Osteoarthritis is a chronic disease characterised by irreversible damage to joint structures, including loss of articular cartilage, osteophyte formation, alterations in the subchondral bone and synovial inflammation. Pain, functional disability and impairment of health-related quality of life are major complaints in patients with osteoarthritis. Several compounds have been investigated for their positive effects on the relief of clinical symptoms and improvement of structural changes in osteoarthritis. It has been shown that chondroitin sulphate interferes with the progression of structural changes in joint tissues and is used in the management of patients with osteoarthritis. This review summarises data from relevant reports describing the mechanisms of action of chondroitin sulphate involved in the beneficial effects of the drug.
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Osteoarthritis (OA) is a progressive degenerative joint disease and the most common form of arthritis, especially in older persons. The disease is thought to result from biochemical changes and biomechanical stresses affecting articular tissue structure. The disorder is multifactorial, increases with ageing, and has genetic and hormonal influences. Articular tissues, including the cartilage, synovial membrane and subchondral bone, are major sites of changes during the course of the disease.
Exposure of the articular cartilage to localised physical or chemical stresses, including mechanical, inflammatory and oxidative phenomena may contribute to degradation of cartilage matrix macromolecules.1–6 From a biochemical perspective, OA may be considered as a breakdown of the extracellular matrix, in which homeostasis depends on the balance between catabolic and anabolic events. During the early stages of the disease process, there is a “hypermetabolic” response in the cartilage considered to be a reactive feedback response of chondrocytes to proteoglycan depletion in the matrix. This response is believed to be a protective attempt to counteract the effects of environmental stress and agents, and may retard the progression of cartilage degeneration.
Although destruction of the articular cartilage characterises OA, inflammation of the synovial membrane plays an important part in the progression of joint tissue lesions. However, synovial inflammation is a secondary phenomenon related to multiple factors, such as cartilage matrix degradation, and excess amounts of nitric oxide (NO) and reactive oxygen species (ROS), rather than the primary cause of the disease. In addition, it has been shown that the subchondral bone is the site of several dynamic morphological changes that are involved in the disease process.7 These changes are associated with a number of locally abnormal biochemical pathways related to the altered metabolism of osteoblasts.8–10
Some compounds have been shown to have a slow-acting symptomatic effect in OA and were termed “symptomatic slow-acting drugs for OA (SySADOA)”. Most of the compounds suggested as SySADOA are physiological molecules contained in the articular tissues and include glucosamine sulphate (GS) and chondroitin sulphate (CS). CS is a major component of the extracellular matrix of many connective tissues, including cartilage, bone, skin, ligaments and tendons. It is a sulphated glycosaminoglycan composed of a long unbranched polysaccharide chain with a repeating disaccharide structure of N-acetylgalactosamine and glucuronic acid. Most of the N-acetylgalactosamine residues are sulphated, particularly in the 4- or 6-position (CS4 and CS6), making CS a strongly charged polyanion. In the articular cartilage, aggrecan binds collagen fibrils and confines water content by limiting the degree to which glycosaminoglycans can separate. The high content of CS in the aggrecan plays a major part in allowing cartilage to resist tensile stresses during various loading conditions by providing this tissue with resistance and elasticity. It has been shown that changes occur in the structure of CS in OA tissues11 and a diminished ratio of CS6:CS4 has been found in OA synovial fluid.12 This could then constitute a way by which cartilage degenerates during this disease. Therefore, knowing the mechanism of action of this molecule is of particular importance.
BIOAVAILABILITY AND PHARMACOKINETICS OF CHONDROITIN SULPHATE
CS used in studies is mainly obtained from bovine, porcine and marine cartilage; however, most studies use a CS purified from bovine trachea (95% purity), which is the same as that used in clinical trials.13 The naturally occurring CS has a molecular weight of 50–100 kDa, and after the extraction process, its molecular weight is 10–40 kDa, depending on the raw material.
Pharmacokinetic studies performed on humans and experimental animals after oral administration of CS revealed that it can be absorbed orally.14–17 More specifically, Volpi18 showed, in a study on oral bioavailability of CS in healthy male volunteers, a significant increase in plasma levels of CS compared with pre-dose levels over a 24 h period. CS plasma levels increased (more than 200%) in all subjects, with a peak concentration after 2 h, the increase reaching significance from 2 to 6 h. Absorbed CS reaches the blood compartment as high, intermediate and low molecular mass derivatives. Moreover, from the above studies, CS shows first-order kinetics up to single doses of 3000 mg. Multiple doses of 800 mg in patients with OA do not alter the kinetics of CS. The bioavailability of CS ranges from 15% to 24% of the orally administered dose. Of the absorbed fraction of CS, 10% is in the form of CS and 90% in the form of depolymerised derivatives with a lower molecular weight. More particularly, on the articular tissue, Ronca et al19 reported that CS is rapidly absorbed in the gastrointestinal tract and a high content of labelled CS is found in the synovial fluid and cartilage.
Of note is that in in vitro studies, a large range of CS concentrations has been used (e.g. 12.5–2000 μg/ml, but generally ⩽200 μg/ml) and one could question the relevance of using the highest drug concentrations. Possible explanations could be as follows. CS is a slow-acting drug for the treatment of OA, characterised by a slow onset of action, with a maximal effect being attained after several months of treatment with a carryover effect that persists after cessation of therapy. Drugs of biological origin, such as CS, are difficult to measure in the biological fluids and to differentiate from endogenous molecules. Moreover, due to the rapid degradation of this drug to compounds with lower molecular weight, the relationship between response and plasma concentration is not obvious. Therefore, although in vivo concentrations of CS are low, the disaccharide concentrations are consequently high, so the in vivo effect of CS may be the sum of CS and disaccharide concentrations. As a consequence of the above, it is difficult to calculate the Cmax of this type of drug. However, du Souich and Vergés20 used an alternative approach to classical methods in which they suggested to predict the Emax of the drug. The authors estimated that in humans the half-life of CS and its derivatives in plasma is approximately 15 h (ie, steady state is attained in 3–4 days); however, approximately 3–6 months may be needed to obtain the maximal effect. Moreover, it was predicted that in patients with knee OA of moderate severity, 50% of Emax is achieved in approximately 35 days. Hence, in order to reproduce in vitro an effect observed in vivo obtained after several weeks of treatment, it appears necessary to increase the drug concentrations used in vitro. This could then provide a rationale as to why, in in vitro experiments, CS is sometimes at a high concentration.
EFFECTS OF CHONDROITIN SULPHATE ON ARTICULAR TISSUES
Anti-apoptotic effects of chondroitin sulphate
Chondrocytes synthesise components of the extracellular matrix and regulate cartilage metabolism; therefore, the number and function of chondrocytes preserves the morphological and biological characteristics of cartilage. Death of chondrocytes is considered an important factor contributing to the depletion of cartilage matrix in OA. Cell death includes necrosis and apoptosis. In OA cartilage, a higher number of chondrocytes showing signs of apoptosis were found compared with normal cartilage.21–23 In a study performed on an experimental mouse model, which spontaneously developed OA,24 the effects of orally administered CS (0.3 mg/day for 12 consecutive days) on the apoptotic index of chondrocytes was evaluated after 30 days of treatment. The results showed that CS induced a significant reduction in apoptotic chondrocytes. In vitro, it was also reported that CS (200 μg/ml) decreased the chondrocyte susceptibility to single nucleotide polymorphism-induced apoptosis, which appears to be concurrent with CS diminishing the activation of p38 mitogen-activated protein kinase.25
Chondroitin sulphate increases the synthesis of proteoglycans
Studies have shown that, in vitro, CS significantly induces the production of proteoglycans by human OA chondrocytes as well as the interleukin (IL)-1β depletion of proteoglycan production.26 27 In vivo, CS was also shown to induce proteoglycans. Indeed, in a model of chymopapain-induced articular cartilage injury, oral treatment and intramuscular injection with CS (80 mg/day) showed a protective effect on the damaged cartilage, with a significant increase in the synthesis of articular cartilage proteoglycans.28 CS is believed to provide building blocks for the synthesis of proteoglycans and increase the sulphate incorporation in OA proteoglycans;29 therefore, increasing its concentration could favour proteoglycan production and account for its beneficial effects.
Chondroitin sulphate reduces the effects of proteases
Extracellular matrix components modulate cellular behaviour by creating an influential cellular environment. Thus, the turnover of extracellular matrix components is an integral part of development, morphogenesis and tissue remodelling. While various types of proteases participate in matrix turnover, one group of key enzymes has specifically been related to articular tissues, this being the matrix metalloproteases (MMP). This enzyme family of calcium-dependent zinc-containing endopeptidases is known to play important roles in tissue remodelling during physiological as well as pathological processes. In cartilage, MMPs are the principal proteases capable of degrading a wide variety of the extracellular matrix components.30 Metalloprotease activity is regulated by specific inhibitors named tissue inhibitors of MMPs (TIMPs).31 The balance between MMPs and TIMPs regulates tissue remodelling under normal conditions. A deregulation of this balance is found in pathological conditions, such as OA.32 33 It has been shown that stromelysin-1, gelatinases and collagenases are of major importance in cartilage degradation. Thus, decreasing the effects of the MMPs by reducing their synthesis and/or activity could then account for the beneficial effects of CS treatment.
Hence, data show that in chondrocytes from normal human knee femoral cartilage, the addition of chondroitin polysulphate (10 μg/ml) to the culture media stimulated the accumulation of molecules such as aggrecan, hyaluronan and type II collagen in the cell-associated matrix, due in part to the downregulation of MMPs.27
Stromelysin-1 or MMP-3 is known to play an important role in proteoglycan cleavage and is critical for cartilage proteoglycan homeostasis. Furthermore, MMP-3 is involved in the activation of other pro-MMPs, including pro-collagenases.34 35 In human OA chondrocytes, CS (150 μg/ml) was shown to inhibit by 28% the IL-1β-induced MMP-3 synthesis.36
Release and activation of the gelatinase B or MMP-9 triggers bone and cartilage degradation, which can exacerbate joint degeneration.37–39 In an arthritis rat model using Freund’s adjuvant, treatment with CS in a dietary bar formulation at a concentration of 18 mg CS/g alone and in combination with GS (22.5 mg/g) prevented the increased joint MMP-9 levels associated with arthritis.40
Collagenase-3 or MMP-13 is a major enzyme involved in cartilage degradation in OA. It preferentially cleaves type II collagen and is five to 10 times more active on this substrate than collageanse-1 or MMP-1.41 Recently, it was shown that CS (25 μg/ml) down-regulated lipopolysaccharide-induced MMP-13 in chondrocytes, which was concurrent with a reduction of p38 mitogen-activated protein kinase and extracellular signal-regulated kinase 1/2 activation.42 In another study, CS (20 μg/ml) alone or in combination with GS (5 μg/ml) also decreased the IL-1β-induced MMP-13 expression by chondrocytes.43
Chondroitin sulphate has anti-inflammatory properties
Joint lubrication is naturally provided, at least in part, by hyaluronic acid in the synovial fluid. Hyaluronic acid is present in abundance in normal young and healthy joints. In degenerative OA, a dilution of high molecular weight hyaluronate has been reported.44 It is considered that an increase in hyaluronate contributes to reducing inflammation in the articular tissues. The addition of CS (10–400 μg/ml) to monolayer cultures of synovial lining cells during the log phase of growth stimulates the synthesis of hyaluronate by 11%.45 Moreover, when added during the stationary phase of growth, hyaluronate synthesis was increased by about 88%.45
Recent evidence has implicated a number of cytokines, and more particularly IL-1β in the OA pathological process. It is believed that IL-1β is the principal cytokine responsible for the degeneration of extracellular matrix components in the articular tissues of patients with OA.6 This cytokine induces a cascade of catabolic events including the upregulation of expression of other proinflammatory cytokines such as tumour necrosis factor-α, IL-6, MMPs, inducible nitric oxide synthase, cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES-1), and the release of NO and prostaglandin E2 (PGE2).
Among the different catabolic pathways that are activated by inflammatory factors, NO is of interest for at least two main reasons. First, NO and its by-products are capable of inducing the inflammatory component of OA. Secondly, it may induce tissue damage and tissue destruction; therefore, it could be responsible not only for the symptoms but also the disease process per se (table 1).
It is well established that PGE2 plays a crucial part in the pathogenesis of arthritis. PGE2 induces pain and increases the production of catabolic factors, including proinflammatory cytokines, MMPs and ROS, which in turn contribute to alterations in cartilage, synovial membrane and subchondral bone. Moreover, in addition to exerting inflammatory effects on its own, PGE2 can also potentiate the effects of other mediators of inflammation. The critical role of PGE2 in the pathology of arthritis has been demonstrated in animal models of arthritis and mice lacking COX-2 or PGE2 receptors.46–49
Until recently, COX activity had been considered the key step in prostaglandin synthesis. However, a terminal enzyme responsible for isomerisation of the COX-derived PGH2 (inactive prostaglandin precursor of PGE2) into PGE2 has been identified; this enzyme was designated PGES. To date, three distinct PGES isoforms have been identified (table 2). From the three isoforms, it has been shown that the mPGES-1 is expressed in human articular tissues, and its level is increased upon IL-1β stimulation and is upregulated in OA tissues.50–52
In vitro, bovine articular cartilage explant treatment with CS (20 μg/ml) was effective in suppressing IL-1β-induced inducible NO synthase, COX-2 and mPGES-1 gene expression.53 54 Furthermore, the effect of CS (20 μg/ml) in combination with GS (5 μg/ml) was more effective in the reduction of all these genes, and their expression levels reverted to normal values. The same authors showed that CS also reduced the production of PGE2, but only the combination of CS and GS reverted the IL-1β-induced PGE2 synthesis to control levels as well as the IL-1β-induced NO production. These data showing that the combination of both CS and GS is more efficient at reducing the level of expression of genes involved in inflammatory conditions, could account for the increased therapeutic effectiveness observed when both compounds were used together in a recent clinical trial of patients with knee OA.55
Similar data were reported for human articular chondrocytes. Indeed, in a model of chondrocyte cultivated in clusters,26 IL-1β induced an increase in PGE2 synthesis with a stronger effect during the first 2 weeks of the experiment than thereafter (from day 16 to day 32). With this model, addition of CS (500–1000 μg/ml) significantly reduced the IL-1β-induced PGE2 during the first 16 days of culture but not during the last 2 weeks. Recently it was reported that CS (200 μg/ml) reduces IL-1β-induced extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase phosphorylation and the nuclear transactivation of nuclear factor-κB, which may contribute to the anti-catabolic effects of this compound.25
However, these results could also be explained by the fact that CS has been found to inhibit several markers of inflammation in vitro and in vivo in animal models. In the murine collagen-induced arthritis model, the effect of CS on joint histopathology was evaluated. CS at a dose of 1000 mg/kg significantly diminished parameters of synovitis, including cell infiltration, fibrosis and proliferation of synovial lining cells.56 In another study using a rat model with Freund’s adjuvant arthritis,40 treatment with CS in a dietary bar formulation at a concentration of 18 mg CS/g significantly reduced IL-1β levels in joint tissues but not in serum. However, the combination of CS and GS (22.5 mg/g bar) reduced IL-1β levels both in serum and tissue.
In the same animal model, treatment with CS (1 ml/kg, intraperitoneally once a day starting at the onset of arthritis and for 10 days) caused a significant reduction in malonaldehyde (an indicator to estimate the extent of lipid peroxidation in the damaged cartilage) and blunted the depletion of endogenous antioxidant reduced glutathione and superoxide dismutase, probably by competing in scavenging for free radicals and therefore contributing to preserve the integrity of cellular membranes in the injured cartilage.57 The production of oxygen-free radicals that occurs with the development of arthritis in articular cartilage leads to decreased glutathione and superoxide dismutase levels as a consequence of their consumption during oxidative stress and cellular lysis.58 59 This contributes to increased cellular damage by favouring attack by free radicals. Therefore, CS could protect against hydrogen peroxide formation and superoxide anions.
Interaction of ROS with DNA can induce a multiplicity of products of varying structures and with differing biological impacts. The antioxidant cell defence system intercepts ROS and normally inhibits cellular and nuclear damage. When the amount of ROS produced overwhelms these endogenous defences, an increase in oxidative DNA injury occurs.60 Various studies have shown that CS4 acts as an antioxidant, thereby protecting cells from ROS damage.57 61 62
A recent study has demonstrated that the marked increases in tumour necrosis factor-α levels and myeloperoxidase activity in the plasma of collagen-induced arthritis rats were significantly reduced by treatment with CS (25 mg/kg).57 The reduction in the myeloperoxidase strongly suggests that CS induces a decrease in polymorphonuclear cell infiltration that occurs in the joint synovial tissue. This decrease and the other biochemical parameters (influx of inflammatory cells, synovial hyperplasia and erosion of bone and cartilage) were evaluated by histological analysis of joints from hind limbs and confirmed the protective effects of the CS4 and hyaluronic acid polymers.
Finally, Cho et al63 showed, using the same animal model (collagen-induced arthritis), that treatment with CS (1200 mg/kg) also significantly reduced serum IL-6 levels. However, the high doses of CS used in this study should be taken into account when considering the results.
Effect of chondroitin sulphate on subchondral bone alterations
OA is considered a complex illness in which tissues of the joint play significant roles in the initiation and/or progression of the pathophysiology. We still do not completely understand what initiates the degradation and loss of cartilage. However, recent evidence suggests a key role for the subchondral bone in the progression and/or initiation of OA and that these changes are related to altered osteoblast metabolism.7–10 Recently, three major factors that play a role in bone metabolism have been identified. These are osteoprotegerin (OPG), the receptor activator of nuclear factor-κB ligand (RANKL) and RANK. The first two factors are synthesised by osteoblasts and RANKL is essential for osteoclast differentiation and bone loss. OPG is a decoy receptor that blocks the binding of RANKL to its receptor RANK (on osteoclasts), thereby preventing osteoclastogenesis and, as a result, inhibiting bone resorption. Recently, the effect of CS (200 μg/ml) on the bone remodelling factors, RANKL and OPG, before and after stimulation with vitamin D3 (50 nM) was evaluated on human OA subchondral bone osteoblast.64 Data showed that CS upregulates OPG expression and production under basal conditions and in the presence of vitamin D3. CS significantly inhibited RANKL expression under basal conditions, and although vitamin D3 drastically upregulated its expression, the drug under vitamin D3 downregulated RANKL levels. Consequently, under basal conditions, CS significantly upregulated the expression ratio of OPG:RANKL, vitamin D3 decreased this ratio, but CS in the presence of vitamin D3 reversed this decrease. These data are of great importance, as the expression of RANKL is increased in abnormal osteoblasts65 and thereby affects the balance of OPG:RANKL resulting in bone destruction. Therefore, by increasing the OPG:RANKL ratio, CS could exert a positive effect on OA structural changes at the subchondral bone level.
This paper reviewed several mechanisms of action of CS that could be responsible for the symptomatic relief properties of this drug in patients with OA. CS has been shown to reduce proinflammatory factors, modify the cellular death process and improve the anabolism/catabolism balance of extracellular cartilage matrix. At the same time it has proven to have a positive effect on some of the pathological processes involving the synovial tissue and subchondral bone. These mechanisms could, therefore, account for the beneficial results observed in some clinical trials.66–72 In a recent study,55 CS was shown to have a significant advantage over placebo by decreasing the incidence of joint swelling, and/or effusion. The same clinical trial55 also showed that the combination of both CS and GS appears more effective than either alone, and this combination could explain the induced higher in vivo effect on symptoms in the subset of patients with moderate to severe knee pain. In addition, a recent meta-analysis73 has suggested the possibility that CS alone has a clinically relevant effect in patients with low-grade OA.
Competing interests: None.
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