Adenosine A₁ receptors (A₁Rs) play a critical role in osteoclast formation and function

Adenosine regulates a wide variety of physiological processes via interaction with one or more G-protein-coupled receptors (A₁R, A _2AR, A_2BR, and A₃R). Because A₁R occupancy promotes fusion of human monocytes to form giant cells in vitro, we determined whether A₁R occupancy similarly promotes osteoclast function and formation. Bone marrow cells (BMCs) were harvested from C57Bl/6 female mice or A₁R-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-κB ligand in the presence or absence of the A₁R 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 A₁R-knockout mice form fewer osteoclasts than BMCs from WT mice, and the A₁R antagonist DPCPX inhibits osteoclast formation (IC₅₀=1 nM), with altered morphology and reduced ability to resorb bone. A₁R 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 A₁R and further suggest that A₁R may be a novel pharmacologic target to prevent the bone loss associated with inflammatory diseases and menopause.