Lipid hydroperoxide-derived DNA-adducts in colon polyps

Abstract

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Numerous studies have demonstrated that cyclooxygenases-2 [COX-2] is upregulated in colon polyps as compared with unaffected tissue; however, the role of COX-2 in polyp formation is still very controversial. The ability of NSAIDs to inhibit polyp formation suggests that inhibition of eicosanoid production may be responsible for this effect. Eicosanoids modulate various steps involved in the progression of carcinogenesis such as apoptosis and angiogenesis. However, eicosanoids are also produced by the COX-1, which is present in high abundance in both affected and unaffected tissue. Therefore, we investigated a pathway independent of eicosanoid formation, i.e., the formation of genotoxic bifunctional electrophiles as a result of COX-mediated lipid hydroperoxide formation. In settings of oxidative stress, where reducing pathways are compromised, 15(S)-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acid [15(S)-HPETE], the major lipid hydroperoxide produced from COX when arachidonic acid is the substrate, may survive long enough to induce DNA damage. We have shown that 15(S)-HPETE undergoes homolytic decomposition to three DNA-reactive bifunctional electrophiles, 4-hydroperoxy-2(E)-nonenal 4-oxo-2(E)-nonenal, and 4,5-epoxy-2(E)-decenal. We have demonstrated that 4-hydroperoxy-2(E)-nonenal formed unsubstituted etheno-adducts and heptanone-etheno -adducts with double stranded DNA; 4,5-epoxy-2(E)-decenal formed only unsubstituted etheno-adducts; and 4-oxo-2(E)-nonenal formed only heptanone-etheno-adducts. Unsubstituted etheno-adducts are known to be highly mutagenic in mammalian cells and have been detected in human DNA. Rat intestinal epithelial cells transfected with COX-2 formed heptanone-etheno-adducts. DNA-adduct formation, as well as 15(S)-HETE biosynthesis, could be inhibited with a specific COX-2 inhibitor. We now report that colon polyps and their surrounding mucosa from humans with colorectal cancer contain the substituted heptanone-etheno-adducts. Also, tissue from the small intestine of Min mice contain increased levels of both the substituted and un-substituted adducts as opposed to wild type or COX-2 knockout mice. These studies provide evidence for a link between polyp formation and COX up-regulation through a pathway that does not involve the biosynthesis of eicosanoids. Supported by NIH grant RO1 CA 91016