Full text of DOI:10.1038/sj.tpj.6500036

Oxidative stress and redox imbalance
H Masutani
165
8
9
Granjeaud
781–790.
S
et al. Bioassays 1999; 21:
Biotechniques Books Division: Natick,
2000.
11 Fodor SP et al. Science 1991; 251: 767–773.
12 Pease AC et al. Proc Nat Acad Sci USA 1994;
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10 Rose D. In: Schena
M (ed). Microarray
Biochip Technology. Eaton Publishing,
Nrf2
controls
oxidative
stress-
Oxidative stress and redox imbalance
in acetaminophen toxicity
inducible proteins such as heme oxy-
genase-1, thioredoxin dependent per-
oxidase (peroxiredoxin, MSP23) and
cystine membrane transport activity,
and showed that Nrf2-deficient cells
were more sensitive to toxic electro-
philes.9 The thioredoxin and gluta-
thione systems constitute the major
cellular reducing systems.10 Thiore-
doxin is an oxidoreductase and a key
component of the thioredoxin system.
Thioredoxin plays protective roles
against oxidative stress. Interestingly,
we have shown recently the hemin-
induced activation of the thioredoxin
gene by Nrf2 through the ARE.11 In
addition, thioredoxin is also upregul-
ated by electrophiles through ARE
(manuscript in preparation). There-
fore, these redox enzymes of the gluta-
thione system and thioredoxin all
H Masutani
Institute for Virus Research, Kyoto University, Kawahara-cho, Sakyo, Kyoto, Japan
Acetaminophen
[N-acetyl-p-amino
sive element (ARE).5 The optimal
recognition sequence of v-Maf and
phenol (APAP)] is one of the most
widely used analgesic drugs world-
wide. It is a major cause of liver failure
and causes death when taken in
excess. Therefore, elucidation of the
mechanism of acetaminophen toxicity
is an important matter for medical and
pharmaceutical research. Although the
precise mechanism is still unknown,
oxidative damage and redox imbal-
ance have been suggested as the mech-
anism of acetaminophen toxicity.
Chan et al clearly demonstrated the
importance of Nrf2 in the detox-
ification of acetaminophen in their
recent paper entitled ‘An important
function of Nrf2 in combating oxidat-
ive stress: detoxification of acetamino-
phen’.1 They showed that the toxicity
of acetaminophen is markedly aug-
mented in Nrf2 knock out mice. The
most striking data are that the Nrf2
knock out mice died rapidly between
6 and 12 h after injection, whereas the
NF-E2p45
is
similar
to
the
ARE/electrophile-responsive element
(EpRE)6 which is identified as an
element to respond to electrophile tar-
geting xenobiotics. Nrf2 is reported to
regulate genes such as the human and
rat NAD(P)H: quinone oxidoreductase
genes, the rat and murine glutathione
S-transferase Ya genes and the human
␥-glutamylcysteine synthetase (GCS)
subunit genes.7,8 Indeed, they revealed
the decrease in gene expression of ␥-
glutamylcysteine synthetase, the rate-
limiting enzyme in the synthetic path-
way of glutathione, and glutathione S-
transferease in the liver of Nrf2 knock
out mice. In addition, they showed
have
a common regulatory mech-
anism and may have co-ordinated
roles against oxidative stress.
This study suggests the importance
of glutathione depletion and oxidative
stress
in
acetaminophen-induced
cellular injury. Oxidative stress triggers
death or apoptotic pathway by several
distinct mechanisms. Among these,
the mitochondrial death pathway
induced by oxidative stress has lately
drawn considerable attention. The
death is either apoptotic or necrotic
depending on the cellular redox status.
Depressed mitochondrial function is
implicated in acetaminophen toxicity.
Glutathione and redox systems are
known to be involved in regulating
the mitochondrial apoptotic pathway.
For example, depletion of glutathione
by an inhibitor of glutathione syn-
thesis often accelerates oxidative
stress-induced apoptosis.12 Further
analysis using cells from the Nrf2
knock out mice will be of great benefit
to clarify the mechanism of acetamin-
ophen cellular injury.
that
expression
of
detoxifying
enzymes such as glutathione S-trans-
ferase pi (GSTpi) and UDP-glucurono-
syltransferase (UGT) was significantly
lower in Nrf2 knock out mice, indicat-
ing the contribution of the conju-
gation process. Hence, this study and
these accumulating works strongly
support the idea that Nrf2 is a regu-
lator of the phase II enzyme genes.
It is interesting to note that these
detoxifying enzymes overlap with
redox regulating enzymes. Using N-
acetylcysteine (NAC), a precursor of
glutathione, they demonstrated the
impaired capacity of glutathione
replenishment in the mice, strongly
suggesting the protective role of the
thiol system against acetaminophen
cytotoxicity. Ishii and co-workers
showed that using Nrf2-deficient cells,
wild-type mice died over
extended period, with the last deaths
occurring between 36 and 48 h post
a more
injection. The LD50 was 235 and 320
Ϫ1
mg kg
for male and female Nrf2
knock out mice, respectively, and 400
and 540 mg kg
female wild-type mice, respectively.
Ϫ1
for the male and
Nrf2
is
a
member
leucine
of
the
zipper
Cap‘n’Collar/basic
(CNC-bZIP) family of transcription
factors including NF-E2p45, Nrf1,
Nrf2,2–4 Bachl, and Bach2. Nrf2 forms
heterodimers with small Maf proteins,
binding to the antioxidant respon-
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Oxidative stress and redox imbalance
H Masutani
166
ARE may be regulated by several
mRNA level in K562 cells. The acti-
vation mechanism of ARE is an
important issue and should be further
clarified to understand the mechanism
elucidation of the mechanism of
chemical-induced carcinogenesis.
mechanisms. Although the liberation
of Nrf2 from Keapl and subsequent
nuclear translocation of Nrf2 is
reported to be an important mech-
anism of activation through the ARE,13
the precise upstream event to Nrf2
activation and regulation of ARE bind-
ing remains undetermined. A report
suggested the involvement of protein
synthesis in ARE activation. Induction
of small Maf protein synthesis is
reported in ␤-naphthoflavone-induced
activation of the ␥-glutamylcysteine
synthetase subunit gene.8 ARE is also
regulated at the level of nuclear
export. Bach2 is regulated by an oxi-
dative stress-sensitive conditional
nuclear export. We showed a mech-
anism of a differential regulation of
ARE through a switch of its binding
factors including CNC-bZIP/ small Maf
transcription factors and Jun/ Fos fam-
ilies of proteins, depending on differ-
ent stimuli.11 In the current study,
Chan et al indicated a mechanism of
ARE regulation by the augmentation
of the Nrf2 mRNA level in the liver in
response to acetaminophen. However,
previous studies showed that exposure
to electrophilic agents does not
change the Nrf2 steady-state mRNA
level and liberates Nrf2 from Keap1,
leading to Nrf2 nuclear translo-
cation.13 We also observed that hemin
treatment does not change the Nrf2
DUALITY OF INTEREST
None declared.
of drug toxicity.
A recent report
showed that the heme oxygenase-1
gene is activated through the ARE by
MEKK1, TAK1, and ASK-1, but not by
p38 MAP kinase.14 Another report
showed that protein kinase C-
mediated Nrf2 phosphorylation is
involved in the PMA-induced acti-
vation of the ARE.15 These works indi-
cate that several distinct signaling
pathways lead to the ARE, depending
on each stimulus and cell type. Jun
and Fos families of proteins have been
reported to be involved in the binding
complex to the ARE. In some situ-
ations, the Jun/ Fos families of pro-
teins might be important for the regu-
lation of cellular detoxification
processes. Elucidation of the upstream
pathway to the ARE may open a novel
approach to drug development and
shed a light on genetic variability and
drug efficacy. Finally, the involvement
of Nrf2 gene mutation in carcinogen-
esis has not been fully investigated. As
the author suggested, the absence of
Nrf2 regulation and subsequent
chronic exposure to oxidative stress
may lead to DNA damage. Analysis of
drug-induced oxidative stress using
Nrf2 knock out mice may facilitate
Correspondence should be sent to
H Masutani, Institute for Virus Research, Kyoto
University, 53 Shogoin, Kawahara-cho, Sakyo,
Kyoto 606-8397, Japan.
Tel: ϩ81 75 751 4026
Fax: ϩ81 75 761 5766
E-mail: hmasutanȰvirus1.virus.kyoto-u.ac.jp
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