Ribonuclease inhibits Ah receptor transformation in vitro

The aryl hydrocarbon (Ah) receptor undergoes a ligand-dependent transformation to a heteromeric structure which has the ability to bind DNA sequence-specifically with high affinity. By this mechanism, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related xenobiotics modify gene expression. We observed that transformation was inhibited in vitro by the presence of ribonuclease A (RNAase) during incubation of rat hepatic cytosol with TCDD. This effect was detected as a decreased ability of the TCDD-receptor complex to bind to calf thymus DNA covalently linked to Sepharose, and to a dioxin-responsive enhancer which is upstream of the cytochrome P450IA1 structural gene. RNAase had no effect on previously transformed TCDD-receptor complexes. These observations indicated that RNAase acted during ligand binding and/or transformation of the Ah receptor. Saturation binding analyses demonstrated that RNAase decreased the receptor affinity for TCDD without changing the total number of binding sites. RNAase also inhibited transformation of the TCDD-bound, partially purified, untransformed, receptor. Thus RNAase does not interfere with ligand binding, but inhibits the subsequent transformation of the receptor monomer to the heteromeric, transcriptionally active, form.