Inhibitory effects of cadmium on in vitro calcification of a clonal osteogenic cell, MC3T3-E1

To examine an inhibitory mechanism of Cd on bone formation, the effects of Cd on calcification were investigated in a culture of a clonal osteogenic cell line, MC3T3-E1. At 3 days after inoculation, Cd was added to the medium containing 7 mM beta-glycerophosphate, and culture was continued for 8 days. Cd at 1.78 microM and above caused a significant decrease in 45Ca accumulation. The decrease in mineralization by Cd was similar to that in collagen content or alkaline phosphatase (ALP) activity. Histologically, the cell density and the mineralization degree were lower than those of the controls. Ultrastructurally, degenerated cells were observed with undifferentiated cells which had fewer rough-surfaced endoplasmic reticulum and many mitochondria. This suggests that Cd may inhibit the differentiation into osteoblasts as well as the cell function. On the other hand, calcification of cells at 8 days after inoculation was inhibited by Cd at 1.78 microM and above. The decrease in collagen content and ALP activity by Cd was much lower than that in calcification. Cd-treated cells were well differentiated into osteoblasts morphologically, but the mineralization degree was lower than that of the controls. Ultrastructurally, cell damage was not recognized so strongly compared with long-term Cd treatment. The mineralization of osteoblasts was also inhibited by Zn levels which left both collagen content and ALP activity unaffected. From these results, it was suggested that the inhibitory effect of Cd on in vitro calcification of MC3T3-E1 cells may be due to both a depression of cell-mediated calcification and a decrease in physiochemical mineral deposition.