Purification of megakaryocyte differentiation activity from a human fibrous histiocytoma cell line: N-terminal sequence homology with activin A

Systemic Activation of Activin A Signaling Causes Chronic Kidney Disease-Mineral Bone Disorder

The high cardiovascular mortality associated with chronic kidney disease (CKD) is caused in part by the CKD-mineral bone disorder (CKD-MBD) syndrome. The CKD-MBD consists of skeletal, vascular and cardiac pathology caused by metabolic derangements produced by kidney disease. The prevalence of osteopenia/osteoporosis resulting from the skeletal component of the CKD-MBD, renal osteodystrophy (ROD), in patients with CKD exceeds that of the general population and is a major public health concern. That CKD is associated with compromised bone health is widely accepted, yet the mechanisms underlying impaired bone metabolism in CKD are not fully understood. Therefore, clarification of the molecular mechanisms by which CKD produces ROD is of crucial significance. We have shown that activin A, a member of the transforming growth factor (TGF)-β super family, is an important positive regulator of receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis with Smad-mediated signaling being crucial for inducing osteoclast development and function. Recently, we have demonstrated systemic activation of activin receptors and activin A levels in CKD mouse models, such as diabetic CKD and Alport (AL) syndrome. In these CKD mouse models, bone remodeling caused by increased osteoclast numbers and activated osteoclastic bone resorption was observed and treatment with an activin receptor ligand trap repaired CKD-induced-osteoclastic bone resorption and stimulated individual osteoblastic bone formation, irrespective of parathyroid hormone (PTH) elevation. These findings have opened a new field for exploring mechanisms of activin A-enhanced osteoclast formation and function in CKD. Activin A appears to be a strong candidate for CKD-induced high-turnover ROD. Therefore, the treatment with the decoy receptor for activin A might be a good candidate for treatment for CKD-induced osteopenia or osteoporosis, indicating that the new findings from in these studies will lead to the identification of novel therapeutic targets for CKD-related and osteopenia and osteoporosis in general. In this review, we describe the impact of CKD-induced Smad signaling in osteoclasts, osteoblasts and vascular cells in CKD.

Activin A inhibits RANKL-mediated osteoclast formation, movement and function in murine bone marrow macrophage cultures

The process of osteoclastic bone resorption is complex and regulated at multiple levels. The role of osteoclast (OCL) fusion and motility in bone resorption are unclear, with the movement of OCL on bone largely unexplored. RANKL (also known as TNFSF11) is a potent stimulator of murine osteoclastogenesis, and activin A (ActA) enhances that stimulation in whole bone marrow. ActA treatment does not induce osteoclastogenesis in stroma-free murine bone marrow macrophage cultures (BMM), but rather inhibits RANKL-induced osteoclastogenesis. We hypothesized that ActA and RANKL differentially regulate osteoclastogenesis by modulating OCL precursors and mature OCL migration. Time-lapse video microscopy measured ActA and RANKL effects on BMM and OCL motility and function. ActA completely inhibited RANKL-stimulated OCL motility, differentiation and bone resorption, through a mechanism mediated by ActA-dependent changes in SMAD2, AKT1 and inhibitor of nuclear factor κB (IκB) signaling. The potent and dominant inhibitory effect of ActA was associated with decreased OCL lifespan because ActA significantly increased activated caspase-3 in mature OCL and OCL precursors. Collectively, these data demonstrate a dual action for ActA on murine OCLs.

Regulation of osteoblastogenesis and osteoclastogenesis by the other reproductive hormones, Activin and Inhibin

There is both cellular and physiological evidence demonstrating that both Activins and Inhibins regulate osteoblastogenesis and osteoclastogenesis, and regulate bone mass in vivo. Although Activins and Inhibins were initially isolated from the gonad, Activins are also produced and stored in bone, whereas Inhibins exert their regulation on bone cell differentiation and metabolism via endocrine effects. The accumulating data provide evidence that reproductive hormones, distinct from classical sex steroids, are important regulators of bone mass and bone strength. Given the well described dominant antagonism of Inhibin over Activin, as well as over BMPs and TGFbeta, the gonadally derived Inhibins are important regulators of locally produced osteotrophic factors. Thus, the cycling Inhibins in females and diurnal changes in Inhibin B in males elicit temporal shifts in Inhibin levels (tone) that de-repress the pituitary. This fundamental action has the potential to de-repress locally stimulated changes in osteoblastogenesis and osteoclastogenesis, thereby altering bone metabolism.

Inhibin A is an endocrine stimulator of bone mass and strength

Gonadal function plays a major role in bone homeostasis. It is widely held that the skeletal consequences of hypogonadism are solely due to a loss of sex steroids; however, increases in bone turnover begin during perimenopause before decreases in serum estradiol levels. These data and our demonstration that inhibins acutely regulate bone cell differentiation in vitro led us to test whether inhibin A (InhA) regulates bone mass in vivo. Using a transgenic model of inducible human InhA expression, InhA increased total body bone mineral density, increased bone volume, and improved biomechanical properties at the proximal tibia in intact mice and also prevented the loss of BMD and bone volume and strength associated with gonadectomy at both the spine and proximal tibia. In addition, InhA increased mineral apposition rate, double-labeled surface, and serum osteocalcin levels in vivo and osteoblastogenesis ex vivo without affecting osteoclast number or activity. Together these results demonstrate novel stimulatory effects of InhA on the skeleton in vivo. These studies provide in vivo evidence demonstrating that gonadal factors other than sex steroids play an important role in regulating bone mass and strength and, combined with our previous clinical data, suggest that gonadal InhA may be a component of the normal endocrine repertoire that regulates bone quality in both the axial and appendicular skeleton.

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.

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.

Inhibin suppresses and activin stimulates osteoblastogenesis and osteoclastogenesis in murine bone marrow cultures

Using primary murine bone marrow cell cultures, we demonstrate that inhibin suppresses osteoblastogenesis and osteoclastogenesis. In contrast, activin supports osteoblast formation (by alkaline phosphatase-positive and mineralized colony formation); and activin also stimulates osteoclast formation (as measured by staining tartrate-resistant acid phosphatase-positive multinucleated cells). Inhibin, the activin antagonist follistatin, and the bone morphogenetic protein antagonist noggin can all suppress endogenous activin accumulation in bone marrow cultures. Associated with this decrease in activin is the loss of mineralized osteoblastic colony formation (colony forming unit-osteoblast; CFU-OB). However, exogenous activin administration, even in the presence of noggin, permits both alkaline phosphatase-positive and CFU-OB colony formation in vitro. In contrast, the stimulatory effects of locally produced activin on osteoblast and osteoclast development are not likely to be dominant over the suppressive effects of gonadally derived inhibin. The suppressive effect of inhibin is maintained in the presence of either activin or bone morphogenetic protein, suggesting the presence of a distinct inhibin-specific receptor. Taken together, the direct regulation of osteoblastogenesis and osteoclastogenesis by inhibin and activin in vitro suggest that changes in the inhibin/activin ratio detected by bone marrow cells, during the perimenopausal transition, contribute to altered cell differentiation and may be associated with the increased bone resorption observed at this time.

Genes coding for mouse activin beta C and beta E are closely linked and exhibit a liver-specific expression pattern in adult tissues

Five activin beta subunits have been isolated to date, and a comparative analysis of amino acid identity has suggested that the activin beta C, beta D and beta E subunits represent a distinct subset. Based on genomic cloning studies, we now report that the mouse activin beta C and beta E genetic loci are closely linked-i.e, the coding sequences are separated by 5.5-kbp. These genes also show similarities in structural organization as well as a unique liver-restricted pattern of expression in adult mice. Our results suggest that tandem duplication of an ancestral gene generated the mouse activin beta C and beta E genetic loci, and they provide further evidence for the postulate that the beta C-beta E subunits form a distinct subset of related activins. To our knowledge, this report is the first to demonstrate close chromosomal linkage between members of the TGF-beta superfamily as well as a liver-restricted expression pattern for a TGF-beta-like gene.

Expression and localization of inhibin/activin subunits and activin receptors in MCF-7 cells, a human breast cancer cell line

Inhibins and activins are members of the transforming growth factor-beta (TGF-beta) superfamily. Since TGF beta has been shown to be a potent proliferation-inhibiting agent for the breast cancer cell line MCF-7, we determined whether this cell line (a) transcribes messenger RNAs coding inhibin/activin alpha-, beta A-, and beta B-subunits and activin receptors, and (b) produces inhibin and/or activin proteins. Messenger RNAs for alpha- and beta-subunits of inhibin/activin and activin receptor II in MCF-7 cells were detected and localized using the reverse transcription-polymerase chain reaction (RT-PCR) analysis and in situ hybridization, respectively. The identity of the RT-PCR products was confirmed by DNA sequencing of PCR products. Immunocytochemically, inhibin and activin were localized in these cells. Our findings that messenger RNAs encoding inhibin alpha-subunit, inhibin/activin beta A-subunit, and activin receptor II were expressed, and inhibin/activin proteins were produced by MCF-7 cells, imply that these gonadal growth factors may have paracrine/autocrine functions in human breast cancer. Further, these observations suggest that these growth factors may be involved in regulating the growth and differentiation of human breast cancer cells.

Expression and localization of inhibin alpha-subunit in rat retinal photoreceptor cells

To determine whether rat retinal photoreceptor cells produce inhibin, a molecule closely related to activin, a multifunctional growth factor in the transforming growth factor beta superfamily (TGF beta), we have conducted immunohistochemistry using specific antibodies for inhibin which were raised against a synthetic N-terminal fragment of the alpha-subunit of inhibin. The mature inhibin molecule was identified at both the inner and outer segments of photoreceptor cells. To determine if messenger RNA for the alpha-subunit of inhibin is expressed in the retinal cells, both in situ hybridization with a specific probe and the reverse transcription-polymerase chain reaction (RT-PCR) technique with primers specific for the alpha-subunit of inhibin were used. Messenger RNA expression of the alpha-subunit of inhibin was detected by RT-PCR and localized in the photoreceptor cells as determined by in situ hybridization. In addition, the identity of the cDNA product of RT-PCR was verified with Southern analysis and DNA sequencing. The localization of mature inhibin protein and its corresponding message to photoreceptor cells suggest that inhibin may have a paracrine function in the retina, perhaps in the photoreceptor cells themselves.

Detection of functional and dimeric activin A in human marrow microenvironment. Implications for the modulation of erythropoiesis

Activin A, which was initially recognized as a gonadal protein, was implicated in the modulation of erythropoiesis through a paracrine control in the bone marrow microenvironment. Present studies demonstrate that, in contrast to T lymphocytes and cultured skin fibroblasts, human marrow stromal cells produce a functional and dimeric beta A beta A molecule (i.e., activin A). RT-PCR further indicates that both alpha and beta A mRNAs of inhibin A/activin A are produced in human stromal cells. The level of beta A subunit mRNAs, however, is in large excess over that of alpha subunit mRNAs, suggesting the predominant production of beta A beta A dimers, as well as some inhibin A (alpha beta A). It should be noted, however, that the beta A subunit can form dimeric proteins other than activin A, such as activin AB (beta A beta B) and inhibin A (alpha beta A). Hence, the presence of the beta A subunit may not necessarily indicate the production of the activin A molecule in any tissue. Therefore, a special quantitative sandwich ELISA assay specific for the dimeric beta A beta A molecule was developed for the measurement of activin A. With this assay, production of activin A in marrow stromal cells is found to be greatly enhanced by cytokines and inflammatory mediators such as TNF-alpha, IL-1 alpha, and lipopolysaccharide. These studies thus suggest that inflammatory cytokines are the inducers for activin A, probably serving a role of up-regulating activin A production locally in bone marrow microenvironment. At present, activin A is not known to play any role in inflammatory reaction; this study may thus raise the possibility that activin A performs more functions than are currently recognized. Alternatively, the enhanced production of this molecule in the bone marrow microenvironment may be regarded as a compensatory mechanism in host defenses, countering inflammatory mediators that are known to suppress erythropoiesis.

Autoinduction of activin genes in early Xenopus embryos

Activin exhibits a potent mesoderm inducing activity towards the ectodermal tissue (animal cap) of Xenopus laevis blastulae. Thus in order to investigate the role of activin in morphogenesis of early Xenopus embryos, activation of genes for activin beta A and beta B was examined by the reverse transcription polymerase chain reaction. In vivo, activin beta B mRNA appears to be present in embryonic stage 1 whereas beta A mRNA is undetectable prior to gastrulation. beta B and beta A mRNAs were noted to accumulate after stages 9 and 15 respectively. Activin gene expression in Xenopus animal caps was examined after treatment with various concentrations of activin A. Under these treatment conditions, both activin beta A and beta B mRNAs accumulated in a dose-dependent fashion after 24 h. The same effect was noted for treatment with similar concentrations of activin B. Accumulation of mRNAs was inhibited by the addition of cycloheximide to the culture medium, consistent with the proposition that activin gene expression requires certain protein factors. In total, therefore, these data suggest that an autoinduction mechanism is involved in the regulation of activin mRNA levels in normal Xenopus embryos and that this mechanism may play a pivotal role during early embryonic development.

Tumor necrosis factor and interleukin-1 induce activin A gene expression in a human bone marrow stromal cell line

Activin A, a homodimer of the beta A chain, regulates hematopoiesis. In a human bone marrow-derived stromal cell line, KM-102, phorbol myristate acetate, tumor necrosis factor-alpha and interleukin-1 beta induced great increases in beta A chain mRNA levels and production of activin A activities. The phorbol ester-induced beta A chain gene expression was inhibited by cycloheximide and down regulation of protein kinase C, whereas the cytokine-induced expression was little affected by these treatments. These results indicate that the inflammatory cytokines directly stimulate beta A chain gene expression via protein kinase C-independent pathways.

Cloning of the human activin receptor cDNA reveals high evolutionary conservation

The entire coding region of the human activin receptor was obtained from a human testis cDNA library. Analysis of the 1539 nucleotide (513 amino acid) sequence of the receptor reveals that there are only 83 nucleotide differences compared to the coding sequence of the mouse activin receptor. Similar to its ligands, the amino acid sequence of the activin receptor is highly conserved with only two conservative amino acid differences (Lys-39 and Val-92 in human versus Arg-39 and Ile-92 in the mouse). This high degree of conservation of the activin receptor illustrates a strong evolutionary selection and confirms that activin and its receptor play an important role in development.

Effect of erythroid differentiation factor on megakaryocytic differentiation of L8057, a murine megakaryoblastic leukemia cell line

To assess the potent effect of erythroid differentiation factor (EDF) on megakaryocytopoiesis, effect of EDF on megakaryocytic differentiation of L8057, a murine megakaryoblastic cell line, was examined. EDF potentiated AchE induction of L8057 in a dose dependent manner. The potency of EDF on megakaryocytic differentiation is comparable to that on erythroid differentiation reported previously. The present results suggest that EDF may play a regulatory role in megakaryocytopoiesis as well as in erythropoiesis.