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

The A₃ 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 A₃ receptor (A₃R-CHO) to selective A₃ receptor ligands. At micromolar concentrations, the A₃ agonists N⁶-(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 A₁ receptor. Cl-IB-MECA also reduced cell number in the human HEK293 cell line transfected with the human A₃ receptor cDNA as opposed to the respective untransfected wild-type cells. In A₃R-CHO, agonist-induced effects were antagonized by nanomolar concentrations of A₃ antagonists, including the triazoloquinazoline derivative MRS1220, the dihydropyridine derivative MRS1191, and the triazolonaphthyridine derivative L-249,313. A₃ 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 A₃ receptor in A₃R-CHO results in markedly impaired cell cycle progression, suggesting an important role for this adenosine receptor subtype in cell cycle regulation and cell growth.