URB is abundantly expressed in adipose tissue and dysregulated in obesity

Dysregulated production of adipocytokines in obesity is involved in the development of metabolic syndrome. URB/DRO1 contains N-terminal signal sequence and is thought to play a role in apoptosis of tumor cells. In the present study, we investigated the expression pattern of URB mRNA in adipose tissue and secretion from cultured adipocytes. In human and mouse, URB mRNA was predominantly expressed in adipose tissue and was downregulated in obese mouse models, such as ob/ob, KKAy, and diet-induced obese mice. In 3T3L1 adipocytes, insulin, TNF-α, H₂O₂ and hypoxia decreased URB mRNA level. This regulation was similar to that for adiponectin and opposite to MCP-1. URB protein was secreted in media of URB cDNA-stably transfected cells and endogenous URB was detected in media of cultured human adipocytes. In conclusion, the expression pattern of URB suggests its role in obesity and the results suggest that URB is secreted, at least in part, from adipocytes.     

C1q/TNF-related protein-12 (CTRP12), a novel adipokine that improves insulin sensitivity and glycemic control in mouse models of obesity and diabetes

Despite the prevalence of insulin resistance and type 2 diabetes mellitus, their underlying mechanisms remain incompletely understood. Many secreted endocrine factors and the intertissue cross-talk they mediate are known to be dysregulated in type 2 diabetes mellitus. Here, we describe CTRP12, a novel adipokine with anti-diabetic actions. The mRNA and circulating levels of CTRP12 were decreased in a mouse model of obesity, but its expression in adipocytes was increased by the anti-diabetic drug rosiglitazone. A modest rise in circulating levels of CTRP12 by recombinant protein administration was sufficient to lower blood glucose in wild-type, leptin-deficient ob/ob, and diet-induced obese mice. A short term elevation of serum CTRP12 by adenovirus-mediated expression improved glucose tolerance and insulin sensitivity, normalized hyperglycemia and hyperinsulinemia, and lowered postprandial insulin resistance in obese and diabetic mice. CTRP12 improves insulin sensitivity in part by enhancing insulin signaling in the liver and adipose tissue. Further, CTRP12 also acts in an insulin-independent manner; in cultured hepatocytes and adipocytes, CTRP12 directly activated the PI3K-Akt signaling pathway to suppress gluconeogenesis and promote glucose uptake, respectively. Collectively, these data establish CTRP12 as a novel metabolic regulator linking adipose tissue to whole body glucose homeostasis through insulin-dependent and independent mechanisms.     

高脂饮食诱导的C57BL／6J肥胖小鼠脂肪组织RIP140基因表达水平的研究

目的：探讨高脂饮食致肥胖小鼠脂肪组织RIP140mRNA表达水平的变化及其与胰岛素抵抗的关系。方法：将C57BL／6J雄性小鼠随机分为正常饮食（NFD）组、高脂饮食（HFD）纽分别喂养14周后,测量两组小鼠体重,以NFD组小鼠体重作为对照,选取HFD组中体重大于对照组小鼠平均体重20％的小鼠作为肥胖组小鼠。对照组和肥胖组小鼠取血测甘油三酯（TG）、总胆固醇（TC）、空腹血糖（FBG）、空腹胰岛素水平（FIns）,计算稳态模型胰岛素抵抗指数（HOMA-IR）;采用RT—PCR技术检测两组小鼠附睾脂肪组织RIP140 mRNA的表达水平,并进行统计学分析。结果：HDF组小鼠中有12只符合标准计入肥胖组。肥胖组小鼠TG、TC、FBG、Fins（P〈0．05）,HOMA-1R（P〈0．01）均明显高于对照组;肥胖组小鼠脂肪组织RIP140mRNA的表达高于对照组,差异具有统计学意义（P〈0．05）;相关分析显示小鼠脂肪组织R1P140 mRNA表达水平与TG水平呈正相关（r=0．536,P〈0．05）,与胰岛素抵抗指数呈正相关（r=0．465,P〈0．05）,而与TC、FBG、Fins水平相关分析无统计学意义（P〉0．05）。结论：高脂饮食诱导的肥胖小鼠脂肪组织RIP140 mRNA表达增加,并与胰岛素抵抗程度呈正相关。     

Enhanced muscle by myostatin propeptide increases adipose tissue adiponectin, PPAR-alpha, and PPAR-gamma expressions

Muscle tissue utilizes a large portion of metabolic energy for its growth and maintenance. Previously, we demonstrated that transgenic over-expression of myostatin propeptide in mice fed a high-fat diet enhanced muscle mass and circulating adiponectin while the wild-type mice developed obesity and insulin resistance. To understand the effects of enhanced muscle growth on adipose tissue metabolism, we analyzed adiponectin, PPAR-α, and PPAR-γ mRNA expressions in several fat tissues. Results indicated muscled transgenic mice fed a high-fat diet displayed increased epididymal adiponectin mRNA expression by 12 times over wild-type littermates. These transgenic mice fed either a high or normal fat diet also displayed significantly high levels of PPAR-α and PPAR-γ expressions above their wild-type littermates in epididymal fat while their expressions in mesenteric fats were not significantly different between transgenic mice and their littermates. This study demonstrates that enhanced muscle growth has positive effects on fat metabolisms through increasing adiponectin expression and its regulations.     

高脂饮食诱导的C57BL/6J肥胖小鼠脂肪组织RIP140基因表达水平的研究(英文)

目的:探讨高脂饮食致肥胖小鼠脂肪组织RIP140mRNA表达水平的变化及其与胰岛素抵抗的关系。方法:将C57BL/6J雄性小鼠随机分为正常饮食(NFD)组、高脂饮食(HFD)组分别喂养14周后,测量两组小鼠体重,以NFD组小鼠体重作为对照,选取HFD组中体重大于对照组小鼠平均体重20%的小鼠作为肥胖组小鼠。对照组和肥胖组小鼠取血测甘油三酯(TG)、总胆固醇(TC)、空腹血糖(FBG)、空腹胰岛素水平(FIns),计算稳态模型胰岛素抵抗指数(HOMA-IR);采用RT-PCR技术检测两组小鼠附睾脂肪组织RIP140 mRNA的表达水平,并进行统计学分析。结果:HDF组小鼠中有12只符合标准计入肥胖组。肥胖组小鼠TG、TC、FBG、FIns(P0.05),HOMA-IR(P0.01)均明显高于对照组;肥胖组小鼠脂肪组织RIP140 mRNA的表达高于对照组,差异具有统计学意义(P0.05);相关分析显示小鼠脂肪组织RIP140 mRNA表达水平与TG水平呈正相关(r=0.536,P0.05),与胰岛素抵抗指数呈正相关(r=0.465,P0.05),而与TC、FBG、FIns水平相关分析无统计学意义(P0.05)。结论:高脂饮食诱导的肥胖小鼠脂肪组织RIP140 mRNA表达增加,并与胰岛素抵抗程度呈正相关。     

Endothelin-1 inhibits resistin secretion in 3T3-L1 adipocytes

Resistin is an adipocyte-derived hormone whose role in the development of insulin resistance is controversial. Endothelin-1 (ET-1) is a 21 amino acid peptide demonstrated to possess vasoconstrictor, positive inotropic, mitogenic, and metabolic properties. In numerous disease states, including congestive heart failure, obesity, and diabetes, elevated levels of ET-1 have been reported and are thought to contribute to the pathology of the disease. A recent study demonstrated that ET-1 induces the expression and stimulates the secretion of the adipose tissue-derived hormone leptin. However, the effect of ET-1 on resistin secretion has not been determined. To characterize the effect of ET-1 on resistin secretion, 3T3-L1 fibroblasts were differentiated into adipocytes and allowed to mature for 14 days. Cells were incubated for 24h with ET-1 (1-100 nM), insulin (1-100 nM), insulin+ET-1 (100 nM I+E) or the appropriate vehicle or antagonist. At the end of the incubation period, resistin secretion was determined in the media by immunoblotting and densitometric analysis. ET-1 (1-100 nM) significantly decreased basal resistin secretion by 49% (1 nM), 43% (10nM), and 59% (100 nM). Insulin (1-100 nM) produced a concentration-dependent increase in resistin secretion from 3T3-L1 adipocytes (1 nM-42%, 10nM-55%, and 100 nM-86% vs. control). Insulin-stimulated resistin secretion (100 nM) was almost completely inhibited (94%) by ET-1 (100 nM). The effects of ET-1 on resistin protein secretion were inhibited by co-incubation with the ET(A) receptor antagonist BQ-610. In conclusion, our studies demonstrate that basal and hormonal stimulation of resistin secretion by insulin are inhibited by ET-1. Such findings demonstrate that resistin secretion is regulated in a similar manner to other adipose tissue factors, including leptin, in 3T3-L1 adipocytes. In addition, our findings suggest that vascular factors such as ET-1 may regulate whole body energy metabolism through adipocyte-derived hormones, including leptin and resistin.     

Peptidyl-prolyl cis/trans isomerase NIMA-interacting 1 associates with insulin receptor substrate-1 and enhances insulin actions and adipogenesis

Peptidyl-prolyl cis/trans isomerase NIMA-interacting 1 (Pin1) is a unique enzyme that associates with the pSer/Thr-Pro motif and catalyzes cis-trans isomerization. We identified Pin1 in the immunoprecipitates of overexpressed IRS-1 with myc and FLAG tags in mouse livers and confirmed the association between IRS-1 and Pin1 by not only overexpression experiments but also endogenously in the mouse liver. The analysis using deletion- and point-mutated Pin1 and IRS-1 constructs revealed the WW domain located in the N terminus of Pin1 and Ser-434 in the SAIN (Shc and IRS-1 NPXY binding) domain of IRS-1 to be involved in their association. Subsequently, we investigated the role of Pin1 in IRS-1 mediation of insulin signaling. The overexpression of Pin1 in HepG2 cells markedly enhanced insulin-induced IRS-1 phosphorylation and its downstream events: phosphatidylinositol 3-kinase binding with IRS-1 and Akt phosphorylation. In contrast, the treatment of HepG2 cells with Pin1 siRNA or the Pin1 inhibitor Juglone suppressed these events. In good agreement with these in vitro data, Pin1 knock-out mice exhibited impaired insulin signaling with glucose intolerance, whereas adenoviral gene transfer of Pin1 into the ob/ob mouse liver mostly normalized insulin signaling and restored glucose tolerance. In addition, it was also demonstrated that Pin1 plays a critical role in adipose differentiation, making Pin1 knock-out mice resistant to diet-induced obesity. Importantly, Pin1 expression was shown to be up-regulated in accordance with nutrient conditions such as food intake or a high-fat diet. Taken together, these observations indicate that Pin1 binds to IRS-1 and thereby markedly enhances insulin action, essential for adipogenesis.     

Obesity as a clinical and public health problem: Is there a need for a new definition based on lipotoxicity effects?

The risk functions for obesity (defined as the quantitative relation between degree of obesity throughout its range and the risk of health problems) have been used to define ‘obesity’ as an excess storage of fat in the body to such an extent that it causes health problems leading to increased mortality. The lipotoxicity theory implies that the fat stored in droplets of triglycerides in the cells are biologically inert and that the metabolic dysfunctions are primarily due to the increased exposure of the cells to fatty acids. If this is true, it has profound implications for the interpretations of the multiple epidemiological studies of the risk functions. It is obvious from all these studies that the sizes of the fat depots are risk indicators of health effects in various ways. Paradoxically, the sizes of the fat stores are also indicators of the preceding implementation of the ability of the body to protect itself against the toxic effects of the free fatty acids. The current risk of metabolic dysfunctions appears to be determined by the balance between the rate of loading of the body with fatty acids and the rate of eliminating the fatty acids by either triglyceride storage or oxidation. The progress in the development of the dysfunction then depends on the persistence of the imbalance leading to future cumulative exposure of the cells to the toxic effects of the fatty acids rather than on the current size of the fat depots. This may be considered as a reason for changing the definition of obesity to one based on better estimates of future risks of health problems derived from later metabolic dysfunctions rather than on the past coping with the exposure to the fatty acids by storage as triglycerides. Implementation of such definition would require a test that measures this residual capacity to avoid excess exposure of the cells to the fatty acids before the metabolic dysfunctions have emerged. In analogy with the glucose tolerance test, a fatty acid tolerance test may be needed to identify individuals who are at a level of risk for developing lipotoxicity induced metabolic dysfunctions such that they require intervention. This test would ideally be a single biomarker that would determine residual capacity for adipose expansion, fatty acid oxidation and safe ectopic lipid deposition.     

Endurance training increases lactate transport in male Zucker fa/fa rats

The purpose of this study was to investigate the effect of endurance training (10 weeks) on previously reported alterations of lactate exchange in obese Zucker fa/fa rats. We used sarcolemmal vesicles to measure lactate transport capacity in control sedentary rats, Zucker (fa/fa), and endurance trained Zucker (fa/fa) rats. Monocarboxylate transporter (MCT) 1 and 4 content was measured in sarcolemmal vesicles and skeletal muscle. Training increased citrate synthase activity in soleus and in red tibialis anterior, and improved insulin sensitivity measured by intraperitoneal glucose tolerance test. Endurance training increased lactate influx in sarcolemmal vesicles at 1 mM of external lactate concentration and increased MCT1 expression on sarcolemmal vesicles. Furthermore, muscular lactate level was significantly decreased after training in red tibialis anterior and extensor digitorum longus. This study shows that endurance training improves impairment of lactate transport capacity that is found in insulin resistance state like obesity and type 2 diabetes.     

Stimulation of PAI-1 and adipokines by glucose in human adipose tissue in vitro

Adipokines such as Plasminogen activator inhibitor-1 (PAI-1), interleukin (IL)-8, and tumor necrosis factor (TNF)-alpha are elevated in patients with obesity, insulin resistance, and type 2 diabetes. In the present study, we investigated whether glucose affected the production of these adipokines in human adipose tissue in vitro. Glucose (up to 35mM) increased secretion of PAI-1 (p<0.01) and IL-8 (p<0.01), but not TNF-alpha, in a dose- and time-dependent manner. Half-maximal stimulatory concentration of glucose was about 1mM. Glucosamine (5mM) decreased production of PAI-1 (p<0.05) and IL-8 (p<0.05), indicating that the hexosamine biosynthesis pathway is not involved in the glucose-induced increment in adipokine secretion. The present data demonstrate that glucose increases PAI-1 and IL-8 secretion. However, glucose concentrations above 5mM had no additional effects on adipokine secretion, suggesting that mechanisms other than diabetes/insulin resistance-related hyperglycemia may be involved in the observed elevation of these adipokines.     

Moderate caloric restriction in lactating rats programs their offspring for a better response to HF diet feeding in a sex-dependent manner

We aimed to assess the lasting effects of moderate caloric restriction in lactating rats on the expression of key genes involved in energy balance of their adult offspring (CR) and their adaptations under high-fat (HF) diet. Dams were fed with either ad libitum normal-fat (NF) diet or a 30% caloric restricted diet throughout lactation. After weaning, the offspring were fed with NF diet until the age of 15 weeks and then with an NF or a HF diet until the age of 28 weeks, when they were sacrificed. Body weight and food intake were followed. Blood parameters and the expression of selected genes in hypothalamus and white adipose tissue (WAT) were analysed. CR ate fewer calories and showed lower body weight gain under HF diet than their controls. CR males were also resistant to the increase of insulin and leptin occurring in their controls under HF diet, and HF diet exposed CR females showed lower circulating fasting triglyceride levels than controls. In the hypothalamus, CR males had higher ObRb mRNA levels than controls, and CR females displayed greater InsR mRNA levels than controls and decreased neuropeptide Y mRNA levels when exposed to HF diet. CR males maintained WAT capacity of fat uptake and storage and of fatty-acid oxidation under HF diet, whereas these capacities were impaired in controls; female CR showed higher WAT ObRb mRNA levels than controls. These results suggest that 30% caloric restriction in lactating dams ameliorates diet-induced obesity in their offspring by enhancing their sensitivity to insulin and leptin signaling, but in a gender-dependent manner.     

C1q/TNF-related protein-3 (CTRP3), a novel adipokine that regulates hepatic glucose output

Adipose tissue-derived adipokines play important roles in controlling systemic insulin sensitivity and energy balance. Our recent efforts to identify novel metabolic mediators produced by adipose tissue have led to the discovery of a highly conserved family of secreted proteins, designated as C1q/TNF-related proteins 1-10 (CTRP1 to -10). However, physiological functions regulated by CTRPs are largely unknown. Here we provide the first in vivo functional characterization of CTRP3. We show that circulating levels of CTRP3 are inversely correlated with leptin levels; CTRP3 increases with fasting, decreases in diet-induced obese mice with high leptin levels, and increases in leptin-deficient ob/ob mice. A modest 3-fold elevation of plasma CTRP3 levels by recombinant protein administration is sufficient to lower glucose levels in normal and insulin-resistant ob/ob mice, without altering insulin or adiponectin levels. The glucose-lowering effect in mice is linked to activation of the Akt signaling pathway in liver and a marked suppression of hepatic gluconeogenic gene expression. Consistent with its effects in mice, CTRP3 acts directly and independently of insulin to regulate gluconeogenesis in cultured hepatocytes. In humans, alternative splicing generates two circulating CTRP3 isoforms differing in size and glycosylation pattern. The two human proteins form hetero-oligomers, an association that does not require interdisulfide bond formation and appears to protect the longer isoform from proteolytic cleavage. Recombinant human CTRP3 also reduces glucose output in hepatocytes by suppressing gluconeogenic enzyme expression. This study provides the first functional evidence linking CTRP3 to hepatic glucose metabolism and establishes CTRP3 as a novel adipokine.     

骨钙素对糖代谢影响的研究进展

骨钙素,亦称γ-羧基骨蛋白、骨谷氨酸蛋白和骨依赖维生素K蛋白,是一种由非增殖期成骨细胞合成的分泌蛋白,经过翻译后加工生成羧化骨钙素而参与骨骼发育。既往研究认为它是骨形成和骨转化的标志。然而最新研究发现,未发生羧化修饰的骨钙素对糖代谢有一定的影响作用,可促进胰腺β细胞增殖和胰岛素分泌,减弱胰岛素抵抗。多项研究证实高血糖可影响骨钙素合成,骨钙素在糖尿病患者中降低,糖尿病患者因胰岛素分泌和作用缺陷对胰岛素受体作用减弱,影响成骨细胞摄取核酸、氨基酸、胶原纤维合成,使其合成分泌骨钙素减少。这些研究确立了骨钙素为一种可以影响糖代谢的重要激素,拓展了对骨骼功能影响的同时也为新型降血糖药物的开发提供了新靶点。本文针对骨钙素对糖代谢的影响做一综述。     

内脂素与2型糖尿病关系的研究进展

内脂素是新近被发现的主要由内脏脂肪合成的一种脂肪细胞因子,它具有类胰岛素样作用,能降低血糖和促进脂肪组织的分化与合成。内脂素还可以调节血管平滑肌的成熟和影响胰岛细胞的胰岛素的分泌,亦具有调节炎症反应和免疫功能的作用。随着研究的发展,人们对内脂素的结构特性、分布、表达调控及其生物学功能有了更加深入的认识。2型糖尿病是以胰岛素抵抗和糖代谢紊乱为特征的代谢性疾病,研究发现内脂素与2型糖尿病密切相关,其中与肥胖、胰岛素抵抗及胰岛素分泌方面的关系尤为显著,深入研究内脂素的生理和病理生理作用将会有力地促进对2型糖尿病的进一步认识、治疗与预防。