Ataxia telangiectasia mutated impacts insulin-like growth factor 1 signalling in skeletal muscle

Abstract

New findings: • What is the central question of this study? In some cultured cells, ataxia telangiectasia mutated (ATM) is required for activation of Akt by insulin. However, this is not the case in other cell or tissue types, including skeletal muscle. Furthermore, it is not known whether ATM plays a role in skeletal muscle insulin-like growth factor 1 (IGF-1) signalling. • What is the main finding and its importance? We found that IGF-1 caused autophosphorylation of ATM in skeletal muscle. However, IGF-1-stimulated phosphorylation of Akt, p70 S6 kinase and mammalian target of rapamycin (but not insulin receptor substrate 1) was impaired in C2C12 myotubes with reduced ATM expression and/or muscle from ATM-haploinsufficient mice. These findings demonstrate activation of ATM by IGF-1 and a role for ATM in IGF-1 signalling downstream of insulin receptor substrate 1. Reports that ataxia telangiectasia mutated (ATM) is required for full activation of Akt raise the hypothesis that ATM plays a role in insulin-like growth factor 1 (IGF-1) signalling through the Akt/mammalian target of rapamycin (mTOR) pathway. Differentiated C2C12 cells harbouring either ATM-targeting short hairpin RNA (shRNA) or non-targeting shRNA and myotubes from a C2C12 lineage previously exposed to empty vector lentivirus were incubated in the presence or absence of 10 nm IGF-1 followed by Western blot analysis. Parallel experiments were performed in isolated soleus muscles from mice expressing only one functional ATM allele (ATM+/-) compared with muscles from wild-type (ATM+/+) mice. Insulin-like growth factor 1 increased phosphorylation of Akt S473, Akt T308 and p70 S6 kinase (S6K) in myotubes expressing non-targeting shRNA and in empty vector controls, but the IGF-1 effects were significantly reduced in myotubes with shRNA-mediated ATM knockdown. Likewise, IGF-1-stimulated phosphorylation of Akt S473, Akt T308, mTOR and S6K was lower in isolated soleus muscles from ATM+/- mice compared with muscles from ATM+/+ mice. The ATM inhibitor KU55933 prevented stimulation of S6K phosphorylation in C2C12 myotubes exposed to IGF-1, suggesting that decreased IGF-1 action is not limited to chronic conditions of decreased ATM function. Stimulation of insulin receptor substrate 1 tyrosine 612 phosphorylation by IGF-1 was unaffected by ATM deficiency, though IGF-1 phosphatidylinositol 3-kinase activity tended to be lower in muscle from ATM haploinsufficient mice compared with wild-type muscle. The data suggest that ATM is a modulator of IGF-1 signalling downstream of insulin receptor substrate 1 in skeletal muscle. © 2012 The Physiological Society.