Analysis of arsenic metabolites in HepG2 and AS3MT-transfected cells

It has been suggested that arsenic (+3 oxidation state) methyltransferase (AS3MT) plays a critical role in methylation of arsenic, and that arsenic-glutathione conjugate is a substrate for AS3MT-catalyzed methylation of arsenic. However, the mechanism of arsenic methylation in cells is not fully understood. Here, we have constructed T-REx-CHO-hAS3MTtr cells that transiently overexpress human AS3MT in response to tetracycline. The decreases in cell viability after exposure to sodium arsenite were greater in tetracycline-treated cells (tet(+) cells) than in untreated cells (tet(-) cells). Concentration of total cellular arsenic was significantly higher in tet(+) cells than in tet(-) cells. Speciation analyses of arsenic metabolites in whole cell lysates and cell culture medium were performed using both HepG2 cells and T-REx-CHO-hAS3MTtr cells. Speciation analyses of arsenic metabolites in lysates of T-REx-CHO-hAS3MTtr cells revealed that dimethylated arsenicals were the predominant arsenic metabolites in tet(+) cells, while methylated metabolites were not found in tet(-) cells. In contrast, less amount of methylated arsenic metabolites were found in the HepG2 cell lysates, and monomethylated trivalent arsenicals were the predominant methylated arsenic metabolites. Arsenate was found in the culture medium after 24 h culture with arsenite. A larger amount of arsenate was found in the culture medium of tet(+) or tet(-) cells compared to HepG2 cells. These findings indicated that AS3MT expression enhanced the cytotoxic effect of arsenite in tet(+) cells because these cells accumulated more arsenic metabolites than did the tet(-) cells, and accordingly, the tet(+) cells were more susceptible to arsenic than were the tet(-) cells. Oxidation-reduction of arsenic may be implicated in the toxic effects of arsenite.