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Biological and Pharmaceutical Bulletin Advance Publication by
J-STAGE DOI:10.1248/bpb.b16-00971
Ⓒ 2017 The Pharmaceutical Society of Japan
Advance Publication April 6, 2017
Biol. Pharm. Bull.
Regular Article
Anti-oxidative activity of hydrolysate from rice bran protein in
HepG2 cells
Chie Moritania, Kayoko Kawakamia, Akiko Fujitab, Koji Kawakamib,
Tadashi Hatanakac and
Seiji Tsuboi*, a
aDepartment of Biochemistry, School of Pharmacy, Shujitsu
University; 1-6-1 Nishigawara,
Okayama 703-8516, Japan, bSatake Corporation, 2-30 Saijo
Nishihonmachi; Higashi-
Hiroshima-shi, Hiroshima, 739-8602, Japan and cOkayama
Prefectural Technology Center
for Agriculture, Forestry, and Fisheries, Research Institute for
Biological Sciences (RIBS);
7549-1 Yoshikawa, Kibi-chuo, Okayama 716-1241, Japan
*To whom correspondence should be addressed.
Seiji Tsuboi: Department of Biochemistry, School of Pharmacy,
Shujitsu University, 1-6-1
Nishigawara, Okayama 703-8516, Japan
E-mail address: [email protected]
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Biological and Pharmaceutical Bulletin Advance Publication
SUMMARY
Glutathione (GSH) is an ubiquitous thiol-containing tripeptide,
which plays important
roles in cellular protection from oxidative stress. In our
search for a dietary source that can
increase glutathione (GSH) levels, we discovered that a 24 h
treatment of HepG2 cells with
rice bran protein hydrolysate (RBPH), prepared by Umamizyme
G-catalyzed hydrolysis,
increased the GSH content in a dose-dependent manner. RBPH
elevated the expression levels
ofγ-glutamylcysteine synthetase (γ-GCS), which constitutes the
rate-limiting enzyme of
GSH synthesis, and of another two enzymes, hemeoxygenase-1
(HO-1) and NAD(P)H
quinone oxidoreductase1 (NQO1). This induction was preceded by
the accumulation of Nrf2
(nuclear factor erythroid 2-related factor 2) inside the
nucleus, which is a key transcription
factor for the expression of the γ-GCS, HO-1, and NQO1.
Pre-treatment of cells with RBPH
produced a significant protective effect against cytotoxicity
caused by H2O2 or ethanol. These
results indicate that RBPH exerts a protective effect against
oxidative stress by modulating
GSH levels and anti-oxidative enzyme expression via the Nrf2
pathway.
KEYWORDS: rice bran; anti-oxidative; glutathione; nuclear factor
erythroid 2-related factor
2
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Biological and Pharmaceutical Bulletin Advance Publication
INTRODUCTION
Rice is the most important cereal food in Japan. Rice bran (RB),
which constitutes
approximately 10% of the grain, is a major by-product of rice
milling. RB is rich in protein,
lipids, dietary fibers, and vitamins.1, 2) Recently, RB was
recognized as a functional ingredient,
containing such antioxidants as tocopherols, tocotorienols, and
γ-oryzanol.3, 4) Moreover,
enzymatically produced RB protein hydrolysates (RBPHs) were
found to possess various
biological functions with potential medical applications. In a
previous study, we
demonstrated that RBPH produced with Umamizyme G, a commercial
protease from
Aspergillus oryzae, from defatted RB protein exhibited the
inhibitory activity of
dipeptidylpetidase-IV (DPP-IV) that is a key regulator involved
in the prevention and
treatment of type 2 diabetes. 5) RBPHs produced with other
peptidases were shown to have an
anti-proliferative effect on cancer cells,6) and the ability to
reduce micellar cholesterol
levels.7)
Excess generation of reactive oxygen species (ROS) leads to
oxidative stress, a condition
characterized by ROS attacks on proteins, lipids, and DNA,
leading to cell-function disorders.
Oxidative stress is thought to be involved in the pathogenesis
of various diseases, e.g.
cancer,8) diabetes,9) cardiovascular diseases,10) and
neurodegenerative disorders.11) Thus,
maintenance or restoration of the mammalian cell balance between
ROS generation and
detoxification through the action of anti-oxidative molecules
and enzymes that decrease
oxidative stress, may be important in the prevention of these
pathological conditions.12)
Glutathione (GSH), a ubiquitous thiol-tripeptide, is a major
cellular anti-oxidative
molecule, as it has the ability, by itself or in combination
with GSH peroxidase, to scavenge
H2O2, other peroxides and free radicals. GSH is biosynthesized
from glutamate, cysteine, and
glycine through a two-step ATP-dependent reaction. The first
rate-limiting step is catalyzed
by the enzyme γ-glutamylcysteine synthetase (γ-GCS), which is a
dimer consisting of a heavy
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Biological and Pharmaceutical Bulletin Advance Publication
chain (γ-GCSh) and a light chain (γ-GCSl).13) The expression of
γ-GCS and various other
antioxidant and phase-2 enzymes, such as hemeoxygenase 1 (HO-1),
NAD(P)H quinone
oxidoreductase 1 (NQO1), catalase, and GSH S-transferase, is
mainly regulated by the
transcription factor Nrf2 (Nuclear factor erythroid 2-related
factor 2).14, 15) Under normal
oxidation conditions, Nrf2 is located in the cytoplasm, bound to
the Kelch-like ECH
associated protein 1 (Keap1), which inhibits Nrf2 translocation
to the nucleus. In response to
oxidative stress, Nrf2 is released from Keap1, translocates to
the nucleus, and activates the
expression of the aforementioned genes, exerting an
anti-oxidative cytoprotective effect.16)
Therefore, activation of Nrf2 contributes to the regulation of
GSH levels and the maintenance
of normal redox status in cells.
Various hydrolysates derived from dietary proteins have been
shown to exert biological
functions, including anti-oxidative, anti-hypertensive,
anti-diabetic, and immuno-modulating
activities.17, 18) After studying the biological functions of
RBPH, we discovered that it can
increase GSH levels in HepG2 cells, and exerts a protective
effect against cytotoxicity
induced by oxidative stress, through the induction of the Nrf2
pathway.
MATERIALS AND METHODS
Materials Defatted RB was a gift from SATAKE Co. Ltd.
(Higashi-Hiroshima, Japan).
Umamizyme G was obtained from AMANO Enzyme Co. Ltd. (Nagoya,
Japan), soybean
protein (FUJIPRO E) from FUJI OIL Co. Ltd. (Osaka, Japan), and
collagen peptide from
Nippi (Tokyo, Japan). The protein assay kit was purchased from
Bio-Rad Laboratories Inc.
(Hercules, CA, USA). 7-Benzo-2-oxa-1,3-diazole-4-sulfonic acid
(SBD-F) and 4-
(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (ABD-F) were
obtained from Dojindo Labs
(Kumamoto, Japan), while 5,5′-dithiobis-(2-nitrobenzoic acid)
(DTNB) was bought from
Wako Pure Chemical (Osaka, Japan).
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Biological and Pharmaceutical Bulletin Advance Publication
Preparation of RBPH RBPH was prepared as described previously.5)
In brief, defatted RB
was solubilized in distilled water, whose pH had been adjusted
to 12.5 using NaOH, by
stirring for 2 h at 45 °C. After centrifugation at 2,000 × g for
15 min, the supernatant was
collected, and the pH was adjusted to 4.0 with 1 M HCl. After a
new centrifugation, the solid
residue (RB proteins) was dried in a vacuum oven, overnight at
40 °C. The obtained proteins
were hydrolyzed with a 1% (w/w) solution of Umamizyme G,
overnight at 45 °C. After a 30
min incubation at 80 °C for protease inactivation, the
hydrolysate was centrifuged at 2,000 ×
g for 30 min. The supernatant (RBPH) was divided into aliquots
and freeze-dried. The RBPH
was dissolved in 25 mM Hepes-NaOH before further analysis. RB
protein without hydrolysis
was prepared by a similar method to RBPH, except for the absence
of Umamizyme G.
Cell cultures HepG2 and COS7 cells were purchased from the RIKEN
Cell Bank (Tsukuba,
Japan), and maintained in MEM and DMEM, respectively. Both media
were supplemented
with 10% fetal bovine serum (FBS), 100 µg/ml streptomycin, 100
µg/ml penicillin, and 0.56
µg/ml amphotericin B. The cells were cultured at 37 °C in a
humidified atmosphere
containing 5 % CO2.
Measurement of intracellular GSH levels Cells were seeded into
6-well plates at a
concentration of 1.5·105 cells / well. After 48 h, the culture
medium was replaced with
medium containing RBPH. After incubating for the indicated
periods of time, cells were
rinsed twice with PBS and collected. Following homogenization
and deproteinization, the
obtained supernatants were used for measuring the GSH content.
The concentrations of GSH
and its oxidized form, glutathione disulfide (GSSG), were
determined simultaneously by
HPLC-fluorescence detection, after labeling with ABD-F and
SBD-F, respectively.19) The
total GSH content was determined by the enzymatic recycling
method using GSH reductase
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Biological and Pharmaceutical Bulletin Advance Publication
and DTNB.20)
Real-time PCR analysis Total RNA from RBPH-treated cells was
extracted with the
RNeasy Plus Mini Kit (QIAGEN, Hilden, Germany), according to the
manufacturer’s
instructions. First strand cDNA was synthesized from 1 µg of
total RNA using the
PrimeScript RT-PCR kit (Takara, Tokyo, Japan). Real-time PCR was
performed using SYBR
Premix EX Taq (Takara), and fluorescence was quantified with the
ABI PRISM 7700
sequence detection system (Thermo Fisher Scientific, Waltham,
MA, USA). The cDNA
levels of the house keeping gene, β -actin, were used as an
endogenous control. The
sequences of the primers used in the PCR were as follows: β-GCSh
forward primer, 5′-
TGCTGTCTCCAGGTGACATTC-3′ and reverse primer,
5′-cccagcgacaatcaatgtct-3′)21);
β-GCSl forward primer 5′-TCCAGTTCCTGCACATCTACCA-3′ and reverse
primer,
5 ′ -TCATCGCCCCACTTGAGAA-3 ′ ); HO-1 forward primer, 5 ′ -
GCAACCCGACAGCATGC-3′ and reverse primer,
5′-TGCGGTGCAGCTCTTCTG-3′
22); NQO1 (forward primer, 5′-CATGAATGTCATTCTCTGGCCA-3′ and
reverse primer,
5 ′ -CTGGAGTGTGCCCAATGCTA-3 ′ ); Nrf-2 forward primer, 5 ′ -
TGCTTTATAGCGTGCAAACCTCGC-3 ′ and reverse primer, 5 ′ -
ATCCATGTCCCTTGACAGCACAGA-3 ′ 23); β -actin forward primer, 5 ′
-
CCTGGCACCCAGCACAAT-3 ′ and reverse primer, 5 ′ -
GCCGATCCACACGGAGTACT-3′.
Western blotting Treated cells were washed twice with PBS and
harvested using a cell
scraper. Harvested cells were lysed using the RIPA Lysis Buffer
System (Santa Cruz
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Biological and Pharmaceutical Bulletin Advance Publication
Biotechnology, Dallas, TX, USA). Cells were incubated in the
lysis buffer for 30 min on ice.
After centrifugation at 13,000 × g for 15 min at 4 °C, the
supernatants were collected as cell
lysates.
Nuclear extracts were prepared as described.24) Briefly,
harvested cells were suspended in
200 µl of extraction buffer containing 10 mM HEPES, pH 7.5, 150
mM NaCl, 0.6 % Nonidet
P-40, 1 mM EDTA, 5 mM DTT, supplemented with a
proteinase-inhibitor cocktail (Roche
Applied Science, Penzberg, Germany) just before use. After a
20-min incubation on ice,
nuclei were pelleted by centrifugation at 13,000 × g for 15 min
at 4 °C. The nuclear pellet
was extracted with a solution containing 10 mM HEPES, pH 7.9,
420 mM NaCl, 1.5 mM
MgCl2, 0.2 mM EDTA, 5 mM DTT, supplemented with a
proteinase-inhibitor cocktail.
Nuclear fractions were collected after a 15 min centrifugation
at 13,000 g, at 4 °C.
Samples of cell lysates and nuclear extracts containing 20 µg of
protein were separated by
SDS-polyacrylamide gel electrophoresis and transferred onto a
PVDF membrane (Bio-Rad).
The membrane was incubated with primary antibodies against γ
-GCSh (Santa Cruz
Biotechnology), γ-GCSl, Nrf2 (Santa Cruz Biotechnology), HO-1
(Enzo Life Science),
Lamin B2 (Santa Cruz Biotechnology) or β-actin (Sigma, St.
Louis, MO, USA), followed by
incubation with horseradish peroxidase-linked second antibodies.
The immune complexes on
the membrane were detected with the Amersham ECL Prime western
blotting detection
reagent (GE Healthcare, Chicago, IL, USA) in a LAS-1000 imager
(Fuji, Tokyo, Japan).
Band intensities were analyzed using ImageJ software (Public
Domain).
Lactate dehydrogenase (LDH) cytotoxicity assay The activity of
LDH released from
damaged cells into the medium was measured using the Cytotoxic
Detection Kit (Roche
Applied Science). HepG2 cells seeded into 96-well microplates at
a concentration of 1.5 · 104
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Biological and Pharmaceutical Bulletin Advance Publication
cells / well were treated with RBPH for 24 h. After incubation,
the medium was replaced with
FBS-free medium containing H2O2 or ethanol. The culture medium
was collected after
incubation periods of 1 h and 24 h for H2O2 and ethanol,
respectively, and used to measure
the activity of released LDH (sample). The activity of the total
LDH in the culture (LDHhigh
control) was determined by lysing cells in 1 % Triton X-100,
while the LDH activity from the
medium of untreated cells was defined as LDHlow control. After
the subtraction of background
absorbance from all other values, the cytotoxicity was
calculated as follows:
% = – ℎ ℎ – x100
RESULTS
Effect of various protein hydrolysates on the intracellular GSH
level We examined the
effect of hydrolysates of various proteins on the intracellular
GSH levels of HepG2 cells after
24 h incubations with each hydrolysate, at a concentration of 5
mg/ml. Cultures treated with
RBPH displayed about double the amount of GSH compared to the
untreated controls (Fig.
1A). Soybean protein hydrolysate produced by Umamizyme G and
collagen peptide did not
cause statistically significant changes in intracellular GSH
levels. Treatment of the cells with
RB protein without hydrolysis did not increase the intracellular
GSH levels (Fig. 1B). A dose-
response experiment revealed that RBPH elevated the
intracellular GSH levels in HepG2 and
COS7 cells in a dose-dependent fashion (Fig. 2A, 2B). On the
other hand, the ratio of GSH to
GSSG, which reflects the redox status in the cell, was not
significantly affected (Fig. 2C).
The time course analysis indicates that the increase of the
intracellular GSH levels was
relatively slow and tended to reach a plateau at 16 h (Fig.
2D).
Effect of RBPH on the expression of γ-GCSh and γ-GCSl In an
effort to determine the
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Biological and Pharmaceutical Bulletin Advance Publication
mechanism behind RBPH's ability to increase intracellular GSH
levels, we determined the
mRNA levels of the γ-GCSh and γ-GCSl genes, that encode the two
subunits comprising
the rate-limiting enzyme of GSH synthesis, γ-GCS mRNA levels of
both genes began to
increase at 3 h after the addition of RBPH, reaching a
statistically significant increase of
about 50% at 8 h (Fig. 3A, 3B). Protein levels of both subunits
were found to be decreased at
1 and 3 h after addition, but this reduction had disappeared at
the samples collected at 8 h and
the protein levels increased at 24 h though without significance
(Fig. 3C, 3D). In addition, the
increase of the intracellular GSH levels was inhibited by about
35% after treatment with 1
mM methionine sulfoximine, which is a known inhibitor of γ-GCS
(data not shown).25)
Effect of RBPH on NQO1 and HO-1 expression levels To determine
the effect of RBPH
on the expression of anti-oxidant and phase II detoxifying
enzymes, the expression of NQO1
and HO-1 was investigated at various times after RBPH addition.
At 3 h, mRNA levels of
NQO1 were increased by about 50%, and remained roughly at this
level until the end of the
experiment (24 h) (Fig. 4A). RBPH also induced the mRNA levels
of HO-1, but more slowly.
The levels had statistically significantly increased by about
100% at 8 h, and kept up to 24 h
(Fig. 4B). The HO-1 protein levels were also increased by 150%
at 24 h (Fig. 4C).
Effects of RBPH on Nrf2 Nrf2 is known as a key regulator for
anti-oxidant and phase II
detoxifying enzymes such as γ -GCS, HO-1 and NQO1. Since mRNA
levels of these
enzymes were induced by RBPH, we decided to examine the
expression and nuclear
translocation of Nrf2. mRNA levels of Nrf2 appeared increased
about 1.5-fold at 3 h after
RBPH addition. At 8 h they had fallen to about 50 % the initial
value, and remained low until
the end of the experiment (Fig. 5A). The Nrf2 protein levels in
total cell lysate also reached a
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maximum increase (about 9-fold) at 3 h, only to fall afterwards,
correlating with mRNA
expression (Fig. 5B). On the other hand, the nuclear Nrf2
protein levels began to increase at 3
h, indicating that RBPH treatment induces the translocation of
Nrf2 into the nucleus (Fig. 5C).
The levels of Nrf2 in the cytosolic fraction were generally
unchanged, although they
increased slightly after 3 h.
Protective effects of RBPH against cytotoxicity induced by
oxidative stress Cells treated
with RBPH, as well as untreated controls, were incubated with
H2O2 or ethanol. Afterwards,
cytotoxicity was evaluated by measuring the activity of released
LDH in the medium.
Treatment of control cells with 100 μM or 200 μM H2O2 1 h had a
cytotoxicity of 10 % and
25 %, respectively. In contrast, at the RBPH-pretreated cells
cytotoxicity had been decreased
to less than 5 %, and this reduction was statistically
significant fashion. Pretreatment of RB
protein without hydrolysis did not show the cytoprotective
effect (Fig. 6A, 6B). Ethanol
treatment was also included in the study, as ROS are known to be
produced during ethanol
metabolism, especially through the action of CYP 2E1.26)
Pre-treatment of control cultures
with 200 mM or 500 mM of ethanol caused cytotoxicity levels of
20 % and 24 %,
respectively, whereas pre-treatment with RBPH reduced these
levels to less than 1% at 200
mM, and to 7% at 500 mM (Fig. 6C).
DISCUSSION
In this study, we demonstrated that RBPH increases intracellular
GSH levels in a dose-
and time- dependent manner (Fig. 2), whereas soybean protein
hydrolysate or collagen
peptide did not exhibit a similar effect (Fig. 1). The mRNA
levels of both γ-GCS subunits
began to increased at 3 h after addition (Fig. 3). In addition
to γ-GCS, HO-1 and NQO1
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were induced, suggesting that RBPH activated the Nrf2 pathway
(Fig. 4). Nrf2 expression
temporarily increased at 3 h after RBPH treatment. Moreover, the
increased Nrf2 nuclear
accumulation started at 3 h after the addition of RBPH, which
corresponded to the expression
of anti-oxidant enzymes (Fig. 5). As our results indicated that
RBPH was able to activate the
Nrf2 pathway, we assumed that it might protect against cell
damage caused by oxidative
stress. The hypothesis was verified because pre-treatment of
cells with RBPH did offer
significant protection against damage brought about by H2O2 and
ethanol (Fig. 6). HO-1
converts heme into biliverdin, releasing free iron and carbon
monoxide. Biliverdin is rapidly
metabolized to the antioxidant bilirubin.27) NQO1 detoxifies
quinones, which protects the cell
against oxidative stress, and reduces the antioxidants vitamin E
and coenzyme Q to their
active form.28) In addition to the induction of HO-1 and NQO1,
other anti-oxidant enzymes
that are regulated by Nrf2, including catalase and SOD, could
contribute to the protection of
the cell from oxidative stress. Taken together, our results
strongly suggest that RBPH may be
able to suppress oxidative stress in cells not only through the
up-regulation of GSH
biosynthesis, but also by increasing of expression of other
antioxidant enzymes.
Even though our results show that RBPH increase on GSH levels
takes place through a
mechanism related to the Nrf2 pathway, this response is
relatively slow. Furthermore, the
levels of the γ-GCS protein actually decreased during the first
few hours of treatment,
before the induction. In LC2 cells, intracellular GSH levels
were found to increase at 6 h after
exposure to 2,3-dimethoxy-1,4-naphthoquinone, which is known to
generate ROS,29) whereas
ionizing radiation and TNF-α have been shown to increase
intracellular GSH levels with a
peak appearing at 6 or 3 h.30) A 3-h treatment with pyrrolidine
dithiocarbamate induces γ-
GCS expression by an Nrf2-associated mechanism.15) After
treatment with tert-
butylhydroquinone31), the nuclear import of Nrf2 started as
early as 1 h and Nrf2 was present
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in the nucleus between 1 h and 4 h. After stimulation with
dieckol, a hexamer of
phloroglucinol with known anti-oxidant activity, the nuclear
translocation of Nrf2 was
induced at 1 h.32) Changes in GSH levels, γ-GCS levels, and Nrf2
nuclear translocation to
these oxidants and anti-oxidants are faster than those to RBPH.
These observations suggest
that activation of the Nrf2 pathway by RBPH is mediated through
an additional step: RBPH
might induce a weak oxidative stress in cells, such as an
imbalance of the GSH/GSSG ratio,
which subsequently triggered the activation of the Nrf2 pathway.
However, the mechanism of
the RBPH-induced decrease of γ -GCS levels is unclear. As it has
been reported that
inhibition of NO synthesis leads to a decrease in GSH levels
through downregulation of γ-
GCS expression,33) a rational hypothesis would be that RBPH
might inhibit NO synthesis.
Several studies have demonstrated the anti-oxidative effects of
phytochemicals isolated
from dietary sources.34, 35) Protein hydrolysates derived from
soy, egg, milk, whey etc., have
also been reported to exhibit anti-oxidative activity.36) Both,
scavenging of ROS and free
radicals as well as sequestering pro-oxidative metals through
chelation, have been described
as the primary mechanisms of anti-oxidative activity. To this
day, only a few peptides have
been shown to exert anti-oxidative action via increasing the
expression levels or the activity
of anti-oxidative enzymes. Egg-derived peptides have been
reported to increase the
intracellular GSH levels and upregulate anti-oxidative enzymes
in Caco-2 cells37) whereas,
peptides derived from chickpeas have been reported to increase
the expression of anti-
oxidative enzymes including γ-GCS, HO-1, NQO1, and Nrf2, in
Caco-2 and HT-29 cells.38)
Thus, RBPH is one of the few anti-oxidants that exert their
anti-oxidative activity by
regulating the oxidative defense system.
Many studies have shown that oxidative stress is related to the
progression or the
aggravation of diseases including cancer, neurodegenerative
disorders, and diabetes.8-11)
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Supplementation with exogenous GSH has been suggested as a novel
treatment for
Parkinson’s disease,39) psychiatric disorders,40) and
diabetes.41) In our previous study, the
apparent molecular weight of RBPH was estimated around 300 Da,
corresponding with the
molecular weight of di- or tripeptides, and inhibitory peptides
of DPP-IV from RBPH were
successfully identified.5) In this case, even though the
sequences of the bioactive peptides in
RBPH have not yet been identified, our results suggest that
novel anti-oxidative agents
among them may prevent the development and progression of
disorders caused by oxidative
stress such as those mentioned above.
In summary, RBPH increased intracellular GSH levels and induced
the expression levels
of anti-oxidative enzymes such as γ-GCS, HO-1 and NQO1 through
an activation of the
Nrf2 pathway. In addition, RBPH provided the cytoprotective
effect against oxidative stress.
Acknowledgements We would like to acknowledge the contribution
Mr. Yoshikazu Inoue,
who was one of our colleagues and passed away suddenly in 2015.
This project was
financially supported by the Iijima Memorial Foundation for the
promotion of food science
and technology.
Conflict of interest Chie Moritani, Kayoko Kawakami and Seiji
Tsuboi received a research
grant from Satake Corporation. Akiko Fujita and Koji Kawakami
are employees of Satake
Corporation. Tadashi Hatanaka has no conflict of interest.
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Biological and Pharmaceutical Bulletin Advance Publication
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Figure legends
Figure 1. Effects of hydrolysates of various proteins on the
intracellular GSH levels in
HepG2 cells. Cells were treated with 5 mg/ml of RB protein
hydrolysate (RBPH), or soybean
protein hydrolysate (Soybean), or collagen peptide (Collagen)
(A), or RB protein without
hydrolysis (RB/−) (B), for 24 h. Values are the means ± SD
(n=3). *p < 0.05, **p < 0.01 vs.
control group.
Figure 2. Dose- and time-dependent dependent effects of RB
protein hydrolysate on the
intracellular GSH levels. HepG2 (A) or COS7 (B) cells were
incubated with the indicated
concentrations of RB protein hydrolysate for 24 h, followed by
measurement of GSH levels.
(C) Effects of RB protein hydrolysate on GSH/GSSG ratios. HepG2
cells were treated with
the indicated concentrations of RB protein hydrolysate for 24 h.
(D) Time-dependent effect of
RB protein hydrolysate on GSH levels. Cells were treated with 5
mg/ml of RB protein
hydrolysate for the indicated times. Values are the means ± SD
(n=3). *p < 0.05, ***p <
0.001 vs. control group.
Figure 3. Effects of RB protein hydrolysate on γ-GCS expression
levels. mRNA levels of
heavy (A) and light (B) subunits were estimated by real-time PCR
analysis after HepG2 cells
were treated with 5 mg/ml RB protein hydrolysate for the
indicated times. Values are the
means ± SD (n=3). *p < 0.05 vs. control. Protein levels of
each subunit (C, D) were analyzed
by western blotting using corresponding antibodies. Values are
the means ± SD (n=3).
Figure 4. Effects of RB protein hydrolysate on NQO1 and HO-1
expression levels. mRNA
levels of NQO1 (A) and HO-1 (B) were estimated by real-time PCR
analysis after HepG2
cells were treated with 5 mg/ml RB protein hydrolysate for the
indicated times. (C) HO-1
-
Biological and Pharmaceutical Bulletin Advance Publication
protein levels were analyzed by western blotting using
corresponding antibodies. Values are
the means ± SD (n=3). *p < 0.05, **p < 0.01 vs. control
group.
Figure 5. Effects of RB protein hydrolysate on Nrf2 expression.
(A) mRNA levels of Nrf2
were estimated by real-time PCR analysis after HepG2 cells were
treated with 5 mg/ml RB
protein hydrolysate for the indicated times. Values are the
means ± SD (n=3). Nrf2 protein
levels in cell lysate (B) and the cytosolic or nuclear fraction
(C) were analyzed by western
blotting using corresponding antibodies. Values are the means ±
SD (n=3). *p < 0.05 and **p
< 0.01 vs. control group.
Figure 6. Protective effect of RB protein hydrolysate against
cell damage caused by
oxidative stress in HepG2 cells. (A) Cells were treated with 5
mg/mL RB protein with
(RBPH) or without hydrolysis (RB/−) for 24 h and subsequently
exposed to 200 µM H2O2 for
1 h (n=4). (B, C) Cells treated with 5 mg/mL RB protein
hydrolysate for 24 h, as well as
untreated controls, were exposed to various concentrations of
H2O2 for 1 h (n=3-4) and
various concentrations of ethanol for 24 h (n=3-4). The
cytotoxicity was determined by
measurement of LDH activity released from damaged cells into the
medium. Values are the
means ± SD. *p < 0.05 and **p < 0.01 vs. the group that
was not pre-treated with RB protein
hydrolysate.
-
0
100
200
300
400
500
600To
tal G
SH
(nm
ol/m
g pr
otei
n)*
Control CollagenRBPH Soybean
Figure 1. Effects of hydrolysates of various proteins on the
intracellular GSH levels in HepG2 cells.
0
50
100
150
200
Control RBPH RB/-Tot
al G
SH
(nm
ol/m
g pr
otei
n)A
B
**
Biological and Pharmaceutical Bulletin Advance Publication
-
*
0
50
100
150
200
250
300
0 1.25 2.5 5Tot
al G
SH
(nm
ol/m
g pr
otei
n)
RB protein hydrolysate (mg/mL)
AHepG2
0
50
100
150
200
250
300
Tota
l GSH
(nm
ol/m
g pr
otei
n)
RB protein hydrolysate (mg/mL)0 2.5 5
*
***
BCOS7
0
20
40
60
80
100
0 1.25 2.5 5
GSH
/GSSG
RB protein hydrolysate(mg/mL)
0
20
40
60
80
0 10 20
Tota
lGSH
(nm
ol/m
g pr
otei
n)
Time (h)
*** ***C D
Figure 2. Dose- and time-dependent dependent effects of RB
protein hydrolysate on the intracellular GSH levels.
Biological and Pharmaceutical Bulletin Advance Publication
-
C
0.0
0.5
1.0
1.5
2.0
0 3 8 24
Rel
ativ
e ex
pres
sion
leve
l
Time (h)
*
A γ-GCSh B
0.0
0.5
1.0
1.5
2.0
0 3 8 24Rel
ativ
e ex
pres
sion
leve
lTime (h)
*
γ-GCSl
D
0.0
0.5
1.0
1.5
0 1 3 8 24
γ-G
CSh/
β-ac
tin (
fold
)
Time (h)
0 1 3 8 24 (h)
β-actin
γ-GCSh0 1 3 8 24 (h)
β-actin
γ-GCSl
0.0
0.5
1.0
1.5
2.0
0 1 3 8 24
γ-G
CSl/
β-ac
tin (
fold
)
Time (h)
Figure 3. Effects of RB protein hydrolysate on γ-GCS expression
levels.
Biological and Pharmaceutical Bulletin Advance Publication
-
0 1 3 8 24 (h)
0.0
0.5
1.0
1.5
2.0
2.5
0 3 8 24Rel
ativ
e ex
pres
sion
leve
l
Time (h)
HO-1
** **
BA
0.0
0.5
1.0
1.5
2.0
0 3 8 24
Rel
ativ
e ex
pres
sion
leve
l
Time (h)
NQO1
** *
HO-1
β-actin
C
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 1 3 8 24
HO
-1/β
-act
in (
fold
)
Time (h)
**
*
**
Figure 4. Effects of RB protein hydrolysate on NQO1 and HO-1
expression levels.
Biological and Pharmaceutical Bulletin Advance Publication
-
0 1 3 8 24 (h)
0 1 3 8 24 (h)
0.0
0.5
1.0
1.5
2.0
0 3 8 24
Rel
ativ
e ex
pres
sion
leve
l
Time (h)
Nrf2
Nrf2
Lamin B
β-actin
A
C
B
**
**
0
2
4
6
8
10
12
0 1 3 8 24
Tota
l Nrf
2/β-
actin
(fo
ld)
Time (h)
Figure 5. Effects of RB protein hydrolysate on Nrf2
expression.
Nrf2
β-actinCytosol
Nucleus0
2
4
6
8
0 1 3 8 24Rel
ativ
e N
rf2
prot
ein
leve
ls(f
old)
Time (h)
Cytosol
Nucleus*
**
**
Biological and Pharmaceutical Bulletin Advance Publication
-
0
10
20
30
40
0 50 100 150 200
Cyt
otox
icity
(%)
H₂O₂ (µM)
0%
10%
20%
30%
40%
0 200 400 600
Cyt
otox
icity
(%)
Ethanol (mM)
**
Control
RB protein hydrolysate
Control
RB protein hydrolysate40
30
20
10
0
Cyt
otox
icity
(%
)
A
Figure 6. Protective effect of RB protein hydrolysate against
cell damage caused by oxidative stress in HepG2 cells.
0
10
20
30
40
Control RBPH RB/-
Cyt
otox
icity
(%
)
**
C
B
***
Biological and Pharmaceutical Bulletin Advance Publication
-
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