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The Journal of Clinical Endocrinology & Metabolism Vol. 87, No. 2 700-708
Copyright © 2002 by The Endocrine Society

Other Original Articles

Cortisol/Progesterone Antagonism in Regulation of 15-Hydroxysteroid Dehydrogenase Activity and mRNA Levels in Human Chorion and Placental Trophoblast Cells at Term

Falguni A. Patel and John R. G. Challis

Canadian Institutes for Health Research Group in Fetal and Neonatal Health and Development, Departments of Physiology and Obstetrics and Gynaecology, University of Toronto, Toronto, Ontario M5S 1A8, Canada

Address all correspondence and requests for reprints to: Falguni A. Patel, M.D., 1 Kings College Circle, Medical Sciences Building, Room 3344, Department of Physiology, Faculty of Medicine, University of Toronto, Toronto, Ontario M5S 1A8, Canada. E-mail: fal.patel{at}utoronto.ca

Abstract

PGs mediate parturition events. 15-Hydroxyprostaglandin dehydrogenase (PGDH) catalyzes the first step in the metabolism of PGs to render them inactive. We have reported previously that cortisol (F) decreases PGDH activity and progesterone (P4) maintains PGDH in human chorion and placenta at term. To study the interaction of P4 and F on the regulation of PGDH, we treated chorion and placental trophoblast cells in culture with combinations of F, dexamethasone, P4, trilostane, and medroxyprogesterone acetate (MPA). Following a 24-h steroid treatment period and 4-h PGF2{alpha} challenge, culture media and cells were collected for measurement of PGF2{alpha} levels and PGDH mRNA by RIA and Northern blotting analysis. F and dexamethasone decreased PGDH activity and mRNA levels. Exogenous P4 did not significantly alter PGDH activity or mRNA levels; however, MPA significantly stimulated PGDH activity. Trilostane decreased P4 production by more than 90% and also decreased PGDH activity and expression. Coincubation with P4 or MPA reversed trilostane inhibition of PGDH, consistent with a stimulatory role for endogenous P4 on PGDH. MPA significantly reversed F inhibition of PGDH activity and mRNA levels. In the presence of trilostane, P4 at equimolar concentration to F reversed F inhibition of PGDH mRNA levels. These findings suggest that F may be acting as an endogenous inhibitor of P4 action in the regulation of PGDH at term.

PGs ARE INVOLVED in the initiation and progression of parturition in humans and other species (1, 2, 3). PGs have been linked with stimulation of myometrial contractility (4, 5, 6), regulation of the cervical effacement and dilatation (7, 8, 9), up-regulation of the fetal hypothalamic-pituitary- adrenal axis (10), membrane rupture (11, 12), maintenance of uterine and placental blood flow (13, 14, 15), and inhibition of fetal breathing and movement at the time of labor (16, 17).

PG dehydrogenase (PGDH), which is present at high activity in chorion trophoblast cells and placental syncytiotrophoblast throughout gestation, is responsible for the initial inactivation of PGs, catalyzing the conversion of PGE2 and PGF2{alpha} to their biologically inactive 15-keto derivatives (2, 18, 19). PGDH activity and mRNA levels were lower in chorion from patients at term spontaneous labor, compared with term elective cesarean section, and lower in chorion tissue collected from patients at idiopathic preterm labor (20, 21). Regulation of chorion PGDH is multifactorial. PGDH activity was reduced by cortisol, potentially produced locally in chorion cells, through the activity of 11ß-hydroxysteroid dehydrogenase (HSD)-1 (22), and maintained by progesterone, derived systemically, or locally via chorionic 3ß-HSD. We provided some preliminary data that the inhibitory effect of cortisol could be at the level of PGDH mRNA; however, there is no information concerning the possibility of interaction between cortisol and progesterone in the regulation of PGDH activity and/or mRNA expression.

The role of progesterone in maintaining uterine quiescence during pregnancy has been demonstrated clearly in those species in which maternal peripheral plasma concentrations fall before labor and delivery (23, 24). In human pregnancy there is little evidence for a prepartum decline in circulating progesterone (1, 25, 26, 27). Therefore the possibility has been raised of local, intrauterine regulation of effective progesterone levels through altered PR activity (28, 29, 30, 31), antagonism by endogenous antiprogestins such as TGFß (32), or by interaction with glucocorticoids (33, 34, 35). In primary cultures of human placenta, cortisol was able to compete with the action of progesterone in regulation of CRH gene expression (35). This provided a strong rationale for examining the interaction of cortisol and progestins on PGDH. Based on this proposed antagonism of progesterone action by glucocorticoids, we hypothesized that cortisol and progesterone would compete in regulating PG output at the level of PGDH gene expression by human intrauterine tissues at term. To examine this possibility, we studied the interaction of progesterone or the synthetic progestin medroxyprogesterone acetate (MPA) and glucocorticoids on PGDH activity and mRNA levels in cultured human chorion and placental trophoblast cells.

Materials and Methods

Tissue culture

Human choriodecidual tissue (n = 14 patients) and placentae (n = 12 patients) were obtained from uncomplicated normal term pregnancies after elective cesarean section or spontaneous vaginal delivery using protocols approved by Mount Sinai Hospital and the University of Toronto. Trophoblast cells from choriodecidual tissue and placental cotyledons were isolated and cultured using a modification of the technique described by Kliman et al. (36), as published previously (37, 38). Briefly, approximately 60 g of cotyledon tissue were removed randomly from the maternal side of the placenta, pooled, and digested with 0.125% trypsin (Sigma, St. Louis, MO) and 0.02% deoxyribonuclease-I (Sigma) in DMEM (Life Technologies, Inc., Grand Island, NY) containing 0.1% BSA, 0.005% gentamicin, and 0.01% streptomycin three times for 30 min each time. The chorion with adherent decidua was peeled off amnion and digested three times for 60 min each time using the same digestion medium with the addition of 0.2% collagenase (Sigma). The dispersed choriodecidual or placental cells were filtered with a 200 µm nylon gauze and loaded onto a continuous Percoll (Sigma) gradient (5–70% in 5% steps of 3 ml each), then centrifuged at 37 C and 1200x g for 20 min to separate different cell types. Cytotrophoblast cells between the density markers of 1.049 and 1.062 g/ml were collected and plated in 24-well plates (Corning, Inc. Costar Corp., Cambridge, MA) at a density of 106 cells/ml per well in DMEM culture medium containing 10% FCS (Life Technologies, Inc.). Cells were also plated on 8-well chamber slides (Lab-Tek, Nunc Inc., Naperville, IL) at a density of 0.3 x 106 cells/well. The cells were cultured for 3 d at 37 C in 5% CO2 and 95% air before experimentation.

Treatment of cells with steroids

After a 3-d incubation period, the cells were washed with FCS-free culture medium (pH 7.4) and then treated with fresh medium containing one or a combination of progesterone, cortisol, dexamethasone (DEX), MPA, and trilostane (a 3ß-HSD inhibitor synthesized at Schering AG, Berlin, Germany; a generous gift from Dr. M. Novy, Oregon Health Sciences University, Portland, OR). Each treatment was performed in duplicate or triplicate for each preparation of cells for 24 h. The medium was then changed and replaced with fresh medium containing PGF2{alpha} (100 ng/ml; 282 nM) for 4 h without steroids (39). The culture medium was then collected and stored at -80 C for later assessment, by RIA, of PGDH activity by measuring 13,14-dihydro-15-keto PGF2{alpha} (PGFM), the stable metabolite of PGF2{alpha} (40). After treatment, cells were scraped off the Petri dish with a rubber policeman and total RNA extracted using TRIZOL Reagent (Life Technologies, Inc., Grand Island, NY). RNA was stored at -80 C in 70% ethanol for later analysis by Northern blot hybridization.

PGFM RIA

The activity of PGDH was assessed by measuring PGFM content in duplicate aliquots (10 µl and 50 µl) of culture medium using a modification of the RIA technique described by PerSeptive Biosystems Inc. (Framingham, MA). PGFM antisera (100 µl, raised in rabbit; PerSeptive Biosystems), diluted 1:10, and 100 µl of [3H] PGFM (10,000–15,000 cpm of 13,14-dihydro-15-keto-[5,6,8,11,12,14(n)-3H] PGF2{alpha}, Amersham Pharmacia Biotech, Buckinghamshire, UK) were added to each tube. The volumes were adjusted with BGG-phosphate buffer (10 mM PO4, 0.85% (wt/vol) NaCl, 0.02% (wt/vol) KCl, 0.1% (wt/vol) bovine {gamma} globulin (Sigma), 0.1% (wt/vol) NaN3, in deionized water, pH 7.0) to total 0.6 ml in 12 x 75 mm borosilicate tubes (Maple Leaf brand; Sigma). Tubes were vortexed and incubated overnight at 4 C. Intra- and interassay coefficients of variation were 10.5 plus or minus 2.2% and 14.6 plus or minus 3.2% respectively (SEM; n = 14). Preliminary experiments showed negligible cross-reactivity with PGF2{alpha} (2.4%), PGE2 (1.7%), and PGE metabolite (1.9%).

RNA extraction and Northern blot analysis

Cells in Petri dishes were dispersed mechanically by scraping with a rubber policeman for 1 min in the presence of TRIZOL Reagent (Life Technologies) and then incubating for 5 min at room temperature to permit complete dissociation of nucleoprotein complexes. Total RNA was extracted from tissues using a method described by Chomczynski and Sacchi (41). Trial extractions demonstrated that 2 ml TRIZOL Reagent was sufficient to obtain total RNA of suitable purity (OD260/280nm between 1.6 and 1.8) from cultured chorion and placental trophoblast cells plated in Petri dishes. Samples were then stored at -80 C in 75% ethanol.

Northern blot hybridization was carried out using an 800-bp fragment of the PGDH cDNA sequence as a probe (20, 42). Blots were exposed to X-AR film (Kodak, Rochester, NY) with an intensifying screen for 5–7 d. After autoradiographic exposure, the blots were stripped and reprobed with a cDNA for mouse 18S rRNA as an internal standard to allow for correction of variations in gel loading and transfer efficiency. The relative optical densities (RODs) were determined using computerized image analysis (MCID, Imaging Research, Inc., St. Catherines, Canada). We quantified the 3.4-kb transcript because this is the mRNA species thought to be processed to active protein (42, 43). The values for RODs were determined after different exposure times to ensure that values were obtained within the linear range of the autoradiographic film and densitometer. Results were expressed as the ratio of the RODs of the PGDH mRNA:18S rRNA hybridization signals.

Statistical analysis

Results are presented as the mean plus or minus SEM for the number of observations (tissues from different patients) indicated. The effects of treatment on concentrations of PGFM in the culture media were determined by one-way ANOVA corrected for repeated measures. Student-Newman-Keuls multiple-range tests were used to assess the effects of different treatment doses. When treatment effects were not normally distributed with equal variances, the Friedman repeated-measures ANOVA on ranks, a nonparametric test, was used to determine statistical significance of data. ROD determinations were analyzed by the t test at a confidence level of 95%. Statistical significance was set at P less than 0.05. Calculations were performed using SigmaStat (Jandel Scientific Software, San Rafael, CA).

Results

Cell characterization

After 72 h of culture, chorion trophoblast cells remained either as single cells or formed clumps, whereas placental trophoblast cells aggregated to form syncytial clumps that have been suggested to correspond to placental syncytiotrophoblast (36). Both chorion and placental trophoblast cell cultures were predominantly cytokeratin positive (chorion: >75–95%; placenta: >90%) and predominantly vimentin negative suggesting the presence of mainly trophoblast cells and few fibroblast or decidual cells (37). Both chorion and placental trophoblast cell cultures were positive for IR-PGDH and IR-PGHS-2. By trypan blue exclusion staining the percentage viability of cultured cells before and after treatment was determined to be greater than 95%.

Effect of cortisol in the presence of progesterone on PGDH activity

Cultured chorion and placental trophoblast cells were treated with cortisol (0–1 µM), progesterone (0–10 µM), and cortisol (0–1 µM) in the presence of fixed (1 µM or 10 µM) progesterone (n = 4; Fig. 1Go). Cortisol significantly decreased PGFM levels in chorion by 81% (mean basal value of 14.1 ± 1.5 ng/ml) and in placenta by 78% (mean basal value of 11.3 ± 1.2 ng/ml) (P < 0.05). There was no significant effect of exogenous progesterone on mean PGFM output in chorion and placenta. Progesterone (1 µM and 10 µM) did not significantly reverse the inhibition of PGFM output seen with cortisol treatment of either chorion or placental cells.



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  		Figure 1. Effect of increasing concentrations of cortisol ({blacksquare}), increasing concentrations of progesterone (•), increasing concentrations of cortisol in the presence of fixed 1 µM progesterone ({square}), and increasing concentrations of cortisol in the presence of fixed 10 µM progesterone ({circ}) on PGFM formation in cultured term human chorion and placental trophoblast cells. All values are means ± SEM (n = 4).

 
Effect of trilostane and progesterone or medroxyprogesterone acetate on PGDH activity

Cultured chorion and placental trophoblast cells (n = 4) were treated with trilostane (0–1 µM; a 3ß-HSD inhibitor), MPA (0–1 µM), MPA (0–1 µM) in the presence of fixed (1 µM) trilostane, and compared with progesterone (0–10 µM) in the presence of fixed (1 µM) trilostane. As reported previously (37), trilostane treatment significantly decreased progesterone output in both chorion and placental trophoblast cells by 80–85% and significantly decreased PGFM levels by 63% in chorion and by 53% in placenta (P < 0.05). MPA, a stable progestin analog, unlike exogenous progesterone, significantly increased PGFM formation in a dose-dependent manner in both chorion and placenta. MPA (1 µM) stimulated PGFM formation in chorion by 49% and in placenta by 77% (P < 0.05) (Fig. 2Go). The addition of increasing concentrations of progesterone (1–10 µM) in the presence of 1 µM trilostane reestablished basal PGFM levels in both chorion and placenta. The addition of MPA to trilostane treated cells significantly increased PGFM output above basal in both chorion (+36%) and in placenta (+43%). Coincubation of cells with cortisol (0–1 µM) and trilostane (1 µM) maintained decreased PGFM levels with no further significant decrease from that seen with trilostane treatment alone (data not shown).



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  		Figure 2. Effect of increasing concentrations of MPA ({blacksquare}), increasing concentrations of MPA in the presence of fixed 1 µM trilostane ({square}), and increasing concentrations of progesterone in the presence of fixed 1 µM trilostane (•) on PGFM formation in cultured term human chorion and placental trophoblast cells. All values are means ± SEM (n = 4).

 
Effect of cortisol in the presence of progesterone or medroxyprogesterone acetate on PGDH activity

Human chorion and placental trophoblast cells (n = 4) were treated with MPA (0–1 µM) in the presence of fixed (1 µM) cortisol and progesterone (0–10 µM) in the presence of fixed (1 µM) cortisol (Fig. 3Go). The addition of progesterone to chorion or placental trophoblast cells treated with cortisol did not reestablish PGFM output. In contrast, MPA, a potent progestin analog, was able to reestablish basal PGFM formation in both placental and chorion cells pretreated with cortisol (1 µM; Fig. 3Go).



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  		Figure 3. Effect of increasing concentrations of MPA in the presence of fixed 1 µM cortisol (•) and progesterone in the presence of fixed 1 µM cortisol ({blacksquare}) on PGFM formation in cultured term human chorion and placental trophoblast cells. All values are means ± SEM (n = 4).

 
Effect of glucocorticoids and progestins on PGDH mRNA expression in chorion and placental trophoblast cells

In chorion trophoblast cells, treatment with exogenous progesterone (1 µM, n = 10; 10 µM, n = 6) did not alter PGDH mRNA levels (Fig. 4Go, A and B). MPA (n = 4), in contrast to its effects on PGFM output (Fig. 2Go), also did not alter PGDH mRNA levels. Cortisol (10 nM, n = 6, 1 µM, n = 10) decreased PGDH mRNA levels in a dose-dependent manner with a 70% decrease at 1 µM concentrations (P < 0.05). Coincubation of chorion trophoblast cells with cortisol (10 nM) and progesterone (10 µM) or cortisol (1 µM) and progesterone (1 µM) did not alter cortisol inhibition of PGDH mRNA levels (n = 6). In contrast, MPA (n = 4), in accordance with effects on PGFM formation, reestablished basal PGDH mRNA expression after treatment with cortisol (1 µM). Trilostane (1 µM) significantly decreased PGDH mRNA levels by 65% (n = 10) and the addition of cortisol (1 µM) to trilostane treated cells decreased further (24%) PGDH mRNA levels to 89% below basal (n = 6). The addition of progesterone (1 µM) to trilostane-treated cells reestablished basal PGDH mRNA expression (n = 10) and the addition of MPA (1 µM) to trilostane-treated cells increased PGDH mRNA expression to 34% above basal (n = 4). Progesterone (1 µM) also reestablished basal PGDH mRNA levels in cells treated with cortisol (1 µM) in the presence of trilostane (1 µM; n = 4).



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  		Figure 4. A, Effect of glucocorticoids and progestins on PGDH mRNA levels determined by Northern blotting analysis in cultured term human chorion trophoblast cells. All values are means ± SEM; *, P < 0.05 vs. basal. B, Representative Northern blots of PGDH mRNA levels in cultured term human chorion trophoblast cells following treatment with cortisol (F), progesterone (P4), MPA, trilostane (Trilos), DEX, and combinations of these steroids. PGDH mRNA is shown as two bands of 3.4 and 2.0 kb. 18S rRNA is shown as an internal standard to correct for variations in gel loading and transfer.

 
In placental trophoblast cells, treatment effects were generally similar. Treatment with progesterone (1 µM, n = 8; 10 µM, n = 4) did not alter PGDH mRNA levels (Fig. 5Go, A and B). MPA (n = 4), in contrast to its effects on PGFM output (Fig. 2Go), also did not alter PGDH mRNA levels. Cortisol (10 nM, n = 4; 1 µM, n = 8) decreased PGDH mRNA levels in a dose-dependent manner with a 69% decrease at 10 nM and a 93% decrease at 1 µM concentrations (P < 0.05). Coincubation of placental trophoblast cells with cortisol (10 nM) and progesterone (10 µM) or cortisol (1 µM) and progesterone (1 µM) did not alter cortisol inhibition of PGDH mRNA levels (n = 4). In contrast, MPA (n = 4), in accordance with effects on PGFM formation, reestablished basal PGDH mRNA expression after treatment with cortisol (1 µM). Trilostane (1 µM) significantly decreased PGDH mRNA levels by 65% (n = 8) and the addition of cortisol (1 µM) to trilostane-treated cells decreased PGDH mRNA levels to 82% below basal (n = 4). The addition of progesterone (1 µM; n = 8) or MPA (1 µM; n = 4) to trilostane-treated cells reestablished basal PGDH mRNA expression. Progesterone (1 µM) also reestablished basal PGDH mRNA levels in cells treated with cortisol (1 µM) in the presence of trilostane (1 µM; n = 4).



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  		Figure 5. A, Effect of glucocorticoids and progestins on PGDH mRNA levels determined by Northern blotting analysis in cultured term human placental trophoblast cells. All values are means ± SEM; *, P < 0.05 vs. basal. B, Representative Northern blots of PGDH mRNA levels in cultured placental trophoblast cells following treatment with cortisol (F), progesterone (P4), MPA, trilostane (Trilos), DEX, and combinations of these steroids. PGDH mRNA is shown as two bands of 3.4 and 2.0 kb. 18S rRNA is shown as an internal standard to correct for variations in gel loading and transfer.

 
Discussion

In this study we have demonstrated for the first time that in the presence of trilostane, progesterone at equimolar concentration to cortisol reversed cortisol inhibition of PGDH mRNA levels but not activity. Although progesterone was unable to compete with cortisol in regulation of PGDH activity, MPA, a more potent progestin analog, significantly reversed cortisol inhibition of PGDH activity and mRNA levels. Cortisol significantly inhibits PGDH activity and mRNA levels in chorion and placental trophoblast cells in a dose-dependent manner. We have also shown that although exogenous progesterone is unable to stimulate PGDH activity and mRNA expression, the addition of trilostane significantly inhibited PGDH activity and mRNA expression in chorion and placenta. Furthermore, treatment of cells with a progestin analog, MPA, significantly stimulated PGDH activity. Coincubation with progesterone or MPA reversed trilostane inhibition of PGDH activity and mRNA expression, consistent with a stimulatory role for endogenous, locally produced, progesterone on PGDH during pregnancy. These results suggest that glucocorticoids and progestins interact in regulating PG metabolism within placenta and chorion at term.

Although we have achieved a high degree of cell purity and PGDH is expressed predominantly in trophoblast cells, we cannot exclude the possibility of interaction with added steroid and decidual stromal cells or fibroblasts. Further, the design of these experiments precluded examination of time-related steroid effects on PGDH expression/activity. These aspects require further examination, particularly in the context of exogenous vs. endogenous steroid interaction.

There is no demonstrable fall in systemic progesterone concentrations in late human pregnancy (1, 25, 26). Efforts to find other mechanisms of progesterone withdrawal, such as a decrease in 3ß-HSD or a decrease in PR levels, have been largely unsuccessful (28, 29, 30, 31, 44). However, it is possible that either cortisol derived from fetal adrenal or generated locally within the fetal membranes and placenta could act in an autocrine/paracrine manner to mediate a functional progesterone withdrawal. Chorion and placental trophoblast cells have the ability to produce progesterone from pregnenolone (28, 45, 46, 47, 48). These cells also have the ability to interconvert cortisone and cortisol, through the actions of 11ß-HSD isozymes (22). The presence of 11ß-HSD1 in chorion enables predominantly reduction of cortisone to active cortisol (49). In the placenta, the presence of 11ß-HSD-2 leads to conversion of cortisol to inactive cortisone. We have shown previously the effect of alterations in 11ß-HSD activity on PGDH activity in chorion and placental trophoblast cells in vitro (22). A recent study by Schoof et al. (50) has demonstrated a corresponding relationship between 11ß-HSD2 and PGDH in human placenta in vivo. A 3-fold reduction in 11ß-HSD2 mRNA levels was found in preeclamptic patients and this reduction significantly correlated with a 2-fold decrease in PGDH mRNA levels. This would suggest that a diminished conversion of placental cortisol (through reduced 11ß-HSD-2) leads to reduced PGDH mRNA expression by means of an autocrine/paracrine mechanism. Furthermore, studies by Alfaidy and Challis (51) have demonstrated the presence of autocrine/paracrine feed-forward loops in regulation of these enzymes within chorion and placenta. PGs were shown to increase 11ß-HSD1 activity in chorion and decrease 11ß-HSD2 activity in placenta, thereby creating the potential to increase local concentrations of cortisol and PGs in these tissues and creating a cascade between them that could be effective in an autocrine/paracrine manner.

The difference seen in cortisol/progesterone competition in the absence and presence of trilostane may also be owing to local mechanisms. In the presence of trilostane, progesterone was able to compete with cortisol and increase PGDH mRNA levels, whereas in the absence of trilostane, the addition of progesterone did not alter cortisol inhibition of PGDH mRNA expression and activity. We speculate that in the absence of trilostane, endogenous progesterone occupies progesterone-binding sites, thereby preventing exogenous progesterone from competing effectively with added cortisol to regulate PGDH. The effect of exogenous progesterone is demonstrable therefore only when endogenous progesterone output has been reduced. We suggest therefore that endogenous progesterone exerts a tonic stimulatory effect on PGDH, reflected by the maintained levels of mRNA and activity that are reduced in the presence of trilostane.

Steroid hormones act via steroid hormone receptors to mediate changes in gene transcription. Cortisol and progesterone may be competing for binding sites on the same steroid receptor. Alternatively, they may be binding to separate receptors and competing for binding at the GR element on the PGDH promoter (52). MPA has a higher affinity for PR than progesterone, and MPA may be able to bypass cortisol down-regulation more effectively than progesterone. Thus, MPA may compete more effectively with cortisol leading to increases in both PGDH mRNA levels and activity. Although glucocorticoids do not bind to the PR at physiological concentrations, progestins can bind to the GR (53). MPA also has a higher affinity for the GR than progesterone (54, 55). This may also explain why MPA, but not progesterone, is able to effectively compete with cortisol for regulation of PGDH. Two isoforms of the GR have been identified: GR{alpha} and GRß (56, 57, 58). GRß is thought to antagonize GR{alpha} action (58, 59, 60, 61). If progesterone effects are mediated via the GR rather than the PR, the presence of a GRß isoform may antagonize progestin action at the PGDH promoter at term.

In addition, progesterone effects are modulated through multiple isoforms of the PR: PR-A, PR-B, and PR-C (62, 63, 64). PR-A is thought to modify PR-B effects and PR-C is thought to modulate both PR-B and PR-A effects (62, 65, 66, 67, 68). There is little information concerning the expression of these different isoforms in human chorion and placenta. Future experiments will need to focus on identification of the types of steroid receptors present in chorion and placental trophoblast cells and on the mechanism by which glucocorticoids and progesterone competitively regulate PGDH activity and expression in chorion and placenta at term.

In summary, this set of experiments has demonstrated that glucocorticoids and progestins regulate PGDH and may do so in a competitive manner in chorion and placental trophoblast cells in vitro. PGDH activity and mRNA expression in vivo may be a reflection of opposing effects of cortisol and progesterone exerted through a common mechanistic pathway suggesting the possibility of a functional withdrawal of progesterone effects at term in mammalian pregnancy.

Acknowledgments

We thank Cristine Botsford and Dr. S. J. Lye (Mt. Sinai Hospital, Toronto, Ontario, Canada) for their assistance in collecting tissues.

Footnotes

This work was supported by the Canadian Institutes for Health Research.

Abbreviations: DEX, Dexamethasone; HSD, hydroxysteroid dehydrogenase; MPA, medroxyprogesterone acetate; PGDH, PG dehydrogenase; PGFM, 13,14-dihydro-15-keto PGF2{alpha}; ROD, relative optical density.

Received May 31, 2001.

Accepted November 3, 2001.

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