Inhibitory effects of hypoxia on metabolic switch and osteogenic differentiation of human mesenchymal stem cells

Effect of Mesenchymal Stem Cells Overexpressing BMP-9 Primed with Hypoxia on BMP Targets, Osteoblast Differentiation and Bone Repair

Bone formation is driven by many signaling molecules including bone morphogenetic protein 9 (BMP-9) and hypoxia-inducible factor 1-alpha (HIF-1α). We demonstrated that cell therapy using mesenchymal stem cells (MSCs) overexpressing BMP-9 (MSCs+BMP-9) enhances bone formation in calvarial defects. Here, the effect of hypoxia on BMP components and targets of MSCs+BMP-9 and of these hypoxia-primed cells on osteoblast differentiation and bone repair was evaluated. Hypoxia was induced with cobalt chloride (CoCl2) in MSCs+BMP-9, and the expression of BMP components and targets was evaluated. The paracrine effects of hypoxia-primed MSCs+BMP-9 on cell viability and migration and osteoblast differentiation were evaluated using conditioned medium. The bone formation induced by hypoxia-primed MSCs+BMP-9 directly injected into rat calvarial defects was also evaluated. The results demonstrated that hypoxia regulated BMP components and targets without affecting BMP-9 amount and that the conditioned medium generated under hypoxia favored cell migration and osteoblast differentiation. Hypoxia-primed MSCs+BMP-9 did not increase bone repair compared with control MSCs+BMP-9. Thus, despite the lack of effect of hypoxia on bone formation, the enhancement of cell migration and osteoblast differentiation opens windows for further investigations on approaches to modulate the BMP-9-HIF-1α circuit in the context of cell-based therapies to induce bone regeneration.

The Key Role of Mitochondria in Somatic Stem Cell Differentiation: From Mitochondrial Asymmetric Apportioning to Cell Fate

The study of the mechanisms underlying stem cell differentiation is under intensive research and includes the contribution of a metabolic switch from glycolytic to oxidative metabolism. While mitochondrial biogenesis has been previously demonstrated in number of differentiation models, it is only recently that the role of mitochondrial dynamics has started to be explored. The discovery of asymmetric distribution of mitochondria in stem cell progeny has strengthened the interest in the field. This review attempts to summarize the regulation of mitochondrial asymmetric apportioning by the mitochondrial fusion, fission, and mitophagy processes as well as emphasize how asymmetric mitochondrial apportioning in stem cells affects their metabolism, and thus epigenetics, and determines cell fate.

Osteometabolism: Metabolic Alterations in Bone Pathologies

Renewing interest in the study of intermediate metabolism and cellular bioenergetics is brought on by the global increase in the prevalence of metabolic illnesses. Understanding of the mechanisms that integrate energy metabolism in the entire organism has significantly improved with the application of contemporary biochemical tools for quantifying the fuel substrate metabolism with cutting-edge mouse genetic procedures. Several unexpected findings in genetically altered mice have prompted research into the direction of intermediate metabolism of skeletal cells. These findings point to the possibility of novel endocrine connections through which bone cells can convey their energy status to other metabolic control centers. Understanding the expanded function of skeleton system has in turn inspired new lines of research aimed at characterizing the energy needs and bioenergetic characteristics of these bone cells. Bone-forming osteoblast and bone-resorbing osteoclast cells require a constant and large supply of energy substrates such as glucose, fatty acids, glutamine, etc., for their differentiation and functional activity. According to latest research, important developmental signaling pathways in bone cells are connected to bioenergetic programs, which may accommodate variations in energy requirements during their life cycle. The present review article provides a unique perspective of the past and present research in the metabolic characteristics of bone cells along with mechanisms governing energy substrate utilization and bioenergetics. In addition, we discussed the therapeutic inventions which are currently being utilized for the treatment and management of bone-related diseases such as osteoporosis, rheumatoid arthritis (RA), osteogenesis imperfecta (OIM), etc., by modulating the energetics of bone cells. We further emphasized on the role of GUT-associated metabolites (GAMs) such as short-chain fatty acids (SCFAs), medium-chain fatty acids (MCFAs), indole derivates, bile acids, etc., in regulating the energetics of bone cells and their plausible role in maintaining bone health. Emphasis is importantly placed on highlighting knowledge gaps in this novel field of skeletal biology, i.e., "Osteometabolism" (proposed by our group) that need to be further explored to characterize the physiological importance of skeletal cell bioenergetics in the context of human health and bone related metabolic diseases.

Energy Metabolism and Lipidome Are Highly Regulated during Osteogenic Differentiation of Dental Follicle Cells

Dental follicle cells (DFCs) are stem/progenitor cells of the periodontium and give rise to alveolar osteoblasts. However, understanding of the molecular mechanisms of osteogenic differentiation, which is required for cell-based therapies, is delimited. This study is aimed at analyzing the energy metabolism during the osteogenic differentiation of DFCs. Human DFCs were cultured, and osteogenic differentiation was induced by either dexamethasone or bone morphogenetic protein 2 (BMP2). Previous microarray data were reanalyzed to examine pathways that are regulated after osteogenic induction. Expression and activity of metabolic markers were evaluated by western blot analysis and specific assays, relative amount of mitochondrial DNA was measured by real-time quantitative polymerase chain reaction, the oxidative state of cells was determined by a glutathione assay, and the lipidome of cells was analyzed via mass spectrometry (MS). Moreover, osteogenic markers were analyzed after the inhibition of fatty acid synthesis by 5-(tetradecyloxy)-2-furoic acid or C75. Pathway enrichment analysis of microarray data revealed that carbon metabolism was amongst the top regulated pathways after osteogenic induction in DFCs. Further analysis showed that enzymes involved in glycolysis, citric acid cycle, mitochondrial activity, and lipid metabolism are differentially expressed during differentiation, with most markers upregulated and more markedly after induction with dexamethasone compared to BMP2. Moreover, the cellular state was more oxidized, and mitochondrial DNA was distinctly upregulated during the second half of differentiation. Besides, MS of the lipidome revealed higher lipid concentrations after osteogenic induction, with a preference for species with lower numbers of C-atoms and double bonds, which indicates a de novo synthesis of lipids. Concordantly, inhibition of fatty acid synthesis impeded the osteogenic differentiation of DFCs. This study demonstrates that energy metabolism is highly regulated during osteogenic differentiation of DFCs including changes in the lipidome suggesting enhanced de novo synthesis of lipids, which are required for the differentiation process.

Impact of High-Altitude Hypoxia on Bone Defect Repair: A Review of Molecular Mechanisms and Therapeutic Implications

Reaching areas at altitudes over 2,500–3,000 m above sea level has become increasingly common due to commerce, military deployment, tourism, and entertainment. The high-altitude environment exerts systemic effects on humans that represent a series of compensatory reactions and affects the activity of bone cells. Cellular structures closely related to oxygen-sensing produce corresponding functional changes, resulting in decreased tissue vascularization, declined repair ability of bone defects, and longer healing time. This review focuses on the impact of high-altitude hypoxia on bone defect repair and discusses the possible mechanisms related to ion channels, reactive oxygen species production, mitochondrial function, autophagy, and epigenetics. Based on the key pathogenic mechanisms, potential therapeutic strategies have also been suggested. This review contributes novel insights into the mechanisms of abnormal bone defect repair in hypoxic environments, along with therapeutic applications. We aim to provide a foundation for future targeted, personalized, and precise bone regeneration therapies according to the adaptation of patients to high altitudes.

Secretome of Mesenchymal Stem Cells from Consecutive Hypoxic Cultures Promotes Resolution of Lung Inflammation by Reprogramming Anti-Inflammatory Macrophages

The secretome from hypoxia-preconditioned mesenchymal stem cells (MSCs) has been shown to promote resolution of inflammation and alleviate acute lung injury (ALI) through its immunomodulatory function. However, the effects of consecutive hypoxic culture on immunomodulatory function of the MSCs secretome are largely unclarified. Here, we intend to investigate the effects of consecutive hypoxia on therapeutic efficacy of conditioned medium derived from MSCs (MSCs-CM) in alleviating ALI. Human umbilical cord-derived MSCs (UC-MSCs) were consecutively cultured in 21% O₂ (Nor-MSCs) or in 1% O₂ (Hypo-MSCs) from passage 0. Their conditioned medium (Nor-CM and Hypo-CM respectively) was collected and administered into ALI models. Our findings confirmed that Hypo-MSCs exhibited increased proliferation ability and decreased cell senescence compared with Nor-MSCs. Consecutive hypoxia promoted UC-MSCs to secrete immunomodulatory cytokines, such as insulin-like growth factor 1(IGF1), IL10, TNFα-stimulated gene 6(TSG6), TGFβ, and prostaglandin E2 (PGE2). Both Nor-CM and Hypo-CM could effectively limit lung inflammation, promote efferocytosis and modulate anti-inflammatory polarization of lung macrophages in ALI models. Moreover, the effects of Hypo-CM were more potent than Nor-CM. Taken together, our findings indicate that consecutive hypoxic cultures could not only promote both proliferation and quality of UC-MSCs, but also enhance the therapeutic efficacy of their secretome in mitigating lung inflammation by promoting efferocytosis and anti-inflammatory polarization of macrophages.

Update on the Effects of Energy Metabolism in Bone Marrow Mesenchymal Stem Cells Differentiation

Background

As common progenitor cells of osteoblasts and adipocytes, bone marrow mesenchymal (stromal) stem cells (BMSCs) play key roles in bone homeostasis, tissue regeneration, and global energy homeostasis; however, the intrinsic mechanism of BMSC differentiation is not well understood. Plasticity in energy metabolism allows BMSCs to match the divergent demands of osteo-adipogenic differentiation. Targeting BMSC metabolic pathways may provide a novel therapeutic perspective for BMSC differentiation unbalance related diseases.

Scope of review

This review covers the recent studies of glucose, fatty acids, and amino acids metabolism fuel the BMSC differentiation. We also discuss recent findings about energy metabolism in BMSC differentiation.

Major conclusions

Glucose, fatty acids, and amino acids metabolism provide energy to fuel BMSC differentiation. Moreover, some well-known regulators including environmental stress, hormone drugs, and biological and pathological factors may also influence BMSC differentiation by altering metabolism. This offers insight to the significance of metabolism on BMSC fate determination and provides the possibility of treating diseases related to BMSC differentiation, such as obesity and osteoporosis, from a metabolic perspective.

Endothelial Cells Promote Osteogenesis by Establishing a Functional and Metabolic Coupling With Human Mesenchymal Stem Cells

Bone formation involves a complex crosstalk between endothelial cells (EC) and osteodifferentiating stem cells. This functional interplay is greatly mediated by the paracrine and autocrine action of soluble factors released at the vasculature-bone interface. This study elucidates the molecular and functional responses triggered by this intimate interaction. In this study, we showed that human dermal microvascular endothelial cells (HMEC) induced the expression of pro-angiogenic factors in stem cells from human exfoliated deciduous teeth (SHED) and sustain their osteo-differentiation at the same time. In contrast, osteodifferentiating SHED increased EC recruitment and promoted the formation of complex vascular networks. Moreover, HMEC enhanced anaerobic glycolysis in proliferating SHED without compromising their ability to undergo the oxidative metabolic shift required for adequate osteo-differentiation. Taken together, these findings provide novel insights into the molecular mechanism underlying the synergistic cooperation between EC and stem cells during bone tissue renewal.

The Proliferation of Pre-Pubertal Porcine Spermatogonia in Stirred Suspension Bioreactors Is Partially Mediated by the Wnt/β-Catenin Pathway

Male survivors of childhood cancer are at risk of suffering from infertility in adulthood because of gonadotoxic chemotherapies. For adult men, sperm collection and preservation are routine procedures prior to treatment; however, this is not an option for pre-pubertal children. From young boys, a small biopsy may be taken before chemotherapy, and spermatogonia may be propagated in vitro for future transplantation to restore fertility. A robust system that allows for scalable expansion of spermatogonia within a controlled environment is therefore required. Stirred suspension culture has been applied to different types of stem cells but has so far not been explored for spermatogonia. Here, we report that pre-pubertal porcine spermatogonia proliferate more in bioreactor suspension culture, compared with static culture. Interestingly, oxygen tension provides an avenue to modulate spermatogonia status, with culture under 10% oxygen retaining a more undifferentiated state and reducing proliferation in comparison with the conventional approach of culturing under ambient oxygen levels. Spermatogonia grown in bioreactors upregulate the Wnt/ β-catenin pathway, which, along with enhanced gas and nutrient exchange observed in bioreactor culture, may synergistically account for higher spermatogonia proliferation. Therefore, stirred suspension bioreactors provide novel platforms to culture spermatogonia in a scalable manner and with minimal handling.

Ultrathin 2D Titanium Carbide MXene (Ti3 C2 Tx ) Nanoflakes Activate WNT/HIF-1α-Mediated Metabolism Reprogramming for Periodontal Regeneration

Periodontal defect regeneration in severe periodontitis relies on the differentiation and proliferation of periodontal ligament cells (PDLCs). Recently, an emerging 2D nanomaterial, MXene (Ti₃ C₂ T_x ), has gained more and more attention due to the extensive antibacterial and anticancer activity, while its potential biomedical application on tissue regeneration remains unclear. Through a combination of experimental and multiscale simulation schemes, Ti₃ C₂ T_x has exhibited satisfactory biocompatibility and induced distinguish osteogenic differentiation of human PDLCs (hPDLCs), with upregulated osteogenesis-related genes. Ti₃ C₂ T_x manages to activate the Wnt/β-catenin signaling pathway by enhancing the Wnt-Frizzled complex binding, thus stabilizing HIF-1α and altering metabolic reprogramming into glycolysis. In vivo, hPDLCs pretreated by Ti₃ C₂ T_x display excellent performance in new bone formation and osteoclast inhibition with enhanced RUNX2, HIF-1α, and β-catenin in an experimental rat model of periodontal fenestration defects, indicating that this material has high efficiency of periodontal regeneration promotion. It is demonstrated in this work that Ti₃ C₂ T_x has highly efficient therapeutic effects in osteogenic differentiation and periodontal defect repairment.

Inhibition of hypoxia-induced Mucin 1 alters the proteomic composition of human osteoblast-produced extracellular matrix, leading to reduced osteogenic and angiogenic potential

The bone microenvironment is one of the most hypoxic regions of the human body and in experimental models; hypoxia inhibits osteogenic differentiation of mesenchymal stromal cells (MSCs). Our previous work revealed that Mucin 1 (MUC1) was dynamically expressed during osteogenic differentiation of human MSCs and upregulated by hypoxia. Upon stimulation, its C‐terminus (MUC1‐CT) is proteolytically cleaved, translocases to the nucleus, and binds to promoters of target genes. Therefore, we assessed the MUC1‐mediated effect of hypoxia on the proteomic composition of human osteoblast‐derived extracellular matrices (ECMs) and characterized their osteogenic and angiogenic potentials in the produced ECMs. We generated ECMs from osteogenically differentiated human MSC cultured in vitro under 20% or 2% oxygen with or without GO‐201, a MUC1‐CT inhibitor. Hypoxia upregulated MUC1, vascular endothelial growth factor, and connective tissue growth factor independent of MUC1 inhibition, whereas GO‐201 stabilized hypoxia‐inducible factor 1‐alpha. Hypoxia and/or MUC1‐CT inhibition reduced osteogenic differentiation of human MSC by AMP‐activated protein kinase/mTORC1/S6K pathway and dampened their matrix mineralization. Hypoxia modulated ECMs by transforming growth factor‐beta/Smad and phosphorylation of NFκB and upregulated COL1A1, COL5A1, and COL5A3. The ECMs of hypoxic osteoblasts reduced MSC proliferation and accelerated their osteogenic differentiation, whereas MUC1‐CT‐inhibited ECMs counteracted these effects.

In addition, ECMs generated under MUC1‐CT inhibition reduced the angiogenic potential independent of oxygen concentration. We claim here that MUC1 is critical for hypoxia‐mediated changes during osteoblastogenesis, which not only alters the proteomic landscape of the ECM but thereby also modulates its osteogenic and angiogenic potentials.

Myelodysplastic Syndromes and Metabolism

Myelodysplastic syndromes (MDS) are acquired clonal stem cell disorders exhibiting ineffective hematopoiesis, dysplastic cell morphology in the bone marrow, and peripheral cytopenia at early stages; while advanced stages carry a high risk for transformation into acute myeloid leukemia (AML). Genetic alterations are integral to the pathogenesis of MDS. However, it remains unclear how these genetic changes in hematopoietic stem and progenitor cells (HSPCs) occur, and how they confer an expansion advantage to the clones carrying them. Recently, inflammatory processes and changes in cellular metabolism of HSPCs and the surrounding bone marrow microenvironment have been associated with an age-related dysfunction of HSPCs and the emergence of genetic aberrations related to clonal hematopoiesis of indeterminate potential (CHIP). The present review highlights the involvement of metabolic and inflammatory pathways in the regulation of HSPC and niche cell function in MDS in comparison to healthy state and discusses how such pathways may be amenable to therapeutic interventions.

Hypoxia conditioned mesenchymal stem cells in tissue regeneration application

Mesenchymal stem cells (MSCs) have been demonstrated as promising cell sources for tissue regeneration due to their capability of self-regeneration, differentiation and immunomodulation. MSCs also exert extensive paracrine effects through release of trophic factors and extracellular vesicles. However, despite extended exploration of MSCs in pre-clinical studies, the results are far from satisfactory due to the poor engraftment and low level of survival after implantation. Hypoxia preconditioning has been proposed as an engineering approach to improve the therapeutic potential of MSCs. During in vitro culture, hypoxic conditions can promote MSC proliferation, survival and migration through various cellular responses to the reduction of oxygen tension. The multilineage differentiation potential of MSCs is altered under hypoxia, with consistent reports of enhanced chondrogenesis. Hypoxia also stimulates the paracrine activities of MSCs and increases the production of secretome both in terms of soluble factors as well as extracellular vesicles. The secretome from hypoxia preconditioned MSCs play important roles in promoting cell proliferation and migration, enhancing angiogenesis while inhibiting apoptosis and inflammation. In this review, we summarise current knowledge of hypoxia-induced changes in MSCs and discuss the application of hypoxia preconditioned MSCs as well as hypoxic secretome in different kinds of disease models.

Hypoxia in Bone and Oxygen Releasing Biomaterials in Fracture Treatments Using Mesenchymal Stem Cell Therapy: A Review

Bone fractures have a high degree of severity. This is usually a result of the physical trauma of diseases that affect bone tissues, such as osteoporosis. Due to its highly vascular nature, the bone is in a constant state of remodeling. Although those of younger ages possess bones with high regenerative potential, the impact of a disrupted vasculature can severely affect the recovery process and cause osteonecrosis. This is commonly seen in the neck of femur, scaphoid, and talus bone. In recent years, mesenchymal stem cell (MSC) therapy has been used to aid in the regeneration of afflicted bone. However, the cut-off in blood supply due to bone fractures can lead to hypoxia-induced changes in engrafted MSCs. Researchers have designed several oxygen-generating biomaterials and yielded varying degrees of success in enhancing tissue salvage and preserving cellular metabolism under ischemia. These can be utilized to further improve stem cell therapy for bone repair. In this review, we touch on the pathophysiology of these bone fractures and review the application of oxygen-generating biomaterials to further enhance MSC-mediated repair of fractures in the three aforementioned parts of the bone.

Circular RNA Circ_0005564 promotes osteogenic differentiation of bone marrow mesenchymal cells in osteoporosis

Circular RNA (CircRNA) plays a potential role in bone formation. We aimed to study the circRNAs expression profiles and their functions in osteogenic differentiation of human bone marrow stromal cells (BMSCs). Firstly, we established osteogenic differentiation of BMSCs displaying increased mRNA expression of osteogenic differentiation marker (RUNX2, OPN, and OCN), increased ALP activity and protein expression, and increased mineralized nodules formation, as well as morphological alteration. Then, we employed high-throughput sequencing to analyze circRNA expression and found that 3440 and 3893 circRNAs in non-induced and induced groups, respectively. We further validated the 10 differentially expressed circRNAs with the most significant difference between induced and non-induced groups. Among these ten circRNAs, five of them with more than one miRNA binding site were used to construct a ceRNA network exhibiting 81 miRNAs and 182 target mRNAs. Furthermore, among these five circRNAs, we found only circ_0005564 significantly reduced the mRNA expression of RUNX2, OPN, and OCN. The circularity of circ_0005564 was verified. Our results showed that knockdown of circ_0005564 inhibited osteoblast differentiation in BMSCs. Taken together, our study demonstrates that circ_0005564 is a potential positive regulator of osteogenic differentiation of BMSCs.

The Cellular Choreography of Osteoblast Angiotropism in Bone Development and Homeostasis

Interaction between endothelial cells and osteoblasts is essential for bone development and homeostasis. This process is mediated in large part by osteoblast angiotropism, the migration of osteoblasts alongside blood vessels, which is crucial for the homing of osteoblasts to sites of bone formation during embryogenesis and in mature bones during remodeling and repair. Specialized bone endothelial cells that form "type H" capillaries have emerged as key interaction partners of osteoblasts, regulating osteoblast differentiation and maturation and ensuring their migration towards newly forming trabecular bone areas. Recent revolutions in high-resolution imaging methodologies for bone as well as single cell and RNA sequencing technologies have enabled the identification of some of the signaling pathways and molecular interactions that underpin this regulatory relationship. Similarly, the intercellular cross talk between endothelial cells and entombed osteocytes that is essential for bone formation, repair, and maintenance are beginning to be uncovered. This is a relatively new area of research that has, until recently, been hampered by a lack of appropriate analysis tools. Now that these tools are available, greater understanding of the molecular relationships between these key cell types is expected to facilitate identification of new drug targets for diseases of bone formation and remodeling.

Targeting reactive oxygen species in stem cells for bone therapy

Reactive oxygen species (ROS) have emerged as key players in regulating the fate and function of stem cells from both non-hematopoietic and hematopoietic lineages in bone marrow, and thus affect the osteoblastogenesis-osteoclastogenesis balance and bone homeostasis. Accumulating evidence has linked ROS and associated oxidative stress with the progression of bone disorders, and ROS-based therapeutic strategies have appeared to achieve favorable outcomes in bone. We review current knowledge of the multifactorial roles and mechanisms of ROS as a target in bone pathology. In addition, we discuss emerging ROS-based therapeutic strategies that show potential for bone therapy. Finally, we highlight the opportunities and challenges facing ROS-targeted stem cell therapeutics for improving bone health.

SIRT4 regulates rat dental papilla cell differentiation by promoting mitochondrial functions

INTRODUCTION

SIRT4 is a mitochondrial sirtuin. Owing to its dependance on the cofactor nicotinamide adenine dinucleotide (NAD⁺), SIRT4 can act as a mitochondrial metabolic sensor of cellular energy status. We have previously shown that enhancement of mitochondrial functions is vital for the odontogenic diff ;erentiation of dental papilla cells (DPCs) during dentinogenesis. However, whether SIRT4 serves as an effective regulator of DPC diff ;erentiation by affecting mitochondrial functions remains unexplored.

METHODS

Primary DPCs obtained from the first molar dental papilla of neonatal Sprague-Dawley rats were used in this study. The expression pattern of SIRT4 was observed by immunohistochemistry in the first molar of postnatal day 1 (P1) rats. The changes in SIRT4 expression during odontogenic DPC differentiation were evaluated using real-time quantitative polymerase chain reaction (PCR), western blotting, and immunofluorescence. DPCs with loss (small interfering RNA-mediated knockdown) and gain (plasmid transfection-induced overexpression) of SIRT4 function were used to explore the role of SIRT4 in odontogenic differentiation. Mitochondrial function assays were performed using ATP, reactive oxygen species (ROS), and NAD⁺/NADH kits to investigate the potential mechanisms involved in SIRT4-mediated dentinogenesis.

RESULTS

In the present study, we found that SIRT4 expression increased in a time-dependent manner during odontogenic differentiation bothin vivo and in vitro. Sirt4 knockdown resulted in reduced odontogenic differentiation and mineralization, whereas an opposite effect was observed with SIRT4 overexpression. Furthermore, our results verified that in addition to reducing DPC differentiation, Sirt4 knockdown could also significantly reduce ATP levels, elevate the NAD⁺/NADH ratio, and increase ROS levels.

CONCLUSION

SIRT4 regulates mitochondrial functions and the antioxidant capacity of DPCs, thereby influencing dentin formation and tooth development, a phenomenon that may provide a foundation for better understanding the specific molecular mechanisms underlying dentin regeneration.

The role and mechanism of mitochondrial functions and energy metabolism in the function regulation of the mesenchymal stem cells

Mesenchymal stem cells (MSCs) are multipotent cells that show self-renewal, multi-directional differentiation, and paracrine and immune regulation. As a result of these properties, the MSCs have great clinical application prospects, especially in the regeneration of injured tissues, functional reconstruction, and cell therapy. However, the transplanted MSCs are prone to ageing and apoptosis and have a difficult to control direction differentiation. Therefore, it is necessary to effectively regulate the functions of the MSCs to promote their desired effects. In recent years, it has been found that mitochondria, the main organelles responsible for energy metabolism and adenosine triphosphate production in cells, play a key role in regulating different functions of the MSCs through various mechanisms. Thus, mitochondria could act as effective targets for regulating and promoting the functions of the MSCs. In this review, we discuss the research status and current understanding of the role and mechanism of mitochondrial energy metabolism, morphology, transfer modes, and dynamics on MSC functions.

Alterations in Blood Metabolic Parameters of Immature Mice After Subchronic Exposure to Cobalt Chloride

The wide use of cobalt (Co) in food, industry, and medical devices requires full elucidation of its biological effects on tissues and organs. The aim was to assess serum metabolic alterations in immature mice after subchronic exposure to CoCl₂. Pregnant ICR mice were subjected to a daily dose of 75 mg cobalt chloride/kg body weight (CoCl₂x6H₂O) 2-3 days before they gave birth, and treatment continued until days 25 and 30 after delivery. The compound was dissolved in and obtained with regular tap water. ICP-DRC-MS analysis showed significantly elevated serum Co²⁺ and diverse alterations in metabolic parameters of 25- and 30-day-old pups after exposure to CoCl₂. Cholesterol and urea levels were significantly elevated in day 25 mice while HDL-C and LDL-C were reduced. In day 30, Co-exposed mice LDL-C and triglycerides were significantly increased while the total cholesterol level remained unchanged. Alkaline phosphatase was significantly reduced in day 25 Co-exposed mice. Blood glucose level of Co-exposed mice remained close to the untreated controls. Total protein content was slightly increased in day 30 mice. Co-exposure reduced albumin content and albumin/globulin ratio but increased significantly globulin content. Co administration showed strong correlation with cholesterol, urea, and HDL-C in both day 25 and 30 mice. Inverse correlation was found with alkaline phosphatase and albumin for day 25 and with triglycerides, globulin, and total protein content in day 30 Co-exposed mice. Subchronic CoCl₂ exposure of immature mice induced significant changes in key metabolic parameters suggesting possible further disturbances in energy metabolism, osteogenesis, and reproduction.

α-Tocopherol Acetate Attenuates Mitochondrial Oxygen Consumption and Maintains Primitive Cells within Mesenchymal Stromal Cell Population

We present here the data showing, in standard cultures exposed to atmospheric O₂ concentration, that alpha-tocopherol acetate (α-TOA) has a positive impact on primitive cells inside mesenchymal stromal cell (MstroC) population, by maintaining their proliferative capacity. α-TOA decreases the O₂ consumption rate of MStroC probably by impacting respiratory chain complex II activity. This action, however, is not associated with a compensatory increase in glycolysis activity, in spite of the fact that the degradation of HIF-1α was decreased in presence of α-TOA. This is in line with a moderate enhancement of mtROS upon α-TOA treatment. However, the absence of glycolysis stimulation implies the inactivity of HIF-1α which might - if it were active - be related to the maintenance of stemness. It should be stressed that α-TOA might act directly on the gene expression as well as the mtROS themselves, which remains to be elucidated. Alpha-tocopherol acetate (α-TOA), a synthetic vitamin E ester, attenuates electron flow through electron transport chain (ETC) which is probably associated with a moderate increase in mtROS in Mesenchymal Stromal Cells. α-TOA action results in enhancement of the proliferative capacity and maintenance of the differentiation potential of the mesenchymal stem and progenitor cells.

Multilineage Differentiation Potential of Human Dental Pulp Stem Cells-Impact of 3D and Hypoxic Environment on Osteogenesis In Vitro

Human dental pulp harbours unique stem cell population exhibiting mesenchymal stem/stromal cell (MSC) characteristics. This study aimed to analyse the differentiation potential and other essential functional and morphological features of dental pulp stem cells (DPSCs) in comparison with Wharton’s jelly-derived MSCs from the umbilical cord (UC-MSCs), and to evaluate the osteogenic differentiation of DPSCs in 3D culture with a hypoxic microenvironment resembling the stem cell niche. Human DPSCs as well as UC-MSCs were isolated from primary human tissues and were subjected to a series of experiments. We established a multiantigenic profile of DPSCs with CD45⁻/CD14⁻/CD34⁻/CD29⁺/CD44⁺/CD73⁺/CD90⁺/CD105⁺/Stro-1⁺/HLA-DR⁻ (using flow cytometry) and confirmed their tri-lineage osteogenic, chondrogenic, and adipogenic differentiation potential (using qRT-PCR and histochemical staining) in comparison with the UC-MSCs. The results also demonstrated the potency of DPSCs to differentiate into osteoblasts in vitro. Moreover, we showed that the DPSCs exhibit limited cardiomyogenic and endothelial differentiation potential. Decreased proliferation and metabolic activity as well as increased osteogenic differentiation of DPSCs in vitro, attributed to 3D cell encapsulation and low oxygen concentration, were also observed. DPSCs exhibiting elevated osteogenic potential may serve as potential candidates for a cell-based product for advanced therapy, particularly for bone repair. Novel tissue engineering approaches combining DPSCs, 3D biomaterial scaffolds, and other stimulating chemical factors may represent innovative strategies for pro-regenerative therapies.

Oxygen-releasing nanofibers for breathable bone tissue engineering application

Oxygen is a vital molecule for cell and tissue processes. Electrospun fibers have been extensively used as drug loading carriers due to possibility of well control over drug release with modulating fiber properties. However, they have not been used as depots for oxygen release. In the present study, an oxygen-releasing nanofibrous scaffold has been developed by electrospinning of polylactic acid/nano-calcium peroxide suspension with different polylactic acid concentrations (6.5 and 13% w/v). The electrospun fibers with calcium peroxide cargo provided oxygen content of 30–94 mmHg in a period of 14 days which lies well within the oxygen level of osseous tissue. The release profile of 13% polylactic acid fibers was different with that of 6.5% fibers with respects to the initial content of released oxygen and the release rate. Not only did 13% fibers supply oxygen with a slower rate, but also they resulted in a lower burst release of oxygen. Cell culture studies in hypoxia corroborated that 13% polylactic acid fibers better preserve cell viability comparing 6.5% counterparts as perceived by MTT assay. Moreover, they endowed more favored milieu for adherence, arrangement and migration of mesenchymal stem cells as confirmed by microscopy images. The oxygen-releasing fibers equally affected alkaline phosphatase, osteocalcin, and calcium deposition by mesenchymal stem cells most likely due to interplay between topographical and metabolic cues offered by 6.5 and 13% formulations.

Succinate Supplement Elicited "Pseudohypoxia" Condition to Promote Proliferation, Migration, and Osteogenesis of Periodontal Ligament Cells

Most mesenchymal stem cells reside in a niche of low oxygen tension. Iron-chelating agents such as CoCl₂ and deferoxamine have been utilized to mimic hypoxia and promote cell growth. The purpose of the present study was to explore whether a supplement of succinate, a natural metabolite of the tricarboxylic acid (TCA) cycle, can mimic hypoxia condition to promote human periodontal ligament cells (hPDLCs). Culturing hPDLCs in hypoxia condition promoted cell proliferation, migration, and osteogenic differentiation; moreover, hypoxia shifted cell metabolism from oxidative phosphorylation to glycolysis with accumulation of succinate in the cytosol and its release into culture supernatants. The succinate supplement enhanced hPDLC proliferation, migration, and osteogenesis with decreased succinate dehydrogenase (SDH) expression and activity, as well as increased hexokinase 2 (HK2) and 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3), suggesting metabolic reprogramming from oxidative phosphorylation to glycolysis in a normal oxygen condition. The succinate supplement in cell cultures promoted intracellular succinate accumulation while stabilizing hypoxia inducible factor-1α (HIF-1α), leading to a state of pseudohypoxia. Moreover, we demonstrate that hypoxia-induced proliferation was G-protein-coupled receptor 91- (GPR91-) dependent, while exogenous succinate-elicited proliferation involved the GPR91-dependent and GPR91-independent pathway. In conclusion, the succinate supplement altered cell metabolism in hPDLCs, induced a pseudohypoxia condition, and enhanced proliferation, migration, and osteogenesis of mesenchymal stem cells in vitro.

PKM2 suppresses osteogenesis and facilitates adipogenesis by regulating β-catenin signaling and mitochondrial fusion and fission

Bone marrow mesenchymal stem cells (BMSCs) differentiation dysfunction is a common pathological phenotype of several prevalent metabolic and genetic bone diseases. Pyruvate kinase muscle isoenzyme 2 (PKM2) regulates the last step of glycolysis, and its role in BMSCs differentiation is still unknown. In this study, the influence of PKM2 on osteogenesis and adipogenesis was assessed in vitro and in vivo. We found that DASA-58 (the activator of PKM2) reduced the enzymatic activity of ALP, and inhibited the levels of osteogenic marker genes, especially RUNX2, which is a crucial transcription factor for osteogenesis. Besides, we provided evidence that C3k, an inhibitor of PKM2, caused increase in mitochondrial membrane potential and maintained low levels of ROS, and promoted mitochondrial fusion. Furthermore, after treatment with DASA-58, the level of active β-catenin gradually decreased, which also inhibited the transport of active β-catenin into the nucleus, but C3k obviously promoted its nuclear translocation. As for adipogenesis, PKM2 activation increased the expression of adipogenic related genes and decreased active-β-catenin expression, whereas treatment of C3k had the opposite effect. In addition, C3k significantly attenuated ovariectomy-induced trabecular bone loss in vivo. Our findings helped uncover the molecular mechanisms underlying PKM2 regulation of BMSCs differentiation.

Fibroblast growth factor-2 ameliorates tumor necrosis factor-alpha-induced osteogenic damage of human bone mesenchymal stem cells by improving oxidative phosphorylation

Tumor necrosis factor-alpha (TNF-α) has been shown to have an inhibitory effect on the osteogenic differentiation of mesenchymal stem cells. The metabolic switch from glycolysis to oxidative phosphorylation (OXPHOS) is vital for energy supply during osteogenic differentiation. However, the metabolic switch is inhibited under inflammatory stimulation. FGF2 has shown that it can improve osteogenic differentiation and promote autoimmune inflammation. In this study, we investigated whether FGF2 can ameliorate TNF-a-inhibited osteogenic damage by improving OXPHOS. Effects of TNF-α or FGF2 on the proliferation and osteogenic differentiation of hBMSCs were evaluated by MTT assay, qRT-PCR, and ALP activity tests. The function of FGF2 on the TNF-a-inhibited metabolic switch was determined by Mito Stress test. The results showed that TNF-α was able to inhibit the osteogenic differentiation and OXPHOS of hBMSCs. FGF2 has no obvious function in improving the osteogenic-related genes, but it can ameliorate the impaired osteogenesis and OCR value caused by TNF-α. These findings suggest that FGF2 can prevent the impaired osteogenic differentiation and metabolic switch of hBMSCs under inflammatory stimulation, which might enhance the regeneration capacity of hBMSCs.

Sema3A and HIF1α co-overexpressed iPSC-MSCs/HA scaffold facilitates the repair of calvarial defect in a mouse model

Mesenchymal stem/stromal cells (MSCs) play an important role in bone tissue engineering because MSCs possess multilineage potential of differentiation to mesenchymal tissues. Semaphorin 3A (Sema3A) and hypoxia-inducible factor-1α (HIF1α) are proved as important regulatory factors for osteogenesis and angiogenesis. The aim of this study was to investigate the effects of Sema3A and HIF1α co-overexpression on the osteogenesis and angiogenesis in induced pluripotent stem cell-derived mesenchymal stem cells (iPSC-MSCs). Importantly, we assessed the potential osteogenic effectiveness of Sema3A and HIF1α co-overexpressed iPSC-MSCs seeded on hydroxyapatite (HA) scaffold in a mouse calvarial defect model. The overexpression for Sema3A, HIF1α, or Sema3A-HIF1α fusion in iPSC-MSCs was performed by separately infecting with conducted lentiviral vector. We determined the cell proliferation, the expressions of osteogenic, and endothelial markers of iPSC-MSCs cultured in osteogenic or endothelial induction medium in vitro. A mouse model calvarial defect was created and implanted with the Empty implant, HA scaffold alone, HA scaffold combined with iPSC-MSCs that infected with negative control or Sema3A-HIF1α fusion for 8 weeks in vivo. The results showed that Sema3A and HIF1α co-overexpression reversed the reduced cell proliferation that reduced by Sema3A overexpression alone. Importantly, the co-overexpression significantly increased the expressions of osteogenic and angiogenic related-genes compared with negative control after induction. Moreover, the Sema3A-HIF1α co-overexpressed iPSC-MSCs seeded on HA scaffold boosted the new bone and collagen fiber formation and facilitated repair of calvarial defect in a mouse model, which might have the potential application for bone defect reconstruction.

Role of offset and gradient architectures of 3-D melt electrowritten scaffold on differentiation and mineralization of osteoblasts

BACKGROUND

Cell-scaffold based therapies have the potential to offer an efficient osseous regenerative treatment and PCL has been commonly used as a scaffold, however its effectiveness is limited by poor cellular retention properties. This may be improved through a porous scaffold structure with efficient pore arrangement to increase cell entrapment. To facilitate this, melt electrowriting (MEW) has been developed as a technique able to fabricate cell-supporting scaffolds with precise micro pore sizes via predictable fibre deposition. The effect of the scaffold's architecture on cellular gene expression however has not been fully elucidated.

METHODS

The design and fabrication of three different uniform pore structures (250, 500 and 750 μm), as well as two offset scaffolds with different layout of fibres (30 and 50%) and one complex scaffold with three gradient pore sizes of 250-500 - 750 μm, was performed by using MEW. Calcium phosphate modification was applied to enhance the PCL scaffold hydrophilicity and bone inductivity prior to seeding with osteoblasts which were then maintained in culture for up to 30 days. Over this time, osteoblast cell morphology, matrix mineralisation, osteogenic gene expression and collagen production were assessed.

RESULTS

The in vitro findings revealed that the gradient scaffold significantly increased alkaline phosphatase activity in the attached osteoblasts while matrix mineralization was higher in the 50% offset scaffolds. The expression of osteocalcin and osteopontin genes were also upregulated compared to other osteogenic genes following 30 days culture, particularly in offset and gradient scaffold structures. Immunostaining showed significant expression of osteocalcin in offset and gradient scaffold structures.

CONCLUSIONS

This study demonstrated that the heterogenous pore sizes in gradient and fibre offset PCL scaffolds prepared using MEW significantly improved the osteogenic potential of osteoblasts and hence may provide superior outcomes in bone regeneration applications.

Expression of HIF‑1α in cycling stretch‑induced osteogenic differentiation of bone mesenchymal stem cells

During orthodontic treatment, mechanical force is applied to the teeth, and following a series of complex metabolism changes, the position of the teeth in the alveolar bone change. This process is closely associated with primitive bone mesenchymal stem cells (BMSCs), which may differentiate into osteoblasts precursor cell. A hypoxic microenvironment may be caused by orthodontic mechanical forces between the alveolar bone and the root. Hypoxia-inducible factor 1α (HIF-1α) is a specific receptor that adapts to a hypoxic environment. The present study was designed to investigate whether HIF-1α was involved in the osteoblastic differentiation of BMSCs induced by cyclic tensile stress. During this process, HIF-1α mRNA and protein expression were detected using a reverse transcription-quantitative polymerase chain reaction and western blotting. It was revealed that alkaline phosphatase activity increased in a time-dependent manner in three different stretching strength groups, which indicates that cyclic stretch promotes the osteogenic differentiation of BMSCs. The optimal force stage of osteogenesis was an unexpected discovery, which will provide theoretical guidance for selecting the most suitable orthodontic force for tooth movement in clinical orthodontic treatment. Most importantly, all experiments revealed that HIF-1α mRNA and protein were significantly increased following stretching treatment in BMSCs. It was therefore concluded that HIF-1α may be involved in BMSCs modulating osteogenic metabolism during exposure to cyclic stretch and a hypoxic microenvironment, which may prove useful for the reconstruction of a jaw during orthodontic treatment.

BMSC-derived leptin and IGFBP2 promote erlotinib resistance in lung adenocarcinoma cells through IGF-1R activation in hypoxic environment

EGFR-TKIs such as erlotinib and gefitinib have been introduced into the first-line treatment for patients having a mutation of deletion in exon 19 or L858R missense mutations in exon 21. Almost all patients who respond to EGFR-TKIs at first place eventually develop acquired resistance after several months of therapy. The secondary mutations and bypass signaling activation are involved in the generation of the resistance. Hypoxia in non-small cell lung cancer (NSCLC) is an important factor in treatment resistance including radiotherapy, chemotherapy and EGFR-TKI therapy. In this study, the effect of hypoxic cancer microenvironment in the bypass signaling activation was investigated. We found that bone marrow-derived mesenchymal stem cells (BMSCs) residing in the hypoxic solid cancer microenvironment highly produced molecules associated with adipocytes including adipokine leptin and IGFBPs. Leptin could induce the resistance of lung cancer cells to erlotinib through activating IGF-1R signaling. IGFBP2 counteracted the activation role of IGF-1 and induced erlotinib resistance by activating IGF-1R signaling in an IGF-1 independent manner. IGFBP2 had synergistic effect with leptin to induce erlotinib resistance. Leptin and IGFBP2 may be predictive factors for acquired resistance for EGFR-TKIs.

Intercellular mitochondria trafficking highlighting the dual role of mesenchymal stem cells as both sensors and rescuers of tissue injury

Mitochondria are crucial organelles that not only regulate the energy metabolism, but also the survival and fate of eukaryotic cells. Mitochondria were recently discovered to be able to translocate from one cell to the other. This phenomenon was observed in vitro and in vivo, both in physiological and pathophysiological conditions including tissue injury and cancer. Mitochondria trafficking was found to exert prominent biological functions. In particular, several studies pointed out that this process governs some of the therapeutic effects of mesenchymal stem cells (MSCs). In this review, we give an overview of the current knowledge on MSC-dependent intercellular mitochondria trafficking and further discuss the recent findings on the intercellular mitochondria transfer between differentiated and mesenchymal stem cells, their biological significance and the mechanisms underlying this process.

CoCl2 induced hypoxia enhances osteogenesis of rat bone marrow mesenchymal stem cells through cannabinoid receptor 2

OBJECTIVES

This study aims to investigate the role of Cannabinoid receptor 2 (CB2) on osteogenesis of bone marrow-derived mesenchymal stem cells (BMSCs) under hypoxia.

MATERIALS AND METHODS

BMSCs were isolated from Sprague-Dawley rats and cultured in the presence of cobalt chloride (CoCl₂) to induce intracellular hypoxia. Cell proliferation was measured with MTT assay. Quantitative real-time PCR and western blot were applied to evaluate the mRNA and protein expressions of CB2 and osteogenic indicators including osteocalcin, RUNX2, collagen-1 and osterix (SP7). The osteogenic differentiation of BMSCs was further examined by ALP assay and alizarin red S (ARS) staining. Moreover, the activation of MAPKs signaling pathways was analyzed by western blot.

RESULTS

CoCl₂ dose-dependently increased hypoxia inducible factor while higher concentrations (200 and 400 μM) of CoCl₂ markedly inhibited cell proliferation. CoCl₂ induced hypoxia significantly increased the protein and mRNA expressions of osteocalcin, RUNX2, collagen-1 and osterix, along with enhanced ALP and ARS staining. Interestingly, such effects can be inhibited by the addition of CB2 inhibitor AM630. Moreover, AM630 partially inhibited hypoxia-induced p38 and ERK pathways, which may lead to a decrease in the osteogenic transcripts of RUNX2, collagen-1 and osterix.

CONCLUSIONS

CoCl₂ induced hypoxia could promote osteogenesis of rat BMSCs possibly through CB2.

Can Hypoxic Conditioning Improve Bone Metabolism? A Systematic Review

Among other functions, hypoxia-inducible factor plays a critical role in bone–vascular coupling and bone formation. Studies have suggested that hypoxic conditioning could be a potential nonpharmacological strategy for treating skeletal diseases. However, there is no clear consensus regarding the bone metabolism response to hypoxia. Therefore, this review aims to examine the impact of different modes of hypoxia conditioning on bone metabolism. The PubMed and Web of Science databases were searched for experimental studies written in English that investigated the effects of modification of ambient oxygen on bone remodelling parameters of healthy organisms. Thirty-nine studies analysed the effect of sustained or cyclic hypoxia exposure on genetic and protein expression and mineralisation capacity of different cell models; three studies carried out in animal models implemented sustained or cyclic hypoxia; ten studies examined the effect of sustained, intermittent or cyclic hypoxia on bone health and hormonal responses in humans. Different modes of hypoxic conditioning may have different impacts on bone metabolism both in vivo and in vitro. Additional research is necessary to establish the optimal cyclical dose of oxygen concentration and exposure time.

Hypoxia-Induced Metabolomic Alterations in Pancreatic Cancer Cells

Hypoxic conditions in the pancreatic tumor microenvironment lead to the stabilization of hypoxia-inducible factor-1 alpha (HIF-1α), which acts as the master regulator of cancer cell metabolism. HIF-1α-mediated metabolic reprogramming results in large-scale metabolite perturbations. Characterization of the metabolic intermediates and the corresponding metabolic pathways altered by HIF-1α would facilitate the identification of therapeutic targets for hypoxic microenvironments prevalent in pancreatic ductal adenocarcinoma and other solid tumors. Targeted metabolomic approaches are versatile in quantifying multiple metabolite levels in a single platform and, thus, enable the characterization of multiple metabolite alterations regulated by HIF-1α. In this chapter, we describe a detailed metabolomic approach for characterizing the hypoxia-induced metabolomic alterations using pancreatic cancer cell lines cultured in normoxic and hypoxic conditions. We elaborate the methodology of cell culture, hypoxic exposure, metabolite extraction, and relative quantification of polar metabolites from normoxia- and hypoxia-exposed cell extracts, using a liquid chromatography-coupled tandem mass spectrometry approach. Herein, using our metabolomic data, we also present the methods for metabolomic data representation.

CypD-mPTP axis regulates mitochondrial functions contributing to osteogenic dysfunction of MC3T3-E1 cells in inflammation

Bone is a dynamic organ, the bone-forming osteoblasts and bone-resorbing osteoclasts form the physiological basis of bone remodeling process. During pathological process of numerous inflammatory diseases, these two aspects are uncoupled and the balance is usually tipped in favor of bone destruction. Evidence suggests that the inflammatory destruction of bone is mainly attributed to oxidative stress and is closely related to mitochondrial dysfunction. The mechanisms underlying osteogenic dysfunction in inflammation still need further investigation. Reactive oxygen species (ROS) is associated with mitochondrial dysfunction and cellular damage. Here, we reported an unexplored role of cyclophilin D (CypD), the major modulator of mitochondrial permeability transition pore (mPTP), and the CypD-mPTP axis in inflammation-induced mitochondrial dysfunction and bone damage. And the protective effects of knocking down CypD by siRNA interference or the addition of cyclosporin A (CsA), an inhibitor of CypD, were evidenced by rescued mitochondrial function and osteogenic function of osteoblast under tumor necrosis factor-α (TNF-α) treatment. These findings provide new insights into the role of CypD-mPTP-dependent mitochondrial pathway in the inflammatory bone injury. The protective effect of CsA or other moleculars affecting the mPTP formation may hold promise as a potential novel therapeutic strategy for inflammation-induced bone damage via mitochondrial pathways.

Revisiting the Advances in Isolation, Characterization and Secretome of Adipose-Derived Stromal/Stem Cells

Adipose-derived stromal/stem cells (ASCs) seems to be a promising regenerative therapeutic agent due to the minimally invasive approach of their harvest and multi-lineage differentiation potential. The harvested adipose tissues are further digested to extract stromal vascular fraction (SVF), which is cultured, and the anchorage-dependent cells are isolated in order to characterize their stemness, surface markers, and multi-differentiation potential. The differentiation potential of ASCs is directed through manipulating culture medium composition with an introduction of growth factors to obtain the desired cell type. ASCs have been widely studied for its regenerative therapeutic solution to neurologic, skin, wound, muscle, bone, and other disorders. These therapeutic outcomes of ASCs are achieved possibly via autocrine and paracrine effects of their secretome comprising of cytokines, extracellular proteins and RNAs. Therefore, secretome-derivatives might offer huge advantages over cells through their synthesis and storage for long-term use. When considering the therapeutic significance and future prospects of ASCs, this review summarizes the recent developments made in harvesting, isolation, and characterization. Furthermore, this article also provides a deeper insight into secretome of ASCs mediating regenerative efficacy.

Osteoblast-like MC3T3-E1 Cells Prefer Glycolysis for ATP Production but Adipocyte-like 3T3-L1 Cells Prefer Oxidative Phosphorylation

Mesenchymal stromal cells (MSCs) are early progenitors that can differentiate into osteoblasts, chondrocytes, and adipocytes. We hypothesized that osteoblasts and adipocytes utilize distinct bioenergetic pathways during MSC differentiation. To test this hypothesis, we compared the bioenergetic profiles of preosteoblast MC3T3-E1 cells and calvarial osteoblasts with preadipocyte 3T3L1 cells, before and after differentiation. Differentiated MC3T3-E1 osteoblasts met adenosine triphosphate (ATP) demand mainly by glycolysis with minimal reserve glycolytic capacity, whereas nondifferentiated cells generated ATP through oxidative phosphorylation. A marked Crabtree effect (acute suppression of respiration by addition of glucose, observed in both MC3T3-E1 and calvarial osteoblasts) and smaller mitochondrial membrane potential in the differentiated osteoblasts, particularly those incubated at high glucose concentrations, indicated a suppression of oxidative phosphorylation compared with nondifferentiated osteoblasts. In contrast, both nondifferentiated and differentiated 3T3-L1 adipocytes met ATP demand primarily by oxidative phosphorylation despite a large unused reserve glycolytic capacity. In sum, we show that nondifferentiated precursor cells prefer to use oxidative phosphorylation to generate ATP; when they differentiate to osteoblasts, they gain a strong preference for glycolytic ATP generation, but when they differentiate to adipocytes, they retain the strong preference for oxidative phosphorylation. Unique metabolic programming in mesenchymal progenitor cells may influence cell fate and ultimately determine the degree of bone formation and/or the development of marrow adiposity. © 2018 American Society for Bone and Mineral Research.

Chemical Activation of the Hypoxia-Inducible Factor Reversibly Reduces Tendon Stem Cell Proliferation, Inhibits Their Differentiation, and Maintains Cell Undifferentiation

Adult stem cell-based therapeutic approaches for tissue regeneration have been proposed for several years. However, adult stem cells are usually limited in number and difficult to be expanded in vitro, and they usually tend to quickly lose their potency with passages, as they differentiate and become senescent. Culturing stem cells under reduced oxygen tensions (below 21%) has been proposed as a tool to increase cell proliferation, but many studies reported opposite effects. In particular, cell response to hypoxia seems to be very stem cell type specific. Nonetheless, it is clear that a major role in this process is played by the hypoxia inducible factor (HIF), the master regulator of cell response to oxygen deprivation, which affects cell metabolism and differentiation. Herein, we report that a chemical activation of HIF in human tendon stem cells reduces their proliferation and inhibits their differentiation in a reversible and dose-dependent manner. These results support the notion that hypoxia, by activating HIF, plays a crucial role in preserving stem cells in an undifferentiated state in the “hypoxic niches” present in the tissue in which they reside before migrating in more oxygenated areas to heal a damaged tissue.

Enamel matrix proteins regulate hypoxia-induced cellular biobehavior and osteogenic differentiation in human periodontal ligament cells

Hypoxia is a crucial microenvironment for inflamed periodontal tissue and periodontal wound healing. Enamel matrix proteins (EMPs) potentially can promote the formation of new periodontium. The effects of EMPs on periodontal ligament cells under hypoxia, however, remain unclear. We investigated the effects of EMPs on cellular biobehavior and osteogenic differentiation of human periodontal ligament cells (hPDLCs) under hypoxia. Under cobalt chloride (CoCl₂)-induced hypoxia, cellular biobehavior of hPDLCs, including proliferation, attachment, spreading, and migration with or without EMPs, was evaluated by 3-(4, 5-dimethylthiazol- 2-yl)-2, 5-diphenyl tetrazolium bromide (MTT), cell counting, spreading area measurement and wound scratch assay. The osteogenic activity of hPDLCs was assessed using alkaline phosphatase (ALP) and alizarin red S staining (ARS). The expressions of osteogenic genes including runt related transcription factor 2 (Runx2), ALP, osteocalcin (OCN) and collagen type I (Col-I) were detected using real time quantitative PCR, western blot and immunocytochemistry assays. The biobehavior and osteogenic differentiation of hPDLCs were inhibited significantly under hypoxia. EMPs have no effect on cell proliferation under mimicked hypoxia. EMPs partly reversed the inhibitory effects of hypoxia, however, for other cellular biobehavior including attachment, spreading and migration, and markedly up-regulated osteogenic differentiation activities including ALP, mineralization ability and the expressions of osteogenic genes such as Runx2, ALP, osteocalcin, and collagen type I in hPDLCs under hypoxia. EMPs attenuate the hypoxic injury to cellular biobehavior and osteogenic differentiation in hPDLCs under hypoxia.