Human adenosine A(1), A(2A), A(2B), and A(3) 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 cha nges in mitogen-activated protein kinases. In this study we therefore compa red 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), stab ly transfected into Chinese hamster ovary (CHO) cells. The adenosine analog 5'-N-ethylcarboxamidoadenosine (NECA), known to act on all subtypes, had n o effect on untransfected CHO cells, but did cause a substantial time- and dose-dependent phosphorylation in CHO cells transfected with each of the re ceptors. The maximal phosphorylation was highest in A(1) and A(3) receptor- transfected cells, intermediate in A(2A) and low in A(2B) receptor-expressi ng 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 m uch more potent in stimulating ERK1/2 phosphorylation (EC50 = 19 nM) than c AMP formation (EC50 = 1.4 mu M). Stimulation with the endogenous ligand ade nosine resulted in the same pattern of ERK1/2 phosphorylation as NECA. Conc entrations of adenosine that occur physiologically caused an increased phos phorylation after 5 min in CHO cells transfected with any one of the four a denosine receptors. Adenosine at levels reached during ischemia (3 mu M) in duced a more pronounced, but still transient, activation of ERK1/2. In conc lusion, this study shows that all the human adenosine receptors transfected into CHO cells are able to activate ERK1/2 at physiologically relevant con centrations of the endogenous agonist.