Joint Destruction in ArthritisMetalloproteinases the Spotlight.

Arthritis & Rheumatism
Official Journal of the American College of Rheumatology
EDITORIAL
JOINT DESTRUCTION IN ARTHRITIS: METALLOPROTEINASES IN
THE SPOTLIGHT
CONSTANCE E . BRINCKERHOFF
The erosion of connective tissue (cartilage,
tendon, and bone) that accompanies both rheumatoid
arthritis (RA) and osteoarthritis (OA) is, unfortunately, well known to physicians and patients alike.
This destruction is mediated largely by collagenase
and stromelysin, enzymes that are produced by the
synovial fibroblasts (1). Collagenase is rate limiting in
collagen degradation, while stromelysin degrades
noncollagen proteins, e.g., laminin, fibronectin, and
proteoglycans. Both enzymes belong to the gene family of metalloproteinases, which comprises at least 10
members (2). All metalloproteinases are active at
neutral pH, contain zinc as an integral part of their
structure, require Ca++ for activity, and are inhibited
by tissue inhibitor of metalloproteinases (TIMP).
Thus, TIMP, also a product of synovial fibroblasts,
provides a potential mechanism for controlling degradation of the extracellular matrix, but its actual role in
modulating disease has been debated.
Until now, most of our information on the
biochemical and biologic features of these enzymes
has been derived from in vitro studies utilizing either
From the Department of Medicine and the Department of
Biochemistry, Dartmouth Medical School, Hanover, New Hampshire.
Supported by NIH grant AR-26599 and by grants from the
Council for Tobacco Research and the RGK Foundation, Austin,
Texas.
Constance E. Brinckerhoff, PhD: Professor of Medicine
and Biochemistry.
Address reprint requests to Constance E. Brinckerhoff,
PhD, Department of Medicine, Dartmouth Medical School, Hanover, NH 03756.
primary cultures of human synovium taken from RA
and OA tissue, or experimentally stimulated normal
synovial cells (1,3-5). These studies showed that synovial fibroblasts can produce massive amounts of both
collagenase and stromelysin, and that these enzymes,
acting in concert, can degrade essentially all components of the extracellular matrix. Thus, the association
of collagenase and stromelysin with arthritic synovium
and with connective tissue destruction was obvious.
However, knowledge of the subtle details of localization and quantitation of metalloproteinase production
by various subpopulations of arthritic tissue was lacking, and this has prevented precise definition of pathologic mechanisms operating in each disease.
Three papers in this issue of Arthritis and
Rheumatism (6-8) go a long way toward remedying
this situation. Using the technique of in situ hybridization to detect and quantify messenger RNA (mRNA)
present in individual cells, the authors document the
production of collagenase and stromelysin mRNA in
synovial tissue from patients with RA and OA. All 3
studies demonstrated production of these enzymes by
synovial lining cells of fibroblast or macrophage origin,
but not by lymphocytes. Levels of mRNA were
greater in tissue from patients with RA compared with
OA, supporting the observation that RA is generally
the more aggressive disease.
The report by Firestein et al (6) carefully quantitates collagenase mRNA in synovial lining biopsy
specimens before and after intraarticular steroid therapy. The authors found that the expression of collagenase and of several other genes (including TIMP)
Arthritis and Rheumatism, Vol. 34, No. 9 (September 1991)
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correlated with the degree of synovial inflammation.
Of particular interest are the findings that 1) the
intraarticular injection of corticosteroids resulted in
marked clinical improvement which was accompanied
by decreased expression of collagenase mRNA and, 2 )
this clinically meaningful analysis technique could be
used with small amounts of material obtained by a
minimally invasive biopsy.
Gravallese and colleagues (7) extend these findings by demonstrating in vivo what has been observed
in vitro: the coordinate expression of both collagenase
and stromelysin by synovial cells. They go on to show
a quantitative relationship in the activation of the two
genes and suggest that this activation may be linked to
the degree of hyperplasia.
The studies by McCachren (8) concentrate on
the relationship between metalloproteinase and TIMP
mRNA levels. He found that the synovial lining cells
produce all 3 mRNAs simultaneously, providing another mechanism by which cells may regulate the
extent of matrix degradation. This is an important
finding since it demonstrates that the inhibitor is in
close physical proximity to the enzyme, and therefore,
its presence may be physiologically relevant to disease
outcome. Further, he found that TIMP levels were
lower in RA patients than in OA patients, suggesting
that this regulator of connective tissue destruction
may be less effective in RA, where metalloproteinase
levels are higher.
Thus, these 3 papers, for the first time, 1)
document the coordinate expression in vivo of collagenase and stromelysin mRNAs by synovial lining
cells, 2) show that this expression is greater in RA
tissue versus OA tissue, and 3) imply that modulation
of these mRNA levels, either therapeutically by steroids or physiologically by TIMP, may influence disease outcome. The findings are significant because
they validate in vivo the results of previous experiments in vitro. Moreover, they permit histologic and
“geographic” mapping of the cells that are making
these destructive enzymes and begin to delineate the
specific pathophysiologic mechanisms involved in RA
and OA. Equally important, they establish the power
of quantitative in situ hybridization and prove the
feasibility of using modern molecular biology techniques to evaluate disease severity and therapeutic
efficacy.
It is heartening to finally be able to address
questions of why, how, and where these enzymes,
long known to mediate the irrevocable erosion of
BRINCKERHOFF
connective tissue in arthritis, are produced. A better
understanding of the precise molecular mechanisms
regulating the expression of the genes for collagenase
and stromelysin, and of how this knowledge can be
applied to clinical medicine, is still needed. For example, we know that both inducers (9-12) and inhibitors
(13,14) of collagenase production affect transcription
of the collagenase gene by acting on a 9-basepair
sequence of DNA (termed the AP-1 site) located in the
promoter region. We also know that additional DNA
sequences participate in these responses (11,12). Determining just where these regulatory sequences are
and what transcription factors bind to them will require years of intensive research but may, in the long
run, lead to new and better therapeutic approaches. In
the immediate future, however, it is exciting to see
that techniques of modern molecular biology are applicable to clinical rheumatology and that the crucial
role of metalloproteinases in the pathology of arthritic
disease is becoming fully recognized.
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EDITORIAL
collagenase messenger RNA expression in rheumatoid
synovium. Arthritis Rheum 34:1076-1084, 1991
8. McCachren SS: Expression of metalloproteinases and
metalloprotease inhibitor in human arthritic synovium.
Arthritis Rheum 34:1085-1093, 1991
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