Activation of the receptor EphB4 by its specific ligand ephrin B2 in human osteoarthritic subchondral bone osteoblasts

Abstract

Objective. Abnormal subchondral bone metabolism is involved in osteoarthritis (OA). It has been suggested that ephrin B2 and its specific receptor EphB4 participate in bone homeostasis. We previously reported that human OA subchondral bone osteoblasts could be classified into 2 subpopulations: low (L), having proresorption properties, and high (H), having proformation properties. The purpose of this study was to investigate the importance of the ephrin system in OA subchondral bone osteoblasts.Methods. The presence of the EphB4 receptor was determined by immunohistochemistry, and its expression level, modulation upon treatment, and consequences of activation by ephrin B2 were determined by quantitative polymerase chain reaction. The effects of ephrin B2 activation of the EphB4 receptor on bone resorption activity were also determined. EphB4 receptor activation signaling pathways were investigated by specific enzyme-linked immunosorbent assay.Results. EphB4 receptors were present in subchondral bone osteoblasts and osteocytes. Compared with normal and H-OA osteoblasts, EphB4 receptor expression levels were significantly increased in L-OA osteoblasts, with no difference between normal and H-OA osteoblasts. EphB4 receptor levels in L-OA osteoblasts were significantly up-regulated by prostaglandin E 2 (PGE 2 ) and interleukin-17 (IL-17). Ephrin B2, PGE 2 , and IL-17 significantly inhibited bone resorption activity in these cells. EphB4 activation by ephrin B2 significantly inhibited the expression of IL-1␤, IL-6, matrix metalloproteinase 1 (MMP-1), MMP-9, MMP-13, and RANKL, but not MMP-2 and osteoprotegerin. EphB4 receptor activation significantly inhibited the phosphatidylinositol 3-kinase/Akt pathway.Conclusion. This study is the first to provide evidence that EphB4 receptor activation by ephrin B2 in OA subchondral bone could affect abnormal metabolism in this tissue by inhibiting resorption factors and their activities. Ephrin B2 could be targeted as a specific therapeutic approach in the development of a diseasemodifying OA drug.Subchondral bone is made up of a specialized connective tissue formed by a mineralized matrix containing the specific type I collagen, proteoglycans, and several growth factors and cytokines, as well as bonespecific cell types (osteoblasts, osteocytes, and osteoclasts) (1,2). Osteoblast and osteoclast activities, either alone or in combination, contribute to the bone remodeling process, and any disturbance in the activities of these 2 cells is responsible for the development of altered bone metabolism.The ephrin B molecules (ephrin B1, ephrin B2, and ephrin B3) bind in a specific manner to their EphB

Targeting subchondral bone for treating osteoarthritis: what is the evidence?

“…In the other OA osteoblast subpopulation, an increase in these factors would not be required, as these cells are in a pro-formation phase. This concurs with further data showing that in pro-resorptive osteoblasts, treatment with ephrin B2 ligands induced a reduction in the bone resorption process as well as in various catabolic mediators, including the inflammatory factors interleukin (IL)-1β and -6; the MMPs -1, -13 and -9; and RANKL [81]. This process appears to occur through an inhibition of the PI3K/Akt signalling pathway.…”

“…Interestingly, these two subgroups were also found to differentially express EphB4 receptors and ephrin B2. Those showing pro-resorptive properties showed a higher level of EphB4 receptors and ephrin B2, while those in a proformation phase had EphB4 receptor and ephrin B2 ligand expression levels similar to the normal subchondral bone osteoblasts [81].…”

“…human OA subchondral bone and chondrocytes could positively impact on the abnormal metabolism of these diseased cells [81,102] suggest that enhancing this system could lead to a protective effect on the structural changes in these OA tissues.…”

“…The signal through the EphB4 receptor is considered a forward signal whereas the reverse signaling is through the ephrin B2 ligand. In OA subchondral bone osteoblasts, the activation of EphB4 receptor by ephrin B2 results in a decreased activation of PI3K/Akt which, in turn, inhibits some pro-inflammatory cytokines and matrix metalloproteinases as well as RANKL, all of which are involved in the remodeling process of the OA subchondral bone [81]. Table 1 Individual markers of OA progression and initiation.…”

Bone cell communication factors and Semaphorins

“…21,22 Furthermore, the ephrinB2 / EphB4 axis has been shown to be implicated in certain pathological settings such as osteoarthritis and multiple myeloma. 22,23 It is possible that the osteoblastogenic effect induced by ephrinB2 / EphB4 signaling is also exerted through the osteoblast -osteoblast interaction in the bone formation phase.…”