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Introduction
Many dietary phytochemicals exhibit beneficial effects
to health, including the prevention of diseases such as cancer,
as well as neurological, cardiovascular, inflammatory, and
metabolic diseases. Evolutionarily, herbivorous and
omnivorous animals have been ingesting plants. This interaction
between "animal_plant" ecosystems has resulted in an
elaborate system of detoxification and defense mechanisms
evolved by animals including humans. The condition of
stress generated by electrophiles or xenobiotics may be
referred to as electrophilic stress. Mammalian cells, including
human cells, respond to these dietary phytochemicals by a
"non-classical receptor sensing" mechanism of electrophilic
chemical-stress typified by "thiol-modulated" cellular
signaling events primarily leading to the gene expression of
pharmacologically beneficial effects, but sometimes unwanted
cytotoxicity also. Our laboratory has been studying two
groups of dietary phytochemical cancer-chemopreventive
compounds (isothiocyanates and
polyphenols)[1,2], which are effective in chemical-induced, as well as genetically-induced,
animal carcinogenesis models[3,4]. These compounds
typically generate "cellular stress" and modulate gene
expression of phase II detoxifying/antioxidant enzymes.
Multicellullar organisms rely on highly-organized
pathways to orchestrate the many extracellular clues received by
the cells and to convert them into specific physiological
processes. The classical first step in this cascade of
molecular events that are collectively referred to as signal
transduction pathways is the specific interaction of an extracellular
ligand with its receptor at the cell
membrane[5]. Reactive oxygen species (ROS) and reactive nitrogen species (RNS)
have been proposed as second messengers in the activation
of several signaling pathways leading to mitogenesis or
apoptosis[5]. The effective transmission of information
requires specificity, and how ROS signaling occurs with
specificity and without oxidative damage remains poorly
understood[6]. In addition, gene expression responses to
oxidative stress are necessary to ensure cell survival and are largely
attributed to specific redox-sensitive transcription
factors[7]. Redox-sensitive cysteine residues are known to sense and
transduce changes in cellular redox status caused by the
generation of ROS and the presence of oxidized
thiols[8].
Sporn and Liby[9] noted that the principal need in the
chemoprevention of cancer remains the discovery of new
agents that are effective and safe, and the development of
new dose-scheduling paradigms that will allow their
beneficial use over relatively long periods of time, but not
necessarily constantly, in a manner that is essentially free of
undesirable side effects. It has been
reported[10] that human cancers overexpressing genes are specific to a variety of normal
human tissues, including normal tissues other than
those from which the cancer originated, suggesting that
this general property of cancer cells plays a major role in
determining the behavior of the cancers, including their metastatic
potential. Surh[11] observed that the disruption or
deregulation of intracellular-signaling cascades often leads to
malignant transformation of cells, and it is therefore important to
identify the molecules in the signaling network that can be
affected by individual chemopreventive phytochemicals to
allow for better assessment of their underlying molecular
mechanisms. Conney[12] suggested that tailoring the
chemopreventive regimen to the individual or to groups of
individuals living under different environmental conditions,
or with different mechanisms of carcinogenesis, may be an
important aspect of cancer chemoprevention in human
populations. Given the inability to guarantee the long-term
safety of current chemopreventive regimens,
Sporn[9] suggested two approaches: the more widespread use of
low-dose combinations of chemopreventive agents with the goal
of achieving a therapeutic synergy between individual drugs
while reducing their individual toxicities, or the use of
intermittent, rather than constant, chronic dosing of
chemopreventive agents.
In the current review, we will attempt to shed light on
redox-mediated signaling translating into gene expression
and in vivo pharmacological events. We will explore dietary
cancer-chemopreventive phytochemicals, discuss the link
between oxidative/electrophilic stress and the redox circuitry,
and consider a brief overview of redox-sensitive
transcription factors. We will then discuss the kelch-like erythroid
Cap'n'Collar (CNC) homologue-associated
protein 1 (Keap1)-nuclear factor-E2-related factor 2 (Nrf2) axis in redox signaling,
an important target for preventive and possibly therapeutic
intervention in many types of cancers. We will also discuss
the Nrf2 paradigm in gene expression, transcriptome
profiling of disparate gene categories, and the
pharmacotoxico-genomic relevance of redox-sensitive Nrf2. Finally, we will
discuss the redox regulation of cell death mechanisms.
Natural dietary anti-cancer chemopreventive compounds
Because carcinogenesis comprises three different stages:
initiation, promotion, and progression, many potential
cancer-protective (chemopreventive) agents can be categorized
broadly as blocking agents (which impede the initiation stage)
or suppressing agents (which arrest or reverse the
promotion and progression of cancer), presumably by affecting or
perturbing crucial factors that control cell proliferation,
differentiation, senescence, or
apoptosis[2,13]. Dietary chemopreventive compounds functioning as detoxifying
enzyme inducers primarily include phenolic and
sulfur-containing compounds. Phenolic compounds may be classified
into polyphenols and flavonoids, whereas
sulfur-containing compounds may be classified into isothiocyanates and
organosulfur compounds[1]. Representative examples of
polyphenols include epigallocatechin-3-gallate (EGCG) from
green tea, curcumin from turmeric, and resveratrol from grapes;
whereas flavonoids are exemplified by quercetin from citrus
fruits and genistein from soy. Isothiocyanates include,
amongst others, sulforaphane (SFN) from broccoli, phenethyl
isothiocyanate (PEITC) from turnips and watercress, and
allyl isothiocyanate from brussels sprout. Organosulfur
compounds chiefly include diallyl sulfides from garlic oil.
Dietary isothiocyanates are derived in vivo from the
hydrolysis of glucosinolates present in cruciferous vegetables.
Our laboratory has worked extensively towards
understanding the molecular mechanisms in vitro
and determining the chemopreventive efficacy
in vivo of polyphenols (eg EGCG and curcumin), and isothiocyanates (eg SFN and
PEITC), and combinations of these phytochemicals to elicit
maximum efficacy in cancer cells/tumorigenic tissue with
minimum toxicity to normal cells. It is indeed quite puzzling
as to how these compounds can differentiate between
"normal" versus "abnormal tumor" cells in terms of signaling,
gene expression, and pharmacological effects. We have
shown[14] that EGCG treatment in human colon HT-29 cancer
cells causes damage to mitochondria, and that c-Jun
N-terminal kinase (JNK) mediates EGCG-induced apoptotic cell
death. Similarly, it was shown[15] that PEITC can induce
apoptosis in HT-29 cells in a time- and dose-dependent
manner via the mitochondrial caspase cascade, and that the
"activation" of JNK appears to be critical for the initiation of the
apoptotic processes. On the other hand, EGCG, curcumin,
SFN, and PEITC appear to "inhibit" (instead of activate)
lipopolysaccharide (LPS)-induced NF-κB activation in the
same cell type, HT-29 cells, stably transfected with an
NF-κB luciferase reporter construct[16]. In addition, the mechanism
of action of SFN was recently[17] attributed to the inhibition
of p38 mitogen-activated protein kinase (MAPK) isoforms
which contributed to the induction of antioxidant response
element (ARE)-mediated heme oxygenase-1 (HO-1) gene
expression in human hepatoma HepG2 cells. Extracellular
signal-regulated protein kinase (ERK) and JNK pathways were
activated[18] by PEITC treatment in human prostate cancer
PC-3 cells. Interestingly, a combination of PEITC and
curcumin was found to have an additive
effect[19] on the induction of apoptosis in PC-3 cells stably transfected with
an NF-κB luciferase reporter construct and involved an
inhibition of epidermal growth factor receptor (EGFR), its
downstream signaling including PI3K and Akt. In
vivo, PEITC and curcumin alone or in combination exhibited significant
cancer-preventive activities in NCr immunodeficient (nu/nu)
mice bearing subcutaneous xenografts of PC-3
cells[20]. It is tempting to speculate that "abnormal tumor" cells such as
PC-3 and HT-29 cells require the overexpressed or
hyper-activated NF-κB and/or EGFR for cell survival/proliferation,
whereas "normal" cells do not require these signaling
molecules to do so. The blockade of NF-κB and/or EGFR
signaling by these compounds would sensitize tumor cells to die,
but not the normal cells (but, instead, in normal cells these
compounds will redox-dependently affect the Nrf2-mediated
cellular detoxifying/antioxidant defense enzymes, which will
be discussed in greater detail later on), and hence the
potential specificities between "abnormal tumor" versus "normal"
cells. Recently, we[3] and
others[21] showed that SFN inhibited adenoma formation in the gastrointestinal tract of
genetically mutant ApcMin/+ mice, and that the
concentrations of SFN and its metabolite SFN_ glutathione (GSH) were
found to be between 3 and 30 nmol/g (~3_30 µmol/L), which
resembled that of the in vitro cell culture
systems[22]. Interestingly, under such conditions, using
immunohistochemical staining of the adenomas indicated that SFN
significantly suppressed the expression of phosphorylated-JNK,
phosphorylated-ERK and phosphorylated-Akt, which were
found to be highly expressed in the adenomas of ApcMin/+
mice versus normal mucosa[3]. When the acute effect of SFN
on the gene expression profiles was investigated in the small
intestine polyps of the SFN-treated Apc+/Min mice by
using Affymetrix microarray platforms, the results showed that
genes involved in apoptosis, cell growth, and maintenance,
rather than phase II detoxifying genes, were modulated in
the polyps of the SFN-treated Apc/Min+ mice. Some of the
pro-apoptotic genes such as MBD4 and serine/threonine
kinase 17b, TNF receptor superfamily member 7 and TNF
(ligand) superfamily member 11 were upregulated, while some
pro-survival genes such as cyclin-D2, integrin β1, and Wnt
9A were significantly downregulated in adenomas treated
with SFN. Importantly, two important genes involved in
colorectal carcinogenesis, arachidonate 15-lipoxygenase
(15-LOX) and COX-2 were found to be increased and decreased
respectively by SFN[3]. Most recently, we found that in the
colon of the Nrf2 _/_ mice challenged with the classical
inflammatory agent dextran sodium sulfate, the inflammatory
proteins such as COX-2 and iNOS were overexpressed with
a concomitant decreased expression of cellular antioxidant
enzymes such as HO-1 and
NQO1[23]. These results imply that the induction of antioxidant enzymes by dietary cancer
chemopreventive compounds can potentially counteract the
oxidative/inflammatory signaling pathways as observed by
Dinkova-Kostova et al[24].
Redox-mediated signaling
Oxidative stress and redox circuitry Although oxidants
are constantly generated for essential biologic functions,
excess generation or an imbalance between oxidants and
antioxidants can produce a common pathophysiological
condition known as oxidative stress[25]. Mediators of oxidative
stress, that is, ROS, also function as second messengers in
signal transduction. In the light of this knowledge, oxidative
stress has been remarkably defined as perturbations in
redox circuitry[6]. Indeed, proteins present on cell surfaces
and located in extracellular fluids undergo oxidation in
diverse pathophysiological conditions, and a growing body
of evidence suggests that the steady-state oxidation is
responsive to diet[25].
Interestingly, redox compartmentation functions as a
mechanism for specificity in redox signaling and oxidative
stress, with the relative redox
states[6] from most reducing to most oxidizing being
mitochondria>nuclei>cytoplasm>endoplasmic reticulum>extracellular space.
Hansen et al[6] noted that a circuitry model for redox signaling and control has
been developed based on the observations that three major
thiol/disulfide couples, namely glutathione (GSH)/glutathione
disulfide (GSSG), reduced thioredoxin [Trx-(SH)2]/oxidized
thioredoxin (Trx-SS), and cysteine (Cys)/cystine (CySS) are
not in redox equilibrium and therefore can function as
control nodes for many different redox-sensitive processes. In
this model, redox switches and pathways exist in parallel
circuits, with electron flow from NADPH as a central electron
donor to ROS and O2 as electron acceptors.
Given the involvement of ROS and RNS in a multiplicity
of physiological responses through the modulation of
signaling pathways, studies on reactive oxygen and nitrogen
species (RONS) signaling have received considerable
attention in recent times. Forman[5] noted primarily that: (i)
antioxidant enzymes are essential "turn-off" components in
signaling; (ii) spatial relationships are probably more
important in RONS signaling than the overall "redox state" of the
cell; and (iii) deprotonation of the cysteine to form the
thiolate, which can react with RONS, occurs in specific
protein sites providing specificity in signaling. The bacterial
transcription factor OxyR[8] mediates the cellular response
to both ROS and RNS by controlling the expression of the
OxyR regulon, which encodes proteins involved in the
H2O2 metabolism and the cytoplasmic thiol redox response. In
budding yeast, the transcription factor Yap1 (yeast AP1),
which is a basic leucine zipper (bZip) transcription factor,
confers the cellular response to redox stress by controlling
the expression of the regulon that encodes most yeast
antioxidant proteins[8]. Interestingly,
Georgiou[26] noted that in both bacteria and yeast, the redox control networks
exemplified by OxyR and Yap1, respectively, exhibit conserved
dynamic features, namely autoregulation (which in yeast is
accomplished via the reduction of Yap1 by thioredoxin) and
hysteresis. Indeed, the question of whether these features
are intrinsic properties of the regulatory architecture required
for proper adaptation to redox stress remains to be resolved.
Jacob[27] observed that disulfide-S-oxides are formed from
glutathione under oxidizing conditions and specifically
modulate the redox status of thiols, indicating the existence of
specialized cellular oxidative pathways. This supports the
paradigm of oxidative signal transduction and provides a
general pathway whereby ROS can convert thiols into
disulfides.
Redox-sensitive transcription factors Many
transcription factors are redox-sensitive, including activator protein-1
(AP-1), NF-κB, Nrf2, p53, and the glucocorticoid
receptor[6]. Such sensitivity involves at least 2 redox-sensitive steps,
one in the activation of the signaling cascade and another in
DNA binding, and possibly additional redox-sensitive
nuclear processes such as nuclear import and
export[6]. Nuclear and cytoplasmic redox couples perform distinct
functions during redox-sensitive transcription factor regulation.
AP-1 is responsive to low levels of oxidants resulting in
AP-1/DNA binding and an increase in gene expression.
AP-1 activation is due to the induction of JNK activity by
oxidants resulting in the phosphorylation of serine 63 and serine
73 in the c-Jun transactivation
domain[7,28,29]. With high concentrations of oxidants, AP-1 is inhibited and gene
expression is impeded. The inhibition of AP-1/DNA interactions is
attributed to the oxidation of specific cysteine residues in
c-Jun's DNA binding region, namely cysteine
252[7,30]. Xanthoudakis and
Curran[31] reported that a DNA repair
enzyme apurinic/apyrimidinic endonuclease (APE), also termed
redox factor-1 (Ref-1), possessed oxidoreductase activity and
was responsible for the redox regulation of AP-1. Oxidized
AP-1 could be effectively reduced by Ref-1, restoring DNA
binding activity. Trx1 was shown to be a critical player in
AP-1 regulation by virtue of its ability to reduce oxidized
Ref-1[32]. Thus, for AP-1, a nuclear pathway to reduce the
Cys of the DNA-binding domain is
distinct[6] from the upstream redox events that activate the signaling kinase
pathway.
Similarly, NF-κB contains a redox-sensitive critical
cysteine residue (cysteine 62) in the p50 subunit that is involved
in DNA binding[7,33]. NF-κB is normally sequestered in the
cytoplasm by IκB, but under oxidative conditions,
IκB is phosphorylated by IκB kinase (IKK), ubiquitinated, and
subsequently degraded. ROS production appears to be
necessary to initiate the events leading to the dissociation of the
NF-κB/IκB complex[6], but excessive ROS production
(oxidative stress) results in the oxidation of cysteine 62 which
does not affect its translocation to the nucleus, but rather,
interferes with DNA binding and decreases gene
expression[7,34]. Overexpression of Trx1 inhibited
NF-κB activity, but overexpression of nuclear-targeted Trx1 enhanced
NF-κB activity[35]. These findings
suggested[6] that Trx1 plays distinct roles within the cytoplasm (regulation of activation and
dissociation of IκB), and within the nucleus (regulation of
DNA binding).
Pivotal to the antioxidant
response[36_39] typical in mammalian homeostasis and oxidative stress is the important
transcription factor Nrf2 that has been extensively studied by
many research groups[1,36_43]. Nrf2 is indispensable to
cellular defense against many chemical insults of endogenous
and exogenous origin, which play major roles in the
etiopathogenesis of many cancers and inflammation-related
diseases such as inflammatory bowel
disease[23] and Parkinson's
disease[44]. The role of Nrf2 as a critical redox-sensitive
transcription factor will be discussed in greater detail in the
following section devoted to redox signaling with specific
emphasis on the Keap1_Nrf2 axis.
Keap1_Nrf2 axis in redox signaling Under homeostatic
conditions, Nrf2 is mainly sequestered in
the cytoplasm by a cytoskeleton-binding
protein called Keap1[40,45,46]
. Zhang and Hannink[47] have identified 2 critical cysteine residues in
Keap1, namely C273 and C288, that are required for
Keap1-dependent ubiquitination of Nrf2 as shown in Figure 1. They
have also identified[47] a third cysteine residue in Keap1,
namely C151, that is uniquely required for the inhibition of
Keap1-dependent degradation of Nrf2 (Figure 1) by
sulfo-raphane and oxidative stress. It has also been
shown[48] that Keap1 is a redox-regulated substrate adaptor protein for a
Cul3-dependent ubiquitin ligase complex that controls
steady-state levels of Nrf2 in response to cancer-preventive
compounds and oxidative stress. Moreover, it has been
report-ed[49] that Keap1 regulates the oxidation-sensitive shuttling
of Nrf2 into and out of the nucleus via an active
Crm1/exportin-dependent nuclear export mechanism. When
challenged with oxidative stress, Nrf2 is quickly released from
Keap1 retention and translocates to the
nucleus[40,50]. We have recently
identified[40] a canonical redox-insensitive
nuclear export signal (NES)
(537LKKQLSTLYL546) located in
the leucine zipper (ZIP) domain of the Nrf2 protein, as well as
a redox-sensitive NES
(173LLSI-PELQCLNI186) in the
transactivation (TA) domain of Nrf2[51]. Once in the nucleus,
Nrf2 not only binds to the specific consensus
cis-element called ARE present in the promoter region of many
cytoprotective genes[43,46], but
also to other trans-acting factors such as small Maf (MafG and
MafK)[52] that can coordinately
regulate gene transcription with Nrf2. We have also
reported[41] that different segments of the Nrf2
transactivation domain have different transactivation potential and that
different MAPK have differential effects on Nrf2 transcriptional
activity with ERK and JNK pathways playing an
unequivocal role in the positive regulation of Nrf2 transactivation
domain activity[41]. To better understand the biological
basis of signaling through Nrf2, it has also become imperative
to identify possible interacting partners of Nrf2 such as
co-activators or co-repressors apart from trans-acting factors
such as small Maf[52,53].
A salient feature of the canonical redox-insensitive
Nrf2_NES (NESzip,
537LKKQLSTLYL546) alluded to earlier, is its
overlap with the ZIP domain. This overlap implies that when Nrf2
forms heterodimers via its ZIP with other bZIP proteins, such
as its obligatory binding partner small MafG/K proteins, the
NES motif may be simultaneously
masked[40]. Further, we
observed[40] that Nrf2 heterodimerization via ZIP with
MafG/K may not only enhance the DNA binding specificity of Nrf2,
but may also effectively recruit Nrf2 into the nucleus by
simultaneously masking the NES activity. Accumulating
evidence shows that after fulfilling its transactivation function,
Nrf2 is destined for proteasomal degradation in the cytoplasm,
although some weak degradation activity may also exist
within the nucleus. Hence, Nrf2 signaling may be turned on
and off rapidly to match the rapid changes of the cellular
redox status[40]. Interestingly, a bipartite nuclear localization
signal (bNLS,
494RRRGKQKVAANQCRKRK511) as well as an
NES (545LKRRLSTLYL554) have been
identified[54] in the C terminus of Nrf2, further underscoring the importance of
nuclear import and export in controlling the subcellular
localization of Nrf2.
Unlike the NESzip, the new functional redox-sensitive NES
located in the transactivation (TA) domain of Nrf2
(NESTA,
173LLSIPELQCLNI186)
possesses a reactive cysteine residue (C183)
[51] and exhibits a dose-dependent nuclear
translocation when treated with
sulforaphane[51]. The NESTA motif
functions as a redox-sensitive switch that can be turned
on/off by oxidative signals and determines the subcellular
localization of Nrf2[51]. These discoveries suggest that Nrf2 may
be able to transduce redox signals in a Keap-1-independent
manner; however, Keap1 may provide additional regulation
of Nrf2 both in basal and inducible conditions.
Li et al[51] have recently proposed a new model for Nrf2
redox signaling. As depicted in Figure 1, the Nrf2 molecule
possesses multivalent NES/NLS motifs, and their relative
driving forces are represented by the size and direction of
the arrows. Under unstimulated conditions (to the left of
Figure 1), the combined nuclear-exporting forces of
NESTA[51] and
NESzip[51] may counteract the nuclear importing force of
the bNLS[54]. As a result, Nrf2 exhibits a predominantly whole
cell distribution. While the majority of Nrf2 molecules
remain in the cytoplasm, the residual nuclear Nrf2 may
account for the basal or constitutive Nrf2 activities. The
observation of a small percentage of cells exhibiting nuclear
and cytosolic distribution of Nrf2 may reflect the hyper- and
hypo-oxidative condition of individual cells, respectively.
When challenged with oxidative stress (to the right of
Figure 1), the redox-sensitive NESTA is disabled, but the
redox-insensitive NESzip remains
functional[40,51], and the bNLS motif may remain functionally
uninterrupted[51,55]. Since the driving force of the
NESzip is weaker than that of the bipartite
bNLS, the nuclear importing force mediated by the bNLS
prevails and triggers Nrf2 nuclear
translocation[51] as shown in Figure 1.
Gene expression and in vivo pharmacological
effects
Nrf2 paradigm in gene expression Nrf2 knockout mice
are greatly predisposed to chemical-induced DNA damage
and exhibit higher susceptibility towards cancer
development in several models of chemical
carcinogenesis[43]. Observations that Nrf2-deficient mice are refractory to the
protective actions of some chemopreventive
agents[43] highlight the importance of the Keap1_Nrf2_ARE signaling pathway
as a molecular target for prevention. Hence, several studies
have focused on using these Nrf2-deficient mice for
pharma-cogenomic or toxicogenomic profiling of the transcriptome
in response to dietary chemopreventive
agents[53,56_59] and
toxicants[60].
Transcriptome profiling of putative Nrf2 co-activators
and co-repressors In recent times, there has been renewed
interest in dissecting the interacting partners of Nrf2, such
as co-activators and co-repressors, which are co-regulated
with Nrf2 to better understand the biochemistry of Nrf2. In a
recent microarray study[56] with Nrf2 knockout mice, it was
reported that the CREB-binding protein (CBP) was
upregul-ated in mice liver on treatment with EGCG in an
Nrf2-dependent manner. Katoh et
al[61] showed that 2 domains of Nrf2
(Neh4 and Neh5) cooperatively bind CBP and
synergistically activate transcription. It was also
demonstrated[41] previously, using a Gal4-Luc reporter co-transfection assay
system in HepG2 cells, that the nuclear transcriptional
co-activator CBP, which can bind to the Nrf2 transactivation
domain and can be activated by the ERK cascade, showed
synergistic stimulation with Raf on the transactivation
activities of both the chimera Gal4-Nrf2 (1-370) and the
full-length Nrf2. In a recent microarray study with butylated
hydroxyanisole (BHA) treatment in Nrf2 knockout
mice[53], we observed the upregulation of the trans-acting factor
v-maf musculoaponeurotic fibrosarcoma oncogene family
protein G avian (MafG), nuclear receptor co-repressor 1
(Ncor1) and nuclear receptor co-repressor interacting
protein (Nrip1) as well as the downregulation of the nuclear
receptor co-activator 3 (Ncoa3) in an Nrf2-dependent manner.
Similarly, in another transcriptome profiling study in Nrf2
knockout mice with the endoplasmic reticulum (ER) stress
inducer tunicamycin[60], that modulates the unfolded protein
response (UPR), we observed the upregulation of the
P300/CBP-associated factor (P/CAF), trans-acting factor v-maf
musculoaponeurotic fibrosarcoma oncogene family, protein
F (MafF), nuclear receptor co-activator 5 (Ncoa5), Smad
nuclear interacting protein (Snip1) and Nrip1 as well as
downregulation of the src family associated
phosphoprotein 2 (Scap2) in an Nrf2-dependent manner. Although
microarray expression profiling cannot provide evidence of
binding between partners, this could potentially
suggest[53] that co-repressors Ncor1 and Nrip1 and co-activator Ncoa3,
such as CBP in the previous studies, may serve as putative
BHA-regulated nuclear interacting partners of Nrf2 in
eliciting the cancer chemopreventive effects of BHA, and that
co-repressor Nrip1 and co-activators P/CAF and Ncoa5 may
serve as putative tunicamycin-regulated nuclear interacting
partners of Nrf2 in eliciting the UPR-responsive
events[60] in vivo. Furthermore, the induction of Nrip1 was observed in
both the small intestine and liver with BHA suggesting that
the Nrf2/ARE pathway may play an important role in
BHA-elicited regulation of this gene. It has also been shown
recently[62] that the co-activator P/CAF could transcriptionally
activate a chimeric Gal4-Nrf2-luciferase system containing
the Nrf2 transactivation domain in HepG2 cells. In addition,
P/CAF, which is known[63] to be a histoneacetyl transferase
protein, has recently been shown[64] to mediate DNA
damage-dependent acetylation on most promoters of genes
involved in the DNA-damage and ER-stress response, which
validates the observation[60] of P/CAF induction via Nrf2 in
response to tunicamycin-induced ER stress.
Taken together, it is tempting to speculate that dietary
chemopreventive agents such as BHA and EGCG, and
toxicants such as tunicamycin, may elicit their chemopreventive
and pharmacological/toxicological events through the ARE
by means of a multi-molecular
complex[53,60] of co-activators and co-repressors that function in concert with the
redox-sensitive transcription factor Nrf2 as depicted in Figure 2.
The putative multi-molecular complex may involve Nrf2 along
with the transcriptional co-repressors Ncor1 and Nrip1, and
the transcriptional co-activators Ncoa3, Ncoa5, P/CAF, and
CBP, in addition to the currently known trans-acting factors
such as MafG[52], with multiple interactions between the
members of the putative complex as has been shown recently
with the p160 family of proteins[62]. As shown in Figure 2,
chemical signals generated by dietary chemopreventive
agents or toxicants may cause Nrf2 nuclear translocation
that sets in motion a dynamic machinery of co-activators
and co-repressors that may form a multi-molecular complex
with Nrf2 to modulate transcriptional response through the
ARE. Further studies of a biochemical nature would be
needed to substantiate this hypothesis and extend our
current understanding of Nrf2 regulation in chemoprevention
with BHA or EGCG and in tunicamycin-mediated ER stress.
Coordinated regulation of phase I, II, and III drug
metabolizing enzyme/transporter genes via Nrf2 Dietary
chemopreventives such as BHA, EGCG, and curcumin could
coordinately regulate the phase I, II, and III xenobiotic
metabolizing enzyme genes as well as antioxidative stress genes
through Nrf2-dependent pathways in
vivo[53,56,57]. Interes-tingly, a coordinated response involving phase I, II, and III
genes was also observed in vivo on ER stress induction
with the toxicant tunicamycin in an Nrf2-dependent
manner[60]. The array of genes
coordinately[53] modulated included phase I drug metabolizing enzymes, phase II detoxification
and antioxidant genes, and phase III transporter genes. The
regulation of these genes could have significant effects on
the prevention of tumor initiation by enhancing the cellular
defense system, preventing the activation of
procarcinogens/reactive intermediates, and increasing the excretion/efflux of
reactive carcinogens or
metabolites[53,57]. Indeed, the induction of phase II enzymes itself achieves protection against
the toxic and neoplastic effects of many carcinogens. In
addition to enzymes (that conjugate to functionalized
xenobiotics) such as glutathione S-transferases and
UDP-glucuronosyltransferases, a provisional and partial list of
phase II genes might include[65] NAD(P)H:quinone
reductase, epoxide hydrolase, dihydrodiol dehydrogenase,
gamma-glutamylcysteine synthetase, heme oxygenase-1, leukotriene
B4 dehydrogenase, aflatoxin B1 dehydrogenase, and ferritin.
The major phase II genes modulated via Nrf2 by BHA, EGCG,
curcumin, sulforaphane, PEITC, and tunicamycin are
summarized in Table 1.
Spatial and temporal control of Nrf2-mediated gene
expression It has been observed that the regulation of
Nrf2-mediated gene expression takes place at both spatial and
temporal levels in in vivo whole animals. For example, the
dietary chemopreventive BHA did not modulate the
transcription of NAD(P)H:quinine oxidoreductase (NQO1) at 3
h; however, NQO1 was induced by BHA at 12
h[53]. The relatively delayed induction of the NQO1 gene compared with
the other phase II genes in response to BHA points to the
possibility of differential kinetics of BHA-regulated phase II
gene response with temporal or time-dependent specificity.
However, many other genes were modulated differentially at
3 h in response to BHA between the small intestine and
liver[53] indicating certain spatial or tissue/compartment-dependent
control of gene expression. Similar phenomena were also
observed in microarray studies with
EGCG[56], curcumin[57],
SFN[58], and PEITC[59]. This could be attributed to a complex
of physiological factors including partitioning across the
gastrointestinal tract, intestinal transit time, uptake into the
hepatobiliary circulation, exposure parameters such as
Cmax, Tmax and, AUC, and the pharmacokinetics of disposition
after oral administration[53]. Other potential complicating
factors could be the possibility of the differential
tissue/organ-dependent expression of endogenous Keap1 and Nrf2 in
conjunction with other tissue-specific/general nuclear
co-regulators and
co-repressors[41,62,66] which could dynamically shift
the equilibrium locally in favor of or against Nrf2-mediated
transcription of different ARE-driven genes.
Pharmacotoxicogenomic relevance of redox-sensitive
Nrf2 Recently, it was reported[67] that Nrf1, another member
of the CNC family of basic ZIP proteins that is structurally
similar to Nrf2, is normally targeted to the ER membrane, and
that ER stress induced by tunicamycin in
vitro may play a role in modulating Nrf1 function as a transcriptional activator.
As a protein-folding compartment, the ER is exquisitely
sensitive to alterations in homeostasis, and provides stringent
quality control systems to ensure that only correctly folded
proteins transit to the Golgi and unfolded or misfolded
proteins are retained and ultimately degraded. A number of
biochemical and physiological stimuli, including
perturbation in redox status, can disrupt ER homeostasis and impose
stress to the ER, subsequently leading to accumulation of
unfolded or misfolded proteins in the ER
lumen[68]. The ER has evolved highly specific signaling pathways called the
UPR to cope with the accumulation of unfolded or misfolded
proteins[68,69]. ER stress stimulus by thapsigargin has also
been shown[70] to activate the c-JNK or stress-activated
protein kinase that is a member of the MAPK
cascade[71]. More-over, it has been reported that the coupling of ER stress to
JNK activation involves the transmembrane protein kinase
IRE1 by binding to an adaptor protein TRAF2, and that
IRE1α_/_ fibroblasts were impaired in JNK activation by ER
stress[72]. It has been previously reported that PEITC,
contained in large amounts in cruciferous vegetables such as
watercress, activates JNK1[73], and that the activation of the
ARE by PEITC involves both Nrf2 and
JNK1[42] in HeLa cells. It has also been demonstrated that ERK and JNK pathways
play an unequivocal role in the positive regulation of Nrf2
transactivation domain activity in vitro in HepG2 cells.
Although there is growing interest amongst researchers in
targeting the UPR against cancerous tumor
growth[74], there is currently little understanding of the role of Nrf2 in
modulating the UPR in vivo. Interestingly, in a Nrf2-deficient
murine model[60], glutathione peroxidase 3 (Gpx3) is upregulated
and superoxide dismutase 1 (Sod1) is downregulated by
tunica-mycin in an Nrf2-deficient manner, which can have
important implications in oxidative
stress-mediated[22] pathophy-siology or ER stress caused by perturbations in
redox circuitry[22,68,75]. The identification of novel molecular
targets that are regulated by the toxicant tunicamycin via
Nrf2 in vivo raises possibilities for targeting the UPR
proteins in future to augment or suppress the ER stress response
and modulate disease progression. Future
toxicogenomic/toxicokinetic studies may provide new biological insights
into the diverse cellular and physiological processes that
may be regulated by the UPR signaling pathways in cancer
pharmacology and toxicology and the role (s) of Nrf2 in these
processes.
Redox regulation of cellular signaling molecules
leading to cell death mechanisms It has been
demonstrated[76] that the redox regulation of platelet-derived growth factor
receptor (PDGFr) tyrosine autophosphorylation and its
signaling are related to NADPH oxidase activity through
protein kinase C (PKC) and phosphoinositide-3-kinase (PI3K)
activation and H2O2 production. Cheng and
Liu[77] reported that lead (Pb) increased LPS-induced liver damage in rats by
modulation of TNF-alpha and oxidative stress through PKC
and p42/44 MAPK. Recently, it was
shown[78] that Rac upregulated the tissue inhibitor of metalloproteinase-1
expression by redox-dependent activation of ERK signaling.
It has been reported[79] that cancer chemoprevention by the
nitroxide antioxidant tempol acts partially via the p53 tumor
suppressor.
Trachootham et al[80] have recently demonstrated a
selective killing of oncogenically transformed cells by PEITC
via a ROS-mediated mechanism. Oncogenic transformation
of ovarian epithelial cells with H-Ras(V12) or expression of
Bcr-Abl in hematopoietic cells caused elevated ROS
generation and rendered the malignant cells highly sensitive to
PEITC, which effectively disabled the glutathione
antioxidant system and caused severe ROS accumulation
preferentially in the transformed cells due to their active ROS output
resulting in oxidative mitochondrial damage, inactivation of
redox-sensitive molecules, and massive cell
death[80]. Xu et al[81]
demonstrated an inhibition by SFN and PEITC on
NF-κB transcriptional activation as well as downregulation of
NF-κB-regulated VEGF, cyclin D1, and Bcl-X(L) gene
expression primarily through the inhibition of IKK-beta
phos-phorylation, and the inhibition of IκB-alpha
phosphorylation and degradation, and decrease of nuclear translocation
of p65 in human prostate cancer PC-3 cells. Furthermore, in
the same PC-3 cells, PEITC and curcumin substantially
inhibited EGFR, PI3K as well as Akt
activation/phosphoryla-tion, and the combination of PEITC and curcumin was more
effective in the inhibition[19]. These inhibitions can also be
recapitulated in vivo in PC-3 transplanted athymic nude
mice[20]. Shen and
Pervaiz[82] have noted a role for
redox-dependent execution in TNF receptor superfamily-induced
cell death. Moreover, oxidation of the intracellular
environment of hepatocytes by ROS or redox-modulating agents
has been recently shown[83] to sensitize hepatocytes to
TNF-induced apoptosis that seems to occur only in a certain
redox range (in which redox changes can inhibit
NF-κB activity, but not completely inhibit caspase activity) by
interfering with NF-κB signaling pathways. Nitric oxide has
been reported[84] to induce thioredoxin-1 nuclear
translocation that may be associated with the p21Ras survival
path-way. Also, the p53-independent G1 cell cycle arrest of HT-29
human colon carcinoma cells by SFN was
shown[85] to be associated with the induction of
p21CIP1 and the inhibition of expression of cyclin D1. A small dose (10 µmol/L) of
hydrogen peroxide has been shown[86] to enhance TNF-alpha-induced cell apoptosis, upregulate Bax, and downregulate
Bcl-2 expression in the human vascular endothelial cell line
ECV304.
Recently, a new functional class of redox-reactive
thalidomide analogs with distinct and selective antileukemic
activity has been identified[87]. These agents activate the
NF of activated T-cells transcriptional pathways while
simultaneously repressing NF-κB via a rapid intracellular
amplification of ROS that is associated with caspase-
independent cell death[87]. This cytotoxicity is highly
selective for transformed lymphoid cells and preferentially
targets cells in the S-phase of the cell
cycle[87]. Capsaicin has been
shown[88] to selectively induce apoptosis in
mitogen-activated or transformed T cells with rapid increase of ROS;
however, neither production of ROS nor apoptosis is
detected in capsaicin-treated resting T cells suggesting
differential signaling between activated/transformed T cells
versus resting T cells. As noted elsewhere, a combination of
PEITC and curcumin has been
reported[19] to suppress EGFR phosphorylation as well as the phosphorylation of
IκB-alpha and Akt. Furthermore, Kim et
al[22] reported that SFN at low concentrations showed strong induction of phase II
genes that could potentially protect cells from cytotoxic
effects; however, at high concentrations (above 50 µmol/L),
SFN induced cell death with activation of caspase 3 and
JNK1/2, which could be blocked by exogenous glutathione.
The extent of SFN-induced cellular stress may thus decide
the direction of signaling between cell survival and cell
death[22]. The formation of mixed disulfides with GSH, which
is known as S-glutathionylation, is a post-translational
modification that is emerging as an important mode of redox
signaling[89]. Indeed, redox-reactive compounds provide a new
tool through which selective cellular properties of redox
status and intracellular bioactivation of potential redox-
sensitive molecular targets can be leveraged by rational
combinatorial therapeutic strategies and appropriate drug
design to exploit cell-specific vulnerabilities for maximum drug
efficacy[87].
Modulation of apoptosis and cell-cycle control genes via
Nrf2 Several genes related to apoptosis and cell-cycle
control that are critical in the etiopathogenesis of many cancers
have been shown[53,56_60] to be regulated through Nrf2 in
response to several dietary chemopreventive agents and
toxicants, far too many to be discussed here, at least in
normal murine tissues. The reader is referred to
references[53,56_60] for a comprehensive list of these genes. Future studies would
ascertain whether Nrf2 would be required for the regulation
of these apoptosis and cell-cycle control genes in tumor
tissues/cells.
Conclusions
In summary, electrophilic stress and oxidative stress, or
perturbations in redox circuitry can have a profound
influence on the direction of signaling between cell survival and
cell death. The potential involvement of cellular
redox-sensitive transcription factors can modulate gene expression
events and pharmacotoxicologic responses in diseases like
cancer, thus providing new molecular targets for preventive
or therapeutic intervention. Controlled nutritional studies in
future may specifically utilize extracellular redox
measurements to explore mechanistic links between diet, health status,
and diseases[25]. Taken together, redox-mediated signaling
is an important mechanism for the cancer chemopreventive
effects elicited by dietary anticancer chemopreventive
compounds. Future studies will likely provide additional
insights into the intricacies of the redox paradigm in signal
transduction pathways with respect to the
etiopathogenesis of cancer and its potential for chemopreventive
interven-tion.
Acknowledgments
The authors are grateful to all members of the Kong
Laboratory as well as researchers in the scientific community
whose works have been cited here. The authors also regret
the inability to include many other excellent relevant
citations in the interests of comprehensive abridgement.
References
1 Chen C, Kong AN. Dietary chemopreventive compounds and
ARE/EpRE signaling. Free Radic Biol Med 2004; 36: 1505_16.
2 Chen C, Kong AN. Dietary cancer-chemopreventive compounds:
from signaling and gene expression to pharmacological effects.
Trends Pharmacol Sci 2005; 26: 318_26.
3 Hu R, Khor TO, Shen G, Jeong WS, Hebbar V, Chen
C, et al. Cancer chemoprevention of intestinal polyposis in ApcMin/+
mice by sulforaphane, a natural product derived from cruciferous
vegetable. Carcinogenesis 2006; 27: 2038_46.
4 Xu C, Huang MT, Shen G, Yuan X, Lin W, Khor
TO, et al. Inhibition of 7,12-dimethylbenz(a)anthracene-induced skin
tumorigenesis in C57BL/6 mice by sulforaphane is mediated by
nuclear factor E2-related factor 2. Cancer Res 2006; 66:
8293_6.
5 Forman HJ, Torres M, Fukuto J. Redox signaling. Mol Cell
Biochem 2002; 234_5: 49_62.
6 Hansen JM, Go YM, Jones DP. Nuclear and mitochondrial
compartmentation of oxidative stress and redox signaling. Annu
Rev Pharmacol Toxicol 2006; 46: 215_34.
7 Hansen JM, Watson WH, Jones DP. Compartmentation of
Nrf-2 redox control: regulation of cytoplasmic activation by
glutathione and DNA binding by thioredoxin-1. Toxicol Sci 2004;
82: 308_17.
8 Barford D. The role of cysteine residues as redox-sensitive
regulatory switches. Curr Opin Struct Biol 2004; 14: 679_86.
9 Sporn MB, Liby KT. Cancer chemoprevention: scientific
promise, clinical uncertainty. Nat Clin Pract Oncol 2005; 2:
518_25.
10 Lotem J, Netanely D, Domany E, Sachs L. Human cancers
overexpress genes that are specific to a variety of normal human
tissues. Proc Natl Acad Sci USA 2005; 102: 18 556_61.
11 Surh YJ. Cancer chemoprevention with dietary phytochemicals.
Nat Rev Cancer 2003; 3: 768_80.
12 Conney AH. Tailoring cancer chemoprevention regimens to the
individual. J Cell Biochem 2004; 91: 277_86.
13 Chemoprevention Working Group. Prevention of cancer in the
next millennium: Report of the Chemoprevention Working Group
to the American Association for Cancer Research. Cancer Res
1999; 59: 4743_58.
14 Chen C, Shen G, Hebbar V, Hu R, Owuor ED, Kong AN.
Epigal-locatechin-3-gallate-induced stress signals in HT-29 human
colon adenocarcinoma cells. Carcinogenesis 2003; 24: 1369_78.
15 Hu R, Kim BR, Chen C, Hebbar V, Kong AN. The roles of JNK
and apoptotic signaling pathways in PEITC-mediated responses
in human HT-29 colon adenocarcinoma cells. Carcinogenesis
2003; 24: 1361_7.
16 Jeong WS, Kim IW, Hu R, Kong AN. Modulatory properties of
various natural chemopreventive agents on the activation of
NF-kappaB signaling pathway. Pharm Res 2004; 21: 661_70.
17 Keum YS, Yu S, Chang PP, Yuan X, Kim JH, Xu
C, et al. Mechanism of action of sulforaphane: inhibition of p38
mitogen-activated protein kinase isoforms contributing to the induction of
antioxidant response element-mediated heme oxygenase-1 in
human hepatoma HepG2 cells. Cancer Res 2006; 66: 8804_13.
18 Xu C, Yuan X, Pan Z, Shen G, Kim JH, Yu
S, et al. Mechanism of action of isothiocyanates: the induction of ARE-regulated genes
is associated with activation of ERK and JNK and the
phosphorylation and nuclear translocation of Nrf2. Mol Cancer Ther
2006; 5: 1918_26.
19 Kim JH, Xu C, Keum YS, Reddy B, Conney A, Kong AN.
Inhibition of EGFR signaling in human prostate cancer PC-3 cells by
combination treatment with beta-phenylethyl isothiocyanate and
curcumin. Carcinogenesis 2006; 27: 475_82.
20 Khor TO, Keum YS, Lin W, Kim JH, Hu R, Shen
G, et al. Combined inhibitory effects of curcumin and phenethyl isothiocyanate
on the growth of human PC-3 prostate xenografts in
immunodeficient mice. Cancer Res 2006; 66: 613_21.
21 Myzak MC, Dashwood WM, Orner GA, Ho E, Dashwood RH.
Sulforaphane inhibits histone deacetylase in vivo
and suppresses tumorigenesis in Apc-minus mice. Faseb J 2006; 20: 506_8.
22 Kim BR, Hu R, Keum YS, Hebbar V, Shen G, Nair SS,
et al. Effects of glutathione on antioxidant response
element-mediated gene expression and apoptosis elicited by sulforaphane.
Cancer Res 2003; 63: 7520_5.
23 Khor TO, Huang MT, Kwon KH, Reddy BS, Kong AN.
Nrf2-deficient mice have an increased susceptibility to dextran sulfate
sodium-induced colitis. Cancer Res 2006; 66:11580-4.
24 Dinkova-Kostova AT, Liby KT, Stephenson KK, Holtzclaw WD,
Gao X, Suh N, et al. Extremely potent triterpenoid inducers of
the phase 2 response: correlations of protection against oxidant
and inflammatory stress. Proc Natl Acad Sci USA 2005; 102:
4584_9.
25 Moriarty-Craige SE, Jones DP. Extracellular thiols and
thiol/disulfide redox in metabolism. Annu Rev Nutr 2004; 24:
481_509.
26 Georgiou G. How to flip the (redox) switch. Cell 2002; 111:
607_10.
27 Jacob C, Lancaster JR, Giles GI. Reactive sulphur species in
oxidative signal transduction. Biochem Soc Trans 2004; 32:
1015_7.
28 Karin M. The regulation of AP-1 activity by mitogen-activated
protein kinases. J Biol Chem 1995; 270: 16 483_6.
29 Smeal T, Binetruy B, Mercola D, Grover-Bardwick A, Heidecker
G, Rapp UR, et al. Oncoprotein-mediated signalling cascade
stimulates c-Jun activity by phosphorylation of serines 63 and
73. Mol Cell Biol 1992; 12: 3507_13.
30 Abate C, Patel L, Rauscher FJ III, Curran T. Redox regulation of
fos and jun DNA-binding activity in vitro. Science 1990; 249:
1157_61.
31 Xanthoudakis S, Curran T. Redox regulation of AP-1: a link
between transcription factor signaling and DNA repair. Adv Exp
Med Biol 1996; 387: 69_75.
32 Hirota K, Matsui M, Iwata S, Nishiyama A, Mori K, Yodoi J.
AP-1 transcriptional activity is regulated by a direct association
between thioredoxin and Ref-1. Proc Natl Acad Sci USA 1997; 94:
3633_8.
33 Matthews JR, Kaszubska W, Turcatti G, Wells TN, Hay RT. Role
of cysteine62 in DNA recognition by the P50 subunit of
NF-kappa B. Nucleic Acids Res 1993; 21: 1727_34.
34 Galter D, Mihm S, Droge W. Distinct effects of glutathione
disulphide on the nuclear transcription factor kappa B and the
activator protein-1. Eur J Biochem 1994; 221: 639_48.
35 Hirota K, Murata M, Sachi Y, Nakamura H, Takeuchi J, Mori
K, et al. Distinct roles of thioredoxin in the cytoplasm and in the
nucleus. A two-step mechanism of redox regulation of
transcription factor NF-kappaB. J Biol Chem 1999; 274: 27 891_7.
36 Alam J, Stewart D, Touchard C, Boinapally S, Choi AM, Cook JL.
Nrf2, a Cap'n'Collar transcription factor, regulates induction of
the heme oxygenase-1 gene. J Biol Chem 1999; 274: 26 071_8.
37 McMahon M, Itoh K, Yamamoto M, Chanas SA, Henderson CJ,
McLellan LI, et al. The Cap'n'Collar basic leucine zipper
transcription factor Nrf2 (NF-E2 p45-related factor 2) controls both
constitutive and inducible expression of intestinal detoxification
and glutathione biosynthetic enzymes. Cancer Res 2001; 61:
3299_307.
38 Thimmulappa RK, Mai KH, Srisuma S, Kensler TW, Yamamoto
M, Biswal S. Identification of Nrf2-regulated genes induced by
the chemopreventive agent sulforaphane by oligonucleotide
microarray. Cancer Res 2002; 62: 5196_203.
39 Prochaska HJ, De Long MJ, Talalay P. On the mechanisms of
induction of cancer-protective enzymes: a unifying proposal.
Proc Natl Acad Sci USA 1985; 82: 8232_6.
40 Li W, Jain MR, Chen C, Yue X, Hebbar V, Zhou
R, et al. Nrf2 possesses a redox-insensitive nuclear export signal overlapping
with the leucine zipper motif. J Biol Chem 2005; 280: 28
430_8.
41 Shen G, Hebbar V, Nair S, Xu C, Li W, Lin W,
et al. Regulation of Nrf2 transactivation domain activity. The differential
effects of mitogen-activated protein kinase cascades and
synergistic stimulatory effect of Raf and CREB-binding protein. J Biol
Chem 2004; 279: 23 052_60.
42 Keum YS, Owuor ED, Kim BR, Hu R, Kong AN. Involvement of
Nrf2 and JNK1 in the activation of antioxidant responsive
element (ARE) by chemopreventive agent phenethyl isothiocyanate
(PEITC). Pharm Res 2003; 20: 1351_6.
43 Yu X, Kensler T. Nrf2 as a target for cancer chemoprevention.
Mutat Res 2005; 591: 93_102.
44 Burton NC, Kensler TW, Guilarte TR. In vivo
modulation of the Parkinsonian phenotype by Nrf2. Neurotoxicology 2006;
27:1094-100
45 Itoh K, Wakabayashi N, Katoh Y, Ishii T, Igarashi K, Engel JD,
et al. Keap1 represses nuclear activation of antioxidant
responsive elements by Nrf2 through binding to the amino-terminal
Neh2 domain. Genes Dev 1999; 13: 76_86.
46 Dhakshinamoorthy S, Jaiswal AK. Functional characterization
and role of INrf2 in antioxidant response element-mediated
expression and antioxidant induction of NAD(P)H:quinone
oxidoreductase1 gene. Oncogene 2001; 20: 3906_17.
47 Zhang DD, Hannink M. Distinct cysteine residues in Keap1 are
required for Keap1-dependent ubiquitination of Nrf2 and for
stabilization of Nrf2 by chemopreventive agents and oxidative stress.
Mol Cell Biol 2003; 23: 8137_51.
48 Zhang DD, Lo SC, Cross JV, Templeton DJ, Hannink M. Keap1
is a redox-regulated substrate adaptor protein for a
Cul3-dependent ubiquitin ligase complex. Mol Cell Biol 2004; 24: 10
941_53.
49 Velichkova M, Hasson T. Keap1 regulates the
oxidation-sensitive shuttling of Nrf2 into and out of the nucleus via a
Crm1-dependent nuclear export mechanism. Mol Cell Biol 2005; 25:
4501_13.
50 Wakabayashi N, Dinkova-Kostova AT, Holtzclaw WD, Kang
MI, Kobayashi A, Yamamoto M, et al. Protection against
electrophile and oxidant stress by induction of the phase 2
response: fate of cysteines of the Keap1 sensor modified by
inducers. Proc Natl Acad Sci USA 2004; 101: 2040_5.
51 Li W, Yu SW, Kong AN. Nrf2 possesses a redox-sensitive nuclear
exporting signal in the Neh5 transactivation domain. J Biol
Chem 2006; 281: 27 251_63.
52 Dhakshinamoorthy S, Jaiswal AK. Small maf (MafG and MafK)
proteins negatively regulate antioxidant response
element-mediated expression and antioxidant induction of the
NAD(P)H:quinone oxidoreductase1 gene. J Biol Chem 2000; 275: 40
134_41.
53 Nair S, Xu C, Shen G, Hebbar V, Gopalakrishnan A, Hu
R, et al. Pharmacogenomics of phenolic antioxidant butylated
hydroxyanisole (BHA) in the small intestine and liver of Nrf2 knockout
and C57BL/6J mice. Pharm Res 2006; 23: 2621_37.
54 Jain AK, Bloom DA, Jaiswal AK. Nuclear import and export
signals in control of Nrf2. J Biol Chem 2005; 280: 29 158_68.
55 Bloom D, Dhakshinamoorthy S, Jaiswal AK. Site-directed
mutagenesis of cysteine to serine in the DNA binding region of Nrf2
decreases its capacity to upregulate antioxidant response
element-mediated expression and antioxidant induction of
NAD(P)H:quinone oxidoreductase1 gene. Oncogene 2002; 21:
2191_200.
56 Shen G, Xu C, Hu R, Jain MR, Nair S, Lin
W, et al. Comparison of (-)-epigallocatechin-3-gallate elicited liver and small intestine
gene expression profiles between C57BL/6J mice and
C57BL/6J/Nrf2 (-/-) mice. Pharm Res 2005; 22: 1805_20.
57 Shen G, Xu C, Hu R, Jain MR, Gopalkrishnan A, Nair S,
et al. Modulation of nuclear factor E2-related factor 2-mediated gene
expression in mice liver and small intestine by cancer
chemopreven-tive agent curcumin. Mol Cancer Ther 2006; 5:
39_51.
58 Hu R, Xu C, Shen G, Jain MR, Khor TO, Gopalkrishnan A,
et al. Gene expression profiles induced by cancer chemopreventive
isothiocyanate sulforaphane in the liver of C57BL/6J mice and
C57BL/6J/Nrf2 (-/-) mice. Cancer Lett 2006; 243: 170_92.
59 Hu R, Xu C, Shen G, Jain MR, Khor TO, Gopalkrishnan A,
et al. Identification of Nrf2-regulated genes induced by
chemopreven-tive isothiocyanate PEITC by oligonucleotide microarray. Life
Sci 2006; 79: 1944_55.
60 Nair S, Xu C, Shen G, Hebbar V, Gopalakrishnan A, Hu R,
et al. Toxicogenomics of endoplasmic reticulum stress inducer
tunica-mycin in the small intestine and liver of Nrf2 knockout and
C57BL/6J Mice. Toxicol Lett 2006; 168: 21_39.
61 Katoh Y, Itoh K, Yoshida E, Miyagishi M, Fukamizu A, Yamamoto
M. Two domains of Nrf2 cooperatively bind CBP, a CREB
binding protein, and synergistically activate transcription. Genes
Cells 2001; 6: 857_68.
62 Lin W, Shen G, Yuan X, Jain MR, Yu S, Zhang A,
et al. Regulation of Nrf2 transactivation domain activity by p160
RAC3/SRC3 and other nuclear co-regulators. J Biochem Mol Biol 2006;
39: 304_10.
63 Chen H, Tini M, Evans RM. HATs on and beyond chromatin.
Curr Opin Cell Biol 2001; 13: 218_24.
64 Ceribelli M, Alcalay M, Vigano MA, Mantovani R. Repression of
new p53 targets revealed by ChIP on chip experiments. Cell
Cycle 2006; 5: 1102_10.
65 Talalay P. Chemoprotection against cancer by induction of phase
2 enzymes. Biofactors 2000; 12: 5_11.
66 Zhang J, Ohta T, Maruyama A, Hosoya T, Nishikawa K, Maher
JM, et al. BRG1 interacts with Nrf2 to selectively mediate
HO-1 induction in response to oxidative stress. Mol Cell Biol 2006;
26: 7942_52.
67 Wang W, Chan JY. Nrf1 is targeted to the endoplasmic
reticulum membrane by an N-terminal transmembrane domain.
Inhibition of nuclear translocation and transacting function. J Biol
Chem 2006; 281: 19 676_87.
68 Zhang K, Kaufman RJ. Protein folding in the endoplasmic
reticulum and the unfolded protein response. Handb Exp Pharmacol
2006; 69_91.
69 Hetz C, Bernasconi P, Fisher J, Lee AH, Bassik MC, Antonsson
B, et al. Proapoptotic BAX and BAK modulate the unfolded
protein response by a direct interaction with IRE1alpha. Science
2006; 312: 572_6.
70 Srivastava RK, Sollott SJ, Khan L, Hansford R, Lakatta EG,
Longo DL. Bcl-2 and Bcl-X(L) block thapsigargin-induced nitric
oxide generation, c-Jun NH(2)-terminal kinase activity, and
apoptosis. Mol Cell Biol 1999; 19: 5659_74.
71 Kyriakis JM, Banerjee P, Nikolakaki E, Dai T, Rubie EA, Ahmad
MF, et al. The stress-activated protein kinase subfamily of c-Jun
kinases. Nature 1994; 369: 156_60.
72 Urano F, Wang X, Bertolotti A, Zhang Y, Chung P, Harding
HP, et al. Coupling of stress in the ER to activation of JNK protein
kinases by transmembrane protein kinase IRE1. Science 2000;
287: 664_6.
73 Yu R, Jiao JJ, Duh JL, Tan TH, Kong AN. Phenethyl
isothio-cyanate, a natural chemopreventive agent, activates c-Jun
N-terminal kinase 1. Cancer Res 1996; 56: 2954_9.
74 Garber K. Researchers target unfolded protein response in
cancerous tumor growth. J Natl Cancer Inst 2006; 98: 512_4.
75 Jones DP. Extracellular redox state: refining the definition of
oxidative stress in aging. Rejuvenation Res 2006; 9: 169_81.
76 Catarzi S, Biagioni C, Giannoni E, Favilli F, Marcucci T, Iantomasi
T, et al. Redox regulation of
platelet-derived-growth-factor-receptor: role of NADPH-oxidase and c-Src tyrosine kinase.
Biochim Biophys Acta 2005; 1745: 166_75.
77 Cheng YJ, Liu MY. Modulation of tumor necrosis factor-alpha
and oxidative stress through protein kinase C and P42/44
mitogen-activated protein kinase in lead increases
lipopolysaccharide-induced liver damage in rats. Shock 2005; 24: 188_93.
78 Engers R, Springer E, Kehren V, Simic T, Young DA, Beier
J, et al. Rac upregulates tissue inhibitor of metalloproteinase-1
expression by redox-dependent activation of extracellular
signal-regulated kinase signaling. Febs J 2006; 273: 4754_69.
79 Erker L, Schubert R, Yakushiji H, Barlow C, Larson D, Mitchell
JB, et al. Cancer chemoprevention by the antioxidant
tempol acts partially via the p53 tumor suppressor. Hum Mol Genet
2005; 14: 1699_708.
80 Trachootham D, Zhou Y, Zhang H, Demizu Y, Chen Z, Pelicano
H, et al. Selective killing of oncogenically transformed cells
through a ROS-mediated mechanism by beta-phenylethyl
isothiocyanate. Cancer Cell 2006; 10: 241_52.
81 Xu C, Shen G, Chen C, Gelinas C, Kong AN. Suppression of
NF-kappaB and NF-kappaB-regulated gene expression by sulforaphane
and PEITC through IkappaBalpha, IKK pathway in human
prostate cancer PC-3 cells. Oncogene 2005; 24: 4486_95.
82 Shen HM, Pervaiz S. TNF receptor superfamily-induced cell
death: redox-dependent execution. Faseb J 2006; 20: 1589_98.
83 Han D, Hanawa N, Saberi B, Kaplowitz N. Hydrogen peroxide
and redox modulation sensitize primary mouse hepatocytes to
TNF-induced apoptosis. Free Radic Biol Med 2006; 41: 627_39.
84 Arai RJ, Masutani H, Yodoi J, Debbas V, Laurindo FR, Stern
A, et al. Nitric oxide induces thioredoxin-1 nuclear translocation:
possible association with the p21Ras survival pathway. Biochem
Biophys Res Commun 2006; 348: 1254_60.
85 Shen G, Xu C, Chen C, Hebbar V, Kong AN. p53-independent G1
cell cycle arrest of human colon carcinoma cells HT-29 by
sulforaphane is associated with induction of p21CIP1 and
inhibition of expression of cyclin D1. Cancer Chemother Pharmacol
2006; 57: 317_27.
86 Luo T, Xia Z. A small dose of hydrogen peroxide enhances
tumor necrosis factor-alpha toxicity in inducing human vascular
endothelial cell apoptosis: reversal with propofol. Anesth Analg
2006; 103: 110_6.
87 Ge Y, Montano I, Rustici G, Freebern WJ, Haggerty CM, Cui
W, et al. Selective leukemic cell killing by a novel functional class
of thalidomide analogs. Blood 2006; 108: 4126_35.
88 Macho A, Calzado MA, Munoz-Blanco J, Gomez-Diaz C, Gajate
C, Mollinedo F, et al. Selective induction of apoptosis by
capsaicin in transformed cells: the role of reactive oxygen species and
calcium. Cell Death Differ 1999; 6: 155_65.
89 Reynaert NL, Wouters EF, Janssen-Heininger YM. Modulation
of glutaredoxin-1 expression in a mouse model of allergic airway
disease. Am J Respir Cell Mol Biol 2007; 36: 147_51.
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