The Rap-B-Raf signalling pathway is activated by glucose and glucagon-like peptide-1 in human islet cells

Diabetologia. 2005 Aug;48(8):1534-40. doi: 10.1007/s00125-005-1820-5. Epub 2005 Jul 2.


Aims/hypothesis: Glucose and glucagon-like peptide-1 have been shown to activate extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase in beta cells. We examined the contributions of the small GTPases Rap and Ras and the serine-threonine kinases B-Raf and Raf-1 to the activation of these kinases in human islet cells.

Methods: The expression of Rap, Ras, B-Raf and Raf-1 in human islets was examined by immunohistochemistry and immunoblotting. Human islets were incubated in glucose at concentrations of 2.5 and 15 mmol/l and were stimulated with 10 nmol/l glucagon-like peptide-1. The activation of ERK and Raf kinases was examined by phosphorylation-specific antibodies and immuno-complexed kinase assays. The activation of Rap and Ras was determined by pull-down assays. Stimulation of phosphoinositide 3-kinase was detected by immuno-complexed lipid kinase assays.

Results: Extracellular-regulated kinase and protein kinase B (a downstream target of phosphoinositide 3-kinase) were activated in islets stimulated with glucose and glucagon-like peptide-1. In these islets, the Rap-B-Raf signalling pathway was activated preferentially compared with Ras and Raf-1, and activated Rap and B-Raf mediated ERK stimulation in kinase assays in vitro. In addition, Rap rather than Ras mediated activation of phosphoinositide 3-kinase in islets stimulated with glucose and glucagon-like peptide-1.

Conclusions/interpretation: In human islet cells, glucose and glucagon-like peptide-1 activate the Rap and B-Raf signalling module, which mediates ERK activation in assays in vitro. Rap also activates phosphoinositide 3-kinase, delineating central roles for Rap and B-Raf as therapeutic targets for beta cell growth in diabetes mellitus.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cells, Cultured
  • Enzyme Inhibitors / pharmacology
  • Extracellular Signal-Regulated MAP Kinases / antagonists & inhibitors
  • Extracellular Signal-Regulated MAP Kinases / physiology
  • GTP Phosphohydrolases / metabolism
  • Gene Products, vpr / physiology*
  • Glucagon / pharmacology*
  • Glucagon-Like Peptide 1
  • Glucose / pharmacology*
  • Humans
  • Immunohistochemistry
  • Islets of Langerhans / drug effects
  • Islets of Langerhans / metabolism*
  • Peptide Fragments / pharmacology*
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphoinositide-3 Kinase Inhibitors
  • Protein Precursors / pharmacology*
  • Proto-Oncogene Proteins B-raf / physiology*
  • Signal Transduction / drug effects*
  • raf Kinases / antagonists & inhibitors
  • raf Kinases / metabolism


  • Enzyme Inhibitors
  • Gene Products, vpr
  • Peptide Fragments
  • Phosphoinositide-3 Kinase Inhibitors
  • Protein Precursors
  • Glucagon-Like Peptide 1
  • Glucagon
  • BRAF protein, human
  • Proto-Oncogene Proteins B-raf
  • raf Kinases
  • Extracellular Signal-Regulated MAP Kinases
  • GTP Phosphohydrolases
  • Glucose