Heart and extra-embryonic mesodermal defects in mouse embryos lacking the bHLH transcription factor Hand1

The basic helix-loop-helix (bHLH) transcription factors, Hand1 and Hand2 (refs 1,2), also called eHand/Hxt/Thing1 and dHand/Hed/Thing2 (refs 3,4), respectively, are expressed in the heart and certain neural-crest derivatives during embryogenesis. In addition, Hand1 is expressed in extraembryonic membranes, whereas Hand2 is expressed in the deciduum. Previous studies have demonstrated that Hand2 is required for formation of the right ventricle of the heart and the aortic arch arteries. We have generated a germline mutation in the mouse Hand1 gene by replacing the first coding exon with a beta-galactosidase reporter gene. Embryos homozygous for the Hand1 null allele died between embryonic days 8.5 and 9.5 and exhibited yolk sac abnormalities due to a deficiency in extraembryonic mesoderm. Heart development was also perturbed and did not progress beyond the cardiac-looping stage. Our results demonstrate important roles for Hand1 in extraembryonic mesodermal and heart development.

Epidermal growth factor induces differentiation and secretion of human chorionic gonadotropin and placental lactogen in normal human placenta

Human trophoblast differentiates by the fusion of cytotrophoblasts to form syncytiotrophoblast. To determine factors controlling this process, the effects of epidermal growth factor (EGF) on trophoblast differentiation were studied using long term serum-free culture of isolated trophoblast. Only trophoblast was present in the cultures, as demonstrated by positive immunoperoxidase staining with beta hCG, cytokeratin, and trophoblast-specific H315 monoclonal antisera and by the absence of contaminating endothelial cells, fibroblasts, and macrophages, as shown by negative staining with vimentin and OKM1 monoclonal antisera. EGF induced large sustained increases in hCG and human placental lactogen (hPL) secretion in a dose-dependent manner. The minimum effective dose was 0.1 ng/mL, and the maximum effective dose was 1 ng/mL. Light and electron microscopic studies showed EGF-induced differentiation of cytotrophoblast to form syncytiotrophoblast. DNA content and cell number did not change during the process. The formation of syncytia thus probably accounted for the increase in hCG and hPL secretion. We conclude that EGF causes morphological differentiation, but not cell proliferation, of trophoblasts, and the differentiation results in increased hCG and hPL secretion from the syncytia.

Determinants of Maternal Insulin Resistance during Pregnancy: An Updated Overview

Insulin resistance changes over time during pregnancy, and in the last half of the pregnancy, insulin resistance increases considerably and can become severe, especially in women with gestational diabetes and type 2 diabetes. Numerous factors such as placental hormones, obesity, inactivity, an unhealthy diet, and genetic and epigenetic contributions influence insulin resistance in pregnancy, but the causal mechanisms are complex and still not completely elucidated. In this review, we strive to give an overview of the many components that have been ascribed to contribute to the insulin resistance in pregnancy. Knowledge about the causes and consequences of insulin resistance is of extreme importance in order to establish the best possible treatment during pregnancy as severe insulin resistance can result in metabolic dysfunction in both mother and offspring on a short as well as long-term basis.

Structural and mechanistic studies on the inhibition of the hypoxia-inducible transcription factor hydroxylases by tricarboxylic acid cycle intermediates

In humans both the levels and activity of the alpha-subunit of the hypoxia-inducible transcription factor (HIF-alpha) are regulated by its post-translation hydroxylation as catalyzed by iron- and 2-oxoglutarate (2OG)-dependent prolyl and asparaginyl hydroxylases (PHD1-3 and factor-inhibiting HIF (FIH), respectively). One consequence of hypoxia is the accumulation of tricarboxylic acid cycle intermediates (TCAIs). In vitro assays were used to assess non-2OG TCAIs as inhibitors of purified PHD2 and FIH. Under the assay conditions, no significant FIH inhibition was observed by the TCAIs or pyruvate, but fumarate, succinate, and isocitrate inhibited PHD2. Mass spectrometric analyses under nondenaturing conditions were used to investigate the binding of TCAIs to PHD2 and supported the solution studies. X-ray crystal structures of FIH in complex with Fe(II) and fumarate or succinate revealed similar binding modes for each in the 2OG co-substrate binding site. The in vitro results suggest that the cellular inhibition of PHD2, but probably not FIH, by fumarate and succinate may play a role in the Warburg effect providing that appropriate relative concentrations of the components are achieved under physiological conditions.

Targeted deletion of the PRL receptor: effects on islet development, insulin production, and glucose tolerance

PRL and placental lactogen (PL) stimulate beta-cell proliferation and insulin gene transcription in isolated islets and rat insulinoma cells, but the roles of the lactogenic hormones in islet development and insulin production in vivo remain unclear. To clarify the roles of the lactogens in pancreatic development and function, we measured islet density (number of islets/cm(2)) and mean islet size, beta-cell mass, pancreatic insulin mRNA levels, islet insulin content, and the insulin secretory response to glucose in an experimental model of lactogen resistance: the PRL receptor (PRLR)-deficient mouse. We then measured plasma glucose concentrations after ip injections of glucose or insulin. Compared with wild-type littermates, PRLR-deficient mice had 26-42% reductions (P < 0.01) in islet density and beta-cell mass. The reductions in islet density and beta-cell mass were noted as early as 3 wk of age and persisted through 8 months of age and were observed in both male and female mice. Pancreatic islets of PRLR-deficient mice were smaller than those of wild-type mice at weaning but not in adulthood. Pancreatic insulin mRNA levels were 20-30% lower (P < 0.05) in adult PRLR-deficient mice than in wild-type mice, and the insulin content of isolated islets was reduced by 16-25%. The insulin secretory response to ip glucose was blunted in PRLR-deficient males in vivo (P < 0.05) and in isolated islets of PRLR-deficient females and males in vitro (P < 0.01). Fasting blood glucose concentrations in PRLR-deficient mice were normal, but glucose levels after an ip glucose load were 10-20% higher (P < 0.02) than those in wild-type mice. On the other hand, the glucose response to ip insulin was normal. Our observations establish a physiologic role for lactogens in islet development and function.

β-Cell adaptation in pregnancy

Pregnancy in placental mammals places unique demands on the insulin-producing β-cells in the pancreatic islets of Langerhans. The pancreas anticipates the increase in insulin resistance that occurs late in pregnancy by increasing β-cell numbers and function earlier in pregnancy. In rodents, this β-cell expansion depends on secreted placental lactogens that signal through the prolactin receptor. Then at the end of pregnancy, the β-cell population contracts back to its pre-pregnancy size. In the current review, we focus on how glucose metabolism changes during pregnancy, how β-cells anticipate these changes through their response to lactogens and what molecular mechanisms guide the adaptive compensation. In addition, we summarize current knowledge of β-cell adaptation during human pregnancy and what happens when adaptation fails and gestational diabetes ensues. A better understanding of human β-cell adaptation to pregnancy would benefit efforts to predict, prevent and treat gestational diabetes.

Hypoxia impairs cell fusion and differentiation process in human cytotrophoblast, in vitro

During human pregnancy, the trophoblast develops from differentiation of cytotrophoblast cells into an endocrine active syncytiotrophoblast. In culture, isolated mononuclear cytotrophoblasts aggregate and then fuse to form a syncytium, reproducing the in vivo process. In this study, we examined the effect of low oxygen tension (approximately 9%, hypoxia) compared to standard conditions (approximately 19% oxygen, normoxia) on these cellular events. Under hypoxia, syncytial formation was less frequently observed, cell staining and electron microscopy revealed that cytotrophoblasts remain aggregated, with a positive proliferative cell nuclear antigen (PCNA) immunostaining. Desmoplakin and E-cadherin, both known to disappear with cytotrophoblast fusion, showed persistent expression in hypoxic cells after 3 days of culture. In contrast, the expression of actin and ezrin, two cytoskeletal proteins, was unchanged. hCG secretion and hPL expression were both decreased in hypoxic cells, reflecting a reduced syncytial formation. Thus, on day 3, the mean values for hCG secretion were 1,100 +/- 155 and 289 +/- 26 mlU/mL in normoxic and hypoxic conditions, respectively. The reduced cell fusion process as well as hCG secretion and hPL expression under hypoxia were reversed by reoxygenation of the cells. We conclude that under hypoxia, the formation of functional syncytiotrophoblast is impaired due to a defect in the cytotrophoblast fusion process. This may explain the observation of a higher number of cytotrophoblast cells and a reduced syncytial layer in placentas of some pathological pregnancies.

Placental localization of human pregnancy--associated plasma proteins

Frozen sections of human placenta were examined for the presence of four human pregnancy proteins, pregnancy-associated plasma proteins A and C (PAPP-A and PAPP-C), human chorionic somatomammotropin (hCS), and pregnancy zone protein (PZP), by the indirect immunofluorescence technique. Monospecific rabbit antiserums to PAPP-A, PAPP-C and hCS all stained the trophoblast cytoplasm equivalently in a continuous layer, usggesting that the same trophoblast cells synthesize all three pregnancy proteins. In contrast, PZP was localized in blood vessel walls, parenchymal structures within the villous, as well as in the trophoblast cytoplasm. Its distribution in the latter was relatively inhomogeneous, tending to be more intense on the basement membrane side.

ERK1/2 and p38 regulate trophoblasts differentiation in human term placenta

Mitogen-activated protein kinases (MAPKs) control many cellular events from complex programmes, such as embryogenesis, cell differentiation and proliferation, and cell death, to short-term changes required for homeostasis and acute hormonal responses. However, little is known about expression and activation of classical MAPKs, extracellular signal-regulated kinase1/2 (ERK1/2) and p38 in human placenta. Therefore, we examined the expression of ERK1/2 and p38 in trophoblasts from human term placenta, and their implication in differentiation. In vitro, freshly isolated cytotrophoblast cells, cultivated in 10% fetal bovine serum (FBS), spontaneously aggregate and fuse to form multinucleated cells that phenotypically resemble mature syncytiotrophoblasts, that concomitantly produce human chorionic gonadotropin (hCG) and human placental lactogen (hPL). This study shows that the level of ERK1/2 and p38 decreases with increasing days of culture, to reach an undetectable level after 5 days of culture. Moreover, pretreatment of cells with an ERK1/2-specific inhibitor (PD98059) and/or a p38-specific inhibitor (SB203580) suppressed trophoblast differentiation. Our results also demonstrate that the p38 pathway is highly solicited as compared to the ERK1/2 pathway in the differentiation process. Furthermore, ERK1/2 and p38 are rapidly activated upon addition of FBS, but the activation of p38 is delayed compared to that of ERK1/2. In summary, this study showed that ERK1/2 and p38 pathways are essential to mediate initiation of trophoblast differentiation.

Demonstration of functional cytokine-placental interactions: CSF-1 and GM-CSF stimulate human cytotrophoblast differentiation and peptide hormone secretion

The placental syncytiotrophoblast (ST) is a terminally differentiated epithelial cell monolayer that constitutes the outermost boundary between fetal and maternal tissues and performs a variety of synthetic, secretory, and transport functions essential for the maintenance of pregnancy. Although it is known that the ST arises from the underlying germinal layer of mononuclear cytotrophoblasts (Langhans' cells) by a process of cell fusion, the molecular mechanisms involved in this process are unclear. In order to address this question, we have investigated the effects of macrophage colony-stimulating factor (CSF-1) and granulocyte-macrophage colony-stimulating factor (GM-CSF), lymphohemopoietic cytokines implicated in mammalian placental development, on the in vitro morphological and functional differentiation of human trophoblast. Both CSF-1 and GM-CSF stimulated cytotrophoblast aggregation into large multinucleated structures composed of extensive patches of syncytium interspersed with mononuclear cells. Concomitant with this morphological differentiation was upregulation of the production of the placental hormones placental lactogen and chorionic gonadotrophin. Placental fibroblasts derived from the villous stroma that underlies the trophoblastic epithelium were found to produce both GM-CSF and CSF-1 under the control of the trophoblast-derived cytokines IL-1 and TNF alpha. These observations suggest that a network of interrelated cytokines operates within the basal (fetal) aspects of the villous stroma where they are situated to play a significant role in the morphological and functional development of the human placenta.

Clinical counterpoint: the physiology of placental lactogen in human pregnancy

In summary, current evidence strongly suggests that PL may play a pivotal role during pregnancy, acting through distinct PL receptors to regulate and coordinate growth and metabolism in the mother and fetus. In early and midgestation, PL may be secreted preferentially into the fetal circulation, exerting growth-promoting effects at a time when the rate of linear growth of the fetus is maximal. Subsequently, during the latter half of pregnancy, the metabolic actions of PL in the mother and fetus may predominate, ensuring the optimal supply of nutrients to the fetus and utilization of the nutrients by fetal tissues. It therefore appears that PL affects fetal growth both by exerting effects on the fetus and the mother. Although hPL acts as "growth hormone of pregnancy," the regulation of the synthesis and secretion of hPL appears to be markedly different than that of GH.

Placental site nodule and characterization of distinctive types of intermediate trophoblast

Both placental site nodule and exaggerated placental site are described as being composed of intermediate trophoblast (IT), yet their morphological features and clinical presentation differ significantly. This study was undertaken to evaluate the morphological and immunohistochemical features of trophoblastic cells in placental site nodules and compare them with the trophoblastic cells in exaggerated placental sites as well as in different anatomic locations in the developing placenta to evaluate these differences. Forty-two placental site nodules, 20 abortus specimens ranging from 3 to 13 weeks, 8 second- and 10 third-trimester placentas, and 12 exaggerated placental sites were studied by conventional light microscopy and immunohistochemistry. This analysis showed that the trophoblastic cells in the placental site nodule closely resemble those in the chorion laeve. We have designated these cells "chorionic-type IT cells." They are composed of two populations of cells, one with eosinophilic and the other with clear (glycogen-rich) cytoplasm. The eosinophilic cells tended to be larger with more pleomorphic nuclei, whereas the clear cells were smaller with more uniform nuclei. Chorionic-type IT cells in the chorion laeve and placental site nodule were diffusely positive for placental alkaline phosphatase but were only focally positive or negative for human placental lactogen (hPL), Mel-CAM (CD146), and oncofetal fibronectin. In contrast, hPL, Mel-CAM, and oncofetal fibronectin were diffusely expressed in IT cells in the placental site, both normal and exaggerated. The chorionic-type IT cells in placental site nodule and chorion laeve showed mild proliferative activity as indicated by an increased Ki-67 labeling index (3% to 10%). In contrast, the Ki-67 labeling index in normal and exaggerated implantation sites was zero. The morphological and immunohistochemical features of chorionic-type IT cells contrast with the IT cells in the implantation site that we have designated "implantation site IT cells." Both types of IT cells develop from a population of trophoblastic cells in the trophoblastic columns that we have tentatively termed "villous IT cells." Four of 42 placental site nodules were larger (>5 mm) than the remainder and showed transitional features between a typical placental site nodule and an epithelioid trophoblastic tumor, a recently described distinctive gestational trophoblastic tumor. There were no recurrences among the placental site nodules regardless of size. All placental site nodules were immunoreactive for inhibin-alpha and cytokeratin 18, whereas 33 squamous cell carcinomas of the cervix, which can at times be confused with placental site nodules, were negative. In conclusion, there appear to be three subpopulations of IT cells with distinctive morphological and immunohistochemical features. Different subpopulations can be related to different trophoblastic lesions: implantation site IT cells to an exaggerated placental site and its neoplastic counterpart, placental site trophoblastic tumor and chorionic-type IT cells to a placental site nodule and its neoplastic counterpart, epithelioid trophoblastic tumor.

Ectopic production of chorionic gonadotropin and its subunits by islet-cell tumors. A specific marker for malignancy

We measured human chorionic gonadotropin (hCG) and its subunits in 76 patients with islet-cell tumors. Seventeen of 27 patients with functioning islet-cell carcinomas had elevated plasma levels of hCG or one of its subunits (hCG-alpha and hCG-beta). Secretion was often discordant; the most frequent finding was an elevated level of hCG-alpha alone. In one patient responding to streptozocin, changes in hCG-alpha correlated with the clinical course. Studies of tumor extracts suggested that the markers observed in the circulation were being produced in the tumor itself. In contrast, none of the 43 patients with benign disease or the six patients with nonfunctioning malignant tumors had elevated levels of hCG, hCG-alpha or hCG-beta. These data show that hCG and its subunits are prevalent and specific markers for islet-cell carcinoma, and suggest that ectopic secretion results from malignant derepression of the genome rather than overproduction by an aberrant "cell rest."

Termination at midpregnancy of the two daily surges of plasma prolactin initiated by mating in the rat

The pattern of plasma prolactin following uterine cervical stimulation consists of two surges each day, one nocturnal, occurring between 0100-0900 h (lights on 0600-1800 h), and one diurnal, occurring between 1500-2100 h. This pattern of prolactin continued throughout pseudopregnancy; the last surge was observed on the morning of day 11 (day 1 was taken as the first day of diestrus of pregnancy or pseudopregnancy). Prolactin levels remained low thereafter until the spontaneous proestrous surge on the afternoon of day 12, signalling the onset of a new estrous cycle. In contrast, the two daily prolactin surges did not continue throughout pregnancy, and in fact, were terminated sooner in pregnant animals than in pseudopregnant animals. The last diurnal surge was observed on day 8 while the last nocturnal surge was observed on day 10. The early termination of prolactin surges during pregnancy correlated with the increased secretion of rat placental lactogen. However, placental extracts obtained from day 11 of pregnancy and injected in large doses failed to inhibit prolactin surges in pseudopregnant animals. Prolactin surges also continued for a longer period of time in pseudopregnant rats bearing decidualized uteri than in pregnant animals. Thus, the two major components of pregnancy that differed from pseudopregnancy, that is, the presence of rat placental lactogen or decidual tissue, did not appear to account for the early termination of prolactin surges during pregnancy.

The phagocytic activity of human first trimester extravillous trophoblast

It has been suggested previously that phagocytic activity in the human placenta is confined to cells of the macrophage lineage. However, earlier studies were hampered by the paucity and poor viability of cells inherent in primary trophoblast cell cultures, contamination by other cell types which themselves have phagocytic activity, lack of reliable markers of trophoblasts, and by limitations of methods available to demonstrate unequivocally the internalization of particulate material. We have overcome these limitations by using: (i) DNA transfection to provide unlimited supplies of pure trophoblast cell lines; (ii) human placental lactogen as a marker unique to trophoblast; and (iii) confocal microscopy to demonstrate unequivocally the intracellular locality of phagocytosed material. We found that both untransfected primary culture extravillous trophoblast cells, as well as the cell lines, had the capacity to phagocytose sheep red blood cells, Staphylococcus aureus and baker's yeast cells, and that this activity was inhibited by cytochalasin B and by culture at 4 degrees C. Phagocytic activity in trophoblast cells was less avid than that seen in a professional phagocyte. In physiological and pathological situations where tissue remodelling occurs, such as the rapid turnover in the periodontal ligament or during inflammation, epithelial cells and other cells that are not considered professional phagocytes actively phagocytose components of the extracellular matrix. We postulate that phagocytosis by human trophoblasts may play an important role in the extensive tissue remodelling that occurs during trophoblastic invasion of the decidua.

Lactogenic hormone regulation of maternal behavior

Biological factors can profoundly affect a mother's response to her young. For example, it is well known that the hormones of pregnancy act on the maternal brain to stimulate the spontaneous onset of maternal behavior at parturition. Studies in the rat have provided an excellent model to investigate maternal behavior in mammals, since maternal behavior in rats is easily observable and readily quantifiable and it is well-documented that the endocrine state of gestation helps to bring about the onset of maternal behavior around the time of birth. The same response in virgin animals requires a number of days of constant exposure to pups before maternal-like behaviors emerge. To date, research has established that the steroid hormones, estradiol and progesterone, and the lactogenic hormones, prolactin and the placental lactogens, act in concert to stimulate maternal behavior in the pregnant female. Treatment of adult, virgin rats with these hormones can stimulate a rapid onset of maternal care. In the present chapter experiments are described that demonstrate key roles for prolactin and placental lactogens in the onset of maternal behavior. Central sites of action of prolactin and placental lactogens, including the medial preoptic area, appear to be involved in stimulating the onset of maternal care. Other studies are discussed which support the involvement of the prolactin receptor in the endocrine regulation of maternal behavior using prolactin receptor antagonist and 'knock-out' models in rats and mice, respectively. Overall, these studies indicate that during pregnancy the endocrine system primes the mother's brain so that the new mother displays appropriate and successful behaviors toward her newborn at parturition.

Behavioural significance of prolactin signalling in the central nervous system during pregnancy and lactation

The role of prolactin in the regulation of mammary gland development and function during pregnancy and lactation is well established. However, in addition, prolactin appears to have a much wider role in the physiology of lactation. There is widespread expression of prolactin receptors in the hypothalamus during lactation, indicative of a multi-faceted role for prolactin in regulating hypothalamic function. During pregnancy and lactation, the maternal brain undergoes structural and functional modification, allowing the establishment of appropriate behaviour to feed and nurture the offspring, to adjust to the nutritional and metabolic demands of milk production, and to maintain appropriate hormone secretion to allow milk synthesis, secretion and ejection. The coordination of such a range of neurobiological and neuroendocrine adaptations requires an endocrine signalling mechanism, capable of communicating the reproductive state to the brain. Evidence indicates that prolactin is part of this mechanism.

Screening for fetal growth restriction using fetal biometry combined with maternal biomarkers

Fetal growth restriction is a major determinant of perinatal morbidity and mortality. Screening for fetal growth restriction is a key element of prenatal care but it is recognized to be problematic. Screening using clinical risk assessment and targeting ultrasound to high-risk women is the standard of care in the United States and United Kingdom, but the approach is known to have low sensitivity. Systematic reviews of randomized controlled trials do not demonstrate any benefit from universal ultrasound screening for fetal growth restriction in the third trimester, but the evidence base is not strong. Implementation of universal ultrasound screening in low-risk women in France failed to reduce the risk of complications among small-for-gestational-age infants but did appear to cause iatrogenic harm to false positives. One strategy to making progress is to improve screening by developing more sensitive and specific tests with the key goal of differentiating between healthy small fetuses and those that are small through fetal growth restriction. As abnormal placentation is thought to be the major cause of fetal growth restriction, one approach is to combine fetal biometry with an indicator of placental dysfunction. In the past, these indicators were generally ultrasonic measurements, such as Doppler flow velocimetry of the uteroplacental circulation. However, another promising approach is to combine ultrasonic suspicion of small-for-gestational-age infant with a blood test indicating placental dysfunction. Thus far, much of the research on maternal serum biomarkers for fetal growth restriction has involved the secondary analysis of tests performed for other indications, such as fetal aneuploidies. An exemplar of this is pregnancy-associated plasma protein A. This blood test is performed primarily to assess the risk of Down syndrome, but women with low first-trimester levels are now serially scanned in later pregnancy due to associations with placental causes of stillbirth, including fetal growth restriction. The development of "omic" technologies presents a huge opportunity to identify novel biomarkers for fetal growth restriction. The hope is that when such markers are measured alongside ultrasonic fetal biometry, the combination would have strong predictive power for fetal growth restriction and its related complications. However, a series of important methodological considerations in assessing the diagnostic effectiveness of new tests will have to be addressed. The challenge thereafter will be to identify novel disease-modifying interventions, which are the essential partner to an effective screening test to achieve clinically effective population-based screening.

Lactogenic and growth hormone-like activities in pregnancy determined by radioreceptor assays

Factors with growth hormone and lactogenic activities have been identified in serum samples and placental extracts of 9 species using a radioreceptor assay for prolactin or lactogens (RRA-PRL) and one for growth hormone-like activity (RRA-GH). The concentrations of placental lactogen (PL) and growth hormone-like activity (GHLA) in pregnancy were different in each species. The levels of PL began to rise at or before mid-pregnancy and either remained elevated until term (hamster, goat, sheep, monkey, and human), declined gradually after reaching peak concentrations just beyond mid-pregnancy (guinea pig), or had 2 peaks of activity (mouse, rat). The peak concentrations in different species ranged from 350 ng/ml in the cow to 45,000 ng/ml in monkey serum samples. The concentrations of GHLA were similar to PL levels in the guinea pig, goat, cow, and monkey but were lower than PL levels in the sheep and human. The maximal concentrations of PL and GHLA in placental extracts varied from 1 mug/g in guinea pigs to 200 mug/g in sheep. The ratio of lactogenic/growth hormone-like activity of both serum and placental extracts varied greatly among the various species. Gel filtration studies of serum samples revealed that PL and GHLA in the hamster, mouse and rat had different elution volumes, whereas, in the remaining species, the two activities co-eluted.

Real-time kinetic measurements of the interactions between lactogenic hormones and prolactin-receptor extracellular domains from several species support the model of hormone-induced transient receptor dimerization

Interactions of recombinant soluble prolactin receptors-extracellular domains (PRLR-ECDs) from rabbit, rat, and cow and human growth hormone receptor ECD with immobilized human growth hormone, several prolactins, and bovine placental lactogen were studied utilizing surface plasmon resonance. This method enables real-time kinetic measurements of the interactions and calculations of kinetic constants and of the stoichiometry of interaction, even in cases where only transient interactions occur. In contrast to gel filtration or crystallographic studies, where in most cases the interaction of PRLR-ECDs with various lactogenic hormones indicated formation of 1:1 complexes, our surface plasmon resonance experiments indicated in all cases the transient formation of a 2:1 complex. In most of the interactions the 2:1 complex was very unstable and underwent rapid dissociation to a 1:1 complex. This situation was particularly characteristic of homologous interactions involving hormone and receptor from the same species and was mainly attributed to increased dissociation constants. We suggest that as in the case of growth hormone PRLR activation occurs via hormone-induced transient homodimerization of the receptor, lasting only a few seconds, and that this may be sufficient to initiate the biological signal. Once the signal is initiated, the receptor dimer is no longer required. Its rapid dissociation to a 1:1 complex or to its components may even be advantageous in that it permits activation of additional receptors.

Induction of central leptin resistance in hyperphagic pseudopregnant rats by chronic prolactin infusion

Pregnancy in rats is associated with hyperphagia, increased fat deposition, and elevated plasma leptin concentrations. Elevated leptin would be expected to inhibit food intake, but hypothalamic leptin resistance develops around midpregnancy, allowing hyperphagia to be maintained and excess energy to be stored as fat in preparation for future metabolic demands of lactation. To investigate the hormonal mechanisms inducing leptin resistance during pregnancy, the anorectic response to leptin was examined during pseudopregnancy. Pseudopregnant rats have identical hormonal profiles to early pregnancy, but no placenta formation, allowing differentiation of maternal and placental hormone effects on appetite. To investigate the effect of leptin on food intake, d-9 pseudopregnant rats were injected with leptin (4 microg) via an intracerebroventricular (icv) cannula, and then food intake was measured 24 h later. Pseudopregnant rats were hyperphagic but had normal anorectic responses to leptin. We therefore hypothesized that a longer exposure time to high concentrations of progesterone might be required to mimic the leptin resistance that occurs on d 14 of pregnancy. Pseudopregnant rats were given progesterone to prolong pseudopregnancy beyond the time that leptin resistance develops during pregnancy. However, rats remained responsive to icv leptin. To model the placental lactogen secretion that occurs during pregnancy, pseudopregnant rats were given progesterone and chronic icv ovine prolactin infusion. Central icv injection of leptin had no effect on food intake in pseudopregnant rats receiving chronic ovine prolactin. These results suggest that chronically high lactogen levels, secreted by the placenta during the second half of pregnancy, induce central leptin resistance.