miR-708-5p deficiency involves the degeneration of mandibular condylar chondrocytes via the TLR4/NF-κB pathway.

OBJECTIVE: Ageing and aberrant biomechanical stimulation are two major risk factors for osteoarthritis (OA). One of the main characteristics of aged cartilage is cellular senescence. One of the main characteristics of osteoarthritic joints is cartilage degeneration. The cells in the temporomandibular joint (TMJ) cartilage are zonally arranged. The deep zone cells are differentiated from the superficial zone cells (SZCs). The purpose of the present study was to investigate whether degenerative shear stress (SS) stimulates the senescence programme in TMJ SZCs, and to determine which miRNA is involved in this process. METHOD: SZCs were isolated from the TMJ condyles of 3-week-old rats and treated with continuous passaging or SS. RNA sequencing was conducted to identify miRNA(s) that overlap with those involved in the replication senescence process and the SS-induced degeneration programme. Unilateral anterior crossbite (UAC), which is TMJ-OA inducible, was applied to 2-month-old and 12-month-old mice for 3 weeks. The effect of TMJ local injection of agomiR-708-5p was evaluated histologically. RESULTS: Both replication and SS treatment induced SZC senescence. miR-708-5p was identified. Knocking down miR-708-5p in SS-treated SZCs led to more severe senescence by alleviating the inhibitory impact of miR-708-5p on the TLR4/NF-κB pathway. miR-708-5p expression in mouse TMJ cartilage decreased with age. UAC induced more severe osteoarthritic cartilage lesions in 12-month-old mice than in 2-month-old mice. Injection of agomiR-708-5p suppressed UAC-induced osteoarthritic cartilage lesions. CONCLUSIONS: Age-related miR-708-5p deficiency is involved in the mechanically stimulated OA process. Intra-articular administration of agomiR-708-5p is a promising new strategy for OA treatment.

Loss of Twist1 in the Mesenchymal Compartment Promotes Increased Fibrosis in Experimental Lung Injury by Enhanced Expression of CXCL12.

Idiopathic pulmonary fibrosis (IPF) is a disease characterized by the accumulation of apoptosis-resistant fibroblasts in the lung. We have previously shown that high expression of the transcription factor Twist1 may explain this prosurvival phenotype in vitro. However, this observation has never been tested in vivo. We found that loss of Twist1 in COL1A2+ cells led to increased fibrosis characterized by very significant accumulation of T cells and bone marrow-derived matrix-producing cells. We found that Twist1-null cells expressed high levels of the T cell chemoattractant CXCL12. In vitro, we found that the loss of Twist1 in IPF lung fibroblasts increased expression of CXCL12 downstream of increased expression of the noncanonical NF-κB transcription factor RelB. Finally, blockade of CXCL12 with AMD3100 attenuated the exaggerated fibrosis observed in Twist1-null mice. Transcriptomic analysis of 134 IPF patients revealed that low expression of Twist1 was characterized by enrichment of T cell pathways. In conclusion, loss of Twist1 in collagen-producing cells led to increased bleomycin-induced pulmonary fibrosis, which is mediated by increased expression of CXCL12. Twist1 expression is associated with dysregulation of T cells in IPF patients. Twist1 may shape the IPF phenotype and regulate inflammation in fibrotic lung injury.

Extreme Trait Whole-Genome Sequencing Identifies PTPRO as a Novel Candidate Gene in Emphysema with Severe Airflow Obstruction.

RATIONALE: Genetic association studies in chronic obstructive pulmonary disease have primarily tested for association with common variants, the results of which explain only a portion of disease heritability. Because rare variation is also likely to contribute to susceptibility, we used whole-genome sequencing of subjects with clinically extreme phenotypes to identify genomic regions enriched for rare variation contributing to chronic obstructive pulmonary disease susceptibility. OBJECTIVES: To identify regions of rare genetic variation contributing to emphysema with severe airflow obstruction. METHODS: We identified heavy smokers that were resistant (n = 65) or susceptible (n = 64) to emphysema with severe airflow obstruction in the Pittsburgh Specialized Center of Clinically Oriented Research cohort. We filtered whole-genome sequencing results to include only rare variants and conducted single variant tests, region-based tests across the genome, gene-based tests, and exome-wide tests. MEASUREMENTS AND MAIN RESULTS: We identified several suggestive associations with emphysema with severe airflow obstruction, including a suggestive association of all rare variation in a region within the gene ZNF816 (19q13.41; P = 4.5 × 10-6), and a suggestive association of nonsynonymous coding rare variation in the gene PTPRO (P = 4.0 × 10-5). Association of rs61754411, a rare nonsynonymous variant in PTPRO, with emphysema and obstruction was demonstrated in all non-Hispanic white individuals in the Pittsburgh Specialized Center of Clinically Oriented Research cohort. We found that cells containing this variant have decreased signaling in cellular pathways necessary for survival and proliferation. CONCLUSIONS: PTPRO is a novel candidate gene in emphysema with severe airflow obstruction, and rs61754411 is a previously unreported rare variant contributing to emphysema susceptibility. Other suggestive candidate genes, such as ZNF816, are of interest for future studies.

Quantitative Primary Tumor Indocyanine Green Measurements Predict Osteosarcoma Metastatic Lung Burden in a Mouse Model.

BACKGROUND: Current preclinical osteosarcoma (OS) models largely focus on quantifying primary tumor burden. However, most fatalities from OS are caused by metastatic disease. The quantification of metastatic OS currently relies on CT, which is limited by motion artifact, requires intravenous contrast, and can be technically demanding in the preclinical setting. We describe the ability for indocyanine green (ICG) fluorescence angiography to quantify primary and metastatic OS in a previously validated orthotopic, immunocompetent mouse model. QUESTIONS/PURPOSES: (1) Can near-infrared ICG fluorescence be used to attach a comparable, quantitative value to the primary OS tumor in our experimental mouse model? (2) Will primary tumor fluorescence differ in mice that go on to develop metastatic lung disease? (3) Does primary tumor fluorescence correlate with tumor volume measured with CT? METHODS: Six groups of 4- to 6-week-old immunocompetent Balb/c mice (n = 6 per group) received paraphyseal injections into their left hindlimb proximal tibia consisting of variable numbers of K7M2 mouse OS cells. A hindlimb transfemoral amputation was performed 4 weeks after injection with euthanasia and lung extraction performed 10 weeks after injection. Histologic examination of lung and primary tumor specimens confirmed ICG localization only within the tumor bed. RESULTS: Mice with visible or palpable tumor growth had greater hindlimb fluorescence (3.5 ± 2.3 arbitrary perfusion units [APU], defined as the fluorescence pixel return normalized by the detector) compared with those with a negative examination (0.71 ± 0.38 APU, -2.7 ± 0.5 mean difference, 95% confidence interval -3.7 to -1.8, p < 0.001). A strong linear trend (r = 0.81, p < 0.01) was observed between primary tumor and lung fluorescence, suggesting that quantitative ICG tumor fluorescence is directly related to eventual metastatic burden. We did not find a correlation (r = 0.04, p = 0.45) between normalized primary tumor fluorescence and CT volumetric measurements. CONCLUSIONS: We demonstrate a novel methodology for quantifying primary and metastatic OS in a previously validated, immunocompetent, orthotopic mouse model. Quantitative fluorescence of the primary tumor with ICG angiography is linearly related to metastatic burden, a relationship that does not exist with respect to clinical tumor size. This highlights the potential utility of ICG near-infrared fluorescence imaging as a valuable preclinical proof-of-concept modality. Future experimental work will use this model to evaluate the efficacy of novel OS small molecule inhibitors. CLINICAL RELEVANCE: Given the histologic localization of ICG to only the tumor bed, we envision the clinical use of ICG angiography as an intraoperative margin and tumor detector. Such a tool may be used as an alternative to intraoperative histology to confirm negative primary tumor margins or as a valuable tool for debulking surgeries in vulnerable anatomic locations. Because we have demonstrated the successful preservation of ICG in frozen tumor samples, future work will focus on blinded validation of this modality in observational human trials, comparing the ICG fluorescence of harvested tissue samples with fresh frozen pathology.

Lung cells support osteosarcoma cell migration and survival.

BACKGROUND: Osteosarcoma (OS) is the most common primary bone tumor, with a propensity to metastasize to the lungs. Five-year survival for metastatic OS is below 30%, and has not improved for several decades despite the introduction of multi-agent chemotherapy. Understanding OS cell migration to the lungs requires an evaluation of the lung microenvironment. Here we utilized an in vitro lung cell and OS cell co-culture model to explore the interactions between OS and lung cells, hypothesizing that lung cells would promote OS cell migration and survival. The impact of a novel anti-OS chemotherapy on OS migration and survival in the lung microenvironment was also examined. METHODS: Three human OS cell lines (SJSA-1, Saos-2, U-2) and two human lung cell lines (HULEC-5a, MRC-5) were cultured according to American Type Culture Collection recommendations. Human lung cell lines were cultured in growth medium for 72 h to create conditioned media. OS proliferation was evaluated in lung co-culture and conditioned media microenvironment, with a murine fibroblast cell line (NIH-3 T3) in fresh growth medium as controls. Migration and invasion were measured using a real-time cell analysis system. Real-time PCR was utilized to probe for Aldehyde Dehydrogenase (ALDH1) expression. Osteosarcoma cells were also transduced with a lentivirus encoding for GFP to permit morphologic analysis with fluorescence microscopy. The anti-OS efficacy of Disulfiram, an ALDH-inhibitor previously shown to inhibit OS cell proliferation and metastasis in vitro, was evaluated in each microenvironment. RESULTS: Lung-cell conditioned medium promoted osteosarcoma cell migration, with a significantly higher attractive effect on all three osteosarcoma cell lines compared to basic growth medium, 10% serum containing medium, and NIH-3 T3 conditioned medium (p <0.05). Lung cell conditioned medium induced cell morphologic changes, as demonstrated with GFP-labeled cells. OS cells cultured in lung cell conditioned medium had increased alkaline phosphatase staining. CONCLUSIONS: Lung endothelial HULEC-5a cells are attractants for OS cell migration, proliferation, and survival. The SJSA-1 osteosarcoma cell line demonstrated greater metastatic potential than Saos-2 and U-2 cells. ALDH appears to be involved in the interaction between lung and OS cells, and ALP may be a valuable biomarker for monitoring functional OS changes during metastasis.

AP2 suppresses osteoblast differentiation and mineralization through down-regulation of Frizzled-1.

Transcription factor activating protein 2 (AP2) plays an important role in cellular differentiation. Although profound craniofacial and long bone developmental abnormalities have been observed in AP2-knockout mice, the molecular effects of AP2 on osteoblasts are poorly defined. We demonstrated that AP2 regulates the expression of human Frizzled 1 (FZD1), a co-receptor for the Wnt signalling pathway, in human osteoblast cell lines and primary bone marrow stromal cells (BMSCs). We also identified a putative AP2-binding site in the FZD1 proximal promoter in silico and characterized this binding element further in Saos2 in vitro by ChIP, electrophoretic mobility shift and promoter reporter assays. The transcriptional repression of the FZD1 promoter by AP2 was confirmed in normal human fetal osteoblasts (hFOB). Furthermore, overexpression of AP2 resulted in a significant reduction in both differentiation and mineralization of Saos2 cells. Knockdown of FZD1 expression before AP2 up-regulation diminished the AP2-dependent inhibition of Saos2 cell differentiation and mineralization. Similarly, overexpressing FZD1 before AP2 treatment in both Saos2 and BMSCs diminished the inhibitory effect of AP2 on osteoblast differentiation and mineralization. Taken together, these results demonstrate that AP2 is a negative regulator of osteoblast differentiation and mineralization, and its inhibitory effect may be mediated in part through down-regulation of FZD1 expression.

Heritability and genetics of serum dickkopf 1 levels in African ancestry families.

The aim of the study was to determine the heritability of serum dickkopf-1 (DKK1) and its association with DKK1 polymorphisms in African ancestry subjects. Serum DKK1 was measured in 422 Afro-Caribbean men and women aged 18+ from 7 large, multi-generational families (mean family size: 60; 3,215 relative pairs). Twenty-four common single nucleotide polymorphisms (SNPs) were genotyped within an 80 kilobase-pair region encompassing the DKK1 gene. Heritability was estimated and SNPs were tested for association with serum DKK1 using variance components analysis. DKK1 mRNA expression was tested in peripheral blood of 16 individuals from each of the rs7069912 genotypes. Mean serum DKK1 was 1724.1 pg/mL and was significantly lower in women than men (P = 0.043). Residual genetic heritability of serum DKK1 was 0.4460 (P < 0.0001). Six SNPs reached nominal significance with DKK1, with rs7069912 being significant after adjustment for multiple comparisons. Two of these six SNPs represented independent association signals (rs7069912 and rs16928725), which accounted for 4.6% of the phenotypic variation in DKK1. Additionally, carriers of the rs7069912 variant had significantly greater DKK1 expression than non-carriers (P = 0.036). Serum DKK1 levels are highly heritable in the African ancestry families. Two SNPs within the DKK1 region accounted for nearly 5% of the variation in serum DKK1.

ATF4 protein deficiency protects against high fructose-induced hypertriglyceridemia in mice.

Hypertriglyceridemia is the most common lipid disorder in obesity and type 2 diabetes. It results from increased production and/or decreased clearance of triglyceride-rich lipoproteins. To better understand the pathophysiology of hypertriglyceridemia, we studied hepatic regulation of triglyceride metabolism by the activating transcription factor 4 (ATF4), a member of the basic leucine zipper-containing protein subfamily. We determined the effect of ATF4 on hepatic lipid metabolism in Atf4(-/-) mice fed regular chow or provided with free access to fructose drinking water. ATF4 depletion preferentially attenuated hepatic lipogenesis without affecting hepatic triglyceride production and fatty acid oxidation. This effect prevented excessive fat accumulation in the liver of Atf4(-/-) mice, when compared with wild-type littermates. To gain insight into the underlying mechanism, we showed that ATF4 depletion resulted in a significant reduction in hepatic expression of peroxisome proliferator-activated receptor-γ, a nuclear receptor that acts to promote lipogenesis in the liver. This effect was accompanied by a significant reduction in hepatic expression of sterol regulatory element-binding protein 1c (SREBP-1c), acetyl-CoA carboxylase, and fatty-acid synthase, three key functions in the lipogenic pathway in Atf4(-/-) mice. Of particular significance, we found that Atf4(-/-) mice, as opposed to wild-type littermates, were protected against the development of steatosis and hypertriglyceridemia in response to high fructose feeding. These data demonstrate that ATF4 plays a critical role in regulating hepatic lipid metabolism in response to nutritional cues.

Critical role of filamin-binding LIM protein 1 (FBLP-1)/migfilin in regulation of bone remodeling.

Bone remodeling is a complex process that must be precisely controlled to maintain a healthy life. We show here that filamin-binding LIM protein 1 (FBLP-1, also known as migfilin), a kindlin- and filamin-binding focal adhesion protein, is essential for proper control of bone remodeling. Genetic inactivation of FBLIM1 (the gene encoding FBLP-1) in mice resulted in a severe osteopenic phenotype. Primary FBLP-1 null bone marrow stromal cells (BMSCs) exhibited significantly reduced extracellular matrix adhesion and migration compared with wild type BMSCs. Loss of FBLP-1 significantly impaired the growth and survival of BMSCs in vitro and decreased the number of osteoblast (OB) progenitors in bone marrow and OB differentiation in vivo. Furthermore, the loss of FBLP-1 caused a dramatic increase of osteoclast (OCL) differentiation in vivo. The level of receptor activator of nuclear factor κB ligand (RANKL), a key regulator of OCL differentiation, was markedly increased in FBLP-1 null BMSCs. The capacity of FBLP-1 null bone marrow monocytes (BMMs) to differentiate into multinucleated OCLs in response to exogenously supplied RANKL, however, was not different from that of WT BMMs. Finally, we show that a loss of FBLP-1 promotes activating phosphorylation of ERK1/2. Inhibition of ERK1/2 activation substantially suppressed the increase of RANKL induced by the loss of FBLP-1. Our results identify FBLP-1 as a key regulator of bone homeostasis and suggest that FBLP-1 functions in this process through modulating both the intrinsic properties of OB/BMSCs (i.e., BMSC-extracellular matrix adhesion and migration, cell growth, survival, and differentiation) and the communication between OB/BMSCs and BMMs (i.e., RANKL expression) that controls osteoclastogenesis.

Gfi1 expressed in bone marrow stromal cells is a novel osteoblast suppressor in patients with multiple myeloma bone disease.

Protracted inhibition of osteoblast (OB) differentiation characterizes multiple myeloma (MM) bone disease and persists even when patients are in long-term remission. However, the underlying pathophysiology for this prolonged OB suppression is unknown. Therefore, we developed a mouse MM model in which the bone marrow stromal cells (BMSCs) remained unresponsive to OB differentiation signals after removal of MM cells. We found that BMSCs from both MM-bearing mice and MM patients had increased levels of the transcriptional repressor Gfi1 compared with controls and that Gfi1 was a novel transcriptional repressor of the critical OB transcription factor Runx2. Trichostatin-A blocked the effects of Gfi1, suggesting that it induces epigenetic changes in the Runx2 promoter. MM-BMSC cell-cell contact was not required for MM cells to increase Gfi1 and repress Runx2 levels in MC-4 before OBs or naive primary BMSCs, and Gfi1 induction was blocked by anti-TNF-α and anti-IL-7 antibodies. Importantly, BMSCs isolated from Gfi1(-/-) mice were significantly resistant to MM-induced OB suppression. Strikingly, siRNA knockdown of Gfi1 in BMSCs from MM patients significantly restored expression of Runx2 and OB differentiation markers. Thus, Gfi1 may have an important role in prolonged MM-induced OB suppression and provide a new therapeutic target for MM bone disease.

Foxo1 mediates insulin-like growth factor 1 (IGF1)/insulin regulation of osteocalcin expression by antagonizing Runx2 in osteoblasts.

In this study, we determined the molecular mechanisms whereby forkhead transcription factor Foxo1, a key downstream signaling molecule of insulin-like growth factor 1 (IGF1)/insulin actions, regulates Runx2 activity and expression of the mouse osteocalcin gene 2 (Bglap2) in osteoblasts in vitro. We showed that Foxo1 inhibited Runx2-dependent transcriptional activity and osteocalcin mRNA expression and Bglap2 promoter activity in MC-4 preosteoblasts. Co-immunoprecipitation assay showed that Foxo1 physically interacted with Runx2 via its C-terminal region in osteoblasts or when co-expressed in COS-7 cells. Electrophoretic mobility shift assay demonstrated that Foxo1 suppressed Runx2 binding to its cognate site within the Bglap2 promoter. IGF1 and insulin prevented Foxo1 from inhibiting Runx2 activity by promoting Foxo1 phosphorylation and nuclear exclusion. In contrast, a neutralizing anti-IGF1 antibody decreased Runx2 activity and osteocalcin expression in osteoblasts. Chromatin immunoprecipitation assay revealed that IGF1 increased Runx2 interaction with a chromatin fragment of the proximal Bglap2 promoter in a PI3K/AKT-dependent manner. Conversely, knockdown of Foxo1 increased Runx2 interaction with the promoter. This study establishes that Foxo1 is a novel negative regulator of osteoblast-specific transcription factor Runx2 and modulates IGF1/insulin-dependent regulation of osteocalcin expression in osteoblasts.

Parathyroid hormone increases activating transcription factor 4 expression and activity in osteoblasts: requirement for osteocalcin gene expression.

PTH is an important peptide hormone regulator of calcium homeostasis and osteoblast function. However, its mechanism of action in osteoblasts is poorly understood. Our previous study demonstrated that PTH activates mouse osteocalcin (Ocn) gene 2 promoter through the osteoblast-specific element 1 site, a recently identified activating transcription factor-4 (ATF4) -binding element. In the present study, we examined effects of PTH on ATF4 expression and activity as well as the requirement for ATF4 in the regulation of Ocn by PTH. Results show that PTH elevated levels of ATF4 mRNA and protein in a dose- and time-dependent manner. This PTH regulation requires transcriptional activity but not de novo protein synthesis. PTH also increased binding of nuclear extracts to osteoblast-specific element 1 DNA. PTH stimulated ATF4-dependent transcriptional activity mainly through protein kinase A with a lesser requirement for protein kinase C and MAPK/ERK pathways. Lastly, PTH stimulation of Ocn expression was lost by small interfering RNA down-regulation of ATF4 in MC-4 cells and Atf4(-/-) bone marrow stromal cells. Collectively, these studies for the first time demonstrate that PTH increases ATF4 expression and activity and that ATF4 is required for PTH induction of Ocn expression in osteoblasts.

Activating transcription factor 4 is critical for proliferation and survival in primary bone marrow stromal cells and calvarial osteoblasts.

Activating transcription factor 4 (ATF4) is essential for bone formation. However, the mechanism of its actions in bone is poorly understood. The present study examined the role for ATF4 in the regulation of proliferation and survival of primary mouse bone marrow stromal cells (BMSCs) and osteoblasts. Results showed that Atf4(-/-) cells display a severe proliferative defect as measured by multiple cell proliferation assays. Cell cycle progression of Atf4(-/-) BMSCs was largely delayed with significant G1 arrest. Expression of cyclin D1 was decreased both at the mRNA and protein level. A similar proliferation defect was observed in Atf4(-/-) calvarial periosteal osteoblasts when compared with wt control. Knocking down Atf4 mRNA by small interfering RNA in MC3T3-E1 subclone 4 preosteoblasts markedly reduced expression of cyclin D1 and cell proliferation. In contrast, overexpression of ATF4 increased cyclin D1 expression as well as cell proliferation in Atf4(-/-) BMSCs. In addition, apoptosis was significantly increased in Atf4(-/-) BMSCs and calvarial periosteal osteoblasts relative to wt controls. Taken together, these results for the first time demonstrate that ATF4 is a critical regulator of proliferation and survival in BMSCs and osteoblasts in vitro and in vivo.

Rapamycin inhibits osteoblast proliferation and differentiation in MC3T3-E1 cells and primary mouse bone marrow stromal cells.

While the roles of the mammalian target of rapamycin (mTOR) signaling in regulation of cell growth, proliferation, and survival have been well documented in various cell types, its actions in osteoblasts are poorly understood. In this study, we determined the effects of rapamycin, a specific inhibitor of mTOR, on osteoblast proliferation and differentiation using MC3T3-E1 preosteoblastic cells (MC-4) and primary mouse bone marrow stromal cells (BMSCs). Rapamycin significantly inhibited proliferation in both MC-4 cells and BMSCs at a concentration as low as 0.1 nM. Western blot analysis shows that rapamycin treatment markedly reduced levels of cyclin A and D1 protein in both cell types. In differentiating osteoblasts, rapamycin dramatically reduced osteoblast-specific osteocalcin (Ocn), bone sialoprotein (Bsp), and osterix (Osx) mRNA expression, ALP activity, and mineralization capacity. However, the drug treatment had no effect on osteoblast differentiation parameters when the cells were completely differentiated. Importantly, rapamycin markedly reduced levels of Runx2 protein in both proliferating and differentiating but not differentiated osteoblasts. Finally, overexpression of S6K in COS-7 cells significantly increased levels of Runx2 protein and Runx2 activity. Taken together, our studies demonstrate that mTOR signaling affects osteoblast functions by targeting osteoblast proliferation and the early stage of osteoblast differentiation.

Transfection of primary chondrocytes using chitosan-pEGFP nanoparticles.

Chitosan-pEGFP nanoparticles were synthesized through the complex coacervation of the cationic polymer with pEGFP, in order to examine the potential of chitosan as a non-viral gene delivery vector to transfer exogenous gene into primary chondrocytes for the treatment of joint diseases. The nanoparticles were prepared at an N/P ratio of 3.8 and showed a spherical or irregular shape. The mean particle size and zeta potential of the nanoparticles freshly prepared with chitosan of different molecular weight were in the range of 100-300 nm and varied from +1 to +23 mV, respectively. Both the particle size and the zeta potential altered in DMEM of different pH. The transfection of primary chondrocytes was performed in different conditions by varying pH of transfection medium, molecular weight of chitosan and different plasmid dosage. Analysis of FACS demonstrated that the transfection efficiency could reach a much high level and the percentage of positive cells could exceed 50% in certain condition. These results suggest that chitosan-DNA nanoparticles have favorable characteristics for non-viral gene delivery to primary chondrocytes, and have the potential to deliver therapeutic genes directly into joint.

Transcriptional Regulation of Frizzled-1 in Human Osteoblasts by Sp1.

The wingless pathway has a powerful influence on bone metabolism and is a therapeutic target in skeletal disorders. Wingless signaling is mediated in part through the Frizzled (FZD) receptor family. FZD transcriptional regulation is poorly understood. Herein we tested the hypothesis that Sp1 plays an important role in the transcriptional regulation of FZD1 expression in osteoblasts and osteoblast mineralization. To test this hypothesis, we conducted FZD1 promoter assays in Saos2 cells with and without Sp1 overexpression. We found that Sp1 significantly up-regulates FZD1 promoter activity in Saos2 cells. Chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift (EMSA) assays identified a novel and functional Sp1 binding site at -44 to -40 from the translation start site in the FZD1 promoter. The Sp1-dependent activation of the FZD1 promoter was abolished by mithramycin A (MMA), an antibiotic affecting both Sp1 binding and Sp1 protein levels in Saos2 cells. Similarly, down-regulation of Sp1 in hFOB cells resulted in less FZD1 expression and lower alkaline phosphatase activity. Moreover, over-expression of Sp1 increased FZD1 expression and Saos2 cell mineralization while MMA decreased Sp1 and FZD1 expression and Saos2 cell mineralization. Knockdown of FZD1 prior to Sp1 overexpression partially abolished Sp1 stimulation of osteoblast differentiation markers. Taken together, our results suggest that Sp1 plays a role in human osteoblast differentiation and mineralization, which is at least partially mediated by Sp1-dependent transactivation of FZD1.

E2F1 effects on osteoblast differentiation and mineralization are mediated through up-regulation of frizzled-1.

Frizzled homolog 1 (FZD1) is a transmembrane receptor that mediates Wnt signaling. The transcriptional regulation of FZD1 and the role of FZD1 in osteoblast biology are not well understood. We examined the role of E2F1 in FZD1 promoter activation and osteoblast differentiation and mineralization. A putative E2F1 binding site in the FZD1 promoter region was initially identified in silico and characterized further in Saos2 cells in vitro by chromatin immunoprecipitation (ChIP), electrophoretic mobility shift (EMSA) and promoter reporter assays. Over-expression of E2F1 transactivated the FZD1 promoter and increased endogenous FZD1 mRNA and protein levels in Saos2 cells. Over-expression of E2F1 in Saos2 cells up-regulated osteoblast differentiation markers alkaline phosphatase (ALP), type I collagen α (COL1A), and osteocalcin (OCN). Furthermore, E2F1 over-expression enhanced mineralization of differentiated Saos2 cells, whereas siRNA knockdown of FZD1 diminished the effects of E2F1 on osteoblast mineralization. The effects of E2F1 on FZD1 expression and osteoblast mineralization were further confirmed in normal human FOB osteoblasts. Taken together, our experiments demonstrate a role of E2F1 in osteoblast differentiation and mineralization and suggest that FZD1 is required, in part, for E2F1 regulation of osteoblast mineralization.

atf4 promotes ß-catenin expression and osteoblastic differentiation of bone marrow mesenchymal stem cells.

Bone marrow mesenchymal stem cells (MSCs) can differentiate into multiple cell types including osteoblasts. How this differentiation process is controlled, however, is not completely understood. Here we show that activating transcription factor 4 (ATF4) plays a critical role in promoting bone marrow MSC differentiation towards the osteoblast lineage. Ablation of the Atf4 gene blocked the formation of osteoprogenitors and inhibited osteoblast differentiation without affecting the expansion and formation of MSCs in bone marrow cultures. Loss of ATF4 dramatically reduced the level of ß-catenin protein in MSCs in vitro and in osteoblasts/osteoprogenitors located on trabecular and calvarial surfaces. Loss of ATF4 did not decrease the expression of major canonical Wnt/ß-catenin signaling components such as Wnt3a, Wnt7b, Wnt10b, Lrp5, and Lrp6 in MSCs. Furthermore, shRNA knockdown of ATF4 expression decreased the level of ß-catenin protein in MC-4 preosteoblasts. In contrast, overexpression of ATF4 increased ß-catenin protein levels in MC-4 cells. Finally, ATF4 and ß-catenin formed a protein-protein complex in COS-7 cells coexpressing both factors or in MC-4 preosteoblastic cells. This study establishes a new role of ATF4 in controlling the ß-catenin protein levels and MSC differentiation towards the osteoblast lineage.

ADAR1 ablation decreases bone mass by impairing osteoblast function in mice.

Bone mass is controlled through a delicate balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption. We show here that RNA editing enzyme adenosine deaminase acting on RNA 1 (ADAR1) is critical for proper control of bone mass. Postnatal conditional knockout of Adar1 (the gene encoding ADAR1) resulted in a severe osteopenic phenotype. Ablation of the Adar1 gene significantly suppressed osteoblast differentiation without affecting osteoclast differentiation in bone. In vitro deletion of the Adar1 gene decreased expression of osteoblast-specific osteocalcin and bone sialoprotein genes, alkaline phosphatase activity, and mineralization, suggesting a direct intrinsic role of ADAR1 in osteoblasts. ADAR1 regulates osteoblast differentiation by, at least in part, modulation of osterix expression, which is essential for bone formation. Further, ablation of the Adar1 gene decreased the proliferation and survival of bone marrow stromal cells and inhibited the differentiation of mesenchymal stem cells towards osteoblast lineage. Finally, shRNA knockdown of the Adar1 gene in MC-4 pre-osteoblasts reduced cyclin D1 and cyclin A1 expression and cell growth. Our results identify ADAR1 as a new key regulator of bone mass and suggest that ADAR1 functions in this process mainly through modulation of the intrinsic properties of osteoblasts (i.e., proliferation, survival and differentiation).

Activating transcription factor 4 regulates osteoclast differentiation in mice.

Activating transcription factor 4 (ATF4) is a critical transcription factor for osteoblast (OBL) function and bone formation; however, a direct role in osteoclasts (OCLs) has not been established. Here, we targeted expression of ATF4 to the OCL lineage using the Trap promoter or through deletion of Atf4 in mice. OCL differentiation was drastically decreased in Atf4-/- bone marrow monocyte (BMM) cultures and bones. Coculture of Atf4-/- BMMs with WT OBLs or a high concentration of RANKL failed to restore the OCL differentiation defect. Conversely, Trap-Atf4-tg mice displayed severe osteopenia with dramatically increased osteoclastogenesis and bone resorption. We further showed that ATF4 was an upstream activator of the critical transcription factor Nfatc1 and was critical for RANKL activation of multiple MAPK pathways in OCL progenitors. Furthermore, ATF4 was crucial for M-CSF induction of RANK expression on BMMs, and lack of ATF4 caused a shift in OCL precursors to macrophages. Finally, ATF4 was largely modulated by M-CSF signaling and the PI3K/AKT pathways in BMMs. These results demonstrate that ATF4 plays a direct role in regulating OCL differentiation and suggest that it may be a therapeutic target for treating bone diseases associated with increased OCL activity.