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.

The influence of gestational zinc deficiency on the fetal insulin-like growth factor axis in the rat

The insulin-like growth factor (IGF) axis, a key regulator of embryonic growth and development, is exquisitely sensitive to the nutrient status of the animal. In addition to macronutrient deficiencies, zinc deficiency can impact the IGF axis. Gestational zinc deficiency is teratogenic, resulting in intrauterine growth retardation and structural abnormalities. The aim of this study was to investigate the effects of gestational zinc deficiency on the fetal IGF axis in a rat model. From gestation day (GD) 0.5, dams consumed zinc-deficient (ZD, 0.3 mg zinc/kg) or control (25 mg zinc/kg) diet ad libitum, while a third group of dams consumed the control diet in amounts equivalent to the food intake of the ZD dams (Paired group). On GD 19.5 fetal tissue, blood and amniotic fluid were collected. Fetal growth was significantly reduced by zinc deficiency compared with the Paired and Control groups. Fetuses from the Paired group were smaller compared with the Control, but only ZD fetuses had structural malformations. Amniotic fluid IGF-1 concentrations were significantly lower in the Paired group than in the ZD and Control groups. Plasma of ZD fetuses contained lower levels of IGF binding protein-1 when compared with fetuses in the Paired and Control groups. Fetal liver IGF-1 mRNA levels were lower in the ZD fetuses than in the Paired and Control fetuses. These observations suggest that differences in the fetal IGF axis between ZD and Paired groups contribute to the poor pregnancy outcome and enhanced fetal growth retardation observed with zinc deficiency.

Markers of insulin sensitivity in placentas and cord serum of intrauterine growth-restricted newborns

OBJECTIVE

Intrauterine growth restriction (IUGR) has been related to a higher incidence of insulin resistance in adult life, which is associated with low adiponectin and high resistin, insulin and interleukin (IL)-6 serum concentrations. This study assessed cortisol, insulin, total insulin receptor, resistin, adiponectin and IL-6 concentrations, as markers of insulin sensitivity, in placental lysates and cord serum of IUGR and appropriate for gestational age (AGA) newborns, to establish relationships among peptides and with growth parameters at birth.

DESIGN AND PATIENTS

Whole villous tissue and cord serum at birth were collected from 24 AGA and 18 IUGR newborns of comparable gestational age.

MEASUREMENTS

Hormonal and peptide concentrations were assayed in placental lysates and cord serum using specific commercial kits. Concentrations in lysates were adjusted per mg of total protein content.

RESULTS

Cortisol, insulin and resistin concentrations and the total amount of insulin receptor were similar in both groups. IL-6 concentration in lysates was significantly higher in IUGR compared with AGA newborns. Adiponectin was significantly lower in lysates from IUGR compared with AGA newborns. Placental insulin and resistin concentrations were positively correlated. Placental adiponectin concentration was positively correlated with the weight of the placenta, birthweight and head circumference. IL-6 concentration in lysates was negatively correlated with birth length, birthweight and head circumference.

CONCLUSIONS

This study evaluated the markers of insulin sensitivity in the placentas of subjects born IUGR, showing new potential roles for adiponectin and IL-6 in particular, and suggesting a role for the placenta in the programming of these hormones.

Abnormalities of IGF-I signaling in the pathogenesis of diseases of the bone, brain, and fetoplacental unit in humans

IGF-I action is essential for the regulation of tissue formation and remodeling, bone growth, prenatal growth, brain development, and muscle metabolism. Cellular effects of IGF-I are mediated through the IGF-I receptor, a transmembrane tyrosine kinase that phosphorylates intracellular substrates, resulting in the activation of multiple intracellular signaling cascades. Dysregulation of IGF-I actions due to impairment in the postreceptor signaling machinery may contribute to multiple diseases in humans. This article will review current information on IGF-I signaling and illustrate recent results demonstrating how impaired IGF-I signaling and action may contribute to the pathogenesis of human diseases, including osteoporosis, neurodegenerative disorders, and reduced fetal growth in utero.

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.

Effects of uteroplacental restriction on the relaxin-family receptors, Lgr7 and Lgr8, in the uterus of late pregnant rats

The peptide hormone relaxin stimulates uterine growth and endometrial angiogenesis and inhibits myometrial contractions in a variety of species. The receptor for relaxin is a leucine-rich repeat containing G-protein-coupled receptor Lgr7 (RXFP1) that is highly expressed in the myometrium of late pregnant mice, with a significant decrease in receptor density observed at term. The present study first compared the expression of Lgr7 with another relaxin-family receptor Lgr8 (RXFP2) in the uterus and placenta of late pregnant rats. The uterus was separated into endometrial and myometrial components, and the myometrium into fetal and non-fetal sites, for further analysis. We then assessed the response of these receptors to uteroplacental restriction (UPR). Expression of the Lgr7 gene was significantly higher in the uterus compared with the placenta. Within the uterus, on Day 20 of gestation, there was equivalent expression of Lgr7 in fetal and non-fetal sites of the myometrium, as well as in the endometrium v. myometrium. The second receptor investigated, Lgr8, was also expressed in the endometrium and myometrium, but at significantly lower levels than Lgr7. Bilateral ligation of the maternal uterine blood vessels on Day 18 of gestation resulted in uteroplacental restriction, a decrease in fetal weight and litter size, and a significant upregulation in uterine, but not placental, Lgr7 and Lgr8 gene expression in UPR animals compared with controls. These data suggest that both relaxin family receptors are upregulated in response to a reduction in uteroplacental blood flow in rats.

Uteroplacental restriction in the rat impairs fetal growth in association with alterations in placental growth factors including PTHrP

During pregnancy, parathyroid hormone-related protein (PTHrP) is one of many growth factors that play important roles to promote fetal growth and development, including stimulation of placental calcium transport. Angiotensin II, acting through the AT1a receptor, is also known to promote placental growth. We examined the effects of bilateral uterine artery and vein ligation (restriction), which mimics placental insufficiency in humans, on growth, intrauterine PTHrP, placental AT1a, and pup calcium. Growth restriction was surgically induced on day 18 of pregnancy in Wistar-Kyoto female rats by uterine vessel ligation. Uteroplacental insufficiency reduced fetal body weight by 15% and litter size ( P < 0.001) compared with the control rats with no effect on placental weight or amniotic fluid volume. Uteroplacental insufficiency reduced placental PTHrP content by 46%, with increases in PTHrP (by 2.6-fold), parathyroid hormone (PTH)/PTHrP receptor (by 11.6-fold), and AT1a (by 1.7-fold) relative mRNA in placenta following restriction compared with results in control ( P < 0.05). There were no alterations in uterine PTHrP and PTH/PTHrP receptor mRNA expression. Maternal and fetal plasma PTHrP and calcium concentrations were unchanged. Although fetal total body calcium was not altered, placental restriction altered perinatal calcium homeostasis, as evidenced by lower pup total body calcium after birth ( P < 0.05). The increased uterine and amniotic fluid PTHrP ( P < 0.05) may be an attempt to compensate for the induced impaired placental function. The present study demonstrates that uteroplacental insufficiency alters intrauterine PTHrP, placental AT1a expression, and perinatal calcium in association with a reduction in fetal growth. Uteroplacental insufficiency may provide an important model for exploring the early origins of adult diseases.