Should activin betaC be more than a fading snapshot in the activin/TGFbeta family album?

The orphan ligand, activin C, signals through activin receptor-like kinase 7

Activin ligands are formed from two disulfide-linked inhibin β (Inhβ) subunit chains. They exist as homodimeric proteins, as in the case of activin A (ActA; InhβA/InhβA) or activin C (ActC; InhβC/InhβC), or as heterodimers, as with activin AC (ActAC; InhβA:InhβC). While the biological functions of ActA and activin B (ActB) have been well characterized, little is known about the biological functions of ActC or ActAC. One thought is that the InhβC chain functions to interfere with ActA production by forming less active ActAC heterodimers. Here, we assessed and characterized the signaling capacity of ligands containing the InhβC chain. ActC and ActAC activated SMAD2/3-dependent signaling via the type I receptor, activin receptor-like kinase 7 (ALK7). Relative to ActA and ActB, ActC exhibited lower affinity for the cognate activin type II receptors and was resistant to neutralization by the extracellular antagonist, follistatin. In mature murine adipocytes, which exhibit high ALK7 expression, ActC elicited a SMAD2/3 response similar to ActB, which can also signal via ALK7. Collectively, these results establish that ActC and ActAC are active ligands that exhibit a distinct signaling receptor and antagonist profile compared to other activins.

Emerging treatment approaches for myeloma-related bone disease

INTRODUCTION

Multiple myeloma is characterized by the presence of osteolytic lesions that leads to devastating skeletal-related events in the majority of patients. Myeloma bone disease is attributed to increased osteoclastic and suppressed osteoblastic activity. Areas covered: Bisphosphonates remain the main treatment option, however they have limitations on their own. Understanding the pathogenesis of myeloma bone disease may provide a roadmap for new therapeutic approaches. The pathway of RANKRANKLOPG pathway has revealed denosumab, a monoclonal antibody targeting RANKL as a novel emerging therapy for myeloma-related bone disease. Furthermore, the Wnt signaling inhibitors dicckopf-1 and sclerostin that are implicated in the pathogenesis of bone destruction of myeloma are now targeted by novel monoclonal antibodies. Activin-A is a TGF-beta superfamily member which increases osteoclast activity and inhibits osteoblast function in myeloma; sotatercept and other molecules targeting activin-A have entered into clinical development. Several other molecules and pathways that play an important role in the pathogenesis of bone destruction in myeloma, such as periostin, adiponectin, Notch and BTK signaling are also targeted in an attempt to develop novel therapies for myeloma-related bone disease. Expert commentary: We summarize the current advances in the biology of myeloma bone disease and the potential therapeutic targets.

Over-Expression of Activin-βC Is Associated with Murine and Human Prostate Disease

Activins are members of the TGF-β superfamily and have been linked to prostate cancer. There are four mammalian activin subunits (β_A, β_B, β_C, and β_E) that dimerize to form functional proteins. The role of activin-A (β_A-β_A) has been relatively well characterized and has been shown to generally inhibit growth in the prostate. In contrast, little is known about the biological function of the β_C and β_E subunits. Previous work indicated activin-C (β_C-β_C) to be an antagonist of activin-A. This is important because resistance to activin-A growth inhibition occurs during prostate cancer progression. This paradox is not currently well understood. Hence, we hypothesize that local expression of the activin-β_C subunit antagonizes activin-A-dependent growth inhibition and represents a key factor contributing to acquired insensitivity to activin-A observed in prostate cancer progression. To test our hypothesis, we characterized the ventral prostate lobes of 9-month-old transgenic mice over-expressing activin-β_C and examined the expression of activin-β_A, activin-β_C, and the activin intracellular signaling factor, Smad-2, in human prostate diseases. Prostate epithelial cell hyperplasia, low-grade prostatic intraepithelial neoplasia (PIN) lesions, alterations in cell proliferation, and reduced Smad-2 nuclear localization were evident in mice over-expressing activin-β_C. Increased activin-β_A and -β_C subunit immunoreactive scores and decreased Smad-2 nuclear localization were also evident in human prostate cancer. This study suggests that over-expression of activin-β_C is associated with murine and human prostate pathologies. We conclude that the activin-β_C subunit may have therapeutic and/or diagnostic implications in human prostate disease.

Novel therapeutic targets in myeloma bone disease

Multiple myeloma is a neoplastic disorder of plasma cells characterized by clonal proliferation within the bone marrow. One of the major clinical features of multiple myeloma is the destructive osteolytic bone disease that occurs in the majority of patients. Myeloma bone disease is associated with increased osteoclast activity and suppression of osteoblastogenesis. Bisphosphonates have been the mainstay of treatment for many years; however, their use is limited by their inability to repair existing bone loss. Therefore, research into novel approaches for the treatment of myeloma bone disease is of the utmost importance. This review will discuss the current advances in our understanding of osteoclast stimulation and osteoblast suppression mechanisms in myeloma bone disease and the treatments that are under development to target this destructive and debilitating feature of myeloma.

microRNA and non-canonical TGF-β signalling: implications for prostate cancer therapy

The incidence of prostate cancer is increasing worldwide and marks a significant health issue. Paired with this, current therapeutic options for advanced prostate cancer, notably androgen deprivation therapy (ADT), fail to provide a consistent level of efficacy throughout the treatment period, highlighting the need for new robust therapies. Growth factors, such as Transforming Growth Factor-beta (TGF-β), possess the ability to impede cancer development in the early stages, via alterations in either apoptosis, cell proliferation, or the promotion of cellular senescence. However, later in the pathogenesis, advanced prostate cancer cells become insensitive to the previously beneficial effects of TGF-β. The molecular mechanisms behind this acquired insensitivity are not well understood. Thus, the aim of this review is to examine the effects of a class of small non-coding RNA, microRNA (miRNA), on TGF-β signalling. The impact of miRNA on the canonical TGF-β Smad signalling pathway has been well investigated, hence, in this review, we will examine whether miRNA targeting members of non-canonical TGF-β signalling members, such as, Erk, RhoA, PI3K/Akt and JNK/p38 could provide alternate therapeutic options for advanced prostate cancer.

SNPs in the TGF-β signaling pathway are associated with increased risk of brain metastasis in patients with non-small-cell lung cancer

Purpose

Brain metastasis (BM) from non-small cell lung cancer (NSCLC) is relatively common, but identifying which patients will develop brain metastasis has been problematic. We hypothesized that genotype variants in the TGF-β signaling pathway could be a predictive biomarker of brain metastasis.

Patients and Methods

We genotyped 33 SNPs from 13 genes in the TGF-β signaling pathway and evaluated their associations with brain metastasis risk by using DNA from blood samples from 161 patients with NSCLC. Kaplan-Meier analysis was used to assess brain metastasis risk; Cox hazard analyses were used to evaluate the effects of various patient and disease characteristics on the risk of brain metastasis.

Results

The median age of the 116 men and 45 women in the study was 58 years; 62 (39%) had stage IIIB or IV disease. Within 24 months after initial diagnosis of lung cancer, brain metastasis was found in 60 patients (37%). Of these 60 patients, 16 had presented with BM at diagnosis. Multivariate analysis showed the GG genotype of SMAD6: rs12913975 and TT genotype of INHBC: rs4760259 to be associated with a significantly higher risk of brain metastasis at 24 months follow-up (hazard ratio [HR] 2.540, 95% confidence interval [CI] 1.204–5.359, P = 0.014; and HR 1.885, 95% CI 1.086–3.273, P = 0.024), compared with the GA or CT/CC genotypes, respectively. When we analyzed combined subgroups, these rates showed higher for those having both the GG genotype of SMAD6: rs12913975 and the TT genotype of INHBC: rs4760259 (HR 2.353, 95% CI 1.390–3.985, P = 0.001).

Conclusions

We found the GG genotype of SMAD6: rs12913975 and TT genotype of INHBC: rs4760259 to be associated with risk of brain metastasis in patients with NSCLC. This finding, if confirmed, can help to identify patients at high risk of brain metastasis.

Activins and inhibins: expression and role in normal and pathological canine reproductive organs: a review

Activins and inhibins are regulatory proteins of the reproductive function. Inhibins antagonise the activin signalling at different levels and are responsible for the negative feedback in the regulation of the release of pituitary follicle stimulating hormone (FSH), which, in turn, is promoted by locally produced activins. In the canine ovary, both peptides are expressed by developing follicles and corpora lutea. Activins may play a stimulatory role in follicular development, promoting the aromatase function; inhibins modulate these processes and suppress the hyperplasic/neoplastic stimuli. Activins are required for ovulation and corpus luteum formation, while inhibins stimulate progesterone synthesis. The exclusive production of alpha-inhibin by granulosa cells allows the peptide to be used as marker to identify canine ovarian stromal tumours by immunohistochemistry. In the male, activins are powerful morphogenetic factors in the foetal testis. In the adult, they display a modulating action on spermatogenesis and Sertoli cell function. Inhibins, produced mainly by Leydig cells, promote testosterone secretion. Canine testicular tumours, such as Leydig, Sertoli and granulosa cell tumours (GCTs), may express inhibin subunits and produce high circulating levels of these glycoproteins. In the canine prostate, activins inhibit epithelium proliferation, antagonising androgen effects, but they are synthesised under androgenic stimulus.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Testicular signaling is the potential target of perfluorooctanesulfonate-mediated subfertility in male mice

Perfluorooctanesulfonate (PFOS) was produced and used by various industries and in consumer products. Because of its persistence, it is ubiquitous in air, water, soil, wildlife, and humans. Although the adverse effects of PFOS on male fertility have been reported, the underlying mechanisms have not yet been elucidated. Here, for the first time, the effects of PFOS on testicular signaling, such as gonadotropin, growth hormone, insulin-like growth factor, and inhibins/activins were shown to be directly related to male subfertility. Sexually mature 8-wk-old CD1 male mice were administered by gavages in corn oil daily with 0, 1, 5, or 10 mg/kg PFOS for 7, 14, or 21 days. Serum concentrations of testosterone and epididymal sperm counts were significantly lower in the mice after 21 days of the exposure to the highest dose compared with the controls. The expression levels of testicular receptors for gonadotropin, growth hormone, and insulin-like growth factor 1 were considerably reduced on Day 21 in mice exposed daily to 10 or 5 mg/kg PFOS. The transcript levels of the subunits of the testicular factors (i.e., inhibins and activins), Inha, Inhba, and Inhbb, were significantly lower on Day 21 of daily exposure to 10, 5, or 1 mg/kg PFOS. The mRNA expression levels of steroidogenic enzymes (i.e., StAR, CYP11A1, CYP17A1, 3beta-HSD, and 17beta-HSD) were notably reduced. Therefore, PFOS-elicited subfertility in male mice is manifested as progressive deterioration of testicular signaling.

Evidence of inhibin/activin subunit betaC and betaE synthesis in normal human endometrial tissue

BACKGROUND

Inhibins are important regulators of the female reproductive system. Recently, two new inhibin subunits betaC and betaE have been described, although it is unclear if they are synthesized in normal human endometrium.

METHODS

Samples of human endometrium were obtained from 82 premenopausal, non-pregnant patients undergoing gynecological surgery for benign diseases. Endometrium samples were classified according to anamnestic and histological dating into proliferative (day 1-14, n = 46), early secretory (day 15-22, n = 18) and late secretory phase (day 23-28, n = 18). Immunohistochemical analyses were performed with specific antibodies against inhibin alpha (n = 81) as well as inhibin betaA (n = 82), betaB (n = 82), betaC (n = 74) and betaE (n = 76) subunits. RT-PCR was performed for all inhibin subunits. Correlation was assessed with the Spearman factor to assess the relationship of inhibin-subunits expression within the different endometrial samples.

RESULTS

The novel inhibin betaC and betaE subunits were found in normal human endometrium by immunohistochemical and molecular techniques. Inhibin alpha, betaA, betaB and betaE subunits showed a circadian expression pattern, being more abundant during the late secretory phase than during the proliferative phase. Additionally, a significant correlation between inhibin alpha and all inhibin beta subunits was observed.

CONCLUSIONS

The differential expression pattern of the betaC- and betaE-subunits in normal human endometrial tissue suggests that they function in endometrial maturation and blastocyst implantation. However, the precise role of these novel inhibin/activin subunits in human endometrium is unclear and warrants further investigation.

Immunohistochemical labeling of the inhibin/activin betaC subunit in normal human placental tissue and chorionic carcinoma cell lines

Inhibins and activins are important regulators of the female reproductive system. A novel inhibin subunit, named betaC, has been identified and demonstrated to be expressed in several human tissues. We demonstrate here that inhibin betaC is expressed in human placenta. Expression of the inhibin betaC subunit was demonstrated at the protein level by means of immunohistochemical evaluation and at the transcriptional level by an inhibin betaC-specific RT-PCR analysis. Expression of inhibin betaC was detected in the human chorionic carcinoma cell lines JEG and BeWo. Although the precise role of this novel inhibin subunit in human placenta development and homeostasis is unclear, analogies with other inhibin subunits and the strong expression of betaC in normal human trophoblast cells and chorionic carcinoma cells suggest that betaC may be involved in autocrine/paracrine signaling pathways, angiogenesis, decidualization, and tissue remodeling under normal and malignant conditions. Additionally, JEG and BeWo express betaC and, therefore, can be used as a cell culture model for further functional analysis of this subunit in the human placenta.

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.

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.

Deregulation of the activin/follistatin system in hepatocarcinogenesis

BACKGROUND/AIMS

Activins A and E negatively regulate hepatic cell number by inhibiting cell replication and inducing apoptosis. Follistatin and follistatin-like 3 bind activins and antagonise their biological activities. Aim of our study was to investigate, whether activins and follistatins may play a role in hepatocarcinogenesis.

METHODS

Expression levels of follistatin, follistatin-like 3, and activin subunits beta(A) as well as beta(E) were investigated in chemically induced rat and human liver tumours by real-time PCR and immunohistochemistry. In addition, the effects of follistatin and activin A on DNA synthesis of normal as well as preneoplastic hepatocytes and hepatoma cells were analysed.

RESULTS

Follistatin was overexpressed while both activin subunits were downregulated in the majority of rat and human liver tumours. Follistatin-like 3 expression was low in normal but enhanced in malignant rat liver. In human normal liver, in contrast, it was abundantly expressed but downregulated in liver cancer. Administration of follistatin to normal and preneoplastic hepatocytes stimulated DNA synthesis preferentially in preneoplastic rat hepatocytes, whereas activin A repressed it.

CONCLUSIONS

The balanced expression of follistatins and activins becomes deregulated during hepatocarcinogenesis. The sensitivity of preneoplastic hepatocytes to activin signals suggests the activin/follistatin system as promising target for therapeutic intervention.

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.

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

Activins are involved in many physiological and pathological processes and, like other members of the transforming growth factor-beta superfamily, signal via type II and I receptor serine kinases. Ligand residues involved in type II receptor binding are located in the two anti-parallel beta strands of the TGF-beta proteins, also known as the fingers. Activin-A mutants able to bind ActRII but unable to bind the activin type I receptor ALK4 define ligand residues involved in ALK4 binding and could potentially act as antagonists. Therefore, a series of FLAG-tagged activin-A/C chimeras were constructed, in each of which eight residues in the wrist loop and helix region (A/C 46-53, 54-61, 62-69, and 70-78) were replaced. Additionally, a chimera was generated in which the entire wrist region (A/C 46-78) was changed from activin-A to activin-C. The chimeras were assessed for ActRII binding, activin bioactivity, as well as antagonistic properties. All five chimeras retained high affinity for mouse ActRII. Of these, only A/C 46-78 was devoid of significant activin bioactivity in an A3 Lux reporter assay in 293T cells at concentrations up to 40 nM. A/C 46-53, 54-61, 62-69, and 70-78 showed activity comparable with wild type activin-A. When tested for the ability to antagonize ligands that signal via activin type II receptors, such as activin-A and myostatin, only the A/C 46-78 chimera showed antagonism (IC(50), 1-10 nM). Additionally, A/C 46-78 decreased follicle-stimulating hormone release from the LbetaT2 cell line and rat anterior pituitary cells in primary culture in a concentration-dependent manner. These data indicate that activin residues in the wrist are involved in ALK4-mediated signaling. The activin antagonist A/C 46-78 may be useful for the study and modulation of activin-dependent processes.