Dimethyloxaloylglycine improves angiogenic activity of bone marrow stromal cells in the tissue-engineered bone

Effect of Dimethyloxalylglycine on Stem Cells Osteogenic Differentiation and Bone Tissue Regeneration-A Systematic Review

Dimethyloxalylglycine (DMOG) has been found to stimulate osteogenesis and angiogenesis of stem cells, promoting neo-angiogenesis in bone tissue regeneration. In this review, we conducted a comprehensive search of the literature to investigate the effects of DMOG on osteogenesis and bone regeneration. We screened the studies based on specific inclusion criteria and extracted relevant information from both in vitro and in vivo experiments. The risk of bias in animal studies was evaluated using the SYRCLE tool. Out of the 174 studies retrieved, 34 studies met the inclusion criteria (34 studies were analyzed in vitro and 20 studies were analyzed in vivo). The findings of the included studies revealed that DMOG stimulated stem cells' differentiation toward osteogenic, angiogenic, and chondrogenic lineages, leading to vascularized bone and cartilage regeneration. Addtionally, DMOG demonstrated therapeutic effects on bone loss caused by bone-related diseases. However, the culture environment in vitro is notably distinct from that in vivo, and the animal models used in vivo experiments differ significantly from humans. In summary, DMOG has the ability to enhance the osteogenic and angiogenic differentiation potential of stem cells, thereby improving bone regeneration in cases of bone defects. This highlights DMOG as a potential focus for research in the field of bone tissue regeneration engineering.

The potential mediation of hypoxia-inducible factor-1α in heat shock protein 27 translocations, caspase-3 and calpain activities and yak meat tenderness during postmortem aging

The present study aimed to characterize the influence of hypoxia-inducible factor-1α on heat shock protein 27 and cytochrome c translocation, yak meat microstructure destruction, and endogenous enzymes activities, refining the understanding of the tenderization process after slaughter. Postmortem yak longissimus thoracis et lumborum muscles were incubated with 0.9% saline or hypoxia-inducible factor-1α stabilizer dimethyloxaloylglycine at 4 °C for 6, 12, 24, 72, and 120 h. Results showed that hypoxia-inducible factor-1α activation promoted heat shock protein 27 migration and cytochrome c release, facilitating (P < 0.05) caspase-3 activity by mediating the heat shock protein 27/caspase-3 interaction but did not exert (P > 0.05) significant effects on the calpain-1 activity. Additionally, hypoxia-inducible factor-1α activation contributed to the mitochondrial apoptosis cascade, leading to a higher (P < 0.01) apoptosis rate. Therefore, these observations indicate that hypoxia-inducible factor-1α affects caspase-3 activity and tenderness of postmortem muscle through distinct regulatory mechanisms, possibly, in part, with heat shock protein 27 and cytochrome c mediation.

HIF-1α increases the osteogenic capacity of ADSCs by coupling angiogenesis and osteogenesis via the HIF-1α/VEGF/AKT/mTOR signaling pathway

BACKGROUND

Stabilization and increased activity of hypoxia-inducible factor 1-α (HIF-1α) can directly increase cancellous bone formation and play an essential role in bone modeling and remodeling. However, whether an increased HIF-1α expression in adipose-derived stem cells (ADSCs) increases osteogenic capacity and promotes bone regeneration is not known.

RESULTS

In this study, ADSCs transfected with small interfering RNA and HIF-1α overexpression plasmid were established to investigate the proliferation, migration, adhesion, and osteogenic capacity of ADSCs and the angiogenic ability of human umbilical vein endothelial cells (HUVECs). Overexpression of HIF-1α could promote the biological functions of ADSCs, and the angiogenic ability of HUVECs. Western blotting showed that the protein levels of osteogenesis-related factors were increased when HIF-1α was overexpressed. Furthermore, the influence of upregulation of HIF-1α in ADSC sheets on osseointegration was evaluated using a Sprague-Dawley (SD) rats implant model, in which the bone mass and osteoid mineralization speed were evaluated by radiological and histological analysis. The overexpression of HIF-1α in ADSCs enhanced bone remodeling and osseointegration around titanium implants. However, transfecting the small interfering RNA (siRNA) of HIF-1α in ADSCs attenuated their osteogenic and angiogenic capacity. Finally, it was confirmed in vitro that HIF-1α promotes osteogenic differentiation and the biological functions in ADSCs via the VEGF/AKT/mTOR pathway.

CONCLUSIONS

This study demonstrates that HIF-1α has a critical ability to promote osteogenic differentiation in ADSCs by coupling osteogenesis and angiogenesis via the VEGF/AKT/mTOR signaling pathway, which in turn increases osteointegration and bone formation around titanium implants.

Flowerbed-Inspired Biomimetic Scaffold with Rapid Internal Tissue Infiltration and Vascularization Capacity for Bone Repair

The favorable microstructure and bioactivity of tissue-engineered bone scaffolds are closely associated with the regenerative efficacy of bone defects. For the treatment of large bone defects, however, most of them fail to meet requirements such as adequate mechanical strength, highly porous structure, and excellent angiogenic and osteogenic activities. Herein, inspired by the characteristics of a "flowerbed", we construct a short nanofiber aggregates-enriched dual-factor delivery scaffold via 3D printing and electrospinning techniques for guiding vascularized bone regeneration. By the assembly of short nanofibers containing dimethyloxalylglycine (DMOG)-loaded mesoporous silica nanoparticles with a 3D printed strontium-contained hydroxyapatite/polycaprolactone (SrHA@PCL) scaffold, an adjustable porous structure can be easily realized by changing the density of nanofibers, while strong compressive strength will be acquired due to the framework role of SrHA@PCL. Owing to the different degradation performance between electrospun nanofibers and 3D printed microfilaments, a sequential release behavior of DMOG and Sr ions is achieved. Both in vivo and in vitro results demonstrate that the dual-factor delivery scaffold has excellent biocompatibility, significantly promotes angiogenesis and osteogenesis by stimulating endothelial cells and osteoblasts, and effectively accelerates tissue ingrowth and vascularized bone regeneration through activating the hypoxia inducible factor-1α pathway and immunoregulatory effect. Overall, this study has provided a promising strategy for constructing a bone microenvironment-matched biomimetic scaffold for bone regeneration.

Functional Approaches in Promoting Vascularization and Angiogenesis in Bone Critical-Sized Defects via Delivery of Cells, Growth Factors, Drugs, and Particles

Critical-sized bone defects, or CSDs, are defined as bone defects that cannot be regenerated by themselves and require surgical intervention via employing specific biomaterials and a certain regenerative strategy. Although a variety of approaches can be used to treat CSDs, poor angiogenesis and vascularization remain an obstacle in these methods. The complex biological healing of bone defects depends directly on the function of blood flow to provide sufficient oxygen and nutrients and the removal of waste products from the defect site. The absence of vascularization can lead to non-union and delayed-union defect development. To overcome this challenge, angiogenic agents can be delivered to the site of injury to stimulate vessel formation. This review begins by introducing the treatment methods for CSDs. The importance of vascularization in CSDs is subsequently highlighted. Delivering angiogenesis agents, including relevant growth factors, cells, drugs, particles, cell secretion substances, their combination, and co-delivery to CSDs are fully explored. Moreover, the effects of such agents on new bone formation, followed by vessel formation in defect areas, are evaluated.

HIF-1α Regulates Bone Homeostasis and Angiogenesis, Participating in the Occurrence of Bone Metabolic Diseases

In the physiological condition, the skeletal system's bone resorption and formation are in dynamic balance, called bone homeostasis. However, bone homeostasis is destroyed under pathological conditions, leading to the occurrence of bone metabolism diseases. The expression of hypoxia-inducible factor-1α (HIF-1α) is regulated by oxygen concentration. It affects energy metabolism, which plays a vital role in preventing bone metabolic diseases. This review focuses on the HIF-1α pathway and describes in detail the possible mechanism of its involvement in the regulation of bone homeostasis and angiogenesis, as well as the current experimental studies on the use of HIF-1α in the prevention of bone metabolic diseases. HIF-1α/RANKL/Notch1 pathway bidirectionally regulates the differentiation of macrophages into osteoclasts under different conditions. In addition, HIF-1α is also regulated by many factors, including hypoxia, cofactor activity, non-coding RNA, trace elements, etc. As a pivotal pathway for coupling angiogenesis and osteogenesis, HIF-1α has been widely studied in bone metabolic diseases such as bone defect, osteoporosis, osteonecrosis of the femoral head, fracture, and nonunion. The wide application of biomaterials in bone metabolism also provides a reasonable basis for the experimental study of HIF-1α in preventing bone metabolic diseases.

Hypoxia-Inducible Factors Signaling in Osteogenesis and Skeletal Repair

Sufficient oxygen is required to maintain normal cellular and physiological function, such as a creature’s development, breeding, and homeostasis. Lately, some researchers have reported that both pathological hypoxia and environmental hypoxia might affect bone health. Adaptation to hypoxia is a pivotal cellular event in normal cell development and differentiation and in pathological settings such as ischemia. As central mediators of homeostasis, hypoxia-inducible transcription factors (HIFs) can allow cells to survive in a low-oxygen environment and are essential for the regulation of osteogenesis and skeletal repair. From this perspective, we summarized the role of HIF-1 and HIF-2 in signaling pathways implicated in bone development and skeletal repair and outlined the molecular mechanism of regulation of downstream growth factors and protein molecules such as VEGF, EPO, and so on. All of these present an opportunity for developing therapies for bone regeneration.

HIF signaling: A new propellant in bone regeneration

Bone tissue destruction leads to severe pain, physical flaws, and loss of motility. Bone repair using biocompatible and osteo-inductive scaffolds is regarded as a viable and potential therapeutic approach. However, for large-scale bone regeneration, oxygen and nutrient supply have become limiting factors. Further, a considerable need exists for recruited cell activities and blood vessel growth. Hypoxia-inducible factor (HIF) signaling pathways induced by hypoxia are involved in angiogenesis and osteogenesis. As an important transcription factor, HIF-1 functions by modulating vital genes, such as VEGF, PDK1, and EPO, and is a crucial regulator that influences the final fate of bone regeneration. Collectively, to achieve better osteogenesis results, the in-depth molecular mechanisms that underpin the links between materials, cells, and HIF signaling pathways must be determined. This review aimed to provide an in-depth insight into recent progress in HIF-regulated bone regeneration. Hypoxia and cellular oxygen-sensing mechanisms and their correlations with osteogenesis were determined, and recent studies on hypoxia-inducing and hypoxia-mimicking strategies were briefly described. Finally, the potential applications of HIF signaling in bone regeneration were highlighted. This review provides theoretical support for establishing a novel and viable bone repair strategy in the clinic by harnessing HIF signaling.

Preconditioning Methods to Improve Mesenchymal Stromal Cell-Derived Extracellular Vesicles in Bone Regeneration—A Systematic Review

Simple Summary

The evidence of the therapeutic effects of mesenchymal stromal cells (MSCs), so-called stem cells, in several diseases relies mostly on the substances they secrete, including their extracellular vesicles (EVs). EVs are an important component of cell communication and they carry a cargo that is similar to their parent cell. Cells respond differently based on their microenvironment, and so it is expected that the therapeutic potential of these vesicles can be modulated by the enrichment of their parent cell microenvironment. With this in mind, we conducted a systematic search for papers that preconditioned MSCs and collected their EVs to assess their potential to favor bone formation. The results showed different methods for MSC preconditioning, including chemical induction, culture conditions, and genetic modifications. All methods were able to improve the therapeutic effects of the derived EVs for bone formation. However, the heterogeneity among studies—regarding the type of cell, EV concentration, and scaffolds—made it difficult to compare fairly the types of preconditioning methods. In summary, the microenvironment greatly influences MSCs, and using preconditioning methods can potentially improve the therapeutic effects of their derived EVs in bone regeneration and other bone diseases.

Abstract

Mesenchymal stromal cells (MSCs) have long been used in research for bone regeneration, with evidence of their beneficial properties. In the segmental area of MSC-based therapies, MSC-derived extracellular vesicles (EVs) have also shown great therapeutic effects in several diseases, including bone healing. This study aimed to assess whether the conditioning of MSCs improves the therapeutic effects of their derived extracellular vesicles for bone regeneration. Electronic research was performed until February 2021 to recover the studies in the following databases: PubMed, Scopus, and Web of Science. The studies were screened based on the inclusion criteria. Relevant information was extracted, including in vitro and in vivo experiments, and the animal studies were evaluated for risk of bias by the SYRCLE tool. A total of 463 studies were retrieved, and 18 studies met the inclusion criteria (10 studies for their in vitro analysis, and 8 studies for their in vitro and in vivo analysis). The conditioning methods reported included: osteogenic medium; dimethyloxalylglycine; dexamethasone; strontium-substituted calcium silicate; hypoxia; 3D mechanical microenvironment; and the overexpression of miR-375, bone morphogenetic protein-2, and mutant hypoxia-inducible factor-1α. The conditioning methods of MSCs in the reported studies generate exosomes able to significantly promote bone regeneration. However, heterogeneity regarding cell source, conditioning method, EV isolation and concentration, and defect model was observed among the studies. The different conditioning methods reported in this review do improve the therapeutic effects of MSC-derived EVs for bone regeneration, but they still need to be addressed in larger animal models for further clinical application.

[Research progress of antibacterial modification of orthopaedic implants surface]

OBJECTIVE

To summarize the related research progress of antibacterial modification of orthopaedic implants surface in recent years.

METHODS

The domestic and foreign related literature in recent years was extensively consulted, the research progress on antibacterial modification of orthopaedic implants surface was discussed from two aspects of characteristics of infection in orthopedic implants and surface anti-infection modification.

RESULTS

The orthopaedic implants infections are mainly related to aspects of bacterial adhesion, decreased host immunity, and surface biofilm formation. At present, the main antimicrobial coating methods of orthopaedic implants are antibacterial adhesion coating, antibiotic coating, inorganic antimicrobial coating, composite antimicrobial coating, nitric oxide coating, immunomodulation, three-dimensional printing, polymer antimicrobial coating, and "smart" coating.

CONCLUSION

The above-mentioned antibacterial coating methods of orthopedic implants can not only inhibit bacterial adhesion, but also solve the problems of low immunity and biofilm formation. However, its mechanism of action and modification are still controversial and require further research.

Vascularized Tumor Spheroid-on-a-Chip Model Verifies Synergistic Vasoprotective and Chemotherapeutic Effects

Prolyl hydroxylases (PHD) inhibitors have been observed to improve drug distribution in mice tumors via blood vessel normalization, increasing the effectiveness of chemotherapy. These effects are yet to be demonstrated in human cell models. Tumor spheroids are three-dimensional cell clusters that have demonstrated great potential in drug evaluation for personalized medicine. Here, we used a perfusable vascularized tumor spheroid-on-a-chip to simulate the tumor microenvironment in vivo and demonstrated that the PHD inhibitor dimethylallyl glycine prevents the degradation of normal blood vessels while enhancing the efficacy of the anticancer drugs paclitaxel and cisplatin in human esophageal carcinoma (Eca-109) spheroids. Our results point to the potential of this model to evaluate anticancer drugs under more physiologically relevant conditions.

Endothelial Transient Receptor Potential Canonical Channel Regulates Angiogenesis and Promotes Recovery After Myocardial Infarction

Background

Transient receptor potential canonical (TRPC) channels play a role in angiogenesis. However, the involvement of TRPC1 in myocardial infarction (MI) remains unclear. The present study was aimed at investigating whether TRPC1 can improve the recovery of cardiac function via prompting angiogenesis following MI.

Methods and Results

In vitro, coronary artery endothelial cells from floxed TRPC1 mice and endothelial cell‐specific TRPC1 channel knockout mice were cultured to access EC angiogenesis. Both EC tube formation and migration were significantly suppressed in mouse coronary artery endothelial cells from endothelial cell‐specific TRPC1 channel knockout mice. In vivo, coronary artery endothelial cells from floxed TRPC1 and endothelial cell‐specific TRPC1 channel knockout mice were subjected to MI, then echocardiography, triphenyltetrazolium chloride staining and immunofluorescence were performed to assess cardiac repair on day 28. Endothelial cell‐specific TRPC1 channel knockout mice had higher ejection fraction change, larger myocardial infarct size, and reduced capillary density in the infarct area compared with coronary artery endothelial cells from floxed TRPC1 mice. Furthermore, we found underlying regulation by HIF‐1α (hypoxic inducible factor‐1α) and MEK‐ERK (mitogen‐activated protein kinase/extracellular signal‐regulated kinase) that could be the mechanism for the angiogenetic action of TRPC1. Significantly, treatment with dimethyloxaloylglycine, an activator of HIF‐1α, induced cardiac improvement via the HIF‐1α‐TRPC1‐MEK/ERK pathway in MI mice.

Conclusions

Our study demonstrated TRPC1 improves cardiac function after MI by increasing angiogenesis via the upstream regulator HIF‐1α and downstream MEK/ERK, and dimethyloxaloylglycine treatment has protective effect on MI through the HIF‐1α‐TRPC1‐MEK/ERK pathway.

Dental Pulp Mesenchymal Stem Cells Attenuate Limb Ischemia via Promoting Capillary Proliferation and Collateral Development in a Preclinical Model

Critical limb ischemia (CLI), an end-stage manifestation of peripheral artery disease (PAD), still lacks effective therapeutic strategies. Recently, dental pulp-derived mesenchymal stem cells (DP-MSCs) have been attracting more and more attentions in therapeutic applications due to their high proliferation ability, powerful osteogenic differentiation potential, and effective anti-inflammatory effects. In this study, we compared the therapeutic effects of MSCs derived from different sources in a femoral artery-ligated preclinical ischemic model. We found that treatments with MSCs, including bone marrow- (BM-), adipose- (AD-), dental pulp- (DP-), and umbilical cord- (UC-) derived MSCs, improved limb functions, reduced inflammatory responses, increased angiogenesis, and promoted regeneration of muscle fiber. Among them, DP-MSCs and BM-MSCs produced much more impressive effects in restoring limb functions and promoting angiogenesis. The flow velocity restored to nearly 20% of the normal level at 3 weeks after treatments with DP-MSCs and BM-MSCs, and obvious capillary proliferation and collateral development could be observed. Although neovascularization was induced in the ischemic limb after ligation, MSCs, especially DP-MSCs, significantly enhanced the angiogenesis. In vitro experiments showed that serum deprivation improved the expression of angiogenic factors, growth factors, and chemokines in DP-MSCs and UC-MSCs, but not in BM-MSCs and AD-MSCs. However, DP-MSCs produced stronger therapeutic responses than UC-MSCs, which might be due to the higher expression of hepatocyte growth factor (HGF) and hypoxia-inducible factor-1 α (HIF-1α). We speculated that DP-MSCs might stimulate angiogenesis and promote tissue repair via expressing and secreting angiogenic factors, growth factors, and chemokines, especially HGF and HIF-1α. In conclusion, DP-MSCs might be a promising approach for treating CLI.

CaO2/gelatin oxygen slow-releasing microspheres facilitate tissue engineering efficiency for the osteonecrosis of femoral head by enhancing the angiogenesis and survival of grafted bone marrow mesenchymal stem cells

The osteonecrosis of femoral head (ONFH), a common refractory disease, is still not fully understood today. Hypoxia caused by ischemia is not only an important pathogenic factor but also a critical challenge for the survival of seed cells in the tissue engineering therapy of ONFH. To explore an efficient strategy to treat ONFH by targeting hypoxia, newly designed CaO₂/gelatin microspheres were composited with 3D printed polycaprolactone/nano-hydroxyapatite (PCL/nHA) porous scaffold, sodium alginate/gelatin hydrogel, and bone marrow mesenchymal stem cells (BMSCs) to develop a novel tissue engineering scaffold and then transplanted into the core depression area of the ONFH rabbit model. The current data demonstrated that CaO₂/gelatin microspheres can constantly release oxygen for 19 days. In vitro assays with BMSCs illustrated that scaffolds have high biocompatibility and are favorable for cell proliferation in extreme hypoxia (1% O₂). The in vivo study demonstrated that the transplanted scaffold with oxygen-generating microspheres significantly enhanced the osteogenic and angiogenic effects compared to the scaffold without microspheres. Further assessments revealed that microspheres in the scaffold can reduce the local cell apoptosis and enhance the survival of grafted cells in the host. Collectively, the present study developed a novel oxygen slow-releasing composite scaffold, which can facilitate tissue engineering efficiency for treating the osteonecrosis of the femoral head by enhancing the angiogenesis and survival of grafted stem cells.

Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling

Historically, the field of regenerative medicine has aimed to heal damaged tissue through the use of biomaterials scaffolds or delivery of foreign progenitor cells. Despite 30 years of research, however, translation and commercialization of these techniques has been limited. To enable mammalian regeneration, a more practical approach may instead be to develop therapies that evoke endogenous processes reminiscent of those seen in innate regenerators. Recently, investigations into tadpole tail regrowth, zebrafish limb restoration, and the super-healing Murphy Roths Large (MRL) mouse strain, have identified ancient oxygen-sensing pathways as a possible target to achieve this goal. Specifically, upregulation of the transcription factor, hypoxia-inducible factor one alpha (HIF-1α) has been shown to modulate cell metabolism and plasticity, as well as inflammation and tissue remodeling, possibly priming injuries for regeneration. Since HIF-1α signaling is conserved across species, environmental or pharmacological manipulation of oxygen-dependent pathways may elicit a regenerative response in non-healing mammals. In this review, we will explore the emerging role of HIF-1α in mammalian healing and regeneration, as well as attempts to modulate protein stability through hyperbaric oxygen treatment, intermittent hypoxia therapy, and pharmacological targeting. We believe that these therapies could breathe new life into the field of regenerative medicine.

Delivery of dimethyloxalylglycine in calcined bone calcium scaffold to improve osteogenic differentiation and bone repair

As hypoxia plays a vital role in the angiogenic-osteogenic coupling, using proline hydroxylase inhibitors to manipulate hypoxia-inducible factors has become a strategy to improve the osteogenic properties of biomaterials. Dimethyloxallyl glycine (DMOG) is a 2-ketoglutarate analog, a small molecular compound that competes for 2-ketoglutaric acid to inhibit proline hydroxylase. In order to improve the osteogenic ability of calcined bone calcium (CBC), a new hypoxia-mimicking scaffold (DMOG/Collagen/CBC) was prepared by immersing it in the DMOG-Collagen solution, followed by freeze-drying. All coated CBC scaffolds retained the inherent natural porous architecture and showed excellent biocompatibility. A slow release of DMOG by the DMOG-loaded CBC scaffolds for up to one week was observed in in vitro experiments. Moreover, the DMOG/Collagen/CBC composite scaffold was found to significantly stimulate bone marrow stromal cells to express osteogenic and angiogenic genes in vitro. In addition, the osteogenic properties of three kinds of scaffolds, raw CBC, Collagen/CBC, and DMOG/Collagen/CBC, were evaluated by histology using the rabbit femoral condyle defect model. Histomorphometric analyses showed that the newly formed bone (BV/TV) in the DMOG/Collagen/CBC group was significantly higher than that of the Collagen/CBC group. However, immunostaining of CD31 and Runx2 expression between these two groups showed no significant difference at this time point. Our results indicate that DMOG-coated CBC can promote osteogenic differentiation and bone healing, and show potential for clinical application in bone tissue engineering.

Enhanced angiogenic activity of dimethyloxalylglycine-treated canine adipose tissue-derived mesenchymal stem cells

The paracrine function of mesenchymal stem cells (MSCs) during transplantation has been recently studied due to its poor differentiation ratio. Dimethyloxalylglycine (DMOG) has been used to promote angiogenesis in experimental animal models, however, comparable approaches for canine MSCs are not sufficient. In the present study, we assessed whether DMOG improves angiogenesis in canine adipose tissue-derived mesenchymal stem cells (cAT-MSCs). cAT-MSCs were treated with DMOG and their effect on angiogenesis was investigated by cell proliferation assay, western blotting, and tube formation assay. Dimethyloxalylglycine preconditioning enhanced the expression of vascular endothelial growth factor (VEGF) among pro-angiogenic factors in cAT-MSCs via hypoxia-inducible factor-1α stabilization. Dimethyloxalylglycine primed-cAT-MSC-conditioned media increased angiogenesis in human umbilical vein endothelial cells. These results suggest that DMOG conditioning of cAT-MSCs augmented the secretion of VEGF, which acted as a prominent pro-angiogenic factor during angiogenesis. DMOG-primed cAT-MSCs may have the potential to induce beneficial effects in ischemic diseases in clinical trials.

Hypoxia-induced upregulation of angiogenic factors in immortalized human periodontal ligament fibroblasts

Hypoxia induces complex cellular responses that are mediated by a key transcription factor, hypoxia-inducible factor-1 (HIF-1). HIF-1 promotes production of cytokines and angiogenic factors and contributes to recovery of injured tissues. In the present study, expressions of angiogenin (ANG) and vascular endothelial growth factor (VEGF), which are potent angiogenic factors in mammalian tissues, were examined in immortalized fibroblasts exposed to hypoxia. After 24 h of exposure to hypoxia, ANG and VEGF mRNAs expressions were significantly elevated in periodontal ligament (PDL) fibroblasts but not in embryonic fibroblasts. Hypoxia also increased productions of ANG and VEGF proteins in PDL fibroblasts. HIF-1α mRNA expression was not affected by hypoxia in either fibroblast, although HIF-1α protein expression was enhanced after exposure to hypoxia. Treatment of PDL fibroblasts with dimethyloxaloylglycine, a prolyl hydroxylase inhibitor that stabilizes the HIF-1α protein, significantly increased expressions of ANG and VEGF mRNAs under normoxia. This suggests that stabilization of HIF-1α is crucial for upregulation of ANG and VEGF in PDL fibroblasts. These results indicate that, under hypoxic conditions, HIF-1α upregulates synthesis of ANG and VEGF in PDL fibroblasts and promotes angiogenesis.

Hypoxia mimicking hydrogels to regulate the fate of transplanted stem cells

Controlling the phenotype of transplanted stem cells is integral to ensuring their therapeutic efficacy. Hypoxia is a known regulator of stem cell fate, the effects of which can be mimicked using hypoxia-inducible factor (HIF) prolyl hydroxylase inhibitors such as dimethyloxalylglycine (DMOG). By releasing DMOG from mesenchymal stem cell (MSC) laden alginate hydrogels, it is possible to stabilize HIF-1α and enhance its nuclear localization. This correlated with enhanced chondrogenesis and a reduction in the expression of markers associated with chondrocyte hypertrophy, as well as increased SMAD 2/3 nuclear localization in the encapsulated MSCs. In vivo, DMOG delivery significantly reduced mineralisation of the proteoglycan-rich cartilaginous tissue generated by MSCs within alginate hydrogels loaded with TGF-β3 and BMP-2. Together these findings point to the potential of hypoxia mimicking hydrogels to control the fate of stem cells following their implantation into the body. STATEMENT OF SIGNIFICANCE: There are relatively few examples where in vivo delivery of adult stem cells has demonstrated a true therapeutic benefit. This may be attributed, at least in part, to a failure to control the fate of transplanted stem cells in vivo. In this paper we describe the development of hydrogels that mimic the effects of hypoxia on encapsulated stem cells. In vitro, these hydrogels enhance chondrogenesis of MSCs and suppress markers associated with chondrocyte hypertrophy. In an in vivo environment that otherwise supports progression along an endochondral pathway, we show that these hydrogels will instead direct mesenchymal stem cells (MSCs) to produce a more stable, cartilage-like tissue. In addition, we explore potential molecular mechanisms responsible for these phenotypic changes in MSCs.

Promoting vascularization for tissue engineering constructs: current strategies focusing on HIF-regulating scaffolds

INTRODUCTION

Vascularization remains one of the greatest yet unmet challenges in tissue engineering. When engineered tissues are scaled up to therapeutically relevant dimensions, their demand of oxygen and nutrients can no longer be met by diffusion. Thus, there is a need for perfusable vascular structures. Hypoxia-inducible factors (HIF) act as transcriptional oxygen sensors and regulate a multitude of genes involved in adaptive processes to hypoxia, including angiogenesis. Thus, targeting HIFs is a promising strategy to induce vascularization of engineered tissues.

AREAS COVERED

Here we review current vascularization strategies and summarize the present knowledge regarding activation of HIF signaling by ions, iron chelating agents, α-Ketoglutarate (αKG) analogues, and the lipid-lowering drug simvastatin to induce angiogenesis. Specifically, we focus on the incorporation of HIF-activating agents into biomaterials and scaffolds for controlled release.

EXPERT OPINION

Vascularization of tissue constructs through activation of upstream regulators of angiogenesis offers advantages but also suffers from drawbacks. HIFs can induce a complete angiogenic program; however, this program appears to be too slow to vascularize larger constructs before cell death occurs. It is therefore crucial that HIF-activation is combined with cell protective strategies and prevascularization techniques to obtain fully vascularized, vital tissues of therapeutically relevant dimensions.

Repair of Critical-Sized Mandible Defects in Aged Rat Using Hypoxia Preconditioned BMSCs with Up-regulation of Hif-1α

The repair of bone defects in the geriatric population remains a challenge for modern medicine. Transplantation of bone marrow mesenchymal stem cells (BMSCs) combined with or without biomaterials has been a promising approach to bone restoration and regeneration. Typically, the transplanted BMSCs are cultured under normoxic conditions (21% O₂ and 10% serum medium) in vitro. However, the micro-environment of bone defect area is much more severe, in which lower physiological oxygen tension (<1%) and tissue ischemia were present. Therefore, how to improve the survival rate and osteogenesis of transplanted BMSCs at the low oxygenic and ischemic region in vivo is critical. Hypoxia inducible factor-1α (HIF-1α) plays an important role in the tolerance, angiogenesis and osteogenesis of BMSCs during bone regeneration after transplantation. Previous studies have demonstrated that Dimethyloxaloylglycine (DMOG) improves the angiogenic activity of BMSCs. Typically, angiogenesis and osteogenesis are coupled with each other. Therefore, we detected that hypoxia preconditioned BMSCs with the combined treatment of 1% O₂ and 0.5mM DMOG showing up-regulation of Hif-1α could enhance the survival rate of BMSCs under severe condition (serum-free medium and 1% O₂) in vitro and enhances the angiogenesis and osteogenesis potential of BMSCs under 1% O₂ microenvironment in vitro. The hypoxia preconditioned BMSCs were transplanted into critical-sized mandible defects in aged SD rats to test the effectiveness of hypoxic preconditioning approach. We found that hypoxia preconditioned BMSCs improved the repair of critical-sized mandible defects in vivo. These data showed that hypoxia preconditioned BMSCs with the up-regulation of Hif-1α have the potential of enhancing the bone healing process in geriatric individuals.

Bioactive Molecules Release and Cellular Responses of Alginate-Tricalcium Phosphate Particles Hybrid Gel

In this article, a hybrid gel has been developed using sodium alginate (Alg) and α-tricalcium phosphate (α-TCP) particles through ionic crosslinking process for the application in bone tissue engineering. The effects of pH and composition of the gel on osteoblast cells (MC3T3) response and bioactive molecules release have been evaluated. At first, a slurry of Alg and α-TCP has been prepared using an ultrasonicator for the homogeneous distribution of α-TCP particles in the Alg network and to achieve adequate interfacial interaction between them. After that, CaCl2 solution has been added to the slurry so that ionic crosslinked gel (Alg-α-TCP) is formed. The developed hybrid gel has been physico-chemically characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and a swelling study. The SEM analysis depicted the presence of α-TCP micro-particles on the surface of the hybrid gel, while cross-section images signified that the α-TCP particles are fully embedded in the porous gel network. Different % swelling ratio at pH 4, 7 and 7.4 confirmed the pH responsiveness of the Alg-α-TCP gel. The hybrid gel having lower % α-TCP particles showed higher % swelling at pH 7.4. The hybrid gel demonstrated a faster release rate of bovine serum albumin (BSA), tetracycline (TCN) and dimethyloxalylglycine (DMOG) at pH 7.4 and for the grade having lower % α-TCP particles. The MC3T3 cells are viable inside the hybrid gel, while the rate of cell proliferation is higher at pH 7.4 compared to pH 7. The in vitro cytotoxicity analysis using thiazolyl blue tetrazolium bromide (MTT), bromodeoxyuridine (BrdU) and neutral red assays ascertained that the hybrid gel is non-toxic for MC3T3 cells. The experimental results implied that the non-toxic and biocompatible Alg-α-TCP hybrid gel could be used as scaffold in bone tissue engineering.

Evaluation of MC3T3 Cells Proliferation and Drug Release Study from Sodium Hyaluronate-1,4-butanediol Diglycidyl Ether Patterned Gel

A pattern gel has been fabricated using sodium hyaluronate (HA) and 1,4-butanediol diglycidyl ether (BDDGE) through the micro-molding technique. The cellular behavior of osteoblast cells (MC3T3) in the presence and absence of dimethyloxalylglycine (DMOG) and sodium borate (NaB) in the pattern gel (HA-BDDGE) has been evaluated for its potential application in bone regeneration. The Fourier transform infrared spectroscopy (FTIR), 13C-nuclear magnetic resonance spectroscopy (13C NMR), and thermogravimetric analysis (TGA) results implied the crosslinking reaction between HA and BDDGE. The scanning electron microscopy (SEM) analysis confirmed the formation of pattern on the surface of HA-BDDGE. The gel property of the crosslinked HA-BDDGE has been investigated by swelling study in distilled water at 37 °C. The HA-BDDGE gel releases DMOG in a controlled way for up to seven days in water at 37 °C. The synthesized gel is biocompatible and the bolus drug delivery results indicated that the DMOG containing patterned gel demonstrates a better cell migration ability on the surface than NaB. For local delivery, the pattern gel with 300 µM NaB or 300 µM DMOG induced cell clusters formation, and the gel with 150 µM NaB/DMOG showed high cell proliferation capability only. The vital role of NaB for bone regeneration has been endorsed from the formation of cell clusters in presence of NaB in the media. The in vitro results indicated that the pattern gel showed angiogenic and osteogenic responses with good ALP activity and enhanced HIF-1α, and Runx2 levels in the presence of DMOG and NaB in MC3T3 cells. Hence, the HA-BDDGE gel could be used in bone regeneration application.

Synergistic effects of dimethyloxallyl glycine and recombinant human bone morphogenetic protein-2 on repair of critical-sized bone defects in rats

In bone remodeling, osteogenesis is closely coupled to angiogenesis. Bone tissue engineering using multifunctional bioactive materials is a promising technique which has the ability to simultaneously stimulate osteogenesis and angiogenesis for repair of bone defects. We developed mesoporous bioactive glass (MBG)-doped poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) composite scaffolds as delivery vehicle. Two bioactive molecules, dimethyloxalylglycine (DMOG), a small-molecule angiogenic drug, and recombinant human bone morphogenetic protein-2 (rhBMP-2), an osteoinductive growth factor, were co-incorporated into the scaffold. The synergistic effects of DMOG and rhBMP-2 released in the composite scaffolds on osteogenic and angiogenic differentiation of hBMSCs were investigated using real-time quantitative polymerase chain reaction and western blotting. Moreover, in vivo studies were conducted to observe bone regeneration and vascular formation of critical-sized bone defects in rats using micro-computed tomography, histological analyses, Microfil^® perfusion, fluorescence labeling, and immunohistochemical analysis. The results showed that DMOG and rhBMP-2 released in the MBG-PHBHHx scaffolds did exert synergistic effects on the osteogenic and angiogenic differentiation of hBMSCs. Moreover, DMOG and rhBMP-2 produced significant increases in newly-formed bone and neovascularization of calvarial bone defects in rats. It is concluded that the co-delivery strategy of both rhBMP-2 and DMOG can significantly improve the critical-sized bone regeneration.

Dimethyloxaloylglycine increases bone repair capacity of adipose-derived stem cells in the treatment of osteonecrosis of the femoral head

Mesenchymal stem cells have been widely studied to promote local bone regeneration of osteonecrosis of the femoral head (ONFH). Previous studies observed that dimethyloxaloylglycine (DMOG) enhanced the angiogenic and osteogenic activity of mesenchymal stem cells by activating the expression of hypoxia inducible factor-1α (HIF-1α), thereby improving the bone repair capacity of mesenchymal stem cells. In the present study, it was investigated whether DMOG could increase the bone repair capacity of adipose-derived stem cells (ASCs) in the treatment of ONFH. Western blot analysis was performed to detect HIF-1α protein expression in ASCs treated with different concentrations of DMOG. The results showed DMOG enhanced HIF-1α expression in ASCs in a dose-dependent manner at least for 7 days. Furthermore, DMOG-treated ASCs were transplanted into the necrotic area of a rabbit model of ONFH to treat the disease. Four weeks later, micro-computed tomography (CT) quantitative analysis showed that 58.8±7.4% of the necrotic area was regenerated in the DMOG-treated ASCs transplantation group, 45.5±3.4% in normal ASCs transplantation group, 25.2±2.8% in only core decompression group and 10.6±2.6% in the untreated group. Histological analysis showed that transplantation of DMOG-treated ASCs clearly improved the bone regeneration of the necrotic area compared with the other three groups. Micro-CT and immunohistochemical analysis demonstrated the revasculation of the necrotic area were also increased significantly in the DMOG-treated ASC group compared with the control groups. Thus, it is hypothesized that DMOG could increase the bone repair capacity of ASCs through enhancing HIF-1α expression in the treatment of ONFH.

Catechol-Functionalized Hyaluronic Acid Hydrogels Enhance Angiogenesis and Osteogenesis of Human Adipose-Derived Stem Cells in Critical Tissue Defects

Over the last few decades, stem cell therapies have been highlighted for their potential to heal damaged tissue and aid in tissue reconstruction. However, materials used to deliver and support implanted cells often display limited efficacy, which has resulted in delaying translation of stem cell therapies into the clinic. In our previous work, we developed a mussel-inspired, catechol-functionalized hyaluronic acid (HA-CA) hydrogel that enabled effective cell transplantation due to its improved biocompatibility and strong tissue adhesiveness. The present study was performed to further expand the utility of HA-CA hydrogels for use in stem cell therapies to treat more clinically relevant tissue defect models. Specifically, we utilized HA-CA hydrogels to potentiate stem cell-mediated angiogenesis and osteogenesis in two tissue defect models: critical limb ischemia and critical-sized calvarial bone defect. HA-CA hydrogels were found to be less cytotoxic to human adipose-derived stem cells (hADSCs) in vitro compared to conventional photopolymerized HA hydrogels. HA-CA hydrogels also retained the angiogenic functionality of hADSCs and supported osteogenic differentiation of hADSCs. Because of their superior tissue adhesiveness, HA-CA hydrogels were able to mediate efficient engraftment of hADSCs into the defect regions. When compared to photopolymerized HA hydrogels, HA-CA hydrogels significantly enhanced hADSC-mediated therapeutic angiogenesis (promoted capillary/arteriole formation, improved vascular perfusion, attenuated ischemic muscle degeneration/fibrosis, and reduced limb amputation) and bone reconstruction (mineralized bone formation, enhanced osteogenic marker expression, and collagen deposition). This study proves the feasibility of using bioinspired HA-CA hydrogels as functional biomaterials for improved tissue regeneration in critical tissue defects.

Dimethyloxaloylglycine Promotes the Angiogenic Activity of Mesenchymal Stem Cells Derived from iPSCs via Activation of the PI3K/Akt Pathway for Bone Regeneration

The vascularization of tissue-engineered bone is a prerequisite step for the successful repair of bone defects. Hypoxia inducible factor-1α (HIF-1α) plays an essential role in angiogenesis-osteogenesis coupling during bone regeneration and can activate the expression of angiogenic factors in mesenchymal stem cells (MSCs). Dimethyloxaloylglycine (DMOG) is an angiogenic small molecule that can inhibit prolyl hydroxylase (PHD) enzymes and thus regulate the stability of HIF-1α in cells at normal oxygen tension. Human induced pluripotent stem cell-derived MSCs (hiPSC-MSCs) are promising alternatives for stem cell therapy. In this study, we evaluated the effect of DMOG on promoting hiPSC-MSCs angiogenesis in tissue-engineered bone and simultaneously explored the underlying mechanisms in vitro. The effectiveness of DMOG in improving the expression of HIF-1α and its downstream angiogenic genes in hiPSC-MSCs demonstrated that DMOG significantly enhanced the gene and protein expression profiles of angiogenic-related factors in hiPSC-MSCs by sustaining the expression of HIF-1α. Further analysis showed that DMOG-stimulated hiPSC-MSCs angiogenesis was associated with the phosphorylation of protein kinase B (Akt) and with an increase in VEGF production. The effects could be blocked by the addition of the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002. In a critical-sized calvarial defect model in rats, DMOG-treated hiPSC-MSCs showed markedly improved angiogenic capacity in the tissue-engineered bone, leading to bone regeneration. Collectively, the results indicate that DMOG, via activation of the PI3K/Akt pathway, promotes the angiogenesis of hiPSC-MSCs in tissue-engineered bone for bone defect repair and that DMOG-treated hiPSC-MSCs can be exploited as a potential therapeutic tool in bone regeneration.

Cell-free scaffolds with different stiffness but same microstructure promote bone regeneration in rabbit large bone defect model

To promote bone healing, bone repair biomaterials are increasingly designed to incorporate growth factors. However, the impact of matrix mechanics of cell-free scaffold independent of microstructure on the osteogenic differentiation of endogenous osteoprogenitor cells orchestrating bone repair and regeneration remains not to be fully understood. In our recent study, three-dimensional (3D) scaffolds with different stiffness but same microstructure have been successfully fabricated by coating decellularized bone with collagen/hydroxyapatite (HA) mixture with different collagen rations. It has been demonstrated that the scaffold with optimal stiffness can induce the osteogenic differentiation of MSCs in vitro and in the subcutaneous tissue. The present in vivo study further investigated the repair efficiency of these scaffolds in a rabbit radius with a critical-sized segmental defect model and its potential mechanism. Micro-computed tomography (μ-CT), X-ray and histological analysis were carried out to evaluate the repair capacity of these scaffolds. The results demonstrated that the cell-free scaffold with optimal stiffness incorporation of endogenous osteoprogenitor cells significantly promoted the repair and reconstruction quality of mass bone defect. One of the crucial mechanisms was that hypoxia and stromal cell-derived factor-1α (SDF-1α) mediated mesenchymal stem cells (MSCs) migration by which matrix mechanics exerted influence on bone fracture healing. These findings suggested that only modulating the matrix stiffness of cell-free scaffold can be one of the most attractive strategies for promoting the progression of bone healing.

Prolyl hydroxylase inhibitors act as agents to enhance the efficiency of cell therapy

INTRODUCTION

In stem cell-based therapy as a subtype of regenerative medicine, stem cells can be used to replace or repair injured tissue and cells in order to treat disease. Stem cells have the ability to integrate into injured areas and produce new cells via processes of proliferation and differentiation. Several studies have demonstrated that hypoxia increases self-renewal, proliferation and post-homing differentiation of stem cells through the regulation of hypoxia-inducible factor-1 (HIF-1)-mediated gene expression. Thus, pharmacological interventions including prolyl hydroxylase (PHD) inhibitors are considered as promising solutions for stem cell-based therapy. PHD inhibitors stabilize the HIF-1 and activate its pathway through preventing proteasomal degradation of HIF-1.

AREAS COVERED

This review focuses on the role of hypoxia, HIF-1 and especially PHD inhibitors on cell therapy. PHD structure and function are discussed as well as their inhibitors. In addition, we have investigated several preclinical studies in which PHD inhibitors improved the efficiency of cell-based therapies.

EXPERT OPINION

The data reviewed here suggest that PHD inhibitors are effective operators in improving stem cell therapy. However, because of some limitations, these compounds should be properly examined before clinical application.

Differential hypoxic regulation of the microRNA-146a/CXCR4 pathway in normal and leukemic monocytic cells: impact on response to chemotherapy

High expression of the chemokine receptor 4, CXCR4, associated with a negative prognosis in acute myeloid leukemia, is related to hypoxia. Because CXCR4 expression is under the post-transcriptional control of microRNA-146a in normal and leukemic monocytic cells, we first investigated the impact of hypoxia on microRNA-146a and CXCR4 expression during monocytopoiesis and in acute monocytic leukemia. We then analyzed the effects of hypoxia on drug sensitivity of CXCR4-expressing leukemic cells. We found that microRNA-146a is a target of hypoxia-inducible factor-1α or -2α in relation to the stage of monocytopoiesis and the level of hypoxia, and demonstrated the regulation of the microRNA-146a/CXCR4 pathway by hypoxia in monocytes derived from CD34(+) cells. Thus, in myeloid leukemic cell lines, hypoxia-mediated control of the microRNA-146a/CXCR4 pathway depends only on the capacity of hypoxia-inducible factor-1α to up-regulate microRNA-146a, which in turn decreases CXCR4 expression. However, at variance with normal monocytic cells and leukemic cell lines, in acute monocytic leukemia overexpressing CXCR4, hypoxia up-modulates microRNA-146a but fails to down-modulate CXCR4 expression. We then investigated the effect of hypoxia on the response of leukemic cells to chemotherapy alone or in combination with stromal-derived factor-1α. We found that hypoxia increases stromal-derived factor-1α-induced survival of leukemic cells by decreasing their sensitivity to anti-leukemic drugs. Altogether, our results demonstrate that hypoxia-mediated regulation of microRNA-146a, which controls CXCR4 expression in monocytic cells, is lost in acute monocytic leukemia, thus contributing to maintaining CXCR4 overexpression and protecting the cells from anti-leukemic drugs in the hypoxic bone marrow microenvironment.

Human TM9SF4 Is a New Gene Down-Regulated by Hypoxia and Involved in Cell Adhesion of Leukemic Cells

Background

The transmembrane 9 superfamily protein member 4, TM9SF4, belongs to the TM9SF family of proteins highly conserved through evolution. TM9SF4 homologs, previously identified in many different species, were mainly involved in cellular adhesion, innate immunity and phagocytosis. In human, the function and biological significance of TM9SF4 are currently under investigation. However, TM9SF4 was found overexpressed in human metastatic melanoma and in a small subset of acute myeloid leukemia (AMLs) and myelodysplastic syndromes, consistent with an oncogenic function of this gene.

Purpose and Results

In this study, we first analyzed the expression and regulation of TM9SF4 in normal and leukemic cells and identified TM9SF4 as a gene highly expressed in human quiescent CD34⁺ hematopoietic progenitor cells (HPCs), regulated during monocytic and granulocytic differentiation of HPCs, both lineages giving rise to mature myeloid cells involved in adhesion, phagocytosis and immunity. Then, we found that TM9SF4 is markedly overexpressed in leukemic cells and in AMLs, particularly in M2, M3 and M4 AMLs (i.e., in AMLs characterized by the presence of a more or less differentiated granulocytic progeny), as compared to normal CD34⁺ HPCs. Proliferation and differentiation of HPCs occurs in hypoxia, a physiological condition in bone marrow, but also a crucial component of cancer microenvironment. Here, we investigated the impact of hypoxia on TM9SF4 expression in leukemic cells and identified TM9SF4 as a direct target of HIF-1α, downregulated in these cells by hypoxia. Then, we found that the hypoxia-mediated downregulation of TM9SF4 expression is associated with a decrease of cell adhesion of leukemic cells to fibronectin, thus demonstrating that human TM9SF4 is a new molecule involved in leukemic cell adhesion.

Conclusions

Altogether, our study reports for the first time the expression of TM9SF4 at the level of normal and leukemic hematopoietic cells and its marked expression at the level of AMLs displaying granulocytic differentiation.