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1 From the Laboratory for Surgical Research, Childrens Hospital, Boston, Massachusetts; the 2 Department of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts; the 3 Schepens Eye Research Institute, Boston, Massachusetts; and 4 Brigham and Womens Hospital, Boston, Massachusetts.
| Abstract |
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METHODS. The expression of ICAM-1, vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and P-selectin on human brain capillary endothelial cell monolayers exposed to VEGF was quantitated by immunoassay. The effect of VEGF on retinal vascular ICAM-1 expression was determined in ICAM-1 immunofluorescence studies of retinal flat-mounts and in RNase protection assays.
RESULTS. VEGF increased capillary endothelial cell ICAM-1 levels in a dose- and time-dependent manner (624 hours, plateau after 6 hours; EC50, 25 ng/ml). VEGF failed to alter E-selectin, P-selectin, or VCAM-1 levels under the conditions tested. Intravitreal injections of pathophysiologically relevant concentrations of VEGF increased ICAM-1 protein and mRNA levels in the retinal vasculature.
CONCLUSIONS. VEGF increases retinal vascular ICAM-1 expression. VEGF-induced increases in ICAM-1 may promote retinal leukostasis in diabetic eyes.
| Introduction |
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Fluorescein angiographic studies in humans have demonstrated that some diabetic retinal ischemia is reversible.5 However, in most eyes a large proportion of the diabetic retinal ischemia that develops is irreversible and results from the formation of acellular capillaries.6 Diabetic retinal leukostasis7 may be operative in the development of both types of retinal ischemia. Increased numbers of monocytes and granulocytes have been demonstrated to occupy the lumens of retinal capillaries and postcapillary venules in diabetic rats, with some leukocytes lying adjacent to dying endothelial cells.8 Increased numbers of granulocytes have also been demonstrated in the retinas of diabetic humans.9 One potential mechanism mediating this phenomenon is the increased adherence of leukocytes to the vascular endothelium. Intercellular adhesion molecule-1 (ICAM-1) mediates the adhesion of neutrophils and monocytes to vascular endothelium, and increased ICAM-1 levels have been found in the retinal vasculature of human diabetics.9 In addition, human diabetic neutrophils have been shown to be more prone to upregulate ICAM-1 ligand CD18.10
Because of these observations, studies were undertaken to characterize the effect of VEGF on the expression of endothelial cell adhesion molecules in vitro and in retina in vivo.
| Methods |
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Adhesion Molecule Quantitation via EIA
Endothelial cell adhesion molecule levels were measured by EIA in
HBCECs. HBCECs were grown to confluence in 96-well plates and treated
with serum-free medium with or without various concentrations of
recombinant human VEGF165 (a gift of
Napoleone Ferrara, Genentech, South San Francisco, CA; endotoxin < 0.03 EU/ml) or recombinant human tumor necrosis factor-
(TNF-
;
R&D Systems, Minneapolis MN; positive control) for 24 hours. Cell
surface EIAs were completed using mouse anti-human monoclonal
antibodies against ICAM-1 (Ab HU 5/3), VCAM-11 (Ab E1/6), and
E-selectin (Ab H18/7).11
The EIAs were carried out by
incubating monolayers first with saturating concentrations of specific
monoclonal antibodies against the target molecule, followed by
biotinylated goat anti-mouse IgG, and finally with
streptavidin-alkaline phosphatase. The surface expression of each
protein was quantified spectrophotometrically, reading the optical
density of the wells (410 nm) 15 to 60 minutes after the addition of a
chromogen (p-nitrophenylphosphate), as described
previously.11
The data were normalized against protein
concentrations and expressed as mean ± SEM.
ICAM-1 Immunofluorescence
Animals were cared for in accordance with the ARVO Resolution on
the Use of Animals in Ophthalmic and Vision Research.. The experiments
were approved by the Schepens Eye Research Institute Animal Care and
Use Committee. Normal C57B/6 mice (male, 6 to 8 weeks old) were
examined for retinal ICAM-1expression after intravitreal injections of
recombinant murine VEGF164 (R&D Systems,
Minneapolis, MN) or vehicle alone (n = 6;
endotoxin < 0.1 EU/mg). After anesthesia with a mixture of
ketamine (Ketalar; ParkeDavis, Morris Plains, NJ; [150
mg/kg]) and xylazine (Rompun; HarverLockhart, Morris Plains, NJ;
[60 mg/kg]), 2 µl concentrated VEGF or phosphate-buffered
saline (PBS) solvent was injected slowly into the vitreous cavity to
attain a final concentration of 100 ng/ml VEGF. After 24 hours, the
animals were killed by cervical dislocation, and the eyes were
enucleated and immersion-fixed in 0.5% (wt/vol) paraformaldehyde in
0.1 M PBS (pH 7.4) for 30 minutes. The intact retinas were dissected
out under a surgical microscope as previously described12
and further fixed in 70% ethanol at room temperature for an additional
30 minutes. The retinas were washed in 0.1 M PBS (pH 7.4) with 0.1%
(vol/vol) Triton X-100 for 30 minutes and then incubated with
biotinylated anti-mouse ICAM-1 (1:50 diluted in 0.1 M PBS containing
1% [wt/vol] bovine serum albumin, BSA) overnight at
4oC. After washing at room temperature with PBS
containing 1% BSA, the retinas were further incubated with
streptavidinfluorescein isothiocyanate (1:100 diluted in PBS
containing 1% BSA; Amersham, Arlington Heights, IL) for 2
hours at room temperature. An isotype-matched IgG was used as a
negative control. The retinas were co-stained with 2 ng/ml ethidium
bromide for 10 seconds and mounted. Endothelial cell nuclei were
counted per unit length of vessel.
RNase Protection Assay for ICAM-1
The experiments were approved by Childrens Hospital Animal Care
and Use Committee. Male SpragueDawley rats weighing 200 to 250 g
were anesthetized with 0.1 mg/kg sodium amobarbital. Pupils were
dilated with 0.5% tropicamide and 2.5% phenylephrine hydrochloride
(Alcon, Humancao, Puerto Rico). Murine VEGF165 or
human TNF-
, in a total volume of 5 µl PBS, was injected into the
right vitreous at a site 1 mm posterior to the limbus using a Hamilton
syringe with a 30-gauge needle. The final concentrations of VEGF and
TNF-
were calculated to be 100 and 10 ng/ml, respectively. The
contralateral control eyes received 5-µl PBS injections. The eyes
were enucleated 2.5 hours later. The rats were euthanatized with an
anesthetic overdose followed by CO2 incubation.
All injections were done under direct observation using a surgical
microscope. Any eyes that had damage to the lens or retina were not
used for analyses. The retinas were gently dissected free and cut at
the disc. The tissue was placed in an Eppendorf tube, snap-frozen in
liquid nitrogen, and stored at -80oC. The
retinas were homogenized in 1 ml RNAzol (Biotecx Laboratories, Houston,
TX) at 4oC and prepared for RNase protection
assays.
The ICAM-1 riboprobe was produced by subcloning the coding sequence of the rat ICAM-1 cDNA into the EcoRIBamHI site of the pBluescript II KS vector. Transcription by T7 RNA polymerase after linearization by EcoRI resulted in a probe of 225 nucleotides (nt). This probe protects a 166-nt fragment of ICAM-1. All samples were simultaneously hybridized with an 18S riboprobe (Ambion, Austin, TX) to normalize for variations in loading and recovery of RNA. Full-length protection of this probe results in an 80-nt fragment. The assay was performed as previously described.13 Ten micrograms of total cellular RNA was hybridized with 32P-labeled antisense ICAM-1 and 18S riboprobes (200,000 cpm of each) overnight at 42oC in 30 µl hybridization buffer. Hybridized RNA was digested with nuclease P1 (20 µg/ml) and RNase T1 (2 µg/ml) for 1 hour at 25oC in 300 µl digestion buffer. Digestions were terminated by the addition of 20 µl of 10% sodium dodecyl sulfate and 50 µg proteinase K for 15 minutes at 37oC. After phenol/chloroform extraction and ethanol precipitation, the protected fragments were resolved on 6% polyacrylamide, 7 M urea gels and quantified with a PhosphorImager (Molecular Dynamics, Sunnyvale, CA).
Statistics
Significance was calculated using the paired Students
t-test. P < 0.05 was deemed significant.
| Results |
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(positive control) produced significant increases in VCAM-1 (2.4 ± 0.1fold, n = 8, P < 0.05 versus
control), E-selectin (17.5 ± 1.3fold, n = 8,
P < 0.05 versus control), P-selectin (10.7 ±
1.2fold, n = 8, P < 0.01, data not
shown), and ICAM-1 (2.5 ± 0.2fold, n = 8,
P < 0.05 versus control, data not shown).
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induced a 3.1 ± 0.8fold
increase of ICAM-1 mRNA (n = 5, P <
0.05).
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| Discussion |
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In addition to VEGF, other pathophysiologically relevant stimuli may
increase ICAM-1 in the diabetic retinal vasculature. Factors relevant
to diabetes, such as the cytokine TNF-
,15
have been
shown to increase ICAM-1 in other systems. TNF-
levels are increased
in the serum of diabetics,16
and as confirmed by our data,
TNF-
is a potent inducer of endothelial cell ICAM-1 expression. As
in other tissues, the induction of ICAM-1 may be causal for the
leukostasis observed in the diabetic retina. In the brain, ICAM-1
mediates leukocyte adhesion to postcapillary venules and is associated
with capillary occlusion and vascular endothelial cell
damage.17
Retinal VEGF levels appear to be elevated early in diabetes, before there is any histopathologic evidence of retinal ischemia.18 This makes it more likely that endogenous VEGF serves to increase retinal vascular ICAM-1 in vivo. We hypothesize that VEGF, in whole or in part, increases retinal ICAM-1 expression and that ICAM-1 mediates the binding of leukocytes to the vasculature, accounting for the early reversible phase of retinal ischemia. With time, the retinal leukostasis becomes chronic and widespread, hastening the death of endothelial cells and pericytes. With the development of more widespread ischemia, retinal VEGF levels are further upregulated, triggering greater ischemia and ultimately neovascularization.
This hypothetical sequence of events remains to be proven. Proof requires the specific inhibition of VEGF and ICAM-1 in the diabetic retina and the direct monitoring of its functional and anatomic consequences. These studies are under way.
| Footnotes |
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5 These authors contributed equally to the work presented here and should therefore be regarded as equivalent senior authors. ![]()
Supported by the National Eye Institute (APA), the Massachusetts Lions, and the Roberta W. Siegel Fund (APA).
Presented in part at the 1997 and 1998 annual meetings of the Association for Research in Vision and Ophthalmology, Ft. Lauderdale, Florida, May 1997 and May 1998.
Submitted for publication June 30, 1998; revised January 8, 1999; accepted February 16, 1999.
Proprietary interest category: N.
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