Pompe disease (glycogen storage disease type II) is an autosomal recessive neuromuscular disorder arising from a deficiency of lysosomal acid α-glucosidase (GAA). Accumulation of autophagosomes is a key pathological change in skeletal muscle fibers and fibroblasts from patients with Pompe disease and is implicated in the poor response to enzyme replacement therapy (ERT). We previously found that mutant GAA-induced endoplasmic reticulum (ER) stress initiated autophagy in patient fibroblasts. However, the mechanism of induction of autophagy in fibroblasts from Pompe disease patients lacking ER stress remains unclear. In this study, we show that inactivated Akt induces ER stress-independent autophagy via mTOR suppression in patient fibroblasts. Activated autophagy as evidenced by increased levels of LC3-II and autophagic vesicles was observed in patient fibroblasts, whereas PERK phosphorylation reflecting the presence of ER stress was not observed in them. These patient fibroblasts showed decreased levels of not only phosphorylated Akt, but also phosphorylated p70 S6 kinase. Treatment with insulin, which acts as an activator of the Akt signaling pathway, resulted in increased phosphorylation of both Akt and p70 S6 kinase and suppression of autophagy in patient fibroblasts. In addition, following combination treatment with recombinant human GAA plus insulin, enhanced localization of the enzymes with lysosomes was observed in patient fibroblasts. These findings define a critical role of Akt suppression in the induction of autophagy in fibroblasts from patients with Pompe disease carrying an ER stress non-inducible mutation, and they provide evidence that insulin may potentiate the effect of ERT.
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