HIF-1α regulates bone formation after osteogenic mechanical loading

Fueling recovery: The importance of energy coupling between angiogenesis and osteogenesis during fracture healing

Approximately half of bone fractures that do not heal properly (non-union) can be accounted to insufficient angiogenesis. The processes of angiogenesis and osteogenesis are spatiotemporally regulated in the complex process of fracture healing that requires a substantial amount of energy. It is thought that a metabolic coupling between angiogenesis and osteogenesis is essential for successful healing. However, how this coupling is achieved remains to be largely elucidated. Here, we will discuss the most recent evidence from literature pointing towards a metabolic coupling between angiogenesis and osteogenesis. We will describe the metabolic profiles of the cell types involved during fracture healing as well as secreted products in the bone microenvironment (such as lactate and nitric oxide) as possible key players in this metabolic crosstalk.

Idiosyncratic bone responses to blood flow restriction exercise: new insights and future directions

Applying blood flow restriction (BFR) during low-load exercise induces beneficial adaptations of the myotendinous and neuromuscular systems. Despite the low mechanical tension, BFR exercise facilitates a localized hypoxic environment and increase in metabolic stress, widely regarded as the primary stimulus for tissue adaptations. First evidence indicates that low-load BFR exercise is effective in promoting an osteogenic response in bone, although this has previously been postulated to adapt primarily during high-impact weight-bearing exercise. Besides studies investigating the acute response of bone biomarkers following BFR exercise, first long-term trials demonstrate beneficial adaptations in bone in both healthy and clinical populations. Despite the increasing number of studies, the physiological mechanisms are largely unknown. Moreover, heterogeneity in methodological approaches such as biomarkers of bone metabolism measured, participant and study characteristics, and time course of measurement renders it difficult to formulate accurate conclusions. Furthermore, incongruity in the methods of BFR application (e.g., cuff pressure) limits the comparability of datasets and thus hinders generalizability of study findings. Appropriate use of biomarkers, effective BFR application, and befitting study design have the potential to progress knowledge on the acute and chronic response of bone to BFR exercise and contribute toward the development of a novel strategy to protect or enhance bone health. Therefore, the purpose of the present synthesis review is to 1) evaluate current mechanistic evidence; 2) discuss and offer explanations for similar and contrasting data findings; and 3) create a methodological framework for future mechanistic and applied research.

HIF-1α and periodontitis: Novel insights linking host-environment interplay to periodontal phenotypes

Periodontitis, the sixth most prevalent epidemic disease globally, profoundly impacts oral aesthetics and masticatory functionality. Hypoxia-inducible factor-1α (HIF-1α), an oxygen-dependent transcriptional activator, has emerged as a pivotal regulator in periodontal tissue and alveolar bone metabolism, exerts critical functions in angiogenesis, erythropoiesis, energy metabolism, and cell fate determination. Numerous essential phenotypes regulated by HIF are intricately associated with bone metabolism in periodontal tissues. Extensive investigations have highlighted the central role of HIF and its downstream target genes and pathways in the coupling of angiogenesis and osteogenesis. Within this concise perspective, we comprehensively review the cellular phenotypic alterations and microenvironmental dynamics linking HIF to periodontitis. We analyze current research on the HIF pathway, elucidating its impact on bone repair and regeneration, while unraveling the involved cellular and molecular mechanisms. Furthermore, we briefly discuss the potential application of targeted interventions aimed at HIF in the field of bone tissue regeneration engineering. This review expands our biological understanding of the intricate relationship between the HIF gene and bone angiogenesis in periodontitis and offers valuable insights for the development of innovative therapies to expedite bone repair and regeneration.

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.

Toll-like receptor 4 signaling in osteoblasts is required for load-induced bone formation in mice

In mature bone, NGF is produced by osteoblasts following mechanical loading and signals through resident sensory nerves expressing its high affinity receptor, neurotrophic tyrosine kinase receptor type 1 (TrkA), to support bone formation. Here, we investigated whether osteoblastic expression of Toll-like receptor 4 (TLR4), a key receptor in the NF-κB signaling pathway, is required to initiate NGF-TrkA signaling required for load-induced bone formation. Although Tlr4 conditional knockout mice have normal skeletal mass and strength in adulthood, the loss of TLR4 signaling significantly reduced lamellar bone formation following loading. Inhibition of TLR4 signaling reduced Ngf expression in primary osteoblasts and RNA sequencing of bones from Tlr4 conditional knockout mice and wild-type littermates revealed dysregulated inflammatory signaling three days after osteogenic mechanical loading. In total, our study reveals an important role for osteoblastic TLR4 in the skeletal adaptation of bone to mechanical forces.

Silencing NTPDase3 activity rehabilitates the osteogenic commitment of post-menopausal stem cell bone progenitors

BACKGROUND

Endogenously released adenine and uracil nucleotides favour the osteogenic commitment of bone marrow-derived mesenchymal stromal cells (BM-MSCs) through the activation of ATP-sensitive P2X7 and UDP-sensitive P2Y₆ receptors. Yet, these nucleotides have their osteogenic potential compromised in post-menopausal (Pm) women due to overexpression of nucleotide metabolizing enzymes, namely NTPDase3. This prompted us to investigate whether NTPDase3 gene silencing or inhibition of its enzymatic activity could rehabilitate the osteogenic potential of Pm BM-MSCs.

METHODS

MSCs were harvested from the bone marrow of Pm women (69 ± 2 years old) and younger female controls (22 ± 4 years old). The cells were allowed to grow for 35 days in an osteogenic-inducing medium in either the absence or the presence of NTPDase3 inhibitors (PSB 06126 and hN3-B3_s antibody); pre-treatment with a lentiviral short hairpin RNA (Lenti-shRNA) was used to silence the NTPDase3 gene expression. Immunofluorescence confocal microscopy was used to monitor protein cell densities. The osteogenic commitment of BM-MSCs was assessed by increases in the alkaline phosphatase (ALP) activity. The amount of the osteogenic transcription factor Osterix and the alizarin red-stained bone nodule formation. ATP was measured with the luciferin-luciferase bioluminescence assay. The kinetics of the extracellular ATP (100 µM) and UDP (100 µM) catabolism was assessed by HPLC RESULTS: The extracellular catabolism of ATP and UDP was faster in BM-MSCs from Pm women compared to younger females. The immunoreactivity against NTPDase3 increased 5.6-fold in BM-MSCs from Pm women vs. younger females. Selective inhibition or transient NTPDase3 gene silencing increased the extracellular accumulation of adenine and uracil nucleotides in cultured Pm BM-MSCs. Downregulation of NTPDase3 expression or activity rehabilitated the osteogenic commitment of Pm BM-MSCs measured as increases in ALP activity, Osterix protein cellular content and bone nodule formation; blockage of P2X7 and P2Y₆ purinoceptors prevented this effect.

CONCLUSIONS

Data suggest that NTPDase3 overexpression in BM-MSCs may be a clinical surrogate of the osteogenic differentiation impairment in Pm women. Thus, besides P2X7 and P2Y₆ receptors activation, targeting NTPDase3 may represent a novel therapeutic strategy to increase bone mass and reduce the osteoporotic risk of fractures in Pm women.

Intracellular glucose starvation inhibits osteogenic differentiation in human periodontal ligament cells

BACKGROUND

Periodontal ligament cells (PDLCs), as mesenchymal cells in the oral cavity, are closely linked to periodontal tissue regeneration. However, the effect of local glucose deficiency on periodontal tissue regeneration, such as immediately post-surgery, remains unknown.

OBJECTIVE

In the present study, we investigated the effect of a low-glucose environment on the proliferation and osteogenic differentiation of PDLCs.

MATERIALS AND METHODS

We used media with five glucose concentrations (100, 75, 50, 25, and 0 mg/dL) and focused on the effects of a low-glucose environment on the proliferation, osteogenic differentiation, and autophagy of PDLCs. Additionally, we focused on changes in lactate production in a low-glucose environment and investigated the involvement of lactate with AZD3965, a monocarboxylate transporter-1 (MCT-1) inhibitor.

RESULTS

The low-glucose environment inhibited PDLCs proliferation, migration, and osteogenic differentiation, and induced the expression of the autophagy-related factors LC3 and p62. Lactate and ATP production were decreased under low-glucose conditions. The addition of AZD3965 (MCT-1 inhibitor) in normal glucose conditions caused a similar trend as in low-glucose conditions on PDLCs.

CONCLUSION

Our results suggest lactate production through glucose metabolism in the osteogenic differentiation of PDLCs. A low-glucose environment decreased lactate production, inhibiting cell proliferation, migration, and osteogenic differentiation and inducing autophagy in PDLCs.

Hypoxia Pathway in Osteoporosis: Laboratory Data for Clinical Prospects

The hypoxia pathway not only regulates the organism to adapt to the special environment, such as short-term hypoxia in the plateau under normal physiological conditions, but also plays an important role in the occurrence and development of various diseases such as cancer, cardiovascular diseases, osteoporosis. Bone, as a special organ of the body, is in a relatively low oxygen environment, in which the expression of hypoxia-inducible factor (HIF)-related molecules maintains the necessary conditions for bone development. Osteoporosis disease with iron overload endangers individuals, families and society, and bone homeostasis disorder is linked to some extent with hypoxia pathway abnormality, so it is urgent to clarify the hypoxia pathway in osteoporosis to guide clinical medication efficiently. Based on this background, using the keywords "hypoxia/HIF, osteoporosis, osteoblasts, osteoclasts, osteocytes, iron/iron metabolism", a matching search was carried out through the Pubmed and Web Of Science databases, then the papers related to this review were screened, summarized and sorted. This review summarizes the relationship and regulation between the hypoxia pathway and osteoporosis (also including osteoblasts, osteoclasts, osteocytes) by arranging the references on the latest research progress, introduces briefly the application of hyperbaric oxygen therapy in osteoporosis symptoms (mechanical stimulation induces skeletal response to hypoxic signal activation), hypoxic-related drugs used in iron accumulation/osteoporosis model study, and also puts forward the prospects of future research.

Local YB-1, Epo, and EpoR concentrations in fractured bones: results from a porcine model of multiple trauma

Little is known about the impact of multiple trauma (MT)-related systemic hypoxia on osseous protein concentration of the hypoxia transcriptome. To shed light on this issue, we investigated erythropoietin (Epo), erythropoietin receptor (EpoR), and Y-box binding protein 1 (YB-1) concentrations in the fracture zone in a porcine MT + traumatic hemorrhage (TH) model. Sixteen male domestic pigs were randomized into two groups: an MT + TH group and a sham group. A tibia fracture, lung contusion, and TH were induced in the MT + TH group. The total observation period was 72 h. YB-1 concentrations in bone marrow (BM) were significantly lower in the fracture zone of the MT + TH animals than in the sham animals. Significant downregulation of BM-localized EpoR concentration in both unfractured and fractured bones was observed in the MT + TH animals relative to the sham animals. In BM, Epo concentrations were higher in the fracture zone of the MT + TH animals compared with that in the sham animals. Significantly higher Epo concentrations were detected in the BM of fractured bone compared to that in cortical bone. Our results provide the first evidence that MT + TH alters hypoxia-related protein concentrations. The impacts of both the fracture and concomitant injuries on protein concentrations need to be studied in more detail to shed light on the hypoxia transcriptome in fractured and healthy bones after MT + TH.

Baduanjin exercise: A potential promising therapy toward osteoporosis

Purpose

Baduanjin (BDJ) exercise is a traditional exercise that combines breathing, body movement, meditation and awareness to help delay the onset and progression of senile degenerative musculoskeletal diseases, such as osteoporosis (OP). The aim of this meta-analysis is to evaluate the efficacy of BDJ exercise, and preliminarily infer its effective mechanism in the treatment of OP.

Methods

We identified relevant randomized controlled trials (RCTs) through eight databases, and compared BDJ exercise with the control groups (including blank control and conventional treatment intervention). The main outcome measure was bone mineral density (BMD), the additional outcome measures were visual analogue scale (VAS), Berg balance scale (BBS), serum Calcium (Ca), serum Phosphorus (P), serum Alkaline phosphatase (ALP), and serum bone gla protein (BGP). Meta-analysis and trial sequence analysis (TSA) were performed using RevMan 5.4, Stata 16.0, and TSA 0.9.

Results

In total, 13 RCTs involving 919 patients were included in the analysis. For postmenopausal osteoporosis, BDJ exercise alone and BDJ exercise combined with conventional treatment can improve the BMD of lumbar spine. BDJ exercise alone can influence serum Ca and ALP. BDJ exercise combined with conventional treatment can improve balance (BBS) and influence serum BGP. For senile osteoporosis, BDJ exercise alone and BDJ exercise combined with conventional treatment can improve balance (BBS). BDJ exercise combined with conventional treatment can improve the BMD of hip and pain relieve (VAS). For primary osteoporosis, BDJ exercise combined with conventional treatment can improve the BMD of lumbar spine and femoral neck.

Conclusion

Baduanjin exercise may be beneficial to improve BMD, relieve pain, improve balance ability, influence serum BGP and serum ALP in patients with OP, but differences occur due to various types of OP. Due to the low quality of research on the efficacy and mechanism of BDJ exercise in the treatment of OP, high-quality evidence-based research is still needed to provide reliable supporting evidence.

Systematic Review Registration

[http://www.crd.york.ac.uk/PROSPERO], identifier [CRD42022329022].

Moderate tibial loading and treadmill running, but not overloading, protect adult murine bone from destruction by metastasized breast cancer

Osteolytic bone lesions, which develop in many metastatic breast cancer patients, impair bone integrity and lead to adverse skeletal related events that are difficult to treat and sometimes fatal. Moderate mechanical loading has been shown to suppress osteolysis in young mice with breast cancer. In this study, we aimed to investigate the dose-dependent effects of mechanical loading on protecting the integrity of adult skeletons with breast cancer. Localized tibial loading and aerobic treadmill running with three levels of varying intensity were tested in a syngeneic mammary tumor bone metastasis model. Adult C57BL/6J female mice (14-week-old, N = 88 mice) received intra-tibial injections of Py8119 triple-negative murine breast cancer cells or PBS and underwent 4 to 5 weeks of exercise or acted as sedentary/non-loaded controls. The bone structure was monitored longitudinally with weekly in vivo micro-computed tomography imaging, while the cellular responses in bone and marrow were examined using immunohistochemistry. Moderate treadmill running (16 m/min, 50 min/day, 5 days/week, and 5 weeks) and tibial loading (4.5 N, 630 με, 4 Hz, 300 cycles/day, 5 days/week, and 4 weeks) suppressed tumor-induced bone destruction, as evaluated by full-thickness perforation of tibial cortex and the volume of osteolytic lesions in the cortex. In contrast, tibial loading at higher magnitude (8 N, 1100 με) induced woven bone and accelerated bone destruction, compared with the non-loaded controls. The three exercise regimens differentially affected osteocyte apoptosis, osteocyte hypoxia, osteoclast activity, bone marrow vasculature, and tumor proliferation. In conclusion, the relationship between exercise intensity and the risk of breast cancer-induced osteolysis was found to follow a J-shaped curve in a preclinical model, suggesting the need to optimize exercise parameters in order to harness the skeletal benefits of exercise in metastatic breast cancers.

PLGA/β-TCP composite scaffold incorporating cucurbitacin B promotes bone regeneration by inducing angiogenesis

Objectives

Vascularization is an essential step in successful bone tissue engineering. The induction of angiogenesis in bone tissue engineering can be enhanced through the delivery of therapeutic agents that stimulate vessel and bone formation. In this study, we show that cucurbitacin B (CuB), a tetracyclic terpene derived from Cucurbitaceae family plants, facilitates the induction of angiogenesis in vitro.

Methods

We incorporated CuB into a biodegradable poly (lactide-co-glycolide) (PLGA) and β-tricalcium phosphate (β-TCP) biomaterial scaffold (PT/CuB) Using 3D low-temperature rapid prototyping (LT-RP) technology. A rat skull defect model was used to verify whether the drug-incorporated scaffold has the effects of angiogenesis and osteogenesis in vivo for the regeneration of bone defect. Cytotoxicity assay was performed to determine the safe dose range of the CuB. Tube formation assay and western blot assay were used to analyze the angiogenesis effect of CuB.

Results

PT/CuB scaffold possessed well-designed bio-mimic structure and improved mechanical properties. CuB was linear release from the composite scaffold without affecting pH value. The results demonstrated that the PT/CuB scaffold significantly enhanced neovascularization and bone regeneration in a rat critical size calvarial defect model compared to the scaffold implants without CuB. Furthermore, CuB stimulated angiogenic signaling via up-regulating VEGFR2 and VEGFR-related signaling pathways.

Conclusion

CuB can serve as promising candidate compound for promoting neovascularization and osteogenesis, especially in tissue engineering for repair of bone defects.

The translational potential of this article

This study highlights the potential use of CuB as a therapeutic agent and strongly support its adoption as a component of composite scaffolds for tissue-engineering of bone repair.

Physical Activity and Bone Vascularization: A Way to Explore in Bone Repair Context?

Physical activity is widely recognized as a biotherapy by WHO in the fight and prevention of bone diseases such as osteoporosis. It reduces the risk of disabling fractures associated with many comorbidities, and whose repair is a major public health and economic issue. Bone tissue is a dynamic supportive tissue that reshapes itself according to the mechanical stresses to which it is exposed. Physical exercise is recognized as a key factor for bone health. However, the effects of exercise on bone quality depend on exercise protocols, duration, intensity, and frequency. Today, the effects of different exercise modalities on capillary bone vascularization, bone blood flow, and bone angiogenesis remain poorly understood and unclear. As vascularization is an integral part of bone repair process, the analysis of the preventive and/or curative effects of physical exercise is currently very undeveloped. Angiogenesis–osteogenesis coupling may constitute a new way for understanding the role of physical activity, especially in fracturing or in the integration of bone biomaterials. Thus, this review aimed to clarify the link between physical activities, vascularization, and bone repair.

Electrospun artificial periosteum loaded with DFO contributes to osteogenesis via the TGF-β1/Smad2 pathway

Deferoxamine (DFO), an iron chelator regarded as a hypoxic analogue, has been reported to be involved in angiogenesis and osteogenic differentiation. In this study, DFO was loaded into nanospheres, Then, DFO-loaded NPs and free DFO were co-encapsulated in nanofibers through coaxial electrospinning and its effects on cell viability, migration, and osteogenic differentiation, and the potential mechanisms were investigated. The results suggested that DFO maintained cell viability and promoted the migration of human mesenchymal stem cells (hMSCs) and MC3T3-E1 cells. ALP activity, calcium deposition, and expression of osteogenesis-related markers, including collagen, osteocalcin, and osteopontin, were all increased with DFO. Moreover, hypoxia-inducible factor-1α, transforming growth factor-β, and Smad2 were upregulated with DFO, which indicated activation of the TGF-β1/Smad2 signalling pathway. This may contribute to osteogenic differentiation of cells.

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

Postnatal intramembranous bone regeneration plays an important role during a wide variety of musculoskeletal regeneration processes such as fracture healing, joint replacement and dental implant surgery, distraction osteogenesis, stress fracture healing, and repair of skeletal defects caused by trauma or resection of tumors. The molecular basis of intramembranous bone regeneration has been interrogated using rodent models of most of these conditions. These studies reveal that signaling pathways such as Wnt, TGFβ/BMP, FGF, VEGF, and Notch are invoked, reminiscent of embryonic development of membranous bone. Discoveries of several skeletal stem cell/progenitor populations using mouse genetic models also reveal the potential sources of postnatal intramembranous bone regeneration. The purpose of this review is to compare the underlying molecular signals and progenitor cells that characterize embryonic development of membranous bone and postnatal intramembranous bone regeneration.

MiR-34a suppresses osteoblast differentiation through glycolysis inhibition by targeting lactate dehydrogenase-A (LDHA)

Bone formation (osteogenesis) is mediated through recruitment of bone marrow mesenchymal stem cells (MSCs) with capacity to differentiate into osteoblasts, a process which is regulated by transcriptional and post-transcriptional mechanisms. Multiple studies have suggested that miRNAs might have important roles in osteoblast differentiation. Expressions of miR-34a were detected by qRT-PCR. Cellular glucose metabolism was assessed by measurements of glucose uptake and lactate production. mRNA expressions of glycolysis enzymes were detected by qRT-PCR. Osteogenic differentiation of human MSCs (hMSCs) was analyzed by alkaline phosphatase (ALP) activity and Alizarin red staining. Here, we report that microRNA-34a is upregulated during the osteoblast differentiation from hMSCs. miR-34a overexpressing inhibited late osteoblast differentiation of hMSCs in vitro. The ALP activity and Alizarin red staining were significantly decreased by miR-34a in hMSCs. Target prediction analysis reveals that the lactate dehydrogenase-A (LDHA) is a potential target of miR-34a. We hypothesized that miR-34a inhibits osteoblast differentiation through targeting the LDHA-mediated cellular glycolysis. Results from Western blotting and luciferase assay validated that miR-34a could directly target 3'UTR of LDHA mRNA. In addition, we demonstrated that overexpression of miR-34a inhibits cellular anaerobic glycolysis through targeting LHDA. The protein and mRNA expressions of glycolysis enzymes, Hexokinase 2 (HK2), glucose transporter 1 (GLUT1), and LDHA were significantly downregulated by miR-34a overexpression in hMSCs. Furthermore, we showed that LDHA restoration in miR-34a overexpressing hMSCs successfully rescued the osteoblast differentiation of hMSCs. This study demonstrated the roles of miR-34a in regulating osteoblast differentiation, suggesting that miR-34a inhibition could be a new therapeutic strategy for improving bone formation.

Platelet Lysate Induces in Human Osteoblasts Resumption of Cell Proliferation and Activation of Pathways Relevant for Revascularization and Regeneration of Damaged Bone

To understand the regenerative effect of platelet-released molecules in bone repair one should investigate the cascade of events involving the resident osteoblast population during the reconstructive process. Here the in vitro response of human osteoblasts to a platelet lysate (PL) stimulus is reported. Quiescent or very slow dividing osteoblasts showed a burst of proliferation after PL stimulation and returned to a none or very slow dividing condition when the PL was removed. PL stimulated osteoblasts maintained a differentiation capability in vitro and in vivo when tested in absence of PL. Since angiogenesis plays a crucial role in the bone healing process, we investigated in PL stimulated osteoblasts the activation of hypoxia-inducible factor 1-alpha (HIF-1α) and signal transducer and activator of transcription 3 (STAT3) pathways, involved in both angiogenesis and bone regeneration. We observed phosphorylation of STAT3 and a strong induction, nuclear translocation and DNA binding of HIF-1α. In agreement with the induction of HIF-1α an enhanced secretion of vascular endothelial growth factor (VEGF) occurred. The double effect of the PL on quiescent osteoblasts, i.e., resumption of proliferation and activation of pathways promoting both angiogenesis and bone formation, provides a rationale to the application of PL as therapeutic agent in post-traumatic bone repair.

HIF-1α induces hypoxic apoptosis of MLO-Y4 osteocytes via JNK/caspase-3 pathway and the apoptotic-osteocyte-mediated osteoclastogenesis in vitro

Apoptotic osteocytes were found in the hypoxic bone microenvironment in osteoporosis, osteotomy, orthodontic tooth movement and periodontitis, and played a key role in bone remolding and the differentiation of osteoclasts. Hypoxia inducible factor-1α(HIF-1α), as a transcription factor under hypoxic conditions, has been confirmed to participate in cell apoptosis. However, the effect of HIF-1α on osteocytes apoptosis and the osteocyte-mediated osteoclast formation remains elusive. Here, we hypothesized that HIF-1α was involved in osteocytes apoptosis. Our results showed that CoCl₂ increased the MLO-Y4 cells apoptosis by upregulating the proapoptotic gene expression of caspase-3. Moreover, siRNA-mediated knockdown of HIF-1α decreased the phosphorylation by JNK and the activation of caspase-3 to inhibit the cell apoptosis in MLO-Y4. Furthermore, SP600125, an inhibitor of JNK, reversed CoCl₂-induced the increased apoptosis of MLO-Y4 cells in term of reducing the expression of caspase-3. These findings revealed that HIF-1α served as a pro-apoptotic factor in the apoptosis of MLO-Y4 cells cultured with CoCl₂, by activating the JNK/caspase-3 signaling pathway. Besides, the osteocyte-mediated osteoclastogenesis was reduced with the decline of the expression of HIF-1α and caspase-3 in MLO-Y4 cells. Our study provided an idea for a more comprehensive understanding of HIF-1α and the process of bone remodeling.

Three-dimensional silk fibroin scaffolds enhance the bone formation and angiogenic differentiation of human amniotic mesenchymal stem cells: a biocompatibility analysis

Silk fibroin (SF) is a fibrous protein with unique mechanical properties, adjustable biodegradation, and the potential to drive differentiation of mesenchymal stem cells (MSCs) along the osteogenic lineage, making SF a promising scaffold material for bone tissue engineering. In this study, hAMSCs were isolated by enzyme digestion and identified by multiple-lineage differentiation. SF scaffold was fabricated by freeze-drying, and the adhesion and proliferation abilities of hAMSCs on scaffolds were determined. Osteoblast differentiation and angiogenesis of hAMSCs on scaffolds were further evaluated, and histological staining of calvarial defects was performed to examine the cocultured scaffolds. We found that hAMSCs expressed the basic surface markers of MSCs. Collagen type I (COL-I) expression was observed on scaffolds cocultured with hAMSCs. The scaffolds potentiated the proliferation of hAMSCs and increased the expression of COL-I in hAMSCs. The scaffolds also enhanced the alkaline phosphatase activity and bone mineralization, and upregulated the expressions of osteogenic-related factors in vitro. The scaffolds also enhanced the angiogenic differentiation of hAMSCs. The cocultured scaffolds increased bone formation in treating critical calvarial defects in mice. This study first demonstrated that the application of 3D SF scaffolds co-cultured with hAMSCs greatly enhanced osteogenic differentiation and angiogenesis of hAMSCs in vitro and in vivo. Thus, 3D SF scaffolds cocultured with hAMSCs may be a better alternative for bone tissue engineering.

HIF-1α-TWIST pathway restrains cyclic mechanical stretch-induced osteogenic differentiation of bone marrow mesenchymal stem cells

Aim: Mechanical strain plays a crucial role in bone formation and remodeling. Hypoxia-inducible factor (HIF)-1α and TWIST are upstream of master regulators of osteogenesis, including runt-related transcription factor 2 (RUNX2) and bone morphogenetic proteins (BMPs). This study investigated the effect of the HIF-1α-TWIST pathway on cyclic mechanical stretch-induced osteogenic differentiation of rat bone marrow mesenchymal stem cells (BMSCs) and the underlying mechanism. Materials and Methods: BMSCs were isolated from bone marrow derived from the femurs and humeri of Sprague-Dawley rats. Osteogenic differentiation of BMSCs was induced by applying cyclic mechanical stretch using the Flexcell Tension System. HIF-1α and TWIST were knocked down using recombinant lentiviral vectors. Osteogenic differentiation was evaluated by real-time qPCR, western blotting, and the alkaline phosphatase (ALP) activity assay. Results: Cyclic mechanical stretch increased ALP activity and expression of HIF-1α and TWIST in BMSCs. Knockdown of HIF-1α decreased TWIST expression in stretched BMSCs. Moreover, knockdown of HIF-1α or TWIST enhanced cyclic mechanical stretch-induced osteogenic differentiation of BMSCs. In addition, knockdown of TWIST increased expression of RUNX2 and BMP2 in stretched BMSCs. Conclusions: The HIF-1α-TWIST signaling pathway inhibits cyclic mechanical stretch-induced osteogenic differentiation of BMSCs. This finding may facilitate cell and tissue engineering for clinical applications.

Naproxen impairs load-induced bone formation, reduces bone toughness, and diminishes woven bone formation following stress fracture in mice

Debilitating stress fractures are surprisingly common in physically active individuals, including athletes, military recruits, and dancers. These individuals are overrepresented in the 30 million daily users of non-steroidal anti-inflammatory drugs (NSAIDs). We hypothesized that regular use of NSAIDs would predispose habitually loaded bones to stress fracture and delay the repair of these injuries. In this project, we used repetitive axial forelimb compression in mice as a model to test these hypotheses. First, adult mice were subjected to six bouts of forelimb compression over a period of two weeks, with aspirin, naproxen, or vehicle continuously administered through drinking water. Naproxen-treated mice had diminished load-induced bone formation as well as a significant loss in toughness in non-loaded bone, which were not observed in aspirin-treated mice. Furthermore, there were no differences in RANKL/OPG ratio or cortical bone parameters. Picrosirius red staining and second harmonic generation imaging revealed that alterations in bone collagen fibril size and organization were driving the loss of toughness in naproxen-treated mice. Separately, adult mice were subjected to an ulnar stress fracture generated by a single bout of fatigue loading, with NSAIDs provided 24 h before injury. Both aspirin-treated and naproxen-treated mice had normal forelimb use in the week after injury, whereas control mice favored the injured forelimb until day 7. However, woven bone volume was only significantly impaired by naproxen. Both NSAIDs were found to significantly inhibit Ptgs2 and Ngf expression following stress fracture, but only naproxen significantly affected serum PGE2 concentration. Overall, our results suggest that naproxen, but not aspirin, may increase the risk of stress fracture and extend the healing time of these injuries, warranting further clinical evaluation for patients at risk for fatigue injuries.

The Effect of Exercise on the Prevention of Osteoporosis and Bone Angiogenesis

Physical activity or appropriate exercise prevents the development of osteoporosis. However, the exact mechanism remains unclear although it is well accepted that exercise or mechanical loading regulates the hormones, cytokines, signaling pathways, and noncoding RNAs in bone. Accumulating evidence has shown that bone is a highly vascularized tissue, and dysregulation of vasculature is associated with many bone diseases such as osteoporosis or osteoarthritis. In addition, exercise or mechanical loading regulates bone vascularization in bone microenvironment via the modulation of angiogenic mediators, which play a crucial role in maintaining skeletal health. This review discusses the effects of exercise and its underlying mechanisms for osteoporosis prevention, as well as an angiogenic and osteogenic coupling in response to exercise.

Preservation of alveolar ridge after tooth extraction with hypoxia-inducible factor-1α protein in a dog model

Hypoxia-inducible factor (HIF)-1α protein, which is upregulated by hypoxia, serves an important role in angiogenesis during osteogenesis. The aim of the present study was to investigate the effect of HIF-1α on alveolar ridge preservation in a dog tooth extraction model. Six beagle dogs were used in the present study. The second and fourth premolar teeth of the lower jaws on both sides were extracted. Two unilateral extraction sockets were randomly selected and filled with Bio-Oss and Bio-Oss + HIF-1α. The contralateral sockets remained unfilled and served as the negative control. Micro-computed tomography examination and histological staining were performed to examine the difference of new bone formation among the three groups. Western blotting and reverse transcription-quantitative polymerase chain reaction analysis were used to detect the expression levels of osteogenesis- and angiogenesis-associated genes in the bone tissues of the three groups. Twelve weeks post-surgery, trabecular bone formation in the Bio-Oss + HIF-1α group was significantly increased compared with the other groups. The expression levels of osteogenesis-associated genes (runt-related transcription factor 2, osteoblast-specific transcription factor osterix and osteocalcin) and angiogenesis-associated genes (HIF-1α and vascular endothelial growth factor) were all significantly increased in the Bio-Oss + HIF-1α group compared with the other two groups (P<0.05). The present results indicated that Bio-Oss with HIF-1α can promote osteogenesis and angiogenesis in vivo and may be used as an effective treatment for the preservation of the alveolar ridge.

Vhl deficiency in osteocytes produces high bone mass and hematopoietic defects

Tissue oxygen (O₂) levels vary during development and disease; adaptations to decreased O₂ (hypoxia) are mediated by hypoxia-inducible factor (HIF) transcription factors. HIFs are active in the skeleton, and stabilizing HIF-α isoforms cause high bone mass (HBM) phenotypes. A fundamental limitation of previous studies examining the obligate role for HIF-α isoforms in the skeleton involves the persistence of gene deletion as osteolineage cells differentiate into osteocytes. Because osteocytes orchestrate skeletal development and homeostasis, we evaluated the influence of Vhl or Hif1a disruption in osteocytes. Osteocytic Vhl deletion caused HBM phenotype, but Hif1a was dispensable in osteocytes. Vhl cKO mice revealed enhanced canonical Wnt signaling. B cell development was reduced while myelopoiesis increased in osteocytic Vhl cKO, revealing a novel influence of Vhl/HIF-α function in osteocytes on maintenance of bone microarchitecture via canonical Wnt signaling and effects on hematopoiesis.

Von Hippel-Lindau (VHL) protein antagonist, VH298, promotes functional activities of tendon-derived stem cells and accelerates healing of entheses in rats by inhibiting ubiquitination of hydroxy-HIF-1α

Enthesis is the region where a tendon attaches to a bone. It is a relatively vulnerable position, and in most cases surgical treatment is required upon rupture. The reconstructed enthesis is usually weaker compared to the original, and is prone to rupture again. Hypoxia-inducible factor-1 α (HIF-1α) is known to be involved in extensive activities in cells. It is inhibited under normoxic conditions, and undergoes two essential processes, hydroxylation and ubiquitination, the latter of which has been largely unexplored. Herein, we measured the levels of HIF-1α and hydroxy-HIF-1α in VH298-treated rat tendon-derived stem cells (TDSCs) by immunoblotting. We also detected the proliferation of TDSCs using CCK-8 assay and the mRNA levels of related genes by quantitative RT-PCR. The TDSCs were observed to be induced and the chondrogenic differentiation related genes were found to be enhanced. We also simulated in-vitro wounding in a scratch test and reconstructed the enthesis in a rat model of Achilles tendon by classical surgery followed by administration of phosphate buffer saline (PBS) injection or VH298 injection. We observed that HIF-1α and hydroxy-HIF-1α levels were increased in VH298-treated TDSCs in a dose- and time-dependent manner. Thirty micromolar VH298 could significantly increase cell proliferation, migration, and expression of collagen-1α, collagen-3α, decorin, tenomodulin, tenascin C genes, and chondrogenic differentiation-related genes, collagen-2α, SRY-box9, aggrecan. VH298-treated enthesis could tolerate more load-to-failure, had a better healing pattern, and activation of HIF signaling pathway. VH298 can thus enhance the functional activities of TDSCs, enhance their chondrogenic differentiation potential, and accelerate enthesis healing by inhibiting the ubiquitination of hydroxy-HIF-1α.

Lactate induces osteoblast differentiation by stabilization of HIF1α

Aerobic glycolysis is involved in osteoblast differentiation induced by Wnt signaling or PTH treatment. However, it is still unclear whether lactate, the end product of aerobic glycolysis, plays any role in osteoblast differentiation. Herein we report that in cultures of osteoblast-lineage cells, lactate promoted alkaline phosphatase-positive cell formation, increased the activity of alkaline phosphatase, and induced the expression of osteocalcin. This osteoblast differentiation-inducing effect of lactate can be inhibited by blocking its entry into cells with MCT1 siRNA or inhibitors, and by interfering with its metabolism by using specific siRNAs for LDHB and PDH. Moreover, lactate stabilized HIF1α expression and inhibited HIF1α activity, with BAY87-2243 lowering the osteoblast differentiation-inducing effect of lactate. Thus, these findings reveal an unrecognized role for aerobic glycolysis in osteoblast differentiation via its end product, lactate.

The zebrafish miR-125c is induced under hypoxic stress via hypoxia-inducible factor 1α and functions in cellular adaptations and embryogenesis

Hypoxia is a unique environmental stress. Hypoxia inducible factor-lα (HIF-lα) is a major transcriptional regulator of cellular adaptations to hypoxic stress. MicroRNAs (miRNAs) as posttranscriptional gene expression regulators occupy a crucial role in cell survival under low-oxygen environment. Previous evidences suggested that miR-125c is involved in hypoxia adaptation, but its precise biological roles and the regulatory mechanism underlying hypoxic responses remain unknown. The present study showed that zebrafish miR-125c is upregulated by hypoxia in a Hif-lα-mediated manner in vitro and in vivo. Dual-luciferase assay revealed that cdc25a is a novel target of miR-125c. An inverse correlation between miR-125c and cdc25a was further confirmed in vivo, suggesting miR-125c as a crucial physiological inhibitor of cdc25a which responds to cellular hypoxia. Overexpression of miR-125c suppressed cell proliferation, led to cell cycle arrest at the G1 phase in ZF4 cells and induced apoptotic responses during embryo development. More importantly, miR-125c overexpression resulted in severe malformation and reduction of motility during zebrafish embryonic development. Taken together, we conclude that miR-125c plays a pivotal role in cellular adaptations to hypoxic stress at least in part through the Hif-1α/miR-125c/cdc25a signaling and has great impact on zebrafish early embryonic development.

NGF-TrkA signaling in sensory nerves is required for skeletal adaptation to mechanical loads in mice

Sensory nerves emanating from the dorsal root extensively innervate the surfaces of mammalian bone, a privileged location for the regulation of biomechanical signaling. Here, we show that NGF-TrkA signaling in skeletal sensory nerves is an early response to mechanical loading of bone and is required to achieve maximal load-induced bone formation. First, the elimination of TrkA signaling in mice harboring mutant TrkA^F592A alleles was found to greatly attenuate load-induced bone formation induced by axial forelimb compression. Next, both in vivo mechanical loading and in vitro mechanical stretch were shown to induce the profound up-regulation of NGF in osteoblasts within 1 h of loading. Furthermore, inhibition of TrkA signaling following axial forelimb compression was observed to reduce measures of Wnt/β-catenin activity in osteocytes in the loaded bone. Finally, the administration of exogenous NGF to wild-type mice was found to significantly increase load-induced bone formation and Wnt/β-catenin activity in osteocytes. In summary, these findings demonstrate that communication between osteoblasts and sensory nerves through NGF-TrkA signaling is essential for load-induced bone formation in mice.

Defective signaling, osteoblastogenesis and bone remodeling in a mouse model of connexin 43 C-terminal truncation

In skeletal tissue, loss or mutation of the gap junction protein connexin 43 (Cx43, also known as GJA1) in cells of the osteoblast lineage leads to a profound cortical bone phenotype and defective tissue remodeling. There is mounting evidence in bone cells that the C-terminus (CT) of Cx43 is a docking platform for signaling effectors and is required for efficient downstream signaling. Here, we examined this function, using a mouse model of Cx43 CT-truncation (Gja1 K258Stop). Relative to Gja1+/- controls, male Gja1-/K258Stop mice have a cortical bone phenotype that is remarkably similar to those reported for deletion of the entire Cx43 gene in osteoblasts. Furthermore, we show that the Cx43 CT binds several signaling proteins that are required for optimal osteoblast function, including PKCδ, ERK1 and ERK2 (ERK1/2, also known as MAPK3 and MAPK1, respectively) and β-catenin. Deletion of the Cx43 CT domain affects these signaling cascades, impacting osteoblast proliferation, differentiation, and collagen processing and organization. These data imply that, at least in bone, Cx43 gap junctions not only exchange signals, but also recruit the appropriate effector molecules to the Cx43 CT in order to efficiently activate signaling cascades that affect cell function and bone acquisition.