Adenosine A2A Receptor Ligation Inhibits Osteoclast Formation

Abstract

Adenosine is generated in increased concentrations at sites of injury/hypoxia and mediates a variety of physiological and pharmacological effects via G protein-coupled receptors (A(1), A(2A), A(2B), and A(3)). Because all adenosine receptors are expressed on osteoclasts, we determined the role of A(2A) receptor in the regulation of osteoclast differentiation. Differentiation and bone resorption were studied as the macrophage colony-stimulating factor-1-receptor activator of NF-κB ligand formation of multinucleated tartrate-resistant acid phosphatase (TRAP)-positive cells from primary murine bone marrow-derived precursors. A(2A) receptor and osteoclast marker expression levels were studied by RT-PCR. Cytokine secretion was assayed by enzyme-linked immunosorbent assay. In vivo examination of A(2A) knockout (KO)/control bones was determined by TRAP staining, micro-computed tomography, and electron microscopy. The A(2A) receptor agonist, CGS21680, inhibited osteoclast differentiation and function (half maximal inhibitory concentration, 50 nmol/L), increased the percentage of immature osteoclast precursors, and decreased IL-1β and tumor necrosis factor-α secretion, an effect that was reversed by the A(2A) antagonist, ZM241385. Cathepsin K and osteopontin mRNA expression increased in control and ZM241385-pretreated osteoclasts, and this was blocked by CGS21680. Micro-computed tomography of A(2A)KO mouse femurs showed a significantly decreased bone volume/trabecular bone volume ratio, decreased trabecular number, and increased trabecular space. A(2A)KO femurs showed an increased TRAP-positive osteoclast. Electron microscopy in A(2A)KO femurs showed marked osteoclast membrane folding and increased bone resorption. Thus, adenosine, acting via the A(2A) receptor, inhibits macrophage colony-stimulating factor-1-receptor activator of NF-κB ligand-stimulated osteoclast differentiation and may regulate bone turnover under conditions in which adenosine levels are elevated.

Lack of effect of adenosine on the function of rodent osteoblasts and osteoclasts in vitro

“…These results are in agreement with some previous investigations [16,17] but differ from others. For example, A 2A receptor agonists have been shown to both inhibit [33] and stimulate [25] the formation and activity of osteoclasts. Kara and colleagues [51] found that A 1 receptor antagonists decreased osteoclast formation and resorption.…”

“…However, more recent work has indicated that adenosine, acting through the A 2A receptor, may stimulate the formation of osteoclasts from human peripheral blood cells [25]. In contrast, Mediero et al (2012) [33] found that A 2A receptor agonists inhibited mouse osteoclast formation in vitro. Blockade or deletion of the A 1 receptor has additionally been reported to reduce the formation of mouse osteoclasts in culture [23]; however, the same group also found that stimulation of the A 1 receptor had no effect on mouse osteoclasts [34].…”

Purinergic signalling in the musculoskeletal system

“…ATP released through P2X7 receptors acts as the major source of adenosine that enhances mature osteoclast function via A 2A receptors [273]. However, a later study showed that adenosine, acting through A 2A receptors, inhibited osteoclast differentiation [285]. Thus, the authors suggested that adenosine may regulate bone turnover under conditions when levels are elevated [285].…”

“…However, a later study showed that adenosine, acting through A 2A receptors, inhibited osteoclast differentiation [285]. Thus, the authors suggested that adenosine may regulate bone turnover under conditions when levels are elevated [285]. A 2A receptor activation has also been shown to prevent the osteolysis that occurs during prosthesis loosening with particle-induced inflammation, a common cause of joint implant failure [286].…”

Does adenosine play a role in bone formation, resorption and repair?

“…Since bone remodelling is orchestrated by osteoblasts, osteoclasts and osteocytes, other studies from Cronstein's laboratory [8][9][10][11] have recently demonstrated a role for the A1 and A2A ARs in osteoclast differentiation and function, again using KO mouse as study models. Furthermore, there is much recent data on the role of adenosine in joint disease such as arthritis (see e.g., [12,13]).…”