Abstract
Extracellular nucleotides play important trophic roles in development and central nervous system (CNS) injury, but the functions of distinct purinergic receptors and related signaling pathways have not been fully elucidated. In the present study we identified opposing effects of P2X and P2Y receptors on the ability of FGF2 to induce proliferation in primary cultures of rat cortical astrocytes. Low concentrations of ATP enhanced DNA synthesis induced by FGF2, whereas high concentrations inhibited FGF2-induced proliferation. Comparison of concentration-response experiments with ATP and 2',3'-O-(4-benzoyl)-benzoyl-ATP (BzATP) indicated that the inhibitory effect was mediated by P2X(7) receptors. Interestingly, activation of P2X(7) receptors led to a state of reversible growth arrest rather than cell death. Selectivity studies showed that proliferation evoked by epidermal growth factor and platelet-derived growth factor was also inhibited by P2X(7) receptors, but P2X(1) or P2X(3) receptors did not inhibit proliferation induced by FGF2. A marker of mitosis, phosphohistone-3, was reduced by BzATP and increased by UTP, suggesting that the enhancing effect of ATP on FGF2-induced proliferation was mediated by P2 purine/pyrimidine receptors. Phosphorylation of the growth arrest-related protein kinases p38/MAPK and SAPK/JNK was strongly increased by BzATP but only weakly affected by UTP. We conclude that P2Y purine/pyrimidine receptors enhance proliferation induced by FGF2 in astrocytes, whereas stimulation of P2X(7) receptors inhibits proliferation by shifting cells to a state of reversible growth arrest that may be mediated by protein kinase signaling. These trophic actions of P2X(7) and P2Y purine/pyrimidine receptors may contribute to the regulation of CNS development, adult neurogenesis, and the response of astrocytes to injury.
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