In vitro studies of PCDD and PCDF action

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and related compounds elicit a broad range of biochemical and toxic responses. One aspect of this dissertation was to assess the utility of an in vitro induction bioassay to accurately estimate the toxicity of a complex environmental mixture of polychlorinated dibenzo-p-dioxin (PCDDs) and polychlorinated dibenzofurans (PCDFs). The bioassay analysis of Great Lakes fish extracts and extracts from pyrolysed brominated flame retardants gave results comparable to those obtained by gas chromatography-mass spectrometry (GC-MS) chemical analysis and in vivo toxicity studies, thus validating the utility of this bioassay for assessing the potential toxicity of complex environmental mixtures. The in vitro induction of aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) activity is a receptor-mediated response analogous to the regulation of genes by steroid hormones and their receptors. This structure-dependent response appears to require ligand activation of Ah receptors and is not the result of differences in: 1) molecular properties, 2) capability to initiate transcription and/or 3) rates of processing of nuclear Ah receptor complexes. Like the steroid hormone receptors, the accumulation of nuclear Ah receptor complexes is enhanced in a dose-, time-, structure- and cell-dependent manner by DNA intercalators and protein synthesis inhibitors. These studies revealed a high level of nuclear Ah receptors and suggest that these effects were the result of both stabilization of receptor complex-DNA interactions and inhibition of protein synthesis. The induction of AHH and EROD activity by 2,3,7,8-TCDD can also be antagonized by 6-methyl-l,3,8- trichlorodibenzofuran (MCDF) and its iodinated analog 6-methyl-8-iodo-l,3- dichlorodibenzofuran in a dose- and time-dependent manner. Antagonism by MCDF is not the result of competitive inhibition but appears to to be a post-transcriptionally regulated effect. Finally, the effects 2,3,7,8-TCDD and related compounds on the downregulation of nuclear estrogen receptor (ER) levels was investigated...