(Chest. 2000;117:295S-299S.)
© 2000
American College of Chest Physicians
Protease Injury in the Development of COPD*
Thomas A. Neff Lecture
Harold A. Chapman, Jr., MD and
Guo-Ping Shi, DSc
*
From the Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA.
Correspondence to: Harold A. Chapman, Jr., MD, Respiratory Division, Brigham and Women’s Hospital, 75 Francis St, Boston, MA 02115; e-mail: hchapman{at}rics.bwh.harvard.edu
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Introduction
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Abbreviations: AAT = 
1-antitrypsin;
MME = macrophage metalloelastase; SMC = smooth muscle cells
Proteases
are a class of hydrolytic enzymes mediating irreversible disruption of
protein amide bonds. Based on the key amino acids used by proteases to
attack the targeted bond, these enzymes are commonly classified into
four distinct groups: serine, metallo, aspartate, and
cysteine.1
Proteolysis mediated by these enzymes is vital
to many aspects of normal cell function, including regulation of the
interface between cells and the extracellular matrices in which they
reside. And yet, unchecked extracellular proteolytic activity is linked
to excessive destruction of extracellular matrices and untoward
clinical events such as bleeding, joint destruction, and
osteoporosis.2
3
4
Since the discovery that deficiency of a
serine protease inhibitor, 1-antitrypsin
(AAT), is a cause of panlobular lung destruction, the pathogenesis of
emphysema has been strongly linked to the excessive action of
proteolytic enzymes.5
This discovery prompted the
widespread belief that COPD, and smokers’ emphysema in particular, is
due to excessive activity of the major serine protease inhibited by
AAT, neutrophil elastase. However numerous attempts in the ensuing 35
years to prove this hypothesis has instead left considerable doubt as
to the role of neutrophil elastase and possibly proteases in general in
the development of smokers’ emphysema. This discussion will attempt to
provide a current perspective on the role of proteases in the
pathogenesis of COPD.
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Proteases Potentially Involved in the Pathophysiology of COPD
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Is excessive extracellular proteolysis actually involved in the
pathogenesis of emphysema? The reason for asking this question is that
morphologic abnormalities developing during the course of emphysema
show progressive loss of alveolar tissue with little evidence of
necrosis. Loss of tissue without necrosis is typical of an apoptotic
process, a process not envisioned by the early framers of the
protease/protease inhibitor imbalance hypothesis. However, the fact
that patients with AAT deficiency are susceptible to emphysema and mice
with metalloprotease deficiency are protected sustains the notion that
proteases play a critical role in the pathophysiology of
COPD.6
It should be noted in this regard that a major
signal for cellular apoptosis is loss of extracellular matrix
contact.7
Most cells seem ready to commit to programmed
cell death (apoptosis) when faced with an unsolvable loss of matrix
attachment. Indeed, cellular survival may depend on signals from the
integrin family of adhesion receptors continuously sensing the
extracellular milieu.8
Thus it is likely that the
development of emphysema involves apoptosis of cells of the alveolar
wall following focal proteolytic damage to their underlying matrix (or
the cells themselves). To what extent excessive proteolysis actually
promotes emphysematous changes by initiating an apoptotic program
within the lung remains to be defined.
The major difficulty with the idea that a relative deficiency of AAT,
and by inference excessive neutrophil elastase activity, underlies the
majority of emphysema is that most patients are not demonstrably
deficient in AAT.9
10
This does not mean that excessive
and likely highly focal neutrophil elastase activity in lungs is
unimportant to collagen and elastin degradation leading to COPD.
Rather, this observation suggests that enzymes in addition to
neutrophil elastase may be important and in some cases critical.
Experiments in animals indicate that elastases are the most potent
enzymes in causing emphysematous changes in the lung.5
The
known mammalian elastases are listed in Table 1 . These enzymes include members of the serine, cysteine, and
metalloprotease families, indicating many enzymes in addition to
neutrophil elastase have the potential to damage the delicate elastin
network of lungs. This point is illustrated by recent studies of mice
with targeted deficiency of the enzyme macrophage metalloelastase
(MME).6
Such mice are protected from emphysematous changes
induced in C57/J129 mice by daily cigarette smoke exposure for 4 to 6
months. Indeed MME -/- mice show defective alveolar macrophage
accumulation in response to cigarette smoke; but even when macrophages
accumulate in their lungs, emphysema is not evident in the absence of
this metalloelastase. This observation verifies the importance of
macrophages to smoke-induced emphysema and implicates metallo in
addition to serine proteases in lung destruction. Whether this
mechanism of matrix destruction is important to smokers’ emphysema in
humans, however, is still uncertain. MME is much less robustly
expressed in humans than in mice.11
A third enzyme system also potentially relevant to the pathogenesis of
emphysema is the papain family of cysteine proteases (especially
cathepsins L, S, and K). These enzymes are among the most potent of
mammalian elastases and collagenases and are released in active form by
osteoclasts and stimulated monocyte-derived macrophages in the context
of inflammation.4
12
13
14
Thus, these enzymes also have the
potential to contribute to excessive extracellular matrix remodeling.
Although lung macrophages constitutively express these enzymes, there
is currently no direct evidence these enzymes are important to
smoking-related emphysema. In this discussion, we will focus on recent
studies examining the role of papain-family cysteine proteases in
extracellular matrix degradation, as these enzymes are a major focus of
our laboratories and the results may have implications for the
development of COPD.
|
The Biology of Cysteine Proteases
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In the last 10 years, the application of molecular biology
techniques to the search for new proteases has revealed the presence of
several previously unrecognized enzymes of the papain family in the
human genome. Currently there are 11 members of the papain family of
human cysteine proteases. These are listed along with their chromosomal
assignments and patterns of tissue expression in Table 2
. Inspection of this tabulation reveals several patterns. First, while
the genes of this enzyme family are dispersed throughout the genome,
several enzymes have apparently arisen by gene duplication from more
ancient family members. Secondly, the various enzymes have distinctive
patterns of tissue distribution. Several enzymes, such as
cathepsins S, W, and V are normally expressed primarily or exclusively
in cells involved in immunity, whereas cathepsin K is almost
exclusively expressed in osteoclasts. These distinguishing features
imply specific functions for these cysteine proteases, in
contradistinction to prior thinking that these endosomal/lysosomal
enzymes merely operated to terminally degrade endocytized proteins.
Thirdly, the substrate specificity of these enzymes are overlapping but
distinct. For example, cathepsins B and H are primarily carboxy- and
aminopeptidases, respectively, whereas cathepsins L, S, K, and F are
potent endoproteases.15
16
These differences reflect
marked changes in the accessibility of the substrate-binding cleft of
the enzyme active sites for proteins, as revealed by the recently
reported crystal structures of cathepsins B, L, and
H.17
18
19
Even among the potent endoproteases, there are
important substrate distinctions, cathepsin S being much more potent
than L or K in degradation of the major histocompatibility complex
class II chaperone, the invariant chain, while cathepsin K is
the more active collagenase.20
21
Although cysteine proteases have important roles in endosomal/lysosomal
proteolysis, do these enzymes appreciably contribute to extracellular
matrix remodeling? The best, and perhaps only, example of this type of
physiology is that of osteoclasts. Osteoclasts secrete active lysosomal
cathepsins, especially cathepsin K, into their pericellular space in
order to resorb bone collagen. This is important for long bone growth
during development and in adults for normal bone
turnover.12
Recent studies suggest that in the setting of
inflammation, such as that induced by cigarette smoking, cells that do
not normally express cathepsins K and S begin to do so, and in so doing
may act like osteoclasts. Our recent studies using human vascular
smooth muscle cells exemplify this point. These studies will be
discussed in the following section.
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Role for Cysteine Proteases in Pathologic Extracellular Matrix
Remodeling
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Vascular smooth muscle cells (SMC) do not normally
express either cathepsin S or K. In the setting of evolving
atherosclerosis, however, there is marked upregulation of both enzymes
in medial and neointimal SMC.22
Further, in
vitro, 
–interferon-stimulated SMC express cathepsin S
messenger RNA and become elastolytic by virtue of secretion of active
cathepsin S. SMC bearing the marker of -interferon stimulation
in vivo, major histocompatibility complex class II
expression, are well described in atherosclerotic
lesions.23
24
These observations are potentially relevant
to COPD because smokers with overt vascular disease (primarily
myocardial infarction) appear much more susceptible to smoking-related
COPD than smokers without overt vascular disease.25
This
epidemiologic association raises the possibility that atherosclerosis
(and/or aneurysm development) and emphysema share common pathogenetic
mechanisms. Aneurysm development and emphysema both involve extensive
elastin breakdown in the context of a chronic inflammatory process.
Recent studies of patients with atherosclerosis and aneurysms shed
light on a mechanism that could underlie susceptibility to cysteine
protease-mediated elastin/collagen damage in the setting of such
inflammation.
The major extracellular inhibitor of papain-type cysteine proteases is
cystatin C.26
27
Normal SMC highly express and secrete
cystatin C, raising the possibility that this inhibitor could
counterbalance augmented cysteine protease expression and secretion
within atherosclerotic vessel walls. Surprisingly, however,
immunohistochemical analyses of sections of atherosclerotic plaques
reveal virtually no cystatin C antigen within plaques.28
Whereas resident medial SMC stain positively for cystatin C antigen,
SMC migrating into and proliferating within the neointima have
virtually no detectable cystatin C. Extracts of aneurysmal vascular
tissue, in contrast to normal vessel walls, also contain little or no
cystatin C. Further, altered pericellular cystatin C levels appear to
be relevant to matrix remodeling. Cytokine-stimulated vascular SMC
secrete active elastolytic cathepsins, and this elastolytic activity
can be blocked by pericellular cystatin C (Fig 1
). Transforming growth factor-ß1, a cytokine whose circulating level
is reportedly low among patients with atherosclerosis,29
induces cystatin C secretion and thereby regulates pericellular
elastase activity. Based on these observations, we examined the
relationship between circulating cystatin C antigen and aortic diameter
in a cohort of patients examined in an outpatient cardiology clinic.
Increased abdominal aortic diameter (> 2.5 cm) among 122 patients
screened by ultrasonography correlated inversely with serum cystatin C
levels (p < 0.03).28
These findings highlight a
potentially important role for imbalance between cysteine proteases and
cystatin C in arterial wall remodeling, and establish cystatin C
deficiency in vascular disease.
|
Implications for COPD
|
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The pathophysiology of vascular wall remodeling may have
implications for the development of smokers’ emphysema. First,
evidence that an imbalance between cysteine proteases and cystatin C is
important to the breakdown of collagen and elastin in the vessel wall
suggests that a similar imbalance may apply at other sites of chronic
inflammation where elastin breakdown is prominent, ie,
emphysema. The association between susceptibility to COPD and vascular
disease supports this possibility.23
If so, then clearly
more than one type of elastolytic/collagenolytic protease contributes
significantly to the development of COPD. Indeed in different patients
the major protease system mediating damage could be different. Are
patients with relatively low cystatin C levels more susceptible to
COPD? This will be an interesting question for future studies. Similar
reasoning could also apply to injury mediated by metalloproteases.
Second, the finding that vascular SMC upregulate and secrete active
cathepsin S in response to -interferon raises the possibility that,
within the lung, cells not normally thought of as inflammatory cells
could in fact be perpetrators of excessive matrix breakdown. What
appears to be needed is a prodegradative cytokine microenvironment.
Recent studies of COPD patients document the presence of lymphocytes
within the lungs of COPD patients and indicate an association between
the extent of lymphocyte accumulation and reduction in
FEV1.30
31
Are cytokines released by
these lymphocytes driving matrix degradation by mesenchymal cells
surrounding lung elastin and collagen? To our knowledge, this
possibility has not been explored.
In summary, the involvement of proteases in the pathogenesis of COPD
appears increasingly complex. Both macrophages and neutrophils are now
clearly implicated in the development of smokers’ emphysema. Indeed
mesenchymal cells, in addition to inflammatory cells, may also
contribute to matrix degradation. These cells express distinct
proteases, implying more than one enzyme system is clinically relevant
to the injury incurred in COPD. Distinguishing features of patients
with COPD may reveal subsets of individuals in whom one or another
protease system is dominant. This could prove to be the basis for
targeted drug therapy to prevent progression to end-stage lung disease,
especially for individuals who quit smoking at later stages of
emphysematous damage. However, other than AAT deficiency, at the
present time no such distinguishing features among COPD patients are
known. The elucidation of additional mechanisms of injury, and
additional identifiers of risk, are vital for new approaches to this
important clinical problem.
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Acknowledgements
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Many primary references for the work cited in this
discussion were omitted because of space limitations. The authors
apologize for any oversights in this respect.
|
Footnotes
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Supported by NIH grants HL48716 and HL60942.
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TOP
Introduction
Proteases Potentially Involved...
