Effect of interleukin-17 on nitric oxide production and osteoclastic bone resorption: is there dependency on nuclear factor-kappaB and receptor activator of nuclear factor kappaB (RANK)/RANK ligand signaling?

Interleukin-17 (IL-17) is a proinflammatory cytokine produced exclusively by activated memory T cells and has recently been found to stimulate osteoclastic resorption. Like other proinflammatory cytokines, IL-17 may affect osteoclastic bone resorption indirectly via osteoblasts, possibly by mechanisms previously reported for chondrocytes that respond in very similarly to osteoblasts. As in chondrocytes, but only in combination with tumor necrosis factor-alpha (TNF-alpha), IL-17 induced nitric oxide (NO) production in osteoblastic cells and fetal mouse metatarsals by a nuclear factor-kappaB (NF-kappaB)-dependent mechanism. This effect was associated with elevated mRNA levels of the NF-kappaB isoforms RelA and p50. In fetal mouse metatarsals, IL-17 stimulated osteoclastic bone resorption only in combination with TNF-alpha. The pathway by which the cytokine combination exerts this effect was examined using inhibitors of NO synthesis and NF-kappaB activation. Although both inhibitors used abolished NO production, they did not prevent the stimulatory effect of the cytokine combination on osteoclastic resorption. In contrast, the inhibitors slightly increased osteoclastic resorption, suggesting a suppressive rather than stimulatory effect of NO on cytokine-induced bone resorption. In addition, we showed that IL-17 + TNF-alpha stimulated osteoclastic resorption independent of NF-kappaB signaling. To further examine the pathway by which osteoclastic resorption was stimulated, we used osteoprotegerin, a specific inhibitor of the receptor activator of NF-kappaB (RANK)/receptor activator of the NF-kappaB ligand (RANKL) pathway. Osteoprotegerin partially inhibited IL-17 + TNF-alpha-stimulated osteoclastic resorption only at the high concentration of 1000 ng/mL, whereas it completely blocked parathyroid hormone-related peptide-stimulated resorption at 300 ng/mL. In conclusion, IL-17 stimulated NO production by an NF-kappaB-dependent pathway in osteoblastic cells and fetal mouse metatarsals only in combination with TNF-alpha. Neither NO production nor NF-kappaB signaling, and only partly the RANK/RANKL pathway, were involved in the stimulatory effect of the cytokine combination on osteoblastic bone resorption in these long bones, suggesting the existence of other pathways by which osteoclastic resorption can be stimulated.