An activin-A/C chimera exhibits activin and myostatin antagonistic properties

Activins as Dual Specificity TGF-β Family Molecules: SMAD-Activation via Activin- and BMP-Type 1 Receptors

Activins belong to the transforming growth factor (TGF)-β family of multifunctional cytokines and signal via the activin receptors ALK4 or ALK7 to activate the SMAD2/3 pathway. In some cases, activins also signal via the bone morphogenetic protein (BMP) receptor ALK2, causing activation of the SMAD1/5/8 pathway. In this study, we aimed to dissect how activin A and activin B homodimers, and activin AB and AC heterodimers activate the two main SMAD branches. We compared the activin-induced signaling dynamics of ALK4/7-SMAD2/3 and ALK2-SMAD1/5 in a multiple myeloma cell line. Signaling via the ALK2-SMAD1/5 pathway exhibited greater differences between ligands than signaling via ALK4/ALK7-SMAD2/3. Interestingly, activin B and activin AB very potently activated SMAD1/5, resembling the activation commonly seen with BMPs. As SMAD1/5 was also activated by activins in other cell types, we propose that dual specificity is a general mechanism for activin ligands. In addition, we found that the antagonist follistatin inhibited signaling by all the tested activins, whereas the antagonist cerberus specifically inhibited activin B. Taken together, we propose that activins may be considered dual specificity TGF-β family members, critically affecting how activins may be considered and targeted clinically.

Structural insights into BMP receptors: Specificity, activation and inhibition

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-β family (TGFβ), which signal through hetero-tetrameric complexes of type I and type II receptors. In humans there are many more TGFβ ligands than receptors, leading to the question of how particular ligands can initiate specific signaling responses. Here we review structural features of the ligands and receptors that contribute to this specificity. Ligand activity is determined by receptor-ligand interactions, growth factor prodomains, extracellular modulator proteins, receptor assembly and phosphorylation of intracellular signaling proteins, including Smad transcription factors. Detailed knowledge about the receptors has enabled the development of BMP-specific type I receptor kinase inhibitors. In future these may help to treat human diseases such as fibrodysplasia ossificans progressiva.

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.

Engineering TGF-β superfamily ligands for clinical applications

TGF-β superfamily ligands govern normal tissue development and homeostasis, and their dysfunction is a hallmark of many diseases. These ligands are also well defined both structurally and functionally. This review focuses on TGF-β superfamily ligand engineering for therapeutic purposes, in particular for regenerative medicine and musculoskeletal disorders. We describe the key discovery that structure-guided mutation of receptor-binding epitopes, especially swapping of these epitopes between ligands, results in new ligands with unique functional properties that can be harnessed clinically. Given the promising results with prototypical engineered TGF-β superfamily ligands, and the vast number of such molecules that remain to be produced and tested, this strategy is likely to hold great promise for the development of new biologics.

The activins and their binding protein, follistatin-Diagnostic and therapeutic targets in inflammatory disease and fibrosis

The activins, as members of the transforming growth factor-β superfamily, are pleiotrophic regulators of cell development and function, including cells of the myeloid and lymphoid lineages. Clinical and animal studies have shown that activin levels increase in both acute and chronic inflammation, and are frequently indicators of disease severity. Moreover, inhibition of activin action can reduce inflammation, damage, fibrosis and morbidity/mortality in various disease models. Consequently, activin A and, more recently, activin B are emerging as important diagnostic tools and therapeutic targets in inflammatory and fibrotic diseases. Activin antagonists such as follistatin, an endogenous activin-binding protein, offer considerable promise as therapies in conditions as diverse as sepsis, liver fibrosis, acute lung injury, asthma, wound healing and ischaemia-reperfusion injury.

Growth factors and myometrium: biological effects in uterine fibroid and possible clinical implications

BACKGROUND

Growth factors are proteins secreted by a number of cell types that are capable of modulating cellular growth, proliferation and cellular differentiation. It is well accepted that uterine cellular events such as proliferation and differentiation are regulated by sex steroids and their actions in target tissues are mediated by local production of growth factors acting through paracrine and/or autocrine mechanisms. Myometrial mass is ultimately modified in pregnancy as well as in tumour conditions such as leiomyoma and leiomyosarcoma. Leiomyomas, also known as fibroids, are benign tumours of the uterus, considered to be one of the most frequent causes of infertility in reproductive years in women.

METHODS

For this review, we searched the database MEDLINE and Google Scholar for articles with content related to growth factors acting on myometrium; the findings are hereby reviewed and discussed.

RESULTS

Different growth factors such as epidermal growth factor (EGF), transforming growth factor-α (TGF-α), heparin-binding EGF (HB-EGF), acidic fibroblast growth factor (aFGF), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF), platelet-derived growth factor (PDGF) and TGF-β perform actions in myometrium and in leiomyomas. In addition to these growth factors, activin and myostatin have been recently identified in myometrium and leiomyoma.

CONCLUSIONS

Growth factors play an important role in the mechanisms involved in myometrial patho-physiology.

Activins and cell migration

Activins are the members of transforming growth factor β superfamily and act as secreted proteins; they were originally identified with a reproductive function, acting as endocrine-derived regulators of pituitary follicular stimulating hormone. In recent years, additional functions of activins have been discovered, including a regulatory role during crucial phases of growth, differentiation, and development such as wound healing, tissue repair, and regulation of branching morphogenesis. The functions of activins through activin receptors are pleiotrophic, while involving in the etiology and pathogenesis of a variety of diseases and being cell type-specific, they have been identified as important players in cancer metastasis, immune responses, inflammation, and are most likely involved in cell migration. In this chapter, we highlight the current knowledge of activin signaling and discuss the potential physiological and pathological roles of activins acting on the migration of various cell types.

The regulation and functions of activin and follistatin in inflammation and immunity

The activins are members of the transforming growth factor β superfamily with broad and complex effects on cell growth and differentiation. Activin A has long been known to be a critical regulator of inflammation and immunity, and similar roles are now emerging for activin B, with which it shares 65% sequence homology. These molecules and their binding protein, follistatin, are widely expressed, and their production is increased in many acute and chronic inflammatory conditions. Synthesis and release of the activins are stimulated by inflammatory cytokines, Toll-like receptor ligands, and oxidative stress. The activins interact with heterodimeric serine/threonine kinase receptor complexes to activate SMAD transcription factors and the MAP kinase signaling pathways, which mediate inflammation, stress, and immunity. Follistatin binds to the activins with high affinity, thereby obstructing the activin receptor binding site, and targets them to cell surface proteoglycans and lysosomal degradation. Studies on transgenic mice and those with gene knockouts, together with blocking studies using exogenous follistatin, have established that activin A plays critical roles in the onset of cachexia, acute and chronic inflammatory responses such as septicemia, colitis and asthma, and fibrosis. However, activin A also directs the development of monocyte/macrophages, myeloid dendritic cells, and T cell subsets to promote type 2 and regulatory immune responses. The ability of both endogenous and exogenous follistatin to block the proinflammatory and profibrotic actions of activin A has led to interest in this binding protein as a potential therapeutic for limiting the severity of disease and to improve subsequent damage associated with inflammation and fibrosis. However, the ability of activin A to sculpt the subsequent immune response as well means that the full range of effects that might arise from blocking activin bioactivity will need to be considered in any therapeutic applications.

Antagonism of activin by activin chimeras

Activins are pluripotent hormones/growth factors that belong to the TGF-β superfamily of growth and differentiation factors (GDFs). They play a role in cell growth, differentiation and apoptosis, endocrine function, metabolism, wound repair, immune responses, homeostasis, mesoderm induction, bone growth, and many other biological processes. Activins and the related bone morphogenic proteins (BMPs) transduce their signal through two classes of single transmembrane receptors. The receptors possess intracellular serine/threonine kinase domains. Signaling occurs when the constitutively active type II kinase domain phosphorylates the type I receptor, which upon activation, phosphorylates intracellular signaling molecules. To generate antagonistic ligands, we generated chimeric molecules that disrupt the receptor interactions and thereby the phosphorylation events. The chimeras were designed based on available structural data to maintain high-affinity binding to type II receptors. The predicted type I receptor interaction region was replaced by residues present in inactive homologs or in related ligands with different type I receptor affinities.

Activin and related proteins in inflammation: not just interested bystanders

Activin A, a member of the transforming growth factor-beta superfamily, is released rapidly into the circulation during inflammation. This review examines the evidence that activin is a critical mediator of inflammation and immunity. Activin modulates several aspects of the inflammatory response, including release of pro-inflammatory cytokines, nitric oxide production and immune cell activity. Crucially, inhibiting activin with follistatin, a high affinity binding protein, alters the pattern of cytokines released and improves survival in a mouse model of endotoxic shock. Serum and tissue concentrations of activin are elevated in a wide range of pathological conditions. The utility of activin as a diagnostic marker of clinical inflammation and the use of follistatin to block activin actions therapeutically are also discussed.

Advances in research of activins C and E

Activins, which consist of two disulfide-linked β subunits, are members of the transforming growth factor β (TGF-β) superfamily of growth factors. Four mammalian activin β subunits, termed as βA, βB, βC, and βE respectively, have been identified. Activin A, the homodimer of two βA subunits, is a pleiotropic cytokine and is expressed in many tissues and cells. There has been compelling evidence that activin A is involved in the regulation of reproductive biology, embryonic development, erythroid differentiation, systemic inflammation, induced apoptosis, tissue repair, fibrogenesis and so on, through classic activin signaling pathway. βC and βE subunits, which are almost exclusively expressed in the liver, are still quite incompletely understood. In this review, we summarize and discuss the function of βC and βE subunits in liver. Further research should be made to understand the biological role of the βC and βE subunits.

Activins and activin antagonists in hepatocellular carcinoma

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

Activin A/bone morphogenetic protein (BMP) chimeras exhibit BMP-like activity and antagonize activin and myostatin

Activins and bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta family of growth and differentiation factors that induce signaling in target cells by assembling type II and type I receptors at the cell surface. Ligand residues involved in type II binding are located predominantly in the C-terminal region that forms an extended beta-sheet, whereas residues involved in type I binding are located in the alpha-helical and preceding loop central portion of the molecule. To test whether the central residues are sufficient to determine specificity toward type I receptors, activin A/BMP chimeras were constructed in which the central residues (45-79) of activin A were replaced with corresponding residues of BMP2 and BMP7. The chimeras were assessed for activin type II receptor (Act RII) binding, activin-like bioactivity, and BMP-like activity as well as antagonistic properties toward activin A and myostatin. ActA/BMP7 chimera retained Act RII binding affinity comparable with wild type activin A, whereas ActA/BMP2 chimera showed a slightly reduced affinity toward Act RII. Both the chimeras were devoid of significant activin bioactivity in 293T cells in the A3 Lux reporter assay up to concentrations 10-fold higher than the minimal effective activin A concentration (approximately 4 nM). In contrast, these chimeras showed BMP-like activity in a BRE-Luc assay in HepG2 cells as well as induced osteoblast-like phenotype in C2C12 cells expressing alkaline phosphatase. Furthermore, both the chimeras activated Smad1 but not Smad2 in C2C12 cells. Also, both the chimeras antagonized ligands that signal via activin type II receptor, such as activin A and myostatin. These data indicate that activin residues in the central region determine its specificity toward type I receptors. ActA/BMP chimeras can be useful in the study of receptor specificities and modulation of transforming growth factor-beta members, activins, and BMPs.

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.

The structural basis of TGF-β, bone morphogenetic protein, and activin ligand binding

The transforming growth factor-β (TGF-β) superfamily is a large group of structurally related growth factors that play prominent roles in a variety of cellular processes. The importance and prevalence of TGF-β signaling are also reflected by the complex network of check points that exist along the signaling pathway, including a number of extracellular antagonists and membrane-level signaling modulators. Recently, a number of important TGF-β crystal structures have emerged and given us an unprecedented clarity on several aspects of the signal transduction process. This review will highlight these latest advances and present our current understanding on the mechanisms of specificity and regulation on TGF-β signaling outside the cell.