Susanne Lager

Regulation of nutrient transport across the placenta.

Abnormal fetal growth, both growth restriction and overgrowth, is associated with perinatal complications and an increased risk of metabolic and cardiovascular disease later in life. Fetal growth is dependent on nutrient availability, which in turn is related to the capacity of the placenta to transport these nutrients. The activity of a range of nutrient transporters has been reported to be decreased in placentas of growth restricted fetuses, whereas at least some studies indicate that placental nutrient transport is upregulated in fetal overgrowth. These findings suggest that changes in placental nutrient transport may directly contribute to the development of abnormal fetal growth. Detailed information on the mechanisms by which placental nutrient transporters are regulated will therefore help us to better understand how important pregnancy complications develop and may provide a foundation for designing novel intervention strategies. In this paper we will focus on recent studies of regulatory mechanisms that modulate placental transport of amino acids, fatty acids, and glucose.

Activation of Placental mTOR Signaling and Amino Acid Transporters in Obese Women Giving Birth to Large Babies

CONTEXT Babies of obese women are often large at birth, which is associated with perinatal complications and metabolic syndrome later in life. The mechanisms linking maternal obesity to fetal overgrowth are largely unknown. OBJECTIVE We tested the hypothesis that placental insulin/IGF-I and mammalian target of rapamycin (mTOR) signaling is activated and amino acid transporter activity is increased in large babies of obese women. DESIGN AND SETTING Pregnant women were recruited prospectively for collection of placental tissue at a university hospital and academic biomedical center. PATIENTS OR OTHER PARTICIPANTS Twenty-three Swedish pregnant women with first trimester body mass index ranging from 18.5 to 44.9 kg/m(2) and with uncomplicated pregnancies participated in the study. INTERVENTIONS There were no interventions. MAIN OUTCOME MEASURES We determined the phosphorylation of key signaling molecules (including Akt, IRS-1, S6K1, 4EBP-1, RPS6, and AMPK) in the placental insulin/IGF-I, AMPK, and mTOR signaling pathways. The activity and protein expression of the amino acid transporter systems A and L were measured in syncytiotrophoblast microvillous plasma membranes. RESULTS Birth weights (range, 3025-4235 g) were positively correlated to maternal body mass index (P < 0.05). The activity of placental insulin/IGF-I and mTOR signaling was positively correlated (P < 0.001), whereas AMPK phosphorylation was inversely (P < 0.05) correlated to birth weight. Microvillous plasma membrane system A, but not system L, activity and protein expression of the system A isoform SNAT2 were positively correlated to birth weight (P < 0.001). CONCLUSIONS Up-regulation of specific placental amino acid transporter isoforms may contribute to fetal overgrowth in maternal obesity. This effect may be mediated by activation of insulin/IGF-I and mTOR signaling pathways, which are positive regulators of placental amino acid transporters.

Increasing Maternal Body Mass Index Is Associated with Systemic Inflammation in the Mother and the Activation of Distinct Placental Inflammatory Pathways

ABSTRACT Obese pregnant women have increased levels of proinflammatory cytokines in maternal circulation and placental tissues. However, the pathways contributing to placental inflammation in obesity are largely unknown. We tested the hypothesis that maternal body mass index (BMI) was associated with elevated proinflammatory cytokines in maternal and fetal circulations and increased activation of placental inflammatory pathways. A total of 60 women of varying pre-/early pregnancy BMI, undergoing delivery by Cesarean section at term, were studied. Maternal and fetal (cord) plasma were collected for analysis of insulin, leptin, IL-1beta, IL-6, IL-8, monocyte chemoattractant protein (MCP) 1, and TNFalpha by multiplex ELISA. Activation of the inflammatory pathways in the placenta was investigated by measuring the phosphorylated and total protein expression of p38-mitogen-activated protein kinase (MAPK), c-Jun-N-terminal kinase (JNK)-MAPK, signal transducer-activated transcription factor (STAT) 3, caspase-1, IL-1beta, IkappaB-alpha protein, and p65 DNA-binding activity. To determine the link between activated placental inflammatory pathways and elevated maternal cytokines, cultured primary human trophoblast (PHT) cells were treated with physiological concentrations of insulin, MCP-1, and TNFalpha, and inflammatory signaling analyzed by Western blot. Maternal BMI was positively correlated with maternal insulin, leptin, MCP-1, and TNFalpha, whereas only fetal leptin was increased with BMI. Placental phosphorylation of p38-MAPK and STAT3, and the expression of IL-1beta protein, were increased with maternal BMI; phosphorylation of p38-MAPK was also correlated with birth weight. In contrast, placental NFkappaB, JNK and caspase-1 signaling, and fetal cytokine levels were unaffected by maternal BMI. In PHT cells, p38-MAPK was activated by MCP-1 and TNFalpha, whereas STAT3 phosphorylation was increased following TNFalpha treatment. Maternal BMI is associated with elevated maternal cytokines and activation of placental p38-MAPK and STAT3 inflammatory pathways, without changes in fetal systemic inflammatory profile. Activation of p38-MAPK by MCP-1 and TNFalpha, and STAT3 by TNFalpha, suggests a link between elevated proinflammatory cytokines in maternal plasma and activation of placental inflammatory pathways. We suggest that inflammatory processes associated with elevated maternal BMI may influence fetal growth by altering placental function.

Effect of IL-6 and TNF-α on fatty acid uptake in cultured human primary trophoblast cells

Maternal obesity and gestational diabetes (GDM) are conditions associated with fetal overgrowth and excessive fat accumulation in the fetus, implicating an increased placental nutrient transfer in these pregnancies. Obese and GDM mothers have altered metabolism and hormone levels, including elevation of maternal circulatory lipids and pro-inflammatory cytokines. We tested the hypothesis that interleukin (IL)-6 and tumor necrosis factor (TNF)-α stimulate placental fatty acid transport, as these pro-inflammatory cytokines have been shown to affect lipid metabolism in other tissues. In cultured primary human trophoblast cells IL-6, but not TNF-α, stimulated fatty acid accumulation, as measured by BODIPY fluorescence. The increased fatty acid accumulation could not be explained by an increased expression of key components in placental fatty acid transport, such as adipophilin, fatty acid transport protein (FATP)1, FATP4, or lipoprotein lipase. In a cohort of lean and overweight/obese pregnant women, increasing maternal third trimester IL-6 plasma concentrations correlated with decreasing placental lipoprotein lipase activity. However, as no effect on lipoprotein lipase activity was observed in cultured trophoblast cells after exposure to either IL-6 or TNF-α, the correlation between maternal circulatory IL-6 levels and placental lipoprotein lipase activity at term is unlikely to represent a cause-and-effect relationship. In conclusion, high levels of IL-6 stimulate trophoblast fatty acid accumulation, which could contribute to an excessive nutrient transfer in conditions associated with elevated maternal IL-6 such as obesity and gestational diabetes.

Placental transport in response to altered maternal nutrition

The mechanisms linking maternal nutrition to fetal growth and programming of adult disease remain to be fully established. We review data on changes in placental transport in response to altered maternal nutrition, including compromized utero-placental blood flow. In human intrauterine growth restriction and in most animal models involving maternal undernutrition or restricted placental blood flow, the activity of placental transporters, in particular for amino acids, is decreased in late pregnancy. The effect of maternal overnutrition on placental transport remains largely unexplored. However, some, but not all, studies in women with diabetes giving birth to large babies indicate an upregulation of placental transporters for amino acids, glucose and fatty acids. These data support the concept that the placenta responds to maternal nutritional cues by altering placental function to match fetal growth to the ability of the maternal supply line to allocate resources to the fetus. On the other hand, some findings in humans and mice suggest that placental transporters are regulated in response to fetal demand signals. These observations are consistent with the idea that fetal signals regulate placental function to compensate for changes in nutrient availability. We propose that the placenta integrates maternal and fetal nutritional cues with information from intrinsic nutrient sensors. Together, these signals regulate placental growth and nutrient transport to balance fetal demand with the ability of the mother to support pregnancy. Thus, the placenta plays a critical role in modulating maternal-fetal resource allocation, thereby affecting fetal growth and the long-term health of the offspring.

Oleic acid stimulates system A amino acid transport in primary human trophoblast cells mediated by toll-like receptor 4

Obese women have an increased risk to deliver large babies. However, the mechanisms underlying fetal overgrowth in these pregnancies are not well understood. Obese pregnant women typically have elevated circulating lipid levels. We tested the hypothesis that fatty acids stimulate placental amino acid transport, mediated via toll-like receptor 4 (TLR4) and mammalian target of rapamycin (mTOR) signaling pathways. Circulating NEFA levels and placental TLR4 expression were assessed in women with varying prepregnancy body mass index (BMI). The effects of oleic acid on system A and system L amino acid transport, and on the activation of the mTOR (4EBP1, S6K1, rpS6), TLR4 (IĸBɑ, JNK, p38 MAPK), and STAT3 signaling pathways were determined in cultured primary human trophoblast cells. Maternal circulating NEFAs (n = 33), but not placental TLR4 mRNA expression (n = 16), correlated positively with BMI (P < 0.05). Oleic acid increased trophoblast JNK and STAT3 phosphorylation (P < 0.05), whereas mTOR activity was unaffected. Furthermore, oleic acid doubled trophoblast system A activity (P < 0.05), without affecting system L activity. siRNA-mediated silencing of TLR4 expression prevented the stimulatory effect of oleic acid on system A activity. Our data suggest that maternal fatty acids can increase placental nutrient transport via TLR4, thereby potentially affecting fetal growth.

Downregulation of cilia-localized Il-6Rα by 17β-estradiol in mouse and human fallopian tubes

The action of interleukin-6 (IL-6) impacts female reproduction. Although IL-6 was recently shown to inhibit cilia activity in human fallopian tubes in vitro, the molecular mechanisms underlying IL-6 signaling to tubal function remain elusive. Here, we investigate the cellular localization, regulation, and possible function of two IL-6 receptors (IL-6R alpha and gp130) in mouse and human fallopian tubes in vivo. We show that IL-6R alpha is restricted to the cilia of epithelial cells in both mouse and human fallopian tubes. Exogenous 17beta-estradiol (E(2)), but not progesterone (P(4)), causes a time-dependent decrease in IL-6R alpha expression, which is blocked by the estrogen receptor (ER) antagonist ICI-182,780. Exposure of different ER-selective agonists propyl-(1H)-pyrazole-1,3,5-triyl-trisphenol or 2,3-bis-(4-hydroxyphenyl)-propionitrile demonstrated an ER subtype-specific regulation of IL-6R alpha in mouse fallopian tubes. In contrast to IL-6R alpha, gp130 was detected in tubal epithelial cells in mice but not in humans. In humans, gp130 was found in the muscle cells and was decreased in the periovulatory and luteal phases during the reproductive cycles, indicating a species-specific expression and regulation of gp130 in the fallopian tube. Expression of tubal IL-6R alpha and gp130 in IL-6 knockout mice was found to be normal; however, E(2) treatment increased IL-6R alpha, but not gp130, in IL-6 knockout mice when compared with wild-type mice. Furthermore, expression levels of IL-6R alpha, but not gp130, decreased in parallel with estrogenic accelerated oocyte-cumulus complex (OCC) transport in mouse fallopian tubes. Our findings open the possibility that cilia-specific IL-6R alpha may play a role in the regulation of OCC transport and suggest an estrogen-regulatory pathway of IL-6R alpha in the fallopian tube.

Diet‐induced obesity in mice reduces placental efficiency and inhibits placental mTOR signaling

As in humans, obesity during pregnancy in mice results in elevated maternal insulin levels and metabolic programming of offspring. mTOR signaling regulates amino acid transport and may function as a placental nutrient sensor. Because obesity is a condition with increased nutrient availability, we hypothesized that diet‐induced obesity activates placental mTOR signaling. To test this hypothesis, female C57BL/6J mice were fed an obesogenic diet or standard chow prior to and throughout pregnancy. Fetuses and placentas were collected at gestational day 18. Using Western blot analysis, placental mTOR activity was determined along with energy, inflammatory, and insulin signaling pathways (upstream modulators of mTOR). At gestational day 18, fetal and placental weights did not differ, however, in obese dams, the fetal/placental weight ratio was lower (P < 0.01). In placentas from obese dams, mTOR signaling was inhibited, as determined by decreased Rheb and S6K1 expression, and lower rpS6 phosphorylation (P < 0.05). In contrast, energy, inflammatory, and insulin signaling pathways were unaffected. Contrary to our hypothesis, diet‐induced obesity in pregnant mice was associated with inhibition of placental mTOR signaling. However, this finding is consistent with the lower fetal/placental weight ratio, indicating reduced placental efficiency.

Expression and functional characterisation of System L amino acid transporters in the human term placenta

BackgroundSystem L transporters LAT1 (SLC7A5) and LAT2 (SLC7A8) mediate the uptake of large, neutral amino acids in the human placenta. Many System L substrates are essential amino acids, thus representing crucial nutrients for the growing fetus. Both LAT isoforms are expressed in the human placenta, but the relative contribution of LAT1 and LAT2 to placental System L transport and their subcellular localisation are not well established. Moreover, the influence of maternal body mass index (BMI) on placental System L amino acid transport is poorly understood. Therefore the aims of this study were to determine: i) the relative contribution of the LAT isoforms to System L transport activity in primary human trophoblast (PHT) cells isolated from term placenta; ii) the subcellular localisation of LAT transporters in human placenta; and iii) placental expression and activity of System L transporters in response to maternal overweight/obesity.MethodsSystem L mediated leucine uptake was measured in PHT cells after treatment with si-RNA targeting LAT1 and/or LAT2. The localisation of LAT isoforms was studied in isolated microvillous plasma membranes (MVM) and basal membranes (BM) by Western blot analysis. Results were confirmed by immunohistochemistry in sections of human term placenta. Expression and activity System L transporters was measured in isolated MVM from women with varying pre-pregnancy BMI.ResultsBoth LAT1 and LAT2 isoforms contribute to System L transport activity in primary trophoblast cells from human term placenta. LAT1 and LAT2 transporters are highly expressed in the MVM of the syncytiotrophoblast layer at term. LAT2 is also localised in the basal membrane and in endothelial cells lining the fetal capillaries. Measurements in isolated MVM vesicles indicate that System L transporter expression and activity is not influenced by maternal BMI.ConclusionsLAT1 and LAT2 are present and functional in the syncytiotrophoblast MVM, whereas LAT2 is also expressed in the BM and in the fetal capillary endothelium. In contrast to placental System A and beta amino acid transporters, MVM System L activity is unaffected by maternal overweight/obesity.

Differential regulation of placental amino acid transport by saturated and unsaturated fatty acids

Fatty acids are critical for normal fetal development but may also influence placental function. We have previously reported that oleic acid (OA) stimulates amino acid transport in primary human trophoblasts (PHTs). In other tissues, saturated and unsaturated fatty acids have distinct effects on cellular signaling, for instance, palmitic acid (PA) but not OA reduces IκBα expression. We hypothesized that saturated and unsaturated fatty acids differentially affect trophoblast amino acid transport and cellular signaling. To test this hypothesis, PHTs were cultured in docosahexaenoic acid (DHA; 50 μM), OA (100 μM), or PA (100 μM). DHA and OA were also combined to test whether DHA could counteract the OA stimulatory effect on amino acid transport. The effects of fatty acids were compared against a vehicle control. Amino acid transport was measured by isotope-labeled tracers. Activation of inflammatory-related signaling pathways and the mechanistic target of rapamycin (mTOR) pathway were determined by Western blot analysis. Exposure of PHTs to DHA for 24 h reduced amino acid transport and phosphorylation of p38 MAPK, STAT3, mTOR, eukaryotic initiation factor 4E-binding protein 1, and ribosomal protein (rp)S6. In contrast, OA increased amino acid transport and phosphorylation of ERK, mTOR, S6 kinase 1, and rpS6. The combination of DHA with OA increased amino acid transport and rpS6 phosphorylation. PA did not affect amino acid transport but reduced IκBα expression. In conclusion, these fatty acids differentially regulated placental amino acid transport and cellular signaling. Taken together, these findings suggest that dietary fatty acids could alter the intrauterine environment by modifying placental function, thereby having long-lasting effects on the developing fetus.