Cyclooxygenase-2-mediated DNA damage

Rat intestinal epithelial cells that express the cyclooxygenase-2 (COX-2) gene permanently (RIES cells) were used as a model of in vivo oxidative stress. A targeted lipidomics approach showed that 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE) was the major hydroxylated non-esterified lipid formed in unstimulated intact cells. The corresponding hydroperoxide, 15(S)- hydroperoxyeicosatetraenoic acid (15(S)-HPETE) undergoes homolytic decomposition to the DNA-reactive bifunctional electrophile 4-oxo-2(E)-nonenal, a precursor of heptanone-etheno-2′-deoxyguanosine. This etheno adduct was identified in the DNA of RIES cells. A dose-dependent increase in adduct levels was observed in the presence of vitamin C. This suggested that vitamin C increased lipid hydroperoxide-mediated 4-oxo-2(E)-nonenal formation in the cells. The selective COX-2 inhibitor NS-398 was protective against cellular DNA damage but was less effective if vitamin C was present. Prostaglandin E₂ and 15(S)-HETE biosynthesis were completely inhibited by 110 μM NS-398 in the intact RIES cells. No inhibition of COX-1 was detected in the wild-type RIE cells at this concentration of NS-398. Arachidonic acid treatment of RIES cell lysates and ionophore stimulation of intact RIES cells produced significantly more 15(R)-HETE than the untreated intact cells. These preparations also both produced 11(R)-HETE, which was not detected in the intact cells. Aspirin treatment of the intact unstimulated RIES cells resulted in the exclusive formation of 15(R)-HETE in amounts that were slightly higher than the original 15(S)-HETE observed in the absence of aspirin, implying that significant amounts of 15(R)-HPETE had also been formed. 15(R)-HPETE should give exactly the same amount of heptanone-etheno-2′-deoxyguanosine as its 15(S)-enantiomer. However, no increase in heptanone-etheno adduct formation occurred in the aspirin-treated cells. The present study suggests a potential mechanism of tumorigenesis that involves DNA adduct formation from COX-2-mediated lipid peroxidation rather than prostaglandin formation. Therefore, inhibition of COX-2-mediated lipid hydroperoxide formation offers a potential therapeutic alternative to COX-2 inhibitors in chemoprevention strategies.