Vascular Biology |
From the Departments of Internal Medicine (Y.C., D.D.H., B.L.D.), Neurology (B.L.D.), and Pharmacology (D.D.H.), the Cardiovascular Center (D.D.H.), the Center on Aging (D.D.H.), and the Program in Gene Therapy (D.D.H., B.L.D.), University of Iowa College of Medicine, Iowa City; the Veterans Administration Medical Center (D.D.H.), Iowa City, Iowa; and the Department of Laboratory Medicine and Pathobiology (M.I.C.), The Toronto Hospital, University of Toronto, Toronto, Ontario, Canada.
Correspondence to Beverly L. Davidson, PhD, Roy J. Carver Associate Professor in Internal Medicine and Neurology, Director, Gene Transfer Vector Core, University of Iowa College of Medicine, Iowa City, IA 52242. E-mail beverly-davidson{at}uiowa.edu
| Abstract |
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Key Words: vascular cell adhesion molecule-1 coxsackie and adenovirus receptor adenovirus-mediated gene transfer atherosclerosis
| Introduction |
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Vascular endothelium is altered by atherosclerosis, with changes in a variety of intracellular and cell surface molecules.7 8 Because increased viral binding often translates into improved transduction efficiency,9 10 we considered candidate molecules on the surface of endothelium that could behave as surrogate receptors, allowing increased binding of adenovirus to the surface.
The receptor for adenovirus types 2 and 5 has been
identified as the coxsackie-adenovirus receptor
(CAR).11 12 13 14
The receptor is expressed most abundantly on human heart and mouse
liver and to varying degrees in other
tissues.11 12 13
In addition to the high-affinity receptor, the adenovirus
may15 or may
not9 16 require the
v-integrins as coreceptors.
Vascular cell adhesion molecule-1 (VCAM-1), which is not
expressed on normal endothelium, is expressed on the
endothelium of atherosclerotic
vessels.17 18 19
VCAM-1 functions as an adhesion receptor for leukocytes bearing very
late antigen 420 21
and may contribute to leukocyte recruitment at the early stages of
atherogenesis. Proatherogenic factors, including
hypercholesterolemia, cytokines (eg,
interleukin-1 and tumor necrosis factor-
), and advanced glycation
end products, induce the expression of
VCAM-1.17 18 22
Like CAR, VCAM-1 belongs to the immunoglobulin gene superfamily, and we observed that there is homology between VCAM-1 and CAR. Therefore, we speculated that augmented adenovirus-mediated gene transfer to the endothelium of atherosclerotic blood vessels5 may result in part from the surface expression of VCAM-1. In the present study, we tested the hypothesis that VCAM-1 could serve as an auxiliary receptor for adenovirus by comparing adenovirus infection and binding in VCAM-1+ and VCAM-1- cells.
| Methods |
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Adenovirus Infection of
VCAM-1+ and Parental NIH 3T3 Cells
VCAM-1+ and parental NIH
3T3 cells were plated at 15 000 to 20 000 cells per well on a 96-well
plate (Costar), which was pretreated with 2% gelatin (Sigma) to
enhance cell adherence. At 90% confluence (24 to 48 hours after
plating), triplicate wells were infected with recombinant adenovirus 5
(Ad5) containing the reporter ß-galactosidase gene (Ad5ßgal) at a
multiplicity of infection of 0, 5, 25, and 125 infectious units per
cell, in a volume of 40 µL per well. Two hours after incubation in a
37°C incubator, unbound virus was removed and replaced with 100 µL
of fresh DMEM containing 2% FBS. Twenty-four hours after infection,
cell lysates were prepared and assayed for ß-galactosidase activity
by using a chemiluminescent assay kit (Tropix), with total activity
normalized to protein content. Protein content was measured by using a
modified Lowry method with BSA as a standard (Dc protein assay kit,
Bio-Rad).
Histochemistry for ß-Galactosidase
VCAM-1+ and parental NIH
3T3 cells were infected with Ad5ßgal at a multiplicity of infection
of 0, 5, 25, and 125 infectious units per cell. Twenty-four hours after
infection, cells were assayed for ß-galactosidase expression by
5-bromo-4-chloro-3-indolyl-ß-D-galactopyranoside
histochemistry.25 Positive
cells were counted in 5 random fields at a magnification of x200 with
an inverted microscope. Values represent the mean±SE from 3
independent experiments, in each of which triplicates were used for
each sample.
Adenovirus Binding Detected by Southern
Blotting
VCAM-1+ and parental NIH
3T3 cells were plated on 60-mm plates. At confluence, the cells were
equilibrated to 4°C for 1 hour. At this low temperature, virus binds
cells through receptors but cannot enter the
cells.26 Fifty microliters of
Ad5ßgal (1x1012 particles
per milliliter) was added to each plate after first being pooled in
ice-cold DMEM/2% FBS. The plates were incubated at 4°C for 2 hours
for virus binding, followed by washing 3 times with ice-cold PBS and
isolation of viral DNA.27
Viral DNA was electrophoresed on a 1% agarose gel together with a
series of 2-fold increasing amounts of viral DNA standard. Southern
blotting with a probe specific for adenovirus was
performed,28 and the viral
band was quantified by densitometry with the use of Volume Trace Motif
version 1.21 (University of Iowa Image Analysis
Facility).
Competition of Viral Infection or Binding to
VCAM-1+ Cells by Soluble VCAM-1
Soluble VCAM-1 protein (sVCAM-1, human 7D form with
deleted transmembrane and cytoplasmic domains) was kindly provided by
Dr Roy Lobb (Biogen Inc, Cambridge,
Mass).29 sVCAM-1 was
incubated at a final concentration of 0.1, 1, or 10 µg/mL with
Ad5ßgal at a multiplicity of infection of 125 infectious units per
cell at 37°C for 15 minutes before infection (37°C for 2 hours) or
binding (on ice for 30 minutes) to VCAM-1+
and parental NIH 3T3 cells. Cells were washed twice with DMEM/2% FBS
after infection or binding to remove unbound virus. After 24 hours of
incubation at 37°C, cell lysate was prepared and assayed for
ß-galactosidase activity as described in the previous section. Three
independent experiments were performed with duplicates used for each
sample.
| Results |
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NIH 3T3 cells, which lack a detectable receptor for
adenovirus
fiber,13 33 34
were cotransfected with pVCAM-1 and pMC1neoPolyA. pVCAM-1 expresses
VCAM-1 on the cell surface under control of the RSV LTR promoter.
pMC1neoPolyA expresses the neomycin-resistance product, conferring
G418 resistance in transfected cells. FACS analysis indicated
that stable transfectants constitutively expressed VCAM-1 on the cell
surface
(Figure 2
). Mean fluorescence in
VCAM-1+ cells was
20-fold higher than
that found in parental NIH 3T3 cells. The
VCAM-1+ cells, like parental NIH 3T3 cells,
lack CAR expression, as detected by reverse transcriptionpolymerase
chain reaction (data not shown).
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Replication-deficient Ad5 containing the
Escherichia coli lacZ gene
driven by the RSV LTR promoter (Ad5ßgal) was used for comparison of
adenovirus infection in VCAM-1+ and
VCAM-1- NIH 3T3 cells. Cells were infected
with Ad5ßgal at 5, 25, or 125 infectious units per cell for 2 hours
at 37°C, followed by removal of unbound virus. Twenty-four hours
after infection, cell lysates were assayed for ß-galactosidase
activity, with total activity normalized to protein content. Total
ß-galactosidase activity was 6- to 10-fold higher in
VCAM-1+ than in parental NIH 3T3 cells
(0.21±0.02 versus 0.019±0.003 at 5 IU per cell, 0.99±0.10 versus
0.17±0.08 at 25 IU per cell, and 7.38±1.12 versus 0.81±0.11 at 125
IU per cell;
Figure 3
). Three independent
VCAM-1+ cell lines were tested and showed
similar results. This finding indicates augmented adenovirus infection
in VCAM-1+ cells.
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Histochemistry for ß-galactosidase activity was performed
to determine whether increased levels of gene transfer as measured by
enzyme activity were the result of the transduction of more cells. When
VCAM-1+ and parental NIH 3T3 cells were
infected with Ad5ßgal and then stained for ß-galactosidase
expression by
5-bromo-4-chloro-3-indolyl-ß-D-galactopyranoside
histochemistry,
5-fold more cells expressed ß-galactosidase in
VCAM-1+ than in parental NIH 3T3 cells
(Figure 4
).
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The results above are best explained by increased binding of
virus to the cells when VCAM-1 is present on the cell surface. To
test this possibility, cells were equilibrated at 4°C for 2 hours,
unbound virus was removed, and bound viral DNA was
isolated.26 The amount of DNA
bound was quantified by Southern
blotting.27 There was
3-fold more adenovirus DNA bound to
VCAM-1+ cells (2.7±0.8, n=3) than to
parental NIH 3T3 cells (1±0, n=3;
Figure 5
).
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If VCAM-1 acts as an auxiliary receptor for adenovirus, then
soluble VCAM-1 protein should compete with adenovirus to bind to
VCAM-1expressing cells. sVCAM-1 was incubated with Ad5ßgal, and the
mixture was added to VCAM-1+ and parental
NIH 3T3 cells. Cells were incubated either on ice (for binding) or at
37°C (for infection) and were washed to remove unbound virus. After
24 hours of incubation at 37°C, sVCAM-1 reduced ß-galactosidase
activity in the lysate of VCAM-1+ cells
(Figure 6
). In control experiments using BSA, instead of
sVCAM-1, BSA did not inhibit or augment ß-galactosidase activity
(data not shown). This finding supports the conclusion that VCAM-1
binds Ad5 in VCAM-1+
cells.
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| Discussion |
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The increase in absolute enzyme activity after Adßgal incubation with VCAM-1+ cells compared with the parental line was greater than the increase in histochemically positive cells and bound viral DNA. This difference could reflect the increased sensitivity of the activity assay compared with the other methods. However, competition with soluble VCAM-1 in VCAM-1+ cells was not complete, inasmuch as ß-galactosidase activity plateaued at levels significantly higher than levels in parental cells. Because 3 independent VCAM-1+ cell lines have been tested and showed similar results, it is unlikely that the augmentation in these cells resulted from an insertional event in the generation of stable cell lines that acts independently of VCAM-1. We cannot exclude an alternative explanation, ie, that VCAM-1 acts to augment transgene expression in addition to acting as a receptor. Nonetheless, the present study provides a mechanism to explain in part augmented adenovirus-mediated gene transfer to atherosclerotic blood vessels.5 6
CAR, the high-affinity receptor for
adenovirus,11 12 13
when introduced to CAR-lacking cells such as Chinese hamster ovary
cells and NIH 3T3 cells, increased adenovirus-mediated gene transfer by
2 to 3 orders of magnitude.11
The first immunoglobulin domain is sufficient for the
adenovirus-binding activity of intact
CAR.30 31 In
comparison, we found that augmentation in
VCAM-1+ cells is
3-fold for viral binding
and 6- to 10-fold for transgene expression. The modest increase can
best be explained by the fact that VCAM-1 is only moderately homologous
to CAR. We have been unable to demonstrate direct binding of Ad5 to
pure sVCAM-1 protein using ELISA-based assays. As such, binding
affinities of Ad5 fiber to VCAM-1 could be substantially lower than for
CAR.
Like VCAM-1, other immunoglobulin superfamily members also
share some homology with CAR. We studied VCAM-1 because VCAM-1 is
expressed on the surface of endothelium of
atherosclerotic vessels. It is possible that other molecules may also
mediate weak binding of adenovirus. Whether a molecule mediates virus
binding depends on the context of the molecule. The major
histocompatibility complex class I
2 domain
is a high-affinity receptor for Ad2 and
Ad5.35 36 When
expressed in hamster cells, however, it is not a high-affinity receptor
for Ad5.37
Factors other than VCAM-1 also may contribute to the augmentation of adenovirus-mediated gene transfer in the endothelium of atherosclerotic vessels. The present study, which demonstrated the effect of VCAM-1 on adenovirus-mediated gene transfer to NIH 3T3 cells, provides an example of a cell surface molecule that is moderately homologous to CAR and that can mediate the binding of adenovirus. Our results suggest that auxiliary low-affinity receptors may be used to direct or improve adenovirus-mediated gene transfer to CAR-negative cells.
| Acknowledgments |
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| Footnotes |
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Received May 15, 2000; accepted November 8, 2000.
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