Bidirectional Signaling through EphrinA2-EphA2 Enhances Osteoclastogenesis and Suppresses Osteoblastogenesis

Abstract

Bone is remodeled constantly throughout life by bone-resorbing osteoclasts and bone-forming osteoblasts. To maintain bone volume and quality, differentiation of osteoclasts and osteoblasts is tightly regulated through communication between and within these two cell lineages. Previously we reported that cellcell interaction mediated by ephrinB2 ligand on osteoclasts and EphB4 receptor on osteoblasts generates bidirectional anti-osteoclastogenic and pro-osteoblastogenic signals into respective cells and presumably facilitates transition from bone resorption to bone formation. Here we show that bidirectional ephrinA2-EphA2 signaling regulates bone remodeling at the initiation phase. EphrinA2 expression was rapidly induced by receptor activator of NF-B ligand in osteoclast precursors; this was dependent on the transcription factor c-Fos but independent of the c-Fos target gene product NFATc1. Receptor EphA2 was expressed in osteoclast precursors and osteoblasts. Overexpression experiments revealed that both ephrinA2 and EphA2 in osteoclast precursors enhanced differentiation of multinucleated osteoclasts and that phospholipase C␥2 may mediate ephrinA2 reverse signaling. Moreover, ephrinA2 on osteoclasts was cleaved by metalloproteinases, and ephrinA2 released in the culture medium enhanced osteoclastogenesis. Interestingly, differentiation of osteoblasts lacking EphA2 was enhanced along with alkaline phosphatase, Runx2, and Osterix expression, indicating that EphA2 on osteoblasts generates anti-osteoblastogenic signals presumably by up-regulating RhoA activity. Therefore, ephrinA2-EphA2 interaction facilitates the initiation phase of bone remodeling by enhancing osteoclast differentiation and suppressing osteoblast differentiation.Bone remodeling maintains bone mass constant during adulthood (1, 2). Resorption of old mineralized bone by osteoclasts is followed by new bone formation by osteoblasts. These processes, consisting of the initiation, transition, and termination phases, are tightly regulated by communication between osteoclasts and osteoblasts (3). Bone resorption is excessive in the most common skeletal diseases such as osteoporosis, but molecular mechanisms that balance bone remodeling are only partially understood.Osteoclasts are multinucleated cells (MNCs) 3 responsible for bone resorption. They originate from the fusion of hematopoietic precursor cells of the monocyte/macrophage lineage. Osteoblasts express the two major membrane-bound proteins required for osteoclast differentiation, macrophage-colony stimulating factor (M-CSF), and the receptor activator of NF-B ligand (RANKL). Soluble forms of M-CSF and RANKL allow us to generate osteoclasts from M-CSF-dependent macrophages (MDMs) in cultures. Downstream signaling pathways ultimately activate critical osteoclastogenic transcription factors such as c-Fos (4) and NFATc1 (5-7). NFATc1 is a target gene product of c-Fos (8) and activates gene expression of tartrate-resistant acid phosphatase (TRAP), calcitonin receptor, and ephrinB2.Ephrin ligands and Ep...

Alendronate Affects Osteoblast Functions by Crosstalk through EphrinB1-EphB

“…Moreover, the blockage of ephrinB2/EphB4 interaction reduces mineralization of osteoblasts in vitro (Allan et al, 2008). Irie et al (2009) have recently reported that ephrinA2-EphA2 interaction promotes the initiation phase of bone remodeling by enhancing osteoclast differentiation and suppressing osteoblast differentiation. Thus, a communicating interaction between osteoclasts and osteoblasts regulates differentiation in both cells through their exchange of ephrin/ Eph signals.…”

“…In contrast, a reverse signal from EphB4 on the osteoblast inhibits osteoclast formation (Zhao et al, 2006). Interestingly, the interaction of ephrinA2-EphA2 inhibits osteoblast differentiation at the initiation phase of bone remodeling, whereas ephrinA2-EphA2 stimulate osteoclast formation (Irie et al, 2009).…”

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“…One possible explanation for this result lies in a report by Irie et al (2009) that EphA2 forward signaling in osteoblasts inhibits differentiation of cultured neonatal osteoblasts. In this scenario, unclustered EphA4/Fc might in fact counter both osteogenic EphB signals and inhibitory EphA2 signals, thus canceling each other out.…”

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“…Ephs and ephrins are cell-surface molecules that signal bidirectionally and play major roles in tumor progression, angiogenesis, cell migration, and morphogenesis (16). Irie et al reported that bidirectional signaling through ephrinA2/EphA2 enhances osteoclastogenesis and suppresses OB differentiation (17). In contrast, ephrinB and EphB Paget's disease (PD) is characterized by focal and dramatic bone resorption and formation.…”