Differential cation regulation of the alpha 5 beta 1 integrin-mediated adhesion of leukemic cells to the central cell-binding domain of fibronectin

Normal and neoplastic leukocytes interact with the central cell-binding and carboxyl-terminal regions of fibronectin (FN) primarily via the alpha(4) beta(1) and alpha(5) beta(1) integrins. By using a unique centrifugation-based cell adhesion assay and affinity chromatography of the cell surface-labeled integrins, we show in this study that the constitutive alpha(5) beta(1)-dependent attachment of three leukemic cell lines, BV-173, K562, and Nalm-6, to FN or to a 110-kDa central cell-binding fragment of FN was totally inhibited at 37 degrees C by preincubation of the substrate with either antibodies to the arginine-glycine-aspartic acid-containing region (3Fn-9 module) or to the synergistic region (3Fn-9 module) of FN. Similar results were obtained when assays were carried out at 4 degrees C, suggesting that energy-dependent events were not involved. On the other hand, only the antibody against the 3Fn-10 module was able to detach most firmly adherent cells. Constitutive cell attachment to the 110-kDa fragment was cation dependent, with the order of efficacy of the cation being Mn2+ > Mg2+ > Ca2+, and Ca2+ was only effective for BV-173 cells. Antibodies against the alpha(5) beta(1) integrin or the alpha(5) subunit completely impaired cell attachment of all cell lines, whereas several blocking anti-beta(1) subunit antibodies, including 4B4, P4C10, and AIIB2, differentially perturbed cell adhesion of BV-173, depending on which cation was present. These anti-beta(1) blocking antibodies, whose epitopes map to a region distant from the one expected to contain the beta(1) putative cation binding sites, seemed to lock the higher activation state of this integrin attained in the presence of Mg2+ and to preserve it during the subsequent adhesion events irrespectively of the presence of the low avidity state-inducing Ca2+ ion. Because the higher binding avidity displayed by BV-173 could not be explained by a higher degree of preclustering of alpha(5) beta(1) integrin in this cell line compared to K562, these findings suggest that cations might act as allosteric activators of integrin function. Whether this novel cation-dependent parameter of alpha(5) beta(1) integrin-FN interaction might contribute to the growth control and/or tissue dissemination of leukemic cells remains to be determined.