Troglitazone (CS-045) inhibits β-cell proliferation rate following stimulation of insulin secretion in HIT-T 15 cells

Thiazolidinedione analogs are new antidiabetic agents that attenuate peripheral insulin resistance in noninsulin-dependent diabetic patients; however, the effects of these agents on insulin secretion are not known. We determined the short-term and long-term effects of troglitazone (CS-045) on insulin secretion in a Syrian hamster clonal beta-cell line, HIT-T 15 cells. The direct effect of troglitazone (CS-045: 10(-6)-10(-4) M) on insulin secretion was examined in F-12 K incubation medium containing 7 mM glucose. CS-045 significantly stimulated insulin secretion within 10 min at the concentration of 10(-4) M and dose dependently stimulated insulin secretion within 60 min at the concentration of 10(-6)-10(-4) M. The addition of 10(-5) M CS-045 showed an immediate increase of cytoplasmic free Ca2+ concentrations ([Ca2+](i)). Removal of extracellular Ca2+ by the addition of 1.5 mM EGTA completely abolished the 10(-4) M CS-045-induced insulin secretion for 10-min. Long-term incubation (24 h) with 10(-4) M CS-045 significantly decreased beta-cell insulin content and inhibited insulin secretion. During a 5-day incubation, CS-045 showed a dose-dependent reduction of insulin secretion measured during the final 24 h. Long-term incubation with CS-045 over 3 days inhibited the beta-cell proliferation rate, assessed with [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] (MTT) assay. CS-045 dose dependently increased the amount of DNA fragmentation measured by ELISA. The addition of nifedipine failed to attenuate the reduction of beta-cell proliferation rate and insulin secretion by CS-045, nifedipine antagonized an increase in the amount of DNA fragmentation caused by 10(-4) M CS-045. The present studies provide evidence that CS-045 inhibits beta-cell function following an acute stimulation of insulin secretion in HIT-T 15 cells. The immediate stimulation of insulin secretion by CS-045 maybe mediated by an increase in Ca2+ influx from extracellular space. The induction of apoptosis may partially involves the reduction of beta-cell number by CS-045.