Hah J S, Ryu J W, Lee W, Kim B S, Lachaal M, Spangler R A, Jung C Y
Department of Physiology and Biophysics, State University of New York at Buffalo, School of Medicine and Biomedical Sciences, and VA Medical Center, Medical Research Service, Buffalo, New York 14215, USA.
Biochemistry. 2002 Dec 3;41(48):14364-71. doi: 10.1021/bi026474n.
In rat adipocytes, insulin-induced GLUT4 recruitment to the plasma membrane (PM) is associated with characteristic changes in the GLUT4 contents of three distinct endosomal fractions, T, H, and L. The organelle-specific marker distribution pattern suggests that these endosomal GLUT4 compartments are sorting endosomes (SR), GLUT4-storage endosomes (ST), and GLUT4 exocytotic vesicules (EV), respectively, prompting us to analyze GLUT4 recycling based upon a four-compartment kinetic model. Our analysis revealed that insulin modulates GLUT4 trafficking at multiple steps, including not only the endocytotic and exocytotic rates, but also the two rate coefficients coupling the three intracellular compartments. This analysis assumes that GLUT4 cycles through PM T, H,L, and back to PM, in that order, with transitions characterized by four first-order coefficients. Values assigned to these coefficients are based upon the four steady-state GLUT4 pool sizes assessed under both basal and insulin stimulated states and the transition time courses observed in the plasma membrane GLUT4 pool. Here we present the first reported experimental measurements of transient changes in each of the four GLUT4 compartments during the insulin-stimulated to basal transition in rat adipocytes and compare these experimental results with the corresponding model simulations. The close correlation of these results offers clear support for the general validity of the assumed model structure and the assignment of the T compartment to the sorting endosome GLUT4 pool. Variations in the recycling pathway from that of an unbranched cyclic topography are also considered in the light of these experimental observations. The possibility that H is a coupled GLUT4 storage compartment lying outside the direct cyclic pathway is contraindicated by the data. Okadaic acid-induced GLUT4 recruitment is accompanied by modulation of the rate coefficients linking individual endosomal GLUT4 compartments, further demonstrating a significant role of the endosomal pathways in GLUT4 exocytosis.
在大鼠脂肪细胞中，胰岛素诱导的葡萄糖转运蛋白4（GLUT4）向质膜（PM）的募集与三个不同内体组分（T、H和L）中GLUT4含量的特征性变化相关。细胞器特异性标记物分布模式表明，这些内体GLUT4区室分别是分拣内体（SR）、GLUT4储存内体（ST）和GLUT4胞吐囊泡（EV），这促使我们基于四组分动力学模型分析GLUT4的循环。我们的分析表明，胰岛素在多个步骤调节GLUT4的运输，不仅包括内吞和胞吐速率，还包括连接三个细胞内区室的两个速率系数。该分析假设GLUT4按此顺序循环通过质膜、T、H、L，然后回到质膜，其转变由四个一级系数表征。赋予这些系数的值基于在基础状态和胰岛素刺激状态下评估的四个稳态GLUT4池大小以及在质膜GLUT4池中观察到的转变时间进程。在这里，我们展示了首次报道的大鼠脂肪细胞从胰岛素刺激到基础状态转变过程中四个GLUT4区室中每个区室瞬态变化的实验测量结果，并将这些实验结果与相应的模型模拟进行比较。这些结果的密切相关性为假设的模型结构的普遍有效性以及将T区室分配给分拣内体GLUT4池提供了明确支持。根据这些实验观察结果，还考虑了与无分支循环拓扑结构的回收途径的差异。数据表明H是位于直接循环途径之外的耦合GLUT4储存区室的可能性不大。冈田酸诱导的GLUT4募集伴随着连接各个内体GLUT4区室速率系数的调节，进一步证明了内体途径在GLUT4胞吐中的重要作用。
