Post-translational Regulation of Runx2 in Bone and Cartilage

Comparative study of hydroxyapatite, fluor-hydroxyapatite and Si-substituted hydroxyapatite nanoparticles on osteogenic, osteoclastic and antibacterial ability

This study compared the effects of hydroxyapatite (HA), fluor-hydroxyapatite (FHA) and Si-substituted hydroxyapatite (SiHA) on osteogenic differentiation, osteoclastic activity and antibacterial properties. HA, FHA and SiHA were prepared via a sol-gel reaction and characterized by scanning electron microscopic analysis (SEM), transmission electron microscopic analysis (TEM), and X-ray photoelectron spectrometry. Cell proliferation was evaluated using an MTT assay and cytoskeletal morphology was observed by fluorescence microscopy. Osteogenic differentiation was evaluated using alkaline phosphatase activity and Alizarin red staining. Quantitative real-time PCR was used to evaluate the mRNA expression of runt-related transcription factor 2 (Runx2) and osteopontin (OPN). New bone formation was tested using μCT, haematoxylin and eosin staining and TRAP staining. The antibacterial actions against Porphyromonas gingivalis (P. g) were evaluated through plate counting and live-dead bacterial staining. The results demonstrated that HA, FHA and SiHA can promote proliferation of bone mesenchymal stem cells (BMSCs). ALP activity in FHA extract with a concentration of 625 μg mL-1 was the highest after 14 days osteogenic induction; similar results were observed for Runx2 and OPN mRNA expression. HA, FHA and SiHA decreased trabecular space in bone defects, but FHA reduced osteoclastic activity and inhibited P. g growth. In conclusion, FHA can promote osteogenic activity, reduce osteoclastic activity and enhance antibacterial effects.

RUNX family: Oncogenes or tumor suppressors (Review)

Runt-related transcription factor (RUNX) proteins belong to a transcription factors family known as master regulators of important embryonic developmental programs. In the last decade, the whole family has been implicated in the regulation of different oncogenic processes and signaling pathways associated with cancer. Furthermore, a suppressor tumor function has been also reported, suggesting the RUNX family serves key role in all different types of cancer. In this review, the known biological characteristics, specific regulatory abilities and experimental evidence of RUNX proteins will be analyzed to demonstrate their oncogenic potential and tumor suppressor abilities during oncogenic processes, suggesting their importance as biomarkers of cancer. Additionally, the importance of continuing with the molecular studies of RUNX proteins' and its dual functions in cancer will be underlined in order to apply it in the future development of specific diagnostic methods and therapies against different types of cancer.

TMCO1-mediated Ca2+ leak underlies osteoblast functions via CaMKII signaling

Transmembrane and coiled-coil domains 1 (TMCO1) is a recently identified Ca²⁺ leak channel in the endoplasmic reticulum. TMCO1 dysfunction in humans is associated with dysmorphism, mental retardation, glaucoma and the occurrence of cancer. Here we show an essential role of TMCO1 in osteogenesis mediated by local Ca²⁺/CaMKII signaling in osteoblasts. TMCO1 levels were significantly decreased in bone from both osteoporosis patients and bone-loss mouse models. Tmco1^−/− mice exhibited loss of bone mass and altered microarchitecture characteristic of osteoporosis. In the absence of TMCO1, decreased HDAC4 phosphorylation resulted in nuclear enrichment of HADC4, which leads to deacetylation and degradation of RUNX2, the master regulator of osteogenesis. We further demonstrate that TMCO1-mediated Ca²⁺ leak provides local Ca²⁺ signals to activate the CaMKII-HDAC4-RUNX2 signaling axis. The establishment of TMCO1 as a pivotal player in osteogenesis uncovers a novel potential therapeutic target for ameliorating osteoporosis.

TMCO1 is a recently described endoplasmic reticular Ca²⁺ channel. Here, the authors show it is important for osteoblast function and bone formation in mice, and identify a novel pathway linking local increases in Ca²⁺ at the ER surface with the posttranslational modification of RUNX2.

Identification of a Novel Transcription Factor Required for Osteogenic Differentiation of Mesenchymal Stem Cells

Osteogenic differentiation is a complex and still poorly understood biological process regulated by intrinsic cellular signals and extrinsic microenvironmental cues. Following appropriate stimuli, mesenchymal stem cells (MSCs) differentiate into osteoblasts through a tightly regulated multistep process driven by several transcription factors and characterized by the expression of a number of bone-specific proteins. In this study, we describe a novel transcription factor that we named osteoblast inducer (ObI)-1, involved in MSC differentiation toward the osteogenic lineage. ObI-1 encodes for a nuclear protein subjected to proteasomal degradation and expressed during osteoblast differentiation both in a murine multipotent mesenchymal cell line (W20-17) and in primary murine MSCs. RNA interference-mediated knockdown of ObI-1 expression significantly impairs osteoblast differentiation and matrix mineralization with reduced expression of the osteogenic markers, Runt-related transcription factor 2 (Runx2) and osteopontin. Conversely, ObI-1 overexpression enhances osteogenic differentiation and bone-specific markers expression. ObI-1 stimulates bone morphogenetic protein (BMP)-4 expression and the consequent activation of the Smad pathway; treatment with a BMP receptor type I antagonist completely abolishes ObI-1-mediated stimulation of osteogenic differentiation. Collectively, our findings suggest that ObI-1 modulates osteogenic differentiation, at least in part, through the BMP signaling pathway, increasing Runx2 activation and leading to osteoblast commitment and maturation.

Suppression of SIRT1 in Diabetic Conditions Induces Osteogenic Differentiation of Human Vascular Smooth Muscle Cells via RUNX2 Signalling

Vascular calcification is associated with significant morbidity and mortality within diabetes, involving activation of osteogenic regulators and transcription factors. Recent evidence demonstrates the beneficial role of Sirtuin 1 (SIRT1), an NAD⁺ dependant deacetylase, in improved insulin sensitivity and glucose homeostasis, linking hyperglycaemia and SIRT1 downregulation. This study aimed to determine the role of SIRT1 in vascular smooth muscle cell (vSMC) calcification within the diabetic environment. An 80% reduction in SIRT1 levels was observed in patients with diabetes, both in serum and the arterial smooth muscle layer, whilst both RUNX2 and Osteocalcin levels were elevated. Human vSMCs exposed to hyperglycaemic conditions in vitro demonstrated enhanced calcification, which was positively associated with the induction of cellular senescence, verified by senescence-associated β-galactosidase activity and cell cycle markers p16 and p21. Activation of SIRT1 by SRT1720 reduced Alizarin red staining by a third, via inhibition of the RUNX2 pathway and prevention of senescence. Conversely, inhibition of SIRT1 via Sirtinol and siRNA increased RUNX2 by over 50%. These findings demonstrate the key role that SIRT1 plays in preventing calcification in a diabetic environment, through the inhibition of RUNX2 and senescence pathways, suggesting a downregulation of SIRT1 may be responsible for perpetuating vascular calcification in diabetes.

Systems biology reveals how altered TGFβ signalling with age reduces protection against pro-inflammatory stimuli

Osteoarthritis (OA) is a degenerative condition caused by dysregulation of multiple molecular signalling pathways. Such dysregulation results in damage to cartilage, a smooth and protective tissue that enables low friction articulation of synovial joints. Matrix metalloproteinases (MMPs), especially MMP-13, are key enzymes in the cleavage of type II collagen which is a vital component for cartilage integrity. Transforming growth factor beta (TGFβ) can protect against pro-inflammatory cytokine-mediated MMP expression. With age there is a change in the ratio of two TGFβ type I receptors (Alk1/Alk5), a shift that results in TGFβ losing its protective role in cartilage homeostasis. Instead, TGFβ promotes cartilage degradation which correlates with the spontaneous development of OA in murine models. However, the mechanism by which TGFβ protects against pro-inflammatory responses and how this changes with age has not been extensively studied. As TGFβ signalling is complex, we used systems biology to combine experimental and computational outputs to examine how the system changes with age. Experiments showed that the repressive effect of TGFβ on chondrocytes treated with a pro-inflammatory stimulus required Alk5. Computational modelling revealed two independent mechanisms were needed to explain the crosstalk between TGFβ and pro-inflammatory signalling pathways. A novel meta-analysis of microarray data from OA patient tissue was used to create a Cytoscape network representative of human OA and revealed the importance of inflammation. Combining the modelled genes with the microarray network provided a global overview into the crosstalk between the different signalling pathways involved in OA development. Our results provide further insights into the mechanisms that cause TGFβ signalling to change from a protective to a detrimental pathway in cartilage with ageing. Moreover, such a systems biology approach may enable restoration of the protective role of TGFβ as a potential therapy to prevent age-related loss of cartilage and the development of OA.

Osteogenic Induction of Wharton's Jelly-Derived Mesenchymal Stem Cell for Bone Regeneration: A Systematic Review

Wharton's jelly-derived mesenchymal stem cells (WJ-MSCs) are emerging as a promising source for bone regeneration in the treatment of bone defects. Previous studies have reported the ability of WJ-MSCs to be induced into the osteogenic lineage. The purpose of this review was to systematically assess the potential of WJ-MSC differentiation into the osteogenic lineage. A comprehensive search was conducted in Medline via Ebscohost and Scopus, where relevant studies published between 1961 and 2018 were selected. The main inclusion criteria were that articles must be primary studies published in English evaluating osteogenic induction of WJ-MSCs. The literature search identified 92 related articles, but only 18 articles met the inclusion criteria. These include two animal studies, three articles containing both in vitro and in vivo assessments, and 13 articles on in vitro studies, all of which are discussed in this review. There were two types of osteogenic induction used in these studies, either chemical or physical. The studies demonstrate that WJ-MSCs are able to differentiate into osteogenic lineage and promote osteogenesis. In light of these observations, it is suggested that WJ-MSCs can be a potential source of stem cells for osteogenic induction, as an alternative to bone marrow-derived mesenchymal stem cells.

Oral implant osseointegration model in C57Bl/6 mice: microtomographic, histological, histomorphometric and molecular characterization

Despite the successful clinical application of titanium (Ti) as a biomaterial, the exact cellular and molecular mechanisms responsible for Ti osseointegration remains unclear, especially because of the limited methodological tools available in this field.

Integrin subunits αV and β3 promote the osteogenic differentiation of umbilical cord blood mesenchymal stem cells

Human umbilical cord-derived mesenchymal stem cells (hUCMSCs) are multipotent cells that have self-renewal properties and can differentiate into osteocytes, adipocytes, cartilage and extoderm. Bone regeneration and repair are important for the repair of bone injury, skeletal development or continuous remodeling throughout adult life. Thus, investigating the factors influencing osteocyte regeneration from hUCMSCs could be conducive to advancements in skeletal repair and the repair of bone injury. Previous reports have demonstrated that single integrin subunits (αV, β3, α5) and collagen I contribute to the osteogenic differentiation of human mesenchymal stem cells (hMSCs). However, the functions of the vitronectin receptor αV and β3 in the osteogenic differentiation of hUCMSCs and bone regeneration remain unclear. Run-related transcription factor 2 (RUNX2) is considered to be an early osteoblastic gene that is upregulated during the osteogenic differentiation of hUCMSCs. Meanwhile, bone sialoprotein (BSP) and collagen I are the most common early markers of osteoblast differentiation. Herein, we found that the mRNA and protein expression of αV, β3, RUNX2 and collagen I were upregulated during the osteogenic differentiation of hUCMSCs. Overexpression of αV and β3 in hMSCs increased the levels of RUNX2, BSP, and collagen I, decreased the number of adipocytes and promoted the osteogenic differentiation of hUCMSCs. Meanwhile, downregulation of αV and β3 decreased the levels of RUNX2, BSP, and collagen I, increased the number of adipocytes and blocked the osteogenic differentiation of hUCMSCs. In conclusion, the integrin subunits αV and β3 can promote the osteogenic differentiation of hUCMSCs and encourage bone formation.

Depletion of pro-oncogenic RUNX2 enhances gemcitabine (GEM) sensitivity of p53-mutated pancreatic cancer Panc-1 cells through the induction of pro-apoptotic TAp63

Recently, we have described that siRNA-mediated silencing of runt-related transcription factor 2 (RUNX2) improves anti-cancer drug gemcitabine (GEM) sensitivity of p53-deficient human pancreatic cancer AsPC-1 cells through the augmentation of p53 family TAp63-dependent cell death pathway. In this manuscript, we have extended our study to p53-mutated human pancreatic cancer Panc-1 cells. According to our present results, knockdown of mutant p53 alone had a marginal effect on GEM-mediated cell death of Panc-1 cells. We then sought to deplete RUNX2 using siRNA in Panc-1 cells and examined its effect on GEM sensitivity. Under our experimental conditions, RUNX2 knockdown caused a significant enhancement of GEM sensitivity of Panc-1 cells. Notably, GEM-mediated induction of TAp63 but not of TAp73 was further stimulated in RUNX2-depleted Panc-1 cells, indicating that, like AsPC-1 cells, TAp63 might play a pivotal role in the regulation of GEM sensitivity of Panc-1 cells. Consistent with this notion, forced expression of TAp63α in Panc-1 cells promoted cell cycle arrest and/or cell death, and massively increased luciferase activities driven by TAp63-target gene promoters such as p21^WAF1 and NOXA. In addition, immunoprecipitation experiments indicated that RUNX2 forms a complex with TAp63 in Panc-1 cells. Taken together, our current observations strongly suggest that depletion of RUNX2 enhances the cytotoxic effect of GEM on p53-mutated Panc-1 cells through the stimulation of TAp63-dependent cell death pathway even in the presence of a large amount of pro-oncogenic mutant p53, and might provide an attractive strategy to treat pancreatic cancer patients with p53 mutations.

Pseudoshikonin I enhances osteoblast differentiation by stimulating Runx2 and Osterix

Pseudoshikonin I (PSI), a novel biomaterial isolated from Lithospermi radix, has been recognized as an herbal medicine for the treatment of infectious and inflammatory diseases. Bone remodeling maintains a balance through bone resorption (osteoclastogenesis) and bone formation (osteoblastogenesis). Bone formation is generally attributed to osteoblasts. However, the effects of PSI on the bone are not well known. In this study, we found that the ethanol extracts of PSI induced osteoblast differentiation by increasing the expression of bone morphogenic protein 4 (BMP 4). PSI positively regulates the transcriptional expression and osteogenic activity of osteoblast-specific transcription factors such as Runx2 and Osterix. To identify the signaling pathways that mediate PSI-induced osteoblastogenesis, we examined the effects of serine-threonine kinase inhibitors that are known regulators of Osterix and Runx2. PSI-induced upregulation of Osterix and Runx2 was suppressed by treatment with AKT and PKA inhibitors. These results suggest that PSI enhances osteoblast differentiation by stimulating Osterix and Runx2 via the AKT and PKA signaling pathways. Thus, the activation of Runx2 and Osterix is modulated by PSI, thereby demonstrating its potential as a treatment target for bone disease.

KDM6A promotes chondrogenic differentiation of periodontal ligament stem cells by demethylation of SOX9

OBJECTIVES

KDM6A has been demonstrated critical in the regulation of cell fates. However, whether KDM6A is involved in cartilage formation remains unclear. In this study, we investigated the role of KDM6A in chondrogenic differentiation of PDLSCs, as well as the underlying epigenetic mechanisms.

METHODS

KDM6A shRNA was transfected into PDLSCs by lentivirus. The chondrogenic differentiation potential of PDLSCs was assessed by Alcian blue staining. Immunofluorescence was performed to demonstrate H3K27me3 and H3K4me3 levels during chondrogenesis. SOX9, Col2a1, ACAN and miRNAs (miR-29a, miR-204, miR-211) were detected by real-time RT-PCR. Western blot was performed to evaluate SOX9, H3K27me3 and H3K4me3.

RESULTS

The production of proteoglycans in PDLSCs was decreased after knockdown of KDM6A. Depletion of KDM6A inhibited the expression of SOX9, Col2a1, ACAN and resulted in increased H3K27me3 and decreased H3K4me3 levels. EZH2 inhibitor rescued the chondrogenic potential of PDLSCs after knockdown of KDM6A by regulating H3K27me3. Additionally, miR-29a, miR-204 and miR-211 were also involved in the process of PDLSCs chondrogenesis.

CONCLUSIONS

KDM6A is required in chondrogenic differentiation of PDLSCs by demethylation of H3K27me3, and EZH2 inhibitor could rescue chondrogenesis of PDLSCs after knockdown of KDM6A. It could be inferred that upregulation of KDM6A or application of EZH2 inhibitor might improve mesenchymal stem cell mediated cartilage regeneration in inflammatory tissue destruction such as osteoarthritis.

Characterization of CADD522, a small molecule that inhibits RUNX2-DNA binding and exhibits antitumor activity

The RUNX2 transcription factor promotes breast cancer growth and metastasis through interactions with a variety of cofactors that activate or repress target genes. Using a direct drug discovery approach we identified CADD522 as a small molecule that inhibits the DNA binding of the runt box domain protein, RUNX2. The current study defines the effect of CADD522 on breast cancer growth and metastasis, and addresses the mechanisms by which it exerts its anti-tumor activity.

CADD522 treatment resulted in significant growth inhibition, clonogenic survival, tumorsphere formation, and invasion of breast cancer cells. CADD522 negatively regulated transcription of RUNX2 target genes such as matrix metalloproteinase-13, vascular endothelial growth factor and glucose transporter-1, but upregulated RUNX2 expression by increasing RUNX2 stability. CADD522 reduced RUNX2-mediated increases in glucose uptake and decreased the level of CBF-β and RUNX2 phosphorylation at the S451 residue. These results suggest several potential mechanisms by which CADD522 exerts an inhibitory function on RUNX2-DNA binding; interference with RUNX2 for the DNA binding pocket, inhibition of glucose uptake leading to cell cycle arrest, down-regulation of CBF-β, and reduction of S451-RUNX2 phosphorylation.

The administration of CADD522 into MMTV-PyMT mice resulted in significant delay in tumor incidence and reduction in tumor burden. A significant decrease of tumor volume was also observed in a CADD522-treated human triple-negative breast cancer-patient derived xenograft model. CADD522 impaired the lung retention and outgrowth of breast cancer cells in vivo with no apparent toxicity to the mice. Therefore, by inhibiting RUNX2-DNA binding, CADD522 may represent a potential antitumor drug.

DNA methylation of the RUNX2 P1 promoter mediates MMP13 transcription in chondrocytes

The Runt-related transcription factor 2 (RUNX2) is critical for bone formation as well as chondrocyte maturation. Matrix metalloproteinase (MMP)-13 is a major contributor to cartilage degradation in osteoarthritis (OA). We and others have shown that the abnormal MMP13 gene expression in OA chondrocytes is controlled by changes in the DNA methylation status of specific CpG sites of the proximal promoter, as well as by the actions of different transactivators, including RUNX2. The present study aimed to determine the influence of the methylation status of specific CpG sites in the RUNX2 promoter on RUNX2-driven MMP13 gene expression in OA chondrocytes. We observed a significant correlation between MMP13 mRNA levels and RUNX2 gene expression in human OA chondrocytes. RUNX2 overexpression enhanced MMP13 promoter activity, independent of the MMP13 promoter methylation status. A significant negative correlation was observed between RUNX2 mRNA levels in OA chondrocytes and the percentage methylation of the CpG sites in the RUNX2 P1 promoter. Accordingly, the activity of the wild type RUNX2 promoter was decreased upon methylation treatment in vitro. We conclude that RUNX2 gene transcription is regulated by the methylation status of specific CpG sites in the promoter and may determine RUNX2 availability in OA cartilage for transactivation of genes such as MMP13.

Bone morphogenetic protein-5, a key molecule that mediates differentiation in MC3T3E1 osteoblast cell line

Bone morphogenetic protein-5 (BMP-5) is a member of the TGF receptor-β family with osteoinductive property. However, its physiological role in osteoblast differentiation is not defined. This study highlights the importance of BMP-5 in MC3T3E1 osteoblast differentiation. Pre-osteoblasts exposed to osteogenic media (ascorbic acid, 50 µg/ml and β-glycerophosphate, 10 mM) showed high protein expression of BMP-5 in cell lysates and cell culture supernatants, which peaked during early time-points of differentiation and declined with onset of mineralization. Attenuation of endogenous BMP-5 protein expression by RNA interference downregulated the expression of type I collagen (COLIA1), an early osteoblast differentiation marker but not osteocalcin, a late osteoblast differentiation marker. Further experiments to analyze the cell signaling components revealed that BMP-5 modulates COLIA1 expression via p38-Runx2 axis involving Runx2 (Ser19) phosphorylation. These effects were also observed when recombinant BMP-5 was added to pre-osteoblast cultures reinforcing the fact that BMP-5 is a modulator of COLIA1 expression. We conclude that BMP-5 has stage-specific role to play during MC3T3E1 osteoblast differentiation in part by autocrine p38/Runx2/COLIA1 signaling. © 2017 BioFactors, 43(4):558-566, 2017.

Atrophic Mandible Fractures: Are Bone Grafts Necessary? An Update

PURPOSE

The management of atrophic mandibular fractures poses a challenge because of anatomic variations and medical comorbidities associated with elderly patients. The purpose of this article is to review and update the literature regarding the management of atrophic mandible fractures using load-bearing reconstruction plates placed without bone grafts.

MATERIALS AND METHODS

We performed a review of the English-language literature looking for atrophic mandibular fractures with or without continuity defects and reconstruction without bone grafts. Included are 2 new patients from our institution who presented with fractures of their atrophic mandibles and had continuity defects and infections. Both patients underwent reconstruction with a combination of a reconstruction plate, recombinant human bone morphogenetic protein 2, and tricalcium phosphate. This study was approved as an "exempt study" by the Institutional Review Board at the University of Kentucky. This investigation observed the Declaration of Helsinki on medical protocol and ethics.

RESULTS

Currently, the standard of care to manage atrophic mandibular fractures with or without a continuity defect is a combination of a reconstruction plate plus autogenous bone graft. However, there is a need for an alternative option for patients with substantial comorbidities. Bone morphogenetic proteins, with or without additional substances, appear to be a choice. In our experience, successful healing occurred in patients with a combination of a reconstruction plate, recombinant human bone morphogenetic protein 2, and tricalcium phosphate.

CONCLUSIONS

Whereas primary reconstruction of atrophic mandibular fractures with reconstruction plates supplemented with autogenous bone graft is the standard of care, in selected cases in which multiple comorbidities may influence local and/or systemic outcomes, bone morphogenetic proteins and tricalcium phosphate can be used as a predictable alternative to autogenous grafts.

Comparative analysis of osteoblast gene expression profiles and Runx2 genomic occupancy of mouse and human osteoblasts in vitro

Fast progress of the next generation sequencing (NGS) technology has allowed global transcriptional profiling and genome-wide mapping of transcription factor binding sites in various cellular contexts. However, limited number of replicates and high amount of data processing may weaken the significance of the findings. Comparative analyses of independent data sets acquired in the different laboratories would greatly increase the validity of the data. Runx2 is the key transcription factor regulating osteoblast differentiation and bone formation. We performed a comparative analysis of three published Runx2 data sets of chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) analysis in osteoblasts from mouse and human origin. Moreover, we assessed the similarity of the corresponding transcription data of these studies available online. The ChIP-seq data analysis confirmed general features of Runx2 binding, including location at genic vs intergenic regions and abundant Runx2 binding on promoters of the highly expressed genes. We also found high frequency of Runx2 DNA binding without a consensus Runx2 motif at the binding site. Importantly, mouse and human Runx2 showed moderately similar binding patterns in terms of peak-associated closest genes and their associated genomic ontology (GO) pathways. Accordingly, the gene expression profiles were highly similar and osteoblastic phenotype was prominent in the differentiated stage in both species. In conclusion, ChIP-seq method shows good reproducibility in the context of mature osteoblasts, and mouse and human osteoblast models resemble each other closely in Runx2 binding and in gene expression profiles, supporting the use of these models as adequate tools in studying osteoblast differentiation.

Pulsed Electromagnetic Field Regulates MicroRNA 21 Expression to Activate TGF-β Signaling in Human Bone Marrow Stromal Cells to Enhance Osteoblast Differentiation

Pulsed electromagnetic fields (PEMFs) have been documented to promote bone fracture healing in nonunions and increase lumbar spinal fusion rates. However, the molecular mechanisms by which PEMF stimulates differentiation of human bone marrow stromal cells (hBMSCs) into osteoblasts are not well understood. In this study the PEMF effects on hBMSCs were studied by microarray analysis. PEMF stimulation of hBMSCs' cell numbers mainly affected genes of cell cycle regulation, cell structure, and growth receptors or kinase pathways. In the differentiation and mineralization stages, PEMF regulated preosteoblast gene expression and notably, the transforming growth factor-beta (TGF-β) signaling pathway and microRNA 21 (miR21) were most highly regulated. PEMF stimulated activation of Smad2 and miR21-5p expression in differentiated osteoblasts, and TGF-β signaling was essential for PEMF stimulation of alkaline phosphatase mRNA expression. Smad7, an antagonist of the TGF-β signaling pathway, was found to be miR21-5p's putative target gene and PEMF caused a decrease in Smad7 expression. Expression of Runx2 was increased by PEMF treatment and the miR21-5p inhibitor prevented the PEMF stimulation of Runx2 expression in differentiating cells. Thus, PEMF could mediate its effects on bone metabolism by activation of the TGF-β signaling pathway and stimulation of expression of miR21-5p in hBMSCs.

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.

Application of tissue-engineered bone grafts for alveolar cleft osteoplasty in a rodent model

OBJECTIVES

The clinical standard for alveolar cleft osteoplasty is augmentation with autologous bone being available in limited amounts and might be associated with donor site morbidity. The aim of the present study was the creation of tissue-engineered bone grafts and their in vivo evaluation regarding their potential to promote osteogenesis in an alveolar cleft model.

MATERIALS AND METHODS

Artificial bone defects with a diameter of 3.3 mm were created surgically in the palate of 84 adult Lewis rats. Four experimental groups (n = 21) were examined: bovine hydroxyl apatite/collagen (bHA) without cells, bHA with undifferentiated mesenchymal stromal cells (MSC), bHA with osteogenically differentiated MSC. In a control group, the defect remained empty. After 6, 9 and 12 weeks, the remaining defect volume was assessed by cone beam computed tomography. Histologically, the remaining defect width and percentage of bone formation was quantified.

RESULTS

After 12 weeks, the remaining defect width was 60.1% for bHA, 74.7% for bHA with undifferentiated MSC and 81.8% for bHA with osteogenically differentiated MSC. For the control group, the remaining defect width measured 46.2% which was a statistically significant difference (p < 0.001).

CONCLUSIONS

The study design was suitable to evaluate tissue-engineered bone grafts prior to a clinical application. In this experimental set-up with the described maxillary defect, no promoting influence on bone formation of bone grafts containing bHA could be confirmed.

CLINICAL RELEVANCE

The creation of a sufficient tissue-engineered bone graft for alveolar cleft osteoplasty could preserve patients from donor site morbidity.

The effect of mesoporous bioglass on osteogenesis and adipogenesis of osteoporotic BMSCs

This study evaluated the effect of mesoporous bioglass (MBG) dissolution on the differentiation of bone marrow mesenchymal stem cells (BMSCs) derived from either sham control or ovariectomized (OVX) rats. MBG was fabricated by evaporation-induced self-assembly method. Cell proliferation was tested by Cell Counting Kit-8 assay, and cytoskeletal morphology was observed by fluorescence microscopy. Osteogenic differentiation was evaluated by alkaline phosphatase (ALP) staining and activity, Alizarin Red staining, while adipogenic differentiation was assessed by Oil Red-O staining. Quantitative real-time PCR and Western blot analysis were taken to evaluate the expression of runt-related transcription factor 2 (Runx2) and proliferator-activated receptor-γ (PPARγ). We found that MBG dissolution (0, 25, 50, 100, 200 µg/mL) was nontoxic to BMSCs growth. Sham and OVX BMSCs exhibited the highest ALP activity in 50 µg/mL of MBG osteogenic dissolution, except that sham BMSCs in 100 µg/mL showed the highest ALP activity on day 14. Runx2 was significantly upregulated after 100 µg/mL of MBG stimulation in sham and OVX BMSCs for 7 and 14 days, except that 25 µg/mL showed highest upregulation effect on OVX BMSCs at day 7. PPARγ was downregulated after MBG stimulation. The protein level of Runx2 from the sham BMSCs group was significantly upregulated after lower doses (25 and 50 µg/mL) of MBG stimulation, whereas PPARγ was downregulated in the sham and OVX BMSCs group. Thus, both the osteogenic and adipogenic abilities of BMSCs were damaged under OVX condition. Moreover, lower concentration of MBG dissolution can promote osteogenesis but inhibit adipogenesis of the sham and OVX BMSCs. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 3004-3014, 2016.

Ablation of Adenosine Monophosphate-Activated Protein Kinase α1 in Vascular Smooth Muscle Cells Promotes Diet-Induced Atherosclerotic Calcification In Vivo

RATIONALE

Atherosclerotic calcification is highly linked with plaque rapture. How calcification is regulated is poorly characterized.

OBJECTIVE

We sought to determine the contributions of AMP-activated protein kinase (AMPK) in atherosclerotic calcification.

METHODS AND RESULTS

Aortic calcification was evaluated in aortic roots and brachiocephalic arteries of atherosclerotic prone ApoE(-/-) mice or in mice with dual deficiencies of ApoE and AMPKα isoforms in whole body (ApoE(-/-)/AMPKα1(-/-) and ApoE(-/-)/AMPKα2(-/-)) or vascular smooth muscle cell (VSMC)-specific or macrophage-specific knockout of AMPKα1 fed with Western diet for 24 weeks. Genetic deficiency of AMPKα1 but not of AMPKα2 promoted atherosclerotic calcification and the expression of Runx2 (Runt-related transcription factor). Conversely, chronic administration of metformin, which activated AMPK, markedly reduced atherosclerotic calcification and Runx2 expression in ApoE(-/-) mice but had less effects in ApoE(-/-)/AMPKα1(-/-) mice. Furthermore, VSMC-specific but not macrophage-specific ablation of AMPKα1 promoted aortic calcification in vivo. Ablation of AMPKα1 in VSMC prevented Runx2 from proteasome degradation in parallel with aberrant osteoblastic differentiation of VSMC, whereas AMPK activation promoted Runx2 post-translational modification by small ubiquitin-like modifier (SUMO, SUMOylation), which is associated with its instability. Mechanically, we found that AMPKα1 directly phosphorylated protein inhibitor of activated STAT-1 (PIAS1), the SUMO E3-ligase of Runx2, at serine 510, to promote its SUMO E3-ligase activity. Finally, mutation of protein inhibitor of activated STAT-1 at serine 510 suppressed metformin-induced Runx2 SUMOylation and subsequently prevented metformin's effect on reducing oxidized low-density lipoprotein-triggered Runx2 expression in VSMC.

CONCLUSIONS

AMPKα1 phosphorylated protein inhibitor of activated STAT-1 to promote Runx2 SUMOylation and subsequently lead to its instability. AMPKα1 deficiency in VSMC increased Runx2 expression and promoted atherosclerotic calcification in vivo.

Bioactive vapor deposited calcium-phosphate silica sol-gel particles for directing osteoblast behavior

Silica-based materials are being developed and used for a variety of applications in orthopedic tissue engineering. In this work, we characterize the ability of a novel silica sol vapor deposition system to quickly modify biomaterial substrates and modulate surface hydrophobicity, surface topography, and composition. We were able to show that surface hydrophobicity, surface roughness, and composition could be rapidly modified. The compositional modification was directed towards generating apatitic-like surface mineral compositions (Ca/P ratios ∼1.30). Modified substrates were also capable of altering cell proliferation and differentiation behavior of preosteoblasts (MC3T3) and showed potential once optimized to provide a simple means to generate osteo-conductive substrates for tissue engineering. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2135-2148, 2016.

MiRNA 34a: a therapeutic target for castration-resistant prostate cancer

INTRODUCTION

Development of a therapy for bone metastases is of paramount importance for castration-resistant prostate cancer (CRPC). The osteomimetic properties of CRPC confer a propensity to metastasize to osseous sites. Micro-ribonucleic acid (miRNA) is non-coding RNA that acts as a post-transcriptional regulator of multiple proteins and associated pathways. Therefore identification of miRNAs could reveal a valid third generation therapy for CRPC.

AREAS COVERED

miR34a has been found to play an integral role in the progression of prostate cancer, particularly in the regulation of metastatic genes involved in migration, intravasation, extravasation, bone attachment and bone homeostasis. The correlation between miR34a down-regulation and metastatic progression has generated substantial interest in this field.

EXPERT OPINION

Examination of the evidence reveals that miR34a is an ideal target for gene therapy for metastatic CRPC. We also conclude that future studies should focus on the effects of miR34a upregulation in CRPC with respect to migration, translocation to bone micro-environment and osteomimetic phenotype development. The success of miR34a as a therapeutic is reliant on the development of appropriate delivery systems and targeting to the bone micro-environment. In tandem with any therapeutic studies, biomarker serum levels should also be ascertained as an indicator of successful miR34a delivery.

Mesenchymal stem cell proliferation and mineralization but not osteogenic differentiation are strongly affected by extracellular pH

Osteomyelitis is a serious complication in oral and maxillofacial surgery affecting bone healing. Bone remodeling is not only controlled by cellular components but also by ionic and molecular composition of the extracellular fluids in which calcium phosphate salts are precipitated in a pH dependent manner.

OBJECTIVE

To determine the effect of pH on self-renewal, osteogenic differentiation and matrix mineralization of mesenchymal stem cells (MSCs).

METHODS

We selected three different pH values; acidic (6.3, 6.7), physiological (7.0-8.0) and severe alkaline (8.5). MSCs were cultured at different pH ranges, cell viability measured by WST-1, apoptosis detected by JC-1, senescence was analyzed by β-galactosidase whereas mineralization was detected by Alizarin Red and osteogenic differentiation analyzed by Real-time PCR.

RESULTS

Self-renewal was affected by pH as well as matrix mineralization in which pH other than physiologic inhibited the deposition of extracellular matrix but did not affect MSCs differentiation as osteoblast markers were upregulated. The expression of osteocalcin and alkaline phosphatase activity was upregulated whereas osteopontin was downregulated under acidic pH.

CONCLUSION

pH affected MSCs self-renewal and mineralization without influencing osteogenic differentiation. Thus, future therapies, based on shifting acid-base balance toward the alkaline direction might be beneficial for prevention or treatment of osteomyelitis.

Articular cartilage tissue engineering: the role of signaling molecules

Effective early disease modifying options for osteoarthritis remain lacking. Tissue engineering approach to generate cartilage in vitro has emerged as a promising option for articular cartilage repair and regeneration. Signaling molecules and matrix modifying agents, derived from knowledge of cartilage development and homeostasis, have been used as biochemical stimuli toward cartilage tissue engineering and have led to improvements in the functionality of engineered cartilage. Clinical translation of neocartilage faces challenges, such as phenotypic instability of the engineered cartilage, poor integration, inflammation, and catabolic factors in the arthritic environment; these can all contribute to failure of implanted neocartilage. A comprehensive understanding of signaling molecules involved in osteoarthritis pathogenesis and their actions on engineered cartilage will be crucial. Thus, while it is important to continue deriving inspiration from cartilage development and homeostasis, it has become increasingly necessary to incorporate knowledge from osteoarthritis pathogenesis into cartilage tissue engineering.

Skp2 inhibits osteogenesis by promoting ubiquitin-proteasome degradation of Runx2

Osteogenic transcription factor Runx2 is essential for osteoblast differentiation. The activity of Runx2 is tightly regulated at transcriptional as well as post-translational level. However, regulation of Runx2 stability by ubiquitin mediated proteasomal degradation by E3 ubiquitin ligases is little-known. Here, for the first time we demonstrate that Skp2, an SCF family E3 ubiquitin ligase negatively targets Runx2 by promoting its polyubiquitination and proteasome dependent degradation. Co-immunoprecipitation studies revealed that Skp2 physically interacts with Runx2 both in a heterologous as well as physiologically relevant system. Functional consequences of Runx2-Skp2 physical interaction were then assessed by promoter reporter assay. We show that Skp2-mediated downregulation of Runx2 led to reduced Runx2 transactivation and osteoblast differentiation. On the contrary, inhibition of Skp2 restored Runx2 levels and promoted osteoblast differentiation. We further show that Skp2 and Runx2 proteins are co-expressed and show inverse relation in vivo such as in lactating, ovariectomized and estrogen-treated ovariectomized animals. Together, these data demonstrate that Skp2 targets Runx2 for ubiquitin mediated degradation and hence negatively regulate osteogenesis. Therefore, the present study provides a plausible therapeutic target for osteoporosis or cleidocranial dysplasia caused by the heterozygous mutation of Runx2 gene.

Ordinary and Activated Bone Grafts: Applied Classification and the Main Features

Bone grafts are medical devices that are in high demand in clinical practice for substitution of bone defects and recovery of atrophic bone regions. Based on the analysis of the modern groups of bone grafts, the particularities of their composition, the mechanisms of their biological effects, and their therapeutic indications, applicable classification was proposed that separates the bone substitutes into “ordinary” and “activated.” The main differential criterion is the presence of biologically active components in the material that are standardized by qualitative and quantitative parameters: growth factors, cells, or gene constructions encoding growth factors. The pronounced osteoinductive and (or) osteogenic properties of activated osteoplastic materials allow drawing upon their efficacy in the substitution of large bone defects.

E3 Ubiquitin Ligase Fbw7 Negatively Regulates Osteoblast Differentiation by Targeting Runx2 for Degradation

Runx2, a master regulator of osteoblast differentiation, is tightly regulated at both transcriptional and post-translational levels. Post-translational modifications such as phosphorylation and ubiquitination have differential effects on Runx2 functions. Here, we show that the reduced expression and functions of Runx2 upon its phosphorylation by GSK3β are mediated by its ubiquitin-mediated degradation through E3 ubiquitin ligase Fbw7α. Fbw7α through its WD domain interacts with Runx2 both in a heterologous (HEK293T cells) system as well as in osteoblasts. GSK3β was also present in the same complex as determined by co-immunoprecipitation. Furthermore, overexpression of either Fbw7α or GSK3β was sufficient to down-regulate endogenous Runx2 expression and function; however, both failed to inhibit endogenous Runx2 when either of them was depleted in osteoblasts. Fbw7α-mediated inhibition of Runx2 expression also led to reduced Runx2 transactivation and osteoblast differentiation. In contrast, inhibition of Fbw7α restored Runx2 levels and promoted osteoblast differentiation. We also observed reciprocal expression levels of Runx2 and Fbw7α in models of bone loss such as lactating (physiological bone loss condition) and ovariectomized (induction of surgical menopause) animals that show reduced Runx2 and enhanced Fbw7α, whereas this was reversed in the estrogen-treated ovariectomized animals. In addition, methylprednisolone (a synthetic glucocorticoid) treatment to neonatal rats showed a temporal decrease in Runx2 with a reciprocal increase in Fbw7 in their calvarium. Taken together, these data demonstrate that Fbw7α negatively regulates osteogenesis by targeting Runx2 for ubiquitin-mediated degradation in a GSK3β-dependent manner and thus provides a plausible explanation for GSK3β-mediated bone loss as described before.

Dysregulation of Mitochondrial Functions and Osteogenic Differentiation in Cisd2-Deficient Murine Induced Pluripotent Stem Cells

Wolfram syndrome 2 (WFS2) is a premature aging syndrome caused by an irreversible mitochondria-mediated disorder. Cisd2, which regulates mitochondrial electron transport, has been recently identified as the causative gene of WFS2. The mouse Cisd2 knockout (KO) (Cisd2(-/-)) recapitulates most of the clinical manifestations of WFS2, including growth retardation, osteopenia, and lordokyphosis. However, the precise mechanisms underlying osteopenia in WFS2 and Cisd2 KO mice remain unknown. In this study, we collected embryonic fibroblasts from Cisd2-deficient embryos and reprogrammed them into induced pluripotent stem cells (iPSCs) via retroviral transduction with Oct4/Sox2/Klf4/c-Myc. Cisd2-deficient mouse iPSCs (miPSCs) exhibited structural abnormalities in their mitochondria and an impaired proliferative capability. The global gene expression profiles of Cisd2(+/+), Cisd2(+/-), and Cisd2(-/-) miPSCs revealed that Cisd2 functions as a regulator of both mitochondrial electron transport and Wnt/β-catenin signaling, which is critical for cell proliferation and osteogenic differentiation. Notably, Cisd2(-/-) miPSCs exhibited impaired Wnt/β-catenin signaling, with the downregulation of downstream genes, such as Tcf1, Fosl1, and Jun and the osteogenic regulator Runx2. Several differentiation markers for tridermal lineages were globally impaired in Cisd2(-/-) miPSCs. Alizarin red S staining and flow cytometry analysis further revealed that Cisd2(-/-) miPSCs failed to undergo osteogenic differentiation. Taken together, our results, as determined using an miPSC-based platform, have demonstrated that Cisd2 regulates mitochondrial function, proliferation, intracellular Ca(2+) homeostasis, and Wnt pathway signaling. Cisd2 deficiency impairs the activation of Wnt/β-catenin signaling and thereby contributes to the pathogeneses of osteopenia and lordokyphosis in WFS2 patients.

Histological examination and clinical evaluation of the jawbone of an adult patient with cleidocranial dysplasia: a case report

OBJECTIVES

Cleidocranial dysplasia (CCD) is a rare congenital malformation syndrome, inherited autosomal-dominantly. During a course of treatment including surgical, implantological and restorative procedures, an opportunity arose to histologically examine biopsies of the maxilla and mandible of a CCD patient 47 years of age.

CASE REPORT

The aim of this case report is to present the results of the histological evaluation of the alveolar bone and the surgical pretreatment for and placement of six implants each in the maxilla and the mandible. The implants were inserted in a minimally invasive procedure using 3D template guidance. Following uneventful healing of the implants, ceramically veneered bridges were cemented on individual titanium abutments. Since the patient had not received orthodontic treatment in childhood-which would have been the treatment modality of choice-this implantological and prosthodontic approach was necessary. Biopsies were taken from the maxilla and the mandible before placing the implants. Histological evaluation showed bone with strong, coarsely interconnecting trabeculae, especially in the maxilla. Both the bone and the gingiva otherwise exhibited a normal structure without pathological features or anomalies.

CONCLUSION

The clinical parameters and histological evaluation of this one clinical case suggest that the concepts familiar from general oral implantology in terms of surgical and prosthetic procedures can be adopted for older patients with CCD.

Src enhances osteogenic differentiation through phosphorylation of Osterix

Osterix, a zinc-finger transcription factor, is required for osteoblast differentiation and new bone formation during embryonic development. The c-Src of tyrosine kinase is involved in a variety of cellular signaling pathways, leading to the induction of DNA synthesis, cell proliferation, and cytoskeletal reorganization. Src activity is tightly regulated and its dysregulation leads to constitutive activation and cellular transformation. The function of Osterix can be also modulated by post-translational modification. But the precise molecular signaling mechanisms between Osterix and c-Src are not known. In this study we investigated the potential regulation of Osterix function by c-Src in osteoblast differentiation. We found that c-Src activation increases protein stability, osteogenic activity and transcriptional activity of Osterix. The siRNA-mediated knockdown of c-Src decreased the protein levels and transcriptional activity of Osterix. Conversely, Src specific inhibitor, SU6656, decreased the protein levels and transcriptional activity of Osterix. The c-Src interacts with and phosphorylates Osterix. These results suggest that c-Src signaling modulates osteoblast differentiation at least in part through Osterix.

PACAP and VIP signaling in chondrogenesis and osteogenesis

Skeletal development is a complex process regulated by multifactorial signaling cascades that govern proper tissue specific cell differentiation and matrix production. The influence of certain regulatory peptides on cartilage or bone development can be predicted but are not widely studied. In this review, we aimed to assemble and overview those signaling pathways which are modulated by PACAP and VIP neuropeptides and are involved in cartilage and bone formation. We discuss recent experimental data suggesting broad spectrum functions of these neuropeptides in osteogenic and chondrogenic differentiation, including the canonical downstream targets of PACAP and VIP receptors, PKA or MAPK pathways, which are key regulators of chondro- and osteogenesis. Recent experimental data support the hypothesis that PACAP is a positive regulator of chondrogenesis, while VIP has been reported playing an important role in the inflammatory reactions of surrounding joint tissues. Regulatory function of PACAP and VIP in bone development has also been proved, although the source of the peptides is not obvious. Crosstalk and collateral connections of the discussed signaling mechanisms make the system complicated and may obscure the pure effects of VIP and PACAP. Chondro-protective properties of PACAP during oxidative stress observed in our experiments indicate a possible therapeutic application of this neuropeptide.

Dental Follicle Cells Participate in Tooth Eruption via the RUNX2-MiR-31-SATB2 Loop

Cleidocranial dysplasia (CCD) is characterized by the runt-related transcription factor 2 (RUNX2) mutation, which results in delayed tooth eruption due to disturbed functions of dental follicle. Accumulating evidence has revealed a key regulatory circuit, including RUNX2, miR-31, and special AT-rich binding protein 2 (SATB2) acting in concert in mesenchymal stem cell homeostasis and functions. However, whether such a regulatory loop works in dental follicle cells (DFCs) remains unknown. Herein, we investigated the roles of RUNX2-miR-31-SATB2 in DFCs from patients with CCD (DFCs-CCD) to advance our understanding regarding physical tooth eruption. We identified a novel mutation on exon 5 (c.634T>G, p.T212P) in RUNX2 via exome sequencing in the CCD patient with typical clinical presentations. Compared with DFCs from healthy donors, DFCs-CCD displayed significantly lower osteogenic, osteoclast-inductive, and matrix-degrading capacities and had lower RUNX2 (a transcriptional inhibitor of miR-31), higher miR-31, and downregulated SATB2. Lower ratios of RANKL/OPG and RANKL/RANK, as well as decreased expression of matrix metalloproteinase 9 (MMP9) and matrix metalloproteinase 2 (MMP2), would lead to inactivation of osteoclasts and suppression of bone matrix remodeling in DFCs-CCD. Furthermore, the roles of the RUNX2-miR-31-SATB2 loop in DFCs-CCD were revealed by endogenous miR-31 knockdown, which resulted in increased SATB2 and RUNX2, as well as osteoclast-inductive and matrix degradation capacities. Conversely, SATB2, RUNX2, MMP9, MMP2, and osteoclast-inductive factors expression declined upon ectopic miR-31 overexpression in normal DFCs. Importantly, neonatal mice with in vivo siRUNX2 delivery exhibited less activated osteoclasts around dental follicles and delayed tooth eruption. Together, these results suggest that RUNX2 mutation/haploinsufficiency disturbs osteoclast-inductive signaling in DFCs, which may be responsible for delayed tooth eruption in CCD patients. Manipulation of the RUNX2-miR-31-SATB2 loop may be a potential way to facilitate tooth eruption in CCD patients.

Osteoarthritis pathogenesis: a review of molecular mechanisms

Osteoarthritis (OA), the most prevalent chronic joint disease, increases in prevalence with age, and affects majority of individuals over the age of 65 and is a leading musculoskeletal cause of impaired mobility in the elderly. Because the precise molecular mechanisms which are involved in the degradation of cartilage matrix and development of OA are poorly understood and there are currently no effective interventions to decelerate the progression of OA or retard the irreversible degradation of cartilage except for total joint replacement surgery. In this paper, the important molecular mechanisms related to OA pathogenesis will be summarized and new insights into potential molecular targets for the prevention and treatment of OA will be provided.

The osteogenic or adipogenic lineage commitment of human mesenchymal stem cells is determined by protein kinase C delta

BACKGROUND

Mesenchymal stem cells (MSCs) have the potential to differentiate into specialized cell lineages such as osteoblasts and adipocytes in vitro. There exists a reciprocal relationship between osteogenic and adipogenic differentiation of MSCs that an osteogenic phenotype occurs at the expense of an adipogenic phenotype and vice versa, which in turn influence one another's phenotype through negative feedback loops. Thus, it is important to understand what signaling molecules modulate the lineage commitment of MSCs. Protein kinase C (PKC) plays a central role in cellular signal transduction for mediating diverse biological functions, and dysregulation of PKC activity is involved in various metabolic diseases including cancer, diabetes, and heart disease. Although the role of individual PKC isoforms has been investigated in various fields, the potential role of PKC in bone metabolism is not completely understood. In this study, we investigated the potential role of PKCδ in osteogenic lineage commitment of human bone marrow-derived mesenchymal stem cells (hBMSCs).

RESULTS

We observed that expression and phosphorylation of PKCδ were increased during osteogenic differentiation of hBMSCs. Pharmacological inhibition and genetic ablation of PKCδ in hBMSCs resulted in a significant attenuation of osteogenic differentiation as evidenced by reduced ALP activity and ECM mineralization, as well as down-regulation of the expression of osteoblast-specific genes. These effects were also accompanied by induction of adipogenic differentiation and up-regulation of the expression of adipocyte-specific genes involved in lipid synthesis in osteogenic induction of hBMSCs. Additionally, the activation of AMPK, which is a key cellular energy sensor, induced osteogenesis of hBMSCs. However, the inhibition of AMPK activity by compound C did not affect the activation of PKCδ at all, indicating that there is no direct correlation between AMPK and PKCδ in osteogenesis of hBMSCs.

CONCLUSIONS

These results suggest that PKCδ is a critical regulator for the balance between osteogenesis and adipogenesis of hBMSCs and thus has a potential novel therapeutic target for the treatment of metabolic bone diseases.

Inhibition of FOXO1/3 promotes vascular calcification

OBJECTIVE

Vascular calcification is a characteristic feature of atherosclerosis, diabetes mellitus, and end-stage renal disease. We have demonstrated that activation of protein kinase B (AKT) upregulates runt-related transcription factor 2 (Runx2), a key osteogenic transcription factor that is crucial for calcification of vascular smooth muscle cells (VSMC). Using mice with SMC-specific deletion of phosphatase and tensin homolog (PTEN), a major negative regulator of AKT, the present studies uncovered a novel molecular mechanism underlying PTEN/AKT/FOXO (forkhead box O)-mediated Runx2 upregulation and VSMC calcification.

APPROACH AND RESULTS

SMC-specific PTEN deletion mice were generated by crossing PTEN floxed mice with SM22α-Cre transgenic mice. The PTEN deletion resulted in sustained activation of AKT that upregulated Runx2 and promoted VSMC calcification in vitro and arterial calcification ex vivo. Runx2 knockdown did not affect proliferation but blocked calcification of the PTEN-deficient VSMC, suggesting that PTEN deletion promotes Runx2-depedent VSMC calcification that is independent of proliferation. At the molecular level, PTEN deficiency increased the amount of Runx2 post-transcriptionally by inhibiting Runx2 ubiquitination. AKT activation increased phosphorylation of FOXO1/3 that led to nuclear exclusion of FOXO1/3. FOXO1/3 knockdown in VSMC phenocopied the PTEN deficiency, demonstrating a novel function of FOXO1/3, as a downstream signaling of PTEN/AKT, in regulating Runx2 ubiquitination and VSMC calcification. Using heterozygous SMC-specific PTEN-deficient mice and atherogenic ApoE(-/-) mice, we further demonstrated AKT activation, FOXO phosphorylation, and Runx2 ubiquitination in vascular calcification in vivo.

CONCLUSIONS

Our studies have determined a new causative effect of SMC-specific PTEN deficiency on vascular calcification and demonstrated that FOXO1/3 plays a crucial role in PTEN/AKT-modulated Runx2 ubiquitination and VSMC calcification.

Plant homeodomain finger protein 2 promotes bone formation by demethylating and activating Runx2 for osteoblast differentiation

Plant homeodomain finger protein 2 (PHF2), which contains a plant homeodomain and a Jumonji-C domain, is an epigenetic regulator that demethylates lysine 9 in histone 3 (H3K9me2). On the other hand, runt-related transcription factor 2 (Runx2) plays essential roles in bone development and regeneration. Given previous reports that the PHF2 mutation can cause dwarfism in mice and that PHF2 expression is correlated with that of Runx2 in differentiating thymocytes, we investigated whether PHF2 regulates Runx2-mediated bone formation. Overexpression of PHF2 facilitated bone development in newborn mice, and viral shRNA-mediated knockdown of PHF2 delayed calvarial bone regeneration in adult rats. In primary osteoblasts and C2C12 precursor cells, PHF2 enhances osteoblast differentiation by demethylating Runx2, while suppressor of variegation 3-9 homolog 1 (SUV39H1) inhibits bone formation by methylating it. The PHF2-Runx2 interaction is mediated by the Jumonji-C and Runt domains of the two proteins, respectively. The interaction between Runx2 and osteocalcin promoter is regulated by the methylation status of Runx2, i.e., the interaction is augmented when Runx2 is demethylated. Our results suggest that SUV39H1 and PHF2 reciprocally regulate osteoblast differentiation by modulating Runx2-driven transcription at the post-translational level. This study may provide a theoretical basis for the development of new therapeutic modalities for patients with impaired bone development or delayed fracture healing.

O-GlcNAc modification of the runt-related transcription factor 2 (Runx2) links osteogenesis and nutrient metabolism in bone marrow mesenchymal stem cells

Runx2 is the master switch controlling osteoblast differentiation and formation of the mineralized skeleton. The post-translational modification of Runx2 by phosphorylation, ubiquitinylation, and acetylation modulates its activity, stability, and interactions with transcriptional co-regulators and chromatin remodeling proteins downstream of osteogenic signals. Characterization of Runx2 by electron transfer dissociation tandem mass spectrometry revealed sites of O-linked N-acetylglucosamine (O-GlcNAc) modification, a nutrient-responsive post-translational modification that modulates the action of numerous transcriptional effectors. O-GlcNAc modification occurs in close proximity to phosphorylated residues and novel sites of arginine methylation within regions known to regulate Runx2 transactivation. An interaction between Runx2 and the O-GlcNAcylated, O-GlcNAc transferase enzyme was also detected. Pharmacological inhibition of O-GlcNAcase (OGA), the enzyme responsible for the removal of O-GlcNAc from Ser/Thr residues, enhanced basal (39.9%) and BMP2/7-induced (43.3%) Runx2 transcriptional activity in MC3T3-E1 pre-osteoblasts. In bone marrow-derived mesenchymal stem cells differentiated for 6 days in osteogenic media, inhibition of OGA resulted in elevated expression (24.3%) and activity (65.8%) of alkaline phosphatase (ALP) an early marker of bone formation and a transcriptional target of Runx2. Osteogenic differentiation of bone marrow-derived mesenchymal stem cells in the presence of BMP2/7 for 8 days culminated in decreased OGA activity (39.0%) and an increase in the abundance of O-GlcNAcylated Runx2, as compared with unstimulated cells. Furthermore, BMP2/7-induced ALP activity was enhanced by 35.6% in bone marrow-derived mesenchymal stem cells differentiated in the presence of the OGA inhibitor, demonstrating that direct or BMP2/7-induced inhibition of OGA is associated with increased ALP activity. Altogether, these findings link O-GlcNAc cycling to the Runx2-dependent regulation of the early ALP marker under osteoblast differentiation conditions.

Akt enhances Runx2 protein stability by regulating Smurf2 function during osteoblast differentiation

Runx2 plays essential roles in bone formation and chondrocyte maturation. Akt promotes osteoblast differentiation induced by the bone morphogenetic proteins BMP2 and enhances the function and transcriptional activity of Runx2. However, the precise molecular mechanism underlying the relationship between Runx2 and Akt is not well understood. In this study, we examined the role of Akt in regulating Runx2 function. We found that Akt increases the stability of Runx2 protein. However, the level of Runx2 mRNA was not affected by Akt, and we did not find any evidence for direct modification of Runx2 by Akt. Instead, we found evidence that Akt induces the phosphorylation of the Smad ubiquitination regulatory factor Smurf2 and decreases the level of Smurf2 protein through ubiquitin/proteasome-mediated degradation of Smurf2. Akt also alleviates Smurf2-mediated suppression of Runx2 transcriptional activity. Taken together, our results suggest that Akt regulates osteoblast differentiation, at least in part, by enhancing the protein stability and transcriptional activity of Runx2 through regulation of ubiquitin/proteasome-mediated degradation of Smurf2.

The RUNX2 cistrome in osteoblasts: characterization, down-regulation following differentiation, and relationship to gene expression

RUNX2 is a transcription factor that is first expressed in early osteoblast-lineage cells and represents a primary determinant of osteoblastogenesis. While numerous target genes are regulated by RUNX2, little is known of sites on the genome occupied by RUNX2 or of the gene networks that are controlled by these sites. To explore this, we conducted a genome-wide analysis of the RUNX2 cistrome in both pre-osteoblastic MC3T3-E1 cells (POB) and their mature osteoblast progeny (OB), characterized the two cistromes and assessed their relationship to changes in gene expression. We found that although RUNX2 was widely bound to the genome in POB cells, this binding profile was reduced upon differentiation to OBs. Numerous sites were lost upon differentiation, new sites were also gained; many sites remained common to both cell states. Additional features were identified as well including location relative to potential target genes, abundance with respect to single genes, the frequent presence of a consensus TGTGGT RUNX2 binding motif, co-occupancy by C/EBPβ and the presence of a typical epigenetic histone enhancer signature. This signature was changed quantitatively following differentiation. While RUNX2 binding sites were associated extensively with adjacent genes, the distal nature of the majority of these sites prevented assessment of whether they represented direct targets of RUNX2 action. Changes in gene expression, however, revealed an abundance of genes that contained RUNX2 binding sites and were regulated in concert. These studies establish a basis for further analysis of the role of RUNX2 activity and its function during osteoblast lineage maturation.