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Short Communication |
From the Department of Surgery, University of Louisville School of Medicine, Louisville, Kentucky
| Abstract |
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(TNF
). These
mediators stimulate a cascade of events including up-regulation of CXC
chemokines and vascular endothelial adhesion molecules, leading
to hepatic neutrophil recruitment and tissue injury. Interleukin-13
(IL-13) is a cytokine that has been shown to suppress macrophage
production of proinflammatory mediators. The objective of the current
study was to determine whether IL-13 could regulate the liver
inflammatory injury induced by ischemia and reperfusion. C57BL/6 mice
underwent 90 minutes of partial hepatic ischemia followed by
reperfusion with or without intravenous administration of recombinant
murine IL-13. Hepatic ischemia/reperfusion increased expression of
TNF
and macrophage inflammatory protein-2 (MIP-2), leading
to hepatic neutrophil recruitment, hepatocellular
injury, and liver edema. Administration of IL-13 reduced the
production of TNF
and MIP-2 mRNA and protein. IL-13 suppressed liver
neutrophil recruitment by up to 72% and hepatocellular injury and
liver edema were each reduced by >60%. Administration of IL-13 had no
effect on liver NF
B activation, but greatly increased the
activation of STAT6. The data suggest that the hepatoprotective effects
of IL-13 may be a result of STAT6 activation.
| Introduction |
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(TNF
).5-7
The later phase of injury
is dependent upon neutrophils. Enhanced hepatic production of TNF
plays a critical role in the initiation of a mediator cascade
responsible for hepatic recruitment of neutrophils and subsequent liver
injury. TNF
up-regulates intercellular adhesion molecule-1 (ICAM-1)
expression on the hepatic vascular endothelium.8,9
In
addition, TNF
induces the hepatic
expression of CXC chemokines, including macrophage inflammatory
protein-2 (MIP-2) and epithelial neutrophil-activating protein-78
(ENA-78).10,11
The combined effects of ICAM-1 and CXC
chemokines result in adhesion and transmigration of neutrophils from
the intravascular space to liver parenchyma. Neutrophil accumulation in
hepatic sinusoids obstructs blood flow and contributes to hepatic
hypoperfusion.12
The ensuing liver injury is caused by
neutrophil-derived oxidants and proteases, which directly damage
endothelial cells and hepatocytes.13
Interleukin-13 (IL-13) was originally identified as a product of
activated Th2 cells which suppressed in vitro monocyte
production of proinflammatory cytokines, including TNF
, IL-1, IL-6,
IL-8, and MIP-1
.14-17
In vivo, IL-13
prevents lipopolysaccharide-induced lethality and IgG immune
complex-induced lung injury.18-20
The mechanism(s) of the
anti-inflammatory effects of IL-13 have been linked with inhibition of
the transcription factor nuclear factor
B (NF
B).21,22
However, in vitro, IL-13 potently activates the
transcription factor, signal transducer and activator of transcription
6 (STAT6).23,24
In the current studies, we evaluated the
effects of IL-13 on the activation state of NF
B and STAT6 in liver
during I/R injury and assessed whether these effects of IL-13 precluded
development of liver inflammatory injury induced by hepatic ischemia
and reperfusion.
| Materials and Methods |
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Male C57BL/6 mice (Charles River Laboratories, Wilmington, MA) weighing 2228 g were used in all experiments. Partial hepatic ischemia was induced as described previously.10 Briefly, mice were anesthetized with sodium pentobarbital (60 mg/kg i.p.). Mice received either sterile saline or recombinant murine IL-13 (1 µg; R&D Systems Inc., Minneapolis, MN) via the lateral tail vein before the induction of ischemia. A midline laparotomy was performed and an atraumatic clip was used to interrupt blood supply to the cephalad lobes of the liver. After 90 minutes of partial hepatic ischemia, mice again received either sterile saline or IL-13 (1 µg) via the lateral tail vein, and the clip was removed, initiating hepatic reperfusion. Sham control mice underwent the same protocol, but without vascular occlusion. Mice were sacrificed after 4 hours of reperfusion, and liver tissues and blood samples were taken for analysis. This project was approved by the University of Louisville Animal Care and Use Committee and was in compliance with National Institutes of Health guidelines.
Electrophoretic Mobility Shift Assay
Nuclear extracts of liver tissue were prepared by the method of
Deryckere and Gannon25
and analyzed by electrophoretic
mobility shift assay (EMSA). Briefly, double-stranded NF
B consensus
oligonucleotide (Promega, Madison, WI) or STAT6 consensus
oligonucleotide (Santa Cruz Biotechnology, Santa Cruz, CA) were
end-labeled with
[32
P] ATP (3,000 Ci/mmol at 10
mCi/ml, Amersham, Arlington Heights, IL). Binding reactions containing
equal amounts of nuclear protein extract (10 µg) and 35 fmol
(~50,000 cpm, Cherenkov counting) of oligonucleotide were incubated
at room temperature for 30 minutes. Reaction volumes were held constant
at 15 µl. For competition and supershift assays of STAT6 binding,
unlabeled NF
B or STAT6 oligonucleotide (50-molar excess) or
antibodies to STAT4 or STAT6 (Santa Cruz Biotechnology) were added to
the reaction mixtures. Binding reaction products were separated in a
4% polyacrylamide gel and analyzed by autoradiography.
Reverse Transcription-Polymerase Chain Reaction (RT-PCR)
Total RNA from liver tissue was extracted using RNeasy Mini Kit
(Qiagen Inc., Valencia, CA). RNA (1 µg) was reverse transcribed to
cDNA using random hexamers. cDNA products were coamplified by PCR (30
cycles of 95°C for 60 seconds, 59°C for 90 seconds, and 72°C for
10 seconds). Primers for TNF
(446-bp product), MIP-2 (205-bp
product), and ß-actin (245-bp product) have been described
elsewhere.26
PCR products were electrophoresed in a 3.5%
agarose gel, stained with ethidium bromide, and photographed.
Liver Neutrophil Accumulation
Liver myeloperoxidase (MPO) content was determined by methods described elsewhere.10 Briefly, liver tissue (50 mg) was homogenized, sonicated, and centrifuged for 20 minutes at 10,000 x g. Supernatants were reacted with 3,3', 3,5'-tetramethylbenzidine (Sigma Chemical Co., St. Louis, MO) and read at 655 nm.
Liver neutrophil morphometric analysis was performed on frozen liver sections fixed in 0.5% glutaraldehyde and stained with hematoxylin and eosin. The number of neutrophils were counted in 5 separate high-power fields for each liver section. The results are expressed as neutrophils/high-power field.
Liver Edema
The extent of liver edema was measured by tissue wet-to-dry weight ratios. After dissection, liver samples were weighed and placed in a drying oven at 55°C until a constant weight was obtained. In this determination, liver edema is represented by an increase in the wet-to-dry weight ratios.
Blood Analyses
Blood was obtained by cardiac puncture at the time of sacrifice.
Serum was analyzed for TNF
and MIP-2 by enzyme-linked immunosorbent
assays (ELISA) according to manufacturer's instructions (BioSource
International, Camarillo, CA). Serum was also analyzed for alanine
aminotransferase (ALT) as an indicator of hepatocellular injury.
Measurements of serum ALT were made using a diagnostic kit from Sigma
Chemical Co.
Statistical Analyses
All data are expressed as mean ± SE. Data were analyzed with a one-way analysis of variance with subsequent Student-Newman-Keuls test. Differences were considered significant when P < 0.05. To calculate percentage change, negative control values were subtracted from positive control and treatment group values.
| Results |
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B and STAT6 in Liver
Because IL-13 is known to suppress activation of
NF
B,21
and because IL-13-induced activation of STAT6 is
associated with anti-inflammatory effects of this
cytokine,24
we sought to determine whether IL-13 altered
activation of NF
B and/or STAT6 in the liver during I/R injury.
Nuclear extracts from liver tissue obtained from sham controls and
livers undergoing ischemia and 1 or 4 hours of reperfusion were
analyzed by EMSA. NF
B activation was greatly increased in liver
after hepatic ischemia and 1 or 4 hours of reperfusion, compared to
that in liver from sham controls (Figure 1A)
. The administration of IL-13 had no
effect on NF
B activation at either time point. In contrast, hepatic
ischemia and reperfusion did not result in detectable increases in
STAT6 activation (Figure 1A)
. However, intravenous treatment with IL-13
resulted in a large increase in activation of STAT6 after 1 hour of
reperfusion. IL-13-induced STAT6 activation was also present after 4
hours of reperfusion.
|
B oligonucleotide had no
effect on DNA binding to the STAT6 oligonucleotide. Competition with
excess unlabeled STAT6 oligonucleotide completely abolished DNA binding
to the labeled STAT6 oligonucleotide. Addition of polyclonal rabbit
anti-STAT6, but not anti-STAT4, resulted in almost complete
disappearance of the STAT6 band. These data demonstrate that IL-13
administration causes hepatic activation of STAT6.
Effects of IL-13 on Production of TNF
and MIP-2
To further investigate the effects of IL-13, including activation
of STAT6, on the development of liver injury induced by I/R, we
assessed whether IL-13 administration altered the generation of TNF
and MIP-2 mRNA in liver tissue. Liver RNA extracts were analyzed by
RT-PCR. Hepatic ischemia and 4 hours of reperfusion resulted in large
increases in the amount of TNF
(Figure 2A)
and MIP-2 mRNA (Figure 2B)
.
Administration of IL-13 greatly reduced the generation of TNF
and
MIP-2 mRNA in liver tissue. Results shown are representative of three
independent experiments.
|
and MIP-2 have been shown to be
closely associated with the hepatic production of these
mediators.6,7,10
ELISA analysis of serum demonstrated that
TNF
was significantly increased after ischemia and 4 hours of
reperfusion (Figure 2C)
in serum by 75% (P < 0.001). Serum
levels of MIP-2 were also increased after hepatic ischemia and
reperfusion (Figure 2D)Effects of IL-13 on Hepatic Neutrophil Accumulation and Inflammatory Injury
Because TNF
and MIP-2 have been shown to be critical mediators
for the hepatic recruitment of neutrophils, we assessed whether the
suppressive effects of IL-13 on TNF
and MIP-2 production were
associated with reduced liver accumulation of neutrophils. Hepatic
neutrophil accumulation was measured by liver MPO content. After
hepatic ischemia and 4 hours of reperfusion there was a significant
increase in liver MPO content compared to the sham controls (Figure 3A)
. In the presence of IL-13, liver MPO
content was reduced by 72% (P = 0.019).
Similarly, hepatic I/R caused a significant increase in the number of
neutrophils in liver sections assessed by morphometric analysis (Figure 3B)
. The administration of IL-13 resulted in a 62% decrease in the
number of neutrophils observed in liver sections
(P = 0.003).
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| Discussion |
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, then cause the up-regulation of CXC
chemokines and adhesion molecules, leading to hepatic recruitment of
neutrophils and ensuing inflammatory tissue injury.7,9,11
The administration of IL-13 reduced I/R-induced production of TNF
and MIP-2. Reduction of these mediators resulted in reduced
accumulation of neutrophils in the liver and limited the development of
liver injury. To determine the mechanism(s) of the protective effects
of IL-13, we assessed the effects of IL-13 on the activation of NF
B
and STAT6 in the liver.
Although a distinct role for STAT6 in the inflammatory response
has not been defined, it has recently been shown that STAT6
activation by IL-13 enhances the production of IL-1 receptor antagonist
production in hepatocytes.27
Furthermore, the
anti-inflammatory activity of IL-13 is compromised in macrophages from
STAT6-deficient mice.24
In the current studies, hepatic
ischemia and up to 4 hours of reperfusion did not result in detectable
activation of STAT6 in liver. Similarly, others have reported that in a
murine model of partial hepatectomy STAT6 is not activated in liver
during or after liver resection.28
However, we show that
administration of IL-13 caused substantial activation of STAT6 within 1
hour after hepatic reperfusion. This activation was associated with
greatly reduced production of TNF
and MIP-2, suppressed neutrophil
accumulation, and decreased liver injury. Although our data provide
evidence suggesting an anti-inflammatory role for STAT6, little is
known about the manner in which this transcription factor may exert
these effects. Activation of STAT6 has been reported to suppress
TNF
-induced adhesion molecule expression by antagonizing the
DNA-binding of NF
B.29
More recently, it has been shown
that STAT6 regulates the expression of Fc
RI in mast
cells30
as well as an inhibitor of cyclin-dependent kinase,
p27Kip1.31
Thus, there is evidence that STAT6 may function
as a negative regulator of transcription. Furthermore, a recent report
demonstrated that STAT6 activation was required for IL-4-induced
suppression of TNF
production in macrophages.32
The
precise mechanism by which STAT6 may suppress the production of
proinflammatory mediators, such as TNF
and MIP-2, is currently
unknown.
We have previously shown that IL-13 suppresses NF
B activation by
preventing the degradation of the NF
B-inhibiting protein
I
B
.21
However, it was recently demonstrated that
liver activation of NF
B during hepatic ischemia/reperfusion occurs
without degradation of I
B
.33
Furthermore, those
studies showed that tyrosine phosphorylation of I
B
occurred, a
mechanism of NF
B activation that does not require I
B
degradation.34
Our current studies suggest that the
protective effects of IL-13 in liver injury induced by I/R are
unrelated to its effects on NF
B. The administration of
IL-13 greatly suppressed proinflammatory cytokine and chemokine
production, liver neutrophil recruitment and development of
liver injury induced by ischemia and reperfusion. These data suggest
that IL-13 may represent a potential therapeutic strategy for the
treatment of inflammatory liver disease. The protective effects of
IL-13 were preceded by activation of STAT6 in liver, suggesting that
STAT6 may operate as a negative regulator of proinflammatory mediator
production. Combined with the recent findings of
others,29-32
these studies provide evidence for an
antiinflammatory role for STAT6 in vivo. Further studies of
the function of STAT6 should provide a better understanding of the role
of this transcription factor in the regulation of acute inflammatory
responses.
| Footnotes |
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Supported by The Jewish Hospital Foundation.
Accepted for publication June 30, 1999.
| References |
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in the pathophysiologic alternations after hepatic ischemia/reperfusion injury in the rat. J Clin Invest 1990, 85:1936-1943
or IL-10. J Immunol 1993, 151:6370-6381[Abstract]
production from human alveolar macrophages and monocytes. Am J Respir Cell Mol Biol 1996, 15:382-389[Abstract]
, IFN-
, and IL-12 production. J Immunol 1997, 158:2898-2903[Abstract]
. Eur J Immunol 1994, 24:1441-1445[Medline]
B and preservation of I
B
by interleukin-10 and interleukin-13. J Clin Invest 1997, 100:2443-2448[Medline]
B, activation protein-1, and apoptosis. J Immunol 1998, 161:2863-2872
B. Hepatology 1999, 30:203-208[Medline]
-stimulated E-selectin gene transcription is mediated by STAT6 antagonism of NF-
B. J Biol Chem 1997, 272:10212-10219
RI expression through a STAT6-dependent mechanism. J Immunol 1998, 161:6915-6923
and IL-12 by STAT6-dependent and -independent mechanisms. J Immunol 1999, 162:5224-5229
B independently of I
B degradation. Hepatology 1998, 28:1022-1030[Medline]
B-
activates NF-
B without proteolytic degradation of I
B-
. Cell 1996, 86:787-798[Medline]
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