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

N-3 polyunsaturated fatty acids may affect the course of COVID-19

The highly infectious coronavirus disease (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is a new coronavirus that has been spreading since late 2019 and has caused millions of deaths worldwide. COVID-19 continues to spread rapidly worldwide despite high vaccination coverage; therefore, it is crucial to focus on prevention. Most patients experience only mild symptoms of COVID-19. However, in some cases, serious complications can develop mainly due to an exaggerated immune response; that is, a so-called cytokine storm, which can lead to acute respiratory distress syndrome, organ failure, or, in the worst cases, death. N-3 polyunsaturated fatty acids and their metabolites can modulate inflammatory responses, thus reducing the over-release of cytokines. It has been hypothesized that supplementation of n-3 polyunsaturated fatty acids could improve clinical outcomes in critically ill COVID-19 patients. Some clinical trials have shown that administering n-3 polyunsaturated fatty acids to critically ill patients can improve their health and shorten the duration of their stay in intensive care. However, previous clinical studies have some limitations; therefore, further studies are required to confirm these findings.

Coordinate Regulation of Yeast Sterol Regulatory Element-binding Protein (SREBP) and Mga2 Transcription Factors

The Mga2 and Sre1 transcription factors regulate oxygen-responsive lipid homeostasis in the fission yeast Schizosaccharomyces pombe in a manner analogous to the mammalian sterol regulatory element-binding protein (SREBP)-1 and SREBP-2 transcription factors. Mga2 and SREBP-1 regulate triacylglycerol and glycerophospholipid synthesis, whereas Sre1 and SREBP-2 regulate sterol synthesis. In mammals, a shared activation mechanism allows for coordinate regulation of SREBP-1 and SREBP-2. In contrast, distinct pathways activate fission yeast Mga2 and Sre1. Therefore, it is unclear whether and how these two related pathways are coordinated to maintain lipid balance in fission yeast. Previously, we showed that Sre1 cleavage is defective in the absence of mga2 Here, we report that this defect is due to deficient unsaturated fatty acid synthesis, resulting in aberrant membrane transport. This defect is recapitulated by treatment with the fatty acid synthase inhibitor cerulenin and is rescued by addition of exogenous unsaturated fatty acids. Furthermore, sterol synthesis inhibition blocks Mga2 pathway activation. Together, these data demonstrate that Sre1 and Mga2 are each regulated by the lipid product of the other transcription factor pathway, providing a source of coordination for these two branches of lipid synthesis.

The integration of epigenetics and genetics in nutrition research for CVD risk factors

There is increasing evidence documenting gene-by-environment (G × E) interactions for CVD related traits. However, the underlying mechanisms are still unclear. DNA methylation may represent one of such potential mechanisms. The objective of this review paper is to summarise the current evidence supporting the interplay among DNA methylation, genetic variants, and environmental factors, specifically (1) the association between SNP and DNA methylation; (2) the role that DNA methylation plays in G × E interactions. The current evidence supports the notion that genotype-dependent methylation may account, in part, for the mechanisms underlying observed G × E interactions in loci such asAPOE, IL6and ATP-binding cassette A1. However, these findings should be validated using intervention studies with high level of scientific evidence. The ultimate goal is to apply the knowledge and the technology generated by this research towards genetically based strategies for the development of personalised nutrition and medicine.

Stearoyl CoA desaturase-1: New insights into a central regulator of cancer metabolism

The processes of cell proliferation, cell death and differentiation involve an intricate array of biochemical and morphological changes that require a finely tuned modulation of metabolic pathways, chiefly among them is fatty acid metabolism. The critical participation of stearoyl CoA desaturase-1 (SCD1), the fatty acyl Δ9-desaturing enzyme that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), in the mechanisms of replication and survival of mammalian cells, as well as their implication in the biological alterations of cancer have been actively investigated in recent years. This review examines the growing body of evidence that argues for a role of SCD1 as a central regulator of the complex synchronization of metabolic and signaling events that control cellular metabolism, cell cycle progression, survival, differentiation and transformation to cancer.

Palmitate induces insulin resistance without significant intracellular triglyceride accumulation in HepG2 cells

Previous studies showed that increased release of free fatty acids from adipocytes leads to insulin resistance and triglyceride (TG) accumulation in the liver, which may progress into hepatic steatohepatitis. We and other investigators have previously reported that palmitate induces endoplasmic reticulum stress-mediated toxicity in several tissues. This work investigated whether palmitate could induce insulin resistance and steatosis in HepG2 cells. We treated cells with either saturated fatty acid (palmitate) or unsaturated fatty acid (oleate), and observed that palmitate significantly activated c-jun N-terminal kinase and inactivated protein kinase B. Both 4-phenylbutyric acid and glycerol significantly activated protein kinase B, confirming the involvement of endoplasmic reticulum stress in palmitate-mediated insulin resistance. Oleate, but not palmitate, significantly induced intracellular TG deposition and activated sterol regulatory element binding protein-1. Instead, diacylglycerol level and protein kinase C epsilon activity were significantly increased by palmitate, suggesting the possible role of diacylglycerol in palmitate-mediated lipotoxicity. Therefore, the present study clearly showed that palmitate impairs insulin resistance, but does not induce significant TG accumulation in HepG2 cells.

An allostatic control of membrane lipid composition by SREBP1

The maintenance of membrane lipid composition within strict limits is critical to maintain optimum cellular function. The biophysical properties of the membrane can be influenced among other factors by the saturation/unsaturation of the phospholipid fatty acyl chain. The rate-limiting enzyme in unsaturated fatty acid biosynthesis is the desaturase enzyme which in turn is regulated by the lipid transcription factor sterol regulatory element binding protein (SREBP1). In this review, we collect some evidence suggesting SREBP1 network as an important allostatic regulator necessary to maintain the pool of unsaturated fatty acid lipid species that can be incorporated into biological membranes.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

24S-hydroxycholesterol effects on lipid metabolism genes are modeled in traumatic brain injury

Membrane damage during traumatic brain injury (TBI) alters the brain homeostasis of cholesterol and other lipids. Cholesterol 24S-hydroxylase (Cyp46) is a cholesterol metabolic enzyme that is increased after TBI. Here, we systematically examined the effects of the enzymatic product of Cyp46, 24S-hydroxycholesterol, on the cholesterol regulatory genes, SREBP-1 and 2, their posttranslational regulation, and their effects on gene transcription. 24S-hydroxycholesterol increased levels of SREBP-1 mRNA and full-length protein but did not change levels of cleaved SREBP-1, consistent with the role of 24-hydroxycholesterol as an LXR agonist. In contrast, 24S-hydroxycholesterol decreased levels of LXR-independent SREBP-2 mRNA, full-length protein, and SREBP-2 active cleavage product. We examined the downstream effects of changes to these lipid regulatory factors by studying cholesterol and fatty acid synthesis genes. In neuroblastoma cells, 24S-hydroxycholesterol decreased mRNA levels of the cholesterol synthesis genes HMG CoA reductase, squalene synthase, and FPP synthase but did not alter levels of the mRNA of fatty acid synthesis genes acetyl CoA carboxylase or fatty acid synthase. After TBI, as after 24S-hydroxycholesterol treatment in vitro, SREBP-1 mRNA levels were increased while SREBP-2 mRNA levels were decreased. Also similar to the in vitro results with 24S-hydroxycholesterol, HMG CoA reductase and squalene synthase mRNA levels were significantly decreased. Fatty acid synthase mRNA levels were not altered but acetyl CoA carboxylase mRNA levels were significantly decreased. Thus, changes to transcription of cholesterol synthesis genes after TBI were consistent with increases in Cyp46 activity, but changes to fatty acid synthesis genes must be regulated by other mechanisms.

Identification of novel genes and pathways regulating SREBP transcriptional activity

BACKGROUND

Lipid metabolism in mammals is orchestrated by a family of transcription factors called sterol regulatory element-binding proteins (SREBPs) that control the expression of genes required for the uptake and synthesis of cholesterol, fatty acids, and triglycerides. SREBPs are thus essential for insulin-induced lipogenesis and for cellular membrane homeostasis and biogenesis. Although multiple players have been identified that control the expression and activation of SREBPs, gaps remain in our understanding of how SREBPs are coordinated with other physiological pathways.

METHODOLOGY

To identify novel regulators of SREBPs, we performed a genome-wide cDNA over-expression screen to identify proteins that might modulate the transcription of a luciferase gene driven from an SREBP-specific promoter. The results were verified through secondary biological assays and expression data were analyzed by a novel application of the Gene Set Enrichment Analysis (GSEA) method.

CONCLUSIONS/SIGNIFICANCE

We screened 10,000 different cDNAs and identified a number of genes and pathways that have previously not been implicated in SREBP control and cellular cholesterol homeostasis. These findings further our understanding of lipid biology and should lead to new insights into lipid associated disorders.

Essential fatty acid deficiency induces fatty acid desaturase expression in rat epididymis, but not in testis

On the molecular level, essential fatty acid deficiency (EFAD) has been associated with induced fatty acid (FA) desaturase expression and activity in several tissues. However, there seem to be exceptions. In the present study, we examine the effects of EFAD in the male rat genital tract, combining FA analysis, gene expression studies, and morphological evaluation of epididymal spermatozoa. When feeding 21-day-old Wistar rats, a fat-free diet for 6 weeks, an increase in 18:1n-9 and 20:3n-9 and a concomitant decrease in the 18:2n-6 and 20:4n-6 species are seen in testis, as well as in liver. However, with regard to desaturase expression the rat testis seems to be unresponsive to EFAD conditions, in contrast to other organs studied. In the sexually mature testis none of the desaturases (SCD1, SCD2, D5D, or D6D) are induced in response to lowered contents of polyunsaturated FAs. This also applies tocaputepididymis, while EFAD sensitivity is regained incaudaepididymis, where the desaturases are upregulated. The FA profile of epididymal spermatozoa is increasingly affected by EFAD during the transport from testis tocaudaepididymis. Furthermore, a significant increase in the number of abnormal spermatozoa is observed incaudaepididymis.

n-3 fatty acids and gene expression

Accumulating evidence in both humans and animal models clearly indicates that a group of very-long-chain polyunsaturated fatty acids, the n-3 fatty acids (or omega-3), have distinct and important bioactive properties compared with other groups of fatty acids. n-3 Fatty acids are known to reduce many risk factors associated with several diseases, such as cardiovascular diseases, diabetes, and cancer. The mechanisms whereby n-3 fatty acids affect gene expression are complex and involve multiple processes. As examples, n-3 fatty acids regulate 2 groups of transcription factors, such as sterol-regulatory-element binding proteins and peroxisome proliferator-activated receptors, that are critical for modulating the expression of genes controlling both systemic and tissue-specific lipid homeostasis. Modulation of specific genes by n-3 fatty acids and cross-talk between these genes are responsible for many effects of n-3 fatty acids.

Cholesterogenic lanosterol 14alpha-demethylase (CYP51) is an immediate early response gene

Lanosterol 14alpha-demethylase (CYP51) responds to cholesterol feedback regulation through sterol regulatory element binding proteins (SREBPs). The proximal promoter of CYP51 contains a conserved region with clustered regulatory elements: GC box, cAMP-response elements (CRE-like), and sterol regulatory element (SRE). In lipid-rich (SREBP-poor) conditions, the CYP51 mRNA drops gradually, the promoter activity is diminished, and no DNA-protein complex is observed at the CYP51-SRE1 site. The majority of cAMP-dependent transactivation is mediated through a single CRE (CYP51-CRE2). Exposure of JEG-3 cells to forskolin, a mediator of the cAMP-dependent signaling pathway, provokes an immediate early response of CYP51, which has not been described before for any cholesterogenic gene. The CYP51 mRNA increases up to 4-fold in 2 h and drops to basal level after 4 h. The inducible cAMP early repressor (ICER) is involved in attenuation of transcription. Overexpressed CRE-binding protein (CREB)/CRE modulator (CREM) transactivates the mouse/human CYP51 promoters containing CYP51-CRE2 independently of SREBPs, and ICER decreases the CREB-induced transcription. Besides the increased CYP51 mRNA, forskolin affects the de novo sterol biosynthesis in JEG-3 cells. An increased consumption of lanosterol, a substrate of CYP51, is observed together with modulation of the postlanosterol cholesterogenesis, indicating that cAMP-dependent stimuli cross-talk with cholesterol feedback regulation. CRE-2 is essential for cAMP-dependent transactivation, whereas SRE seems to be less important. Interestingly, when CREB is not limiting, the increasing amounts of SREBP-1a fail to transactivate the CYP51 promoter above the CREB-only level, suggesting that hormones might have an important role in regulating cholesterogenesis in vivo.

Bioactive small molecules reveal antagonism between the integrated stress response and sterol-regulated gene expression

Phosphorylation of translation initiation factor 2alpha (eIF2alpha) coordinates a translational and transcriptional program known as the integrated stress response (ISR), which adapts cells to endoplasmic reticulum (ER) stress. A screen for small molecule activators of the ISR identified two related compounds that also activated sterol-regulated genes by blocking cholesterol biosynthesis at the level of CYP51. Ketoconazole, a known CYP51 inhibitor, had similar effects, establishing that perturbed flux of precursors to cholesterol activates the ISR. Surprisingly, compound-mediated activation of sterol-regulated genes was enhanced in cells with an ISR-blocking mutation in the regulatory phosphorylation site of eIF2alpha. Furthermore, induction of the ISR by an artificial drug-activated eIF2alpha kinase reduced the level of active sterol regulatory element binding protein (SREBP) and sterol-regulated mRNAs. These findings suggest a mechanism by which interactions between sterol metabolism, the ISR, and the SREBP pathway affect lipid metabolism during ER stress.

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

In the process of membrane biogenesis several dozen proteins must operate in precise concert to generate approximately 100 lipids at appropriate concentrations. To study the regulation of bilayer assembly in a cell cycle-independent manner, we have exploited the fact that phagocytes replenish membranes expended during particle engulfment in a rapid phase of lipid synthesis. In response to phagocytosis of latex beads, human embryonic kidney 293 cells synthesized cholesterol and phospholipids at amounts equivalent to the surface area of the internalized particles. Lipid synthesis was accompanied by increased transcription of several lipogenic proteins, including the low-density lipoprotein receptor, enzymes required for cholesterol synthesis (3-hydroxy-3-methylglutaryl CoA synthase, 3-hydroxy-3-methylglutaryl CoA reductase), and fatty acid synthase. Phagocytosis triggered the proteolytic activation of two lipogenic transcription factors, sterol regulatory element binding protein-1a (SREBP-1a) and SREBP-2. Proteolysis of SREBPs coincided with the appearance of their transcriptionally active N termini in the nucleus and 3-fold activation of an SREBP-specific reporter gene. In previous studies with cultured cells, proteolytic activation of SREBP-1a and SREBP-2 has been observed in response to selective starvation of cells for cholesterol and unsaturated fatty acids. However, under the current conditions, SREBP-1a and SREBP-2 are induced without lipid deprivation. SREBP activation is inhibited by high levels of the SREBP-interacting proteins Insig1 or the cytosolic domain of SREBP cleavage-activating protein. Upon overexpression of these proteins, phagocytosis-induced transcription and lipid synthesis were blocked. These results identify SREBPs as essential regulators of membrane biogenesis and provide a useful system for further studies on membrane homeostasis.

Mitochondrial glycerol-3-phosphate acyltransferase-1 is essential in liver for the metabolism of excess acyl-CoAs

In vitro studies suggest that the mitochondrial glycerol-3-phosphate acyltransferase-1 (mtGPAT1) isoform catalyzes the initial and rate-controlling step in glycerolipid synthesis and aids in partitioning acyl-CoAs toward triacylglycerol synthesis and away from degradative pathways. To determine whether the absence of mtGPAT1 would increase oxidation of acyl-CoAs and restrict the development of hepatic steatosis, we fed wild type and mtGPAT1-/- mice a diet high in fat and sucrose (HH) for 4 months to induce the development of obesity and a fatty liver. Control mice were fed a diet low in fat and sucrose (LL). With the HH diet, absence of mtGPAT1 resulted in increased partitioning of acyl-CoAs toward oxidative pathways, demonstrated by 60% lower hepatic triacylglycerol content and 2-fold increases in plasma beta-hydroxybutyrate, acylcarnitines, and hepatic mRNA expression of mitochondrial HMG-CoA synthase. Despite the increase in fatty acid oxidation, liver acyl-CoA levels were 3-fold higher in the mtGPAT1-/- mice fed both diets. A lack of difference in CPT1 and FAS mRNA expression between genotypes suggested that the increased acyl-CoA content was not because of increased de novo synthesis, but instead, to an impaired ability to use long-chain acyl-CoAs derived from the diet, even when the dietary fat content was low. Hyperinsulinemia and reduced glucose tolerance on the HH diet was greater in the mtGPAT1-/- mice, which did not suppress the expression of the gluconeogenic genes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. This study demonstrates that mtGPAT1 is essential for normal acyl-CoA metabolism, and that the absence of hepatic mtGPAT1 results in the partitioning of fatty acids away from triacylglycerol synthesis and toward oxidation and ketogenesis.

Regulation of adipocyte differentiation and function by polyunsaturated fatty acids

A diet enriched in PUFAs, in particular of the n-3 family, decreases adipose tissue mass and suppresses development of obesity in rodents. Although several nuclear hormone receptors are identified as PUFA targets, the precise molecular mechanisms underlying the effects of PUFAs still remain to be elucidated. Here we review research aimed at elucidating molecular mechanisms governing the effects of PUFAs on the differentiation and function of white fat cells. This review focuses on dietary PUFAs as signaling molecules, with special emphasis on agonistic and antagonistic effects on transcription factors currently implicated as key players in adipocyte differentiation and function, including peroxisome proliferator activated receptors (PPARs) (alpha, beta and gamma), sterol regulatory element binding proteins (SREBPs) and liver X receptors (LXRs). We review evidence that dietary n-3 PUFAs decrease adipose tissue mass and suppress the development of obesity in rodents by targeting a set of key regulatory transcription factors involved in both adipogensis and lipid homeostasis in mature adipocytes. The same set of factors are targeted by PUFAs of the n-6 family, but the cellular/physiological responses are dependent on the experimental setting as n-6 PUFAs may exert either an anti- or a proadipogenic effect. Feeding status and hormonal background may therefore be of particular importance in determining the physiological effects of PUFAs of the n-6 family.

Transgenic mice overexpressing SREBP-1a under the control of the PEPCK promoter exhibit insulin resistance, but not diabetes

Sterol regulatory element-binding protein-1 (SREBP-1) is a transcription factor which regulates genes involved in the synthesis of fatty acids and triglycerides. The overexpression of nuclear SREBP-1a in transgenic mice under the control of the PEPCK promoter (TgSREBP-1a) caused a massively enlarged fatty liver and disappearance of peripheral white adipose tissue. In the current study, we estimated the impact of this lipid transcription factor on plasma glucose/insulin metabolism in vivo. TgSREBP-1a exhibited mild peripheral insulin resistance as evidenced by hyperinsulinemia both at fasting and after intravenous glucose loading, and retarded glucose reduction after insulin injection due to decreased plasma leptin levels. Intriguingly, hyperinsulinemia in TgSREBP-1a mice was markedly exacerbated in a fed state and sustained after intravenous glucose loading, and paradoxically decreased after the portal injection of glucose. TgSREBP-1a mice consistently showed very small plasma glucose increases after portal glucose loading because of a large capacity for hepatic glucose uptake. These data suggested that hepatic insulin resistance emerges postprandially. In addition, pancreatic islets from TgSREBP-1a were enlarged. These data demonstrate that SREBP-1a activation in the liver has a strong impact on plasma insulin levels, implicating the potential role of SREBPs in hepatic insulin metabolism relating to insulin resistance.

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

Stearoyl-CoA desaturase (SCD) synthesizes oleate necessary for the biosynthesis of triglycerides and other lipids. Mice with a targeted disruption of the SCD1 gene are deficient in tissue oleate and have reduced expression of the sterol regulatory element-binding protein (SREBP) and its target genes. The SREBP-1c isoform is a known mediator of insulin action on hepatic gene expression, but its transcriptional effects due to glucose or fructose are still unclear. We found that fructose compared with glucose is a stronger inducer of SREBP-1c and lipogenic gene expression, causing a dramatic increase in hepatic triglyceride levels. However, when fed to the SCD1-/- mice, fructose failed to induce SREBP-1 or lipogenic genes and the triglyceride levels were not increased. Instead fructose feeding caused a decrease in hepatic glycogen and plasma glucose levels. The induction of SREBP-1 and lipogenic gene expression as well as the levels of liver triglycerides, glycogen, and plasma glucose was partially restored when the fructose diet was supplemented with very high levels of oleate (20% by weight) but not with palmitate, stearate, or linoleate. Fructose in a long term feeding induced the expression of SCD1 and that of other lipogenic genes in the liver of SREBP-1c-/- mice, and a further increase in expression of these genes occurred when the fructose diet was supplemented with oleate. Our observations demonstrated that oleate produced by SCD is necessary for fructose-mediated induction of lipogenic gene expression through SREBP-1c-dependent and -independent mechanisms and suggested that SCD1 gene expression is important in lipid and carbohydrate homeostasis.

Regulation of fatty acid and cholesterol synthesis: co-operation or competition?

Fatty acids and sterols originally evolved symbiotically as structural components of cell membranes. In some respects, control of their biosynthetic pathways reflects their mutual interdependence in defining changes in the physicochemical properties of the membranes in response to the changing internal and external cellular environments. In some tissues of higher animals, however, cholesterol and fatty acids have multifunctional roles. In particular, the liver synthesizes these lipids for export as multimolecular complexes in the form of micellar bile components and lipoproteins. Intrahepatic fatty acid and cholesterol synthesis is dependent upon the balance between hepatic output of these complexes and dietary input of fat and cholesterol. Thus physiological control of these synthetic processes is often co-ordinated at both the transcriptional and post-translational levels. On the other hand, changes in flux through major metabolic pathways, particularly during physiological transitions and as a result of genetic manipulation, affects substrate availability for these pathways. Under these circumstances, regulation reflects a compensatory response to ensure that flux through the lipid pathways remains unchanged. These regulatory changes can best be interpreted in terms of a Metabolic Control Analysis approach. In summary, flux through the fatty acid and cholesterol pathways reflects (a) cellular demand for these lipids, (b) a variable availability of substrates, (c) a combination of (a) and (b).

Conjugated linoleic acid isomers and mammary cancer prevention

There is increasing evidence that individual isomers of conjugated linoleic acid (CLA) may have unique biological or biochemical effects. A primary objective of this study was to determine whether there might be differences in the anticancer activity of 9,11-CLA and 10,12-CLA. This was achieved by evaluating the reduction in premalignant lesions and carcinomas in the mammary gland of rats that had been treated with a single dose of methylnitrosourea and given 0.5% of either highly purified CLA isomer in the diet. Our results showed that the anticancer efficacies of the two isomers were very similar. At 6 wk after carcinogen administration, the total number of premalignant lesions was reduced by 33-36%. At 24 wk, the total number of mammary carcinomas was reduced by 35-40%. The concentration of each CLA isomer and its respective metabolites was analyzed in the mammary fat pad. Tissue level of 10,12-CLA was much lower than that of 9,11-CLA. The pool of metabolites from each isomer was very similar between the two groups and represented only a small fraction of total conjugated diene fatty acids. Feeding of 9,11-CLA resulted in minimal changes in other unsaturated fatty acids. In contrast, feeding of 10,12-CLA produced a wider spectrum of perturbations. Small but significant increases in 16:1 and 16:2 were detected; these were accompanied by decreases in 20:2, 20:3, 20:4, 22:4, and 22:6. The above observation suggests that 10,12-CLA might be more potent than 9,11-CLA in interfering with elongation and desaturation of linoleic and linolenic acids. In summary, our study showed that, at the 0.5% dose level, the anticancer activity of 9,11-CLA and 10,12-CLA was very similar, even though accumulation of 10,12-CLA in the mammary tissue was considerably less than that of 9,11-CLA. These confounding changes of the other unsaturated fatty acids in contributing to the effect of 10,12-CLA need to be clarified.

Role of oleic acid as a neurotrophic factor is supported in vivo by the expression of GAP-43 subsequent to the activation of SREBP-1 and the up-regulation of stearoyl-CoA desaturase during postnatal development of the brain

We have recently reported that albumin, a serum protein present in the developing brain, stimulates the synthesis of oleic acid by cultured astrocytes by inducing stearoyl-CoA 9-desaturase, the rate-limiting enzyme in oleic acid synthesis, through activation of the sterol regulatory element-binding protein-1. In this work, we offer evidence supporting the in vivo occurrence of this process during the postnatal development of the rat brain. Our results show that albumin reaches maximal brain level by day 1 after birth, coinciding with activation of the sterol response element binding protein-1, which is responsible for the transcription of the enzymes required for oleic acid synthesis. In addition, the developmental profile of stearoyl-CoA 9-desaturase-1 mRNA expression follows that of sterol regulatory element-binding protein-1 activation, indicating that these phenomena are tightly linked. In a previous work, we showed that oleic acid induces neuronal differentiation, as indicated by the expression of growth associated protein-43. Here, we report that the expression of growth associated protein-43 mRNA peaks at about day 7 after birth, following the maximal expression of stearoyl-CoA 9-desaturase-1 mRNA that occurs between days 3 and 5 postnatally. In conclusion, our results support the hypothesis that the synthesis of oleic acid is linked to neuronal differentiation during rat brain development.

Sterol regulatory element-binding protein family as global regulators of lipid synthetic genes in energy metabolism

Sterol regulatory element-binding proteins (SREBPs) have been established as lipid synthetic transcription factors for cholesterol and fatty acid synthesis. SREBPs are synthesized as membrane-bound precursors with their N-terminal active portions entering the nucleus to activate target genes after proteolytic cleavage in a sterol-regulated manner. This cleavage step is regulated by a putative sterol-sensing molecule, SREBP-activating protein (SCAP), that forms a complex with SREBPs and traffics between the rough endoplasmic reticulum and Golgi. DNA cis-elements that SREBPs bind, originally identified as sterol-regulatory elements (SREs), now expands to a variety of SRE-like sequences and some of E-boxes, which makes SREBPs eligible to regulate a wide range of lipid genes. Animal experiments including transgenic and knockout mice suggest that three isoforms, SREBP-1a, -1c, and -2, have different roles in lipid synthesis. In differentiated tissues and organs, SREBP-1c is involved in fatty acid, whereas SREBP-2 plays a major role in regulation of cholesterol synthesis. SREBP-1a is expressed in growing cells, providing both cholesterol and fatty acids that are required for membrane synthesis. SREBP-1c seems to be a mediator for insulin/glucose signaling to lipogenesis, and could be involved in insulin resistance, remnant lipoproteins, and fatty livers. Future studies in this field will certainly focus on understanding the molecular mechanisms sensing cellular sterol and energy states leading to the activation of SREBP-mediated gene transcription.

Polyunsaturated fatty acids decrease the expression of sterol regulatory element-binding protein-1 in CaCo-2 cells: effect on fatty acid synthesis and triacylglycerol transport

Regulation of sterol regulatory element-binding proteins (SREBPs) by fatty acid flux was investigated in CaCo-2 cells. Cells were incubated with 1 mM taurocholate with or without 250 microM 18:0, 18:1, 18:2, 20:4, 20:5 or 22:6 fatty acids. Fatty acid synthase (FAS) and acetyl-CoA carboxylase mRNA levels and gene and protein expression of SREBPs were estimated. 18:2, 20:4, 20:5 and 22:6 fatty acids decreased the amount of mature SREBP-1 and mRNA levels of SREBP-1c, SREBP-1a, FAS and acetyl-CoA carboxylase. SREBP-2 gene or mature protein expression was not altered. Liver X receptor (LXR) activation by T0901317 increased gene expression of SREBP-1c, SREBP-1a, FAS and acetyl-CoA carboxylase without altering SREBP-2. 20:5, but not 18:1, prevented the full expression of SREBP-1c mRNA by T0901317. T0901317 increased SREBP-1 mass without altering the mass of mature SREBP-2. Although only 18:2, 20:4, 20:5 and 22:6 suppressed SREBP-1, acetyl-CoA carboxylase and FAS expression, all fatty acids decreased the rate of fatty acid synthesis. T0901317 increased endogenous fatty acid synthesis yet did not increase secretion of triacylglycerol-rich lipoproteins. In CaCo-2 cells, polyunsaturated fatty acids decrease gene and protein expression of SREBP-1 and FAS mRNA, probably through interference with LXR activity. Since all fatty acids decreased fatty acid synthesis, mechanisms other than changes in SREBP-1c expression must be entertained. Increased endogenous fatty acid synthesis does not promote triacylglycerol-rich lipoprotein secretion.

Oxidized products of cholesterol: dietary and metabolic origin, and proatherosclerotic effects (review)

Cholesterol oxidation products, termed oxysterols, are increasingly considered of potential interest in the pathogenesis of atherosclerotic lesions. Of dietary or endogenous origin, oxysterols may occur in significant amounts in low density lipoprotein (LDL) particles, especially in hypercholesterolemic subjects. They likely contribute to the uptake of modified LDL by scavenger receptors and some of them finally accumulate in the subintimal space of major arteries; here cholesterol oxides may favor the perpetuation of a chronic inflammatory state, through their ability to trigger irreversible damage of vascular cells with consequent activation of phagocytes. Furthermore, practically all oxysterols of major pathophysiologic interest have been shown to markedly up-regulate expression and synthesis of adhesion molecules, inflammatory cytokines and chemokines. Cholesterol oxidation thus appears to be an important biochemical pathway through which it exerts toxic, inflammatory and finally atherogenic effects.

Effects of fatty acids on gene expression: role of peroxisome proliferator-activated receptor alpha, liver X receptor alpha and sterol regulatory element-binding protein-1c

Dietary fatty acids have numerous effects on cellular function, many of which are achieved by altering the expression of genes. The present paper reviews recent data on the mechanisms by which fatty acids influence DNA transcription, and focus specifically on the importance of three transcription factors: peroxisome proliferator-activated receptor alpha; liver X receptor alpha; sterol regulatory element-binding protein 1c. These data indicate that fatty acids induce or inhibit the mRNA expression of a variety of different genes by acting both as agonists and as antagonists for nuclear hormone receptors.

Astrocyte-synthesized oleic acid behaves as a neurotrophic factor for neurons

Unlike in the adult brain, the newborn brain specifically takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we shall summarize our knowledge about the role played by albumin in brain development. The role of this protein in brain development is intimately related to its ability to carry fatty acids. Thus, albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis, which is followed by transcytosis, including passage through the endoplasmic reticulum (ER). The presence of albumin in the ER activates the sterol regulatory element-binding protein-1 (SREBP-1) and increases stearoyl-CoA 9-desaturase (SCD) mRNA, the key enzyme in oleic acid synthesis. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is specifically incorporated into growth cones. In addition, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth associated protein, GAP-43. All of these observations indicate neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C. The expression of GAP-43 is significantly increased by the presence of albumin in neurons co-cultured with astrocytes, indicating that neuronal differentiation takes place by the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, thereby inducing neuronal differentiation.

Transcytosis of albumin in astrocytes activates the sterol regulatory element-binding protein-1, which promotes the synthesis of the neurotrophic factor oleic acid

We have recently reported that albumin, a serum protein present in the developing brain, stimulates the synthesis of oleic acid by astrocytes, which promotes neuronal differentiation. In this work, we gain insight into the mechanism by which albumin induces the synthesis of this neurotrophic factor. Our results show that astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis. Albumin uptake was followed by transcytosis, including passage through the endoplasmic reticulum, which was required to induce the synthesis of oleic acid. Oleic acid synthesis is feedback-regulated by the sterol regulatory element-binding protein-1, which induces the transcription of stearoyl-CoA 9-desaturase, the key rate-limiting enzyme for oleic acid synthesis. In our research, the presence of albumin activated the sterol regulatory element-binding protein-1 and increased stearoyl-CoA 9-desaturase mRNA. Moreover, when the activity of sterol regulatory element-binding protein-1 was inhibited by overexpression of a truncated form of this protein, albumin did not affect stearoyl-CoA 9-desaturase mRNA, indicating that the effect of albumin is mediated by this transcription factor. The effect of albumin was abolished when traffic to the endoplasmic reticulum was prevented or when albumin was accompanied with oleic acid. In conclusion, our results suggest that the transcytosis of albumin includes passage through the endoplasmic reticulum, where oleic acid is sequestrated, initiating the signal cascade leading to an increase in its own synthesis.

The SREBP pathway in Drosophila: regulation by palmitate, not sterols

In mammals, synthesis of cholesterol and unsaturated fatty acids is controlled by SREBPs, a family of membrane-bound transcription factors. Here, we show that the Drosophila genome encodes all components of the SREBP pathway, including a single SREBP (dSREBP), SREBP cleavage-activating protein (dSCAP), and the two proteases that process SREBP at sites 1 and 2 to release the nuclear fragment. In cultured Drosophila S2 cells, dSREBP is processed at sites 1 and 2, and the liberated fragment increases mRNAs encoding enzymes of fatty acid biosynthesis, but not sterol or isoprenoid biosynthesis. Processing requires dSCAP, but is not inhibited by sterols as in mammals. Instead, dSREBP processing is blocked by palmitic acid. These findings suggest that the ancestral SREBP pathway functions to maintain membrane integrity rather than to control cholesterol homeostasis.

Acetyl-coenzyme A synthetase is a lipogenic enzyme controlled by SREBP-1 and energy status

DNA microarray analysis on upregulated genes in the livers from transgenic mice overexpressing nuclear sterol regulatory element-binding protein (SREBP)-1a, identified an expressed sequence tag (EST) encoding a part of murine cytosolic acetyl-coenzyme A synthetase (ACAS). Northern blot analysis of the livers from transgenic mice demonstrated that this gene was highly induced by SREBP-1a, SREBP-1c, and SREBP-2. DNA sequencing of the 5' flanking region of the murine ACAS gene identified a sterol regulatory element with an adjacent Sp1 site. This region was shown to be responsible for SREBP binding and activation of the ACAS gene by gel shift and luciferase reporter gene assays. Hepatic and adipose tissue ACAS mRNA levels in normal mice were suppressed at fasting and markedly induced by refeeding, and this dietary regulation was nearly abolished in SREBP-1 knockout mice, suggesting that the nutritional regulation of the ACAS gene is controlled by SREBP-1. The ACAS gene was downregulated in streptozotocin-induced diabetic mice and was restored after insulin replacement, suggesting that diabetic status and insulin also regulate this gene. When acetate was administered, hepatic ACAS mRNA was negatively regulated. These data on dietary regulation and SREBP-1 control of ACAS gene expression demonstrate that ACAS is a novel hepatic lipogenic enzyme, providing further evidence that SREBP-1 and insulin control the supply of acetyl-CoA directly from cellular acetate for lipogenesis. However, its high conservation among different species and the wide range of its tissue distribution suggest that this enzyme might also play an important role in basic cellular energy metabolism.

Sterol regulatory element-binding proteins (SREBPs): transcriptional regulators of lipid synthetic genes

Roles of sterol regulatory element-binding proteins (SREBPs) have been established as lipid synthetic transcription factors especially for cholesterol and fatty acid synthesis. SREBPs have unique characteristics. Firstly, they are membrane-bound proteins and the N-terminal active portions enter nucleus to activate their target genes after proteolytic cleavage, which requires sterol-sensing molecule, SREBP-activating protein (SCAP) and is crucial for sterol-regulation. Secondly, they bind and activate sterol-regulatory (SREs) containing promoters as well as some E-boxes, which makes SREBPs eligible to regulate a wide range of lipid genes. Finally, three isoforms, SREBP-1a-1c, and have different roles in lipid synthesis. In vivo studies using transgenic and knockout mice suggest that SREBP-1 seems to be involved in energy metabolism including fatty acid and glucose/insulin metabolism, whereas SREBP-2 is specific to cholesterol synthesis. Future studies will be focused on understanding molecular mechanisms sensing cellular sterol and energy states where SREBPs are deeply involved.

Isolation and characterization of the human stearoyl-CoA desaturase gene promoter: requirement of a conserved CCAAT cis-element

Stearoyl-CoA desaturase is the rate-limiting enzyme in the production of mono-unsaturated fatty acids. We have recently cloned and characterized the human Scd cDNA and SCD (the stearoyl-CoA desaturase structural gene) on chromosome 10, as well as the non-transcribed pseudogene on chromosome 17. In order to further define SCD regulation and function, we have isolated and characterized the promoter of the structural gene. Screening of chromosome-10-specific libraries resulted in the isolation of 4.1 kb of SCD sequence upstream of the translation start site. Binding sites for transcription factors critical for mouse Scd1 and Scd2 promoter activity, such as sterol-regulated-element-binding protein and nuclear factor Y, were present in the human SCD promoter (Scd is the mouse stearoyl-CoA desaturase gene). Deletion analysis in HaCaT keratinocytes identified a critical region for promoter activity between nts 496-609 upstream of the translation start site. Site-directed mutagenesis of binding sites in this region identified the CCAAT box as the critical cis-element for SCD promoter activity. An electrophoretic mobility-shift assay confirmed that this element binds nuclear proteins from HaCaT keratinocytes. The polyunsaturated-fatty-acid (PUFA) response element, previously identified in the promoters of mouse Scd1 and Scd2, was found to be conserved in the human SCD promoter, and contained the critical CCAAT cis-element. A minimal promoter construct including this region was responsive to fatty acids, with oleate and linoleate decreasing transcription and stearate increasing it. These studies indicate that CCAAT-box-binding proteins activate SCD transcription in cultured keratinocytes and that fatty acids modulate transcription, most likely through the conserved PUFA response element.

2000 George Lyman Duff Memorial Lecture: atherosclerosis is a liver disease of the heart

The production of apolipoprotein B (apoB)-containing lipoproteins by the liver is regulated by a complex series of processes involving apoB being cotranslationally translocated across the endoplasmic reticulum and assembled into a lipoprotein particle. The translocation of apoB across the endoplasmic reticulum is facilitated by the intraluminal chaperone, microsomal triglyceride transfer protein (MTP). MTP facilitates the translocation and folding of apoB, as well as the addition of lipid to lipid-binding domains (which consist of amphipathic beta sheets and alpha helices). In the absence of MTP or sufficient lipid, apoB exhibits translocation arrest. Thus, apoB translation, translocation, and assembly with lipids to form a core-containing lipoprotein particle occur as concerted processes. Abrogation of >/=1 of these processes diverts apoB into a degradation pathway that is dependent on conjugation with ubiquitin and proteolysis by the proteasome. The nascent core-containing lipoprotein particle that forms within the lumen of the endoplasmic reticulum can be "enlarged" to form a mature very low density lipoprotein particle. Additional studies show that the assembly and secretion of apoB-containing lipoproteins are linked to the cholesterol/bile acid synthetic pathway controlled by cholesterol 7alpha-hydroxylase. Studies in cultured cells and transgenic mice indicate that the expression of cholesterol 7alpha-hydroxylase indirectly regulates the expression of lipogenic enzymes through changes in the cellular content of mature sterol response element binding proteins. Oxysterols and bile acids may also act via the ligand-activated nuclear receptors LXR and FXR to link the metabolic pathways controlling energy balance and lipid metabolism to nutritional state.

Identification of liver X receptor-retinoid X receptor as an activator of the sterol regulatory element-binding protein 1c gene promoter

In an attempt to identify transcription factors which activate sterol-regulatory element-binding protein 1c (SREBP-1c) transcription, we screened an expression cDNA library from adipose tissue of SREBP-1 knockout mice using a reporter gene containing the 2.6-kb mouse SREBP-1 gene promoter. We cloned and identified the oxysterol receptors liver X receptor (LXRalpha) and LXRbeta as strong activators of the mouse SREBP-1c promoter. In the transfection studies, expression of either LXRalpha or -beta activated the SREBP-1c promoter-luciferase gene in a dose-dependent manner. Deletion and mutation studies, as well as gel mobility shift assays, located an LXR response element complex consisting of two new LXR-binding motifs which showed high similarity to an LXR response element recently found in the ABC1 gene promoter, a reverse cholesterol transporter. Addition of an LXR ligand, 22(R)-hydroxycholesterol, increased the promoter activity. Coexpression of retinoid X receptor (RXR), a heterodimeric partner, and its ligand 9-cis-retinoic acid also synergistically activated the SREBP-1c promoter. In HepG2 cells, SREBP-1c mRNA and precursor protein levels were induced by treatment with 22(R)-hydroxycholesterol and 9-cis-retinoic acid, confirming that endogenous LXR-RXR activation can induce endogenous SREBP-1c expression. The activation of SREBP-1c by LXR is associated with a slight increase in nuclear SREBP-1c, resulting in activation of the gene for fatty acid synthase, one of its downstream genes, as measured by the luciferase assay. These data demonstrate that LXR-RXR can modify the expression of genes for lipogenic enzymes by regulating SREBP-1c expression, providing a novel link between fatty acid and cholesterol metabolism.

Mechanisms of nutritional and hormonal regulation of lipogenesis

Fat build-up is determined by the balance between lipogenesis and lipolysis/fatty acid oxidation. In the past few years, our understanding of the nutritional, hormonal and particularly transcriptional regulation of lipogenesis has expanded greatly. Lipogenesis is stimulated by a high carbohydrate diet, whereas it is inhibited by polyunsaturated fatty acids and by fasting. These effects are partly mediated by hormones, which inhibit (growth hormone, leptin) or stimulate (insulin) lipogenesis. Recent research has established that sterol regulatory element binding protein-1 is a critical intermediate in the pro- or anti-lipogenic action of several hormones and nutrients. Another transcription factor implicated in lipogenesis is the peroxisome proliferator activated receptor gamma. Both transcription factors are attractive targets for pharmaceutical intervention of disorders such as hypertriglyceridemia and obesity.

Progesterone stimulates adipocyte determination and differentiation 1/sterol regulatory element-binding protein 1c gene expression. potential mechanism for the lipogenic effect of progesterone in adipose tissue

Fatty acid synthase (FAS), a nutritionally regulated lipogenic enzyme, is transcriptionally controlled by ADD1/SREBP1c (adipocyte determination and differentiation 1/sterol regulatory element-binding protein 1c), through insulin-mediated stimulation of ADD1/SREBP1c expression. Progesterone exerts lipogenic effects on adipocytes, and FAS is highly induced in breast tumor cell lines upon progesterone treatment. We show here that progesterone up-regulates ADD1/SREBP1c expression in the MCF7 breast cancer cell line and the primary cultured preadipocyte from rat parametrial adipose tissue. In MCF7, progesterone induced ADD1/SREBP1c and Metallothionein II (a well known progesterone-regulated gene) mRNAs, with comparable potency. In preadipocytes, progesterone increased ADD1/SREBP1c mRNA dose-dependently, but not SREBP1a or SREBP2. Run-on experiments demonstrated that progesterone action on ADD1/SREBP1c was primarily at the transcriptional level. The membrane-bound and mature nuclear forms of ADD1/SREBP1 protein accumulated in preadipocytes cultured with progesterone, and FAS induction could be abolished by adenovirus-mediated overexpression of a dominant negative form of ADD1/SREBP1 in these cells. Finally, in the presence of insulin, progesterone was unable to up-regulate ADD1/SREBP1c mRNA in preadipocytes, whereas its effect was restored after 24 h of insulin deprivation. Together these results demonstrate that ADD1/SREBP1c is controlled by progesterone, which, like insulin, acts by increasing ADD1/SREBP1c gene transcription. This provides a potential mechanism for the lipogenic actions of progesterone on adipose tissue.

Unsaturated fatty acids down-regulate srebp isoforms 1a and 1c by two mechanisms in HEK-293 cells

Sterol regulatory element-binding proteins (SREBPs) are membrane-bound transcription factors that increase the synthesis of fatty acids as well as cholesterol in animal cells. All three SREBP isoforms (SREBP-1a, -1c, and -2) are subject to feedback regulation by cholesterol, which blocks their proteolytic release from membranes. Previous data indicate that the SREBPs are also negatively regulated by unsaturated fatty acids, but the mechanism is uncertain. In the current experiments, unsaturated fatty acids decreased the nuclear content of SREBP-1, but not SREBP-2, in cultured human embryonic kidney (HEK)-293 cells. The potency of unsaturated fatty acids increased with increasing chain length and degree of unsaturation. Oleate, linoleate, and arachidonate were all effective, but the saturated fatty acids palmitate and stearate were not effective. Down-regulation occurred at two levels. The mRNAs encoding SREBP-1a and SREBP-1c were markedly reduced, and the proteolytic processing of these SREBPs was inhibited. When SREBP-1a was produced by a cDNA expressed from an independent promoter, unsaturated fatty acids reduced nuclear SREBP-1a without affecting the mRNA level. There was no effect when the cDNA encoded a truncated version that was not membrane-bound. When administered together, sterols and unsaturated fatty acids potentiated each other in reducing nuclear SREBP-1. In the absence of fatty acids, sterols did not cause a sustained reduction of nuclear SREBP-1, but they did reduce nuclear SREBP-2. We conclude that unsaturated fatty acids, as well as sterols, can down-regulate nuclear SREBPs and that unsaturated fatty acids have their greatest inhibitory effects on SREBP-1a and SREBP-1c, whereas sterols have their greatest inhibitory effects on SREBP-2.

Sterol regulatory element-binding protein-1 participates in the regulation of fatty acid synthase expression in colorectal neoplasia

Endogenous fatty acid synthesis has been observed in certain rapidly proliferating normal and neoplastic tissues. Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate the expression of lipogenic genes including fatty acid synthase (FAS), the major biosynthetic enzyme for fatty acid synthesis. We have previously shown that SREBP-1, FAS, and Ki-67, a proliferation marker, colocalized in the crypts of the fetal gastrointestinal tract epithelium. This study sought to determine whether SREBP-1 participates in the regulation of proliferation-associated fatty acid synthesis in colorectal neoplasia. An immunohistochemical analysis of SREBP-1, FAS, and Ki-67 expression in 25 primary human colorectal carcinoma specimens showed colocalization in 22 of these. To elucidate a functional linkage between SREBP-1 activation and proliferation-associated FA synthesis, SREBP-1 and FAS content were assayed during the adaptive response of cultured HCT116 colon carcinoma cells to pharmacological inhibition of FA synthesis. Cerulenin and TOFA each inhibited the endogenous synthesis of fatty acids in a dose-dependent manner and each induced increases in both precursor and mature forms of SREBP-1. Subsequently, both the transcriptional activity of the FAS promoter in a luciferase reporter gene construct and the FAS expression increased. These results demonstrate that tumor cells recognize and respond to a deficiency in endogenous fatty acid synthesis by upregulating both SREBP-1 and FAS expression and support the model that SREBP-1 participates in the transcriptional regulation of lipogenic genes in colorectal neoplasia.

Promoter analysis of the mouse sterol regulatory element-binding protein-1c gene

Recent data suggest that sterol regulatory-binding protein (SREBP)-1c plays a key role in the transcriptional regulation of different lipogenic genes mediating lipid synthesis as a key regulator of fuel metabolism. SREBP-1c regulates its downstream genes by changing its own mRNA level, which led us to sequence and analyze the promoter region of the mouse SREBP-1c gene. A cluster of putative binding sites of several transcription factors composed of an NF-Y site, an E-box, a sterol-regulatory element 3, and an Sp1 site were located at -90 base pairs of the SREBP-1c promoter. Luciferase reporter gene assays indicated that this SRE complex is essential to the basal promoter activity and confers responsiveness to activation by nuclear SREBPs. Deletion and mutation analyses suggest that the NF-Y site and SRE3 in the SRE complex are responsible for SREBP activation, although the other sites were also involved in the basal activity. Gel mobility shift assays demonstrate that SREBP-1 binds to the SRE3. Taken together, these findings implicate a positive loop production of SREBP-1c through the SRE complex, possibly leading to the overshoot in induction of SREBP-1c and its downstream genes seen in the livers of refed mice. Furthermore, reporter assays using larger upstream fragments indicated another region that was inducible by addition of sterols. The presence of the SRE complex and a sterol-inducible region in the same promoter suggests a novel regulatory link between cholesterol and fatty acid synthesis.

Sterol regulatory element-binding protein-1 is regulated by glucose at the transcriptional level

In vivo studies suggest that sterol regulatory element-binding protein (SREBP)-1 plays a key role in the up-regulation of lipogenic genes in the livers of animals that have consumed excess amounts of carbohydrates. In light of this, we sought to use an established mouse hepatocyte cell line, H2-35, to further define the mechanism by which glucose regulates nuclear SREBP-1 levels. First, we show that these cells transcribe high levels of SREBP-1c that are increased 4-fold upon differentiation from a prehepatocyte to a hepatocyte phenotype, making them an ideal cell culture model for the study of SREBP-1c induction. Second, we demonstrate that the presence of precursor and mature forms of SREBP-1 protein are positively regulated by medium glucose concentrations ranging from 5. 5 to 25 mm and are also regulated by insulin, with the amount of insulin in the fetal bovine serum being sufficient for maximal stimulation of SREBP-1 expression. Third, we show that the increase in SREBP-1 protein is due to an increase in SREBP-1 mRNA. Reporter gene analysis of the SREBP-1c promoter demonstrated a glucose-dependent induction of transcription. In contrast, expression of a fixed amount of the precursor form of SREBP-1c protein showed that glucose does not influence its cleavage. Fourth, we demonstrate that the glucose induction of SREBP could not be reproduced by fructose, xylose, or galactose nor by glucose analogs 2-deoxy glucose and 3-O-methyl glucopyranose. These data provide strong evidence for the induction of SREBP-1c mRNA by glucose leading to increased mature protein in the nucleus, thus providing a potential mechanism for the up-regulation of lipogenic genes by glucose in vivo.

Sterols and isoprenoids: signaling molecules derived from the cholesterol biosynthetic pathway

Compounds derived from the isoprenoid/cholesterol biosynthetic pathway have recently been shown to have novel biological activities. These compounds include certain sterols, oxysterols, farnesol, and geranylgeraniol, as well as the diphosphate derivatives of isopentenyl, geranyl, farnesyl, geranylgeranyl, and presqualene. They regulate transcriptional and post-transcriptional events that in turn affect lipid synthesis, meiosis, apoptosis, developmental patterning, protein cleavage, and protein degradation.

Transcriptional homeostatic control of membrane lipid composition

Plasma membranes have a structural property, commonly referred to as membrane fluidity, that is compositionally regulated. The two main features of plasma membrane lipid composition that determine membrane fluidity are the ratio of cholesterol to phospholipids and the ratio of saturated to unsaturated fatty acids that are incorporated into the phospholipids. These ratios are determined, at least in part, by regulation of membrane lipid biosynthesis-particularly that of cholesterol and oleate. It now appears that cholesterol and oleate biosynthesis are feedback regulated by a common transcriptional mechanism which is governed by the maturation of the SREBP transcription factors. In this article, we briefly review our current understanding of transcriptional regulation of plasma membrane lipid biosynthesis by sterols and oleate. We also discuss studies related to the mechanism by which the physical state of membrane lipids signals the transcriptional regulatory machinery to control the rates of synthesis of these structural components of the lipid bilayer.

Cross-talk between fatty acid and cholesterol metabolism mediated by liver X receptor-alpha

LXR alpha (liver X receptor, also called RLD-1) is a nuclear receptor, highly expressed in tissues that play a role in lipid homeostasis. In this report we show that fatty acids are positive regulators of LXR alpha gene expression and we investigate the molecular mechanisms underlying this regulation. In cultured rat hepatoma and primary hepatocyte cells, fatty acids and the sulfur-substituted fatty acid analog, tetradecylthioacetic acid, robustly induce LXR alpha (up to 3.5- and 7-fold, respectively) but not LXR beta (also called OR-1) mRNA steady state levels, with unsaturated fatty acids being more effective than saturated fatty acids. RNA stability and nuclear run-on studies demonstrate that changes in the transcription rate of the LXR alpha gene account for the major part of the induction of LXR alpha mRNA levels. A similar induction of protein level was also seen after treatment of primary hepatocytes with the same fatty acids. Consistent with such a transcriptional effect, transient transfection studies with a luciferase reporter gene, driven by 1.5 kb of the 5'-flanking region of the mouse (m)LXR alpha gene, show a peroxisome proliferator-activated receptor-alpha-dependent increase in luciferase activity upon treatment with tetradecylthioacetic acid and the synthetic peroxisome proliferator-activated receptor-alpha activator, Wy 14.643, suggesting that the mLXR alpha 5'-flanking region contains the necessary sequence elements for fatty acid responsiveness. In addition, in vivo LXR alpha expression was induced by fatty acids, consistent with the in vitro cell culture data. These observations demonstrate that LXR alpha expression is controlled by fatty acid signaling pathways and suggest an important cross-talk between fatty acid and cholesterol regulation of lipid metabolism.

Isolation of a somatic cell mutant resistant to the induction of apoptosis by oxidized low density lipoprotein

Oxidized low density lipoprotein (oxLDL) induces apoptosis in macrophages, smooth muscle cells, and endothelial cells. To elucidate the molecular mechanism of oxLDL-induced cytotoxicity and determine its tissue specificity, we have used Chinese hamster ovary (CHO)-K1 cells expressing human CD36 (CHO/CD36). Expression of CD36 rendered these cells susceptible to killing by oxLDL. This cytotoxicity was due to the induction of apoptosis. Therefore, CD36 expression is the only requirement for oxLDL-induced apoptosis. Oxysterols apparently mediate the cytotoxicity of oxLDL in macrophage foam cells and endothelial cells. 25-Hydroxycholesterol, at concentrations higher than 1 microg/ml, killed CHO-K1 cells, by apoptosis, in medium supplemented with serum as a source of cholesterol. These effects were not seen in a 25-hydroxycholesterol-resistant CHO/CD36 mutant (OX(R)), which was otherwise capable of undergoing apoptosis in response to staurosporine. This mutant was also resistant to killing by oxLDL, suggesting that oxysterols are at least partially responsible for the toxic effects of oxLDL. Oxysterol-induced apoptosis did not involve regulation of sterol regulatory element-binding protein proteolysis or the cholesterol biosynthetic pathway. 25-Hydroxycholesterol stimulated calcium uptake by CHO-K1 cells within 2 min after addition. Treatment of CHO or THP-1 (macrophage) cells with the calcium channel blocker nifedipine prevented 25-hydroxycholesterol induction of apoptosis. OX(R) showed no enhanced calcium uptake in response to 25-hydroxycholesterol.

Different sterol regulatory element-binding protein-1 isoforms utilize distinct co-regulatory factors to activate the promoter for fatty acid synthase

Sterol regulatory element-binding proteins (SREBPs) activate genes of cholesterol and fatty acid metabolism. In each case, a ubiquitous co-regulatory factor that binds to a neighboring recognition site is also required for efficient promoter activation. It is likely that gene- and pathway-specific regulation by the separate SREBP isoforms is dependent on subtle differences in how the individual proteins function with specific co-regulators to activate gene expression. In the studies reported here we extend these observations significantly by demonstrating that SREBPs are involved in both sterol regulation and carbohydrate activation of the FAS promoter. We also demonstrate that the previously implicated Sp1 site is largely dispensable for sterol regulation in established cultured cells, whereas a CCAAT-binding factor/nuclear factor Y is critically important. In contrast, carbohydrate activation of the FAS promoter in primary hepatocytes is dependent upon SREBP and both the Sp1 and CCAAT-binding factor/nuclear factor Y sites. Because 1c is the predominant SREBP isoform expressed in hepatocytes and 1a is more abundant in sterol depleted established cell lines, this suggests that the different SREBP isoforms utilize distinct co-regulatory factors to activate target gene expression.