New Insights into the Pro-Inflammatory and Osteoclastogenic Profile of Circulating Monocytes in Osteoarthritis Patients.

Osteoarthritis (OA) is a degenerative condition of the articular cartilage with chronic low-grade inflammation. Monocytes have a fundamental role in the progression of OA, given their implication in inflammatory responses and their capacity to differentiate into bone-resorbing osteoclasts (OCLs). This observational-experimental study attempted to better understand the molecular pathogenesis of OA through the examination of osteoclast progenitor (OCP) cells from both OA patients and healthy individuals (25 OA patients and healthy samples). The expression of osteoclastogenic and inflammatory genes was analyzed using RT-PCR. The OA monocytes expressed significantly higher levels of CD16, CD115, TLR2, Mincle, Dentin-1, and CCR2 mRNAs. Moreover, a flow cytometry analysis showed a significantly higher surface expression of the CD16 and CD115 receptors in OA vs. healthy monocytes, as well as a difference in the distribution of monocyte subsets. Additionally, the OA monocytes showed a greater osteoclast differentiation capacity and an enhanced response to an inflammatory stimulus. The results of this study demonstrate the existence of significant differences between the OCPs of OA patients and those of healthy subjects. These differences could contribute to a greater understanding of the molecular pathogenesis of OA and to the identification of new biomarkers and potential drug targets for OA.

Connection between Mesenchymal Stem Cells Therapy and Osteoclasts in Osteoarthritis.

The use of mesenchymal stem cells constitutes a promising therapeutic approach, as it has shown beneficial effects in different pathologies. Numerous in vitro, pre-clinical, and, to a lesser extent, clinical trials have been published for osteoarthritis. Osteoarthritis is a type of arthritis that affects diarthritic joints in which the most common and studied effect is cartilage degradation. Nowadays, it is known that osteoarthritis is a disease with a very powerful inflammatory component that affects the subchondral bone and the rest of the tissues that make up the joint. This inflammatory component may induce the differentiation of osteoclasts, the bone-resorbing cells. Subchondral bone degradation has been suggested as a key process in the pathogenesis of osteoarthritis. However, very few published studies directly focus on the activity of mesenchymal stem cells on osteoclasts, contrary to what happens with other cell types of the joint, such as chondrocytes, synoviocytes, and osteoblasts. In this review, we try to gather the published bibliography in relation to the effects of mesenchymal stem cells on osteoclastogenesis. Although we find promising results, we point out the need for further studies that can support mesenchymal stem cells as a therapeutic tool for osteoclasts and their consequences on the osteoarthritic joint.

Extracellular Vesicles Do Not Mediate the Anti-Inflammatory Actions of Mouse-Derived Adipose Tissue Mesenchymal Stem Cells Secretome.

Adipose tissue represents an abundant source of mesenchymal stem cells (MSC) for therapeutic purposes. Previous studies have demonstrated the anti-inflammatory potential of adipose tissue-derived MSC (ASC). Extracellular vesicles (EV) present in the conditioned medium (CM) have been shown to mediate the cytoprotective effects of human ASC secretome. Nevertheless, the role of EV in the anti-inflammatory effects of mouse-derived ASC is not known. The current study has investigated the influence of mouse-derived ASC CM and its fractions on the response of mouse-derived peritoneal macrophages against lipopolysaccharide (LPS). CM and its soluble fraction reduced the release of pro-inflammatory cytokines, adenosine triphosphate and nitric oxide in stimulated cells. They also enhanced the migration of neutrophils or monocytes, in the absence or presence of LPS, respectively, which is likely related to the presence of chemokines, and reduced the phagocytic response. The anti-inflammatory effect of CM may be dependent on the regulation of toll-like receptor 4 expression and nuclear factor-κB activation. Our results demonstrate the anti-inflammatory effects of mouse-derived ASC secretome in mouse-derived peritoneal macrophages stimulated with LPS and show that they are not mediated by EV.

Microvesicles from Human Adipose Tissue-Derived Mesenchymal Stem Cells as a New Protective Strategy in Osteoarthritic Chondrocytes.

BACKGROUND/AIMS: Chronic inflammation contributes to cartilage degeneration during the progression of osteoarthritis (OA). Adipose tissue-derived mesenchymal stem cells (AD-MSC) show great potential to treat inflammatory and degradative processes in OA and have demonstrated paracrine effects in chondrocytes. In the present work, we have isolated and characterized the extracellular vesicles from human AD-MSC to investigate their role in the chondroprotective actions of these cells. METHODS: AD-MSC were isolated by collagenase treatment from adipose tissue from healthy individuals subjected to abdominal lipectomy surgery. Microvesicles and exosomes were obtained from conditioned medium by filtration and differential centrifugation. Chondrocytes from OA patients were used in primary culture and stimulated with 10 ng/ml interleukin(IL)-1ß in the presence or absence of AD-MSC microvesicles, exosomes or conditioned medium. Protein expression was investigated by ELISA and immunofluorescence, transcription factor-DNA binding by ELISA, gene expression by real-time PCR, prostaglandin E2 (PGE2) by radioimmunoassay, and matrix metalloproteinase (MMP) activity and nitric oxide (NO) production by fluorometry. RESULTS: In OA chondrocytes stimulated with IL-1ß, microvesicles and exosomes reduced the production of inflammatory mediators tumor necrosis factor-α, IL-6, PGE2 and NO. The downregulation of cyclooxygenase-2 and microsomal prostaglandin E synthase-1 would lead to the decreased PGE2 production while the effect on NO could depend on the reduction of inducible nitric oxide synthase expression. Treatment of OA chondrocytes with extracellular vesicles also decreased the release of MMP activity and MMP-13 expression whereas the production of the anti-inflammatory cytokine IL-10 and the expression of collagen II were significantly enhanced. The reduction of inflammatory and catabolic mediators could be the consequence of a lower activation of nuclear factor-κB and activator protein-1. The upregulation of annexin A1 specially in MV may contribute to the anti-inflammatory and chondroprotective effects of AD-MSC. CONCLUSIONS: Our data support the interest of AD-MSC extracellular vesicles to develop new therapeutic approaches in joint conditions.

Paracrine in vivo inhibitory effects of adipose tissue-derived mesenchymal stromal cells in the early stages of the acute inflammatory response.

BACKGROUND AIMS: Excessive or unresolved inflammation leads to tissue lesions. Adipose tissue-derived mesenchymal stromal cells (AMSCs) have shown protective effects that may be dependent on the modulation of inflammation by secreted factors. METHODS: We used the zymosan-induced mouse air pouch model at two time points (4 h and 18 h) to evaluate the in vivo effects of AMSCs and their conditioned medium (CM) on key steps of the early inflammatory response. We assessed the effects of AMSCs and CM on leukocyte migration and myeloperoxidase activity. The levels of chemokines, cytokines and eicosanoids in exudates were measured by use of enzyme-linked immunoassay or radio-immunoassay. In addition, the expression of cyclooxygenase-2 and microsomal prostaglandin E synthase-1 (mPGES-1) was studied by use of Western blotting and the phosphorylation of p65 nuclear factor-κB (NF-κB) by immunofluorescence. RESULTS: All inflammatory parameters were significantly reduced by CM and AMSCs to a similar extent at 4 h after zymosan injection with lower effects at 18 h. The observed inhibition of leukocyte migration was associated with reduced levels of chemokines and leukotriene B4. Interleukin-1ß, interleukin-6, tumor necrosis factor-α and tumor necrosis factor-stimulated gene 6 levels were significantly decreased. The downregulation of mPGES-1 was associated with inhibition of prostaglandin E2 production. Our results suggest that these anti-inflammatory effects are related, in part, to the inhibition of NF-κB activation. CONCLUSIONS: AMSCs dampen the early process of inflammation in the zymosan-induced mouse air pouch model through paracrine mechanisms. These results support the potential utility of these cells as a source of novel treatment approaches for inflammatory pathologies.

Haem oxygenase-1 induction reverses the actions of interleukin-1ß on hypoxia-inducible transcription factors and human chondrocyte metabolism in hypoxia.

HO-1 (haem oxygenase-1) catalyses the degradation of haem and possesses anti-inflammatory and cytoprotective properties. The role of inflammatory mediators in the pathogenesis of OA (osteoarthritis) is becoming increasingly appreciated. In the present study, we investigated the effects of HO-1 induction in OA and healthy HACs (human articular chondrocytes) in response to inflammatory cytokine IL-1 ß (interleukin-1ß) under hypoxic conditions. Hypoxia was investigated as it is a more physiological condition of the avascular cartilage. Hypoxic signalling is mediated by HIFs (hypoxia-inducible factors), of which there are two main isoforms, HIF-1α and HIF-2α. Normal and OA chondrocytes were stimulated with IL-1ß. This cytokine suppresses HO-1 expression and exerts both catabolic and anti-anabolic effects, while increasing HIF-1α and suppressing HIF-2α protein levels in OA chondrocytes in hypoxia. Induction of HO-1 by CoPP (cobalt protoporphyrin IX) reversed these IL-1ß actions. The hypoxia-induced anabolic pathway involving HIF-2α, SOX9 [SRY (sex determining region Y)-box 9] and COL2A1 (collagen type II α1) was suppressed by IL-1ß, but importantly, levels were restored by HO-1 induction, which down-regulated TNFα (tumour necrosis factor α), MMP (matrix metalloproteinase) activity and MMP-13 protein levels. Depletion of HO-1 using siRNA (small interfering RNA) abolished the CoPP effects, further demonstrating that these were due to HO-1. The results of the present study reveal the different mechanisms by which HO-1 exerts protective effects on chondrocytes in physiological levels of hypoxia.

Conditioned media from adipose-tissue-derived mesenchymal stem cells downregulate degradative mediators induced by interleukin-1ß in osteoarthritic chondrocytes.

Osteoarthritis (OA) is the most frequent joint disorder and an important cause of disability. Recent studies have shown the potential of adipose-tissue-derived mesenchymal stem cells (AD-MSC) for cartilage repair. We have investigated whether conditioned medium from AD-MSC (CM) may regulate in OA chondrocytes a number of key mediators involved in cartilage degeneration. CM enhanced type II collagen expression in OA chondrocytes while decreasing matrix metalloproteinase (MMP) activity in cell supernatants as well as the levels of MMP-3 and MMP-13 proteins and mRNA in OA chondrocytes stimulated with interleukin- (IL-) 1ß. In addition, CM increased IL-10 levels and counteracted the stimulating effects of IL-1ß on the production of tumor necrosis factor-α, IL-6, prostaglandin E2, and NO measured as nitrite and the mRNA expression of these cytokines, CCL-2, CCL-3, CCL-4, CCL-5, CCL-8, CCL-19, CCL-20, CXCL-1, CXCL-2, CXCL-3, CXCL-5, CXCL-8, cyclooxygenase-2, microsomal prostaglandin E synthase-1, and inducible NO synthase. These effects may be dependent on the inhibition of nuclear factor-κB activation by CM. Our data demonstrate the chondroprotective actions of CM and provide support for further studies of this approach in joint disease.

Haem oxygenase-1 counteracts the effects of interleukin-1ß on inflammatory and senescence markers in cartilage-subchondral bone explants from osteoarthritic patients.

IL (interleukin)-1ß plays an important role in cartilage extracellular matrix degradation and bone resorption in OA (osteoarthritis) through the induction of degradative enzymes and pro-inflammatory mediators. In the present study, we have determined the consequences of HO-1 (haem oxygenase-1) induction on markers of inflammation and senescence in the functional unit cartilage-subchondral bone stimulated with IL-1ß. Cartilage-subchondral bone specimens were obtained from the knees of osteoarthritic patients. Treatment with the HO-1 inducer CoPP (cobalt protoporphyrin IX) counteracted the stimulatory effects of IL-1ß on IL-6, nitrite, PGE2 (prostaglandin E2), TGF (transforming growth factor) ß2, TGFß3 and osteocalcin. Immunohistochemical analyses indicated that CoPP treatment of explants down-regulated iNOS (inducible nitric oxide synthase), COX-2 (cyclooxygenase-2) and mPGES-1 (microsomal prostaglandin E synthase-1) induced by IL-1ß. In contrast, the expression of HMGB1 (high-mobility group box 1) was not significantly modified. In addition, CoPP decreased the expression of iNOS and mPGES-1 in cells isolated from the explants and stimulated with IL-1ß, which was counteracted by an siRNA (small interfering RNA) specific for human HO-1. In isolated primary chondrocytes, we determined senescence-associated ß-galactosidase activity and the expression of senescence markers by real-time PCR. We have found that HO-1 induction could regulate senescence markers in the presence of IL-1ß and significantly affected telomerase expression, as well as ß-galactosidase activity and hTERT (human telomerase reverse transcriptase) and p21 expression in chondrocytes. The findings of the present study support the view that HO-1 induction results in the down-regulation of inflammatory and senescence responses in OA articular tissues.

Cellular and Molecular Targets of Extracellular Vesicles from Mesenchymal Stem/Stromal Cells in Rheumatoid Arthritis.

Rheumatoid arthritis (RA) is a chronic autoimmune disease that causes progressive joint destruction. Despite the advances in the treatment of this condition there remains a clinical need for safe therapies leading to clinical remission. Mesenchymal stem/stromal cells (MSCs) play immunomodulatory and regenerative roles which can be partly mediated by their secretome. In recent years, the important contribution of extracellular vesicles (EVs) to MSC actions has received an increasing interest as a new therapeutic approach. We provide an extensive overview of the immunomodulatory properties of MSC EVs and their effects on articular cells such as fibroblast-like synoviocytes that play a central role in joint destruction. This review discusses the anti-arthritic effects of MSC EVs in vitro and in animal models of RA as well as their potential mechanisms. Recent preclinical data suggest that transfer of non-coding RNAs by MSC EVs regulates key signaling pathways involved in the pathogenesis of RA. We also examine a number of EV modifications for improving their anti-arthritic efficacy and carrier ability for drug delivery.

Evaluation of Extracellular Vesicles from Adipose Tissue-Derived Mesenchymal Stem Cells in Primary Human Chondrocytes from Patients with Osteoarthritis.

Adipose tissue-derived mesenchymal stem cells (AD-MSCs) offer great therapeutic potential for osteoarthritis (OA) treatment. Recent investigations have revealed the contribution of extracellular vesicles (EVs) to AD-MSC actions. Here, we describe a method to study the in vitro effects of EVs from AD-MSCs in OA chondrocytes. This chapter includes the isolation and analysis of human AD-MSCs and their EVs as well as the isolation and treatment of OA chondrocytes.

Role of peroxiredoxin 6 in the chondroprotective effects of microvesicles from human adipose tissue-derived mesenchymal stem cells.

BACKGROUND: Osteoarthritis (OA) is a joint disease characterized by cartilage degradation, low-grade synovitis and subchondral bone alterations. In the damaged joint, there is a progressive increase of oxidative stress leading to disruption of chondrocyte homeostasis. The modulation of oxidative stress could control the expression of inflammatory and catabolic mediators involved in OA. We have previously demonstrated that extracellular vesicles (EVs) present in the secretome of human mesenchymal stem cells from adipose tissue (AD-MSCs) exert anti-inflammatory and anti-catabolic effects in OA chondrocytes. In the current work, we have investigated whether AD-MSC EVs could regulate oxidative stress in OA chondrocytes as well as the possible contribution of peroxiredoxin 6 (Prdx6). METHODS: Microvesicles (MV) and exosomes (EX) were isolated from AD-MSC conditioned medium by differential centrifugation with size filtration. The size and concentration of EVs were determined by resistive pulse sensing. OA chondrocytes were isolated from knee articular cartilage of advanced OA patients. 4-Hydroxynonenal adducts, IL-6 and MMP-13 were determined by enzyme-linked immunosorbent assay. Expression of Prdx6 and autophagic markers was assessed by immunofluorescence and Western blotting. Prdx6 was downregulated in AD-MSCs by transfection with a specific siRNA. RESULTS: MV and to a lesser extent EX significantly reduced the production of oxidative stress in OA chondrocytes stimulated with IL-1ß. Treatment with MV resulted in a dramatic upregulation of Prdx6. MV also enhanced the expression of autophagy marker LC3B. We downregulated Prdx6 in AD-MSCs by using a specific siRNA and then MV were isolated. These Prdx6-silenced MV failed to modify oxidative stress and the expression of autophagy markers. We also assessed the possible contribution of Prdx6 to the effects of MV on IL-6 and MMP-13 production. The reduction in the levels of both mediators induced by MV was partly reverted after Prdx6 silencing. CONCLUSION: Our results indicate that EVs from AD-MSCs regulate the production of oxidative stress in OA chondrocytes during inflammation. Prdx6 may mediate the antioxidant and protective effects of MV.The translational potential of this article: This study gives insight into the protective properties of EVs from AD-MSCs in OA chondrocytes. Our findings support the development of novel therapies based on EVs to prevent or treat cartilage degradation.

Extracellular Vesicles from Mesenchymal Stem Cells as Novel Treatments for Musculoskeletal Diseases.

Mesenchymal stem/stromal cells (MSCs) represent a promising therapy for musculoskeletal diseases. There is compelling evidence indicating that MSC effects are mainly mediated by paracrine mechanisms and in particular by the secretion of extracellular vesicles (EVs). Many studies have thus suggested that EVs may be an alternative to cell therapy with MSCs in tissue repair. In this review, we summarize the current understanding of MSC EVs actions in preclinical studies of (1) immune regulation and rheumatoid arthritis, (2) bone repair and bone diseases, (3) cartilage repair and osteoarthritis, (4) intervertebral disk degeneration and (5) skeletal muscle and tendon repair. We also discuss the mechanisms underlying these actions and the perspectives of MSC EVs-based strategies for future treatments of musculoskeletal disorders.

Emerging therapeutic agents in osteoarthritis.

Osteoarthritis (OA) is the most common joint disorder and a leading cause of disability. Current treatments for OA can improve symptoms but do not delay the progression of disease. In the last years, much effort has been devoted to developing new treatments for OA focused on pain control, inflammatory mediators or degradation of articular tissues. Although promising results have been obtained in ex vivo studies and animal models of OA, few of these agents have completed clinical trials. Available clinical data support the interest of nerve growth factor as a target in pain control as well as the disease-modifying potential of inhibitors of Wnt signaling or catabolic enzymes such as aggrecanases and cathepsin K, and anabolic strategies like fibroblast growth factor-18 or cellular therapies. Carefully controlled studies in patients selected according to OA phenotypes and with a long follow-up will help to confirm the relevance of these new approaches as emerging therapeutic treatments in OA.

The carbon monoxide-releasing molecule tricarbonyldichlororuthenium(II) dimer protects human osteoarthritic chondrocytes and cartilage from the catabolic actions of interleukin-1beta.

We have investigated the effects of a carbon monoxide-releasing molecule, tricarbonyldichlororuthenium(II) dimer (CORM-2), on catabolic processes in human osteoarthritis (OA) cartilage and chondrocytes activated with interleukin-1beta. In these cells, proinflammatory cytokines induce the synthesis of matrix metalloproteinases (MMPs) and aggrecanases, including members of a disintegrin and metalloproteinase with thrombospondin domain (ADAMTS) family, which may contribute to cartilage loss. CORM-2 down-regulated MMP-1, MMP-3, MMP-10, MMP-13, and ADAMTS-5 in OA chondrocytes, and it inhibited cartilage degradation. These effects were accompanied by increased aggrecan synthesis and collagen II expression in chondrocytes. Our results also indicate that the inhibition of extracellular signal-regulated kinase 1/2 and p38 activation by CORM-2 may contribute to the maintenance of extracellular matrix homeostasis. These observations suggest that CORM-2 could exert chondroprotective effects due to the inhibition of catabolic activities and the enhancement of aggrecan synthesis.

Extracellular vesicles: A new therapeutic strategy for joint conditions.

Extracellular vesicles (EVs) are attracting increasing interest since they might represent a more convenient therapeutic tool with respect to their cells of origin. In the last years much time and effort have been expended to determine the biological properties of EVs from mesenchymal stem cells (MSCs) and other sources. The immunoregulatory, anti-inflammatory and regenerative properties of MSC EVs have been demonstrated in in vitro studies and animal models of rheumatoid arthritis or osteoarthritis. This cell-free approach has been proposed as a possible better alternative to MSC therapy in autoimmune conditions and tissue regeneration. In addition, EVs show great potential as biomarkers of disease or delivery systems for active molecules. The standardization of isolation and characterization methods is a key step for the development of EV research. A better understanding of EV mechanisms of action and efficacy is required to establish the potential therapeutic applications of this new approach in joint conditions.

Extracellular Vesicles from Adipose-Derived Mesenchymal Stem Cells Downregulate Senescence Features in Osteoarthritic Osteoblasts.

Osteoarthritis (OA) affects all articular tissues leading to pain and disability. The dysregulation of bone metabolism may contribute to the progression of this condition. Adipose-derived mesenchymal stem cells (ASC) are attractive candidates in the search of novel strategies for OA treatment and exert anti-inflammatory and cytoprotective effects on cartilage. Chronic inflammation in OA is a relevant factor in the development of cellular senescence and joint degradation. In this study, we extend our previous observations of ASC paracrine effects to study the influence of conditioned medium and extracellular vesicles from ASC on senescence induced by inflammatory stress in OA osteoblasts. Our results in cells stimulated with interleukin- (IL-) 1ß indicate that conditioned medium, microvesicles, and exosomes from ASC downregulate senescence-associated ß-galactosidase activity and the accumulation of γH2AX foci. In addition, they reduced the production of inflammatory mediators, with the highest effect on IL-6 and prostaglandin E2. The control of mitochondrial membrane alterations and oxidative stress may provide a mechanism for the protective effects of ASC in OA osteoblasts. We have also shown that microvesicles and exosomes mediate the paracrine effects of ASC. Our study suggests that correction of abnormal osteoblast metabolism by ASC products may contribute to their protective effects.

Anti-senescence and Anti-inflammatory Effects of the C-terminal Moiety of PTHrP Peptides in OA Osteoblasts.

Osteoarthritis (OA) is characterized by degenerative changes in the whole joint leading to physical disability in the elderly population. This condition is associated with altered bone metabolism in subchondral areas suggesting that therapeutic strategies aimed at modifying bone cell metabolism may be of interest. We have investigated the effects of several parathyroid hormone-related protein (PTHrP)-derived peptides (1-37): (N-terminal), (107-111) and (107-139) (C-terminal) on senescence features induced by inflammatory stress in human OA osteoblasts. Incubation of these primary cells with interleukin(IL)-1ß led to an increased expression of senescence markers senescence-associated-ß-galactosidase activity, γH2AX foci, p16, p21, p53, and caveolin-1. PTHrP (107-111) and PTHrP (107-139) significantly reduced all these parameters. Both peptides decreased the production of IL-6 and prostaglandin E2 which was the consequence of cyclo-oxygenase-2 downregulation. PTHrP (107-139) also reduced tumor necrosis factor-α release. These anti-inflammatory effects would be related to the reduction of nuclear factor-κB activation by both peptides and activator protein-1 by PTHrP (107-139). The three PTHrP peptides favored osteoblastic function although the C-terminal domain of PTHrP was more efficient than its N-terminal domain. Our data support an anti-senescence and anti-inflammatory role for the C-terminal moiety of PTHrP with potential applications in chronic inflammatory conditions such as OA.

Paracrine effects of human adipose-derived mesenchymal stem cells in inflammatory stress-induced senescence features of osteoarthritic chondrocytes.

Aging and exposure to stress would determine the chondrocyte phenotype in osteoarthritis (OA). In particular, chronic inflammation may contribute to stress-induced senescence of chondrocytes and cartilage degeneration during OA progression. Recent studies have shown that adipose-derived mesenchymal stem cells exert paracrine effects protecting against degenerative changes in chondrocytes. We have investigated whether the conditioned medium (CM) from adipose-derived mesenchymal stem cells may regulate senescence features induced by inflammatory stress in OA chondrocytes. Our results indicate that CM down-regulated senescence markers induced by interleukin-1ß including senescence-associated ß-galactosidase activity, accumulation of γH2AX foci and morphological changes with enhanced formation of actin stress fibers. Treatment of chondrocytes with CM also decreased the production of oxidative stress, the activation of mitogen-activated protein kinases, and the expression of caveolin-1 and p21. The effects of CM were related to the reduction in p53 acetylation which would be dependent on the enhancement of Sirtuin 1 expression. Therefore, CM may exert protective effects in degenerative joint conditions by countering the premature senescence of OA chondrocytes induced by inflammatory stress.

Heme oxygenase-1 induction and regulation in unstimulated mouse peritoneal macrophages.

Heme oxygenase-1 (HO-1) is a stress protein induced by a variety of stimuli in inflammatory cells. This study was set up to investigate the induction of this protein in unstimulated macrophages. Resident mouse peritoneal macrophages purified by adhesion and cultured in basal conditions strongly induced HO-1 in a time-dependent manner, with a peak at 20 hr. At the same time, low levels of nitrite accumulated in the culture medium and expression of nitric oxide synthase-2 (NOS-2) and NOS-3 protein was detected. Inhibition of NO production and/or NOS expression by incubating macrophages with different drugs inhibiting NOS activity or modulating the redox state of the cell, such as N-acetylcysteine (NAC) resulted in inhibition of HO-1 expression, suggesting that NO is an endogenous mediator of this stress response. In conclusion, mouse peritoneal macrophages cultured in basal conditions develop an adaptive response with up-regulation of HO-1 as a very sensitive marker of oxidative stress.

Expression of heme oxygenase-1 and regulation by cytokines in human osteoarthritic chondrocytes.

Heme oxygenase-1 (HO-1) is implicated in the protection against tissue injury. We investigated the expression of this protein in cartilage sections and chondrocytes obtained from osteoarthritic patients. HO-1 was immunodetected in preparations from cartilage and also in chondrocytes cultured in the absence of stimulation. We found that HO-1 can be modulated by cytokines since the pro-inflammatory cytokines interleukin (IL)-1beta, IL-17 and tumour necrosis factor-alpha (TNF-alpha) down-regulated this protein, whereas the anti-inflammatory cytokine IL-10 exerted the opposite effect. Our results suggest a role for HO-1 as part of protective mechanisms against tissue injury in human cartilage.