A Defect in Cell–to–cell Adhesion via Integrin–Fibronectin Interactions in a Highly Metastatic Tumor Cell Line

We investigated the role of integrin–fibronectin (FN) interactions in tumor cell adhesion. Two cloned tumor cell lines designated OV–LM (low–metastatic) and OV–HM (high–metastatic) were isolated from a murine ovarian carcinoma, OV2944. OV–LM and OV–HM cells exhibited high and low RGDS–sequence–dependent adhesiveness to FN, respectively. Both lines expressed comparable levels of α5 and αv integrins, which are capable of reacting with RGDS on FN. To compare the functions of these integrins between the two tumor lines, the signaling mechanism following FN stimulation was examined. Significant levels of phosphorylation of focal adhesion kinase(FAK)were detected in bothOV–LM and OV–HM cells before FN stimulation. Whereas the level of FAK phosphorylation was appreciably enhanced in OV–LM cells stimulated with FN, stimulation of OV–HM cells with FN induced a reduction in the FAK phosphorylation in association with a significant decrease in theamount of FAK protein in the soluble compartment of cell lysates. A difference in the deposition of FN on the cell surfacewas also observed between the two types of tumor lines; OV–HM cells had an appreciably smaller amount of FN than OV–LM. Consistent with the functional abnormality of the integrin–FAK system and the smaller amount of FN on OV–HM, this clone exhibited a reduced cell–cell adhesion in the in vitro cell aggregation assay. Namely, OV–LM cells displayed a time–dependent increase in the formation of cell aggregates, whereas most OV–HM cells remained single. The formation of aggregates by OV–LM cells was inhibited by addition of RGDS peptide. These results indicate that the highly metastatic clone, OV–HM, exhibits a decreased capacity of cell–cell adhesion mediated by integrin–FN interactions and suggest that this defect is mainly due to the dysfunction of integrins/FAK rather than a decrease in the amount of integrins expressed on tumor cells.