Activation of stress-activated protein kinase in osteoarthritic cartilage: evidence for nitric oxide dependence

Endoplasmic reticulum stress-mediated apoptosis and autophagy in osteoarthritis: From molecular mechanisms to therapeutic applications

Osteoarthritis (OA) is characterized primarily by the degeneration of articular cartilage, with a high prevalence and disability rate. The functional phenotype of chondrocytes, as the sole cell type within cartilage, is vital for OA progression. Due to the avascular nature of cartilage and its limited regenerative capacity, repair following injury poses significant challenges. Various cellular stressors, including hypoxia, nutrient deprivation, oxidative stress, and collagen mutations, can lead to the accumulation of misfolded proteins in the endoplasmic reticulum (ER), resulting in ER stress (ERS). In response to restore ER homeostasis as well as cellular vitality and function, a series of adaptive mechanisms are triggered, including the unfolded protein response, ER-associated degradation, and ER-phagy. Prolonged or severe ERS may exceed the adaptive capacity of cells, leading to dysregulation in apoptosis and autophagy-key pathogenic factors contributing to chondrocyte damage and OA progression. This review examines the relationship between ERS in OA chondrocytes and both apoptosis and autophagy in order to identify potential therapeutic targets and strategies for prevention and treatment of OA.

Regulation of chondrocyte apoptosis in osteoarthritis by endoplasmic reticulum stress

Osteoarthritis (OA), a common degenerative joint disease, is characterized by the apoptosis of chondrocytes as a primary pathophysiological change, with endoplasmic reticulum stress (ERS) playing a crucial role. It has been demonstrated that an imbalance in endoplasmic reticulum (ER) homeostasis can lead to ERS, activating three cellular adaptive response pathways through the unfolded protein response to restore ER homeostasis. Mild ERS exerts a protective effect on cells, while prolonged ERS that disrupts the self-regulatory balance of the ER activates apoptotic signaling pathways, leading to chondrocyte apoptosis and hastening OA progression. Hence, controlling the ERS signaling pathway and its apoptotic factors has become a critical focus for preventing and treating OA. This review aims to elucidate the key mechanisms of ERS pathway-induced apoptosis, associated targets, and regulatory pathways, offering valuable insights to enhance the mechanistic understanding of OA. It also reviews the mechanisms studied for ERS-related drugs or compounds for the treatment of OA.

Shikonin Derivatives Inhibit Inflammation Processes and Modulate MAPK Signaling in Human Healthy and Osteoarthritis Chondrocytes

Osteoarthritis (OA) is the most common joint disorder and is characterized by the degeneration of articular cartilage. To develop new therapeutic approaches, we investigated the effect of shikonin derivatives on inflammation, MMP expression, and the regulation of MAPK signaling in human healthy (HC) and OA chondrocytes (pCH-OA). Viability was analyzed using the CellTiter-Glo^® Assay. Inflammatory processes were investigated using a proteome profiler™ assay. Furthermore, we analyzed the effects of the shikonin derivatives by protein expression analysis of the phosphorylation pattern and the corresponding downstream gene regulation using RT-qPCR. Both HC and pCH-OA showed a dose-dependent decrease in viability after treatment. The strongest effects were found for shikonin with IC₅₀ values of 1.2 ± 0.1 µM. Shikonin counteracts the inflammatory response by massively reducing the expression of the pro-inflammatory mediators. The phosphorylation level of ERK changed slightly. pJNK and pp38 showed a significant increase, and the downstream targets c/EBPs and MEF2c may play a role in the cartilage homeostasis. STAT3 phosphorylation decreased significantly and has a chondroprotective function through the regulation of cyclin D1 and Sox9. Our results demonstrate for the first time that shikonin derivatives have extensive effects on the inflammatory processes, MAPKs, and IL6/STAT3 downstream regulation in healthy and OA chondrocytes.

Physalin A Inhibits MAPK and NF-κB Signal Transduction Through Integrin αVβ3 and Exerts Chondroprotective Effect

Osteoarthritis (OA) is a common articular ailment presented with cartilage loss and destruction that is common observed in the elderly population. Physalin A (PA), a natural bioactive withanolide, exerts anti-inflammatory residences in more than a few diseases; however, little is known about its efficacy for OA treatment. Here, we explored the therapeutic effects and potential mechanism of PA in mouse OA. After the in vitro administration of PA, the expression of inflammation indicators including inducible nitric oxide synthase and cyclooxygenase-2 was low, indicating that PA could alleviate the IL-1β-induced chondrocyte inflammation response. Moreover, PA reduced IL-1β-induced destruction of the extracellular matrix by upregulating the gene expression of anabolism factors, including collagen II, aggrecan, and sry-box transcription factor 9, and downregulating the gene expression of catabolic factors, including thrombospondin motif 5 and matrix metalloproteinases. In addition, the chondroprotective effect of PA was credited to the inhibition of mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) signaling pathways. Furthermore, in vivo experiments showed that intra-articular injection of PA could alleviate cartilage destruction in a mouse OA model. However, the anti-inflammatory, anabolism enhancing, catabolism inhibiting, and MAPK and NF-κB signaling pathway inhibiting properties of PA on IL-1β-induced chondrocytes could be reversed when integrin αVβ3 is knocked down by siRNA. In conclusion, our work demonstrates that PA exhibits a chondroprotective effect that may be mediated by integrin αVβ3. Thus, PA or integrin αVβ3 might be a promising agent or molecular target for the treatment of OA.

Substance P and Alpha-Calcitonin Gene-Related Peptide Differentially Affect Human Osteoarthritic and Healthy Chondrocytes

Osteoarthritis (OA) is a degenerative joint disease that not only causes cartilage loss but also structural damage in all joint tissues. Joints are innervated by alpha-calcitonin gene-related peptide (αCGRP) and substance P (SP)-positive sensory nerve fibers. Alteration of sensory joint innervation could be partly responsible for degenerative changes in joints that contribute to the development of OA. Therefore, our aim was to analyze and compare the molecular effects of SP and αCGRP on the metabolism of articular chondrocytes from OA patients and non-OA cartilage donors. We treated the cells with SP or αCGRP and analysed the influence of these neuropeptides on chondrocyte metabolism and modulation of signaling pathways. In chondrocytes from healthy cartilage, SP had minimal effects compared with its effects on OA chondrocytes, where it induced inflammatory mediators, inhibited chondrogenic markers and promoted apoptosis and senescence. Treatment with αCGRP also increased apoptosis and senescence and reduced chondrogenic marker expression in OA chondrocytes, but stimulated an anabolic and protective response in healthy chondrocytes. The catabolic influence of SP and αCGRP might be due to activation of ERK signaling that could be counteracted by an increased cAMP response. We suggest that a switch between the G-subunits of the corresponding receptors after binding their ligands SP or αCGRP plays a central role in mediating the observed effects of sensory neuropeptides on chondrocytes.

Etomidate ameliorated advanced glycation end-products (AGEs)-induced reduction of extracellular matrix genes expression in chondrocytes

Osteoarthritis (OA) is a rheumatic disease common in the elderly. AGEs are the end products of glycation reactions and play an important role in the development of OA. Etomidate is a general anesthesia-inducing agent recently reported to exert significant anti-inflammatory effects. The present study aims to explore the protective effect of Etomidate against advanced glycation end-products (AGEs)-induced reduction of extracellular matrix gene expression in chondrocytes. In the present study, we found that AGEs significantly reduced the expression of Collagen II (COL2A1) and Aggrecan (ACAN) at the gene level. Furthermore, AGEs inhibited the expression of SRY-related high mobility group-box gene 9 (SOX-9), promoting the expression of COL2A1 and ACAN. COL2A1, ACAN, and SOX-9 in chondrocytes were significantly elevated by treatment with Etomidate alone. Consistently, Etomidate ameliorated AGEs-induced downregulation of COL2A1, ACAN, and SOX-9 in a dose-dependent manner. Importantly, we found that knockdown of SOX-9 eliminated the beneficial effects of Etomidate against AGEs-induced decrease in COL2A1 and ACAN genes. Based on these findings, we demonstrated that Etomidate could ameliorate AGEs-induced reduction of extracellular matrix gene expression in chondrocytes by upregulating SOX-9.

Intra-Articular Injection of (-)-Epigallocatechin 3-Gallate (EGCG) Ameliorates Cartilage Degeneration in Guinea Pigs with Spontaneous Osteoarthritis

Osteoarthritis (OA) is the most prevalent joint disease that causes an enormous burden of disease worldwide. (-)-Epigallocatechin 3-gallate (EGCG) has been reported to reduce post-traumatic OA progression through its anti-inflammatory property. Aging is the most crucial risk factor of OA, and the majority of OA incidences are related to age and not trauma. In this study, we assess whether EGCG can ameliorate cartilage degradation in primary OA. In an in-vitro study, real-time PCR was performed to assess the expression of genes associated with human articular chondrocyte homeostasis. A spontaneously occurring OA model in guinea pigs was used to investigate the effect of EGCG in vivo. OA severity was evaluated using Safranin O staining and Osteoarthritis Research Society International (OARSI) scores, as well as by immunohistochemical (IHC) analysis to determine the protein level of type II collagen (Col II), matrix metalloproteinase 13 (MMP-13), and p16 ink4a in articular cartilage. In the in-vitro study, EGCG increased the gene expression of aggrecan and Col II and decreased the expression of interleukin-1, cyclooxygenase 2, MMP-13, alkaline phosphatase, Col X, and p16 Ink4a; EGCG treatment also attenuated the degraded cartilage with a lower OARSI score. Meanwhile, IHC results showed that EGCG exerted an anti-OA effect by reducing ECM degradation, cartilage inflammation, and cell senescence with a less-immunostained Col II, MMP-13, and p16 Ink4a. In conclusion, these findings suggest that EGCG may be a potential disease-modifying OA drug for the treatment of primary OA.

DUSP5 suppresses interleukin-1β-induced chondrocyte inflammation and ameliorates osteoarthritis in rats

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by deterioration of articular cartilage. Dual specificity phosphatase 5 (DUSP5), a member of the DUSP subfamily, is known to regulate cellular inflammation. Here, we studied the relationship between DUSP5 and OA by knockdown and overexpression DUSP5, respectively. Results from in vitro experiments demonstrated that the knockdown of DUSP5 increased interleukin-1β (IL-1β)-induced expression of inflammatory genes, such as inducible nitric oxide synthase (iNOS), cyclooxygenase 2 (COX2), and matrix metalloproteinases (MMPs) in chondrocytes, whereas it decreased the expression of anti-inflammatory genes, such as tissue inhibitor of metalloproteinase 3 (TIMP3) and IL-10. Conversely, the overexpression of DUSP5 suppressed the IL-1β-induced expression of iNOS, COX-2, and MMPs, and upregulated the expression of TIMP3 and IL-10. Moreover, knockdown of DUSP5 enhanced the IL-1β-induced activation of NF-κB and ERK pathways, whereas its overexpression inhibited these pathways. DUSP5 overexpression prevented cartilage degeneration in a rat OA model, while its knockdown reversed that effect. Our findings reveal that DUSP5 suppresses IL-1β-induced chondrocyte inflammation by inhibiting the NF-κB and ERK signaling pathways and ameliorates OA.

Review: ER stress-induced cell death in osteoarthritic cartilage

In cartilage, chondrocytes are responsible for the biogenesis and maintenance of the extracellular matrix (ECM) composed of proteins, glycoproteins and proteoglycans. Various cellular stresses, such as hypoxia, nutrient deprivation, oxidative stress or the accumulation of advanced glycation end products (AGEs) during aging, but also translational errors or mutations in cartilage components or chaperone proteins affect the synthesis and secretion of ECM proteins, causing protein aggregates to accumulate in the endoplasmic reticulum (ER). This condition, referred to as ER stress, interferes with cartilage cell homeostasis and initiates the unfolded protein response (UPR), a rescue mechanism to regain cell viability and function. Chronic or irreversible ER stress, however, triggers UPR-initiated cell death. Due to unresolved ER stress in chondrocytes, diseases of the skeletal system, such as chondrodysplasias, arise. ER stress has also been identified as a contributing factor to the pathogenesis of cartilage degeneration processes such as osteoarthritis (OA). This review provides current knowledge about the biogenesis of ECM components in chondrocytes, describes possible causes for the impairment of involved processes and focuses on the ER stress-induced cell death in articular cartilage during OA. Targeting of the ER stress itself or intervention in UPR signaling to reduce death of chondrocytes may be promising for future osteoarthritis therapy.

Deletion of JNK Enhances Senescence in Joint Tissues and Increases the Severity of Age-Related Osteoarthritis in Mice

OBJECTIVE

To determine the role of JNK signaling in the development of osteoarthritis (OA) induced by joint injury or aging in mice.

METHODS

In the joint injury model, 12-week-old wild-type control, JNK1^-/- , JNK2^-/- , and JNK1^fl/fl JNK2^-/- aggecan-Cre^{ERT 2} double-knockout mice were subjected to destabilization of the medial meniscus (DMM) (n = 15 mice per group) or sham surgery (n = 9-10 mice per group), and OA was evaluated 8 weeks later. In the aging experiment, wild-type control, JNK1^-/- , and JNK2^-/- mice (n = 15 per group) were evaluated at 18 months of age. Mouse knee joints were evaluated by scoring articular cartilage structure, toluidine blue staining, osteophytes, and synovial hyperplasia, by histomorphometric analysis, and by immunostaining for the senescence marker p16^{INK 4a} . Production of matrix metalloproteinase 13 (MMP-13) in cartilage explants in response to fibronectin fragments was measured by enzyme-linked immunosorbent assay.

RESULTS

There were no differences after DMM surgery between the wild-type and the JNK-knockout mouse groups in articular cartilage structure, toluidine blue, or osteophyte scores or in MMP-13 production in explants. All 3 knockout mouse groups had increased subchondral bone thickness and area of cartilage necrosis compared to wild-type mice. Aged JNK-knockout mice had significantly worse articular cartilage structure scores compared to the aged wild-type control mice (mean ± SD 52 ± 24 in JNK1^-/- mice and 60 ± 25 in JNK2^-/- mice versus 32 ± 18 in controls; P = 0.02 and P = 0.004, respectively). JNK1^-/- mice also had higher osteophyte scores. Deletion of JNK resulted in increased expression of p16^{INK 4a} in the synovium and cartilage in older mice.

CONCLUSION

JNK1 and JNK2 are not required for the development of OA in the mouse DMM model. Deletion of JNK1 or JNK2 is associated with more severe age-related OA and increased cell senescence, suggesting that JNK may act as a negative regulator of senescence in the joint.

The Role of Chondrocyte Hypertrophy and Senescence in Osteoarthritis Initiation and Progression

Osteoarthritis (OA) is the most common joint disease that causes pain and disability in the adult population. OA is primarily caused by trauma induced by an external force or by age-related cartilage damage. Chondrocyte hypertrophy or chondrocyte senescence is thought to play a role in the initiation and progression of OA. Although chondrocyte hypertrophy and cell death are both crucial steps during the natural process of endochondral bone formation, the abnormal activation of these two processes after injury or during aging seems to accelerate the progression of OA. However, the exact mechanisms of OA progression and these two processes remain poorly understood. Chondrocyte senescence and hypertrophy during OA share various markers and processes. In this study, we reviewed the changes that occur during chondrocyte hypertrophy or senescence in OA and the attempts that were made to regulate them. Regulation of hypertrophic or senescent chondrocytes might be a potential therapeutic target to slow down or stop OA progression; thus, a better understanding of the processes is required for management.

Nobiletin Inhibits IL-1β-Induced Inflammation in Chondrocytes via Suppression of NF-κB Signaling and Attenuates Osteoarthritis in Mice

Osteoarthritis (OA), a common degenerative joint disease, is principally characterized by inflammation and destruction of cartilage. Nobiletin, an extract of the peel of citrus fruits, is known to have anti-inflammatory properties. However, the mechanisms by which nobiletin plays a protective role in osteoarthritis (OA) are not completely understood. In the present study, we investigated the anti-inflammatory effects of nobiletin in the progression of OA in both in vitro and in vivo experiments. Mouse chondrocytes were pretreated with nobiletin (0, 10, 20, 40 μM) for 24 h and then incubated with IL-1β (10 ng/ml, 24 h) in vitro. The generation of PGE2 and NO was evaluated by the Griess reaction and ELISAs. The protein expression of inducible nitric oxide synthase, matrix metalloproteinase-3, matrix metalloproteinase-13, A disintegrin and metalloproteinase with thrombospondin motifs-5 (ADAMTS5), cyclooxygenase-2, collagen II, and aggrecan was analyzed by Western blotting. Immunofluorescence and Western blot analysis were used to detect nuclear factor-κB (NF-κB) signaling molecules. Induction of proinflammatory and catabolic mediators by IL-1β stimulation of mouse chondrocytes could be partially blocked by treatment with nobiletin or ammonium pyrrolidine dithiocarbamate (an NF-κB inhibitor). Furthermore, our results indicated that nobiletin exhibited a therapeutic effect through active inhibition of the NF-κB signaling pathway. In a mouse model of OA, injection of nobiletin (20 mg/kg) every 2 days for 8 weeks after surgery inhibited cartilage destruction and synovitis. Taken together, our findings suggest that nobiletin may be a potential therapeutic agent for the treatment of OA.

Impaired Annulus Fibrosus Development and Vertebral Fusion Cause Severe Scoliosis in Mice with Deficiency of c-Jun NH2-Terminal Kinases 1 and 2

Mitogen-activated protein kinases, including c-Jun NH2-terminal kinase (JNK), play an important role in the development and function of a large variety of tissues. The skeletal phenotype of JNK1 and JNK2 double-knockout (dKO) mice (JNK1^fl/flCol2-Cre/JNK2^-/-) and control genotypes were analyzed at different embryonic and postnatal stages. JNK1/2 dKO mice displayed a severe scoliotic phenotype beginning during development that was grossly apparent around weaning age. Alcian blue staining at embryonic day 17.5 showed abnormal fusion of the posterior spinal elements. In adult mice, fusion of vertebral bodies and of spinous and transverse processes was noted by micro-computed tomography, Alcian blue/Alizarin red staining, and histology. The long bones developed normally, and histologic sections of growth plate and articular cartilage revealed no significant abnormalities. Histologic sections of the vertebral column at embryonic days 15.5 and 17.5 revealed an abnormal organization of the annulus fibrosus in the dKOs, with chondrocyte-like cells and fusion of dorsal processes. Spinal sections in 10-week-old dKO mice showed replacement of intervertebral disk structures (annulus fibrosus and nucleus pulposus) by cartilage and bone tissues, with cells staining for markers of hypertrophic chondrocytes, including collagen X and runt-related transcription factor 2. These findings demonstrate a requirement for both JNK1 and JNK2 in the normal development of the axial skeleton. Loss of JNK signaling results in abnormal endochondral bone formation and subsequent severe scoliosis.

H2O2 oxidation of cysteine residues in c-Jun N-terminal kinase 2 (JNK2) contributes to redox regulation in human articular chondrocytes

Reactive oxygen species (ROS), in particular H₂O₂, regulate intracellular signaling through reversible oxidation of reactive protein thiols present in a number of kinases and phosphatases. H₂O₂ has been shown to regulate mitogen-activated protein kinase (MAPK) signaling depending on the cellular context. We report here that in human articular chondrocytes, the MAPK family member c-Jun N-terminal kinase 2 (JNK2) is activated by fibronectin fragments and low physiological levels of H₂O₂ and inhibited by oxidation due to elevated levels of H₂O₂ The kinase activity of affinity-purified, phosphorylated JNK2 from cultured chondrocytes was reversibly inhibited by 5-20 μm H₂O₂ Using dimedone-based chemical probes that react specifically with sulfenylated cysteines (RSOH), we identified Cys-222 in JNK2, a residue not conserved in JNK1 or JNK3, as a redox-reactive site. MS analysis of human recombinant JNK2 also detected further oxidation at Cys-222 and other cysteines to sulfinic (RSO₂H) or sulfonic (RSO₃H) acid. H₂O₂ treatment of JNK2 resulted in detectable levels of peptides containing intramolecular disulfides between Cys-222 and either Cys-213 or Cys-177, without evidence of dimer formation. Substitution of Cys-222 to alanine rendered JNK2 insensitive to H₂O₂ inhibition, unlike C177A and C213A variants. Two other JNK2 variants, C116A and C163A, were also resistant to oxidative inhibition. Cumulatively, these findings indicate differential regulation of JNK2 signaling dependent on H₂O₂ levels and point to key cysteine residues regulating JNK2 activity. As levels of intracellular H₂O₂ rise, a switch occurs from activation to inhibition of JNK2 activity, linking JNK2 regulation to the redox status of the cell.

DUSP19, a downstream effector of leptin, inhibits chondrocyte apoptosis via dephosphorylating JNK during osteoarthritis pathogenesis

Increased mitogen-activated protein kinase (MAPK) activity has been found in human osteoarthritis (OA). Dual specificity protein phosphatase 19 (DUSP19), a member of mitogen-activated protein kinase (MAPK) phosphatases (MKPs), controls the activity of various MAPKs. This study was aimed to explore the function of DUSP19 during OA pathogenesis. Here, OA and healthy control data were downloaded from the NCBI Gene Expression Omnibus database (GSE57218). Forty-five patients with OA and 25 healthy donors were enrolled in this study. A rat OA model was induced by anterior cruciate ligament transection. Primary cultured chondrocytes were treated with leptin (10 ng mL(-1)). Cell survival, cell apoptosis and reactive oxygen species (ROS) were identified by CCK-8 and flow cytometry, respectively. In the cartilage of OA patients, DUSP19 was expressed in a lower level than in the cartilage of healthy control. The DUSP19 level was negatively correlated with leptin, which was confirmed by experiments in the rat OA model. Moreover, cell apoptosis and JNK activation in the rat cartilage were increased with the increasing of leptin levels and the decreasing of DUSP19 mRNA levels. In primary culture chondrocytes, exogenous leptin suppressed DUSP19 expression. The ectopic expression of DUSP19 significantly ameliorated leptin-induced apoptosis in damaged chondrocytes, accompanied by the reduced production of ROS. Moreover, the activity of JNK stimulated by leptin was suppressed by DUSP19 overexpression. The present study indicated that DUSP19, a downstream of leptin, inhibited apoptosis of chondrocytes through dephosphorylating JNK.

Apoptosis signaling pathways in osteoarthritis and possible protective role of melatonin

Osteoarthritis (OA) is a degenerative joint disease characterized by progressive erosion of articular cartilage. As chondrocytes are the only cell type forming the articular cartilage, their gradual loss is the main cause of OA. There is a substantial body of published research that suggests reactive oxygen species (ROS) are major causative factors for chondrocyte damage and OA development. Oxidative stress elicited by ROS is capable of oxidizing and subsequently disrupting cartilage homeostasis, promoting catabolism via induction of cell death and damaging numerous components of the joint. IL-1β and TNF-α are crucial inflammatory factors that play pivotal roles in the pathogenesis of OA. In this process, the mitochondria are the major source of ROS production in cells, suggesting a role of mitochondrial dysfunction in this type of arthritis. This may also be promoted by inflammatory cytokines such as IL-1β and TNF-α which contribute to chondrocyte death. In patients with OA, the expression of endoplasmic reticulum (ER) stress-associated molecules is positively correlated with cartilage degeneration. Melatonin and its metabolites are broad-spectrum antioxidants and free radical scavengers which regulate a variety of molecular pathways such as inflammation, proliferation, apoptosis, and metastasis in different pathophysiological situations. Herein, we review the effects of melatonin on OA, focusing on its ability to regulate apoptotic processes and ER and mitochondrial activity. We also evaluate likely protective effects of melatonin on OA pathogenesis.

Regulation of senescence associated signaling mechanisms in chondrocytes for cartilage tissue regeneration

Adult articular chondrocytes undergo slow senescence and dedifferentiation during in vitro expansion, restricting successful cartilage regeneration. A complete understanding of the molecular signaling pathways involved in the senescence and dedifferentiation of chondrocytes is essential in order to better characterize chondrocytes for cartilage tissue engineering applications. During expansion, cell fate is determined by the change in expression of various genes in response to aspects of the microenvironment, including oxidative stress, mechanical stress, and unsuitable culture conditions. Rapid senescence or dedifferentiation not only results in the loss of the chondrocytic phenotype but also enhances production of inflammatory mediators and matrix-degrading enzymes. This review focuses on the two groups of genes that play direct and indirect roles in the induction of senescence and dedifferentiation. Numerous degenerative signaling pathways associated with these genes have been reported. Upregulation of the genes interleukin 1 beta (IL-1β), p53, p16, p21, and p38 mitogen-activated protein kinase (MAPK) is responsible for the direct induction of senescence, whereas downregulation of the genes transforming growth factor-beta (TGF-β), bone morphogenetic protein-2 (BMP-2), SRY (sex determining region Y)-box 9 (SOX9), and insulin-like growth factor-1 (IGF-1), indirectly induces senescence. In senescent and dedifferentiated chondrocytes, it was found that TGF-β, BMP-2, SOX9, and IGF-1 are downregulated, while the levels of IL-1β, p53, p16, p21, and p38 MAPK are upregulated followed by inhibition of the normal molecular functioning of the chondrocytes. This review helps to elucidate the underlying mechanism in degenerative cartilage disease, which may help to improve cartilage tissue regeneration techniques.

TGFβ signaling in cartilage development and maintenance

Members of the transforming growth factor beta (TGFβ) superfamily of secreted factors play essential roles in nearly every aspect of cartilage formation and maintenance. However, the mechanisms by which TGFβs transduce their effects in cartilage in vivo remain poorly understood. Mutations in several TGFβ family members, their receptors, extracellular modulators, and intracellular transducers have been described, and these usually impact the development of the cartilaginous skeleton. Furthermore, genome-wide association studies have linked components of the (TGFβ) superfamily to susceptibility to osteoarthritis. This review focuses on recent discoveries from genetic studies in the mouse regarding the regulation of TGFβ signaling in developing growth plate and articular cartilage, as well as the different modes of crosstalk between canonical and noncanonical TGFβ signaling. These new insights into TGFβ signaling in cartilage may open new prospects for therapies that maintain healthy articular cartilage.

A review of crosstalk between MAPK and Wnt signals and its impact on cartilage regeneration

Chondrogenesis is a developmental process that is controlled and coordinated by many growth and differentiation factors, in addition to environmental factors that initiate or suppress cellular signaling pathways and the transcription of specific genes in a temporal-spatial manner. As key signaling molecules in regulating cell proliferation, homeostasis and development, both mitogen-activated protein kinases (MAPK) and the Wnt family participate in morphogenesis and tissue patterning, playing important roles in skeletal development, especially chondrogenesis. Recent findings suggest that both signals are also actively involved in arthritis and related diseases. Despite the implication that crosstalk between MAPK and Wnt signaling has a significant function in cancer, few studies have summarized this interaction and its regulation of chondrogenesis. In this review, we focus on MAPK and Wnt signaling, referencing their relationships in various types of cells and particularly to their influence on chondrogenesis and cartilage development. We also discuss the interactions between MAPK and Wnt signaling with respect to cartilage-related diseases such as osteoarthritis and explore potential therapeutic targets for disease treatments.

Intra-articular injection of the selective cyclooxygenase-2 inhibitor meloxicam (Mobic) reduces experimental osteoarthritis and nociception in rats

OBJECTIVE

To study the effect of intra-articular injection of meloxicam (Mobic) on the development of osteoarthritis (OA) in rats and examine concomitant changes in nociceptive behavior and the expression of mitogen-activated protein kinases (MAPKs) in articular cartilage chondrocytes.

METHODS

OA was induced in Wistar rats by right anterior cruciate ligament transection (ACLT); the left knee was not treated. The OA + meloxicam (1.0 mg) group was injected intra-articularly in the ACLT knee with 1.0 mg of meloxicam once a week for 5 consecutive weeks starting 5 weeks after ACLT. The OA + meloxicam (0.25 mg) group was treated similarly with 0.25 mg meloxicam. The sham group underwent arthrotomy only and received vehicle of 0.1 mL sterile 0.9% saline injections, whereas the naive rats in meloxicam-only groups were treated similarly with 1.0- and 0.25-mg meloxicam. Nociception was measured as secondary mechanical allodynia and hind paw weight-bearing distribution at before (pre-) and 5, 10, 15, and 20 weeks post-ACLT. Histopathology of the cartilage and synovia was examined 20 weeks after ACLT. Immunohistochemical analysis was performed to examine the effect of meloxicam on MAPKs (p38, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK)) expression in the articular cartilage chondrocytes.

RESULTS

OA rats receiving intra-articular meloxicam treatment showed significantly less cartilage degeneration and synovitis than saline-treated controls. Nociception were improved in the OA + meloxicam groups compared with the OA group. Moreover, meloxicam attenuated p38 and JNK but enhanced ERK expression in OA-affected cartilage.

CONCLUSIONS

Intra-articular injection of meloxicam (1) attenuates the development of OA, (2) concomitantly reduces nociception, and (3) modulates chondrocyte metabolism, possibly through inhibition of cellular p38 and JNK, but enhances ERK expression.

Static mechanical stress induces apoptosis in rat endplate chondrocytes through MAPK and mitochondria-dependent caspase activation signaling pathways

Mechanical stress has detrimental effects on cartilaginous endplate chondrocytes due to apoptosis in vivo and in vitro. In this study, we investigated the possible apoptosis signaling pathways induced by mechanical stress in cultured rat cervical endplate chondrocytes. Static mechanical load significantly reduced cell viability in a time- and load-dependent manner, as demonstrated by the Cell Counting Kit-8 (CCK-8) assay. Chondrocyte apoptosis induced by mechanical stress was confirmed by annexin V/propidium iodide (PI) staining and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL). Western blot analysis revealed that static load-induced chondrocyte apoptosis was accompanied by increased phosphorylation of c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 mitogen-activated protein kinase (MAPK). The loss of mitochondrial membrane potential (ΔΨm), increased Cytochrome c release, and activated Caspase-9 and Caspase-3, indicating that the mitochondrial pathway is involved in mechanical stress-induced chondrocyte apoptosis. Treatment with inhibitors of JNK (SP600125), p38 MAPK (SB203580), and ERK (PD98059) prior to mechanical stimulation reversed both the static load-induced chondrocyte apoptosis and the activation of JNK, p38 MAPK, and ERK. Taken together, the data presented in this study demonstrate that mechanical stress induces apoptosis in rat cervical endplate chondrocytes through the MAPK-mediated mitochondrial apoptotic pathway.

Effects of regenerative radioelectric asymmetric conveyer treatment on human normal and osteoarthritic chondrocytes exposed to IL-1β. A biochemical and morphological study

Purpose

Osteoarthritis (OA) is a degenerative disease characterized by a progressive loss of articular cartilage extracellular matrix and is due to functional impairments occurring in chondrocytes. In previous works, we highlighted that Regenerative Tissue Optimization (TO-RGN) treatment with radioelectric asymmetric conveyer (REAC) technology influenced the gene expression profiles controlling stem cell differentiation and the pluripotency of human skin-derived fibroblasts in vitro. Since interleukin-1 beta signaling has been implicated in the induction and progression of this disease (through metalloproteinase-3 synthesis and nitric oxide production), we investigated whether REAC TO-RGN might influence the biochemical and morphological changes induced by interleukin-1 beta in normal and OA chondrocytes.

Methods

The induction of metalloproteinase-3 and proteoglycan synthesis was evaluated by a solid-phase enzyme-amplified sensitivity immunoassay, and nitric oxide production was evaluated with the Griess method. Ultrastructural features were observed by transmission electron microscopy.

Results

REAC TO-RGN treatment decreased nitric oxide and metalloproteinase-3 production in normal and OA chondrocytes, while inducing an increase in proteoglycan synthesis. OA chondrocytes were more affected by REAC TO-RGN treatment than were normal chondrocytes. Ultrastructural changes confirmed that REAC TO-RGN may counteract the negative effects of interleukin-1 beta incubation.

Conclusion

The results of this in vitro study suggest that REAC TO-RGN treatment may represent a new, promising approach for the management of OA.

c-Jun N-Terminal Kinase in Inflammation and Rheumatic Diseases

The c-Jun N-terminal kinases (JNKs) are members of the mitogen-activated protein kinase (MAPK) family and are activated by environmental stress. JNK is also activated by proinflammatory cytokines, such as TNF and IL-1, and Toll-like receptor ligands. This pathway, therefore, can act as a critical convergence point in immune system signaling for both adaptive and innate responses. Like other MAPKs, the JNKs are activated via the sequential activation of protein kinases that includes two dual-specificity MAP kinase kinases (MKK4 and MKK7) and multiple MAP kinase kinase kinases. MAPKs, including JNKs, can be deactivated by a specialized group of phosphatases, called MAP kinase phosphatases. JNK phosphorylates and regulates the activity of transcription factors other than c-Jun, including ATF2, Elk-1, p53 and c-Myc and non-transcription factors, such as members of the Bcl-2 family. The pathway plays a critical role in cell proliferation, apoptosis, angiogenesis and migration. In this review, an overview of the functions that are related to rheumatic diseases is presented. In addition, some diseases in which JNK participates will be highlighted.

Aggravation of ADAMTS and matrix metalloproteinase production and role of ERK1/2 pathway in the interaction of osteoarthritic subchondral bone osteoblasts and articular cartilage chondrocytes -- possible pathogenic role in osteoarthritis

OBJECTIVE

Degradative enzymes, such as A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) and matrix metalloproteinases (MMP), play key roles in development of osteoarthritis (OA). We investigated if crosstalk between subchondral bone osteoblasts (SBO) and articular cartilage chondrocytes (ACC) in OA alters the expression and regulation of ADAMTS5, ADAMTS4, MMP-1, MMP-2, MMP-3, MMP-8, MMP-9, and MMP-13, and also tested the possible involvement of mitogen-activated protein kinase (MAPK) signaling pathway during this process.

METHODS

ACC and SBO were isolated from normal and OA patients. An in vitro coculture model was developed to study the regulation of ADAMTS and MMP under normal and OA joint crosstalk conditions. The MAPK-ERK inhibitor PD98059 was applied to delineate the involvement of specific pathways during this interaction process.

RESULTS

Indirect coculture of OA SBO with normal ACC resulted in significantly increased expression of ADAMTS5, ADAMTS4, MMP-2, MMP-3, and MMP-9 in ACC, whereas coculture of OA ACC led to increased MMP-1 and MMP-2 expression in normal SBO. Upregulation of ADAMTS and MMP under these conditions was correlated with activation of the MAPK-ERK1/2 signaling pathway, and addition of the MAPK-ERK inhibitor PD98059 reversed the overexpression of ADAMTS and MMP in cocultures.

CONCLUSION

These results add to the evidence that in human OA, altered bidirectional signals between SBO and ACC significantly influence the critical features of both cartilage and bone by producing abnormal levels of ADAMTS and MMP. We have demonstrated for the first time that this altered crosstalk was mediated by the phosphorylation of MAPK-ERK1/2 signaling pathway.

The immunosuppressant FTY720 (fingolimod) enhances glycosaminoglycan depletion in articular cartilage

Background

FTY720 (Fingolimod) is a novel immunosuppressive drug investigated in clinical trials for organ transplantation and multiple sclerosis. It acts as a functional sphingosine-1-phosphate (S1P) receptor antagonist, thereby inhibiting the egress of lymphocytes from secondary lymphoid organs. As S1P is able to prevent IL-1beta induced cartilage degradation, we examined the direct impact of FTY720 on cytokine induced cartilage destruction.

Methods

Bovine chondrocytes were treated with the bioactive phosphorylated form of FTY720 (FTY720-P) in combination with IL-1beta or TNF-alpha. Expression of MMP-1,-3.-13, iNOS and ADAMTS-4,-5 and COX-2 was evaluated using quantitative real-time PCR and western blot. Glycosaminoglycan depletion from cartilage explants was determined using a 1,9-dimethylene blue assay and safranin O staining.

Results

FTY720-P significantly reduced IL-1beta and TNF-alpha induced expression of iNOS. In contrast FTY720-P increased MMP-3 and ADAMTS-5 mRNA expression. Furthermore depletion of glycosaminoglycan from cartilage explants by IL-1beta and TNF-alpha was significantly enhanced by FTY720-P in an MMP-3 dependent manner.

Conclusions

Our results suggest that FTY720 may enhance cartilage degradation in pro-inflammatory environment.

The role of mitogen activated protein kinase signaling in microglia and neurons in the initiation and maintenance of chronic pain

Effective treatments for patients suffering from chronic pain remain an area of intense focus within the pharmaceutical industry, as the development of novel therapies would help to treat an area of significant unmet medical need. The successful development of pharmacological agents to treat inflammatory and neuropathic pain conditions relies on a thorough understanding of the mechanisms that underlie the development and maintenance of chronic pain states. The goal of this review is to highlight recent discoveries regarding the intracellular signaling mechanisms that appear to play a critical role in persistent inflammatory and neuropathic pain. The review will focus on the mitogen activated protein kinase family of enzymes and the data suggesting that treatments designed to inhibit the activation of these enzymes may lead to significant advancements in the treatment of chronic pain. The review will also highlight the important interplay between neurons and non-neuronal cells (i.e., microglia and astrocytes) within the dorsal horn of the spinal cord in the generation and maintenance of chronic inflammatory and neuropathic pain.

Enhanced apoptotic and reduced protective response in chondrocytes following endoplasmic reticulum stress in osteoarthritic cartilage

Endoplasmic reticulum (ER) stress has been shown to participate in many disease pathologies. Although recent reports have demonstrated that ER stress in chondrocytes is present in human osteoarthritis (OA), its role in the pathology of cartilage degeneration, such as chondrocyte apoptosis, remains unclear. In the present study, we investigated the expression of phosphorylated PERK (pPERK), ubiquitin (Ub), GRP78, CHOP, phosphorylated JNK (pJNK) and cleaved caspase-3 (C-CASP3) and the mRNA splicing of XBP1 (XBP1 splicing) in human OA cartilage by immunohistochemistry and RT-PCR. Additionally, human chondrocytes were treated with several concentrations of tunicamycin, an ER stress inducer, to assess the impact of ER stress on the mRNA expression of CHOP, XBP1 splicing and apoptosis, as determined by real-time PCR, RT-PCR and ELISA analyses respectively. In human OA cartilage, the number of chondrocytes expressing pPERK, Ub, CHOP and pJNK positively correlated with cartilage degeneration and the number of C-CASP3-positive chondrocytes. XBP1 splicing and GRP78 expression in severe OA containing the greatest number of C-CASP3-positive chondrocytes were similar to the levels in mild OA, however, XBP1 splicing was higher in moderate OA than in mild and severe OA. Tunicamycin dose dependently increased CHOP expression and apoptosis of cultured chondrocytes. Although tunicamycin upregulated XBP1 splicing in cultured chondrocytes, its impact on XBP1 splicing was weakened at higher concentrations. In conclusion, the present results indicate that ER stress may contribute to chondrocyte apoptosis along with OA progression, which was closely associated with an enhanced apoptotic response and a reduced protective response by the cells.

Herbal medicinal products target defined biochemical and molecular mediators of inflammatory autoimmune arthritis

Rheumatoid arthritis (RA) is a chronic debilitating disease characterized by synovial inflammation, damage to cartilage and bone, and deformities of the joints. Several drugs possessing anti-inflammatory and immunomodulatory properties are being used in the conventional (allopathic) system of medicine to treat RA. However, the long-term use of these drugs is associated with harmful side effects. Therefore, newer drugs with low or no toxicity for the treatment of RA are actively being sought. Interestingly, several herbs demonstrate anti-inflammatory and anti-arthritic activity. In this review, we describe the role of the major biochemical and molecular mediators in the pathogenesis of RA, and highlight the sites of action of herbal medicinal products that have anti-arthritic activity. With the rapidly increasing use of CAM products by patients with RA and other inflammation-related disorders, our review presents timely information validating the scientific rationale for the use of natural therapeutic products.

Leg amputation accelerates senescence of rat lumbar intervertebral discs

STUDY DESIGN

Several senescence biomarkers were observed to investigate cell senescence in degenerative intervertebral lumbar discs of foreleg-amputated rats.

OBJECTIVE

To determine if cell senescence is accelerated in degenerative intervertebral lumbar disc cells in an upright-rat model.

SUMMARY OF BACKGROUND DATA

Cellular senescence was accelerated in human and sand rat degenerative intervertebral disc (IVD) cells. Repeated use of upright posture by rats contributed to degenerative disc changes. No convincing evidence of cell senescence was observed in the lumbar disc of the foreleg amputated rat.

METHODS

The forelimbs of 20 rats were amputated at 1 month of age such that they maintained an upright stance; rats were housed in custom-made cages. Nonamputated rats, also 1 month of age, were kept in regular cages and served as a control group. The lumbar IVDs were harvested from rats in 2 groups, at 5 or 9 months following amputation. Senescence-associated-β-galactosidase-positive staining was used to detect cell senescence. p16INK4a and p27KIP were assessed by immunohistochemistry analysis. Total RNA isolated from these samples was used to measure the gene expression of p16INK4a, RB, cyclin D1, CDK4, PTEN, p27KIP, p19ARF, p21, TERT, and RAGE by real-time polymerase chain reaction assay.

RESULTS

The highest levels of SA-β-GAL activity were detected in 9-month amputated rats. Quantitative immunohistochemical analysis showed that there were highest rates of p16INK4a and p27KIP protein expression in the cartilage endplate and anulus fibrosus of 9-month amputated rats. The mRNA levels of p16INK4a, RB, PTEN, p27KIP, p19ARF, and RAGE were upregulated. The increased cyclin D1 mRNA level was statistically significant only at the ninth month following amputation; CDK4 and TERT mRNA levels were downregulated to a similar extent at both points compared with nonamputated controls. mRNA expression of p21 was significantly downregulated.

CONCLUSION

Accelerated cell senescence was associated with forelimb amputation that causes abnormal loading in rat lumbar IVDs.

Glucosamine sulfate reduces experimental osteoarthritis and nociception in rats: association with changes of mitogen-activated protein kinase in chondrocytes

OBJECTIVE

To study the effects of oral glucosamine sulfate on the development of osteoarthritis (OA) and to examine concomitant changes in the nociceptive behavior of rats.

METHODS

OA was induced in Wistar rats by anterior cruciate ligament transection (ACLT) of the right knee; the left knee was untreated. The OA+glucosamine group received oral glucosamine sulfate (250 mg/kg/day) in a 2-g wafer once a day for 10 consecutive weeks starting at week 5 after ACLT. The OA group was treated as above with 2-g wafers (placebo). The control group of naïve rats received 2-g wafers only. The glucosamine alone group comprised naïve rats receiving glucosamine sulfate only. Nociceptive behavior (mechanical allodynia and weight-bearing distribution of hind paws) during OA development was analyzed pre- and 3, 6, 9, 12, 15, and 18 weeks post-ACLT. Macroscopic and histologic studies were then performed on the cartilage and synovia. Immunohistochemical analysis was performed to examine the effect of glucosamine on expression of mitogen-activated protein kinases (MAPKs) in the articular cartilage chondrocytes.

RESULTS

OA rats receiving glucosamine showed a significantly lower degree of cartilage degeneration than the rats receiving placebo. Glucosamine treatment also suppressed synovitis. Mechanical allodynia and weight-bearing distribution studies showed significant improvement in the OA+glucosamine group as compared to the OA group. Moreover, glucosamine attenuated p38 and c-Jun N-terminal kinase (JNK) but increased extracellular signal-regulated kinase 1/2 (ERK) expression in OA-affected cartilage.

CONCLUSION

Our results indicate that treatment with oral glucosamine sulfate in a rat OA model (1) attenuates the development of OA, (2) concomitantly reduces nociception, and (3) modulates chondrocyte metabolism, possibly through inhibition of cell p38 and JNK and increase of ERK expression.

Biology and pathology of Rho GTPase, PI-3 kinase-Akt, and MAP kinase signaling pathways in chondrocytes

Chondrocytes provide the framework for the developing skeleton and regulate long-bone growth through the activity of the growth plate. Chondrocytes in the articular cartilage, found at the ends of bones in diarthroidial joints, are responsible for maintenance of the tissue through synthesis and degradation of the extracellular matrix. The processes of growth, differentiation, cell death and matrix remodeling are regulated by a network of cell signaling pathways in response to a variety of extracellular stimuli. These stimuli consist of soluble ligands, including growth factors and cytokines, extracellular matrix proteins, and mechanical factors that act in concert to regulate chondrocyte function through a variety of canonical and non-canonical signaling pathways. Key chondrocyte signaling pathways include, but are not limited to, the p38, JNK and ERK MAP kinases, the PI-3 kinase-Akt pathway, the Jak-STAT pathway, Rho GTPases and Wnt-beta-catenin and Smad pathways. Modulation of the activity of any of these pathways has been associated with various pathological states in cartilage. This review focuses on the Rho GTPases, the PI-3 kinase-Akt pathway, and some selected aspects of MAP kinase signaling. Most studies to date have examined these pathways in isolation but it is becoming clear that there is significant cross-talk among the pathways and that the overall effects on chondrocyte function depend on the balance in activity of multiple signaling proteins.

Oxidative stress inhibits insulin-like growth factor-I induction of chondrocyte proteoglycan synthesis through differential regulation of phosphatidylinositol 3-Kinase-Akt and MEK-ERK MAPK signaling pathways

The ability of insulin-like growth factor I (IGF-I) to stimulate cartilage matrix synthesis is reduced in aged and osteoarthritic cartilage. Aging and osteoarthritis are associated with an increase in reactive oxygen species, which we hypothesized would interfere with normal IGF-I signaling. We compared IGF-I signaling in normal and osteoarthritic human articular chondrocytes and investigated the effects of oxidative stress induced by tert-butylhydroperoxide (tBHP). In normal human chondrocytes, IGF-I initiated a strong and sustained phosphorylation of IRS-1 (Tyr-612) and Akt (Ser-473) and transient ERK phosphorylation. In contrast, in osteoarthritic chondrocytes, which possessed elevated basal IRS-1 (Ser-312) and ERK phosphorylation, IGF-I failed to stimulate IRS-1 (Tyr-612) or Akt phosphorylation. In normal human chondrocytes, tBHP triggered strong IRS-1 (Ser-312 and Ser-616) and ERK phosphorylation and inhibited IGF-I-induced IRS-1 (Tyr-612) and Akt phosphorylation. Lentivirus-mediated overexpression of constitutively active (CA) Akt significantly enhanced proteoglycan synthesis, whereas both dominant negative Akt and CA MEK inhibited proteoglycan synthesis. CA Akt also promoted type II collagen and Sox9 expression, whereas tBHP treatment and CA MEK inhibited aggrecan, collagen II, and Sox9 mRNA expression. In osteoarthritic chondrocytes, the antioxidants Mn(III) tetrakis(4-benzoic acid)porphyrin and N-acetylcysteine increased the ratio of Akt to ERK phosphorylation and promoted IGF-I-mediated proteoglycan synthesis. Chemical inhibition of ERK significantly enhanced IGF-I phosphorylation of Akt and alleviated tBHP inhibition of Akt phosphorylation. These results demonstrate opposing roles for phosphatidylinositol 3-kinase-Akt and MEK-ERK in cartilage matrix synthesis and suggest that elevated levels of reactive oxygen species cause chondrocyte IGF-I resistance by altering the balance of Akt to ERK activity.

Heme oxygenase-1 induction modulates microsomal prostaglandin E synthase-1 expression and prostaglandin E(2) production in osteoarthritic chondrocytes

Pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) may participate in the pathogenesis of cartilage damage in osteoarthritis (OA) through the production of catabolic enzymes and inflammatory mediators. Induction of heme oxygenase-1 (HO-1) has previously been shown to exert anti-inflammatory effects in different cell types. We have investigated whether HO-1 induction may modify chondrocyte viability and the production of relevant mediators such as oxidative stress and prostaglandin E(2) (PGE(2)) elicited by IL-1beta in OA chondrocytes. Chondrocytes were isolated from OA cartilage and used in primary culture. Cells were stimulated with IL-1beta in the absence or presence of the HO-1 inducer cobalt protoporphyrin IX (CoPP). Gene expression was assessed by quantitative real-time PCR, protein levels by ELISA and Western blot, apoptosis by laser scanning cytometry using annexin V-FITC and TUNEL assays, and oxidative stress by LSC with dihydrorhodamine 123. HO-1 induction by CoPP enhanced chondrocyte viability and aggrecan content while inhibiting apoptosis and oxidative stress generation. PGE(2) is produced in OA chondrocytes stimulated by IL-1beta by the coordinated induction of cyclooxygenase-2 and microsomal PGE synthase 1 (mPGES-1). The production of PGE(2) was decreased by HO-1 induction as a result of diminished mPGES-1 protein and mRNA expression. Transfection with HO-1 small interfering RNA counteracted CoPP effects. In addition, the activation of nuclear factor-kappaB and early growth response-1 was significantly reduced by CoPP providing a basis for its anti-inflammatory effects. These results confirm the protective role of HO-1 induction in OA chondrocytes and suggest the potential interest of this strategy in degenerative joint diseases.

Mitogen-activated protein kinases as therapeutic targets in osteoarthritis

PURPOSE OF REVIEW

The mitogen-activated protein (MAP) kinases are intracellular signaling proteins which play a central role in controlling the activity of pathways that regulate production and activity of multiple mediators of joint tissue destruction. The therapeutic potential of MAP kinase inhibition in osteoarthritis was reviewed.

RECENT FINDINGS

Results from basic research studies support the role of MAP kinases as central mediators that regulate expression of proinflammatory cytokines and metalloproteinases but also as potential pain mediators as well. Cell culture and animal model studies suggest that inhibition of MAP kinases might slow progression of osteoarthritis but trials of MAP kinase inhibitors in humans with osteoarthritis have not yet been reported. Safety concerns of the currently available inhibitors have limited their initial use to trials in conditions considered more severe than osteoarthritis.

SUMMARY

MAP kinase inhibition has the potential to slow disease progression in osteoarthritis and also might reduce pain; however, safety concerns have limited the use of general MAP kinase inhibitors in humans. Further understanding of the function of specific isoforms of the MAP kinases as well as upstream and downstream effectors may lead to the development of more specific inhibitors with less toxicity that could eventually be used as structure-modifying drugs for osteoarthritis.

Interleukin-1beta and interleukin-6 disturb the antioxidant enzyme system in bovine chondrocytes: a possible explanation for oxidative stress generation

OBJECTIVE

Beside matrix metalloproteinases, reactive oxygen species (ROS) are the main biochemical factors of cartilage degradation. To prevent ROS toxicity, chondrocytes possess a well-coordinated enzymatic antioxidant system formed principally by superoxide dismutases (SODs), catalase (CAT) and glutathione peroxidase (GPX). This work was designed to assess the effects of interleukin (IL)-1beta and IL-6 on the enzymatic activity and gene expression of SODs, CAT and GPX in bovine chondrocytes.

METHODS

Bovine chondrocytes were cultured in monolayer for 4-96 h in the absence or in the presence of IL-1beta (0.018-1.8ng/ml) or IL-6 (10-100 ng/ml). To study signal transduction pathway, inhibitors of mitogen-activated protein kinases (MAPK) (PD98059, SB203580 and SP600125) (5-20 microM) and nuclear factor (NF)-kappaB inhibitors [BAY11-7082 (1-10 microM) and MG132 (0.1-10 microM)] were used. SODs, CAT and GPX enzymatic activities were evaluated in cellular extract by using colorimetric enzymatic assays. Mn SODs, Cu/Zn SOD, extracellular SOD (EC SOD), CAT and GPX gene expressions were quantified by real-time and quantitative polymerase chain reaction (PCR).

RESULTS

Mn SOD and GPX activities were dose and time-dependently increased by IL-1beta. In parallel, IL-1beta markedly enhanced Mn SOD and GPX gene expressions, but decreased Cu/Zn SOD, EC SOD and CAT gene expressions. Induction of SOD enzymatic activity and Mn SOD mRNA expression were inhibited by NF-kappaB inhibitors but not by MAPK inhibitors. IL-6 effects were similar but weaker than those of IL-1beta.

CONCLUSIONS

In conclusion, IL-1beta, and to a lesser extend IL-6, dysregulates enzymatic antioxidant defenses in chondrocyte. These changes could lead to a transient accumulation of H(2)O(2) in mitochondria, and consequently to mitochondria damage. These changes contribute to explain the mitochondrial dysfunction observed in osteoarthritis chondrocytes.

Osteoarthritis and nitric oxide

Osteoarthritis (OA) is caused by both biochemical and mechanical factors. While the mechanisms that underlie the disease are not completely understood, investigators have characterized a number of catabolic and protective factors that have a role in the disease process. Nitric oxide (NO) and its redox derivatives appear to have a number of different functions in both normal and pathophysiological joint conditions. Until recently, NO was considered a catabolic factor that was responsible for perpetuating the OA disease process by mediating the expression of proinflammatory cytokines, inhibiting the synthesis of collagen and proteoglycans and inducing apoptosis. However, recent studies suggest that NO and its redox derivatives may also have protective effects on cartilage. This review will summarize the literature on the effects of NO on cartilage and chondrocytes as well as discuss some evidence that suggests potential protective effects of NO and/or its derivatives on other cell types. More research is needed to elucidate the role of NO and its derivatives on both normal and osteoarthritis cartilage.

Spingosine-1-phosphate stimulates proliferation and counteracts interleukin-1 induced nitric oxide formation in articular chondrocytes

OBJECTIVE

Sphingosine-1-phosphate (S1P) is a messenger molecule, with important functions in inflammation and wound healing. The present study was performed to elucidate a possible role of S1P signaling in articular chondrocytes.

METHODS

Human and bovine primary chondrocytes were cultured in monolayer. Reverse transcriptase polymerase chain reaction (RT-PCR) was performed to detect S1P receptor mRNA. Proliferation of S1P stimulated chondrocytes was measured by 3H-thymidine uptake. Supernatants of cultured bovine chondrocytes stimulated with S1P alone or in combination with interleukin-1beta (IL-1beta) were tested for nitric oxide (NO) formation and expression of inducible nitric oxide synthase (iNOS). Matrixmetalloprotease-13 (MMP-13) and aggrecanase-1 (ADAMTS-4) were evaluated using real-time PCR. Glycosaminoglycan (GAG) loss from bovine cartilage explants was evaluated using the dimethylene blue method.

RESULTS

S1P1, S1P2 and S1P3 but not S1P4 and S1P5 receptor mRNA were detected in human and bovine chondrocytes. S1P dose dependently induced proliferation in bovine and human chondrocytes. S1P significantly reduced NO formation and iNOS mRNA and protein expression, both in un-stimulated and IL-1beta stimulated bovine chondrocytes. Furthermore, S1P dose dependently inhibited IL-1beta induced expression of ADAMTS-4 and MMP-13 and diminished IL-1beta mediated GAG depletion from cartilage explants.

CONCLUSION

These results suggest that S1P provides an anti-catabolic signal in articular chondrocytes.

Cyclic equibiaxial tensile strain induces both anabolic and catabolic responses in articular chondrocytes

Mechanical disturbance is directly implicated in the development of osteoarthritis (OA) but the precise mode for degenerative changes is still largely unknown because of the complexity of the biomechanical and biochemical milieu in the articular joint. To investigate the effects of tensile strain on articular cartilage, cyclic equibiaxial tensile strain (CTS, 0.5 Hz, 10% strain) was applied to monolayer cultures of porcine articular chondrocytes by using a Flexercell strain unit. Overproduction of proinflammatory mediators and imbalanced expression of anabolic and catabolic genes were induced. The cellular secretion of nitric oxide (NO) and prostaglandin E(2) (PGE(2)), as well as the mRNA level of cyclooxygenase-2 (COX-2) were up-regulated in response to mechanical stimuli. Additionally, CTS resulted in an initial peak of anabolic response at 3 h of stretch with respect to the expression of type II collagen and aggrecan. After 12 h of CTS, the expression for these two cartilage-specific matrix proteins fell to control levels. A distinct catabolic response developed after 24 h of stretch with an increase in matrix metalloproteinase-1 (MMP-1). Interestingly, a parallel increase in transforming growth factor (TGF) beta3 was associated with the anabolic changes while an increase in expression of TGF beta1, the predominant isoform of the TGF family, appeared at 24 h. The expression at 24 h of MMP-1, an enzyme that degrades interstitial collagens as well as other cartilage matrix proteins and TGF beta1, may signify a shift towards matrix remodeling and potentially a change in matrix composition as a consequence of continuous CTS.

Functional genomics, evo-devo and systems biology: a chance to overcome complexity?

PURPOSE OF REVIEW

This review addresses the key question of how to integrate a high complexity of processes and data to a unifying picture of disease processes and progression relevant for osteoarthritis.

RECENT FINDINGS

Many research efforts in the last few years have resulted in the accumulation of a huge amount of data. To date, however, these data have not led to a unifying concept of the pathogenesis and progression of the osteoarthritic disease process. Methods to integrate a lot of information are needed, therefore, in order to progress from experimental findings to practical knowledge. Several such strategies have been followed up in the past: in-vitro models, large-scale gene expression analysis/functional genomics, and an attempt to interpret gene expression patterns on the basis of developmental chondrocyte differentiation. A novel approach is systems biology, which promises to overcome issues of complexity using appropriate models and quantitative simulation.

SUMMARY

Efforts are required to integrate a continuously growing high complexity of experimental data into an understanding of the joint system and its derangement in osteoarthritis. Modelling of the 'whole' picture appears to be needed so that we do not get lost in the plethora of details.

Activation of interleukin-1 signaling cascades in normal and osteoarthritic articular cartilage

Interleukin (IL)-1 is one of the most important catabolic cytokines in rheumatoid arthritis. In this study, we were interested in whether we could identify IL-1 expression and activity within normal and osteoarthritic cartilage. mRNA expression of IL-1beta and of one of its major target genes, IL-6, was observed at very low levels in normal cartilage, whereas only a minor up-regulation of these cytokines was noted in osteoarthritic cartilage, suggesting that IL-1 signaling is not a major event in osteoarthritis. However, immunolocalization of central mediators involved in IL-1 signaling pathways [38-kd protein kinases, phospho (P)-38-kd protein kinases, extracellular signal-regulated kinase 1/2, P-extracellular signal-regulated kinase 1/2, c-Jun NH(2)-terminal kinase 1/2, P-c-Jun NH(2)-terminal kinase 1/2, and nuclear factor kappaB] showed that the four IL-1 signaling cascades are functional in normal and osteoarthritic articular chondrocytes. In vivo, we found that IL-1 expression and signaling mechanisms were detectible in the upper zones of normal cartilage, whereas these observations were more pronounced in the upper portions of osteoarthritic cartilage. Given these expression and distribution patterns, our data support two roles for IL-1 in the pathophysiology of articular cartilage. First, chondrocytes in the upper zone of osteoarthritic articular cartilage seem to activate catabolic signaling pathways that may be in response to diffusion of external IL-1 from the synovial fluid. Second, IL-1 seems to be involved in normal cartilage tissue homeostasis as shown by identification of baseline expression patterns and signaling cascade activation.

Heat-shock proteins and their role in chondrocyte protection, an application for autologous transplantation

Articular cartilage injury presents a unique therapeutic challenge. As cartilage possesses no blood or nerve supply of its own it has a particular susceptibility to early injury and a poor capacity for self-repair. Treatment options are limited and injury can eventually lead to osteoarthritis. Autologous chondrocyte transplantation is an exciting therapeutic development, but despite initial encouraging results, graft failure and formation of fibro- as opposed to hyaline cartilage remain problematic. Bleeding is an inevitable consequence of surgery, and blood-induced cartilage damage is well documented. It is hypothesised here that protecting chondrocytes against blood could significantly improve results. Heat-shock protein induction may confer chondroprotection. The expression of heat-shock proteins in human chondrocytes and rat femoral head cartilage following heat shock was analysed by Western blotting, and red-blood-cell-induced chondrocyte death was assessed by cell viability and apoptosis by flow cytometry. We demonstrate that heat-shock induced expression of heat-shock protein 70 (HSP70) (rat and human) and HSP32 (human). Blood and blood products reduced rat cartilage proteoglycan synthesis and human chondrocyte viability, and induced human chondrocyte apoptosis at concentrations considerably lower than those reported previously. The induction of HSP70 in rat cartilage was ineffective in reducing chondrocyte death in the absence or presence of red blood cells or red cell products. Heat shock to human chondrocytes reduced low levels of apoptosis (<20%) and cell death induced by low levels of blood products, but not higher levels. Induction of HSP32 with diacetylrhein appeared to be more effective and may hold greater promise. Blood has potent adverse effects on chondrocytes and the induction and chondroprotective effects of heat-shock proteins could be applied to increase the initial success of implanted chondrocytes improving the outcome of autologous chondrocyte transplantation.

Effects of hyaluronan on nitric oxide levels and superoxide dismutase activities in synovial fluid in knee osteoarthritis

The aim of the present study was to evaluate the effects of hyaluronan (HA) on nitric oxide (NO) levels and superoxide dismutase (SOD) enzyme activities in synovial fluid (SF) in the treatment of patients with knee osteoarthritis (OA). SF samples were aspirated from OA patients before the commencement of the treatment (n=23) and 6 weeks after they were treated with HA products. NO levels and SOD activities were compared between the pre- and post-treatment of OA patients and of the control group (n=10). SF NO levels were significantly higher in patients with OA before the commencement of the treatment compared with the post-treatment (p<0.001) and the control groups. The SF SOD activity of patients before the commencement of the treatment was lower than the values in the controls and post-treatment (p<0.001). There is no significant correlation between SF NO and SOD levels and the radiographic changes of the OA knee according to Kellgren-Lawrence grading (p>0.05). Also, the Western Ontario and McMaster Universities Osteoarthritis index (WOMAC) pain scores and physical function scores were gradually improved. These findings made us think that SF NO was a potent mediator in cartilage damage in OA, whereas SOD was an antioxidant mediator in the same process. Exogenous HA injections might reduce the NO levels and increase SOD activities in synovial fluid. These effects also do not seem to be dependent on the radiographic grading of the OA knee. More comprehensive studies are needed to clarify a possible clinical significance of this topic, and we suggest that this is an important area for further research into new treatment options.