β-Catenin is essential for E-cadherin–mediated cell adhesion in epithelial cells, but it also forms nuclear complexes with high mobility group transcription factors. Using a mouse mammary epithelial cell system, we have shown previously that conversion of epithelial cells to a fibroblastoid phenotype (epithelial-mesenchymal transition) involves downregulation of E-cadherin and upregulation of β-catenin transcriptional activity. Here, we demonstrate that transient expression of exogenous E-cadherin in both epithelial and fibroblastoid cells arrested cell growth or caused apoptosis, depending on the cellular E-cadherin levels. By expressing E-cadherin subdomains, we show that the growth-suppressive effect of E-cadherin required the presence of its cytoplasmic β-catenin interaction domain and/or correlated strictly with the ability to negatively interfere with β-catenin transcriptional activity. Furthermore, coexpression of β-catenin or lymphoid enhancer binding factor-1 or T cell factor 3 with E-cadherin rescued β-catenin transcriptional activity and counteracted E-cadherin–mediated cell cycle arrest. Stable expression of E-cadherin in fibroblastoid cells decreased β-catenin activity and reduced cell growth. Since proliferating cells had a higher β-catenin activity than G1 phase–arrested or contact-inhibited cells, we conclude that β-catenin transcriptional activity is essential for cell proliferation and can be controlled by E-cadherin in a cell adhesion-independent manner.

Lamina-associated polypeptide (LAP) 2 of the inner nuclear membrane (now LAP2β) and LAP2α are related proteins produced by alternative splicing, and contain a common 187 amino acid N-terminal domain. We show here that, unlike LAP2β, LAP2α behaved like a nuclear non-membrane protein in subcellular fractionation studies and was localized throughout the nuclear interior in interphase cells. It co-fractionated with LAP2β in nuclear lamina/matrix-enriched fractions upon extraction of nuclei with detergent, salt and nucleases. During metaphase LAP2α dissociated from chromosomes and became concentrated around the spindle poles. Furthermore, LAP2α was mitotically phosphorylated, and phosphorylation correlated with increased LAP2α solubility upon extraction of cells in physiological buffers. LAP2α relocated to distinct sites around chromosomes at early stages of nuclear reassembly and intermediarily co-localized with peripheral lamin B and intranuclear lamin A structures at telophase. During in vitro nuclear assembly LAP2α was dephosphorylated and assembled into insoluble chromatin-associated structures, and recombinant LAP2α was found to interact with chromosomes in vitro. Some LAP2α may also associate with membranes prior to chromatin attachment. Altogether the data suggest a role of LAP2α in post-mitotic nuclear assembly and in the dynamic structural organization of the nucleus.

Mouse mammary epithelial cells expressing a fusion protein of c-Fos and the estrogen receptor (FosER) formed highly polarized epithelial cell sheets in the absence of estradiol. β-Catenin and p120ctn were exclusively located at the lateral plasma membrane in a tight complex with the adherens junction protein, E-cadherin. Upon activation of FosER by estradiol addition, cells lost epithelial polarity within two days, giving rise to a uniform distribution of junctional proteins along the entire plasma membrane. Most of the β-catenin and p120ctn remained in a complex with E-cadherin at the membrane, but a minor fraction of uncomplexed cytoplasmic β-catenin increased significantly. The epithelial–mesenchymal cell conversion induced by prolonged estradiol treatment was accompanied by a complete loss of E-cadherin expression, a 70% reduction in β-catenin protein level, and a change in the expression pattern of p120ctn isoforms. In these mesenchymal cells, β-catenin and p120ctn were localized in the cytoplasm and in defined intranuclear structures. Furthermore, β-catenin colocalized with transcription factor LEF-1 in the nucleus, and coprecipitated with LEF-1–related proteins from cell extracts. Accordingly, β-catenin– dependent reporter activity was upregulated in mesenchymal cells and could be reduced by transient expression of exogenous E-cadherin. Thus, epithelial mesenchymal conversion in FosER cells may involve β-catenin signaling.