Analysis of the function of activin betaC subunit using recombinant protein

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.

Production, Isolation, and Structural Analysis of Ligands and Receptors of the TGF-β Superfamily

The ability to understand the molecular mechanisms by which secreted signaling proteins of the TGF-β superfamily assemble their cell surface receptors into complexes to initiate downstream signaling is dependent upon the ability to determine atomic-resolution structures of the signaling proteins, the ectodomains of the receptors, and the complexes that they form. The structures determined to date have revealed major differences in the overall architecture of the signaling complexes formed by the TGF-βs and BMPs, which has provided insights as to how they have evolved to fulfill their distinct functions. Such studies, have however, only been applied to a few members of the TGF-β superfamily, which is largely due to the difficulty of obtaining milligram-scale quantities of highly homogenous preparations of the disulfide-rich signaling proteins and receptor ectodomains of the superfamily. Here we describe methods used to produce signaling proteins and receptor ectodomains of the TGF-β superfamily using bacterial and mammalian expression systems and procedures to purify them to homogeneity.

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.

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.