REGULATION OF ALPHA-4 INTEGRIN AVIDITY IN HUMAN B-CELLS - REQUIREMENTFOR DEPHOSPHORYLATION EVENTS FOR HIGH AVIDITY VCAM-1 BINDING

The authors compared the phosphorylation-dependent signal transduction pathways involved in the regulation of adhesiveness of integrin LFA-1 with that of alpha 4 integrins binding to VCAM-1. The authors develop ed an in vitro method to monitor changes in adhesiveness using a VCAM- 1 fusion protein coupled to magnetic beads. For LFA-1, a similar metho d has previously been established using an ICAM-1 fusion protein. Bind ing of cells was monitored and found to be strictly integrin alpha 4 a nd VCAM-1 dependent. The serine/threonine phosphatase inhibitors okada ic acid and calyculin A were equally potent in inhibiting binding to V CAM-1 as to ICAM-1, and inhibition of protein phosphatase-1 (PP1) is p roposed to be the important denominator. Similarly, the phorbol ester PDBu, potentially stimulating protein phosphatase-1 and staurosporine, an inhibitor of serine/threonine kinases, enhanced adhesion to VCAM-1 as has previously been shown for ICAM-1. A major difference was that a significant portion of the binding to VCAM-1 was not susceptible to inhibition by drugs while binding to ICAM-1 could be completely inhibi ted. We propose that the adhesiveness of the alpha 4 integrins for VCA M-1 and of LFA-1 for ICAM-1 is regulated by similar or identical prote in kinases and phosphatases.