2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN INDUCES THE NUCLEAR TRANSLOCATIONOF 2 XRE BINDING-PROTEINS IN MICE

The administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3 -methylcholanthrene (3MC) to mice results in their binding to the liga nd binding portion of the cytosolic dioxin-(Ah)-receptor, followed by translocation of the Ah receptor complex to the nucleus where the DNA binding form of the receptor can be measured by gel retardation analys is. In this report, extended electrophoresis of the nuclear DNA bindin g proteins isolated from liver demonstrate that TCDD and 3MC induce tw o nuclear DNA binding proteins in Ah-responsive C57BL/6 mice, while on ly TCDD induces these proteins in the Ah-nonresponsive DBA/2 mice. The two TCDD inducible (TI) nuclear DNA binding proteins, identified as T I-1 and TI-2, bind specifically to the Cypla-1 gene dioxin-(Ah)-recept or enhancer sequences (XREs) concordant with the properties of the Ah receptor. TI-1 is the predominant inducible form that is present in li ver and extrahepatic tissues and most likely represents what is though t to be the Ah receptor, while TI-2 represents a minor form that is fo und only in liver. The nuclear induction of the Ah receptor by TCDD ca n be inhibited by phorbol esters such as TPA (Okino et al., 1992), but analysis of nuclear TI-I and TI-2 shows that TPA can selectively inhi bit the appearance of TI-1. The results of differential expression wit h regard to tissue and also inhibition by TPA suggests that TI-1 and T I-2 are under different modes of regulation. We are proposing that the heterogeneity observed with TI-1 and TI-2 represent two unique subtyp es of the Ah receptor, possibly resulting from a single ligand-binding subunit in association with different partner proteins.