Hyperglycemia and hyperinsulinemia have additive effects on activation and proliferation of pancreatic stellate cells: possible explanation of islet-specific fibrosis in type 2 diabetes mellitus

Pancreatic islet fibrosis observed in Type 2 diabetes is one of the major factors leading to progressive beta-cell loss and dysfunction. Despite its importance, the mechanism of islet-restricted fibrogenesis associated with pancreatic stellate cell (PSC) activation and proliferation remains to be defined. Therefore, we studied whether the islet-specific environment represented by hyperglycemia and hyperinsulinemia had additive effects on the activation and proliferation of cultured rat PSCs. Cells were stimulated to activate and proliferate with glucose and insulin, either individually or concomitantly. Both stimuli promoted PSC proliferation and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation independently, but an additive effect was also demonstrated. Blockade of ERK signaling by the mitogen-activated protein kinase kinase (MEK) inhibitor, U0126, suppressed both glucose- and insulin-induced ERK 1/2 phosphorylation and PSC proliferation. Glucose and insulin-induced ERK 1/2 phosphorylation also stimulated connective tissue growth factor gene expression. Thus, hyperglycemia and hyperinsulinemia are two crucial mitogenic factors that activate and proliferate PSCs, and the presence of both states will amplify this response.