Received for publication January 20, 2006.
Revised May 12, 2006.
Accepted for publication May 17, 2006.

Cross-talk between Gs and Gq-coupled Pathways in Regulation of IL-4 by A_2B Adenosine Receptors in Human Mast Cells

Abstract

Human mast cells express functional A_2A and A_2B adenosine receptors. However, only stimulation of A_2B but not A_2A leads to secretion of IL-4, an important step in adenosine receptor-mediated induction of IgE synthesis by B-cells. Here we studied intracellular pathways that link stimulation of A_2B receptors to IL-4 upregulation in HMC-1 mast cells. Both A_2A and A_2B receptors couple to Gs proteins and stimulate adenylate cyclase, but only A_2B stimulates phospholipase C through coupling to Gq proteins leading to activation of protein kinase C and calcium mobilization. Inhibition of phospholipase C with U73122 completely blocked A_2B receptor-dependent IL-4 secretion. The protein kinase C inhibitor Ro-32-0432 had no effect on A_2B-mediated, but inhibited PMA-stimulated IL-4 secretion. In contrast, chelation of intracellular Ca²⁺ inhibited both A_2B receptor-mediated and ionomycin-induced IL-4 secretion. This Ca²⁺-sensitive pathway most likely includes calcineurin and nuclear factor of activated T-cells (NFAT), because A_2B receptor-dependent IL-4 secretion was blocked with cyclosporine A or 11R-VIVIT peptide. Gs-linked pathways also play a role in the A_2B-dependent stimulation of IL-4 secretion; inhibition of adenylate cyclase with 2,'5'-dideoxyadenosine, or protein kinase A with Rp-cAMP or H-89, attenuated A_2B receptor-dependent IL-4 secretion. Although stimulation of adenylate cyclase with forskolin did not increase IL-4 secretion on its own, it potentiated the effect of Pasteurella multocida toxin by 2-fold and ionomycin by 3-fold. Both forskolin and stimulation of A_2B receptors upregulated NFATc1 protein levels. We conclude that A_2B receptors upregulate IL-4 through Gq signaling that is potentiated via crosstalk with Gs-coupled pathways.

Key words: Adenosine, Purinergic

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