Unsaturated fatty acids inhibit transcription of the sterol regulatory element-binding protein-1c (SREBP-1c) gene by antagonizing ligand-dependent activation of the LXR

Octanoate attenuates adipogenesis in 3T3-L1 preadipocytes

Preadipocytes exposed to octanoate accumulate less lipid than cells exposed to long-chain fatty acids. This effect of octanoate involves significant attenuation of expression of key adipogenic transcription factors, including peroxisome proliferator-activated receptor (PPAR)gamma, steroid regulatory binding element protein (SREBP)-1c and CCAAT element binding protein (C/EBPalpha) at both the mRNA and protein levels. Expression of differentiation markers, including adipocyte fatty acid binding protein (ALBP), glycerol-3-phosphate dehydrogenase (GPDH) and leptin, was also significantly diminished by octanoate. However, octanoate did not prevent the decrease in preadipocyte factor-1 (Pref-1) expression that occurs during adipogenesis, nor did it inhibit the early induction of C/EBPbeta,delta. Treatment with synthetic PPARgamma ligands partially offset the inhibitory effect of octanoate on differentiation. Ectopic expression of PPARgamma2 in 3T3-L1 cells partially restored lipid accretion and GPDH activity in octanoate-treated cells. Adding octanoate together with troglitazone attenuated the effects of troglitazone on adipocyte differentiation in both normal 3T3-L1 cells and engineered 3T3-L1 cells that expressed ectopic PPARgamma2, implying that octanoate might compete against troglitazone for its binding to PPARgamma. These results suggest that octanoate may block adipogenesis at least in part by its influence on the expression/activation of PPARgamma.

Oxysterols: friends, foes, or just fellow passengers?

Oxysterols are oxygenated derivatives of cholesterol that are intermediates or even end products in cholesterol excretion pathways. Because of their ability to pass cell membranes and the blood-brain barrier at a faster rate than cholesterol itself, they are also important as transport forms of cholesterol. In addition, oxysterols have been ascribed a number of important roles in connection with cholesterol turnover, atherosclerosis, apoptosis, necrosis, inflammation, immunosuppression, and the development of gallstones. According to current concepts, oxysterols are physiological mediators in connection with a number of cholesterol-induced metabolic effects. However, most of the evidence for this is still indirect, and there is a discrepancy between the documented potent effects of oxysterols under in vitro conditions and the studies demonstrating that they are of physiological importance in vivo. Oxysterol-binding proteins, such as liver X receptor-alpha (a nuclear receptor), do have a regulatory role in cholesterol turnover, but the physiological ligand of the protein has not yet been defined with certainty. Recently developed genetically engineered mouse models with markedly reduced or increased concentration of some of the oxysterols have exhibited surprisingly small changes in cholesterol turnover and homeostasis. The present review is a critical evaluation of the literature on oxysterols, in particular, the in vivo evidence for a role of oxysterols as physiological regulators of cholesterol homeostasis and as atherogenic factors.

Dietary polyunsaturated fatty acids and regulation of gene transcription

Dietary polyunsaturated fatty acids (PUFAs) are a source of energy and structural components for cells. PUFAs also have dramatic effects on gene expression by regulating the activity or abundance of four families of transcription factor, including peroxisome proliferator activated receptor (PPAR) (alpha, beta and gamma), liver X receptors (LXRs) (alpha and beta), hepatic nuclear factor-4 (HNF-4)alpha and sterol regulatory element binding proteins (SREBPs) 1 and 2. These transcription factors play a major role in hepatic carbohydrate, fatty acid, triglyceride, cholesterol and bile acid metabolism. Non-esterified fatty acids or fatty acid metabolites bind to and regulate the activity of PPARs, LXRs and HNF-4. In contrast, PUFAs regulate the nuclear abundance of SREBPs by controlling the proteolytic processing of SREBP precursors, or regulating transcription of the SREBP-1c gene or turnover of mRNA(SREBP-1c). The n3 and n6 PUFAs are feed-forward activators of PPARs, while these same fatty acids are feedback inhibitors of LXRs and SREBPs. Saturated fatty acyl coenzyme A thioesters activate HNF-4 alpha, while coenzyme A thioesters of PUFAs antagonize HNF-4 alpha action. Understanding how fatty acids regulate the activity and abundance of these and other transcription factors will likely provide insight into the development of novel therapeutic strategies for better management of whole body lipid and cholesterol metabolism.

Liver X receptors as insulin-mediating factors in fatty acid and cholesterol biosynthesis

The nuclear receptor liver X receptor (LXR) alpha, an important regulator of cholesterol and bile acid metabolism, was analyzed after insulin stimulation in liver in vitro and in vivo. A time- and dose-dependent increase in LXRalpha steady-state mRNA level was seen after insulin stimulation of primary rat hepatocytes in culture. A maximal induction of 10-fold was obtained when hepatocytes were exposed to 400 nm insulin for 24 h. Cycloheximide, a potent inhibitor of protein synthesis, prevented induction of LXRalpha mRNA expression by insulin, indicating that the induction is dependent on de novo synthesis of proteins. Stabilization studies using actinomycin D indicated that insulin stimulation increased the half-life of LXRalpha transcripts in cultured primary hepatocytes. Complementary studies where rats and mice were injected with insulin induced LXRalpha mRNA levels and confirmed our in vitro studies. Furthermore, deletion of both the LXRalpha and LXRbeta genes (double knockout) in mice markedly suppressed insulin-mediated induction of an entire class of enzymes involved in both fatty acid and cholesterol metabolism. The discovery of insulin regulation of LXR in hepatic tissue as well as gene targeting studies in mice provide strong evidence that LXRs plays a central role not only in cholesterol homeostasis, but also in fatty acid metabolism. Furthermore, LXRs appear to be important insulin-mediating factors in regulation of lipogenesis.

Differential effects of sterol regulatory binding proteins 1 and 2 on sterol 12 alpha-hydroxylase. SREBP-2 suppresses the sterol 12 alpha-hydroxylase promoter

The most important pathway for the catabolism and excretion of cholesterol in mammals is the formation of bile acids. Improper regulation of this pathway has implications for atherosclerosis, cholesterol gallstone formation, and some lipid storage diseases. Sterol 12 alpha-hydroxylase (12 alpha-hydroxylase) is required for cholic acid biosynthesis. The alpha(1)-fetoprotein transcription factor FTF is crucial for the expression and the bile acid-mediated down-regulation of 12 alpha-hydroxylase. Cholesterol, on the other hand, down-regulates expression of the 12 alpha-hydroxylase gene. In this study, we show that the two sterol regulatory binding proteins (SREBPs) have opposite effects on the 12 alpha-hydroxylase promoter. SREBP-1 activated the 12 alpha-hydroxylase promoter, as it does with many other cholesterol-regulated genes. In contrast, SREBP-2 suppressed 12 alpha-hydroxylase promoter activity. SREBP-1 mediates the cholesterol-down-regulation of 12 alpha-hydroxylase promoter by binding to two inverted sterol regulatory elements found approximately 300 nucleotides from the transcriptional initiation site. SREBP-2 mediated suppression of 12 alpha-hydroxylase without binding to its promoter. Data are presented suggesting that SREBP-2 suppresses the 12 alpha-hydroxylase promoter by interacting with FTF. This is the first report of a promoter responding oppositely to two members of the SREBP family of transcription factors. These studies provide a novel function and mode of action of a SREBP protein.

Unsaturated fatty acids inhibit cholesterol efflux from macrophages by increasing degradation of ATP-binding cassette transporter A1

Abnormal high density lipoprotein metabolism may contribute to the increased atherosclerosis associated with diabetes and insulin resistance. The ATP-binding cassette transporter ABCA1 mediates cholesterol transport from tissue macrophages to apoA-I, the major high density lipoprotein protein component. Because fatty acids are elevated in diabetes, we examined the effects of fatty acids on ABCA1 activity in cultured macrophages. Results showed that unsaturated fatty acids markedly inhibited ABCA1-mediated cholesterol and phospholipid efflux from macrophages when ABCA1 was induced by a cAMP analog. This was accompanied by a reduction in the membrane content of ABCA1 and a decrease in apoA-I binding to whole cells and to ABCA1. In contrast, saturated fatty acids had no effect on these processes. Fatty acids did not alter ABCA1 mRNA abundance or incorporation of methionine into ABCA1, indicating that decreased ABCA1 transcription, enhanced mRNA decay, or impaired translation efficiency did not account for these inhibitory effects. Unsaturated fatty acids, however, increased ABCA1 turnover when protein synthesis was blocked by cycloheximide. We conclude that unsaturated fatty acids reduce the macrophage ABCA1 content by enhancing its degradation rate. These findings raise the possibility that an increased supply of unsaturated fatty acids in the artery wall promotes atherogenesis by impairing the ABCA1 cholesterol secretory pathway in macrophages.

Unsaturated fatty acid-mediated decreases in sterol regulatory element-mediated gene transcription are linked to cellular sphingolipid metabolism

A major physiological feedback mechanism of cholesterol in transcription of a number of lipid metabolism-related genes is mediated by sterol regulatory elements (SREs) and their binding proteins (SREBPs). Polyunsaturated free fatty acids alone, as well as synergistically with sterols, decrease SRE-mediated gene expression up to 80% in a dose-dependent manner by decreasing levels of the active transcription factor SREBP. We investigated potential mechanisms for this effect. We hypothesized that free fatty acids reduce SREBP-mediated gene transcription by increasing intracellular cholesterol content through the hydrolysis of cellular sphingomyelin, which has a high affinity for free cholesterol. We also questioned whether the lipid second messenger ceramide, a product of sphingomyelin hydrolysis, can decrease SRE-mediated gene transcription. First we investigated the effect of fatty acids on sphingomyelin hydrolysis. Incubation of [(3)H]choline-labeled cells with unsaturated (but not saturated) fatty acids induced hydrolysis of [(3)H]choline-labeled sphingomyelin. Also, incubation of cell extracts from fatty acid-treated cells with [(3)H]sphingomyelin increased generation of [(3)H]ceramide compared with control cells in vitro. We found that addition of ceramide analogs alone and additively with fatty acids decreased SRE expression and that ceramide analogs reduced levels of the transcriptionally active forms of SREBP-1 and SREBP-2. Increasing intracellular ceramide levels by exogenous sphingomyelinase or inhibition of ceramidase decreased SRE-mediated gene expression. None of the above conditions induced apoptosis. Incubation with U18666A, a compound that inhibits intracellular cholesterol movement, increased SRE-mediated gene transcription. C(2)-ceramide abrogated the effect of U18666A on SRE-mediated gene transcription, suggesting cholesterol-independent regulation of SREBP. We provide evidence that sphingomyelin hydrolysis and intermediates of sphingomyelin metabolism (in addition to cholesterol and fatty acids) contribute to regulation of SRE-mediated gene transcription.

NF-Y involvement in the polyunsaturated fat inhibition of fatty acid synthase gene transcription

Dietary polyunsaturated fats (PUFA) reduce the hepatic content of SREBP-1 65-75%, and this is paralleled by a comparable decrease in the expression of fatty acid synthase (FAS) gene. The close association between the nuclear content of SREBP-1 and FAS transcription has led to the conclusion that PUFA inhibit lipogenic gene transcription by suppressing SREBP-1 expression, but this conclusion is based upon correlative data. When in fact the SREBP-1/USF sites of the insulin response element of FAS were mutated, only 25% of the PUFA inhibition of FAS promoter activity was lost. On the other hand, mutating the -99/-93 NF-Y site reduced overall promoter activity 85%, and eliminated 50% of the PUFA suppression of FAS promoter activity. In addition, extended cloning and transfection-reporter assays revealed that the FAS gene contains a second PUFA response region (PUFA-RR) in the distal area of -7382/-6970. Interestingly, the distal PUFA-RR(FAS) has many similarities to the PUFA-RR of l-pyruvate kinase gene while the proximal PUFA-RR(FAS) is comparable to the PUFA-RR of the S14 and stearoyl-CoA desaturase genes.

Polyunsaturated fatty acids suppress sterol regulatory element-binding protein 1c promoter activity by inhibition of liver X receptor (LXR) binding to LXR response elements

Previous studies have demonstrated that polyunsaturated fatty acids (PUFAs) suppress sterol regulatory element-binding protein 1c (SREBP-1c) expression and, thus, lipogenesis. In the current study, the molecular mechanism for this suppressive effect was investigated with luciferase reporter gene assays using the SREBP-1c promoter in HEK293 cells. Consistent with previous data, the addition of PUFAs to the medium in the assays robustly inhibited the SREBP-1c promoter activity. Deletion and mutation of the two liver X receptor (LXR)-responsive elements (LXREs) in the SREBP-1c promoter region eliminated this suppressive effect, indicating that both LXREs are important PUFA-suppressive elements. The luciferase activities of both SREBP-1c promoter and LXRE enhancer constructs induced by co-expression of LXRalpha or -beta were strongly suppressed by the addition of various PUFAs (arachidonic acid > eicosapentaenoic acid > docosahexaenoic acid > linoleic acid), whereas saturated or mono-unsaturated fatty acids had minimal effects. Gel shift mobility and ligand binding domain activation assays demonstrated that PUFA suppression of SREBP-1c expression is mediated through its competition with LXR ligand in the activation of the ligand binding domain of LXR, thereby inhibiting binding of LXR/retinoid X receptor heterodimer to the LXREs in the SREBP-1c promoter. These data suggest that PUFAs could be deeply involved in nutritional regulation of cellular fatty acid levels by inhibiting an LXR-SREBP-1c system crucial for lipogenesis.

Multidrug permeases and subcellular cholesterol transport

Studies of Niemann-Pick C (NPC) and Tangier diseases have led to the identification of the causative genes, NPC1 and ABCA1, respectively. Characterization of their protein products shows that NPC1 and ABCA1 are permeases that belong to two different superfamilies of efflux pumps, which might be important in subcellular lipid and cholesterol transport.