Regulation of gonadotropin subunit transcription after ovariectomy in the rat: measurement of subunit primary transcripts reveals differential roles of GnRH and inhibin

Regulation of reproduction via tight control of gonadotropin hormone levels

Mammalian reproduction is controlled by the hypothalamic-pituitary-gonadal axis. GnRH from the hypothalamus regulates synthesis and secretion of gonadotropins, LH and FSH, which then control steroidogenesis and gametogenesis. In females, serum LH and FSH levels exhibit rhythmic changes throughout the menstrual or estrous cycle that are correlated with pulse frequency of GnRH. Lack of gonadotropins leads to infertility or amenorrhea. Dysfunctions in the tightly controlled ratio due to levels slightly outside the normal range occur in a larger number of women and are correlated with polycystic ovaries and premature ovarian failure. Since the etiology of these disorders is largely unknown, studies in cell and mouse models may provide novel candidates for investigations in human population. Hence, understanding the mechanisms whereby GnRH regulates gonadotropin hormone levels will provide insight into the physiology and pathophysiology of the reproductive system. This review discusses recent advances in our understanding of GnRH regulation of gonadotropin synthesis.

Maximal expression of Foxl2 in pituitary gonadotropes requires ovarian hormones

Gonadotropin-releasing hormone (GnRH) and activin regulate synthesis of FSH and ultimately fertility. Recent in vivo studies cast SMAD4 and FOXL2 as master transcriptional mediators of activin signaling that act together and independently of GnRH to regulate Fshb gene expression and female fertility. Ovarian hormones regulate GnRH and its receptor (GNRHR) through negative and positive feedback loops. In contrast, the role of ovarian hormones in regulating activin, activin receptors, and components of the activin signaling pathway, including SMAD4 and FOXL2, remains understudied. The widespread distribution of activin and many of its signaling intermediates complicates analysis of the effects of ovarian hormones on their synthesis in gonadotropes, one of five pituitary cell types. We circumvented this complication by using a transgenic model that allows isolation of polyribosomes selectively from gonadotropes of intact females and ovariectomized females treated with or without a GnRH antagonist. This paradigm allows assessment of ovarian hormonal feedback and distinguishes responses that are either independent or dependent on GnRH. Surprisingly, our results indicate that Foxl2 levels in gonadotropes decline significantly in the absence of ovarian input and independently of GnRH. Expression of the genes encoding other members of the activin signaling pathway are unaffected by loss of ovarian hormonal feedback, highlighting their selective effect on Foxl2. Expression of Gnrhr, a known target of FOXL2, also declines upon ovariectomy consistent with reduced expression of Foxl2 and loss of ovarian hormones. In contrast, Fshb mRNA increases dramatically post-ovariectomy due to increased compensatory input from GnRH. Together these data suggest that ovarian hormones regulate expression of Foxl2 thereby expanding the number of genes controlled by the hypothalamic-pituitary-gonadal axis that ultimately dictate reproductive fitness.

The Antidepressant-like Effects of Estrogen-mediated Ghrelin

Ghrelin, one of the brain-gut peptides, stimulates food-intake. Recently, ghrelin has also shown to play an important role in depression treatment. However, the mechanism of ghrelin's antidepressant-like actions is unknown. On the other hand, sex differences in depression, and the fluctuation of estrogens secretion have been proved to play a key role in depression. It has been reported that women have higher level of ghrelin expression, and ghrelin can stimulate estrogen secretion while estrogen acts as a positive feedback mechanism to up-regulate ghrelin level. Ghrelin may be a potential regulator of reproductive function, and estrogen may have additional effect in ghrelin's antidepressantlike actions. In this review, we summarize antidepressant-like effects of ghrelin and estrogen in basic and clinical studies, and provide new insight on ghrelin's effect in depression.

Endocrine disrupting effects of dichlorodiphenyltrichloroethane analogues on gonadotropin hormones in pituitary gonadotrope cells

It has been shown that exposure to dichlorodiphenyltrichloroethane (DDT) analogues leads to disharmony of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). However, the effects and mechanisms of DDT analogues on the expression of gonadotropin genes (FSHβ, LHβ and Cgα), which is the rate-limiting step of FSH and LH biosynthesis, remain unknown. In this study, we assessed the effects of p,p'-DDT, o,p'-DDT, p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) and methoxychlor (MXC) on gonadotropin genes expression and hormones synthesis in gonadotrope cells. p,p'-DDT and MXC at test concentrations ranging from 10(-9) to 10(-7)mol/L, stimulated gonadotropin genes expression and hormones synthesis in a dose-dependent manner. The activation of extracellular signal-regulated kinase (ERK) was required for the induction of gonadotropin genes expression and hormones synthesis by p,p'-DDT or MXC exposure. This study showed for the first time that p,p'-DDT and MXC regulated gonadotropin genes expression and hormones synthesis through ERK pathway in gonadotrope cells.

Translational control of gene expression in the gonadotrope

The study of gene expression in gonadotropes has largely focused on the variety of mechanisms regulating transcription of the gonadotropin genes and ancillary factors that contribute to the overall phenotype and function of these cells in reproduction. However, there are aspects of the response to GNRH signaling that are not readily explained by changes at the level of transcription. As our understanding of regulation at the level of mRNA translation has increased, it has become evident that GNRH receptor signaling engages multiple aspects of translational regulation. This includes activation of cap-dependent translation initiation, translational pausing caused by the unfolded protein response and RNA binding protein interaction. Gonadotropin mRNAs and the mRNAs of other factors that control the transcriptional and signaling responses to GNRH have been identified as targets of regulation at the level of translation. In this review we examine the impact of translational control of the expression of gonadotropin genes and other genes relevant to GNRH-mediated control of gonadotrope function.

Unsaturated fatty acids disrupt Smad signaling in gonadotrope cells leading to inhibition of FSHβ gene expression

Reproductive function is highly dependent on nutritional input. We recently provided evidence that the unsaturated ω6 fatty acid (FA), linoleic acid (linoleic), interferes with transcription and secretion of the gonadotropin LH, highlighting the existence of a lipid sensing in pituitary gonadotropes. Here, we show, using a combination of in vivo and in vitro models, that linoleic differentially regulates Lhb and Fshb expression. Central exposure of rats to linoleic over 7 days was associated with increase of Lhb but not Fshb transcript levels. Consistently, exposure of rat pituitary cells or LβT2 cells to linoleic increased Lhb, whereas it dramatically decreased Fshb transcript levels without affecting its stability. This effect was also induced by ω9 and ω3-polyunsaturated FA but not by saturated palmitic acid. Analysis of the underlying mechanisms in LβT2 cells using small interfering RNA revealed that early growth response protein 1 mediates linoleic stimulation of Lhb expression. Furthermore, we demonstrated that linoleic counteracts activin and bone morphogenetic protein-2 stimulation of Fshb expression. Using Western blotting and Smad-responsive reporter gene assays, linoleic was shown to decrease basal Smad2/3 phosphorylation levels as well as activin- and bone morphogenetic protein-2-dependent activation of Smad, uncovering a new FA-sensitive signaling cascade. Finally, the protein phosphatase magnesium-dependent 1A was shown to mediate linoleic inhibition of basal Smad phosphorylation and Fshb expression, identifying protein phosphatase magnesium-dependent 1A as a new target of FA in gonadotropes. Altogether, this study provides a novel mechanism by which FAs target gene expression and underlines the relevant role of pituitary gonadotropes in mediating the effects of nutritional FA on reproductive function.

Pituitary expression of LHbeta- and FSHbeta-subunit mRNA, cellular distribution of LHbeta-subunit mRNA and LH and FSH synthesis during and after treatment with a gonadotrophin-releasing hormone agonist in heifers

The aim was to examine transcriptional and post-transcriptional regulation of LH and FSH biosynthesis. Female cattle were allocated to three groups: (i) Group 1, control (n = 6), synchronized to be at around Day 11 of the oestrous cycle on Day 31; (ii) Group 2 (n = 6), treated with gonadotrophin-releasing hormone (GnRH) agonist (deslorelin) for 31 days; and (iii) Group 3 (n = 6), treated with deslorelin for 28 days. All animals were slaughtered on Day 31. For animals in Group 2, pituitary content of LHbeta-subunit mRNA was suppressed 60% (P < 0.001) and LH 95% (P < 0.001), whereas FSHbeta-subunit mRNA was suppressed 25% (P > 0.05) and FSH 90% (P < 0.001). Three days after treatment with deslorelin (Group 3) LHbeta-subunit mRNA and LH remained suppressed (50% and 95%, respectively; P < 0.001). At the same time, FSHbeta-subunit mRNA did not differ from controls (P > 0.05) whereas FSH remained reduced by 80% (P < 0.001). The ratio of LHbeta-subunit mRNA present in the nucleus versus cytoplasm of gonadotroph cells was reduced (P < 0.05) in heifers during treatment with deslorelin (0.59 +/- 0.05) compared with the ratio in control heifers (1.31 +/- 0.22) and heifers 3 days after discontinuation of treatment (1.01 +/- 0.05). The findings indicated that treatment with GnRH agonist can suppress LHbeta-subunit mRNA expression without any significant effect on FSHbeta-subunit mRNA. As LH and FSH contents were suppressed to a greater degree than their beta-subunit mRNAs, it would appear that treatment with a GnRH agonist might influence gonadotrophin biosynthesis by a post-transcriptional mechanism(s). For LHbeta-subunit mRNA, this would appear not to be reduced export of message from the nucleus.

Luteinizing hormone beta promoter stimulation by adenylyl cyclase and cooperation with gonadotropin-releasing hormone 1 in transgenic mice and LBetaT2 Cells

Rat luteinizing hormone beta (Lhb) gene transcription is stimulated by hypothalamic gonadotropin-releasing hormone 1 (GnRH1), and this response may be modulated by other signaling pathways such as cAMP. Here we characterize the ability of cAMP, alone or with GnRH1, to stimulate Lhb gene transcription in mouse pituitary and clonal gonadotroph cells. Both cAMP and pituitary adenylyl cyclase-activating peptide increase GnRH1 stimulation of luciferase activity in pituitaries of mice expressing the rat Lhb-luciferase transgene, suggesting cAMP and GnRH1 pathways interact in vivo. cAMP stimulation of the Lhb-luciferase transgene was similar between females in metestrus and proestrus, but GnRH1 stimulation was greater at proestrus. Additive effects with combined treatments were observed at metestrus and proestrus. Elevated intracellular cAMP stimulated Lhb promoter activity in LbetaT2 clonal gonadotroph cells, alone and with GnRH1. In LbetaT2 cells, cAMP stimulation of the Lhb promoter was eliminated by inhibition of protein kinase A (PKA); GnRH1 stimulation was partially suppressed by either PKA or protein kinase C inhibitors. Only the proximal GnRH1-responsive region of the promoter was required for cAMP stimulation, and mutation of the 3' NR5A1 site diminished the response. Regulation of primary mRNA transcripts from the endogenous Lhb gene by cAMP and GnRH1 correlated with results from the Lhb-luciferase transgene or transfected promoter. Occupancy of the endogenous promoter by EGR1 was increased by GnRH1 with or without forskolin, but forskolin alone had little effect. Thus, cAMP stimulation of Lhb promoter activity, and enhancement of GnRH1 stimulation, occurs in multiple physiological states independent of steroid status, via a PKA-dependent mechanism.

The regulation of FSHbeta transcription by gonadal steroids: testosterone and estradiol modulation of the activin intracellular signaling pathway

Recent reports suggest that androgens increase FSHbeta transcription directly via the androgen receptor and by modulating activin signaling. Estrogens may also regulate FSHbeta transcription in part through the activin system. Activin signaling can be regulated extracellularly via activin, inhibin, or follistatin (FS) or intracellularly via the Smad proteins. We determined the effects of androgen and estrogen on FSHbeta primary transcript (PT) concentrations in male and female rats, and we correlated those changes with pituitary: activin betaB mRNA, FS mRNA, the mRNAs for Smads2, -3, -4, and -7, and the phosphorylation (p) status of Smad2 and -3 proteins. In males, testosterone (T) increased FSHbeta PT two- to threefold between 3 and 24 h and was correlated with reduced FS mRNA, transient increases in Smad2, -4, and -7 mRNAs, and a six- to 10-fold increase in pSmad2, and activin betaB mRNA was unchanged. In females, T also increased FSHbeta PT twofold and pSmad2 threefold but had no effect on activin betaB, FS, or the Smad mRNAs. Androgen also increased Smad2 phosphorylation in gonadotrope-derived alphaT3 cells. In contrast, estradiol had no effect on FSHbeta PT but transiently increased activin betaB mRNA and suppressed FS mRNA before increasing FS mRNA at 24 h and increased Smads2, -3, and -7 mRNAs and pSmad2 threefold. In conclusion, T acts on the pituitary to increase FSHbeta PT in both sexes and modulates FS mRNA, Smad mRNAs, and/or Smad2 phosphorylation. These findings suggest that T regulates FSHbeta transcription, in part, through modulation of various components of the activin-signaling system.

Luteinizing hormone, a reproductive regulator that modulates the processing of amyloid-beta precursor protein and amyloid-beta deposition

Hormonal changes associated with the dysregulation of the hypothalamic-pituitary-gonadal (HPG) axis following menopause/andropause have been implicated in the pathogenesis of Alzheimer's disease (AD). Experimental support for this has come from studies demonstrating an increase in amyloid-beta (Abeta) deposition following ovariectomy/castration. Because sex steroids and gonadotropins are both part of the HPG feedback loop, any loss in sex steroids results in a proportionate increase in gonadotropins. To assess whether Abeta generation was due to the loss of serum 17beta-estradiol or to the up-regulation of serum gonadotropins, we treated C57Bl/6J mice with the anti-gonadotropin leuprolide acetate, which suppresses both sex steroids and gonadotropins. Leuprolide acetate treatment resulted in a 3.5-fold (p < 0.0001) and a 1.5-fold (p < 0.024) reduction in total brain Abeta1-42 and Abeta1-40 concentrations, respectively, after 8 weeks of treatment. To further explore the role of gonadotropins in promoting amyloidogenesis, M17 neuroblastoma cells were treated with the gonadotropin luteinizing hormone (LH) at concentrations equivalent to early adulthood (10 mIU/ml) or post-menopause/andropause (30 mIU/ml). LH did not alter amyloid-beta precursor protein (AbetaPP) expression but did alter AbetaPP processing toward the amyloidogenic pathway as evidenced by increased secretion and insolubility of Abeta, decreased alphaAbetaPP secretion, and increased AbetaPP-C99 levels. These results suggest the marked increases in serum LH following menopause/andropause as a physiologically relevant signal that could promote Abeta secretion and deposition in the aging brain. Suppression of the age-related increase in serum gonadotropins using anti-gonadotropin agents may represent a novel therapeutic strategy for AD.

Gonadotropin-Releasing Hormone Receptor Gene Expression During Pubertal Development of Female Rats1

Appropriate expression of the GnRH receptor (GnRH-R) in gonadotrophs is critical for GnRH signaling and hence for gonadotropin secretion and sexual development. In the present work, we have studied the ontogeny of the steady-state GnRH-R mRNA levels in pituitaries of female rats from Day 5 to Day 55, when sexual maturity is attained. Developmental changes of gonadotropin subunit (alpha, FSHbeta, and LHbeta) mRNA levels were also assessed. In addition, the role of the endogenous GnRH on the maturational changes of GnRH-R and gonadotropin subunit gene expression was investigated. Messenger RNA levels were determined by Northern blot analysis of total RNA from anterior pituitaries. Amounts of the most abundant (5.0 kilobase [kb]) GnRH-R mRNA increased slowly from Day 5 through the infantile period, to peak at Day 20 ( approximately 4-fold increase vs. Day 5). Thereafter the levels of the GnRH-R mRNA decline abruptly by Day 25 (75% decrease vs. Day 20) and then fell slightly until Day 35. Parallel changes were observed on the 4.5-kb transcript of the GnRH-R gene. Alpha subunit mRNA was easily detected at Day 5 and its levels increased quickly through the beginning of the infantile period to peak at Day 10 (3.2-fold increase vs. Day 5); then it decreased by 85% at Day 35. FSHbeta and LHbeta mRNA levels rose slowly until Days 15-20, a short time before GnRH-R. Thereafter, the levels of both mRNAs fell until Day 35 (90% decrease vs. Day 15 for FSHbeta and 50% decrease vs. Day 20 for LHbeta). To ascertain whether developmental activation of the GnRH-R and gonadotropin subunit gene expression is GnRH dependent, we have studied the effect of blocking the endogenous GnRH action by treating developing female rats with the specific GnRH antagonist cetrorelix (1.5 mg/kg body weight/wk, s.c.) through the infantile (Days 5-20) and the juvenile period (Days 20-35). Cetrorelix completely blocked the rise of levels of the two most abundant species, 5.0 kb and 4.5 kb, of GnRH-R mRNA during the infantile phase and dropped them to almost undetectable levels during the juvenile prepubertal period. Cetrorelix also abolished the developmental rise of gonadotropin beta subunit mRNAs during the two periods of the study. In contrast, alpha subunit gene expression tended to decrease, but not significantly, with cetrorelix treatment during the two periods. These data demonstrate that sexual maturation of female rats is advanced by an early and strong induction of GnRH-R and gonadotropin subunit gene expression during the infantile period, followed by weaker persistent activation during puberty. Developmental GnRH-R and gonadotropin beta subunit gene expression is almost entirely GnRH dependent, not only in the juvenile prepubertal stage but also during the infantile period.

Pituitary Follistatin Gene Expression in Female Rats: Evidence That Inhibin Regulates Transcription1

Follistatin (FS), along with the members of the transforming growth factor beta family activin and inhibin, are important regulators of FSH secretion and messenger RNA production. While activin and inhibin appear to function as tonic modulators of FSH (stimulatory and inhibitory, respectively), dynamic changes in FS are noted through the estrous cycle and under varying physiological experimental paradigms. This suggests that FS is a major contributor to the precisely coordinated secretion of FSH that maintains reproductive function. The aim of this study was to investigate changes in FS, in particular the early (<12 h) rise observed after ovariectomy (OVX), and to determine whether these changes were as a consequence of variations in gene transcription rates. FS primary transcript (PT) and mRNA were found to increase 3-fold 12 h post-OVX, indicating increased gene transcription during this time period. Replacement with estradiol and/or blockade of GnRH had only modest effects on FS PT concentration. Inhibin immunoneutralization of intact rats resulted in a 3-fold increase in FS PT 12 h after administration of inhibin alpha antisera. Significant increases in FS mRNA at both 2 and 12 h also suggested that inhibin also may have effects on message stability. After administration of recombinant human inhibin A, there was a prompt decline in both FS PT and mRNA. These results indicate that inhibin is a major regulator of FS, both by transcriptional and nontranscriptional mechanisms.

Regulation of luteinizing hormone-beta and follicle-stimulating hormone (FSH)-beta gene transcription by androgens: testosterone directly stimulates FSH-beta transcription independent from its role on follistatin gene expression

The gonadotropin beta-subunit mRNAs are differentially regulated by androgens. Testosterone (T) suppresses LH-beta and increases FSH-beta. We aimed to determine whether androgens regulate LH-beta and FSH-beta transcription [as measured by changes in primary transcript (PT)] and to determine whether androgens act directly on FSH-beta or via the intrapituitary activin/follistatin (FS) system. In castrate + GnRH antagonist-treated rats, T increased FSH-beta PT between 3 and 48 h. In contrast, T suppressed LH-beta PT. The increases in FSH-beta mRNA and PT were associated with reduced FS mRNA. Activin betaB mRNA was modestly suppressed. The increase in FSH-beta PT after T was androgen specific. Both T and dihydrotestosterone (DHT) increased FSH-beta PT 2-fold and decreased both FS and betaB mRNA. Estradiol suppressed FSH-beta PT 3-fold and had no effect on FS or betaB mRNAs. LH-beta PT was suppressed by DHT. To determine whether T stimulation of FSH-beta PT reflected a decrease in pituitary FS, we gave androgen in the presence of exogenous FS in vitro. T and DHT increased FSH-beta PT 2- to 3-fold. FS alone decreased FSH-beta PT 40% but did not diminish the increase FSH-beta PT in response to T. T, DHT, and FS did not affect FS mRNA, betaB mRNA, or LH-beta PT. In conclusion, androgens acting directly on the pituitary increase FSH-beta and decrease LH-beta transcription. The increase in FSH-beta PT in response to T was androgen specific and occurs in the presence of excess FS, suggesting that T stimulates FSH-beta transcription independently of modulation of FS.

Gonadotropin-releasing hormone-desensitized LbetaT2 gonadotrope cells are refractory to acute protein kinase C, cyclic AMP, and calcium-dependent signaling

Sustained exposure of gonadotropes to GnRH causes a pronounced desensitization of gonadotropin release, but the mechanisms involved are poorly understood. It is known that desensitization is associated with decreased GnRH receptor and Gq/11 levels in alphaT3-1 cells, but it is not known whether downstream signaling is impaired. We have shown previously that chronic stimulation of signaling via expression of an active form of Galphaq causes GnRH resistance in LbetaT2 cells. In this study we investigated whether chronic GnRH treatment could down-regulate protein kinase C (PKC), cAMP, or Ca2+-dependent signaling in LbetaT2 cells. We found that chronic GnRH treatment desensitizes cells to acute GnRH stimulation not only by reducing GnRH receptor and Gq/11 expression but also by down-regulating PKC, cAMP, and calcium-dependent signaling. Desensitization was observed for activation of ERK and p38 MAPK and induction of c-fos and LHbeta protein expression. Activation of individual signaling pathways was able to partially mimic the desensitizing effect of GnRH on ERK, p38 MAPK, c-fos, and LHbeta but not on Gq/11. Chronic stimulation with phorbol esters reduced GnRH receptor expression to the same extent as chronic GnRH. Sustained GnRH also desensitized PKC signaling by down-regulating the delta, epsilon, and theta isoforms of PKC. We further show that chronic GnRH treatment causes heterologous desensitization of other Gq-coupled receptors.

Gonadotropin-releasing hormone receptor gene expression during pubertal development of male rats

Appropriate expression of the GnRH receptor (GnRH-R) in gonadotropes is critical for GnRH signaling and hence for gonadotropin secretion and sexual development. In the present work, we have studied the ontogeny of the steady-state GnRH-R mRNA levels in pituitaries of male rats from Day 5 to Day 55, when sexual maturity is attained. Developmental changes of gonadotropin subunit (alpha, FSHbeta, and LHbeta) mRNA levels were also assessed. In addition, the role of the endogenous GnRH on the maturational changes of GnRH-R and gonadotropin subunit gene expression was investigated. Messenger RNA levels were determined by Northern blot analysis of total RNA from anterior pituitaries. Amounts of the most abundant (5.0 kb) GnRH-R mRNA increased slowly from Day 5 through the infantile and the juvenile periods, to peak at Day 35 (12-fold increase vs. Day 5). Thereafter, the levels of the GnRH-R mRNA decline slightly until Day 55 (33% decrease vs. Day 35). Parallel changes were observed on the 4.5-kb transcript of the GnRH-R gene. Alpha subunit mRNA was easily detected at Day 5, and its levels increased progressively through the infantile period (2.5-fold increase) and peaked at Day 25 (3.3-fold increase vs. Day 5) with a smooth nonstatistically significant increment until Day 35; then it decreased by 41.5% at Day 55. FSHbeta and LHbeta mRNA levels rose slowly until Day 25. A sharp rise occurred thereafter to reach maximum levels at Day 35 (5.8-fold for FSHbeta and 3.8-fold for LHbeta vs. Day 25). Thereafter, the levels of both mRNAs fell until Day 55 (44.1% decrease for FSHbeta and 37.1% decrease for LHbeta vs. Day 35). To ascertain whether developmental activation of the GnRH-R and gonadotropin subunit gene expression is GnRH dependent, we have studied the effect of blocking the endogenous GnRH action by treating developing male rats with the specific GnRH antagonist cetrorelix (1.5 mg/kg body weight/week, s.c.) through the infantile (Days 5-20) and the juvenile periods (Days 20-35). Cetrorelix completely blocked the rise of levels of the two most abundant species, 5.0 kb and 4.5 kb, of the GnRH-R mRNA, during both the infantile and the juvenile periods. Cetrorelix also abolished the developmental rise of the gonadotropin beta subunit mRNAs during the two periods of the study. In contrast, the alpha subunit gene expression was not altered by cetrorelix treatment during any of the two periods. These data demonstrate that sexual maturation of male rats is accompanied by a progressive and concerted induction of GnRH-R and gonadotropin subunit gene expression. Developmental activation of GnRH-R and gonadotropin beta subunit genes is GnRH dependent. The apparent GnRH-independent regulation of the alpha-glycoprotein subunit mRNA levels may be due to the contribution of thyrotropes and perhaps to the presence of exclusive regulatory signals for this gene.

Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium

It has been 70 years since the name inhibin was used to describe a gonadal factor that negatively regulated pituitary hormone secretion. The majority of this period was required to achieve purification and definitive characterization of inhibin, an event closely followed by identification and characterization of activin and follistatin (FS). In contrast, the last 15-20 years saw a virtual explosion of information regarding the biochemistry, physiology, and biosynthesis of these proteins, as well as identification of activin receptors, and a unique mechanism for FS action-the nearly irreversible binding and neutralization of activin. Many of these discoveries have been previously summarized; therefore, this review will cover the period from the mid 1990s to present, with particular emphasis on emerging themes and recent advances. As the field has matured, recent efforts have focused more on human studies, so the endocrinology of inhibin, activin, and FS in the human is summarized first. Another area receiving significant recent attention is local actions of activin and its regulation by both FS and inhibin. Because activin and FS are produced in many tissues, we chose to focus on a few particular examples with the most extensive experimental support, the pituitary and the developing follicle, although nonreproductive actions of activin and FS are also discussed. At the cellular level, it now seems that activin acts largely as an autocrine and/or paracrine growth factor, similar to other members of the transforming growh factor beta superfamily. As we discuss in the next section, its actions are regulated extracellularly by both inhibin and FS. In the final section, intracellular mediators and modulators of activin signaling are reviewed in detail. Many of these are shared with other transforming growh factor beta superfamily members as well as unrelated molecules, and in a number of cases, their physiological relevance to activin signal propagation remains to be elucidated. Nevertheless, taken together, recent findings suggest that it may be more appropriate to consider a new paradigm for inhibin, activin, and FS in which activin signaling is regulated extracellularly by both inhibin and FS whereas a number of intracellular proteins act to modulate cellular responses to these activin signals. It is therefore the balance between activin and all of its modulators, rather than the actions of any one component, that determines the final biological outcome. As technology and model systems become more sophisticated in the next few years, it should become possible to test this concept directly to more clearly define the role of activin, inhibin, and FS in reproductive physiology.