PAX 8 regulates human WT1 transcription through a novel DNA binding site

The Wilms' tumor gene (WT1) is an essential gene for kidney and gonadal development, although how WT1 expression is induced in these tissues is not known. One kidney transcription factor likely to play a role in this regulation is PAX 8. The co-expression of WT1 and PAX 8 during kidney development and in Wilms' tumors with an epithelium predominant histology suggested a possible interaction, and indeed, we identified potential core PAX-binding sites in the WT1 promoter. Endogenous PAX 8 plays an important role in the activation of the WT1 promoter, since promoter activity is much stronger in cells with PAX 8 than without. Using binding assays, we searched for evidence of PAX 8-DNA interactions throughout the 652-base pair human WT1 promoter and found only one functional PAX 8 site with DNA binding activity, located 250 base pairs 5' of the minimal promoter. The responsiveness of the PAX 8 site was confirmed by assessing its ability to function as an enhancer significantly activating the minimal promoter in a position- and orientation-independent manner. Using transfection assays, we demonstrated that either endogenous or exogenously added PAX 8 activated the WT1 promoter and that this promoter up-regulation depended upon the presence of an intact PAX 8-binding site. In contrast, the previously reported core PAX 8-binding sites identified by computer analysis of the WT1 promoter failed to specifically bind in vitro translated PAX 8 protein or activate the minimal promoter. Thus, we identified a novel functional binding site for the transcription factor PAX 8, suggesting that part of its role in kidney development may be as a modulator of WT1 expression in the kidney.