Page 846 - Adams and Stashak's Lameness in Horses, 7th Edition

812 Chapter 7

proteoglycans (aggrecan, decorin, fibromodulin, link pro­ demonstrated cleavage of HA by free radicals. 32,116 There
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tein) and type IV, V, VII, IX, and XI collagen. It also is also evidence that superoxide can degrade the alpha
VetBooks.ir of IL‐1‐induced cartilage degeneration has revealed evi­ treatment inhibits gelatin. Proteoglycans may be
chains of collagen based on the finding that superoxide
cleaves type II collagen in nonhelical sites. In vitro study
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cleaved by free radicals. Increased free radicals in the
dence of collagen degradation that could be attributed to
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stromelysin in addition to that attributed to collagenase. synovial fluid of cases of equine joint disease have been
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Molecular cloning and cartilage gene expression of equine demonstrated more recently. 20
stromelysin 1 (MMP‐3) has been described by Balkman Nitric oxide has been recognized as an important
and Nixon. Stromelysin cleaves the protein backbone of physiologic mediator. It combines avidly with superoxide
2
cartilage proteoglycans at the asparagine 341–phenylala­ anion, and although this was originally thought to pro­
nine 342 bond. Cleavage due to aggrecanase, on the other vide a protective function, it has been suggested that this
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hand, occurs at the GLU373‐ALA373 site. Based on pre­ reaction can generate further destructive species, includ­
liminary work in the horse using markers that differentiate ing peroxynitrite anion and hydroxyl radicals. A recent
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these two cleavage sites, it appears that aggrecanase is the review has noted that the specific effect of a radical on a
principal enzyme degrading proteoglycan in equine joint given cell function very much depends on experimental
disease. 51 context. Specifically, the simultaneous presence of super­
Equine MMPs 2 and 9 are two gelatinases that have oxide, which leads to the formation of peroxynitrite (due
been characterized in the horse. 11,12 It is known that the to interaction of NO and supraoxide), a stronger and
one‐fourth and three‐fourth fragments generated by highly reactive radical, is likely to lead to cell and tissue
cleavage by fibrin or collagens by collagenases can damage. However, NO has certain protective effects in
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unwind and are then susceptible to further cleavage by cartilage and other tissues, and inhibition of NO in vitro
MMP‐2 and MMP‐9. MMPs 2 and 9 are produced by a or in vivo can potentially lead to undesired exacerbation
variety of equine cell types, and these enzymes have been of inflammation and tissue destruction under certain con­
demonstrated in elevated levels in synovial fluids from ditions. Selective scavengers of peroxynitrite must be
horses with joint disease. 11,12 more promising candidates for the treatment of OA than
The metalloproteinases are inhibited by two tissue specific inhibition of NO. The role of nitric oxide in joint
inhibitors of metalloproteinase and these are known disease needs, and is receiving, further attention.
commonly as TIMP (TIMP‐1 and TIMP‐2). 75,102 TIMP is
found in many connective tissues and may be the most
important inhibitor found in articular cartilage. This Cytokines and Articular Cartilage Degradation
current belief is that the balance between MMPs and Cytokines are soluble peptides produced by one cell
TIMP is important to prevent progression of articular affecting the activity of other cell types. Studies of
cartilage degradation. cytokines in joint tissues suggest that IL‐1 and TNFα
In summary, MMPs are considered to play a major modulate the synthesis of metalloproteinases by both
role in articular cartilage degradation. They are secreted chondrocytes and synovial cells 16,17,117 and are therefore
as latent proenzymes and activated extracellularly by important mediators in joint disease. Because IL‐1 and
serine proteinases. Plasmin may activate stromelysin, TNFα may be produced by synovial cells, they may
17
which in turn is an important activator of collagenase. therefore be of importance in the deleterious effects of
Upregulation of these enzymes in synovial membrane synovitis on articular cartilage. It is thought that the
and articular cartilage samples from traumatic equine normal turnover of the extracellular matrix of the artic­
joint disease has been demonstrated. It is also known ular cartilage is regulated by the chondrocytes under the
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that the production of MMPs is upregulated by IL‐1. control and influence of cytokines and mechanical stim­
uli. Articular cartilage degradation in association with
disease represents an exacerbation of these normal pro­
Prostaglandins
cesses. It is widely accepted that the cytokine IL‐1
Prostaglandins (primarily E group) are produced in induces proteoglycan depletion in articular cartilage by
inflamed joints and can cause a decrease in the proteo­ either increasing the rate of degradation or decreasing
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glycan content of the cartilage matrix. The presence of synthesis in association with the release of proteinases
prostaglandin E in synovial fluid from inflamed joints and prostaglandins from chondrocytes (Figure 7.10).
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has been demonstrated in the horse. In the author’s IL‐1 produces its effects by binding with an IL‐1 receptor
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laboratory, PGE measurements are used as an objective on the cell. The presence of IL‐1 in equine osteoarthritic
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index of the level of synovitis. 26,43 Actions of PGE in joints was first reported by Morris et al. An equine
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joints include vasodilation, enhancement of pain percep­ IL‐1 containing extract was produced by May in 1990. 59
tion, proteoglycan depletion from cartilage (by both The significant role of equine IL‐1 has been further
degradation and inhibition of synthesis), bone deminer­ consolidated, starting with the cDNA sequences for
alization, and promotion of plasminogen activator IL‐1α and IL‐1β being constructed. 34,35 After generation
secretion. PGE is released from chondrocytes on stimu­ of these DNA sequences, the IL‐1α and IL‐1β recombi­
2
lation of these cells by IL‐1 and TNFα. nant proteins were purified. Prior to that, only human
recombinant IL‐1 protein was available. Using equine
articular cartilage explants, significant proteoglycan
Oxygen‐Derived Free Radicals
release was induced by both rEq‐IL‐1α and rEq‐IL‐1β at
Oxygen‐derived free radicals, including superoxide concentrations greater than or equal to 0.01 ng/mL with
anion, hydroxyl radicals, and hydrogen peroxide, may 38%–76% and 88%–90% of total GAG released by
be released from injured joint tissues. Studies have 4 and 6 days, respectively. 42,104 Significant inhibition of