Prostaglandins PGs in particular PGE and PGF play
Prostaglandins (PGs), in particular PGE2 and PGF2α, play pivotal roles in human parturition by stimulating cervical ripening, myometrial contraction and fetal membrane rupture (Challis et al., 1997). In human pregnancy, the amnion is the principle source of PGE2 (Duchesne et al., 1978). In the amnion, fibroblasts produce at least 5 times more PGE2 per cell than epithelial SB202190 as parturition approaches (Sun et al., 2003). Moreover, the expression of cyclooxygenase-2 (COX-2), a rate-limiting enzyme in prostaglandin synthesis and the production of PGE2 increase dramatically in the amnion toward the end of pregnancy with a further increase during labor (Fuentes et al., 1996). The effects of PGE2 are mediated through specific membrane-bound G-protein-coupled EP receptors. The EP receptor family is composed of 4 distinct subtypes, i.e. EP1-4 (Narumiya et al., 1999). While EP1 and EP3 signal through Gαq, which mobilizes Ca2+ and activates protein kinase C (PKC) (Narumiya et al., 1999), EP2 and EP4 signal through Gαs to increase cyclic adenosine monophosphate (cAMP) levels and activate protein kinase A (PKA) (Narumiya et al., 1999). In addition, EP3 also signals through Gαi to decrease cAMP levels (Narumiya et al., 1999). All of the four EP receptor subtypes have been found to be expressed in a variety of cell types in the fetal membranes including the amnion fibroblasts (Grigsby et al., 2006). Activation of the EP2/EP4 receptor-coupled cAMP/PKA pathway by PGE2 has been shown to up-regulate COX-2 expression (Pino et al., 2005, Faour et al., 2008) and attenuate the up-regulation of LOX expression by transforming growth factor-β (TGF-β) in human lung fibroblasts (Roy et al., 1996). Given these findings, it is tempting to speculate that the increasing production of PGE2 caused by COX-2 induction may account, at least in part, for the down-regulation of LOX expression toward the end of pregnancy in human amnion. The reciprocal regulation of LOX and COX-2 by PGE2 may thus be the result of a feed-forward loop of PGE2 production thereby diminishing the cross-linking of collagen fibrils in the amnion, which ultimately leads to the rupture of fetal membranes. We have tested this hypothesis in cultured primary human amnion fibroblasts as well as in human amnion tissue collected from pregnancies with or without labor. The role of different EP receptor subtypes and signaling pathways in the regulation of LOX by PGE2 were also examined.
Materials and methods
Discussion In this study, we showed that PGE2, a major prostaglandin derived from the amnion, concurrently induced COX-2 and reduced LOX expression via the EP2/EP4 receptor-coupled cAMP/PKA signaling pathway. Since PGE2 is a product of COX-2, this reciprocal regulation of COX-2 and LOX by PGE2 may initiate a feed-forward cascade of events leading to the breakdown of collagen cross linkage in the amnion towards the end of pregnancy. Because the cross-linking of collagen catalyzed by LOX not only strengthens the amnion and also prevents the breakdown of collagens by MMPs (Buerzle et al., 2013, Stuart et al., 2005), the inhibition of LOX by the feed-forward production of PGE2 may thus constantly expose uncross-linked collagen fibrils for degradation by MMPs themselves induced by PGE2 (McLaren et al., 2000). The involvement of reciprocal changes in COX-2 and LOX levels in the rupture of fetal membranes is further supported by the observation of increased COX-2 and decreased LOX expression in amnion tissue obtained from deliveries after labor. We believe that the results of this study may explain, at least in part, feed-forward mechanisms underlying the rupture of the fetal membranes in parturition. Based on a previous study showing that COX-2 expression increases along with gestational age (Hirst et al., 1995), we propose that a threshold of PGE2 accumulation in the amnion may be required to trigger the onset of this feed-forward loop. In addition to the regulation by PGE2, the expression of COX-2 in amnion fibroblasts is known to be controlled by a number of other factors including cortisol regenerated locally by 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) and pro-inflammatory cytokines produced in large quantities during chorioamnionitis (Zhu et al., 2009, Perkins and Kniss, 1997). It is of interest that the expression of 11β-HSD1 is also under feed-forward control by cortisol as well as by pro-inflammatory cytokines in the amnion (Sun and Myatt, 2003), which may act in concert with the feed-forward induction of COX-2 by PGE2 resulting in rapid accumulation of PGE2 in the amnion and a dramatic decrease in LOX expression towards the end of pregnancy (Roy et al., 1996). As pro-inflammatory cytokines produced during chorioamnionitis potently induce COX-2 expression and MMP activity (So, 1993), the findings that PGE2 inhibits LOX expression in this study may provide an alternative explanation why infection causes PROM (Furuta et al., 2000).