Roles of Bcl-2 and caspase 3 in the adenosine A₃ receptor-induced apoptosis

Selective A₃ adenosine receptor agonists have been shown to induce apoptosis in a variety of cell types. In this study we examined the effects of adenosine receptor agonists selective for A₁, A_2A, or A₃ receptors on the induction of apoptosis in primary cultures of rat astrocytes and in C6 glial cells. Treatment of the cells with the A₃ receptor agonist Cl-IB-MECA (10 μM) induced apoptosis in both cell types. The effects of Cl-IB-MECA were partially antagonized by the A₃ receptor-selective antagonist MRS 1191. In contrast, the A₁ and A_2A receptor agonists, CPA and CGS 21680, respectively, did not have significant effects on apoptosis in these cells. Cl-IB-MECA reduced the expression of endogenous Bcl-2, whereas it did not affect the expression of Bax. Overexpression of Bcl-2 in C6 cells abrogated the induction of apoptosis induced by the A₃ agonist. CMB-MECA also induced an increase in caspase 3 activity and caspase inhibitors decreased the apoptosis induced by the A₃ agonist. These findings suggest that intense activation of the A₃ receptor is pro-apoptotic in glial cells via bcl2 and caspase-3 dependent pathways.