From St George's Hospital Medical School, Histopathology
Department, London, UK.
Correspondence to M.J. Davies, MD, FRCP, FRCPath, FACC, St George's Hospital Medical School, BHF Cardiovascular Pathology Unit, Histopathology Department, Cranmer Terrace, London SW17 ORE, UK.
Understanding
of the factors that lead to atherosclerotic plaque instability causing
thrombosis is increasing rapidly. The morphological characteristics of
plaques that are unstable, ie, complicated by recent thrombosis, lend
insight into the structural and cellular features of presently
stable plaques that are vulnerable, ie, at high risk of becoming
unstable in the future. The risk of any individual with
coronary atherosclerosis developing an acute
ischemic event depends on the number of such vulnerable plaques
present in that individual rather than the number of plaques
overall. One factor in the variation in risk for a further acute event
after, for example, an acute myocardial infarction is the variation
from individual to individual in the number of vulnerable plaques.
The features found in unstable plaques compared with stable
plaques have been shown by study of both necropsy and atherectomy
material to be a large core of extracellular lipid, a high density of
macrophages containing lipid, a reduced smooth muscle content,
and a thin cap.1 2 The majority of episodes of
major thrombi, particularly in white men with high plasma lipids, are
due to plaque rupture.3 4 In plaque rupture, the
fibrous cap of a plaque tears, exposing the highly thrombogenic lipid
core to blood in the lumen of the artery. The mechanical strength of
the plaque cap is therefore a vital component of plaque stability and
depends on the amount and organization of collagen and other connective
tissue proteins. Smooth muscle cells exist in lacunae in the plaque
cap, where they produce and maintain the connective tissue matrix on
which the cap integrity depends. The cap tissue is dynamic, with
production of connective tissue matrix proteins by smooth
muscle cells being balanced by degradation of the
matrix.5 Both sides of this equation are
detrimentally altered by inflammatory processes within the plaque. A
reduction in the smooth muscle cell density will inevitably lead to a
decline in connective tissue synthesis. There is growing evidence that
smooth muscle cell death by apoptosis occurs in plaques,
perhaps related to a decline in growth factors needed for their
maintenance or to the activity of macrophages in the
vicinity producing ROSs.6 7 8 9 Interferon-
Observational studies on human plaque tissue have used either in situ
hybridization to show metalloproteinase mRNA or immunohistochemistry to
show the metalloproteinase itself.11 12 The
difficulty of such observational studies is that most of the antibodies
used recognize both the active and inactive forms of the
metalloproteinase and therefore give no indication of the balance of
the active enzyme with its inhibitor. Nevertheless, these
observational studies have shown large amounts of, in particular, MMP-9
(gelatinase B)13 and MMP-3 (stromelysin) in
macrophages in unstable plaques. A biological assay of dynamic
enzyme activity within the plaque can be made by placing the tissue
section on a gelatin sheet and observing where lysis
occurs.14 This approach has confirmed that an
excess of active enzyme over its inhibitor is present
in unstable human plaques and is maximal at vulnerable areas in the
cap.15
These data suggest that one potential way of inhibiting or
preventing atherosclerotic plaque progression and clinical events is to
reduce metalloproteinase production or activation. Although
MMP-9 is emerging as a major member of the metalloproteinase family in
the context of plaque events, it must be remembered that its proenzyme
is constitutively expressed by monocytes and macrophages, for
example in fatty streaks, long before any question of instability of
the plaque arises. Mechanisms must therefore exist for the
upregulation of expression, enhanced release of the proenzyme, or
increased extracellular activation. Tumor necrosis factor-
Animal models of atherosclerosis induced by high-lipid
diets give a system in which the control of metalloproteinases can be
explored more fully. In this issue of Circulation, such a
rabbit model is described20 that suggests new
ways in which metalloproteinase expression is induced and enhanced.
Studies of the plaques in situ in the rabbit showed that gelatinase B
(MMP-9) was plentiful within the lipid-filled
macrophage-derived foam cells. The activity assay carried
out by laying the plaque on a sheet of gelatin and looking for lysis
showed an excess of the active enzyme over its inhibitors
(TIMPS). The model thus far confirmed much of what was already known
about MMP-9 in atherosclerosis. The novel aspect is
that the plaques themselves and macrophage-derived foam
cells taken from the lesions into culture in vitro showed a marked
reduction in both the expression of the precursor gelatinase B (MMP-9)
and its activated form if the animal had been treated with an
ROS scavenger, N-acetyl-L-cysteine. The
potential benefit of antioxidant therapy is usually regarded as
protecting LDL from the modification and oxidation that convert it into
an inflammatory mediator. The data in this article provide another
rational explanation of why antioxidant therapy might be effective
against atherosclerosis. The data further enhance the
concept that the lipid-filled macrophage-derived foam cell
internally drives the production of metalloproteinases by its
own production of ROSs after its reactions with modified lipid
in the plaque. Recently, another article in
Circulation21 described a similar rabbit
model of atherogenesis induced by a high-lipid diet that was used to
study the activity of metalloproteinases in plaques after reduction of
the plasma lipid levels by reversion to a normal diet. A marked
reduction in MMP-9 content and activity with the plaque was also
recorded. Taken together, these studies provide a rationale for
using a combination of lipid lowering and an antioxidant to inhibit
metalloproteinases in human atherosclerosis and thereby
reduce the risk of further acute events.
Selected Abbreviations and Acronyms
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
References
© 1998 American Heart Association, Inc.
Editorials
Reactive Oxygen Species, Metalloproteinases, and Plaque Stability
Key Words: Editorials • metalloproteinases • free radicals
production by lymphocytes depresses collagen synthesis by
smooth muscle cells.5 Enhancement of the
catabolic side of the equation of connective tissue synthesis, however,
is probably more important. Connective tissue matrix proteins are
degraded by a range of proteases, the most widely studied of which are
the metalloproteinase family.10 There are at
least 12 members of this family, with a large range of molecular
weights and with considerable individual variation in their affinity
for different components of the connective tissue matrix. One form
(MT-MMP) is bound to cell membranes, and its activation plays a role in
cell migration. Those with the ability to initiate or enhance the
degradation of collagen include interstitial
collagenase (MMP-1), gelatinase B (MMP-9), and stromelysin
(MMP-3). Although several cell lines in the plaque, including smooth
muscle cells and basophils, produce metalloproteinases, the major
source is the macrophage. A feature common to all these
metalloproteinases is that they are secreted into the extracellular
milieu as an inactive precursor that is then converted to an active
lower-molecular-weight enzyme. The same cell type, although not
necessarily the same cell, also produces TIMPS, which bind to and
neutralize the active enzyme. Control of the catabolism of connective
tissue is potentially exerted at three levels. The first is in the
transcription and secretion of metalloproteinases by the
macrophage, the second is at the activation point, and the
third is at the level of inhibition because of binding of TIMPS to the
active enzyme. 
and
interleukin-1 are known to upregulate metalloproteinase activity by
macrophages in culture16 and are one way
in which enhanced inflammatory activity in the plaque leads to a
detrimental effect. The interaction of macrophages with
lymphocytes using CD40 and its ligand also upregulates
metalloproteinases.17 Although the classic
activation pathway for metalloproteinases in the tissues is by
plasmin,18 there is also now evidence that active
metalloproteinases can further activate the proenzymes in the
adjacent tissue, that MMP-MT will induce activation, that mast cells
may play a role, and finally, that ROSs can lead to direct activation
of the proenzyme.19
MMP
=
matrix metalloproteinases
MT-MMP
=
membrane-type MMP
ROS
=
reactive oxygen species
TIMPS
=
tissue inhibitors of metalloproteinases
and IL-ß
selectively induce expression of 92-kDa gelatinase by human
macrophages. J Immunol. 1996;157:4159–4165.[Abstract]
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