Molecular Pharmacology for Diabetes Laboratory, Health Innovations Research Institute, School of Health Sciences, RMIT University, Bundoora, Victoria 3083, Australia
Insulin resistance is a hallmark of type 2 diabetes. Liver exhibits earlier signs of impairment of insulin action compared to peripheral tissues like skeletal muscle and fat. Endoplasmic reticulum (ER) stress caused by accumulation of misfolded proteins, has been shown to play an important role in the development of hepatic insulin resistance. The aim of this study was to investigate the relationship between the activation of ER stress pathways and insulin-dependent signalling cascades in hepatic cells. FAO hepatoma cells were used as in vitro experimental model. ER stress was induced with tunicamycin (2mg/ml for 20 h) as indicated by increases in both IRE1 (2.5-fold, p<0.05) and PERK (4-fold, p<0.01) phosphorylation. Consistent with impairment in insulin sensitivity, tunicamycin decreased by 50% (p<0.01) the phosphorylation of Akt induced by insulin (10 nM for 15 min). Interestingly, thapsigargin (0.2 mM for 2 h), another ER stressor, increased eIF2a phosphorylation (2-2.5 fold, p<0.01) whilst no increases in IRE-1 phosphorylation or reductions in insulin-dependent phosphorylation of Akt were observed. Unexpectedly, progesterone (10 mM, 20 h), a hormone associated with whole-body insulin resistance as well as a modulator of molecular chaperones that stabilise ER-residing proteins, reversed both the reduction of insulin-dependent phosphorylation of Akt and the increased IRE-1 phosphorylation induced by tunicamycin, without affecting the activation of the PERK pathway. However, progesterone was not able to reverse the decrease in Akt phosphorylation in insulin resistant cells induced by chronic insulin (10 nM for 48h). Our results suggest that the activation of the IRE-1 pathway is the key contributing factor to the impairment of insulin action in hepatic cells specific to ER stress. Progesterone has protective effect against ER stress-induced insulin resistance, but not against chronic insulin treatment, suggesting specific beneficial effects of progesterone on ER chaperone activity.