Induction of metallothionein synthesis in cultured human trophoblasts by cadmium and zinc.

A system of primary cultures of human chorionic trophoblasts has been used for studying the effects of heavy metals on human reproductive tissue. Using this system, changes in cellular concentration of metallothionein (MT) in response to exposure to Cd or Zn were determined. Trophoblasts were isolated from term chorion leave, grown in RPMI-1640 medium, and exposed to Cd or Zn. Cellular content of MT was measured using the Cd/heme radioassay. MT increased in a concentration- and time-dependent manner after exposure to either metal. Cd increased the content of MT in trophoblasts at concentrations as low as 0.5 microM during a 24-h exposure. Moreover, extending exposure to Cd (2 microM) to 72 h resulted in a 3-4-fold increase in the concentration of MT. On a molar basis, Zn was not as potent a stimulus for MT synthesis as Cd, and required a concentration of 2.5 microM to increase the concentration of MT over a 24-h period. However, a 48- or 72-h exposure to Zn (10 microM) increased concentrations of MT nearly 8-fold over control values. Simultaneous exposure to Cd (2 microM) and inhibitors of protein synthesis, cycloheximide and actinomycin D, prevented the typical increase in MT concentration, suggesting that the metals act to increase the synthesis of MT. In another series of experiments, trophoblasts were exposed to Cd (2 microM) for 24 h, after which the cells were challenged with cytotoxic concentrations of Cd. Cells pretreated with Cd and then challenged with toxic concentrations of Cd had higher levels of MT and showed less toxicity, as indicated by leakage of lactic dehydrogenase. These results suggest that MT serves to sequester the metals in trophoblasts and reduce the toxicity of heavy metals. Thus, this system should be useful for studying the effects of heavy metals and characterizing the induction of MT in human reproductive tissue in vitro.