Combined effects of okadaic acid and cadmium on lipid peroxidation and DNA bases modifications (m5dC and 8-(OH)-dG) in Caco-2 cells

Okadaic acid (OA) is a marine toxin, a tumour promoter and an inducer of apoptosis. It mainly inhibits protein-phosphatases, protein synthesis and enhances lipid peroxidation. Cadmium (Cd) is known to be carcinogenic in animals and humans (group 1 according to the International Agency for Research on Cancer (IARC) classification). Cd also induces oxidative stress in living organisms. Since they are sometimes found simultaneously in mussels, we have evaluated in the present investigation, the lipid peroxidation, as malondialdehyde (MDA) production, in the variation of the ratios of 8-(OH)-dG/10⁵dG and m⁵dC/ (dC + m⁵dC) induced by OA and/or Cd in Caco-2 cells. When cells were treated exclusively by OA (15 ng/ml) or Cd (0.625 and 5μg/ml) 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 toxicant induced modified bases in DNA; increases in oxidised bases and methylated dC. The combination of OA and cadmium was more cytotoxic and caused more DNA base modifications; the ratio m⁵dC/(m⁵dC+dC) was increased from 3 ± 0.15 to 9 ± 0.15 and the ratio 8-(OH)-dG/10⁵ dG also (from 36 ± 2 to 76 ± 6). The combination of OA and Cd also increased the level of MDA (16874 ± 2189 pmole/mg protein). The present results strongly suggest that DNA damage resulting from the oxidative stress induced by these two toxicants may significantly contribute to increasing their carcinogenicity via epigenetic processes. Received: 28 September 1999 / Accepted: 10 January 2000