Combined effects of okadaic acid and cadmium on lipid peroxidation and DNAbases modifications (m(5)dC and 8-(OH)-dG) in Caco-2 cells

Okadaic acid (OA) is a marine toxin, a tumour promoter and an inducer of ap optosis. It mainly inhibits protein-phosphatases, protein synthesis and enh ances lipid peroxidation. Cadmium (Cd) is known to be carcinogenic in anima ls and humans (group 1 according to the International Agency for Research o n Cancer (IARC) classification). Cd also induces oxidative stress in living organisms. Since they are sometimes found simultaneously in mussels, we ha ve evaluated in the present investigation, the lipid peroxidation, as malon dialdehyde (MDA) production, in the variation of the ratios of 8-(OH)-dG/10 (5)dG and m(5)dC/(dC + m(5)dC) induced by OA and/or Cd in Caco-2 cells. Whe n cells were treated exclusively by OA (15 ng/ml) or Cd (0.625 and 5 mu g/m l) for 24 h, protein synthesis was inhibited (by 42 +/- 5%, 18 +/- 13%, and 90 +/- 4% respectively) while MDA production was 2235 +/- 129, 1710 +/- 20 , and 11496 +/- 1624 pmol/mg protein respectively. In addition, each toxica nt induced modified bases in DNA; increases in oxidised bases and methylate d dC. The combination of OA and cadmium was more cytotoxic and caused more DNA base modifications; the ratio m(5)dC/(m(5)dC + dC) was increased from 3 +/- 0.15 to 9 +/- 0.15 and the ratio 8-(OH)-dG/10(5) dG also (from 36 +/- 2 to 76 +/- 6). The combination of OA and Cd also increased the level of MD A (16874 +/- 2189 pmole/mg protein). The present results strongly suggest t hat DNA damage resulting from the oxidative stress induced by these two tox icants may significantly contribute to increasing their carcinogenicity via epigenetic processes.