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BOR - Papers in Press, published online ahead of print April 6, 2005.
Biol Reprod 2005, 10.1095/biolreprod.105.039925
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BIOLOGY OF REPRODUCTION 73, 230–236 (2005)
DOI: 10.1095/biolreprod.105.039925
© 2005 by the Society for the Study of Reproduction, Inc.

Expression of Prostaglandin Transporter in the Bovine Uterus and Fetal Membranes During Pregnancy1

S.K. Banu 3 4, J.A. Arosh 3 4, P. Chapdelaine 4, and M.A. Fortier 2 4,5 

Unité d'Ontogénie et Reproduction,4 Centre Hospitalier Universitaire de Québec, and Centre de Recherche en Biologie de la Reproduction, and Département d'Obstétrique et Gynécologie,5 Université Laval, Québec, Canada G1K 7P4


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Uteroplacental prostaglandins (PGs) play pivotal roles in the maintenance and termination of pregnancy in mammals. In the present study, we have characterized the expression of prostaglandin transporter (PGT) in placentome caruncles, intercaruncular tissues, fetal membranes, and utero-ovarian plexus during pregnancy in cattle. Pregnant bovine uteri were collected and classified into six groups covering the entire gestational length. In caruncles and intercaruncular tissues, PGT mRNA (also known as SLC02A1) and PGT protein were highly expressed at the late stage of pregnancy compared to the early and mid stages, whereas the level of expression is constant and low in fetal membranes throughout pregnancy. PGT mRNA and PGT protein were expressed at a constant level in the utero-ovarian plexus both ipsilateral and contralateral to corpus luteum throughout the course of pregnancy. Overall, the relative expression of PGT mRNA and PGT protein were higher in caruncles than in intercaruncular tissue and fetal membranes, whereas no differences were detected between intercaruncular tissues and fetal membranes at any stage of gestation. Immunohistochemistry indicated that PGT was preferentially expressed in caruncular epithelial cells of placentomes and endometrial luminal epithelial and myometrial smooth muscle cells of the intercaruncular regions. The level of PGT expression was comparatively higher in maternal components than in fetal components. In conclusion, differential spatiotemporal tissue-specific expression of PGT in uterine and intrauterine tissues suggests a role for this transporter in the exchange of PGs between the maternal and the fetal compartments, as well as for intrauterine metabolism of PGs during pregnancy.

bovine, female reproductive tract, fetal membrane, parturition, placental transport, pregnancy, prostaglandin, prostaglandin transporter, uterus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Prostaglandins (PGs) play major roles in female reproductive functions such as ovulation, luteolysis, establishment and maintenance of pregnancy, and parturition [1–3]. Arachidonic acid stored in membrane phospholipids is the primary precursor for PGs. Arachidonic acid is converted into PGH2 by cyclooxygenase (COX) -1 and -2 (also known as PTGS1 and PTGS2). PGH2 is then converted into different PGs, including PGE2, PGF2{alpha}, PGD2, PGI2, and TxA2 by specific PG synthases [4]. PGs act mainly through G-protein-coupled receptors, and recently, nuclear receptors have been identified [5, 6].

In mammals, PGF2{alpha} was identified as the luteolytic hormone [7], and as a myometrial stimulant [3]. PGE2 has been proposed to have multiple roles as a temporary luteotropic, luteostatic, or luteoprotective signal at the time that pregnancy is established [8, 9]. Moreover, PGE2 acts as an immunomodulatory signal at the fetal-maternal interface during pregnancy [10] and has mitogenic, antiapoptotic, and angiogenic effects in several biological systems [11– 14]. PGE2 can also act as a myometrial relaxant [15] or stimulant [16]. In ruminants, uterine PGF2{alpha}, PGE2, or both drain into the utero-ovarian vein and are transported from the uterine vein to the ovarian artery through a local, unique veno-arterial pathway known as the utero-ovarian plexus (UOP). This local transport system allows uterine PGs to exert their luteotropic or luteolytic action on the ovary without passing through the systemic circulation where 65%–99% would be metabolized in one passage through lungs, depending on the species [7].

PGs are polar and exist as anions that diffuse poorly through plasma membranes in spite of their lipid nature [17, 18]. The transport of PGs through plasma membrane is poorly understood. Various mechanisms have been proposed such as simple diffusion, passive transport, active transport, countercurrent transfer, as well as carrier-mediated transport [7, 17, 18]. It has been shown that although anions cross the cell membrane by simple diffusion, the estimated flow rate would be too low to maintain biological functions [19]. Recently, a novel prostaglandin transporter (PGT) also known as SLCO2A1 was identified in rat, mouse, and human. PGT belongs to the solute-carrier organic anion transporter 2A1 family, or OATP2A1 [17, 18]. It has been proposed that PGT mediates both the efflux of newly synthesized PGs to effect their biological actions through their cell surface receptors, as well as the influx of PGs from the extracellular milieu for their cellular inactivation. We have recently cloned a bovine PGT (bPGT) and studied its dynamic expression in the endometrium, myometrium, UOP, and corpus luteum (CL) during the estrous cycle and during establishment of pregnancy [20–22]. Functional characterization of bPGT in transiently transfected HeLa cells indicated that it transports PGE2 and PGF2{alpha} with equal affinity in a competitive manner. We have proposed the novel hypothesis that PGT is involved in the cellular and compartmental transport of PGs during reproductive processes [20–22].

Uteroplacental PGs play pivotal roles in maintenance or termination of pregnancy (or both) in mammals [23–27]. In ruminants, endometrial caruncles take part in the formation of placentomes with fetal cotyledons and are involved in fetal-maternal communication and maintenance of pregnancy [23–27]. The intercaruncular (ICAR) region has been primarily associated with maintenance of uterine quiescence, and this region is also involved in other essential fetal-maternal interactions [23–27]. Several studies have documented the selective expression of cyclooxygenases, and PG relaxant and contractile receptors in uterine and intrauterine tissues at the time that pregnancy is established [28, 29]. Likewise, an up-regulation of these components has been reported at term pregnancy and parturition in various species [3, 30, 31]. We have recently shown a temporal and tissue-specific expression of PGE2 and PGF2{alpha} receptors and COX-1 and COX-2 in uteroplacental tissues, suggesting selective roles for PGE2 and PGF2{alpha} in uterine activities during pregnancy in bovine [32]. Despite the fact that PGs are involved in various functions in the regulation and maintenance of pregnancy the mechanism responsible for the trafficking of PGs across the maternal and fetal compartments is completely unknown. Therefore, we analyzed the distribution and expression of the PGT gene and PGT protein within the fetal and maternal compartments during pregnancy. Our objectives were 1) to study the expression of the PGT gene and PGT protein in caruncular (CAR), ICAR, and fetal membrane (FM) tissues throughout bovine pregnancy; and 2) to study the expression of the PGT gene and PGT protein in the UOP during pregnancy.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The reagents used in this study and their suppliers were as follows: TRIzol (Invitrogen Life Technologies Inc., Burlington, ON, Canada), Ready-To-Go DNA labeling kit (Amersham Pharmacia Biotech, Montreal, QC, Canada), prestained protein markers (New England Biolabs Inc., Mississauga, ON, Canada), Bright Star-plus nylon membrane and UltraHyb (Ambion Inc., Austin, TX), Trans-Blot nitrocellulose membrane (Bio-Rad Laboratories, Hercules, CA), [{gamma}32P] ATP and [{alpha}32P] dCTP (Perkin-Elmer Life Sciences, Markham, ON, Canada), Renaissance (Life Science Products Inc., Boston, MA), BioMax film (Eastman Kodak Corp., New York, NY), Mayer hematoxylin (Sigma-Aldrich Canada Ltd., Oakville, ON, Canada), and Vectastain Elite ABC kit (Vector Laboratories Inc., Burlingame, CA). The other chemicals used were of molecular biological grade available from Laboratoire Mat or Fisher Biotech (Quebec, QC, Canada). The antibodies and their sources were as follows: goat anti-rabbit biotinylated immunoglobulin (Ig) (DAKO Diagnostics of Canada Inc., Mississauga, ON, Canada), goat anti-rabbit or mouse IgG conjugated with horseradish peroxidase (Jackson Immunoresearch Laboratories, West Grove, PA), and monoclonal anti-mouse ß-actin antibody (Cayman Chemicals, Ann Arbor, MI). Anti-bovine PGT was produced in our laboratory as described previously [20].

Tissue Collection and Experimental Design

Bovine pregnant uteri were collected at a local abattoir immediately after slaughter. Care was taken to eliminate potential pathological conditions as described previously [33]. Uteri were opened longitudinally along the greater curvature, and the day of pregnancy was determined by measurement of crown-rump length of the fetus present in the uterus [32]. CAR, ICAR, and FM tissues were collected. CAR was composed of endometrial caruncles and fetal cotyledons (chorion and chorioallantois); ICAR tissues consisted of endometrium and myometrium (full thickness of uterus); and FM was composed of intercotyledonary portions of amnion, chorion, and chorioallantois. UOP was collected from sites ipsilateral (near) and contralateral (opposite) to the CL. Cross-sections of tissues were prepared and processed for immunohistochemistry as described below. Additionally, tissues were cut into small pieces and snap-frozen in liquid nitrogen and stored at –80°C until used. Total RNA was extracted from each tissue sample using TRIzol according to the manufacturer's protocol. Total protein was extracted as described previously [34].

Based on the days of pregnancy the CAR, ICAR, and FM tissues were classified into six groups as Days <50 (n = 3), 51–100 (n = 6), 101–150 (n = 4), 151–200 (n = 5), 201–250 (n = 4), and >251 (n = 3). Expression of bPGT mRNA and PGT protein were analyzed using Northern blot and Western blot, respectively. Cellular localization of bPGT was performed by immunohistochemistry.

Northern Blot Analysis

Northern blotting and hybridization were performed as described [20, 33]. Briefly, total RNA (~20 µg) was loaded in each lane and electrophoresed on a 1.2% formaldehyde-agarose gel. RNA was transferred overnight onto a nylon membrane in 10x saline-sodium citrate. The bPGT cDNA probe was labeled with [{alpha}32P] dCTP (3000 Ci/mmol) using the Ready-To-Go DNA labeling kit. Prehybridization (2–3 h) and hybridization (overnight) were carried out at 45°C using UltraHyb. The blots were stripped off by boiling in 1% SDS for 30 min and rehybridized with {gamma}-32P[ATP] labeled oligoprobe specific to 18S ribosomal RNA. The blots were exposed to BioMax film and densitometry of autoradiograms was performed using an Alpha Imager (Alpha Innotec Corporation, Montreal, QC, Canada).

Western Blot Analysis

Western blot analysis was performed as we described previously [20, 33]. Briefly, total proteins (~20 µg) were loaded in each lane and electrophoresed on 10% SDS-PAGE followed by electro-transfer onto nitrocellulose membrane. Anti-bPGT was used as the primary antibody (1: 1000). Goat anti-rabbit IgG conjugated with horseradish peroxidase was used as the secondary antibody (1:20 000). Chemiluminescent substrate was applied according to the manufacturer's instructions. The blots were exposed to BioMax film and densitometry of the autoradiogram was performed using an Alpha Imager. As an internal standard ß-actin (1:5000) was measured.

Immunohistochemistry

Cross-sections were made in the CAR and ICAR regions of the uterus, FM, and UOP. Tissues were fixed in 4% buffered paraformaldehyde for 4 h at 4°C and then paraffin sections (6 µm) were prepared. Immunocytolocalization of bPGT protein was performed using the Vectastain Elite ABC kit according to the manufacturer's protocol [20]. Endogenous peroxidase activity was removed by fixing the sections in 0.3% hydrogen peroxide in methanol. Tissue sections were blocked in 10% goat serum for 1 h at room temperature. Incubation with the primary antibody (anti-bPGT, 1:1000) was performed overnight at 4°C. The sections were further incubated with the second antibody (goat anti-rabbit IgG biotinylated, 1: 200) for 30 min at room temperature. For the negative control, preimmune rabbit serum (1:1000) was used instead of bPGT antibody. Between each step, sections were washed in PBS. Finally, sections were stained with Mayer hematoxylin.

Statistical Analysis

All numerical data were presented as the mean ± SEM. Data were analyzed using analysis of variance followed by Scheff' tests (SUPER ANOVA, Abacus Concepts, Inc., Berkeley, CA). Differences were considered as statistically significant at a 95% confidence level (P < 0.05).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Expression of bPGT in Placentome Caruncles

In caruncles, PGT mRNA and PGT protein were expressed and showed a steady increase during pregnancy (Fig. 1). The level of PGT mRNA was higher (P < 0.05) from Days 151 to ≥251 than other days of pregnancy. PGT protein followed a similar but slightly delayed pattern of expression. PGT protein was more highly (P < 0.05) expressed between Days 201 and ≥251 than at early and mid stages of pregnancy. Immunohistochemistry (Fig. 2) indicated that PGT protein was preferentially expressed in epithelial cells of caruncular crypts than in stromal cells of caruncular septum and fetal trophoblastic cells. The level of PGT expression was higher in the maternal part than in the fetal part of the placentome caruncle.



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  		FIG. 1. Expression of PGT mRNA and PGT protein in caruncles during bovine pregnancy. A) Northern blot analysis. As an internal standard, 18S RNA was used. B) Densitometry analysis of bPGT mRNA. a: Days 151 to ≥251 vs. others (P < 0.05). C) Western blot analysis. As an internal standard, ß-actin was measured. D) Densitometry analysis of bPGT protein. b: Days 201 to ≥251 vs. others (P < 0.05). Numerical values are presented as the mean ± SEM of 3–6 samples



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  		FIG. 2. Cellular localization of bPGT protein in placentome CAR and ICAR tissues and FM during bovine pregnancy (200–250 days). A) Normal structure of placentome caruncle. B) Immunohistochemistry. In CAR, PGT is preferentially expressed in epithelial cells of caruncular crypts than in stromal cells of caruncular septum and fetal trophoblastic cells. In ICAR, PGT expression is comparatively higher in endometrial luminal epithelium and myometrial smooth muscle cells, lower in endometrial stroma, and undetectable in glandular epithelium. In FM, PGT expression is more evident in the secondary branches of chorioallantois (the deeper part of the cotyledon interdigited with caruncular tissue) than in the primary branches (superficial parts of the cotyledon). PGT expression is undetectable in uninucleated, binucleated, and giant cells of trophoblast. PGT is also expressed in smooth muscle cells of blood vessels. BV, blood vessels; CEP, caruncular epithelial cells; CC, caruncular crypts; CS, caruncular stalk; CSE, caruncular septum; CST, caruncular stromal cells; GLE, glandular epithelial cells; LE, luminal epithelium; LU, lumen; PCAV, primary chorioallantoic villus; SCAV, secondary chorioallantoic villus; SMC, smooth muscle cells; ST, stromal cells; TRV, trophoblastic villus; UW, uterine wall. Trophectoderm: 1, mononuclear; 2, binuclear; 3, giant cells

Expression of bPGT in Intercaruncular Tissue

Figure 3 illustrates that in ICAR tissues, PGT mRNA expression was higher (P < 0.05) in the later part of pregnancy from Day 151 than during early and mid stages (<150 days). PGT protein followed a similar but slightly delayed pattern of expression. PGT protein expression increased (P < 0.05) only at later stages (≥201 days) of pregnancy (Fig. 3). The relative levels of PGT mRNA and PGT protein were ~2-fold to ~3-fold (P < 0.05) lower in ICAR than in CAR at each time point evaluated. Immunohistochemistry (Fig. 2) revealed that in ICAR, PGT protein expression was comparatively high in endometrial luminal epithelium and myometrial smooth muscle cells, but lower in endometrial stroma. PGT protein was undetectable in glandular epithelium.



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  		FIG. 3. Expression of bPGT mRNA and PGT protein in intercaruncular tissues during the bovine pregnancy. A) Northern blot analysis. As an internal standard, 18S RNA was used. B) Densitometry analysis of bPGT mRNA. a: Days 201 to ≥251 vs. ≤50 to 150 (P < 0.05). C) Western blot analysis. As an internal standard, ß-actin was measured. D) Densitometry analysis of bPGT protein. b: Days ≥251 vs. others (P < 0.05). Numerical values are presented as the mean ± SEM of 3–6 samples

Expression of bPGT in Fetal Membrane Tissues

In FM tissues, PGT mRNA was expressed at a constant low level throughout pregnancy (Fig. 4). Immunohistochemical (Fig. 2) localization indicated that PGT protein was expressed at a very low level in mononuclear, binuclear, and giant cells of the trophectoderm. PGT protein expression was more evident in the secondary branches of chorioallantois (a deeper part of the cotyledon interdigited with caruncular tissue) compared to that of the primary branches (superficial part of cotyledon). PGT protein was also expressed in smooth muscle cells of blood vessels.



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  		FIG. 4. Expression of bPGT mRNA and PGT protein in FM during bovine pregnancy. A) Northern blot analysis. As an internal standard, 18S RNA was used. B) Densitometry analysis of bPGT mRNA. C) Western blot analysis. As an internal standard, ß-actin was measured. D) Densitometry analysis of bPGT protein. Numerical values are presented as the mean ± SEM of 3–6 samples

Expression of bPGT in the UOP

PGT mRNA expression was studied in the UOPs ipsilateral and contralateral to the CL during the early (<100 days), mid (Days 101–200), and late (Days ≥200) trimesters of pregnancy (Fig. 5). Our results indicated that PGT mRNA and PGT protein were expressed in both the ipsilateral and contralateral UOPs at relatively low and constant levels throughout the different stages of pregnancy.



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  		FIG. 5. Expression of bPGT mRNA and PGT protein in UOP during bovine pregnancy. A) Northern blot analysis. As an internal standard, 18S RNA was used. B) Western blot analysis with ß-actin measured as an internal standard. C) Densitometry analysis of bPGT mRNA. D) Densitometry analysis of bPGT protein. Numerical values are presented as the mean ± SEM of 3–6 samples


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In cattle, the duration of pregnancy is approximately 270 days. The placenta is epitheliochorial (or syndesmochorial), cotyledonary, and relatively noninvasive. Caruncular and intercaruncular tissues of the pregnant uterus play selective and distinct roles in the maintenance of a successful pregnancy [25–27]. Uteroplacental PGE2 and PGF2{alpha} are considered to be important mediators involved in recognition and maintenance of pregnancy [3, 25–27]. In a previous study we have characterized the expression of PG receptors EP2, EP3, and FP (also known as PTGER2, PTGER3, and PTGFR) and biosynthetic enzymes COX-1 and COX-2 in uteroplacental tissues during bovine pregnancy [32]. In the present study, we studied the temporal and tissue-specific expression of PGT in uteroplacental tissues and we now propose a role for PGT in facilitated transport of PGs between cells and tissues during pregnancy.

In CAR, PGT mRNA and PGT protein are highly expressed at the later stages of pregnancy. The level of expression was higher in CAR than in ICAR and FM at all time points tested. This observation suggests that the regulation of PG transit is particularly critical at the preferential site of exchange between the fetal and maternal compartments. PGT is primarily expressed in maternal caruncular epithelial and stromal cells, and absent in primary chorioallantoic villi but slightly expressed in its secondary branches. The relative expression pattern of PGT in CAR at later stages of pregnancy closely matches the expression of PGE2 receptor (EP2) and COX-2 [32]. However, the cellular localization of PGT is different from that of EP2 and COX-2, which are preferentially localized in fetal trophoblast cells during bovine pregnancy [32]. It has been postulated that in ruminants the exchange of oxygen and nutrients from the maternal to the fetal compartments, and placental and fetal secretory products from the fetal to the maternal compartments, primarily occur at the placentome caruncle through concurrent and cross-current mechanisms [23, 24]. It is believed that most of the transfer mechanisms across the ruminant placenta are similar to those of lung and kidneys [23]. Placenta continuously secretes PGs during pregnancy [3, 25–27], but they have to be metabolized before reaching the maternal side to ensure maintenance of pregnancy until term. Therefore, it is plausible that high expression of PGT in caruncular epithelial and stromal cells is involved in metabolic clearance of PGs. In our previous study we have shown that bPGT accelerates the influx of PGE2 and PGF2{alpha} 20-fold compared with that of control [20]. Other studies have demonstrated a role for PGT in local transport of PGs for catabolism in the lung, kidney [17, 18, 35], and different cell types [17, 18, 35, 36]. PGT mRNA is highly expressed in bovine lung [20]. In ruminants, 65%–99% of PGs are metabolized in the pulmonary vascular bed after a single passage [7]. Further, caruncles undergo morphogenesis throughout bovine pregnancy. The size of the caruncle increases with the day of gestation and size of the fetus [23, 24]. It is possible that PGT plays a role in facilitated efflux of PGs across the caruncular epithelial and stromal cells and contributes to growth and differentiation of caruncular tissues with the advancement of gestation. It has been shown in other systems that PGE2 acts as a mitogenic, angiogenic, and anti-apoptotic factor [11, 12], whereas PGF2{alpha} may be involved in tissue remodeling [37].

In ICAR, the expression patterns of bPGT mRNA and PGT protein parallel that of CAR, with higher expression toward the end of pregnancy. Cellular localization reveals that in ICAR, PGT is highly but diffusely expressed in endometrial luminal epithelium and myometrial smooth muscle cells. PGT expression is very low in endometrial stroma and not expressed in glandular epithelium. We have recently shown that in cyclic bovine endometrium, PGT mRNA and PGT protein are highly expressed during the luteolysis-pregnancy recognition window [20] in which selective localization of PGT at the basal region of the luminal epithelial cells suggested that PGs produced in the epithelial cells are transported toward the uterine compartment rather than into the uterine lumen. In the present study, diffused expression of PGT at the apical region and absence of selective localization at the basal region of the luminal epithelial cells suggest that PGs produced by the endometrium are directed into the uterine lumen but not toward the vascular system and ovary/CL during pregnancy. The very low expression of PGT in stroma also supports this notion. PGT is expressed uniformly in myometrium and diffusely in both inner circular and outer longitudinal smooth muscle cells. This suggests that the local transport of PGs may contribute to uterine relaxation, contraction, or both. In a previous study we documented the presence of biosynthetic components of PGs (COX-1 and COX-2) and signaling components (EP2, EP3, and FP) in ICAR during different stages of pregnancy [32]. In pigs, the action of endometrial PGF2{alpha} is explained by an endocrine and exocrine theory in which endometrial PGF2{alpha} is released toward the circulation to effect luteolysis or toward the uterine lumen at the time of recognition of pregnancy [38]. This theory requires a selective active transport of PGs rather than passive transport. Our findings on selective localization of PGT in bovine endometrium support the endocrine/exocrine theory, and also suggest that it may occur not only in pigs but also in cattle.

In FM, PGT mRNA is expressed at low, constant levels throughout pregnancy. Cellular localization indicates that expression of PGT protein is more evident in the secondary branches than in the primary branches of chorioallantois. However, PGT expression in mononuclear, binuclear, and giant cells of trophectoderm is not evident. In general, PGT is more highly expressed in maternal caruncle than in fetal membrane tissues during bovine pregnancy. Many studies have documented that fetal membrane produces PGs that are then transported to the maternal part where they alter maternal physiology [3, 39]. Placentome is the only part in which maternal caruncle and fetal cotyledons closely oppose each other, allowing exchange of nutrients and metabolites. PGs produced by any part of FM should be transported toward placentome where PGs could enter into maternal circulation. High expression of PGT in maternal caruncle may favor the transport of PGs from the fetal side to the maternal side, and thereby play a role in intrauterine PG catabolism to maintain successful pregnancy. Comparatively low expression of PGT in FM might be associated with transport of PGs in fetal compartments.

In ruminants, uterine PGs are transferred from the uterine to the ovarian compartment through the UOP to bring forth their endocrine actions [7]. We have recently shown that PGT mRNA and PGT protein are more highly expressed in UOP ipsilateral to the CL during the luteolysis-pregnancy recognition window, which coincides with the high production of endometrial PGs and the presence of high levels of PGs in the uterine venous effluent [7]. By contrast, in the present study, PGT mRNA and PGT protein are expressed at only just detectable levels in both UOPs ipsilateral and contralateral to the CL throughout pregnancy. The physiological significance of these observations remains to be elucidated, but it may represent a mechanism for protecting the CL of pregnancy against premature regression.

We and others have reported that lactate facilitates the transport of PGs through PGT. PGT exchanges lactate with PGs as a classical antiporter. This phenomenon has been proved in human and bovine PGT [18, 20, 40]. In ruminants, the uteroplacental system produces and uses lactate [41], and the human endometrium produces high levels of lactate during the implantation window [42]. In humans, deficiency in lactate dehydrogenase, an enzyme responsible for the production of lactate, leads to the lack of uterine smooth muscle contraction and parturition failure [43]. These studies, together with the known effects of PGs in reproductive processes, indirectly support a significant contribution of PGT to the maintenance of pregnancy.

Uteroplacental PGE2 and PGF2{alpha} are considered as important mediators involved in the establishment and maintenance of pregnancy [23–27]. PGE2 is considered as a mitogenic, angiogenic, and antiapoptotic factor in different cell types [13, 14]. In ruminants, PGE2 has long been proposed as a temporary luteostatic factor, luteoprotective factor (or both), and as an immunomodulatory mediator at the fetal/maternal interface. PGE2 regulates its own production in uteroplacental tissues depending on the stage of bovine gestation [44]. PGE2 is involved in placental functions and maintenance of pregnancy in ewes [45, 46]. Our present findings demonstrate that PGT is expressed in a temporal and tissue-specific pattern in CAR, ICAR, and FM during pregnancy in cattle. Based on these findings, and the previous data regarding the expression of PG biosynthetic enzymes and receptors in uteroplacental tissues during bovine pregnancy [32], and available information on PGT in bovine uterus and other tissues of different species [17, 18, 20], we propose that paracrine and autocrine actions of PGs are involved in the regulation of growth, differentiation, and function of uteroplacental tissues in cattle. Given the strategic role of PGs in the female reproductive system, improper transport and metabolism of uteroplacental PGs could result in pregnancy failure. Although PG enzymes and receptors are essential for PG biosynthesis and signaling, the trafficking of PGs across the cell membrane seems to be highly facilitated by PGT. Further studies are required to unravel the molecular control of the PGT gene and PGT protein expression in uteroplacental tissues during pregnancy. To our knowledge, the characterization of PGT in uterine and intrauterine tissues associated with pregnancy has not been reported previously in any species.

In conclusion, the present study describes the spatiotemporal tissue-specific expression of PGT mRNA and PGT protein in CAR, ICAR, FM, and UOP during pregnancy. Taken together, our present findings along with our previous studies [20, 32] suggest that PGT might have roles in 1) the transport of PGs between the maternal and fetal compartments and 2) the intrauterine metabolism of PGs.


   FOOTNOTES
 
1 Supported by grants from the Natural Sciences and Engineering Research Council of Canada to M.A.F. Back

2 Correspondence: Michel A. Fortier, Unité d'Ontogénie et Reproduction, Centre de Recherche du CHUQ, CHUL, 2705 Boul Laurier, Ste-Foy, Qué bec, Canada GIV 4G2. FAX: 418 654 2765; mafortier{at}crchul.ulaval.ca Back

3 Current address: Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843-4458 Back

Received: 13 January 2005.

First decision: 26 January 2005.

Accepted: 17 March 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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