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Focus on TGF-ß Signalling

Pituitary actions of ligands of the TGF-ß family: activins and inhibins

The Clayton Foundation Laboratories for Peptide Biology, The Salk Institute, 10010 N. Torrey Pines Road, La Jolla, California 92037, USA

Correspondence should be addressed to L M Bilezikjian; Email: bilezikjian{at}salk.edu

	Abstract

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
Activins, as members of the transforming growth factor-ß superfamily, control and orchestrate many physiological processes and are vital for the development, growth and functional integrity of most tissues, including the pituitary. Activins produced by pituitary cells work in conjunction with central, peripheral, and other local factors to influence the function of gonadotropes and maintain a normal reproductive axis. Follistatin, also produced by the pituitary, acts as a local buffer to bind activin and modulate its bioactivity. On the other hand, inhibins of gonadal origin provide an endocrine feedback signal to antagonize activin signaling in cells that express the inhibin co-receptor, betaglycan, such as gonadotropes. This review highlights the pituitary roles of activin and the mechanisms through which these actions are modulated by inhibin and follistatin.

	Introduction

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
The growth and differentiation factors (GDFs), comprising the transforming growth factor-ß (TGF-ß) family, control many biological processes by orchestrating and modulating the developmental program, growth and differentiation profile, and functional homeostasis of most cell types. They act as morphogens to regulate asymmetric cell division and cell fate determination during embryogenesis and have profound effects on reproductive function, immune responses, bone formation, liver growth and regeneration, tissue remodeling and repair, erythropoiesis, and angiogenesis throughout adult life (Vale et al. 1990, Massague 2000). In general, members of the TGF-ß family are not only viewed as important suppressors of primary tumorigenesis but, remarkably, are also capable of promoting tumor growth depending on circumstances and cellular context (Roberts & Wakefield 2003). Recent studies have revealed that members of the TGF-ß family are also involved in regulating pathways that help to maintain embryonic stem cells in an undifferentiated state (Ying et al. 2003, Beattie et al. 2005). The factors that are represented in this large family of structurally related polypeptide ligands include the activins, inhibins, TGF-ß isoforms, bone morphogenetic proteins (BMPs), GDFs, as well as Nodal and Lefty (Vale et al. 1990, Massague 2000). With few exceptions, these protein factors are expressed and secreted near their targets, where they act as autocrine and/or paracrine factors to locally modify diverse cellular functions of those cells that express appropriate receptors. Cellular responses to individual ligands, in turn, are tempered through extracellular mechanisms of receptor antagonism and ligand inactivation, as well as intracellular events that terminate signaling (Harrison et al. 2005).

Numerous in vitro and in vivo animal studies, as well as data from clinical settings have substantiated the importance of activins and inhibins as modulators of the reproductive axis and normal reproductive function (de Kretser et al. 2002, Welt et al. 2002, Tong et al. 2003, Muttukrishna et al. 2004, Luisi et al. 2005). Inhibins and, shortly thereafter, activins were identified as components of gonadal fluids that exhibit the ability to suppress or stimulate follicle-stimulating hormone (FSH) secretion from pituitary gonadotropes respectively (Vale et al. 1990). The characterization of these factors, as members of the TGF-ß superfamily, initiated a cascade of novel ideas regarding their role in the regulation of the reproductive axis and expanded our appreciation of the scope of their function. It is now clear that activins and inhibins regulate reproductive function by exerting effects at all levels of the reproductive axis (Welt et al. 2002). Thus, not surprisingly, genetic manipulations or mutations that either delete or alter the expression of the relevant ligands of the TGF-ß family, their binding proteins or components of their signaling pathways are associated with varying degrees of reproductive anomalies (Chang et al. 2002). In the pituitary, as in many other tissues, activins are produced and act locally to regulate the function of their targets, including gonadotropes and other cell types (Bilezikjian et al. 2004). Inhibins, on the other hand, have a more established role as endocrine feedback modulators of the pituitary, although several lines of evidence suggest that they also act locally as autocrine or paracrine factors (Welt et al. 2002, Kumar et al. 2003). In addition to activins and inhibins, some of the TGF-ß and BMP isoforms also exert cell-specific effects within the pituitary and are implicated in the regulation of the reproductive axis in some species (Welt et al. 2002). Beyond their reported actions on differentiated pituitary cells, the stage-specific opposing gradients of BMP4 and FGF-8 or BMP2 and Wnt4 (Wingless-type MMTV integration site family, member 4) are critical for the invagination and formation of Rathke’s pouch and the normal development of the pituitary (Rosenfeld et al. 2000). The purification efforts that culminated in the discovery of gonadal activins and inhibins also led to the discovery of the FSH-suppressing activity of the follistatins, later shown to stem from their activin-binding function (Nakamura et al. 1990). Further investigations demonstrated the ubiquitous nature of follistatin distribution and established the importance of the local function of follistatins in the modulation of activin and BMP signaling (Nakamura et al. 1990). This review summarizes the evidence for the physiologic roles of activins and their two functional antagonists, inhibins and follistatin, in the modulation of gonadotropes.

	Signaling mechanisms of activin

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
With few exceptions, the biologically active ligands of the TGF-ß family, including the activins, BMPs, and TGF-ßs, are generally homodimers of protein subunits that display a common structural feature shared by the members of the larger family of cystine-knot growth factors (Vale et al. 1990, Massague 2000, Vitt et al. 2001). Activins are sulfhydryl-linked dimers of inhibin/ activin-ß subunits (Vale et al. 1990). Of the five ß-subunits that have been identified to date, homo- or heterodimers of ßA and ßB are the only ones with demonstrated biological function as receptor ligands (Vale et al. 1990). The other ß-subunits, on the other hand, may have alternative modes of actions (Mellor et al. 2003). The ligands of the TGF-ß superfamily initiate downstream signaling events by sequentially interacting with cognate type-II and type-I receptors belonging to an extended family of single-transmembrane Ser/Thr kinase proteins (Lebrun et al. 1997, Massague 2000). Structural studies have revealed some of the important features of these interactions and provided evidence for the ‘six-chain complex’ generated by the assembly of two type-II and two type-I receptors by the dimeric nature of the ligands (Greenwald et al. 2004, Lin et al. 2006). The assembly of two types of receptors serves an important function to enable the constitutively active kinase domains of the type-II receptors to trans-phosphorylate and activate the type-I receptors and, in turn, allow them to phosphorylate downstream-signaling substrates (Lebrun et al. 1997, Feng & DAllaerts et al. 2005, Massague et al. 2005). Activins initiate downstream-signaling events by sequentially interacting with one of the two known type-II activin receptors, ActRII or ActRIIB, and the type-I receptor known as activin receptor-like kinase (ALK) 4 (Lebrun et al. 1997, Massague 2000). Further analysis has suggested that in addition to ALK4, the activin B and activin AB isoforms may preferentially signal via ALK7 in tissues that express this type-I receptor (Tsuchida et al. 2004). Interestingly, activins and TGF-ßs share the same downstream-signaling substrates (Lebrun et al. 1997, ten Dijke et al. 2000, Feng & Derynck 2005, Massague et al. 2005). The ligand-specific activation of ALK4 by activin or ALK5 by TGF-ß facilitates the direct phosphorylation of Smad2 and/or Smad3 by the kinase domain of these receptors and promotes their ability to enter the nucleus and modify target gene activity in co-operation with Smad4/ DPC4 (deleted in pancreatic carcinoma) and other partners. The activation of the Smad-signaling pathway by activin or TGF-ß also initiates an intracellular mechanism of feedback modulation by inducing the inhibitory Smad7 and preventing urther Smad2/3 activation (Lebrun et al. 1997, Massague et al. 2005).

	Modes of inhibin and follistatin action

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
Inhibins function as potent cell selective antagonists of activins and, under certain circumstances, of certain BMPs (Lewis et al. 2000, Wiater & Vale 2003). Inhibin A and inhibin B are formed by the heterodimerization of the corresponding inhibin/activin ß-subunits with the structurally related inhibin/activin -subunit (Vale et al. 1990, Welt et al. 2002). The molecular basis of inhibin antagonism of either activin or BMP was elucidated by the realization that the type-III TGF-ß receptor (TßRIII or betaglycan) has a dual function and serves as a high-affinity co-receptor for inhibin as well as TGF-ß (Lewis et al. 2000). Binding and functional studies support a model of antagonism in which the interaction of inhibin with betaglycan facilitates the recruitment of ActRII or ActRIIB into a complex and sequesters them away from activins. In a similar manner, inhibin-bound betaglycan is able to antagonize BMP actions that are dependent on ActRII or ActRIIB, as well as BMPRII (Wiater & Vale 2003). The dual co-receptor function of betaglycan, on the other hand, may allow TGF-ß to interfere with the suppressive actions of inhibin and, indirectly, promote activin bioactivity (Ethier et al. 2002). Although betaglycan displays high affinity for both inhibin and TGF-ß, recent studies have shown that a single mutation within the inhibin-binding extracellular domain of betaglycan disrupts inhibin and TGF-ß binding to this site, but does not affect TGF-ß binding to the second TGF-ß-binding domain, which does not bind inhibin (Wiater et al. 2006). Betaglycan mRNA and protein are present in inhibin-responsive cells throughout the hypothalamo-pituitary–gonadal axis including gonadotropes (MacConell et al. 2002, Chapman & Woodruff 2003). The broader distribution of betaglycan beyond these known inhibin-responsive sites probably reflects its additional function as a co-receptor for promoting TGF-ß2 action, as well as its unexpected role in facilitating the antagonism of selective BMP actions by inhibin (MacConell et al. 2002, Chapman & Woodruff 2003, Wiater & Vale 2003). The functional versatility of betaglycan and the broader actions of inhibin are likely reflected in the phenotypes of mice that are deficient either in inhibin or its co-receptor, betaglycan. In the inhibin-deficient mice, cell-selective perturbations and dysregulated signaling by both activin and BMP may contribute to the formation of gonadal and adrenal tumors of inhibin-deficient mice (Chang et al. 2002). In the betaglycan-deficient mice, on the other hand, embryonic lethality due to heart and liver defects could reflect diminished TGF-ß2 and/or unrestrained BMP signaling (Stenvers et al. 2003).

Follistatins are recognized as activin-binding proteins that can bio-neutralize and thereby modulate all actions of activins, and, at higher concentrations those of certain BMPs (Michel et al. 1993, Phillips & deKretser 1998, Balemans & Van Hul 2002, Shimasaki et al. 2004). Although first identified as FSH-suppressing components of gonadal fluids, numerous studies have documented the presence of follistatin in many tissues, including most cell types of the anterior pituitary (Bilezikjian et al. 2004). Mutagenesis studies had previously indicated that follistatin interferes with activin function by masking its type-II binding site (Fischer et al. 2003). The recent elucidation of the crystal structure of a follistatin:activin complex has extended this model and revealed the manner in which follistatin masks the binding sites on activin for both type-I and type-II receptors (Thompson et al. 2005). These structural studies have also provided a model that would explain the basis for the differential binding modes of follistatin to activin and BMP isoforms (Thompson et al. 2005). Two follistatin isoforms (FS315 and FS288) that differ in their abilities to associate with cell-surface proteoglycans are encoded by two alternatively spliced mRNA products of the follistatin gene (Michel et al. 1990, Schneyer et al. 2004). A third isoform (FS303) that has been identified in porcine follicular fluids is generated from the proteolytic cleavage of FS315 (Schneyer et al. 2004). The relative abundance of the various follistatin isoforms has been difficult to evaluate, but recent measurements confirm that FS315 is the form that circulates, while the C-terminally truncated FS288 form is presumed to act locally because of its greater affinity for cell-surface proteoglycans (Welt et al. 2002, Keutmann et al. 2004, Schneyer et al. 2004). Genetic models of altered follistatin expression have established the importance of this modulator in restraining the actions of activins, BMPs, and possibly other related ligands (Chang et al. 2002). Mice deficient in follistatin develop embryonic defects and die shortly after birth (Matzuk et al. 1995). Follistatin overexpression, on the other hand, is associated with infertility probably resulting from disrupted activin and BMP signaling at the level of both the gonads and the pituitary (Guo et al. 1998).

	Local pituitary actions of activin

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
The various components required for activin signaling are present in the anterior pituitary and activins have been reported to exert effects on multiple pituitary cell types, the best-characterized of which are the gonadotropes (Bilezikjian et al. 2004). The regulated and the differential production of FSH from pituitary gonadotropes through different stages of the reproductive cycle is achieved by the integrated actions of multiple factors that originate from central, peripheral, or local sources. Activins play a central role in this process and numerous studies have demonstrated that the differential production of FSH is exquisitely dependent on the actions of this TGF-ß family member to induce FSH-ß synthesis (Vale et al. 1990, Bilezikjian et al. 2004). Early studies demonstrated that activins exert their effects by differentially stimulating FSH-ß mRNA accumulation (Carroll et al. 1989). Recent data, however, raise the possibility that activins, under certain circumstances, can also regulate luteinizing hormone (LH) production by modifying the expression of the LH-ß-subunit (Yamada et al. 2004, Coss et al. 2005). Genetic knockout or overexpression models, while provide some answers regarding the function of the activin circuitry in reproductive function, have also highlighted the complexity of the system (Chang et al. 2002).

The autocrine or paracrine function of activin in the pituitary became appreciated by the results of a series of studies that confirmed that inhibin/activin - and ß-subunit mRNAs and proteins are expressed in the pituitary and that anterior pituitary cell preparations secrete activins A and B (Bilezikjian et al. 2004). Experiments with an immunoneutralizing MAB to activin B subsequently provided convincing evidence that activin B produced by rat pituitary cells serves as a positive signal and locally drives the expression of FSH-ß and the secretion of FSH, in vitro, and mediates the hypersecretory FSH response to ovariectomy, in vivo (Corrigan et al. 1991, DePaolo et al. 1992). The fact that pituitaries of ovariectomized rats, when grafted under the kidney capsule, continue to overproduce FSH is consistent with the existence of a local mechanism that operates independent of the hypothalamic and gonadal inputs (DePaolo et al. 1992). More recent studies with castrated ActRII null mice have provided additional support for not only a local activin tone, but also an inhibin tone (Kumar et al. 2003). Activin B originating from gonadotropes, the primary sites of inhibin/activin - and ßB-subunit expression, works in concert with factors known to modulate this cell type, including gonadotrophin-releasing hormone (GnRH), gonadal steroids, inhibin, and local follistatin, and, in turn, is influenced by them (Bilezikjian et al. 2004). Through a paracrine mechanism, activin A originating from other pituitary cell types, including folliculostellate cells, may also participate in modulating the responses of gonadotropes (Bilezikjian et al. 2003). Activins are permissive for the actions of GnRH on FSH production and GnRH pulse frequency, in turn, can alter activin B production by modulating inhibin/activin ßB mRNA levels (Weiss et al. 1992, Burger et al. 2002). The availability of suitable cell lines (LßT2 and T3-1), derived from the gonadotrope lineage of the mouse pituitary, has permitted direct examination of the mechanisms underlying the actions of activin in gonadotropes. Transcriptional studies of FSH-ß and GnRH receptor promoters in these cell lines have revealed that both are modulated by activin (Fernandez-Vazquez et al. 1996, Duval et al. 1999, Pernasetti et al. 2001, Norwitz et al. 2002, Suszko et al. 2003, Bernard 2004) and targets the Smad2/3 pathway used by activin (Pernasetti et al. 2001, Norwitz et al. 2002, Suszko et al. 2003, Bernard 2004). These studies have shown that activin modulates gonadotrope sensitivity to GnRH by facilitating the action of GnRH to promote the transcription of the FSH-ß and GnRH receptor genes (Pernasetti et al. 2001, Gregory et al. 2005). The precise cellular mechanisms used by activins to facilitate the expression of their multiple targets and coordinate gonadotrope responsiveness are not known at this time, but are likely to be achieved through differential partnerships between Smads and other transcription factors that are formed in response to different cellular inputs and sensitive to different thresholds of activin signaling.

	Pituitary actions of inhibin

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
The inhibin/activin -subunit mRNA and protein are expressed in the anterior pituitary, but the endocrine feedback function of gonadal inhibin seems to be its principal mode of action to differentially modulate FSH production (de Kretser et al. 2002, Welt et al. 2002). The importance of circulating inhibin has been substantiated by a number of in vivo studies using wild-type animals, as well as genetic models. Immunoneutralization of circulating inhibin with an antibody to the inhibin/ activin -subunit was shown to increase plasma FSH levels and pituitary FSH-ß mRNA levels, while injections of recombinant inhibin A had the opposite effect (Vale et al. 1990). In inhibin/activin -subunit knockout mice, liver-specific expression of exogenous inhibin A lowered circulating FSH levels and resulted in reproductive defects, demonstrating that circulating inhibin can reach the pituitary to regulate FSH production (Pierson et al. 2000). Gonadotropes are the main pituitary targets of inhibin and the majority of these cells express the inhibin co-receptor, betaglycan (Lewis et al. 2000, MacConell et al. 2002, Chapman & Woodruff 2003). Inhibins modulate the function of gonadotropes by antagonizing the actions of activins on FSH secretion and FSH-ß expression (Bilezikjian et al. 2004, Gray et al. 2004, Phillips & Woodruff 2004) and by modulating gonadotrope sensitivity to GnRH receptors (Wang et al. 1989, Braden et al. 1990, Sealfon et al. 1990, Gregg et al. 1991). Whether the local production of inhibin within the pituitary contributes to the feedback actions of circulating inhibin to modulate the function of gonadotropes or other pituitary cell types remains an open question for future studies (Kumar et al. 2003, Bilezikjian et al. 2004).

	Local pituitary actions of follistatin

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
Measurable levels of follistatins also circulate, but their endocrine role as feedback modulators of activins in the pituitary remains questionable. Quite a number of studies, on the other hand, has substantiated the importance of autocrine or paracrine function of follistatins as local modulators of the actions of activins in the pituitary and many other tissues (Welt et al. 2002, Bilezikjian et al. 2004). Most pituitary cell types express follistatin mRNA and cultured rat anterior pituitary cells secrete the protein (Kogawa et al. 1991, Kaiser et al. 1992, Bilezikjian et al. 1993, Lan-Lee et al. 1993). The observation that folliculostellate cells isolated from bovine pituitaries secrete substantial amounts of follista-tin protein provided one of the first clues regarding the potential role of this protein as an autocrine or paracrine factor of the pituitary (Gospodarowicz & Lau 1989). Folliculostellate cells isolated from rat anterior pituitaries have also been reported to produce large amounts of follistatin (Bilezikjian et al. 2003). Experimental manipulation of the local availability of free follistatin in cultured rat anterior pituitary cells later confirmed that the local follistatin tone of the pituitary may be an important buffer that controls the potency and the efficacy of locally secreted activins to drive basal FSH secretion from gonadotropes (Bilezikjian et al. 1993, Fischer et al. 2003). These observations emphasize the importance of maintaining a delicate balance between activin and follistatin tone to achieve physiologically relevant levels, patterns of FSH production, and prevent reproductive anomalies due to excessive activin signaling in gonadotropes.

Activins are potent inducers of pituitary follistatin expression and the level of local follistatin expression, to some extent, is determined by the intrapituitary activin B tone, which also drives FSH-ß expression (Bilezikjian et al. 1993). These actions of activins, however, are self-limiting and sensitive to the intrinsic follistatin tone through a reciprocal feedback loop (Bilezikjian et al. 2004). A substantial portion of pituitary follistatin is likely to originate from gonadotropes and, therefore, is subject to regulation by factors that influence this cell type including GnRH, the autocrine action of activin B, inhibins, and gonadal steroids. Experimental data suggest that this may be an important feature underlying the ability of these factors to control follistatin production and thereby exert an indirect control on the actions of activin on gonadotropes. For example, a number of studies has suggested that GnRH and gonadal factors exert their effects on FSH production in part by modulating follistatin and inhibin/activin ßB subunit expression in the pituitary and thus altering the local availability and ratio of activin B and follistatin (DePaolo et al. 1993, Besecke et al. 1996, Dalkin et al. 1998). Moreover, differential FSH and LH production may be achieved, in part, through the differential effects of different patterns of GnRH pulses on inhibin/activin ßB and follistatin expression (Kirk et al. 1994, Dalkin et al. 1999). Rapid GnRH frequencies have been shown to support maximal follistatin expression with no change in FSH-ß, while slower frequencies produce a selective rise in FSH-ß, but no change in follistatin (Kirk et al. 1994). Moreover, the fluctuations in pituitary follistatin mRNA levels may partly account for the rise and fall in FSH production across the reproductive cycle (Halvorson et al. 1994, Bauer-Dantoin et al. 1996, Besecke et al. 1997).

Follistatin production from the folliculostellate cells provides another level of control on the actions of activins in the pituitary. The exact function of these S100 positive non-endocrine cells of the pituitary is not fully appreciated yet. They have been reported to be a major source of several paracrine factors of the pituitary to form a network that facilitates the synchronization of the pituitary gland or to represent a group of progenitor cells with the potential to differentiate into specialized endocrine cells (Allaerts et al. 1990, Renner et al. 1996, Stojilkovic 2001, Inoue et al. 2002). Folliculo-stellate cell lines derived from rat, mouse, and human anterior pituitaries have been characterized in recent years and experiments with these cells are beginning to shed some light into the function of this cell type (Inoue et al. 1992, Danila et al. 2000b, Bilezikjian et al. 2003). As a major source of intrapituitary follistatin, they have an important function to exert local control on the pituitary actions of activin (Gospodarowicz & Lau 1989, Bilezikjian et al. 2003). Follistatin production from folliculostellate cells is negatively correlated with FSH-ß expression and a paracrine modulator of FSH secretion (Fujii et al. 2002, Kawakami et al. 2002, Bilezikjian et al. 2003). Recently, characterized rat anterior pituitary folliculostellate cells (FS/D1h) have provided some useful and surprising information. These cells produce substantial amounts of follistatin (Bilezikjian et al. 2003). They express activin receptors and are responsive to activin as measured by the induction of Smad7 mRNA levels and inhibition of proliferation (Bilezikjian et al. 2003). Surprisingly, activin has no effect on follistatin mRNA levels in FS/D1h, unlike its effects on gonadotropes (Bilezikjian et al. 1999, 2003). The FS/D1h cells, however, respond to interleukin-1ß (IL-1ß) and dexamethasone by a dramatic increase in follistatin production both at the mRNA and protein level (Bilezikjian et al. 2003). Both activin A and follistatin have been implicated as participants of the systemic inflammatory response and the evidence suggests that the dramatic rise in circulating follistatin may be secondary to cytokine-induced increases in activin A (Phillips et al. 2001). Because FS/D1h cells also express inhibin/activin ßA mRNA and, therefore, might secrete some activin A, the effect of IL-1ß on follistatin production could have also been indirectly mediated by an autocrine action of activin A. An inhibitor of ALK4,5,7 (a generous gift from John Saunders; Neurocrine Biosciences, Inc., La Jolla, CA, USA), which suppresses signaling in response to activins and other ligands that utilize these type-I receptors, was used to address this possibility (Inman et al. 2002). These experiments indicated that unlike the case seen in systemic inflammation, cytokine-induced follistatin production from FS/D1h cells was probably not mediated by activins (Fig. 1). Both IL-1ß and glucocorticoids have been reported to modulate FSH production from gonadotropes (Bohnsack et al. 2000, Leal et al. 2003). The observations that both these agents can modulate follistatin production from FS/D1h cells suggest that their actions on FSH may, in part, be mediated by the paracrine action of follistatin derived primarily from folliculostellate cells. Interestingly, recent observations raise the possibility that folliculostellate cells may also be targets of BMPs, as indicated by the dose-dependent effects of BMP4 on follistatin secretion from FS/D1h cells (Fig. 2). The significance of this BMP4 effect remains to be evaluated, but could reflect another mode of paracrine control of the pituitary. Altogether, these observations have revealed the existence of diverse mechanisms through which cell-selective inputs influence the intrapituitary follistatin tone and modify activin signaling within the pituitary by autocrine or paracrine mechanisms (Fig. 3).

View larger version (17K): [in this window] [in a new window]   Figure 1 Follistatin secretion from FS/D1h folliculostellate cells in response to interleukin-1ß (IL-1ß) is not affected by co-incubation with an inhibitor of ALK4,5,7 to block activin signaling. Follistatin levels were quantified using an indirect immunoassay that measures follistatin:[125I]activin A complexes that are immunoprecipitated with anti-FS288 (#5542), as previously described (Bilezikjian et al. 2003).

View larger version (21K): [in this window] [in a new window]   Figure 2 The dose-dependent effects of BMP4 and IL-1ß on follistatin secretion from FS/D1h folliculostellate cells. Follistatin measurements were performed as previously described (Bilezikjian et al. 2003).

View larger version (7K): [in this window] [in a new window]   Figure 3 A model of the autocrine and paracrine actions of pituitary follistatin. In this model, activin modulates its own bioactivity by an autocrine mechanism, promoting follistatin production from gonadotropes and preventing further signaling. Follistatin secretion from folliculostellate cells in response to agents, such as IL-1ß, glucocorticoids, BMP4, and possibly other BMP isoforms modulates activin bioactivity by a paracrine mechanism.

	Conclusions

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

	Acknowledgements

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 
Work in the laboratory of W W V is supported in part by NIH grant HD13527 and the Foundation for Medical Research. W W V is a senior Foundation for Medical Research Investigator. The work contributed by L M B is supported in part by NIH grant HD46941 and the Foundation for Medical Research. We thank Debbie Doan, Sandra Guerra and Dave Dalton for help in finalizing the illustrations and the manuscript. The authors declare that there is no conflict of interest that would prejudice the impartiality of this scientific work.

	Footnotes

 
Received 24 January 2006
First decision 7 April 2006
Revised manuscript received 1 May 2006
Accepted 18 May 2006

	References

Top Abstract Introduction Signaling mechanisms of activin Modes of inhibin and... Local pituitary actions of... Pituitary actions of inhibin Local pituitary actions of... Conclusions Acknowledgements References

 

Allaerts W, Carmeliet P & Denef C 1990 New perspectives in the function of pituitary folliculo-stellate cells. Molecular and Cellular Endocrinology 71 73–81.[CrossRef][Web of Science][Medline]

Balemans W & Van Hul W 2002 Extracellular regulation of BMP signaling in vertebrates: a cocktail of modulators. Developmental Biology 250 231–250.[CrossRef][Web of Science][Medline]

Bauer-Dantoin AC, Weiss J & Jameson JL 1996 Gonadotropin-releasing hormone regulation of pituitary follistatin gene expression during the primary follicle-stimulating hormone surge. Endocrinology 137 1634–1639.[Abstract]

Beattie GM, Lopez AD, Bucay N, Hinton A, Firpo MT, King CC & Hayek A 2005 Activin A maintains pluripotency of human embryonic stem cells in the absence of feeder layers. Stem Cells 23 489–495.[CrossRef][Web of Science][Medline]

Bernard DJ 2004 Both Smad2 and Smad3 mediate activin-stimulated expression of the follicle-stimulating hormone beta subunit in mouse gonadotrope cells. Molecular Endocrinology 18 606–623.[Abstract/Free Full Text]

Besecke LM, Guendner MJ, Schneyer AL, Bauer-Dantoin AC, Jameson JL & Weiss J 1996 Gonadotropin-releasing hormone regulates follicle-stimulating hormone- ß gene expression through an activin/follistatin autocrine or paracrine loop. Endocrinology 137 3667–3673.[Abstract]

Besecke LM, Guendner MJ, Sluss PA, Polak AG, Woodruff TK, Jameson JL & Bauer-Dantoin AC 1997 Pituitary follistatin regulates activin-mediated production of follicle-stimulating hormone during the rat estrous cycle. Endocrinology 138 2841–2848.[Abstract/Free Full Text]

Bilezikjian LM, Corrigan AZ, Vaughan JM & Vale WW 1993 Activin-A regulates follistatin secretion from cultured rat anterior pituitary cells. Endocrinology 133 2554–2560.[Abstract/Free Full Text]

Bilezikjian LM, Blount AL & Vale WW 1999 Follistation expression is transcriptiionally regualted by activin-A in ßT3-1 gonadotropes. In 81st Annual Meeting of the Endocrine Society. San Diego, CA.

Bilezikjian LM, Corrigan AZ, Blount AL, Chen Y & Vale WW 2001 Regulation and actions of Smad7 in the modulation of activin, inhibin and TGFß signaling in anterior pituitary cells. Endocrinology 142 1065–1072.[Abstract/Free Full Text]

Bilezikjian LM, Leal AMO, Blount A, Corrigan AZ, Turnbull AV & Vale W 2003 Rat anterior pituitary folliculo-stellate cells are targets of interleukin-1ß and a major source of intrapituitary follistatin. Endocrinology 144 732–740.[Abstract/Free Full Text]

Bilezikjian LM, Blount A, Leal AMO, Donaldson C, Fischer W & Vale W 2004 Autocrine/paracrine regulation of pituitary function by activin, inhibin and follistatin. Molecular and Cellular Endocrinology 225 29–36.[CrossRef][Web of Science][Medline]

Bohnsack BL, Szabo M, Kilen SM, Tam DHY & Schwartz NB 2000 Follistatin suppresses steroid-enhanced follicle-stimulating hormone release in vitro in rats. Biology of Reproduction 62 636–641.[Abstract/Free Full Text]

Braden TD, Farnworth PG, Burger HG & Conn PM 1990 Regulation of the synthetic rate of gonadotropin-releasing hormone receptors in rat pituitary cell cultures by inhibin. Endocrinology 127 2387–2392.[Abstract/Free Full Text]

Burger LL, Dalkin AC, Aylor KW, Haisenleder DJ & Marshall JC 2002 GnRH pulse frequency modulation of gonadotropin subunit gene transcription in normal gonadotropes-assessment by primary transcript assay provides evidence for roles of GnRH and follistatin. Endocrinology 143 3243–3249.[Abstract/Free Full Text]

Carroll RS, Corrigan AZ, Gharib SD, Vale WW & Chin WW 1989 Inhibin, activin and follistatin: regulation of follicle-stimulating hormone messenger ribonucleic acid levels. Molecular Endocrinology 3 1969–1976.[Abstract/Free Full Text]

Chang H, Brown CW & Matzuk MM 2002 Genetic analysis of the mammalian transforming growth factor-beta superfamily. Endocrine Reviews 23 787–823.[Abstract/Free Full Text]

Chapman SC & Woodruff TK 2003 Betaglycan localization in the female rat pituitary: implications for the regulation of follicle-stimulating hormone by inhibin. Endocrinology 144 5640–5649.[Abstract/Free Full Text]

Corrigan AZ, Bilezikjian LM, Carroll RS, Bald LN, Schmelzer CH, Fendly BM, Mason AJ, Chin WW, Schwall RH & Vale WW 1991 Evidence for an autocrine role of activin B within rat anterior pituitary cultures. Endocrinology 128 1682–1684.[Abstract/Free Full Text]

Coss D, Thackray VG, Deng CX & Mellon PL 2005 Activin regulates luteinizing hormone beta-subunit gene expression through Smad-binding and homeobox elements. Molecular Endocrinology 19 2610–2623.[Abstract/Free Full Text]

Dalkin AC, Haisenleder DJ, Gilrain JT, Aylor K, Yasin M & Marshall JC 1998 Regulation of pituitary follistatin and inhibin/activin subunit messenger ribonucleic acids (mRNAs) in male and female rats: evidence for inhibin regulation of follistatin mRNA in females. Endocrinology 139 2818–2823.[Abstract/Free Full Text]

Dalkin AC, Haisenleder DJ, Gilrain JT, Aylor K, Yasin M & Marshall JC 1999 Gonadotropin-releasing hormone regulation of gonadotropin subunit gene expression in female rats: actions on follicle-stimulating hormone ß messenger ribonucleic acid (mRNA) involve differential expression of pituitary activin (ß-B) and follistatin mRNAS. Endocrinology 140 903–908.[Abstract/Free Full Text]

Danila DC, Inder WJ, Zhang X, Alexander JM, Swearingen B, Hedley-Whyte ET & Klibanski A 2000a Activin effects on neoplastic proliferation of human pituitary tumors. Journal of Clinical Endocrinology and Metabolism 85 1009–1015.[Abstract/Free Full Text]

Danila DC, Zhang X, Zhou Y, Dickersin GR, Fletcher JA, Hedley-Whyte ET, Selig MK, Johnson SR & Klibanski A 2000b A human pituitary tumor-derived folliculostellate cell line. Journal of Clinical Endocrinology and Metabolism 85 1180–1187.[Abstract/Free Full Text]

de Kretser DM, Hedger MP, Loveland KL & Phillips DJ 2002 Inhibins, activins and follistatin in reproduction. Human Reproduction Update 8 529–541.[Abstract/Free Full Text]

DePaolo LV, Bald LN & Fendly BM 1992 Passive immunoneutralization with a monoclonal antibody reveals a role for endogenous activin-B in mediating FSH hypersecretion during estrus and following ovariectomy of hypophysectomized, pituitary-grafted rats. Endocrinology 130 1741–1743.[Abstract/Free Full Text]

DePaolo LV, Mercado M, Guo Y & Ling N 1993 Increased follistatin (activin-binding protein) gene expression in rat anterior pituitary tissue after ovariectomy may be mediated by pituitary activin. Endocrinology 132 2221–2228.[Abstract/Free Full Text]

Duval DL, Ellsworth BS & Clay CM 1999 Is gonadotrope expression of the gonadotropin releasing hormone receptor gene mediated by autocrine/paracrine stimulation of an activin response element?. Endocrinology 140 1949–1952.[Abstract/Free Full Text]

Ethier JF, Farnworth PG, Findlay JK & Ooi GT 2002 Transforming growth factor-ß modulates inhibin A bioactivity in the LßT2 gonadotrope cell line by competing for binding to betaglycan. Molecular Endocrinology 16 2754–2763.[Abstract/Free Full Text]

Ezzat S 2001 The role of hormones, growth factors and their receptors in pituitary tumorigenesis. Brain Pathology 11 356–370.[Web of Science][Medline]

Feng XH & Derynck R 2005 Specificity and versatility in TGF-ß signaling through Smads. Annual Reviews in Cellular and Developmental Biology 21 659–693.

Fernandez-Vazquez G, Kaiser UB, Albarracin CT & Chin WW 1996 Transcriptional activation of the gonadotropin-releasing hormone receptor gene by activin A. Molecular Endocrinology 10 356–366.[Abstract/Free Full Text]

Fischer WH, Park M, Donaldson C, Wiater E, Vaughan J, Bilezikjian LM & Vale W 2003 Residues in the C-terminal region of activin-A determine specificity for follistatin and type II receptor binding. Journal of Endocrinology 176 61–68.[Abstract]

Fujii Y, Okada Y, Moore JP Jr, Dalkin AC & Winters SJ 2002 Evidence that PACAP and GnRH down-regulate follicle-stimulating hormone-beta mRNA levels by stimulating follistatin gene expression: effects on folliculostellate cells, gonadotrophs and LbetaT2 gonadotroph cells. Molecular and Cellular Endocrinology 192 55–64.[CrossRef][Web of Science][Medline]

Gospodarowicz D & Lau K 1989 Pituitary follicular cells secrete both vascular endothelial growth factor and follistatin. Biochemical and Biophysical Research Communications 165 292–298.[CrossRef][Web of Science][Medline]

Gray P, Bilezikjian LM, Harrison C, Wiater E, & Vale W 2004 Activins and inhibins: physiologic roles, signaling mechanisms and regulation. In Hormones and the Brain, pp 1–28. Eds C Kordan, RC Galliard & Y Christen. Springer-Verlag: Paris, France.

Greenwald J, Vega M, Allendorph GP, Fischer W, Vale W & Choe S 2004 A flexible activin can explain the membrane-dependent cooperative assembly of TGF-beta family receptors. Molecular Cell 15 485–489.[CrossRef][Web of Science][Medline]

Gregg DW, Schwall RH & Nett TM 1991 Regulation of gonadotropin secretion and number of gonadotropin-releasing hormone receptors by inhibin, activin-A, and estradiol. Biology of Reproduction 44 725–732.[Abstract]

Gregory SJ, Lacza CT, Detz AA, Xu S, Petrillo LA & Kaiser UB 2005 Synergy between activin A and gonadotropin-releasing hormone in transcriptional activation of the rat follicle-stimulating hormone-beta gene. Molecular Endocrinology 19 237–254.[Abstract/Free Full Text]

Guo Q, Kumar TR, Woodruff T, Hadsell LA, DeMayo FJ & Matzuk MM 1998 Overexpression of mouse follistatin causes reproductive defects in transgenic mice. Molecular Endocrinology 12 96–106.[Abstract/Free Full Text]

Halvorson LM, Weiss J, Bauer-Dantoin AC & Jameson JL 1994 Dynamic regulation of pituitary follistatin messenger ribonucleic acids during the rat estrous cycle. Endocrinology 134 1247–1253.[Abstract/Free Full Text]

Harrison CA, Gray PC, Vale WW & Robertson DM 2005 Antagonists of activin signaling: mechanisms and potential biological applications. Trends in Endocrinology and Metabolism 16 73–78.[CrossRef][Web of Science][Medline]

Inman GJ, Nicolas FJ, Callahan JF, Harling JD, Gaster LM, Reith AD, Laping NJ & Hill CS 2002 SB-431542 is a potent and specific inhibitor of transforming growth factor-beta superfamily type I activin receptor-like kinase (ALK) receptors ALK4, ALK5, and ALK7. Molecular Pharmacology 62 65–74.[Abstract/Free Full Text]

Inoue K, Matsumoto H, Koyama C & Shibata K 1992 Establishment of a folliculo-stellate-like cell line from a murine thyrotropic pituitary tumor. Endocrinology 131 3110–3116.[Abstract/Free Full Text]

Inoue K, Mogi C, Ogawa S, Tomida M & Miyai S 2002 Are folliculo-stellate cells in theanterior pituitary gland supportivecells or organ-specificstem cells? Archives in Physiology and Biochemistry 110 50–53.[CrossRef]

Kaiser UB, Lee BL, Carroll RS, Unabia G, Chin WW & Childs GV 1992 Follistatin gene expression in the pituitary: localization in gonadotropes and folliculostellate cells in diestrous rats. Endocrinology 130 3048–3056.[Abstract/Free Full Text]

Kawakami S, Fujii Y, Okada Y & Winters SJ 2002 Paracrine regulation of FSH by follistatin in folliculostellate cell-enriched primate pituitary cell cultures. Endocrinology 143 2250–2258.[Abstract/Free Full Text]

Keutmann HT, Schneyer AL & Sidis Y 2004 The role of follistatin domains in follistatin biological action. Molecular Endocrinology 18 228–240.[Abstract/Free Full Text]

Kirk SE, Dalkin AC, Yasin M, Haisenleder DJ & Marshall JC 1994 Gonadotropin-releasing hormone pulse frequency regulates expression of pituitary follistatin messenger ribonucleic acid: a mechanism for differential gonadotrope function. Endocrinology 135 876–880.[Abstract]

Kogawa K, Nakamura T, Sugino K, Takio K, Titani K & Sugino H 1991 Activin-binding protein is present in pituitary. Endocrinology 128 1434–1440.[Abstract/Free Full Text]

Kumar TR, Agno J, Janovick JA, Conn PM & Matzuk MM 2003 Regulation of FSHß and GnRH receptor gene expression in activin receptor II knockout male mice. Molecular and Cellular Endocrinology 212 19–27.[CrossRef][Web of Science][Medline]

Lan-Lee B, Unabia G & Childs G 1993 Expression of follistatin mRNA by somatotropes and mammotropes early in the rat estrous cycle. Journal of Histochemistry and Cytochemistry 41 955–960.[Abstract]

Leal AMO, Blount AL, Donaldson C, Bilezikjian LM & Vale WW 2003 Regulation of follicle-stimulating hormone secretion by the interactions of activin A, dexamethasone and testosterone in anterior pituitary cell cultures of male rats. Neuroendocrinology 77 298–304.[CrossRef][Web of Science][Medline]

Lebrun J-J, Chen Y & Vale WW 1997 Receptor serine kinases and signaling by activins and inhibins. In Inhibin, Activin and Follistatin –Recent Advances and Future Views, pp 1–20. Ed. T Aono. Springer-Verlag: Tokushima, Japan.

Lewis KA, Gray PC, Blount AL, MacConell LA, Wiater E, Bilezikjian LM & Vale W 2000 Betaglycan binds inhibin and can mediate functional antagonism of activin signaling. Nature 404 411–414.[CrossRef][Medline]

Lin SJ, Lerch TF, Cook RW, Jardetzky TS & Woodruff TK 2006 The structural basis of TGF-ß, BMP and activin ligand binding. Reproduction 132 179–190.[Abstract/Free Full Text]

Luisi S, Florio P, Reis FM & Petraglia F 2005 Inhibins in female and male reproductive physiology: role in gametogenesis, conception, implantation and early pregnancy. Human Reproduction Update 11 123–135.[Abstract/Free Full Text]

MacConell LA, Leal A & Vale W 2002 The distribution of betaglycan protein and mRNA in rat brain, pituitary and gonads: implications for a role in inhibin-mediated reproductive functions. Endocrinology 143 1066–1075.[Abstract/Free Full Text]

Massague J 2000 How cells read TGF-beta signals. Nature Reviews Molecular and Cellular Biology 1 169–178.

Massague J, Seoane J & Wotton D 2005 Smad transcription factors. Genes and Development 19 2783–2810.[Abstract/Free Full Text]

Matzuk MM, Lu N, Vogel H, Sellheyer K, Roop DR & Bradley A 1995 Multiple defects and perinatal death in mice deficient in follistatin. Nature 374 360–363.[CrossRef][Medline]

Mellor SL, Ball EM, O’Connor AE, Ethier JF, Cranfield M, Schmitt JF, Phillips DJ & Groome NP 2003 Activin ßC-subunit heterodimers provide a new mechanism of regulating activin levels in the prostate. Endocrinology 144 4410–4419.[Abstract/Free Full Text]

Michel U, Albiston A & Findlay JK 1990 Rat follistatin: gonadal and extragonadal expression and evidence for alternative splicing. Biochemical and Biophysical Research Communications 173 401–407.[CrossRef][Web of Science][Medline]

Michel U, Farnworth P & Findlay JK 1993 Follistatins: more than follicle-stimulating hormone suppressing proteins. Molecular and Cellular Endocrinology 91 1–11.[CrossRef][Web of Science][Medline]

Muttukrishna S, Tannetta D, Groome N & Sargent I 2004 Activin and follistatin in female reproduction. Molecular and Cellular Endocrinology 225 45–56.[CrossRef][Web of Science][Medline]

Nakamura T, Takio K, Eto Y, Shibai H, Titani K & Sugino H 1990 Activin-binding protein from rat ovary is follistatin. Science 247 836–838.[Abstract/Free Full Text]

Norwitz ER, Xu S, Jeong KH, Bedecarrats GY, Winebrenner LD, Chin WW & Kaiser UB 2002 Activin A augments GnRH-mediated transcriptional activation of the mouse GnRH receptor gene. Endocrinology 143 985–997.[Abstract/Free Full Text]

Pernasetti F, Vasilyev VV, Rosenberg SB, Bailey JS, Huang HJ, Miller WL & Mellon PL 2001 Cell-specific transcriptional regulation of follicle-stimulating hormone-beta by activin and gonadotropin-releasing hormone in the LbetaT2 pituitary gonadotrope cell model. Endocrinology 142 2284–2295.[Abstract/Free Full Text]

Phillips DJ & deKretser DM 1998 Follistatin: A multifunctional regulatory protein. Frontiers in Neuroendocrinology 19 287–322.[CrossRef][Web of Science][Medline]

Phillips DJ & Woodruff TK 2004 Inhibin: actions and signalling. Growth Factors 22 13–18.[CrossRef][Web of Science][Medline]

Phillips DJ, Jones KL, Scheerlinck JY, Hedger MP & de Kretser DM 2001 Evidence for activin A and follistatin involvement in the systemic inflammatory response. Molecular and Cellular Endocrinology 180 155–162.[CrossRef][Web of Science][Medline]

Pierson TM, Wang Y, DeMayo FJ, Matzuk MM, Tsai SY & Omalley BW 2000 Regulable expression of inhibin A in wild-type and inhibin alpha null mice. Molecular Endocrinology 14 1075–1085.[Abstract/Free Full Text]

Renner U, Pagotto U, Arzt E & Stalla G 1996 Autocrine and paracrine roles of polypeptide growth factors, cytokines and vasogenic substances in normal and tumorous pituitary function and growth: a review. European Journal of Endocrinology 135 515–532.[Abstract/Free Full Text]

Risbridger GP, Schmitt JF & Robertson DM 2001 Activins and inhibins in endocrine and other tumors. Endocrine Reviews 22 836–858.[Abstract/Free Full Text]

Roberts AB & Wakefield LM 2003 The two faces of transforming growth factor beta in carcinogenesis. PNAS 100 8621–8623.[Free Full Text]

Rosenfeld MG, Briata P, Dasen J, Gleiberman AS, Kioussi C, Lin C, O’Connell SM, Ryan A, Szeto DP, Szeto DP & Treier M 2000 Multistep signaling and transcriptional requirements for pituitary organogenesis in vivo. Recent Progress in Hormone Research 55 1–13.[Web of Science][Medline]

Schneyer AL, Wang Q, Sidis Y & Sluss PM 2004 Differential distribution of follistatin isoforms: application of a new FS315-specific immunoassay. Journal of Clinical Endocrinology and Metabolism 89 5067–5075.[Abstract/Free Full Text]

Sealfon SC, Laws SC, Wu JC, Gillo B & Miller WL 1990 Hormonal regulation of gonadotropin-releasing hormone receptors and messenger RNA activity in ovine pituitary culture. Molecular Endocrinology 4 1980–1987.[Abstract/Free Full Text]

Shimasaki S, Moore RK, Otsuka F & Erickson GF 2004 The bone morphogenetic protein system in mammalian reproduction. Endocrine Reviews 25 72–101.[Abstract/Free Full Text]

Stenvers KL, Tursky ML, Harder KW, Kountouri N, Amatayakul-Chantler S, Grail D, Small C, Weinberg RA, Sizeland AM & Zhu HJ 2003 Heart and liver defects and reduced transforming growth factor beta2 sensitivity in transforming growth factor beta type III receptor-deficient embryos. Molecular and Cellular Biology 23 4371–4385.[Abstract/Free Full Text]

Stojilkovic SS 2001 A novel view of the function of pituitary folliculo-stellate cell network. Trends in Endocrinology and Metabolism 12 378–380.[CrossRef][Web of Science][Medline]

Suszko MI, Lo DJ, Suh H, Camper SA & Woodruff TK 2003 Regulation of the rat follicle-stimulating hormone beta-subunit promoter by activin. Molecular Endocrinology 17 318–332.[Abstract/Free Full Text]

ten Dijke P, Miyazono K & Heldin CH 2000 Signaling inputs converge on nuclear effectors in TGF-beta signaling. Trends in Biochemical Science 25 64–70.[CrossRef]

Thompson TB, Lerch TF, Cook RW, Woodruff TK & Jardetzky TS 2005 The structure of the follistatin:activin complex reveals antagonism of both type I and type II receptor binding. Developmental Cell 9 535–543.[CrossRef][Web of Science][Medline]

Tong S, Wallace EM & Burger HG 2003 Inhibins and activins: clinical advances in reproductive medicine. Clinical Endocrinology 58 115–127.[CrossRef][Medline]

Tsuchida K, Nakatani M, Yamakawa N, Hashimoto O, Hasegawa Y & Sugino H 2004 Activin isoforms signal through type I receptor serine/threonine kinase ALK7. Molecular and Cellular Endocrinology 220 59–65.[CrossRef][Web of Science][Medline]

Vale W, Hsueh A, Rivier C & Yu J 1990 The inhibin/activin family of growth factors. In Peptide Growth Factors and Their Receptors, Handbook of Experimental Pharmacology, pp 211–248. Eds MA Sporn & AB Roberts. Heidelberg: Springer-Verlag.

Vitt UA, Hsu SY & Hsueh AJ 2001 Evolution and classification of cystine knot-containing hormones and related extracellular signaling molecules. Molecular Endocrinology 15 681–694.[Abstract/Free Full Text]

Wang QF, Farnworth PG, Findlay JK & Burger HG 1989 Inhibitory effect of pure 31-kilodalton bovine inhibin on gonadotropin-releasing hormone (GnRH)-induced up-regulation of GnRH binding sites in cultured rat anterior pituitary cells. Endocrinology 124 363–368.[Abstract/Free Full Text]

Weiss J, Harris PE, Halvorson LM, Crowley WF Jr & Jameson JL 1992 Dynamic regulation of follicle-stimulating hormone-ß messenger ribonucleic acid levels by activin and gonadotropin-releasing hormone in perifused rat pituitary cells. Endocrinology 131 1403–1408.[Abstract/Free Full Text]

Welt C, Sidis Y, Keutmann H & Schneyer A 2002 Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium. Experimental Biology and Medicine 227 724–752.[Abstract/Free Full Text]

Wiater E & Vale W 2003 Inhibin is an antagonist of bone morphogenetic protein signaling. Journal of Biological Chemistry 278 7934–7941.[Abstract/Free Full Text]

Wiater E, Harrison CA, Lewis KA, Gray PC & Vale WW 2006 Identification of distinct inhibin and TGFß binding sites on betaglycan: functional separation of betaglycan co-receptor actions. Journal of Biological Chemistry 281 17011–17022.[Abstract/Free Full Text]

Yamada Y, Yamamoto H, Yonehara T, Kanasaki H, Nakanishi H, Miyamoto E & Miyazaki K 2004 Differential activation of the luteinizing hormone beta-subunit promoter by activin and gonado-tropin-releasing hormone: a role for the mitogen-activated protein kinase signaling pathway in LbetaT2 gonadotrophs. Biology of Reproduction 70 236–243.[Abstract/Free Full Text]

Ying QL, Nichols J, Chambers I & Smith A 2003 BMP induction of Id proteins suppresses differentiation and sustains embryonic stem cell self-renewal in collaboration with STAT3. Cell 115 281–292.[CrossRef][Web of Science][Medline]

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