A sensitive and specific in vitro bioassay for activin using a mouse plasmacytoma cell line, MPC-11

High-level production and purification of bioactive recombinant human activin A in Chinese hamster ovary cells

Activin A, a member of the TGF-β superfamily, is a homodimer of the inhibin βΑ subunit that plays a diversity of roles in biological processes. Because of its multiple functions, significant efforts have been made to produce activin A, however, unsatisfactory results were obtained due to its low level of expression. In this study, a stable CHO cell line exhibiting high expression of rhActivin A was isolated and production of rhActivin A was achieved using the cell line from 11-day fed-batch cultures in a 7.5 L bioreactor. The production rate was 0.22 g/L, substantially higher than those reported in previous studies. The culture supernatant of the bioreactor was used to purify rhActivin A (purity: >99%, recovery rate: 47%). The purified rhActivin A exhibited biological activity, with an EC50 of 3.893 ng/mL and a specific activity of 1.38 × 103 IU/mg. The control of process-related impurities in the purified rhActivin A was successful and met the USP recommendations for use in cell therapy. Thus, our production and purification methods were appropriate for large-scale GMP-grade rhActivin A production, which can be used for various purposes including cell therapy.

An Easy and Fast Method for Production of Chinese Hamster Ovary Cell Line Expressing and Secreting Human Recombinant Activin A

Objective

Growth factors are key elements of embryonic stem cell (ESC) research. Cell line development in eukaryotes is a time-consuming procedure which usually takes 12-18 months. Here, we report an easy and fast method with which production of Chinese hamster ovary (CHO) cells that express and secrete recombinant Activin A, as a major growth factor in endo/mesoderm differentiation of embryonic stem cells is achieved within 3-4 weeks.

Materials and Methods

In this experimental study, we cloned human Activin A into the pDONR/Zeo gateway entry vector using the BP reaction. Activin A was subcloned next into the pLIX_403 and pLenti6.3/TO/V5-DEST destination vectors by the LR reaction. The result was the production of constructs with which 293T cells were finally transfected for virus production. CHO cells were transduced using viral particles to produce a cell line that secretes the His6- Activin A fusion protein.

Results

We developed a quick protocol which saves up to 3-4 weeks of time for producing recombinant proteins in CHO cells. The recombinant cell line produced 90 mg/L of functional Activin A measured in human ESC line Royan H5 (RH5), during in vitro differentiation into meso-endoderm and definitive endoderm.

Conclusion

Our results showed no significant differences in functionality between commercial Activin A and the one produced using our novel protocol. This approach can be easily used for producing recombinant proteins in CHO.

Role of Activins in Hepcidin Regulation during Malaria

Epidemiological observations have linked increased host iron with malaria susceptibility, and perturbed iron handling has been hypothesized to contribute to the potentially life-threatening anemia that may accompany blood-stage malaria infection. To improve our understanding of these relationships, we examined the pathways involved in regulation of the master controller of iron metabolism, the hormone hepcidin, in malaria infection. We show that hepcidin upregulation in Plasmodium berghei murine malaria infection was accompanied by changes in expression of bone morphogenetic protein (BMP)/sons of mothers against decapentaplegic (SMAD) pathway target genes, a key pathway involved in hepcidin regulation. We therefore investigated known agonists of the BMP/SMAD pathway and found that Bmp gene expression was not increased in infection. In contrast, activin B, which can signal through the BMP/SMAD pathway and has been associated with increased hepcidin during inflammation, was upregulated in the livers of Plasmodium berghei-infected mice; hepatic activin B was also upregulated at peak parasitemia during infection with Plasmodium chabaudi Concentrations of the closely related protein activin A increased in parallel with hepcidin in serum from malaria-naive volunteers infected in controlled human malaria infection (CHMI) clinical trials. However, antibody-mediated neutralization of activin activity during murine malaria infection did not affect hepcidin expression, suggesting that these proteins do not stimulate hepcidin upregulation directly. In conclusion, we present evidence that the BMP/SMAD signaling pathway is perturbed in malaria infection but that activins, although raised in malaria infection, may not have a critical role in hepcidin upregulation in this setting.

Combined administration of testosterone plus an ornithine decarboxylase inhibitor as a selective prostate-sparing anabolic therapy

Because of its anabolic effects on muscle, testosterone is being explored as a function-promoting anabolic therapy for functional limitations associated with aging; however, concerns about testosterone's adverse effects on prostate have inspired efforts to develop strategies that selectively increase muscle mass while sparing the prostate. Testosterone's promyogenic effects are mediated through upregulation of follistatin. We show here that the administration of recombinant follistatin (rFst) increased muscle mass in mice, but had no effect on prostate mass. Consistent with the results of rFst administration, follistatin transgenic mice with constitutively elevated follistatin levels displayed greater muscle mass than controls, but had similar prostate weights. To elucidate signaling pathways regulated differentially by testosterone and rFst in prostate and muscle, we performed microarray analysis of mRNAs from prostate and levator ani of castrated male mice treated with vehicle, testosterone, or rFst. Testosterone and rFst shared the regulation of many transcripts in levator ani; however, in prostate, 593 transcripts in several growth-promoting pathways were differentially expressed after testosterone treatment, while rFst showed a negligible effect with only 9 transcripts differentially expressed. Among pathways that were differentially responsive to testosterone in prostate, we identified ornithine decarboxylase (Odc1), an enzyme in polyamine biosynthesis, as a testosterone-responsive gene that is unresponsive to rFst. Accordingly, we administered testosterone with and without α-difluoromethylornithine (DFMO), an Odc1 inhibitor, to castrated mice. DFMO selectively blocked testosterone's effects on prostate, but did not affect testosterone's anabolic effects on muscle. Co-administration of testosterone and Odc1 inhibitor presents a novel therapeutic strategy for prostate-sparing anabolic therapy.

Serum activin A and B levels predict outcome in patients with acute respiratory failure: a prospective cohort study

INTRODUCTION

30 day mortality in patients with Acute Respiratory Failure (ARF) is approximately 30%, defined as patients requiring ventilator support for more than 6 hours. Novel biomarkers are needed to predict patient outcomes and to guide potential future therapies. The activins A and B, members of the Transforming Growth Factor β family of proteins, and their binding protein, follistatin, have recently been shown to be important regulators of inflammation and fibrosis but no substantial data are available concerning their roles in ARF.

METHODS

Specific assays for activin A, B and follistatin were used and the results analyzed according to diagnostic groups as well as according to standard measures in intensive care. Multivariable logistic regression was used to create a model to predict death at 90 days and 12 months from the onset of the ARF.

RESULTS

Serum activin A and B were significantly elevated in most patients and in most of the diagnostic groups. Patients who had activin A and/or B concentrations above the reference maximum were significantly more likely to die in the 12 months following admission [either activin A or B above reference maximum: Positive Likelihood Ratio [LR+] 1.65 [95% CI 1.28-2.12, P = 0.00013]; both activin A and B above reference maximum: LR + 2.78 [95% CI 1.96-3.95, P < 0.00001]. The predictive model at 12 months had an overall accuracy of 80.2% [95% CI 76.6-83.3%].

CONCLUSIONS

The measurement of activin A and B levels in these patients with ARF would have assisted in predicting those at greatest risk of death. Given the existing data from animal studies linking high activin A levels to significant inflammatory challenges, the results from this study suggest that approaches to modulate activin A and B bioactivity should be explored as potential therapeutic agents.

Systemic administration of follistatin288 increases muscle mass and reduces fat accumulation in mice

The present study describes the physiological response associated with daily subcutaneous injection of mice with recombinant follistatin288. This systemic administration of follistatin288 increases the follistatin levels in serum, indicating that the protein enters the circulation. The data suggest that a dose-dependent increase in body lean mass also occurs, together with an increase in muscle mass, possibly as a result of an increase in the size of the muscle fibers. After thirteen weeks of treatment, metabolic changes were observed; additionally, the switching of muscle fiber types was also apparent through myosin heavy chain remodeling, implying that changes are occurring at the molecular level. Furthermore, an increase in the muscle mass was associated with a significant decrease in the body fat mass. Overall, this study raises the possibility for the use of follistatin288 as an agent to treat muscle wasting diseases and/or to restrict fat accumulation by systemic administration of the protein.

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.

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.

Structure analysis of bone morphogenetic protein-2 type I receptor complexes reveals a mechanism of receptor inactivation in juvenile polyposis syndrome

Bone morphogenetic proteins regulate many developmental processes during embryogenesis as well as tissue homeostasis in the adult. Signaling of bone morphogenetic proteins (BMPs) is accomplished by binding to two types of serine/threonine kinase transmembrane receptors termed type I and type II. Because a large number of ligands signal through a limited number of receptors, ligand-receptor interaction in the BMP superfamily is highly promiscuous, with a ligand binding to various receptors and a receptor binding many different BMP ligands. In this study we investigate the interaction of BMP-2 with its two high affinity type I receptors, BMP receptors IA (BMPR-IA) and BMPR-IB. Interestingly, 50% of the residues in the BMP-2 binding epitope of the BMPR-IA receptor are exchanged in BMPR-IB without a decrease in binding affinity or specificity for BMP-2. Our structural and functional analyses show that promiscuous binding of BMP-2 to both type I receptors is achieved by inherent backbone and side-chain flexibility as well as by variable hydration of the ligand-receptor interface enabling the BMP-2 surface to adapt to different receptor geometries. Despite the high degree of amino acid variability found in BMPR-IA and BMPR-IB binding equally to BMP-2, three single point missense mutations in the ectodomain of BMPR-IA cannot be tolerated. In juvenile polyposis syndrome these mutations have been shown to inactivate BMPR-IA. On the basis of our biochemical and biophysical analyses, we can show that the mutations, which are located outside the ligand binding epitope, alter the local or global fold of the receptor, thereby inactivating BMPR-IA and causing a loss of the BMP-2 tumor suppressor function in colon epithelial cells.

Execution of BMP-4-induced apoptosis by p53-dependent ER dysfunction in myeloma and B-cell hybridoma cells

Bone morphogenic protein (BMP)-4 inhibits proliferation and induces the apoptosis of myeloma cells. However, little is known about the molecular mechanisms of how BMP-4 executes this apoptosis. In this report, we investigated the roles of p53 and the endoplasmic reticulum (ER) in BMP-4-induced apoptosis of mouse hybridoma HS-72 cells. We found that 3 ng/ml of BMP-4 is sufficient to induce the expression of proapoptotic proteins, puma and bax, in a p53-dependent mechanism, and facilitate Ca(2+) release from the ER to the cytosol, resulting in the activation of caspase-12 and ER dysfunction. Similarly to HS-72 cells, multiple myeloma cells with wild-type p53 genes show much higher sensitivity to BMP-4-induced apoptosis than cells without wild-type p53 genes, suggesting that wild-type p53 status is required for dysfunction of the ER during BMP-4-induced apoptosis in ER-enriched cells, such as hybridoma and myeloma cells. These findings demonstrate that the presence of wild-type p53 genes and enrichment of the ER determines the sensitivity to effective apoptosis by BMP-4, and suggest that ER stress-inducing agents would be valuable in the treatment of multiple myeloma.

A molecular recognition paradigm: promiscuity associated with the ligand-receptor interactions of the activin members of the TGF-β superfamily

The structure-function properties of the pleiotropic activins and their relationship to other members of the transforming growth factor-beta superfamily of proteins are described. In order to highlight the molecular promiscuity of these growth factors, emphasis has been placed on molecular features associated with the recognition by activin A and the bone morphogenic proteins of the corresponding extracellular domains of the ActRI and ActRII receptors. The available evidence suggests that the homodimeric activin A in its various functional roles has the propensity to fulfill key tasks in the regulation of mammalian cell behaviour, through coordination of numerous transcriptional and translational processes. Because of these profound effects, under physiologically normal conditions, activin A levels are closely controlled by a variety of binding partners, such as follistatin-288 and follistatin-315, alpha(2)-macroglobulin and other proteins. Moreover, the subunits of other members of the activin subfamily, such as activin B or activin C, are able to form heterodimers with the activin A subunit, thus providing a further avenue to positively or negatively control the physiological concentrations of activin A that are available for interaction with specific receptors and induction of cell signaling events. Based on data from X-ray crystallographic studies and homology modeling experiments, the molecular architecture of the ternary receptor-activin ligand complexes has been dissected, permitting rationalization in structural terms of the pattern of interactions that are the hallmark of this protein family.

The role of activin a in regulation of hemopoiesis

Activin A, a cytokine member of the transforming growth factor-beta superfamily, is expressed locally by the mesenchymal component of the hemopoietic microenvironment. Its expression is regulated on the mRNA level by different cytokines, and the biological activity of the protein is tightly controlled by several inhibitory molecules. Activin A affects hemopoietic cells of various lineages, as evidenced by in vitro studies of leukemia and lymphoma cell lines, which were used to elucidate the mechanism of its action. In the B-cell lineage, activin A is a cell cycle inhibitor, a mediator of apoptosis, and a cytokine antagonist. Limited information is available on the effects of activin A on normal hemopoietic cells. Recent studies suggest that it might be a negative regulator of normal B lymphopoiesis. Whereas the functions of activin A in vitro are well established, further research tools are needed to elucidate its role within specific hemopoietic microenvironments in vivo.

Activin maintains the condensed type of mitochondria in germ cells

Development of germ cells during spermatogenesis is characterized by a complex series of differentiation events finally leading to the production of spermatozoa. Beside the main hormonal regulators, paracrine interactions are thought to play a major role in this process. Mitochondria in germ cells pass through unique alterations ranging from the 'typical' cristae-rich mitochondria found in spermatogonia to the 'condensed' form in pachytene spermatocytes. This study provides further support that paracrine factors produced by Sertoli cells, most likely activin A, are involved in germ cell differentiation as monitored by the maintenance of the physiological 'condensed' mitochondrial phenotype in primary spermatocytes. Culture of primary spermatocytes in Sertoli cell conditioned medium (SCCM) for 18 h resulted in the maintenance of a high percentage of 'condensed-type' mitochondria in comparison to cells cultured in Dulbecco's minimum essential medium (DMEM). Activin A, a product of Sertoli cells, showed at subnanogram concentrations a similar ability to SCCM to maintain a high percentage of spermatocyte mitochondria in the 'condensed' state, while inhibin had no effect. The addition of an antiserum specific for activin A resulted in a neutralization of the effect caused by activin A or SCCM. This strongly suggested that the active substance in SCCM was activin A. Taken together these data indicate that activin A is the first Sertoli cell product that has been identified to influence differentiation of male meiotic germ cells.

Divergent cell-specific effects of activin-A on thymocyte proliferation stimulated by phytohemagglutinin, and interleukin 1beta or interleukin 6 in vitro

Activin-A is a member of the transforming growth factor-beta (TGF-beta) cytokine family. Based on studies in several cell systems, activin-A has been postulated to be a specific inhibitor of the actions of the inflammatory cytokine, interleukin 6. In cultures of adult rat thymocytes, activin-A inhibited sub-optimal phytohemagglutinin-induced and interleukin 1beta-stimulated proliferation, as measured by [(3)H]-thymidine incorporation in vitro. In contrast with TGF-beta1, which exerted similar inhibitory effects on thymocyte proliferation, activin-A activity was reduced by increasing the concentration of phytohemagglutinin or addition of the reducing agent, beta-mercaptoethanol. Both activin-A and TGF-beta1 inhibited the in vitro production of interleukin 6 by thymocytes in the presence of phytohemagglutinin and interleukin 1beta. In the presence of exogenous interleukin 6, however, both activin-A and TGF-beta1 stimulated thymocyte proliferation. These data suggest that activin-A inhibits thymocyte growth and differentiation, at least in part, by inhibiting endogenous production of interleukin 6, but stimulates thymocyte growth when exogenous interleukin 6 is present in vitro. These data indicate that activin interacts with other cytokines to exert complex regulation of T cell development, and is not an inhibitor of interleukin 6 action in all cell systems.

Uterine milk protein, a novel activin-binding protein, is present in ovine allantoic fluid

Activins are pluripotent growth factors that have recently been shown to be present in placental and fetal membrane preparations. Our previous studies have identified and purified activin A from ovine amniotic and allantoic fluids. In this study, ligand blots of side fractions from the isolation of activin A from allantoic fluid suggested the presence of activin-binding proteins other than follistatin. Further purification of one of these fractions involved two sequential reverse phase HPLC steps and a Superose 12HR fractionation. SDS-PAGE revealed a single protein band of 55 kDa, which was identified by NH2-terminal sequencing as ovine uterine milk protein (UTMP), a member of the serine protease inhibitor (serpin) superfamily of proteins. Further binding studies, using ligand blot techniques and Superose 12HR fractionation in the presence of [125I]activin, demonstrated UTMP to be an activin-binding protein with a lower affinity for activin than that of follistatin. A study of the specific binding behavior of UTMP to activin, using surface plasmon resonance, revealed an apparent equilibrium dissociation constant (Kd) of 49 +/- 25 nM, compared with the follistatin-activin Kd of 379 +/- 51 pM. Similar to another activin-binding protein, alpha2-macroglobulin, UTMP was unable to neutralize the bioactivity of activin in a bioassay based on the capacity of activin to inhibit the proliferation of an MPC-11 plasmacytoma cell line. The high concentrations of this protein in uterine fluid during pregnancy and its ability to bind activin suggest that UTMP may act as a low affinity, high capacity binding protein to sequester activin in the local uterine environment.