LXR-alpha/SREBP-1c通路参与HCV NS5A诱导的肝细胞脂质代谢紊乱

肝细胞脂肪变性是丙型肝炎患者的突出病理特征,但丙肝病毒（HCV）诱导脂肪变性的分子机制尚不清楚.为探究HCV非结构蛋白5A（NS5A）参与诱导脂肪变性的可能分子机制,本研究以HCV NS5A转基因小鼠为研究对象,采集3～16月龄NS5A转基因小鼠和同窝非转基因小鼠的肝组织,进行病理学检测,并用气相色谱 质谱（GC-MS）法分析脂质主要成分胆固醇酯的含量.采用RT-PCR法检测肝细胞中与脂质代谢密切相关基因肝X受体（LXR-alpha）、固醇调节元件结合蛋白（SREBP-1c）、脂肪酸合成酶（FAS）、硬脂酰辅酶A去饱和酶1（SCD1）、过氧化物酶体增殖物受体alpha（PPAR-alpha）的mRNA表达水平.结果表明,与同窝非转基因对照小鼠相比,3～5月龄NS5A转基因小鼠的肝组织没有发生显著的病理性变化,但6～16月龄的NS5A转基因小鼠的肝脏发生了显著的脂肪变性（47.1% vs 130%;P=0.003）.与此相一致,胆固醇酯的含量在NS5A转基因小鼠的肝脏中显著升高（P < 0.01）.RT PCR检测结果表明,与对照小鼠相比,14月龄NS5A转基因小鼠肝组织中与脂质代谢相关的基因LXR.alpha、SREBP.1c、FAS、SCD1的mRNA表达水平显著增高（P < 0.05）,而PPAR alpha的表达则没有显著变化（P > 0.05）. 以上结果提示,NS5A在小鼠肝细胞中可能通过调高LXR.alpha/SREBP.1c信号通路相关基因的表达,进而促进脂质重新合成,诱导脂肪变性.     

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.     

HIV protease inhibitor induces fatty acid and sterol biosynthesis in liver and adipose tissues due to the accumulation of activated sterol regulatory element-binding proteins in the nucleus

The mechanism by which human immunodeficiency virus (HIV) protease inhibitor therapy adversely induces lipodystrophy and hyperlipidemia has not been defined. This study explored the mechanism associated with the adverse effects of the prototype protease inhibitor ritonavir in mice. Ritonavir treatment increased plasma triglyceride and cholesterol levels through increased fatty acid and cholesterol biosynthesis in adipose and liver. Ritonavir treatment also resulted in hepatic steatosis and hepatomegaly. These abnormalities, which were especially pronounced after feeding a Western type high fat diet, were due to ritonavir-induced accumulation of the activated forms of sterol regulatory binding protein (SREBP)-1 and -2 in the nucleus of liver and adipose, resulting in elevated expression of lipid metabolism genes. Interestingly, protease inhibitor treatment did not alter SREBP mRNA levels in these tissues. Thus, the adverse lipid abnormalities associated with protease inhibitor therapy are caused by the constitutive induction of lipid biosynthesis in liver and adipose tissues due to the accumulation of activated SREBP in the nucleus.     

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.     

Synthetic LXR agonist attenuates plaque formation in apoE-/- mice without inducing liver steatosis and hypertriglyceridemia

Liver X receptors (LXRs) are important regulators of cholesterol and lipid metabolism. LXR agonists have been shown to limit the cellular cholesterol content by inducing reverse cholesterol transport, increasing bile acid production, and inhibiting intestinal cholesterol absorption. Most of them, however, also increase lipogenesis via sterol regulatory element-binding protein-1c (SREBP1c) and carbohydrate response element-binding protein activation resulting in hypertriglyceridemia and liver steatosis. We report on the antiatherogenic properties of the steroidal liver X receptor agonist N,N-dimethyl-3beta-hydroxy-cholenamide (DMHCA) in apolipoprotein E (apoE)-deficient mice. Long-term administration of DMHCA (11 weeks) significantly reduced lesion formation in male and female apoE-null mice. Notably, DMHCA neither increased hepatic triglyceride (TG) levels in male nor female apoE-deficient mice. ATP binding cassette transporter A1 and G1 and cholesterol 7alpha-hydroxylase mRNA abundances were increased, whereas SREBP1c mRNA expression was unchanged in liver, and even decreased in macrophages and intestine. Short-term treatment revealed even higher changes on mRNA regulation. Our data provide evidence that DMHCA is a strong candidate as therapeutic agent for the treatment or prevention of atherosclerosis, circumventing the negative side effects of other LXR agonists.     

Hepatocyte-specific deletion of BAP31 promotes SREBP1C activation,promotes hepatic lipid accumulation,and worsens IR in mice

Conditional knockout mice with targeted disruption of B-cell associated protein (BAP)31 in adult mouse liver were generated and challenged with a high-fat diet (HFD) for 36 or 96 days and markers of obesity, diabetes, and hepatic steatosis were determined. Mutant mice were indistinguishable from WT littermates, but exhibited increased HFD-induced obesity. BAP31-deletion in hepatocytes increased the expression of SREBP1C and the target genes, including acetyl-CoA carboxylase 1 and stearoyl-CoA desaturase-1, and increased hepatic lipid accumulation and HFD-induced liver steatosis. Immunoprecipitation assay showed that BAP31 interacts with SREBP1C and insulin-induced gene 1 (INSIG1), and BAP31-deletion reduces INSIG1 expression, suggesting that BAP31 may regulate SREBP1C activity by modulating INSIG1 protein levels. Additionally, BAP31-deletion induced glucose and insulin intolerance, decreased Akt and glycogen synthase kinase 3β phosphorylation, and enhanced hepatic glucose production in mice. Expression of endoplasmic reticulum (ER) stress markers was significantly induced in BAP31-mutant mice. HFD-induced inflammation was aggravated in mutant mice, along with increased c-Jun N-terminal kinase and nuclear factor-κB activation. These findings demonstrate that BAP31-deletion induces SREBP activation and promotes hepatic lipid accumulation, reduces insulin signaling, impairs glucose/insulin tolerance, and increases ER stress and hepatic inflammation, explaining the protective roles of BAP31 in the development of liver steatosis and insulin resistance in HFD-induced obesity in animal models.     

Consumption of Soy Protein Isolate Reduces Hepatic SREBP-1c and Lipogenic Gene Expression in Wild-Type Mice,but Not in FXR-Deficient Mice

We examined to determine whether hepatic gene expression is affected in mice in which blood lipid levels remain unchanged fed soy protein isolate (SPI) for a short time. We also examined SPI-mediated effects in farnesoid X receptor (FXR)-deficient mice. Compared with casein, SPI affected the expression of various hepatic genes related to lipid metabolism in the wild-type mice. No effects of SPI were observed in the FXR-deficient mice, suggesting the importance of FXR. Hepatic peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) gene expression was reduced by SPI, and this might be associated with a decrease in FXR expression. Decreased FXR led to decreased expression of its target, the bile-salt export pump necessary for bile acid secretion and dietary lipid absorption. The earliest response to SPI was a decrease in hepatic sterol regulatory element-binding protein (SREBP)-1c mRNA, on day 3. SPI activated hepatic adenosine monophosphate-activated protein kinase (AMPK), which can lead to a reduction in SREBP-1c mRNA. These data indicate the importance of SREBP-1c and PGC-1α/FXR in SPI-mediated alterations in hepatic gene expression.     

Consumption of soy protein isolate reduces hepatic SREBP-1c and lipogenic gene expression in wild-type mice, but not in FXR-deficient mice

We examined to determine whether hepatic gene expression is affected in mice in which blood lipid levels remain unchanged fed soy protein isolate (SPI) for a short time. We also examined SPI-mediated effects in farnesoid X receptor (FXR)-deficient mice. Compared with casein, SPI affected the expression of various hepatic genes related to lipid metabolism in the wild-type mice. No effects of SPI were observed in the FXR-deficient mice, suggesting the importance of FXR. Hepatic peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) gene expression was reduced by SPI, and this might be associated with a decrease in FXR expression. Decreased FXR led to decreased expression of its target, the bile-salt export pump necessary for bile acid secretion and dietary lipid absorption. The earliest response to SPI was a decrease in hepatic sterol regulatory element-binding protein (SREBP)-1c mRNA, on day 3. SPI activated hepatic adenosine monophosphate-activated protein kinase (AMPK), which can lead to a reduction in SREBP-1c mRNA. These data indicate the importance of SREBP-1c and PGC-1α/FXR in SPI-mediated alterations in hepatic gene expression.