Impaired Akt phosphorylation in insulin-resistant human muscle is accompanied by selective and heterogeneous downstream defects

Abstract

Aims/hypothesis Muscle insulin resistance, one of the earliest defects associated with type 2 diabetes, involves changes in the phosphoinositide 3-kinase/Akt network. The relative contribution of obesity vs insulin resistance to perturbations in this pathway is poorly understood. Methods We used phosphospecific antibodies against targets in the Akt signalling network to study insulin action in muscle from lean, overweight/obese and type 2 diabetic individuals before and during a hyperinsulinaemic-euglycaemic clamp. Results Insulin-stimulated Akt phosphorylation at Thr309 and Ser474 was highly correlated with whole-body insulin sensitivity. In contrast, impaired phosphorylation of Akt substrate of 160 kDa (AS160; also known as TBC1D4) was associated with adiposity, but not insulin sensitivity. Neither insulin sensitivity nor obesity was associated with defective insulin-dependent phosphorylation of forkhead box O (FOXO) transcription factor. In view of the resultant basal hyperinsulinaemia, we predicted that this selective response within the Akt pathway might lead to hyperactivation of those processes that were spared. Indeed, the expression of genes targeted by FOXO was downregulated in insulin-resistant individuals. Conclusions/interpretation These results highlight nonlinearity in Akt signalling and suggest that: (1) the pathway from Akt to glucose transport is complex; and (2) pathways, particularly FOXO, that are not insulin-resistant, are likely Electronic supplementary material The online version of this article (doi:10.1007/s00125-012-2811-y) contains peer-reviewed but unedited supplementary material, which is available to authorised users.

Selective Insulin Resistance in Adipocytes

“…This study provides further evidence in support of the emerging view that insulin resistance is highly selective (14,16,17,49,53) and that this selectivity occurs in all insulin target tissues. In all insulin resistance models, with the exception of DEX-treated 3T3-L1 adipocytes, we observed defective glucose uptake but no defect in insulin-regulated protein synthesis or lipolysis (Table 1).…”

“…Increased basal FoxO1 phosphorylation was recently suggested as a potential consequence of selective insulin resistance because of concomitant hyperinsulinemia (16). Consistent with this, phosphorylation of FoxO1 was significantly increased in white adipose tissue prepared from 4-h fasted ob/ob mice compared with lean controls (Fig.…”

“…The recent observation of selective insulin resistance in certain conditions (14,16,17,49) has added to the complexity of this phenomenon. In view of the central role of the adipocyte in whole body insulin sensitivity (18,50), in the present study, we have analyzed the selectivity of insulin resistance in various adipose models.…”

“…4, A and B). It has been shown that Akt phosphorylation at these sites does not always correlate with Akt activity (16), and considerable sparseness was reported for Akt (42). To test this, we also assessed phosphorylation of a range of Akt substrates and the mTORC1 substrate S6K, downstream of Akt.…”

“…Fibroblasts from patients with polycystic ovary syndrome exhibit defective insulin regulation of glycogen synthesis without any detectable defect in insulin-regulated mitogenesis (15). We recently reported that insulin-dependent regulation of the transcription factor FoxO1 in skeletal muscle of humans with type 2 diabetes (T2D) 6 is normal despite impaired insulin regulation of glucose uptake (16). Insulin-mediated FoxO1 translocation to the cytosol was not affected in chronic insulintreated 3T3-L1 adipocytes at physiological insulin doses, whereas GLUT4 translocation to the cell surface was impaired (17).…”