Titanium particles stimulate COX-2 expression in synovial fibroblasts through an oxidative stress-induced, calpain-dependent, NF-κB 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-κB 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-κB signaling. Inhibitor of NF-κB (IκBα) is degraded following particle treatment, permitting active NF-κB to translocate to the nucleus where it interacts with the COX-2 promoter and drives transcription. NF-κB activation is dependent on reactive oxygen species since antioxidants block the NF-κB signaling induced by particles. Surprisingly, IκBα degradation is independent of IKK (IκB kinase) and the 26S proteasome. Instead, calpain inhibitor can block the IκBα degradation induced by particles. Furthermore, the calpaintargeted COOH-terminal PEST sequence of IκBα 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-κB 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.