Glibenclamide activates translation in rat pancreatic beta cells through calcium-dependent mTOR, PKA and MEK signalling pathways

Abstract

Aims/hypothesis: Prolonged exposure of rat beta cells to the insulin secretagogue glibenclamide has been found to induce a sustained increase in basal insulin synthesis. This effect was calcium-dependent and localised in cells that had been degranulated by the drug. Since it was blocked by the translation inhibitor cycloheximide, we examined whether sustained exposure to glibenclamide activates translational factors by calcium-dependent signalling pathways. Methods: Purified rat beta cells were cultured with and without glibenclamide in the presence or absence of inhibitors of calcium-dependent signalling pathways before measurement of basal and stimulated protein and insulin synthesis, and assessment of abundance of (phosphorylated) translation factors. Results: A 24 h exposure to glibenclamide induced activation of four translation factors, i.e. phosphorylation of eukaryotic initiation factor (eIF) 4e binding protein 1 and ribosomal protein S6 (rpS6), and dephosphorylation of eIF-2α and eukaryotic elongation factor 2. The rise in phospho-rpS6 intensity was localised to a subpopulation of beta cells with low insulin content. This activation of translational factors and the associated elevation of insulin synthesis were completely blocked by the calcium channel blocker verapamil and partially blocked by the mammalian target of rapamycin (mTOR) inhibitor rapamycin, the protein kinase A (PKA) inhibitor Rp-8-Br-cAMPs and the mitogen-activated protein kinase/ extracellular signal-regulated kinase kinase (MEK) inhibitor U0126; a combination of inhibitors exhibited additive effects. Conclusions/interpretation: Prolonged exposure to glibenclamide activates protein translation in pancreatic beta cells through the calcium-regulated mTOR, PKA and MEK signalling pathways. The observed intercellular differences in translation activation are proposed as underlying mechanism for functional heterogeneity in the pancreatic beta cell population. © 2008 Springer-Verlag.