To study amalgam-related toxicity in a primary target cell type, human oral fibroblasts were grown in a low-serum medium containing 1.25% fetal bovine serum and exposed to Hg2+, a corrosion product of amalgam. A 1-h exposure to various concentrations of Hg2+ resulted in a dose-dependent loss of colony forming efficiency. Removal of the low-molecular-weight thiol cysteine from the medium increased the toxicity of Hg2+ almost 50-fold in comparison with complete medium or medium without fetal bovine serum. Accordingly, fetal bovine serum was not found to contain detectable levels of low-molecular-weight thiols. The levels of cellular free protein thiols were shown to be depleted by Hg2+ at significantly lower concentrations of the metal ion than those required to decrease the levels of the major cellular low-molecular weight thiol glutathione. These decreases were dependent on the exposure conditions, i.e. the presence of serum and thiols, in a manner similar to the effect on colony forming efficiency. Other functions commonly related to cell viability, including the accumulation of the vital dye neutral red, the cytosolic retention of deoxyglucose and the mitochondrial reduction of tetrazolium were also inhibited by Hg2+, albeit at higher concentrations. Finally, the depletion of cellular glutathione, by pre-exposure of the cells to the glutathione synthesis inhibitor buthionine sulfoximine, somewhat increased the toxicity of Hg2+ and potentiated the depletion of protein thiols. Taken together, the toxicity of Hg2+ in human oral fibroblasts was demonstrated in several assays of which colony forming efficiency was the most sensitive, cell killing by this agent was related to its high affinity for protein thiols, whereas glutathione showed a significant, but limited, ability to protect the cells from Hg2+ toxicity.
THE MECHANISM OF HG2+ TOXICITY IN CULTURED HUMAN ORAL FIBROBLASTS - THE INVOLVEMENT OF CELLULAR THIOLS
ATZORI, LUIGI;
1992-01-01
Abstract
To study amalgam-related toxicity in a primary target cell type, human oral fibroblasts were grown in a low-serum medium containing 1.25% fetal bovine serum and exposed to Hg2+, a corrosion product of amalgam. A 1-h exposure to various concentrations of Hg2+ resulted in a dose-dependent loss of colony forming efficiency. Removal of the low-molecular-weight thiol cysteine from the medium increased the toxicity of Hg2+ almost 50-fold in comparison with complete medium or medium without fetal bovine serum. Accordingly, fetal bovine serum was not found to contain detectable levels of low-molecular-weight thiols. The levels of cellular free protein thiols were shown to be depleted by Hg2+ at significantly lower concentrations of the metal ion than those required to decrease the levels of the major cellular low-molecular weight thiol glutathione. These decreases were dependent on the exposure conditions, i.e. the presence of serum and thiols, in a manner similar to the effect on colony forming efficiency. Other functions commonly related to cell viability, including the accumulation of the vital dye neutral red, the cytosolic retention of deoxyglucose and the mitochondrial reduction of tetrazolium were also inhibited by Hg2+, albeit at higher concentrations. Finally, the depletion of cellular glutathione, by pre-exposure of the cells to the glutathione synthesis inhibitor buthionine sulfoximine, somewhat increased the toxicity of Hg2+ and potentiated the depletion of protein thiols. Taken together, the toxicity of Hg2+ in human oral fibroblasts was demonstrated in several assays of which colony forming efficiency was the most sensitive, cell killing by this agent was related to its high affinity for protein thiols, whereas glutathione showed a significant, but limited, ability to protect the cells from Hg2+ toxicity.
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