The role of adenosine A 2A and A 2B receptors in the regulation of TNF-α production by human monocytes

Abstract

Adenosine is an endogenous nucleoside that regulates many physiological processes through the activation of its four receptors: A 1, A 2A, A 2B and A 3. Previous studies have identified the involvement of A 2 receptors in the inhibitory activity of adenosine analogues on tumor necrosis factor-α (TNF-α) production by lipopolysaccharide (LPS) activated monocytes, but the relative contributions of A 2A versus A 2B receptors have not been determined in human primary monocytes. Nor has the role of A 1 and A 3 been clearly identified in the system. The lack of such information impacts on the selection of adenosine receptor agonists for disease intervention. Using LPS-stimulated human primary monocytes, we found that the adenosine receptor agonist, 5′- N-ethylcarboxamidoadenosine (NECA) or the A 2A receptor agonist, 4-[2-[[6-amino-9-( N-ethyl- b- d-ribofuranuronamidosyl)-9 H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS21680) produced a concentration-dependent inhibition of TNF-α production, with IC 50s of 58.4 nM (32.7–104.5 nM, 95% confidence interval) and 49.2 nM (22.7–105.9 nM, 95% confidence interval), respectively. The selective A 2A receptor blocker, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylaminso]ethyl)phenol (ZM241385, 30 nM), antagonized the effects of NECA and CGS21680 ( pK B estimates were 8.7 ± 0.1 and 8.9 ± 0.1, respectively), while the selective A 2B antagonist, N-(4-cyano-phenyl)-2-[4-(2,6-dioxo-1,3-dipropyl-2,3,4,5,6,7-hexahydro-1H-purin-8-yl)-phenoxy]-acetamide (MRS1754, 100 nM), failed to antagonize the effects of either agonist. Furthermore, neither the A 1 receptor agonist, 2-chloro- N 6-cyclopentyladenosine (CCPA) nor the A 3 receptor agonist, 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy- N-methyl- b- d-ribofuranuronamide (2-Cl-IB-MECA) showed significant inhibitory activity at concentrations that effectively bind to their respective receptors. We conclude that A 2A receptor activation is predominantly responsible for the inhibitory effects of adenosine receptor agonists on TNF-α production from LPS-stimulated monocytes.