ANTIBODY-INDUCED ENGAGEMENT OF BETA(2) INTEGRINS ON ADHERENT HUMAN NEUTROPHILS TRIGGERS ACTIVATION OF P21(RAS) THROUGH TYROSINE PHOSPHORYLATION OF THE PROTOONCOGENE PRODUCT VAV

It is known that beta(2) integrins are crucial for leukocyte cell-cell and cell-matrix interactions, and accumulating evidence now suggests that integrins serve not only as a structural link but also as a signa l-transducing unit that controls adhesion-induced changes in cell func tions, In the present study, we plated human neutrophils on surface-bo und anti-beta 2 (CD18) antibodies and found that the small GTP-binding protein p21(ras) is activated by beta 2 integrins. Pretreatment of th e cells with genistein, a tyrosine kinase inhibitor, led to a complete block of p21(ras) activation, an effect that was not achieved with ei ther U73122, which abolishes the beta(2) integrin-induced Ca2+ signal, or wortmannin, which totally inhibits the phosphatidylinositol 3 kina se activity, Western blot analysis revealed that antibody-induced enga gement of beta(2) integrins causes tyrosine phosphorylation of several proteins in the cells. One of these tyrosine-phosphorylated proteins had an apparent molecular mass of 95 kDa and was identified as the pro tooncogene product Vav, a p21(ras) guanine nucleotide exchange factor that is specifically expressed in cells of hematopoietic lineage. A ro le for Vav in the activation of p21(ras) is supported by the observati ons that antibody-induced engagement of beta(2) integrins causes an as sociation of Vav with p21(ras) and that the effect of genistein on p21 (ras) activation coincided with its ability to inhibit both the tyrosi ne phosphorylation of Vav and the Vav-p21(ras) association. Taken toge ther, these results indicate that antibody-induced engagement of beta( 2) integrins on neutrophils triggers tyrosine phosphorylation of Vav a nd, possibly through its association, a downstream activation of p21(r as).