First trimester levels of inhibins and activin A in normal and failing pregnancies

Activin A and pathologies of pregnancy: a review

Activin A is a two-subunit protein belonging to the transforming growth factor β superfamily. First discovered almost three decades ago, it has since been implicated in diverse physiological roles, ranging from wound repair to reproduction. After 30 years of research, altered activin A levels are now understood to be associated with the development of various diseases, making activin A a potential therapeutic target. In pregnancy, the placenta and fetal membranes are major producers of activin A, with significantly enhanced serum concentrations now recognised as a contributor to numerous gestational disorders. Evidence now suggests that circulating levels of activin A may be clinically relevant in the early detection of pregnancy complications, including miscarriage and preeclampsia. This review aims to summarise our current understanding of activin A as a potential diagnostic marker in common pregnancy pathologies.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Activins in reproductive biology and beyond

BACKGROUND

Activins are members of the pleiotrophic family of the transforming growth factor-beta (TGF-β) superfamily of cytokines, initially isolated for their capacity to induce the release of FSH from pituitary extracts. Subsequent research has demonstrated that activins are involved in multiple biological functions including the control of inflammation, fibrosis, developmental biology and tumourigenesis. This review summarizes the current knowledge on the roles of activin in reproductive and developmental biology. It also discusses interesting advances in the field of modulating the bioactivity of activins as a therapeutic target, which would undoubtedly be beneficial for patients with reproductive pathology.

METHODS

A comprehensive literature search was carried out using PUBMED and Google Scholar databases to identify studies in the English language which have contributed to the advancement of the field of activin biology, since its initial isolation in 1987 until July 2015. 'Activin', 'testis', 'ovary', 'embryonic development' and 'therapeutic targets' were used as the keywords in combination with other search phrases relevant to the topic of activin biology.

RESULTS

Activins, which are dimers of inhibin β subunits, act via a classical TGF-β signalling pathway. The bioactivity of activin is regulated by two endogenous inhibitors, inhibin and follistatin. Activin is a major regulator of testicular and ovarian development. In the ovary, activin A promotes oocyte maturation and regulates granulosa cell steroidogenesis. It is also essential in endometrial repair following menstruation, decidualization and maintaining pregnancy. Dysregulation of the activin-follistatin-inhibin system leads to disorders of female reproduction and pregnancy, including polycystic ovary syndrome, ectopic pregnancy, miscarriage, fetal growth restriction, gestational diabetes, pre-eclampsia and pre-term birth. Moreover, a rise in serum activin A, accompanied by elevated FSH, is characteristic of female reproductive aging. In the male, activin A is an autocrine and paracrine modulator of germ cell development and Sertoli cell proliferation. Disruption of normal activin signalling is characteristic of many tumours affecting reproductive organs, including endometrial carcinoma, cervical cancer, testicular and ovarian cancer as well as prostate cancer. While activin A and B aid the progression of many tumours of the reproductive organs, activin C acts as a tumour suppressor. Activins are important in embryonic induction, morphogenesis of branched glandular organs, development of limbs and nervous system, craniofacial and dental development and morphogenesis of the Wolffian duct.

CONCLUSIONS

The field of activin biology has advanced considerably since its initial discovery as an FSH stimulating agent. Now, activin is well known as a growth factor and cytokine that regulates many aspects of reproductive biology, developmental biology and also inflammation and immunological mechanisms. Current research provides evidence for novel roles of activins in maintaining the structure and function of reproductive and other organ systems. The fact that activin A is elevated both locally as well as systemically in major disorders of the reproductive system makes it an important biomarker. Given the established role of activin A as a pro-inflammatory and pro-fibrotic agent, studies of its involvement in disorders of reproduction resulting from these processes should be examined. Follistatin, as a key regulator of the biological actions of activin, should be evaluated as a therapeutic agent in conditions where activin A overexpression is established as a contributing factor.

Inhibin at 90: from discovery to clinical application, a historical review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

Activin A and follistatin as biomarkers for ectopic pregnancy and missed abortion

Activin A as a predictor of pregnancy failure has been the focus of heated debate, but the value of a combined activin A and follistatin (FS) measurement in serum to predict pregnancy failure has not been reported yet. We assessed whether a single serum measurement of the two physiological antagonists at 6–8 weeks gestation could differentiate ectopic pregnancies (EP) or missed abortions (MA) from healthy intrauterine pregnancies (IUP). activin A concentrations were significantly lower in women with EP (n = 30, median value of 264 pg/mL) and women with MA (n = 30, median value of 350 pg/mL) compared to IUP (n = 33, median value of 788 pg/mL); P < 0.001. At a threshold value of 505 pg/mL, activin A had 87.9% sensitivity and 100% specificity and negative predictive value of 0.974 for discriminating an ectopic pregnancy from viable pregnancies. FS was able to discriminate IUP from EP (ROC curve P < 0.001) as was their ratio (ROC curve P = 0.008), but was unable to discriminate a MA from an EP. In EP, activin A did not correlate with beta HCG levels. The present findings support the thesis that activin A or FS could be considered promising biomarkers for the discrimination between an IUP and a failed pregnancy (MA or EP).

Predictive power of activin A levels in the prognosis of first trimester in vitro fertilization pregnancies

BACKGROUND

The present study aims to determine the predictive power of activin A levels in the prognosis of first trimester pregnancies conceived by in vitro fertilization (IVF).

METHODS

The study cohort included 23 biochemical, and 23 normal ongoing pregnancies conceived via IVF. Serum β-human chorionic gonadotropin (β-hCG), progesterone, and activin A levels were assessed 14 days after embryo transfer.

RESULTS

Serum activin A levels were significantly lower in biochemical pregnancies compared to normal ongoing pregnancies (0.57 vs. 0.81 ng/mL, p<0.001). The ability of activin A to predict normal ongoing pregnancies at a cutoff level of 0.695 ng/mL gave a sensitivity of 91.3%, a specificity of 100%, a positive predictive value of 100%, and a negative predictive value of 92%. Significant correlation was found between β-hCG and progesterone, β-hCG and activin A, and progesterone and activin A. Activin A, β-hCG, and progesterone were all found to be efficacious in prediction of early IVF pregnancies.

CONCLUSIONS

The present study indicates that single measurement of activin A can be suggested as a marker for the progress and outcome of early pregnancies conceived via IVF. However, further large-scale studies are required to determine the efficacy and reliability of activin A in prediction of early pregnancies achieved by assisted reproductive techniques (ART).

The synthesis and secretion of inhibins

Inhibin A and B, dimeric glycoproteins comprising an α- and β((A/B))-subunit, negatively regulate follicle stimulating hormone (FSH) synthesis by the pituitary. The expression of α- and β-subunits within Sertoli cells of the testis and granulosa cells of the ovary is controlled by a range of transcription factors, including CREB, SP-1, Smads, and GATA factors. The inhibin α- and β-subunits are synthesized as precursor molecules consisting of an N-terminal propeptide and a C-terminal mature domain. Recently, we showed that hydrophobic residues within the propeptides of the α- and β-subunits interact noncovalently with their mature domains, maintaining the molecules in a conformation competent for dimerization. Dimeric precursors are cleaved by proprotein convertases and mature inhibins are secreted from the cell noncovalently associated with their propeptides. Propeptides may increase the half-life of inhibin A and B in circulation, but they are readily displaced in the presence of the high-affinity receptors, betaglycan, and ActRII.

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.

TGF-β superfamily expression and actions in the endometrium and placenta

Transforming growth factor β (TGFβ) superfamily members are closely associated with tissue remodelling events and reproductive processes. This review summarises the current state of knowledge regarding the expression and actions of TGFβ superfamily members in the uterus, during the menstrual cycle and establishment of pregnancy. TGFβs and activin β subunits are abundantly expressed in the endometrium, where roles in preparation events for implantation have been delineated, particularly in promoting decidualisation of endometrial stroma. These growth factors are also expressed by epithelial glands and secreted into uterine fluid, where interactions with preimplantation embryos are anticipated. Knockout models and embryo culture experiments implicate activins, TGFβs, nodal and bone morphogenetic proteins (BMPs) in promoting pre- and post-implantation embryo development. TGFβ superfamily members may therefore be important in the maternal support of embryo development. Following implantation, invasion of the decidua by fetal trophoblasts is tightly modulated. Activin promotes, whilst TGFβ and macrophage inhibitory cytokine-1 (MIC-1) inhibit, trophoblast migration in vitro, suggesting the relative balance of TGFβ superfamily members participate in modulating the extent of decidual invasion. Activins and TGFβs have similar opposing actions in regulating placental hormone production. Inhibins and activins are produced by the placenta throughout pregnancy, and have explored as a potential markers in maternal serum for pregnancy and placental pathologies, including miscarriage, Down’s syndrome and pre-eclampsia. Finally, additional roles in immunomodulation at the materno-fetal interface, and in endometrial inflammatory events associated with menstruation and repair, are discussed.

Human chorionic gonadotropin in cervical fluid as a predictor of miscarriage

OBJECTIVE

To determine whether the concentration of hCG in cervical fluid can predict miscarriage during early pregnancy.

DESIGN

Prospective clinical study.

SETTING

University hospital.

PATIENT(S)

Ninety-nine women with singleton pregnancies, 17 of whom suffered a miscarriage.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Miscarriage and the cervical fluid/serum hCG concentration ratio.

RESULT(S)

With a cutoff value of 1.0, the sensitivity, specificity, and positive and negative predictive values of the cervical fluid/serum hCG concentration ratio for predicting miscarriage were 88.2%, 94.9%, 83.3%, and 96.6%, respectively.

CONCLUSION(S)

The cervical fluid/serum hCG concentration ratio might be a useful predictor of miscarriage during early pregnancy.

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.

Significance of inhibin in reproductive pathophysiology and current clinical applications

The human reproductive process is regulated by complex mechanisms that involve many organs, including the brain, gonads and endocrine system. It has been more than 70 years since the name 'inhibin' was used to describe a substance produced in the gonads that negatively regulates pituitary secretion. Inhibin B controls FSH secretion via a negative feedback mechanism. It is a glycoprotein hormone secreted by the Sertoli cells of the testis and granulosa and theca cells of the ovary. Serum inhibin B concentrations are positively related to testicular volume and sperm counts. Current understanding of inhibin physiology and pathology in the human suggests that inhibin B may be of importance as a marker of Sertoli cell function in men with infertility and as a prognostic indicator in women undergoing ovulation induction therapy. Inhibin concentrations are elevated in patients with granulosa cell tumours and in post-menopausal women with mucinous ovarian cancers. Immunoreactivity against the inhibin alpha-subunit was identified in all cases of adrenal cortical adenoma and carcinoma, and levels are suppressed in the malignant prostate disease. This article discusses the structure, regulation and clinical use of inhibin and other related substances.

Inhibin, activin, follistatin, activin receptors and β-glycan gene expression in the villous tissue of miscarriage patients

Maternal circulating levels of inhibin A are significantly lower in patients with clinical symptoms of miscarriage. The objective of this study was to quantify relative expression of inhibin alpha, inhibin/activin betaA, betaB, betaC, follistatin, activin receptors and beta-glycan genes and content of inhibin A, activin A and follistatin protein in villous tissue of first trimester miscarriages and gestation-matched normal pregnancies. Twelve women with clinical symptoms of miscarriage were matched with 12 normal pregnancies for gestational age. Total RNA was isolated from placental samples. Complementary DNA produced by reverse transcription was used in the real-time PCR to quantify the expression of the genes. The ratio between the target and rRNA 18S was calculated to provide relative gene expression. Villous tissue homogenates were used for the determination of the content of inhibin A, activin A and follistatin protein. Maternal serum was assayed for inhibin A, activin A and follistatin. All villous samples expressed inhibin alpha, inhibin/activin betaA, betaB, betaC, follistatin, activin receptors (ACTRIA, ACTRIB, ACTRIIA, ACTRIIB) and beta-glycan genes. There was no significant difference in the relative expression of these genes between the groups. Villous content of inhibin A, activin A and follistatin were also not different between the two groups. Maternal serum levels of inhibin A were significantly lower in the miscarriage group compared to the controls. The decreased maternal levels of inhibin A in miscarriage patients could be due to a decrease in placental mass prior to embryonic demise. This finding also confirms that the trophoblast is the major source of inhibin A after the luteo-placental shift in early pregnancy.