Spatial polarization of insulin-like growth factor receptors on the human syncytiotrophoblast

Regulation of placental amino acid transport in health and disease

Abnormal fetal growth, i.e., intrauterine growth restriction (IUGR) or fetal growth restriction (FGR) and fetal overgrowth, is associated with increased perinatal morbidity and mortality and is strongly linked to the development of metabolic and cardiovascular disease in childhood and later in life. Emerging evidence suggests that changes in placental amino acid transport may contribute to abnormal fetal growth. This review is focused on amino acid transport in the human placenta, however, relevant animal models will be discussed to add mechanistic insights. At least 25 distinct amino acid transporters with different characteristics and substrate preferences have been identified in the human placenta. Of these, System A, transporting neutral nonessential amino acids, and System L, mediating the transport of essential amino acids, have been studied in some detail. Importantly, decreased placental Systems A and L transporter activity is strongly associated with IUGR and increased placental activity of these two amino acid transporters has been linked to fetal overgrowth in human pregnancy. An array of factors in the maternal circulation, including insulin, IGF-1, and adiponectin, and placental signaling pathways such as mTOR, have been identified as key regulators of placental Systems A and L. Studies using trophoblast-specific gene targeting in mice have provided compelling evidence that changes in placental Systems A and L are mechanistically linked to altered fetal growth. It is possible that targeting specific placental amino acid transporters or their upstream regulators represents a novel intervention to alleviate the short- and long-term consequences of abnormal fetal growth in the future.

Maternal obesity and placental function: impaired maternal-fetal axis

The prevalence of maternal obesity rapidly increases, which represents a major public health concern worldwide. Maternal obesity is characteristic by metabolic dysfunction and chronic inflammation. It is associated with health problems in both mother and offspring. Increasing evidence indicates that the placenta is an axis connecting maternal obesity with poor outcomes in the offspring. In this brief review, we have summarized the current data regarding deregulated placental function in maternal obesity. The data show that maternal obesity induces numerous placental defects, including lipid and glucose metabolism, stress response, inflammation, immune regulation and epigenetics. These placental defects affect each other and result in a stressful intrauterine environment, which transduces and mediates the adverse effects of maternal obesity to the fetus. Further investigations are required to explore the exact molecular alterations in the placenta in maternal obesity, which may pave the way to develop specific interventions for preventing epigenetic and metabolic programming in the fetus.

Made in the Womb: Maternal Programming of Offspring Cardiovascular Function by an Obesogenic Womb

Obesity incidence has been increasing at an alarming rate, especially in women of reproductive age. It is estimated that 50% of pregnancies occur in overweight or obese women. It has been described that maternal obesity (MO) predisposes the offspring to an increased risk of developing many chronic diseases in an early stage of life, including obesity, type 2 diabetes, and cardiovascular disease (CVD). CVD is the main cause of death worldwide among men and women, and it is manifested in a sex-divergent way. Maternal nutrition and MO during gestation could prompt CVD development in the offspring through adaptations of the offspring's cardiovascular system in the womb, including cardiac epigenetic and persistent metabolic programming of signaling pathways and modulation of mitochondrial metabolic function. Currently, despite diet supplementation, effective therapeutical solutions to prevent the deleterious cardiac offspring function programming by an obesogenic womb are lacking. In this review, we discuss the mechanisms by which an obesogenic intrauterine environment could program the offspring's cardiovascular metabolism in a sex-divergent way, with a special focus on cardiac mitochondrial function, and debate possible strategies to implement during MO pregnancy that could ameliorate, revert, or even prevent deleterious effects of MO on the offspring's cardiovascular system. The impact of maternal physical exercise during an obesogenic pregnancy, nutritional interventions, and supplementation on offspring's cardiac metabolism are discussed, highlighting changes that may be favorable to MO offspring's cardiovascular health, which might result in the attenuation or even prevention of the development of CVD in MO offspring. The objectives of this manuscript are to comprehensively examine the various aspects of MO during pregnancy and explore the underlying mechanisms that contribute to an increased CVD risk in the offspring. We review the current literature on MO and its impact on the offspring's cardiometabolic health. Furthermore, we discuss the potential long-term consequences for the offspring. Understanding the multifaceted effects of MO on the offspring's health is crucial for healthcare providers, researchers, and policymakers to develop effective strategies for prevention and intervention to improve care.

Placental lipid transport and content in response to maternal overweight and gestational diabetes mellitus in human term placenta

BACKGROUND AND AIMS

Placental lipid transport is altered in women with high prepregnancy body mass index (pre-BMI) or gestational diabetes (GDM), which consequently affects foetal growth. However, the interaction of maternal overweight (OW) and GDM on placental lipid metabolism and possible adaptations are less studied. We aimed to examine whether maternal OW or GDM is the main factor disrupting placental lipid processing in human term placenta.

METHODS AND RESULTS

A total of 152 lean (18.5 ≤ pre-BMI ≤ 23.9 kg/m²) and OW (24 ≤ pre-BMI ≤ 27.9 kg/m²) pregnant women with or without GDM with a scheduled delivery by caesarean section were recruited. Maternal venous blood samples were used to measure metabolic parameters during pregnancy. Term placentas and cord blood were collected at delivery to determine placental lipid metabolism and foetal circulating lipid levels. Maternal OW significantly increased the placental mRNA expression of genes involved in lipid metabolism (FAT/CD36, FATP1, FATP4, FATP6, and PPAR-α), elevated placental lipid content (triglyceride, cholesterol), enhanced placental mTORC1-rpS6 and ERK1/2 signalling, increased cord blood insulin levels and birth weight. Neonatal birth weight was positively correlated with maternal pre-BMI, placental ERK1/2 signalling and cord blood insulin. There was an interaction between OW and GDM in regulating key placental fuel transport and storage gene expression (LPL, FATP6, FABP7, PPAR-α, PPAR-β, PPAR-γ, IR-β, GLUT1, SNAT2, SNAT4, and LAT1).

CONCLUSION

Maternal OW mainly affects placental lipid metabolism, which may contribute to foetal overgrowth and may impact long-term offspring health. GDM plays a less significant role in affecting placental lipid transfer and other mechanisms may be involved.

Maternal Diet Quality Is Associated with Placental Proteins in the Placental Insulin/Growth Factor, Environmental Stress, Inflammation, and mTOR Signaling Pathways: The Healthy Start ECHO Cohort

BACKGROUND

Maternal nutritional status affects placental function, which may underlie the intrauterine origins of obesity and diabetes. The extent to which diet quality is associated with placental signaling and which specific pathways are impacted is unknown.

OBJECTIVES

To examine sex-specific associations of maternal diet quality according to the Healthy Eating Index (HEI)-developed to align with recommendations from the Dietary Guidelines for Americans-with placental proteins involved in metabolism and mediators of environmental stress, inflammation, and growth factors.

METHODS

Among 108 women from the Healthy Start cohort with a mean ± SD age of 29.0 ± 6.1 y and a prepregnancy BMI (in kg/m2) of 24.8 ± 5.3, we conducted multivariable linear regression analysis stratified by offspring sex. We adjusted for maternal race or ethnicity, age, education, prenatal smoking habits, and physical activity and tested for an association of maternal HEI >57 compared with ≤57 and the abundance and phosphorylation of key proteins involved in insulin/growth factor signaling; mediators of environmental stress, inflammation, and growth factors; mechanistic target of rapamycin signaling proteins; and energy sensing in placental villus samples. HEI >57 was chosen given its prior relevance among Healthy Start mother-child dyads.

RESULTS

In adjusted models, HEI >57 was associated with greater abundance of insulin receptor β (0.80; 95% CI: 0.11, 1.49) in placentas of females. In males, maternal HEI >57 was associated with greater activation and abundance of select placental nutrient-sensing proteins and environmental stress, inflammation, and growth factor proteins (S6K1Thr389/S6K1: 0.81; 95% CI: 0.21, 1.41; JNK1Thr183/Tyr185/JNK1: 0.82; 95% CI: 0.27, 1.37; JNK2Thr183/Tyr185/JNK2: 0.57; 95% CI: 0.02, 1.11).

CONCLUSIONS

Higher-quality diet had sex-specific associations with placental protein abundance/phosphorylation. Given that these proteins have been correlated with neonatal anthropometry, our findings provide insight into modifiable factors and placental pathways that should be examined in future studies as potential links between maternal diet and offspring metabolic health. This trial was registered at clinicaltrials.gov as NCT02273297.

Placental function in maternal obesity

Maternal obesity is associated with pregnancy complications and increases the risk for the infant to develop obesity, diabetes and cardiovascular disease later in life. However, the mechanisms linking the maternal obesogenic environment to adverse short- and long-term outcomes remain poorly understood. As compared with pregnant women with normal BMI, women entering pregnancy obese have more pronounced insulin resistance, higher circulating plasma insulin, leptin, IGF-1, lipids and possibly proinflammatory cytokines and lower plasma adiponectin. Importantly, the changes in maternal levels of nutrients, growth factors and hormones in maternal obesity modulate placental function. For example, high insulin, leptin, IGF-1 and low adiponectin in obese pregnant women activate mTOR signaling in the placenta, promoting protein synthesis, mitochondrial function and nutrient transport. These changes are believed to increase fetal nutrient supply and contribute to fetal overgrowth and/or adiposity in offspring, which increases the risk to develop disease later in life. However, the majority of obese women give birth to normal weight infants and these pregnancies are also associated with activation of inflammatory signaling pathways, oxidative stress, decreased oxidative phosphorylation and lipid accumulation in the placenta. Recent bioinformatics approaches have expanded our understanding of how maternal obesity affects the placenta; however, the link between changes in placental function and adverse outcomes in obese women giving birth to normal sized infants is unclear. Interventions that specifically target placental function, such as activation of placental adiponectin receptors, may prevent the transmission of metabolic disease from obese women to the next generation.

Human placental GLUT1 glucose transporter expression and the fetal insulin-like growth factor axis in pregnancies complicated by diabetes

We have previously described regulation of syncytial GLUT1 glucose transporters by IGF-I. Despite this, it is not clear what signal regulates transplacental glucose transport. In this report we asked whether changes in GLUT1 expression and glucose transport activity in diabetic pregnancies were associated with alterations in the fetal IGF axis. Cord blood samples and paired syncytial microvillous and basal membranes were isolated from normal term pregnancies and pregnancies characterized by gestational diabetes type A2 (GDM A2) and pre-existing insulin-dependent diabetes mellitus (IDDM). Circulating IGF-I, basal membrane GLUT1 expression and glucose transporter activity were correlated with birth weight, but only in control, not diabetic groups. Basal membrane GLUT1 and transporter activity were correlated with IGF-I concentrations in control, but not diabetic groups. IGF binding protein (IGFBP) binding capacity showed a ≥50% reduction in the diabetic groups compared to control; both showed a higher level of free IGF-I. The absence of a correlation between birth weight and factors such as fetal IGF-I or GLUT1 expression in the diabetic groups suggests that IGF-I-stimulated effects may have reached a limiting threshold, such that further increases in IGF-I (or GLUT1) are without effect. These data support that fetal IGF-I acts as a fetal nutritional signal, modulating placental GLUT1 expression and birth weight via altered levels of fetal circulating IGFBPs. Diabetes appears to exert its effects on fetal and placental factors prior to the third trimester and, despite good glycemic control immediately prior to, and in the third trimester, these effects persist to term.

Complex, coordinated and highly regulated changes in placental signaling and nutrient transport capacity in IUGR

The most common cause of intrauterine growth restriction (IUGR) in the developed world is placental insufficiency, a concept often used synonymously with reduced utero-placental and umbilical blood flows. However, placental insufficiency and IUGR are associated with complex, coordinated and highly regulated changes in placental signaling and nutrient transport including inhibition of insulin and mTOR signaling and down-regulation of specific amino acid transporters, Na⁺/K⁺-ATPase, the Na^+/H⁺-exchanger, folate and lactate transporters. In contrast, placental glucose transport capacity is unaltered and Ca²⁺-ATPase activity and the expression of proteins involved in placental lipid transport are increased in IUGR. These findings are not entirely consistent with the traditional view that the placenta is dysfunctional in IUGR, but rather suggest that the placenta adapts to reduce fetal growth in response to an inability of the mother to allocate resources to the fetus. This new model has implications for the understanding of the mechanisms underpinning IUGR and for the development of intervention strategies.

Placental Responses to Changes in the Maternal Environment Determine Fetal Growth

Placental responses to maternal perturbations are complex and remain poorly understood. Altered maternal environment during pregnancy such as hypoxia, stress, obesity, diabetes, toxins, altered nutrition, inflammation, and reduced utero-placental blood flow may influence fetal development, which can predispose to diseases later in life. The placenta being a metabolically active tissue responds to these perturbations by regulating the fetal supply of nutrients and oxygen and secretion of hormones into the maternal and fetal circulation. We have proposed that placental nutrient sensing integrates maternal and fetal nutritional cues with information from intrinsic nutrient sensing signaling pathways to balance fetal demand with the ability of the mother to support pregnancy by regulating maternal physiology, placental growth, and placental nutrient transport. Emerging evidence suggests that the nutrient-sensing signaling pathway mechanistic target of rapamycin (mTOR) plays a central role in this process. Thus, placental nutrient sensing plays a critical role in modulating maternal-fetal resource allocation, thereby affecting fetal growth and the life-long health of the fetus.

The role of placental nutrient sensing in maternal-fetal resource allocation

The placenta mediates maternal-fetal exchange and has historically been regarded as a passive conduit for nutrients. However, emerging evidence suggests that the placenta actively responds to nutritional and metabolic signals from the mother and the fetus. We propose that the placenta integrates a multitude of maternal and fetal nutritional cues with information from intrinsic nutrient-sensing signaling pathways to match fetal demand with maternal supply by regulating maternal physiology, placental growth, and nutrient transport. This process, which we have called placental nutrient sensing, ensures optimal allocation of resources between the mother and the fetus to maximize the chances for propagation of parental genes without jeopardizing maternal health. We suggest that these mechanisms have evolved because of the evolutionary pressures of maternal undernutrition, which result in decreased placental growth and down-regulation of nutrient transporters, thereby limiting fetal growth to ensure maternal survival. These regulatory loops may also function in response to maternal overnutrition, leading to increased placental growth and nutrient transport in cases of maternal obesity or gestational diabetes. Thus, placental nutrient sensing modulates maternal-fetal resource allocation to increase the likelihood of reproductive success. This model implies that the placenta plays a critical role in mediating fetal programming and determining lifelong health.

Down-regulation of placental mTOR, insulin/IGF-I signaling, and nutrient transporters in response to maternal nutrient restriction in the baboon

The mechanisms by which maternal nutrient restriction (MNR) causes reduced fetal growth are poorly understood. We hypothesized that MNR inhibits placental mechanistic target of rapamycin (mTOR) and insulin/IGF-I signaling, down-regulates placental nutrient transporters, and decreases fetal amino acid levels. Pregnant baboons were fed control (ad libitum, n=11) or an MNR diet (70% of controls, n=11) from gestational day (GD) 30. Placenta and umbilical blood were collected at GD 165. Western blot was used to determine the phosphorylation of proteins in the mTOR, insulin/IGF-I, ERK1/2, and GSK-3 signaling pathways in placental homogenates and expression of glucose transporter 1 (GLUT-1), taurine transporter (TAUT), sodium-dependent neutral amino acid transporter (SNAT), and large neutral amino acid transporter (LAT) isoforms in syncytiotrophoblast microvillous membranes (MVMs). MNR reduced fetal weights by 13%, lowered fetal plasma concentrations of essential amino acids, and decreased the phosphorylation of placental S6K, S6 ribosomal protein, 4E-BP1, IRS-1, Akt, ERK-1/2, and GSK-3. MVM protein expression of GLUT-1, TAUT, SNAT-2 and LAT-1/2 was reduced in MNR. This is the first study in primates exploring placental responses to maternal undernutrition. Inhibition of placental mTOR and insulin/IGF-I signaling resulting in down-regulation of placental nutrient transporters may link maternal undernutrition to restricted fetal growth.

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.

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.

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.

Ontogeny of growth-regulating genes in the placenta

BACKGROUND

Placental nutrient flow is the primary determinant of fetal growth. This key function of the placenta depends on several growth-promoting or -suppressing imprinted genes including Insulin-like growth factor [IGF] axis genes, which regulate nutrient transfer across the placenta. However whether changes in the placental expression of these genes parallel increased fetal growth observed in the second and third trimester remains unknown.

OBJECTIVE

The aim of our study was to determine the ontogeny of key IGF axis genes and other growth regulating imprinted genes in the placenta and to characterize patterns of placental gene expression associated with intrauterine growth restriction (IUGR).

STUDY DESIGN

Real time RT-PCR analysis of 11 genes using specific probes were performed in the placental tissue collected at the time of delivery from 63 subjects with live birth pregnancies from 24 to 40 weeks gestation between 2009 -2010.

RESULTS

We found that paternally expressed gene ZNF127 (p < 0.001) was upregulated whereas IGF1 (p = 0.001) and maternally expressed gene PHLDA2 (p = 0.001) were downregulated with advancing gestational age. ROC analysis revealed a significant change in the expression of the above genes early in the third trimester. When compared to age-matched appropriate for gestational age (AGA) infants, expression of PHLDA2 (p = 0.03) IGF2R (p < 0.05) was upregulated in IUGR infants. Maternal age was also a significant predictor for IUGR (p = 0.05).

CONCLUSION

We found increased placental expression of growth-promoting imprinted genes and decreased expression of growth-suppressive imprinted genes with advancing gestational age. These changes in placental gene expression could potentially explain accelerated fetal growth seen in the third trimester. Upregulation of maternally expressed imprinted genes in IUGR population supports the "parental conflict hypothesis".

Placental malaria-associated inflammation disturbs the insulin-like growth factor axis of fetal growth regulation

BACKGROUND

The pathogenetic mechanisms of fetal growth restriction associated with placental malaria are largely unknown. We sought to determine whether placental malaria and related inflammation were associated with disturbances in the insulin-like growth factor (IGF) axis, a major regulator of fetal growth.

METHOD

We measured IGF-1 and IGF-2 concentrations in plasma from 88 mother-neonate pairs at delivery and IGF binding proteins 1 and 3 (IGFBP-1 and IGFBP-3, respectively) in cord plasma from a cohort of Papua New Guinean women with and without placental malaria. Messenger RNA levels of IGF-1, IGF-2, and the IGF receptors were measured in matched placental biopsy specimens.

RESULTS

Compared with those for uninfected pregnancies, IGF-1 levels were reduced by 28% in plasma samples from women with placental Plasmodium falciparum infection and associated inflammation (P = .007) and by 25% in their neonates (P = .002). Levels of fetal IGFBP-1 were elevated in placental malaria with and without inflammation (P = .08 and P = .006, respectively) compared with uninfected controls. IGF-2 and IGFBP-3 plasma concentrations and placental IGF ligand and receptor messenger RNA transcript levels were similar across groups.

CONCLUSION

Placental malaria-associated inflammation disturbs maternal and fetal levels of IGFs, which regulate fetal growth. This may be one mechanism by which placental malaria leads to fetal growth restriction.

IGF2 actions on trophoblast in human placenta are regulated by the insulin-like growth factor 2 receptor, which can function as both a signaling and clearance receptor

Insulin-like growth factor 2 (IGF2) enhances proliferation and survival of human first-trimester cytotrophoblasts (CTB) by signaling through the insulin-like growth factor 1 receptor (IGF1R). However, the role of the IGF2 receptor (IGF2R) in regulating trophoblast kinetics is unclear: It could act as a clearance receptor for trafficking excess ligand to lysosomes for degradation and/or directly mediate IGF2 signaling. We used an IGF2R knockdown strategy in BeWo cells and placental villous explants to investigate trophoblast proliferation and survival in response to stimulation by IGF. Both IGF1 and IGF2 significantly (P < 0.001) increased mitosis and reduced apoptosis in serum-starved BeWo cells. Small interfering RNA (siRNA)-mediated knockdown of IGF2R further enhanced IGF2-stimulated mitosis (P < 0.01), and IGF2-mediated rescue of apoptosis (P < 0.001) in these cells. Leu(27)IGF2, an IGF2 analogue that binds to IGF2R but not IGF1R, also protected IGF2R-expressing BeWo cells from apoptosis but did not increase mitosis. IGF treatment of term placental villous explants with reduced syncytial expression of IGF2R increased CTB proliferation (P < 0.001) and decreased apoptosis (P < 0.01) compared to untreated controls. Moreover, IGF2-mediated rescue of CTB apoptosis was significantly greater than that in tissue with normal IGF2R expression. Leu(27)IGF2 promoted mitogenesis and survival only in explants with intact IGF2R expression. Given that altered CTB turnover is observed in pregnancies complicated by fetal growth restriction, the development of strategies to manipulate the IGF2R signaling axis in the syncytiotrophoblast may provide a therapeutic avenue for treating this condition.

The neglected role of insulin-like growth factors in the maternal circulation regulating fetal growth

Maternal insulin-like growth factors (IGFs) play a pivotal role in modulating fetal growth via their actions on both the mother and the placenta. Circulating IGFs influence maternal tissue growth and metabolism, thereby regulating nutrient availability for the growth of the conceptus. Maternal IGFs also regulate placental morphogenesis, substrate transport and hormone secretion, all of which influence fetal growth either via indirect effects on maternal substrate availability, or through direct effects on the placenta and its capacity to supply nutrients to the fetus. The extent to which IGFs influence the mother and/or placenta are dependent on the species and maternal factors, including age and nutrition. As altered fetal growth is associated with increased perinatal morbidity and mortality and a greater risk of developing degenerative diseases in adult life, understanding the role of maternal IGFs during pregnancy is essential in order to identify mechanisms underlying altered fetal growth and offspring programming.

The protein-tyrosine phosphatase, SRC homology-2 domain containing protein tyrosine phosphatase-2, is a crucial mediator of exogenous insulin-like growth factor signaling to human trophoblast

Adequate fetal growth depends on placental transfer of nutrients and gases from the mother; thus, as pregnancy progresses, the placenta must grow to meet the increasing demands of the developing fetus. IGFs control proliferation, differentiation, and survival of trophoblast in first-trimester placenta via intracellular tyrosine kinase signaling cascades, the activation of which is also regulated by tyrosine phosphatases. The protein-tyrosine phosphatase, Src homology-2 domain containing protein tyrosine phosphatase (SHP)-2, is crucial for mouse placental development and is known to mediate IGF actions in other systems. In this study we examined the role of SHP-2 in regulating IGF-mediated proliferation in human trophoblast. Immunohistochemical analysis demonstrated that SHP-2 is expressed strongly in cytotrophoblast and only weakly in syncytium. After small interfering RNA-mediated knockdown of SHP-2 in BeWo choriocarcinoma cells and human first-trimester placental explants, IGF-induced trophoblast proliferation, examined using immunohistochemical analysis of Ki67 and 5-bromo-2'-deoxyuridine incorporation, was significantly reduced (P < 0.05). Kinase activation assays suggested that SHP-2 interacts with the MAPK pathway to mediate these effects. Markers of trophoblast differentiation were elevated after SHP-2 knockdown. This study demonstrates a role for tyrosine phosphatases in human trophoblast and establishes SHP-2 as a component of the IGF signaling pathway that is required for normal placental growth.

Insulin-like growth factor I and II regulate the life cycle of trophoblast in the developing human placenta

The main disorders of human pregnancy are rooted in defective placentation. Normal placental development depends on proliferation, differentiation, and fusion of cytotrophoblasts to form and maintain an overlying syncytiotrophoblast. There is indirect evidence that the insulin-like growth factors (IGFs), which are aberrant in pregnancy disorders, are involved in regulating trophoblast turnover, but the processes that control human placental growth are poorly understood. Using an explant model of human first-trimester placental villus in which the spatial and ontological relationships between cell populations are maintained, we demonstrate that cytotrophoblast proliferation is enhanced by IGF-I/IGF-II and that both factors can rescue cytotrophoblast from apoptosis. Baseline cytotrophoblast proliferation ceases in the absence of syncytiotrophoblast, although denuded cytotrophoblasts can proliferate when exposed to IGF and the rate of cytotrophoblast differentiation/fusion and, consequently, syncytial regeneration, increases. Use of signaling inhibitors suggests that IGFs mediate their effect on cytotrophoblast proliferation/syncytial formation through the MAPK pathway, whereas effects on survival are regulated by the phosphoinositide 3-kinase pathway. These results show that directional contact between cytotrophoblast and syncytium is important in regulating the relative amounts of the two cell populations. However, IGFs can exert an exogenous regulatory influence on placental growth/development, suggesting that manipulation of the placental IGF axis may offer a potential therapeutic route to the correction of inadequate placental growth.

The IGF axis and placental function. a mini review

It is well known that the insulin-like growth factor (IGF) axis is an important regulator of foetal growth and in recent years, it has been suggested that the ligands IGF-I and IGF-II may, in part, mediate this effect by promoting proper placental development and function. In other tissues, IGF effects on metabolism, proliferation and differentiation are primarily mediated via IGF binding protein-regulated interaction of IGFs with the type 1 IGF receptor and therefore here, we review the placental expression and postulated role, of each of the IGF axis components and discuss the cellular mechanisms through which these effects are exerted.

The IGF axis in baboon pregnancy: placental and systemic responses to feeding 70% global ad libitum diet

Information on the influence of poor maternal nutrition on the regulation of responses to pregnancy, placental and fetal growth and development is critical to a better understanding of pregnancy physiology and pathophysiology. We determined normal changes and effects of controlled and monitored moderate nutrient restriction (NR) (global nutrient intake reduced to 70% of food consumed by mothers feeding ad libitum from 0.16 to 0.5 of gestation) in the baboon, on important hematological, biochemical, and hormonal indices of fetal growth and placental function. Serum IGF-I:IGFBP-3 ratio was lower in pregnant than control non-pregnant baboons feeding ad libitum. Serum concentrations of total and free IGF-I were decreased in NR mothers compared with controls (p<0.05). The decrease in fetal IGF-I did not reach significance (p=0.057). Serum IGF-I: IGFBP-3 ratio was decreased by NR in both mothers and fetuses. Maternal serum IGF-II was unchanged by NR. Placental IGF-I mRNA and protein abundance were similarly reduced whereas IGF-II mRNA increased in placental tissue of NR compared to control mothers. Systemic (maternal) and local (placental) IGFBP-1 and IGFBP-3 mRNA and protein abundance were unchanged by NR. Type 1 IGF receptor protein in the syncytiotrophoblast increased in NR. Type 2 IGF receptor protein was present in the stem villi core, and decreased after NR. We conclude that moderate NR in this important non-human primate model significantly disrupts the maternal and placental IGF-IGFBP axis and influences placental expression of this key system at the gene and protein level. Changes observed appear to be directed toward preserving placental growth.

Insulin-like growth factor-1 receptor expression in the placentae of diabetic and normal pregnancies

BACKGROUND

Septal hypertrophic cardiomyopathy (sHCM) is a characteristic anomaly of the infant of diabetic mother (IDM). Insulin-like growth factor-1 (IGF-1) has been identified as a mediator of tissue overgrowth and we have previously shown that maternal IGF-1 levels were significantly elevated among neonates with asymmetrical sHCM. IGF-1 does not cross the placenta; it exerts physiologic action through binding to the IGF-1 receptor (IGF-1R). Localisation and expression of IGF-1R in term diabetic pregnancies are largely unclear. We have studied IGF-1R in the placentae of diabetic and normal pregnancies and this receptor expression in association with neonates with sHCM.

METHODS

IGF-1R localization and expression in the placentae of six diabetic pregnancies associated with neonatal sHCM were compared with six each of randomly selected diabetic and normal pregnancies without neonatal sHCM by immunohistochemistry. The staining for IGF-1R in the deciduas, cytotrophoblasts, syncytiotrophoblasts and villous endothelium for these 18 samples were assessed and scored by two pathologists who were blinded to the respective diagnoses.

RESULTS

Placental IGF-1R staining was negative in the villous endothelium for all three groups. IGF-1R staining was present in deciduas, cytotrophoblasts and syncytiotrophoblasts but the staining was weaker in the entire group of infants with sHCM compared to those without sHCM.

CONCLUSIONS

IGF-1R is localized in all cell types of the placenta except in villous endothelium. Weaker placental IGF-1R staining in the placentae of diabetic pregnancies associated with sHCM suggests reduced expression of IGF-1R. This may be a down-regulatory response to elevated maternal IGF with neonatal sHCM outcome.

The PLAC1 protein localizes to membranous compartments in the apical region of the syncytiotrophoblast

PLAC1 is a trophoblast-specific gene that maps to a locus on the X-chromosome important to placental development. We have previously shown that PLAC1 gene expression is linked to trophoblast differentiation. The objective of this study was to define the localization of the PLAC1 polypeptide as a prerequisite to understanding its function. Polyclonal antibodies specific for the putative PLAC1 polypeptide were generated. The subcellular localization of PLAC1 in the trophoblast was examined by immunohistochemical analysis of human placenta complemented by immunoblot analysis of subcellular fractions. Brightfield immunohistochemical analysis of placental tissue indicated that the PLAC1 protein localizes to the differentiated syncytiotrophoblast in the apical region of the cell. Deconvlution immunofluorescence microscopy confirmed localization to the apical region of the syncytiotrophoblast. Its distribution included both intracellular compartments as well as loci in close association with the maternal-facing, microvillous brush border membrane (MVM). These findings were supported by immunoblot analysis of subcellular fractions. A 30 kDa band was associated with the microsomal fraction of placental lysates but not the mitochondrial, nuclear, or soluble fractions, suggesting PLAC1 is targeted to a membrane location. Plasma membranes were obtained from the fetal-facing, basal surface (BM) and the maternal-facing, MVM of the syncytiotrophoblast membrane. PLAC1 immunoreactivity was only detected in membrane fractions derived from the apical MVM consistent with immunohistochemical analyses. These data demonstrate that the PLAC1 protein is restricted primarily to the differentiated trophoblast, localizing to intracellular membranous compartment(s) in the apical region of the syncytiotrophoblast and associated with its apical, microvillous membrane surface.

IGF regulation of neutral amino acid transport in the BeWo choriocarcinoma cell line (b30 clone): evidence for MAP kinase-dependent and MAP kinase-independent mechanisms

OBJECTIVE

IGF-1 and IGF-1 receptors are major determinants of fetal growth and are expressed primarily on the maternal-facing surface of the syncytiotrophoblast cell membrane in the human placenta. IGF-1 regulates fetal growth, in part, by regulating amino acid transport across the placenta. The objective of these studies was to study the role of IGF-1 and its signaling pathway in regulating neutral amino acid transport in a human trophoblast cell culture model.

DESIGN

The regulation of neutral amino acid transport by IGF-1 was studied in cultured BeWo(b30) choriocarcinoma cells using the non-metabolizing amino acid analog, [(3)H]-alpha-aminoisobutyric acid (AIB). Transport in the absence of Na was used to distinguish system L from total AIB transport. Similarly, Na-dependent transport in the presence of excess methyl-AIB (MeAIB) permitted discrimination of systems A (MeAIB-sensitive) and ASC (MeAIB-insensitive). Specific inhibitors of intracellular signaling pathways were then used to determine the signaling pathway utilized by IGFs to regulate each amino acid transport system. Specificity of inhibition was assessed using specific markers of p70 S6 kinase activity and MAP kinase activation.

RESULTS

Maximal stimulating concentrations of IGF-I (100 ng/ml) stimulated AIB transport by 30-40% exclusively through system A. Wortmannin (100 nM), an inhibitor of PI-3-kinase activity, inhibited all IGF-I-stimulated transport. Rapamycin (100 ng/ml), an inhibitor of p70 S6 kinase, and bisindolylmaleimide, an inhibitor of protein kinase C (PKC), had no effect. PD-098059 (50 miccroM), an inhibitor of MAP kinase activation, inhibited 20-30% of basal AIB transport but did not inhibit IGF-I-stimulated transport under the conditions studied. IGF-1 did not increase steady state mRNA levels of the system A transporters, SNAT1 and SNAT2, suggesting IGF-1 stimulates transport via post-transcriptional mechanisms.

CONCLUSIONS

These data demonstrate that IGF-I stimulates neutral amino acid transport system A by a PI3-kinase dependent, post-transcriptional pathway in the BeWo(b30) cell line. Additionally, system A activity appear to be sensitive to MAP kinase-dependent pathways not regulated by IGFs.

Maternal insulin-like growth factors-I and -II act via different pathways to promote fetal growth

The placenta transports substrates and wastes between the maternal and fetal circulations. In mice, placental IGF-II is essential for normal placental development and function but, in other mammalian species, maternal circulating IGF-II is substantial and may contribute. Maternal circulating IGFs increase in early pregnancy, and early treatment of guinea pigs with either IGF-I or IGF-II increases placental and fetal weights by mid-gestation. We now show that these effects persist to enhance placental development and fetal growth and survival near term. Pregnant guinea pigs were infused with IGF-I, IGF-II (both 1 mg/kg.d), or vehicle sc from d 20-38 of pregnancy and killed on d 62 (term = 69 d). IGF-II, but not IGF-I, increased the mid-sagittal area and volume of placenta devoted to exchange by approximately 30%, the total volume of trophoblast and maternal blood spaces within the placental exchange region (+29% and +46%, respectively), and the total surface area of placenta for exchange by 39%. Both IGFs reduced resorptions, and IGF-II increased the number of viable fetuses by 26%. Both IGFs increased fetal weight by 11-17% and fetal circulating amino acid concentrations. IGF-I, but not IGF-II, reduced maternal adipose depot weights by approximately 30%. In conclusion, increased maternal IGF-II abundance in early pregnancy promotes fetal growth and viability near term by increasing placental structural and functional capacity, whereas IGF-I appears to divert nutrients from the mother to the conceptus. This suggests major and complementary roles in placental and fetal growth for increased circulating IGFs in early to mid-pregnancy.

Endocrine regulation of human fetal growth: the role of the mother, placenta, and fetus

The environment in which the fetus develops is critical for its survival and long-term health. The regulation of normal human fetal growth involves many multidirectional interactions between the mother, placenta, and fetus. The mother supplies nutrients and oxygen to the fetus via the placenta. The fetus influences the provision of maternal nutrients via the placental production of hormones that regulate maternal metabolism. The placenta is the site of exchange between mother and fetus and regulates fetal growth via the production and metabolism of growth-regulating hormones such as IGFs and glucocorticoids. Adequate trophoblast invasion in early pregnancy and increased uteroplacental blood flow ensure sufficient growth of the uterus, placenta, and fetus. The placenta may respond to fetal endocrine signals to increase transport of maternal nutrients by growth of the placenta, by activation of transport systems, and by production of placental hormones to influence maternal physiology and even behavior. There are consequences of poor fetal growth both in the short term and long term, in the form of increased mortality and morbidity. Endocrine regulation of fetal growth involves interactions between the mother, placenta, and fetus, and these effects may program long-term physiology.

Expression of IGR-IR and VEGF and trophoblastic proliferative activity in placentas from pregnancies complicated by IUGR

Intrauterine growth retardation (IUGR) is recognized as an important cause of low birth weight and elective preterm delivery. IUGR is associated with multiple causative factors, including placental dysfunction. The aim of this prospective study was to investigate the role of trophoblastic proliferative activity and type I insuline-like growth factor receptor (IGF-IR) and vascular endothelial growth factor (VEGF) expressions in the pathogenesis of IUGR. Immunohistochemistry using VEGF, IGF-IR, and Ki-67 antibodies was performed on formalin-fixed placental tissues of third-trimester pregnancies complicated by IUGR (n = 19) and pregnancies with appropriately grown fetuses (n = 27). In addition, histopathological examination of the placentas was performed, and histological findings were categorized into three groups: utero-placental vascular pathologies (UPVP), coagulation-related pathologies, and chronic inflammation. Statistical analysis revealed that villous trophoblastic IGF-IR immunostaining was significantly weaker in placentas with IUGR (p < 0.001), whereas trophoblastic Ki-67 proliferative index and VEGF immunoscoring did not show any significant difference. Histologically, UPVP and chronic inflammation were significant findings in placentas with IUGR (p = 0.04 and p = 0.04, respectively). In addition, placentas were significantly smaller in the IUGR group (p < 0.001). We conclude that villous trophoblastic IGF-IR expression may play a significant role in the pathogenesis of IUGR, and histopathological examination of placentas in pregnancies complicated by IUGR may yield significant findings. In contrast, based on our findings, trophoblastic proliferation and VEGF expression are unlikely to be significant parameters in the pathogenesis of IUGR.

Insulin and insulin-like growth factors in human development: implications for the perinatal period

The physiologic and cellular mechanisms regulating fetal growth cannot be adequately described by regulatory mechanisms important postnatally. This review summarizes recent advances in clinical medicine, cell and molecular biology, and physiology showing the central and essential roles of insulin and the insulin-like growth factor family of peptides in regulating fetal growth. Moreover, the importance of insulin-like growth factors in tissue-specific growth regulation during critical periods of development suggest that these mechanisms may also be relevant to the pathogenesis of tissue injury in the preterm infant, and may offer therapeutic strategies aimed at reducing morbidity associated with prematurity. Illustrations of how the insulin-like growth factor axis may represent potential therapeutic targets for specific clinical problems facing the newborn are briefly discussed.

Glucose transporters in the human placenta

The availability of antibodies and cDNA probes specific for the various members of the facilitated-diffusion glucose transporter (GLUT) family has enabled researchers to obtain a much clearer picture of the mechanisms for placental uptake and transplacental transport of glucose. This review examines studies of human placental glucose transport with the aim of providing a model which describes the transporter isoforms present in the placenta, their cellular localization and functional significance. The GLUT1 glucose transporter, present on both the microvillous and basal membranes of the syncytial barrier, is the primary isoform involved in the transplacental movement of glucose. Although GLUT3 mRNA is widely distributed, GLUT3 protein is localized to the arterial component of the vascular endothelium, where it may play a role in enhancing transplacental glucose transport. This data is in contrast to the situation in other mammalian species, such as the mouse, rat and sheep, where GLUT3 protein is not only present in those epithelial cells which carry out transplacental transport but becomes an increasingly prominent isoform as gestation progresses. The asymmetric distribution of GLUT1 in the human syncytiotrophoblast (microvillous>basal) means that basal GLUT1 acts as the rate limiting step in transplacental transfer. Changes in basal GLUT1 therefore have the potential to cause alterations in transplacental transport of glucose. Although there appear to be no changes in syncytial GLUT1 expression in intrauterine growth retardation, in diabetic pregnancies increases in basal GLUT1 expression and activity have been observed, with significant consequences for the maternal-fetal flux of glucose. Little is known of glucose transporter regulation in the placenta save for the effects of hyper- and hypoglycemia. GLUT1 expression and activity appear to be inversely related to extracellular glucose concentration, however within the physiological range, GLUT1 expression is relatively refractory to glucose concentration. Information is still needed on gestational development, on the expression and activity in well-defined conditions of intrauterine growth retardation, on the mechanisms and consequences of the changes observed in diabetic pregnancy and on the role of external agents other than glucose in regulating placental glucose transport.

IGF binding protein-1 (IGFBP-1) is preferentially associated with the fetal-facing basal surface of the syncytiotrophoblast in the human placenta

IGF receptors are expressed in a spatially polarized manner on the syncytiotrophoblast cell membrane. We therefore examined the hypothesis that IGFBPs expressed at the maternal-fetal interface interact with distinct surfaces of the syncytiotrophoblast membrane to modulate IGF function. Membrane vesicles were prepared specifically from the maternal-facing, microvillous membrane (MVM) and the fetal-facing, basal membrane (BM) surfaces of the syncytiotrophoblast. The association of IGFBPs with each membrane preparation was determined by ligand blot analysis. A doublet migrating at 38/42 kD was detected in both MVM and BM preparations. Selective immunoprecipitation followed by ligand blot analysis identified this IGF binding species as IGFBP-3. Additionally, a protein migrating at approximately 29 kD was associated primarily with the BM. This protein was identified as IGFBP-1 by both immunoprecipitation and ligandblotting techniques. Non-denaturing PAGE revealed five distinct bands corresponding to different degrees of phosphorylation. The phosphorylation pattern of BM-associated IGFBP-1 was identical to that of native IGFBP-1 in amniotic fluid. Immunohistological analysis of term placenta revealed IGFBP-1-specific staining of the syncytiotrophoblast and the fetal capillary/pericapillary bed. The localization of IGFBP-1 to a distinct compartment within the fetal placenta, not in proximity to the syncytiotrophoblast type I IGF receptor, suggests it may play a role in regulating/targeting IGF activity within the stromal compartment or by exerting IGF-independent effects on the basal surface of the syncytiotrophoblast. The nature of its binding to the BM has not been determined.