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.