(Investigative Ophthalmology and Visual Science. 2001;42:3325-3330.)
© 2001
by The Association for Research in Vision and Ophthalmology, Inc.
Angiostatin Inhibits Pathological but Not Physiological Retinal Angiogenesis
Tom A. Drixler1,2,
Inne H. M. Borel Rinkes1,2,
Ewan D. Ritchie1,
Frits W. Treffers3,
Theo J. M. V. van Vroonhoven1,
Martijn F. B. G. Gebbink2 and
Emile E. Voest2
1 From the Department of Surgery; the
2 Laboratory of Medical Oncology, Division of Medical Oncology; and the
3 Donders Institute of Ophthalmology, University Medical Center, Utrecht, The Netherlands.
 |
Abstract
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PURPOSE. Antiangiogenic treatment is a promising new therapy for
angiogenesis-dependent diseases. In the current study, the biologic
effects on pathologic and physiological angiogenesis in the retina of
angiostatin, a very potent angiogenesis inhibitor were determined. In
addition, the effects of angiostatin on the growth and development of
newborn mice were examined.
METHODS. Oxygen-induced retinopathy was induced by subjecting mice postnatal day
(P)7 to hyperoxic conditions (5 days) followed by normoxic conditions
(relative hypoxia). Mice were treated with angiostatin (intravitreal or
systemic). Retinal blood vessels were visualized by fluorescein
angiography. Retinal neovascularization was assessed by counting
intravitreal endothelial cell nuclei. Growth and organogenesis were
determined between P0 and P14.
RESULTS. Relative hypoxia resulted in intravitreal proliferation of retinal
blood vessels. However, proliferation was inhibited completely by
systemic administration of angiostatin without affecting normal retinal
vascularization. After intravitreal injection of angiostatin,
pathologic proliferation of the retinal blood vessels was impaired by
62%. Neither systemic nor intravitreal treatment impaired the
development or growth of organs throughout the body.
CONCLUSIONS. Angiostatin inhibits oxygen-induced intravitreal pathologic retinal
angiogenesis without affecting the development of physiological retinal
vascularization, development, and growth of newborn mice. Therefore,
antiangiogenic treatment may be a useful tool in the treatment of
proliferative retinopathies.
 |
Introduction
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Retinopathy of prematurity (ROP) is a disease associated
with visual impairment as a result of excessive retinal
neovascularization. This pathologic retinal neovascularization is
initiated when premature infants are exposed to high concentrations of
oxygen. The mechanism underlying retinal neovascularization is
considered to be mediated by hyperoxic damage to immature retinal blood
vessels, resulting in retinal ischemia, hypoxia, and a compensatory
induction of new, leaky blood vessels, often resulting in retinal
detachment and loss of vision. Several growth factors have been
implicated in ischemia-induced retinal neovascularization. Vascular
endothelial growth factor (VEGF), a specific endothelial cell mitogen
and chemotactic factor, is upregulated by hypoxia.1
2
3
In
a mouse model of oxygen-induced retinopathy VEGF mRNA is upregulated in
the inner nuclear layer of the retina. Furthermore, it was demonstrated
that VEGF acts as a survival factor for newly formed retinal blood
vessels by preventing endothelial cell apoptosis. Although VEGF is an
important angiogenic factor, it is likely that additional angiogenic
factors play a role as well.4
5
6
7
8
Because of the debilitating effects of retinal neovascularization,
several antiangiogenic treatment strategies have been explored in
animals.9
10
11
12
13
14
Vitamin E has been evaluated in early
randomized trials.15
The currently used treatment
modalities in patients include laser coagulation and
cryotherapy.16
17
Limited information is available on the effects of antiangiogenic
agents on physiological angiogenesis. However, this is of major
importance in considering the use of antiangiogenic drugs in the
treatment of ROP. It has been shown that TNP-470, a potent inhibitor of
angiogenesis, impairs fetal development when administered during
pregnancy.18
Thalidomide is also thought to have this
effect.19
Angiostatin, a novel and very potent inhibitor
of angiogenesis, has predominantly been studied in murine tumor models,
in which it was shown to inhibit the growth of a variety of
tumors.20
21
22
23
Although its mechanism of action is
presently unclear, it does not seem to involve the neutralization of a
specific angiogenic factor.
In the present study the effects of angiostatin were evaluated in an
oxygen-induced retinopathy model in newborn mice. The objectives of
this study were to determine the effects of systemic and local
treatment with angiostatin on pathologic retinal neovascularization and
to evaluate the effects of angiostatin on the development of neonatal
mice. In this report we provide evidence that treatment with human
angiostatin completely abolished pathologic angiogenesis in the retina
without affecting normal retinal vessel development and without
affecting the normal growth of newborn mice.
 |
Methods
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Purification of Human Angiostatin
Human angiostatin was generated as described by OReilly et
al.,20
with several modifications. Briefly, recovered
outdated human plasma was diluted 2:1 with PBS, supplemented with 3 mM
EDTA filtered (0.1 mm) at 37°C. The plasma was then applied to a
lysine-Sepharose column (Pharmacia & Upjohn, Uppsala, Sweden) at room
temperature. After washing the column with 0.5 M phosphate buffer,
plasminogen was eluted with 0.2 M prewarmed (37°C)
-aminocaproic
acid (
-ACA) at pH 7.4. SDS-polyacrylamide gel electrophoresis
(SDS-PAGE) of the eluant revealed one apparent band of 92 kDa,
corresponding to plasminogen. The eluant was dialyzed against demiwater
(molecular weight cut off [MWCO]: 68000; >4 x
107 dilution; 4°C) in dialysis tubing
(Spectra/Por; Spectrum Europe, Breda, The Netherlands) followed by
proteolytic digestion (12 hours; 37°C; 120 rpm) with porcine
pancreatic elastase in a concentration of 0.8 U/mg plasminogen
(Calbiochem, San Diego, CA), using a shaker (37°C; 120 rpm;
overnight). Next, the solution was applied to a
lysine-Sepharose column that had been equilibrated with a salt solution
(pH 7.4; 0.5 M NaCl, 0.2 M
-ACA, 0.03 M
NaH2PO4, 0.02 M
NaN3, and 0.1% Triton X-100). The column was
then re-equilibrated with 30 mM phosphate buffer at pH 7.4. Finally,
angiostatin was eluted with 0.2 mM
-ACA and dialyzed against
demiwater. SDS-PAGE revealed three distinct bands of approximately 40
kDa, 42 kDa, and 45 kDa, resembling the triplet first described by
OReilly et al.20
Antiangiogenic Activity of Human Angiostatin
To demonstrate antiangiogenic activity of the purified human
angiostatin in mice, the mouse cornea; neovascularization assay was
used. Briefly, a pellet containing basic fibroblast growth factor was
inserted into the cornea of 6- to 8-week-old BALB/c mice. Mice were
treated with subcutaneous injections of increasing concentrations of
angiostatin (0.5, 5, and 50 mg/kb body weight, twice daily). A
dose-dependent inhibition of corneal neovascularization was
observed.24
A dose of 50 mg/kg body weight administered in
twice-daily injections was considered an effective treatment schedule
and was used in the oxygen-induced retinopathy model.
Mouse Model of Oxygen-Induced Retinopathy
Animal experiments were approved by and performed according to
the guidelines of the Committee of Experimental Animals and adhered to
the ARVO Statement for the Use of Animals in Ophthalmic and Vision
Research. The mouse model of oxygen-induced retinopathy has
been described previously and has been a reproducible model of retinal
angiogenesis.25
Briefly, postnatal day (P)7 C57/Bl mice,
obtained from breeding colonies maintained at the Common Animal
Laboratory (GDL; Utrecht, The Netherlands) together with their nursing
mothers were exposed to hyperoxic conditions in an incubator
continuously for 5 days (75% O2 1.5 l/min; <300
lux of 12-hour cyclic broad-spectrum light; 23 ± 2°C). All
litters and nursing mothers survived the incubation in 75% oxygen. As
soon as mother and litter had returned (P12) to normoxic conditions
(room air), treatment was initiated. In the systemic treatment group
twice-daily subcutaneous injections with angiostatin (50 mg/kg body
weight; n = 18) or saline (n = 14) were begun and
continued until P17. The local-treatment groups were subjected to a
single intravitreal injection (1 µl) of angiostatin (n =
6; 60 µg angiostatin/1.0 µl PBS) or PBS (1.0 µl; n =
6) at P12. Furthermore, a control group was included that received an
intravitreal injection only (trauma group; n = 6) without
injection of fluid. In the local-treatment groups, the transcorneal
puncture (in the fourth quadrant of the cornea) was performed by a
32-gauge needle (Hamilton, Reno, NV). The needle penetrated the vitreal
corpus without damaging the lens.
Retinal Fluorescein Angiogram
Mice were deeply anesthetized, a median laparotomy was
performed, and the portal vein was exposed. One milliliter PBS
containing 25 mg 2 x 106 molecular weight
fluorescein isothiocyanate-dextran (FD-2000S; Sigma-Aldrich,
Zwijndrecht, The Netherlands) dye was injected intraportally. Eyes were
subsequently removed and fixed in 4% (wt/vol) formaldehyde for 24
hours. The cornea was removed, the sclera was cut sagittally
(bilateral, ventral, and caudal), and the eye was exposed and mounted
on a glass slide. Fluorescent micrographs were taken.
Quantification of Neovascular Proliferative Retinopathy
At P17 the eyes of mice were enucleated and fixed in 4%
(wt/vol) formaldehyde for 24 hours and embedded in paraffin. Serial
sagittal sections (6 µm) of whole eyes were stained with hematoxylin.
Fifteen consecutive sections per eye (30 per mouse) were used to count
endothelial cell nuclei that were located on the vitreal side of the
internal limiting membrane. The mean number of nuclei per eye was used
in subsequent comparisons.
Effects of Angiostatin on Normal Neonatal Development
To exclude inhibitory side effects of angiostatin on normal
vascular development and growth, neonates not subjected to hyperoxia
were subjected to systemic treatment of angiostatin (50 mg/kg body
weight twice daily) from P0 to P14. All animals were monitored for gain
of body weight and general health (toxicity). Tail length and organ
weight (liver, kidney, spleen, heart) were used as indicators of
vasculogenesis. Litters treated with saline served as the control for
these experiments.
Statistical Analysis
Data were expressed as mean ± SEM unless otherwise stated.
The significance of differences among groups was determined by the
unpaired Students t-test. P < 0.05 was
considered to be statistically significant.
 |
Results
|
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Effect of Angiostatin on Pathologic Retinal Neovascularization
Inhibition of oxygen-induced pathologic retinal neovascularization
by systemic administration of angiostatin and locally administered
angiostatin is shown in Figure 1
. In normal neonates, few intravitreal endothelial nuclei (IEN)
were present (0.35 ± 0.07; Fig. 2a ). In mice subjected to hyperoxic conditions, pathologic angiogenesis
was represented by the penetration of newly formed endothelial nuclei
(tufts) into corpus vitreous (21.9 ± 0.77 IEN; Fig. 2b
).
Functionality (i.e., perfusion) of these newly formed pathologic blood
vessels was demonstrated by the presence of red blood cells in the
lumen of the tufts (Fig. 2c) . Systemic administration of angiostatin
completely suppressed oxygen-induced retinal neovascularization
(0.54 ± 0.06 IEN) compared with mice subjected to hyperoxic
conditions only (P < 0.001). The number of IEN was not
different between normal neonates and neonates subjected to hyperoxia
subsequently treated with angiostatin (not significant;
P = 0.06). A single intravitreal injection of
angiostatin (±60 µg/1.0 µl PBS) at P12 resulted in a 62.9%
reduction of IEN (7.65 ± 0.53) compared with control animals
treated by puncture alone (trauma group; IEN count, 20.61 ± 1.06;
P < 0.0001) and a 62.5% reduction compared with
control animals subjected to intravitreal injection of PBS (IEN,
20.39 ± 0.89; P < 0.0001). Trauma of the
vitreous body, either by puncture or by injection of PBS, did not
reduce the number of endothelial nuclei when compared with mice treated
with systemic PBS (Fig. 1)
.

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Figure 1. Inhibition of oxygen-induced pathologic retinal neovascularization by
systemic administration of angiostatin (left) and
locally administered angiostatin (right). Total number
of vascular nuclei, extending from the internal limiting membrane into
the vitreous was counted. Bars represent mean number of nuclei ±
SE (*P < 0.001). PBS, twice-daily subcutaneous
injection with 100 µl PBS; AS, twice-daily injection with 50 µg/kg
body weight AS in 100 µl PBS; Trauma, transcorneal puncture with a
32-gauge needle followed by injection of 60 µg angiostatin in 1.0
µl PBS.
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Figure 2. Comparison of retinas of the normal (a), oxygen-induced
retinopathy (b, c), and oxygen-induced
retinopathy, angiostatin-treated (d) groups 17 days after
birth. Newly formed intravitreal pathologic blood vessels, so-called
tufts (b, d; arrowheads), were in
contact with preexisting normal intraretinal blood vessels. Perfusion
of new blood vessels (c) is demonstrated by the presence of
intraluminal erythrocytes (higher magnification of b).
Hematoxylin and eosin; magnification, (a, b,
d) x100; (c) x400.
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Effect of Angiostatin on the Physiological Development of the
Retinal Vasculature
To determine whether angiostatin had an effect on normal retinal
vessel development, neonates were treated with twice daily subcutaneous
injections of angiostatin for 14 days (P0P14) at a dose of 100 mg/kg
· d. Vessel development was analyzed by retinal fluorescein
angiograms (tortuosity, vessel dilatation, leakiness, hemorrhages). No
qualitative differences were observed between angiograms of mice
treated with PBS and those treated with angiostatin (Figs. 3a
3b)
. Retinal cryosections stained with hematoxylin revealed no
morphologic differences between either group (i.e., no IEN in either
group and no cellular changes in the ganglion cell layer).

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Figure 3. Effect of angiostatin on the physiological development of the retinal
vasculature. Comparison of FITC-dextranperfused retinas of normal
mice and mice supplemented with twice-daily administration of
angiostatin. Normal mice (a) show large superficial radial
vessels (arrows) from which collateral vessels have
developed. In mice treated with angiostatin (b), no
differences in development of superficial radial or collateral vessels
(arrows) were observed. No differences in structure
(tortuosity, vessel dilatation) between both groups were observed, and
no hemorrhages were present. Original magnification, x100.
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Effect of Angiostatin on Growth and Development of Neonatal Mice
In addition to the analysis of the retinal vasculature, other
parameters of development were analyzed. Neonates were treated from P0
until P14 as described. Treatment was well tolerated and did not result
in any obvious toxicity. There was no difference in weight gain (Fig. 4a)
. As a general marker of vascular development, tail length was used.
The tail development was identical for the angiostatin- and PBS-treated
groups (Fig. 4b)
. Angiostatin treatment did not affect murine organ
development, as judged by weight (Figs. 4c
4d
4e)
.

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Figure 4. Effects of angiostatin on normal murine growth and development: body
weight (a) and tail length (b) were measured from
birth until P14. The development of vascularized organs, such as liver,
kidney, and spleen, was evaluated by measuring wet liver weight
(c), kidney weight (d), and spleen weight
(e) at P14. No significant differences were observed between
control groups and angiostatin-treated groups. Bars, mean ± SE.
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 |
Discussion
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Angiostatin is one of the most promising antiangiogenic agents
available.20
In the present study we demonstrate that
systemic administration of angiostatin completely prevents retinal
neovascularization in a mouse model of oxygen-induced retinopathy,
without affecting physiological angiogenesis. The development of the
normal retinal vasculature, tail length, and gain in body weight were
not different between control and angiostatin-treated animals. In
contrast, even a single intraocular injection with angiostatin had a
substantial inhibitory effect on retinal neovascularization.
ROP is a debilitating disease for which no effective treatment is
available. Because the disease is associated with exposure to highly
concentrated oxygen, it has been suggested that reactive oxygen
intermediates may play a role in its pathogenesis. We have recently
shown that reactive oxygen intermediates (both hydrogen peroxide and
superoxide) induce the production of VEGF in retinal pigment epithelial
cells, both in vitro and in vivo.26
Based on randomized
clinical trials, it has been advocated that vitamin E, a naturally
occurring antioxidant, should be given as a prophylactic agent to a
high-risk population of preterm infants.15
The rapidly increasing knowledge on angiogenesis and the
characterization of a large number of antiangiogenic agents has opened
new avenues for the treatment of diseases associated with retinal
neovascularization, such as ROP. Currently, more than 20 antiangiogenic
agents are tested in clinical trials.27
However, with the
exception of one TNP-470 phase I trial, all these are tested in adults.
In contrast with growing children, the majority of endothelial cells in
adults are quiescent.28
Because inhibitors of angiogenesis
specifically inhibit proliferating endothelial cells, it could be
speculated that antiangiogenic treatment may adversely affect normal
vascular development as well. Contradictory to this hypothesis, we
demonstrate in this study that systemic administration of angiostatin
markedly suppressed excessive retinal neovascularization in newborn
mice, without any apparent adverse effect on the development of blood
vessels or organs in the newborn mice during the first 2 weeks of life.
The mechanism(s) responsible for the antiangiogenic properties of
angiostatin are presently unknown. However, some investigators have
hypothesized about its mechanisms. Stack et al.29
suggest
that tissue plasminogen activator (t-PA), when bound to angiostatin,
cannot participate in a ternary complex formation between t-PA,
plasminogen, and matrix protein. This results in the inhibition of
plasminogen activation and a reduced cellular migration and invasion.
Moser et al.30
attribute its antiangiogenic effects to
binding to the
- and ß-subunits of adenosine triphosphate
(ATP)-synthase on the cell surface of endothelial cells, resulting in
the downregulation of endothelial cell proliferation and migration.
Lucas et al.31
claim that the antiangiogenic activity of
angiostatin can be ascribed to its apoptotic effect on endothelial
cells. However, no consensus has been reached, so far. Obviously, the
mechanism of angiostatin requires further investigation.
Initially, we intended to avoid systemic administration of
angiostatin and used intravitreal injections. This approach was
reported to be successful in studies in which soluble VEGF receptor
chimeric proteins and VEGF antisense oligonucleotides were
used.9
Although it was clearly shown that inhibition of
VEGF inhibited angiogenesis over control intravitreal injections, no
comparison was made with the number of retinal vessels in uninjected
eyes. In our study, a single intravitreal injection with angiostatin
significantly inhibited angiogenesis, thereby confirming the efficacy
of such an approach. Its inhibitory effect on retinal
neovascularization is not as high as that obtained with systemic
administration of angiostatin: 62.9% and 96.9% inhibition,
respectively. The strong inhibition of a single intravitreal injection
with angiostatin may be explained by the composition of the vitreous.
This may act as a slow-release compartment.
In the current investigation, we did not perform pharmacokinetic
studies. This prohibits our drawing conclusions on the optimal route of
administration. Control experiments demonstrated that a limited trauma
in the vitreous did not contribute to the antiangiogenic
effect. Ophthalmologists have used vitrectomies as a treatment
modality in patients with ROP and proliferative diabetic retinopathy.
This procedure temporarily improves the vascular abnormalities in the
retina,32
but long-term effects are limited. The mechanism
by which this manipulation of the vitreous inhibits angiogenesis has
not been elucidated. However, such a procedure can hardly be called a
limited trauma. Therefore, these empiric clinical data are not in
contradiction to our control experiments, in which we used a simple
intravitreal needle puncture.
In summary, the present study suggests that angiostatin may be
used safely to treat diseases associated with retinal angiogenesis.
That there were no adverse effects on normal development in the newborn
mice indicates that angiostatin may be used in cases of ROP. Obviously,
further toxicity studies in newborn primates are required to support
clinical testing.
 |
Acknowledgements
|
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The authors thank Colinda Aarsman for expert technical assistance.
 |
Footnotes
|
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Supported by the Fisher Foundation (EEV), the Wijnand M. Pon Foundation
(TJMVvV), Grant WS 96-29 from the Netherlands Digestive Diseases
Foundation (EEV, IHMBR), and Grant 99-2114 from the Dutch Cancer
Society (EEV, MFBGG).
Submitted for publication November 10, 2000; revised June 8, 2001;
accepted July 10, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Emile E. Voest, Department of Internal Medicine,
Division of Medical Oncology, Laboratory of Medical Oncology,
University Medical Center, PO Box 85500, 3508 GA Utrecht, The
Netherlands. e.e.voest{at}digd.azu.nl
 |
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