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

Abstract

channels, and was blocked by treatment with nimodipine, thapsigargin, or ryanodine, but not by the IP 3 receptor antagonist xestospongin C.3. Treatment with the cAMP antagonist 8-Br-Rp-cAMPS blocked CICR in response to exendin-4, whereas the PKA inhibitor H-89 was ineffective when tested at a concentration demonstrated to inhibit PKA-dependent gene expression. RT-PCR of INS-1 cells demonstrated expression of mRNA coding for the type-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 inactivating mutations were introduced into the exchange factor's two cAMP-binding domains.6. It is concluded that CICR in INS-1 cells results from GLP-1 receptor-mediated sensitization of the intracellular Ca 2+ release mechanism, a signal transduction pathway independent of PKA, but which requires cAMP-GEF-II.

Ca2+-induced Ca2+ release by activation of inositol 1,4,5-trisphosphate receptors in primary pancreatic β-cells

“…In several studies of primary - In most studies of CICR in insulin-secreting cells, the phenomenon has been attributed to RyRs [12,14,[17][18][19]. Both type 2 RyR mRNA and RyR protein are expressed in primary as well as clonal -cells [13][14][15].…”

“…Both type 2 RyR mRNA and RyR protein are expressed in primary as well as clonal -cells [13][14][15]. Convincing evidence for functional RyRs come only from the clonal -cells RINm-5F [54], HIT-T15 [14], MIN6 [15] and INS-1 [17,18,55] in which caffeine exhibits the characteristic acute effect on Ca 2+ mobilization. The present data confirm functional ryanodine receptors in INS-1 cells [17] by showing a Ca 2+ -mobilizing effect of caffeine and that ryanodine pre-treatment abolishes such mobilization.…”

“…Convincing evidence for functional RyRs come only from the clonal -cells RINm-5F [54], HIT-T15 [14], MIN6 [15] and INS-1 [17,18,55] in which caffeine exhibits the characteristic acute effect on Ca 2+ mobilization. The present data confirm functional ryanodine receptors in INS-1 cells [17] by showing a Ca 2+ -mobilizing effect of caffeine and that ryanodine pre-treatment abolishes such mobilization. However, these drugs had no corresponding effects on primary -cells from mice, rats and human subjects under identical conditions.…”

“…Even though RyRs are expressed in -cells [13][14][15], the physiological role of cADPr and RyRs remains controversial [16]. Nevertheless, there are several suggestions that RyRs mediate CICR in -cells [12,14,[17][18][19].…”

Glucagon-like peptide-1 prevents beta cell glucolipotoxicity

“…GLP-1 increases the DNA binding activity and expression level of the beta-cell-specific transcription factor pancreatic and duodenal homeobox gene-1 (PDX-1) [30,31], which is implicated in regulating expression of the insulin, GLUT2 and glucokinase genes, and in the regulation of beta cell differentiation [32,33,34]. GLP-1 mobilises intracellular Ca 2+ [35,36] via cyclic AMP guanine nucleotide exchange factor 2 (Epac) [37], an effect that may contribute to the insulinotropic action of the peptide. Moreover, this gluco-incretin hormone increases islet mass in mouse pancreas in vivo [31,38] and causes in vitro beta cell proliferation via transactivation of the epidermal growth factor receptor and subsequent activation of the phosphatidylinositol-3 kinase (PI-3K) and protein kinase C-ζ (PKC-ζ) signalling pathway in beta-(INS-1)-cells [30,39,40].…”