BMP2 protein regulates osteocalcin expression via Runx2-mediated Atf6 gene transcription

Impaired bone remodeling in children with osteogenesis imperfecta treated and untreated with bisphosphonates: the role of DKK1, RANKL, and TNF-α

In this study, we investigated the bone cell activity in patients with osteogenesis imperfecta (OI) treated and untreated with neridronate. We demonstrated the key role of Dickkopf-1 (DKK1), receptor activator of nuclear factor-κB ligand (RANKL), and tumor necrosis factor alpha (TNF-α) in regulating bone cell of untreated and treated OI subjects. These cytokines could represent new pharmacological targets for OI.

INTRODUCTION

Bisphosphonates are widely used in the treatment of children with osteogenesis imperfecta (OI) with the objective of reducing the risk of fractures. Although bisphosphonates increase bone mineral density in OI subjects, the effects on fracture incidence are conflicting. The aim of this study was to investigate the mechanisms underlying bone cell activity in subjects with mild untreated forms of OI and in a group of subjects with severe OI treated with cycles of intravenous neridronate.

METHODS

Sclerostin, DKK1, TNF-α, RANKL, osteoprotegerin (OPG), and bone turnover markers were quantified in serum of 18 OI patients (12 females, mean age 8.86 ± 3.90), 8 of which were receiving cyclic intravenous neridronate, and 21 sex- and age-matched controls. The effects on osteoblastogenesis and OPG expression of media conditioned by the serum of OI patients and anti-DKK1 neutralizing antibody were evaluated. Osteoclastogenesis was assessed in cultures from patients and controls.

RESULTS

DKK1 and RANKL levels were significantly increased both in untreated and in treated OI subjects with respect to controls. The serum from patients with high DKK1 levels inhibited both osteoblast differentiation and OPG expression in vitro. High RANKL and low OPG messenger RNA (mRNA) levels were found in lymphomonocytes from patients. High amounts of TNF-α were expressed by monocytes, and an elevated percentage of circulating CD11b-CD51/CD61+ osteoclast precursors was observed in patients.

CONCLUSIONS

Our study demonstrated the key role of DKK1, RANKL, and TNF-α in regulating bone cell activity of subjects with OI untreated and treated with bisphosphonates. These cytokines could represent new pharmacological targets for OI patients.

Grass carp (Ctenopharyngodon idella) ATF6 (activating transcription factor 6) modulates the transcriptional level of GRP78 and GRP94 in CIK cells

ATF transcription factors are stress proteins containing alkaline area-leucine zipper and play an important role in endoplasmic reticulum stress. ATF6 is a protective protein which regulates the adaptation of cells to ER stress by modulating the transcription of UPR (Unfolded Protein Response) target genes, including GRP78 and GRP94. In the present study, a grass carp (Ctenopharyngodon idella) ATF6 full-length cDNA (named CiATF6, KT279356) has been cloned and identified. CiATF6 is 4176 bp in length, comprising 159 nucleotides of 5'-untranslated sequence, a 1947 nucleotides open reading frame and 2170 nucleotides of 3'-untranslated sequences. The largest open reading frame of CiATF6 translates into 648 aa with a typical DNA binding domain (BRLZ domain) and shares significant homology to the known ATF6 counterparts. Phylogenetic reconstruction confirmed its closer evolutionary relationship with other fish counterparts, especially with Zebrafish ATF6. RT-PCR showed that CiATF6 was ubiquitously expressed and significantly up-regulated after stimulation with thermal stress in all tested grass carp tissues. In order to know more about the role of CiATF6 in ER stress, recombinant CiATF6N with His-tag was over-expressed in Rosetta Escherichia coli, and the expressed protein was purified by affinity chromatography with Ni-NTA His-Bind Resin. In vitro, gel mobility shift assays were employed to analyze the interaction of CiATF6 protein with the promoters of grass carp GRP78 and GRP94, respectively. The result has shown that CiATF6 could bind to these promoters with high affinity by means of its BRLZ mainly. To further study the transcriptional regulatory mechanism of CiATF6, Dual-luciferase reporter assays were applied. Recombinant plasmids of pGL3-GRP78P and pGL3-CiGRP94P were constructed and transiently co-transfected with pcDNA3.1-CiATF6 (pcDN3.1-CiATF6-nBRLZ, respectively) into C. idella kidney (CIK) cells. The result has shown that CiATF6 could activate CiGRP78 and CiGRP94 promoters.

Nuclear Matrix Protein 4 Is a Novel Regulator of Ribosome Biogenesis and Controls the Unfolded Protein Response via Repression of Gadd34 Expression

The unfolded protein response (UPR) maintains protein homeostasis by governing the processing capacity of the endoplasmic reticulum (ER) to manage ER client loads; however, key regulators within the UPR remain to be identified. Activation of the UPR sensor PERK (EIFAK3/PEK) results in the phosphorylation of the α subunit of eIF2 (eIF2α-P), which represses translation initiation and reduces influx of newly synthesized proteins into the overloaded ER. As part of this adaptive response, eIF2α-P also induces a feedback mechanism through enhanced transcriptional and translational expression of Gadd34 (Ppp1r15A),which targets type 1 protein phosphatase for dephosphorylation of eIF2α-P to restore protein synthesis. Here we describe a novel mechanism by which Gadd34 expression is regulated through the activity of the zinc finger transcription factor NMP4 (ZNF384, CIZ). NMP4 functions to suppress bone anabolism, and we suggest that this occurs due to decreased protein synthesis of factors involved in bone formation through NMP4-mediated dampening of Gadd34 and c-Myc expression. Loss of Nmp4 resulted in an increase in c-Myc and Gadd34 expression that facilitated enhanced ribosome biogenesis and global protein synthesis. Importantly, protein synthesis was sustained during pharmacological induction of the UPR through a mechanism suggested to involve GADD34-mediated dephosphorylation of eIF2α-P. Sustained protein synthesis sensitized cells to pharmacological induction of the UPR, and the observed decrease in cell viability was restored upon inhibition of GADD34 activity. We conclude that NMP4 is a key regulator of ribosome biogenesis and the UPR, which together play a central role in determining cell viability during endoplasmic reticulum stress.

ATF6a, a Runx2-activable transcription factor, is a new regulator of chondrocyte hypertrophy

Our previous research has shown that the spliced isoform of XBP1 (XBP1s) is an important downstream mediator of BMP2 and is involved in BMP2-stimulated chondrocyte differentiation. Herein, we report that ATF6 and its cleaved N-terminal cytoplasmic domain (known as ATF6a) are expressed in growth plate chondrocytes. We find that these proteins are differentially induced during BMP2-triggered chondrocyte differentiation. This differential expression probably results from the activation of the ATF6 gene by Runx2 and its repression by the Sox6 transcription factor. Runx2 and Sox6 act through their respective binding elements on the ATF6 gene. When overexpressed, ATF6 and ATF6a intensify chondrogenesis; our studies demonstrate that under the stimulation of ATF6 and ATF6a, chondrocytes tend to be hypertrophied and mineralized, a process leading to bone formation. By contrast, lowering expression of ATF6a by use of its specific siRNA suppresses chondrocyte differentiation. Moreover, ATF6a interacts with Runx2 and augments the Runx2-mediated hypertrophication of chondrocytes. Importantly, overexpression and knockdown of ATF6a during the chondrocyte hypertrophy process also led to altered expressions of IHH and PTHrP (also known as PTHLH). Taken together, these findings indicate that ATF6a favorably controls chondrogenesis and bone formation (1) by acting as a co-factor of Runx2 and enhancing Runx2-incited hypertrophic chondrocyte differentiation, and (2) by affecting IHH and PTHrP signaling.

Transmission of ER stress response by ATF6 promotes endochondral bone growth

Background

We reported earlier that X-box binding protein1 spliced (XBP1S), a key regulator of the unfolded protein response (UPR), as a bone morphogenetic protein 2 (BMP2)-inducible transcription factor, positively regulates endochondral bone formation by activating granulin-epithelin precursor (GEP) chondrogenic growth factor. Under the stress of misfolded or unfolded proteins in the endoplasmic reticulum (ER), the cells can be protected by the mammalian UPR. However, the influence of activating transcription factor 6 (ATF6), another transcriptional arm of UPR, in BMP2-induced chondrocyte differentiation has not yet been elucidated. In the current study, we investigate and explore the role of ATF6 in endochondral bone formation, focus on associated molecules of hypertrophic chondrocyte differentiation, as well as the molecular events underlying this process.

Methods

High-cell-density micromass cultures were used to induce ATDC5 and C3H10T1/2 cell differentiation into chondrocytes. Quantitative real-time PCR, immunoblotting analysis, and immunohistochemistry were performed to examine (1) the expression of ATF6, ATF6α, collagen II, collagen X, and matrix metalloproteinase-13 (MMP13) and (2) whether ATF6 stimulates chondrogenesis and whether ATF6 enhances runt-related transcription factor 2 (Runx2)-mediated chondrocyte hypertrophy. Culture of fetal mouse bone explants was to detect whether ATF6 stimulates chondrocyte hypertrophy, mineralization, and endochondral bone growth. Coimmunoprecipitation was employed to determine whether ATF6 associates with Runx2 in chondrocyte differentiation.

Results

ATF6 is differentially expressed in the course of BMP2-triggered chondrocyte differentiation. Overexpression of ATF6 accelerates chondrocyte differentiation, and the ex vivo studies reveal that ATF6 is a potent stimulator of chondrocyte hypertrophy, mineralization, and endochondral bone growth. Knockdown of ATF6 via a siRNA approach inhibits chondrogenesis. Furthermore, ATF6 associates with Runx2 and enhances Runx2-induced chondrocyte hypertrophy. And, the stimulation effect of ATF6 is reduced during inhibition of Runx2 via a siRNA approach, suggesting that the promoting effect is required for Runx2.

Conclusions

Our observations demonstrate that ATF6 positively regulates chondrocyte hypertrophy and endochondral bone formation through activating Runx2-mediated hypertrophic chondrocyte differentiation.

Different Roles of GRP78 on Cell Proliferation and Apoptosis in Cartilage Development

Eukaryotic cells possess several mechanisms to adapt to endoplasmic reticulum (ER) stress and thereby survive. ER stress activates a set of signaling pathways collectively termed as the unfolded protein response (UPR). We previously reported that Bone morphogenetic protein 2 (BMP2) mediates mild ER stress and activates UPR signal molecules in chondrogenesis. The mammalian UPR protects the cell against the stress of misfolded proteins in the endoplasmic reticulum. Failure to adapt to ER stress causes the UPR to trigger apoptosis. Glucose regulated protein 78 (GRP78), as an important molecular chaperone in UPR signaling pathways, is responsible for binding to misfolded or unfolded protein during ER stress. However the influence on GRP78 in BMP2-induced chondrocyte differentiation has not yet been elucidated and the molecular mechanism underlyng these processes remain unexplored. Herein we demonstrate that overexpression of GRP78 enhanced cell proliferation in chondrocyte development with G1 phase advance, S phase increasing and G2-M phase transition. Furthermore, overexpression of GRP78 inhibited ER stress-mediated apoptosis and then reduced apoptosis in chondrogenesis induced by BMP2, as assayed by cleaved caspase3, caspase12, C/EBP homologous protein (CHOP/DDIT3/GADD153), p-JNK (phosphorylated c-Jun N-terminal kinase) expression during the course of chondrocyte differentiation by Western blot. In addition, flow cytometry (FCM) assay, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling (TUNEL) assay and immune-histochemistry analysis also proved this result in vitro and in vivo. It was demonstrated that GRP78 knockdown via siRNA activated the ER stress-specific caspase cascade in developing chondrocyte tissue. Collectively, these findings reveal a novel critical role of GRP78 in regulating ER stress-mediated apoptosis in cartilage development and the molecular mechanisms involved.

Haploinsufficiency for either one of the type-II regulatory subunits of protein kinase A improves the bone phenotype of Prkar1a+/- mice

Carney Complex (CNC), a human genetic syndrome predisposing to multiple neoplasias, is associated with bone lesions such as osteochondromyxomas (OMX). The most frequent cause for CNC is PRKAR1A deficiency; PRKAR1A codes for type-I regulatory subunit of protein kinase A (PKA). Prkar1a(+/-) mice developed OMX, fibrous dysplasia-like lesions (FDL) and other tumors. Tumor tissues in these animals had increased PKA activity due to an unregulated PKA catalytic subunit and increased PKA type II (PKA-II) activity mediated by the PRKAR2A and PRKAR2B subunits. To better understand the effect of altered PKA activity on bone, we studied Prkar2a and Prkar2b knock out (KO) and heterozygous mice; none of these mice developed bone lesions. When Prkar2a(+/-) and Prkar2b(+/-) mice were used to generate Prkar1a(+/-)Prkar2a(+/-) and Prkar1a(+/-)Prkar2b(+/-) animals, bone lesions formed that looked like those of the Prkar1a(+/-) mice. However, better overall bone organization and mineralization and fewer FDL lesions were found in both double heterozygote groups, indicating a partial restoration of the immature bone structure observed in Prkar1a(+/-) mice. Further investigation indicated increased osteogenesis and higher new bone formation rates in both Prkar1a(+/-)Prkar2a(+/-) and Prkar1a(+/-)Prkar2b(+/-) mice with some minor differences between them. The observations were confirmed with a variety of markers and studies. PKA activity measurements showed the expected PKA-II decrease in both double heterozygote groups. Thus, haploinsufficiency for either of PKA-II regulatory subunits improved bone phenotype of mice haploinsufficient for Prkar1a, in support of the hypothesis that the PRKAR2A and PRKAR2B regulatory subunits were in part responsible for the bone phenotype of Prkar1a(+/-) mice.

Osteogenic changes in kidneys of hyperoxaluric rats

Many calcium oxalate (CaOx) kidney stones develop attached to renal papillary sub-epithelial deposits of calcium phosphate (CaP), called Randall's plaque (RP). Pathogenesis of the plaques is not fully understood. We hypothesize that abnormal urinary environment in stone forming kidneys leads to epithelial cells losing their identity and becoming osteogenic. To test our hypothesis male rats were made hyperoxaluric by administration of hydroxy-l-proline (HLP). After 28days, rat kidneys were extracted. We performed genome wide analyses of differentially expressed genes and determined changes consistent with dedifferentiation of epithelial cells into osteogenic phenotype. Selected molecules were further analyzed using quantitative-PCR and immunohistochemistry. Genes for runt related transcription factors (RUNX1 and 2), zinc finger protein Osterix, bone morphogenetic proteins (BMP2 and 7), bone morphogenetic protein receptor (BMPR2), collagen, osteocalcin, osteonectin, osteopontin (OPN), matrix-gla-protein (MGP), osteoprotegrin (OPG), cadherins, fibronectin (FN) and vimentin (VIM) were upregulated while those for alkaline phosphatase (ALP) and cytokeratins 10 and 18 were downregulated. In conclusion, epithelial cells of hyperoxaluric kidneys acquire a number of osteoblastic features but without CaP deposition, perhaps a result of downregulation of ALP and upregulation of OPN and MGP. Plaque formation may additionally require localized increases in calcium and phosphate and decrease in mineralization inhibitory potential.

Phytomolecule icaritin incorporated PLGA/TCP scaffold for steroid-associated osteonecrosis: Proof-of-concept for prevention of hip joint collapse in bipedal emus and mechanistic study in quadrupedal rabbits

Steroid-associated osteonecrosis (SAON) may lead to joint collapse and subsequent joint replacement. Poly lactic-co-glycolic acid/tricalcium phosphate (P/T) scaffold providing sustained release of icaritin (a metabolite of Epimedium-derived flavonoids) was investigated as a bone defect filler after surgical core-decompression (CD) to prevent femoral head collapse in a bipedal SAON animal model using emu (a large flightless bird). The underlying mechanism on SAON was evaluated using a well-established quadrupedal rabbit model. Fifteen emus were established with SAON, and CD was performed along the femoral neck for the efficacy study. In this CD bone defect, a P/T scaffold with icaritin (P/T/I group) or without icaritin (P/T group) was implanted while no scaffold implantation was used as a control. For the mechanistic study in rabbits, the effects of icaritin and composite scaffolds on bone mesenchymal stem cells (BMSCs) recruitment, osteogenesis, and anti-adipogenesis were evaluated. Our efficacy study showed that P/T/I group had the significantly lowest incidence of femoral head collapse, better preserved cartilage and mechanical properties supported by more new bone formation within the bone tunnel. For the mechanistic study, our in vitro tests suggested that icaritin enhanced the expression of osteogenesis related genes COL1α, osteocalcin, RUNX2, and BMP-2 while inhibited adipogenesis related genes C/EBP-ß, PPAR-γ, and aP2 of rabbit BMSCs. Both P/T and P/T/I scaffolds were demonstrated to recruit BMSCs both in vitro and in vivo but a higher expression of migration related gene VCAM1 was only found in P/T/I group in vitro. In conclusion, both efficacy and mechanistic studies show the potential of a bioactive composite porous P/T scaffold incorporating icaritin to enhance bone defect repair after surgical CD and prevent femoral head collapse in a bipedal SAON emu model.

Cyclic AMP Response Element-binding Protein H (CREBH) Mediates the Inhibitory Actions of Tumor Necrosis Factor α in Osteoblast Differentiation by Stimulating Smad1 Degradation

Endoplasmic reticulum (ER) stress transducers, such as old astrocyte specifically induced substance (OASIS) and activating transcription factor 6 (ATF6), which are induced by bone morphogenetic protein 2 (BMP2), regulate bone formation and osteoblast differentiation. Here, we examined the role of cAMP response element-binding protein H (CREBH), a member of the same family of ER membrane-bound basic leucine zipper (bZIP) transcription factors as OASIS and ATF6, in osteoblast differentiation and bone formation. Proinflammatory cytokine TNFα increased CREBH expression by up-regulating the nuclear factor-κB (NF-κB) signaling pathway in osteoblasts, increased the level of N-terminal fragment of CREBH in the nucleus, and inhibited BMP2 induction of osteoblast specific gene expression. Overexpression of CREBH suppressed BMP2-induced up-regulation of the osteogenic markers runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), and osteocalcin (OC) in MC3T3-E1 cells and primary osteoblasts, as well as BMP2-induced ALP activity and OC protein production. In contrast, knockdown of CREBH attenuated the inhibitory effect of TNFα on BMP2-induced osteoblast differentiation. Mechanistic studies revealed that CREBH increased the expression of Smad ubiquitination regulatory factor 1 (Smurf1), leading to ubiquitin-dependent degradation of Smad1, whereas knockdown of CREBH inhibited TNFα-mediated degradation of Smad1 by Smurf1. Consistent with these in vitro findings, administration of Ad-CREBH inhibited BMP2-induced ectopic and orthotopic bone formation in vivo. Taken together, these results suggest that CREBH is a novel negative regulator of osteoblast differentiation and bone formation.

Mineral trioxide aggregate induces osteoblastogenesis via Atf6

Mineral trioxide aggregate (MTA) has been recommended for various uses in endodontics. To understand the effects of MTA on alveolar bone, we examined whether MTA induces osteoblastic differentiation using MC3T3-E1 cells. MTA enhanced mineralization concomitant with alkaline phosphatase activity in a dose- and time-dependent manner. MTA increased production of collagens (Type I and Type III) and matrix metalloproteinases (MMP-9 and MMP-13), suggesting that MTA affects bone matrix remodeling. MTA also induced Bglap (osteocalcin) but not Bmp2 (bone morphogenetic protein-2) mRNA expression. We observed induction of Atf6 (activating transcription factor 6, an endoplasmic reticulum (ER) stress response transcription factor) mRNA expression and activation of Atf6 by MTA treatment. Forced expression of p50Atf6 (active form of Atf6) markedly enhanced Bglap mRNA expression. Chromatin immunoprecipitation assay was performed to investigate the increase in p50Atf6 binding to the Bglap promoter region by MTA treatment. Furthermore, knockdown of Atf6 gene expression by introduction of Tet-on Atf6 shRNA expression vector abrogated MTA-induced mineralization. These results suggest that MTA induces in vitro osteoblastogenesis through the Atf6–osteocalcin axis as ER stress signaling. Therefore, MTA in endodontic treatment may affect alveolar bone healing in the resorbed region caused by pulpal infection.

Role of calcium in the regulation of bone morphogenetic protein 2, runt-related transcription factor 2 and Osterix in primary renal tubular epithelial cells by the vitamin D receptor

The aim of the present study was to investigate the effect of 1,25(OH)2D3/vitamin D receptor (VDR) and calcium on the expression levels of osteogenic factors in primary renal tubular epithelial cells (RTECs) using genetic hypercalciuric rats. The basal levels of osteogenic factors were detected in Sprague Dawley and genetic hypercalciuric rats. The gene and protein levels of bone morphogenetic protein 2 (BMP2), runt-related transcription factor 2 (Runx2) and osterix were detected in the RTECs transduced with Lenti-VDR-sh and were incubated with calcium. Using the o-cresolphthalein complexone method, the calcium levels of the primary RTECs cultured with Lenti-VDR-sh and with 1,25(OH)2D3 were assessed. The basal levels of BMP2, Runx2 and Osterix in the cells were significantly higher in the genetic hypercalciuric rats compared with the control rats. VDR knockdown in the RTECs reduced the expression levels of BMP2, Runx2 and Osterix. The calcium depositions in the primary RTECs were also decreased following exposure to Lenti-VDR-sh, but increased following treatment with 1,25(OH)2D3. The expression levels of BMP2, Runx2 and Osterix were markedly increased in the cells incubated with calcium compared with the cells treated with normal saline and the untreated cells. These findings indicated that osteogenic factors, including BMP2, Runx2 and Osterix may be important in renal stone formation in idiopathic hypercalciuria. VDR may mediate the increased expression levels of BMP2, Runx2 and Osterix by positively regulating calcium levels in primary RTECs.

Nanoscale stimulation of osteoblastogenesis from mesenchymal stem cells: nanotopography and nanokicking

AIM

Mesenchymal stem cells (MSCs) have large regenerative potential to replace damaged cells from several tissues along the mesodermal lineage. The potency of these cells promises to change the longer term prognosis for many degenerative conditions currently suffered by our aging population. We have endeavored to demonstrate our ability to induce osteoblatogenesis in MSCs using high-frequency (1000-5000 Hz) piezo-driven nanodisplacements (16-30 nm displacements) in a vertical direction.

MATERIALS & METHODS

Osteoblastogenesis has been determined by the upregulation of osteoblasic genes such as osteonectin (ONN), RUNX2 and Osterix, assessed via quantitative real-time PCR; the increase of osteocalcin (OCN) and osteopontin (OPN) at the protein level and the deposition of calcium phosphate determined by histological staining.

RESULTS

Intriguingly, we have observed a relationship between nanotopography and piezo-stimulated mechanotransduction and possibly see evidence of two differing osteogenic mechanisms at work. These data provide confidence in nanomechanotransduction for stem cell differentiation without dependence on soluble factors and complex chemistries.

CONCLUSION

In the future it is envisaged that this technology may have beneficial therapeutic applications in the healthcare industry, for conditions whose overall phenotype maybe characterized by weak or damaged bones (e.g., osteoporosis and bone fractures), and which can benefit from having an increased number of osteoblastic cells in vivo.

Bone morphogenetic protein 7 (BMP-7) influences tendon-bone integration in vitro

Introduction

Successful graft ingrowth following reconstruction of the anterior cruciate ligament is governed by complex biological processes at the tendon-bone interface. The aim of this study was to investigate in an in vitro study the effects of bone morphogenetic protein 7 (BMP-7) on tendon-bone integration.

Materials and Methods

To study the biological effects of BMP-7 on the process of tendon-bone-integration, two independent in vitro models were used. The first model involved the mono- and coculture of bovine tendon specimens and primary bovine osteoblasts with and without BMP-7 exposure. The second model comprised the mono- and coculture of primary bovine osteoblasts and fibroblasts. Alkaline phosphatase (ALP), lactate dehydrogenase (LDH), lactate and osteocalcin (OCN) were analyzed by ELISA. Histological analysis and electron microscopy of the tendon specimens were performed.

Results

In both models, positive effects of BMP-7 on ALP enzyme activity were observed (p<0.001). Additionally, similar results were noted for LDH activity and lactate concentration. BMP-7 stimulation led to a significant increase in OCN expression. Whereas the effects of BMP-7 on tendon monoculture peaked during an early phase of the experiment (p<0.001), the cocultures showed a maximal increase during the later stages (p<0.001). The histological analysis showed a stimulating effect of BMP-7 on extracellular matrix formation. Organized ossification zones and calcium carbonate-like structures were only observed in the BMP-stimulated cell cultures.

Discussion

This study showed the positive effects of BMP-7 on the biological process of tendon-bone integration in vitro. Histological signs of improved mineralization were paralleled by increased rates of osteoblast-specific protein levels in primary bovine osteoblasts and fibroblasts.

Conclusion

Our findings indicated a role for BMP-7 as an adjuvant therapeutic agent in the treatment of ligamentous injuries, and they emphasized the importance of the transdifferentiation process of tendinous fibroblasts at the tendon-bone interface.

Osteoinduction of umbilical cord and palate periosteum-derived mesenchymal stem cells on poly(lactic-co-glycolic) acid nanomicrofibers

The need for tissue-engineered bone to treat complex craniofacial bone defects secondary to congenital anomalies, trauma, and cancer extirpation is sizeable. Traditional strategies for treatment have focused on autologous bone in younger patients and bone substitutes in older patients. However, the capacity for merging new technologies, including the creation of nanofiber and microfiber scaffolds with advances in natal sources of stem cells, is crucial to improving our treatment options. The advantages of using smaller diameter fibers for scaffolding are 2-fold: the similar fiber diameters mimic the in vivo extracellular matrix construct and smaller fibers also provide a dramatically increased surface area for cell-scaffold interactions. In this study, we compare the capacity for a polymer with Federal Drug Administration approval for use in humans, poly(lactic-co-glycolic) acid (PLGA) from Delta polymer, to support osteoinduction of mesenchymal stem cells (MSCs) harvested from the umbilical cord (UC) and palate periosteum (PP). Proliferation of both UC- and PP-derived MSCs was improved on PLGA scaffolds. The PLGA scaffolds promoted UC MSC differentiation (indicated by earlier gene expression and higher calcium deposition), but not in PP-derived MSCs. Umbilical cord-derived MSCs on the PLGA nanomicrofiber scaffolds have potential clinical utility in providing solutions for craniofacial bone defects, with the added benefit of earlier availability.

Tumor suppressor long non-coding RNA, MT1DP is negatively regulated by YAP and Runx2 to inhibit FoxA1 in liver cancer cells

Recent studies are indicative for strong carcinogenetic roles of Runt related transcription factor 2 (Runx2) and Yes associated protein (YAP) in several cancer types. However, whether and how the interaction between Runx2 and YAP plays a role in liver tumorigenesis still remain illusive. Here, we identified a close relationship between Runx2 and YAP in liver cancer cells. Runx2 had a positive role on YAP expression and vice versa. We also found that Rux2 and YAP were capable of inhibiting long non-coding RNA (lncRNA), Metallothionein 1D, Pseudogene (MT1DP) expression through direct promoter binding. Overexpression of MT1DP resulted in reduced cell proliferation and colony formation in soft agar, but increased apoptosis in liver cancer cells, whereas knockdown of this lncRNA had the opposite effect, indicating that MT1DP acts as a tumor suppressor. Furthermore, MT1DP was revealed as a negative regulator of Alfa-fetoprotein (AFP), a classic liver cancer tumor marker, through inhibiting protein synthesis of Forkhead box A1 (FoxA1), an important transcription factor in liver development and cancer progression. Furthermore, we found that FoxA1 plays a positive role on YAP and Runx2 expression. Specially, opening the compacted chromatin by FoxA1 around CREB binding site within the YAP promoter facilitates CREB-mediated YAP transcription. Finally, MT1DP-inhibited in vivo liver cancer cell growth could be rescued by a combination of overexpression of FoxA1, Runx2 and YAP. Taken together, the close relationship between Rnux2 and YAP plays a pro-carcinogenetic role in liver cancer cells through inhibiting tumor suppressor lncRNA, MT1DP in a FoxA1 dependent manner.

Osteoblast maturation on microtextured titanium involves paracrine regulation of bone morphogenetic protein signaling

Osteoblasts are sensitive to surface microtopography and chemistry. Osteoblast differentiation and maturation are higher in vitro and bone formation and osseointegration enhanced in vivo on microstructured titanium (Ti) compared to smooth surfaces. Cells increased BMP2 expression on microtextured Ti alloy, suggesting a paracrine role in regulating osteoblast maturation. However, recent studies show that exogenous BMP2 inhibits osteoblast production of anti-inflammatory cytokines and osteocalcin, indicating that control of BMP-signaling may be involved. This study examined whether cells modulate BMP ligands, receptors, and inhibitors during osteoblast maturation on Ti, specifically focusing on the roles of BMP2 and Noggin (NOG). mRNA and protein for BMP2, BMP4, and BMP7 and receptors BMPR1A, BMPR1B, and BMPR2, and BMP inhibitors were upregulated on microtextured surfaces in comparison to smooth surfaces. Maturation on microstructured Ti was slightly enhanced with exogenous BMP2 while NOG addition inhibited osteoblast maturation. Cells with NOG knocked down significantly increased osteoblast maturation. These results demonstrate that BMP-related molecules are controlled during osteoblast maturation on microstructured Ti surfaces and that endogenous NOG is an important regulator of the process. Modifying paracrine BMP signaling may yield more robust bone formation than application of exogenous BMPs.

A role for PERK in the mechanism underlying fluoride-induced bone turnover

While it has been well-documented that excessive fluoride exposure caused the skeletal disease and osteoblasts played a critical role in the advanced skeletal fluorosis, the underlying mechanism that mediated these effects remain poorly understood. The present study was undertaken to examine the effect of fluoride on bone of rats and MC3T3-E1 cells in vitro. Herein we found pathological features of high bone turnover in fluoride-treated rats, which was supported by an increase of osteogenic and osteoclastogenic genes expression in different stages of fluoride exposure. The skeletal toxicity of fluoride was accompanied by activation of endoplasmic reticulum (ER) stress and subsequent unfolded protein response (UPR). A novel finding of this study was that expression of PKR-like endoplasmic reticulum kinase (PERK) was the same trend with receptor activator for nuclear factor-κ B ligand (RANKL), and NF-E2 p45-related factor 2 (Nrf2) was the same trend with Runt-related transcription factor 2 (Runx2) in bones of rats exposed to varied fluoride condition. Based on these data, we hypothesized that up-regulation of PERK probably played a role in mediating bone turnover induced by fluoride. Action of fluoride on MC3T3-E1 cells differentiation was demonstrated through analysis of alkaline phosphatase (ALP) activity and mineralized nodules formation. Meantime, an increase of binding immunoglobulin protein (BiP) expression indicated the active ER stress in cells exposed to various dose of fluoride. Blocking PERK expression using siRNA showed the obvious decrease of osteogenic and osteoclastogenic factors expression in MC3T3-E1 cells exposed to certain dose of fluoride that could positively stimulate osteoblastic viability. In conclusion these findings underscore the importance of PERK in modulating fluoride induced bone formation and bone resorption. Understanding the link between PERK and bone turnover could probe into the mechanism underlying different bone lesion of skeletal fluorosis.

Multiple myeloma mesenchymal stromal cells: Contribution to myeloma bone disease and therapeutics

Multiple myeloma is a hematological malignancy in which clonal plasma cells proliferate and accumulate within the bone marrow. The presence of osteolytic lesions due to increased osteoclast (OC) activity and suppressed osteoblast (OB) function is characteristic of the disease. The bone marrow mesenchymal stromal cells (MSCs) play a critical role in multiple myeloma pathophysiology, greatly promoting the growth, survival, drug resistance and migration of myeloma cells. Here, we specifically discuss on the relative contribution of MSCs to the pathophysiology of osteolytic lesions in light of the current knowledge of the biology of myeloma bone disease (MBD), together with the reported genomic, functional and gene expression differences between MSCs derived from myeloma patients (pMSCs) and their healthy counterparts (dMSCs). Being MSCs the progenitors of OBs, pMSCs primarily contribute to the pathogenesis of MBD because of their reduced osteogenic potential consequence of multiple OB inhibitory factors and direct interactions with myeloma cells in the bone marrow. Importantly, pMSCs also readily contribute to MBD by promoting OC formation and activity at various levels (i.e., increasing RANKL to OPG expression, augmenting secretion of activin A, uncoupling ephrinB2-EphB4 signaling, and through augmented production of Wnt5a), thus further contributing to OB/OC uncoupling in osteolytic lesions. In this review, we also look over main signaling pathways involved in the osteogenic differentiation of MSCs and/or OB activity, highlighting amenable therapeutic targets; in parallel, the reported activity of bone-anabolic agents (at preclinical or clinical stage) targeting those signaling pathways is commented.

Comparative in vitro study regarding the biocompatibility of titanium-base composites infiltrated with hydroxyapatite or silicatitanate

BACKGROUND

The development of novel biomaterials able to control cell activities and direct their fate is warranted for engineering functional bone tissues. Adding bioactive materials can improve new bone formation and better osseointegration. Three types of titanium (Ti) implants were tested for in vitro biocompatibility in this comparative study: Ti6Al7Nb implants with 25% total porosity used as controls, implants infiltrated using a sol-gel method with hydroxyapatite (Ti HA) and silicatitanate (Ti SiO2). The behavior of human osteoblasts was observed in terms of adhesion, cell growth and differentiation.

RESULTS

The two coating methods have provided different morphological and chemical properties (SEM and EDX analysis). Cell attachment in the first hour was slower on the Ti HA scaffolds when compared to Ti SiO2 and porous uncoated Ti implants. The Alamar blue test and the assessment of total protein content uncovered a peak of metabolic activity at day 8-9 with an advantage for Ti SiO2 implants. Osteoblast differentiation and de novo mineralization, evaluated by osteopontin (OP) expression (ELISA and immnocytochemistry), alkaline phosphatase (ALP) activity, calcium deposition (alizarin red), collagen synthesis (SIRCOL test and immnocytochemical staining) and osteocalcin (OC) expression, highlighted the higher osteoconductive ability of Ti HA implants. Higher soluble collagen levels were found for cells cultured in simple osteogenic differentiation medium on control Ti and Ti SiO2 implants. Osteocalcin (OC), a marker of terminal osteoblastic differentiation, was most strongly expressed in osteoblasts cultivated on Ti SiO2 implants.

CONCLUSIONS

The behavior of osteoblasts depends on the type of implant and culture conditions. Ti SiO2 scaffolds sustain osteoblast adhesion and promote differentiation with increased collagen and non-collagenic proteins (OP and OC) production. Ti HA implants have a lower ability to induce cell adhesion and proliferation but an increased capacity to induce early mineralization. Addition of growth factors BMP-2 and TGFβ1 in differentiation medium did not improve the mineralization process. Both types of infiltrates have their advantages and limitations, which can be exploited depending on local conditions of bone lesions that have to be repaired. These limitations can also be offset through methods of functionalization with biomolecules involved in osteogenesis.

Explore on the effect of ATF6 on cell growth and apoptosis in cartilage development

We previously report that BMP2 mediates mild ER stress-activated ATF6 and directly regulates XBP1S splicing in the course of chondrogenesis. The mammalian unfolded protein response (UPR) protects the cell against the stress of misfolded proteins in the endoplasmic reticulum (ER). Failure to adapt to ER stress causes the UPR to trigger apoptosis. The transcription factor activating transcription factor 6 (ATF6), a key regulator of the UPR, is known to be important for ER stress-mediated apoptosis and cell growth, but the molecular mechanism underlying these processes remains unexplored. In this study, we demonstrate that ATF6 is differentially expressed during BMP2-stimulated chondrocyte differentiation and exhibits prominent expression in growth plate chondrocytes. ATF6 can enhance the level of IRE1a-spliced XBP1S protein in chondrogenesis. IRE1a and ATF6 can synergistically regulate endogenous XBP1S gene expression in chondrogenesis. Furthermore, overexpression ATF6 inhibited, while ATF6-knockdown enhanced, the cell proliferation in chondrocyte development with G1 phase arresting, S phase reducing and G2-M phase delaying. Besides, Ad-ATF6 can activate, whereas knockdown ATF6 by an siRNA-silencing approach inhibited, ER stress-mediated apoptosis in chondrogenesis induced by BMP2, as assayed by cleaved caspase3, CHOP, p-JNK expression in the course of chondrocyte differentiation. On the other hand, FCM, TUNEL assay and immunohistochemistry analysis also proved this result in vitro and in vivo. It was demonstrated that Ad-ATF6 activation of the ER stress-specific caspase cascade in developing chondrocyte tissue. Collectively, these findings reveal a novel critical role of ATF6 in regulating ER stress-mediated apoptosis in chondrocyte differentiation and the molecular mechanisms involved.

IRE1a constitutes a negative feedback loop with BMP2 and acts as a novel mediator in modulating osteogenic differentiation

Bone morphogenetic protein 2 (BMP2) is known to activate unfolded protein response (UPR) signaling molecules, such as BiP (IgH chain-binding protein), PERK (PKR-like ER-resistant kinase), and IRE1α. Inositol-requiring enzyme-1a (IRE1a), as one of three unfolded protein sensors in UPR signaling pathways, can be activated during ER stress. Granulin-epithelin precursor (GEP) is an autocrine growth factor that has been implicated in embryonic development, tissue repair, tumorigenesis, and inflammation. However, the influence on IRE1a in BMP2-induced osteoblast differentiation has not yet been elucidated. Herein we demonstrate that overexpression of IRE1a inhibits osteoblast differentiation, as revealed by reduced activity of alkaline phosphatase (ALP) and osteocalcin; however, knockdown of IRE1a via the RNAi approach stimulates osteoblastogenesis. Mechanistic studies revealed that the expression of IRE1a during osteoblast was a consequence of JunB transcription factor binding to several AP1 sequence (TGAG/CTCA) in the 5'-flanking regulatory region of the IRE1a gene, followed by transcription. In addition, GEP induces IRE1a expressions and this induction of IRE1a by GEP depends on JunB. Furthermore, IRE1a inhibition of GEP-induced osteoblastogenesis relies on JunB. Besides, GEP is required for IRE1a inhibition of BMP2-induced bone formation. Collectively, these findings demonstrate that IRE1a negatively regulates BMP2-induced osteoblast differentiation and this IRE1a inhibition effect depends on GEP growth factor. Thus, IRE1a, BMP2, GEP growth factor, and JunB transcription factor form a regulatory loop and act in concert in the course of osteoblastogenesis.

Transcriptional factor ATF6 is involved in odontoblastic differentiation

ATF6 is an endoplasmic reticulum (ER) membrane-bound transcription factor that regulates various cellular functions. The purpose of this study was to investigate the role of ATF6 in odontoblast differentiation. Rat tooth germs were isolated, changes in gene expression were evaluated over time, and localization of ATF6 was determined by immunohistochemistry. Human dental pulp cells (HDPCs) were cultured with 50 µg/mL ascorbic acid and 5 mmol/L β-glycerophosphate or 100 ng/mL bone morphogenetic protein 2 to induce differentiation. Translocation of ATF6 was observed by immunofluorescence and confocal microscopy. Overexpression of ATF6 was performed with an adenoviral vector. Matrix mineralization was evaluated by alizarin red staining. Immunoreactivity to anti-ATF6 was observed in the odontoblastic layer of the molar tooth germ, and expressions of ATF6, dentin sialophosphoprotein (DSPP) and dentin matrix protein 1 (DMP1) increased gradually during tooth germ development. When HDPCs were cultured in differentiation media, ATF6, DSPP, and DMP1 expression increased with the expression of unfolded protein response (UPR) markers, BiP and CHOP. Immunofluorescence results showed that ATF6 protein moved from cytoplasm to nucleus when cells were exposed to differentiation media. Notably, overexpression of ATF6 increased DSPP and DMP1 expression, alkaline phosphatase (ALP) activity, and matrix mineralization in HDPC cultures. Inhibition of ATF6 decreased ALP activity and mineralization. These results suggest that ER membrane-bound transcriptional factor ATF6 may be involved in odontoblastic differentiation.

Isoquercitrin and polyphosphate co-enhance mineralization of human osteoblast-like SaOS-2 cells via separate activation of two RUNX2 cofactors AFT6 and Ets1

Isoquercitrin, a dietary phytoestrogen, is a potential stimulator of bone mineralization used for prophylaxis of osteoporotic disorders. Here we studied the combined effects of isoquercitrin, a cell membrane permeable 3-O-glucoside of quercetin, and polyphosphate [polyP], a naturally occurring inorganic polymer inducing bone formation, on mineralization of human osteoblast-like SaOS-2 cells. Both compounds isoquercitrin and polyP induce at non-toxic concentrations the mineralization process of SaOS-2 cells. Co-incubation experiments revealed that isoquercitrin (at 0.1 and 0.3μM), if given simultaneously with polyP (as Ca(2+) salt; at 3, 10, 30 and 100μM) amplifies the mineralization-enhancing effect of the inorganic polymer. The biomineralization process induced by isoquercitrin and polyP is based on two different modes of action. After incubation of the cells with isoquercitrin or polyP the expression of the Runt-related transcription factor 2 [RUNX2] is significantly upregulated. In addition, isoquercitrin causes a strong increase of the steady-state-levels of the two co-activators of RUNX2, the activating transcription factor 6 [ATF6] and the Ets oncogene homolog 1 [Ets1]. The activating effect of isoquercitrin occurs via a signal transduction pathway involving ATF6, and by that, is independent from the induction cascade initiated by polyP. This conclusion is supported by the finding that isoquercitrin upregulates the expression of the gene encoding for osteocalcin, while polyP strongly increases the expression of the Ets1 gene and of the alkaline phosphatase. We show that the two compounds, polyP and isoquercitrin, have a co-enhancing effect on bone mineral formation and in turn might be of potential therapeutic value for prevention/treatment of osteoporosis.

SMILE inhibits BMP-2-induced expression of osteocalcin by suppressing the activity of the RUNX2 transcription factor in MC3T3E1 cells

Small heterodimer partner interacting leucine zipper protein (SMILE) is an orphan nuclear receptor and a member of the bZIP family of proteins. Several recent studies have suggested that SMILE is a novel co-repressor that is involved in nuclear receptor signaling; however, the role of SMILE in osteoblast differentiation has not yet been elucidated. This study demonstrates that SMILE inhibits osteoblast differentiation by regulating the activity of Runt-related transcription factor-2 (RUNX2). Tunicamycin, an inducer of endoplasmic reticulum stress, stimulated SMILE expression. Bone morphogenetic protein-2-induced expression of alkaline phosphatase and osteocalcin, both of which are osteogenic genes, was suppressed by SMILE. The molecular mechanism by which SMILE affects osteocalcin expression was also determined. An immunoprecipitation assay revealed a physical interaction between SMILE and RUNX2 that significantly impaired the RUNX2-dependent activation of the osteocalcin gene. A ChIP assay revealed that SMILE repressed the ability of RUNX2 to bind to the osteocalcin gene promoter. Taken together, these findings demonstrate that SMILE negatively regulates osteocalcin via a direct interaction with RUNX2.

XBP1S, a BMP2-inducible transcription factor, accelerates endochondral bone growth by activating GEP growth factor

We previously reported that transcription factor XBP1S binds to RUNX2 and enhances chondrocyte hypertrophy through acting as a cofactor of RUNX2. Herein, we report that XBP1S is a key downstream molecule of BMP2 and is required for BMP2-mediated chondrocyte differentiation. XBP1S is up-regulated during chondrocyte differentiation and demonstrates the temporal and spatial expression pattern during skeletal development. XBP1S stimulates chondrocyte differentiation from mesenchymal stem cells in vitro and endochondral ossification ex vivo. In addition, XBP1S activates granulin-epithelin precursor (GEP), a growth factor known to stimulate chondrogenesis, and endogenous GEP is required, at least in part, for XBP1S-stimulated chondrocyte hypertrophy, mineralization and endochondral bone formation. Furthermore, XBP1S enhances GEP-stimulated chondrogenesis and endochondral bone formation. Collectively, these findings demonstrate that XBP1S, a BMP2-inducible transcription factor, positively regulates endochondral bone formation by activating GEP chondrogenic growth factor.

Sox9 potentiates BMP2-induced chondrogenic differentiation and inhibits BMP2-induced osteogenic differentiation

Bone morphogenetic protein 2 (BMP2) is one of the key chondrogenic growth factors involved in the cartilage regeneration. However, it also exhibits osteogenic abilities and triggers endochondral ossification. Effective chondrogenesis and inhibition of BMP2-induced osteogenesis and endochondral ossification can be achieved by directing the mesenchymal stem cells (MSCs) towards chondrocyte lineage with chodrogenic factors, such as Sox9. Here we investigated the effects of Sox9 on BMP2-induced chondrogenic and osteogenic differentiation of MSCs. We found exogenous overexpression of Sox9 enhanced the BMP2-induced chondrogenic differentiation of MSCs in vitro. Also, it inhibited early and late osteogenic differentiation of MSCs in vitro. Subcutaneous stem cell implantation demonstrated Sox9 potentiated BMP2-induced cartilage formation and inhibited endochondral ossification. Mouse limb cultures indicated that BMP2 and Sox9 acted synergistically to stimulate chondrocytes proliferation, and Sox9 inhibited BMP2-induced chondrocytes hypertrophy and ossification. This study strongly suggests that Sox9 potentiates BMP2-induced MSCs chondrogenic differentiation and cartilage formation, and inhibits BMP2-induced MSCs osteogenic differentiation and endochondral ossification. Thus, exogenous overexpression of Sox9 in BMP2-induced mesenchymal stem cells differentiation may be a new strategy for cartilage tissue engineering.

ER stress-inducible ATF3 suppresses BMP2-induced ALP expression and activation in MC3T3-E1 cells

Endoplasmic reticulum (ER) stress suppresses osteoblast differentiation. Activating transcription factor (ATF) 3, a member of the ATF/cAMP response element-binding protein family of transcription factors, is induced by various stimuli including cytokines, hormones, DNA damage, and ER stress. However, the role of ATF3 in osteoblast differentiation has not been elucidated. Treatment with tunicamycin (TM), an ER stress inducer, increased ATF3 expression in the preosteoblast cell line, MC3T3-E1. Overexpression of ATF3 inhibited bone morphogenetic protein 2-stimulated expression and activation of alkaline phosphatase (ALP), an osteogenic marker. In addition, suppression of ALP expression by TM treatment was rescued by silencing of ATF3 using shRNA. Taken together, these data indicate that ATF3 is a novel negative regulator of osteoblast differentiation by specifically suppressing ALP gene expression in preosteoblasts.

ATF6 upregulates XBP1S and inhibits ER stress-mediated apoptosis in osteoarthritis cartilage

As we previously reported, transcription factor XBP1S enhances BMP2-induced chondrocyte differentiation and acts as a positive mediator of chondrocyte hypertrophy. The purpose of this study was to determine (1) whether XBP1S influences ER stress-mediated apoptosis in osteoarthritis (OA); (2) whether ATF6 regulates IRE1/XBP1 signal pathway in OA cartilage; (3) what are the associated molecules affecting apoptosis in osteoarthritis and the molecular events underlying this process. Herein, we examined and found that ER stress-associated molecules were activated in OA patients, specifically XBP1S splice and expression were increased markedly by TNF-α and IL-1β treatments. Transcription factor ATF6 can specifically bind to the promoter of XBP1 gene and enhance the expression of XBP1S spliced by IRE1α in osteoarthritis cartilage. Furthermore, siXBP1S can enhance ER stress-mediated apoptosis and main matrix degradation in osteoarthritis. Whereas AdXBP1S can inhibit ER stress-mediated apoptosis and TNFα induced nitrite production in OA cartilage. In a word, our observations demonstrate the importance of XBP1S in osteoarthritis. ATF6 and IRE1α can regulate endogenous XBP1S gene expression synergistically in OA cartilage. More significantly, XBP1S was a negative regulator of apoptosis in osteoarthritis by affecting caspase 3, caspase 9, caspase 12, p-JNK1, and CHOP.

Gene expression profiles in human and mouse primary cells provide new insights into the differential actions of vitamin D3 metabolites

1α,25-Dihydroxyvitamin D₃ (1α,25(OH)₂D₃) had earlier been regarded as the only active hormone. The newly identified actions of 25-hydroxyvitamin D₃ (25(OH)D₃) and 24R,25-dihydroxyvitamin D₃ (24R,25(OH)₂D₃) broadened the vitamin D₃ endocrine system, however, the current data are fragmented and a systematic understanding is lacking. Here we performed the first systematic study of global gene expression to clarify their similarities and differences. Three metabolites at physiologically comparable levels were utilized to treat human and mouse fibroblasts prior to DNA microarray analyses. Human primary prostate stromal P29SN cells (hP29SN), which convert 25(OH)D₃ into 1α,25(OH)₂D₃ by 1α-hydroxylase (encoded by the gene CYP27B1), displayed regulation of 164, 171, and 175 genes by treatment with 1α,25(OH)₂D₃, 25(OH)D₃, and 24R,25(OH)₂D₃, respectively. Mouse primary Cyp27b1 knockout fibroblasts (mCyp27b1^−/−), which lack 1α-hydroxylation, displayed regulation of 619, 469, and 66 genes using the same respective treatments. The number of shared genes regulated by two metabolites is much lower in hP29SN than in mCyp27b1^−/−. By using DAVID Functional Annotation Bioinformatics Microarray Analysis tools and Ingenuity Pathways Analysis, we identified the agonistic regulation of calcium homeostasis and bone remodeling between 1α,25(OH)₂D₃ and 25(OH)D₃ and unique non-classical actions of each metabolite in physiological and pathological processes, including cell cycle, keratinocyte differentiation, amyotrophic lateral sclerosis signaling, gene transcription, immunomodulation, epigenetics, cell differentiation, and membrane protein expression. In conclusion, there are three distinct vitamin D₃ hormones with clearly different biological activities. This study presents a new conceptual insight into the vitamin D₃ endocrine system, which may guide the strategic use of vitamin D₃ in disease prevention and treatment.

Role of endoplasmic reticulum stress in aberrant activation of fluoride-treated osteoblasts

The aberrant activation of osteoblasts in the early stage is one of the critical steps during the pathogenesis of skeletal fluorosis. The endoplasmic reticulum (ER) stresses and unfolded protein response (UPR) are initiated to alleviate the accumulation of unfolded proteins against cell injury. The previous researches had demonstrated that fluoride induced ER stress in other cells or tissues. In this study, we determined the ER stress and UPR to investigate their roles in aberrant activation of fluoride-treated osteoblasts. The gene expression of bone markers and UPR factors in MC3T3-E1 cells treated with varying doses of fluoride administration was analyzed. Meantime, levels of glutathione and glutathione disulfide were tested by the ultraperformance liquid chromatography-tandem mass spectrometry applications. Our results indicated that a certain dose and period of fluoride administration induced cell proliferation and differentiation, and Runx2 was involved in the regulation of osteoblastic differentiation of MC3T3-E1 cells. Increase trend of Runx2 expression was consistent with change of marker of ER stress. Fluoride caused ER stress and stimulated UPR during the process of osteoblast maturation, while oxidative stress was also active in the occurrence of ER stress. These data indicated that ER stress and UPR were possibly involved in the action of fluoride on osteoblasts.

In vitro proliferation and osteogenic differentiation of mesenchymal stem cells on nanoporous alumina

Cell adhesion, migration, and proliferation are significantly affected by the surface topography of the substrates on which the cells are cultured. Alumina is one of the most popular implant materials used in orthopedics, but few data are available concerning the cellular responses of mesenchymal stem cells (MSCs) grown on nanoporous structures. MSCs were cultured on smooth alumina substrates and nanoporous alumina substrates to investigate the interaction between surface topographies of nanoporous alumina and cellular behavior. Nanoporous alumina substrates with pore sizes of 20 nm and 100 nm were used to evaluate the effect of pore size on MSCs as measured by proliferation, morphology, expression of integrin β1, and osteogenic differentiation. An MTT assay was used to measure cell viability of MSCs on different substrates, and determined that cell viability decreased with increasing pore size. Scanning electron microscopy was used to investigate the effect of pore size on cell morphology. Extremely elongated cells and prominent cell membrane protrusions were observed in cells cultured on alumina with the larger pore size. The expression of integrin β1 was enhanced in MSCs cultured on porous alumina, revealing that porous alumina substrates were more favorable for cell growth than smooth alumina substrates. Higher levels of osteoblastic differentiation markers such as alkaline phosphatase, osteocalcin, and mineralization were detected in cells cultured on alumina with 100 nm pores compared with cells cultured on alumina with either 20 nm pores or smooth alumina. This work demonstrates that cellular behavior is affected by variation in pore size, providing new insight into the potential application of this novel biocompatible material for the developing field of tissue engineering.

Low osteogenic differentiation potential of placenta-derived mesenchymal stromal cells correlates with low expression of the transcription factors Runx2 and Twist2

Recent studies indicated that mesenchymal stromal cells from bone marrow (bmMSC) differ in their osteogenic differentiation capacity compared to MSC from term placenta (pMSC). We extended these studies and investigated the expression of factors involved in regulation of bone metabolism in both cell types. To this end, MSC were expanded in vitro and characterized. The total transcriptome was investigated by microarrays, and for selected genes, the differences in gene expression were explored by quantitative reverse transcriptase-polymerase chain reaction, immunocytochemistry, and flow cytometry. We report that bmMSC and pMSC share expression of typical lineage surface markers, including CD73, CD90, CD105, and lack of CD14, CD34, and CD45. However, according to transcriptome analyses, they differ significantly in their expression of more than 590 genes. Factors involved in bone metabolism, including alkaline phosphatase (P<0.05), osteoglycin (P<0.05), osteomodulin (P<0.05), runt-related transcription factor 2 (Runx2) (P<0.04), and WISP2 (P<0.05), were expressed at significantly lower levels in pMSC, but twist-related protein 2 (Twist2) (P<0.0002) was expressed at significantly higher levels. The osteogenic differentiation capacity of pMSC was very low. The adipogenic differentiation was somewhat more prominent in bmMSC, while the chondrogenic differentiation seemed not to differ between bmMSC and pMSC, as determined by histochemical staining. However, expression and induction of peroxisome proliferator-activated receptor gamma-2 (PPARγ2) and Sox9, factors involved in early adipogenesis and chondrogenesis, respectively, were higher in bmMSC. We conclude that despite many similarities between bmMSC and pMSC, when expanded under identical conditions, they vary considerably with respect to their in vitro differentiation potential. For regenerative purposes, the choice of MSC may therefore influence the outcome of a treatment considerably.

IRE1α dissociates with BiP and inhibits ER stress-mediated apoptosis in cartilage development

Bone morphogenetic protein 2 is known to activate unfolded protein response signaling molecules, including XBP1S, BiP and IRE1α. Endoplasmic reticulum stress is induced in chondrogenesis and activates IRE1α signal pathway, which is associated with ER stress-mediated apoptosis. However, the influence on IRE1α and BiP in BMP2-induced chondrocyte differentiation has not yet been elucidated; the molecular mechanism remains unexplored. In this study, we demonstrate that IRE1α interacts with BiP in unstressed cells and dissociates from BiP in the course of cartilage development. Induction of ER stress-responsive proteins (XBP1S, IRE1α, BiP) was also observed in differentiating cells. IRE1α inhibition ER stress-mediated apoptosis lies in the process of chondrocyte differentiation. Furthermore, knockdown of IRE1α expression by way of the RNAi approach accelerates ER stress-mediated apoptosis in chondrocyte differentiation induced by BMP2, as revealed by enhanced expressions of cleaved caspase3, CHOP and p-JNK1; and this IRE1α inhibition effect on ER stress-mediated apoptosis is required for BiP in chondrogenesis. Collectively, the ER stress sensors were activated during apoptosis in cartilage development, suggesting that selective activation of ER stress signaling was sufficient for induction of apoptosis. These findings reveal a novel critical role of IRE1α in ER stress-mediated apoptosis and the molecular mechanisms involved. These results suggest that activation of p-JNK1, caspase3 and CHOP was detected in developing chondrocytes and that specific ER stress signaling leads to naturally occurring apoptosis during cartilage development.

Fluid shear stress and melatonin in combination activate anabolic proteins in MC3T3-E1 osteoblast cells

In this study, we investigated whether fluid shear stress and melatonin in combination stimulate the anabolic proteins through the phosphorylation of extracellular signal-regulated kinase (p-ERK) in MC3T3-E1 osteoblast cells. First, we researched why fluid shear stress and melatonin in combination influence cell survival. Fluid shear stress (1 hr) and melatonin (1 mM) in combination reduced autophagic marker LC3-II compared with fluid shear stress (1 hr) and/or melatonin (0.1 mM). Under the same conditions for fluid shear stress, markers of cell survival signaling pathway p-ERK, phosphorylation of serine-threonine protein kinase (p-Akt), phosphorylation of mammalian target of rapamycin (p-mTOR), and p85-S6K were investigated. p-Akt, p-mTOR (Ser 2481) expressions increased with the addition of 1 mM melatonin prior to 0.1 mM melatonin treatment. However, p-S6K expression did not change significantly. Next, mitochondria activity including Bcl-2, Bax, catalase, and Mn-superoxide dismutase (Mn-SOD) were studied. Expressions of Bcl-2, Bax, and catalase proteins were low under fluid shear stress plus 1 mM melatonin compared with only fluid shear stress alone, whereas Mn-SOD expression was high compared with conditions of no fluid shear stress. Finally, the anabolic proteins of bone, osteoprotegerin, type I collagen (collagen I), and bone sialoprotein II (BSP II) were checked. These proteins increased with combined fluid shear stress (1, 4 hr) and melatonin (0.1, 1 mM). Together, these results suggest that fluid shear stress and melatonin in combination may increase the expression of anabolic proteins through the p-ERK in MC3T3-E1 osteoblast cells. Therefore, fluid shear stress in combination with melatonin may promote the anabolic response of osteoblasts.

Osteogenesis of mesenchymal stem cells by nanoscale mechanotransduction

It is likely that mesenchymal stem cells will find use in many autologous regenerative therapies. However, our ability to control cell stem growth and differentiation is presently limited, and this is a major hurdle to the clinical use of these multipotent cells especially when considering the desire not to use soluble factors or complex media formulations in culture. Also, the large number of cells required to be clinically useful is currently a hurdle to using materials-based (stiffness, chemistry, nanotopography, etc.) culture substrates. Here we give a first demonstration of using nanoscale sinusoidal mechanotransductive protocols (10-14 nm displacements at 1 kHz frequency), "nanokicking", to promote osteoblastogenesis in human mesenchymal stem cell cultures. On the basis of application of the reverse piezo effect, we use interferometry to develop the optimal stem cell stimulation conditions, allowing delivery of nanoscale cues across the entire surface of the Petri dishes used. A combination of immunofluorescence, PCR, and microarray has then been used to demonstrate osteoblastogenesis, and the arrays implicate RhoA as central to osteoblastic differentiation in agreement with materials-based strategies. We validate this with pharmacological inhibition of RhoA kinase. It is easy to envisage such stimulation protocols being up-scaled to form large-scale osteoblast bioreactors as standard cell culture plates and incubators are used in the protocol.

High dickkopf-1 levels in sera and leukocytes from children with 21-hydroxylase deficiency on chronic glucocorticoid treatment

Children with 21-hydroxylase deficiency (21-OHD) need chronic glucocorticoid (cGC) therapy to replace congenital deficit of cortisol synthesis, and this therapy is the most frequent and severe form of drug-induced osteoporosis. In this study, we enrolled 18 patients (9 females) and 18 sex- and age-matched controls. We found in 21-OHD patients high serum and leukocyte levels of dickkopf-1 (DKK1), a secreted antagonist of the Wnt/β-catenin signaling pathway known to be a key regulator of bone mass. In particular, we demonstrated by flow cytometry, confocal microscopy, and real-time PCR that monocytes, T lymphocytes, and neutrophils from patients expressed high levels of DKK1, which may be related to the cGC therapy. In fact, we showed that dexamethasone treatment markedly induced the expression of DKK1 in a dose- and time-dependent manner in leukocytes. The serum from patients containing elevated levels of DKK1 can directly inhibit in vitro osteoblast differentiation and receptor activator of NF-κB ligand (RANKL) expression. We also found a correlation between both DKK1 and RANKL or COOH-terminal telopeptides of type I collagen (CTX) serum levels in 21-OHD patients on cGC treatment. Our data indicated that DKK1, produced by leukocytes, may contribute to the alteration of bone remodeling in 21-OHD patients on cGC treatment.

Effect of bone morphogenetic protein-2 on tendon-bone integration in an in vitro cell culture

The goal of this study was to evaluate the influence of bone morphogenetic protein-2 (BMP-2) on tendon-bone integration in a bovine in vitro cell culture. Seventy-two bovine tendons were cultivated over 3 months. The effects of BMP-2 were evaluated by generation in 4 subgroups. The groups differed in 2 parameters: the application of BMP-2 and the application of primary bovine osteoblasts. Results were analyzed biochemically by determining alkaline phosphatase activity and histologic tendon calcification, both markers for graft incorporation. Histological analysis demonstrated a positive effect of BMP-2 on the production of extracellular matrix and therefore the induction of osteogenesis. In addition, the results showed a superior cell ingrowth on the tendon in the BMP-2-stimulated groups. Calcium carbonate-like structures and organized ossification zones could only be detected in the BMP-2-stimulated tendons. The histological results matched those of the biochemical alkaline phosphatase analysis. The highest alkaline phosphatase activity was detected using BMP-2 stimulation in the first month (P<.001). High alkaline phosphatase values suggest high osteoblast activity and a high potential for mineralization. Furthermore, a positive effect of BMP-2 on fibroblasts existed with regard to the overall integration process. These results confirm the positive influence and triggering effect of BMP-2 on the mineralization process. Bone morphogenetic protein-2 seems to accelerate and optimize tendon-bone integration in the early process of graft incorporation. Besides the influence of BMP-2 on bovine osteoblasts, an additional positive effect of BMP-2 on bovine fibroblasts was detected; therefore, graft incorporation may be carried out by osteoblasts and fibroblasts.

Inorganic polyphosphates: biologically active biopolymers for biomedical applications

Inorganic polyphosphate (polyP) is a widely occurring but only rarely investigated biopolymer which exists in both prokaryotic and eukaryotic organisms. Only in the last few years, this polymer has been identified to cause morphogenetic activity on cells involved in human bone formation. The calcium complex of polyP was found to display a dual effect on bone-forming osteoblasts and bone-resorbing osteoclasts. Exposure of these cells to polyP (Ca(2+) complex) elicits the expression of cytokines that promote the mineralization process by osteoblasts and suppress the differentiation of osteoclast precursor cells to the functionally active mature osteoclasts dissolving bone minerals. The effect of polyP on bone formation is associated with an increased release of the bone morphogenetic protein 2 (BMP-2), a key mediator that activates the anabolic processes leading to bone formation. In addition, polyP has been shown to act as a hemostatic regulator that displays various effects on blood coagulation and fibrinolysis and might play an important role in platelet-dependent proinflammatory and procoagulant disorders.

PARM-1 promotes cardiomyogenic differentiation through regulating the BMP/Smad signaling pathway

PARM-1, prostatic androgen repressed message-1, is an endoplasmic reticulum (ER) molecule that is involved in ER stress-induced apoptosis in cardiomyocytes. In this study, we assessed whether PARM-1 plays a role in the differentiation of stem cells into cardiomyocytes. While PARM-1 was not expressed in undifferentiated P19CL6 embryonic carcinoma cells, PARM-1 expression was induced during cardiomyogenic differentiation. This expression followed expression of mesodermal markers, and preceded expression of cardiac transcription factors. PARM-1 overexpression did not alter the expression of undifferentiated markers and the proliferative property in undifferentiated P19CL6 cells. Expression of cardiac transcription factors during cardiomyogenesis was markedly enhanced by overexpression of PARM-1, while expression of mesodermal markers was not altered, suggesting that PARM-1 is involved in the differentiation from the mesodermal lineage to cardiomyocytes. Furthermore, overexpression of PARM-1 induced BMP2 mRNA expression in undifferentiated P19CL6 cells and enhanced both BMP2 and BMP4 mRNA expression in the early phase of cardiomyogenesis. PARM-1 overexpression also enhanced phosphorylation of Smads1/5/8. Thus, PARM-1 plays an important role in the cardiomyogenic differentiation of P19CL6 cells through regulating BMP/Smad signaling pathways, demonstrating a novel role of PARM-1 in the cardiomyogenic differentiation of stem cells.

Mechanical strain promotes osteoblast ECM formation and improves its osteoinductive potential

BACKGROUND

The extracellular matrix (ECM) provides a supportive microenvironment for cells, which is suitable as a tissue engineering scaffold. Mechanical stimulus plays a significant role in the fate of osteoblast, suggesting that it regulates ECM formation. Therefore, we investigated the influence of mechanical stimulus on ECM formation and bioactivity.

METHODS

Mouse osteoblastic MC3T3-E1 cells were cultured in cell culture dishes and stimulated with mechanical tensile strain. After removing the cells, the ECMs coated on dishes were prepared. The ECM protein and calcium were assayed and MC3T3-E1 cells were re-seeded on the ECM-coated dishes to assess osteoinductive potential of the ECM.

RESULTS

The cyclic tensile strain increased collagen, bone morphogenetic protein 2 (BMP-2), BMP-4, and calcium levels in the ECM. Compared with the ECM produced by unstrained osteoblasts, those of mechanically stimulated osteoblasts promoted alkaline phosphatase activity, elevated BMP-2 and osteopontin levels and mRNA levels of runt-related transcriptional factor 2 (Runx2) and osteocalcin (OCN), and increased secreted calcium of the re-seeded MC3T3-E1 cells.

CONCLUSION

Mechanical strain promoted ECM production of osteoblasts in vitro, increased BMP-2/4 levels, and improved osteoinductive potential of the ECM. This study provided a novel method to enhance bioactivity of bone ECM in vitro via mechanical strain to osteoblasts.

The unfolded protein response in secretory cell function

The endoplasmic reticulum (ER) controls many important aspects of cellular function, including processing of secreted and membrane proteins, synthesis of membranes, and calcium storage. Maintenance of ER function is controlled through a network of signaling pathways collectively known as the unfolded protein response (UPR). The UPR balances the load of incoming proteins with the folding capacity of the ER and allows cells to adapt to situations that disrupt this balance. This disruption is referred to as ER stress. Although ER stress often arises in pathological situations, the UPR plays a central role in the normal development and function of cells specializing in secretion. Many aspects of this response are conserved broadly across eukaryotes; most organisms use some subset of a group of ER transmembrane proteins to signal to the nucleus and induce a broad transcriptional upregulation of genes involved in ER function. However, new developments in metazoans, plants, and fungi illustrate interesting variations on this theme. Here, we summarize mechanisms for detecting and counteracting ER stress, the role of the UPR in normal secretory cell function, and how these pathways vary across organisms and among different tissues and cell types.

XBP1S associates with RUNX2 and regulates chondrocyte hypertrophy

BMP2 (bone morphogenetic protein 2) is known to activate unfolded protein response signaling molecules, including XBP1S and ATF6. However, the influence on XBP1S and ATF6 in BMP2-induced chondrocyte differentiation has not yet been elucidated. In this study, we demonstrate that BMP2 mediates mild endoplasmic reticulum stress-activated ATF6 and directly regulates XBP1S splicing in the course of chondrogenesis. XBP1S is differentially expressed during BMP2-stimulated chondrocyte differentiation and exhibits prominent expression in growth plate chondrocytes. This expression is probably due to the activation of the XBP1 gene by ATF6 and splicing by IRE1a. ATF6 directly binds to the 5'-flanking regulatory region of the XBP1 gene at its consensus binding elements. Overexpression of XBP1S accelerates chondrocyte hypertrophy, as revealed by enhanced expression of type II collagen, type X collagen, and RUNX2; however, knockdown of XBP1S via the RNAi approach abolishes hypertrophic chondrocyte differentiation. In addition, XBP1S associates with RUNX2 and enhances RUNX2-induced chondrocyte hypertrophy. Altered expression of XBP1S in chondrocyte hypertrophy was accompanied by altered levels of IHH (Indian hedgehog) and PTHrP (parathyroid hormone-related peptide). Collectively, XBP1S may be a novel regulator of hypertrophic chondrocyte differentiation by 1) acting as a cofactor of RUNX2 and 2) affecting IHH/PTHrP signaling.