Undercarboxylated Osteocalcin Improves Insulin-Stimulated Glucose Uptake in Muscles of Corticosterone-Treated Mice

Short-term administration of glucocorticoids (GCs) impairs muscle insulin sensitivity at least in part via the reduction of undercarboxylated osteocalcin (ucOC). However, whether ucOC treatment reverses the GC-induced muscle insulin resistance remains unclear. To test the hypothesis that ucOC directly ameliorates impaired insulin-stimulated glucose uptake (ISGU) induced by short-term GC administration in mice muscle and to identify the molecular mechanisms, mice were implanted with placebo or corticosterone (CS) slow-release pellets. Two days post-surgery, insulin-tolerance tests (ITTs) were performed. On day 3, serum was collected and extensor digitorum longus (EDL) and soleus muscles were isolated and treated ex vivo with vehicle, ucOC (30 ng/mL), insulin (60 mu U/mL), or both. Circulating hormone levels, muscle glucose uptake, and muscle signaling proteins were assessed. CS administration reduced both serum osteocalcin and ucOC levels, whole-body insulin sensitivity, and muscle ISGU in EDL. Ex vivo ucOC treatment restored ISGU in CS-affected muscle, without increasing non-insulin-stimulated glucose uptake. In CS-affected EDL muscle, ucOC enhanced insulin action on phosphorylated (p-)protein kinase B (Akt)(Ser473)and the p-extracellular signal-regulated kinase isoform 2 (ERK2)(Thr202/Tyr204)/total (t)ERK2 ratio, which correlated with ISGU. In CS-affected soleus muscle, ucOC enhanced insulin action on p-mammalian target of rapamycin (mTOR)(Ser2481), the p-mTOR(Ser2481)/tmTOR ratio, p-Akt substrate of 160kD (AS160)(Thr642), and p-protein kinase C (PKC) (pan)(Thr410), which correlated with ISGU. Furthermore, p-PKC (pan)(Thr410) correlated with p-Akt(Ser473) and p-AS160(Thr642). ucOC exerts direct insulin-sensitizing effects on CS-affected mouse muscle, likely through an enhancement in activity of key proteins involved in both insulin and ucOC signaling pathways. Furthermore, these effects are muscle type-dependent. (c) 2019 American Society for Bone and Mineral Research.