The Adenosine A2B Receptor Drives Osteoclast-Mediated Bone Resorption in Hypoxic Microenvironments

Abstract

Osteoclast-mediated bone destruction is amplified in the hypoxic synovial microenvironment of rheumatoid arthritis (RA). This increased bone resorption is driven by the hypoxia-inducible transcription factor HIF. We identified hypoxic induction of the HIF-regulated adenosine A2B receptor in primary human osteoclasts (mRNA, 3.8-fold increase, p < 0.01) and sought to identify the role(s) of purinergic signaling via this receptor in the bone resorption process. Primary human osteoclasts were differentiated from CD14+ monocytes and exposed to hypoxia (2% O2) and A2B receptor inhibitors (MRS1754, PSB603). The hypoxic increase in bone resorption was prevented by the inhibition of the A2B receptor, at least partly by the attenuation of glycolytic and mitochondrial metabolism via inhibition of HIF. A2B receptor inhibition also reduced osteoclastogenesis in hypoxia by inhibiting early cell fusion (day 3–4, p < 0.05). The A2B receptor is only functional in hypoxic or inflammatory environments when the extracellular concentrations of adenosine (1.6-fold increase, p < 0.05) are sufficient to activate the receptor. Inhibition of the A2B receptor under normoxic conditions therefore did not affect any parameter tested. Reciprocal positive regulation of HIF and the A2B receptor in a hypoxic microenvironment thus enhances glycolytic and mitochondrial metabolism in osteoclasts to drive increased bone resorption. A2B receptor inhibition could potentially prevent the pathological osteolysis associated with hypoxic diseases such as rheumatoid arthritis.

“Take My Bone Away?” Hypoxia and bone: A narrative review

“…One explanation may be that PHD3 depletion stimulates PHD2 expression to inhibit HIF-1α stabilisation and negate the catabolic effects, but this remains speculative. It is possible that the hypoxic-induced increase in osteoclast number is the result of osteoblast-osteoclast crosstalk (Shirakura et al, 2010;Xian et al, 2016), although studies examining osteoclasts alone were able to demonstrate increased differentiation, suggesting mediation by downstream effects (Arnett et al, 2003; Brandao-Burch, Kato & Matsushita, 2014;Knowles, 2019;Utting et al, 2010).…”

Osteocytes and Bone Metastasis

“…The solid tumor microenvironment is usually hypoxic and/or inflammatory, and the extracellular concentration of nucleotides (ATP/adenosine) is higher in comparison to normal tissue ( 92 , 103 , 104 ). In this hypoxic/inflammatory microenvironment, adenosine promotes cancer cell migration and chemotaxis in breast cancer and melanoma cells, along with an increase in osteoclastic activity and bone resorption ( Figure 1 , ⑥ and ⑦) ( 29 ). This microenvironment also results in poor immune surveillance with high lymphocytic tolerance ( Figure 1 , steps ⑥and ⑧) ( 92 , 105 , 106 ).…”

“…The mechanistic comprehension of bone turnover in tumor growth has led to the clinical use of osteoclast inhibiting bisphosphonates, and Denosumab (anti-RANKL antibody) in patients with bone metastasis, and has become the standard of care to improve quality of life by limiting bone turnover ( Figure 1 , step ②) ( 21 ). In addition to molecules directly involved in bone resorption, other factors involved in bone resorption include interleukins-6 and 11 ( 22 ), parathyroid hormone-related peptide (PTHrP) ( 23 , 24 ), soluble intercellular adhesion molecule 1 (ICAM-1) ( 25 ), Wnt molecules ( 26 , 27 ), macrophage-stimulating protein (MSP) ( 28 ), and extracellular adenosine ( Figure 1 , step ③) ( 29 ).…”

Adenosine-Related Mechanisms in Non-Adenosine Receptor Drugs

“…Conversely, in other disease states, the removal of extracellular adenosine, for example, as an indirect result of the pathological upregulation of intracellular adenosine kinase in the brain, can exacerbate epilepsy [5]. In bone, reduced A 2A AR or A 2B AR signaling has a deleterious effect in models of osteo-and rheumatoid arthritis [17,18]. The four AR subtypes (A 1 , A 2A , A 2B , and A 3 ; Figure 1) are important in the body's adaptation to stress [1,3], often in the context of increasing local blood flow by vasodilation.…”