Role for ICAT in beta-catenin-dependent nuclear signaling and cadherin functions.

Inhibitor of beta-catenin and TCF-4 (ICAT) is a 9-kDa polypeptide that inhibits beta-catenin nuclear signaling by binding beta-catenin and competing its interaction with the transcription factor TCF (T cell factor), but basic characterization of the endogenous protein and degree to which it alters other beta-catenin functions is less well understood. At the subcellular level, we show that ICAT localizes to both cytoplasmic and nuclear compartments. In intestinal tissue, ICAT is upregulated in the mature, nondividing enterocyte population lining intestinal villi and is absent in the beta-catenin/TCF signaling-active crypt region, suggesting that its protein levels may be inversely related with beta-catenin signaling activity. However, ICAT protein levels are not altered by activation or inhibition of Wnt signaling in cultured cells, suggesting that ICAT expression is not a direct target of the Wnt/beta-catenin pathway. In cells where beta-catenin levels are elevated by Wnt, a fraction of this beta-catenin pool is associated with ICAT, suggesting that ICAT may buffer the cell from increased levels of beta-catenin. Distinct from TCF and cadherin, ICAT does not protect the soluble pool of beta-catenin from degradation by the adenomatous polyposis coli containing "destruction complex." Although ICAT inhibits beta-catenin binding to the cadherin as well as TCF in vitro, stable overexpression of ICAT in Madin-Darby canine kidney (MDCK) epithelial cells shows no obvious alterations in the cadherin complex, suggesting that the ability of ICAT to inhibit beta-catenin binding to the cadherin may be restricted in vivo. MDCK cells overexpressing ICAT do, however, exhibit enhanced cell scattering on hepatocyte growth factor treatment, suggesting a possible role in the regulation of dynamic rather than steady-state cell-cell adhesions. These findings confirm ICAT's primary role in beta-catenin signaling inhibition and further suggest that ICAT may have consequences for cadherin-based adhesive function in certain circumstances, implying a broader role than previously described.