Osteoblastic Responses to TGF-β during Bone Remodeling

Bone remodeling depends on the spatial and temporal coupling of bone formation by osteoblasts and bone resorption by osteoclasts; however, the molecular basis of these inductive interactions is unknown. We have previously shown that osteoblastic overexpression of TGF-β2 in transgenic mice deregulates bone remodeling and leads to an age-dependent loss of bone mass that resembles high-turnover osteoporosis in humans. This phenotype implicates TGF-β2 as a physiological regulator of bone remodeling and raises the question of how this single secreted factor regulates the functions of osteoblasts and osteoclasts and coordinates their opposing activities in vivo. To gain insight into the physiological role of TGF-β in bone remodeling, we have now characterized the responses of osteoblasts to TGF-β in these transgenic mice. We took advantage of the ability of alendronate to specifically inhibit bone resorption, the lack of osteoclast activity in c-fos^−/− mice, and a new transgenic mouse line that expresses a dominant-negative form of the type II TGF-β receptor in osteoblasts. Our results show that TGF-β directly increases the steady-state rate of osteoblastic differentiation from osteoprogenitor cell to terminally differentiated osteocyte and thereby increases the final density of osteocytes embedded within bone matrix. Mice overexpressing TGF-β2 also have increased rates of bone matrix formation; however, this activity does not result from a direct effect of TGF-β on osteoblasts, but is more likely a homeostatic response to the increase in bone resorption caused by TGF-β. Lastly, we find that osteoclastic activity contributes to the TGF-β–induced increase in osteoblast differentiation at sites of bone resorption. These results suggest that TGF-β is a physiological regulator of osteoblast differentiation and acts as a central component of the coupling of bone formation to resorption during bone remodeling.     

HLA-B-associated transcript 3 (Bat3/Scythe) negatively regulates Smad phosphorylation in BMP signaling

Members of the transforming growth factor-β (TGF-β) superfamily participate in numerous biological phenomena in multiple tissues, including in cell proliferation, differentiation, and migration. TGF-β superfamily proteins therefore have prominent roles in wound healing, fibrosis, bone formation, and carcinogenesis. However, the molecular mechanisms regulating these signaling pathways are not fully understood. Here, we describe the regulation of bone morphogenic protein (BMP) signaling by Bat3 (also known as Scythe or BAG6). Bat3 overexpression in murine cell lines suppresses the activity of the Id1 promoter normally induced by BMP signaling. Conversely, Bat3 inactivation enhances the induction of direct BMP target genes, such as Id1, Smad6, and Smad7. Consequently, Bat3 deficiency accelerates the differentiation of primary osteoblasts into bone, with a concomitant increase in the bone differentiation markers Runx2, Osterix, and alkaline phosphatase. Using biochemical and cell biological analyses, we show that Bat3 inactivation sustains the C-terminal phosphorylation and nuclear localization of Smad1, 5, and 8 (Smad1/5/8), thereby enhancing biological responses to BMP treatment. At the mechanistic level, we show that Bat3 interacts with the nuclear phosphatase small C-terminal domain phosphatase (SCP) 2, which terminates BMP signaling by dephosphorylating Smad1/5/8. Notably, Bat3 enhances SCP2–Smad1 interaction only when the BMP signaling pathway is activated. Our results demonstrate that Bat3 is an important regulator of BMP signaling that functions by modulating SCP2–Smad interaction.     

Dasatinib as a bone-modifying agent: anabolic and anti-resorptive effects

Background

Bone loss, in malignant or non-malignant diseases, is caused by increased osteoclast resorption and/or reduced osteoblast bone formation, and is commonly associated with skeletal complications. Thus, there is a need to identify new agents capable of influencing bone remodeling. We aimed to further pre-clinically evaluate the effects of dasatinib (BMS-354825), a multitargeted tyrosine kinase inhibitor, on osteoblast and osteoclast differentiation and function.

Methods

For studies on osteoblasts, primary human bone marrow mensenchymal stem cells (hMSCs) together with the hMSC-TERT and the MG-63 cell lines were employed. Osteoclasts were generated from peripheral blood mononuclear cells (PBMC) of healthy volunteers. Skeletally-immature CD1 mice were used in the in vivo model.

Results

Dasatinib inhibited the platelet derived growth factor receptor-β (PDGFR-β), c-Src and c-Kit phosphorylation in hMSC-TERT and MG-63 cell lines, which was associated with decreased cell proliferation and activation of canonical Wnt signaling. Treatment of MSCs from healthy donors, but also from multiple myeloma patients with low doses of dasatinib (2–5 nM), promoted its osteogenic differentiation and matrix mineralization. The bone anabolic effect of dasatinib was also observed in vivo by targeting endogenous osteoprogenitors, as assessed by elevated serum levels of bone formation markers, and increased trabecular microarchitecture and number of osteoblast-like cells. By in vitro exposure of hemopoietic progenitors to a similar range of dasatinib concentrations (1–2 nM), novel biological sequelae relative to inhibition of osteoclast formation and resorptive function were identified, including F-actin ring disruption, reduced levels of c-Fos and of nuclear factor of activated T cells 1 (NFATc1) in the nucleus, together with lowered cathepsin K, αVβ3 integrin and CCR1 expression.

Conclusions

Low dasatinib concentrations show convergent bone anabolic and reduced bone resorption effects, which suggests its potential use for the treatment of bone diseases such as osteoporosis, osteolytic bone metastasis and myeloma bone disease.     

Osteogenic differentiation of C2C12 myogenic progenitor cells requires the Fos-related antigen Fra-1 – A novel target of Runx2

Bone formation is a developmental process requiring the differentiation of mesenchymal stem cells into osteoblasts. It is established that Runx2 tightly regulates osteoblast differentiation and bone formation. Fos-related antigen Fra-1 is an essential factor for bone formation. Current evidence does not support a relationship between Fra-1 and Runx2 in osteogenesis. Here, we explored the possibility that Runx2 regulates Fra-1 expression during osteogenic differentiation of C2C12 myogenic progenitor cells. Expression of Fra-1 was induced rapidly after activation of Runx2 in a Tet-on stable C2C12 cell-line (C2C12/Runx2^Dox sub-line). Transient transfection assay showed that Runx2 activates Fra-1 promoter-reporter activity, suggesting that Fra-1 may be a direct target of Runx2. To determine the minimal region of the Fra-1 promoter that was activated by Runx2, a series of Fra-1 promoter deletion constructs were made. By transient transfection assay, we defined the minimal region to the proximal 342 bp (?84 to +258). Two potential Runx2-binding sites (at positions +139 and +208) were predicted within this region. Mutation of the Runx2 motif at position +208 significantly decreased Fra-1 promoter activity compared to wild type, whereas mutation of Runx2 at position +139 had no effect. Electrophoretic mobility shift assay (EMSA) demonstrated the existence of one atypical Runx2-binding element at position +208, and chromatin immunoprecipitation (ChIP) assay revealed that Runx2 bound to the native Fra-1 promoter in vivo via this site. Finally, forced expression of Fra-1 resulted in upregulation of alkaline phosphatase (ALP), a marker of early osteoblast differentiation. Together, these results indicate that Fra-1 is a direct target of Runx2 during osteogenic differentiation of C2C12 myogenic progenitor cells.     

Osteoblast Menin Regulates Bone Mass in Vivo

Menin, the product of the multiple endocrine neoplasia type 1 (Men1) tumor suppressor gene, mediates the cell proliferation and differentiation actions of transforming growth factor-β (TGF-β) ligand family members. In vitro, menin modulates osteoblastogenesis and osteoblast differentiation promoted and sustained by bone morphogenetic protein-2 (BMP-2) and TGF-β, respectively. To examine the in vivo function of menin in bone, we conditionally inactivated Men1 in mature osteoblasts by crossing osteocalcin (OC)-Cre mice with floxed Men1 (Men1^f/f) mice to generate mice lacking menin in differentiating osteoblasts (OC-Cre;Men1^f/f mice). These mice displayed significant reduction in bone mineral density, trabecular bone volume, and cortical bone thickness compared with control littermates. Osteoblast and osteoclast number as well as mineral apposition rate were significantly reduced, whereas osteocyte number was increased. Primary calvarial osteoblasts proliferated more quickly but had deficient mineral apposition and alkaline phosphatase activity. Although the mRNA expression of osteoblast marker and cyclin-dependent kinase inhibitor genes were all reduced, that of cyclin-dependent kinase, osteocyte marker, and pro-apoptotic genes were increased in isolated Men1 knock-out osteoblasts compared with controls. In contrast to the knock-out mice, transgenic mice overexpressing a human menin cDNA in osteoblasts driven by the 2.3-kb Col1a1 promoter, showed a gain of bone mass relative to control littermates. Osteoblast number and mineral apposition rate were significantly increased in the Col1a1-Menin-Tg mice. Therefore, osteoblast menin plays a key role in bone development, remodeling, and maintenance.     

Canonical Wnt signaling inhibits osteoclastogenesis independent of osteoprotegerin

Although Wnt signaling is considered a key regulatory pathway for bone formation, inactivation of β-catenin in osteoblasts does not affect their activity but rather causes increased osteoclastogenesis due to insufficient production of osteoprotegerin (Opg). By monitoring the expression pattern of all known genes encoding Wnt receptors in mouse tissues and bone cells we identified Frizzled 8 (Fzd8) as a candidate regulator of bone remodeling. Fzd8-deficient mice displayed osteopenia with normal bone formation and increased osteoclastogenesis, but this phenotype was not associated with impaired Wnt signaling or Opg production by osteoblasts. The deduced direct negative influence of canonical Wnt signaling on osteoclastogenesis was confirmed in vitro and through the generation of mice lacking β-catenin in the osteoclast lineage. Here, we observed increased bone resorption despite normal Opg production and a resistance to the anti-osteoclastogenic effect of Wnt3a. These results demonstrate that Fzd8 and β-catenin negatively regulate osteoclast differentiation independent of osteoblasts and that canonical Wnt signaling controls bone resorption by two different mechanisms.     

Phosphorylated c-Jun and Fra-1 induce matrix metalloproteinase-1 and thereby regulate invasion activity of 143B osteosarcoma cells

Osteosarcoma is the most common primary malignancy of bone and patients often develop pulmonary metastases. Despite the advances in surgical and medical management, the mechanisms underlying human osteosarcoma progression and metastasis remain to be elucidated. Gene expression profiles were compared by the cDNA microarray technique between two different human osteosarcoma sublines, MNNG/HOS and 143B, which differ greatly in spontaneous pulmonary metastatic potential. Here we report an enhanced expression of matrix metalloproteinase (MMP)-1 in the highly metastatic human osteosarcoma cell line 143B. Moreover, the in vitro invasion activity of 143B cells was MMP-1-dependent. The activator protein (AP)-1 binding site in the MMP-1 gene promoter was required for the constitutive expression of MMP-1 in 143B cells. Two AP-1 components, c-Jun and Fra-1, were phosphorylated, and bound to the AP-1 binding site of the MMP-1 promoter in 143B cells. Activated c-Jun and Fra-1 were essential for MMP-1 gene expression in 143B cells. Mitogen-activated protein kinase pathways including the c-Jun NH₂-terminal kinase and the extracellular signal-regulated kinase activate c-Jun and Fra-1 and thereby regulate c-Jun/Fra-1 mediated events, establishing the mitogen-activated protein kinase/AP-1/MMP-1 axis as important in 143B cells. These data suggest that MMP-1 plays a central role in osteosarcoma invasion. Accordingly, MMP-1 might be a biomarker and therapeutic target for invasive osteosarcomas and pulmonary metastases.     

Activation of invariant NKT cells exacerbates experimental visceral leishmaniasis

We report that natural killer T (NKT) cells play only a minor physiological role in protection from Leishmania donovani infection in C57BL/6 mice. Furthermore, attempts at therapeutic activation of invariant NKT (iNKT) cells with α-galactosylceramide (α-GalCer) during L. donovani infection exacerbated, rather than ameliorated, experimental visceral leishmaniasis. The inability of α-GalCer to promote anti-parasitic immunity did not result from inefficient antigen presentation caused by infection because α-GalCer–loaded bone marrow–derived dendritic cells were also unable to improve disease resolution. The immune-dampening affect of α-GalCer correlated with a bias towards increased IL-4 production by iNKT cells following α-GalCer stimulation in infected mice compared to naïve controls. However, studies in IL-4–deficient mice, and IL-4 neutralisation in cytokine-sufficient mice revealed that α-GalCer–induced IL-4 production during infection had only a minor role in impaired parasite control. Analysis of liver cell composition following α-GalCer stimulation during an established L. donovani infection revealed important differences, predominantly a decrease in IFNγ⁺ CD8⁺ T cells, compared with control-treated mice. Our data clearly illustrate the double-edged sword of NKT cell–based therapy, showing that in some circumstances, such as when sub-clinical or chronic infections exist, iNKT cell activation can have adverse outcomes.     

Apigenin inhibits osteoblastogenesis and osteoclastogenesis and prevents bone loss in ovariectomized mice

Polyphenol have been reported to have physiological effects with respect to alleviating diseases such as osteoporosis and osteopetrosis. We recently reported that the olive polyphenol hydroxytyrosol accelerates bone formation both in vivo and in vitro. The present study was designed to evaluate the in vivo and in vitro effects of apigenin (4′,5,7-trihydroxyflavone), one of the major polyphenols in olives and parsley, on bone formation by using cultured osteoblasts and osteoclasts and ovariectomized (OVX) mice, respectively. Apigenin markedly inhibited cell proliferation and indices of osteoblast differentiation, such as collagen production, alkaline phosphatase activity, and calcium deposition in osteoblastic MC3T3-E1 cells at concentrations of 1–10 μM. At 10 μM, apigenin completely inhibited the formation of multinucleated osteoclasts from mouse splenic cells. Moreover, injection of apigenin at 10 mg kg⁻¹ body weight significantly suppressed trabecular bone loss in the femurs of OVX mice. Our findings indicate that apigenin may have critical effects on bone maintenance in vivo.     

Reduced expression of transcription factor AP-2α is associated with gastric adenocarcinoma prognosis

Background

This study aims to investigate the expression and prognostic significance of activator protein 2α (AP-2α) in gastric adenocarcinoma.

Methodology/Principal Findings

AP-2α expression was analyzed using real-time quantitative PCR (RT-qPCR), western blotting, and immunohistochemical staining methods on tissue samples from a consecutive series of 481 gastric adenocarcinoma patients who underwent resections between 2003 and 2006. The relationship between AP-2α expression, clinicopathological factors, and patient survival was investigated. RT- qPCR results showed that the expression of AP-2α mRNA was reduced in tumor tissue samples, compared with expression in matched adjacent non-tumor tissue samples (P = 0.009); this finding was confirmed by western blotting analysis (P = 0.012). Immunohistochemical staining data indicated that AP-2α expression was significantly decreased in 196 of 481 (40.7%) gastric adenocarcinoma cases; reduced AP-2α expression was also observed in patients with poorly differentiated tumors (P = 0.001) and total gastric carcinomas (P = 0.002), as well as in patients who underwent palliative tumor resection (P = 0.004). Additionally, reduced expression of AP-2α was more commonly observed in tumors that were staged as T4a/b (P = 0.018), N3 (P = 0.006), and M1 (P = 0.008). Kaplan-Meier survival curves revealed that reduced expression of AP-2α was associated with poor prognosis in gastric adenocarcinoma patients (P<0.001). Multivariate Cox analysis identified AP-2α expression as an independent prognostic factor for overall survival (HR = 1.512, 95% CI = 1.127–2.029, P = 0.006).

Conclusions/Significance

Our data suggest that AP-2α plays an important role in tumor progression and that reduced AP-2α expression independently predicts an unfavorable prognosis in gastric adenocarcinoma patients.     

Choline Kinase β Mutant Mice Exhibit Reduced Phosphocholine,Elevated Osteoclast Activity,and Low Bone Mass

The maintenance of bone homeostasis requires tight coupling between bone-forming osteoblasts and bone-resorbing osteoclasts. However, the precise molecular mechanism(s) underlying the differentiation and activities of these specialized cells are still largely unknown. Here, we identify choline kinase β (CHKB), a kinase involved in the biosynthesis of phosphatidylcholine, as a novel regulator of bone homeostasis. Choline kinase β mutant mice (flp/flp) exhibit a systemic low bone mass phenotype. Consistently, osteoclast numbers and activity are elevated in flp/flp mice. Interestingly, osteoclasts derived from flp/flp mice exhibit reduced sensitivity to excessive levels of extracellular calcium, which could account for the increased bone resorption. Conversely, supplementation of cytidine 5′-diphosphocholine in vivo and in vitro, a regimen that bypasses CHKB deficiency, restores osteoclast numbers to physiological levels. Finally, we demonstrate that, in addition to modulating osteoclast formation and function, loss of CHKB corresponds with a reduction in bone formation by osteoblasts. Taken together, these data posit CHKB as a new modulator of bone homeostasis.