| Abstract: | 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aryl hydrocarbons (HAHs) are industrial chemicals or by-products of industrial and combustion processes. This class of compounds elicits a diverse spectrum of toxic and biochemical responses in animals and humans. TCDD and related compounds diffuse into target cells and bind to an intracellular cytosolic aryl hydrocarbon (Ah) receptor protein. The liganded cytosolic Ah receptor undergoes transformation and translocates into the nucleus, binds to specific genomic target sequences (i.e. dioxin responsive elements, DREs) and induces gene transcription. The nuclear Ah receptor from human and rodent cell lines was characterized in this study. Photoaffinity labeling of the Ah receptor utilizing high affinity ligands [3H]TCDD and 7-[125I]iodo-2,3-dibromodibenzo-p-dioxin was carried out and the apparent molecular weight for the nuclear Ah receptor was shown to be 110-, 100- and 95-kDa in human Hep G2, rat H-4-II E and mouse Hepa 1c1c7 hepatoma cells, respectively. Partial peptide mapping of the nuclear Ah receptor from human Hep G2 and mouse Hepa 1c1c7 cells revealed that although the human Ah receptor was more resistant to proteolysis than the mouse receptor, there were several apparent common structural features based on the SDS-PAGE of their ligand-bound proteolytic fragments (i.e. 48- and 45-kDa proteolytic fragments). The heterogeneity of the human Ah receptor was investigated by determining the molecular properties and apparent molecular masses (Mr) of the nuclear Ah receptor complexes from 7 different human cancer cell lines. The Mr values were highly variable (e.g., from 221- to 175-kDa) and the Ah-nonresponsive MDA-MB-231 breast cancer cell line expressed the lowest Mr Ah receptor complex. The results of photoaffinity labeling studies suggested that intracellular differences were due to the variability of the Ah receptor nuclear translocator (Arnt) protein. This was confirmed by studies in which transient and stable expression of the arnt gene in MDA-MB-231 cells restored Ah-responsiveness... |
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