Hexosamines regulate leptin production in human subcutaneous adipocytes

The hexosamine biosynthetic pathway has recently been proposed as a mechanism through which cells "sense" nutrient flux to regulate leptin release. This study was undertaken to examine the regulation of leptin production by hexosamines in human adipocytes. Adipose tissue UDP-N-acetylglucosamine, an end product of hexosamine biosynthesis, was elevated 3.2-fold, and ob messenger ribonucleic acid was elevated 2-fold in the sc adipose tissue of 17 obese [body mass index (BMI), 41.3+/-12.0 kg/m2; age, 31+/-5 yr] subjects compared to 14 lean (BMI, 23.4+/-1.6 kg/m2; age, 33+/-11 yr) subjects. Serum leptin was increased 2.7-fold in the obese subjects. A significant positive relationship was found between adipose tissue UDP-N-acetylglucosamine and BMI (Spearman correlation = 0.576; P = 0.0007) and between UDP-N-acetylglucosamine and serum leptin (Spearman correlation = 0.4650; P = 0.0145). Treatment of isolated sc adipocytes with 1 mmol/L glucosamine, an intermediate product in UDP-N-acetylglucosamine biosynthesis, increased leptin release 21.4+/-17.6% (mean +/- SD) over control (P = 0.0365) and 74.5+/-82.8% over control (P = 0.0271) in adipocytes from lean (BMI, 23.2+/-1.6 kg/m2; n = 6) and obese (BMI, 55.4+/-13.0 kg/m2,; n = 9) subjects, respectively, by 48 h of culture. Inhibition of UDP-N-acetylglucosamine biosynthesis with 6-diazo-5-oxo-norleucine reduced glucose-stimulated leptin release from cultured adipocytes 21.8+/-32.4% (P = 0.0395; n = 12) and ob gene expression 19.9+/-18.9% (P = 0.0208; n = 8) by 48 h of treatment. These findings suggest that hexosamine biosynthesis regulates leptin production in human adipose tissue.     

Galanin inhibits leptin expression and secretion in rat adipose tissue and 3T3-L1 adipocytes

In addition to serving as a fat depot, adipose tissue is also considered as an important endocrine organ that synthesizes and secretes a number of factors. Leptin is an adipocyte-derived hormone that plays a vital role in energy balance. Expression of leptin is regulated by dietary status and hormones. In the present study, we report that galanin, an orexigenic peptide, inhibits leptin expression and secretion in rat adipose tissue and in 3T3-L1 adipocytes. Treatment with galanin (25 micro g/animal) induced approximately 46% down-regulation of leptin secretion at 15 min, followed by 40, 37 and 47% decreases in leptin secretion at 1, 2 and 4 h respectively. Although Northern blot analysis of adipose tissue from the same animals showed that leptin mRNA expression in adipose tissue was unaffected by galanin treatment for 2 h, galanin treatment for 4 h led to decline of leptin mRNA expression in a dose-dependent manner. Meanwhile, treating the rats with galanin had no effect on leptin mRNA expression in the hypothalamus. The inhibitory action of the galanin on leptin mRNA and protein levels was also observed in vitro. When incubated with 10 nM galanin for 48 h, leptin mRNA expression and protein secretion also decreased in 3T3-L1 adipocytes. On the other hand, galanin was found not only to express in rat adipose tissue, but also to increase about 8-fold after fasting. Based on these data, we speculate that increased galanin expression in rat adipose tissue after fasting may be involved in reducing leptin expression and secretion in fasting rats.     

Ginsenoside Rb1 stimulates glucose uptake through insulin-like signaling pathway in 3T3-L1 adipocytes

A series of clinical trials and animal experiments have demonstrated that ginseng and its major active constituent, ginsenosides, possess glucose-lowering action. In our previous study, ginsenoside Rb(1) has been shown to regulate peroxisome proliferator-activated receptor gamma activity to facilitate adipogenesis of 3T3-L1 cells. However, the effect of Rb(1) on glucose transport in insulin-sensitive cells and its molecular mechanism need further elucidation. In this study, Rb(1) significantly stimulated basal and insulin-mediated glucose uptake in a time- and dose-dependent manner in 3T3-L1 adipocytes and C2C12 myotubes; the maximal effect was achieved at a concentration of 1 microM and a time of 3 h. In adipocytes, Rb(1) promoted GLUT1 and GLUT4 translocations to the cell surface, which was examined by analyzing their distribution in subcellular membrane fractions, and enhanced translocation of GLUT4 was confirmed using the transfection of GLUT4-green fluorescence protein in Chinese Hamster Ovary cells. Meanwhile, Rb(1) increased the phosphorylation of insulin receptor substrate-1 and protein kinase B (PKB), and stimulated phosphatidylinositol 3-kinase (PI3K) activity in the absence of the activation of the insulin receptor. Rb(1)-induced glucose uptake as well as GLUT1 and GLUT4 translocations was inhibited by the PI3K inhibitor. These results suggest that ginsenoside Rb(1) stimulates glucose transport in insulin-sensitive cells by promoting translocations of GLUT1 and GLUT4 by partially activating the insulin signaling pathway. These findings are useful in understanding the hypoglycemic and anti-diabetic properties of ginseng and ginsenosides.     

Chemerin过表达致3T3-L1脂肪细胞葡萄糖消耗减少

目的 应用重组慢病毒构建3T3-L1脂肪细胞chemerin过表达模型并进一步探讨其对糖代谢的影响及可能机制.方法 构建鼠chemerin过表达重组慢病毒,并设对照慢病毒,感染3T3-L1细胞,实时定量聚合酶链反应（RT-PCR）法检测转染后chemerin表达水平;应用胰岛素、3-异丁基1-甲基黄嘌呤、地塞米松诱导3T3-L1前脂肪细胞分化为成熟脂肪细胞,油红O染色鉴定;诱导分化第8天加入慢病毒重组体,继续培养5d,葡萄糖氧化酶法检测各组葡萄糖消耗;RT-PCR法检测各组胰岛素受体底物1（IRS1）、胰岛素受体底物2（IRS2）、蛋白激酶B1 （Akt1）、叉头状转录因子O1 （FoxO1）基因表达水平;Western-blotting检测chemerin、丝氨酸/苏氨酸蛋白激酶（Akt）、磷酸化丝氨酸/苏氨酸蛋白激酶（pAkt）、FoxO1、磷酸化叉头状转录因子O1 （pFoxO1）蛋白水平.两组数据比较应用t检验.结果 Chemerin过表达慢病毒感染3T3-L1细胞72 h后细胞中可见红色荧光,RT-PCR结果显示：过表达组与空载对照组相比chemerin基因表达明显增加（分别为3.04±0.19比1.01±0.11,t=15.65,P＜0.05）;chemerin过表达组葡萄糖消耗减少[分别为（3.30± 1.44）比（6.07±1.15） mmol/L,t=-0.35,P＜0.05];RT-PCR结果显示：IRS1、IRS2基因水平无明显变化（均P＞0.05）,Akt1基因表达下降（分别为0.76±0.08比1.07±0.15,t=-3.11,P＜0.05）,FoxO1基因表达上调（分别为1.53±0.30与1.03±0.21,t=2.34,P＜0.05）.Western-blotting结果显示：Chemerin过表达后chemerin蛋白水平增加（相对表达量分别为1.08±0.06比0.72±0.03,t=-10.12;P＜0.05）;Akt、pAkt蛋白水平均降低（分别为0.74±0.21比1.23±0.20,0.58±0.17比0.92±0.07;t=2.81、3.17,均P＜0.05）,FoxO1蛋白水平升高（分别为1.04±0.09比0.76±0.14,t=-2.91,P＜0.05）、pFoxO1蛋白水平降低（分别为0.61±0.13比0.89±0.10,t=2.93,P＜0.05）.结论 Chemerin可能通过下调Akt1 mRNA使3T3-L1脂肪细胞葡萄糖消耗减少.     

Hexosamines stimulate leptin production in transgenic mice

Hexosamine flux has been shown to mediate aspects of nutrient sensing in insulin sensitive tissues and has been hypothesized to represent a satiety signal that results in shunting of fuel toward storage as fat. It has been recently reported that in vitro treatment of fat and muscle cells with hexosamines and acute glucosamine infusion in intact rats stimulate leptin secretion. In order to investigate the effects of chronic, physiologic increases in hexosamine flux on leptin we have examined leptin mRNA and serum leptin in mice overexpressing the rate-limiting enzyme for hexosamine synthesis, GFA, in muscle and fat. Increased levels of UDP-N-acetylglucosamine, the principal end-product of the hexosamine pathway were seen in transgenic fat, consistent with the overexpression of GFA. After overnight fasting, the transgenic mice were hyperleptinemic compared to littermate controls (4.5+/-0.5 ng/ml in transgenic, 2.8+/-0.2 in control, p = 0.005) despite equal body weights. In the random-fed state, the leptin levels of control mice increased to 4.1+/-0.5 ng/ml (p = 0.01) whereas the leptin levels in the transgenics did not increase any further (3.7+/-0.4 ng/ml). Leptin mRNA levels were also increased in transgenic fat (2.7+/-0.6 in transgenic compared to 0.8+/-0.2 in control, arbitrary units normalized to actin, p < 0.007). Despite increased leptin, the transgenic animals did not have lower body fat content. We conclude that hexosamine flux in fat regulates leptin synthesis and secretion.     

Regulated expression of the obese gene product (leptin) in white adipose tissue and 3T3-L1 adipocytes.

A mutation within the obese gene was recently identified as the genetic basis for obesity in the ob/ob mouse. The obese gene product, leptin, is a 16-kDa protein expressed predominantly in adipose tissue. Consistent with leptin's postulated role as an extracellular signaling protein, human embryonic kidney 293 cells transfected with the obese gene secreted leptin with minimal intracellular accumulation. Upon differentiation of 3T3-L1 preadipocytes into adipocytes, the leptin mRNA was expressed concomitant with mRNAs encoding adipocyte marker proteins. A factor(s) present in calf serum markedly activated expression of leptin by fully differentiated 3T3-L1 adipocytes. A 16-hr fast decreased (by approximately 85%) the leptin mRNA level of adipose tissue of lean (ob/+ or +/+) mice but had no effect on the approximately 4-fold higher level in obese (ob/ob) littermates. Since the mutation at the ob locus fails to produce the functional protein, yet its cognate mRNA is overproduced, it appears that leptin is necessary for its own downregulation. Leptin mRNA was also suppressed in adipose tissue of rats during a 16-hr fast and was rapidly induced during a 4-hr refeeding period. Insulin deficiency provoked by streptozotocin also markedly down-regulated leptin mRNA and this suppression was rapidly reversed by insulin. These results suggest that insulin may regulate the expression of leptin.     

Nitric oxide stimulates glucose transport through insulin-independent GLUT4 translocation in 3T3-L1 adipocytes

OBJECTIVE: It is well known that nitric oxide synthase (NOS) is expressed and that it modulates glucose transport in skeletal muscles. Recent studies have shown that adipose tIssues also express inducible and endothelial nitric oxide synthase (eNOS). In the present study, we investigated whether nitric oxide (NO) induces glucose uptake in adipocytes, and the signaling pathway involved in the NO-stimulated glucose uptake in 3T3-L1 adipocytes. METHODS: First, we determined the expression of eNOS in 3T3-L1 adipocytes, and then these cells were treated with the NO donor sodium nitroprusside (SNP) and/or insulin, and glucose uptake and phosphorylation of insulin receptor substrate (IRS)-1 and Akt were evaluated. Moreover, we examined the effects of a NO scavenger, a guanylate cyclase inhibitor or dexamethasone on SNP-stimulated glucose uptake and GLUT4 translocation. RESULTS: SNP at a concentration of 50 mmol/l increased 2-deoxyglucose uptake (1.8-fold) without phosphorylation of IRS-1 and Akt. Treatment with the NO scavenger or guanylate cyclase inhibitor decreased SNP-stimulated glucose uptake to the basal level. Dexamethasone reduced both insulin- and SNP-stimulated glucose uptake with impairment of GLUT4 translocation. CONCLUSION: NO is capable of stimulating glucose transport through GLUT4 translocation in 3T3-L1 adipocytes, via a mechanism different from the insulin signaling pathway.     

Heat shock modulates adipokines expression in 3T3-L1 adipocytes

Studies have demonstrated that heat shock is associated with alteration in energy metabolism. In this study, we investigated the effect of heat shock on gene expression and secretion of adiponectin and leptin, and gene expression of Hspa2 and Ppargamma in 3T3-L1 adipocytes. Compared with 37 degrees C, adiponectin mRNA was higher at 39 degrees C, and lower at 41 degrees C. Leptin mRNA was higher when adipocytes were exposed to 41 degrees C compared with 37 and 39 degrees C. Secretion of adiponectin increased at 39 degrees C, and when cells were exposed to 41 degrees C it was not detectable. Leptin secretion increased significantly at 41 degrees C, compared with 37 and 39 degrees C. Hspa2 mRNA was increased at 39 degrees C, and the highest level was reached at 41 degrees C. Ppargamma mRNA exhibited a substantial increase in a temperature-dependent manner. The study provides the first evidence of a possible direct effect of heat shock on adiponectin and leptin gene expression and secretion, and demonstrates that the expression of the two adipokines is differentially regulated at the temperatures tested.     

Interleukin-18 enhances glucose uptake in 3T3-L1 adipocytes

In order to characterize the potential causative effects of interleukin-18 (IL-18) on insulin resistance, we measured glucose uptake in 3T3-L1 adipocytes treated with mouse recombinant IL-18. IL-18 surprisingly enhanced, rather than reduced insulin-mediated glucose uptake in adipocytes. Moreover IL-18 could counteract the glucose uptake suppression caused by tumor necrosis factor α in 3T3-L1 adipocytes. The mechanism dissection showed that the IL-18 upregulated phosphorylated Akt and downregulated phosphorylated P38 MAPK. These findings indicated that the elevated serum IL-18 levels in obesity and diabetes might be a compensatory response to insulin resistance.     

Amylin stimulates fatty acid esterification in 3T3-L1 adipocytes

Amylin is co-localized and co-secreted with insulin, however its direct effects on adipocytes are unexplored. In 3T3-L1 preadipocytes, amylin increased thymidine incorporation (174%; p < 0.05) and Myc mRNA expression (378%; p < 0.01). Amylin supplementation during differentiation enhanced triglyceride accumulation (272%; p < 0.001). In 3T3-L1 adipocytes, amylin increased fatty acid uptake (238%; p < 0.01) and further potentiated the effects of insulin (insulin 158%; p < 0.01, amylin + insulin 335%; p < 0.001 vs CTL, p < 0.001 vs insulin). By contrast, amylin inhibited glycerol release in 3T3-L1 adipocytes (−50%; p < 0.05) and primary adipocytes (−34%; p < 0.05). Amylin stimulated cytokine secretion (monocyte chemotactic protein-1 + 166%, keratinocyte-derived chemokine + 174%; both p < 0.05) and mRNA expression of PPARγ (163%; p < 0.01), C/EBPβ (121%, p < 0.05), DGAT1 (157%; p < 0.01), FABP4 (122%; p < 0.01), and CD36 (122%; p < 0.05). In human adipose tissue, mRNA expression of amylin receptor genes (CALCR and RAMP3) correlated with numerous lipid and insulin signaling genes, plasma glucose and HOMA. Altogether amylin directly stimulates fat cells, potentiates the effects of insulin and may influence insulin resistance.     

Reactive oxygen species up-regulates SOCS-3 in 3T3-L1 adipocytes

We investigated the effect of increased intracellular reactive oxygen species(ROS) on SOCS-3 expression in 3T3-L1 adipocytes. Increased intracellular ROS levels in 3T3-L1 adipocytes were achieved by two methods of exposure to H2O2 and the occurrence of oxidative stress in cells was assessed by flow cytometry . Expression of SOCS-3 mRNA and that of some adipokines were measured by real time PCR. The level of SOCS-3 protein was determined by western blot. The effect of the antioxidant alpha-lipoic acid was also investigated. Both the relatively mild increased intracellular ROS and the acute but transient increased ROS elevated the levels of SOCS-3 mRNA and protein in 3T3-L1 adipocytes, acompanied with elevated levels of TNF-α mRNA and resistin mRNA and the decreased levels of adiponectin mRNA and secretory adiponectin in culture medium. α-lipoic acid could attenuate the effects of ROS on 3T3-L1 adipocytes. We hypothesized that in mature 3T3-L1 adipocytes, SOCS-3 could be upregulated directly by the induction of increased intracellular ROS and to some extent which was up-regulated by adipokine modulation.

     

Estrogen receptor alpha regulates insulin sensitivity through IRS-1 tyrosine phosphorylation in mature 3T3-L1 adipocytes

There are many clinical and experimental reports demonstrating that estrogens and insulin interact when affecting their target organs. Estrogen receptors consist of two isoforms, estrogen receptors-alpha (ER-alpha) and -beta (ER-beta), but their roles in insulin-induced glucose uptake in mature adipose tissue have yet to be clarified. To evaluate the roles of ER-alpha, expressed predominantly in adipocytes, we have investigated the effects of estradiol (E2), an ER-alpha selective agonist (PPT), and its selective antagonist (MPP) on glucose uptake and insulin action in 3T3-L1 adipocytes. 3T3-L1 adipocytes were exposed to E2 or PPT and/or MPP at different concentrations. The cells were then subjected to 2-deoxy-D-glucose transport assay, western blot analysis, or RT-PCR analysis. Treatment of these cells with E2 or PPT resulted in biphasic effects on glucose transport, that is high (10(-5) M or 3 x 10(-6) M each) and low (10(-8) M) doses produced inhibition and stimulation, respectively. The favorable effect observed at 10(-8) M of E2 was diminished by treatment with MPP. Western bolt analysis revealed that these effects of E2, PPT and MPP paralleled the level of IRS-1 tyrosine phosphorylation. However, IRS-1 serine phosphorylation, suppressor of cytokine signaling (SOCS)-1,-2,-3 and protein tyrosine phosphatase 1B (PTP1B) expression did not change compared to control subjects. Our data clearly show that ER-alpha contributes to insulin stimulated glucose uptake through regulation of the tyrosine phosphorylation of IRS-1 protein.     

Latent insulin receptors and possible receptor precursors in 3T3-L1 adipocytes.

Cell surface and cryptic insulin receptors were solubilized from the particulate fraction of murine 3T3-L1 adipocytes with buffer containing 1% Triton X-100. Solubilized receptors were affinity crosslinked with 125I-labeled insulin and disuccinimidyl suberate and characterized by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and autoradiography after specific immunoprecipitation. Two insulin-binding polypeptides were identified: the more abundant protein had a Mr of 130,000, corresponding to the size of the hormone-binding subunit of insulin receptors on the surface of target cells; the second polypeptide exhibited a Mr of 200,000 and appears to be a component of the latent pool because it was unaffected when 3T3-L1 adipocytes were exposed to trypsin under conditions that result in a 95% reduction in cell surface insulin-binding activity and the loss of the Mr 130,000 polypeptide in crosslinking experiments. Unexpectedly, the population of Mr 200,000 molecules in intact cells was accessible for limited cleavage by chymotrypsin, yielding a Mr 195,000 insulin-binding polypeptide. When 3T3-L1 adipocytes received a 15-min pulse of [35S]methionine, the predominant immunoprecipitated polypeptide had a Mr of 180,000. During a 1.5-hr chase, radioactivity in the Mr 180,000 species rapidly declined while the latent Mr 200,000 polypeptide became intensely labeled. After a 5-hr chase period, broad protein bands with Mrs of 130,000 and 90,000 were visualized as the major immunoprecipitated radioactive polypeptides. Thus, the Mr 180,000 species may be a very early biosynthetic precursor that may be subsequently processed to a Mr 200,000 form and one or both of the smaller receptor subunits at the cell surface.     

Effects of leukemia inhibitory factor on 3T3-L1 adipocytes

Leukemia inhibitory factor (LIF) is a member of the gp130 cytokine family and signals through the receptor complex of gp130 and the LIF receptor (LIFR) to activate the JAK/STAT signaling cascade. Since LIF activates STATs 1 and 3 in adipocytes, we examined the effects of LIF on 3T3-L1 adipocytes. Our studies clearly demonstrate that LIF treatment had minimal effects on adipocyte differentiation as judged by marker gene expression, but did inhibit triacylglyceride (TAG) accumulation during adipogenesis. Acute treatment with LIF resulted in increased expression of suppressors of cytokine signaling-3 (SOCS3) and CCAAT/enhancer-binding protein-delta (C/EBPdelta) mRNA in 3T3-L1 adipocytes. Moreover, the upregulation of C/EBPdelta correlated with binding to three sites in the C/EBPdelta promoter by LIF-activated protein complexes that contained STAT1 and not STAT3. Chronic treatment with LIF resulted in decreased protein levels of sterol regulatory element binding protein-1 (SREBP1) and fatty acid synthase (FAS), but had no effect on the expression of other adipocyte marker proteins or on TAG levels in mature 3T3-L1 adipocytes. LIF had a small effect on insulin-stimulated glucose uptake in 3T3-L1 adipocytes, but did not cause insulin resistance following chronic treatment. These findings indicate that LIF has similar and distinct effects in comparison with the effects of other gp130 cytokines on cultured fat cells. In summary, our results support a role for LIF in the regulation of proteins involved in lipid synthesis and in the modulation of signal transduction pathways in 3T3-L1 adipocytes.     

小檗碱对3T3-L1脂肪细胞内脂素表达的影响

目的观察小檗碱对3T3-L1脂肪细胞内脂素（visfatin）表达的影响和探讨小檗碱改善胰岛素抵抗的机制。方法采用RT—PCR法检测不同浓度小檗碱和不同作用时间后，3T3-L1脂肪细胞内脂素mRNA的表达，并以Western印迹方法检测其蛋白水平的表达。结果0～10μmol／L小檗碱剂量依赖性地增加内脂素mRNA的表达，其中10μmol／L时最明显，是空白对照组的4．96倍，其后随着小檗碱浓度的进一步增加，内脂素mRNA的表达反而降低；10μmol／L的小檗碱作用3h后内脂素mRNA的表达开始明显增强，至作用12h时表达增强最明显，是基础组的4．57倍，随着作用时间的延长内脂素mRNA的表达虽然有所下降，但到48h时内脂素mRNA的表达仍比空白对照组明显增高；5、10、20μmol／L的小檗碱均可使内脂素蛋白的表达增加1．13、2．46、2．34倍，与空白对照组相比差异有统计学意义（P〈0．05或P〈0．01）。结论在一定浓度范围内小檗碱可剂量和时间依赖性地促进离体脂肪细胞内脂素mRNA及蛋白表达。小檗碱对内脂素表达的调节作用可能是其改善机体胰岛素抵抗、降低血糖的机制之一。     

持续激活的Akt减少了 3T3-L1脂肪细胞脂联素蛋白的表达

目的 研究蛋白激酶B（Akt／PKB）的持续激活与灭活对3T3-L1脂肪细胞内脂联素蛋白表达的影响。方法 通过腺病毒表达系统将持续激活的Akt（myrAkt）和无酶活性的Akt（Akt-AA）导入3T3-L1脂肪细胞内，应用免疫印迹法检测3T3-L1脂肪细胞脂联素蛋白的表达。结果 表达myrAkt的3T3-L1脂肪细胞中脂联素明显减少，表达Akt-AA的3T3-L1脂肪细胞中脂联素无明显变化。结论Akt的激活抑制了3T3-L1脂肪细胞中脂联素蛋白的表达，且Akt的激活是影响脂联素的充分条件，而不是必要条件。