To elucidate the physiological role of sterol regulatory element-binding protein-1 (SREBP-1), the hepatic mRNA levels of genes encoding various lipogenic enzymes were estimated in SREBP-1 gene knockout mice after a fasting-refeeding treatment, which is an established dietary manipulation for the induction of lipogenic enzymes. In the fasted state, the mRNA levels of all lipogenic enzymes were consistently low in both wildtype and SREBP-1 ؊/؊ mice. However, the absence of SREBP-1 severely impaired the marked induction of hepatic mRNAs of fatty acid synthetic genes, such as acetyl-CoA carboxylase, fatty acid synthase, and stearoyl-CoA desaturase, that was observed upon refeeding in the wild-type mice. Furthermore, the refeeding responses of other lipogenic enzymes, glycerol-3-phosphate acyltransferase, ATP citrate lyase, malic enzyme, glucose-6-phosphate dehydrogenase, and S14 mRNAs, were completely abolished in SREBP-1 ؊/؊ mice. In contrast, mRNA levels for cholesterol biosynthetic genes were elevated in the refed SREBP-1 ؊/؊ livers accompanied by an increase in nuclear SREBP-2 protein. When fed a high carbohydrate diet for 14 days, the mRNA levels for these lipogenic enzymes were also strikingly lower in SREBP-1 ؊/؊ mice than those in wild-type mice. These data demonstrate that SREBP-1 plays a crucial role in the induction of lipogenesis but not cholesterol biosynthesis in liver when excess energy by carbohydrates is consumed.Cholesterol and fatty acids are the primary lipids synthesized in liver. However, biosynthetic pathways for cholesterol and fatty acids are under distinct and separate regulation (for a review, see Ref. 1). In contrast to cholesterol synthesis, which is tightly regulated by a feedback system to maintain cellular cholesterol levels, fatty acid synthesis is driven primarily by the availability of carbohydrates and the actions of hormones such as insulin. Despite these different patterns of regulation, recent evidence suggests that both biosynthetic pathways can be controlled by a common family of transcription factors designated sterol regulatory element binding proteins (SREBPs)

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 (LXR␣) and LXR␤ as strong activators of the mouse SREBP-1c promoter. Sterol-regulatory element (SRE)-binding proteins (SREBPs) are transcription factors which belong to the basic helix-loophelix leucine zipper family (3-5). In contrast to other members of this family, SREBPs are synthesized as precursor proteins which remain bound to the endoplasmic reticulum and the nuclear envelope in the presence of sufficient sterol concentrations. Upon sterol deprivation, the precursor protein undergoes a sequential two-step cleavage process to release the NH 2 -terminal portion (28). This mature SREBP then enters the nucleus and activates the transcription of genes involved in cholesterol and fatty acid synthesis by binding to SREs or to palindromic sequences called E boxes within their promoter regions (20, 39). Three forms of SREBP have been characterized: SREBP-1a and -1c (also known as ADD1) (14, 38, 43) and SREBP-2. It has been shown that all of the cultured cells analyzed to date express primarily SREBP-2 and the SREBP1a isoform, whereas most organs, including the liver and adipose tissue, express predominantly SREBP-2 and the SREBP1c isoform (36). SREBP-1a is a stronger activator than SREBP-1c due to its longer transactivation domain and has a wider range of target genes involved in both cholesterol and fatty acid synthesis (30, 31).Lipogenic enzymes, which are involved in energy storage through synthesis of fatty acids and triglycerides, are coordinately regulated at the transcriptional level during different metabolic states (9,11). Recent in vivo studies demonstrated that SREBP-1c plays a crucial role in the dietary regulation of most hepatic lipogenic genes, whereas SREBP-2 is actively involved in the transcription of cholesterogenic enzymes (13). These include studies of the effects of the absence or overexpression of SREBP-1 on hepatic lipogenic gene expression (30,31,33), as well as physiological changes of SREBP-1c protein in normal mice after dietary manipulation such as placement on high-carbohydrate diets, polyunsaturated fatty acid-enriched diets, and fasting-refeeding regimens (12,17,37,40,41). The similar coordinated changes in SREBP-1c and lipogenic gene expression upon fasting and refeeding were also observed in adipose tissue (18). In fat tissue, SREBP-1c (ADD1) appears to be involved in adipocyte differentiation and insulin resistance (19,35). Recent studies suggest that insulin or insulin-facilitated glucose uptake mediates lipogenesis through SREBP-1c induction (7,8,10,21,34). Previous reports on the regulation of SREBP-1c have all demonstrated the induction to be at the mRNA level. Up-regulation of hepatic SREBP-1 mRNA was observed in the livers...

The liver, the principal lipogenic organ, is responsible for the conversion of excess dietary carbohydrates to triglycerides. A high carbohydrate diet induces the synthesis of several lipogenic and glycolytic enzymes including acetyl-CoA carboxylase (ACC), 1 fatty acid synthase (FAS), stearoyl-CoA desaturase, ATP citrate lyase, malic enzyme, glucose-6-phosphate dehydrogenase, and pyruvate kinase (PK) (1-3). This coordinate induction of enzymes is due to increased mRNA levels, resulting primarily from the accelerated transcription.Dietary polyunsaturated fatty acids (PUFA) have been well established as negative regulators of hepatic lipogenesis. Allmann and Gibson (4) discovered that adding 2% linoleate to a high carbohydrate fat-free diet suppressed the rate of hepatic fatty acid biosynthesis and the activities of FAS and glucose-6-phosphate dehydrogenase by nearly 70% in mice. In contrast, supplementing the high carbohydrate diet with palmitate, oleate, or cholesterol had no effect on hepatic lipogenesis or the activity of lipogenic enzymes. Since then, several investigators have demonstrated that dietary PUFA of the n-6 and n-3 families suppress hepatic lipogenesis. This anti-lipogenic action of PUFA reflects decreases in mRNA levels of hepatic enzymes including ACC, FAS, stearoyl-CoA desaturase, ATP citrate lyase, malic enzyme, glucose-6-phosphate dehydrogenase, and PK. The regulation by PUFA has been shown to be primarily at the transcriptional level; however, the precise mechanism for this action remains unknown (5-7).Sterol regulatory element-binding proteins (SREBPs) are transcription factors that belong to the basic helix-loop-helixleucine zipper family and regulate enzymes responsible for the synthesis of cholesterol, fatty acids, and triglycerides (8). Unlike other members of the basic helix-loop-helix-leucine zipper family, SREBPs are synthesized as precursors bound to the endoplasmic reticulum and nuclear envelope. Upon activation, SREBPs are released from the membrane into the nucleus as a mature protein by a sequential two-step cleavage process. To date, three SREBP isoforms, SREBP-1a, -1c and -2, have been identified and characterized. The predominant SREBP-1 isoform in the liver is SREBP-1c. Whereas SREBP-2 is relatively selective in transcriptionally activating cholesterol biosynthetic genes, SREBP-1c has a greater role in regulating fatty acid synthesis than cholesterol synthesis in the liver (9 -11, 30).The role of SREBP-1 for the regulation of hepatic lipogenesis has been recently established. Changes in hepatic mature SREBP-1c protein levels were shown to parallel those of mRNAs for lipogenic genes in the liver using a dietary manipulation and a transgenic technology (12). Moreover, SREBP-1 has been demonstrated to be crucial for the carbohydrate stimulation of lipogenic genes in mice with a targeted disruption of SREBP-1 (30).

Previous studies have demonstrated that polyunsaturated fatty acids (PUFAs) suppress sterol regulatory element-binding protein 1c (SREBP-1c) expression and, thus, lipogenesis. In the current study, the molecular mechanism for this suppressive effect was investigated with luciferase reporter gene assays using the SREBP-1c promoter in HEK293 cells. Consistent with previous data, the addition of PUFAs to the medium in the assays robustly inhibited the SREBP-1c promoter activity. Deletion and mutation of the two liver X receptor (LXR)-responsive elements (LXREs) in the SREBP-1c promoter region eliminated this suppressive effect, indicating that both LXREs are important PUFAsuppressive elements. The luciferase activities of both SREBP-1c promoter and LXRE enhancer constructs induced by co-expression of LXR␣ or -␤ were strongly suppressed by the addition of various PUFAs (arachidonic acid > eicosapentaenoic acid > docosahexaenoic acid > linoleic acid), whereas saturated or monounsaturated fatty acids had minimal effects. Gel shift mobility and ligand binding domain activation assays demonstrated that PUFA suppression of SREBP-1c expression is mediated through its competition with LXR ligand in the activation of the ligand binding domain of LXR, thereby inhibiting binding of LXR/retinoid X receptor heterodimer to the LXREs in the SREBP-1c promoter. These data suggest that PUFAs could be deeply involved in nutritional regulation of cellular fatty acid levels by inhibiting an LXR-SREBP-1c system crucial for lipogenesis. Sterol regulatory element (SRE)1 -binding proteins (SREBPs) are membrane-bound transcription factors that belong to the basic helix-loop-helix leucine zipper family (1-3). In the absence of sterols, by means of sterol-regulated cleavage, SREBP enters the nucleus and activates the transcription of genes involved in cholesterol and fatty acid synthesis by binding to an SRE or its related sequences including SRE-like sequences and E-boxes, within their promoter regions (4, 5). There are three forms of SREBP, SREBP-1a and -1c (also known as ADD1) and -2 (6 -8). Most organs, including the liver and adipose tissue, predominantly express SREBP-2 and the -1c isoform of SREBP-1 (9). Recent in vivo studies demonstrate that SREBP-1c plays a crucial role in the dietary regulation of most hepatic lipogenic genes, whereas SREBP-2 is actively involved in the transcription of cholesterogenic enzymes (10). These include studies of the effects of the absence or overexpression of SREBP-1 on hepatic lipogenic gene expression (10 -12) as well as physiological changes of SREBP-1c protein in normal mice refed after fasting (13-17). Polyunsaturated fatty acid (PUFA) administration has been well established as a negative regulator of hepatic lipogenesis as well as an activator of peroxisome proliferator-activated receptor (PPAR) ␣, which is crucial for lipid degradation. Consistent with the notion that SREBP-1c is a dominant regulator for lipogenesis, there are several reports demonstrating that administration of PUFA suppresses ...

Obesity is a common nutritional problem often associated with diabetes, insulin resistance, and fatty liver (excess fat deposition in liver). Leptin-deficient Lep ob / Lep ob mice develop obesity and those obesity-related syndromes. Increased lipogenesis in both liver and adipose tissue of these mice has been suggested. We have previously shown that the transcription factor sterol regulatory element-binding protein-1 (SREBP-1) plays a crucial role in the regulation of lipogenesis in vivo. To explore the possible involvement of SREBP-1 in the pathogenesis of obesity and its related syndromes, we generated mice deficient in both leptin and SREBP-1. In doubly mutant Lep ob/ob ؋ Srebp-1 ؊/؊ mice, fatty livers were markedly attenuated, but obesity and insulin resistance remained persistent. The mRNA levels of lipogenic enzymes such as fatty acid synthase were proportional to triglyceride accumulation in liver. In contrast, the mRNA abundance of SREBP-1 and lipogenic enzymes in the adipose tissue of Lep ob /Lep ob mice was profoundly decreased despite sustained fat, which could explain why the SREBP-1 disruption had little effect on obesity. In conclusion, SREBP-1 regulation of lipogenesis is highly involved in the development of fatty livers but does not seem to be a determinant of obesity in Lep ob /Lep ob mice.