CLONING AND CHARACTERIZATION OF CDNAS ENCODING MOUSE UGT1.6 AND RABBIT UGT1.6 - DIFFERENTIAL INDUCTION BY 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN

In this report, cDNAs for mouse liver Ugt1.6 and rabbit liver UGT1.6 have been cloned and characterized. The predicted amino acid sequence of mouse Ugt1.6 is 93% and 78% similar to the rat and human UGT1.6 sequences, respectively, while the rabbit UGT1.6 is 79% and 83% similar to the rat and human UGT1.6 sequences, respectively. To examine the substrate specificities of the proteins encoded by the mouse Ugt1.6 and rabbit UGT1.6 cDNAs, the recombinants were expressed in monkey kidney COS-1 cells. Transfection of the mouse and rabbit recombinants allowed for the expression of the UGT1.6 proteins as determined by immunoprecipitation of newly synthesized protein. The expressed UGTs conjugated small planar phenolic molecules such as 4-nitrophenol, 1-naphthol, and 4-methylumbelliferone. While the bulky phenol 4-hydroxybiphenyl was not a substrate for the enzymes, 2-hydroxybiphenyl was an excellent substrate. Androgens and estrogens were not conjugated by either mouse Ugt1.6 or rabbit UGT1.6. In rodents, UGT1.6 mRNA is expressed constitutively and induced when the animals are treated with the Ah receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Using wild-type mouse hepatoma cells and the Ah receptor deficient class II cells, it was demonstrated that induction of mouse Ugt1.6 was dependent upon a functional Ah receptor complex. However, when New Zealand white rabbits were treated with TCDD and liver mRNA was examined by Northern blot analysis, it was shown that TCDD had no effect on the induction of UGT1.6 mRNA. These results indicate that the Ah receptor is involved in the induction of Ugt1.6 mRNA by TCDD in rodents, but there exist species differences related to the induction and expression of the UGT1*6 gene by TCDD.