Sterol Response Element-binding Protein 1c (SREBP1c) Is Involved in the Polyunsaturated Fatty Acid Suppression of Hepatic S14 Gene Transcription

Abstract

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.

Nutrient-gene interactions: a single nutrient and hundreds of target genes

“…In contrast, unsaturated fatty acids suppress the nuclear SREBP-1c levels and when nSREBP-1c is over-expressed in primary hepatocytes it eliminates the effects of polyunsaturated fatty acids (PUFA) on several lipogenic genes. This indicates that SREBP-1c is a key target for PUFA action on de novo lipogenesis (Mater et al, 1999;Moon et al, 2002). In Zndeficient rats, the fatty acid composition of liver lipids changes substantially with a two-to three-fold rise in particular of a-linolenic, g-linolenic and eicosapentanoic acid concentrations (unpublished data) and similar changes have been observed in previous studies on zincdeficient rats (Eder et al, 1994).…”

The Role of Liver X Receptor-α in the Fatty Acid Regulation of Hepatic Gene Expression

“…Primary Hepatocytes and Transfections-Male Sprague-Dawley rats maintained ad libitum on a Teklad chow diet were used for primary hepatocyte preparation (19). For RNA analysis, cells were plated onto 100-mm type I collagen-coated plates or Primaria plastic dishes (BD Biosciences) at 10 7 cells/plate in Williams E with 10 mM lactate, 10 nM dexamethasone, 100 nM insulin, and 10% fetal calf serum.…”

“…Animals were either maintained on this diet or switched to a high carbohydrate diet supplemented with fish oil (10%, w/w) (Dyets, Bethlehem, PA). After 5 days on the olive oil or fish oil diets, animals were euthanized ϳ2 h after completing the meal, and livers were removed for RNA analysis (19).…”

“…In primary hepatocytes, unsaturated fatty acids have little impact on SREBP-1c gene transcription (18,19) but enhance mRNA SREBP-1c turnover (16). Over-expression of nSREBP-1c in primary hepatocytes eliminates the polyunsaturated fatty acid (PUFA) effects on several lipogenic genes, indicating that SREBP-1c is a key target for PUFA action on de novo lipogenesis (19,20).…”

“…In established cell lines, unsaturated fatty acids inhibit transcription of the SREBP-1 gene (15), enhance mRNA SREBP-1 turnover (16), and interfere with proteolytic processing of SREBPs (17). In primary hepatocytes, unsaturated fatty acids have little impact on SREBP-1c gene transcription (18,19) but enhance mRNA SREBP-1c turnover (16). Over-expression of nSREBP-1c in primary hepatocytes eliminates the polyunsaturated fatty acid (PUFA) effects on several lipogenic genes, indicating that SREBP-1c is a key target for PUFA action on de novo lipogenesis (19,20).…”