Imoxin Prevents Dexamethasone‐Induced Promotion of Muscle‐Specific E3 Ubiquitin Ligases and Stimulates Anabolic Signaling in C2C12 Myotubes

Abstract

Muscle atrophy is the loss of skeletal muscle mass during several pathological conditions such as long‐term fasting, aging, cancer, diabetes, sepsis and immune disorders. Dexamethasone (DEX), a synthetic glucocorticoid, induces skeletal muscle atrophy by suppression of protein synthesis and promotion of protein degradation. Specifically, DEX increases the levels of muscle specific ubiquitin E3 ligases including MAFbx and MuRF1. The double‐stranded RNA (dsRNA)‐activated protein kinase R (PKR) plays a significant role in mediating inflammation and glucose homeostasis. However, pathological roles of PKR in muscle atrophy are not fully understood. The current study aimed to investigate the effect of imoxin, a PKR inhibitor, on DEX‐induced muscle atrophy in C2C12 myotubes. Myotubes were incubated with imoxin at different concentrations with or without 5 μM DEX for 24 h. Western blotting was performed to measure proteins involved in muscle protein synthesis as well as degradation. DEX significantly augmented the protein levels of MuRF1 and MAFbx (275 ± 5 % and 694 ± 36 %, respectively) compared with serum free control (CON). However, 5 μM imoxin treatment significantly reduced protein levels of MuRF1 by 88 ± 2 % and MAFbx by 99 ± 0 % compared with DEX, respectively. In addition, DEX suppressed FoxO4 phosphorylation (33 ± 9 %) compared to CON, but 5 μM imoxin treatment promoted FoxO4 phosphorylation (735 ± 44 %) compared to DEX. Additionally, DEX stimulated protein ubiquitination compared with control (124 ± 1 %) but 5 μM imoxin treatment reduced protein ubiquitination by 42 ± 4 % compared to DEX. At the same time, 5 μM imoxin treatment stimulated Akt phosphorylation (195 ± 5 %), mTOR phosphorylation (171 ± 21%) and p70S6K1 phosphorylation (314 ± 31 %) under DEX‐treated condition even though DEX treatment did not suppressed Akt/mTOR/p70S6K1 axis. These findings suggest that imoxin protects against DEX‐induced skeletal muscle atrophy by alleviating muscle specific E3 ubiquitin ligases and promoting protein synthesis via Akt/mTOR/S6K1 axis in muscle cells.Support or Funding InformationNone