Hypoxia-inducible factor-1 alpha maintains mouse articular cartilage through suppression of NF-κB signaling

Phenotype and energy metabolism differ between osteoarthritic chondrocytes from male compared to female patients: Implications for sexual dimorphism in osteoarthritis development?

OBJECTIVES

The prevalence and severity of knee osteoarthritis (OA) are greater in females than males. The purpose of this study was to determine whether there is an underlying difference in the biology of OA chondrocytes between males and females.

METHODS

Chondrocytes were obtained following knee arthroplasty from male and female patients with primary OA. Phenotype marker expression, glucose and fat consumption, and rates of glycolysis and oxidative phosphorylation were compared between females and males. RNAi was used to determine the consequences of differential expression of Sry-box transcription factor 9 (SOX9) and PGC1α between males and females.

RESULTS

OA chondrocytes from male donors showed elevated ribonucleic acid (RNA) and protein levels of SOX9, elevated COL2A1 protein synthesis, higher glucose consumption, and higher usage of glycolysis compared to females. OA chondrocytes from females had higher PGC1α protein levels, higher fat consumption, and higher oxidative energy metabolism than males. Knockdown of SOX9 reduced expression of COL2A1 to a greater extent in male OA chondrocytes than females whereas knockdown of PGC1α reduced COL2A1 expression in females but not males. Expression of ACAN and the glycolytic enzyme PGK1 was also reduced in males but not females following SOX9 knockdown.

CONCLUSIONS

OA chondrocyte phenotype and energy metabolism differ between males and females. Our results indicate transcriptional control of COL2A1 differs between the two. Differences in chondrocyte biology between males and females imply the underlying mechanisms involved in OA may also differ, highlighting the need to consider sex and gender when investigating pathogenesis and potential treatments for OA.

Mitochondria: a breakthrough in combating rheumatoid arthritis

As a chronic autoimmune disease with complex aetiology, rheumatoid arthritis (RA) has been demonstrated to be associated with mitochondrial dysfunction since mitochondrial dysfunction can affect the survival, activation, and differentiation of immune and non-immune cells involved in the pathogenesis of RA. Nevertheless, the mechanism behind mitochondrial dysfunction in RA remains uncertain. Accordingly, this review addresses the possible role and mechanisms of mitochondrial dysfunction in RA and discusses the potential and challenges of mitochondria as a potential therapeutic strategy for RA, thereby providing a breakthrough point in the prevention and treatment of RA.

Promoting Articular Cartilage Regeneration through Microenvironmental Regulation

In recent years, as the aging population continues to grow, osteoarthritis (OA) has emerged as a leading cause of disability, with its incidence rising annually. Current treatments of OA include exercise and medications in the early stages and total joint replacement in the late stages. These approaches only relieve pain and reduce inflammation; however, they have significant side effects and high costs. Therefore, there is an urgent need to identify effective treatment methods that can delay the pathological progression of this condition. The changes in the articular cartilage microenvironment, which are complex and diverse, can aggravate the pathological progression into a vicious cycle, inhibiting the repair and regeneration of articular cartilage. Understanding these intricate changes in the microenvironment is crucial for devising effective treatment modalities. By searching relevant research articles and clinical trials in PubMed according to the keywords of articular cartilage, microenvironment, OA, mechanical force, hypoxia, cytokine, and cell senescence. This study first summarizes the factors affecting articular cartilage regeneration, then proposes corresponding treatment strategies, and finally points out the future research direction. We find that regulating the opening of mechanosensitive ion channels, regulating the expression of HIF-1, delivering growth factors, and clearing senescent cells can promote the formation of articular cartilage regeneration microenvironment. This study provides a new idea for the treatment of OA in the future, which can promote the regeneration of articular cartilage through the regulation of the microenvironment so as to achieve the purpose of treating OA.

T-2 toxin-induced damage to articular cartilage in rats coincided with impaired autophagy linked to the HIF-1α/AMPK signaling axis

T-2 toxin is one of the most toxic mycotoxins. People are primarily exposed to T-2 toxin through the consumption of spoiled food, typically over extended periods and at low doses. T-2 toxin can cause damage to articular cartilage. However, the exact mechanism is not fully understood. In this experiment, 36 male rats were divided into a control group, a solvent control group, and a T-2 toxin group. The rats in the T-2 toxin group were orally administered the toxin at a dosage of 100 ng/g BW/Day. The damage to articular cartilage and key proteins associated with the autophagy process and the HIF-1α/AMPK signaling axis was assessed at 4, 8, 12, and 16 weeks. Our findings indicate that T-2 toxin-induced damage to articular cartilage in rats coincided with impaired autophagy linked to the HIF-1α/AMPK signaling pathway. This study offers novel insights into the precise mechanism underlying T-2 toxin-induced damage to articular cartilage.

Specific isoforms of the ubiquitin ligase gene WWP2 are targets of osteoarthritis genetic risk via a differentially methylated DNA sequence

BACKGROUND

Transitioning from a genetic association signal to an effector gene and a targetable molecular mechanism requires the application of functional fine-mapping tools such as reporter assays and genome editing. In this report, we undertook such studies on the osteoarthritis (OA) risk that is marked by single nucleotide polymorphism (SNP) rs34195470 (A > G). The OA risk-conferring G allele of this SNP associates with increased DNA methylation (DNAm) at two CpG dinucleotides within WWP2. This gene encodes a ubiquitin ligase and is the host gene of microRNA-140 (miR-140). WWP2 and miR-140 are both regulators of TGFβ signaling.

METHODS

Nucleic acids were extracted from adult OA (arthroplasty) and foetal cartilage. Samples were genotyped and DNAm quantified by pyrosequencing at the two CpGs plus 14 flanking CpGs. CpGs were tested for transcriptional regulatory effects using a chondrocyte cell line and reporter gene assay. DNAm was altered using epigenetic editing, with the impact on gene expression determined using RT-qPCR. In silico analysis complemented laboratory experiments.

RESULTS

rs34195470 genotype associates with differential methylation at 14 of the 16 CpGs in OA cartilage, forming a methylation quantitative trait locus (mQTL). The mQTL is less pronounced in foetal cartilage (5/16 CpGs). The reporter assay revealed that the CpGs reside within a transcriptional regulator. Epigenetic editing to increase their DNAm resulted in altered expression of the full-length and N-terminal transcript isoforms of WWP2. No changes in expression were observed for the C-terminal isoform of WWP2 or for miR-140.

CONCLUSIONS

As far as we are aware, this is the first experimental demonstration of an OA association signal targeting specific transcript isoforms of a gene. The WWP2 isoforms encode proteins with varying substrate specificities for the components of the TGFβ signaling pathway. Future analysis should focus on the substrates regulated by the two WWP2 isoforms that are the targets of this genetic risk.

Hypoxic Conditions Modulate Chondrogenesis through the Circadian Clock: The Role of Hypoxia-Inducible Factor-1α

Hypoxia-inducible factor-1 (HIF-1) is a heterodimer transcription factor composed of an alpha and a beta subunit. HIF-1α is a master regulator of cellular response to hypoxia by activating the transcription of genes that facilitate metabolic adaptation to hypoxia. Since chondrocytes in mature articular cartilage reside in a hypoxic environment, HIF-1α plays an important role in chondrogenesis and in the physiological lifecycle of articular cartilage. Accumulating evidence suggests interactions between the HIF pathways and the circadian clock. The circadian clock is an emerging regulator in both developing and mature chondrocytes. However, how circadian rhythm is established during the early steps of cartilage formation and through what signaling pathways it promotes the healthy chondrocyte phenotype is still not entirely known. This narrative review aims to deliver a concise analysis of the existing understanding of the dynamic interplay between HIF-1α and the molecular clock in chondrocytes, in states of both health and disease, while also incorporating creative interpretations. We explore diverse hypotheses regarding the intricate interactions among these pathways and propose relevant therapeutic strategies for cartilage disorders such as osteoarthritis.

Hypoxia Potentiated Lung Cancer Cell Migration and Invasion by up-regulating HIF1α/JAK2/STAT3 Axis and Activating MMP13 Transcription

Excessive aggressive migration and invasion are important factors that increase the mortality of cancer patients. Matrix metalloproteinase 13 (MMP13) expression is positively correlated with lung cancer malignancy. However, the mechanism underlying an elevated MMP13 expression is not clearly defined. In this study, we demonstrated that hypoxia induced by CoCl2 enhanced the expression of HIF1α, JAK2, STAT3 and MMP13 in A549 cells. A positive correlation between HIF1α and MMP13 expression was observed in lung adenocarcinoma patients. Mechanically, hypoxia upregulated HIF1α/JAK2/STAT3 signal axis, promoted transcription factor STAT3 to bind to MMP13 promoter region, and activated MMP13 transcription, finally promoted cell invasion and migration. However, stattic (STAT3 inhibitor) could reverse this effect caused by STAT3 in A549 cells. Together our data indicated that hypoxia might promote lung cancer cell migration and invasion through the HIF1α/JAK2/STAT3 axis by activating MMP13 transcription. MMP13 could be a promising therapeutic target for lung adenocarcinoma metastasis.

Metabolic reprogramming of glycolysis favors cartilage progenitor cells rejuvenation

Osteoarthritis (OA), the leading cause of disability in the elderly, still lacks effective treatment due to the unelucidated mechanisms of pathogenesis and progression. In cartilage, although the solo cell type of chondrocytes is resident, cartilage progenitor cells (CPCs) are identified. Chondrocytes in cartilage mainly utilize glycolysis because of the low oxygen tension. Until now, whether the metabolic pathway changes are associated with OA initiation or progression, as well as the biology of CPCs, remains fully clarified. By reviewing relevant literature from previous functional studies, we further mined recently published mouse and human chondrocytes single-cell RNA-sequencing datasets to explore gene expression profiles shift in OA initiation or during OA progression, regarding metabolism. In this review, we demonstrated that chondrocytes' metabolic shift from glycolysis to oxidative phosphorylation (OXPHOS) in OA initiation or during OA progression. Genes that related to OXPHOS, electron transport, mitochondrial translation, and mitochondrial respiratory chain complex assembly were upregulated in chondrocytes of injured cartilage or during OA progression. In addition, compared to OXPHOS, glycolysis facilitates CPC expansion and chondrogenic potential. The collated information suggests a potential therapeutic for OA through metabolic reprogramming of glycolysis to interrupt OA pathology and favor CPCs rejuvenation to restore healthy cartilage.

Inflammatory and Metabolic Signaling Interfaces of the Hypertrophic and Senescent Chondrocyte Phenotypes Associated with Osteoarthritis

Osteoarthritis (OA) is a complex disease of whole joints with progressive cartilage matrix degradation and chondrocyte transformation. The inflammatory features of OA are reflected in increased synovial levels of IL-1β, IL-6 and VEGF, higher levels of TLR-4 binding plasma proteins and increased expression of IL-15, IL-18, IL-10 and Cox2, in cartilage. Chondrocytes in OA undergo hypertrophic and senescent transition; in these states, the expression of Sox-9, Acan and Col2a1 is suppressed, whereas the expression of RunX2, HIF-2α and MMP-13 is significantly increased. NF-kB, which triggers many pro-inflammatory cytokines, works with BMP, Wnt and HIF-2α to link hypertrophy and inflammation. Altered carbohydrate metabolism and the upregulation of GLUT-1 contribute to the formation of end-glycation products that trigger inflammation via the RAGE pathway. In addition, a glycolytic shift, increased rates of oxidative phosphorylation and mitochondrial dysfunction generate reactive oxygen species with deleterious effects. An important surveyor mechanism, the YAP/TAZ signaling system, controls chondrocyte differentiation, inhibits ageing by protecting the nuclear envelope and suppressing NF-kB, MMP-13 and aggrecanases. The inflammatory microenvironment and synthesis of key matrix components are also controlled by SIRT1 and mTORc. Senescent chondrocytes represent the functional end stage of hypertrophic differentiation and characteristically upregulate p16 and p21, but also a variety of inflammatory cytokines, chemokines and metalloproteinases, developing the senescence-associated secretory phenotype. Senolysis with dendrobin, miR29b-5p and other agents has been shown to be efficient under experimental conditions, and appears to be a promising tool for the treatment of OA, as it restores COL2A1 and aggrecan synthesis, suppressing NF-kB and destructive metalloproteinases.

Metabolic rewiring controlled by c-Fos governs cartilage integrity in osteoarthritis

OBJECTIVES

The activator protein-1 (AP-1) transcription factor component c-Fos regulates chondrocyte proliferation and differentiation, but its involvement in osteoarthritis (OA) has not been functionally assessed.

METHODS

c-Fos expression was evaluated by immunohistochemistry on articular cartilage sections from patients with OA and mice subjected to the destabilisation of the medial meniscus (DMM) model of OA. Cartilage-specific c-Fos knockout (c-FosΔCh) mice were generated by crossing c-fosfl/fl to Col2a1-CreERT mice. Articular cartilage was evaluated by histology, immunohistochemistry, RNA sequencing (RNA-seq), quantitative reverse transcription PCR (qRT-PCR) and in situ metabolic enzyme assays. The effect of dichloroacetic acid (DCA), an inhibitor of pyruvate dehydrogenase kinase (Pdk), was assessed in c-FosΔCh mice subjected to DMM.

RESULTS

FOS-positive chondrocytes were increased in human and murine OA cartilage during disease progression. Compared with c-FosWT mice, c-FosΔCh mice exhibited exacerbated DMM-induced cartilage destruction. Chondrocytes lacking c-Fos proliferate less, have shorter collagen fibres and reduced cartilage matrix. Comparative RNA-seq revealed a prominent anaerobic glycolysis gene expression signature. Consistently decreased pyruvate dehydrogenase (Pdh) and elevated lactate dehydrogenase (Ldh) enzymatic activities were measured in situ, which are likely due to higher expression of hypoxia-inducible factor-1α, Ldha, and Pdk1 in chondrocytes. In vivo treatment of c-FosΔCh mice with DCA restored Pdh/Ldh activity, chondrocyte proliferation, collagen biosynthesis and decreased cartilage damage after DMM, thereby reverting the deleterious effects of c-Fos inactivation.

CONCLUSIONS

c-Fos modulates cellular bioenergetics in chondrocytes by balancing pyruvate flux between anaerobic glycolysis and the tricarboxylic acid cycle in response to OA signals. We identify a novel metabolic adaptation of chondrocytes controlled by c-Fos-containing AP-1 dimers that could be therapeutically relevant.

Bioinformatics and system biology approach to identify potential common pathogenesis for COVID-19 infection and osteoarthritis

A growing of evidence has showed that patients with osteoarthritis (OA) had a higher coronavirus 2019 (COVID-19) infection rate and a poorer prognosis after infected it. Additionally, scientists have also discovered that COVID-19 infection might cause pathological changes in the musculoskeletal system. However, its mechanism is still not fully elucidated. This study aims to further explore the sharing pathogenesis of patients with both OA and COVID-19 infection and find candidate drugs. Gene expression profiles of OA (GSE51588) and COVID-19 (GSE147507) were obtained from the Gene Expression Omnibus (GEO) database. The common differentially expressed genes (DEGs) for both OA and COVID-19 were identified and several hub genes were extracted from them. Then gene and pathway enrichment analysis of the DEGs were performed; protein-protein interaction (PPI) network, transcription factor (TF)-gene regulatory network, TF-miRNA regulatory network and gene-disease association network were constructed based on the DEGs and hub genes. Finally, we predicted several candidate molecular drugs related to hub genes using DSigDB database. The receiver operating characteristic curve (ROC) was applied to evaluate the accuracy of hub genes in the diagnosis of both OA and COVID-19. In total, 83 overlapping DEGs were identified and selected for subsequent analyses. CXCR4, EGR2, ENO1, FASN, GATA6, HIST1H3H, HIST1H4H, HIST1H4I, HIST1H4K, MTHFD2, PDK1, TUBA4A, TUBB1 and TUBB3 were screened out as hub genes, and some showed preferable values as diagnostic markers for both OA and COVID-19. Several candidate molecular drugs, which are related to the hug genes, were identified. These sharing pathways and hub genes may provide new ideas for further mechanistic studies and guide more individual-based effective treatments for OA patients with COVID-19 infection.

Natural compounds protect against the pathogenesis of osteoarthritis by mediating the NRF2/ARE signaling

Osteoarthritis (OA), a chronic joint cartilage disease, is characterized by the imbalanced homeostasis between anabolism and catabolism. Oxidative stress contributes to inflammatory responses, extracellular matrix (ECM) degradation, and chondrocyte apoptosis and promotes the pathogenesis of OA. Nuclear factor erythroid 2-related factor 2 (NRF2) is a central regulator of intracellular redox homeostasis. Activation of the NRF2/ARE signaling may effectively suppress oxidative stress, attenuate ECM degradation, and inhibit chondrocyte apoptosis. Increasing evidence suggests that the NRF2/ARE signaling has become a potential target for the therapeutic management of OA. Natural compounds, such as polyphenols and terpenoids, have been explored to protect against OA cartilage degeneration by activating the NRF2/ARE pathway. Specifically, flavonoids may function as NRF2 activators and exhibit chondroprotective activity. In conclusion, natural compounds provide rich resources to explore the therapeutic management of OA by activating NRF2/ARE signaling.

Differential Effects of Hypoxia versus Hyperoxia or Physoxia on Phenotype and Energy Metabolism in Human Chondrocytes from Osteoarthritic Compared to Macroscopically Normal Cartilage

Chondrocyte phenotype and energy metabolism are altered in osteoarthritis (OA). However, most studies characterising the change in human chondrocyte behaviour in OA have been conducted in supraphysiological oxygen concentrations. The purpose of this study was to compare phenotype and energy metabolism in chondrocytes from macroscopically normal (MN) and OA cartilage maintained in 18.9% (standard tissue culture), 6% (equivalent to superficial zone of cartilage in vivo) or 1% oxygen (equivalent to deep zone of cartilage in vivo). MMP13 production was higher in chondrocytes from OA compared to MN cartilage in hyperoxia and physoxia but not hypoxia. Hypoxia promoted SOX9, COL2A1 and ACAN protein expression in chondrocytes from MN but not OA cartilage. OA chondrocytes used higher levels of glycolysis regardless of oxygen availability. These results show that differences in phenotype and energy metabolism between chondrocytes from OA and MN cartilage differ depending on oxygen availability. OA chondrocytes show elevated synthesis of cartilage-catabolising enzymes and chondrocytes from MN cartilage show reduced cartilage anabolism in oxygenated conditions. This is relevant as a recent study has shown that oxygen levels are elevated in OA cartilage in vivo. Our findings may indicate that this elevated cartilage oxygenation may promote cartilage loss in OA.

Maintaining hypoxia environment of subchondral bone alleviates osteoarthritis progression

Abnormal subchondral bone remodeling featured by overactivated osteoclastogenesis leads to articular cartilage degeneration and osteoarthritis (OA) progression, but the mechanism is unclear. We used lymphocyte cytosolic protein 1 (Lcp1) knockout mice to suppress subchondral osteoclasts in a mice OA model with anterior cruciate ligament transection (ACLT), and Lcp1^-/- mice showed decreased bone remodeling in subchondral bone and retarded cartilage degeneration. For mechanisms, the activated osteoclasts in subchondral bone induced type-H vessels and elevated oxygen concentration, which ubiquitylated hypoxia-inducible factor 1 alpha subunit (HIF-1α) in chondrocytes and led to cartilage degeneration. Lcp1 knockout impeded angiogenesis, which maintained hypoxia environment in joints and delayed the OA progression. Stabilization of HIF-1α delayed cartilage degeneration, and knockdown of Hif1a abolished the protective effects of Lcp1 knockout. Last, we showed that Oroxylin A, an Lcp1-encoded protein l-plastin (LPL) inhibitor, could alleviate OA progression. In conclusion, maintaining hypoxic environment is an attractive strategy for OA treatment.

Hypoxia-Inducible Factor-2α Signaling in the Skeletal System

Hypoxia-inducible factors (HIFs) are oxygen-dependent heterodimeric transcription factors that mediate molecular responses to reductions in cellular oxygen (hypoxia). HIF signaling involves stable HIF-β subunits and labile, oxygen-sensitive HIF-α subunits. Under hypoxic conditions, the HIF-α subunit is stabilized, complexes with nucleus-confined HIF-β subunit, and transcriptionally regulates hypoxia-adaptive genes. Transcriptional responses to hypoxia include altered energy metabolism, angiogenesis, erythropoiesis, and cell fate. Three isoforms of HIF-α-HIF-1α, HIF-2α, and HIF-3α-are found in diverse cell types. HIF-1α and HIF-2α serve as transcriptional activators, whereas HIF-3α restricts HIF-1α and HIF-2α. The structure and isoform-specific functions of HIF-1α in mediating molecular responses to hypoxia are well established across a wide range of cell and tissue types. The contributions of HIF-2α to hypoxic adaptation are often unconsidered if not outrightly attributed to HIF-1α. This review establishes what is currently known about the diverse roles of HIF-2α in mediating the hypoxic response in skeletal tissues, with specific focus on development and maintenance of skeletal fitness. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Hypoxia and Wnt signaling inversely regulate expression of chondroprotective molecule ANP32A in articular cartilage

OBJECTIVES

ANP32A is a key protector of cartilage health, via preventing oxidative stress and Wnt hyper-activation. We aimed to unravel how ANP32A is regulated in cartilage.

METHODS

A bioinformatics pipeline was applied to identify regulators of ANP32A. Pathways of interest were targeted to study their impact on ANP32A in in vitro cultures of the human chondrocyte C28/I2 cell-line and primary human articular chondrocytes (hACs) from up to five different donors, using Wnt-activator CHIR99021, hypoxia-mimetic IOX2 and a hypoxia chamber. ANP32A was evaluated using real-time quantitative polymerase chain reaction (RT-qPCR) and Western blot. In vivo, the effect of hypoxia was examined by immunohistochemistry in mice injected intra-articularly with IOX2 after destabilization of the medial meniscus. Effects of Wnt hyper-activation were investigated using Frzb-knockout mice and wild-type mice treated intra-articularly with CHIR99021. Wnt inhibition effects were assessed upon intra-articular injection of XAV939.

RESULTS

The hypoxia and Wnt signaling pathways were identified as networks controlling ANP32A expression. In vitro and in vivo experiments demonstrated increases in ANP32A upon hypoxic conditions (1.3-fold in hypoxia in C28/I2 cells with 95% confidence interval (CI) [1.11-1.54] and 1.90-fold in hACs [95% CI: 1.56-2] and 1.67-fold in ANP32A protein levels after DMM surgery with IOX2 injections [95% CI: 1.33-2.08]). Wnt hyper-activation decreased ANP32A in chondrocytes in vitro (1.23-fold decrease [95% CI: 1.02-1.49]) and in mice (1.45-fold decrease after CHIR99021 injection [95% CI: 1.22-1.72] and 1.41-fold decrease in Frzb-knockout mice [95% CI: 1.00-1.96]). Hypoxia and Wnt modulated ataxia-telangiectasia mutated serine/threonine kinase (ATM), an ANP32A target gene, in hACs (1.89-fold increase [95% CI: 1.38-2.60] and 1.41-fold decrease [95% CI: 1.02-1.96]).

CONCLUSIONS

Maintaining hypoxia and limiting Wnt activation sustain ANP32A and protect against osteoarthritis.

Small-molecule photoswitches for fluorescence bioimaging: engineering and applications

Fluorescence microscopy has revolutionised our understanding of biological systems, enabling the visualisation of biomolecular structures and dynamics in complex systems. The possibility to reversibly control the optical or biochemical properties of fluorophores can unlock advanced applications ranging from super-resolution microscopy to the design of multi-stimuli responsive and functional biosensors. In this Highlight, we review recent progress in small-molecule photoswitches applied to biological imaging with an emphasis on molecular engineering strategies and promising applications, while underlining the main challenges in their design and implementation.

Mechanism of HIFs in osteoarthritis

Osteoarthritis (OA) is a common disabling disease which has a high incidence rate in the elderly. Studies have found that many factors are involved in the pathogenesis of OA. Hypoxia-inducible factors (HIFs) are core regulators that induce hypoxia genes, repair the cellular oxygen environment, and play an important role in the treatment of OA. For example, HIF-1α can maintain the stability of the articular cartilage matrix, HIF-2α is able to cause chondrocyte apoptosis and intensify in-flammatory response, and HIF-3α may be the target gene of HIF-1α and HIF-2α, thereby playing a negative regulatory role. This review examines the mechanism of HIFs in cartilage extracellular matrix degradation, apoptosis, inflammatory reaction, autophagy and then further expounds on the roles of HIFs in OA, consequently providing theoretical support for the pathogenesis of OA and a new target for OA treatment.

Oxidative Stress Response's Kinetics after 60 Minutes at Different (30% or 100%) Normobaric Hyperoxia Exposures

Oxygen is a powerful trigger for cellular reactions and is used in many pathologies, including oxidative stress. However, the effects of oxygen over time and at different partial pressures remain poorly understood. In this study, the metabolic responses of normobaric oxygen intake for 1 h to mild (30%) and high (100%) inspired fractions were investigated. Fourteen healthy non-smoking subjects (7 males and 7 females; age: 29.9 ± 11.1 years, height: 168.2 ± 9.37 cm; weight: 64.4 ± 12.3 kg; BMI: 22.7 ± 4.1) were randomly assigned in the two groups. Blood samples were taken before the intake at 30 min, 2 h, 8 h, 24 h, and 48 h after the single oxygen exposure. The level of oxidation was evaluated by the rate of reactive oxygen species (ROS) and the levels of isoprostane. Antioxidant reactions were observed by total antioxidant capacity (TAC), superoxide dismutase (SOD), and catalase (CAT). The inflammatory response was measured using interleukin-6 (IL-6), neopterin, creatinine, and urates. Oxidation markers increased from 30 min on to reach a peak at 8 h. From 8 h post intake, the markers of inflammation took over, and more significantly with 100% than with 30%. This study suggests a biphasic response over time characterized by an initial "permissive oxidation" followed by increased inflammation. The antioxidant protection system seems not to be the leading actor in the first place. The kinetics of enzymatic reactions need to be better studied to establish therapeutic, training, or rehabilitation protocols aiming at a more targeted use of oxygen.

African swine fever virus infection activates inflammatory responses through downregulation of the anti-inflammatory molecule C1QTNF3

African swine fever (ASF) is the most dangerous pig disease, and causes enormous economic losses in the global pig industry. However, the mechanisms of ASF virus (ASFV) infection remains largely unclear. Hence, this study investigated the host response mechanisms to ASFV infection. We analyzed the differentially expressed proteins (DEPs) between serum samples from ASFV-infected and uninfected pigs using quantitative proteomics. Setting the p-value < 0.05 and |log2 (fold change)| > 1.5, we identified 173 DEPs, comprising 57 upregulated and 116 downregulated proteins, which belonged to various biological processes and pathways based on the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses. The enriched pathways include immune responses, metabolism, and inflammation signaling pathways. Western blot analysis validated the DEPs identified using quantitative proteomics. Furthermore, our proteomics data showed that C1QTNF3 regulated the inflammatory signaling pathway. C1QTNF3 knockdown led to the upregulation of pro-inflammatory factors IL-1β, IL-8, and IL-6, thus inhibiting ASFV replication. These results indicated that C1QTNF3 was critical for ASFV infection. In conclusion, this study revealed the molecular mechanisms underlying the host-ASFV interaction, which may contribute to the development of novel antiviral strategies against ASFV infection in the future.

Hypoxia-Inducible Factors Signaling in Osteogenesis and Skeletal Repair

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

Meta-analysis Integrated With Multi-omics Data Analysis to Elucidate Pathogenic Mechanisms of Age-Related Knee Osteoarthritis in Mice

Increased mechanistic insight into the pathogenesis of knee osteoarthritis (KOA) is needed to develop efficacious disease-modifying treatments. Though age-related pathogenic mechanisms are most relevant to the majority of clinically presenting KOA, the bulk of our mechanistic understanding of KOA has been derived using surgically induced posttraumatic OA (PTOA) models. Here, we took an integrated approach of meta-analysis and multi-omics data analysis to elucidate pathogenic mechanisms of age-related KOA in mice. Protein-level data were integrated with transcriptomic profiling to reveal inflammation, autophagy, and cellular senescence as primary hallmarks of age-related KOA. Importantly, the molecular profiles of cartilage aging were unique from those observed following PTOA, with less than 3% overlap between the 2 models. At the nexus of the 3 aging hallmarks, advanced glycation end product (AGE)/receptor for AGE (RAGE) emerged as the most statistically robust pathway associated with age-related KOA. This pathway was further supported by analysis of mass spectrometry data. Notably, the change in AGE-RAGE signaling over time was exclusively observed in male mice, suggesting sexual dimorphism in the pathogenesis of age-induced KOA in murine models. Collectively, these findings implicate dysregulation of AGE-RAGE signaling as a sex-dependent driver of age-related KOA.

HIF-1α in Osteoarthritis: From Pathogenesis to Therapeutic Implications

Osteoarthritis is a common age-related joint degenerative disease. Pain, swelling, brief morning stiffness, and functional limitations are its main characteristics. There are still no well-established strategies to cure osteoarthritis. Therefore, better clarification of mechanisms associated with the onset and progression of osteoarthritis is critical to provide a theoretical basis for the establishment of novel preventive and therapeutic strategies. Chondrocytes exist in a hypoxic environment, and HIF-1α plays a vital role in regulating hypoxic response. HIF-1α responds to cellular oxygenation decreases in tissue regulating survival and growth arrest of chondrocytes. The activation of HIF-1α could regulate autophagy and apoptosis of chondrocytes, decrease inflammatory cytokine synthesis, and regulate the chondrocyte extracellular matrix environment. Moreover, it could maintain the chondrogenic phenotype that regulates glycolysis and the mitochondrial function of osteoarthritis, resulting in a denser collagen matrix that delays cartilage degradation. Thus, HIF-1α is likely to be a crucial therapeutic target for osteoarthritis via regulating chondrocyte inflammation and metabolism. In this review, we summarize the mechanism of hypoxia in the pathogenic mechanisms of osteoarthritis, and focus on a series of therapeutic treatments targeting HIF-1α for osteoarthritis. Further clarification of the regulatory mechanisms of HIF-1α in osteoarthritis may provide more useful clues to developing novel osteoarthritis treatment strategies.

Deciphering Pro-angiogenic Transcription Factor Profiles in Hypoxic Human Endothelial Cells by Combined Bioinformatics and in vitro Modeling

Background

Constant supply of oxygen is crucial for multicellular tissue homeostasis and energy metabolism in cardiac tissue. As a first response to acute hypoxia, endothelial cells (ECs) promote recruitment and adherence of immune cells to the dysbalanced EC barrier by releasing inflammatory mediators and growth factors, whereas chronic hypoxia leads to the activation of a transcription factor (TF) battery, that potently induces expression of growth factors and cytokines including platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF). We report a hypoxia-minded, targeted bioinformatics approach aiming to identify and validate TFs that regulate angiogenic signaling.

Results

A comprehensive RNA-Seq dataset derived from human ECs subjected to normoxic or hypoxic conditions was selected to identify significantly regulated genes based on (i) fold change (normoxia vs. hypoxia) and (ii) relative abundancy. Transcriptional regulation of this gene set was confirmed via qPCR in validation experiments where HUVECs were subjected to hypoxic conditions for 24 h. Screening the promoter and upstream regulatory elements of these genes identified two TFs, KLF5 and SP1, both with a potential binding site within these regions of selected target genes. In vitro, siRNA experiments confirmed SP1- and KLF5-mediated regulation of identified hypoxia-sensitive endothelial genes. Next to angiogenic signaling, we also validated the impact of TFs on inflammatory signaling, both key events in hypoxic sensing. Both TFs impacted on inflammatory signaling since endogenous repression led to increased NF-κB signaling. Additionally, SP1 silencing eventuated decreased angiogenic properties in terms of proliferation and tube formation.

Conclusion

By detailed in silico analysis of promoter region and upstream regulatory elements for a list of hypoxia-sensitive genes, our bioinformatics approach identified putative binding sites for TFs of SP or KLF family in vitro. This strategy helped to identify TFs functionally involved in human angiogenic signaling and therefore serves as a base for identifying novel RNA-based drug entities in a therapeutic setting of vascularization.

Hypoxia-inducible factor expression is related to apoptosis and cartilage degradation in temporomandibular joint osteoarthritis

BACKGROUND

It remains unclear etiology of cartilaginous tissues in osteoarthritis (OA) lesions. In this study, we hypothesized the accumulation of hypoxia-inducible factor (HIF) and activated apoptosis relate to condylar cartilage degeneration in vivo.

METHODS

Malocclusion stress was applied for 2 weeks, 4 weeks and 8 weeks to induce an OA-like lesion animal model in rats. Histological analysis was performed by H&E staining and Safranin O/fast green staining. The expression levels of protein in condylar cartilage were examined by immunostaining to evaluate cartilage degeneration.

RESULTS

We found apparent histological phenotypes associated with degeneration in the occlusion disorder (OD) stress group. The OD group at 4 weeks and 8 weeks had obviously reduced expression of Aggrecan (Acan) and type II collagen (Col II) in cartilage. In contrast, the OD groups had higher levels of ADAM metallopeptidase with thrombospondin type 5 (ADAMTS5) and matrix metallopeptidase 13 (MMP13) in the condylar cartilage than the control group. Moreover, the OD group cartilage had prominent degenerative changes with reduced levels of hypoxia inducible factor 1 alpha (HIF1α) and increased levels of hypoxia inducible factor 2 alpha (HIF2α) and the apoptosis factor Caspase3 in condylar cartilage at 8 weeks.

CONCLUSION

Thus, abnormal hypoxic conditions inducing Occlusion disorder stress results in cartilage degeneration. opposite expression patterns of HIF1α and HIF2α could be involved in the pathogenesis of condylar cartilage degeneration and chondrocyte apoptosis. HIF2α may provide a potential negative feedback mechanism for HIF1α during cartilage damage.

The role of disrupted iron homeostasis in the development and progression of arthropathy

Arthropathy or joint disease leads to significant pain and disability irrespective of etiology. Clinical and experimental evidence point to the presence of considerable links between arthropathy and iron overload. Previous work has suggested that iron accumulation in the joints is often associated with increased oxidative stress, disrupted matrix metabolism, and cartilage degeneration. However, key issues regarding the role of iron overload in the pathogenesis of arthropathy remain ambiguous. For example, significant gaps in our knowledge of the primary cellular targets of iron overload-induced damage and the exact molecular mechanism through which disrupted iron homeostasis leads to joint damage still exist. The exact signaling pathway that links iron metabolism and cellular damage in arthropathy also remains largely unmapped. In this review, we focus on the relationship between iron overload and arthropathy with special emphasis on the adversarial relationship between iron that accumulates in the joints over time and cartilage homeostasis. A better understanding of the mechanisms and pathways underlying iron-induced cartilage degeneration may help in defining new prognostic markers and therapeutic targets in arthropathy.

Remodeling Osteoarthritic Articular Cartilage under Hypoxic Conditions

Osteoarthritis (OA) is one of the leading joint diseases induced by abnormalities or inflammation in the synovial membrane and articular cartilage, causing severe pain and disability. Along with the cartilage malfunction, imbalanced oxygen uptake occurs, changing chondrocytes into type I collagen- and type X collagen-producing dedifferentiated cells, contributing to OA progression. However, mounting evidence suggests treating OA by inducing a hypoxic environment in the articular cartilage, targeting the inhibition of several OA-related pathways to bring chondrocytes into a normal state. This review discusses the implications of OA-diseased articular cartilage on chondrocyte phenotypes and turnover and debates the hypoxic mechanism of action. Furthermore, this review highlights the new understanding of OA, provided by tissue engineering and a regenerative medicine experimental design, modeling the disease into diverse 2D and 3D structures and investigating hypoxia and hypoxia-inducing biomolecules and potential cell therapies. This review also reports the mechanism of hypoxic regulation and highlights the importance of activating and stabilizing the hypoxia-inducible factor and related molecules to protect chondrocytes from mitochondrial dysfunction and apoptosis occurring under the influence of OA.

Strategies for Articular Cartilage Repair and Regeneration

Articular cartilage is an avascular tissue, with limited ability to repair and self-renew. Defects in articular cartilage can induce debilitating degenerative joint diseases such as osteoarthritis. Currently, clinical treatments have limited ability to repair, for they often result in the formation of mechanically inferior cartilage. In this review, we discuss the factors that affect cartilage homeostasis and function, and describe the emerging regenerative approaches that are informing the future treatment options.

Hypoxia induces DOT1L in articular cartilage to protect against osteoarthritis

Osteoarthritis is the most prevalent joint disease worldwide, and it is a leading source of pain and disability. To date, this disease lacks curative treatment, as underlying molecular mechanisms remain largely unknown. The histone methyltransferase DOT1L protects against osteoarthritis, and DOT1L-mediated H3K79 methylation is reduced in human and mouse osteoarthritic joints. Thus, restoring DOT1L function seems to be critical to preserve joint health. However, DOT1L-regulating molecules and networks remain elusive, in the joint and beyond. Here, we identified transcription factors and networks that regulate DOT1L gene expression using a potentially novel bioinformatics pipeline. Thereby, we unraveled a possibly undiscovered link between the hypoxia pathway and DOT1L. We provide evidence that hypoxia enhanced DOT1L expression and H3K79 methylation via hypoxia-inducible factor-1 α (HIF1A). Importantly, we demonstrate that DOT1L contributed to the protective effects of hypoxia in articular cartilage and osteoarthritis. Intra-articular treatment with a selective hypoxia mimetic in mice after surgical induction of osteoarthritis restored DOT1L function and stalled disease progression. Collectively, our data unravel a molecular mechanism that protects against osteoarthritis with hypoxia inducing DOT1L transcription in cartilage. Local treatment with a selective hypoxia mimetic in the joint restores DOT1L function and could be an attractive therapeutic strategy for osteoarthritis.

Mechanical stimulation of chondrocytes regulates HIF-1α under hypoxic conditions

We investigated the effects of hypoxia-inducible factor (HIF)-1α on articular cartilage under mechanical stimulation and the associated mechanisms. Chondrocytes, isolated from articular cartilage from the knee, hip, and shoulder joints of Wistar rats, were subjected to 20 % tensile stress under hypoxic (5% O₂) conditions for 24 h. HIF-1α and aggrecan expression was significantly enhanced with mechanical stimulation under hypoxia but not significantly altered with mechanical stimulation under normoxia. The nuclear translocation of HIF-1α was enhanced by mechanical stress under hypoxia. Under both normoxia and hypoxia, a disintegrin and metalloproteinase with thrombospondin motifs (ADAM-TS) 5 expression was significantly reduced with mechanical stimulation compared to that in the group without mechanical stimulation. However, HIF-1α knockdown mitigated changes in aggrecan and ADAM-TS5 expression mediated by mechanical stimulation under hypoxia. The effects of treadmill running on HIF-1α production in the articular cartilage of rat knee joints were also analyzed. HIF-1α production increased in the moderate running group and decreased to the same levels as those in the control group in the excessive running group. This suggests that HIF-1α regulates aggrecan and ADAM-TS5 expression in response to mechanical stimulation under hypoxia and general mechanical stimulation in articular cartilage under hypoxia, while controlling cartilage homeostasis.

The Cellular Composition of Bovine Coccygeal Intervertebral Discs: A Comprehensive Single-Cell RNAseq Analysis

Intervertebral disc (IVD) degeneration and its medical consequences is still one of the leading causes of morbidity worldwide. To support potential regenerative treatments for degenerated IVDs, we sought to deconvolute the cell composition of the nucleus pulposus (NP) and the annulus fibrosus (AF) of bovine intervertebral discs. Bovine calf tails have been extensively used in intervertebral disc research as a readily available source of NP and AF material from healthy and young IVDs. We used single-cell RNA sequencing (scRNAseq) coupled to bulk RNA sequencing (RNAseq) to unravel the cell populations in these two structures and analyze developmental changes across the rostrocaudal axis. By integrating the scRNAseq data with the bulk RNAseq data to stabilize the clustering results of our study, we identified 27 NP structure/tissue specific genes and 24 AF structure/tissue specific genes. From our scRNAseq results, we could deconvolute the heterogeneous cell populations in both the NP and the AF. In the NP, we detected a notochordal-like cell cluster and a progenitor stem cell cluster. In the AF, we detected a stem cell-like cluster, a cluster with a predominantly fibroblast-like phenotype and a potential endothelial progenitor cluster. Taken together, our results illustrate the cell phenotypic complexity of the AF and NP in the young bovine IVDs.

The Cellular Composition of Bovine Coccygeal Intervertebral Discs: A Comprehensive Single-Cell RNAseq Analysis

Intervertebral disc (IVD) degeneration and its medical consequences is still one of the leading causes of morbidity worldwide. To support potential regenerative treatments for degenerated IVDs, we sought to deconvolute the cell composition of the nucleus pulposus (NP) and the annulus fibrosus (AF) of bovine intervertebral discs. Bovine calf tails have been extensively used in intervertebral disc research as a readily available source of NP and AF material from healthy and young IVDs. We used single-cell RNA sequencing (scRNAseq) coupled to bulk RNA sequencing (RNAseq) to unravel the cell populations in these two structures and analyze developmental changes across the rostrocaudal axis. By integrating the scRNAseq data with the bulk RNAseq data to stabilize the clustering results of our study, we identified 27 NP structure/tissue specific genes and 24 AF structure/tissue specific genes. From our scRNAseq results, we could deconvolute the heterogeneous cell populations in both the NP and the AF. In the NP, we detected a notochordal-like cell cluster and a progenitor stem cell cluster. In the AF, we detected a stem cell-like cluster, a cluster with a predominantly fibroblast-like phenotype and a potential endothelial progenitor cluster. Taken together, our results illustrate the cell phenotypic complexity of the AF and NP in the young bovine IVDs.

The role of metabolism in chondrocyte dysfunction and the progression of osteoarthritis

Osteoarthritis (OA) is a degenerative joint disease characterized by low-grade inflammation and high levels of clinical heterogeneity. Aberrant chondrocyte metabolism is a response to changes in the inflammatory microenvironment and may play a key role in cartilage degeneration and OA progression. Under conditions of environmental stress, chondrocytes tend to adapt their metabolism to microenvironmental changes by shifting from one metabolic pathway to another, for example from oxidative phosphorylation to glycolysis. Similar changes occur in other joint cells, including synoviocytes. Switching between these pathways is implicated in metabolic alterations that involve mitochondrial dysfunction, enhanced anaerobic glycolysis, and altered lipid and amino acid metabolism. The shift between oxidative phosphorylation and glycolysis is mainly regulated by the AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) pathways. Chondrocyte metabolic changes are likely to be a feature of different OA phenotypes. Determining the role of chondrocyte metabolism in OA has revealed key features of disease pathogenesis. Future research should place greater emphasis on immunometabolism and altered metabolic pathways as a means to understand the pathophysiology of age-related OA. This knowledge will advance the development of new drugs against therapeutic targets of metabolic significance.

Human engineered meniscus transcriptome after short-term combined hypoxia and dynamic compression

This study investigates the transcriptome response of meniscus fibrochondrocytes (MFCs) to the low oxygen and mechanical loading signals experienced in the knee joint using a model system. We hypothesized that short term exposure to the combined treatment would promote a matrix-forming phenotype supportive of inner meniscus tissue formation. Human MFCs on a collagen scaffold were stimulated to form fibrocartilage over 6 weeks under normoxic (NRX, 20% O₂) conditions with supplemented TGF-β3. Tissues experienced a delayed 24h hypoxia treatment (HYP, 3% O₂) and then 5 min of dynamic compression (DC) between 30 and 40% strain. Delayed HYP induced an anabolic and anti-catabolic expression profile for hyaline cartilage matrix markers, while DC induced an inflammatory matrix remodeling response along with upregulation of both SOX9 and COL1A1. There were 41 genes regulated by both HYP and DC. Overall, the combined treatment supported a unique gene expression profile favouring the hyaline cartilage aspect of inner meniscus matrix and matrix remodeling.

INSR mediated by transcription factor KLF4 and DNA methylation ameliorates osteoarthritis progression via inactivation of JAK2/STAT3 signaling pathway

OBJECTIVE

To probe into the role and regulatory mechanisms of INSR in pathogenesis of osteoarthritis (OA).

METHODS

KLF4 and INSR expression was detected in cartilage tissues of 40 OA patients and 10 controls using RT-qPCR. IL-1β-induced OA chondrocytes and anterior cruciate ligament transection (ACLT)-induced OA models were respectively constructed. After overexpressing or silencing KLF4 or INSR, flow cytometry assay was utilized to detect chondrocyte apoptosis. Furthermore, JAK2/STAT3, cartilage markers and OA-related markers were examined by western blot. Dual luciferase report and CHIP assay were carried out to verify the interactions between KLF4 and INSR, followed by functional gain and loss assay. INSR promoter methylation was assessed by MS-PCR.

RESULTS

Both KLF4 and INSR were down-regulated both in OA chondrocytes and cartilage tissues. Knockdown of KLF4 or INSR accelerated apoptosis of IL-1β-induced OA chondrocytes. However, overexpression of KLF4 or INSR ameliorated OA progression both in OA chondrocytes and OA mouse models. Moreover, INSR inactivated JAK2/STAT3 pathway in OA chondrocytes. Dual luciferase report and CHIP assay results confirmed that INSR was transcriptionally regulated by KLF4. As shown in MS-PCR results, INSR expression was mediated by DNA methylation in OA.

CONCLUSION

Our findings suggested that INSR, as a key regulator for OA, was regulated by transcription factor KLF4 and DNA methylation, thereby mediating the activation of JAK2/STAT3 signaling, which was considered as an underlying therapeutic target for OA.