Susceptibility of insulinoma cells to cadmium and modulation by L-type calcium channels

Cadmium (Cd), a toxic metal that induces apoptosis and necrosis in a variety of cells, accumulates in pancreas and may be a cause of diabetes in humans. In the insulinoma cells line HIT-T15 ( HIT), we measured internal calcium (Ca) and Cd levels by the fluorescent dye Fura-2 and confirm that L-type voltage-dependent calcium channels ( VDCC) play a major role in glucose response and represent a pathway of Cd influx in these cells. Therefore we examined the role of VDCC in acute Cd poisoning by comparing its accumulation and cytotoxic effect in HIT cells and in epithelial-like VDCC-free HeLa cells. Cultures were incubated with 10 - 300 mu M Cd for 15 min - 6 h. While negligible at the end of the treatment, HIT cell death was evident after 18 - 24 h, and it was time-, dose- and serum-dependent. Short ( <= 60 min) Cd treatments with lower doses (<= 100 mu M in serum-free medium) induced delayed apoptotic cell death, as demonstrated by DNA fragmentation on agarose gels and segmentation of DAPI-stained nuclei. Longer incubations and/or higher concentrations caused mainly necrosis. The same treatments were largely harmless in HeLa cells, in which neither death nor DNA fragmentation was observed. The Ca antagonist nimodipine was capable to prevent HIT cell death at lower doses of Cd and to restore the apoptotic condition at higher doses, indicating that reduction of Cd flux through VDCC modulates Cd toxicity. These data demonstrate a specific sensitivity to Cd of insulinoma cells that can be significant for pancreatic beta-cell pathology.