Targeting NF-κB and NFATc1 signalling to inhibit bone resorption in peri-prosthetic osteolysis.

Abstract

Peri-prosthetic osteolysis is a bone loss disease involving granulomatous inflammation in the soft tissues around prostheses characterised by excessive bone resorption adjacent to implants. Macrophages phagocytose particles of prosthetic material, thereby inducing persistent release of pro-inflammatory osteoclastogenic cytokines, such as, receptor activator of NF-κB ligand (RANKL). RANKL interacts with its receptor RANK, to activate key transcription factor in osteoclastogenesis, NFATc1. This activates an inflammatory response leading to bone erosion at the implant bone interface and subsequent prosthetic failure. Other bone cells are affected by particles. For example, osteoblasts have reduced bone formation activity, and osteocytes, undergo apoptosis, whereby in this process they release cytokines that stimulate bone loss. Apart from the RANKL/RANK-NFATc1 system, immunoreceptor tyrosine-based activation motif (ITAM)-dependent pathway has been identified as a co-stimulatory pathway in osteoclasts. Osteoclast-associated receptor (OSCAR) and TREM2 are ITAM containing receptors pairing with adaptor molecules FcRγ and DAP12, respectively. Our group has demonstrated the increased expression of NFATc1 and ITAM-related molecules adjacent to sites of bone loss in human peri-prosthetic tissues and polyethylene (PE) particle-stimulated osteoclasts in vitro. Soluble OSCAR has been proposed as a potential regulator of osteoclast activity in osteolysis. Considering the importance of NF-κB and NFATc1 we hypothesize that their inhibition of will abrogate osteoclast bone resorption. The data presented here showed that inhibition of NFATc1 by calcineurin-NFAT inhibitors, FK506 and VIVIT, and inhibition of both NFATc1 and NF-κB by NC100, significantly suppressed osteoclast formation and activity in normal human-derived RANKL-induced osteoclast differentiation in vitro. mRNA expression analysis showed OSCAR was inhibited by FK506, VIVIT and NC100 at later stages of osteoclastogenesis. Together, this demonstrates potential benefits of targeting NFATc1 and NF-κB to suppress osteoclastogenesis and modulate ITAM-containing molecules in human osteoclasts. In the murine calvarial model of PE-induced peri-prosthetic osteolysis, live animal micro-computed tomography analyses showed that PE particles significantly induced localised osteolysis in mice implanted with PE particles compared to controls. PAR strongly reduced surface bone resorption but not local bone volume. However, CAPE treatment reduced local PE-induced calvarial osteolysis at both surface resorption and volumetric change. Additionally, PE particles significantly increased serum levels of bone resorptive marker CTX-1 and soluble OSCAR. However, neither PAR nor CAPE regulated CTX-1 and OSCAR in PE-implanted mice. Neither PE nor CAPE affected the gastrointestinal tract, a potential side effect of their treatments. Interestingly, PE particles strongly enhanced osteocyte death, supporting previous reports that osteocytes undergo apoptosis in response to PE particles. PAR significantly decreased osteocyte apoptosis in PE-implanted mice, as assessed by reduced number of osteocytes with empty lacunae within the calvarial tissues. It could be important to determine the possible mechanism by which PE particles activate NF-κB in osteocytes as this could be an important mechanism inducing osteolysis in response to prosthetic particles. Overall the results demonstrate that targeting NFATc1 and NF-κB signalling suppresses osteoclast differentiation and resorption induced by PE particles. Future studies are necessary to fully understand this pathology.