Alterations in insulin signalling pathway induced by prolonged insulin treatment of 3T3-L1 adipocytes

Summary Insulin-induced glucose transport stimulation, which results from the translocation of glucose transporter 4 (GLUT 4)-containing vesicles, is completely blocked after prolonged insulin treatment of 3T3-L1 adipocytes. Since GLUT 4 expression was reduced by only 30%, we looked at the insulin signalling pathway in this insulin-resistant model. Insulin-induced tyrosine phosphorylation of the major insulin receptor substrate IRS 1 was reduced by 50±7%, while its expression was decreased by 70±4%. When cells were treated with wortmannin (a PI3-kinase inhibitor) together with insulin, the expression of IRS 1 diminished to a much lower extent. Associated with the decrease in IRS 1 expression and phosphorylation, the activation by insulin of antiphosphotyrosine immunoprecipitable PI3-kinase activity and of p44^mapk and p42^mapk activities was altered. However, the expression of these proteins was normal and p44^mapk activity remained responsive to the tumour promoter TPA. Those results indicate that prolonged insulin treatment of 3T3-L1 adipocytes induces an insulin-resistant state with a reduced ability of insulin to stimulate the PI3-kinase and the MAP-kinases and a blockade of glucose transporter translocation.Abbreviations GLUT Glucose transporter - TPA tumour promoter - MAPK mitogen-activated protein kinase - IRS insulin receptor substrate - SH2 src homology 2 - GRB GRB: Growth factor Receptor bound protein - PVDF polyvinyliden difluoride - HDM/LDM high density/low density microsomes - MBP myelin basic protein - DMEM Dulbecco's modified Eagle's medium - PMSF phenylmethanesulphonyl fluoride - PI3-kinase phosphatidylinositol 3-kinase     

GTPase activating protein activity for Rab4 is enriched in the plasma membrane of 3T3-L1 adipocytes. Possible involvement in the regulation of Rab4 subcellular localization

Summary The small guanosine 5-triphosphate (GTP)ase Rab4 has been suggested to play a role in insulin-induced GLUT4 translocation. Under insulin stimulation, GLUT4 translocates to the plasma membranes, while Rab4 leaves the GLUT4-containing vesicles and becomes cytosolic. Rab proteins cycle between a GTP-bound active form and a guanosine 5-diphosphate (GDP)-bound inactive form. The intrinsic GTPase activity of Rab proteins is low and the interconversion between the two forms is dependent on accessory factors. In the present work, we searched for a GTPase activating protein (GAP) for Rab4 in 3T3-L1 adipocytes. We used a glutathione-S-transferase (GST)-Rab4 protein which possesses the properties of a small GTPase (ability to bind GDP and GTP and to hydrolyse GTP) and can be isolated in a rapid and efficient way. This GAP activity was observed in 3T3-L1 adipocyte lysates, and was able to accelerate the hydrolysis of the [-³²P]GTP bound to GST-Rab4 into [-³²P]GDP. This activity, tentatively called Rab4-GAP, was also present in 3T3-L1 fibroblasts. The Rab4-GAP activity was present in total membrane fractions and nearly undetectable in cytosol. Following subcellular fractionation, Rab4-GAP was found to be enriched in plasma membranes when compared to internal microsomes. Insulin treatment of the cells had no effect on the total Rab4-GAP activity or on its subcellular localization. Taking our results together with the accepted model of Rab cycling in intracellular traffic, we propose that Rab4-GAP activity plays a role in the cycling between the GTP- and GDP-bound forms of Rab4, and thus possibly in the traffic of GLUT4-containing vesicles.Abbreviations GAP GTPase activating protein - GDI guanosine dissociation inhibitor - GDS guanosine dissociation stimulator - GDF GDI dissociation factor - GEF GDP exchange factor - GST glutathione-S-transferase - p44^mapk MAP-kinase isoform with an M_r 44000 - PM plasma membranes - HLDM high and low density microsomes - DMEM Dulbecco's modified Eagle's medium - BSA bovine serum albumin - PVDF polyvinylidene difluoride - KLH Keyhole limpet haemocyanin - CHAPS 3-[(3-cholamidopropyl)dimethylammonic]-1-propane sulphonate - AS subunit of G_i1,2     

高密度脂蛋白对3T3-L1脂肪细胞分泌白细胞介素8的影响

分别用不同浓度高密度脂蛋白(HDL,0、10、50和100μg/ml)孵育3T3-L1脂肪细胞16 h,再加入100 ng/ml脂多糖共同孵育6 h.用酶联免疫吸附法(ELISA)检测各组脂肪细胞培养液中的白细胞介素8水平,半定量逆转录多聚酶链式反应(RT-PCR)测定脂肪细胞PPARγmRNA的表达.结果 显示,脂多糖刺激使脂肪细胞分泌白细胞介素8增加(P＜0.05).不同浓度HDL干预的脂肪细胞分泌的白细胞介素8水平均低于脂多糖刺激组,并且呈剂量依赖性.不同浓度HDL干预的脂肪细胞PPARγmRNA表达较单用脂多糖刺激组显著升高.这些结果提示HDL可能通过上调PPARγ的表达、抑制脂肪细胞分泌白细胞介素8分泌.     

TNF-α和吡格列酮对3T3-L1脂肪细胞adiponutrin mRNA表达的影响

目的探讨肿瘤坏死因子α（TNF-α）和吡格列酮（PIO）对3T3-L1脂肪细胞中，脂肪滋养蛋白（adiponutrinADPN） mRNA表达的影响及时间效应。方法用100Fmol／L的PIO和100ng／ml的TNE-α处理不同阶段的3T3-L1脂肪细胞，RT-PCR检测ADPN的表达水平。结果在分化过程中和分化成熟的3T3-L1细胞中，TNF-α均增加ADPN的表达，PIO则可明显抑制其mRNA的表达。结论PIO和TNF-α可影响3T3-L1脂肪细胞的ADPN的表达。     

C反应蛋白对3T3-L1脂肪细胞脂联素表达和分泌的影响

目的 观察C反应蛋白(CRP)对313-L1脂肪细胞脂联素的表达和分泌的影响,并探讨其致胰岛素抵抗的作用机制.方法 分别用Northem印迹、Western印迹等方法观察CRP对脂联素表达及分泌的影响.结果 (1)Northern印迹显示25和50μg/ml CRP作用24 h分别使脂联素mRNA表达下降约31%和52%(均P<0.01),呈剂量依赖趋势;50 μg/ml CRP干预12和24 h分别使脂联素的表达下降约42%和52%(均P<0.01),呈时间依赖趋势.(2)Western印迹显示25和50μg/ml CRP作用24 h分别使脂联素的分泌下降约19%和41%(均P<0.01),呈剂量依赖趋势;50μs/ml CRP干预12和24 h分别使脂联素的分泌下降约29%和41%(均P<0.01).(3)用10 μmol/L磷脂酰肌醇3激酶(PDK)抑制剂LY294002与50μg/ml CRP干预313-L1细胞24 h,可部分逆转CRP对脂联素mRNA表达的抑制作用,使脂联素的表达恢复到对照组的77%.结论 CRP通过PDK途径抑制脂肪细胞脂联索的表达和分泌,可能是其导致胰岛素抵抗机制之一.     

Long-term insulin treatment of 3T3-L1 adipocytes results in mis-targeting of GLUT4: implications for insulin-stimulated glucose transport

Aims/hypothesis. Insulin stimulates glucose transport in adipose and muscle tissue by the translocation of a specialised pool of intracellular GLUT4-containing vesicles to the cell surface. It is well established that defective insulin-stimulated GLUT4 translocation is associated with insulin resistance. Long-term insulin treatment (500 nmol/l for 24 h) of 3T3-L1 adipocytes has previously been shown to decrease cellular GLUT4 content and reduce insulin-stimulated GLUT4 translocation. Here, we test the hypothesis that the insulin resistance observed after long-term insulin treatment arises by the selective loss of GLUT4 from a specific intracellular compartment.¶Methods. Using iodixanol gradient centrifugation we have separated intracellular GLUT4 containing membranes into two distinct populations corresponding to recycling endosomes and a distinct intracellular compartment which probably represents GLUT4 storage vesicles (GSVs).¶Results. A short-term insulin stimulation reduced the content of GLUT4 in the GSV fraction (51 ± 3.5 %) with only a modest decrease from the endosomal fraction (23 ± 2.6 %). Long-term insulin treatment decreased cellular GLUT4 content by about 40 % and diminished the ability of a short-term insulin challenge to promote GLUT4 translocation. We further show that this depletion of cellular GLUT4 is selectively from the GSV fraction (68 ± 7 % decrease compared to untreated cells).¶Conclusions/interpretation. Such data argue that long-term insulin treatment results in the mis-targeting of GLUT4 such that it no longer accesses the GSV compartment. These data imply that defective targeting of GLUT4 away from the GSV compartment plays an important role in the aetiology of insulin resistance. [Diabetologia (2000) 43: 1273–1281]     

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.     

胰岛素上调3T3-L1细胞apelin基因表达的初步观察

Northern印迹法证实apelin在分离的小鼠脂肪细胞上有表达，其表达量随3T3-L1细胞分化而渐增。胰岛素上调3T3-L1脂肪细胞apelin的表达，提示脂肪细胞apelin的表达可能与肥胖、胰岛素抵抗、高血压相关。     

GLP-1 amplifies insulin signaling by up-regulation of IRbeta, IRS-1 and Glut4 in 3T3-L1 adipocytes

Glucagon-like peptide-1 (7–36) amide (GLP-1) is an insulin secretagogue. Recently, many studies have shown GLP-1 can improve insulin resistance in peripheral tissues. In the present study, we investigated glucose uptake in 3T3-L1 adipocytes in either basal or insulin resistant state and dissected insulin signaling pathway in order to elucidate the molecular mechanisms of GLP-1 mediated improvement of insulin resistance. We found GLP-1 and its long lasting analogue, exendin 4 up-regulated basal IR, IRS-1 and Glut 4 expressions although they did not increase basal glucose uptake alone. However, GLP-1 and exendin-4 increased insulin mediated glucose uptake in intact and TNF-α treated 3T3-L1 adipocytes by up-regulation of phophorylated IRβ, IRS-1, Akt and GSK-3β. These results indicate that GLP-1 and its analogue exendin-4 can amplify insulin signaling in 3T3-L1 adipocytes by up-regulation of some crucial insulin signaling molecules. Hong Gao and Xinjun Wang equally contributed to this work.     

Lipoic acid protects against oxidative stress induced impairment in insulin stimulation of protein kinase B and glucose transport in 3T3-L1 adipocytes

Aims/hypothesis. Oxidative stress has been shown to impair insulin-stimulated glucose transporter 4 translocation in 3T3-L1 adipocytes. This study explores the potential of the antioxidant lipoic acid to protect the cells against the induction of insulin resistance when given before exposure to oxidative stress. Methods. 3T3-LI were exposed for 16 h to lipoic acid after which cells were exposed for 2 h to continuous production of H₂O₂ by adding glucose oxidase to the culture medium. Results. These conditions resulted in a 50–70 % reduction in insulin-stimulated glucose transport activity associated with a decrease in reduced glutathione content from 37.4 ± 3.1 to 26.4 ± 4.9 nmol/mg protein, (p < 0.005). Lipoic acid pretreatment increased insulin-stimulated glucose transport following oxidative stress, reaching 84.8 ± 4.4 % of the control, associated with an increase in reduced glutathione content. Oxidation impaired the 4.89 ± 0.36-fold insulin-stimulated increase in glucose transporter 4 content in plasma membrane lawns of control cells. Lipoic acid pretreatment was, however, associated with preserved insulin-induced glucose transporter 4 translocation in cells exposed to oxidation, yielding 80 % of its content in controls. Although tyrosine phosphorylation patterns were not affected by lipoic acid pretreatment, insulin-stimulated protein kinase B/Akt serine 473 phosphorylation and activity were considerably impaired by oxidation but protected by lipoic acid pretreatment. A protective effect was not observed with either troglitazone, its isolated vitamin E moiety, or with vitamin C. Conclusion/interpretation. This study shows the ability of lipoic acid to provide partial protection against the impaired insulin-stimulated glucose transporter 4 translocation and protein kinase B/Akt activation induced by oxidative stress, potentially by its capacity to maintain intracellular redox state. [Diabetologia (1999) 42: 949–957] Received: 8 January 1999 and in revised form: 3 March 1999     

氧化型低密度脂蛋白诱导分子伴侣GRP78在3T3-L1脂肪细胞中的表达

目的观察氧化型低密度脂蛋白(ox-LDL)诱导3T3-L1脂肪细胞内质网应激相关分子伴侣葡萄糖调节蛋白78(GRP78)的表达,探讨ox-LDL对脂肪细胞内质网应激的诱导作用。方法体外培养3T3-L1脂肪细胞,分别给予不同浓度ox-LDL(50,100,200μg/ml)及内质网应激阳性对照诱导剂衣霉素(Tun)处理。用RT-PCR检测GRP78 mRNA的表达,筛选出最佳干预浓度,用最佳浓度的ox-LDL分别干预脂肪细胞6、12、24 h,分别用RT-PCR、Western印迹法检测GRP78 mRNA、蛋白表达情况。结果 ox-LDL呈浓度依赖方式诱导脂肪细胞GRP78表达,其表达强度随时间延长而增强,24 h达高峰(P<0.05)。结论 ox-LDL可诱导3T3-L1脂肪细胞发生内质网应激,促使未折叠蛋白反应信号通路的分子伴侣表达增加。     

胰岛素、葡萄糖对3T3-L1脂肪细胞脂肪水孔蛋白表达的调节

利用半定量RT PCR技术及Western印迹法研究胰岛素、葡萄糖对成熟脂肪细胞脂肪水孔蛋白 (AQPap)基因表达的影响。结果表明 ,胰岛素对AQPap的表达具有抑制作用 ;而高浓度葡萄糖则对AQPap的表达具有促进作用     

外源性硫化氢对胰岛素抵抗脂肪细胞葡萄糖转运体4表达的影响

目的观察外源性硫化氢(H2S)对3T3-L1脂肪细胞胰岛素抵抗(IR)的影响,并探讨其机制。方法用高糖高胰岛素培养3T3-L1脂肪细胞,建立IR细胞模型,外源性H2S供体NaHS(10-5、10-4和10-3mol/L)处理IR 3T3-L1细胞12、24和48 h。MTT法检测细胞活力,葡萄糖氧化酶法检测培养液中的葡萄糖消耗量,2-脱氧-[3H]-D-葡萄糖摄入法检测葡萄糖的摄取。实时定量PCR和Western blot检测葡萄糖转运体4(Glut4)的表达。结果与对照组比较,IR模型组细胞葡萄糖消耗和摄取量以及Glut4 mRNA和蛋白的表达显著降低(均为P<0.05)。与对照组比较,所有浓度的NaHS均未影响细胞活力。与IR模型组比较,NaHS(10-4和10-3mol/L)处理24和48 h显著增加细胞葡萄糖消耗和摄取量以及Glut4 mRNA和蛋白的表达(均P<0.05)。结论外源性H2S改善了高糖高胰岛素诱导的脂肪细胞的IR,其机制可能与H2S上调Glut4的表达有关。     

葡萄糖和胰岛素对3T3-L1脂肪细胞内脏脂肪素mRNA表达的影响

目的研究不同浓度葡萄糖和胰岛素对3T3-L1脂肪细胞中内脏脂肪素（Visfatin）mRNA表达的影响。方法通过real—time RT-PCR方法检测不同浓度葡萄糖和胰岛素培养下3T3-L1脂肪细胞Visfatin mRNA的表达。结果葡萄糖增加了3T3-L1脂肪细胞Visfatin mRNA的表达;胰岛素降低其表达。结论葡萄糖和胰岛素对3T3-L1脂肪细胞中Visfatin mRNA的表达有凋控作用。     

Reduced insulin-stimulated GLUT4 bioavailability in stroke-prone spontaneously hypertensive rats

Aims/hypothesis Insulin-stimulated glucose transport is impaired in a genetic model of hypertension, the stroke-prone spontaneously hypertensive rat (SHRSP), yet the molecular mechanisms that underlie this defect in the animals remain unclear.Methods We examined the effects of insulin on the trafficking of the insulin-responsive glucose transporter GLUT4 to the plasma membrane in isolated adipocytes from SHRSP and normotensive control Wistar–Kyoto (WKY) rats.Results Treatment of isolated adipocytes with insulin resulted in trafficking of GLUT4 to the plasma membrane. There was no significant difference in the magnitude of insulin-stimulated GLUT4 trafficking from intracellular membranes to the plasma membrane between strains. In contrast, we demonstrated that there is a significant reduction in GLUT4 accessible to the glucose photolabel Bio-LC-ATB-BGPA at the plasma membrane of SHRSP adipocytes compared with control rats.Conclusions/interpretation We propose that a large proportion of GLUT4 translocated to the plasma membrane in response to insulin is not able to bind substrate and catalyse transport in the SHRSP. Therefore, there is a reduction in bioavailable GLUT4 in SHRSP animals that is likely to account, at least in part, for the reduced insulin-stimulated glucose uptake.     

HDL和阿托伐他汀对oxLDL刺激下脂肪细胞炎症反应的影响

目的 观察高密度脂蛋白（HDL）及阿托伐他汀对氧化型低密度脂蛋白（oxLDL）刺激下3T3-L1脂肪细胞肿瘤坏死因子-α（TNFα）分泌及mRNA表达的影响,并探讨其可能的作用机制.方法 3T3-L1脂肪细胞促分化成熟后,oxLDL刺激脂肪细胞,给予不同浓度的HDL（10～100 μg/mL）和阿托伐他汀（0.1～10 μM）,及H-89（10 μM）＋HDL（100 μg/mL）干预,收集细胞,测定脂肪细胞TNFα水平、TNFα mRNA表达水平、核因子-κB（NF-κB）活性及NF-κB抑制单位（IκB）蛋白浓度.结果 oxLDL刺激使3T3-L1脂肪细胞TNFα分泌、mRNA表达水平及NF-κB活性明显增强.阿托伐他汀浓度依赖性降低TNFα 分泌及mRNA表达,抑制NF-κB活化.10 μM阿托伐他汀使oxLDL诱导的脂肪细胞TNFα mRNA表达降低56.5%,NF-κB活性减少41.2%.HDL也呈浓度依赖性抑制TNFα分泌及mRNA表达,降低NF-κB活性,减少IκB降解.与oxLDL刺激组比较,100 μg/ml HDL使TNFα mRNA表达降低64.5%,NF-κB活性减少49%,并明显增加IκB蛋白水平.HDL的这些抗炎效应能被蛋白激酶A（PKA）抑制剂（应放在H89第一次出现之处）H-89部分抑制.结论 HDL能抑制oxLDL诱导的3T3-L1脂肪细胞TNFα分泌和mRNA表达,PKA-IκB-NF-κB信号通路可能是其中作用途径之一,该效应不需要HDL与oxLDL的直接接触作用.阿托伐他汀亦通过NF-κB途径抑制oxLDL诱导的3T3-L1脂肪细胞TNFα分泌和mRNA表达.HDL的抗炎作用强度与阿托伐他汀相似.