Connective Tissue Part 4: Glycosaminoglycans
Source: Mesomorphosis
Author: Elzi Volk
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What are glycosaminoglycans?
Proteoglycans are very large molecules consisting of
proteins with attached chains of polysaccharides called
glycosaminoglycans (GAGs)(see Part 1). 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 2-3 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 2-3). Following secretion by the chondrocytes and fibroblasts, PGs
are continuously but slowly turned over (see Part 1). 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 (Part 2), 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.
Chondroprotective Agents
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
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.
Hyaluronate
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.
Glucosamine
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 in Part 3 of 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
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.
Please send us your feedback on
this article.
Elzi Volk
elzi@thinkmuscle.com
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31. Nutramax Laboratories, Inc. Baltimore, Maryland. www.cosamin.com.
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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