Possible involvement of protein kinases and Smad2 signaling pathways on osteoclast differentiation enhanced by activin A

Single-cell transcriptomics of human cholesteatoma identifies an activin A-producing osteoclastogenic fibroblast subset inducing bone destruction

Cholesteatoma, which potentially results from tympanic membrane retraction, is characterized by intractable local bone erosion and subsequent hearing loss and brain abscess formation. However, the pathophysiological mechanisms underlying bone destruction remain elusive. Here, we performed a single-cell RNA sequencing analysis on human cholesteatoma samples and identify a pathogenic fibroblast subset characterized by abundant expression of inhibin βA. We demonstrate that activin A, a homodimer of inhibin βA, promotes osteoclast differentiation. Furthermore, the deletion of inhibin βA /activin A in these fibroblasts results in decreased osteoclast differentiation in a murine model of cholesteatoma. Moreover, follistatin, an antagonist of activin A, reduces osteoclastogenesis and resultant bone erosion in cholesteatoma. Collectively, these findings indicate that unique activin A-producing fibroblasts present in human cholesteatoma tissues are accountable for bone destruction via the induction of local osteoclastogenesis, suggesting a potential therapeutic target.

Deletion of activin A in mesenchymal but not myeloid cells ameliorates disease severity in experimental arthritis

Objective

The aim of this study was to assess the extent and the mechanism by which activin A contributes to progressive joint destruction in experimental arthritis and which activin A-expressing cell type is important for disease progression.

Methods

Levels of activin A in synovial tissues were evaluated by immunohistochemistry, cell-specific expression and secretion by PCR and ELISA, respectively. Osteoclast (OC) formation was assessed by tartrat-resistant acid phosphatase (TRAP) staining and activity by resorption assay. Quantitative assessment of joint inflammation and bone destruction was performed by histological and micro-CT analysis. Immunoblotting was applied for evaluation of signalling pathways.

Results

In this study, we demonstrate that fibroblast-like synoviocytes (FLS) are the main producers of activin A in arthritic joints. Most significantly, we show for the first time that deficiency of activin A in arthritic FLS (ActβA^d/d ColVI-Cre) but not in myeloid cells (ActβA^d/d LysM-Cre) reduces OC development in vitro, indicating that activin A promotes osteoclastogenesis in a paracrine manner. Mechanistically, activin A enhanced OC formation and activity by promoting the interaction of activated Smad2 with NFATc1, the key transcription factor of osteoclastogenesis. Consistently, ActβA^d/d LysM-Cre hTNFtg mice did not show reduced disease severity, whereas deficiency of activin A in ColVI-Cre-expressing cells such as FLS highly diminished joint destruction reflected by less inflammation and less bone destruction.

Conclusions

The results highly suggest that FLS-derived activin A plays a crucial paracrine role in inflammatory joint destruction and may be a promising target for treating inflammatory disorders associated with OC formation and bone destruction like rheumatoid arthritis.

Influence of the TGF-β Superfamily on Osteoclasts/Osteoblasts Balance in Physiological and Pathological Bone Conditions

The balance between bone forming cells (osteoblasts/osteocytes) and bone resorbing cells (osteoclasts) plays a crucial role in tissue homeostasis and bone repair. Several hormones, cytokines, and growth factors-in particular the members of the TGF-β superfamily such as the bone morphogenetic proteins-not only regulate the proliferation, differentiation, and functioning of these cells, but also coordinate the communication between them to ensure an appropriate response. Therefore, this review focuses on TGF-β superfamily and its influence on bone formation and repair, through the regulation of osteoclastogenesis, osteogenic differentiation of stem cells, and osteoblasts/osteoclasts balance. After introducing the main types of bone cells, their differentiation and cooperation during bone remodeling and fracture healing processes are discussed. Then, the TGF-β superfamily, its signaling via canonical and non-canonical pathways, as well as its regulation by Wnt/Notch or microRNAs are described and discussed. Its important role in bone homeostasis, repair, or disease is also highlighted. Finally, the clinical therapeutic uses of members of the TGF-β superfamily and their associated complications are debated.

Activin A Expressed in Rheumatoid Synovial Cells Downregulates TNFα-Induced CXCL10 Expression and Osteoclastogenesis

OBJECTIVE

Activin A is known to be highly expressed in rheumatoid synovium. In the present study, we investigated the effect of inflammatory cytokines on activin A production and its role in rheumatoid inflammation using freshly prepared rheumatoid synovial cells (fresh-RSC).

METHODS

Fresh-RSC from patients with rheumatoid arthritis were obtained and stimulated with multiple cytokines for activin A production. Gene expression levels of activin A and inflammatory cytokines were determined by quantitative PCR (qPCR) analysis. An enzyme-linked immunosorbent assay (ELISA) was used to measure activin A and CXCL10 in culture supernatants. The osteoclasts generated from human peripheral monocytes by RANKL stimulation were identified by tartrate-resistant acid phosphatase staining and bone resorption assay using Osteo plate. The expression levels of NFATc1 and cathepsin K, critical intracellular proteins for osteoclastogenesis, were determined by Western blotting.

RESULTS

Activin A production in fresh-RSC was markedly enhanced by the synergistic effect of TGF-β1 with inflammatory cytokines, including TNFα, IL-1β, and IL-6. Activin A inhibited TNFα-induced CXCL10, an important chemoattractant for pathogen-activated T cells and monocytes of osteoclast precursors, but it did not affect the expression of inflammatory cytokines and chemokines. In addition, activin A directly inhibited the expression of NFATc1 and cathepsin K, as well as osteoclast formation in human samples.

CONCLUSION

Our data indicated that TGF-β1 is involved in the expression of activin A at inflamed joints. Activin A mainly exerts an anti-inflammatory action, which prevents joint damage via the regulation of CXCL10 and osteoclastogenesis.

Skeletal Response to Soluble Activin Receptor Type IIB in Mouse Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder primarily due to mutations in the type I collagen genes (COL1A1 and COL1A2), leading to compromised biomechanical integrity in type I collagen-containing tissues such as bone. Bone is inherently mechanosensitive and thus responds and adapts to external stimuli, such as muscle mass and contractile strength, to alter its mass and shape. Myostatin, a member of the TGF-β superfamily, signals through activin receptor type IIB to negatively regulate muscle fiber growth. Because of the positive impact of myostatin deficiency on bone mass, we utilized a soluble activin receptor type IIB-mFc (sActRIIB-mFc) fusion protein in two molecularly distinct OI mouse models (G610C and oim) and evaluated their bone properties. Wild-type (WT), +/G610C, and oim/oim mice were treated from 2 to 4 months of age with either vehicle (Tris-buffered saline) or sActRIIB-mFc (10 mg/kg). Femurs of sActRIIB-mFc-treated mice exhibited increased trabecular bone volume regardless of genotype, whereas the cortical bone microarchitecture and biomechanical strength were only improved in WT and +/G610C mice. Dynamic histomorphometric analyses suggest the improved cortical bone geometry and biomechanical integrity reflect an anabolic effect due to increased mineral apposition and bone formation rates, whereas static histomorphometric analyses supported sActRIIB-mFc treatment also having an anti-catabolic impact with decreased osteoclast number per bone surface on trabecular bone regardless of sex and genotype. Together, our data suggest that sActRIIB-mFc may provide a new therapeutic direction to improve both bone and muscle properties in OI. © 2018 American Society for Bone and Mineral Research.

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.

Mechanisms involved in enhancement of osteoclast formation by activin-A

Several growth factors in bone tissues are reported to be associated with osteoclastogenesis. Activin-A, a member of the transforming growth factor-β (TGF-β) family is known to be present in bone tissues and an important regulator in osteoclastogenesis with SMAD-mediated signaling being crucial for inducing osteoclast differentiation. In the present study, we examined the effect and underlying mechanisms of activin-A on osteoclast formation in vitro culture systems. Activin-A enhanced osteoclast formation in both mouse bone marrow cells and monocyte/macrophage cell line RAW 264.7 cells induced by receptor activator of nuclear factor kappa B (NF-κB) ligand (RANKL) and/or macrophage stimulating factor (M-CSF). We also found that activin-A stimulated bone resorption and actin ring formation induced by RANKL and/or M-CSF. Furthermore, activin-A enhanced RANKL-induced expression of nuclear factor of activated T cell cytoplasmic 1 (NFATc1), a key regulator of osteoclastogenesis, thereby increasing osteoclastogenesis-related marker gene expression, including tartrate-resistant acid phosphatase, osteoclast stimulatory transmembrane protein, and cathepsin K. Blockage of receptor binding by follistatin, an activing-binding protein suppressed the activin-A-mediated stimulation of NFATc1. In addition, activin-A increased RANKL-induced c-fos expression without significantly affecting the NF-κB and mitogen-activated protein kinase (MAPK) signaling pathway. Pre-treatment of the cells with a specific inhibitor of SMAD2/3 attenuated the activin-A-induced expression of NFATc1 and co-immunoprecipitation assay revealed that treatment with activin-A increased physical interaction of phosphorylated-c-fos and phosphorylated-SMAD2 protein induced by RANKL. These results suggest that activin-A enhances RANKL-induced osteoclast formation mediated by interaction of c-fos and smad2/3.

Smad-dependent mechanisms of inflammatory bone destruction

Homeostatic bone remodelling becomes disturbed in a variety of pathologic conditions that affect the skeleton, including inflammatory diseases. Rheumatoid arthritis is the prototype of an inflammatory arthritis characterised by chronic inflammation, progressive cartilage destruction and focal bone erosions and is a prime example for a disease with disturbed bone homeostasis. The inflammatory milieu favours the recruitment and activation of osteoclasts, which have been found to be the cells that are primarily responsible for bone erosions in many animal models of inflammatory arthritis. Among the inflammatory modulators, members of the transforming growth factor (TGF)-β super family are shown to be important regulators in osteoclastogenesis with Smad-mediated signalling being crucial for inducing osteoclast differentiation. These findings have opened a new field for exploring mechanisms of osteoclast differentiation under inflammatory conditions. Recent studies have shown that the TGF-β superfamily members TGF-β1, myostatin and activin A directly regulate osteoclast differentiation through mechanisms that depend on the RANKL–RANK interplay. These growth factors transduce their signals through type I and II receptor serine/threonine kinases, thereby activating the Smad pathway. In this review, we describe the impact of inflammation-induced Smad signalling in osteoclast development and subsequently bone erosion in rheumatoid arthritis.

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.

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.

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.

Serum follistatin in patients with prostate cancer metastatic to the bone

The clinical significance of circulating follistatin (FLST), an inhibitor of the multifunctional cytokine activin A (Act A), was investigated in patients with prostate cancer (PCa). The serum concentrations of this molecule were determined by an enzyme-linked immunosorbent assay (ELISA) in PCa patients with (M+) or without (M0) bone metastases, in patients with benign prostate hyperplasia (BPH) and in healthy subjects (HS). The effectiveness of FLST in detecting PCa patients with skeletal metastases was determined by the receiver operating characteristic (ROC) curve analysis. Serum FLST was significantly higher in PCa patients than in BPH patients (P = 0.001) or HS (P = 0.011). Conversely, in BPH patients, FLST levels resulted lower than in HS (P = 0.025). In cancer patients the serum concentrations of FLST significantly correlated with the presence of bone metastases (P = 0.0005) or increased PSA levels (P = 0.04). Interestingly, significant differences in the ratio between FLST and Act A serum concentrations (FLST/Act A) were observed between HS and BPH patients (P = 0.001) or PCa patients (P = 0.0005). Finally, ROC curve analysis, highlighted a sound diagnostic performance of FLST in detecting M+ patients (P = 0.0001). However, the diagnostic effectiveness of FLST did not result significantly superior to that of Act A or PSA. These findings suggest that FLST may be regarded as a potential, molecular target in the treatment of metastatic bone disease while its clinical role as soluble marker in the clinical management of PCa patients with bone metastases needs to be better defined.

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.

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.

Activin subunit and receptor expression in normal and cleft human fetal palate tissues

Craniofacial malformations, such as cleft palate, present serious complications in the newborn and are often of unknown etiology. Activin BA subunit deletion leads to cleft palate in mice, but the expression of this protein in the human palate has not been explored. Our goal was to determine the spatial and temporal expression of inhibin/activin subunits; the binding protein, follistatin; and activin receptors in the human fetal palate. Residual human fetal palate tissues, with or without cleft, were collected during routine autopsy at Women and Infants Hospital. Inhibin/activin alpha and beta subunits, follistatin, and activin receptor protein and mRNA expression were studied by immunocytochemistry and reverse-transcriptase polymerase chain reaction (RT-PCR) experiments, respectively. Dimeric activin A levels were compared in cleft and normal palate tissue homogenates by immunoassay. Activin BA, follistatin, and activin receptor type IIA proteins were observed in normal and cleft palate tissues throughout pregnancy (gestational weeks 11 to 40). Proteins were predominantly found in developing bone cells, with no significant group differences. Inhibin/activin BA subunit, follistatin, and activin receptor mRNAs were also detected in normal and cleft fetal palate tissues, but inhibin alpha and BB subunit were absent. Inhibin/activin BA subunit expression was consistent with the presence of dimeric activin A, but levels did not differ significantly between cleft and control tissues. Inhibin/activin BA subunit, follistatin, and activin receptor proteins and mRNAs are present in the human fetal palate. These data suggest that activin signalling has the potential to be associated with human palate development.

Activin A is a critical component of the inflammatory response, and its binding protein, follistatin, reduces mortality in endotoxemia

Activin A is a member of the transforming growth factor-beta superfamily, which we have identified as having a role in inflammatory responses. We show that circulating levels of activin increase rapidly after LPS-induced challenge through activation of Toll-like receptor 4 and the key adaptor protein, MyD88. Treatment with the activin-binding protein, follistatin, alters the profiles of TNF, IL-1beta, and IL-6 after LPS stimulation, indicating that activin modulates the release of several key proinflammatory cytokines. Further, mice administered one 10-mug dose of follistatin to block activin effects have increased survival after a lethal dose of LPS, and the circulating levels of activin correlate with survival outcome. These findings demonstrate activin A's crucial role in the inflammatory response and show that blocking its actions by the use of follistatin has significant therapeutic potential to reduce the severity of inflammatory diseases.

Effects of tacrolimus, cyclosporin A and sirolimus on MG63 cells

The reduction in bone mineral density after organ transplantation results in increased morbidity (post-transplantation bone disease) and remains an unsolved problem. A connection with the long-term application of nonglucocorticoidal immunosuppressants is the subject of controversial discussion. We hypothesized that such substances have an influence on the skeletal system on the cellular level by modulating osteoblast differentiation. Therefore, we investigated the effects of tacrolimus, cyclosporin A and sirolimus as representative substances of nonglucocorticoidal immunosuppressants on cell proliferation and expression of bone tissue-specific genes of human osteoblasts (MG63). None of the examined substances affected cell proliferation, but all influenced the gene expression pattern towards change in cell differentiation. In detail, collagen III and XII, matrix metalloproteinase 2, SMAD2, epithelial growth factor receptor, annexin V and osteonectin expression were increased by all of the examined substances. Tacrolimus, cyclosporin A and sirolimus influence intracellular signalling pathways, transmembranous receptors and bone-specific matrix synthesis. They do not have antiproliferative or toxic effects. We postulate that the shown changes of osteoblast differentiation cause post-transplantation disease.

Ameloblastoma induces osteoclastogenesis: a possible role of ameloblastoma in expanding in the bone

Ameloblastoma, a tumor located in bone, when neglected, can perforate the bone and, ultimately, spread into the soft tissues. To expand in the bone, ameloblastoma must have a mechanism of resorbing the surrounding bone. However, the mechanism for bone resorption is poorly understood. In the present study, we found that RANKL and TNFalpha were expressed and secreted by ameloblastoma cells, and was proven to induce osteoclastogenesis. Our present results also showed that phosphorylation of p38, SAPK, p44/42 and Akt were upregulated under treatment of 10xCM (concentrated conditioned media of AM-1 cells). We also noticed formation of resorption lacunae on dentin slice by 10xCM-induced osteoclast-like MNCs. These results suggested that ameloblastoma by secreting RANKL and TNFalpha could induce osteoclastogenesis.

Activin A stimulates IkappaB-alpha/NFkappaB and RANK expression for osteoclast differentiation, but not AKT survival pathway in osteoclast precursors

Recent studies have reported that activin A enhances osteoclastogenesis in cultures of mouse bone marrow cells stimulated with receptor activator of nuclear factor-kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). However, the exact mechanisms by which activin A functions during osteoclastogenesis are not clear. RANKL stimulation of RANK/TRAF6 signaling increases nuclear factor-kappaB (NFkappaB) nuclear translocation and activates the Akt/PKB cell survival pathway. Here we report that activin A alone activates IkappaB-alpha, and stimulates nuclear translocation of NFkappaB and receptor activator of nuclear factor-kappaB (RANK) expression for osteoclastogenesis, but not Akt/PKB survival signal transduction including BAD and mammalian target of rapamycin (mTOR) for survival in osteoclast precursors in vitro. Activin A alone failed to activate Akt, BAD, and mTOR by immunoblotting, and it also failed to prevent apoptosis in osteoclast precursors. While activin A activated IkappaB-alpha and induced nuclear translocation of phosphorylated-NFkappaB, and it also enhanced RANK expression in osteoclast precursors. Moreover, activin A enhanced RANKL- and M-CSF-stimulated nuclear translocation of NFkappaB. Our data suggest that activin A enhances osteoclastogenesis treated with RANKL and M-CSF via stimulation of RANK, thereby increasing the RANKL stimulation. Activin A alone activated the NFkappaB pathway, but not survival in osteoclast precursors in vitro, but it is, thus, insufficient as a sole stimulus to osteoclastogenesis.

Transcriptional profiling of human osteoblast differentiation

Osteoblast differentiation is a key aspect of bone formation and remodeling. To further our understanding of the differentiation process, we have developed a collection of conditionally immortalized adult human osteoblast cell lines representing discrete stages of differentiation. To evaluate changes in gene expression associated with differentiation, polyA((+)) RNA from pre-osteoblasts, early and late osteoblasts, and pre-osteocytes was subjected to gene chip analysis using the Affymetrix Hu6800 chip in conjunction with an Affymetrix custom chip enriched in bone and cartilage cDNAs. Overall, the expression of 47 genes was found to change threefold or more on both chips between the pre-osteoblastic and pre-osteocytic stages of differentiation. Many of the observed differences, including down-regulation of collagen type I and collagen-processing enzymes, reflect expected patterns and support the relevance of our results. Other changes have not been reported and offer new insight into the osteoblast differentiation process. Thus, we observed regulation of factors controlling cell cycle and proliferation, reflecting decreased proliferation, and increased apoptosis in pre-osteocytic cells. Elements maintaining the cytoskeleton, extracellular matrix, and cell-cell adhesion also changed with differentiation reflecting profound alterations in cell architecture associated with the differentiation process. We also saw dramatic down-regulation of several components of complement and other immune response factors that may be involved in recruitment and differentiation of osteoclasts. The decrease in this group of genes may provide a mechanism for controlling bone remodeling of newly formed bone. Our screen also identified several signaling proteins that may control osteoblast differentiation. These include an orphan nuclear receptor DAX1 and a small ras-related GTPase associated with diabetes, both of which increased with increasing differentiation, as well as a high mobility group-box transcription factor, SOX4, that was down-regulated during differentiation. In summary, our study provides a comprehensive transcriptional profile of human osteoblast differentiation and identifies several genes of potential importance in controlling differentiation of osteoblasts.