Parallel signaling pathways of melatonin in the pancreatic beta-cell.
Previous results demonstrated that melatonin inhibits cAMP production and stimulates IP(3) liberation in rat insulinoma INS1 cells, a model for the pancreatic beta-cell. This study addresses the impact of melatonin on insulin release. Insulin, cAMP and IP(3) levels of INS1 cells in a superfusion system were measured. Initially, forskolin was used to stimulate cAMP and subsequently insulin release. Incubation of forskolin (5 micromol/L)-stimulated cells with melatonin (100 nmol/L) inhibited cAMP and insulin levels (down to 60% of insulin and cAMP release). The G(i)alpha-protein-inhibitor pertussis toxin (PTX) was used to distinguish between the G(i)alpha-dependent cAMP pathway and the G(i)alpha-independent IP(3) pathway. In our experiments we employed a specific stimulation pattern to prove proper inhibition of G(i)alpha-proteins by PTX. In INS1 cells incubated with 250 ng/mL PTX for 24 hr, melatonin was no longer able to inhibit the forskolin-induced cAMP and insulin release. In a study, carbachol was used to stimulate IP(3) and subsequently insulin release. Surprisingly, incubation of carbachol (300 micromol/L)-stimulated cells with melatonin (100 nmol/L) inhibited insulin release (down to 75% of insulin release). Finally, in PTX-incubated INS1 cells, melatonin (100 nmol/L) increased carbachol (300 micromol/L)-induced insulin release (up to 124% of insulin release). In conclusion, we found that the melatonin MT(1)-receptor on pancreatic beta-cells is coupled to parallel signaling pathways, with opposite influences on insulin secretion. The cAMP- and subsequently insulin-inhibiting signaling pathway involves PTX-sensitive G(i)alpha-proteins and is predominant in terms of insulin release.
Peschke
Elmar
E
Institute of Anatomy and Cell Biology, Martin Luther University Halle-Wittenberg, Halle/Saale, Germany. elmar.peschke@medizin.uni-halle.de
Bach
Andreas G
AG
Mühlbauer
Eckhard
E
D000818:Animals; D045744:Cell Line, Tumor; D000242:Cyclic AMP / Q000378:metabolism; D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D007328:Insulin / Q000378:metabolism; D050417:Insulin-Secreting Cells / Q000502:physiology*; D008550:Melatonin / Q000502:physiology*; D037342:Pertussis Toxin; D051381:Rats; D015290:Second Messenger Systems / Q000502:physiology*
D007328:Insulin; D015544:Inositol 1,4,5-Trisphosphate; D000242:Cyclic AMP; D037342:Pertussis Toxin; D020962:Heterotrimeric GTP-Binding Proteins; D008550:Melatonin