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BOR - Papers in Press, published online ahead of print July 21, 2004.
Biol Reprod 2004, 10.1095/biolreprod.104.031112
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BIOLOGY OF REPRODUCTION 71, 1746–1752 (2004)
DOI: 10.1095/biolreprod.104.031112
© 2004 by the Society for the Study of Reproduction, Inc.

Pregnancy

Two Isoforms of the Leptin Receptor Are Enhanced in Pregnancy-Specific Tissues and Soluble Leptin Receptor Is Enhanced in Maternal Serum with Advancing Gestation in the Baboon1

Deborah E. Edwards3,6, Rudolf P. Bohm, Jr7, Jeanette Purcell7, Marion S. Ratterree7, Kenneth F. Swan3, V. Daniel Castracane8, and Michael C. Henson2,3,4,5,6,7

Departments of Obstetrics and Gynecology,3 Structural and Cellular Biology,4 Physiology5; Interdisciplinary Program in Molecular and Cellular Biology6; and Tulane National Primate Research Center,7 Tulane University Health Sciences Center, New Orleans, Louisiana 70112 Department of Obstetrics and Gynecology8 and the Women's Health Research Center of Amarillo, Texas Tech University Health Sciences Center, Amarillo, Texas 79106


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Leptin is a polypeptide hormone produced by adipose and other endocrine tissues. Although it has been linked to receptor-mediated pathways that directly influence human conceptus development, mechanisms that regulate the leptin receptor in pregnancy-specific tissues remain unclear. Therefore, we assessed leptin-receptor ontogeny and regulation in the baboon (Papio sp.), a primate model for human pregnancy. Placentae, decidua, and amniochorion were collected from baboons in early (Days 54–63, n = 4), mid (Days 98–103, n = 4), and late (Days 159–165, n = 4) gestation. Regulation by estrogen was assessed by elimination of androgen precursors via removal of the fetus (fetectomy) at midgestation and collection of tissues in late gestation (n = 4; term, ~184 days). Maternal serum was sampled with advancing gestation, and the abundance of soluble leptin receptor (solLepR), a potential mediator of gestational hyperleptinemia, was determined. Two placental leptin-receptor isoforms (130 and 150 kDa) increased (P < 0.04 and P < 0.02, respectively) in abundance with advancing gestation. Similarly, the 130-kDa isoform increased approximately fourfold (P < 0.0025) in decidua and approximately 10-fold (P < 0.015) in amniochorion between early and late gestation. Following fetectomy, maternal serum estradiol levels declined approximately 85% (P < 0.03), and the 150-kDa placental leptin-receptor isoform was reduced by more than half (P < 0.002). Maternal serum solLepR concentrations were correlated with gestational age (r = 0.52, P < 0.01) and were unaffected by fetectomy. The presence of leptin-receptor isoforms in pregnancy-specific tissues further denoted leptin's potential to directly influence conceptus development, whereas the 130-kDa solLepR identified in maternal serum suggested a means to facilitate the hyperleptinemia typical of primate pregnancy. Although estrogen did not appear to be the principal regulator of solLepR, it and other factors linked to advancing gestation may be implicated in the regulation of leptin-receptor synthesis.

conceptus, estrogen, leptin, leptin receptor, pregnancy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Leptin is the protein product of the LEP gene and has been implicated in the regulation of developmental processes important in human gestation, including implantation [1], angiogenesis [2], fetal development [3], and fetoplacental endocrinology [4, 5]. Because leptin also regulates metabolism and energy expenditure by inhibiting food intake thereby signaling the status of adipose stores to the brain [6], the high levels of maternal serum leptin typically associated with pregnancy [7] are perplexing, because this is a period generally associated with increased nutritional demands [4, 8]. To this end, the soluble isoform of the leptin receptor (solLepR), which is the main leptin-binding component in human serum [9], may serve to potentiate leptin resistance in pregnancy as well as facilitate the maternal hyperleptinemia that is characteristic of normal gestation [10] by sequestering leptin within the maternal circulation and preventing its clearance [11]. Intriguingly, recent evidence also suggests that transplacental passage of leptin in rats is facilitated by solLepR, which is a mechanism consistent with the hormone's role as a direct modulator of mammalian conceptus development [12].

The leptin receptor is a member of the class I cytokine-receptor superfamily and exists in as many as five isoforms [13, 14]. Alternative splicing of a single transcriptional message produces variations in intracellular domain length and confers both isoform identity and signaling capability [14, 15]. The LepRL is the isoform with the longest intracellular domain and is considered to be the primary signaling isoform, although a category of short isoforms with truncated intracellular domains (LepRS) may also contribute to leptin signal transduction via alternate pathways [16, 17]. Production of the solLepR in humans is proposed to result from proteolytic cleavage of membrane-bound leptin receptors [18]. This isoform lacks signaling capability and is composed of only the extracellular leptin-binding domain [19]. Both leptin-receptor mRNA [20] and protein [21] have been detected in the human placenta, and we have similarly determined the presence of both LepRL and LepRS mRNA transcripts in the nonhuman primate placenta, decidua, and amniochorion [22]. These findings document the presence of leptin receptor in pregnancy-specific tissues, and they suggest that receptor synthesis may be regulated by hormones and/or specific physiological conditions that are inherent (or enhanced) during gestation. Estrogens, which are mainly of placental origin and increase in the maternal peripheral circulation with advancing gestation, have been linked to this process [23–25], and we have previously reported a reduction in placental LepRS transcript abundance when maternal estrogens were inhibited [26].

The baboon (Papio sp.) is an established model of human pregnancy [20, 27, 28], and like the human, it possesses a true maternal fetoplacental unit that is dependent on androgen precursors from the fetal adrenal for optimal placental estrogen biosynthesis. The removal of the fetus (fetectomy) at midgestation eliminates these precursors, and although the placenta remains intact and endocrinologically functional following the procedure, fetectomy induces a decline in placental estrogen production that is evidenced by a dramatic reduction in maternal peripheral estrogen concentrations [20, 26, 29]. Therefore, in the present study, we employed both fetectomized and pregnancy-intact baboons to investigate the previously proposed impact of increasing maternal estrogen concentrations, which are typical of advancing primate pregnancy [4, 5, 26, 30], on leptin-receptor synthesis in pregnancy-specific tissues. Because enhanced solLepR levels may potentiate maternal hyperleptinemia and be directly regulated by the enhanced levels of maternal estrogen that are common to gestation, maternal serum solLepR concentrations in both intact and fetectomized pregnancies were also assessed.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals

Baboons were maintained in strict accordance with U.S. Department of Agriculture regulations and the Guide for the Care and Use of Laboratory Animals (National Institutes of Health Publication 86-23). Protocols were approved by the Institutional Care and Use Committee of the Tulane National Primate Research Center. As we have previously described [22, 26], female baboons (Papio sp.) were housed individually in stainless-steel cages, and constant temperature and 12L:12D photoperiod (lights-on, 0600–1800 h) were maintained. Animals were provided a maintenance ration with fresh fruit daily and water ad libitum. For mating, females were transferred to indoor/outdoor enclosures with males for 4–5 days at the anticipated time of ovulation as determined by daily menstrual cycle records and visible turgescence of external sex skin.

Blood Collection

Blood samples were obtained from pregnant baboons at Days 30 and 60 of gestation as well as from pregnant and fetectomized baboons at intervals of approximately 5 days between Day 80 and Day 160 of gestation (term, ~184 days). At approximately 0800–0900 h, animals were briefly restrained and anesthetized with an i.m. injection of ketamine HCl (10–15 mg/kg body weight; Ketalar; Fort Dodge Pharmaceuticals, Fort Dodge, IA), and a 6-ml blood sample was drawn from an antecubital vein.

Tissue Collection

Decidua, amniochorion, and placental villous tissues were collected on cesarean section performed under isofluorane anesthesia after induction with ketamine HCl/atropine, as we have previously described [22, 31]. Deliveries were conducted at three gestational stages approximating Days 60, 100, and 160 (n = 4 per stage). Additionally, four baboons were fetectomized on approximately Day 100 of gestation in accordance with previous procedures utilized by our laboratory [20, 26]. Thus, following laparotomy and hysterotomy, the fetus was withdrawn and the umbilical cord severed between ligatures placed near the midpoint of the fetal and placental attachment. The placenta remained in situ for 60 days following fetectomy, at which time baboons were again anesthetized and placentae retrieved. All placental villous samples were collected from the maternal surface of the placenta. Decidual tissue was detached from the maternal uterine wall via curettage, and samples of the amniochorionic membrane were excised from intact regions of the amniochorion. All tissue samples were flash-frozen in liquid nitrogen and stored at –70°C until further processing.

Homogenization of Tissues for Immunoblotting

Tissues were weighed and homogenized in a glass-Teflon homogenizer (Glas-Col, Terre Haute, IN) with 2.5 volumes of homogenization buffer (40 mM KH2PO4, 10 mM sucrose, 50 mM KCl, 30 mM EDTA, 2 mM EGTA, 1 mM PMSF, 25 µg/ml of aprotinin, and 25 µg/ml of leupeptin), as previously described [26]. Centrifugation at 2000 x g for 15 min at 4°C removed particulates, and the supernatant was analyzed for total protein content according to the method described by Bradford [32].

Immunoblotting

Serum samples and tissue homogenates were stored at –20°C and –70°C, respectively, until leptin-receptor abundance was determined by immunoblotting. Samples were denatured and reduced by adding lithium dodecyl sulfate and ß-mercaptoethanol, respectively. Fifty micrograms of total protein were separated on 4–12% gradient Bis-Tris gels (Invitrogen) and transferred to nitrocellulose for immunodetection, utilizing an antibody against human leptin receptor. This antibody is specific for epitopes within the first 20 amino acids of the N-terminus for the extracellular domain of the leptin receptor and, thus, detects all leptin-receptor isoforms (N-20; Santa Cruz Biotechnology, Inc., Santa Cruz, CA). Visualization of leptin-receptor immunodetection was achieved utilizing an anti-goat immunoglobulin G antibody labeled with horseradish peroxidase (Santa Cruz) along with the ECL Western Blotting detection system (Amersham Pharmacia Biotech, Piscataway, NJ). The reactive signal was captured on radiographic film (Hyperfilm; Amersham), and band intensity was analyzed with the Alpha Imager 2000 (Alpha Innotech Corp., San Leandro, CA).

Tissue samples were analyzed in such a manner that at least one sample from each group was included on each immunoblot, with leptin-receptor abundance expressed as relative densitometric units (RDUs) of band intensity. However, the large quantity of serum samples analyzed required an alternate method of quantitation. Therefore, one control serum sample was included in duplicate on each immunoblot, and an RDU ratio of sample to control was calculated as a measure of protein abundance. The intra- and interassay coefficients of variation for this method were 5.8% and 13.3%, respectively.

Estradiol Immunoassay

Serum estradiol was quantitated via an automated chemiluminescent assay (Immulite; Diagnostics Products Corporation, Los Angeles, CA), as we have previously described [26]. The interassay coefficient of variation was 11.8%.

Statistical Analyses

Both ANOVA and Student t-tests were performed with SigmaStat statistical analysis software (Version 5.0; SPSS, Inc., Richmond, CA). A statistically significant difference between data groups was defined as P < 0.05. Linear regression models for the abundance of solLepR in serum, as determined by Western blot analysis, were constructed using SigmaPlot 5.0 scientific graphing software (Jandel Scientific Software, San Rafael, CA).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Leptin-Receptor Ontogeny

As depicted in Figure 1A, two isoforms of the leptin receptor (130 and 150 kDa) were detected in placental villous tissue. Figure 1B depicts the 12-fold increase (P < 0.04) in protein abundance that was exhibited by the 130-kDa isoform from early (approximately Day 60) to late (approximately Day 160) gestation. Levels (mean ± SEM) rose from 1.25 ± 1.25 RDU to 15.50 ± 4.91 RDUs during this period. As illustrated in Figure 1C, levels of the 150-kDa isoform increased (P < 0.02) somewhat earlier, between Day 60 (8.50 ± 6.03 RDUs) and Day 100 (29.00 ± 1.08 RDUs) of gestation.



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  		FIG. 1. Relative abundances of leptin-receptor isoforms detected in placental villous tissue at approximately Day 60 (n = 2), Day 100 (n = 2), and Day 160 (n = 2) gestation (A). Relative densitometric units were determined for band intensities from immunoblots, and these values are presented as the mean ± SEM for each isoform. A 130-kDa isoform (n = 4; B) increased in abundance between Day 60 and Day 160 of gestation Note the representative immunoblot with conditions optimized for imaging the isoform at early and midgestation (B inset). Levels of a 150-kDa isoform (n = 4; C) increased between Day 60 and Day 100 of gestation. Different lowercase letters indicate significant differences between means (abP < 0.04 and P < 0.02 for B and C, respectively)

A single leptin-receptor isoform of 130 kDa was detected in both the decidua and the amniochorion by immunoblotting. Densitometric analysis revealed an increase in receptor abundance with advancing gestation in both tissues. Thus, levels in the decidua (Fig. 2A) rose from 9.25 ± 3.54 RDUs at approximately Day 60 of gestation to 35.75 ± 3.64 RDUs at approximately Day 160 of gestation, demonstrating a nearly fourfold increase (P < 0.0025). Leptin-receptor protein was enhanced more than 10-fold (P < 0.015) in the amniochorion (Fig. 2B) between Day 60 (6.00 ± 3.03 RDUs) and Day 160 (66.50 ± 16.86 RDUs) of gestation.



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  		FIG. 2. A 130-kDa leptin-receptor isoform in decidua (A) and amniochorion (B) at approximately Day 60 (n = 4), Day 100 (n = 4), and Day 160 (n = 4) gestation. Relative densitometric units of immunoblot band intensities are presented as the mean ± SEM. Different lowercase letters indicate significant differences (abP < 0.03). Insets depict representative immunoblots of tissues collected at Day 60, Day 100, and Day 160 gestation

As depicted in Figure 3A, evaluation of pregnancy serum revealed a single protein with a molecular weight similar to that leptin receptors detected in our tissue analyses. The protein was not detected in serum from nonpregnant female baboons. Because of its presence in maternal serum, this 130-kDa protein was identified as solLepR. A comprehensive analysis of its abundance in maternal serum from intact pregnancies is presented in Figure 3B, with each symbol representing one sample and illustrating the correlation between maternal solLepR and gestational age (r = 0.52, P < 0.01). Therefore, at Day 30 of gestation, the receptor was undetectable, but by Day 60, solLepR was not only detectable but had begun a progressive increase in abundance that continued throughout gestation.



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  		FIG. 3. Representative immunoblot of solLepR in serum from nonpregnant female baboons (NP, n = 2), and in maternal serum at Day 60 (n = 2), Day 100 (n = 2), and Day 160 (n = 2) gestation (A). Levels of maternal solLepR in intact pregnancies (n = 4) were correlated (r = 0.52, P < 0.01) with gestational age (B). The solLepR was detected by immunoblot, and RDUs were normalized to an interassay control

Impact of Reduced Maternal Estrogen Availability on Leptin-Receptor Production

As illustrated in Figure 4, maternal serum estradiol concentrations from Day 110 to Day 160 of gestation were reduced (P < 0.03) in fetectomized baboons to approximately 15% of the values from intact pregnancies (intact, 3.67 ± 0.42 ng/ml; fetectomy, 0.58 ± 0.04 ng/ml). Placental villous tissue was collected on approximately Day 160 of gestation from both intact and fetectomized pregnancies, and protein abundance for the two placental leptin-receptor isoforms was determined. With respect to the impact of reduced maternal estrogen availability on placental leptin-receptor biosynthesis, levels of the 130-kDa isoform (Fig. 5A) remained relatively (P > 0.05) unaffected following fetectomy, whereas the abundance of the larger, 150-kDa isoform (Fig. 5B) was substantially (P < 0.002) less. Thus, the levels of 150-kDa leptin-receptor protein were 45.80 ± 3.12 RDUs in the intact group and 20.50 ± 2.90 RDUs in the fetectomized group. Leptin-receptor protein abundance was also evaluated in the decidua and amniochorion of fetectomized baboons. However, neither tissue exhibited a reduction in leptin-receptor levels following fetectomy (data not shown).



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  		FIG. 4. Following fetectomy (FetX, n = 4), estradiol concentrations were significantly reduced in maternal serum in comparison to intact pregnancies (n = 4). Values are presented as the mean ± SEM for Days 110–160 of gestation. Different lowercase letters indicate significant differences (abP < 0.03)



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  		FIG. 5. Levels of placental leptin-receptor protein at Day 160 of gestation from intact (n = 4) and fetectomized (FetX, n = 4) pregnancies. Densitometric analysis of immunoblot band intensity demonstrated a reduction in levels of the 150-kDa isoform following FetX (B), although levels of the 130-kDa isoform remained relatively unaffected (A). Values are presented as the mean ± SEM. Different lowercase letters indicate significant differences (abP < 0.002)

Maternal solLepR was also monitored in maternal serum from intact and fetectomized pregnancies. Figure 6 depicts solLepR concentrations between Days 80 and 160 of gestation. Receptor abundance was roughly equivalent in both groups, demonstrating that the enhanced concentrations of solLepR typical of normal intact pregnancies were maintained in fetectomized baboons.



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  		FIG. 6. The enhanced concentrations of maternal peripheral solLepR from intact pregnancies (n = 4) were maintained in fetectomized baboons (FetX, n = 4) following fetectomy on Day 100 of gestation. Values represent the sample RDUs in comparison to a control (RDU:Control) and are presented as the means ± SEM on Days 80–160 of gestation


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The leptin-receptor plays an integral role in leptin function, because the polypeptide must interact with specific receptor isoforms to initiate signaling pathways. Additionally, solLepR may help to maintain maternal serum leptin levels during pregnancy as well as engender gestational leptin resistance and/or facilitate transplacental transport. Although relatively few studies have assessed the factors regulating leptin-receptor synthesis, Lindell et al. [24] identified multiple alternative 5'-untranslated regions (UTRs) in leptin-receptor mRNA transcripts. In an upstream region surrounding one of the most commonly utilized UTRs, they located a putative estrogen-response element and two SP-1 sites. This finding suggested that estrogen may exert direct regulation over leptin-receptor production, at least at the level of transcription. In addition to the potential for estrogen to regulate leptin-receptor synthesis directly, estrogen may also regulate receptor abundance indirectly, via the regulation of leptin biosynthesis [7, 33, 34] and/or ligand-induced receptor down-regulation [35, 36].

Extensive evidence suggests that mechanisms regulating leptin dynamics in pregnancy are both species- and tissue-specific [4, 5, 37]. Therefore, in the present study, we determined the effects of both advancing gestation and fetectomy-induced estrogen deprivation on leptin-receptor abundance in the placenta, decidua, and amniochorion of a well-characterized, nonhuman primate model of human pregnancy, the baboon. Previously, we reported an elevation in maternal leptin concentrations in baboon pregnancy [28] that was analogous to the hyperleptinemia seen in human gestation, and we identified leptin-receptor transcripts in the baboon placental trophoblast [22] that were similar to those in human placental tissue [21, 38]. However, to our knowledge, no account of leptin-receptor protein or isoform identity has been reported in nonhuman primate placenta to date. To this end, we report here the presence of two leptin-receptor isoforms in baboon placental tissue and their relative abundances at specific gestational intervals. The smaller, 130-kDa isoform was observed at very low levels until late in pregnancy, whereas the larger, 150-kDa isoform increased in abundance by midgestation. Our identification of two placental isoforms in the baboon is reminiscent of the report by Schulz et al. [21], who identified two receptor isoforms in term human placenta. Differences in leptin-receptor isoform ontogeny in the baboon, a relationship that to our knowledge is as yet unreported in the human, suggest preferential production of particular receptor isoforms at specific gestational stages. This finding implies similarities with leptin-receptor synthesis in the rat placenta, where Ob-Ra, Ob-Rb, and Ob-Re isoforms are produced in varying abundances with respect to advancing gestational age [39].

The specific mRNA transcripts that we previously identified for LepRS and LepRL in the baboon placenta [22] suggest identities for the two immunoreactive isoforms detected in the present study. However, because we previously reported constitutive placental expression of receptor transcripts throughout pregnancy, our present finding of an increase in placental receptor protein with gestational age suggests that transcript abundance is not correlated with leptin-receptor production in this tissue. Certainly, mRNA transcript abundance is not always an index of protein production, as is the case with leptin in both the baboon [26] and the human [40]. Interestingly, localization of leptin-receptor transcripts to the baboon placental syncytiotrophoblast [22] mimics the location of immunoreactive leptin receptor in human pregnancy [41]. This determination places the receptor in a position of vital physiological influence, because the syncytium represents the placental interface between maternal and fetal circulations. Localization of the leptin receptor to the syncytiotrophoblast strongly suggests a responsiveness of the primate placenta to the effects of fetal, maternal, and/or placental-derived leptin. Additionally, we previously reported leptin-receptor mRNA transcripts in the decidua and amniochorion [22], and we now have identified in these tissues a single leptin-receptor isoform with a molecular weight (130 kDa) identical to that of the smaller isoform detected in placenta. As in the placenta, receptor protein abundance increased with advancing gestation in both decidua and amniochorion.

Pregnancy is a physiological state marked by profound alterations in maternal metabolism that are necessary to accommodate the growth of both the fetus and the placenta. As previously documented in the human [7], we have reported that maternal leptin levels also increase with advancing gestation in the baboon [28]. In this nonhuman primate model, leptin production by maternal adipose tissue appears to be up-regulated by estrogen with advancing gestation [26]. In human pregnancy, increases in maternal peripheral leptin concentrations have been attributed, at least in part, to an increase in the percentage of serum-bound leptin [10, 42, 43], although one recent report suggests a lack of unanimity on this issue [44]. With respect to this disagreement, the correlation observed between gestational age and solLepR abundance in the present study adds additional credence to the proposed enhancement in the leptin-binding capacity [19] of maternal serum that is suggested to occur with advancing gestation. This hypothesis is also supported by the identification of solLepR as the main leptin-binding component in the human periphery [9]. Therefore, because solLepR production [10, 21] has been proposed to result from proteolytic cleavage of membrane-bound isoforms [18], the increase that we have observed in leptin-receptor abundance in pregnancy-specific tissues may provide a comparative index of solLepR bioavailability. However, a definitive understanding of any enhanced physiological role for a leptin-solLepR complex in primate pregnancy, such as an effect on transplacental passage (as proposed in the rodent [12]) or a general diminution in the bioavailability of that leptin sequestered in the maternal serum [9–11], must await further study.

The enhanced maternal estrogen concentrations of intact primate pregnancy provided an opportunity to assess the impact of elevated estrogen at specific time points during normal gestation, but fetectomy provided a novel opportunity to compare receptor protein abundance following a period of reduced maternal estrogen. Thus, the present findings are in accordance with our previous results [20, 26] that demonstrated a decline in maternal estradiol concentrations to approximately 15% of those in pregnancy-intact baboons. Although maternal estrogen concentrations were dramatically reduced, the reduction in the 150-kDa placental receptor marked the only significant effect of fetectomy with regard to leptin-receptor abundance, whereas levels of the 130-kDa leptin receptor in the placenta, decidua, and amniochorion remained essentially unaltered. Likewise, maternal serum concentrations of solLepR were comparable between pregnancy-intact and fetectomized animals, suggesting no effect of estrogen on the synthesis of this isoform. However, it must be acknowledged that the identification of leptin-receptor protein based on molecular weight precluded us from determining a specific isoform identity. Therefore, if the effects of estrogen are truly isoform specific, then our present findings do not allow us to definitively assign isoform identities to estrogen regulation. It is worth noting, however, that such focused effects may have been recently observed with respect to a decline in Ob-Rb isoform abundance following ovariectomy in the rat, which was reversed by subsequent administration of estradiol [45].

Additionally, it should be recognized that although a putative estrogen-response element has been localized to an upstream region surrounding one of the most commonly utilized UTRs for leptin-receptor transcription [24], the tissue analyzed in that study was not specific to pregnancy (e.g., placenta, amniochorion). Therefore, it may not represent a common UTR for production of the leptin receptor in pregnancy-specific tissues. In fact, the presence of multiple UTRs often indicates tissue-specific or cell type-specific regulation and confers the ability to be transcriptionally activated by multiple promoters [46]. Thus, as primate pregnancy represents a unique physiological state, processes regulating leptin-receptor synthesis in tissues specific to gestation may differ vastly from those of synthesis in other tissues, as previously proposed in murine pregnancy [47]. In a similar fashion, variations in serum solLepR concentrations that are engendered by differences in physiological conditions may significantly alter the effects of leptin in target tissues [48]. To this end, the gestational increase that we report in maternal solLepR abundance may indicate a preferential targeting of leptin bioactivity to pregnancy-specific tissues. Finally, if estrogen, indeed, does not function as the principal regulator of solLepR biosynthesis, other placental products might well be postulated to function in this capacity. In this regard, growth hormone and insulin-like growth factor (IGF)-I have been reported to enhance solLepR concentrations in individuals with growth hormone deficiency [49]. Therefore, it could be postulated that growth hormone/IGF-I might drive the production of solLepR and/or, in the absence of any secondary influence of estrogen (as in our fetectomized baboons), maintain synthesis.

In summary, leptin-receptor abundance increased in the present study with advancing baboon pregnancy in both maternal serum and pregnancy-specific tissues. However, the fetectomy-induced reduction in maternal estrogen availability did not universally alter the abundance of leptin-receptor isoforms in pregnancy-specific tissues or the level of maternal serum solLepR. Perhaps universal regulation of leptin-receptor isoforms should not be expected, however, in light of the diverse signaling pathways utilized by individual receptor isoforms and the variety of functions implied for leptin action. Collective results suggest that mechanisms regulating the synthesis of leptin-receptor isoforms in primate pregnancy are, instead, of an isoform-specific nature and may be regulated in a manner that is dependent on the interplay of multiple factors specific to pregnancy.


   ACKNOWLEDGMENTS
 
Appreciation is expressed to Dr. April G. O'Quinn, past Chair; Dr. Gabriella Pridjian, current Chair; and the Department of Obstetrics and Gynecology and to Mrs. Nathlynn Dellande for her assistance in manuscript and figure preparation.


   FOOTNOTES
 
1 Supported by NIH P51 RR00164 awarded to the Tulane National Primate Research Center. Back

2 Correspondence: Michael C. Henson, Department of Obstetrics and Gynecology, Tulane University Health Sciences Center, 1430 Tulane Avenue, New Orleans, LA 70112-2699. FAX: 504 988 1846; henson{at}tulane.edu Back

Received: 20 April 2004.

First decision: 18 May 2004.

Accepted: 15 July 2004.


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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