FLRG, an activin-binding protein, is a new target of TGFbeta transcription activation through Smad proteins

FSTL3 promotes tumor immune evasion and attenuates response to anti-PD1 therapy by stabilizing c-Myc in colorectal cancer

Programmed cell death 1 ligand 1 (PDL1)/programmed cell death 1 (PD1) blockade immunotherapy provides a prospective strategy for the treatment of colorectal cancer (CRC), but various constraints on the effectiveness of the treatment are still remaining. As reported in previous studies, follistatin-like 3 (FSTL3) could mediate inflammatory response in macrophages by induction lipid accumulation. Herein, we revealed that FSTL3 were overexpressed in malignant cells in the CRC microenvironment, notably, the expression level of FSTL3 was related to tumor immune evasion and the clinical efficacy of anti-PD1 therapy. Further studies determined that hypoxic tumor microenvironment induced the FSTL3 expression via HIF1α in CRC cells, FSTL3 could bind to the transcription factor c-Myc (354-406 amino acids) to suppress the latter's ubiquitination and increase its stability, thereby to up-regulated the expression of PDL1 and indoleamine 2,3-dioxygenase 1 (IDO1). The results in the immunocompetent tumor models verified that FSLT3 knockout in tumor cells increased the proportion of CD8+ T cells in the tumor microenvironment, reduced the proportion of regulatory T cells (CD25+ Foxp3+) and exhausted T cells (PD1+ CD8+), and synergistically improved the anti-PD1 therapy efficacy. To sum up, FSTL3 enhanced c-Myc-mediated transcriptional regulation to promote immune evasion and attenuates response to anti-PD1 therapy in CRC, suggesting the potential of FSTL3 as a biomarker of immunotherapeutic efficacy as well as a novel immunotherapeutic target in CRC.

The Activin/FLRG pathway associates with poor COVID-19 outcomes in hospitalized patients

A subset of hospitalized COVID-19 patients, particularly the aged and those with comorbidities, develop the most severe form of the disease, characterized by acute respiratory disease syndrome (ARDS), coincident with experiencing a “cytokine storm.” Here, we demonstrate that cytokines which activate the NF-κB pathway can induce activin A. Patients with elevated activin A, activin B, and FLRG at hospital admission were associated with the most severe outcomes of COVID-19, including the requirement for mechanical ventilation, and all-cause mortality. A prior study showed that activin A could decrease viral load, which indicated there might be a risk to giving COVID-19 patients an inhibitor of activin. To evaluate this, the role for activin A was examined in a hamster model of SARS-CoV-2 infection, via blockade of activin A signaling. The hamster model demonstrated that use of an anti-activin A antibody did not worsen the disease and there was no evidence for increase in lung viral load and pathology. The study indicates blockade of activin signaling may be beneficial in treating COVID-19 patients experiencing ARDS.

Activin type II receptor signaling in cardiac aging and heart failure

Activin type II receptor (ActRII) ligands have been implicated in muscle wasting in aging and disease. However, the role of these ligands and ActRII signaling in the heart remains unclear. Here, we investigated this catabolic pathway in human aging and heart failure (HF) using circulating follistatin-like 3 (FSTL3) as a potential indicator of systemic ActRII activity. FSTL3 is a downstream regulator of ActRII signaling, whose expression is up-regulated by the major ActRII ligands, activin A, circulating growth differentiation factor-8 (GDF8), and GDF11. In humans, we found that circulating FSTL3 increased with aging, frailty, and HF severity, correlating with an increase in circulating activins. In mice, increasing circulating activin A increased cardiac ActRII signaling and FSTL3 expression, as well as impaired cardiac function. Conversely, ActRII blockade with either clinical-stage inhibitors or genetic ablation reduced cardiac ActRII signaling while restoring or preserving cardiac function in multiple models of HF induced by aging, sarcomere mutation, or pressure overload. Using unbiased RNA sequencing, we show that activin A, GDF8, and GDF11 all induce a similar pathologic profile associated with up-regulation of the proteasome pathway in mammalian cardiomyocytes. The E3 ubiquitin ligase, Smurf1, was identified as a key downstream effector of activin-mediated ActRII signaling, which increased proteasome-dependent degradation of sarcoplasmic reticulum Ca2+ ATPase (SERCA2a), a critical determinant of cardiomyocyte function. Together, our findings suggest that increased activin/ActRII signaling links aging and HF pathobiology and that targeted inhibition of this catabolic pathway holds promise as a therapeutic strategy for multiple forms of HF.

The human NUPR1/P8 gene is transcriptionally activated by transforming growth factor β via the SMAD signalling pathway

NUPR1 (nuclear protein 1), also called P8 (molecular mass 8 kDa) or COM1 (candidate of metastasis 1), is involved in the stress response and in cancer progression. In the present study, we investigated whether human NUPR1 expression was regulated by TGFβ (transforming growth factor β), a secreted polypeptide largely involved in tumorigenesis. We demonstrate that the expression of NUPR1 was activated by TGFβ at the transcriptional level. We show that this activation is mediated by the SMAD proteins, which are transcription factors specifically involved in the signalling of TGFβ superfamily members. NUPR1 promoter analysis reveals the presence of a functional TGFβ-response element binding the SMAD proteins located in the genomic DNA region corresponding to the 5'-UTR (5'-untranslated region). Altogether, the molecular results of the present study, which demonstrate the existence of a TGFβ/SMAD/NUPR1 activation cascade, open the way to consider and investigate further a new mechanism enabling TGFβ to promote tumorigenesis by inducing stress resistance.

Follistatin-like 3 mediates paracrine fibroblast activation by cardiomyocytes

Follistatins are extracellular inhibitors of the TGF-β family ligands including activin A, myostatin and bone morphogenetic proteins. Follistatin-like 3 (FSTL3) is a potent inhibitor of activin signalling and antagonises the cardioprotective role of activin A in the heart. FSTL3 expression is elevated in patients with heart failure and is upregulated in cardiomyocytes by hypertrophic stimuli, but its role in cardiac remodelling is largely unknown. Here, we show that the production of FSTL3 by cardiomyocytes contributes to the paracrine activation of cardiac fibroblasts, inducing changes in cell adhesion, promoting proliferation and increasing collagen production. We found that FSTL3 is necessary for this response and for the induction of cardiac fibrosis. However, full activation requires additional factors, and we identify connective tissue growth factor as a FSTL3 binding partner in this process. Together, our data unveil a novel mechanism of paracrine communication between cardiomyocytes and fibroblasts that may provide potential as a therapeutic target in heart remodelling.

Cell-type specific modulation of pituitary cells by activin, inhibin and follistatin

Activins are multifunctional proteins and members of the TGF-β superfamily. Activins are expressed locally in most tissues and, analogous to the actions of other members of this large family of pleiotropic factors, play prominent roles in the regulation of diverse biological processes in both differentiated and embryonic stem cells. They have an essential role in maintaining tissue homeostasis in the adult and are known to contribute to the developmental programs in the embryo. Activins are further implicated in the growth and metastasis of tumor cells. Through distinct modes of action, inhibins and follistatins function as antagonists of activin and several other TGF-β family members, including a subset of BMPs/GDFs, and modulate cellular responses and the signaling cascades downstream of these ligands. In the pituitary, the activin pathway is known to regulate key aspects of gonadotrope functions and also exert effects on other pituitary cell types. As in other tissues, activin is produced locally by pituitary cells and acts locally by exerting cell-type specific actions on gonadotropes. These local actions of activin on gonadotropes are modulated by the autocrine/paracrine actions of locally secreted follistatin and by the feedback actions of gonadal inhibin. Knowledge about the mechanism of activin, inhibin and follistatin actions is providing information about their importance for pituitary function as well as their contribution to the pathophysiology of pituitary adenomas. The aim of this review is to highlight recent findings and summarize the evidence that supports the important functions of activin, inhibin and follistatin in the pituitary.

The role of activin in mammary gland development and oncogenesis

TGFβ contributes to mammary gland development and has paradoxical roles in breast cancer because it has both tumor suppressor and tumor promoter activity. Another member of the TGFβ superfamily, activin, also has roles in the developing mammary gland, but these functions, and the role of activin in breast cancer, are not well characterized. TGFβ and activin share the same intracellular signaling pathways, but divergence in their signaling pathways are suggested. The purpose of this review is to compare the spatial and temporal expression of TGFβ and activin during mammary gland development, with consideration given to their functions during each developmental period. We also review the contributions of TGFβ and activin to breast cancer resistance and susceptibility. Finally, we consider the systemic contributions of activin in regulating obesity and diabetes; and the impact this regulation has on breast cancer. Elevated levels of activin in serum during pregnancy and its influence on pregnancy associated breast cancer are also considered. We conclude that evidence demonstrates that activin has tumor suppressing potential, without definitive indication of tumor promoting activity in the mammary gland, making it a good target for development of therapeutics.

The Local Control of the Pituitary by Activin Signaling and Modulation

The pituitary gland plays a prominent role in the control of many physiological processes. This control is achieved through the actions and interactions of hormones and growth factors that are produced and secreted by the endocrine cell types and the non-endocrine constituents that collectively and functionally define this complex organ. The five endocrine cell types of the anterior lobe of the pituitary, somatotropes, lactotropes, corticotropes, thyrotropes and gonadotropes, are defined by their primary product, growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH) and follicle stimulating hormone (FSH)/luteinizing hormone (LH). They are further distinguishable by the presence of cell surface receptors that display high affinity and selectivity for specific hypothalamic hormones and couple to appropriate downstream signaling pathways involved in the control of cell type specific responses, including the release and/or synthesis of pituitary hormones. Central control of the pituitary via the hypothalamus is further fine-tuned by the positive or negative actions of peripheral feedback signals and of a variety of factors that originate from sources within the pituitary. The focus of this review is the latter category of intrinsic factors that exert local control. Special emphasis is given to the TGF-β family of growth factors, in particular activin effects on the gonadotrope population, because a considerable body of evidence supports their contribution to the local modulation of the embryonic and postnatal pituitary as well as pituitary pathogenesis. A number of other substances, including members of the cytokine and FGF families, VEGF, IGF1, PACAP, Ghrelin, adenosine and nitric oxide have also been shown or implicated to function as autocrine/paracrine factors, though, definitive proof remains lacking in some cases. The ever-growing list of putative autocrine/paracrine factors of the pituitary nevertheless has highlighted the complexity of the local network and its impact on pituitary functions.

Generation of mice with conditionally activated transforming growth factor beta signaling through the TbetaRI/ALK5 receptor

We generated a transgenic mouse strain (LSL-TbetaRI(CA)) containing a latent constitutively active TGFbeta type I receptor (TbetaRI/ALK5) by using a knock-in strategy into the X chromosome-linked hypoxanthine phosphoribosyl-transferase (Hprt) locus. Transgene expression, under the control of the ubiquitous CAG (human cytomegalovirus enhancer and chicken beta-actin) promoter, is repressed by a floxed transcriptional "Stop" (LSL, Lox-Stop-Lox). In the presence of cre-recombinase, the "Stop" is excised to allow TbetaRI(CA) transgene expression. We showed that restricted expression of TbetaRI(CA) in T lymphocytes efficiently activates TGFbeta signaling and rescues the T-cell autoimmune disorders of TGFbetaRII conditional knockouts. Unexpectedly, our study reveals that TGFbeta signaling upregulation controls T-cell activation but does not impair their development or their peripheral homeostasis. In addition to the information provided on TGFbeta effects on T-cell biology, LSL-TbetaRI(CA) mouse constitutes an attractive tool to address the effect of TGFbeta signaling upregulation in any cell type expressing the cre-recombinase.

Expression of follistatin-related genes is altered in heart failure

Follistatins play roles in diverse biological processes including cell proliferation, wound healing, inflammation, and skeletal muscle growth, yet their role in the heart is currently unknown. We have investigated the myocardial expression profile and cellular distribution of follistatin (FST) and the FST-like genes FSTL1 and FSTL3 in the normal and failing heart. Expression was further analyzed in the novel setting of recovery from heart failure in myocardium obtained from patients who received combined mechanical (left ventricular assist device) and pharmacological therapy. Real-time PCR revealed that FSTL1 and FSTL3 expression was elevated in heart failure but returned to normal after recovery. FSTL3 expression levels correlated with molecular markers of disease severity and FSTL1 with the endothelial cell marker CD31, suggesting a potential link with vascularization. FSTL1 levels before treatment correlated with cardiac function after recovery, suggesting initial levels may influence long-term outcome. Immunohistochemistry revealed that FST was primarily localized to fibroblasts and vascular endothelium within the heart, whereas FSTL1 was localized to myocytes, endothelium, and smooth muscle cells and FSLT3 to myocytes and endothelium. Microarray analysis revealed that FST and FSTL1 were associated with extracellular matrix-related and calcium-binding proteins, whereas FSTL3 was associated mainly with cell signaling and transcription. These data show for the first time that elevated myocardial expression of FST-like genes is a feature of heart failure and may be linked to both disease severity and mechanisms underlying recovery, revealing new insight into the pathogenesis of heart failure and offering novel therapeutic targets.

Peroxisome proliferator-activated receptor gamma down-regulates follistatin in intestinal epithelial cells through SP1

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) down-regulates the expression of follistatin mRNA in intestinal epithelial cells in vivo. The mechanism of PPARgamma-mediated down-regulation of follistatin was investigated using non-transformed, rat intestinal epithelial cells (RIE-1). RIE cells expressed activin A, the activin receptors ActRI and ActRII, and the follistatin-315 mRNA. RIE-1 cells responded to endogenous activin A, and this response was antagonized by follistatin, as evidenced by changes in cell growth and regulation of an activin-responsive reporter. Using RIE-1 cells, we show that activation of PPARgamma by rosiglitazone reduced follistatin mRNA levels in a dose- and concentration-dependent manner. Down-regulation of follistatin by rosiglitazone required the DNA binding domain of PPARgamma and was dependent upon dimerization with the retinoid X receptor. Inhibition of follistatin expression by rosiglitazone was not associated with decreased follistatin mRNA stability, suggesting that regulation may be at the promoter level. Analysis of the follistatin promoter revealed consensus binding sites for AP-1, AP-2, and Sp1. Targeting the AP-1 pathway with SP600125, an inhibitor of JNK, and TAM67, a dominant negative c-Jun, had no effect on PPARgamma-mediated down-regulation of follistatin. However, the follistatin promoter was dramatically regulated by Sp1, and this regulation was inhibited by PPARgamma expression. Knockdown of Sp1 expression relieved repression of follistatin levels by rosiglitazone. Moreover, PPARgamma was found to interact with Sp1 and repress its transcriptional activation function. Collectively, our data indicate that repression of Sp1 transcriptional activity by PPARgamma is the underlying mechanism responsible for PPARgamma-mediated regulation of follistatin expression.

AF10-dependent transcription is enhanced by its interaction with FLRG

BACKGROUND INFORMATION

FLRG (follistatin-related gene) is a secreted glycoprotein which is very similar to follistatin. As observed for follistatin, FLRG is involved in the regulation of various biological processes through its binding to members of the TGFbeta (transforming growth factor beta) superfamily, activin, BMPs (bone morphogenetic proteins) and myostatin. Unlike follistatin, FLRG has been found to be both secreted and localized within the nucleus of many FLRG-producing cells, suggesting the existence of specific intracellular functions of the protein.

RESULTS

In order to analyse the function of the nuclear form of FLRG, we performed a yeast two-hybrid screen, in which we identified AF10 [ALL1 (acute lymphoblastic leukaemia) fused gene from chromosome 10], a translocation partner of the MLL (mixed-lineage leukaemia) oncogene in human leukaemia, as a FLRG-interacting protein. This interaction was confirmed by far-Western-blot analysis and co-immunoprecipitation with transfected COS-7 cells. The N-terminal region of AF10, including the PHD (plant homeodomain), is sufficient to mediate this interaction, and has been shown to be involved in AF10 homo-oligomerization. By immunoprecipitation experiments, we showed that FLRG enhances the homo-oligomerization of AF10. Functional studies demonstrated that FLRG enhances the transactivation properties of the AF10 protein fused to Gal4 DNA-binding domains in transient transfection assays.

CONCLUSIONS

Our present study provides novel insights into the function of the nuclear form of the FLRG protein, which is revealed as a novel regulator of transcription. The nuclear isoform of FLRG lacks an intrinsic transactivation domain, but enhances AF10-mediated transcription, probably through promoting the homo-oligomerization of AF10, thus facilitating the recruitment of co-activators.

Hypoxia enhances the expression of follistatin-like 3 in term human trophoblasts

Hypoxic injury hinders placental differentiation and alters trophoblast gene expression. We tested the hypothesis that the expression of follistatin-like 3 (FSTL3), a member of the follistatin family of proteins, is modulated by hypoxia in primary human trophoblast (PHT). Using immunofluorescence of human term placental villi we detected the expression of FSTL3 protein in placental villi, primarily in trophoblasts. We verified this finding in cultured term PHT cells. Basal expression of FSTL3 transcript in cultured PHT cells, determined using quantitative PCR, was stable over the culture period. Importantly, when compared to culture in FiO(2)=20% or FiO(2)=8%, PHT cells cultured in FiO(2) <1% exhibited a 4-6 fold increase in FSTL3 mRNA expression as early as 4h in hypoxia. Whereas cellular FSTL3 protein was unchanged in hypoxia, we found that hypoxia increased the level of FSTL3 in the medium. Lastly, the exposure of PHT cells to either the hypoxia-mimetic cobalt chloride or the proline hydroxylase inhibitor dimethyloxaloylglycine upregulated the expression of FSTL3 transcript. Our data indicate that hypoxia enhances the expression of FSTL3 and its release from PHT cells. Our finding that hypoxia-mimetic agents enhance FSTL3 expression implicates HIF1alpha in this process.

Patterns of expression of the Follistatin and Follistatin-like1 genes during chicken heart development: a potential role in valvulogenesis and late heart muscle cell formation

The regulation of concentration and function of growth factors is of crucial importance to proper embryonic development of the heart. The patterns of expression of three extracellular modulators of the transforming growth factor-beta superfamily of growth factors, Follistatin, Follistatin-like1, and Follistatin-like3, are described with respect to heart development. Follistatin is highly localized in the endocardium covering the developing cardiac valves. Follistatin-like1 is localized in the mesenchymal filling of the pharyngeal arches and broadly expressed in cells directly bordering myocardium. Follistatin-like3 is not expressed in the heart. Taken together, these observations are suggestive for a role for Follistatin in cardiac valvulogenesis and a role for Follistatin-like1 in controlling late heart muscle cell formation.

Identification of NF-kappaB responsive elements in follistatin related gene (FLRG) promoter

Follistatin related gene (FLRG) has been previously identified from a chromosomal translocation observed in a B-cell chronic lymphocytic leukemia (B-CLL). FLRG (alternative names: follistatin-related protein, FSRP/follistatin-like-3, FSTL3) is a secreted glycoprotein highly similar to follistatin. Like follistatin, FLRG is involved in the regulation of various biological effects through its binding to members of the transforming growth factor beta (TGFbeta) superfamily such as activin A and myostatin. We have previously shown that TGFbeta and activin A are potent inducers of FLRG transcriptional activation through the Smad proteins. Using a biochemical approach, we investigated whether tumor necrosis factor alpha (TNFalpha) could regulate FLRG expression since TNFalpha plays a critical role in hematopoietic malignancies. We demonstrate that TNFalpha activates FLRG expression at the transcriptional level. This activation depends on a promoter region containing four 107-108 bp DNA repeats, which are evolutionary conserved in primates. These repeats carry a strong phylogenetic signal, which is not common among non-coding sequences. Each DNA repeat contains one TNFalpha responsive element (5'-GGGAGAG/TTCC-3') able to bind nuclear factor kappaB (NF-kappaB) transcription factors. We also show that TGFbeta, through the Smad proteins, potentates the effect of TNFalpha on FLRG expression. This cooperation is unexpected since TGFbeta and TNFalpha usually have opposite biological effects. In all, this work brings new insights in the understanding of FLRG regulation by cytokines and growth factors. It opens attractive perspectives of research that should allow us to better understand the role of FLRG during tumorigenesis.

Activin signaling and its role in regulation of cell proliferation, apoptosis, and carcinogenesis

Activins, cytokine members of the transforming growth factor-beta superfamily, have various effects on many physiological processes, including cell proliferation, cell death, metabolism, homeostasis, differentiation, immune responses endocrine function, etc. Activins interact with two structurally related serine/threonine kinase receptors, type I and type II, and initiate downstream signaling via Smads to regulate gene expression. Understanding how activin signaling is controlled extracellularly and intracellularly would not only lead to more complete understanding of cell growth and apoptosis, but would also provide the basis for therapeutic strategies to treat cancer and other related diseases. This review focuses on the recent progress on activin-receptor interactions, regulations of activin signaling by ligand-binding proteins, receptor-binding proteins, and nucleocytoplasmic shuttling of Smad proteins.

The human Cyr61 gene is a transcriptional target of transforming growth factor beta in cancer cells

Cyr61 is a multifunctional protein that can stimulate angiogenesis and tumor growth. Its expression by many cancers and breast cancers increases with tumor grade. Cyr61 is closely related to connective tissue growth factor, CTGF. Both proteins regulate skeletal development, suggesting that they could contribute to breast cancer metastases to bone, a process regulated by TGFbeta. We show that Cyr61 transcription is activated by TGFbeta and that the human Cyr61 promoter contains consensus sequences that bind Smad proteins. TGFbeta in the tumor microenvironment may stimulate cancer metastases to sites such as bone by increasing Cyr61 expression and secretion.

A novel role for fibronectin type I domain in the regulation of human hematopoietic cell adhesiveness through binding to follistatin domains of FLRG and follistatin

FLRG and follistatin belong to the family of follistatin proteins involved in the regulation of various biological effects, such as hematopoiesis, mediated by their binding to activin and BMP, both members of the TGFbeta family. To further characterize the function of FLRG, we searched for other possible functional partners using a yeast two-hybrid screen. We identified human fibronectin as a new partner for both FLRG and follistatin. We also demonstrated that their physical interaction is mediated by type I motifs of fibronectin and follistatin domains. We then analyzed the biological consequences of these protein interactions on the regulation of hematopoiesis. For the first time, we associated a biological effect with the regulation of human hematopoietic cell adhesiveness of both the type I motifs of fibronectin and the follistatin domains of FLRG and follistatin. Indeed, we observed a significant and specific dose-dependent increase of cell adhesion to fibronectin in the presence of FLRG or follistatin, using either a human hematopoietic cell line or primary cells. In particular, we observed a significantly increased adhesion of immature hematopoietic precursors (CFC, LTC-IC). Altogether these results highlight a new mechanism by which FLRG and follistatin regulate human hematopoiesis.

FLRG, a new ADAM12-associated protein, modulates osteoclast differentiation

BACKGROUND INFORMATION

FLRG (follistatin-related gene) is a secreted glycoprotein that is highly homologous with follistatin. These proteins are involved in the regulation of various biological effects mediated by their binding to TGF-beta (transforming growth factor-beta) superfamily members, activin A and bone morphogenetic proteins. To characterize further the function of FLRG, we used a yeast two-hybrid screen to look for other possible functional partners.

RESULTS

We report a direct interaction between the cysteine-rich domain of FLRG and ADAM12 (a disintegrin and metalloprotease 12). ADAMs are metalloprotease-disintegrin proteins that have been implicated in cell adhesion, protein ectodomain shedding, matrix protein degradation and cell fusion. Several studies have reported that ADAM12 protein, as well as activin A, are important regulators of osteoclast differentiation. We observed that the expressions of ADAM12 and activin A are modulated during osteoclast formation, whereas the FLRG expression seemed to remain quite constant. We showed that the FLRG protein inhibits osteoclast differentiation from murine primary spleen cells and macrophage RAW264.7 cells cultured in the presence of RANK-L (receptor activator of nuclear factor kappaB ligand) and M-CSF (macrophage colony-stimulating factor). Addition of FLRG protein to precursors significantly reduces the number of osteoclasts, as well as the average number of nuclei in each osteoclast.

CONCLUSIONS

Our study indicates that the FLRG protein may contribute to bone formation by inhibiting osteoclast differentiation.

Novel factors in regulation of activin signaling

Activin type II receptors (ActRIIs) are the primary receptors that transmit the activin signal to intracellular signaling pathways. Binding of activins to ActRIIs recruits the activin type I receptor and initiates downstream signaling. We have found that PDZ proteins, named activin receptor-interacting proteins (ARIPs), specifically associate with ActRIIs. We have studied the mechanism that ARIPs regulate cell surface expression and cellular localization of ActRIIs. ARIP2 interacts with both ActRIIs and RalBP1 (Ral binding protein 1) through different domains to dramatically change the localization of ActRIIs. Overexpression of ARIP2 enhances endocytosis of ActRIIs. These data indicate that ARIP2 is a novel factor regulating cell surface ActRII expression and activin function. A novel activin binding protein, follistatin-related gene (FLRG) was identified. FLRG protein binds activin and myostatin with a high affinity. The biological activity of FLRG is similar to those of follistatin, however, the regulation and expression patterns of follistatin and FLRG differ. Immunohistochemical analysis shows that FLRG is distributed in spermatogenic cells of the testis, renal tubules, epithelial cells of the lung, and myocardium. Thus, although structurally and functionally similar, follistatin and FLRG likely play distinct roles as activin/GDF binding proteins in vivo.

FLRG, member of the follistatin family, a new player in hematopoiesis

Several years ago, we cloned and characterized from a B cell leukemia a new secreted protein which, on the basis of its high degree of structural homology with follistatin, was defined as a member of the follistatin family and accordingly named follistatin-related gene (FLRG). However, follistatin and FLRG revealed non-overlapping patterns of expression in various tissues thereby indicating the existence of non-redundant functional roles for these proteins throughout the organism. As known for a long time, follistatin is a biological regulator of activin and bone morphogenetic protein (BMP) function in various cellular systems: in particular, it inhibits the effects of activin on hematopoiesis. We therefore investigated the expression and effects of FLRG during human hematopoiesis with particular focus on the effect of this soluble glycoprotein in the regulation of erythropoiesis. For this purpose, we have for the first time, compared the role of Activin A, BMP2 and BMP4 during erythropoiesis, in primary human cells. Our results indicate that, BMP2 acts on early erythroid cells while Activin A acts on a more differentiated population. We report the induction by Activin A and BMP2 of cell commitment towards erythropoiesis in the absence of EPO. This induction involves two key events: increase of EPO-R and the decrease of GATA2 expression. Our results indicate that despite their high structural homology, follistatin and FLRG do not regulate the same signaling targets, therefore highlighting distinct functions and mechanisms for these two proteins in the human hematopoietic system. We thus propose a working model for the regulation of activin or BMP-induced human erythropoiesis by follistatin/FLRG.

Purification, cDNA cloning, and secretory properties of FLRG protein from PC12 cells and the distribution of FLRG mRNA and protein in rat tissues

A 35 kD protein was isolated and purified from conditioned media of Bcl-2 cDNA-transfected PC12 cells and its cDNA cloned. A database analysis showed that the 35 kD protein is a rat homologue of the human FLRG protein. The biochemical as well as morphological properties of the rat FLRG protein in PC12 cells were examined and its distribution in rat tissues determined. The levels of FLRG mRNA expressed were low during the fetal period, compared with those of follistatin mRNA. The distribution of FLRG and follistatin mRNAs differed from each other after birth; the expression levels of FLRG mRNA were abundant in the adrenal gland and testis, whereas those of follistatin mRNA and activin A were markedly high in the ovary. The presence of FLRG mRNA and/or protein was confirmed in spermatocytes at various differentiating stages andin endocrine cells of both the adrenal cortex and medulla. When overexpressed in PC12 cells, the FLRG protein was found to be stored in secretory granules of the cells and largely secreted by a regulated pathway, while activin A enhancedthe constitutive secretion of the FLRG protein from wild-typpe PC12 cells, indicating that the FLRG protein possesses dualproperties in secretory pathways. The different distribution between FLRG and follistatin mRNA suggests that, like follistatin in the ovary, the FLRG protein may be involved in the maintenance of spermatogenesis in the testis and the growth and function of adrenal tissue cells, probably by regulating the functions of its binding partners such as the TGF-beta ( superfamily members.

Human placenta and fetal membranes express follistatin-related gene mRNA and protein

Activin A is a placental glycoprotein and possible biological actions during pregnancy, suggested by experimental data, are the modulation of cytotrophoblast differentiation, placental hormonogenesis and uterotonins secretion. Follistatin-related gene (FLRG) is a 70 amino acids protein which binds activin A with high affinity, and which modulates its biological effects on target tissues by preventing the activin A interaction with its receptors. The present study investigated whether trophoblast, decidua and fetal membranes express FLRG mRNA (by RT-PCR) and peptide (by immunohistochemistry). Tissue specimens were collected at first and third trimester of pregnancy, from patients undergoing voluntary pregnancy interruption (no.=6; from 8 to 12 gestational weeks) and elective caesarean section at term (no.=6; 39-40 weeks of pregnancy). FLRG mRNA was expressed by the various gestational tissues both at early gestation and at term pregnancy. Immunoreactive protein was found in the trophoblast cells, epithelial amniotic and chorionic cells and maternal decidua; nevertheless, the most intense FLRG stain was detected in the walls of decidual and placental blood vessels. In conclusion, FLRG mRNA and peptide are expressed by placenta and fetal membranes. Its different immunolocalization with respect to follistatin and activin A supports a different role for FLRG in modulating activin A actions into gestational tissues.

Regulation of FLRG expression in rat primary astroglial cells and injured brain tissue by transforming growth factor-beta 1 (TGF-beta 1)

Follistatin-related gene (FLRG) is a member of the follistatin family of proteins and interacts with transforming growth factor (TGF) superfamily proteins like follistatin. To understand the expression level of FLRG in brain tissue, we examined whether primary neurons and glial cells from rat embryos express FLRG mRNA and produce its protein product. FLRG and follistain mRNAs were mainly expressed in astroglial cells, while activin A mRNA was abundant in primary neurons. TGF-beta1 highly enhanced expression levels of FLRG mRNA in astroglial cells, compared with those of follistatin and activin A mRNAs. Particularly, TGF-beta1 facilitated the secretion of FLRG protein from primary astroglial cells in a dose-dependent manner. Moreover, changes in expression levels of FLRG mRNA and protein in brain tissue were also analyzed after a penetrating injury, using quantitative polymerase chain reactin (PCR) and immunohistochemical methods. Expression levels of FLRG mRNA were significantly increased in damaged regions after penetrating injury together with those of activin A and TGF-beta1 mRNAs. Immunohistochemical observations showed that positive signals of FLRG protein were colocalized in glial fibrillary acidic protein-positive reactive astroglial cells located in damaged regions after a penetrating injury. The expression of follistatin mRNA rather decreased in damage regions after the brain injury. These results suggest that FLRG is synthesized in and secreted from astroglial cells. In particular, FLRG, but not follistatin, may play a role in the regulation of activin A in brain wound healing in response to TGF-beta1.

Regulation of human erythropoiesis by activin A, BMP2, and BMP4, members of the TGFbeta family

Activin A, BMP2, and BMP4, members of the TGFbeta family, have been implicated in the regulation of hematopoiesis. Here we explore and compare, for the first time in human primary cells, the role of activin A, BMP2, and BMP4 during erythropoiesis. Using in vitro erythroid differentiation of CD34(+) primary cells, we obtained the main stages of early erythropoiesis, characterized at the molecular, biochemical, and functional levels. Our results indicate that BMP2 acts on early erythroid cells and activin A on a more differentiated population. We report an insight into the mechanism of commitment of erythropoiesis by activin A and BMP2 involving two key events, increase in EPO-R and decrease in GATA2 expression. Simultaneous addition of activin A with BMP molecules suggests that BMP2 and BMP4 differently affect activin A induction of erythropoiesis. Follistatin and FLRG proteins downmodulate the effects of activin A and BMP2 on erythroid maturation.

Characterization of rat follistatin-related gene: effects of estrous cycle stage and pregnancy on its messenger RNA expression in rat reproductive tissues

Follistatin-related gene (FLRG) was first identified as a target of a chromosomal translocation in a human B-cell leukemia. Because FLRG protein binds to activins and bone morphogenetic proteins, FLRG is postulated to be a regulator of these growth factors. However, physiological aspects of FLRG are unclear. To elucidate the physiology of FLRG, we examined expression of FLRG in reproductive tissues of the rat. FLRG mRNA was abundantly expressed in the placenta. FLRG mRNA was also expressed in the ovary, uterus, testis, lung, adrenal gland, pituitary, kidney, small intestine, and heart. During the second half of pregnancy, expression of FLRG in the placenta continuously increased, whereas follistatin mRNA levels decreased from Day 12 to Day 14 and remained low thereafter. FLRG was also expressed in decidua. Levels of decidual FLRG mRNA remained low from Day 12 to Day 16 and then noticeably increased until Day 20. In contrast, follistatin mRNA was highly expressed in the decidua on Day 12, continuously decreased until Day 16, and then remained at relatively low levels thereafter. During the rat estrous cycle, levels of ovarian FLRG mRNA fluctuated diurnally, with highest levels during daytime, and did not change relative to the day of the estrous cycle. The present results suggest that FLRG may play a role in the regulation of reproductive events.

The myostatin propeptide and the follistatin-related gene are inhibitory binding proteins of myostatin in normal serum

Myostatin, also known as growth and differentiation factor 8, is a member of the transforming growth factor beta superfamily that negatively regulates skeletal muscle mass (1). Recent experiments have shown that myostatin activity is detected in serum by a reporter gene assay only after activation by acid, suggesting that native myostatin circulates as a latent complex (2). We have used a monoclonal myostatin antibody, JA16, to isolate the native myostatin complex from normal mouse and human serum. Analysis by mass spectrometry and Western blot shows that circulating myostatin is bound to at least two major proteins, the myostatin propeptide and the follistatin-related gene (FLRG). The myostatin propeptide is known to bind and inhibit myostatin in vitro (3). Here we show that this interaction is relevant in vivo, with a majority (>70%) of myostatin in serum bound to its propeptide. Studies with recombinant V5-His-tagged FLRG protein confirm a direct interaction between mature myostatin and FLRG. Functional studies show that FLRG inhibits myostatin activity in a reporter gene assay. These experiments suggest that the myostatin propeptide and FLRG are major negative regulators of myostatin in vivo.

Transcription activation of FLRG and follistatin by activin A, through Smad proteins, participates in a negative feedback loop to modulate activin A function

Signaling of TGFbeta family members such as activin is tightly regulated by soluble binding proteins. Follistatin binds to activin A with high affinity, and prevents activin binding to its own receptors, thereby blocking its signaling. We previously identified FLRG gene from a B-cell leukemia carrying a t(11;19)(q13;p13) translocation. We and others have already shown that FLRG, which is highly homologous to follistatin, may be involved in the regulation of the activin function through its binding to activin. In this study, we found that, like follistatin, FLRG protein inhibited activin A signaling as demonstrated by the use of a transcriptional reporter assay, and blocked the activin A-induced growth inhibition of HepG2 cells. We have recently shown that the TGFbeta-induced expression of FLRG occurs at a transcriptional level through the action of Smad proteins. Here we show that activin A increases FLRG and follistatin at both the mRNA and protein levels. We found that Smad proteins are involved in the activin A-induced transcription activation of FLRG and follistatin. Finally we demonstrate that FLRG protein regulates its own activin-induced expression. In conclusion, activin A induces FLRG and follistatin expression. This observation, in conjunction with the antagonistic effect of FLRG and follistatin on activin signaling, indicates that these two proteins participate in a negative feedback loop which regulates the activin function.

Genomic organization and promoter analysis of mouse follistatin-related gene (FLRG)

Follistatin (FS) is well characterized as an activin-binding protein. Recently, a novel follistatin-like protein called follistatin-related gene (FLRG) that has a similar domain organization to that of follistatin has been identified. Like follistatins, FLRG binds activins and bone morphogenetic proteins (BMPs). To study the regulation of FLRG expression, we have analyzed the genomic organization and promoter of the mouse FLRG gene. The mouse FLRG gene consists of five exons, and each encodes discrete functional regions. The overall genomic structure of FLRG is similar to that of FS except that the FLRG gene is missing one exon that codes a third FS domain found in FS. The promoter that covers 2.5 kbp and is linked to a luciferase reporter construct is active in human cervical carcinoma HeLa cells as well as in human embryonic kidney (HEK293) cells. Deletion analysis of the promoter regions indicates that a proximal 550 base pairs are enough for basal FLRG promoter activity in the cell lines. FLRG promoter activity is significantly augmented by phorbol 12-myristate 13-acetate (PMA) treatment, but not by cAMP stimulation. By contrast, FS promoter is activatable either by cAMP or PMA. Thus, although FS and FLRG are structurally and functionally related, their modes of regulation by external stimuli are different.