Cellular adaptation to chronic cadmium exposure: Intracellular localization of metallothionein protein in human trophoblast cells (JAr)

Trophoblast cells are the first embryonic cells that modulate the transfer of a variety of compounds (oxygen, amino acids, xenobiotics, metals) from the maternal to the fetal circulation in the human placenta. Human placental exposure to the toxic metal, cadmium (Cd) results in a decrease in the production of human chorionic gonadotropin (hCG), a decrease in the maternal to fetal transport of zinc (Zn), and trophoblastic necrosis. Thus, the ability of trophoblast cells to adapt to exposure to the toxic metal Cd has been considered crucial. In this study, the expression and intracellular localization of metallothionein (MT), a small molecular weight, metal binding protein, was examined in trophoblast cells (JAr) grown in normal media and in cells exposed chronically (6 months) to 2 μM CdCl2. Conventional and confocal fluorescence microscopy were used to examine the intracellular localization of MT protein in control cells and cells grown chronically in Cd. In unexposed trophoblast cells, MT protein was primarily perinuclear with low level, punctate expression in the cytosol. Following both chronic and 24 hour exposure to Cd, MT protein levels were increased (at least 3‐fold in both chronic and acute exposures) and the protein was now concentrated inside the nucleus with a lacy, cytoskeletal pattern of expression in the cytosol. To determine if the nuclear accumulation of MT protein was dependent on new protein synthesis, control cells were exposed to CdCl2(2 μM) and cycloheximide (2 μg/ml) for 24 hours. Following this exposure to Cd and cycloheximide, MT protein was perinuclear, suggesting that it was high levels of newly synthesized MT protein that accumulated in the nucleus of Cd treated cells. Cd exposed cells were also treated with colchicine (10 μM) to disrupt the cytoskeleton. Cytosolic MT protein expression in these cells was different from cells treated with Cd alone, suggesting that MT protein is associated with elements of the cytoskeleton. Thus, human trophoblast cells (JAr) exposed chronically will modulate cellular defense mechanisms to adapt to Cd. The specific redistribution of MT protein to the nucleus and to the cytoskeleton of challenged cells may be an important mechanism for cell defense against Cd and other toxicants.