ADD1/SREBP1c activates the PGC1-α promoter in brown adipocytes

Cold adaptation elicits a paradoxical simultaneous induction of fatty acid synthesis and β-oxidation in brown adipose tissue. We show here that cold exposure coordinately induced liver X receptor α (LXRα), adipocyte determination and differentiation-dependent factor 1 (ADD1)/sterol regulatory element-binding protein-1c (SREBP1c) and peroxisome proliferator-activated receptor γ coactivator-1α (PGC1α) in brown and inguinal white adipose tissues, but not in epididymal white adipose tissue. Using in vitro models of white and brown adipocytes we demonstrate that β-adrenergic stimulation induced expression of LXRα, ADD1/SREBP1c and PGC1α in cells with a brown-like adipose phenotype. We demonstrate that ADD1/SREBP1c is a powerful inducer of PGC1α expression via a conserved E box in the proximal promoter and that β-adrenergic stimulation led to recruitment of ADD1/SREBP1c to this E box. The ability of ADD1/SREBP1c to activate the PGC1α promoter exhibited a striking cell type dependency, suggesting that additional cell type-restricted factors contribute to ADD1/SREBP1c-mediated activation. In conclusion, our data demonstrate a novel role of ADD1/SREBP1c as a regulator of PGC1α expression in brown adipose tissue.     

Activated liver X receptors stimulate adipocyte differentiation through induction of peroxisome proliferator-activated receptor gamma expression

Liver X receptors (LXRs) are nuclear hormone receptors that regulate cholesterol and fatty acid metabolism in liver tissue and in macrophages. Although LXR activation enhances lipogenesis, it is not well understood whether LXRs are involved in adipocyte differentiation. Here, we show that LXR activation stimulated the execution of adipogenesis, as determined by lipid droplet accumulation and adipocyte-specific gene expression in vivo and in vitro. In adipocytes, LXR activation with T0901317 primarily enhanced the expression of lipogenic genes such as the ADD1/SREBP1c and FAS genes and substantially increased the expression of the adipocyte-specific genes encoding PPARγ (peroxisome proliferator-activated receptor γ) and aP2. Administration of the LXR agonist T0901317 to lean mice promoted the expression of most lipogenic and adipogenic genes in fat and liver tissues. It is of interest that the PPARγ gene is a novel target gene of LXR, since the PPARγ promoter contains the conserved binding site of LXR and was transactivated by the expression of LXRα. Moreover, activated LXRα exhibited an increase of DNA binding to its target gene promoters, such as ADD1/SREBP1c and PPARγ, which appeared to be closely associated with hyperacetylation of histone H3 in the promoter regions of those genes. Furthermore, the suppression of LXRα by small interfering RNA attenuated adipocyte differentiation. Taken together, these results suggest that LXR plays a role in the execution of adipocyte differentiation by regulation of lipogenesis and adipocyte-specific gene expression.     

The LXR ligand T0901317 induces severe lipogenesis in the db/db diabetic mouse

Liver X receptor (LXR) ligands are currently being evaluated as potential therapeutic agents for the treatment of low HDL. The LXR ligand T0901317 elevates ATP binding cassette transporter A1 (ABCA1) and HDL levels in animal models and induces moderate lipogenesis through upregulation of sterol regulatory element binding protein 1c (SREBP1c). Because insulin may also regulate lipogenesis through SREBP1c and fatty acid synthase (FAS), we investigated the effect of an LXR ligand in hyperinsulinemic mice. Administration of T0901317 to male db/db mice for 12 days resulted in a more severe hypertriacylglycerolemia and hepatic triacylglycerol accumulation than observed in nondiabetic mice. The LXR target genes ABCA1, SREBP1c, FAS, and stearoyl-CoA desaturase 1 were upregulated by T0901317 treatment in both diabetic db/db and nondiabetic C57BLKS mice. Changes in lipogenic gene expression were independent of mouse strain, indicating that the severe lipogenesis observed in LXR ligand-treated db/db mice was not due to additive effects of insulin on lipogenic gene expression. Phosphoenolpyruvate carboxykinase expression was suppressed, suggesting that a shift from gluconeogenesis toward lipogenesis could partially explain our observations in db/db mice. Our data suggest that LXR ligands that have effects on both fatty acid and carbohydrate metabolism should be carefully evaluated in obesity, insulin, and leptin resistance.     

SREBP1 is required for the induction by glucose of pancreatic beta-cell genes involved in glucose sensing

Previous studies have reported both positive and negative effects of culture of islets at high glucose concentrations on regulated insulin secretion. Here, we have reexamined this question in mouse islets and determined the role of changes in lipid synthesis in the effects of glucose. Glucose-stimulated insulin secretion (GSIS) and gene expression were examined in islets from C57BL/6 mice or littermates deleted for sterol-regulatory element binding protein-1 (SREBP1) after 4 days of culture at high glucose concentrations. Culture of control islets at 30 versus 8 mmol/l glucose led to enhanced secretion at both basal (3 mmol/l) and stimulatory (17 mmol/l) glucose concentrations and to enhanced triacylglycerol accumulation. These changes were associated with increases in the expression of genes involved in glucose sensing (glucose transporter 2, glucokinase, sulfonylurea receptor 1, inwardly rectifying K(+) channel 6.2), differentiation (pancreatic duodenal homeobox 1), and lipogenesis (Srebp1, fatty acid synthase, acetyl-coenzyme A carboxylase 1, stearoyl-coenzyme A desaturase 1). When cultured at either 8 or 30 mmol/l glucose, SREBP1-deficient (SREBP1(-/-)) islets displayed reduced GSIS and triacylglycerol content compared with normal islets. Correspondingly, glucose induction of the above genes in control islets was no longer observed in SREBP1(-/-) mouse islets. We conclude that enhanced lipid synthesis mediated by SREBP1c-dependent genes is required for the adaptive changes in islet gene expression and insulin secretion at high glucose concentrations.     

Repression of endometrial tumor growth by targeting SREBP1 and lipogenesis

The aberrantly increased lipogenesis is a universal metabolic feature of proliferating tumor cells. Although most normal cells acquire the bulk of their fatty acids from circulation, tumor cells synthesize more than 90% of required lipids de novo. The sterol regulatory element-binding protein 1 (SREBP1), encoded by SREBF1 gene, is a master regulator of lipogenic gene expression. SREBP1 and its target genes are overexpressed in a variety of cancers; however, the role of SREBP1 in endometrial cancer is largely unknown. We have screened a cohort of endometrial cancer (EC) specimen for their lipogenic gene expression and observed a significant increase of SREBP1 target gene expression in cancer cells compared with normal endometrium. By using immunohistochemical staining, we confirmed SREBP1 protein overexpression and demonstrated increased nuclear distribution of SREBP1 in EC. In addition, we found that knockdown of SREBP1 expression in EC cells suppressed cell growth, reduced colonigenic capacity and slowed tumor growth in vivo. Furthermore, we observed that knockdown of SREBP1 induced significant cell death in cultured EC cells. Taken together, our results show that SREBP1 is essential for EC cell growth both in vitro and in vivo, suggesting that SREBP1 activity may be a novel therapeutic target for endometrial cancers.     

Repression of endometrial tumor growth by targeting SREBP1 and lipogenesis

The aberrantly increased lipogenesis is a universal metabolic feature of proliferating tumor cells. Although most normal cells acquire the bulk of their fatty acids from circulation, tumor cells synthesize more than 90% of required lipids de novo. The sterol regulatory element-binding protein 1 (SREBP1), encoded by SREBF1 gene, is a master regulator of lipogenic gene expression. SREBP1 and its target genes are overexpressed in a variety of cancers; however, the role of SREBP1 in endometrial cancer is largely unknown. We have screened a cohort of endometrial cancer (EC) specimen for their lipogenic gene expression and observed a significant increase of SREBP1 target gene expression in cancer cells compared with normal endometrium. By using immunohistochemical staining, we confirmed SREBP1 protein overexpression and demonstrated increased nuclear distribution of SREBP1 in EC. In addition, we found that knockdown of SREBP1 expression in EC cells suppressed cell growth, reduced colonigenic capacity and slowed tumor growth in vivo. Furthermore, we observed that knockdown of SREBP1 induced significant cell death in cultured EC cells. Taken together, our results show that SREBP1 is essential for EC cell growth both in vitro and in vivo, suggesting that SREBP1 activity may be a novel therapeutic target for endometrial cancers.     

Oleoyl-estrone affects lipid metabolism in adrenalectomized rats treated with corticosterone through modulation of SREBP1c expression

Oleoyl-estrone (OE) elicits a decrease in body fat, which is blocked by glucocorticoids. In order to analyze this counterregulatory effect, we studied the effects of oral OE on adrenalectomized female rats simultaneously receiving corticosterone (subcutaneous pellets). Circulating corticosteroids, liver glycogen, lipids and the expressions in whole liver, soleus muscle, interscapular brown adipose tissue (BAT), and the inguinal and periovaric white adipose tissue (WAT) of genes controlling lipid metabolism were analyzed. Corticosterone reversed OE lipid mobilization, storing fat in liver and subcutaneous WAT. This was not simply the predominance of corticosteroid enhancement of lipogenesis against OE inhibition, but a synergy to enhance lipogenesis. Periovaric WAT showed a different effect, with corticosterone inhibiting OE arrest of lipogenic gene expressions. The data presented suggests that interaction of OE and glucocorticoids (and the metabolic response) depends on the organ or WAT site; there was a direct relationship on the direction and extent of change of SREBP1c expression with those of important energy and lipid handling genes. Our results confirm that corticosterone blocks – and even reverses – OE effects on body lipids in a dose-dependent way, a process mediated, at least in part, by modulation of SREBP1c expression.