Long-Term Exposure of Pancreatic β-Cells to Palmitate Results in SREBP-1C-Dependent Decreases in GLP-1 Receptor Signaling via CREB and AKT and Insulin Secretory Response

Endocrinology. 2016 Jun;157(6):2243-58. doi: 10.1210/en.2015-2003. Epub 2016 Apr 1.

Abstract

The effects of prolonged exposure of pancreatic β-cells to high saturated fatty acids on glucagon-like peptide-1 (GLP-1) action were investigated. Murine islets, human pancreatic 1.1B4 cells, and rat INS-1E cells were exposed to palmitate for 24 hours. mRNA and protein expression/phosphorylation were measured by real-time RT-PCR and immunoblotting, respectively. Specific short interfering RNAs were used to knockdown expression of the GLP-1 receptor (Glp1r) and Srebf1. Insulin release was assessed with a specific ELISA. Exposure of murine islets, as well as of human and INS-1E β-cells, to palmitate reduced the ability of exendin-4 to augment insulin mRNA levels, protein content, and release. In addition, palmitate blocked exendin-4-stimulated cAMP-response element-binding protein and v-akt murine thymoma viral oncogene homolog phosphorylation, whereas phosphorylation of MAPK-ERK kinase-1/2 and ERK-1/2 was not altered. Similarly, RNA interference-mediated suppression of Glp1r expression prevented exendin-4-induced cAMP-response element-binding protein and v-akt murine thymoma viral oncogene homolog phosphorylation, but did not impair exendin-4 stimulation of MAPK-ERK kinase-1/2 and ERK-1/2. Both islets from mice fed a high fat diet and human and INS-1E β-cells exposed to palmitate showed reduced GLP-1 receptor and pancreatic duodenal homeobox-1 (PDX-1) and increased sterol regulatory element-binding protein (SREBP-1C) mRNA and protein levels. Furthermore, suppression of SREBP-1C protein expression prevented the reduction of PDX-1 and GLP-1 receptor levels and restored exendin-4 signaling and action. Finally, treatment of INS-1E cells with metformin for 24 h resulted in inhibition of SREBP-1C expression, increased PDX-1 and GLP-1 receptor levels, consequently, enhancement of exendin-4-induced insulin release. Palmitate impairs exendin-4 effects on β-cells by reducing PDX-1 and GLP-1 receptor expression and signaling in a SREBP-1C-dependent manner. Metformin counteracts the impairment of GLP-1 receptor signaling induced by palmitate.

MeSH terms

  • Animals
  • Cell Line
  • Cyclic AMP Response Element-Binding Protein / metabolism*
  • Exenatide
  • Glucagon-Like Peptide-1 Receptor / metabolism*
  • Humans
  • Insulin / metabolism*
  • Insulin-Secreting Cells / drug effects
  • Insulin-Secreting Cells / metabolism*
  • Islets of Langerhans / drug effects
  • Islets of Langerhans / metabolism*
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Palmitates / pharmacology*
  • Peptides / pharmacology
  • Proto-Oncogene Proteins c-akt / metabolism*
  • Rats
  • Signal Transduction / drug effects
  • Sterol Regulatory Element Binding Protein 1 / metabolism*
  • Venoms / pharmacology

Substances

  • Cyclic AMP Response Element-Binding Protein
  • Glucagon-Like Peptide-1 Receptor
  • Insulin
  • Palmitates
  • Peptides
  • Sterol Regulatory Element Binding Protein 1
  • Venoms
  • Exenatide
  • Proto-Oncogene Proteins c-akt