Induction of apoptosis in cells by cadmium: Quantitative negative correlation between basal or induced metallothionein concentration and apoptotic rate

Metallothionein (MT) often reduces the adverse effects of cadmium (Cd), but how it may alter Cd-induced apoptosis is unclear. The goal of this study w as to define the role of MT in Cd-induced apoptosis using cell lines with w idely varying sensitivity to Cd. Effects of Cd on growth of human hepatocel lular carcinoma cell lines (HepG2 and PLC/PRF/5) were investigated and comp ared with Chang cells. These cells were cultured with 0, 5, 10, 20, 40, 80, and 120 muM of Cd for 3, 6, 12, and 24 h. Significant cytolethality was ob served in HepG2 and PLC/PRF/5 cells in a time- and concentration-dependent manner, with LC50 values of 24 muM and 13 muM, respectively. However, Chang cells were much less sensitive to Cd-induced cytotoxicity (LC50, 64 muM). Apoptotic cell death occurring at cytolethal concentrations was demonstrate d in all cell lines by DNA fragmentation on agarose gel electrophoresis or by ELISA. When MT was measured, there was a highly significant negative lin ear correlation between the basal cellular MT concentration or Cd-induced M T and the rate of apoptosis induced by Cd in these cell lines. Treating Hep G2 cells with zinc (Zn) made the relatively sensitive HepG2 cell line resis tant to Cd-induced apoptosis, likely due to Zn-induced MT. In fact, there w as also a significant negative linear correlation between the amount of Zn- induced MT in HepG2 cells and the rate of Cd-induced apoptosis. These findi ngs revealed that basal or induced MT perturbs Cd-induced apoptotic cell de ath in various cell lines, and a strong negative correlation exists between cellular MT content and the rate of apoptosis induced by Cd.