alpha 2-Macroglobulin (alpha 2-M), a major serum glycoprotein, has been implicated as a low-affinity binding protein for inhibin and activin. In serum, alpha 2-M exists as two major species, a native form that is abundant and stable, and a transformed ('fast') species that is rapidly cleared from the circulation via alpha 2-M receptors. In this study inhibin, activin and their major binding protein follistatin were investigated for their ability to bind to the native or transformed species of purified human alpha 2-M. Using native PAGE and size exclusion chromatography, radiolabelled inhibin, activin and follistatin bound to the transformed alpha 2-M. Inhibin and follistatin did not bind significantly to native alpha 2-M, whereas activin was able to bind to the native species but with a lower capacity compared with that to transformed alpha 2-M. Under reducing conditions, binding of these hormones to alpha 2-M was abolished. These findings implicate alpha 2-M as a mechanism whereby inhibin, activin and follistatin may be removed from the circulation through alpha 2-M receptors, but also whereby activin can be maintained in the circulation through its ability to bind to native alpha 2-M.
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-1, -2 and -3 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 ED 50 response of 3·5 ng/ml, and an intraassay 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.
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.
Five pituitary preparations of follicle stimulating hormone (FSH), namely NIDDK-oFSH-17, Bioscan oFSH, Ovagen, Folltropin-V and F.S.H.-P., were examined for biological activity in terms of their potency in an in vitro bioassay, receptor assay and heterologous radioimmunoassay and in terms of their metabolic clearance rates. In the three assays, Bioscan oFSH was the most potent (P < 0.05) (3- to 5-fold the potency of NIDDK-oFSH-17), with Ovagen being 25-50% the potency of the NIDDK standard (P < 0.05). Folltropin-V and F.S.H.-P. had the lowest potencies in all three assays. For each preparation, the ratio of activities between the assays was not consistent, suggesting that the preparations behaved differently in each assay. In 9 of 10 cases, potency estimates in the heterologous radioimmunoassay were greater than those in the in vitro bioassay or receptor assay. Polyacrylamide gel electrophoresis of the preparations showed banding consistent with the molecular weight of FSH, but also indicated that the preparations were contaminated with other proteins to varying extents. The half-lives of these preparations when injected into the bloodstream of mature female mice were 28.0, 8.6, 13.4, 11.6 and 17.4 min for NIDDK-oFSH-17, Bioscan oFSH, Ovagen, Folltropin-V and F.S.H.-P. respectively. The slopes of the decay rates were significantly different from each other (P < 0.05) except between Ovagen and Folltropin-V. The results of these studies show that a number of widely available FSH preparations have differing biopotencies. Moreover, the biopotency of a preparation in vitro is not related to its metabolic clearance rate, and not all FSH preparations behave identically in different assays. Measures of biopotency in vitro combined with those of metabolic clearance rate may provide useful information on the properties of FSH preparations used for research purposes and for superovulation of farmed livestock.
To further our understanding of how gonadal steroids impact sleep biology, we sought to address the mechanism by which proestrus levels of cycling ovarian steroids, particularly estradiol (E2), suppress sleep in female rats. We showed that steroid replacement of proestrus levels of E2 to ovariectomized female rats, suppressed sleep to similar levels as those reported by endogenous ovarian hormones. We further showed that this suppression is due to the high levels of E2 alone, and that progesterone did not have a significant impact on sleep behavior. We found that E2 action within the Median Preoptic Nucleus (MnPN), which contains estrogen receptors (ERs), is necessary for this effect; antagonism of ERs in the MnPN attenuated the E2-mediated suppression of both non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM) sleep. Finally, we found E2 action at the MnPN is also sufficient for sleep suppression, as direct infusion of E2 into the MnPN suppressed sleep. Based on our findings, we predict proestrus levels of E2 alone, acting at the MnPN, mediate sex-hormone driven suppression of sleep in female rats.
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