Both AP-1 and Cbfa1-like factors are required for the induction of interstitial collagenase by parathyroid hormone

Whole Aspect of Runx2 Functions in Skeletal Development

Runt-related transcription factor 2 (Runx2) is a fundamental transcription factor for bone development. In endochondral ossification, Runx2 induces chondrocyte maturation, enhances chondrocyte proliferation through Indian hedgehog (Ihh) induction, and induces the expression of vascular endothelial growth factor A (Vegfa), secreted phosphoprotein 1 (Spp1), integrin-binding sialoprotein (Ibsp), and matrix metallopeptidase 13 (Mmp13) in the terminal hypertrophic chondrocytes. Runx2 inhibits the apoptosis of the terminal hypertrophic chondrocytes and induces their transdifferentiation into osteoblasts and osteoblast progenitors. The transdifferentiation is required for trabecular bone formation during embryonic and newborn stages but is dispensable for acquiring normal bone mass in young and adult mice. Runx2 enhances the proliferation of osteoblast progenitors and induces their commitment to osteoblast lineage cells through the direct regulation of the expressions of a hedgehog, fibroblast growth factor (Fgf), Wnt, and parathyroid hormone-like hormone (Pthlh) signaling pathway genes and distal-less homeobox 5 (Dlx5), which all regulate Runx2 expression and/or protein activity. Runx2, Sp7, and Wnt signaling further induce osteoblast differentiation. In immature osteoblasts, Runx2 regulates the expression of bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and bone gamma carboxyglutamate protein (Bglap)/Bglap2, and induces osteoblast maturation. Osteocalcin (Bglap/Bglap2) is required for the alignment of apatite crystals parallel to the collagen fibers; however, it does not physiologically work as a hormone that regulates glucose metabolism, testosterone synthesis, or muscle mass. Thus, Runx2 exerts multiple functions essential for skeletal development.

A Molecular Cascade Underlying Articular Cartilage Degeneration

Preserving of articular cartilage is an effective way to protect synovial joints from becoming osteoarthritic (OA) joints. Understanding of the molecular basis of articular cartilage degeneration will provide valuable information in the effort to develop cartilage preserving drugs. There are currently no disease-modifying OA drugs (DMOADs) available to prevent articular cartilage destruction during the development of OA. Current drug treatments for OA focus on the reduction of joint pain, swelling, and inflammation at advanced stages of the disease. However, based on discoveries from several independent research laboratories and our laboratory in the past 15 to 20 years, we believe that we have a functional molecular understanding of articular cartilage degeneration. In this review article, we present and discuss experimental evidence to demonstrate a sequential chain of the molecular events underlying articular cartilage degeneration, which consists of transforming growth factor beta 1, high-temperature requirement A1 (a serine protease), discoidin domain receptor 2 (a cell surface receptor tyrosine kinase for native fibrillar collagens), and matrix metalloproteinase 13 (an extracellularmatrix degrading enzyme). If, as we strongly suspect, this molecular pathway is responsible for the initiation and acceleration of articular cartilage degeneration, which eventually leads to progressive joint failure, then these molecules may be ideal therapeutic targets for the development of DMOADs.

The Transcription Factor HAND1 Is Involved in Cortical Bone Mass through the Regulation of Collagen Expression

Temporal and/or spatial alteration of collagen family gene expression results in bone defects. However, how collagen expression controls bone size remains largely unknown. The basic helix-loop-helix transcription factor HAND1 is expressed in developing long bones and is involved in their morphogenesis. To understand the functional role of HAND1 and collagen in the postnatal development of long bones, we overexpressed Hand1 in the osteochondroprogenitors of model mice and found that the bone volumes of cortical bones decreased in Hand1^Tg/+;Twist2-Cre mice. Continuous Hand1 expression downregulated the gene expression of type I, V, and XI collagen in the diaphyses of long bones and was associated with decreased expression of Runx2 and Sp7/Osterix, encoding transcription factors involved in the transactivation of fibril-forming collagen genes. Members of the microRNA-196 family, which target the 3' untranslated regions of COL1A1 and COL1A2, were significantly upregulated in Hand1^Tg/+;Twist2-Cre mice. Mass spectrometry revealed that the expression ratios of alpha 1(XI), alpha 2(XI), and alpha 2(V) in the diaphysis increased during postnatal development in wild-type mice, which was delayed in Hand1^Tg/+;Twist2-Cre mice. Our results demonstrate that HAND1 regulates bone size and morphology through osteochondroprogenitors, at least partially by suppressing postnatal expression of collagen fibrils in the cortical bones.

MEF2C Interacts With c-FOS in PTH-Stimulated Mmp13 Gene Expression in Osteoblastic Cells

Parathyroid hormone (PTH) regulates the transcription of many genes in the osteoblast. One of these genes is Mmp13, which is involved in bone remodeling and early stages of endochondral bone formation. Previously, we reported that PTH induces Mmp13 transcription by regulating the dissociation of histone deacetylase 4 (HDAC4) from runt-related transcription factor 2 (Runx2), and the association of the HATs, p300, and p300/CREB binding protein (CBP)-associated factor. It is known that, in addition to Runx2, HDAC4 binds to the transcription factor, myocyte-specific enhancer factor 2c (MEF2C), and represses its activity. In this work, we investigated whether MEF2C participates in PTH-stimulated Mmp13 gene expression in osteoblastic cells and how it does so. Knockdown of Mef2c in UMR 106-01 cells repressed Mmp13 messenger RNA expression and promoter activity with or without PTH treatment. Chromatin immunoprecipitation (ChIP) assays showed that MEF2C associated with the Mmp13 promoter; this increased after 4 hours of PTH treatment. ChIP-reChIP results indicate that endogenous MEF2C associates with HDAC4 on the Mmp13 promoter; after PTH treatment, this association decreased. From gel shift, ChIP, and promoter-reporter assays, MEF2C was found to associate with the activator protein-1 (AP-1) site without directly binding to DNA and had its stimulatory effect through interaction with c-FOS. In conclusion, MEF2C is necessary for Mmp13 gene expression at the transcriptional level and participates in PTH-stimulated Mmp13 gene expression by increased binding to c-FOS at the AP-1 site in the Mmp13 promoter. The observation of MEF2C interacting with a member of the AP-1 transcription factor family provides knowledge of the functions of HDAC4, c-FOS, and MEF2C.

Deletion of Runx2 in Articular Chondrocytes Decelerates the Progression of DMM-Induced Osteoarthritis in Adult Mice

Runx2 may play an important role in development of osteoarthritis (OA). However, the specific role of Runx2 in articular chondrocyte function and in OA development in adult mice has not been fully defined. In this study, we performed the destabilization of the medial meniscus (DMM) surgery at 12-week-old mice to induce OA in adult Runx2^Agc1CreER mice, in which Runx2 was specifically deleted in Aggrecan-expressing chondrocytes by administering tamoxifen at 8-weeks of age. Knee joint samples were collected 8- and 12-weeks post-surgery and analyzed through histology, histomorphometry and micro-computed tomography (μCT). Our results showed that severe OA-like defects were observed after DMM surgery in Cre-negative control mice, including articular cartilage degradation and subchondral sclerosis, while the defects were significantly ameliorated in Runx2^Agc1CreER KO mice. Immunohistochemical (IHC) results showed significantly reduced expression of MMP13 in Runx2^Agc1CreER KO mice compared to that in Cre-negative control mice. Results of quantitative reverse-transcription PCR (qRT-PCR) demonstrated that expression of the genes encoding for matrix degradation enzymes was significantly decreased in Runx2^Agc1CreER KO mice. Thus, our findings suggest that inhibition of Runx2 in chondrocytes could at least partially rescue DMM-induced OA-like defects in adult mice.

Lectin-like, oxidized low-density lipoprotein receptor-1-deficient mice show resistance to age-related knee osteoarthritis

The lectin-like, oxidized low-density lipoprotein (ox-LDL) receptor-1 (LOX-1)/ox-LDL system contributes to atherosclerosis and may be involved in cartilage degeneration. The purpose of this study was to determine whether the LOX-1/ox-LDL system contributes to age-related osteoarthritis (OA) in vivo, using LOX-1 knockout (LOX-1 KO) mice. Knee cartilage from 6, 12, and 18-month old (n = 10/group) C57Bl/6 wild-type (WT) and LOX-1 KO mice was evaluated by determining the Osteoarthritis Research Society International (OARSI) score of Safranin-O stained samples. The prevalence of knee OA in both mouse strains was also investigated. Expression levels of LOX-1, ox-LDL, runt-related transcription factor-2 (Runx2), type-X collagen (COL X), and matrix metalloproteinase-13 (MMP-13) in the articular chondrocytes were analyzed immunohistologically. No significant difference was observed in the mean scores of WT (2.00±0.61) and LOX-1 KO mice (2.00±0.49) at 6 months of age (P=1.00, n=10). At 12 and 18 months of age, the mean scores of LOX-1 KO mice (3.75±0.93 and 5.50±0.78) were significantly lower than those of WT mice (5.25±1.14 and 9.00±1.01; P<0.001 in both cases; n=10). The prevalence of OA in LOX-1 KO mice was lower than that in WT mice at 12 and 18 months of age (40 vs 70%, 70 vs 90%, respectively; n=10). The expression levels of Runx2, COL X, and MMP-13 in articular chondrocytes significantly decreased in LOX-1 KO, mice compared with those in WT mice. The study indicated that the LOX-1/ox-LDL system in chondrocytes plays a role in the pathogenesis of age-related knee OA, which is potentially a target for preventing OA progression.

Cytokine-induced MMP13 Expression in Human Chondrocytes Is Dependent on Activating Transcription Factor 3 (ATF3) Regulation

Irreversible breakdown of cartilage extracellular matrix (ECM) by the collagenase matrix metalloproteinase 13 (MMP13) represents a key event in osteoarthritis (OA) progression. Although inflammation is most commonly associated with inflammatory joint diseases, it also occurs in OA and is thus relevant to the prevalent tissue destruction. Here, inflammation generates a cFOS AP-1 early response that indirectly affects MMP13 gene expression. To ascertain a more direct effect on prolonged MMP13 production we examined the potential molecular events occurring between the rapid, transient expression of cFOS and the subsequent MMP13 induction. Importantly, we show MMP13 mRNA expression is mirrored by nascent hnRNA transcription. Employing ChIP assays, cFOS recruitment to the MMP13 promoter occurs at an early stage prior to gene transcription and that recruitment of transcriptional initiation markers also correlated with MMP13 expression. Moreover, protein synthesis inhibition following early FOS expression resulted in a significant decrease in MMP13 expression thus indicating a role for different regulatory factors modulating expression of the gene. Subsequent mRNA transcriptome analyses highlighted several genes induced soon after FOS that could contribute to MMP13 expression. Specific small interfering RNA-mediated silencing highlighted that ATF3 was as highly selective for MMP13 as cFOS. Moreover, ATF3 expression was AP-1(cFOS/cJUN)-dependent and expression levels were maintained after the early transient cFOS response. Furthermore, ATF3 bound the proximal MMP13 AP-1 motif in stimulated chondrocytes at time points that no longer supported binding of FOS Consequently, these findings support roles for both cFOS (indirect) and ATF3 (direct) in effecting MMP13 transcription in human chondrocytes.

Roles of Runx2 in Skeletal Development

Runx2 is the most upstream transcription factor essential for osteoblast differentiation. It regulates the expression of Sp7, the protein of which is a crucial transcription factor for osteoblast differentiation, as well as that of bone matrix genes including Spp1, Ibsp, and Bglap2. Runx2 is also required for chondrocyte maturation, and Runx3 has a redundant function with Runx2 in chondrocyte maturation. Runx2 regulates the expression of Col10a1, Spp1, Ibsp, and Mmp13 in chondrocytes. It also inhibits chondrocytes from acquiring the phenotypes of permanent cartilage chondrocytes. It regulates chondrocyte proliferation through the regulation of Ihh expression. Runx2 enhances osteoclastogenesis by regulating Rankl. Cbfb, which is a co-transcription factor for Runx family proteins, plays an important role in skeletal development by stabilizing Runx family proteins. In Cbfb isoforms, Cbfb1 is more potent than Cbfb2 in Runx2-dependent transcriptional regulation; however, the expression level of Cbfb2 is three-fold higher than that of Cbfb1, demonstrating the requirement of Cbfb2 in skeletal development. The expression of Runx2 in osteoblasts is regulated by a 343-bp enhancer located upstream of the P1 promoter. This enhancer is activated by an enhanceosome composed of Dlx5/6, Mef2, Tcf7, Ctnnb1, Sox5/6, Smad1, and Sp7. Thus, Runx2 is a multifunctional transcription factor that is essential for skeletal development, and Cbfb regulates skeletal development by modulating the stability and transcriptional activity of Runx family proteins.

MMP-13 is one of the critical mediators of the effect of HDAC4 deletion on the skeleton

Histone deacetylase 4 (Hdac4) regulates chondrocyte hypertrophy. Hdac4(-/-) mice are runted in size and do not survive to weaning. This phenotype is primarily due to the acceleration of onset of chondrocyte hypertrophy and, as a consequence, inappropriate endochondral mineralization. Previously, we reported that Hdac4 is a repressor of matrix metalloproteinase-13 (Mmp13) transcription, and the absence of Hdac4 leads to increased expression of MMP-13 both in vitro (osteoblastic cells) and in vivo (hypertrophic chondrocytes and trabecular osteoblasts). MMP-13 is thought to be involved in endochondral ossification and bone remodeling. To identify whether the phenotype of Hdac4(-/-) mice is due to up-regulation of MMP-13, we generated Hdac4/Mmp13 double knockout mice and determined the ability of deletion of MMP-13 to rescue the Hdac4(-/-) mouse phenotype. Mmp13(-/-) mice have normal body size. Hdac4(-/-)/Mmp13(-/-) double knockout mice are significantly heavier and larger than Hdac4(-/-) mice, they survive longer, and they recover the thickness of their growth plate zones. In Hdac4(-/-)/Mmp13(-/-) double knockout mice, alkaline phosphatase (ALP) expression and TRAP-positive osteoclasts were restored (together with an increase in Mmp9 expression) but osteocalcin (OCN) was not. Micro-CT analysis of the tibiae revealed that Hdac4(-/-) mice have significantly decreased cortical bone area compared with the wild type mice. In addition, the bone architectural parameter, bone porosity, was significantly decreased in Hdac4(-/-) mice. Hdac4(-/-)/Mmp13(-/-) double knockout mice recover these cortical parameters. Likewise, Hdac4(-/-) mice exhibit significantly increased Tb.Th and bone mineral density (BMD) while the Hdac4(-/-)/Mmp13(-/-) mice significantly recovered these parameters toward normal for this age. Taken together, our findings indicate that the phenotype seen in the Hdac4(-/-) mice is partially derived from elevation in MMP-13 and may be due to a bone remodeling disorder caused by overexpression of this enzyme.

GnRH regulates trophoblast invasion via RUNX2-mediated MMP2/9 expression

STUDY HYPOTHESIS

We hypothesized that Runt-related transcription factor 2 (RUNX2), matrix metalloproteinase (MMP)2 and MMP9 are involved in basal and gonadotrophin-releasing hormone (GnRH)-induced human extravillous trophoblast (EVT) cell invasion.

STUDY FINDING

Our finding indicates that GnRH-induced RUNX2 expression enhances the invasive capacity of EVT cells by modulating the expression of MMP2 and MMP9.

WHAT IS KNOWN ALREADY

GnRH is expressed in first-trimester placenta and exerts pro-invasive effects on EVT cells in vitro. RUNX2 regulates MMP2 and MMP9 expression and is often associated with invasive phenotypes.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

First-trimester human placenta (n = 9) was obtained from women undergoing elective termination of pregnancy. The localization of RUNX2, MMP2 and MMP9 in first-trimester human placenta was examined by immunohistochemistry. Primary or immortalized (HTR-8/SVneo) EVT cells were treated alone or in combination with GnRH, GnRH antagonist Antide, MAPK kinase inhibitor PD98095, phosphatidylinositol 3-kinase inhibitor LY294002, MMP2/9 inhibitor or small interfering RNAs (siRNAs) targeting RUNX2, MMP2 and/or MMP9. Protein and mRNA levels were measured by western blot and RT-PCR, respectively. Cell invasiveness was evaluated by transwell Matrigel or collagen I invasion assays.

MAIN RESULTS AND THE ROLE OF CHANCE

RUNX2, MMP2 and MMP9 were detected in the cell column regions of human first-trimester placental villi. GnRH treatment increased RUNX2 mRNA and protein levels in HTR-8/SVneo cells and primary EVTs, and these effects were attenuated by co-treatment with Antide, PD98095 or LY294002. Down-regulation of RUNX2 by siRNA reduced basal and GnRH-induced MMP2/9 expression and cell invasion. Moreover, pharmacological inhibition or siRNA-mediated knockdown of MMP2/9 reduced basal and GnRH-induced cell invasion.

LIMITATIONS, REASONS FOR CAUTION

The lack of an in vivo model is the major limitation of our in vitro study.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings provide important insight into the functions of the GnRH - GnRH receptor system in early implantation and placentation.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This research was supported by Canadian Institutes of Health Research (Grant #143317) to P.C.K.L. The authors have nothing to disclose.

Expression array analysis of the hepatocyte growth factor invasive program

Signaling by human hepatocyte growth factor (hHGF) via its cell surface receptor (MET) drives mitogenesis, motogenesis and morphogenesis in a wide spectrum of target cell types and embryologic, developmental and homeostatic contexts. Oncogenic pathway activation also contributes to tumorigenesis and cancer progression, including tumor angiogenesis and metastasis, in several prevalent malignancies. The HGF gene encodes full-length hHGF and two truncated isoforms known as NK1 and NK2. NK1 induces all three HGF activities at modestly reduced potency, whereas NK2 stimulates only motogenesis and enhances HGF-driven tumor metastasis in transgenic mice. Prior studies have shown that mouse HGF (mHGF) also binds with high affinity to human MET. Here we show that, like NK2, mHGF stimulates cell motility, invasion and spontaneous metastasis of PC3M human prostate adenocarcinoma cells in mice through human MET. To identify target genes and signaling pathways associated with motogenic and metastatic HGF signaling, i.e., the HGF invasive program, gene expression profiling was performed using PC3M cells treated with hHGF, NK2 or mHGF. Results obtained using Ingenuity Pathway Analysis software showed significant overlap with networks and pathways involved in cell movement and metastasis. Interrogating The Cancer Genome Atlas project also identified a subset of 23 gene expression changes in PC3M with a strong tendency for co-occurrence in prostate cancer patients that were associated with significantly decreased disease-free survival.

Cbfb regulates bone development by stabilizing Runx family proteins

Runx family proteins, Runx1, Runx2, and Runx3, play important roles in skeletal development. Runx2 is required for osteoblast differentiation and chondrocyte maturation, and haplodeficiency of RUNX2 causes cleidocranial dysplasia, which is characterized by open fontanelles and sutures and hypoplastic clavicles. Cbfb forms a heterodimer with Runx family proteins and enhances their DNA-binding capacity. Cbfb-deficient (Cbfb(-/-) ) mice die at midgestation because of the lack of fetal liver hematopoiesis. We previously reported that the partial rescue of hematopoiesis in Cbfb(-/-) mice revealed the requirement of Cbfb in skeletal development. However, the precise functions of Cbfb in skeletal development still remain to be clarified. We deleted Cbfb in mesenchymal cells giving rise to both chondrocyte and osteoblast lineages by mating Cbfb(fl/fl) mice with Dermo1 Cre knock-in mice. Cbfb(fl/fl/Cre) mice showed dwarfism, both intramembranous and endochondral ossifications were retarded, and chondrocyte maturation and proliferation and osteoblast differentiation were inhibited. The differentiation of chondrocytes and osteoblasts were severely inhibited in vitro, and the reporter activities of Ihh, Col10a1, and Bglap2 promoter constructs were reduced in Cbfb(fl/fl/Cre) chondrocytes or osteoblasts. The proteins of Runx1, Runx2, and Runx3 were reduced in the cartilaginous limb skeletons and calvariae of Cbfb(fl/fl/Cre) embryos compared with the respective protein in the respective tissue of Cbfb(fl/fl) embryos at E15.5, although the reduction of Runx2 protein in calvariae was much milder than that in cartilaginous limb skeletons. All of the Runx family proteins were severely reduced in Cbfb(fl/fl/Cre) primary osteoblasts, and Runx2 protein was less stable in Cbfb(fl/fl/Cre) osteoblasts than Cbfb(fl/fl) osteoblasts. These findings indicate that Cbfb is required for skeletal development by regulating chondrocyte differentiation and proliferation and osteoblast differentiation; that Cbfb plays an important role in the stabilization of Runx family proteins; and that Runx2 protein stability is less dependent on Cbfb in calvariae than in cartilaginous limb skeletons.

An update on diabetes related skeletal fragility

There are several mechanisms by which diabetes could affect bone mass and strength. These mechanisms include insulin deficiency; hyperglycemia; the accumulation of advanced glycation end products that may influence collagen characteristics; marrow adiposity and bone inflammation. Furthermore, associated diabetic complications and treatment with thaizolidinediones may also increase risk of fracturing. The following article provides its readers with an update on the latest information pertaining to diabetes related bone skeletal fragility. In the authors' opinion, future studies are needed in order to clarify the impact of different aspects of diabetes metabolism, glycemic control, and specific treatments for diabetes on bone. Given that dual energy x-ray absorptiometry is a poor predictor of bone morbidity in this group of patients, there is a need to explore novel approaches for assessing bone quality. It is important that we develop a better understanding of how diabetes affects bone in order to improve our ability to protect bone health and prevent fractures in the growing population of adults with diabetes.

MiR-126-5p regulates osteolysis formation and stromal cell proliferation in giant cell tumor through inhibition of PTHrP

Parathyroid hormone-related protein (PTHrP) has been identified to play a crucial role in osteolysis formation and stromal cell (GCTSC) proliferation in giant cell tumor (GCT). MiR-126-5p is an intronic miRNA identified as tumor suppressor in many tumors, but its role in GCT is poorly understood. We found that miR-126-5p was decreased in GCT and could directly regulate PTHrP expression. Furthermore, miR-126-5p could control osteoclast (OC) differentiation, GCTSC proliferation and osteolysis formation in GCT through negative regulation of PTHrP. Thus, these results suggest that miR-126-5p could directly target PTHrP and have a tumor suppressor function in GCT.

Kruppel-like factor 4 expression in osteoblasts represses osteoblast-dependent osteoclast maturation

Kruppel-like factor 4 (KLF4) is a zinc-finger-type transcription factor with a restricted expression pattern during skeletal development. We have previously shown that KLF4 represses osteoblast mineralization concomitant with a down-regulation in the expression of a number of osteoblastic genes, both in vivo and in vitro. In addition to the cell-autonomous effects of KLF4 in osteoblasts, transgenic osteoblastic-KLF4 mice show severe defects in osteoclast maturation. Wild-type bone-marrow-derived macrophages co-cultured with KLF4-expressing osteoblasts exhibit reduced formation of multinuclear osteoclasts as compared with control cultures overexpressing green fluorescent protein. Significantly, the transduction of Runx2, a master regulator of osteoblastogenesis, together with KLF4 into osteoblasts restores the reduction in osteoclastogenesis induced by KLF4 alone. Various extracellular matrix molecules are down-regulated by KLF4 overexpression but this down-regulation can be partially restored by the co-transduction of Runx2. These results suggest that osteoblastic-KLF4 affects osteoclast maturation by regulating cell-matrix interactions and reinforce the importance of the regional down-regulation of KLF4 expression in the subset of osteoblasts for normal skeletal modeling and remodeling.

Genetic expression of MMP-Matrix-mettalo-proteinases (MMP-1 and MMP-13) as a function of anterior mandibular repositioning appliance on the growth of mandibular condylar cartilage with and without administration of Insulin like growth factor (IGF-1) and Transforming growth factor-B (TGF-β)

OBJECTIVE

To determine if the mandibular condylar cartilage (MCC) will grow with and without mandibular anterior repositioning appliances with the administration of insulin-like growth factor (IGF-1) and transforming growth factor-β (TGF-β).

MATERIALS AND METHODS

Twenty-four growing New Zealand rabbits were divided into three groups: a group with saline injection in the temporomandibular joint, a group that received anterior positioning appliance, and a group that received injection of growth factors as well as mandibular repositioning appliance. Real-time reverse transcription polymerase chain reaction technique was used to study gene expression supported by histomorphometry.

RESULTS

Administration of growth factors along with mandibular repositioning appliances has induced 5.70-fold expression of matrix metalloproteinase-1 (MMP-1) (P < .0005) and 1.29-fold expression of MMP-13 (P < .0005). In contrast, administration of mandibular repositioning appliances only has induced 2.33-fold expression of MMP-1 (P < .0005) and 0.83-fold expression of MMP-13 (P < .0005). Histomorphometric analysis revealed increased proliferation of the condylar cartilage in the appliance and injection group as compared to the control group.

CONCLUSION

The administration of growth factors along with the use of mandibular advancement appliance has increased genetic expression of MMP-1 and MMP-13 supported by histomorphometric evidence indicating growth of condylar cartilage.

Changes in serum runt-related transcription factor 2 levels after a 6-month treatment with recombinant human parathyroid hormone in patients with osteoporosis

BACKGROUND

The mechanisms regulating the anabolic response of the skeleton for recombinant human PTH (1- 34) [rhPTH (1-34)] administration has not been fully elucidated.

AIM

The aim of this study was to evaluate the effect of rhPTH (1-34) on serum levels of runt-related transcription factor 2 (Runx2) in women with osteoporosis.

METHODS

Sixty post-menopausal women with osteoporosis (EO group) and 45 control subjects (NC group) were enrolled in this study. The EO group received daily injection of 20 μg rhPTH (1-34) plus oral 500 mg elemental calcium and 400 IU vitamin D3 for 6 months. Runx2 and Matrix metalloproteinase 13 (MMP-13) were measured with commercially available enzyme-linked immunosorbent assay kits. Bone mineral density (BMD) was also measured before and 6 months after rhPTH (1-34) treatment.

RESULTS

Serum total Ca2+, phosphate, and bone-specific alkaline phosphatase were significantly increased (p<0.05 or p<0.01), and the lumbar spine BMD (LS-BMD) was also increased by 4% in patients with osteoporosis after treatment with rhPTH (1-34) (p<0.05). On the contrary, serum Runx2 and MMP-13 were significantly decreased at post treatment (13.1% and 36.6%, respectively, p<0.05 and p<0.01). At baseline, serum Runx2 positively correlated with MMP-13 (r=0.74, p<0.01), the correction remained after adjusting for age and body mass index.

CONCLUSION

The daily injection of rhPTH (1-34) was able to stimulate bone formation. The therapy of 20 μg rhPTH (1- 34) for 6 months resulted in decrease of serum Runx2 and MMP-13. These changes might reflect the increase of active osteoblasts and the better bone homeostasis.

Review: The diabetic bone: a cellular and molecular perspective

With the increasing worldwide prevalence of diabetes the resulting complications, their consequences and treatment will lead to a greater social and financial burden on society. One of the many organs to be affected is bone. Loss of bone is observed in type 1 diabetes, in extreme cases mirroring osteoporosis, thus a greater risk of fracture. In the case of type 2 diabetes, both a loss and an increase of bone has been observed, although in both cases the quality of the bone overall was poorer, again leading to a greater risk of fracture. Once a fracture has occurred, healing is delayed in diabetes, including nonunion. The reasons leading to such changes in the state of the bone and fracture healing in diabetes is under investigation, including at the cellular and the molecular levels. In comparison with our knowledge of events in normal bone homeostasis and fracture healing, that for diabetes is much more limited, particularly in patients. However, progress is being made, especially with the use of animal models for both diabetes types. Identifying the molecular and cellular changes in the bone in diabetes and understanding how they arise will allow for targeted intervention to improve diabetic bone, thus helping to counter conditions such as Charcot foot as well as preventing fracture and accelerating healing when a fracture does occur.

Developmental mechanisms in articular cartilage degradation in osteoarthritis

Osteoarthritis is the most common arthritic condition, which involves progressive degeneration of articular cartilage. The most recent accomplishments have significantly advanced our understanding on the mechanisms of the disease development and progression. The most intriguing is the growing evidence indicating that extracellular matrix destruction in osteoarthritic articular cartilage resembles that in the hypertrophic zone of fetal growth plate during endochondral ossification. This suggests common regulatory mechanisms of matrix degradation in OA and in the development and can provide new approaches for the treatment of the disease by targeting reparation of chondrocyte phenotype.

Sumoylation and nuclear translocation of S100A4 regulate IL-1beta-mediated production of matrix metalloproteinase-13

S100A4, a member of the S100 family of proteins, plays an important role in matrix remodeling by up-regulating the expression of matrix metalloproteinases (MMPs). We have previously shown that S100A4 is overexpressed in diseased cartilage and that extracellular S100A4 stimulates MMP-13 production, a major type II collagen-degrading enzyme, via activation of receptor for advanced glycation end product signaling. In the present study, using human articular chondrocytes, we show that intracellular S100A4 translocated into the nucleus upon interleukin-1β (IL-1β) stimulation and translocation required post-translational modification of S100A4 by the sumo-1 protein. Two sumoylation sites were identified on the S100A4 molecule, Lys(22) and Lys(96). Mutation of these lysine residues abolished the ability of S100A4 to be sumoylated and to translocate into the nucleus. Blocking of sumoylation and nuclear transport of S100A4 inhibited the IL-1β-induced production of MMP-13. Nuclear S100A4 was bound to the promoter region of MMP-13 in IL-1β-treated cells. Thus, we demonstrate a novel mechanism for sumoylated S100A4 as a regulator of expression of the MMP-13 gene.

SOX9 is a major negative regulator of cartilage vascularization, bone marrow formation and endochondral ossification

SOX9 is a transcription factor of the SRY family that regulates sex determination, cartilage development and numerous other developmental events. In the foetal growth plate, Sox9 is highly expressed in chondrocytes of the proliferating and prehypertrophic zone but declines abruptly in the hypertrophic zone, suggesting that Sox9 downregulation in hypertrophic chondrocytes might be a necessary step to initiate cartilage-bone transition in the growth plate. In order to test this hypothesis, we generated transgenic mice misexpressing Sox9 in hypertrophic chondrocytes under the control of a BAC-Col10a1 promoter. The transgenic offspring showed an almost complete lack of bone marrow in newborns, owing to strongly retarded vascular invasion into hypertrophic cartilage and impaired cartilage resorption, resulting in delayed endochondral bone formation associated with reduced bone growth. In situ hybridization analysis revealed high levels of Sox9 misexpression in hypertrophic chondrocytes but deficiencies of Vegfa, Mmp13, RANKL and osteopontin expression in the non-resorbed hypertrophic cartilage, indicating that Sox9 misexpression in hypertrophic chondrocytes inhibits their terminal differentiation. Searching for the molecular mechanism of SOX9-induced inhibition of cartilage vascularization, we discovered that SOX9 is able to directly suppress Vegfa expression by binding to SRY sites in the Vegfa gene. Postnatally, bone marrow formation and cartilage resorption in transgenic offspring are resumed by massive invasion of capillaries through the cortical bone shaft, similar to secondary ossification. These findings imply that downregulation of Sox9 in the hypertrophic zone of the normal growth plate is essential for allowing vascular invasion, bone marrow formation and endochondral ossification.

Parathyroid hormone induction of cyclooxygenase-2 in murine osteoblasts: role of the calcium-calcineurin-NFAT pathway

Murine MC3T3-E1 and MC-4 cells were stably transfected with -371/+70 bp of the murine cyclooxygenase-2 (COX-2) promoter fused to a luciferase reporter (Pluc371) or with Pluc371 carrying site-directed mutations. Mutations were made in (1) the cAMP response element (CRE) at -57/-52 bp, (2) the activating protein-1 (AP-1)-binding site at -69/-63 bp, (3) the nuclear factor of activated T-cells (NFAT)-binding site at -77/-73 bp, and (4) both the AP-1 and NFAT sites, which comprise a composite consensus sequence for NFAT/AP-1. Single mutation of CRE, AP-1, or NFAT sites decreased parathyroid hormone (PTH)-stimulated COX-2 promoter activity 40% to 60%, whereas joint mutation of NFAT and AP-1 abrogated the induction. On electrophoretic mobility shift analysis, PTH stimulated binding of phosphorylated CREB to an oligonucleotide spanning the CRE and binding of NFATc1, c-Fos, and c-Jun to an oligonucleotide spanning the NFAT/AP-1 composite site. Mutation of the NFAT site was less effective than mutation of the AP-1 site in competing binding to the composite element, suggesting that cooperative interactions of NFATc1 and AP-1 are more dependent on NFAT than on AP-1. Both PTH and forskolin, an activator of adenylyl cyclase, stimulated NFATc1 nuclear translocation. PTH- and forskolin-stimulated COX-2 promoter activity was inhibited 56% to 80% by calcium chelation or calcineurin inhibitors and 60% to 98% by protein kinase A (PKA) inhibitors. These results indicate an important role for the calcium-calcineurin-NFAT signaling pathway in the PTH induction of COX-2 and suggest that cross-talk between the cAMP/PKA pathway and the calcium-calcineurin-NFAT pathway may play a role in other functions of PTH in osteoblasts.

HDAC4 represses matrix metalloproteinase-13 transcription in osteoblastic cells, and parathyroid hormone controls this repression

Parathyroid hormone (PTH) is a hormone regulating bone remodeling through its actions on both bone formation and bone resorption. Previously we reported that PTH induces matrix metalloproteinase-13 (MMP-13) transcription in osteoblastic cells. Here, we show that histone deacetylase 4 (HDAC4) interacts with Runx2, binds the MMP-13 promoter, and suppresses MMP-13 gene transcription in the rat osteoblastic cell line, UMR 106-01. PTH induces the rapid cAMP-dependent protein kinase-dependent release of HDAC4 from the MMP-13 promoter and subsequent transcription of MMP-13. Knock-out of HDAC4 either by siRNA in vitro or by gene deletion in vivo leads to an increase in MMP-13 expression, and overexpression of HDAC4 decreases the PTH induction of MMP-13. All of these observations indicate that HDAC4 represses MMP-13 gene transcription in bone. Moreover, PTH stimulates HDAC4 gene expression and enzymatic activity at times corresponding to the reassociation of HDAC4 with the MMP-13 promoter and a decline in its transcription. Thus, HDAC4 is a basal repressor of MMP-13 transcription, and PTH regulates HDAC4 to control MMP-13 promoter activity. These data identify a novel and discrete mechanism of regulating HDAC4 levels and, subsequently, gene expression.

Agonist-regulated cleavage of the extracellular domain of parathyroid hormone receptor type 1

The receptor for parathyroid hormone (PTHR) is a main regulator of calcium homeostasis and bone maintenance. As a member of class B of G protein-coupled receptors, it harbors a large extracellular domain, which is required for ligand binding. Here, we demonstrate that the PTHR extracellular domain is cleaved by a protease belonging to the family of extracellular metalloproteinases. We show that the cleavage takes place in a region of the extracellular domain that belongs to an unstructured loop connecting the ligand-binding parts and that the N-terminal 10-kDa fragment is connected to the receptor core by a disulfide bond. Cleaved receptor revealed reduced protein stability compared with noncleaved receptor, suggesting degradation of the whole receptor. In the presence of the agonistic peptides PTH(1-34), PTH(1-14), or PTH(1-31), the processing of the PTHR extracellular domain was inhibited, and receptor protein levels were stabilized. A processed form of the PTHR was also detected in human kidney. These findings suggest a new model of PTHR processing and regulation of its stability.

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.

Regulation of bone development and extracellular matrix protein genes by RUNX2

RUNX2 is a multifunctional transcription factor that controls skeletal development by regulating the differentiation of chondrocytes and osteoblasts and the expression of many extracellular matrix protein genes during chondrocyte and osteoblast differentiation. This transcription factor plays a major role at the late stage of chondrocyte differentiation: it is required for chondrocyte maturation and regulates Col10a1 expression in hypertrophic chondrocytes and the expression of Spp1, Ibsp, and Mmp13 in terminal hypertrophic chondrocytes. It is essential for the commitment of pluripotent mesenchymal cells to the osteoblast lineage. During osteoblast differentiation, RUNX2 upregulates the expression of bone matrix protein genes including Col1a1, Spp1, Ibsp, Bglap, and Fn1 in vitro and activates many promoters including those of Col1a1, Col1a2, Spp1, Bglap, and Mmp13. However, overexpression of Runx2 inhibits osteoblast maturation and reduces Col1a1 and Bglap expression. The inhibition of RUNX2 in mature osteoblasts does not reduce the expression of Col1a1 and Bglap in mice. Thus, RUNX2 directs pluripotent mesenchymal cells to the osteoblast lineage, triggers the expression of major bone matrix protein genes, and keeps the osteoblasts in an immature stage, but does not play a major role in the maintenance of the expression of Col1a1 or Bglap in mature osteoblasts. During bone development, RUNX2 induces osteoblast differentiation and increases the number of immature osteoblasts, which form immature bone, whereas Runx2 expression has to be downregulated for differentiation into mature osteoblasts, which form mature bone. During dentinogenesis, Runx2 expression is downregulated, and RUNX2 inhibits the terminal differentiation of odontoblasts.

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.

Podoplanin is a novel fos target gene in skin carcinogenesis

Expression and function of the oncogenic transcription factor activator protein (AP-1; mainly composed of Jun and Fos proteins) is required for neoplastic transformation of keratinocytes in vitro and tumor promotion as well as malignant progression in vivo. Here, we describe the identification of 372 differentially expressed genes comparing skin tumor samples of K5-SOS-F transgenic mice (Fos(f/f) SOS(+)) with samples derived from animals with a specific deletion of c-Fos in keratinocytes (Fos(Deltaep) SOS(+)). Fos-dependent transcription of selected genes was confirmed by quantitative real-time PCR analysis using tumor samples and mouse back skin treated with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). One of the most differentially expressed genes encodes the small mucin-like glycoprotein Podoplanin (Pdpn), whose expression correlates with malignant progression in mouse tumor model systems and human cancer. We found Pdpn and Fos expression in chemically induced mouse skin tumors, and detailed analysis of the Pdpn gene promoter revealed impaired activity in Fos-deficient mouse embryonic fibroblasts, which could be restored by ectopic Fos expression. Direct Fos protein binding to the Pdpn promoter was shown by chromatin immunoprecipitation and a TPA-induced complex at a TPA-responsive element-like motif in the proximal promoter was identified by electrophoretic mobility shift assays. In summary, we could define a Fos-dependent genetic program in a well-established model of skin tumors. Systematic analysis of these novel target genes will guide us in elucidating the molecular mechanisms of AP-1-regulated pathways that are critically implicated in neoplastic transformation and/or malignant progression.

Matrix metalloproteinases and bone

Matrix metalloproteinases (MMPs) are members of a family of zinc-dependent proteolytic enzymes. Several of the MMPs are expressed at high levels in bone and cartilage in mammals including humans and mice and are capable of cleaving native, undenatured collagens with long uninterrupted triple helices; these MMPs therefore potentially function as collagenases in vivo. Several MMPs expressed in the skeleton appear to function in endochondral ossification during embryonic development and in modeling and remodeling of bone postnatally and later in life. Different functions of MMPs have been elucidated through observations of spontaneous mutations in MMP genes in humans and of targeted mutations in Mmp genes and collagen (substrate) genes in mice. Potential mechanisms to account for effects of these mutations are considered in this review.

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

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

Runx2- and histone deacetylase 3-mediated repression is relieved in differentiating human osteoblast cells to allow high bone sialoprotein expression

Bone sialoprotein (BSP) is a bone matrix glycoprotein whose expression coincides with terminal osteoblastic differentiation and the onset of mineralization. In this study we show that BSP expression is considerably increased in confluent Saos-2 human osteosarcoma cells and in differentiating normal human osteoblasts, concomitantly with the decrease of Runx2, a key transcription factor controlling bone formation. Therefore, we investigated the role of Runx2 in the regulation of BSP expression in Saos-2 cells. Using a mobility shift assay, we demonstrated that Runx2 binds to the BSP promoter only in preconfluent cells. Histone deacetylase 3 (HDAC3) has been recently shown to act as a Runx2 co-repressor. Chromatin immunoprecipitation assays demonstrated that both Runx2 and HDAC3 are detectable at the BSP promoter in preconfluent Saos-2 cells but not when they are confluent and overexpress BSP. Consistently, nuclear Runx2 protein level is down-regulated, whereas Saos-2 cells became increasingly confluent. Finally, the suppression of HDAC3, Runx2, or both by RNA interference induced the expression of BSP at both mRNA and protein levels in Saos-2 cells. Our data demonstrate that Runx2 and HDAC3 repress BSP gene expression and that this repression is suspended upon osteoblastic cell differentiation. Both the nuclear disappearance of Runx2 and the non-recruitment of HDAC3 represent new means to relieve Runx2-mediated suppression of BSP expression, thus allowing the acquisition of a fully differentiated and mineralization-competent phenotype by osteoblast cells.

[Use of bone marrow mesenchymal stem cells for ex vivo cartilage regeneration]

Articular cartilage disorders and injuries often result in lifelong chronic pain and compromised quality of life. When it comes to local articular cartilage defects, modern medicine is limited to short-term pain relief and inflammation control. In extreme cases the affected tissue is surgically removed and replaced by a synthetic prosthesis of limited durability. Cell-based therapies to regenerate articular cartilage have been in use since 1994. Such therapies provide a healthy population of cells to the injured site and require differentiated chondrocytes from the uninjured site as base material. Their usage often leads to donor site morbidity and they generate rigid fibrous cartilage where more flexible hyaline cartilage is required. The major restrictive factors for such methods are inadequate number and limited proliferation capacity of chondrocytes in vitro. Tissue engineering of adult marrow stromal cells/mesenchymal stem cells (MSCs) with their almost unlimited proliferation potential and proven capability to differentiate into chondrocytes for ex vivo generation of cartilage tissue still remains a vision. For optimal harnessing of MSCs as chondroprogenitor cells, basic background information regarding commitment to the lineage, cartilage differentiation and the regulatory factors and molecules involved is essential.

Soluble signalling factors derived from differentiated cartilage tissue affect chondrogenic differentiation of rat adult marrow stromal cells

BACKGROUND

Chondral defects show lack of proper regeneration whereas osteochondral lesions display limited regeneration capacity. Latter is probably due to immigration of chondroprogenitor cells from the subchondral bone. Known chondroprogenitor cells for cartilage tissues are multi-potent adult marrow stromal or mesenchymal stem cells (MSCs). In vitro chondrogenic differentiation of these precursor cells usually require cues from growth and signalling factors provided in vivo by surrounding tissues and cells. We hypothesise that signalling factors secreted by differentiated cartilage tissue can initiate and maintain chondrogenic differentiation status of MSCs.

METHODS

To study such paracrine communication between allogenic rat articular cartilage and rat MSCs embedded in alginate beads a novel coculture system without addition of external growth factors has been established.

RESULTS

Impact of cartilage on differentiating MSCs was observed at two different time points. Firstly, sustained expression of Sox9 was observed at an early stage which indicated induction of chondrogenic differentiation. Secondly, late stage repression of collagen X indicated pre-hypertrophic arrest of differentiation. In the culture supernatant we have identified vascular endothelial growth factor alpha (VEGF-164 alpha), matrix metalloproteinase (MMP) -13 and tissue inhibitors of MMPs (TIMP-1 and TIMP-2) which could be traced back either to the cartilage explant or to the MSCs under the influence of cartilage.

CONCLUSION

The identified factors might be involved in regulation of collagen X gene and protein expression and therefore, may have an impact on the control and regulation of MSCs differentiation.

CEBPbeta, JunD and c-Jun contribute to the transcriptional activation of the metastasis-associated C4.4A gene

The glycosylphosphatidylinositol-anchored molecule C4.4A, which shares structural features with uPAR, is frequently expressed on carcinomas with upregulated expression during tumor progression. Moreover, rare expression on nontransformed epithelial cells is strongly increased during tissue remodeling, e.g., during wound healing. This strictly regulated expression prompted us to define transcriptional activation of the C4.4A gene. C4.4A transcription was analyzed in 2 syngenic rat tumor cell lines with low or high metastatic potential, respectively. Though genomic C4.4A DNA was present in both lines, C4.4A mRNA and transcription of a reporter construct containing the C4.4A promoter was only observed in the metastasizing subline. Deletions and point mutations in the C4.4A promoter-driven reporter construct revealed that activation of the TATA-less, GC-rich core promoter (-1 to -50 bp) does not suffice to initiate transcription that requires coactivation of a proximal response element (-71 to -88 bp) and can be further increased by more distal response elements (-89 to -133 bp). Mobility-shift and cotransfection studies showed that Sp3 binding enhances C4.4A transcription, whereas potential Sp1 binding sites were ineffective. C4.4A transcription essentially requires C/EBPbeta binding to a TRE/CCAAT composite element (-71 to -88 bp) as measured by ChIP assay. C4.4A transcription is strikingly enhanced by cotransfection with JunD or c-Jun, such that C4.4A is most strongly transcribed even in the C4.4A-negative tumor cell line after cotransfection with C/EBPbeta plus JunD or c-Jun. Thus, upregulation of C/EBPbeta during tumor progression and wound repair may well provide a sufficient trigger for transcription of the C4.4A gene.

Expression of MMP-13 in osteoblast cells and rat tibia after exposure to gamma rays or accelerated carbon ions

In past research, we found that carbon ion irradiation increased bone volume in rats, and a significant amount of cartilage remained inside the carbon ion-irradiated trabeculae. The amounts of matrix metalloproteinase 13 (MMP-13) mRNA in osteoblast-like MC3T3-E1 cells tended to decrease after carbon ion irradiation. The level of MMP-13 mRNA in non-irradiated cells was stable during the experimental period, but in gamma ray-irradiated cells it tended to increase. When localization of MMP-13 in locally irradiated experimental rats was investigated, it was found in the marginal trabeculae in both non-irradiated and gamma ray-irradiated animals. MMP-13 was detected in osteoid and neogenetic bone in the trabeculae surface. The trabeculae in carbon ion-irradiated bone remained cartilaginous. Carbon ion-irradiated rats exhibited weak expression of MMP-13 around the cartilage inside the trabeculae. We conclude that carbon ion irradiation reduced expression of MMP-13, thus suppressing both chondrocyte maturation and cartilage resorption. Increases in hyperplasia of the bone trabeculae and of bone volume were caused by ongoing bone addition and calcification in the absence of sufficient cartilage resorption.

Cbfa-1 mediates nitric oxide regulation of MMP-13 in osteoblasts

During bone development, osteoblast differentiation requires remodeling of the extracellular matrix. Although underlying mechanisms have not been elucidated, evidence points to the participation of the nitric oxide (NO) and cyclic guanosine 3',5'-monophosphate (cGMP) system. Here, we detected increased matrix metalloproteinase (MMP)-13 mRNA, protein and activity, as well as increased inducible NO synthase (iNOS) and NO production during the differentiation of MC3T3-E1 osteoblasts. Transcriptional activity of the MMP-13 promoter was augmented by NO, 8-bromo-cGMP (8-Br-cGMP), and by a dominant-positive form of protein kinase G (PKG1-alpha). The stimulatory effect on the MMP-13 promoter was partially inhibited by mutation of the osteoblast-specific element 2 (OSE-2) binding site. Core binding factor-1 (Cbfa-1) expression peaked at 7 days of differentiation, and was phosphorylated by PKG in vitro. Cbfa-1 was localized to cell nuclei, and its translocation was inhibited by the iNOS inhibitor 1400W. Immunohistological examination revealed that MMP-13 and Cbfa-1 expression levels are both reduced in 17-day-old embryos of iNOS-deficient mice. Silencing of Cbfa-1 mRNA blocked MMP-13 expression without interfering with endogenous NO production, confirming its role in NO-induced MMP-13 expression by MC3T3-E1 cells. The results described here suggest a mechanism by which NO regulates osteogenesis.

Over-expression of TIMP-1 in osteoblasts increases the anabolic response to PTH

Intermittent PTH treatment induces structural changes that affect cancellous bone mass and have led to its indication for the treatment of osteoporosis. PTH is also known to upregulate the expression of matrix metalloproteinases (MMP) in osteoblasts. We wanted to find out whether inhibiting osteoblastic MMPs can affect the anabolic action of PTH in vivo. We had shown previously that mice over-expressing TIMP-1 (tissue inhibitor of MMPs) specifically in osteoblasts display an increase in bone mineral density and bone mass combined with an overall decrease in bone turnover. In the present study, 10-week-old wild-type (WT) and transgenic (TG) mice were treated with PTH at 40 microg/kg/day for 1.5 months. DEXA analysis was performed before and after treatment, and histomorphometric and molecular analysis were carried out at the end of the experiment. Our findings indicate that the transgene boosted the anabolic action of PTH. The femurs of PTH-treated TG mice displayed a greater increase in bone mineral density and trabecular bone volume than treated WT mice. Interestingly, the positive effect of the transgene on the action of PTH resulted from both reduced bone resorption activity and an increase in the bone formation rate. Osteoclastic surfaces that were increased in PTH-treated WT mice remained unchanged in TG mice, suggesting a decrease in osteoclastic differentiation. Histomorphometric data also indicate that PTH administration increased osteoblast activity in TG mice and affected the number of osteoblasts in WT mice. In conclusion, we demonstrate that inhibiting osteoblastic MMPs can potentiate the anabolic effect of PTH by decreasing osteoclast activity and increasing osteoblast activity. Our data also suggest that osteoblastic MMPs have some role in mediating the anabolic effects of PTH in vivo and indicate that inhibitors of MMPs could constitute a new therapy for degenerative diseases.

JunB is required for endothelial cell morphogenesis by regulating core-binding factor beta

The molecular mechanism triggering the organization of endothelial cells (ECs) in multicellular tubules is mechanistically still poorly understood. We demonstrate that cell-autonomous endothelial functions of the AP-1 subunit JunB are required for proper endothelial morphogenesis both in vivo in mouse embryos with endothelial-specific ablation of JunB and in in vitro angiogenesis models. By cDNA microarray analysis, we identified core-binding factor beta (CBFbeta), which together with the Runx proteins forms the heterodimeric core-binding transcription complex CBF, as a novel JunB target gene. In line with our findings, expression of the CBF target MMP-13 was impaired in JunB-deficient ECs. Reintroduction of CBFbeta into JunB-deficient ECs rescued the tube formation defect and MMP-13 expression, indicating an important role for CBFbeta in EC morphogenesis.

Role of Cbfa1/Runx2 in the fluid shear stress induction of COX-2 in osteoblasts

Induction of cyclooxygenase-2 (COX-2) is thought to be important for the anabolic effects of mechanical loading. The transcription factor Cbfa1/Runx2 is essential for osteoblastic differentiation. We examined the role of Cbfa1 in the fluid shear stress (FSS) induction of COX-2 in MC3T3-E1 cells stably transfected with a COX-2 promoter-luciferase reporter. Cells were subjected to FSS for 30 min and returned to static culture (post-FSS). COX-2 mRNA and promoter activity peaked 0.5-1h and 2-3h, respectively, post-FSS. Mutation of the Cbfa1 consensus sequence at -267/-261 bp decreased the FSS fold-induction of luciferase activity by 50%. On electrophoretic mobility shift assay (EMSA), proteins binding to an oligonucleotide spanning the Cbfa1 site were supershifted by specific antibody to Cbfa1. FSS did not increase Cbfa1 binding on EMSA or Cbfa1 mRNA or protein levels. These data suggest that transcriptional activity of Cbfa1, independent of its level of expression, is necessary for maximal FSS induction of COX-2 in osteoblasts.