Characterization of 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated decreases in dexamethasone binding to rat hepatic cytosolic glucocorticoid receptor.

An investigation of the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the liver cytosolic glucocorticoid receptor (GRc) in intact and adrenalectomized (ADX) rats, using equilibrium binding analysis, sucrose gradient sedimentation, and affinity labeling experiments, clearly demonstrated that TCDD significantly reduced the binding capacity (Bmax) of the hepatic GRc but did not alter the apparent equilibrium dissociation constant (Kd). This effect was maximal after 24 hr and was still present 22 days after treatment. Western blot analysis revealed that TCDD treatment did not cause a comparable decrease in the levels of immunodetectable receptor protein, which suggests that the steroid-binding properties of the hepatic GRc are altered, rather than the absolute concentration of receptor protein. Studies of TCDD effects on the uptake of GRc by nuclei indicated that TCDD treatment did not alter the ability of the steroid-GRc complex to be taken up by nuclei; however, TCDD treatment did increase the total capacity of liver nuclei to bind steroid-GRc complexes. TCDD dose-response studies that compared the hepatic GRc steroid binding of ADX and intact rats indicated that adrenalectomy markedly enhanced the response to TCDD. Significant effects on the GRc binding in ADX animals were induced at TCDD doses that were 10,000 times lower than those required for a response in intact rats. Analysis of two other biochemical markers demonstrated that ADX rats were 10-fold more sensitive to the induction of microsomal benzo[a]pyrene hydroxylase but of similar sensitivity to reduction of epidermal growth factor receptor binding, when compared with the responses of intact animals. These data indicate that adrenal status may be important in modulating the responses of the animals to TCDD and that the alteration of the hepatic GRc pathway may have a role in some of the actions of TCDD.