Insulin-like growth factor I in inclusion-body myositis and human muscle cultures

Possible pathogenic mechanisms of sporadic inclusion-body myositis (sIBM) include abnormal production and accumulation of amyloid β (Aβ), muscle aging, and increased oxidative stress. Insulin-like growth factor I (IGF-I), an endocrine and autocrine/paracrine trophic factor, provides resistance against Aβ toxicity and oxidative stress in vitro and promotes cell survival. In this study we analyzed the IGF-I signaling pathway in sIBM muscle and found that 16.2% ± 2.5% of nonregenerating fibers showed increased expression of IGF-I, phosphatidylinositide 3′OH-kinase, and Akt. In the majority of sIBM abnormal muscle fibers, increased IGF-I mRNA and protein correlated with the presence of Aβ cytoplasmic inclusions. To investigate a possible relationship between Aβ toxicity and IGF-I upregulation, normal primary muscle cultures were stimulated for 24 hours with the Aβ(25-35) peptide corresponding to the biologically active domain of Aβ. This induced an increase of IGF-I mRNA and protein in myotubes at 6 hours, followed by a gradual reduction thereafter. The level of phosphorylated Akt showed similar changes. We suggest that in sIBM, IGF-I overexpression represents a reactive response to Aβ toxicity, possibly providing trophic support to vulnerable fibers. Understanding the signaling pathways activated by IGF-I in sIBM may lead to novel therapeutic strategies for the disease.