Synthesis of junctional proteins in metastasizing colon cancer cells

Various authors have reported reduced synthesis of epithelial junctional proteins during dedifferentiation, tumorigenesis and metastasis in a great variety of tumors. Consequently, it is generally accepted that loss of adhesive molecules and adhesion structures is implicated in the development of an invasive phenotype and poor patient prognosis. Colon carcinomas, on the other hand, were shown to behave differently as synthesis of main adhesive proteins continues despite the development of an invasive phenotype. In this study we used cultured cells grown under conditions that inhibited intercellular adhesion (low Ca2+ concentration) and compared these results with data obtained from metastasizing colon cancer cells (signet ring cell carcinoma). Characterization of these proteins and their structures were performed by immunoprecipitations, Western blot analysis, immunohistochemistry, pre-embedding immuno-electron microscopy, and a new method to perform immuno-electron microscopy on paraffin-embedded material, which we present in this paper. We demonstrate that synthesis carries on for both, the desmosomal and the proteins of the zonula adhaerens. While, however, the assembly of desmosomal structures in the form of half-desmosomes at the cell surface continues, those of the zonula adhaerens did not. Instead E-cadherin was found, although associated with alpha-catenin, beta-catenin, and plakoglobin, evenly distributed at the plasma membrane of the cultured cells and also at the surface of the dissociated tumor cells. We conclude from our observations that continued expression and synthesis of junctional proteins do not necessarily contribute to the suppression of tumor invasion and metastasis of colon cancer.