2010 RWISO Journal - Roth Williams International Society of ...

Figure 4-c The extracellular activity of MMPs is controlled by the
presence of inhibitory proteins called tissue inhibitors of
metalloproteinases, or TIMPs. TIMPs bind directly to the
MMPs, causing conformational changes that prevent the
destruction of matrix proteins.
MMPs and Arthritis
The hallmark sign of arthritis is articular bone loss. In the
past, clinicians have differentiated between inflammatory arthritis
and osteoarthritis (OA). Recently, however, the cellular
processes that result in bone and cartilage loss in both forms
of arthritis have been shown to be quite similar. 11 While inflammatory
arthritis is promoted by a systemic problem, the
result is an inflammatory cytokine cascade, which ultimately
results in osteoclastic activity and bone loss at the articular
surface. OA is not a systemic problem but a local one, secondary
to oxidation reactions, free radical production, or sheer
stress—all three of which result from overuse. 12, 13 Despite
the localized nature of OA, the cascade of cellular events that
cause articular surface loss is the same as the systemically induced
cascade. An increase in TNF-α and IL-1β increases the
number of osteoclasts and their activity. TNF-α, IL-1β, IL-
6, and RANKL all cause increased expression of the MMP
genes. The end result is destruction of cartilage, bone, and
connective tissue in both arthritis models. 14-18
MMPs also respond to systemic hormones such as estrogen,
vitamin D, and parathyroid hormones. We found an association
between low estrogen levels and low vitamin D levels
in patients with severe condylar resorption. 3 All of these
hormones and cytokines are intimately involved in osteoclast
differentiation and activation. This makes sense: MMPs are
osteoclast produced and are responsible for bone and cartilage
destruction.
MMPs and the TMJ
There is substantial evidence indicating that MMPs play an
important role in bone and cartilage degradation associated
with degenerative temporomandibular joint (TMJ) arthriti-
des. This evidence supports the presence of 6 of the known
28 matrix metalloproteinases (MMP-1, MMP-2, MMP-3,
MMP-8, MMP-9, and MMP-13) in fluid or tissue samples
obtained from diseased human TMJs. 13, 16, 17, 19-34 Some cases
of degenerative joint disease also result from an imbalance
between the activities of MMPs and TIMPs, favoring unreg-
35, 36
ulated degradation of tissue by MMPs.
Tetracyclines
Because MMPs are found to be elevated in patients with
TMJ arthritis and are so destructive to articular tissues, finding
a way to reduce their activity or their production would
be helpful in treating patients with arthritis and condylar
resorption.
From 1972-1982, at the School of Dental Medicine in
Stony Brook New York, Ramurmathy and Golub discovered
that tetracyclines have anti-collagenolytic properties.
In 1998, Golub and colleagues showed that tetracyclines
inhibit bone resorption in two ways—by controlling the expression
and activity of MMPs and by regulating osteoclasts
and their activity. 37
Controlling MMPs With Tetracyclines
Tetracyclines inhibit MMPs by chelating zinc and by regulating
MMP gene expression. As noted above, MMPs need
zinc to actively cleave collagen proteins. Tetracyclines bind
divalent ions, such as zinc. By reducing the amount of free
zinc in tissues, tetracyclines reduce the number of MMPs
available. 38 In addition, tetracyclines bind to the MMP itself,
which causes a conformational change in the enzyme, inactivating
it (Figure 5). 39 Tetracyclines have also been shown to
decrease the transcription of MMPs by blocking both pro-
40, 41
tein kinase C and calmodulin pathways.
Figure 5 Tetracycline binds directly to the zinc of the MMP.
This deactivates the enzyme and protects the matrix
from degradation. Tetracycline also controls osteoclastic
activity and MMP transcription.
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