Preventive effects of zinc on cadmium-induced inhibition of alkaline phosphatase activity and mineralization activity in osteoblast-like cells, MC3T3-E1.

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Abstract
In order to determine the specific action of cadmium on bone metabolism, the effect of cadmium on alkaline phosphatase activity, a marker enzyme of osteoblasts, was compared with that of other divalent heavy metal ions, i.e., zinc, manganese, lead, copper, nickel and mercury (10 .mu.M each), using cloned osteoblast-like cells, MC3T3-E1. Cadmium had the strongest inhibitory effect on alkaline phosphate activity of the cells among the metals tested. At the same dose, however, cadmium failed to inhibit cellular glucose-6-phosphatase and lactate dehydrogenase activities, suggesting that the inhibitory effect of cadmium on alkaline phosphatase was specific and was not dependent on cell injury. Cadmium treatment caused a significant decrease in cellular zinc level, but mercury treatment had no such effect at the dose inhibiting alkaline phosphate activity. There was a good correlation between decrease of cellular zinc level and inhibition of alkaline phosphatase activity in cadmium-treated cells. Concomitant treatment of the cells with zinc prevented the cadmium-induced inhibition of alkaline phosphatase activity. However, this was not the case in the mercury-induced inhibition. Cadmium also inhibited the mineralization of osteoblasts. When 10 or 20 .mu.M zinc was concomintantly added to the cultures, the inhibition of mineralization was prevented. These data suggest that the inhibitory effect of cadmium in osteoblasts may be closely related to its influence on the cellular zinc metabolism.