Polyunsaturated Fatty Acids Decrease Expression of Promoters with Sterol Regulatory Elements by Decreasing Levels of Mature Sterol Regulatory Element-binding Protein

Abstract

Membrane physiology, plasma lipid levels, and intracellular sterol homeostasis are regulated by both fatty acids and cholesterol. Sterols regulate gene expression of key enzymes of cholesterol and fatty acid metabolism through proteolysis of the sterol regulatory elementbinding protein (SREBP), which binds to sterol regulatory elements (SRE) contained in promoters of these genes. We investigated the effect of fatty acids on SREdependent gene expression and SREBP. Consistent results were obtained in three different cell lines (HepG2, Chinese hamster ovary, and CV-1) transfected with SREcontaining promoters linked to the luciferase expression vector. We show that micromolar concentrations of oleate and other polyunsaturated fatty acids (C18:2-C22:6) dose-dependently (0.075-0.6 mmol) decreased transcription of SRE-regulated genes by 20 -75%. Few or no effects were seen with saturated free fatty acids. Fatty acid effects on SRE-dependent gene expression were independent and additive to those of exogenous sterols. Oleate decreased levels of the mature sterol regulatory element-binding proteins SREBP-1 and -2 and HMG-CoA synthase mRNA. Oleate had no effect in sterol regulation defective Chinese hamster ovary cells or in cells transfected with mutant SRE-containing promoters. We hypothesize that unsaturated fatty acids increase intracellular regulatory pools of cholesterol and thus affect mature SREBP levels and expression of SREdependent genes.Cholesterol and fatty acids interact in cholesterol homeostasis and modulate plasma and intracellular lipid levels. Both fatty acids and cholesterol stimulate cholesterol esterification via activation of acyl-coenzyme A:cholesterol acyltransferase (ACAT), 1 a reaction that is important in regulating intracellular cholesterol pools (1-3). Fatty acids and cholesterol also exert negative feedback control on their respective biosynthetic pathways (4 -6).Sterol regulatory elements (SREs) are cis-acting promoter elements present in genes that perform key steps in lipid related metabolic pathways (7). In sterol depletion, the sterol regulatory element-binding proteins (SREBPs) are cleaved and activate SREs in the nucleus. Recent studies show that sterols regulate important genes of fatty acid as well as sterol metabolism (8 -10). Overexpression of SREBP-1 promotes fatty acid and triglyceride synthesis-related pathways as opposed to cholesterol pathways, whereas SREBP-2 overexpression affects cholesterol over triglyceride pathways (11).Although fatty acids have been demonstrated to be important regulators of plasma LDL cholesterol levels due to alterations in LDL receptor activity (12,13). Precise molecular mechanisms of regulation are not defined. Like cholesterol, fatty acids also modulate mechanisms important to cell function and cholesterol homeostasis (1-3, 14, 15).Because of the close interaction between fatty acid and cholesterol metabolism, we questioned whether fatty acids directly affect SREBP maturation and expression of genes that are controlled by SREs. Our experiments demo...

Regulation of gene transcription by fatty acids

“…Cell culture studies have clearly shown that PUFA have the ability to reduce SREBP1a and 1c mRNA and SREBP1 protein concentrations (Worgall et al, 1998). In our own experiments in the hamster feeding PUFA reduces mRNA for all three SREBPs in a manner that is dependent on the nature of the PUFA (Figure 3a).…”

Transcriptional regulation of phagocytosis-induced membrane biogenesis by sterol regulatory element binding proteins

“…In 293 and other cultured mammalian cells, proteolysis of SREBP-1a and SREBP-2 has been shown to be induced by cholesterol withdrawal (21,29,30). Moreover, growth in medium lacking unsaturated fatty acids promotes the activation of SREBP-1a and SREBP-1c͞ADD1 (24,31,32).…”

Stearoyl-CoA Desaturase 1 Gene Expression Is Necessary for Fructose-mediated Induction of Lipogenic Gene Expression by Sterol Regulatory Element-binding Protein-1c-dependent and -independent Mechanisms

“…The mechanism by which oleate mediates or induces SREBP-1c and lipogenic gene expression remains to be explored. Several in vitro studies have shown that oleate suppresses the proteolytic processing of SREBP-1 protein and reduces SREBP-1 and lipogenic gene expression (52)(53)(54). The inhibitory effect has been proposed to be attributed to the increase in the release of ceramide from sphingomyelin (55).…”