Abstract

Glucocorticoid excess induces hyperglycemia, which may result in diabetes. The present experiments explored whether glucocorticoids trigger apoptosis in insulin-secreting cells. Treatment of mouse beta-cells or INS-1 cells with the glucocorticoid dexamethasone (0.1 micromol/l) over 4 days in cell culture increased the number of fractionated nuclei from 2 to 7 and 14%, respectively, an effect that was reversed by the glucocorticoid receptor antagonist RU486 (1 micromol/l). In INS-1 cells, dexamethasone increased the number of transferase-mediated dUTP nick-end labeling-staining positive cells, caspase-3 activity, and poly-(ADP-) ribose polymerase protein cleavage; decreased Bcl-2 transcript and protein abundance; dephosphorylated the proapoptotic protein of the Bcl-2 family (BAD) at serine155; and depolarized mitochondria. Dexamethasone increased PP-2B (calcineurin) activity, an effect abrogated by FK506. FK506 (0.1 micromol/l) and another calcineurin inhibitor, deltamethrin (1 micromol/l), attenuated dexamethasone-induced cell death. The stable glucagon-like peptide 1 analog, exendin-4 (10 nmol/l), inhibited dexamethasone-induced apoptosis in mouse beta-cells and INS-1 cells. The protective effect of exendin-4 was mimicked by forskolin (10 micromol/l) but not mimicked by guanine nucleotide exchange factor with the specific agonist 8CPT-Me-cAMP (50 micromol/l). Exendin-4 did not protect against cell death in the presence of cAMP-dependent protein kinase (PKA) inhibition by H89 (10 micromol/l) or KT5720 (5 micromol/l). In conclusion, glucocorticoid-induced apoptosis in insulin-secreting cells is accompanied by a downregulation of Bcl-2, activation of calcineurin with subsequent dephosphorylation of BAD, and mitochondrial depolarization. Exendin-4 protects against glucocorticoid-induced apoptosis, an effect mimicked by forskolin and reversed by PKA inhibitors.