Sjöholm Ake, Lehtihet Mikael, Efanov Alexandre M, Zaitsev Sergei V, Berggren Per-Olof, Honkanen Richard E
Department of Internal Medicine, Karolinska Institutet, Stockholm South Hospital, SE 118 83 Stockholm, Sweden.
Endocrinology. 2002 Dec;143(12):4592-8. doi: 10.1210/en.2002-220672.
In human type 2 diabetes mellitus, loss of glucose-sensitive insulin secretion is an early pathogenetic event. Glucose is the cardinal physiological stimulator of insulin secretion from the pancreatic beta-cell, but the mechanisms involved in glucose sensing are not fully understood. Specific ser/thr protein phosphatase (PPase) inactivation by okadaic acid promotes Ca(2+) entry and insulin exocytosis in the beta-cell. We now show that glycolytic and Krebs cycle intermediates, whose concentrations increase upon glucose stimulation, not only dose dependently inhibit ser/thr PPase enzymatic activities, but also directly promote insulin exocytosis from permeabilized beta-cells. Thus, fructose-1,6-bisphosphate, phosphoenolpyruvate, 3-phosphoglycerate, citrate, and oxaloacetate inhibit PPases and significantly enhance insulin exocytosis, nonadditive to that of okadaic acid, at micromolar Ca2+ concentrations. In contrast, the effect of GTP is potentiated by okadaic acid, suggesting that the action of GTP does not require PPase inactivation. We conclude that specific glucose metabolites and GTP inhibit beta-cell PPase activities and directly stimulate Ca2+-independent insulin exocytosis. The glucose metabolites, but not GTP, seem to require PPase inactivation for their stimulatory effect on exocytosis. Thus, an increase in phosphorylation state, through inhibition of protein dephosphorylation by metabolic intermediates, may be a novel regulatory mechanism linking glucose sensing to insulin exocytosis in the beta-cell.
在人类2型糖尿病中，葡萄糖敏感性胰岛素分泌的丧失是早期发病事件。葡萄糖是胰腺β细胞胰岛素分泌的主要生理刺激物，但参与葡萄糖感应的机制尚未完全了解。冈田酸使特定的丝氨酸/苏氨酸蛋白磷酸酶（PPase）失活，可促进β细胞中的Ca（2+）内流和胰岛素胞吐作用。我们现在表明，糖酵解和三羧酸循环中间体在葡萄糖刺激后浓度增加，它们不仅剂量依赖性地抑制丝氨酸/苏氨酸PPase酶活性，还直接促进通透化β细胞的胰岛素胞吐作用。因此，在微摩尔Ca2+浓度下，1,6-二磷酸果糖、磷酸烯醇丙酮酸、3-磷酸甘油酸、柠檬酸和草酰乙酸抑制PPases并显著增强胰岛素胞吐作用，与冈田酸的作用无相加性。相比之下，冈田酸增强了GTP的作用，表明GTP的作用不需要PPase失活。我们得出结论，特定的葡萄糖代谢产物和GTP抑制β细胞PPase活性并直接刺激不依赖Ca2+的胰岛素胞吐作用。葡萄糖代谢产物而非GTP，似乎需要PPase失活才能对胞吐作用产生刺激作用。因此，通过代谢中间体抑制蛋白质去磷酸化来增加磷酸化状态，可能是一种将葡萄糖感应与β细胞胰岛素胞吐作用联系起来的新型调节机制。
