Reactive oxygen species‐induced changes in glucose and lipid metabolism contribute to the accumulation of cholesterol in the liver during aging

Aging is a major risk factor for many chronic diseases due to increased vulnerability to external stress and susceptibility to disease. Aging is associated with metabolic liver disease such as nonalcoholic fatty liver. In this study, we investigated changes in lipid metabolism during aging in mice and the mechanisms involved. Lipid accumulation was increased in liver tissues of aged mice, particularly cholesterol. Increased uptake of both cholesterol and glucose was observed in hepatocytes of aged mice as compared with younger mice. The mRNA expression of GLUT2, GK, SREBP2, HMGCR, and HMGCS, genes for cholesterol synthesis, was gradually increased in liver tissues during aging. Reactive oxygen species (ROS) increase with aging and are closely related to various aging‐related diseases. When we treated HepG2 cells and primary hepatocytes with the ROS inducer, H₂O₂, lipid accumulation increased significantly compared to the case for untreated HepG2 cells. H₂O₂ treatment significantly increased glucose uptake and acetyl‐CoA production, which results in glycolysis and lipid synthesis. Treatment with H₂O₂ significantly increased the expression of mRNA for genes related to cholesterol synthesis and uptake. These results suggest that ROS play an important role in altering cholesterol metabolism and consequently contribute to the accumulation of cholesterol in the liver during the aging process.     

PACAP ameliorates hepatic metabolism and inflammation through up‐regulating FAIM in obesity

Obesity is considered a chronic inflammatory disease, the inflammatory factors, such as interleukin 6 (IL‐6), monocyte chemoattractant protein 1 (MCP‐1) and small inducible cytokine A5 (RANTES), are elevated in obese individuals. Pituitary adenylate cyclase‐activating polypeptide (PACAP) suppresses anti‐inflammatory cytokines and ameliorates glucose and lipid metabolism. Our previous study showed that Fas apoptosis inhibitory molecule (FAIM) is a new mediator of Akt2 signalling, increases the insulin signalling pathway and lipid metabolism. In this study, we found that PACAP promoted the expression of FAIM protein in a human hepatocyte cell line (L02). Overexpression of FAIM with lentivirus suppressed the expression of the inflammatory factor interleukin 6 (IL‐6), monocyte chemoattractant protein 1 (MCP‐1) and tumour necrosis factor alpha (TNF‐α). Following treatment of obese mice with FAIM or PACAP for 2 weeks, inflammation was alleviated and the bodyweight and blood glucose levels were decreased. Overexpression of FAIM down‐regulated the expression of adipogenesis proteins, including SREBP1, SCD1, FAS, SREBP2 and HMGCR, and up‐regulated glycogen synthesis proteins, including Akt2 (Ser474) phosphorylation, GLUT2 and GSK‐3β, in the liver of obese mice. However, down‐regulation of FAIM with shRNA promotes obesity. Altogether, our data identified that FAIM mediates the function of PACAP in anti‐inflammation, glucose regulation and lipid metabolism in obese liver.     

Resistin overexpression impaired glucose tolerance in hepatocytes

Resistin is a 12.5-kDa cysteine-rich protein secreted from adipose tissue and is an important factor linking obesity with insulin resistance. Here, we investigated the effect of resistin on glucose tolerance in adult human hepatocytes (L-02 cells). In this study, resistin cDNA was transfected into L-02 cells, and glucose concentration and glucokinase activity were determined subsequently. The data indicated resistin impaired, insulin-stimulated glucose utilization, which implied liver was a target tissue of resistin. To understand its molecular mechanism, mRNA levels of key genes in glucose metabolism and insulin signaling pathway were analyzed. The results demonstrated resistin-stimulated expression of glucose-6-phosphatase (G6Pase), sterol regulatory element-binding protein 1c (SREBP1c) and suppressor of cytokine signaling 3 (SOCS-3), repressed expression of peroxisome proliferator-activated receptor gamma (PPARgamma) as well as insulin receptor substrate 2 (IRS-2). Given that glucokinase (GK) activity and glucose transporter 2 (GLUT2) expression were not altered, we presumed that resistin did not effect them. Moreover, resistin lowered mRNA levels of IRS-2 while stimulating SOCS-3 expression, which suggests it impairs glucose tolerance by blocking the insulin signal transduction pathway.     

成纤维细胞生长因子21对1型糖尿病动物模型的肝糖代谢影响及机制研究

成纤维细胞生长因子(FGF)-21是FGF家族的成员之一.作为近年发现的一种新的糖代谢调节因子,大量研究表明,FGF-21是一种不依赖胰岛素,能够独立降糖的2型糖尿病治疗潜力型药物.但是,能否应用于1型糖尿病的治疗,国内外目前尚无报道.通过改良传统造模方法,诱导小鼠缓慢产生糖耐量异常,研究FGF-21对此类模型的糖代谢影响及肝糖代谢机制.通过检测FGF-21短期注射和长期注射后模型动物血糖的变化,研究FGF-21在模型动物上对血糖的调控效果.采用实时定量PCR检测FGF-21对模型动物肝脏中葡萄糖转运蛋白(GLUT)1、4 mRNA的表达影响.利用蒽酮法检测模型动物肝脏中糖原合成量.实验结果显示,FGF-21能够调节1型糖尿病动物的血糖水平,并呈剂量依赖性.同时,首次在1型糖尿病动物模型上证实了低剂量FGF-21(0.125 mg/kg)与胰岛素的协同作用效果优于相同剂量FGF-21和胰岛素单独注射的效果.治疗结果表明,FGF-21能够维持1型糖尿病动物模型血糖在正常范围,效果优于胰岛素.实时定量PCR结果发现,与胰岛素上调GLUT4 mRNA表达量不同的是,FGF-21作用动物模型8周后,GLUT1 mRNA表达量显著提高,长期的FGF-21与胰岛素协同注射使GLUT1、4 mRNA表达量同时显著提高.长期FGF-21与胰岛素协同注射组和高剂量FGF-21注射均可显著提高模型动物肝糖原的合成.结果表明,FGF-21促进动物模型糖代谢机制与增加GLUT1表达、增加糖原合成作用有关.为临床应用FGF-21治疗1型糖尿病,增加胰岛素敏感性提供了理论依据.     

成纤维细胞生长因子(FGF)-21改善胰岛素抵抗肝细胞对葡萄糖的吸收和肝糖原的合成

在体外建立胰岛素抵抗肝细胞模型,探讨在胰岛素抵抗状态下成纤维细胞生长因子(FGF)-21对模型细胞糖代谢的影响及机制．将HepG2细胞置于10^-7 mol/L 的胰岛素培养基中培养24 h,建立胰岛素抵抗细胞模型．分别用不同浓度的胰岛素和FGF-21处理模型细胞,采用葡萄糖氧化酶-过氧化物酶(GOD-POD)法检测细胞对葡萄糖的摄取情况,并检查胰岛素与FGF-21的协同作用．利用实时荧光定量PCR检测FGF-21对模型细胞葡萄糖转运蛋白1(GLUT1)mRNA表达的影响,蒽酮法检测模型细胞糖原合成量,探讨FGF-21对胰岛素抵抗细胞模型葡萄糖摄取的影响及机制．结果发现,用高浓度胰岛素处理HepG2细胞24 h后,细胞对胰岛素的敏感性显著降低,说明成功建立了胰岛素抵抗细胞模型,抵抗状态可维持48 h,未发现细胞形态学变化．FGF-21能改善胰岛素抵抗模型细胞的葡萄糖摄取,参与肝糖原的合成,并与胰岛素产生协同作用．实时荧光定量PCR结果发现,FGF-21作用模型细胞后,细胞的GLUT1 mRNA表达量显著增加,说明FGF-21促进模型细胞摄取葡萄糖的作用机制与其增加GLUT1的表达有关．     

Branched-chain amino acids improve glucose metabolism in rats with liver cirrhosis

It is well established that impaired glucose metabolism is a frequent complication in patients with hepatic cirrhosis. We previously showed that leucine, one of the branched-chain amino acids (BCAA), promotes glucose uptake under insulin-free conditions in isolated skeletal muscle from normal rats. The aim of the present study was to evaluate the effects of BCAA on glucose metabolism in a rat model of CCl(4)-induced cirrhosis (CCl(4) rats). Oral glucose tolerance tests were performed on BCAA-treated CCl(4) rats. In the CCl(4) rats, treatment with leucine or isoleucine, but not valine, improved glucose tolerance significantly, with the effect of isoleucine being greater than the effect of leucine. Glucose uptake experiments using isolated soleus muscle from the CCl(4) rats revealed that leucine and isoleucine, but not valine, promoted glucose uptake under insulin-free conditions. To clarify the mechanism of the blood glucose-lowering effects of BCAA, we collected soleus muscles from BCAA-treated CCl(4) rats with or without a glucose load. These samples were used to determine the subcellular location of glucose transporter proteins and glycogen synthase (GS) activity. Oral administration of leucine or isoleucine without a glucose load induced GLUT4 and GLUT1 translocation to the plasma membrane. GS activity was augmented only in leucine-treated rats and was completely inhibited by rapamycin, an inhibitor of mammalian target of rapamycin. In summary, we found that leucine and isoleucine improved glucose metabolism in CCl(4) rats by promoting glucose uptake in skeletal muscle. This effect occurred as a result of upregulation of GLUT4 and GLUT1 and also by mammalian target of rapamycin-dependent activation of GS in skeletal muscle. From these results, we consider that BCAA treatment may have beneficial effects on glucose metabolism in cirrhotic patients.