Structure and expression of the Ah receptor repressor gene

The aryl hydrocarbon receptor (AhR) repressor (AhRR) gene has been isolated and characterized from a mouse genomic library. The gene is distributed as 11 exons in a total length of about 60 kilobase pairs. Fluorescence in sit u hybridization analysis has shown that the AhRR gene is located at mouse c hromosome 13C2, at rat chromosome 1p11.2, and at human chromosome 5p15.3. T he AhRR gene has a TATA-less promoter and several transcription start sites . In addition, putative regulatory DNA sequences such as xenobiotic respons ive element (XRE), GC box, and NF-kappaB-binding sites have been identified in the 5'-upstream region of the AhRR gene. Transient transfection analyse s of HeLa cells with reporter genes that contain deletions and point mutati ons in the AhRR promoter revealed that all three XREs mediated the inducibl e expression of the AhRR gene by 3-methylcholanthrene treatment, and furthe rmore, GC box sequences were indispensable for a high level of inducible ex pression and for constitutive expression. Moreover, by using gel mobility s hift assays we were able to show that the AhR/Arnt heterodimer binds to the XREs with very low affinity, which is due to three varied nucleotides outs ide the XRE core sequence. We have also shown that Sp1 and Sp3 can bind to the GC boxes. Finally, both transient transfection analysis and gel mobilit y shift assay revealed that the AhRR gene is up-regulated by a p65/p50 hete rodimer that binds to the NF-kappaB site when the cells has been exposed to 12-O-tetradecanoylphorbol-13-acetate, and this inducible expression was fu rther enhanced by cotreatment of 12-O-tetradecanoylphorbol-13-acetate and 3 -methylcholanthrene.