Download Involvement of Rad51 in cytotoxicity induced by

Carcinogenesis vol.29 no.7 pp.1448–1458, 2008
doi:10.1093/carcin/bgn130
Advance Access publication June 9, 2008
Involvement of Rad51 in cytotoxicity induced by epidermal growth factor receptor
inhibitor (gefitinib, IressaR) and chemotherapeutic agents in human lung cancer cells
Jen-Chung Ko, Shih-Ci Ciou1, Chau-Ming Cheng1,
Lyu-Han Wang1, Jhao-Hao Hong1, Ming-Yan Jheng1,
Szu-Ting Ling1 and Yun-Wei Lin1,
Department of Internal Medicine, Hsinchu Hospital, Department of Health,
The Executive Yuan, Taiwan and 1Molecular Oncology Laboratory,
Department of Biochemical Science and Technology, National Chiayi
University, Chiayi 600, Taiwan
To whom correspondence should be addressed. Tel: þ886 5 271 7770;
Fax: þ886 5 271 7780;
Email: [email protected]
Gefitinib (IressaR, ZD1839) is a selective epidermal growth factor
receptor (EGFR) tyrosine kinase inhibitor that blocks growth
factor-mediated cell proliferation and extracellular signalregulated kinases 1/2 (ERK1/2) signaling activation. Rad51 is an
essential component of the homologous recombination repair
pathway. High level of Rad51 expression has been reported in
chemo- or radioresistant carcinomas. We hypothesized that gefitinib may enhance the effects of the alkylating agent cisplatin- or
the antitumor antibiotic mitomycin C (MMC)-mediated cytotoxicity by decreasing ERK1/2 activation and Rad51 expression.
Exposure of human non-small lung cancer cells to gefitinib decreased cisplatin- or MMC-elicited ERK1/2 activation and Rad51
protein induction. Neither cisplatin nor MMC treatment affected
Rad51 messenger RNA (mRNA). However, gefitinib cotreatment
with cisplatin or MMC significantly decreased Rad51 mRNA levels. In addition, gefitinib decreased cisplatin- or MMC-elicited
Rad51 protein levels by increasing Rad51 protein instability. Enhancement of ERK1/2 signaling by constitutively active mitogenactivated protein kinase kinase 1/2 (MKK1/2-CA) increased
Rad51 protein levels and protein stability in gefitinib and cisplatin
or MMC cotreated cells. Moreover, the synergistic cytotoxic effects induced by gefitinib cotreatment with cisplatin or MMC
were remarkably decreased by MKK1-CA-mediated enhancement of ERK1/2 activation. Depletion of endogenous Rad51 expression by si-Rad51 RNA transfection significantly enhanced
lung cancer cell death upon treatment with cisplatin or MMC.
We conclude that Rad51 protein protects lung cancer cells from
synergistic cytotoxic effects induced by gefitinib and chemotherapeutic agents. Suppression of Rad51 expression may be a novel
lung cancer therapeutic modality to overcome drug resistance to
EGFR inhibitors and chemotherapeutic agents.
Introduction
Lung cancer is one of the most commonly occurring malignancies in
the USA and has the highest mortality rate for all cancers in both men
and women (1). Non-small-cell lung cancer (NSCLC) accounts for
85% of all lung cancers (2). The current first-line therapeutic option
for patients with advanced NSCLC includes chemotherapy with
a platinum-containing compound such as cisplatin or carboplatin in
combination with a second- or third-generation cytotoxic agent such
as mitomycin C (MMC), paclitaxel, gemcitabine, vinorelbine, irinotecan or docetaxel (3). The use of cytotoxic chemotherapy improves
Abbreviations: ALLN, N-acetyl-Leu-Leu-norleucinal; CFA, colony-forming
ability; CI, combination index; EGFR, epidermal growth factor receptor;
ERK1/2, extracellular signal-regulated kinase 1/2; MAPK, mitogen-activated
protein kinase; MKK1/2-CA, constitutively active form of mitogen-activated
protein kinase kinase; MMC, mitomycin C; mRNA, messenger RNA; NSCLC,
non-small-cell lung cancer; PTEN, phosphatase and tensin homolog deleted on
chromosome 10; siRNA, small interfering RNA; TKI, tyrosine kinase inhibitor.
in both median overall survival and 1 year survival rates compared
with best supportive care (4).
It has been shown that the epidermal growth factor receptor
(EGFR) is commonly expressed at high levels in NSCLC (5). Several
studies have indicated that a high level of EGFR expression correlates
with poor prognosis and reduced survival (6,7). The approach of
treating NSCLC by inhibiting EGFR signaling is inherently different
from commonly used cytotoxic chemotherapies. Whereas chemotherapeutic drugs affect all dividing cells, agents that target the EGFR act
selectively on malignant cells due to the limited role of the EGFR in
normal tissue. These agents therefore have the potential for considerably reduced toxicity compared with non-specific cytotoxic agents
(8,9).
The inhibitors of tyrosine kinase (TKIs) are small-molecule agents
that block EGFR activity by interfering with the adenosine
triphosphate-binding site on the intracellular region of the receptor
(10). A variety of TKIs have been developed for advanced NSCLC.
Gefitinib (IressaR, ZD1839) is a quinazoline derivative that inhibits
EGFR tyrosine kinase activity by competitively inhibiting the adenosine triphosphate-binding domain within EGFR’s catalytic domain
(8,11). Gefitinib has demonstrated anticancer efficacy in vivo in
NSCLC (12). Specific mutant versions of EGFR have been identified
in NSCLC cells that may determine different response to treatment
with gefitinib (13,14).
Cisplatin, a platinum analog that is classified as an alkylating agent,
can covalently bind to DNA with preferential binding to the N-7
position of guanine and adenine; binding interferes with DNA replication by inducing DNA adducts, which preludes cell death (15).
Platinum compounds are commonly used chemotherapeutic agents
that are an integral part of the treatment of lung, gynecological, gastrointestinal and hematological malignancies in combination with
other drugs. Platinum-based therapy remains the standard of care
for the first-line treatment of patients with advanced NSCLC. When
combined with a third-generation agent, platinum-based doublets
improve survival compared with the third-generation agent given
alone (16).
MMC has a unique chemical structure that includes aziridine and
aminoquinone groups. These groups lead to a mono- and bifunctional
alkylating reaction, and the latter forms cross-links in double-stranded
DNA, resulting in the inhibition of DNA synthesis and cell division
(17). MMC is typically used as first- or second-line regimen to treat
NSCLC and is often combined with other chemotherapeutic agents
for advanced NSCLC treatment (18,19).
Rad51 is a DNA repair nuclear protein that is involved in homologous recombination (20,21). When cells are exposed to genotoxic
treatments, the Rad51 protein relocalizes and concentrates in nuclear
foci (21). The foci are believed to be sites of repair, either at DNA
lesions or at stalled replication forks (21,22). Abnormal expression of
Rad51 has been reported in various carcinomas (23,24). Overexpression of Rad51 can enhance spontaneous recombination frequency and
increase resistance to ionizing radiation treatment in mammalian cells
(25,26). Moreover, high level of Rad51 expression in tumor tissue is
associated with an unfavorable prognosis in lung cancer (27,28), and
overexpression of the Rad51 protein correlate with resistance to chemotherapeutic agents and poor prognostic outcome (25,29). However,
the role of Rad51 in resistance to chemotherapeutic agents and gefitinib in lung cancer therapy is unclear.
The mammalian mitogen-activated protein kinases (MAPKs) compose the extracellular signal-regulated kinases 1/2 (ERK1/2), the p38
MAPK kinases and the c-JUN N-terminal kinases. MAPKs are vital
intracellular signaling transducers differentially activated in response
to a wide variety of extracellular stimuli (30,31). MAPKs are activated
through a three-kinase module composed of MAPK, mitogen-activated
Ó The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
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Involvement of Rad51 in cytotoxicity induced by epidermal growth factor receptor inhibitor and chemotherapeutic agents in human lung cancer cells
protein kinase kinase and mitogen-activated protein kinase kinase
kinase. Activation of MAPKs requires a dual phosphorylation of the
Thr and Tyr residues in the subdomain VIII of the catalytic domain
(32,33). Activated ERK1/2 are vital in the control of cell survival,
proliferation, differentiation, apoptosis, cell transformation and tumor
invasion through phosphorylation of many downstream substrates including transcription factors, intracellular protein kinases, phosphatases, enzymes and cytoskeletal proteins (34,35).
In this study, we hypothesized that gefitinib may enhance cisplatinor MMC-mediated cytotoxicity by decreasing the activation of ERK1/2
and Rad51 expression in human NSCLC lines. We evaluated the
role of Rad51 in cell survival in human lung cancer cells exposed
to gefitinib and chemotherapeutic agents. The results may be the
basis of a novel and important therapeutic modality in lung cancer
to overcome drug resistance to EGFR inhibitors and chemotherapeutic agents via the regulation of Rad51 protein expression.
Materials and methods
Materials
Cycloheximide was purchased from Sigma–Aldrich (St Louis, MO). N-acetylLeu-Leu-norleucinal (ALLN), MG132 and U0126 were purchased from
Calbiochem–Novabiochem (San Diego, CA). Gefitinib (IressaR, ZD1839) was
purchased from AstraZeneca (London, UK). Cisplatin and MMC were obtained
from Bristol-Myers Squibb (New York, NY) and were prepared in saline prior
to use. Cell culture media were from Invitrogen (Carlsbad, CA). Gefitinib,
ALLN, MG132 and U0126 were dissolved in dimethyl sulfoxide. Cycloheximide was dissolved in MilliQ-purified water (Millipore, Billerica, MA). Plasmid expressions of MKK1-CA (a constitutively active form of MKK1, DN3/
S218E/S222D) and MKK2-CA (DN4/S222E/S226D) were achieved as described previously (36). The specific phospho-ERK(Thr202/Tyr204) antibody
was from Cell Signaling (Beverly, MA). Rabbit polyclonal antibodies against
ERK2(C-14) (sc-154), Rad51(H-92) (sc-8349), HA(F-7) (sc-7392) and actin
(I-19) (sc-1616) were from Santa Cruz Biotechnology (Santa Cruz, CA).
Cell culture
A549 human lung carcinoma cells derived from human alveolar type 2 cells
(CCL-185; American Type Culture Collection, Manassas, VA) and human lung
adenocarcinoma H1650 cells (CRL-5883; American Type Culture Collection)
were cultured at 37°C in a humidified atmosphere containing 5% CO2 in
RPMI-1640 complete medium supplemented with sodium bicarbonate
(2.2%, wt/vol), L-glutamine (0.03%, wt/vol), penicillin (100 units/ml), streptomycin (100 lg/ml) and fetal calf serum (10%).
Western blot
After different treatments, cells were rinsed twice with cold phosphate-buffered
saline and lysed in whole-cell extract buffer (20 mM N-2-hydroxyethylpiperazineN#-2-ethanesulfonic acid pH 7.6, 75 mM NaCl, 2.5 mM MgCl2, 0.1 mM ethylenediaminetetraacetic acid, 0.1% Triton X-100, 0.1 mM Na3VO4, 50 mM NaF,
1 lg/ml leupeptin, 1 lg/ml aprotinin, 1 lg/ml pepstatin and 1 mM 4-(2aminoethyl)benzenesulfonyl fluoride). The cell lysates were rotated at 4°C for
30 min, centrifuged at 10 000 r.p.m. for 15 min and the precipitates were discarded. A BCA protein assay kit (Pierce, Rockford, IL) was used to determine
protein concentrations using bovine serum albumin as a standard control. Equal
amounts of proteins from each set of experiments were subjected to western blot
analysis as described previously (36). Antibodies were stripped from polyvinyl
difluoride membranes (Millipore) by a solution containing 2% sodium dodecyl
sulfate, 62.5 mM Tris–HCl pH 6.8 and 0.7% (wt/wt) b-mercaptoethanol at
50°C for 15 min before reprobing with another primary antibody.
Treatments
Exponentially growing A549 or H1650 cells were plated for 18 h before a 1 h
exposure to gefitinib and 24 h cotreatment with gefitinib and either cisplatin or
MMC in serum-free media. To determine the effects of gefitinib–cisplatin or
gefitinib–MMC on de novo protein synthesis, cycloheximide (60 lg/ml), an
inhibitor of de novo protein synthesis, was coadded with gefitinib–cisplatin or
gefitinib–MMC for 2–6 h. To determine the effect of MKK1/2-ERK1/2 signaling on Rad51, MKK1/2-CA expression vectors were transfected to A549 or
H1650 cells prior to gefitinib–cisplatin or gefitinib–MMC cotreatments.
Transfection
The sense-strand sequences of small interfering RNA (siRNA) duplexes used
for Rad51 and lamin (as a control) were 5#-UGUAGCAUAUGCUCGAGCGdTdT-3# and 5#-CUGGACUUCCAGAAGAACAdTdT-3# (Dharmacon
Research, Lafayette, CO). siRNA duplexes (200 nM) were transfected into
A549 cells using lipofectamine2000 (Invitrogen) for 1 day. The siRNAtransfected cells were treated with gefitinib–cisplatin or gefitinib–MMC for
24 h, Rad51 and phospho-ERK1/2 protein levels were determined by western blotting analysis and the levels of cytotoxicity were determined by
colony-forming ability (CFA) assay described subsequently.
CFA assay
Immediately after the different chemotherapeutic agents and gefitinib treatments, A549 cells were washed with phosphate-buffered saline and trypsinized
for the determination of cell numbers. The cells were plated at a density of
500–2000 cells on a 60 mm diameter Petri dish in triplicate for each treatment.
The cells were cultured for 10–14 days, and the cell colonies were stained with
1% crystal violet solution in 30% ethanol. Cytotoxicity was determined by the
number of colonies in the treated cells divided by the number of colonies in the
untreated control (37).
Drugs combination effect analysis
The cytotoxicity of the gefitinib–cisplatin or gefitinib–MMC combination was
compared with the cytotoxicity of each drug alone using the combination index
(CI), where CI , 0.9, CI 5 0.9–1.1 and CI . 1.1 indicated synergistic, additive
and antagonistic effects, respectively (38). The combination effect analysis
was performed using CalcuSyn software (Biosoft, Oxford, UK). The CI values
at fraction affected of 0.5, 0.75 and 0.9 were averaged for each experiment,
and the value was used to calculate the mean between four independent
experiments.
Cell viability analysis
Cell viability was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. In brief, cells (2 104) were plated in 96-well cell
culture plates in RPMI containing 10% fetal bovine serum in a final volume of
0.2 ml. When the cells reached 50% confluence, they were treated with cisplatin or MMC for 24 h. Cell survival was assessed by directly adding 100 ll of
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (500 lg/ml) to
the medium for another 3 h and then cells were solubilized in dimethyl sulfoxide (100 ll/well) on a shaker at room temperature for 15 min before reading
the absorbance at 562 nm using a Biorad Technologies Microplate Reader.
Reverse transcription–polymerase chain reaction
RNA was isolated from cultured cells using TRIzol (Invitrogen) as detailed by
the manufacturer. Reverse transcription–polymerase chain reaction was performed
on 2 lg of total RNA using random hexamers following the Moloney Murine
Leukemia Virus reverse transcriptase cDNA synthesis system (Invitrogen).
The final complementary DNA was used for subsequent polymerase chain
reactions. Rad51 was amplified by using the primers with the sequence
5#-CTTTGGCCCACAACCCATTTC-3# (forward) and 5#-ATGGCCTTTCCTTCACCTCCAC-3# (reverse) in conjunction with thermal cycling program
consisting of 26 cycles of 95°C for 30 s, 61°C for 30 s and 72°C for 60 s.
Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was amplified as an internal control. The GAPDH primer was 5#-CTACATGGTTTACATGTTCC-3#
(forward) and 5#-GTGAGCTTCCCGTTCAGCTCA-3# (reverse). The samples were loaded in triplicate, and the results of each sample were normalized
to GAPDH.
Determination of cell death
A549 cells transfected with Rad51 siRNA were trypsinized and 1 105 cells
were cultured in a 60 mm diameter Petri dish. After 18 h, cells were treated
with cisplatin or MMC for 24 h or remained untreated. After treatment, unattached and attached cells were collected and stained by trypan blue solution.
The stain is excluded from living cells but will penetrate dead cells. The proportion of dead A549 cells was determined by counting the cells stained by
trypan blue using a hemocytometer.
Statistical analysis
For each protocol, three or four independent experiments were performed.
Results were expressed as mean ± SEM. Statistical calculations were performed using the SigmaPlot 2000 software (Systat Software, San Jose, CA).
Differences in measured variables between experimental and control groups
were assessed using an unpaired t-test. P , 0.05 was considered statistically
significant.
Results
Influence of gefitinib treatment on phospho-ERK1/2 and Rad51
protein levels in cisplatin- or MMC-treated lung cancer cells
Human lung cancer cell lines, A549 and H1650, were exposed to
gefitinib (2–10 lM) in serum-free medium for 1 h prior to cotreatment
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with cisplatin for 24 h. Phosphorylation of ERK1/2 was determined
by western blot analysis using antibodies specific to phospho-ERK1/2.
Gefitinib treatment decreased cellular phospho-ERK1/2 levels and
also decreased cisplatin-elicited ERK1/2 phosphorylation (Figure
1A and B), whereas endogenous ERK2 protein levels did not change
in each cell extract. Cisplatin increased cellular Rad51 protein and
phospho-ERK1/2 levels in A549 or H1650 cells (Figure 1C and D).
Gefitinib pretreatment decreased cisplatin-induced Rad51 protein expression and ERK1/2 phosphorylation (Figure 1C and D). Moreover,
gefitinib also decreased MMC-induced Rad51 protein and phosphoERK1/2 levels (Figure 1E–H).
Influence of gefitinib and chemotherapeutic agents on cellular Rad51
messenger RNA expression
To elucidate whether the observed cisplatin stimulation of Rad51
protein expression occurred at the transcriptional level, various concentrations of cisplatin were added to A549 or H1650 cells for 24 h.
Total RNA was isolated and subjected to reverse transcription–
polymerase chain reaction analysis for Rad51. Rad51 messenger
RNA (mRNA) did not change after treatment with various concentrations of cisplatin (Figure 2A and D) or MMC (Figure 2B and E) in
A549 or H1650 cells. In contrast, Rad51 mRNA was gradually downregulated by various concentrations of gefitinib treatment in A549 cells
(Figure 2C). Gefitinib (10 lM) alone or in combination with cisplatin
(10 lg/ml) or MMC (10 lM) produced a lowered level of Rad51
mRNA in comparison with untreated A549 or H1650 cells (Figure 2).
Influence of gefitinib treatment on cisplatin- and MMC-enhanced
Rad51 protein stability
To investigate whether Rad51 protein expression affected by treatment with gefitinib in combination with cisplatin or MMC was also
regulated at the posttranslational level, cycloheximide (60 lg/ml), an
inhibitor of de novo protein synthesis, was coadded with cisplatin
(10 lM) for 2–6 h. Rad51 protein levels were progressively reduced
Fig. 1. Gefitinib treatment decreases cisplatin- or MMC-induced phospho-ERK1/2 and Rad51 protein levels in human lung cancer cells. (A and B) A549 or
H1650 cells (1 106) were cultured in complete medium for 18 h and then exposed to gefitinib (2–10 lM) for 1 h before cotreatment with cisplatin (5 lg/ml) for
24 h in serum-free medium. (C and D) Cells were pretreated with gefitinib (10 lM) in serum-free media for 1 h before treatment with cisplatin (2–10 lg/ml) for 24 h.
(E and F) Cells were exposed to gefitinib (2–10 lM) for 1 h before cotreatment with MMC (5 lM) for 24 h in serum-free medium. (G and H) Cells were pretreated
with gefitinib (10 lM) in serum-free media for 1 h before treatment with MMC (2–10 lM) for 24 h. After treatment, cells extracts were examined by western blot
for the determination of phospho-ERK1/2, Rad51 and ERK2 protein levels.
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Involvement of Rad51 in cytotoxicity induced by epidermal growth factor receptor inhibitor and chemotherapeutic agents in human lung cancer cells
proteasome-mediated degradation of Rad51 induced by gefitinib and
the chemotherapeutic agents, the 26S proteasome inhibitors MG132
and ALLN were added for the final 4 h before harvesting gefitinib–
cisplatin or gefitinib–MMC coexposed A549 or H1650 cells. As
shown in Figure 3F–I, both MG132 and ALLN partially restored
the decreased Rad51 protein expressions induced by cotreatment with
gefitinib and cisplatin or MMC. Together, gefitinib-enhanced Rad51
protein instability in cells treated with chemotherapeutic agents was
through 26S proteasome-mediated proteolysis mechanism. The decrease of Rad51 protein levels by gefitinib treatment with cisplatin
or MMC resulted from the decrease in its mRNA expression and
protein stability.
Fig. 2. Gefitinib decreases Rad51 mRNA in cisplatin- or MMC-treated A549
cells. (A and D) A549 or H1650 cells (1 106) were cultured in complete
medium for 18 h and then treated with cisplatin (2–10 lg/ml) for 24 h in serumfree medium. Gefitinib (10 lM) was added to cells for 1 h prior to cotreatment
with cisplatin (10 lg/ml) for 24 h. (B and E) A549 or H1650 cells were
exposed to MMC (2–10 lM) for 24 h in serum-free medium. Gefitinib (10 lM)
was added to cells for 1 h prior to cotreatment with MMC (10 lM) for 24 h. (C)
A549 cells (1 106) were cultured in complete medium for 18 h and then
treated with gefitinib (2–10 lM) for 24 h in serum-free medium. Total RNA
was isolated and used for reverse transcription–polymerase chain reaction.
with time in the presence of cycloheximide. Cisplatin treatment significantly prevented Rad51 degradation after cycloheximide treatment
compared with untreated cells (Figure 3A). However, in the presence
of cycloheximide for 6 h, there was still 54% of the Rad51 remaining in cisplatin-treated cells, whereas only 4% of the Rad51 remained
in the gefitinib and cisplatin cotreated cells (Figure 3B), indicating
Rad51 was less stable after gefitinib cotreatment. Moreover, Rad51
was more stable in MMC-treated cells than in untreated cells (Figure
3C). In contrast, cells treated with MMC following gefitinib had
a significantly lower proportion of Rad51 protein remaining after
cycloheximide treatment as compared with cells treated with MMC
alone (Figure 3D). Moreover, gefitinib treatment promoted Rad51
degradation after cycloheximide treatment compared with untreated
A549 cells (Figure 3E). These results suggested that downregulation
of Rad51 protein levels by gefitinib treatment with cisplatin or MMC
resulted from the decrease in its protein stability. Taken together,
downregulation of Rad51 protein levels by gefitinib cotreatment with
cisplatin or MMC resulted from the decrease in its protein stability
and mRNA expression.
Association of gefitinib-induced Rad51 protein instability with 26S
proteasome-mediated proteolysis of Rad51
Rad51 is degraded via the ubiquitin-mediated proteasome pathway
upon exposure to ionizing radiation (39). To investigate the role of
Influence of ERK1/2 activation on the regulation of Rad51 protein
expression and stability
To determine whether the ERK1/2 signaling pathway was involved
in regulation of Rad51 protein expression, A549 or H1650 cells were
transient transfected with a plasmid carrying MKK1-CA or MKK2-CA.
Expression of MKK1-CA or MKK2-CA could increase cellular ERK1/
2 phosphorylation and Rad51 protein content (Figure 4A–C). In addition, expression of MKK1/2-CA partially rescued the decrease of
Rad51 protein and phosphorylation of ERK1/2 in cells cotreated with
gefitinib cisplatin or gefitinib–MMC in A549 or H1650 cells (Figure
4A–C). Furthermore, when A549 cells were treated with various concentrations of MKK1/2 inhibitors (U0126), cellular ERK1/2 activation and Rad51 protein decreased significantly in a dose-dependent
manner (Figure 4D). In addition, the stability of Rad51 affected
by MKK1–ERK1/2 signaling pathway was investigated by cycloheximide chase analysis. MKK1-CA or MKK2-CA plasmid transfection increased Rad51 protein stability in A549 cells cotreated
with gefitinib and cisplatin or MMC as compared with pcDNA3transfected cells (Figure 4E–H), consistent with ERK1/2 signaling
pathway as the upstream signal for regulation of Rad51 protein
stability.
Effect of cotreatment with gefitinib and chemotherapeutic agents on
cytotoxicity and cell death of A549 cells
Gefitinib applied in combination with cytotoxic drugs such as cisplatin, carboplatin, oxaliplatin, paclitaxel, docetaxel, doxorubicin,
etoposide or topotecan produce an additive growth inhibitory effect
and apoptosis in ovarian, breast and colon cancer cells (40). It was,
therefore, of great interest to determine whether this also occurred
with lung cancer cells. A549 cells were treated with gefitinib for 1 h
before exposure to various concentrations of cisplatin or MMC for
24 h in a serum-free medium. Cytotoxicity was determined by CFA
assay. In Figure 5A and B, when A549 cells were exposed with
gefitinib (2 lM) alone did not cause significantly cytotoxicity
(92.20% survival). However, the cytotoxicity of combination treatment of gefitinib (2 lM) and cisplatin (5 lg/ml) was higher than
cisplatin treatment alone (5.70% survival compared with 17.48%
survival, respectively; P , 0.01). The cytotoxicity of gefitinib
(2 lM) and MMC (5 lM) cotreatment was higher than MMC treatment alone (2.88% survival compared with 12.87% survival, respectively; P , 0.01). The drugs combination effect analysis revealed
synergistic effects in gefitinib–cisplatin or gefitinib–MMC treatment.
For instance, in Figure 5E, the average CI value for the gefitinib–
cisplatin is 0.44 and the gefitinib–MMC is 0.24, respectively, displaying the synergistic effect in A549 cells. Moreover, assessment of
A549 cell death using the trypan blue exclusion assay revealed that
gefitinib in combination with cisplatin or MMC produced synergistic
effect in cell death (Figure 5C and D).
Effect of Rad51 expression knockdown on the cytotoxicity effect
of chemotherapeutic agents
To determine the role of Rad51 in the cytotoxicity induced by cisplatin and MMC, Rad51 protein expression was knocked down using
specific siRNA duplexes. As shown in Figure 6A and B, transfection
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J.-C.Ko et al.
Fig. 3. Gefitinib decreases Rad51 protein stability in cisplatin- or MMC-treated A549 cells. (A and B) A549 cells were pretreated with or without gefitinib
(10 lM) for 1 h and then cotreated with cisplatin (10 lg/ml) for 6 h, followed by cotreatment with cycloheximide (CHX, 60 lg/ml) for 2–6 h. (C and D) Cells were
pretreated with or without gefitinib (10 lM) for 1 h and then added with MMC (10 lM) for 6 h, followed by cotreated with cycloheximide (60 lg/ml) for 2–6 h.
Western blot analysis was performed using specific antibodies against Rad51 or actin. (E) A549 cells were pretreated with or without gefitinib (10 lM) for 6 h and
then cotreated cycloheximide for 2–6 h. (F–I) Gefitinib and cisplatin or MMC cotreatment triggers 26S proteasome-mediated proteolysis of Rad51. Gefitinib
(10 lM) was added to A549 or H1650 cells for 1 h before treatment with cisplatin (2 lg/ml) or MMC (2 lM) for 20 h as described in the legend to Figure 1A. Cells
were then cotreated with MG132 (25 lM) or ALLN (10 lM) for 4 h. Whole-cell extracts were collected for western blot analysis.
of si-Rad51 RNA duplex suppressed Rad51 protein expression in
cisplatin- or MMC-treated A549 cells. Interestingly, knock down of
Rad51 expression had no effect on the ERK1/2 activation, implying
that ERK1/2 may be the upstream signal for inducing Rad51 expression upon cisplatin or MMC exposure. Cisplatin or MMC treatment
resulted in cytotoxicity determined by CFA (Figure 6C and D), and
cell viability determined by 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide assay in a dose-dependent manner
(Figure 6E and F). In addition, the cell death was determined by
trypan blue staining in a dose-dependent manner (Figure 6G and
H). As shown in Figure 6C–H, cisplatin or MMC treatment increased
cytotoxicity and cell death in a does-dependent manner. Interestingly,
suppression of Rad51 protein expression by si-Rad51 RNA resulted in
A549 or H1650 cells significantly enhanced the cytotoxicity caused
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by cisplatin or MMC as compared with si-lamin-transfected cells
(Figure 6C–H, gray and white bar, respectively).
Influence of MKK1-CA overexpression on cytotoxicity induced by
gefitinib and cisplatin or MMC cotreatment in A549 cells
To evaluate the effects of ERK1/2 on synergistic cytotoxicity induced by
gefitinib–cisplatin or gefitinib–MMC cotreatment, A549 cells were
transfected with MKK1-CA vectors followed by treatment with gefitinib
and cisplatin or MMC prior to assessment of CFA assay. Transfection of
MKK1-CA vector could enhance the cell survival that was previously
suppressed by cotreatment with gefitinib–cisplatin or gefitinib–MMC
(Figure 6I and J, black and white bar, respectively), consistent with
the synergistic cytotoxic effect from gefitinib and the chemotherapeutic
agents being mainly due to suppression of ERK1/2 activation.
Involvement of Rad51 in cytotoxicity induced by epidermal growth factor receptor inhibitor and chemotherapeutic agents in human lung cancer cells
Fig. 4. Overexpression of MKK1/2 restores gefitinib-suppressed ERK1/2 activation, Rad51 expression and Rad51 protein stability induced by cisplatin or MMC.
(A–C) MKK1-CA expression vectors (3 lg) were transfected into A549 or H1650 cells using lipofectamine. After expression for 1 day, the cells were treated with
gefitinib (10 lM) and cisplatin (2 lg/ml) or MMC (2 lM) as described in the legend to Figure 1. (D) A549 cells were treated with U0126 (2–10 lM) for 1 h.
Western blot analysis was performed using specific antibodies against Rad51, phopho-ERK1/2 and ERK2. (E–H) MKK1-CA or MKK2-CA expression vectors
were transfected into A549 cells using lipofectamine. After expression for 1 day, the cells were treated with gefitinib (10 lM) for 1 h prior to exposure to cisplatin
(2 lg/ml) or MMC (2 lM) for 6 h, followed by the addition of cycloheximide (60 lg/ml) for 2–6 h. Western blot analysis was performed using specific antibodies
against Rad51, HA or actin.
Discussion
In this study, gefitinib could decrease cisplatin- or MMC-elicited
ERK1/2 activation and Rad51 protein induction. Downregulation of
Rad51 protein levels by gefitinib treatment with cisplatin or MMC
resulted from the decrease in its mRNA expression and protein
stability. Rad51 protein instability induced by cotreatment with gefitinib–cisplatin or gefitinib–MMC was derived from 26S proteasomemediated protein degradation. Expression of the MKK1/2-CA vectors
could restore ERK1/2 activation and Rad51 protein stability in
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Fig. 5. Gefitinib cotreated with cisplatin or MMC synergistically enhances cytotoxicity. (A and B) Gefitinib (2 lM) was added to A549 cells for 1 h, followed by
treatment with cisplatin (2–10 lg/ml) or MMC (2–10 lM) for 24 h. Cytotoxicity was determined by assessment of CFA. (C and D) At the end of treatment,
unattached and attached cells were collected and stained with trypan blue dye. The numbers of stained cells (dead) were manually counted. Columns, percentage of
trypan blue-positive cells, representing a population of dead cells; bar, standard error (SE) from three independent experiments. The and , respectively, denote
P , 0.05 and 0.01 using Student’s t-test for the comparison between the cells treated with cisplatin or MMC alone or cotreated with gefitinib. (E) The mean CI
values of gefitinib–cisplatin or gefitinib–MMC combination in A549 cells. CI values at fraction affected of 0.5, 0.75 and 0.9 were averaged for each experiment,
and the value was used to calculate the mean between experiments, as described under Materials and Methods. Points and columns, mean values obtained from
four independent experiments; bars, SE.
gefitinib–cisplatin or gefitinib–MMC cotreated lung cancer cells. In
addition, gefitinib enhances cisplatin- and MMC-induced cytotoxicity. Enhancement of the ERK1/2 activation by expression of the
MKK1-CA vector could decrease the synergistic cytotoxicity induced by gefitinib–cisplatin or gefitinib–MMC. Interestingly, suppression of Rad51 protein expression by si-Rad51 RNA resulted in
cells significantly enhanced the cytotoxicity caused by cisplatin or
MMC as compared with si-lamin-transfected cells (Figure 6K).
Moreover, knock down of Rad51 protein expression by si-Rad51
RNA also significantly enhanced the cytotoxicity induced by gefitinib in human lung cancer cells (Jen-Chung Ko, Jhao-Hao Hong,
Lyu-Han Wang, Chau-Ming Cheng, Shih-Ci Ciou, Szu-Ting Ling,
Ming-Yan Jheng and Yun-Wei Lin, unpublished data). Together,
these results indicate that the Rad51 is essential for maintaining
cancer cell viability when these cells are exposed to various cytotoxic agents and EGFR TKIs.
1454
Gefitinib, a potent and selective inhibitor of the EGFR tyrosine
kinase, has antitumor activity in mouse xenograft models and tumor
cell lines (41,42). It has been shown that gefitinib inhibit cell growth
and survival through blockage of downstream signaling pathways
such as ERK1/2 and phophatidylinositol-3 kinase/protein kinase B
(43). It has been suggested that gefitinib prevents cell survival by
activating the proapoptotic protein BCL2-antagonist of cell death
protein (9). It has shown that antitumor responses to gefitinib occur
more frequently in women than in men, are higher with adenocarcinoma than other histological types and occur more often in those
patients who have never smoked (44). Specific mutant types of EGFR
in NSCLC cells determine the response to treatment with gefitinib
(13). However, the mechanism of resistance to gefitinib has remained
unclear.
Platinum-based therapy remains the standard of care for the firstline treatment of patients with advanced NSCLC (45). The platinum
Involvement of Rad51 in cytotoxicity induced by epidermal growth factor receptor inhibitor and chemotherapeutic agents in human lung cancer cells
Fig. 6. Knock down of Rad51 expression by siRNA transfection enhances cell death induced by cisplatin or MMC. (A and B) A549 cells were transfected with
siRNA duplexes (200 nM) specific to Rad51 or lamin (control) in complete medium for 48 h prior to treatment with cisplatin (2–10 lg/ml) or MMC (2–10 lM) in
serum-free medium for 24 h. Whole-cell extracts were collected for western blot analysis using specific antibodies against Rad51, phospho-ERK1/2 and ERK2. (C
and D) After treatment as above, cytotoxicity was determined by assessment of CFA assay. (E and F) H1650 cells were transfected with si-Rad51 RNA or si-lamin
(as control) and then the cells were treated as above. Cell survival was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. (G and
H) After treatment as above, both unattached and attached cells were collected and stained with trypan blue dye, and the numbers of dead cells were manually
counted. Columns, percentage of trypan blue-positive cells, representing a population of dead cells; bar, standard error from three independent experiments.
The and , respectively, denote P , 0.05 and 0.01 using Student’s t-test for the comparison between the cells pretreated with cisplatin or MMC in si-Rad51
RNA or si-lamin RNA-transfected cells. (I and J) Overexpression of MKK1-CA rescues the levels of cytotoxicity induced by cotreatment with gefitinib and
cisplatin or MMC in A549 cells. A549 cells were transfected with MKK1-CA vectors for 1 day, then the cells were treated with gefitinib (10 lM) for 1 h, followed
by adding cisplatin (2 lg/ml) or MMC (0.5 lM) for 24 h. The cytotoxicity affected by MKK1-CA vector transfection was determined by CFA. The results (mean ±
SEM) were obtained from four independent experiments. The and , respectively, denote P , 0.05 and 0.01 using Student’s t-test for the comparisons between
the cells transfected with MKK1-CA or pcDNA3 vectors. (K) A diagram for Rad51 protein protects lung cancer cells from synergistic cytotoxic effects induced by
gefitinib and chemotherapeutic agents. See text for the details.
analog cisplatin binds to DNA, particularly to linker regions via histone H1 protein, and forms inter- and intrastrand DNA adducts (46).
Cells treated with cisplatin undergo repair of DNA adducts, cell cycle
arrest, inhibition of replication or apoptotic cell death pathways
(15,47). Depending on the tumor node metastasis stage at diagnosis,
response rates to chemotherapy for inoperable NSCLC vary from 30
1455
J.-C.Ko et al.
to 60% with platinum combined with gemcitabine, vinorelbine or
a taxane (45). When combined with a second- or third-generation
chemotherapeutic agent, platinum-based doublets improve survival
compared with the single agent alone (16).
In preclinical models, additive or synergistic activities of gefitinib
and several chemotherapeutic drugs have been reported (41,42). However, the combined treatment of patients with advanced NSCLC with
either gefitinib or erlotinib (Tarceva), the other EGFR-selective TKI
and standard two-drug (gemcitabine/cisplatin and carboplatin/
paclitaxel) chemotherapy regimens did not result in any improvement
in overall survival over chemotherapy alone (48–51). The reasons for
the disappointing efficacy results are still unclear. The lack of an
additive or synergistic effect with gefitinib or erlotinib and chemotherapy may relate to a mechanistic interaction. EGFR TKIs and
gemcitabine–cisplatin have different mechanisms of action (cytostatic
and cytotoxic, respectively). Another possible explanation for the
negative results may be the lack of patient selection. For example,
the finding of mutations in the EGFR tyrosine kinase domain in advanced NSCLC patients may be one example of the potential to
suitably select for a subset of patients who may benefit from EGFR
TKIs. Because no additive effect was observed by administering gefitinib continuously in combination with chemotherapy, the doses and
schedules of EGFR TKIs and chemotherapeutic agents cotreatment
may affect antitumor efficacy. Preclinical data suggest that alternative
dosing schedules, such as sequential or pulse dosing of erlotinib, may
prove more effective than concurrent administration. For example,
Giovannetti et al. proved that additive–synergistic cytotoxic effects
in the simultaneous pemetrexed (an inhibitor of thymidylate synthase,
dihydrofolate reductase and glycinamide ribonucleotide formyltransferase) and erlotinib treatment in NSCLC cell lines. However, strong
synergism in the pemetrexed and erlotinib cotreatment followed
by erlotinib treatment (52). In our study, Rad51 protein protects
lung cancer cells from synergistic cytotoxic effects induced by gefitinib and chemotherapeutic agents. Therefore, a significant correlation may be found between Rad51 overexpression and gefitinib
sensitivity.
The antitumor antibiotic MMC is also used in clinical chemotherapy regimens for the treatment of the various carcinomas (53). MMC
cytotoxicity is due primarily to the formation of the DNA adducts, in
particular DNA interstrand cross-links (54). It usually is used as firstor second-line regimen to treat NSCLC and is often combined with
other chemotherapeutic agents for advanced NSCLC treatment
(18,19,55). Unfortunately, some cancer cells are able to resist the
action of chemotherapeutic agents through increased DNA repair capacity (56). For example, excision repair cross-complementation
group 1 (ERCC1) is a DNA repair protein in the nucleotide excision
repair pathway that specifically removes platinum adducts of DNA
(57). DNA repair capacity is considered both a barrier to carcinogenesis and a crucial molecular pathway implicated in the resistance to
cisplatin-based chemotherapy (56,58). Platinum resistance arises
from decreased tumor blood flow, extracellular conditions, reduced
platinum uptake, increased efflux, intracellular detoxification by glutathione, DNA repair mechanism, decreased mismatch repair, defective apoptosis, antiapoptotic factors, effects of several signaling
pathways or presence of quiescent non-cycling cells (59,60). Gene
expression signatures can predict drug sensitivity to cisplatin (61) and
Bcl-2 downregulation can sensitize NSCLC to cisplatin (62). In this
study, we show that depletion of endogenous Rad51 expression significantly enhanced lung cancer cell death upon treatment with cisplatin or MMC. Rad51 protein protects lung cancer cells from
synergistic cytotoxic effects induced by gefitinib and chemotherapeutic agents. Suppression of Rad51 expression may be a novel lung
cancer therapeutic modality to overcome drug resistance to EGFR
inhibitors and chemotherapeutic agents. However, the detail mechanism of Rad51 in overcoming drug resistance to chemotherapeutic
agents deserves further investigation.
Rad51 is essential for embryonic survival in response to exogenous
DNA damaging agents (63) and in the repair of spontaneously occurring chromosome breaks in proliferating cells (64). Thus, Rad51 is
1456
very important in the maintenance of genomic stability in eukaryotic
cells. Rad51 is also essential for tumor progression and is associated
with resistance to radiation or chemotherapy (65,66). The combined
expression of Rad51 and ERCC1 was observed to be associated with
resistance to platinum agents in clinical NSCLC (28). In contrast,
Rad51 siRNA enhances the sensitivity to cisplatin in cancer cells both
in vitro and in vivo (67). Elevated expression of wild-type Rad51
protein is correlated with histological grading of invasive breast
carcinoma and overexpression of Rad51 has been observed in a number
of tumor cells with elevated rates of homologous recombination
(23,25). It has been reported that Rad51 is also involved in the
sensitivity of cancers to other anticancer drugs, such as etoposide
(VP16) and imatinib mesylate (Gleevec) (68,69). Consistent with
our results, we suggest that the combination of cisplatin and Rad51
siRNA will be an effective anticancer modality. However, high-level
expression of Rad51 was demonstrated to be an independent prognostic marker of survival in NSCLC patients (27). Another study that
utilized immunohistochemical staining of ERCC1, hRad51 and
BRCA1 in tumor biopsies from NSCLC patients determined that
these were not predictive for tumor response and survival after
chemotherapy (70).
p53 has been shown to modulate homologous recombination by transcriptional repression of Rad51 gene (71,72) and impaired Rad51
repression by p53 mutation proteins may contribute to malignant
transformation (72). Moreover, decreased Rad51 gene expression by
hypoxia was specifically mediated by repressive E2F4–p130 complexes that bind to a single E2F site in the proximal promoter of the
Rad51 gene (73). Moreover, Shen et al. (74) found that Pten null MEF
exhibit spontaneous DNA double strand breaks and the Rad51 transcript and protein is much lower in Pten/ mouse embryonic fibroblasts (MEFs) than in Ptenþ/þ MEFs. They propose that phosphatase
and tensin homolog deleted on chromosome 10 (PTEN) is necessary
for the basal expression of Rad51 and synergistic regulation of Rad51
by PTEN and E2F-1 play an essential role in DNA repair and chromosomal stabilization. However, the detail mechanism of signaling
regulate Rad51 gene expression is unclear. Our findings indicate that
inhibition of ERK1/2 by gefitinib significantly decreased the cisplatin- or MMC-induced Rad51 protein level that was resulting from
decreased Rad51 mRNA expression and protein stability. The 26S
proteasome inhibitors cotreatment or MKK1/2-CA vectors transfection into cells partially restored the decreased Rad51 protein expressions induced by cotreatment with gefitinib and cisplatin or MMC. It
is maybe because of the repression action of gefitinib on Rad51 transcription. However, we cannot rule out the possibility of ERK singling
or other signaling pathways participated in regulating the Rad51 gene
expression. The role of gefitinib in the repression of Rad51 transcription is under investigation.
Our results reveal that chemotherapeutic agents such as cisplatin
and MMC can enhance Rad51 expression and ERK1/2 activation,
which may be the reasons for treatment failure from drug resistance
to NSCLC. However, blocking cisplatin- and MMC-elicited ERK1/2
activation by gefitinib could decrease Rad51 expression. The synergistic cytotoxic effects of gefitinib and cisplatin or MMC are mediated
by blocking ERK1/2 activation and suppression of Rad51 expression.
Blockage of endogenous Rad51 expression by si-Rad51 RNA transfection can further enhance the cytotoxicity induced by chemotherapeutic agents (Figure 6K).
In conclusion, the present results indicate that Rad51 protein can
protect lung cancer cells from synergistic cytotoxic effects induced by
gefitinib and chemotherapeutic agents. Suppression of Rad51 protein
expression might prove to be a novel and important therapeutic modality to human lung cancer, which is resistant to EGFR inhibitor and
cytotoxic agents.
Funding
National Science Council of Taiwan (NSC 96-2311-B-415-004) and
Hsinchu Hospital, Department of Health, Taiwan.
Involvement of Rad51 in cytotoxicity induced by epidermal growth factor receptor inhibitor and chemotherapeutic agents in human lung cancer cells
Acknowledgements
We thank Dr Jia-Ling Yang for providing us expression plasmids for
transfection.
Conflict of Interest Statement: None declared.
References
1. Silvestri,G.A. et al. (2005) Targeted therapy for the treatment of advanced
non-small cell lung cancer: a review of the epidermal growth factor
receptor antagonists. Chest, 128, 3975–3984.
2. Landis,S.H. et al. (1999) Cancer statistics, 1999. CA Cancer J. Clin., 49,
8–31, 1.
3. Pfister,D.G. et al. (2004) American Society of Clinical Oncology treatment
of unresectable non-small-cell lung cancer guideline: update 2003. J. Clin.
Oncol., 22, 330–353.
4. Schiller,J.H. et al. (2002) Comparison of four chemotherapy regimens for
advanced non-small-cell lung cancer. N. Engl. J. Med., 346, 92–98.
5. Salomon,D.S. et al. (1995) Epidermal growth factor-related peptides and
their receptors in human malignancies. Crit. Rev. Oncol. Hematol., 19,
183–232.
6. Arteaga,C.L. (2003) EGF receptor as a therapeutic target: patient selection
and mechanisms of resistance to receptor-targeted drugs. J. Clin. Oncol.,
21, 289s–291s.
7. Raymond,E. et al. (2000) Epidermal growth factor receptor tyrosine kinase
as a target for anticancer therapy. Drugs, 60 (suppl. 1), 15–23; discussion,
41–42.
8. Morin,M.J. (2000) From oncogene to drug: development of small molecule
tyrosine kinase inhibitors as anti-tumor and anti-angiogenic agents.
Oncogene, 19, 6574–6583.
9. Gilmore,A.P. et al. (2002) Activation of BAD by therapeutic inhibition of
epidermal growth factor receptor and transactivation by insulin-like growth
factor receptor. J. Biol. Chem., 277, 27643–27650.
10. Mendelsohn,J. (2001) The epidermal growth factor receptor as a target for
cancer therapy. Endocr. Relat. Cancer, 8, 3–9.
11. Wakeling,A.E. et al. (2002) ZD1839 (Iressa): an orally active inhibitor of
epidermal growth factor signaling with potential for cancer therapy. Cancer
Res., 62, 5749–5754.
12. Sridhar,S.S. et al. (2003) Inhibitors of epidermal-growth-factor receptors:
a review of clinical research with a focus on non-small-cell lung cancer.
Lancet Oncol., 4, 397–406.
13. Lynch,T.J. et al. (2004) Activating mutations in the epidermal growth factor
receptor underlying responsiveness of non-small-cell lung cancer to gefitinib. N. Engl. J. Med., 350, 2129–2139.
14. Rosell,R. et al. (2006) Epidermal growth factor receptor activation: how
exon 19 and 21 mutations changed our understanding of the pathway. Clin.
Cancer Res., 12, 7222–7231.
15. Furuta,T. et al. (2002) Transcription-coupled nucleotide excision repair as
a determinant of cisplatin sensitivity of human cells. Cancer Res., 62,
4899–4902.
16. Scagliotti,G. (2007) Optimizing chemotherapy for patients with advanced
non-small cell lung cancer. J. Thorac. Oncol., 2 (suppl. 2), S86–S91.
17. Rockwell,S. et al. (1993) Cellular pharmacology of quinone bioreductive
alkylating agents. Cancer Metastasis Rev., 12, 165–176.
18. Booton,R. et al. (2006) A phase III trial of docetaxel/carboplatin versus
mitomycin C/ifosfamide/cisplatin (MIC) or mitomycin C/vinblastine/
cisplatin (MVP) in patients with advanced non-small-cell lung cancer:
a randomised multicentre trial of the British Thoracic Oncology Group
(BTOG1). Ann. Oncol., 17, 1111–1119.
19. Babiak,A. et al. (2007) Mitomycin C and vinorelbine for second-line chemotherapy in NSCLC–a phase II trial. Br. J. Cancer, 96, 1052–1056.
20. Shinohara,A. et al. (1992) Rad51 protein involved in repair and recombination in S. cerevisiae is a RecA-like protein. Cell, 69, 457–470.
21. Haaf,T. et al. (1995) Nuclear foci of mammalian Rad51 recombination
protein in somatic cells after DNA damage and its localization in synaptonemal complexes. Proc. Natl Acad. Sci. USA, 92, 2298–2302.
22. Tashiro,S. et al. (1996) S phase specific formation of the human Rad51
protein nuclear foci in lymphocytes. Oncogene, 12, 2165–2170.
23. Maacke,H. et al. (2000) Over-expression of wild-type Rad51 correlates
with histological grading of invasive ductal breast cancer. Int. J. Cancer,
88, 907–913.
24. Yoshikawa,K. et al. (2000) Abnormal expression of BRCA1 and BRCA1interactive DNA-repair proteins in breast carcinomas. Int. J. Cancer, 88, 28–36.
25. Vispe,S. et al. (1998) Overexpression of Rad51 protein stimulates homologous recombination and increases resistance of mammalian cells to
ionizing radiation. Nucleic Acids Res., 26, 2859–2864.
26. Arnaudeau,C. et al. (1999) The RAD51 protein supports homologous recombination by an exchange mechanism in mammalian cells. J. Mol. Biol.,
289, 1231–1238.
27. Qiao,G.B. et al. (2005) High-level expression of Rad51 is an independent
prognostic marker of survival in non-small-cell lung cancer patients.
Br. J. Cancer, 93, 137–143.
28. Takenaka,T. et al. (2007) Combined evaluation of Rad51 and ERCC1 expressions for sensitivity to platinum agents in non-small cell lung cancer.
Int. J. Cancer, 121, 895–900.
29. Schneider,S. et al. (2006) Gene expression in tumor-adjacent normal tissue
is associated with recurrence in patients with rectal cancer treated with
adjuvant chemoradiation. Pharmacogenet. Genomics, 16, 555–563.
30. Chang,L. et al. (2001) Mammalian MAP kinase signalling cascades.
Nature, 410, 37–40.
31. Kolch,W. (2005) Coordinating ERK/MAPK signalling through scaffolds
and inhibitors. Nat. Rev. Mol. Cell Biol., 6, 827–837.
32. Galanis,A. et al. (2001) Selective targeting of MAPKs to the ETS domain
transcription factor SAP-1. J. Biol. Chem., 276, 965–973.
33. Murphy,L.O. et al. (2004) A network of immediate early gene products
propagates subtle differences in mitogen-activated protein kinase signal
amplitude and duration. Mol. Cell. Biol., 24, 144–153.
34. Kolch,W. (2000) Meaningful relationships: the regulation of the Ras/Raf/
MEK/ERK pathway by protein interactions. Biochem. J., (351 Pt), 2,
289–305.
35. Marshall,C.J. (1995) Specificity of receptor tyrosine kinase signaling: transient versus sustained extracellular signal-regulated kinase activation. Cell,
80, 179–185.
36. Lin,Y.W. et al. (2003) ERK1/2 achieves sustained activation by stimulating
MAPK phosphatase-1 degradation via the ubiquitin-proteasome pathway.
J. Biol. Chem., 278, 21534–21541.
37. Lin,Y.W. et al. (2003) Persistent activation of ERK1/2 by lead acetate
increases nucleotide excision repair synthesis and confers anti-cytotoxicity
and anti-mutagenicity. Carcinogenesis, 24, 53–61.
38. Peters,G.J. et al. (2000) Basis for effective combination cancer chemotherapy with antimetabolites. Pharmacol. Ther., 87, 227–253.
39. Bennett,B.T. et al. (2005) Cellular localization of human Rad51C and
regulation of ubiquitin-mediated proteolysis of Rad51. J. Cell. Biochem.,
96, 1095–1109.
40. Ciardiello,F. et al. (2000) Antitumor effect and potentiation of cytotoxic
drugs activity in human cancer cells by ZD-1839 (Iressa), an epidermal
growth factor receptor-selective tyrosine kinase inhibitor. Clin. Cancer
Res., 6, 2053–2063.
41. Sirotnak,F.M. et al. (2000) Efficacy of cytotoxic agents against human
tumor xenografts is markedly enhanced by coadministration of ZD1839
(Iressa), an inhibitor of EGFR tyrosine kinase. Clin. Cancer Res., 6,
4885–4892.
42. Ciardiello,F. (2000) Epidermal growth factor receptor tyrosine kinase
inhibitors as anticancer agents. Drugs, 60 (suppl. 1), 25–32; discussion,
41–42.
43. Ono,M. et al. (2004) Sensitivity to gefitinib (Iressa, ZD1839) in non-small
cell lung cancer cell lines correlates with dependence on the epidermal
growth factor (EGF) receptor/extracellular signal-regulated kinase 1/2
and EGF receptor/Akt pathway for proliferation. Mol. Cancer Ther., 3,
465–472.
44. Kris,M.G. et al. (2003) Efficacy of gefitinib, an inhibitor of the epidermal
growth factor receptor tyrosine kinase, in symptomatic patients with nonsmall cell lung cancer: a randomized trial. JAMA, 290, 2149–2158.
45. Spira,A. et al. (2004) Multidisciplinary management of lung cancer.
N. Engl. J. Med., 350, 379–392.
46. Yaneva,J. et al. (1997) The major chromatin protein histone H1 binds
preferentially to cis-platinum-damaged DNA. Proc. Natl Acad. Sci. USA,
94, 13448–13451.
47. Benhar,M. et al. (2001) Enhanced ROS production in oncogenically transformed cells potentiates c-Jun N-terminal kinase and p38 mitogen-activated
protein kinase activation and sensitization to genotoxic stress. Mol. Cell.
Biol., 21, 6913–6926.
48. Gatzemeier,U. et al. (2007) Phase III study of erlotinib in combination with
cisplatin and gemcitabine in advanced non-small-cell lung cancer: the Tarceva Lung Cancer Investigation Trial. J. Clin. Oncol., 25, 1545–1552.
49. Giaccone,G. (2004) The role of gefitinib in lung cancer treatment. Clin.
Cancer Res., 10, 4233s–4237s.
1457
J.-C.Ko et al.
50. Herbst,R.S. et al. (2004) Gefitinib in combination with paclitaxel and carboplatin in advanced non-small-cell lung cancer: a phase III trial–INTACT
2. J. Clin. Oncol., 22, 785–794.
51. Herbst,R.S. et al. (2005) TRIBUTE: a phase III trial of erlotinib
hydrochloride (OSI-774) combined with carboplatin and paclitaxel chemotherapy in advanced non-small-cell lung cancer. J. Clin. Oncol., 23,
5892–5899.
52. Giovannetti,E. et al. (2008) Molecular mechanisms underlying the synergistic interaction of erlotinib, an epidermal growth factor receptor tyrosine
kinase inhibitor, with the multitargeted antifolate pemetrexed in non-smallcell lung cancer cells. Mol. Pharmacol., 73, 1290–1300.
53. Verweij,J. et al. (1990) Mitomycin C: mechanism of action, usefulness and
limitations. Anticancer Drugs, 1, 5–13.
54. Dorr,R.T. et al. (1985) Interactions of mitomycin C with mammalian DNA
detected by alkaline elution. Cancer Res., 45, 3510–3516.
55. Feliu,J. et al. (2006) Docetaxel and mitomycin as second-line treatment in
advanced non-small cell lung cancer. Cancer Chemother. Pharmacol., 58,
527–531.
56. Kelland,L. (2007) The resurgence of platinum-based cancer chemotherapy.
Nat. Rev. Cancer, 7, 573–584.
57. Zamble,D.B. et al. (1996) Repair of cisplatin–DNA adducts by the mammalian excision nuclease. Biochemistry, 35, 10004–10013.
58. Wei,Q. et al. (2000) DNA repair: a double-edged sword. J. Natl Cancer
Inst., 92, 440–441.
59. Stewart,D.J. (2007) Mechanisms of resistance to cisplatin and carboplatin.
Crit. Rev. Oncol. Hematol., 63, 12–31.
60. Camps,C. et al. (2007) Gene expression and polymorphisms of DNA repair
enzymes: cancer susceptibility and response to chemotherapy. Clin. Lung
Cancer, 8, 369–375.
61. Hsu,D.S. et al. (2007) Pharmacogenomic strategies provide a rational approach to the treatment of cisplatin-resistant patients with advanced cancer.
J. Clin. Oncol., 25, 4350–4357.
1458
62. Losert,D. et al. (2007) Bcl-2 downregulation sensitizes nonsmall cell lung
cancer cells to cisplatin, but not to docetaxel. Anticancer Drugs, 18, 755–761.
63. Tsuzuki,T. et al. (1996) Targeted disruption of the Rad51 gene leads to
lethality in embryonic mice. Proc. Natl Acad. Sci. USA, 93, 6236–6240.
64. Sonoda,E. et al. (1998) Rad51-deficient vertebrate cells accumulate chromosomal breaks prior to cell death. EMBO J., 17, 598–608.
65. Slupianek,A. et al. (2002) Fusion tyrosine kinases induce drug resistance
by stimulation of homology-dependent recombination repair, prolongation
of G(2)/M phase, and protection from apoptosis. Mol. Cell. Biol., 22, 4189–
4201.
66. Bello,V.E. et al. (2002) Homologous recombinational repair vis-a-vis
chlorambucil resistance in chronic lymphocytic leukemia. Biochem. Pharmacol., 63, 1585–1588.
67. Ito,M. et al. (2005) Rad51 siRNA delivered by HVJ envelope vector enhances the anti-cancer effect of cisplatin. J. Gene Med., 7, 1044–1052.
68. Russell,J.S. et al. (2003) Gleevec-mediated inhibition of Rad51 expression
and enhancement of tumor cell radiosensitivity. Cancer Res., 63, 7377–
7383.
69. Hansen,L.T. et al. (2003) The role of RAD51 in etoposide (VP16) resistance in small cell lung cancer. Int. J. Cancer, 105, 472–479.
70. Wachters,F.M. et al. (2005) ERCC1, hRad51, and BRCA1 protein expression in relation to tumour response and survival of stage III/IV NSCLC
patients treated with chemotherapy. Lung Cancer, 50, 211–219.
71. Arias-Lopez,C. et al. (2006) p53 modulates homologous recombination by
transcriptional regulation of the RAD51 gene. EMBO Rep., 7, 219–224.
72. Lazaro-Trueba,I. et al. (2006) Double bolt regulation of Rad51 by p53:
a role for transcriptional repression. Cell Cycle, 5, 1062–1065.
73. Bindra,R.S. et al. (2007) Repression of RAD51 gene expression by E2F4/
p130 complexes in hypoxia. Oncogene, 26, 2048–2057.
74. Shen,W.H. et al. (2007) Essential role for nuclear PTEN in maintaining
chromosomal integrity. Cell, 128, 157–170.
Received January 17, 2008; revised April 30, 2008; accepted May 22, 2008