Chen Tzu-Chieh, Kuo Taiyi, Dandan Mohamad, Lee Rebecca A, Chang Maggie, Villivalam Sneha D, Liao Szu-Chi, Costello Damian, Shankaran Mahalakshmi, Mohammed Hussein, Kang Sona, Hellerstein Marc K, Wang Jen-Chywan
Metabolic Biology Graduate Program, University of California Berkeley, Berkeley, California, USA; Department of Nutritional Sciences & Toxicology, University of California Berkeley, Berkeley, California, USA.
Department of Nutritional Sciences & Toxicology, University of California Berkeley, Berkeley, California, USA; Endocrinology Graduate Program, University of California Berkeley, Berkeley, California, USA.
J Biol Chem. 2021 Jan-Jun;296:100395. doi: 10.1016/j.jbc.2021.100395. Epub 2021 Feb 7.
Chronic glucocorticoid exposure causes insulin resistance and muscle atrophy in skeletal muscle. We previously identified phosphoinositide-3-kinase regulatory subunit 1 (Pik3r1) as a primary target gene of skeletal muscle glucocorticoid receptors involved in the glucocorticoid-mediated suppression of insulin action. However, the in vivo functions of Pik3r1 remain unclear. Here, we generated striated muscle-specific Pik3r1 knockout (MKO) mice and treated them with a dexamethasone (DEX), a synthetic glucocorticoid. Treating wildtype (WT) mice with DEX attenuated insulin activated Akt activity in liver, epididymal white adipose tissue, and gastrocnemius (GA) muscle. This DEX effect was diminished in GA muscle of MKO mice, therefore, resulting in improved glucose and insulin tolerance in DEX-treated MKO mice. Stable isotope labeling techniques revealed that in WT mice, DEX treatment decreased protein fractional synthesis rates in GA muscle. Furthermore, histology showed that in WT mice, DEX treatment reduced GA myotube diameters. In MKO mice, myotube diameters were smaller than in WT mice, and there were more fast oxidative fibers. Importantly, DEX failed to further reduce myotube diameters. Pik3r1 knockout also decreased basal protein synthesis rate (likely caused by lower 4E-BP1 phosphorylation at Thr37/Thr46) and curbed the ability of DEX to attenuate protein synthesis rate. Finally, the ability of DEX to inhibit eIF2α phosphorylation and insulin-induced 4E-BP1 phosphorylation was reduced in MKO mice. Taken together, these results demonstrate the role of Pik3r1 in glucocorticoid-mediated effects on glucose and protein metabolism in skeletal muscle.
长期暴露于糖皮质激素会导致骨骼肌出现胰岛素抵抗和肌肉萎缩。我们之前已确定磷脂酰肌醇-3-激酶调节亚基1（Pik3r1）是骨骼肌糖皮质激素受体的主要靶基因，其参与糖皮质激素介导的胰岛素作用抑制。然而，Pik3r1在体内的功能仍不清楚。在此，我们构建了横纹肌特异性Pik3r1基因敲除（MKO）小鼠，并用地塞米松（DEX，一种合成糖皮质激素）对其进行处理。用DEX处理野生型（WT）小鼠会减弱胰岛素在肝脏、附睾白色脂肪组织和腓肠肌（GA）中激活的Akt活性。因此，这种DEX效应在MKO小鼠的GA肌肉中减弱，从而使接受DEX处理的MKO小鼠的葡萄糖和胰岛素耐受性得到改善。稳定同位素标记技术显示，在WT小鼠中，DEX处理降低了GA肌肉中的蛋白质合成率。此外，组织学检查表明，在WT小鼠中，DEX处理减小了GA肌管直径。在MKO小鼠中，肌管直径小于WT小鼠，且有更多的快氧化纤维。重要的是，DEX未能进一步减小肌管直径。Pik3r1基因敲除还降低了基础蛋白质合成率（可能是由于4E-BP1在Thr37/Thr46处的磷酸化水平较低所致），并抑制了DEX减弱蛋白质合成率的能力。最后，在MKO小鼠中，DEX抑制eIF2α磷酸化和胰岛素诱导的4E-BP1磷酸化的能力降低。综上所述，这些结果证明了Pik3r1在糖皮质激素介导的骨骼肌葡萄糖和蛋白质代谢效应中的作用。
