What are glycosaminoglycans?
Proteoglycans are very large molecules consisting of proteins with attached chains of polysaccharides called glycosaminoglycans (GAGs)(see). GAG chains contain repeating units of modified sugars: one of two amino sugars (glucosamine or galactosamine) and a uronic acid. Many of these chains attach to a protein core and are collectively referred to as a proteoglycan (PG) monomer. Imagine, if you will, a bottlebrush with the bristles as GAGs. The molecular weight of a PG monomer may be one million. In articular cartilage, up to a hundred of these monomers can link to a hyaluronic acid chain to form a PG aggregate. The molecular weight of the aggregate may be as much as 100,000,000.
The major GAGs are classified according to the saccharide-uronic acid subunits:
- Hyaluronic acid (HA), which is unsulfated, is a large molecule that is found in the synovial fluid of articular joints.
- Dermatan sulfate, a relatively small GAG, is widely distributed in the body (skin, blood vessels and heart valves). It is found in small amounts in cartilage and dense connective tissue.
- Chondroitin sulfate (CS), a very large molecule, often aggregates with HA. CS is the most abundant GAG in the body and predominant in cartilage, tendons and ligaments.
- Heparin is an intracellular component of mast cells and exists in the liver, lung and skin. It is used clinically as an anti-coagulant and lipid-clearing agent.
- Keratan sulfate is found along with chondroitin sulfates in several types of connective tissue, including cartilage.
With the exception of hyaluronic acid, GAG units are sulfated and, consequently, highly negatively charged (due to high density of SO4- and COO-), allowing attraction and binding of water. The nature of the high density of negative charges imparts the physical properties to PGs. Because of their great attraction for water, PGs are viscous, making them ideal for lubricating fluid in joints. The charges repel each other, which gives them an open structure and is space-filling. These biochemical traits contribute to the mechanical properties of PGs in articular cartilage, such as absorption and distribution of compressive weight, and protect structures in the joints from mechanical damage.
PGs carry two or more types of GAG chains, whose size and composition change with species, age or disease. Part- of the series discussed the alterations in connective tissue during the aging process and in some pathophysiologies, due to nutritional deficits, overuse, inflammation, diabetes, and by several pharmaceuticals (see Part - ). Following secretion by the chondrocytes and fibroblasts, PGs are continuously but slowly turned over ( ). Recall from Part 1 that mechanisms of PG turnover must be synchronized with synthesis so that PG content is maintained at a constant level. Since PGs contribute to regulation of collagen synthesis by tissue cells, sustained PG loss may precede major loss of collagen.
As mentioned previously (), animals serve as an important model for connective tissue (CT) research. Therefore, most of our understanding of CT metabolism and physiology is derived from animal and in vitro studies. Obtaining intact human CT tissue for histological and biochemical studies is often prohibitive. However, intact human CT tissues for studies have been obtained from fresh cadavers or patients with orthopedic surgery. Some studies have used urine concentrations of biochemical markers (indices) to measure collagen breakdown in exercise (1). Although less predominant in the literature, measurements of cartilage PG metabolites may be used as markers, but require extractions of joint fluid (2). Arguably, these indirect indices are not as reliable as actual tissue studies; therefore, human CT studies in vivo rely mostly on clinical cases. Also, methodology problems have plagued literature of human CT. Therefore, extreme care must be used in interpretation and extrapolation of the literature.
A recent non-conventional modality for some CT injuries and diseases is a group of naturally occurring compounds referred to as ‘chondroprotective agents’ (CAs). Often grouped in a larger class of substances called ‘nutriceuticals’, these agents are non-pharmaceuticals as well. Because the literature on these substances has been contradictory, most mainstream physicians and therapy for CT rehabilitation and diseases have not acknowledged the benefits of (CAs). Although claims that CAs alone may cure joint diseases or completely prevent injury are surely misguided, they may indeed prove worthy as adjunct therapy. Procuring and maintaining control of other factors, such as weight control and diet changes, is a crucial action. As well, other modalities, such as physical therapy, thermotherapy and appropriate pharmaceuticals, may be implemented in conjunction with the aforementioned factors and CAs. The various CAs available are discussed here with reference to other modalities where applicable.
Interestingly, CAs have been used for veterinary purposes for several decades. Only within the last 10 years have these compounds been studied for human therapy. Nearly all of the human research focuses on osteoarthritis (OA), with little, if any, addressing other CT such as ligament and tendon maladies. Currently, some veterinarians are using CAs as an adjunct modality for equine tendon and ligament injuries (personal communications). Hopefully, such treatments will eventually be studied in humans as well. Until then, extrapolation of CA literature to tendon and ligament metabolism is limited.
Several classes of compounds are referred to as CAs, with varied chemical structure and effectiveness. The OA research focuses mostly on delayed cartilage breakdown and stimulation of cartilage regeneration, with concomitant alleviation of symptoms such as pain, stiffness, etc. These compounds may be administered by injecting into the articular joint (intra-articular), intramuscularly (into the muscle), or orally. A few of those compounds delivered intra-articularly and intramuscularly are discussed here; however, emphasis will be on the orally administered compounds.
Pentosan polysulfate (PPS) is a semisynthetic product derived from beech trees. As a polysulfated polysaccharide, it is similar to heparin. Although some athletic ‘gurus’ tout its use for athletes with CT injuries, human use of PPS is approved in the US only for the management of interstitial cystitis (inflammation of the bladder) (5). It is used clinically here and in Europe as an antithrombotic agent.
A few research studies have shown some benefit in treating 0A in animals, but results are inconclusive because of administration route. Intra-articular injections of PPS reduced hyaluronic acid loss induced by concomitant intra-articular injections of hydrocortisone in rabbit joint cartilage (3). Additionally, intra-articular injections of PPS reduced cartilage erosion in canines with artificially induced OA (4). However, several cases have been reported in humans of PPS-induced thrombocytopenia, a condition of abnormally small number of platelets in circulating blood and which may lead to stroke (6,7). The literature suggests that PPS may induce this condition if the route of administration is intramuscular or subcutaneous and regardless of dose (prophylactic or curative). Although the literature does not address the safety of intra-articular injections of PPS, this may be the only safe route of administration to treat OA.
Sodium hyaluronate (HA) is a high-molecular-weight polysaccharide manufactured from bacterial fermentation. It differs from other GAGs in that it is unsulfated. Recall form Part 1-2 of this series that normal synovial fluid contains hyaluronic acid as a natural lubricating and cushioning substance. It is also a very integral component of articular cartilage PGs.
Long used in treatment of OA in horses, HA and derivatives have also been administered for use in treatment of human OA. Having been used clinically for several decades in Europe, most of the studies with HA originate from overseas. Because HA is not well absorbed orally, intra-articular injections of highly-purified HA aim to restore the fluid properties of the extracellular matrix in arthritic joints. Although the mechanisms of action are not clear, scientists posit that HA modulates several cellular functions thereby reducing inflammation and pain response (8).
Glycosaminoglycan polysulfates (Arteparon)
A group of over-sulfated chondroitin GAGs have been studied and used extensively in Europe for several decades. Arteparon is the trade name for the most commonly used glycosaminoglycan polysulfate in human administration, whereas Adequan is used for veterinary cases in dogs and horses. Arteparon is commonly administered intra-articularly, but the intramuscular route has also proven to be therapeutic as well. Several clinical studies have demonstrated Arteparon’s effectiveness in treatment of cartilage calcification, chondromalacia (softening of the cartilage) and other degenerative joint diseases. Proposed mechanisms include anabolic (increases synthesis of PGs and collagen) and anticatabolic (inhibition of degradative enzymes and inflammatory mediators) effects.
When GAGs are injected into the joint, the commencement of action is rapid and pain relief may appear after a few days. However, treatment with the compounds above requires many (3-5) weekly injections for several weeks. Consequently, treatment utilizing this route of administration necessitates several office visits and a high cost. Alternatively, oral administration of GAGs (glucosamine and chondroitin sulfate) may offer a less expensive and easily administered modality for joint injuries and degenerative diseases.
Until recently physicians have relied mostly on symptom alleviation to restore a degree of normal mobility and function to patients with OA and other joint degenerative diseases. Conventional treatment generally includes non-steroidal anti-inflammatory drugs (NSAIDs) or corticosteroids. However, as discussed inof this series, use of these pharmaceuticals is not devoid of side effects. Additionally, research has shown that long-term use of these substances may inhibit synthesis of collagen and GAGs, depressing the repair mechanisms. Search for new treatments has focused on substances that might enhance synthesis and inhibit catabolism of matrix components. Affordability and ease of administration have also been strong criteria. An increasing number of research and clinical studies support that oral GAGs (glucosamine and chondroitin sulfates) may be likely candidates.
Glucosamine (GA) is a naturally occurring amino-sugar synthesized by chondrocytes from glucose. Most GAGs contain glucosamine: heparin, hyaluronate, keratan sulfates. As well, GA easily converts to galactosamine (by enzymes), which is incorporated into chondroitin and dermatan sulfates. Since glucosamine availability is the rate-limiting step in GAG and PG synthesis, increases in availability of GA may augment synthesis of these macromolecules. Conceivably, enhanced synthesis of GAGs and PGs may overcome or possibly reverse some of the degradation that occurs with joint injuries and diseases.
Unlike other GAGs, studies have used intramuscular, intravenous and oral routes of administration of GA in animal models years prior to studies in humans. Similarly, clinical veterinary use of GA in canines and equines has been prevalent for decades. Many authors report good to excellent efficacy of GAGs for OA and other degenerative joint diseases in these animal species (9-11).
Results from human trials demonstrate that GA may produce a gradual and progressive reduction in joint pain as well as an increase in joint mobility and function with no toxicity (12,13). In fact, some studies show that GA may be equal to treatment with some NSAIDs in controlling symptoms with less side effects (14,15). Based on several recent short-term studies, there is increasing evidence suggesting that GA may provide therapeutic benefits for individuals with OA.
Much of the information available on absorption, bioavailability, and efficacy in animal models has laid the foundation for human pharmacokinetics and therapeutic effects. GA administered as a salt (hydrochloride, sulfate, or hydroiodide) is well absorbed in animals and humans (16,17). Moreover, studies show the pharmacokinetics of GA in humans does not differ significantly from that in rats and dogs. Approximately 87% of orally administered radiolabeled GA was absorbed with approximately 26% bioavailability after first-pass metabolism in humans. Radiolabeled GA absorbed from the gut is well distributed in the plasma and subsequently into tissues throughout the body. Articular cartilage is one of the tissues with highest concentrations.
Chondroitin sulfate (CS) is found in many tissues in the body such as tendon, bone, and eye cornea. Additionally, CS is the most abundant GAG in articular cartilage. CS has been demonstrated in vitro to inhibit several degradative enzymes that destroy cartilage and exhibit anti-inflammatory activity. Therefore, authors postulate that CS has a protective effect rather than an anabolic effect as seen in GA.
Similar to GA studies, CS has been demonstrated in clinical trials to increase movement as well as decrease pain and use of NSAIDs in human OA patients (23-25). As in the case of GA, the therapeutic response to CS is gradual, appearing weeks after beginning of therapy. Exogenous GAGs require prolonged periods of treatment because the compounds must enter into the metabolism of the joint cartilage. Nevertheless, the clinical improvements persist after stopping treatment. As well, patients report few side effects.
The literature on bioavailability studies is conflicting. Baici et al reported statistically little change in serum GAG concentration after oral administration of CS in humans (18). However, the validity of methodology used in the Baici et al study was questioned (19). Conte et al demonstrated in two studies, of which one included radiolabeled CS, that 70% of oral doses were absorbed in rats and dogs (20,21). Radioactivity was associated with high, intermediate and low molecular mass polysaccharide compounds. Because CS is a large molecule, authors posit that it is partially absorbed in the gut after digestion with smaller molecules being preferentially absorbed (21,22). Conte et al also demonstrated an increased (10-20%) steady-state plasma level of CS when administered daily to human subjects after 2-3 days (21). After 5 days of daily CS administration, increases in hyaluronate and changes in GAG size were observed in synovial fluid samples from human subjects. Such results demonstrate that polysaccharides originating from oral GAGs are incorporated into tissues.
Synergy of GA and CS
Although studies report beneficial results from using the two GAGs singly, some authors speculate that combining the two GAGs are synergistic. Because glucosamine and chondroitin sulfate have beneficial but different mechanisms of action, combining these compounds produces a synergistic response in articular cartilage. Using the two GAGs together will:
- stimulate chondrocyte and synoviocyte metabolism,
- inhibit degradative enzymes.
Consequently, concomitant use of both GAGs may result in a net increase in cartilage synthesis thereby slowing progression of OA as well as reducing disease symptoms. Several studies in both animal and human models have administered CS and GA combined, but no comparisons to singularly administered GAGs have been made.
The available published studies offer promising results of improvement in affliction from osteoarthritis with treatment of exogenous GAGs. However, the evidence is met with controversy from mainstream medical practitioners. For example, GAGs are not recommended by the Arthritis Foundation. Despite mounting testimony that GAGs may have a role in management of osteoarthritis, their use is not recommended based on the available scientific studies due to serious design flaws or insufficient details. Studies are criticized for their small sample populations. Additionally, although no short-term toxicity has been reported, long-term safety of GAGs needs to be investigated. Although few side-effects in humans have been reported, GAG effects on patients with underlying diseases should be examined, especially diseases affecting coagulation. Thirdly, no studies have examined their use in other forms of arthritis or other connective tissue maladies.
Historically, most of the data on use of GAGs has been derived from European studies. Until last year, no studies had been published from the US. Das et al conducted the first clinical investigation in the USA of GAG use for treatment of OA (27). Philippi et al performed the first study of GAG use to treat degenerative joint disease of the knee or low back in athletic populations (28). Both studies demonstrate effectiveness of GAGs in treatment of OA in the knee. The second study did not show any statistical benefit in spinal degenerative joint disease. There was, however, a trend for some benefit and the authors suggest a follow-up trial with a larger sample base over a longer time period for further elucidation.
A protocol has been established for design and conduct of clinical trials in studies of OA (26). Hopefully, more studies will examine the most effective dose and long-term effects of GAGs as well as their combination with traditional OA treatments. On-going animal research may demonstrate GAG efficacy in other forms of connective tissue diseases. In the interim, individuals are advised to follow standard treatment recommendations, such as weight control, exercise, adequate nutrition and thermotherapy. Nonetheless, use of GAGs, in conjunction with proper use of other medications, may provide additional relief from symptoms and protect cartilage from degradation.
Glucosamine and chondroitin sulfate are available over the counter in many commercial products: singly and combined. Because they are considered a natural product and a dietary supplement, GAGs are not evaluated nor regulated by the Federal Drug Administration (FDA) for purity which can vary tremendously depending on extraction techniques and analysis technology. Purity can determine effectiveness, especially considering that all research and clinical studies used purified substances. Information from University of Maryland School of Pharmacy has shown that analysis of several commercial GAG products do not meet label claims (29). Purchasers should therefore be careful to buy from a reputable manufacturer that uses pure substances and can validate their finished product. Nutramax Laboratories Inc (31) currently offers the only patented GAG combination product for animal and human use in the US. Their products are used extensively in both animal and human clinical trials.
According to the studies, the standard daily dosage for glucosamine is 1000-1500 mg and 800-1200 mg of chondroitin sulfate divided into 2-3 dosages. A loading dose is recommended for a minimum of two months. Most individuals should see an improvement in eight weeks or less. Thereafter, daily maintenance dosages may be reduced to 500 mg GA and 400 mg of CS or more, depending on disease status. Two other compounds that are frequently used with GAGs are manganese and ascorbic acid. Manganese is a mineral that serves as a cofactor in biochemical reactions in joint connective tissue metabolism, such as GAG synthesis. Deficiencies of manganese result in formation of abnormal bone and cartilage. However, evidence of efficacy of manganese in osteoarthritis is lacking. Recall from previous sections of this series that ascorbic acid (vitamin C) is an important cofactor in collagen synthesis and deficiencies result in poor wound healing (see Part 3).
Questions raised by individuals with diabetes address the safety of GAG use. Although GA and CS are classed as carbohydrates, the body does not break them down into glucose. Consequently, they will not raise blood sugar levels by providing a source of glucose. However, since many factors can affect insulin secretion and blood glucose levels in diabetic patients, those who use GAGs are advised to check their glucose levels frequently.
The general media has recently proclaimed GAGs as the “cure” for arthritis. The evidence supporting this claim is less than impressive. Critical review of the literature with few well-controlled studies thus far does not support GAGs as a cure. However, mounting evidence provided by in vitro studies and improved (larger study size, consistent treatment regime, randomized and double-blind protocols) clinical trials in animals and humans demonstrate that GAGs may be effective as adjunct therapy for OA. As well, future studies will hopefully investigate their usefulness in therapy with other types of arthritis and connective tissue diseases and injuries. Thus supplementation of GAGs may be of importance to athletes, considering the stress to connective tissue during sports activities and the mounting frequency of soft tissue injuries over the last two decades (30). Nonetheless, consumers are urged to be cautious when choosing a GAG product. Studies tend to support the synergism of GA and CS and not all commercial products contain both GAGs. Additionally, because they are classified as a dietary supplement, the strength and purity of GAG products are not subject to FDA regulation or control. Therefore, look for a product from a reputable manufacturer that can provide analysis of quality. Meanwhile; eat right, train sensibly, and supplement with only that which is needed.
Note: CosaminDS is the human product; Cosaquin is the product for horses, dogs and cats. They are formulated differently based on absorption by specific species, despite recent claims by an internet athletic guru’s recommendation for athletes to dose with the animal product.
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