Inhibin suppresses human chorionic gonadotropin secretion in term, but not first trimester, placenta

Placental production and maternal serum and urine levels of inhibin A and activin A are modified by antihypertensive therapy in hypertensive disorders of pregnancy

OBJECTIVE

Levels of inhibin A and activin A are raised in pre-eclampsia (PE) but it is not known if antihypertensive therapy can affect their levels. Our aim was to investigate the effect of the antihypertensive drug alpha-methyldopa on serum, urine and placental concentrations of inhibin A and activin A in women presenting with hypertensive disorders of pregnancy.

DESIGN

This was a cross-sectional study.

PATIENTS

We recruited 65 women presenting with PE, 39 with gestational hypertension (GH) and 104 normotensive controls matched for maternal age, gestational age and parity.

MEASUREMENTS

Using specific validated ELISAs, serum and urine levels of inhibin A and activin A, and uterine artery Doppler indices, were measured before and 24-48 h after initiating alpha-methyldopa therapy in women with PE, with GH and controls. Protein extracts were obtained from samples of placental tissue from another group of women with PE, GH and controls for the same analysis.

RESULTS

In PE, but not GH, alpha-methyldopa therapy was associated with significantly (P < 0.05) lower levels of both serum and urine inhibin A and activin A. Similarly, in PE but not GH, alpha-methyldopa therapy was associated with lower placental levels of both markers (P < 0.05). There was no significant difference in pulsatility index following treatment in either PE or GH.

CONCLUSIONS

Our data indicate that antihypertensive therapy with alpha-methyldopa may have an effect on the synthesis and/or release of placental proteins in pregnancies complicated by PE and that this effect may be independent of its known antihypertensive action.

Maternal serum hormone concentrations for prediction of adverse outcome in threatened miscarriage

Many serum markers have been investigated in attempts to predict the outcome of pregnancy in the first trimester, with varying degrees of success. The objective of this study was to investigate whether they can be related to pregnancy outcome in women presenting with first trimester threatened miscarriage. A cohort study of women attending the Early Pregnancy Unit of a London teaching hospital was studied. A total of 122 women presenting with bleeding in the first trimester and an ongoing pregnancy, and 33 women undergoing termination of pregnancy, were recruited. The main outcome measures were gestation at delivery, birth weight and the incidence of adverse pregnancy outcome. Inhibin A, activin A, human chorionic gonadotrophin (HCG), pregnancy-associated plasma protein-A and follistatin concentrations were all significantly lower in women who subsequently miscarried when compared with live births. Serum HCG concentrations were significantly higher in cases of threatened miscarriage compared with controls (P = 0.0009). Logistic regression analysis indicated that inhibin A alone provided the best predictor for first trimester miscarriage. This pilot study suggests that placental hormone concentrations could be useful in predicting adverse pregnancy outcome in women presenting with threatened miscarriage. Inhibin A was best at predicting the likelihood of subsequent miscarriage in this group.

Hormonal monitoring of the first trimester of pregnancy

The hormonal changes and maternal adaptations of human pregnancy are among the most remarkable phenomena in nature. Endocrinologic parameters in the early gestation period have been used to predict abnormal pregnancies and to identify fetuses that have chromosomal aberrations. This article focuses on the changes in hormones that are secreted by the maternal-fetal-placental unit that are unique for the first trimester of pregnancy and their impact on clinical outcome.

Activin and follistatin in female reproduction

Activin and follistatin were initially identified in the follicular fluid based on their effects on pituitary FSH secretion in the mid-1980s. It is now evident that activin, follistatin and activin receptors are widely expressed in many tissues where they function as autocrine/paracrine regulators of a variety of physiological processes including reproduction. The major function of follistatin is to bind to activin with high affinity and block activin binding to its receptors. Total activin A and follistatin are also found in the maternal circulation throughout pregnancy. Activin A levels are increased in abnormal pregnancies such as pre-eclampsia, fetal growth restriction and gestational hypertension. The placenta, vascular endothelial cells and activated peripheral mononuclear cells (PBMC) may all contribute to the raised levels of activin A in pre-eclampsia with unaltered follistatin in pre-eclamptic placenta, PBMCs or vascular endothelial cells suggesting the availability of 'free' activin A that could be biologically active in these cells.

Predicting complications of pregnancy with first-trimester maternal serum free-?hCG, PAPP-A and inhibin-A

OBJECTIVE

To find whether fbetahCG, PAPP-A and inhibin-A levels in maternal serum or fetal nuchal translucency (NT) thickness at the first-trimester screening for trisomy 21 (T21) might detect women at high risk for adverse pregnancy outcomes.

METHODS

A retrospective analysis of 1136 women with singleton pregnancy between 10 and 14 weeks. Women with pregnancy complications were allotted to five subgroups: small for gestational age (SGA), large for gestational age (LGA), gestational diabetes (GDM), hypertensive disorders, preterm delivery; women with normal pregnancy represented the control group. NT, maternal serum fbetahCG, PAPP-A and inhibin-A were measured. Mann-Whitney test was used for the comparison of fbetahCG, PAPP-A, inhibin-A and NT between a subgroup of a certain pregnancy complication and the control group. Multivariate logistic regression models were built to explore the relationship among different variables and the occurrence of pregnancy complications.

RESULTS

PAPP-A values were significantly lower in women who delivered SGA babies (n=51, 0.76 MoM; p=0.002) and significantly higher in women who delivered LGA babies (n=120, 1.12 MoM; p=0.036). In women with GDM (n=27), fbetahCG, PAPP-A and inhibin-A were insignificantly lower than in controls, whereas in women with hypertensive disorders (n=56) no significant differences between the groups were found. In women with a preterm delivery (<34 weeks) (n=17), inhibin-A levels were significantly higher (1.25 MoM; p=0.015).

CONCLUSION

Low PAPP-A level is associated with the delivery of an SGA baby and high PAPP-A with the delivery of an LGA baby. High inhibin-A is associated with preterm delivery before 34 weeks. Feto-placental products in the first trimester do not prove to be useful as a screening tool for predicting pregnancy complications.

Immunohistochemical expression analysis of inhibin-alpha and -beta subunits in partial and complete moles, trophoblastic tumors, and endometrial decidua

The expression of inhibin-alpha subunit has been described in normal placentas, hydatidiform moles, and trophoblastic tumors. We performed a double immunohistochemical expression analysis of inhibin-alpha and inhibin-beta subunits in a cytogenetically well characterized series of 21 complete and 22 partial hydatidiform moles, 2 placental site trophoblastic tumors, and one choriocarcinoma. Syncytiotrophoblastic cells were consistently inhibin-alpha and inhibin-beta positive in all hydatidiform moles and in the one choriocarcinoma. Cytotrophoblast was negative for both subunits in all trophoblastic lesions studied. While villous intermediate trophoblastic cells were consistently inhibin-alpha negative in all hydatidiform moles, focal inhibin-beta immunoreactivity was detected in villous intermediate trophoblast in approximately one third of complete and partial hydatidiform moles. Decidual stromal cells in 40 hydatidiform moles were inhibin-alpha and inhibin-beta positive in approximately one third of cases. Both placental site trophoblastic tumors were inhibin-alpha positive but inhibin-beta negative. Our findings indicate that inhibin-alpha and -beta subunits are consistently coexpressed in syncytiotrophoblast in complete and partial moles. Immunohistochemical detection of inhibin subunits may be useful in the differential diagnosis of trophoblastic lesions.

Maternal serum levels of dimeric inhibin A in pregnancies affected by trisomy 21 in the first trimester

Dimeric inhibin A was measured in maternal serum samples from 45 pregnancies affected by trisomy 21 and 493 samples from unaffected pregnancies at 10-14 weeks of gestation. Inhibin A levels in affected pregnancies were compared with levels of free beta-hCG and PAPP-A in the same series. In the trisomy 21 group, the median multiple of the median (MoM) inhibin A was not significantly elevated (1.28 vs 1.00) with only 15.5% being above the 95th centile. In contrast, the median MoM free beta-hCG was significantly increased (2.05 vs 1.00) with 36% above the 95th centile and PAPP-A was significantly reduced (0.49 vs 1.00) with 42% below the 5th centile. Inhibin A levels in the trisomy 21 group were significantly correlated with gestational age such that median levels rose from 1.04 at 11 weeks to 1.30 at 12 weeks and 1.67 at 13 weeks. These findings suggest that first trimester biochemical screening for trisomy 21, which is currently optimised using maternal serum free beta-hCG and PAPP-A and fetal nuchal translucency, will not benefit from the inclusion of inhibin A.

Placental and maternal serum inhibin-A and activin-A levels in Down's syndrome pregnancies

The objective of this study was to analyse the levels of inhibin-A and activin-A in maternal serum and placental tissue from Down's syndrome (DS) pregnancies. Inhibin-A and activin-A levels were determined by specific immunoassays and individual results were expressed as multiples of the control median (MoM) at the appropriate gestation. Immunohistochemistry was used to localize inhibin alpha and beta(A)-subunits in a selection of placental sections. In DS pregnancies, median inhibin-A levels were found to be significantly elevated to 1.46 MoM (P< 0.05) in placental extracts, and 2.06 MoM (P< 0.0001) in maternal serum, when compared with uncomplicated pregnancies. Median activin-A MoMs were also elevated in placental extracts and maternal serum to 1.62 MoM (P< 0.01), and 1.26 MoM (P< 0.05), respectively. Immunohistochemistry revealed that the alpha subunit of inhibin-A and the beta(A)subunit of inhibin-A and activin-A were mainly localized to the trophoblastic layer of placental villi. Semiquantitative studies of staining intensity revealed a trend towards stronger staining of placental trophoblasts and stroma of DS tissues, although this was statistically significant only for beta(A)subunit staining of trophoblasts (P< 0.05). These results support the hypothesis that maternal serum levels of inhibin-A and activin-A are elevated due to increased production in the placenta, and increased immunostaining of trophoblasts suggests that this may be due to increased production in the trophoblasts.

High concentrations of serum inhibin in pre-eclampsia

OBJECTIVE

To evaluate maternal serum immunoreactive inhibin (ir-inhibin) concentrations in women with pre-eclampsia, and assess the correlation between serum ir-inhibin and HCG.

METHODS

The subjects comprised 28 pregnant women with suspected intrauterine growth retardation (IUGR) during the third trimester. Serum concentrations of ir-inhibin and HCG were measured in 13 women with pre-eclampsia and 15 pregnant women as control subjects. Serum ir-inhibin was determined by a double antibody radioimmunoassay, and HCG by a solid-phase immunoradiometric assay.

RESULTS

There were no significant differences in maternal characteristics between the pre-eclamptic group and control group. The pre-eclamptic group had significantly higher concentrations of serum ir-inhibin and HCG compared with the control group. The serum concentrations of ir-inhibin correlated positively with those of HCG.

CONCLUSION

The pre-eclamptic patients displayed high serum levels of ir-inhibin and HCG, and this might reflect hyperplasia of trophoblastic cells.

Advance in the study of inhibin, activin and follistatin production in pregnant women

This review summarizes the new information on the studies of inhibin, activin, and follistatin production in the placenta during human pregnancy. Inhibin and activin exert suppressive and stimulatory effects, respectively, on the release of FSH in the pituitary. Follistatin is bound to inhibin and activin and indirectly modulates the FSH release. The placenta produces these three proteins. The serum levels of inhibin, activin, and follistatin are elevated in pregnant women and decrease after delivery. The trophoblast cells from term placenta secrete inhibin and activin in the primary cultures. The production and mRNA expression of inhibin and activin are regulated by several stimulatory and suppressive hormones and growth factors in placental tissues. cAMP, Ca2+, and protein kinase-C may be involved in intracellular signal transduction in trophoblasts. Activin receptors are present on placental cells. Follistatin inhibits the binding of activin to ActRII receptor. Abnormal levels of inhibin and activin in maternal serum are observed in problem pregnancies and gestational diseases. Inhibin, activin, and follistatin may play roles in the regulation of reproductive endocrinology in pregnant women and the embryo/fetal development.

Equine inhibin/activin beta A-subunit mRNA is expressed in the endometrial gland, but not in the trophoblast, during pregnancy

The expression of both inhibin alpha- and inhibin/activin beta A-subunit mRNA was examined in equine uteroplacental tissues collected during pregnancy (days 90 to 300). Northern blot analysis revealed that 5 transcripts (7.0, 4.1, 3.4, 2.6, 1.5 kb) of beta A-subunit were present, and the most abundantly expressed transcript was the 1.5 kb one. Relatively high levels of the 1.5 kb transcript were seen in the second trimester of pregnancy compared to what was found in the third trimester. To identify the tissue localization of beta A-subunit mRNA, in situ hybridization was performed, and the positive signal was observed exclusively in the endometrial glands, but not in the fetal placental tissue (trophoblast) at days 150, 210, and 300 of pregnancy. On the other hand, inhibin alpha-subunit transcript could not be detected at any stage of pregnancy examined either by Northern blot analysis or in situ hybridization. Although the factor(s) regulating the gene expression of beta A-subunit in this equine tissue is currently unknown, these results suggest that activin, but not inhibin, is predominantly produced in the endometrial glands of the pregnant mare, and thus produced activin may play a paracrine or endocrine role during pregnancy in this species.

Maternal serum inhibin-A in pregnancies complicated by insulin dependent diabetes mellitus

We have examined whether insulin dependent diabetes mellitus (IDDM) affects maternal serum levels of inhibin-A, a recently described prenatal marker of Down's syndrome, by comparing levels in 169 women with IDDM with levels in 432 nondiabetic pregnant women between 15 and 20 weeks of gestation. There was a small but significant increase in the inhibin-A level in the diabetic women only when levels were corrected for maternal weight: median MoM 1.17 (P < 0.01 vs controls, Student's t test). The underlying mechanism for this elevation in pregnancies complicated by IDDM currently remains obscure.

Inhibin, activin and follistatin in the human placenta--a new family of regulatory proteins

The family of inhibin-related proteins has been investigated extensively in the last decade. It is composed of three members: inhibin, activin and follistatin. Inhibin and activin are chemically related, while follistatin acts as an activin-binding protein. Initially identified as regulators of pituitary follice stimulating hormone (FSH) secretion, inhibin, activin and follistatin have more recently been characterized as growth factors, embryo modulators and immune factors. Human placenta, amnion, chorion and maternal decidua express mRNAs for inhibin, activin and follistatin, and the presence of both immunoreactive and bioactive proteins has been demonstrated. The proteins are present in maternal and fetal circulation, and are measurable in amniotic fluid with changes related to gestational age and to the occurrence of gestational diseases. Various biological actions have been described in embryo and intrauterine tissues, which suggest a role for these proteins in the development of the gestational unit. However, several questions remain to be elucidated. The chemical forms of inhibin, activin and follistatin produced by human placenta and the mechanisms involved in the regulation of their secretion are largely unknown. The nature of the receptors for these proteins and the physiological implications of receptor activation have not yet been elucidated and this will require further investigation.

Steroid-protein interaction in human placenta

Human placenta produces a large variety of bioactive substances with endocrine and neural competence: pituitary and gonadal hormones, hypothalamic-like releasing or inhibiting hormones, growth factors, cytokines and neuropeptides. The most recent findings indicate that locally produced hormones regulate the secretion of other placental hormones supporting a paracrine/autocrine regulation. In placental endocrinology, a particular relevance is played by steroid hormones. In fact, a specific gonadotropin-releasing hormone (GnRH)-human chorionic gonadotropin (hCG) regulation of placental steroidogenesis has been proposed as a placental internal regulatory system acting on steroids production from human placenta. In addition, activin and inhibin have been proposed as further regulatory substances of the synthesis and secretion of steroids; the addition of activin A to placental culture augments GnRH, hCG and progesterone, and this effect can be significantly reduced by the addition of inhibins. Finally, a steroid-steroid interaction is suggested by the evidence that placental estrogen has a positive role in the regulation of progesterone biosynthesis. Other steroid-protein interactions have been observed in human placenta. In fact, recent data indicate that progesterone inhibits placental corticotropin-releasing factor (CRF) and estrogens act on placental conversion of cortisol to cortisone, activating cortisol secretion by the fetal adrenal and enhancing fetal adrenal function with advancing gestation.

Inhibin in normal and abnormal pregnancy: maternal serum concentration and partial characterization

OBJECTIVE

This study evaluated maternal serum inhibin concentration and molecular distribution in normal and abnormal pregnancies.

STUDY DESIGN

Serum inhibin levels were measured by radioimmunoassay in 33 abnormal pregnancies (23 singleton and 10 multiple gestations) and 33 matched controls and were compared with pregnancy variables.

RESULTS

In the controls serum inhibin concentrations rose with gestational age (correlation coefficient 0.358, p < 0.01) and were higher in mothers with female fetuses (p < 0.05). Inhibin levels were higher in the abnormal singleton pregnancies (2.69 +/- 1.41 ng/ml) than in controls (1.26 +/- 0.32 ng/ml, p < 0.0001) and increased more rapidly with gestational age in multiple pregnancies (p < 0.05). Gel filtration chromatographic analysis of immunoreactive inhibin revealed three peaks with the major form having an apparent molecular weight of 55 to 60 kd.

CONCLUSION

Maternal serum inhibin concentrations are affected by gestational age, fetal gender, and complications of pregnancy. Most of the immunoreactive inhibin in maternal circulation is a large-molecular-weight form.

Gonadotropin-releasing hormone (GnRH) and cyclic AMP positively regulate inhibin subunit messenger RNA levels in human placental cells

Bioactive and immunodetectable levels of both inhibin and activin are present in the placenta, raising questions as to the regulatory control of their synthesis. This study was designed to determine the effect of cyclic AMP (cAMP) and gonadotropin-releasing hormone (GnRH) on inhibin subunit gene expression in short-term incubations of placental cells. A semi-quantitative polymerase chain reaction (PCR) technique was used after isolation of total RNA and first strand cDNA synthesis from mechanically dispersed trophoblast-enriched cells obtained from human placentae at term. The level of gene expression of inhibin subunits was higher for beta A and alpha-subunits mRNA compared to the beta B-subunit mRNA as determined by PCR in combination with Southern blotting or Northern hybridization. Steady-state levels of beta-actin mRNA did not change throughout the 6-h incubation period and was used as a control of PCR amplification of the respective inhibin subunit gene transcripts following treatments with 8-bromo cAMP or GnRH. 8-bromo cAMP dose-dependently increased all three inhibin subunit gene transcripts with maximal responses seen at 150 microM (alpha-subunit mRNA 2.3-fold, beta A-subunit mRNA 1.8-fold and beta B-subunit mRNA 2.8-fold over control). GnRH (100 nM) significantly increased inhibin alpha and beta B-subunit mRNA levels 2.9-fold and 2.0-fold, respectively (P < 0.01), but not beta A-subunit mRNA. Collectively, the present findings demonstrate that in human term placental cells, gene expression of all inhibin subunits is under the direct influence of cAMP and further support a modulatory role of local GnRH in placental trophoblasts during late pregnancy.

The relationship between peripheral immunoactive inhibin, human chorionic gonadotrophin, oestradiol and progesterone during human pregnancy

OBJECTIVE

The objective of this study was to investigate serial changes in the plasma concentration of inhibin in both the very early days of pregnancy following implantation and in late pregnancy. The timing of the changes in inhibin concentration relative to changes in the concentrations of other hormones of pregnancy was also investigated.

DESIGN

Serial observations of the peripheral concentrations of inhibin and other hormones in two groups of healthy volunteers in (a) early pregnancy and (b) late pregnancy.

PATIENTS

(a) Four healthy women recruited on cessation of contraception prior to conception. (b) Nine healthy women recruited at the antenatal clinic.

MEASUREMENTS

In the early pregnancy subjects, the concentrations of inhibin, progesterone, oestradiol and hCG were measured in plasma samples obtained three times per week from day 8 to day 10 of each menstrual cycle until 11 weeks after the last menstrual period in the conception cycle. In the late pregnancy subjects, plasma samples were obtained at 4-week intervals from 12 weeks until term.

RESULTS

The concentration of inhibin, progesterone and oestradiol in conception cycles were similar to those in the preceding cycles until the mid/to late-luteal phase of the cycle when hCG was first measureable. By day 12 of the luteal phase the concentration of inhibin was significantly higher in the pregnancy cycle than in the non-pregnancy cycle (P < 0.05) and progressively increased after the time of the missed menstrual period. The concentration of inhibin reached a peak (513.0 U/l, CI 442.1-595.3) by day 47 when the concentration of hCG was maximal. In early pregnancy the concentration of inhibin was correlated with that of hCG (r = 0.361; P < 0.01) as well as progesterone (r = 0.584, P < 0.001) and oestradiol (r = 0.602, P < 0.001). After 12 weeks there was no significant correlation between hCG and inhibin although significant correlations persisted with progesterone (r = 0.553, P < 0.001) and oestradiol (r = 0.361, P < 0.01).

CONCLUSIONS

The corpus luteum makes a significant contribution to the production of inhibin in early pregnancy while after 12 weeks the placenta is the major source.