Effects of secreted frizzled-related protein 3 on osteoblasts in vitro

The role of dexamethasone in mediating the contradictory effects of Wnt antagonists SFRP2 and SFRP3 on human hair follicle growth

Stress can be one of the leading causes of hair loss. Stress related hormones, glucocorticoids (GCs), secretion by hair follicle have been mentioned in literature and proven to exert an inhibitory effect on hair follicle cells growth by modulating the expression of target genes related to cell proliferation and cycling. The gene modulating effect of the synthetic GC, dexamethasone (DEX), in human dermal papilla (DP) cells has been outlined in this study by mediating a contradictory effect on the expression of secreted frizzled related protein 2 (SFRP2) and SFRP3. The SFRP2 and SFRP3 possess a regulating effect on wnt signaling pathway. Their structural similarities to the cysteine-rich-domain of the frizzled receptors (FZD) allow their binding to the wnt ligands causing the blocking of the wnt ligands-receptors complex. The SFRP family members have been known as inhibitors of the wnt signaling modulating the proliferation and development of various cells. In hair follicle cells, SFRP2 activity has been reported positively on the proliferation of keratinocytes. However, the SFRP3 effect hasn't been well addressed. Under stress, the investigation of the mRNA and protein expressions of SFRP members in human DP cells revealed opposite expressions where SFRP2 decreased while SFRP3 increased by DEX. The proliferation rate of hair keratinocytes outer root sheath was detected via immunofluorescence highlighting the stimulatory effect of SFRP2 and the inhibitory effect of SFRP3. Here, we sought to determine the effect of GC agonist on SFRPs expression and their effect on hair follicle growth.

Wnt Pathway Extracellular Components and Their Essential Roles in Bone Homeostasis

The Wnt pathway is involved in several processes essential for bone development and homeostasis. For proper functioning, the Wnt pathway is tightly regulated by numerous extracellular elements that act by both activating and inhibiting the pathway at different moments. This review aims to describe, summarize and update the findings regarding the extracellular modulators of the Wnt pathway, including co-receptors, ligands and inhibitors, in relation to bone homeostasis, with an emphasis on the animal models generated, the diseases associated with each gene and the bone processes in which each member is involved. The precise knowledge of all these elements will help us to identify possible targets that can be used as a therapeutic target for the treatment of bone diseases such as osteoporosis.

Wnt3a and ASCs are capable of restoring mineralization in staph aureus-infected primary murine osteoblasts

INTRODUCTION

Bone infections are one of the main reasons for impaired bone regeneration and non-union formation. In previous experimental animal studies we could already demonstrate that bone defects due to prior infections showed a markedly reduced healing capacity, which could effectively be enhanced via application of Wnt3a and Adipose-derived stromal cells (ASCs). For a more in-depth analysis, we investigated proliferation and mineralization of cultured osteoblasts infected with staph aureus and sought to investigate effects of Wnt3a and ASCs on infected osteoblasts.

MATERIALS AND METHODS

Primary murine osteoblasts were isolated from calvariae and infected with staph aureus. Infected osteoblasts received treatment via application of recombinant Wnt3a, ASC conditioned medium and were furthermore cocultured with ASCs. Osteoblasts were evaluated by Alamar blue assay for metabolic activity, TUNEL-assay for apoptosis, ALP and Alizarin Red staining for mineralization. In addition, immunoflourescent staining (IF) and qRT-PCR analyses were performed.

RESULTS

Infected osteoblasts showed a markedly reduced ability for mineralization and increased apoptosis, which could be restored to physiological levels by Wnt3a and ASC treatment. Interestingly, metabolic activity of osteoblasts seemed to be unaffected by staph aureus infection. Additional analyses of Wnt-pathway activity revealed effective enhancement of canonical Wnt-pathway activity in Wnt3a-treated osteoblasts.

CONCLUSIONS

In summary, we gained further osteoblast-related insights into pathomechanisms of reduced bone healing capacity upon infections.

Regulation of the mesenchymal stem cell fate by interleukin-17: Implications in osteogenic differentiation

Bone regeneration is a tightly regulated process that ensures proper repair and functionality after injury. The delicate balance between bone formation and resorption is governed by cytokines and signaling molecules released during the inflammatory response. Interleukin (IL)-17A, produced in the early phase of inflammation, influences the fate of osteoprogenitors. Due to their inherent capacity to differentiate into osteoblasts, mesenchymal stem/stromal cells (MSCs) contribute to bone healing and regeneration. This review presents an overview of IL-17A signaling and the leading cellular and molecular mechanisms by which it regulates the osteogenic differentiation of MSCs. The main findings demonstrating IL-17A’s influence on osteoblastogenesis are described. To this end, divergent information exists about the capacity of IL-17A to regulate MSCs’ osteogenic fate, depending on the tissue context and target cell type, along with contradictory findings in the same cell types. Therefore, we summarize the data showing both the pro-osteogenic and anti-osteogenic roles of IL-17, which may help in the understanding of IL-17A function in bone repair and regeneration.

Dehydrated human amniotic membrane modulates canonical Wnt signaling in multiple cell types in vitro

Canonical Wnt signaling is a major pathway known to regulate diverse physiological processes in multicellular organisms. Signaling is tightly regulated by feedback mechanisms; however, persistent dysregulation of this pathway is implicated in the progression of multiple disease states. In this study, proteomic analysis identified endogenous Wnt antagonists in micronized dehydrated human amnion/chorion membrane (μdHACM); thereby, prompting a study to further characterize the intrinsic properties of μdHACM as it relates to Wnt activity, in vitro. A TCF/LEF reporter cell line demonstrated the general ability of μdHACM to inhibit β-catenin induced transcription activity. Furthermore, in vitro systems, modeling elevated Wnt signaling, were developed in relevant cell types including tenocytes, synoviocytes, and human dermal fibroblasts (HDFs). Stimulation of these cells with Wnt3A resulted in translocation of β-catenin to the nucleus and increased expression of Wnt related genes. The subsequent addition of μdHACM, in the continued presence of Wnt-stimulus, mitigated the downstream effects of Wnt3A in tenocytes, synoviocytes, and HDFs. Nuclear localization of β-catenin was abated with corresponding reduction of Wnt related gene expression. These data demonstrate the in vitro regulation of canonical Wnt signaling as an inherent property of μdHACM and a novel mechanism of action.

1,25(OH)2D3 ameliorates palmitate-induced lipotoxicity in human primary osteoblasts leading to improved viability and function

Contributing to bone loss with aging is a progressive reduction in osteoblast number and function leading to decreased bone formation. In aging bone, mesenchymal stem cells decrease in number and their differentiation potential into osteoblasts is reduced. Instead, there is a shift towards adipogenic differentiation and increased lipid accumulation in the marrow of osteoporotic bones. Bone marrow adipocytes produce palmitic acid (PA), a saturated fatty acid, which is toxic to osteoblasts in vitro. Vitamin D (1,25(OH)₂D₃) stimulates osteoblastogenesis and has known anti-apoptotic effects on osteoblasts, as such it may protect human primary osteoblasts from PA-induced lipotoxicity. Here, the effects of PA (250 μM) or 1,25(OH)₂D₃ (10^-8 M), alone or in combination, on osteoblast differentiation and mineralization, viability and autophagy were investigated. In PA-treated osteoblasts, 1,25(OH)₂D₃ ameliorated the decrease in the mRNA transcript abundance of representative palmitoylation (ZDHHC1, ZDHHC2 and ZDHHC12) and osteogenic (alkaline phosphatase and osteocalcin) genes. Collectively these gene regulate signaling pathways pertinent to osteoblastogenesis. In osteoblasts treated with PA and 1,25(OH)₂D₃, the capacity to undergo differentiation and mineralization was recovered and cell viability was increased when compared to osteoblasts treated with PA alone. 1,25(OH)₂D₃, irrespective of PA treatment, increased the expression of key osteogenic signaling proteins; specifically, SMAD1-3,5, Runx2 and β-catenin. 1,25(OH)₂D₃ also attenuated the high level of impaired autophagy induced by PA and potentiated a shift towards activated, functional autophagy and increased flux through autolysosomes. Altogether, these findings provide in vitro evidence regarding the potential of 1,25(OH)₂D₃ to protect osteoblasts from lipotoxicity by modulating autophagy and facilitating cell differentiation, which may enhance bone formation in an osteoporotic microenvironment with a high level of marrow adipose tissue.

Expression profiles of the Wnt/β-catenin signaling-related extracellular antagonists during proliferation and differentiation in human osteoblast-like cells

Bone formation is a dynamic process directed by osteoblast activity. The transition from the proliferation to differentiation stage during osteoblast maturation involves the downregulation of the Wnt/β-catenin signaling pathway, and extracellular antagonists are important for the regulation of Wnt signaling. However, the expression levels of Wnt antagonists in these stages of human osteoblast maturation have not been fully elucidated. Therefore, the aim of the present study was to investigate the expression levels of extracellular Wnt antagonists during proliferation and differentiation in osteoblast-like cell lines. The results demonstrated an overlap between the differential expression of secreted Frizzled-related protein (SFPR)2, SFRP3, SFRP4 and Dickkopf (DKK) 2 genes during the differentiation stage in the MG-63 and Saos-2 cells. Furthermore, high expression levels of DKK3 in MG-63 cells, Wnt inhibitory factor 1 (WIF1) in Saos-2 cells and DKK4 in hFOB 1.19 cells during the same stage (differentiation), were observed. The upregulated expression levels of Wnt antagonists were also correlated with the high expression of anxin 2 during the differentiation stage. These findings suggested that Wnt-related antagonists could modulate the Wnt/β-catenin signaling pathway. By contrast, DKK1 was the only gene that was found to be upregulated during the proliferation stage in hFOB 1.19 and Saos-2 cells. To the best of our knowledge, the present study provides, for the first time, the expression profile of Wnt antagonists during the proliferation stage and the initial phases of differentiation in osteoblast-like cell lines. The current results offer a basis to investigate potential targets for bone-related Wnt-signaling modulation in bone metabolism research.

Effect of puerarin on osteogenic differentiation of human periodontal ligament stem cells

Objective

To investigate the effects of the flavonoid, puerarin, on osteogenic differentiation of human periodontal ligament stem cells (PDLSCs).

Methods

Human PDLSCs were isolated from patients undergoing orthodontic treatment, and the cell surface markers CD146, CD34, CD45, and STRO-1 were identified by immunofluorescence. Cell proliferation was detected by MTT assay; alkaline phosphatase (ALP) activity was measured, and calcium deposition was detected by alizarin red staining. PCR was then used to detect the distributions of COL-I, OPN, Runx2, and OCN, genes related to osteogenic differentiation.

Results

Staining was positive for cytokines CD146, CD34, CD45, and STRO-1 in the experimental group; staining was also positive for silk protein, but negative for keratin. After 7 days of culture, exposure to puerarin significantly promoted the level of intracellular ALP; increased puerarin concentration led to increased intracellular ALP. Red mineralized nodules appeared upon exposure to puerarin and the number of nodules was concentration-dependent. PCR analysis revealed that COL-I, OPN, Runx2, and OCN expression levels increased as puerarin concentration increased.

Conclusions

Exposure to puerarin can promote proliferation and ALP activity in human PDLSCs, thus promoting both molecular and osteogenic differentiation; these findings may provide a theoretical basis for the clinical treatment of periodontal disease with puerarin.

The effects of mechanically loaded osteocytes and inflammation on bone remodeling in a bisphosphonate-induced environment

Bisphosphonate-related osteonecrosis of the jaw is a disease appearing after tooth removal in patients undergoing bisphosphonate treatment for metastasizing cancers and osteoporosis. The complexity of the condition requires a multicellular model to address the net effects of two key risk factors: mechanical trauma (pathologic overload) and inflammation. In this work, a system comprised of a polydimethylsiloxane chip and mechanical loading device is used to expose bisphosphonate-treated osteocytes to mechanical trauma. Specifically, osteocytes are treated with the potent nitrogen-containing bisphosphonate, zoledronic acid, and exposed to short-term pathologic overload via substrate stretch. During bone remodeling, osteocyte apoptosis plays a role in attracting pre-osteoclasts to sites of damage; as such, lactate dehydrogenase activity, cell death and protein expression are evaluated as functions of load. Additionally, the effects of osteocyte soluble factors on osteoclast and osteoblast functional activity are quantified. Osteoclast activity and bone resorption are quantified in the presence and absence of inflammatory components, lipopolysaccharide and interferon gamma. Results suggest that inflammation associated with bacterial infection may hinder bone resorption by osteoclasts. In addition, osteocytes may respond to overload by altering expression of soluble signals that act on osteoblasts to attenuate bone formation. These findings give insight into the multicellular interactions implicated in bisphosphonate-related osteonecrosis of the jaw.

Ectodermal-Neural Cortex 1 Isoforms Have Contrasting Effects on MC3T3-E1 Osteoblast Mineralization and Gene Expression

The importance of Wnt pathway signaling in development of bone has been well established. Here we investigated the role of a known Wnt target, ENC1 (ectodermal-neural cortex 1; NRP/B), in osteoblast differentiation. Enc1 expression was detected in mouse osteoblasts, chondrocytes, and osteocytes by in situ hybridization, and osteoblastic expression was verified in differentiating primary cultures and MC3T3-E1 pre-osteoblast cells, with 57 kDa and 67 kDa ENC1 protein isoforms detected throughout differentiation. Induced knockdown of both ENC1 isoforms reduced alkaline phosphatase staining and virtually abolished MC3T3-E1 mineralization. At culture confluence, Alpl (alkaline phosphatase liver/bone/kidney) expression was markedly reduced compared with control cells, and there was significant and coordinated alteration of other genes involved in cellular phosphate biochemistry. In contrast, with 67 kDa-selective knockdown mineralized nodule formation was enhanced and there was a two-fold increase in Alpl expression at confluence. There was enhanced expression of Wnt/β-catenin target genes with knockdown of both isoforms at this time-point and a five-fold increase in Frzb (Frizzled related protein) with 67 kDa-selective knockdown at mineralization, indicating possible ENC1 interactions with Wnt signaling in osteoblasts. These results are the first to demonstrate a role for ENC1 in the control of osteoblast differentiation. Additionally, the contrasting mineralization phenotypes and transcriptional patterns seen with coordinate knockdown of both ENC1 isoforms vs selective knockdown of 67 kDa ENC1 suggest opposing roles for the isoforms in regulation of osteoblastic differentiation, through effects on Alpl expression and phosphate cellular biochemistry. This study is the first to report differential roles for the ENC1 isoforms in any cell lineage. J. Cell. Biochem. 118: 2141-2150, 2017. © 2016 Wiley Periodicals, Inc.

IL-17A deficiency promotes periosteal bone formation in a model of inflammatory arthritis

BACKGROUND

Interleukin-17A (IL-17A) plays a pathogenic role in several rheumatic diseases including spondyloarthritis and, paradoxically, has been described to both promote and protect from bone formation. We therefore examined the effects of IL-17A on osteoblast differentiation in vitro and on periosteal bone formation in an in vivo model of inflammatory arthritis.

METHODS

K/BxN serum transfer arthritis was induced in IL-17A-deficient and wild-type mice. Clinical and histologic inflammation was assessed and periosteal bone formation was quantitated. Murine calvarial osteoblasts were differentiated in the continuous presence of IL-17A with or without blockade of secreted frizzled related protein (sFRP)1 and effects on differentiation were determined by qRT-PCR and mineralization assays. The impact of IL-17A on expression of Wnt signaling pathway antagonists was also assessed by qRT-PCR. Finally, regulation of Dickkopf (DKK)1 expression in murine synovial fibroblasts was evaluated after treatment with IL-17A, TNF, or IL-17A plus TNF.

RESULTS

IL-17A-deficient mice develop significantly more periosteal bone than wild-type mice at peak inflammation, despite comparable severity of inflammation and bone erosion. IL-17A inhibits calvarial osteoblast differentiation in vitro, inducing mRNA expression of the Wnt antagonist sFRP1 in osteoblasts, and suppressing sFRP3 expression, both potentially contributing to inhibition of osteoblast differentiation. Furthermore, a blocking antibody to sFRP1 reduced the inhibitory effect of IL-17A on differentiation. Although treatment with IL-17A suppresses DKK1 mRNA expression in osteoblasts, IL-17A plus TNF synergistically upregulate DKK1 mRNA expression in synovial fibroblasts.

CONCLUSIONS

IL-17A may limit the extent of bone formation at inflamed periosteal sites in spondyloarthritis. IL-17A inhibits calvarial osteoblast differentiation, in part by regulating expression of Wnt signaling pathway components. These results demonstrate that additional studies focusing on the role of IL-17A in bone formation in spondyloarthritis are indicated.

Osteogenesis induced by frizzled-related protein (FRZB) is linked to the netrin-like domain

Abnormal Wnt signaling is associated with bone mass disorders. Frizzled-related protein (FRZB, also known as secreted frizzled-related protein-3 (SFRP3)) is a Wnt modulator that contains an amino-terminal cysteine-rich domain (CRD) and a carboxy-terminal Netrin-like (NTN) motif. Frzb(-/-) mice show increased cortical thickness. However, the direct effect of FRZB on osteogenic differentiation and the involvement of the structural domains herein are not fully understood. In this study, we observed that stable overexpression of Frzb in MC3T3-E1 cells increased calcium deposition and osteoblast markers compared with control. Western blot analysis showed that the increased osteogenesis was associated with reduced canonical, but increased non-canonical Wnt signaling. On the contrary, loss of Frzb induced the opposite effects on osteogenesis and Wnt signaling. To translationally validate the positive effects of FRZB on primary human cells, we treated human periosteal and human bone marrow stromal cells with conditioned medium from MC3T3-E1 cells overexpressing Frzb and observed an increase in Alizarin red staining. We further studied the effect of the domains. FrzbNTN overexpression induced similar effects on osteogenesis as full-length Frzb, whereas FrzbCRD overexpressing cells mimicked loss of Frzb experiments. The CRD is considered as the Wnt binding domain, but the NTN domain also has important effects on bone biology. FRZB and other SFRPs or their specific domains may hold surprising potential as therapeutics for bone and joint disorders considering that excess of SFRPs has effects that are not expected under physiological, endogenous expression conditions.

Induction of CXC chemokines in human mesenchymal stem cells by stimulation with secreted frizzled-related proteins through non-canonical Wnt signaling

AIM: To investigate the effect of secreted frizzled-related proteins (sFRPs) on CXC chemokine expression in human mesenchymal stem cells (hMSCs).

METHODS: CXC chemokines such as CXCL5 and CXCL8 are induced in hMSCs during differentiation with osteogenic differentiation medium (OGM) and may be involved in angiogenic stimulation during bone repair. hMSCs were treated with conditioned medium (CM) from L-cells expressing non-canonical Wnt5a protein, or with control CM from wild type L-cells, or directly with sFRPs for up to 10 d in culture. mRNA expression levels of both CXCL5 and CXCL8 were quantitated by real-time reverse transcriptase-polymerase chain reaction and secreted protein levels of these proteins determined by ELISA. Dose- (0-500 ng/mL) and time-response curves were generated for treatment with sFRP1. Signal transduction pathways were explored by western blot analysis with pan- or phosphorylation-specific antibodies, through use of specific pathway inhibitors, and through use of siRNAs targeting specific frizzled receptors (Fzd)-2 and 5 or the receptor tyrosine kinase-like orphan receptor-2 (RoR2) prior to treatment with sFRPs.

RESULTS: CM from L-cells expressing Wnt5a, a non-canonical Wnt, stimulated an increase in CXCL5 mRNA expression and protein secretion in comparison to control L-cell CM. sFRP1, which should inhibit both canonical and non-canonical Wnt signaling, surprisingly enhanced the expression of CXCL5 at 7 and 10 d. Dickkopf1, an inhibitor of canonical Wnt signaling prevented the sFRP-stimulated induction of CXCL5 and actually inhibited basal levels of CXCL5 expression at 7 but not at 10 d post treatment. In addition, all four sFRPs isoforms induced CXCL8 expression in a dose- and time-dependent manner with maximum expression at 7 d with treatment at 150 ng/mL. The largest increases in CXCL5 expression were seen from stimulation with sFRP1 or sFRP2. Analysis of mitogen-activated protein kinase signaling pathways in the presence of OGM showed sFRP1-induced phosphorylation of extracellular signal-regulated kinase (ERK) (p44/42) maximally at 5 min after sFRP1 addition, earlier than that found in OGM alone. Addition of a phospholipase C (PLC) inhibitor also prevented sFRP-stimulated increases in CXCL8 mRNA. siRNA technology targeting the Fzd-2 and 5 and the non-canonical Fzd co-receptor RoR2 also significantly decreased sFRP1/2-stimulated CXCL8 mRNA levels.

CONCLUSION: CXC chemokine expression in hMSCs is controlled in part by sFRPs signaling through non-canonical Wnt involving Fzd2/5 and the ERK and PLC pathways.

Loss of SFRP4 Alters Body Size, Food Intake, and Energy Expenditure in Diet-Induced Obese Male Mice

Secreted frizzled-related protein 4 (SFRP4) is an extracellular regulator of the wingless-type mouse mammary tumor virus integration site family (WNT) pathway. SFRP4 has been implicated in adipocyte dysfunction, obesity, insulin resistance, and impaired insulin secretion in patients with type 2 diabetes. However, the exact role of SFRP4 in regulating whole-body metabolism and glucose homeostasis is unknown. We show here that male Sfrp4(-/-) mice have increased spine length and gain more weight when fed a high-fat diet. The body composition and body mass per spine length of diet-induced obese Sfrp4(-/-) mice is similar to wild-type littermates, suggesting that the increase in body weight can be accounted for by their longer body size. The diet-induced obese Sfrp4(-/-) mice have reduced energy expenditure, food intake, and bone mineral density. Sfrp4(-/-) mice have normal glucose and insulin tolerance and β-cell mass. Diet-induced obese Sfrp4(-/-) and control mice show similar impairments of glucose tolerance and a 5-fold compensatory expansion of their β-cell mass. In summary, our data suggest that loss of SFRP4 alters body length and bone mineral density as well as energy expenditure and food intake. However, SFRP4 does not control glucose homeostasis and β-cell mass in mice.

Multi-lineage differentiation of mesenchymal stem cells - To Wnt, or not Wnt

Mesenchymal stem cells (MSCs) are multipotent precursor cells originating from several adult connective tissues. MSCs possess the ability to self-renew and differentiate into several lineages, and are recognized by the expression of unique cell surface markers. Several lines of evidence suggest that various signal transduction pathways and their interplay regulate MSC differentiation. To that end, a critical player in regulating MSC differentiation is a group of proteins encoded by the Wnt gene family, which was previously known for influencing various stages of embryonic development and cell fate determination. As MSCs have gained significant clinical attention for their potential applications in regenerative medicine, it is imperative to unravel the mechanisms by which molecular regulators control differentiation of MSCs for designing cell-based therapeutics. It is rather coincidental that the functional outcome(s) of Wnt-induced signals share similarities with cellular redox-mediated networks from the standpoint of MSC biology. Furthermore, there is evidence for a crosstalk between Wnt and redox signalling, which begs the question whether Wnt-mediated differentiation signals involve the intermediary role of reactive oxygen species. In this review, we summarize the impact of Wnt signalling on multi-lineage differentiation of MSCs, and attempt to unravel the intricate interplay between Wnt and redox signals.

Expression of osteoblast and osteoclast regulatory genes in the bone marrow microenvironment in multiple myeloma: only up-regulation of Wnt inhibitors SFRP3 and DKK1 is associated with lytic bone disease

Multiple myeloma (MM) lytic bone disease (LBD) is caused by osteoclast activation and osteoblast inhibition. RANK/RANKL/OPG play central roles in osteoclast activation and Wnt inhibitor DKK1 in osteoblast inhibition. The role of other Wnt inhibitors is less clear. We evaluated gene expression of osteoclast regulators (RANK, RANKL, OPG, TRAIL, MIP1A), Wnt inhibitors (DKK1, SFRP2, SFRP3, sclerostin, WIF1) and osteoblast transcription factors (RUNX2, osterix) by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) in the bone marrow (BM) microenvironment using snap-frozen BM biopsies, thereby achieving minimal post-sampling manipulation, and gene expression profiling (GEP) data, reflecting the in vivo situation. We analyzed 110 biopsies from newly diagnosed patients with MM and monoclonal gammopathy of unknown significance (MGUS) and healthy volunteers. LBD was evaluated using standard radiographs and the bone resorption marker CTX-1. Protein levels were evaluated by enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry. Among Wnt inhibitors, only SFRP3 and DKK1 were significantly overexpressed in advanced LBD, correlating with protein levels. SFRP3 correlated with CTX-1. Our findings support osteoblast inhibition as the driving force behind MM LBD.

Dynamic expression of secreted Frizzled-related protein 3 (sFRP3) in the developing mouse spinal cord and dorsal root ganglia

Wnt proteins have been implicated in regulating a variety of developmental processes in the CNS. Secreted Frizzled-related protein 3 (sFRP3) is a member of the sFRP family that can inhibit the Wnt signaling by binding directly to Wnts via their regions of homology to the Wnt-binding domain of Frizzleds. Recent studies suggested that sFRP3 plays an important role in cell proliferation and differentiation in various tissues. To understand the role of sFRP3 in neural development, we carried out detailed studies on the expression of sFRP3 in the developing nervous system. Our results revealed that sFRP3 is initially expressed in the ventricular zone of the spinal cord and dorsal root ganglia (DRG), and later in the dorsal horn of spinal cord and subpopulation of DRG neurons. The spatiotemporally dynamic expression ofsFRP3 strongly suggests that sFRP3 has potential functions in the sensory neuron genesis and sensory circuitry formation.

A systems biology approach to identify molecular pathways altered by HDAC inhibition in osteosarcoma

Osteosarcoma (OS) is the most common primary tumor in humans and dogs affecting the skeleton, and spontaneously occurring OS in dogs serves as an extremely useful model. Unacceptable toxicities using current treatment protocols prevent further dose-intensification from being a viable option to improve patient survival and thus, novel treatment strategies must be developed. Histone deacetylase inhibitors (HDACi) have recently emerged as a promising class of therapeutics demonstrating an ability to enhance the anti-tumor activity of traditional chemotherapeutics. To date, gene expression analysis of OS cell lines treated with HDACi has not been reported, and evaluation of the resultant gene expression changes may provide insight into the mechanisms that lead to success of HDACi. Canine OS cells, treated with a clinically relevant concentration of the HDACi valproic acid (VPA), were used for expression analysis on the Affymetrix canine v2.0 genechip. Differentially expressed genes were grouped into pathways based upon functional annotation; pathway analysis was performed with MetaCore and Ingenuity Pathways Analysis software. Validation of microarray results was performed by a combination of qRT-PCR and functional/biochemical assays revealing oxidative phosphorylation, cytoskeleton remodeling, cell cycle, and ubiquitin-proteasome among those pathways most affected by HDACi. The mitomycin C-bioactivating enzyme NQ01 also demonstrated upregulation following VPA treatment, leading to synergistic reductions in cell viability. These results provide a better understanding of the mechanisms by which HDACi exert their effect in OS, and have the potential to identify biomarkers that may serve as novel targets and/or predictors of response to HDACi-containing combination therapies in OS.

Wnt3a regulates tumor necrosis factor-α-stimulated interleukin-6 release in osteoblasts

It is recognized that Wnt pathways regulate bone metabolism. We have previously shown that tumor necrosis factor-α (TNF-α) stimulates synthesis of interleukin-6 (IL-6), a potent bone resorptive agent, via p44/p42 mitogen-activated protein (MAP) kinase and phosphatidylinositol 3-kinase (PI3-kinase)/Akt in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of Wnt3a on TNF-α-stimulated IL-6 synthesis in these cells. Wnt3a, which alone did not affect the IL-6 levels, significantly suppressed the TNF-α-stimulated IL-6 release. Lithium Chloride (LiCl), which is an inhibitor of GSK3β, markedly reduced the TNF-α-stimulated IL-6 release, similar to the results with Wnt3a. The suppression by Wnt3a or LiCl was also observed in the intracellular protein levels of IL-6 elicited by TNF-α. Wnt3a failed to affect the TNF-α-induced phosphorylation of p44/p42 MAP kinase, Akt, IκB or NFκB. Either Wnt3a or LiCl failed to reduce, rather increased the IL-6 mRNA expression stimulated by TNF-α. Lactacystin, a proteasome inhibitor, and bafilomycin A1, a lysosomal protease inhibitor, significantly restored the suppressive effect of Wnt3a on TNF-α-stimulated IL-6 release. Taken together, our results strongly suggest that Wnt3a regulates IL-6 release stimulated by TNF-α at post-transcriptional level in osteoblasts.

Quality control and monitoring for the isolation process of mesenchymal stem cells and their differentiation into osteoblasts

We developed a method of quality control and monitoring for the isolation of mesenchymal stem cells (MSCs) from bone marrow and their differentiation into osteoblasts. After dividing the cell culture process into five groups based on cell types such as MSCs and osteoblasts, we used microarray analysis to select genes with expression profiles characteristic of each group and quantitative polymerase chain reaction for confirming the expression profiles of these genes. Comparing multiple gene expression profiles per cell from quantitative polymerase chain reaction permitted us to distinguish (1) different groups of cell culture including MSCs and osteoblasts; (2) MSCs that had differentiated cells other than osteoblasts such as chondroblasts, adipocytes, or skin-derived fibroblasts; and (3) desirable MSCs from undesirable MSCs occurring under different culture conditions. These findings suggest that it is possible to standardize MSCs and osteoblasts on the basis of multiple gene expression profiles and to check the quality of these cells. We believe that our methods can be applied to cells cultured for transplants.

Mechanical stretching induces osteoprotegerin in differentiating C2C12 precursor cells through noncanonical Wnt pathways

Mechanical loading is known to be important for maintaining the formation and resorption rates of bone. To study the mechanisms by which mechanical loading regulates osteogenesis, we investigated the role of the Wnt pathway in C2C12 cells committed to osteogenic differentiation in response to cyclic mechanical stretching. Osteoprotegerin (OPG) acts as a decoy receptor for RANKL to inhibit osteoclastogenesis and resorption of bone. Our results demonstrate that stretching leads to a sustained increase in OPG expression in C2C12 cells. The expression of osteogenic marker genes, such as osteocalcin and alkaline phosphatase, was transiently decreased by stretching at 24 hours and returned to control levels at 48 hours. The addition of inhibitors of the canonical Wnt/beta-catenin pathways, such as the secreted FZD-related peptide sRFP2, as well as siRNA-mediated knockdown, did not inhibit the effect of stretching on OPG expression. In contrast, treatment with inhibitors of noncanonical Wnt signaling, including KN93, and siRNA for Nemo-like kinase (NLK) blocked most of the mechanical inductive effect on OPG. Furthermore, stretching-induced OPG production in the culture medium was able to inhibit the osteoclast formation of bone marrow macrophages. These results suggest that mechanical stretching may play an important role in bone remodeling through the upregulation of OPG and that the mechanical signaling leading to OPG induction involves the noncanonical Wnt pathway.

Targeting subchondral bone for treating osteoarthritis: what is the evidence?

Over the past few decades, significant progress has been made with respect to new concepts about the pathogenesis of osteoarthritis (OA). This article summarises some of the knowledge we have today on the involvement of the subchondral bone in OA. It provides substantial evidence that changes in the metabolism of the subchondral bone are an integral part of the OA disease process and that these alterations are not merely secondary manifestations, but are part of a more active component of the disease. Thus, a strong rationale exists for therapeutic approaches that target subchondral bone resorption and/or formation, and data evaluating the drugs targeting bone remodelling raise the hope that new treatment options for OA may become available.

Osteocytes and WNT: the Mechanical Control of Bone Formation

Mechanical loading is of pivotal importance in the maintenance of skeletal homeostasis, but the players involved in the transduction of mechanical stimuli to promote bone maintenance have long remained elusive. Osteocytes, the most abundant cells in bone, possess mechanosensing appendices stretching through a system of bone canaliculi. Mechanical stimulation plays an important role in osteocyte survival and hence in the preservation of bone mechanical properties, through the maintenance of bone hydratation. Osteocytes can also control the osteoblastic differentiation of mesenchymal precursors in response to mechanical loading by modulating WNT signaling pathways, essential regulators of cell fate and commitment, through the protein sclerostin. Mutations of Sost, the sclerostin-encoding gene, have dramatic effects on the skeleton, indicating that osteocytes may act as master regulators of bone formation and localized bone remodeling. Moreover, the development of sclerostin inhibitors is opening new possibilities for bone regeneration in orthopedics and the dental field.

Wnt inhibitory factor (WIF)-1 inhibits osteoblastic differentiation in mouse embryonic mesenchymal cells

Wnt inhibitory factor (WIF)-1 belongs to the members of secreted modulators of Wnt proteins. Secreted frizzled-related proteins (sFRPs), another member of Wnt modulators, have been shown to play differential roles in Wnt signaling depending on the subtypes and cell models. This study was undertaken to investigate the functional role of WIF-1 in osteoblastic differentiation of mouse mesenchymal C3H10T1/2 cells. C3H10T1/2 cells express endogenous WIF-1 and its expression level decreases during osteoblastogenesis. Treatment of C3H10T1/2 cells with WIF-1 significantly reduced alkaline phosphatase (ALP) activities induced by either osteogenic medium (OM, ascorbic acid and beta-glycerophosphate) or Wnt-3a conditioned medium (CM) in a dose-dependent manner. In contrast, the expression level of endogenous WIF-1 increased during adipogenesis and WIF-1 treatment resulted in increased adipogenesis. C3H10T1/2 cells transduced with WIF-1 retrovirus also exhibited reduced ALP activity and decreased mRNA expression of Runx2, collagen type 1, ALP and osteocalcin during osteoblastic differentiation compared to empty virus-transduced cells. Moreover, treatment with WIF-1 dose-dependently attenuates beta-catenin/T-cell factor (TCF) transcriptional activity in this cell line. Finally, knockdown of WIF-1 in C3H10T1/2 cells by RNA interference leads to increase in ALP activities. Collectively, these results indicate that WIF-1 plays as a negative regulator of osteoblastic differentiation in mouse mesenchymal C3H10T1/2 cells in vitro.

Secreted frizzled related protein (sFRP)-2 inhibits bone formation and promotes cell proliferation in ameloblastoma

Secreted frizzled related protein (sFRP)-2, a Wnt antagonist, was strongly expressed by both stromal and tumor cells of ameloblastoma. The aim of this study is to evaluate whether sFRP-2 secreted from tumor cells have any direct role in suppressed bone formation or not. A pre-osteoblastic cell line, KUSA/A1 cells, cultured in conditioned medium of an ameloblastoma-derived cell line (AM-1CM) was used in the study. Alkaline phosphatase (ALP) activity, alizarin red staining, mineral quantification and MTS assay was performed. Wnt-canonical pathway is a major pathway for osteoblasts. Antagonists of this pathway, sFRP-1, 2 and 3, were detected by immunohistochemistry and western blot analysis. KUSA/A1 cells cultured in AM-1CM showed high cell proliferation, low ALP activity without mineralized matrix deposition. sFRP-2 was strongly expressed in ameloblastoma tissue and AM-1 cells. After sFRP-2 depletion, the cells showed diffuse mineralization. In this study, it was confirmed that ameloblastoma cells have a major role in decreased bone formation by secreting sFRP-2 in cell culture model. Though, sFRP-2 has great effect on tumor progression, inhibition of sFRP-2's anti-bone formation activity and cell proliferative activity may reduce the invasive property of ameloblastoma and possibility of recurrence rate.

Targeting the Wnt signaling pathway to augment bone formation

Recent discoveries in humans and mice have revealed that the Wnt (Wingless and Int-1) signaling pathway is responsible for a complex array of functions in maintaining bone homeostasis. The Wnt proteins are key modulators of mesenchymal lineage specification and regulate most aspects of osteoblast physiology and postnatal bone acquisition by controlling the differentiation and activity of osteoblasts and osteoclasts. Initial reports have indicated that activators of Wnt signaling are potent promoters of osteogenesis; however, systemic hyperactivation of the canonical Wnt pathway could potentially accelerate neoplastic transformation and subsequent tumor growth. Alternatively, recent investigations of natural soluble antagonists of Wnt signaling in bone suggest the possibilities of bone-specific therapies targeting the negative regulators of Wnt pathway, especially sclerostin. With this new knowledge, novel pharmacologic interventions that alter Wnt signaling are being evaluated for the management of osteoporosis. In this article, we briefly describe the Wnt signaling elements, their characterized role in bone, and summarize the current knowledge on the potential to enhance bone formation through the manipulation of Wnt signaling antagonists.

Binding of sFRP-3 to EGF in the extra-cellular space affects proliferation, differentiation and morphogenetic events regulated by the two molecules

Background

sFRP-3 is a soluble antagonist of Wnts, widely expressed in developing embryos. The Wnt gene family comprises cysteine-rich secreted ligands that regulate cell proliferation, differentiation, organogenesis and oncogenesis of different organisms ranging from worms to mammals. In the canonical signal transduction pathway Wnt proteins bind to the extracellular domain of Frizzled receptors and consequently recruit Dishevelled (Dsh) to the cell membrane. In addition to Wnt membrane receptors belonging to the Frizzled family, several other molecules have been described which share homology in the CRD domain and lack the putative trans-membrane domain, such as sFRP molecules (soluble Frizzled Related Protein). Among them, sFRP-3 was originally isolated from bovine articular cartilage and also as a component of the Spemann organizer. sFRP-3 blocks Wnt-8 induced axis duplication in Xenopus embryos and binds to the surface of cells expressing a membrane-anchored form of Wnt-1. Injection of sFRP-3 mRNA blocks expression of XMyoD mRNA and leads to embryos with enlarged heads and shortened trunks.

Methodology/Principal Findings

Here we report that sFRP-3 specifically blocks EGF-induced fibroblast proliferation and foci formation. Over-expression of sFRP-3 reverts EGF-mediated inhibition of hair follicle development in the mouse ectoderm while its ablation in Xenopus maintains EGF-mediated inhibition of ectoderm differentiation. Conversely, over-expression of EGF reverts the inhibition of somitic myogenesis and axis truncation in Xenopus and mouse embryos caused by sFRP-3. In vitro experiments demonstrated a direct binding of EGF to sFRP-3 both on heparin and on the surface of CHO cells where the molecule had been membrane anchored.

Conclusions/Significance

sFRP-3 and EGF reciprocally inhibit their effects on cell proliferation, differentiation and morphogenesis and indeed are expressed in contiguous domains of the embryo, suggesting that in addition to their canonical ligands (Wnt and EGF receptor, respectively) these molecules bind to each other and regulate their activities during embryogenesis.

Microarray gene expression profiling of osteoarthritic bone suggests altered bone remodelling, WNT and transforming growth factor-beta/bone morphogenic protein signalling

Osteoarthritis (OA) is characterized by alterations to subchondral bone as well as articular cartilage. Changes to bone in OA have also been identified at sites distal to the affected joint, which include increased bone volume fraction and reduced bone mineralization. Altered bone remodelling has been proposed to underlie these bone changes in OA. To investigate the molecular basis for these changes, we performed microarray gene expression profiling of bone obtained at autopsy from individuals with no evidence of joint disease (control) and from individuals undergoing joint replacement surgery for either degenerative hip OA, or fractured neck of femur (osteoporosis [OP]). The OP sample set was included because an inverse association, with respect to bone density, has been observed between OA and the low bone density disease OP. Compugen human 19K-oligo microarray slides were used to compare the gene expression profiles of OA, control and OP bone samples. Four sets of samples were analyzed, comprising 10 OA-control female, 10 OA-control male, 10 OA-OP female and 9 OP-control female sample pairs. Print tip Lowess normalization and Bayesian statistical analyses were carried out using linear models for microarray analysis, which identified 150 differentially expressed genes in OA bone with t scores above 4. Twenty-five of these genes were then confirmed to be differentially expressed (P < 0.01) by real-time PCR analysis. A substantial number of the top-ranking differentially expressed genes identified in OA bone are known to play roles in osteoblasts, osteocytes and osteoclasts. Many of these genes are targets of either the WNT (wingless MMTV integration) signalling pathway (TWIST1, IBSP, S100A4, MMP25, RUNX2 and CD14) or the transforming growth factor (TGF)-β/bone morphogenic protein (BMP) signalling pathway (ADAMTS4, ADM, MEPE, GADD45B, COL4A1 and FST). Other differentially expressed genes included WNT (WNT5B, NHERF1, CTNNB1 and PTEN) and TGF-β/BMP (TGFB1, SMAD3, BMP5 and INHBA) signalling pathway component or modulating genes. In addition a subset of genes involved in osteoclast function (GSN, PTK9, VCAM1, ITGB2, ANXA2, GRN, PDE4A and FOXP1) was identified as being differentially expressed in OA bone between females and males. Altered expression of these sets of genes suggests altered bone remodelling and may in part explain the sex disparity observed in OA.

Osteoblast-targeted expression of Sfrp4 in mice results in low bone mass

Transgenic mice overexpressing Sfrp4 in osteoblasts were established. These mice exhibited low bone mass caused by a decrease in bone formation.

INTRODUCTION

We recently reported that single nucleotide polymorphisms in the secreted frizzled-related protein 4 (Sfrp4) gene are responsible for low peak BMD in senescence-accelerated mouse (SAM) P6. In vitro studies revealed inhibition of osteoblast proliferation by Sfrp4, which is supposed to be mediated by canonical Wnt signaling.

MATERIALS AND METHODS

We examined the expression of Sfrp4 in neonate long bones by in situ hybridization and generated transgenic mice in which Sfrp4 was specifically overexpressed in osteoblasts under the control of a 2.3-kb Col1a1 osteoblast-specific promoter. Next, we compared the phenotype of Sfrp4 transgenic (Sfrp4 TG) mice with that of mice in which one allele of beta-catenin was conditionally disrupted in osteoblasts (betaChet), and administered lithium chloride (LiCl) to Sfrp4 TG mice.

RESULTS

Hemizygous Sfrp4 TG mice exhibited a 30% reduction of trabecular bone mass compared with that in wildtype littermates at 8 wk of age, and histomorphometrical analysis showed decreases in both osteoblast numbers and bone formation rate. betaChet mice exhibited a 17% reduction of trabecular bone mass in distal femora caused by an increase in the osteoclast number and a decrease in bone formation rate. Furthermore, LiCl administration rescued the bone phenotype of Sfrp4 TG mice.

CONCLUSIONS

Expression of Sfrp4 in periosteum and bone tissues suggested the role of Sfrp4 in osteoblasts, and we identified that overexpression of Sfrp4 in osteoblasts suppressed osteoblast proliferation, resulting in a decrease in bone formation in vivo. Partial suppression of beta-catenin/canonical Wnt signaling also impaired bone formation, and activation of the signaling restored low bone mass of Sfrp4 TG mice. Thus, these results indicate that Sfrp4 decreases bone formation at least in part by attenuating canonical Wnt signaling in vivo.

Differential effects of secreted frizzled-related proteins (sFRPs) on osteoblastic differentiation of mouse mesenchymal cells and apoptosis of osteoblasts

Secreted frizzled-related proteins (sFRPs) are modulators of Wnt signaling. This study was undertaken for definitive assessment of contribution of different sFRPs in osteoblastic differentiation of mesenchymal progenitor cells and apoptosis of osteoblasts. Treatment of C3H10T1/2 cells with sFRP-2 at concentrations of 10, 50, and 100nM and sFRP-4 at low concentrations (5nM) significantly increased Wnt-3A-induced alkaline phosphatase (ALP) activities, whereas sFRP-1 or 3 did not. Retroviral transduction of the sFRP-2 but not other sFRPs also significantly enhanced ALP activity induced by beta-glycerophosphate and ascorbic acid. Furthermore, transfection of all the sFRP expression vectors significantly increased beta-catenin/TCF reporter activity and the effects were most prominent with sFRP-2 and -4. In osteoblast apoptosis assay, only sFRP-3 increased etoposide-induced apoptosis in MC3T3-E1 mouse osteoblasts. In conclusion, we found that different repertoires of sFRPs exert differential effects on osteoblastic differentiation of mouse mesenchymal cells and cellular apoptosis of mouse osteoblasts in vitro.

Variation in the secreted frizzled-related protein-3 gene and risk of osteolysis and heterotopic ossification after total hip arthroplasty

Secreted frizzled-related protein-3 (sFRP3) antagonizes ligands that promote new bone formation in adult tissues. We examined whether variation in the FRZB gene that encodes sFRP3 is associated with development of osteolysis or heterotopic ossification (HO) after total hip arthroplasty (THA). Genomic DNA was extracted from 609 subjects (osteolysis group n = 268) at a mean of 11 years following cemented THA for idiopathic osteoarthritis and genotyped for the FRZB Arg200Trp and Arg324Gly polymorphisms. The Brooker classification was used to assess HO following primary THA in 563 of the subjects. The carriage rate of the FRZB 200Trp allele was 14.2% in subjects with osteolysis versus 21.0% in controls (p = 0.041). The carriage rate of this allele was 21.7% in subjects with HO (n = 299) versus 12.0% in those without HO (p = 0.063). The odds ratio for osteolysis with carriage of FRZB 200Trp was 0.62 (95% CI 0.38 to 0.99; p = 0.049) and for HO was 1.64 (1.05 to 2.54; p = 0.028), after adjustment for the effects of other risk factors associated with the development of osteolysis or HO. Variants in the FRZB 324 locus alone were not associated with osteolysis or HO. However, the most frequent haplotype (FRZB 200Arg:324Arg) was associated with osteolysis (OR 1.50, 95% CI 1.09 to 2.07; p = 0.014). Our data suggest that the FRZB Arg200Trp locus may be a marker for pro-osteoblastic activity after THA. Carriage of the FRZB 200Trp allele is associated with a "positive" bone balance phenotype (osteolysis -: HO+).

Nuclear factor-E2-related factor-1 mediates ascorbic acid induction of osterix expression via interaction with antioxidant-responsive element in bone cells

We recently found that deletion of the gulonolactone oxidase gene, which is involved in the synthesis of ascorbic acid (AA), was responsible for the fracture phenotype in spontaneous fracture mice. To explore the molecular mechanisms by which AA regulates osteoblast differentiation, we examined the effect of AA on osterix expression via Nrf1 (NF-E2-related factor-1) binding to antioxidant-responsive element (ARE) in bone marrow stromal (BMS) cells. AA treatment caused a 6-fold increase in osterix expression in mutant BMS cells at 24 h, which was unaffected by pretreatment with protein synthesis inhibitor. Sequence analyses of mouse osterix promoter revealed a putative ARE located at -1762 to -1733 upstream of the transcription start site to which Nrf potentially binds. A gel mobility shift assay revealed that nuclear proteins from AA-treated BMS cells bound to radiolabeled ARE much more strongly than nuclear extracts from AA-untreated cells. A chromatin immunoprecipitation assay with Nrf1 antibody confirmed the interaction of Nrf1 with the mouse osterix promoter. A reporter assay demonstrated that the promoter activity of mouse osterix containing an ARE was stimulated 4-fold by a 48-h treatment with AA in spontaneous fracture BMS cells. Treatment of mutant BMS cells with AA resulted in a 3.9-fold increase in the nuclear accumulation of Nrf1. Transfection of mutant BMS cells with Nrf1 small interfering RNA decreased Nrf1 protein by 4.5-fold, blocked AA induction of osterix expression, and impaired BMS cell differentiation. Our data provided the first experimental evidence that AA modulated osterix expression via a novel mechanism involving Nrf1 nuclear translocation and Nrf1 binding to ARE to activate genes critical for cell differentiation.

Secreted frizzled-related protein 4 is a negative regulator of peak BMD in SAMP6 mice

We segregated a QTL for peak BMD on Chr 13 by generating congenic sublines of the senescence-accelerated mouse SAMP6. Sfrp 4 within this locus was responsible for lower BMD of SAMP6.

INTRODUCTION

Our genome-wide linkage study using SAMP6 and SAMP2 showed a significant quantitative trait locus (QTL) for peak BMD on chromosome (Chr) 13. To verify the gene that regulates peak BMD, we generated a congenic strain, P6.P2-Pbd2(b), which carried a 15-cM SAMP2 interval on an osteoporotic SAMP6 background, and showed that this Pbd2 locus increased peak BMD in SAMP6.

MATERIALS AND METHODS

To narrow down this interval, we generated a new congenic subline P6.P2-13. We studied the effect of this locus on morphological and histomorphological features in vivo and on osteoblasts in vitro. The levels of expression of all genes in the segregated interval were examined, and we clarified the effect of the candidate gene, secreted frizzled-related protein (Sfrp4), on osteoblasts in vitro.

RESULTS

The new congenic strain, P6.P2-13, retained the 2.4-Mb SAMP2 interval on the SAMP6 background, and 11 genes existed in this interval. In morphometrical analysis, P6.P2-13 increased the bone area fraction (BA/TA) by 6.6% at the diaphysial cortex (p < 0.001) and increased the trabecular bone volume (BV/TV) by 54.2% at the distal metaphysis (p < 0.05) in the femora compared with those of SAMP6. The bone formation rate of P6.P2-13 was markedly increased at the periosteal surface of femoral cortex and that was caused by a higher proliferation rate of osteoblasts in P6.P2-13 compared with those in SAMP6. Quantitative RT-PCR analysis of calvaria tissue showed approximately 40-fold higher levels of expression of Sfrp4 in SAMP6 than in P6.P2-13. Taken together with the result that recombinant Sfrp4 suppressed the proliferation of osteoblasts, we hypothesized that Sfrp4 inhibited the proliferation of osteoblasts through its antagonistic effect on Wnt signaling. TCF/beta-catenin-dependent reporter activity in osteoblasts derived from SAMP6 showed lower responsiveness for the Wnt ligand, Wnt3A, than that in osteoblasts from P6.P2-13.

CONCLUSIONS

In SAMP6 mice, Sfrp4 negatively regulates bone formation and decreases BMD through the inhibition of Wnt signaling.

The Wnt antagonist secreted frizzled-related protein-1 controls osteoblast and osteocyte apoptosis

Mechanisms controlling human bone formation remain to be fully elucidated. We have used differential display-polymerase chain reaction analysis to characterize osteogenic pathways in conditionally immortalized human osteoblasts (HOBs) representing distinct stages of differentiation. We identified 82 differentially expressed messages and found that the Wnt antagonist secreted frizzled-related protein (sFRP)-1 was the most highly regulated of these. Transient transfection of HOBs with sFRP-1 suppressed canonical Wnt signaling by 70% confirming its antagonistic function in these cells. Basal sFRP-1 mRNA levels increased 24-fold during HOB differentiation from pre-osteoblasts to pre-osteocytes, and then declined in mature osteocytes. This expression pattern correlated with levels of cellular viability such that the pre-osteocytes, which had the highest levels of sFRP-1 mRNA, also had the highest rate of cell death. Basal sFRP-1 mRNA levels also increased 29-fold when primary human mesenchymal stem cells were differentiated to osteoblasts supporting the developmental regulation of the gene. Expression of sFRP-1 mRNA was induced 38-fold following prostaglandin E2 (PGE2) treatment of pre-osteoblasts and mature osteoblasts that had low basal message levels. In contrast, sFRP-1 expression was down-regulated by as much as 80% following transforming growth factor (TGF)-beta1 treatment of pre-osteocytes that had high basal mRNA levels. Consistent with this, treatment of pre-osteoblasts and mature osteoblasts with PGE(2) increased apoptosis threefold, while treatment of pre-osteocytes with TGF-beta1 decreased cell death by 50%. Likewise, over-expression of sFRP-1 in HOBs accelerated the rate of cell death threefold. These results establish sFRP-1 as an important negative regulator of human osteoblast and osteocyte survival.

Bone-related gene profiles in developing calvaria

Generating a comprehensive understanding of osteogenesis-related gene profiles is very important in the development of new treatments for osteopenic conditions. Developing calvaria undergoes a typical intramembranous bone-forming process. To identify genes associated with osteoblast differentiation, we isolated total RNAs from parietal bones, that represent active osteoblasts, and sutural mesenchyme, that represents osteoprogenitor cells, and comprehensively analyzed their gene expression profiles using an oligo-based Affymetrix microarray chip containing 22,690 probes. About 2100 genes with "Present" calls had more than 2-fold higher expression in bone compared to sutures while 73 of these genes had more than 8-fold expression. Some of these genes are already known to be bone-related biomarkers: VitD receptor, bone sialoprotein, osteocalcin, osteopontin, MMP13, etc. Eight genes were selected and subjected to confirmation by quantitative real-time RT-PCR analyses. All the genes tested showed higher expression in bones, ranging from 5- to 140-fold. Several of these genes are ESTs while others are already known but their functions in osteogenesis were not previously known. Most genes of the BMP and FGF families probed in the Genechip analysis were more highly expressed in bone tissues compared to suture. All differentially-expressed Runx and Dlx family genes also showed higher expression in bone. These results imply that our data is valid and can be used as a good standard for the mining of osteogenesis-related genes.

Regulation of insulin-like growth factor binding protein-5, four and a half lim-2, and a disintegrin and metalloprotease-9 expression in osteoblasts

The roles of insulin-like growth factors (IGFs) in regulating growth and their modulation by six IGF binding proteins (IGFBP) are well established. IGFBP-5, the most abundant IGFBP stored in bone, is an important regulator of bone formation via IGF-dependent and -independent mechanisms. Two new proteins, four and a half lim (FHL)-2, a transcription modulator that interacts with IGFBP-5, and a disintegrin and metalloprotease (ADAM)-9, an IGFBP-5 protease, have been identified as potential regulators of IGFBP-5 action in bone. We tested the hypothesis that agents which modulate bone formation by regulating IGFBP-5 expression would also regulate FHL-2 and ADAM-9 expression in a coordinated manner. We evaluated the expression of IGFBP-5, FHL-2, and ADAM-9 by real-time reverse transcriptase (RT)-PCR during differentiation of mouse bone marrow stromal cells into osteoblasts and in response to treatment with bone formation modulators in the LSaOS human osteosarcoma cell line. IGFBP-5 and FHL-2 increased 4.3- and 3.0-fold (P < or = 0.01), respectively, during osteoblast differentiation. Dexamethasone (Dex), an inhibitor of bone formation, decreased IGFBP-5 and FHL-2 and increased ADAM-9 in LSaOS cells (P < or = 0.05). Bone morphogenic protein (BMP)-7, a stimulator of bone formation, increased IGFBP-5 and decreased ADAM-9 (P<0.01). To determine if BMP-7 would eliminate Dex inhibition of IGFBP-5, cells were treated with Dex+BMP-7. The BMP-7-induced increase in IGFBP-5 was reduced, but not eliminated, in the presence of Dex (P < or = 0.01), indicating that BMP-7 and Dex may regulate IGFBP-5 via different mechanisms. Transforming growth factor (TGF)-beta, a stimulator of bone formation, increased IGFBP-5 and FHL-2 expression (P < or = 0.01). IGF-I and TNF-alpha decreased expression of ADAM-9 (P<0.05). In conclusion, our findings are consistent with the hypothesis that FHL-2 and ADAM-9 are important modulators of IGFBP-5 actions and are, in part, regulated in a coordinated manner in bone.

Frizzled-related protein variants are risk factors for hip osteoarthritis

OBJECTIVE

To examine the association of the Arg200Trp and Arg324Gly variants of FRZB with the risk and phenotype of radiographic osteoarthritis (OA) of the hip and serum levels of Frizzled-related protein (FRP) in a prospective cohort of elderly Caucasian women.

METHODS

Radiographic hip OA status of patients was defined by the presence of severe joint space narrowing (JSN) (feature grade>or=3), a summary grade>or=3, or definite osteophytes (grade>or=2) and JSN (grade>or=2) in the same hip. Genotypes were obtained in 569 patients with radiographic OA of the hip and in 1,317 and 4,136 controls for the Arg200Trp and Arg324Gly variants, respectively. Serum FRP levels were measured by enzyme-linked immunosorbent assay. Multivariate logistic regression was performed.

RESULTS

The minor allele frequency for the Arg200Trp polymorphism was 0.12 in the control group compared with 0.14 in the group with radiographic OA of the hip (P=0.12), and the minor allele frequency for the Arg324Gly variant was 0.083 in the control group compared with 0.088 in the group with radiographic OA of the hip (P=0.63). The multilocus genotypes available in 1,886 subjects suggested that inheritance of both minor alleles was a risk factor for developing OA characterized by JSN (P<0.01). Patients with radiographic OA of the hip who were homozygous for the Arg200Trp minor allele had higher serum FRP levels than controls who were homozygous for the major allele.

CONCLUSION

Our data confirm findings of another study, that a rare haplotype with both Arg200Trp and Arg324Gly FRZB variants contributes to the genetic susceptibility to hip OA among Caucasian women, and that these polymorphisms may contribute to increased serum levels of proteins as biomarkers of OA.

Molecular bases of the regulation of bone remodeling by the canonical Wnt signaling pathway

Osteoporosis is a common, prevalent, and debilitating condition, particularly in postmenopausal women. Genetics play a major role in determining peak bone mass and fracture risk, but few genes have been demonstrated conclusively to be involved, much less the signaling pathways with which they are affiliated. The identification of mutations in the gene Lrp5, a Wnt coreceptor, as the cause for both osteoporotic and high-bone mass disorders implicated the canonical Wnt signaling pathway in bone mass regulation. Since Lrp5, other Wnt components have been identified as being regulators of bone mass, and Wnt target genes affecting bone homeostasis have begun to be elucidated. This chapter looks at the various components of the canonical Wnt signaling pathway and the data indicating that this pathway plays a major role in the control of both bone formation and bone resorption, the two key aspects of bone remodeling.

Downregulation of Wnt signaling by increased expression of Dickkopf-1 and -2 is a prerequisite for late-stage osteoblast differentiation of KS483 cells

We examined the role of Wnt/beta-catenin signaling in successive stages of osteoblast differentiation. It has been shown that Wnt signaling in mature osteoblasts needs to be downregulated to enable the formation of a mineralized matrix. Using RNA interference, we showed that this is, at least in part, accomplished by upregulation of the Wnt antagonists Dickkopf-1 and -2.

INTRODUCTION

The role of Wnt signaling in the initiation of osteoblast differentiation has been well studied. However, the role during late-stage differentiation is less clear. We have examined the role of Wnt/beta-catenin signaling in successive stages of osteoblast differentiation.

MATERIALS AND METHODS

We treated murine bone marrow and mesenchymal stem cell-like KS483 cells with either LiCl or Wnt3A during several stages of osteoblast differentiation. In addition, we generated stable KS483 cell lines silencing either the Wnt antagonist Dkk-1 or -2

RESULTS

Activation of Wnt signaling by LiCl inhibits the formation of a mineralized bone matrix in both cell types. Whereas undifferentiated KS483 cells respond to Wnt3A by inducing nuclear beta-catenin translocation, differentiated cells do not. This is at least in part accomplished by upregulated expression of Dkk-1 and -2 during osteoblast differentiation. Using RNA interference, we showed that Dkk-1 plays a crucial role in blunting the BMP-induced alkaline phosphatase (ALP) response and in the transition of an ALP+ osteoblast in a mineralizing cell. In contrast, Dkk-2 plays a role in osteoblast proliferation and the initiation of osteoblast differentiation.

CONCLUSIONS

Our data suggest that Wnt signaling in maturing osteoblasts needs to be downregulated to enable the formation of a mineralized bone matrix. Furthermore, they suggest that Dkk-1 and Dkk-2 may have distinct functions in osteoblast differentiation.

Spontaneous fractures in the mouse mutant sfx are caused by deletion of the gulonolactone oxidase gene, causing vitamin C deficiency

Using a mouse mutant that fractures spontaneously and dies at a very young age, we identified that a deletion of the GULO gene, which is involved in the synthesis of vitamin C, is the cause of impaired osteoblast differentiation, reduced bone formation, and development of spontaneous fractures.

INTRODUCTION

A major public health problem worldwide, osteoporosis is a disease characterized by inadequate bone mass necessary for mechanical support, resulting in bone fracture. To identify the genetic basis for osteoporotic fractures, we used a mouse model that develops spontaneous fractures (sfx) at a very early age.

MATERIALS AND METHODS

Skeletal phenotype of the sfx phenotype was evaluated by DXA using PIXImus instrumentation and by dynamic histomorphometry. The sfx gene was identified using various molecular genetic approaches, including fine mapping and sequencing of candidate genes, whole genome microarray, and PCR amplification of candidate genes using cDNA and genomic DNA as templates. Gene expression of selected candidate genes was performed using real-time PCR analysis. Osteoblast differentiation was measured by bone marrow stromal cell nodule assay.

RESULTS

Femur and tibial BMD were reduced by 27% and 36%, respectively, in sfx mice at 5 weeks of age. Histomorphometric analyses of bones from sfx mice revealed that bone formation rate is reduced by >90% and is caused by impairment of differentiated functions of osteoblasts. The sfx gene was fine mapped to a 2 MB region containing approximately 30 genes in chromosome 14. By using various molecular genetic approaches, we identified that deletion of the gulonolactone oxidase (GULO) gene, which is involved in the synthesis of ascorbic acid, is responsible for the sfx phenotype. We established that ascorbic acid deficiency caused by deletion of the GULO gene (38,146-bp region) contributes to fractures and premature death because the sfx phenotype can be corrected in vivo by treating sfx mice with ascorbic acid and because osteoblasts derived from sfx mice are only able to form mineralized nodules when treated with ascorbic acid. Treatment of bone marrow stromal cells derived from sfx/sfx mice in vitro with ascorbic acid increased expression levels of type I collagen, alkaline phosphatase, and osteocalcin several-fold.

CONCLUSION

The sfx is a mutation of the GULO gene, which leads to ascorbic acid deficiency, impaired osteoblast cell function, and fractures in affected mice. Based on these and other findings, we propose that ascorbic acid is essential for the maintenance of differentiated functions of osteoblasts and other cell types.

Wnt Influence on Chondrocyte Differentiation and Cartilage Function

The Wnt signaling network regulates chondrocyte differentiation, proliferation, and maturation during embryonic limb development. In this review, we summarize studies of Wnt signaling during the chondrocyte life cycle in avian and mammalian systems, both before and after birth. Recent reports that implicate abnormal Wnt signaling as a contributing factor to pathogenic joint conditions are also discussed. In addition, we show new data that suggests Wnt signaling is active in adult cartilage. Overall, it appears that the Wnt network has dual roles in cartilage, as has been described in other tissues: it is an important regulator of chondrocyte development, but deregulated signaling is detrimental to mature tissues and may lead to disease.

Myeloma cells block RUNX2/CBFA1 activity in human bone marrow osteoblast progenitors and inhibit osteoblast formation and differentiation

Decreased bone formation contributes to the development of bone lesions in multiple myeloma (MM) patients. In this study, we have investigated the effects of myeloma cells on osteoblast formation and differentiation and the potential role of the critical osteoblast transcription factor RUNX2/CBFA1 (Runt-related transcription factor 2/core-binding factor Runt domain alpha subunit 1) in the inhibition of osteoblastogenesis in MM. We found that human myeloma cells suppress the formation of human osteoblast progenitors in bone marrow (BM) cultures. Moreover, an inhibitory effect on osteocalcin, alkaline phosphatase, collagen I mRNA, protein expression, and RUNX2/CBFA1 activity by human preosteoblastic cells was observed in cocultures with myeloma cells. The inhibitory effect was more pronounced in the cell-to-cell contact conditions compared with those without the contact and involved the very late antigen 4 (VLA-4) integrin system. Among the soluble osteoblast inhibitors screened, we show the potential contribution of interleukin-7 (IL-7) in the inhibitory effect on osteoblast formation and RUNX2/CBFA1 activity by human myeloma cells in coculture. Finally, our in vitro results were supported in vivo by the finding of a significant reduction in the number of Runx2/Cbfa1-positive cells in the BM biopsies of patients with MM who had osteolytic lesions compared with those who did not have bone lesions, suggesting the critical involvement of RUNX2/CBFA1 in the decreased bone formation in MM.