FIBRINOGEN ACTIVATES NF-KAPPA-B TRANSCRIPTION FACTORS IN MONONUCLEAR PHAGOCYTES

Adhesion to extracellular matrices is known to modulate leukocyte acti vation, although the mechanisms are not fully understood. Mononuclear phagocytes are exposed to fibrinous provisional matrix throughout migr ation into inflammatory foci, so this study was undertaken to determin e whether fibrinogen triggers activation of NF-kappa B transcription f actors, U937 cells differentiated with PMA in nonadherent culture were shown to express two fibrinogen-binding integrins, predominately CD11 b/CD18, and to a lesser extent, CD11c/CD18, Cells stimulated with fibr inogen (10-100 mu g/ml)/Mn2+ (50 mu M) for 2 h were examined by electr ophoretic mobility shift assay. NF-kappa B activation, minimal in unst imulated cells, was substantially up-regulated by fibrinogen, Fibrinog en also caused activation of AP-1, but not SP1 or cAMP response elemen t-binding protein (CREB) factors. Blocking mAbs against CD18 and CD11b abrogated fibrinogen-induced NF-kappa B activation, To determine the effects on transcriptional regulation, U937 cells were transfected wit h a plasmid containing the HIV-1 enhancer (bearing two NF-kappa B site s) coupled to a chloramphenicol acetyltransferase (CAT) reporter. Cell s were subsequently stimulated with 1) PMA for 24 h, inducing CAT acti vity by 2.6-fold, 2) fibrinogen/Mn2+ for 2 h, inducing CAT activity by 3.2-fold, or 3) costimulation with fibrinogen and PMA, inducing 5.7-f old the CAT activity induced by PMA alone. We conclude that contact wi th fibrinogen-derived proteins may contribute to mononuclear phagocyte activation by signaling through CD11b/CD18, resulting in selective ac tivation of transcriptional regulatory factors, including NF-kappa B.