From the Department of Pharmaceutical Sciences,
School of Pharmacy and Cancer Center, University of Colorado Health
Sciences Center, Denver, Colorado 80262 and the § Department
of Biochemistry, Center in Molecular Toxicology, and the
Vanderbilt-Ingram Cancer Center, Vanderbilt University School of
Medicine, Nashville, Tennessee 37232-6305
Received for publication, November 25, 2002, and in revised form, January 10, 2003
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ABSTRACT |
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NAD(P)H:quinone oxidoreductase 1 (NQO1)
has been proposed to stabilize p53 via a redox mechanism involving
oxidation of NAD(P)H as a consequence of the catalytic activity of
NQO1. We report that treatment of HCT-116 human colon carcinoma cells
with the NQO1 inhibitor ES936 had no effect on the levels of p53
protein. ES936 is a mechanism-based inhibitor of NQO1 that irreversibly blocks the catalytic function of the enzyme. This suggests that a redox
mechanism involving NQO1-mediated NAD(P)H oxidation is not responsible
for the stabilization of p53. We also examined the ability of the NQO1
protein to associate with p53 using co-immunoprecipitation experiments.
Results from these experiments demonstrated co-immunoprecipitation of
NQO1 with p53 and vice versa. The association between p53 and NQO1 was
not affected by treatment of HCT-116 cells with ES936, demonstrating
that the association was not dependent on the catalytic activity of
NQO1. A comparison of isogenic HCT-116 p53+/+ and HCT-116 p53 The p53 gene is one of the major tumor suppressor genes in humans
(1, 2), and p53 mutations are one of the most common genetic events
that occur in human cancers (3). When normal cells are subjected to
stress signals, such as DNA damage or oxidative stress, p53 is
activated, resulting in transcription of downstream genes that
coordinate either growth arrest of the cell or apoptosis (4) preventing
proliferation and clonal expansion of damaged cells. Mutations or
deletions in p53 and a consequent loss of p53-dependent
function leads to increased susceptibility to neoplasia (1). The
identification of small molecules and proteins that increase p53
stability and, as a result, protect cells against cancer progression is
an active area of current research (3).
NAD(P)H:quinone oxidoreductase 1 (NQO1, DT-diaphorase, EC
1.6.99.2)1 is a cytosolic
flavoenzyme that catalyzes the two-electron reduction of a broad range
of substrates (5). NQO1 is an obligate two-electron reductase and is
classified as a detoxification enzyme primarily because of its ability
to reduce quinone substrates directly to their less toxic hydroquinone
derivatives bypassing the redox-cycling semiquinone radical (6, 7).
NQO1 can also function as an antioxidant enzyme reducing ubiquinone and
vitamin E quinone to their antioxidant forms (8, 9). We have
characterized a polymorphism in NQO1 (NQO1*2) (10, 11) and demonstrated
that individuals homozygous for the NQO1*2 polymorphism (NQO1*2/*2) have no measurable NQO1 activity (12). The null phenotype of individuals carrying the homozygous NQO1*2 polymorphism is due to rapid
proteasomal degradation of the mutant NQO1*2 protein (13). A lack of
NQO1 protein due to the homozygous NQO1*2 polymorphism has been
associated with an increased risk of various cancers including renal,
urothelial, and cutaneous basal cell carcinomas (14, 15). Of particular
interest are five separate epidemiological studies that have identified
the NQO1*2 polymorphism as a significant risk factor for development of
leukemias of diverse origin (16-20). NQO1-knock-out mice have been
shown to be more susceptible to chemical-induced skin cancer (21, 22),
and inducers of NQO1 have long been recognized as chemoprotective
agents against a range of animal tumors (23, 24).
Recently Asher et al. (25) proposed that NQO1, which is
activated by many of the same stresses that activate p53, stabilized wild type p53 protein (25). Experiments were performed by examining p53
stability and function in cells transfected with human NQO1. Transfected wild type NQO1*1 protein, but not the mutant NQO1*2 protein, stabilized wild type p53 in HCT-116 cells (26). Experiments were also performed in the presence and absence of the competitive and
relatively nonspecific NQO1 inhibitor dicumarol (27), which was found
to increase proteasomal degradation of p53 and modulate p53-dependent apoptosis (25). Very recent work by the same
group has demonstrated that transfected wild type NQO1, but not the mutant NQO1*2 protein, stabilizes wild type p53 in HCT-116 cells (26).
A redox mechanism was proposed for p53 stabilization that relied upon
NQO1-dependent NAD(P)H oxidation (25), although these
authors have commented that physical interaction of p53 and NQO1
represents an attractive possibility as a mechanism of stabilization
(26).
NQO1 is known to bind other proteins such as Hsp70 and Hsp40 (28).
Consequently, we examined whether NQO1 could interact with p53 via a
protein-protein interaction. In this work, we demonstrate that a novel
suicide inhibitor of NQO1 developed in our laboratory, which
irreversibly blocks the catalytic function of NQO1, has no effect on
p53 stability. This suggests that a redox mechanism of stabilization is
unlikely. However, for the first time, we demonstrate that NQO1 is able
to physically associate with p53 suggesting that a protein-protein
interaction may be responsible for the stabilization of p53 by NQO1. In
addition to the roles of NQO1 in direct detoxification of quinones and
in antioxidant defense, the interaction of NQO1 with p53 may represent
an additional mechanism that contributes to the chemoprotective
activity of NQO1.
Chemicals and Reagents--
Dichlorophenol-indophenol, NADH and
dicumarol (3,3'-methylene-bis(4-hydroxycoumarin)) were obtained
from Sigma. ES936
(5-methoxy-1,2-dimethyl-3-[(4-nitrophenol)methyl]indole-4,7-dione) was synthesized as described previously (29). MG132 was obtained from
Biomol Research Laboratories (Plymouth Meeting, PA). Anti-p53 mouse
monoclonal antibody clone PAb421 (Ab-1) and protein G plus/protein A-agarose were obtained from Calbiochem. Anti-p53 mouse monoclonal antibody DO-1, anti-p53 mouse monoclonal antibody DO-1 directly conjugated to horseradish peroxidase (HRP), and anti-mdm-2 mouse monoclonal antibody (SMP14) were obtained from Santa Cruz Biotechnology (Santa Cruz, CA). Anti-actin mouse monoclonal antibody (Ab-1, IgM) was
obtained from Oncogene (La Jolla, CA). Anti-NQO1 mouse monoclonal
antibodies (clones A180 and B771) were developed at the Cancer Center,
University of Colorado Health Sciences Center, Denver, CO and supplied
as a hybridoma supernatant (RPMI1640 containing 5% (v/v) fetal bovine
serum. Horseradish peroxidase-conjugated secondary antibodies were
purchased from Jackson ImmunoResearch Laboratories (West Grove, PA).
rhNQO1 was purified from Escherichia coli as described
previously (30), and the purified protein was resolved as a single band
on SDS-PAGE with a molecular mass of 31 kDa.
Cell Lines--
The human colorectal carcinoma cell line HCT-116
was obtained from American Tissue Culture Collection (Manassas, VA).
The cell lines HCT-116 p53+/+ and HCT-116 p53 ES936 Inhibition Experiments--
Exponentially growing HCT-116
cells were treated with ES936 (dissolved in Me2SO)
for the indicated times after which cells were washed free of drug and
then scrapped into freshly prepared RIPA lysis buffer (50 mM Tris-HCl, pH 8, 150 mM NaCl, 1 mM dithiothreitol, 1% (v/v) Nonidet P-40, 0.5%
(w/v) AB-deoxycholate, 0.1% SDS (w/v) containing 1 µg/ml leupeptin,
aprotinin, and peptstatin. The lysate was sonicated on ice for 5 s
and then centrifuged at 13,000 rpm for 10 min at 4 °C. Protein
concentrations were determined by the method of Lowry (33). NQO1
activity was measured in HCT-116 sonicates using dicumarol-sensitive
inhibition of DCPIP reduction (34). p53 protein levels were determined
in HCT-116 sonicates by immunoblot analysis with anti-p53 mouse
monoclonal antibody DO-1 and HRP-labeled secondary antibodies as
described below. Following p53 immunoblot analysis, membranes were
stripped in 62.5 mM Tris-HCl, pH 6.7, 2% (w/v) SDS, and
100 mM 2-mercaptoethanol for 1 h at 55 °C.
Following stripping, membranes were reincubated in blocking buffer (10 mM Tris, pH 8, 150 mM NaCl, 0.2% (v/v) Tween
20 containing 5% (w/v) nonfat dry milk) for 1 h before the addition of anti- Co-immunoprecipitation--
To examine the physical interaction
between NQO1 and p53, we utilized three different
co-immunoprecipitation methodologies: 1) immunoprecipitation with
anti-p53 antibodies followed by NQO1 immunoblot analysis, 2)
immunoprecipitation with anti-NQO1 antibodies (A180) cross-linked to
protein A-Sepharose beads followed by p53 immunoblot analysis, and 3)
immunoprecipitation with anti-NQO1 antibodies (A180) followed by
immunoblot analysis with anti-p53 antibodies directly coupled to
horseradish peroxidase.
Co-immunoprecipitation Studies in HCT-116 Cells--
For
co-immunoprecipitation experiments, an equal number of exponentially
growing cells (5 × 106) were harvested by
trypsinization, washed in phosphate-buffered saline, and then lysed in
freshly prepared RIPA buffer with protease inhibitors. Cells were lysed
in RIPA buffer on ice for 15 min followed by centrifugation at 13,000 rpm for 5 min. Equal aliquots of supernatant (6 mg) were added to tubes
containing anti-Bcl-2 (isotyped matched control, 2 µg), anti-p53 (2 µg), or anti-NQO1 (200 µl) antibodies overnight at 4 °C while
gently agitating. Protein A/G-agarose beads (40 µl) were then added,
and the incubations were continued for an additional 90 min at 4 °C.
Protein A/G-agarose beads were collected by centrifugation and then
washed four times with 50 mM Tris-HCl, pH 8.0, containing
150 mM NaCl, 1% (v/v) Nonidet P-40 followed by a single
wash (1 ml) with 50 mM Tris-HCl, pH 8.0. The protein
A/G-agarose beads were then suspended in 2× Laemmli SDS sample buffer
and heated to 75 °C for 5 min in preparation for SDS-PAGE and
subsequent immunoblot analysis.
Co-immunoprecipitation Studies in Human Primary Cell
Cultures--
Immunoblot analysis of proteins isolated from primary
epithelial cell cultures were performed as described previously (32) using the anti-p53 monoclonal antibody DO-1 and the anti-NQO1 monoclonal antibody A180. For the experiments described in Fig. 3, an
alternative methodology was employed where the anti-NQO1 antibody
(A180) was cross-linked to protein A-Sepharose (PAS) to prevent
interference with IgG heavy chain. For cross-linking, the PAS and
antibodies were incubated overnight while mixing at 4 °C. The
antibody-bound PAS was washed twice with 500 mM sodium borate, pH 9.0 (NaB), resuspended in NaB containing 100 mM
dimethyl pimelimidate (Pierce), and the pH readjusted to 8.2. The
antibody-bound PAS was mixed with dimethyl pimelimidate for 2 h at
25 °C, washed twice with 200 mM ethanolamine, pH 8.0, and then resuspended in ethanolamine and mixed for 2 h at
25 °C. The cross-linked PAS was washed twice with phosphate-buffered
saline and the incubation with dimethyl pimelimidate and subsequent
steps repeated to achieve complete cross-linking of the antibodies to
the PAS. After the final phosphate-buffered saline wash, the
cross-linked, antibody-bound PAS was resuspended in phosphate-buffered
saline, and chemical cross-linking was verified by immunoblot analysis
prior to use in immunoprecipitation studies with primary cultured cell
lines. To assure complete chemical cross-linking, DO-1-PAS was was
processed by SDS-PAGE, and the presence of uncross-linked antibody was
assessed by immunoblot analysis using a goat anti-mouse horseradish
peroxidase-conjugated antibody (Pierce). In all DO1-PAS preparations
used, free antibody was not detected by immunoblot analysis.
Co-immunoprecipitation Studies Using in Vitro
Transcription/Translation Reactions--
p53 in
vitro transcription/translation was carried out using SP6 quick
coupled transcription/translation system (TNT-RRL, Promega, Madison,
WI) according to the manufacturer's instructions using 1 µg of
plasmid p53SP64 poly(A). Reactions (50 µl) were carried out for 90 min at 32 °C, after which a small aliquot of reaction mixture was
removed, and p53 protein translation was monitored by immunoblot
analysis. Reactions were terminated by the addition of 200 µl of RIPA
buffer containing protease inhibitors, and immunoprecipitation was
carried out using anti-p53 monoclonal antibody PAb421 followed by NQO1
immunoblot analysis. The reverse immunoprecipitation was also performed
using anti-NQO1 antibodies (A180/B771) followed by immunoblot analysis
using anti-p53 antibodies (DO-1) directly coupled to horseradish
peroxidase (see SDS-PAGE). The human wild type p53 coding region in
pBSK-SN2 was a generous gift from Dr. Bert Vogelstein, Johns
Hopkins University, Baltimore, MD and subcloned into the pSP64 poly(A)
expression vector (Promega) using polymerase chain reaction (PCR)
amplification of the full coding region. The following oligomers
5'-cccaagcttATGGAGGAGCCGCAGTCAGATCC-3' and
5'-tgctctagaAGATCAGTCTGAGTCAGGCCCTTCTG-3' containing
HindIII and XbaI restriction sites, respectively,
were used to amplify the p53 coding region. The temperature cycles used
for this PCR amplification were as follows: 94 °C for 5 min, 25 cycles of 94 °C for 1 min, 63 °C for 1 min, followed by 72 °C
for 1.5 min. After the final cycle the reaction was kept at 4 °C.
The p53SP64 construct was verified by complete sequencing of the coding region.
SDS-PAGE and Immunoblot Analysis--
Proteins were separated by
12% SDS-PAGE and transferred to polyvinylidene difluoride membranes in
25 mM Tris, 192 mM glycine containing 20%
(v/v) methanol at 110 volts for 1 h. Following transfer, membranes
were placed overnight in blocking buffer (see above). Immunoblot
analysis of co-immunoprecipitated proteins was performed as follows: 1)
for NQO1 detection following p53 immunoprecipitation, membranes were
probed first with 20 ml of blocking buffer containing 500 µl of
anti-NQO1 monoclonal antibodies (A180/B771) for 1 h at 27 °C
followed by 20 ml of blocking buffer containing HRP-conjugated
secondary antibody (1:7,500) for 30 min at 27 °C. 2) For p53
detection following NQO1 immunoprecipitation samples were prepared in
2-mercaptoethanol-free 2× Laemmli buffer and heated to 75 °C for 5 min. Membranes were probed with 20 ml of blocking buffer containing
HRP-conjugated DO-1 diluted 1:2,000 for 45 min at 27 °C. Protein
bands were visualized using luminol-based enhanced chemiluminescence as
described by the manufacturer (PerkinElmer Life Sciences).
p53 Protein Levels Do Not Change in HCT-116 Cells in Response to
Inhibition of NQO1--
We have recently developed ES936 as a suicide
inhibitor of NQO1 (29). ES936 inhibits NQO1 at low concentrations in an
irreversible manner and has advantages over dicumarol, since the latter
is a competitive rather than an irreversible inhibitor of NQO1 and is
known to inhibit many other enzymes (27, 35). Treatment of HCT-116
cells with ES936 had little effect on the stability of p53 (Fig.
1A), suggesting that
inhibition of the catalytic function of NQO1 does not affect p53
stability. Inhibition of greater than 99% of NQO1 catalytic activity
in HCT-116 cells by ES936 (100 nM) was verified by NQO1
activity assays (Fig. 1B). Additional control experiments
demonstrated that treatment of HCT-116 cells with ES936 (100, 250, or
500 nM) had little effect on NQO1 protein levels (Fig.
1C).
Co-immunoprecipitation of NQO1 and p53--
To examine a potential
interaction between NQO1 and p53 proteins in HCT-116 cells, we utilized
a co-immunoprecipitation approach. Experiments were performed using p53
antibodies for immunoprecipitation and subsequent immunoblotting with
antibodies to NQO1 (Fig. 2A). In Fig. 2B, we demonstrate in HCT-116 cells that
pretreatment with ES936, a suicide inhibitor of NQO1, had little effect
on the association of p53 and NQO1, suggesting that the interaction of
the two proteins is not dependent on the catalytic activity of NQO1. An
isogenic HCT-116 cell line without p53 (p53
The experiments in Fig. 2 were performed using a p53 antibody for
immunoprecipitation. The reverse immunoprecipitation using an NQO1
antibody for immunoprecipitation and a p53 antibody for immunoblotting
performed in the same manner resulted in co-migration of IgG heavy
chain and interference with the p53 signal. To overcome this problem we
utilized immunoprecipitating antibodies directly coupled to agarose
beads, which prevents IgG heavy chain co-migration with p53. Using this
technique (Fig. 3), an association of
NQO1 and p53 could be detected in HCT-116 cells and also in primary HEKs and primary HMECs. Note the slower migrating species of p53 protein present in both the HEKs and HMECs. The shift in migration is
likely due to differential phosphorylation of p53 in the primary cultures of normal cells versus the transformed cell line
HCT-116. We have used an additional approach of employing an anti-NQO1 antibody for immunoprecipitation followed by immunoblotting using an
anti-p53 antibody (DO-1) directly coupled to HRP. This technique eliminates the need for the use of a secondary antibody and thus prevents interference of IgG heavy chain with p53. Interaction of NQO1
and p53 was also evident in HCT-116 cells using this method (data not
shown).
The association of NQO1 with p53 was confirmed in
co-immunoprecipitation experiments in which p53 was generated in an
in vitro transcription/translation system (RRL). Expression
of p53 in the RRL system using a p53 coding region inserted into the
SP64 poly(A) plasmid was confirmed by immunoblotting (Fig.
4A). Recombinant NQO1 (0.5 µg) was added to the RRL system as purified protein and its presence
confirmed also by immunoblotting (Fig. 4A).
Immunoprecipitation was performed using an anti-p53 antibody (PAb421)
followed by immunoblotting using anti-NQO1 monoclonal antibodies (Fig.
4B). Recombinant NQO1 was added to the RRL system either
before or after synthesis of p53, and in either case an association
between NQO1 and p53 was observed (Fig. 4B). An association
of NQO1 and p53 was also apparent in a reverse immunoprecipitation
employing either of two monoclonal antibodies to NQO1 (B771 or A180)
followed by immunoblotting with anti p53 antibody DO-1 directly coupled to HRP (Fig. 4C).
mdm-2 Does Not Associate with NQO1 in HCT-116
Cells--
Interaction of mdm-2 with p53 leads to rapid proteasomal
degradation of p53, and detection of the p53-mdm-2 interaction is facilitated during proteasomal inhibition. In Fig.
5A, we demonstrate interaction
of p53 with mdm-2 after treatment of HCT-116 cells with the proteasomal
inhibitor MG132. No interaction of NQO1 with mdm-2 could be detected in
HCT-116 cells either in the presence or the absence of MG132 (Fig.
5B). MG132 treatment had no effect on the levels of mdm-2 or
NQO1 in HCT-116 cells as determined by immunoblot analysis (data not
shown).
In this work, we demonstrate that wild type p53 associates with
NQO1 in a variety of human tumor cells, in primary human cell types,
and in cell-free systems. Experiments were performed using a
co-immunoprecipitation approach employing an anti-p53 monoclonal antibody followed by immunoblotting with an anti-human NQO1 antibody. Reverse immunoprecipitations were also performed employing either an
NQO1 antibody directly coupled to agarose beads or an anti-NQO1 monoclonal antibody followed by immunoblotting with a p53 antibody directly coupled to HRP. Both of these techniques avoided any co-migration of IgG heavy chain with p53. Initially, HCT-116 cells were
used to demonstrate the protein-protein interaction of NQO1 and p53
using both forward and reverse immunopreceipitations. We also utilized
an isogenic pair of HCT-116 cell lines differing only in their p53
status to explore the association of NQO1 and p53. NQO1 could be
detected by immunoblotting in a p53 immunoprecipitate in HCT-116 p53+/+
cells but not in HCT-116 p53/
cells demonstrated an interaction of NQO1 and p53 only in the p53+/+
cells. Experiments performed in an in vitro transcription/translation system utilizing rabbit reticulocyte lysates confirmed the interaction of NQO1 and p53. In these experiments a full-length p53 coding region was used to express p53 in the presence
of recombinant NQO1 protein. An association of p53 and NQO1 was also
observed in primary human keratinocytes and mammary epithelial cells.
In studies where mdm-2 co-immunoprecipitated with p53, no association
of mdm-2 with NQO1 was observed. These data demonstrate an association
between p53 and NQO1 that may represent an alternate mechanism of p53
stabilization by NQO1 in a wide variety of human cell types.
INTRODUCTION
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
MATERIALS AND METHODS
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
/
were obtained from Dr. B. Vogelstein, Johns Hopkins University (31). All HCT-116 cell
lines were genotyped as homozygous wild type for the NQO1*1 allele
(11). Cells were grown as monolayers at 37 °C in 5% CO2 in minimal essential medium supplemented with 10% (v/v) fetal bovine
serum, 10 units/ml penicillin/streptomycin, and 2 mM
L-glutamine. Second passage primary human epidermal
keratinocytes (HEK) were obtained from the Vanderbilt Skin Disease
Research Core. HEKs were isolated as described previously (32) and were
cultured in EpiLife M-EPI-500 keratinocyte growth media (Cascade
Biologics, Portland, OR) supplemented with human keratinocyte growth
supplement S-001-5 (Cascade Biologics) and 0.06 mM
CaCl2. Primary cultures of human mammary epithelial cells
(HMECs) were obtained from Clonetics (San Diego, CA) and passaged
according to manufacturer's instructions.
-actin monoclonal antibodies.
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
p53 and NQO1 protein levels and NQO1 activity
following treatment of HCT-116 cells with ES936. A, p53
and -actin protein levels were measured by immunoblot analysis in
HCT-116 cells (50 µg lysate) following treatment with the NQO1
mechanism-based inhibitor ES936 (100, 250 nM) for 2 and
8 h. B, NQO1 activity was measured using the rate of
dicumarol-sensitive reduction of DCPIP in HCT-116 cells following
treatment with ES936 (100 nM) for 2 h. C,
NQO1 and
-actin protein levels were measured by immunoblot analysis
in HCT-116 cells (50 µg of lysate) following treatment with the NQO1
mechanism-based inhibitor ES936 (0-500 nM) for 2 h.
/
) has been derived by
deletion of the p53 allele (31), and an interaction of NQO1 and p53
could be detected only in HCT-116 p53+/+ and not in HCT-116 p53
/
cells (Fig. 2C). Immunoblot analysis of HCT-116 p53+/+ and
p53
/
cell lines confirmed the absence of p53 in the p53
/
cell
line (Fig. 2D). These data also demonstrated that both
HCT-116 p53+/+ and HCT-116 p53
/
cells had marked levels of NQO1 as
indicated by immunoblotting (Fig. 2D). Although HCT-116 cells from ATCC and HCT-116 p53+/+ cells obtained as part of the p53
isogenic pair of cell lines (see "Materials and Methods") would be
expected to be essentially identical, we also verified that ES936 had
little effect on p53 or NQO1 protein levels in the HCT 116 p53+/+ cell
line (Fig. 2E), confirming the data in Fig. 1. Inhibition of
NQO1 catalytic activity by ES936 was verified by activity measurements
at the indicated time points and was 90% at 1 min and greater than
99% at all subsequent time points.
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Fig. 2.
Co-immunoprecipitation of NQO1 with p53 in
HCT-116 colon cancer cells. A, NQO1 was detected by
immunoblot analysis following immunoprecipitation of p53 from HCT-116
lysates. Lane 1, rhNQO1 standard (1 ng); lane 2,
anti-p53 (PAb 421); lane 3, anti-p53 (DO-1); lane
4, minus lysate control; lane 5, isotyped matched,
nonspecific antibody control; lane 6, minus antibody
control. B, NQO1 was detected by immunoblot analysis
following immunoprecipitation of p53 from HCT-116 cells grown in the
presence and absence of ES936 (250 nM) for 2 h prior
to immunoprecipitation with anti-p53 DO-1 antibody. Lane 1,
rhNQO1 standard (1 ng); lane 2, ES936 treated cells;
lane 3, untreated cells; lane 4, minus lysate
control; lane 5, isotyped matched, nonspecific antibody
control; lane 6, minus antibody control. C, NQO1
was detected by immunoblot analysis following immunoprecipitation of
p53 from HCT-116 p53+/+ lysates but not HCT-116 p53 /
lysates.
Lanes 2-4 are the results from immunoprecipitations
performed in HCT-116 p53+/+ lysates, while lanes 5-7 are
the results from imunoprecipitations preformed in HCT-116 p53
/
lysates. Lane 1, rhNQO1 standard (1 ng); lane 2,
anti-p53 (DO-1); lane 3, minus lysate control; lane
4, isotyped matched, nonspecific antibody control; lane
5, anti-p53 (DO-1); lane 6, minus lysate control;
lane 7, isotyped matched, nonspecific antibody control.
D, immunoblot analysis of p53 and NQO1 in HCT-116 p53+/+ and
p53
/
lysates (25 µg). E, immunoblot analysis of p53
and NQO1 in HCT-116 p53+/+ cells treated with ES936. p53, NQO1, and
-actin protein levels were measured at the indicated times following
ES936 treatment (100 nM) in HCT-116 p53+/+ lysates (50 µg). Following p53 immunblot analysis the membrane was stripped and
re-probed for NQO1 and
-actin. NT, no ES936
treatment.
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Fig. 3.
Co-immunoprecipitation of p53 with NQO1 in
HCT-116 and primary human epithelial cells. p53 was detected by
immunoblot analysis following immunoprecipitation of NQO1 from HCT-116,
HEK, and HMEC lysates. Lanes 1-3, immunoblot analysis of
NQO1 and p53 in HCT-116, HEK, and HMEC lysates (25 µg). Lanes
4-6, lysates from HCT-116 and HEK cells and HMECs were
immunoprecipitated using anti-NQO1 monoclonal antibody A180 followed by
p53 immunoblot analysis (DO-1).
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Fig. 4.
Co-immunoprecipitation of recombinant NQO1
with p53 generated in an in vitro
transcription/translation system. A, immunoblot
analysis for p53 and NQO1 prior to immunoprecipitation of in
vitro transcription/translation reactions. *, rhNQO1 (0.5 µg)
added before p53 synthesis; **, rhNQO1 (0.5 µg) added after p53
synthesis. B, NQO1 was detected by immunoblot analysis
following immunoprecipitation of p53 (PAb 421) generated in an in
vitro transcription/translation system. C, in the
reverse immunoprecipitation, p53 was detected by immunoblot analysis
following immunoprecipitation of NQO1 with anti-NQO1 monoclonal
antibodies (A180 or B771).
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Fig. 5.
Co-immunoprecipitation of MDM-2 with p53 but
not NQO1 in HCT-116 cells treatment with proteasome inhibitor
MG132. A, mdm-2 was detected by immunoblot analysis
following immunoprecipitation of p53 from HCT-116 cells pretreated with
MG132 (25 µM) for 6 h. In the absence of MG132
pretreatment no association of mdm-2 with p53 was observed.
B, no mdm-2 protein was observed by immunoblot analysis
following immunoprecipitation of NQO1 from HCT-116 cells grown in the
presence and absence of MG132 (25 µM) for 6 h.
Lane 1, isotyped matched, nonspecific control; lane
2, minus MG132 treatment; lane 3, plus MG132 treatment;
lane 4, HCT-116 lysate (100 µg), mdm-2-positive
control.
DISCUSSION
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
/
cells. An interaction of p53 and NQO1
was confirmed in a cell-free system where p53 was expressed in a
coupled transcription/translation system containing recombinant hNQO1
protein. Importantly the interaction of NQO1 and p53 could also be
detected in two primary human cell types, human epidermal keratinocytes
and human mammary epithelial cells, suggesting that this
protein-protein association occurs in a wide variety of human cell
types. The interaction of NQO1 and p53 in HMECs was less efficient than
in other cell types with similar levels of p53 and NQO1. This suggests
that interaction of NQO1 and p53 may be cell type-specific and may
reflect the predominance of other p53 interacting proteins in certain
cell types. Previous work performed on the stabilization of p53 by NQO1
had suggested a redox mechanism of stabilization dependent on the
catalytic function of NQO1 (25). In our experiments ES936, a specific suicide inactivator of NQO1, had no effect on the stability of p53.
These data suggest that the physical association of NQO1 with p53
should be considered as an alternate mechanism of NQO1-mediated stabilization of p53 (Scheme 1).
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Scheme 1.
Proposed non-catalytic mechanism of
stabilization of p53 by NQO1.
One of the major mechanisms of degradation of p53 involves association with mdm-2, which targets p53 for proteasomal degradation (36, 37). A p53-mdm-2 interaction could be observed after treating cells with the proteasomal inhibitor MG132, but no interaction of NQO1 with mdm-2 could be observed either in the presence or absence of proteasomal inhibitors. The NQO1*1 wild type protein is not subject to proteasomal degradation (13), and as expected, pretreatment of HCT-116 cells with MG132 did not result in any detectable interaction of NQO1 with mdm-2. These data suggest that NQO1 does not interact with mdm-2; therefore, the potential exists for competition of mdm-2 and NQO1 for p53. In addition to interacting with p53, we have previously demonstrated that wild type NQO1 is able to associate with Hsp70 and Hsp40 but not Hsp90 (28). p53 also interacts with proteins of the Hsp70 and Hsp40 families (38), and therefore, one possibility to consider is that p53, Hsp70/40, and NQO1 co-exist in a multiprotein complex. However, data obtained in the present study demonstrate that recombinant exogenous NQO1 when added to RRLs can interact with p53 transcribed and translated using a plasmid expression system. In previous work, we have shown that Hsp70 interacts with early, immature forms of NQO1 and does not interact with the mature protein (28). Since Hsp70 does not interact with the mature recombinant form of NQO1 (28), the participation of Hsp70 in the protein complex of p53 and the mature form of recombinant NQO1 added to the RRL system can be ruled out. Whether other proteins participate in the complex containing NQO1 and p53 remains to be elucidated (Scheme 1).
Proteins other than mdm2 are known to interact with p53 (39, 40), so
NQO1 is certainly not unique in its propensity to associate with p53.
Such proteins include CBP/p300 (41), HIF1 (42), ZBP-89 (43), WOX-1
(44), protein kinase 2 (45), nuclear actin (46), ARF (47), PIAS/Sumo
(48), and heat shock proteins (38, 49). Protein-protein interactions
may lead to either stabilization or degradation of p53 (39, 40), and
our data taken together with the previous work of Asher et
al. (25, 26) suggests that NQO1 may be another important protein
involved in stabilization of p53. Different stress signals utilize
multiple pathways to stabilize p53 such as down-regulation of mdm2,
modification of mdm2 activity by ARF binding, phosphorylation, or
regulation of the localization of p53 and/or mdm-2 (50). The growing
list of proteins that interact with p53 may well reflect multiple
mechanisms of p53 regulation that differ depending on the particular
stress response.
An important question that arises is whether the known metabolic functions of NQO1, the ability to detoxify quinones and protection against oxidative stress because of its role in ubiquinone and vitamin E metabolism, could be responsible for the protective effects of NQO1 against such a wide range of human solid tumors and leukemias (16-20). Interestingly in recent work, disruption of the NQO1 gene in mice led to the development of myelogenous hyperplasia, and NQO1 null mice were found to have markedly decreased bone marrow p53 content relative to wild type animals (51). Given the wide range of cancers that have been associated with a lack of NQO1 due to the NQO1*2 polymorphism, it is conceivable that NQO1 is functioning via a non-catalytic mechanism to protect against neoplasia. There is precedent for proteins normally considered as metabolic enzymes playing other roles in the cell. For example, glutathione S-transferase has recently been demonstrated to physically associate with the c-Jun-N-terminal kinase (JNK1) leading to inhibition of kinase activity (52) and modulation of c-Jun-N-terminal kinase signaling and cellular proliferation (53, 54).
In summary, we report a physical association of p53 and NQO1, which we
propose as a non-catalytic mechanism influencing p53 stability. This
may provide a mechanism for the increased susceptibility of individuals
lacking NQO1 due to the NQO1*2 polymorphism to various forms of cancer.
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FOOTNOTES |
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* This work was supported by Health and Human Services RO1 Grants CA51210, ES09554, and CA70856. This work was presented in part at the 93rd annual meeting of the American Association of Cancer Research, April 6-10th 2002, San Francisco, CA.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed: Dept. of Pharmaceutical Sciences, Box C-238, School of Pharmacy, University of Colorado Health Sciences Center, 4200 East 9th Ave., Denver, CO 80262. Tel.: 303-315-6077; Fax: 303-315-0274; E-mail: david.ross@uchsc.edu.
Published, JBC Papers in Press, January 14, 2003, DOI 10.1074/jbc.M211981200
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ABBREVIATIONS |
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The abbreviations used are: NQO1, NAD(P)H:quinone oxidoreductase 1; rhNQO1, recombinant human NAD(P)H:quinone oxidoreductase 1; HRP, horseradish peroxidase; RRL, rabbit reticulocyte lysate; MG132, Z-Leu-Leu-Leu-CHO; HEK, human epidermal keratinocyte; HMEC, human mammary epithelial cell; PAS, protein A-Sepharose.
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