Metabolism and toxicity of arsenicals in cultured cells

The metabolism and toxicities of arsenite, arsenate and bivalent and pentavalent methylated arsenicals have been examined in primary rat hepatocytes and in cells derived from human liver, skin, urinary bladder, and cervix. Among the cell lines examined, primary rat hepatocytes exhibited the greatest capacity for methylation of arsenicals. Trivalent arsenicals, arsenite, diiodomethylarsine or methylarsine oxide were better substrates for the methylation reactions than were pentavalent arsenate and methylarsenate. Compared to primary rat hepatocytes, the capacity for methylation of arsenicals was significantly lower in primary human hepatocytes. Even lower capacity for arsenic methylation was found in HeLa (human cervical adenocarcinoma cells) and normal human epidermal keratinocytes. The Urotsa cell line, an SV-40 transformed human urinary bladder cell line, did not methylate any arsenical tested. In primary rat hepatocytes incubated with 0.1 to 1 mu M arsenite, dimethylarsenic (DMAs) was the major methylated metabolite and was found mainly in culture media. Small amounts of monomethylarsenic (MAs), were detected in cells. Incubation of primary rat hepatocytes with 4 to 20 CIM arsenite resulted in partial inhibition of the methylation reactions, a decreased DMAs/MAs ratio, and the release of significant amounts of MAs from cells. In cell lines with low capacities for arsenic methylation, inorganic arsenic and/or MAs accumulated in the cells, suggesting that complete methylation (dimethylation) is a prerequisite for clearance of arsenic from cells. Addition of glutathione, glutathione ethyl ester or N-acetylcysteine to culture media stimulated the efflux of MAs from cells decreasing the DMAs/MAs ratio. For all cell lines examined, bivalent mono- and dimethylated arsenicals were more toxic than was arsenite. There was no correlation between methylation capacity of cell lines and resistance to the cytotoxicity of bivalent arsenicals. These results suggest that (i) for human tissues, capacity of cells for methylation of arsenicals varies significantly; (ii) methylation is inhibited by high concentrations of inorganic arsenic; (iii) bivalent methylated metabolites are more cytotoxic than inorganic arsenicals, and (iv) high methylation capacity does not protect cells from the acute toxicity of bivalent arsenicals.