Arang Kwon

TNF-α Upregulates Sclerostin Expression in Obese Mice Fed a High-Fat Diet

Sclerostin decreases bone mass by antagonizing the Wnt signaling pathway. We examined whether obesity‐induced bone loss is associated with the expression of sclerostin. Five‐week‐old male mice were assigned to one of two groups (n = 10 each) and fed either a control diet (10% kcal from fat; CON) or a high‐fat diet (60% kcal from fat; HF) for 12 weeks. Thex final body weight and whole body fat mass of the HF mice were higher than those of the CON mice. The distal femur cancellous bone mineral density and bone formation rate was lower in HF mice than in CON mice. The percent erosion surface was higher in the HF mice than the CON mice. The serum levels and femoral osteocytic protein expression levels of tumor necrosis factor‐α (TNF‐α) were significantly higher in HF mice than in CON mice. Sclerostin mRNA levels and osteocytic sclerostin protein levels in femoral cortex were also higher in HF mice than in CON mice. Sclerostin expression in MLO‐Y4 osteocytes increased with TNF‐α treatment, and TNF‐α‐induced sclerostin expression was blocked by the inhibition of NF‐κB activation. Chromatin immunoprecipitation and a luciferase reporter assay demonstrated that NF‐κB directly binds to the NF‐κB binding elements on the mouse sost promoter and stimulates sclerostin expression. These results support a model in which, in the context of obesity or other inflammatory diseases that increase the production of TNF‐α, TNF‐α upregulates the expression of sclerostin through NF‐κB signaling pathway, thus contributing to bone loss. J. Cell. Physiol. 229: 640–650, 2014.

High extracellular calcium-induced NFATc3 regulates the expression of receptor activator of NF-κB ligand in osteoblasts

Nuclear factor of activated T cell (NFAT) is a key transcription factor for receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation. However, it is unclear whether NFAT plays a role in the expression of RANKL in osteoblasts. High extracellular calcium ([Ca(2+)](o)) increases intracellular calcium, enhances RANKL expression in osteoblasts/stromal cells, and induces osteoclastogenesis in a coculture of osteoblasts and hematopoietic bone marrow cells. Because intracellular calcium signaling activates the calcineurin/NFAT pathway, we examined the role of NFAT activation on high [Ca(2+)](o)-induced RANKL expression in MC3T3-E1 subclone 4 (MC4) cells. Among the family of NFAT transcription factors, expression of NFATc1 and NFATc3, but not NFATc2, NFATc4 or NFAT5, was observed in MC4 cells. High [Ca(2+)](o) increased the expression levels of NFATc1, NFATc3 and RANKL. Cyclosporin A and FK506, inhibitors of calcineurin phosphatase, blocked high [Ca(2+)](o)-induced expression of NFAT and RANKL. Knockdown of NFATc1 and NFATc3 by siRNA prevented high [Ca(2+)](o)-induced RANKL expression, whereas overexpression of NFATc1 and NFATc3 induced RANKL expression. Furthermore, overexpressed NFATc1 upregulated NFATc3 expression, but NFATc1 knockdown decreased NFATc3 expression. Chromatin immunoprecipitation and reporter assay results showed that NFATc3, but not NFATc1, directly binds to the RANKL promoter and stimulates RANKL expression. In summary, these results demonstrate that high [Ca(2+)](o) increases expression of RANKL via activation of the calcineurin/NFAT pathway in osteoblasts. In addition, high [Ca(2+)](o) induces the activation and expression of NFATc1; NFATc3 expression and activity are subsequently increased; and NFATc3 directly binds to the RANKL promoter to increase its expression.

Hypoxia inducible factor-1α directly induces the expression of receptor activator of nuclear factor-κB ligand in periodontal ligament fibroblasts

During orthodontic tooth movement, local hypoxia and enhanced osteoclastogenesis are observed in the compression side of periodontal tissues. The receptor activator of nuclear factor-κB ligand (RANKL) is an osteoblast/stromal cell-derived factor that is essential for osteoclastogenesis. In this study, we examined the effect of hypoxia on RANKL expression in human periodontal ligament fibroblasts (PDLFs) to investigate the relationship between local hypoxia and enhanced osteoclastogenesis in the compression side of periodontal tissues. Hypoxia significantly enhanced the levels of RANKL mRNA and protein as well as hypoxia inducible factor-1α (HIF-1α) protein in PDLFs. Constitutively active HIF-1α alone significantly increased the levels of RANKL expression in PDLFs under normoxic conditions, whereas dominant negative HIF-1α blocked hypoxia-induced RANKL expression. To investigate further whether HIF-1α directly regulates RANKL transcription, a luciferase reporter assay was performed using the reporter vector containing the RANKL promoter sequence. Exposure to hypoxia or overexpression of constitutively active HIF-1α significantly increased RANKL promoter activity, whereas dominant negative HIF-1α blocked hypoxia-induced RANKL promoter activity. Furthermore, mutations of putative HIF-1α binding elements in RANKL promoter prevented hypoxia-induced RANKL promoter activity. The results of chromatin immunoprecipitation showed that hypoxia or constitutively active HIF-1α increased the DNA binding of HIF-1α to RANKL promoter. These results suggest that HIF-1α mediates hypoxia-induced up-regulation of RANKL expression and that in compression side periodontal ligament, hypoxia enhances osteoclastogenesis, at least in part, via an increased RANKL expression in PDLFs.

Tumor necrosis factor-α enhances the transcription of smad ubiquitination regulatory factor 1 in an activating protein-1- and runx2-dependent manner†

Smad ubiquitination regulatory factor 1 (Smurf1) is an E3 ubiquitin ligase that negatively regulates osteoblast differentiation. Although tumor necrosis factor‐α (TNF‐α) has been shown to increase Smurf1 expression, the details of the regulatory mechanisms remain unclear. Here, we investigated the molecular mechanism by which TNF‐α stimulates Smurf1 expression in C2C12 and primary cultured mouse calvarial cells. TNF‐α treatment rapidly induced the activation of NF‐κB and MAPKs. Smurf1 induction by TNF‐α was blocked by the inhibition of JNK or ERK, while the inhibition of NF‐κB and p38 MAPK had no effect on Smurf1 induction. TNF‐α treatment or c‐Jun overexpression enhanced the activity of a luciferase reporter that contained a 2.7 kb mouse Smurf1 promoter sequence. Site‐directed mutagenesis of the Smurf1 reporter and chromatin immunoprecipitation analysis demonstrated that the activating protein‐1 (AP‐1) binding motif at −922 bp on the mouse Smurf1 promoter mediated TNF‐α/JNK/AP‐1‐stimulated Smurf1 transcription. Interestingly, Smurf1 expression was not observed in Runx2‐null mouse calvarial cells. When Runx2 was ectopically expressed in these cells, the basal and TNF‐α‐induced expression of Smurf1 was restored. Overexpression of Runx2 transactivated the Smurf1 promoter in a dose‐dependent manner. Reporter and chromatin immunoprecipitation assays demonstrated that the Runx2‐binding motif at −202 bp functioned in Runx2‐mediated Smurf1 expression. ERK activation by TNF‐α treatment or constitutively active MEK1 overexpression increased Smurf1 expression in a Runx2‐dependent manner. These results suggest that the JNK/AP‐1 and ERK/Runx2 signaling pathways mediate TNF‐α‐dependent Smurf1 transcription. J. Cell. Physiol.

SMURF1 Plays a role in EGF-induced breast cancer cell migration and invasion

Epidermal growth factor (EGF) is a well-known growth factor that induces cancer cell migration and invasion. Previous studies have shown that SMAD ubiquitination regulatory factor 1 (SMURF1), an E3 ubiquitin ligase, regulates cell motility by inducing RhoA degradation. Therefore, we examined the role of SMURF1 in EGF-induced cell migration and invasion using MDA-MB-231 cells, a human breast cancer cell line. EGF increased SMURF1 expression at both the mRNA and protein levels. All ErbB family members were expressed in MDA-MB-231 cells and receptor tyrosine kinase inhibitors specific for the EGF receptor (EGFR) or ErbB2 blocked the EGF-mediated induction of SMURF1 expression. Within the signaling pathways examined, ERK1/2 and protein kinase C activity were required for EGF-induced SMURF1 expression. The overexpression of constitutively active MEK1 increased the SMURF1 to levels similar to those induced by EGF. SMURF1 induction by EGF treatment or by the overexpression of MEK1 or SMURF1 resulted in enhanced cell migration and invasion, whereas SMURF1 knockdown suppressed EGF- or MEK1-induced cell migration and invasion. EGF treatment or SMURF1 overexpression decreased the endogenous RhoA protein levels. The overexpression of constitutively active RhoA prevented EGF- or SMURF1-induced cell migration and invasion. These results suggest that EGFinduced SMURF1 plays a role in breast cancer cell migration and invasion through the downregulation of RhoA.

The Effect of Antioxidants on the Production of Pro-Inflammatory Cytokines and Orthodontic Tooth Movement

Orthodontic force causes gradual compression of the periodontal ligament tissues, which leads to local hypoxia in the compression side of the tissues. In this study, we investigated whether antioxidants exert a regulatory effect on two factors: the expression of pro-inflammatory cytokines in human periodontal ligament fibroblasts (PDLFs) that were exposed to mechanical compression and hypoxia and the rate of orthodontic tooth movement in rats. Exposure of PDLFs to mechanical compression (0.5–3.0 g/cm2) or hypoxic conditions increased the production of intracellular reactive oxygen species. Hypoxic treatment for 24 h increased the mRNA levels of IL-1β, IL-6 and IL-8 as well as vascular endothelial growth factor (VEGF) in PDLFs. Resveratrol (10 nM) or N-acetylcysteine (NAC, 20 mM) diminished the transcriptional activity of hypoxiainducible factor-1 and hypoxia-induced expression of VEGF. Combined treatment with mechanical compression and hypoxia significantly increased the expression levels of IL-1β, IL-6, IL-8, TNF-α and VEGF in PDLFs. These levels were suppressed by NAC and resveratrol. The maxillary first molars of rats were moved mesially for seven days using an orthodontic appliance. NAC decreased the amount of orthodontic tooth movement compared to the vehicle-treated group. The results from immunohistochemical staining demonstrated that NAC suppressed the expression of IL-1β and TNF-α in the periodontal ligament tissues compared to the vehicle-treated group. These results suggest that antioxidants have the potential to negatively regulate the rate of orthodontic tooth movement through the down-regulation of pro-inflammatory cytokines in the compression sides of periodontal ligament tissues.

Smurf1 plays a role in EGF inhibition of BMP2-induced osteogenic differentiation

It has been demonstrated that epidermal growth factor (EGF) plays a role in supporting the proliferation of bone marrow stromal cells in bone but inhibits their osteogenic differentiation. However, the mechanism underlying EGF inhibition of osteoblast differentiation remains unclear. Smurf1 is an E3 ubiquitin ligase that targets Smad1/5 and Runx2, which are critical transcription factors for bone morphogenetic protein 2 (BMP2)-induced osteoblast differentiation. In this study, we investigated the effect of EGF on the expression of Smurf1, and the role of Smurf1 in EGF inhibition of osteogenic differentiation using C2C12 cells, a murine myoblast cell line. EGF increased Smurf1 expression, which was blocked by inhibiting the activity of either JNK or ERK. Chromatin immunoprecipitation and Smurf1 promoter assays demonstrated that c-Jun and Runx2 play roles in the EGF induction of Smurf1 transcription. EGF suppressed BMP2-induced expression of osteogenic marker genes, which were rescued by Smurf1 knockdown. EGF downregulated the protein levels of Runx2 and Smad1 in a proteasome-dependent manner. EGF decreased the transcriptional activity of Runx2 and Smurf1, which was partially rescued by Smurf1 silencing. Taken together, these results suggest that EGF increases Smurf1 expression via the activation of JNK and ERK and the subsequent binding of c-Jun and Runx2 to the Smurf1 promoter and that Smurf1 mediates the inhibitory effect of EGF on BMP2-induced osteoblast differentiation.

Suberoylanilide Hydroxamic Acid Enhances Odontoblast Differentiation

Previous studies have shown that histone deacetylase (HDAC) inhibitors stimulate osteoblast differentiation in vitro and bone formation in vivo. However, the effects of HDAC inhibitors on odontoblasts have not been elucidated. Therefore, in this study, we examined the effect of suberoylanilide hydroxamic acid (SAHA), an HDAC inhibitor, on odontoblast differentiation using an MDPC23 odontoblast-like cell line. SAHA significantly enhanced matrix mineralization and the expression levels of odontoblast marker genes. SAHA increased the expression levels of nuclear factor I/C (Nfic) and dentin sialophosphoprotein (Dspp). Nfic bound directly to the Dspp promoter and stimulated Dspp transcription. SAHA increased both basal and Nfic-induced Dspp promoter activity. SAHA-induced Dspp promoter activity disappeared when mutations were introduced within the Nfic binding element of the Dspp promoter. Nfic knockdown by siRNA blocked SAHA stimulation of Dspp expression. These results indicate that SAHA enhances odontoblast differentiation and that SAHA increases Dspp expression, at least in part, by increasing the expression level of Nfic.

Propranolol, a β-adrenergic antagonist, attenuates the decrease in trabecular bone mass in high calorie diet fed growing mice

We investigated the effects of high calorie and low calorie diets on skeletal integrity, and whether β-adrenergic blockade (BB) attenuates bone loss induced by dietary calorie alteration. Male 6-week-old C57BL/6 mice were assigned to either an ad-lib fed control diet (CON), a high calorie diet (HIGH), or a low calorie diet (LOW) group. In each diet group, mice were treated with either vehicle (VEH) or propranolol, a β-adrenergic antagonist. Over 12-weeks, β-blockade mitigated body weight and fat mass increases induced by the high calorie diet. Femoral trabecular bone mineral density and the expression levels of osteogenic marker genes in bone marrow cells were reduced in HIGHVEH and LOWVEH mice, and BB significantly attenuated this decline only in HIGH mice. In summary, the magnitude of bone loss induced by low calorie diet was greater than that caused by high calorie diet in growing mice, and β-blockade mitigated high calorie diet-induced bone loss. [BMB Reports 2014; 47(9): 506-511]

Myeloid Elf-1-like factor stimulates adipogenic differentiation through the induction of peroxisome proliferator-activated receptor γ expression in bone marrow†

Myeloid Elf‐1 like factor (MEF) is one of the Ets transcription factors known to regulate cell proliferation and differentiation. A previous report has shown that osteoblast‐specific MEF transgenic mice (Col1a1‐MEF TG mice) have low bone mass but high bone marrow adiposity. In the present study, we explored a previously unappreciated mechanism whereby MEF promotes adipogenesis in bone marrow. An adipogenic colony‐forming unit assay showed that bone marrow cells derived from Col1a1‐MEF TG mice had a higher adipogenic differentiation potential compared to those from wild‐type. The levels of adipogenic marker genes expression in 3T3L1 cells were higher when co‐cultured with Col1a1‐MEF TG bone marrow cells than with wild‐type cells. MC3T3‐E1 preosteoblasts transfected with MEF secreted higher levels of 15‐deoxy‐delta (12, 14)‐prostaglandin J2, a potent endogenous ligand of peroxisome proliferator‐activated receptor γ (PPARγ), under adipogenic conditions. MEF overexpression increased the adipogenic marker genes expression including PPARγ and lipid droplet accumulation in MC3T3‐E1 preosteoblasts and 3T3L1 preadipocytes. Endogenous MEF expression levels increased as adipocyte differentiation proceeded. Knockdown of MEF by siRNA suppressed expression levels of adipogenic marker genes including PPARγ. MEF directly bound to the MEF binding element on the mouse PPARγ promoter, transactivating promoter activity. Immunohistochemical staining of tibia sections demonstrated that bone lining cells and bone marrow cells express higher levels of PPARγ protein in Col1a1‐MEF TG mice than in wild‐type mice. These results suggest that MEF transactivates PPARγ expression, which, in turn, enhances adipogenic differentiation. Furthermore, MEF overexpressing osteoblasts secrete higher levels of adipogenic factors, creating a marrow microenvironment that favors adipogenesis. J. Cell. Physiol. 227: 3603–3612, 2012.