Contrasting insulin dose-dependent defects in activation of atypical protein kinase C and protein kinase B/Akt in muscles of obese diabetic humans

Aims/hypothesis Insulin-stimulated glucose transport in muscle is impaired in obesity and type 2 diabetes, but alterations in levels of relevant signalling factors, i.e. atypical protein kinase C (aPKC) and protein kinase B (PKB/Akt), are still uncertain. Clamp studies using maximal insulin concentrations have revealed defects in activation of aPKC, but not PKB, in both obese non-diabetic and obese diabetic subjects. In contrast, clamp studies using submaximal insulin concentrations revealed defects in PKB activation/phosphorylation in obese non-diabetic and diabetic subjects, but changes in aPKC were not reported. The aim of this study was to test the hypothesis that dose-related effects of insulin may account for the reported differences in insulin signalling to PKB in diabetic muscle. Subjects and methods We compared enzymatic activation of aPKC and PKB, and PKB phosphorylation (threonine-308 and serine-473) during hyperinsulinaemic-euglycaemic clamp studies using both submaximal (400-500 pmol/l) and maximal (1400 pmol/l) insulin levels in non-diabetic control and obese diabetic subjects. Conclusions/interpretations In lean control subjects, the submaximal insulin concentration increased aPKC activity and glucose disposal to approximately 50% of the maximal level and PKB beta activity to 25% of the maximal level, but PKB alpha activity was not increased. In these subjects, phosphorylation of PKB alpha and PKB beta was increased to near-maximal levels at submaximal insulin concentrations. In obese diabetic subjects, whereas aPKC activation was defective at submaximal and maximal insulin concentrations, PKB beta activation and the phosphorylation of PKB beta and PKB alpha were defective at submaximal, but not maximal, insulin concentrations. Defective PKB beta activation/phosphorylation, seen on submaximal insulin stimulation in diabetic muscle, may largely reflect impaired activation of insulin signalling factors present in concentrations greater than those needed for full PKB activation/phosphorylation. Defective aPKC activation, seen at all insulin levels, appears to reflect, at least partly, an impaired action of distal factors needed for aPKC activation, or poor aPKC responsiveness.