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

Investigating a suitable model for the study of vitamin D mediated regulation of human placental gene expression

Transfer and metabolism of vitamin D across the human placenta is required for fetal development. However, these fundamental mechanisms are not well understood and model systems are required to help understand them. The BeWo choriocarcinoma cell line is derived from extravillous trophoblast but is used as a model for villous syncytiotrophoblast and the placental barrier. Questions have been raised about the suitability of the BeWo cell line as a model for villous trophoblast. This study compares the expression of amino acid transporters and vitamin D related genes in human term placenta with the BeWo and human embryonic kidney (HEK)293 cell lines. HEK293 cells, as transporting epithelium may be more similar to placenta. Gene expression in term placenta was much more similar to HEK293 than BeWo. This study provides further evidence that the BeWo cell line is not an appropriate model for villous trophoblast and a model that more closely represents the human placenta is now required to investigate the effects of vitamin D on the placenta ex-vivo.

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.

Novel roles of mechanistic target of rapamycin signaling in regulating fetal growth†

Mechanistic target of rapamycin (mTOR) signaling functions as a central regulator of cellular metabolism, growth, and survival in response to hormones, growth factors, nutrients, energy, and stress signals. Mechanistic TOR is therefore critical for the growth of most fetal organs, and global mTOR deletion is embryonic lethal. This review discusses emerging evidence suggesting that mTOR signaling also has a role as a critical hub in the overall homeostatic control of fetal growth, adjusting the fetal growth trajectory according to the ability of the maternal supply line to support fetal growth. In the fetus, liver mTOR governs the secretion and phosphorylation of insulin-like growth factor binding protein 1 (IGFBP-1) thereby controlling the bioavailability of insulin-like growth factors (IGF-I and IGF-II), which function as important growth hormones during fetal life. In the placenta, mTOR responds to a large number of growth-related signals, including amino acids, glucose, oxygen, folate, and growth factors, to regulate trophoblast mitochondrial respiration, nutrient transport, and protein synthesis, thereby influencing fetal growth. In the maternal compartment, mTOR is an integral part of a decidual nutrient sensor which links oxygen and nutrient availability to the phosphorylation of IGFBP-1 with preferential effects on the bioavailability of IGF-I in the maternal-fetal interface and in the maternal circulation. These new roles of mTOR signaling in the regulation fetal growth will help us better understand the molecular underpinnings of abnormal fetal growth, such as intrauterine growth restriction and fetal overgrowth, and may represent novel avenues for diagnostics and intervention in important pregnancy complications.

Placental phenotype and the insulin-like growth factors: resource allocation to fetal growth

The placenta is the main determinant of fetal growth and development in utero. It supplies all the nutrients and oxygen required for fetal growth and secretes hormones that facilitate maternal allocation of nutrients to the fetus. Furthermore, the placenta responds to nutritional and metabolic signals in the mother by altering its structural and functional phenotype, which can lead to changes in maternal resource allocation to the fetus. The molecular mechanisms by which the placenta senses and responds to environmental cues are poorly understood. This review discusses the role of the insulin-like growth factors (IGFs) in controlling placental resource allocation to fetal growth, particularly in response to adverse gestational environments. In particular, it assesses the impact of the IGFs and their signalling machinery on placental morphogenesis, substrate transport and hormone secretion, primarily in the laboratory species, although it draws on data from human and other species where relevant. It also considers the role of the IGFs as environmental signals in linking resource availability to fetal growth through changes in the morphological and functional phenotype of the placenta. As altered fetal growth is associated with increased perinatal morbidity and mortality and a greater risk of developing adult-onset diseases in later life, understanding the role of IGFs during pregnancy in regulating placental resource allocation to fetal growth is important for identifying the mechanisms underlying the developmental programming of offspring phenotype by suboptimal intrauterine growth.

SLC38A1 promotes proliferation and migration of human colorectal cancer cells

Current studies have demonstrated that SLC38A1 proteins play a causal role in neoplastic cell transformation. The twofold aim of this study was to provide insight into whether a variance in the expression of SLC38A1 exists between human colorectal cancer and healthy human tissues and to determine how silencing or overexpressing the SLC38A1 gene could affect the proliferation, viability and migration of colorectal cancer cells. Immunohistochemical staining was used to analyze the expression of SLC38A1 in colorectal cancer tissues and adjacent normal mucosa in 77 patients who underwent surgical resection. The expression of SLC38A1 in colorectal cancer tissues and cell lines was detected using RT-PCR and Western blotting. Two colorectal cancer cell lines SW480 and HCT116 were used to examine whether silencing SLC38A1 with siRNA and overexpressing SLC38A1 with shRNA could affect cell viability and migration. As a result, the SLC38A1 protein was very low or undetectable in the normal colon mucosa. In contrast, strong staining of SLC38A1 protein was found in the cytoplasm in 79.2% colorectal cancer samples. More pronounced SLC38A1 expression in colorectal cancer tissues was significantly associated with tumor node metastasis (TNM) stage. Inhibition of SLC38A1 reduced tumour growth and suppressed proliferation and migration of SW480 cells. In contrast, overexpression of SLC38A1 had the opposite effects on HCT116 cells. SLC38A1 is overexpressed in colorectal cancer, which suggests that it is associated with tumour progression. These results encourage the exploration of SLC38A1 as a target for intervention in colorectal cancer.

Maternal-fetal nutrient transport in pregnancy pathologies: the role of the placenta

Appropriate in utero growth is essential for offspring development and is a critical contributor to long-term health. Fetal growth is largely dictated by the availability of nutrients in maternal circulation and the ability of these nutrients to be transported into fetal circulation via the placenta. Substrate flux across placental gradients is dependent on the accessibility and activity of nutrient-specific transporters. Changes in the expression and activity of these transporters is implicated in cases of restricted and excessive fetal growth, and may represent a control mechanism by which fetal growth rate attempts to match availability of nutrients in maternal circulation. This review provides an overview of placenta nutrient transport with an emphasis on macro-nutrient transporters. It highlights the changes in expression and activity of these transporters associated with common pregnancy pathologies, including intrauterine growth restriction, macrosomia, diabetes and obesity, as well as the potential impact of maternal diet. Molecular signaling pathways linking maternal nutrient availability and placenta nutrient transport are discussed. How sexual dimorphism affects fetal growth strategies and the placenta’s response to an altered intrauterine environment is considered. Further knowledge in this area may be the first step in the development of targeted interventions to help optimize fetal growth.

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.

Regulation of amino acid transporters by adenoviral-mediated human insulin-like growth factor-1 in a mouse model of placental insufficiency in vivo and the human trophoblast line BeWo in vitro

Previous work in our laboratory demonstrated that over-expression of human insulin-like growth factor-11 (hIGF-1) in the placenta corrects fetal weight deficits in mouse, rat, and rabbit models of intrauterine growth restriction without changes in placental weight. The underlying mechanisms of this effect have not been elucidated. To investigate the effect of intra-placental IGF-1 over-expression on placental function we examined amino acid transporter expression and localization in both a mouse model of placental Insufficiency (PI) and a model of human trophoblast, the BeWo Choriocarcinoma cell line. For in vitro human studies, BeWo Choriocarcinoma cells were maintained in F12 complete medium + 10%FBS. Cells were incubated in serum-free control media ± Ad-IGF-1 or Ad-LacZ for 48 h. MOIs of 10:1 and 100:1 were utilized. In BeWo, transfection efficiency was 100% at an MOI of 100:1 and Ad-IGF-1 significantly increased IGF-1 secretion, proliferation and invasion but reduced apoptosis compared to controls. In vitro, amino acid uptake was increased following Ad-IGF-1 treatment and associated with significantly increased RNA expression of SNAT1, 2, LAT1 and 4F2hc. Only SNAT2 protein expression was increased but LAT1 showed relocalization from a perinuclear location to the cytoplasm and cell membrane. For in vivo studies, timed-pregnant animals were divided into four groups on day 18; sham-operated controls, uterine artery branch ligation (UABL), UABL + Ad-hIGF-1 (10(8) PFU), UABL + Ad-LacZ (10(8) PFU). At gestational day 20, pups and placentas were harvested by C-section. Only LAT1 mRNA expression changed, showing that a reduced expression of the transporter levels in the PI model could be partially rectified with Ad-hIGF1 treatment. At the protein level, System L was reduced in PI but remained at control levels following Ad-hIGF1. The System A isoforms were differentially regulated with SNAT2 expression diminished but SNAT1 increased in PI and Ad-hIGF1 groups. Enhanced amino acid isoform transporter expression and relocalization to the membrane may be an important mechanism contributing to Ad-hIGF-1 mediated correction of placental insufficiency.

Solute carriers (SLCs) in cancer

During tumor progression cells acquire an altered metabolism, either as a cause or as a consequence of an increased need of energy and nutrients. All four major classes of macromolecules are affected: carbohydrates, proteins, lipids and nucleic acids. As a result of the changed needs, solute carriers (SLCs) which are the major transporters of these molecules are differently expressed. This renders them important targets in the treatment of cancer. Blocking or activating SLCs is one possible therapeutic strategy. For example, some SLCs are upregulated in tumor cells due to the increased demand for energy and nutritional needs. Thus, blocking them and turning off the delivery of fuel or nutrients could be one way to interfere with tumor progression. Specific drug delivery to cancer cells via transporters is another approach. Some SLCs are also interesting as chemosensitizing targets because blocking or activating them may result in an altered response to chemotherapy. In this review we summarize the roles of SLCs in cancer therapy and specifically their potential as direct or indirect targets, as drug carriers or as chemosensitizing targets.

Oxidative stress decreases uptake of neutral amino acids in a human placental cell line (BeWo cells)

Increased oxidative stress (OS) is implicated in the pathophysiology of several pregnancy disorders. We aimed to investigate the effect of tert-butylhydroperoxide (TBHP)-induced OS upon the placental transport of the neutral amino acids L-methionine (L-Met) and L-alanine (L-Ala), by using a human trophoblast cell model (BeWo cells). TBHP reduced both total and Na(+)-independent (14)C-L-Met intracellular steady-state accumulation over time (Amax), by reducing non-system L-mediated uptake - most probably system y(+) - while having no effect on system L. Moreover, TBHP reduced total (14)C-L-Ala Amax through an inhibition of system A. The effect of TBHP upon total, but not system A-mediated, (14)C-L-Ala uptake was dependent upon phosphoinositide 3-kinase (PI3K) and protein kinase C (PKC) activation, and was completely prevented by the polyphenol quercetin. In conclusion, a reduction in placental uptake of neutral amino acids may contribute to the deleterious effects of pregnancy disorders associated with OS.

L-methionine placental uptake: characterization and modulation in gestational diabetes mellitus

Our aim was to investigate the influence of gestational diabetes mellitus (GDM) and GDM-associated conditions upon the placental uptake of (14)C-l-methionine ((14)C-l-Met). The (14)C-l-Met uptake by human trophoblasts (TBs) obtained from normal pregnancies (normal trophoblast [NTB] cells) is mainly system l-type amino acid transporter 1 (LAT1 [L])-mediated, although a small contribution of system y(+)LAT2 is also present. Comparison of (14)C-l-Met uptake by NTB and by human TBs obtained from GDM pregnancies (diabetic trophoblast [DTB] cells) reveals similar kinetics, but a contribution of systems A, LAT2, and b(0+) and a greater contribution of system y(+)LAT1 appears to exist in DTB cells. Short-term exposure to insulin and long-term exposure to high glucose, tumor necrosis factor-α, and leptin decrease (14)C-l-Met uptake in a human TB (Bewo) cell line. The effect of leptin was dependent upon phosphoinositide 3-kinase, extracellular-signal-regulated kinase 1/2 (ERK/MEK 1/2), and p38 mitogen-activated protein kinase. In conclusion, GDM does not quantitatively alter (14)C-l-Met placental uptake, although it changes the nature of transporters involved in that process.

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.

Weekly intra-amniotic IGF-1 treatment increases growth of growth-restricted ovine fetuses and up-regulates placental amino acid transporters

Frequent treatment of the growth-restricted (IUGR) ovine fetus with intra-amniotic IGF-1 increases fetal growth. We aimed to determine whether increased growth was maintained with an extended dosing interval and to examine possible mechanisms. Pregnant ewes were allocated to three groups: Control, and two IUGR groups (induced by placental embolization) treated with weekly intra-amniotic injections of either saline (IUGR) or 360 µg IGF-1 (IGF1). IUGR fetuses were hypoxic, hyperuremic, hypoglycemic, and grew more slowly than controls. Placental glucose uptake and SLC2A1 (GLUT2) mRNA levels decreased in IUGR fetuses, but SLC2A3 (GLUT3) and SLC2A4 (GLUT4) levels were unaffected. IGF-1 treatment increased fetal growth rate, did not alter uterine blood flow or placental glucose uptake, and increased placental SLC2A1 and SLC2A4 (but not SLC2A3) mRNA levels compared with saline-treated IUGR animals. Following IGF-1 treatment, placental mRNA levels of isoforms of the system A, y⁺, and L amino acid transporters increased 1.3 to 5.0 fold, while the ratio of phosphorylated-mTOR to total mTOR also tended to increase. Weekly intra-amniotic IGF-1 treatment provides a promising avenue for intra-uterine treatment of IUGR babies, and may act via increased fetal substrate supply, up-regulating placental transporters for neutral, cationic, and branched-chain amino acids, possibly via increased activation of the mTOR pathway.

Biology and significance of signalling pathways activated by IGF-II

Insulin-like growth factor-II (IGF-II) affects many aspects of cellular function through its ability to activate several different receptors and, consequently, numerous intracellular signalling molecules. Thus, IGF-II is a key regulator of normal foetal development and growth. However, abnormalities in IGF-II function are associated with cardiovascular disease and cancer. Here, we review the cellular mechanisms by which IGF-II's physiological and pathophysiological actions are exerted by discussing the involvement of the type 1 and type 2 IGF receptors (IGF1R and IGF2R), the insulin receptor and the downstream MAP kinase, PI-3 kinase and G-protein-coupled signalling pathways in mediating IGF-II stimulated cellular proliferation, survival, differentiation and migration.

Promoter characterization and role of CRE in the basal transcription of the rat SNAT2 gene

Small neutral amino acid transporter 2 (SNAT2) is the most abundant and ubiquitous transporter for zwitterionic short-chain amino acids. The activity of this amino acid transporter is stimulated in vivo or in vitro by glucagon or cAMP analogs. However, it is not known whether the increase in activity at the protein level is due to an increase in SNAT2 gene transcription. Thus, the aim of the present work was to study whether cAMP was able to stimulate SNAT2 gene expression and to localize and characterize the presence of cAMP response elements (CRE) in the promoter that controls the expression of the rat SNAT2 gene. We found that consumption of a high-protein diet that increased serum glucagon concentration or the administration of glucagon or incubation of hepatocytes with forskolin increased the SNAT2 mRNA level. We then isolated the 5' regulatory region of the SNAT2 gene and determined that the transcriptional start site was located 970 bp upstream of the translation start codon. We identified two potential CRE sites located at -354 and -48 bp. Our results, using deletion analysis of the 5' regulatory region of the SNAT2 gene, revealed that the CRE site located at -48 bp was fully responsible for SNAT2 regulation by cAMP. This evidence was strongly supported by mutation of the CRE site and EMSA and ChIP analysis. Alignment of rat, mouse, and human sequences revealed that this CRE site is highly conserved among species, indicating its essential role in the regulation of SNAT2 gene expression.

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.

Regulation of amino acid transporters by glucose and growth factors in cultured primary human trophoblast cells is mediated by mTOR signaling

Inhibition of mammalian target of rapamycin (mTOR) signaling in cultured human primary trophoblast cells reduces the activity of key placental amino acid transporters. However, the upstream regulators of placental mTOR are unknown. We hypothesized that glucose, insulin, and IGF-I regulate placental amino acid transporters by inducing changes in mTOR signaling. Primary human trophoblast cells were cultured for 24 h with media containing various glucose concentrations, insulin, or IGF-I, with or without the mTOR inhibitor rapamycin, and, subsequently, the activity of system A, system L, and taurine (TAUT) transporters was measured. Glucose deprivation (0.5 mM glucose) did not significantly affect Thr172-AMP-activated protein kinase phosphorylation or REDD1 expression but decreased S6 kinase 1 phosphorylation at Thr389. The activity of system L decreased in a dose-dependent manner in response to decreasing glucose concentrations. This effect was abolished in the presence of rapamycin. Glucose deprivation had two opposing effects on system A activity: 1) an "adaptive" upregulation mediated by an mTOR-independent mechanism and 2) downregulation by an mTOR-dependent mechanism. TAUT activity was increased after incubating cells with glucose-deprived media, and this effect was largely independent of mTOR signaling. Insulin and IGF-I increased system A activity and insulin stimulated system L activity, effects that were abolished by rapamycin. We conclude that the mTOR pathway represents an important intracellular regulatory link between nutrient and growth factor concentrations and amino acid transport in the human placenta.

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.

Early pregnancy maternal endocrine insulin-like growth factor I programs the placenta for increased functional capacity throughout gestation

In early pregnancy, the concentrations of IGFs increase in maternal blood. Treatment of pregnant guinea pigs with IGFs in early to midpregnancy enhances placental glucose transport and fetal growth and viability near term. In the current study, we determined whether exogenous IGFs altered placental gene expression, transport, and nutrient partitioning during treatment, which may then persist. Guinea pigs were infused with IGF-I, IGF-II (both 1 mg/kg x d) or vehicle sc from d 20-35 of pregnancy and killed on d 35 (term is 70 d) after administration of [(3)H]methyl-D-glucose (MG) and [(14)C]amino-isobutyric acid (AIB). IGF-I increased placental and fetal weights (+15 and +17%, respectively) and MG and AIB uptake by the placenta (+42 and +68%, respectively) and fetus (+59 and +90%, respectively). IGF-I increased placental mRNA expression of the amino acid transporter gene Slc38a2 (+780%) and reduced that of Igf2 (-51%), without altering the glucose transporter Slc2a1 or Vegf and Igf1 genes. There were modest effects of IGF-I treatment on MG and AIB uptake by individual maternal tissues and no effect on plasma glucose, total amino acids, free fatty acids, triglycerides, and cholesterol concentrations. IGF-II treatment of the mother did not alter any maternal, fetal or placental parameter. In conclusion, exogenous IGF-I, but not IGF-II, in early pregnancy increases placental transport of MG and AIB, enhancing midgestational fetal nutrient uptake and growth. This suggests that early pregnancy rises in maternal circulating IGF-I play a major role in regulating placental growth and functional development and thus fetal growth throughout gestation.

Regulation of Placental Nutrient Transport – A Review

Fetal growth is primarily determined by nutrient availability, which is intimately related to placental nutrient transport. Detailed information on the regulation of placental nutrient transporters is therefore critical in order to understand the mechanisms underlying altered fetal growth and fetal programming. After briefly summarizing the cellular mechanisms for placental transport of glucose, amino acids and free fatty acids, we will discuss factors shown to regulate placental nutrient transporters and review the data describing how these factors are altered in pregnancy complications associated with abnormal fetal growth. We propose an integrated model of regulation of placental nutrient transport by maternal and placental factors in IUGR.