plik

Postępy Biochemii 59 (4) 2013

415

Serafim Kiriakidis

1,2,*

Ewa M. Paleolog

1,2

Kennedy

Institute

Rheumatolo-

gy, Nuffield Department of Orthopa-

edics,

Rheumatology

and

Musculo-

skeletal Sciences, University of Oxford

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

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,

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

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

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

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

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

Kennedy Institute of Rheumatology, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford,

Roosevelt Drive, Headington, Oxford, OX3 7FY, UK

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.

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