title: Effect of the AMP-kinase modulators AICAR, metformin and compound C on insulin secretion of INS-1E rat insulinoma cells under standard cell culture conditions.
abstract: OBJECTIVE
  The function of β-cells is regulated by nutrient uptake and metabolism. The cells' metabolic state can be expressed as concentration ratios of AMP, ADP and ATP. Relative changes in these ratios regulate insulin release. An increase in the intracellular ATP concentration causes closure of K(ATP) channels and cell membrane depolarization, which triggers stimulus-secretion coupling (SSC). In addition to K(ATP) channels, the AMP-dependent protein kinase (AMPK), a major cellular fuel sensor in a variety of cells and tissues, also affects insulin secretion and β-cell survival. In a previous study we found that the widely used AMPK inhibitor compound C retards proliferation and induces apoptosis in the rat β-cell line INS-1E. We therefore tested the effects of AMPK activators (AICAR and metformin), and compound C on AMPK phosphorylation, insulin secretion, K(ATP) channel currents, cell membrane potential, intracellular calcium concentration, apoptosis and cell cycle distribution of INS-1E cells under standard cell culture conditions (11 mM glucose).
  
  
  METHODS
  Western blotting, ELISA, patch-clamp, calcium imaging and flow cytometry.
  
  
  RESULTS
  We found that basal AMPK phosphorylation is enhanced by AICAR (1 mM) and metformin (1 mM) but remained unaffected by compound C (10 μM). Both AICAR and compound C stimulated basal insulin secretion whereas metformin had no effect. Pre-incubation with AICAR (1 mM) caused an inhibition of K(ATP) currents but did not significantly alter the average cell membrane potential (Vm) or the threshold potential of electrical activity. Acute administration of AICAR (300 μM) led to a depolarization of Vm, which was not due to an inhibition of the basal- or glucose-induced chloride conductance, and was not accompanied by elevations of intracellular calcium (Ca(i)). AICAR had no additive blocking effect on K(ATP) currents when applied together with tolbutamide. Compound C applied over 24 hours induced an increase in the percentage of cells positive for caspase activity, whereas AICAR (1 mM) applied for 48 hours was without effect. Medium glucose concentration <3 mM caused cell cycle arrest, caspase activation and an increase of cell granularity.
  
  
  CONCLUSIONS
  We conclude that under standard cell culture conditions the AMPK modulators AICAR and compound C, but not metformin, stimulate insulin secretion by AMPK-independent mechanisms.
authors.affiliation: Institute of Physiology and Pathophysiology, Paracelsus Medical University, Salzburg, Austria.
authors.forename: Christian
authors.initials: C
authors.lastname: Langelueddecke
authors.forename: Martin
authors.initials: M
authors.lastname: Jakab
authors.forename: Nina
authors.initials: N
authors.lastname: Ketterl
authors.forename: Lukas
authors.initials: L
authors.lastname: Lehner
authors.forename: Clemens
authors.initials: C
authors.lastname: Hufnagl
authors.forename: Sabine
authors.initials: S
authors.lastname: Schmidt
authors.forename: John P
authors.initials: JP
authors.lastname: Geibel
authors.forename: Johannes
authors.initials: J
authors.lastname: Fuerst
authors.forename: Markus
authors.initials: M
authors.lastname: Ritter
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000737:chemistry* / Q000378:metabolism; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D002118:Calcium / Q000378:metabolism; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D005947:Glucose / Q000494:pharmacology; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007340:Insulinoma / Q000503:physiopathology; D054086:KATP Channels / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D008687:Metformin / Q000494:pharmacology*; D010766:Phosphorylation; D011720:Pyrazoles / Q000494:pharmacology*; D011743:Pyrimidines / Q000494:pharmacology*; D051381:Rats; D012265:Ribonucleotides / Q000494:pharmacology*
chemical_list: D007004:Hypoglycemic Agents; D007328:Insulin; D054086:KATP Channels; D011720:Pyrazoles; D011743:Pyrimidines; D012265:Ribonucleotides; C516138:dorsomorphin; D000620:Aminoimidazole Carboxamide; D008687:Metformin; D055372:AMP-Activated Protein Kinases; C031143:AICA ribonucleotide; D005947:Glucose; D002118:Calcium