Structural and functional analyses of a novel Ig-like cell adhesion molecule, hepaCAM, in the human breast carcinoma MCF7 cells

We have recently identified a novel gene, hepaCAM, in liver that encodes a cell adhesion molecule of the immunoglobulin superfamily. In this study, we examined the characteristics of hepaCAM protein and the relationship between its structure and function, in particular its adhesive properties. The wild-type and the cytoplasmic domain-truncated mutants of hepaCAM were transfected into the human breast carcinoma MCF7 cells, and the physiological and biological properties were assessed. Biochemical analyses revealed that hepaCAM is an N-linked glycoprotein phosphorylated in the cytoplasmic domain and that it forms homodimers through cis-interaction on the cell surface. The subcellular localization of hepaCAM appears density-dependent; in well spread cells, hepaCAM is distributed to cell protrusions, whereas in confluent cells, hepaCAM is predominantly accumulated at the sites of cell-cell contacts on the cell membrane. In polarized cells, hepaCAM is recruited to the lateral and basal membranes, and lacking physical interaction, hepaCAM is shown to co-localize with E-cadherin at the lateral membrane. Cell adhesion and motility assays demonstrated that hepaCAM increased cell spreading on the matrices fibronectin and matrigel, delayed cell detachment, and enhanced wound healing. Furthermore, when the cytoplasmic domain was deleted, hepaCAM mutants did not affect cell surface localization and dimer formation. Cell-matrix adhesion, however, was less significantly increased, and cell motility was almost unchanged when compared with the effect of the wild-type hepaCAM. Taken together, the cytoplasmic domain of hepaCAM is essential to its function on cell-matrix interaction and cell motility.