Nutritional and insulin regulation of fatty acid synthetase and leptin gene expression through ADD1/SREBP1

The FIRE3-mediated sterol response of the FAS promoter requires NF-Y/CBF as a coactivator

The transcription of the fatty acid synthase (FAS) gene is regulated by the sterol status of the cell via cleavage of the sterol regulatory element-binding protein (SREBP). When human HepG2 hepatoma cells were cotransfected with an expression plasmid for mature SREBP-1a together with FAS promoter/reporter constructs significant increases in reporter activity were observed. Deletion analysis of the FAS promoter between -151 and -52 relative to the transcription start site pinpoint two cis-elements important in sterol regulation of the FAS gene. One element, FIRE3, between -71 and -52 can bind in vitro translated and transcribed SREBP-1a whereas the other element, the inverted CCAAT element ICE(-97/-92), binds the trimeric transcription factor NF-Y/CBF as shown with rat liver extract and reconstituted, recombinant NF-Y. The results clearly show that the coactivator for SREBP-1a in this cell line is NF-Y. This finding was confirmed by using a dominant negative form of NF-YA, NF-YAm29, which interferes with the effect of ectopically expressed SREBP-1a on FAS reporter activity.

The roles of PPARs in adipocyte differentiation

Adipose tissue development takes place primarily around birth but adipose cell number can increase throughout life in response to nutritional changes. At the molecular level, adipogenesis is the result of transcriptional remodeling that leads to activation of a considerable number of genes. Several transcription factors act cooperatively and sequentially in this process. This article attempts to review the roles of peroxisome proliferator-activated receptors gamma and delta in the control of preadipocyte proliferation and differentiation during adipose tissue development or during the adaptive response of adipose tissue mass to high-fat feeding.

Insulin and glucocorticoids differentially regulate leptin transcription and secretion in brown adipocytes

Leptin, the ob gene product, is produced by adipose tissue and is submitted to a complex hormonal and metabolic regulation. Leptin plays a critical role in the balance of body weight. Here we report on secretion and hormonal regulation of leptin by brown adipocytes. Using the recently established T37i cell line, we show that leptin expression and secretion occurred as a function of cell differentiation. In differentiated T37i cells, insulin induced leptin release ( approximately 0.25 ng/10(6) cells/h) in a concentration-dependent manner (EC50=0.1 nM), and this was totally suppressed by beta3-adrenergic ligand, thiazolidinedione, cycloheximide, or actinomycin D. Insulin induced a strong, rapid (within 2 h) but transient fivefold increase in leptin mRNA levels. This transcriptional control of ob gene expression by insulin involved both phosphatidylinositol 3-kinase- and MAP kinase-dependent pathways. Glucocorticoids inhibited both insulin-stimulated leptin secretion and ob gene expression without affecting leptin mRNA stability (t(1/2)=3h05). Altogether, our results demonstrate that brown adipocytes express and secrete leptin, whose hormonal regulation clearly differs from that described in white adipose tissue. These findings point to tissue-specific molecular mechanisms and suggest that leptin might exert direct effects on energy homeostasis through an autocrine mechanism.

Antiretroviral therapy-associated hyperlipidaemia in HIV disease

Widespread utilization of highly active antiretroviral therapy (HAART) for HIV-infection, primarily protease inhibitors in combination with nucleoside analogue reverse transcriptase inhibitors, has recently led to a sustained reduction in the morbidity and mortality of this disease. However, administration of HAART is frequently associated with the development of lipid disorders. The severity and prevalence of dyslipidaemia vary, depending on the type of HAART, nutritional status, HIV disease stage, and concomitant presence of lipodystrophy and insulin resistance (two additional adverse effects of HAART). The mechanism that is responsible for HAART-associated dyslipidaemia remains incompletely understood. Recent data indicate that this effect may be, at least in part, accounted for by protease inhibitor-mediated inhibition of the proteasome activity and accumulation of the active portion of sterol regulatory element-binding protein-1c in liver cells and adipocytes. Whether lipid disorders in HIV-infected patients receiving HAART translate into an increased cardiovascular risk, and the indications for lipid-lowering interventions in this population, remain to be established.

Cholesterol, a cell size-dependent signal that regulates glucose metabolism and gene expression in adipocytes

Enlarged fat cells exhibit modified metabolic capacities, which could be involved in the metabolic complications of obesity at the whole body level. We show here that sterol regulatory element-binding protein 2 (SREBP-2) and its target genes are induced in the adipose tissue of several models of rodent obesity, suggesting cholesterol imbalance in enlarged adipocytes. Within a particular fat pad, larger adipocytes have reduced membrane cholesterol concentrations compared with smaller fat cells, demonstrating that altered cholesterol distribution is characteristic of adipocyte hypertrophy per se. We show that treatment with methyl-beta-cyclodextrin, which mimics the membrane cholesterol reduction of hypertrophied adipocytes, induces insulin resistance. We also produced cholesterol depletion by mevastatin treatment, which activates SREBP-2 and its target genes. The analysis of 40 adipocyte genes showed that the response to cholesterol depletion implicated genes involved in cholesterol traffic (caveolin 2, scavenger receptor BI, and ATP binding cassette 1 genes) but also adipocyte-derived secretion products (tumor necrosis factor alpha, angiotensinogen, and interleukin-6) and proteins involved in energy metabolism (fatty acid synthase, GLUT 4, and UCP3). These data demonstrate that altering cholesterol balance profoundly modifies adipocyte metabolism in a way resembling that seen in hypertrophied fat cells from obese rodents or humans. This is the first evidence that intracellular cholesterol might serve as a link between fat cell size and adipocyte metabolic activity.

Glucose regulation of the acetyl-CoA carboxylase promoter PI in rat hepatocytes

The rat acetyl-CoA carboxylase (ACC) alpha gene is transcribed from two promoters, denoted PI and PII, that direct regulated expression in a tissue-specific manner. Induction of ACC, the rate-controlling enzyme of fatty acid biosynthesis, occurs in the liver in response to feeding of a high carbohydrate, low fat diet, conditions that favor enhanced lipogenesis. This induction is mainly due to increases in PI promoter activity. We have used primary cultured hepatocytes from the rat to investigate glucose regulation of ACC expression. Glucose and insulin synergistically activated expression of ACC mRNAs transcribed from the PI promoter with little or no effect on PII mRNAs. Glucose treatment stimulated PI promoter activity in transfection assays and a glucose-regulated element was identified (-126/-102), homologous to those previously described in other responsive genes, including l-type pyruvate kinase, S(14) and fatty acid synthase. Mutation of this element eliminated the response to glucose. This region of the ACC PI promoter was able to bind a liver nuclear factor designated ChoRF that interacts with other conserved glucose-regulated elements. This ACC PI element is also capable of conferring a strong response to glucose when linked to a heterologous promoter. We conclude that induction of ACC gene expression under lipogenic conditions in hepatocytes is mediated in part by the activation of a glucose-regulated transcription factor, ChoRF, which stimulates transcription from the PI promoter. Similar mechanisms operate on related genes permitting the coordinate induction of the lipogenic pathway.

Transcriptional regulation of the leptin promoter by insulin-stimulated glucose metabolism in 3t3-l1 adipocytes

Insulin-stimulated glucose metabolism plays a key role in the regulation of leptin mRNA expression and protein secretion. However, it is not known whether stimulation of leptin production by glucose metabolism is regulated at the level of promoter activation or at a step distal to the promoter. Therefore, in order to investigate the transcriptional regulation of the leptin promoter by insulin-stimulated glucose metabolism, 3T3-L1 cells were transfected with a plasmid containing the leptin promoter driving a luciferase reporter gene. Leptin promoter activity was increased after 48 hours of treatment by 219 +/- 64 (p = 0.028) and 225 +/- 69% (p = 0.046) at insulin concentrations of 16 and 160 nM, respectively. The activation of the leptin promoter induced by insulin (16 nM) was markedly inhibited by 2-deoxy-D-glucose (2-DG, 50 mg/dl), a competitive inhibitor of glucose metabolism. The increment of insulin-stimulated leptin promoter activation was reduced by 52 +/- 11% (p = 0.028 vs insulin alone). The activity of a control plasmid (pGL2-Control) was unaffected by insulin or 2-DG. These results provide strong evidence that insulin-stimulated glucose metabolism, and not insulin per se, mediates the effects of insulin to increase the transcriptional activity of the leptin promoter.

Proteolytic Activation of SREBPs during Adipocyte Differentiation

A member of sterol regulatory element-binding protein (SREBP) family, SREBP-1, is a key regulator of adipocyte differentiation. Expression of the SREBP-1 gene is induced during adipocyte differentiation, but proteolytic activation of the synthesized precursor form of SREBP-1 has not been well analyzed. The proteolytic processing of SREBPs is severely suppressed in sterol loaded culture cells. Here we report that a splicing isoform, SREBP-1a, is predominantly expressed in 3T3-L1 preadipocytes and adipocytes, and that the nuclear active form of SREBP-1 protein increases in adipocyte differentiation. We further show that the amount of nuclear SREBP-2 protein also increases despite no increase in SREBP-2 mRNA, suggesting that proteolytic cleavage of SREBPs is induced in lipid loaded adipocytes. Northern blot analyses reveal that mRNA levels for SREBP cleavage-activating protein (SCAP), Site-1 protease (S1P), and Site-2 protease (S2P), which participate in the proteolytic processing of SREBPs, are relatively unaffected in adipogenesis. These results demonstrate that SREBP-2 appears to promote adipocyte differentiation as well as SREBP-1 and that the proteolytic activation of SREBPs may be induced by an as-yet unidentified mechanism in lipid loaded adipocytes.

Regulation by insulin of gene expression in human skeletal muscle and adipose tissue. Evidence for specific defects in type 2 diabetes

Defective regulation of gene expression may be involved in the pathogenesis of type 2 diabetes. We have characterized the concerted regulation by insulin (3-h hyperinsulinemic clamp) of the expression of 10 genes related to insulin action in skeletal muscle and in subcutaneous adipose tissue, and we have verified whether a defective regulation of some of them could be specifically encountered in tissues of type 2 diabetic patients. Basal mRNA levels (determined by reverse transcriptase-competitive polymerase chain reaction) of insulin receptor, insulin receptor substrate-1, p85alpha phosphatidylinositol 3-kinase (PI3K), p110alphaPI3K, p110betaPI3K, GLUT4, glycogen synthase, and sterol regulatory-element-binding protein-1c (SREBP-1c) were similar in muscle of control (n = 17), type 2 diabetic (n = 9), type 1 diabetic (n = 9), and nondiabetic obese (n = 9) subjects. In muscle, the expression of hexokinase II was decreased in type 2 diabetic patients (P < 0.01). In adipose tissue, SREBP-1c (P < 0.01) mRNA expression was reduced in obese (nondiabetic and type 2 diabetic) subjects and was negatively correlated with the BMI of the subjects (r = -0.63, P = 0.02). Insulin (+/-1,000 pmol/l) induced a two- to threefold increase (P < 0.05) in hexokinase II, p85alphaPI3K, and SREBP-1c mRNA levels in muscle and in adipose tissue in control subjects, in insulin-resistant nondiabetic obese patients, and in hyperglycemic type 1 diabetic subjects. Upregulation of these genes was completely blunted in type 2 diabetic patients. This study thus provides evidence for a specific defect in the regulation of a group of important genes in response to insulin in peripheral tissues of type 2 diabetic patients.

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.

Polyunsaturated fatty acids suppress hepatic sterol regulatory element-binding protein-1 expression by accelerating transcript decay

The reduction in hepatic abundance of sterol regulatory element binding protein-1 (SREBP-1) mRNA and protein associated with the ingestion of polyunsaturated fatty acids (PUFA) appears to be largely responsible for the PUFA-dependent inhibition of lipogenic gene transcription. Our initial studies indicated that the induction of SREBP-1 expression by insulin and glucose was blocked by PUFA. Nuclear run-on assays suggested PUFA reduced SREBP-1 mRNA by post-transcriptional mechanisms. In this report we demonstrate that PUFA enhance the decay of both SREBP-1a and -1c. When rat hepatocytes in monolayer culture were treated with albumin-bound 20:4(n-6) or 20:5(n-3) the half-life of total SREBP-1 mRNA was reduced by 50%. Ribonuclease protection assays revealed that the decay of SREBP-1c mRNA was more sensitive to PUFA than was SREBP-1a, i.e. the half-life of SREBP-1c and -1a was reduced from 10.0 to 4.6 h and 11.6 to 7.6 h, respectively. Interestingly, treating the hepatocytes with the translational inhibitor, cycloheximide, prevented the PUFA-dependent decay of SREBP-1. This suggests that SREBP-1 mRNA may need to undergo translation to enter the decay process, or that the decay process requires the synthesis of a rapidly turning over protein. Although the mechanism by which PUFA accelerate SREBP-1 mRNA decay remains to be determined, cloning and sequencing of the 3'-untranslated region for the rat SREBP-1 transcript revealed the presence of an A-U-rich region that is characteristic of a destablizing element.

Glucose and insulin function through two distinct transcription factors to stimulate expression of lipogenic enzyme genes in liver

Transcription of a number of genes involved in lipogenesis is stimulated by dietary carbohydrate in the mammalian liver. Both insulin and increased glucose metabolism have been proposed to be initiating signals for this process, but the pathways by which these effectors act to alter transcription have not been resolved. We have previously defined by electrophoretic mobility shift assay a factor in nuclear extracts from rat liver, designated the carbohydrate-responsive factor (Cho- RF), that binds to liver-type pyruvate kinase and S(14) promoters at sites critical for regulation by carbohydrate. The sterol regulatory element binding protein-1c (SREBP-1c) has also emerged as a major transcription factor involved in this nutritional response. In this study, we examined the relationship between SREBP-1c and ChoRF in lipogenic gene induction. The two factors were found to possess distinct DNA binding specificities both in vitro and in hepatocytes. Reporter constructs containing binding sites for ChoRF were responsive to glucose but not directly to insulin. On the other hand, reporter constructs with an SREBP-1c site responded directly to insulin. The S(14) gene possesses binding sites for both ChoRF and SREBP, and both sites were found to be functionally important for the response of this promoter to glucose and insulin in hepatocytes. Consequently, we propose that SREBP-1c and ChoRF are independent transcription factors that mediate signals generated by insulin and glucose, respectively. For many lipogenic enzyme genes, these two factors may provide an integrated signaling system to support the overall nutritional response to dietary carbohydrate.

Molecular regulation of adipogenesis

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.

Transcriptional control of adipogenesis

The major transcriptional factors involved in the adipogenic process include proteins belonging to the CCAAT/enhancer binding protein family, peroxisome proliferator-activated receptor gamma, and adipocyte determination and differentiation dependent factor 1, also known as sterol regulatory element-binding protein 1. This process has been characterized with the aid of cell lines that represent various stages in the path of adipocyte commitment, ranging from pluripotent mesodermal fibroblasts to preadipocytes. Molecular analyses have led to a cascade model for adipogenesis based on timed expression of CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptor gamma. Gene targeting and transgenic-mouse technologies, which allow the manipulation of endogenous genes for these transcription factors, have also contributed to the understanding of adipogenesis. This review aims to integrate this information to gain an understanding of the transcriptional regulation of fat cell formation.

Antecedent protein restriction and high-fat feeding interactively sensitise the leptin response to elevated insulin

Using a rat model of moderate (8 vs. 20% protein) isocaloric protein restriction initiated in early life (low protein, LP), we examined the possible basis for the association between impaired early growth and elevated leptin levels in later life in man by examining the acute leptin response to insulin and its relationship with glucose utilisation. We placed subsets of LP rats on a high-saturated-fat (HF) diet containing 20% protein for 4 weeks (LP-4HF) or 8 weeks (LP-8HF), making comparison with age-matched control (C) groups (C, C-4HF, C-8HF). At ambient insulin concentrations, LP was not associated with altered leptinaemia compared with C, despite a more active lipolytic programme as inferred from increased adipocyte sensitivity to norepinephrine. HF feeding led to insulin resistance with respect to whole-body glucose disposal (R(d)) (measured using [3-(3)H] glucose at steady state) in both LP and C in vivo and impaired suppression of agonist-stimulated lipolysis by insulin in LP but not C in vitro. Whereas insulin infusion for 2 h (while maintaining euglycaemia) only modestly increased plasma leptin levels in vivo in C, C-4HF, C-8HF and LP groups, the leptin response to insulin was greatly enhanced in the HF-fed LP groups. A close positive correlation (r = 0.96) existed between plasma leptin levels and R(d) in the C groups (viz. C, C-4HF, C-8HF) whereas a close inverse correlation (r = 0.95) existed between plasma leptin levels and insulin-stimulated R(d) in the LP groups (viz. LP, LP-4HF, LP-8HF). Glucose utilisation (estimated from 2-deoxy-D-[1-(3)H] glucose 6-phosphate accumulation) in vivo in two intra-abdominal and two superficial adipose-tissue depots was consistently higher in the LP group. After HF feeding, glucose utilisation by the superficial adipose-tissue depots was threefold higher in the LP than in the C group. We conclude that protein restriction from conception to adulthood followed by high-fat feeding sensitizes the acute leptin response to insulin, an adaptation associated with enhanced glucose utilisation by adipose tissue. This effect is observed despite impaired insulin sensitivity, both at the level of whole-body glucose disposal and adipocyte anti-lipolysis, and increased lipolytic activity (although the latter is not in itself sufficient to influence the leptin response). We propose that associations between a low birthweight and elevated leptin concentrations in later life may reflect long-term modulation of adipocyte glucose handling.

Regulation of metabolism and body fat mass by leptin

The relative stability of body weight over the long term and under a variety of environmental conditions that alter short-term energy intake and expenditure provides strong evidence for the regulation of body energy content. The lipostatic theory of energy balance regulation proposed 40 years ago that circulating factors, generated in proportion to body fat stores, acted as signals to the brain, eliciting changes in energy intake and expenditure. The discovery of leptin and its receptors has now provided a molecular basis for this theory. Leptin functions as much more than an adipocyte-derived signal of lipid stores, however. Although suppression of food intake is an important centrally mediated effect of leptin, considerable evidence indicates that leptin also functions both directly and indirectly, via the brain, to orchestrate complex metabolic changes in a number of organs and tissues, altering nutrient flux to favor energy expenditure over energy storage.

Physiological and nutritional regulation of enzymes of triacylglycerol synthesis

Although triacylglycerol stores play the critical role in an organism's ability to withstand fuel deprivation and are strongly associated with such disorders as diabetes, obesity, and atherosclerotic heart disease, information concerning the enzymes of triacylglycerol synthesis, their regulation by hormones, nutrients, and physiological conditions, their mechanisms of action, and the roles of specific isoforms has been limited by a lack of cloned cDNAs and purified proteins. Fortunately, molecular tools for several key enzymes in the synthetic pathway are becoming available. This review summarizes recent studies of these enzymes, their regulation under varying physiological conditions, their purported roles in synthesis of triacylglycerol and related glycerolipids, the possible functions of different isoenzymes, and the evidence for specialized cellular pools of triacylglycerol and glycerolipid intermediates.

Expression of sterol regulatory element-binding protein 1c (SREBP-1c) mRNA in rat hepatoma cells requires endogenous LXR ligands

The current paper describes a line of cultured rat hepatoma cells (McA-RH7777 cells) that mimics the behavior of rat liver by producing an excess of mRNA for sterol regulatory element-binding protein 1c (SREBP-1c) as opposed to SREBP-1a. These two transcripts are derived from a single gene by use of alternative promoters that are separated by many kilobases in the genome. The high level of SREBP-1c mRNA is abolished when cholesterol synthesis is blocked by compactin, an inhibitor of 3-hydroxy-3-methylglutaryl CoA (HMG CoA) reductase that inhibits cholesterol synthesis. Levels of SREBP-1c mRNA are restored by mevalonate, the product of the HMG CoA reductase reaction, and by ligands for the nuclear hormone receptor LXR, including 22(R)-hydroxycholesterol and T0901317. These data suggest that transcription of the SREBP-1c gene in hepatocytes requires tonic activation of LXR by an oxysterol intermediate in the cholesterol biosynthetic pathway. Reduction of this intermediate lowers SREBP-1c levels, and this in turn is predicted to lower the rates of fatty acid biosynthesis in liver.

Nutrient sensing, leptin and insulin action

The glucose-fatty acid cycle as proposed four decades ago by Randle suggests that insulin resistance develops in consequence of alterations of the metabolic pressure of lipids. The more recently published 'hexosamine pathway theory' and the 'malonyl-CoA hypothesis' depict insulin resistance as a consequence of an imbalance between utilization of lipids and carbohydrates. The latter is finely tuned by entry of fatty acids into the mitochondria and/or by entry of glucose to the hexosamine pathway. A significant body of evidence has also been accumulated which points to the complex effects of leptin, an adipocyte-derived signal of lipid stores, on the storage and metabolism of fats and carbohydrates. These are mediated either directly, through actions on specific tissues, or indirectly, via CNS, endocrine and neural mechanisms. The available literature also provides good evidence that leptin orchestrates the metabolic changes in a number of organs and tissues, and alters nutrient fluxes to favor energy expenditure over energy storage. In this article, the proposed lipopenic effects of leptin as studied in various animal models of diet-induced insulin resistance, and possible regulations of leptin production and action by marine fish oil feeding are reviewed.

Insulin effects on sterol regulatory-element-binding protein-1c (SREBP-1c) transcriptional activity in rat hepatocytes

The transcription factor sterol regulatory-element-binding protein-1c (SREBP-1c) plays a major role in the effect of insulin on the transcription of hepatic genes such as glucokinase and fatty acid synthase. We show here in cultured rat hepatocytes that insulin, through activation of the phosphatidylinositol 3-kinase pathway increases the abundance of the precursor form of SREBP-1c in endoplasmic reticulum. This precursor form is then rapidly cleaved, possibly irrespective of the continuous presence of insulin, leading to an increased content of the nuclear mature form of SREBP-1c. Nevertheless, the increased amount of the mature form of SREBP-1c in the nucleus is not a prerequisite for the rapid effect of insulin on the transcription of genes such as glucokinase, suggesting that additional actions of the hormone are involved, such as the activation of the nuclear form of SREBP-1c or of an unidentified SREBP-1c partner.

Diseases of liporegulation: new perspective on obesity and related disorders

Obesity-related diseases now threaten to reach epidemic proportions in the United States. Here we review in a rodent model of genetic obesity, the fa/fa Zucker diabetic fatty (ZDF) rat, the mechanisms involved in the most common complications of diet-induced human obesity, i.e., noninsulin-dependent diabetes mellitus, and myocardial dysfunction. In ZDF rats, hyperphagia leads to hyperinsulinemia, which up-regulates transcription factors that stimulate lipogenesis. This causes ectopic deposition of triacylglycerol in nonadipocytes, providing fatty acid (FA) substrate for damaging pathways of nonoxidative metabolism, such as ceramide synthesis. In beta cells and myocardium, the resulting functional impairment and apoptosis cause diabetes and cardiomyopathy. Interventions that lower ectopic lipid accumulation or block nonoxidative metabolism of FA and ceramide formation completely prevent these complications. Given the evidence for a similar etiology for the complications of human obesity, it would be appropriate to develop strategies to avert the predicted epidemic of lipotoxic disorders.

Phosphatidylinositol-3,4,5-trisphosphate is required for insulin-like growth factor 1-mediated survival of 3T3-L1 preadipocytes

Adipocyte number, a determinant of adipose tissue mass, reflects the balance between the rates of proliferation/differentiation vs. apoptosis of preadipocytes. The percentage of 3T3-L1 preadipocytes undergoing cell death following serum deprivation was reduced by 10 nM insulin-like growth factor (IGF)-1 (from 50.0 +/- 0.7% for control starved cells to 27.5 +/- 3.1%). TUNEL staining confirmed the apoptotic nature of the cell death. The protective effect of IGF-1 was blocked by phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin, and LY294002, but was unaffected by rapamycin, PD98059, or SB203580, which inhibit mammalian target of rapamycin (mTOR), ERK kinase (MEK1), and p38 MAPK respectively. Exogenous PI(3,4,5)P3 (10 microM), the principal product of IGF-1-stimulated PI3K in 3T3-L1 preadipocytes, had a modest survival effect on its own, reducing cell death from 47.9 +/- 3.4% to 35.6 +/- 3.5%. When added to the combination of IGF-1 and LY294002, PI(3,4,5)P3 reversed most of the inhibitory effect of LY294002 on IGF-1-dependent cell survival, protein kinase B/Akt phosphorylation, and caspase-3 activity. Taken together, these results implicate PI(3,4,5)P3 as a necessary signal for the anti-apoptotic action of IGF-1 on 3T3-L1 preadipocytes.

Role of Sp1 and Sp3 in the Transcriptional Regulation of the Rat Fatty Acid Synthase Gene

Inspection of the 5' region of the sequence of the rat fatty acid synthase (FAS) gene revealed a high GC content between -900 and +500, implying several binding sites for members of the Sp1 family of transcription factors. Using SL2 and H4IIE cells in conjunction with FAS promoter/luciferase constructs either successively deleted or containing defined deletions we characterized six GC boxes--GC-I to GC-VI--located between -557 and -83 and discovered a seventh, GC-VII, in the first intron. In vitro DNAse I-footprinting, electrophoretic mobility shift assays, and the yeast one-hybrid system indicated that Sp1 as well as Sp3 interacts with GC-I to GC-VII. Each of the GC boxes conferred Sp1-dependent transcription on the FAS-Mini promoter and in the case of GC-I, Sp1, and Sp3 exert an additive effect on FAS promoter activity.

Suppression of preadipocyte differentiation and promotion of adipocyte death by HIV protease inhibitors

Many human immunodeficiency virus (HIV)-infected patients taking combination antiretroviral therapy that includes HIV protease inhibitors experience atrophy of peripheral subcutaneous adipose tissue. We investigated the effects of HIV protease inhibitors on adipogenesis and adipocyte survival using the 3T3-L1 preadipocyte cell line. Several HIV protease inhibitors were found either to inhibit preadipocyte differentiation or to promote adipocyte cell death. One protease inhibitor, nelfinavir, elicited both of these effects strongly. When induced to differentiate in the presence of nelfinavir, 3T3-L1 preadipocytes failed to accumulate cytoplasmic triacylglycerol and failed to express normal levels of the adipogenic transcription factors CCAAT/enhancer-binding protein alpha and peroxisome proliferator-activated receptor gamma. The level of the proteolytically processed, active 68-kDa form of sterol regulatory element-binding protein-1, a transcription factor known to promote lipogenic gene expression, also was reduced markedly in nelfinavir-treated cells, whereas the level of the 125-kDa precursor form of this protein was unaffected. The inhibitory effect of nelfinavir occurred subsequent to critical early events in preadipocyte differentiation, expression of CCAAT/enhancer-binding protein beta and completion of the mitotic clonal expansion phase, because these events were unaffected by nelfinavir treatment. In addition, nelfinavir treatment of fully differentiated 3T3-L1 adipocytes resulted in DNA strand cleavage and severe loss of cell viability. In contrast, cell proliferation and viability of preadipocytes were unaffected by nelfinavir treatment. Thus, molecular or cellular changes that occur during acquisition of the adipocyte phenotype promote susceptibility to nelfinavir-induced cell death. When considered together, these results suggest that nelfinavir may promote adipose tissue atrophy by compromising adipocyte viability and preventing replacement of lost adipocytes by inhibiting preadipocyte differentiation.

Induction of LPL gene expression by sterols is mediated by a sterol regulatory element and is independent of the presence of multiple E boxes

Overexpression of the adipocyte differentiation and determination factor-1 (ADD-1) or sterol regulatory element binding protein-1 (SREBP-1) induces the expression of numerous genes involved in lipid metabolism, including lipoprotein lipase (LPL). Therefore, we investigated whether LPL gene expression is controlled by changes in cellular cholesterol concentration and determined the molecular pathways involved. Cholesterol depletion of culture medium resulted in a significant induction of LPL mRNA in the 3T3-L1 preadipocyte cell line, whereas addition of cholesterol reduced LPL mRNA expression to basal levels. Similar to the expression of the endogenous LPL gene, the activity of the human LPL gene promoter was enhanced by cholesterol depletion in transient transfection assays, whereas addition of cholesterol caused a reversal of its induction. The effect of cholesterol depletion upon the human LPL gene promoter was mimicked by cotransfection of expression constructs encoding the nuclear form of SREBP-1a, -1c (also called ADD-1) and SREBP-2. Bioinformatic analysis demonstrated the presence of 3 potential sterol regulatory elements (SRE) and 3 ADD-1 binding sequences (ABS), also known as E-box motifs. Using a combination of in vitro protein-DNA binding assays and transient transfection assays of reporter constructs containing mutations in each individual site, a sequence element, termed LPL-SRE2 (SRE2), was shown to be the principal site conferring sterol responsiveness upon the LPL promoter. These data furthermore underscore the importance of SRE sites relative to E-boxes in the regulation of LPL gene expression by sterols and demonstrate that sterols contribute to the control of triglyceride metabolism via binding of SREBP to the LPL regulatory sequences.

Regulation of mouse sterol regulatory element-binding protein-1c gene (SREBP-1c) by oxysterol receptors, LXRalpha and LXRbeta

The liver X receptors (LXRs) are members of the nuclear hormone receptor superfamily that are bound and activated by oxysterols. These receptors serve as sterol sensors to regulate the transcription of gene products that control intracellular cholesterol homeostasis through catabolism and transport. In this report, we describe a novel LXR target, the sterol regulatory element-binding protein-1c gene (SREBP-1c), which encodes a membrane-bound transcription factor of the basic helix-loop-helix-leucine zipper family. SREBP-1c expression was markedly increased in mouse tissues in an LXR-dependent manner by dietary cholesterol and synthetic agonists for both LXR and its heterodimer partner, the retinoid X receptor (RXR). Expression of the related gene products, SREBP-1a and SREBP-2, were not increased. Analysis of the mouse SREBP-1c gene promoter revealed an RXR/LXR DNA-binding site that is essential for this regulation. The transcriptional increase in SREBP-1c mRNA by RXR/LXR was accompanied by a similar increase in the level of the nuclear, active form of the SREBP-1c protein and an increase in fatty acid synthesis. Because this active form of SREBP-1c controls the transcription of genes involved in fatty acid biosynthesis, our results reveal a unique regulatory interplay between cholesterol and fatty acid metabolism.

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.

The roles of sterol regulatory element-binding proteins in the transactivation of the rat ATP citrate-lyase promoter

ATP citrate-lyase (ACL) is a key enzyme supplying acetyl-CoA for fatty acid and cholesterol synthesis. Its expression is drastically up-regulated when an animal is fed a low fat, high carbohydrate diet after prolonged fasting. In this report, we describe the role of sterol regulatory element-binding proteins (SREBPs) in the transactivation of the rat ACL promoter. ACL promoter activity was markedly stimulated by the overexpression of SREBP-1a and, to a lesser extent, by SREBP-2 in Alexander human hepatoma cells. The promoter elements responsive to SREBPs were located within the 55-base pair sequences from -114 to -60. The gel mobility shift assay revealed four SREBP-1a binding sites in this region. Of these four elements, the -102/-94 region, immediately upstream of the inverted Y-box, and the -70/-61 region, just adjacent to Sp1 binding site, played critical roles in SREBPs-mediated stimulation. The mutation in the inverted Y-box and the coexpression of dominant negative nuclear factor-Y (NF-Y) significantly attenuated the transactivation by SREBP-1a, suggesting that NF-Y binding is a prerequisite for SREBPs to activate the ACL promoter. However, the multiple Sp1 binding sites did not affect the transactivation of the ACL promoter by SREBPs. The binding affinity of SREBP-1a to SREs of the ACL promoter also was much higher than that of SREBP-2. The transactivation potencies of the chimeric SREBPs, of which the activation domains (70 amino acids of the amino terminus) were derived from the different species of their carboxyl-terminal region, were similar to those of SREBPs corresponding to their carboxyl termini. Therefore, it is suggested that the carboxyl-terminal portions of SREBPs containing DNA binding domains are important in determining their transactivation potencies to a certain promoter.

Defect in peroxisome proliferator-activated receptor alpha-inducible fatty acid oxidation determines the severity of hepatic steatosis in response to fasting

Fasting causes lipolysis in adipose tissue leading to the release of large quantities of free fatty acids into circulation that reach the liver where they are metabolized to generate ketone bodies to serve as fuels for other tissues. Since fatty acid-metabolizing enzymes in the liver are transcriptionally regulated by peroxisome proliferator-activated receptor alpha (PPARalpha), we investigated the role of PPARalpha in the induction of these enzymes in response to fasting and their relationship to the development of hepatic steatosis in mice deficient in PPARalpha (PPARalpha(-/-)), peroxisomal fatty acyl-CoA oxidase (AOX(-/-)), and in both PPARalpha and AOX (double knock-out (DKO)). Fasting for 48-72 h caused profound impairment of fatty acid oxidation in both PPARalpha(-/-) and DKO mice, and DKO mice revealed a greater degree of hepatic steatosis when compared with PPARalpha(-/-) mice. The absence of PPARalpha in both PPARalpha(-/-) and DKO mice impairs the induction of mitochondrial beta-oxidation in liver following fasting which contributes to hypoketonemia and hepatic steatosis. Pronounced steatosis in DKO mouse livers is due to the added deficiency of peroxisomal beta-oxidation system in these animals due to the absence of AOX. In mice deficient in AOX alone, the sustained hyperactivation of PPARalpha and up-regulation of mitochondrial beta-oxidation and microsomal omega-oxidation systems as well as the regenerative nature of a majority of hepatocytes containing numerous spontaneously proliferated peroxisomes, which appear refractory to store triglycerides, blunt the steatotic response to fasting. Starvation for 72 h caused a decrease in PPARalpha hepatic mRNA levels in wild type mice, with no perceptible compensatory increases in PPARgamma and PPARdelta mRNA levels. PPARgamma and PPARdelta hepatic mRNA levels were lower in fed PPARalpha(-/-) and DKO mice when compared with wild type mice, and fasting caused a slight increase only in PPARgamma levels and a decrease in PPARdelta levels. Fasting did not change the PPAR isoform levels in AOX(-/-) mouse liver. These observations point to the critical importance of PPARalpha in the transcriptional regulatory responses to fasting and in determining the severity of hepatic steatosis.

Nutritional regulation of the fatty acid synthase promoter in vivo: sterol regulatory element binding protein functions through an upstream region containing a sterol regulatory element

The transcription of fatty acid synthase (FAS), a central enzyme in de novo lipogenesis, is dramatically induced by fasting/refeeding and insulin. We reported that upstream stimulatory factor binding to the -65 E-box is required for induction of the FAS transcription by insulin in 3T3-L1 adipocytes. On the other hand, we recently found that two upstream 5' regions are required for induction in vivo by fasting/refeeding and insulin; one at -278 to -131 albeit at a low level, and the other at -444 to -278 with an E-box at -332 where upstream stimulatory factor functions for maximal induction. Here, we generated double transgenic mice carrying the chloramphenicol acetyltransferase reporter driven by the various 5' deletions of the FAS promoter region and a truncated active form of the sterol regulatory element (SRE) binding protein (SREBP)-1a. We found that SREBP participates in the nutritional regulation of the FAS promoter and that the region between -278 and -131 bp is required for SREBP function. We demonstrate that SREBP binds the -150 canonical SRE present between -278 and -131, and SREBP can function through the -150 SRE in cultured cells. These in vivo and in vitro results indicate that SREBP is involved in the nutritional induction of the FAS promoter via the -278/-131 region and that the -150 SRE is the target sequence.

Transcriptional regulation of the adipocyte fatty acid synthase gene by agouti: interaction with insulin

Mice carrying dominant mutations at the agouti locus exhibit ectopic expression of agouti gene transcripts, obesity, and type II diabetes through unknown mechanisms. To gain insight into the role of agouti protein in modulating adiposity, we investigated regulation of a key lipogenic gene, fatty acid synthase (FAS) by agouti alone and in combination with insulin. Both agouti and insulin increase FAS activity in 3T3-L1 and in human adipocytes. Agouti and insulin independently and additively increase FAS activity in 3T3-L1 adipocytes. We further investigated the mechanism responsible for the agouti-induced FAS expression in these cells and demonstrated that both insulin (3-fold increase) and agouti (2-fold) increased FAS gene expression at the transcriptional level. Furthermore, insulin and agouti together exerted additive effects (5-fold increase) on FAS gene transcription. Transfection assays of FAS promoter-luciferase fusion gene constructs into 3T3-L1 adipocytes indicated that the agouti response element(s) is (are) located in the -435 to -415 region (-435/-415) of the FAS promoter. Nuclear proteins binding to this novel sequence are adipocyte specific. Thus the agouti response sequences mapped to a region upstream of the insulin-responsive element (which we previously reported to be located at -67/-52), consistent with additive effects of these two factors on FAS gene transcription.

Insulin effects on sterol regulatory-element-binding protein-1c (SREBP-1c) transcriptional activity in rat hepatocytes

The transcription factor sterol regulatory-element-binding protein-1c (SREBP-1c) plays a major role in the effect of insulin on the transcription of hepatic genes such as glucokinase and fatty acid synthase. We show here in cultured rat hepatocytes that insulin, through activation of the phosphatidylinositol 3-kinase pathway increases the abundance of the precursor form of SREBP-1c in endoplasmic reticulum. This precursor form is then rapidly cleaved, possibly irrespective of the continuous presence of insulin, leading to an increased content of the nuclear mature form of SREBP-1c. Nevertheless, the increased amount of the mature form of SREBP-1c in the nucleus is not a prerequisite for the rapid effect of insulin on the transcription of genes such as glucokinase, suggesting that additional actions of the hormone are involved, such as the activation of the nuclear form of SREBP-1c or of an unidentified SREBP-1c partner.

Loss of insulin signaling in hepatocytes leads to severe insulin resistance and progressive hepatic dysfunction

The liver plays a central role in the control of glucose homeostasis and is subject to complex regulation by substrates, insulin, and other hormones. To investigate the effect of the loss of direct insulin action in liver, we have used the Cre-loxP system to inactivate the insulin receptor gene in hepatocytes. Liver-specific insulin receptor knockout (LIRKO) mice exhibit dramatic insulin resistance, severe glucose intolerance, and a failure of insulin to suppress hepatic glucose production and to regulate hepatic gene expression. These alterations are paralleled by marked hyperinsulinemia due to a combination of increased insulin secretion and decreased insulin clearance. With aging, the LIRKO liver exhibits morphological and functional changes, and the metabolic phenotype becomes less severe. Thus, insulin signaling in liver is critical in regulating glucose homeostasis and maintaining normal hepatic function.

Diurnal variation in circulating leptin is dependent on gender, food intake and circulating insulin in mice

Leptin is an adipocyte hormone involved in the regulation of energy homeostasis. Its circulating levels show a diurnal rhythm with a nocturnal peak. We examined the influences of gender, feeding state, and plasma insulin and glucose on the diurnal rhythm in normal mice. Plasma was sampled at 4-h interval for 24 h in female (n=80) and male (n=80) mice, which were freely fed or fasted. In both genders, plasma leptin displayed a diurnal rhythm with a nadir at 8 or 10 AM and a nocturnal peak at 10 PM to 2 AM. The nocturnal increase in leptin was higher in females (+160+/-18%) than in males (61+/- 16%; P<0.001), completely abolished by fasting, and correlated significantly to the diurnal variation in plasma insulin both in females (r=0.44, P=0.003) and males (r=0.46, P<0.001). Baseline plasma leptin in non-fasted animals were not different between the genders, whereas during fasting, the reduction in leptin was more pronounced in males than in females, resulting in a higher plasma leptin after fasting in females. Plasma insulin was higher in males under non-fasted conditions (P=0.003), but not significantly different between genders in fasted animals. In conclusion, plasma leptin displays a nocturnal increase in mice, which is more pronounced in female mice than in male mice, is completely abolished by fasting and correlates to the diurnal variation in circulating insulin. It is suggested that the nocturnal rise in leptin shows gender dependency and is caused by the increase in plasma insulin caused by food intake.

Leptin, troglitazone, and the expression of sterol regulatory element binding proteins in liver and pancreatic islets

Overaccumulation of lipids in nonadipose tissues of obese rodents may lead to lipotoxic complications such as diabetes. To assess the pathogenic role of the lipogenic transcription factor, sterol regulatory element binding protein 1 (SREBP-1), we measured its mRNA in liver and islets of obese, leptin-unresponsive fa/fa Zucker diabetic fatty rats. Hepatic SREBP-1 mRNA was 2.4 times higher than in lean +/+ controls, primarily because of increased SREBP-1c expression. mRNA of lipogenic enzymes ranged from 2.4- to 4.6-fold higher than lean controls, and triacylglycerol (TG) content was 5.4 times higher. In pancreatic islets of fa/fa rats, SREBP-1c was 3.4 times higher than in lean +/+ Zucker diabetic fatty rats. The increase of SREBP-1 in liver and islets of untreated fa/fa rats was blocked by 6 weeks of troglitazone therapy, and the diabetic phenotype was prevented. Up-regulation of SREBP-1 also occurred in livers of Sprague-Dawley rats with diet-induced obesity. Hyperleptinemia, induced in lean +/+ rats by adenovirus gene transfer, lowered hepatic SREBP-1c by 74% and the lipogenic enzymes from 35 to 59%. In conclusion, overnutrition increases and adenovirus-induced hyperleptinemia decreases SREBP-1c expression in liver and islets. SREBP-1 overexpression, which is prevented by troglitazone, may play a role in the ectopic lipogenesis and lipotoxicity complicating obesity in Zucker diabetic fatty rats.

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.

Nutritional and insulin regulation of leptin gene expression

The leptin and lipogenic enzyme genes contain the common DNA sequences of binding sites for Sp1 proteins. These sites appear to be responsible for glucose/insulin stimulation and polyunsaturated fatty acid suppression. In rat adipose tissue leptin and lipogenic gene expression is similarly regulated by nutritional manipulation. Interestingly, leptin has the ability to down-regulate lipogenic enzyme expression.

An indirect role for upstream stimulatory factor in glucose-mediated induction of pyruvate kinase and S14 gene expression

Transcription of the L-type pyruvate kinase (L-PK) and S14 genes is induced in hepatocytes in response to increased glucose metabolism. The regulatory sequences of these genes responsible for induction by glucose have been mapped to related E-box containing motifs in the promoters. Similarly, L-PK promoter activity is stimulated in a differentiated pancreatic beta-cell line, INS-1, in response to elevated glucose. By mutational analysis, we demonstrate that the sequence requirements for glucose induction in the INS-1 cell are identical to those observed in the hepatocyte, suggesting that the same transcriptional factor(s) is responsible for regulation of L-PK expression in the two cell types. One nuclear factor that binds to the glucose regulatory sequences of both of these genes is the Upstream Stimulatory Factor (USF), a ubiquitous E-box binding protein. Mice deleted for the USF2 gene display a severely delayed response to carbohydrate feeding (Vallet et al. [26]). This observation, however, does not differentiate between a direct and an indirect role for USF in the process. To gain further insight into the possible involvement of USF in glucose signaling, we have used a recombinant adenoviral construct that expresses a dominant negative form of USF. This dominant negative can dimerize with endogenous USF and is shown to inhibit DNA binding of USF in hepatocytes and INS-1 cells. However, expression of the dominant negative USF did not block the ability of glucose to stimulate L-PK or S14 gene expression in hepatocytes or L-PK promoter activity in INS-1 cells. We conclude that USF does not act by binding to the glucose regulatory sequences of the S14 or L-PK genes and the role of USF in the process of glucose induction is indirect.