title: 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: OBJECTIVE
  The transcription factor forkhead box O1 (FOXO1) induces pancreatic islet beta cell endoplasmic reticulum stress and is involved in fatty-acid-induced insulin-secretion defects. Cask is a downstream target gene of FOXO1. Using INS-1 cells with palmitate-induced insulin-release 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
  The induction of beta cell insulin-secretion 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.
authors: Department of Endocrinology, Zhongda Hospital, Institute of Diabetes, Southeast University, Nanjing, People's Republic of China.
authors: Yao
authors: Y
authors: Wang
authors: Haiyan
authors: H
authors: Lin
authors: Nana
authors: N
authors: Hao
authors: Zhengqiu
authors: Z
authors: Zhu
authors: Dong
authors: D
authors: Wang
authors: Yuan
authors: Y
authors: Li
authors: Hong
authors: H
authors: Chen
authors: Yunxia
authors: Y
authors: Zhu
authors: Xiao
authors: X
authors: Han
mesh_terms: D000818:Animals; D003594:Cytoplasmic Granules / Q000378:metabolism*; D015536:Down-Regulation; D059865:Endoplasmic Reticulum Stress; D005089:Exocytosis; D005227:Fatty Acids / Q000378:metabolism; D051858:Forkhead Transcription Factors / Q000378:metabolism*; D005786:Gene Expression Regulation; D005947:Glucose / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D051528:Guanylate Kinases / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000166:cytology; D007340:Insulinoma / Q000378:metabolism; D007515:Islets of Langerhans / Q000378:metabolism; D008297:Male; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D010168:Palmitates / Q000494:pharmacology*; D034622:RNA Interference; D051381:Rats; D017207:Rats, Sprague-Dawley; D000071161:Forkhead Box Protein O1
chemical_list: D005227:Fatty Acids; D051858:Forkhead Transcription Factors; D005947:Glucose; D049452:Green Fluorescent Proteins; D051528:Guanylate Kinases; D007328:Insulin; D009419:Nerve Tissue Proteins; D010168:Palmitates; C074639:Foxo1 protein, rat; D000071161:Forkhead Box Protein O1; C106868:CASK kinases