| Current
Drug Targets
ISSN: 1389-4501
Current Drug Targets
Volume 8, Number 2, February 2007
Contents
Highlights on Important Signaling Pathways as Drug Targets
in Hematological Malignancies
Guest Editors: H. Serve and H.C. Hasselbalch
Editorial Pp. 203
Tyrosine Kinases as Therapeutic Targets in BCR-ABL
Negative Chronic Myeloproliferative Disorders Pp.
205-216
A. Reiter, C. Walz and N.C.P. Cross
[Abstract]
Targeting the RAS Signaling Pathway in Malignant
Hematologic Diseases Pp. 217-235
M.A. Morgan, A. Ganser and C.W.M. Reuter
[Abstract]
Oncogenic Signaling in Acute Myeloid Leukemia
Pp. 237-246
C.H. Brandts, W.E. Berdel and H. Serve
[Abstract]
The Mevalonate Pathway as a Therapeutic Target in
the Ph-Negative Chronic Myeloproliferative Disorders
Pp. 247-256
H.C. Hasselbalch and C.H. Riley
[Abstract]
Vascular Endothelial Growth Factor and Its Receptor
as Drug Targets in Hematological Malignancies Pp.
257-268
T. Kessler, F. Fehrmann, R. Bieker, W.E. Berdel and R.M.
Mesters
[Abstract]
Current Drug Targets in Degenerative Joint Disease
Guest Editors: T. Aigner and E. Bartnik
Editorial Pp. 269-270
Pathobiology of Osteoarthritis: Pathomechanisms and
Potential Therapeutic Targets Pp. 271-282
H.I. Roach, T. Aigner, S. Soder, J. Haag and H. Welkerling
[Abstract]
Cytokine Targeting in Osteoarthritis Pp.
283-292
A.B. Blom, P.M. van der Kraan and W.B. van den Berg
[Abstract]
Molecular Targets in Osteoarthritis: Metalloproteinases
and Their Inhibitors Pp. 293-303
P.S. Burrage and C.E. Brinckerhoff
[Abstract]
Inflammatory Signaling in Cartilage: MAPK and NF-κB
Pathways in Chondrocytes and the Use of Inhibitors for Research
into Pathogenesis and Therapy of Osteoarthritis Pp.
305-313
J. Saklatvala
[Abstract]
Role of Cathepsin K in Normal Joints and in the Development
of Arthritis Pp. 315-323
H.J. Salminen-Mankonen, J. Morko and E. Vuorio
[Abstract]
Osteoarthritis: Aging of Matrix and Cells - Going
for a Remedy Pp. 325-331
T. Aigner, J. Haag, J. Martin and J. Buckwalter
[Abstract]
Cell Death and Apoptosis in Ostearthritic Cartilage
Pp. 333-345
H.A. Kim and F.J. Blanco
[Abstract]
Antioxidant to Treat Osteoarthritis: Dream or Reality?
Pp. 347-357
Y. Henrotin and B. Kurz
[Abstract]
Anabolic Factors in Degenerative Joint Disease
Pp. 359-365
L.J. Sandell
[Abstract]
In Vivo Osteoarthritis Target Validation
Utilizing Genetically-Modified Mice Pp. 367-376
S.S. Glasson
[Abstract]
Growth Plate Cartilage as Developmental Model in Osteoarthritis
Research – Potentials and Limitations Pp. 377-385
T. Aigner and N. Gerwin
[Abstract]
Abstracts
[Back to top]
Editorial
In recent years, the dramatic progress in basic cancer
research has finally reached clinical practice. Five years
after the introduction of imatinib for the treatment of Bcr-Abl
driven diseases, several novel targeted treatment modalities
have been introduced into clinical medicine. For various reasons,
hematological malignancies remain amongst the most promising
candidates to be successfully treated by such modalities.
This Theme Issue, entitled “Highlights on Important
Signaling Pathways in Hematological Malignancies” describes
some of the reasons, why this hope for our patients is realistic,
especially for those suffering from Ph-negative myeloproliferative
disorders. Here, major breakthroughs have been the discovery
of various activating tyrosine kinases, including the most
recent finding of a single, recurrent point mutation in the
Janus kinase 2 (JAK2) in the large majority of patients with
polycythemia vera, but also in about 50 % of patients with
essential thrombocythemia and idiopathic myelofibrosis. Among
other questions, Reiter and co-workers address this surprising
result of a single mutation causing a variety of related diseases
in their description of tyrosine kinases as therapeutic targets
in Bcr-Abl negative chronic myeloproliferative disorders.
However, the future will not only be bright. We will also
have to face that the astounding success of imatinib in the
treatment of chronic myeloid leukemia will not be easily copied
to the other, often more complex, fully malignant and multigenic
hematological malignancies. Here, it will be important to
identify the pathways into the disease, to define their contribution
to the initiation, progression and maintenance, to define
molecular targets, and finally to combine specific therapeutic
modalities with our cytotoxic armamentarium. It Is the second
purpose of this Theme Issue to describe recent progress along
these lines.
In a comprehensive review, Morgan et al. thoroughly
describe the Ras-signaling pathway and ways of therapeutic
interference with this pathway by inhibition of the mevalonate
pathway in malignant hematological diseases, including farnesyl
transferase inhibitor (FTI) treatment. The mevalonate pathway
as a novel therapeutic target in polycythemia vera and related
diseases is separately described by Hasselbalch & Riley
who focus on the potential beneficial effects of statins and
zoledronic acid in these disorders. This novel concept is
based upon in vitro and in vivo studies
showing that statins – in addition to the well-known
cholesterol lowering effect – display anticancer potential
as evidenced by their antiproliferative, proapoptotic, and
antiangiogeneic properties. Furthermore, statins also have
antithrombotic effects. All these drug effects may be beneficial
in clonal myeloid diseases that feature myeloproliferation,
myeloaccumulation (decreased apoptosis), increased angiogenesis
and thrombotic complications.
The vascular endothelial growth factor (VEGF) is considered
a key mediator of the abnormal angiogenesis in many hematological
malignancies. Also, it is a survival factor for endothelial
cells and leukemic cells as well. Thus, in the myelodysplastic
syndrome and in acute myeloid leukaemia (AML), VEGF is thought
to play a major role in autocrine and paracrine loops that
drive leukemic cell and endothelial cell proliferation. Kessler
and co-workers comprehensively describe these aspects of leukaemia-stroma
interactions and the potential role of VEGF as a drug target
in these diseases.
In an extensive review on oncogenic signalling in AML, Brandts
et al. discuss the fundamental genetic events that
determine the pathogenesis of AML. In particular, they describe
the oncogenic signalling events resulting from the translocation
of myeloid transcription factors and mutations in receptor
tyrosine kinases. By describing the pathways that so far have
not been successfully targeted with small molecules, but are
undoubtedly central to AML development they show that kinase
inhibitors can only be a start of molecularly targeted therapy
and that much has to be done to develop new ways to specifically
target phosphatases, transcription factors, DNA-modifying
enzymes and other proteins that cooperatively turn a normal
bone marrow into a leukemic blast.
We would like to thank all the contributors for their efforts
in providing their excellent reviews, which we hope may shed
some light on the most important signalling pathways in hematological
malignancy, and their potential to be intelligently targeted
for the better of our patients. Many of the contributors of
this Theme Issue are also members of the European LeukemiaNet
(ELN). In one of the projects of this EU-funded “Network
of Excellence”, European basic scientists and clinicians
gather to discuss problems and progress in the identification
of new targets and the development of novel, targeted drugs
for the treatment of leukemias (WP16). The discussions in
this group on the ELN meetings greatly helped in putting together
this Theme Issue.
Hubert Serve, MD
Professor of Medicine
Department of Medicine,
Hematology/Oncology
University of Munster
Munster, Germany
Hans Carl Hasselbalch, MD
Professor of Hematology
Department of Hematology
Odense University Hospital
Odense
Denmark
[Back to top]
Tyrosine Kinases as Therapeutic Targets in BCR-ABL
Negative Chronic Myeloproliferative Disorders
A. Reiter, C. Walz and N.C.P. Cross
Acquired constitutive activation of protein tyrosine
kinases is a central feature in the pathogenesis of chronic
myeloproliferative disorders (CMPDs). The most commonly involved
genes are the receptor tyrosine kinases PDGFRA, PDGFRB,
FGFR1 or c-KIT and the non-receptor tyrosine
kinases JAK2 and ABL. Activation occurs
as a consequence of specific point mutations or fusion genes
generated by chromosomal translocations, insertions or deletions.
Mutant kinases are constitutively active in the absence of
the natural ligands resulting in deregulation of haemopoiesis
in a manner analogous to BCR-ABL in chronic myeloid leukaemia.
With the advent of targeted signal transduction therapy with
tyrosine kinase inhibitors, an accurate diagnosis of CMPDs
by morphology, karyotyping and molecular genetics has become
increasingly important. Imatinib induces high response rates
in patients associated with constitutive activation of ABL,
PDGFRα,
PDGFRβ
and some KIT mutants. Other inhibitors under development are
promising candidates for effective treatment of patients with
constitutive activation of JAK2, FGFR1 and imatinib-resistant
KIT mutants.
[Back to top]
Targeting the RAS Signaling Pathway in Malignant
Hematologic Diseases
M.A. Morgan, A. Ganser and C.W.M. Reuter
Molecularly targeting signaling pathways that are involved
in the pathogenesis of hematopoietic malignancies may lead
to more specific and efficacious therapies. Activation of
the RAS signal transduction cascade is a common feature in
the molecular pathogenesis of hematologic malignancies. A
number of novel agents targeting RAS signaling have been developed
over the past decade. This review will focus on these agents,
which include inhibitors of RAS post-translational modification
(farnesyl transferase (FTase)-, geranylgeranyl transferase-I
(GGTase-I)-, isoprenylcysteine carboxylmethyltransferase (ICMTase)-inhibitors,
statins, bisphosphonates), and inhibitors of RAF and MEK activity.
Although some of these inhibitors (e.g. FTase, RAF and MEK
inhibitors) were developed to specifically inhibit RAS signaling,
it has become clear that RAS may not be the only critical
target of these compounds. This review provides a background
on RAS signaling in hematologic malignancies and discusses
opportunities to exploit aberrant cancer cell signaling in
order to develop better treatment options for patients suffering
from these diseases.
[Back to top]
Oncogenic Signaling in Acute Myeloid Leukemia
C.H. Brandts, W.E. Berdel and H. Serve
Acute myeloid leukemia (AML) is a malignant disease of the
bone marrow. Despite intensive treatment only one third of
AML patients are cured. Numerous genetic events have been
identified in the last years that have shed light into the
mechanisms dictating increased self-renewal, proliferation,
survival and block in differentiation. It is increasingly
recognized that AML represents a hierarchical disease, originating
from a leukemia stem cell population.
Sophisticated animal models have helped to elucidate rate-limiting
steps in initiation, development and maintenance of AML. This
review discusses the fundamental genetic events identified
to date that determine the pathogenesis of AML, with particular
emphasis on the oncogenic signaling events that result from
translocation of myeloid transcription factors and mutations
in receptor tyrosine kinases. Our current understanding of
the biology of AML has fueled the development of promising
anti-leukemic agents, which may improve the treatment of the
disease in the future.
[Back to top]
The Mevalonate Pathway as a Therapeutic Target in
the Ph-Negative Chronic Myeloproliferative Disorders
H.C. Hasselbalch and C.H. Riley
The Ph-negative chronic myeloproliferative disorders (CMPDs)
polycythaemia vera, essential thrombocytosis and idiopathic
myelofibrosis are acquired stem cell disorders, which pathophysiologically
are featured by clonal myeloproliferation and accumulation
of myeloid cells, the latter being consequent to decreased
apoptosis. Myelofibrosis and neoangiogenesis in the bone marrow
and spleen are the histopathological hallmarks of idiopathic
myelofibrosis but may develop in the other diseases as well.
In patients with myelofibrosis elevated levels of circulating
CD34+ cells are highly characteristic being partly explained
by a proteolytic bone marrow mileu owing to excessive release
of various proteases with ensuing extracellular matrix degradation
and constitutive mobilisation of CD34+ cells into the peripheral
blood. Thrombohaemorrhagic complications are major clinical
problems contributing significantly to morbidity and mortality.
Based upon in vitro and in vivo studies
of the effects of statins (antithrombotic, antiproliferative,
antiangiogenic, antiproteolytic) and zoledronic acid (antiproliferative
antiproliferative, antiangiogenic, antiproteolytic) this review
focusses on the translation of these effects into potential
clinical benefits of combinational therapy with these agents
in patients with CMPDs.
[Back to top]
Vascular Endothelial Growth Factor and Its Receptor
as Drug Targets in Hematological Malignancies
T. Kessler, F. Fehrmann, R. Bieker, W.E. Berdel and R.M.
Mesters
Angiogenesis is defined as formation of new blood vessels
from the preexisting vasculature, a process which is essential
for malignant tumor growth. While this has been accepted for
solid forms of cancer there is now emerging evidence that
progression of hematological malignancies also requires the
induction of new blood vessels. Vascular endothelial growth
factor (VEGF) is known to be an essential regulator of physiological
and pathological angiogenesis. Numerous preclinical and clinical
studies have validated VEGF as target for antiangiogenesis
and anticancer therapy. With regard to hematological malignancies
a stimulating effect of VEGF for proliferation, survival and
migration of leukemia cells could be demonstrated. Bone marrow
of leukemia patients shows an increased microvessel density
as well as VEGF expression. Complete remissions in acute myeloid
leukemia (AML) have been reported by targeting the receptor
tyrosine kinase system of VEGF. While the pathophysiology
behind the contribution of VEGF to leukemia progression is
not yet completely understood, VEGF and its receptors may
provide promising targets not only in solid tumors but also
hematological malignancies such as AML.
[Back to top]
Editorial
The more the baby-boomer generation ages, the more patients
will be suffering from chronic, degenerative diseases, like
osteoarthritis. This means they will experience pain, swelling,
weakness and loss of functional ability in their diseased
joints. These symptoms will progress to significantly impact
the quality of life and the productivity of the affected people.
In the US Centers for Disease Control report from 2002 “Prevalence
of self-reported Arthritis of chronic joint symptoms among
adults – United States, 2001” the prevalance of
arthritis/chronic joint symptoms were given as 33%, representing
69.9 million adults. This translates directly into a significant
economic burden, which comprises both the direct costs (diagnosis,
hospitalization, therapies) and the indirect costs including
premature mortality, chronic and short-term disability and
not to forget losses of productivity. Not surprisingly, indirect
costs far exceed direct costs.
Despite tremendous efforts to understand pathomechanisms of
osteoarthritic joint changes both at the level of basic and
at the level of applied research, the major unmet medical
need for osteoarthritis remain disease-modifying agents and
better diagnostic tools. With the COX-2 inhibitors we thought
to have alternatives to NSAIDs with their gastrointestinal
side effects to treat at least symptoms of osteoarthritis,
but the withdrawal of rofecoxib (Vioxx; Merck) in September
2004 and the subsequent discussion on a class effect of coxibs
in respect to cardiovascular risks, depredated us of our illusions.
But did not biology advance tremendously in recent years with
all its genomics, proteomics, metabolomics and other ‘omics’
to show hundreds of new molecules to explain the academic
researchers the intricacies of disease pathways and at the
same time to quench the thirst of the industry researcher
for novel drug targets? Reading through the collected reviews,
a tremendous amount of knowledge on individual targets has
been gained. But quite notably, an other notion emanates from
most of the reviews: Understanding individual targets does
not seem to be sufficient to develop effective therapeutic
strategies. Roach et al. set the stage by describing
OA as a disease of an organ system, implying that a true understanding
of the disease must eventually be based on ‘molecular
portraits’ of the affected tissues including articular
cartilage, bone, synovium, capsule, ligaments and muscles.
Burrage and Brinckerhoff summarise our understanding of apparently
the most obvious drug targets for OA, the proteases that are
directly responsible for the degradation of the cartilage
matrix components. Although some very attractive targets (MMP-13,
ADAMTS aggrecanases and TACE) are known, they speculate, that
we need to follow the concept of “molecular polypharmacy”
i.e. to target several proteases at the same time to achieve
clinical effects.
Salminen-Mankonen et al. report on Cystein cathepsins,
most notably on Cathepsin K, whose role as a protease responsible
for degradative events in bone and in cartilage earns more
attention than in the past, although here the development
of tissue specific inhibitors poses problems. Turning to the
level of communication between cells, Bloom et al.
point towards cytokines with IL-1 inhibition shown to ameliorate
OA-like pathology and TGFb providing an approach to promote
cartilage integrity and repair, but also in the context of
a combination therapy. Our hopes to find therapeutic approaches
at the level of regulation of the MAPK and NFkB signaling
pathways are dampened by Saklatvala with the arguments that
it is not clear which pathway would have to be blocked for
most effective MMP suppression and what effects it may have
on normal cartilage turnover to block these central pathways.
Sandel and Fukui continue with cartilage-specific anabolic
pathways, whose knowledge should eventually allow to define
markers of specific stages of OA and to identify mechanisms
to stimulate cartilage-specific neosynthesis of matrix molecules
and to inhibit inappropriate synthesis. The regulatory pathways
securing the homeostasis between catabolic and anabolic events
are probably as diverse and complicated like the ones leading
to apoptotic cell death as described by Kim and Blanco. Although
we know a host of potentially attractive targets for pharmaceutical
interventions to interfere with apoptosis, we lack specific
inhibitors to provide evidence from experimental models of
OA that inhibition of chondrocyte apoptosis would lead to
any structural effects in OA. Henrotin and Kurz review the
attracative, yet speculative approach to use antioxidants
to treat osteoarthritis but have to state that currently there
is no consistent evidence that additional antioxidant supply
is efficient to relieve OA symptoms or to prevent structural
changes in OA. Aigner et al. remind us that aging,
the most prominent risk factor for the initiation and progression
of OA should not be seen as inevitable fate, but should be
taken as a challenge for therapeutic intervention.
In a very thorough analysis Glasson summarizes all genetically
modified transgenics in the field to validate, i.e. to confirm
therapeutic targets in vivo. Although she states,
that results in the mouse will not always transpose to the
human condition, much has been learnt from these models and
that their further evaluation will continue to ad to our understanding
of genes contributing to OA. Aigner and Gerwin add to the
genomics perspective the importance of developmental models
to study the function of genes involved in the pathophysiology
of OA.
We are fascinated by the intricacies of individual gene products,
their interaction partners and their multilayered regulation.
But life did not evolve to let us dissect mechanisms of chronic
diseases that easily. “The hope of the rapid translation
of ‘genes to drugs’ has foundered on the reality
that disease biology is complex, and that drug development
must be driven by insights into biological responses.”
(cited from Butcher et al. 2004 [1]).
Let alone to find that single drug target, the magic bullet,
to treat osteoarthritis.
Peeling through layer and layer of unexpected complications
to finally reveal the understanding of disease causes, we
have to realize that single gene products always act in concert
with many other ones. Regulatory pathways are crosslinked,
they crosstalk and thus if you block one signalling pathway,
another one takes over its function. More and more a new paradigm
enters the arena: systems biology. Most of us will probably
feel that systems biology is still far-off, but in the end
it will be the only way to provide detailed, dynamic simulations
of physiology both during the normal as well as during the
diseased status. Such simulations will require the analysis
of activity of hundreds or thousands of genes or proteins
simultaneously, instead of one gene at a time. We will need
to study all sorts of networks, gene regulatory networks,
protein interaction networks and signaling networks. And not
in a static sense, but under the incorporation of time as
the key dimension. Finally computational modeling and simulation
may give us a model to describe pathomechanisms of osteoarthritis
and with this true drug targets. Thus we will have to leave
yet another paradigm: one drug one target.
Current drug discovery researchers are focussed on drugs composed
of single chemical entities, yet todays clinical practice
already employs therapeutic regimens with more than one active
ingredient. This is most evident in the treatment of HIV,
cancer and neurodegenerate diseases, and also in the treatment
of rheumatoid arthritis, where pharmacologic therapy often
consists of combinations of NSAIDs, DMARDs and/or glucocorticoids
(American College of Rheumatology 2002 [2]).
Instead a multicomponent drug would offer a concerted pharmacological
intervention with several compounds that interact with multiple
targets [3]. The hope is, that such an approach would allow
to affect disease relevant pathways more efficiently with
reduced side effects. Araujo et al. [4] used a mathematical
model of the EGF signaling network to investigate combination
therapy to show that with a novel type of combination therapy
in which multiple nodes in a signaling cascade are targeted
simultaneously with selective inhibitors, one may induce a
desired signal attenuation with lower doses, then when only
one node would be targeted in isolation. Thus the inhibition
of serially-connected processes should have supra-additive
(synergistic) effects on the downstream signals. Why not apply
this to osteoarthritis ? The compilation of reviews teach
us a lot about individual seemingly attractive drug targets.
But since we only try to affect the highly networked system
at one point at a time, we may only see effects in isolated
in vitro systems, but not in more complex in
vivo models.
References
[1] Butcher, E.C.; Berg, E.L.; and Kunkel, E.J. (2004) Nat.
Biotechnol., 22(10), 1253-1259.
[2] American College of Rheumatology Subcommittee on Rheumatoid
Arthritis Guidelines (2002) Arthritis Rheum, 46(2),
328-346.
[3] Keith, C.T.; Borisy, A.A., and Stockwell, B.R. (2005)
Nat. Rev. Drug Discov., 4(1), 71-78.
[4] Araujo, R.P.; Petricoin, E.F., and Liotta, L.A. (2005)
Biosystems, 80(1), 57-69.
Thomas Aigner, MD, DSc,
(Theme Editor)
Institute of Pathology, University of Leipzig,
Liebigstr. 26,
04103 Leipzig, Germany
E-mail: thomas.aigner@medizin.uni-leipzig.de
Eckart Bartnik, PhD
(Theme Editor)
TD Thrombosis&Angiogenesis
Sanofi-Aventis Pharma Deutschland GmbH
Industriepark Höchst/Building H821
65926 Frankfurt/Main, Germany
E-mail: Eckart.Bartnik@Sanofi-Aventis.com
[Back to top]
Pathobiology of Osteoarthritis: Pathomechanisms and
Potential Therapeutic Targets
H.I. Roach, T. Aigner, S. Soder, J. Haag and H. Welkerling
Osteoarthritis, a degenerative joint disease, is the most
disabling condition of the Western world. It affects first
and foremost the articular cartilages and leads to a molecular
and supramolecular destruction of the extracellular cartilage
matrix. In addition, the cells, the chondrocytes, show severe
alterations of their phenotype: they get anabolically and
catabolically activated, change accordingly their gene expression
pattern, lose their differentiated phenotype, and undergo
focally cell death and cell degeneration. All these processes
represent potential targets for therapeutic intervention and
drug development. Apart from the cartilage itself, however,
other joint tissues are also involved in the disease: thus,
the synovial capsule and membrane as well as the subchondral
bone account not only for most of the symptoms of the disease,
but are also presumably involved in the progression of the
degenerative process. Both, inflammation and stiffening within
the joint capsule accelerate joint destruction. Stiffening
of the subchondral bone increases the mechanical stress over
the overlying cartilage during physiological movement.
Altogether, there is a plethora of tissues, disease processes
and targets for treating osteoarthritic joint degeneration,
which will need to be followed up systematically in the future.
[Back to top]
Cytokine Targeting in Osteoarthritis
A.B. Blom, P.M. van der Kraan and W.B. van den Berg
Cytokines are involved in osteoarthritis (OA) at several levels.
They are involved in primary cartilage damage, but also in
synovial activation that is observed in osteoarthritic joints.
From in vitro studies and animal models for OA, several
cytokines have been identified that are potential targets
for OA therapy. Two promising targets are the destructive
cytokine Interleukin-1 (IL-1) and the anabolic growth factor
transforming growth factor (TGF)β
and these will be discussed in more detail. Inhibition of
IL-1 has been proven to result in amelioration of osteoarthritis-like
pathology in animal models and the role of IL-1 is substantiated
in studies in IL-1 deficient mice. In contrast, application
of the anabolic growth factor TGFβ
may provide an alternative approach to promote cartilage integrity
and repair. TGFβ
is a potent stimulator of chondrocyte matrix production, and
therefore has a potency to repair already damaged cartilage.
However, TGFβ
induces tissue fibrosis and osteophytes at the joint margins
and can only be applied to promote cartilage repair when these
side effects can be blocked. This appears possible with concomitant,
compartmentalized application of selective inhibitors of TGFβ
in soft tissues, using local gene therapy with inhibitory
Smad 6 and 7.
Since OA is often limited to a few joints, local gene therapy
may provide a suitable way to treat OA patients. Depending
on the phenotype of a particular OA patient, e.g. with or
without marked synovial activation, treatment may be focused
mainly on suppression of catabolism or stimulation of anabolism,
but combination therapy seems most warranted.
[Back to top]
Molecular Targets in Osteoarthritis: Metalloproteinases
and Their Inhibitors
P.S. Burrage and C.E. Brinckerhoff
The debilitating destruction of joint tissues seen in osteoarthritis
(OA) is due, in large part, to the degradative activity of
metalloproteinase (MP) enzymes that target extracellular matrix
(ECM) components within articular cartilage. Although successful
in suppressing the pain and inflammation associated with this
disease, conventional OA therapeutics do not inhibit the underlying
tissue catabolism, allowing the disease to progress into irreversible
ECM loss and chronic disability. Therapeutic inhibition of
metalloproteinase activity is not a new concept, however,
its transfer into clinical use has been frustrating. Disappointing
results from clinical trials with small molecule inhibitors
of metalloproteinases have highlighted the critical importance
of inhibitor specificity, and the need to identify the individual
metalloproteinases responsible for joint destruction. We discuss
strategies of inhibition using small molecule inhibitors and
tissue inhibitors of metalloproteinases (TIMPs) engineered
to increase inhibitory specificity, and present new data using
of new reagents such as ribozymes and inhibitory RNAs that
repress expression of specific enzymes. Recent data has implicated
the disease stage-dependent involvement of matrix metalloproteinase-1,
-2, -3, -9, -13, ADAM-17/TACE (tumor-necrosis factor-α
converting enzyme), and ADAMTS-5 (a disintegrin and metalloproteinase
with thrombospondin 1 motifs) as major in vivo mediators
of the ECM degradation seen in OA, and as such, they represent
promising therapeutic targets. We conclude that the concept
of molecular polypharmacy, in which the relevant enzymes are
selectively targeted with multiple directed therapies, may
offer a new therapeutic strategy that prevents joint destruction
and minimizes toxicities.
[Back to top]
Inflammatory Signaling in Cartilage: MAPK and NF-κB
Pathways in Chondrocytes and the Use of Inhibitors for Research
into Pathogenesis and Therapy of Osteoarthritis
J. Saklatvala
Osteoarthritis is characterised by degeneration of articular
cartilage. It is thought to be primarily a disease of cartilage.
Inflammatory response genes, such as proteinases, cyclooxygenase,
and cytokines are implicated in its pathogenesis. The evidence
for expression of these genes in articular cartilage in osteoarthritis
is reviewed. The expression of inflammatory response genes
is controlled by four major intracellular signalling pathways.
These lead to activation of the three mitogen-activated protein
kinases (MAPK) and the transcriptional regulator nuclear factor
kappa (NFκ)-B.
The current state of knowledge of the structure of these pathways
is summarized. Pharmacological inhibitors of the protein kinases
of the pathways in current use are described, and insights
into chondrocyte gene expression obtained with them are discussed.
Very limited use of these inhibitors has yet been made in
animal models of osteoarthritis.
The main use of the inhibitors in the near future will be
in investigation of pathogenetic mechanisms in osteoarthritis,
both in experimental animals and in vitro, with a
view to identifying therapeutic targets. Prospects for using
signalling pathway inhibitors for therapy in osteoarthritis
are distant.
[Back to top]
Role of Cathepsin K in Normal Joints and in the Development
of Arthritis
H.J. Salminen-Mankonen, J. Morko and E. Vuorio
Cysteine cathepsins are a large family of proteolytic enzymes
active at acidic pH as found in lysosomes. Since its discovery
in 1990’s, cathepsin K has been shown to be a key enzyme
in osteoclastic bone resorption through its activity in the
resorption lacuna. Although characteristic to osteoclasts,
the expression of cathepsin K has also been observed at other
sites in skeleton. Several recent observations have demonstrated
up-regulation of cathepsin K in osteoarthritic cartilage and
inflamed synovial tissue. As cathepsin K is one of the few
extracellular proteolytic enzymes capable of degrading native
fibrillar collagen, it may play an important role in the progressive
destruction of articular cartilage both in osteoarthritis
and in inflammatory arthritides. Also transgenic mouse models
have provided evidence supporting the important role of cathepsin
K in both groups of arthritides. The aim of this chapter is
to review the accumulating evidence for the role of cathepsin
K in degradation of articular cartilage regardless of its
pathogenic background, and to discuss the potential efficacy
of cathepsin K inhibitors to slow down or prevent articular
cartilage degradation.
[Back to top]
Osteoarthritis: Aging of Matrix and Cells - Going
for a Remedy
T. Aigner, J. Haag, J. Martin and J. Buckwalter
It has been known for a very long time that aging is the most
prominent risk factor for the initiation and progression of
osteoarthritis. This might be related to continuous mechanical
wear and tear and/or result from time/age-related modifications
of cartilage matrix components. Also a mere loss of viable
cells over time, due to apoptosis or any other mechanism,
might contribute. More recent evidence, however, supports
that stressful conditions for the cells might promote chondrocyte
senescence and might be in particular important for the progression
of the osteoarthritic disease process. One of the most important
implications of this hypothesis is that it points to issues
of cellular degeneration as the basis for understanding of
the initiation and the progression of osteoarthritis. Equally
important, it emphasizes that whatever treatment we envisage
for osteoarthritis, we must take into account that we are
dealing with aged/(pre) senescent cells which no longer have
the abilities of their juvenile counterparts to respond to
the many mechanical, inflammatory, and traumatic assaults
to the tissue. Thirdly, this directs treatment options to
deal with the senescence of cells, which are only conceptually
available today.
Clearly, if accumulation of wear and tear over time is the
major scenario of osteoarthritis, any therapy will largely
be hopeless as moving and loading the joints is unavoidable
as implication of their use. However, this review intends
to open up the idea that age-related changes are less a fate,
but rather a challenge for therapeutic intervention which
can be taken.
[Back to top]
Cell Death and Apoptosis in Ostearthritic Cartilage
H.A. Kim and F.J. Blanco
Osteoarthritis (OA) is the most common chronic joint disease
in the elderly population, causing significant pain and disability.
Because the cardinal feature of OA is a progressive loss of
articular cartilage, a great portion of the research endeavour
into the pathogenesis of OA has been focused on the regulation
of matrix synthesis and degradation. The phenotypic stability
and survival of the chondrocytes are essential for the maintenance
of a proper cartilage matrix. This has lead to the long-standing
assumption that cell death is a central feature in OA cartilage
degeneration. The important role of apoptosis in OA has been
demonstrated in in vitro and in vivo models.
However, it should be noted that the relative contribution
of apoptotic cell death in the pathogenesis of OA is still
difficult to assess because of the chronic nature of the disease
process. Therefore, the apoptosis of chondrocytes seems to
be a potential target for therapeutic interventions in OA.
The death receptor, mitochondrial and endoplasmic reticulum
pathways are the major cellular pathways of apoptosis. Of
all these elements involved in the apoptosis of chondrocytes,
caspase inhibition has been studied with the most detail.
Other molecules with the capacity to modulate mitochondria
function, phosphatase (PP-1A/B) activity and pro-apoptosis
stimuli (NO, prostaglandins, cytokines, ROS) could be excellent
targets to block apoptosis of chondrocytes. Finally, the regulation
of the natural inhibitors of apoptosis (c-FLIP, BAR, ARC and
HC-gp39) could complement the other strategies to reduce cartilage
degradation.
[Back to top]
Antioxidant to Treat Osteoarthritis: Dream or Reality?
Y. Henrotin and B. Kurz
Osteoarthritis is one of the most common chronic diseases
that causes pain and physical disability in patient. Although
OA is considered as a global disease affecting all joint tissues,
cartilage degradation is the end point. The degradation of
cartilage results of the combination of mechanical stress
and biochemical factors, mainly metalloproteinases and reactive
oxygen species (ROS). The activity of reactive oxygen species
is balanced by enzymatic and non-enzymatic antioxidants, that
act by inhibiting oxidative enzymes, scavenging free radicals
or chelating ion metals. Until now, few information is available
on the antioxidative status of chondrocytes. Further, the
modification of the antioxidative system in osteoarthritis
remains unknown. Some antioxidant supplements or drugs with
antioxidant properties have been developed to reinforce the
cellular antioxidant status. However, until now, there is
no consistent evidence that additional antioxidant supply
is efficient to relieve OA symptoms or to prevent structural
changes in OA cartilage.
[Back to top]
Anabolic Factors in Degenerative Joint Disease
L.J. Sandell
While a great deal of information is available on the cellular
and molecular biology of cartilage degradation, less is known
about anabolism in normal cartilage and degenerating cartilage.
A consistent amount of evidence is now available on the neo-synthesis
of matrix molecules and enzymes in OA: the entire cell metabolism
appears to be increased leading to the hypothesis that chondrocytes
in OA are actually “activated”.
This chapter will focus on anabolic events that are now known
to occur in articular cartilage. We will begin to view articular
cartilage as a complex three-dimensional tissue in which local
events may be different. We will also be interested in viewing
the development of degenerative arthritis as a continuum from
functionally normal tissue to degeneration.
[Back to top]
In Vivo Osteoarthritis Target Validation
Utilizing Genetically-Modified Mice
S.S. Glasson
Osteoarthritis (OA) is a progressive disease of cartilage
degradation that significantly impacts quality of life. There
are currently no effective treatments and, while a large number
of potential therapeutic targets exist, most have not been
validated in vivo. The range of OA models in the
mouse has dramatically expanded in the last decade, beyond
spontaneous models, to include genetically modified transgenic,
knockout (KO) and knock-in (KI) mice that can develop premature
cartilage degeneration reminiscent of OA. In addition, instability
models of OA, either induced by intra-articular (IA) collagenase
or surgery, are providing a set of tools to assist in the
identification of disease-modifying OA drug (DMOAD) targets.
These models are now vital tools to dissect the pathways essential
to the pathogenesis of OA. Two targets, ADAMTS (a disintegrin
and metalloproteinase with thrombospondin-like motifs)-5 and
IL-1β
(interleukin-1 beta), have been validated in the surgical
destabilization of the medial meniscus model (DMM) in KO mice.
Other potential targets evaluated in instability models, either
showed no disease modification or a worsening of disease,
suggesting that those targets have no role, a protective role
or that other, more destructive enzymes etc., can overcompensate.
Development of small molecule or protein antagonist inhibitors
of therapeutic targets require many years to bring to clinical
trials and often confront potency and safety issues which
impede successful progress. Validation, or confirmation of
therapeutic targets in vivo is most clearly and efficiently
obtained by using KO studies, than by creating potent and
selective DMOADs to multiple potential targets. While the
results in the mouse will not always transpose to the human
condition, the track record of mouse knockouts corresponding
to the human phenotype have been excellent. These results
indicate that the evaluation of genetically modified mice
will become increasingly important as we unravel the genes
contributing to OA.
[Back to top]
Growth Plate Cartilage as Developmental Model in Osteoarthritis
Research – Potentials and Limitations
T. Aigner and N. Gerwin
Gene expression analysis including large scale gene expression
profiling has become a very basic tool for investigating the
pathogenesis of degenerative joint diseases as well as for
the search of new drug targets. However, gene expression analysis
so far revealed very complex expression patterns rather than
a clear picture of molecular changes occurring during the
initiation and progression of the disease. To elucidate the
molecular changes in osteoarthritis the analysis of the fetal
growth plate as a developmental model for phenotypic changes
in chondrocytes occurring in osteoarthritis can help in three
ways: it allows to interpret gene expression patterns in the
context of disease-relevant processes also occurring in developing
cartilage (e.g. cell differentiation, proliferation, matrix
synthesis, catabolism and calcification), it offers the chance
to investigate gene function in these functional contexts
by knocking out or overex-pressing genes in animals, and it
provides a suitable model for testing the effect of therapeutic
compounds on these processes within the growing cartilage.
|
