FORCE REQUIRED TO BREAK ALPHA(5)BETA(1) INTEGRIN-FIBRONECTIN BONDS ININTACT ADHERENT CELLS IS SENSITIVE TO INTEGRIN ACTIVATION STATE

Binding of integrin receptors to extracellular ligands is a complex pr ocess involving receptor-ligand interactions at the cell-substrate int erface, signals activating the receptors, and assembly of cytoskeletal and adhesion plaque proteins at the cytoplasmic face. To analyze the contribution of these elements to overall cell adhesion, Fee have deve loped a model system that characterizes the functional binding charact eristic for adhesion receptors as the force required to separate the i ntegrin-ligand bond. A spinning disk device was used to apply a range of controlled hydrodynamic forces to adherent cells. The adhesion of K 562 erythroleukemia cells, a cell line expressing a single fibronectin receptor, integrin alpha(5) beta(1), which was uniformly activated wi th the monoclonal antibody TS2/16, to defined fibronectin surface dens ities was examined. Cell adhesion strength increased linearly with rec eptor and ligand densities. Eased on chemical equilibrium principles, it is shown that adhesion strength is directly proportional to the num ber of receptor-ligand bonds. This analysis provides for the definitio n of a new physical parameter, the adhesion constant psi, which is rel ated to the bond strength and binding equilibrium constant and has uni ts of force-length(2). This parameter can be measured by the experimen tal system presented and is governed by the activation state of integr in receptors, This simplified model isolates the integrin receptor-lig and binding parameters and provides a basis for analysis of the functi ons of signaling and cytoskeletal elements in the adhesion process.