Interleukin-8 Regulation of the Ras/Raf/Mitogen-activated Protein Kinase Pathway in Human Neutrophils

Abstract

Interleukin-8 (IL-8), the prototypic member of the CXC subfamily of chemokines, induces in neutrophils chemotaxis, the respiratory burst, granule release, and increased cell adhesion. The IL-8 receptor is a seven-transmembrane spanning receptor coupled to specific heterotrimeric G proteins including Gi and G16. IL-8 stimulation of its receptor on neutrophils activates Ras GTP loading and the mitogen-activated protein kinase (MAPK) pathway including Raf-1 and B-Raf. The properties of IL-8 stimulation of the MAPK pathway differ from those observed for chemoattractants such as C5a. Even though Ras GTP loading is similar for IL-8 and C5a, the maximal activation of Raf-1 and B-Raf is approximately 2-fold and 3-7-fold, respectively, less for IL-8 than that observed for C5a. Raf-1 activation is rapid but transient, returning to near basal levels by 10 min. B-Raf activation is slower in onset and does not return to basal levels for nearly 30 min. IL-8 activation of MAPK follows a time course suggesting an involvement of both Raf-1 and B-Raf. Surprisingly, wortmannin, at low concentrations, inhibits Raf-1, B-Raf, and MAPK activation in response to IL-8 and C5a demonstrating a role for phosphatidylinositol 3-kinase in the activation of Raf kinases in G protein-coupled receptor systems in human neutrophils. Furthermore, wortmannin inhibits IL-8 stimulated granule release and neutrophil adherence. These findings demonstrate the control of Raf kinases, the MAPK pathway and specific neutrophil functions by phosphatidylinositol 3-kinase enzymes.