Physicochemical Differences in the AH Receptors of the Most TCDD-Susceptible and the Most TCDD-Resistant Rat Strains

• Published in: Toxicology and applied pharmacology 1999-02, Vol.155 (1), p.82-95
• Main Authors: Pohjanvirta, Raimo, Viluksela, Matti, Tuomisto, Jouni T., Unkila, Mikko, Karasinska, Joanna, Franc, Monique-Andrée, Holowenko, Mary, Giannone, John V., Harper, Patricia A., Tuomisto, Jouko, Okey, Allan B.
• Format: Article
• Language: English
• Subjects: Animals; Aryl Hydrocarbon Receptor Nuclear Translocator; Biological and medical sciences; Chemical and industrial products toxicology. Toxic occupational diseases; Cytochrome P-450 CYP1A1 - biosynthesis; DNA - metabolism; DNA-Binding Proteins; Drug Resistance; Female; Male; Medical sciences; Mice; Mice, Inbred C57BL; Molecular Weight; Polychlorinated Dibenzodioxins - toxicity; Rats; Rats, Long-Evans; Rats, Sprague-Dawley; Rats, Wistar; Receptors, Aryl Hydrocarbon - chemistry; Receptors, Aryl Hydrocarbon - metabolism; Sodium Chloride - pharmacology; Species Specificity; Toxicology; Transcription Factors - chemistry; Various organic compounds
• ISSN: 0041-008X; 1096-0333
• DOI: 10.1006/taap.1998.8565

Long–Evans rats (strain Turku AB; L–E) are at least 1000-fold more sensitive (LD50about 10 μg/kg) to the acute lethal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) than are Han/Wistar (Kuopio; H/W) rats (LD50> 9600 μg/kg). The AH receptor (AHR) is believed to mediate the toxic effects of TCDD and related halogenated aromatic hydrocarbons. We compared the AHRs of L–E and H/W rats to determine if there were any structural or functional receptor differences that might be related to the dramatic difference in the sensitivity of these two strains to the lethal effects of TCDD. Cytosols from liver and lung of the sensitive L–E rats contained about twofold higher levels of specific binding sites for [3H]TCDD than occurred in H/W rats; theKdfor binding of [3H]TCDD to AHR in hepatic cytosols was similar between the two strains. Addition of the oxyanions, molybdate or tungstate (20 mM), had little effect upon ligand binding to AHR in hepatic cytosols from L–E rats whereas in cytosols from H/W rats these agents substantially diminished or totally abolished TCDD binding. The AHR in H/W cytosols also lost ligand-binding function when NaCl (20 to 400 mM) was added to the buffer whereas, in cytosols from L–E rats, the addition of 400 mM NaCl caused the receptor complex to shift from 9S to 6S during velocity sedimentation but did not destroy ligand binding function. AHR from hepatic cytosol of both the L–E and H/W rats could be transformed to the DNA-binding state in the presence of TCDD or other dioxin congeners as assessed by gel mobility shift assays. The most dramatic difference in AHR properties between L–E and H/W rats is molecular mass. Immunoblotting of cytosolic proteins revealed that the AHR in L–E rats has an apparent mass of ≈106 kDa, similar to the mass of the receptor previously reported in several other common laboratory rat strains. In contrast, the mass of the AHR in H/W rats is ≈98 kDa, significantly smaller than the mass of receptor reported in any other rat strains. F1offspring of a cross between L–E and H/W rats expressed both the 106- and the 98-kDa protein. There was no apparent difference in the mass of the AHR nuclear translocator protein (ARNT) between the two strains, but the hepatic concentration of ARNT was about three times as high in L–E as in H/W rats. It will be interesting to find out how the altered structure of the AHR in H/W rats is related to their remarkable resistance to the lethal effects of TCDD.