Titanium particles stimulate COX-2 expression in synovial fibroblasts through an oxidative stress-induced, calpain-dependent, NF-[kappa]B pathway

In prosthetic loosening, bone resorption is induced by wear debris particles generated from the artificial joint articulation. Our prior work showed that synovial-like fibroblasts respond to titanium particles by producing receptor activator of NF-[kappa]B ligand (RANKL), a critical activator of osteoclastogenesis. While this effect occurs through a cyclooxygenase-2 (COX-2)-dependent pathway, the mechanism of COX-2 stimulation by titanium particles is not clear. Here we show that titanium particles induce COX-2 gene expression by activating NF-[kappa]B signaling. Inhibitor of NF-[kappa]B (I[kappa]B[alpha]) is degraded following particle treatment, permitting active NF-[kappa]B to translocate to the nucleus where it interacts with the COX-2 promoter and drives transcription. NF-[kappa]B activation is dependent on reactive oxygen species since antioxidants block the NF-[kappa]B signaling induced by particles. Surprisingly, I[kappa]B[alpha] degradation is independent of IKK (I[kappa]B kinase) and the 26S proteasome. Instead, calpain inhibitor can block the I[kappa]B[alpha] degradation induced by particles. Furthermore, the calpain-targeted COOH-terminal PEST sequence of I[kappa]B[alpha] is necessary for phosphorylation and degradation, consistent with a proteasome-independent mechanism of catabolism. Altogether, the data demonstrate a signaling pathway by which titanium particles induce oxidative stress, stimulate calpain-mediated NF-[kappa]B activation, and activate target gene expression, including COX-2. These findings define important targets for osteolysis but may also have importance in other diseases where fibroblasts respond to environmental particles, including pulmonary diseases. cyclooxygenase-2; osteolysis; inflammation; cell signaling; reactive oxygen species