Sterol Regulatory Element-binding Protein 1 Target Gene Research Articles

The Niemann-Pick C1 gene interacts with a high-fat diet to promote weight gain through differential regulation of central energy metabolism pathways.

A genome-wide association study (GWAS) reported that common variation in the human Niemann-Pick C1 gene (NPC1) is associated with morbid adult obesity. This study was confirmed using our BALB/cJ Npc1 mouse model, whereby heterozygous mice (Npc1+/- ) with decreased gene dosage were susceptible to weight gain when fed a high-fat diet (HFD) compared with homozygous normal mice (Npc1+/+ ) fed the same diet. The objective for our current study was to validate this Npc1 gene-diet interaction using statistical modeling with fitted growth trajectories, conduct body weight analyses for different measures, and define the physiological basis responsible for weight gain. Metabolic phenotype analysis indicated no significant difference between Npc1+/+ and Npc1+/- mice fed a HFD for food and water intake, oxygen consumption, carbon dioxide production, locomotor activity, adaptive thermogenesis, and intestinal lipid absorption. However, the livers from Npc1+/- mice had significantly increased amounts of mature sterol regulatory element-binding protein-1 (SREBP-1) and increased expression of SREBP-1 target genes that regulate glycolysis and lipogenesis with an accumulation of triacylglycerol and cholesterol. Moreover, white adipose tissue from Npc1+/- mice had significantly decreased amounts of phosphorylated hormone-sensitive lipase with decreased triacylglycerol lipolysis. Consistent with these results, cellular energy metabolism studies indicated that Npc1+/- fibroblasts had significantly increased glycolysis and lipogenesis, in addition to significantly decreased substrate (glucose and endogenous fatty acid) oxidative metabolism with an accumulation of triacylglycerol and cholesterol. In conclusion, these studies demonstrate that the Npc1 gene interacts with a HFD to promote weight gain through differential regulation of central energy metabolism pathways.

LMNA p.R482W mutation related to FPLD2 alters SREBP1-A type lamin interactions in human fibroblasts and adipose stem cells

SREBP1 (Sterol regulatory element binding protein 1), transcription factor that regulates hundreds of genes involved in lipid metabolism and adipocyte differentiation, is a direct partner of A-type lamins. We show that i) in vitro, the tail regions of prelamin A, lamin A and lamin C bind a polypeptide of SREBP1 and ii) within cells, interactions between wild-type A-type lamins and SREBP1 occur mainly at the nuclear periphery but also within the nucleoplasm. While A-type lamin R482W mutation is responsible for Dunnigan type familial partial lipodystrophy (FPLD2), we show that both overexpression of LMNA p.R482W in primary human preadipocytes and endogenous expression of A-type lamins p.R482W in FPLD2 patient fibroblasts, reduce A-type lamins-SREBP1 in situ interactions and upregulates a large number of SREBP1 target genes[1]. As this LMNA mutant was previously shown to inhibit adipogenic differentiation, we propose that deregulation of SREBP1 by mutated A-type lamins constitutes one underlying mechanism of the physiopathology of FPLD2.

Genome-Wide Analysis of SREBP1 Activity around the Clock Reveals Its Combined Dependency on Nutrient and Circadian Signals

In mammals, the circadian clock allows them to anticipate and adapt physiology around the 24 hours. Conversely, metabolism and food consumption regulate the internal clock, pointing the existence of an intricate relationship between nutrient state and circadian homeostasis that is far from being understood. The Sterol Regulatory Element Binding Protein 1 (SREBP1) is a key regulator of lipid homeostasis. Hepatic SREBP1 function is influenced by the nutrient-response cycle, but also by the circadian machinery. To systematically understand how the interplay of circadian clock and nutrient-driven rhythm regulates SREBP1 activity, we evaluated the genome-wide binding of SREBP1 to its targets throughout the day in C57BL/6 mice. The recruitment of SREBP1 to the DNA showed a highly circadian behaviour, with a maximum during the fed status. However, the temporal expression of SREBP1 targets was not always synchronized with its binding pattern. In particular, different expression phases were observed for SREBP1 target genes depending on their function, suggesting the involvement of other transcription factors in their regulation. Binding sites for Hepatocyte Nuclear Factor 4 (HNF4) were specifically enriched in the close proximity of SREBP1 peaks of genes, whose expression was shifted by about 8 hours with respect to SREBP1 binding. Thus, the cross-talk between hepatic HNF4 and SREBP1 may underlie the expression timing of this subgroup of SREBP1 targets. Interestingly, the proper temporal expression profile of these genes was dramatically changed in Bmal1 −/− mice upon time-restricted feeding, for which a rhythmic, but slightly delayed, binding of SREBP1 was maintained. Collectively, our results show that besides the nutrient-driven regulation of SREBP1 nuclear translocation, a second layer of modulation of SREBP1 transcriptional activity, strongly dependent from the circadian clock, exists. This system allows us to fine tune the expression timing of SREBP1 target genes, thus helping to temporally separate the different physiological processes in which these genes are involved.

Sterol regulatory element binding protein 1 and its target gene networks

Sterol regulatory element binding protein 1 (SREBP-1) is one of the important nuclear transcription factors. SREBP-1 can maintain lipids dynamic equilibrium by regulating the expression of enzymes required for synthesis of endogenous cholesterol, fatty acids, triglycerides and phospholipids. Anomalies of SREBP-1 and its target genes can cause a series of metabolic diseases such as insulin resistance, type Ⅱ diabetes, heart dysfunction, vascular complications and hepatic steatosis. In these years, the development of high-throughput technologies has greatly expanded our knowledge about SREBP-1 target genes and the pattern of transcriptional regulation. Here we reviewed recent research progress of SREBP-1, with a focus on the protein structure, activation process, DNA binding sites and target genes. Most importantly, we showed the transcriptional regulatory networks based on omics datasets, which will contribute to a better understanding of the role of SREBP-1 in lipid metabolism and provide new clues for the treatment of lipid metabolism disorders.

Betulinic acid alleviates non-alcoholic fatty liver by inhibiting SREBP1 activity via the AMPK–mTOR–SREBP signaling pathway

Non-alcoholic fatty liver disease (NAFLD) is emerging as the most common liver disease in industrialized countries. The discovery of food components that can ameliorate NAFLD is therefore of interest. Betulinic acid (BA) is a triterpenoid with many pharmacological activities, but the effect of BA on fatty liver is as yet unknown. To explore the possible anti-fatty liver effects and their underlying mechanisms, we used insulin-resistant HepG2 cells, primary rat hepatocytes and liver tissue from ICR mice fed a high-fat diet (HFD). Oil Red O staining revealed that BA significantly suppressed excessive triglyceride accumulation in HepG2 cells and in the livers of mice fed a HFD. Ca+2-calmodulin dependent protein kinase kinase (CAMKK) and AMP-activated protein kinase (AMPK) were both activated by BA treatment. In contrast, the protein levels of sterol regulatory element-binding protein 1 (SREBP1), mammalian target of rapamycin (mTOR) and S6 kinase (S6K) were all reduced when hepatocytes were treated with BA for up to 24h. We found that BA activates AMPK via phosphorylation, suppresses SREBP1 mRNA expression, nuclear translocation and repressed SREBP1 target gene expression in HepG2 cells and primary hepatocytes, leading to reduced lipogenesis and lipid accumulation. These effects were completely abolished in the presence of STO-609 (a CAMKK inhibitor) or compound C (an AMPK inhibitor), indicating that the BA-induced reduction in hepatic steatosis was mediated via the CAMKK–AMPK–SREBP1 signaling pathway. Taken together, our results suggest that BA effectively ameliorates intracellular lipid accumulation in liver cells and thus is a potential therapeutic agent for the prevention of fatty liver disease.

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.

Activation of liver X receptor (LXR) enhances de novo fatty acid synthesis in bovine mammary epithelial cells

Liver X receptor (LXR) is a nuclear receptor and a known regulator of lipid synthesis in rodents; however, the role of LXR in the regulation of fatty acid synthesis in bovine mammary epithelial cells has not yet been defined. The objective of the study was to evaluate the effect of LXR activation on the de novo synthesis of fatty acids in bovine mammary epithelial cells (BME-UV). Bovine mammary epithelial cells were treated with T0901317 (T09), an LXR agonist. Treatment of BME-UV with T09 increased the transcription of ATP-binding cassette transporter-G1, an LXR target gene, without modifying LXRα mRNA abundance. Acute and chronic treatment of BME-UV with T09 dramatically increased de novo fatty acid synthesis. Activation of LXR resulted in the upregulation of transcription, translation, and proteolytic cleavage of sterol regulatory element-binding protein-1 (SREBP1), a lipogenic transcription factor expressed in the bovine mammary gland. Additionally, the mRNA abundance of fatty acid synthase, an LXR and SREBP1 target gene, increased in response to LXR activation. Our data indicate that SREBP1 is regulated by LXR activation in BME-UV. Controlling LXR activation may prove useful in regulating milk fat production in lactating dairy cows.

High density lipoprotein inhibits the activation of sterol regulatory element-binding protein-1 in cultured cells

A link between cellular uptake of high density lipoprotein (HDL) and regulation of sterol regulatory element-binding protein-1 (SREBP-1) was investigated in vitro. HDL decreased nuclear SREBP-1 levels as well as SREBP-1 target gene expression in HepG2 and HEK293 cells. However, HDL did not repress an exogenously expressed, constitutively active form of SREBP-1. HDL increased cellular cholesterol levels, and cellular cholesterol depletion by methyl-β-cyclodextrin abolished the effects of HDL. These results suggest that HDL inhibits the activation of SREBP-1 through a cholesterol-dependent mechanism, which may play an important role in regulating lipid synthetic pathways mediated by SREBP-1.

Sterol Regulatory Element-Binding Protein 1 Mediates Liver X Receptor-β-Induced Increases in Insulin Secretion and Insulin Messenger Ribonucleic Acid Levels

Liver X receptors (LXRalpha and LXRbeta) regulate glucose and lipid metabolism. Pancreatic beta-cells and INS-1E insulinoma cells express only the LXRbeta isoform. Activation of LXRbeta with the synthetic agonist T0901317 increased glucose-induced insulin secretion and insulin content, whereas deletion of the receptor in LXRbeta knockout mice severely blunted insulin secretion. Analysis of gene expression in LXR agonist-treated INS-1E cells and islets from LXRbeta-deficient mice revealed that LXRbeta positively regulated expression of ATP-binding cassette transporter A1 (ABCA1), sterol regulatory element-binding protein 1 (SREBP-1), insulin, PDX-1, glucokinase, and glucose transporter 2 (Glut2). Down-regulation of SREBP-1 expression with the specific small interfering RNA blocked basal and LXRbeta-induced expression of pancreatic duodenal homeobox 1 (PDX-1), insulin, and Glut2 genes. SREBP-1 small interfering RNA also prevented an increase in insulin secretion and insulin content induced by T0901317. Moreover, 5-(tetradecyloxy)-2-furoic acid, an inhibitor of the SREBP-1 target gene acetyl-coenzyme A carboxylase, blocked T0901317-induced stimulation of insulin secretion. In conclusion, activation of LXRbeta in pancreatic beta-cells increases insulin secretion and insulin mRNA expression via SREBP-1-regulated pathway. These data support the role of LXRbeta, SREBP-1, and cataplerosis/anaplerosis pathways in the control of insulin secretion in pancreatic beta-cells.