2866
Bortezomib induces apoptosis in esophageal squamous
cell carcinoma cells through activation of the p38
mitogen-activated protein kinase pathway
further suggested that bortezomib could be added to
existing ESCC therapeutic regimens. [Mol Cancer Ther
2008;7(9):2866 – 75]
1
The Wistar Institute; 2Fox Chase Cancer Center; 3Department
of Cancer Biology, Abramson Family Cancer Research Institute;
and 4Gastroenterology Division, Department of Medicine,
Department of Genetics, Abramson Cancer Center, University of
Pennsylvania School of Medicine, Philadelphia, Pennsylvania
Introduction
Abstract
Esophageal squamous cell carcinoma (ESCC) is an
exceptionally drug-resistant tumor with a 5-year survival
rate <5%. From an initial drug screen, we identified
bortezomib as having robust activity in ESCC lines.
Mechanistically, bortezomib induced a G2-M-phase cell
cycle arrest and p53-independent apoptosis associated
with caspase cleavage and Noxa induction. Bortezomib
also showed excellent activity in organotypic culture and
in vivo models of ESCC. Biochemically, bortezomib
treatment activated the p38 and c-Jun NH2-termnial
kinase stress-activated mitogen-activated protein kinase
(MAPK) pathways and induced phospho-H2AX activity.
Although H2AX is known to cooperate with c-Jun
NH 2 -termnial kinase to induce apoptosis following
UV irradiation, knockdown of H2AX did not abrogate
bortezomib-induced apoptosis. Instead, blockade of p38
MAPK signaling, using either small interfering RNA or a
pharmacologic inhibitor, reversed bortezomib-induced
apoptosis and the up-regulation of Noxa. Radiation
therapy is known to activate the p38 MAPK pathway
and is a mainstay of ESCC treatment strategies. In a final
series of studies, we showed that the coadministration of
bortezomib with irradiation led to enhanced p38 MAPK
activity and a significant reduction in colony formation.
We therefore suggest that p38 MAPK pathway activation
is an excellent potential therapeutic strategy in ESCC. It is
Received 5/1/08; revised 7/11/08; accepted 7/13/08.
Grant support: National Cancer Institute grant P01CA098101 (M. Herlyn).
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.
Requests for reprints: Keiran S.M. Smalley, The Wistar Institute, 3601
Spruce Street, Philadelphia, PA 19104. Phone: 215-898-0002;
Fax: 215-898-0890. E-mail: k.smalley@mac.com; or Meenhard Herlyn.
E-mail: herlynm@wistar.org
Copyright C 2008 American Association for Cancer Research.
doi:10.1158/1535-7163.MCT-08-0391
Esophageal cancers are among the most aggressive tumors
known with over 75% of newly diagnosed patients dying
within the first year. Five-year survival rates are also dismal
(5-10%), with over 50% of patients harboring distant
metastases at the time of presentation (1). One source for
optimism is the current revolution in molecularly targeted
cancer therapy. Recently, there has been a great deal of
progress in the identification of key ‘‘driver’’ oncogenic
mutations and signaling pathway activities that can be
targeted by small-molecule pharmacologic inhibitors. In
particular, striking results have been observed with imatinib
(Gleevec) in chronic myeloid leukemia and gastrointestinal
stromal tumors and gefitinib (Iressa) and erlotinib (Tarceva)
in non-small cell lung carcinoma (2 – 5). To date, such
‘‘targeted’’ strategies have been not been explored extensively in esophageal squamous cell carcinoma (ESCC). As
tumors often posses multiple genetic and cell signaling
lesions, the inhibition of one signaling pathway is often
therapeutically ineffective (6). Rather, more success can be
envisioned with agents targeted against multiple cellular
pathways. One such multipathway inhibitor is the novel
proteasome inhibitor bortezomib (Velcade). Bortezomib is a
dipeptidyl boronic acid that inhibits the 26S proteasome, a
large multisubunit protein complex that degrades polyubiquitinated target proteins, such as the cyclins, apoptosis
regulators, and p53 (7, 8). Although the proteasome
performs many important housekeeping functions in normal cells, the increased metabolic activity and rapid cycling
seen in transformed cells makes proteasome function critical
for tumor cell survival (7). The mechanisms of bortezomibinduced cell death remain poorly defined and appear to be
cell type specific or context dependent. In melanoma and
head and neck carcinoma cell lines, increased Noxa
expression appears to be critical for apoptosis induction (9,
10).There is also evidence that the generation of cellular
stress leading to reactive oxygen species (ROS) is a critical
feature of the anticancer activity of bortezomib (11). In the
current study, we show that bortezomib is strongly
apoptotic in ESCC lines and blocks the growth and survival
of these cells in three-dimensional organotypic cultures and
animal xenograft models, thereby providing complementary in vitro and in vivo findings. We further show that
bortezomib exerts its proapoptotic effects through a novel
mechanism involving the activation of the p38 mitogenactivated protein kinase (MAPK) pathway.
Mol Cancer Ther 2008;7(9). September 2008
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Mercedes Lioni,1 Kazuhiro Noma,1
Andrew Snyder,1 Andres Klein-Szanto,2
J. Alan Diehl,3 Anil K. Rustgi,4 Meenhard Herlyn,1
and Keiran S.M. Smalley1
Molecular Cancer Therapeutics
Materials and Methods
Mol Cancer Ther 2008;7(9). September 2008
Results
Bortezomib Inhibits the Proliferation of Human ESCC
Cell Lines through G2-M Cell Cycle Arrest and Induction
of Apoptosis
As targeted therapeutic approaches have not been
attempted in ESCC in an extensive fashion, we initiated
our studies by screening a panel of ESCC lines against
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Cell Lines
Esophageal cancer cells, TE cell lines (TE1, TE3, TE8,
TE10, TE11, and TE12), and FEF3 (fetal esophageal
fibroblasts) are available commercially and through the
NIH/National Institute of Diabetes Digestive and Kidney Diseases Center for Molecular Studies in the
Digestive and Liver Diseases’ Cell Culture Core Facility
(University of Pennsylvania) and were cultured as
described previously (12). Human microvascular endothelial cells (HMVEC) are available commercially
through Cascade Biologics and were cultured as described previously (13).
Antibodies and Reagents
H2AX, phospho-H2AX, p53, p21, and Noxa antibodies
were purchased from Calbiochem. p38 MAPK, phosphop38 MAPK, caspase-3, and poly(ADP-ribose) polymerase
(PARP) antibodies were purchased from Cell Signaling.
Mouse monoclonal anti-human CD31 antibody was
purchased from DAKO North America. Texas redconjugated anti-phalloidin antibody was purchased from
Molecular Probes. Texas red-conjugated anti-mouse
secondary antibody and fluorescein-conjugated anti-rabbit secondary antibody were from Vector Laboratories.
TUNEL, In situ Cell Death Detection Kit, was used from
Boehringer Mannheim/Roche. Ki-67 staining was done
using Ki-67 antibody from Abcam. For in vitro assays,
stock solutions (10 mmol/L) of the c-Jun NH2-termnial
kinase (JNK) inhibitor VIII (Calbiochem), proteasome
inhibitor-1 (Calbiochem), MG-132 (Calbiochem), and
SB230580 (Calbiochem) were prepared in DMSO and
stored at -20jC. Stock solutions of 1 Amol/L bortezomib
(Millennium Pharmaceuticals) were prepared in normal
saline and stored at -20jC. Stock solutions of 10 Amol/L
z-VAD-FMK (Sigma) were prepared in DMSO and
stored at -20jC.
In vitro Three-Dimensional Network Formation Assay
and Fluorescence Imaging
Reconstruction of vessel-like structure in three-dimensional collagen gels and subsequent fluorescent staining
of networks/cords in whole-mount gels were done as
described previously (13).
Western Blotting Analysis
Experiments were done as described previously (6).
Irradiation Experiments
Genotoxic stress was induced by exposing the U2OS
cells to ionizing radiation at a dose of 3 Gy for a 3 min
period (IR; J.L. Shepherd Mark 1 Model 30, 137Ce
Irradiator; J.L. Shepherd and Associates). TE12 cells
were plated in four groups (control, control irradiated,
bortezomib alone, and bortezomib + radiation) at 30 or
60 cells per well in 96-well plates (n = 6). Bortezomibtreated plates received 10 nmol/L of drug for 4 h before
irradiation. The radiation plates received 2 Gy radiation.
Plates were examined for the presence of colonies after 2
weeks, and the number of colonies per well was scored.
Transfection
To achieve transient suppression of gene expression of
H2AX and p38 MAPK, Dharmacon SMARTpool small
interfering RNAs (siRNA) were used as described previously (12).
Organotypic Cell Culture
Reconstructs of human ESCC were grown as described
(12, 14).
Immunofluorescence Microscopy
TE12 cells were seeded onto glass coverslips in six-well
plates and incubated overnight. Cells were then fixed and
analyzed as described previously (12) Immunofluorescence
detection of Ki-67 and TUNEL on the esophageal reconstructs were done as described (15).
Generation of ROS
TE12 cells were treated with 500 Amol/L H2O2 for 1 h,
10 nmol/L bortezomib for 24 h, and normal saline 24 h. The
cells were trypsinized and incubated with 100 nmol/L CMH2DCFDA (Invitrogen) before being washed and resuspended in PBS. Cellular ROS was measured by flow
cytometry. Data shown are representative of three independent experiments.
Adherent Cell Proliferation Assay
Cells were plated into a 96-well plate at a density of
2.5 104/mL and left to grow overnight. Inhibition of
proliferation was analyzed by the MTT assay as described
previously (6).
Cell Cycle Analysis
Cell cycle analysis was done after treatment with
bortezomib 10 nmol/L (0, 8, 24, and 48 h) as described (6).
Three-Dimensional Spheroid Growth
Esophageal carcinoma spheroids were prepared using
the liquid overlay method as described (6).
In vivo experiments
The study protocol was approved by the Wistar Institute
Animal Care and Use Committee. Each group consisted of
eight NOD/SCID mice. Sixteen mice were injected s.c. with
TE11 cells (2 106) into the lower back. When animals
developed nodules of about 5 mm in diameter, the study
drug administration was initiated (day1). The NOD/SCID
mice were assigned randomly to two experimental groups of
eight animals each: (a) 200 AL normal saline and (b) 1 mg/kg
bortezomib (in 200 AL normal saline) by i.p. injection twice a
week. The dose chosen in the present study were based on
preliminary dose-finding experiments. During the experiment, tumor volumes were assessed twice weekly by caliper
measurements. At treatment day 16, 6 h after the final drug
application, all animals were euthanized. Data show the
mean F SE of the treated and untreated groups from one
experiment.
2867
2868 Bortezomib Induces Apoptosis through p38 MAPK Pathway
5
Supplementary material for this article is available at Molecular Cancer
Therapeutics Online (http://mct.aacrjournals.org/).
control (hydrogen peroxide) induced a rightward shift in
the curve, there was no equivalent increase in fluorescence
intensity seen following bortezomib treatment, indicating
that this was not inducing apoptosis through ROS
generation (Supplementary Fig. S2).5
Bortezomib Induces Apoptosis in aThree-Dimensional
Organotypic Model of ESCC and Inhibits Angiogenesis
We next tested bortezomib in two more elaborate ESCC
models that allowed us to assess its effects on both stromal
fibroblasts and endothelial cells. In the first model, ESCC
were layered on top of a tissue-like matrix consisting of
esophageal fibroblasts and collagen. Here, we found that
treatment with bortezomib (500 nmol/L or 1 Amol/L) led to
a concentration-dependent increase in the level of apoptosis
as seen by the enhanced TUNEL staining (Fig. 2A and B).
Interestingly, the effects of the bortezomib were relatively
tumor cell specific, and there was little positive TUNEL
staining seen in the underlying fibroblast layer.
Many targeted therapies also have unintended beneficial
effects on angiogenesis. To investigate this, we used a
model where the interaction of ESCC and human esophageal fibroblasts cooperate to induce vascular network
formation of HMVEC. In control cultures, the interaction of
ESCC and fibroblasts induced the HMVEC to lift off the
bottom of the plate and grow upwards into the acellular
collagen layer, where they formed organized vascular
networks, as shown by the increased CD31 staining
(Fig. 2C). Treatment of the cultures with bortezomib
(0.5 Amol/L) completely inhibited the organized vascular
network formation, and only a sparse layer of unorganized
HMVEC was observed (Fig. 2C).
Bortezomib Induces Regression of Established Human
ESCC Xenografts through Inhibition of Proliferation and
Apoptosis Induction
Next, we grew TE11 cells as tumor xenografts in NOD/
SCID mice. After tumor establishment (5 5 mm), mice
were dosed twice weekly with 1 mg/kg bortezomib by i.p.
injection. After 14 days, it was found that bortezomib
treatment had significantly suppressed tumor growth
(vehicle treated: 4.2 F 0.2-fold; bortezomib treated: 0.6 F
0.1-fold) and led to significant (P < 0.05) tumor regression
(Fig. 3A and B). To assess the mechanism of action of
bortezomib in the xenografts, sections were taken from
control and treated tumors and stained for either proliferation (Ki-67) or apoptosis induction (TUNEL; Fig. 3C). It
was noted that 14-day bortezomib treatment led to a
significant (P < 0.05) reduction in Ki-67 positivity while
concurrently increasing TUNEL staining (Fig. 3D).
Treatment of ESCC Lines with Bortezomib Induces
Phospho-H2AX Foci Downstream of Caspase-3
Activation
It is known that, during the DNA damage response,
activated H2AX localizes to discrete nuclear foci at the site
of DNA double-strand breaks. Treatment of the TE12 cells
with bortezomib (10 nmol/L) caused a massive upregulation of phospho-H2AX in the nucleus, with highpower magnification revealing the discrete focal expression
of the H2AX (Fig. 4A, inset). As recent studies have shown
Mol Cancer Ther 2008;7(9). September 2008
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inhibitors of MEK (U0126), PI3K (LY294002), glycogen
synthase kinase-3h (DW1/2; ref. 16), the proteasome (MG132, proteasome inhibitor-1, and bortezomib), Hedgehog
(cyclopamine), and cyclooxygenase-2 (NS-398) (Supplementary Table S1).5 In our initial screen, compounds were tested
both in a two-dimensional cell culture assay (MTT) and in a
three-dimensional collagen implanted spheroid assay (6).
Two-dimensional and three-dimensional models were used
as the pharmacologic profiles of drugs in two-dimensional
cell culture are not predictive of response in threedimensional cell culture or in vivo (6). Of the compounds
tested, only the proteasome inhibitors MG-132, proteasome
inhibitor-1, and bortezomib continued to show good
anticancer activity under both model systems (Fig. 1;
Supplementary Fig. S1).5
Treatment of six human ESCC lines (TE1, TE3, TE8, TE10,
TE11, and TE12) with increasing concentrations of bortezomib (1 nmol/L-10 Amol/L) led to a concentration-dependent
decrease in cell growth (Fig. 1A). Cell cycle analysis on the
TE12 cell line showed that treatment with bortezomib (10
nmol/L) led to a time-dependent (0, 8, 24, and 48 h) increase
in the G2-M, S, and sub-G1 phases of the cell cycle (Fig. 1B).
The increase in the sub-G1 population following bortezomib
treatment was indicative of the cells undergoing apoptosis.
Similar proapoptotic effects were also noted in the TE1, TE3,
and TE10 cell lines (Supplementary Fig. S1A).5 Bortezomib
was also highly effective in our three-dimensional spheroid
model and reduced both the cell viability (as witnessed by
loss of green staining and increased red fluorescence) and
invasion of the TE12 line in a concentration-dependent
manner (Fig. 1C). Other proteasome inhibitors (proteasome
inhibitor-1 and MG-132) were also found to have similar
effects on the invasion and survival of two other ESCC lines
(TE1 and TE10) in our spheroid model (Supplementary Fig.
S1B;5 not shown).
Treatment of TE12 cells with bortezomib (10 nmol/L)
induced the rapid (<12 h) cleavage of caspase-3 and PARP
(Fig. 1D). Similar caspase-3 and PARP cleavage was also
seen in TE1 cells treated with the proteasome inhibitor MG132 (Supplementary Fig. S1C).5 As the TE12 cell line has
mutated p53 (17), we did not observe any increase in the
expression of p53 or its downstream target p21 following
bortezomib administration. However, both p53 and p21
expression was increased in 1205Lu cells, a melanoma cell
line that retains functional p53 (16). The proapoptotic
protein Noxa was shown to be rapidly up-regulated (<8 h)
in both the TE12 ESCC line and the 1205Lu melanoma line
(Fig. 1D). There is some suggestion that increased Noxa
expression is a consequence of ROS generation. To
investigate this, we treated the TE12 cell line with
bortezomib and measured the increase in fluorescence of
the cell-permeable ROS probe CM-H2DCFDA (Supplementary Fig. S2).5 It was found that, although the positive
Molecular Cancer Therapeutics
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Figure 1. Bortezomib inhibits the proliferation of human ESCC cell lines through G2-M cell cycle arrest and induction of apoptosis. A, in two-dimensional
adherent cell culture, bortezomib reduces growth of the ESCC cell lines in a concentration-dependent fashion. Adherent TE1, TE3, TE8, TE10, TE11, and
TE12 cells were treated with increasing concentrations of bortezomib (1 nmol/L-10 Amol/L) for 72 h before being treated with MTT. The resulting changes
in absorbance were read in a plate reader at 490 nm and expressed as a percentage of control absorbance. Mean F SE of three independent experiments. B,
inhibition of cell growth by bortezomib is associated with G2-M cell cycle arrest. Adherent TE12 cells were treated with saline or 10 nmol/L bortezomib for
8, 24, or 48 h. Cells treated with bortezomib were found to enter G2-M phase cell cycle arrest with increasing periods. The cell cycle profile was obtained
using 15,000 cells. Mean F SE of three independent experiments. C, bortezomib blocks growth of TE cells in three-dimensional culture. TE12 cells were
grown under nonadherent conditions for 72 h until spheroids had formed. Spheroids were then harvested and embedded into a collagen matrix before being
treated with normal saline or bortezomib (10 nmol/L-10 Amol/L). After 72 h, spheroids were treated with calcein-AM (viable cells; green ) and ethidium
bromide (dead cells; red ). Representative of three independent experiments. Magnification, 4. Bar, 100 Am. D, Western blot showing induction of
apoptosis in cells treated with bortezomib. TE12 (ESCC) and 1205Lu (melanoma) cells were treated with 10 nmol/L bortezomib for increasing periods
(0-48 h) followed by protein extraction and probing for expression of cleaved caspase-3/PARP, p53, p21, and Noxa. h-Actin is shown as a loading control.
that H2AX activation may play an important role in
apoptosis induction (18), we transfected TE12 cells with a
siRNA against H2AX, which led to near-total protein
knockdown after 4 days of treatment (Fig. 4B). Any possible
Mol Cancer Ther 2008;7(9). September 2008
role of H2AX in bortezomib-induced apoptosis induction
was discounted by the fact that protein knockdown did
not reduce either caspase-3 cleavage or DNA laddering
(Fig. 4C; data not shown). Next, we investigated whether
2870 Bortezomib Induces Apoptosis through p38 MAPK Pathway
the increase in phospho-H2AX occurred as a downstream
consequence of DNA strand breaks following caspase
cleavage. In these studies, the TE12 cells were pretreated
using the pan-caspase inhibitor z-VAD-FMK (1 Amol/L)
before bortezomib treatment. Here, the inhibition of
bortezomib-induced caspase activation completely blocked
the phosphorylation of H2AX, indicating that this occurred
following caspase cleavage (Fig. 4D).
p38 MAPK Activity Is Critical to Bortezomib-Induced
Apoptosis and Interaction with RadiationTreatment
Previous work has shown that p38 MAPK is critical for
radiation-induced G2-M arrest (19). As bortezomib induces
a G2-M cell cycle arrest in the ESCC lines (Fig. 1B), the
potential role of p38 MAPK in the activity of bortezomib
was addressed. Treatment of the cells with bortezomib
(10 nmol/L) induced the phosphorylation (<4 h) of p38
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Figure 2. Bortezomib induces apoptosis in a three-dimensional organotypic model of ESCC and inhibits vascular network formation. A, threedimensional organotypic cultures were formed in a Transwell system using a collagen and fibroblast matrix on the bottom and ESCC (TE12 cells) seeded on
top. After 14 d, the cultures were treated with bortezomib (0, 0.5, and 1 Amol/L). After 72 h, the reconstructs were harvested and paraffin-embedded.
TUNEL staining was carried out to quantify apoptotic cells after treatment. Green, TUNEL; blue, DAPI. Magnification, 10. Bar, 100 Am. B, quantification
of apoptotic TE12 cells in treated reconstructs (as shown in A). Total area of positive TUNEL staining (Amol/L 103) was assessed using ProImage 6.0
software. Representative of three independent experiments. *, P < 0.05. C, vascular networks were created using HMVEC that were cultured and
overlaid with collagen I followed by a second overlay of collagen containing TE12 cells and esophageal fibroblasts (FEF3). After solidification of the
collagen matrix, cells were treated with 0 (normal saline) or 0.5 Amol/L bortezomib for 72 h. Three-dimensional angiogenesis models were harvested and
immunofluorescence staining was done, showing green fluorescent protein-tagged FEF3 esophageal fibroblasts (green ), HMVEC (CD31; red), and total
nuclei (DAPI; blue ). All representative images are shown as a three-color merge, with the monochrome images of CD31 staining. Bortezomib treated threedimensional angiogenesis model showed decreased vascular network formation in comparison with the control. Magnification, 20. Bar, 100 Am.
Mol Cancer Ther 2008;7(9). September 2008
Molecular Cancer Therapeutics
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Figure 3.
Bortezomib induces regression of established human ESCC xenografts through inhibition of proliferation and apoptosis induction TE11 cells
were grown as tumor xenografts in NOD/SCID mice. After tumor establishment, mice were dosed twice weekly with 0 (normal saline) or 1 mg/kg
bortezomib i.p. (8 mice per group) for 16 days. A, photographs of representative tumors for each group at day 16. B, growth curves normalized to the start
volumes. Bortezomib treatment led to significant (*, P < 0.05) levels of tumor regression. C, immunofluorescent staining of tumors from each group.
TUNEL and Ki-67 staining was done in paraffin-embedded sections [TUNEL (red ), Ki-67 (green ), and DAPI (blue )]. Magnification, 10. Bar, 50 Am.
D, quantification of immunofluorescence for treated and untreated tumors. Percentage of positive cells in each group. *, P < 0.04.
MAPK (Fig. 5A). A critical role for p38 MAPK in
bortezomib-induced apoptotic response was shown by
the ability of the specific p38 MAPK inhibitor SB230580
(10 Amol/L) to block caspase-3 cleavage (Fig. 5B). Pretreatment of the cells with SB230580 also blocked the bortezoMol Cancer Ther 2008;7(9). September 2008
mib-induced activation of both Noxa and phospho-H2AX
(Fig. 5B), showing that p38 MAPK is a critical pathway
necessary for the anticancer activity of bortezomib in ESCC
lines. We further showed that knockdown of p38 MAPKa
protein expression using a siRNA was similarly able to
2872 Bortezomib Induces Apoptosis through p38 MAPK Pathway
coadministration of the drug with irradiation significantly
increased the level of p38 MAPK activity. The enhanced p38
MAPK activity also translated into increased cytotoxicity
with the administration of bortezomib (4 h, 10 nmol/L)
before irradiation (2 Gy), significantly (P < 0.0001) reducing
the formation of TE12 colonies over a 14-day period
(Fig. 5D).
Discussion
In the current study, we describe the possible therapeutic
utility of the proteasome inhibitor bortezomib in ESCC and
elucidate a completely novel mechanism of action for this
drug involving p38 MAPK-dependent apoptosis. In our
initial screen, it was noted that only proteasome inhibitor1/MG-132/bortezomib had good anticancer activity in
both two-dimensional and three-dimensional models.
Figure 4. Knockdown of H2AX expression does not inhibit bortezomib-induced apoptosis. A, immunofluorescence showing g-H2AX foci formation after
bortezomib treatment. TE12 cells were grown on glass slides and treated with normal saline or 10 nmol/L bortezomib for 12 h. Slides were harvested,
fixed, and permeabilized before being stained for phospho-H2AX [g-H2AX (red ) and DAPI (blue )]. Magnification, 20. Bar, 50 Am. Inset, magnification,
60. Bar, 50 Am. B, Western blot showing the down-regulation of H2AX after day 4 of siRNA transfection. Proteins were extracted and resolved followed
by probing for H2AX, ATM, and h-actin. C, H2AX knockdown TE12 cells and control cells were treated with 10 nmol/L bortezomib for 24 and 48 h. Cells
were harvested, lysed, and probed for caspase-3 cleavage. No difference in caspase-3 cleavage was observed in H2AX knockdown cells compared with
control cells. D, TE12 cells were treated with z-VAD-FMK (1 Amol/L), bortezomib, or a combination of both for 48 h. Cells lysates were then harvested,
resolved by Western blotting, and probed for cleaved-PARP (c-PARP ), cleaved caspase-3 (c-Caspase-3 ), total caspase-3 (Caspase-3 ), total PARP (PARP ),
and phospho-H2AX (p-H2AX ). Equal protein loading was confirmed by stripping of the blot and reprobing for actin expression.
Mol Cancer Ther 2008;7(9). September 2008
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block bortezomib-induced caspase cleavage (Fig. 5C). Next,
it was shown that bortezomib also activated another stressactivated MAPK, the JNK pathway (Supplementary
Fig. S3). However, unlike p38 MAPK, there was little
evidence for this pathway being directly involved in
bortezomib-induced apoptosis, as inhibition of the pathway, using the small-molecule inhibitor JNK inhibitor VIII
(1 Amol/L), did not attenuate the level of bortezomibinduced caspase-3 cleavage.
It is known that irradiation activates the p38 MAPK
pathway. Given that preoperative radiotherapy is a
standard treatment for ESCC, we next explored whether
there was any positive interaction between bortezomib and
irradiation. Treatment of the TE12 cells with either
bortezomib (10 nmol/L) or radiation (2 Gy) led to a modest
increase in p38 MAPK activity (Fig. 5D), whereas
Molecular Cancer Therapeutics
Bortezomib was antiproliferative and strongly proapoptotic
in two-dimensional ESCC cell cultures, three-dimensional
organotypic cultures, and a human ESCC xenograft mouse
model. The concentrations of bortezomib required to
induce regression of established ESCC xenografts are lower
than those reported for other solid tumors (20) and seem to
Mol Cancer Ther 2008;7(9). September 2008
be readily achievable in the clinic (21). Unlike in other
squamous cell carcinoma lines, such as PAM212, we did
not find any effects on nuclear factor-nB signaling
following bortezomib treatment (20). At least part of the
potential activity of bortezomib against ESCC may be the
result of impaired angiogenesis, as bortezomib was found
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Figure 5. p38 MAPK activity is critical to bortezomib-induced apoptosis and interaction with radiation treatment. A, bortezomib treatment led to the
rapid up-regulation of phospho-p38 MAPK (p-p38 ) activity in TE12 cells. Irradiation of the human osteosarcoma line U2OS also led to the rapid upregulation of p38 activity. Equal protein expression was confirmed by stripping and probing the blot for actin expression. B, inhibition of p38 MAPK
reverses bortezomib-induced apoptosis. TE12 cells were pretreated with 10 Amol/L SB230580 for 24 h before being treated with bortezomib (10 nmol/L)
for 48 h. Cells were harvested, lysed, and probed for caspase-3 cleavage, total caspase-3, phospho-H2AX, total H2AX, and Noxa expression. Equal protein
loading was confirmed by stripping the blot and probing for actin expression. C, knockdown of p38 MAPK inhibits bortezomib-induced caspase-3 cleavage.
TE12 cells were transiently transfected using SMART pool siRNA p38 MAPK (siRNAp38 ). Control cells were transfected with a scrambled siRNA sequence
(siRNAcontrol) After 4 days, cells were treated with bortezomib (10 nmol/L, 48 h). Protein was then extracted from the cells, resolved, and probed for
expression of caspase-3, cleaved caspase-3 (c-caspase-3 ), and total p38 MAPK (p38 ). D, Western blot showing the positive interaction of bortezomib and
radiation in the activation of p38 MAPK signaling. TE12 cells plated in 10 cm dishes were treated with control, 2 Gy radiation alone, bortezomib alone, and
bortezomib + radiation (2 Gy). Plates were pretreated for 4 h with saline or bortezomib (10 nmol/L) before irradiation. Cells were left to grow overnight
before the extraction of protein; extracts were then resolved and probed for expression of phospho-p38 MAPK, total p38 MAPK, and h-actin. Radiation
survival. Cells were plated in 96-well plates at the indicated number per well. Cells were then treated with either saline (control) or bortezomib (10 nmol/L)
for 4 h before radiation. Cells were left to grow for 2 weeks, after this time the total number of colonies were counted. Columns, mean of six experiments.
*, P < 0.0001.
2873
2874 Bortezomib Induces Apoptosis through p38 MAPK Pathway
Another possible explanation for the observed DNA
damage response involved the direct role of caspase
activation in the cleavage of genomic DNA leading to
subsequent H2AX/ATM activation. It was found that
inhibition of caspase activity using z-VAD-FMK blocked
the phosphorylation of H2AX, suggesting that the DNA
damage response occurred following caspase cleavage.
Our initial studies showed activation of the G2-M
checkpoint following bortezomib treatment. UV irradiation
is known to induce a G2-M arrest through p38 MAPKmediated inhibition of cdc25B activity (19). We next asked
whether bortezomib activated the p38 MAPK pathway and
whether this led to apoptosis in the ESCC cells. Bortezomib
treatment was found to induce p38 MAPK activity and
inhibition of p38 MAPK signaling, using an inhibitor or a
siRNA, markedly inhibited bortezomib-induced caspase-3
cleavage. p38 MAPK activation was also required for the
bortezomib-induced increase in Noxa expression and the
phosphorylation of H2AX, as this could be also reversed by
SB230580 treatment. Thus, it was shown that activation of
p38 MAPK is a critical step for bortezomib-induced
apoptosis in ESCC cells. Thus far, no direct link has been
made between the activation of p38 MAPK and increased
Noxa expression. Previously, it was shown that the p38
MAPK activated cofactor p18HAMLET increases Noxa
expression following treatment with either UV or cisplatin
(29). However, in this instance, Noxa is regulated through a
p53-dependent mechanism. As our ESCC lines lack any
functional p53 activity, this current study provides the first
clues that p38 MAPK activity may regulate Noxa expression independently of p53.
The essential role for p38 MAPK in bortezomib-induced
apoptosis in ESCC cells is in stark contrast to findings
reported for multiple myeloma cells. Here, bortezomib also
activates the p38 MAPK pathway but instead leads to drug
resistance through increased expression of Hsp27 (30, 31).
In multiple myeloma cells, it was shown that p38 MAPK
pathway inhibition actually enhanced the proapoptotic
effect of bortezomib through the down-regulation of
Hsp27, Mcl-1, and Bcl-XL expression while up-regulating
p53 activity (31, 32). Again, in contrast to our findings, this
study also reported that increased JNK activity leads to
enhanced caspase cleavage (31).
Preoperative radiotherapy is a front-line treatment for
ESCC. As irradiation induces p38 MAPK, we investigated
whether bortezomib-induced p38 activity would enhance
responses to radiation treatment. Combined treatment of
the ESCC cells with radiation and a single 4 h treatment
with bortezomib enhanced both p38 MAPK activity and
cytotoxicity in a colony formation assay. We therefore
suggest that bortezomib treatment could be a potential
radiosensitizing agent for ESCC. In summary, we have
shown that bortezomib is a promising potential treatment
for ESCC. We also found that bortezomib induces
apoptosis in ESCC lines through a novel mechanism
involving the p38 MAPK-induced Noxa activation, leading
to caspase cleavage, which is depicted as a model in
Supplementary Fig. S4.5
Mol Cancer Ther 2008;7(9). September 2008
Downloaded from http://aacrjournals.org/mct/article-pdf/7/9/2866/1880485/2866.pdf by guest on 11 June 2022
to inhibit tumor-induced vascular network formation in a
three-dimensional model of ESCC-induced angiogenesis.
It is however, difficult to conclude how bortezomib was
exerting its antiangiogenic effects, particularly as the
concentrations of bortezomib used were likely to induce
apoptosis of the ESCC cells and may have in fact been the
result of impaired ESCC/HMVEC interaction. Any potential antiangiogenic activity of bortezomib is likely to be of
great significance to ESCC, as this is known to be a highly
angiogenic tumor. Several studies have shown that
angiogenesis, as measured by the extent of tumor microvessel density, as well as VEGF expression, are prognostic
factors for ESCC (22 – 24). It was, however, difficult to
confirm the bortezomib-driven inhibition of angiogenesis
within our ESCC xenografts, as the tumors are typically not
highly vascularized when grown in this model.
Treatment of human ESCC lines with bortezomib is
strongly growth inhibitory, associated with induction of a
G2-M-phase cell cycle arrest and apoptosis. Induction of
G2-M arrest is typically associated with induction of p53
activity and transcription of its downstream target p21 (25).
Previous studies have shown that almost all human ESCC
lines have defects in p53 function, arising as a result of
missense mutations in the coding sequence (17). In
agreement with a lack of functional p53 in the ESCC lines,
bortezomib was found to induce cleavage of caspase-3 and
PARP without the induction of either p53 or its downstream target p21. In contrast, bortezomib did induce the
activation of p53 and p21 in the 1205Lu melanoma cells
(which are p53 wild-type). Bortezomib treatment was
found to induce the rapid up-regulation of the proapoptotic
protein Noxa in both the ESCC and the melanoma cell lines
(10). Although Noxa is known to be a p53 target gene (26),
the p53-independent induction of Noxa reported here is
consistent with that observed in other studies using p53mutated cancer cell lines (Jurkat, Sk-MEL-28, and MDAMB-231; refs. 9, 10, 27).
Previous studies have shown that the induction of ROS
can activate Noxa in response to bortezomib treatment and
that this may be independent of p53 function (27). It has
been also shown that ROS generation is associated with
bortezomib activity in mantle cell lymphoma and small cell
lung carcinoma (27, 28). Here, we found that bortezomib
treatment was not associated with ROS generation. Instead,
we observed an increase in the expression of phosphoH2AX, a marker of the DNA damage response. Recent
work has shown that H2AX can mediate apoptosis directly
where JNK directly phosphorylates H2AX leading to
caspase-3 cleavage and caspase-activated DNase activity
(18). As bortezomib both activates JNK signaling and
induces H2AX phosphorylation, we next asked whether
H2AX activity was required for bortezomib-induced
apoptosis in ESCC lines. Although a siRNA against
H2AX yielded good levels of protein knockdown, we did
not observe any change in the magnitude of apoptosis
induction or DNA laddering following 48 h bortezomib
treatment, suggesting that the interaction of JNK and H2AX
was not responsible for the apoptotic effects observed.
Molecular Cancer Therapeutics
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Acknowledgments
We thank Millenium Pharmaceuticals for supplying the bortezomib
(Velcade) and all the members of the Herlyn laboratory for enthusiasm
and support.
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