Signaling pathways involved in IL-18 induced VCAM-1 expression in RA synovial fibroblasts

In a recent report, we suggested the implication of a phosphoinositide-3 kinase (PI3 kinase) pathway in IL-18-induced VCAM-1 expression in rheumatoid arthritis (RA) synovial fibroblasts. Here, we demonstrated for the first time that IL-18 rapidly activates PI3 kinase and its downstream effector Akt. In our investigation to elucidate the signaling events downstream and upstream of PI3-kinase, we found that IL-18 induces a parallel pathway involving extracellular regulated kinases (ERK) 1/2. Using specific kinases inhibitors LY294002 and PP2, we showed that ERK1/2 activation was independent of PI3 kinase but dependent of Src kinase with an approximately 63% (P < 0.05) decrease of phosphorylated ERK. IL-18 also showed a time dependent activation of both c-Src, Ras and Raf-1 suggesting the implication of this signaling cascade in ERK activation. Electrophorectic mobility shift assay demonstrate that activator protein-1 (AP-1) is activated by IL-18 through ERK and Src but not through PI3 kinase. Finally, the Src kinase, which is known to activate PI-3 kinase may represent the early event in IL-18-activated PI3 kinase because the specific inhibitors of these two kinases both affect IL-18 induced VCAM-1 expression in RA synovial fibroblasts by 50% (P < 0.05). In contrast, PD98059 and pertussis toxin had no impact on VCAM-1 expression excluding the participation of ERK and Go/i proteins in the IL-18-mediated VCAM-1 production. These data demonstrate the activation of two novel pathways in IL-18-stimulated RA synovial fibroblasts and they indicate that it is possible to selectively inhibit the expression of VCAM-1. Targeting PI3 kinase may represent an interesting approach to the development of selective drugs in inflammation and RA.