In summary, the present study indicates clearly that chronic arsenic exposure in vivo induces hepatic DNA hypomethylation, and inappropriate expression of genes potentially important in oncogenesis. The induction of DNA hypomethylation after chronic arsenic exposure in mice is consistent with cell systems associating arsenic-induced malignant transformation with DNA hypomethylation in liver cells.
Thus, arsenic induced errors in DNAmethylation could be an early molecular lesion with the potential for impacting oncogenic growth in the liver, and therefore a plausible mechanism of hepatocarcinogenesis. This should not be seen as the only potential mechsnism of arsenic carcinogenesis and it is quite likely arsenic acts through multiple mechanisms in a tissue-specific manner. EVIDENCE THAT FREE RADICALS MEDIATE ARSENICGENOTOXICITY Effects of depleted intracellular glutathione level onarsenic mutagenesis Cellular nonprotein sulfhydryls (NPSHs) consist essentiallyof glutathione (f95%) and other low-molecular-weight aminothiols such as cysteine and cysteamine37.
As cellular sulfhydryls have significant free radicalscavenging abilities, the effect of glutathione depletionon the mutagenicity of arsenic has been examined usingbuthionine S,R-sulfoximine (BSO), a competitive inhibitorof the enzyme g-glutamyl cysteine synthetase used inthe biosynthesis of glutathione. Treatment of cells withBSO (25 AM) for 24 h decreases the NPSH level to lessthan 2 nmol per 107 AL, a level that is less than 5% of thenormal level 38,39. Pretreatment of cells with BSOenhances both the cytotoxicity and mutagenicity ofarsenic to similar extents. In contrast, pretreatment ofcells with glutathione and cysteine protects mammaliancells against the toxic effects of arsenite40.
Furthermore,low concentrations of arsenite have been shown toinduce a transient increase in cellular glutathione levelsin bovine vascular endothelial cells 41. The upregulationis thought to be a ”secondary” stress responsedirectly regulated by the thiol reactivity of arsenite42. These findings are consistent with the observationthat arsenite activates the transcription factor nuclearfactor nh, which regulates response genes intrinsic tooxidative stress 43. Effects of antioxidants on arsenic-induced genotoxicity A second, complementary approach that delineates thecontribution of ROS in the genotoxicity of arseniteincludes the use of the antioxidants, superoxide dismutase,and catalase. The deleterious effect of oxygentoxicity is normally held in check by the delicate balancebetween the rate of generation of these radicals and therate of their removal by various antioxidant enzymes.
Superoxide dismutase catalyzes the dismutation of superoxideanions, whereas catalase removes hydrogenperoxides and prevents the subsequent formation ofhydroxyl radicals 44 for review. Addition of eithersuperoxide dismutase (400 U/ml) or catalase (5000 U/ml) can partially suppress both the toxicity and themutagenic potential of sodium arsenite45. In contrast,treatment with heat-inactivated catalase results in essentiallyno protection. The findings that catalase and SODcan reduce the mutagenic potential of arsenic are consistentwith data obtained with other genotoxic endpoints.For example, using CHO cells and an x-ray- hypersensitive DNA repair-deficient mutant, XRS5,Wang and Huang have shown that arsenite induces adose-dependent increase in micronuclei that is blockedby exogenous catalase 46.
In addition, heme oxygenase,an oxidative stress protein, and peroxidase areinduced by sodium