ABSTRACT␤-Catenin plays a dual role in the cell: one in linking the cytoplasmic side of cadherin-mediated cell-cell contacts to the actin cytoskeleton and an additional role in signaling that involves transactivation in complex with transcription factors of the lymphoid enhancing factor (LEF-1) family. Elevated ␤-catenin levels in colorectal cancer caused by mutations in ␤-catenin or by the adenomatous polyposis coli molecule, which regulates ␤-catenin degradation, result in the binding of ␤-catenin to LEF-1 and increased transcriptional activation of mostly unknown target genes. Here, we show that the cyclin D1 gene is a direct target for transactivation by the ␤-catenin͞LEF-1 pathway through a LEF-1 binding site in the cyclin D1 promoter. Inhibitors of ␤-catenin activation, wild-type adenomatous polyposis coli, axin, and the cytoplasmic tail of cadherin suppressed cyclin D1 promoter activity in colon cancer cells. Cyclin D1 protein levels were induced by ␤-catenin overexpression and reduced in cells overexpressing the cadherin cytoplasmic domain. Increased ␤-catenin levels may thus promote neoplastic conversion by triggering cyclin D1 gene expression and, consequently, uncontrolled progression into the cell cycle.

Transcriptional repression of E-cadherin, characteristic of epithelial to mesenchymal transition, is often found also during tumor cell invasion. At metastases, migratory fibroblasts sometimes revert to an epithelial phenotype, by a process involving regulation of the E-cadherin–β-catenin complex. We investigated the molecular basis of this regulation, using human colon cancer cells with aberrantly activated β-catenin signaling. Sparse cultures mimicked invasive tumor cells, displaying low levels of E-cadherin due to transcriptional repression of E-cadherin by Slug. Slug was induced by β-catenin signaling and, independently, by ERK. Dense cultures resembled a differentiated epithelium with high levels of E-cadherin and β-catenin in adherens junctions. In such cells, β-catenin signaling, ErbB-1/2 levels, and ERK activation were reduced and Slug was undetectable. Disruption of E-cadherin–mediated contacts resulted in nuclear localization and signaling by β-catenin, induction of Slug and inhibition of E-cadherin transcription, without changes in ErbB-1/2 and ERK activation. This autoregulation of E-cadherin by cell–cell adhesion involving Slug, β-catenin and ERK could be important in tumorigenesis.

β-Catenin and plakoglobin are homologous proteins that function in cell adhesion by linking cadherins to the cytoskeleton and in signaling by transactivation together with lymphoid-enhancing binding/T cell (LEF/TCF) transcription factors. Here we compared the nuclear translocation and transactivation abilities of β-catenin and plakoglobin in mammalian cells. Overexpression of each of the two proteins in MDCK cells resulted in nuclear translocation and formation of nuclear aggregates. The β-catenin-containing nuclear structures also contained LEF-1 and vinculin, while plakoglobin was inefficient in recruiting these molecules, suggesting that its interaction with LEF-1 and vinculin is significantly weaker. Moreover, transfection of LEF-1 translocated endogenous β-catenin, but not plakoglobin to the nucleus. Chimeras consisting of Gal4 DNA-binding domain and the transactivation domains of either plakoglobin or β-catenin were equally potent in transactivating a Gal4-responsive reporter, whereas activation of LEF-1– responsive transcription was significantly higher with β-catenin. Overexpression of wild-type plakoglobin or mutant β-catenin lacking the transactivation domain induced accumulation of the endogenous β-catenin in the nucleus and LEF-1–responsive transactivation. It is further shown that the constitutive β-catenin–dependent transactivation in SW480 colon carcinoma cells and its nuclear localization can be inhibited by overexpressing N-cadherin or α-catenin. The results indicate that (a) plakoglobin and β-catenin differ in their nuclear translocation and complexing with LEF-1 and vinculin; (b) LEF-1–dependent transactivation is preferentially driven by β-catenin; and (c) the cytoplasmic partners of β-catenin, cadherin and α-catenin, can sequester it to the cytoplasm and inhibit its transcriptional activity.

We studied the effect of N-cadherin, and its free or membrane-anchored cytoplasmic domain, on the level and localization of ␤-catenin and on its ability to induce lymphocyte enhancer-binding factor 1 (LEF-1)-responsive transactivation. These cadherin derivatives formed complexes with ␤-catenin and protected it from degradation. N-cadherin directed ␤-catenin into adherens junctions, and the chimeric protein induced diffuse distribution of ␤-catenin along the membrane whereas the cytoplasmic domain of N-cadherin colocalized with ␤-catenin in the nucleus. Cotransfection of ␤-catenin and LEF-1 into Chinese hamster ovary cells induced transactivation of a LEF-1 reporter, which was blocked by the N-cadherin-derived molecules. Expression of N-cadherin and an interleukin 2 receptor͞cadherin chimera in SW480 cells relocated ␤-catenin from the nucleus to the plasma membrane and reduced transactivation. The cytoplasmic tails of N-or E-cadherin colocalized with ␤-catenin in the nucleus, and suppressed the constitutive LEF-1-mediated transactivation, by blocking ␤-catenin-LEF-1 interaction. Moreover, the 72 C-terminal amino acids of N-cadherin stabilized ␤-catenin and reduced its transactivation potential. These results indicate that ␤-catenin binding to the cadherin cytoplasmic tail either in the membrane, or in the nucleus, can inhibit ␤-catenin degradation and efficiently block its transactivation capacity.