Adenosine A1 and A3 receptors protect astrocytes from hypoxic damage

Brain levels of adenosine are elevated during hypoxia. Through effects on adenosine receptors (A1, A2A, A2B and A3) on astrocytes, adenosine can influence functions such as glutamate uptake, reactive gliosis, swelling, as well as release of neurotrophic and neurotoxic factors having an impact on the outcome of metabolic stress. We have studied the roles of these receptors in astrocytes by evaluating their susceptibility to damage induced by oxygen deprivation or exposure to the hypoxia mimic cobalt chloride (CoCl2). Hypoxia caused ATP breakdown and purine release, whereas CoCl2 (0.8 mM) mainly reduced ATP by causing cell death in human D384 astrocytoma cells. Further experiments were conducted in primary astrocytes prepared from specific adenosine receptor knock-out (KO) and wild type (WT) mice. In WT cells purine release following CoCl2 exposure was mainly due to nucleotide release, whereas hypoxia-induced intracellular ATP breakdown followed by nucleoside efflux. N-ethylcarboxamidoadenosine (NECA), an unselective adenosine receptor agonist, protected from cell death following hypoxia. Cytotoxicity was more pronounced in A1R KO astrocytes and tended to be higher in WT cells in the presence of the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Genetic deletion of A2A receptor resulted in less prominent effects. A3R KO glial cells were more affected by hypoxia than WT cells. Accordingly, the A3 receptor agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5′-carbamoyladenosine (CL-IB-MECA) reduced ATP depletion caused by hypoxic conditions. It also reduced apoptosis in human astroglioma D384 cells after oxygen deprivation. In conclusion, the data point to a cytoprotective role of adenosine mediated by both A1 and A3 receptors in primary mouse astrocytes.