Itani S I, Zhou Q, Pories W J, MacDonald K G, Dohm G L
Department of Biochemistry, School of Medicine, East Carolina University, Greenville, North Carolina 27858, USA.
Diabetes. 2000 Aug;49(8):1353-8. doi: 10.2337/diabetes.49.8.1353.
This study was conducted to investigate the possible involvement of protein kinase C (PKC) and serine/threonine phosphorylation of the insulin receptor in insulin resistance and/or obesity. Insulin receptor tyrosine kinase activity was depressed in muscle from obese insulin-resistant patients compared with lean insulin-responsive control subjects. Alkaline phosphatase treatment resulted in a significant 48% increase in in vitro insulin-stimulated receptor tyrosine kinase activity in obese but not lean muscle. To investigate the involvement of PKC in skeletal muscle insulin resistance and/or obesity, membrane-associated PKC activity and the protein content of various PKC isoforms were measured in human skeletal muscle from lean, insulin-responsive, and obese insulin-resistant patients. Membrane-associated PKC activity was not changed; however, PKC-beta protein content, assayed by Western blot analysis, was significantly higher, whereas PKC-theta, -eta, and -mu were significantly lower in muscle from obese patients compared with muscle from lean control subjects. Incubation of muscle strips with insulin significantly increased membrane-associated PKC activity in muscle from obese but not lean subjects. PKC-delta, -beta, and -theta were translocated from the cytosol to the membrane fraction in response to insulin treatment. These results suggest that in skeletal muscle from insulin-resistant obese patients, insulin receptor tyrosine kinase activity was reduced because of hyperphosphorylation on serine/threonine residues. Membrane-associated PKC-beta protein was elevated under basal conditions, and membrane-associated total PKC activity was increased under insulin-stimulated conditions in muscle from obese insulin-resistant patients. Thus, we postulate that the decreased tyrosine kinase activity of the insulin receptor may be caused by serine/threonine phosphorylation by PKC.
本研究旨在探讨蛋白激酶C（PKC）及胰岛素受体的丝氨酸/苏氨酸磷酸化在胰岛素抵抗和/或肥胖症中可能发挥的作用。与体型消瘦且胰岛素反应敏感的对照受试者相比，肥胖且胰岛素抵抗患者肌肉中的胰岛素受体酪氨酸激酶活性降低。碱性磷酸酶处理后，肥胖者而非体型消瘦者的肌肉在体外胰岛素刺激下的受体酪氨酸激酶活性显著增加了48%。为研究PKC在骨骼肌胰岛素抵抗和/或肥胖症中的作用，我们检测了体型消瘦、胰岛素反应敏感及肥胖且胰岛素抵抗患者的人体骨骼肌中膜相关PKC活性及各种PKC亚型的蛋白质含量。膜相关PKC活性未发生变化；然而，通过蛋白质印迹分析检测发现，肥胖患者肌肉中的PKC-β蛋白质含量显著更高，而PKC-θ、-η和-μ则显著低于体型消瘦的对照受试者的肌肉。用胰岛素孵育肌肉条后，肥胖者而非体型消瘦者的肌肉中膜相关PKC活性显著增加。胰岛素处理后，PKC-δ、-β和-θ从胞质溶胶转位至膜部分。这些结果表明，在胰岛素抵抗的肥胖患者的骨骼肌中，胰岛素受体酪氨酸激酶活性因丝氨酸/苏氨酸残基的过度磷酸化而降低。在基础条件下，肥胖且胰岛素抵抗患者的肌肉中膜相关PKC-β蛋白质升高，在胰岛素刺激条件下，膜相关总PKC活性增加。因此，我们推测胰岛素受体酪氨酸激酶活性降低可能是由PKC介导的丝氨酸/苏氨酸磷酸化所致。
