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Juvenile hormone induction of glutathione S-transferase activity in the larval fat body of the common cutworm Spodoptera litura LepidopteraNoctuidae.

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232
Wu and Lu
Archives of Insect Biochemistry and Physiology 68:232–240 (2008)
Juvenile Hormone Induction of Glutathione
S-Transferase Activity in the Larval Fat Body of the
Common Cutworm, Spodoptera litura
(Lepidoptera: Noctuidae)
Ming-Cheng Wu and Kuang-Hui Lu*
The effect of pyriproxyfen, a juvenile hormone analog (JHA), on the pupation of S. litura was examined. A topical application
of 100 µg JHA/larva on the newly ecdysed (0-day) sixth instar larvae resulted in more than 80% pupation, while most of the
1- or 2-day-old larvae similarly treated developed into supernumerary larvae. Glutathione S-transferse (GST) activity in the fat
body of 0-day-old sixth instar larvae was significantly induced within 12 h of JHA (100 µg/larva) treatment. In contrast, no
such induction was found when 1- and 2-day-old sixth instar larvae were similarly treated. This induction phenomenon was
also observed when 0-day-old sixth instar larvae were treated with the natural JH III. The suppressive effects of α-amanitin
and cycloheximide suggest that JHA induction of GST activity in these cutworm larvae presumably occurred at the gene transcription level. Arch. Insect Biochem. Physiol. 68:232–240, 2008. © 2008 Wiley-Liss, Inc.
Keywords: Glutathione S-transferase (GST); fat body; juvenile hormone; Spodoptera litura
INTRODUCTION
Glutathione S-transferases (GSTs; E.C. 2.5.1.18)
are a large family of multi-functional enzymes
that catalyze the conjugation of glutathione with
a wide spectrum of endogenous and xenobiotic
compounds (Mannervik and Danielson, 1988;
Clark, 1989). They are involved in the detoxification of xenobiotics, protection from oxidative
damage, isomerization, and intracellular transportation (Hayes and Pulford, 1995; Yu, 1996). In
insects, cytosolic GSTs are classified into six
classes, i.e., sigma, zeta, theta, delta, epsilon,
omega, plus several unclassified genes (Ranson et
al., 2001; Ding et al., 2003; Enayati, et al., 2005).
Among them, the delta and epsilon classes, which
are unique to insects, are involved in the xenobiotic metabolism (Ranson, et al., 2001). Most
studies related to insect GSTs have emphasized
their roles of detoxification and induction by
allelochemicals, drugs, insecticides, and other
xenobiotics (Yu, 1984; Kao et al., 1989; Fournier
et al., 1992; Yu, 1999; Ranson et al., 2001). Feng
et al. (1999) first reported a dramatic increase of
GST during diapause in Choristoneura fumiferana;
subsequently, they demonstrated a fluctuating pattern of GST expression throughout its development and implicated the involvement of insect
Department of Entomology, National Chung-Hsing University, Taiwan, ROC
Contract grant sponsor: ATU Plan of the Ministry of Education, Taiwan, ROC.
*Correspondence to: Kuang-Hui Lu, Department of Entomology, National Chung-Hsing University, 250, Kuo-Kuang Road, Taichung 40227 Taiwan, ROC.
E-mail: khlu@dragon.nchu.edu.tw
© 2008 Wiley-Liss, Inc.
DOI: 10.1002/arch.20257
Published online in Wiley InterScience (www.interscience.wiley.com)
Archives of Insect Biochemistry and Physiology
August 2008
JH Induction of S. litura GST Activity
molting hormone in GST induction (Feng et al.,
2001). Strode et al. (2006) stated that GSTs from
Anopheles gambiae were differentially regulated
during different stages of development. These results show that GSTs may play roles in certain
physiological processes in insect development.
Juvenile hormone analogs (JHAs) act as JH agonists on insects and influence their growth and
metamorphosis (Riddiford et al., 2003; Wilson,
2004). During the transition of larval to pupal
stages, the level of JH in insect will drop and
thereby commit the cells to pupal development
(Riddiford et al., 2003). At this stage, the function
of fat body shifts from synthesis and storage to consumption and excretion (Tsuchida and Wells, 1988;
Riddiford, 1994), and then the fat body is lysed
and rebuilt during metamorphosis. Application of
exogenous JHA in this transition stage results in
supernumerary larvae (Hatakoshi et al., 1986). In
our preliminary studies, we applied a PCR-based
cDNA subtraction technique to construct a fat body
cDNA library of the common cutworm, Spodoptera
litura, to identify genes that are regulated by JH
and specifically expressed during metamorphosis
(Wu and Lu, unpublished data). In the survey of
the expressed sequence tags (ESTs) in this library,
several putative GST cDNA fragments have been
isolated and sequenced. This result led us to hypothesize that JH may influence the expression of
some forms of the GSTs.
MATERIALS AND METHODS
Experimental Insects
S. litura egg masses were collected from taro
fields and reared on artificial diet (Ou-Yang and
Chu, 1988) in the environmental chamber at 25°C
with a 12:12-h L:D photoperiod. Newly hatched
larvae were reared together in plastic cups (9.5-cm
diameter × 5.5-cm height) until pre-ecdysis of the
fifth instar. The pre-ecdysed larvae were randomly
chosen and maintained individually in a 30-well
rearing plate without any diet for molting. After
ecdysis, as the beginning of day 0, diet was added
again to each well to feed the larvae.
Archives of Insect Biochemistry and Physiology
August 2008
233
Juvenile Hormone Treatment
Since the life span of the sixth instar larvae is
approximately three days, they were divided into
three groups, namely 0-, 1-, and 2-day-old, to receive a topical treatment of 0.01, 0.1, 1, 10, and
100 µg pyriproxyfen (a JHA, a gift from Sumitomo
Chemical Taiwan Co., LTD, Taipei, Taiwan)/larva,
respectively. The development of each larval group
consisted of 10 individuals and was observed and
recorded for 2 weeks.
For induction experiments, each of the 0-, 1-,
and 2-day-old sixth instar larvae was topically
treated with 100 µg JHA in 1 µl acetone on the
dorsal cuticle of abdomen by a microliter syringe
(Hamilton Co., Reno, NV) and the control group
was treated with 1 µl acetone. Fat body of female
larvae collected at various time intervals after treatment was homogenized for enzyme assays.
In a dose-response experiment, 0-day-old sixth
instar larvae were topically treated with different
doses of JHA, i.e., 1, 6.25, 12.5, 25, 50, and 100
µg/larva in 1 µl acetone. Twenty-four hours after
treatment, fat body of the females was collected
and homogenized for enzyme assays. In another
experiment, 0-day-old sixth instar larvae were
treated with different concentrations of JH-III
(Sigma Chemical Company, St Louis, MO), i.e., 1,
10, and 100 µg/larva in 1 µl of mineral oil, with
the control larvae receiving an equivalent volume
of the solvent. Homogenate of the fat body of female larvae was prepared 24 hours after treatment
for enzyme assays.
Treatment of Inhibitors of mRNA and
Protein Biosynthesis
To determine the origin of GST activity induced
by JH, 0-day-old sixth instar larvae were treated
with α-amanitin and cycloheximide (CHXM)
(Sigma), inhibitors of transcription and translation,
respectively, prior to JHA application. Alpha-amanitin, in two different concentrations (2.5 and 5 µg/
larva in 10 µl Ringer’s solution), was injected into
the abdomen and then the larvae were topically
treated with 100 µg JHA/larva (Lu et al., 1995).
234
Wu and Lu
Statistics
The fat body was collected for enzyme assays 12
and 24 h later. CHXM (100 µg/larva in 10 µl
Ringer’s solution) was injected into the larval abdomen 1 h prior to topical application of 100 µg
JHA, and a second dose of 100 µg CHXM was administered 4 h after JHA treatment to ensure continued suppression of protein synthesis (Lu et al.,
1995). Fat body homogenate was prepared 10 and
24 h after the first cycloheximide injection. Control
insects were treated with 10 µl Ringer solution.
Statistical analysis was conducted using SigmaPlot 9.0 (Systat Software Inc., San Jose, CA). Means
of experimental and control groups were based on
at least three independent replicates with each
sample consisting of pooled fat bodies from at least
five larvae. Data were examined with Turkey multiple comparison by an SAS system viewer 9.1 (SAS
Institute Inc., Cary, NC) for statistical significance.
Glutathione S-Transferase Assay
RESULTS
Fat body of 5 female larvae was dissected, rinsed
in 1.15% KCl, pooled and homogenized in 2 ml
of ice-cold 100 mM Tris-Cl buffer (pH 8.0, with 1
mM EDTA and 10 mM GSH) by a motor-driven
tissue grinder for 2 min. The crude homogenate
was centrifuged (20,000g, 20 min, 4°C) and the
supernatant was used for assay. Fat body of 1-dayold sixth instar female larvae was used for optimization of GST assay conditions.
Glutathione S-transferase activity was assayed
with 1-chloro-2, 4-dinitrobenzene (CDNB) as substrate using the method of Yu (1982) with slight
modification. The 1.98-ml reaction mixture containing 100 mM Tris-HCl buffer (pH 8.0, with 1
mM EDTA), reduced glutathione (10 mM), and the
supernatant of fat body homogenate (containing
50 µg total protein) was first incubated for 3 min at
room temperature. Then the reaction was initiated
by adding 20 µl of 20 mM CDNB and the absorbance of 340 nm was measured 2 min later. A unit
of enzyme activity was defined as nanomoles of
CDNB conjugated per minute per milligram of
protein using an extinction coefficient of 9.6 mM–1
cm–1 for S-(2-chloro-4-nitrophenyl) glutathione.
For each treatment, at least three assays were carried out.
Protein concentration was estimated by BCA
protein assay reagent kit (Pierce, Rockford, IL) and
a 96-well microplate was used to measure absorbance at 570 nm by an ELISA spectrophotometer
(Multiskan EX, Labsystem, Finland). Bovine serum
albumin was used as the standard.
Effect of JHA on the Metamorphosis
of S. litura Larvae
As shown in Figure 1, both 100 and 10 µg of
JHA topically applied onto the sixth instar larvae
of S. litura significantly interfered with the process
of pupation. Both doses caused supernumerary
molts in more than 70% and nearly 30% of the 1and 2-day-old sixth instar larvae, respectively. Yet,
treatments of the 0-day-old sixth instar larvae with
the same dosages allowed, correspondingly, approximately 80% and nearly 100% of the larvae to
pupate normally. Treatments of JHA lower than 1
µg/larva did not have significant effects on the pupation of these larvae.
As 100 µg JHA/larva showed the most notable
effect on the metamorphosis of the sixth instar larvae, this dosage was chosen to perform the following experiments.
Effect of JHA and JH-III Treatments on
Glutathione S-Transferase Activity
At the optimum pH of 8.0 (Fig. 2a), GST activity was linear up to 70 µg of total protein for 5
min (Fig. 2b and c). Accordingly, an assay of larval fat body GST was carried out with 50 µg of
total protein in the enzyme solution and the absorbance at 340 nm was recorded at 2 min after
incubation in pH 8.0 Tris-HCl buffer.
While fat body GST activity of control larvae increased moderately with time, a significant induction of this enzyme occurred starting 12 h after
Archives of Insect Biochemistry and Physiology
August 2008
JH Induction of S. litura GST Activity
235
Fig. 1. Effect of JHA pyriproxyfen on the metamorphosis of the sixth instar larvae of
S. litura. Various doses, i.e.,
0.01, 0.1, 1, 10, and 100 µg
JHA /larva, were topically applied onto the abdomens of
0-, 1-, and 2-day-old larvae, respectively. Each bar represents
mean ± SEM for eight replicates, and each replicate consisted of 10 individuals. Bars
with different letters are significantly different (P < 0.05)
by LSD.
treatment of 0-day-old sixth instar larvae with 100
µg JHA/larva and continued up to the end of the
observation, namely 72 h (Fig. 3a). The final GST
activity of treated larvae, more than 10-fold higher
than at the beginning, was 2- to 3-fold higher than
that of control larvae. However, no such induction
of GST activity was observed when 1- and 2-dayold sixth instar larvae received similar treatment
(Fig. 3b,c). This induction of GST by JHA in 0day-old sixth instar larvae was dose-dependent, i.e.,
a linear increase of GST activity with JHA doses
(log scale) ranging from 1 to 100 µg/larva (Fig. 4).
Twenty-four hours after treatment, fat body GST
activity of female larvae receiving 100 µg JHA was
approximately 3-fold higher than that of larvae
treated with 1 µg JHA.
Figure 5 indicates that the fat body GST activity of 0-day-old sixth instar larvae was induced by
JH-III, a natural JH, in a dose-dependent manner,
reaching a maximum of 1,175 nmol of CDNB conjugated/min/mg protein for 100 µg JH-III treatment.
Effect of Transcriptional and Translational
Inhibitors on JHA-Induced GST Activity
Alpha-amanitin, a transcriptional inhibitor, applied along with JHA onto 0-day-old sixth instar
larvae produced only slight suppression of JH inArchives of Insect Biochemistry and Physiology
August 2008
duction of fat body GST activity after 12 h, and
yet, this effect became more apparent, with approximately 40% suppression 24 h after treatment
(Fig. 6). A similar effect, with approximately 35%
suppression of GST activity (Fig. 7), was also observed 24 h after treatment with the translational
inhibitor, cycloheximide.
DISCUSSION
In lepidopterans, a decline of JH titer to an undetectable level within the first third of final larval
instar initiates metamorphosis (Riddiford et al.,
2003). Hatakoshi et al. (1986) reported that application of exogenous JHA interrupts the normal
transformation of S. litura larvae. In our study, similar results have been obtained, namely, JHA induced the final instar S. litura larvae to form
supernumerary larvae in a dose-dependent manner; additionally, this JHA-effect exists in an agedependent manner as well. This observation is
generally consistent with the work done by Tojo
et al. (1985) on the same insect.
The absence of exogenous JHA effect on the pupation of 0-day-old sixth instar S. litura implies
some mechanism could help remove or compensate for the excessive JH present. Feng et al. (1999,
2001) measured the expression of CfGST in dia-
236
Wu and Lu
Fig. 2. Effect of reaction conditions on the fat body GST
activity in S. litura larvae. GST activity in the fat body of
1-day-old sixth instar larvae was assayed using 1-chloro2, 4-dinitrobenzene (CDNB) as substrate under different
conditions, including pH (a), enzyme level (b), and incubation time (c). Each point represents the mean ± SEM
of three replicates with each replicate consisting of pooled
fat bodies from five larvae.
pause and its induction by ecdysteroids and proposed that GST might play certain roles in diapause
and larval development of C. fumiferana. We have
identified several putative GST cDNA fragments in
the JH-affected subtractive cDNA library of S. litura
Fig. 3. Effect of JHA on the fat body GST activity in S.
litura larvae. One hundred micrograms of pyriproxyfen/
larva was topically applied onto the abdomens of 0- (a),
1- (b), and 2-day-old (c) sixth instar larvae for various
time periods. The GST activity (Y axis) represents nmol
of CDNS conjugated/min/mg protein. Each point represents the mean ± SEM of three replicates with each replicate consisting of pooled fat bodies from five larvae.
(Wu and Lu, unpublished data). These results
prompted us to focus on investigating if GST might
play some role in decreasing the effect of endogenous JH required for larval pupation.
JHA elevated the fat body GST activity in 0-dayold sixth instar S. litura larvae. The same phenomenon occurred when the natural JH-III was applied.
These results imply that the induction of GST activity in fat body is very possibly a hormonal effect. It
Archives of Insect Biochemistry and Physiology
August 2008
JH Induction of S. litura GST Activity
237
Fig. 4. Dose-dependent effect of JHA on the fat body
GST activity in S. litura larvae.
The 0-day-old sixth instar
larvae were topically treated
with various doses of JHA.
The GST activity (Y axis) represents nmol of CDNS conjugated/min/mg protein. Each
point represents the mean ±
SEM of three replicates with
each replicate consisting of
pooled fat bodies from five
larvae.
has been considered that all GSTs can bind with
a wide range of neutral and anionic lipophilic
components, like steroid hormone, bile acids, and
fatty acids (Hayes and Pulford, 1995), and are involved in the transport of a variety of hormones
and endogenous metabolites (Maruyama and
Listowsky, 1984; Listowsky et al., 1988; Ishigaki,
1989). Perhaps the JH-inducible GST in this cutworm might serve as a binding protein for JH to
eliminate its effect on metamorphosis. A significant suppression of JHA elevation of fat body
GST-activity by the transcription inhibitor αamanitin suggests that this increase of GST activity
could be due to JH stimulation of its transcription.
On the other hand, the translation inhibitor
CHXM also significantly blocked the JHA elevation of the fat body GST activity, indicating that
protein synthesis was required for the increase in
the fat body GST activity. These results of experiments with transcriptional and translational inhibitors suggest that the increase of fat body GST
in response to JHA treatment was due to either
an enhancement of GST transcription or its mRNA
stability as described by Tang and Tu (1995). They
imply that JHA-induced activation of GST in S.
Archives of Insect Biochemistry and Physiology
August 2008
litura larval fat body was not due to post-translational modification, such as phosphorylation.
From the foregoing discussion, it is concluded
that a class of GST could be induced by JHA and
Fig. 5. Effect of JH-III on the fat body GST activity in S.
litura. The 0-day-old sixth instar larvae were topically
treated with various doses of JH-III. The GST activity (Y
axis) represents nmol of CDNS conjugated/min/mg protein. Each bar represents the mean ± SEM of five replicates with each replicate consisting of pooled fat bodies
from five larvae. Bars with different letters are significantly
different (P < 0.05) by LSD.
238
Wu and Lu
Fig. 6. Inhibitory effect of αamanitin on JHA-induced fat
body GST activity in S. litura.
The 0-day-old sixth instar larvae were topically treated with
100 µg of JHA along with αamanitin (2.5 and 5 µg) for
12 and 24 h, respectively. The
GST activity (Y axis) represents nmol of CDNS conjugated/min/mg protein. Each
bar represents the mean ±
SEM of three replicates with
each replicate consisting of
pooled fat bodies from five
larvae. Bars with different letters are significantly different
(P < 0.05) by LSD.
JH-III only in 0-day-old of sixth instar of S. litura,
and this induction profile suggests that GST may
play an important role in metamorphosis. This investigation reports the induction of GST activity by
JHA in the fat body of an insect.
ACKNOWLEDGMENTS
We are grateful to the referees’ detailed reviews
of this report, which were valuable in producing
an excellent publication. We thank Dr. Chih-Ning
Fig. 7. Inhibitory effect of
cycloheximide on JHA-induced fat body GST activity in
S. litura. The 0-day-old sixth
instar larvae were topically
treated with 100 µg of JHA
along with 100 µg of cycloheximide for 10 and 24 h, respectively. The GST activity
(Y axis) represents nmol of
CDNS conjugated/min/mg
protein. Each bar represents
the mean ± SEM of three replicates with each replicate
consisting of pooled fat bodies from five larvae. Bars
with different letters are significantly different (P < 0.05)
by LSD.
Archives of Insect Biochemistry and Physiology
August 2008
JH Induction of S. litura GST Activity
Sun and Dr. Sundararaju Jothi Saraswathi for critical reading and useful discussions of the article and
Mr. Chun-Cheng Fanjiang for helpful assistance on
the statistic analysis. This study was partially supported by the ATU plan of the Ministry of Education for publication.
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