Leptin in pregnancy: an update

Leptin influences satiety, adiposity, and metabolism and is associated with mechanisms regulating puberty onset, fertility, and pregnancy in various species. Maternal hyperleptinemia is a hallmark of mammalian pregnancy, although both the roles of leptin and the mechanisms regulating its synthesis appear to be taxa specific. In pregnant humans and nonhuman primates, leptin is produced by both maternal and fetal adipose tissues, as well as by the placental trophoblast. Specific receptors in the uterine endometrium, trophoblast, and fetus facilitate direct effects of the polypeptide on implantation, placental endocrine function, and conceptus development. A soluble isoform of the receptor may be responsible for inducing maternal leptin resistance during pregnancy and/or may facilitate the transplacental passage of leptin for the purpose of directly regulating fetal development. The steroid hormones are linked to the regulation of leptin and the leptin receptor and probably interact with other pregnancy-specific, serum-borne factors to regulate leptin dynamics during pregnancy. In addition to its effects on normal conceptus development, leptin is linked to mechanisms affecting a diverse array of pregnancy-specific pathologies that include preeclampsia, gestational diabetes, and intrauterine growth restriction. Association with these anomalies and with mechanisms pointing to a fetal origin for a range of conditions affecting the individual's health in adult life, such as obesity, diabetes mellitus, and cardiovascular disease, reiterate the need for continued research dedicated to elucidating leptin's roles and regulation throughout gestation.     

Serum leptin in nonpregnant and pregnant women and in old and new world nonhuman primates

Leptin is a hormone that is produced during mammalian pregnancy in the placental trophoblast and other tissues, including! fetal and maternal adipocytes. Synthesis of the polypeptide and the presence of its specific receptors throughout the human maternal fetoplacental unit suggest direct effects on conceptus growth and development. However, both the physiologic roles of leptin and the mechanisms regulating leptin synthesis in human pregnancy differ from those in laboratory and domestic species, necessitating the development of non-human primate research models. Therefore, we compared serum leptin concentrations in nonpregnant and pregnant women with those in both old world nonhuman primates (i.e., baboon, rhesus monkey, cynomolgus monkey) and new world nonhuman primates (i.e., squirrel monkey, titi monkey). As expected, maternal leptin levels were elevated in human and baboon pregnancies (P < 0.05 and P < 0.001, respectively). Levels in both species of old world monkeys were also greatly enhanced (P < 0.001). Although maternal serum concentrations were slightly elevated compared to nonpregnant levels in both species of new world monkeys, overall concentrations were dramatically lower than for either old world primates or humans. Results provide comparisons of serum leptin concentrations in pregnant and nonpregnant humans and baboons with those in both old and new world monkeys and further characterize these nonhuman primates as models for the investigation of leptin dynamics in pregnancy.     

Placental growth hormone and lactogen production by perifused ovine placental explants: regulation by growth hormone-releasing hormone and glucose

The factors controlling normal placental development are poorly understood. We have previously reported the presence of ovine placental growth hormone (oPGH) and growth hormone receptors in ovine placenta, and oPGH production by the trophectoderm and syncitium during the second month of pregnancy. To identify factors regulating oPGH production, we developed a perifusion system to measure oPGH and ovine placental lactogen (oPL) production by Day 45 ovine placental explants. The mRNAs for both hormones were quantitated by real-time polymerase chain reaction in explants collected after perifusion periods of up to 8 h. Ovine PGH and oPL were released into the medium at mean rates of 2.45 +/- 0.2 and 353.6 +/- 13.6 ng/g/h, respectively. Ovine placenta produces growth hormone-releasing hormone (GHRH), but addition of GHRH to the perifusion medium did not modify either oPGH or oPL production. In vivo, oPGH production occurs between Days 30 and 60 of pregnancy. Because modulation of the maternal diet during this period affects placental development, the potential regulation of oPGH and oPL production by glucose was evaluated. Glucose supplementation of the perifusion medium resulted in a concentration-dependent decrease in oPGH release after 4 h, but oPGH mRNA levels were not affected. Production of oPL was not affected by glucose. Thus, oPGH and oPL belong to the same growth hormone/prolactin family but are differentially regulated by glucose. Ovine PGH modulations should be taken into account in metabolic experiments performed during the first trimester of pregnancy in sheep.     

Late but not early gestational maternal growth hormone treatment increases fetal adiposity in overnourished adolescent sheep

In the overnourished adolescent sheep, maternal tissue synthesis is promoted at the expense of placental growth and leads to a major decrease in lamb birth weight at term. Maternal growth hormone (GH) concentrations are attenuated in these pregnancies, and it was recently demonstrated that exogenous GH administration throughout the period of placental proliferation stimulates uteroplacental and fetal development by Day 81 of gestation. The present study aimed to determine whether these effects persist to term and to establish whether GH affects fetal growth and body composition by increasing placental size or by altering maternal metabolism. Adolescent recipient ewes were implanted with singleton embryos on Day 4 postestrus. Three groups of ewes offered a high dietary intake were injected twice daily with recombinant bovine GH from Days 35 to 65 of gestation (high intake plus early GH) or from Days 95 to 125 of gestation (high intake plus late GH) or remained untreated (high intake only). A fourth moderate-intake group acted as optimally nourished controls. Pregnancies were terminated at Day 130 of gestation (6 per group) or were allowed to progress to term (8-10 per group). GH administration elevated maternal plasma concentrations of GH, insulin, glucose, and nonesterified fatty acids during the defined treatment windows, while urea concentrations were decreased. At Day 130, GH treatment had reduced the maternal adiposity score, percentage of fat in the carcass, and internal fat depots and leptin concentrations, predominantly in the high-intake plus late GH group. Placental weight was lower in high-intake vs. control dams but independent of GH treatment. In contrast, fetal weight was elevated by late GH treatment, and these fetuses had higher relative carcass fat content, perirenal fat mass, and liver glycogen concentrations than all other groups. Expression of leptin mRNA in fetal perirenal fat and fetal plasma leptin concentrations were not significantly altered by maternal nutritional intake or GH. In pregnancies proceeding to term, the duration of gestation, fetal placental mass, and lamb birth weight were reduced in high-intake compared with control dams but were not significantly affected by GH treatment. In conclusion, exogenous GH has profound effects on maternal endocrinology, metabolism, and body composition when administered during early and late pregnancy. Treatment during late pregnancy has a modest effect on fetal growth independent of placental size and a profound effect on fetal adiposity, which may have implications beyond the fetal period.     

Effect of iron deficiency on placental cytokine expression and fetal growth in the pregnant rat.

Iron deficiency anemia is the most common nutritional disorder in the world. Anemia is especially serious during pregnancy, with deleterious consequences for both the mother and her developing fetus. We have developed a model to investigate the mechanisms whereby fetal growth and development are affected by maternal anemia. Weanling rats were fed a control or iron-deficient diet before and throughout pregnancy and were killed at Day 21. Dams on the deficient diet had lower hematocrits, serum iron concentrations, and liver iron levels. Similar results were recorded in the fetus, except that the degree of deficiency was markedly less, indicating compensation by the placenta. No effect was observed on maternal weight or the number and viability of fetuses. The fetuses from iron-deficient dams, however, were smaller than controls, with higher placental:fetal ratios and relatively smaller livers. Iron deficiency increased levels of tumor necrosis factor alpha (TNFalpha) only in the trophoblast giant cells of the placenta. In contrast, levels of type 1 TNFalpha receptor increased significantly in giant cells, labyrinth, cytotrophoblast, and fetal vessels. Leptin levels increased significantly in labyrinth and marginally (P = 0.054) in trophoblast giant cells. No change was observed in leptin receptor levels in any region of the placentas from iron-deficient dams. The data show that iron deficiency not only has direct effects on iron levels and metabolism but also on other regulators of growth and development, such as placental cytokines, and that these changes may, in part at least, explain the deleterious consequences of maternal iron deficiency during pregnancy.     

Steroid-dependent up-regulation of adipose leptin secretion in vitro during pregnancy in mice

Circulating leptin levels are elevated during the later stages of pregnancy in mammals, suggesting that maternal leptin may play a role in maintenance of pregnancy and/or preparation for parturition and lactation. The regulation and source of circulating leptin during pregnancy remains undetermined, but leptin mRNA levels increase in adipose tissue during this time in some species. Considerable controversy exists whether placenta is also a leptin-secreting tissue during pregnancy. Here, we directly demonstrate that leptin secretion rates from mouse adipose tissue in vitro are decreased during early pregnancy and up-regulated during late pregnancy and lactation. Changes in leptin secretion rates in vitro paralleled those of circulating leptin in vivo during gestation. Subcutaneous implants of estradiol or corticosterone into lactating mice for 48 h stimulated adipose leptin secretion rates in vitro to the level of that in pregnant mice. However, corticosterone, but not estradiol, increased leptin secretion when added to isolated adipose tissue in vitro. Placentae obtained at two stages of pregnancy did not secrete leptin in vitro, either when acutely isolated or when dissociated into cells for long-term cultures. Placental tissue (or cells) secreted progesterone, however, demonstrating placental viability. We conclude that hyperleptinemia during late pregnancy in mice primarily results from corticosterone-dependent up-regulation of leptin secretion from adipose tissue, and that the placenta does not contribute to leptin secretion. The initial decrease in leptin secretory rates from adipose tissue during early pregnancy may facilitate energy storage for the subsequent, increased metabolic demands of later pregnancy and lactation.     

The effect of leptin on mouse trophoblast cell invasion

The hormone leptin is produced by adipose tissue and can function as a signal of nutritional status to the reproductive system. The expression of leptin receptor and, in some species, leptin, in the placenta suggests a role for leptin in placental development, but this role has not been elucidated. Leptin is required at the time of embryo implantation in the leptin-deficient ob/ ob mouse and has been shown to upregulate expression of matrix metalloproteinases (MMPs), enzymes involved in trophoblast invasion, in cultured human trophoblast cells. This led us to the hypothesis that leptin promotes the invasiveness of trophoblast cells crucial to placental development. We found that leptin stimulated mouse trophoblast cell invasion through a matrigel-coated insert on Day 10, but not Day 18 of pregnancy. Optimal stimulation occurred at a concentration of 50 ng/ml leptin, similar to the peak plasma leptin concentration during pregnancy in the mouse. Leptin treatment did not stimulate proliferation of mouse trophoblast cells in primary culture. Leptin stimulation of invasion was prevented by 25 muM GM6001, an inhibitor of MMP activity. Our results suggest that leptin may play a role in the establishment of the placenta during early pregnancy and that this function is dependent on MMP activity. This effect of leptin may represent one mechanism by which body condition affects placental development.     

Hormonal regulation of placental nitric oxide and pathogenesis of pre-eclampsia

The placenta is central to foetal growth and development in mammalian pregnancy. Compromised placental function (as found in pre-eclampsia) often results in life-threatening situations for both mother and foetus. The nitric-oxide (NO) signalling cascade is important for placental function, in particular for the development of the vascular network and for maintaining vascular tone. This pathway seems to be regulated by multiple hormonal signals. Emerging evidence suggests that pathogenic mechanisms that are involved in abnormal placental function target specific molecules, such as hormone receptors, that regulate NO release and have subsequent dramatic consequences. Here, we discuss the current knowledge of NO function in the placenta, its hormonal regulation in normal pregnancy and in the pathophysiology of pre-eclampsia, its potential pathogenic mechanisms and possible use as a therapeutic target.     

Steroid hormone metabolism by the chorioallantoic placenta of the mountain spiny lizard Sceloporus jarrovi as a possible mechanism for buffering maternal-fetal hormone exchange

The placenta provides a maternal-fetal exchange interface that maximizes the diffusion of gases, nutrients, and wastes. However, the placenta also may permit diffusion of lipid-soluble steroid hormones that influence processes such as sex-specific fetal development and maternal pregnancy maintenance. In mammals, placental steroid metabolism contributes to regulation of maternal and fetal hormone levels. Such mechanisms may be less highly developed in species that have recently evolved placentation, such as many reptiles. We therefore chose to investigate placental metabolism of steroids in the viviparous lizard Sceloporus jarrovi. In vitro tissue incubations tested the abilities of the chorioallantoic placenta to clear progesterone and corticosterone by converting them to other metabolites and to synthesize progesterone. Placental tissue rapidly cleared progesterone and corticosterone added to the incubation media, indicating that the tissue had converted the steroids to other products. Placental tissue also synthesized substantial concentrations of progesterone from the prohormone pregnenolone. Thus, even in a species with a simple, recently evolved placenta, steroid metabolism appears to be highly developed and could be critical for regulation of maternal and fetal hormone levels. This finding suggests that placental hormone metabolism may be critical to the successful evolution of placentation.     

Correlation between birth weight, leptin, zinc and copper levels in maternal and cord blood

Leptin and zinc are involved in the regulation of appetite. Copper is a trace element regulating the functions of several cuproenzymes that are essential for life. To evaluate the relationship between zinc and copper status and the leptin system in humans, we examined whether leptin concentrations in the mother and the newborn correlate with the weight of mother, placenta and newborn. A total of 88 pregnant women at 38-42 weeks' gestation were studied. All infants were categorized as small for gestational age (SGA) (n = 16), average for gestational age (AGA) (n = 59) or large for gestational age (LGA) (n = 13). Leptin, zinc, and copper levels were measured in maternal and cord serum at birth. Maternal BMI and placental weight of the LGA groups were significantly higher than those of the SGA and AGA groups. Cord and maternal leptin levels of the SGA groups were significantly lower than those of the AGA and LGA groups. Maternal serum leptin levels were positively correlated with BMI and maternal zinc levels in all groups. Cord serum leptin levels of all groups were positively correlated with birth weight and placental weight. Birth weight was negatively correlated with maternal and cord copper level of all groups. Umbilical leptin concentrations of SGA newborns correlated with leptin concentrations of their mothers. In all pregnancies, birth weight increases in association with increase in cord leptin level. Our results suggest that maternal zinc but not copper level has an effect on maternal serum leptin levels. The increase in copper level in both maternal and cord blood may contribute to restriction in fetal growth.     

Corticotropin-releasing hormone stimulates expression of leptin, 11beta-HSD2 and syncytin-1 in primary human trophoblasts

ABSTRACT: BACKGROUND: The placental syncytiotrophoblast is the major source of maternal plasma corticotropin-releasing hormone (CRH) in the second half of pregnancy. Placental CRH exerts multiple functions in the maternal organism: It induces the adrenal secretion of cortisol via the stimulation of adrenocorticotropic hormone, regulates the timing of birth via its actions in the myometrium and inhibits the invasion of extravillous trophoblast cells in vitro. However, the auto- and paracrine actions of CRH on the syncytiotrophoblast itself are unknown. Intrauterine growth restriction (IUGR) is accompanied by an increase in placental CRH, which could be of pathophysiological relevance for the dysregulation in syncytialisation seen in IUGR placentas. METHODS: We aimed to determine the effect of CRH on isolated primary trophoblastic cells in vitro. After CRH stimulation the trophoblast syncytialisation rate was monitored via syncytin-1 gene expression and beta-hCG (beta-human chorionic gonadotropine) ELISA in culture supernatant. The expression of the IUGR marker genes leptin and 11beta-hydroxysteroid dehydrogenase 2 (11beta-HSD2) was measured continuously over a period of 72 h. We hypothesized that CRH might attenuate syncytialisation, induce leptin, and reduce 11beta-HSD2 expression in primary villous trophoblasts, which are known features of IUGR. RESULTS: CRH did not influence the differentiation of isolated trophoblasts into functional syncytium as determined by beta-hCG secretion, albeit inducing syncytin-1 expression. Following syncytialisation, CRH treatment significantly increased leptin and 11beta-HSD2 expression, as well as leptin secretion into culture supernatant after 48 h. CONCLUSION: The relevance of CRH for placental physiology is underlined by the present in vitro study. The induction of leptin and 11beta-HSD2 in the syncytiotrophoblast by CRH might promote fetal nutrient supply and placental corticosteroid metabolism in the phase before labour induction.     

Effects of glucose, glycerol, 3-hydroxybutyrate, insulin, and leptin on placental growth hormone secretion in placental explants

Placental growth hormone (PGH) is secreted from the syncytiotrophoblast in increasing amounts during pregnancy. The physiology and regulation of PGH is not well known; however, low glucose levels appear to stimulate PGH liberation IN VITRO and IN VIVO. PGH appears to have lipolytic effects, and inverse correlations between maternal body mass index and serum PGH levels have been reported. Therefore, substances related to maternal adipose tissue metabolism could influence PGH secretion. The effect of insulin, glycerol, 3-hydroxybutyrate (3-OHB), and leptin on PGH and human placental lactogen (hPL) secretion from cultured placental explants was studied. In glucose-free media, PGH content increased upto 237.5+/-28.4% of control media (p<0.001, ANOVA). Insulin levels were without effect on PGH secretion, as were 3-OHB, leptin, and glycerol at 0.02 mmol/l. Glycerol at 0.2 mmol/l increased PGH in all of the placental explants studied (n=8; mean increase 27.3+/-7.1%), and this difference was significantly different from the control explants (p=0.004). The liberation of hPL to culture media was different from PGH and was influenced by glucose and insulin. In conclusion, the absence of glucose profoundly increased PGH secretion in cultured placental explants. Addition of glycerol in physiologically relatively high concentrations showed a less pronounced stimulatory effect.     

Adipokines underlie the early origins of obesity and associated metabolic comorbidities in the offspring of women with pregestational obesity

Maternal pregestational obesity is a well-known risk factor for offspring obesity, metabolic syndrome, cardiovascular disease and type 2 diabetes. The mechanisms by which maternal obesity can induce alterations in fetal and later neonatal metabolism are not fully elucidated due to its complexity and multifactorial causes. Two adipokines, leptin and adiponectin, are involved in fetal and postnatal growth trajectories, and both are altered in women with pregestational obesity. The placenta synthesizes leptin, which goes mainly to the maternal circulation and in lesser amount to the developing fetus. Maternal pregestational obesity and hyperleptinemia are associated with placental dysfunction and changes in nutrient transporters which directly affect fetal growth and development. By the other side, the embryo can produce its own leptin from early in development, which is associated to fetal weight and adiposity. Adiponectin, an insulin-sensitizing adipokine, is downregulated in maternal obesity. High molecular weight (HMW) adiponectin is the most abundant form and with most biological actions. In maternal obesity lower total and HMW adiponectin levels have been described in the mother, paralleled with high levels in the umbilical cord. Several studies have found that cord blood adiponectin levels are related with postnatal growth trajectories, and it has been suggested that low adiponectin levels in women with pregestational obesity enhance placental insulin sensitivity and activation of placental amino acid transport systems, supporting fetal overgrowth. The possible mechanisms by which maternal pregestational obesity, focusing in the actions of leptin and adiponectin, affects the fetal development and postnatal growth trajectories in their offspring are discussed.     

Leptin concentrations in cord blood in normal newborn infants and offspring of diabetic mothers.

Leptin has been implicated in the regulation of body weight and energy balance; Leptin is produced by adipocytes and placental tissue. Chronic fetal hyperinsulinemia and accelerated fetal growth with increased amounts of body fat are frequent findings in the offspring of diabetic mothers. In this study, we examined whether leptin levels in cord blood of infants of type 1 diabetic mothers (n = 29), gestational diabetic mothers (n = 6 and controls (n = 96) correlated with level of maternal glucose control, maternal leptin level at delivery, gender, fetal and placental size, and C-peptide in cord blood at birth. Leptin was significantly elevated in infants of type 1 diabetic (24.7 ng/ml) and gestational diabetic mothers (29.3 ng/ml) as compared to controls (7.9 ng/ml). C-peptide was also significantly higher in infants of type 1 diabetic (0.91 nmol/l) and gestational diabetic mothers (0.99 nmol/l) vs controls (0.34 nmol/l). Infants of type 1 diabetic mothers with a leptin level in cord blood above the upper normal range, i.e. > 30 ng/ml (n = 13), had an average maternal HbA1c level of 5.4% (normal < 5.5%) that was not different from 5.2% in infants with a leptin level < 30 ng/ml (n = 15). In both neonatal groups of diabetic mothers, leptin in cord blood did not correlate with maternal leptin concentrations, placental weight, birthweight, gender and cord blood C-peptide. In controls, leptin in cord blood was higher in girls than in boys (p = 0.044) and correlated significantly with birthweight (p = 0.41, p < 0.001) and cord blood C-peptide (p = 0.44, p < 0.001) but not with maternal leptin level or placental weight. The 3-4 times higher leptin levels in the offspring of diabetic mothers than normal could reflect increased adipose tissue mass and/or increased contribution from other sources such as placental tissue.     

The Influence of Overweight and Obesity on Longitudinal Trends in Maternal Serum Leptin Levels During Pregnancy

Maternal obesity influences a number of metabolic factors that can affect the course of pregnancy. Among these factors, leptin plays an important role in energy metabolism and fetal development during pregnancy. Our objective was to estimate the influence of maternal overweight/obesity on variation in the maternal serum leptin profile during pregnancy. In a prospective cohort of 143 adult gravidas with singleton pregnancies presenting for general prenatal care, we measured serum leptin levels at 6–10, 10–14, 16–20, 22–26, and 32–36 weeks' gestation. The longitudinal effects of maternal prepregnancy BMI, categorized as nonoverweight (≤26.0 kg/m²) and overweight/obese (>26.0 kg/m²), on serum leptin concentration were analyzed using linear mixed models. Overweight/obese women had significantly higher serum leptin concentrations than their nonoverweight counterparts throughout pregnancy (P < 0.01). Although these concentrations increased significantly across gestation for both groups, the rate of increase was significantly smaller for overweight/obese women (P < 0.05). To investigate whether these differences merely reflected differences in weight‐gain patterns between the two groups, we examined an index of leptin concentration per unit body weight (leptin (ng/ml)/weight (kg)). Overweight/obese women had a significantly higher index throughout pregnancy (P < 0.01). However, although this index increased significantly across pregnancy for nonoverweight women, it actually decreased significantly for overweight/obese women (P < 0.01). Our results suggest that factors other than fat mass alone influence leptin concentrations in overweight/obese women compared to normal‐weight women during pregnancy. Such factors may contribute to differences in the intrauterine environment and its influence on pregnancy outcomes in the two groups.     

Maternal omega-3 fatty acids maintained positive maternal lipids and cytokines profile,and improved pregnancy outcomes of C57BL/6 mice

Omega (n)-3 polyunsaturated fatty acids (PUFA) are known to regulate lipid metabolism and inflammation; however, the regulation of maternal lipid metabolism and cytokines profile by n-3 PUFA during different gestation stages, and its impact on fetal sustainability is not known. We investigated the effects of maternal diet varying in n-3 PUFA prior to, and during gestation, on maternal metabolic profile, placental inflammatory cytokines, and fetal outcomes. Female C57BL/6 mice were fed either a high, low or very low (9, 3 or 1% w/w n-3 PUFA) diet, containing n-6:n-3 PUFA of 5:1, 20:1 and 40:1, respectively for two weeks before mating, and throughout pregnancy. Animals were sacrificed prior to mating (NP), and during pregnancy at gestation days 6.5, 12.5 and 18.5. Maternal metabolic profile, placental cytokines and fetal outcomes were determined. Our results show for the first time that a maternal diet high in n-3 PUFA prevented dyslipidemia in NP mice, and maintained the expected lipid profile during pregnancy. However, females fed the very low n-3 PUFA diet became hyperlipidemic prior to pregnancy, and carried this profile into pregnancy. Maternal diet high in n-3 PUFA maintained maternal plasma progesterone and placental pro-inflammatory cytokines profile, and sustained fetal numbers throughout pregnancy, while females fed the low and very-low n-3 PUFA diet had fewer fetuses. Our findings demonstrate the importance of maternal diet before, and during pregnancy, to maintain maternal metabolic profile and fetus sustainability. These findings are important when designing dietary strategies to optimize maternal metabolism during pregnancy for successful pregnancy outcome.     

Spatial and developmental regulation of leptin in fetal sheep

To better understand the biology of leptin during prenatal life, the developmental and spatial regulation of leptin was studied in ovine fetuses. Fetal plasma leptin increased steadily between days 40 and 143 postcoitus (PC), but it was unrelated to fetal weight or placental weight at day 135 PC. Leptin gene expression was detected in fetal brain and liver during most of gestation and in fetal adipose tissue after day 100 PC. At day 130 PC, expression in fetal perirenal adipose tissue was approximately 10% of maternal expression. In contrast, leptin gene expression was never detected in the placenta and other uteroplacental tissues. When ewes were fed 55% of requirements between days 122 and 135 PC, fetal plasma leptin remained constant despite acute reduction in maternal concentration. We conclude that fetal plasma leptin originates mostly from nonadipose tissue in early pregnancy and, in addition, from fetal adipose tissue near term. The role of fetal plasma leptin remains uncertain given the lack of nutritional regulation and association with fetal growth.