Isoform-specific defects of insulin stimulation of Akt/protein kinase B (PKB) in skeletal muscle cells from type 2 diabetic patients

Abstract

Aims/hypothesis: The serine/threonine kinase Akt/protein kinase B (PKB) is required for the metabolic actions of insulin. Controversial data have been reported regarding Akt defective activation in the muscle of type 2 diabetic patients. Because three Akt isoforms exist, each having a distinct physiological role, we investigated the contribution of isoform-specific defects to insulin signalling in human muscle. Methods: The phosphorylation pattern and kinase activity of each Akt isoform were compared in primary myotubes from healthy control participants and type 2 diabetic patients. Phosphorylation of Ser 473 and of Thr308 in each isoform was determined after immunoprecipitation in myotubes treated or not with insulin. Results: Muscle cells from diabetic patients displayed defective insulin action and a drastic reduction of insulin-stimulated activity of all Akt isoforms. This was associated with specific defects of their phosphorylation pattern in response to insulin, with impaired Akt2- (and to a lower extent Akt3-) Ser473 phosphorylation, and with altered Akt1-Thr308 phosphorylation. These defects were not due to faulty phosphoinositide-dependent protein kinase 1 (PDK1) production or activation. Rather, we found higher levels of the Akt2-Ser473-specific protein phosphatase PH domain leucine-rich repeat protein phosphatase 1 (PHLPP1) in muscle from diabetic patients, which may contribute to the alteration of Akt2-Ser473 phosphorylation. Conclusions/interpretation: These results suggest that several mechanisms affecting Akt isoforms, including deregulated production of PHLPP1, could underlie the alterations of skeletal muscle insulin signalling in type 2 diabetes. Taking into account the recently described isoform-specific metabolic functions of Akt, our results provide mechanistic insight that may contribute to the defective regulation of glucose and lipid metabolisms in the muscle of diabetic patients. © 2007 Springer-Verlag.