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.

Antepartum Aspirin Administration Reduces Activin A and Cardiac Global Longitudinal Strain in Preeclamptic Women

Background Approximately 60% of women have Stage B heart failure 1 year after a preeclamptic delivery. Emerging evidence suggests that the profibrotic growth factor activin A, which has been shown to induce cardiac fibrosis and hypertrophy, is elevated in preeclampsia and may be inhibited by aspirin therapy. We hypothesized that preeclamptic women receiving aspirin would have lower activin A levels and reduced global longitudinal strain (GLS), a sensitive measure of cardiac dysfunction, than women who do not receive aspirin. To test our hypothesis, we performed a cohort study of women with preeclampsia or superimposed preeclampsia and compared activin A levels and GLS in parturients who did or did not receive aspirin. Methods and Results Ninety-two parturients were enrolled, of whom 25 (27%) received aspirin (81 mg/day) therapy. GLS, plasma activin A, and follistatin, which inactivates activin A, were measured. Women receiving aspirin therapy had lower median (interquartile range) levels of activin A (8.17 [3.70, 10.36] versus 12.77 [8.37, 31.25] ng/mL; P=0.001) and lower activin/follistatin ratio (0.59 [0.31, 0.93] versus 1.01 [0.64, 2.60] P=0.002) than women who did not receive aspirin, which also remained significant after multivariable analysis. Furthermore, GLS was worse in patients who did not receive aspirin (-19.84±2.50 versus -17.77±2.60%; P=0.03) despite no differences in blood pressure between groups. Conclusions Our study suggests that antepartum aspirin therapy reduced serum activin A levels and improved GLS in preeclamptic patients, suggesting that aspirin may mitigate the postpartum cardiac dysfunction seen in women with preeclampsia.

Activin and inhibin signaling: From regulation of physiology to involvement in the pathology of the female reproductive system

Activins and inhibins - comprising activin A, B, AB, C and E, and inhibin A and B isoforms - belong to the transforming growth factor beta (TGFβ) superfamily. They regulate several biological processes, including cellular proliferation, differentiation and invasiveness, to enhance the formation and functioning of many human tissues and organs. In this review, we have discussed the role of activin and inhibin signaling in the physiological and female-specific pathological events that occur in the female reproductive system. The up-to-date evidence indicates that these cytokines regulate germ cell development, follicular development, ovulation, uterine receptivity, decidualization and placentation through the activation of several signaling pathways; and that their dysregulated expression is involved in the pathogenesis and pathophysiology of the numerous diseases, including pregnancy complications, that disturb reproduction. Hence, some of the isoforms have been suggested as potential biomarkers and therapeutic targets for the management of some of these diseases. Tackling the research directions highlighted in this review will enhance a detailed comprehension and the clinical utility of these cytokines.

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.

Serum activin A and follistatin levels in gestational diabetes and the association of the Activin A-Follistatin system with anthropometric parameters in offspring

CONTEXT

The Activin A-Follistatin system has emerged as an important regulator of lipid and glucose metabolism with possible repercussions on fetal growth.

OBJECTIVE

To analyze circulating activin A, follistatin and follistatin-like-3 (FSTL3) levels and their relationship with glucose metabolism in pregnant women and their influence on fetal growth and neonatal adiposity.

DESIGN AND METHODS

A prospective cohort was studied comprising 207 pregnant women, 129 with normal glucose tolerance (NGT) and 78 with gestational diabetes mellitus (GDM) and their offspring. Activin A, follistatin and FSTL3 levels were measured in maternal serum collected in the early third trimester of pregnancy. Serial fetal ultrasounds were performed during the third trimester to evaluate fetal growth. Neonatal anthropometry was measured to assess neonatal adiposity.

RESULTS

Serum follistatin levels were significantly lower in GDM than in NGT pregnant women (8.21±2.32 ng/mL vs 9.22±3.41, P = 0.012) whereas serum FSTL3 and activin A levels were comparable between the two groups. Serum follistatin concentrations were negatively correlated with HOMA-IR and positively with ultrasound growth parameters such as fractional thigh volume estimation in the middle of the third trimester and percent fat mass at birth. Also, in the stepwise multiple linear regression analysis serum follistatin levels were negatively associated with HOMA-IR (β = -0.199, P = 0.008) and the diagnosis of gestational diabetes (β = -0.138, P = 0.049). Likewise, fractional thigh volume estimation in the middle of third trimester and percent fat mass at birth were positively determined by serum follistatin levels (β = 0.214, P = 0.005 and β = 0.231, P = 0.002, respectively).

CONCLUSIONS

Circulating follistatin levels are reduced in GDM compared with NGT pregnant women and they are positively associated with fetal growth and neonatal adiposity. These data suggest a role of the Activin-Follistatin system in maternal and fetal metabolism during pregnancy.

Anti-inflammatory therapy in an ovine model of fetal hypoxia induced by single umbilical artery ligation

Perinatal morbidity and mortality are significantly higher in pregnancies complicated by chronic hypoxia and intrauterine growth restriction (IUGR). Clinically, placental insufficiency and IUGR are strongly associated with a fetoplacental inflammatory response. To explore this further, hypoxia was induced in one fetus in twin-bearing pregnant sheep (n = 9) by performing single umbilical artery ligation (SUAL) at 110 days gestation. Five ewes were administered the anti-inflammatory drug sulfasalazine (SSZ) daily, beginning 24 h before surgery. Fetal blood gases and inflammatory markers were examined. In both SSZ- and placebo-treated ewes, SUAL fetuses were hypoxic and growth-restricted at 1 week (P < 0.05). A fetoplacental inflammatory response was observed in SUAL pregnancies, with elevated pro-inflammatory cytokines, activin A and prostaglandin E2. SSZ did not mitigate this inflammatory response. It is concluded that SUAL induces fetal hypoxia and a fetoplacental inflammatory response and that SSZ does not improve oxygenation or reduce inflammation. Further studies to explore whether alternative anti-inflammatory treatments may improve IUGR outcomes are warranted.

Activin A as a marker of intrauterine infection in women with preterm prelabour rupture of membranes

OBJECTIVE

To assess maternal serum activin A, an early phase response protein in systemic infection, as an early marker of intrauterine infection in women with preterm prelabour rupture of membranes (PPROM).

STUDY DESIGN

A prospective cohort study of women with singleton pregnancies complicated by PPROM at 24 to 34 weeks' gestation. Serum was collected for activin A and cytokine measurements. Activin A was measured using commercial enzyme-linked immunosorbent assay. Cytokines were measured using commercial multiplex assay. Pregnancy outcomes including infection were determined by case-record review.

RESULT

Eighteen women with PPROM were studied, with seven developing intrauterine infection. Serum activin A in women with and without infection did not differ. Peripheral white cell count, interleukin (IL)-6 and IL-10 were higher (P=0.03, 0.05 and 0.009, respectively) and IL-7 lower (P=0.04) 72 h before delivery in women with infection.

CONCLUSION

Activin A is not a clinically useful marker of intrauterine infection in women with PPROM.

First-trimester maternal serum activin A in pre-eclampsia and fetal growth restriction

OBJECTIVE

To investigate whether the reported increase in maternal serum activin A concentration in pre-eclampsia is evident from the first trimester.

DESIGN

This was a case-control study carried out in antenatal clinics among singleton pregnancies at 10-14 weeks of gestation.

METHODS

Activin A concentration was measured in stored maternal serum samples obtained at 11-14 weeks of gestation from 131 women who subsequently developed pre-eclampsia, 77 who developed non-proteinuric pregnancy-induced hypertension, 141 with fetal growth restriction in the absence of hypertensive complications and from 494 normotensive controls.

RESULTS

Compared to the median activin A level in the control group (1.00 MoM), the median MoM in the patients who subsequently developed pre-eclampsia and pregnancy-induced hypertension (1.49 MoM and 1.32 MoM, respectively) was significantly increased (p < 0.001), and in patients with fetal growth restriction (1.02 MoM) it was not significantly different (p = 0.57). In the pre-eclampsia group (n = 131) the disease was considered to be sufficiently severe to necessitate iatrogenic delivery before 35 weeks in 25 patients, and in this group the median MoM was 1.92.

CONCLUSION

Maternal serum activin A concentration at 12 weeks of gestation in pregnancies which subsequently develop hypertensive disease is increased, whereas in those complicated by fetal growth restriction it is normal.

Single serum activin a testing to predict ectopic pregnancy

CONTEXT

Ectopic pregnancy (EP) is an important cause of maternal deaths in early pregnancy because most fatal cases result from delayed diagnosis and inappropriate investigation.

OBJECTIVE

We evaluated whether the measurement of activin A may be useful in the diagnosis of EP in women with unknown pregnancy location.

DESIGN

The study was designed as an open observational study.

SETTING

The study was set in a tertiary referral center for obstetric care.

PATIENTS

Patients were women with unknown pregnancy location (n = 536) who had complaints of bleeding, pain, or cramping.

INTERVENTIONS

Interventions included clinical examination; transvaginal ultrasound scan; human chorionic gonadotropin (hCG), progesterone, and activin A measurements; laparoscopy; uterine curettage; and histological examination.

MAIN OUTCOME MEASURES

Main outcome measures were pregnancy outcomes and evaluation of sensitivity, specificity, and predictive values of hCG, progesterone, and activin A as diagnostic tests for the detection of EP.

RESULTS

Pregnancy outcomes included 155 (28.9%) viable intrauterine pregnancies (IUP), 305 (56.9%) first-trimester spontaneous abortion (SAB), and 76 (14.2%) EP. SAB had the lowest (P < 0.0001) hCG and progesterone concentrations, significantly lower than EP (P < 0.001) and IUP (P < 0.001). In EP, levels were significantly (P < 0.001) lower than in IUP. On the contrary, activin A levels were lowest (P < 0.0001) in EP, significantly lower than in SAB (P < 0.001) and IUP (P < 0.001). IUP had significantly (P < 0.001) lower activin A levels than SAB. When evaluated by the receiver operating curve analysis, activin A at the cutoff of 0.37 ng/ml combined a sensitivity and a specificity of 100 and 99.6%, respectively, for prediction of EP. When activin A concentrations were below the cutoff, the positive predictive value for EP was 97.43%, and 0% for concentrations higher than 0.37 ng/ml.

CONCLUSIONS

Activin A measurement may identify patients at risk of EP with a high sensibility and specificity.

Serum and urine inhibin A but not free activin A are endocrine biomarkers of severe pre-eclampsia

OBJECTIVE

Elevation of total serum inhibin A and activin A has been interpreted as evidence of placental dysfunction in women who develop pre-eclampsia. We sought to evaluate serum and urine levels of inhibin A and free activin A in normal and hypertensive pregnancies.

STUDY DESIGN

Inhibin A and free activin A were measured by immunoassay in simultaneously collected serum and urine samples from 75 women: (1) severe pre-eclampsia (n = 30); (2) mild pre-eclampsia (n = 11); (3) chronic hypertension (n = 9); (4) pregnant control women (n = 16); and (5) nonpregnant control women (n = 9). Urine levels were normalized to milligrams urine creatinine, and fractional excretions were calculated.

RESULTS

Serum and urine inhibin A were increased and fractional excretion was decreased in pregnancy. Serum, urine, and fractional excretion of inhibin A were increased in severe pre-eclampsia, compared with other gravidas. The only difference observed in free activin A was a decrease in serum free activin A in chronic hypertension, compared with severe pre-eclampsia and pregnant control women. Urine inhibin A showed the greatest discrimination between severe pre-eclampsia and pregnant control women: a cut-off of 45 pg/mg urine creatinine had 96.8% sensitivity, 87.5% specificity, and 93.6% accuracy. Women with urine inhibin A greater than 90 pg/mg urine creatinine had a 17-fold relative risk (95% confidence interval 9.7-459.5) of a clinically indicated delivery due to pre-eclampsia.

CONCLUSION

Serum and urine levels of inhibin A are altered in severe pre-eclampsia. Urine inhibin A may have application in the diagnosis and management of pre-eclampsia. Those with chronic hypertension have lower serum but not urine free activin A levels, compared with severe pre-eclampsia and mild pre-eclampsia.

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.

Expression of Inhibin/activin Subunits alpha (-α), beta A (-β A) and beta B (-β B) in Placental Tissue of Normal and Intrauterine Growth Restricted (IUGR) Pregnancies

During human pregnancy the placenta produces a variety of proteins like steroid hormones and their receptors that are responsible for the establishment and ongoing of the feto-placental unit. Inhibins are dimeric glycoproteins, composed of an alpha-subunit and one of two possible beta-subunits (beta (A) or beta (B)). Aims of the present study were the determination of the frequency and tissue distribution patterns of the inhibin/activin subunits in human placental tissue of normal pregnancies and pregnancies complicated with fetal growth restriction (IUGR). Slides of paraffin embedded placental tissue were obtained after delivery from patients diagnosed with IUGR (n = 6) and normal term placentas (n = 8). Tissue samples were fixed and incubated with monoclonal antibodies inhibin/activin-subunits -alpha, -beta (A), -beta (B). Intensity of immunohistochemical reaction on the slides was analysed using a semi-quantitative score and statistical analysis was performed (P<0.05). A significant lower expression of the inhibin-alpha subunit in IUGR extravillous trophoblast compared to normal pregnancies was observed, while the inhibin-alpha immunostaining was significantly upregulated in syncytiotrophoblast. Additionally, a significant down-regulation of inhibin-beta (B) subunit in extravillous trophoblast cells in IUGR syncytiotrophoblast cells was demonstrated. A co-localisation of inhibin-alpha and the beta-subunits was also observed, suggesting a production and secretion of intact inhibin A and inhibin B. Although the precise role of these inhibin/activin subunits in human placenta and IUGR pregnancies is still unclear, they could be involved in autocrine/paracrine signalling, contributing to several aspects like angiogenesis and tissue remodelling.

Umbilical Cord Serum Activin A Levels are Increased in Pre-eclampsia with Impaired Blood Flow in the Uteroplacental and Fetal Circulation

The aims of the present study were to evaluate the umbilical cord serum activin A concentrations in complicated pregnancies and also to explore the relationship between activin A levels and blood flow velocity in fetal arteries. Umbilical cord blood samples were obtained postpartum after a full term uneventful gestation (control group, n=40), and from pregnancies complicated by gestational diabetes (n=13), preterm labour (n=18), or pre-eclampsia (n=19). Cord serum activin A levels were three-fold higher in pregnancies complicated by pre-eclampsia (1.17+/-0.14 ng/ml, p<0.01) than in the control group (0.43+/-0.03 ng/ml), but were unaltered in the diabetes and preterm labour groups. The pre-eclampsia group had a marked increase of umbilical artery pulsatility index (PI) and also a decrease of middle cerebral artery PI (p<0.01). Furthermore, activin A concentration correlated directly with the umbilical artery PI (r=0.540, p=0.021), with the length of stay in the Neonatal Intensive Care Unit (r=0.857, p<0.001) and also with cord blood pH (r=-0.886, p<0.001). In conclusion, umbilical cord serum activin A levels are increased in the presence of pre-eclampsia and provide an indirect marker of impaired blood flow in the uteroplacental and fetal circulation.

Activin A, hypoxia and the prediction of obstetric outcomes

On the back of sheep experiments showing that uterine hypoxia induces a rapid and sustained elevation of fetal activin A levels, we undertook two prospective studies to explore whether this novel observation could be exploited clinically. The first was a prospective labour ward study investigating whether umbilical arterial activin A levels at delivery correlated with either neonatal hypoxic ischaemic encephalopathy or pH. Unfortunately, we were unable to demonstrate a link with either, but found that levels were significantly depressed among those who had an emergency Caesarean section, suggesting a possible role in active labour. Second, we investigated the link between activin A and intrauterine growth restriction (IUGR), a condition of fetoplacental hypoxia, by measuring levels in women presenting for antenatal ultrasound biometry with clinical suspicion of a small baby. We found that, compared to pregnancies with a baby that was small for gestational age (SGA) but otherwise healthy, levels were 2.4 and 8 times higher, respectively, in pregnancies complicated by IUGR, and those complicated by both IUGR and preeclampsia. However, a single blood sample of activin A was unable to distinguish between IUGR and SGA pregnancies with sufficient sensitivity to be clinically useful. Our studies were unable to demonstrate clinical utility for the experimental observation linking activin A and hypoxia.

Maternal serum activin A and the prediction of intrauterine growth restriction

BACKGROUND

Differentiating between the small healthy fetus and the high risk growth restricted fetus remains a significant obstetric challenge. It has been previously shown that maternal activin A levels are increased in association with fetal growth restriction.

AIM

To evaluate maternal serum activin A as a marker of fetal growth restriction.

METHODS

Prospective cohort study of 62 women referred for fetal assessment because of a clinical suspicion of a small for gestation fetus. Maternal serum levels of activin A were measured with an ELISA.

RESULTS

Activin A levels, expressed as median (95% CI) MoMs, were similar in the women with a normal-sized fetus and in those with a healthy small for gestational age fetus, 1.14 (95% CI 1.0-1.5) and 1.31 (95% CI 0.8-2.1), respectively (P = 0.97). Compared to the women with a normal-sized fetus or a healthy small fetus, activin A levels were significantly elevated in the women who had an intrauterine fetal growth restriction fetus 2.37 (95% CI 1.6-3.7; P = 0.01 compared to normal and P = 0.04 compared to healthy small).

CONCLUSIONS

These data confirm that circulating activin A is increased in association with fetal growth restriction. However, a single blood sample for activin A will not efficiently discern between healthy and compromised small fetuses.

Chronic fetal hypoxia increases activin A concentrations in the late-pregnant sheep

OBJECTIVE

To determine whether activin A concentrations are altered in chronic fetal hypoxemia and intrauterine fetal growth restriction (IUGR).

DESIGN

In vivo animal experimental model.

SETTING

Department of Physiology, Monash University.

POPULATION

Chronically catherised fetal sheep in late pregnancy.

METHODS

Chronic fetal hypoxia and IUGR were experimentally induced by single umbilical artery ligation (SUAL) in catheterised fetal sheep. Maternal and fetal blood samples and amniotic fluid (AF) samples were collected during surgery and thereafter on alternate days, until the time of delivery for analyte measurement. Fetal blood gas parameters were measured daily.

MAIN OUTCOME MEASURES

Plasma and AF was used to analyse activin A, prostaglandin E2 (PGE2) and cortisol and fetal blood gas analysis was undertaken in whole blood.

RESULTS

SUAL produced asymmetric IUGR and non-acidaemic chronic fetal hypoxia and resulted in preterm labour (129 [3] days). AF activin A concentrations were 10-fold higher in the SUAL group than in controls whereas levels in the fetal and maternal circulations were similar between groups.

CONCLUSIONS

SUAL-induced IUGR and fetal hypoxaemia increases AF activin A. This may be an important adaptive or protective response to IUGR.

Hypoxia induced activin secretion by the fetoplacental unit: differential responses related to gestation

OBJECTIVE

To determine whether activin A levels reflect oxygen availability in basal and hypoxic conditions in the late pregnant fetus and newborn lamb.

DESIGN

In vivo animal experimental study.

SETTING

Department of Physiology, Monash University.

POPULATION

Chronically catheterised fetal sheep in late gestation.

METHODS

Fetal hypoxia was induced at 125 (n = 4), 135 (n = 4) or 145 days ('term'; n = 3) gestational age by maternal nitrogen exposure, for 4 hours, during which maternal and fetal arterial, and amniotic fluid samples were collected. Lambs (age one, five and eight days; n = 3) were exposed to 1 hour of hypoxia via nitrogen exposure.

MAIN OUTCOME MEASURES

Activin A, prostaglandin E2 (PGE2) and cortisol were analysed in plasma and amniotic fluid, and whole blood was used to determine Pao2, Paco2, %O2, lactate and pH.

RESULTS

Basal activin A concentrations in the fetal arterial circulation remained unchanged between 125 days (0.230 [0.10] ng/mL) and term (0.28 [0.10] ng/mL), as did fetal oxygen saturation (59.11% [4.74%] to 52.25% [4.84%]) and pH (7.35 [0.02] to 7.37 [0.02]). Moderate fetal hypoxia (50% fall in fetal arterial %O2) produced a significant increase in circulating activin A (2.05 [0.67] ng/mL) and a significant decrease in pH (7.27 [0.03]) at 125 days of gestation, however, at 135 and 145 days, activin A and pH remained unchanged. Fetal activin A concentration was significantly correlated with pH (P = 0.036) but not %O2 (P = 0.072). Hypoxia in the lambs did not alter circulating activin A.

CONCLUSIONS

In response to hypoxia, activin A is increased in the circulation of 125-day-old fetuses, but not in older fetuses. Fetal arterial activin A levels sensitively reflect pH but not oxygen saturation, with increasing activin A in conditions of metabolic acidosis.

Inhibins and activins in pregnancy

Human placenta, decidua, and fetal membranes are the major sites of production and secretion of inhibin A and activin A in maternal serum, amniotic fluid, and umbilical cord blood. These tissues also express follistatin-related gene and betaglycan, the binding proteins of activin A and inhibin A, respectively, recently identified. They show a different expression throughout pregnancy, suggesting new functional roles into gestational tissues. The availability of suitable assays for measuring inhibin A and activin A lead us the possibility to investigate their secretion in healthy pregnancy. In addition, several evidences underline the potential role and the clinical usefulness of their measurement in the diagnosis, prevention, prognosis and follow-up of different gestational pathologies such as: threatened abortion, placental tumors, hypertensive disorders of pregnancy, intrauterine growth restriction, fetal hypoxia. The measurement of inhibin A and activin A into the biological fluids of pregnancy will offer in the future further possibilities in early diagnosis, prediction, and monitoring pregnancy diseases.

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.

Fetal activin A: associations with labour, umbilical artery pH and neonatal outcome

OBJECTIVE

To define the ontogeny of umbilical artery activin A at term and to evaluate activin A as a potential marker of perinatal hypoxia.

DESIGN

A cohort study.

SETTING

A university teaching hospital delivery suite.

POPULATION

A convenience sample of 141 term pregnancies.

METHODS

At delivery, umbilical artery and vein bloods were collected for blood gas measurements and subsequent measurement of activin A. Activin A levels were correlated with blood gas measurements and with labour and neonatal outcomes.

MAIN OUTCOME MEASURES

Umbilical arterial activin A and pH.

RESULTS

The median (95% CI) umbilical arterial activin A level at delivery was 1.38 (1.34-1.70) ng/mL. Levels varied significantly across gestation (P= 0.03), increasing from 36 to 38 weeks, thereafter decreasing to a nadir at 41 weeks. In 60 matched samples, the median (95% CI) venous and arterial activin A levels were 0.89 (0.81-1.06) ng/mL and 1.38 (1.21-1.61) ng/mL, respectively (P < 0.0001). Mean umbilical arterial pH was 7.20 (7.06-7.38; 5-95th centiles) and was not significantly correlated with log10 activin A (r=- 0.01; P= 0.68). Compared with healthy controls, there was no difference in arterial activin A in neonates identified as having suffered significant intrapartum asphyxia (P= 0.96). Fetal activin A levels were significantly lower in cases delivered by emergency caesarean section for complications during the first stage of labour compared with cases delivered vaginally (P= 0.003).

CONCLUSIONS

Umbilical artery activin A does not appear to be a sensitive marker of fetal oxygenation or of risk of hypoxic-ischaemic encephalopathy.

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

OBJECTIVE

Miscarriage is the commonest complication of human pregnancy. We undertook this study to assess whether inhibin A, pro-alphaC inhibin and/or activin A, products of the corpus luteum and placenta, might be useful in either the prediction or diagnosis of miscarriage.

DESIGN

Case-control study.

PATIENTS

Ninety-eight asymptomatic women at 6-13 weeks gestation who subsequently had a miscarriage and 198 gestation-matched women with a normal singleton pregnancy.

MEASUREMENTS

Maternal serum levels of inhibin A, pro-alphaC inhibin, activin A and human chorionic gonadotrophin (hCG) were measured.

RESULTS

Inhibin A, pro-alphaC and hCG, expressed as multiples of the normal median (MoM; +/-95% confidence intervals) in the miscarriage cases were significantly lower than in the viable controls, 0.56 (0.48-0.69) MoM vs. 1.00 (0.98-1.13) MoM, 0.55 (0.51-0.84) MoM vs. 1.0 (0.86-1.22) MoM and 0.34 (0.23-0.36) MoM vs. 1.00 (0.94-1.08) MoM, respectively (P < 0.0001 for all). Of the three analytes, hCG was the most discriminating between cases and controls. Levels of activin A in the miscarriage cases were not significantly different from controls, 0.96 (0.86-1.07) vs. 1.0 (0.95-1.08).

CONCLUSIONS

These data suggest that inhibin A, pro-alphaC inhibin and activin A will not be useful in either the prediction or diagnosis of early pregnancy miscarriage.

Effect of graded hypoxia on activin A, prostaglandin E2 and cortisol levels in the late-pregnant sheep

The aim of the present study was to determine whether activin A concentrations are dependent on feto–placental oxygen availability and to investigate the temporal relationship of activin A with prostaglandin (PG) E2 and cortisol. Nine fetal sheep (six hypoxic and three control) were instrumented and catheterised at 0.8 gestation. Reduced uterine blood flow was used to achieve three levels of hypoxia (mild = fetal SaO2 40–50%; moderate = fetal SaO2 30–40%; severe = fetal SaO2 20–30%), for 4 h on 3 consecutive days. Activin A, PGE2 and cortisol levels were determined in maternal and fetal blood and amniotic fluid. Moderate and severe hypoxia produced a significant (P < 0.05) increase in fetal plasma activin A concentrations. The amniotic fluid activin A concentrations were 15-fold higher than those in the fetal circulation, but were unchanged by hypoxia. The fetal PGE2 response reflected the degree of hypoxia over the 3 days, with moderate and severe hypoxia producing a significant (P < 0.05) increase in PGE2 concentrations. Fetal plasma cortisol concentrations were increased (P < 0.05) during all levels of hypoxia. Fetal arterial activin A was increased in response to moderate and severe hypoxia, but levels were not maintained over the hypoxic period. The increases in activin A and cortisol concentrations preceded the increase in PGE2.