Toxicity of cadmium in human trophoblast cells (JAr choriocarcinoma): role of calmodulin and the calmodulin inhibitor, zaldaride maleate.

Cadmium (Cd), the heavy metal, is toxic to the placenta. The objectives of this study were to determine if Cd toxicity is due to inhibition of placental or trophoblast cell proliferation through interactions with the intracellular calcium binding protein, calmodulin (CaM). Cd can replace calcium and thus interfere with CaM's function. Also, CaM inhibitors reverse selected toxic effects of Cd. The CaM inhibitor, zaldaride maleate, was used to determine if Cd inhibits trophoblast cell proliferation through interactions with CaM. JAr choriocarcinoma cells, a neoplastic trophoblast cell line which is similar to early human trophoblast cells, were selected to study this question. Cd (20 and 40 microM) inhibits JAr cell proliferation, as measured by cell number and BrdU incorporation. Zaldaride (10 and 20 microM) inhibits proliferation to a lesser extent; 100 microM is lethal. To determine if zaldaride alters actions of Cd, zaldaride and Cd are added simultaneously. Zaldaride (20 microM) and Cd (20 microM) together inhibit proliferation less than Cd alone, thus partially protecting cells. Metallothionein is induced in cells exposed to Cd, while zaldaride does not cause induction of this cellular defense mechanism protein. To determine if Cd inhibits proliferation through alterations of cell cycle, JAr cells enriched for G0/G1 phase were exposed to 20 microM Cd, 20 microM zaldaride, or 20 microM Cd plus 20 microM zaldaride for 24 hr. Cells remain in G0/G1 following Cd exposure; cells treated with 20 microM zaldaride progress through S phase and into G2. Zaldaride and Cd together allow JAr cells to leave G1 and enter S phase, partially relieving the cycle block produced by Cd. This study demonstrates a role for calmodulin in mediating the toxicity of Cd in trophoblast cell proliferation.