The blood vessels of healthy, vascularized tissue are lined 

with a monolayer of quiescent endothelial cells organ-

ized as a ‘phalanx’ aligned in the direction of blood flow.  

As the blood vessels mature with an intact pericyte layer, 

the endothelial cells remain quiescent, and so mini-

mal leukocyte migration occurs; in addition, the blood 

vessel is stable, and blood perfusion and the supply of 

oxygen are efficient. During inflammation, however, 

activated endothelial cells can lose their polarity, detach 

and protrude into the vessel lumen, thereby disrupting 

the pericyte layer. The resultant poorly organized vessel 

is dysfunctional; it increases stromal oedema and limits 

the delivery of nutrients and oxygen, causing hypoxia. 

Hypoxia is defined as inadequate oxygenation. Oxygen 

levels vary under physiological conditions, ranging from 

150 mm Hg in the lung to 40–100 mm Hg in other organs

1

Oxygen-sensing mechanisms have evolved to regulate 

endothelial cell and vessel morphogenesis to maximize 

perfusion and oxygen delivery to surrounding tissues

2

Hypoxia has been implicated in the pathogenesis of rheu-

matoid arthritis (RA), as synovial hypoxia causes inflam-

matory cells to switch on invasive mechanisms, accelerate 

cell proliferation and enhance migration, and the levels 

of in vivo synovial oxygen correlate negatively with  

macro scopic synovitis and measures of disease activity

3–5

.  

Our knowledge of the partial pressure of oxygen (pO

2

) in 

inflamed synovial tissue has, until a few years ago, been 

limited 

(BOX 1)

.

In the case of inflamed joints, activated endothelial 

vessels provide the gateway for leukocyte infiltration 

into the synovium 

(FIG. 1)

. The resultant increase in meta-

bolic turnover of the expanding synovial pannus out-

paces the dysfunctional oxygen supply, which, in turn, 

increases hypoxia and subsequent metabolic demand. 

Additionally, the increased distance between the vessels 

and the cellular stromal infiltrate increases the distance 

that molecular oxygen is required to diffuse

6

. Raised 

intra-articular pressures owing to movement and joint 

swelling caused by inflammation might further compro-

mise the vascular supply to the inflamed joint, thereby 

exacerbating the hypoxic environment.

The complementary actions of vascular endothelial  

growth factor (VEGF) and members of the angio poietin 

(Ang) family are critical for the maintenance of vessel 

stability, vascularization and regression during the for-

mation of the RA vasculature

7–9

. The expression of Ang1, 

Ang2 and their receptor Tie2 is significantly increased 

in whole paws during disease progression in collagen- 

induced arthritis (CIA) mouse models, and the blockade 

of Tie2 ameliorates bone destruction

10,11

; Tie2 has also 

1

The Department of 

Molecular Rheumatology, 
Trinity College Dublin,  
The University of Dublin, 
College Green, Dublin 2, 
Ireland.

2

The Centre for Arthritis and 

Rheumatic Disease, Dublin 
Academic Medical Centre,  
St. Vincent’s University 
Hospital, Elm Park,  
Dublin 4, Ireland.

Correspondence to D.J.V. 

douglas.veale@ucd.ie

doi:10.1038/nrrheum.2016.69
Published online 26 May 2016

Hypoxia, mitochondrial dysfunction 

and synovial invasiveness  

in rheumatoid arthritis

Ursula Fearon

1,2

, Mary Canavan

2

, Monika Biniecka

2

 and Douglas J. Veale

2

Abstract | Synovial proliferation, neovascularization and leukocyte extravasation transform  
the normally acellular synovium into an invasive tumour-like ‘pannus’. The highly dysregulated 
architecture of the microvasculature creates a poor oxygen supply to the synovium, which, 
along with the increased metabolic turnover of the expanding synovial pannus, creates a hypoxic 
microenvironment. Abnormal cellular metabolism and mitochondrial dysfunction thus ensue 
and, in turn, through the increased production of reactive oxygen species, actively induce 
inflammation. When exposed to hypoxia in the inflamed joint, immune-inflammatory cells show 
adaptive survival reactions by activating key proinflammatory signalling pathways, including 
those mediated by hypoxia-inducible factor-1

α (HIF-1α), nuclear factor κB (NF-κB), Janus  

kinase–signal transducer and activator of transcription (JAK–STAT) and Notch, which contribute 
to synovial invasiveness. The reprogramming of hypoxia-mediated pathways in synovial cells, 
such as fibroblasts, dendritic cells, macrophages and T cells, is implicated in the pathogenesis  
of rheumatoid arthritis and other inflammatory conditions, and might therefore provide an 
opportunity for therapeutic intervention.

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been shown to mediate angiogenesis induced by Toll-like 

receptor 2 (TLR2) in RA

12

. Platelet-derived growth fac-

tor (PDGF) and transforming growth factor-β (TGF-β) 

also regulate vessel stability and can induce the invasion 

of fibroblast-like synoviocytes (FLSs)

13,14

. Unstable ves-

sels in the inflamed joint are associated with the presence 

of incomplete interactions between endothelial cells and 

pericytes, hypoxia and increased oxidative damage

4,15

 

(FIG. 1)

. Oxidative damage, which arises in RA syno-

vial tissue through the detrimental effects of hypoxia 

on mitochondria, might mediate the activation of 

endothelial cells as well as promoting angiogenesis and 

cartilage damage by upregulating matrix degradation

16,17

Oxidative damage might also induce the expression of 

adhesion molecules that influence both vessel stability 

and leukocyte migration.

In this Review, we examine the emerging evidence for 

the role of hypoxia in immune-inflammatory responses 

in RA, focusing on mitochondrial dysfunction, the acti-

vation of proinflammatory signalling pathways and the 

reprogramming of metabolic activity. Understanding 

the regulation of hypoxia and metabolic perturbation 

in inflammation, and resolution, might provide a basis 

for novel therapies.

Hypoxia and mitochondrial damage

Mitochondria carry out a central role in the regulation of 

cellular bioenergetics and metabolism, and facilitate cell-

ular stress responses. Hypoxia induces a wide spectrum 

of alterations in mitochondrial structure, dynamics and 

genome stability, resulting in reduced mitochondrial res-

piration, excessive production of reactive oxygen species 

(ROS), loss of ATP, increased oxidative damage and the 

accumulation of mitochondrial (mt)DNA mutations

18,19

Damaged mitochondria also release molecules that can 

translocate outside the organelle and promote immune 

responses

20

.

ROS, oxidative stress and DNA damage

ROS, oxidative stress and mitochondrial alterations have 

been implicated in the pathogenesis of joint inflamma-

tion

21

. ROS stimulate FLSs to secrete matrix metallopro-

teinases, inhibit the synthesis of cartilage proteoglycans 

and accelerate bone resorption

16,22

. Higher levels of oxi-

dative DNA damage, as measured by 8-oxo-dG, were 

detected in mononuclear cells and granulocytes, as well 

as in serum, synovial fluid and urine, from arthritic 

patients when compared with healthy controls

23–25

, and 

the presence of vascular 8-oxo-dG is associated with 

synovial immature vessel status and with low levels of 

synovial oxygen

4

. Similarly, levels of lipid peroxidation 

are high in the inflamed joint and correlate inversely 

with synovial pO

2

 levels, which reflects mitochondrial 

damage within the inflamed joint

26

.

Mitochondrial dysfunction in normal synoviocytes 

induces the expression of cyclooxygenase-2 (COX-2), 

prostaglandin E2 (PGE2) and IL-8, and amplifies 

the responsiveness to cytokine-induced chondrocyte 

inflammation through the production of ROS and 

activation of nuclear factor κB (NF-κB)

27,28

. Similarly, 

ROS induce the expression of COX-2 in FLSs through 

phosphorylation of mitogen-activated protein kinases 

and NF-κB, providing a functional link between mito-

chondrial dysfunction and synovial cell activation

29

.  

The immediate-early response gene X-1 (IEX‑1, also known  

as IER3) is involved in preventing the production of 

ROS in mitochondria and, consequently, null mutation 

of IER3 increases the production of mitochondrial ROS. 

This increase subsequently facilitates the differentiation 

of T helper 17 (T

H

17) cells, the increased production of 

IL-17, and more severe arthritis in Ier3 null mice than in 

wild-type mice after immunization with collagen, indicat-

ing that mitochondrial alterations contribute substantially 

to the dominant T

H

 cell effector phenotype

30

.

The mitochondrial genome is highly susceptible to 

mutagenesis, and elevated oxidative stress contributes 

to somatic mtDNA mutations. We examined syno-

vial tissue from patients with RA for the presence of 

mtDNA mutations and demonstrated an increase 

in mitochondrial mutagenesis associated with lower 

pO

2

 in the synovium, suggesting that the accumula-

tion of random mitochondrial mutations is driven by 

the hypoxia-induced overproduction of ROS

31

. The 

mutations detected were mainly transitions, which are 

characteristic of oxidative stress

32

. The accumulation of 

random synovial mitochondrial mutations in response 

Key points

• 

Hypoxia, arising as a consequence of the increased cellular demand for oxygen 
during the inflammatory response, is a powerful trigger for the activation, 
proliferation and survival of endothelial cells and fibroblast-like synoviocytes

• 

Impaired mitochondrial function and oxidative damage caused by hypoxia further 
exacerbate the inflammatory response through metabolic perturbation

• 

Hypoxia induces immune cell dysfunction, resulting in an altered metabolic profile

• 

The hypoxic environment induces activation of a complex crosstalk of signalling 
pathways, providing a feedback loop leading to further activation and inflammation

• 

Targeting synovial metabolic pathways through inhibition of hypoxia-induced 
signalling pathways might have therapeutic benefit for rheumatoid arthritis and 
other inflammatory diseases

Box 1 | 

Measuring hypoxia in the rheumatoid joint

Originally, studies used surrogate markers to measure hypoxia in the synovial fluid  
of patients with rheumatoid arthritis (RA)

54,150

. Then, in 1970, Lund-Olesen used a 

Clark-type electrode to demonstrate significantly lower partial pressure of oxygen

 

(

pO

2

) levels in the synovial fluid of patients with RA compared to those with 

osteoarthritis

151

. Low oxygen levels have also been reported in the tenosynovium of 

RA patients undergoing repair surgery

152

. In 2008, we developed a method to directly 

measure pO

2

 in synovial tissue using a 2.7 mm needle arthroscope (Wolf, Illinois)  

and a LICOX® combined pO

and temperature probe (CC1.P1, Integra Life Sciences 

Corporation, New Jersey, USA). Oxygen that diffuses from the tissue through the 
polyethylene wall into the inner electrolyte chamber of the probe is transformed  
to OH

 ions at a negatively charged polarized precious metal electrode, the 

polarographic cathode. In this way, following synovial biopsy under direct 
visualization, the LICOX® probe registers a stable

 in vivo measurement of pO

2

 and 

temperature in the synovial tissue

3

.

 The measurements are taken every 5 min for 

approximately 20 min until a steady state is achieved. This method, which has been 
extensively validated, demonstrates profound hypoxia in inflamed synovial joints 
(median synovial oxygen levels 3.2% [range 0.46–7%]).

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Pericyte

Endothelial cell

Nature Reviews | 

Rheumatology

a

b

Macrophage

B cell

T cell

Dendritic cell

Secretion of proinflammatory mediators

Leukocyte infiltration

and activation

Synovial

hyperplasia

Osteoclastogenesis

Fibroblast

Osteoclast

progenitor

Bone marrow-

derived

haematopoietic

progenitors

Osteoclast

to hypoxia suggests the promotion of a mitochondrial 

mutator phenotype that might be involved in regulat-

ing inflammatory responses. This notion is consistent 

with previous studies demonstrating increased DNA 

damage and dysfunctional mitochondria in synovial 

tissue and peripheral lymphocytes from patients with 

RA

4,33

. An increased frequency of clonal mtDNA muta-

tion in MT‑ND1, which encodes mitochondrial NADH 

dehydro genase-1, has been detected in RA FLSs, with 

potential mutation sites in the MHC epitope in patients 

with RA, but not those with osteoarthritis. This increase 

in somatic mutations might influence cellular function 

by contributing to the mechanisms involved in the trans-

formed phenotype of RA FLSs, which, along with their 

association with the MHC epitope, might further perpet-

uate the immune response

34

. Furthermore, synovial tis-

sue pO

2

 levels were reported to increase in patients who 

responded to biologic therapy, leading to a less hypoxic 

microenvironment and a significant decrease in mtDNA 

mutations and disease activity

35

. Fibroblasts from mice 

that lack TNF receptor (TNFR)1 have been described 

as having altered mitochondrial function that results 

in enhanced oxidative capacity as well as the increased 

generation of mitochondrial ROS and proinflammatory 

cytokines

36

. Moreover, using RA FLSs, we have shown 

in vitro that the stability of mtDNA and mitochondrial 

function are altered by the presence of TNF or hypoxia, 

thus recapitulating the in vivo responses

37

.

Mitochondrial DAMPs

Inflammation is characterized by elevated levels of damage- 

associated molecular pattern (DAMP) molecules, which 

are released as a result of tissue injury. The released 

mitochondrial DAMPs can induce innate or adaptive 

immune responses by activating cell surface receptors 

(such as the P2X purinoceptor 7 (P2X7R) or N-formyl 

Figure 1 | 

Blood vessel activation in the rheumatoid joint. a | Direct visualization by arthroscopy of a joint affected 

by inflammatory arthritis reveals a swollen, oedematous synovial lining tissue that is red from angiogenesis (arrow, upper 
panel) and hyperaemia and often forms villi (arrows, lower panel). 

| Within an inflamed rheumatoid joint, a mixture  

of immature and mature blood vessels are present, suggesting blood vessels are probably undergoing simultaneous 
angiogenesis, pericyte recruitment and stabilization. The activation of immature synovial vessels facilitates the 
recruitment and migration of immune cells into the synovial membrane. These cells release proinflammatory mediators, 
which activate fibroblast-like synoviocytes, resulting in synovial hyperplasia, which, in turn, leads to osteoclastogenesis, 
bone resorption and, ultimately, joint destruction.

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peptide receptors (FPRs)). Damaged mitochondria have 

been reported to contain at least two unique molecular 

signatures, including free mtDNA and N-formyl pep-

tides, which are known to contribute to proinflamma-

tory responses 

(FIG. 2a)

. Furthermore, Nod-like receptors 

(NLRs) comprise a large family of intracellular proteins 

that are involved in the innate immune responses to 

microbial pathogens through the recognition of con-

served pathogen-associated molecular patterns. The 

NLRP3 inflammasome is very well studied. Upon inflam-

masome activation, caspase-1 controls the activation and 

cellular release of active IL-1β and IL-18, both of which 

have been implicated in RA. Dysfunctional mitochon-

dria have been implicated in the activation of the NLRP3 

inflammasome through the generation of mitochondrial 

ROS and the release of mtDNA.

Free mtDNA. Mitochondrial DNA contains inflammato-

genic unmethylated CpG DNA repeats, which function 

as ligands for TLR9, a member of the highly conserved 

pattern-recognition receptors (PRRs)

20

. Bacterial DNA 

containing unmethylated CpG motifs has been shown to 

induce arthritis in mice

38

, as has endogenously oxidized 

mtDNA, which displays immunostimulatory properties 

in vitro and in vivo owing to the presence of unmethylated 

CpG sequences

39

. Not only have significantly higher levels 

of extracellular mtDNA been detected in the plasma and 

synovial fluid of patients with RA compared with non- 

arthritic controls, but a strong correlation between the 

levels of both extracellular mtDNA and 8-oxo-dG and the 

presence of rheumatoid factor has been shown in the syno-

vial fluid of patients with RA

25

. Accordingly, a vicious cycle 

between mitochondrial dysfunction and inflammation 

exists, in which cell-free mtDNA activates phagocytes to 

produce tissue-destructive enzymes and proinflammatory 

cytokines, which contribute to inflammation and thereby 

accelerate the release of additional endogenous inflam-

matory mtDNA. mtDNA induces inflammation and lung 

injury in rats and increases the expression of TLR9 and 

NF-κB in both rat lung tissue and macrophage cultures 

derived from rat peritoneum

40

. Treatment of plasmacytoid 

dendritic cells (DCs) with either native mtDNA (con-

taining unmethylated CpG DNA repeats) or oxidatively 

modified mtDNA upregulated the cell-surface expression 

of a co-stimulatory molecule (CD86), maturation marker 

(CD83) and antigen-presenting molecule (HLA-DQ), as 

well as increasing the production of TNF and IL-8 

(REF. 41)

.

N‑formyl peptides. Mitochondria use an N-formyl-

methionyl-tRNA as an initiator of protein synthesis, and 

N-formyl peptides constitute another molecular signature 

of damaged mitochondria. N-formyl peptide sequences, 

such as N-formyl-methionyl-leucyl-phenylalanine 

(fMLF), are potent chemoattractants. fMLF activates neu-

trophil chemotaxis and mediates antimicrobial responses 

by binding to FPRs and, whereas knockout of FPR1 and 

FPR2 increased the severity of infection, FPR agonists 

inhibited the secretion of proinflammatory cytokines 

after microbial infection in mice

42

. In a K/B×N mouse 

model of RA, an FPR agonist significantly reduced clinical  

disease severity and attenuated osteoclastogenesis

43

.

The NLRP3 inflammasome and autophagy. Cytosolic 

mtDNA released from damaged mitochondria can  

activate the NLRP3 inflammasome, leading to the 

sub sequent activation of caspase-1 and secretion of 

IL-1β and IL-18 

(REF. 44)

. NLRP3 is also activated by 

ATP, which is currently considered not only as a mito-

chondrially produced intracellular energy source, but 

also as an important extracellular signalling molecule. 

Extracellular ATP promotes neutrophil chemotaxis 

through the release of CXCL8, the adhesion of neu-

trophils to endothelial cells, and the secretion of IL-1β 

and IL-18 through NLRP3 inflammasome activation in 

macrophages

45,46

. Furthermore, mitochondrial ROS have 

the ability to induce NLRP3 inflammasome activation. 

However, although NLRP3 is expressed in RA synovium, 

to date no study has demonstrated redox activation of 

inflammasome components in RA.

Autophagy is a process of lysosome-mediated deg-

radation of organelles and proteins that is activated by 

conditions of cellular stress, including hypoxia, ROS, 

endoplasmic reticulum (ER) stress and microbial infec-

tion. A dual role for autophagy in the regulation of cell 

death pathways has been demonstrated in RA FLSs: 

autophagy promoted ER-stress-induced cell death but 

also protected against apoptosis induced by protea some 

inhibition. Furthermore, autophagy and resistance to 

apoptosis were more pronounced in methotrexate- 

treated FLSs from patients with RA compared with 

synovial fibroblasts from patients with osteoarthritis. 

Inhibition of autophagy in RA FLSs increased their sus-

ceptibility to methotrexate, inducing cell death, thereby 

confirming an apoptosis-resistant phenotype of RA 

FLSs

47,48

. Defective mitochondria that overproduce ROS 

are eliminated by autophagy to restore redox homeo-

stasis. In a study by Zhou et al.

49

, pharmacologic or 

genetic inhibition of autophagy caused the accumulation 

of dysfunctional ROS-generating mitochondria, which 

was accompanied by spontaneous activation of the 

NLRP3 inflammasome and the release of IL-1β. Nakahira 

et al.

44

 propose that autophagic proteins are impor-

tant for the regulation of inflammasome-dependent  

inflammation, as depletion of autophagic proteins pro-

moted the accumulation of dysfunctional mitochondria, 

cytosolic translocation of mtDNA, activation of caspase-1 

and the secretion of IL-1β and IL-18.

Hypoxia and metabolism

The increased proliferation and rapid activation of 

immune cells during inflammation causes them to 

undergo a metabolic switch in favour of glycolysis 

over oxidative phosphorylation. This metabolic shift, 

which enables energy to be produced independently of 

the oxygen supply, occurs in many hypoxia-associated 

inflammatory conditions, including RA

50

. The acti-

vation of the transcription factor hypoxia-inducible  

factor (HIF)-1α in response to low pO

2

 (discussed 

below) modulates the activity of a number of genes and, 

by inducing genes that encode glucose transporters and 

glycolytic enzymes, HIF can promote the production 

of glycolytic energy 

(FIG. 2b)

. Studies have shown an 

increase in the levels of the glucose transporter GLUT1 

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Inflammation

 

GLUTs

 

Glycolytic

   enzymes

 

LDHA

 

PDK

 

TCA cycle

 

ATP

Hypoxia

 

ROS-induced

   oxidative damage

Nature Reviews | 

Rheumatology

Glucose

Glucose

Pyruvate

Acetyl CoA

Dysfunctional

mitochondrion

Damaged

mitochondrion

Lactate

I

II III IV V

PHDs

HIF-1α

HIF-1α

target

gene

Nucleus

pro IL-1β

pro IL-18

IL-1β/IL-18

Nucleus

Pro-inflammatory genes

Cytosol

Cytosol

ROS

ETC

mtDNA

a

  Inflammatory cell

b

Oxygen

deprivation

20%

10%

0%

TLR9

FPR1

N-formyl peptide

mtDNACpG

Oxidized

mtDNA

ATP

P2RX7

NLRP3 inflammasome

Caspase-1

Endosome

Figure 2 | 

Hypoxia and mitochondrial dysfunction. a | Cellular injury and necrosis trigger the release of mitochondrial 

damage-associated molecular patterns (DAMPs), such as N-formyl peptides, mitochondrial (mt)DNA and ATP, from 
damaged mitochondria, which are potent immunological activators.  The released mtDAMPs can initiate innate or 
adaptive immune responses by activating cell surface receptors (such as the P2X7 receptor P2X7R, or formyl peptide 
receptors) or intracellular receptors (such as Toll-like receptor 9 (TLR9) or NLRP3). 

b | Reduced oxygen conditions stabilize 

hypoxia-inducible factor (HIF)-1, which targets genes involved in hypoxic cell metabolism. HIF-1 promotes the activation 
of glycolysis by upregulating the expression of glucose transporters (GLUTs), therefore increasing glucose uptake into 
the cell; intracellular glucose is metabolized by the HIF-dependent increase of the glycolytic enzymes. Elevated glycolysis 
generates pyruvate, which is largely converted to lactate by HIF-inducible lactate dehydrogenase A (LDHA). HIF-1 also 
induces pyruvate dehydrogenase kinase (PDK), diminishing pyruvate entry into the tricarboxylic acid (TCA) cycle and 
subsequently decreasing mitochondrial oxidative phosphorylation. Hypoxia increases the production of reactive oxygen 
species (ROS), the primary source of mitochondrial genomic instability, leading to respiratory chain dysfunction.  
Excess ROS cause oxidative damage to DNA, lipids, proteins, altering cell functions. ETC, electron transport chain.

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in articular chondrocytes in response to hypoxia

51

 and 

in glyceraldehyde 6-phosphate dehydrogenase in carti-

lage explants exposed to oxidative damage

52

. Pyruvate 

dehydrogenase kinase (PDK) is an enzyme that inacti-

vates pyruvate dehydrogenase resulting in the conversion 

of pyruvate to acetyl co-A which is then oxidized through 

the Krebs cycle to produce energy.

HIF-1α also increases the activity of lactate dehydro-

genase A (LDHA), which converts pyruvate to lactate; 

the resultant acidic environment promotes cell prolif-

eration, invasiveness and mutations. In patients with 

RA, increased activity of lactate dehydrogenase was 

demonstrated in the synovium

53

, elevated lactate levels 

correlated with diminished levels of glucose in synovial 

fluids

54,55

 and the degree of synovial lactic acidosis was 

associated with the level of synovial inflammation

56

Increased acidosis in tissues/cells is known to cause 

mutations, resulting in defects in DNA repair mecha-

nisms, which can lead to transformation of normal cells 

and prevent apoptosis. The switch to glycolysis in the 

inflamed joint and the acidic microenvironment might 

contribute to the transformed phenotype of RA FLSs 

and to altered expression of p53 tumour suppressor 

gene, which is known to be dysregulated in the RA syno-

vium

57,58

. In turn, this altered expression might inhibit 

DNA repair, thus interfering with apoptotic mechanisms 

and promoting cell survival

57,58

. In human and mouse 

macrophages and an in vivo mouse model of atheroscle-

rosis, hypoxia potentiated the glycolytic flux induced by 

upregulation of proinflammatory activity in a manner 

that was dependent on both HIF-1α and 6-phospho-

fructo-2-kinase

59

. Furthermore, high concentrations 

of glucose increased the secretion of IL-1β from RA  

monocytes through an NLRP3-dependent mechanism

60

.

The effect of hypoxia on immune cells

As immune cells develop, differentiate and migrate to 

different tissues, they encounter fluctuations in oxygen 

tension; accordingly, they have developed the ability to 

adapt to this adverse environment.

Hypoxia and T cells

Low pO

2

 can promote the survival of T cells by stabi-

lizing HIF-1α

61

. Gaber et al. showed that the expression 

of HIF-1α was increased in the synovium of patients 

with RA and that CD4

+

 T cells adapt to hypoxic condi-

tions through activation of HIF-1α-driven pathways

62

Hypoxia can also further enhance the stabilization of 

HIF-1α that is induced by T-cell-receptor-mediated acti-

vation of the phosphatidylinositol 3-kinase–mechanistic 

target of rapamycin (PI3K–mTOR) pathway

63

. Although 

HIF-1α activation has been identified in T cells at sites 

of inflammation and is thought to facilitate the ability of  

T cells to maintain immunity in hypoxic tissues, evi-

dence has emerged that HIF-1α

+

 T cells also contribute 

to the progression of inflammatory disease, and several 

T-cell subsets with distinct functions in immunity and 

inflammation have been identified: T

H

1, T

H

2, T

H

17, 

T

REG

, T

H

9 and T

FH

 cells

64–67

. These subsets retain a level of 

plasticity and are not terminally differentiated. FOXP3

+

  

regulatory T cells (T

REG

 cells) have been shown to convert 

to pathogenic T

H

17 cells in the context of RA

68

, which 

might, in part, be driven by the hypoxic microenviron-

ment of the joint. Moran et al.

69

 demonstrated that 

patients with RA with lower pO

2

 levels in synovial tissue  

had significantly higher numbers of IL-17A

+

 mono-

nuclear cells than patients with higher pO

2

 levels. Dang 

et al.

70

 reported that HIF-1α could induce the transcrip-

tion of retinoic acid-receptor-related orphan receptor-γt 

(RORγt) while simultaneously targeting FOXP3 for deg-

radation, thereby promoting the generation of T

H

17 cells 

over T

REG

 cells.

Other studies have also highlighted the role of HIF 

signalling in mediating T

H

17–T

REG

 cell-fate decisions, 

providing evidence that hypoxia can alter T-cell lineage 

decisions

71

. CD161

+

 T

H

17 cells that were resistant to sup-

pression by T

REG

 cells, polyfunctional in their cytokine 

production, highly proliferative and capable of acti-

vating FLSs were identified in the hypoxic RA joint

72

Conversely, hypoxia can also control T

REG

 cell develop-

ment through induction of FOXP3 in models of intes-

tinal inflammation

73

. It is therefore likely that oxygen 

gradients and the cytokine milieu at the site of inflam-

mation might together determine T-cell differentiation.

The hypoxic environment itself can also induce 

metabolites and growth factors which, in turn, can affect 

T-cell function. Lactic acid enhances the secretion by 

human monocytes and macrophages of IL-6 and IL-23, 

which are required to maintain T

H

17 cells

74

. Lactic 

acid has also been shown to inhibit T-cell motility and 

enhance IL-17 production

75

.

Hypoxia and macrophages

Macrophages are found in abundance within the 

inflamed synovium

76

. The ability of monocytes to traffic  

from blood to inflammatory tissues is mediated by 

chemokines and possibly hypoxic gradients

77

. Hypoxia 

induces the translocation of HIF-1α and HIF-2α into the 

nucleus in isolated macrophages

78

, and the expression 

of matrix metalloproteinase-7 (MMP-7), neuromedin 

B receptor and DNA-binding protein inhibitor (Id2), as 

well as known hypoxia-inducible genes such as VEGF 

and GLUT1 

(REF. 79)

. Furthermore, numerous studies 

have shown that hypoxia can alter the morphology,  

survival, phagocytosis, metabolic activity, cytokine 

secretion and cell-surface protein expression by macro-

phages

80

, which, in turn, can induce neovascularization 

and further activation of immune cell responses, thus 

perpetuating the inflammatory response.

Conditional-knockout experiments in mice indicated 

that HIF-1α is essential for the inflammatory activity of 

myeloid cells. Targeted deletion of Hif1a in myeloid cells 

in arthritic mice reduced macrophage mobility, invasive-

ness and bactericidal activity, resulting in reduced syno-

vial infiltration of macrophages and a marked reduction 

in joint swelling

77

. The expression of CD68

+

 macro-

phages in synovial tissue is inversely related to in vivo 

synovial pO

2

 levels and correlates with disease activity; 

these effects are reversed in patients who respond to 

treatment with TNF inhibitors

3,5

. Unsurprisingly, TNF 

in combination with low pO

2

 levels can further enhance 

the survival of macrophages

81

.

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a  

Acute hypoxia: DC maturation

 

IFNγ

 

CD40

 

CD80

 

CD86

 

CCR7

 

CCR5

 

Proliferation

 

Phagocytosis

 

Homing to lymph nodes

 

In chemokines

 

In chemokine receptors

 

TNF, IL-1β,

VEGF

TCR

TCR

MHCII

MoDC

CD4

+

T cell

b  

Chronic hypoxia: DC differentiation

MoDC

 

CCR5

 

CXCR4

 

IFNγ

T

H

 polarization

 

IL-4

 

IL-17

via TREM1

CD4

+

T cell

TREM1

 

MHCII

Ag

Nature Reviews | 

Rheumatology

In myeloid cells, HIF-1α increases the transcription  

of glycolytic enzymes, thereby increasing glucose uptake 

and the glycolytic rate, consequently limiting oxidative 

phosphorylation

77

. As M1 and M2 polarized macrophages 

have been shown to preferentially use different metabolic 

pathways, with M1 relying on glycolysis and M2 relying 

on oxidative phosphorylation for energy, these results  

suggest that hypoxia might drive an M1 macrophage  

phenotype

82

. Furthermore, studies have demonstrated that 

different signalling mechanisms can occur in monocytes 

and macrophages as they adapt to a hypoxic environment: 

whereas HIF-1α activation predominates in macrophages, 

monocytes use NF-κB1 to regulate the expression of 

hypoxia-induced genes

78

.

Hypoxia and dendritic cells

During the differentiation of monocytes into immature 

DCs under hypoxic conditions in vitro, the expression of 

over 2,000 genes was reported to be induced

83

. Among 

these genes were those involved in glycolysis, the pentose 

phosphate pathway, antigen processing and presentation 

pathways, and cell migration

83

. DCs that mature under 

conditions of acute hypoxia show a decreased migra-

tory capacity and increased inflammatory capabilities

84

,  

whereas those that mature under chronic hypoxia 

develop a migratory phenotype through the modula-

tion of genes that encode chemokines and chemokine 

receptors, and those that are involved in cell adhesion 

and tissue remodelling

83–86

 

(FIG. 3)

. Mature DCs express 

an array of cell-surface receptors, including CD86, CD80 

and CD40, that induce T-cell proliferation and activa-

tion via co-stimulation. These T cells can then produce 

a variety of proinflammatory and anti-inflammatory 

cytokines such as TNF, IFNγ, IL-17 and IL-4. Mancino 

et al.

84

 demonstrated that acute hypoxia upregulates the 

expression of CCR5, which encodes a protein involved 

in the responsiveness of DCs to inflammatory chemo-

kines. Moreover, hypoxic DCs injected into the footpads 

of wild-type mice showed defective homing to draining 

lymph nodes, whereas leukocyte recruitment to the site 

of injection was enhanced

84

, suggesting that exposure of 

DCs to hypoxia promotes a dissociation between their 

inflammatory and tissue repair functions, which has 

implications for tissue homeostasis. By contrast, chronic 

hypoxia can induce chemotactic responses via upregu-

lation of chemokine receptors and increases in ligand 

responsiveness, while simultaneously inhibiting the 

production of inflammatory chemokines

87

. Yang et al.

88

 

reported decreased maturation, migration and antigen 

uptake in DCs in the presence of hypoxia. Furthermore, 

studies have also emerged highlighting the effect of 

hypoxia-treated DCs on T-cell differentiation, with 

implications for cell migration and activation in tumours 

and at sites of inflammation

88,89

. Finally, the expression 

Figure 3 | 

Acute and chronic hypoxia regulates dendritic cell function. a | Monocyte-derived dendritic cells 

(MoDCs) differentiated under normoxia and subsequently activated or matured under acute hypoxia display altered 
inflammatory capabilities as observed by decreased expression of the co-stimulatory markers CD86, CD80 and CD40, 
increased production of TNF, IL-1

β and vascular endothelial growth factor (VEGF), and a decreased ability to induce T-cell 

proliferation.  MoDCs matured under acute hypoxia also have altered chemotactic responses, promoting cell migration  
to inflamed tissues in addition to impairing cellular homing to lymph nodes. 

b | Exposure of monocytes to hypoxia during 

their differentiation into DCs (chronic hypoxia) can decrease phagocytosis and antigen (Ag) uptake. Some reports suggest 
these cells have decreased expression of maturation markers whereas other groups have reported no change in these 
markers. These cells also express the hypoxia-inducible marker triggering receptor expressed on myeloid cells 1 (TREM1), 
which, upon ligation, promotes the differentiation of T

H

17 cells. In co‑culture models, T cells cultured with hypoxia‑ 

differentiated MoDCs secrete lower levels of IFN

γ but increased levels of IL-4. These cells have high sensitivity to 

chemoattractants, ensuring they are ready to leave hypoxic tissue, migrate to lymph nodes and initiate adaptive immune 
responses. DCs express a wide variety of chemokine receptors (CCR2, CCR5, CCR7 and CXCR4) that respond to their 
respective chemokine ligands (MCP-1, RANTES, CXCL19 and CXCL12, respectively) in order to facilitate specialised 
migration. In order for DCs to effectively function, they utilise these chemotactic properties to traffic to inflamed sites 
within the body and subsequently migrate as mature DCs to the draining lymph node to induce adaptive immune 
responses. TCR, T-cell receptor.

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of TREM1, a hypoxia-inducible gene that encodes a  

protein involved in amplifying the immune response, 

has been detected on DCs isolated from the synovial 

fluid of patients with juvenile idiopathic arthritis

85

.

Hypoxia signalling pathways in RA
Hypoxia-inducible factor

As the master regulator of oxygen homeostasis, HIF  

executes the cellular response to altered levels of oxygen

90

HIF is a heterodimeric transcription factor composed of 

two subunits: HIF-1α, the expression of which is regu-

lated by oxygen availability, and HIF-1β, which is con-

stitutively expressed in the cell nucleus. HIF-1α senses 

oxygen through the activity of the oxygen-dependent 

hydroxylase enzymes prolyl hydroxylases 1–3 (PHD1–3)  

and asparagine hydroxylase factor inhibiting HIF 

(FIH)

91

. Under normoxic conditions, the prolyl hydroxy-

lases hydroxylate two prolyl residues on HIF-1α (pro402 

and pro564), which generates a binding site for the von 

Hippel–Lindau tumour suppressor protein (VHL)

92

VHL is an E3 ubiquitin ligase, and subsequent poly-

ubiquitylation of HIF targets it for proteasomal degra-

dation. Under hypoxic conditions, however, hydroxylase 

activity is inhibited, so HIF-1α subunits accumulate; 

they translocate to the nucleus where they dimerize 

with HIF-1β and its cofactor p300/CBP. This HIF-1α 

complex binds to specific DNA motifs and regulates 

the transcription of hundreds of genes, each containing 

hypoxia-responsive elements (HREs) 

(FIG. 4)

, involved in 

survival, metabolism, angiogenesis and invasion.

Studies have shown that the expression of HIF-1α 

and HIF-2α is increased in the RA synovium, in both 

the lining layer and sublining, and that this increase is 

associated with increased synovial vascularization and 

inflammation

93,94

. HIF-1α overexpression enhances the 

expansion of inflammatory T

H

1 and T

H

17 cells medi-

ated by RA FLSs, resulting in the increased production 

of IFNγ and IL-17 

(REFS 95,96)

. IL-17 and TNF synergis-

tically induce a HIF-1α-dependent invasive phenotype 

in RA FLSs

97

. Hypoxia potentiates the effects of IL-17A, 

IL-1β and TNF on angiogenic and invasive mechanisms 

in RA through the activation of HIF-1α, as well as 

NF-κB

98,99

. Hypoxia, through HIF-1α, has also been seen 

to potentiate the TLR-induced expression of cytokines, 

metalloproteinases and VEGF

95

.

In animal models of arthritis, HIF-1α expression has 

been detected in the synovium as early as 1 week after 

collagen injection, before clinical evidence of arthritis, 

suggesting that HIF-1α responses are involved in RA 

pathogenesis at a very early stage of disease

100

. Decreased 

infiltration of myeloid cells to the joint, reduced paw 

swelling and decreased disease development were 

observed in CIA models using HIF-1α-deficient macro-

phages

94

. FLSs enhance angiogenesis and the subsequent 

recruitment of myeloid cells through the activation of 

HIF-1α in immunodeficient mice

101

. Pretreatment of 

mice with chronic intermittent hypobaric hypoxia con-

fers a protective effect against CIA through the down-

regulation of HIF-1α and inhibition of both TNF and 

IL-17 and the ratio of T

H

1/T

H

2 lymphocytes

102

. Finally, 

amino-terminal mutation of the FOXP3 transcription 

factor in T

REG

 cells blocks interactions with HIF-1α but 

increases those with interferon regulatory factor (IRF)4, 

which confers protection against antibody-mediated 

arthritis in the K/B×N model

103

. In summary, HIF-1α 

expression is increased in the synovium of patients with 

RA, where it is associated with markers of inflamma-

tion and disease activity. In synovial cells and animal 

models of arthritis, overexpression of HIF-1α signal-

ling increases angiogenesis, activation of immune cells, 

secretion of proinflammatory mediators and invasive 

mechanisms, whereas blockade of HIF-1α promotes 

resolution of inflammation.

Prolyl hydroxylases

Although prolyl hydroxylases regulate HIF activation 

and stability, little is known about their expression and 

regulation in the inflamed joint. Transient silencing of 

these enzymes in fibroblasts not surprisingly induces 

the expression of many pro-angiogenic and inflamma-

tory mediators by increasing the levels of HIF

104

. Muz 

et al.

105

 demonstrated that PHD2 was the prominent 

prolyl hydroxylase in RA FLSs, and that its knock-

down augmented HIF-1α-induced gene expression. 

Similarly, upregulation of miR-210 by connective tissue 

growth factor through PI3K–AKT and NF-κB pathways 

repressed the activity of prolyl hydroxylases and subse-

quently promoted HIF-1α-dependent VEGF expression 

in synoviocytes from patients with osteoarthritis

106

Treatment of FLSs with dimethyloxalylglycine, a pan- 

hydroxylase inhibitor

107

, results in HIF-1α activation and 

induces mitochondrial dysfunction, which is reflected by 

an increase in ROS, mitochondrial membrane potential 

and mass. In parallel, dimethyloxalylglycine (DMOG) 

induces a switch to glycolysis and promotes RA FLS 

invasiveness. Manipulation of prolyl hydroxy lases in vas-

cular models has highlighted their critical involvement 

in the regulation of the maturity, stability and survival of 

blood vessels

108

. Owing to its avascular nature, cartilage is 

considered to be an oxygen-deprived tissue. Nevertheless, 

human chondrocytes are able to enhance matrix synthe-

sis in hypoxic conditions through the HIF-1α-mediated 

upregulation of SOX9 

(REF. 109)

; HIF-1α stability, in turn, 

is mediated by the inhibition of PHD2 in response to 

low pO

2

. These data are consistent with the protective 

effects conferred by prolyl hydroxylase inhibitors in other  

diseases such as colitis

110

.

NF-κB signalling

As one of the key transcriptional pathways in RA, NF-κB 

signalling primarily regulates the levels of proinflamma-

tory mediators and components of anti-apoptotic path-

ways. In the canonical pathway, the activity of NF-κB is 

controlled by inhibitor of NF-κB (IκB) kinase (IKK), which 

mediates the serine phosphorylation and degradation of 

IκBα, the endogenous inhibitor of NF-κB; degradation  

of IκBα induces nuclear translocation of NF-κB.

Previous studies have shown that NF-κB can pro-

mote HIF-1α activation in response to proinflammatory 

cytokines

111,112

. NF-κB binding sites are present within 

the HIF-1α promoter, the mutation of which results  

in the loss of HIF-1α activation in response to hypoxia

113

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Hypoxia

TNF

IL1B

VCAM1

IL8

Nature Reviews | 

Rheumatology

Degradation

Transcription

Nucleus

IKK complex

p65

p50

p65

p50

b  

NF-κB signalling

 

PHD/FIH

VEGF

EPO

STATs

Notch

PKM

 

PHD/FIH

 

PHD

HIF-1α

Degradation

Transcription

Nucleus

OH

HIF-1α

IκBα

IκBα

pVHL

p300/CBP

HIF-1α

HRE

HIF-1β

Hypoxia

Normoxia

a  

HIF-1α signalling

Furthermore, activation of the NF-κB p65 subunit is 

associated with low in vivo synovial pO

2

 levels in RA 

synovial biopsy samples

114

. NF-κB–HIF-1α signalling also 

mediates the synergistic effects of IL-17A and hypoxia on 

the invasiveness of FLSs

98

. In osteoclasts, the expression 

of VEGF induced by receptor activator of NF-κB ligand 

(RANKL) is mediated through NF-κB and subsequent 

induction of HIF-1α. The increase in VEGF promotes 

angiogenesis, facilitating leukocyte infiltration and fur-

ther promoting an hypoxic environment

115

. Furthermore, 

inhibiting NF-κB in a CIA model inhibits HIF-1α activation,  

angiogenesis and synovial inflammation

116

.

The mechanism(s) by which hypoxia influences 

NF-κB in the inflamed joint is unclear, but studies in other 

cell types have shown that, under hypoxic conditions, pro-

lyl hydroxylases show decreased hydroxylation activity 

towards IKKβ, which thereby relieves the repression of 

NF-κB

117

 

(FIG. 4)

. Consistent with this mechanism, IKK2 

contains an evolutionarily conserved LxxLAP consensus 

motif within its activation loop, which can potentially 

be hydroxylated by prolyl hydroxylases. Furthermore, 

the ankyrin-repeat domains of the p105 precursor of the 

p50 subunit of NF-κB and IκBα are hydroxylated by FIH,  

indicating that prolyl hydroxylases are directly involved in 

the NF-κB transcriptional response

118

.

Notch signalling

Notch receptors and their ligands are transmembrane  

proteins that regulate cell fate decisions, proliferation, 

differentiation and apoptosis. Following cleavage by 

γ-secretase, the intracellular domain of Notch, NICD, 

translocates to the nucleus, where it stimulates the 

transcription of the ‘hairy/enhancer-of-split’ (Hes) and 

Hes-related transcription factor (Hrt) families of tran-

scriptional repressors

119

. Under hypoxic conditions, Notch 

signalling is a key mechanism involved in the differenti-

ation, in angiogenesis, of vascular-tip versus stalk cells, 

during which the tip cells guide the vascular sprouts fol-

lowed by the stalk cell, which forms a functional vascular 

branch, and then the phalanx cells, which are involved 

in the developing plexus

120

. The Notch ligands DLL4 and 

Jagged-1, NICD, and downstream target genes have all 

been detected in RA synovial tissue

107

. In the inflamed 

joint, NICD signalling mediates angiogenesis induced 

Figure 4 | 

Hypoxia-induced signalling in the rheumatoid joint. a | Under normoxic conditions, hydroxylation of 

hypoxia-inducible factor (HIF)-1

α by prolyl hydroxylases (PHDs) generates a binding site for the von Hippel Lindau 

tumour suppressor protein (VHL), thereby promoting the ubiquitylation and subsequent proteasomal degradation 
of HIF-1

α.  However, under hypoxic conditions, such as in the inflamed joint, the hydroxylation activity of PHDs is 

reduced, resulting in the accumulation and activation of HIF-1

α, which can then translocate into the nucleus and 

associate with HIF-1

β and the cofactor p300/CBP. This complex binds to, and induces the transcription of, genes 

containing hypoxia-responsive elements (HRE), such as VEGFEPO, STAT3Notch and PKM2

b | Nuclear factor κB 

(NF-

κB) is one of the main transcription factors in the inflamed joint. Both inflammatory stimuli, such as cytokines, and 

hypoxia can activate NF-

κB signalling. Under hypoxic conditions, the PHD-mediated repression of inhibitor of κB 

kinase (IKK

β) is suppressed, which leads to increased IKKβ activity and enhanced phosphorylation of inhibitor of  

κBα (IκBα); IκBα phosphorylation causes its degradation, enabling NF-κB subunits (p65 and p50) to translocate  

to the nucleus, where they activate the transcription of inflammatory genes such as TNFIL1BVCAM and IL8.  
EPO, erythropoietin; FIH, asparagine hydroxylase factor inhibiting HIF; PKM2, pyruvate kinase M2; STAT, signal 
transducer and activator of transcription.

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PHD/FIH

NICD

STAT3/HES1

PIAS

VEGF

EPO

STATs

Notch

PKM

 

PHD/FIH

Nature Reviews | 

Rheumatology

HIF-1α

Degradation

Transcription

Nucleus

OH

HIF-1α

pVHL

p300/CBP

HIF-1α

HIF-1β

Hypoxia

Normoxia

HRE

by VEGF or Ang2 as well as regulating chondrocyte  

proliferation. High levels of NICD are associated with low 

levels of pO

2

 in synovial tissue in vivo, and its levels, along 

with the expression of DLL4HEY1 (encoding HRT-1), 

and HEY2 (encoding HRT-2), are induced in RA FLSs and 

human dermal microvascular endothelial cells in response 

to hypoxia

107

, which is consistent with studies showing the  

presence of HIF-1α binding domains and HREs in  

the promoters of HEY1HEY2 and DLL4 

(REF. 121)

. In turn,  

HIF-1α activation can be inhibited in the presence of the 

Notch inhibitor DAPT in RA cells

107

. HIF-1α and NICD 

have been demonstrated to interact, increasing the sta-

bility of HIF-1α under hypoxic conditions

122

. However, 

the HIF-1 inhibitor FIH not only binds HIF-1α, but can 

also bind Notch, and shows a higher affinity for Notch 

than it does for HIF, suggesting that Notch might regulate 

HIF-1α activation by sequestering FIH

123

 

(FIG. 5)

.

JAK–STAT signalling

The Janus kinase (JAK) family of receptor-associated 

tyrosine kinases has been implicated in RA; once acti-

vated, JAKs recruit and activate signal transducers and 

activators of transcription (STATs), which, in turn, drive 

gene transcription

124

. In RA, STAT activation correlates 

with synovitis, promotes FLS survival

125

 and mediates 

RANKL-dependent osteoclastogenesis

126

, whereas JAK2 

inhibition ameliorated disease in CIA and in collagen- 

antibody-induced arthritis models

127

In vivo, synovial 

pO

2

 levels induced STAT3 activation and promoted 

migration and invasion in RA FLSs. However, although 

HIF-1α silencing inhibited these effects, STAT3 blockade 

could also inhibit hypoxia-induced HIF-1α expression, 

which suggests the existence of bi-directional inter-

actions between STAT3 and HIF-1α

128

These results are 

consistent with studies showing that HIF-1α facilitates 

the binding of STAT3 to the haptoglobin promoter in 

human hepatoma cells

129

. Hypoxia-induced STAT3 acti-

vation can accelerate the accumulation and activation of 

HIF-1α

130

; STAT3 binding to HIF-1α inhibits the inter-

action of HIF-1α with, and subsequent ubiquitylation 

by, VHL

131

 

(FIG. 4)

. PIAS, a negative regulator of STAT 

activation that is known to be expressed in the inflamed 

synovium, can also interact with VHL and induce VHL 

SUMOylation, thereby inactivating it

132

. Further studies  

have shown that activation of hypoxia signalling in 

cancer cells is Hes1/STAT3 dependent

133

, that the JAK 

inhibitor WP1066 significantly decreases hypoxia- 

induced HIF-1α–NICD signalling in RA

131

, and that 

inhibition of STAT3 reduces hypoxia-induced Notch 

activation in glioblastoma stem cells

134

. Furthermore, 

Jagged-1–Notch-mediated regulation of inflammatory 

responses is mediated through both the NF-κB and 

JAK–STAT–SOCS signalling pathways

135

. Together, 

these studies demonstrate complex interactions between 

key signalling pathways within the inflamed hypoxic 

synovial microenvironment, and provide evidence for 

both positive and negative feedback loops at many dif-

ferent levels. Increased efficiency of the oxygen supply 

to the synovium might therefore lead to inhibition of 

these pathways and promote resolution of inflammation.

Therapeutic implications

Several studies have examined the consequence of block-

ing angiogenic pathways such as those mediated by VEGF, 

its receptors or the angiopoietins in models of arthritis, 

both in vitro and in vivo, and have reported a delayed onset 

of arthritis, reduced number of synovial blood vessels and 

cellular infiltrates, and decreased joint damage

10,11,137,140

Bevacizumab, an anti-VEGF monoclonal antibody

141

has been approved for the treatment of colon, kidney and 

lung cancer; however, its efficacy has not been demonstr-

ated in RA. Several pharmacologic agents that activate or 

inhibit HIF signalling have also been examined in many 

disease states. Activators of HIF, including hydroxylase 

inhibitors such as DMOG, FG-4497 and JNJ1935, have 

been shown to have beneficial effects in in vitro and 

in vivo studies on epithelial and endothelial cell function 

in vascular and colitis models

120,142,143

. In the context of RA,  

however, molecules that promote, rather than inhibit, 

Figure 5 | 

Interplay between HIF-1α, Notch and STAT3 

signalling. Interplay exists between hypoxia-inducible 
factor (HIF)-1

α, the Notch intracellular domain (NICD) and 

signal transducer and activator of transcription (STAT)3 
signalling at various levels.  Hypoxia-induced STAT3 can,  
in turn, accelerate the activation of HIF-1

α, an effect 

mediated through competition of STAT3 with HIF-1

α for 

binding to the von Hippel Lindau (pVHL) protein, which 
might involve the Notch-1 downstream target gene Hes1
Protein inhibitor of activated STAT (PIAS), a negative 
regulator of STAT activation, also has the ability to interact 
with pVHL to induce VHL SUMOylation and subsequent 
inactivation, thereby preventing pVHL from promoting the 
ubiquitylation and proteasomal degradation of HIF-1

α. The 

prolyl hydroxylase FIH can hydroxylate the ankyrin repeat 
domain of Notch and displays a higher affinity for Notch 
than for HIF, suggesting that Notch might sequester FIH 
activity, thereby allowing the accumulation and activation 
of HIF-1

α. FIH, asparagine hydroxylase factor inhibiting HIF.

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