Development of cell-based high-throughput assays for the identification of inhibitors of receptor activator of nuclear factor-kappa B signaling

C/EBPα and PU.1 exhibit different responses to RANK signaling for osteoclastogenesis

The transcription factors C/EBPα and PU.1 are upregulated by RANKL through activation of its receptor RANK during osteoclastogenesis and are critical for osteoclast differentiation. Herein we investigated the mechanisms underlying how C/EBPα and PU.1 regulate osteoclast differentiation in response to RANK signaling. We showed that C/EBPα or PU.1 overexpression could initiate osteoclastogenesis and upregulate the expressions of the osteoclast genes encoding the nuclear factor of activated T-cells, C1, cathepsin K, and tartrate-resistant acid phosphatase independently of RANKL. However, while PU.1 upregulated C/EBPα, C/EBPα could not upregulate PU.1. RANK has a unique cytoplasmic domain, 535IVVY538 motif, which is crucial for osteoclast differentiation. We demonstrated that mutational inactivation of RANK IVVY motif blocked osteoclast differentiation and significantly attenuated C/EBPα, but not PU.1, expression, indicating that RANK-IVVY-induced signaling is dispensable to PU.1 upregulation during osteoclastogenesis. However, C/EBPα or PU.1 overexpression failed to promote osteoclastogenesis in cells expressing mutated RANK IVVY motif. We noted that RANK-IVVY-motif inactivation significantly repressed osteoclast genes as compared with a vector control, suggesting that IVVY motif might also negatively regulate osteoclast inhibitors during osteoclastogenesis. Consistently, IVVY-motif inactivation triggered upregulation of RBP-J, a potent osteoclast inhibitor, during osteoclastogenesis. Notably, C/EBPα or PU.1 overexpression in cells expressing mutated RANK IVVY motif failed to control the deregulated RBP-J expression, resulting in repression of osteoclast genes. Accordingly, RBP-J silencing in the mutant cells rescued osteoclastogenesis with C/EBPα or PU.1 overexpression. In conclusion, we revealed that while PU.1 and C/EBPα are critical for osteoclastogenesis, they respond differently to RANKL-induced activation of RANK IVVY motif.

C/EBPα transcription factor is regulated by the RANK cytoplasmic 535IVVY538 motif and stimulates osteoclastogenesis more strongly than c-Fos

Binding of receptor activator of NF-κB ligand (RANKL) to its receptor RANK on osteoclast (OC) precursors up-regulates c-Fos and CCAAT/enhancer-binding protein-α (C/EBPα), two critical OC transcription factors. However, the effects of c-Fos and C/EBPα on osteoclastogenesis have not been compared. Herein, we demonstrate that overexpression of c-Fos or C/EBPα in OC precursors up-regulates OC genes and initiates osteoclastogenesis independently of RANKL. However, although C/EBPα up-regulated c-Fos, c-Fos failed to up-regulate C/EBPα in OC precursors. Consistently, C/EBPα overexpression more strongly promoted OC differentiation than did c-Fos overexpression. RANK has a cytoplasmic ⁵³⁵IVVY⁵³⁸ (IVVY) motif that is essential for osteoclastogenesis, and we found that mutation of the IVVY motif blocked OC differentiation by partly inhibiting expression of C/EBPα but not expression of c-Fos. We therefore hypothesized that C/EBPα overexpression might rescue osteoclastogenesis in cells expressing the mutated IVVY motif. However, overexpression of C/EBPα or c-Fos failed to stimulate osteoclastogenesis in the mutant cells. Notably, the IVVY motif mutation abrogated OC gene expression compared with a vector control, suggesting that the IVVY motif might counteract OC inhibitors during osteoclastogenesis. Consistently, the IVVY motif mutant triggered up-regulation of recombinant recognition sequence-binding protein at the Jκ site (RBP-J) protein, a potent OC inhibitor. Mechanistically, C/EBPα or c-Fos overexpression in the mutant cells failed to control the up-regulated RBP-J expression, leading to suppression of OC genes. Accordingly, RBP-J silencing in the mutant cells rescued osteoclastogenesis with C/EBPα or c-Fos overexpression with C/EBPα exhibiting a stronger osteoclastogenic effect. Collectively, our findings indicate that C/EBPα is a stronger inducer of OC differentiation than c-Fos, partly via C/EBPα regulation by the RANK ⁵³⁵IVVY⁵³⁸ motif.

Fish as a model to assess chemical toxicity in bone

Environmental toxicology has been expanding as growing concerns on the impact of produced and released chemical compounds over the environment and human health are being demonstrated. Among the toxic effects observed in organisms exposed to pollutants, those affecting skeletal tissues (osteotoxicity) have been somehow overlooked in comparison to hepato-, immune-, neuro- and/or reproductive toxicities. Nevertheless, sub-lethal effects of toxicants on skeletal development and/or bone maintenance may result in impaired growth, reduced survival rate, increased disease susceptibility and diminished welfare. Osteotoxicity may occur by acute or chronic exposure to different environmental insults. Because of biologically and technically advantagous features - easy to breed and inexpensive to maintain, external and rapid rate of development, translucent larvae and the availability of molecular and genetic tools - the zebrafish (Danio rerio) has emerged in the last decade as a vertebrate model system of choice to evaluate osteotoxicity. Different experimental approaches in fish species and analytical tools have been applied, from in vitro to in vivo systems, from specific to high throughput methodologies. Current knowledge on osteotoxicity and underlying mechanisms gained using fish, with a special emphasis on zebrafish systems, is reviewed here. Osteotoxicants have been classified into four categories according to the pathway involved in the transduction of the osteotoxic effects: activation/inhibition of membrane and/or nuclear receptors, alteration of redox condition, mimicking of bone constituents and unknown pathways. Knowledge on these pathways is also reported here as it may provide critical insights into the development, production and release of future chemical compounds with none or low osteotoxicity, thus promoting the green/environmental friendly chemistry.

WSS25, a sulfated polysaccharide, inhibits RANKL-induced mouse osteoclast formation by blocking SMAD/ID1 signaling

AIM

WSS25 is a sulfated polysaccharide extracted from the rhizome of Gastrodia elata BI, which has been found to bind to bone morphogenetic protein 2 (BMP-2) in hepatocellular cancer cells. Since BMP-2 may regulate both osteoclasts and osteoblasts, here we investigated the effects of WSS25 on osteoclastogenesis in vitro and bone loss in ovariectomized mice.

METHODS

RAW264.7 cells or mouse bone marrow macrophages (BMMs) were treated with RANKL to induce osteoclastogenesis, which was assessed using TRAP staining, actin ring formation and pit formation assays, as well as bone resorption assay. Cell viability was detected with MTT assay. The mRNA levels of osteoclastogenesis-related genetic markers (TRAP, NFATc1, MMP-9 and cathepsin K) were detected using RT-PCR, while the protein levels of p-Smad1/5/8 and Id1 were measure with Western blotting. WSS25 was administered to ovariectomized mice (100 mg·kg(-1)·d(-1), po) for 3 months. After the mice were euthanized, total bone mineral density and cortical bone density were measured.

RESULTS

In RAW264.7 cells and BMMs, WSS25 (2.5, 5, 10 μg/mL) did not affect the cell viability, but dose-dependently inhibited RANKL-induced osteoclastogenesis. Furthermore, WSS25 potently suppressed RANKL-induced expression of TRAP, NFATc1, MMP-9 and cathepsin K in RAW264.7 cells. Treatment of RAW264.7 cells with RANKL increased BMP-2 expression, Smad1/5/8 phosphorylation and Id1 expression, which triggered osteoclast differentiation, whereas co-treatment with WSS25 or the endogenous BMP-2 antagonist noggin suppressed the BMP-2/Smad/Id1 signaling pathway. In RAW264.7 cells, knockdown of Id1 attenuated RANKL-induced osteoclast differentiation, which was partially rescued by Id1 overexpression. In conformity to the in vitro experiments, chronic administration of WSS25 significantly reduced the bone loss in ovariectomized mice.

CONCLUSION

WSS25 inhibits RANKL-induced osteoclast formation in RAW264.7 cells and BMMs by blocking the BMP-2/Smad/Id1 signaling pathway. WSS25 administration reduces bone loss in ovariectomized mice, suggesting that it may be a promising therapeutic agent for osteoporosis.

Yu Ping Feng San, an ancient Chinese herbal decoction containing Astragali Radix, Atractylodis Macrocephalae Rhizoma and Saposhnikoviae Radix, regulates the release of cytokines in murine macrophages

Yu Ping Feng San (YPFS), a Chinese herbal decoction, is composed of Astragali Radix (AR; Huangqi), Atractylodis Macrocephalae Rhizoma (AMR; Baizhu) and Saposhnikoviae Radix (SR; Fangfeng) in a weight ratio of 1∶2∶1. Clinically, YPFS has been widely used to regulate immune functions; however, the action mechanism of it is not known. Here, we addressed this issue by providing detail analyses of chemical and biological properties of YPFS. By using rapid resolution liquid chromatography coupled with mass spectrometry, fifteen chemicals deriving from different herbs of YPFS were determined, and which served as a control for the standardization of the herbal extract of YPFS. In general, the amounts of chosen chemical markers were higher in a preparation of YPFS as compared to that of single herb or two-herb compositions. In order to reveal the immune functions of YPFS, the standardized extract was applied onto cultured murine macrophages. The treatment of YPFS stimulated the mRNA and protein expressions of pro-inflammatory cytokines via activation of NF-κB by enhancing IκBα degradation. In contrast, the application of YPFS suppressed the expressions of pro-inflammatory cytokines significantly in the lipopolysaccharide (LPS)-induced chronic inflammation model. In addition, YPFS could up regulate the phagocytic activity in cultured macrophages. These results therefore supported the bi-directional immune-modulatory roles of YPFS in regulating the releases of cytokines from macrophages.

Abrogated RANKL expression in properdin-deficient mice is associated with better outcome from collagen-antibody-induced arthritis

Introduction

Properdin amplifies the alternative pathway of complement activation. In the present study, we evaluated its role in the development of collagen antibody-induced arthritis (CAIA).

Methods

Arthritis was induced by intraperitoneal injection of a collagen antibody cocktail into properdin-deficient (KO) and wild-type (WT) C57BL/6 mice. Symptoms of disease were evaluated daily. The degree of joint damage was assessed histologically and with immunostaining for bone-resorption markers. Phenotypes of cell populations, their receptor expression, and intracellular cytokine production were determined with flow cytometry. Osteoclast differentiation of bone marrow (BM) precursors was evaluated by staining for tartrate-resistant acid phosphatase (TRAP).

Results

Properdin-deficient mice developed less severe CAIA than did WT mice. They showed significantly improved clinical scores and downregulated expression of bone-resorption markers in the joints at day 10 of disease. The frequencies of Ly6G⁺CD11b⁺ cells were fewer in BM, blood, and synovial fluid (SF) of KO than of WT CAIA mice. The receptor activator of nuclear factor κB ligand (RANKL) was downregulated on arthritic KO neutrophils from BM and the periphery. Decreased C5a amounts in KO SF contributed to lower frequencies of CD5aR⁺-bearing neutrophils. In blood, surface C5aR was detected on KO Ly6G⁺ cells as a result of low receptor engagement. Circulating CD4⁺ T cells had an altered ability to produce interleukin (IL)-17 and interferon (IFN)-γ and to express RANKL. In KO CAIA mice, decreased frequencies of CD4⁺ T cells in the spleen were related to low CD86 expression on Ly6G^highCD11b⁺ cells. Arthritic KO T cells spontaneously secreted IFN-γ but not IL-17 and IL-6, and responded to restimulation with less-vigorous cytokine production in comparison to WT cells. Fewer TRAP-positive mature osteoclasts were found in KO BM cell cultures.

Conclusions

Our data show that the active involvement of properdin in arthritis is related to an increased proinflammatory cytokine production and RANKL expression on immune cells and to a stimulation of the RANKL-dependent osteoclast differentiation.