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We have examined the expression of prostaglandin endoperoxide H synthase (PGHS) isoenzymes in the amnion and the decidua during gestation, and the abundance of PGHS mRNA in the amnion at idiopathic preterm labour. PGHS-1 and -2 mRNA abundance in the amnion, determined with ribonuclease protection assays, was significantly (P< 0.05) higher at term than earlier during pregnancy. In contrast, neither PGHS-1 and -2 mRNA values, nor PGHS-specific activity, measured with a cell-free assay, was different in the decidua at term as compared to earlier gestational ages. In individual term patients, PGHS-2 mRNA values in the amnion were positively correlated with PGHS-2 mRNA values in the chorion laeve. PGHS-1 and -2 mRNA abundance was higher (P < 0.05) in the amnion after idiopathic preterm labour than in the absence of labour at the same gestational age (28-35 weeks). Thus, PGHS-1 and -2 are induced in the amnion at term. Furthermore, amniotic PGHS-2 changes in co-ordination with PGHS-2 concentrations in the chorion laeve. PGHS is not induced in the decidua at term. Increased amniotic PGHS expression may contribute to the enhanced intrauterine prostaglandin synthesis before term labour. Both PGHS isoenzymes may participate in the increase of PGHS activity in the amnion at preterm birth.

Human amnion cells in primary culture respond to glucocorticoids in a characteristic fashion by the increased expression of the inducible prostaglandin endoperoxide H synthase isoenzyme, PGHS-2. Since PGHS-2 induction by agonists generally involves tyrosine kinases, we examined the possibility that the glucocorticoid stimulation of PGHS-2 in the amnion cells is tyrosine kinase dependent. PGHS-2 expression was stimulated in confluent, serum-starved amnion cells with dexamethasone, and the effect of the tyrosine kinase inhibitors herbimycin A and tyrphostins AG126, AG1288, and A1 on enzyme activity induction was determined. All four inhibitors blocked the increase of PGHS activity in a concentration-dependent manner with IC50 values of 0.077 +/- 0.05, 15.38 +/- 5.14, 20.91 +/- 3.1, and 29.77 +/- 8.21 microM, respectively (mean +/- SE, n = 4). Dexamethasone increased (approximately twofold) the tyrosine phosphorylation of 120-, 110-, and 77-kDa proteins in cell extracts, and herbimycin A selectively blocked the phosphorylation of the 110-kDa phosphoprotein. The stimulation of the steady-state level of PGHS-2 mRNA by dexamethasone was also inhibited by herbimycin A. These results suggest that glucocorticoids induce PGHS-2 expression in amnion cells with the involvement of tyrosine kinase(s). The role of tyrosine kinase dependent mechanisms in the control of amnion cell responsiveness to corticosteroids remains to be established.

Prostaglandin H2 synthase (PGHS)-1 and PGHS-2 expression was examined in primary cultures of human amnion cells, an in vitro model of amnion tissue. Epidermal growth factor (EGF), the protein kinase C (PKC) activating phorbol ester TPA, and the protein phosphatase inhibitor, okadaic acid (OA), stimulated PGHS activity and the level of PGHS-2 mRNA, but did not affect the level of PGHS-1 mRNA. In situ hybridization suggested that the same population of cells responded to EGF, TPA and OA. Okadaic acid promoted PGHS activity independently of PKC. EGF stimulated the activity of extracellular signal-regulated protein kinase (Erk) and N-terminal c-Jun kinase (Jnk). OA increased Jnk activity but had no effect on Erk activity, while TPA had no influence on either Erk or Jnk activity. PD098059, a selective inhibitor of the Erk-activating kinase MEK, blocked the stimulation of PGHS expression by EGF, but did not decrease stimulation in response to OA. Herbimycin A, a tyrosine kinase inhibitor, suppressed the stimulation of PGHS activity and PGHS-2 mRNA abundance by all three stimulants, and blocked signalling via the Erk and Jnk mitogen-activated protein kinase pathways. Thus, growth factor stimulation, PKC activation and protein phosphatase inhibition induced the expression of PGHS-2 in primary amnion cells by distinct regulatory mechanisms involving tyrosine kinase(s). Tyrosine kinase inhibitors may constitute a new category of PGHS-2 inhibitors that act by blocking the expression of the enzyme., (Copyright 1998 Elsevier Science B.V.)

Corticosteroids increase the production of prostaglandin E2 (PGE2) and the activity of prostaglandin endoperoxide H synthase (PGHS) in cultured amnion cells, although they inhibit prostanoid biosynthesis in numerous other cell types. This suggests that glucocorticoids control the level of PGHS in amnion cells by a hitherto unexplored, positive regulatory mechanism. We have tested the possibility that corticosteroids act by stimulating the expression of messenger RNAs (mRNAs) encoding one or both isoforms of PGHS. Ribonuclease protection assays were used to determine the levels of PGHS-1 and -2 mRNAs and, for reference, gamma-actin mRNA levels in confluent primary cultures of human amnion cells. In untreated cultures, PGHS-1 and -2 mRNA levels were low, often not reaching the level of detection. Dexamethasone (DEX) treatment for 4 h resulted in a measurable level of PGHS-2 mRNA, which increased further 10-fold and 20-fold after incubation with the glucocorticoid for 8 h and 16 h, respectively. The stimulation was dependent on DEX concentration, and was concomitant with an increase in the capacity of the cells to metabolize arachidonic acid to PGE2. PGHS-1 mRNA levels remained low in DEX-treated cells, while the gamma-actin message level showed no change. Estradiol and progesterone had no influence on PGHS-2 mRNA expression, but cortisol increased the PGHS-2 mRNA abundance. The glucocorticoid antagonist RU486 blocked the effect of DEX. Conditioned media of DEX-treated cells did not contain steroid-induced factor(s) stimulating PGE2 production. Inhibition of protein synthesis by cycloheximide potentiated the effect of DEX, and raised the abundance of PGHS-1, PGHS-2, and gamma-actin mRNAs in untreated cells. DEX did not affect the stability of the PGHS-2 mRNA. These results show that glucocorticoids promote PGE2 synthesis by amnion cells by stimulating the expression of PGHS-2 mRNA in a receptor-dependent, selective, and immediate fashion.