Paradoxical effect of rapamycin on inflammatory stress-induced insulin resistance in vitro and in vivo.

Ping Yang,Lei Zhao,Jun Yuan,Zac Varghese,John F Moorhead,Yao Chen,Yunfei Zhao,Xiong Z Ruan,Yaxi Chen

doi:10.1038/srep14959

Abstract

Insulin resistance is closely related to inflammatory stress and the mammalian target of rapamycin/S6 kinase (mTOR/S6K) pathway. The present study investigated whether rapamycin, a specific inhibitor of mTOR, ameliorates inflammatory stress-induced insulin resistance in vitro and in vivo. We used tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) stimulation in HepG2 hepatocytes, C2C12 myoblasts and 3T3-L1 adipocytes and casein injection in C57BL/6J mice to induce inflammatory stress. Our results showed that inflammatory stress impairs insulin signaling by reducing the expression of total IRS-1, p-IRS-1 (tyr632), and p-AKT (ser473); it also activates the mTOR/S6K signaling pathway both in vitro and in vivo. In vitro, rapamycin treatment reversed inflammatory cytokine-stimulated IRS-1 serine phosphorylation, increased insulin signaling to AKT and enhanced glucose utilization. In vivo, rapamycin treatment also ameliorated the impaired insulin signaling induced by inflammatory stress, but it induced pancreatic β-cell apoptosis, reduced pancreatic β-cell function and enhanced hepatic gluconeogenesis, thereby resulting in hyperglycemia and glucose intolerance in casein-injected mice. Our results indicate a paradoxical effect of rapamycin on insulin resistance between the in vitro and in vivo environments under inflammatory stress and provide additional insight into the clinical application of rapamycin.

Highlights

Insulin resistance is closely related to inflammatory stress and the mammalian target of rapamycin/ S6 kinase pathway
Our data showed that glucose (2-NBDG) uptake and glucose consumption were decreased in the TNF-α and IL-6-treated groups compared with controls in the three types of cell lines in the absence and presence of insulin (Fig. 1a–f), suggesting that inflammatory stress impaired glucose utilization in vitro
Glucose uptake and consumption were improved in the rapamycin plus TNF-α or IL-6 groups compared with cells treated with TNF-α or IL-6 alone in the absence and presence of insulin (Fig. 1a–f), suggesting that rapamycin ameliorates inflammatory stress-induced insulin resistance in vitro

Summary

Introduction

Insulin resistance is closely related to inflammatory stress and the mammalian target of rapamycin/ S6 kinase (mTOR/S6K) pathway. Classic insulin signaling that involves insulin receptor, insulin receptor substrates (IRS), phosphatidylinositol-3-kinase (PI3K) and the protein kinase AKT plays a central role in the metabolic actions of insulin in many types of cells and tissues[3,4] In pathophysiological states, such as inflammatory stress, free fatty acid loading and oxidative stress, uncontrolled inhibitory serine phosphorylation of IRS-restrained tyrosine phosphorylation and hijacked insulin signaling[5,6]. The inhibition of mTOR by rapamycin was found to attenuate serine phosphorylation of IRS-1, restore insulin action on the PI3K/AKT pathway and prevent the insulin-resistant effects of excess nutrients on insulin-mediated glucose transport in muscle and adipose cells[25,26,27,28] It appears that a significant side effect of chronic rapamycin treatment is deranged glucose metabolism[29]. The effects of rapamycin on insulin action and glucose metabolism under the condition of inflammatory stress remain totally unknown in vivo

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