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Biology of Reproduction 59, 1378-1384 (1998)
©Copyright 1998 Society for the Study of Reproduction, Inc.

11ß-Hydroxysteroid Dehydrogenase Type 2 Is the Predominant Isozyme in the Guinea Pig Placenta: Decreases in Messenger Ribonucleic Acid and Activity at Term1

R. Sampath-Kumara, S.G. Matthewsb, and K. Yang2,a

a The Lawson Research Institute, St. Joseph's Hospital, Departments of Obstetrics&Gynecology and Physiology, University of Western Ontario, London, Ontario, Canada N6A 4V2 b Departments of Physiology and Obstetrics&Gynecology, University of Toronto, Toronto, Ontario, Canada M5S 1A8


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The type 2 isozyme of 11ß-hydroxysteroid dehydrogenase (11ß-HSD2) is responsible for inactivating physiologically active glucocorticoids to their inert metabolites. This is the predominant 11ß-HSD isozyme in the human placenta, where it is believed to protect the fetus from high levels of maternal cortisol. Given the similarity in placental structure between the human and the guinea pig (hemomonochorial), we have evaluated the potential of utilizing the guinea pig as a model to study the function and regulation of placental 11ß-HSD2 in fetal development. In this study, we characterized the intrinsic properties of 11ß-HSD in the guinea pig placenta during late pregnancy. The 11ß-HSD activity in the placenta was characteristic of 11ß-HSD2 in that it possessed only dehydrogenase activity that was NAD-dependent and had a high affinity for cortisol (Km = 134 nM). Moreover, the level of the 11ß-HSD2-like activity decreased significantly at term. To verify the expression of 11ß-HSD2 gene and to determine whether corresponding changes in 11ß-HSD2 mRNA occur at term, we also cloned the cDNA encoding guinea pig placental 11ß-HSD2. The deduced guinea pig 11ß-HSD2 enzyme contains 395 amino acids and shares over 80% sequence identity with other mammalian 11ß-HSD2 proteins. Northern blot analyses demonstrated the presence of the mRNA for 11ß-HSD2 but not that for 11ß-HSD1. Moreover, the level of 11ß-HSD2 mRNA decreased significantly at term. The parallel decrease in levels of 11ß-HSD2 activity and mRNA at term is consistent with, and provides a plausible molecular basis for, the previously reported increase in the rate of placental transfer of cortisol between mother and fetus at that time. In conclusion, the present study demonstrates that the guinea pig resembles the human in that 11ß-HSD2 is the predominant, if not exclusive, isozyme expressed in the placenta. Therefore, the guinea pig appears to represent a suitable model in which to study the role of placental 11ß-HSD2 in human fetal development.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
During human pregnancy, the level of cortisol in maternal circulation is several-fold higher than that in the fetus [1]. This large maternal-fetal cortisol gradient is believed to be critical in protecting the fetus from high maternal cortisol levels since excessive exposure to glucocorticoids in utero is known to cause intrauterine growth restriction (IUGR) [2]. The biochemical process responsible for the maintenance of this gradient remained unknown until Murphy [3] postulated that this may be a result of the rapid conversion of maternal cortisol to its inactive metabolite cortisone by the 11ß-hydroxysteroid dehydrogenase enzyme (11ß-HSD) in the placenta.

To date, two distinct isozymes of 11ß-HSD, known as 11ß-HSD1 and 11ß-HSD2, have been characterized and cloned [4]. 11ß-HSD1 possesses both dehydrogenase (cortisol to cortisone) and reductase (cortisone to cortisol) activities, prefers NADP(H) as cofactor, and has a low affinity for glucocorticoids (Km in micromolar range) [5, 6]. Moreover, this enzyme is widely expressed in mammalian tissues, most notably the liver [7, 8]. In contrast, 11ß-HSD2, under physiological conditions, exhibits only dehydrogenase activity that is NAD-dependent [9, 10]. Furthermore, it has a much higher affinity for glucocorticoids (Km in nanomolar range), and its expression is restricted to the placenta and aldosterone-target organs such as the kidney [11]. In the kidney, 11ß-HSD2 helps to confer aldosterone specificity for the nonselective mineralocorticoid receptors (MR) by inactivating glucocorticoids locally [12, 13]. Thus, deficiencies in this enzyme activity, either congenital or acquired through liquorice ingestion, lead to the syndrome of apparent mineralocorticoid excess in which cortisol acts as a mineralocorticoid causing hypertension and hypokalemia [4].

In the human placenta, the expression of 11ß-HSD2 is predominant over that of 11ß-HSD1 [14, 15]. This is consistent with Murphy's hypothesis that the placental 11ß-HSD system serves to protect the fetus from high levels of maternal cortisol [3]. Although it remains controversial whether placental 11ß-HSD2 activity correlates positively with birth weight at term in uncomplicated pregnancies [14, 16], there is evidence that placental 11ß-HSD2 activity is attenuated in pregnancies complicated with IUGR [17]. There have been numerous studies on the placental 11ß-HSD system using several animal models [18], most notably the baboon [19, 20] and the rat [21, 22]. The results from these animal studies seem to provide additional support for Murphy's hypothesis. However, our understanding of the role of placental 11ß-HSD2 in fetal development is incomplete. One of the reasons is that the relative expression of 11ß-HSD1 and 11ß-HSD2 isozymes in the placenta differs greatly between species [18]. For instance, the placenta of the rat [22], pig [23], and sheep [18] expresses predominantly 11ß-HSD1. Moreover, there is limited information on the regulation of placental 11ß-HSD2 [24].

Given the known similarity in placental structure (hemomonochorial) [25] and maternal-fetal cortisol gradient between the human [26] and the guinea pig [1], we have evaluated the potential of using this animal as a model to study the role of placental 11ß-HSD2 in fetal development. Our results demonstrate that the guinea pig resembles the human in that 11ß-HSD2 is the predominant, if not exclusive, isozyme expressed in the placenta.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Reagents and Supplies

[1,2,6,7-3H(N)]Cortisol (80 Ci/mmol) was purchased from DuPont Canada Inc. (Markham, ON, Canada). [1,2,6,7-3H(N)]Cortisone was prepared from [1,2,6,7-3H(N)]cortisol in our laboratory as described previously [10]. Nonradioactive steroids were obtained from Steraloids Inc. (Wilton, NH). Cofactors (NAD and NADPH) were purchased from Sigma Chemical Co. (St. Louis, MO). Polyester-backed thin-layer chromatography (TLC) plates were obtained from Fisher Scientific Ltd. (Unionville, ON, Canada). All solvents used were OmniSolv grade from BDH Inc. (Toronto, ON, Canada). General molecular biology reagents were from Gibco BRL (Burlington, ON, Canada) or Pharmacia Canada Inc. (Baie D'Urte, PQ, Canada). The cDNAs used in this study, including a mouse 18S rRNA cDNA (Dr. D.T. Denhardt, Rutgers University, Newark, NJ), were labeled with [32P]dCTP (Du Pont Canada; 3000 Ci/mmol) by random priming. Oligonucleotides were synthesized using a Pharmacia Gene Assembler and purified using NAP-50 columns (Pharmacia) according to the manufacturer's instructions.

Collection of Placental Tissues

Placental tissues were collected from time-dated pregnant guinea pigs of the Hartley strain (purchased from Charles River Canada, St-Constant, PQ, Canada) at Days 40–45 (n = 5), 50–60 (n = 7), and at term (approximately 68 days; n = 4). The animals were killed with an overdose of euthanyl (MTC Pharmaceuticals, Cambridge, ON, Canada), and the placental tissues were collected rapidly, snap-frozen in liquid nitrogen, and stored at -80°C until analysis.

Assay of 11ß-HSD Activity

Preparation of tissue homogenates Placental tissues (0.1–0.2 g) were homogenized in 20 volumes of ice-cold 10 mM sodium phosphate buffer, pH 7.0, containing 0.25 M sucrose. The homogenate was used immediately in assays as described below.

Protein estimation Protein concentration was determined by the Bradford method using a Bio-Rad (Mississauga, ON, Canada) protein assay kit with BSA as standard.

Assay of 11ß-HSD dehydrogenase activity The 11ß-HSD dehydrogenase activity was determined by measuring the rate of conversion of cortisol to cortisone, as described previously [10]. Briefly, the assay tubes contained placental tissue homogenate (20–50 µg protein), approximately 100 000 cpm of the labeled cortisol, and final concentrations of nonradioactive cortisol and cofactor (NAD or NADP) at 0.5 µM and 250 µM, respectively. After incubation in a water bath at 37°C for 30 min (preliminary studies indicated that the rate of reaction was linear with time from 10 to 60 min), the reaction was arrested, and the steroids were extracted. The extracts were dried and the residues resuspended. A fraction of the resuspension was spotted on a TLC plate that was developed in chloroform/methanol (9:1, v:v). The bands containing the labeled cortisol and cortisone were identified by UV light of the cold carriers, cut out into scintillation vials, and counted in Scintisafe Econol 1 (Fisher Scientific, Toronto, ON, Canada). The rate of cortisol-to-cortisone conversion was calculated from the specific activity of the labeled cortisol and the radioactivity of cortisone, and results were expressed as the amount of cortisone (picomoles) formed per minute per milligram protein.

Assay of 11-oxoreductase activity The 11-oxoreductase activity was determined similarly except that cortisone was used as substrate, and NADH or NADPH as cofactor.

Kinetic Analysis

Since under conditions of the present study, no appreciable 11-oxoreductase activity was detected (the limit of detection was defined as greater than twice the blank), kinetic studies were performed for the 11ß-HSD dehydrogenase activity only, as described previously [10]. Briefly, conversion assays were conducted using a fixed amount of NAD (250 µM; preliminary studies indicated that this was at saturating concentrations), enzyme preparation (30–100 µg protein), and reaction time (30 min), but with varying amounts of cortisol (0.05–1.0 µM). Each experiment was done in duplicate, and a total of 5 independent experiments using 5 different placental tissues (Days 50–60) were conducted (preliminary studies indicated that there was no change in the Km at the three gestational ages studied).

Cloning and Sequencing of Guinea Pig 11ß-HSD2 cDNA

The 11ß-HSD activity results indicated the presence of 11ß-HSD2 rather than 11ß-HSD1 in the guinea pig placenta. In order to confirm the placental expression of 11ß-HSD2 gene, we cloned the cDNA encoding guinea pig 11ß-HSD2 by a concerted strategy of reverse transcription-polymerase chain reaction (RT-PCR) and gene cloning, based on sequence homology among the other mammalian 11ß-HSD2 cDNAs.

RNA extraction Total RNA was extracted from the placental tissues using lithium chloride/urea [27] and quantified spectrophotometrically at 260 nm. Prior to use, samples (10 µg) were checked by agarose gel electrophoresis in the presence of formaldehyde, and the integrity of the RNA was assessed by the presence of two sharp bands representing 28S and 18S rRNA after staining with ethidium bromide. For RT-PCR, total RNA samples were purified using RNeasy kit (Qiagen Inc., Mississauga, ON, Canada).

RT-PCR To obtain the 3'-end 11ß-HSD2 cDNA, 3 µg of a placental total RNA sample was subjected to the 3'-RACE (rapid amplification of cDNA ends) protocol [28], modified as described below. Briefly, the first-strand cDNA was synthesized using an in-house-designed oligo dT-adapter primer (5'-GTCGAC GGTACC GATATC T17) in a total volume of 20 µl. An aliquot (2 µl) was subjected to a standard PCR (95°C, 55 sec; 50°C, 55 sec; 72°C, 2 min; 30 cycles) using the adapter primer and a gene-specific primer (5'-CTGAAGCTGC TGCAGATGGA) that corresponds to nucleotides 502–521 in the published human 11ß-HSD2 cDNA [11]. The positive PCR products were selected by Southern blot analysis using 32P-labeled sheep 11ß-HSD2 cDNA and cloned into pBluescript KS. An upstream portion of the cDNA was then synthesized by a standard RT-PCR protocol using a reverse primer specific for the guinea pig (5'-TAATGTCCTC TGGCTTCATC) and a forward primer (5'-CCGCGCTCGA CTGGCTGTGC) corresponding to nucleotides 233–252 in the published human 11ß-HSD2 cDNA [11]. We then tried various strategies including the 5'-RACE but failed to obtain the missing 5' end. To get the 5'-end guinea pig 11ß-HSD2 cDNA, we therefore resorted to conventional gene cloning.

Isolation of 11ß-HSD2 gene A guinea pig genomic library (Stratagene, La Jolla, CA) was screened by plaque hybridization using the cloned 3'-end guinea pig 11ß-HSD2 cDNA as probe. Positive plaques were isolated, and their inserts were subjected to restriction digestion followed by Southern blotting. The restriction fragments containing 5' and 3' ends of the gene were then subcloned into pBluescript KS.

DNA sequencing All the cloned DNA sequences were determined on denatured double-strand plasmid DNA by the chain termination method using Quick-Denature Sequenase 2.0 Kit (US Biochemical Corp., Cleveland, OH).

Northern Blot Analysis

To verify the expression of 11ß-HSD2 gene and to determine whether changes occur in the level of 11ß-HSD2 mRNA in the guinea pig placenta during late gestation, total RNA samples were subjected to Northern blot analysis as described previously [8, 29]. Briefly, denatured RNA samples (30 µg) were subjected to agarose gel (1%) electrophoresis in the presence of formaldehyde and transferred overnight by capillary blotting to a Zeta-Probe membrane (Bio-Rad). The RNA was fixed by UV cross-linking (Gene Cross-Linker; Bio-Rad) to the membrane, which was then baked under vacuum at 80°C for 60 min. The blot was hybridized with 32P-labeled guinea pig 11ß-HSD2 cDNA at 42°C for 16 h in the presence of formamide (50%). The same blot was then stripped and reprobed with 32P-labeled guinea pig 11ß-HSD1 cDNA [30]. We used a cDNA for mouse 18S rRNA as an internal control for gel loading and efficiency of RNA transfer, as described previously [8].

To determine the relative abundance of 11ß-HSD2 mRNA and 18S rRNA, the relative optical density of the corresponding signals on autoradiographic films was measured by scanning with a laser densitometer (LKB 2222–020 UltraScan XL; LKB Produkter AB, Bromma, Sweden), as previously described [8]. In all cases, the signals were detected within the linear scan range of the densitometer. For each RNA sample, the ratio of 11ß-HSD2 mRNA signal to 18S rRNA signal was calculated, and group means were obtained.

Data Analysis

Statistical analyses of 11ß-HSD2 mRNA and 11ß-HSD dehydrogenase activity data were performed using one-way ANOVA, followed by LSD (least-square difference) test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Characteristics of Placental 11ß-HSD Activity

As shown in Figure 1, the 11ß-HSD dehydrogenase activity in placental tissue homogenates at all three gestational ages was clearly NAD-dependent. Moreover, the NAD-dependent activity decreased significantly at term. Under conditions of the present study, there was no detectable 11-oxoreductase activity at any of the ages (data not shown). The results from kinetic studies revealed that the guinea pig placental 11ß-HSD dehydrogenase activity had a Km of 134 ± 5 nM for cortisol and a Vmax of 10 ± 3 pmol/min per milligram protein. Collectively, these characteristics of 11ß-HSD enzyme activity in the guinea pig placenta are indicative of 11ß-HSD2.



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  		FIG. 1. Changes in 11ß-HSD dehydrogenase activity in the guinea pig placenta during late gestation. Tissue homogenates were incubated with 0.5 µM cortisol in the presence of NAD or NADP as described in Materials and Methods. Each bar represents group mean ± SEM. ** p < 0.01 when compared with the other two groups.

Guinea Pig 11ß-HSD2 cDNA Sequence and the Deduced Primary Structure

The 3'-end 11ß-HSD2 cDNA obtained by the modified 3'-RACE protocol was 1.5 kilobases (kb) in length, including a poly A tail of approximately 200 base pairs (bp). The 11ß-HSD2 cDNA synthesized by the standard RT-PCR was 274 bp long. Together, these two overlapping 11ß-HSD2 cDNA fragments yielded a partial cDNA of 1.7 kb that lacks 5' end when aligned with other known mammalian 11ß-HSD2 cDNAs. A 2.5-kb HindIII fragment derived from gene cloning was found to contain sequences corresponding to exon 1 of the known mammalian 11ß-HSD2 genes [31–33]. The region encompassing the cDNA stop codon was also verified by sequencing the corresponding region of the gene. Together, they resulted in a cDNA of 1.9 kb, which included a 41-bp 5'-noncoding region, followed by an 1188-bp open reading frame that encodes a protein of 395 amino acids, and a 3'-noncoding region of 520 bp (Fig. 2). The deduced guinea pig 11ß-HSD2 polypeptide has a molecular weight of 43.5 kDa and displays more than 80% sequence identity to the predicted other mammalian 11ß-HSD2 proteins (Fig. 3).



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  		FIG. 2. Nucleotide sequence of guinea pig 11ß-HSD2 cDNA, and the deduced amino acid sequence of 11ß-HSD2 polypeptide.



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  		FIG. 3. Comparison of the deduced human, guinea pig, and mouse 11ß-HSD2 amino acid sequences. Hyphens indicate identical amino acids with respect to the human sequence; # indicates gaps that are required to align the sequences. The putative cofactor-binding site (A) and catalytic site (B) are underlined.

Expression of 11ß-HSD2 mRNA in the Guinea Pig Placenta

When total RNA samples from placental tissues at various gestational ages were analyzed by Northern blotting using the cloned 1.5-kb guinea pig 11ß-HSD2 cDNA probe, a 2.0-kb transcript was detected in all the samples. Moreover, the relative abundance of 11ß-HSD2 mRNA decreased significantly (p < 0.01) at term (Fig. 4). When the same blot was reprobed with guinea pig 11ß-HSD1 cDNA probe, no signals were detectable at any of the ages (data not shown).



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  		FIG. 4. Changes in 11ß-HSD2 mRNA in the guinea pig placenta during late gestation. Total RNA samples (30 µg) were analyzed. The top panel shows autoradiographs of the blot probed sequentially with 32P-labeled guinea pig 11ß-HSD2 cDNA, and with 32P-labeled mouse 18S rRNA cDNA used as a control. The bottom panel illustrates changes in the relative abundance of 11ß-HSD2 mRNA in the placenta at discrete times during late pregnancy. Each bar represents mean ± SEM. ** p < 0.01 when compared with the other two groups.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In the present study we have, for the first time, characterized the 11ß-HSD system in the guinea pig placenta and also cloned the cDNA encoding guinea pig 11ß-HSD2. Our results demonstrate that the guinea pig placenta, much like the human, expresses predominantly 11ß-HSD2 gene. Given the known similarity in placental structure and maternal-fetal cortisol gradient between the human and the guinea pig, the present findings suggest that the guinea pig may represent a suitable model in which to study the role and regulation of placental 11ß-HSD2 in human fetal development.

The human placenta is known to express high levels of 11ß-HSD2 mRNA and protein, while only very low levels of 11ß-HSD1 mRNA and protein are detectable [14, 15]. This is consistent with earlier studies demonstrating that the dehydrogenase activity was predominant in human placental tissues [34, 35], which led Murphy to propose that the placental 11ß-HSD enzyme system may serve as a barrier to protect the fetus from high maternal levels of cortisol [3]. Indeed, within the human placenta, 11ß-HSD2 is well positioned to fulfill this putative function since it is localized exclusively to the syncytiotrophoblast, the site of maternal-fetal exchange [36]. In uncomplicated human pregnancies at term, it has been found that placental 11ß-HSD2 activity correlates positively with birth weight [14], although a more recent study failed to confirm such a correlation [16]. Recent studies from our laboratory indicate that placental 11ß-HSD2 activity is attenuated in pregnancies complicated with IUGR [17]. Moreover, IUGR is a characteristic feature of apparent mineralocorticoid excess [37], a syndrome resulting from 11ß-HSD2 dysfunction [4]. However, the precise pathophysiological significance of attenuated placental 11ß-HSD2 in IUGR remains to be defined.

The placental 11ß-HSD system in baboons has been studied extensively by Pepe, Albrecht, and colleagues [19]. There is an estrogen-induced change in placental 11ß-HSD activity from predominantly reduction (cortisone to cortisol) at midgestation to oxidation (cortisol to cortisone) at term. It has been proposed that this change in the placental enzyme activity may play an important role in regulating the activation of the fetal hypothalamic-pituitary-adrenal axis. More recently, it has been shown that both 11ß-HSD1 and 11ß-HSD2 mRNAs are present in the baboon placenta from gestational Day 50 to term, and levels of both types increase progressively during this time [20]. In the rat, placental 11ß-HSD dehydrogenase activity correlates positively with birth weight [38]. Furthermore, a reduction in birth weight was noted when pregnant rats were treated with dexamethasone; this being a poor substrate for 11ß-HSD2, most would cross the placenta unmetabolized. Similar results were obtained with carbenoxolone, a potent inhibitor of 11ß-HSD1 and 11ß-HSD2 [39]. Collectively, these results provide additional support for Murphy's hypothesis.

Our results demonstrate that the guinea pig placental 11ß-HSD is characteristic of 11ß-HSD2 in that it possesses only dehydrogenase activity that is NAD-dependent and has a high affinity for cortisol. In order to confirm the placental expression of 11ß-HSD2 gene, we have cloned the cDNA encoding guinea pig 11ß-HSD2. The deduced guinea pig 11ß-HSD2 enzyme displays over 80% sequence identity to that of the human [11], mouse [32], rat [40], rabbit [41], and sheep [33, 42]. The putative cofactor-binding and catalytic motifs are also conserved in the guinea pig. However, the predicted amino acid sequence of the guinea pig (395 amino acids [aa]) is shorter than those of the human (405 aa), rabbit (406 aa), sheep (404 aa), and rat (400 aa) proteins. This is owing to the presence of a premature stop codon (TAA) in the guinea pig 11ß-HSD2 open reading frame, although a second stop codon (TGA) corresponding to the one in other known mammalian 11ß-HSD2 cDNAs is also present. The premature stop codon was confirmed by sequence analysis of the corresponding exon derived from the genomic clone. The relatively short guinea pig 11ß-HSD2 enzyme is unlikely to have any functional significance, since the deduced mouse 11ß-HSD2 enzyme (396 aa) is similar in size to that of the guinea pig. Moreover, the C-terminal sequences of all known mammalian 11ß-HSD2 proteins show the highest degree of diversity. Using the cloned guinea pig 11ß-HSD2 cDNA and the cDNA encoding guinea pig 11ß-HSD1 as probes, we have demonstrated the presence of 11ß-HSD2, but not 11ß-HSD1, mRNA in the guinea pig placenta. This is consistent with the characteristics of placental 11ß-HSD enzyme activity. Therefore, the guinea pig placenta, much like the human, expresses predominantly, if not exclusively, 11ß-HSD2.

The guinea pig is known as a glucocorticoid-resistant species having high levels of free cortisol in circulation, which results from a more potent ACTH and a low-affinity corticosteroid binding globulin. In addition, guinea pigs possess an abnormal glucocorticoid receptor with a reduced affinity for cortisol [43, 44]. However, the MR seems unremarkable in that its binding characteristics are similar to those of the rat [45]. Given the crucial role of 11ß-HSD2 in protecting the renal MR [12, 13], it has been proposed that guinea pigs may possess a super-efficient 11ß-HSD2 (with enhanced catalytic activity) in order to deal effectively with much higher circulating levels of cortisol. Alternatively, guinea pigs may have distinct MR-protective mechanisms other than 11ß-HSD2 [43, 44]. The present findings neither prove nor discount the former possibility, since no apparent gross abnormalities were identifiable in the deduced guinea pig 11ß-HSD2 primary structure. The answer to this question will come to light when the intrinsic properties of the purified and/or the expressed guinea pig 11ß-HSD2 have been determined.

Previous studies in the guinea pig [46] demonstrated that the rate of transplacental transfer of cortisol between the mother and her fetuses increased near term. To determine whether this increase can be explained by a reduction in the level of placental 11ß-HSD2, we collected placental tissues at discrete times during late pregnancy and sought changes in the tissue level of 11ß-HSD2 activity and mRNA. Our results revealed a progressive decrease in both 11ß-HSD2 activity and mRNA in the guinea pig placenta from Day 45, with the decrease being significant at term. Thus, the temporal change in placental 11ß-HSD2 expression during late pregnancy is consistent with, and provides a plausible molecular basis for, the previously reported increase in the transplacental transfer of cortisol in this species. In the human placenta, a similar decrease in 11ß-HSD dehydrogenase activity was noted at term [34, 47]. Therefore, the guinea pig displays remarkable resemblance to the human with respect to the placental 11ß-HSD system and appears to represent a suitable model in which to study the role of placental 11ß-HSD2 in fetal development.


   ACKNOWLEDGMENTS
 
We thank Drs. T. Drysdale, G. Fong, and J. Peng for their help in the subcloning of genomic fragments. We are also grateful to Ms. X. Pu for her useful discussions during the course of this study.


   FOOTNOTES
 
1 This work was supported by the Canadian MRC (Grant MT-12100 to K.Y.) and NSERC (Operating Grant to S.G.M.). K.Y. is an Ontario Ministry of Health Career Scientist. Back

2 Correspondence: K. Yang, Lawson Research Institute, 268 Grosvenor Street, London, ON, Canada N6A 4V2. FAX: 519 646 6110; kyang{at}julian.uwo.ca Back

Accepted: July 16, 1998.

Received: May 6, 1998.


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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