Keywords
matrix metalloproteinases - platelet physiology - atherosclerosis
Introduction
Matrix metalloproteinases (MMPs) are a family of zinc-dependent enzymes involved in
many physiologic and pathologic phenomena regulated by extracellular matrix degradation,
including tissue remodeling, cell migration, and angiogenesis.[1] MMPs also affect several cell functions either by modifying chemokines and cytokines
or by directly acting on cell surface receptors triggering cell signaling.[1]
[2] While their role in disease has been widely explored, showing a central function
in embryonic and skeletal disorders, cancer and metastasis, arthritis and central
nervous system disorders, and cardiovascular disease, their interaction with blood
platelets has been much less studied. Indeed, MMPs were first identified in the early
1960s by studies on the resorption of the tadpole tail and already in 1974 a collagenase
activity was found in platelets,[3] but it was not until a quarter of a century later that their effects on platelet
function were discovered[4] and only in the past 15 years their role in several pathophysiologic phenomena regulated
by platelets have started to be unraveled.[5] In parallel, the last few decades have witnessed to the great expansion of the understanding
of the central role of platelets not only in hemostasis but also in immune response,
inflammation and allergy, atherosclerosis, and cancer development,[6] and MMPs seem to contribute importantly to this role. Here, we will shortly review
the presence of MMPs in platelets, their role in platelet functions, and the effect
of platelet MMPs on other cells and tissues in disease.
Matrix Metalloproteinases and Platelet Function
Matrix Metalloproteinases and Platelet Function
Megakaryocytes carry mRNA transcripts for up to 10 different MMPs and platelets contain
several MMPs and tissue inhibitors which are implicated in hemostasis modulating platelet
function ([Table 1]).[5]
[7]
[8]
[9]
Table 1
Interactions between MMPs and platelets
|
MMP
|
Alternative name
|
Localization in platelets
|
Role in platelet function
|
References
|
|
MMP-1
|
Collagenase type I
|
Granules/Cytoplasm
|
Primes platelets, cleaves PAR1 (at site D39↓P40), activates platelet signaling, increases thrombus formation
|
[10]
[11]
[12]
|
|
MMP-2
|
Gelatinase A;
72 kDa Gelatinase; type IV collagenase
|
Granules/Cytoplasm
|
Primes platelets, cleaves PAR1 (at TL38↓D39PR), activates platelet signaling, increases
thrombus formation
|
[4]
[13]
[17]
[38]
|
|
MMP-3
|
Stromelysin-1; proteoglycanase
|
Granules/Cytoplasm
|
No effects
|
[10]
|
|
MMP-9
|
Gelatinase B;
92 kDa gelatinase
|
Plasma-derived
|
Decreases activation. Reduces Ca2+ mobilization
|
[21]
[22]
[23]
[24]
|
|
MMP-12
|
Macrophage metalloelastase
|
Granules/Cytoplasm
|
Cleaves CEACAM1, facilitates adhesion to type I collagen, platelet aggregation, and
α granule secretion
|
[25]
|
|
MMP-13
|
Collagenase type III
|
Plasma-derived (?)
|
Impaired platelet aggregation to low-dose collagen, CRP.
Reduced thrombus formation
|
[26]
[27]
[28]
|
|
MMP-14
|
MT1-MMP
|
Membrane
|
Inhibits thrombus growth and stability
|
[29]
|
Abbreviations: MMP, matrix metalloproteinase; MT, membrane-type; PAR, protease-activated
receptor.
Resting platelets constitutively express MMP-1 (16.5 ± 7.2 ng per 1 × 109 cells), primarily as pro-MMP-1, which is released and transformed into the active
form upon thrombin stimulation.[10] Released MMP-1 colocalizes with β3 integrins on activated platelets at cell-to-cell contact sites, and also binds to
the α2I domain of integrin α2β1 through its linker and hemopexin motifs.[11] MMP-1 regulates outside-in signaling in platelets by clustering β3 integrins, inducing tyrosine phosphorylation of intracellular proteins and priming
platelets for aggregation.[10] MMP-1 promotes platelet thrombus formation on collagen-coated surfaces at arterial
flow rates, a phenomenon blocked by MMP-1 or PAR1 inhibitors.[12]
Platelets also contain pro-MMP-2 (17.3 ± 3.7 ng/108 platelets) which upon stimulation translocates to the platelet surface where it gets
activated and in part released.[4]
[13] The presence of MMP-2 in platelets of Gray platelet syndrome patients, which lack
α-granules, suggests that this protease is not granular[14] but probably cytoplasmic.[15] Active MMP-2 does not induce platelet aggregation but amplifies the activation response
to a wide range of agonists, suggesting that its effects are mediated by the activation
of a common, post-receptorial signaling pathway.[4]
[13] The concentrations of MMP-2 exerting this priming activity (0.1–50 ng/mL, i.e.,
0.0015–0.75 nM) are in the range of those secreted by activated human platelets in
vitro and in vivo.[13]
[16]
Platelet adhesion to fibrinogen is associated with the release of MMP-2 and phenanthroline,
an unspecific MMP inhibitor, reduces platelet adhesion suggesting that MMP-2 promotes
adhesion.[17] Indeed, active MMP-2 enhances shear stress–induced platelet activation and thrombus
formation on collagen and it acts as an adhesive substrate per se.[18] MMP-2 is thus likely to play a relevant role in thrombus formation at the sites
of increased shear stress in vivo, like in stenosed atherosclerotic coronary arteries.[19]
On the other hand, a recent study suggested that MMP-2 could blunt NOX2 activity and
ROS formation in platelets possibly downregulating reactivity in oxidative stress
conditions.[20]
Conflicting results exist concerning the presence of MMP-9 in platelets. Activated
platelets bind MMP-9, suggesting that when MMP-9 is detected in platelets it is probably
plasma-derived.[21] Moreover, contamination by white blood cells, which are very rich in MMP-9, may
explain the presence of MMP-9 in platelet preparations.[22]
[23] Active MMP-9 was reported to counteract the platelet-potentiating effects of MMP-2[17] at concentrations (15–90 ng/mL) in the range of those found in plasma (30–50 ng/mL).[24]
There are discordant findings concerning the presence of MMP-3 in platelets and megakaryocytes,
and no effects on platelet function were reported; thus, its role for platelets is
still awaiting clarification.[10]
Expression of MMP-12 in human platelets was also recently reported and shown to mediate
carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) shedding from
their surface generating a peptide which facilitates adhesion to type I collagen,
aggregation, and α-granule secretion.[25]
MMP-13, a collagenolytic metalloproteinase not present in platelets but upregulated
in atherosclerotic and inflammatory tissue,[26] was reported to reduce thrombus formation on fibrillar collagen under flow through
the partial digestion of collagen monomers, suggesting that MMP-13 may inhibit platelet
recruitment at ruptured plaques.[27] However, these findings were obtained with rather high concentrations of MMP-13
(80 nM) and are thus of uncertain physiological meaning.[28]
Platelets express MT1-MMP (MMP-14) which forms a trimolecular complex with pro-MMP-2
and TIMP-2 (the physiologic inhibitor of MMP-2) on the platelet surface allowing the
generation of active MMP-2.[29]
Recently, it has been shown that resting platelets express on their surface the extracellular
MMP inducer EMMPRIN (CD147), an immunoglobulin-like receptor known for its ability
to induce MMPs expression. Its expression is upregulated in vitro upon platelet activation
with several stimuli.[30]
Biochemical Mechanisms
Catalytically active MMP-1 cleaves the platelet PAR1 exodomain at LD39/P40RSFL, two amino acid residues upstream the thrombin cleavage site (R41–S42), triggering G12/13-Rho-GTP signaling. MMP-1 also enhances protein tyrosine phosphorylation,
and namely of p38MAPK and its substrate MAPKAP-K2 involved in cytoskeletal reorganization.[12]
Also active MMP-2 enhances platelet activation by enzymatically cleaving PAR1 at a
specific, noncanonical extracellular site, different from that of both MMP-1 and thrombin,
by an αIIbβ3-facilitated mechanism. The cleavage of PAR1 generates a tethered ligand different
from that produced by thrombin that in turn triggers biased PAR1 signaling. This explains
why MMP-2, although initiating intraplatelet signaling, requires Gi activation triggered by other agonists to start aggregation.[31] Active MMP-2 amplifies platelet activation by triggering the post-receptorial signaling
pathway phosphatidyl-inositol 3-kinase (PI-3-K).[13] A direct interaction between αvβ3 and MMP-2 was shown on the surface of melanoma cells with the formation of a stable
MMP-2 integrin αvβ3 complex dependent on the C-terminus of MMP-2.[32] MMP-2 interacts also with integrin αIIbβ3 on activated platelets via the C‐terminal hemopexin‐like domain and this interaction
is required for platelet activation.[31]
[33]
MMP-2 was also reported to play an intracellular function in platelets by hydrolyzing
talin, a cytoskeletal protein required for the inside-out activation of αIIbβ3.[34] Finally, MMP-2 upregulates glycoprotein (GP)Ib receptor expression, thus potentiating
the adhesion to VWF, but also the affinity of VWF for GPIb by proteolytic modification
of the former.[35]
The platelet-inhibitory activity of activated MMP-9 may be due to the inhibition of
phospholipase C, with consequent suppression of phosphoinositide breakdown, protein
kinase C activation, thromboxane A2 formation, and intracellular Ca2+ mobilization,[36] and to the formation of nitric oxide which acts as a negative feedback regulator
of platelet activation.[37]
Relatively high MMP-13 concentrations were reported to bind platelet αIIbβ3 and GPVI but without triggering platelet degranulation or αIIbβ3 activation, thus not starting intraplatelet signaling.[28] On the other hand, we showed that physiologic concentrations of active MMP-13 stimulate
human platelets and thrombus formation in mice.[38]
Finally, resting platelets express latent MT1-MMP on their surface which is activated
upon collagen stimulation, suggesting that MT1-MMP may contribute to collagen-induced
platelet aggregation.[29]
Platelet stimulation with recombinant EMMPRIN-Fc induced surface expression of CD40L
and P-selectin, suggesting that EMMPRIN–EMMPRIN interaction activates platelets.[30] Soluble CD147 binds to platelet GPVI with high affinity and this interaction mediates
platelet rolling.[39] Moreover, CD147 is the major receptor of cyclophilin A, a proinflammatory cytokine
expressed in a wide variety of cell types, including platelets, and tissues. Extracellular
cyclophilin A activates platelets via EMMPRIN, inducing platelet degranulation depending
on phosphoinositide-3-kinase/Akt signaling.[40]
Studies in Animals
In an ex vivo model of platelet activation on collagen under flow, platelet thrombi
were smaller when blood from MMP-2−/− mice was employed as compared with blood from wild-type mice.[21] In contrast, perfusion of blood from MMP-9−/− mice resulted in thrombi covering a larger surface, and blood from MMP-3−/− mice did not behave differently from wild-type mice ([Table 2]).[21]
Table 2
Summary of the results concerning platelet-related phenomena obtained in animal models
|
Mouse strain
|
Platelet-related effects
|
References
|
|
MMP-2−/−
|
• Reduced platelet thrombi ex vivo
• Reduced collagen + epinephrine-induced lung thromboembolism
• Reduced photochemically induced arterial thrombosis
• Enhanced abdominal aortic aneurysms
|
[21]
[41]
[41]
[41]
[41]
[48]
|
|
MMP-9−/−
|
• Increased platelet thrombi ex vivo
• Enhanced abdominal aortic aneurysms
|
[21]
[44]
[45]
|
|
MMP-3−/−
|
• Normal platelet thrombi ex vivo
|
[21]
|
Abbreviation: MMP, matrix metalloproteinase.
Platelet pulmonary thromboembolism induced by the i.v. injection of collagen + epinephrine
and femoral artery thrombosis induced by photochemical damage were reduced in MMP-2−/− mice. Thrombus formation was delayed also in chimeric mice lacking MMP-2 only in
platelets, indicating that it is platelet-derived MMP-2 that facilitates thrombus
formation. Finally, arterial thrombus formation at the site of mild vascular injury
in mice was triggered by platelet-released MMP-2 which may thus transform a normal
hemostatic response to vessel injury into thrombosis.[41] This observation might explain why in the coronary bed of patients dying from acute
myocardial infarction (MI), several fissured or eroded plaques are found, but only
one occluding thrombus forms and is ultimately responsible for the acute ischemic
event ([Table 2]).[42]
MMPs play a crucial role in atherogenesis. MMP-2 degrades elastin-generating peptides
which accelerate low-density lipoprotein (LDL) oxidation and calcification.[43] MMP-2 and MMP-9 produced by macrophages and mesenchymal-derived cells in the adventitia
and media of the aorta contribute also to the initiation and progression of abdominal
aortic aneurysms (AAAs) by degrading elastin fibers.[44]
[45] The deletion of MMP-2 and MT1-MMP genes reduced AAA formation in mice ([Table 2]).[45]
[46]
[47] Treatment with aspirin and clopidogrel, inhibiting platelet activation, significantly
decreased aortic tissue MMP-2 in a mouse model of angiotensin II–induced AAA, suggesting
that circulating activated platelets play a role in MMP-2 accumulation in the aortic
wall.[48]
Recently, we generated a novel mouse model of spontaneous AAA formation (LDL receptor
[LDL-R]/endothelial nitric oxide synthase (eNOS)−/− mice), strictly recapitulating human AAA, showing that platelets are essential for
the migration of inflammatory cells into the aortic vessel wall and that a significant
fraction of the MMP-2 found in AAA extracts derives from an enhanced production by
vascular smooth muscle cells generated by the contact with platelets infiltrating
aorta.[49]
Platelet-derived CD40L is a potent inducer of lung neutrophil infiltration in abdominal
sepsis-induced lung injury. In turn, neutrophil-derived MMP-9 induces CD40L shedding
from platelets triggering a vicious circle crucial for the pathogenic consequences
of sepsis in mice.[50]
Platelets are involved in tumor metastasis in bone. Platelets regulate bone formation
triggered by tumor cells through the uptake of tumor-derived proteins, including several
MMPs (MMP-1, MMP-3, MMP-13, TIMP-1, and TIMP-2), and through the secretion of α-granule
proteins favoring osteoblast differentiation. In a xenograft tumor model of human
prostate cancer in immunocompromised mice, the neutralization of MMP-1, MMP-3, and
MMP-13 released by platelets using specific antibodies or marimastat inhibited bone
formation in response to tumor growth.[51]
The role of MMP-1 and MMP-14 in regulating platelet function and the interactions
with other cells in vivo has not been investigated in mice because murine platelets
do not express MMP-1,[52] while MMP-14–deficient mice are not vital.
EMMPRIN gene silencing is associated with aberrant extracellular matrix remodeling,
characterized by a striking reduction of age-associated fibrosis resulting in dilated
cardiomyopathy in aging mice.[53]
Studies in Humans
Platelet MMP-1 is released upon interaction with Streptococcus sanguinis, a predominant bacterium of the oral cavity associated with the development of infective
endocarditis; thus, it may participate in the cardiovascular complications related
to this infection.[54]
Platelets release MMP-2 in vivo in healthy humans during primary hemostasis, suggesting
that MMP-2 plays a physiological role in the regulation of the platelet response to
vessel wall damage. MMP-2 concentrations in shed blood were significantly higher than
in venous blood and increased progressively, consistent with ongoing platelet activation.
Active MMP-2 in shed blood was in the range of concentrations (around 1 ng/108 platelets) found to potentiate platelet activation.[16] Patients with acute coronary syndromes (ACSs) showed enhanced concentrations of
total and active MMP-2 in blood from the coronary artery carrying the culprit lesion
compared with peripheral blood, and plasma obtained from coronary blood potentiated
the activation of control platelets, an effect suppressed by TIMP-2, suggesting a
role for MMP-2 in sustained platelet activation during ACS.[19] Atherosclerotic plaques contain high amounts of MMP-2[55]
[56]
[57] and recently we showed that human carotid plaque extracts promote platelet aggregation
due to their content of active MMP-2, an effect prevented by specific MMP-2 inhibitors.
Moreover, elevated MMP-2 activity in plaques and high aggregation–potentiating plaques
were associated with a higher rate of subsequent ischemic cerebrovascular events,
suggesting that MMP-2 contained in plaques participates in platelet thrombus formation.[58]
MMP-3 plays an important role in several pathologic processes such as rheumatoid arthritis,
systemic lupus erythematosus, and atherosclerosis,[59]
[60]
[61] in which platelets are also involved[62]
[63]
[64]; thus, future studies should focus on the role of platelet-derived MMP-3 in these
disorders.
Platelet-derived MMP-9 has been implicated in inflammatory disorders. MMP-9 platelet
content and release are increased in Crohn's disease,[65] a chronic inflammatory bowel disorder, and Behçet's disease,[66] an autoimmune vasculitis, and induce the shedding of platelet CD40L (sCD40L) which
in turn causes endothelial activation.[65]
[66]
[67] Finally, a significant correlation exists between plasma concentrations of CRP and
MMP-9 in the coronary circulation of ACS patients, suggesting a link between inflammation
and plaque rupture.[68]
EMMPRIN has been shown to participate in the induction of proinflammatory and prothrombotic
effects in patients with MI and in ApoE−/− mice: recent studies have highlighted a role for platelet CD147 in plaque formation,
monocyte recruitment, cytokine release, and foam cell formation.[69]
[70]
[71] CD147 expression has also been observed in atheromatous plaques in association with
MMP-9 expression.[72] Moreover, circulating levels of soluble CD147 correlated with soluble glycoprotein
VI in plasma, a platelet-specific marker in healthy subjects and patients with coronary
artery disease.[73] Thus, EMMPRIN is considered a novel potential target to reduce vascular inflammation
and atherosclerotic lesion development.
Inhibition of MMPs as a Therapy for Cardiovascular Disease
Inhibition of MMPs as a Therapy for Cardiovascular Disease
The studies summarized earlier lend compelling evidence to the hypothesis that MMP
inhibition may represent a novel therapeutic approach to the prevention of atherosclerotic
plaque instability, MI, and stroke. Numerous synthetic selective and nonselective
MMP inhibitors (MMPi) have indeed been created and pursued as therapeutic agents.
Some have been tested in animal models, but few have made their way to clinical trials
and mostly in the oncology field.[74]
The potential of MMPi for the treatment of cardiovascular disease has been evaluated
in several studies ([Table 3]).[75]
Table 3
MMPi as potential tools for the treatment of cardiovascular disease
|
MMPi
|
Specificity
|
Animal models
|
Human studies
|
Effects
|
References
|
|
CGS 27023A
|
Broad-spectrum
|
Progression of atherosclerosis, aneurysm, and restenosis in LDL-R−/− mice
|
|
No prevention of plaque development or progression; retardation of the progression
of aneurysm
|
[76]
|
|
RS-130830
|
Broad-spectrum
|
Plaque development and stability in ApoE −/− mice
|
|
No change in the incidence of plaque rupture
|
[77]
|
|
PG-116800
|
MMP-2, -3, -8, -9, -13, and -14
|
|
PREMIER (Prevention of Myocardial Infarction Early Remodeling) in post-MI patients
|
No improvement in echocardiographic or clinical outcomes
|
[78]
|
|
Doxycycline
|
Broad-spectrum
|
Ischemic heart in Sprague–Dawley rats
|
|
Improves endothelial dysfunction post-MI
|
[80]
|
|
|
AngII-induced atherosclerosis in LDL-R−/− mice
|
|
Reduces the incidence of AAA
|
[81]
|
|
|
|
Pilot clinical trials in patients with symptomatic carotid artery disease
|
No positive effects on plaque phenotype and atheroma progression
|
[82]
|
|
|
|
MIDAS pilot trial (in patients with coronary artery disease)
|
No prevention of plaque rupture events
|
[83]
|
|
|
|
TIPTOP trial (in patients with myocardial infarction)
|
Reduces end-diastolic volumes, infarct size, and infarct severity
|
[84]
|
|
Minocycline
|
MMP-9
|
|
MINOS trial (in patients with stroke treated with rt-PA)
|
Decreases plasma MMP-9 levels (associated with the risk of tPA-related hemorrhage)
|
[85]
|
|
RXP470.1
|
MMP-12
|
Progression of atherosclerosis in ApoE −/− mice
|
|
Retards atherosclerotic plaque development
|
[86]
|
|
SB-3CT
|
MMP-2, MMM-9
|
Cerebral ischemia in mice
|
|
Prevention of brain damage
|
[89]
[90]
|
Abbreviations: AAA, abdominal aortic aneurysm; LDL-R, low-density lipoprotein receptor;
MI, myocardial infarction; MMP, matrix metalloproteinases; MMPi, MMP inhibitors.
Direct Matrix Metalloproteinase Inhibitors
Zinc group-chelating inhibitors (such as thiol or hydroxamate or tetracycline derivatives)
have given results far from encouraging. Nonselective hydroxamic acid–based MMPi did
not prevent plaque development or progression in LDL-R or ApoE knockout mice.[76]
[77]
PG-116800, an oral MMPi of the hydroxamic acid class with high affinity for MMP-2,
-3, -8, -9, -13, and -14, was studied in the phase II double-blind, multicenter randomized
control trial PREMIER (Prevention of Myocardial Infarction Early Remodeling) in post-MI
patients, but it did not show improvement in echocardiographic or clinical outcomes.[78]
Other studies were performed with the widely used antibiotic doxycycline which, at
sub-antimicrobial doses, displays broad-spectrum MMPi properties.[79] In a study in rats, doxycycline reduced MMP-2 activity in left ventricular extracts
and improved endothelial dysfunction post-MI.[80] Another study testing the effect of doxycycline on the development of angiotensin
II–induced atherosclerosis and AAA formation in LDL-R−/− mice showed no effect on the extent of atherosclerosis but a markedly reduced incidence
of AAA, showing that MMPs are crucially involved in AAA formation.[81]
In two independent, prospective placebo-controlled pilot clinical trials in patients
with symptomatic carotid[82] or coronary artery disease (MIDAS pilot trial)[83] undergoing intervention, treatment with doxycycline failed to exert positive effects
on plaque phenotype, atheroma progression,[82] or clinical outcome.[84] However, some limitations of these studies should be considered: patients received
doxycycline for variable times before surgery and only subjects with recent ACS were
studied.[83] Moreover, although the MIDAS trial did not show differences in the primary clinical
endpoint, 6 months of doxycycline reduced heart dysfunction and CRP and IL-6, which
are markers of inflammation.[83]
In the phase II TIPTOP trial in patients with MI, treatment with doxycycline (100 mg
twice daily) reduced end-diastolic volumes, infarct size, and infarct severity in
comparison to standard treatment.[84]
Minocycline, a semisynthetic tetracycline able to bind MMPs due to its affinity for
Zn2+, administered to patients with stroke treated with rt-PA decreased plasma MMP-9 levels,
which are associated with the risk of tPA-related hemorrhage, showing promise for
the prevention of the adverse consequences of thrombolytic therapy.[85]
A phosphinic peptide (RXP470.1) that is a potent, selective murine MMP-12 inhibitor,
significantly retarded atherosclerotic plaque development in ApoE knockout mice.[86]
A highly selective small molecule inhibitor of MMP-9, JNJ0966, which prevents conversion
of the MMP-9 zymogen into the catalytically active enzyme, showed effectiveness in
reducing disability scores in a mouse model of neuroinflammation.[87]
SB-3CT, a selective inhibitor of MMP-2 and -9 which binds the active site of gelatinases,[88] showed promise in the prevention of brain damage caused by cerebral ischemia in
mice.[89]
[90]
Finally, SP-8356, a synthetic small molecule with anti-inflammatory and antioxidative
activities, reduces plaque progression and stabilizes vulnerable plaques in ApoE-deficient
mice, inhibiting CD147–cyclophilin A interactions[91] and reducing neointimal hyperplasia through inhibition of MMP-9 activity in Sprague–Dawley
rats.[92]
Indirect Matrix Metalloproteinase Inhibitors
Several cardiovascular drugs act as indirect inhibitors of MMPs.
The catalytic domain of ACE is similar to that of MMPs; thus, ACE inhibitors display
an inhibitory effect on some MMPs.[93] For example, captopril and lisinopril inhibit MMP-2 activity at concentrations greater
than 4 and 1 mmol/L, respectively, whereas MMP-9 was inhibited by captopril at 87
nmol/L.[94]
[95] ACE inhibitors improve post-MI outcomes[96] and part of this action might be due to MMP inhibition.[97]
Similarly to ACE inhibitors, angiotensin II receptor antagonists inhibit MMPs and
improve ECM remodeling. Rats treated with losartan showed reduced mRNA transcription
and protein expression of MMP-2 and MMP-9 in atherosclerotic lesions.[98] Treatment with valsartan decreased levels of MMP-2, -3, and -9 post-MI in rats.[99] Moreover, patients treated with trandolapril and valsartan showed reduced MMP-9
plasma levels and LV remodeling post-MI.[100]
The β-adrenergic receptor antagonist atenolol decreased MMP activity and improved
LV stiffness in an experimental heart failure model in dogs.[101] Rats treated with metoprolol showed decreased MMP-2 mRNA levels and decreased cardiac
oxidative stress markers post-MI.[102] Similar results were observed in post-MI pigs treated with carvedilol or metoprolol,
both of which decreased MMP-2 activity, MCP-1 expression, and macrophage infiltration.[103] Finally, patients with heart failure treated with carvedilol showed reduced MMP-9
activity in plasma.[104]
Statins (hydroxymethylglutaryl coenzyme A [HMG-CoA] reductase inhibitors) exert a
variety of pleiotropic effects, including the inhibition of expression of various
MMPs (e.g., MMP-2 and MMP-9) in atheromatous plaques by reducing vascular inflammation.[105] Patients with MI treated with pravastatin showed decreased MMP-2 and MMP-9 serum
levels.[106]
[107]
[108] Also, in a rat model of heart failure, pravastatin suppressed myocardial MMP-2 and
MMP-9.[109]
Very few data are available on the effects of MMP inhibition on platelets. Platelet-derived
MMP-1 secretion is inhibited by pretreatment with aspirin and GPIb and GPIIb/IIIa
antagonists.[12]
Neutralization of MMP-2 by blocking antibodies, recombinant TIMP-2, or MMPi reduced
collagen-induced platelet aggregation, indicating that platelet-released MMP-2 mediates
aggregation.[4]
[13] On the other hand, aspirin did not inhibit in vivo release of MMP-2 in humans[16] and did not prevent MMP-2-induced platelet potentiation.[13]
[16]
[19]
[110]
Recently, by the application of the nanobody technology, we generated a highly selective
inhibitor of MMP-2 that completely abolished the potentiating activity of MMP-2 on
human platelet activation.[111] This new nanotechnological tool may show promise for the study of the role of MMP-2
in cardiovascular pathophysiology.
Conclusions
Blood platelets have phylogenetically evolved as a highly specialized cell devoted
to the maintenance of hemostasis, a vital function of blood, but they retain several
other functions of their ancestor cells, the hemocytes which played both the role
of arresting hemorrhage and of fighting invading pathogens.[112] MMPs are a highly conserved protein family originated probably before the emergence
of vertebrates from invertebrates,[113] with multiple functions in organism defense. It is thus reasonable to expect that
the interaction between platelets and MMPs regulates multiple pathophysiologic phenomena
related to the hemostatic and immunologic systems in humans ([Fig. 1]).[5]
[114]
Fig. 1 Role of platelet-derived MMPs in disease. Platelets contain, release, and/or express
on their surface upon activation several MMPs, including MMP-1, -2, -3, -9, -12, -13,
and MT1-MMP (MMP-14). Platelet-derived MMP-1, -3, and -13 regulate bone formation
triggered by tumor cells, stimulating osteoblasts proliferation and differentiation.
MMP-2 contained in atherosclerotic plaques contributes to platelet activation, thus
further stimulating thrombus formation. On the other hand, platelet-released MMP-2
facilitates thrombus formation at mild injury sites. MT1-MMP (MMP-14) allows the generation
of active MMP-2 on the platelet surface. Platelet-derived active MMP-2, acting on
PAR-1 of endothelial cells, induces endothelial activation with consequent increase
of VCAM-1 expression triggering monocyte transmigration, thus promoting atherosclerotic
plaque formation. Similarly, platelet-derived MMP-2 induces abdominal aortic aneurysm
formation, enhancing MMP-2 levels in the abdominal aorta. Platelets contain and release
MMP-12, and MMP-12 has a role in atherosclerosis. Platelet-released MMP-9 induces
the shedding of platelet CD40L in Crohn's disease, a chronic inflammatory bowel disease,
and might be responsible, at least in part, for the high state of activation of platelets
from these patients. The release of platelet MMP-1 in response to Streptococcus sanguinis, a bacterium associated with the development of infective endocarditis, may link
platelet activation with the cardiovascular complications related to this infection.
chol, cholesterol; PAR-1, protease-activated receptor-1; TIMP-2, tissue inhibitor
of metalloproteinases-2; VSMCs, vascular smooth muscle cells.
While great progress in the understanding of these interactions has been made in the
last few years, the translation into clinical applications is lagging behind and MMP
biomarkers for cardiovascular disease or MMPi as antiplatelet or antiatherosclerotic
therapies have not entered clinical use yet.
Further insight into the molecular mechanisms regulating the interactions between
platelets and MMPs and innovative approaches to the inhibition of their pathogenic
effects may lead to significant advances in the treatment of cardiovascular, inflammatory,
and tumor disorders.
