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 (promotion) 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 effects of TCDD (1.5 pM) and of the reference tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA; 0.4 mM) on carcinogen (N-methyl-N'-nitro-N-nitrosoguanidine or 3-methylcholanthrene)-pre-treated fibroblasts was abolished by cotreatment with indomethacin, hydrocortisone, caffeic acid or nordihydroguaiaretic acid. A differential inhibition was found with N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide, a selective inhibitor of the cyclooxygenase 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(1alpha), which were identified as major metabolites of arachidonic acid in C3H/M2 cell cultures, was enhanced ( approximately 2-fold) following long-term treatment with TCDD (0.15 and 1.5 pM). The results suggest that the stimulation of arachidonic acid metabolism caused by a sustained cyclooxygenase induction is a critical event in the promoting action of TCDD in mouse fibroblasts in vitro. However, in contrast to TPA, the TCDD-mediated enhancement of malignant cell transformation may not specifically depend on the induction of COX-2 but, additionally, the induction of COX-1 activity may be necessary.
Wölfle
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Department of Toxicology, University of Hamburg Medical School, D-20146 Hamburg, Germany. d.woelfle@gbvv.de
Marotzki
S
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Dartsch
D
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Schäfer
W
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Marquardt
H
H
D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism; D002471:Cell Transformation, Neoplastic / Q000139:chemically induced*; D004790:Enzyme Induction / Q000187:drug effects; D005347:Fibroblasts / Q000187:drug effects; D051379:Mice; D008809:Mice, Inbred C3H; D000072317:Polychlorinated Dibenzodioxins / Q000633:toxicity*; D011451:Prostaglandin-Endoperoxide Synthases / Q000096:biosynthesis* / Q000235:genetics; D011453:Prostaglandins / Q000096:biosynthesis; D012333:RNA, Messenger / Q000032:analysis; D013755:Tetradecanoylphorbol Acetate / Q000633:toxicity
D000072317:Polychlorinated Dibenzodioxins; D011453:Prostaglandins; D012333:RNA, Messenger; D016718:Arachidonic Acid; D011451:Prostaglandin-Endoperoxide Synthases; D013755:Tetradecanoylphorbol Acetate