Adenosine A₂ receptor‐induced inhibition of leukotriene B₄ synthesis in whole blood ex vivo

Abstract

Engagement of adenosine A2 receptors suppresses several leukocyte functions. In the present study, we examined the effect of adenosine on the inhibition of leukotriene B4 (LTB4) synthesis in heparinized human whole blood, pretreated with lipopolysaccharide (LPS) and tumour necrosis factor α (TNF‐α) and stimulated with the chemotactic peptide, N‐formyl‐Met‐Leu‐Phe (FMLP). The FMLP‐induced synthesis of LTB4 in whole blood pretreated with LPS and TNF‐α was dose‐dependently inhibited by adenosine analogues in the following order of potency; 5′(N‐ethyl)carbox‐ amidoadenosine (NECA) ≅ CGS 21680 > 2‐Cl‐adenosine > N6‐cyclopentyladenosine (CPA), indicating the involvement of the adenosine A2 receptor subtype. The IC50 values for NECA, CGS 21680, 2‐Cl‐ adenosine, and CPA were 6nM, 9nM, 180nM, and 990 nM, respectively. Dipyridamole, an agent that blocks the cellular uptake of adenosine by red cells and causes its accumulation in plasma, also inhibited the synthesis of LTB4 in LPS and TNF‐α‐treated whole blood stimulated by FMLP; moreover, this inhibition was reversed upon addition of adenosine deaminase. A highly selective antagonist of the adenosine A2 receptor, 8‐(3‐chlorostyryl)caffeine (CSC), reversed the inhibition of LTB4 synthesis by 2‐Cl‐adenosine and dipyridamole in LPS and TNF‐α‐treated whole blood, stimulated by FMLP. LTB4 synthesis in whole blood originates predominantly from neutrophils and to a lesser extent from monocytes. 2‐Cl‐adenosine also inhibited the synthesis of LTB4 induced by FMLP in these isolated LPS and TNF‐α‐treated cells; however, 2‐Cl‐adenosine was a more potent inhibitor of LTB4 synthesis in neutrophils than monocytes. The present data demonstrate that adenosine, acting through A2 receptors, exerts a potent inhibitory effect on the synthesis of LTB4 and thus contribute to the understanding of its anti‐inflammatory properties.

The effects of phosphodiesterase type 4 inhibitors on tumour necrosis factor‐α and leukotriene B4 in a novel human whole blood assay

“…The non-selective PDE inhibitors and the PDE III-selective inhibitors were not potent against either mediator and the PDE V-selective inhibitors were not active. The activity observed with dipyridamole was probably due to its inhibition on adenosine uptake by red blood cells rather than its inhibition of PDE V (Krump et al, 1996;. The 5-LO inhibitor L-739,010 had no e ect on TNF-a but was very potent against LTB 4 .…”

Asthma and leukotrienes: antileukotrienes as novel anti‐asthmatic drugs

“…A [46,77,79]. It has been shown that adenosine is an inhibitor of transcellular metabolism of LTA 4 , released by leucocytes upon activation with physiological agonists [79]. The relative role of transcellular metabolism in the formation of LTC 4 in asthma and other pathophysiological conditions remains to be clarified.…”

“…Several lines of evidence suggest that stimulated leucocytes can release arachidonic acid or LTA 4 , which can be further metabolized by other surrounding cells (so-called transcellular metabolism) [74][75][76][77][78]. A [46,77,79]. It has been shown that adenosine is an inhibitor of transcellular metabolism of LTA 4 , released by leucocytes upon activation with physiological agonists [79].…”

Caffeine Modulates TNF-α Production by Cord Blood Monocytes: The Role of Adenosine Receptors

“…At therapeutic range (5 to 15 g/mL), caffeine blocks A 1 and A 2a adenosine receptors (ARs) stimulating ventilation (2-4). Recently, caffeine has also been linked to a decrease in the incidence of bronchopulmonary dysplasia and cerebral palsy in extremely premature infants (5,6), although the mechanisms explaining these findings have not been elucidated.The natural ligand for ARs, adenosine, has a crucial role in multiple biologic processes including inflammation (7,8). The increase in tumor necrosis factor-alpha (TNF-␣) release by adult peripheral blood monocytes (PBM) in response to lipopolysaccharide (LPS) exposure can be abolished by pretreatment with A 2a R agonists (9,10).…”

“…The natural ligand for ARs, adenosine, has a crucial role in multiple biologic processes including inflammation (7,8). The increase in tumor necrosis factor-alpha (TNF-␣) release by adult peripheral blood monocytes (PBM) in response to lipopolysaccharide (LPS) exposure can be abolished by pretreatment with A 2a R agonists (9,10).…”