Defects in TLR3 expression and RNase L activation lead to decreased MnSOD expression and insulin resistance in muscle cells of obese people

O Fabre,M Kitzmann,C Breuker,C Amouzou,T Salehzada,C Bisbal,J Mercier

doi:10.1038/cddis.2014.104

Abstract

Obesity is associated with chronic low-grade inflammation and oxidative stress that blunt insulin response in its target tissues, leading to insulin resistance (IR). IR is a characteristic feature of type 2 diabetes. Skeletal muscle is responsible for 75% of total insulin-dependent glucose uptake; consequently, skeletal muscle IR is considered to be the primary defect of systemic IR development. Interestingly, some obese people stay insulin-sensitive and metabolically healthy. With the aim of understanding this difference and identifying the mechanisms responsible for insulin sensitivity maintenance/IR development during obesity, we explored the role of the latent endoribonuclease (RNase L) in skeletal muscle cells. RNase L is a regulator of innate immunity, of double-stranded RNA sensors and of toll-like receptor (TLR) 4 signaling. It is regulated during inflammation by interferons and its activity is dependent on its binding to 2-5A, an oligoadenylate synthesized by oligoadenylate synthetases (OAS). Increased expression of RNase L or downregulation of its inhibitor (RLI) improved insulin response in mouse myogenic C2C12 cells and in primary human myotubes from normal-weight subjects treated with palmitate, a saturated free fatty acid (FFA) known to induce inflammation and oxidative stress via TLR4 activation. While RNase L and RLI levels remained unchanged, OAS level was decreased in primary myotubes from insulin-resistant obese subjects (OB-IR) compared with myotubes from insulin-sensitive obese subjects (OB-IS). TLR3 and mitochondrial manganese superoxide dismutase (MnSOD) were also underexpressed in OB-IR myotubes. Activation of RNase L by 2-5A transfection allowed to restore insulin response, OAS, MnSOD and TLR3 expression in OB-IR myotubes. Due to low expression of OAS, OB-IR myotubes present a defect in RNase L activation and TLR3 regulation. Consequently, MnSOD level is low and insulin sensitivity is reduced. These results support that RNase L activity limits FFA/obesity-induced impairment of insulin response in muscle cells via TLR3 and MnSOD expression.

Highlights

Since the 50s, several studies have led to develop the idea that insulin resistance (IR) is a major consequence of the establishment of a chronic low-grade inflammatory state and oxidative stress.[4,5]
It results in production and release of numerous cytokines such as type I interferons (IFNI), interleukin (IL)-1b, IL-6 and tumor necrosis factor a (TNFa), and chemokines such as monocyte chemoattractant protein-1 (MCP-1), which activate inflammation and oxidative stress
We previously demonstrated the major role of RNase L and RLI in lipids accumulation, cell fate and insulin response during myogenesis and adipogenesis via regulation of MyoD and C/ EBP (CCAAT-enhancer-binding protein)-homologous protein 10 (CHOP10) mRNA expression.[28,29,30]

Summary

Introduction

Since the 50s, several studies have led to develop the idea that IR is a major consequence of the establishment of a chronic low-grade inflammatory state and oxidative stress.[4,5]. Chronic FFA exposure is associated with increased reactive oxygen species (ROS) production (the main source being the mitochondria), through a TLR4-dependent mechanism that involves innate immunity response and the release of pro-inflammatory cytokines.[11,12,13] Besides, ROS and in particular hydrogen peroxide (H2O2) are essential for many biological processes including regulation of insulin signal transduction.[4,14] small amount of H2O2 is produced during insulin receptor stimulation and facilitates normal insulin signal transduction to the different proteins of the signaling cascade via tyrosine phosphorylation (e.g., insulin receptor, IRS1) or serine phosphorylation (protein kinase B (PKB)/Akt).[15,16] If ROS are indispensable for the proper insulin response, their excess may cause its disruption and contribute to IR and T2D, in particular because they lead to JNK activation.[17,18] ROS level must be tightly regulated. The mechanism of this positive effect of IFNI on insulin signaling has not yet been studied in depth

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