Triggering receptor expressed on myeloid cells-1 (TREM-1) is a recently identified cell surface molecule that is expressed by neutrophils and monocytes. TREM-1 expression is modulated by various ligands for TLRs in vitro and in vivo. However, the influence of PGE(2), a potential mediator of inflammation, on TREM-1 expression has not been elucidated. In this study, we examined the effects of PGE(2) on LPS-induced TREM-1 expression by resident murine peritoneal macrophages (RPM) and human PBMC. PGE(2) significantly induced murine TREM-1 (mTREM-1) expression by RPM. Up-regulation of TREM-1 expression was specific to PGE(2) among arachidonic acid metabolites, while ligands for chemoattractant receptor-homologous molecule expressed on Th2 cells and the thomboxane-like prostanoid receptor failed to induce mTREM-1 expression. PGE(2) also increased expression of the soluble form of TREM-1 by PBMC. LPS-induced TREM-1 expression was regulated by endogenous PGE(2) especially in late phase (>2 h after stimulation), because cyclooxygenase-1 and -2 inhibitors abolished this effect at that points. A synthetic EP4 agonist and 8-Br-cAMP also enhanced mTREM-1 expression by RPM. Furthermore, protein kinase A, PI3K, and p38 MAPK inhibitors prevented PGE(2)-induced mTREM-1 expression by RPM. Activation of TREM-1 expressed on PGE(2)-pretreated PBMC by an agonistic TREM-1 mAb significantly enhanced the production of IL-8 and TNF-alpha. These findings indicate that LPS-induced TREM-1 expression on macrophages is mediated, at least partly, by endogenous PGE(2) followed by EP4 and cAMP, protein kinase A, p38 MAPK, and PI3K-mediated signaling. Regulation of TREM-1 and the soluble form of TREM-1 expression by PGE(2) may modulate the inflammatory response to microbial pathogens.