Cardiovascular
& Hematological Agents in Medicinal Chemistry
ISSN: 1871-5257
Cardiovascular & Hematological
Agents in Medicinal Chemistry
Volume 6, Number 3, July 2008
Contents
Structural Basis for Variable Lytic Susceptibility
of Fibrin
Bridging Structure with Function in Fibrinolysis
Guest Editor: K. Kolev
Editorial Pp. 159-160
The Biochemical and Physical Process of Fibrinolysis
and Effects of Clot Structure and Stability on the Lysis Rate
Pp. 161-180
J.W. Weisel and R.I. Litvinov
[Abstract]
Searching for Differences between Fibrinogen and
Fibrin that Affect the Initiation of Fibrinolysis Pp.
181-189
Russell F. Doolittle
[Abstract]
The Involvement of Blood Coagulation Factor XIII in
Fibrinolysis and Thrombosis Pp. 190-205
L. Muszbek, Z. Bagoly, Z. Bereczky
and É. Katona
[Abstract]
Alterations of Fibrinogen Structure in Human Disease
Pp. 206-211
M. Hoffman
[Abstract]
Fibrin Binding and the Regulation of Plasminogen Activators
during Thrombolytic Therapy Pp. 212-223
C. Longstaff, S. Williams and C.
Thelwell
[Abstract]
Role of Cellular Elements in Thrombus Formation and
Dissolution Pp. 224-228
N. Wohner
[Abstract]
Abstracts
[Back to top]
Editorial : Structural Basis for Variable
Lytic Susceptibility of Fibrin Bridging Structure with Function
in Fibrinolysis
Cardiovascular and cerebrovascular diseases continue
to be a major public health burden worldwide. According to
the WHO data the mortality related to atherothrombosis is
the leading cause of death responsible for 22.3% of the total
deaths in the world preceding infectious diseases (19.1%)
and neoplasms (12.5%) [1]. Some of the improvements in death
rates seen in many developed countries may be partly attributable
to the application of thrombolytic therapy for myocardial
infarction or ischemic stroke [2, 3], designed to lyse the
clots blocking the arteries and re-store blood flow as quickly
as possible. This therapeutic approach is based on the administration
of plasminogen activators (urokinase, streptokinase, tissue-type
plasminogen activator and its recombinant variants), which
convert the plasminogen in blood plasma, on the surface of
or inside the thrombi to plasmin which then dissolves fibrin,
the solid matrix of thrombi. However, persistent recanalization
of the occluded blood vessels often fails (in 15 to 40% of
patients) and efficient doses of current fibrinolytic agents
have significant bleeding side-effects [4]. These limitations
of fibrinolytic therapy maintain a continuous interest in
the basic research of the molecular mechanisms underlying
fibrin dissolution, which could help the development of more
efficient and safe clot busting agents. The present collection
of reviews explores recent advances in understanding the interconnections
between the structure of the fibrin clots and the action of
the enzymes destroying it.
In their review entitled “The biochemical and physical
process of fibrinolysis and effects of clot structure and
stability on the lysis rate” Weisel and Litvinov (University
of Pennsylvania, Philadelphia) present a comprehensive overview
of all participants in the fibrinolytic process, which helps
the reader to unify the more specialized aspects of the molecular
mechanisms discussed in the accompanying papers [5]. A special
focus of this review is the microscopic structure of the fibrin
mesh and its impact on the enzymatic steps of the dissolution
process.
The minireview entitled “Searching for differences between
fibrinogen and fibrin that affect the initiation of fibrinolysis”
contributed by Doolittle (University of California, San Diego)
summarizes the ultrastructural changes accompanying the conversion
of fibrinogen to fibrin [6]. This minireview addresses the
structural determinants of the self-destructing nature of
fibrin as opposed to fibrinogen, which co-exists in peace
with all participants in fibrinolysis.
Muszbek, Bagoly, Bereczky and Katona (University of Debrecen
Medical and Health Science Center, Debrecen) explain the basic
biochemical function of factor XIII in their minireview entitled
“The involvement of blood coagulation factor XIII in
fibrinolysis and thrombosis” [7]. The stability of fibrin
conferred by factor XIIIa is discussed in the context of clinical
states of thrombosis related to variations in factor XIII
level and its genetic polymorphisms.
In the minireview entitled “Alterations of fibrinogen
structure in human disease” Hoffman (Duke University
Medical Center, Durham) explores the consequences of post-translational
modification of fibrinogen through oxidation, nitration, homocysteinylation
and glycation [8]. This minireview helps the understanding
of the altered fibrinolysis in disease states with increased
rate of covalent modification of proteins (e.g. diabetes).
Longstaff, Williams and Thelwell (National Institute for Biological
Standards and Control, South Mimms) approach the structure-function
relationships in fibrinolysis from the side of plasminogen
activators in their minireview entitled “Fibrin binding
and the regulation of plasminogen activators during thrombolytic
therapy” [9]. A special focus in this minireview is
the methodology for evaluating and modeling fibrinolysis in
vitro as a tool for assessing new thrombolytic agents.
In the last review of the series, entitled “Role of
cellular elements in thrombus formation and dissolution”
Wohner (Semmelweis University, Budapest) presents a cellular
view of fibrinolysis [10]. This minireview outlines the contribution
of platelets, leukocytes and red blood cells to the modification
of fibrin structure as well as their direct effects on discrete
steps of fibrinolysis.
REFERENCES
[1] The World Health Report 2004;
WHO: http://www.who.int/whr/ 2004/annex/en/ Statistical Annex;
pp. 120-125.
[2] de Boer, M.J.; Zijlstra, F. Pharmacoeconomic,
1997, 12, 427.
[3] Baker W.F. Hematol. Oncol. Clin. N. Am., 2005,
19, 147.
[4] Verstraete, M. In New therapeutic agents in thrombosis
and thrombolysis, Sasahara, A.A.; Loscalzo, J.L., Eds.;
Marcel Dekker, Inc.: New York, 2003; pp.
477-478.
[5] Weisel, J.W.; Litvinov, R.I. Cardiovasc. Hematol.
Agents Med. Chem., 2008, 6,
161.
[6] Doolittle, R.F. Cardiovasc. Hematol. Agents Med. Chem.,
2008, 6, 181.
[7] Muszbek, L.; Bagoly, Z.; Bereczky, Z.; Katona, É.
Cardiovasc. Hematol. Agents Med. Chem., 2008,
6, 190.
[8] Hoffman, M. Cardiovasc. Hematol. Agents Med. Chem.,
2008, 6, 206.
[9] Longstaff, C.; Williams, S.; Thelwell, C. Cardiovasc.
Hematol. Agents Med. Chem., 2008,
6, 212.
[10] Wohner, N. Cardiovasc. Hematol. Agents Med. Chem.,
2008, 6, 224.
K. KOLEV
Department of Medical Biochemistry
Semmelweis University, Budapest
Hungary 1088, Puskin u. 9, Hungary
Tel: +36-1-2661030
Fax: +36-1-2670031
E-mail: kale@puskin.sote.hu
[Back to top]
The Biochemical and Physical Process of Fibrinolysis
and Effects of Clot Structure and Stability on the Lysis Rate
J.W. Weisel and R.I. Litvinov
The effectiveness of fibrinolysis results from the combination
of regulated enzymatic activity and the physical properties
of the fibrin scaffold. Physiologically, clots or thrombi
are dissolved from within via internal lysis. In
contrast, with therapeutic thrombolysis, lytic agents are
introduced at one surface and lysis proceeds across the thrombus.
In the latter case, there are complex changes that take place
at the lysis front in a narrow zone. However, at the microscopic
level the mechanisms for either general type of fibrinolysis
appear to be similar. Fibrinolysis proceeds by fibers being
transected laterally, rather than digestion of fibers by surface
erosion from the outside. A molecular model to account for
these observations together with what is known from the biochemical
characterization of fibrinolysis involves the movement of
plasmin laterally across fibers, binding to sites created
by its own proteolytic activity. Fibrin clots can have a great
diversity of structural, biological, physical, and chemical
properties depending on the conditions of formation, and the
rate and nature of fibrinolysis is related to these properties.
In general, the rate of lysis appears to be faster for clots
made up of thicker fibers than for clots made up of thinner
fibers, but the lysis rate is not simply a function of fiber
diameter and also depends on other physical properties of
the clot. Platelet aggregation and clot retraction have a
dramatic effect on the structure of fibrin and hence on fibrinolysis.
[Back to top]
Searching for Differences between Fibrinogen and
Fibrin that Affect the Initiation of Fibrinolysis
Russell F. Doolittle
Although in a gross sense fibrin is merely a collection
of fibrinogen molecules packed together in bundles, numerous
small structural differences can arise as a result of the
conversion of the soluble precursor into the gelled product.
Some of the consequences are obvious, others more subtle.
In one way or another, all these changes are the result of
a sequence of events that includes the release of the fibrinopeptides
A and B, the formation of protofibrils, the cross-linking
of γ
chains, the assembly into mature fibers and the cross-linking
of α
chains. Numerous immunologic differences between fibrinogen
and fibrin have been cataloged, and putative sites for fibrin
enhancing the activity of plasminogen activators have been
identified. Although some conformational changes have been
found by X-ray crystallography, the structural changes leading
to the exposure of sites thought to bind t-PA and/or plasminogen
remain to be demonstrated.
[Back to top]
The Involvement of Blood Coagulation Factor XIII in
Fibrinolysis and Thrombosis
L. Muszbek, Z. Bagoly, Z. Bereczky
and É. Katona
It has been known for a long time that blood coagulation
factor XIII (FXIII) is essential for maintaining haemostasis,
its deficiency leads to severe bleeding complication. Biochemical
studies have revealed that FXIII is a key regulator of fibrinolysis
and, in addition to its role in haemostasis, it has also been
implicated in the pathology of arterial and venous thrombosis.
Most recently, the polymorphisms in the FXIII subunit genes
and their influence on the risk of thrombotic diseases have
stirred a lot of interest. This review, besides including
the basic biochemistry of FXIII, mainly concentrates on the
biochemical and clinical aspects of the involvement of FXIII
in fibrinolysis and thrombosis.
Biochemical aspects: Basics on the structure and
activation of plasma and cellular FXIII. The enzymological
features of activated FXIII and its main substrates. The interaction
of FXIIIa with fibrinogen/fibrin and with components of the
fibrinolytic system. The impact of cross-linked fibrin clot
formation on the fibrinolytic processes. The down-regulation
of FXIIIa within the fibrin clot. FXIII polymorphisms and
their biochemical consequences.
Clinical Aspects: FXIII level and the risk of arterial
thrombosis (coronary artery disease, peripheral artery disease,
ischemic stroke). The effect of FXIII subunit polymorphisms
on the risk of arterial thrombotic diseases. The interplay
between FXIII polymorphisms and other factors influencing
the risk of arterial thrombosis. FXIII and venous thromboembolism.
[Back to top]
Alterations of Fibrinogen Structure in Human Disease
M. Hoffman
Products of normal and pathologic metabolism can react
with proteins to cause covalent modification. When such modifications
affect fibrinogen they can potentially alter fibrinogen function.
Those that have been best studied are oxidation, nitration,
homocysteinylation and glycation. It appears that the clottability
of fibrinogen is maintained unless the degree of modification
is extensive. However, modest degrees of fibrinogen modification
can alter the rate of assembly of fibrin monomers into a fibrin
clot and the fiber structure and packing. In addition, some
types of modification affect lysine residues that are critical
to binding, activation and activity of fibrinolytic enzymes.
Any of these alterations could potentially affect the susceptibility
of fibrin clots to fibrinolysis, and have been shown to do
so in vitro. In the case of homocysteinylation and
glycation, good evidence exists that fibrinogen modification
affects clot stability in vivo. However, direct evidence
is still lacking that these modifications contribute to the
increased atherothrombotic risk associated with hyperhomocysteinemia
and diabetes.
[Back to top]
Fibrin Binding and the Regulation of Plasminogen Activators
during Thrombolytic Therapy
C. Longstaff, S. Williams and C.
Thelwell
First generation thrombolytics (streptokinase and urokinase)
had no fibrin binding capabilities and caused systemic plasminogen
activation with concomitant destruction of haemostatic proteins.
A primary driving force behind the development of the second
generation plasminogen activator tissue plasminogen activator
(tPA or alteplase) was its ability to bind to fibrin and target
thrombolysis. Although in vitro assays highlighted
advantages of fibrin binding, clinical trials were disappointing,
showing only small benefits in mortality with tPA versus streptokinase,
but also with some increase in haemorrhagic stroke. Third
generation thrombolytic agents (reteplase, tenecteplase and
pamiteplase) are variants of tPA engineered to have improved
structure/function, such as longer half life and resistance
to inhibitors. However, clear therapeutic advantages of third
generation thrombolytics in clinical trials have also been
difficult to demonstrate. Although fibrin binding is critical
in regulating the activity of tPA, it is not clear how important
it is for thrombolytic treatment. Advances are needed in our
understanding of the relationship between structure/binding
and activity of PAs in vivo under normal conditions
and when administered in pharmacological doses. Clearly the
impact of fibrin structure and the other components in fibrin
clots must also be considered. Ultimately these studies may
lead to better engineered therapeutics or optimised mixtures
of molecules. With a more detailed understanding of the regulation
of plasminogen activation and fibrinolysis it might be possible
to tailor thrombolytic therapy to different situations such
as myocardial or cerebrovascular treatment or to the patient’s
age and sex and other characteristics.
[Back to top]
Role of Cellular Elements in Thrombus Formation and
Dissolution
N. Wohner
Although fibrin forms the core matrix of thrombi, their
structure depends also on the cellular elements embedded in
its meshwork. Platelets are essential in the initial stages
of thrombus formation, because they adhere and aggregate at
sites of blood vessel wall injury and then serve as a surface
for coagulation reactions, the overall rate of which determines
the final structure of fibrin. In addition, platelets affect
fibrinolysis through their proteins and phospholipids, which
modulate plasmin activity. Leukocytes form mixed aggregates
with platelets and thus influence the structure of thrombi.
After activation they secrete different proteases (elastase,
cathepsin G, matrix metalloproteinases) that enhance the von
Willebrand factor-dependent platelet adhesion. Leukocyte-derived
enzymes, first of all elastase, effect fibrinolysis by direct
digestion of fibrin or indirectly modulate it by partial degradation
of zymogens and inhibitors of coagulation and fibrinolytic
proteases.
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