Endothelin-1 down-regulates the expression of vascular endothelial growth factor-A associated with osteoprogenitor proliferation and differentiation

Effect of Polydopamine-Coated Strontium-Doped Hydroxyapatite Nanowires on Bone Marrow Mesenchymal Stem Cells and Umbilical Vein Endothelial Cells

Hydroxyapatite nanowires (HAW) can effectively improve the bone repair ability in bone engineered tissue. However, due to their single function, the application of HAWs in biological tissue engineering materials is limited. In this study, strontium-doped hydroxyapatite nanowires (SrHAW) were synthesized by a hydrothermal method and coated with polydopamine (PDA) to improve the function of HAWs. The material structure, biocompatibility evaluation, and differentiation capability testing of PDA-coated strontium-doped hydroxyapatite (SrHAW@PDA) nanowires were conducted. Then, the nanowires were co-cultured with rat bone marrow mesenchymal stem cells (BMSCs) and rat umbilical vein endothelial cells (UVECs) to prepare cell spheroids. Compared with the undoped and uncoated HAW, the SrHAW@PDA nanowires enhanced the cell activity and their angiogenesis and osteogenesis abilities. In addition, their performance in the three-dimensional spheroid also played a positive role in the cells in the spheroid. Due to the presence of PDA, the adhesion between the cells in the three-dimensional spheroid and the nanowires were enhanced. In summary, these results show that SrHAW@PDA has the potential to be used as an alternative material to regulate cell biological activity in three-dimensional cell spheroids.

New Insights to the Crosstalk between Vascular and Bone Tissue in Chronic Kidney Disease-Mineral and Bone Disorder

Vasculature plays a key role in bone development and the maintenance of bone tissue throughout life. The two organ systems are not only linked in normal physiology, but also in pathophysiological conditions. The chronic kidney disease–mineral and bone disorder (CKD-MBD) is still the most serious complication to CKD, resulting in increased morbidity and mortality. Current treatment therapies aimed at the phosphate retention and parathyroid hormone disturbances fail to reduce the high cardiovascular mortality in CKD patients, underlining the importance of other factors in the complex syndrome. This review will focus on vascular disease and its interplay with bone disorders in CKD. It will present the very late data showing a direct effect of vascular calcification on bone metabolism, indicating a vascular-bone tissue crosstalk in CKD. The calcified vasculature not only suffers from the systemic effects of CKD but seems to be an active player in the CKD-MBD syndrome impairing bone metabolism and might be a novel target for treatment and prevention.

Osteogenic and Angiogenic Synergy of Human Adipose Stem Cells and Human Umbilical Vein Endothelial Cells Cocultured in a Modified Perfusion Bioreactor

Synergistic promotion of angiogenesis and osteogenesis in bone tissue-engineered constructs remains a crucial clinical challenge, which might be overcome by simultaneous employment of superior techniques including coculture systems, differentiation-stimulated factors, combinatorial scaffolds and bioreactors.Current study investigated the effect of flow perfusion along with coculture of human adipose stem cells (hASCs) and human umbilical vein endothelial cells (HUVECs) on osteogenic and angiogenic differentiation.Pre-treated hASCs with 1,25-dihydroxyvitamin D₃ were seeded onto poly(lactic-co-glycolic acid)/β-tricalcium phosphate/polycaprolactone (PLGA/β-TCP/PCL) scaffold with/without HUVECs, and cultured for 14 days within a flask or modified perfusion bioreactor. Analysis of osteogenic and angiogenic gene expression, alkaline phosphatase (ALP) activity and ALP staining indicates a synergistic effect of perfusion flow and coculture system on osteogenic and angiogenic differentiation. The advantage of modified perfusion bioreactor is its five-branch flow distributor which directly connect to the porous PCL hollow fibers embedded in the 3D scaffold to improve flow and flow-induced shear stress uniformity.Dynamic coculture increased VEGF₁₆₅ by 6-fold, VEGF₁₈₉ by 2-fold, and Endothelin-1 by 4-fold, relative to dynamic monoculture. Static coculture enhanced osteogenic and angiogenic differentiation, compared with static monoculture. Although dynamic coculture is in preference to static coculture due to significant increase in ALP activity and promoted angiogenic marker expression. Our finding is the first to indicate that the modified perfusion bioreactor combined with the beneficial cell-cell crosstalk in pre-treated hASC/HUVEC cocultures provides a synergy between osteogenic and angiogenic differentiation of the accumulation of cells, suggesting that it represents a promising approach for regeneration of critical-sized bone defects.

Coupling Osteogenesis and Vasculogenesis in Engineered Orthopedic Tissues

Inadequate vascularization of engineered tissue constructs is a main challenge in developing a clinically impactful therapy for large, complex, and recalcitrant bone defects. It is well established that bone and blood vessels form concomitantly during development, as well as during repair after injury. Endothelial cells (ECs) and mesenchymal stromal cells (MSCs) are known to be key players in orthopedic tissue regeneration and vascularization, and these cell types have been used widely in tissue engineering strategies to create vascularized bone. Coculture studies have demonstrated that there is crosstalk between ECs and MSCs that can lead to synergistic effects on tissue regeneration. At the same time, the complexity in fabricating, culturing, and characterizing engineered tissue constructs containing multiple cell types presents a challenge in creating multifunctional tissues. In particular, the timing, spatial distribution, and cell phenotypes that are most conducive to promoting concurrent bone and vessel formation are not well understood. This review describes the processes of bone and vascular development, and how these have been harnessed in tissue engineering strategies to create vascularized bone. There is an emphasis on interactions between ECs and MSCs, and the culture systems that can be used to understand and control these interactions within a single engineered construct. Developmental engineering strategies to mimic endochondral ossification are discussed as a means of generating vascularized orthopedic tissues. The field of tissue engineering has made impressive progress in creating tissue replacements. However, the development of larger, more complex, and multifunctional engineered orthopedic tissues will require a better understanding of how osteogenesis and vasculogenesis are coupled in tissue regeneration. Impact statement Vascularization of large engineered tissue volumes remains a challenge in developing new and more biologically functional bone grafts. A better understanding of how blood vessels develop during bone formation and regeneration is needed. This knowledge can then be applied to develop new strategies for promoting both osteogenesis and vasculogenesis during the creation of engineered orthopedic tissues. This article summarizes the processes of bone and blood vessel development, with a focus on how endothelial cells and mesenchymal stromal cells interact to form vascularized bone both during development and growth, as well as tissue healing. It is meant as a resource for tissue engineers who are interested in creating vascularized tissue, and in particular to those developing cell-based therapies for large, complex, and recalcitrant bone defects.

The impact of implant abutment surface treatment with TiO2 on peri-implant levels of angiogenesis and bone-related markers: a randomized clinical trial

The goal of this randomized, blinded, split-mouth controlled clinical trial was to assess the influence of abutment surface treatment on tissue healing. Fifteen patients received two implants distributed randomly to two groups: test (TiO₂ abutment surface), control (standard abutment surface). Levels of epidermal growth factor (EGF), bone morphogenetic protein 9 (BMP-9), endothelin 1 (ET-1), fibroblast growth factor (FGF), placental growth factor (PlGF), and vascular endothelial growth factor (VEGF) were quantified in the peri-implant fluid after 3, 14, 30, and 60 days. Inter-group comparisons indicated higher levels of EGF, BMP-9, ET-1, FGF, and PlGF in the test group after 30days (P<0.05). PlGF levels were also higher in the test group after 60 days. In the test group, intra-group analysis revealed different levels of ET-1 and FGF between days 3 and 30, and days 3 and 60 (P<0.05); furthermore, VEGF levels were significantly higher on day 60 than on day 3 (P <0.05). In the control group, intra-group analysis demonstrated significantly different levels of ET-1, FGF, and PlGF between days 3 and 60 and of PlGF between days 14 and 60 (P<0.05). In conclusion, abutment surfaces treated with TiO₂ influenced the levels of angiogenesis and bone-related markers.

In vivo and in vitro study of osteogenic potency of endothelin-1 on bone marrow-derived mesenchymal stem cells

Bone marrow-derived mesenchymal stem cells (BMSCs) are a major source of osteoblasts and are crucial for bone remolding and repair and thus they are widely used for tissue engineering applications. Tissue engineering in combination with gene therapy is considered as a promising approach in new bone regeneration. Endothelin-1（EDN-1）is produced by vascular endothelial cells which plays an important role during bone development. However, its role in BMSCs remains largely unknown. We established EDN-1 overexpressed BMSCs, proliferation ability and osteogenesis differentiation were detected respectively. Transduced BMSCs were then combined with CPC-scaffold to repair calvarial defects in rats to evaluate the in-vivo osteogenic potential of EDN-1. EDN-1 overexpressed BMSCs showed increased proliferation and significantly increased osteogenesis potential ability than vector transfected control. The in-vivo data also revealed more new bone formation with higher bone mineral density and number of trabeculae in EDN-1 overexpressed BMSCs. These findings have demonstrated the influence of EDN-1 on differentiation potential of BMSCs, which suggest that EDN-1 may be a new promising agent for bone tissue engineering.

TiN x O y coatings facilitate the initial adhesion of osteoblasts to create a suitable environment for their proliferation and the recruitment of endothelial cells

Titanium-nitride-oxide coatings (TiN _x O _y ) improve osseointegration of endosseous implants. The exact mechanisms by which these effects are mediated are poorly understood except for an increase of osteoblast proliferation while a high degree of differentiation is maintained. One hypothesis holds that TiN _x O _y facilitates the initial spreading and adhesion of the osteoblasts. The aim of this work was to investigate the molecular mechanisms of osteoblast adhesion on TiN _x O _y as compared to microrough titanium SLA. A global view of the osseointegrative process, that is, taking into account other cell groups, especially endothelial cells, is also presented. To this aim, gene expression and focal adhesion analysis, cocultures and wound assays were performed early after seeding, from 6 h to 3 days. We demonstrated that TiN _x O _y coatings enhance osteoblast adhesion and spreading when compared to the standard microrough titanium. The integrin β1, either in association with α1 or with α2 plays a central role in these mechanisms. TiN _x O _y coatings optimize the process of osseointegration by acting at several levels, especially by upregulating osteoblast adhesion and proliferation, but also by supporting neovascularization and the development of a suitable inflammatory environment.

Interactions between osteopontin and vascular endothelial growth factor: Implications for skeletal disorders

Osteopontin (OPN) and vascular endothelial growth factor (VEGF) are characterized by a convergence in function for maintaining the homeostasis of the skeletal and renal systems (the bone-renal-vascular axis regulates bone metabolism). The two cytokines contribute to bone remodeling, dental healing, kidney function, and the adjustment to microgravity. Often, they are co-expressed or one molecule induces the other, however, in some settings OPN-associated pathways and VEGF-associated pathways are distinct. In bone remodeling, OPN and VEGF are regulated under the influence of growth factors and hormones, hypoxia and inflammation, the micro-environment, and various physical forces. Their abundance can be affected by drug treatment. OPN and VEGF are variably associated with kidney disease. Their balanced levels are critical for restoring endothelial cell function and ameliorating the adverse effects of microgravity. Here, we review the relevant 83 papers of 257 articles published, and listed in PubMed under the key words OPN and VEGF.

Umbilical cord and bone marrow mesenchymal stem cell seeding on macroporous calcium phosphate for bone regeneration in rat cranial defects

Human umbilical cord mesenchymal stem cells (hUCMSCs) are inexhaustible and can be harvested at a low cost without an invasive procedure. However, there has been no report on comparing hUCMSCs with human bone marrow MSCs (hBMSCs) for bone regeneration in vivo. The aim of this study was to investigate hUCMSC and hBMSC seeding on macroporous calcium phosphate cement (CPC), and to compare their bone regeneration in critical-sized cranial defects in rats. Cell attachment, osteogenic differentiation and mineral synthesis on RGD-modified macroporous CPC were investigated in vitro. Scaffolds with cells were implanted in 8-mm defects of athymic rats. Bone regeneration was investigated via micro-CT and histological analysis at 4, 12, and 24 weeks. Three groups were tested: CPC with hUCMSCs, CPC with hBMSCs, and CPC control without cells. Percentage of live cells and cell density on CPC in vitro were similarly good for hUCMSCs and hBMSCs. Both cells had high osteogenic expressions of alkaline phosphatase, osteocalcin, collagen I, and Runx2. Bone mineral density and trabecular thickness in hUCMSC and hBMSC groups in vivo were greater than those of CPC control group. New bone amount for hUCMSC-CPC and hBMSC-CPC constructs was increased by 57% and 88%, respectively, while blood vessel density was increased by 15% and 20%, than CPC control group at 24 weeks. hUCMSC-CPC and hBMSC-CPC groups generally had statistically similar bone mineral density, new bone amount and vessel density. In conclusion, hUCMSCs seeded on CPC were shown to match the bone regeneration efficacy of hBMSCs in vivo for the first time. Both hUCMSC-CPC and hBMSC-CPC constructs generated much more new bone and blood vessels than CPC without cells. Macroporous RGD-grafted CPC with stem cell seeding is promising for craniofacial and orthopedic repairs.

Blood vessels are concentrated within the implant surface concavities: a histologic study in rabbit tibia

Angiogenesis plays a key role in bone formation and maintenance. Bone formation has been reported to initiate in the concavities rather than the convexities in a hydroxyapatite substratum and the implant threads of dental implants. The aim of the present study was to evaluate the number of the blood vessels inside the concavities and around the convexities of the threads of implants in a rabbit tibia model. A total of 32 thread-shaped implants blasted with apatitic calcium phosphate (TCP/HA blend) (Resorbable Blast Texturing, RBT) (Maestro, BioHorizons(®), Birmingham, AL, USA) were inserted in 8 rabbits. Each rabbit received 4 implants, 2 in the right and 2 in left tibia. Implants were retrieved after 1, 2, 4, and 8 weeks and treated to obtain thin ground sections. Statistically significant differences were found in the number of vessels that had formed in the concavities rather than the convexities of the implants after 1 (p = 0.000), and 2 weeks (p = 0.000), whilst no significant differences after 4 (p = 0.546) and 8 weeks (p = 0.275) were detected. The present results supported the hypothesis that blood vessel formation was stimulated by the presence of the concavities, which may provide a suitable environment in which mechanical forces, concentrations and gradients of chemotactic molecules and blood clot retention may all drive vascular and bone cell migration.

Localized delivery of growth factors for angiogenesis and bone formation in tissue engineering

Angiogenesis is a key component of bone formation. Delivery of growth factors for both angiogenesis and osteogenesis is about to gain important potential as a future therapeutic tool. This review focuses on these growth factors that have dual functions in angiogenesis and osteogenesis, and their localized application. A major hurdle in the clinical development of growth factor therapy so far is how to assure safe and efficacious therapeutic use of such factors and avoid unwanted side effects and toxicity. It is now firmly established from the available information that the type, dose, combinations and delivery kinetics of growth factors all play a decisive role for the success of growth factor therapy. All of these parameters have to be adapted and optimized for each animal model or clinical case. In this review we discuss some important parameters associated with growth factor therapy and present an overview of selected preclinical studies, followed by a conceptual description of both established and proposed delivery strategies meeting therapeutic needs.

Altered VEGF-A and receptor mRNA expression profiles, and identification of VEGF144 in foetal rat calvaria cells, in coculture with microvascular endothelial cells

Cellular proliferation and differentiation during angiogenesis and osteogenesis require the communication of different cell types through growth factors and their receptors. Vascular endothelial growth factor (VEGF-A) plays an important role in osteoblast and endothelial cell intercommunication. We have investigated the effect of monocultures and indirect coculture of foetal rat calvarial (FRC) osteoblasts and microvascular endothelial cells (ECs) on nodule formation, proliferation, and mRNA-expression of VEGF-A and its receptors during culturing. Despite increased nodule formation in the presence of dexamethasone (Dex) in monocultures, the number of nodules and alkaline phosphatise activity were decreased in cocultured FRCs. VEGF mRNA expression over the differentiation period showed the expression of most Vegf isoforms is biphasic in both FRCs and ECs, whereas receptor expression was quite variable; however, that of Np-2 in FRCs increased steadily and significantly from 8 h to 14 days after an initial drop in expression. Significant changes in the proportion of Vegfa by Day 14 were noted mainly in the matrix-bound variants Vegf144 and Vegf188 in ECs and osteoblasts, respectively. Less striking results were seen in the expression of the soluble isoforms in either cell type. These results have identified expression of Vegf144 in osteoblasts, suggesting a possible autocrine and/or paracrine role that is affecting osteoblast mineralisation along with Vegf188, as well as possible early roles of these isoforms in initial cell attachment. Further study of VEGF expression in coculture and Vegf144 will lead to better understanding of its role in cell-cell communication and bone development.

Bone vascularization in normal and disease conditions

Bone vasculature is essential for many processes, such as skeletal development and growth, bone modeling and remodeling, and healing processes. Endothelium is an integral part of bone tissue, expressing a physiological paracrine function via growth factors and chemokines release, and interacting with several cellular lines. Alterations of the complex biochemical interactions between vasculature and bone cells may lead to various clinical manifestations. Two different types of pathologies result: a defect or an excess of bone vasculature or endothelium metabolism. Starting from the molecular basis of the interactions between endothelial and bone cells, the Authors present an overview of the recent acquisitions in the physiopathology of the most important clinical patterns, and the modern therapeutic strategies for their treatments.

Copolymer cell/scaffold constructs for bone tissue engineering: co-culture of low ratios of human endothelial and osteoblast-like cells in a dynamic culture system

The aim of this study was to compare the effect of different ratios of human umbilical vein endothelial cells (HUVECs) on osteogenic activity of human osteoblast-like cells (HOB) and capillary-like structure (CLS), seeded into copolymer scaffolds in a dynamic culture system. HOB and HUVEC were co-cultured into poly(L-lactide)-co-(1,5-dioxepan-2-one) [poly(LLA-co-DXO)] scaffolds at ratios of 5:1 (5:1 group) and 2:1 (2:1 group). Samples were collected after 5, 15, and 25 days. Cross-sections were processed and the CLS from HUVEC was disclosed in both groups. Cell viability was determined by dsDNA assay. Cells seeded at the ratio of 5:1 had good viability. Total RNA was isolated and the reverse transcription reaction was performed. The influences on the expression of several osteogenic genes were various with regarding to different ratios of HUVEC demonstrated by the PCR array. The RT-PCR results was in consistent with the PCR array results that several osteogenesis related genes had higher expression in the 5:1 group than in the 2:1 group, especially at day 25, such as alkaline phosphatase, insulin-like growth factor 1 (IGF1), and so forth. ELISA showed that the production of IGF1 after 25 days of incubation were higher in cells co-cultured at the 5:1 ratio than at the 2:1 ratio. The results show that under dynamic culture conditions, co-culture of HOB with a low ratio of HUVEC in copolymer scaffolds results in CLS formation and significantly influenced the expression of osteogenic markers.

Cellular mechanisms of bone remodeling

Bone remodeling is a tightly regulated process securing repair of microdamage (targeted remodeling) and replacement of old bone with new bone through sequential osteoclastic resorption and osteoblastic bone formation. The rate of remodeling is regulated by a wide variety of calcitropic hormones (PTH, thyroid hormone, sex steroids etc.). In recent years we have come to appreciate that bone remodeling proceeds in a specialized vascular structure,--the Bone Remodeling Compartment (BRC). The outer lining of this compartment is made up of flattened cells, displaying all the characteristics of lining cells in bone including expression of OPG and RANKL. Reduced bone turnover leads to a decrease in the number of BRCs, while increased turnover causes an increase in the number of BRCs. The secretion of regulatory factors inside a confined space separated from the bone marrow would facilitate local regulation of the remodeling process without interference from growth factors secreted by blood cells in the marrow space. The BRC also creates an environment where cells inside the structure are exposed to denuded bone, which may enable direct cellular interactions with integrins and other matrix factors known to regulate osteoclast/osteoblast activity. However, the denuded bone surface inside the BRC also constitutes an ideal environment for the seeding of bone metastases, known to have high affinity for bone matrix. Circulating osteoclast- and osteoblast precursor cells have been demonstrated in peripheral blood. The dominant pathway regulating osteoclast recruitment is the RANKL/OPG system, while many different factors (RUNX, Osterix) are involved in osteoblast differentiation. Both pathways are modulated by calcitropic hormones.

Effect of human vascular endothelial growth factor gene transfer on endogenous vascular endothelial growth factor mRNA expression in a rat fibroblast and osteoblast culture model

PURPOSE

Vascular endothelial growth factor (VEGF) plays an important role in promoting angiogenesis and osteogenesis during fracture repair. Our previous studies have shown that cell-based VEGF gene therapy enhances bone healing of a rabbit tibia segmental bone defect in vivo. The aim of this project was to examine the effect of exogenous human VEGF on the endogenous rat VEGF messenger RNA (mRNA) expression in a cell-based gene transfer model.

METHODS

Rat fibroblasts and osteoblasts were harvested from the dermal tissue and periosteum, respectively, of Fisher 344 rats. The cells were then cultured and transfected with pcDNA-human VEGF using Superfect reagent (Qiagen). Four experimental groups were created: 1) fibroblast-VEGF; 2) osteoblast-VEGF; 3) nontransfected fibroblast controls; and 4) nontransfected osteoblast controls. The cultured cells were harvested at 1, 3, and 7 days after the gene transfection. The total mRNA was extracted (Trizol; Invitrogen); both human VEGF and rat VEGF mRNA were measured by reverse transcriptase-polymerase chain reaction and quantified by VisionWorksLS.

RESULTS

The human VEGF165 mRNA was detected by reverse transcriptase-polymerase chain reaction from transfected fibroblasts and osteoblasts at 1, 3, and 7 days after gene transfection. The human VEGF165 levels peaked at Day 1 and then gradually reduced expression in both transfected fibroblasts and osteoblasts. Two endogenous rat VEGF isoforms were detected in this cell culture model: rat VEGF120 and rat VEGF164. We compared the rat VEGF120 and rat VEGF164 expression level of the fibroblasts or osteoblasts that were transfected with human VEGF165, with nontransfected control cells. Both the transfected fibroblasts and osteoblasts showed greater expression of rat VEGF164 than nontransfected controls at Day 1 (peak level) and Day 3, but not at Day 7. The expression of rat VEGF120 was lower in transfected fibroblasts, but higher in transfected osteoblasts, than the relevant control groups at any time point after transfection. In addition, human VEGF gene transfection increased osteoblast cell proliferation after 3 days.

CONCLUSION

These in vitro results suggest that cell-based human VEGF gene therapy is not only effective at causing human VEGF expression, but also enhances endogenous rat VEGF mRNA expression in both fibroblasts and osteoblasts, particularly the rat VEGF164 isoform.

VEGF-A expression in osteoclasts is regulated by NF-kappaB induction of HIF-1alpha

Large osteoclasts (10+ nuclei), predominant in rheumatoid arthritis and periodontal disease, have higher expression of proteases and activating receptors and also have increased resorptive activity when compared to small (2-5 nuclei) osteoclasts. We hypothesized that large and small osteoclasts activate different signaling pathways. A Signal Transduction Pathway Finder Array was used to compare gene expression of large and small osteoclasts in RAW 264.7-derived osteoclasts. Expression of vascular endothelial growth factor A (Vegfa) was higher in large osteoclasts and this result was confirmed by RT-PCR. RT-PCR further showed that RANKL treatment of RAW cells induced Vegfa expression in a time-dependent manner. Moreover, VEGF-A secretion in conditioned media was also increased in cultures with a higher proportion of large osteoclasts. To investigate the mechanism of Vegfa induction, specific inhibitors for the transcription factors NF-kappaB, AP-1, NFATc1, and HIF-1 were used. Dimethyl bisphenol A, the HIF-1alpha inhibitor, decreased Vegfa mRNA expression, whereas blocking NF-kappaB, AP-1, and NFATc1 had no effect. Furthermore, the NF-kappaB inhibitor gliotoxin inhibited Hif1alpha mRNA expression. In conclusion, VEGF-A gene and protein expression are elevated in large osteoclasts compared to small osteoclasts and this increase is regulated by HIF-1. In turn, Hif1alpha mRNA levels are induced by RANKL-mediated activation of NF-kappaB. These findings reveal further differences in signaling between large and small osteoclasts and thereby identify novel therapeutic targets for highly resorptive osteoclasts in inflammatory bone loss.

Sonic hedgehog promotes angiogenesis and osteogenesis in a coculture system consisting of primary osteoblasts and outgrowth endothelial cells

A number of previous studies documented the angiogenic potential of outgrowth endothelial cells in vitro and in vivo and provided evidence that therapeutic success could depend on coculture or coimplantation strategies. Thus, deeper insight into the molecular mechanisms underlying this pro-angiogenic effect of cocultures might provide new translational options for tissue engineering and regenerative medicine. One promising signaling pathway in bone repair involved in neoangiogenesis and bone formation is the sonic hedgehog (Shh) pathway. In this article, we focus on the effect of Shh on the formation of microvessel-like structures and osteoblastic differentiation in cocultures of primary osteoblasts and outgrowth endothelial cells. Already after 24 h of treatment, Shh leads to a massive increase in microvessel-like structures compared with untreated cocultures. Increased formation of angiogenic structures seems to correlate with the upregulation of vascular endothelial growth factor or angiopoietins (Ang-1 and Ang-2) studied at both the mRNA and protein levels. In addition, treatment with cyclopamine, an inhibitor of hedgehog signaling, blocked the formation of microvessel-like structures in the cocultures. However, exogenous Shh also resulted in the upregulation of several osteogenic differentiation markers in real-time polymerase chain reaction, as well as in an increased mineralization and alkaline phosphatase activity. The present data highlight the central role of the Shh pathway in bone regeneration and vascularization. Further, Shh might have the potential to improve both angiogenesis and osteogenesis in clinical applications in the future.

Vascularization in bone tissue engineering: physiology, current strategies, major hurdles and future challenges

The lack of a functional vascular supply has, to a large extent, hampered the whole range of clinical applications of 'successful' laboratory-based bone tissue engineering strategies. To the present, grafts have been dependent on post-implant vascularization, which jeopardizes graft integration and often leads to its failure. For this reason, the development of strategies that could effectively induce the establishment of a microcirculation in the engineered constructs has become a major goal for the tissue engineering research community. This review addresses the role and importance of the development of a vascular network in bone tissue engineering and provides an overview of the most up to date research efforts to develop such a network.

Transient muscle paralysis disrupts bone homeostasis by rapid degradation of bone morphology

We have previously shown that transient paralysis of murine hindlimb muscles causes profound degradation of both trabecular and cortical bone in the adjacent skeleton within 3 weeks. Morphologically, the acute loss of bone tissue appeared to arise primarily due to osteoclastic bone resorption. Given that the loss of muscle function in this model is transient, we speculated that the stimulus for osteoclastic activation would be rapid and morphologic evidence of bone resorption would appear before 21 days. We therefore utilized high-resolution in vivo serial micro-CT to assess longitudinal alterations in lower hindlimb muscle volume, proximal tibia trabecular, and tibia mid-diaphysis cortical bone morphology in 16-week-old female C57 mice following transient calf paralysis from a single injection of botulinum toxin A (BtA; 2U/100 g body weight). In an acute study, we evaluated muscle and bone alterations at days 0, 3, 5, and 12 following transient calf paralysis. In a chronic study, following day 0 imaging, we assessed the recovery of these tissues following the maximum observed trabecular degradation (day 12) through day 84 post-paralysis. The time course and degree of recovery of muscle, trabecular, and cortical bone varied substantially. Significant atrophy of lower limb muscle was evident by day 5 of paralysis, maximal at day 28 (-34.1+/-0.9%) and partially recovered by day 84. Trabecular degradation within the proximal tibia metaphysis occurred more rapidly, with significant reduction in BV/TV by day 3, maximal loss at day 12 (-76.8+/-2.9%) with only limited recovery by day 84 (-51.7+/-5.1% vs. day 0). Significant cortical bone volume degradation at the tibia mid-diaphysis was first identified at day 12, was maximal at day 28 (-9.6+/-1.2%), but completely recovered by day 84. The timing, magnitude, and morphology of the observed bone erosion induced by transient muscle paralysis were consistent with a rapid recruitment and prolific activation of osteoclastic resorption. In a broader context, understanding how brief paralysis of a single muscle group can precipitate such rapid and profound bone resorption in an adjacent bone is likely to provide new insight into how normal muscle function modulates bone homeostasis.

Cell-to-cell communication between osteogenic and endothelial lineages: implications for tissue engineering

There have been extensive research efforts to develop new strategies for bone tissue engineering. These have mainly focused on vascularization during the development and repair of bone. It has been hypothesized that pre-seeding a scaffold with endothelial cells could improve angiogenesis and bone regeneration through a complex dialogue between endothelial cells and bone-forming cells. Here, we focus on the paracrine signals secreted by both cell types and the effects they elicit. We discuss the other modes of cell-to-cell communication that could explain their cell coupling and reciprocal interactions. Endothelial cell-derived tube formation in a scaffold and the dialogue between endothelial cells and mesenchymal stem cells provide promising means of generating vascular bone tissue-engineered constructs.

Role of MT1-MMP in the osteogenic differentiation

Metalloproteinase MT1-MMP is induced and Pro-MMP-2 up modulated early in rat preosteoblasts (ROB) set to differentiate. We here show that the induction of MMPs, accompanied by activation of Pro-MMP-2, occurs by 6 h of adhesion on endogenous extracellular matrix (ECM), Fibronectin (FN) and Collagen type I (CI). These events do not occur after adhesion on Collagen III (CIII), Vitronectin (VN) or BSA. Within the first hour on inducing substrata or plastic, FAK is unchanged and ERK(1,2), is activated, but this activation is not sufficient for MT1-MMP induction. The function of p38 MAPK and PTKs is not required for the induction by substrata of MMPs. Six hours after plating preosteoblasts on MMP-inducing substrata, complexes of beta1 integrin with MT1-MMP are formed, that contain integrin dimers specifically engaged by the substratum, alpha4 and alpha5 chains for cells plated on FN, and alpha2 chain for cells plated on CI and ECM. Induction of MT1-MMP and its expression during osteogenesis pleiotropically regulate alkaline phosphatase (AP) expression. During differentiation, variant clones derived from preosteoblasts and MMPs-over-expressing osteoblasts show high MT1-MMP level associated with high AP level both persisting in time, while inhibition of MMPs is accompanied by inhibition of AP. Up or down modulation of AP, transcriptionally or by inhibition of the enzyme activity, has no effect on level or timing of expression of MT1-MMP and Pro-MMP-2. The persistence in expression of MT1-MMP during differentiation, and the associated persistence in expression of AP, as well as their inhibition, both impair the formation of nodules and mineral deposition. A transient pattern of expression of MT1-MMP is required for the establishment of nodules, and MT1-MMP decrease is permissive for nodule mineralization. The expression of AP is required for nodule formation and its level modulates the mineralization. MT1-MMP has multiple functions and is implicated in multiple steps of the differentiation process, acting to regulate homeostasis of the osteogenic differentiation.

Bone remodeling: A review of the bone microenvironment perspective for fragility fracture (osteoporosis) of the hip

Bone remodeling is an active process throughout the skeleton. The concept of bone turnover surface has been developed and reported in the peer reviewed literature as the quotient of formation surface/resorption surface and is significantly lower in hip fracture. It is necessary to identify the molecular drivers of these changes in bone turnover. Factors that have been strongly implicated in bone metabolism are therefore divided into three categories, "Vascular", "Anabolic" and "Catabolic". Further data is required from the bone tissue-level microenvironment, of fragility fracture patients, on the variation in molecular signals, and the associated changes in bone structure and remodeling.

Microvessel density in sinus augmentation procedures using anorganic bovine bone and autologous bone: 3 months results

PURPOSE

This study was an immunohistochemical evaluation of microvessel density (MVD) in sinus augmentation procedures with autologous bone and anorganic bone (Bio-Oss).

MATERIALS AND METHODS

Twenty-four patients (14 men and 10 women - mean age of 48 years with a range from 34 to 53 years) participated in this study. All the patients presented a maxillary partial unilateral edentulism involving the premolar/molar areas, with a residual alveolar ridge height of about 4 to 5 mm. Twelve patients received sinus augmentation procedures with 100% autologous bone; 100% Bio-Oss was used in the other 12 patients. Endosseous implants were inserted after a mean period of 3 months. As control, the portions of preexisting subantral bone were used. The mean value of the MVD in control bone was 23.4 +/- 1.3. The mean value of the MVD in the sinuses augmented with autologous bone was 29.0 +/- 2.4. The mean value of the MVD in the sinuses augmented with Bio-Oss was 23.8 +/- 2.2.

RESULTS

The statistical analysis showed that the differences of the MVD between control bone and sinuses augmented with Bio-Oss were not statistically significant (P = 0.52), while the difference of the MVD between sinuses augmented with autologous bone and those augmented with Bio-Oss was statistically significant (P = 0.0008).

CONCLUSIONS

Autologous bone may act not only as a passive filling material in bone defects but may also release osteogenic growth factors; and particles of autologous bone seem to contain vital osteoprogenitor cells.

Microvessel-Like Structures from Outgrowth Endothelial Cells from Human Peripheral Blood in 2-Dimensional and 3-Dimensional Co-Cultures with Osteoblastic Lineage Cells

Tissue regeneration involves complex processes in the interaction between different cell types that control the process of neo-vascularization. In bone, osteoblasts and bone marrow stem cells provide cue elements for the proliferation of endothelial cells, differentiation of endothelial precursors, and the maturation of a vascular network. In this study, we investigated outgrowth endothelial cells (OECs), a potential source of autologous endothelial cells derived from human peripheral blood, in direct 2-dimensional (2-D) and 3-D co-culture systems with cells relevant for the regeneration of bone tissue, such as osteoblasts. In the co-cultures, OECs were evaluated in terms of their stability as an endothelial population at the single cell level using flow cytometry and their ability to establish a pre-vascular network at the light-microscopical and ultra-structural level. In co-cultures with the osteoblast cell line MG63 and with human primary osteoblasts (pOBs), OECs, in contrast to human umbilical vein endothelial cells, formed highly organized microvessel-like structures. These microvessel-like structures included the formation of a vascular lumen with tight junctional complexes at intercellular contacts of endothelial cells. In the co-culture, the formation of this vascular network was achieved in the standard growth medium for OECs. Furthermore, using a rotating culture vessel system, 3-D co-cultures consisting of OECs and pOBs were generated. Based on these observations, we conclude that OECs could provide a valuable source of autologous endothelial cells for the generation of complex tissue-engineered tissues.

Remodeling and vascular spaces in bone

In recent years, we have come to appreciate that the close association between bone and vasculature plays a pivotal role in the regulation of bone remodeling and fracture repair. In 2001, Hauge et al. characterized a specialized vascular structure, the bone remodeling compartment (BRC), and showed that the outer lining of this compartment was made up of flattened cells, displaying all the characteristics of lining cells in bone. A decrease in bone turnover leads to a decrease in surfaces covered with remodeling compartments, whereas increased turnover causes an increase. Immunoreactivity for all major osteotropic growth factors and cytokines including osteoprotegerin (OPG) and RANKL has been shown in the cells lining the BRC, which makes the BRC the structure of choice for coupling between resorption and formation. The secretion of these factors inside a confined space separated from the bone marrow would facilitate local regulation of the remodeling process without interference from growth factors secreted by blood cells in the marrow space. The BRC creates an environment where cells inside the structure are exposed to denuded bone, which may enable direct cellular interactions with integrins and other matrix factors known to regulate osteoclast/osteoblast activity. However, the denuded bone surface inside the BRC also constitutes an ideal environment for the seeding of bone metastases, known to have high affinity for bone matrix. Reduction in BRC space brought about by antiresorptive therapies such as bisphosphonates reduce the number of skeletal events in advanced cancer, whereas an increase in BRC space induced by remodeling activators like PTH may increase the bone metastatic burden. The BRC has only been characterized in detail in trabecular bone; there is, however, evidence that a similar structure may exist in cortical bone, but further characterization is needed.

Targeted approaches for the management of metastatic prostate cancer

Huggins and Hodges described the first systemic targeted therapy for prostate cancer in 1941 with their report on the effects of androgen ablation in men with metastatic disease. Since that time, researchers have identified multiple additional "targets" that may be important in prostate cancer tumorigenesis. These areas include continued emphasis on the androgen receptor in the androgen-independent state, parallel growth pathways such as AKT and HER2 that may act in conjunction or independently of the androgen receptor, the supporting environment that allows for the development of metastatic disease, and standard cytotoxic targets, such as the microtubule. This review is intended to highlight these potential targets and several of the agents that are under development in the treatment of prostate cancer.

Effects of endothelin antagonist tezosentan on orthodontic tooth movement in rats

INTRODUCTION

Endothelin is a cytokine peptide present in the periodontal ligament in physiological conditions. Its concentration in the periodontal ligament doubles after 3 hours of axial loading of a tooth. The role of endothelin in orthodontic tooth movement has not yet been explained. We tried to determine the effect of tezosentan, a specific dual endothelin receptor antagonist, on orthodontic tooth movement in rats.

METHODS

Twenty-one rats were divided into 3 groups (n = 7). In the tezosentan group, closed-coil springs were placed, and the animals were treated daily with tezosentan; in the coil-spring group, closed-coil springs were placed, and the animals were treated with an inert physiological solution. The appliance was placed between the maxillary left first molar and the incisor. The third group was the control group. On days 0, 7, 14, and 25, the distances between the maxillary molars and incisors were measured at the experimental and contralateral sides.

RESULTS

Changes in measured distances at the experimental sides were significantly different between the tezosentan and the coil-spring groups on day 25 (P < .001), with the tezosentan group showing greater changes in the measured distances. There were no differences between the tezosentan and the coil-spring groups at the contralateral side. In the control group, the measured distance increased at both sides (P < .001).

CONCLUSIONS

Tezosentan, an endothelin dual antagonist, enhanced tooth movements in rats.

Role of osterix in endothelin-1-induced downregulation of vascular endothelial growth factor in osteoblastic cells

Endothelin-1 (ET-1) is produced by vascular endothelial cells to play an important role during bone development, remodeling and repair. ET-1 promotes osteoblastic cell proliferation and differentiation, but has the unique effect of downregulating vascular endothelial growth factor (VEGF) and may thereby control angiogenesis during bone production. Our objectives were to identify the intracellular mechanisms by which ET-1 controls VEGF expression during osteoblastic proliferation and differentiation. ET-1 induced osteoblastic differentiation in rat SBMC-D8 osteoblastic cells, but downregulated expression of VEGF mRNA isoforms (VEGF120, 164 and 188) as demonstrated by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and by use of a luciferase reporter construct containing the promoter region of the VEGF gene. Co-transfection with the endothelin receptor A (ETRA) had the same effect. ET-1 and ETRA both upregulated the transcription factor osterix (Osx). RNA silencing of Osx resulted in an upregulation of VEGF. This study supports the novel inhibitory role for ET-1, via Osx, on VEGF synthesis in osteoblastic cells as a possible mechanism in the temporal and spatial feedback of angiogenesis to bone formation and resorption.

Origins of endothelial and osteogenic cells in the subcutaneous collagen gel implant

The interdependent relationship between vascular endothelial cells and osteoblasts during bone formation and fracture healing has been long appreciated. This paper reports a heterotopic implant model using FGF-2-expanded bone marrow stromal cells (BMSC) derived from Tie2eGFP (endothelial marker) and pOBCol3.6GFPcyan or topaz (early osteoblast marker) transgenic mice to appreciate the host/donor relationships of cells participating in the process of heterotopic bone formation. The study included various combinations of Tie2eGFP and pOBCol3.6GFPcyan and topaz transgenics as BMSC or whole bone marrow (WBM) donors and also as recipients. Rat tail collagen was used as a carrier of donor cells and implantation was done in lethally irradiated mice rescued with WBM injection. Development of ossicles in the implants was followed weekly during the 4- to 5-week long post-implantation period. By 4-5 weeks after total body irradiation (TBI) and implantation, a well-formed bone spicule had developed that was invested with bone marrow. Experiments showed absolute dominance of donor-derived cells in the formation of endothelial-lined vessels inside the implants as well as the marrow stromal-derived osteogenic cells. Host-derived fibroblasts and osteogenic cells were confined to the fibrous capsule surrounding the implant. In addition, cells lining the endosteal surface of newly formed marrow space carrying a pOBCol3.6GFP marker were observed that were contributed by WBM donor cells and the host. Thus, FGF-2-expanded BMSC appear to be a source of endothelial and osteogenic progenitor cells capable of eliciting heterotopic bone formation independent of cells from the host. This model should be useful for understanding the interactions between these two cell types that control osteogenic differentiation in vivo.

New blood vessel formation and expression of VEGF receptors after implantation of platelet growth factor-enriched biodegradable nanocrystalline hydroxyapatite

Vascular endothelial growth factor (VEGF) plays a key role for the interaction of osteoblasts and endothelial cells and, therefore, is an important factor for the osteointegration of bone substitutes. The aim of the current work was to study the effects of platelet growth factors (PLF) on new blood vessel formation and VEGF-receptors expression pattern in bone defects filled with nanocrystalline hydroxyapatite (HA) paste in miniature-pigs. Conventional histology, RT-PCR for VEGF and receptors mRNA, Western blot analysis, immunohistochemical staining and quantitative assessment of newly formed vessels was performed. HA enriched with platelet growth factor (HA/PLF+) led to an up-regulation of VEGF-R1 synthesis, a slightly enhanced number of newly formed vessels with higher sprouting activity compared with HA without PLF (HA/PLF-) filling defects. These observation are most likely attributable to a stimulating effect of TGF-ss from the platelet factor on VEGF expression in osteoblasts.