Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
Proc Natl Acad Sci U S A. 2010 Dec 21;107(51):22314-9. doi: 10.1073/pnas.1016630108. Epub 2010 Dec 6.
In pancreatic β cells, ERK1 and ERK2 participate in nutrient sensing, and their activities rise and fall as a function of glucose concentration over the physiologic range. Glucose metabolism triggers calcium influx and release of calcium from intracellular stores to activate ERK1/2. Calcium influx also activates the calcium-dependent phosphatase calcineurin, which is required for maximal ERK1/2 activation by glucose. Calcineurin controls insulin gene expression by ERK1/2-dependent and -independent mechanisms. Here, we show that, in β cells, glucose activates the ERK1/2 cascade primarily through B-Raf. Glucose activation of B-Raf, like that of ERK1/2, is calcineurin-sensitive. Calcineurin binds to B-Raf in both unstimulated and stimulated cells. We show that B-Raf is a calcineurin substrate; among calcineurin target residues on B-Raf is T401, a site of negative feedback phosphorylation by ERK1/2. Blocking calcineurin activity in β cells prevents dephosphorylation of B-Raf T401 and decreases B-Raf and ERK1/2 activities. We conclude that the major calcineurin-dependent event in glucose sensing by ERK1/2 is the activation of B-Raf.
在胰腺β细胞中，ERK1 和 ERK2 参与营养感应，其活性随生理范围内葡萄糖浓度的变化而上升和下降。葡萄糖代谢触发钙内流和细胞内储存钙的释放，从而激活 ERK1/2。钙内流还激活钙依赖性磷酸酶钙调神经磷酸酶，这是葡萄糖对 ERK1/2 最大激活所必需的。钙调神经磷酸酶通过 ERK1/2 依赖和非依赖机制控制胰岛素基因表达。在这里，我们表明，在β细胞中，葡萄糖主要通过 B-Raf 激活 ERK1/2 级联。与 ERK1/2 一样，葡萄糖对 B-Raf 的激活也是钙调神经磷酸酶敏感的。钙调神经磷酸酶在未刺激和受刺激的细胞中与 B-Raf 结合。我们表明 B-Raf 是钙调神经磷酸酶的底物；钙调神经磷酸酶在 B-Raf 上的靶残基包括 T401，这是 ERK1/2 负反馈磷酸化的位点。在β细胞中阻断钙调神经磷酸酶活性可防止 B-Raf T401 的去磷酸化，并降低 B-Raf 和 ERK1/2 的活性。我们得出结论，钙调神经磷酸酶依赖性事件在 ERK1/2 葡萄糖感应中是 B-Raf 的激活。
