Tumor necrosis factor-alpha induced DNA cleavage in human articular chondrocytes may involve multiple endonucleolytic activities during apoptosis

Targeting Soluble Epoxide Hydrolase and Cyclooxygenases Enhance Joint Pain Control, Stimulate Collagen Synthesis, and Protect Chondrocytes From Cytokine-Induced Apoptosis

Objective: To determine the symptomatic and disease-modifying capabilities of sEH and COX inhibitors during joint inflammation. Methods: Using a blinded, randomized, crossover experimental design, 6 adult healthy horses were injected with lipopolysaccharide (LPS; 3 μg) from E. coli in a radiocarpal joint and concurrently received the non-selective cyclooxygenase (COX) inhibitor phenylbutazone (2 mg/kg), the sEH inhibitor t-TUCB (1 mg/kg) or both (2 mg/kg phenylbutazone and 0.1, 0.3, and 1 mg/kg t-TUCB) intravenously. There were at least 30 days washout between treatments. Joint pain (assessed via inertial sensors and peak vertical forces), synovial fluid concentrations of prostanoids (PGE₂, TxB₂), cytokines (IL-1β, IL-6, TNF-α) and biomarkers of collagen synthesis (CPII) and degradation (C2C) were measured at pre-determined intervals over a 48-h period. The anti-apoptotic effect of COX and sEH inhibitors was determined via ELISA technique in primary equine chondrocytes incubated with TNF-α (10 ng/ml) for 24 h. Apoptosis was also determined in chondrocytes incubated with sEH-generated metabolites. Results: Combined COX and sEH inhibition produced significantly better control of joint pain, prostanoid responses, and collagen synthesis-degradation balance compared to each compound separately. When administered separately, pain control was superior with COX vs. sEH inhibition. Cytokine responses were not different during COX and/or sEH inhibition. In cultured chondrocytes, sEH inhibition alone or combined with COX inhibition, but not COX inhibition alone had significant anti-apoptotic effects. However, sEH-generated metabolites caused concentration-dependent apoptosis. Conclusions: Combined COX and sEH inhibition optimize pain control, attenuate loss of articular cartilage matrix during joint inflammation and cytokine-induced chondrocyte apoptosis.

Expression and activity of AIM2-inflammasome in rheumatoid arthritis patients

INTRODUCTION

AIM2 inflammasome activation leads to the release of IL-β, which plays an important role in rheumatoid arthritis pathogenesis. In this work, we evaluated AIM2 expression and activity in RA patients and healthy controls.

METHODS

AIM2 and RANKL expression were evaluated by flow cytometry. Inflammasome activity was determined in monocyte cultures stimulated with synthetic DNA by measuring IL-1β levels in supernatants using an ELISA assay. The caspase-1 expression in monocytes was measured by western blot, the POP3 expression was analysed by qPCR, and serum levels of IFN-γ were evaluated using ELISA assay.

RESULTS

We observed a diminution of CD14+AIM2+ cells in RA patients, associated with disease activity and evolution. Likewise, the levels of IL-1β were increased in monocyte cultures un-stimulated and stimulated with LPS from RA patients with DAS28 ≥ 4. The Caspase-1 activity and RANKL + monocytes in RA patients were slightly increased. Finally, augmented POP3 expression and diminished IFN-γ serum levels were detected in RA patients.

CONCLUSION

Our results showed that the monocytes from RA patients were prone to release IL-1β in the absence of the AIM2 inflammasome signal. The down-regulation of AIM2 to a systemic level in RA patients might be a consequence of augmented POP3 expression and might imply the survival of pro-inflammatory cells contributing to the inflammation process.

Antagonism of cysteinyl leukotriene receptor 1 (cysLTR1) by montelukast suppresses cell senescence of chondrocytes

Aging is closely associated with osteoarthritis (OA). Although its underlying mechanisms remain unknown, cellular senescence in chondrocytes has become an important therapeutic target for the treatment of OA. Cysteinyl leukotriene receptors (cysLTRs) mediate the pathobiological function of cysteinyl leukotrienes (cysLTs). However, the roles of cysLTRs in the pathogenesis of OA have not been reported before. In the current study, we found that cysLTR1 but not cysLTR2 is expressed in human primary chondrocytes. In addition, stimulation with tumor necrosis factor α (TNF-α) resulted in a significant increase in the expression of cysLTR1. Interestingly, montelukast, a specific cysLTR1 antagonist, attenuated TNF-α-induced up-regulation of the activity of senescence-associated β-galactosidase (SA-β-Gal). In addition, TNF-α led to cell cycle arrest at the G0/G1 phase, which was prevented by treatment with montelukast. Notably, montelukast reduced expression of the senescence markers p53, p21 and PAI-1. In addition, montelukast ameliorated TNF-α-induced K382 acetylation of p53 by promoting the expression of SIRT1. Silencing of SIRT1 using SIRT1 siRNA broke the inhibitory effects of montelukast on K382 acetylation of p53. Importantly, silencing of cysLTR1 reversed the reduction of SIRT1 expression as well as the K382 acetylation of p53. Our findings strongly implicate that cysLTR1 has the capacity to regulate cellular senescence in chondrocytes. It is suggested that montelukast may be a potential therapeutic agent for chondro-protective therapy.

Insights on Molecular Mechanisms of Chondrocytes Death in Osteoarthritis

Osteoarthritis (OA) is a joint pathology characterized by progressive cartilage degradation. Medical care is mainly based on alleviating pain symptoms. Compelling studies report the presence of empty lacunae and hypocellularity in cartilage with aging and OA progression, suggesting that chondrocyte cell death occurs and participates to OA development. However, the relative contribution of apoptosis per se in OA pathogenesis appears complex to evaluate. Indeed, depending on technical approaches, OA stages, cartilage layers, animal models, as well as in vivo or in vitro experiments, the percentage of apoptosis and cell death types can vary. Apoptosis, chondroptosis, necrosis, and autophagic cell death are described in this review. The question of cell death causality in OA progression is also addressed, as well as the molecular pathways leading to cell death in response to the following inducers: Fas, Interleukin-1β (IL-1β), Tumor Necrosis factor-α (TNF-α), leptin, nitric oxide (NO) donors, and mechanical stresses. Furthermore, the protective role of autophagy in chondrocytes is highlighted, as well as its decline during OA progression, enhancing chondrocyte cell death; the transition being mainly controlled by HIF-1α/HIF-2α imbalance. Finally, we have considered whether interfering in chondrocyte apoptosis or promoting autophagy could constitute therapeutic strategies to impede OA progression.

Lentiviral-mediated multiple gene transfer to chondrocytes promotes chondrocyte differentiation and bone formation in rabbit bone marrow-derived mesenchymal stem cells

The aim of the present study was to provide a theoretical and experimental foundation on the differentiation of stem cells through the induction of multiple genes. The lentiviral vector carrying TGF-β1 and IL-10 genes was transfected to bone marrow-derived mesenchymal stem cells (BMSCs) which differentiated into chondrogenesis. Healthy New Zealand white rabbits, 2-3 months of age were used in the present study. A 6-8 ml of bone marrow was isolated from the iliac and tibial shaft of each rabbit. The BMSCs suspension was aspired following centrifugation of the bone marrow by percoll separating medium. The BMSCs were primarily cultured and subcultured in vitro, then divided into four groups according to the difference of lentivirus vectors: group A, receiving transforming growth factor β1 (TGF‑β1); group B, receiving TGF-β1 and Interleukin-10 (IL-10); group C, empty vector transfection; and group D, receiving no cell growth factor. Fluorescence expression was detected 12 h after transfecting the lentiviral vector carrying the TGF-β1 and IL-10 gene to BMSCs. The transfection efficiency was approximately 70% with a MOI=100 after 96 h. Expression of SOX-9 aggrecan and Type Ⅱ collagen in groups A-E on day 7 and 14 was detected by RT-PCR and western blot analysis. The expression level of three genes expressed in groups A and C were higher compared to the expression in groups B, D and E. The expression level of the three genes expressed in group B was higher compared to the expression in group D. The expression level of three genes expressed in group A and C showed no statistical difference. Cytokines therefore play an important role in cell proliferation and chondrogenic differentiation. TGF-β1 has a synergistic effect in the differentiation. In addition, IL-10 may have a protective role in the restoration of cartilaginous tissue.

Biomarkers of Chondrocyte Apoptosis and Autophagy in Osteoarthritis

Cell death with morphological and molecular features of apoptosis has been detected in osteoarthritic (OA) cartilage, which suggests a key role for chondrocyte death/survival in the pathogenesis of OA. Identification of biomarkers of chondrocyte apoptosis may facilitate the development of novel therapies that may eliminate the cause or, at least, slow down the degenerative processes in OA. The aim of this review was to explore the molecular markers and signals that induce chondrocyte apoptosis in OA. A literature search was conducted in PubMed, Scopus, Web of Science and Google Scholar using the keywords chondrocyte death, apoptosis, osteoarthritis, autophagy and biomarker. Several molecules considered to be markers of chondrocyte apoptosis will be discussed in this brief review. Molecular markers and signalling pathways associated with chondroycte apoptosis may turn out to be therapeutic targets in OA and approaches aimed at neutralizing apoptosis-inducing molecules may at least delay the progression of cartilage degeneration in OA.

The ECM-cell interaction of cartilage extracellular matrix on chondrocytes

Cartilage extracellular matrix (ECM) is composed primarily of the network type II collagen (COLII) and an interlocking mesh of fibrous proteins and proteoglycans (PGs), hyaluronic acid (HA), and chondroitin sulfate (CS). Articular cartilage ECM plays a crucial role in regulating chondrocyte metabolism and functions, such as organized cytoskeleton through integrin-mediated signaling via cell-matrix interaction. Cell signaling through integrins regulates several chondrocyte functions, including differentiation, metabolism, matrix remodeling, responses to mechanical stimulation, and cell survival. The major signaling pathways that regulate chondrogenesis have been identified as wnt signal, nitric oxide (NO) signal, protein kinase C (PKC), and retinoic acid (RA) signal. Integrins are a large family of molecules that are central regulators in multicellular biology. They orchestrate cell-cell and cell-matrix adhesive interactions from embryonic development to mature tissue function. In this review, we emphasize the signaling molecule effect and the biomechanics effect of cartilage ECM on chondrogenesis.

Single impact cartilage trauma and TNF-α: interactive effects do not increase early cell death and indicate the need for bi-/multidirectional therapeutic approaches

Blunt trauma of articular cartilage, often resulting from accidents or sports injuries, is associated with local inflammatory reactions and represents a major risk factor for development of post-traumatic osteoarthritis. TNF-α is increased in synovial fluid early after trauma, potentiates injury-induced proteoglycan degradation and may act proapoptotic under permissive conditions. We asked whether TNF-α also influences chondrocyte death, gene expression of catabolic and anabolic markers and the release of proinflammatory mediators in the early post-traumatic phase. Interactive effects of a defined single impact trauma (0.59 J) and TNF-α (100 ng/ml) on human early-stage osteoarthritic cartilage were investigated in vitro over 24 h. Exposure of traumatized cartilage to TNF-α did not increase chondrocyte death. IL-6-synthesis was augmented by trauma, TNF-α and combined treatment. The impact increased the release of PGE2 and PGD2 in the presence and absence of TNF-α to a similar extent while TNF-α alone showed no effect. In contrast, NOS2A-expression and nitric oxide (NO)-release were not affected by trauma but significantly increased by TNF-α. Expression of OPG and RANKL was not affected by TNF-α but modulated by trauma. TNF-α with and without trauma significantly induced MMP1 gene expression. These results indicate that TNF-α does not potentiate early cell death in early-stage osteoarthritic cartilage after blunt injury. However, trauma and TNF-α showed independent and interactive effects concerning prostaglandin and NO release. TNF-α probably contributes to cartilage degradation after trauma by an early induction of MMP1 gene expression. Our study confirms that an anti-TNF-α therapy may have inhibitory effects on catabolic and, partly, on inflammatory processes after a single impact trauma. As TNF-α does not contribute to the loss of chondrocytes in the initial post-traumatic phase, a combination with pharmaco-therapeutic strategies reducing early cell death could be reasonable.

Biochemical and proteomic characterization of alkaptonuric chondrocytes

Alkaptonuria (AKU) is a rare genetic disease associated with the accumulation of homogentisic acid (HGA) and its oxidized/polymerized products which leads to the deposition of melanin-like pigments (ochronosis) in connective tissues. Although numerous case reports have described ochronosis in joints, little is known on the molecular mechanisms leading to such a phenomenon. For this reason, we characterized biochemically chondrocytes isolated from the ochronotic cartilage of AKU patients. Based on the macroscopic appearance of the ochronotic cartilage, two sub-populations were identified: cells coming from the black portion of the cartilage were referred to as “black” AKU chondrocytes, while those coming from the white portion were referred to as “white” AKU chondrocytes. Notably, both AKU chondrocytic types were characterized by increased apoptosis, NO release, and levels of pro-inflammatory cytokines. Transmission electron microscopy also revealed that intracellular ochronotic pigment deposition was common to both “white” and “black” AKU cells. We then undertook a proteomic and redox-proteomic analysis of AKU chondrocytes which revealed profound alterations in the levels of proteins involved in cell defence, protein folding, and cell organization. An increased post-translational oxidation of proteins, which also involved high molecular weight protein aggregates, was found to be particularly relevant in “black” AKU chondrocytes. J. Cell. Physiol. 227: 3333–3343, 2012. © 2011 Wiley Periodicals, Inc.

Chondrocyte apoptosis determined by caspase-3 expression varies with fibronectin distribution in equine articular cartilage

AIM

The purpose of this study was to investigate associations between the extent of chondrocyte apoptosis and expression of the articular cartilage (AC) extracellular matrix (ECM) molecules, cartilage oligomeric matrix protein (COMP) and fibronectin.

METHOD

Cartilage from four sites (when available) on equine left middle carpal joints (n = 12) were used. Expression of COMP and fibronectin was determined using specific polyclonal antibodies and a biotin-streptavidin/peroxidase method. The intensity of staining for matrix molecules was graded (none, mild, moderate, strong) in each cartilage zone. Apoptosis of chondrocytes in AC sections was assessed by their expression of active caspase-3 using immunohistochemistry.

RESULTS

The intensity of fibronectin expression varied significantly according to cartilage depth, with greater expression in the deep zone than in either the superficial or middle layers (P < 0.001). A significant positive association was found overall between intensity of fibronectin expression and chondrocyte apoptosis (r = 0.44, P = 0.0187). The data were also significant for superficial and deep zones (r = 0.44, P = 0.0239 and r = 0.42, P = 0.0279 respectively). Conversely, intensity of COMP expression did not show zonal differences and was un-associated with degree of apoptosis. However, COMP expression was significantly more intense in cartilage than fibronectin (P = 0.0007), and the correlation between overall intensity of COMP and fibronectin was statistically significant (r = 0.56, P = 0.0018).

CONCLUSION

The positive correlation between the incidence of apoptosis and expression of fibronectin, a key ECM molecule involved in communication between the chondrocyte and surrounding matrix, suggests that chondrocyte death by apoptosis may alter cartilage metabolism, supporting the role of this process in the pathogenesis of osteoarthritis.

A 3D cartilage - inflammatory cell culture system for the modeling of human osteoarthritis

Inflammation plays a major role in the destruction of cartilage in osteoarthritis (OA), with the interaction of multiple mediators, immune cells, fibroblasts and chondrocytes. Current 2D studies in vitro with cell lines, as well as animal models, are limited in terms of providing insight into pathogenic mechanisms related to the human system. Hence, an in vitro human 3D cartilage tissue system was established to study the impact of inflammatory mediators on chondrocytes and matrices as an initial approach to emulating early stages of OA. An in vitro 3D human cartilage tissue system was established by culturing primary chondrocytes in silk protein porous scaffolds up to 21 days in static culture, with and without cytokine (IL-1β and TNF-α) exposure or with the use of macrophage conditioned medium (MCM). To assess chondrocyte responses, transcript levels, histology and immunohistochemistry were used to assess changes in cell viability and in cartilage matrix composition, including collagen type II and aggrecan. Chondrocyte hypertrophy and apoptosis were assessed via collagen type X and caspase-3. RT-PCR revealed that the cytokines and the MCM regulated matrix-related gene expression of chondrocytes, but with different outcomes. For anabolic-encoding genes, MCM suppressed collagen type II and upregulated aggrecan. In contrast, the cytokines suppressed aggrecan formation and had no effect on collagen type II. For catabolic-encoded genes, both cytokines and MCM upregulated MMP1, MMP3, MMP13 and ADAMTS4, with cytokines preferentially upregulating MMP13 and MCM upregulating ADMTS4. MCM down-regulated ADAMTS5. In addition, MCM stimulation led to hypertrophy and apoptosis of chondrocytes, outcomes not found with the cytokine treatment group. A decrease in aggrecan content with cytokines and MCM stimulation was found, while MCM resulted in greater reduction than the cytokine treatment. The results demonstrated that OA-like features, such as changes in matrix synthesis gene expression, increase of collagense gene expression and loss of aggrecan, were initiated within this 3D chrondrocyte human tissue system upon stimulation of the cultures with cytokines and MCM. MCM was a better inducer of immune-related features of OA, because besides the features found with cytokine stimulation, the MCM treatment also initiated collagen X expression and deposition and apoptosis of chondrocytes, important features of human OA. The results obtained with this new in vitro tissue model provide an initial step towards the development of an early stage OA system to allow for more systematic study and insight into the origins and outcomes with this disease.

The role of pro-inflammatory and immunoregulatory cytokines in tendon healing and rupture: new insights

Owing to limited self-healing capacity, tendon ruptures and healing remain major orthopedic challenges. Increasing evidence suggests that post-traumatic inflammatory responses, and hence, cytokines are involved in both cases, and also in tendon exercise and homeostasis. This review summarizes interrelations known between the cytokines interleukin (IL)-1β, tumor necrosis factor (TNF)α, IL-6 and vascular endothelial growth factor (VEGF) in tendon to assess their role in tendon damage and healing. Exogenic cytokine sources are blood-derived leukocytes that immigrate in damaged tendon. Endogenous expression of IL-1β, TNFα, IL-6, IL-10 and VEGF was demonstrated in tendon-derived cells. As tendon is a highly mechanosensitive tissue, cytokine homeostasis and cell survival underlie an intimate balance between adequate biomechanical stimuli and disturbance through load deprivation and overload. Multiple interrelations between cytokines and tendon extracellular matrix (ECM) synthesis, catabolic mediators e.g. matrix-degrading enzymes, inflammatory and angiogenic factors (COX-2, PGE2, VEGF, NO) and cytoskeleton assembly are evident. Pro-inflammatory cytokines affect ECM homeostasis, accelerate remodeling, amplify biomechanical adaptiveness and promote tenocyte apoptosis. This multifaceted interplay might both contribute to and interfere with healing. Much work must be undertaken to understand the particular interrelation of these inflammatory and regulatory mediators in ruptured tendon and healing, which has relevance for the development of novel immunoregulatory therapeutic strategies.

Variations in chondrocyte apoptosis may explain the increased prevalence of osteoarthritis in some joints

OBJECTIVES

To investigate whether there are any variations in chondrocyte susceptibility to an apoptotic stimulus between cells of articular cartilage (AC) from equine joints that differ in prevalence of osteoarthritis (OA).

METHODS

Cartilage from macroscopically normal equine metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints was used. Prior to culture, chondrocyte viability was assessed using the fluorescein diacetate (FDA) and propidium iodide paravital staining method. AC explants were subsequently treated with tumour necrosis factor-α (TNF-α) in combination with Actinomycin D to induce apoptosis. Apoptosis of chondrocytes in cartilage sections was assessed by expression of active caspase-3 using indirect immunohistochemistry and sections also histologically graded using a 'modified' Mankin scoring system.

RESULTS

Prior to culture (mean ± standard deviation) chondrocyte viability was 80.7% (3.5). The extent of chondrocyte apoptosis induced by TNF-α/Actinomycin D varied markedly according to the joint type that the cartilage was sampled from. For MCP joints, the extent of overall chondrocyte apoptosis was significantly higher (P < 0.001) in stimulated explants (26.7%, 10.3) than that observed in unstimulated control samples (9.6%, 7.5). Conversely, chondrocytes from PIP and DIP joint cartilage did not respond significantly to apoptotic stimulation (P > 0.05). Significant variations in cellularity and thickness were also evident between cartilages of different joint types.

CONCLUSIONS

Data in this study demonstrate that chondrocytes from three equine joint types with varying prevalences of OA differ significantly in terms of susceptibility to apoptosis induction. This may provide a possible explanation for the joint-specific nature of the disease.

Joint aging and chondrocyte cell death

Articular cartilage extracellular matrix and cell function change with age and are considered to be the most important factors in the development and progression of osteoarthritis. The multifaceted nature of joint disease indicates that the contribution of cell death can be an important factor at early and late stages of osteoarthritis. Therefore, the pharmacologic inhibition of cell death is likely to be clinically valuable at any stage of the disease. In this article, we will discuss the close association between diverse changes in cartilage aging, how altered conditions influence chondrocyte death, and the implications of preventing cell loss to retard osteoarthritis progression and preserve tissue homeostasis.

IL-10 overexpression differentially affects cartilage matrix gene expression in response to TNF-alpha in human articular chondrocytes in vitro

Cartilage-specific extracellular matrix synthesis is the prerequisite for chondrocyte survival and cartilage function, but is affected by the pro-inflammatory cytokine TNF-alpha in arthritis. The aim of the present study was to characterize whether the immunoregulatory cytokine IL-10 might modulate cartilage matrix and cytokine expression in response to TNF-alpha. Primary human articular chondrocytes were treated with either recombinant IL-10, TNF-alpha or a combination of both (at 10ng/mL each) or transduced with an adenoviral vector overexpressing human IL-10 and subsequently stimulated with 10ng/ml TNF-alpha for 6 or 24h. The effects of IL-10 on the cartilage-specific matrix proteins collagen type II, aggrecan, matrix-metalloproteinases (MMP)-3, -13 and pro-inflammatory cytokines were evaluated by real-time RT-PCR and immunohistochemistry. Transduced chondrocytes overexpressed high levels of IL-10 which significantly up-regulated collagen type II expression. TNF-alpha suppressed collagen type II and aggrecan, but increased MMP and cytokine expression in chondrocytes compared to the non-stimulated controls. The TNF-alpha mediated down-regulation of aggrecan expression was significantly antagonized by IL-10 overexpression, whereas the suppression of collagen type II was barely affected. The MMP-13 and IL-1beta expression by TNF-alpha was slightly reduced by IL-10. These results suggest that IL-10 overexpression modulates some catabolic features of TNF-alpha in chondrocytes.

Interleukin-10 modulates pro-apoptotic effects of TNF-alpha in human articular chondrocytes in vitro

The aim of this study is to determine if there is an antagonistic effect between tumour necrosis factor (TNF)-alpha and the immunoregulatory interleukin (IL)-10 on chondrocytes survival. Serum-starved primary human articular chondrocytes were stimulated with either 10 ng/ml recombinant TNF-alpha, IL-10 or a combination of both (at 10 ng/ml each). Chondrocyte apoptosis was determined by measuring caspase-3/7, -8 and -9 activities using caspase assays. Mitochondrial apoptotic inducer bax, and the suppressor bcl-2 were evaluated using western blotting at 48 h. Results indicated that TNF-alpha increased caspase activities and resulted in a significant (p = 0.001) increase in bax/bcl-2 ratio. Stimulation with IL-10 did not alter caspase activities, while co-treatment with IL-10 and TNF-alpha inhibited TNF-alpha induced caspase activities and significantly (p > 0.004) impaired bax/bcl-2 ratio. At 24 h, mRNA levels for collagen type II, TNF-alpha and IL-10 were determined using real-time RT-PCR. Stimulation with TNF-alpha or TNF-alpha and IL-10 significantly inhibited collagen type II and increased IL-10 and TNF-alpha mRNA expression. IL-10 modulated the pro-apoptotic capacity of TNF-alpha in chondrocytes as shown by the decrease in caspase activities and bax/bcl-2 ratio compared to TNF-alpha stimulated chondrocytes, suggesting a mostly antagonistic interplay of IL-10 and TNF-alpha on mitochondrial apoptotic pathways.

Cytokines, tumor necrosis factor-alpha and interleukin-1beta, differentially regulate apoptosis in osteoarthritis cultured human chondrocytes

OBJECTIVE

This study addresses the effects of tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) on cell death in human chondrocytes.

METHODS

Osteoarthritis (OA) human chondrocytes stimulated with Actinomycin-D (ActD) were used as a cellular apoptotic model. Caspase family mRNA expression and protein synthesis were analyzed by the ribonuclease protection assay and Western-blot, respectively. Cell viability and apoptosis were evaluated using the 3-[4,5-dimethylthiazol-2yl] 2,5-diphenyl tetrazolium bromide (MTT) assay and flow cytometry, respectively. Prostaglandin E2 (PGE2) and nitric oxide (NO) were evaluated by enzyme-linked immunosorbent assay (ELISA) and the Griess method, respectively.

RESULTS

TNF-alpha and IL-1beta differentially affected the pattern of caspase mRNA expression by human chondrocytes. TNF-alpha induced a gradual increase in caspase-1 and -8 mRNA levels that was not seen with IL-1beta. The time sequence of caspase-3 and -7 inductions by TNF-alpha differs from that induced by IL-1beta. Cell viability was not modified by TNF-alpha or IL-1beta in cultured chondrocytes. Then, we employed ActD as a model to facilitate cell death. Treatment with TNF-alpha and ActD (TNF-alpha/ActD) increased cell death induced by ActD (23%). Treatment with IL-1beta and ActD (IL-1beta/ActD) did not modulate ActD-induced cell death. Similarly, IL-1beta/ActD did not induce an increase in the activation of caspase-3 and -7 and poly (ADP-ribose) polymerase (PARP) cleavage observed by the incubation with TNF-alpha/ActD. These different effects were not due to bcl-2 or mcl-1 levels. Inhibition of PGE2 synthesis by indomethacin increased the cell death induced by IL-1beta/Act-D (59%). An inhibitor of caspase-8 significantly reduced only the TNF-alpha/ActD-induced cell death (58%).

CONCLUSION

TNF-alpha and IL-1beta differentially regulate the apoptotic pathway in human chondrocytes. This difference is dependent on PGE2 and caspase-8 levels.

Upregulated expression of inducible nitric oxide synthase plays a key role in early apoptosis after anterior cruciate ligament injury

The reason that the anterior cruciate ligament (ACL) has a very poor healing potential after injury is not well understood. In this study, we investigated the role of nitric oxide (NO) in the apoptotic cell death of ACL cells using a rabbit model and in vitro cell culture. The apoptosis of ACL cells in vivo was analyzed by TUNEL assay and electron microscopy. NO synthase (NOS) expression was observed by immunohistochemical analysis. ACL cells were cultured and the susceptibility to NO-induced apoptosis was tested. Inducible NOS (iNOS) expression after treatment with cytokines was examined by immunohistochemical and RT-PCR analyses. Mitogen-activated protein kinase (MAPK) inhibitors were used for the analysis of downstream signals. A significant number of apoptotic cells were observed on days 1 to 3 after injury; the apoptotic rate returned to the control level by day 7. Upregulation of iNOS in the ACL remnant was observed at day 1. Intraarticular injection of NOS inhibitor suppressed the apoptotic rate. Isolated ACL cells showed much higher susceptibility to NO-induced apoptosis than did medial collateral ligament cells. IL-1beta stimulated ACL cells to upregulate iNOS mRNA and increase NO production. p38 MAPK inhibitor decreased NO-induced apoptosis. Rapid iNOS induction after injury contributes to the high apoptotic rate of ACL cells, and this may partly account for the poor healing capacity of this ligament. iNOS and NO production is suggested to be stimulated by IL-1beta, and NO activates the p38 MAPK pathway and triggers an apoptotic signal in ACL cells.

Downregulation of inhibitor of apoptosis proteins in apoptotic human chondrocytes treated with tumor necrosis factor-alpha and actinomycin D

OBJECTIVE

Apoptosis of chondrocytes plays a pivotal role in cartilage degeneration. Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine and has been assumed to cause the degradation of human cartilage. To investigate the mechanisms of TNF-alpha-mediated apoptosis of human chondrocytes from a point of view of the balance between the caspase-cascade and the expression of inhibitor of apoptosis proteins (IAPs), although both of them are induced with TNF-signals.

METHODS

The expression of TNF-receptors (TNF-Rs) in normal human articular chondrocyte (NHAC-kn) was examined with immunocytochemistry. Subconfluent cultures of NHAC-kn were tested with TNF-alpha and/or actinomycin D (actD), and the induction of apoptosis was evaluated by the frequency of apoptotic cells visualized with nuclear staining using Hoechst 33342. The activation of caspases and the expression of IAPs were examined with Western blot analyses.

RESULTS

NHAC-kn expressed TNF-R1 and -R2. When NHAC-kn was treated with TNF-alpha (10 ng/ml) and actD (0.2 microg/ml) for 24 h, the frequency of apoptotic cells increased to more than 25%. TNF-alpha alone, however, induced the apoptosis insufficiently (up to 8.3%), even when used at the concentration of 100 ng/ml for 48 h. In apoptotic human chondrocytes induced with TNF-alpha (10 ng/ml) and actD (0.2 microg/ml), the caspase-3, -8, and -9 were activated and the protein expression of XIAP and c-IAP1 decreased.

CONCLUSIONS

In apoptotic human chondrocytes induced with TNF-alpha and actD, the balance between caspase activation and IAPs' expression lay with the executioner caspase (caspase-3) and led to decreased expression of XIAP and c-IAP1.

Polyamine depletion inhibits apoptosis following blocking of survival pathways in human chondrocytes stimulated by tumor necrosis factor-alpha

Chondrocyte apoptosis can be an important contributor to cartilage degeneration, thereby making it a potential therapeutic target in articular diseases. To search for new approaches to limit chondrocytic cell death, we investigated the requirement of polyamines for apoptosis favored by tumor necrosis factor-alpha (TNF), using specific polyamine biosynthesis inhibitors in human chondrocytes. The combined treatment of C-28/I2 chondrocytes with TNF and cycloheximide (CHX) resulted in a prompt effector caspase activation and internucleosomal DNA fragmentation. Pre-treatment of chondrocytes with alpha-difluoromethylornithine (DFMO), an ornithine decarboxylase (ODC) inhibitor, markedly reduced putrescine and spermidine content as well as the caspase-3 activation and DNA fragmentation induced by TNF and CHX. DFMO treatment also inhibited the increase in effector caspase activity provoked by TNF plus MG132, a proteasome inhibitor. DFMO decreased caspase-8 activity and procaspase-8 content, an apical caspase essential for TNF-induced apoptosis. Although DFMO increased the amount of active, phosphorylated Akt, inhibitors of the Akt pathway failed to restore the TNF-induced increase in caspase activity blunted by DFMO. DFMO also reduced the increase in caspase activity induced by staurosporine, but in this case Akt inhibition prevented the DFMO effect. Pre-treatment with CGP 48664, an S-adenosylmethionine decarboxylase (SAMDC) inhibitor markedly reduced spermidine and spermine levels, and provoked effects similar to those caused by DFMO. Finally DFMO was effective even in primary osteoarthritis (OA) chondrocyte cultures. These results suggest that the intracellular depletion of polyamines in chondrocytes can inhibit both the death receptor pathway by reducing the level of procaspase-8, and the apoptotic mitochondrial pathway by activating Akt.

Induction of ornithine decarboxylase in T/C-28a2 chondrocytes by lysophosphatidic acid: signaling pathway and inhibition of cell proliferation

Among several extracellular messengers tested, lysophosphatidic acid (LPA) was able to cause the most marked induction of ornithine decarboxylase (ODC) in serum-starved human T/C-28a2 chondrocytes. LPA also induced the activation/phosphorylation of Src, Akt and p44/42 MAPK, and the translocation of PKC-delta from cytosol to membrane coupled to its tyrosine phosphorylation. Experiments with selective signaling inhibitors indicate that LPA leads to Src activation through Gi protein-coupled receptors. In turn Src can activate PI3K and PKC-delta, and all these signaling proteins are required for ODC induction. In conclusion these results show that chondrocytes may be a novel target for LPA action. However, although LPA is considered a mitogen for several cell types and ODC induction is generally correlated to cell growth, LPA was not able to stimulate chondrocyte growth, but rather exerted an anti-proliferative effect.

Regulation of apoptosis in osteoclasts and osteoblastic cells

In postnatal life, the skeleton undergoes continuous remodeling in which osteoclasts resorb aged or damaged bone, leaving space for osteoblasts to make new bone. The balance of proliferation, differentiation, and apoptosis of bone cells determines the size of osteoclast or osteoblast populations at any given time. Bone cells constantly receive signals from adjacent cells, hormones, and bone matrix that regulate their proliferation, activity, and survival. Thus, the amount of bone and its microarchitecture before and after the menopause or following therapeutic intervention with drugs, such as sex hormones, glucocorticoids, parathyroid hormone, and bisphosphonates, is determined in part by effects of these on survival of osteoclasts, osteoblasts, and osteocytes. Understanding the mechanisms and regulation of bone cell apoptosis will enhance our knowledge of bone cell function and help us to develop better therapeutics for the management of osteoporosis and other bone diseases.

Hydrostatic pressure induces apoptosis in human chondrocytes from osteoarthritic cartilage through up-regulation of tumor necrosis factor-alpha, inducible nitric oxide synthase, p53, c-myc, and bax-alpha, and suppression of bcl-2

Hydrostatic pressure (HP) is thought to increase within cartilage extracellular matrix as a consequence of fluid flow inhibition. The biosynthetic response of human articular chondrocytes to HP in vitro varies with the load magnitude, load frequency, as well as duration of loading. We found that continuous cyclic HP (5 MegaPascals (MPa) for 4 h; 1 Hz frequency) induced apoptosis in human chondrocytes derived from osteoarthritic cartilage in vitro as evidenced by reduced chondrocyte viability which was independent of initial cell densities ranging from 8.1 x 10(4) to 1.3 x 10(6) cells ml(-1). HP resulted in internucleosomal DNA fragmentation, activation of caspase-3, and cleavage of poly-ADP-ribose polymerase (PARP). At the molecular level, induction of apoptosis by HP was characterized by up-regulation of p53, c-myc, and bax-alpha after 4 h with concomitant down-regulation of bcl-2 after 2 h at 5 MPa as measured by RT-PCR. In contrast, beta-actin expression was unchanged. Real-time quantitative RT-PCR confirmed a HP-induced (5 MPa) 1.3-2.6 log-fold decrease in bcl-2 mRNA copy number after 2 and 4 h, respectively, and a significant increase (1.9-2.5 log-fold) in tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) mRNA copy number after 2 and 4 h, respectively. The up-regulation of p53 and c-myc, and the down-regulation of bcl-2 caused by HP were confirmed at the protein level by Western blotting. These results indicated that HP is a strong inducer of apoptosis in osteoarthritic human chondrocytes in vitro.

Pathophysiological mechanisms in osteoarthritis lead to novel therapeutic strategies

Osteoarthritis (OA) is a debilitating, progressive disease of diarthrodial joints associated with aging. At the molecular level, OA is characterized by an imbalance between anabolic (i.e. extracellular matrix biosynthesis) and catabolic (i.e. extracellular matrix degradation) pathways in which articular cartilage is the principal site of tissue injury responses. The pathophysiology of OA also involves the synovium in that 'nonclassical' inflammatory synovial processes contribute to OA progression. Chondrocytes are critical to the OA process in that the progression of OA can be judged by the vitality of chondrocytes and their ability to resist apoptosis. Growth factors exemplified by insulin-like growth factor-1, its binding proteins and transforming growth factor-beta contribute to anabolic pathways including compensatory biosynthesis of extracellular matrix proteins. Catabolic pathways are altered by cytokine genes such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) which are upregulated in OA. In addition, IL-1 and TNF-alpha downregulate extracellular matrix protein biosynthesis while concomitantly upregulating matrix metalloproteinase (MMP) gene expression. When MMPs are activated, cartilage extracellular matrix degradation ensues apparently because levels of endogenous cartilage MMP inhibitors cannot regulate MMP activity. Therapeutic strategies designed to modulate the imbalance between anabolic and catabolic pathways in OA may include neutralizing cytokine activity or MMP gene expression or inhibiting signaling pathways which result in apoptosis dependent on mature caspase activity or mitogen-activated protein kinase (MAPK) activity. MAPK activity appears critical for regulating chondrocyte and synoviocyte apoptosis and MMP genes.

TRAIL induces apoptosis of chondrocytes and influences the pathogenesis of experimentally induced rat osteoarthritis

OBJECTIVE

To investigate whether TRAIL influences the pathogenesis of osteoarthritis (OA).

METHODS

A recombinant adenoviral vector system (Ad-TRAIL) was used. Expression of TRAIL in a rat chondrocyte cell line (RCJ3.1C.18) and alterations in the expression of death and decoy receptors after Ad-TRAIL infection were measured by Western blot assay. To explore the underlying mechanism, Western blot assays (to detect caspase 8, poly[ADP-ribose] polymerase [PARP], and caspase 3 activation), mitochondrial membrane potential (DeltaPsim) measurement, Hoechst staining, and DNA electrophoresis were conducted. Next, expression of TRAIL and death and decoy receptors was examined by immunochemistry in primary cultured chondrocytes and on cartilage obtained from rats with experimentally induced OA.

RESULTS

Ad-TRAIL infection induced expression of TRAIL in RCJ3.1C.18 cells, increased expression of death receptor 4 (DR4), and decreased expression of DR5 and decoy receptor 1 (DcR1). Ad-TRAIL, at doses of 10 and 100 multiplicities of infection, decreased the viability of chondrocytes 4 days after infection. Reduction of DeltaPsim, cytochrome c release, nuclear condensation, activation of caspase 3 and PARP, and DNA fragmentation proved the induction of apoptosis. Activation of caspase 8 was also observed. Ad-TRAIL also induced apoptosis in primary cultured chondrocytes, in which alterations in expression of TRAIL and death receptors were similar to those observed in RCJ3.1C.18 cells. Cartilage obtained from rats with experimentally induced OA showed increased expression of TRAIL and DR4 and decreased expression of DR5 and DcR1 compared with control cartilage.

CONCLUSION

TRAIL induces chondrocyte apoptosis, and TRAIL-induced chondrocyte apoptosis may play a role in the pathogenesis of OA.

Short-term exposure of cartilage to blood results in chondrocyte apoptosis

Studies have shown that joint bleeding leads to cartilage degradation independent of concurrent synovitis. We hypothesized that the blood-induced cartilage damage is because of increased chondrocyte apoptosis after short-term exposure of whole blood or isolated mononuclear cells plus red blood cells to cartilage. Human cartilage tissue samples were co-cultured for 4 days with whole blood (50% v/v) or with mononuclear cells plus red blood cells (50% v/v equivalents). Cartilage matrix proteoglycan synthesis ((35)SO(4)(2-) incorporation) was determined after 4 days as well as at day 16 (after a 12-day recovery period in the absence of any additions). To test the involvement of apoptosis a specific caspase-3 inhibitor (acDEVDcho, 0 to 500 micro mol/L) as well as a pan-caspase inhibitor (zVADfmk, 0 to 500 micro mol/L) were added. Chondrocyte apoptosis was evaluated by immunohistochemical staining of single-strand DNA and by terminal dUTP nick-end labeling. Cartilage co-cultured with whole blood as well as mononuclear cells plus red blood cells induced a long-term inhibition of proteoglycan synthesis (74% and 78% inhibition on day 16, respectively). Immunohistochemistry showed a threefold increase in apoptotic chondrocytes in cultures with 50% whole blood as well as with mononuclear cells plus red blood cells. Both the specific caspase-3 inhibitor and the pan-caspase inhibitor partially restored proteoglycan synthesis in the cartilage after blood exposure. This effect was accompanied by a decrease in the number of apoptotic chondrocytes. These data suggest that a single joint hemorrhage (a 4-day exposure of cartilage to 50% v/v blood) results in induction of chondrocyte apoptosis, responsible for the observed inability of the chondrocytes to restore the proteoglycan synthesis during recovery from a short-term exposure to blood. This reduced restoration could eventually lead to cartilage degeneration and ultimately joint destruction.

Growth-related oncogene alpha induction of apoptosis in osteoarthritis chondrocytes

OBJECTIVE

To evaluate the apoptotic effect of the chemokine growth-related oncogene alpha (GROalpha), which we recently reported to be up-regulated in osteoarthritis (OA) chondrocytes. Chondrocyte apoptosis is considered to be a major determinant of cartilage damage in OA, a disease resulting from the aberrant production of inflammatory mediators (cytokines and chemokines) and effectors (matrix metalloproteinases and reactive oxygen and nitrogen species) by chondrocytes.

METHODS

We investigated the apoptotic effect of GROalpha on isolated human cells and on in vitro-cultured cartilage explants by conventional methods (morphology, detection of DNA fragmentation in situ and in solution, exposure of phosphatidylserine) and by analysis of "early" biochemical events (plasma membrane depolarization, activation of caspase 3, and phosphorylation of c-Jun N-terminal kinase/stress-activated protein kinase).

RESULTS

We clearly demonstrated that GROalpha was able to initiate a series of morphologic, biochemical, and molecular changes that led to chondrocyte apoptosis. Moreover, we found that additional signals delivered from the extracellular matrix (ECM) were essential in the control of chondrocyte susceptibility to GROalpha-induced apoptosis, since cell death was detected only when cells were stimulated after reestablishment of their proper interactions with the ECM, or in cartilage explant samples with reduced ECM, as indicated by decreased Safranin O staining.

CONCLUSION

GROalpha can induce apoptosis in articular chondrocytes, and the induction is dependent upon additional signals from the ECM. These findings are relevant to understanding the pathogenesis of OA, in view of the availability of the GROalpha chemokine in the joint space in the course of this rheumatic disease.

TNF-alpha protects human primary articular chondrocytes from nitric oxide-induced apoptosis via nuclear factor-kappaB

TNF-alpha plays a key role in rheumatoid arthritis, but its effect on chondrocyte survival is still conflicting. In the present study, we tested how TNF-alpha influences chondrocyte survival in response to nitric oxide (NO)-related apoptotic signals, which are abundant during rheumatoid arthritis. Human primary articular chondrocytes or cartilage explants were pretreated with TNF-alpha for 24 hours and then treated with the proapoptotic NO donor sodium-nitro-prusside (SNP) for an additional 24 hours. TNF-alpha pretreatment markedly protected chondrocytes from SNP-induced cell death. Preincubation of chondrocytes with TNF-alpha inhibited both SNP-induced high-molecular weight DNA fragmentation and annexin V-FITC binding. Of interest, TNF-alpha induced persistent nuclear factor-kappaB (NF-kappaB)-DNA binding activity even in the presence of SNP, mirroring apoptosis protection effects. Both the TNF-alpha antiapoptotic effect and NF-kappaB-DNA binding activity were significantly inhibited by NF-kappaB inhibitors, Bay 11-7085, MG-132, and adenovirus-expressing mutated IkappaB-alpha. Phosphatidylinositol-3 kinase inhibitor LY 294002 also markedly inhibited the antiapoptotic effect of TNF-alpha. In primary chondrocytes, TNF-alpha induced expression of the antiapoptotic protein Cox-2, which persisted in the presence of SNP, and a specific Cox-2 inhibitor significantly blocked the TNF-alpha protective effect. We therefore conclude that TNF-alpha-mediated protection of chondrocytes from NO-induced apoptosis acts through NF-kappaB and requires Cox-2 activity.

Tumor necrosis factor-related apoptosis-inducing ligand induces apoptosis in human articular chondrocytes in vitro

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is produced by immune cells and by mediating apoptosis, TRAIL plays an important role in tumor surveillance. TRAIL binds four different membrane-bound receptors: DR4, DR5, DcR1, and DcR2. The DR4- and DR5-receptors mediate apoptosis, whereas the others do not. We demonstrated by reverse transcriptase-polymerase chain reaction and flow cytometry that, in vitro, normal human articular chondrocytes express the receptors mediating apoptosis (DR4 and DR5) and one of the decoy receptors (DcR2). Also, we demonstrated that chondrocytes were subjected to cell death within few hours after challenge with TRAIL and that cytotoxicity was dose-dependent. Treated cells had apoptotic morphology accompanied by active caspase-3 immunoreactivity. These data indicate that normal human articular chondrocytes are susceptible to TRAIL-mediated apoptosis, which otherwise is typical for transformed cells, and also that death receptors and their respective ligands may have a crucial role in cartilage generation and destruction.

Induction of apoptosis in chondrocytes by tumor necrosis factor-alpha

Tumor necrosis factor alpha (TNF-alpha) induces apoptosis in a number of cell types and plays an essential role in bone remodeling, both stimulating the proliferation of osteoblasts and activating osteoclasts. During endochondral ossification, apoptosis of chondrocytes occurs concurrently with new bone formation and the resorption and replacement of mineralized cartilage with woven bone. In the present study, the role of TNF-alpha in promoting chondrocyte apoptosis was examined. Chondrocyte cell populations, enriched in either hypertrophic or non-hypertrophic cells, were isolated from the cephalic and caudal portions of 17-day chick embryo sterna, respectively, and treated in vitro with 0.1-10 nM recombinant human TNF-alpha. As a positive control, apoptosis was also induced by Fas receptor antibody binding. Dye exclusion assays of the live/dead ratios of cells showed that TNF-alpha caused a dose-dependent 1.5- and 2.0-fold increase in the number of dead cells in both hypertrophic and non-hypertrophic chondrocytes. Induction of apoptosis was independently assayed by measurement of interleukin-1beta-converting enzyme (ICE) activity, and analyzed by a semi-quantitative determination of DNA fragmentation. When compared to untreated cells, these analyses also showed dose-dependent increases in TNF-alpha induced apoptosis in both chondrocyte populations, with increases in the levels of ICE activity for all doses of TNF-alpha (from approximately 5 to approximately 20 fold). Osteoblasts, however, were not affected by treatment with TNF-alpha or by Fas antibody/protein G induction. Immunostaining of chondrocytes for Fas receptor and caspase-2 protein expression showed that most of the chondrocytes expressed these two markers of apoptosis after treatment with TNF-alpha. Although cell killing and ICE induction were higher in the more hypertrophic cells, TNF-alpha induced apoptosis in both hypertrophic and non-hypertrophic chondrocyte populations. These results demonstrate that apoptosis may be induced in both hypertrophic and non-hypertrophic chondrocytes through both Fas and TNF-alpha receptor mediated signaling, and suggest that chondrocytes are more sensitive to apoptotic effects of TNF-alpha within the skeletal lineage than are osteoblasts.