Background/Purpose: The Wnt/β-catenin signaling pathway plays a key role in regulating bone formation and maintaining bone hemostasis. Wnt activates a pathway that leads to stabilization of β-catenin and its translocation to the nucleus. Osteoblast differentiation and proliferation are also regulated by adenosine receptors, among other signals. We recently reported that A2aR signaling promotes Wnt/β-catenin signaling in fibroblasts via activation of Akt and p38MAPK. In the present study we sought to determine whether there is a similar interaction between these pathways in osteoblasts.

Methods: We studied murine osteoblast cell line (MC3T3-E1) and primary osteoblasts derived from bone marrow-derived mesenchymal stem cells of mice. The cells were treated with CGS21680, a selective A2aR agonist, at doses ranging from 0 to 10µM, and for varying incubation periods up to 240 minutes. Levels of phosphorylated β-catenin at Ser552 (p-Ser552), a β-catenin isoform with enhanced transcriptional activity, were measured by Western Blot assays before and after A2aR activation. We also analyzed nuclear translocation of p-Ser552 β-catenin in the osteoblastoid cell line and primary cell cultures using immunofluorescence (IF) staining. Cellular levels of activated AKT were measured by immunoblotting assays before and following administration of CGS21680.

Results: We observed a significant increase in p-Ser552 β-catenin levels in the osteoblastoid cells treated with 1 µM CGS21680 compared to the control, starting at 15 minutes following A2aR activation (253±122%, p<0.05, n=5). Western blot analysis showed a significant increase in nuclear translocation of p-Ser552 β-catenin at 15 minutes after treatment with A2aR agonist in MC3T3-E1 cells (153±37%, p<0.05, n=4), and primary osteoblasts (148±31%, p<0.05, n=4). Similarly, immunofluorescence revealed approximately a 40% increase in nuclear accumulation of p-Ser552 β-catenin in CGS21680-treated MC3T3-E1 cells as well as in primary osteoblasts. We also found a significant increase in the levels of phosphorylated AKT at Ser473 among osteoblastoid cells following A2aR stimulation (203±47%, p<0.05, n=4).

Conclusion: These findings demonstrate cross-talk between A2aR and Wnt/β-catenin signaling pathways in osteoblasts. Moreover, our results suggest that A2aR activation can bypass blockade of Wnt ligands at the cell surface and thereby maintain bone homeostasis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/signaling-at-adenosine-a2a-receptor-a2ar-in-osteoblasts-crosstalk-with-wnt-%ce%b2-catenin-signaling-pathway/