An inhibitory factor in rat thymus which interferes with binding of cytosol Ah receptor to xenobiotic responsive...

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to induce aryl hydrocarbon hydroxylase in several tissues in the rat. This effect of TCDD is mediated by the Ah receptor. However, TCDD does not significantly enhance aryl hydrocarbon hydroxylase activity in the rat thymus. Analysis of 3H-TCDD bound Ah receptor on sucrose gradients revealed the presence of radioactivity sedimenting at 8-9S and 5-6S in the cytosol and nuclei of thymus respectively. Incubation of rat thymic cytosol with TCDD increased protein kinase C dependent phosphorylation, which reached saturation levels at high concentrations of TCDD, but the TCDD bound receptor did not appreciably retard 32P-labeled XRE-3 as determined by electrophoretic mobility shift assay. Incubation of rat hepatic cytosol with TCDD also increased protein kinase C dependent phosphorylation in a concentration dependent manner as well as retarded the mobility of 32P-labeled XRE far more than that observed with thymic cytosol. The ability of the Ah receptor to bind to XRE was temperature dependent. Incubation of thymic cytosol either with okadaic acid or with sodium fluoride did not increase the binding of the Ah receptor to XRE. On mixing hepatic cytosol with thymic cytosol prior to the electrophoretic mobility shift assay, the binding of the hepatic Ah receptor to XRE was reduced in a concentration-dependent manner. A similar effect was observed when renal cytosol was used instead of hepatic cytosol. Thymic cytosol also reduced the binding of the hepatic glucocorticoid receptor to its cognate responsive element. Thymic cytosol did not alter the topology/integrity of XRE. These data suggest that the thymus contains a factor(s) which interferes with the binding of AhR to its cognate responsive element.