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Activin A and follistatin are normally present in relatively low amounts in the circulation. Heparin administration elicits a rapid and robust release of these proteins, although this phenomenon is poorly defined. In the present studies, the response to heparin administration was evaluated in the plasma of adult ewes in terms of whether it was dose-dependent, could be neutralized, was responsive to multiple stimulation, and the nature of the activin A and follistatin released. Activin A and follistatin were rapidly released by heparin in a dose-dependent manner (25, 100 or 250 IU/kg), with differences in the response as adjudged by peak concentration, timing of the peak and area under the curve. The heparin response could be blocked by pretreatment with protamine; conversely protamine injection alone (2 mg/kg) elicited release of follistatin but not activin A. Repeat administration of heparin at three-hourly intervals resulted in activin and follistatin responses to each injection, but each subsequent stimulation increased and extended the responses, consistent with saturation of the heparin clearance mechanism. Size exclusion chromatography of plasma samples confirmed that the majority of activin and follistatin released by heparin was a complex, whereas follistatin released by protamine was unbound. These data are consistent with a large pool of activin A and follistatin resident on extracellular matrices, with the rapid response implicating the vascular endothelium as the prime site of release following administration of these commonly used anticoagulant therapies.
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The effects on plasma follistatin concentrations of an inflammatory episode, induced by the intrathoracic injection of yeast, were examined in growing lambs; this model results in acute loss of appetite, food intake and liveweight and the activation of the acute-phase pathway for several weeks as adjudged by the production of haptoglobin and other acute-phase proteins. In these animals (n = 8) there was a biphasic response in follistatin concentrations, with an initial 200% increase (P < 0.001) in follistatin within 24 h of injection of yeast. Thereafter, follistatin concentrations were depressed to 70% of pretreatment levels 48 h after injection (P < 0.01), followed by a gradual recovery of concentrations to pretreatment values. In another group of lambs (n = 16) that were feed-restricted to mimic the reduced food intakes and liveweight changes in the yeast-injected group, plasma follistatin was also reduced to around 70% of pretreatment levels (P < 0.01) within 1 day of the dietary regimen being implemented, followed by a gradual return to pretreatment values as food intakes were increased. Plasma follistatin correlated significantly (r = 0.57, P < 0.0001) with food intake, but not with liveweight changes. Plasma follistatin concentrations were unchanged in a third group fed ad libitum (n = 8), except during two periods when food intakes were significantly (P < 0.05) reduced, when follistatin concentrations also decreased (P < 0.01). Plasma follicle-stimulating hormone (FSH) concentrations in the three groups of lambs were not significantly affected by the treatment regimes or changes in follistatin concentrations. These findings indicate that peripheral follistatin concentrations are modulated by both inflammatory and nutritional mechanisms, and that significant fluctuations in follistatin levels can occur without detectable perturbations in FSH secretion.
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A new in vitro bioassay for activin was developed using the mouse plasmacytoma cell line, MPC-11. Human recombinant (hr) activin A dose-dependently inhibited the proliferation of these cells, whereas a range of other factors, including inhibin, follistatin and transforming growth factor-beta1, -beta2 and -beta3 had no effect. Conditioned medium containing activin B induced an inhibition similar to hr-activin A. The inhibitory influence of activin A could be blocked by follistatin, but not by hr-inhibin A. This bioassay had a sensitivity for activin A of around 0.4 ng/ml, an ED50 response of 3.5 ng/ml, and an intra-assay coefficient of variation of <11%. It offers substantial advantages over existing in vitro activin bioassays in terms of ease of use, specificity and throughput. The utility of the MPC-11 bioassay was demonstrated in the purification of activin from amniotic fluid, where an almost identical profile of bioactive activin A was detected compared with the pituitary cell bioassay of activin. Bioactive activin could also be detected in unpurified ovine allantoic and amniotic fluids and bovine follicular fluid. Measuring activin in untreated and heat-treated human sera or seminal plasma was hampered by a non-specific inhibitory effect, so that several serum samples did not run parallel with the hr-activin A standard. This inhibitory effect by serum could not be overcome by addition of follistatin, suggesting it is not activin-like bioactivity. This new bioassay for activin demonstrates widespread applicability for monitoring of purified or partially purified samples during purification procedures, bioactivity measurements, receptor-binding studies and assays of cell culture medium.
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A series of experiments were conducted in adult ewes to delineate the release profile of activin A and its relationship to other cytokines following an i.v. injection of the bacterial cell wall component, lipopolysaccharide (LPS). Following this challenge, plasma activin A increased rapidly and appeared to be released in a biphasic manner, slightly preceding the release of tumour necrosis factor-alpha (TNFalpha) and before elevation of interleukin (IL)-6 and follistatin levels. The concentration of activin A was correlated with body temperature during the response to LPS. A second experiment compared cytokine concentrations in matched blood and cerebrospinal fluid (CSF) samples. This revealed that activin A was not released centrally in the CSF following a peripheral LPS injection, nor was TNFalpha or the activin binding protein, follistatin, but IL-6 showed a robust elevation. In a third experiment, the stimulus for activin A release was examined by blocking prostaglandin synthesis. Flurbiprofen, a prostaglandin synthesis inhibitor, effectively attenuated the fever response to LPS and partly inhibited cortisol release, but the cytokine profiles were unaffected. Finally, the bioactivity of TNFalpha and/or IL-1 was blocked using soluble receptor antagonists. These treatments did not affect the initial release of activin A, but blockade of TNFalpha depressed the second activin peak. These studies define more rigorously the release of activin A into the circulation following acute inflammatory challenge. The response is rapid and probably biphasic, independent of prostaglandin- mediated pathways and does not depend upon stimulation by TNFalpha or IL-1. The data suggest that activin A release is an early event in the inflammatory cascade following the interaction of LPS with its cellular receptor.
