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Advances in Brief |
Surgery Branch, National Cancer Institute, Bethesda, Maryland 20892 [A. L. F., N. P. R., H. R. A., D. L. B., P. H., S. K. L.]; Human Gene Therapy Research Institute, Des Moines, IA 50309 [P. S.]
| ABSTRACT |
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| Introduction |
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| Materials and Methods |
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cells
(Life Technologies), and the signal sequence-murine
endostatin (ss-mEndo) sequence was confirmed (ABI Prism 310
autosequencer; PE Applied Biosystems, Foster City, CA).
Synthesis of Adenoviral Vectors.
The ss-mEndo construct was digested with EcoRI and cloned by
blunt-end ligation into the multiple cloning site of the adenoviral
shuttle plasmid pAd/CMV.1. The resulting plasmid was recombined with
type 5 E1A/B-deleted
Ad2
as described previously (6
, 7)
and used to infect 293
cells (American Type Culture Collection, Manassas, VA). Plaque DNA was
extracted using proteinase K digestion, phenol extraction, and ethanol
precipitation and screened for ss-mEndo by PCR. The resulting virus,
Ad-ss-mEndo, was amplified in 293 cells. A similar strategy was used to
create control recombinant viruses containing the genes for ß-gal
(Ad-ß-gal) and firefly luc (Ad-luc). Viruses were titered using a
standard plaque-forming assay in 293 cells.
In Vitro Infection with Recombinant Ad.
Cells were grown in complete medium consisting of DMEM with 10% FCS,
100 units/ml penicillin, 100 µg/ml streptomycin, 50 µg/ml
gentamicin, 0.5 µg/ml Fungizone, and 4 mM glutamine
(Biofluids, Rockville, MD). Cells were infected at MOIs ranging from
0.1 to 100 (105 to 108 pfu
per 106 cells in 1.0 ml of complete media) with
Ad-ss-mEndo, Ad-luc, or no virus and incubated at 37°C for 24 h.
Supernatants were centrifuged at 2 x g for 5
min and assayed for endostatin using a competitive EIA (Cytimmune
Sciences, College Park, MD), according to the manufacturers
instructions. 293 cell supernatants were concentrated 10-fold in
cellulose columns (Centricon YM-10; Millipore, Bedford, MA) and
analyzed by Western blotting (NuPAGE; Novex, San Diego, CA) using 570
ng/ml rabbit antimurine endostatin polyclonal IgG antibody (gift of
Cytimmune Sciences). The EIA murine endostatin standard was used as a
positive control. The susceptibility of the murine colon adenocarcinoma
cell line MC38 (developed in the Surgery Branch, National Cancer
Institute) to adenoviral infection was tested by infecting cells with
Ad-ß-gal as described above and assaying for ß-gal 24 h later
using a staining kit (Boehringer Mannheim, Indianapolis, IN).
Susceptibility of the murine hepatocyte line NMuLi (American Type
Culture Collection) to Ad-ß-gal infection was used as a positive
control.
Functional Assay of Virally Generated Endostatin.
The human melanoma cell line 501 Mel (Z-H. Wang, National Cancer
Institute) was infected as above with varying MOIs of Ad-ss-mEndo or
Ad-luc. Supernatants were harvested 24 h later and centrifuged as
above. Supernatants were analyzed for their ability to inhibit
endothelial cell proliferation, as described previously
(8)
, with slight modifications. Briefly, 1000 bovine
capillary endothelial cells (EJG; American Type Culture Collection)
were plated in complete medium in each well of collagen I-coated
96-well plates (Biocoat, Becton Dickinson, Bedford, MA). After
overnight incubation at 37°C, the medium was aspirated and replaced
with 20 µl of the supernatant sample to be tested. Eight samples of
each supernatant were tested. After 20 min incubation at 37°C, 80
µl of modified complete medium containing 5% FCS and 1 ng/ml basic
fibroblast growth factor (R&D Systems, Minneapolis, MN) were added.
After 72 h incubation at 37°C, proliferation was analyzed by
WST-1 assay (Boehringer Mannheim), according to the manufacturers
instructions. Inhibition of proliferation in each sample was calculated
according to the formula:
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In Vivo Production of Endostatin.
Animal experiments were conducted according to protocols approved by
the NIH Animal Care and Use Committee. Eight-week-old female nude mice
(Charles River Laboratories) received injections i.v. by lateral tail
vein with 109 or 1010 pfu
Ad-ss-mEndo or no virus in 100 µl of PBS (n = 5 animals/group) to assess dose response and toxicity. Animal
well-being was monitored for 6 days, and surviving mice were euthanized
and autopsied. To assess the duration and amount of endostatin
expression, mice were injected with 109 pfu of
Ad-ss-mEndo or Ad-luc, or no virus, as above on day 0. On days 2, 4, 8,
and 13, mice were euthanized with carbon dioxide, and blood samples
were obtained by cardiac puncture (n = 6
animals/treatment group per time point). Samples were centrifuged in
EDTA-containing tubes (Microtainer; Becton Dickinson, Franklin Lakes,
NJ), and plasma endostatin levels were determined by EIA.
In Vivo Activity of Endostatin.
In a treatment experiment, mice were injected s.c. with
106 MC38 cells in 200 µl of PBS. Two days later
(treatment day 0) and again 1 week later (treatment day 7), mice
received injections of 109 pfu of Ad-ss-mEndo or
Ad-luc, or no virus, as described above (n = 10 animals/treatment group). Tumors were measured in two dimensions
using calipers on days 0, 2, 5, 7, 9, 11, and 13 by an investigator
blinded to the treatment groups. Tumor volume was calculated according
to the formula: volume = width2 x length x 0.52.
Statistical Analysis.
Data are presented as the mean ± SE. Comparisons
between groups were performed using the Mann-Whitney U test
(Instat 2.01, GraphPad Software), and two-tailed P < 0.05 was considered significant.
| Results and Discussion |
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Endostatin gene therapy may obviate these difficulties by using the
protein production capacity of the host to generate endostatin in
vivo, rather than delivering exogenous recombinant protein. Two
previous reports of endostatin gene therapy in mice using nonviral
vectors (8
, 11)
have demonstrated an antitumor effect in
the absence of significantly elevated circulating endostatin levels.
Chen et al. (11)
demonstrated a peak serum
endostatin concentration of 33 ng/ml using a liposomal gene delivery
system. We believe the antitumor effect noted may have been in part
attributable to tumor-specific gene delivery, because we have found
serum endostatin levels in untreated nude mice to be
30 ng/ml
(12)
. Blezinger et al. (8)
used
i.m. delivery of polymerized plasmid DNA carrying the endostatin gene
and also demonstrated an antitumor effect despite peak serum levels of
only 8 ng/ml. Differences between the murine endostatin immunoassay
used by Blezinger et al. (8)
and that used by
ourselves and Chen et al. (11)
may explain a
systemic endostatin effect despite apparently low circulating levels.
Tumor-specific gene therapy designed to produce high local concentrations of an antiangiogenic agent may be a useful strategy, particularly for tumors with minimal metastatic potential. On the basis of the work of Mauceri et al. (13) , radiation therapy may augment this antitumor effect. Although tumor-specific expression of an antiangiogenic gene offers clear benefits, a systemic approach avoids the difficult problem of developing tumor-specific gene delivery vectors while taking advantage of the apparent absence of systemic toxicity of most antiangiogenic agents (3) . Furthermore, elevation of circulating levels may allow the transgene product access to micrometastatic tumors in a prevascular stage of development, whereas tumor-targeted therapy is likely to require an established tumor blood supply to deliver the vector directly to the tumor.
We selected an adenoviral vector to deliver the murine endostatin gene because of previously reported high circulating levels of transgene product (5) , as well as known biodistribution mostly to the liver (14 , 15) . This model allowed us to investigate the hypothesis that the host, in particular the host liver, can be used as a "factory" for generating angiogenesis inhibitors.
We first cloned the murine endostatin gene preceded by the
E3/19K signal sequence (16)
into an adenoviral shuttle
plasmid (Fig. 1a)
and generated recombinant Ad-ss-mEndo. Infection of 293
cells (in which E1-deleted Ad can replicate) yielded dose-dependent
elevation of supernatant endostatin concentrations, up to 920 ± 33 ng/ml at a MOI of 100. Western blotting of the
Ad-ss-mEndo-infected cell supernatant revealed a single band with
mobility identical to that of recombinant murine endostatin (Fig. 1c)
. Human melanoma cells (in which E1-deleted Ad cannot
replicate) then were infected with Ad-ss-mEndo or Ad-luc. Supernatant
endostatin levels (Fig. 2a)
and the ability of the supernatant samples to inhibit
endothelial cell proliferation (Fig. 2b)
were evaluated. A
dose-dependent increase in inhibition was observed, up to 61 ± 4% (versus 25 ± 4% in supernatant
from Ad-luc-infected cells; P = 0.0006).
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| FOOTNOTES |
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1 To whom requests for reprints should be
addressed, at Surgery Branch, National Cancer Institute, NIH, 9000
Rockville Pike, Bethesda, MD 20892. Phone: (301) 496-6457; Fax:
(301) 402-1788. ![]()
2 The abbreviations used are: Ad, adenovirus;
ß-gal, ß-galactosidase; luc, luciferase; MOI, multiplicity of
infection; pfu, plaque-forming unit(s); EIA, enzyme immunoassay. ![]()
Received 10/ 5/99. Accepted 2/ 3/00.
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