Differential Effects of PTH (1-34), PTHrP (1-36), and Abaloparatide on the Murine Osteoblast Transcriptome

Teriparatide (PTH (1-34)), PTHrP (1-36), and abaloparatide (ABL) have been used for the treatment of osteoporosis, but their efficacy long term is significantly limited. The 3 peptides exert time- and dose-dependent differential responses in osteoblasts, leading us to hypothesize they may also differentially modulate the osteoblast transcriptome. Treatment of mouse calvarial osteoblasts with 1 nM of the peptides for 4 hours results in RNA sequencing data with PTH (1-34) regulating 367 genes, including 194 unique genes; PTHrP (1-36) regulating 117 genes, including 15 unique genes; and ABL regulating 179 genes, including 20 unique genes. There were 83 genes shared among all 3 peptides. Gene ontology analyses showed similarities in Wnt signaling, cAMP-mediated signaling, ossification, but differences in morphogenesis of a branching structure in biological processes; receptor ligand activity, transcription factor activity, and cytokine receptor/binding activity in molecular functions. The peptides increased Vdr, Cited1, and Pde10a messenger RNAs (mRNAs) in a pattern similar to Rankl, that is, PTH (1-34) greater than ABL greater than PTHrP (1-36). mRNA abundance of other genes, including Wnt4, Wnt7, Wnt11, Sfrp4, Dkk1, Kcnk10, Hdac4, Epn3, Tcf7, Crem, Fzd5, Ppp2r2a, and Dvl3, showed that some genes were regulated similarly by all 3 peptides; others were not. Finally, small interfering RNA knockdowns of SIK1/2/3 and CRTC1/2/3 in PTH (1-34)-treated cells revealed that Vdr and Wnt4 genes are regulated by salt-inducible kinases (SIKs) and CREB-regulated transcriptional coactivators (CRTCs), while others are not. Although many studies have examined PTH signaling in the osteoblast/osteocyte, ours is the first to compare the global effects of these peptides on the osteoblast transcriptome or to analyze the roles of the SIKs and CRTCs.

Differential effects of PTH (1-34), PTHrP (1-36) and abaloparatide on the murine osteoblast transcriptome

Teriparatide (PTH(1-34)) and its analogs, PTHrP(1-36) and abaloparatide (ABL) have been used for the treatment of osteoporosis, but their efficacy over long-term use is significantly limited. The 3 peptides exert time- and dose-dependent differential responses in osteoblasts, leading us to hypothesize that they may also differentially modulate the osteoblast transcriptome. We show that treatment of mouse calvarial osteoblasts with 1 nM of the 3 peptides for 4 h results in RNA-Seq data with PTH(1-34) regulating 367 genes, including 194 unique genes; PTHrP(1-36) regulating 117 genes, including 15 unique genes; and ABL regulating 179 genes, including 20 unique genes. There were 83 genes shared among all 3 peptides. Gene ontology analyses showed differences in Wnt signaling, cAMP-mediated signaling, bone mineralization, morphogenesis of a branching structure in biological processes; receptor ligand activity, transcription factor activity, cytokine receptor/binding activity and many other actions in molecular functions. The 3 peptides increased Vdr, Cited1 and Pde10a mRNAs in a pattern similar to Rankl , i.e., PTH(1-34) > ABL > PTHrP(1-36). mRNA abundance of other genes based on gene/pathway analyses, including Wnt4, Wnt7, Wnt11, Sfrp4, Dkk1, Kcnk10, Hdac4, Epha3, Tcf7, Crem, Fzd5, Pp2r2a , and Dvl3 showed that some genes were regulated similarly by all 3 peptides; others were not. Finally, siRNA knockdowns of SIK1/2/3 and CRTC1/2/3 in PTH(1-34)-treated cells revealed that Vdr and Wnt4 genes are regulated by SIKs and CRTCs, while others are not. Although many studies have examined PTH signaling in the osteoblast/osteocyte, ours is the first to examine the global effects of these peptides on the osteoblast transcriptome. Further delineation of which signaling events are attributable to PTH(1-34), PTHrP(1-36) or ABL exclusively and which are shared among all 3 will help improve our understanding of the effects these peptides have on the osteoblast and lead to the refinement of PTH-derived treatments for osteoporosis.

Differential effects of parathyroid hormone, parathyroid hormone-related protein, and abaloparatide on collagen 1 expression by mouse cementoblasts and mouse tooth root density

INTRODUCTION

Parathyroid hormone (PTH) plays an important role in maintaining mineral homeostasis by regulating calcium and phosphate levels. Clinical trials have shown that peptides of PTH (1-34), PTH-related protein (PTHrP 1-36), and the new peptide modeled on PTHrP, abaloparatide, can have different anabolic effects on osteoporotic subjects, but the underlying mechanisms are still unclear. The prevalence of moderate and major gingival recession has been shown to be higher in postmenopausal women with osteoporosis. In addition, there is a significant association between osteoporosis and tooth loss.

METHODS

We investigated the actions of these peptides on the cementoblasts and teeth of mice. The murine cementoblast line, OCCM-30, known to express collagen I (Col1a1), was treated with intermittent PTH (1-34), PTHrP (1-36), or abaloparatide for 6 h/d for 3 days. Microcomputed tomography was performed on the teeth of mice receiving daily injections of phosphate-buffered saline, PTH (1-34), or abaloparatide. Statistical differences were analyzed by a 2-way or 1-way analysis of variance followed by a Tukey's post-hoc test. Results are expressed as mean ± standard deviation, and P <0.05 was considered significant.

RESULTS

Gene expression showed regulation of Bsp, Col1a1, Opg, Rankl, and Mmp13 by the 3 peptides in these cells. Western blots revealed that after intermittent treatment for 3 days, PTH (1-34) caused an increase in COL1A1 protein immediately after treatment. In contrast, abaloparatide showed a latent effect in increasing COL1A1 protein 18 hours after treatment. PTHrP had no effect on COL1A1 expression. Immunofluorescence confirmed the same result as the Western blots. Microcomputed tomography of teeth showed PTH (1-34) injections increased molar root mineral density in mice, whereas abaloparatide increased density in roots of incisors and molars.

CONCLUSIONS

This study reveals the differential anabolic effects of intermittent PTH (1-34), PTHrP (1-36), and abaloparatide on cementoblasts, as revealed by COL1A1 expression and root mineral density. Abaloparatide may be a potential therapeutic approach for achieving improved cementogenesis.

Abaloparatide Has the Same Catabolic Effects on Bones of Mice When Infused as PTH (1-34)

Abaloparatide is a peptide analog of parathyroid hormone-related protein (PTHrP 1-34) and was approved in 2017 as the second osteoanabolic peptide for treating osteoporosis. We previously showed that intermittent abaloparatide is equally as effective as PTH (1-34). This study was designed to compare the catabolic effects of PTH (1-34) and abaloparatide on bone in young female wild-type mice. Two-month-old C57Bl/6J female mice were continuously infused with human PTH (1-34) or abaloparatide at 80 μg/kg BW/day or vehicle for 2 weeks. At euthanasia, DEXA-PIXImus was performed to assess bone mineral density (BMD) in the whole body, femurs, tibiae, and vertebrae. Bone turnover marker levels were measured in sera, femurs were harvested for micro-computer tomography (μCT) analyses and histomorphometry, and tibiae were separated into cortical and trabecular fractions for gene expression analyses. Our results demonstrated that the infusion of abaloparatide resulted in a similar decrease in BMD as infused PTH (1-34) at all sites. μCT and histomorphometry analyses showed similar decreases in cortical bone thickness and BMD associated with an increase in bone turnover from the increased bone formation rate found by in vivo double labeling and serum P1NP and increased bone resorption as shown by osteoclast numbers and serum cross-linked C-telopeptide. Trabecular bone did not show major changes with either treatment. Osteoblastic gene expression analyses of trabecular and cortical bone revealed that infusion of PTH (1-34) or abaloparatide led to similar and different actions in genes of osteoblast differentiation and activity. As with intermittent and in vitro treatment, both infused PTH (1-34) and abaloparatide similarly regulated downstream genes of the PTHR1/SIK/HDAC4 pathway such as Sost and Mmp13 but differed for those of the PTHR1/SIK/CRTC pathway. Taken together, at the same dose, infused abaloparatide causes the same high bone turnover as infused PTH (1-34) with a net resorption in female wild-type mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

OR21-4 Comparison of the Effects of PTH (1-34), PTHrP (1-36) and Abaloparatide (ABL) on the Murine Osteoblast Transcriptome

Osteoporosis is a prevalent disease with substantial morbidity/mortality among the aging population. Due to gaps in knowledge, current therapeutics are limited in their ability to prevent degeneration of bone while also stimulating formation of new bone. Teriparatide (PTH (1-34)) and its analogs PTHrP (1-36) and abaloparatide (ABL), have been utilized for treatment of osteoporosis but have significant limitations in efficacy over long-term use. Research from our laboratory has shown PTH (1-34), PTHrP (1-36), and ABL exert time and dose-dependent differential responses in the osteoblast, leading us to hypothesize they may also differentially modulate the osteoblast transcriptome. In this study we show that treatment of mouse calvarial osteoblasts with 1 nM of these peptides for 4 h results in differing effects on the osteoblast transcriptome by performing gene enrichment analysis of RNA-Seq data. Genes were selected with a Log2 fold change >1 and a false discovery rate <0.05. These data were analyzed and compiled into heat maps for each peptide and smear/volcano plots. RNA-Sequencing revealed that PTH (1-34) regulated 367 genes, 194 were unique; PTHrP (1-36) regulated 117 genes, 15 were unique; ABL regulated 179 genes, 20 were unique. There were 74 genes shared only among PTH(1-34) and ABL; 16 genes shared only among PTH (1-34) and PTHrP; and 83 genes shared among all three peptides. Data collected show pathway-specific differences including, 1) cAMP/PKA, 2) Wnt/β-catenin, 3) Transcriptional regulation 4) Inflammatory response 5) Transmembrane transport, 6) Metabolism, and 7) NF-kB. Further analysis of the data illuminated that the three peptides increased Vitamin D receptor (VDR) and Cbp/p300-interacting transactivator 1 (CITED1) mRNAs similar to RankL expression. These findings were confirmed via qRT-PCR of additional cultured samples of mouse calvarial osteoblasts, treated with 1 nM of PTH (1-34), PTHrP (1-36), and ABL for 4 h prior to harvest. RankL mRNA, VDR mRNA, and CITED1 mRNA were measured for each sample and statistical differences were analyzed via Kruskal Wallis p<0.05. Additionally, we analyzed mRNA levels of several genes of interest, including WNT7b, WNT11, TCF7, SFRP4, FZD5, PP2R2A, and DVL3 mRNA. Pathway analysis and subsequent qRT-PCR confirmation has shown that PTH (1-34) and ABL lead to a significant increase in Wnt11 mRNA, while PTHrP (1-36) does not. Our findings highlight the complexity of the genetic and functional events triggered by PTH (1-34) and its analogs. Many studies have examined PTH signaling in the osteoblast/osteocyte; ours is the first to examine global effects of these peptides on the osteoblast transcriptome. Further delineation of which signaling events are attributable to PTH (1-34), PTHrP (1-36), and ABL exclusively and which are shared among all three will further our understanding of the effects these peptides have on the osteoblast and lead to refinement of PTH-derived treatments for osteoporosis.

Presentation: Monday, June 13, 2022 11:45 a.m. - 12:00 p.m.

Bioactive, full-length parathyroid hormone delivered using an adeno-associated viral vector

Delivering the parathyroid hormone (PTH) gene has been attempted preclinically in a handful of studies, but delivering full-length PTH (1-84) using adeno-associated viral (AAV) vectors has not. Given the difficulty in achieving therapeutic levels of secreted proteins using gene therapy, this study seeks to determine the feasibility of doing so with PTH. An AAV vector was used to deliver human PTH driven by a strong promoter. We demonstrate the ability to secrete full-length PTH from various cell types in vitro. PTH secretion from hepatocytes was measured over time and a fluorescent marker was used to compare the secretion rate of PTH in various cell types. Potency was measured by the ability of PTH to act on the PTH receptors of osteosarcoma cells and induced proliferation. PTH showed potency in vitro by inducing proliferation in two osteosarcoma cell lines. In vivo, AAV was administered systemically in immunocompromised mice which received xenografts of osteosarcoma cells. Animals that received the highest dose of AAV-PTH had higher liver and plasma concentrations of PTH. All dosing groups achieved measurable plasma concentrations of human PTH that were above the normal range. The high-dose group also had significantly larger tumors compared to control groups on the final day of the study. The tumors also showed dose-dependent differences in morphology. When looking at endocrine signaling and endogenous bone turnover, we observed a significant difference in tibial growth plate width in animals that received the high-dose AAV as well as dose-dependent changes in blood biomarkers related to PTH. This proof-of-concept study shows promise for further exploration of an AAV gene therapy to deliver full-length PTH for hypoparathyroidism. Additional investigation will determine efficacy in a disease model, but data shown establish bioactivity in well-established models of osteosarcoma.

Ca2+-activated chloride channel ANO1: A new regulator of osteoclast function

A paper by Sun et al. identified the Ca2+-activated Cl- channel anoctamin 1 or ANO1 (TMEM16A) as an important regulator of osteoclast function by interacting with RANKL activating signaling pathways involved in bone resorption. Although Cl- transporters (e.g. ClC7, CLIC5) have been known to be involved in the active process of bone resorption, ANO1 appears to control osteoclast differentiation and function to levels beyond those of other Cl-transporters. Regulating ANO1 function might be a useful target for therapeutics in osteoporosis.

The Critical Role of MMP13 in Regulating Tooth Development and Reactionary Dentinogenesis Repair Through the Wnt Signaling Pathway

Matrix-metalloproteinase-13 (MMP13) is important for bone formation and remodeling; however, its role in tooth development remains unknown. To investigate this, MMP13-knockout (Mmp13^−/−) mice were used to analyze phenotypic changes in the dentin–pulp complex, mineralization-associated marker-expression, and mechanistic interactions. Immunohistochemistry demonstrated high MMP13-expression in pulp-tissue, ameloblasts, odontoblasts, and dentin in developing WT-molars, which reduced in adults, with human-DPC cultures demonstrating a >2000-fold increase in Mmp13-expression during mineralization. Morphologically, Mmp13^−/− molars displayed critical alterations in the dentin-phenotype, affecting dentin-tubule regularity, the odontoblast-palisade and predentin-definition with significantly reduced dentin volume (∼30% incisor; 13% molar), and enamel and dentin mineral-density. Reactionary-tertiary-dentin in response to injury was reduced at Mmp13^−/− molar cusp-tips but with significantly more dystrophic pulpal mineralization in MMP13-null samples. Odontoblast differentiation-markers, nestin and DSP, reduced in expression after MMP13-loss in vivo, with reduced calcium deposition in MMP13-null DPC cultures. RNA-sequencing analysis of WT and Mmp13^−/− pulp highlighted 5,020 transcripts to have significantly >2.0-fold change, with pathway-analysis indicating downregulation of the Wnt-signaling pathway, supported by reduced in vivo expression of the Wnt-responsive gene Axin2. Mmp13 interaction with Axin2 could be partly responsible for the loss of odontoblastic activity and alteration to the tooth phenotype and volume which is evident in this study. Overall, our novel findings indicate MMP13 as critical for tooth development and mineralization processes, highlighting mechanistic interaction with the Wnt-signaling pathway.

Regulation of transforming growth factor-β1-stimulation of Runx2 acetylation for matrix metalloproteinase 13 expression in osteoblastic cells

Transforming growth factor beta 1 (TGF-β1) functions as a coupling factor between bone development and resorption. Matrix metalloproteinase 13 (MMP13) is important in bone remodeling, and skeletal dysplasia is caused by a deficiency in MMP13 expre-ssion. Runx2, a transcription factor is essential for bone development, and MMP13 is one of its target genes. TGF-β1 promoted Runx2 phosphorylation, which was necessary for MMP13 production in osteoblastic cells, as we previously shown. Since the phosphorylation of some proteins causes them to be degraded by the ubiquitin/proteasome pathway, we hypothesized that TGF-β1 might stabilize the phosphorylated Runx2 protein for its activity by other post-translational modification (PTM). This study demonstrated that TGF-β1-stimulated Runx2 acetylation in rat osteoblastic cells. p300, a histone acetyltransferase interacted with Runx2, and it promoted Runx2 acetylation upon TGF-β1-treatment in these cells. Knockdown of p300 decreased the TGF-β1-stimulated Runx2 acetylation and MMP13 expression in rat osteoblastic cells. TGF-β1-treatment stimulated the acetylated Runx2 bound at the MMP13 promoter, and knockdown of p300 reduced this effect in these cells. Overall, our studies identified the transcriptional regulation of MMP13 by TGF-β1 via Runx2 acetylation in rat osteoblastic cells, and these findings contribute to the knowledge of events presiding bone metabolism.

Osteoblastic monocyte chemoattractant protein-1 (MCP-1) mediation of parathyroid hormone's anabolic actions in bone implicates TGF-β signaling

Parathyroid hormone (PTH) is necessary for the regulation of calcium homeostasis and PTH (1-34) was the first approved osteoanabolic therapy for osteoporosis. It is well established that intermittent PTH increases bone formation and that bone remodeling and several cytokines and chemokines play an essential role in this process. Earlier, we had established that the chemokine, monocyte chemoattractant protein-1 (MCP-1/CCL2), was the most highly stimulated gene in rat bone after intermittent PTH injections. Nevertheless, MCP-1 function in bone appears to be complicated. To identify the primary cells expressing MCP-1 in response to PTH, we performed in situ hybridization of rat bone sections after hPTH (1-34) injections and showed that bone-lining osteoblasts are the primary cells that express MCP-1 after PTH treatment. We previously demonstrated MCP-1's importance by showing that PTH's anabolic effects are abolished in MCP-1 null mice, further implicating a role for the chemokine in this process. To establish whether rhMCP-1 peptide treatment could rescue the anabolic effect of PTH in MCP-1 null mice, we treated 4-month-old wild-type (WT) mice with hPTH (1-34) and MCP-1^-/- mice with rhMCP-1 and/or hPTH (1-34) for 6 weeks. Micro-computed tomography (μCT) analysis of trabecular and cortical bone showed that MCP-1 injections for 6 weeks rescued the PTH anabolic effect in MCP-1^-/- mice. In fact, the combination of rhMCP-1 and hPTH (1-34) has a synergistic anabolic effect compared with monotherapies. Mechanistically, PTH-enhanced transforming growth factor-β (TGF-β) signaling is abolished in the absence of MCP-1, while MCP-1 peptide treatment restores TGF-β signaling in the bone marrow. Here, we have shown that PTH regulates the transcription of the chemokine MCP-1 in osteoblasts and determined how MCP-1 affects bone cell function in PTH's anabolic actions. Taken together, our current work indicates that intermittent PTH stimulates osteoblastic secretion of MCP-1, which leads to increased TGF-β signaling, implicating it in PTH's anabolic actions.

Abaloparatide at the Same Dose Has the Same Effects on Bone as PTH (1-34) in Mice

Abaloparatide, a novel analog of parathyroid hormone-related protein (PTHrP 1-34), became in 2017 the second osteoanabolic therapy for the treatment of osteoporosis. This study aims to compare the effects of PTH (1-34), PTHrP (1-36), and abaloparatide on bone remodeling in male mice. Intermittent daily subcutaneous injections of 80 μg/kg/d were administered to 4-month-old C57Bl/6J male mice for 6 weeks. During treatment, mice were followed by DXA-Piximus to assess changes in bone mineral density (BMD) in the whole body, femur, and tibia. At either 4 or 18 hours after the final injection, femurs were harvested for μCT analyses and histomorphometry, sera were assayed for bone turnover marker levels, and tibias were separated into cortical, trabecular, and bone marrow fractions for gene expression analyses. Our results showed that, compared with PTH (1-34), abaloparatide resulted in a similar increase in BMD at all sites, whereas no changes were found with PTHrP (1-36). With both PTH (1-34) and abaloparatide, μCT and histomorphometry analyses revealed similar increases in bone volume associated with an increased trabecular thickness, in bone formation rate as shown by P1NP serum level and in vivo double labeling, and in bone resorption as shown by CTX levels and osteoclast number. Gene expression analyses of trabecular and cortical bone showed that PTH (1-34) and abaloparatide led to different actions in osteoblast differentiation and activity, with increased Runx2, Col1A1, Alpl, Bsp, Ocn, Sost, Rankl/Opg, and c-fos at different time points. Abaloparatide seems to generate a faster response on osteoblastic gene expression than PTH (1-34). Taken together, abaloparatide at the same dose is as effective as PTH (1-34) as an osteoanabolic, with an increase in bone formation but also an increase in bone resorption in male mice. © 2019 American Society for Bone and Mineral Research.

Parathyroid hormone-stimulation of Runx2 during osteoblast differentiation via the regulation of lnc-SUPT3H-1:16 (RUNX2-AS1:32) and miR-6797-5p

Parathyroid hormone (PTH) acts as a regulator of calcium homeostasis and bone remodeling. Runx2, an essential transcription factor in bone, is required for osteoblast differentiation. Noncoding RNAs such as long noncoding RNAs (lncRNAs) and microRNAs (miRNAs) play crucial roles in regulating gene expression in osteoblasts. In this study, we investigated the effects of PTH on osteoblast differentiation via Runx2, lncRNA, and miRNA expression in human bone marrow stromal cells (hBMSCs) and human osteoblastic cells (MG63). PTH-treatment of hBMSCs for 24 h, 7 days, and 14 days stimulated Runx2 mRNA expression. Using bioinformatics tools, we identified 17 lncRNAs originating from human Runx2 gene. Among these, lnc-SUPT3H-1:16 (RUNX2-AS1:32) expression was highly up-regulated by the 7 d PTH-treatment in hBMSCs. We also identified miR-6797-5p as the putative target of lnc-SUPT3H-1:16 and Runx2 using bioinformatics tools. PTH-treatment increased the expression of miR-6797-5p in hBMSCs, and overexpression of miR-6797-5p decreased osteoblast differentiation in MG63 cells, suggesting a role for lnc-SUPT3H-1:16 as sponge molecule. A luciferase gene reporter assay identified direct targeting of miR-6797-5p with lnc-SUPT3H-1:16 and 3'UTR Runx2 in MG63 cells. Thus, PTH stimulated the expression of lnc-SUPT3H-1:16, miR-6797-5p and Runx2, and due to the sponging mechanism of lnc- SUPT3H-1:16 towards miR-6797-5p, Runx2 was protected, resulting in the promotion of osteoblast differentiation.

Parathyroid hormone(1-34) and its analogs differentially modulate osteoblastic Rankl expression via PKA/SIK2/SIK3 and PP1/PP2A-CRTC3 signaling

Osteoporosis can result from the loss of sex hormones and/or aging. Abaloparatide (ABL), an analog of parathyroid hormone-related protein (PTHrP(1-36)), is the second osteoanabolic therapy approved by the United States Food and Drug Administration after teriparatide (PTH(1-34)). All three peptides bind PTH/PTHrP receptor type 1 (PTHR1), but the effects of PTHrP(1-36) or ABL in the osteoblast remain unclear. We show that, in primary calvarial osteoblasts, PTH(1-34) promotes a more robust cAMP response than PTHrP(1-36) and ABL and causes a greater activation of protein kinase A (PKA) and cAMP response element-binding protein (CREB). All three peptides similarly inhibited sclerostin (Sost). Interestingly, the three peptides differentially modulated two other PKA target genes, c-Fos and receptor activator of NF-κB ligand (Rankl), and the latter both in vitro and in vivo Knockdown of salt-inducible kinases (SIKs) 2 and 3 and CREB-regulated transcription coactivator 3 (CRTC3), indicated that all three are part of the pathway that regulates osteoblastic Rankl expression. We also show that the peptides differentially regulate the nuclear localization of CRTC2 and CRTC3, and that this correlates with PKA activation. Moreover, inhibition of protein phosphatases 1 and 2A (PP1/PP2A) activity revealed that they play a major role in both PTH-induced Rankl expression and the effects of PTH(1-34) on CRTC3 localization. In summary, in the osteoblast, the effects of PTH(1-34), PTHrP(1-36), and ABL on Rankl are mediated by differential stimulation of cAMP/PKA signaling and by their downstream effects on SIK2 and -3, PP1/PP2A, and CRTC3.

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.

Pulsed electromagnetic fields inhibit human osteoclast formation and gene expression via osteoblasts

Pulsed electromagnetic fields (PEMFs) can be effective in promoting the healing of delayed union or nonunion fractures. We previously reported that PEMF (Spinal-Stim® by Orthofix, Inc., Lewisville, TX) stimulated proliferation, differentiation and mineralization of rat calvarial osteoblastic cells in culture. In the present work we investigated the effects of PEMF (Physio-Stim® by Orthofix, Inc., Lewisville, TX) on human bone marrow macrophages (hBMMs) differentiated to osteoclasts. PEMF had striking inhibitory effects on formation of osteoclasts from hBMMs from both younger and older women. There were significantly greater changes in gene expression as ascertained by RNAseq from cells from older women. Interestingly, all of the genes identified by RNAseq were upregulated, and all were genes of mesenchymal or osteoblastic cells and included members of the TGF-β signaling pathway and many extracellular matrix proteins, as well as RANKL and osteoprotegerin, indicating the mixed nature of these cultures. From these results, we suggest that PEMF can inhibit osteoclast formation via action on osteoblasts. Thus, PEMF may be very effective for bone mass maintenance in subjects with osteoporosis.

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.

The Histone-Deacetylase-Inhibitor Suberoylanilide Hydroxamic Acid Promotes Dental Pulp Repair Mechanisms Through Modulation of Matrix Metalloproteinase-13 Activity

Direct application of histone-deacetylase-inhibitors (HDACis) to dental pulp cells (DPCs) induces chromatin changes, promoting gene expression and cellular-reparative events. We have previously demonstrated that HDACis (valproic acid, trichostatin A) increase mineralization in dental papillae-derived cell-lines and primary DPCs by stimulation of dentinogenic gene expression. Here, we investigated novel genes regulated by the HDACi, suberoylanilide hydroxamic acid (SAHA), to identify new pathways contributing to DPC differentiation. SAHA significantly compromised DPC viability only at relatively high concentrations (5 μM); while low concentrations (1 μM) SAHA did not increase apoptosis. HDACi-exposure for 24 h induced mineralization-per-cell dose-dependently after 2 weeks; however, constant 14d SAHA-exposure inhibited mineralization. Microarray analysis (24 h and 14 days) of SAHA exposed cultures highlighted that 764 transcripts showed a significant >2.0-fold change at 24 h, which reduced to 36 genes at 14 days. 59% of genes were down-regulated at 24 h and 36% at 14 days, respectively. Pathway analysis indicated SAHA increased expression of members of the matrix metalloproteinase (MMP) family. Furthermore, SAHA-supplementation increased MMP-13 protein expression (7 d, 14 days) and enzyme activity (48 h, 14 days). Selective MMP-13-inhibition (MMP-13i) dose-dependently accelerated mineralization in both SAHA-treated and non-treated cultures. MMP-13i-supplementation promoted expression of several mineralization-associated markers, however, HDACi-induced cell migration and wound healing were impaired. Data demonstrate that short-term low-dose SAHA-exposure promotes mineralization in DPCs by modulating gene pathways and tissue proteases. MMP-13i further increased mineralization-associated events, but decreased HDACi cell migration indicating a specific role for MMP-13 in pulpal repair processes. Pharmacological inhibition of HDAC and MMP may provide novel insights into pulpal repair processes with significant translational benefit. J. Cell. Physiol. 231: 798-816, 2016. © 2015 Wiley Periodicals, Inc.

Physiological Bone Remodeling: Systemic Regulation and Growth Factor Involvement

Bone remodeling is essential for adult bone homeostasis. It comprises two phases: bone formation and resorption. The balance between the two phases is crucial for sustaining bone mass and systemic mineral homeostasis. This review highlights recent work on physiological bone remodeling and discusses our knowledge of how systemic and growth factors regulate this process.

CCL2/Monocyte Chemoattractant Protein 1 and Parathyroid Hormone Action on Bone

Chemokines are small molecules that play a crucial role as chemoattractants for several cell types, and their components are associated with host immune responses and repair mechanisms. Chemokines selectively recruit monocytes, neutrophils, and lymphocytes and induce chemotaxis through the activation of G protein-coupled receptors. Two well-described chemokine families (CXC and CC) are known to regulate the localization and trafficking of immune cells in cases of injury, infection, and tumors. Monocyte chemoattractant protein 1 (MCP-1/CCL2) is one of the important chemokines from the CC family that controls migration and infiltration of monocytes/macrophages during inflammation. CCL2 is profoundly expressed in osteoporotic bone and prostate cancer-induced bone resorption. CCL2 also regulates physiological bone remodeling in response to hormonal and mechanical stimuli. Parathyroid hormone (PTH) has multifaceted effects on bone, depending on the mode of administration. Intermittent PTH increases bone in vivo by increasing the number and activity of osteoblasts, whereas a continuous infusion of PTH decreases bone mass by stimulating a net increase in bone resorption. CCL2 is essential for both anabolic and catabolic effects of PTH. In this review, we will discuss the pharmacological role of PTH and involvement of CCL2 in the processes of PTH-mediated bone remodeling.

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.

DMP-1-mediated Ghr gene recombination compromises skeletal development and impairs skeletal response to intermittent PTH

Bone minerals are acquired during growth and are key determinants of adult skeletal health. During puberty, the serum levels of growth hormone (GH) and its downstream effector IGF-1 increase and play critical roles in bone acquisition. The goal of the current study was to determine how bone cells integrate signals from the GH/IGF-1 to enhance skeletal mineralization and strength during pubertal growth. Osteocytes, the most abundant bone cells, were shown to orchestrate bone modeling during growth. We used dentin matrix protein (Dmp)-1-mediated Ghr knockout (DMP-GHRKO) mice to address the role of the GH/IGF axis in osteocytes. We found that DMP-GHRKO did not affect linear growth but compromised overall bone accrual. DMP-GHRKO mice exhibited reduced serum inorganic phosphate and parathyroid hormone (PTH) levels and decreased bone formation indices and were associated with an impaired response to intermittent PTH treatment. Using an osteocyte-like cell line along with in vivo studies, we found that PTH sensitized the response of bone to GH by increasing Janus kinase-2 and IGF-1R protein levels. We concluded that endogenously secreted PTH and GHR signaling in bone are necessary to establish radial bone growth and optimize mineral acquisition during growth.

Sirtuin 1 is a negative regulator of parathyroid hormone stimulation of matrix metalloproteinase 13 expression in osteoblastic cells: role of sirtuin 1 in the action of PTH on osteoblasts

Parathyroid hormone (PTH) is the only current anabolic treatment for osteoporosis in the United States. PTH stimulates expression of matrix metalloproteinase 13 (MMP13) in bone. Sirtuin 1 (SIRT1), an NAD-dependent deacetylase, participates in a variety of human diseases. Here we identify a role for SIRT1 in the action of PTH in osteoblasts. We observed increased Mmp13 mRNA expression and protein levels in bone from Sirt1 knock-out mice compared with wild type mice. PTH-induced Mmp13 expression was significantly blocked by the SIRT1 activator, resveratrol, in osteoblastic UMR 106-01 cells. In contrast, the SIRT1 inhibitor, EX527, significantly enhanced PTH-induced Mmp13 expression. Two h of PTH treatment augmented SIRT1 association with c-Jun, a component of the transcription factor complex, activator protein 1 (AP-1), and promoted SIRT1 association with the AP-1 site of the Mmp13 promoter. This binding was further increased by resveratrol, implicating SIRT1 as a feedback inhibitor regulating Mmp13 transcription. The AP-1 site of the Mmp13 promoter is required for PTH stimulation of Mmp13 transcriptional activity. When the AP-1 site was mutated, EX527 was unable to increase PTH-stimulated Mmp13 promoter activity, indicating a role for the AP-1 site in SIRT1 inhibition. We further showed that SIRT1 deacetylates c-Jun and that the cAMP pathway participates in this deacetylation process. These data indicate that SIRT1 is a negative regulator of MMP13 expression, SIRT1 activation inhibits PTH stimulation of Mmp13 expression, and this regulation is mediated by SIRT1 association with c-Jun at the AP-1 site of the Mmp13 promoter.

Runx2, a target gene for activating transcription factor-3 in human breast cancer cells

Activating transcription factor (ATF-3) is a stress response gene and is induced by transforming growth factor beta 1 (TGF-β1) in breast cancer cells. In this study, we dissected the functional role of ATF-3 gene in vitro by knocking down its expression stably in human bone metastatic breast cancer cells (MDA-MB231). Knockdown of ATF-3 expression in these cells decreased cell number, altered cell cycle phase transition, and decreased mRNA expression of cell cycle genes. Knockdown of ATF-3 expression in MDA-MB231 cells also decreased cell migration, and the expression levels of invasive and metastatic genes such as MMP-13 and Runx2 were found to be decreased in these cells. Most importantly, ATF-3 was associated with Runx2 promoter in MDA-MB231 cells and knockdown of ATF-3 expression decreased its association with Runx2 promoter. Hence, our results suggested that ATF-3 plays a role in proliferation and invasion of bone metastatic breast cancer cells in vitro and we identified for the first time that Runx2 is a target gene of ATF-3 in MDA-MB231 cell line.

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.

Parathyroid hormone regulates histone deacetylase (HDAC) 4 through protein kinase A-mediated phosphorylation and dephosphorylation in osteoblastic cells

Histone deacetylases (HDACs) are crucial regulators of gene expression in transcriptional co-repressor complexes. Previously, we reported that HDAC4 was a basal repressor of matrix metalloproteinase-13 (MMP-13) transcription and parathyroid hormone (PTH) regulates HDAC4 to control MMP-13 promoter activity through dissociation from Runx2. Here, we show that PTH induces the protein kinase A (PKA)-dependent phosphorylation of HDAC4 in the nucleus of the rat osteoblastic cell line, UMR 106-01. We demonstrate that PKA-dependent phosphorylated HDAC4 is released from Runx2 bound to the MMP-13 promoter in these cells. Point mutation of Ser-740 in rHDAC4 prevents the release of HDAC4 from Runx2 on the MMP-13 promoter and also prevents the PTH stimulation of MMP-13 transcription. Thus, PTH-induced phosphorylation of rHDAC4 at Ser-740 is crucial for regulating MMP-13 transcription in osteoblasts. PTH causes degradation of HDAC4, and this product appears in the cytoplasm. The cytoplasmic degradation of HDAC4 is blocked by PKA and lysosomal inhibitors, but is not affected by proteasome, caspase-3, or serine and aspartic protease inhibitors. In addition, the phosphatase inhibitor, okadaic acid, prevents degradation indicating that dephosphorylation is associated with degradation. These mechanisms regulating HDAC4 and their roles in such processes are crucial for bone and chondrocyte development. Our data support a link between PTH regulating HDAC4 phosphorylation by PKA, trafficking, partial degradation, and the control of MMP-13 transcription through association with Runx2.

Monocyte chemoattractant protein-1 is a mediator of the anabolic action of parathyroid hormone on bone

Parathyroid hormone (PTH) has a significant role as an anabolic hormone in bone when administered by intermittent injection. Previous microarray studies in our laboratory have shown that the most highly regulated gene, monocyte chemoattractant protein-1 (MCP-1), is rapidly and transiently induced when hPTH(1-34) is injected intermittently in rats. Through further in vivo studies, we found that rats treated with hPTH(1-34) showed a significant increase in serum MCP-1 levels 2 hours after PTH injection compared with basal levels. Using immunohistochemistry, increased MCP-1 expression in osteoblasts and osteocytes is evident after PTH treatment. PTH also increased the number of marrow macrophages. MCP-1 knockout mice injected daily with hPTH(1-34) showed less trabecular bone mineral density and bone volume compared with wild-type mice as measured by peripheral quantitative computed tomography (pQCT) and micro-computed tomography (µCT). Histomorphometric analysis revealed that the increase in osteoclast surface and osteoclast number observed with intermittent PTH treatment in the wild-type mice was completely eliminated in the MCP-1 null mice, as well as much lower numbers of macrophages. Consequently, the lack of osteoclast and macrophage activity in the MCP-1 null mice was paralleled by a reduction in bone formation. We conclude that osteoblast and osteocyte MCP-1 expression is an important mediator for the anabolic effects of PTH on bone.

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.

Triclosan blocks MMP-13 expression in hormone-stimulated osteoblasts

BACKGROUND

Matrix metalloproteinase-13 (MMP-13) is an important enzyme for the modulation of bone turnover and gingival recession. Elevated levels of MMP-13 are associated with alveolar bone resorption, periodontal ligament breakdown, and gingival attachment loss, which are the clinical symptoms of periodontal disease. Evidence continues to suggest that periodontal disease contributes to oral tissue breakdown and is linked to numerous systemic conditions. Triclosan (TCN) is a long-standing, proven antibacterial and anti-inflammatory agent found in the only Food and Drug Administration-approved dentifrice for the treatment of plaque and gingivitis.

METHODS

This study examines the inhibitory effects of TCN on lipopolysaccharide-, parathyroid hormone (PTH)-, and prostaglandin E2 (PGE2)-induced expression of MMP-13 in UMR 106-01 cells, an osteoblastic osteosarcoma cell line. The cells were stimulated with PTH or PGE2 to induce MMP-13 mRNA expression, and real-time reverse transcription-polymerase chain reaction was performed to determine gene expression levels. Western blot analysis assessed the presence or absence of protein degradation or inhibition of protein synthesis. MMP-13 promoter reporter assay was used to explore possible direct effects of TCN on the MMP-13 promoter.

RESULTS

TCN significantly reduced PTH or PGE2 elevated expression of MMP-13 in osteoblastic cells without affecting basal levels of the mRNA. Surprisingly, TCN enhanced the expression of c-fos and amphiregulin mRNA. A promoter assay indicated that TCN directly inhibits the activation of the PTH-responsive minimal promoter of MMP-13.

CONCLUSION

The present study appears to have identified a nuclear mechanism of action of TCN that accounts for the ability of TCN to inhibit PTH- or PGE2-induced MMP-13 expression in osteoblastic cells.

Alendronate affects osteoblast functions by crosstalk through EphrinB1-EphB

Bisphosphonates are therapeutic agents in the treatment of post-menopausal osteoporosis. Although they have been associated with delayed healing in injured tissues, inappropriate femoral fractures, and osteonecrosis of the jaw (ONJ), the pathophysiological mechanisms involved are not clear. Our hypothesis is that alendronate, a member of the N-containing bisphosphonates, indirectly inhibits osteoblast function through the coupling of osteoclasts to osteoblasts by ephrinB-EphB interaction. We found that alendronate increased gene and protein expression of ephrinB1 and EphB1, as well as B3, in femurs of adult mice injected with alendronate (10 µg/100 g/wk) for 8 weeks. Alendronate suppressed the expression of bone sialoprotein (BSP) and osteonectin in both femurs and bone marrow osteoblastic cells of mice. After elimination of pre-osteoclasts from bone marrow cells, alendronate did not affect osteoblast differentiation, indicating the need for pre-osteoclasts for alendronate's effects. Alendronate stimulated EphB1 and EphB3 protein expression in osteoblasts, whereas it enhanced ephrinB1 protein in pre-osteoclasts. In addition, a reverse signal by ephrinB1 inhibited osteoblast differentiation and suppressed BSP gene expression. Thus, alendronate, through its direct effects on the pre-osteoclast, appears to regulate expression of ephrinB1, which regulates and acts through the EphB1, B3 receptors on the osteoblast to suppress osteoblast differentiation.

Bio-composite scaffolds containing chitosan/nano-hydroxyapatite/nano-copper-zinc for bone tissue engineering

The current study involves fabrication and characterization of bio-composite scaffolds containing chitosan (CS), nano-hydroxyapatite (nHAp) and Cu-Zn alloy nanoparticles (nCu-Zn) by freeze drying technique. The fabricated composite scaffolds (CS/nHAp and CS/nHAp/nCu-Zn) were characterized by SEM, EDX, XRD and FT-IR studies. The addition of nCu-Zn in the CS/nHAp scaffolds significantly increased swelling, decreased degradation, increased protein adsorption, and increased antibacterial activity. The CS/nHAp/nCu-Zn scaffolds had no toxicity towards rat osteoprogenitor cells. So the developed CS/nHAp/nCu-Zn scaffolds have advantageous and potential applications over the CS-nHAp scaffolds for bone tissue engineering.

EGFR signaling suppresses osteoblast differentiation and inhibits expression of master osteoblastic transcription factors Runx2 and Osterix

The epidermal growth factor receptor (EGFR) and its ligands regulate key processes of cell biology, such as proliferation, survival, differentiation, migration, and tumorigenesis. We previously showed that, EGFR signaling pathway is an important bone regulator and it primarily plays an anabolic role in bone metabolism. In this study, we demonstrated that EGF-like ligands strongly inhibited osteoblast differentiation and mineralization in several lines of osteoblastic cells. Real-time RT-PCR and promoter reporter assays revealed that EGF-like ligands suppressed the expression of both early and late bone marker genes at the transcriptional level in the differentiating osteoblasts via an EGFR-dependent manner. This inhibitory effect of EGFR signaling was not dependent on its mitogenic activity. Furthermore, we demonstrated that EGFR signaling reduced the expression of two major osteoblastic transcription factors Runx2 (type II) and Osterix in osteoblast differentiating cells. EGFR-induced decrease in Runx2 transcriptional activity was confirmed by Runx2 reporter and chromatin immunoprecipitation assays. EGFR signaling increased the protein amounts of transcription co-repressors HDAC4 and 6 and over-expression of HDAC4 decreased Runx2 amount in differentiating osteoblasts, implying that HDACs contribute to the down-regulation of Runx2 by EGFR. Moreover, activation of EGFR in undifferentiated osteoprogenitors attenuated the expression of early bone markers and Osterix and decreased Runx2 protein amounts. Together with our previous data, that EGFR stimulates osteoprogenitor proliferation and that blocking EGFR activity in osteoblast lineage cells results in fewer osteoprogenitors and an osteopenic phenotype, we conclude that EGFR signaling is important for maintaining osteoprogenitor population at an undifferentiated stage.

The high serum monocyte chemoattractant protein-1 in obesity is influenced by high parathyroid hormone and not adiposity

CONTEXT

Chronic high levels of PTH may be associated with up-regulation of proteases and cytokines. Monocyte chemoattractant protein-1 (MCP-1) is an inflammatory cytokine, produced predominantly by macrophages and endothelial cells, and is expressed in adipose tissue. More recently it has been shown that PTH administration increases MCP-1 expression in osteoblasts.

OBJECTIVES

Because both PTH and MCP-1 levels are higher in obesity, the goal was to determine whether the high MCP-1 occurs only in the presence of high serum PTH and is independent of adiposity and examine its relationship with bone mineral density (BMD) and turnover.

DESIGN, SETTING, AND PARTICIPANTS

In this case-control clinical design, 111 eligible women were categorized into four groups: leaner women [body mass index (BMI) 23 ± 2 kg/m(2)] with normal or higher PTH and obese (BMI 44 ± 7 kg/m(2)) with normal or higher PTH.

RESULTS

Serum MCP-1 levels were higher (P < 0.01) in the high (PTH = 74.9 ± 27.0 pg/ml, MCP-1 = 421.5 ± 157.0 pg/ml) compared with normal PTH (PTH = 32.5 ± 10.4 pg/ml, MCP-1 = 322.5 ± 97.8 pg/ml) group, independent of BMI. C-reactive protein and adiponectin were influenced only by BMI and not PTH. MCP-1 was positively associated with osteocalcin and propeptide of type 1 collagen in the leaner (r > 0.3, P < 0.05) but not the obese women and was not associated with BMD in either group.

CONCLUSIONS

Together these data suggest that MCP-1 is higher only in the presence of increased PTH and that adiposity alone cannot explain the higher MCP-1 levels in obesity.

Epidermal growth factor receptor plays an anabolic role in bone metabolism in vivo

While the epidermal growth factor receptor (EGFR)-mediated signaling pathway has been shown to have vital roles in many developmental and pathologic processes, its functions in the development and homeostasis of the skeletal system has been poorly defined. To address its in vivo role, we constructed transgenic and pharmacologic mouse models and used peripheral quantitative computed tomography (pQCT), micro-computed tomography (µCT) and histomorphometry to analyze their trabecular and cortical bone phenotypes. We initially deleted the EGFR in preosteoblasts/osteoblasts using a Cre/loxP system (Col-Cre Egfr(f/f)), but no bone phenotype was observed because of incomplete deletion of the Egfr genomic locus. To further reduce the remaining osteoblastic EGFR activity, we introduced an EGFR dominant-negative allele, Wa5, and generated Col-Cre Egfr(Wa5/f) mice. At 3 and 7 months of age, both male and female mice exhibited a remarkable decrease in tibial trabecular bone mass with abnormalities in trabecular number and thickness. Histologic analyses revealed decreases in osteoblast number and mineralization activity and an increase in osteoclast number. Significant increases in trabecular pattern factor and structural model index indicate that trabecular microarchitecture was altered. The femurs of these mice were shorter and smaller with reduced cortical area and periosteal perimeter. Moreover, colony-forming unit-fibroblast (CFU-F) assay indicates that these mice had fewer bone marrow mesenchymal stem cells and committed progenitors. Similarly, administration of an EGFR inhibitor into wild-type mice caused a significant reduction in trabecular bone volume. In contrast, Egfr(Dsk5/+) mice with a constitutively active EGFR allele displayed increases in trabecular and cortical bone content. Taken together, these data demonstrate that the EGFR signaling pathway is an important bone regulator and that it primarily plays an anabolic role in bone metabolism.

Parathyroid hormone activation of matrix metalloproteinase-13 transcription requires the histone acetyltransferase activity of p300 and PCAF and p300-dependent acetylation of PCAF

Parathyroid hormone (PTH) regulates the transcription of many genes involved in bone remodeling in osteoblasts. One of these genes is matrix metalloproteinase-13 (MMP-13), which is involved in bone remodeling and early stages of endochondral bone formation. We have previously shown that Mmp-13 gene expression is highly induced by PTH treatment in osteoblastic UMR 106-01 cells, as well as primary osteoblasts. Here, we show that p300/CBP-associated factor (PCAF), in addition to p300 and Runx2, is required for PTH activation of Mmp-13 transcription. PCAF was increasingly recruited to the MMP-13 proximal promoter region after PTH treatment, and this was associated with an increase in RNA polymerase II recruitment and histone acetylation. In addition, PTH treatment increased the acetylation of PCAF, a process that required p300. Knockdown of PCAF, p300, or Runx2 by siRNA decreased Mmp-13 mRNA expression after PTH treatment in both UMR 106-01 cells and primary osteoblasts. We found that there is a mutual dependence between p300 and PCAF to be recruited to the Mmp-13 promoter after PTH treatment. In promoter-reporter assays, p300 and PCAF had an additive effect on PTH stimulation of MMP-13 promoter activity, and this required their histone acetyltransferase activity. Our findings demonstrate that PCAF acts downstream of PTH signaling as a transcriptional coactivator that is required for PTH stimulation of MMP-13 transcription. PCAF cooperates with p300 and Runx2 to mediate PTH activation of MMP-13 transcription.

Cellular and molecular mechanisms of bone remodeling

Physiological bone remodeling is a highly coordinated process responsible for bone resorption and formation and is necessary to repair damaged bone and to maintain mineral homeostasis. In addition to the traditional bone cells (osteoclasts, osteoblasts, and osteocytes) that are necessary for bone remodeling, several immune cells have also been implicated in bone disease. This minireview discusses physiological bone remodeling, outlining the traditional bone biology dogma in light of emerging osteoimmunology data. Specifically discussed in detail are the cellular and molecular mechanisms of bone remodeling, including events that orchestrate the five sequential phases of bone remodeling: activation, resorption, reversal, formation, and termination.

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.

Transforming growth factor-beta1 regulation of ATF-3 and identification of ATF-3 target genes in breast cancer cells

Transforming growth factor-beta1 (TGF-beta1) is a crucial molecule for stimulation of breast cancer invasion and formation of bone metastases. The molecular mechanisms of how TGF-beta1 mediates these effects have yet to be completely determined. We have found that activating transcription factor-3 (ATF-3) is strongly stimulated and its level is sustained by TGF-beta1 in highly invasive and metastatic human breast cancer (MDA-MB231) and in mouse mammary pad tumor cells (r3T). ATF-3 is also overexpressed in human primary breast cancer tissue. Overexpression of ATF-3 increased normal human mammary epithelial cell number and DNA synthesis suggesting a role for ATF-3 in cell proliferation. The functional role of ATF-3 in breast cancer progression was determined by the RNA interference technique. Knockdown of ATF-3 by ATF-3 shRNA in MDA-MB231 cells decreased expression of cell cycle gene, cyclin A1 in MDA-MB231 cells. ATF-3 shRNA also decreased expression of an invasive and metastatic gene, matrix metalloproteinase-13 (MMP-13; collagenase-3) in these cells. Chromatin immunoprecipitation experiments identified the direct physical interaction of ATF-3 protein on the human MMP-13 promoter. Thus, the dysregulation of ATF-3 by TGF-beta1 is likely to activate cyclin A1 and MMP-13 genes in breast cancer cells and that would be key to the subsequent cancer cell invasion and metastasis.

Toxicity induced enhanced extracellular matrix production in osteoblastic cells cultured on single-walled carbon nanotube networks

A central effort in biomedical research concerns the development of materials for sustaining and controlling cell growth. Carbon nanotube based substrates have been shown to support the growth of different kinds of cells (Hu et al 2004 Nano Lett. 4 507-11; Kalbacova et al 2006 Phys. Status Solidi b 13 243; Zanello et al 2006 Nano Lett. 6 562-7); however the underlying molecular mechanisms remain poorly defined. To address the fundamental question of mechanisms by which nanotubes promote bone mitosis and histogenesis, primary calvariae osteoblastic cells were grown on single-walled carbon nanotube thin film (SWNT) substrates. Using a combination of biochemical and optical techniques we demonstrate here that SWNT networks promote cell development through two distinct steps. Initially, SWNTs are absorbed in a process that resembles endocytosis, inducing acute toxicity. Nanotube-mediated cell destruction, however, induces a release of endogenous factors that act to boost the activity of the surviving cells by stimulating the synthesis of extracellular matrix.

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.

Mitogen activated protein kinase-dependent inhibition of osteocalcin gene expression by transforming growth factor-beta1

TGF-beta (transforming growth factor-beta) plays a key role in osteoblast differentiation and bone development. While the ability of TGF-beta to inhibit the expression of osteoblast differentiation genes has been well documented, the mechanism of this inhibition is not yet completely characterized. Runx2, a transcription factor necessary for expression of osteoblast differentiation genes is a central target of inhibition by TGF-beta. In this study, we found that TGF-beta1 inhibits expression of osteoblast differentiation genes without altering expression of Runx2. Transient transfection experiments determined that TGF-beta1 inhibited osteocalcin promoter activity and this effect is mediated through Runx2. We further identified that there was no change in protein expression, cellular localization, or DNA binding affinity of Runx2 after TGF-beta1-treatment of osteoblasts, suggesting that Runx2 undergoes post-translational modifications following TGF-beta1 treatment. Co-immunoprecipitation experiments identified increased phosphorylation of Runx2 when differentiating osteoblasts were treated with TGF-beta1. Mitogen activated protein kinase (MAPK) inhibitors relieved the TGF-beta1-inhibitory effect of Runx2-mediated osteocalcin expression. Thus, our results suggest that TGF-beta1-inhibition of osteoblast differentiation is dependent on the MAPK pathway and this effect is most likely mediated by post-translational modification of Runx2 such as phosphorylation rather than other regulatory mechanisms.

Parathyroid hormone stimulation of noncanonical Wnt signaling in bone

In bone, parathyroid hormone (PTH) exerts either a catabolic or an anabolic effect depending on its method of administration. This paradoxical action has led to the use of PTH as an effective treatment for osteoporosis. The Wnt family of signaling proteins has a critical role in multiple events, which are necessary for proper animal development and survival, yet their exact method of action in bone remains elusive. We have uncovered a novel link between Wnt-4 and PTH. We think that, in bone, Wnt-4 signaling in response to PTH implicates cross-talk of multiple signaling pathways. This work hopes to further elucidate Wnt signaling in bone and provide greater understanding of PTH's anabolic effects in bone.

Interleukin-18 is regulated by parathyroid hormone and is required for its bone anabolic actions

Interleukin-18 (IL-18) can regulate osteoblast and osteoclast function. We have identified, using cDNA microarray technology, that IL-18 expression is increased in UMR 106-01 rat osteoblastic cells in response to parathyroid hormone (PTH) treatment. Confirmation of these data using real-time reverse transcription-PCR showed that steady-state levels of IL-18 mRNA increased by 2 h (3-fold), peaked by 4 h (10-fold), and had diminished after 12 h (4.4-fold) and that this regulation was via the protein kinase A signaling pathway and did not involve activation of the PKC signal cascade. PTH regulation of IL-18 was confirmed at the protein level, and analysis of differentiating primary rat calvarial osteoblasts verified that both IL-18 mRNA and protein are regulated by PTH in primary rat osteoblasts. Promoter reporter assays revealed that PTH regulated the upstream IL-18 promoter and induced the exon 1 containing 1.1-kb IL-18 mRNA transcript in primary osteoblast cells. The in vivo physiological role of IL-18 in the anabolic actions of PTH on bone was then assessed using IL-18 knock-out mice. Female IL-18 null mice and wild-type littermate controls were injected with vehicle or 8 microg/100 g of human 1-38 PTH for 4 weeks. In IL-18 knock-out animals the anabolic effect of PTH (determined by bone mineral density changes in the proximal tibia) was abolished in trabecular bone but not in the cortical component. These data characterize the PTH regulation of IL-18 expression in osteoblastic cells and suggest that this cytokine is involved in the anabolic actions of PTH.

Parathyroid Hormone Stimulates Osteoblastic Expression of MCP-1 to Recruit and Increase the Fusion of Pre/Osteoclasts

The clinical findings that alendronate blunted the anabolic effect of human parathyroid hormone (PTH) on bone formation suggest that active resorption is involved and enhances the anabolic effect. PTH signals via its receptor on the osteoblast membrane, and osteoclasts are impacted indirectly via the products of osteoblasts. Microarray with RNA from rats injected with human PTH or vehicle showed a strong association between the stimulation of monocyte chemoattractant protein-1 (MCP-1) and the anabolic effects of PTH. PTH rapidly and dramatically stimulated MCP-1 mRNA in the femora of rats receiving daily injections of PTH or in primary osteoblastic and UMR 106-01 cells. The stimulation of MCP-1 mRNA was dose-dependent and a primary response to PTH signaling via the cAMP-dependent protein kinase pathway in vitro. Studies with the mouse monocyte cell line RAW 264.7 and mouse bone marrow proved that osteoblastic MCP-1 can potently recruit osteoclast monocyte precursors and facilitate receptor activator of NF-kappaB ligand-induced osteoclastogenesis and, in particular, enhanced fusion. Our model suggests that PTH-induced osteoblastic expression of MCP-1 is involved in recruitment and differentiation at the stage of multinucleation of osteoclast precursors. This information provides a rationale for increased osteoclast activity in the anabolic effects of PTH in addition to receptor activator of NF-kappaB ligand stimulation to initiate greater bone remodeling.