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

Abstract

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 > 2,3,7,8-TCDF > 1,2,3,7,8-PeCDD ~ 1,2,3,7,8-PeCDF > 1,2,7,8-TCDF > 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 (>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.