Cyclooxygenase-2, malondialdehyde and pyrimidopurinone adducts of deoxyguanosine in human colon cells

Cyclooxygenases (COX) catalyse the oxygenation of arachidonic acid to prost aglandin (PG) endoperoxides. Activity of one of the COX isoforms, COX-2, re sults in production of prostaglandin E-2 (PGE(2)) via the endoperoxide PGH( 2). COX-2 has been implicated in the pathogenesis of colorectal cancer. Mal ondialdehyde (MDA) is a mutagen produced by spontaneous and enzymatic break down of PGH(2). MDA reacts with DNA to form adducts, predominantly the pyri midopurinone adduct of deoxyguanosine (M(1)G). Here the hypothesis was test ed that COX-2 activity in human colon cells results in formation of MDA and generation of M(1)G adducts. M(1)G was detected in basal cultures of human non-malignant colon epithelial (HCEC) and malignant SW48, SW480, HT29 and HCA-7 colon cells, at levels from 77 to 148 adducts/10(8) nucleotides. Only HCA-7 and HT29 cells expressed COX-2 protein. Levels of M(1)G correlated s ignificantly (r = 0.98, P < 0.001) with those of intracellular MDA determin ed colorimetrically in the four malignant cell types, but neither parameter correlated with expression of COX-2 or PG biosynthesis. Induction of COX-2 expression by phorbol 12-myristate 13-acetate in HCEC cells increased PGE( 2) production 20-fold and MDA concentration 3-fold. Selective inhibition of COX-2 activity in HCA-7 cells by NS-398 significantly inhibited PGE2 produ ction, but altered neither MDA nor M(1)G levels. Malondialdehyde treatment of HCEC cells resulted in a doubling of MIG levels. These results show for the first time in human colon cells that COX-2 activity is associated with formation of the endogenous mutagen, MDA. Moreover, they demonstrate the co rrelation between,MDA concentration and M(1)G adduct levels in malignant ce lls.