Background/Purpose: Communication between osteoclasts and osteoblasts is essential for bone homeostasis. Semaphorin4D (Sema4D), expressed on the surface of and secreted by osteoclasts, macrophages and T cells, is a potent inhibitor of bone formation; Sema4D binds to its receptor (PlexinB1) on osteoblasts which induces RhoA activation which suppresses osteoblast differentiation by attenuating insulin-like growth factor-1 (IGF-1) signaling. Adenosine, a nucleoside released at sites of injury and hypoxia, modulates cell function by interacting with specific cell-surface receptors (A₁R, A_2AR, A_2BR, A₃R). We have recently demonstrated that A_2AR stimulation inhibits wear particle-mediated bone destruction and sought to determine whether A_2AR-mediated regulation of Sema4D expression might play a role in preventing bone destruction.

Methods: C57Bl/6 mice mice age 6-8 weeks were anesthetized and a 1cm midline sagittal incision was made over the calvaria. 4 mice received no particles (Control), and the rest received 3mg of polyethylene particles (UHMWPE) together with 20µl of saline 0.9% or  CGS21680 1µM (n=4 each) at the surgical site every day. Animals were sacrificed after 14 days and calvaria were prepared for histology. Sema4D expression was studied by RT-PCR and Western Blot in RAW267.4 cell-derived osteoclasts in the presence/absence of CGS21680 and ZM241385 (A_2AR antagonist) 1µM each.

Results: As previously described, exposure to UHMWPE particles induce inflammatory infiltration that is significantly decreased in the presence of CGS21680. Exposure to UHMWPE particles significantly increased expression of both Sema4D (224±5cells/Ipf compare to 40±2cells/Ipf in control, p<0.001, n=4) and its receptor PlexinB1 (206±5cells/Ipf compare to 86±4cells/Ipf in control, p<0.001, n=4) on bone surfaces close to inflammatory infiltrates and CGS21680 treatment markedly diminished overexpression of both Sema4D and PlexinB1 (34±3cells/Ipf and 85±3cells/Ipf respectively, p<0.001, n=4). In in vitrostudies, RANKL induced a 2.5±0.1 fold increase in Sema4D mRNA expression (p<0.001. n=4) that is completely blocked by CGS21680 in a protein kinase A-dependent fashion.

Conclusion: Inflammation promotes Sema4D secretion and PlexinB1 activation. Targeting osteoclasts via A_2AR activation prevents wear particle-stimulated bone resorption and the inhibition of Sema4D and PlexinB1 expression likely bone resorption by permitting greater osteoblast differentiation and bone formation. Moreover, these results suggest a novel approach to prevent bone resorption in inflammatory arthritis or infectious arthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/adenosine-a2a-receptor-diminishes-bone-destruction-at-inflamed-sites-in-part-via-downregulating-semaphorin4d-plexinb1-communication-between-osteoclasts-and-osteoblasts/