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.

Inhibition of Na/K-ATPase promotes myocardial tumor necrosis factor-alpha protein expression and cardiac dysfunction via calcium/mTOR signaling in endotoxemia

“…In islet beta cells during glibenclamide stimulation, Ca 2? activates mTOR via PKA/PKBdependent and -independent pathways [51]. In PC-12 cells, hypoxia-stimulated influx of Ca 2?…”

“…is an important regulator of mTOR activation in various types of cells [26,30,51]. In primary mouse Fig.…”

“…It is sensitive to tuberous sclerosis complex (TSC)1/TSC2 activation and intracellular Ca 2? concentration [23,26,30,51]. Studies have shown that mTOR contributes to cardiac hypertrophy [11,35] and ventricular remodeling after myocardial infarction [6].…”

Interaction of glibenclamide and metformin at the level of translation in pancreatic cells

“…We have recently reported that glibenclamide, a second-generation sulfonylurea drug, also exerts stimulatory effects on b-cell protein and insulin synthesis when present over a prolonged period both in vivo and in vitro (Ling et al 2006). This chronic effect was achieved through an activation of translation at the steps of initiation and elongation, as evidenced by phosphorylation of the eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and of ribosomal protein S6 (rpS6), and dephosphorylation of eIF2a and of eukaryotic elongation factor 2 (eEF2; Wang et al 2008). It was shown to be calcium dependent and mediated through the mammalian target of rapamycin (mTOR; also known as mechanistic target of rapamycin, MTOR), protein kinase A (PKA), and MAPK/ERK kinases (MEKs; Wang et al 2008).…”

“…This chronic effect was achieved through an activation of translation at the steps of initiation and elongation, as evidenced by phosphorylation of the eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and of ribosomal protein S6 (rpS6), and dephosphorylation of eIF2a and of eukaryotic elongation factor 2 (eEF2; Wang et al 2008). It was shown to be calcium dependent and mediated through the mammalian target of rapamycin (mTOR; also known as mechanistic target of rapamycin, MTOR), protein kinase A (PKA), and MAPK/ERK kinases (MEKs; Wang et al 2008). Thus, glibenclamide, which is used for its acute stimulatory effect on insulin release, exerts an mTOR-dependent stimulation of insulin synthesis when present for prolonged periods .…”