Induction of cytochrome P450-dependent monooxygenase activities in rat hepatoma H-4-IIE cells in culture by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds: mechanistic studies using radiolabeled congeners.
Treatment of rat hepatoma H-4-IIE cells in culture with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF), 1,2,7,8-TCDF, and 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) resulted in the structure-dependent induction of aryl hydrocarbon hydroxylase and ethoxyresorufin O-deethylase activities. The induction potencies followed the order 2,3,7,8-TCDD greater than 2,3,7,8-TCDF greater than 1,2,3,7,8-PeCDD approximately 1,2,3,7,8-PeCDF greater than 1,2,7,8-TCDF greater than 2,3,7-TrCDD and were comparable to structure-toxicity relationships which have previously been reported. In contrast, many of the properties of these compounds were structure-independent. For example, using tritiated congeners of high specific activity (greater than 30 Ci/mmol) the sedimentation coefficients (S) for the nuclear and cytosolic aryl hydrocarbon (Ah) receptor complexes were 5-6 and 9-10 S, respectively, for all the radioligands. Moreover, examination of the processing of nuclear Ah receptor complexes for the radiolabeled congeners showed that after 6 h, the rates of nuclear processing were very low and varied between 0.006 and 0.0385 fmol degraded/mg protein/mg total DNA. These results were consistent with the reported stability and persistence of the nuclear Ah receptor complexes and in addition, there were no apparent structure-dependent differences in the processing rates. Inspection of the nuclear receptor levels and the corresponding induced enzyme activities for the congeners showed that there was a linear correlation between average nuclear receptor complex levels (18-42 h) and induced enzyme activities (32-42 h) for all six radioligands; these data indicated that the rates of cytochrome P450-dependent gene expression correlated with the levels of nuclear Ah receptor complex. In contrast, the accumulation of occupied nuclear receptor complexes in rat hepatoma H-4-IIE cells was structure-dependent and appeared to be one of the factors which governed the observed structure-induction and the previously reported structure-toxicity relationships for 2,3,7,8-TCDD and related halogenated aryl hydrocarbons.
Zacharewski
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Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine, Texas A&M University, College Station 77843-4466.
Harris
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Safe
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D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000096:biosynthesis; D002467:Cell Nucleus / Q000378:metabolism; D019363:Cytochrome P-450 CYP1A1; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis*; D003600:Cytosol / Q000378:metabolism; D004147:Dioxins / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004790:Enzyme Induction / Q000187:drug effects; D005786:Gene Expression Regulation / Q000187:drug effects; D008114:Liver Neoplasms, Experimental / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000096:biosynthesis*; D010088:Oxidoreductases / Q000096:biosynthesis; D000072317:Polychlorinated Dibenzodioxins / Q000031:analogs & derivatives / Q000494:pharmacology*; D051381:Rats; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug / Q000378:metabolism; D013329:Structure-Activity Relationship
D004147:Dioxins; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D010088:Oxidoreductases; D001189:Aryl Hydrocarbon Hydroxylases; D019363:Cytochrome P-450 CYP1A1