Inhibin antagonizes inhibition of liver cell growth by activin by a dominant-negative mechanism

Review: Role and regulatory mechanism of inhibin in animal reproductive system

Inhibin (INH) is a glycoprotein hormone secreted by the gonads that inhibit the synthesis and secretion of follicle-stimulating hormone (FSH). Increasing evidence indicates that INH plays a significant role in the development of the reproductive system including follicle development, ovulation rate, corpus luteum formation and ablation, steroid hormone synthesis and spermatogenesis, subsequently affecting the reproductive capacity of animals such as litter size and egg production. There are currently three main views on how INH inhibits FSH synthesis and secretion: influencing the activity of adenylate cyclase, the expression of follicle-stimulating hormone receptor or gonadotropin-releasing hormone receptor, and the competition system of inhibin-activin. This review discusses the current findings on the structure, function, and mechanism of action of INH in the reproductive system of animals.

TGFBR3L is an inhibin B co-receptor that regulates female fertility

Follicle-stimulating hormone (FSH), a key regulator of ovarian function, is often used in infertility treatment. Gonadal inhibins suppress FSH synthesis by pituitary gonadotrope cells. The TGFβ type III receptor, betaglycan, is required for inhibin A suppression of FSH. The inhibin B co-receptor was previously unknown. Here, we report that the gonadotrope-restricted transmembrane protein, TGFBR3L, is the elusive inhibin B co-receptor. TGFBR3L binds inhibin B but not other TGFβ family ligands. TGFBR3L knockdown or overexpression abrogates or confers inhibin B activity in cells. Female Tgfbr3l knockout mice exhibit increased FSH levels, ovarian follicle development, and litter sizes. In contrast, female mice lacking both TGFBR3L and betaglycan are infertile. TGFBR3L’s function and cell-specific expression make it an attractive new target for the regulation of FSH and fertility.

Inhibin-positive hepatic carcinoma: proposal for a solid-tubulocystic variant of intrahepatic cholangiocarcinoma

Inhibin-positive hepatic carcinoma is a rare primary liver neoplasm that resembles sex cord-stromal tumor and thyroid follicular tumors. The term "cholangioblastic variant of intrahepatic cholangiocarcinoma" has been proposed. This study describes the clinicopathologic, immunophenotypic, and molecular features of a small series (n = 6) of this rare tumor. Albumin in situ hybridization (ISH) and capture-based next-generation sequencing (NGS) were also performed. All tumors occurred in young women (mean age 32.5 years, range 19-44 years) as a solitary large mass (mean 15.8 cm, range 6.9-23.5 cm). All tumors showed a highly distinctive morphology with sheets and large nests of tumor cells alternating with tubular and cystic areas imparting a sex cord-like or thyroid follicle-like morphology. Cytologic atypia was mild, and mitotic activity was low. All cases were positive for inhibin, as well as pancytokeratin, CK7, CK19, and albumin ISH. Synaptophysin and chromogranin showed focal or patchy staining, whereas INSM1 was negative. Markers for hepatocellular differentiation, thyroid origin, and sex cord-stromal tumor were negative. There were no recurrent genomic changes based on capture-based NGS of ∼500 cancer genes. Recurrence and/or metastasis was seen in three (50%) cases (follow-up time range for all cases: 5 months to 2 years). In conclusion, this series describes the distinctive morphology, immunophenotypic features, and diffuse albumin staining in six cases of a rare inhibin-positive primary liver carcinoma that runs an aggressive course similar to intrahepatic cholangiocarcinoma. Genomic changes typical of cholangiocarcinoma or hepatocellular carcinoma were not identified, and there were no recurrent genetic abnormalities. We propose the term "solid-tubulocystic variant of intrahepatic cholangiocarcinoma" to reflect the spectrum of morphologic patterns observed in this tumor.

Dual Roles of the Activin Signaling Pathway in Pancreatic Cancer

Activin, a member of the TGF-β superfamily, is involved in many physiological processes, such as embryonic development and follicle development, as well as in multiple human diseases including cancer. Genetic mutations in the activin signaling pathway have been reported in many cancer types, indicating that activin signaling plays a critical role in tumorigenesis. Recent evidence reveals that activin signaling may function as a tumor-suppressor in tumor initiation, and a promoter in the later progression and metastasis of tumors. This article reviews many aspects of activin, including the signaling cascade of activin, activin-related proteins, and its role in tumorigenesis, particularly in pancreatic cancer development. The mechanisms regulating its dual roles in tumorigenesis remain to be elucidated. Further understanding of the activin signaling pathway may identify potential therapeutic targets for human cancers and other diseases.

Receptor binding competition: A paradigm for regulating TGF-β family action

The transforming growth factor (TGF)-β family is a group of structurally related, multifunctional growth factors, or ligands that are crucially involved in the development, regulation, and maintenance of animal tissues. In humans, the family counts over 33 members. These secreted ligands typically form multimeric complexes with two type I and two type II receptors to activate one of two distinct signal transduction branches. A striking feature of the family is its promiscuity, i.e., many ligands bind the same receptors and compete with each other for binding to these receptors. Although several explanations for this feature have been considered, its functional significance has remained puzzling. However, several recent reports have promoted the idea that ligand-receptor binding promiscuity and competition are critical features of the TGF-β family that provide an essential regulating function. Namely, they allow a cell to read and process multi-ligand inputs. This capability may be necessary for producing subtle, distinctive, or adaptive responses and, possibly, for facilitating developmental plasticity. Here, we review the molecular basis for ligand competition, with emphasis on molecular structures and binding affinities. We give an overview of methods that were used to establish experimentally ligand competition. Finally, we discuss how the concept of ligand competition may be fundamentally tied to human physiology, disease, and therapy.

A Tale of Two Proteins: Betaglycan, IGSF1, and the Continuing Search for the Inhibin B Receptor

Inhibins are gonadal hormones that suppress follicle-stimulating hormone (FSH) synthesis by pituitary gonadotrope cells. The structurally related activins stimulate FSH by signaling through complexes of type I and type II receptors. Two models of inhibin action were proposed in 2000. First, inhibins function as competitive receptor antagonists, binding activin type II receptors with high affinity in the presence of the TGF-β type III coreceptor, betaglycan. Second, immunoglobulin superfamily, member 1 (IGSF1, then called p120) was proposed to mediate inhibin B antagonism of activin signaling via its type I receptor. These ideas have been challenged over the past few years. Rather than playing a role in inhibin action, IGSF1 is involved in the central control of the thyroid gland. Betaglycan binds inhibin A and inhibin B with high affinity, but only functions as an obligate inhibin A coreceptor in murine gonadotropes. There is likely to be a distinct, but currently unidentified coreceptor for inhibin B.

Inhibin B suppresses anoikis resistance and migration through the transforming growth factor-β signaling pathway in nasopharyngeal carcinoma

Inhibin B (INHBB), a heterodimer of a common α‐subunit and a βB‐subunit, is a glycoprotein belonging to the transforming growth factor‐β (TGF‐β) family. In this study, we observed INHBB expression was reduced in nasopharyngeal carcinoma (NPC) tissues compared to non‐tumor nasopharyngeal epithelium tissues, and INHBB was associated with lymph node metastasis, stage of disease, and clinical progress. Positive expression of INHBB in NPC predicted a better prognosis (overall survival, P = 0.038). However, the molecular mechanisms of INHBB have not been addressed in NPC. We induced anoikis‐resistant cells in NPC cell lines under anchorage‐independent conditions, then found epithelial‐mesenchymal transition markers changed, cell apoptosis decreased, cell cycle was modified, and invasion strengthened in anoikis‐resistant NPC cells. These anoikis‐resistant NPC cells showed decreased expression of INHBB compared with adhesion cells. Furthermore, INHBB was found to influence the above‐mentioned changes. In the anoikis‐resistant NPC cells with INHBB overexpression, apoptotic cells increased, S phase cells weakened, vimentin, matrix metallopeptidase‐9, and vascular endothelial growth factor A expression were downregulated, and E‐cadherin expression was upregulated, and vice versa in knockdown of INHBB (INHBB shRNA) anoikis‐resistant NPC cells. Diminished INHBB expression could activate the TGF‐β pathway to phosphorylate Smad2/3 and form complexes in the nucleus, which resulted in the above changes. Thus, our results revealed for the first time that INHBB could suppress anoikis resistance and migration of NPC cells by the TGF‐β signaling pathway, decrease p53 overexpression, and could serve as a potential biomarker for NPC metastasis and prognosis as well as a therapeutic application.

Bone morphogenetic protein 2 stimulates noncanonical SMAD2/3 signaling via the BMP type 1A receptor in gonadotrope-like cells: implications for FSH synthesis

FSH is an essential regulator of mammalian reproduction. Its synthesis by pituitary gonadotrope cells is regulated by multiple endocrine and paracrine factors, including TGFβ superfamily ligands, such as the activins and inhibins. Activins stimulate FSH synthesis via transcriptional regulation of its β-subunit gene (Fshb). More recently, bone morphogenetic proteins (BMPs) were shown to stimulate murine Fshb transcription alone and in synergy with activins. BMP2 signals via its canonical type I receptor, BMPR1A (or activin receptor-like kinase 3 [ALK3]), and SMAD1 and SMAD5 to stimulate transcription of inhibitor of DNA binding proteins. Inhibitor of DNA binding proteins then potentiate the actions of activin-stimulated SMAD3 to regulate the Fshb gene in the gonadotrope-like LβT2 cell line. Here, we report the unexpected observation that BMP2 also stimulates the SMAD2/3 pathway in these cells and that it does so directly via ALK3. Indeed, this novel, noncanonical ALK3 activity is completely independent of ALK4, ALK5, and ALK7, the type I receptors most often associated with SMAD2/3 pathway activation. Induction of the SMAD2/3 pathway by ALK3 is dependent upon its own previous activation by associated type II receptors, which phosphorylate conserved serine and threonine residues in the ALK3 juxtamembrane glycine-serine-rich domain. ALK3 signaling via SMAD3 is necessary for the receptor to stimulate Fshb transcription, whereas its activation of the SMAD1/5/8 pathway alone is insufficient. These data challenge current dogma that ALK3 and other BMP type I receptors signal via SMAD1, SMAD5, and SMAD8 and not SMAD2 or SMAD3. Moreover, they suggest that BMPs and activins may use similar intracellular signaling mechanisms to activate the murine Fshb promoter in immortalized gonadotrope-like cells.

Proliferation of rat small hepatocytes requires follistatin expression

Small hepatocytes (SHs) are a subpopulation of hepatocytes that have high growth potential in culture and can differentiate into mature hepatocytes (MHs). The activin (Act)/follistatin (Fst) system critically contributes to homeostasis of cell growth in the normal liver. ActA and ActB consist of two disulfide-linked Inhibin (Inh)β subunits, InhβA and InhβB, respectively. Fst binds to Act and blocks its bioactivity. In the present study we carried out the experiments to clarify how Fst regulates the proliferation of SHs. The gene expression was analyzed using DNA microarray analysis, reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR, and protein expression was examined by western blots, immunocytochemistry, and enzyme-linked immunosorbent assay. RT-PCR showed that Fst expression was high in SHs and low in MHs. Although the ActA expression was opposite to that of Fst, ActB expression was high in SHs and low in MHs and increased with time in culture. Fst protein was detected in the cytoplasm of SHs and secreted into the culture medium. ActB protein was also secreted into the medium. Although the exogenous administration of ActA and ActB apparently suppressed the proliferation of SHs, apoptosis of SHs was not induced by treatment with ActA or ActB. On the other hand, Fst treatment did not affect the colony formation of SHs but prevented the inhibitory effect of ActA. Neutralization by the anti-Fst antibody resulted in the suppression of DNA synthesis in SHs, and small hairpin RNA against Fst suppressed the expansion of SH colonies. In conclusion, Fst expression is necessary for the proliferation of SHs.

Inhibin α-subunit N terminus interacts with activin type IB receptor to disrupt activin signaling

Inhibin is a heterodimeric peptide hormone produced in the ovary that antagonizes activin signaling and FSH synthesis in the pituitary. The inhibin β-subunit interacts with the activin type II receptor (ActRII) to functionally antagonize activin. The inhibin α-subunit mature domain (N terminus) arose relatively early during the evolution of the hormone, and inhibin function is decreased by an antibody directed against the α-subunit N-terminal extension region or by deletion of the N-terminal region. We hypothesized that the α-subunit N-terminal extension region interacts with the activin type I receptor (ALK4) to antagonize activin signaling in the pituitary. Human or chicken free α-subunit inhibited activin signaling in a pituitary gonadotrope-derived cell line (LβT2) in a dose-dependent manner, whereas an N-terminal extension deletion mutant did not. An α-subunit N-terminal peptide, but not a control peptide, was able to inhibit activin A signaling and decrease activin-stimulated FSH synthesis. Biotinylated inhibin A, but not activin A, bound ALK4. Soluble ALK4-ECD bioneutralized human free α-subunit in LβT2 cells, but did not affect activin A function. Competitive binding ELISAs with N-terminal mutants and an N-terminal region peptide confirmed that this region is critical for direct interaction of the α-subunit with ALK4. These data expand our understanding of how endocrine inhibin achieves potent antagonism of local, constitutive activin action in the pituitary, through a combined mechanism of competitive binding of both ActRII and ALK4 by each subunit of the inhibin heterodimer, in conjunction with the co-receptor betaglycan, to block activin receptor-ligand binding, complex assembly, and downstream signaling.

Inhibin/activin expression in human and rodent liver: subunits α and βB as new players in human hepatocellular carcinoma?

Background:

Activins and inhibins belong to the TGFβ-superfamily, which controls cell proliferation and differentiation in many organs. Activin A, the dimer of inhibin βA subunit, acts strongly anti-proliferative in hepatocytes. Little is known on the other activin/inhibin subunits in human liver and hepatocellular carcinoma (HCC).

Methods:

We studied the expression of the complete inhibin family α, βA, βB, βC, βE in normal liver, tumour-adjacent and HCC tissue, 12 additional organs and rodent liver. A total of 16 HCC and 10 disease-free livers were analysed. Expression of inhibin subunits was determined by qRT–PCR, normalised to RNA input and by geNorm algorithm, and confirmed by immunohistochemistry.

Results:

Remarkably, βA expression was not decreased in HCC. Similarly, βC and βE exhibited no major changes. In contrast, inhibin α, barely detectable in normal liver, was strongly increased in tumour-adjacent liver and dramatically enhanced in HCC. βB was strongly enhanced in some HCC. At variance with human liver, rodent liver showed higher inhibin α and βC expression, but βA was somewhat, and βB dramatically lower.

Conclusions:

Upregulation of inhibin α – and possibly of βB – may shield HCC cells from anti-proliferative effects of activin A. Dramatic variations between humans and rodents may reflect different functions of some inhibins/activins.

Genetic evidence that SMAD2 is not required for gonadal tumor development in inhibin-deficient mice

BACKGROUND

Inhibin is a tumor-suppressor and activin antagonist. Inhibin-deficient mice develop gonadal tumors and a cachexia wasting syndrome due to enhanced activin signaling. Because activins signal through SMAD2 and SMAD3 in vitro and loss of SMAD3 attenuates ovarian tumor development in inhibin-deficient females, we sought to determine the role of SMAD2 in the development of ovarian tumors originating from the granulosa cell lineage.

METHODS

Using an inhibin alpha null mouse model and a conditional knockout strategy, double conditional knockout mice of Smad2 and inhibin alpha were generated in the current study. The survival rate and development of gonadal tumors and the accompanying cachexia wasting syndrome were monitored.

RESULTS

Nearly identical to the controls, the Smad2 and inhibin alpha double knockout mice succumbed to weight loss, aggressive tumor progression, and death. Furthermore, elevated activin levels and activin-induced pathologies in the liver and stomach characteristic of inhibin deficiency were also observed in these mice. Our results indicate that SMAD2 ablation does not protect inhibin-deficient females from the development of ovarian tumors or the cachexia wasting syndrome.

CONCLUSIONS

SMAD2 is not required for mediating tumorigenic signals of activin in ovarian tumor development caused by loss of inhibin.

Activin receptor signaling regulates prostatic epithelial cell adhesion and viability

Mutational changes coupled with endocrine, paracrine, and/or autocrine signals regulate cell division during carcinogenesis. The hormone signals remain undefined, although the absolute requirement in vitro for fetal serum indicates the necessity for a fetal serum factor(s) in cell proliferation. Using prostatic cancer cell (PCC) lines as a model of cancer cell proliferation, we have identified the fetal serum component activin A and its signaling through the activin receptor type II (ActRII), as necessary, although not sufficient, for PCC proliferation. Activin A induced Smad2 phosphorylation and PCC proliferation, but only in the presence of fetal bovine serum (FBS). Conversely, activin A antibodies and inhibin A suppressed FBS-induced PCC proliferation confirming activin A as one of multiple serum components required for PCC proliferation. Basic fibroblast growth factor was subsequently shown to synergize activin A-induced PCC proliferation. Inhibition of ActRII signaling using a blocking antibody or antisense-P decreased mature ActRII expression, Smad2 phosphorylation, and the apparent viability of PCCs and neuroblastoma cells grown in FBS. Suppression of ActRII signaling in PCC and neuroblastoma cells did not induce apoptosis as indicated by the ratio of active/inactive caspase 3 but did correlate with increased cell detachment and ADAM-15 expression, a disintegrin whose expression is strongly correlated with prostatic metastasis. These findings indicate that ActRII signaling is required for PCC and neuroblastoma cell viability, with ActRII mediating cell fate via the regulation of cell adhesion. That ActRII signaling governs both cell viability and cell adhesion has important implications for developing therapeutic strategies to regulate cancer growth and metastasis.

Endogenous betaglycan is essential for high-potency inhibin antagonism in gonadotropes

Inhibins are endocrine hormones that regulate gametogenesis and reproduction through a negative feedback loop with FSH. Inhibin action involves antagonism of signaling by activin or other TGFbeta family ligands. In transfection assays, antagonism by inhibin can be potentiated by betaglycan, a coreceptor for selected TGFbeta family ligands. We tested whether betaglycan is an obligate inhibin coreceptor through disruption of betaglycan function by RNA interference-mediated knockdown and immunoneutralization. Betaglycan knockdown and anti-betaglycan IgG each independently prevented inhibin-A binding to betaglycan and reversed functional effects of transfected betaglycan. Neither betaglycan immunoneutralization nor knockdown affected activin responsiveness in cell lines or in rat anterior pituitary cultures. Betaglycan knockdown decreased the potency of inhibin antagonism of activin-induced FSH secretion in primary gonadotropes. Similarly, anti-betaglycan IgG decreased the potency of inhibin antagonism in primary gonadotropes in a dose-dependent manner, with a reduction in the sensitivity to inhibin-A of greater than 1000-fold. These data establish that betaglycan is an endogenous inhibin coreceptor required for high-sensitivity inhibin antagonism of activin signaling in rat anterior pituitary gonadotropes.

Activin, TGF-beta and menin in pituitary tumorigenesis

Pituitary adenomas are common monoclonal neoplasms accounting for approximately one-fifth of primary intracranial tumors. Prolactin-secreting pituitary adenomas (prolactinomas) are the most common form of pituitary tumors in humans. They are associated with excessive release of the hormone prolactin and increased tumor growth, giving rise to severe endocrine disorders and serious clinical concerns for the patients. Recent studies indicated that the activin/TGF-beta family of growth factors plays a prominent role in regulating pituitary tumor growth and prolactin secretion from anterior pituitary lactotrope cells. Furthermore, these studies highlighted the tumor suppressor menin and the protein Smads as central regulators of these biological processes in the pituitary. Alterations in the activin/TGF-beta downstream signaling pathways are critical steps towards tumor formation and progression. This chapter will review the role and intracellular molecular mechanisms of action by which activin, TGF-beta, Smads and menin act in concert to prevent pituitary tumor cell growth and control hormonal synthesis by the anterior pituitary.

Activins and activin antagonists in hepatocellular carcinoma

In many parts of the world hepatocellular carcinoma (HCC) is among the leading causes of cancer-related mortality but the underlying molecular pathology is still insufficiently understood. There is increasing evidence that activins, which are members of the transforming growth factor β (TGFβ) superfamily of growth and differentiation factors, could play important roles in liver carcinogenesis. Activins are disulphide-linked homo- or heterodimers formed from four different β subunits termed βA, βB, βC, and βE, respectively. Activin A, the dimer of two βA subunits, is critically involved in the regulation of cell growth, apoptosis, and tissue architecture in the liver, while the hepatic function of other activins is largely unexplored so far. Negative regulators of activin signals include antagonists in the extracellular space like the binding proteins follistatin and FLRG, and at the cell membrane antagonistic co-receptors like Cripto or BAMBI. Additionally, in the intracellular space inhibitory Smads can modulate and control activin activity. Accumulating data suggest that deregulation of activin signals contributes to pathologic conditions such as chronic inflammation, fibrosis and development of cancer. The current article reviews the alterations in components of the activin signaling pathway that have been observed in HCC and discusses their potential significance for liver tumorigenesis.

The activin A-follistatin system: potent regulator of human extracellular matrix mineralization

Bone quality is an important determinant of osteoporosis, and proper osteoblast differentiation plays an important role in the control and maintenance of bone quality. We investigated the impact of activin signaling on human osteoblast differentiation, extracellular matrix formation, and mineralization. Activins belong to the transforming growth factor-beta superfamily and activin A treatment strongly inhibited mineralization in osteoblast cultures, whereas the activin antagonist follistatin increased mineralization. Osteoblasts produced activin A and follistatin in a differentiation-dependent manner, leading to autocrine regulation of extracellular matrix formation and mineralization. In addition, mineralization in a vascular smooth muscle cell-based model for pathological calcification was inhibited. Comparative activin A and follistatin gene expression profiling showed that activin signaling changes the expression of a specific range of extracellular matrix proteins prior to the onset of mineralization, leading to a matrix composition with reduced or no mineralizing capacity. These findings demonstrate the regulation of osteoblast differentiation and matrix mineralization by the activin A-follistatin system, providing the possibility to control bone quality as well as pathological calcifications such as atherosclerosis by using activin A, follistatin, or analogs thereof.

Interaction of testosterone with inhibin alpha and betaA subunits to regulate prostate gland growth

Testosterone regulation of prostate gland growth has been shown to involve reciprocal interaction with inhibin and activin. This study was therefore conducted to correlate the effect of testosterone on prostate gland proliferation and differentiation with the level of expression of inhibin alpha and betaA subunits. Immature dogs were treated with testosterone for 0, 3, 7, and 14 days and prostate gland growth was assessed by morphological and immunohistological localization of differentiation and proliferation markers. The results showed that testosterone treatment resulted in an initial significant increase in PCNA proliferation index by days 3 and 7, followed by a significant decrease by day 14 post-treatment. Interestingly, the decrease of cell proliferation was associated with structural and biochemical changes characteristic of glandular and stromal differentiation of the prostate gland. These changes include progressive glandular ductal canalization and inter-ductal stroma differentiation which were apparent from a gradual shift from vimentin expression to vimentin and alpha-actin expression. Testosterone also had a differential effect on inhibin alpha and beta subunits. Although testosterone treatment resulted in significant and constant inhibition of alpha subunit mRNA expression, it resulted in a significant increase of betaA mRNA expression by day 3, followed by a decrease by days 7 and 14. These results indicated that testosterone acts first to drive proliferation of undifferentiated prostatic cells and then to maintain a low proliferation turnover of differentiated cells. Because it has been shown that activin is an antagonistic regulator of androgens, the attenuated stimulatory effect of testosterone on cell proliferation by day 14 might be mediated, at least in part, by interplay between testosterone and activin.

The role of TGF-beta superfamily during T cell development: new insights

Members of the transforming growth factor beta (TGF-beta) superfamily are soluble factors that regulate a variety of functional responses including proliferation, differentiation, apoptosis and cell cycle, among others, depending not only on the cell type and its differentiation state, but also on the milieu of cytokines present. All three members of this superfamily: TGF-betas, bone morphogenetic proteins (BMPs) and Activins, have been shown to be expressed in the thymus suggesting their potential role as regulators of the T lymphocyte differentiation process. Although initial reports described the role of TGF-beta in controlling specific checkpoints during thymocyte development, recent data has provided new evidence on the role of BMPs and Activins in this process. This review provides new insights on the function of members of the TGF-beta superfamily at different stages of thymocyte development.

The activin axis in liver biology and disease

Activins are a closely related subgroup within the TGFbeta superfamily of growth and differentiation factors. They consist of two disulfide-linked beta subunits. Four mammalian activin beta subunits termed beta(A), beta(B), beta(C), and beta(E), respectively, have been identified. Activin A, the homodimer of two beta(A) subunits, has important regulatory functions in reproductive biology, embryonic development, inflammation, and tissue repair. Several intra- and extracellular antagonists, including the activin-binding proteins follistatin and follistatin-related protein, serve to fine-tune activin A activity. In the liver there is compelling evidence that activin A is involved in the regulation of cell number by inhibition of hepatocyte replication and induction of apoptosis. In addition, activin A stimulates extracellular matrix production in hepatic stellate cells and tubulogenesis of sinusoidal endothelial cells, and thus contributes to restoration of tissue architecture during liver regeneration. Accumulating evidence from animal models and from patient data suggests that deregulation of activin A signaling contributes to pathologic conditions such as hepatic inflammation and fibrosis, acute liver failure, and development of liver cancer. Increased production of activin A was suggested to be a contributing factor to impaired hepatocyte regeneration in acute liver failure and to overproduction of extracellular matrix in liver fibrosis. Recent evidence suggests that escape of (pre)neoplastic hepatocytes from growth control by activin A through overexpression of follistatin and reduced activin production contributes to hepatocarcinogenesis. The role of the activin subunits beta(C) and beta(E), which are both highly expressed in hepatocytes, is still quite incompletely understood. Down-regulation in liver tumors and a growth inhibitory function similar to that of beta(A) has been shown for beta(E). Contradictory results with regard to cell proliferation have been reported for beta(C). The profound involvement of the activin axis in liver biology and in the pathogenesis of severe hepatic diseases suggests activin as potential target for therapeutic interventions.

Autoimmune targeted disruption of the pituitary-ovarian axis causes premature ovarian failure

Premature ovarian failure (POF) is characterized by amenorrhea and high serum levels of follicle-stimulating hormone (FSH). POF causes female infertility and represents a substantial women's health risk affecting 1% of women by age 40. Although ovarian autoimmunity has been associated with POF, the identity of ovarian Ags recognized is unknown. In this study, we show that autoimmune-targeted disruption of the pituitary-ovarian axis leads to POF. Immunization of SWXJ female mice with the p215-234 peptide derived from mouse inhibin-alpha activates CD4(+) T cells and induces experimental autoimmune oophoritis with a unique biphasic phenotype characterized by an early stage of enhanced fertility followed by a delayed stage of POF. Affected mice show high serum levels of inhibin-alpha-neutralizing Abs that prevent inhibin-mediated down-regulation of activin-induced pituitary FSH release. The loss of activin/FSH down-regulation leads to prolonged metestrus-diestrus, superovulation, increased numbers of mature follicles, increased offspring, accelerated depletion of primordial follicles, and ultimately premature infertility. Thus, inhibin-alpha-targeted experimental autoimmune oophoritis is initiated by CD4(+) Th1 T cells that stimulate B cells to produce inhibin-alpha-neutralizing Abs directly capable of mediating POF and transferring disease into naive recipients. Our inhibin-alpha autoimmune model of POF shows how premature infertility may develop in the context of elevated FSH levels thereby closely mimicking the hallmark features of human POF.

A crucial function of PDGF in TGF-beta-mediated cancer progression of hepatocytes

Polarized hepatocytes expressing hyperactive Ha-Ras adopt an invasive and metastatic phenotype in cooperation with transforming growth factor (TGF)-beta. This dramatic increase in malignancy is displayed by an epithelial to mesenchymal transition (EMT), which mimics the TGF-beta-mediated progression of human hepatocellular carcinomas. In culture, hepatocellular EMT occurs highly synchronously, facilitating the analysis of molecular events underlying the various stages of this process. Here, we show that in response to TGF-beta, phosphorylated Smads rapidly translocated into the nucleus and activated transcription of target genes such as E-cadherin repressors of the Snail superfamily, causing loss of cell adhesion. Within the TGF-beta superfamily of cytokines, TGF-beta1, -beta2 and -beta3 were specific for the induction of hepatocellular EMT. Expression profiling of EMT kinetics revealed 78 up- and 235 downregulated genes, which preferentially modulate metabolic activities, extracellular matrix composition, transcriptional activities and cell survival. Independent of the genetic background, platelet-derived growth factor (PDGF)-A ligand and both PDGF receptor subunits were highly elevated, together with autocrine secretion of bioactive PDGF. Interference with PDGF signalling by employing hepatocytes expressing the dominant-negative PDGF-alpha receptor revealed decreased TGF-beta-induced migration in vitro and efficient suppression of tumour growth in vivo. In conclusion, these results provide evidence for a crucial role of PDGF in TGF-beta-mediated tumour progression of hepatocytes and suggest PDGF as a target for therapeutic intervention in liver cancer.

Differential regulation of follicle stimulating hormone by activin A and TGFB1 in murine gonadotropes

BACKGROUND

Activins stimulate the synthesis of follicle stimulating hormone (FSH) in pituitary gonadotropes, at least in part, by inducing transcription of its beta subunit (Fshb). Evidence from several laboratories studying transformed murine LbetaT2 gonadotropes indicates that activins signal through Smad-dependent and/or Smad-independent pathways, similar to those used by transforming growth factor beta-1 (TGFB1) in other cell types. Therefore, given common intracellular signaling mechanisms of these two ligands, we examined whether TGFBs can also induce transcription of Fshb in LbetaT2 cells as well as in purified primary murine gonadotropes.

METHODS

Murine Fshb promoter-reporter (-1990/+1 mFshb-luc) activity was measured in LbetaT2 cells treated with activin A or TGFB1, and in cells transfected with either activin or TGFB receptors. The ability of the ligands to stimulate phosphorylation of Smads 2 and 3 in LbetaT2 cells was measured by western blot analysis, and expression of TGFB type I and II receptors was assessed by reverse transcriptase polymerase chain reaction in both LbetaT2 cells and primary gonadotropes purified from male mice of different ages. Finally, regulation of endogenous murine Fshb mRNA levels by activin A and TGFB1 in purified gonadotropes and whole pituitary cultures was measured using quantitative RT-PCR.

RESULTS

Activin A dose-dependently stimulated -1990/+1 mFshb-luc activity in LbetaT2 cells, but TGFB1 had no effect at doses up to 5 nM. Similarly, activin A, but not TGFB1, stimulated Smad 2 and 3 phosphorylation in these cells. Constitutively active forms of the activin (Acvr1b-T206D) and TGFB (TGFBR1-T204D) type I receptors strongly stimulated -1990/+1 mFshb-luc activity, showing that mechanisms down stream of Tgfbr1 seem to be intact in LbetaT2 cells. RT-PCR analysis of LbetaT2 cells and whole adult murine pituitaries indicated that both expressed Tgfbr1 mRNA, but that Tgfbr2 was not detected in LbetaT2 cells. When cells were transfected with a human TGFBR2 expression construct, TGFB1 acquired the ability to significantly stimulate -1990/+1 mFshb-luc activity. In contrast to LbetaT2 cells, primary murine gonadotropes from young mice (8-10 weeks) contained low, but detectable levels of Tgfbr2 mRNA and these levels increased in older mice (1 yr). A second surprise was the finding that treatment of purified primary gonadotropes with TGFB1 decreased murine Fshb mRNA expression by 95% whereas activin A stimulated expression by 31-fold.

CONCLUSION

These data indicate that TGFB1-insensitivity in LbetaT2 cells results from a deficiency in Tgfbr2 expression. In primary gonadotropes, however, expression of Tgfbr2 does occur, and its presence permits TGFB1 to inhibit Fshb transcription, whereas activin A stimulates it. These divergent actions of activin A and TGFB1 were unexpected and show that the two ligands may act through distinct pathways to cause opposing biological effects in primary murine gonadotropes.

Structural basis for a functional antagonist in the transforming growth factor beta superfamily

Within the transforming growth factor beta superfamily, the agonist-antagonist relationship between activin and inhibin is unique and critical to integrated reproductive function. Activin acts in the pituitary to stimulate follicle-stimulating hormone, and is antagonized by endocrine acting, gonadally derived inhibin. We have undertaken a mutational analysis of the activin betaA subunit to determine the precise structural aspects that contribute to inhibin antagonism of activin. By substituting specific amino acid residues in the activin betaA subunit with similarly aligned amino acids from the alpha subunit, we have pinpointed the residues required for activin receptor binding and activity, as well as for inhibin antagonism of activin through its receptors. Additionally, we have identified an activin mutant with a higher affinity for the activin type I receptor that provides structural evidence for the evolution of ligand-receptor interactions within the transforming growth factor beta superfamily.

Activin induces hepatocyte cell growth arrest through induction of the cyclin-dependent kinase inhibitor p15INK4B and Sp1

In this report, we examined the role of activin in the regulation of cell growth inhibition of human hepatocarcinoma cells. Using RNase protection assay for various cell cycle regulators and Western blotting experiments, we show that activin treatment of HepG2 cells leads to increased gene expression of the cyclin-dependent kinase inhibitor (CDKI) p15INK4B. Furthermore, transient co-transfection studies of the p15INK4B promoter/luciferase construct performed in HepG2 cells demonstrates that activin induction of the p15INK4B promoter is mediated through the Smad pathway. p15INK4B gene promoter mapping analysis revealed a 66-bp region within the proximal domain of the promoter, which contains a consensus site for the transcription factor Sp1, as critical for mediating the activin effect on p15INK4B gene expression. Finally, gel mobility shift experiments, using the Sp1 consensus site, revealed increased DNA binding of Sp1 in response to activin treatment of HepG2 cells, further confirming the involvement of Sp1 in activin-mediated p15INK4B gene promoter activation. Together, our data indicates an important role for the cyclin-dependent kinase inhibitor p15INK4B in activin-induced cell cycle arrest in liver cells.

Cancer progression: is inhibin alpha from Venus or Mars?

The inhibin field has been perplexed by the information that inhibin alpha is a tumour suppressor in mice yet is elevated in women with ovarian cancer. Furthermore, we have consistently observed a down-regulation or loss of inhibin alpha in prostate cancer patient samples and cell lines. However, our latest data have prompted us to re-evaluate the role of inhibin alpha in prostate and other cancers. Using the analogy of TGF-beta as a springboard for our hypothesis, we offer a unifying model whereby the previously conflicting observations in mice, men and women can be explained. We propose that initially inhibin alpha is tumour-suppressive and is expressed in benign and early-stage primary cancers. Tumour-suppressive inhibin alpha is then silenced as the tumour progresses but is reactivated as a pro-metastatic factor in advanced, aggressive cancers.

Messenger RNA and Protein Expression Analysis of Betaglycan in the Pituitary and Ovary of the Domestic Hen1

Betaglycan was originally characterized as the type III receptor for TGFbeta, yet recent research has indicated that betaglycan can serve as an accessory receptor for inhibin. To understand better the action of inhibin in avian follicular development, we have investigated the expression of betaglycan in the pituitary gland and ovary of the hen. In experiments 1 and 2, betaglycan mRNA was detected at 6 kilobases (kb) by Northern blot analysis (n = 5) in chicken pituitary, granulosa, and theca layers and whole ovary. Expression of betaglycan was greatest in the pituitary gland in experiment 1 and greater in the granulosa layer of small yellow follicles (SYF) compared with the granulosa layer of larger follicles. In experiment 2, betaglycan mRNA was more abundantly expressed in the theca layer compared with the granulosa layer for all follicle sizes, although there was no significant difference in betaglycan expression in the theca layer among follicle sizes. In experiment 3, immunohistochemical analysis revealed betaglycan protein in the anterior pituitary as well as in the ovary (n = 4) and SYF (n = 4). Colocalization studies revealed a high abundance of cells within the anterior pituitary expressing both betaglycan and FSH (n = 4). Betaglycan protein was found in the granulosa layer; however, markedly enhanced staining was observed in the theca layer of ovarian follicles. Our results provide evidence for expression of betaglycan mRNA and protein colocalization with FSH in the anterior pituitary, consistent with known inhibin effects. Ovarian localization of betaglycan, particularly in the theca layer, suggests a paracrine role for inhibin in the hen.

Beta A versus beta B: is it merely a matter of expression?

Activins are members of the transforming growth factor (TGF) beta (beta) superfamily of proteins that function in a wide array of physiological processes. Like other TGFbeta ligands, activins are biologically active as dimers. An activin molecule is comprised of two beta-subunits, of which four isoforms have been identified: betaA, betaB, betaC, and betaE. The most widely studied activins to date are activin A (betaA/betaA), activin B (betaB/betaB), and activin AB (betaA/betaB). Inhibin is a naturally occurring activin antagonist that consists of an alpha-subunit disulfide-linked to one of the activin beta-subunits, producing inhibin A (alpha/betaA), or inhibin B (alpha/betaB). The development of assays distinguishing between different forms of activins and inhibins, along with knock-in and knock-out models, have provided evidence that the betaA- and betaB-subunits have independent and separate roles physiologically. Additionally, evaluation of ligand-receptor interactions indicates significant differences in receptor affinity between activin isoforms, as well as between inhibin isoforms. In this review we explore the differences between activin/inhibin betaA- and betaB-subunits, including expression patterns, binding properties, and the specific structural aspects of each. From the growing pool of knowledge regarding activins and inhibins, the emerging data support the hypothesis that betaA- and betaB-subunits are functionally differently.

Intra-pituitary regulation of gonadotrophs in male rodents and primates

Paracrine and autocrine regulation is well established in many organs including the gonads, but the notion of communication among pituitary cells is a relatively new concept. The FSH-beta and GnRH-receptor genes are up-regulated by pituitary activin and down-regulated by pituitary follistatin, and circulating inhibin disrupts this local regulation by functioning as an endogenous competitor of the activin receptor. Activin and follistatin production by folliculostellate cells may play a central role in these responses. alpha-Subunit expression is maintained at high levels in the absence of GnRH through unknown mechanisms. There is evidence that the intra-pituitary regulation of FSH-beta and GnRH-receptor gene expression may activate pubertal maturation in male rats. Finally, there are marked differences in follistatin expression and its regulation by GnRH and androgens in male primates and rats that appear to explain species differences in the differential secretion of FSH and LH, although the physiological significance of these differences is not yet known.

Activin and follistatin in rat mammary gland

Mammary gland morphogenesis and differentiation are mediated through the combined activities of systemic hormones and locally synthesized growth factors. Activin, a member of the transforming growth factor (TGF)-beta superfamily, is known to regulate the growth and differentiation of several cell types. In the present study, we investigated the role of activin in rat mammary gland on different stages of development. We found that activin A in vitro inhibits the proliferation of isolated acini, and this effect increases with the development of the gland. This factor also produces in vitro an inhibition of the final differentiation of acini obtained from 19th day pregnant rats. We also report the expression of activin receptors IIA and IIB mRNA in whole rat mammary gland and acini, with decreased levels of expression of type IIA (in both compartments) and IIB (in acini) during pregnancy and lactogenesis. In addition, we show that activin betaB-subunit mRNA decreases throughout pregnancy, and that the mRNA levels of follistatin (Fst) (its ligand protein) are high in cycling rats and at the beginning of pregnancy and diminish thereafter, having the acini higher levels of expression. Our data show that activin betaB-subunit, follistatin and ActRIIA and IIB transcripts are expressed in rat mammary gland at appropriate times and locations during development, allowing an interplay that might regulate activin action on growth and differentiation of the gland.

Ovarian regulation of endometrial gland morphogenesis and activin-follistatin system in the neonatal ovine uterus

Postnatal development of the ovine uterus between birth and Postnatal Day (PND) 56 involves differentiation of the endometrial glandular epithelium from the luminal epithelium followed by tubulogenesis and branching morphogenesis. Previous results indicated that ovariectomy of ewes at birth did not affect uterine growth or initial stages of endometrial gland genesis on PND 14 but did affect uterine growth after PND 28. Available evidence from a number of species supports the hypothesis that the ovary does not affect endometrial gland morphogenesis in the postnatal uterus. To test this hypothesis in our sheep model, ewes were assigned at birth to a sham surgery as a control or bilateral ovariectomy (OVX) on PND 7. Uteri were removed and weighed on PND 56. Ovariectomy did not affect circulating levels of estradiol-17beta. Uterine weight was 52% lower in OVX ewes. Histomorphological analyses indicated that the thickness of the endometrium and myometrium, total number of endometrial glands, and endometrial gland density in the stratum spongiosum stroma was reduced in uteri of OVX ewes. In contrast, the number of superficial ductal gland invaginations and gland density in the stratum compactum stroma was not affected by ovariectomy. The uteri of OVX ewes contained lower levels of betaA subunit, activin receptor (ActR) type IA, ActRIB, and follistatin protein expression but higher levels of betaB subunit. In the neonatal ovary, follistatin, inhibin alpha subunit, betaA subunit, and betaB subunit were expressed in antral follicles between PNDs 0 and 56. These results led to rejection of the hypothesis that the ovary does not influence endometrial adenogenesis. Rather, the ovary and, thus, an ovarian-derived factor regulates, in part, the coiling and branching morphogenetic stage of endometrial gland development after PND 14 and expression of specific components of the activin-follistatin system in the neonatal ovine uterus that appear to be important for that critical process.

The activin-follistatin system in the neonatal ovine uterus

Uterine gland development or adenogenesis in the neonatal ovine uterus involves budding and tubulogenesis followed by coiling and branching morphogenesis of the glandular epithelium (GE) from the luminal epithelium (LE) between birth (Postnatal Day [PND] 0) and PND 56. Activins, which are members of the transforming growth factor beta superfamily, and follistatin, an inhibitor of activins, regulate epithelial branching morphogenesis in other organs. The objective of the present study was to determine effects of postnatal age on expression of follistatin, inhibin alpha subunit, betaA subunit, betaB subunit, activin receptor (ActR) type IA, ActRIB, and ActRII in the developing ovine uterus. Ewes were ovariohysterectomized on PND 0, 7, 14, 21, 28, 35, 42, 49, or 56. The uterus was analyzed by in situ hybridization and immunohistochemistry. Neither inhibin alpha subunit mRNA or protein was detected in the neonatal uterus. Expression of betaA and betaB subunits was detected predominantly in the endometrial LE and GE and myometrium between PND 0 and PND 56. In all uterine cell types, ActRIA, ActRIB, and ActRII were expressed, with the highest levels observed in the endometrial LE and GE and myometrium. Between PND 0 and PND 14, follistatin was detected in all uterine cell types. However, between PND 21 and PND 56, follistatin was only detected in the stroma and myometrium and not in the developing GE. Collectively, the present results indicate that components of the activin-follistatin system are expressed in the developing neonatal ovine uterus and are potential regulators of endometrial gland morphogenesis.

Normal reproductive function in InhBP/p120-deficient mice

The inhibins are gonadal transforming growth factor beta superfamily protein hormones that suppress pituitary follicle-stimulating hormone (FSH) synthesis. Recently, betaglycan and inhibin binding protein (InhBP/p120, also known as the product of immunoglobulin superfamily gene 1 [IGSF1]) were identified as candidate inhibin coreceptors, shedding light on the molecular basis of how inhibins may affect target cells. Activins, which are structurally related to the inhibins, act within the pituitary to stimulate FSH production. Betaglycan increases the affinity of inhibins for the activin type IIA (ACVR2) receptor, thereby blocking activin binding and signaling through this receptor. InhBP/p120 may not directly bind inhibins but may interact with the activin type IB receptor, ALK4, and participate in inhibin B's antagonism of activin signaling. To better understand the in vivo functions of InhBP/p120, we characterized the InhBP/p120 mRNAs and gene in mice and generated InhBP/p120 mutant mice by gene targeting in embryonic stem cells. InhBP/p120 mutant male and female mice were viable and fertile. Moreover, they showed no alterations in FSH synthesis or secretion or in ovarian or testicular function. These data contribute to a growing body of evidence indicating that InhBP/p120 does not play an essential role in inhibin biology.

Inhibin is an antagonist of bone morphogenetic protein signaling

Inhibins are endogenous antagonists of activin signaling, long recognized as important regulators of gonadal function and pituitary FSH release. Inhibin, in concert with its co-receptor, betaglycan, can compete with activin for binding to type II activin receptors and, thus, prevent activin signaling. Because bone morphogenetic proteins (BMPs) also utilize type II activin receptors, we hypothesized that BMP signaling might also be sensitive to inhibin blockade. Here we show that inhibin blocks cellular responses to diverse BMP family members in a variety of BMP-responsive cell types. Inhibin abrogates BMP-induced Smad signaling and transcription responses. Inhibin competes with BMPs for type II activin receptors, and this competition is facilitated by betaglycan. Betaglycan also enables inhibin to bind to and compete with BMPs for binding to the BMP-specific type II receptor BMPRII, which does not bind inhibin in the absence of betaglycan. Betaglycan can confer inhibin responsiveness on cells that are otherwise insensitive to inhibin. These findings demonstrate that inhibin, acting through betaglycan, can function as an antagonist of BMP responses, suggesting a broader role for inhibin and betaglycan in restricting and refining a wide spectrum of transforming growth factor beta superfamily signals.

Transforming growth factor-beta modulates inhibin A bioactivity in the LbetaT2 gonadotrope cell line by competing for binding to betaglycan

Activin stimulates expression of GnRH receptor (GnRHR) and FSH beta-subunit in gonadotropes. Inhibin antagonizes activin actions on the gonadotropes, but its molecular mechanism of action remains poorly understood. It has been suggested that inhibin exerts its antagonistic effects by competing with activin for the binding of the activin receptor complex. Betaglycan has recently been identified as an inhibin-binding accessory protein in this process. Because both inhibin and TGFbeta bind betaglycan, we examined whether TGFbeta can modify inhibin's antagonism of activin-induced transcription in gonadotrope cells. Two activin-responsive reporter constructs were used, the first containing 5.5 kb of the ovine FSHbeta promoter (oFSHbetaluc), and the second containing three copies of the activin-responsive sequence of the GnRHR promoter (3XGRAS-PRL-lux). These constructs were transfected into the gonadotrope cell line LbetaT2. The oFSHbetaluc and 3XGRAS-PRL-lux activities stimulated by 0.5 nM activin A were decreased by up to 50% in a dose-dependent manner by inhibin A. TGFbeta(1) and TGFbeta(2) (0-4 nM), alone or in the presence of activin A, did not significantly affect the promoter elements. However, with increasing doses of TGFbeta(1) or TGFbeta(2), inhibin A antagonism of activin A activity was partly or completely reversed. Competition studies with radiolabeled inhibin A showed that TGFbeta(1) and TGFbeta(2) competed with [(125)I]inhibin for the binding to LbetaT2 cells (IC(50) = 280 pM and 72 pM, respectively). Immunoprecipitation studies of [(125)I]inhibin A cross-linked receptor complexes confirmed that TGFbeta(1) and TGFbeta(2) competed with inhibin A for the binding of betaglycan. These results suggest that TGFbeta competition with inhibin for binding to betaglycan interferes with inhibin's suppression of activin-induced FSHbeta and GnRHR promoters in LbetaT2 cells. We propose that under certain circumstances, TGFbeta may facilitate activin biological activity by hindering the access of inhibin to its coreceptor betaglycan.

Properties of inhibin binding to betaglycan, InhBP/p120 and the activin type II receptors

Activin-stimulated FSH synthesis and release by the pituitary gonadotrope is antagonized by gonadally derived inhibins. The two isoforms of inhibin, inhibin A and B, bind to the activin type II receptors, though at a lower affinity than the activins, but do not stimulate intracellular signaling. Theoretically, therefore, inhibins can prevent activin signaling through competitive binding if present at higher concentrations than the activins. In reality, the inhibins have been shown to antagonize activin signaling when the two ligand types are present at equimolar concentrations. These observations led to the hypothesis that inhibin binding proteins or co-receptors exist that either increase the affinity of the inhibins for the activin receptors or propagate inhibin-specific intracellular signals. Two candidate inhibin co-receptors, betaglycan and InhBP/p120, interact with activin receptors and augment inhibin antagonism of activin action. Here, we report the effect of betaglycan and InhBP/p120 on both inhibin A and inhibin B binding to the activin receptors ActRIIA and ActRIIB2. InhBP/p120 did not bind inhibin A or B when expressed alone or in combination with activin receptors, requiring a re-examination of the role of this protein in inhibin biology. Both inhibins bound the activin type II receptor, ActRIIB2. Inhibin B had a higher affinity for this receptor than inhibin A but an approximately 10-fold lower affinity than that of activin A. Inhibin A and B bound betaglycan with high affinity; however, only inhibin A binding to ActRIIB2 was significantly enhanced in the presence of betaglycan. Both inhibin isoforms showed slight but significant binding to ActRIIA, yet this binding was potentiated in the presence of betaglycan. Additionally, the complex formed between the inhibins, ActRIIA, and betaglycan was resistant to disruption by activin A, whereas activin A potently competed for inhibin binding to ActRIIB2 and betaglycan. Collectively, these data show that the inhibin isoforms have different affinities for the activin type II receptors but bind betaglycan with high affinity. A recently developed model of inhibin action proposes that inhibins form a high affinity, activin-resistant ternary complex with activin type II receptors and betaglycan, thereby providing a mechanism for inhibin antagonism of activin signaling. Importantly, the results presented here clearly show that this model does not apply equally to both forms of inhibin nor to the different activin type II receptor isoforms. Thus, it appears that the mechanisms of inhibin action may vary depending on the ligand and receptor types involved.

The physiology and pathophysiology of inhibin, activin and follistatin in female reproduction

In the last 2 years, major advances have been made in the understanding of inhibin physiology. Discovery of an inhibin receptor and binding protein has expanded our knowledge of the mechanism whereby inhibin antagonizes activin action. Controlled experimental studies have clarified the regulation and physiology of inhibin A and inhibin B, providing evidence for their use as markers of ovarian function. Clinical studies continue to uphold the use of inhibin as a marker for ovarian cancer, but have not generally supported its use over standard prognostic markers in assisted reproductive technologies. Finally, ongoing work suggests alterations in inhibin and follistatin that may be linked to the pathophysiology of polycystic ovary syndrome. Thus, the mechanism of inhibin action and its role in normal and abnormal ovarian function continues to emerge.

Recruitment and development of the follicle; the roles of the transforming growth factor-beta superfamily

Peripheral endocrine hormones and local paracrine and autocrine factors contribute, in a coordinated fashion, to the processes of recruitment, development or atresia, selection and ovulation of follicles. Among the local ovarian factors, there is growing evidence from genetic and experimental data that many members of the transforming growth factor (TGFbeta) superfamily have a biological role to play in folliculogenesis. These members include activin, inhibin, TGFbeta, BMP, GDF9 and perhaps MIS. In this review, we discuss the potential roles of the TGFbeta superfamily members, in particular activin, during folliculogenesis. Since the actions of these factors are determined by ligand availability, receptor expression and modulation of their signal transduction pathways, we also collate information on the expression of their signalling components in the follicle. We conclude that the TGFbeta superfamily signalling pathways, in particular activin's pathway, reside in the ovary. Furthermore, follistatin and beta-glycan-components of the accessory binding protein system that modifies activin action-are also present in follicles. In the post-natal rat ovary, the changes in receptor/Smad expression coincide with granulosa cell proliferation and antrum formation. We hypothesise that these pathway components are expressed in a temporal and cell-specific manner to meet the changing demands of cells during follicular development. The analysis of the components of the signal transduction pathways of the TGFbeta family members in populations of defined follicles and the identification of activated pathways in individually stimulated follicles should help clarify the roles of the TGFbeta members in folliculogenesis.

Neuroendocrine control of follicle-stimulating hormone (FSH) secretion: II. Is follistatin-induced suppression of FSH secretion mediated via changes in activin availability and does it involve changes in gonadotropin-releasing hormone secretion?

The objective of the present study was to determine to what extent activin participates in setting the level of FSH secretion and if this regulation includes mediation via changes in GnRH secretion. We administered follistatin, the high-affinity binding protein for activin, to five ovariectomized sheep; we reasoned that the resultant binding of follistatin to activin should lower activin bioavailability and FSH secretion. Hypophyseal portal and peripheral blood samples were collected simultaneously at 10-min intervals for 18 h to measure GnRH, LH, FSH, and both activin-free and total follistatin. Six hours into collection, each ewe received 150 microg/kg i.v. of recombinant human follistatin-288. A week later, the same ewes were subjected to a second series of blood collections of similar length (time control). The FSH levels in pituitary portal blood were approximately 8-fold higher than those in the peripheral circulation. The FSH secretory patterns changed minimally during the time-control period. In contrast, follistatin had profound suppressive effects on FSH secretion. Maximal FSH suppression after FS-288 administration occurred at 5-6 h in the pituitary portal (65% suppression) and 9-10 h in the peripheral (48% suppression) circulation. Follistatin had no effect on GnRH or LH secretory patterns. Disappearance of total follistatin (i.e., free follistatin plus activin-bound follistatin) from the circulation was slower (P < 0.05) than that of free follistatin alone, suggesting that some of the follistatin was complexed with circulating activin, thus reducing the bioavailability of activin. The slower clearance of total follistatin and the lack of follistatin effects on GnRH secretion suggest that changes in activin bioavailability dictate the level of pituitary FSH secretion and that this is a pituitary-specific effect.

The distribution of betaglycan protein and mRNA in rat brain, pituitary, and gonads: implications for a role for betaglycan in inhibin-mediated reproductive functions

Betaglycan was reported by our laboratory to serve as an inhibin binding protein and to facilitate the antagonism of activin signaling. Although an accessory receptor for TGFbeta and inhibin, its distribution within reproductive tissues remains largely unexplored. Histochemical analyses reveal betaglycan protein and mRNA distributed throughout the rat reproductive axis. In the brain, betaglycan mRNA is localized in discrete regions of the forebrain and brain stem, including olfactory, septal, and hypothalamic nuclei. In the pituitary, moderate levels of betaglycan protein and mRNA were observed in the anterior and intermediate lobes. Betaglycan immunoreactivity was colocalized with all the pituitary cell subtypes, to the greatest extent with the gonadotrope population. In the gonads, betaglycan mRNA was localized in cellular compartments, coinciding with its protein for the most part. Moderate levels of mRNA were observed in ovarian granulosa cells, with lower expression in the thecal layer and the oocyte. In the testes, betaglycan mRNA was observed in the Leydig and tubule-specific germ cells. This is the first comprehensive report detailing the distribution of betaglycan in mammalian reproductive tissues. The present findings illustrate and support the hypothesis of a modulatory role for betaglycan in TGFbeta and/or inhibin effects in these tissues.

Regulation of cell proliferation, apoptosis, and carcinogenesis by activin

The aim of this review is to provide insight into the molecular mechanisms by which activin A modulates cell proliferation, apoptosis, and carcinogenesis in vitro and in vivo. Activin A, a member of the TGFbeta superfamily, has various effects on diverse biological systems, including cell growth inhibition in many cell types. However, the mechanism(s) by which activin exerts its inhibitory effects are not yet understood. This review highlights activin's effects on activin receptors and signaling pathway, modulation of activin signaling, and regulation of cell proliferation and apoptosis by activin. Based on the experiences of all the authors, we emphasized cell cycle inhibitors such as p16 and p21 and regulators of apoptosis such as p53 and members of the bcl-2 family. Aside from activin's inhibition of cell proliferation and enhancement of apoptosis, other newly developed methods for molecular studies of apoptosis by activin were briefly presented that support the role of activin as an inhibitor of carcinogenesis and cancer progression. These methods include subtractive hybridization based on covalent bonding, a simple and accurate means to determine molecular profile of as few as 20 cells based on an RNA-PCR approach, and a messenger RNA-antisense DNA interference phenomenon (D-RNAi), resulting in a long-term gene knockout effects.

Inhibin binding protein (InhBP/p120), betaglycan, and the continuing search for the inhibin receptor

Betaglycan (the TGFbeta type III receptor) and InhBP/p120 (a membrane-tethered proteoglycan) were recently identified as putative inhibin receptors. Here, we review the current state of knowledge regarding these two proteins with respect to their potential roles in inhibin biology. Importantly, neither protein appears to satisfy all of the criteria required for classification as a bona fide inhibin receptor. Betaglycan does not appear to be expressed in pituitary gonadotropes, the primary target of circulating inhibins, and InhBP/p120 does not bind inhibins in conventional receptor binding assays. While both proteins appear capable of promoting inhibin-mediated antagonism of activin signaling, neither appears to generate inhibin-specific intracellular signals. Recently, additional inhibin binding proteins were identified in inhibin target tissues, including pituitary and Leydig cells. Characterization of these proteins, coupled with ongoing investigations of betaglycan and InhBP/p120, will lead to a clearer understanding of mechanisms of inhibin action.

Intra-follicular activin availability is altered in prenatally-androgenized lambs

Prenatal exposure of sheep to testosterone (T) disrupts ovarian cyclicity and leads to anovulation in adulthood. We propose that the disruption of ovarian function in prenatally-androgenized sheep is mediated via follicular defects stemming from reduced intrafollicular activin availability/action. The intra-follicular activin availability/action that facilitates follicular development is dictated by the relative proportions of activins, inhibins (antagonists of activin action) and follistatins (FS; binding proteins of activin and negator of activin action). Inhibin alpha, beta A, beta B, and FS mRNA expression were determined by in situ hybridization in 5 week-old ovaries from control (C) lambs or those exposed to testosterone (T) or DHT from 30-90 days of gestation. In utero exposure to T, but not DHT, increased total ovarian weight (0.4+/-0.1,1.5+/-0.5 and 0.3+/-0.1 g, C, T and DHT, respectively) and total number of follicles (16.5+/-2.8,37.8+/-7.9, and 18.8+/-3.0). With the exception of two follicles in T animals, all follicles were < or = 2 mm in diameter. All follicles < or = 2 mm in all groups expressed FSH receptor mRNA in the granulosa cells and LH receptor only in the thecal cells. The percentage of follicles expressing FS mRNA was increased (P<0.05) in sheep prenatally-androgenized with either T (80.4+/-8) or DHT (80.3+/-5.5) as compared to C (50.8+/-8.2). In contrast, the percentage of follicles expressing activin beta B mRNA tended to be lower (P=0.06) in the T (30.9+/-7.1) and DHT (40.5+/-3.3) groups as compared to C (66.1+/-15.6). Increased expression of FS along with the reduced expression of activin beta B mRNA provides evidence for compromised intra-follicular activin availability in the majority of follicles in the androgenized groups. The increase in ovarian weight and follicular number in the T, but not in the DHT group, suggests that the effects of T are mediated through the action of estrogen. We speculate that the decrease in relative abundance of activin may contribute to the selection defects in prenatally-androgenized sheep. If true, this may be a useful model to understand the etiology of polycystic ovarian syndrome.

Activins and inhibins in endocrine and other tumors

Inhibin and activin are members of the TGF beta superfamily of growth and differentiation factors. They were first identified as gonadal-derived regulators of pituitary FSH and were subsequently assigned multiple actions in a wide range of tissues. More recently, the inhibin alpha subunit was considered as a tumor suppressor based on functional studies employing transgenic mouse models. This review evaluates the functional and molecular evidence that the inhibin alpha subunit is a tumor suppressor in endocrine cancers. The evaluation highlights the discrepant results from the human and mouse studies, as well as the differences between endocrine tumor types. In addition, we examine the evidence that the activin-signaling pathway is tumor suppressive and identify organ-specific differences in the actions and putative roles of this pathway in endocrine tumors. In summary, there is a considerable body of evidence to support the role of inhibins and activins in endocrine-related tumors. Future studies will define the mechanisms by which inhibins and activins contribute to the process of initiation, promotion, or progression of endocrine-related cancers.

The functional significance of FSH in spermatogenesis and the control of its secretion in male primates

The aim of this review is to provide an integrative analysis of the role of FSH in the control of testicular function in higher primates, including man. Attention is focused on the action of FSH during neonatal development, puberty, and adulthood. Whether FSH is the major determinant of the adult complement of Sertoli cells and whether FSH is obligatory for the initiation, maintenance, and restoration of spermatogenesis is evaluated. The mechanism whereby the circulating concentration of FSH regulates spermatogonial proliferation to dictate the sperm production rate under physiological conditions in the adult is discussed in detail. Inhibin B is the major component of the testicular negative feedback signal governing FSH beta gene expression and FSH secretion, and the evidence for this view is presented. The review concludes with the presentation of a model for the operation of the FSH-inhibin B feedback control system regulating sperm production postpubertally in monkey and man, and with speculation on issues of clinical interest.

Activins as regulators of branching morphogenesis

Development of glandular organs such as the kidney, lung, and prostate involves the process of branching morphogenesis. The developing organ begins as an epithelial bud that invades the surrounding mesenchyme, projecting dividing epithelial cords or tubes away from the site of initiation. This is a tightly regulated process that requires complex epithelial-mesenchymal interactions, resulting in a three-dimensional treelike structure. We propose that activins are key growth and differentiation factors during this process. The purpose of this review is to examine the direct, indirect, and correlative lines of evidence to support this hypothesis. The expression of activins is reviewed together with the effect of activins and follistatins in the development of branched organs. We demonstrate that activin has both negative and positive effects on cell growth during branching morphogenesis, highlighting the complex nature of activin in the regulation of proliferation and differentiation. We propose potential mechanisms for the way in which activins modify branching and address the issue of whether activin is a regulator of branching morphogenesis.

Transforming growth factor beta signalling in vitro and in vivo: activin ligand-receptor interaction, Smad5 in vasculogenesis, and repression of target genes by the deltaEF1/ZEB-related SIP1 in the vertebrate embryo

The identification and characterization of components of the transforming growth factor beta (TGFbeta) signalling pathway are proceeding at a very fast pace. To illustrate a number of our activities in this field, we first summarize our work aiming at the selection from a large collection of single residue substitution mutants of two activin A polypeptides in which D27 and K102, respectively, have been modified. This work has highlighted the importance of K102 and its positive charge for binding to activin type II receptors. Activin K102E, which did not bind to high-affinity receptor complexes, may be a valuable beta chain, when incorporated in recombinant inhibin to unambiguously detect novel inhibin binding sites at the cell surface. We then illustrate how Smad5 knockout mice and an overexpression approach with a truncated TGFbeta type II receptor in the mouse embryo can contribute to the identification of a novel TGFbeta-->TbetaRII/ALK1-->Smad5 pathway in endothelial cells in the embryo proper and the yolk sac vasculature. We conclude with a summary of our results with a Smad-interacting transcriptional repressor but focus on its biological significance in the vertebrate embryo.

An emerging role for co-receptors in inhibin signal transduction

While many transforming growth factor-beta (TGFbeta) superfamily ligands such as TGFbeta, activin, and the bone morphogenic proteins (BMPs) are critical to the control of growth, differentiation, and cell fate, inhibin has a more limited role and is primarily responsible for the regulation of one hormone from one cell-type in the anterior pituitary. Inhibin is an endocrine hormone, produced by the gonads, that inhibits follicle stimulating hormone (FSH) release from the pituitary gonadotrope. The other hormones in the superfamily do not appear to act in an endocrine fashion, but rather control cell function in a paracrine or autocrine manner. Many components of the TGFbeta/activin/BMP signal transduction pathway have been elegantly defined; however, the mechanism of inhibin action has not been completely dissected. Several cell surface proteins that associate with inhibin have been identified recently, and these molecules may provide the clues necessary to understand how inhibin regulates reproductive function.

Antagonism of activin by inhibin and inhibin receptors: a functional role for betaglycan-glycan

Activin and inhibin research has provided important insight into reproductive physiology as well as many areas involving regulation of cell growth, differentiation and function. Progress in understanding the roles of these hormones in various cell and tissue types has been complimented by novel discoveries at the molecular level that have shed light on ligand/receptor interactions, signaling mechanisms and regulation. While the receptors and signaling pathway for activin are now well characterized, the molecular basis for inhibin action has remained relatively unclear. Here we summarize recent advances in understanding inhibin's mode of action focusing on our recent identification of betaglycan-glycan as an inhibin co-receptor capable of mediating inhibin action.

Inhibin binding sites and proteins in pituitary, gonadal, adrenal and bone cells

Activin signals via complexes of type I (50-55 kDa) and II (70-75 kDa) activin receptors, but the mechanism of inhibin action is unclear. Proposed models range from an anti-activin action at the type II activin receptor to independent actions involving putative inhibin receptors. Two membrane-embedded proteoglycans, betaglycan and p120, have recently been implicated in inhibin binding, but neither appears to be a signalling receptor. The present studies on primary cultures of rat pituitary and adrenal cells, and several murine and human cell lines were undertaken to characterise inhibin binding to its physiological targets. High affinity binding of inhibin to the primary cultures and several of the cell lines, like that previously described for ovine pituitary cells, was saturable and reversible. Scatchard analysis revealed two classes of binding sites (K(d) of 40-400 and 500-5000 pM, respectively). Affinity labelling identified [125I]inhibin binding proteins with apparent molecular weights of 41, 74, 114 and >170 kDa in all cell types that displayed high affinity, high capacity binding of inhibin. Additional labelling of a 124 kDa species was evident in gonadal TM3 and TM4 cell lines. In several cases, activin (> or =20 nM) competed poorly or not at all for binding to these proteins. The 74, 114 and >170 kDa inhibin binding proteins in TM3 and TM4 cells were immunoprecipitated by an anti-betaglycan antiserum. These three proteins correspond in size to the activin receptor type II and the core protein and glycosylated forms of betaglycan, respectively, that have been proposed to mediate anti-activin actions of inhibin, but the identity of the 74 kDa species is yet to be confirmed. Studies of [125I]inhibin binding kinetics and competition for affinity labelling of individual binding proteins in several cell lines suggest these three species and the 41 and 124 kDa proteins form a high affinity inhibin binding complex. In summary, common patterns of inhibin binding and affinity labelling were observed in inhibin target cells. Novel inhibin binding proteins of around 41 and 124 kDa were implicated in the high affinity binding of inhibin to cells from several sources.