Cell-specific increases in metallothionein expression in the human placenta perfused with cadmium

Metallothionein (MT) is a cysteine-rich protein that may have both a nutritional and a protective role by binding either physiologic or toxic concentrations of metals in cells. The objective of this investigation was to measure the inducibility of MT mRNA and protein, and to determine their specific cellular localization following exposure to 20 microM cadmium (Cd) in the perfused human placenta for periods up to 8 h. MT mRNA was quantitated using slot blot hybridization with an MT IIa genomic clone and MT transcripts were localized via in situ hybridization. MT protein was measured using a 109Cd-binding assay and localized by immunocytochemistry using a monoclonal antibody to MT in fresh placental tissue from nonsmoking mothers and in tissue perfused for 4 or 8 h with CdCl2 (20 microM). Perfusions were performed on placentae from which fresh samples were also obtained. In fresh term placentae, MT mRNA was barely detectable using both slot blot and in situ hybridization. Slot blot hybridization for MT message demonstrated a dramatic increase of at least 70-fold above fresh after 8 h of perfusion with 20 microM CdCl2. In this time frame, however, statistically significant increases in MT protein were not detected. Following perfusion for 4 and 8 h with 20 microM CdCl2, accumulation of MT transcripts was shown, in situ, to occur in stromal and endothelial cells with a small but detectable increase in trophoblast cells. These results were consistent with the localization of the MT protein after perfusion with Cd and also correlated with areas of stromal edema. Thus, while the trophoblast cells are in direct contact with Cd in maternal perfusate, the cells of the villous core and fetal endothelial cells are most responsive to Cd exposure with regard to MT expression. Lower levels of MT expression in trophoblast cells may leave this cell type more susceptible to Cd toxicity, while higher levels of MT expression in cells of the villous core and endothelial cells may provide these cells with more protection.