Abstract
Arsenic and cadmium are well-reconized human carcinogens, buttheir mechanismsare not entirely clear. Recent studiessuggested that reactive oxygen species(ROS) and/or reactivenitrogen species (RNS) are involved in the clastogenici-ty ofarsenic and cadmium. Since chromosome aberrations mainly comefrom DNA damage, and DNA damages may require several cellularsteps to become chromoso-me aberrations, a more simple questionto be asked is: how DNA damages are in-duced by arsenic andcadmium? The aim of this invesgation is to find out whatkindsof ROS and RNS are involved in arsenic- and cadmium-induced DNAstrand breaks. DNA strand breaks in sodium arsenite (SA) andcadmium chloride (Cd) treated bovine aortic endothelialcellswere analyzed by single-cell alkaline electrophoresis. Theresults indicate that inhibitors of nitric oxide syntha-se, Nw-nitro-L-arginine methyl ester (NAME) and S-methyl-L-thiocitrulline (MTC),scavengers of hydroxyl radicals, dimethylsulfoxide and D-mannitol, could decrease DNA strand breaks inSA- but not in Cd-treated cells. On the contray, catalase, ascavenger of hydrogen peroxide, could decrease DNA strand breaksin Cd- but not in SA-treated cells. However, superoxidedimutase,a scavenger of superoxide, uric acid and Trolox,scavengers of peroxynitrite, could decrea-sse DNA strand breaksbreaks in both SA- and in Cd-treated cells. These rsultssuggest that SA may induce DNAstrand breaks via nitric oxideandperoxynitrite; and Cd induce DNA strand breaks via superoxide,hydrogenperoxide,and hydroxyl radicals. Moreover, calcium seemsto be involved in both SA- and Cd-induced DNA breaks, sincecalcium chelators, ethylene glycol-bis (beta-ami-ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), Quin 2, 1,2-Bis(2-aminophe-noxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA), supressed SA-as well as Cd- induced DNA strand breaks. calcium ionophore,A23187, increased SA- but not Cd-induced comet. Inorganictrivalent arsenic compounds are generally recognized to be moretoxic than their metabolite, methylated arsenic compounds.However, SA has recently been shown not to induce DNA strandbreaks in human diploid pulmonary epithelial cells, but itsmetabolite, dimethylarsinic acid (DMA) does. We have comparedthe induction of DNA strand breaks by SA and DMA in a humanembryo lung cell line, MRC-5, and in bovine aortic endothelialcells by single-cell alkaline electrophoresis. The resultsindicate that SA could induce DNA strand breaks in both celllines with or wihtout the blockage of DNA repair synthesis byhydroxyurea (Hu) plus cytosine-beta-D-arabinofuranoside (AraC).Whereas, DMA did not induce DNA strand breaks without Hu/AraC.Induction of DNA strand breaks by SA plus Hu/AraC, reached aplateau by 6 h of treatment, wheras, DNA strand breaks inducedby DMA plus Hu/AraC contiously increased beyound 10 h. The SA-induced DNA strand breaks were also repaired in a faster ratethan that of DMA. Moreover, while the SA-induced DNA strandbreaks were mediated by the nitric oxide production and weresensitive to calcium modulation, the DMA-induced DNA strandbreaks were insensitive to nitric oxide synthase inhibitors andcalcium modulators. These results indicate that SA and DMAinduce DNA strand breaks through distinct different mechanisms.