Effect of simvastatin and pitavastatin on glucose-stimulated insulin secretion in clonal pancreatic beta-cells (INS-1) cultured with low glucose

OBJECTIVE: Investigate and compare the effects of simvastatin and pitavastatin exposure on glucose stimulated insulin secretion in INS-1 pancreatic β-cells cultured with 4 mM glucose (low glucose) to elucidate possible mechanisms that may contribute to the increased risk of developing Type 2 Diabetes that has been associated with taking statins. METHODS: Simvastatin was prepared to resemble the intracellular active form of the compound by converting the lactone to its active acid form. Simvastatin hydroxyacid (SVA) and pitavastatin were then incubated over a wide range of concentrations (0-1000 nM) with clonal pancreatic β-cells (INS-1) for 3 days. A concentration of 50 nM statins was then chosen for extended incubations (10 and 17 days). Glucose-stimulated insulin secretion was measured in cells cultured at low (4 mM) with and without statin incubation. Results were measured using a homogenous time-resolved fluorescence (HTRF) insulin assay kit (Cisbio). RESULTS: Simvastatin Hydroxyacid (SVA) and pitavastatin both impaired GSIS after the 3 day incubation in INS-1 cells that had been cultured chronically with low glucose. Simvastatin depleted insulin content in a dose-dependent manner (25-1000 nM) after the 3 day exposure. However, when normalized for insulin content, inhibition of GSIS was not observed. In contrast, pitavastatin did not decrease insulin content in a dose-dependent manner. 50 nM incubation with SVA for 10 days showed impairment of GSIS and reduction in content when cells were exposed to high glucose (12 mM) for 1 hour, whereas pitavastatin at that concentration had no effect on GSIS or content. After extending the incubation to 17 days, INS-1 cells treated with 50 nM SVA exhibited increased sensitivity to glucose with hypersecretion occurring at submaximal glucose concentrations and impairment of GSIS at high glucose. In contrast, pitavastatin treated cells did not display hypersensitivity to glucose, but did show a reduction in GSIS at high glucose. CONCLUSION: Diabetogenic effects of SVA in INS-1 cells cultured at low glucose include reduced insulin secretion, reduced insulin content and a left-shift in the glucose dependence of insulin secretion resulting in hypersecretion at basal (3mM) glucose. Pitavastatin also reduced insulin secretion, though in contrast did not deplete insulin content, and did not induce hypersensitivity to glucose. These results are consistent with previous results from our lab that demonstrate SVA but not pitavastatin treated cells exhibit increased oscillation frequency and amplitude of intracellular calcium ([Ca2+])i at basal glucose. The variable effect of statins to induce basal insulin hypersecretion may contribute to their relative risk of use for developing type 2 diabetes