The breast cancer β4 integrin and endothelial human CLCA2 mediate lung metastasis

Adhesion of blood-borne cancer cells to the endothelium is a critical determinant of organ-specific metastasis. Here we show that colonization of the lungs by human breast cancer cells is correlated with cell surface expression of the alpha (6)beta (4) integrin and adhesion to human CLCA2 (hCLCA2), a Ca2+-sensitive chloride channel protein that is expressed on the endothelial cell luminal surface of pulmonary arteries, arterioles, and venules, Tumor cell adhesion to endothelial hCLCA2 is mediated by the beta (4) integrin, establishing for the first time a cell-cell adhesion property for this integrin that involves an entirely new adhesion partner. This adhesion is augmented by an increased surface expression of the alpha (6)beta (4) integrin in breast cancer cells selected in vivo for enhanced lung colonization but abolished by the specific cleavage of the beta (4) integrin with matrilysin. beta (4) integrin/hCLCA2 adhesion-blocking antibodies directed against either of the two interacting adhesion molecules inhibit lung colonization, while overexpression of the beta (4) integrin in a model murine tumor cell line of modest lung colonization potential significantly increases the lung metastatic performance. Our data clearly show that the beta (4)/hCLCA2 adhesion is critical for lung metastasis, yet expression of the beta (4) integrin in many benign breast tumors shows that this integrin is insufficient to bestow metastatic competence on cells that lack invasiveness and other established properties of metastatic cells.