TISSUE-SPECIFIC EXPRESSION OF THE RAT AH-RECEPTOR AND ARNT MESSENGER-RNAS

The Ah-receptor (AHR) is a ligand activated transcription factor that mediates the biological effects of agonists such as 2,3,7,8-tetrachlor odibenzo-p-dioxin. Upon binding agonists, the AHR dimerizes with a str ucturally related protein known as ARNT and this heterodimer then bind s cognate enhancer elements and activates the expression of target gen es. In this report we describe the cloning of the rat AHR cDNA and a f ragment of the rat ARNT cDNA for use as probes in ribonuclease protect ion analysis. Ribonuclease protection analysis indicated that the rat AHR mRNA is expressed at the highest levels in the lung > thymus > kid ney > liver while lower levels were expressed in heart and spleen. The rat AHR and ARNT mRNAs were expressed in a largely coordinate manner across the eight tissues examined with the exception of the placenta w here AHR levels were relatively low compared to ARNT. In these experim ents, a rare splice variant of the AHR was cloned that encoded a prote in with a deletion in the ligand binding domain. In vitro expression s tudies demonstrated that in contrast to the full length AHR, the splic e variant did not bind ligand nor did it bind to a cognate enhancer el ement in the presence of ARNT.