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The expression and regulation of the PGE receptors, EP2 and EP4, both of which are coupled to the stimulation of adenylate cyclase, were examined in peritoneal resident macrophages from C3H/HeN mice. mRNA expression of EP4 but not EP2 was found in nonstimulated cells, but the latter was induced by medium change alone, and this induction was augmented by LPS. mRNA expression of EP4 was down-regulated by LPS but not by medium change. PGE2 increased the cAMP content of both LPS-treated and nontreated cells. ONO-604, an EP4 agonist, also increased cAMP content in nonstimulated cells and in cells treated with LPS for 3 h, but not for 6 h. Butaprost, an EP2 agonist, was effective only in the cells treated with LPS for 6 h. The inhibitory effects of ONO-604 on TNF-α and IL-12 production were equipotent with PGE2 at any time point, but the inhibitory effects of butaprost were only seen from 14 h after stimulation. PGE2 or dibutyryl cAMP alone, but not butaprost, reduced EP4 expression, and indomethacin reversed the LPS-induced down-regulation of EP4, indicating that the down-regulation of EP4 is mediated by LPS-induced PG synthesis and EP4 activation. Indeed, when we used C3H/HeJ (LPS-hyporesponsive) macrophages, such reduction in EP4 expression was found in the cells treated with PGE2 alone, but not in LPS-treated cells. In contrast, up-regulation of EP2 expression was again observed in LPS-treated C3H/HeJ macrophages. These results suggest that EP4 is involved mainly in the inhibition of cytokine release, and that the gene expression of EP2 and EP4 is differentially regulated during macrophage activation.

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