Physicochemical differences in the AH receptors of the most TCDD-susceptible and the most TCDD-resistant rat strains

Long-Evans rats (strain Turku AB; L-E) are at least 1000-fold more sensitiv e (LD50 about 10 mu g/kg) to the acute lethal effects of 2,3,7,8-tetrachlor odibenzo-p-dioxin (TCDD) than are Han/Wistar (Kuopio; H/W) rats (LD50 > 960 0 mu g/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 WW rats to determine if there were any structural or functional receptor differences that might be related to the dramatic difference in t he 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 high er levels of specific binding sites for [H-3]TCDD than occurred in H/W rats ; the K-d for binding of [H-3]TCDD to AHR in hepatic cytosols was similar b etween the two strains. Addition of the oxyanions, molybdate or tungstate ( 20 mM), had little effect upon ligand binding to AHR in hepatic cytosols fr om GE rats whereas in cytosols from WW rats these agents substantially dimi nished 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 w hereas, in cytosols from GE rats, the addition of 400 mM NaCl caused the re ceptor 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 WW rats could be transformed to the DNA-binding state in the presen ce of TCDD or other dioxin congeners as assessed by gel mobility shift assa ys. The most dramatic difference in AHR properties between L-E and WW rats is molecular mass. Immunoblotting of cytosolic proteins revealed that the A HR in GE rats has an apparent mass of approximate to 106 kDa, similar to th e mass of the receptor previously reported in several other common laborato ry rat strains. In contrast, the mass of the AHR in H/W rats is approximate to 98 kDa, significantly smaller than the mass of receptor reported in any other rat strains. F-1 offspring of a cross between L-E and H/W rats expre ssed 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 hi gh in L-E as in WW 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. (C) 1999 Academic Press.