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

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Abstract
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 32P-labeled DNA fragments obtained from human tumor suppressor genes and a proto-oncogene. There were two mechanisms by which homocysteine caused DNA damage in the presence of Cu(II). A low concentration of homocysteine (20 M) frequently induced piperidine-labile sites at thymine residues, whereas a high concentration of homocysteine (100 M) resulted in damage principally to guanine residues. Catalase inhibited DNA damage by 20 M homocysteine, indicating the participation of H2O2, but was ineffective in preventing DNA damage by 100 M homocysteine. Experiments using a singlet oxygen probe showed that 100 M 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.