A placental clock controlling the length of human pregnancy

Activation of L-type calcium channels induces corticotropin-releasing factor secretion from human placental trophoblasts

The ultimate outcome of pregnancy, parturition, is a well orchestrated process in which placental corticotropin-releasing factor (CRF) seems to play an important role. The objective of the present study was to investigate the involvement of L-type calcium channels and calcium-dependent signaling in the depolarization-induced CRF release from human syncytiotrophoblast. The basal secretion of CRF by trophoblastic cells, isolated from human term placenta, was maximal after their functional differentiation, which was monitored by hCG measurements. On the fourth day of culture, the basal CRF secretion of the cells in serum-free medium was linear between 2 and 8 h. Incubation of the trophoblasts with KCl, a depolarizing stimulus, or with Bay K8644, an L-type calcium channel agonist, for 3 or 8 h led to an increase in CRF secretion, but was without effect on its synthesis. This stimulated CRF release was calcium dependent, as it could be prevented by loading cells with 1,2-bis(0-Aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl) ester. Furthermore, the KCl-induced CRF secretion involved L-type calcium channels activation, as 2 micromol/L nitrendipine, an L-type specific blocker, abolished the stimulation. In trophoblasts, where we have previously shown calcium-dependent protein kinase C (cPKCs) activity, incubation with Bay K8644 also stimulated calcium calmodulin kinase II (CaMKII) and extracellular regulated kinase activities. In the present study we observed that CaMKII and cPKCs were linked to the Bay K8644-induced secretion of CRF, as only the autocamtide-2 related inhibitory peptide, a CaMKII inhibitor, and Gö6976, an inhibitor of mu and cPKCs partially prevented (30-78%) the activation of CRF release by Bay K8644. The use of PD 098056, an inhibitor of the ERKs kinases, showed no effect on CRF release. Taken together, these results support a depolarization-induced and calcium-dependent exocytotic-like secretion of CRF from human placental trophoblasts. In addition, CaMKII and cPKCs seem to be potential modulators or mediators of these calcium effects on CRF secretion.

Corticotropin-releasing hormone, corticotropin-releasing hormone-binding protein, and activin A in maternal serum: prediction of preterm delivery and response to glucocorticoids in women with symptoms of preterm labor

OBJECTIVE

The aim of this study was to determine prospectively whether serum concentrations of corticotropin-releasing hormone, corticotropin-releasing hormone-binding protein, and activin A (1) predict preterm birth within 10 days of hospital admission or at <37 weeks' gestation among women with symptoms of preterm labor and (2) are affected by glucocorticoid therapy.

STUDY DESIGN

Serum concentrations of corticotropin-releasing hormone and activin A were measured in 94 women with symptoms of preterm labor between 24 and 34 weeks' gestation, and delivery outcomes were monitored. Corticotropin-releasing hormone-binding protein concentrations were measured in 71 of these women. In a subgroup of 15 women the serum analytes were assayed in conjunction with estriol before and 12 to 24 hours after administration of dexamethasone.

RESULTS

Forty-six percent (6/13) of the women who were delivered within 10 days of hospital admission had a raised serum corticotropin-releasing hormone level, but the predictive relationship was not significant (chi(2) = 1.7; P =.2). Among the 31 women (including the 6 previously mentioned) who were delivered at <37 weeks' gestation, 39% (12/31) had a raised corticotropin-releasing hormone level. Although a raised corticotropin-releasing hormone concentration was positively associated with delivery at <37 weeks' gestation (chi(2) = 9; P =.003), the predictive diagnostic value was poor, with sensitivity, specificity, and positive and negative predictive values of 39%, 90%, 67%, and 75%, respectively. The serum concentrations of corticotropin-releasing hormone-binding protein and activin A were unrelated to gestational age at delivery. Dexamethasone markedly lowered the serum estriol level (P <.001) but had no effect on concentrations of corticotropinreleasing hormone, corticotropin-releasing hormone-binding protein, and activin A.

CONCLUSION

Serum concentrations of corticotropin-releasing hormone, corticotropin-releasing hormone-binding protein, and activin A are not clinically useful for the prediction of preterm delivery among women with symptoms of preterm labor and are not affected by administration of glucocorticoids.

Maternal serum total activin A and follistatin in pregnancy and parturition

OBJECTIVE

To examine changes in maternal serum levels of activin A and follistatin during pregnancy and labour.

DESIGN

In three cross sectional and three longitudinal studies venous blood was collected from women during pregnancy, spontaneous labour, labour induction and prior to elective caesarean section for the measurement of activin A and follistatin.

SETTING

Monash Medical Centre, Clayton, Victoria, Australia.

POPULATION

One hundred and twenty-three women participated in a cross sectional study in pregnancy, 18 women in two longitudinal pregnancy studies, 36 women in a cross sectional labour study, nine women in a longitudinal study of labour induction. Ten women undergoing elective caesarean section were also studied.

METHODS

Activin A and follistatin were measured using two sensitive and specific enzyme-linked immunosorbent assays.

RESULTS

In the cross sectional study of pregnancy, mean (SEM) maternal serum activin A and follistatin levels increased towards term (2.4 ng/mL (0.3) and 1.8 ng/mL (0.3) in first trimester to 18.9 ng/mL (3.8) and 5.3 ng/mL (0.9) at term, respectively), but the longitudinal study revealed that levels plateau in the last three weeks of pregnancy (16.0 ng/mL (2.6) and 6.2 ng/mL (1.4) at 37 weeks and 16.6 ng/mL (3.5) and 6.2 ng/mL (0.5) before labour for activin A and follistatin, respectively). There was no difference in levels of activin A and follistatin between women delivered by caesarean section and labouring women at term (14.9 ng/mL (2.8) vs 11.0 ng/mL (0.93) and 5.95 ng/mL (0.67) vs 5.71 ng/mL (0.63), respectively) and levels of both proteins did not alter throughout spontaneous or induced labour.

CONCLUSIONS

We believe that these data argue against activin A playing an acute role in the initiation or regulation of human parturition.

Characterization of neuropeptide Y-mediated corticotropin-releasing factor synthesis and release from human placental trophoblasts

Neuropeptide Y (NPY) is a CRF secretagogue for human placental cells in culture. We have studied the involvement of intracellular calcium and calcium-dependent signaling in the NPY-induced CRF release in trophoblastic cells. The incubation of trophoblasts with NPY for 3 and 8 h led to a dose-dependent increase in CRF secretion. Also, NPY stimulated synthesis of this peptide hormone upon an 8-h incubation period. BIBP3226, a selective Y1 receptor antagonist, and pertussis toxin (PTX) eliminated these effects. NPY-stimulated CRF secretion was mostly prevented by loading cells with BAPTA-AM, suggesting that elevation of intracellular calcium is responsible for the increase of CRF secretion. However, this calcium chelator had no effect on CRF synthesis. Furthermore, U-73122, a phospholipase C-betas (PLC) inhibitor or xestospongin C, an inositol triphosphate receptor (InsP3-R) blocker, have partially prevented the effect of NPY on CRF synthesis and secretion. Therefore, the increase in CRF synthesis and secretion rely in part on the release of calcium from intracellular store. Interestingly, SKF 96365, an inhibitor of store operated calcium (SOC) influx, also partially blocked the NPY stimulatory effect on CRF release but not its synthesis, suggesting that calcium influx is also involved in this stimulation. In the syncytiotrophoblast, known to possess a NPY-activated protein kinase C (PKCs) activity, NPY also stimulated calcium calmodulin kinase II (CaMKII) and extracellular regulated kinase (ERK1/2) activities. In the present study, we observed that bisindolylmaleimide (BIM), a nonspecific PKCs inhibitor partially prevented the NPY-induced CRF release. On the other hand, autocamtide-2 related inhibitory peptide (AIP), a CaMKII inhibitor, prevented most of the stimulatory effect of NPY on both CRF synthesis and release. Go6976, an inhibitor of the conventional and mu PKCs and PD 098059, an inhibitor of the ERK cascade, had no effect on neither CRF synthesis nor release. Altogether, these results support a Y1 receptor-mediated PTX-sensitive induction on CRF synthesis and release by NPY from human placental trophoblasts. The stimulation of CRF synthesis by NPY seems to depend mainly on a PLC-beta to InsP3-R axis and on CaMKII activity. Also, the release of CRF depends on the PLC-beta to InsP3-R axis and CaMKII activity but also entails the participation of a calcium-independent PKCs.

Corticotropin releasing hormone and proopiomelanocortin involvement in the cutaneous response to stress

The skin is a known target organ for the proopiomelanocortin (POMC)-derived neuropeptides alpha-melanocyte stimulating hormone (alpha-MSH), beta-endorphin, and ACTH and also a source of these peptides. Skin expression levels of the POMC gene and POMC/corticotropin releasing hormone (CRH) peptides are not static but are determined by such factors as the physiological changes associated with hair cycle (highest in anagen phase), ultraviolet radiation (UVR) exposure, immune cytokine release, or the presence of cutaneous pathology. Among the cytokines, the proinflammatory interleukin-1 produces important upregulation of cutaneous levels of POMC mRNA, POMC peptides, and MSH receptors; UVR also stimulates expression of all the components of the CRH/POMC system including expression of the corresponding receptors. Molecular characterization of the cutaneous POMC gene shows mRNA forms similar to those found in the pituitary, which are expressed together with shorter variants. The receptors for POMC peptides expressed in the skin are functional and include MC1, MC5 and mu-opiate, although most predominant are those of the MC1 class recognizing MSH and ACTH. Receptors for CRH are also present in the skin. Because expression of, for example, the MC1 receptor is stimulated in a similar dose-dependent manner by UVR, cytokines, MSH peptides or melanin precursors, actions of the ligand peptides represent a stochastic (predictable) nonspecific response to environmental/endogenous stresses. The powerful effects of POMC peptides and probably CRH on the skin pigmentary, immune, and adnexal systems are consistent with stress-neutralizing activity addressed at maintaining skin integrity to restrict disruptions of internal homeostasis. Hence, cutaneous expression of the CRH/POMC system is highly organized, encoding mediators and receptors similar to the hypothalamic-pituitary-adrenal (HPA) axis. This CRH/POMC skin system appears to generate a function analogous to the HPA axis, that in the skin is expressed as a highly localized response which neutralizes noxious stimuli and attendant immune reactions.

Maternal hypothalamic-pituitary-adrenal axis in pregnancy and the postpartum period. Postpartum-related disorders

During pregnancy, placenta-derived CRH increases exponentially in the plasma. Circulating levels of CRH-binding protein decrease considerably in the last trimester of pregnancy, resulting in further elevation of bioavailable plasma CRH. The adrenal glands during pregnancy gradually become hypertrophic because of the increase in ACTH, which parallels that of CRH. Thus, pregnancy is a transient period of relative hypercortisolism. The activation of the hypothalamic-pituitary-adrenal axis during pregnancy has been proposed to function as a biological clock. In this model, the placenta is perceived as a stress-sensitive organ and placental CRH as a timing starter, determining a preterm, term, or postterm labor. During pregnancy, as well as during the immediate postpartum period, the hypothalamic maternal CRH secretion is suppressed, because of the circulating levels of cortisol. Hypothalamic CRH secretion normalizes within 12 weeks. This transient postpartum maternal hypothalamic CRH suppression, together with the steroid withdrawal that follows parturition, might be causally related to the mood disorders and the vulnerability to autoimmune diseases such as thyroiditis or rheumatoid arthritis often observed during the postpartum period.

Placental corticotropin-releasing factor. An update

Corticotropin-releasing factor (CRF) produced in placenta has paracrine effects within placenta, decidua, and myometrium and endocrine effects on mother and fetus. CRF is a potent local regulator of myometrial contractility and of prostaglandin release, Recently, urocortin, a new member of the CRF family, has been localized in human placenta and membranes. Urocortin mimics some of the local effects of CRF in intrauterine tissues, that is, increase of adrenocorticotrophic hormone (ACTH) and prostagiandin release and myometrial contractility. A local CRF-BP modulates the paracrine effects of CRF and urocortin. The various CRF receptor subtypes are well distributed in placenta and membranes. CRH also acts on placental blood vasculature and has an action on fetal adrenal gland to stimulate the production of the steroid DHEA-S. In nonpregnant women, plasma CRF levels are low; they become higher during the first and second trimesters of pregnancy. A clear increase is evident at term and when CRF-BP levels decrease. Women with preterm labor show high CRF and low CRF-BP levels, supporting an involvement of this pathway in mechanism of parturition.

Intrauterine infection and preterm delivery: evidence for activation of the fetal hypothalamic-pituitary-adrenal axis

OBJECTIVE

We studied pregnant women in preterm labor with and without intrauterine infection to determine whether fetal hypothalamic-pituitary-adrenal axis activation occurs in the setting of infection-induced preterm parturition.

STUDY DESIGN

Amniotic fluid collected by amniocentesis and maternal blood from patients in preterm labor with intact membranes at 24 to 34 weeks' gestation were analyzed by radioimmunoassay for the steroid hormones estrone, estradiol, progesterone, androstenedione, dehydroepiandrosterone, dehydroepiandrosterone sulfate, and cortisol. Amniotic fluid was also obtained for microbial culture and for interleukin 6 measurements by enzyme immunoassay.

RESULTS

Patients with intrauterine infection (n = 11) had significantly higher amniotic fluid concentrations of dehydroepiandrosterone (539 +/- 79 pg/mL) and of cortisol (5.28 +/- 1.0 microg/dL) than did patients with preterm labor and preterm delivery without infection (n = 11; 273 +/- 82 pg/mL and 1.61 +/- 1.05 microg/dL, respectively) or patients with preterm labor and subsequent term delivery (n = 11; 202 +/- 79 pg/mL and 1.82 +/- 1.0 microg/dL, respectively). Furthermore those patients who were delivered within 7 days after enrollment (who were also more likely to have intrauterine infection) had higher amniotic fluid concentrations than did those who were not delivered within 7 days of both estrone (586 +/- 101 pg/mL vs 314 +/- 98 pg/mL) and estradiol (238 +/- 44 pg/mL vs 91 +/- 43 pg/mL).

CONCLUSION

Intrauterine infection was associated with increased fetal adrenal androgen and cortisol biosynthesis, and delivery within 7 days after the onset of preterm labor was associated with increased placental estrogen synthesis. These data are consistent with fetal hypothalamic-pituitary-adrenal axis activation in the setting of infection-associated preterm delivery.

Corticotropin releasing hormone: therapeutic implications and medicinal chemistry developments

Corticotropin releasing hormone (CRH, sometimes known as CRF) is an endogenous 41 amino acid peptide that has been implicated in the onset of pregnancy, the 'fight or flight' response, in addition to a large number of physiological disorders. Recently, medicinal chemists have developed a number of potent and selective compounds that show promise in a vast array of therapeutic uses. Herein we review the current status of research.

Cell type-specific regulation of fetal fibronectin expression in amnion: conservation of glucocorticoid responsiveness in human and nonhuman primates

The appearance of oncofetal fibronectin (FFN) in cervical and vaginal secretions is predictive of human labor. Levels of FFN in amnion increase with the onset of labor in rhesus monkeys. Since glucocorticoid (GC) levels in serum and amniotic fluid increase in association with parturition, we compared GC-mediated regulation of FFN expression in cultures of amnion epithelial cells and fibroblasts isolated from human and baboon amnions. Cells were maintained with and without dexamethasone (DEX), and levels of FFN in the conditioned media were determined by ELISA. We observed that DEX treatment suppressed FFN levels in both human and baboon amnion epithelial cells, whereas it increased FFN levels in amnion fibroblasts. DEX treatment reduced FFN levels in cytotrophoblasts from human placenta and increased FFN levels in placental fibroblasts. Northern blots revealed that DEX reduced levels of fibronectin (FN) mRNA in amnion epithelial cells and cytotrophoblasts, whereas it increased FN mRNA in amnion and placental fibroblasts. We conclude that GC differentially regulates FFN expression in epithelial and mesenchymal cells from amnion and placenta. In addition, this pattern of cell type-specific FFN regulation by GC is conserved in human and nonhuman primates and may be responsible for parturition-dependent changes in FFN expression in gestational tissues.

Signal transduction characteristics of the corticotropin-releasing hormone receptors in the feto-placental unit

Placentally derived CRH plays a major role in the mechanisms controlling human pregnancy and parturition. In this study, we sought to investigate the signal transduction mechanisms of CRH Type-1 receptors in the feto-placental unit. To clarify the signal transduction components in placenta and fetal membranes, we investigated the expression of G proteins and adenylate cyclase. Using the nonhydrolysable photoreactive analog [alpha-32P] GTP-azidoanilide and peptide antisera raised against G protein alpha-subunits, we studied coupling of CRH receptors to G proteins in both placental and fetal membranes. Treatment of placental membranes with human CRH (100 nM) increased the labeling of Gq, Go, and Gz but not Gi and Gs. Treatment of fetal membranes with human CRH (100 nM) increased the labeling of Go and Gq but not Gi, Gs, and Gz. These results were supported by experiments that showed that CRH failed to activate adenylate cyclase in these tissues, but induced an increase in inositol phosphates instead. These findings provide new insights into the components of the signal transduction machinery in both fetal and placental membranes and suggest that CRH Type-1 receptors can couple to different G proteins in different tissues. The physiological significance of these observations remains to be elucidated.

Glucocorticoid stimulation of corticotropin-releasing hormone gene expression requires a cyclic adenosine 3',5'-monophosphate regulatory element in human primary placental cytotrophoblast cells

Production of placental CRH, which is identical to the peptide synthesized and secreted in the hypothalamus, has been linked to human parturition. Glucocorticoids stimulate placental CRH secretion and messenger ribonucleic acid expression, in contrast to their inhibition of CRH synthesis in the hypothalamus. A positive feedforward loop involving glucocorticoid-CRH-ACTH-glucocorticoid is thought to drive the exponential increase in placental CRH leading to delivery. Tissue-specific effects of glucocorticoids on CRH expression are therefore of interest. Using human primary placental cells, we investigated the mechanism by which glucocorticoids stimulate placental CRH gene expression. Nuclear run-on transcription shows that in human placental cells glucocorticoids up-regulate transcription of human CRH (hCRH). Using transient transfection assays we demonstrate that dexamethasone up-regulates both basal and cAMP-stimulated hCRH promoter activity, correlating well with the increase in endogenous CRH peptide levels. Through mutagenesis and deletion analyses we show that dexamethasone stimulation of hCRH gene transcription requires a functional cAMP regulatory element (CRE); this CRE is adequate to confer dexamethasone stimulation upon a heterologous promoter, and electrophoretic mobility shift assay studies show that a placental nuclear protein specifically binds to the hCRH CRE.

Physiology of the menstrual cycle

The normal female life cycle is associated with a number of hormonal milestones: menarche, pregnancy, contraceptive use, menopause, and the use of replacement sex hormones. All these events and interventions alter the levels and cycling of sex hormones and may cause a change in the prevalence or intensity of headache. The menstrual cycle is the result of a carefully orchestrated sequence of interactions among the hypothalamus, pituitary, ovary, and endometrium, with the sex hormones acting as modulators and effectors at each level. Oestrogen and progestins have potent effects on central serotonergic and opioid neurons, modulating both neuronal activity and receptor density. The primary trigger of menstrual migraine appears to be the withdrawal of oestrogen rather than the maintenance of sustained high or low oestrogen levels. However, changes in the sustained oestrogen levels with pregnancy (increased) and menopause (decreased) appear to affect headaches. Headaches that occur with premenstrual syndrome appear to be centrally generated, involving the inherent rhythm of CNS neurons, including perhaps the serotonergic pain-modulating systems.

Genetic analysis of fetal development and parturition control in the mouse

The application of targeted gene inactivation methodologies to the study of late fetal development and control of the timing for parturition in mice has yielded insight into the mechanisms that enhance fetal survival. An essential role for glucocorticoids in promoting lung maturation sufficient for viability ex utero before the onset of normal parturition has been demonstrated in corticotropin-releasing hormone-deficient mice. In contrast, maternal deficiency in the prostaglandin synthetic enzyme cyclooxygenase-1 results in the markedly delayed onset of labor and fetal demise because of postdates gestation. The complex interplay of factors that govern the onset of labor is highlighted by mice deficient in both cyclooxygenase-1 and oxytocin. Whereas mice deficient in oxytocin demonstrate normal parturition, simultaneous cyclooxygenase-1 and oxytocin deficiency rescues the delayed onset of labor found in cyclooxygenase-1 knockout mice but results in the prolonged duration of labor. The consequences of complete deficiency of molecules involved in parturition in mice suggest novel interventions for human preterm labor.

Corticotrophin-releasing hormone and fetal responses in human pregnancy

During human pregnancy, maternal and fetal compartments of the human placenta produce and release corticotrophic-releasing hormone (CRH). Elevations of placental CRH are associated with decreased gestational length (including preterm delivery). The effects of elevated placental CRH on human fetal neurological development are not known. Pregnant women in the 31st and 32nd week of gestation consented to procedures for collection of blood and measurement of fetal heart rate (FHR) in response to a series of 40 vibro-acoustic stimuli (VAS). Measures of habituation and dishabituation were calculated from the FHR. All subjects were followed to delivery. Fetuses (N = 33) of women with highly elevated CRH were least responsive (p < .03) to stimulation after presentation of a novel (dishabituating) stimulus with control for parity, fetal gender, medical (antepartum) risk, and gestational length at term. In a larger sample (N = 156) a polynomial model predicted the pattern of FHR reactivity for the first 15 trials. Placental CRH concentration significantly predicted FHR reactivity after controlling for the effects of trial number, baseline FHR, inter-trial interval, and presence of uterine contractions. Increased maternal CRH levels were significantly related to the length of gestation after controlling for the effects of fetal gender, parity, and medical risk (p = .05). The relationship between length of gestation and FHR was not significant suggesting separate actions of CRH on these events. Elevated placental CRH appears to accelerate certain developmental events (gestational length) and may influence the fetal nervous system. The impaired fetal responses to novelty and increased arousal observed in this study suggest that neurological systems may be targets for placental CRH during sensitive developmental periods.

Corticotrophin-releasing hormone and CRH-binding protein. Differences between patients at risk for preterm birth and hypertension

BACKGROUND

During pregnancy in the second and third trimester there is a progressive rise in plasma CRH thought to be secreted by the placenta. Plasma CRH-BP inactivates CRH, which may prevent its peripheral action on the maternal pituitary and myometrium. In the last few weeks of pregnancy CRH-BP decreases, thereby causing an increase in free CRH or a CRH/CRH-BP complex available to play a role in the onset of parturition.

OBJECTIVE

We tested the hypothesis that differences in CRH, CRH-BP, or a CRH/CRH-BP complex in patients at risk for preterm birth (PTB) and hypertension (HYP) account for the differences in the timing of parturition.

METHODS

From a Behavior in Pregnancy Study database, we identified 18 patients who had spontaneous PTB and 23 patients who developed HYP. Both groups were case controlled and matched with patients who delivered at term (Normal). Maternal plasma samples had been appropriately collected from these patients at 18-20, 28-30, and 35-36 weeks gestational age. CRH levels were measured by double antibody RIA kit and the CRH-BP by a immunoradiometric technique. A CRH-BP/CRH dimer complex index was calculated. Statistical analysis was done using Kruskal-Wallis test for two cases.

RESULTS

Maternal CRH (pg/ml) in the PTB cases compared to the HYP cases was significantly elevated at all three time periods. Maternal CRH-BP (pg/ml) in the PTB versus HYP cases was significantly lower at all three time periods in the PTB cases compared to the HYP cases. Maternal CRH-BP/CRH dimer complex index was significantly lower in the PTB cases at all three time periods than either the controls or the HYP cases, suggesting excessive CRH. The mean GA at delivery for the PTB cases was significantly lower than the control or HYP cases.

CONCLUSIONS

These results suggest that those patients at risk for PTB have significantly elevated CRH, lower CRH-BP, and a reduced CRH-BP/CRH dimer complex index at all three time periods of assessment.

Detection of N-terminal pro-corticotrophin releasing hormone (CRH) and a 'novel' CRH in human maternal plasma and placenta

Corticotrophin-releasing hormone (CRH) is released from the human placenta in high concentrations towards the end of the third trimester of pregnancy, but relative concentrations of other cleavage products derived from the CRH prohormone are unknown. We have measured CRH and N-terminal (1-100) and (1-127) proCRH peptides in maternal plasma in the second and third trimesters of pregnancy and in term placental extract using immunoradiometric assays (IRMAs) specific to different regions of the CRH precursor. Levels of N-terminal proCRH (amino acid residues 1-100) rose from 24+/-4 pmol/l (mean+/-s.e.) in the second trimester to 378.8+/-65 pmol/l (mean+/-s.e.) at term. Levels of intact proCRH and/or (1-127) proCRH remained relatively constant throughout the second and third trimesters, with a concentration of 29.3+/-3.8 pmol/l (mean+/-s.e.). In the course of this work a novel form of CRH that cross-reacts within specific CRH immunoassays was observed. The use of two IRMAs developed for CRH (1-41) having different C-terminal epitope specificities provided evidence for two types of CRH coexisting in maternal plasma. Separation of term placental extract by HPLC and application of the two CRH IRMAs revealed two peaks of immunoreactivity one of which coeluted with synthetic CRH (1-41).

Corticotropin-releasing hormone gene expression in primary placental cells is modulated by cyclic adenosine 3',5'-monophosphate

CRH, the principal neuropeptide regulator of pituitary ACTH secretion, is also expressed in placenta. Placental CRH has been linked to the process of human parturition. However, the mechanisms regulating transcription of the CRH gene in placenta remain unclear. cAMP signaling pathways play important roles in regulating the expression of a diverse range of endocrine genes in the placenta. Therefore, we have explored the effect of cAMP on CRH promoter activity in primary cultures of human placental cells. Both forskolin and 8-bromo-cAMP, activators of protein kinase A, can increase CRH promoter activity 5-fold in transiently transfected human primary placental cells, in a manner that parallels the increase in endogenous CRH peptide. Maximal stimulation of CRH promoter activity occurs at 500 micromol/L 8-bromo-cAMP and 10 micromol/L forskolin. Electrophoretic mobility shift assay and mutation analysis combined with transient transfection demonstrate that in placental cells cAMP stimulates CRH gene expression through a cAMP regulatory element in the proximal CRH promoter region and involves a placental nuclear protein interacting specifically with the cAMP regulatory element.

The molecular basis of premature adrenarche: an hypothesis

Adrenarche is characterized by a prepubertal rise in adrenal secretion of dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS) that is independent of the gonads or gonadotropins. Adrenopause is the corresponding diminution in DHEA and DHEAS concentrations in later life. The mechanisms by which adrenarche and adrenopause are induced and regulated are unknown. Early work focused on identifying hypothetical adrenal androgen regulatory hormones that would induce DHEA in much the same way that adrenocorticotropin induces cortisol, but no such factors have been found. Current studies of adrenarche focus on intra-adrenal events, particularly those concerning 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and 17alpha-hydroxylase/17,20-lyase (P450c17). Molecular data implicate a decrease in 3beta-HSD specifically in the adrenal zona reticularis. However, a decrease in 3beta-HSD is insufficient to explain why the reticularis catalyzes 17,20-lyase activity and hence makes DHEA, rather than catalyzing only 17alpha-hydroxylase activity, as does the zona fasciculata. P450c17 appears to catalyze 17,20-lyase activity only if P450c17 has undergone serine phosphorylation and has access to cytochrome b5 as an allosteric cofactor. Although these two factors have not yet been investigated in adrenarche, it appears that both a zone-specific diminution in 3beta-HSD and a zone-specific induction of 17,20-lyase activity are required to account for the physiological data. Exaggerated premature adrenarche appears to be an early sign of polycystic ovary syndrome (PCOS). Mechanistic considerations of PCOS suggest a key role for serine phosphorylation of P450c17 in both adrenarche and some forms of heritable PCOS.

Role of corticotropin-releasing hormone in onset of labour

Corticotropin-releasing hormone (CRH) derived from the placenta is secreted into the maternal circulation in large amounts during the third trimester of human pregnancy and may have an important role in the onset of labour. Although the biological role of CRH remains enigmatic, the presence of functional CRH receptors in the myometrium suggests that CRH may modulate myometrial contractility and hence parturition. CRH action is mediated via multiple receptor subtypes and pregnancy results in differential receptor expression. These receptors are primarily linked to the adenylate cyclase second messenger system, which would help the intracellular microenvironment to maintain the required myometrial quiescence. CRH can exert further actions such as inhibition of prostaglandin production to prevent contractions. At term under the influence of oxytocin there is a modification in the coupling mechanisms that leads to a decrease in the biological activity of the CRH receptor and in the generation of cyclic adenosine monophosphate which favours myometrial contractions. CRH, via distinct receptor subtypes, is then able to enhance the contractile response of the myometrium. This hypothesis places CRH in a central role in coordinating the smooth transition from a state of relaxation to one of contraction.

Elevated mid-trimester maternal corticotrophin-releasing hormone levels in pregnancies that delivered before 34 weeks

OBJECTIVE

To test whether maternal corticotrophin-releasing hormone levels are elevated in the mid- trimester for those women who subsequently had spontaneous preterm delivery and to assess the clinical utility of the measurement in the prediction of preterm delivery.

DESIGN

A prospective observational study.

SETTING

Department of Obstetrics and Gynaecology, Prince of Wales Hospital, Hong Kong.

POPULATION

1047 low risk pregnant women recruited at 15-20 weeks of gestation.

METHODS

Venous samples were assayed for levels of corticotrophin-releasing hormone. The investigators responsible for the laboratory assay were blinded to the obstetric outcome.

MAIN OUTCOME MEASURES

Incidence of preterm, term and post-term pregnancies.

RESULTS

Those who were delivered spontaneously at a preterm gestational age (before 34 weeks) had significantly higher corticotrophin-releasing hormone levels in the mid-trimester, compared with those who were delivered at term or post-term. There was a trend towards lower corticotrophin-releasing hormone levels with more advanced gestational age at delivery. When the measurement of corticotrophin- releasing hormone was used to predict delivery before 34 weeks, the best cut off was 1.9 MoM, which produced a sensitivity of 72.7% and specificity of 78.4%. This translated to a positive predictive value of 3.6%, negative predictive value of 99.6% and relative risk of 9.4 when the background prevalence of spontaneous preterm delivery before 34 weeks was 1.1%. The likelihood ratio was 3.4.

CONCLUSIONS

Mid-trimester maternal corticotrophin-releasing hormone levels are elevated in pregnancies destined to deliver preterm before 34 weeks. When used alone in a low risk population, the measurement has a low predictive power for preterm delivery. However, the likelihood ratio of 3 4 implies that in high risk populations the test may be considerably more valuable.

Psychoneuroimmunology and preterm birth. A holistic model for obstetrical nursing practice and research

This article presents a framework through which nurses can conceptualize premature labor and birth for both practice and research. Use of the psychoneuroimmunology (PNI) model may guide the study of the problem of preterm birth in a more holistic manner, discovering relationships between the body and the mind that may affect how nurses can intervene to prevent premature birth. Nursing assessment of risk needs to include those situations that may lead to increased stress or anxiety, as is supported by research based on the PNI model. Reduction of stressors that lead to physiological changes related to the stress response can affect the incidence of preterm labor. Interventions to decrease stress and poor coping behaviors need to be tested and integrated into practice.

Predicting risk of preterm delivery by second-trimester measurement of maternal plasma corticotropin-releasing hormone and alpha-fetoprotein concentrations

OBJECTIVE

Many women who have preterm labor have abnormally high plasma concentrations of the placental peptide corticotropin-releasing hormone and the fetal product alpha-fetoprotein in early pregnancy. This study was designed to evaluate the ability of these biochemical tests and a clinical risk factor score to prospectively discriminate pregnancies at high risk for preterm delivery.

STUDY DESIGN

Eight hundred sixty women were studied prospectively from the early second trimester until delivery. A risk factor score for preterm delivery was calculated from the clinical history and maternal plasma corticotropin-releasing hormone and alpha-fetoprotein concentrations were measured by radioimmunoassay between 17 and 30 weeks' gestation. The risk factor score, corticotropin-releasing hormone concentration, and alpha-fetoprotein concentration for each woman were expressed as individual odds or likelihood ratios for preterm delivery and as a combined risk estimate derived from the 3 tests.

RESULTS

Sixty women had preterm deliveries (n = 37 spontaneous labor, n = 23 planned deliveries), and these women had significantly higher concentrations of corticotropin-releasing hormone (median 1.92 multiples of the median) and alpha-fetoprotein (median 1.32 multiples of the median) than did women with term deliveries (median 1.00 multiples of the median, P <.001 for both tests), with these abnormalities being evident from early in the second trimester. The risk factor score was >/=10 in 28% of women with preterm delivery and 7% of women with term delivery. The combination of all 3 markers resulted in a higher detection rate and a higher positive predictive value than the risk factor score, corticotropin-releasing hormone concentration, or alpha-fetoprotein concentration alone, correctly discriminating 37% of women who would have preterm deliveries with a false-positive rate of 5%. The positive predictive value was also 37% (odds of being affected given a positive result were 1:1.7).

CONCLUSIONS

The combination of measurement of maternal plasma corticotropin-releasing hormone and alpha-fetoprotein concentrations in the second trimester with risk factor scoring provides a more accurate indicator of the risk of preterm delivery than does risk factor scoring alone. This method of risk assessment may therefore be of use in targeting prevention strategies.

Corticotropin-releasing Hormone in Human Pregnancy and Parturition

The role of corticotropin-releasing hormone (CRH) in the regulation of pituitary adrenocorticotropin secretion and the stress response is well established. However, in recent years this peptide has been found to serve a number of functions outside the classic neuroendocrine domain. During pregnancy, CRH derived from the placenta is thought to play a crucial role in the regulation of foetal maturation and the timing of delivery, and CRH has also been implicated in the control of foetal-placental bloodflow. Abnormalities of the placental CRH system might be involved in the pathogenesis of preterm labour, foetal growth retardation and pre-eclampsia, which are the three leading causes of perinatal morbidity and mortality in developed countries.

The production of interleukin-1beta from human fetal membranes is not obligatory for increased prostaglandin output

Bacterial endotoxin increased the expression of mRNA (maximal after 4 hr) for interleukin-1beta (IL-1beta) and the release of mature protein from intact human fetal membranes. In contrast, the change in expression of mRNA for type 2 cyclo-oxygenase (COX-2) was biphasic, with peaks after 0.5-1 hr and after 8 hr of culture. An antibody to IL-1beta was without effect after 4 hr of culture, inhibited endotoxin-stimulated prostaglandin E2 (PGE2) production after 8 hr of culture, and caused a parallel decrease in the expression of mRNA for COX-2. We conclude that endotoxin induced the expression of COX-2 through IL-1beta-independent and IL-1beta-dependent mechanisms, and these differences are time dependent. Corticotrophin-releasing hormone (CRH) or platelet-activating factor (PAF) also increased the expression of mRNA for IL-1beta and the release of IL-1beta from some, but not all, fetal membranes. The antibody to IL-1beta did not affect CRH-stimulated or PAF-stimulated PGE2 production or COX-2 expression. We conclude that CRH and PAF can induce the expression of IL-1beta, but this is not obligatory for increased PGE2 release, and the effect of these stimuli on COX-2 expression is a direct, IL-1beta-independent effect.

Markers of risk for preterm delivery

Clinical and experimental evidence indicate that PTD results from four primary pathogenic mechanisms: activation of the maternal or fetal HPA axis; amniochorionic-decidual or systemic inflammation; decidual hemorrhage; and, pathologic distention of the myometrium. Each of these four pathways has a distinct epidemiological and clinical profile, and unique biochemical and biophysical pathways initiating parturition, but shares a common final biochemical pathway involving myometrial activation and stimulation, and enhanced genital tract protease activity promoting PPROM and cervical change. Traditional methods of predicting women at risk relying on obstetrical history or symptoms and epidemiological risk factors are neither sensitive nor specific. Recent approaches to predicting PTD, including sonographic measurement of cervical length and biochemical assays for hCG, cytokines, fFN, MMPs, estrogens, and CRH, are more sensitive than traditional methods. Moreover, given the heterogeneous, interactive etiopathogeneses of PTD, multiple biochemical markers should not only increase sensitivity and specificity, but also permit the detection of the relative contribution of each pathogenesis to the overall risk of PTD.

Role of nitric oxide in the uterus and cervix: implications for the management of labor

The results of our experimental studies indicate that that parturition is composed principally of two major phases, a relatively long conditioning (preparatory) phase followed by a short and probably irreversible active labor phase. The major events of conditioning phase are: (i) the progression of uterine contractility from an inactive to a vigorously active state, (ii) cervical ripening, and (iii) activation of fetal membranes. Our experimental and molecular studies indicate that nitric oxide (NO) is an important element in controlling uterine and cervical functions during the transition from pregnancy to the conditioning phase of labor. These studies show that NO acts in concert with progesterone to regulate uterine quiescence and cervical rigidity. In the rat uterus and cervix NO is produced mainly by the cytokine-inducible NO synthase (iNOS) which expression is gestationally-regulated and progesterone-dependent. During pregnancy iNOS expression is up-regulated in the uterus and down-regulated in the cervix. Opposite changes occur during term and antiprogestin-induced preterm labor. Our studies with NOS-inhibitors and NO donors suggest that in the uterus a decrease in NO production contributes to the initiation of labor, whereas in the cervix NO acts as a final metabolic mediator of cervical ripening. The evidence also indicates that NO donors have therapeutic potential as tocolytic agents. On the other hand, locally applied NO donors can be used to induce cervical ripening. In addition iNOS, may represent a new target for novel therapeutic agents capable of promoting or inhibiting uterine of both contractility and cervical ripening. There is an urgent need for controlled studies demonstrating clinical efficacy and safety of NO donors in obstetrics.

Effects of corticotrophin releasing hormone on contractile activity of myometrium from pregnant women

OBJECTIVE

To determine whether corticotrophin releasing hormone plays a role in the regulation of tone in term nonlabouring human myometrium.

SETTING

A teaching hospital research laboratory.

SAMPLE

Thirty-seven women undergoing elective nonlabour caesarean section under regional anaesthesia.

METHODS

Human corticotrophin releasing hormone (1, 10, 100 nmol/L) was added to strips of term, nonlabouring myometrium mounted in an organ bath, and the effect on spontaneous, oxytocin (1 nmol/L) or prostaglandin F2alpha (100 nmol/L) stimulated contractions determined. Cyclic adenosine monophosphate (cAMP) content of the tissue was also determined by enzyme immunoassay.

RESULTS

Corticotrophin releasing hormone did not affect myometrial tension development in any of the experimental protocols. cAMP increased transiently after addition of corticotrophin releasing hormone (166.7 +/- 12.7% at 1 minute) but this was not reflected by any change in tension.

CONCLUSIONS

This study suggests that despite high maternal plasma concentrations of corticotrophin releasing hormone in pregnancy at term, this peptide is unlikely to play a direct role in the control of myometrial contractility in nonlabouring myometrium.

Maturation of body and breathing movements in 24-33 week-old fetuses threatening to deliver prematurely

Maturation of spontaneous fetal body and breathing movements of 24- to 33-week-old fetuses in 168 pregnancies threatening to deliver prematurely were examined on the basis of newborn outcome (premature compromised, premature healthy, term healthy). Maturation of fetuses in 60 low-risk pregnancies delivering as healthy full-term infants served as a normative comparison group. Each fetus was observed for 30 min; the amount of body and breathing movements were noted and an estimation of amniotic fluid volume was made. The pattern of behavioural maturation was similar for all outcome groups; with advancing gestation there was a decrease in body movements and an increase in breathing movements. Both reduced activity levels and advanced behaviours were observed in the high-risk outcome groups. The high-risk fetuses had reduced levels of body movements which increased with better outcome and, an earlier onset of increased amounts of breathing, occurring at 30 weeks in contrast to 33 weeks for the comparison group. In the presence of ruptured membranes, those high-risk fetuses who were born prematurely had less breathing compared to those who delivered at term. Similar maturation patterns among high- and low-risk outcome groups suggests normal/typical functional development in the high-risk fetal groups. The observed differential behaviours were associated with prematurity and most likely associated with events leading to premature labour.

Maternal corticotropin-releasing hormone and habituation in the human fetus

Elevated concentrations of maternal corticotrophin-releasing hormone (CRH) during the 2nd and early 3rd trimester of human pregnancy are associated with spontaneous preterm birth, but the effects of maternal CRH on the fetus are unknown. Maternal plasma was collected for analysis of CRH concentration, m = 156.24 +/- 130.91 pg/ml, from 33 pregnant women during Weeks 31-33 of gestation. Immediately after collection of plasma, fetal heart rate (FHR) measures were obtained in response to a challenge with a series of vibroacoustic stimuli. Fetuses of mothers with highly elevated CRH did not respond significantly to the presence of a novel stimulus in a repeated series, p = 0.016. These effects on the FHR response were not related to parity, fetal gender, medical (antepartum) risk, or eventual birth outcomes. Impaired dishabituation in these fetuses of mothers with high concentrations of CRH suggests that neurological systems rich with CRH receptors that support learning and memory, such as parahippocampal regions, may be targets for maternal/placental CRH, with implications for fetal neurological development.

Cortisol in fetal fluids and the fetal adrenal at parturition in the tammar wallaby (Macropus eugenii)

Glucocorticoid hormones may play a critical role in initiating parturition in tammar wallabies. In this study, we investigated the concentration of cortisol in fetal fluids and cortisol production by fetal adrenals over the last 3 days of the 26-day pregnancy and within 24 h postpartum. The fetal adrenals almost doubled in size between Days 24 and 26 of pregnancy, and their cortisol content increased over 10-fold during this period, from 10 pg to over 100 pg per adrenal pair. After birth, neonatal adrenals continued to grow, but cortisol content fell dramatically to 20 pg. The prepartum increase in adrenal cortisol was reflected by a substantial rise in cortisol concentrations in yolk sac fluid, allantoic fluid, and fetal blood, which were below 10 ng/ml on Day 24 and rose to over 40 ng/ml by Day 26. Cortisol concentrations in neonatal blood decreased postpartum, mirroring decreased cortisol content in neonatal adrenals. Cortisol production by the fetal adrenal was stimulated in vitro by ACTH and prostaglandin E2, suggesting that the in vivo increase may be stimulated by release of ACTH from the fetal hypothalamic-pituitary axis and prostaglandin E2 from the placenta. These results indicate that increasing cortisol production by the fetal adrenal is a characteristic of late pregnancy in the tammar wallaby and support the suggestion that fetal cortisol may trigger the initiation of parturition in this marsupial species.

Cervical length in uncomplicated pregnancy: A study of sociodemographic predictors of cervical changes across gestation

OBJECTIVE

We conducted this study to evaluate sociodemographic factors associated with changes in the length of the cervix across gestation in pregnancies that resulted in term deliveries.

STUDY DESIGN

This study is an observational cohort design of sonographically determined cervical length measured at 3- to 4-week intervals in 125 women with a singleton pregnancy between 20 and 32 weeks gestation. We developed a structured questionnaire to collect psychosocial and sociodemographic characteristics. We used bivariate analysis, analysis of variance, and regression analysis to study variation in cervical length.

RESULTS

Overall, cervical length decreased minimally as gestational age progressed. However, among black women cervical length decreased significantly with increasing gestational age (P =.006). In addition, high psychosocial stress was associated with significantly shorter cervices later in gestation, independent of race (P =.003). Finally, women whose occupations involved skilled manual labor had shorter cervices (P =.02).

CONCLUSIONS

Women who are black, under stress, or working as skilled manual laborers demonstrate significant shortening of the cervix during gestation. Given that a shorter cervix predisposes to preterm delivery, our findings provide new insights into the well-described association of these psychosocial and sociodemographic factors with an increased risk of preterm delivery.

Maternal endocrine adaptations to placental hormones in humans

The remarkable endocrine alterations that are characteristic of human pregnancy are attributable to the placenta. In this tissue, steroid and peptide hormones are produced in extraordinary amounts. In addition, the haemomonochorioendothelial placentation of human pregnancy contributes to the unique distribution of products formed in trophoblasts into maternal and fetal compartments. In this review, the partial control exerted by the trophoblast on maternal metabolism is illustrated by the replacement in the maternal compartment of pituitary growth hormone (GH) with the trophoblast's own product, human placental GH. Placental GH differs from pituitary GH by 13 amino acids, has high somatogenic and low lactogenic activities and is secreted by the syncytiotrophoblast in a non-pulsatile manner. This continuous secretion appears to have important implications for the control of maternal levels of insulin-like growth factor I. Placental GH secretion is inhibited by glucose in vitro and in vivo, and is significantly decreased in the maternal circulation in cases of pregnancies with intrauterine growth retardation.

High precursor level in maternal blood results from the alternate mode of proopiomelanocortin processing in human placenta

OBJECTIVE

ACTH-producing non-pituitary tumours are often associated with altered precursor processing, particularly in the most aggressive ones. Since placental tissue is characterized by its ability to express the proopiomelanocortin (POMC) gene and rapid cellular proliferation, we examined whether intact POMC could be released physiologically during human gestation.

SUBJECTS

One hundred and fifty six normal pregnant women, 12 with multiple pregnancies, and 23 non-pregnant controls. Twenty-eight women were studied in the immediate postpartum period.

MEASUREMENTS

We measured plasma POMC levels with a specific immunoradiometric assay (IRMA) using a combination of antibodies directed against ACTH and beta endorphin. Results obtained with this first IRMA were confirmed in 22 subjects with a second assay using the same beta endorphin antibody and a more distal antibody directed against the N-terminal fragment of POMC. Reverse transcription-PCR detected full length, pituitary-like, POMC mRNA in human placenta.

RESULTS

Plasma POMC was undetectable (< 60 U/ml) in 23 normal subjects. In normal monofetal pregnancies, POMC became detectable in most women by the third month and then increased steadily until midgestation: 168 +/- 108 (U/ml; mean +/- SD) between 12 and 15 weeks, 190 +/- 103 between 16 and 19 weeks, 324 +/- 180 between 20 and 23 weeks, 276 +/- 171 between 24 and 27 weeks, 292 +/- 177 between 28 and 31 weeks, 290 +/- 235 between 32 and 35 weeks and 308 +/- 210 between 36 weeks and parturition. Plasma POMC was significantly higher in multiple pregnancies with very high levels in three triplet-bearing mothers: 671, 941, and 1731 U/ml at 31, 33 and 32 weeks, respectively. POMC levels felt quickly in post partum, becoming undetectable in five of 13 women on day 1, seven of eight on day 2 and five of six on day 3. Plasma POMC displayed no diurnal variation, was not suppressed by glucocorticoid administration and did not correlate with plasma ACTH or cortisol. In contrast, plasma POMC positively correlated with plasma CRH.

CONCLUSIONS

Pregnancy is the only condition in which POMC is produced and released physiologically, similar in some respects to the ectopic ACTH syndrome. POMC is derived solely from the placenta, with no interference from maternal pituitary secretion, and is thus a new and specific placental marker.

Maternal plasma corticotropin-releasing hormone associated with stress at 20 weeks' gestation in pregnancies ending in preterm delivery

OBJECTIVE

This study tested the hypothesis that maternal stress is associated with elevated maternal levels of corticotropin releasing hormone and activation of the placental-adrenal axis before preterm birth.

STUDY DESIGN

In a behavior in pregnancy study, 524 ethnically and socioeconomically diverse women were followed up prospectively and evaluated at 3 gestational ages: 18 to 20 weeks, 28 to 30 weeks, and 35 to 36 weeks. Maternal variables included demographic data, medical conditions, perceived stress level, and state anxiety. Maternal plasma samples were collected at each gestational age. Eighteen case patients with spontaneous onset of preterm labor were matched against 18 control subjects who were delivered at term, and their samples were assayed for corticotropin-releasing hormone, adrenocorticotropic hormone, and cortisol by means of radioimmunoassay. Statistical tests were used to examine mean differences in these hormones. In addition, the relationship between stress level and each hormone was tested with a Pearson correlation coefficient and hierarchic multiple regressions in each group.

RESULTS

Patients who had preterm delivery had significantly higher plasma corticotropin-releasing hormone levels than did control subjects at all 3 gestational ages (P <.0001). Analyses did not find any differences in reported levels of stress between 18 to 20 weeks' gestation and 28 to 30 weeks' gestation. A hierarchic multiple regression indicated that maternal stress level at 18 to 20 weeks' gestation and maternal age accounted for a significant amount of variance in corticotropin-releasing hormone at 28 to 30 weeks' gestation, after controlling for corticotropin-releasing hormone at 18 to 20 weeks' gestation (P <. 001). In addition, patients who were delivered preterm had significantly elevated plasma levels of adrenocorticotropic hormone at all 3 gestational ages (P <.001) and significantly elevated cortisol levels at 18 to 20 weeks' gestation and 28 to 30 weeks' gestation (P <.001).

CONCLUSION

Maternal plasma levels of corticotropin-releasing hormone are significantly elevated at as early as 18 to 20 weeks' gestation in women who are subsequently delivered preterm. Changes in corticotropin-releasing hormone between 18 to 20 weeks' gestation and 28 to 30 weeks' gestation are associated with maternal age and stress level at 18 to 20 weeks' gestation. Maternal stress and corticotropin-releasing hormone levels may be potential markers for the patient at risk for preterm birth. Activation of the placental maternal pituitary-adrenal axis is consistent with the classic endocrine response to stress.

A central theory of preterm and term labor: putative role for corticotropin-releasing hormone

Near the end of human pregnancy the concentration of placental corticotropin-releasing hormone in maternal blood rises exponentially. The rate of elevation of corticotropin-releasing hormone and its duration through time have been linked to the time of onset of labor. Paradoxically, although glucocorticoids are known to inhibit corticotropin-releasing hormone production within the hypothalamic-pituitary-adrenal axis, cortisol actually increases corticotropin-releasing hormone levels in several areas outside the hypothalamus, including the placenta. Placental corticotropin-releasing hormone may be an important component of a system that controls the normal maturation of the fetus and signals the initiation of labor. Abnormal elevations in corticotropin-releasing hormone, which may be a hormonal response to stressors arising in either the mother, placenta, or fetus, may prove to participate in the premature onset of parturition.

Placental corticotropin-releasing hormone: function and regulation

Corticotropin-releasing hormone is a neuropeptide placentally expressed among mammals only in primates. Its expression increases as much as 100 times during the last 6 to 8 weeks of pregnancy and is paradoxically stimulated by glucocorticoids. Increasing evidence suggests that placental corticotropin-releasing hormone may have evolved in primates to stimulate fetal adrenocorticotropin release and adrenal steroidogenesis, thus satisfying the high demand for synthesis of dehydroepiandrosterone, the predominant source of placental estradiol. Concomitant stimulation by placental corticotropin-releasing hormone of fetal cortisol and dehydroepiandrosterone would couple the glucocorticoid effects on fetal organ maturation with the timing of parturition, an obvious benefit in postnatal survival.

Regulation of uterine smooth muscle function during gestation

The uterus is unique among smooth muscular organs in that, during pregnancy, it undergoes profound, largely reversible, changes orchestrated by the ovarian hormones. These changes facilitate uterine adaptation to the stretch induced by the growing fetus such that a state of myometrial contractile quiescence can be maintained. This quiescent state usually is maintained until fetal development is sufficient for extrauterine life, at which point unknown mechanisms precipitate conversion to a highly contractile state. Throughout pregnancy, signaling mechanisms for myometrial contractility are altered--first to promote quiescence and then again to promote contractions. The mechanisms responsible for these changes are only partially understood. This review attempts to summarize salient features of many of the changes in uterine contractile signaling and the current state of ongoing investigations of their mechanisms. We have also highlighted some newer information and concepts from nonuterine tissues, which we believe may provide insight into the control of uterine smooth muscle function. Some detail has been omitted, and can be found in the many excellent reviews cited. We hope that this discussion may stimulate the interests of other investigators. The diverse areas of inquiry offer hope that this decade will lead to a fuller understanding of myometrial function and the development of vastly improved approaches for the control of preterm labor.

Maternal corticotropin-releasing hormone levels in the early third trimester predict length of gestation in human pregnancy

OBJECTIVE

Corticotropin releasing hormone, a hypothalamic neuropeptide, plays a major role in regulating pituitary-adrenal function and the physiologic response to stress. During pregnancy corticotropin-releasing hormone is synthesized in large amounts by the placenta and released into the maternal and fetal circulations. Various endocrine, autocrine, and paracrine roles have been suggested for placental corticotropin-releasing hormone. The aim of this study was to prospectively assess the relationship between maternal plasma concentrations of corticotropin-releasing hormone in the early third trimester of pregnancy and the length of gestation in two groups of deliveries, with and without spontaneous labor.

STUDY DESIGN

In a sample of 63 women with singleton intrauterine pregnancies, maternal plasma samples were collected between 28 and 30 weeks' gestation and corticotropin-releasing hormone concentrations were determined by radioimmunoassay. Each pregnancy was dated on the basis of last menstrual period and early ultrasonography. Parity, antepartum risk conditions, presence or absence of spontaneous labor, and birth outcomes were abstracted from the medical record.

RESULTS

Maternal corticotropin-releasing hormone levels between 28 and 30 weeks' gestation significantly and negatively predicted gestational length (P < .01) after adjustment for antepartum risk. Moreover, subjects who were delivered preterm had significantly higher corticotropin-releasing hormone levels in the early third trimester (P < .01) than did those who were delivered at term. In deliveries preceded by spontaneous onset of labor, maternal third-trimester corticotropin-releasing hormone levels significantly and independently predicted earlier onset of labor (P < .01) and preterm labor (P < .05), whereas in deliveries effected by induction of labor or cesarean delivery, maternal corticotropin-releasing hormone levels were a marker of antepartum risk but not a statistically independent predictor of gestational length.

CONCLUSION

These findings support the premise that placental corticotropin-releasing hormone is potentially implicated in the timing of human delivery in at least two ways. First, placental corticotropin-releasing hormone may play a role in the physiology of parturition. Premature or accelerated activation of the placental corticotropin-releasing hormone system, as reflected by precocious elevation of maternal corticotropin-releasing hormone levels, may therefore be associated with earlier onset of spontaneous labor and resultant delivery. Second, placental corticotropin-releasing hormone may be a marker of antepartum risk for preterm delivery and therefore an indirect predictor of earlier delivery. The implications of these findings are discussed in the context of the neuroendocrinology of placental corticotropin-releasing hormone and human parturition. Furthermore, the role of corticotropin-releasing hormone as a possible effector of prenatal stress in producing alterations in the timing of normal delivery is detailed.

Chronic antagonism of nuclear factor-kappaB activity in cytotrophoblasts by dexamethasone: a potential mechanism for antiinflammatory action of glucocorticoids in human placenta

Circulating glucocorticoids are present in increasing quantities as human gestation progresses, peaking during labor whether it occurs before or at term. Although the precise role of glucocorticoids in pregnancy is not well defined, it is clear that glucocorticoids suppress inflammation in many cell types by antagonizing the acute stimulatory actions of members of the Rel/nuclear factor-kappaB (NF-kappaB) family on cytokine gene expression. In the present study we tested the hypothesis that during pregnancy, glucocorticoids chronically suppress inflammation in the human placenta. Cytotrophoblasts obtained from human term placentas were maintained for 48 h in culture medium supplemented with 10% charcoal-stripped calf serum with and without 100 nmol/L dexamethasone (DEX). Enzyme-linked immunosorbent assay studies revealed that cytotrophoblasts constitutively express interleukin-8 (IL-8), a known mediator of placental inflammation, between 24-96 h of culture. A 48-h treatment of cytotrophoblasts with 100 nmol/L DEX significantly reduced the production of IL-8 to 24+/-1% of control levels (P < 0.01). DEX and cortisol mediated a dose-dependent inhibition of IL-8 expression, with ED50 values of 5 and 50 nmol/L, respectively. DEX treatment also significantly reduced levels of IL-6 and tumor necrosis factor-alpha in culture medium, suggesting that glucocorticoids coordinately reduce cytokine levels in cytotrophoblasts. As cytokine expression is regulated by NF-kappaB and activator protein-1 (AP-1) transcription factors, electrophoretic mobility shift assays (n = 4) were used to determine whether DEX treatment altered the binding of nuclear proteins from cytotrophoblasts to labeled oligonucleotides corresponding to the kappaB and AP-1 response elements. We observed that a 48-h treatment of cytotrophoblasts with 100 nmol/L DEX markedly reduced binding of nuclear extracts from cytotrophoblasts to the kappaB response element. DEX treatment promoted a relatively smaller reduction of binding to the AP-1 response element. Northern blotting experiments revealed that DEX treatment did not alter the level of IkappaB, p50, or p65 messenger ribonucleic acid, suggesting that the antiinflammatory action of glucocorticoid in cytotrophoblasts did not directly involve alterations in the level of NF-kappaB proteins. Our results demonstrate a novel chronic suppressive action of glucocorticoid on cytokine production and nuclear binding of NF-kappaB and AP-1 proteins in cytotrophoblasts, providing a potential mechanism through which glucocorticoids may suppress inflammation at maternal-fetal interfaces across gestation.

Corticotropin-releasing hormone directly and preferentially stimulates dehydroepiandrosterone sulfate secretion by human fetal adrenal cortical cells

Estrogens produced by the placenta play a pivotal role in the endocrine control of pregnancy and induce many of the key changes involved in parturition. The placentae of humans and higher primates use the C19 androgen dehydroepiandrosterone sulfate (DHEA-S) supplied by the fetal adrenals as the principal substrate for estrogen synthesis. Thus, secretion of androgens by the fetal adrenals may be central to the process of primate parturition. The timing of human parturition also is correlated with placental CRH concentrations in the maternal circulation. Because the mechanisms that regulate DHEA-S production by the fetal adrenals are incompletely understood, we examined whether there is a functional relationship between CRH and steroid production by human fetal adrenal cortical cells. Using Northern blot analysis, we detected messenger RNA transcripts (2.7 kb) encoding the type-1 CRH receptor in total RNA extracted from midgestation human fetal adrenals, suggesting that the fetal adrenal cortex may be directly responsive to CRH. To test this, primary cultures of human fetal adrenal cortical cells were exposed to human CRH. Human CRH increased DHEA-S production by cultured human fetal adrenal cortical cells in a dose-dependent fashion, with an ED50 of 10-100 pmol/L. Human CRH was as effective as ACTH at stimulating DHEA-S production; however, it was 70% less potent than ACTH at stimulating cortisol production, indicating that its actions were preferentially directed toward increasing DHEA-S synthesis. Consistent with this thesis, we found that CRH increased abundance of messenger RNA encoding cytochrome P450 cholesterol side-chain cleavage and 17alpha-hydroxylase/17,20 lyase but not 3beta-hydroxysteroid dehydrogenase in adrenal cells. CRH did not alter cell number, indicating that it is not mitogenic for fetal adrenal cortical cells. These data demonstrate a direct functional interaction between CRH and the fetal adrenal. Therefore, placental CRH production, which rises exponentially during human pregnancy, may play a key role in promoting DHEA-S production by the fetal adrenals, which could lead to increasing placental estrogen synthesis and contribute to the process of parturition in humans.

Characterization of a human-specific regulator of placental corticotropin-releasing hormone

The hypothalamic hormone CRH is also expressed in the placentas of humans and higher primates and may play an important role in the regulation of labor. In choriocarcinoma cell lines, activation of cAMP-dependent pathways increases human (h)CRH reporter gene expression. A cAMP-responsive region distinct from the cAMP response element at -220 bp, has been identified between -200 and -99 bp, and a candidate transcription factor was identified in nuclear extracts of human, but not rodent, choriocarcinoma cell lines. This region, which does not contain a canonical cAMP response element (CRE), transfers protein kinase A responsiveness to a heterologous promoter. Electromobility shift assays and methylation and uracil interference studies localized factor binding to a 20-bp region from -128 to -109 bp of the hCRH promoter. This 20-bp fragment exhibited a similar shift in nuclear extracts from both human term placenta and from human JEG-3 cells. Base contacts, identified in interference studies, were confirmed as critical for binding, as a mutation of these bases abolished factor binding. Furthermore, a CRH promoter containing this mutation exhibited a diminished response to forskolin. UV cross-linking demonstrated the protein in nuclear extracts from human, but not rodent, choriocarcinoma cell lines and estimated its size as 58 kDa. Although this factor participates in cAMP-regulated gene expression, competition electrophoretic mobility assays demonstrated that the factor does not bind to a CRE. Furthermore, neither anti-CREB nor anti-ATF2 antibodies alter factor binding. These data identify this 58-kDa protein as the human-specific CRH activator previously identified as a candidate factor contributing to the species-specific expression of CRH in human placenta.

Urocortin in pregnancy

OBJECTIVES

The aim of this study was to investigate the possibility that urocortin is the ligand that displaces corticotropin-releasing hormone from its binding protein in the maternal circulation during pregnancy and, if so, to determine whether urocortin, like corticotropin-releasing hormone, is synthesized in substantial quantities in the placenta.

STUDY DESIGN

A radioimmunoassay specific for urocortin was developed and used for measurement of the peptide in chorionic villi and fetal membranes (amnion and chorion) from normal and preeclamptic pregnancies. These tissues were also assayed for corticotropin-releasing hormone. Assays for urocortin were also carried out on normal term pregnant and nonpregnant myometrium and on plasma from nonpregnant individuals, and assays for both peptides were performed on sequential normal pregnancy plasma samples taken from mid gestation until term.

RESULTS

Corticotropin-releasing hormone was present in normal term (1904 +/- 489 pg/g) and preeclamptic placentas (5897 +/- 1526 pg/g) and in normal term fetal membranes (645 +/- 155 pg/g, n = 6 in all cases). Urocortin was not detected in any of the tissues studied, nor was it found in the normal human plasma samples. Unlike the situation for corticotropin-releasing hormone, no pregnancy-related pattern was seen for urocortin in the plasma from pregnant women.

CONCLUSIONS

Urocortin is not translated to any great extent in the pregnancy tissues investigated, nor is it present in the circulation of pregnant women in detectable amounts. Furthermore, it is unlikely that urocortin is responsible for the high maternal plasma levels of free corticotropin-releasing hormone circulating in the latter stages of pregnancy, but this does not preclude the possibility that another, as yet uncharacterized, corticotropin-releasing hormone-like peptide may be.