VLA-4 INTEGRIN CROSS-LINKING ON HUMAN MONOCYTIC THP-1 CELLS INDUCES TISSUE FACTOR EXPRESSION BY A MECHANISM INVOLVING MITOGEN-ACTIVATED PROTEIN-KINASE

Adhesion molecules such as VLA 4 are important not only for monocyte a dhesion to extracellular matrix proteins, but also for subsequent cell activation. Monocyte adherence to fibronectin or engagement of VLA-4 has been demonstrated to stimulate production of potent inflammatory m ediators such as tumor necrosis factor-alpha, interleukin-1, and the p rocoagulant tissue factor protein, However, the intracellular signalin g cascades leading to gene expression have not been elucidated, Using the human monocytic THP-1 cell line, VLA-4 cross-linking by monoclonal antibodies directed against its alpha(4) and beta(1) subunits produce d a time-dependent increase in tyrosine phosphorylation of a broad ran ge of cellular proteins, Using Western blot analysis directed against the phosphorylated form of the extracellular signal-related kinase (ER K) mitogen-activated protein (MAP) kinase proteins, as well as immunop recipitation and in vitro kinase assays, we found that VLA-4 crosslink ing increased ERK1/ERK2 tyrosine phosphorylation and activity, In conj unction, integrin cross-linking also increased NF-kappa B nuclear tran slocation and 4-h expression of tissue factor, Inhibition of tyrosine kinase activity with genistein (10 mu g/ml) as well as selective MAP k inase inhibition with the MEK-1 inhibitor PD98059 abolished the VLA-4- dependent ERK tyrosine phosphorylation, inhibited NF kappa B nuclear b inding, and abrogated tissue factor expression induced by both VLA-4 c ross-linking and adhesion to fibronectin in THP-1 cells and human peri pheral blood monocytes, These studies point to the involvement of the MAP kinase pathway in the activation of monocytic cells during transmi gration to inflammatory sites.