Protein kinase C activation reduces microglial cyclic AMP response to prostaglandin E2 by interfering with EP2 receptors

Abstract

We studied the modulation by protein kinase C (PKC) of the cyclic AMP (cAMP) accumulation induced by prostaglandin (PG) E2 in rat neonatal microglial cultures. Short pretreatment of microglia with phorbol 12- myristate 13-acetate (PMA) or 4β-phorbol 12,13-didecanoate, which activate PKC, but not with the inactive 4α-phorbol 12,13-didecanoate, substantially reduced cAMP accumulation induced by 1 μM PGE2. The action of PMA was dose and time dependent, and the maximal inhibition (~85%) was obtained after 10- min preincubation with 100 nM PMA. The inhibitory effect of PMA was mimicked by diacylglycerol and was prevented by the PKC inhibitor calphostin C. As PMA did not affect isoproterenol- or forskolin-stimulated cAMP accumulation, we investigated whether activation of PKC decreased cAMP production by acting directly at PGE2 EP receptors. Neither sulprosione (10-9-10-5 M), a potent agonist at EP3 receptors (coupled to adenylyl cyclase inhibition), nor 17-phenyl-PGE2 (10-6-10-5 M), an agonist of EP1 receptors, modified cAMP accumulation induced by forskolin. On the contrary, 11-deoxy-16,16-dimethyl PGE2, which does not discriminate between EP2 and EP4 receptors, both coupled to the activation of adenylyl cyclase, and butaprost, a selective EP2 agonist, induced a dose-dependent elevation of cAMP that was largely reduced by PMA pretreatment, as in the case of PGE2. These results indicated EP2 receptors as a possible target of PKC and suggest that PKC-activating agents present in the pathological brain may prevent the cAMP-mediated microglia- deactivating function of PGE2.