
Clinical Cancer Research Vol. 6, 1655-1663, May 2000
© 2000 American Association for Cancer Research
Antisense TRPM-2 Oligodeoxynucleotides Chemosensitize Human Androgen-independent PC-3 Prostate Cancer Cells Both in Vitro and in Vivo1
Hideaki Miyake,
Kim N. Chi and
Martin E. Gleave2
The Prostate Centre, Vancouver General Hospital, Vancouver, British Columbia, V6H 3Z6 Canada [H. M., K. N. C., M. E. G.], and Division of Urology, University of British Columbia, Vancouver, British Columbia, V5Z 3J5 Canada [M. E. G.]
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ABSTRACT
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Although
numerous chemotherapeutic regimens have been evaluated for patients
with hormone-refractory prostate cancer, none has improved survival.
Testosterone-repressed prostate message-2 (TRPM-2), which is highly
up-regulated after androgen withdrawal and during androgen-independent
progression in prostate cancer, has been shown to inhibit apoptosis
induced by various kinds of stimuli. The objectives in this study were
to test whether antisense (AS) oligodeoxynucleotides (ODNs) targeted
against TRPM-2 enhance chemosensitivity in human
androgen-independent prostate cancer PC-3 cells both in
vitro and in vivo. Initially, the potency of 10
AS ODNs targeting various regions of the TRPM-2 mRNA were evaluated,
and the AS ODN targeted to the TRPM-2 translation
initiation site (AS ODN#2) was found to be the most potent sequence for
inhibiting TRPM-2 expression in PC-3 cells. Despite significant
dose-dependent and sequence-specific suppression of TRPM-2 expression,
AS ODN#2 had no effect on growth of PC-3 cells both in
vitro and in vivo. However, pretreatment of PC-3
cells with AS ODN#2 significantly enhanced chemosensitivity of Taxol
(paclitaxel) and mitoxantrone in vitro. Characteristic
apoptotic DNA laddering and cleavage of poly(ADP-ribose) polymerase
were observed after combined treatment with AS ODN#2 plus paclitaxel or
mitoxantrone but not with either agent alone. In vivo
administration of AS ODN#2 plus either paclitaxel or mitoxantrone
significantly decreased PC-3 tumor volume by 80 or 60%, respectively,
compared with mismatch control ODN plus either paclitaxel or
mitoxantrone. In addition, terminal deoxynucleotidyl
transferase-mediated nick end labeling staining revealed increased
apoptotic cells in tumors treated with AS ODN#2 plus paclitaxel or
mitoxantrone. These findings confirm that TRPM-2 overexpression confers
resistance to cytotoxic chemotherapy in prostate cancer cells and
illustrates the potential utility of combined treatment with AS TRPM-2
ODN plus chemotherapeutic agents for patients with hormone-refractory
prostate cancer.
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Introduction
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Prostate cancer is now the most commonly diagnosed malignancy and
the second leading cause of cancer mortality in men in Western
industrialized countries. Androgen withdrawal remains the only
effective therapy for patients with advanced disease. Approximately
80% of patients achieve symptomatic and/or objective response after
androgen ablation; however, progression to androgen independence
ultimately occurs in almost all cases (1)
. Although
numerous nonhormonal agents have been evaluated in patients with
hormone-refractory prostate cancer, these agents have limited antitumor
activity with an objective response rate of <20% and no demonstrated
survival benefit (2)
. Therefore, novel therapeutic
strategies targeting molecular mechanisms mediating resistance to
conventional agents must be developed to make a significant impact on
survival.
Advances in the field of nucleic acid chemistry offers one attractive
strategy to design AS3
ODN-based
therapeutic agents that specifically hybridize with complementary mRNA
regions of a target gene and thereby inhibit gene expression by forming
RNA/DNA duplexes (3)
. Rapid intracellular degradation of
ODNs is a potential disadvantage of AS ODN therapy, but this problem
can be overcome by substituting a nonbridging phosphoryl oxygen of DNA
with a sulfur to create a phosphorothioate backbone, which stabilizes
the ODN to nuclease digestion (4)
. Recently, several
antisense ODNs targeted against specific genes involved in neoplastic
progression have been evaluated as potential therapeutic agents
(5, 6, 7, 8)
. Collectively, these findings identify AS ODNs as a
novel class of antineoplastic agents when designed for appropriate
molecular targets. However, because numerous genes are involved in
tumor progression, inhibition of a single target gene will likely be
insufficient to inhibit tumor progression in a meaningful way. In fact,
combined use of AS ODN with other compounds, such as chemotherapeutic
agents, have been demonstrated to produce more potent antineoplastic
effects in some tumor model systems (9, 10, 11, 12)
.
TRPM-2, also known as clusterin or sulfated glycoprotein-2, was first
isolated from ram rete testes fluid (13)
and plays
important roles in various pathophysiological processes, including
tissue remodeling, reproduction, lipid transport, complement
regulation, and apoptosis (14)
. Because TRPM-2 expression
is increased in various benign and malignant tissues undergoing
apoptosis, it has been regarded as a marker for cell death
(15, 16, 17, 18)
. Recent studies, however, provide conflicting
findings regarding the relationship between TRPM-2 up-regulation and
increased apoptotic activity (19, 20, 21)
. Similarly, TRPM-2
expression increases in regressing normal prostatic epithelial cells
(22
, 23) and prostate cancer xenografts (24
, 25)
after treatment with various apoptotic stimuli and is
associated with cell survival and disease progression in prostate
cancer (26, 27, 28, 29, 30)
. We have reported recently that an
increase in TRPM-2 expression after androgen ablation accelerates tumor
progression by inhibiting castration (31)
- and
chemotherapy4
-induced apoptosis.
However, the functional significance of TRPM-2 expression in
established AI human prostate cancer has not been examined.
In the present study, we designed and screened 10 phosphorothioate AS
ODNs targeted against the human TRPM-2 gene to identify
potent ODN sequences that specifically inhibit TRPM-2 expression in
human AI prostate cancer PC-3 cells. We then tested whether AS TRPM-2
ODN can enhance the response of PC-3 cells to either Taxol (paclitaxel)
or mitoxantrone both in vitro and in vivo.
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Materials and Methods
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Tumor Cell Line.
PC-3, derived from human prostate cancer, was purchased from the
American Type Culture Collection (Rockville, MD). Cells were maintained
in DMEM (Life Technologies, Inc., Gaithersburg, MD) supplemented with
5% heat-inactivated FCS.
Chemotherapeutic Agents.
Paclitaxel and mitoxantrone were purchased from Sigma Chemical Co. (St.
Louis, MO) and Wyeth-Ayerst, Inc. (Montreal, Canada), respectively.
Stock solutions of paclitaxel and mitoxantrone (1 mg/ml) were prepared
with DMSO and diluted with PBS to the required concentrations before
each in vitro experiment. Polymeric micellar paclitaxel used
in these in vivo studies was generously supplied by Dr.
Helen M. Burt (Faculty of Pharmaceutical Sciences, University of
British Columbia, Vancouver, Canada).
AS TRPM-2 ODN.
Phosphorothioate ODNs used in this study were obtained from the Nucleic
Acid-Protein Service Unit, University of British Columbia (Vancouver,
Canada). The sequences of 10 AS TRPM-2 ODNs (AS ODN#1 to AS ODN#10) and
a two-base TRPM-2 MM ODN used as a control (MM Co. ODN) were as
follows: AS ODN#1, 5'-TGGAGTCTTTGCACGCCTCGG-3'; AS ODN#2,
5'-CAGCAGCAGAGTCTTCATCAT-3'; AS ODN #3, 5'-ATTGTCTGAG-ACCGTCTGGTC-3';
AS ODN#4, 5'-CCTTCAGCTTTGT-CTCTGATT-3'; AS ODN#5,
5'-AGCAGGGAGTCGATG-CGGTCA-3'; AS ODN#6,
5'-ATCAAGCTGC-GGACGATG-CGG-3'; AS ODN#7,
5'-GCAGGCAGCCCGTGGAGTTGT-3'; AS ODN#8, 5'-TTCAGCTGCTCCAGCAAGGAG-3'; AS ODN#9, 5'-AATTTAGGGTT- CTTCCTGGAG-3'; AS ODN#10,
5'-GCTGGGCGGAGTTGGGGGCCT-3'; and MM Co. ODN,
5'-CAGCAGCAGAGTATTTATCAT-3'.
Treatment of Cells with ODN.
Lipofectin, a cationic lipid (Life Technologies, Inc.), was used to
increase the ODN uptake of cells. PC-3 cells were treated with various
concentrations of ODN after a preincubation for 20 min with 3 µg/ml
lipofectin in serum-free OPTI-MEM (Life Technologies, Inc.). Four h
after the beginning of the incubation, the medium containing ODN and
lipofectin was replaced with standard culture medium described above.
Northern Blot Analysis.
Total RNA was isolated from cultured PC-3 cells and PC-3 tumor tissues
using the acid-guanidinium thiocyanate-phenol-chloroform method.
Electrophoresis, hybridization, and washing conditions were carried out
as reported previously (8)
. Human TRPM-2 and GAPDH cDNA
probes were generated by reverse transcription-PCR from total RNA
of human kidney using primers 5'-AAGGAAATTCAAAATGCTGTCAA-3' (sense) and
5'-ACAGACAAGATCTCCCGGCACTT-3' (AS) for TRPM-2, and
5'-TGC-TTTTAAC-TCTGGTAAAGT-3' (sense) and
5'-ATATTTGG-CAGGTTTTTCTAGA-3' (AS) for GAPDH. The density of bands
for TRPM-2 was normalized against that of GAPDH by densitometric
analysis.
Western Blot Analysis.
Samples containing equal amounts of protein (15 µg) from lysates of
the cultured PC-3 cells and PC-3 tumors were electrophoresed on a
SDS-polyacrylamide gel and transferred to a nitrocellulose filter. The
filters were blocked in PBS containing 5% nonfat milk powder at 4°C
overnight and then incubated for 1 h with a 1:400-diluted
antihuman TRPM-2 goat polyclonal antibody (Santa Cruz Biotechnology,
Inc.), 1:10000-diluted antirat ß-tubulin mouse monoclonal antibody
(Chemicon International Inc., Tumecula, CA), or 1:600-diluted antihuman
PARP mouse monoclonal antibody (PharMingen, Mississauga, Ontario,
Canada). The filters were then incubated for 30 min with
horseradish peroxidase-conjugated antigoat or mouse IgG antibody
(Amersham Life Science, Arlington Heights, IL), and specific proteins
were detected using an enhanced chemiluminescence Western blotting
analysis system (Amersham Life Science).
MTT Assay.
The in vitro growth-inhibitory effects of AS TRPM-2 ODN plus
paclitaxel or mitoxantrone on PC-3 cells were assessed using the MTT
assay as described previously (32)
. Briefly, 1 x
104 cells were seeded in each well of 96-well
microtiter plates and allowed to attach overnight. Cells were then
treated once daily with 500 nM ODN for 2 days.
After ODN treatment, cells were treated with various concentrations of
paclitaxel or mitoxantrone. After 48 h of incubation, 20 µl of 5
mg/ml MTT (Sigma Chemical Co.) in PBS were added to each well, followed
by incubation for 4 h at 37°C. The formazan crystals were then
dissolved in DMSO. The absorbance was determined with a microculture
plate reader (Becton Dickinson Labware, Lincoln Park, NJ) at 540 nm.
Absorbance values were normalized to the values obtained for the
vehicle-treated cells to determine the percentage of survival. Each
assay was performed in triplicate.
DNA Fragmentation Analysis.
Nucleosomal DNA degradation was analyzed as described previously with a
minor modification (32)
. Briefly, 1 x
105 PC-3 cells were seeded in 5-cm culture dishes
and allowed to adhere overnight. After treatment with ODN plus
paclitaxel or mitoxantrone using the same schedule described above,
cells were harvested and then lysed in a solution containing 100
mM NaCl, 10 mM Tris (pH 7.4), 25 mM
EDTA, and 0.5% SDS. After the centrifugation, the supernatants were
incubated with 300 µg/ml proteinase K for 5 h at 65°C and
extracted with phenol-chloroform. The aqueous layer was treated with
0.1 volume of 3 M sodium acetate, and the DNA was
precipitated with 2.5 volumes of 95% ethanol. After treatment with 100
µg/ml RNase A for 1 h at 37°C, the sample was electrophoresed
on a 2% agarose gel and stained with ethidium bromide.
Assessment of in Vivo Tumor Growth.
Approximately 1 x 106 PC-3 cells were
inoculated s.c. with 0.1 ml of Matrigel (Becton Dickinson Labware,
Bedford, MA) in the flank region of male athymic nude mice (BALB/c
strain; Charles River Laboratory, Montreal, Quebec, Canada),
68 weeks of age, under methoxyfluorane anesthesia. When PC-3 tumors
grew to 1 cm in diameter, usually 23 weeks after injection, mice were
randomly selected for treatment with AS TRPM-2 ODN alone, MM Co. ODN
alone, AS TRPM-2 ODN plus paclitaxel, MM Co. ODN plus paclitaxel, AS
TRPM-2 ODN plus mitoxantrone, or MM Co. ODN plus mitoxantrone. Each
experimental group consisted of eight mice. After randomization, 10
mg/kg AS TRPM-2 or MM Co. ODN was injected i.p. once daily into each
mouse for 28 days. From days 10 to 14 and from days 24 to 28, 0.5 mg
polymeric micellar paclitaxel or 0.03 mg mitoxantrone was administered
once daily by i.v. injection. Tumor volume was measured once weekly and
calculated by the formula: length x width x depth x
0.5236 (33)
. Data points were reported as average tumor
volumes ± SD.
TUNEL Staining.
A modified TUNEL technique (34)
was used to detect
apoptotic cells in PC-3 tumors using the ApopTag In situ
Apoptosis Detection System (Oncor, Gaithersburg, MD), according to the
manufacturers protocol. The number of positively stained cells/high
power field in five random fields was counted and averaged.
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Results
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Screening for Active AS ODN Sequences Targeting the Human
TRPM-2 Gene in PC-3 Cells.
To identify effective AS ODNs capable of inhibiting TRPM-2
gene expression in PC-3 cells, 10 phosphorothioate ODNs designed to
hybridize with various regions of TRPM-2 mRNA were synthesized (Fig. 1A)
. Northern blot analyses
were used to evaluate the effects of treatment with these AS ODNs on
TRPM-2 mRNA expression in PC-3 cells. As shown in Fig. 1, B and C
, after daily treatment of PC-3 cells with 1
µM AS TRPM-2 ODN for 2 days, the different ODNs
exhibited varied degrees of activity. Seven AS ODNs had little or no
effect on TRPM-2 mRNA expression levels, whereas three ODNs had
moderate effects. The most potent AS sODN identified from this series
was AS ODN#2 (5'-CAGCAGCAGAGTCTTCATCAT-3'), which targets the human
TRPM-2 translation initiation site, reducing TRPM-2
expression levels by 80% compared with MM Co. ODN treatment. AS ODN#2
was used in all subsequent experiments.

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Fig. 1. Effects of various AS human TRPM-2 ODNs on
TRPM-2 mRNA expression in PC-3 cells. A, schematic
representation of the relative position of 10 phosphorothioate AS ODNs
designed to hybridize with various regions of the TRPM-2
gene. UTR, untranslated region. B, PC-3
cells were treated daily with AS TRPM-2 ODN or a two-base TRPM-2 MM ODN
for 2 days. Total RNA was extracted from cultured cells and analyzed
for TRPM-2 and GAPDH levels by Northern blotting. No Tx,
untreated cells. C, quantitative analysis of TRPM-2 mRNA
levels after normalization to GAPDH mRNA levels in PC-3 cells with AS
TRPM-2 or MM Co. ODN was performed using a laser densitometer.
Columns, means; bars, SD. *, differs
from control (P < 0.01) by Students
t test.
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Sequence-specific and Dose-dependent Inhibition of TRPM-2
Expression by AS TRPM-2 ODN.
To further define the specificity and potency of AS ODN#2-mediated
inhibition of TRPM-2 gene expression, the effects of AS
ODN#2 and MM Co. ODN on TRPM-2 mRNA and protein levels were determined
by Northern and Western blot analyses, respectively. As shown in Fig. 2, A and B
, daily
treatment of PC-3 cells with AS ODN#2 (100, 500, or 1000
nM) for 2 days reduced TRPM-2 mRNA levels by 17,
39, or 78%, respectively, whereas TRPM-2 mRNA expression was not
affected by the MM Co. ODN at any of the used concentrations.
Inhibition of TRPM-2 protein levels in PC-3 cells was also observed
after daily treatment with AS ODN#2 for 4 consecutive days (Fig. 2C)
.

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Fig. 2. Sequence-specific and dose-dependent
inhibition of TRPM-2 expression by AS TRPM-2 ODN in PC-3 cells.
A, PC-3 cells were treated daily with various
concentrations of AS TRPM-2 ODN (AS ODN#2: CAGCAGCAGA-GTCTTCATCAT)
or a two-base TRPM-2 MM ODN (MM Co. ODN: CAGCA-GCAGAGTATTTATCAT) as a
control for 2 days; total RNA was extracted from culture cells, and
TRPM-2 and GAPDH levels were analyzed by Northern blotting. No
Tx, untreated cells. B, quantitative analysis of
TRPM-2 mRNA levels after normalization to GAPDH mRNA levels in PC-3
cells after treatment with various concentrations of AS ODN#2 or MM Co.
ODN was performed by using laser densitometry. Points,
means of triplicate analyses; bars, SD. ** and *,
differ from control (P < 0.01 and
P < 0.05, respectively) by Students
t test. C, PC-3 cells were treated daily
with 1 µM AS ODN#2 or MM Co. ODN for 4 days, protein was
extracted from culture cells, and TRPM-2 and ß-tubulin protein levels
were analyzed by Western blotting. No Tx, untreated
cells.
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Effects of AS TRPM-2 ODN Treatment on the Growth of PC-3 Cells Both
in Vitro and in Vivo.
To determine whether the reduction of TRPM-2 expression affects the
growth of PC-3 cells in vitro, the growth rates of PC-3
cells after treatment with various concentrations of AS ODN#2 or MM Co.
ODN once daily for 2 days were examined using the MTT assay. No
significant difference in PC-3 cell growth was observed between AS
ODN#2 and MM Co. ODN treatment (data not shown).
We then evaluated the effects of AS ODN#2 treatment on the growth of
PC-3 tumors in vivo. Male nude mice bearing PC-3 tumors
1
cm in diameter were randomly selected for treatment with AS ODN#2
versus MM Co. ODN, and 10 mg/kg ODN was administered once
daily by i.p. injection for 28 days. As shown in Fig. 3
, there was no significant difference in
PC-3 tumor growth between these two groups.

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Fig. 3. Effects of AS TRPM-2 ODN administration on PC-3
tumor growth. When PC-3 tumors became 1 cm in diameter, 10 mg/kg AS
ODN#2 or MM Co. ODN were injected i.p. once daily for 28 days into each
mouse. Tumor volume was measured once weekly and calculated by the
formula: length x width x depth x 0.5236.
Points, mean tumor volume in each experimental group
containing eight mice; bars, SD.
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To determine whether AS ODN#2 treatment inhibits TRPM-2 expression in
PC-3 tumors in vivo, each of three tumor-bearing nude mice
were given 10 mg/kg AS ODN#2 or MM Co. ODN i.p. once daily for 5 days,
and TRPM-2 mRNA expression levels in harvested tumor tissues were then
analyzed by Northern blotting. Treatment with AS ODN#2 resulted in a
68% reduction in TRPM-2 mRNA levels in PC-3 tumors compared with MM
Co. ODN-treated tumors (Fig. 4, A and B)
.

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Fig. 4. Effects of AS TRPM-2 ODN administration on
TRPM-2 mRNA levels in PC-3 tumors in vivo. A, each of 3
PC-3 tumor-bearing mice were daily treated with AS ODN#2 or MM Co. ODN
at a dose of 10 mg/kg for 5 days; total RNA was extracted from PC-3
tumors 6 days after the initiation of treatment, and TRPM-2 and GAPDH
mRNA levels were analyzed by Northern blotting. Lanes
13, PC-3 tumors in mice administered MM Co. ODN; Lanes
46, PC-3 tumors in mice administered AS ODN #2.
B, quantitative analysis of TRPM-2 mRNA levels after
normalization to GAPDH mRNA levels in PC-3 tumors after treatment with
AS ODN #2 or MM Co. ODN was performed using a laser densitometer.
Columns, means; bars, SD. *, differs
from control (P < 0.01) by Students
t test.
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Enhanced Chemosensitivity of PC-3 Cells in Vitro
with AS TRPM-2 ODN Treatment.
To determine whether treatment with AS ODN#2 enhances the cytotoxic
effects of paclitaxel and mitoxantrone, PC-3 cells were treated with
500 nM AS ODN#2 or MM Co. ODN once daily for 2 days and
then incubated with medium containing various concentrations of either
paclitaxel or mitoxantrone for 2 days. The MTT assay was then performed
to determine cell viability. As shown in Fig. 5, A and B
, AS ODN#2 treatment significantly enhanced
chemosensitivity of paclitaxel and mitoxantrone in a dose-dependent
manner, reducing the IC50 of paclitaxel and
mitoxantrone by more than 60 and 50%, respectively.

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Fig. 5. Effect of combined treatment with AS TRPM-2 ODN
and chemotherapy on PC-3 cell growth and apoptosis. A,
PC-3 cells were treated daily with 500 nM AS ODN#2 or MM
Co. ODN for 2 days. After ODN treatment, the medium was replaced with
medium containing various concentrations of Taxol (paclitaxel). After
48 h of incubation, cell viability was determined by the MTT
assay. Points, means of triplicate analyses;
bars, SD. ** and *, differ from control
(P < 0.01 and P < 0.05,
respectively) by Students t test. B,
PC-3 cells were treated daily with 500 nM AS ODN#2 or MM
Co. ODN for 2 days. After ODN treatment, the medium was replaced with
medium containing various concentrations of mitoxantrone. After 48 h of incubation, cell viability was determined by the MTT assay.
Points, means of triplicate analyses;
bars, SD. ** and *, differ from control
(P < 0.01 and P < 0.05,
respectively) by Students t test. C,
after the same treatment schedule as described A and
B, DNA was extracted from PC-3 cells, electrophoresed in
a 2% agarose gel, and visualized by ethidium bromide staining and UV
transillumination. D, proteins were extracted from PC-3
cells after the same treatment as described in A and
B and analyzed by Western blotting with an anti-PARP
antibody. Uncleaved intact PARP, Mr 116,000;
cleaved PARP, Mr 85,000.
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The DNA fragmentation assay was performed to compare induction of
apoptosis after treatment with 500 nM AS ODN#2 either alone
or in combination with 5 nM paclitaxel or 1 nM
mitoxantrone. Using the same treatment schedule described above, the
characteristic apoptotic DNA ladder was observed only with combined
treatment of AS ODN#2 plus paclitaxel or mitoxantrone (Fig. 5C)
. We further evaluated the effects of combined AS ODN#2
plus chemotherapy by using Western blot analysis to identify cleavage
of PARP protein, a substrate of the caspases activated during the
process of apoptotic execution (35)
. The
Mr 116,000 intact form of PARP was
observed in all samples examined, whereas the
Mr 85,000 PARP cleavage fragment was
detected only after combined treatment with AS ODN#2 plus paclitaxel or
mitoxantrone (Fig. 5D)
.
Enhanced Cytotoxic Effects of Chemotherapy in PC-3 Tumors in
Vivo by Systemic Administration of AS TRPM-2 ODN.
Athymic male mice bearing PC-3 tumors
1 cm in diameter were randomly
selected for treatment with AS ODN#2 plus paclitaxel, MM Co. ODN plus
paclitaxel, AS ODN#2 plus mitoxantrone, or MM Co. ODN plus
mitoxantrone. Mean tumor volume was similar at the beginning of
treatment in each of these groups. After randomization, 10 mg/kg AS
ODN#2 or MM Co. ODN were injected i.p. once daily for 28 days. From
days 10 to 14 and from days 24 to 28, 0.5 mg polymeric micellar
paclitaxel or 0.3 mg mitoxantrone was administered once daily by i.v.
injection. As shown in Fig. 6, A and B
, AS ODN#2 significantly enhanced the
apoptotic effects of micellar paclitaxel and mitoxantrone in PC-3
tumors, reducing mean tumor volume by more than 80 and 60%,
respectively, by 8 weeks after initiation of treatment. In addition,
TUNEL staining detected a 5- or 3-fold increase in the numbers of
apoptotic cells in the PC-3 tumors treated with AS ODN#2 plus micellar
paclitaxel or mitoxantrone, respectively, compared with those treated
with MM Co. ODN plus micellar paclitaxel or mitoxantrone. Under the
experimental conditions used in the above in vivo
experiments, no side effects associated with ODN treatment and/or
chemotherapy were observed.
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Fig. 6. Effects of combined treatment with AS
TRPM-2 ODN plus chemotherapy on PC-3 tumor growth. A and
B, mice bearing PC-3 tumors were randomly selected for
treatment with AS ODN#2 plus micellar Taxol (paclitaxel) or MM Co. ODN
plus micellar paclitaxel (A), or AS ODN#2 plus
mitoxantrone or MM Co. ODN plus mitoxantrone (B). When
PC-3 tumors became 1 cm in diameter, 10 mg/kg AS ODN#2 or MM Co. ODN
were daily injected i.p. for 28 days. From days 10 to 14 and from days
24 to 28, 0.5 mg micellar paclitaxel or 0.3 mg mitoxantrone was daily
administered by i.v. injection. Tumor volume was measured once weekly
and calculated by the formula: length x width x depth x 0.5236. Points, mean tumor volume in each
experimental group containing eight mice; bars, SD.
** and *, differ from control (P < 0.01 and
P < 0.05, respectively) by Students
t test. CF, after completion of the
same treatment schedule described in A and
B, PC-3 tumors were harvested from each treatment group
for detection of apoptosis using TUNEL staining. Sections of
paraffin-embedded PC-3 tumors were stained with digoxigenin-dUTP
antibody to identify apoptotic cells. C, PC-3 tumor
after treatment with AS ODN#2 and micellar paclitaxel.
D, PC-3 tumor after treatment with MM Co. ODN and
micellar paclitaxel. E, PC-3 tumor after treatment with
AS ODN#2 and mitoxantrone. F, PC-3 tumor after treatment
with MM Co. ODN and mitoxantrone.\.
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Discussion
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In the prostate gland, despite the original hypothesis that TRPM-2
is a marker of programmed cell death (12
, 23, 24, 25)
, several
experimental and clinical studies have provided conflicting findings
showing the dissociation of TRPM-2 expression from apoptosis
(26, 27, 28, 29, 30)
. For example, increased TRPM-2 expression in
dysplastic lesions of rat prostate was not associated with enhanced
apoptotic activity (29)
. Introduction of the
TRPM-2 gene into LNCaP prostate cancer cells renders them
highly resistant to tumor necrosis factor-
-induced apoptosis
(27)
. Furthermore, a close correlation between
intracellular levels of TRPM-2 and tumor grade in human prostate cancer
specimens has been reported (28)
. We also demonstrated
previously that overexpression of TRPM-2 helps mediate AI progression
against castration (31)
- and
chemotherapy4-induced apoptosis in
androgen-dependent prostate cancer models. Collectively, these findings
suggest that TRPM-2 up-regulation plays a protective role in normal and
malignant prostate tissues against apoptosis induced by various kinds
of stimuli and thereby may confer an aggressive phenotype during
prostate cancer progression.
The limited efficacy of cytotoxic chemotherapy remains a major problem
for the treatment of patients with advanced hormone refractory prostate
cancer.
The lack of survival benefits with traditional cytotoxic chemotherapy
in patients with hormone-refractory prostate cancer results from
intrinsic chemoresistance and the limitation of toxicity on an elderly
population. The chemoresistant phenotype in hormone-refractory prostate
cancer is attributable, in part, to high levels of antiapoptotic
genes, including bcl-2, Bcl-xL, and
TRPM-2, all of which are increased after androgen ablation
and remain constitutively overexpressed in AI tumors (18
, 36
, 37)
.
Although no chemotherapeutic agent has demonstrated improved survival
in patients with advanced prostate cancer, recent Phase II reports are
documenting improved response rates in hormone-refractory disease
(2
, 36, 37, 38)
. For example, >50% of patients responded in
one Phase II study of combined paclitaxel plus estramustine treatment
(38)
. In two randomized trials, mitoxantrone in
combination with prednisone was shown to produce significant palliative
benefit compared with steroids alone (39
, 40)
.
Furthermore, recent preclinical studies have provided proof of
principle evidence that targeting antiapoptotic genes using AS ODN
enhances apoptosis induced by conventional cytotoxic chemotherapy
(9, 10, 11, 12)
. Therefore, in this study, we set out to screen
for potent and sequence-specific ODNs targeted against human TRPM-2.
The AS TRPM-2 ODN corresponding to the human TRPM-2
translation initiation site was the most potent sequence of the 10 AS
ODN targeting various regions of TRPM-2 gene. To clarify the
functional role of TRPM-2 expression in AI prostate cancer, we then
tested the effects of AS TRPM-2 ODN on human prostate cancer PC-3 cell
growth and whether AS TRPM-2 ODN could enhance the cytotoxic effects of
paclitaxel and mitoxantrone in this model system.
Phosphorothioate AS TRPM-2 ODN used in this study significantly
inhibited expression of TRPM-2 mRNA and protein in PC-3 cells both
in vitro and in vivo. Sequence specificity was
confirmed using a two-base TRPM-2 MM ODN, which had no effects on
TRPM-2 expression in PC-3 cells. Despite a significant decrease in
TRPM-2 expression after AS ODN treatment, no differences in PC-3 cell
growth in vitro or PC-3 tumor growth in vivo was
observed after AS TRPM-2 ODN treatment. These findings suggest that
targeting and inhibiting TRPM-2 expression have no significant effects
on cell proliferation in the absence of other apoptotic stimuli or cell
death signals.
The administration of AS TRPM-2 ODN with chemotherapeutic agents,
however, inhibited PC-3 cell growth both in vitro and
in vivo through enhanced apoptosis. Pretreatment of PC-3
cells with AS enhanced apoptosis induced by these agents.
Consistent with these in vitro studies, synergistic effects
of combined use of AS TRPM-2 ODN plus chemotherapeutic agents were also
observed in in vivo studies. Systemic administration of AS
TRPM-2 ODN plus polymeric micellar paclitaxel or mitoxantrone
suppressed the PC-3 tumor growth by 90 and 50%, respectively, compared
with treatment with MM Co. ODN plus either agent. Detection of
increased apoptotic cells after combined AS ODN and chemotherapy by
TUNEL staining in PC-3 tumors suggests that decreased tumor progression
rates after combined AS TRPM-2 ODN plus paclitaxel or mitoxantrone
resulted from enhanced chemotherapy-induced apoptosis rather than
decreased cell proliferation.
The results in the present study suggest that increased TRPM-2 helps
mediate prostate cancer progression by inhibiting apoptotic cell death
induced by several kinds of therapy, including cytotoxic chemotherapy.
Decreasing TRPM-2-mediated chemoresistance by AS TRPM-2 ODN may provide
a feasible and safe strategy to enhance chemosensitivity in
hormone-refractory prostate cancer. The preclinical data presented here
provide proof of principle support for designing clinical studies with
combined AS TRPM-2 plus paclitaxel and/or mitoxantrone therapy for
patients with hormone-refractory disease.
 |
ACKNOWLEDGMENTS
|
|---|
We thank Mary Bowden and Howard Tearle for excellent technical
assistance.
 |
FOOTNOTES
|
|---|
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 This work was supported by Grant 009002 from the
National Cancer Institute of Canada. 
2 To whom requests for reprints should be
addressed, at Division of Urology, University of British Columbia, D-9,
2733 Heather Street, Vancouver, British Columbia, V5Z 3J5 Canada. 
3 The abbreviations used are: AS, antisense; ODN,
oligodeoxynucleotide; TRPM-2, testosterone repressed prostate
message-2; MM, mismatch; Co., control; AI, androgen independent;
PARP, poly(ADPribose) polymerase; GAPDH, glyceraldehyde-3-phosphate
dehydrogenase; MTT,
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; TUNEL,
terminal deoxynucleotidyl transferase-mediated nick end labeling. 
4 H. Miyake, C. Nelson, P. S. Rennie, and M. E.
Gleave. Acquisition of chemoresistant phenotype by overexpression of
the antiapoptotic gene, TRPM-2, in prostate cancer
xenograft models. Cancer Res., in press, 2000. 
Received 11/18/99;
revised 2/ 2/00;
accepted 2/ 2/00.
 |
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