Abstract

BACKGROUND
High glucose-induced apoptosis has been implicated in the loss of beta-cells of the pancreatic islets in animal models of type 2 diabetes. GLP-1 has been shown to reduce apoptosis by the cAMP-dependent mechanism in beta-cells. Other studies have also shown that elevated levels of intracellular cyclic AMP delayed apoptosis in other types of cells. We investigated whether cAMP-elevating agents could suppress the high glucose-induced apoptosis of isolated rat islets. METHODS: Pancreatic islets were isolated from Sprague-Dawley (SD) rats. The expression of phosphodiesterase (PDE) 3 subtypes was investigated by using extracts of freshly isolated islets and analyzing them by RT-PCR. After 2 days of isolation, the islets were cultured in RPMI-1640 media containing 5% FBS with various glucose concentrations (11.1, 16.7 and 27.8 mM), 5x10-6 M forskolin, 2x10-4 M 3-isobutyl-1-methylxanthine (IBMX), 10-5 M cilostazol, and 10-6, 5x10-6 and 10-5 M H-89 for 5 days. The islet apoptosis was measured by a sandwich enzyme-immunoassay using antihistone antibody. RESULTS: Apoptosis was lowest at 11.1 mM glucose concentration, and increased at higher glucose concentrations (1.00 +/- 0.04 A.U. (arbitrary unit) at 11.1 mM, 1.17 +/- 0.12 A.U. at 16.7 mM, and 1.65 +/-0.13 A.U. at 27.8 mM (P <0.05 for 11.1 mM). Both PDE 3A and 3B mRNA were expressed in the islet extracts. In 16.7 and 27.8 mM glucose concentrations, forskolin (P <0.01), IBMX (P <0.05) and cilostazol (P < 0.05) suppressed apoptosis of the islet cells. Protein kinase A (PKA) nhibitor, H-89, did not prevent the inhibition of apoptosis by forskolin. CONCLUSION: These results show that high glucose-induced apoptosis of the cells in rat islet is attenuated by such cAMP-elevating agents as cilostazol. However, cyclic AMP regulation of islet apoptosis may occur via a PKA-independent signaling pathway.

• Keywords: Glucose toxicity;Beta-cell;Apoptosis;cAMP;Cilostazol;Protein kinase A;