Postępy Biochemii 59 (4) 2013
415
Serafim Kiriakidis
1,2,*
Ewa M. Paleolog
1,2
1
Kennedy
Institute
of
Rheumatolo-
gy, Nuffield Department of Orthopa-
edics,
Rheumatology
and
Musculo-
skeletal Sciences, University of Oxford
2
Department of Medicine, Imperial College
London
*
Kennedy Institute of Rheumatology, Nuffield
Department of Orthopaedics, Rheumatology
and Musculoskeletal Sciences, University of
Oxford, Roosevelt Drive, Headington, Oxford,
OX3 7FY, UK; e-mail: serafim.kiriakidis@
kennedy.ox.ac.uk
Received: September 14, 2013
Accepted: September 14, 2013
Key words: endothelium, inflammation, arthri-
tis, cytokines, angiogenesis
Abbreviations: CRP — C-reactive protein; FIH-
1 — factor inhibiting HIF-1; HIF — hypoxia-in-
ducible factor; ICAM-1 — intercellular adhe-
sion molecule-1; IL — interleukin; PHD — pro-
lyl hydroxylase domain-containing enzyme;
RA — rheumatoid arthritis; TNFα —tumour
necrosis factor α; VCAM-1 — vascular cell ad-
hesion molecule-1; VEGF — vascular endothe-
lial growth factor
Vascular endothelium — role in chronic inflammatory disease
ABSTRACT
T
he vascular endothelial lining of blood vessels plays a key ‘target-effector’ role in vivo,
integrating the body’s response to inflammatory cytokines, chemokines and growth fac-
tors (derived from both endothelial cells themselves and from other cells such as leukocytes
and fibroblasts), to allow leukocyte activation, adhesion and extravasation from the flowing
blood into underlying tissue. Endothelial proliferation, through the process of angiogene-
sis, results in an increased cell surface area for these events to occur, and further functions
to deliver oxygen and nutrients, and to remove waste products. In addition to playing an
important role in physiology, the endothelium is thus an active participant in inflammatory
pathologies. One of the best understood diseases in which inflammation and angiogenesis
play a part is rheumatoid arthritis (RA). Blockade of the inflammatory cascade in RA has
significant consequences for the vasculature, highlighting the links between reducing endo-
thelial activation and therapeutic benefit in chronic inflammatory disorders.
INTRODUCTION
The human vascular endothelial lining of blood vessels, which covers the
body’s network of arteries, veins, capillaries and lymphatics, was in the past
thought to be merely an inactive barrier between the circulation and underlying
tissues. However, it is now clear that despite its relatively small total mass, en-
dothelium actively participates in physiology and pathology in vivo. Endothelial
cells produce mediators regulating blood flow, and influence coagulation and
fibrinolysis, usually presenting a non-thrombogenic surface to flowing blood.
Furthermore, the endothelium plays a role in the process of cell recruitment,
through expression of cytokines and chemokines, thus affecting the activation
status of leukocytes. Finally, endothelial cells play a central role in the process of
angiogenesis, which is vital for efficient supply of oxygen and nutrients to tissue,
and for removal of waste products.
Vascular endothelium thus fulfills a vital homeostatic function and acts as a
rapid response facility in situations of inflammation, injury or infection. Indeed
endothelium plays an important target-effector role in many diseases associated
with inflammation. Such diseases include diabetes type 1, where microvascular
and macrovascular complications combine with activation of the immune sys-
tem and inflammation. In chronic airway disease, inflammation leads to chan-
ges in the airways and obstruction of airflow, but other events include vascular
remodelling and angiogenesis. Other disorders, such as systemic lupus erythe-
matous, atherosclerosis and inflammatory bowel diseases, also involve immune
system activation and enhanced blood coagulation in association with pro-in-
flammatory cytokine expression.
Rheumatoid arthritis (RA) is a prototypical inflammatory disease, in which
angiogenesis and changes in oxygen tension interact with inflammation to cul-
minate in the features of joint and cartilage destruction. Since the pathogenesis
of RA is relatively well understood, RA can serve as a paradigm for understan-
ding the role of the vasculature in inflammation, particularly in the light of ob-
servations using therapies targeting aspects of the inflammatory process in RA,
such as anti-tumour necrosis factor α (TNFα) biologicals. The involvement of
the vasculature in RA pathogenesis will be discussed in detail in the following
sections.
RHEUMATOID ARTHRITIS — A PROTOTYPICAL INFLAMMATORY
DISEASE WITH VASCULAR INVOLVEMENT
RA is a common human disease, affecting about 1% of the population in
most parts of the world, and is characterized by inflammation of the synovial
membrane which lines the joint spaces, leading to the localized invasion and
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destruction of underlying cartilage and bone. Every year in
the United Kingdom there are approximately 20 000 new
cases of RA, which is more common in women than men
by a factor of 3:1 [1]. The clinical presentation can vary in
terms of severity and the age of onset, although the peak of
RA onset occurs between the fifth and sixth decades of life.
Patients display painful, stiff and swollen joints, and usual-
ly present with a symmetrical polyarthritis, predominantly
involving the small joints of the hands and wrists, as well as
the metatarsophalangeal joints, ankles and knees. RA is as-
sociated with a range of non-articular symptoms, including
inflammatory nodules, vasculitis and pericarditis, together
with involvement of the lungs and nervous system, depres-
sion and anaemia. Furthermore, the standardized mortality
ratio for patients with RA is more than 1.5–2.5-fold higher
than for the general population. The major cause of mor-
tality (more than 40% of deaths) is cardiovascular disease,
including ischemic heart disease and heart failure [2]. A to-
tal of 10 million working days were lost in 2006–2007 in the
UK due to musculoskeletal conditions such as RA, second
only to stress, depression and anxiety, representing a signi-
ficant economic impact due to lost production. RA patients
of working age are significantly more likely to stop work on
health grounds than matched controls. RA thus imposes a
significant social and economic burden, due to loss of ear-
nings and medical expenses, apart from adversely affecting
quality of life.
In spite of many years of intensive investigation, the
cause of RA remains unknown, although current thinking
favours the concept of a multi-factorial disease, in which
contributory genetic factors, including shared epitope alle-
les of the human leukocyte antigen and a polymorphism of
protein tyrosine phosphatase N22 [3,4], combine with envi-
ronmental factors (such as smoking), sex hormones, and
perhaps an infectious agent or other immune-activating fac-
tor, to initiate an autoimmune response that culminates in a
disease with inflammatory and destructive features [5]. As
mentioned, the primary site of inflammation is the synovial
lining of the closed spaces of articular joints. The normal sy-
novium is generally 1–3 cell layers thick and is composed
of loosely associated macrophage- and fibroblast-like cells,
as well as vascular endothelial cells. In RA, the synovium
is altered to a thickened tissue several cell layers thick, and
becomes infiltrated by blood-derived cells, including T cells,
B cells and macrophages. Subsequently the synovium beco-
mes locally invasive at the synovial interface with cartila-
ge and bone. Progressive destruction of cartilage and bone
eventually combine to produce deformities and functional
deterioration and profound disability in the long term [6].
Of relevance to this review, the term “vascular rheumato-
logy” has been coined, to emphasise the importance of mi-
crovascular and macrovascular endothelium in RA and in
other rheumatic disorders.
ROLE OF ENDOTHELIUM IN INFLAMMATORY DISEASE
RA, as an autoimmune disease,
is
characterized by the
presence of circulating auto-
antibodies, including some
that bind with high affinity to human endothelial
cells [7].
Integration of antigen-presentation, amplification of lym-
phocytes and generation of mediators of humoral and cellu-
lar immunity needs to occur in the peripheral lymphoid or-
gans, primarily lymph nodes and spleen. T lymphocytes cir-
culate between non-lymphoid tissues and the lymph nodes,
entering through the afferent lymphatic blood vessels and
across the high endothelial venules, and exiting via efferent
lymphatic vessels. This continuous lymphocyte trafficking
across endothelium enables the antigen-sensitive cells to be
exposed to their specific antigen, prompting clonal expan-
sion. Blood vessels thus allow recruitment of the activated
lymphocytes to specific sites, which is promoted further by
vasodilation. An increase in vessel density through angioge-
nesis further increases the endothelial surface area available
for the ingress of cells and molecules to the site of inflam-
mation, amplifying and maintaining the immune response.
The involvement of endothelium in the pathogenesis of
RA can also be inferred from observations that RA is as-
sociated with vascular and haematological abnormalities
(such as anaemia). The swollen joints which occur in RA pa-
tients suggest altered vascular permeability, with increased
plasma extravasation and oedema formation. Interestingly,
a recent study has documented the presence of immature
blood vessels in RA synovium. Comparison of the staining
patterns for CD31 and the pericyte marker α-smooth musc-
le cell actin revealed a significant fraction of CD31-positive
but α-smooth muscle cell actin-negative cells in RA tissue
when compared to osteoarthritis or control tissue [8]. This
altered vascular signal in RA synovium was also detected
using Doppler ultrasound [9,10]. Studies using mouse mo-
dels of RA have also shown vascularised synovium in arth-
ritic mice [11] (Fig. 1).
Therefore, a
ctivation of an effective immune response,
enhanced leukocyte activation and extravasation, expres-
sion of chemokines, and increased angiogenesis, possibly
leading to formation of immature and highly permeable
Figure 1. Synovial vascularity in an in vivo model of arthritis. Arthritis was indu-
ced in mice using bovine collagen and arthritis development and severity were
monitored daily [11]. Serial sections of paw tissue from healthy (A, C) and severe-
ly arthritic (B, D) mice were stained with either (A, B) haematoxylin and eosin or
(C, D) with anti-CD31 antibody and counter-stained with haematoxylin. Images
show the metatarsal joint of mouse paws. The healthy section demonstrates a nor-
mal joint architecture without signs of inflammation or bone destruction. In the
arthritic joint, synovial hyperplasia, blood vessels (CD31-positive), inflammatory
cell infiltration, and loss of cartilage and bone are evident. Abbreviations: B —
bone; C — cartilage; S — synovium; JS — joint space.
Postępy Biochemii 59 (4) 2013
417
blood vessels, all point to the underlying role of inflamma-
tion-mediated vascular endothelial activation in RA, which
will be discussed in more detail in subsequent sections.
LEUKOCYTE-ENDOTHELIAL INTERACTIONS IN RA
The cellular infiltration which is a characteristic of RA
synovium suggests that activation of endothelium, together
with expression of leukocyte adhesion molecules, as well as
of cytokines and chemokines, is likely to be involved in RA
pathogenesis. Adhesion of leukocytes to vascular endothe-
lium in vivo must overcome the normal vascular mobility
of circulating cells and result in a localized arrest of leu-
kocytes at relevant sites. Endothelial cells resembling high
endothelial venules, which control lymphocyte migration
into organized lymphoid tissues, have been observed in RA
synovium. These
s
ynovial microvascular endothelial cells
acquire a cuboidal morphology, and become fenestrated
[12], suggesting specialised mechanism(s) regulating leuko-
cyte extravasation into joint tissue.
Inflammatory cytokines such as TNFα and interleukin
(IL)-1, which play a central role in RA pathogenesis, have
the potential capacity to regulate many of the events occur-
ring in the RA microvasculature, including leukocyte extra-
vasation and chemotaxis [13]. TNFα is a fundamental indu-
cer of endothelial cell responses, and both TNFα receptors
CD120a/TNF-R1 and CD120b/TNF-R2 have been detected
on RA synovial endothelial cells. In RA, endothelial cells
express numerous cytokine-inducible adhesion molecules,
including E-selectin [14], vascular cell adhesion molecule
(VCAM)-1 and intercellular adhesion molecule (ICAM)-1.
For example, scintigraphy utilizing a
99m
Tc-anti-E-selectin-
-Fab was used to image synovitis in RA, and demonstra-
ted improved specificity compared to a conventional tracer
for bone and joint, and specificity for targeting active joint
inflammation [15]. In our laboratory, we recently demon-
strated that imaging of anti-E-selectin in vivo could detect
endothelial activation in models of arthritis and could be
applied to quantify disease and investigate the effects of no-
vel therapies [16]. Elevated levels in RA of soluble forms of
E-selectin, ICAM-1 and VCAM-1 have also been described.
Synovial membrane and synovial fluid T cells display an
enhanced capacity to interact with purified E-selectin and
VCAM-1, relative to peripheral blood lymphocytes from
either the same patients or from healthy donors, due to
increased levels of VLA-4α, the counter-ligand for VCAM-
1. In addition, synovial fluid lymphocytes show higher
expression of other integrins such as CD29, VLA-1α, VLA-
5α and VLA-6α [17,18]. Accumulation of T cells in RA sy-
novium thus appears to result from elevated expression of
adhesion receptors on synovial microvascular endothelium,
leading to the selective emigration of memory T lympho-
cytes, which may bear enhanced levels of ligands for these
adhesion molecules as a result of a previous activation step.
Other adhesion molecules present on synovial endothelial
cells include CD31, vascular adhesion proteins (VAP)-1 and
VAP-2 and CD146 [19].
Moreover, endothelial cells are a source of a range of pro-
-inflammatory cytokines, including IL-1, IL-6 and granu-
locyte macrophage colony-stimulating factor. Many of the
features of the rheumatoid synovial environment suggest
possible roles for chemoattractant cytokines, in that the lar-
ge number of infiltrating leukocytes, especially the accumu-
lation of monocyte/macrophages and lymphocytes, could
in part be a response to the elaboration of chemokines. En-
dothelial cells secrete and present on cell surface proteogly-
cans chemokines of both CC and
CXC sub-sets, in particular IL-8/
CXCL8, monocyte chemoattrac-
tant protein-1/CCL2, RANTES/
CCL5 and Groα/CXCL1 [20]. The
ability of endothelium to captu-
re chemokines ensures that me-
diators become anchored on the
endothelial surface, to enable a
relatively high concentration of
chemoattractants close to the ves-
sel wall, and hence to temporally
and spatially regulate activation
of circulating cells (Fig. 2).
ENDOTHELIUM AND
ANGIOGENESIS IN RA
Another feature of the syno-
vium in RA is altered density of
sub-lining capillaries and post-
-capillary venules, supporting
an important role for angioge-
nesis. Endothelial cells, exposed
to inflammatory cytokines and
growth factors, respond by alte-
ring their proliferation rate and
cellular metabolism, to form new
Figure 2. The vasculature in RA: cause and consequence. In RA, infiltration by blood-derived cells, hyperproliferation
of synovial fibroblasts and angiogenesis occur. A self-perpetuating cascade of events is triggered, due to expression of
cytokines, growth factors and chemokines. Fibroblast proliferation results in synovial hypoxia, resulting in expression
of hypoxia-derived factors such as VEGF. Increased angiogenesis further amplifies the inflammatory cascade.
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blood vessels. Blood vessels therefore fulfil an important
role in RA, fuelling synovial expansion and infiltration by
inflammatory cells from the blood, by supplying oxygen
and nutrients necessary for cell metabolism and division, as
well as by bringing in leukocytes and signalling mediators
such as cytokines and growth factors [21-25]. The number of
synovial blood vessels has been found to correlate with sy-
novial cell hyperplasia and indices of joint tenderness [26].
Endothelial cells lining blood vessels within RA synovium
express cell cycle-associated antigens, and indices of endo-
thelial turnover are increased in synovia from patients with
RA compared with non-inflamed controls. A morphometric
study also suggested that capillaries are distributed more
deeply in RA synovium [27], and endothelial proliferation
was shown to be increased in synovium from patients with
RA [28].
Many of the cytokines and growth factors expressed
in RA have the potential to stimulate endothelial proli-
feration [21]. For example, serum levels of vascular en-
dothelial growth factor (VEGF) are markedly elevated in
RA, relative to either patients with osteoarthritis or nor-
mal controls, and correlate with levels of inflammatory
markers such as C-reactive protein (CRP). Serum VEGF
levels have also been shown to correlate with blood flow
in wrist synovium of patients with RA [29]. Expression
of VEGF by RA lining layer cells has been reported, and
microvascular endothelial cells in the vicinity of VEGF-
-positive cells express VEGF receptors [30]. Conditioned
medium from synovial tissue explants was shown to be
mitogenic for endothelial cells, an activity reduced by
anti-VEGF antibody [30], further supporting the concept
of an important role for endothelium and VEGF in RA.
In addition to synovial expression of VEGF, circulating
(serum) levels of VEGF are increased, and correlate with
inflammatory response markers [31,32]. Treatment of
murine arthritis using anti-VEGF antibody delayed dise-
ase onset and reduced symptoms of arthritis [33,34]. Tar-
geting VEGF receptors, specifically VEGF receptor 1, also
resulted in disease amelioration [35-37]. In summary, the
invasive synovium in RA is highly vascularised, and mo-
lecules such as VEGF are expressed, and are thus likely
to significantly modulate endothelial activation (Fig. 2).
CARDIOVASCULAR DISEASE IN RA — ROLE
OF ENDOTHELIAL DYSFUNCTION
The mortality of patients with severe RA is equivalent to
that of individuals with triple vessel coronary artery dise-
ase, with the major cause of mortality (more than 40% of
deaths) being cardiovascular disease, including ischemic
heart disease and heart failure [38]. Endothelial dysfunction
is known to occur in RA, providing a possible link betwe-
en these seemingly disparate pathologies. The endothelial
lining of blood vessels has a critical function in atheroscle-
rosis, serving as the site of initial injury and leukocyte ad-
hesion/migration. Maintenance of an intact vascular lining
and an uninterrupted vascular supply is thus critical in the
prevention of the cascade of events which trigger acute
coronary syndromes such as myocardial infarction. Many
studies have reported an association between RA and tra-
ditional cardiovascular risk factors such as cholesterol and
low density lipoprotein levels. The acute phase response in-
flammatory marker CRP is a risk factor for atherosclerosis,
and CRP levels are markedly elevated in RA, as part of the
ongoing systemic inflammatory processes, suggesting that
such an augmented inflammatory burden may be linked to
the increased cardiovascular risk in RA [39,40].
Interestingly, the process of vasodilation is altered in
RA. Herbrig et al., who studied blood flow in the forearm
following infusion of acetylcholine, showed that vaso-
dilatation was significantly reduced in RA patients [41].
A more recent study examined the relationship betwe-
en flow-mediated endothelium-dependent vasodilata-
tion and carotid artery intima-media wall in RA patients
without clinically evident cardiovascular disease, and
found that carotid intima-media thickness values were
higher and flow-mediated vasodilatation were lower in
individuals with long-standing RA compared to those
with shorter disease duration [42]. Another recent study
reported arterial stiffness to be associated with endothe-
lial dysfunction and atherosclerosis in patients with auto-
immune diseases such as RA [43].
Furthermore, although blood vessel density is altered in
RA, and angiogenesis has generally been thought to un-
derlie these changes, endothelial progenitor cells may also
play a role. Endothelial progenitor cells have been found
to differentiate into endothelial cells, express classic en-
dothelial cell markers, including CD31, CD34 and VEGF
receptor 2 and to exhibit endothelial cell properties, such
as expression of the endothelial-specific isoform of nitric
oxide synthase (eNOS) and the adhesion molecule E-selec-
tin. The endothelial cells present in the circulation are ca-
pable of integrating into vessel walls, and it is these endo-
thelial progenitor cells which may link RA with increased
cardiovascular morbidity and mortality. In RA synovium,
CD34/VEGF receptor 2-positive cells have been described
found in apposition to synovial blood vessels [44]. Bone
marrow-derived CD34-positive cells, which can expand
into CD31- and von Willebrand factor-expressing cells,
have been reported to be generated at a higher rate from
bone marrow samples taken from RA patients, compared
to normal subjects. Furthermore, the capacity of bone mar-
row-derived cells from RA patients to progress into endo-
thelial cells correlated with synovial microvessel density
[45]. In a parallel to the situation seen with coronary artery
disease and ischemic heart disease patients, endothelial
progenitor cells numbers are decreased in the peripheral
blood of RA patients compared with healthy individuals.
Circulating endothelial progenitor cells (CD34/VEGF re-
ceptor 2-positive) were lower in patients with active RA
than in individuals with inactive disease or healthy con-
trols [46]. The observation of reduced circulating endo-
thelial progenitor cells in RA patients was confirmed in
another study, which demonstrated reduced migration of
endothelial progenitor cells from RA patients in response
to VEGF, suggesting that the functional capacity of these
cells may be attenuated in RA. Endothelial progenitor cells
from RA patients exhibited only modest adhesion to en-
dothelial cells stimulated with TNFα, compared with cells
from healthy subjects, despite comparable levels of adhe-
Postępy Biochemii 59 (4) 2013
419
sion to unstimulated endothelial cells or matrix proteins
such as fibronectin or laminin [41].
The above data suggest enhanced recruitment from pe-
ripheral blood of endothelial progenitor cells to RA syno-
vium. This might then lead to increased RA synovial blood
vessel formation, perpetuating disease. Furthermore, incre-
ased endothelial progenitor cell trafficking to the synovium
would be paralleled by reduced peripheral blood endothe-
lial progenitor cells in RA, which could be a significant fac-
tor in the increased cardiovascular morbidity and mortality
seen in RA.
INSIGHTS FROM STUDIES USING TNFα INHIBITORS
Over the last 25 years, major advances in the understan-
ding of the pathogenesis of RA, based on bench-bedside stu-
dies of human tissue and animal models of disease, have led
to the identification of a number of new molecular targets
for intervention. TNFα mediates many inflammatory and
immunoregulatory activities relevant in RA. The concept of
TNFα as a therapeutic target was put forward by Feldmann
and Maini in the late 1980s, and to date several biological in-
hibitors of this cytokine have been approved for use in RA.
Clinical trials of these inhibitors, which commenced in
the late 1990s, have shed considerable light on the role of
the endothelium in RA. The first Phase I/II study was an
open-label trial of a single intra-venous infusion of inflixi-
mab (Remicade™), a chimeric mouse Fv-human IgG1k an-
tibody that binds both soluble and membrane-bound TNFα
with high affinity, in long-standing active RA patients who
had failed all prior therapy. The results were striking, sho-
wing reductions in pain and morning stiffness, swollen and
tender joint counts, and CRP levels. Since trafficking into
the synovium of blood-borne cells is a feature of RA, and
since TNFα is one of the most potent regulators of leukocyte
trafficking, it seemed reasonable to hypothesize that anti-
TNFα antibody treatment might regulate synovial infiltra-
tion. This question has been addressed over the years with
increasingly sophisticated studies, which started with the
measurement of soluble adhesion molecules, which could
be quantified in serially acquired serum samples. Levels of
serum E-selectin and ICAM-1 were decreased after infusion
of anti-TNFα antibody, and the extent of the decrease was
significantly higher in patients in whom a clinical benefit
of anti-TNFα was observed. Moreover, a significant reduc-
tion was observed in CD3- and CD68-positive cells in the
synovium, as well as a decrease in synovial expression of
VCAM-1 and E-selectin [47,48]. Later studies showed that
synovial and serum chemokines (IL-8/CXCL8 and mono-
cyte chemoattractant protein-1/CCL2) were decreased [47].
These reductions correlated with a rapid and sustained in-
crease in blood lymphocyte counts [48].
Direct evidence of an effect on cell trafficking came from
elegant studies using gamma camera imaging of radioacti-
vely labelled polymorphonuclear cells. Patients with long-
standing RA received a single infusion of anti-TNFα anti-
body, and the articular localization of autologous polymor-
phonuclear cells, separated in vitro and labelled with
111
In,
was studied before and 2 weeks after treatment. Anti-TNFα
therapy in RA significantly reduced
111
In-labelled granulo-
cyte migration into affected joints. There was a simultane-
ous reduction in the numbers of infiltrating synovial CD3-
-positive T cells, CD22-positive B cells, and CD68-positive
macrophages [47].
In later studies, ultrasonography has been used to me-
asure synovial inflammation and vascularity. These inve-
stigations have shown that assessment of synovial thicke-
ning and vascularity at baseline was an early and sensitive
measure of response to treatment and radiological changes
to anti-TNFα antibody [10]. Indeed ultrasonographic me-
asures of synovial thickening and vascularity were able to
discriminate between RA patients receiving anti-TNFα or
placebo with greater sensitivity than conventionally used
outcome measures of change in disease activity that depend
on the use of clinical evaluation, such as the numbers of ten-
der and swollen joints.
In the context of effects on endothelial progenitor cells,
RA patients with active disease treated with TNFα inhibi-
tors showed a restoration of circulating endothelial progeni-
tor cells levels to those seen in healthy control subjects. This
effect was not seen in patients with active RA but receiving
conventional disease-modifying drugs [46]. A more recent
study directly examined the effect of anti-TNFα antibody
on endothelial progenitor cell numbers and function. A si-
gnificant increase in endothelial progenitor cell levels and
adhesion was seen after 2 weeks of anti-TNFα treatment,
together with an improvement in the disease activity score.
Interestingly, a significant correlation was seen between the
extent of clinical response and the degree of increase in en-
dothelial progenitor cell numbers [49]. The cardiovascular
risk profile is also altered after TNFα blockade. Treatment
with anti-TNFα antibody significantly increased concentra-
tions of fibrinogen and HDL-cholesterol, whereas LDL and
triglyceride levels were unchanged, and no changes in lipid
profile were seen in the placebo group [50]. Similarly, treat-
ment with TNFα inhibitors has been reported to reduce the
incidence of first cardiovascular events in patients with RA
[51].
Given that serum VEGF levels were elevated in patients
with RA, it seemed reasonable to suppose that treatment of
RA with anti-cytokine biologicals might modulate VEGF
expression. To examine this hypothesis, we measured se-
rum VEGF levels in RA patients treated with anti-TNFα an-
tibody. In patients receiving a single infusion of anti-TNFα,
serum VEGF levels were markedly reduced. Treatment of
RA patients with a combination of multiple infusions of
anti-TNFα and methotrexate resulted in a more prolonged
decrease in serum VEGF levels relative to patients who re-
ceived anti-TNFα antibody alone [32]. As discussed, the
presence or density of immature vessels is significantly in-
creased in RA patients, and i
nterestingly,
immature vessels
were depleted in response to anti-TNF therapy, highligh-
ting the co-dependency of angiogenesis and inflammation
[8]. Furthermore, as mentioned earlier, endothelial dysfunc-
tion is a feature of RA. Impaired flow-mediated vasodila-
tion in RA patients was reversed following TNFα inhibition
[52,53].
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Taken together, these observations suggest that at least
part of the clinical effectiveness of TNFα blockade in RA is
due to deactivation of vascular endothelium, leading to re-
duced inflammation, cell trafficking (and, as a consequence,
diminished synovial cellularity) and angiogenesis, and nor-
malisation of coagulation and fibrinolytic systems (summa-
rised in Tab. 1). This is supported by a recent finding regar-
ding Certolizumab pegol, a humanised anti-TNFα antibody
approved for clinical use for RA. Certolizumab significantly
blocked TNFα-induced E-selectin, VCAM-1 and ICAM-1
expression on microvascular endothelial cells, as well as
chemokine expression and endothelial tube formation, and
adhesion of HL60 leukaemia cells to endothelial cells [54].
These data demonstrate that blockade of the inflammatory
cascade in RA, using approaches such as TNFα inhibitors,
diminishes endothelial activation and is associated with cli-
nical benefit.
ENDOTHELIUM AND INFLAMMATION
— INTERACTION WITH HYPOXIA
Mammalian cells and tissues are exposed to various oxy-
gen tensions, depending on their location, frequently as low
as 5% in the case of venular endothelial cells [55]. A complex
interplay between altered oxygen levels and inflammation
is involved in the pathogenesis of inflammatory diseases
such as RA. The micro-environment in the inflamed joint
is characterised by a low partial pressure of oxygen, first
demonstrated more than 40 years ago. Mean synovial fluid
O
2
in knee joints was reported to be lower in RA patients
than in osteoarthritis patients or in healthy controls. Despite
these intriguing observations, it was only recently that we
were able to directly measure synovial oxygen tension in
RA patients using a highly sensitive gold microelectrode.
We observed that synovial tissue in RA patients was inde-
ed hypoxic, with O
2
lower than in non-inflamed synovium
in patients without RA [56]. This hypoxic milieu leads to a
cascade of enhanced expression of hypoxia-regulated trans-
cription factors and hypoxia-responsive genes, and incre-
ased levels of pro-inflammatory cytokines and angiogenic
factors, establishing a link between synovial hypoxia and
inflammation in RA [25,57].
Cellular responses in situations of reduced ava-
ilability of oxygen are coordinated by the hypoxia-
-inducible factor (HIF) transcription factor family.
Activation of HIF signalling leads to extensive chan-
ges in gene expression, to allow adaption of cells and
tissues to reduced oxygenation. The HIF complex
consists of a constitutively expressed b subunit, and
an oxygen-responsive α subunit. Regulation of HIF-
-dependent gene expression requires α-subunit ac-
cumulation in the cytoplasm and translocation into
the nucleus, which enables HIF-α to dimerise with
HIF-β and bind HIF co-activators, prior to binding
hypoxia-response elements in the target gene to ini-
tiate transcription. Hydroxylation by FIH-1 of aspa-
ragine residues in HIF-α prevents recruitment of
co-activators p300/CBP and thereby HIF-mediated
gene transcription. In contrast, prolyl hydroxylase
domain (PHD) enzymes (PHD1-3) modify HIF-α by
hydroxylation of specific proline residues in HIF-α,
enabling capture by an E3 ubiquitin ligase complex, leading
to proteasomal destruction of HIF-α. FIH-1 and PHD1-3 be-
long to a superfamily of 2-oxoglutarate and iron dependent
dioxygenases, which require molecular oxygen as a co-sub-
strate [58]. Thus, under conditions where O
2
supply limited,
as is the case in RA synovium, HIF-α subunits accumulate
and activate gene transcription. In RA synovial tissue, HIF-
1α-positive cells correlate with the number of blood vessels
and with inflammatory endothelial cell infiltration, prolife-
ration and synovitis [59].
Hypoxia alters the expression of a number of endothe-
lial genes, including those involved in the inflammatory
response. For example, increased expression of chemokines
such as CCL15 and IL-8/CXCL8 has been described in en-
dothelial cells exposed to hypoxia, suggesting that altered
oxygen tension may influence leukocyte activation [60,61].
Increased leukocyte adhesion to endothelial cells exposed
to low oxygen tension has also been described [62,63], and
hypoxia may synergise with inflammatory cytokines such
as TNFα to upregulate E-selectin and ICAM-1 [64]. Trans-
criptomic and proteomic analyses have shown that hypoxia
activates endothelial cells to express cytokines, growth fac-
tors, extracellular matrix protein genes, collagens and mem-
bers of the PHD family in a HIF-1-dependent manner, and
that hypoxia increased basement membrane invasion and
tube formation by endothelial cells [65-67]. Of potential re-
levance to RA, hypoxia increases endothelial permeability,
affecting adhesion molecules such as VE-cadherin and Rho
GTPases regulating the actin cytoskeleton, such as RhoA
and Rac1 [68].
Hypoxia may also affect endothelial activation indirectly,
by activating synovial cells to express factors which stimula-
te endothelial cell responses. Hypoxia increases expression
by synovial cells of pro-angiogenic factors such as VEGF
[32], as well as chemokines IL-8/CXCL8 [69], CCL20 [70]
and SDF-1/CXCL12 [71,72]. Increased levels of pro-inflam-
matory cytokines such as IL-6, and of matrix-metalloprote-
ase (MMP) enzymes MMP-1 and MMP-3 [69] together with
enhanced synovial cell invasiveness, in response to hypoxia
have also been reported. In a recent study, interaction be-
Table 1. Summary of the effects of anti-TNFα on the vasculature in RA.
Parameter
Observed effect of anti-tnfα
Leukocyte adhesion
reduced serum adhesion molecules
reduced synovial adhesion molecules
reduced synovial CD3- and CD68-positive cells
reduced leukocyte trafficking
increased circulating lymphocytes
Chemokine expression
reduced synovial chemokines
reduced serum chemokines
Angiogenesis
reduced serum VEGF
reduced synovial vascularity
reduced synovial thickening
reduced joint vascularity
Haematological markers
reduction in elevated fibrinogen
reduction in elevated platelet counts
restoration of reduced hemoglobin
Postępy Biochemii 59 (4) 2013
421
tween hypoxia, HIF and the Notch signalling pathway was
shown to play an important role in hypoxia-induced angio-
genesis. Notch-1 was highly expressed in inflamed synovial
tissue and was localized predominantly to perivascular/
vascular regions, and inhibition of Notch-1 by RNA interfe-
rence significantly attenuated hypoxia-induced angiogene-
sis and endothelial cell function [73].
Synovial hypoxia is therefore likely to contribute to RA
by promoting inflammation, angiogenesis, cellular infiltra-
tion and cartilage degradation. However, recent emerging
evidence suggests the opposite, adding some controversy
to the previous well established dogma, and increasing the
need of further studies for a better understanding of the role
of hypoxia/HIF in RA. A very good example is the role of
HIF in anaemia, which as in many chronic inflammatory
diseases, is one of the most common extra-articular manife-
stations of RA, estimated to occur in 30–60% of RA patients
[74]. Anaemic patients have more severe RA and also have
more affected joints and higher levels of functional disabi-
lity and pain. Studies have shown that treating anaemia in
RA patients leads to reduced joint swelling and had a po-
sitive effect on patients’ quality of life. Α key mediator of
anaemia in RA is hepcidin, a regulatory hormone that limits
iron availability and suppresses erythropoiesis under con-
ditions of inflammation. Expression of hepcidin is induced
by IL-6, a major player in the pathogenesis of RA and incre-
ased levels of serum hepcidin were directly linked to the
occurrence of coronary artery atherosclerosis in RA patients
[75]. Under conditions of hypoxia however, the expression
of hepcidin is repressed to permit physiological adaptation
to tissue oxygen tension. It has been reported that inhibition
of the PHD enzymes by deferoxamine or dimethyloxaloyl-
glycine
was also able to down-regulate hepcidin expression,
independently of HIF [76]. However, recently Liu and col-
leagues has shown that suppression of hepcidin was me-
diated by HIF, indirectly through erythropoietin-induced
erythropoiesis [77].
CONCLUSIONS
The response of vascular endothelium to cytokines, che-
mokines and growth factors governs subsequent resolution
or perpetuation of the inflammatory cascade in vivo. Inap-
propriate or excessive responses result in consequences
such as leukocyte extravasation, immune activation and an-
giogenesis, thus contributing to certain diseases such as RA.
Some questions regarding the function of endothelium
in chronic inflammatory disorders such as RA still remain
unanswered, for example regarding the relative roles of the
different adhesion molecules and chemokines. However, it
is not unreasonable to suggest that targeting the vasculature
in RA, for example using angiogenesis inhibitors, in combi-
nation with other therapies such as anti-TNFα, may lead to
a more persistent reduction in synovial volume and hence
modify disease progression, but confirmation of this hypo-
thesis requires appropriate clinical trials. It is likely that our
understanding of RA is a model of chronic inflammatory
disease will allow elucidation of the potential of developing
new therapeutic approaches for treatment of other disor-
ders in which inflammation, hypoxia and the vasculature
are involved, such as atherosclerosis, psoriasis, diabetes and
cancer.
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Postępy Biochemii 59 (4) 2013
423
Śródbłonek wyścielający naczynia krwionośne —
rola w przewlekłych chorobach zapalnych
Serafim Kiriakidis
1,2,*
, Ewa M. Paleolog
1,2
1
Kennedy Institute of Rheumatology, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford,
Roosevelt Drive, Headington, Oxford, OX3 7FY, UK
2
Department of Medicine, Imperial College London, UK
*
e-mail: serafim.kiriakidis@kennedy.ox.ac.uk
Słowa kluczowe: śródbłonek, zapalenie, zapalenie stawów, cytokiny, angiogeneza
STRESZCZENIE
Śródbłonek wyścielający naczynia krwionośne odgrywa kluczową rolę w regulacji odpowiedzi organizmu na cytokiny prozapalne, chemoki-
ny i czynniki wzrostu (pochodzące zarówno z samych komórek śródbłonkowych jak i innych komórek — np. leukocytów czy fibroblastów),
wpływając na aktywację leukocytów, ich adhezję oraz migrację ze światła naczynia do przylegających tkanek. Proliferacja śródbłonka i two-
rzenie nowych naczyń w procesie angiogenezy zwiększa powierzchnię oddziaływania z leukocytami, a jednocześnie ułatwia dostarczanie
tlenu i usuwanie zbędnych produktów przemiany materii. Oprócz ważnej roli w fizjologii, śródbłonek bierze aktywny udział w patogenezie
chorób związanych z zapaleniem. Jednym z najlepiej poznanych schorzeń, w których istotną rolę odgrywa reakcja zapalna i nasilona angio-
geneza jest reumatoidalne zapalenie stawów (RA, ang.
rheumatoid arthritis). Zablokowanie odpowiedzi zapalnej w RA znacząco wpływa
na unaczynienie, potwierdzając współzależność między zahamowaniem aktywacji śródbłonka i leczeniem chronicznych stanów zapalnych.
between synovial and endothelial inflammation. Int J Immunopathol
Pharmacol 23: 255-262
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