Cd2+-induced synthesis of metallothionein in HeLa cells

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Abstract
HeLa cells synthesize metallothioneins [which are involved in heavy metal detoxification] in response to Cd2+. The kinetics of thionein (apoprotein of metallothionein) synthesis was studied by pulse-labeling the cells with [35S]cysteine and measuring relative amounts of the labeled thioneins separated by electrophoresis. Thionein synthesis rapidly inreased in the first 6-8 h after exposure to 0.5 .mu.g of Cd2+/ml, and beings to decrease in a few hours after reaching the maximum. The rate of synthesis never returns to the basal value at least in 30 h after Cd2+ exposure; instead, the second increase in thionein synthesis occurs at 16-18 h. A possible regulatory mechanism of thionein synthesis is discussed from these results, together with the data on intracellular accumulation and subcellular distribution of 109Cd2+. The initial increase of thionein synthesis is accompanied by an increase of mRNA coding for thioneins (MT-mRNA). The induction of MT-mRNA is sensitive to actinomycin D, but not to cycloheximide, sugesting transcriptional regulation without any mediating protein synthesis. Two thionein isomers are coded for by mRNA molecules of almost the same size, which is simlar to that of hepatic MT-mRNA of mouse and rat.

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