Role of HIF-1alpha in skeletal development

Osteogenic differentiation of adipose-derived stem cells is hypoxia-inducible factor-1 independent

Tissue engineering is a promising approach to repair critical-size defects in bone. Damage to vasculature at the defect site can create a lower O2 environment compared with healthy bone. Local O2 levels influence stem cell behavior, as O2 is not only a nutrient, but also a signaling molecule. The hypoxia-inducible factor-1 (HIF-1) is a transcription factor that regulates a wide range of O2-related genes and its contribution in bone repair/formation is an important area that can be exploited. In this study, we examined the effect of low O2 environments (1% and 2% O2) on the osteogenic differentiation of adipose-derived stem cells in both two-dimensional (2-D) and three-dimensional (3-D) culture systems. To determine the role of HIF-1 in the differentiation process, an inhibitor was used to block the HIF-1 activity. The samples were examined for osteogenesis markers as measured by quantification of the alkaline phosphatase (ALP) activity, mineral deposition, and expression of osteonectin (ON) and osteopontin (OPN). Results show a downregulation of the osteogenic markers (ALP activity, mineralization, ON, OPN) in both 1% and 2% O2 when compared to 20% O2 in both 2-D and 3-D culture. Vascular endothelial growth factor secretion over 28 days was significantly higher in low O2 environments and HIF-1 inhibition reduced this effect. The inhibition of the HIF-1 activity did not have a significant impact on the expression of the osteogenic markers, suggesting HIF-1-independent inhibition of osteogenic differentiation in hypoxic conditions.

BMP9 signaling in stem cell differentiation and osteogenesis

Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily and play a critical role in skeletal development, bone formation and stem cell differentiation. Disruptions in BMP signaling result in a variety of skeletal and extraskeletal anomalies. BMP9 is a poorly characterized member of the BMP family and is among the most osteogenic BMPs, promoting osteoblastic differentiation of mesenchymal stem cells (MSCs) both in vitro and in vivo. Recent findings from various in vivo and molecular studies strongly suggest that the mechanisms governing BMP9-mediated osteoinduction differ from other osteogenic BMPs. Many signaling pathways with diverse functions have been found to play a role in BMP9-mediated osteogenesis. Several of these pathways are also critical in the differentiation of other cell lineages, including adipocytes and chondrocytes. While BMP9 is known to be a potent osteogenic factor, it also influences several other pathways including cancer development, angiogenesis and myogenesis. Although BMP9 has been demonstrated as one of the most osteogenic BMPs, relatively little is known about the specific mechanisms responsible for these effects. BMP9 has demonstrated efficacy in promoting spinal fusion and bony non-union repair in animal models, demonstrating great translational promise. This review aims to summarize our current knowledge of BMP9-mediated osteogenesis by presenting recently completed work which may help us to further elucidate these pathways.

Cross-talk between skeletal muscle and immune cells: muscle-derived mediators and metabolic implications

Skeletal muscles contain resident immune cell populations and their abundance and type is altered in inflammatory myopathies, endotoxemia or different types of muscle injury/insult. Within tissues, monocytes differentiate into macrophages and polarize to acquire pro- or anti-inflammatory phenotypes. Skeletal muscle macrophages play a fundamental role in repair and pathogen clearance. These events require a precisely regulated cross-talk between myofibers and immune cells, involving paracrine/autocrine and contact interactions. Skeletal muscle also undergoes continuous repair as a result of contractile activity that involves participation of myokines and anti-inflammatory input. Finally, skeletal muscle is the major site of dietary glucose disposal; therefore, muscle insulin resistance is essential to the development of whole body insulin resistance. Notably, muscle inflammation is emerging as a potential contributor to insulin resistance. Recent reports show that inflammatory macrophage numbers within muscle are elevated during obesity and that muscle cells in vitro can mount autonomous inflammatory responses under metabolic challenge. Here, we review the nature of skeletal muscle inflammation associated with muscle exercise, damage, and regeneration, endotoxin presence, and myopathies, as well as the new evidence of local inflammation arising with obesity that potentially contributes to insulin resistance.

Oxygenated environment enhances both stem cell survival and osteogenic differentiation

Osteogenesis of mesenchymal stem cells (MSCs) is highly dependent on oxygen supply. We have shown that perfluorotributylamine (PFTBA), a synthetic oxygen carrier, enhances MSC-based bone formation in vivo. Exploring this phenomenon's mechanism, we hypothesize that a transient increase in oxygen levels using PFTBA will affect MSC survival, proliferation, and differentiation, thus increasing bone formation. To test this hypothesis, MSCs overexpressing bone morphogenetic protein 2 were encapsulated in alginate beads that had been supplemented with an emulsion of PFTBA or phosphate-buffered saline. Oxygen measurements showed that supplementation of PFTBA significantly increased the available oxygen level during a 96-h period. PFTBA-containing beads displayed an elevation in cell viability, which was preserved throughout 2 weeks, and a significantly lower ratio of dead cells throughout the experiment. Furthermore, the cells from the control group expressed significantly more hypoxia-related genes such as VEGF, DDIT3, and PKG1. Additionally, PFTBA supplementation led to an increase in the osteogenic differentiation and to a decrease in chondrogenic differentiation of MSCs. In conclusion, PFTBA increases the oxygen availability in the vicinity of the MSCs, which suffer oxygen exhaustion shortly after encapsulation in alginate beads. Consequently, cell survival is increased, and hypoxia-related genes are downregulated. In addition, PFTBA promotes osteogenic differentiation over chondrogeneic differentiation, and thereby can accelerate MSC-based bone regeneration.

Synergistic inhibition of Wnt pathway by HIF-1α and osteoblast-specific transcription factor osterix (Osx) in osteoblasts

Osterix (Osx) is an osteoblast-specific transcription factor required for osteoblast differentiation. Inhibition of Wnt pathway by Osx highlights the potential for feedback control mechanisms involved in bone formation. Hypoxia-inducible factor-1α (HIF-1α) is a master regulator of hypoxia. HIF-1α has been reported to couple angiogenesis to osteogenesis. Our recent study has demonstrated that Osx and HIF-1α cooperatively regulate VEGF expression in osteoblasts. Effects of hypoxia/HIF-1α on osteoblast proliferation and related mechanisms are not well understood. In this study, osteoblast growth under hypoxia was examined. We observed that osteoblast growth was inhibited under hypoxia. To explore possible mechanisms for hypoxia/HIF-1α to inhibit osteoblast proliferation, we tested the effect of hypoxia/HIF-1α on Wnt pathway. Quantitative RT-PCR results revealed that Wnt target genes such as cyclin D1 and c-Myc were downregulated under hypoxia while HIF-1α was upregulated. Treatment of desferrioxamine, a HIF-1α activator, led to further downregulation of expressions of cyclin D1 and c-Myc in osteoblasts. On the contrary, the inhibition of HIF-1α by siRNA in osteoblasts led to the expression increase of cyclin D1 and c-Myc. These data suggest that HIF-1α inhibits Wnt pathway in osteoblasts. To examine the effect of HIF-1α on Wnt pathway, HIF-1α was cotransfected with β-catenin along with Topflash reporter in transient transfection assay. Our results showed that HIF-1α inhibited β-catenin-induced Topflash reporter activity. Interestingly, a synergistic interplay was observed between Osx and HIF-1α in the inhibition of β-catenin-induced Topflash expression. Our findings indicate that Osx and HIF-1α cooperatively inhibit Wnt pathway. This study revealed additional new information of the cooperation between HIF-1α and Osx in osteoblasts.

BMP9-regulated angiogenic signaling plays an important role in the osteogenic differentiation of mesenchymal progenitor cells

Mesenchymal stromal progenitor cells (MSCs) are multipotent progenitors that can be isolated from numerous tissues. MSCs can undergo osteogenic differentiation under proper stimuli. We have recently demonstrated that bone morphogenetic protein 9 (BMP9) is one of the most osteogenic BMPs. As one of the least studied BMPs, BMP9 has been shown to regulate angiogenesis in endothelial cells. However, it is unclear whether BMP9-regulated angiogenic signaling plays any important role in the BMP9-initiated osteogenic pathway in MSCs. Here, we investigate the functional role of hypoxia-inducible factor 1α (HIF1α)-mediated angiogenic signaling in BMP9-regulated osteogenic differentiation of MSCs. We find that BMP9 induces HIF1α expression in MSCs through Smad1/5/8 signaling. Exogenous expression of HIF1α potentiates BMP9-induced osteogenic differentiation of MSCs both in vitro and in vivo. siRNA-mediated silencing of HIF1α or HIF1α inhibitor CAY10585 profoundly blunts BMP9-induced osteogenic signaling in MSCs. HIF1α expression regulated by cobalt-induced hypoxia also recapitulates the synergistic effect between HIF1α and BMP9 in osteogenic differentiation. Mechanistically, HIF1α is shown to exert its synergistic effect with BMP9 by inducing both angiogenic signaling and osteogenic signaling in MSCs. Thus, our findings should not only expand our understanding of the molecular basis behind BMP9-regulated osteoblastic lineage-specific differentiation, but also provide an opportunity to harness the BMP9-induced synergy between osteogenic and angiogenic signaling pathways in regenerative medicine.

Role of Slug transcription factor in human mesenchymal stem cells

The pathways that control mesenchymal stem cells (MSCs) differentiation are not well understood, and although some of the involved transcription factors (TFs) have been characterized, the role of others remains unclear. We used human MSCs from tibial plateau (TP) trabecular bone, iliac crest (IC) bone marrow and Wharton’s jelly (WJ) umbilical cord demonstrating a variability in their mineral matrix deposition, and in the expression levels of TFs including Runx2, Sox9, Sox5, Sox6, STAT1 and Slug, all involved in the control of osteochondroprogenitors differentiation program. Because we reasoned that the basal expression level of some TFs with crucial role in the control of MSC fate may be correlated with osteogenic potential, we considered the possibility to affect the hMSCs behaviour by using gene silencing approach without exposing cells to induction media. In this study we found that Slug-silenced cells changed in morphology, decreased in their migration ability, increased Sox9 and Sox5 and decreased Sox6 and STAT1 expression. On the contrary, the effect of Slug depletion on Runx2 was influenced by cell type. Interestingly, we demonstrated a direct in vivo regulatory action of Slug by chromatin immunoprecipitation, showing a specific recruitment of this TF in the promoter of Runx2 and Sox9 genes. As a whole, our findings have important potential implication on bone tissue engineering applications, reinforcing the concept that manipulation of specific TF expression levels may elucidate MSC biology and the molecular mechanisms, which promote osteogenic differentiation.

The role of Connexin 46 promoter in lens and other hypoxic tissues

Gap junctions are multimeric membrane protein channels that connect the cytoplasm of one cell to another. Much information about connexins regards electrophysiology and channel function but relatively little information is known about non-channel functions of connexins. Lens connexins, Cx43, Cx46 and Cx50, have been extensively studied for their role in lens homeostasis. Connexins allow the movement of small metabolically relevant molecules and ions between cells and this action in the lens prevents cataract formation. Interruption of Cx46 channel function leads to cataract formation due to dysregulation of lens homeostasis. The loss of Cx46 upregulates Cx43 in lens cell culture and suppresses tumor growth in breast and retinoblastoma tumor xenografts. Upregulation of Cx46 in hypoxic tissues has been noted and may be due in part to the effects of hypoxia and HIF activators. Here, we report that the Cx46 promoter is regulated by hypoxia and also offer speculation about the role of Cx46 in lens differentiation and solid tumor growth.

Hypoxia promotes osteogenesis but suppresses adipogenesis of human mesenchymal stromal cells in a hypoxia-inducible factor-1 dependent manner

BACKGROUND

Bone fracture initiates a series of cellular and molecular events including the expression of hypoxia-inducible factor (HIF)-1. HIF-1 is known to facilitate recruitment and differentiation of multipotent human mesenchymal stromal cells (hMSC). Therefore, we analyzed the impact of hypoxia and HIF-1 on the competitive differentiation potential of hMSCs towards adipogenic and osteogenic lineages.

METHODOLOGY/PRINCIPAL FINDINGS

Bone marrow derived primary hMSCs cultured for 2 weeks either under normoxic (app. 18% O(2)) or hypoxic (less than 2% O(2)) conditions were analyzed for the expression of MSC surface markers and for expression of the genes HIF1A, VEGFA, LDHA, PGK1, and GLUT1. Using conditioned medium, adipogenic or osteogenic differentiation as verified by Oil-Red-O or von-Kossa staining was induced in hMSCs under either normoxic or hypoxic conditions. The expression of HIF1A and VEGFA was measured by qPCR. A knockdown of HIF-1α by lentiviral transduction was performed, and the ability of the transduced hMSCs to differentiate into adipogenic and osteogenic lineages was analyzed. Hypoxia induced HIF-1α and HIF-1 target gene expression, but did not alter MSC phenotype or surface marker expression. Hypoxia (i) suppressed adipogenesis and associated HIF1A and PPARG gene expression in hMSCs and (ii) enhanced osteogenesis and associated HIF1A and RUNX2 gene expression. shRNA-mediated knockdown of HIF-1α enhanced adipogenesis under both normoxia and hypoxia, and suppressed hypoxia-induced osteogenesis.

CONCLUSIONS/SIGNIFICANCE

Hypoxia promotes osteogenesis but suppresses adipogenesis of human MSCs in a competitive and HIF-1-dependent manner. We therefore conclude that the effects of hypoxia are crucial for effective bone healing, which may potentially lead to the development of novel therapeutic approaches.

Osteoblast-specific transcription factor Osterix (Osx) and HIF-1α cooperatively regulate gene expression of vascular endothelial growth factor (VEGF)

Bone formation is a highly regulated process involving the differentiation of mesenchymal stem cells to osteoblasts. Angiogenesis and osteogenesis are tightly coupled during bone formation. Vascular endothelial growth factor (VEGF) is involved in both processes. Relatively little is known about VEGF gene regulation in osteoblasts. Osterix (Osx) is a bone morphogenetic protein 2 (BMP-2) inducible osteoblast-specific transcription factor required for osteoblast differentiation and bone formation. Our recent study has demonstrated that Osx controls VEGF expression in osteoblasts. Here, we further characterized Osx regulation of VEGF. To address which domain of Osx is responsible for VEGF regulation, the deletion mutant analysis and transfection assay were carried out to show that proline-rich region (PRR) is required for Osx activation of VEGF promoter activity. Hypoxia-inducible factor-1α (HIF-1α) has been reported to couple angiogenesis to osteogenesis, and to upregulate VEGF. Effect of Osx on HIF-1α expression was examined in this study. Quantitative RT-PCR results revealed that HIF-1α remained unchanged between wild type and Osx knockout calvaria at E18.5 in mouse embryos. Overexpression of Osx in stable C2C12 mesenchymal cells using Tet-off system did not affect HIF-1α expression. HIF-1α level did not change after Osx inhibition by siRNA in osteoblasts. Moreover, BMP-2 stimulation led to upregulation of Osx and VEGF, but not HIF-1α. These results demonstrate that HIF-1α is not a downstream target of Osx in osteoblasts, suggesting that Osx regulation of VEGF is independent of HIF-1α expression level. Interestingly, synergistic interplays were observed between Osx and HIF-1α in VEGF promoter activation in transfection assay. Our findings indicate that Osx and HIF-1α cooperatively regulate VEGF expression.

New bone formation in a true bone ceramic scaffold loaded with desferrioxamine in the treatment of segmental bone defect: a preliminary study

BACKGROUND

Desferrioxamine (DFO), an iron chelator, can stimulate osteogenesis and angiogenesis by stabilizing hypoxia-inducible factor 1α. We postulate that a bone graft substitute combined with DFO is beneficial to the reconstruction of bone defects.

METHODS

We implanted pure true bone ceramic (TBC) and DFO-loaded TBC (DFO/TBC) scaffolds into 15-mm rabbit radial defects for 8 weeks. The bone segments were examined with X-ray, micro-CT and histology.

RESULTS

Radiographs showed that the DFO/TBC scaffold became radiopaque, and the gaps between the scaffold and radial cut ends were often invisible. Variables from micro-CT, including the bone volume fraction (BV/TV), trabecular thickness (Tb.Th) and trabecular number (Tb.N), were significantly increased in pure TBC and DFO/TBC scaffolds that had been implanted for 8 weeks compared to unimplanted TBC scaffolds (p values <0.05-0.001). Between the former two groups, BV/TV and Tb.Th were significantly increased in DFO/TBC scaffolds (p < 0.001), but Tb.N did not show significant differences. Histological examinations showed considerably increased new bone and decreased TBC trabecular remnants in DFO/TBC scaffolds compared to pure TBC scaffolds. Many cavities in the new bone area in DFO/TBC scaffolds were occupied by bone marrow elements and blood vessels. Percent of new bone with tetracycline labeling was significantly greater in DFO/TBC scaffolds than in pure TBC scaffolds (p < 0.001).

CONCLUSION

This preliminary study reveals that DFO can effectively induce new bone growing into TBC scaffolds, suggesting that the DFO/TBC composite is a promising bone graft substitute for the treatment of bone defects.

Sonic Hedgehog-activated engineered blood vessels enhance bone tissue formation

Large bone defects naturally regenerate via a highly vascularized tissue which progressively remodels into cartilage and bone. Current approaches in bone tissue engineering are restricted by delayed vascularization and fail to recapitulate this stepwise differentiation toward bone tissue. Here, we use the morphogen Sonic Hedgehog (Shh) to induce the in vitro organization of an endothelial capillary network in an artificial tissue. We show that endogenous Hedgehog activity regulates angiogenic genes and the formation of vascular lumens. Exogenous Shh further induces the in vitro development of the vasculature (vascular lumen formation, size, distribution). Upon implantation, the in vitro development of the vasculature improves the in vivo perfusion of the artificial tissue and is necessary to contribute to, and enhance, the formation of de novo mature bone tissue. Similar to the regenerating callus, the artificial tissue undergoes intramembranous and endochondral ossification and forms a trabecular-like bone organ including bone-marrow-like cavities. These findings open the door for new strategies to treat large bone defects by closely mimicking natural endochondral bone repair.

Repair of critical-sized rat calvarial defects using genetically engineered bone marrow-derived mesenchymal stem cells overexpressing hypoxia-inducible factor-1α

The processes of angiogenesis and bone formation are coupled both temporally and spatially during bone repair. Bone marrow-derived mesenchymal stem cells (BMSCs) have been effectively used to heal critical-size bone defects. Enhancing their ability to undergo angiogenic and osteogenic differentiation will enhance their potential use in bone regeneration. Hypoxia-inducible factor-1α (HIF-1α) has recently been identified as a major regulator of angiogenic-osteogenic coupling. In this study, we tested the hypothesis that HIF-1α gene therapy could be used to promote the repair of critical-sized bone defects. Using lentivirus-mediated delivery of wild-type (HIF) or constitutively active HIF-1α (cHIF), we found that in cultured BMSCs in vitro, HIF and cHIF significantly enhanced osteogenic and angiogenic mRNA and protein expression when compared with the LacZ group. We found that HIF-1α-overexpressing BMSCs dramatically improved the repair of critical-sized calvarial defects, including increased bone volume, bone mineral density, blood vessel number, and blood vessel area in vivo. These data confirm the essential role of HIF-1α modified BMSCs in angiogenesis and osteogenesis in vitro and in vivo.

Osteogenic differences in cultured rat periosteal cells under hypoxic and normal conditions

The aim of the present study was to investigate the osteogenic capability of rat calvarial periosteal cells in hypoxic conditions in vitro. Periosteum was obtained from the calvarial bone of Sprague-Dawley rats. Following primary tissue culture, subcultured cells were used in hypoxic or normal conditions. On days 1, 2, 3 and 4 following the cell culture, cell proliferation and mRNA and protein expression levels were evaluated. No significant difference in the cell proliferation rate was found between the normal and hypoxic condition groups. The hypoxic condition group exhibited a stronger expression of hypoxia-inducible factor (HIF)1α, vascular endothelial growth factor (VEGF), Runx2, alkaline phosphatase (ALP), bone sialoprotein (BSP), osteocalcin (OCN) and periostin at the mRNA level compared to that of the normal condition group. The hypoxic condition group also exhibited a stronger expression of HIF1α, VEGF, bone morphogenetic protein (BMP)2, Runx2, ALP and BSP at the protein level compared to that of the normal condition group. In conclusion, periosteal cells cultured in hypoxic conditions demonstrated activated osteogenic capability in vitro.

Oxidized low-density lipoprotein and β-glycerophosphate synergistically induce endothelial progenitor cell ossification

AIM

To investigate the ability of ox-LDL to induce ossification of endothelial progenitor cells (EPCs) in vitro and explored whether oxidative stress, especially hypoxia inducible factor-1α (HIF-1α) and reactive oxygen species (ROS), participate in the ossific process.

METHODS

Rat bone marrow-derived endothelial progenitor cells (BMEPCs) were cultured in endothelial growth medium supplemented with VEGF (40 ng/mL) and bFGF (10 ng/mL). The cells were treated with oxidized low-density lipoprotein (ox-LDL, 5 μg/mL) and/or β-glycerophosphate (β-GP, 10 mmol/L). Calcium content and Von Kossa staining were used as the measures of calcium deposition. Ossific gene expression was determined using RT-PCR. The expression of osteocalcin (OCN) was detected with immunofluorescence. Alkaline phosphatase (ALP) activity was analyzed using colorimetric assay. Intercellular reactive oxygen species (ROS) were measured with flow cytometry.

RESULTS

BMEPCs exhibited a spindle-like shape. The percentage of cells that expressed the cell markers of EPCs CD34, CD133 and kinase insert domain-containing receptor (KDR) were 46.2%±5.8%, 23.5%±4.0% and 74.3%±8.8%, respectively. Among the total cells, 78.3%±4.2% were stained with endothelial-specific fluorescence. Treatment of BMEPCs with ox-LDL significantly promoted calcium deposition, which was further significantly enhanced by co-treatment with β-GP. The same treatments significantly increased the gene expression of core-binding factor a-1 (cbfa-1) and OCN, while decreased the gene expression of osteoprotegerin (OPG). The treatments also significantly enhanced the activity of ALP, but did not affect the number of OCN(+) cells. Furthermore, the treatments significantly increased ROS and activated the hypoxia inducible factor-1α (HIF-1α). In all these effects, ox-LDL acted synergistically with β-GP.

CONCLUSION

Ox-LDL and β-GP synergistically induce ossification of BMEPCs, in which an oxidizing mechanism is involved.

Transcriptional regulation of Vascular Endothelial Growth Factor (VEGF) by osteoblast-specific transcription factor Osterix (Osx) in osteoblasts

Osterix (Osx) is an osteoblast-specific transcription factor required for bone formation and osteoblast differentiation. The critical step in bone formation is to replace the avascular cartilage template with vascularized bone. Osteogenesis and angiogenesis are associated with each other, sharing some essential regulators. Vascular endothelial growth factor (VEGF) is involved in both angiogenesis and osteogenesis. Transcriptional regulation of VEGF expression is not well known in osteoblasts. In this study, quantitative real-time RT-PCR results revealed that VEGF expression was down-regulated in Osx-null calvarial cells and that osteoblast marker osteocalcin expression was absent. Overexpression of Osx in stable C2C12 mesenchymal cells using a Tet-off system resulted in up-regulation of both osteocalcin and VEGF expression. The inhibition of Osx by siRNA led to repression of VEGF expression in osteoblasts. These results suggest that Osx controls VEGF expression. Transfection assays demonstrated that Osx activated VEGF promoter activity. A series of VEGF promoter deletion mutants were examined and the minimal Osx-responsive region was defined to the proximal 140-bp region of the VEGF promoter. Additional point mutants were used to identify two GC-rich regions that were responsible for VEGF promoter activation by Osx. Gel shift assay showed that Osx bound to the VEGF promoter sequence directly. Chromatin immunoprecipitation assays indicated that endogenous Osx associated with the native VEGF promoter in primary osteoblasts. Moreover, immunohistochemistry staining showed decreased VEGF protein levels in the tibiae of Osx conditional knock-out mice. We provide the first evidence that Osx controlled VEGF expression, suggesting a potential role of Osx in coordinating osteogenesis and angiogenesis.

Developmental effects related to angiogenesis and osteogenic differentiation in Salmon larvae continuously exposed to dioxin-like 3,3',4,4'-tetrachlorobiphenyl (congener 77)

We have studied the effects of dioxin-like 3,3',4,4'-tetrachlorobiphenyl (PCB-77) on developmental effects related to angiogenesis and osteogenesis during early life-stages of salmon. Larvae were kept at 6°C and continuously exposed to waterborne PCB-77 (1 or 10 ng/L) initiated at the egg stage or 416-day degrees (dd) and throughout yolk-sac stage (716 dd) and for a total duration of 50 days (or 300 dd). Gene transcription analysis was performed on whole larvae total RNA at 548, 632, 674 and 716 dd using real-time PCR. Bone morphogenetic protein (bmp2 and bmp4), transforming growth factor β (TGF-β), estrogen receptors (ERα and ERβ), runx2, sox9 and collagen type 2 alpha 1 (col2a1) and vascular endothelial growth factor (VEGF) genes were studied. Effect on VEGF gene transcription was related to observation of heart rate, arrhythmia and anemia, demonstrating effects on vascular system development. Alizarine-red staining and quantification of ossified bone structures showed that PCB-77 produced concentration-dependent increases in the rate of osteogenic tissue formation. PCB-77 produced increases in col2a1 and runx2 transcription with subsequent induction of chondrogenesis and osteogenesis, respectively. The transcription of TGF-β gene was associated with ERβ transcription. Transcripts of AhR gene battery were differentially modulated by PCB-77 and these effects were dependent on concentration and larval age. Evidence of vascular system disruption by PCB-77 was observed as cardiac edema, anemia and arrhythmia in exposed individuals and as decreased level of VEGF gene transcription at early age. In general, our data indicate that PCB-77 produced developmental effects related to angiogenesis and osteogenic differentiation and disruption of vascular system development.

Hypoxia induces osteogenesis-related activities and expression of core binding factor α1 in mesenchymal stem cells

Mesenchymal stem sells (MSCs) have received much attention in the field of bone tissue engineering due to their biological capability to differentiate into osteogenic lineage cells. Hypoxia-inducible factor 1alpha (HIF-1α) plays an important role in the MSC-related bone regeneration during hypoxia, while core binding factor alpha 1 (Cbfα1) is a transcription regulator that is involved in the chondrocyte differentiation and ossification. In the present study, we investigated the effects of hypoxia on biological capability of MSCs. MSCs were isolated from adult rabbit bone marrow, and were cultured in vitro under normoxia (air with 5% CO(2)) or hypoxia (5% CO(2) and 95% N(2)). The proliferation of MSCs, alkaline phosphatase (ALP) activity, and production of collagens type I and type III (Col I/III) were examined. The expression levels of HIF-1α and Cbfα1 were measured by real-time PCR and western blot analyses. We found that hypoxia significantly induced the proliferation of MSCs and increased ALP activity and the production of Col I/III. Moreover, hypoxia increased the expression of Cbfα1 mRNA after 12 h, whereas the expression of HIF-1α mRNA was increased after 1 h of hypoxia. Knockdown of HIF-1α expression with a small interfering RNA significantly increased the expression levels of Cbfα1 protein either under the normoxia or hypoxia condition. Our results indicate that hypoxia enhances MSCs to differentiate into osteogenic lineage cells and suggest that Cbfα1 may be negatively regulated by HIF-1α.

In vitro study of enhanced osteogenesis induced by HIF-1α-transduced bone marrow stem cells

OBJECTIVES

Hypoxia-inducible factor 1α (HIF-1α) is a pivotal regulator of hypoxic and ischaemic vascular responses that drives transcriptional activation of hundreds of genes involved in vascular reactivity, angiogenesis and arteriogenesis. Previous reports based on gene knockout technology have demonstrated that HIF-1α can promote osteogenesis. However, this protein is easily degraded in a normoxic state, which makes in vitro studies of HIF-1α-induced mesenchymal stem cell (MSC) osteogenesis difficult. For better understanding of HIF-1α promoting osteogenesis, the role of HIF-1α-induced MSC osteogenesis in the normoxic state has been investigated here.

MATERIALS AND METHODS

HIF-1α was made to overexpress using a lentiviral vector, and its effects on bone marrow-derived mesenchymal stem cell (BMSC) osteogenesis were investigated. Real-time quantitative and western blotting (to assess expression levels of angiogenic and osteogenic related genes regulated by Lenti-HIF-1α), alkaline phosphatase (ALP) and alizarin red-S staining analyses, were performed.

RESULTS

In HIF-1α gene-transfected BMSCs, expression levels of angiogenic, cartilaginous and osteogenic genes were all increased significantly compared to Lenti LacZ-transfected cells, at both mRNA and protein levels. ALP activity and alizarin red-S staining were significantly enhanced in HIF-1α transduced cells compared to control cells, on day 21.

CONCLUSIONS

These results indicate that Lenti-HIF-1α can induce BMSC overexpression levels of angiogenic and osteogenic genes in vitro in the normoxic state. Further study will be focused on whether HIF-1α can also improve bone repair in vivo.

Hsp90 and angiogenesis in bone disorders—lessons from the avian growth plate

Thiram-induced tibial dyschondroplasia (TD) and vitamin-D deficiency rickets are avian bone disorders of different etiologies characterized by abnormal chondrocyte differentiation, enlarged and unvascularized growth plates, and lameness. Heat-shock protein 90 (Hsp90) is a proangiogenic factor in mammalian tissues and in tumors; therefore, Hsp90 inhibitors were developed as antiangiogenic factors. In this study, we evaluated the association between Hsp90, hypoxia, and angiogenesis in the chick growth plate. Administration of the Hsp90 inhibitor to TD- and rickets-afflicted chicks at the time of induction resulted in reduction in growth-plate size and, contrary to its antiangiogenic effect in tumors, a major invasion of blood vessels occurred in the growth plates. This was the result of upregulation of the VEGF receptor Flk-1, the major rate-limiting factor of vascularization in TD and rickets. In addition, the abnormal chondrocyte differentiation, as characterized by collagen type II expression and alkaline phosphatase activity, and the changes in hypoxia-inducible factor-1α (HIF-1α) in both disorders were restored. All these changes resulted in prevention of lameness. Inhibition of Hsp90 activity reduced growth-plate size, increased vascularization, and mitigated lameness also in TD chicks with established lesions. In summary, this is the first reported demonstration of involvement of Hsp90 in chondrocyte differentiation and growth-plate vascularization. In contrast to the antiangiogenic effect of Hsp90 inhibitors observed in mammals, inhibition of Hsp90 activity in the unvascularized TD- and rickets-afflicted chicks resulted in activation of the angiogenic switch and reinstated normal growth-plate morphology.

Differential roles of hypoxia inducible factor subunits in multipotential stromal cells under hypoxic condition

Cell therapy with bone marrow multipotential stromal cells (MSCs) represents a promising approach to promote wound healing and tissue regeneration. MSCs expanded in vitro lose early progenitors with differentiation and therapeutic potentials under normoxic condition, whereas hypoxic condition promotes MSC self-renewal through preserving colony forming early progenitors and maintaining undifferentiated phenotypes. Hypoxia inducible factor (HIF) pathway is a crucial signaling pathway activated in hypoxic condition. We evaluated the roles of HIFs in MSC differentiation, colony formation, and paracrine activity under hypoxic condition. Hypoxic condition reversibly decreased osteogenic and adipogenic differentiation. Decrease of osteogenic differentiation depended on HIF pathway; whereas decrease of adipogenic differentiation depended on the activation of unfolded protein response (UPR), but not HIFs. Hypoxia-mediated increase of MSC colony formation was not HIF-dependent also. Hypoxic exposure increased secretion of VEGF, HGF, and basic FGF in a HIF-dependent manner. These findings suggest that HIF has a limited, but pivotal role in enhancing MSC self-renewal and growth factor secretions under hypoxic condition.

A central role for hypoxic signaling in cartilage, bone, and hematopoiesis

Hypoxic signaling plays an essential role in maintaining oxygen homeostasis and cell survival. Hypoxia-inducible transcription factors HIF-1 and HIF-2 are central mediators of the cellular response to hypoxia by regulating the expression of genes controlling metabolic adaptation, oxygen delivery, and survival in response to oxygen deprivation. Recent studies have identified an important role for HIF-1 and HIF-2 in the regulation of skeletal development, bone formation, and regeneration, as well as joint formation and homeostasis. In addition, overexpression of HIF-1 and HIF-2 is clinically associated with osteosarcoma and osteoarthritis. Together, these findings implicate hypoxic signaling as a central regulator of bone biology and disease.

Synergistic inhibition of endochondral bone formation by silencing Hif1α and Runx2 in trauma-induced heterotopic ossification

Angiogenesis and osteogenesis are tightly coupled during bone development. We studied the effect of inhibition of Hif1α and Runt-related protein 2 (Runx2) on the formation of heterotopic ossification (HO). We constructed lentivirus vectors expressing Hif1α small interfering RNA (siRNA) and Runx2 siRNA. The inhibition of Hif1α function impaired osteoblast proliferation while osteoblasts differentiated normally. Osteoblasts lacking Runx2 proliferated normally while the differentiation was impaired. The osteoblast differentiation was significantly inhibited by co-Runx2 and Hif1α siRNA treatment. The formation of HO by inhibiting Runx2 and Hif1α in an animal model induced by Achilles tenotomy was investigated. The results showed that lacking of Runx2 and Hif1α could inhibit HO formation. Inhibition of Hif1α prevented HO formation only at the initial step and inhibition of Runx2 worked both at the initial step and after chondrogenesis. Angiogenesis and the expressions of osteogenic genes were downregulated in the Hif1α siRNA group. We found synergistic inhibition of endochondral bone formation by silencing Hif1α and Runx2. Our study provided new insight into the roles of Hif1α and Runx2 during the processes of endochondral bone formation, and had important implications for the new therapeutic methods to inhibit HO or to enhance bone formation.

Ascorbic acid regulates osterix expression in osteoblasts by activation of prolyl hydroxylase and ubiquitination-mediated proteosomal degradation pathway

Mouse genetic studies reveal that ascorbic acid (AA) is essential for osteoblast (OB) differentiation and that osterix (Osx) was a key downstream target of AA action in OBs. To determine the molecular pathways for AA regulation of Osx expression, we evaluated if AA regulates Osx expression by regulating production and/or actions of local growth factors and extracellular matrix (ECM) proteins. Inhibition of actions of IGFs by inhibitory IGFBP-4, BMPs by noggin, and ECM-mediated integrin signaling by RGD did not block AA effects on Osx expression in OBs. Furthermore, blockade of components of MAPK signaling pathway had no effect on AA-induced Osx expression. Because AA is required for prolyl hydroxylase domain (PHD) activity and because PHD-induced prolyl-hydroxylation targets proteins to proteosomal degradation, we next tested if AA effect on Osx expression involves activation of PHD to hydroxylate and induce ubiquitin-proteosome-mediated degradation of transcriptional repressor(s) of Osx gene. Treatment of OBs with dimethyloxallyl glycine and ethyl 3, 4-dihydroxybenzoate, known inhibitors of PHD, completely blocked AA effect on Osx expression and OB differentiation. Knockdown of PHD2 expression by Lentivirus-mediated shRNA abolished AA-induced Osx induction and alkaline phosphatase activity. Furthermore, treatment of OBs with MG115, inhibitor of proteosomal degradation, completely blocked AA effects on Osx expression. Based on these data, we conclude that AA effect on Osx expression is mediated via a novel mechanism that involves PHD2 and proteosomal degradation of a yet to be identified transcriptional repressor that is independent of BMP, IGF-I, or integrin-mediated signaling in mouse OBs.

Bone metastases: molecular mechanisms and novel therapeutic interventions

It has been long recognized that skeleton represents one of the most favored metastatic sites for common cancers like breast and prostate. During the last decade the molecular mechanisms that are responsible for the development of bone metastasis have been gradually illuminated. It appears that the bone microenvironment has a pivotal role in this process. Metastatic tumor cells interact with bone triggering a cascade of molecular events that produce osteolytic and/or osteoblastic phenomena. In this review, we summarize and discuss the most significant factors and signaling pathways implicated in bone colonization. Moreover, based on the recent literature and data, we foresee the need for designing novel agents that will efficiently disrupt these interactions among cancer cells and bone microenvironment, bringing hope for more effective treatments.