The engagement of β₁ integrins on promonocytic cells promotes phosphorylation of Syk and formation of a protein complex containing Lyn and β₁ integrin

The protein-tyrosine kinase Syk participates in signal transduction pathways downstream from multiple immune recognition receptors. Recent evidence indicates that Syk is also functionally coupled to cell surface integrins, which mediate interactions between the actin cytoskeleton and extracellular matrix proteins. The interactions of undifferentiated, promonocytic HL60 or U937 cells with fibronectin or anti-β, integrin antibodies leads to an apparent activation and tyrosine phosphorylation of Syk that is independent of tight cellular adhesion and spreading. In response to fibronectin or anti-β₁ integrin antibodies, β₁ integrins become associated with a complex of proteins that include the Lyn protein tyrosine kinase and endogenous kinase substrates of 29 and 75-80 kDa. Lyn becomes transiently activated following integrin engagement and co-localizes with the actin cytoskeleton. These studies suggest a major role for Lyn in coupling β₁ integrins to the activation of Syk.