Role of α₅β₁ integrin in determining malignant properties of colon carcinoma cells

We characterized the expression of α₅β₁ integrin in two distinct phenotypes of colon carcinoma cell lines. Highly invasive colon cell lines (designated Group I cell lines) expressed higher levels of integrin α₅β₁ mRNA and protein than did poorly invasive colon cell lines (designated Group III cell lines). The relatively high expression of integrin α₅β₁ in Group I cell lines resulted in strong enhancement of cell adhesion to fibronectin (FN) tissue culture plates, whereas Group III cell lines showed little or no enhancement of cell adhesion by coating. There was no significant difference between Group I and Group III cell lines with respect to cell adhesion to laminin and collagen IV. Cell adhesion to FN in Group I cells was mainly mediated by integrin α₅β₁ because a monoclonal antiα₅ subunit antibody could block cell adhesion to FN, whereas anti-α₂ and anti-α₃ antibodies had no effect on cell adhesion to FN. The divergence of α₅β₁ expression in these two distinct colon carcinoma phenotypes suggested that high expression of α₅β₁ might contribute to malignant progression in this model system. To test this hypothesis, GEO cells, a Group III cell line that did not express α₅ integrin, were transfected with the α₅ subunit. Stable transfection of α₅ sense cDNA into a typical GEO-limiting dilution clone led to the expression of α₅ subunit mRNA and cell surface α₅β₁ protein. The α₅ sense transfectants showed enhanced attachment to FN-coated plates and were more tumorigenic when the cells were injected into athymicnude mice. These results indicate that inappropriately high α₅β₁ integrin expression contributes to malignant progression in colon carcinoma.