THE ROLE OF PROTEIN-TYROSINE PHOSPHORYLATION IN INTEGRIN-MEDIATED GENE INDUCTION IN MONOCYTES

Integrin-mediated cell adhesion, or cross-linking of integrins using a ntibodies, often results in the enhanced tyrosine phosphorylation of c ertain intracellular proteins, suggesting that integrins may play a ro le in signal transduction processes. In fibroblasts, platelets, and ca rcinoma cells, a novel tyrosine kinase termed pp125(FAK) has been impl icated in integrin-mediated tyrosine phosphorylation. In some cell typ es, integrin ligation or cell adhesion has also been shown to result i n the increased expression of certain genes. Although it seems reasona ble to hypothesize that integrin-mediated tyrosine phosphorylation and integrin-mediated gene induction are related, until now, there has be en no direct evidence supporting this hypothesis. In the current repor t, we explore the relationship between integrin-mediated tyrosine phos phorylation and gene induction in human monocytes. We demonstrate that monocyte adherence to tissue culture dishes or to extracellular matri x proteins is followed by a rapid and profound increase in tyrosine ph osphorylation, with the predominant phosphorylated component being a p rotein of 76 kD (pp76). Tyrosine phosphorylation of pp76 and other mon ocyte proteins can also be triggered by incubation of monocytes with a ntibodies to the integrin beta 1 subunit, or by F(ab')(2) fragments of such antibodies, but not by F(ab) fragments. The ligation of beta 1 i ntegrins with antibodies or F(ab')(2) fragments also induces the expre ssion of immediate-early (IE) genes such as IL-1 beta. When adhering m onocytes are treated with the tyrosine kinase inhibitors genistein or herbimycin, both phosphorylation of pp76 and induction of IL-1 beta me ssage are blocked in a dose-dependent fashion. Similarly, treatment wi th genistein or herbimycin can block tyrosine phosphorylation of pp76 and IL-1 beta message induction mediated by ligation of beta 1 integri n-mediated IE gene induction in monocytes. The cytoplasmic tyrosine ki nase pp125(FAK), although important in integrin signaling in other cel l types, seems not to play a role in monocytes because this protein co uld not be detected in these cells.