
[Cancer Research 60, 1793-1796, April 1, 2000]
© 2000 American Association for Cancer Research
Antitumor Activity of Endostatin against Carcinogen-induced Rat Primary Mammary Tumors1
Gianpaolo Perletti,
Paola Concari,
Roberto Giardini,
Emanuela Marras,
Francesco Piccinini,
Judah Folkman2 and
Longchuan Chen
Dipartimento di Biologia Strutturale e Funzionale, Università degli Studi dellInsubria, 21100 Varese, Italy [G. P., P. C., E. M., F. P.]; Divisione di Anatomia Patologica, Istituto Nazionale per lo Studio e la Cura dei Tumori, 20133 Milano, Italy [R. G.]; Department of Surgery, Childrens Hospital, Harvard Medical School, Boston, Massachusetts 02115 [J. F., L. C.]
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ABSTRACT
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Endostatin, a Mr 20,000 fragment
of collagen XVIII, potently inhibits the growth of experimental tumors
implanted in mice. Here we report the cloning, expression, and
antitumor activity of the rat form of endostatin. When tested on breast
carcinomas arising in female virgin rats after intragastric
administration of 9,10-dimethyl-1,2-benzanthracene (DMBA), endostatin
induced significant inhibition of mammary tumor growth in all of the
treated rats during a 4-week treatment period without signs of systemic
toxicity. Interestingly, this arrest of tumor growth persisted
throughout a four-week off-therapy period. Moreover, endostatin was
effective in counteracting the development of multiple primary tumors.
These results confirm that rat endostatin is a potent anticancer agent
in a carcinogen-induced, spontaneously arising rat breast cancer model.
It not only stops the growth of existing tumors but also decreases the
incidence of the development of multiple neoplastic lesions.
 |
Introduction
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When cells are exposed to carcinogens, oncogenesis can occur,
leading to the formation of microscopic foci of proliferating
neoplastic cells. The progression of these lesions to larger, invasive
tumors requires the formation of new blood vessels (1)
.
This process is regulated by stimulators and inhibitors of angiogenesis
(2)
. In recent years, research has been devoted to
characterize the therapeutic potential of angiogenesis inhibitors as
antitumor agents. Endostatin, a specific angiogenesis inhibitor
produced through enzymatic cleavage of a protein precursor, the
multiplexin
1-collagen XVIII, is a potent anticancer agent in animal
models. The Mr 20,000 endostatin protein was shown to
specifically inhibit the proliferation of endothelial cells and to stop
the growth of Lewis lung carcinoma experimental metastases in mice
without signs of drug toxicity (3)
. Furthermore,
endostatin suppressed, in a dose-dependent fashion, the growth of a
panel of murine primary tumors (3)
. When therapy was
discontinued, tumors regrew at their primary sites. These tumors
regressed again at resumption of endostatin treatment. Tumors could be
subjected to repeated treatment cycles, without acquired resistance to
the therapy (4)
. Interestingly, after multiple cycles
specific for each tumor type, tumors ceased to regrow and remained as
dormant microscopic nodules, without further recurrence
(4)
. All of these studies were performed using implanted
tumors in mice. Thus, we extended our study to a different animal
species and to a model consisting of primary mammary carcinomas arising
in rats after administration of
DMBA,3
a potent carcinogen and environmental pollutant. This model of
carcinogen-induced predisposition to mammary carcinoma has the
advantage of bearing many similarities with human breast cancer. We
cloned the rat form of endostatin and demonstrated its ability to
inhibit tumor formation in this model.
 |
Materials and Methods
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When not specified, reagents were purchased from Sigma-Aldrich
(Milan, Italy).
Cloning of Rat Endostatin.
Rat liver cDNA (Clontech, Palo Alto, CA) was used as a template for a
first round of 25 cycles of PCR amplification with the following
primers: GAG GTG CCG GAG GGC TGG CTC ATC TT (forward); and ACT CTA GAG
CTT CCT TTT ATT TCT TGA GGA TTA CAT (reverse). The reverse primer was
designed to add an XbaI restriction site at the 3' end of
the amplified PCR product, which was digested with XbaI,
directionally ligated into the StuI-XbaI sites of
a pFastBac 1 vector, and sequenced. This construct was then used as
template for a second round of PCR aimed to obtain the exact sequence
of rat endostatin. The primers used for this second amplification
(forward: CAT ATG CAT ACT CAC CAG GAC TTT CAC; reverse: GCT AGC CAG AGG
CCC TAT TTG GAG A) were designed to introduce NdeI and
NheI restriction sites at the 5' and 3' extremities of the
PCR product, respectively. The PCR fragment was subcloned into the pCR
2.1 vector (Invitrogen, De Schelp, the Netherlands) and sequenced. The
rat endostatin cDNA was finally excised from the pCR 2.1 vector by
digestion with NdeI-NheI, and subcloned in-frame
into a pET24a-derived bacterial expression plasmid pCTB42#5 (kindly
provided by Dr. Thomas Boehm, Childrens Hospital, Boston, MA),
between NdeI and NheI restriction sites. Correct
in-frame insertion of rat endostatin cDNA was confirmed by automated
DNA sequencing.
Purification of Rat Endostatin.
Recombinant rat endostatin was purified from Escherichia
coli cultures according to the procedure described by OReilly
et al. (3)
. At the end of purification,
recombinant endostatin precipitated into a white protein flocculate,
which was subsequently concentrated in PBS by centrifugation at a
protein concentration of 8 mg/ml, aliquoted, and used for treatment of
tumor-bearing rats in its suspension form. The yield of recombinant rat
endostatin from E. coli cultures was low (2.6 mg/l),
compared with the yield of E. coli recombinant mouse
endostatin routinely with murine endostatin (3040 mg/l;
3
). Because of the low yield, a single purification batch
was not sufficient for treatment of an individual animal, and several
batches had to be randomly pooled. The dose of 20 mg/kg/day was
selected on the basis of the maximum antitumor efficacy shown by mouse
endostatin on murine tumors (3)
.
Chemical Carcinogenesis.
Fifteen virgin Sprague Dawley rats were used for our experiments
(5
, 6) . At the age of 50 days they were given 20 mg DMBA
dissolved at 45°C in 1 ml of corn oil through a stomach catheter.
Thereafter, rats were examined to monitor the outgrowth of mammary
tumors at weekly intervals for the first 3 weeks, and then twice weekly
until the first tumor was detected in each rat. Rats showing signs of
persistent toxicity due to DMBA administration (diarrhea, fur ruffling,
poor mobility) were excluded from the experiment.
Treatment of Rat Mammary Tumors.
At the onset of the first palpable tumors, between 40 and 60 days after
DMBA administration, rats were allotted to the endostatin or vehicle
groups alternating between the two. When the tumor volume reached
100200 mm3, the treated group received 20
mg/kg/day rat endostatin in PBS, whereas the control group received
equivalent volumes of PBS daily by s.c. injections for a period of 28
days. s.c. injections were given in the flanks of animals alternatively
and rotated at different positions. The injection sites were at a
distance at least 30 mm from the tumors.
The treatment period was followed by an off-therapy observation period
of 28 days. Tumors were measured weekly with a caliper throughout both
treatment and off-therapy periods; tumor volumes were calculated as
previously described (3)
.
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Results and Discussion
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Rat endostatin sequence was cloned by PCR based on sequence
homology between the NC1 region of human and mouse collagen
XVIII cDNA (5)
. Rat endostatin shares a high degree of
homology with the murine (95% identity) and human (85% identity)
proteins (Fig. 1
). The antitumor activity of E. coli-produced recombinant rat
endostatin was tested in a well-established rat mammary tumor model
("Huggins model"; Ref. 6
). A single intragastric
dose of 20 mg of DMBA, administered to immature, virgin rats at 50 days
of age, was sufficient to induce the onset within 4050 days of large,
fast-growing tumors, localized in the mammary epithelial area, which
have been classified as mammary adenocarcinomas (7)
. In
carcinogen-fed rats, the onset of the first tumor nodule was detectable
by palpation as early as 40 days after DMBA administration, followed by
other primary tumors appearing within 714 days in the mammary area of
the rats, reaching the maximum number of five tumors per animal. At the
onset of their first palpable tumor (average volume, 100200
mm3), rats were randomly divided in two groups:
one group of four rats was treated with 20 mg/kg/day rat recombinant
endostatin, and a second group of six control animals was treated with
equivalent volumes of PBS.

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Fig. 1. A, the nucleotide sequence of the
COOH-terminal region of rat collagen XVIII containing endostatin
(GenBank accession no AF189709). The rat endostatin coding region
(underlined) was cloned into an E. coli
expression vector. In B, the amino acid sequence of the
region containing rat endostatin is aligned with corresponding regions
of murine and human sequences. , the start of endostatin coding
region. *, stop codon. C, SDS PAGE analysis of rat
endostatin column purification steps. Lane 1, E.
coli lysate before applying to Ni-NTA column; Lane
2, Ni-NTA column flow-through; Lane 3,
Ni-NTA column eluate. The Mr 20,000 band
indicated by arrows in Lane 3 corresponds to
the expected size of recombinant rat endostatin.
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Endostatin showed a powerful inhibitory activity on mammary cancer
growth (Fig. 2A
). From the second week of treatment (starting at day 7) up
to the end of the off-therapy follow-up period, tumor burden values in
the endostatin-treated group were significantly different from those in
controls (P < 0.05 at day 7;
P < 0.001 from day 1456; Students
t test). At the end of treatment, the ratio between treated
tumor volumes and control tumor volumes (T:C
ratio) was 0.07. Throughout the whole experiment, no sign of toxicity
from endostatin was detected. During the first week of treatment with
endostatin, tumors grew very slowly before they arrested by
approximately 8 days (Fig. 2A
). This pattern is similar to
mouse models treated with endostatin (3)
. Although no data
are available about the pharmacokinetics of endostatin, it is possible
that slow release of the protein from a s.c. compartment required a
long lag time to reach a therapeutic threshold. An alternative
hypothesis could be that a significant amount of endostatin must first
accumulate in the tumor bed.

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Fig. 2. A, The effect of rat endostatin treatment
on the growth of DMBA-induced rat mammary tumors. The tumor
burdens ± SE of endostatin-treated () and
vehicle-treated ( ) animals during treatment (days 028) and
off-therapy (days 2956) periods are compared. Tumor burdens are
calculated as the sum of total measured tumor volumes per single
animal. In the endostatin-treated group, tumors that had been exposed
to endostatin remained growth-arrested even in the off-therapy period
as indicated by the dotted line. In contrast, two new tumors
that developed after treatment stopped grew normally, accounting for
the rise of the tumor volume curve during the off-therapy period. In
B, pictures show mammary tumors in an endostatin-treated
rat (left) and in a control, vehicle-treated animal
(right). Both of the pictures were taken at the end of
the 28-days treatment cycle.
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Interestingly, endostatin that was administered at the onset of the
first palpable tumor was able to counteract the outgrowth of multiple
primary tumors in treated rats. Fig. 3
summarizes the number of tumors in treated and control groups during
the time course of the experiment. Vehicle-treated control rats
developed 34 tumors during the treatment period (days 028). In
contrast, three of four endostatin-treated rats had only one tumor
throughout the treatment period (days 028). In a fourth rat, a second
nodule appeared 3 days after the beginning of endostatin therapy.
During the off-therapy period (days 2956), one tumor underwent
complete regression in one rat and was no longer palpable 7 days after
the end of endostatin treatment. Two rats developed a second tumor
during the off-therapy period. Interestingly, whereas, in these two
animals, second tumors grew with kinetics similar to those observed for
control cancers, the early-onset, "treated" tumors did not regrow
throughout the whole experiment period in either of the cases. It,
therefore, seems that, after the termination of endostatin treatment,
tumors that were exposed to endostatin remained growth-arrested,
whereas the later, untreated tumors could grow independently. In a
completely different model, a similar pattern was observed by Boehm
et al. (4)
. When endostatin-treated mice
bearing dormant, microscopic tumors were injected with fresh tumor
cells at distant sites while mice were off of endostatin therapy, new
tumors developed with growth kinetics similar to controls. Our
experiments also show, for the first time, that antiangiogenic therapy
with endostatin can reduce the incidence of primary tumors in
carcinogen-exposed rats. Moreover, once a tumor became dormant under
endostatin therapy, it did not resume growth after endostatin was
discontinued. New tumors could still form in the same
carcinogen-exposed rat after endostatin was discontinued, but these
tumors were always at remote sites. These results suggest that the
inhibition of an existing tumor by endostatin produces a local
antitumor effect in that tumor bed that persists after endostatin
therapy is discontinued. Endostatin may induce tumor dormancy only in
those tumors that are already angiogenic inasmuch as new tumors that
arose after discontinuation of endostatin did not enter a dormant state
in the absence of further endostatin therapy. The mechanism of
persistence of tumor dormancy after discontinuation of endostatin in
both our rat model and in the mouse model of Boehm et al.
(4)
is unclear. However, it can be speculated that
prolonged endostatin therapy induces some changes in the stroma of the
tumor bed, or that nonangiogenic tumor cells are preferentially
selected during treatment.

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Fig. 3. The average number of tumors detected in
endostatin-treated (n = 4) and control,
vehicle-treated (n = 6) rats during the
treatment (days 028) and off-therapy period (days 2956). The
occurrence and regression of tumors in the endostatin-treated group are
indicated by asterisks. *, a second tumor module appeared 3 days
after the beginning of endostatin therapy in one rat. **, one new
tumor arose in one rat and one treated tumor regressed completely in
another rat. ***, one new tumor arose in one rat.
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In summary, to our knowledge, this is the first demonstration of
antitumor efficacy of endostatin in carcinogen-induced primary tumors.
Secondly, it is the first confirmation in a different animal species of
tumor dormancy after endostatin therapy is discontinued. Thirdly, this
is the first demonstration that the carcinogenic effect of DMBA can be
suppressedalthough not completely eliminatedby short-term
endostatin therapy. Fourthly, the dormant state that occurs when
therapy is discontinued seems to be localized to the tumor bed
previously exposed to endostatin. In contrast, the fact that new tumors
could still arise at remote sites after discontinuation of endostatin
therapy suggests that endostatin-induced tumor dormancy may require
that therapy be initiated after the angiogenic process has begun. If
this latter speculation is valid, it would provide additional evidence
that endostatin is acting specifically against growing endothelium.
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Acknowledgments
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We thank Karen Keough and Rolf Christofferson for advice and
Armen Tashjian, Jr., and Shmuel Ben-Sasson for reading the manuscript.
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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 in part by grants from the Italian
government (Ministero dellUniversità e Ricerca Scientifica e
Tecnologica, 60%), from AIRC (Associazione Italiana per la Ricerca sul
Cancro), and from NIH (RO1-CA64481), and by a grant from Entremed to
Childrens Hospital. 
2 To whom requests for reprints should be
addressed, at Laboratory of Surgery Research, Childrens Hospital, 300
Longwood Avenue, Boston, MA 02115. 
3 The abbreviation used is: DMBA,
9,10-dimethyl-1,2-benzanthracene. 
Received 11/ 9/99.
Accepted 2/16/00.
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