A homodimer of the beta-subunits of inhibin A stimulates the secretion of pituitary follicle stimulating hormone

Identification of ActivinβA and Gonadotropin Regulation of the Activin System in the Ovary of Chinese Sturgeon Acipenser sinensis

Activin is a dimeric growth factor with diverse biological activities in vertebrates. This study aimed to investigate the regulatory role of the activin signaling pathway in the ovary of the endangered, cultured sturgeon species Acipenser sinensis. One activinβA subunit was identified, with a full-length complementary DNA (cDNA) sequence of 1572 base pairs. Multiple sequence alignment suggested that ActivinβA shared high sequence identities with its counterparts in four other sturgeon species. Phylogenetic analysis indicated the conserved evolution of ActivinβA among vertebrates from mammals to fish species. Transcripts of activinβA were distributed ubiquitously in the liver, kidney, intestine, ovary, midbrain, hypothalamus, and pituitary, with the highest transcription found in the pituitary. In Chinese sturgeon ovarian cells, in vitro human recombinant Activin A incubation stimulated the activin system-related gene transcriptions of activinβA, follistatin, its receptors -activinRIIA and activinRIIB, and drosophila mothers against decapentaplegic proteins (smads) smad2, smad3, and smad4. Ovary development-related mRNA levels of cyp19a1a and aromatase receptors of erα and erβ were enhanced by Activin A or human chorionic gonadotropin (hCG) incubation. Furthermore, 15 IU/mL hCG treatment increased the transcription levels of activinβA, follistatin, activinRIIA, and smad2. This suggested that the activin system was functional for the regulation of ovary development in Chinese sturgeon, possibly under the regulation of gonadotropin, by recruiting activinβA, follistatin, activinRIIA, and smad2. These results were helpful for the molecular exploration of activin signaling in fish species, as well as the ovarian maturation regulation of A. sinensis.

Evolving roles of activins and inhibins in ovarian cancer pathophysiology

Activins and inhibins are unique members of the transforming growth factor-β (TGFβ) family of growth factors, with the ability to exert autocrine, endocrine, and paracrine effects in a wide range of complex physiologic and pathologic processes. Although first isolated within the pituitary, emerging evidence suggests broader influence beyond reproductive development and function. Known roles of activin and inhibin in angiogenesis and immunity along with correlations between gene expression and cancer prognosis suggest potential roles in tumorigenesis. Here, we present a review of the current understanding of the biological role of activins and inhibins as it relates to ovarian cancers, summarizing the underlying signaling mechanisms and physiologic influence, followed by detailing their roles in cancer progression, diagnosis, and treatment.

Transcription factor GATA2 may potentiate follicle-stimulating hormone production in mice via induction of the BMP antagonist gremlin in gonadotrope cells

Mammalian reproduction depends on the gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone, which are secreted by pituitary gonadotrope cells. The zinc-finger transcription factor GATA2 was previously implicated in FSH production in male mice; however, its mechanisms of action and role in females were not determined. To directly address GATA2 function in gonadotropes, we generated and analyzed gonadotrope-specific Gata2 KO mice using the Cre-lox system. We found that while conditional KO (cKO) males exhibited ∼50% reductions in serum FSH levels and pituitary FSHβ subunit (Fshb) expression relative to controls, FSH production was apparently normal in cKO females. In addition, RNA-seq analysis of purified gonadotropes from control and cKO males revealed a profound decrease in expression of gremlin (Grem1), a bone morphogenetic protein (BMP) antagonist. We show Grem1 was expressed in gonadotropes, but not other cell lineages, in the adult male mouse pituitary. Furthermore, Gata2, Grem1, and Fshb mRNA levels were significantly higher in the pituitaries of WT males relative to females but decreased in males treated with estradiol and increased following ovariectomy in control but not cKO females. Finally, we found that recombinant gremlin stimulated Fshb expression in pituitary cultures from WT mice. Collectively, the data suggest that GATA2 promotes Grem1 expression in gonadotropes and that the gremlin protein potentiates FSH production. The mechanisms of gremlin action have not yet been established but may involve attenuation of BMP binding to activin type II receptors in gonadotropes, facilitating induction of Fshb transcription by activins or related ligands.

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.

Activin a inhibits foam cell formation and up-regulates ABCA1 and ABCG1 expression through Alk4-Smad signaling pathway in RAW 264.7 macrophages

BACKGROUND

Activin A has been reported to play important roles in the pathogenesis of atherosclerosis. The purpose of this study is to investigate the effects of activin A on oxidized low-density lipoprotein (ox-LDL)-induced foam cell formation and explore the underlying molecular mechanisms in murine macrophage-like cell line RAW 264.7.

METHODS

The effects of activin A on Dil-labeled ox-LDL uptake were examined by confocal microscopy and flow cytometry analysis. The mRNA and protein levels of cholesterol receptors were analyzed by RT-qPCR and western blot analysis, respectively. To investigate whether activin receptor-like kinase 4 (Alk4) is required for activin A-mediated cellular effects, cells were pre-treated with SB-431542. The involvement of Smad2, Smad3 and Smad4 was confirmed by transfection with specific small interfering RNAs (siRNAs).

RESULTS

Activin A inhibits ox-ldl-induced foam cell formation and class A scavenger receptors (SR-A) expression, while up-regulates ATP-binding cassette transporter A1 (ABCA1) and ABCG1 expression in RAW 264.7 macrophages. Pre-treatment with SB-431542 abolished activin A-mediated anti-atherogenic effect. Knockdown of Smad2 reversed activin A-induced inhibition of ox-LDL uptake and SR-A expression. However, knockdown of Smad3 or Smad4 did not have such effect. Meanwhile, knockdown of either Smad2, Smad3 or Smad4 reversed the activin A-induced up-regulation of ABCA1 and ABCG1.

CONCLUSIONS

Our study provides novel evidence that activin A may exert anti-atherogenic effects through Alk4-Smad signaling pathway in RAW 264.7 macrophages.

Circulating activin A during equine gestation and immunolocalization of its receptors system in utero-placental tissues and fetal gonads

Although equine gestation is unique from the standpoint of fetal gonadal enlargement and regression, the activator of this process is still unknown. The present study aimed to show a possible role of activin during equine gestation. In the first experiment, weekly plasma samples from six pregnant mares were used to measure activin A. In the second experiment, eight pregnant mares carrying female (gestational days 110, 140, 180, and 270) and male fetuses (gestational days 120, 180, 225, and 314) were used for immunohistochemistry of activin receptors (IA, IB, IIA, IIB), and their intracellular mediators (Smad2, Smad3, Smad4). Activin A levels in maternal circulation remained low until fourth weeks of gestation, thereafter, started to increase, and peaked first at 11 weeks of gestation. The second significant peak was observed on the day of parturition. Activin receptors type IA, IB, IIA, and IIB were immunostained in interstitial and germ cells of fetal ovaries and testes along with utero-placental tissues. Smad2, Smad3, and Smad4 were also immunolocalized in all these organs. These results demonstrated the activin-producing capacity of utero-placental tissues, and also evidenced the existence of activin receptors and functional signaling molecules in these organs. The first increment in circulating activin A in maternal circulation coinciding with the timing of initiation of fetal gonadal enlargement suggests that activin from the utero-placental tissues may have a stimulatory role in fetal gonad enlargement and utero-placental development in mares, whereas the second peak could be important to follicular development in the maternal ovary for foal heat.

A bioinformatic analysis of the inhibin-betaglycan-endoglin/CD105 network reveals prognostic value in multiple solid tumors

Inhibins and activins are dimeric ligands belonging to the TGFβ superfamily with emergent roles in cancer. Inhibins contain an α-subunit (INHA) and a β-subunit (either INHBA or INHBB), while activins are mainly homodimers of either βA (INHBA) or βB (INHBB) subunits. Inhibins are biomarkers in a subset of cancers and utilize the coreceptors betaglycan (TGFBR3) and endoglin (ENG) for physiological or pathological outcomes. Given the array of prior reports on inhibin, activin and the coreceptors in cancer, this study aims to provide a comprehensive analysis, assessing their functional prognostic potential in cancer using a bioinformatics approach. We identify cancer cell lines and cancer types most dependent and impacted, which included p53 mutated breast and ovarian cancers and lung adenocarcinomas. Moreover, INHA itself was dependent on TGFBR3 and ENG/CD105 in multiple cancer types. INHA, INHBA, TGFBR3, and ENG also predicted patients' response to anthracycline and taxane therapy in luminal A breast cancers. We also obtained a gene signature model that could accurately classify 96.7% of the cases based on outcomes. Lastly, we cross-compared gene correlations revealing INHA dependency to TGFBR3 or ENG influencing different pathways themselves. These results suggest that inhibins are particularly important in a subset of cancers depending on the coreceptor TGFBR3 and ENG and are of substantial prognostic value, thereby warranting further investigation.

IGSF1 Does Not Regulate Spermatogenesis or Modify FSH Synthesis in Response to Inhibins or Activins

Loss-of-function mutations in the X-linked immunoglobulin superfamily, member 1 (IGSF1) gene result in central hypothyroidism, often associated with macroorchidism. Testicular enlargement in these patients might be caused by increases in follicle-stimulating hormone (FSH) levels, as IGSF1 has been proposed to function as an inhibin B receptor or as an inhibitor of activin type I receptor (ALK4) activity in pituitary gonadotrope cells. If true, loss of IGSF1 should lead to reduced inhibin B action or disinhibition of activin signaling, thereby increasing FSH synthesis. Here, we show that FSH levels and sperm counts are normal in male Igsf1 knockout mice, although testis size is mildly increased. Sperm parameters are also normal in men with IGSF1 deficiency, although their FSH levels may trend higher and their testes are enlarged. Inhibin B retains the ability to suppress FSH synthesis in pituitaries of Igsf1-knockout mice and IGSF1 does not interact with ALK4 or alter activin A/ALK4 stimulation of FSHβ (Fshb/FSHB) subunit transcription or expression. In light of these results, it is unlikely that macroorchidism in IGSF1 deficiency derives from alterations in spermatogenesis or inhibin/activin regulation of FSH.

Potential treatment of keloid pathogenesis with follistatin 288 by blocking the activin molecular pathway

Keloids are benign tumours caused by abnormal wound healing driven by increased expression of cytokines, including activin A. This study compared effects of activins on normal and keloid-derived human dermal fibroblasts and investigated a novel treatment for keloids using follistatin. Normal skin and keloid tissue samples from 11 patients were used to develop primary fibroblast cultures, which were compared in terms of their histology and relevant gene (qRT-PCR and RNAseq) and protein (ELISA) expression. Activin A (INHBA) and connective tissue growth factor (CTGF) gene expression were significantly upregulated in keloid fibroblasts, as was activin A protein expression in cell lysates and culture medium. Activator protein 1 inhibitor (SR11302) significantly decreased INHBA and CTGF expression in keloid fibroblasts and a single treatment of follistatin over 5 days significantly inhibited activin and various matrix-related genes in keloid fibroblasts when compared to controls. Follistatin, by binding activin A, suppressed CTGF expression suggesting a novel therapeutic role in managing keloids and perhaps other fibrotic diseases.

Involvement of activin signal pathway in cyclic apoptosis of the oviductal isthmic epithelium in cows

Activin (ACV) A induces various cellular functions via activin receptor type 2 (ACVR2A/2B)-activin receptor-like kinase (ALK) 4 -Smad 2/3 pathway. Although the production of ACVA is indicated in bovine oviducts, its role on the oviduct is unclear. Oviductal isthmus needs to change its function rapidly at peri-fertilization, however, the mechanism is unknown. This study was aimed to clarify the role of ACVA in the morphological changes of oviductal isthmus in cows. First, mRNA expressions of INHBA (ACVA component) and its receptors (ALK4, ACVR2A and ACVR2B) in the isthmic tissues were examined throughout the estrous cycle. INHBA was the highest, however, ACVR2A was the lowest on the day of ovulation, suggesting reduced ACV signal transduction in the isthmus just after ovulation. Proteins of ACVRs and Smad2/3 were clearly detected in the cultured epithelial cells. It is known that ACVA regulates cellular apoptosis. Our data showed that the number of cleaved caspase-3-positive epithelial cells was largest at 2-3 days after ovulation in the isthmus. Interestingly, our study demonstrated that follistatin (ACV/TGFB/BMP inhibitor) significantly decreased the BCL2/BAX ratio in the cultured isthmic epithelial cells. To clarify which ALK pathway is involved in the regulation of BCL2/BAX ratio, the effects of SB431542 (ACV signaling (ALK4) and TGFB signaling (ALK5) inhibitor), SB525334 (ALK5 inhibitor) and LDN193189 (BMP signaling (ALK2/3) inhibitor) were investigated in the next study. The results showed that only SB431542 significantly decreased BCL2/BAX and the others had no effects. These results suggest that decreased ACVA-ACVR2A-ALK4 signal at the post-ovulation induces cyclic apoptosis of isthmic epithelial cells in bovine oviducts.

Cisplatin-Induced Skeletal Muscle Dysfunction: Mechanisms and Counteracting Therapeutic Strategies

Among the severe side effects induced by cisplatin chemotherapy, muscle wasting is the most relevant one. This effect is a major cause for a clinical decline of cancer patients, since it is a negative predictor of treatment outcome and associated to increased mortality. However, despite its toxicity even at low doses, cisplatin remains the first-line therapy for several types of solid tumors. Thus, effective pharmacological treatments counteracting or minimizing cisplatin-induced muscle wasting are urgently needed. The dissection of the molecular pathways responsible for cisplatin-induced muscle dysfunction gives the possibility to identify novel promising therapeutic targets. In this context, the use of animal model of cisplatin-induced cachexia is very useful. Here, we report an update of the most relevant researches on the mechanisms underlying cisplatin-induced muscle wasting and on the most promising potential therapeutic options to preserve muscle mass and function.

The systemic activin response to pancreatic cancer: implications for effective cancer cachexia therapy

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is a particularly lethal malignancy partly due to frequent, severe cachexia. Serum activin correlates with cachexia and mortality, while exogenous activin causes cachexia in mice.

METHODS

Isoform-specific activin expression and activities were queried in human and murine tumours and PDAC models. Activin inhibition was by administration of soluble activin type IIB receptor (ACVR2B/Fc) and by use of skeletal muscle specific dominant negative ACVR2B expressing transgenic mice. Feed-forward activin expression and muscle wasting activity were tested in vivo and in vitro on myotubes.

RESULTS

Murine PDAC tumour-derived cell lines expressed activin-βA but not activin-βB. Cachexia severity increased with activin expression. Orthotopic PDAC tumours expressed activins, induced activin expression by distant organs, and produced elevated serum activins. Soluble factors from PDAC elicited activin because conditioned medium from PDAC cells induced activin expression, activation of p38 MAP kinase, and atrophy of myotubes. The activin trap ACVR2B/Fc reduced tumour growth, prevented weight loss and muscle wasting, and prolonged survival in mice with orthotopic tumours made from activin-low cell lines. ACVR2B/Fc also reduced cachexia in mice with activin-high tumours. Activin inhibition did not affect activin expression in organs. Hypermuscular mice expressing dominant negative ACVR2B in muscle were protected for weight loss but not mortality when implanted with orthotopic tumours. Human tumours displayed staining for activin, and expression of the gene encoding activin-βA (INHBA) correlated with mortality in patients with PDAC, while INHBB and other related factors did not.

CONCLUSIONS

Pancreatic adenocarcinoma tumours are a source of activin and elicit a systemic activin response in hosts. Human tumours express activins and related factors, while mortality correlates with tumour activin A expression. PDAC tumours also choreograph a systemic activin response that induces organ-specific and gene-specific expression of activin isoforms and muscle wasting. Systemic blockade of activin signalling could preserve muscle and prolong survival, while skeletal muscle-specific activin blockade was only protective for weight loss. Our findings suggest the potential and need for gene-specific and organ-specific interventions. Finally, development of more effective cancer cachexia therapy might require identifying agents that effectively and/or selectively inhibit autocrine vs. paracrine activin signalling.

Retinoblastoma protein represses E2F3 to maintain Sertoli cell quiescence in mouse testis

Maintenance of the differentiated state and cell cycle exit in adult Sertoli cells depends on tumor suppressor retinoblastoma protein (RB, also known as RB1). We have previously shown that RB interacts with transcription factor E2F3 in the mouse testis. Here, we investigated how E2f3 contributes to adult Sertoli cell proliferation in a mouse model of Sertoli cell-specific knockout of Rb by crossing these mice with an E2f3 knockout mouse line. In the presence of intact RB, E2f3 was redundant in Sertoli cells. However, in the absence of RB, E2f3 is a key driver for cell cycle re-entry and loss of function in adult Sertoli cells. Knockout of E2f3 in Sertoli cells rescued the breakdown of Sertoli cell function associated with Rb loss, prevented proliferation of adult Sertoli cells and restored fertility of the mice. In summary, our results show that RB-mediated repression of E2F3 is critical for the maintenance of cell cycle exit and terminal differentiation in adult mouse Sertoli cells.

Molecular Mechanisms and Signaling Pathways Involved in Sertoli Cell Proliferation

Sertoli cells are somatic cells present in seminiferous tubules which have essential roles in regulating spermatogenesis. Considering that each Sertoli cell is able to support a limited number of germ cells, the final number of Sertoli cells reached during the proliferative period determines sperm production capacity. Only immature Sertoli cells, which have not established the blood-testis barrier, proliferate. A number of hormonal cues regulate Sertoli cell proliferation. Among them, FSH, the insulin family of growth factors, activin, and cytokines action must be highlighted. It has been demonstrated that cAMP/PKA, ERK1/2, PI3K/Akt, and mTORC1/p70SK6 pathways are the main signal transduction pathways involved in Sertoli cell proliferation. Additionally, c-Myc and hypoxia inducible factor are transcription factors which participate in the induction by FSH of various genes of relevance in cell cycle progression. Cessation of proliferation is a pre-requisite to Sertoli cell maturation accompanied by the establishment of the blood-testis barrier. With respect to this barrier, the participation of androgens, estrogens, thyroid hormones, retinoic acid and opioids has been reported. Additionally, two central enzymes that are involved in sensing cell energy status have been associated with the suppression of Sertoli cell proliferation, namely AMPK and Sirtuin 1 (SIRT1). Among the molecular mechanisms involved in the cessation of proliferation and in the maturation of Sertoli cells, it is worth mentioning the up-regulation of the cell cycle inhibitors p21Cip1, p27Kip, and p19INK4, and of the gap junction protein connexin 43. A decrease in Sertoli cell proliferation due to administration of certain therapeutic drugs and exposure to xenobiotic agents before puberty has been experimentally demonstrated. This review focuses on the hormones, locally produced factors, signal transduction pathways, and molecular mechanisms controlling Sertoli cell proliferation and maturation. The comprehension of how the final number of Sertoli cells in adulthood is established constitutes a pre-requisite to understand the underlying causes responsible for the progressive decrease in sperm production that has been observed during the last 50 years in humans.

Betaglycan (TGFBR3) Functions as an Inhibin A, but Not Inhibin B, Coreceptor in Pituitary Gonadotrope Cells in Mice

Inhibins are gonadal hormones that act on pituitary gonadotrope cells to suppress FSH synthesis and secretion. Inhibin A and B are heterodimers of the inhibin ⍺-subunit disulfide-linked to one of two inhibin β-subunits. Homodimers or heterodimers of the inhibin β-subunits form the activins, which stimulate FSH production. Activins signal through complexes of type I and II receptor serine/threonine kinases to increase transcription of the FSHβ subunit gene. According to in vitro observations, inhibins impair FSH synthesis by competitively binding to activin type II receptors, particularly in the presence of the TGFβ type III receptor (TGFBR3, or betaglycan). The role of TGFBR3 in inhibin action in vivo has not been determined. Here, we ablated Tgfbr3 specifically in murine gonadotropes. Conditional knockout females were supra-fertile, exhibiting enhanced folliculogenesis, numbers of ovulated eggs per cycle, and litter sizes relative to control mice. Despite these phenotypes, FSH levels appeared to be unaltered in knockout mice, and the mechanisms underlying their enhanced fertility remain unexplained. Inhibin B is the predominant form of the hormone in males and in females during most stages of the estrous cycle. Remarkably, inhibin A, but not inhibin B, suppression of FSH synthesis was impaired in cultured pituitaries of knockout mice, which may explain the absence of discernible changes in FSH levels in vivo. Collectively, these data challenge current dogma by demonstrating that TGFBR3 (betaglycan) functions as an inhibin A, but not an inhibin B, coreceptor in gonadotrope cells in vivo. Mechanisms of inhibin B action merit further investigation.

Activin/follistatin system in grass carp pituitary cells: - Regulation by local release of growth hormone and luteinizing hormone and its functional role in growth hormone synthesis and secretion

Gonadotrophin regulation by activin/follistatin system is well-documented, but the corresponding effect on growth hormone (GH) has not been fully characterized and with little information available in lower vertebrates, especially in fish models. In grass carp, local interactions of GH and luteinizing hormone (LH) can induce GH release and gene expression at pituitary level via autocrine/paracrine mechanisms. To shed light on the role of activin/follistatin system in GH regulation by local actions of GH and LH, grass carp activin βA and βB were cloned, shown to be single-copy genes expressed in the pituitary, and confirmed to encode activin proteins capable of transactivating promoter with activin-responsive elements. In grass carp pituitary cells, activin A and B were effective in reducing GH secretion and GH cell content with concurrent drop in GH mRNA level whereas the opposite was true for follistatin, the activin-binding protein known to neutralize the effects of endogenous activin. Treatment with activin A and B not only could suppress basal but also inhibit GH mRNA expression induced by GH and human chorionic gonadotropin (hCG), a functional analogue of LH in fish model. Apparently, down-regulation of GH mRNA by activin was mediated by reducing GH transcript stability with concurrent inhibition on GH promoter activity via the SMAD pathway. In reciprocal experiments, GH treatment was found to up-regulate activin βA, activin βB and follistatin mRNA levels in carp pituitary cells but the opposite was noted by removing endogenous GH with GH antiserum. Interestingly, parallel treatment with hCG could also inhibit basal as well as GH-induced activin βA, activin βB and follistatin gene expression. These results, as a whole, indicate that the pituitary activin/follistatin system can serve as a regulatory target for local interactions of GH and LH and contribute to GH regulation by autocrine/paracrine mechanisms in the carp pituitary.

Activins and Inhibins: Roles in Development, Physiology, and Disease

Since their original discovery as regulators of follicle-stimulating hormone (FSH) secretion and erythropoiesis, the TGF-β family members activin and inhibin have been shown to participate in a variety of biological processes, from the earliest stages of embryonic development to highly specialized functions in terminally differentiated cells and tissues. Herein, we present the history, structures, signaling mechanisms, regulation, and biological processes in which activins and inhibins participate, including several recently discovered biological activities and functional antagonists. The potential therapeutic relevance of these advances is also discussed.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

Activin A expression in esophageal carcinoma and its association with tumor aggressiveness and differentiation

The aim of the present study was to investigate the expression of activin A in esophageal carcinoma and its association with tumor differentiation and metastasis. A total of 57 esophageal carcinoma patients and 36 controls were included in the current study. The mRNA and protein expression levels of activin A in esophageal tumors or normal esophageal tissues were determined by reverse transcription-quantitative polymerase chain reaction and immunohistochemical analysis. In addition, the association of activin A expression with esophageal carcinoma differentiation, metastasis and recurrence postoperatively was analyzed. The mRNA and protein expression levels of activin A in esophageal carcinoma were significantly higher compared with those in normal esophageal tissues (P<0.05). The expression of activin A was higher in poorly-/moderately-differentiated esophageal tumor tissues compared with that of well-differentiated or control tissues (P<0.05). Furthermore, the expression of activin A in poorly-differentiated esophageal tumor tissues was higher compared with that of moderately-differentiated tissues (P<0.05). A positive correlation was also observed between differentiation degree and activin A expression. The expression of activin A was higher in patients with lymph node metastasis compared with those without metastasis (P<0.05). The cumulative survival rate of patients with a high expression of activin A at 1, 2 and 3 years postoperatively was significantly decreased compared with that of patients with a lower expression of activin A (P<0.05); by contrast, the cumulative recurrence rate was significantly higher in patients with a lower activin A expression (P<0.05). In conclusion, abnormal expression of activin A was detected in esophageal tumor tissues, which was correlated with the tumor differentiation, metastasis, survival and recurrence. In conclusion, activin A may be used as an auxiliary index in the diagnosis and prognosis of clinical esophageal carcinoma.

Minireview: Activin Signaling in Gonadotropes: What Does the FOX say… to the SMAD?

The activins were discovered and named based on their abilities to stimulate FSH secretion and FSHβ (Fshb) subunit expression by pituitary gonadotrope cells. According to subsequent in vitro observations, activins also stimulate the transcription of the GnRH receptor (Gnrhr) and the activin antagonist, follistatin (Fst). Thus, not only do activins stimulate FSH directly, they have the potential to regulate both FSH and LH indirectly by modulating gonadotrope sensitivity to hypothalamic GnRH. Moreover, activins may negatively regulate their own actions by stimulating the production of one of their principal antagonists. Here, we describe our current understanding of the mechanisms through which activins regulate Fshb, Gnrhr, and Fst transcription in vitro. The activin signaling molecules SMAD3 and SMAD4 appear to partner with the winged-helix/forkhead transcription factor, forkhead box L2 (FOXL2), to regulate expression of all 3 genes. However, in vivo data paint a different picture. Although conditional deletion of Foxl2 and/or Smad4 in murine gonadotropes produces impairments in FSH synthesis and secretion as well as in pituitary Fst expression, Gnrhr mRNA levels are either unperturbed or increased in these animals. Surprisingly, gonadotrope-specific deletion of Smad3 alone or with Smad2 does not impair FSH production or fertility; however, mice harboring these mutations may express a DNA binding-deficient, but otherwise functional, SMAD3 protein. Collectively, the available data firmly establish roles for FOXL2 and SMAD4 in Fshb and Fst expression in gonadotrope cells, whereas SMAD3's role requires further investigation. Gnrhr expression, in contrast, appears to be FOXL2, SMAD4, and, perhaps, activin independent in vivo.

Activins and Follistatin in Chronic Hepatitis C and Its Treatment with Pegylated-Interferon-α Based Therapy

Pegylated-interferon-α based therapy for the treatment of chronic hepatitis C (CHC) is considered suboptimal as not all patients respond to the treatment and it is associated with several side effects that could lead to dose reduction and/or termination of therapy. The currently used markers to monitor the response to treatment are based on viral kinetics and their performance in the prediction of treatment outcome is moderate and does not combine accuracy and their values have several limitations. Hence, the development of new sensitive and specific predictor markers could provide a useful tool for the clinicians and healthcare providers, especially in the new era of interferon-free therapy, for the classification of patients according to their response to the standard therapy and only subscribing the novel directly acting antiviral drugs to those who are anticipated not to respond to the conventional therapy and/or have absolute contraindications for its use. The importance of activins and follistatin in the regulation of immune system, liver biology, and pathology has recently emerged. This review appraises the up-to-date knowledge regarding the role of activins and follistatin in liver biology and immune system and their role in the pathophysiology of CHC.

Inhibin at 90: from discovery to clinical application, a historical review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

Acute modulation of synaptic plasticity of pyramidal neurons by activin in adult hippocampus

Activin A is known as a neuroprotective factor produced upon acute excitotoxic injury of the hippocampus (in pathological states). We attempt to reveal the role of activin as a neuromodulator in the adult male hippocampus under physiological conditions (in healthy states), which remains largely unknown. We showed endogenous/basal expression of activin in the hippocampal neurons. Localization of activin receptors in dendritic spines (=postsynapses) was demonstrated by immunoelectron microscopy. The incubation of hippocampal acute slices with activin A (10 ng/mL, 0.4 nM) for 2 h altered the density and morphology of spines in CA1 pyramidal neurons. The total spine density increased by 1.2-fold upon activin treatments. Activin selectively increased the density of large-head spines, without affecting middle-head and small-head spines. Blocking Erk/MAPK, PKA, or PKC prevented the activin-induced spinogenesis by reducing the density of large-head spines, independent of Smad-induced gene transcription which usually takes more than several hours. Incubation of acute slices with activin for 2 h induced the moderate early long-term potentiation (moderate LTP) upon weak theta burst stimuli. This moderate LTP induction was blocked by follistatin, MAPK inhibitor (PD98059) and inhibitor of NR2B subunit of NMDA receptors (Ro25-6981). It should be noted that the weak theta burst stimuli alone cannot induce moderate LTP. These results suggest that MAPK-induced phosphorylation of NMDA receptors (including NR2B) may play an important role for activin-induced moderate LTP. Taken together, the current results reveal interesting physiological roles of endogenous activin as a rapid synaptic modulator in the adult hippocampus.

Novel aspects of the endocrinology of the menstrual cycle

Ovarian control of gonadotrophin secretion is normally achieved via the feedback mechanisms mediated by oestradiol and progesterone. Evidence has been provided that nonsteroidal substances, such as inhibin A and B, participate in the negative feedback control of FSH secretion. Another nonsteroidal ovarian substance is gonadotrophin surge-attenuating factor (GnSAF), the activity of which is particularly evident in women undergoing ovulation induction. Accumulating evidence has suggested that GnSAF plays a physiological role during the menstrual cycle. In particular, this factor antagonizes the sensitizing effect of oestradiol on the pituitary response to gonadotrophin-releasing hormone during the follicular phase of the cycle. A hypothesis has been developed that, in the late follicular phase, the activity of GnSAF is reduced and this facilitates the sensitizing effect of oestradiol on the pituitary, thus enforcing the massive discharge of gonadotrophins at the midcycle LH surge. The interaction of oestradiol, progesterone and GnSAF on the hypothalamic-pituitary system provides a novel approach to explain the mechanisms which control LH secretion during the normal menstrual cycle. The ovarian control of gonadotrophin secretion during the normal menstrual cycle is achieved via negative and positive feedback mechanisms. The steroids oestradiol and progesterone are the main regulators; however, nonsteroidal substances, such as inhibin A and inhibn B, also participate. Accumulating evidence has demonstrated that another nonsteroidal ovarian substance, gonadotrophin surge-attenuating factor (GnSAF), plays a key role in the control of LH secretion during the follicular phase and at midcycle, providing thus a novel aspect in the ovarian control of gonadotrophin secretion during the human menstrual cycle. The ovarian control of gonadotrophin secretion during the normal menstrual cycle is achieved via negative and positive feedback mechanisms. The steroids oestradiol and progesterone are the main regulators; however, nonsteroidal substances, such as inhibin A and inhibn B, also participate. Accumulating evidence has demonstrated that another nonsteroidal ovarian substance, gonadotrophin surge-attenuating factor (GnSAF), plays a key role in the control of LH secretion during the follicular phase and at midcycle, providing thus a novel aspect in the ovarian control of gonadotrophin secretion during the human menstrual cycle.

Activin A as a mediator of NK-dendritic cell functional interactions

The interaction of NK cells with dendritic cells (DCs) results in reciprocal cell activation through the interaction of membrane proteins and the release of soluble factors. In this article, we report that in NK-DC cocultures, among a set of 84 cytokines investigated, activin A was the second highest induced gene, with CXCL8 being the most upregulated one. Activin A is a member of the TGF-β superfamily and was previously shown to possess both proinflammatory and anti-inflammatory activities. In NK-DC cocultures, the induction of activin A required cell contact and was dependent on the presence of proinflammatory cytokines (i.e., IFN-γ, TNF-α, and GM-CSF), as well as on NK cell-mediated DC killing. CD1(+) DCs were the main activin A producer cells among myeloid blood DC subsets. In NK-DC cocultures, inhibition of activin A by follistatin, a natural inhibitory protein, or by a specific blocking Ab, resulted in the upregulation of proinflammatory cytokine release (i.e., IL-6, IL-8, TNF-α) by DCs and in the increase of DC maturation. In conclusion, our study reports that activin A, produced during NK-DC interactions, represents a relevant negative feedback mechanism that might function to prevent excessive immune activation by DCs.

DNA methylation and histone modifications are associated with repression of the inhibin α promoter in the rat corpus luteum

The transition from follicle to corpus luteum after ovulation is associated with profound morphological and functional changes and is accompanied by corresponding changes in gene expression. The gene encoding the α subunit of the dimeric reproductive hormone inhibin is maximally expressed in the granulosa cells of the preovulatory follicle, is rapidly repressed by the ovulatory LH surge, and is expressed at only very low levels in the corpus luteum. Although previous studies have identified transient repressors of inhibin α gene transcription, little is known about how this repression is maintained in the corpus luteum. This study examines the role of epigenetic changes, including DNA methylation and histone modification, in silencing of inhibin α gene expression. Bisulfite sequencing reveals that methylation of the inhibin α proximal promoter is low in preovulatory and ovulatory follicles but is elevated in the corpus luteum. Increased methylation during luteinization is observed within the cAMP response element in the promoter, and EMSA demonstrate that methylation of this site inhibits cAMP response element binding protein binding in vitro. Chromatin immunoprecipitation reveals that repressive histone marks H3K9 and H3K27 trimethylation are increased on the inhibin α promoter in primary luteal cells, whereas the activation mark H3K4 trimethylation is decreased. The changes in histone modification precede the alterations in DNA methylation, suggesting that they facilitate the recruitment of DNA methyltransferases. We show that the DNA methyltransferase DNMT3a is present in the ovary and in luteal cells when the inhibin α promoter becomes methylated and observe recruitment of DNMT3a to the inhibin promoter during luteinization.

Activins and inhibins in mammalian testis development: new models, new insights

The discovery of activin and inhibins as modulators of the hypothalamic-pituitary-gonadal axis has set the foundation for understanding their central importance to many facets of development and disease. This review contains an overview of the processes and cell types that are central to testis development and spermatogenesis and then provides an update focussed on information gathered over the past five years to address new concepts about how these proteins function to control testis development in fetal and juvenile life. Current knowledge about the interactive nature of the transforming growth factor-β (TGFβ) superfamily signalling network is applied to recent findings about activins and inhibins in the testis. Information about the regulated synthesis of signalling components and signalling regulators in the testis is integrated with new concepts that demonstrate their functional significance. The importance of activin bioactivity levels or dosage in controlling balanced growth of spermatogonial cells and their niche at different stages of testis development is highlighted.

Identification of Smad Response Elements in the Promoter of Goldfish FSHβ Gene and Evidence for Their Mediation of Activin and GnRH Stimulation of FSHβ Expression

As an essential hormone regulating gonads in vertebrates, the biosynthesis and secretion of follicle-stimulating hormone (FSH) is controlled by a variety of endocrine and paracrine factors in both mammalian and non-mammalian vertebrates. Activin was initially discovered in the ovary for its specific stimulation of FSH secretion by the pituitary cells. Our earlier studies in fish have shown that activin stimulates FSHβ but suppresses LHβ expression in both the goldfish and zebrafish. Further experiments showed that the regulation of FSHβ in fish occurred at the promoter level involving Smads, in particular Smad3. To further understand the mechanisms by which activin/Smad regulates FSHβ transcription, the present study was undertaken to analyze the promoter of goldfish FSHβ gene (fshb) with the aim to identify potential cis-regulatory elements responsible for activin/Smad stimulation. Both serial deletion and site-directed mutagenesis were used, and the promoter activity was tested in the LβT-2 cells, a murine gonadotroph cell line. The reporter constructs of goldfish FSHβ promoter-SEAP (secreted alkaline phosphatase) were co-transfected with an expression plasmid for Smads (2 or 3) followed by measurement of SEAP activity in the medium. Two putative Smad responsive elements were identified in the promoter at distal and proximal regions, respectively. The distal site contained a consensus Smad binding element (AGAC, -1675/-1672) whereas the proximal site (GACCTTGA, -212/-205) was identical to an SF-1 binding site reported in humans, which was preceded by a sequence (AACACTGA) highly conserved between fish and mammals. The proximal site also seemed to be involved in mediating stimulation of FSHβ expression by gonadotropin-releasing hormone and its potential interaction with activin. In conclusion, we have identified two potential cis-regulatory elements in the promoter of goldfish FSHβ that are responsible for activin-induced expression of the gene. Since activin stimulation of FSHβ expression is functionally conserved in fish and mammals, our findings contribute to the understanding of the fundamental mechanisms of this regulation across vertebrates.

Activin-A: a novel critical regulator of allergic asthma

Activin-A is a pleiotropic cytokine that belongs to the TGF-β superfamily and plays an important role in fundamental biological processes, such as development and tissue repair. Growing evidence proposes a crucial role for activin-A in immune-mediated responses and associated diseases, with both enhancing and suppressive effects depending on the cell type, the cytokine micromilieu and the context of the response. Several recent studies have demonstrated a striking increase in activin-A expression in experimental models of asthma, as well as, in the asthmatic airway in humans. Importantly, a strong immunoregulatory role for activin-A in allergic airway disease, with suppression of T helper (Th) type 2 cell-driven allergic responses and protection against the development of cardinal features of the asthmatic phenotype was revealed by in vivo functional studies. Activin-A-mediated immunosuppression is associated with induction of functional allergen-specific regulatory T cells. In human asthma, although activin-A levels are increased in the airway epithelium and submucosal cells, the expression of its signalling components is markedly decreased, pointing to decreased regulation. Nevertheless, a rapid activation of the activin-A signalling pathway is observed in the airway of individuals with asthma following inhalational allergen challenge, suggestive of an inherent protective mechanism to control disease. In support, in vitro studies using human airway epithelial cells have demonstrated that endogenous activin-A suppresses the release of inflammatory mediators, while it induces epithelial repair. Collectively, compelling evidence suggests that activin-A orchestrates the regulation of key events involved in the pathogenesis of allergic asthma. The critical role of activin-A in allergic airway responses places this cytokine as an exciting new therapeutic target for asthma.

Transforming growth factor beta family expression at the bovine feto-maternal interface

BACKGROUND

Endometrial remodelling is necessary for implantation in all mammalian species. The TGF beta super-family plays a crucial role in this event in humans and mice. However, the role of TGF beta super-family members during implantation is still unclear in ruminants. In the present study, the spacio-temporal expression of TGF beta super-family members including activin was explored in bovine trophoblasts and endometrial tissue during the peri-implantation period in order to elucidate whether it is essential for promoting cell proliferation at the implantation site.

METHODS

Gene expression in the fetal membrane and endometrium of the gravid and non-gravid horn around Day 35 of gestation were analyzed with a custom-made oligo-microarray in cattle. The expression of activin and its related genes was also analyzed with quantitative RT-PCR. Activin-like activity in trophoblastic tissue and BT-1 cells was examined using a fibroblast cell proliferation test and Western blotting.

RESULTS

The expression of various TGF beta super-family related genes including activin was detected in trophoblasts and the endometrium in cattle. The most intensive activin expression was found in the gravid horn endometrium, and rather intense expression was detected in the non-gravid trophoblastic tissue. Extracts from the fetal membrane including trophoblasts and purified activin both stimulated fibroblast proliferation effectively, and activin was immunologically detected in BT-1 cells, which have trophoblastic features.

CONCLUSIONS

Specific expression of the activin gene (gene name: inhibin beta A) was found in the gravid horn endometrium during peri-implantation. An activin-like molecule, which was derived from the endometrium and trophoblasts, stimulated the proliferation of fibroblast cells. These results suggested that as in other species, the activity of TGF beta super-family members including activin-like molecules plays a pivotal role in endometrial remodelling, which is an essential process in implantation and placentogenesis during the peri-implantation period in cattle.

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 inhibits cell growth and induces differentiation in human retinoblastoma y79 cells

PURPOSE

Activin is a member of the transforming growth factor-beta (TGF-beta) superfamily and exerts certain effects on differentiation and apoptosis. We investigated the effects of activin on retinoblastoma cell line.

MATERIALS AND METHODS

We used retinoblastoma cell line Y79. Intracellular signal transduction of activin was investigated with RT-PCR, immunofluorescence study, and luciferase reporter assay. The effect of activin on cell growth was examined with fluorescence cell viability assays. To determine the effect of activin on apoptosis, a TUNEL assay and an immunofluorescence study of cleaved PARP were performed. The effect of activin on cell differentiation was examined with RT-PCR and Western blotting.

RESULTS

Intracellular signal transduction of activin was confirmed in Y79 cells. Activin inhibited Y79 cell growth. Activin induced the expression of neural retina leucine zipper (Nrl) at the mRNA and protein levels.

CONCLUSIONS

Nrl is a specific gene in rod photoreceptor development and is a gene indispensable to differentiation into rod photoreceptors, so the present results suggest that activin affects the differentiation of retinoblastoma cells into rod photoreceptor cells.

Activins and follistatins: Emerging roles in liver physiology and cancer

Activins are secreted proteins belonging to the TGF-β family of signaling molecules. Activin signals are crucial for differentiation and regulation of cell proliferation and apoptosis in multiple tissues. Signal transduction by activins relies mainly on the Smad pathway, although the importance of crosstalk with additional pathways is increasingly being recognized. Activin signals are kept in balance by antagonists at multiple levels of the signaling cascade. Among these, follistatin and FLRG, two members of the emerging family of follistatin-like proteins, can bind secreted activins with high affinity, thereby blocking their access to cell surface-anchored activin receptors. In the liver, activin A is a major negative regulator of hepatocyte proliferation and can induce apoptosis. The functions of other activins expressed by hepatocytes have yet to be more clearly defined. Deregulated expression of activins and follistatin has been implicated in hepatic diseases including inflammation, fibrosis, liver failure and primary cancer. In particular, increased follistatin levels have been found in the circulation and in the tumor tissue of patients suffering from hepatocellular carcinoma as well as in animal models of liver cancer. It has been argued that up-regulation of follistatin protects neoplastic hepatocytes from activin-mediated growth inhibition and apoptosis. The use of follistatin as biomarker for liver tumor development is impeded, however, due to the presence of elevated follistatin levels already during preceding stages of liver disease. The current article summarizes our evolving understanding of the multi-faceted activities of activins and follistatins in liver physiology and cancer.

Synthesis, purification and bioactivity of recombinant human activin A expressed in the yeast Pichia pastoris

The transforming growth factor-beta (TGF-beta) superfamily member, activin A, plays a central role in the regulation of multiple physiological processes including cell differentiation, mitogenesis, embryogenesis, apoptosis and inflammation. In normal cells, activin A signalling is regulated to maintain cellular and tissue health and suppress tumour growth. Disruption of activin A signalling has been implicated in tumour formation and progression. Hence, the availability of activin A is an important target for the development of diagnostics and drugs for therapeutic intervention. To this end, we have expressed human activin A in Pichia pastoris, permitting its secretion into culture medium and purification as the mature homodimer. A construct was engineered encoding the monomeric precursor protein with a N-terminal FLAG affinity tag (DYKDDDDK) and a cleavage site (EKR) for Kex2p protease. Procedures for the two-step purification of human activin A by ion-exchange and anti-FLAG antibody affinity chromatography, and for the removal of the FLAG affinity tag from purified recombinant human activin A by enteropeptidase, are described. The molecular weights of the FLAG-tagged and de-tagged human activin A were confirmed by MALDI-TOF mass spectroscopy. The biological activity of these recombinant activins was assessed for their effects on modulating the secretion of Endothelin-1 (ET-1) by human umbilical vein endothelial cells (HUVECs). The recombinant human activin A containing the intact FLAG tag resulted in a reduced ET-1 secretion from HUVECs, whereas upon removal of this affinity purification tag the purified recombinant human activin A restored ET-1 secretion to levels comparable to the positive control. These results document an approach of considerable potential for the simple, large-scale expression and purification of this important human growth factor for use in diagnostic and therapeutic purposes.

Inhibins as diagnostic markers in human reproduction

Over the past 75 years, many publications have focused on measurement of inhibin concentration and/or activity in biological samples in order to understand its role in physiology and disease. This chapter highlights the accomplishments within this area of research over the past decade including development of specific inhibin assays. Inhibin A is a marker of dominant follicle and corpus luteum activity and decreases in polycystic ovary syndrome (PCOS). Inhibin A increases in gestational diseases such as pre-eclampsia and fetal Down's syndrome, and this increase in inhibin A improves early diagnosis of both conditions. The measurement of inhibin A in women with threatened abortion provides useful information about the likelihood of pregnancy loss. Inhibin B increases markedly in women with granulosa cell tumor and appears closely related to gametogenesis in men, that is, reflecting Sertoli cell activity. On the contrary, Inhibin B decreases in women with declining ovarian function and correlates with female response to ovulation induction. This review evaluates the biochemical significance ofinhibins including their use in clinical practice.

Activin A Causes Cancer Cell Aggressiveness in Esophageal Squamous Cell Carcinoma Cells

BACKGROUND

Expression of activin A is associated with lymph node metastasis and clinical stage in esophageal cancer.

METHODS

To clarify the aggressive behavior of tumors with high activin A expression, we used the beta subunit of activin A to establish stable activin betaA (Act-betaA)-transfected carcinoma cells in two human esophageal carcinoma cell lines, KYSE110 and KYSE140. The biological behavior of these cells was compared with that in mock-transfected cells from the same cell lines. We focused our attention on cell growth and tumorigenesis, and proliferation and apoptosis.

RESULTS

Both Act-betaA-transfected carcinoma cell lines showed a higher growth rate than the mock-transfected carcinoma cells. In an in vitro invasion assay and a xenograft analysis, the Act-betaA-transfected carcinoma cells showed far higher proliferation in vitro and a higher potency for tumorigenesis in vivo, respectively. Moreover, in an analysis of apoptosis via Fas stimulation, the Act-betaA-transfected carcinoma cells showed a higher tolerance to apoptosis compared with the mock-transfected carcinoma cells. Moreover, anti-activin-neutralizing antibody-treated squamous cell cancer cell lines inhibited their migration.

CONCLUSIONS

Collectively, these data indicate that continuous high expression of activin A in esophageal carcinoma cells is not related to tumor suppression, but rather to tumor progression in vitro and in vivo. The inhibition of activin might be one of the methods to attenuate tumor aggressiveness.

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.

Ovarian feedback, mechanism of action and possible clinical implications

The secretion of gonadotrophins from the pituitary in women is under ovarian control via negative and positive feedback mechanisms. Steroidal and non-steroidal substances mediate the ovarian effects on the hypothalamic-pituitary system. During the follicular phase of the cycle, estradiol (E(2)) plays a key role, while circulating progesterone (at low concentrations) and inhibin B contribute to the control of LH and FSH secretion respectively. During the luteal phase, both E(2) and progesterone regulate secretion of the two gonadotrophins, while inhibin A plays a role in FSH secretion. The intercycle rise of FSH is related to changes in the levels of the steroidal and non-steroidal substances during the luteal-follicular transition. In terms of the positive feedback mechanism, E(2) is the main component sensitizing the pituitary to GnRH. Activity of a non-steroidal ovarian substance, named gonadotrophin surge-attenuating factor (GnSAF), has been detected after ovarian stimulation. It is hypothesized that GnSAF, by antagonizing the sensitizing effect of E(2) on the pituitary, regulates the amplitude of the endogenous LH surge at midcycle. Disturbances in the feedback mechanisms can occur in various abnormal conditions or after treatment with pharmaceutical compounds that interfere with the production or the action of endogenous hormones.

Switching of NR5A proteins associated with the inhibin alpha-subunit gene promoter after activation of the gene in granulosa cells

The inhibin alpha-subunit gene is transcriptionally activated by FSH in ovarian granulosa cells during follicular growth. We have investigated the roles of the NR5A family nuclear receptors steroidogenic factor 1 (SF-1) and liver receptor homolog 1 (LRH-1) in transcriptional activation of the inhibin alpha-subunit gene. Transfection assays using an inhibin alpha-subunit promoter reporter in GRMO2 granulosa cells show that LRH-1 and SF-1 act similarly to increase promoter activity, and that the activity of both transcription factors is augmented by the coactivators cAMP response element-binding protein-binding protein and steroid receptor coactivator 1. However, chromatin immunoprecipitation experiments illustrate differential dynamic association of LRH-1 and SF-1 with the alpha-subunit inhibin promoter in both primary cells and the GRMO2 granulosa cell line such that hormonal stimulation of transcription results in an apparent replacement of SF-1 with LRH-1. Transcriptional stimulation of the inhibin alpha-subunit gene is dependent on MAPK kinase activity, as is the dynamic association/disassociation of SF-1 and LRH-1 with the promoter. Inhibition of the phosphatidylinositol 3-kinase signaling pathway influences promoter occupancy and transcriptional activation by SF-1 but not LRH-1, suggesting a possible mechanistic basis for the distinct functions of these NR5A proteins in inhibin alpha-subunit gene regulation.

INHA promoter polymorphisms are associated with premature ovarian failure

Inhibin is an important glycoprotein that is involved in folliculogenesis. INHA, the gene encoding the inhibin alpha subunit, was recently proposed as a candidate for premature ovarian failure (POF), a syndrome that leads to the cessation of ovarian function under the age of 40 years. 70 POF patients and 70 controls were screened for the previously identified INHA -16C>T transition mutation. The T allele was found in 31/70 (44.3%) of controls, but only 18/70 (25.7%) of POF patients. This result indicates that the T allele is significantly underrepresented in the POF patient population (Fisher's exact test, two-tail: P = 0.033). Sequence analysis of the INHA promoter in 50 POF patients and 50 controls identified a highly polymorphic imperfect TG repeat at approximately -300 bp, that consisted of four common haplotypes (A, B, C and D). The -16T allele is linked to the shortest repeat haplotype (haplotype C). Despite the association between haplotype C and POF, no significant difference was found between the promoter activity of a luciferase reporter construct containing haplotype C, and most of the other haplotypes tested. Interestingly, haplotype B failed to show any promoter activity. We conclude that the inheritance of specific INHA promoter haplotypes predispose to the development of premature ovarian failure.

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.

Modulation of activin/bone morphogenetic protein signaling by follistatin is required for the morphogenesis of mouse molar teeth

Teeth form as ectodermal appendages, and their morphogenesis is regulated by conserved signaling pathways. The shape of the tooth crown results from growth and folding of inner dental epithelium, and the cusp patterning is regulated by transient signaling centers, the enamel knots. Several signal proteins in the transforming growth factor-beta (TGF beta) superfamily are required for tooth development. Follistatin is an extracellular inhibitor of TGF beta signaling. To investigate the roles of follistatin during tooth development, we analyzed in detail the expression patterns of follistatin, activin beta A, as well as Bmp2, Bmp4, and Bmp7 during tooth morphogenesis. We also examined the tooth phenotypes of follistatin knockout mice and of transgenic mice overexpressing follistatin in the epithelium under the keratin 14 (K14) promoter. The folding of the dental epithelium was aberrant in the molars of follistatin knockout mice, and the cusps were shallow with reduced cell proliferation and lack of anteroposterior polarization. The functions of both primary and secondary enamel knots were apparently disturbed. In K14-follistatin transgenic mice, the molar cusp pattern was also seriously affected (although different from the follistatin knockouts) and the occlusal surfaces of the molars were whorled. Their enamel was prematurely worn. In addition, all of the third molars were missing. Our results indicate that follistatin regulates morphogenesis and shaping of the tooth crown. We propose that finely tuned antagonistic effects between follistatin and TGF beta superfamily signals are critical for enamel knot formation and function, as well as for patterning of tooth cusps.

Follistatin suppresses FSHbeta but increases LHbeta expression in the goldfish - evidence for an activin-mediated autocrine/paracrine system in fish pituitary

Our previous study demonstrated that recombinant goldfish activin B stimulated goldfish FSHbeta but inhibited LHbeta expression. Similar to activin B, activin A also exhibited the inverse effects on the expression of the two gonadotropins. The novel dual effects of activins on FSH and LH in the goldfish raise an interesting question as to where the activin comes from in vivo. In the present study, we first demonstrated the expression of activin, its receptors and binding protein follistatin in the goldfish pituitary, leading to a suggestion that an autocrine/paracrine regulatory system involving activin is operative in fish pituitary. To investigate the functionality of the pituitary-derived activin system in the regulation of gonadotropin biosynthesis, we further examined the effects of follistatin, an activin-binding protein, on goldfish FSHbeta and LHbeta expression. Follistatin not only reversed the effects of exogenous activin on FSHbeta and LHbeta expression but also had inverse effects on the basal expression of the genes; and its effects were opposite to those of activin. This suggests that the endogenous activin plays roles in controlling the expression of both FSHbeta and LHbeta genes. It is conceivable that any factors that influence the intrapituitary activin system in vivo will likely affect the biosynthesis of the two gonadotropins in the goldfish.

Spatial expression patterns of activin and its signaling system in the zebrafish ovarian follicle: evidence for paracrine action of activin on the oocytes

We have previously demonstrated that activin is likely an ovarian mediator of pituitary gonadotropin(s) and local epidermal growth factor in their stimulating oocyte maturation and maturational competence in the zebrafish. However, the downstream events controlled by activin remain unknown. One possible mechanism is that activin may directly work on the oocytes to promote the development of oocyte maturational competence. To substantiate this hypothesis, we performed the present study to demonstrate the expression of the activin system in different compartments of zebrafish follicles, namely, the follicle cells and oocytes. The proteins examined include activin subunits (betaA and betaB), activin-binding protein (follistatin), activin type II receptors (type IIA and IIB), the type I activin receptor-like kinases (ALK1-like, ALK2-like, and ALK4-like), and the intracellular activin signaling molecules (Smad2, Smad3, Smad4, and Smad7). The results showed that the entire activin signaling system is expressed by the full-grown immature zebrafish oocytes ( approximately 0.65 mm in diameter), including ALK4-like (ActRIB), ALK2-like (ActRIA), ActRIIA, ActRIIB, Smad2, Smad3, Smad4, and Smad7, therefore supporting our hypothesis that the oocytes are one of the direct targets of activin actions in the zebrafish ovary. In contrast, activin itself (betaA and betaB) and ALK1-like type I receptor are predominantly expressed in the follicle cells surrounding the oocytes. Interestingly, although follistatin is expressed in both the follicle cells and oocytes, its level of expression is significantly higher in the oocytes than the follicle cells, implying that follistatin may serve as a signal from the oocytes to modulate the activity of activin produced by the follicle cells. Taken together, the present study provides convincing evidence that although all members of the activin system are expressed in the whole follicle, they exhibit distinct spatial patterns of expression among different compartments of the follicle. It is likely that activin works directly on the oocytes in a paracrine manner to promote oocyte maturation and maturational competence. On the other hand, instead of being controlled passively by the follicle cells, the oocytes may actively participate in the regulation of follicle development by releasing various modulating molecules such as follistatin.

Regulation of the rat follicle-stimulating hormone beta-subunit promoter by activin

FSH is controlled by a variety of positive and negative stimuli, and the unique FSHbeta-subunit is a major target for this regulation. Activin is a key modulator of FSHbeta transcription and hormone secretion. The signal transduction pathway leading to FSH expression was previously unknown. Here, we show that the transcription factors Smad3 and Smad4 mediate activin-stimulated activity of the rat FSHbeta promoter in a pituitary-derived cell line, LbetaT2. Cells were transiently transfected with the rat FSHbeta promoter fused to a luciferase reporter gene (-338rFSHbeta-Luc), and a minimal activin-responsive region was identified. Transfection of Smad3, but not the highly related Smad2, led to a ligand-independent stimulation of the FSHbeta promoter activity. As expected, activin caused an additional increase of luciferase expression, which was blocked by cotreatment with follistatin. Although Smad4 alone had no effect on FSHbeta transcription, it significantly augmented Smad3 and activin-mediated stimulation of the promoter. A palindromic consensus Smad-binding element in the proximal promoter was found to bind Smad4, and elimination of the region resulted in a loss of activin-mediated FSHbeta transcription. The activin signaling pathway is conserved in a number of cells, but FSHbeta expression is restricted to gonadotropes. A pituitary-specific transcription factor necessary for activin-dependent induction of the FSHbeta promoter has been identified that permits FSHbeta expression in nongonadotrope cells. Pitx2 is a member of Pitx subfamily of bicoid-related homeodomain factors that is required for pituitary development and is present in the adult pituitary. This factor was transfected into LbetaT2 cells, where it caused up-regulation of basal and activin-mediated FSHbeta promoter activity. Furthermore, cotransfection of Pitx2c with Smad3 in kidney-derived TSA cells resulted in activin-regulated FSHbeta response, suggesting its important role in tissue-restricted regulation of FSHbeta by activin. A Pitx2c binding site was identified within the proximal promoter, and elimination of this region also resulted in a loss of activin-regulated FSHbeta promoter activity. Taken together, these studies suggest that the regulation of FSHbeta is dependent on activin-mediated signaling factors in concert with pituitary-derived nuclear regulatory proteins.

Effect of intracellular interactions on the processing and secretion of bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9. Implication of the aberrant ovarian phenotype of BMP-15 mutant sheep

Bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9 (GDF-9) are members of the transforming growth factor-beta superfamily. Both molecules are closely related in their primary structures and share a nearly identical spatiotemporal expression pattern in the oocyte during folliculogenesis in mammals. Here we have established a series of cell lines, which express recombinant BMP-15, GDF-9, or both, and investigated whether they form homodimers and/or heterodimers. We demonstrate the first evidence that both BMP-15 and GDF-9 can form non-covalent homodimers when expressed individually, while when both are co-expressed BMP-15/GDF-9 heterodimers are produced. Interestingly, when GDF-9 and BMP-15 are co-expressed the processing of both proproteins are significantly impaired as compared with that of the singly expressed proproteins, suggesting that the proprotein heterodimer is less susceptible to proteolytic cleavage than the individual homodimers. Since BMP-15 mutant sheep, called Inverdale, exhibit severe defects in ovarian function we have also established stable transformants expressing the mutant BMP-15 (InvBMP-15) alone or together with GDF-9. Although InvBMP-15 was previously predicted to be unable to form homodimers, we show here that it does form non-covalent dimers; however, the processing efficiency of InvBMP-15 proprotein is significantly lower than wild-type BMP-15. Surprisingly, when GDF-9 is co-expressed, the processing and secretion of InvBMP-15 is abolished, and the processing of GDF-9 is also severely impaired, suggesting that the heterodimers of InvBMP-15/GDF-9 proproteins are not susceptible to proteolytic cleavage and thus degrade in the cells. Based on these findings we propose a novel hypothesis that a decrease in GDF-9 secretion may be involved in causing infertility in homozygous Inverdale ewes.