Plakoglobin induces desmosome formation and epidermoid phenotype in N-cadherin-expressing squamous carcinoma cells deficient in plakoglobin and E-cadherin.

Pg is a homologue of beta-catenin and Armadillo, the product of the Drosophila segment polarity gene and has been shown to have both adhesive and signaling functions. It interacts with both classic and desmosomal cadherins. Pg interaction with the desmosomal cadherins is essential for desmosome assembly. Its precise role in the classic cadherin complexes is unclear, although Pg-E-cadherin interaction appears to be necessary for the formation of desmosomes. In addition to cadherins in adhesion complexes, Pg interacts with a number of proteins involved in regulation of cell differentiation and proliferation such as Lef-1/Tcf-1 transcription factors and the tumor suppressor protein APC. In this study, we have introduced Pg cDNA into SCC9 cells, a Pg- and E-cadherin-deficient squamous cell carcinoma line, which also lacks desmosomes. These cells have both alpha-catenin and beta-catenin, display unusual expression of N-cadherin, and have the typical fibroblastic phenotype of transformed cells. Pg-expressing SCC9 cells (SCC9P) formed desmosomes. Desmosome formation coincided with the appearance of an epidermoid phenotype, with increased adhesiveness and a contact-dependent decrease in growth. Biochemical characterization of SCC9P cells showed an increase in the expression and stability of N-cadherin and a decrease in level and stability of beta-catenin, without any apparent effects on alpha-catenin. These results show that, in the absence of E-cadherin, Pg can efficiently use N-cadherin to induce desmosome formation and epidermoid phenotype. They also suggest a role for Pg as one of the regulators of the intracellular beta-catenin levels and underscore the pivotal role of this protein in regulating cell adhesion and differentiation.