Forkhead box O1 mediates defects in palmitate-induced insulin granule exocytosis by downregulation of calcium/calmodulin-dependent serine protein kinase expression in INS-1 cells

Abstract

Aims/hypothesis The transcription factor forkhead box O1 (FOXO1) induces pancreatic islet beta cell endoplasmic reticulum stress and is involved in fatty-acid-induced insulinsecretion defects. Cask is a downstream target gene of FOXO1. Using INS-1 cells with palmitate-induced insulinrelease defects, we investigated the relationship between FOXO1 and Cask. Methods The expression levels and location of calcium/ calmodulin-dependent serine protein kinase (CASK) and FOXO1 were evaluated by real-time PCR, western blotting and immunofluorescence. The regulation of Cask by FOXO1 was examined using chromatin immunoprecipitation (ChIP) and luciferase assays. Potassium-stimulated insulin-secretion assays were used to verify the function of INS-1 cells and islets. Electron microscopy was used to establish the anchoring process of the insulin granules after CASK knockdown in islets. Results Palmitic acid reduced CASK levels and increased FOXO1 levels. ChIP and luciferase assays demonstrated FOXO1 binding with the Cask promoter, which was enhanced by palmitate treatment. CASK knockdown reduced insulin release in INS-1 cells and primary islets, and Cask overexpression reversed the palmitate-induced insulin reduction. CASK knockdown attenuated forskolin-enhanced insulin release, but Cask overexpression did not change the insulin-secretion suppression induced by nifedipine. In pancreatic islet beta cells, CASK knockdown reduced the anchoring of insulin vesicles to cell membranes. Conclusions/interpretation The induction of beta cell insulinsecretion defects by fatty acids is mediated, at least in part, by FOXO1 via downregulation of Cask expression. It is characterised mainly as an obstruction of the anchoring of insulin granules to beta cell membranes.

Islet Stellate Cells Regulate Insulin Secretion via Wnt5a in Min6 Cells

“…In our future studies, we will aim at identifying the mechanisms underlying this difference. Previous studies have shown that FoxO1 inhibits insulin granule exocytosis through downregulation of Cask expression in INS-1 cells [45]. Our study confirmed that the Wnt5a protein can suppress the expression of pFoxO1, but the expression of Cask did not increase accordingly.…”

“…ese experimental results are consistent with those of previous studies. Many genes are related to the insulin secretion from β-cells, for example, β-cell transcription factors (PDX1, Rfx6, and NeuroD1); insulin gene transcription factors; genes mediating GSIS, such as FoxO1, PDX1, Glut2, Gck, Abcc8, and Kcnj11; and genes mediating insulin granule exocytosis (Cask) [42][43][44][45].…”

A role for alternative splicing in circadian control of exocytosis and glucose homeostasis

“…CASK was of particular interest as a member of the membrane-associated guanylate kinase (MAGUK) protein family that helps initiate assembly of the presynaptic secretory machinery. CASK is involved in exocytosis of neurotransmitters in the brain (Atasoy et al 2007) and of insulin in the endocrine pancreas (Suckow et al 2008;Wang et al 2015). Furthermore, Uniprot and Interpro annotations show that exon 11 of Cask, which is skipped more frequently in the circadian mutants, encodes a domain that facilitates interactions with synaptic membrane proteins ( Fig.…”

How the stimulus defines the dynamics of vesicle pool recruitment, fusion mode, and vesicle recycling in neuroendocrine cells

“…In pancreatic islet beta cells, CASK knockdown reduced the anchoring of insulin vesicles to cell membranes and insulin release (Wang et al . ).…”

“…There is also data suggesting that the disruption of Mint1-Munc18-1 binding decreases secretion in chromaffin cells (Graham et al 2011). In pancreatic islet beta cells, CASK knockdown reduced the anchoring of insulin vesicles to cell membranes and insulin release (Wang et al 2015).…”