[Induction of Cd and Zn-thionein in a clonal osteogenic cell, MC3T3-E1]

Clone MC3T3-E1 cells at various differentiation stages were exposed to 0.44-13.3 microM Cd or 50-175 microM Zn in culture medium. After a 2-h culture period, the amount of Zn accumulated in the cells was shown to be larger than that for Cd, but the production of Zn-thionein was much less compared with that of Cd-thionein. After a 24-h incubation period, the synthesis of Zn-thionein increased markedly at levels of 150 microM Zn or greater, Cd induced metallothionein (MT) synthesis in a dose-dependent manner at 0.44 microM Cd or greater. Since calcified cells differentiated into osteoblastic cells also produced MT, osteoblasts were confirmed to have an ability to induce MT synthesis. Initiation of production of Cd-or Zn-thionein in the cells occurred at an accumulation of about 0.4 nmol Cd and 2.5 nmol Zn/mg cytosol protein. The ratios of thionein-binding Cd/Cd accumulated in the cytosol and thionein-binding Zn/Zn accumulated in the cytosol were 0.11 mol/mol and 0.067 mol/mol, respectively. These results show that the concentration of accumulated Zn necessary for initiating production of MT is about six times that of Cd and one molecule of Cd induces thionein about 1.6 times as effectively as one molecule of Zn does.