Activation of the A3 adenosine receptor affects cell cycle progression and cell growth

The A(3) adenosine receptor has been implicated in modulation of cell growth. As a first step to the characterization of the underlying mechanisms, we exposed Chinese hamster ovary (CHO) cells transfected with the human A(3) receptor (A(3)R-CHO) to selective A(3) receptor ligands. At micromolar concentrations, the A(3) agonists N-6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) and its 2-chloro derivative Cl-IB-MECA reduced cell number, with no effects on either parental CHO cells (not expressing any adenosine receptor), or CHO cells transfected with the human A(1) receptor. Cl-IB-MECA also reduced cell number in the human HEK293 cell line transfected with the human A(3) receptor cDNA as opposed to the respective untransfected wild-type cells. In A(3)R-CHO, agonist-induced effects were antagonized by nanomolar concentrations of A(3) antagonists, including the triazoloquinazoline derivative MRS1220, the dihydropyridine derivative MRS 1191, and the triazolonaphthyridine derivative L-249,313, A(3) agonist-induced effects were not due to modulation of cell adhesion, nor to necrosis or apoptosis. Growth curves revealed highly impeded growth, and flow-cytometric analysis showed markedly reduced bromodeoxyuridine incorporation into nuclei. The effect on cell cycle was completely antagonized by MRS1191. Hence, activation of the human A(3) receptor in A(3)R-CHO results in markedly impaired cell cycle progression, suggesting an important role for this adenosine receptor subtype in cell cycle regulation and cell growth.