Chao Hongxia, Li Haochen, Grande Rebecca, Lira Vitor, Yan Zhen, Harris Thurl E, Li Chien
Departments of Pharmacology (H.C., H.L., R.G., Z.Y., T.H., C.L.), Medicine (V.L., Z.Y.), and Molecular Physiology and Biophysics (Z.Y.), and Center for Skeletal Muscle Research at Robert M. Berne Cardiovascular Research Center (Z.Y.), University of Virginia Health System, Charlottesville, Virginia 22908.
Mol Endocrinol. 2015 Jun;29(6):831-41. doi: 10.1210/me.2014-1245. Epub 2015 Apr 15.
Type 2 corticotropin-releasing factor receptor (CRFR2) is expressed in skeletal muscle and stimulation of the receptor has been shown to inhibit the effect of insulin on glucose uptake in muscle cells. Currently, little is known about the mechanisms underlying this process. In this study, we first showed that both in vivo and in vitro CRFR2 expression in muscle was closely correlated with insulin sensitivity, with elevated receptor levels observed in insulin resistant muscle cells. Stimulation of CRFR2 by urocortin 2 (Ucn 2), a CRFR2-selective ligand, in C2C12 myotubes greatly attenuated insulin-induced glucose uptake. The inhibitory effect of CRFR2 signaling required cAMP production and is involved the mammalian target of rapamycine pathway, as rapamycin reversed the inhibitory effect of CRFR2 stimulation on insulin-induced glucose uptake. Moreover, stimulation of CRFR2 failed to inhibit glucose uptake in muscle cells induced by platelet-derived growth factor, which, similar to insulin, signals through Akt-mediated pathway but is independently of insulin receptor substrate (IRS) proteins to promote glucose uptake. This result argues that CRFR2 signaling modulates insulin's action likely at the levels of IRS. Consistent with this notion, Ucn 2 reduced insulin-induced tyrosine phosphorylation of IRS-1, and treatment with rapamycin reversed the inhibitory effect of Ucn 2 on IRS-1 and Akt phosphorylation. In conclusion, the inhibitory effect of CRFR2 signaling on insulin action is mediated by cAMP in a mammalian target of rapamycine-dependent manner, and IRS-1 is a key nodal point where CRFR2 signaling modulates insulin-stimulated glucose uptake in muscle cells.
2型促肾上腺皮质激素释放因子受体（CRFR2）在骨骼肌中表达，并且已证明刺激该受体会抑制胰岛素对肌肉细胞葡萄糖摄取的作用。目前，对于这一过程的潜在机制知之甚少。在本研究中，我们首先表明，肌肉中CRFR2的体内和体外表达均与胰岛素敏感性密切相关，在胰岛素抵抗的肌肉细胞中观察到受体水平升高。用CRFR2选择性配体urocortin 2（Ucn 2）刺激C2C12肌管中的CRFR2，可大大减弱胰岛素诱导的葡萄糖摄取。CRFR2信号传导的抑制作用需要cAMP的产生，并且涉及雷帕霉素哺乳动物靶点途径，因为雷帕霉素可逆转CRFR2刺激对胰岛素诱导的葡萄糖摄取的抑制作用。此外，刺激CRFR2未能抑制血小板衍生生长因子诱导的肌肉细胞葡萄糖摄取，血小板衍生生长因子与胰岛素类似，通过Akt介导的途径发出信号，但独立于胰岛素受体底物（IRS）蛋白来促进葡萄糖摄取。这一结果表明，CRFR2信号传导可能在IRS水平调节胰岛素的作用。与此观点一致，Ucn 2降低了胰岛素诱导的IRS-1酪氨酸磷酸化，雷帕霉素处理可逆转Ucn 2对IRS-1和Akt磷酸化的抑制作用。总之，CRFR2信号传导对胰岛素作用抑制作用是由cAMP以雷帕霉素哺乳动物靶点依赖性方式介导的，并且IRS-1是CRFR2信号传导调节肌肉细胞中胰岛素刺激的葡萄糖摄取的关键节点。
