Ectopic expression of PA2.26 antigen in epidermal keratinocytes leads to destabilization of adherens junctions and malignant progression

PA2.26 antigen is a small mucin-type transmembrane glycoprotein induced in mouse epidermal keratinocytes during carcinogenesis. It is located at plasm a membrane projections, such as microvilli and ruffles, where it interacts with the actin cytoskeleton. Previous studies revealed that ectopic express ion of PA2.26 in epidermal MCA3D keratinocytes induces cell surface extensi ons and increased motility. Here, we show that PA2.26-expressing MCA3D (3D2 .26) cell transfectants undergo a phenotypic conversion linked to the acqui sition of malignant characteristics. The 3[)2.26 cells down-regulate basal keratin K14 and up-regulate vimentin and keratin K8 expression. Immunofluor escence analysis in 3D2.26 cell cultures showed loss of cortical actin fila ments and destabilization of adherens junctions mediated by E- and P-cadher in, although both cadherin mRNAs were expressed in the transfectants. When the cadherin protein levels were analyzed in Western blots, no P-cadherin p rotein or smaller polypeptide E-cadherin forms were detected, suggesting th at E- and P-cadherin synthesized in 3D2.26 cells was unstable and proteolyt ically degraded. Transplantation of 3[)2.26 cells into athymic nude mice in duced tumors, whereas MCA3D cells and control (3DN) transfectants were not tumorigenic after 72 days postinjection. The phenotype of the tumors was un differentiated, with mixed regions exhibiting a glandular differentiation p attern in which the presence of numerous surface microvilli was observed at the ultrastructural level. Interestingly, PA2.26 antigen was highly expres sed in these microvillous cell surfaces. Tumor cells were vimentin- and K8- positive and showed an aberrant pattern of E-cadherin protein expression in which large cytoplasmic aggregates were found close to the nucleus. infilt ration of tumor cells into lymphatic vessels and the presence of frequent r egional lymph node metastases were also observed in the tumors. These resul ts indicate that expression of PA2.26 antigen in premalignant keratinocytes induces a fully transformed and metastatic phenotype, and they suggest an involvement of PA2.26 in malignant progression.