Original Article

Yonago Acta medica 2016;59:223–231

Preconditioning by Low Dose LPS Prevents Subsequent LPS-Induced Severe
Liver Injury via Nrf2 Activation in Mice
Masato Nakasone,*† Kazuhiro Nakaso,* Yosuke Horikoshi,* Takehiko Hanaki,*‡ Yoshinori Kitagawa,*† Toru
Takahashi,* Yoshimi Inagaki† and Tatsuya Matsura**Division of Medical Biochemistry, Department of Pathophysiological and Therapeutic Science, School of Medicine, Tottori University
Faculty of Medicine, Yonago 683-8503, Japan, †Division of Anesthesiology and Clinical Care Medicine, Department of Surgery, Schoolof Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan and ‡Division of Surgical Oncology, Department of Surgery, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan

ABSTRACT

jection of a lethal level of LPS (20 mg/kg body weight).
DNA array revealed that the gene ontology term “antioxidant activity” as one of the candidate mechanismsof the PC effect by LPS. In Nrf2 reporter mice, PC
immediately and intensely enhanced luminescence thatindicated Nrf2 activation after subsequent LPS injection.
The induction of HO-1 by LPS was also enhanced bypreceding PC, and its induction was observed mainlyin Kupffer cells of the liver. In Nrf2-deficient mice, theinduction of HO-1 in Kupffer cells and the hepatoprotective effect of PC were decreased as compared with wildtype mice.
Conclusion Our results suggest that activation of theNrf2 system is, at least in part, one of the mechanismsof a PC effect in the mouse liver in the case of repeated
LPS stimulation.

Background

Sepsis is a syndrome triggered by endotoxin lipopolysaccharide (LPS) during bacterial infection. Sepsis sometimes recurs, with the second sepsis
giving rise to a different phenotype because of disease
modification by the preceding sepsis. Such a protective
modification is called a preconditioning (PC) effect. PC
is an endogenous protective mechanism by which sublethal damage confers tolerance to a subsequent lethal
load. Oxidative stress is one of the important pathogenetic mechanisms that occur in sepsis. The nuclear factor erythroid 2 (NF-E2)-related factor-2 (Nrf2) system isa key regulatory transcription factor that protects organs
and cells against oxidative stress and may be associatedwith the PC effect in repeated sepsis.
Methods The effect of PC induced by low-dose LPS
on survival rate and liver injury against subsequent highdose LPS stimulation was examined using a mousemodel of sepsis. In order to understand the detailed
mechanism(s) involved in the PC effect within the liver,gene expression array was performed. As a candidate
mechanism of PC, the activation of the Nrf2 system wasanalyzed using Nrf2 reporter mice. Furthermore, theinduction of heme oxygenase-1 (HO-1), one of the main
targets of Nrf2, in the liver was examined by immunoblotting and immunohistochemistry. The PC effect onliver injury induced by LPS was further examined using
Nrf2-deficient mice.
Results PC by LPS (1.7 or 5.0 mg/kg body weight,
intraperitoneally) increased the survival rate of mice anddecreased liver injury in response to a subsequent in-

Key words heme oxygenase-1; Kupffer cells; lipopoly-saccharide; NF-E2-related factor 2; preconditioning
Sepsis is a complex syndrome characterized by an enhanced innate immune response to bacterial infection.
Lipopolysaccharide (LPS) endotoxin within the bacterial wall triggers a release of proinflammatory mediatorsfrom macrophages and neutrophils through a Toll-likereceptor 4 (TLR4) signaling pathway that mediates hostdamage.1 Sepsis sometimes recurs where the second
sepsis may differ from the first sepsis on the type of organ disorder induced.2 In this regard, the first sepsis may
modify the protective reaction of organs to the pathogenesis of the second sepsis.
Preconditioning (PC) is an endogenous protective
mechanism for organs by which a sublethal stress, including ischemia,3–5 metals,6 and inflammation,7, 8 confers tolerance to a subsequent lethal stress. The detailed
mechanisms of PC are not yet fully understood, however, experimental and clinical evidence from various
sources, especially in the ischemic preconditioning field,
have highlighted alterations in gene expression3–5 andepigenomic modifications9 as candidate mechanisms of
the PC phenomenon.

Corresponding author: Kazuhiro Nakaso, MD, PhD
kazuhiro@med.tottori-u.ac.jp
Received 2016 June 24
Accepted 2016 July 6
Abbreviations: ALT, alanine aminotransferase; ARE, antioxidant
response element; AST, aspartate aminotransferase; BCA, Bicinchonic acid; BW, body weight; DAB, 3,3-Diamiobenzidine; GO,gene ontology; HE, hematoxylin-eosin; HO-1, heme oxygenase-1;
i.p., intraperitoneally; LPS, lipopolysaccaride; MS, main stimulation; NF-E2, nuclear factor erythroid 2; Nrf2, NF-E2-related factor-2; PC, preconditioning; TLR4, Toll-like receptor 4223

M. Nakasone et al.

Oxidative stress contributes, at least in part, to thepathogenesis of LPS-induced sepsis and inflammation.10
Therefore, reduction of oxidative stress via gene expression of antioxidative molecules may be associated with
PC mechanisms. Nuclear factor erythroid 2 (NF-E2)-related factor-2 (Nrf2) is known as a key regulator of anantioxidative response.11 Nrf2 usually binds to Keap1,
an E3 ubiquitin ligase, and is located in the extranuclear
space of cells. Under normal conditions, Nrf2 is unstablebecause it is proteolyzed by an ubiquitin-proteasome
system; however, once cells are exposed to oxidative
stress, Nrf2 translocates to the nucleus and acts as atranscription factor. Nrf2 binds to the antioxidant response element (ARE) and upregulates ARE-driven
genes such as heme oxygenase-1 (HO-1).11 Indeed, LPS
induced Nrf2-related gene expression and Nrf2-deficient
mice are vulnerable to LPS-induced sepsis.12–17
In the present study, we focused on the role of Nrf2
in PC under recurrent LPS exposure.
MATERIALS AND METHODS
Chemicals and antibodies

LPS from Salmonella enterica serotype enteritidis waspurchased from Sigma (St. Louis, MO). Beetle luciferin for Nrf2 reporter mice was obtained from Promega(Madison, WI). Antibodies against HO-1 and CD163
were obtained from Santa Cruz (Dallas, TX), and anti-β-actin antibody was obtained from Cell Signaling
Technology (Danvers, MA). Horseradish peroxidase(HRP)-linked anti-rabbit antibody for western blotting
was obtained from GE Healthcare (Buckinghamshire,UK).

Fig. 1. Experimental protocol performed in the present study. (A)PC (0, 1.7, or 5.0 mg/kg BW LPS, i.p.) was carried out at day –5 in
C57BL/6 mice. At day 0, a high dose (20 mg/kg BW) of LPS was
injected i.p. as a MS. Survival rates were followed for 7 days (days
0–7). Tissue sampling for DNA array was carried out 12 h after
MS. Western blotting (WB), histological analyses (HA), and plasmaaminotransferase assay (ALT and AST) were carried out at days
1, 1, and 2, respectively. Abbreviations of each condition and how
LPS was injected are described in the figure. (B) Time course of invivo Nrf2 reporter assay. Luminescence (Lumi) was detected 24 hafter PC (on day –4) and 16 h after MS by LPS. ALT, alanine aminotransferase; AST, aspartate aminotransferase; BW, body weight;
HA, histological analyses; i.p., intraperitoneal injection; LPS, lipopolysaccharide; Lumi, luminescence; MS, main stimulation; Nrf2,
NF-E2-related factor-2; PC, preconditioning; WB, western blotting.

Animals

C57BL/6 mice (n = 61, male) were obtained from CLEA
Japan (Tokyo, Japan). Nrf2-deficient mice (n = 12, male)and Nrf2 reporter mice (OKD48 mice)18 (n = 3, male)
were kindly provided by Dr. Warabi (University ofTsukuba, Tsukuba, Japan) and Dr. Iwawaki (Gunma University, Maebashi, Japan), respectively. The mice werehoused under standardized conditions of light (06:00–
18:00), temperature (25 °C), and humidity (approximately50%), and were allowed free access to food and water.
The mice used in this study were treated in accordancewith the Guidelines for Animal Experimentation of Tottori University (No.14-Y-01). All surgical procedures andcollecting blood samples were performed under deep
anesthesia with pentobarbital, and in vivo Nrf2 reporter
assay was carried out under inhalation anesthesia with
isoflurane, and all efforts were made to minimize suffering.
Experimental procedures are shown in Fig. 1. Brief-ly, the paired administration of LPS, PC and main stimulation (MS), was carried out. Firstly, 0 (vehicle), 1.7, or
5.0 mg/kg body weight (BW) of LPS dissolved in PBS
was administered intraperitoneally (i.p.) in C57BL/6
mice as PC at day –5. Subsequently, after 5 days (day0), 0 or 20 mg/kg BW of LPS was administered i.p. toeach PC group as the MS. Hereafter, these five groups:i) 0 mg/kg BW LPS as PC and 0 mg/kg BW LPS as
MS; ii) 0 mg/kg BW LPS as PC and 20 mg/kg BW LPS
as MS; iii) 1.7 mg/kg BW LPS as PC and 0 mg/kg BW
LPS as MS; iv) 1.7 mg/kg BW LPS as PC and 20 mg/
kg BW LPS as MS, and v) 5.0 mg/kg BW LPS as PC
and 20 mg/kg BW LPS as MS, were referred to as: i)
PC0-LPS0; ii) PC0-LPS20; iii) PC1.7-LPS0; iv) PC1.7LPS20, and v) PC5.0-LPS20, respectively (Fig. 1A). Af224

Preconditioning by LPS prevents liver injury

ter the time point shown in Fig. 1, DNA array (Fig. 1A),
an in vivo Nrf2 reporter assay (Fig. 1B), westernblotting(Fig. 1A), histological analyses (Fig. 1A), and aspartate
aminotransferase (AST)/alanine aminotransferase (ALT)
assays in blood (Fig. 1A) were performed. Survival rates
were followed until day 7.

molecular size on 12% polyacrylamide gels, transferred
onto polyvinylidene difluoride membranes (Hybond-P;
GE Healthcare), and hybridized with anti-HO-1 or β-actin antibody in PBS with 0.1% Tween 20 at room temperature for 1 h. The immunoreactive signal was detected using HRP-linked anti-rabbit or mouse IgG and ECL
detection reagents (GE Healthcare). The protein content
of each sample was measured using a bicinchonic acid
(BCA) protein assay system (Thermo Fisher Scientific,
Rockford, IL).

DNA microarray

Total RNAs were prepared from liver tissues using an
SV Total RNA isolation kit (Promega). In order to examine the effect of PC with 5.0 mg/kg BW of LPS on
the alteration of gene expression, especially in the earlystages, RNAs were isolated from liver tissue collectedfrom PC0-LPS20 mice (n = 1) and PC5.0-LPS20 mice(n = 1) 12 h after MS. We compared gene expression
between PC0-LPS20 and PC5.0-LPS20 groups using
DNA microarray (GeneChip Mouse Gene 2.0 ST array,
Filgen, Nagoya, Japan). Using microarray data, gene ontology (GO) analysis (Filgen) was performed to classify
the genes by function.

Histological studies

Hematoxylin-eosin (HE) staining and immunohistochemistry were performed on paraffin-embedded sections of mouse liver, brain, lung, and kidney 24 after
MS. A portion of liver tissue was fixed by perfusion
using 4% paraformaldehyde (Wako), and embedded inparaffin. Paraffin sections (4 μm) were deparaffinizedby placing slides into three changes of xylene, followedby rehydration in a graded ethanol series. Sections weretreated with methanol containing 3% H2O2 at room temperature for 30 min to reduce endogenous peroxidase activity. HE staining was performed following a standard
protocol.
HO-1 and CD163, as a marker of Kupffer cells, weredetected by incubating sections in HO-1 antibody at a
1:1000 dilution or CD163 antibody at a 1:1000 dilution in
PBS at 4 °C overnight. The labeled antigens were visualized using a Histofine kit (Nichirei Biosciences, Tokyo,
Japan), followed by followed by manufacturer’s protocol
using HRP/3,3-Diamiobenzidine (DAB) and/or alkaline
phosphatase/fuchsin reaction. For double staining forHO-1 and CD163, the signal of HO-1 was visualized byalkaline phosphatase/fuchsin reaction (red) and that of
CD163 was visualized by HRP/DAB reaction (brown).
Finally, sections were counterstained with hematoxylin
and observed by microscopy (BZ-9000; Keyence, Tokyo, Japan).

In vivo Nrf2 reporter assay (OKD48 mice)

In order to clarify when and where Nrf2 was activatedin mice after LPS injection, an in vivo Nrf2 reporter assay was performed. For this experiment, Nrf2 reporter
mice, known as OKD48 mice, were used. OKD48 mice
were treated with 0, 1.7, and 5.0 mg/kg BW of LPS (i.p.)
as PC, and after 24 h, D-luciferin (0.15 mg/kg BW)
dissolved in PBS was administered intraperitoneally.
Luminescence was detected using an in vivo imaging
system IVIS (Xenogen, Alameda, CA). In a similarmanner, Nrf2 activation in PC0-LPS20, PC1.7-LPS20,
and PC5.0-LPS20 mice groups was examined 16 h afterMS.
AST and ALT assays

Blood samples (approximately 200 μL each) from PC0LPS20 and PC1.7-LPS20 mice were collected from theheart using 25G needles at day 2, and bloods were centrifuged at 1,200 × g to separate plasma and blood cells.
Using AST and ALT assay kits (Transaminase CII-test;
Wako, Osaka, Japan), plasma AST and ALT levels were
measured as an index of liver injury.

Statistics

Quantitative data were assessed by ANOVA using theStat View software. The criterion for statistical significance was P < 0.05 for the AST/ALT assay All values
are expressed as means ± S.D. Survival curves werecompared by the use of Kaplan–Meiyer analysis and
Log–rank test. Values of P < 0.05 were considered statistically significant. P-value in GO analysis was calculatedby two tailed Fisher’s extract test using manufacturer’s
software (Filgen).

Western blotting

Liver tissues for immunoblot analyses were collectedfrom PC0-LPS0, PC1.7-LPS0, PC0-LPS20, and PC1.7LPS20 groups of mice 24 h after MS. Tissues werelysed in SDS sample buffer (50 mM Tris–HCl, pH 6.8,
2% SDS, 10% glycerol, 1 mM phenylmethylsulfonyl
fluoride, and 2 mM ethylenediaminetetraacetic acid).
Protein aliquots (20 μg) were separated on the basis of225

M. Nakasone et al.

Fig. 2. PC by low-dose LPS induces tolerance
against later high-dose LPS stimulation. (A) Life
spans of LPS-injected mice (n = 8, each group).
The life spans of PC1.7-LPS20 and PC5.0-LPS20
mice groups are significantly longer than that of
the PC0-LPS20 group. *: P < 0.01 vs. PC0-LPS20,**: P < 0.05 vs. PC0-LPS20. (B, C) Plasma ALT
and AST levels in PC0-LPS0 (n = 3), PC0-LPS20(n = 4), and PC1.7-LPS20 groups (n = 7). *: P <0.05 vs. PC0-LPS0, **: P < 0.05 vs. PC0-LPS20.
(ANOVA) (D–H) HE staining of liver tissues. (D–H) HE staining of the liver. (D) The liver from a representative PC0-LPS0 mouse. (E)The liver from a representative PC0-LPS20 mouse. Sites of focal necrosis are scattered within the liver (arrowheads). (F) The liver from a
representative PC0-LPS20 mouse at higher magnification. Neutrophil infiltration is observed in the sinusoids (arrows). (G) The liver froma representative PC1.7-LPS20 mouse. A small area of focal necrosis (arrowhead) is observed in the liver. (H) The liver from a representative PC1.7-LPS20 mouse at higher magnification. A small number of infiltrating neutrophils is observed in a sinusoid (arrow). Scale bars
(D)(E)(G) represent 200 μm and (F)(H) 100 μm. ALT, alanine aminotransferase; AST, aspartate aminotransferase; CV, central vein; HE,
hematoxylin-eosin; LPS, lipopolysaccharide; PC, preconditioning; PV, portal vein.

RESULTS

LPS, we measured plasma ALT and AST levels in mice
2 days after MS. Although the plasma ALT level was
significantly elevated in the PC0-LPS20 mice group, the
ALT level in the PC1.7-LPS20 mice group was significantly decreased compared with that of the PC0-LPS20
group (Fig. 2B). The plasma AST level showed a similar
pattern to the ALT level, however the difference between
the PC0-LPS20 and PC1.7-LPS20 group was not statistically significant (Fig. 2C).
We performed histological analyses of liver tissues
24 after MS (Figs. 2D–H). In livers from the PC0LPS20 mice group, small areas of focal necrosis (Fig.
2E) and infiltrating neutrophils were observed in the

PC by low-dose LPS gives the tolerance against
subsequent stimulation by high-dose LPS

We examined the effect of PC by LPS on survival rate
after the subsequent administration of a high- dose (20mg/kg BW) of LPS. Although the administration ofhigh-dose LPS (20 mg/kg BW) decreased the survival
rate of mice (to approximately 12.5% after 1.5 days), PC
using medium or low doses of LPS (5.0 or 1.7 mg/kg
BW, respectively) improved the survival rate to 87.5 %and 100%, respectively (Fig. 2A).
In order to clarify the effect of PC by low-dose LPS
on liver injury in response to a subsequent high dose of226

Preconditioning by LPS prevents liver injury

sinusoids of the liver (Fig. 2F). On the other hand, livers
from the PC1.7-LPS20 mice group showed milder focal necrosis and cellular infiltration compared with the
PC0-LPS20 group (Figs. 2G and H). We also investigated the effect of PC on its protective role against sepsis in
the brain, lungs, and kidneys. Microhemorrhages in the
cerebral cortex 7 days after MS were reduced in brainsfrom PC1.7-LPS20 mice compared with those in PC0LPS20 mice (data not shown). In lungs from PC1.7LPS20 mice, the number of inflammatory cells 24 h after MS was reduced compared with that for PC0-LPS20
mice (data not shown). In kidneys from PC1.7-LPS20
mice, kidney tubular necrosis 7 days after MS was milder than that in PC0-LPS20 mice (data not shown).

cal analyses at the same time of western blotting. The
inducible expression of HO-1 was observed mainly in
the CD163-positive Kupffer cells but not in hepatocytes
in liver from PC1.7-LPS20 mice (Fig 3D). In liver from
PC0-LPS20, the expression of HO-1 in the CD163 positive cells was faint (Fig. 3C).

PC by low-dose LPS enhances Nrf2 activation
during later stimulation by high-dose LPS

Table 1 . GO terms that are upregulated in PC5.0LPS20 compared with PC0-LPS20
Z-scoreP-value

Antioxidant activity

2.458

0.0395

Molecular function (GO term)

Binding

2.547

0.0003

Catalytic activity

6.788< 0.0001

Electron carrier activity

2.668

0.0196

Transporter activity

3.084

0.0036GO, gene ontology; LPS, lipopolysaccharide; PC, preconditioning.

In order to identify the mechanism(s) which contributed
to a PC effect, we performed gene expression array andcompared gene expression patterns between PC0-LPS20
and PC5.0-LPS20 groups. GO analysis using microarray data highlighted several classified genes that wereactivated by PC. We chose 5 molecular functions in GO
terms by cutting off the line that met both a Z-score > 0
and a P value < 0.05: antioxidant activity, binding activity, catalytic activity, electron carrier activity, and transporter activity (Table 1). We then focused on “antioxidant
activity” in GO terms, and investigated Nrf2 as a key
regulator of antioxidant molecules.
Using Nrf2-reporter mice (OKD48 mice), we analyzed Nrf2 activation on the point of time and spatial
elements. PC by intraperitoneal injection of 1.7 or 5.0
mgk/g BW LPS mildly activated Nrf2 in the abdominal
cavity; luminescence was detected 24 h after PC (Fig.
3A; upper panel). An intraperitoneal administration of
20 mg/kg BW LPS as a MS enhanced the activation of
Nrf2 not only in the abdominal cavity but also in thelungs 16 h after MS (Fig. 3A; lower panel, right). Furthermore, activation of Nrf2 in PC1.7-LPS20 and PC5.0LPS20 mice was more intense and more immediate thanthat in the case of a single LPS injection (shown in theupper panel; Fig. 3A, middle and left).
We next examined the induction of HO-1 protein in
the liver by western blotting since this enzyme is one of
the major targets of Nrf2 24 h after MS. The expressionof HO-1 in liver from a single LPS injection of PC1.7LPS0 or PC0-LPS20 mice was minimal (Fig. 3B). In
contrast, the induction of HO-1 was more intense in liverfrom PC1.7-LPS20 mice, than that from other groups,
24 h after MS (Fig. 3B).
In order to confirm what type of cells express
HO-1 in the liver, we performed immunohistochemi-

Nrf2 deficiency cancels the protective effect of PC
on liver disturbances

We further analyzed the PC effect on liver injury using
Nrf2-deficient mice. In these mice, the induction of
HO-1 in livers from PC1.7-LPS20 group mice was abolished 24 h after MS as compared with the PC1.7-LPS20
wild-type mice (Fig. 4A). Immunohistochemistry also
showed a reduced expression of HO-1 in Kupffer cells(Figs. 4B and C). In Nrf2-deficient mice with PC0LPS20 stimulation, liver tissue was severely damaged
with microhemorrhage, and numerous neutrophils infiltrated the liver, 24 h after MS (Fig. 4D). In contrast, thePC1.7-LPS20 group showed a mild improvement in histological changes in the liver in terms of focal necrosis,
microhemorrhage, and neutrophil infiltration (Figs. 4E
and F), however, the protective effect of PC in Nrf2-deficient mice (Figs. 4E and F) was weaker than what wasseen in wild-type mice (Figs. 2G and H).
DISCUSSION
Recently, increased attention has been given to theprotective effect of PC against adverse effects such as
sublethal ischemia, exposure to mild toxin, and inflammation.3–8 Numerous reports suggest that PC prevents
severe organ damage induced by later, more deleterious
events.3–8
Sepsis is a syndrome characterized by an enhanced
innate immune response to bacterial infection. LPS
usually triggers a release of proinflammatory mediators,
such as interleukins and nitric oxide, from macrophages
and neutrophils through a TLR4/NF-κB signaling pathway that mediates host damage.1 Sepsis often occurs
227

M. Nakasone et al.

A	
24	
  h	
  a-er	  PC	

B	HO-­‐1	β-­‐ac9n	

16	
  h	
  a-er	
  MS	

C	

D	

Fig. 3. PC by low-dose LPS enhances Nrf2 activation during subsequent stimulation by high-dose LPS. (A) An in vivo Nrf2 reporter assay using OKD48 mice. The upper photograph shows luminescence 24 h after PC (day –4). The lower photograph shows luminescence
16 h after MS with LPS (20 mg/kg BW). An immediate and intense luminescence is observed in PC1.7-LPS20 and PC5.0-LPS20 mice. (B)The induction of HO-1 protein is shown. Immunoblots are representative of three independent experiments. HO-1 protein bands in liver
samples from PC0-LPS0, PC1.7-LPS0, and PC0-LPS20 mice appear faint. The induced expression of HO-1 protein in liver from PC1.7LPS20 mice is intense. (C, D) Immunohistochemistry using anti-HO-1 and anti-CD163 antibodies in the liver from PC0-LPS20 (C) andPC1.7-LPS20 mice (D). The expression of HO-1 (brown) is observed mainly in CD163-positive Kupffer cells. Small window in (D) represents a double staining for HO-1 (red) and CD163 (brown). Scale bars represent 100 μm. BW, body weight; HO-1, heme oxygenase-1;
LPS, lipopolysaccharide; MS, main stimulation; Nrf2, NF-E2-related factor-2; PC, preconditioning.

repeatedly, with a second sepsis that may differ from thefirst sepsis in terms of organ disturbance.2 For example,
organs may sometimes gain a tolerance against sepsis
when exposed to an initial, LPS-induced sepsis, which
prevents severe organ disorders on the occurrence ofa second sepsis.2, 7, 8 Such a phenomenon, whereby atolerance is gained by a preceding mild load, has beentermed PC. Several PC phenomena have been reported in various organs and animal species.3–8 Ischemia
and ischemia–reperfusion are the most investigated PC
events to confer tolerance against later, more severe
ischemia.3–5 Research on a PC effect by ischemia or
ischemia–reperfusion may be useful for understanding
underlying mechanisms in cases of organ transplantationin various clinical stages. In a similar manner, research
on a PC effect by low-dose LPS may also be useful forunderstanding how to prevent severe organ damage inrepeated sepsis.
It is well known that Nrf2 and antioxidant molecules

are upregulated by single LPS stimulation, however,it has not been clarified whether Nrf2 and antioxidant
molecules are also associated with PC effects by low
dose LPS.12–17 In order to clarify which molecularmechanism(s) was associated with such a PC effect, we
performed DNA microarray and compared gene expression between PC0-LPS20 and PC5.0-LPS20 treatedmice. Since the microarray highlighted the inductionof genes associated with antioxidant activity as a pointof difference between the two mice groups examined,
we hypothesized that increased antioxidant activity was
one of the candidate mechanisms whereby PC induced
a protective effect in the liver. Oxidative stress is one of
the most important factors in the pathogenesis of sepsis.10 Therefore, we focused on Nrf2, a key transcription
factor for the induction of endogenous antioxidant molecules under oxidative stress,11 as one of the candidate
targets for PC mechanisms. DNA array revealed an
induction of HO-1 in PC5.0-LPS20 mice (more than two
228

Preconditioning by LPS prevents liver injury

A	
B	
Nrf2	

+/+	

−/−	

−/−	

−/−	

C	

−/−	

HO-­‐1	β-­‐ac/n	

CV	

D	

PV	

CV	

CV	

F	

E	

Fig. 4. Nrf2 deficiency abolished the protective effect of PC against liver damage by high-dose LPS in mice. (A) Immunoblot using anti-HO-1 antibody. The immunoblot shown is representative of three independent experiments. In Nrf2-deficient mice, the expression of
HO-1 protein bands in liver samples from PC0-LPS0, PC1.7-LPS0, and PC0-LPS20 mice are faint, unlike the case for wild-type mice (farleft lane). Liver samples from PC1.7-LPS20 treated Nrf2-deficient mice show a lower induction of HO-1 protein (far right lane) than PC1.7LPS20 treated wild-type mice (far left lane). (B, C) Immunohistochemistry using anti-HO-1 antibody at the same time of western blotting.
(B) The liver from the Nrf2-deficient mice with PC0-LPS20, (C) The liver from the Nrf2-deficient mice with PC1.7-LPS20. In Nrf2-deficient mice, the induction of HO-1 in Kupffer cells is reduced even if samples are from PC1.7-LPS20 treatment group. (D–F) HE stainingof the liver from Nrf2-deficient mice. (D) The liver from Nrf2-deficient mice with PC0-LPS20 treatment shows severe necrosis in a large
area (arrowheads), severe neutrophil infiltration (arrows), and microhemorrhage (white arrowhead). (E, F) In a liver from an Nrf2-deficientmouse after PC1.7-LPS20 treatment, the protective effect of PC is abolished, and focal necrosis (arrowheads), neurotrophil infiltration insinusoids (arrows), and microhemorrhage (white arrowhead) are observed. Scale bars (D)(E) represent 200 μm and (B, C, F) 100 μm. CV,
central vein; HE, hematoxylin-eosin; LPS, lipopolysaccharide; MS, main stimulation; Nrf2, NF-E2-related factor-2; PC, preconditioning.

Kupffer cells are the resident macrophages of theliver. These cells are located in sinusoidal spaces and arein contact with portal blood draining the intestinal tract,during which they are exposed to bacteria and bacterial
products such as LPS.19 Kupffer cells play an importantrole in clearing LPS.20 Moreover, the activation and induction of Nrf2/HO-1 in Kupffer cells and hepatocytes
is important for hepatoprotection against LPS-induced
toxicity.21 Previous reports and our present studies suggest that PC by low-dose LPS enhances Nrf2 activationand prevents severe damage of the liver.
The Nrf2/Keap1 system is the primary sensor in thecellular response to oxidative stress. The cytoplasmic
protein and ubiquitin E3 ligase, Keap1, interacts with
Nrf2 and continuously leads it to an ubiquitin–proteasome-dependent proteolysis.11 However, once cells are
exposed to oxidative stress, signals associated with thisprocess disrupt binding between Nrf2 and Keap1, resulting in the transcription of Nrf2 in the nucleus. Withintimes vs. PC0-LPS20). Therefore, we used the inductionof HO-1 as an index of the activation of Nrf2.
In the present study, we demonstrated the enhanced
activation of Nrf2 by PC with low-dose LPS administration in vivo using Nrf2 reporter mice (Fig. 3A). An
in vivo Nrf2 reporter assay using OKD48 mice showedthat an intense and immediate activation of Nrf2 wasobserved throughout the whole body, including in thelungs and liver, in PC1.7-LPS20 or PC5.0-LPS20 mice.
This suggests that PC by low-dose LPS induced the enhanced activation of Nrf2 when mice were exposed tosevere LPS-induced toxicity after 5 days. Indeed, HO-1,
one of the major targets of Nrf2, was dramatically induced in the PC1.7-LPS20 mouse liver. Furthermore, we
also demonstrated that an enhanced induction of HO-1
was observed, predominantly in Kupffer cells, but not in
hepatocytes or neutrophils 24 h after MS by LPS. Thissuggests that Kupffer cells may play an important role in
the PC effect via an Nrf2 system.
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the nucleus, Nrf2 forms a heterodimer with a smallMaf protein and activates the transcription of oxidative
stress-related genes via an ARE.11 Recently, PC by mild
ischemia in the brain was found to confer tolerance
against later severe ischemic events via epigenomic
modification on the promotor area of the Nrf2 gene.9
Moreover, evidence suggests that Nrf2 and its related
systems are controlled by epigenomic modifications following several conditions.22, 23 These facts suggest thatan epigenomic change of the Nrf2 gene may also be included in an LPS-induced PC phenomenon. Intense andimmediate upregulation of Nrf2 by PC may bring theresistance against oxidative stress. Furthermore, it has
been reported recently that Nrf2 suppresses macrophageinflammatory response by blocking proinflammatory
cytokine transcription.24 These facts suggest that PC effect against LPS-induced injury in our experiments may
include not only antioxidant activity but also suppressionof cytokine transcription.
In order to confirm the role of Nrf2 on the protective effect of PC, we examined this using Nrf2-deficientmice. A severe vulnerability against a preceding stimulation by LPS was noted in Nrf2-deficient mice, withmore than a half of the mice dying after a single stimulation for PC (data not shown). Therefore, we could notcompare survival rates of PC0-LPS20, PC1.7-LPS20,
and PC5.0-LPS20 groups after MS. Since we used tissue
samples from the limited numbers of mice that survived
during our experimental procedures, we should consider
that other mechanisms for tolerance may have been enhanced in the surviving Nrf2-deficient mice.
The decreased induction of HO-1 (Fig. 4A), andincreased focal necrosis (Fig. 4E) and neutrophil infiltration (Fig. 4F) in the liver were observed in surviving
Nrf2-deficient mice, even in the PC group as cpmparedwith wild-type mice. These results strongly suggest that
Nrf2 plays an important role in hepatoprotection during
PC and subsequent LPS stimulation. Even in Nrf2-deficient mice, a mild induction of HO-1 was still observedin Kupffer cells (Fig. 4C). Since the expression of HO-1
is also controlled by other transcription factors such asAP-1 in addition to Nrf2,25 this may explain why HO-1
continued to be induced in the liver by LPS in Nrf2-deficient mice.
Our results suggest that Nrf2 plays key role in thehepatoprotective effect induced by PC with low-dose
LPS, however, the detailed mechanism of how Nrf2 isenhanced by PC remains unknown. Therefore, further
studies, such as on the epigenomic modifications of theNrf2 gene, are required in future.

HI 15K09315 (KN), 15K00816 (TM), and 16K20096 (YK). We
thank Dr. Iwawaki for the kind gift of OKD48 mice, and also
thank Dr. Warabi for the kind gift of Nrf2 deficient mice.
The authors declare no conflict of interest.

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