In vitro study of placental trophoblast calcium uptake using JEG-3 human choriocarcinoma cells

During human fetal development, placental syncytiotrophoblasts actively transport calcium from the maternal to the fetal circulation. Two functional components, a cytosolic Ca²⁺-binding protein (CaBP) and a Ca²⁺-ATPase have been identified in the syncytiotrophoblasts of the chorionic villi. We report here the calcium uptake properties of a human choriocarcinoma cell line, JEG-3, which was used as an in vitro model cell system for the syncytiotrophoblasts. In culture, JEG-3 proliferated as large syncytial aggregates expressing typical syncytiotrophoblast markers. ⁴⁵Ca uptake by JEG-3 was a substrate- and temperature-dependent, membrane-mediated active process that exhibited linear kinetics for up to 7 min. Both the CaBP and the Ca²⁺-ATPase were expressed by JEG-3, on the basis of biochemical, histochemical, immunochemical and/or mRNA asssays. Immunohistochemistry and in situ hybridization revealed that JEG-3 cells were heterogeneous with respect to the expression of the CaBP. The Ca²⁺-ATPase activity of JEG-3 was similar to the placental enzyme in terms of sensitivity to specific inhibitors, and was detected histochemically along the cell membrane. Fura-2 Ca²⁺ imaging revealed that calcium uptake by JEG-3 was not accompanied by a concomitant increase in cytosolic [Ca²⁺], suggesting a specific Ca²⁺ sequestration mechanism. The involvement of calciotropic hormonal regulation was evaluated by studying the response of JEG-3 to 1,25-dihydroxy vitamin D₃. Calcium uptake was significantly stimulated in a dose-dependent manner by a 24-h treatment of the cells with 1,25-dihydroxy vitamin D₃ (optimal dose ∼0.5 nM); the CaBP level doubled whereas steady-state CaBP mRNA did not, suggesting that CaBP expression was regulated by 1,25-dihydroxy vitamin D₃. These observations strongly suggest that the JEG-3 human choriocarcinoma cells should serve as a convenient in vitro model system for studying the cellular mechanism and regulation of transplacental calcium transport.