Divergent AKT Signalling Mechanisms Regulate GLUT4 Translocation and Glucose Uptake in Skeletal Muscle and Adipose Tissue.

BACKGROUND

The serine/threonine kinase AKT is a key regulator of glucose and energy metabolism. Prevailing dogma suggests that AKT is an obligate intermediate for glucose uptake in all metabolic tissues and that impaired AKT signalling is a major molecular driver of insulin resistance in obesity.

However, whether AKT is universally required for insulin-stimulated glucose uptake across tissues in vivo has remained unresolved.

METHOD

Several mouse models of adipose-specific AKT2 deletion (F-AKT2KO) and skeletal muscle-specific AKT1, AKT2 and combined AKT1/AKT2 knockout mice (M-AKT1KO, M-AKT2KO and M-AKTDKO) were generated. Skeletal muscle and adipose tissues were analysed following in vivo administration of insulin (2 U/kg), using Western blotting, phosphoproteomics, PI(3,4,5)P3 ELISA and mitochondrial respiration assays.

Glucose metabolism was assessed using [ 3H]-2-deoxyglucose uptake, hyperinsulinemic-euglycemic clamps, glucose and insulin tolerance tests. Global phosphoproteomics was performed in insulin-stimulated skeletal muscle lacking AKT isoforms.

RESULTS

Loss of AKT2 in adipose tissue impaired insulin signalling, including reduced pAS160 Thr649, and markedly decreased insulin-stimulated glucose uptake (~2-3 fold reduction in F-AKT2KO vs F-Control, p < 0.001, n = 7-11), resulting in systemic insulin resistance.

In contrast, M-AKTDKO mice exhibited a robust increase in insulin-stimulated glucose uptake (~3-4 fold increase) despite complete loss of AKT signalling, including pAS160 Thr649. Phosphoproteomic analysis of M-AKTDKO (n = 3-4) identified ~7088 phosphosites, with 795 uniquely upregulated in insulin-stimulated M-AKTDKO muscle (fold change > 2, p < 0.05), enriched in PI3K and AMPK pathways.

Consistently, ~8-fold (p < 0.05) increase in PIP3 levels was observed in M-AKTDKO muscle in response to insulin. Additionally, AKT deficiency was associated with reduced complex I-dependent mitochondrial respiration (~37% decrease in state 3 respiration), consistent with altered energetic status and AMPK activation.

Genetic epistasis experiments demonstrated that both AKT and AMPK activity are required for insulin-stimulated glucose uptake, systemic glucose homeostasis and whole body insulin sensitivity.

CONCLUSION

These findings challenge the long-standing assumption that AKT is universally required for insulin-stimulated glucose uptake in vivo. The study demonstrates that while AKT is essential in adipose tissue, it is dispensable for insulin-stimulated glucose uptake in skeletal muscle.

AKT exerts negative feedback on PI3K signalling in both tissues; however, only skeletal muscle engages AMPK in the abscence of AKT to preserve glucose uptake. These findings redefine tissue-specific insulin signalling mechanisms and identify AMPK as a critical downstream target of PI3K that coordinates with AKT to regulate glucose uptake.