COX inhibitor, flurbiprofen, did not affect the 20:4,n–6 regulation of lipogenic gene expression in primary hepatocytes. Despite the absence of COX-1 and -2 expression in primary hepatocytes, prostaglandins (PGE 2 and PGF 2 a ) suppressed fatty acid synthase, L -pyruvate kinase, and the S14 protein mRNA, while having no effect on acyl-CoA oxidase or CYP4A2 mRNA. Using PGE 2 receptor agonist, the PGE 2 effect on lipogenic gene expression was linked to EP3 receptors. PGE 2 inhibited S14CAT activity in transfected primary hepatocytes and targeted the S14 PUFA-response region located 2 220 to 2 80 bp upstream from the transcription start site. Taken together, these studies show that COX-1 and COX-2 do not contribute to the n–6 PUFA suppression of hepatocyte lipogenic gene expression. However, cyclooxygenase products from non-parenchymal cells can act on parenchymal cells through a paracrine process and mimic the effects of n–6 PUFA on lipogenic gene expression. —Mater, M. K., A. P. Thelen, and D. B. Jump. Arachidonic acid and PGE 2 regulation of hepatic lipogenic gene expression. J. Lipid Res . 1999. 40: 1045–1052. Supplementary key words S14 protein • fatty acid synthase • l -pyruvate kinase • cyclooxygenase • gene transcription The effect of dietary n–6 polyunsaturated fatty acids (PUFA) on hepatic de novo lipogenesis was first reported by Allman and Gibson (1) while studying essential fatty acid deficiency. Those studies showed that removal of n–6 PUFA from the diet led to a rise in hepatic de novo lipogenesis. In subsequent studies, Flick, Chen, and Vagalos (2) reported that in vivo administration of indomethacin failed to block the n–6 PUFA suppression of fatty acid synthase (FAS) activity. N–6 PUFA are precursors of prostaglandins and indomethacin inhibits prostaglandin synthesis. These observations led investigators to suggest that n– 6 PUFA regulation of hepatic lipogenesis did not involve prostaglandins. However, a recent study examining the effects of n–6 PUFA on lipogenic gene expression in cultured adipocytes implicated the involvement of cyclooxygenase (3). Briefly, treatment of 3T3-L1 adipocytes with 20:4,n–6 suppressed (ED 50 , 50 m m ) mRNAs encoding two proteins involved in lipogenesis, FAS and the S14 protein (S14). The 20:4,n–6 effect on lipogenic gene expression was blocked by flurbiprofen, a non-specific cyclooxygenase inhibitor. PGE 2 and PGF 2 a also suppressed mRNA S14 (ED 50 , 10 m m ) and the PGE 2 effect on adipocyte gene expression was blocked by pertussis toxin, implicating a G i -linked regulatory pathway. Like adipocytes, n–6 PUFA suppression of hepatic lipogenic gene transcription leads to a decline in de novo lipogenesis (4–7). The liver contains Kupffer and endothelial cells, cells known to generate prostanoids under appropriate stimulation (8). Moreover, primary hepatocytes respond to prostanoids through specific G-protein-linked receptors leading to changes in carbohydrate metabolism and DNA synthesis (8–13). In this report, we have reexamined the requirement for cyclooxygenase in the n–6 PUFA regulation of hepatic lipogenic gene expression and have extended this analysis to include effects of prostaglandins on lipogenic gene expression. We report that 20:4,n–6 suppression of lipogenic gene expression in hepatic parenchymal cells does not involve metabolism through a cyclooxygenase-dependent pathway. However, PGE 2 and PGF 2 a suppress mRNAs encoding FAS, S14, and l -pyruvate kinase (L-PK) as well as the activity of S14CAT reporter genes in transfected primary hepatocytes. These studies suggests that PUFA and prostanoids may utilize a common mechanism at the genomic level to control lipogenic gene