Human adenosine A1, A2A, A2B, and A3 receptors expressed in Chinese hamster ovary cells all mediate the phosphorylation of extracellular-regulated kinase 1/2

The known diverse effects of adenosine on mitogenesis may be related to changes in mitogen-activated protein kinases. In this study we therefore compared the phosphorylation of extracellular-regulated kinase 1/2 (ERK1/2) via the four known human adenosine receptors A(1), A(2A), A(2B), and A(3), stably transfected into Chinese hamster ovary (CHO) cells. The adenosine analog 5'-N-ethylcarboxamidoadenosine (NECA), known to act on all subtypes, had no effect on untransfected CHO cells, but did cause a substantial time- and dose-dependent phosphorylation in CHO cells transfected with each of the receptors. The maximal phosphorylation was highest in A(1) and A(3) receptor-transfected cells, intermediate in A(2A) and low in A(2B) receptor-expressing CHO cells. For all receptors the half-maximal ERK1/2 phosphorylation was observed at 19-115 nM NECA. NECA acting on adenosine A(2B) receptors was much more potent in stimulating ERK1/2 phosphorylation (EC50 = 19 nM) than cAMP formation (EC50 = 1.4 mu M). Stimulation with the endogenous ligand adenosine resulted in the same pattern of ERK1/2 phosphorylation as NECA. Concentrations of adenosine that occur physiologically caused an increased phosphorylation after 5 min in CHO cells transfected with any one of the four adenosine receptors. Adenosine at levels reached during ischemia (3 mu M) induced a more pronounced, but still transient, activation of ERK1/2. In conclusion, this study shows that all the human adenosine receptors transfected into CHO cells are able to activate ERK1/2 at physiologically relevant concentrations of the endogenous agonist.