Induction of cyclooxygenase expression and enhancement of malignant cell transformation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The potential role of arachidonic acid metabolism in the enhancement (promo tion) of malignant transformation of C3H/M2 mouse fibroblasts by the tumor promoter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was investigated using inhibitors of cyclooxygenase and lipoxygenase activities. The promoting eff ects of TCDD (1.5 pM) and of the reference tumor promoter 12-O-tetradecanoy lphorbol-13-acetate (TPA; 0.4 mM) on carcinogen (N-methyl-N'-nitro-N-nitros oguanidine or 3-methylcholanthrene)-pre-treated fibroblasts was abolished b y cotreatment with indomethacin, hydrocortisone, caffeic acid or nordihydro guaiaretic acid, A differential inhibition was found with N-(2-cyclohexylox y-4-nitrophenyl)methanesulfonamide a selective inhibitor of the cyclooxygen ase isoenzyme COX-2: the promoting effect of TPA, but not that of TCDD, was abolished, Therefore, the role of the cyclooxygenase isoenzymes COX-1 and COX-2 during chronic exposure to TCDD was studied in more detail. Long-term treatment with TCDD (4-7 weeks) induced the expression of COX-1 and COX-2 mRNA in C3H/M2 fibroblasts (up to 2-fold). The enhanced expression of COX-2 protein in TCDD-treated fibroblasts was confirmed by western blot analysis . Concomitantly, the accumulation of the prostaglandins (PGs) PGE(2) and 6- keto-PGF(1 alpha), which were identified as major metabolites of arachidoni c acid in C3H/M2 cell cultures, was enhanced (similar to 2-fold) following long-term treatment with TCDD (0.15 and 1.5 pM), The results suggest that t he stimulation of arachidonic acid metabolism caused by a sustained cycloox ygenase induction is a critical event in the promoting action of TCDD in mo use fibroblasts in vitro. However, in contrast to TPA, the TCDD-mediated en hancement of malignant cell transformation may not specifically depend on t he induction of COX-2 but, additionally, the induction of COX-1 activity ma y be necessary.