Background/Purpose Semaphorins (Sema), axonal guidance proteins, play a role in communication between osteoclast and osteoblast. Thus, sema4D, secreted by osteoclasts, binds to its receptor PlexinB1 on osteoblasts to inhibit osteoblast differentiation and function whereas sema3A, produced by osteoblasts, binds to PlexinA1/Neuropilin-1 to both inhibit RANKL-induced osteoclast differentiation and stimulate osteoblast differentiation and function. Because stimulation of A2AR diminishes osteolysis we asked whether A_2AR activation regulates bone homeostasis by regulating osteoclast and osteoblast expression of semaphorins.

Methods Osteoclast and osteoblast differentiation were studied in primary murine bone marrow culture as the number of TRAP-positive or Alizarin Red-positive cells, respectively, after challenge in the presence/absence of CGS21680 (A_2A agonist) 1µM and ZM241385 (A_2AR antagonist) 1µM together with recombinant Sema4D or Sema3A 10ng/ml each. Sema3A/PlexinA1/Neuropilin-1 and Sema4D/PlexinB1 expression were studied by RT-PCR and Western Blot in bone marrow-derived osteoclasts and osteoblasts in the presence/absence of CGS21680 and ZM241385 1µM each. RANKL and Osteoprotegerin (OPG) levels were studied by RT-PCR. β-catenin activation was studied in primary osteoblast culture. Cytoskeleton changes were studied in osteoclasts.

Results RANKL induced a 2.5±0.1 fold increase in Sema4D mRNA (p<0.001,n=4) in osteoclasts which was blocked by CGS21680 (1.3±0.3 fold change, p<0.001,n=4). In contrast, PlexinA1 mRNA was enhanced by CGS21680 (9.3±0.7 fold increase vs 4.9±0.6 for RANKL, p<0.001,n=4) but Neuropilin-1 mRNA was unchanged. Sema3A mRNA increased 3.5±0.5 fold during osteoblast differentiation and CGS21680 enhanced this increase (8.7±0.2 fold, p<0.001,n=4); PlexinB1 mRNA was increased 2 fold during osteoblast differentiation and was not altered by CGS21680. Similar changes were observed at the protein level. CGS21680 decreased RANKL expression and increased OPG expression in osteoblasts. Total and nuclear β-catenin expression were increased in osteoblasts after CGS21680 treatment and this increase was abrogated by ZM241385. Sema4D increased RhoA phosphorylation and FAK activation in osteoclast precursors and these effectswere reversed in the presence of CGS21680.

Conclusion A2AR activation diminishes secretion of Sema4D by osteoclasts and enhances secretion of Sema3A by osteoblasts leading to an increase in osteoblast differentiation and function, and, in combination with the suppressive effects of A_2AR on osteoclast differentiation and function, diminishes bone osteolysis.

---

« Back to 2014 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/adenosine-a2a-receptor-a2ar-stimulation-inhibits-osteoclast-differentiation-and-promotes-osteoblast-formation-by-regulation-of-axon-guidance-proteins/