The Niemann-Pick C1 gene is downregulated by feedback inhibition of the SREBP pathway in human fibroblasts

The Niemann-Pick C1 (NPC1) protein regulates the transport of cholesterol from late endosomes/lysosomes to other compartments responsible for maintaining intracellular cholesterol homeostasis. The present study examined the expression of the NPC1 gene and the distribution of the NPC1 protein that resulted from the transport of LDL-derived cholesterol through normal human fibroblasts. A key finding was that the transport of cholesterol from late endosomes/lysosomes to the sterol-regulatory pool at the endoplasmic reticulum, as determined by feedback inhibition of the sterol-regulatory element binding protein (SREBP) pathway, was associated with the downregulation of the NPC1 gene. Consistent with these results, fibroblasts incubated with LDL had decreased amounts of SREBP protein that interacted with sterol-regulatory element (SRE) sequences positioned within the NPC1 gene promoter region. Finally, partial colocalization of the NPC1 protein with late endosomes/lysosomes and distinct regions of the endoplasmic reticulum suggested that the NPC1 protein may facilitate the transport of cholesterol directly between these two compartments. Together, these results indicate that the transport of LDL-derived cholesterol from late endosomes/lysosomes to the sterol-regulatory pool, known to be regulated by the NPC1 protein, is responsible for promoting feedback inhibition of the SREBP pathway and downregulation of the NPC1 gene.     

固醇调节元件结合蛋白2信号通路与非酒精性脂肪性肝病

非酒精性脂肪性肝病（non-alcoholic fatty liver disease,NAFLD）是以肝细胞内甘油三酯和胆固醇等脂毒性脂肪过度沉积为主要特征的一种临床获得性代谢综合征。最新研究表明,NAFLD向非酒精性脂肪肝炎(NASH)进展时,肝内胆固醇积累可能较甘油三酯更具有细胞毒性风险。固醇调节元件结合蛋白2（sterol regulatory element-binding protein 2,SREBP2）是脂质代谢重要的核转录因子之一,主要调控胆固醇的生物合成和体内平衡。SREBP2及其靶基因调控的胆固醇异常是引起非酒精性脂肪肝病发生发展的重要因素之一。因此,认识SREBP2信号通路中,上下游各因素的表达调控作用与NAFLD发病机制之间关系,就显得非常重要。本文总结了受SREBP2调控表达的靶基因的特点,着重介绍SREBP2调控胆固醇体内合成与平衡的信号通路与NAFLD发病机制之间关系,为研究和指导治疗NAFLD及其代谢性疾病提供新的思路。     

肝细胞中活化转录因子ATF6抑制SREBP1的转录活性

内质网膜定位的活化转录因子ATF6和SREBP1均是经过蛋白酶切水解激活,激活后的ATF6(N)和SREBP1(N)进入细胞核内,分别指导内质网膜未折叠蛋白聚集反应相关基因和脂肪酸合成相关基因的表达.研究发现,肝细胞内葡萄糖饥饿激活ATF6并抑制SREBP1的转录活性及其靶基因的表达.过表达ATF6(N)能够抑制SREBP1介导的转录及其下游基因的表达.免疫共沉淀实验显示,ATF6(N)在细胞核内结合SREBP1(N),这种结合在无糖状况下增强.不同功能区缺失分析表明,ATF6和SREBP1通过亮氨酸拉链(leucinezipper)功能区相互作用.在葡萄糖饥饿状况下,ATF6对SREBP1转录活性的抑制保证了细胞基本生命活动所需要的能量.     

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.     

Mechanistic insights into the effects of SREBP1c on hepatic stellate cell and liver fibrosis

Sterol regulatory element‐binding protein 1c (SREBP1c) plays key roles in maintenance of hepatic stellate cell (HSC) quiescence. The present researches investigated the mechanisms underlying the effects of SREBP1c on HSCs and liver fibrogenesis by HSC‐targeted overexpression of the active SREBP1c using adenovirus in vitro and in vivo. Results demonstrated that SREBP1c exerted inhibitory effects on TAA‐induced liver fibrosis. SREBP1c down‐regulated TGFβ1 level in liver, reduced the receptors for TGFβ1 and PDGFβ, and interrupted the signalling pathways of Smad3 and Akt1/2/3 but not ERK1/2 in HSCs. SREBP1c also led to the decreases in the protein levels of the bromodomain‐containing chromatin‐modifying factor bromodomain protein 4, methionine adenosyltransferase 2B (MAT2B) and TIMP1 in HSCs. In vivo activated HSCs did not express cyclin D1 and cyclin E1 but SREBP1c down‐regulated both cyclins in vitro. SREBP1c elevated PPARγ and MMP1 protein levels in the model of liver fibrosis. The effect of SREBP1c on MAT2B expression was associated with its binding to MAT2B1 promoter. Taken together, the mechanisms underlying the effects of SREBP1c on HSC activation and liver fibrosis were involved in its influences on TGFβ1 level, the receptors for TGFβ1 and PDGFβ and their downstream signalling, and the molecules for epigenetic regulation of genes.     

Use of in Vivo Models to Study the Role of Cholesterol in the Etiology of Alzheimer's Disease

Cholesterol has been implicated in the pathogenesis of Alzheimer's disease, both through intracellular effects, and through an extracellular effect due to its physical interaction with plaque associated amyloid. Epidemiology studies have implicated high cholesterol as a risk factor for AD, and have shown that the use of cholesterol reducing agents (statins) can be protective against the disease. We, and others have shown that cholesterol levels modulate the processing of the amyloid precursor protein (APP) both in vivo and in vitro, affecting the accumulation of Abeta (A) peptides which may directly impact the risk of AD. This review describes the biology of sterols, and identifies how cholesterol may exacerbate the pathogenesis of AD. Data from in vivo and in vitro studies will then be presented to describe how treatments aimed at modulating lipid levels may be efficacious in treating AD.     

An anti-PCSK9 antibody reduces LDL-cholesterol on top of a statin and suppresses hepatocyte SREBP-regulated genes

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a promising therapeutic target for treating coronary heart disease. We report a novel antibody 1B20 that binds to PCSK9 with sub-nanomolar affinity and antagonizes PCSK9 function in-vitro. In CETP/LDLR-hemi mice two successive doses of 1B20, administered 14 days apart at 3 or 10 mpk, induced dose dependent reductions in LDL-cholesterol (≥ 25% for 7-14 days) that correlated well with the extent of PCSK9 occupancy by the antibody. In addition, 1B20 induces increases in total plasma antibody-bound PCSK9 levels and decreases in liver mRNA levels of SREBP-regulated genes PCSK9 and LDLR, with a time course that parallels decreases in plasma LDL-cholesterol (LDL-C). Consistent with this observation in mice, in statin-responsive human primary hepatocytes, 1B20 lowers PCSK9 and LDLR mRNA levels and raises serum steady-state levels of antibody-bound PCSK9. In addition, mRNA levels of several SREBP regulated genes involved in cholesterol and fatty-acid synthesis including ACSS2, FDPS, IDI1, MVD, HMGCR, and CYP51A1 were decreased significantly with antibody treatment of primary human hepatocytes. In rhesus monkeys, subcutaneous (SC) dosing of 1B20 dose-dependently induces robust LDL-C lowering (maximal ~70%), which is correlated with increases in target engagement and total antibody-bound PCSK9 levels. Importantly, a combination of 1B20 and Simvastatin in dyslipidemic rhesus monkeys reduced LDL-C more than either agent alone, consistent with a mechanism of action that predicts additive effects of anti-PCSK9 agents with statins. Our results suggest that antibodies targeting PCSK9 could provide patients powerful LDL lowering efficacy on top of statins, and lower cardiovascular risk.     

Nur77 modulates hepatic lipid metabolism through suppression of SREBP1c activity

NR4A nuclear receptors are induced in the liver upon fasting and regulate hepatic gluconeogenesis. Here, we studied the role of nuclear receptor Nur77 (NR4A1) in hepatic lipid metabolism. We generated mice expressing hepatic Nur77 using adenoviral vectors, and demonstrate that these mice exhibit a modulation of the plasma lipid profile and a reduction in hepatic triglyceride. Expression analysis of >25 key genes involved in lipid metabolism revealed that Nur77 inhibits SREBP1c expression. This results in decreased SREBP1c activity as is illustrated by reduced expression of its target genes stearoyl-coA desaturase-1, mitochondrial glycerol-3-phosphate acyltransferase, fatty acid synthase and the LDL receptor, and provides a mechanism for the physiological changes observed in response to Nur77. Expression of LXR target genes Abcg5 and Abcg8 is reduced by Nur77, and may suggest involvement of LXR in the inhibitory action of Nur77 on SREBP1c expression. Taken together, our study demonstrates that Nur77 modulates hepatic lipid metabolism through suppression of SREBP1c activity.