Adenosine A₁receptors (A₁Rs) play a critical role in osteoclast formation and function

Abstract

Adenosine regulates a wide variety of physiological processes via interaction with one or more G-protein-coupled receptors (A(1)R, A(2A)R, A(2B)R, and A(3)R). Because A(1)R occupancy promotes fusion of human monocytes to form giant cells in vitro, we determined whether A(1)R occupancy similarly promotes osteoclast function and formation. Bone marrow cells (BMCs) were harvested from C57Bl/6 female mice or A(1)R-knockout mice and their wild-type (WT) littermates and differentiated into osteoclasts in the presence of colony stimulating factor-1 and receptor activator of NF-kappaB ligand in the presence or absence of the A(1)R antagonist 1,3-dipropyl-8-cyclopentyl xanthine (DPCPX). Osteoclast morphology was analyzed in tartrate-resistant acid phosphatase or F-actin-stained samples, and bone resorption was evaluated by toluidine blue staining of dentin. BMCs from A(1)R-knockout mice form fewer osteoclasts than BMCs from WT mice, and the A(1)R antagonist DPCPX inhibits osteoclast formation (IC(50)=1 nM), with altered morphology and reduced ability to resorb bone. A(1)R blockade increased ubiquitination and degradation of TRAF6 in RAW264.7 cells induced to differentiate into osteoclasts. These studies suggest a critical role for adenosine in bone homeostasis via interaction with adenosine A(1)R and further suggest that A(1)R may be a novel pharmacologic target to prevent the bone loss associated with inflammatory diseases and menopause.

The role of purinergic signalling in the musculoskeletal system

“…Pellegatti et al (2011) reported that the A1 receptor was only weakly expressed by osteoclasts and activation of the A1 receptor was recently shown to have no effect on mouse osteoclasts (Pellegatti et al, 2011;He et al, 2012). In contrast, blockade or deletion of the A1 receptor can reduce the formation of mouse osteoclasts in culture (Kara et al, 2010a) and A1 receptor knockout mice display increased bone mineral density and resistance to ovariectomy-induced bone loss (Kara et al, 2010b) Adenosine has also been reported to stimulate osteoclastogenesis indirectly. Evans et al (2006) suggested that the adenosine produced from the hydrolysis of released ATP acts on P1 receptors to cause IL-6 release and inhibition of osteoprotegerin secretion (Evans et al, 2006).…”

Adenosine A1 receptor regulates osteoclast formation by altering TRAF6/TAK1 signaling

“…Recent studies in our laboratory have revealed a novel role for adenosine/A 1 receptor (A 1 R) in osteoclastogenesis: A 1 R activation is required for both osteoclast formation and function in vitro and only function in vivo, as demonstrated using pharmacologic inhibitors and mice lacking adenosine A 1 receptors [29,30]. The disparity between in vitro and in vivo osteoclast formation is reminiscent of a similar disparity in osteoclast formation in vitro and in vivo in mice lacking either TRAF6 or Atp6v0d2 in which osteoclasts are present in vivo, although functionally defective [31] and do not form from precursors in vitro [32,33]).…”

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

“…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, we found that mouse osteoclasts only express low levels of A 1 receptor protein.…”