Adenosine A3 receptor-mediated regulation of p38 and extracellular-regulated kinase ERK1/2 via phosphatidylinositol-3'-kinase.

The adenosine A(3) receptor generally couples to the G(i) class of heterotrimeric G proteins, thereby decreasing cAMP levels and also mediating signaling via release of betagamma subunits. Here we describe the central role of phosphatidylinositol-3'-kinase (PI3K) for adenosine A(3) receptor-induced intracellular signaling to the stress-activated protein kinase p38 and the extracellular signal-regulated protein kinases ERK1/2. We used Chinese hamster ovary cells expressing the human adenosine A(3) receptor, phospho-specific antibodies and different pharmacological tools to dissect the signaling pathways involving PI3K. The adenosine receptor agonist 5'N-ethylcarboxamidoadenosine induced a time- and dose-dependent increase in p38 and ERK1/2 phosphorylation, two signaling pathways that appeared also to be activated in the immortalized microglia cell line N13, which expressed endogenous adenosine A(3) receptors. The 5'N-ethylcarboxamidoadenosine-induced effects on p38 and ERK1/2 in CHO cells were blocked by pertussis toxin pretreatment and were sensitive to pharmacological inhibition of PI3K. In addition, inhibition of Rac/Cdc42, small GTPases of the Rho family, by clostridium toxin B, diminished p38 phosphorylation but did not affect ERK1/2. Furthermore, we identified the serine 727 site of signal transducer and activator of transcription STAT3 as a probable downstream target of ERK1/2, and thereby provide evidence that adenosine A(3) receptor mediated ERK1/2 activation has functional consequences.