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

Inhibitor of β-catenin and TCF-4 (ICAT) is a 9-kDa polypeptide that inhibits β-catenin nuclear signaling by binding β-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 β-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 β-catenin/TCF signaling-active crypt region, suggesting that its protein levels may be inversely related with β-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/β-catenin pathway. In cells where β-catenin levels are elevated by Wnt, a fraction of this β-catenin pool is associated with ICAT, suggesting that ICAT may buffer the cell from increased levels of β-catenin. Distinct from TCF and cadherin, ICAT does not protect the soluble pool of β-catenin from degradation by the adenomatous polyposis coli containing “destruction complex.” Although ICAT inhibits β-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 β-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 β-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.

δ-catenin, an Adhesive Junction–associated Protein Which Promotes Cell Scattering

The classical adherens junction that holds epithelial cells together consists of a protein complex in which members of the cadherin family linked to various catenins are the principal components. δ-catenin is a mammalian brain protein in the Armadillo repeat superfamily with sequence similarity to the adherens junction protein p120ctn. We found that δ-catenin can be immunoprecipitated as a complex with other components of the adherens junction, including cadherin and β-catenin, from transfected cells and brain. The interaction with cadherin involves direct contact within the highly conserved juxtamembrane region of the COOH terminus, where p120ctn also binds. In developing mouse brain, staining with δ-catenin antibodies is prominent towards the apical boundary of the neuroepithelial cells in the ventricular zone. When transfected into Madin-Darby canine kidney (MDCK) epithelial cells δ-catenin colocalized with cadherin, p120ctn, and β-catenin. The Arm domain alone was sufficient for achieving localization and coimmunoprecipitation with cadherin. The ectopic expression of δ-catenin in MDCK cells altered their morphology, induced the elaboration of lamellipodia, interfered with monolayer formation, and increased scattering in response to hepatocyte growth factor treatment. We propose that δ-catenin can regulate adhesion molecules to implement the organization of large cellular arrays necessary for tissue morphogenesis.