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

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.

Activin A Release into Cerebrospinal Fluid in a Subset of Patients with Severe Traumatic Brain Injury

Activin A is a member of the transforming growth factor-beta superfamily and has been demonstrated to be elevated during inflammation and to have neuroprotective properties following neural insults. In this study, we examined whether traumatic brain injury (TBI) induced a response in activin A or in the concentrations of its binding protein, follistatin. Thirty-nine patients with severe TBI had daily, matched cerebrospinal fluid (CSF) and serum samples collected post-TBI and these were assayed for activin A and follistatin using specific immunoassays. Concentrations of both molecules were assessed relative to a variety of clinical parameters, such as the Glasgow Coma Score, computer tomography classification of TBI, measurement of injury markers, cell metabolism and membrane breakdown products. In about half of the patients, there was a notable increase in CSF activin A concentrations in the first few days post-TBI. There were only minor perturbations in either serum activin or in either CSF or serum follistatin concentrations. The CSF activin A response was not related to any of the common TBI indices, but was strongly correlated with two common markers of brain damage, neuronal specific enolase and S100-beta. Further, activin A levels were also associated with indices of metabolism, such as lactate and pyruvate, excitotoxicity (glutamate) and membrane lipid breakdown products such as glycerol. In one of the two patients who developed a CSF infection, activin A concentrations in CSF became markedly elevated. Thus, some TBI patients have an early release of activin A into the CSF that may result from activation of inflammatory and/or neuroprotective pathways.

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.