Activation of P2X(7) receptors stimulates the expression of P2Y(2) receptor mRNA in astrocytes cultured from rat brain.

Under pathological conditions brain cells release ATP at concentrations reported to activate P2X,
ionotropic receptor subtypes expressed in both neuronal and glial cells. In the present study we report
that the most potent P2X, receptor agonist BzATP stimulates the expression of the metabotropic ATP
receptor P2Y2 in cultured rat brain astrocytes. In other cell types several kinds of stimulation, including
stress or injury, induce P2Y2 expression that, in turn, is involved in different cell reactions. Similarly, it
has recently been found that in astrocytes and astrocytoma cells P2Y2 sites can trigger neuroprotective
pathways through the activation of several mechanisms, including the induction of genes for antiapoptotic
factors, neurotrophins, growth factors and neuropeptides. Here we present evidence that P2Y2 mRNA
expression in cultured astrocytes peaks 6 h after BzATP exposure and returns to basal levels after 24 h.
This effect was mimicked by high ATP concentrations (1 mM) and was abolished by P2X7-antagonists
oATP and BBG. The BzATP-evoked P2Y2 receptor up-regulation in cultured astrocytes was coupled
to an increased UTP-mediated intracellular calcium response. This effect was inhibited by oATP and
BBG and by P2Y2siRNA, thus supporting evidence of increased P2Y2 activity. To further investigate the
mechanisms by which P2X7 receptors mediated the P2Y2 mRNA up-regulation, the cells were pre-treated
with the chelating agent EGTA, or with inhibitors of mitogen-activated kinase (MAPK) (PD98059) or
protein kinase C, (GF109203X). Each inhibitor signi6cantly reduced the extent to which BzATP induced
P2Y2 mRNA. Both BzATP and ATP (1 mM) increased ERKI/2 activation. P2X,-induced ERKI/2
phosphorylation was unaffected by pre-treatment of astrocytes with EGTA whereas it was inhibited by
GF109203X. Phorbol-12-myristate-13-acetate (PMA), an activator of PKCs, rapidly increased ERKI/2
activation. We conclude that activation of P2X, receptors in astrocytes enhances P2Y2 mRNA expression
by a mechanism involving both calcium inOux and PKC/MAPK signalling pathways.