Nitric oxide regulates mitochondrial respiration and functions of articular chondrocytes

NOS inhibition reverses TLR2-induced chondrocyte dysfunction and attenuates age-related osteoarthritis

Osteoarthritis (OA) is a joint disease featuring cartilage breakdown and chronic pain. Although age and joint trauma are prominently associated with OA occurrence, the trigger and signaling pathways propagating their pathogenic aspects are ill defined. Following long-term catabolic activity and traumatic cartilage breakdown, debris accumulates and can trigger Toll-like receptors (TLRs). Here we show that TLR2 stimulation suppressed the expression of matrix proteins and induced an inflammatory phenotype in human chondrocytes. Further, TLR2 stimulation impaired chondrocyte mitochondrial function, resulting in severely reduced adenosine triphosphate (ATP) production. RNA-sequencing analysis revealed that TLR2 stimulation upregulated nitric oxide synthase 2 (NOS2) expression and downregulated mitochondria function-associated genes. NOS inhibition partially restored the expression of these genes, and rescued mitochondrial function and ATP production. Correspondingly, Nos2-/- mice were protected from age-related OA development. Taken together, the TLR2-NOS axis promotes human chondrocyte dysfunction and murine OA development, and targeted interventions may provide therapeutic and preventive approaches in OA.

Metabolic Health, Mitochondrial Fitness, Physical Activity, and Cancer

Cancer continues to be a significant global health issue. Traditional genetic-based approaches to understanding and treating cancer have had limited success. Researchers are increasingly exploring the impact of the environment, specifically inflammation and metabolism, on cancer development. Examining the role of mitochondria in this context is crucial for understanding the connections between metabolic health, physical activity, and cancer. This study aimed to review the literature on this topic through a comprehensive narrative review of various databases including MedLine (PubMed), Cochrane (Wiley), Embase, PsychINFO, and CinAhl. The review highlighted the importance of mitochondrial function in overall health and in regulating key events in cancer development, such as apoptosis. The concept of "mitochondrial fitness" emphasizes the crucial role of mitochondria in cell metabolism, particularly their oxidative functions, and how proper function can prevent replication errors and regulate apoptosis. Engaging in high-energy-demanding movement, such as exercise, is a powerful intervention for improving mitochondrial function and increasing resistance to environmental stressors. These findings support the significance of considering the role of the environment, specifically inflammation and metabolism, in cancer development and treatment. Further research is required to fully understand the mechanisms by which physical activity improves mitochondrial function and potentially reduces the risk of cancer.

Time-dependently Appeared Microenvironmental Changes and Mechanism after Cartilage or Joint Damage and the Influences on Cartilage Regeneration

Cartilage and joint damage easily degenerates cartilage and turns into osteoarthritis (OA), which seriously affects human life and work, and has no cure currently. The temporal and spatial changes of multiple microenvironments upon the damage of cartilage and joint are noticed, including the emergences of inflammation, bone remodeling, blood vessels, and nerves, as well as alterations of extracellular and pericellular matrix, oxygen tension, biomechanics, underneath articular cartilage tissues, and pH value. This review summarizes the existing literatures on microenvironmental changes, mechanisms, and their negative effects on cartilage regeneration following cartilage and joint damage. We conclude that time-dependently rebuilding the multiple normal microenvironments of damaged cartilage is the key for cartilage regeneration after systematic studies for the timing and correlations of various microenvironment changes.

Mitochondrial transfer from bone-marrow-derived mesenchymal stromal cells to chondrocytes protects against cartilage degenerative mitochondrial dysfunction in rats chondrocytes

Background

Previous studies have reported that mitochondrial dysfunction participates in the pathological process of osteoarthritis (OA). However, studies that improve mitochondrial function are rare in OA. Mitochondrial transfer from mesenchymal stem cells (MSCs) to OA chondrocytes might be a cell-based therapy for the improvement of mitochondrial function to prevent cartilage degeneration. This study aimed to determine whether MSCs can donate mitochondria and protect the mitochondrial function and therefore reduce cartilage degeneration.

Methods

Bone-marrow-derived mesenchymal stromal cells (BM-MSCs) were harvested from the marrow cavities of femurs and tibia in young rats. OA chondrocytes were gathered from the femoral and tibial plateau in old OA model rats. BM-MSCs and OA chondrocytes were co-cultured and mitochondrial transfer from BM-MSCs to chondrocytes was identified. Chondrocytes with mitochondria transferred from BM-MSCs were selected by fluorescence-activated cell sorting. Mitochondrial function of these cells, including mitochondrial membrane potential (Δψm), the activity of mitochondrial respiratory chain (MRC) enzymes, and adenosine triphosphate (ATP) content were quantified and compared to OA chondrocytes without mitochondrial transfer. Chondrocytes proliferation, apoptosis, and secretion ability were also analyzed between the two groups.

Results

Mitochondrial transfer was found from BM-MSCs to OA chondrocytes. Chondrocytes with mitochondrial from MSCs (MSCs + OA group) showed increased mitochondrial membrane potential compared with OA chondrocytes without mitochondria transfer (OA group) (1.79 ± 0.19 vs. 0.71 ± 0.12, t = 10.42, P < 0.0001). The activity of MRC enzymes, including MRC complex I, II, III, and citrate synthase was also improved (P < 0.05). The content of ATP in MSCs + OA group was significantly higher than that in OA group (161.90 ± 13.49 vs. 87.62 ± 11.07 nmol/mg, t = 8.515, P < 0.0001). Meanwhile, we observed decreased cell apoptosis (7.09% ± 0.68% vs.15.89% ± 1.30%, t = 13.39, P < 0.0001) and increased relative secretion of type II collagen (2.01 ± 0.14 vs.1.06 ± 0.11, t = 9.141, P = 0.0008) and proteoglycan protein (2.08 ± 0.20 vs. 0.97 ± 0.12, t = 8.227, P = 0.0012) in MSCs + OA group, contrasted with OA group.

Conclusions

Mitochondrial transfer from BM-MSCs provided protection for OA chondrocytes against mitochondrial dysfunction and degeneration through improving mitochondrial function, cell proliferation, and inhibiting apoptosis in chondrocytes. This finding may offer a new therapeutic direction for OA.

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.

Pyrroloquinoline Quinone Slows Down the Progression of Osteoarthritis by Inhibiting Nitric Oxide Production and Metalloproteinase Synthesis

Osteoarthritis (OA) is the most common arthritis and also one of the major causes of joint pain in elderly people. The aim of this study was to investigate the effects of pyrroloquinoline quinone (PQQ) on degenerated-related changes in osteoarthritis (OA). SW1353 cells were stimulated with IL-1β to establish the chondrocyte injury model in vitro. PQQ was administrated into SW1353 cultures 1 h before IL-1β treatment. Amounts of MMP-1, MMP-13, P65, IκBα, ERK, p-ERK, P38, and p-P38 were measured via western blot. The production of NO was determined by Griess reaction assay and reflected by the iNOS level. Meniscal-ligamentous injury (MLI) was performed on 8-week-old rats to establish the OA rat model. PQQ was injected intraperitoneally 3 days before MLI and consecutively until harvest, and the arthritis cartilage degeneration level was assessed. The expressions of MMP-1 and MMP-13 were significantly downregulated after PQQ treatment compared with that in IL-1β alone group. NO production and iNOS expression were decreased by PQQ treatment compared with control group. Amounts of nucleus P65 were upregulated in SW1353 after stimulated with IL-1β, while PQQ significantly inhibited the translocation. In rat OA model, treatment with PQQ markedly decelerated the degeneration of articular cartilage. These findings suggested that PQQ could inhibit OA-related catabolic proteins MMPs expression, NO production, and thus, slow down the articular cartilage degeneration and OA progression. Owing to its beneficial effects, PQQ is expected to be a novel pharmacological application in OA clinical prevention and treatment in the near future.

A Network Pharmacology Approach to Uncover the Pharmacological Mechanism of XuanHuSuo Powder on Osteoarthritis

As the most familiar type of arthritis and a chronic illness of the joints, Osteoarthritis (OA) affects a great number of people on the global scale. XuanHuSuo powder (XHSP), a conventional herbal formula from China, has been extensively applied in OA treatment. Nonetheless, its pharmacological mechanism has not been completely expounded. In this research, a network pharmacology approach has been chosen to study the pharmacological mechanism of XHSP on OA, and the pharmacology networks were established based on the relationship between four herbs found in XHSP, compound targets, and OA targets. The pathway enrichment analysis revealed that the significant bioprocess networks of XHSP on OA were regulation of inflammation, interleukin-1β (IL-1β) production and nitric oxide (NO) biosynthetic process, response to cytokine or estrogen stimuli, and antiapoptosis. These effects have not been reported previously. The comprehensive network pharmacology approach developed by our research has revealed, for the first time, a connection between four herbs found in XHSP, corresponding compound targets, and OA pathway systems that are conducive to expanding the clinical application of XHSP. The proposed network pharmacology approach could be a promising complementary method by which researchers might better evaluate multitarget or multicomponent drugs on a systematic level.

The effects of oxygen level and glucose concentration on the metabolism of porcine TMJ disc cells

OBJECTIVE

To determine the combined effect of oxygen level and glucose concentration on cell viability, ATP production, and matrix synthesis of temporomandibular joint (TMJ) disc cells.

DESIGN

TMJ disc cells were isolated from pigs aged 6-8 months and cultured in a monolayer. Cell cultures were preconditioned for 48 h with 0, 1.5, 5, or 25 mM glucose DMEM under 1%, 5%, 10%, or 21% O2 level, respectively. The cell viability was measured using the WST-1 assay. ATP production was determined using the Luciferin-Luciferase assay. Collagen and proteoglycan synthesis were determined by measuring the incorporation of [2, 3-(3)H] proline and [(35)S] sulfate into the cells, respectively.

RESULTS

TMJ disc cell viability significantly decreased (P < 0.0001) without glucose. With glucose present, decreased oxygen levels significantly increased viability (P < 0.0001), while a decrease in glucose concentration significantly decreased viability (P < 0.0001). With glucose present, decreasing oxygen levels significantly reduced ATP production (P < 0.0001) and matrix synthesis (P < 0.0001). A decreased glucose concentration significantly decreased collagen synthesis (P < 0.0001). The interaction between glucose and oxygen was significant in regards to cell viability (P < 0.0001), ATP production (P = 0.00015), and collagen (P = 0.0002) and proteoglycan synthesis (P < 0.0001).

CONCLUSIONS

Although both glucose and oxygen are important, glucose is the limiting nutrient for TMJ disc cell survival. At low oxygen levels, the production of ATP, collagen, and proteoglycan are severely inhibited. These results suggest that steeper nutrient gradients may exist in the TMJ disc and it may be vulnerable to pathological events that impede nutrient supply.

Autophagy activation and protection from mitochondrial dysfunction in human chondrocytes

OBJECTIVE

Autophagy is a key pathway of cellular homeostasis for removing damaged macromolecules and organelles, including mitochondria. Recent studies indicate that activation of autophagy is defective in aging and osteoarthritis (OA), contributing to cell death and tissue damage. In addition, there is increasing evidence that mitochondrial dysfunction plays an important role in OA pathogenesis. The objective of this study was to determine whether activation of autophagy protects against mitochondrial dysfunction in human chondrocytes.

METHODS

Human chondrocytes were treated with oligomycin, an inhibitor of mitochondrial respiratory chain complex V. Autophagy activation was analyzed by determination of light chain 3 membrane-bound form II (LC3-II), a marker of autophagosome formation. To investigate whether autophagy protects from mitochondrial dysfunction, autophagy was induced by rapamycin, the selective inhibitor of mammalian target of rapamycin complex 1 (mTORC-1), and by torin 1, the inhibitor of mTORC-1 and mTORC-2. Small interfering autophagy-related 5 was used to evaluate the role of autophagy in mitochondrial dysfunction.

RESULTS

Mitochondrial dysfunction was induced by treatment with oligomycin, which significantly decreased mitochondrial membrane potential (ΔΨm). This was associated with increased production of reactive oxygen species and cell death. Autophagy activation, as reflected by LC3-II, was decreased in a time-dependent manner. To evaluate whether autophagy regulates mitochondrial function, chondrocytes were pretreated with rapamycin and torin 1 before oligomycin. Autophagy activation significantly protected against mitochondrial dysfunction. Conversely, genetic inhibition of autophagy induced significant mitochondrial function defects.

CONCLUSION

Our data highlight the role of autophagy as a critical protective mechanism against mitochondrial dysfunction. Pharmacologic interventions that enhance autophagy may have chondroprotective activity in cartilage degenerative processes such as OA.

Mitochondrial respiration and redox coupling in articular chondrocytes

Introduction

Chondrocytes rely primarily on glycolysis to meet cellular energy needs, but recent studies implicate impaired mitochondrial function in osteoarthritis (OA) pathogenesis. Our objectives were to investigate the ability of chondrocytes to upregulate mitochondrial respiration when challenged with a nutrient stress and determine the effect on mediators of chondrocyte oxidative homeostasis.

Methods

Primary bovine chondrocytes were isolated and cultured in alginate beads. Mitochondrial respiration was stimulated by culturing cells with galactose-supplemented media for a period of 1 or 5 days. Metabolic flexibility was assessed by measuring metabolite and enzymatic biomarkers of glycolytic and mitochondrial metabolism. Oxidative homeostasis was assessed by measuring (1) cellular glutathione content and redox homeostasis, (2) rates of nitric oxide and superoxide production, and (3) the abundance and activity of cellular anti-oxidant proteins, especially the mitochondrial isoform of superoxide dismutase (SOD2). The regulatory role of hypoxia-inducible factor 2α (HIF-2α) in mediating the metabolic and redox responses was evaluated by chemical stabilization with cobalt chloride (CoCl₂).

Results

After 5 days of galactose culture, lactate production and lactate dehydrogenase activity were reduced by 92% (P <0.0001) and 28% (P = 0.051), respectively. Conversely, basal oxygen consumption increased 35% (P = 0.042) without increasing mitochondrial content. Glutathione redox homeostasis was unaffected by galactose culture. However, the production of nitric oxide and superoxide and the expression and activity of SOD2 were significantly reduced after 5 days in galactose culture. Nuclear protein expression and gene expression of HIF-2α, a transcription factor for SOD2, were significantly downregulated (more than twofold; P <0.05) with galactose culture. CoCl₂-mediated stabilization of HIF-2α during the initial galactose response phase attenuated the reduction in SOD2 (P = 0.028) and increased cell death (P = 0.003).

Conclusions

Chondrocyte metabolic flexibility promotes cell survival during a nutrient stress by upregulating mitochondrial respiration and reducing the rate of reactive nitrogen and oxygen species production. These changes are coupled to a substantial reduction in the expression and activity of the mitochondrial anti-oxidant SOD2 and its pro-catabolic transcription factor HIF-2α, suggesting that an improved understanding of physiologic triggers of chondrocyte metabolic flexibility may provide new insight into the etiology of OA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0566-9) contains supplementary material, which is available to authorized users.

Nitric oxide compounds have different effects profiles on human articular chondrocyte metabolism

Introduction

The pathogenesis of osteoarthritis (OA) is characterized by the production of high amounts of nitric oxide (NO), as a consequence of up-regulation of chondrocyte-inducible nitric oxide synthase (iNOS) induced by inflammatory cytokines. NO donors represent a powerful tool for studying the role of NO in the cartilage in vitro. There is no consensus about NO effects on articular cartilage in part because the differences between the NO donors available. The aim of this work is to compare the metabolic profile of traditional and new generation NO donors to see which one points out the osteoarthritic process in the best way.

Methods

Human healthy and OA chondrocytes were isolated from patients undergoing joint replacement surgery, and primary cultured. Cells were stimulated with NO donors (NOC-12 or SNP). NO production was evaluated by the Griess method, and apoptosis was quantified by flow cytometry. Mitochondrial function was evaluated by analysing respiratory chain enzyme complexes, citrate synthase (CS) activities by enzymatic assay, mitochondrial membrane potential (Δψm) by JC-1 using flow cytometry, and ATP levels were measured by luminescence assays. Glucose transport was measured as the uptake of 2-deoxy-[³H]glucose (2-[³H]DG). Statistical analysis was performed using the Mann-Whitney U test.

Results

NOC-12 liberates approximately ten times more NO₂^- than SNP, but the level of cell death induced was not as profound as that produced by SNP. Normal articular chondrocytes stimulated with NOC-12 had reduced activity from complexes I, III y IV, and the mitochondrial mass was increased in these cells. Deleterious effects on ΔΨm and ATP levels were more profound with SNP, and this NO donor was able to reduce 2-[³H]DG levels. Both NO donors had opposite effects on lactate release, SNP diminished the levels and NOC-12 lead to lactate accumulation. OA chondrocytes incorporate significantly more 2-[³H]DG than healthy cells.

Conclusions

These findings suggest that the new generation donors, specifically NOC-12, mimic the OA metabolic process much better than SNP. Previous results using SNP have to be considered prudently since most of the effects observed can be induced by the interactions of secondary products of NO.

Fuyuan Decoction inhibits nitric oxide production via inactivation of nuclear factor-κB in SW1353 chondrosarcoma cells

ETHNOPHARMACOLOGICAL RELEVANCE

Fuyuan Decoction (FYD) is an empirical formula of treating Bi Zheng in traditional Chinese medicine (TCM). Despite the fact that the efficiency of FYD on treating osteoarthritis has been verified in clinic, the underlying mechanisms are not totally understood. This study was to investigate the effects and mechanisms of FYD on nitric oxide (NO) production and nuclear factor (NF)-κB activation in interleukin (IL)-1β-stimulated chondrocytes.

MATERIALS AND METHODS

SW1353 human chondrosarcoma cells were pretreated with various concentrations of FYD-containing serum (FYD-CS), and then were stimulated by IL-1β. Amounts of NO were determined by Griess reaction assay. Inducible NO synthase (iNOS) expression, inhibitor-κBα (IκBα) degradation and nuclear translocation of p65 protein were determined by Western blot assay. DNA binding activity of NF-κB was determined by ELISA assay using Trans AM(™) kit for p65.

RESULTS

10% and 20% (v/v) FYD-CS significantly decreased NO production in a concentration-dependent manner (p<0.05 or p<0.01) as compared to control in IL-1β-induced SW1353 cells. Besides, 10% and 20% FYD-CS also significantly reduced iNOS protein expression by about 60% and 70% (both p<0.01), respectively. Furthermore, 10% and 20% FYD-CS markedly decreased IκBα degradation by about 45% and 26% (p<0.01 or p<0.05), lessened P65 content in the nucleus by about 28% and 60% (both p<0.01), and repressed DNA binding activity of P65 by about 30% and 45% (both p<0.01) in IL-1β-induced SW1353 cells.

CONCLUSIONS

These findings suggested that FYD could inhibit NO production and iNOS expression in IL-1β-induced chondrocytes through suppressing NF-κB activation.

Mitochondrial dysfunction and the inflammatory response

Inflammation has been linked to multiple degenerative and acute diseases as well as the aging process. Moreover, mitochondrial alterations play a central role in these processes. Mitochondria have an important role in pro-inflammatory signaling; similarly, pro-inflammatory mediators may also alter mitochondrial function. Both of these processes increase mitochondrial oxidative stress, promoting a vicious inflammatory cycle. Additionally, damage-associated molecular patterns derived from mitochondria could contribute to inflammasome formation and caspase-1 activation, while alterations in mitochondrial autophagy may cause inflammation. Strategies aimed at controlling excessive oxidative stress within mitochondria may represent both preventive and therapeutic interventions in inflammation.

Mechanical loading affects the energy metabolism of intervertebral disc cells

Research has shown that mechanical loading affects matrix biosynthesis of intervertebral disc (IVD) cells; however, the pathway(s) to this effect is currently unknown. Cellular matrix biosynthesis is an energy demanding process. The objective of this study was to investigate the effects of static and dynamic compressive loading on energy metabolism of IVD cells. Porcine annulus fibrosus (AF) and nucleus pulposus (NP) cells seeded in 2% agarose were used in this experiment. Experimental groups included 15% static compression and 0.1 and 1 Hz dynamic compression at 15% strain magnitude for 4 h. ATP, lactate, glucose, and nitric oxide (NO) contents in culture media, and ATP content in cell-agarose construct were measured using biochemical assays. While the total ATP content of AF cells was promoted by static and dynamic loading, only 1 Hz dynamic loading increased total ATP content of NP cells. Increases in lactate production and glucose consumption of AF cells suggest that ATP production via glycolysis is promoted by dynamic compression. ATP release and NO production of AF and NP cells were significantly increased by dynamic loading. Thus, this study clearly illustrates that static and dynamic compressive loading affect IVD cell energy production while cellular responses to mechanical loading were both cell type and compression type dependent.

Effect of nitric oxide on mitochondrial activity of human synovial cells

Background

Nitric oxide (NO) is a messenger implicated in the destruction and inflammation of joint tissues. Cartilage and synovial membrane from patients with rheumatoid arthritis (RA) and osteoarthritis (OA) have high levels of NO. NO is known to modulate various cellular pathways and, thus, inhibit the activity of the mitochondrial respiratory chain (MRC) of chondrocytes and induce the generation of reactive oxygen species (ROS) and cell death in multiple cell types. For these reasons, and because of the importance of the synovial membrane in development of OA pathology, we investigated the effects of NO on survival, mitochondrial function, and activity of fibroblastic human OA synovial cells.

Methods

Human OA synovia were obtained from eight patients undergoing hip joint replacement. Sodium nitroprusside (SNP) was used as a NO donor compound and cell viability was evaluated by MTT assays. Mitochondrial function was evaluated by analyzing the mitochondrial membrane potential (Δψm) with flow cytometry using the fluorofore DePsipher. ATP levels were measured by luminescence assays, and the activities of the respiratory chain complexes (complex I: NADH CoQ₁ reductase, complex II: succinate dehydrogenase, complex III: ubiquinol-cytochrome c reductase, complex IV: cytochrome c oxidase) and citrate synthase (CS) were measured by enzymatic assay. Protein expression analyses were performed by western blot.

Results

SNP at a concentration of 0.5 mM induced cell death, shown by the MTT method at different time points. The percentages of viable cells at 24, 48 and 72 hours were 86.11 ± 4.9%, 74.31 ± 3.35%, and 43.88 ± 1.43%, respectively, compared to the basal level of 100% (*p < 0.05). SNP at 0.5 mM induced depolarization of the mitochondrial membrane at 12 hours with a decrease in the ratio of polarized cells (basal = 2.48 ± 0.28; SNP 0.5 mM = 1.57 ± 0.11; *p < 0.01). The time course analyses of treatment with SNP at 0.5 mM demonstrated that treatment reliably and significantly reduced intracellular ATP production (68.34 ± 14.3% vs. basal = 100% at 6 hours; *p < 0.05). The analysis of the MRC at 48 hours showed that SNP at 0.5 mM increased the activity of complexes I (basal = 36.47 ± 3.92 mol/min/mg protein, SNP 0.5 mM = 58.08 ± 6.46 mol/min/mg protein; *p < 0.05) and III (basal = 63.87 ± 6.93 mol/min/mg protein, SNP 0.5 mM = 109.15 ± 30.37 mol/min/mg protein; *p < 0.05) but reduced CS activity (basal = 105.06 ± 10.72 mol/min/mg protein, SNP at 0.5 mM = 66.88 ± 6.08 mol/min/mg protein.; *p < 0.05), indicating a decrease in mitochondrial mass. Finally, SNP regulated the expression of proteins related to the cellular cycle; the NO donor decreased bcl-2, mcl-1 and procaspase-3 protein expression.

Conclusions

This study suggests that NO reduces the survival of OA synoviocytes by regulating mitochondrial functionality, as well as the proteins controlling the cell cycle.

The role of mitochondria in osteoarthritis

Mitochondria are important regulators of cellular function and survival that may have a key role in aging-related diseases. Mitochondrial DNA (mtDNA) mutations and oxidative stresses are known to contribute to aging-related changes. Osteoarthritis (OA) is an aging-associated rheumatic disease characterized by articular cartilage degradation and elevated chondrocyte mortality. Articular cartilage chondrocytes survive and maintain tissue integrity in an avascular, low-oxygen environment. Recent ex vivo studies have reported mitochondrial dysfunction in human OA chondrocytes, and analyses of mitochondrial electron transport chain activity in these cells show decreased activity of Complexes I, II and III compared to normal chondrocytes. This mitochondrial dysfunction may affect several pathways that have been implicated in cartilage degradation, including oxidative stress, defective chondrocyte biosynthesis and growth responses, increased cytokine-induced chondrocyte inflammation and matrix catabolism, cartilage matrix calcification, and increased chondrocyte apoptosis. Mitochondrial dysfunction in OA chondrocytes may derive from somatic mutations in the mtDNA or from the direct effects of proinflammatory mediators such as cytokines, prostaglandins, reactive oxygen species and nitric oxide. Polymorphisms in mtDNA may become useful as biomarkers for the diagnosis and prognosis of OA, and modulation of serum biomarkers by mtDNA haplogroups supports the concept that mtDNA haplogroups may define specific OA phenotypes in the complex OA process.

Rotenone prevents impact-induced chondrocyte death

Mechanical insult to articular cartilage kills chondrocytes, an event that may increase the risk of posttraumatic osteoarthritis. Recent reports indicate that antioxidants decrease impact-induced chondrocyte death, but the source(s) of oxidants, the time course of oxidant release, and the identity of the oxidative species generated in response to injury are unknown. A better understanding of these processes could lead to new treatments of acute joint injuries. To that end, we studied the kinetics and distribution of oxidant production in osteochondral explants subjected to a single, blunt-impact injury. We followed superoxide production by measuring the time-dependent accumulation of chondrocyte nuclei stained with the superoxide-sensitive probe dihydroethidium. The percentage of chondrocytes that were dihydroethidium-positive was 35% above baseline 10 min after impact, and 65% above baseline 60 min after impact. Most positive cells were found within and near areas contacted directly by the impact platen. Rotenone, an electron transport chain inhibitor, was used to test the hypothesis that mitochondria contribute to superoxide release. Rotenone treatment significantly reduced dihydroethidium staining, which remained steady at 15% above baseline for up to 60 min postimpact. Moreover, rotenone reduced chondrocyte death in impact sites by more than 40%, even when administered 2 h after injury (p < 0.001). These data show that much of the acute chondrocyte mortality caused by in vitro impact injuries results from superoxide release from mitochondria, and suggest that brief exposure to free radical scavengers could significantly improve chondrocyte viability following joint injury.

Hyaluronic acid inhibits nitric oxide-induced apoptosis and dedifferentiation of articular chondrocytes in vitro

OBJECTIVE

Nitric oxide is an important mediator in Osteoarthritis (OA), and causes apoptosis and dedifferentiation in articular chondrocytes. Protein kinase Calpha is involved in modulating apoptosis and dedifferentiation of articular chondrocytes induced by nitric oxide. Hyaluronic acid is widely used in the treatment of osteoarthritis and exerts significant chondroprotective effects. The exact mechanisms of its chondroprotective action are not yet fully elucidated. The present study was performed to investigate the effects and mechanisms of hyaluronic acid in NO-induced apoptosis and dedifferentiation of chondrocytes.

METHODS

The ratio of apoptotic cell and cell viability was surveyed by TUNEL, MTT assay and flow cytometry. The expression of aggrecan, type II collagen, and PKCalpha were determined by real-time PCR and Western blot. The expression changes of caspase-3 and bcl-2 was detected by Western blot. The mitochondrial membrane potential (DeltaPsim) was evaluated by Rhodamine-123 fluorescence.

RESULTS

HA reduces the TUNEL positive cell, nuclei fragment and the impairment of DeltaPsim. NO-induced chondrocyte dedifferentiation was blocked by HA, which restores expression of aggrecan and type II collagen of chondrocytes and cell viability. HA can block inhibition of PKC-alpha by NO.

CONCLUSION

Our results show that HA blocks NO-induced apoptosis and dedifferentiation of articular chondrocytes by modulation of DeltaPsim and PKCalpha.

Low oxygen reduces the modulation to an oxidative phenotype in monolayer-expanded chondrocytes

Autologous chondrocyte implantation requires a phase of in vitro cell expansion, achieved by monolayer culture under atmospheric oxygen levels. Chondrocytes reside under low oxygen conditions in situ and exhibit a glycolytic metabolism. However, oxidative phosphorylation rises progressively during culture, with concomitant reactive oxygen species production. We determine if the high oxygen environment in vitro provides the transformation stimulus. Articular chondrocytes were cultured in monolayer for up to 14 days under 2%, 5%, or 20% oxygen. Expansion under 2% and 5% oxygen reduced the rate at which the cells developed an oxidative phenotype compared to 20% oxygen. However, at 40 +/- 4 fmol cell(-1) h(-1) the oxygen consumption by chondrocytes expanded under 2% oxygen for 14 days was still 14 times the value observed for freshly isolated cells. Seventy-five to 78% of the increased oxygen consumption was accounted for by oxidative phosphorylation (oligomycin sensitive). Expansion under low oxygen also reduced cellular proliferation and 8-hydroxyguanosine release, a marker of oxidative DNA damage. However, these parameters remained elevated compared to freshly isolated cells. Thus, expansion under physiological oxygen levels reduces, but does not abolish, the induction of an oxidative energy metabolism. We conclude that simply transferring chondrocytes to low oxygen is not sufficient to either maintain or re-establish a normal energy metabolism. Furthermore, a hydrophobic polystyrene culture surface which promotes rounded cell morphology had no effect on the development of an oxidative metabolism. Although the shift towards an oxidative energy metabolism is often accompanied by morphological changes, this study does not support the hypothesis that it is driven by them.

Effect of Obstructive Jaundice and Nitric Oxide on the Profiles of Intestinal Bacterial Flora in Wild and iNOS Mice

We previously reported that the plasma level of endotoxin and colonic expression of IgA in the mouse increased with obstructive jaundice induced by bile duct ligation (BDL). To elucidate the mechanism of the BDL-induced increase, we analyzed the effect of BDL on intestinal flora in wild type and inducible nitric oxide synthase (iNOS)-deficient mice (iNOS(-/-)) using the terminal restriction fragment length polymorphism analysis (T-RFLP) and 16S rDNA clone libraries. The amounts of bacterial DNA detected in fecal samples from both animal groups pretreated with antibiotics were extremely low as compared with untreated groups. We found that the profiles of enteric bacteria changed markedly after BDL. The bacterial composition is significantly different between not only wild type and iNOS(-/-) mice but also those before and after BDL, respectively. Among enteric bacteria examined, Lactobacillus murinus was found to increase markedly after BDL in rectum of both animal groups. However, Escherichia coli markedly increased after BDL in the iNOS(-/-) mice. These findings suggest that profiles of enteric flora change markedly in animals during obstructive jaundice by some mechanism that is affected by bile constituents and iNOS-derived NO.

Mitochondrial dysregulation of osteoarthritic human articular chondrocytes analyzed by proteomics: a decrease in mitochondrial superoxide dismutase points to a redox imbalance

Mitochondria are involved in many cellular processes; mitochondrial dysfunctions have been associated with apoptosis, aging, and a number of pathological conditions, including osteoarthritis (OA). Mitochondrial proteins are attractive targets for the study of metabolism of the chondrocyte, the unique cell type present in mature cartilage, and its role in tissue degradation. Using a proteomics approach based on two-dimensional DIGE and MALDI-TOF/TOF mass spectrometric identification of mitochondria- enriched protein fractions from human articular chondrocytes, we analyzed mitochondrial protein changes that are characteristic of OA chondrocytes. A total of 73 protein forms were unambiguously identified as significantly altered in OA; 23 of them have been previously described as mitochondrial. An extensive statistical and cluster analysis of the data revealed a mitochondrial protein profile characteristic for OA. This pattern includes alterations in energy production, maintenance of mitochondrial membrane integrity, and free radical detoxification. Real time PCR, Western blot, and immunohistofluorescence assays confirmed a significant decrease of the major mitochondrial antioxidant protein manganese-superoxide dismutase (SOD2) in the superficial layer of OA cartilage. As possible outputs for this antioxidant deficiency, we found an increase of intracellular reactive oxygen species generation in OA chondrocytes and also verified an OA-dependent increase in the mitochondrial tumor necrosis factor-alpha receptor-associated protein 1 (TRAP1), a chaperone with a reported reactive oxygen species antagonist role. Our results describe the differences between the mitochondrial protein profiles of normal and OA chondrocytes, demonstrating that mitochondrial dysregulation occurs in cartilage cells during OA and highlighting redox imbalance as a key factor in OA pathogenesis.

Intra-articular injection of interleukin-4 decreases nitric oxide production by chondrocytes and ameliorates subsequent destruction of cartilage in instability-induced osteoarthritis in rat knee joints

OBJECTIVE

To investigate the in vitro and in vivo effects of interleukin (IL)-4 on mechanical stress-induced nitric oxide (NO) expression by chondrocytes, and destruction of cartilage and NO production in an instability-induced osteoarthritis (OA) model in rat knee joints, respectively.

MATERIALS AND METHODS

Cyclic tensile stress (CTS; 0.5Hz and 7% elongation) was applied to cultured normal rat chondrocytes with or without pre-incubation with recombinant rat IL-4 (rrIL-4). Inducible NO synthase (iNOS) mRNA expression and NO production were examined with real-time polymerase chain reaction and the Griess reaction, respectively. OA was induced in rat knee joints by transection of the anterior cruciate and medial collateral ligaments and resection of the medial meniscus. rrIL-4 (10, 50, and 100 ng/joint/day) was injected intra-articularly, and knee joint samples were collected 2, 4, and 6 weeks after surgery. Cartilage destruction was evaluated by the modified Mankin score and Osteoarthritis Research Society International scoring system on paraffin-embedded sections stained with safranin O. Cleavage of aggrecan and NO production were examined by immunohistochemistry for aggrecan neoepitope (NITEGE) and of nitrotyrosine (NT), respectively.

RESULTS

rrIL-4 down-regulated CTS-induced iNOS mRNA expression and NO production by chondrocytes. The intra-articular injection of rrIL-4 gave rise to a limited, but significant amelioration of cartilage destruction, prevention of loss of aggrecan, and decrease in the number of NT-positive chondrocytes, an effect that was not dose-dependent.

CONCLUSION

The present study suggests that IL-4 may exert chondroprotective properties against mechanical stress-induced cartilage destruction, at least in part, by inhibiting NO production by chondrocytes.

The chemical biology of nitric oxide--an outsider's reflections about its role in osteoarthritis

Excess formation of nitric oxide (NO) has been invoked in the development of osteoarthritis and blamed for triggering chondrocyte apoptosis and matrix destruction. Much of the evidence for a deleterious role of NO in disease progression has been obtained indirectly and inferred from the measurement of nitrite/nitrate and nitrotyrosine concentrations as well as iNOS expression in biopsy specimen, cartilage explants and cytokine-stimulated cells in culture. While these results clearly indicate an involvement of NO and suggest additional contributions from oxidative stress-related components they do not necessarily establish a cause/effect relationship. Many NO metabolites are not mere dosimeters of local NO production but elicit potent down-stream effects in their own right. Moreover, oxygen tension and other experimental conditions typical of many in vitro studies would seem to be at odds with the particular situation in the joint. Recent insight into the chemical biology of NO, in particular with regard to cellular redox-regulation, mitochondrial signaling and nitration reactions, attest to a much richer network of chemical transformations and interactions with biological targets than hitherto assumed. In conjunction with the emerging biology of nitrite and nitrate this information challenges the validity of the long-held view that "too much NO" is contributing to disease progression. Instead, it suggests that part of the problem is a shift from NO to superoxide-dominated chemistries triggering changes in thiol-dependent redox signaling, hypoxia-induced gene expression and mitochondrial function. This essay aims to provide a glimpse into research areas that may hold promise for future investigations into the underlying causes of osteoarthritis.

A compact computational model for cell construct development in perfusion culture

A problem nowadays tissue engineers encounter in developing sizable tissue implants is the nonuniform spread of cells and/or extracellular matrices. Research shows such a nutrients transport restriction may be improved by employing hydrodynamic culture systems. We propose a compact model for the simulation of cell growth in a porous construct under direct perfusion. Unlike the previous model proposed in the literature, which composes a cellular scaffold sandwiched between two culture media layers, the current model includes only the scaffold layer to simplify the mathematical and computational complex. Results show the present single-layer model can predict cell spreads and the nutrient and metabolic waste distribution as accurately as does the three-layer model. Only if the hydrodynamic aspects such as the pressure and viscous stress are prominent to know, should the more sophisticated analyses with the three-layer model be employed. The compact model provides comparable investigations for the tissue-engineering construct developments.

Reversal of suppressed metabolism in prolonged cold preserved cartilage

Chondrocytes in cold preserved cartilage are metabolically suppressed. The goal of this study was to address this metabolic suppression and seek ways to reverse it. Specifically, we examined the roles of rewarming protocols and nitric oxide (NO) in this metabolic suppression. Bovine and canine full-thickness articular cartilage explants were cultured under various temperature conditions, and NO production, proteoglycan (PG) synthesis, and cell viability were measured. Nitric oxide was shown to be negatively correlated with PG synthesis following abrupt rewarming of cold preserved osteochondral allografts. Gradual rewarming of the allograft tissue decreased NO production with higher PG synthesis. Inhibition of nitric oxide synthases (NOS) led to a decrease in NO production and a concomitant increase in PG synthesis. We were able to partially reverse metabolic suppression of cold preserved osteochondral allograft material with gradual rewarming and decrease NO production with NOS inhibition. Chondrocytes in cold preserved allograft material may be metabolically suppressed predisposing the graft to failure in vivo. Minimizing this loss of metabolic function by gradual graft rewarming and decreasing NO production by NOS inhibition at the time of graft implantation may have implications on graft survival in vivo.

Synovial fluid and peripheral blood immune complexes of patients with rheumatoid arthritis induce apoptosis in cytokine-activated chondrocytes

The destruction of cartilage is an important characteristic of rheumatoid arthritis (RA). Immune complexes (IC) are usually found in high amounts in RA synovial fluids (SF) and in the superficial layers of RA cartilage. The objective of this study was to investigate if IC have a direct influence on proliferation, survival and production of nitric oxide (NO) of cytokine-activated chondrocytes. Primary bovine chondrocytes were incubated with cytokines (huIL-1alpha, bovIFN-gamma, huTNF-alpha) and IC containing precipitates of peripheral blood (PB) and/or synovial fluid (SF) of 14 RA patients, 5 osteoarthritis (OA) patients and 10 healthy age and sex-matched controls. After 48 h, chondrocyte viability, proliferation, apoptosis, NO production and oxygen radical levels were measured. Staining with May-Grünwald-Giemsa after incubation with IC of RA PB and SF, showed apoptotic chondrocytes with condensation of the nuclei. The proliferation rates of cytokine-activated chondrocytes, incubated with sera and SF IC of RA patients were significantly decreased compared to chondrocytes, incubated with sera and SF IC of OA patients and compared to sera of controls. Quantitative evaluation of apoptotic cells by annexin-V/propidium iodide and TUNEL assays revealed a significant increase after incubation with sera and SF IC of RA patients, compared to control sera and OAs sera and SF. In all TUNEL positive samples, active-caspase-3-positive cells were found. There was a significant increase of chondrocyte NO production, after incubation with SF IC of RA patients, compared to OA SF. These results support the hypothesis that IC, present in serum and SF of RA patients, have a profound influence on chondrocyte growth, NO production and apoptosis, contributing to cartilage destruction in RA.

Anti-apoptotic Bcl-2 gene transfection of human articular chondrocytes protects against nitric oxide-induced apoptosis

We stably transfected early passage chondrocytes with an anti-apoptotic Bcl-2 gene in vitro using a retrovirus vector. Samples of articular cartilage were obtained from 11 patients with a mean age of 69 years (61 to 75) who were undergoing total knee replacement for osteoarthritis. The Bcl-2-gene-transfected chondrocytes were compared with non-transfected and lac-Z-gene-transfected chondrocytes, both of which were used as controls. All three groups of cultured chondrocytes were incubated with nitric oxide (NO) for ten days. Using the Trypan Blue exclusion assay, an enzyme-linked immunosorbent assay and flow cytometric analysis, we found that the number of apoptotic chondrocytes was significantly higher in the non-transfected and lac-Z-transfected groups than in the Bcl-2-transfected group (p < 0.05). The Bcl-2-transfected chondrocytes were protected from NO-induced impairment of proteoglycan synthesis.

We conclude that NO-induced chondrocyte death involves a mechanism which appears to be subject to regulation by an anti-apoptotic Bcl-2 gene. Therefore, Bcl-2 gene therapy may prove to be of therapeutic value in protecting human articular chondrocytes.

Mitochondrial proteomic characterization of human normal articular chondrocytes

OBJECTIVE

Mitochondrial dysfunctions have been associated with apoptosis, aging and osteoarthritis (OA). Chondrocyte mitochondrial proteins are attractive targets for the study of the metabolism of cartilage degradation. The copurification of "contaminating" proteins has been the major problem in all phases of mitochondrial proteome research. Therefore, we set up a procedure for the proteomic analysis of human chondrocyte mitochondrial proteins.

METHOD

Four types of protein extracts were obtained from primary cultured chondrocytes isolated from healthy donors: (1) initial total chondrocyte extract (CE), (2) cytosol-enriched supernatant fraction (CY), (3) crude mitochondria fraction (CM), and (4) pure mitochondria fraction (PM). Mitochondria were purified by density gradient ultracentrifugation. Mitochondrial proteins were separated by means of two-dimensional gel electrophoresis (2-DE) and silver stained. Protein spots were then identified by mass spectrometry using MALDI-TOF/TOF technology.

RESULTS

The best 2-DE reference map of mitochondrial proteome was constructed employing PM fraction. Thirty-nine percent of the identified proteins were functionally distributed in the mitochondria, 14% in the endoplasmic reticulum and 36% in the cytoplasm. Examining their biological function, 22% are involved in protein targeting, 12% in signaling, 12% in glycolysis, 10% in RNA, DNA or protein synthesis, 10% in oxidative phosphorylation and 4% in redox. The analysis of mitochondrial Mn-superoxide dismutase (SODM) revealed an age-dependent decrease of this protein.

CONCLUSION

PM fraction allowed the obtention of a high quality proteomic map for the study of mitochondrial proteins in human articular chondrocytes. This proteomic approach may be also efficient to analyze both quantitative and qualitative modulations of the mitochondrial proteome in human chondrocytes during aging and pathological conditions such as OA.

Effects of CNI-1493 on human granulocyte functions

During acute bacterial infections such as sepsis and meningitis, activation of inflammatory mediators such as nitric oxide (NO) plays a crucial role in both pathogenesis and host defense. We have previously reported that CNI-1493, a macrophage deactivator, reduced mortality in infant rats infected with Haemophilus influenzae type b (Hib) with associated decrease in the number of granulocytes in the infected tissue. The aim of the present study was to investigate how CNI-1493 affects granulocytes and macrophages in vitro. Murine macrophages (RAW 264.7) pre-incubated with CNI-1493 prior to activation with lipopolysaccharide (LPS)/interferon gamma (IFNgamma) had decreased NO production measured as NO(2)(-)/NO(3)(-) levels and reduction in inducible NO-synthase (iNOS) expression. Reactive oxygen species (ROS) production was increased in formylmethionyl-leucyl-phenylalanine (FMLP)-stimulated granulocytes following CNI-1493 treatment, whereas F-actin content, motility and chemotaxis were decreased under the same conditions. The effects of CNI-1493 on both NO production in LPS/IFNgamma-activated macrophages and ROS production, F-actin content, motility and chemotaxis in granulocytes, may contribute to the reduced inflammatory response and increased survival in Hib-infected animals treated with CNI-1493.

Computational study of culture conditions and nutrient supply in cartilage tissue engineering

Different culture conditions for cartilage tissue engineering were evaluated with respect to the supply of oxygen and glucose and the accumulation of lactate. A computational approach was adopted in which the culture configurations were modeled as a batch process and transport was considered within constructs seeded at high cell concentrations and of clinically relevant dimensions. To assess the extent to which mass transfer can be influenced theoretically, extreme cases were distinguished in which the culture medium surrounding the construct was assumed either completely static or well mixed and fully oxygenated. It can be concluded that severe oxygen depletion and lactate accumulation can occur within constructs for cartilage tissue engineering. However, the results also indicate that transport restrictions are not insurmountable, providing that the medium is well homogenized and oxygenated and the construct's surfaces are sufficiently exposed to the medium. The large variation in uptake rates of chondrocytes indicates that for any specific application the quantification of cellular utilization rates, depending on the cell source and culture conditions, is an essential starting point for optimizing culture protocols.

Differential effects of hyperosmotic challenge on interleukin-1-activated pathways in bovine articular cartilage

Chondrocytes in situ experience fluctuations in extracellular osmolarity resulting from mechanical loading. The objective of this study was to determine whether hyperosmotic stress causes or exacerbates interleukin-1 (IL-1)-mediated effects in bovine articular cartilage. Disks of cartilage cut from the articular surface of calf radiocarpal joints were incubated for 24h in the presence or absence of IL-1 in Dulbecco's modified Eagle's medium adjusted to various osmolalities with sucrose or NaCl. Cyclooxygenase (COX)-2 levels in the cartilage were examined by Western blot. Culture media were assayed for prostaglandin E(2) (PGE(2)), nitrite as an indicator of nitric oxide (NO) production, and sulfated glycosaminoglycan as an indicator of proteoglycan degradation. We report the osmolality-dependent potentiation of COX-2 and PGE(2) production, and the osmolality-dependent inhibition of NO production and proteoglycan degradation in IL-1-activated cartilage. The data demonstrate that osmotic and cytokine signaling interact to differentially modulate IL-1-stimulated effects in calf articular cartilage.

Nutrient Utilization by Bovine Articular Chondrocytes: A Combined Experimental and Theoretical Approach

A combined experimental-numerical approach was adopted to characterize glucose and oxygen uptake and lactate production by bovine articular chondrocytes in a model system. For a wide range of cell concentrations, cells in agarose were supplemented with either low or high glucose medium. During an initial culture phase of 48h, oxygen was monitored noninvasively using a biosensor system. Glucose and lactate were determined by medium sampling. In order to quantify glucose and oxygen uptake, a finite element approach was adopted to describe diffusion and uptake in the experimental model. Numerical predictions of lactate, based on simple relations for cell metabolism, were found to agree well for low glucose, but not for high glucose medium. Oxygen did not play a role in either case. Given the close association between chondrocyte energy metabolism and matrix synthesis, a quantifiable prediction of utilization can present a valuable contribution in the optimization of tissue engineering conditions.

Effect of nitric oxide on mitochondrial respiratory activity of human articular chondrocytes

OBJECTIVE

To investigate the effect of nitric oxide (NO) on mitochondrial activity and its relation with the apoptosis of human articular chondrocytes.

MATERIALS AND METHODS

Mitochondrial function was evaluated by analysing respiratory chain enzyme complexes, citrate synthase (CS) activities, and mitochondrial membrane potential (Delta psi m). The activities of the mitochondrial respiratory chain (MRC) complexes (complex I: NADH CoQ(1) reductase, complex II: succinate dehydrogenase, complex III: ubiquinol cytochrome c reductase, complex IV: cytochrome c oxidase) and CS were measured in human articular chondrocytes isolated from normal cartilage. The Delta psi m was measured by 5,5',6,6'-tetracholoro-1,1',3,3'-tetraethylbenzimidazole carbocyanide iodide (JC-1) using flow cytometry. Apoptosis was analysed by flow cytometry. The mRNA expression of caspases was analysed by ribonuclease protection analysis and the detection of protein synthesis by western blotting. Sodium nitroprusside (SNP) was used as an NO compound donor.

RESULTS

SNP at concentrations higher than 0.5 mmol/l for 24 hours induced cellular changes characteristic of apoptosis. SNP elicited mRNA expression of caspase-3 and caspase-7 and down regulated bcl-2 synthesis in a dose and time dependent manner. Furthermore, 0.5 mM SNP induced depolarisation of the mitochondrial membrane at 5, 12, and 24 hours. Analysis of the MRC showed that at 5 hours, 0.5 mM SNP reduced the activity of complex IV by 33%. The individual inhibition of mitochondrial complex IV with azide modified the Delta psi m and induced apoptosis.

CONCLUSIONS

This study suggests that the effect of NO on chondrocyte survival is mediated by its effect on complex IV of the MRC.

Nitric oxide decreases IGF-1 receptor function in vitro; glutathione depletion enhances this effect in vivo

Insulin-like growth factor-1 (IGF) helps maintain healthy articular cartilage; however, arthritic cartilage becomes less responsive to the anabolic actions of IGF. We previously showed that high concentrations of nitric oxide (NO) decrease IGF receptor tyrosine phosphorylation and response to IGF in intact chondrocytes. The current studies evaluate direct effects of NO on IGF receptor kinase (IGF-RK) in vitro. NO from S-nitroso-N-acetyl-d,l-penicillamine (SNAP) or 1-hydroxy-2-oxo-3-(N-3-methyl-aminopropyl)-3-methyl-1-triazene (NOC-7) inhibits IGF-RK auto- and substrate phosphorylation in a dose and time dependent manner. There is a linear correlation between inhibition of auto- and substrate phosphorylation (r(2)=0.98). Increasing either dithiothreitol or reduced glutathione (GSH) content of the phosphorylation buffer to protect thiol groups blocks NO inhibition of IGF-RK substrate phosphorylation. Increased S-nitrosylation of cysteines in IGF-RK after exposure to SNAP suggests that NO may react with sulfhydryl groups, form S-nitrosothiols, which may result in functional modifications. NO blockade of IGF-1 stimulated proteoglycan synthesis in intact cells is enhanced when chondrocyte glutathione is depleted. The in vitro system shows that there can be direct effects of NO on IGF-RK that modify receptor function; the intact cell studies suggest that the mechanisms identified in vitro may be important in intact chondrocyte insensitivity to IGF-1 in cells exposed to NO.

Mediation of spontaneous knee osteoarthritis by progressive chondrocyte ATP depletion in Hartley guinea pigs

OBJECTIVE

Because articular chondrocytes reside in a hypoxic milieu, anaerobic glycolysis is central in generating ATP to support chondrocyte matrix synthesis and viability, with mitochondrial oxidative phosphorylation possibly providing physiologic reserve ATP generation. Nitric oxide (NO) potently suppresses mitochondrial oxidative phosphorylation. Because enhanced cartilage NO generation occurs in osteoarthritis (OA), we systematically tested for mitochondrial dysfunction in the pathogenesis of OA.

METHODS

We assessed chondrocytes for ATP depletion and for in situ changes in mitochondrial ultrastructure prior to and during the evolution of spontaneous knee OA in male Hartley guinea pigs, a model in which chondrocalcinosis also supervenes.

RESULTS

Spontaneous NO release from knee cartilage samples in organ culture doubled between ages 2 months and 8 months as knee OA developed. Concomitantly, chondrocyte intracellular ATP levels declined by approximately 50%, despite a lack of mitochondrial ultrastructure abnormalities in knee chondrocytes. As ATP depletion progressed with aging in knee chondrocytes, an increased ratio of lactate to pyruvate was observed, consistent with an adaptive augmentation of glycolysis to mitochondrial dysfunction. Furthermore, we observed progressive elevation of chondrocyte ATP-scavenging nucleotide pyrophosphatase/phosphodiesterase (NPP) activity and extracellular levels of the NPP enzymatic end product inorganic pyrophosphate (PPi), which stimulate chondrocalcinosis.

CONCLUSION

Profound chondrocyte ATP depletion develops in association with heightened NO generation in guinea pig knee OA. Increased NPP activity and concordant increases in extracellular PPi, which are strongly associated with human aging-associated degenerative arthropathy and directly stimulate chondrocalcinosis, may be primarily driven by chondrocyte ATP depletion. Our findings implicate a decreased mitochondrial bioenergetic reserve as a pathogenic factor in both degenerative arthropathy and chondrocalcinosis in aging.

Critical O2 and NO concentrations in NO-induced cell death in a rat liver sinusoidal endothelial cell line

Nitric oxide (NO) plus oxygen (O2) are known to cause cell damage via formation of reactive nitrogen species. NO itself directly inhibits cytochrome oxidase of the mitochondrial respiratory chain in competition with O2, thus inducing a hypoxic-like injury. To assess the critical NO and O2 concentrations for both mechanisms of NO-induced cell injury, cells of a rat liver sinusoidal endothelial cell line were incubated in the presence of the NO donor spermineNONOate at different O2 concentrations, and their loss of viability was determined by the release of lactate dehydrogenase. Protection by ascorbic acid was used as indication for the involvement of reactive nitrogen species, whereas a hypoxic-like injury was indicated by the protective effects of glycine and glucose and the increase in NAD(P)H fluorescence. High concentrations of NO (approx. 10 microM NO) and O2 (21% O2) were required to induce endothelial cell death mediated by formation of reactive nitrogen species. On the other hand, pathophysiologically relevant NO concentrations at low but physiological O2 concentrations (ca. 2 microM NO at 5% O2 and about 1 microM NO at 2% O2) induced hypoxic-like cell death in the endothelial cells that was prevented by the presence of glucose.

Influence of pro-inflammatory (IL-1 alpha, IL-6, TNF-alpha, IFN-gamma) and anti-inflammatory (IL-4) cytokines on chondrocyte function

OBJECTIVE

Cytokines produced by inflammatory cells play a pivotal role in synovial inflammation and joint destruction in rheumatoid arthritis. To investigate the influence of pro-inflammatory cytokines (IL-1 alpha, IL-6, TNF-alpha, IFN-gamma) and subsequently the possible beneficial role of an anti-inflammatory cytokine (IL-4) on chondrocyte viability (necrosis/apoptosis), proliferation and nitric oxide (NO) production.

METHODS

Primary bovine chondrocytes were cultured until monolayers were obtained. Cells were incubated with cytokines (IL-1 alpha, IFN-gamma, TNF-alpha, IL-4, IL-6) at 0.1, 1, 10 and 100 ng/mL. After 48 h, the viability of the chondrocytes was measured flow cytometrically with propidium iodide. Proliferation was determined by the incorporation of tritiated thymidine. The morphology of the chondrocytes, including presence of apoptotic nuclei, was evaluated by a May-Grünwald-Giemsa staining. In addition, the number of apoptotic chondrocytes was detected flow cytometrically with a TUNEL technique and annexin-V/propidium iodide staining. NO production was evaluated using a spectrophotometric assay, based upon the Griess reaction.

RESULTS

The viability and proliferation of bovine chondrocytes decreased after incubation with 100 ng/mL IL-1 alpha, TNF-alpha or IFN-gamma. In contrast, incubation of chondrocytes with IL-4 or IL-6 had no influence on the viability or the proliferation of cells. IL-1 alpha was able to enhance NO production in a dose dependent manner. IFN-gamma and TNF-alpha induced NO production only at the highest concentration (100 ng/mL), whereas IL-4 and IL-6 did not. There was a dose dependent increase in apoptosis of bovine chondrocytes cultured in the presence of IL-1 alpha and TNF-alpha. This effect could not be prevented by preincubation with IL-4. Preincubation with IL-4 diminished IL-1 alpha and TNF-alpha induced NO production and increased proliferation of chondrocytes. In an additional experiment, incubation of human chondrocytes with anti-Fas did not induce apoptosis as measured by annexin-V/propidium iodide staining.

CONCLUSIONS

Pro-inflammatory cytokines are able to induce apoptosis, whereas IL-4 as an anti-inflammatory cytokine can inhibit the effect of IL-1 alpha and TNF-alpha on NO production and proliferation of bovine chondrocytes.

Mitochondrial respiratory activity is altered in osteoarthritic human articular chondrocytes

OBJECTIVE

Osteoarthritis (OA) is a degenerative rheumatic disease that is associated with extracellular matrix degradation and chondrocyte apoptosis in the articular cartilage. The role of mitochondria in degenerative diseases is widely recognized. We undertook this study to evaluate mitochondrial function in normal and OA chondrocytes and to examine age-related changes in mitochondria.

METHODS

Mitochondrial function was evaluated by analyzing respiratory chain enzyme complexes and citrate synthase (CS) activities as well as changes in mitochondrial membrane potential (Delta Psi m). The activities of mitochondrial respiratory chain complexes (complex I: rotenone-sensitive NADH-coenzyme Q(1) reductase; complex II: succinate dehydrogenase; complex III: antimycin-sensitive ubiquinol cytochrome c reductase; and complex IV: cytochrome c oxidase) and CS were measured in human articular chondrocytes isolated from OA and normal cartilage. Delta Psi m was measured by JC-1 using flow cytometry. Statistical analysis was performed using the Mann-Whitney U test and Student's t-test as well as several models of multiple linear regression.

RESULTS

OA articular chondrocytes had reduced activities of complexes II and III compared with cells from normal cartilage. However, the mitochondrial mass was increased in OA. Cultures of OA chondrocytes contained a higher proportion of cells with de-energized mitochondria. We found no relationship between mitochondrial function and donor age either in normal or in OA chondrocytes.

CONCLUSION

These findings suggest the involvement of mitochondrial function in the pathophysiology of OA. Cartilage degradation by OA and cartilage aging may be two different processes.

Effects of an adenosine kinase inhibitor and an adenosine deaminase inhibitor on accumulation of extracellular adenosine by equine articular chondrocytes

OBJECTIVE

To investigate accumulation of extracellular adenosine (ADO) by equine articular chondrocytes and to compare effects of adenosine kinase inhibition and adenosine deaminase inhibition on the amount of nitric oxide (NO) produced by lipopolysaccharide (LPS)-stimulated chondrocytes.

SAMPLE POPULATION

Articular cartilage from metacarpophalangeal and metatarsophalangeal joints of 14 horses.

PROCEDURE

Chondrocytes were cultured as monolayers, and cells were incubated with LPS, the adenosine kinase inhibitor 5'-iodotubercidin (ITU), or the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA). Concentrations of ADO in cell supernatants were measured by use of reverse-phase high-performance liquid chromatography. Effect of inhibition of enzymatic metabolism of ADO on induced NO production was evaluated by exposing cells to a combination of LPS and ITU or LPS and EHNA.

RESULTS

Articular chondrocytes accumulated extracellular ADO when exposed to LPS or ITU. Chondrocytes exposed to ITU accumulated ADO in a time-dependent manner. Unstimulated chondrocytes did not accumulate ADO. Similarly, EHNA alone did not produce detectable ADO concentrations; however, addition of EHNA and ITU resulted in a synergistic effect on accumulation of ADO. Lipopolysaccharide-induced NO production was more effectively suppressed by exposure to ITU than to EHNA CONCLUSIONS AND CLINICAL RELEVANCE: Equine articular chondrocytes release ADO in response to the proinflammatory stimulus of bacterial LPS. Inhibition of the metabolism of ADO increases accumulation of extracellular ADO. Autocrine release of ADO from chondrocytes may play a role in the cellular response to tissue damage in arthritic conditions, and pharmacologic modulation of these pathways in joints of arthritic horses could be a potential method of therapy.

Nitric oxide-mediated chondrocyte cell death requires the generation of additional reactive oxygen species

OBJECTIVE

Chondrocyte cell death, possibly related to increased production of endogenous nitric oxide (NO), has been observed during the pathogenesis of osteoarthritis and rheumatoid arthritis. The purpose of this study was to investigate the potential role of NO in causing chondrocyte cell death and to determine the contribution of other reactive oxygen species (ROS).

METHODS

Cell death and cytotoxicity were evaluated in human articular chondrocytes in response to various NO donor compounds with and without agents that stimulate or inhibit the production of additional ROS using both the alginate bead and the monolayer culture systems. Cell death was quantified by a total cell count with fluorescent labels, and cytotoxicity was measured as a function of cellular NADH- and NADPH-dependent dehydrogenase activity. To determine if the redox status of the chondrocyte could influence the observed effect of NO, cells were preincubated for 24 hours in L-cystine- and glutathione (GSH)-depleted media to reduce intracellular GSH levels, a major defense mechanism against oxidative stress. Apoptosis was analyzed with the quantification of histone-associated DNA fragments.

RESULTS

Treatment of chondrocytes with peroxynitrite (ONOO-), 3-morpholinosydnonimine (SIN-1), and sodium nitroprusside (SNP) resulted in apoptotic cell death at concentrations of 0.5 mM, 1.0 mM, and 0.5 mM, respectively. In contrast, treatment of chondrocytes with diazeniumdiolates (or the "NOC" compounds, NOC-5 and NOC-12) at concentrations as high as 2.0 mM did not cause cell death. Furthermore, NOC-5 and NOC-12, at all concentrations tested (0.125-2.0 mM), could prevent cell death caused by oxidative stress. Selective ROS scavengers protected against cell death caused by either SIN-1 or ONOO-; however, no protection could be afforded against the cytotoxicity of SNP with any of the ROS scavengers tested.

CONCLUSION

These results show that NO by itself is not cytotoxic to cultured chondrocytes and can even be protective under certain conditions of oxidative stress. Chondrocyte cell death from NO occurs under conditions where other ROS are also generated.

The role of nitric oxide in tissue destruction

Nitric oxide (NO) is synthesized via the oxidation of arginine by a family of nitric oxide synthases (NOS), which are either constitutive (ie. endothelial (ec)NOS and neuronal (nc)NOS) or inducible (iNOS). The production of nitric oxide plays a vital role in the regulation of physiological processes, host defence, inflammation and immunity. Pro-inflammatory effects include vasodilation, oedema, cytotoxicity and the mediation of cytokine-dependent processes that can lead to tissue destruction. Nitric oxide-dependent tissue injury has been implicated in a variety of rheumatic diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis and osteoarthritis. Conversely, the production of NO by endothelial cell NOS may serve a protective, or anti-inflammatory, function by preventing the adhesion and release of oxidants by activated neutrophils in the microvasculature. In this chapter we describe the multifaceted role of nitric oxide in inflammation and address the potential therapeutic implications of NOS inhibition.

Nitric oxide and inflammatory mediators in the perpetuation of osteoarthritis

Articular chondrocyte production of nitric oxide (NO) and other inflammatory mediators, such as eicosanoids and cytokines, are increased in human osteoarthritis. The excessive production of nitric oxide inhibits matrix synthesis and promotes its degradation. Furthermore, by reacting with oxidants such as superoxide anion, nitric oxide promotes cellular injury and renders the chondrocyte susceptible to cytokine-induced apoptosis. PGE(2) exerts anabolic and catabolic effects on chondrocytes, depending on the microenvironment and physiologic condition. The increased expression of inducible NOS (iNOS) and cyclo-oxygenase-2 (COX-2) in OA chondrocytes is largely due to the increased expression of pro-inflammatory cytokines, particularly IL-1, which act in an autocrine/paracrine fashion to perpetuate a catabolic state that leads to progressive destruction of articular cartilage. The initiating factors for the production of inflammatory mediators include altered biomechanical forces; their continued production may be augmented by an increase in extracellular matrix proteins acting through ligation of surface integrins.