Overexpression of Runx2 directed by the matrix metalloproteinase-13 promoter containing the AP-1 and Runx/RD/Cbfa sites alters bone remodeling in vivo

Nuciferine-loaded chitosan hydrogel-integrated 3D-printed polylactic acid scaffolds for bone tissue engineering: A combinatorial approach

Critical-sized bone defects resulting from severe trauma and open fractures cannot spontaneously heal and require surgical intervention. Limitations of traditional bone grafting include immune rejection and demand-over-supply issues leading to the development of novel tissue-engineered scaffolds. Nuciferine (NF), a plant-derived alkaloid, has excellent therapeutic properties, but its osteogenic potential is yet to be reported. Furthermore, the bioavailability of NF is obstructed due to its hydrophobicity, requiring an efficient drug delivery system, such as chitosan (CS) hydrogel. We designed and fabricated polylactic acid (PLA) scaffolds via 3D printing and integrated them with NF-containing CS hydrogel to obtain the porous biocomposite scaffolds (PLA/CS-NF). The fabricated scaffolds were subjected to in vitro physicochemical characterization, cytotoxicity assays, and osteogenic evaluation studies. Scanning electron microscopic studies revealed uniform pore size distribution on PLA/CS-NF scaffolds. An in vitro drug release study showed a sustained and prolonged release of NF. The cyto-friendly nature of NF in PLA/CS-NF scaffolds towards mouse mesenchymal stem cells (mMSCs) was observed. Also, cellular and molecular level studies signified the osteogenic potential of NF in PLA/CS-NF scaffolds on mMSCs. These results indicate that the PLA/CS-NF scaffolds could promote new bone formation and have potential applications in bone tissue engineering.

Exosomes derived from miR-338-3p-modified adipose stem cells inhibited inflammation injury of chondrocytes via targeting RUNX2 in osteoarthritis

BACKGROUND

Osteoarthritis (OA) is a chronic degenerative disease that is one of the main causes of disability in middle-aged and elderly people. Adipose stem cell (ASC)-derived exosomes (ASC-Exo) could repair cartilage damage and treat OA. MiRNA-338-3p expression was confirmed to play a role in inhibiting proinflammatory cytokines. Herein, we aimed to explore the mechanism by which exosomes derived from miR-338-3p overexpressing ASCs protects chondrocytes from interleukin (IL)-1β-induced chondrocyte change.

METHODS

Exosomes were extracted from ASCs transfected with miR-338-3p or its antisense inhibitor. The ASC-Exos (miR-338-3p silencing/overexpression) were incubated with IL-1β-induced ATDC5 cells, followed by evaluation of the chondrocyte proliferation, degradation, and inflammation injury.

RESULTS

In vitro results revealed that ASC-Exos inhibited the expression of prostaglandin E2 (PGE2), IL-6, IL-1β, and TNF-α, as well as promoted the proliferation of ATDC5 cells. Moreover, ASC-Exos inhibited inflammation injury and degradation of ATDC5 cells by transferring miR-338-3p. Luciferase reporter assays showed that RUNX2 was a target gene of miR-338-3p. Additionally, RUNX2 overexpression in ATDC5 cells reversed the protective effect of miR-338-3p on chondrocytes. Taken together, this study demonstrated that exosomes secreted from miR-338-3p-modified ASCs were effective in the repair of IL-1β-induced chondrocyte change by inhibiting RUNX2 expression.

CONCLUSIONS

Our result provided valuable data for understanding the mechanism of ASC-Exos in OA treatment.

Regional specific differentiation of integumentary organs: SATB2 is involved in α- and β-keratin gene cluster switching in the chicken

BACKGROUND

Animals develop skin regional specificities to best adapt to their environments. Birds are excellent models in which to study the epigenetic mechanisms that facilitate these adaptions. Patients suffering from SATB2 mutations exhibit multiple defects including ectodermal dysplasia-like changes. The preferential expression of SATB2, a chromatin regulator, in feather-forming compared to scale-forming regions, suggests it functions in regional specification of chicken skin appendages by acting on either differentiation or morphogenesis.

RESULTS

Retrovirus mediated SATB2 misexpression in developing feathers, beaks, and claws causes epidermal differentiation abnormalities (e.g. knobs, plaques) with few organ morphology alterations. Chicken β-keratins are encoded in 5 sub-clusters (Claw, Feather, Feather-like, Scale, and Keratinocyte) on Chromosome 25 and a large Feather keratin cluster on Chromosome 27. Type I and II α-keratin clusters are located on Chromosomes 27 and 33, respectively. Transcriptome analyses showed these keratins (1) are often tuned up or down collectively as a sub-cluster, and (2) these changes occur in a temporo-spatial specific manner.

CONCLUSIONS

These results suggest an organizing role of SATB2 in cluster-level gene co-regulation during skin regional specification.

Identification and characterization of TGF-β1-responsive Runx2 acetylation sites for matrix Metalloproteinase-13 expression in osteoblastic cells

In skeletal tissues, transforming growth factor-beta 1 (TGF-β1) serves a number of activities. For example, in osteoblastic cells, TGF-β1 stimulates the expression of matrix metalloproteinase-13 (MMP-13, a bone remodeling gene), which requires the bone transcription factor Runx2. Although TGF-β1 is known to stimulate Runx2 acetylation, the sites involved in MMP-13 gene activation remain unknown. Mass spectrometry analysis revealed that Runx2 was acetylated at one site (K134) and three sites (K24, K134, and K169) following control and TGF-β1-treatment, respectively, in osteoblastic cells. In addition, we mutated the lysine residues in the Runx2 construct into arginine and transfected the construct into mouse mesenchymal stem cells (C3H10T1/2). Wild-type Runx2 expression and acetylation were significantly increased by TGF-β1-treatment, whereas this effect was decreased in the presence of the Runx2 double mutant construct (K24 + K169) in C3H10T1/2 cells. TGF-β1 enhanced MMP-13 promoter activity in cells transfected with the wild-type Runx2 construct, but this effect was considerably reduced in cells transfected with the Runx2 double mutant construct (K24 + K169), according to a luciferase reporter test. Hence, the stability of Runx2 may be mediated by TGF-β1-induced acetylation at K24 and K169 and is required for MMP-13 expression in osteoblastic cells. These findings add to our knowledge of TGF-β1, Runx2, and MMP-13's physiological roles in bone metabolism.

Species-specific sensitivity to TGFβ signaling and changes to the Mmp13 promoter underlie avian jaw development and evolution

Precise developmental control of jaw length is critical for survival, but underlying molecular mechanisms remain poorly understood. The jaw skeleton arises from neural crest mesenchyme (NCM), and we previously demonstrated that these progenitor cells express more bone-resorbing enzymes including Matrix metalloproteinase 13 (Mmp13) when they generate shorter jaws in quail embryos versus longer jaws in duck. Moreover, if we inhibit bone resorption or Mmp13, we can increase jaw length. In the current study, we uncover mechanisms establishing species-specific levels of Mmp13 and bone resorption. Quail show greater activation of and sensitivity to transforming growth factor beta (TGFβ) signaling than duck; where intracellular mediators like SMADs and targets like Runt-related transcription factor 2 (Runx2), which bind Mmp13, become elevated. Inhibiting TGFβ signaling decreases bone resorption, and overexpressing Mmp13 in NCM shortens the duck lower jaw. To elucidate the basis for this differential regulation, we examine the Mmp13 promoter. We discover a SMAD-binding element and single nucleotide polymorphisms (SNPs) near a RUNX2-binding element that distinguish quail from duck. Altering the SMAD site and switching the SNPs abolish TGFβ sensitivity in the quail Mmp13 promoter but make the duck promoter responsive. Thus, differential regulation of TGFβ signaling and Mmp13 promoter structure underlie avian jaw development and evolution.

RUNX2 Phosphorylation by Tyrosine Kinase ABL Promotes Breast Cancer Invasion

Activity of transcription factors is normally regulated through interaction with other transcription factors, chromatin remodeling proteins and transcriptional co-activators. In distinction to these well-established transcriptional controls of gene expression, we have uncovered a unique activation model of transcription factors between tyrosine kinase ABL and RUNX2, an osteoblastic master transcription factor, for cancer invasion. We show that ABL directly binds to, phosphorylates, and activates RUNX2 through its SH2 domain in a kinase activity-dependent manner and that the complex formation of these proteins is required for expression of its target gene MMP13. Additionally, we show that the RUNX2 transcriptional activity is dependent on the number of its tyrosine residues that are phosphorylated by ABL. In addition to regulation of RUNX2 activity, we show that ABL transcriptionally enhances RUNX2 expression through activation of the bone morphogenetic protein (BMP)-SMAD pathway. Lastly, we show that ABL expression in highly metastatic breast cancer MDA-MB231 cells is associated with their invasive capacity and that ABL-mediated invasion is abolished by depletion of endogenous RUNX2 or MMP13. Our genetic and biochemical evidence obtained in this study contributes to a mechanistic insight linking ABL-mediated phosphorylation and activation of RUNX2 to induction of MMP13, which underlies a fundamental invasive capacity in cancer and is different from the previously described model of transcriptional activation.

Polycaprolactone/polyvinylpyrrolidone coaxial electrospun fibers containing veratric acid-loaded chitosan nanoparticles for bone regeneration

Veratric acid (3,4-dimethoxy benzoic acid) (VA) is a hydrophobic phenolic phytocompound possessing therapeutic potential, but it has not been reported as actuating bone regeneration to date. Furthermore, delivery of hydrophobic compounds is often impeded in the body, thus depreciating their bioavailability. In this study, VA was found to have osteogenic potential and its sustained delivery was facilitated through a nanoparticle-embedded coaxial electrospinning technique. Polycaprolactone/polyvinylpyrrolidone (PCL/PVP) coaxial fibers were electrospun, encasing VA-loaded chitosan nanoparticles (CHS-NP). The fibers showed commendable physiochemical and material properties and were biocompatible with mouse mesenchymal stem cells (mMSCs). When mMSCs were grown on coaxial fibers, VA promoted these cells towards osteoblast differentiation as was reflected by calcium deposits. The mRNA expression of Runx2, an important bone transcriptional regulator, and other differentiation markers such as alkaline phosphatase, collagen type I, and osteocalcin were found to be upregulated in mMSCs grown on the PCL/PVP/CHS-NP-VA fibers. Overall, the study portrays the delivery of the phytocompound, VA, in a sustained manner to promote bone regeneration.

Runx2 plays a central role in Osteoarthritis development

Osteoarthritis (OA) is the most common form of arthritis, is the leading cause of impaired mobility in the elderly, and accounts for more than a third of chronic moderate to severe pain. As a degenerative joint disorder, OA affects the whole joint and results in synovial hyperplasia, degradation of articular cartilage, subchondral sclerosis, osteophyte formation, and chronic pain. Currently, there is no effective drug to decelerate OA progression and molecular targets for drug development have been insufficiently investigated. Anti-OA drug development can benefit from more and precise knowledge of molecular targets for drug development. Runt-related transcription factor 2 (Runx2) is a key transcription factor controlling osteoblast and chondrocyte differentiation and is among the most promising potential therapeutic targets. Notably, Runx2 expression is upregulated in several murine OA models, suggesting a role in disease pathogenesis. In this review article, we summarized recent findings on Runx2 related to OA development and evaluated its potential as a therapeutic target.

The translational potential of this article

A better understanding of the role of Runx2 in osteoarthritis pathogenesis will contribute to the development of novel intervention of osteoarthritis disease.

Cladophora glomerata methanolic extract promotes chondrogenic gene expression and cartilage phenotype differentiation in equine adipose-derived mesenchymal stromal stem cells affected by metabolic syndrome

BACKGROUND

Chondrogenesis represents a highly dynamic cellular process that leads to the establishment of various types of cartilage. However, when stress-related injuries occur, a rapid and efficient regeneration of the tissues is necessary to maintain cartilage integrity. Mesenchymal stem cells (MSCs) are known to exhibit high capacity for self-renewal and pluripotency effects, and thus play a pivotal role in the repair and regeneration of damaged cartilage. On the other hand, the influence of certain pathological conditions such as metabolic disorders on MSCs can seriously impair their regenerative properties and thus reduce their therapeutic potential.

OBJECTIVES

In this investigation, we attempted to improve and potentiate the in vitro chondrogenic ability of adipose-derived mesenchymal stromal stem cells (ASCs) isolated from horses suffering from metabolic syndrome.

METHODS

Cultured cells in chondrogenic-inductive medium supplemented with Cladophora glomerata methanolic extract were experimented for expression of the main genes and microRNAs involved in the differentiation process using RT-PCR, for their morphological changes through confocal and scanning electron microscopy and for their physiological homeostasis.

RESULTS

The different added concentrations of C. glomerata extract to the basic chondrogenic inductive culture medium promoted the proliferation of equine metabolic syndrome ASCs (ASCs_EMS) and resulted in chondrogenic phenotype differentiation and higher mRNA expression of collagen type II, aggrecan, cartilage oligomeric matrix protein, and Sox9 among others. The results reveal an obvious inhibitory effect of hypertrophy and a strong repression of miR-145-5p, miR-146-3p, and miR-34a and miR-449a largely involved in cartilage degradation. Treated cells additionally exhibited significant reduced apoptosis and oxidative stress, as well as promoted viability and mitochondrial potentiation.

CONCLUSION

Chondrogenesis in EqASCs_EMS was found to be prominent after chondrogenic induction in conditions containing C. glomerata extract, suggesting that the macroalgae could be considered for the enhancement of ASC cultures and their reparative properties.

Osteostimulatory effect of biocomposite scaffold containing phytomolecule diosmin by Integrin/FAK/ERK signaling pathway in mouse mesenchymal stem cells

Non-availability of an ideal alternative for autografts in treating critical-size bone defects is a major challenge in orthopedics. Phytocompounds have been proven to enhance osteogenesis via various osteogenic signaling pathways, but its decreased bioavailability and increased renal clearance limit its application. In this study, we designed a biocomposite scaffold comprising gelatin (Gel) and nanohydroxyapatite (nHAp) incorporated with diosmin (DM) and we investigated its bone forming potential in vitro and in vivo. Physiochemical characterization of the scaffold showed that DM had no effect on altering the material characteristics of the scaffold. The addition of DM enhanced the osteoblast differentiation potential of the scaffold in mouse mesenchymal stem cells at both cellular and molecular levels, possibly via the integrin-mediated activation of FAK and ERK signaling components. Using the rat tibial bone defective model, we identified the effect of DM in Gel/nHAp scaffold on enhancing bone formation in vivo. Based on our results, we suggest that Gel/nHAp/DM can be a potential therapeutic agent in scaffold-mediated bone regeneration.

Osteogenic potential of zingerone, a phenolic compound in mouse mesenchymal stem cells

Zingerone, 4-(4-hydroxy-3-methoxyphenyl)-2-butanone (Zg), a phenolic compound isolated from ginger is reported to have anti-inflammatory and antidiabetic properties. However, its role in the promotion of osteogenesis is not known. In this study, we investigated the therapeutic effect of Zg on osteogenesis at the cellular and molecular levels. Zg treatment was nontoxic to mouse mesenchymal stem cells (mMSCs). At the cellular level, it enhanced osteoblast differentiation as evidenced by more calcium deposits. At the molecular level, Zg stimulated the expression of Runx2 (a bone transcription factor) and other marker genes of osteoblast differentiation in mMSCs. Recent studies indicated that microRNAs (miRNAs) regulate bone metabolism, and we identified that Zg treatment in mMSCs upregulated mir-590, a positive regulator of Runx2 by targeting Smad7, an antagonist of TGF-β1 signaling. Thus, the osteogenic potential of Zg would be beneficial for treating bone and bone-related diseases.

Sinapic acid-loaded chitosan nanoparticles in polycaprolactone electrospun fibers for bone regeneration in vitro and in vivo

Sinapic acid (SA) is a plant-derived phenolic compound known for its multiple biological properties, but its role in the promotion of bone formation is not yet well-studied. Moreover, the delivery of SA is hindered by its complex hydrophobic nature, limiting its bioavailability. In this study, we fabricated a drug delivery system using chitosan nanoparticles (nCS) loaded with SA at different concentrations. These were incorporated into polycaprolactone (PCL) fibers via an electrospinning method. nCS loaded with 50 μM SA in PCL fibers promoted osteoblast differentiation. Furthermore, SA treatment activated the osteogenesis signaling pathways in mouse mesenchymal stem cells. A critical-sized rat calvarial bone defect model system identified that the inclusion of SA into PCL/nCS fibers accelerated bone formation. Collectively, these data suggest that SA promoted osteoblast differentiation in vitro and bone formation in vivo, possibly by activating the TGF-β1/BMP/Smads/Runx2 signaling pathways, suggesting SA might have therapeutic benefits in bone regeneration.

Cellular and molecular changes to chondrocytes in an in vitro model of developmental dysplasia of the hip‑an experimental model of DDH with swaddling position

The aim of the present study was to assess the cellular and molecular changes to chondrocytes in a developmental dysplasia of the hip (DDH) model and to investigate the early metabolism of chondrocytes in DDH. Neonatal Wistar rats were used for the DDH model with swaddling position. Primary cultures of chondrocytes were prepared at serial interval stages (2, 4, 6 and 8 weeks) to investigate cellular proliferation. The expression of collagen II and aggrecan mRNA was detected to assess the anabolic ability of chondrocytes. The expression of matrix metallopeptidase (MMP)-13 and ADAM metallopeptidase with thrombospondin type 1 motif 5 (ADAMTS-5) mRNA was measured to investigate the degradation of collagen II and aggrecan, respectively. Morphological changes were observed in coronal dissection samples after the removal of fixation. Primary chondrocytes at serial intervals were assessed using a Cell Counting Kit-8 assay and the results revealed that DDH chondrocytes had more proliferative activity. The expression of collagen II mRNA was upregulated at 2 weeks and was more sensitive to mechanical loading compared with aggrecan. Similar changes occurred at 6 weeks. Furthermore, MMP-13 and ADAMTS-5 mRNA expression levels were upregulated at 2 weeks. It was also demonstrated that DDH chondrocytes exhibited high proliferative activity at the early stages and degeneration later.

The Deletion of Hdac4 in Mouse Osteoblasts Influences Both Catabolic and Anabolic Effects in Bone

Histone deacetylase 4 (Hdac4) is known to control chondrocyte hypertrophy and bone formation. We have previously shown that parathyroid hormone (PTH) regulates many aspects of Hdac4 function in osteoblastic cells in vitro; however, in vivo confirmation was previously precluded by preweaning lethality of the Hdac4-deficient mice. To analyze the function of Hdac4 in bone in mature animals, we generated mice with osteoblast lineage-specific knockout of Hdac4 (Hdac4^ob-/- ) by crossing transgenic mice expressing Cre recombinase under the control of a 2.3-kb fragment of the Col1a1 promoter with mice bearing loxP-Hdac4. The Hdac4^ob-/- mice survive to adulthood and developed a mild skeletal phenotype. At age 12 weeks, they had short, irregularly shaped and stiff tails due to smaller tail vertebrae, with almost no growth plates. The tibial growth plate zone was also thinned, and Mmp13 and Sost mRNAs were increased in the distal femurs of Hdac4^ob-/- mice. Immunohistochemistry showed that sclerostin was elevated in Hdac4^ob-/- mice, suggesting that Hdac4 inhibits its gene and protein expression. To determine the effect of PTH in these mice, hPTH (1-34) or saline were delivered for 14 days with subcutaneously implanted devices in 8-week-old female Hdac4^ob-/- and wild-type (Hdac4^fl/fl ) mice. Serum CTX, a marker of bone resorption, was increased in Hdac4^ob-/- mice with or without PTH treatment. Tibial cortical bone volume/total volume (BV/TV), cortical thickness (Ct.Th), and relative cortical area (RCA) were decreased in Hdac4^ob-/- mice, but PTH caused no further decrease in Hdac4^ob-/- mice. Tibial trabecular BV/TV and thickness were not changed significantly in Hdac4^ob-/- mice but decreased with PTH treatment. These results indicate that Hdac4 inhibits bone resorption and has anabolic effects via inhibiting Mmp13 and Sost/sclerostin expression. Hdac4 influences cortical bone mass and thickness and knockout of Hdac4 prevents the catabolic effect of PTH in cortical bone. © 2018 American Society for Bone and Mineral Research.

Biology and Mechanisms of Action of the Vitamin D Hormone

The central role of hormonal 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] is to regulate calcium and phosphorus homeostasis via actions in intestine, kidney, and bone. These and other actions in many cell types not involved in mineral metabolism are mediated by the vitamin D receptor. Recent studies using genome-wide scale techniques have extended fundamental ideas regarding vitamin D-mediated control of gene expression while simultaneously revealing a series of new concepts. This article summarizes the current view of the biological actions of the vitamin D hormone and focuses on new concepts that drive the understanding of the mechanisms through which vitamin D operates.

Runx1 Activities in Superficial Zone Chondrocytes, Osteoarthritic Chondrocyte Clones and Response to Mechanical Loading

Runx1, the hematopoietic lineage determining transcription factor, is present in perichondrium and chondrocytes. Here we addressed Runx1 functions, by examining expression in cartilage during mouse and human osteoarthritis (OA) progression and in response to mechanical loading. Spared and diseased compartments in knees of OA patients and in mice with surgical destabilization of the medial meniscus were examined for changes in expression of Runx1 mRNA (Q-PCR) and protein (immunoblot, immunohistochemistry). Runx1 levels were quantified in response to static mechanical compression of bovine articular cartilage. Runx1 function was assessed by cell proliferation (Ki67, PCNA) and cell type phenotypic markers. Runx1 is enriched in superficial zone (SZ) chondrocytes of normal bovine, mouse, and human tissues. Increasing loading conditions in bovine cartilage revealed a positive correlation with a significant elevation of Runx1. Runx1 becomes highly expressed at the periphery of mouse OA lesions and in human OA chondrocyte 'clones' where Runx1 co-localizes with Vcam1, the mesenchymal stem cell (MSC) marker and lubricin (Prg4), a cartilage chondroprotective protein. These OA induced cells represent a proliferative cell population, Runx1 depletion in MPCs decreases cell growth, supporting Runx1 contribution to cell expansion. The highest Runx1 levels in SZC of normal cartilage suggest a function that supports the unique phenotype of articular chondrocytes, reflected by upregulation under conditions of compression. We propose Runx1 co-expression with Vcam1 and lubricin in murine cell clusters and human 'clones' of OA cartilage, participate in a cooperative mechanism for a compensatory anabolic function.

Selective Distal Enhancer Control of the Mmp13 Gene Identified through Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) Genomic Deletions

Matrix metalloproteinase 13 (Mmp13, collagenase-3) plays an essential role in bone metabolism and mineral homeostasis. It is regulated by numerous factors, including BMP-2, parathyroid hormone, and 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3), through transcription factors such as Runt-related transcription factor 2 (RUNX2), CCAAT/enhancer-binding protein β (C/EBPβ), OSX, and vitamin D receptor (VDR). During osteoblast maturation, the basal expression of Mmp13 and its sensitivity to 1,25(OH)2D3 are strikingly increased. In this report, ChIP-sequencing analysis in mouse preosteoblasts revealed that the Mmp13 gene was probably regulated by three major enhancers located -10, -20, and -30 kb upstream of the gene promoter, occupied by activated VDR and prebound C/EBPβ and RUNX2, respectively. Initially, bacterial artificial chromosome clone recombineering and traditional mutagenesis defined binding sites for VDR and RUNX2. We then employed a CRISPR/Cas9 gene editing approach to delete the -10 and -30 kb Mmp13 enhancers, a region proximal to the promoter, and VDR or RUNX2. VDR-mediated up-regulation of Mmp13 transcription was completely abrogated upon removal of the -10 kb enhancer, resulting in a 1,25(OH)2D3-directed repression of Mmp13. Deletion of either the -30 kb enhancer or RUNX2 resulted in a complete loss of basal transcript activity and a ChIP-identified destabilization of the chromatin enhancer environment and factor binding. Whereas enhancer deletions only affected Mmp13 expression, the RUNX2 deletion led to changes in gene expression, a reduction in cellular proliferation, and an inability to differentiate. We conclude that the Mmp13 gene is regulated via at least three specific distal enhancers that display independent activities yet are able to integrate response from multiple signaling pathways in a model of activation and suppression.

A positive role of microRNA-15b on regulation of osteoblast differentiation

Osteoblast differentiation is tightly regulated by several factors including microRNAs (miRNAs). In this paper, we report that pre-mir-15b is highly expressed in differentiated osteoblasts. The functional role of miR-15b in osteoblast differentiation was determined using miR-15b mimic/inhibitor and the expression of osteoblast differentiation marker genes such as alkaline phosphatase (ALP), type I collagen genes was decreased by miR-15b inhibitor. Runx2, a bone specific transcription factor is generally required for expression of osteoblast differentiation marker genes and in response to miR-15b inhibitor treatment, Runx2 mRNA expression was not changed; whereas its protein expression was decreased. Even though Smurf1 (SMAD specific E3 ubiquitin protein ligase 1), HDAC4 (histone deacetylase 4), Smad7, and Crim1 were found to be few of miR-15b's putative target genes, there was increased expression of only Smurf1 gene at mRNA and protein levels by miR-15b inhibitor. miR-15b mimic treatment significantly increased and decreased expressions of Runx2 and Smurf1 proteins, respectively. We further identified that the Smurf1 3'UTR is directly targeted by miR-15b using the luciferase reporter gene system. This is well documented that Smurf1 interacts with Runx2 and degrades it by proteasomal pathway. Hence, based on our results we suggest that miR-15b promotes osteoblast differentiation by indirectly protecting Runx2 protein from Smurf1 mediated degradation. Thus, this study identified that miR-15b can act as a positive regulator for osteoblast differentiation.

RUNX2 is overexpressed in melanoma cells and mediates their migration and invasion

In the present study, we investigated the role of the transcription factor RUNX2 in melanomagenesis. We demonstrated that the expression of transcriptionally active RUNX2 was increased in melanoma cell lines as compared with human melanocytes. Using a melanoma tissue microarray, we showed that RUNX2 levels were higher in melanoma cells as compared with nevic melanocytes. RUNX2 knockdown in melanoma cell lines significantly decreased Focal Adhesion Kinase expression, and inhibited their cell growth, migration and invasion ability. Finally, the pro-hormone cholecalciferol reduced RUNX2 transcriptional activity and decreased migration of melanoma cells, further suggesting a role of RUNX2 in melanoma cell migration.

Comparative characteristics of porous bioceramics for an osteogenic response in vitro and in vivo

Porous calcium phosphate ceramics are used in orthopedic and craniofacial applications to treat bone loss, or in dental applications to replace missing teeth. The implantation of these materials, however, does not induce stem cell differentiation, so suitable additional materials such as porous calcium phosphate discs are needed to influence physicochemical responses or structural changes. Rabbit adipose-derived stem cells (ADSC) and mouse osteoblastic cells (MC3T3-E1) were evaluated in vitro by the MTT assay, semi-quantitative RT-PCR, and immunoblotting using cells cultured in medium supplemented with extracts from bioceramics, including calcium metaphosphate (CMP), hydroxyapatite (HA) and collagen-grafted HA (HA-col). In vivo evaluation of the bone forming capacity of these bioceramics in rat models using femur defects and intramuscular implants for 12 weeks was performed. Histological analysis showed that newly formed stromal-rich tissues were observed in all the implanted regions and that the implants showed positive immunoreaction against type I collagen and alkaline phosphatase (ALP). The intramuscular implant region, in particular, showed strong positive immunoreactivity for both type I collagen and ALP, which was further confirmed by mRNA expression and immunoblotting results, indicating that each bioceramic material enhanced osteogenesis stimulation. These results support our hypothesis that smart bioceramics can induce osteoconduction and osteoinduction in vivo, although mature bone formation, including lacunae, osteocytes, and mineralization, was not prominent until 12 weeks after implantation.

Expression of microRNA-30c and its target genes in human osteoblastic cells by nano-bioglass ceramic-treatment

Osteoblast differentiation is tightly regulated by post transcriptional regulators such as microRNAs (miRNAs). Several bioactive materials including nano-bioglass ceramic particles (nBGC) influence differentiation of the osteoblasts, but the molecular mechanisms of nBGC-stimulation of osteoblast differentiation via miRNAs are not yet determined. In this study, we identified that nBGC-treatment stimulated miR-30c expression in human osteoblastic cells (MG63). The bioinformatics tools identified its regulatory network, molecular function, biological processes and its target genes involved in negative regulation of osteoblast differentiation. TGIF2 and HDAC4 were found to be its putative target genes and their expression was down regulated by nBGC-treatment in MG63 cells. Thus, this study advances our understanding of nBGC action on bone cells and supports utilization of nBGC in bone tissue engineering.

Transcriptional regulatory cascades in Runx2-dependent bone development

The development of the musculoskeletal system is a complex process that involves very precise control of bone formation and growth as well as remodeling during postnatal life. Although the understanding of the transcriptional mechanisms of osteogenesis has increased considerably, the molecular regulatory basis, especially the gene regulatory network of osteogenic differentiation, is still poorly understood. This review provides the reader with an overview of the key transcription factors that govern bone formation, highlighting their function and regulation linked to Runt-related transcription factor 2 (Runx2). Runx2 as the master transcription factor of osteoblast differentiation, Twist, Msh homeobox 2 (Msx2), and promyelocytic leukemia zinc-finger protein (PLZF) acting upstream of Runx2, Osterix (Osx) acting downstream of Runx2, and activating transcription factor 4 (ATF4) and zinc-finger protein 521 (ZFP521) acting as cofactors of Runx2 are discussed, and their relevance for tissue engineering is presented. References are provided for more in-depth personal study.

Conditional activation of β-catenin signaling in mice leads to severe defects in intervertebral disc tissue

OBJECTIVE

The incidence of low back pain is extremely high and is often linked to intervertebral disc (IVD) degeneration. The mechanism of this disease is currently unknown. This study was undertaken to investigate the role of β-catenin signaling in IVD tissue function.

METHODS

β-catenin protein levels were measured by immunohistochemical analysis of disc samples obtained from patients with disc degeneration and from normal subjects. To generate β-catenin conditional activation (cAct) mice, Col2a1-CreER(T2) -transgenic mice were bred with β-catenin(fx(Ex3)/fx(Ex3)) mice. Changes in disc tissue morphology and function were examined by micro-computed tomography, histologic analysis, and real-time polymerase chain reaction assays.

RESULTS

β-catenin protein was up-regulated in disc tissue samples from patients with disc degeneration. To assess the effects of increased β-catenin levels on disc tissue, we generated β-catenin cAct mice. Overexpression of β-catenin in disc cells led to extensive osteophyte formation in 3- and 6-month-old β-catenin cAct mice, which were associated with significant changes in the cells and extracellular matrix of disc tissue and growth plate. Gene expression analysis demonstrated that activation of β-catenin enhanced runt-related transcription factor 2-dependent Mmp13 and Adamts5 expression. Moreover, genetic ablation of Mmp13 or Adamts5 on the β-catenin cAct background, or treatment of β-catenin cAct mice with a specific matrix metalloproteinase 13 inhibitor, ameliorated the mutant phenotype.

CONCLUSION

Our findings indicate that the β-catenin signaling pathway plays a critical role in disc tissue function.

Matrix metalloproteinase-13 is required for osteocytic perilacunar remodeling and maintains bone fracture resistance

Like bone mass, bone quality is specified in development, actively maintained postnatally, and disrupted by disease. The roles of osteoblasts, osteoclasts, and osteocytes in the regulation of bone mass are increasingly well defined. However, the cellular and molecular mechanisms by which bone quality is regulated remain unclear. Proteins that remodel bone extracellular matrix, such as the collagen-degrading matrix metalloproteinase (MMP)-13, are likely candidates to regulate bone quality. Using MMP-13-deficient mice, we examined the role of MMP-13 in the remodeling and maintenance of bone matrix and subsequent fracture resistance. Throughout the diaphysis of MMP-13-deficient tibiae, we observed elevated nonenzymatic cross-linking and concentric regions of hypermineralization, collagen disorganization, and canalicular malformation. These defects localize to the same mid-cortical bone regions where osteocyte lacunae and canaliculi exhibit MMP-13 and tartrate-resistant acid phosphatase (TRAP) expression, as well as the osteocyte marker sclerostin. Despite otherwise normal measures of osteoclast and osteoblast function, dynamic histomorphometry revealed that remodeling of osteocyte lacunae is impaired in MMP-13(-/-) bone. Analysis of MMP-13(-/-) mice and their wild-type littermates in normal and lactating conditions showed that MMP-13 is not only required for lactation-induced osteocyte perilacunar remodeling, but also for the maintenance of bone quality. The loss of MMP-13, and the resulting defects in perilacunar remodeling and matrix organization, compromise MMP-13(-/-) bone fracture toughness and postyield behavior. Taken together, these findings demonstrate that osteocyte perilacunar remodeling of mid-cortical bone matrix requires MMP-13 and is essential for the maintenance of bone quality.

Advanced prostate cancer: reinforcing the strings between inflammation and the metastatic behavior

It is currently estimated that inflammatory responses are linked to 15-20% of all deaths from cancer worldwide. Although many studies point to an important role of inflammation in prostate growth, the contribution of inflammation to castration-resistant prostate cancer is not completely understood. The presence of inflammatory mediators in tumor microenvironment raises the question whether genetic events that participate in cancer development and progression are responsible for the inflammatory milieu inside and surrounding tumors. Activated oncogenes, cytokines, chemokines and their receptors, sustained oxidative stress and antioxidant imbalance share the capacity to orchestrate these pro-inflammatory programs; however, the diversity of the inflammatory cell components will determine the final response in the prostate tissue. These observations give rise to the concept that early genetic events generate an inflammatory microenvironment promoting prostate cancer progression and creating a continuous loop that stimulates a more aggressive stage. It is imperative to dissect the molecular pathologic mechanism of inflammation involved in the generation of the castration-resistant phenotype in prostate cancer. Here, we present a hypothesis where molecular signaling triggered by inflammatory mediators may evolve in prostate cancer progression. Thus, treatment of chronic inflammation may represent an important therapeutic target in advanced prostate cancer.

Hypoxia differentially affects IL-1β-stimulated MMP-1 and MMP-13 expression of fibroblast-like synoviocytes in an HIF-1α-dependent manner

OBJECTIVES

To further understand the expression regulation of MMP-1 and MMP-13 under physiological and pathological conditions, we investigated the combined effects of hypoxia and pro-inflammatory stimuli on the expression of MMP-1 and MMP-13 in rheumatoid synovial fibroblasts.

METHODS

Synovial fibroblasts were cultured under either hypoxic or normoxic conditions in the presence of IL-1β stimulation. The culture supernatant was analysed for secreted levels of VEGF, MMP-1 and MMP-13. Their gene expression was quantified with real-time and semi-quantitative PCR. Another group of cells was transfected with small-interfering RNA (siRNA) specific for hypoxia-inducible factor-1 α (HIF-1α). The protein levels of HIF-1α were detected by western blot analysis.

RESULTS

In response to 10 ng/ml of IL-1β under normoxia, the levels of MMP-1 and MMP-13 increased compared with the levels observed under hypoxia. IL-1β stimulation under hypoxia induced a 2-fold increase in the level of MMP-1 and a 2-fold decrease in the level of MMP-13 compared with cells cultured under normoxia. A similar pattern of differential expression for MMP-1 and MMP-13 was observed with 1 and 5 ng/ml IL-1β, but not at 0.1 ng/ml. The differential expression of MMPs under the combined effect of IL-1β and hypoxia was significantly attenuated by silencing HIF-1α with siRNA.

CONCLUSIONS

Hypoxia in arthritic joints may differentially affect the IL-1β-stimulated expression of MMP-1 and MMP-13 in rheumatoid synovial fibroblasts. This effect is dependent on HIF-1α expression. This hypoxia-mediated differential effect should be taken into consideration when testing the efficiency of therapies that target HIF-1α.

C/EBPβ and RUNX2 cooperate to degrade cartilage with MMP-13 as the target and HIF-2α as the inducer in chondrocytes

To elucidate the molecular mechanism underlying the endochondral ossification process during the skeletal growth and osteoarthritis (OA) development, we examined the signal network around CCAAT/enhancer-binding protein-β (C/EBPβ, encoded by CEBPB), a potent regulator of this process. Computational predictions and a C/EBP motif-reporter assay identified RUNX2 as the most potent transcriptional partner of C/EBPβ in chondrocytes. C/EBPβ and RUNX2 were induced and co-localized in highly differentiated chondrocytes during the skeletal growth and OA development of mice and humans. The compound knockout of Cebpb and Runx2 in mice caused growth retardation and resistance to OA with decreases in cartilage degradation and matrix metalloproteinase-13 (Mmp-13) expression. C/EBPβ and RUNX2 cooperatively enhanced promoter activity of MMP13 through specific binding to a C/EBP-binding motif and an osteoblast-specific cis-acting element 2 motif as a protein complex. Human genetic studies failed to show the association of human CEBPB gene polymorphisms with knee OA, nor was there a genetic variation around the identified responsive region in the human MMP13 promoter. However, hypoxia-inducible factor-2α (HIF-2α), a functional and genetic regulator of knee OA through promoting endochondral ossification, was identified as a potent and functional inducer of C/EBPβ expression in chondrocytes by the CEBPB promoter assay. Hence, C/EBPβ and RUNX2, with MMP-13 as the target and HIF-2α as the inducer, control cartilage degradation. This molecular network in chondrocytes may represent a therapeutic target for OA.

Inducible expression of Runx2 results in multiorgan abnormalities in mice

Runx2 is a transcription factor controlling skeletal development, and is also expressed in extraskeletal tissues where its function is not well understood. Existing Runx2 mutant and transgenic mouse models do not allow the necessary control of Runx2 expression to understand its functions in different tissues. We generated conditional, doxycyline-inducible, triple transgenic mice (CMV-Cre;ROSA26-neo(flox/+)-rtTA;Tet-O-Runx2) to investigate the effects of wide spread overexpression of Runx2. Osteoblasts isolated from CMV-Cre;ROSA26-neo(flox/+)-rtTA; Tet-O-Runx2 mice demonstrated a dose-dependent effect of doxycycline to stimulate Runx2 transgene expression. Doxycycline administration to CMV-Cre;ROSA26-neo(flox/+)-rtTA;Tet-O-Runx2 mice induced Runx2 transgene expression in all tissues tested, with the highest levels observed in kidney, ovary, and bone. Runx2 overexpression resulted in deceased body size and reduced viability. With regard to bone, Runx2 overexpressing mice paradoxically displayed profound osteopenia and diminished osteogenesis. Induced expression of Runx2 in extraskeletal tissues resulted in ectopic calcification and induction of the osteogenic program in a limited number of tissues, including lung and muscle. In addition, the triple transgenic mice showed evidence of a myeloproliferative disorder and an apparent inhibition of lymphocyte development. Thus, overexpression of Runx2 both within and outside of the skeleton can have diverse biological effects. Use of tissue specific Cre mice will allow this model to be used to conditionally and inducibly overexpress Runx2 in different tissues and provide a means to study the post-natal tissue- and cell context-dependent functions of Runx2.

Divergent upstream osteogenic events contribute to the differential modulation of MG63 cell osteoblast differentiation by MMP-1 (collagenase-1) and MMP-13 (collagenase-3)

Previously we showed that MMP-1 (collagenase-1) and MMP-13 (collagenase-3) differentially regulate the expression of osteoblastic markers in a heterogenous population of primary human periodontal ligament cells. The mechanisms for these differential responses are not known, but may result from divergence in regulation of early osteogenic transcription factors. The purpose of this study was to elucidate where in the hierarchy of osteoblast-specific transcription factors and markers the differences in MMP-1- and -13-mediated regulation of osteoblastic differentiation arise. We found that the overexpression of MMP-1 resulted in significant decreases in BMP-2, Dlx5, AP, OP and BSP and increases in TGF-β1 and MSX2. In contrast, MMP-13 overexpression resulted in significant decreases in Runx2, OP and BSP, and increases in TGF-β1, MSX2 and OC. The knockdown of MMP-1 caused significant increases in all osteoblastic markers. MMP-13 knockdown produced significant increases only in TGF-β1, MSX2 and Osx, but decreases in Runx2 and OC. Suppression of both MMPs together resulted in significant increases of all osteoblastic markers except Runx2. MMP-1 had a more robust and generalized effect in regulating osteoblast transcription factors and markers than MMP-13. Finally, of the markers and transcription factors assayed, Runx2 is the most early stage transcription factor induced by suppression of MMP-1, while Osx and MSX2 are the most early stage transcription factors regulated by MMP-13. These data show that MMP-1's and -13's differential regulation of osteoblastic markers in MG63 cells likely results from their modulation of divergent signaling pathways involved in osteoblastic differentiation.

AP-1 as a Regulator of MMP-13 in the Stromal Cell of Giant Cell Tumor of Bone

Matrix-metalloproteinase-13 (MMP-13) has been shown to be an important protease in inflammatory and neoplastic conditions of the skeletal system. In particular, the stromal cells of giant cell tumor of bone (GCT) express very high levels of MMP-13 in response to the cytokine-rich environment of the tumor. We have previously shown that MMP-13 expression in these cells is regulated, at least in part, by the RUNX2 transcription factor. In the current study, we identify the expression of the c-Fos and c-Jun elements of the AP-1 transcription factor in these cells by protein screening assays and real-time PCR. We then used siRNA gene knockdown to determine that these elements, in particular c-Jun, are upstream regulators of MMP-13 expression and activity in GCT stromal cells. We conclude that there was no synergy found between RUNX2 and AP-1 in the regulation of the MMP13 expression and that these transcription factors may be independently regulated in these cells.

Regulation of mechanical stress-induced MMP-13 and ADAMTS-5 expression by RUNX-2 transcriptional factor in SW1353 chondrocyte-like cells

OBJECTIVE

To investigate the mechanism of mechanical stress-induced expression and regulation of aggrecanases and examine the role of runt-related transcription factor 2 (RUNX-2) in chondrocyte-like cells.

METHODS

SW1353 cells were seeded onto stretch chambers at a concentration of 5×10⁴ cells/chamber, and a uni-axial cyclic tensile strain (CTS) (0.5 Hz, 10% stretch) was applied for 30 min. Total RNA was extracted, reverse transcribed, and analyzed by polymerase chain reaction (PCR) and real-time PCR. RUNX-2 overexpression and small interfering RNA (siRNA) targeting RUNX-2 were used to investigate the role of RUNX-2 in CTS-induced gene expression. The involvement of diverse mitogen-activated protein kinase (MAPK) pathways in the activation of RUNX-2, MMP-13 and ADAMTS-5 during CTS was examined by Western blotting.

RESULTS

CTS induced expression of RUNX-2, MMP-13, ADAMTS-4, -5, and -9. Overexpression of RUNX-2 up-regulated expression of MMP-13 and ADAMTS-5, whereas RUNX-2 siRNA resulted in significant down-regulation of mechanically-induced MMP-13 and ADAMTS-5 expression. CTS induced activation of p38 MAPK, and CTS induction of RUNX-2, MMP-13 and ADAMTS-5 mRNA was down-regulated by the selective p38 MAPK inhibitor SB203580 but not by the p44/42 MAPK inhibitor U0126, or the JNK MAPK inhibitor JNK inhibitor II.

CONCLUSIONS

RUNX-2 might have a role as a key downstream mediator of p38's ability to regulate mechanical stress-induced MMP-13 and ADAMTS-5 expression.

GADD45beta enhances Col10a1 transcription via the MTK1/MKK3/6/p38 axis and activation of C/EBPbeta-TAD4 in terminally differentiating chondrocytes

GADD45beta (growth arrest- and DNA damage-inducible) interacts with upstream regulators of the JNK and p38 stress response kinases. Previously, we reported that the hypertrophic zone of the Gadd45beta(-/-) mouse embryonic growth plate is compressed, and expression of type X collagen (Col10a1) and matrix metalloproteinase 13 (Mmp13) genes is decreased. Herein, we report that GADD45beta enhances activity of the proximal Col10a1 promoter, which contains evolutionarily conserved AP-1, cAMP-response element, and C/EBP half-sites, in synergism with C/EBP family members, whereas the MMP13 promoter responds to GADD45beta together with AP-1, ATF, or C/EBP family members. C/EBPbeta expression also predominantly co-localizes with GADD45beta in the embryonic growth plate. Moreover, GADD45beta enhances C/EBPbeta activation via MTK1, MKK3, and MKK6, and dominant-negative p38alphaapf, but not JNKapf, disrupts the combined trans-activating effect of GADD45beta and C/EBPbeta on the Col10a1 promoter. Importantly, GADD45beta knockdown prevents p38 phosphorylation while decreasing Col10a1 mRNA levels but does not affect C/EBPbeta binding to the Col10a1 promoter in vivo, indicating that GADD45beta influences the transactivation function of DNA-bound C/EBPbeta. In support of this conclusion, we show that the evolutionarily conserved TAD4 domain of C/EBPbeta is the target of the GADD45beta-dependent signaling. Collectively, we have uncovered a novel molecular mechanism linking GADD45beta via the MTK1/MKK3/6/p38 axis to C/EBPbeta-TAD4 activation of Col10a1 transcription in terminally differentiating chondrocytes.

Regulation of gene expression in osteoblasts

In recent years, much progress has been made in understanding the factors that regulate the gene expression program that underlies the induction, proliferation, differentiation, and maturation of osteoblasts. A large and growing number of transcription factors make important contributions to the precise control of osteoblast formation and function. It has become increasingly clear that these diverse transcription factors and the signals that regulate their activity cannot be viewed as discrete, separate signaling pathways. Rather, they form a highly interconnected, cooperative network that permits gene expression to be closely regulated. There has also been a substantial increase in our understanding of the mechanistic control of gene expression by cofactors such as acetyltransferases and histone deacetylases. The purpose of this review is to highlight recent progress in understanding the major transcription factors and epigenetic coregulators, including histone deacetylases and microRNAs, involved in osteoblastogenesis and the mechanisms that determine their functions as regulators of gene expression.

Runx2 association with progression of prostate cancer in patients: mechanisms mediating bone osteolysis and osteoblastic metastatic lesions

Runx2, a bone-specific transcriptional regulator, is abnormally expressed in highly metastatic prostate cancer cells. Here we identified the functional activities of Runx2 in facilitating tumor growth and osteolysis. Our studies demonstrate that negligible Runx2 is found in normal prostate epithelial and non-metastatic LNCaP prostate cancer cells. In the intra-tibial metastasis model, high Runx2 levels are associated with development of large tumors, increased expression of metastasis-related genes (MMP9, MMP13, VEGF, Osteopontin), and secreted bone resorbing factors (PTHrP, IL-8) promoting osteolytic disease. Runx2 siRNA treatment of PC3 cells decreased cell migration and invasion through Matrigel in vitro, and in vivo shRunx2 expression in PC3 cells blocked their ability to survive in the bone microenvironment. Mechanisms of Runx2 function were identified in co-culture studies demonstrating that PC3 cells promote osteoclastogenesis and inhibit osteoblast activity. The clinical significance of these findings is supported by human tissue microarray studies of prostate tumors at stages of cancer progression, where Runx2 is expressed in both adenocarcinomas and metastatic tumors. Together these findings indicate that Runx2 is a key regulator of events associated with prostate cancer metastatic bone disease.

Dose-dependent effects of Runx2 on bone development

Runx2 controls the commitment of mesenchymal cells to the osteoblastic lineage. Distinct promoters, designated P1 and P2, give rise to functionally similar Runx2-II and Runx2-I isoforms. We postulate that this dual promoter gene structure permits temporal and spatial adjustments in the amount of Runx2 isoforms necessary for optimal bone development. To evaluate the gene dose-dependent effect of Runx2 isoforms on bone development, we intercrossed selective Runx2-II(+/-) with nonselective Runx2-II(+/-)/Runx2-I(+/-) mice to create compound mutant mice: Runx2-II(+/-), Runx2-II(+/-)/Runx2-I(+/-), Runx2-II(-/-), Runx2-II(-/-)/Runx2-I(+/-), Runx2-II(-/-)/Runx2-I(-/-). Analysis of the different Runx2-deficient genotypes showed gene dose-dependent differences in the level of expression of the Runx2 isoforms. In addition, we found that Runx2-I is predominately expressed in the perichondrium and proliferating chondrocytes, whereas Runx2-II is expressed in hypertrophic chondrocytes and metaphyseal osteoblasts. Newborn mice showed impaired development of a mineralized skeleton, bone length, and widening of the hypertrophic zone that were proportionate to the reduction in total Runx2 protein expression. Osteoblast differentiation ex vivo was also proportionate to total amount of Runx2 expression that correlated with reduced Runx2 binding to the osteocalcin promoter by quantitative chromatin immunoprecipitation analysis. Functional analysis of P1 and P2 promoters showed differential regulation of the two promoters in osteoblastic cell lines. These findings support the possibility that the total amount of Runx2 derived from two isoforms and the P1 and P2 promoters, by regulating the time, place, and amount of Runx2 in response to changing environmental cues, impacts on bone development.

Distinct roles for AF-1 and -2 of ER-alpha in regulation of MMP-13 promoter activity

Previous studies have indicated that ER-alpha can influence the activity of the MMP-13 promoter. ER-alpha activity is mediated by two separate transcriptional activation domains (AF-1 and AF-2). The present study focused on analyzing the roles of these domains on the activation of the MMP-13 promoter. Transfection of synoviocytes with an ER-alpha construct lacking the C-terminus AF-2 domain led to significant elevation in MMP-13 promoter activity compared to wild type ER-alpha. Progressive deletions in the N-terminal AF-1 domain led to significant losses in MMP-13 promoter activity. MMP-13 promoter mutagenesis indicated that an AP-1 regulatory site was essential for ER-alpha mutant activity. Thus, both AF-1 and AF-2 domains of ER-alpha are required for regulation of MMP-13 promoter activity. As ER variants and ER related proteins have been implicated in bone and joint disorders, these findings provide understanding of the possible role of ER variants in the development of such conditions.

Upregulation of MMP-13 via Runx2 in the stromal cell of Giant Cell Tumor of bone

Giant Cell Tumor of bone (GCT) is an aggressively osteolytic and cytokine-rich bone tumor. Previous work in our lab has shown that matrix metalloproteinase-13 (MMP-13) is the principal proteinase expressed by the mesenchymal stromal cells of GCT. The Runx2 transcription factor is known to have a binding site in the MMP-13 promoter region, and we have previously found this transcription factor to be constitutively expressed in GCT stromal cells. The purpose of this study was to determine the role of Runx2 in MMP-13 regulation in GCT stromal cells. Following in vitro stimulation of GCT stromal cells with incremental concentrations of cytokine IL-1beta or TNF-alpha, the level of MMP-13 mRNA expression increased dramatically over 100-fold with a concomitant increase in MMP-13 protein expression. Inhibition of the ERK and JNK signaling pathways inhibited the upregulation of MMP-13 in these cells. Runx2 siRNA knockdown resulted in MMP-13 knockdown, and this effect was amplified following cytokine stimulation. Our study provides the first evidence that Runx2 may play a crucial role in cytokine-mediated MMP-13 expression in GCT stromal cells.

Sp proteins and Runx2 mediate regulation of matrix gla protein (MGP) expression by parathyroid hormone

As part of its catabolic action in bone, parathyroid hormone (PTH) inhibits extracellular matrix mineralization. We previously showed that PTH dose-dependently induces matrix gla protein (MGP) expression in osteoblasts and this induction is at least partially responsible for PTH-mediated inhibition of mineralization. Recently, we identified PKA and ERK/MAPK as the key signaling pathways involved in PTH regulation of MGP expression. The goal of this study was to further characterize the mechanism by which PTH stimulates expression of MGP. Deletion analysis of the murine Mgp gene promoter identified a PTH-responsive region between -173 bp and-49 bp. Using gel-mobility shift assays we found that Sp1/Sp3, and Runx2 bind to distinct sites within this region. Mutation of either the Sp or the Runx2 site reduced MGP induction by PTH, while mutation of both sites completely abolished PTH responsiveness. Overexpression of Runx2 or Sp1 activated the Mgp reporter, while Sp3 was a dose-dependent repressor of Sp1 and PTH-induced MGP expression. Collectively, these data show that PTH regulates MGP gene transcription in osteoblasts through altered activities of Sp and Runx2 transcription factors.

Runx2 recruits p300 to mediate parathyroid hormone's effects on histone acetylation and transcriptional activation of the matrix metalloproteinase-13 gene

PTH regulates transcription of a number of genes involved in bone remodeling and calcium homeostasis. We have previously shown that the matrix metalloproteinase-13 (MMP-13) gene is induced by PTH in osteoblastic cells as a secondary response through the protein kinase A pathway requiring the runt domain and activator protein 1 binding sites of the proximal promoter. Here, we investigated the changes PTH causes in histone acetylation in this region (which contains the only deoxyribonuclease-hypersensitive sites in the promoter) leading to MMP-13 gene activation in these cells. Chromatin immunoprecipitation experiments revealed that PTH rapidly increased histone H4 acetylation followed by histone H3 acetylation associated with the different regions of the MMP-13 proximal promoter. The hormone also stimulated p300 histone acetyl transferase activity and increased p300 bound to the MMP-13 proximal promoter, and this required protein synthesis. Upon PTH treatment, Runx2, already bound to the runt domain site of the MMP-13 promoter, interacted with p300, which then acetylated histones H4 and H3. The knockdown of either Runx2 or p300 by RNA interference reduced PTH-induced acetylation of histones H3 and H4, association of p300 with the MMP-13 promoter, and resultant MMP-13 gene transcription. Overall, our studies suggest that without altering the gross chromatin structure, PTH stimulates acetylation of histones H3 and H4 via recruitment of p300 to Runx2 bound to the MMP-13 promoter, resulting in gene activation. This work establishes the molecular basis of transcriptional regulation in osteoblasts by PTH, a hormone acting through a G-protein coupled receptor.

Identification and characterization of Runx2 phosphorylation sites involved in matrix metalloproteinase-13 promoter activation

Matrix metalloproteinase-13 (MMP-13) plays a critical role in parathyroid hormone (PTH)-induced bone resorption. PTH acts via protein kinase A (PKA) to phosphorylate and stimulate the transactivation of Runx2 for MMP-13 promoter activation. We show here that PTH stimulated Runx2 phosphorylation in rat osteoblastic cells. Runx2 was phosphorylated on serine 28 and threonine 340 after 8-bromo cyclic adenosine mono phosphate (8-Br-cAMP) treatment. We further demonstrate that in the presence of 8-Br-cAMP, the wild-type Runx2 construct stimulated MMP-13 promoter activity, while the Runx2 construct having mutations at three phosphorylation sites (S28, S347 and T340) was unable to stimulate MMP-13 promoter activity. Thus, we have identified the Runx2 phosphorylation sites necessary for PKA stimulated MMP-13 promoter activation and this event may be critical for bone remodeling.

Role of matrix metalloproteinase 13 in both endochondral and intramembranous ossification during skeletal regeneration

Extracellular matrix (ECM) remodeling is important during bone development and repair. Because matrix metalloproteinase 13 (MMP13, collagenase-3) plays a role in long bone development, we have examined its role during adult skeletal repair. In this study we find that MMP13 is expressed by hypertrophic chondrocytes and osteoblasts in the fracture callus. We demonstrate that MMP13 is required for proper resorption of hypertrophic cartilage and for normal bone remodeling during non-stabilized fracture healing, which occurs via endochondral ossification. However, no difference in callus strength was detected in the absence of MMP13. Transplant of wild-type bone marrow, which reconstitutes cells only of the hematopoietic lineage, did not rescue the endochondral repair defect, indicating that impaired healing in Mmp13^−/− mice is intrinsic to cartilage and bone. Mmp13^−/− mice also exhibited altered bone remodeling during healing of stabilized fractures and cortical defects via intramembranous ossification. This indicates that the bone phenotype occurs independently from the cartilage phenotype. Taken together, our findings demonstrate that MMP13 is involved in normal remodeling of bone and cartilage during adult skeletal repair, and that MMP13 may act directly in the initial stages of ECM degradation in these tissues prior to invasion of blood vessels and osteoclasts.

Overexpression of the transcriptional factor Runx2 in osteoblasts abolishes the anabolic effect of parathyroid hormone in vivo

There is convincing evidence that Runx2 could be a regulator of the anabolic action of parathyroid hormone (PTH) in bone. We therefore decided to determine how Runx2 overexpression in osteoblasts affects the anabolic response to PTH. Transgenic osteoporotic female mice overexpressing Runx2 (TG) and their wild-type littermates (WT) were treated with PTH (100 microg/kg/day, 7 days a week) or with the vehicle for 6 weeks. Unexpectedly, Runx2 overexpression blunted the increase in the mineral density and volume of bone induced by intermittent PTH in WT mice. Our findings also indicate that PTH failed to increase bone formation in TG mice overexpressing Runx2. This abolition of the effect of PTH by Runx2 overexpression was attributable to a decrease in the differentiation of osteoblastic cells both in vivo and in vitro. Finally, we showed that less cAMP was induced by PTH and that there were fewer PTH binding sites in TG than WT osteoblasts. In conclusion, our findings demonstrate that in vivo a high level of Runx2 abolishes the anabolic effect of PTH, probably via a decrease in the sensitivity of TG osteoblasts to PTH, and that the level of expression of Runx2 is critical if PTH is to produce its anabolic effect on bone in vivo.