Abstract

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 A₁, A_2A, A_2B, and A₃, 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 A₁ and A₃receptor-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 μM). 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 μM) 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.