Abstract
Previous work has shown that the transcriptional regulator beta-catenin can translocate to the nuclei when cells are stimulated with the type 1 insulin-like growth factor (IGF-1). We show by immunocoprecipitation and by confocal microscopy that beta-catenin binds to and co-localizes with the insulin receptor substrate-1 (IRS-1), a docking protein for both the insulin and the IGF-1 receptors. IRS-1 is required for IGF-1-mediated nuclear translocation of beta-catenin, resulting in the activation of the beta-catenin target genes. IGF-1-mediated nuclear translocation of beta-catenin is facilitated by the nuclear translocation of IRS-1. Both IRS-1 and beta-catenin are recruited to the cyclin D1 promoter, an established target for beta-catenin, but only IRS-1 is recruited to the ribosomal DNA (rDNA) promoter. UBF proteins (known to interact with both IRS-1 and beta-catenin) are also detectable in the cyclin D1 and rDNA promoters. These results indicate that IRS-1 (activated by the IGF-1 receptor) is one of several proteins that regulate the subcellular localization and activity of beta-catenin. The ability of IRS-1 to localize to both RNA polymerase II (with beta-catenin) and RNA polymerase I-regulated promoters suggest an explanation for the effect of IRS-1 on both cell growth in size and cell proliferation. This possibility is supported by the demonstration that enforced nuclear localization of IRS-1 causes nuclear translocation of beta-catenin and transformation of normal mouse embryo fibroblasts (colony formation in soft agar).
Highlights
The important roles played by -catenin in adhesion, cancer, and development and its connections to Wnt and APC have been discussed in recent reviews [1,2,3]
Because insulin receptor substrate-1 (IRS-1) is known to interact with upstream binding factor 1 (UBF1), whereas -catenin has been reported to interact with UBF2 in the cyclin D1 promoter [24], we examined the possibility that IGF-1 stimulation may recruit IRS-1 and -catenin together or separately to the cyclin D1 and the ribosomal DNA (rDNA) promoters
IRS-1 and -Catenin Co-immunoprecipitate in the Nuclei and Cytoplasm of RϪ-derived Cells—Our first step was to confirm by co-immunoprecipitation the IRS-1-/-catenin interaction we originally detected by our modified TAPtag technique [10]
Summary
The important roles played by -catenin in adhesion, cancer, and development and its connections to Wnt and APC have been discussed in recent reviews [1,2,3]. IGFs are known to cause translocation of -catenin to the nuclei, where it activates the target genes (5, 19 –21). IRS-1 and -Catenin using a plasmid in which IRS-1 is expressed in fusion to a Nuclear Localization Signal (NLS) Stable expression of this plasmid in growth-regulated, contact-inhibited mouse fibroblast R12 cells (where both IRS-1 and -catenin are normally cytoplasmic) causes both proteins to co-localize to the nuclei and induces the transformation of R12 cells into cells capable of forming colonies in soft agar (the best criteria for in vitro transformation). -catenin can be translocated to the nuclei by different stimuli and pathways (see above), independently of IGF-1R signaling, these results indicate that IRS-1 can be considered one of the proteins that regulate the subcellular localization and activity of -catenin, especially in cells responsive to the mitogenic action of IGF-1
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
