Mechanisms regulating prostaglandin H2 synthase-2 mRNA level in the amnion and chorion during pregnancy.

Increasing prostaglandin H(2) synthase (PGHS)-2 expression in the fetal membranes is implicated in the production of prostaglandins (PGs) that stimulate labour. We have determined the activity of the PGHS-2 gene in the amnion and chorion throughout gestation and defined the contribution of transcriptional and post-transcriptional mechanisms to the increase of PGHS-2 mRNA levels. We also measured PGHS-1 mRNA abundance to assess the participation of the two isoenzymes in fetal membrane PG-production during pregnancy. Amnion and chorion were collected from non-labouring women at 10-19 weeks (early), at 28-36 weeks (preterm) and at term (37-41 weeks). We determined PGHS-1 and -2 mRNA abundance and assessed PGHS-2 gene activity by measuring PGHS-2 heterogeneous nuclear RNA levels using real-time RT-PCR. PGHS-2 gene activity and mRNA levels were up-regulated in both tissues with advancing gestation. Path analysis demonstrated that the PGHS-2 mRNA up-regulation involved both transcriptional and post-transcriptional components. PGHS-2 mRNA abundance increased 9-11 fold between the early (10-19 weeks) and preterm (28-36 weeks) groups and remained high at term. The underlying mechanism was predominantly transcriptional in the amnion and post-transcriptional in the chorion. PGHS-1 mRNA expression precipitously decreased between early gestation and term. Thus, PGHS-2 mRNA abundance is up-regulated well in advance of term and is not a trigger for labour. There is a switch in PGHS mRNA expression during pregnancy with PGHS-1 dominating in the early period and PGHS-2 dominating at term.