AFFINITY MODULATION OF INTEGRIN ALPHA-5-BETA-1 - REGULATION OF THE FUNCTIONAL-RESPONSE BY SOLUBLE FIBRONECTIN

We report that a beta1 integrin (alpha5beta1) can exist in different affinity states for its soluble ligand, fibronectin. The alpha5beta1 expressed by the erythroleukemic cell line K562 binds soluble fibronectin with low affinity (K(d) > 1 muM), but is induced to bind it with 20-fold higher affinity (K(d)-54 nM) in the presence of the anti-beta1 mAb 8A2. This activation seems to be due to direct antibody-induced change in the receptor that does not require intracellular signaling, and is a plausible basis for the 8A2-induced enhancement of beta1-dependent adhesion to fibronectin and other immobilized ligands (Kovach, N. L., T. M. Carlos, E. Yee, and J. M. Harlan. 1992. J. Cell Biol. 116: 499-509). Fab fragments of 8A2 bind with higher affinity to alpha5beta1 receptor that is occupied by the GRG-DSP peptide ligand suggesting that the antibody functions by stabilizing a high affinity (occupied) conformer of the receptor. A functional consequence of the affinity modulation is that soluble fibronectin (at physiological concentrations) occupies the high affinity receptors, and so becomes an effective inhibitor of adhesion to immobilized fibronectin. In contrast, the majority of low affinity receptors remain unoccupied and are still able to mediate cellular adhesion.