A₃ adenosine receptor activation triggers phosphorylation of protein kinase B and protects rat basophilic leukemia 2H3 mast cells from apoptosis

Adenosine accumulates to high levels in inflamed or ischemic tissues and activates A₃ adenosine receptors (ARs) on mast cells to trigger degranulation. Here we show that stimulation of rat basophilic leukemia (RBL)-2H3 mast-like cells with the A₃ AR agonists N⁶-(3-iodo)benzyl-5′-N-methylcarboxamidodoadenosine (IB-MECA; 10 nM) or inosine (10 μM) stimulates phosphorylation of protein kinase B (Akt). IB-MECA (1 μM) also causes a <50% reduction in apoptosis caused by exposure of RBL-2H3 cells to UV light. Akt phosphorylation is not stimulated by 100 nM N⁶-cyclopentyladenosine (A₁-selective) or CGS21680 (A_2A-selective) and is absent in cells pretreated with wortmannin or pertussis toxin. The K_i values of the AR antagonists BW-1433 and 8-sulfophenyltheophylline (8-SPT) were determined in radioligand binding assays for all four subtypes of rat ARs: BW-1433 (A₁, 5.8 ± 1.0 nM; A_2A, 240 ± 37; A_2B, 30 ± 10; A₃, 12,300 ± 3,700); 8-SPT (A₁, 3.2 ± 1.2 μM; A_2A, 57 ± 4; A_2B, 2.2 ± 0.8; A₃, > 100). BW-1433 and the A₃-slective antagonist MRS1523 (5 μM), but not 8-SPT (100 μM), block IB-MECA-induced protection from apoptosis, confirming the A₃ AR as the mediator of the antiapoptotic response. The data suggest that adenosine and inosine activate Gi-coupled A₃ ARs to protect mast cells from apoptosis by a pathway involving the μγ, subunits of Gi, phosphatidylinositol 3-kinase β, and Akt. We speculate that activation of A₃ ARs on mast cells or other cells that express A₃ ARs (e.g., eosinophils) may facilitate their survival and accumulation in inflamed tissues.