Background Ankylosing spondylitis (AS) is a chronic inflammatory arthritis characterized by bony overgrowth. It has been suggested that erosive bone destruction, followed by repair with ossification, leads to new bone formation. However, little is known about the molecular mechanisms underlying proliferative bone formation in AS. Previously, we have reported that IL-32 could affect bone physiology via osteoclast activation.
Objectives To investigate the role of IL-32 on osteoblast differentiation and its association with new bone formation in AS.
Methods The concentration and expression levels of IL-32 and TNF-α were evaluated in the synovial fluid and tissue from patients with AS, rheumatoid arthritis (RA) and osteoarthritis (OA) by enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry. To address whether IL-32 affects bone formation in vivo, we examined osteoblast differentiation from calvarial cells of IL-32 transgenic (Tg) mice and also the bone volume in these mice. To elucidate the mechanism of IL-32 effect on osteoblastogenesis, osteoblast regulators including DKK1, BMPRII, BMP2 and LRP5 were assayed in the IL-32 Tg mice. To determine the role of IL-32 in the interaction of osteoblasts and osteoclasts, the expression of RANKL and OPG was measured with or without IL-32 treatment in human primary osteoblasts.
Results The levels of IL-32 were significantly higher in the synovial fluid of AS patients compared with RA or OA patients. In addition, the expression of IL-32 in AS synovia was increased compared to that in RA or OA synovia. The differentiation of osteoblasts from calvarial cells was more robust and rapid in IL-32 Tg mice than in wild-type mice. Micro-CT analysis further revealed that IL-32 Tg mice, particularly males, have a higher bone volume than wild-type mice. The levels of DKK-1, a negative regulator of Wnt signaling, were decreased in IL-32 Tg mice during osteoblast differentiation. In addition, after IL-32 stimulation, DKK-1 expression was significantly decreased in osteoblast precursor cells from mice and also in human osteoblasts. Furthermore, the expression of RANKL, but not OPG, was enhanced by the stimulation with IL-32, leading to a higher expression ratio of RANKL to OPG.
Conclusions These results indicate that IL-32 is significantly highly expressed in AS and may enhance osteoblast differentiation via its inhibitory effects on DKK-1. Therefore, IL-32 might have a role in osteoproliferation and is a potential biomarker to predict new bone formation in AS.
Disclosure of Interest None declared