To explore the signaling pathways by which the proinflammatory cytokine interleukin-17 (IL-17) may contribute to cartilage catabolism in osteoarthritis (OA) by inducing inducible nitric oxide synthase (iNOS) expression in chondrocytes.
We examined the IL-17-induced NO production in human OA chondrocytes, in combination with the proinflammatory cytokines IL-1beta, tumor necrosis factor alpha (TNF alpha), and leukemia inhibitory factor (LIF); the antiinflammatory cytokines IL-4, IL-10, and IL-13; and IL-1 receptor antagonist (IL-1Ra). Further, we explored the major intracellular signaling pathways through which IL-17 induced iNOS expression and NO production.
Treatment with IL-17 induced a dose-dependent increase in the level of NO. When IL-17 was combined with the above factors, it resulted in a synergistic effect with TNF alpha, an additive effect with LIF, and no further effect than when used alone with IL-1beta. IL-4, IL-10, IL-13, and IL-1Ra had no true effect on IL-17-induced NO production. The cAMP mimetics, 3-isobutyl-1-methyl xanthine plus forskolin, completely blocked IL-17-induced NO production. KT-5720, genistein, and Calphostin C, inhibitors of protein kinase A (PKA), tyrosine kinase, and protein kinase C, respectively, reduced the IL-17-induced NO production by 72%, 56%, and 42%, respectively. Within minutes, IL-17 induced the phosphorylation of mitogen-activated protein kinase kinase-1/2 (MEK-1/2), -3/6 (MKK-3/6), p44/42, p38, and inhibitor of nuclear factor kappaB (I kappaB)-alpha, as well as the activation of mitogen-activated protein kinase-activated protein kinase-1 and -2 (MAPKAPK-1 and -2). Interestingly, IL-17 induced phosphorylation of the stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK) (p54/46) only when PKA was inhibited. Specific protein kinase inhibitors for MEK-1/2 (PD98059), p38 (SB202190), and nuclear factor kappaB (NF-kappaB) (pyrrolidine dithiocarbamate) each markedly decreased the IL-17-increased iNOS level and NO production. Inhibiting MAPK, including MEK-1/2 and p38, had no effect on the IL-17-induced activation of IkappaB-alpha, but reversed the IL-17 activation of MAPKAPK-1 and -2, respectively.
These findings show that the stimulation of NO production by IL-17 is mediated mainly by a complex activation of kinases, especially PKA, NF-kappaB, and MAPK. NF-kappaB appears to require MAPK activation, with downstream activation of MAPKAPK probably acting as a transactivating factor, to induce iNOS expression.