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APPETITE, OBESITY AND METABOLISM
Department of Biology, Boston University, Boston, Massachusetts 02215
Submitted 23 December 2002 ; accepted in final form 24 March 2003
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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bats; adipose tissue; energy homeostasis; reproduction; placenta
In contrast to nonpregnant animals, elevated plasma leptin levels during pregnancy are not always correlated with adiposity (13, 16, 50). This finding led us to propose that leptin secretion from adipose tissue may be upregulated during pregnancy. We demonstrated that upregulation of leptin secretion from adipose tissue occurs in pregnant mice (28) and, to a much lesser extent, in little brown bats, but not in big brown bats (29). The lack of pronounced upregulation of leptin secretion from adipose tissue in vitro in pregnant bats led us to test the hypothesis that the placenta was a source of leptin secretion in bats. The rationale for this hypothesis was based on recent demonstrations that leptin mRNA is present in placentas of humans and baboons (20) and leptin is secreted from cultured human choriocarcinoma cells (37). We demonstrated that leptin was constitutively secreted by placentas of Myotis lucifugus, the little brown bat, but not by mouse placentas incubated in vitro (28, 29). Whether placental leptin secretion is regulated and whether the placenta of M. lucifugus synthesizes leptin transcript, however, remain unknown. In addition, whether the placenta is a site of leptin expression in rodents remains unresolved (23, 27, 28).
In the present study, leptin expression during pregnancy was examined in
several species representing the two most abundant mammalian orders, Rodentia
and Chiroptera, and in human choriocarcinoma cells. Rodents and bats comprise
50% and 20% of all known mammalian species, respectively
(51), and are not closely
related taxonomically (44).
The little brown bat M. lucifugus is one of the most abundant and
widely distributed insectivorous bats in North America
(25). In New England, this
species is active from April through September and hibernates for the rest of
the year. Mating occurs in autumn, and sperm is stored in the female's
reproductive tract during hibernation
(5). Ovulation and
fertilization occur on arousal from hibernation in spring
(25), and gestation lasts
60 days, with one offspring produced each year
(34). During pregnancy,
greatly increased leptin levels are observed in maternal blood
(29,
32,
50). Parturition begins around
mid-June and is highly synchronized within a given colony. Litter size
accounts for 25% of adult body mass
(31), and pups reach 84% of
adult size by the end of lactation
(30). Thus, during pregnancy
and lactation, little brown bats face extraordinary energy demands, and leptin
would be expected to be tightly controlled during these periods.
We have used Mus musculus and M. lucifugus as model species to 1) determine whether the rodent and bat placentas are capable of synthesizing leptin mRNA transcript, 2) determine whether placental leptin expression and secretion are under regulatory control, and 3) establish the gestational profile and tissue distribution of leptin mRNA in M. lucifugus. In part on the basis of our earlier observations (28, 29), we hypothesized that the placenta of M. lucifugus, but not M. musculus, would express leptin transcript, that leptin transcript in the placenta would be expressed throughout gestation, and that leptin expression would be primarily restricted to adipose and placental tissue.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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10–12 h after capture, bats were weighed and then killed by
decapitation. Embryos were removed and weighed to estimate the stage of
gestation on the basis of a regression equation of embryo mass (EM) vs.
gestational stage (GS) in M. lucifugus as follows: GS = -9.6789
x EM2 + 44.381 x EM - 53.358. This equation was derived
by regressing mean embryo mass against date of collection. In some cases,
gestational stages were arbitrarily divided into three trimesters to
facilitate statistical comparisons across gestation: early pregnancy (from
day 60 to day 40 from parturition), middle pregnancy (from
day 40 to day 20 from parturition), and late pregnancy (from
day 20 to day 0 from parturition). Immediately after death,
placenta, liver, kidney, heart, brain, spleen, lung, and subcutaneous fat were
dissected and frozen in liquid nitrogen for mRNA analyses. Timed-pregnant Swiss-Webster mice and Sprague-Dawley rats (Harlan) were individually housed in the Boston University Laboratory Animal Care Facility (lights on between 0700 and 1900) with water and commercial laboratory chow available ad libitum. Animals were killed between 0800 and 1000 by exposure to CO2. Adipose tissue, liver, and placenta were immediately dissected and frozen in liquid nitrogen for RNA analyses. All procedures used in this study were approved by the Boston University Institutional Animal Care and Use Committee.
In vitro incubation. Placentas from bats at 23–48 days
before parturition (as determined by the equation in Collection of animals
and tissues) were weighed and dissected in half, and each half was
separately minced with a razor into 1-mm3 cubes. Placental
fragments were washed with several changes of 1,000 volume equivalents of
cold, serum-free Krebs buffer containing 4% (wt/vol) BSA and 0.1% (wt/vol)
glucose to remove residual extracellular fluid and blood cells. Tissue
fragments (1/2 placenta per tube) were incubated for 4 h in glass test tubes
with 1 ml of serum-free Krebs buffer as described above, supplemented with the
protease inhibitors leupeptin and aprotinin (25 µg/ml each) and 5 mM
dibutyryl cAMP, 1–2 µM corticosterone [a naturally occurring steroid
in M. lucifugus
(48)], 10 nM–10 µM
dexamethasone, or vehicle (dibutyric acid or ethanol). Each placental half
served as its own control in a given experiment. Incubations were at 37°C
in a humidified O2-CO2 chamber with gentle shaking.
Culture media were collected every hour and replaced with 1 ml of fresh media.
Collected media were centrifuged, and the supernates were dehydrated in a
Centrivap concentrator (LabConco) and frozen for future hormone assays. Blank
samples (no tissue) were processed similarly. Placental fragments were
collected at the conclusion of an experiment and processed for RT-PCR analysis
of leptin transcript (see below).
Leptin concentrations in culture media were determined by RIA using a human leptin kit (Linco Research) that we previously validated for M. lucifugus plasma leptin (29, 32, 50). Dehydrated media were first reconstituted to 10% original volume, which was empirically determined to result in leptin measurements that fell on the linear portion of the human leptin standard curve. Immunoreactive leptin was at or below the limit of detection in blank samples of reconstituted culture medium.
Human choriocarcinoma (BeWo) cells were obtained from American Type Culture Collection and grown in RPMI 1640 culture medium (GIBCO) supplemented with 10% heat-inactivated fetal calf serum, 100 U/ml penicillin, 100 µg/ml streptomycin, and 2 mM glutamine. Cells were cultured in the presence of vehicle (butyric acid) or 1 mM dibutyryl cAMP for 6 days. Media were collected and replaced daily. A portion of the collected media from each culture dish was dried in a Centrivap concentrator and reconstituted to 10% of original volume for RIA of leptin using the human leptin RIA kit (Linco Research). Cells were harvested at the end of the 6th day in culture, and total RNA was extracted with TRIzol reagent. RT-PCR analysis of leptin transcripts was performed as described below.
Northern blot analyses. Poly(A) RNA was extracted using the
Oligotex Direct mRNA kit (Qiagen); 10 µg of poly(A) RNA were run for 3 h on
formaldehyde-1% agarose gels at 40 mV and then transferred to nylon membranes
using a vacuum blotter (model 785, Bio-Rad) at 5 mmHg for 4 h. After transfer,
membranes were baked at 80°C for 30 min to fix the mRNA. Membranes were
prehybridized at 45°C with Ambion ULTRAhyb solution for 1 h and hybridized
overnight at 45°C with 32P-labeled probe
(106–107 cpm/ml). Blots were washed twice with
2x saline-sodium citrate (300 mM sodium chloride and 30 mM sodium
citrate) plus 0.1% (wt/vol) SDS at 45°C for 5 min and twice with
0.1x saline-sodium citrate plus 0.1% SDS at 45°C for 15 min.
Hybridized membranes were exposed overnight to X-ray film with intensifying
screen at -80°C. The oligonucleotide probe was 38 nt long
(5'-GACTGCGTGTGTGAAATGTCATTGATCCTGGTGACAAT-3') and complementary
in sequence to a highly conserved region among different mammalian leptin
cDNAs. The 5' end was radiolabeled with [
-32P]ATP
using the KinaseMax 5'-end-labeling kit (Ambion).
RT-PCR and sequencing. The following custom-made primers were used
for RT-PCR and sequencing. Forward primer 1
(5'-GRTTCYTGTGGCTTTGGYCCTATC-3') was a degenerate primer
corresponding to a highly conserved region of human, mouse, rat, dog, and cat
leptin cDNA and extended into the 5'-flanking region. Forward primer
2 (5'-ARRGTCCARGATGACACCAAAACCC-3') was a degenerate primer;
its origin was within the coding region of leptin cDNA. Both forward primers
were used in separate reactions with the same reverse direction primer
(5'-GCCACCACCTCBGTGGAGTAG-3'). Fragments of 415 and 362 bp were
expected to be generated with forward primers 1 and 2,
respectively. No sequence similarity in the primers was found in the database
other than leptin sequences from different species. The primers spanned an
intron of 1.7 kb in the mouse leptin gene
(19).
Total RNA from placenta was extracted using TRIzol reagent. Two micrograms of total RNA were reverse transcribed using the leptin-specific reverse primer (see above) and a reverse transcription kit (Promega) according to the manufacturer's protocol. PCRs were performed in a thermal cycler (model PTC-100, MJ Research) with the following conditions: 94°C for 1 min (step 1), 94°C for 40 s, 55°C for 40 s, and 72°C for 2 min for a total of 38 cycles (step 2), and 72°C for 2 min (step 3). These conditions were determined to be optimal for assessing the presence or absence of leptin transcript, which is not abundantly expressed in most tissues. PCR products were electrophoresed on 1.5% low-melt agarose gels in the presence of ethidium bromide and visualized under ultraviolet light. Products corresponding to the expected leptin cDNA fragment were excised from the gel and subcloned into a TA cloning vector (Invitrogen). Sequencing reactions were performed using BigDye Terminator Cycler Sequencing Ready Reaction Kit (Perkin-Elmer) and an automated DNA sequencer (model 377, Applied Biosystems).
For all other PCRs, forward primer 2 and the reverse primer were used for amplifying leptin transcripts in bat and mouse tissue extracts. In some experiments, control tubes were included in which PCR was performed without RT. No PCR products were observed under these conditions (not shown), suggesting that RNA samples were not significantly contaminated with genomic DNA. Lack of contamination was further suggested by the size of the PCR product relative to the expected size that would include the intervening intron.
Conditions for semiquantitative PCR using 
-actin transcript were the
same as those described above except for the number of amplification cycles.
It was determined that 25 amplification cycles resulted in actin transcript
signal within the exponential increase and before the plateau in reaction
product and that 33 cycles resulted in exponential increase of leptin reaction
product (not shown). The primer pair for -actin was purchased from
Promega; a fragment of 285 bp was expected. PCR products were electrophoresed
on 1% agarose gels in the presence of ethidium bromide and visualized with a
digital imaging system from Molecular Analysis (Bio-Rad). RNA transcript
expression was semiquantitatively assessed using the ratio of leptin to
-actin. Product intensities were analyzed using the NIH ImageJ
software.
Miscellaneous. Cortisol in plasma of M. lucifugus was determined using a commercial RIA (ICN), which we previously validated for bat plasma (48, 49). Immunoblots for detection of glucocorticoid receptor were performed using rabbit anti-human glucocorticoid receptor antiserum (Santa Cruz Biotechnologies) at a dilution of 1:100. Several hypothalami were pooled to obtain sufficient starting material. Detection of signal was performed with goat anti-rabbit IgG (1:4,000) conjugated to horseradish peroxidase (Sigma) and Western Lightening Chemiluminescence Reagent Plus (Perkin-Elmer).
ANOVAs or t-tests were used to test for significant differences among treatment groups using PRISM software (GraphPad); significant differences were considered to exist at P < 0.05.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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2.3 kb in bat adipose tissue (data not shown).
Placental mRNA from early-, middle-, and late-gestation M.
lucifugus was hybridized with the probe described above. The blots
revealed a single placental transcript of 2.3 kb
(Fig. 1A) that was
identical in size to the transcript in M. lucifugus adipose tissue.
After the data were normalized to -actin expression, there was no
significant change in leptin mRNA expression per unit mass of placenta from
early to late gestation (Fig.
1B). Thus leptin transcript was expressed in M.
lucifugus placenta in high amounts throughout gestation.
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Stimulation of leptin secretion from M. lucifugus placenta. Once it was established that leptin transcript was expressed in bat placenta, we examined control of leptin secretion from placental fragments in vitro. Preliminary experiments confirmed our published report (29) that leptin is secreted from placentas of M. lucifugus and also demonstrated that the secretion was increased by cAMP (not shown). In addition, cAMP significantly stimulated secretion of leptin from cultured human choriocarcinoma cells (Fig. 2A) and also significantly increased leptin mRNA in the cells (Fig. 2B).
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Glucocorticoids potently stimulate leptin synthesis and secretion from
adipose tissue cells (28,
39) and are important
metabolic hormones during pregnancy. The hypothesis that glucocorticoids may
stimulate placental leptin production was tested using placental fragments
from M. lucifugus incubated with natural (corticosterone) or
synthetic (dexamethasone) steroids for up to 8 h. Because the gestational
profile of plasma glucocorticoids in any chiropteran species was unknown,
however, we first determined cortisol (the major glucocorticoid in M.
lucifugus) levels in plasma of pregnant bats. Plasma cortisol was
50–350 ng/ml (0.14–1 µM) in pregnant bats over the
course of gestation (n = 41, data not shown) compared with
15–170 ng/ml in postlactational M. lucifugus
(48). Using a range of
concentrations (10 nM–10 µM) spanning and exceeding those measured in
pregnant M. lucifugus, however, we could not detect any effect of
natural or synthetic glucocorticoids on leptin secretion or mRNA production in
placental fragments from M. lucifugus (not shown). Failure to observe
effects of glucocorticoids was not due to the absence of placental
glucocorticoid receptors, because immunoblots revealed the presence of
receptors in M. lucifugus placenta
(Fig. 3).
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Leptin expression profile in M. lucifugus. Tissue distribution of leptin mRNA was examined in M. lucifugus using RT-PCR analysis of total RNA from adipose tissue, placenta, heart, liver, kidney, spleen, brain, and lung from late-gestation females. Leptin transcript was detected in adipose and placental tissue, with barely detectable levels in heart, liver, and spleen and below-detectable levels in lung, kidney, and brain (Fig. 4, A and B). These results were confirmed in a replicate analysis (not shown) with tissue from different animals, except the amount of expression (relative to that in adipose and placental tissue) was greater in heart and spleen than shown in Fig. 4A.
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Because leptin mRNA is expressed in avian liver (46), but not in any known adult mammal, further studies were performed to verify the presence of leptin mRNA in M. lucifugus liver as seen in Fig. 4A. RT-PCR analysis of hepatic tissue from several individual little brown bats and a related species, E. fuscus, revealed expression of a single PCR product that corresponded to the predicted size of the partial leptin transcript (Fig. 4, C and D). No expression of leptin transcript was present in mouse or rat liver (Fig. 4, E and F). There was, however, expression of leptin transcript in rat placenta, but no detectable expression in mouse placenta (Fig. 4E). Northern blot analysis of hepatic mRNA from M. lucifugus detected a single transcript in liver that corresponded in size to that expressed in placenta (Fig. 5). The amount of leptin transcript (assessed by Northern blotting) in the liver of this individual animal collected in 2000 was greater than that identified in replicate animals collected in 2002 (assessed by RT-PCR; Fig. 4); this may have been due to individual or seasonal variability. No leptin expression was detected by Northern blotting in mouse liver or placenta (Fig. 5), confirming the RT-PCR results.
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Light-microscopic examination of cryostat sections of M. lucifugus liver revealed a lobular appearance, with cords of hepatocytes radiating out from a central vein typical of mammalian livers. No evidence for the presence of adipose-storing cells, which might have accounted for the presence of hepatic leptin, was observed when 16-µm sections were stained with Sudan black (not shown).
Because RT-PCR analysis was performed with degenerate primers based on the
mouse leptin cDNA sequence, the results were verified by cloning and
sequencing leptin cDNA from M. lucifugus placenta and using the
sequence to generate a bat-specific oligonucleotide probe. A composite M.
lucifugus leptin cDNA sequence was generated by directly sequencing
several RT-PCR products and several different sub-clones. This sequence
(GenBank accession no. AY055474
[GenBank]
) represented 80% of the predicted coding
region and was 87% homologous to human and 84% homologous at the nucleotide
level to rat and mouse leptin cDNAs. On the basis of this sequence, a
homologous oligonucleotide probe corresponding to the probe used in Figs.
1 and
5 (differing by 2 nt) was
generated for use in Northern blots. As with the degenerate probes, a single
transcript of 2.3 kb was observed on Northern blots of adipose and placental
tissue (data not shown).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Here, we demonstrate that placental expression of leptin transcript is
species specific, even within the same order of mammals. We chose to examine
rodents and bats, because members of these orders collectively comprise
70% of all mammalian species
(51) and are not closely
related taxonomically (44).
Thus patterns of leptin expression or regulation that are similar among
rodents and bats may reveal ubiquitous regulatory mechanisms. Human
choriocarcinoma cells and placentas of little brown bats, big brown bats, and
rats, but not mice, expressed leptin transcript. Absence of expression in the
mouse placenta [and our earlier report that mouse placenta does not secrete
leptin in vitro (28)] supports
previous observations (16,
47) but is in conflict with
observations reported by Hoggard et al.
(23), which may possibly be
explained by the use of different strains of mice between studies
(23,
28). On the basis of these
results and those reported for baboons and humans
(20), we conclude that leptin
expression appears to be a common feature of placental physiology in mammals,
but it is nonetheless not a ubiquitous feature. Thus it appears that
generalities about placental leptin expression must be tempered at this time,
because all related species within a single family may not express and secrete
leptin from the placenta. It is clear that there are at least three mechanisms
in mammals by which maternal leptin levels are increased. First, as in mice,
adipose tissue leptin expression and secretion are greatly upregulated.
Second, as in little brown bats and primates, placental leptin expression is
increased and the placenta secretes leptin (production of adipose leptin may
still be increased in these pregnant animals). Third, as in mice and humans,
the placenta secretes a soluble form of the leptin receptor into the
circulation, which may act to prolong the half-life of leptin in plasma,
contributing to higher circulating levels
(16,
35,
52).
The placentas of certain species, such as humans (20), baboons (20), and little brown bats (unpublished observations), express the active form of the leptin receptor, OB-Rb. Together with the ability of the placenta to produce and secrete leptin, this suggests that the placenta may be a target organ for leptin and that placental leptin may have autocrine or paracrine activities. Indeed, leptin has been suggested to regulate placental growth and angiogenesis (3), function as a local immunomodulator (22), and possibly play a role in implantation by modulating expression of extracellular matrix-degrading enzymes (3).
The size of the leptin mRNA transcript was smaller in M. lucifugus than in the mouse [and other species (27, 37)]. The reason for this difference is unknown. However, leptin mRNA contains a long 3'-untranslated region (53), which may be important for regulating message half-life, as has been reported for other transcripts with such regions [e.g., estradiol receptor transcript (42)]. It is not possible to say at this time, however, whether the shortened 3'-untranslated region of the bat leptin mRNA plays a role in stability of the transcript.
We previously reported that leptin accumulates in media of placental
fragments from M. lucifugus incubated in vitro in the absence of any
stimulus (29). We report here
that the secretion of leptin from M. lucifugus placenta may be
regulated, because it was stimulated by cAMP in preliminary experiments. This
finding is consistent with a previous report in human placentas
(9). Cultured human
choriocarcinoma cells also secreted leptin under the influence of cAMP in the
present study. One difference between the bat and the human tumor cells,
however, was that cAMP stimulated expression of leptin transcript in the tumor
cells, but not in the placental fragments. Although this may be a true
species-specific function of cAMP, it is also possible that the effect of cAMP
on the bat placenta requires a time interval longer than was feasible with the
use of acutely minced placental fragments. Nonetheless, it appears that cAMP
exerts tissue-specific actions on leptin, because cAMP is believed to mediate
the inhibitory effects of 3-adrenergic receptor activation on
adipose leptin secretion (17)
yet stimulates placental leptin secretion. Placental-specific enhancer regions
and stimulatory transcription factors have, in fact, been identified in human
placenta (4).
Glucocorticoids potently stimulate leptin production and secretion from adipose cells (28, 39), and mammalian placentas express glucocorticoid receptors in trophoblast cells (12). Thus we tested the hypothesis that glucocorticoids would stimulate placental leptin expression and secretion but were unable to demonstrate any effect of natural or synthetic glucocorticoids on M. lucifugus placentas. This was true, despite the presence of immunoreactive glucocorticoid receptor and the use of steroid concentrations consistent with and higher than those found in plasma of pregnant M. lucifugus. Thus the results obtained in M. lucifugus are in contrast with those of Sudgen et al. (45), who demonstrated that glucocorticoids stimulated leptin secretion in vivo from rat placenta, and Coya et al. (9), who found that glucocorticoids upregulated expression and secretion of leptin in human trophoblast cells.
Expression of leptin synthesis in bat placenta did not change throughout
gestation when normalized to -actin expression. This agrees with our
previously reported results which demonstrated that the rate at which leptin
accumulated in the media of minced placental fragments was similar throughout
gestation when normalized to leptin secreted per gram of placenta
(29). Thus leptin appears to
be constitutively expressed in the placenta throughout the course of gestation
in M. lucifugus. This suggests that as gestation advances, increased
placental leptin secretion is due to the increase in placental mass, rather
than upregulation of leptin per gram of tissue (or per cell). However, other
studies have demonstrated that placental leptin expression increases as
gestation advances in humans and rats
(10,
15) and decreases from early
to late gestation in baboons
(21), indicating that not only
the expression of placental leptin, but also its temporal pattern of
expression during pregnancy, is species specific.
In addition to adipose and placental tissue, leptin is also expressed in pituitary (26), stomach (8), ovary (43), skeletal muscle (8), mammary gland (8), brain (8), endometrium (18), avian liver (46), and several fetal tissues, such as liver, heart, lung, and hair follicle (22). The present study revealed that leptin transcript was expressed in variable amounts in several tissues in bats, with the strongest and most consistent expression in adipose and placental tissue. Although we did not have sufficient material to further characterize leptin expression in most tissues, we report here that the liver of adult little brown bats and big brown bats, but not rats or mice, expresses leptin transcript. The expression level on Northern blots was greater than that observed using RT-PCR. Although we cannot explain this difference, it may reflect the fact that tissue samples for each analysis were obtained from different sampling seasons, and individual animals from one season may have been exposed to different environmental and metabolic challenges at the time of capture, resulting in variable degrees of hepatic leptin expression. Notwithstanding this individual variability, this is the first report of a healthy adult mammal expressing leptin in the liver. Fetal mice (24) and fetal sheep (6, 13), but not their adult counterparts, have been shown to express hepatic leptin. In both cases, this occurs at a developmental time when adipose stores are minimal or undetectable (1). Similarly, hepatic leptin in chickens is most closely correlated with metabolism during the posthatchling period (2), when subcutaneous adipose tissue is minimal. During pregnancy, M. lucifugus and E. fuscus contain very little detectable adipose stores, which may reflect their very high metabolic rate and energy use at that time. Thus it is possible that hepatic expression of leptin is a widespread phenomenon in vertebrates that compensates for reduced or absent subcutaneous fat stores. It would be interesting to determine whether adult animals that do not normally express leptin in liver do so during periods of prolonged starvation and depletion of body fat. Likewise, during the prehibernatory season, M. lucifugus gains considerable amounts of adipose tissue, and it would be interesting to determine whether hepatic leptin expression decreases at that time.
Leptin has been cloned or partially sequenced in several species, including human, rat and mouse (53), and big brown bats (11). The homology of the nucleotide sequence of M. lucifugus leptin cDNA to reported sequences for other eutherian mammals was 79–95%. Leptin was strongly expressed in adipose and placental tissue of the little brown bat and moderately in liver, heart, and spleen. It is not known whether leptin expression in placenta, liver, heart, and spleen was due to the presence of a small amount of adipocytes in these organs. However, placental leptin expression has been clearly documented within syncytiocytotrophoblast in human and baboon placenta (20), and we could detect no lipid-staining cells in the liver of M. lucifugus. In chickens, liver expression of leptin occurs in hepatocytes (41), but this is unknown for fetal mammals or for adult M. lucifugus.
In summary, we demonstrate that expression of leptin and its secretion from the placenta are species specific, under the acute regulation of cAMP but not glucocorticoids, and that livers of little brown bats and a related species, big brown bats, express leptin mRNA. The latter may represent a common feature of animals, in which stores of adipose tissue are restricted because of the life stage or natural history of the species. Although increased maternal leptin appears to be ubiquitous in pregnant mammals, generalities about the mechanisms that result in increased plasma leptin during pregnancy are not possible.
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DISCLOSURES |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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REFERENCES |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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-hydroxysteroid dehydrogenase isozymes and corticosteroid hormone
receptors in primary cultures of human trophoblast and placental bed biopsies.
Mol Hum Reprod 7:
357–363, 2001.3-adrenergic receptor
inhibits insulin-stimulated leptin secretion from isolated rat adipocytes.
Endocrinology 137:
4054–4057, 1996.[Abstract]
.
J Biol Chem 270:
28887–28891, 1995.This article has been cited by other articles:
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  L. C. Schulz and E. P. Widmaier The Effect of Leptin on Mouse Trophoblast Cell Invasion Biol Reprod, December 1, 2004; 71(6): 1963 - 1967. [Abstract] [Full Text] [PDF]
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D. E. Edwards, R. P. Bohm Jr, J. Purcell, M. S. Ratterree, K. F. Swan, V. D. Castracane, and M. C. Henson 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 Baboon Biol Reprod, November 1, 2004; 71(5): 1746 - 1752. [Abstract] [Full Text] [PDF]
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