
[Cancer Research 60, 3123-3126, June 15, 2000]
© 2000 American Association for Cancer Research
Inhibition of Angiogenesis and Induction of Apoptosis Are Involved in E1A-mediated Bystander Effect and Tumor Suppression1
Ruping Shao,
Weiya Xia and
Mien-Chie Hung2
Department of Molecular and Cellular Oncology, Breast Cancer Basic Research Program, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030
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ABSTRACT
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Adenovirus type 5 E1A has been implicated in mediation of tumor
suppression. Preclinical gene therapy studies have additionally shown
that complete growth suppression can be achieved by incomplete transfer
of E1A into tumors, suggesting that a bystander effect may also be
associated with E1A. In this study, we investigated the E1A-mediated
bystander effect and the mechanisms that may be associated with it. By
s.c. inoculating nude mice with a mixture of E1A transfectants and
parental cells, we found that the E1A transfectants exhibited a
bystander effect on inhibition of tumor growth. We further showed that
E1A mediated suppression of angiogenesis and induction of apoptosis in
the tumors, likely contributing to the bystander effect. In addition,
coculture of E1A transfectants and parental cells in a Transwell unit
led to growth retardation and apoptosis mediated by the supernatant in
the parental cells, indicating that a secreted factor may also
contribute to the bystander effect. Taken together, our results
suggested that E1A mediates a bystander effect on tumor suppression by
inhibiting angiogenesis and inducing apoptosis.
 |
Introduction
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The bystander effect was originally identified through a study in
which cancer cells modified by the herpes simplex virus thymidine
kinase gene caused the death of nearby unmodified cancer cells after
administration of the drug ganciclovir (1)
. In this case,
gap junctional intercellular communication was the mechanism
responsible for bystander killing (2)
. However, recent
studies have shown that tumor cells transfected with genes such as
IFN-ß, interleukin 4, and p53 exhibit bystander effects with
alternative mechanisms, including enhancement of the immune response
and suppression of angiogenesis (3, 4, 5)
, which has been
shown to play an important role in tumor growth and metastasis
(6
, 7)
.
Ad53
E1A has recently been demonstrated to exhibit antioncogenic activity by
suppressing transformation, metastasis, and tumorigenicity via multiple
mechanisms (8, 9, 10, 11, 12, 13)
. The molecular mechanisms that may be
associated with this antioncogenic activity include down-regulation of
transforming and metastasis genes such as HER-2/neu, Axl,
stromelysin, plasminogen activator, urokinase, and type 4 collagenase
(8
, 14)
.
E1A has also been shown to increase the susceptibility of cells
to tumor necrosis factor and
-radiation (15
, 16)
,
enhance their susceptibility to host cellular immune responses, and
sensitize them to apoptosis induced by various stimuli such as high
cell density. E1A can also induce apoptosis in a cell-free system and
in both p53-dependent and p53-independent manners
(17, 18, 19)
. Moreover, local and systemic liposome- and
adenovirus-mediated delivery of E1A results in significant tumor
suppression in cell lines and mouse models (10
, 11)
.
Current methodology for E1A transfection is not 100% successful;
hence, we raised the question as to whether E1A-mediated tumor
suppression might be due in part to a bystander effect. To test this
possibility, we investigated the bystander effect on tumor development
in nude mice inoculated with Ad5 E1A transfectant cells.
 |
Materials and Methods
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Cell Lines and Culture.
Cell lines used were parental human ovarian cancer line SKOV3.ip1,
stable Ad5 E1A transfectants of SKOV3.ip1 (ip1-E1A2), E1A frameshift
mutant transfectants of SKOV3.ip1 (ip1-Efs), and the human breast
cancer cell line MDA-MB-435 (American Type Culture Collection,
Manassas, VA). All cultures were established and grown as described by
Shao et al. (16)
.
Tumor Cell Inoculation.
Female 45-week-old nude mice were purchased from Harlan Bioproducts
for Science, Inc. (Indianapolis, IN) and maintained in the animal
facility of The University of Texas M. D. Anderson Cancer Center. Mice
were divided into six experimental groups containing five mice each.
One group was inoculated s.c. with a cell mixture of 2
x 106 ip1-E1A2 cells and an equal amount of
parental SKOV3.ip1 cells to induce tumor. A second group was inoculated
with 1 x 106 ip1-E1A2 cells and
an equal amount of MDA-MB-435 cells. The remaining groups were
inoculated with equal numbers of SKOV3.ip1, MDA-MB-435, or ip1-E1A2
cells alone. Cells were injected into the flanks of the mice, and each
mouse was inoculated with tumors at two sites. Tumor volumes were
measured and recorded once a week for 47 weeks. Each experiment was
repeated at least one time.
Immunohistochemistry.
Histological sections of ip1-E1A2/SKOV3.ip1-, ip1-E1A2-, and
SKOV3.ip1-induced tumors were taken, fixed with formalin, and embedded
in paraffin. The sections were incubated with rabbit antibody against
factor VIII (1:200; Dako Corp., Carpinteria, CA), followed by
incubation with biotinylated goat antirabbit IgG (1:200; Vector
Laboratories, Inc., Burlingame, CA). Sections were then incubated with
an avidin-biotin complex-horseradish peroxidase conjugate and developed
using aminoethylcarbazole chromogen (Sigma, St. Louis, MO) as a
substrate. Positive signals were visualized by light microscopy at high
power (x400), and the number of microvessels was counted in six x200
fields for each tumor section.
Tumor Apoptosis Assay.
The sections from ip1-E1A2/SKOV3.ip1-, ip1-E1A2-, and SKOV3.ip1-induced
tumors were taken and analyzed for apoptosis by TUNEL assay as
described by Shao et al. (15)
.
[3H]Thymidine Incorporation.
Cells were cocultured in a 24-well Transwell unit containing an upper
and a lower chamber (Costar Corporate, Cambridge, MA). The chambers
were separated by a membrane that allowed the medium, but not the
cells, to penetrate freely. The upper chamber was filled with
2 x 104 ip1-E1A2 cells or
ip1-Efs, SKOV3.ip1, or MDA-MB-435 control cells in 100 µl of medium.
The lower chamber was filled with 1.5 x
104 SKOV3.ip1 or MDA-MB-435 cells in 600 µl of
medium. The supernatant from the upper chamber, which was replaced with
100 µl of fresh medium, was filtered and used to replace 100 µl of
the medium from the lower chamber each day. After 72 h of culture,
3 µCi of [3H]thymidine in 20 µl of medium
were added into the lower chamber, and incubation was continued for
another 16 h. Thymidine incorporation was determined with a
scintillation counter.
In Vitro Apoptosis Assay.
To analyze apoptosis in bystander cells induced by E1A transfectants in
culture, we cultured 3 x 105
ip1-E1A2, ip1-Efs, or SKOV3.ip1 cells in the upper chamber and
1.5 x 104 SKOV3.ip1 cells in the
lower chamber of the Transwell unit under serum starvation conditions.
After 96 h of culture, cells were trypsinized and harvested for
TUNEL assay as described by Shao et al. (15)
.
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Results and Discussion
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E1A has exhibited multiple tumor-suppressive properties
(8)
. To test whether E1A could mediate a bystander effect
on tumor development, a human ovarian cancer cell line derivative
(SKOV3.ip1), the wild-type Ad5 E1A stable transfectant of SKOV3.ip1
(ip1-E1A2), the human breast cancer cell line MDA-MB-435, and
ip1-E1A2/SKOV3.ip1 and ip1-E1A2/MDA-MB-435 mixtures were used to inject
tumors ectopically in nude mice. Tumors induced by the cell mixtures or
ip1-E1A2 cells alone grew more slowly than those induced by either
SKOV3.ip1 or MDA-MB-435 cells alone (Fig. 1, A and B
). These results, consistent with
previous results from our laboratory (8
, 10
, 11)
,
indicated that E1A mediates tumor suppression. In addition, data showed
that ip1-E1A2 cells mediated a bystander effect by suppressing tumor
growth induced by the SKOV.3 ip1 or MDA-MB-435 cells.

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Fig. 1. Bystander effect and tumor suppression mediated by
ip1-E1A2 cells. Nude mice (n = 5) were
injected with various cells and cell mixtures to induce tumors at two
sites in each mouse, A, SKOV3.ip1, ip1-E1A2 +
SKOV3.ip1, or ip1-E1A2 cells; B, MDA-MB-435
(435); ip1-E1A2 + MDA-MB-435
(ip1-E1A2+435), or ip1-E1A2 cells. Tumor volumes were
measured and recorded once a week for 4 (A) and 7 weeks
(B). Each point on the figure represents
the mean volume of 10 tumors.
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Recent studies have shown that angiogenesis is one of the important
factors affecting tumor growth. Factor VIII, which presents on the
blood vessel endothelial cells, has been used as a marker for
angiogenesis (6)
. To examine whether E1A-mediated tumor
suppression involves a change in angiogenesis and whether the change is
involved in the bystander effect, we performed immunohistochemical
staining for factor VIII in tumor tissue. Discrete microvessel counts
and neovascularization in tumors induced by ip1-E1A2/SKOV3.ip1 cell
mixtures (Fig. 2A
) and ip1-E1A2 cells alone (Fig. 2B
) were
dramatically lower than those in tumors induced by SKOV3.ip1 cells
alone (Fig. 2C
). These results suggest that E1A mediates the
antiangiogenic suppression process of blood vessel formation and that
this action is involved in the bystander effect.

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Fig. 2. Inhibition of angiogenesis mediated by ip1-E1A2 cells.
Histological sections from tumors induced by various cell lines were
embedded in paraffin and stained for factor VIII. A,
SKOV3.ip1 plus ip1-E1A2 cells; B, ip1-E1A2 cells;
C, SKOV3.ip1 cells. SDs were calculated after viewing
six independent x200 fields.
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In addition to angiogenesis, apoptosis has been implicated in cancer
pathogenesis and treatment (20)
and as a mechanism for
bystander cell death (2)
. E1A has been shown to induce
apoptosis in vitro; therefore, to determine whether E1A
could mediate apoptotic changes in vivo, we analyzed tumor
sections by TUNEL assay. Tumors induced by ip1-E1A2/SKOV3.ip1 cell
mixtures and by ip1-E1A2 cells alone exhibited apoptosis in 7% and 9%
of the cells examined, respectively (Fig. 3, A and B
), whereas tumors induced by SKOV3.ip1
cells showed apoptosis in <1% of cells examined (Fig. 3C
).
These results indicate that E1A mediates apoptosis not only in tumors
induced by the E1A transfectants but also in tumors induced by the cell
mixtures.

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Fig. 3. Apoptosis in tumors mediated by ip1-E1A2 cells. Sections
were taken from tumors induced by various cells and cell mixtures.
A, ip1-E1A2 plus SKOV3.ip1; B, ip1-E1A2
cells; C, SKOV3.ip1 cells. Sections were analyzed for
apoptosis by TUNEL assay. Arrows point to representative
apoptotic cells. SDs were derived from two separate counts of apoptotic
cells.
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It has been shown that cytokines or other factors secreted by
transfected cells can induce bystander effects (3, 4, 5)
. To
further investigate whether our bystander effect was mediated by a
factor secreted from ip1-E1A2 cells, we cocultured SKOV3.ip1 or
MDA-MB-435 cells with ip1-E1A2 or control ip1-Efs cells in a Transwell
unit. Their mitogenic activity was measured by
[3H]thymidine incorporation. The mitogenic
activity of SKOV3.ip1 or MDA-MB-435 cells cocultured with ip1-E1A2
cells was lower than that of SKOV3.ip1 or MDA-MB-435 cells cocultured
with ip1-Efs cells, which do not produce functional E1A proteins (Fig. 4, A and B
). This indicated that ip1-E1A2 cells
could mediate inhibition of mitogenic activity of SKOV3.ip1 and
MDA-MB-435 cells and that the effect was caused by a secretable
factor(s) present in the ip1-E1A2 cell supernatant. To further examine
whether ip1-E1A2 cells could secrete this factor(s) to induce apoptosis
in vitro, we cultured SKOV3.ip1 cells alone or with either
ip1-E1A2 or control ip1-Efs cells in Transwell units under serum
starvation conditions to induce apoptosis. Twenty percent of SKOV3.ip1
cells underwent apoptosis when cultured with ip1-E1A2 cells as compared
with 5% and 3% of SKOV3.ip1 cells cultured with ip1-Efs cells or
alone, respectively (Fig. 4C
). These results indicate that
cells transfected with E1A mediated a bystander effect by releasing a
secretable factor to inhibit mitosis and enhance apoptosis through the
culture supernatant under serum withdrawal conditions. Taken together,
our results suggest that E1A-mediated inhibition of angiogenesis,
induction of apoptosis, and suppression of mitosis may contribute to
the bystander effect on E1A-mediated tumor suppression.

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Fig. 4. Inhibition of [3H]thymidine incorporation
and enhancement of apoptosis mediated by ip1-E1A2 cells in
vitro. Cells were cultured in the Transwell unit,
[3H]thymidine was added to the lower chambers, and the
radioactivity of harvested cells was determined by scintillation.
A, SKOV3.ip1 cells in the lower chamber and ip1-E1A2,
ip1-Efs, or control SKOV3.ip1 cells in the upper chamber.
B, MDA-MB-435 (435) cells in the lower
chamber and ip1-E1A2, ip1-Efs, or control MDA-MB-435 cells in the upper
chamber. Additional cells were grown under serum stavation conditions,
harvested after 96 h, and analyzed by TUNEL assay.
C, SKOV3.ip1 cells in the lower chamber and ip1-E1A2,
ip1-Efs, or control SKOV3.ip1 cells in the upper chamber.
Bars, SDs determined from three independent
experiments.
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FOOTNOTES
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 Supported by NIH Grants CA58880 and CA77858 and
Ovarian Specialized Programs of Research Excellence Grant P50
CA83639 (to M-C. H.). 
2 To whom requests for reprints should be
addressed, at Department of Molecular and Cellular Oncology, Breast
Cancer Basic Research Program, The University of Texas M. D. Anderson
Cancer Center, 1515 Holcombe Boulevard, Box 108, Houston, TX 77030.
Phone: (713) 792-3630; Fax: (713) 794-0209. 
3 The abbreviations used are: Ad5, adenovirus type
5; TUNEL, terminal deoxynucleotidyl transferase-mediated dUTP-biotin
nick end labeling. 
Received 1/28/00.
Accepted 5/ 4/00.
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