Differential expression of E-type prostanoid receptors 2 and 4 in microglia stimulated with lipopolysaccharide

Background: Cyclooxygenase-2 (COX-2) is induced under inflammatory conditions, and prostaglandin E2 (PGE(2)) is one of the products of COX activity. PGE(2) has pleiotropic actions depending on the activation of specific E-type prostanoid EP1-4 receptors. We investigated the involvement of PGE(2) and EP receptors in glial activation in response to an inflammatory challenge induced by LPS. Methods: Cultures of mouse microglia or astroglia cells were treated with LPS in the presence or absence of COX-2 inhibitors, and the production of PGE(2) was measured by ELISA. Cells were treated with PGE(2), and the effect on LPS-induced expression of TNF-alpha messenger RNA (mRNA) and protein was studied in the presence or absence of drug antagonists of the four EP receptors. EP receptor expression and the effects of EP2 and EP4 agonists and antagonists were studied at different time points after LPS. Results: PGE(2) production after LPS was COX-2-dependent. PGE2 reduced the glial production of TNF-alpha after LPS. Microglia expressed higher levels of EP4 and EP2 mRNA than astroglia. Activation of EP4 or EP2 receptors with selective drug agonists attenuated LPS-induced TNF-alpha in microglia. However, only antagonizing EP4 prevented the PGE(2) effect demonstrating that EP4 was the main target of PGE2 in naive microglia. Moreover, the relative expression of EP receptors changed during the course of classical microglial activation since EP4 expression was strongly depressed while EP2 increased 24 h after LPS and was detected in nuclear/peri-nuclear locations. EP2 regulated the expression of iNOS, NADPH oxidase-2, and vascular endothelial growth factor. NADPH oxidase-2 and iNOS activities require the oxidation of NADPH, and the pentose phosphate pathway is a main source of NADPH. LPS increased the mRNA expression of the rate-limiting enzyme of the pentose pathway glucose-6-phosphate dehydrogenase, and EP2 activity was involved in this effect. Conclusions: These results show that while selective activation of EP4 or EP2 exerts anti-inflammatory actions, EP4 is the main target of PGE(2) in naive microglia. The level of EP receptor expression changes from naive to primed microglia where the COX-2/PGE(2)/EP2 axis modulates important adaptive metabolic changes.