
[Cancer Research 61, 6377-6381, September 1, 2001]
© 2001 American Association for Cancer Research
Targeting Oncolytic Adenoviral Agents to the Epidermal Growth Factor Pathway with a Secretory Fusion Molecule1
Akseli Hemminki2,
Igor Dmitriev,
Bin Liu,
Renee A. Desmond,
Ramon Alemany and
David T. Curiel
Division of Human Gene Therapy, Departments of Medicine, Pathology, and Surgery, and the Gene Therapy Center [A. H., I. D., B. L., R. A., D. T. C.], and Comprehensive Cancer Center Biostatistics Unit [R. A. D.], University of Alabama at Birmingham, Birmingham, Alabama 35294-3300
 |
ABSTRACT
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Cancer gene therapy with conditionally replicating adenoviruses is a
powerfulway of overcoming low tumor transduction. However, one of the main
remaining obstacles is the highly variable level of the
coxsackie-adenovirus receptor expression on human primary cancers. In
contrast, the epidermal growth factor receptor (EGFR) is overexpressed
in various tumor types, and its expression correlates with metastatic
behavior and poor prognosis. We constructed an adenovirus expressing a
secretory adaptor capable of retargeting adenovirus to EGFR, resulting
in a more than 150-fold increase in gene transfer. A
replication-competent dual-virus system secreting the adaptor displayed
increased oncolytic potency in vitro and therapeutic
gain in vivo. This approach could translate into
increased efficacy and specificity in the treatment of EGFR
overexpressing human cancers.
 |
Introduction
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CRADs3
are a promising and novel way of overcoming low tumor transduction,
which is the main obstacle preventing effective gene transfer and
therapeutic effect in clinical applications of cancer gene therapy
(1)
. However, one of the main reasons why the unparalleled
transduction efficacy of Ads has not translated into similar results in
humans is the variable level of the CAR on primary cancers
(2, 3, 4, 5, 6, 7, 8, 9)
in vivo. CAR is ubiquitously expressed
on normal epithelial tissues and is the main receptor mediating binding
of the most commonly used Ad serotypes 2 and 5. Expression of CAR may
be the major factor determining the rate of transduction (4
, 6
, 9, 10, 11)
. Importantly, recent evidence (11)
suggests
that CAR expression may inversely correlate with the malignant
potential of tumors, resulting in low infectivity of highly aggressive
tumors. In contrast to the expression profile of CAR, EGFR, the
prototype of cancer-associated receptors, is commonly overexpressed in
many if not most carcinomas with correlation to metastatic behavior and
poor prognosis (12)
. A powerful approach for increasing
tumor transduction could be combining the tissue-penetrating capability
of CRADs with the transductional control provided by retargeting
moieties. In support of this hypothesis, an artificial receptor system
has been used to demonstrate that the effect of Ad dispersion and
subsequent oncolysis critically depends on receptor expression
(13)
. We have constructed a novel virus that mediates
secretion of a fusion molecule consisting of the extracellular domain
of CAR and EGF. We then explored the capability of the sCAR-EGF to
retarget Ad to EGFR. Finally, we demonstrated that infection of cancer
cells with a sCAR-EGF-retargeted replication-competent dual-virus
system resulted in increased oncolysis in vitro and a
therapeutic benefit in vivo.
 |
Materials and Methods
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Viruses.
For construction of AdsCAR-EGF, a replication-deficient Ad with
sCAR-EGF in E1, the gene coding for sCAR-EGF was cloned from
pFBshCAR-EGF (14)
into pShuttle-CMV (Quantum,
Montreal, Quebec, Canada). Homologous recombination with pAdEasy-1
(Quantum) was performed in Escherichia coli, followed by
confirmation of structure with EcoRV and PacI
digestions, PCR, and sequencing of the transgene (data not shown). The
viral genome was transfected into 293 cells for plaque purification,
followed by cesium chloride purification and standard titering with
OD260 and plaque assay. Resulting titers were 3.8 x
1011 VPs/ml and 1.0 x 1010 plaque-forming units/ml, ratio = 38.4 VPs/plaque-forming unit. Large-scale preparations of
AdCMVLuc (a nonreplicating Ad-expressing luciferase; courtesy of
Dr. Robert Gerard, Texas Southwestern Medical Center,
Dallas, TX), AdsCAR6H (a nonreplicating Ad-mediating secretion of
sCAR6H),4
and
24 (an Ad with a 24-bp deletion in E1A, allowing
selective replication in cells mutant in the Rb-p16 pathway;
Ref. 15
) were performed with standard methods on 293 cells
(or A549 cells for
24).
CRAdsCAR-EGF denotes a replication-competent, sCAR-EGF-secreting
dual-virus system consisting of equal VPs of AdsCAR-EGF and
24 mixed
immediately before administration to cells. CRAdCMVLuc is the
respective combination of
24 and AdCMVLuc.
24 has been
characterized previously (15)
. Validating the dual-virus
strategy, it has been demonstrated that transcomplementation of E1
proteins from a plasmid or replication-competent virus results in
replication of E1-deleted viruses present in the same cell (9
, 16, 17, 18)
.
Cell Lines.
293 cells were purchased from Microbix (Toronto, Ontario, Canada). A549
(lung cancer), HeLa (cervical cancer), U118 (glioma), A431 (squamous
cell skin cancer), BT474, and MB-453 (breast cancer) were obtained from
the American Type Culture Collection (Rockville, MD), and SKOV3.ip1
cells (ovarian cancer) are from Dr. Janet Price (M. D. Anderson Cancer
Center, Houston, TX). Cell lines were propagated in the recommended
conditions.
Protein Detection.
HeLa cells were infected overnight with 50 VP/cell, and BT474 and MB453
cells were infected with 500 VP/cell of AdsCAR-EGF, AdsCAR6H, and
AdCMVLuc. Supernatants were collected at 48 h, and cellular debris
was removed by centrifugation. Dilutions in a volume of 300 µl were
transferred onto a nitrocellulose membrane using the Bio-Dot apparatus
(Bio-Rad). BSA (3%) was used for blocking, followed by detection with
a 1:5000 polyclonal mouse anti-CAR antibody (14)
and
1:2000 goat antimouse alkaline phosphatase (Sigma Chemical Co., St.
Louis, MO) in 3% BSA. Western blot analysis on the supernatants was
performed with standard methodology in a 12% two-phase gel, and
proteins were detected as above. Baculovirus-expressed and -purified
sCAR-EGF and sCAR6H (14)
were used as controls.
Retargeting Assays.
SKOV3.ip1, U118, and A431 cells were infected in duplicate with
AdCMVLuc preincubated for 30 min with aliquots of supernatants (see
above). Twenty VP/cell were used for SKOV3.ip1, whereas 200 VP/cell
were used for U118 and A431. Luciferase assay was performed 48 h
later (Luciferase Assay System; Promega, Madison, WI).
Cell-killing Assays.
SKOV3.ip1 cells were plated in triplicate and infected with
24
or AdCMVLuc for 1 h. Aliquots (25 µl) of AdsCAR-EGF or AdCMVLuc
supernatant (from protein detection assays) were added each day. At 20
days, cells were fixed with formalin and stained with crystal violet.
Next, A431 and SKOV3.ip1 cells were infected with the CRAdCMVLuc or
CRAdsCAR-EGF for 1.5 h. After incubating for 20 days with medium
changes, the remaining cells were fixed and stained as above.
Preliminary toxicity analysis was performed by infecting A431 and
SKOV3.ip1 cells with 50, 100, 200, 500, and 1000 VP/cell of AdsCAR-EGF
and AdsCAR6H, followed by medium changes every 2 days for 20 days.
Animal Experiments.
Initially, A431 cells were infected ex vivo with 50 VP/cell
of CRAdsCAR-EGF for 1 h followed by a 5-h incubation. Cells were
then collected and mixed with uninfected cells, and a total of
107 cells were injected into flanks of athymic
CD-1/nu mice (Charles River Laboratories, Wilmington, MA;
n = five/group). Tumor size was determined as
the mean of the shortest and longest diameter (to avoid variability
attributable to difficulty with estimation of height).
To compare CRAdsCAR-EGF to CRAdCMVLuc in vivo, s.c. tumors
were established by injecting 5 x 106 A431 cell into both flanks of athymic mice
(n = five/group). When tumors were
5 mm
("day 0"), viruses were injected intratumorally in a 15-µl volume
and tumors were measured as above. Each mouse was checked daily for the
absence of pain or distress (19)
.
Statistics.
Upon termination of the experiment, mean tumor size and SDs were
calculated for each group of animals for each time point. The
nonparametric change-point test (20)
was used to show a
systematic change in the pattern of observations as opposed to
fluctuation attributable to chance. The mixed model (21)
was used to longitudinally model the data on each tumor. The variance
covariance structure was determined by using Akaikes Information
Criteria (22)
. The Proc Mixed procedure in SAS
v.6.12 (SAS Institute, Cary, NC) was used to examine the effects of
group and time on tumor growth.
 |
Results
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Infection of Cells with AdsCAR-EGF Results in Secretion of
sCAR-EGF.
Initially, infection of high EGFR HeLa cells (23)
with
AdsCAR-EGF produced no evidence of secretion, whereas sCAR6H was
secreted (Fig. 1)
. With low EGFR-expressing cells MB453 and BT474 (14
, 24) ,
secretion of sCAR-EGF was detected. The amount of protein was estimated
at 110 ng/ml (75-cm2 flask; 12 ml of medium).
Western blot confirmed secretion (Fig. 1B)
. The
altered migration rate of the protein in comparison with
baculovirus-expressed sCAR-EGF perhaps resulted from altered charge
caused by different glycosylation by insect cells. A preliminary
investigation on sCAR-EGF toxicity was performed by infecting SKOV3.ip1
and A431 cells with various amounts of AdsCAR-EGF and AdsCAR6H without
significant differences in cell viability (data not shown).

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Fig. 1. Secretion of sCAR-EGF from human cancer cells infected
with AdsCAR-EGF. A, supernatant from MB453 and BT474
(both low EGFR breast cancer lines) cells infected with AdsCAR6H (codes
for human CAR ectodomain, positive control), AdsCAR-EGF, or AdCMVLuc
(negative control) was centrifuged and then transferred onto a
membrane. Arrow, the signal for sCAR-EGF. When compared
with known amounts of sCAR-EGF (lowest row), the amount
of secretion could be estimated at 110 ng/ml. B, Western
blot suggested that the sCAR-EGF secreted from BT474 and MB453 cells
(Lanes 12) was close in size to baculovirus-expressed
sCAR-EGF (Lane 5). High EGFR HeLa cells (Lane
3) did not show evidence of sCAR-EGF secretion, but the
positive control sCAR6H was secreted (Lane 9). -,
supernatants collected from cells infected with AdCMVLuc (Lanes
4 and 7). These serve as negative controls. +, the
positive controls, including Lane 8, which has
supernatant from BT474 cells infected with AdsCAR6H. sCAR-EGF and
sCAR6H (Lanes 56) are purified baculovirus-expressed
proteins.
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Secreted sCAR-EGF Mediates Retargeting of Ad to EGFR.
Aliquots of supernatant from AdsCAR-EGF-infected BT474 cells were
incubated with AdCMVLuc. The virus-supernatant mix was used for
infection of SKOV3.ip1, U118, and A431 cells, which display moderate
(SKOV3.ip1 and U118) to high (A431) EGFR expression and moderate (A431)
to low (SKOV3.ip1 and U118) CAR expression (4
, 14)
. A
supernatant dose-dependent increase in luciferase expression was seen,
with the highest readings 17.1-, 20.2-, and 158-fold higher than
without retargeting for SKOV3.ip1, U118, and A431 cells, respectively
(curves with triangles in Fig. 2
).
With the highest amounts of supernatant from AdsCAR6H-infected cells,
luciferase expression was reduced to 73%, 48%, and 65% (on
SKOV3.ip1, U118, and A431, respectively) of the highest values for the
series (curves with squares in Fig. 2
). sCAR6H binds to Ad fiber but
does not mediate binding to EGFR, thus modeling blockage of CAR-binding
with sCAR-EGF.
Retargeting Replication-competent Ad to EGFR Results in
Increased Cell Killing in Vitro.
To validate
24 replication in SKOV3.ip1 cells, infections were
performed at 0, 0.01, 0.1, or 1 VP/cell, and aliquots of supernatant
(from BT474 cells infected with AdsCAR-EGF or AdCMVLuc) were added
daily. At 20 days, cell killing and partial loss of monolayer was seen
only with cells that had been infected with 1 VP/cell and subjected to
AdsCAR-EGF/BT474 supernatant (data not shown).
To study the effect of continuous sCAR-EGF secretion on the oncolytic
potential of CRADs, we infected SKOV3.ip1 and A431 cells with the
CRAdsCAR-EGF and CRAdCMVLuc dual-virus systems. On both cell lines,
infection with CRAdsCAR-EGF resulted in cell killing with one to two
orders of magnitude less virus than with CRAdCMVLuc (Fig. 3)
.

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Fig. 3. Targeting oncolytic Ad to EGFR with sCAR-EGF results in an
increased oncolytic effect. (A) SKOV3.ip1 or
(B) A431 cells were infected with CRAdsCAR-EGF, a
replication-competent sCAR-EGF-secreting dual-virus system. Oncolytic
potency was compared with CRAdCMVLuc, which is isogenic in regard to
replicativity but does not secrete a retargeting molecule. A
similar effect on cells was observed with 1 VP/cell of CRAdsCAR-EGF as
with 100 VP/cell of CRAdCMVLuc, suggesting increased oncolysis because
of sCAR-EGF secretion-mediated EGFR targeting.
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Targeting Replicative Ad to EGFR Results in a Therapeutic Advantage
in Vivo.
Various proportions of infected and uninfected A431 cells were mixed
and injected s.c. (Fig. 4A)
. One percent of infected cells was sufficient to inhibit
tumor growth, and 5% or more resulted in healing of tumors. None of
the mice showed signs of illness or distress, suggesting that the
secretion of sCAR-EGF did not cause overt toxicity.

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Fig. 4. sCAR-EGF secretion results in therapeutic efficacy
in vivo. A, various percentages of A431
cells were infected ex vivo with CRAdsCAR-EGF and mixed
with uninfected cells. One percent of infected cells was sufficient to
inhibit tumor growth, whereas 5% or more resulted in healing of mice.
Single intratumoral injections of (B) 109 or
(C) 108 VP of CRAdsCAR-EGF or CRAdCMVLuc
dual-virus systems were performed into established A431 xenografts.
Arrows (solid for CRAdsCAR-EGF,
dotted for CRAdCMVLuc) indicate the change-point in
tumor growth characteristics, which represents the time point when
oncolysis changes the initial pattern of tumor growth. On the basis of
tumor size measurements (observed), the growth patterns
were mathematically modeled for statistical comparison
(modeled), and CRAdsCAR-EGF was found to be more
oncolytic. *, P < 0.05; **,
P < 0.0001.
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To evaluate sCAR-EGF retargeting in vivo, CRAdsCAR-EGF or
CRAdCMVLuc were administered with a single intratumoral injection into
established A431 tumors (Fig. 4, BC)
. The change-point
test (20)
revealed that the tumor growth pattern changed
at 13 days for 109 VP CRAdsCAR-EGF
(P = 0.0045), 21 days for
109 VP CRAdCMVLuc (P = 0.0012), 17 days for 108 VP CRAdsCAR-EGF
(P = 0.0026), and 25 days for
108 VP CRAdCMVLuc (P = 0.0011), i.e., 8 days earlier for CRAdsCAR-EGF with both
doses.
The change-point test and the test of fixed effects (22)
showed that there was a significant correlation between observations of
tumor size and time (P < 0.0001 for all of
the groups). A polynomial equation was fit for each group, thereby
creating a mathematical model for each growth pattern ("modeled" in
Fig. 4
). The mixed model (22)
was used to allow for the
curvature of the plots, which was caused by the opposite effects of
oncolysis and cell division. For the mice that received injections with
109 VP, groups were significantly different from
day 29 onwards (P < 0.05; Fig. 4B
). When 108 VP was used (Fig. 4C)
, the differences were significant from day 25 onwards
(P = 0.0066 and 0.0003 on days 25 and 29,
respectively, and < 0.0001 on days 3353).
 |
Discussion
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In this study, we report construction of the first human Ad
secreting a paracrine adaptor molecule. Secretion of sCAR-EGF was
demonstrated with low EGFR cells (Lanes 1 and 2, Fig. 1
),
but not with high EGFR HeLa cells (23)
. In contrast,
secretion was detected when HeLa cells were infected with AdsCAR6H,
which codes for ectodomain of CAR but not EGF (Lane 9, Fig. 1B
). EGF exhibits high affinity binding to EGFR, which leads
to rapid internalization but no recycling of the receptor-ligand
complex (12)
. Thus, perhaps secreted sCAR-EGF also
internalizes. Alternatively, binding without internalization would also
limit the amount of sCAR-EGF in the supernatant.
Supporting the capacity of sCAR-EGF to mediate binding and subsequent
internalization of Ad, supernatant containing the fusion molecule
resulted in dose-dependent increases in marker gene expression (Fig. 2)
. The shape of the curves suggests that the upper limit of
retargeting potential was not reached. In an in vitro
system, it is difficult to assess the capability of a secreted fusion
molecule to block fiber-CAR interaction, because in the absence of CAR
binding, uptake of Ad into cells can also occur via alternative
mechanisms (10)
. However, we observed up to 52% reduction
in luciferase expression with sCAR6H, which could translate into
partial blocking of CAR-mediated internalization (into normal cells) by
sCAR-EGF in vivo, but additional studies are needed.
We used a dual-virus system (CRAdsCAR-EGF) to evaluate the combination
of oncolysis and EGFR targeting and saw dramatically increased killing
of cells relatively low in CAR but high in EGFR expression, a
combination commonly seen with primary cancer cells (Fig. 3)
. In
vitro, the isogenic control virus (CRAdCMVLuc) is expected to
enter cells even if they are low in CAR (10)
. The observed
difference in oncolysis may result from more rapid internalization of
the retargeted virus because of a higher number of receptors. In a
living organism, extracellular viruses are at risk for neutralization
by immune defenses or being swept away into organs responsible for Ad
clearance. Thus, the advantage of rapid binding and internalization
could be more pronounced.
s.c. xenografts are a stringent model for testing an oncolytic effect,
because viral replication is balanced against rapid tumor growth. Here,
we demonstrated significantly improved therapeutic efficacy of
CRAdsCAR-EGF in comparison with the isogenic control not secreting
sCAR-EGF (Fig. 4)
.
No signs of sCAR-EGF-causing toxicity were evident when cells were
infected with AdsCAR-EGF in comparison with AdsCAR6H. When sCAR-EGF was
added daily to SKOV3.ip1 cells infected with a CRAD, no evidence of
toxicity to cells was seen. Moreover, obvious signs of toxicity were
absent in mice whose xenografts were infected with CRAdsCAR-EGF.
Additional studies will show whether the adaptor molecule has an effect
of cell growth or whether there is toxicity in vivo. Also,
it remains to be seen whether sCAR-EGF mediates Ad internalization via
the EGFR pathway or merely substitutes for CAR in binding Ad for the
native entry mechanism via penton base arginine-glycine-aspartic acid
and cellular integrins.
This is the first report of a retargeting molecule secretory from human
cells, but this strategy could be feasible with various high-affinity,
cancer-specific ligands. Because rapid screening methods allow
recognition of large numbers of cancer-specific features, unlimited
possibilities for retargeting with secretory-targeting moieties may
soon be available. The dual-virus system used here provides a useful
model for rendering AdsCAR-EGF replicative and investigating the
combination of oncolysis and retargeting, but efficacy could be
improved when sCAR-EGF is genetically incorporated into a
CRAD.
In conclusion, we show that retargeting of replicating Ad to a receptor
overexpressed in cancers is a powerful way of increasing tumor
transduction and allows overcoming the lack of the primary Ad receptor.
Clinical translation of this approach may be effective in treatment of
a variety of human cancers that overexpress EGFR.
 |
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 the Damon Runyon-Walter Winchell
Foundation, the Sigrid Juselius Foundation, the Emil Aaltonen
Foundation, the Maud Kuistila Foundation, NCI (RO1 CA83821), U. S.
Army (PC991018, DAmD 17-00-1-0115), the CapCure Foundation, and
the Lustgarten Foundation. Gene transfer assays were performed in part
at the University of Alabama at Birmingham Gene Therapy Center
Correlative Laboratories for Human Clinical Trials. 
2 To whom requests for reprints should addressed,
at Division of Human Gene Therapy, Gene Therapy Center, WTI #602, 1824
6th Avenue S., University of Alabama at Birmingham, Birmingham, AL
35294-3300. Phone: (205) 975-2896; Fax: (205) 975-8565; E-mail: akseli{at}uab.edu 
3 The abbreviations used are: CRAD, conditionally
replicative adenovirus; Ad, adenovirus; CAR, coxsackie-Ad receptor;
EGF, epidermal growth factor; EGFR, EGF receptor; sCAR-EGF, secretory
CAR-EGF fusion molecule; CMV, cytomegalovirus immediate early promoter;
VP, viral particle; sCAR6H, secretory CAR with 6-histidine tag. 
4 I. Dmitriev, unpublished observations. 
Received 5/ 2/01.
Accepted 7/16/01.
 |
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May 1, 2007;
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M. Rajecki, A. Kanerva, U.-H. Stenman, M. Tenhunen, L. Kangasniemi, M. Sarkioja, M. Y. Ala-Opas, H. Alfthan, A. Sankila, E. Rintala, et al.
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L. Kangasniemi, T. Kiviluoto, A. Kanerva, M. Raki, T. Ranki, M. Sarkioja, H. Wu, F. Marini, K. Hockerstedt, H. Isoniemi, et al.
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F. Kuhnel, B. Schulte, T. Wirth, N. Woller, S. Schafers, L. Zender, M. Manns, and S. Kubicka
Protein Transduction Domains Fused to Virus Receptors Improve Cellular Virus Uptake and Enhance Oncolysis by Tumor-Specific Replicating Vectors
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T. Hakkarainen, A. Hemminki, A. V. Pereboev, S. D. Barker, C. K. Asiedu, T. V. Strong, A. Kanerva, J. Wahlfors, and D. T. Curiel
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G. J. Bauerschmitz, J. T. Lam, A. Kanerva, K. Suzuki, D. M. Nettelbeck, I. Dmitriev, V. Krasnykh, G. V. Mikheeva, M. N. Barnes, R. D. Alvarez, et al.
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D. M. Shayakhmetov, Z.-Y. Li, S. Ni, and A. Lieber
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