Type VIII adenylyl cyclase in rat beta cells: coincidence signal detector/generator for glucose and GLP-1

Abstract

Aims/hypothesis. The secretory function of pancreatic beta cells is synergistically stimulated by two signalling pathways which mediate the effects of nutrients and hormones such as glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic peptide (GIP) or glucagon. These hormones are known to activate adenylyl cyclase in beta cells. We examined the type of adenylyl cyclase that is associated with this synergistic interaction. Methods. Insulin release, cAMP production, adenylyl cyclase activity, mRNA and protein expression were measured in fluorescence-activated cell sorter-purified rat beta cells and in the rat beta-cell lines RINm5F, INS-1 832/13 and INS-1 832/2. Results. In primary beta cells, glucagon and GLP-1 synergistically potentiate the stimulatory effect of 20 mmol/l glucose on insulin release and cAMP production. Both effects are abrogated in the presence of the L-type Ca 2+ -channel blocker verapamil. The cAMP-producing activity of adenylyl cyclase in membranes from RINm5F cells is synergistically increased by Ca 2+ -calmodulin and recombinant GTP γ S-activated G sα -protein subunits. This type of regulation is characteristic for type I and type VIII AC isoforms. Consistent with this functional data, AC mRNA analysis shows abundant expression of type VI AC, four splice variants of type VIII AC and low expression level of type I AC in beta cells. Type VIII AC expression at the protein level was observed using immunoblots of RINm5F cell extracts. Conclusion/interpretation. This study identifies type VIII AC in insulin-secreting cells as one of the potential molecular targets for synergism between GLP-1 receptor mediated and glucose-mediated signalling. [Diabetologia (2003)

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…Type VIII AC is synergistically activated by both G s α and calcium/ calmodulin (Cali et al, 1994) and thus acts as a coincidence detector for glucose and GLP-1 in the β cell. Elevation of glucose concentration (from 1.4 mM to 20 mM) alone did not increase cAMP accumulation in isolated rat primary β cells during a 15 min static incubation (Delmeire et al, 2003). However, the addition of GLP-1 (10 nM) at either low (1.4 mM) or high (20 mM) glucose did and this effect was abrogated by the L-type calcium channel blocker verapamil.…”

“…Leech and co-workers performed RT-PCR on extracts from whole rat and human islets showing that AC III, IV,V,VI and VII were present in rat islets and AC V and VI and were found in human islets . A more recent RT-PCR analysis performed on α and β cells from the islets of Wistar rats clearly shows strong expression of transcripts for AC VI and VIII in β cells and AC II, III, IV, V and VI in α cells (Delmeire et al, 2003). Type VIII AC mRNA expression was also found in RINm5F and INS-1 clonal cell lines (Delmeire et al, 2003).…”

“…A more recent RT-PCR analysis performed on α and β cells from the islets of Wistar rats clearly shows strong expression of transcripts for AC VI and VIII in β cells and AC II, III, IV, V and VI in α cells (Delmeire et al, 2003). Type VIII AC mRNA expression was also found in RINm5F and INS-1 clonal cell lines (Delmeire et al, 2003). It must be noted that neither Leech and colleagues nor Delmeire and colleagues probed for the presence of type IX AC in islets or individual cells, possibly because AC IX is the only one not activated by forskolin (FSK), a non-specific AC activator that leads to supra-physiological levels of intracellular cAMP in FSK-treated cells (Yan et al, 1998).…”

Oscillatory control of insulin secretion

“…From the observation that purified β-cells, lacking influence from the glucagon-producing α-cells, show reduced cAMP content and impaired secretory capacity, it was suggested that cAMP has a permissive role in glucose-induced insulin release and that the effect of the sugar on cAMP content represents amplification of glucagon-induced cAMP production (Schuit and Pipeleers, 1985). Among several adenylyl cyclases expressed in islet cells and insulin-secreting cell lines (Leech et al, 1999;Guenifi et al, 2000;Delmeire et al, 2003), the Ca 2+ -sensitive AC8 isoform has been proposed to integrate the cAMP responses to glucose and hormone receptor stimuli (Delmeire et al, 2003).…”

“…This was interpreted as Ca 2+ -dependent activation of the PDE1 family of phosphodiesterases (Landa et al, 2005). Islets and insulinsecreting cells express both Ca 2+ -regulated phosphodiesterases (Pyne and Furman, 2003;Landa et al, 2005) and adenylyl cyclases (Leech et al, 1999;Guenifi et al, 2000;Delmeire et al, 2003), and among the cyclases certain isoforms are stimulated, whereas others are suppressed by Ca 2+ (Willoughby and Cooper, 2007). It is therefore likely that the phase relationship between cAMP and Ca 2+ signals varies depending on the relative expression levels of different adenylyl cyclases and phosphodiesterases as well as on the type of stimulus (Fridlyand et al, 2007).…”