cAMP-regulated guanine nucleotide exchange factor II (Epac2) mediates Ca2+-induced Ca2+ release in INS-1 pancreatic beta-cells

1. The signal transduction pathway responsible for cAMP-dependent Ca2+-indu ced Ca2+ release (CICR) from endoplasmic reticulum. Ca2+ stores was assesse d in the insulin-secreting cell line INS-1. 2. CICR was triggered by the GLP-1 receptor agonist exendin-4, an effect mi micked by caffeine, Sp-cAMPS or forskolin, CICR required influx of Ca2+ thr ough L-type voltage-dependent Ca2+ channels, and was blocked by treatment w ith nimodipine, thapsigargin, or ryanodine, but not by the IP3 receptor ant agonist xestospongin C. 3. Treatment with the cAMP antagonist 8-Br-Rp-cAMPS blocked CICR in respons e to exendin-4, whereas the PKA inhibitor H-89 was ineffective when tested at a concentration demonstrated to inhibit PKA-dependent gene expression. 4. RT-PCR of INS-1 cells demonstrated expression of mRNA coding for the typ e-II isoform of cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF -II, Epac2). 5. CICR in response to forskolin was blocked by transient transfection and expression of a dominant negative mutant isoform of cAMP-GEF-II in which in activating mutations were introduced into the exchange factor's two cAMP-bi nding domains. 6. It is concluded that CICR in INS-1 cells results from GLP-1 receptor-med iated sensitization of the intracellular Ca2+ release mechanism, a signal t ransduction pathway independent of PKA, but which requires cAMP-GEF-II.