Abstract
Human amnion fibroblasts produce abundant prostaglandin E2 (PGE2), which plays a crucial role in parturition by stimulating not only myometrial contraction and cervical ripening but also the expression of the rate-limiting enzyme in PGE2 synthesis-namely, cyclooxygenase-2 (COX-2). This feed-forward induction of COX-2 expression by PGE2 is mediated via its receptors coupled with the cAMP and PKA pathway and subsequent phosphorylation of the transcription factors cAMP-response element binding protein (CREB) and signal transducer and activator of transcription 3 (STAT3). Although prostaglandin E receptor (EP)-2 and EP4 for PGE2 are coupled with activation of the cAMP and PKA pathway, the exact roles of these 2 receptors in the regulation of COX-2 expression in amnion fibroblasts remain to be determined. Here, we clarify this issue by employing human amnion tissue and fibroblasts with the long-term objective of specific targeting of prostaglandin synthesis in prevention of preterm birth. We find that an EP2 agonist caused long-lasting increases in CREB phosphorylation and COX-2 expression, whereas an EP4 agonist induced only transient increases in CREB phosphorylation and COX-2 expression in amnion fibroblasts. Moreover, only EP2 stimulation increased STAT3 phosphorylation, whereas only EP4 stimulation increased PI3K activity. EP4 antagonist or inhibition of PI3K enhanced the induction of CREB and STAT3 phosphorylation and COX-2 expression by PGE2 or EP2 stimulation, which was attenuated by EP4 overexpression. Of interest, PGE2 and cortisol, both well-demonstrated stimulants of COX-2 expression in amnion fibroblasts, increased EP2 but decreased EP4 receptor expression. Furthermore, increased EP2 but decreased EP4 abundance were observed in amnion tissue at parturition. We conclude that EP2 and EP4 receptors play different roles in the regulation of COX-2 expression in human amnion fibroblasts. EP2 is the dominant PGE2 receptor mediating the induction of COX-2 at parturition, which can be attenuated by simultaneous activation of PI3K coupled to the EP4 receptor.-Lu, J.-W., Wang, W.-S., Zhou, Q., Gan, X.-W., Myatt, L., Sun, K. Activation of prostaglandin EP4 receptor attenuates the induction of cyclooxygenase-2 expression by EP2 receptor activation in human amnion fibroblasts: implications for parturition.