The engagement of beta(1) integrins on promonocytic cells promotes phosphorylation of Syk and formation of a protein complex containing Lyn and beta(1) integrin

The protein-tyrosine kinase Syk participates in signal transduction pathway s downstream from multiple immune recognition receptors. Recent evidence in dicates that Syk is also functionally coupled to cell surface integrins, wh ich mediate interactions between the actin cytoskeleton and extracellular m atrix proteins. The interactions of undifferentiated, promonocytic HL60 or U937 cells with fibronectin or anti-beta(1) integrin antibodies leads to an apparent activation and tyrosine phosphorylation of Syk that is independen t of tight cellular adhesion and spreading, in response to fibronectin or a nti-beta(1) integrin antibodies, beta(1) integrins become associated with a complex of proteins that include the Lyn protein tyrosine kinase and endog enous kinase substrates of 29 and 75-80 kDa. Lyn becomes transiently activa ted following integrin engagement and co-localizes with the actin cytoskele ton. These studies suggest a major role for Lyn in coupling beta(1) integri ns to the activation of Syk.