Regulation of insulin-like growth factor I receptor promoter activity by wild-type and mutant versions of the WT1 tumor suppressor

The insulin-like growth factor I (IGF-I) receptor is a transmembrane tyrosine kinase that mediates the growth-promoting effects of IGF-I and IGF-II. Changes in IGF-I receptor messenger RNA levels are reflected in cell surface receptor number, and modulation of IGF-I receptor levels affects tumorigenicity in numerous cellular models; thus, control of IGF-I receptor gene expression appears to be an important level at which cellular proliferation and tumorigenic potential may be regulated. We have previously shown that the product of the WT1 Wilms' tumor suppressor gene represses IGF-I receptor gene expression both in vitro and in vivo, and that decreased WT1 levels are correlated with up-regulation of IGF-I receptor gene expression in Wilms' tumor, benign prostatic hyperplasia, and breast cancer. Gene regulation by WT1 is complex, in that the WT1 gene encodes a variety of products as a result of alternative splicing and RNA editing, and a number of missense point mutations have been characterized in Wilms' tumor-associated syndromes. Additionally, the WT1 protein has been demonstrated to self-associate through its N-terminal domain, although the role of this intermolecular interaction in transcriptional regulation by WT1 is unclear. In this report, we analyze the relative activity of wild-type and mutant versions of the WT1 protein with respect to IGF-I receptor promoter activity in transient transfection assays and assess the potential contribution of WT1 self-association to IGF-I receptor regulation using the yeast two-hybrid system. Of the naturally occurring variations in WT1 structure, only the presence of a three-amino acid KTS insert in the zinc finger domain introduced by alternative splicing of exon 9 had a significant effect on WT1 repression of IGF-I receptor promoter activity. The N- and C-terminal domains of WT1 also exhibited partial repression, as did the most common mutant version of the WT1 protein associated with Denys-Drash syndrome. Mutations in the WT1 N-terminus attenuated WT1 self-association in the yeast two-hybrid system, but did not impair transcriptional repression. Our results suggest that 1) the DNA-binding capacity of WT1 is critical for maximal repression of the IGF-I receptor promoter, but some effects may be mediated through protein-protein interactions involving the N-terminal domain; 2) WT1 self-association may not be required for repression of the IGF-I receptor promoter; and 3) the Denys-Drash syndrome version of the WT1 protein may exhibit residual or possible gain of function activity in some contexts rather than exerting dominant negative effects, as has been proposed previously.