Rheumatoid arthritis (RA) is a chronic disease characterized by both synovial and systemic inflammation, with primary joint involvement. The intense inflammatory process seen in the disease is the most important risk factor for progressive destruction of extracellular matrices of articular cartilage and bone in joints affected by RA [1
]. Three principal forms of bone loss have been described in patients with RA: focal bone erosions, juxta-articular bone loss, and systemic bone loss [4
]. Of these, juxta-articular bone loss represents a common and early feature of RA that affects the trabecular bone adjacent to the inflamed joint [5
]. In spite of this, the pathogenesis of juxta-articular bone loss in RA has not yet been fully elucidated, mainly because of the difficulties in obtaining appropriate human samples for study. Nevertheless, the loss of periarticular bone in RA has been associated with dysregulation of bone remodeling, which is redirected towards the predominance of resorption activity over formation [6
Different hormones, cytokines and chemokines produced by the inflamed synovial membrane have been reported to be involved in juxta-articular osteoporosis in RA [4
]. Also, the pannus directly infiltrates osseous tissue contributing to periarticular bone loss [7
]. Certainly, juxta-articular bone loss is related to the intensity of inflammatory response in the affected joint [2
]. This fact is observed not only in RA, but also in other arthritides associated with a high degree of inflammation [8
]. Macrophages differentiate into bone-resorbing osteoclasts in zones of contact between the inflamed synovium and subchondral bone in RA in the presence of a crucial factor, the receptor activator of nuclear factor-κB ligand (RANKL) [10
RANKL is markedly involved in osteoclastogenesis, osteoclast migration, adherence to bone, and apoptosis regulation due to its binding to the receptor activator of nuclear factor-κB (RANK) [12
], which is expressed on osteoclast precursors and mature osteoclasts. A study by Pettit, et al
showed that tumor necrosis factor-related activation-induced cytokine (TRANCE) knockout mice were deficient in osteoclasts and protected from bone loss in a serum transfer model of arthritis, demonstrating in vivo
the importance of RANKL and osteoclastogenesis in bone loss associated with RA [13
]. The pro-osteoclastogenic actions of RANKL are physiologically regulated by osteoprotegerin (OPG), a soluble non-signalling receptor for RANKL [14
]. Indeed, OPG competitively inhibits RANKL binding to its receptor RANK. In healthy joints, RANKL expression has been described in bone lining cells of osteoblast lineage, synovial T cells, and chondrocytes [9
], whereas in inflamed arthritic joints, it is detected in synovial fibroblasts, T and B cells, osteoclasts, and chondrocytes [6
Proinflammatory cytokines such as tumor necrosis alpha (TNF-α), interleukin (IL)-1β, IL-6, IL-7, and IL-17, as well as macrophage-colony stimulating factor (M-CSF), parathyroid hormone (PTH), 1,25-dihydroxycholecalciferol [1,25(OH)2
], and prostaglandin E2
) increase RANKL synthesis [3
]. More recently, transcriptional repressors that suppress RANKL-induced gene expression and osteoclast differentiation have been described, including IL-4/IL-13 and granulocyte-macrophage colony stimulating factor (GM-CSF), IL-10, IL-27, interferon-γ, TNF apoptosis related inducing ligand (TRAIL), IL-12, IL-18, IL-6, and toll-like receptors [26
]. Thus, the extent of bone destruction in inflammatory arthritis is determined by the balance between osteoclastogenic and anti-osteoclastogenic factors, with the relevant biological mediation of RANKL.
In addition to membrane-bound RANKL in osteoblasts [27
], RANKL secreted by synovial cells actively promotes bone destruction in chronic inflammatory arthritis [21
]. Hence, high local RANKL concentrations may lead to increased osteoclastogenesis at the bone-pannus interface. However, scarce attention has been given to the potential role of the RANKL expressed by chondrocytes in the pathogenesis of RA-related juxta-articular bone loss, although it has been reported to diffuse from the cartilage to subchondral bone in human osteoarthritis (OA) [28
]. Accordingly, we hypothesized that RANKL produced in articular cartilage might contribute to juxta-articular bone loss in chronic arthritis.
Animal models of RA offer a valuable opportunity to enhance our understanding of the pathogenic mechanisms underlying juxta-articular bone loss in the disease. To optimize this potential, we characterized an experimental model of chronic arthritis that represents a more intense and destructive version of the well-established antigen-induced arthritis (AIA) [29
]. This inflammatory arthritis is accompanied by severe juxta-articular bone loss, as estimated by x-ray and bone mineral density (BMD). Thus, our experimental model is suitable to study the role of RANKL, OPG, and RANKL/OPG ratio in the pathogenesis of juxta-articular bone loss in chronic arthritis.
Therefore, we carried out an in vivo study to explore the potential effect of cartilage-synthesized RANKL on juxta-articular bone loss associated with RA.