Klf10 is involved in extracellular matrix calcification of chondrocytes alleviating chondrocyte senescence.

Osteoarthritis (OA) is a chronic degenerative disease resulting joint disability and pain. Accumulating evidences suggest that chondrocyte extracellular matrix calcification plays an important role in the development of OA. Here, we showed that Krüppel-like factor 10 (Klf10) was involved in the regulation of chondrocyte extracellular matrix calcification by regulating the expression of Frizzled9. Knockdown of Klf10 attenuated TBHP induced calcification and reduced calcium content in chondrocytes. Restoring extracellular matrix calcification of chondrocytes could aggravate chondrocyte senescence. Destabilization of a medial meniscus (DMM) mouse model of OA, in vivo experiments revealed that knockdown Klf10 improved the calcification of articular cartilage and ameliorated articular cartilage degeneration. These findings suggested that knockdown Klf10 inhibited extracellular matrix calcification-related changes in chondrocytes and alleviated chondrocyte senescence.

A Novel Role for RILP in Regulating Osteoclastogenesis and Bone Resorption.

Increased bone resorption caused by excessive number or activity of osteoclasts is the main cause of osteoporosis. Osteoclasts are multinucleated cells that are formed by the fusion of precursor cells. Although osteoclasts are primarily characterized by bone resorption, our understanding of the mechanisms that regulate the formation and function of osteoclasts is poor. Here we showed that the expression level of Rab interacting lysosomal protein (RILP) was strongly induced by receptor activator of NF-κB ligand in mouse bone marrow macrophages. Inhibition of RILP expression induced a drastic decrease in the number, size, F-actin ring formation of osteoclasts, and the expression level of osteoclast-related genes. Functionally, inhibition of RILP reduced the migration of preosteoclasts through PI3K-Akt signaling and suppressed bone resorption by inhibiting the secretion of lysosome cathepsin K. Treatments with siRNA-RILP attenuated pathologic bone loss in disease models induced by lipopolysaccharide. Thus, this work indicates that RILP plays an important role in the formation and bone resorption function of osteoclasts and may have a therapeutic potential to treat bone diseases caused by excessive or hyperactive osteoclasts.

Inhibition of Klf10 Attenuates Oxidative Stress-Induced Senescence of Chondrocytes via Modulating Mitophagy.

Osteoarthritis (OA) is the most prevalent degenerative joint disease in the elderly. Accumulation of evidence has suggested that chondrocyte senescence plays a significant role in OA development. Here, we show that Krüppel-like factor 10 (Klf10), also named TGFß inducible early gene-1 (TIEG1), is involved in the pathology of chondrocyte senescence. Knocking down the Klf10 in chondrocytes attenuated the tert-butyl hydroperoxide (TBHP)-induced senescence, inhibited generation of reactive oxygen species (ROS), and maintained mitochondrial homeostasis by activating mitophagy. These findings suggested that knocking down Klf10 inhibited senescence-related changes in chondrocytes and improved cartilage homeostasis, indicating that Klf10 may be a therapeutic target for protecting cartilage against OA.

Leukemia/lymphoma-related factor (LRF) or osteoclast zinc finger protein (OCZF) overexpression promotes osteoclast survival by increasing Bcl-xl mRNA: A novel regulatory mechanism mediated by the RNA binding protein SAM68.

RANKL induces NFATc1, a key transcriptional factor to induce osteoclast-specific genes such as cathepsin K, whereas transcriptional control of osteoclast survival is not fully understood. Leukemia/lymphoma-related factor (LRF) in mouse and osteoclast zinc finger protein (OCZF) in rat are zinc finger and BTB domain-containing protein (zBTB) family of transcriptional regulators, and are critical regulators of hematopoiesis. We have previously shown that differentiation and survival were enhanced in osteoclasts from OCZF-Transgenic (Tg) mice. In the present study, we show a possible mechanism of osteoclast survival regulated by LRF/OCZF and the role of OCZF overexpression in pathological bone loss. In the in vitro cultures, LRF was highly colocalized with NFATc1 in cells of early stage in osteoclastogenesis, but only LRF expression persisted after differentiation into mature osteoclasts. LRF expression was further enhanced in resorbing osteoclasts formed on dentin slices. Osteoclast survival inhibitor such as alendronate, a bisphosphonate reduced LRF expression. Micro CT evaluation revealed that femurs of OCZF-Tg mice showed significantly lower bone volume compared to that of WT mice. Furthermore, OCZF overexpression markedly promoted bone loss in ovariectomy-induced osteolytic mouse model. The expression of anti-apoptotic Bcl-xl mRNA, which is formed by alternative splicing, was enhanced in the cultures in which osteoclasts are formed from OCZF-Tg mice. In contrast, the expression of pro-apoptotic Bcl-xs mRNA was lost in the culture derived from OCZF-Tg mice. We found that the expression levels of RNA binding splicing regulator, Src substrate associated in mitosis of 68 kDa (Sam68) protein were markedly decreased in OCZF-Tg mice-derived osteoclasts. In addition, shRNA-mediated knockdown of Sam68 expression increased the expression of Bcl-xl mRNA, suggesting that SAM68 regulates the expression of Bcl-xl. These results indicate that OCZF overexpression reduces protein levels of Sam68, thereby promotes osteoclast survival, and suggest that LRF/OCZF is a promising target for regulating pathological bone loss.

Socioeconomic status influences on bone mineral density in American men: findings from NHANES 2011-2020.

The association between socioeconomic status (SES) and bone mineral density (BMD) in men remains controversial. We showed that SES was positively associated with BMD in American men. Confounding factors like race/ethnicity and age could affect the association. INTRODUCTION: Based on the data from the National Health and Nutrition Examination Survey (NHANES), 2011-2020, this article aims to investigate the association of SES (poverty income ratio (PIR) and education level) with the BMD in American men. METHODS: We evaluated the association of SES with BMD in 4446 men aged ≥ 20 years (mean age, 41.0 ± 13.4 years) from the NHANES 2011-2020. BMD was measured by dual-energy X-ray absorptiometry (DXA) at the lumbar spine. We used multivariate linear regression models to examine the relationship between SES and total spine BMD, adjusted for a large range of confounding factors. RESULTS: Compared with other PIR quarters, individuals in the highest quarter of PIR were more likely to be older and white and had fewer smoking or drinking behaviors. After adjusting for race/ethnicity, age, drinking and smoking behavior, body mass index (BMI), total protein, serum calcium, serum uric acid, cholesterol, serum phosphorus, and blood urea nitrogen, PIR was positively correlated with total spine BMD (ß = 0.004 95% CI: 0.001-0.007, P = 0.006). Individuals with the highest degree (college degree or above) had a 0.057 g/cm2 greater BMD than that of the lowest degree (less than 9th grade) (ß = 0.057 95% CI: 0.037-0.077, P < 0.001). CONCLUSIONS: Our study indicates that SES was positively associated with the lumbar BMD among American men. Clinicians, healthcare providers, and policymakers should consider the unequal SES of men when implementing osteoporosis prevention and treatment strategies.

PMEPA1 and NEDD4 control the proton production of osteoclasts by regulating vesicular trafficking.

Osteoclast bone resorption activity is critically regulated to maintain bone homeostasis. Osteoclasts resorb bone by producing protons and acid hydrolase via lysosomal secretion, however, a detailed mechanism remains elusive. PMEPA1 is a vesicular membrane protein, which binds to the NEDD4 family member of ubiquitin ligases. We have previously reported that Pmepa1 is highly expressed in bone resorbing osteoclasts, and regulates bone resorption. Here, we investigated the mechanism of bone resorption regulated by PMEPA1. Mutant mice lacking NEDD4-binding domains of PMEPA1 displayed enhanced bone volume, and reduced bone resorption activity in comparison with those of WT mice. Analysis with pH-sensitive fluorescence probe revealed that proton secretion from osteoclasts significantly decreased in Pmepa1 mutant osteoclasts. Immunofluorescence analysis revealed that PMEPA1 was colocalized with NEDD4, V0A3, and V0D2 subunits of vacuolar ATPase, which regulate the proton production of osteoclasts. In addition, Nedd4 knockdown reduced bone resorption and proton secretion of osteoclasts. Furthermore, Pmepa1 mutation and Nedd4 knockdown altered the cytoplasmic distribution of components of V-ATPase and expression of autophagy-related proteins, suggesting that lysosomal secretion is affected. Collectively, these findings indicate that PMEPA1 controls proton secretion from osteoclasts via NEDD4 by regulating vesicular trafficking, and NEDD4 is an important regulator of bone resorption.

TGF-ß1 regulates chondrocyte proliferation and extracellular matrix synthesis via circPhf21a-Vegfa axis in osteoarthritis.

BACKGROUND: The transforming growth factor-beta (TGF-ß) signaling pathway is an important pathway associated with the pathogenesis of osteoarthritis (OA). This study was to investigate the involvement of circRNAs in the TGF-ß signaling pathway. METHODS: Cell Counting Kit-8 (CCK-8) assay and 5-ethynyl-2'-deoxyuridine (EdU) assay were used to detect the proliferation of primary mouse chondrocytes (PMCs). RNA-sequencing together with bioinformatics analysis were used to systematically clarify TGF-ß1 induced alternations of circRNAs in PMCs. The regulatory and functional role of circPhf21a was examined in PMCs. Downstream targets of circPhf21a were explored by RNA-sequencing after overexpression of circPhf21a and verified by RT-qPCR in PMCs. Finally, the role and mechanism of circPhf21a in OA were explored in mouse models. RESULTS: We found that TGF-ß1 promoted the proliferation of PMCs. Meanwhile, RT-qPCR and western blotting indicated that TGF-ß1 promoted extracellular matrix (ECM) anabolism. RNA-sequencing revealed that a total of 36 circRNAs were differentially expressed between PMCs treated with and without TGF-ß1. Of these, circPhf21a was significantly decreased by TGF-ß1. Furthermore, circPhf21a knockdown promoted the proliferation and ECM synthesis of PMCs, whereas overexpression of circPhf21a showed the opposite effects. Mechanically, the expression profiles of the mRNAs revealed that Vegfa may be the target of circPhf21a. Additionally, we found that circPhf21a was significantly upregulated in the mouse OA model, and inhibition of circPhf21a significantly relieved the progression of OA. CONCLUSIONS: Our results found that TGF-ß1 promoted the proliferation and ECM synthesis of PMCs via the circPhf21a-Vegfa axis, which may provide novel therapeutic targets for OA treatment. Video abstract.

Procollagen I N-terminal peptide correlates with inflammation on sacroiliac joint magnetic resonance imaging in ankylosing spondylitis but not in non-radiographic axial spondyloarthritis: A cross-sectional study.

OBJECTIVES: To identify disease activity scores and biomarkers that reflect magnetic resonance imaging (MRI)-determined sacroiliac joint (SIJ) inflammation in ankylosing spondylitis (AS) and non-radiographic axial spondyloarthritis (nr-axSpA). METHODS: Patients who had AS and nr-axSpA were enrolled. All the patients underwent SIJ MRI. SpondyloArthritis Research Consortium of Canada (SPARCC) method was used to score bone marrow edema in the inflammatory lesions on MRI. Radiographic assessment of the spine was performed using modified Stoke Ankylosing Spondylitis Spine Score. Clinical variables, inflammatory markers, serum alkaline phosphatase, osteocalcin (OC), C-terminal telopeptide of type I collagen (CTX-I), and procollagen I N-terminal peptide (PINP) were measured. Correlation analysis between MRI-determined SIJ inflammation scores and disease activity scores and laboratory variables was performed. RESULTS: Thirty-five patients had AS and 36had nr-axSpA. Significant differences were noted between the AS group and the nr-axSpA group in terms of erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), Ankylosing Spondylitis Disease Activity Score (ASDAS)-ESR, ASDAS-CRP, PINP, and SPARCC (p < .001, p = .004, p < .001, p < .001, p = .030, p < .001, respectively). MRI-determined SIJ inflammatory scores correlated with Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Bath Ankylosing Spondylitis Functional Index (BASFI), OC, CTX-I, and PINP in AS (p = .036, p = .023, p = .002, p = .041, p = .004, respectively) and correlated with ESR, CRP, ASDAS-ESR, ASDAS-CRP, BASDAI, and BASFI in nr-axSpA (p = .003, p = .002, p < .001, p < .001, p = .010, p = .007, respectively). Multivariate analysis showed that PINP exhibited a positive correlation independent of the MRI inflammatory score and that age exhibited a negative correlation independent of the MRI inflammatory score. CONCLUSIONS: In AS, PINP and age independently correlated with active inflammation on SIJ MRI. PINP may be useful as a marker of objective inflammation in AS.

IgG immune complexes with Staphylococcus aureus protein A enhance osteoclast differentiation and bone resorption by stimulating Fc receptors and TLR2.

Staphylococcus aureus is a main pathogen of osteomyelitis and protein A is a virulence factor with high affinity for IgG. In this study, we investigated whether S. aureus affects the differentiation and bone resorption of osteoclasts through the IgG-binding capacity of protein A. Staphylococcus aureus pre-treated with serum or IgG showed marked enhancement in osteoclastogenesis and bone resorption compared to non-treated S. aureus or a protein A-deficient mutant. Blocking of the Fc receptor and deletion of the Fcγ receptor gene in osteoclast precursor cells showed that enhanced osteoclastogenesis stimulated by S. aureus IgG immune complexes (ICs) was mediated by the Fc receptor on osteoclast precursor cells. In addition, osteoclastogenesis stimulated by S. aureus ICs but not the protein A-deficient mutant was markedly reduced in osteoclast precursor cells of Myd88-knockout mice. Moreover, NFATc1, Syk and NF-κB signals were necessary for osteoclastogenesis stimulated by S. aureus ICs. The results suggest the contribution of a of Toll-like receptor 2 (TLR2)-Myd88 signal to the activity of S. aureus ICs. We further examined the expression of pro-inflammatory cytokines that is known to be enhanced by FcγR-TLR cross-talk. Osteoclasts induced by S. aureus ICs showed higher expression of TNF-α and IL-1ß, and marked stimulation of proton secretion of osteoclasts activated by pro-inflammatory cytokines. Finally, injection of S. aureus, but not the protein A-deficient mutant, exacerbated bone loss in implantation and intra-peritoneal administration mouse models. Our results provide a novel mechanistic aspect of bone loss induced by S. aureus in which ICs and both Fc receptors and TLR pathways are involved.

Prostate transmembrane protein androgen induced 1 is induced by activation of osteoclasts and regulates bone resorption.

Osteoclasts derived from hematopoietic cells are activated on bone surface. To resorb bone, osteoclasts release acid and lysosome acid hydrolase via membrane transport. Prostate transmembrane protein androgen induced 1 (Pmepa1) is a type I transmembrane protein that regulates proliferation, migration, and metastasis of cancer cells. Because recent reports showed that Pmepa1 is involved in membrane transport in cancer cells, we investigated the role of Pmepa1 in osteoclast function. Pmepa1 expression was barely detected in osteoclasts formed on plastic surfaces in vitro, but was markedly increased in activated osteoclasts formed on calcified matrix. Inhibitors of bone resorption, such as alendronate, bafilomycin A1, and the PI3K inhibitor LY294002, suppressed the expression of Pmepa1 in osteoclasts. Knockdown of Pmepa1 expression impaired bone resorption activity and inhibited formation of a ring-like, actin-rich podosome belt that is essential for osteoclast function. Pmepa1 protein localized to lysosomes in osteoclasts. In addition, in sites of bone destruction observed in rats with adjuvant-induced arthritis, a marked high level of Pmepa1 expression was associated with the osteoclasts' resorbing bone. Our results suggest that Pmepa1 is a critical regulator of bone resorption and is a promising marker for activated osteoclasts and a potential therapeutic target in pathologic bone destruction.-Xu, X., Hirata, H., Shiraki, M., Kamohara, A., Nishioka, K., Miyamoto, H., Kukita, T., Kukita, A. Prostate transmembrane protein androgen induced 1 is induced by activation of osteoclasts and regulates bone resorption.

Deficiency of stress-associated gene Nupr1 increases bone volume by attenuating differentiation of osteoclasts and enhancing differentiation of osteoblasts.

Nuclear protein 1 (NUPR1) is a multifunctional stress-induced protein involved in regulating tumorigenesis, apoptosis, and autophagy. Bone homeostasis is maintained by bone-resorbing osteoclasts and bone-forming osteoblasts and osteocytes. We aimed to determine the role of NUPR1 in bone metabolism. Using microcomputed tomography, we found that mice lacking Nupr1 exhibited increased bone volume. Histologic analysis showed that Nupr1 deficiency decreased osteoclast numbers but increased osteoblast numbers and osteoid formation. In vitro culture of bone marrow macrophages showed that receptor activator of NF-κB ligand-induced osteoclastogenesis was down-regulated in Nupr1-deficient mice. In contrast, primary osteoblasts from Nupr1-deficient mice revealed that proliferation of osteoblasts and expression of bone matrix proteins were markedly enhanced. In addition, expression of autophagy-related genes, formation of autophagosomes, and cell survival were up-regulated in Nupr1-deficient osteoblasts. In contract, deletion of Nupr1 reduced the formation of osteocyte cellular projection, which is an indicator of mature osteocytes. Importantly, we found that the expression of sclerostin (Sost), an inhibitor of bone formation, was down-regulated in the osteoblasts and osteocytes of Nupr1-deficient mice. Conversely, Nupr1 overexpression enhanced Sost expression in primary osteoblasts. Collectively, these results indicate that Nupr1 deficiency increases bone volume by attenuating production of Sost and osteoclastogenesis and enhancing differentiation of osteoblasts.-Shiraki, M., Xu, X., Iovanna, J. L., Kukita, T., Hirata, H., Kamohara, A., Kubota, Y., Miyamoto, H., Mawatari, M., Kukita, A. Deficiency of stress-associated gene Nupr1 increases bone volume by attenuating differentiation of osteoclasts and enhancing differentiation of osteoblasts.

IL-1ß Induces Pathologically Activated Osteoclasts Bearing Extremely High Levels of Resorbing Activity: A Possible Pathological Subpopulation of Osteoclasts, Accompanied by Suppressed Expression of Kindlin-3 and Talin-1.

As osteoclasts have the central roles in normal bone remodeling, it is ideal to regulate only the osteoclasts performing pathological bone destruction without affecting normal osteoclasts. Based on a hypothesis that pathological osteoclasts form under the pathological microenvironment of the bone tissues, we here set up optimum culture conditions to examine the entity of pathologically activated osteoclasts (PAOCs). Through searching various inflammatory cytokines and their combinations, we found the highest resorbing activity of osteoclasts when osteoclasts were formed in the presence of M-CSF, receptor activator of NF-κB ligand, and IL-1ß. We have postulated that these osteoclasts are PAOCs. Analysis using confocal laser microscopy revealed that PAOCs showed extremely high proton secretion detected by the acid-sensitive fluorescence probe Rh-PM and bone resorption activity compared with normal osteoclasts. PAOCs showed unique morphology bearing high thickness and high motility with motile cellular processes in comparison with normal osteoclasts. We further examined the expression of Kindlin-3 and Talin-1, essential molecules for activating integrin ß-chains. Although normal osteoclasts express high levels of Kindlin-3 and Talin-1, expression of these molecules was markedly suppressed in PAOCs, suggesting the abnormality in the adhesion property. When whole membrane surface of mature osteoclasts was biotinylated and analyzed, the IL-1ß-induced cell surface protein was detected. PAOCs could form a subpopulation of osteoclasts possibly different from normal osteoclasts. PAOC-specific molecules could be an ideal target for regulating pathological bone destruction.

Pmepa1 induced by RANKL-p38 MAPK pathway has a novel role in osteoclastogenesis.

Osteoclasts are multinucleated cells formed by fusion of preosteoclasts (POCs) derived from cells of the monocyte/macrophage lineage. We have reported a culture system that supports the formation of POCs from stroma-depleted rat bone marrow cells. Global gene expression analysis of this culture system identified genes highly expressed in POCs. Here, we have analyzed the expression and function of one of these highly expressed genes, prostate transmembrane protein androgen induced 1 (Pmepa1), a target of TGF-ß and binds Nedd4 ubiquitin ligase, which plays a role in intracellular trafficking. We show here that the expression of Pmepa1 was strongly induced by RANKL in mouse bone marrow macrophage and in the osteoclast precursor cell line RAW-D. The expression of Pmepa1 was increased at 24 hr of culture, but was decreased at 72 hr. Pmepa1 protein was localized to intracellular vesicle membrnane of mononuclear cells, some of which were cathepsin-K positive. RANKL-induced expression of Pmepa1 was significantly reduced by inhibitors of p38 MAPK signaling. Pmepa1 siRNA suppressed the formation of osteoclasts in RAW-D cells, and inhibited the expression of cathepsin K and c-fos but not RANK. In addition, inhibition of Pmepa1 expression reduced the surface expression of RANK in RAW-D cells induced by RANKL. These results demonstrate that Pmepa1 is induced by RANK-p38 MAPK pathway signaling, and upregulates cell surface expression of RANK, suggesting that Pmepa1 plays a role in osteoclastogenesis and osteoclast signaling.

Overall survival comparison between pediatric and adult Ewing sarcoma of bone and adult nomogram construction: a large population-based analysis.

Background: Ewing sarcoma (ES) is a common primary bone tumor in children. Our study aimed to compare overall survival (OS) between pediatric and adult bone ES patients, identify independent prognostic factors and develop a nomogram for predicting OS in adult patients with ES of bone. Methods: We retrospectively analyzed data for the 2004-2015 period from the Surveillance, Epidemiology, and End Results (SEER) database. To guarantee well-balanced characteristics between the comparison groups, propensity score matching (PSM) was used. Kaplan-Meier (KM) curves were used to compare OS between pediatric and adult patients with ES of bone. Univariate and multivariate Cox regression analyses were used to screen independent prognostic factors for ES of bone, and a prognostic nomogram was constructed by using the factors identified. The prediction accuracy and clinical benefit were evaluated using receiver operating characteristic (ROC) curves, areas under the curves (AUCs), calibration curves, and decision curve analysis (DCA). Results: Our results showed that adult ES patients had lower OS than younger ES patients. Age, surgery, chemotherapy, and TNM stage were independent risk factors for bone ES in adults and were used to develop a nomogram. AUCs for 3-, 5-, and 10-year OS were 76.4 (67.5, 85.3), 77.3 (68.6, 85.9) and 76.6 (68.6, 84.5), respectively. Calibration curves and DCA results indicated excellent performance for our nomogram. Conclusion: We found that ES pediatric patients have better OS than adult ES patients, and we constructed a practical nomogram to predict the 3-, 5- and 10-year OS of adult patients with ES of bone based on independent prognostic factors (age, surgery, chemotherapy, T stage, N stage and M stage).

Inhibition of Cpt1a alleviates oxidative stress-induced chondrocyte senescence via regulating mitochondrial dysfunction and activating mitophagy.

Osteoarthritis (OA) is an age-related chronic degenerative disease, and chondrocyte senescence has been established to play an important role in the pathological process. There is ample evidence to suggest that lipid metabolism plays an important role in the aging process. However, the effect of lipid metabolism on chondrocyte senescence and OA remains unclear. Accordingly, we constructed a TBHP-induced senescent chondrocytes model and a destabilization of the medial meniscus (DMM) mouse model. We found that lipid accumulation and fatty acid oxidation were enhanced in senescent chondrocytes. Interestingly, carnitine palmitoyltransferase 1A (Cpt1a), the rate-limiting enzyme for fatty acid oxidation, was highly expressed in senescent chondrocytes and murine knee cartilage tissue. Suppressing Cpt1a expression using siRNA or Etomoxir, an inhibitor of Cpt1a, could attenuate oxidative stress-induced premature senescence and OA phenotype of primary murine chondrocytes, decrease cellular ROS levels, restore mitochondrial function, and maintain mitochondrial homeostasis via activating mitophagy. In vivo, pharmacological inhibition of Cpt1a by Etomoxir attenuated cartilage destruction, relieved joint space narrowing and osteophyte formation in the DMM mouse model. Overall, these findings suggested that knockdown of Cpt1a alleviated chondrocyte senescence by regulating mitochondrial dysfunction and promoting mitophagy, providing a new therapeutic strategy and target for OA treatment.

CircHIPK3 prevents chondrocyte apoptosis and cartilage degradation by sponging miR-30a-3p and promoting PON2.

Osteoarthritis (OA) is a common joint disease featured by the deterioration of articular cartilage and chondrocyte death. Emerging evidence has indicated that circular RNAs (circRNAs) play an essential role in OA progress. Here, we found that the expression of circHIPK3 was significantly decreased in human and mouse OA cartilage. Knocking down circHIPK3 increased apoptosis and intracellular ROS level in HC-a chondrocytes. We performed proteomic studies and identified that circHIPK3 regulated chondrocyte apoptosis through the mitochondrial pathway. Results of JC-1 staining and western blot further confirmed that mitochondrial outer membrane permeabilization was promoted in HC-a chondrocytes transfected by circHIPK3 siRNA. In terms of mechanism, we showed that PON2 functioned as a potential target of circHIPK3 to regulate chondrocyte apoptosis. Moreover, we revealed that circHIPK3 interacted with miR-30a-3p to regulate PON2 expression in chondrocytes. Taken together, our findings suggested that circHIPK3 regulated chondrocyte apoptosis by mitochondrial pathway, and targeting the circHIPK3/miR-30a-3p/PON2 axis might be a potential strategy for OA treatment.