Oleate potentiates oxysterol inhibition of transcription from sterol regulatory element-1-regulated promoters and maturation of sterol regulatory element-binding proteins

Long-chain acyl-CoA-dependent regulation of gene expression in bacteria, yeast and mammals

Fatty acyl-CoA thioesters are essential intermediates in lipid metabolism. For many years there have been numerous conflicting reports concerning the possibility that these compounds also serve regulatory functions. In this review, we examine the evidence that long-chain acyl-CoA is a regulatory signal that modulates gene expression. In the bacteria Escherichia coli, long-chain fatty acyl-CoA bind directly to the transcription factor FadR. Acyl-CoA binding renders the protein incapable of binding DNA, thus preventing transcription activation and repression of many genes and operons. In the yeast Saccharomyces cerevisiae, genes encoding peroxisomal proteins are activated in response to exogenously supplied fatty acids. In contrast, growth of yeast cells in media containing exogenous fatty acids results in repression of a number of genes, including that encoding the delta9-fatty acid desaturase (OLE1). Both repression and activation are dependent upon the function of either of the acyl-CoA synthetases Faa1p or Faa4p. In mammals, purified hepatocyte nuclear transcription factor 4alpha (HNF-4alpha) like E. coli FadR, binds long chain acyl-CoA directly. Coexpression of HNF-4alpha and acyl-CoA synthetase increases the activation of transcription of a fatty acid-responsive promoter, whereas coexpression with thioesterase decreases the fatty acid-mediated response. Conflicting data exist in support of the notion that fatty acyl-CoA are natural ligands for peroxisomal proliferator-activated receptor alpha (PPARalpha).

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Sterol response element-binding protein 1c (SREBP1c) is involved in the polyunsaturated fatty acid suppression of hepatic S14 gene transcription

Polyunsaturated fatty acids (PUFA) suppress hepatic lipogenic gene transcription through a peroxisome proliferator activated receptor alpha (PPARalpha)- 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 PPARalpha-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 alpha, 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.

Sterol regulatory element binding protein-1 expression is suppressed by dietary polyunsaturated fatty acids. A mechanism for the coordinate suppression of lipogenic genes by polyunsaturated fats

Polyunsaturated fatty acids (PUFA) coordinately suppress the transcription of a wide array of hepatic lipogenic genes including fatty acid synthase (FAS) and acetyl-CoA carboxylase. Interestingly, the over-expression of sterol regulatory element binding protein-1 (SREBP-1) induces the expression of all of the enzymes suppressed by PUFA. This observation led us to hypothesize that PUFA coordinately inhibit lipogenic gene transcription by suppressing the expression of SREBP-1. Our initial studies revealed that the SREBP-1 and FAS mRNA contents of HepG2 cells were reduced by 20:4(n-6) in a dose-dependent manner (i.e. EC(50) approximately 10 microM), whereas 18:1(n-9) had no effect. Similarly, supplementing a fat-free, high glucose diet with oils rich in (n-6) or (n-3) PUFA reduced the hepatic content of precursor and nuclear SREBP-1 60 and 85%, respectively; however, PUFA had no effect on the nuclear content of upstream stimulatory factor (USF)-1. The PUFA-dependent decrease in nuclear content of mature SREBP-1 was paralleled by a 70-90% suppression in FAS gene transcription. In contrast, dietary 18:1(n-9), i.e. triolein, had no inhibitory influence on the expression of SREBP-1 or FAS. The decrease in hepatic expression of SREBP-1 and FAS associated with PUFA ingestion was mimicked by supplementing the fat-free diet with the PPARalpha-activator, WY 14, 643. Interestingly, nuclear run-on assays revealed that changes in SREBP-1 mRNA abundance were not accompanied by changes in SREBP-1 gene transcription. These results support the concept that PUFA coordinately inhibit lipogenic gene transcription by suppressing the expression of SREBP-1 and that the PUFA regulation of SREBP-1 appears to occur at the post-transcriptional level.

Regulation of peroxisome proliferator-activated receptor gamma expression by adipocyte differentiation and determination factor 1/sterol regulatory element binding protein 1: implications for adipocyte differentiation and metabolism

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor implicated in adipocyte differentiation and insulin sensitivity. We investigated whether PPARgamma expression is dependent on the activity of adipocyte differentiation and determination factor 1/sterol regulatory element binding protein 1 (ADD-1/SREBP-1), another transcription factor associated with both adipocyte differentiation and cholesterol homeostasis. Ectopic expression of ADD-1/SREBP-1 in 3T3-L1 and HepG2 cells induced endogenous PPARgamma mRNA levels. The related transcription factor SREBP-2 likewise induced PPARgamma expression. In addition, cholesterol depletion, a condition known to result in proteolytic activation of transcription factors of the SREBP family, induced PPARgamma expression and improved PPRE-driven transcription. The effect of the SREBPs on PPARgamma expression was mediated through the PPARgamma1 and -3 promoters. Both promoters contain a consensus E-box motif that mediates the regulation of the PPARgamma gene by ADD-1/SREBP-1 and SREBP-2. These results suggest that PPARgamma expression can be controlled by the SREBP family of transcription factors and demonstrate new interactions between transcription factors that can regulate different pathways of lipid metabolism.

Identification of conserved cis-elements and transcription factors required for sterol-regulated transcription of stearoyl-CoA desaturase 1 and 2

We previously identified stearoyl-CoA desaturase 2 (SCD2) as a new member of the family of genes that are transcriptionally regulated in response to changing levels of nuclear sterol regulatory element binding proteins (SREBPs) or adipocyte determination and differentiation factor 1 (ADD1). A novel sterol regulatory element (SRE) (5'-AGCAGATTGTG-3') identified in the proximal promoter of the mouse SCD2 gene is required for induction of SCD2 promoter-reporter genes in response to cellular sterol depletion (Tabor, D. E., Kim, J. B., Spiegelman, B. M., and Edwards, P. A. (1998) J. Biol. Chem. 273, 22052-22058). In this report, we demonstrate that this novel SRE is both present in the promoter of the SCD1 gene and is critical for the sterol-dependent transcription of SCD1 promoter-reporter genes. Two conserved cis elements (5'-CCAAT-3') lie 5 and 48 base pairs 3' of the novel SREs in the promoters of both the SCD1 and SCD2 murine genes. Mutation of either of these putative NF-Y binding sites attenuates the transcriptional activation of SCD1 or SCD2 promoter-reporter genes in response to cellular sterol deprivation. Induction of both reporter genes is also attenuated when cells are cotransfected with dominant-negative forms of either NF-Y or SREBP. In addition, we demonstrate that the induction of SCD1 and SCD2 mRNAs that occurs during the differentiation of 3T3-L1 preadipocytes to adipocytes is paralleled by an increase in the levels of ADD1/SREBP-1c and that the SCD1 and SCD2 mRNAs are induced to even higher levels in response to ectopic expression of ADD1/SREBP-1c. We conclude that transcription of both SCD1 and SCD2 genes is responsive to cellular sterol levels and to the levels of nuclear SREBP/ADD1 and that transcriptional induction requires three spatially conserved cis elements, that bind SREBP and NF-Y. Additional studies demonstrate that maximal transcriptional repression of SCD2 reporter genes in response to an exogenous polyunsaturated fatty acid is dependent upon the SRE and the adjacent CCAAT motif.

Fatty acids: links between genes involved in fatty acid and cholesterol metabolism

Fatty acids are a major constituent of dietary fats and form an integral part of the cellular membrane and lipoproteins. The gene regulatory potential of fatty acids has long been recognized, but the precise regulatory mechanisms are unknown. The regulatory ability of fatty acids on the expression of a number of genes together with potential mechanisms and pathways of regulation are reviewed. In this review, we emphasize a key aspect of regulation mediated by the sterol regulatory element binding-protein, and its effects on sterol regulatory elements.