Ann. N.Y. Acad. Sci. ISSN 0077-8923
ANNALS OF THE NEW YORK ACADEMY OF SCIENCES
Issue: Targeting VEGF-Mediated Tumor Angiogenesis in Cancer Therapy
Angiopoietin-2: a multifaceted cytokine that functions
in both angiogenesis and inflammation
Alexander Scholz,1 Karl H. Plate,2,3,4,5 and Yvonne Reiss2,3,4,5
1
Department of Pathology, Laboratory of Immunology and Vascular Biology, Stanford University School of Medicine, Stanford,
California. 2 Edinger Institute/Institute of Neurology, Frankfurt University Medical School, Frankfurt, Germany. 3 German Center
for Cardiovascular Research (DZHK), Frankfurt, Germany. 4 German Cancer Consortium (DKTK), Frankfurt, Germany.
5
German Cancer Research Center (DKFZ), Heidelberg, Germany
Address for correspondence: Alexander Scholz, Ph.D., Department of Pathology, Laboratory of Immunology and Vascular
Biology, Stanford University School of Medicine, Stanford, California 94304. alexander.scholz@stanford.edu
Angiogenesis and inflammation are two highly linked processes. In the last decade, several factors with dual function
in both of these major pathways have been identified. This review focuses on angiopoietin-2 (Ang-2), an important
proangiogenic factor that has more recently been implicated in mediating inflammatory processes as well. Ang-2 is
upregulated in multiple inflammatory diseases and has been implicated in the direct control of inflammation-related
signaling pathways. As a consequence of its multiple roles, designs for therapeutic targeting of Ang-2 should consider
the dual function of this factor in regulating angiogenesis and inflammation.
Keywords: angiopoietin; Ang-2; inflammation; myeloid cell; innate immune cell
Introduction
The angiopoietins (Ang) are a family of secreted factors comprising Ang-1,1 Ang-2,2 and Ang-3 (Ang-4
in humans).3 While Ang-1 has been widely accepted
as an agonist for the Tyr kinase with Ig and epidermal growth factor homology domains (Tie2),
Ang-2 has been described as a context-dependent
antagonist interfering with Ang-1–induced Tie2
phosphorylation.4 Interestingly, both ligands bind
with similar kinetics to the receptor’s second Ig
domain.5,6 Genetic mouse models demonstrated
that the phenotypes of Ang-1-7 and Tie2-deficient
mice8,9 and mice with endothelial-specific overexpression of Ang-22 are highly similar. Thus, the
concept arose that Ang-1–induced paracrine Tie2
phosphorylation is an important stimulus for vascular integrity and homeostasis and is constantly active
in healthy adult vasculature.1,10 In contrast, Ang-2
expression is tightly regulated, and its expression is
mostly restricted to the vascular endothelium.11–14
Upon secretion from Weibel–Palade bodies,15
Ang- 2 acts on Tie2 in an autocrine manner and
competes with Ang-1 for receptor binding, leading
to disturbances in endothelial junctional integrity,16
pericyte drop-off,2,17 and an overall priming of the
vascular bed for angiogenic sprouting. The latter
is highly dependent on the local cytokine milieu. In
conjunction with vascular endothelial growth factor
(VEGF), Ang-2 has been reported to induce vascular
sprouting, whereas in the absence of VEGF, Ang-2
induces endothelial cell (EC) apoptosis.18 Because of
its proangiogenic properties, Ang-2 has become an
interesting target in antiangiogenic tumor therapy,
with several inhibitors in current clinical trials.19
The role of Ang-2 in the induction of angiogenic
sprouting has been reviewed elsewhere.20–22 This
review will therefore focus on the proinflammatory
effects of Ang-2 and the potential therapeutic implications arising from those.
Ang-2 at the interface between leukocytes
and the endothelium
The link between the angiopoietins and inflammation was hypothesized soon after their discovery.
At first, the anti-inflammatory properties of Ang-1
were assessed and mainly attributed to their effect
on vascular integrity and the reduction in plasma
leakage.23–25 Later, the molecular mechanisms
doi: 10.1111/nyas.12726
C 2015 New York Academy of Sciences.
Ann. N.Y. Acad. Sci. xxxx (2015) 1–7
1
Angiopoietin-2 and inflammation
Scholz et al.
Figure 1. Comparison of autocrine and paracrine Ang-2 signaling during inflammatory responses. Angiogenic and inflammatory stimuli trigger the release of Ang-2 from endothelial
Weibel–Palade bodies. Ang-2 can act either in an autocrine manner on endothelial cells or in a paracrine manner on leukocytes,
in particular cells of the myeloid lineage. While Ang-2–induced
autocrine signaling effects are well understood and entail the
induction of adhesion molecules and vascular leakage, paracrine
Ang-2 signaling is still enigmatic. Paracrine Ang-2 signaling has
been described in monocytes/macrophages and neutrophils and
is either Tie2 dependent (blue receptor) or the responsible receptor has not yet been identified (black receptor). Upon stimulation, innate immune cells can respond with increased adhesion
to the vessel wall, triggered in a 2-integrin–dependent manner.
Furthermore, upregulation of IL-10, downregulation of TNF-␣
and IL-6, and induction of CD206 expression has been described
in monocytes/macrophages, indicative of an M2-polarized phenotype. In addition, IL-8 induction in neutrophils has been
linked to Ang-2 stimulation.
underlying the stabilizing functions of Ang-1 were
further investigated. Besides an increase in pericyte coverage, Ang-1 additionally tightens the EC
junctions by sequestering small kinases to prevent
VEGF-induced VE-cadherin phosphorylation.26
The role of Ang-2 in inflammation was long projected on the basis of the assumption that it counteracts the anti-inflammatory effects of Ang-124,27
and on the finding that it is upregulated in several inflammatory diseases.28–37 The first mechanistic evidence arose in 2006, when it was shown that
Ang-2 sensitized Tie2+ ECs toward inflammatory
stimuli such as tumor necrosis factor ␣ (TNF-␣).38
Following this sensitization, ECs strongly upregulated adhesion molecules such as intercellular adhesion molecule 1 (ICAM-1) or vascular cell adhesion molecule 1 (VCAM-1), thus inducing leukocyte
adhesion (Fig. 1). In fact, using intravital fluorescence videomicroscopy, the authors showed that, in
2
mice lacking Ang-2 (Ang2 knockout (KO)), leukocytes failed to arrest. Consequently, the number
of infiltrating cells in thioglycollate-induced peritonitis was strongly reduced in Ang2 KO mice, but
could be rescued by administration of recombinant Ang-2.38 Supporting the autocrine proinflammatory effects of Ang-2, it has been shown that
Ang-2 is required for the transformation of capillaries into venules, which consequently mediates
leukocyte trafficking in a mouse airway inflammation model.39
Beyond multiple studies that focused on the
autocrine effects of endothelial secreted Ang-2 on
the EC-expressed Tie2 receptor, recent evidence in
the literature suggests an additional direct paracrine
action of Ang-2 on immune cells (Fig. 1). It has
been shown that Ang-2 acts as a chemoattractant,
mediating the in vitro migration of a specific subset of human monocytes that is characterized by
the expression of the Tie2 receptor (Tie2 expressing
monocytes (TEMs)).40,41 Blockade of Tie2 using a
neutralizing antibody was able to abolish Ang-2–
mediated migration.40 Another group has described
the expression of Tie2 on human neutrophils. Tie2expressing neutrophils show increased adhesion
upon Ang-2 stimulation42 and, similar to monocytes, directed migration toward Ang-2.43 Interestingly, in these studies, Ang-1 and Ang-2 triggered
additive effects.42,43 Further evidence for a paracrine
function of Ang-2 is based on the observation that
human neutrophils react to Ang-2 stimulation with
the upregulation of IL-8RB and IL-10RB, whereas
Ang-1 stimulation induced IL-1 and IL1-Ra.44 The
induction of specific cytokines by angiopoietins has
also been shown in IL-10–polarized macrophages,45
indicative of a more general effect of angiopoietins
on innate immune cells. In line with these observations, Ang-2 triggered the infiltration of neutrophils
and monocytes into a murine sponge/matrigel
model.46 In addition, we were able to demonstrate
that Ang-2 generally attracts innate immune cells
in a paracrine, integrin-dependent manner.47 Mice
with inducible, EC-specific overexpression of Ang-2
showed massive influx of myeloid cells into multiple organs in a time-dependent manner. Using an
in vitro flow-chamber model, we demonstrated that
Ang-2 is very potent in inducing human monocyte
adhesion on ICAM-1 and VCAM-1, while it had no
effect on lymphocyte adhesion. Furthermore, Ang-2
was specifically upregulated in the vasculature
C 2015 New York Academy of Sciences.
Ann. N.Y. Acad. Sci. xxxx (2015) 1–7
Scholz et al.
Angiopoietin-2 and inflammation
of human myeloid cell–dominated inflammation,
whereas it is absent in lymphoid-dominated inflammatory diseases.47 This specificity in recruiting cells
of the myeloid lineage is of particular interest in
targeted tumor therapy, as discussed next.
Ang-2 in cancer inflammation
The role of cancer-related inflammation, especially the infiltration of innate immune cells, has
been extensively reviewed.48–51 In most solid cancers, larger numbers of infiltrating tumor-associated
macrophages (TAMs) correlate with worse patient
survival. This could mainly be attributed to the
alternative polarization of TAMs, rendering them
proangiogenic and immunosuppressive. However,
the molecular mechanisms underlying the recruitment and polarization of innate immune cells in
solid tumors are poorly understood. Ang-2 is highly
upregulated in the vasculature of mouse and human
neoplasms and, given its effects on EC activation
and direct myeloid cell recruitment, it represents an
interesting candidate to mediate tumor immune cell
homing. We showed in a subcutaneous Lewis lung
carcinoma model that tumors grown in mice with
EC-specific Ang-2 overexpression show increased
infiltration with innate immune cells, in particular TEMs.52 Circulating TEMs are already innately
proangiogenic; however, stimulation with Ang-2
could further increase their expression of angiogenic
and immunosuppressive genes.52 This is of particular interest, as the recruitment of proangiogenic
myeloid cells has been discussed as a potential mechanism for resistance to antiangiogenic therapy.53
More recently, Mazzieri et al. showed that blockade of Ang-2 in two different spontaneous mouse
tumor models did not impair the recruitment but
rather the close association of TEM with the tumor
vasculature.54 Blocking the EC–TEM interaction
impaired the restoration of tumor neovascularization and thus enhanced antiangiogenic therapy. In
another study, the inhibition of Ang-2 reduced the
amount of infiltrating TEM in a Colo-205 xenograft
model.55 Interestingly, in some tumors, Ang-2 was
upregulated and counteracted antiangiogenic therapy targeting VEGF. Double inhibition of both
VEGF and Ang-2 thus greatly enhanced the efficacy of antiangiogenic therapy in those tumors.56
This type of compensatory mechanism has been
described for Ang-2 as well. In a mouse model
of liver metastasis, the lack of Ang-2 strongly
Figure 2. Contribution of autocrine and paracrine Ang-2 signaling in different inflammatory diseases. On the basis of the
current literature, the contribution of direct effects of Ang-2
on leukocytes (paracrine) and on endothelial cells (autocrine)
in different inflammation-related diseases is shown. Activating or inhibiting arrows indicate how Ang-2 affects the course
of the particular disease, whereas their position in relation to
autocrine or paracrine signaling indicates the expected contribution of these pathways to the inflammatory response. For
example, Ang-2 has been described to worsen disease outcome
in sepsis, and this has been attributed to direct, destabilizing
effects on the endothelium. In contrast, cancer-related inflammation equally entails endothelial stimulation and direct effects
on tumor-infiltrating innate immune cells. IBD, inflammatory
bowel disease.
upregulated granulocyte colony-stimulating factor, leading to the recruitment of CD11b+ Tie2+
immune cells, which restored neoangiogenesis and
tumor growth.57 Nevertheless, Ang-2 is a promising candidate for combined anticancer therapy, as it
targets two major protumorigenic processes, angiogenesis and inflammation (Fig. 2).
Ang-2 in autoimmune diseases
In addition to neoplastic diseases, Ang-2 expression is also upregulated in human autoimmune
diseases, such as psoriasis32 and arthritis.33–35,58
Both types of disease are characterized by extensive
angiogenesis and inflammatory cell recruitment.
In this context, Ang-2 expression has been shown
to correlate with arthritis severity in patients.58
More recently, the functional role of Ang-2 in
arthritis has been addressed using a fully humanized blocking antibody directed against Ang-2.59
The authors showed that anti-Ang-2 therapy significantly improved the clinical score of animals
with psoriatic and rheumatoid arthritis. The rationale for the use of Ang-2 blockade was based on
the observation that Ang-2 and TNF-␣ act cooperatively on synovial macrophages to induce a
proinflammatory phenotype.58 In line with these
C 2015 New York Academy of Sciences.
Ann. N.Y. Acad. Sci. xxxx (2015) 1–7
3
Angiopoietin-2 and inflammation
Scholz et al.
findings, dual inhibition of TNF-␣ and Ang-2 by a
bispecific antibody was superior to targeting TNF-␣
alone.60 Interestingly, sequestering Ang-2 in combination with VEGF using double antiangiogenic
protein in collagen-induced arthritis reduced
the disease severity and also decreased F4/80+
macrophage infiltrate, indicative of similar but distinct roles of these angiogenic factors in arthritis.61
Ang-2 in inflammatory bowel disease
Inflammatory bowel disease (IBD) encompasses
ulcerative colitis and Crohn’s disease (CD) and is
characterized by uncontrolled inflammation of the
intestinal mucosa. Excessive recruitment of neutrophils to the gut wall and inadequate control
of their proinflammatory activation are thought
to be the critical underlying mechanisms in IBD.
However, the role of the Ang/Tie system during IBD is only poorly understood. On the one
hand, studies show that patient serum levels of
Ang-2 are elevated during IBD, while Ang-1 levels are decreased.28,29 Conversely, during colonrestricted CD, Ang-2 levels were significantly lower
in patients as compared to healthy controls,62
indicative of distinct roles of Ang-2 at different IBD
sites. Interestingly, in a mouse model of dextran
sodium sulfate (DSS)-induced colitis, genetic ablation of Angpt2 decreased the influx of inflammatory
cells to the gut wall but failed to improve clinical
symptoms. On the contrary, the clinical outcome in
Angpt2 KO mice in experimental colitis was worse
compared to wild-type littermates.63
Ang-2 in sepsis
Sepsis is a systemic inflammatory response to infection. The high mortality in septic patients is owed to
multiorgan dysfunction caused by a dramatic drop
in blood pressure. ECs are supposed to be the main
mediators of septic shock, reacting to a massive
cytokine storm with the loss of vascular integrity
and increased plasma leakage. A number of studies
have described Ang-2 to be highly upregulated in
the serum of septic patients, and its expression
strongly correlates with severity of the disease.30,31,64
It has been shown that intravenously administered
lipopolysaccharide (LPS) strongly upregulated
Ang-2 expression in several organs, which was
accompanied by reduced Tie2 phosphorylation.65
Currently, evidence in the literature suggests that
4
the vessel-destabilizing functions of Ang-2 are the
main contributors to the septic phenotype (Fig. 2).
Ziegler et al. showed that mice with endothelialspecific, but not cardiomyocyte-specific, Ang-2
overexpression develop sepsis-like hemodynamic
alterations.66 These effects were antagonized by
injecting mice with an Angpt1- or Pdgfb-encoding
adeno-associated virus, suggesting a strong involvement of Ang-2–mediated pericyte detachment
during septic shock. Furthermore, targeting Ang-2
with a neutralizing antibody decreased mortality in
LPS-induced endotoxic shock.66 Another study used
the synthetic Tie2-activating ligand vasculotide in
a cecal ligation–puncture model and demonstrated
a significant enhancement of survival.67 Similar
results were obtained when injecting Mat-Ang-1, a
stable Ang-1 variant, into mice with LPS-induced
septic shock.68 These studies indicate that the tight
balance between vascular-destabilizing Ang-2 and
stabilizing Ang-1 could predict the outcome during
septic shock. This is supported by the fact that even
the loss of one Ang2 allele improved survival in
different sepsis models.64 This finding was accompanied by reduced VCAM-1 expression in Ang2+/–
mice and induced inflammatory cell infiltration.64
Although the vast majority of studies link Ang-2
to a worse disease outcome, Tzepi et al. showed
that injection of recombinant Ang-2 was protective
against septic shock induced in mice by inactivated
Pseudomonas aeruginosa or Escherichia coli. In
contrast, Ang-2 was not able to protect against LPSinduced septic shock, indicating crucial differences
in the septic models that have been used.69
Whether the detrimental role of Ang-2 during septic shock is primarily attributable to the
destabilization of the endothelium or additionally
dependent on the recruitment of proinflammatory
immune cells remains to be investigated.
Concluding remarks
In the past decade, numerous studies convincingly demonstrated a link between Ang-2 signaling
and the recruitment of innate immune cells, both
in a Tie2-dependent and Tie2-independent manner (Fig. 1). Thus, Ang-2 has been widely termed
a proinflammatory molecule. Yet, it is questionable whether Ang-2 is indeed proinflammatory or
might even operate as an anti-inflammatory factor.
For example, we observed massive recruitment of
C 2015 New York Academy of Sciences.
Ann. N.Y. Acad. Sci. xxxx (2015) 1–7
Scholz et al.
myeloid cells into organs of Ang-2–overexpressing
mice without signs of active tissue damage.47 This
is supported by the finding that during DSSinduced colitis, Ang-2 deficiency impaired neutrophil recruitment but did not improve disease
progression.63
In addition, monocytes recruited by Ang-2 into
tumors are also highly immunosuppressive, a characteristic further enhanced by Ang-2 stimulation.52
Furthermore, Ang-2–stimulated TAMs are strong
producers of the anti-inflammatory cytokine IL-10
and promote the expansion of FoxP3+ regulatory
T cells, a lymphoid subset that is highly immunosuppressive and known to correlate with a worse
prognosis.70 In a more recent study, Ang-2 induced
a M2-like phenotype in monocytes while Ang-1
induced a proinflammatory signaling cascade in
monocytes via p38 and Erk1/2, leading to increased
TNF-␣ release71 (Fig. 1). However, whether this
concept holds true in vivo needs to be investigated. Accordingly, Ang-2 deficiency does not protect against LPS-induced acute kidney injury, but
leads to even higher TNF-␣ and IL-6 levels.72
Thus, it remains to be determined whether Ang-2
triggers classic inflammatory responses or rather
attracts myeloid cells without additional obvious
signs of inflammation. Resolution of this question
will have strong implications for the use of Ang-2
as a therapeutic target: depending on the type of disease, it has to be taken into consideration if the antiinflammatory (i.e., during tumor therapy) or proinflammatory (i.e., during IBD) actions of myeloid
cells are the objective of therapeutic targeting.
Acknowledgments
We would like to thank Eugene C. Butcher for
insightful contributions and Cathrin J. Czupalla
and Rebecca Kunder for proofreading and improvements of the manuscript. This work was supported
by the Collaborative Research Center “Vascular differentiation and remodeling” (CRC/Transregio23,
project C1) to Y.R. and K.H.P., a start-up grant from
the Cluster of Excellence 147 “Cardiopulmonary
system” (ECCPS), and a postdoctoral research fellowship (SCHO 1555/1-1) to A.S. by the German
Research Council (DFG).
Conflicts of interest
The authors declare no conflicts of interest.
Angiopoietin-2 and inflammation
References
1. Davis, S. et al. 1996. Isolation of angiopoietin-1, a ligand for
the TIE2 receptor, by secretion-trap expression cloning. Cell
87: 1161–1169.
2. Maisonpierre, P.C. 1997. Angiopoietin-2, a natural antagonist for Tie2 that disrupts in vivo angiogenesis. Science 277:
55–60.
3. Valenzuela, D.M. et al. 1999. Angiopoietins 3 and 4: diverging
gene counterparts in mice and humans. Proc. Natl. Acad. Sci.
USA 96: 1904–1909.
4. Reiss, Y. et al. 2007. Angiopoietin-2 impairs revascularization
after limb ischemia. Circ. Res. 101: 88–96.
5. Fiedler, U. 2002 Angiopoietin-1 and angiopoietin-2 share
the same binding domains in the Tie-2 receptor involving
the first Ig-like loop and the epidermal growth factor-like
repeats. J. Biol. Chem. 278: 1721–1727.
6. Davis, S. et al. 2002. Angiopoietins have distinct modular
domains essential for receptor binding, dimerization and
superclustering. Nat. Struct. Biol. 10: 38–44.
7. Suri, C. et al. 1996. Requisite role of angiopoietin-1, a ligand
for the TIE2 receptor, during embryonic angiogenesis. Cell
87: 1171–1180.
8. Sato, T.N., Y. Qin, C.A. Kozak & K.L. Audus. 1993. Tie-1
and tie-2 define another class of putative receptor tyrosine
kinase genes expressed in early embryonic vascular system.
Proc. Natl. Acad. Sci. USA 90: 9355–9358.
9. Dumont, D.J., T.P. Yamaguchi, R.A. Conlon, et al. 1992. tek,
a novel tyrosine kinase gene located on mouse chromosome
4, is expressed in endothelial cells and their presumptive
precursors. Oncogene 7: 1471–1480.
10. Wong, A.L. et al. 1997. Tie2 expression and phosphorylation
in angiogenic and quiescent adult tissues. Circ. Res. 81: 567–
574.
11. Gale, N.W. et al. 2002. Angiopoietin-2 is required for postnatal angiogenesis and lymphatic patterning, and only the
latter role is rescued by Angiopoietin-1. Dev. Cell 3: 411–
423.
12. Stratmann, A., W. Risau & K.H. Plate. 1998 Cell type-specific
expression of angiopoietin-1 and angiopoietin-2 suggests a
role in glioblastoma angiogenesis. Am. J. Pathol. 153: 1459–
1466.
13. Huang, Y.Q. 2002. Thrombin induces increased expression
and secretion of angiopoietin-2 from human umbilical vein
endothelial cells. Blood 99: 1646–1650.
14. Zhang, L. et al. 2003. Tumor-derived vascular endothelial
growth factor up-regulates angiopoietin-2 in host endothelium and destabilizes host vasculature, supporting angiogenesis in ovarian cancer. Cancer Res. 63: 3403–3412.
15. Fiedler, U. 2004. The Tie-2 ligand angiopoietin-2 is stored
in and rapidly released upon stimulation from endothelial
cell Weibel–Palade bodies. Blood 103: 4150–4156.
16. Scharpfenecker, M., U. Fiedler, Y. Reiss & H.G. Augustin.
2005. The Tie-2 ligand angiopoietin-2 destabilizes quiescent
endothelium through an internal autocrine loop mechanism. J. Cell Sci. 118: 771–780.
17. Hammes, H.-P. et al. 2004. Angiopoietin-2 causes pericyte
dropout in the normal retina: evidence for involvement in
diabetic retinopathy. Diabetes 53: 1104–1110.
C 2015 New York Academy of Sciences.
Ann. N.Y. Acad. Sci. xxxx (2015) 1–7
5
Angiopoietin-2 and inflammation
Scholz et al.
18. Holash, J. 1999. Vessel cooption, regression, and growth in
tumors mediated by angiopoietins and VEGF. Science 284:
1994–1998.
19. Plate, K.H., A. Scholz & D.J. Dumont. 2012. Tumor angiogenesis and anti-angiogenic therapy in malignant gliomas
revisited. Acta Neuropathol. 124: 763–775.
20. Eklund, L. & P. Saharinen. 2013. Angiopoietin signaling in
the vasculature. Exp. Cell Res. 319: 1271–1280.
21. Reiss, Y. 2010. Angiopoietins. Recent Results Cancer Res. 180:
3–13.
22. Augustin, H.G., G. Young Koh, G. Thurston & K. Alitalo.
2009. Control of vascular morphogenesis and homeostasis
through the angiopoietin–Tie system. Nat. Rev. Mol. Cell
Biol. 10: 165–177.
23. Baffert, F. 2005. Angiopoietin-1 decreases plasma leakage by
reducing number and size of endothelial gaps in venules.
Am. J. Physiol. Heart Circ. Physiol. 290: H107–H118.
24. Thurston, G. et al. 2000. Angiopoietin-1 protects the adult
vasculature against plasma leakage. Nat. Med. 6: 460–
463.
25. Jiang, H., F. Zhang, J. Yang & S. Han. 2014. Angiopoietin1 ameliorates inflammation-induced vascular leakage and
improves functional impairment in a rat model of acute
experimental autoimmune encephalomyelitis. Exp. Neurol.
261: 245–257.
26. Gavard, J., V. Patel & J.S. Gutkind. 2008. Angiopoietin-1 prevents VEGF-induced endothelial permeability by sequestering Src through mDia. Dev. Cell 14: 25–36.
27. Anisimov, A. et al. 2013. Vascular endothelial growth factorangiopoietin chimera with improved properties for therapeutic angiogenesis. Circulation 127: 424–434.
28. Koutroubakis, I.E. et al. 2006. Potential role of soluble
angiopoietin-2 and Tie-2 in patients with inflammatory
bowel disease. Eur. J. Clin. Invest. 36: 127–132.
29. Pousa, I.D., J. Maté & X.S. Mora. 2008. Role of vascular
endothelial growth factor and angiopoietin systems in serum
of Crohn’s disease patients. Inflamm Bowel Dis. 14: 61–67.
30. Lukasz, A. et al. 2008. Circulating angiopoietin-1 and
angiopoietin-2 in critically ill patients: development and
clinical application of two new immunoassays. Crit. Care
12: R94.
31. Kümpers, P. et al. 2008. Excess circulating angiopoietin-2 is a
strong predictor of mortality in critically ill medical patients.
Crit. Care 12: R147.
32. Kuroda, K., A. Sapadin, T. Shoji, et al. 2001. Altered expression of angiopoietins and Tie2 endothelium receptor in psoriasis. J. Invest. Dermatol. 116: 713–720.
33. Fearon, U. et al. 2003. Angiopoietins, growth factors, and
vascular morphology in early arthritis. J. Rheumatol. 30:
260–268.
34. Westra, J. et al. 2011. Angiopoietin-2 is highly correlated with
inflammation and disease activity in recent-onset rheumatoid arthritis and could be predictive for cardiovascular disease. Rheumatology (Oxford) 50: 665–673.
35. Scott, B.B. et al. 2002. Constitutive expression of
angiopoietin-1 and -2 and modulation of their expression
by inflammatory cytokines in rheumatoid arthritis synovial
fibroblasts. J. Rheumatol. 29: 230–239.
6
36. Bhandari, V. et al. 2006. Hyperoxia causes angiopoietin 2–
mediated acute lung injury and necrotic cell death. Nat. Med.
12: 1286–1293.
37. Bhandari, V. & J.A. Elias. 2007. The role of angiopoietin 2
in hyperoxia-induced acute lung injury. Cell Cycle 6: 1049–
1052.
38. Fiedler, U. et al. 2006. Angiopoietin-2 sensitizes endothelial
cells to TNF-␣ and has a crucial role in the induction of
inflammation. Nat. Med. 12: 235–239.
39. Fuxe, J. et al. 2010. Angiopoietin/Tie2 signaling transforms
capillaries into venules primed for leukocyte trafficking in
airway inflammation. Am. J. Pathol. 176: 2009–2018.
40. Venneri, M.A. et al. 2007. Identification of proangiogenic
TIE2-expressing monocytes (TEMs) in human peripheral
blood and cancer. Blood 109: 5276–5285.
41. Murdoch, C., S. Tazzyman, S. Webster & C.E. Lewis.
2007. Expression of Tie-2 by human monocytes and their
responses to angiopoietin-2. J. Immunol. 178: 7405–7411.
42. Lemieux, C. 2005. Angiopoietins can directly activate
endothelial cells and neutrophils to promote proinflammatory responses. Blood 105: 1523–1530.
43. Brkovic, A., M. Pelletier, D. Girard & M.G. Sirois. 2007.
Angiopoietin chemotactic activities on neutrophils are regulated by PI-3K activation. J. Leukocyte Biol. 81: 1093–
1101.
44. Haddad, L.E. & M.G. Sirois. 2014. Angiopoietin-1 upregulates de novo expression of IL-1 and Il1-Ra, and the exclusive release of Il1-Ra from human neutrophils. PLoS ONE 9:
e88980.
45. Garcı́a, S. et al. 2014. Tie2 signaling cooperates with TNF
to promote the pro-inflammatory activation of human
macrophages independently of macrophage functional phenotype. PLoS ONE 9: e82088.
46. Sinnathamby, T., T.Y. Jin, M.-É. Clavet-Lanthier, et al.
2015. VEGF and angiopoietins promote inflammatory cell
recruitment and mature blood vessel formation in murine
sponge/matrigel model. J. Cell Biochem. 116: 45–57.
47. Scholz, A. et al. 2011. Angiopoietin-2 promotes myeloid cell
infiltration in a 2 -integrin-dependent manner. Blood 118:
5050–5059.
48. Lewis, C.E. 2006. Distinct role of macrophages in different
tumor microenvironments. Cancer Res. 66: 605–612.
49. Coffelt, S.B. et al. 2010. Elusive identities and overlapping
phenotypes of proangiogenic myeloid cells in tumors. Am.
J. Pathol. 176: 1564–1576.
50. Mantovani, A., P. Allavena, A. Sica & F. Balkwill. 2008.
Cancer-related inflammation. Nature 454: 436–444.
51. Galdiero, M.R., C. Garlanda, S. Jaillon, et al. 2013. Tumor
associated macrophages and neutrophils in tumor progression. J. Cell. Physiol. 228: 1404–1412.
52. Coffelt, S. B. et al. 2010. Angiopoietin-2 regulates gene
expression in TIE2-expressing monocytes and augments
their inherent proangiogenic functions. Cancer Res. 70:
5270–5280.
53. Bergers, G. & D. Hanahan. 2008. Modes of resistance to
anti-angiogenic therapy. Nat. Rev. Cancer 8: 592–603.
54. Mazzieri, R. et al. 2011. Targeting the ANG2/TIE2
axis inhibits tumor growth and metastasis by impairing
C 2015 New York Academy of Sciences.
Ann. N.Y. Acad. Sci. xxxx (2015) 1–7
Scholz et al.
55.
56.
57.
58.
59.
60.
61.
62.
angiogenesis and disabling rebounds of proangiogenic
myeloid cells. Cancer Cell 19: 512–526.
Huang, H. et al. 2011. Specifically targeting angiopoietin2 inhibits angiogenesis, Tie2-expressing monocyte infiltration, and tumor growth. Clin. Cancer Res. 17: 1001–
1011.
Rigamonti, N. et al. 2014. Role of angiopoietin-2 in adaptive
tumor resistance to VEGF signaling blockade. Cell Rep. 8:
696–706.
Im, J.H. et al. 2012. G-CSF rescues tumor growth
and neo-angiogenesis during liver metastasis under host
angiopoietin-2 deficiency. Int. J. Cancer 132: 315–326.
López-Mejı́as, R. et al. 2013. Angiopoietin-2 serum levels
correlate with severity, early onset and cardiovascular disease
in patients with rheumatoid arthritis. Clin. Exp. Rheumatol.
31: 761–766.
Krausz, S. et al. 2012. Angiopoietin-2 promotes inflammatory activation of human macrophages and is essential for
murine experimental arthritis. Ann. Rheum. Dis. 71: 1402–
1410.
Kanakaraj, P. et al. 2012. Simultaneous targeting of TNF and
Ang2 with a novel bispecific antibody enhances efficacy in
an in vivo model of arthritis. MAbs 4: 600–613.
Hah, Y.-S. et al. 2013. Double-antiangiogenic protein DAAP
targeting vascular endothelial growth factor A and angiopoietins attenuates collagen-induced arthritis. Arthritis Res.
Ther. 15: R85.
Oikonomou, K.A. et al. 2011. Angiogenin, angiopoietin-1, angiopoietin-2, and endostatin serum levels in
inflammatory bowel disease. Inflamm. Bowel Dis. 17: 963–
970.
Angiopoietin-2 and inflammation
63. Ganta, V.C., W. Cromer, G.L. Mills & J. Traylor. 2010.
Angiopoietin-2 in experimental colitis. Inflamm. Bowel Dis.
16: 1029–1039.
64. David, S., A. Mukherjee, C.C. Ghosh et al. 2012.
Angiopoietin-2 may contribute to multiple organ dysfunction and death in sepsis. Crit. Care Med. 40: 3034–3041.
65. Mofarrahi, M. et al. 2008. Regulation of angiopoietin expression by bacterial lipopolysaccharide. Am. J. Physiol. Lung
Cell. Mol. Physiol. 294: L955–L963.
66. Ziegler, T. et al. 2013. Angiopoietin 2 mediates microvascular
and hemodynamic alterations in sepsis. J. Clin. Invest. 123:
3436–3445.
67. Kümpers, P. et al. 2011. The synthetic Tie2 agonist peptide
vasculotideprotects against vascular leakage and reducesmortality in murine abdominal sepsis. Crit. Care 15: R261.
68. Alfieri, A. et al. 2012. Angiopoietin-1 variant reduces LPSinduced microvascular dysfunction in a murine model of
sepsis. Crit. Care 16: R182.
69. Tzepi, I.-M. et al. 2012. Angiopoietin-2 enhances survival
in experimental sepsis induced by multidrug-resistant Pseudomonas aeruginosa. J. Pharmacol. Exp. Ther. 343: 278–287.
70. Coffelt, S.B. et al. 2011. Angiopoietin 2 stimulates TIE2expressing monocytes to suppress T cell activation and to
promote regulatory T cell expansion. J. Immunol. 186: 4183–
4190.
71. Seok, S.H. et al. 2013. Angiopoietin-1 elicits proinflammatory responses in monocytes and differentiating
macrophages. Mol. Cells 35: 550–556.
72. Kurniati, N.F. et al. 2013. Pleiotropic effects of angiopoietin2 deficiency do not protect mice against endotoxin-induced
acute kidney injury. Nephrol. Dial. Transplant. 28: 567–575.
C 2015 New York Academy of Sciences.
Ann. N.Y. Acad. Sci. xxxx (2015) 1–7
7