Glibenclamide inhibits islet carnitine palmitoyltransferase 1 activity, leading to PKC-dependent insulin exocytosis

Abstract

Hypoglycemic sulfonylureas such as glibenclamide have been widely used to treat type 2 diabetic patients for 40 yr, but controversy remains about their mode of action. The widely held view is that they promote rapid insulin exocytosis by binding to and blocking pancreatic β-cell ATP-dependent K+ (KATP) channels in the plasma membrane. This event stimulates Ca2+ influx and sets in motion the exocytotic release of insulin. However, recent reports show that >90% of glibenclamide-binding sites are localized intracellularly and that the drug can stimulate insulin release independently of changes in KATP channels and cytoplasmic free Ca2+. Also, glibenclamide specifically and progressively accumulates in islets in association with secretory granules and mitochondria and causes long-lasting insulin secretion. It has been proposed that nutrient insulin secretagogues stimulate insulin release by increasing formation of malonyl-CoA, which, by blocking carnitine palmitoyltransferase 1 (CPT-1), switches fatty acid (FA) catabolism to synthesis of PKC-activating lipids. We show that glibenclamide dose-dependently inhibits β-cell CPT-1 activity, consequently suppressing FA oxidation to the same extent as glucose in cultured fetal rat islets. This is associated with enhanced diacylglycerol (DAG) formation, PKC activation, and KATP-independent glibenclamide-stimulated insulin exocytosis. The fat oxidation inhibitor etomoxir stimulated KATP-independent insulin secretion to the same extent as glibenclamide, and the action of both drugs was not additive. We propose a mechanism in which inhibition of CPT-1 activity by glibenclamide switches β-cell FA metabolism to DAG synthesis and subsequent PKC-dependent and KATP-independent insulin exocytosis. We suggest that chronic CPT inhibition, through the progressive islet accumulation of glibenclamide, may explain the prolonged stimulation of insulin secretion in some diabetic patients even after drug removal that contributes to the sustained hypoglycemia of the sulfonylurea.

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

“…The glibenclamide concentration used in this study is similar to that of previous in vitro studies [23][24][25]. It is, however, significantly higher than that in patients (below 0.4 μmol/l) [26,27].…”

Glibenclamide Treatment Recruits β-Cell Subpopulation Into Elevated and Sustained Basal Insulin Synthetic Activity

“…This sustained effect coincides with cellular accumulation of the drug, primarily in membranes of secretory vesicles and mitochondria (22)(23)(24)(25). It has been attributed to a protein kinase C activation following glibenclamide stimulation of diacylglycerol synthesis (21,26). During these in vitro studies up to 2-h incubation periods, the sulfonylurea stimulation of insulin release was not associated with an increase in insulin synthesis (27)(28)(29)(30).…”

N-Acetylcysteine and α-cyano-4-hydroxycinnamic acid alter protein kinase C (PKC)-δ and PKC-ζ and diminish dysmorphogenesis in rat embryos cultured with high glucose in vitro

“…The densitometric measurement of the membrane fractions and the cytosolic fraction of each sample was normalized by division of the protein content of the whole sample. The ratio of the membrane-bound fraction and the cytosolic fraction was subsequently used as a marker of isoenzyme activity (Alcazar et al 1997, Ceolotto et al 1999, Lehtihet et al 2003.…”