Cbl相关蛋白在高糖刺激下肾小球系膜细胞中的作用

目的：探讨高糖对肾小球系膜细胞（GMC）Cbl相关蛋白（CAP）mRNA表达的影响,以及糖尿病肾病发生发展中葡萄糖转运蛋白4（GLUT4）的下游信号分子CAP的重要作用。方法：系膜细胞株分为8组,正常对照组,生理浓度胰岛素组（10^-9mol／L）,低浓度胰岛素组（10^-9mol／L）,高浓度胰岛素组（10^-6mol／L）,高糖组（30mmol／L）,甘露醇组,高糖加高浓度胰岛素组,高糖加生理浓度胰岛素组。采用RT—PCR法,观察高糖刺激与不同浓度胰岛素作用下,系膜细胞中CAPmRNA的表达及其变化。结果：正常对照组系膜细胞中（CAPmRNA有一定表达。高糖组CAPmRNA表达明显下降;低浓度胰岛素组和高浓度胰岛素组分别为对照组的1．91倍和2．15倍（P〈0．01）;高糖加入高浓度胰岛素组CAPmRNA表达为单纯高糖组的2．14倍（P〈0．01）。结论：（1）正常系膜细胞中CAPmRNA有一定表达;（2）高糖可抑制CAPmRNA表达;（3）胰岛素能部分拮抗高糖导致系膜细胞中CAPmRNA表达的下调作用;（4）CAP在糖尿病肾病发生发展过程中是其重要因子之一。     

葡萄糖转运蛋白4及其下游信号分子在高糖刺激下肾小球系膜细胞中的作用

目的 探讨高糖和胰岛素对肾小球系膜细胞（GMC）葡萄糖转运蛋白4（GLUT4）和Cbl相关蛋白（CAP）的mRNA表达及细胞骨架纤维状肌动蛋白F-actin 的影响,探讨糖尿病肾病发生发展中GLUT4 及其下游分子F-actin和CAP的重要作用。 方法 将细胞分为8组：正常对照组、生理浓度胰岛素（10－9 mol/L）组、低浓度胰岛素（10－8 mol/L）组、高浓度胰岛素（10－6 mol/L）组、高糖（30 mmol/L）组、甘露醇组（25 mmol/L甘露醇＋5 mmol/L葡萄糖）、高糖加高浓度胰岛素组、高糖加生理浓度胰岛素组。采用RT-PCR法和免疫组化法,观察不同情况下GMC中GLUT4蛋白和mRNA以及CAP mRNA 的表达及其变化。Rhodamine-phalloidin染色和激光共聚焦显微镜观察F-actin形态及荧光强度。 结果 正常对照组GMC中GLUT4蛋白和mRNA以及CAP mRNA有一定表达,而生理浓度胰岛素组与正常对照组差异均无统计学意义。高糖组GLUT4蛋白（P < 0.01)和mRNA（P < 0.05）以及CAP mRNA（P < 0.01)表达均显著减少,F-actin解聚增加（P < 0.01）;而甘露醇组以上各指标与对照组差异均无统计学意义。低浓度胰岛素组和高浓度胰岛素组GLUT4 mRNA表达分别为生理浓度胰岛素组的2.06倍和2.66倍,GLUT4蛋白表达分别为对照组的1.93倍和2.83倍,CAP mRNA表达分别为对照组的1.91倍和2.15倍,F-actin荧光强度分别为对照组的1.296倍及1.224倍,均呈一定的浓度依赖性。高糖加高浓度胰岛素组GLUT4 mRNA表达为高糖组的2.15倍（P < 0.05）,GLUT4蛋白表达为高糖组的2.08倍（P < 0.01）,CAP mRNA表达为高糖组的2.14倍（P < 0.01）,F-actin荧光强度为高糖组的1.838倍（P < 0.01）。GLUT4 mRNA与CAP mRNA呈正相关（r = 0.905,P = 0.002）;GLUT4与F-actin呈正相关（r = 0.929,P = 0.001）。 结论 （1）正常GMC中GLUT4 mRNA与蛋白、CAP mRNA有一定表达。（2）高糖可抑制GLUT4的蛋白和mRNA以及CAP mRNA表达,促进F-actin解聚。（3）胰岛素能部分拮抗高糖导致系膜细胞中GLUT4的蛋白和mRNA以及CAP mRNA表达的下调作用。（4）GLUT4、CAP和F-actin是糖尿病肾病发生发展的重要影响因子之一。     

高糖和胰岛素对肾小球系膜细胞葡萄糖转运蛋白4 mRNA表达及细胞骨架纤维状肌动蛋白的影响

目的探讨高糖和胰岛素对肾小球系膜细胞(GMC)葡萄糖转运蛋白4(GluT4)mRNA表达及细胞骨架纤维状肌动蛋白(F-actin)的影响,进一步研究糖尿病肾病发生发展中GluT4及其下游分子F-actin的重要作用.方法将培养的鼠1097系膜细胞分为8组正常对照组,生理浓度胰岛素组(10-9mol/L),低浓度胰岛素组(10-8mol/L),高浓度胰岛素组(10-6mol/L),高糖组(30 mmol/L),甘露醇组,高糖加高浓度胰岛素组,高糖加生理浓度胰岛素组.采用RT-PCR法检测GluT4mRNA含量,rhodamine-phalloidin染色,激光共聚焦显微镜观察F-actln形态并测定荧光强度.结果(1)正常系膜细胞可检测到GluT4 mRNA.(2)高糖组GluT4 mRNA表达为对照组的58.7%(P＜0.05);10-8mol/L胰岛素组、10-6mol/L胰岛素组分别为对照组的230.2%和297.2%(P＜0.01);高糖加10-6mol/L胰岛素组,高糖加10-9mol/L胰岛素组分别为高糖组的170.6%和140.3%(P＜0.05).(3)高糖组F-actin荧光强度为对照组的44.5%;10-8mmol/L胰岛素组、10-6mol/L胰岛素组分别为对照组的122.4%(P＜0.05)和129.6%(P＜0.01);高糖加10-6mol/L胰岛素组为高糖组的183.8%(P＜0.05).(4)GluT4 mRNA表达与F-actin荧光强度呈正相关(r=0.786,P＜0.05).结论(1)正常系膜细胞有GluT4 mRNA表达.(2)高糖可抑制GluT4 mRNA表达及促进F-actin解聚.(3)胰岛素有一定拮抗作用,且呈剂量依赖性.(4)GluT4 mRNA表达与F-actin荧光强度呈正相关.(5)GluT4、F-actin是糖尿病肾病发生发展过程中的重要因子.     

高糖刺激肾小球系膜细胞葡萄糖转运蛋白4及p21的表达及其意义

目的探讨早期糖尿病肾病(DN)肾小球系膜细胞(GMC)中葡萄糖转运蛋白(GLUT)4、p21mRNA表达变化及其与GMC肥大的关系。方法大鼠1097系膜细胞株分为高糖组、甘露醇组、不同浓度胰岛素组、高糖加不同浓度胰岛素组、正常对照组。用RT-PCR法检测各组GLUT4mRNA、p21mRNA的表达。流式细胞仪测各组GMC体积大小。结果正常对照组GMC有一定GLUT4mRNA、p21mRNA表达。高糖组GLUT4mRNA表达明显下降,p21mRNA表达明显增加。胰岛素刺激GMCGLUT4mRNA表达存在浓度依赖关系。p21mRNA表达越高,细胞前向角度散射光(FSC)越强,GMC体积越大。结论高糖刺激导致GMC肥大,GMC的p21mRNA表达上调和GLUT4mRNA表达下调与DN早期GMC肥大-肾小球肥大有关。     

ERK1/2对高糖刺激肾小球系膜细胞葡萄糖转运蛋白1和p27Kip1表达的作用

高糖是引起糖尿病肾病（DN）时肾脏肥大的始动因素。葡萄糖通过葡萄糖转运蛋白（GLUT）进入细胞。肾小球系膜细胞（GMC）主要表达GLUT1。细胞外高糖可以刺激GMC的GLUT1表达以及对糖的摄入。然而，高糖诱导GMC的GLUT1表达机制不甚明确。本研究利用大鼠GMC观察细胞外调节蛋白激酶（ERK1／2）对高糖诱导的GLUT1表达及细胞周期抑制蛋白（CKI）p27^Kip1的影响。旨在给DN时抑制ERK1／2的活性可以阻止GLUT1的表达增高而延缓GMC肥大的假设提供理论依据。     

Ghrelin对大鼠离体胰腺胰岛素分泌及Glut-2蛋白表达的影响

目的探讨Ghrelin对大鼠离体胰腺组织胰岛素分泌及葡萄糖转运蛋白-2（Glut-2）表达的影响。方法将25只Wistar大鼠随机分为正常对照组（NC组）、高糖组（HCG组）、高糖＋高浓度Ghrelin（10-8mol/L）组（HCG＋HCGh组）、高糖＋中浓度Ghrelin（10-9mol/L）组（HCG＋MCGh组）及高糖＋低浓度Ghrelin（10-10mol/L）组（HCG＋LCGh组）,每组5只。建立大鼠离体胰腺灌注模型,经腹主动脉远端相应灌注低糖（5.5 mmol/L）、单纯高糖（33.3 mmol/L）溶液或加入上述不同浓度Ghrelin的高糖（33.3 mmol/L）溶液。采用ELISA法测定门静脉流出液的胰岛素水平,采用免疫组化染色方法半定量测定Glut-2蛋白在离体胰腺组织中的表达水平,采用透射电镜观察胰岛β细胞的超微结构改变。结果 5组大鼠的空腹血浆葡萄糖（FBG）、空腹血浆胰岛素（FINS）、胰岛素抵抗指数（HOMA-IR）及胰岛素分泌指数（HOMA-β）比较差异均无统计学意义（P〉0.05）。用低糖溶液灌注时,5组大鼠门静脉流出液的胰岛素水平比较差异无统计学意义（P〉0.05）;而用高糖溶液灌注后,胰岛素的分泌在3 min和10～12 min时出现了2个高峰（以HCG＋HCGh组最高）。NC组大鼠胰岛β细胞Glut-2蛋白的平均光密度值高于其余4组（P〈0.05）。透射电镜观察结果显示,高糖各组大鼠离体胰腺的凋亡征象较NC组严重,但HCG＋HCGh组的细胞器损害程度较HCG＋MCGh组及HCG＋LCGh组轻。结论在大鼠离体胰腺中,Ghrelin可促进高浓度葡萄糖诱导的胰岛素分泌,对胰岛β细胞起保护作用。     

磷脂酶D对高糖培养的肾小球系膜细胞骨架的影响

目的 研究高糖环境下肾小球系膜细胞（GMC）内磷脂酶D（PLD）的活性改变及其对细胞骨架的影响。方法 对高糖（30mmol/L)刺激48h的大鼠GMC，用酶联比色法测定磷脂酰胆碱专一性磷脂酶D（PC-PLD）活性，底物磷酸化法检测蛋白激酶C（PKC）的活性。用免疫荧光标记和共聚焦显微镜显示并测量F-actin的表达。结果 高糖刺激48h后，GMC内PC-PLD和PKC活性明显增高，而F-actin荧光表达减少，排列紊乱，给予PC-PLD抑制剂后，高糖培养的GMCPKC活性明显下降。F-actin的荧光表达和排列都得到显著改善。结论 高糖时PLD活性增高可以通过PKC途径影响GMC骨架的组装状态。改变系膜细胞收缩功能。     

尿酸活化ERK1/2信号通路刺激大鼠肾小球系膜细胞增殖

目的 探讨尿酸（UA）对大鼠肾小球系膜细胞（GMC）增殖的影响及其可能的机制。 方法 体外培养大鼠GMC,应用不同浓度的UA（50、100、300 μmol/L）刺激或应用细胞外信号调节激酶（ERK1/2）特异性抑制剂U0126（10 μmol/L）、NADPH氧化酶特异性抑制剂夹竹桃麻素（500 μmol/L）、线粒体复合体Ⅰ抑制剂鱼藤酮（10 μmol/L）预处理 30 min 后,再加入UA（300 μmol/L）。于实验终点收集细胞,应用3H-TdR掺入法、细胞计数及流式细胞术测定GMC增殖和细胞周期变化;应用实时定量PCR、Western印迹法检测细胞周期素cyclin D1和cyclin A2的表达及ERK1/2的磷酸化水平;应用荧光探针2,7-二氯二氢荧光素乙酰乙酸（DCFDA）检测细胞内活性氧（ROS）的变化。 结果 （1）与对照组相比,3H-TdR掺入法和细胞计数均显示,UA呈剂量依赖性促进GMC增殖,300 μmol/L UA刺激组其细胞数为对照组的1.5倍以上。（2）流式细胞术显示,UA呈剂量依赖性减少G1/G0期细胞数,增加S期细胞数,300 μmol/L UA刺激组其S期细胞数为对照组的2倍以上。（3）UA呈剂量依赖性促进系膜细胞周期蛋白cyclin D1和cyclin A2的表达。（4）UA呈剂量依赖性促进系膜细胞ERK1/2磷酸化且U0126能够抑制UA诱导的GMC增殖。细胞计数和3H-TdR掺入法分别显示,U0126的抑制率分别是UA 300 μmol/L刺激组的22%和31%（均P < 0.05）。（5）UA呈剂量依赖性促进ROS产生增加,夹竹桃麻素能够明显抑制UA诱导的ROS生成、ERK1/2的磷酸化和系膜细胞增殖（均P < 0.05）,而鱼藤酮对其无明显影响。 结论 UA可促进GMC增殖,其可能的机制为UA诱导NADPH 氧化酶来源的 ROS 产生增加,从而激活ERK1/2信号通路,引起周期蛋白表达增加,促进GMC增殖。     

高糖培养下系膜细胞JAK2／STAT3信号转导通路活性变化及与活性氧簇作用的研究

目的：通过观察高糖培养条件下大鼠肾小球系膜细胞P-STAT3的表达变化,探讨糖尿病状态下JAK2／STAT3途径活性的变化及该通路与活性氧簇（ROS之间的相互作用。方法：用传代培养的大鼠肾小球系膜细胞同步化后分组：（1）正常糖浓度组（含5．5mmol／L）,高糖浓度组（25mmol／L）,甘露醇组（5．5mmot／L糖＋19．5mmol甘露醇）,正常糖＋AG-490（浓度10／μmol／L）组,高糖＋AG-490（浓度10μmol／L）组。继续观察培养后用Westem Blot及细胞免疫化学方法检测系膜细胞STAT3、P-STAT3表达的变化。（2）正常糖浓度组（N）,高糖浓度组（H）,甘露醇组（M）,正常糖＋Apocynin组（N＋A,Apocynin浓度为100μmol／L）,高糖＋Apocynin组（H＋A,Atx）cynin浓度为100μmol／L）,收集上清液,用比色法检测系膜细胞ROS水平。（3）NADPH氧化酶抑制剂Apocynin预处理,分组同（2）,Apocynin提前1h加入,与正常糖或高糖同时培养后,用Westem Blot方法检测系膜细胞P-STAT3表达。结果：（1）高糖培养大鼠肾小球系膜细胞P-STAT3的表达较正常糖浓度组明显升高,甘露醇组与正常糖浓度组相比差异无统计学意义;各组之间STAT3表达差异无统计学意义。（2）高糖条件下,ROS产生明显升高,NADPH氧化酶抑制剂Apocynin可明显降低ROS的产生。（3）高糖条件下,Apocynin经预处理,在正常糖浓度和高糖浓度同时培养48h后,正常糖浓度组和正常糖＋Apocynin组对比P-NTAT3的表达差异无明显区别;高糖十Apocynin组较正常糖浓度组有明显区别,但与高糖组相比明显降低。结论：高糖通过磷酸化方式激活大鼠肾小球系膜细胞JAK2／STAT3信号转导通路;高糖作用下,肾小球系膜细胞ROS产生增加,并具有时间依赖性;高糖状态下ROS可激活肾小球系膜细胞的JAK2／STAL信号传导通路,证明ROS可能参与糖尿病肾病的发生和发展过程。     

缬沙坦对高糖上调系膜细胞细胞外调节蛋白激酶表达的影响

目的观察在高糖刺激下，系膜细胞细胞外调节蛋白激酶（ERKI／2）的活性变化以及缬沙坦对其影响，探讨缬沙坦保护肾脏作用的可能机制。方法原代培养大鼠肾脏系膜细胞，随机分为4组：低糖组（NG，d-葡萄糖5．5mmol／L）、高糖组（HG，d-葡萄糖30mmol／L）、甘露醇组（MG，d-葡萄糖5．5mmol／L＋甘露醇24．5mmol／L）和缬沙坦组（HG＋Val，d-葡萄糖30mmol／L＋缬沙坦10μmol／L）。用免疫细胞化学法及Western印迹法对系膜细胞中磷酸化ERK1／2（p-ERK1／2）的表达进行定位及半定量分析；RT—PCR法检测细胞中TGF-β1 mRNA的表达；放射免疫法测定各组细胞上清中Ⅳ型胶原的含量。结果高糖组系膜细胞中P-ERK1／2蛋白的表达较低糖组明显增高，并由胞质向胞核内转移，呈时间依赖方式（P〈0．01）；TGF-β1 mRNA及细胞上清液中Ⅳ型胶原水平均高于低糖组（P〈0．01）。而缬沙坦组上述指标均较同时相点高糖组显著降低，差异有统计学意义（P〈0．01）。甘露醇组与低糖组各指标间差异均无统计学意义。结论高糖可显著激活系膜细胞ERK信号通路，缬沙坦可抑制高糖的激活作用。     

Sustained exposure of L6 myotubes to high glucose and insulin decreases insulin-stimulated GLUT4 translocation but upregulates GLUT4 activity

Hyperglycemia and hyperinsulinemia are cardinal features of acquired insulin resistance. In adipose cell cultures, high glucose and insulin cause insulin resistance of glucose uptake, but because of altered GLUT4 expression and contribution of GLUT1 to glucose uptake, the basis of insulin resistance could not be ascertained. Here we show that GLUT4 determines glucose uptake in L6 myotubes stably overexpressing myc-tagged GLUT4. Preincubation for 24 h with high glucose and insulin (high Glc/Ins) reduced insulin-stimulated GLUT4 translocation by 50%, without affecting GLUT4 expression. Insulin receptor and insulin receptor substrate-1 tyrosine phosphorylation, phosphatidylinositol 3-kinase activation, and Akt phosphorylation also diminished, as did insulin-mediated glucose uptake. However, basal glucose uptake rose by 40% without any gain in surface GLUT4. High Glc/Ins elevated basal p38 mitogen-activated protein kinase (MAPK) phosphorylation and activity, and a short inhibition of p38 MAPK with SB202190 corrected the rise in basal glucose uptake, suggesting that p38 MAPK activity contributes to this rise. We propose that in a cellular model of skeletal muscle, chronic exposure to high Glc/Ins reduced the acute, insulin-elicited GLUT4 translocation. In addition, basal state GLUT4 activity was augmented to partially compensate for the translocation defect, resulting in a more robust glucose uptake than what would be predicted from the amount of cell surface GLUT4 alone.     

Troglitazone induces GLUT4 translocation in L6 myotubes

A number of studies have demonstrated that insulin resistance in the skeletal muscle plays a pivotal role in the insulin resistance associated with obesity and type 2 diabetes. A decrease in GLUT4 translocation from the intracellular pool to the plasma membranes in skeletal muscles has been implicated as a possible cause of insulin resistance. Herein, we examined the effects of an insulin-sensitizing drug, troglitazone (TGZ), on glucose uptake and the translocation of GLUT4 in L6 myotubes. The prolonged exposure (24 h) of L6 myotubes to TGZ (10(-5) mol/l) caused a substantial increase in the 2-deoxy-[3H]D-glucose (2-DG) uptake without changing the total amount of the glucose transporters GLUT4, GLUT1, and GLUT3. The TGZ-induced 2-DG uptake was completely abolished by cytochalasin-B (10 micromol/l). The ability of TGZ to translocate GLUT4 from light microsomes to the crude plasma membranes was greater than that of insulin. Both cycloheximide treatment (3.5 x 10(-6) mol/l) and the removal of TGZ by washing reversed the 2-DG uptake to the basal level. Moreover, insulin did not enhance the TGZ-induced 2-DG uptake additively. The TGZ-induced 2-DG uptake was only partially reversed by wortmannin to 80%, and TGZ did not change the expression and the phosphorylation of protein kinase B; the expression of protein kinase C (PKC)-lambda, PKC-beta2, and PKC-zeta; or 5'AMP-activated protein kinase activity. a-Tocopherol, which has a molecular structure similar to that of TGZ, did not increase 2-DG uptake. We conclude that the glucose transport in L6 myotubes exposed to TGZ for 24 h is the result of an increased translocation of GLUT4. The present results imply that the effects of troglitazone on GLUT4 translocation may include a new mechanism for improving glucose transport in skeletal muscle.     

Troglitazone not only increases GLUT4 but also induces its translocation in rat adipocytes

Thiazolidinediones, insulin-sensitizing agents, have been reported to increase glucose uptake along with the expression of glucose transporters in adipocytes and cardiomyocytes. Recently, we have further suggested that the translocation of GLUT4 is stimulated by thiazolidinediones in L6 myocytes. However, the direct effects of thiazolidinediones on translocation of glucose transporters have not yet been determined. In this study, using hemagglutinin epitope-tagged GLUT4 (GLUT4-HA), we provide direct evidence of the effect of troglitazone on the translocation of GLUT4 in rat epididymal adipocytes. Primary cultures of rat adipocytes were transiently transfected with GLUT4-HA and overexpressed eightfold compared with endogenous GLUT4 in transfected cells. A total of 24 h of treatment with troglitazone (10(-4) mol/l) increased the cell surface level of GLUT4-HA by 1.5 +/- 0.03-fold (P < 0.01) without changing the total amount of GLUT4-HA, whereas it increased the protein level of endogenous GLUT4 (1.4-fold) without changing that of GLUT1. Thus, the direct effect on the translocation can be detected apart from the increase in endogenous GLUT4 content using GLUT4-HA. Troglitazone not only increased the translocation of GLUT4-HA on the cell surface in the basal state but also caused a leftward shift in the dose-response relations between GLUT4-HA translocation and insulin concentration in the medium (ED(50): from approximately 0.1 to 0.03 nmol/l). These effects may partly contribute to the antidiabetic activity of troglitazone in patients with obesity and type 2 diabetes.     

Need for GLUT4 activation to reach maximum effect of insulin-mediated glucose uptake in brown adipocytes isolated from GLUT4myc-expressing mice

There is a need to understand whether the amount of GLUT4 at the cell surface determines the extent of glucose uptake in response to insulin. Thus, we created a heterozygous mouse expressing modest levels of myc-tagged GLUT4 (GLUT4myc) in insulin-sensitive tissues under the control of the human GLUT4 promoter. Insulin stimulated 2-deoxyglucose uptake 6.5-fold in isolated brown adipocytes. GLUT1 did not contribute to the insulin response. The stimulation by insulin was completely blocked by wortmannin and partly (55 +/- 2%) by the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580. Insulin increased surface exposure of GLUT4myc twofold (determined by fluorescent or enzyme-linked myc immunodetection in intact adipocytes). Such increase was completely blocked by wortmannin but insensitive to SB203580. Insulin increased the kinase activity of the p38 MAPK beta-isoform 1.9-fold without affecting p38-alpha. In summary, the GLUT4myc mouse is a promising model for measuring GLUT4 translocation in intact primary cells. It affords direct comparison between GLUT4 translocation and glucose uptake in similar cell preparations, allowing one to study the regulation of GLUT4 activity. Using this animal model, we found that stimulation of glucose uptake into brown adipocytes involves both GLUT4 translocation and activation.     

High glucose and glucosamine induce insulin resistance via different mechanisms in 3T3-L1 adipocytes

Sustained hyperglycemia induces insulin resistance, but the mechanism is still incompletely understood. Glucosamine (GlcN) has been extensively used to model the role of the hexosamine synthesis pathway (HSP) in glucose-induced insulin resistance. 3T3-L1 adipocytes were preincubated for 18 h in media +/- 0.6 nmol/l insulin containing either low glucose (5 mmol/l), low glucose plus GlcN (0.1-2.5 mmol/l), or high glucose (25 mmol/l). Basal and acute insulin-stimulated (100 nmol/l) glucose transport was measured after re-equilibration in serum and insulin-free media. Preincubation with high glucose or GlcN (1-2.5 mmol/l) inhibited basal and acute insulin-stimulated glucose transport only if insulin was present during preincubation. However, only preincubation with GlcN plus insulin inhibited insulin-stimulated GLUT4 translocation. GLUT4 and GLUT1 protein expression were not affected. GlcN (2.5 mmol/l) increased cellular UDP-N-acetylhexosamines (UDP-HexNAc) by 400 and 900% without or with insulin, respectively. High glucose plus insulin increased UDP-HexNAc by 30%. GlcN depleted UDP-hexoses, whereas high glucose plus insulin increased them. Preincubation with 0.5 mmol/l GlcN plus insulin maximally increased UDP-HexNAc without affecting insulin-stimulated or basal glucose transport. GlcN plus insulin (but not high glucose plus insulin) caused marked GlcN dose-dependent accumulation of GlcN-6-phosphate, which correlated with insulin resistance of glucose transport (r = 0.935). GlcN plus insulin (but not high glucose plus insulin) decreased ATP (10-30%) and UTP (>50%). GTP was not measured, but GDP increased. Neither high glucose plus insulin nor GlcN plus insulin prevented acute insulin stimulation (approximately 20-fold) of insulin receptor substrate 1-associated phosphatidylinositol (PI)-3 kinase. We have come to the following conclusions. 1) Chronic exposure to high glucose or GlcN in the presence of low insulin caused insulin resistance of glucose transport by different mechanisms. 2) GlcN inhibited GLUT4 translocation, whereas high glucose impaired GLUT4 "intrinsic activity" or membrane intercalation. 3) Both agents may act distally to PI-3 kinase. 4) GlcN has metabolic effects not shared by high glucose. GlcN may not model HSP appropriately, at least in 3T3-L1 adipocytes.     

Imaging of insulin signaling in skeletal muscle of living mice shows major role of T-tubules

Insulin stimulates glucose transport in skeletal muscle by glucose transporter GLUT4 translocation to sarcolemma and membrane invaginations, the t-tubules. Although muscle glucose uptake plays a key role in insulin resistance and type 2 diabetes, the dynamics of GLUT4 translocation and the signaling involved are not well described. We have now developed a confocal imaging technique to follow trafficking of green fluorescent protein-labeled proteins in living muscle fibers in situ in anesthetized mice. Using this technique, by imaging the dynamics of GLUT4 translocation and phosphatidylinositol 3,4,5 P(3) (PIP(3)) production in response to insulin, here, for the first time, we delineate the temporal and spatial distribution of these processes in a living animal. We find a 10-min delay of maximal GLUT4 recruitment and translocation to t-tubules compared with sarcolemma. Time-lapse imaging of a fluorescent dye after intravenous injection shows that this delay is similar to the time needed for insulin diffusion into the t-tubule system. Correspondingly, immunostaining of muscle fibers shows that insulin receptors are present throughout the t-tubule system. Finally, PIP(3) production, an early event in insulin signaling, progresses slowly along the t-tubules with a 10-min delay between maximal PIP(3) production at sarcolemma compared with deep t-tubules following the appearance of dye-labeled insulin. Our findings in living mice indicate a major role of the t-tubules in insulin signaling in skeletal muscle and show a diffusion-associated delay in insulin action between sarcolemma and inner t-tubules.     

Tyrosine phosphorylation of specific protein kinase C isoenzymes participates in insulin stimulation of glucose transport in primary cultures of rat skeletal muscle.

Several reports indicate that protein kinase C (PKC) plays a role in insulin-induced glucose transport in certain cells. The precise effects of insulin on specific PKC isoforms are as yet unknown. Utilizing primary cultures of rat skeletal muscle, we investigated the possibility that insulin may influence the activation state of PKC isoenzymes by inducing their translocation and tyrosine phosphorylation. This, in turn, may mediate insulin effects on glucose transport. We identified and determined the glucose transporters and PKC isoforms affected by insulin and 12-O-tetradecanoylphorbol-13-acetate (TPA). Insulin and TPA each caused an increase in glucose uptake. Insulin translocated GLUT3 and GLUT4 without affecting GLUT1. In contrast, TPA translocated GLUT1 and GLUT3 without affecting GLUT4. Insulin translocated and tyrosine phosphorylated and activated PKC-beta2 and -zeta; these effects were blocked by phosphatidylinositol 3-kinase (PI3K) inhibitors. TPA translocated and activated PKC-alpha, -beta2, and -delta; these effects were not noticeably affected by PI3K inhibitors. Furthermore, wortmannin significantly inhibited both insulin and TPA effects on GLUT translocation and glucose uptake. Finally, insulin-induced glucose transport was blocked by the specific PKC-beta2 inhibitor LY379196. These results indicate that specific PKC isoenzymes, when tyrosine-phosphorylated, are implicated in insulin-induced glucose transport in primary cultures of skeletal muscle.