Oxidative damage to cellular and isolated DNA by homocysteine: implications for carcinogenesis

Homocysteine is considered to be an important risk factor for cancer as well as cardiovascular diseases. To clarify whether homocysteine has potential carcinogenicity, we investigated formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), which is known to be correlated with the incidence of cancer, induced by homocysteine in human cultured cell lines. Homocysteine increased the amount of 8-oxodG in human leukemia cell line HL-60, whereas the amount of 8-oxodG in its hydrogen peroxide (H2O2)-resistant clone HP100 was not increased. We investigated the mechanism for oxidative DNA damage by homocysteine using P-32-labeled DNA fragments obtained from human tumor suppressor genes and a protooncogene. There were two mechanisms by which homocysteine caused DNA damage in the presence of Cu(II). A low concentration of homocysteine (20 muM) frequently induced piperidine-labile sites at thymine residues, whereas a high concentration of homocysteine (100 muM) resulted in damage principally to guanine residues. Catalase inhibited DNA damage by 20 muM homocysteine, indicating the participation of H2O2, but was ineffective in preventing DNA damage by 100 muM homocysteine. Experiments using a singlet oxygen probe showed that 100 muM homocysteine enhanced chemiluminescence intensity in deuterium oxide more than that in H2O. These results indicated that the metal-dependent DNA damage through H2O2 is likely to be a more relevant mechanism for homocysteine carcinogenicity.