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From the Department of Surgery, University of Louisville School of Medicine, Louisville, Kentucky
| Abstract |
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B, increased production of tumor necrosis factor-
(TNF
), liver neutrophil accumulation, and
hepatocellular damage. Exogenous administration of interleukin-4 (IL-4)
or IL-13 was recently shown to regulate this inflammatory injury in
association with activation of signal transducer and activator of
transcription-6 (STAT6). The objective of the present study was to
determine whether STAT6 was required for the regulation of liver
inflammation by IL-4 and IL-13. Wild-type and STAT6 knockout mice
underwent 90 minutes of hepatic ischemia followed by 8 hours of
reperfusion. Hepatic ischemia/reperfusion in wild-type and STAT6
knockout mice significantly increased (P < 0.05)
NF-
B activation, serum levels of TNF
, liver
accumulation of neutrophils [measured by myeloperoxidase (MPO)
content], and hepatocellular damage [measured by serum
alanine aminotransferase (ALT)] compared to sham controls. In
wild-type mice, activation of STAT6 was not observed after
ischemia/reperfusion. Administration of 1 µg of IL-4 or IL-13 at
reperfusion reduced serum TNF
, liver neutrophil
accumulation, and hepatocellular injury in wild-type mice.
Treatment with IL-4 or IL-13 had no effect on liver NF-
B activation
but significantly increased activation of STAT6. In STAT6 knockout
mice, neither IL-4 nor IL-13 had any effect on TNF
,
MPO, or ALT values, the regulatory effects of these
cytokines being completely abolished. The data suggest that activation
of STAT6 may regulate liver inflammatory injury.
| Introduction |
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B is a
major regulator of the gene expression of all of these
mediators.7
Recent reports have suggested that activation
of NF-
B may be an essential component in the development of the
hepatic inflammatory response.8
Other studies have shown
that blockade of NF-
B activation may result in reduced
proinflammatory mediator production and reduced liver inflammation
in vivo.9,10
The Th2 cytokines, interleukin-4 (IL-4)
and IL-13, are known to modulate inflammatory responses in part by
down-regulating the production of proinflammatory mediators. Consistent
with these functions, we recently demonstrated that IL-4 and IL-13
protect against liver inflammatory injury induced by
ischemia/reperfusion by preventing production of proinflammatory
mediators, including tumor necrosis factor-
(TNF
) and macrophage
inflammatory protein-2 (MIP-2).11,12
Gene expression of
TNF
and MIP-2 is controlled, at least in part, by
NF-
B.7
However, in previous studies we found that IL-4-
and IL-13-mediated reductions in hepatic production of TNF
and MIP-2
mRNA and protein occurred without suppression of NF-
B nuclear
translocation or DNA binding.11,12
These findings
suggested that the hepatoprotective properties of IL-4 and IL-13 may be
unrelated to effects on NF-
B activation.
Signal transducer and activator of transcription-6 (STAT6) is another rapidly activated transcription factor that is known to be critical for the immunoregulatory functions of IL-4 and IL-13 on macrophages in vitro.13,14 STAT6 is present in the cytoplasm in an unphosphorylated, monomeric form. Binding of IL-4 or IL-13 to their cognate receptors results in phosphorylation of STAT6 by receptor-associated members of the Janus kinase family.15 Phosphorylated STAT6 proteins dimerize and are then able to translocate into the nucleus and bind specific target regions of DNA. We found previously that treatment with IL-4 or IL-13 results in rapid activation (nuclear translocation) of STAT6 in liver during inflammation induced by ischemia/reperfusion.11,12 In the present studies, we sought to determine the role of STAT6 in the regulation of the acute inflammatory response in liver.
| Materials and Methods |
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Male BALB/c (wild-type controls) and BALB/c-Stat6tm1Gru (STAT6 -/-) mice (68 weeks old) were purchased from the Jackson Laboratory (Bar Harbor, ME). Partial hepatic ischemia was induced as described previously.4 Briefly, mice were anesthetized with sodium pentobarbital (60 mg/kg, intraperitoneal). A midline laparotomy was performed, and an atraumatic clip was used to interrupt the blood supply to the cephalad lobes of the liver. After 90 minutes of partial hepatic ischemia, mice received either sterile saline or 1 µg recombinant murine IL-4 or IL-13 (R&D Systems, Minneapolis, MN) via the lateral tail vein, and the vascular clip was removed, initiating hepatic reperfusion. Sham control mice underwent the same protocol, but without vascular occlusion. Mice were sacrificed after 1, 4, or 8 hours of reperfusion, and blood and liver tissues were taken for analysis. This project was approved by the University of Louisville Animal Care and Use Committee and was in compliance with the National Institutes of Health guidelines.
Electrophoretic Mobility Shift Assay
Nuclear extracts of liver tissue were prepared by the method of
Deryckere and Gannon16
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) was
end-labeled with [
-32P]ATP (3000 Ci/mmol at
10 mCi/ml; Amersham, Arlington Heights, IL). Binding reactions
containing equal amounts of nuclear protein extract (20 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. Reaction products were separated in a native
4% polyacrylamide gel and analyzed by autoradiography. Autoradiographs
were digitized, and activation of NF-
B and STAT6 was quantitated
with image analysis software (Adobe Systems, San Jose, CA).
Reverse Transcription-Polymerase Chain Reaction
Total RNA from liver tissue was extracted with an RNeasy Mini Kit
(Qiagen, Valencia, CA). RNA (1 µg) was reverse transcribed to cDNA,
using random hexamers. cDNA products were coamplified by polymerase
chain reaction (PCR) (30 cycles; 95°C for 60 seconds, 59°C for 90
seconds, and 72°C for 10 seconds). Primers for TNF
(446-bp
product) and ß-actin (245-bp product) have been described
elsewhere.9
PCR products were electrophoresed in a 2%
agarose gel, stained with ethidium bromide, and photographed. Digitized
photographs were assessed using image analysis software (Adobe
Systems). Relative TNF
mRNA expression was determined from the ratio
of band intensities and presented graphically as percentage ß-actin.
Myeloperoxidase Assay
Liver myeloperoxidase (MPO) content was assessed by methods similar to those of Schierwagen et al.17 Liver tissue (100 mg) was homogenized in 2 ml of buffer A (3.4 mmol/L KH2HPO4, 16 mmol/L Na2HPO4, pH 7.4). After centrifugation for 20 minutes at 10,000 x g, the pellet was resuspended in 10 volumes of buffer B (43.2 mmol/L KH2HPO4, 6.5 mmol/L Na2HPO4, 10 mmol/L EDTA, 0.5% hexadecyltrimethylammonium, pH 6.0) and sonicated for 10 seconds. After heating for 2 hours at 60°C, the supernatant was reacted with 3,3',3,5'-tetramethylbenzidine (Sigma Chemical Co., St. Louis, MO), and the optical density was determined at 655 nm.
Blood and Tissue Analysis
At the time of sacrifice, blood was obtained from the inferior
vena cava for analysis of serum alanine aminotransferase (ALT) as an
index of hepatocellular injury, using a commercially available
diagnostic kit (Sigma Chemical Co.). Serum levels of TNF
were
measured by sandwich enzyme-linked immunosorbent assay with antibodies
to mouse TNF
purchased from R & D Systems. Liver and lung edema was
determined by organ wet-to-dry weight ratios.
Statistical Analysis
All data are expressed as mean ± SEM. Data were analyzed with a one-way analysis of variance with a subsequent Student-Newman-Keuls test. Differences were considered significant when P < 0.05.
| Results |
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To assess the effects of IL-4 and IL-13 on the activation of STAT6
and NF-
B in liver after hepatic ischemia and reperfusion in BALB/c
mice, either saline, IL-4, or IL-13 was administered intravenously just
before reperfusion. Liver nuclear extracts were obtained over an 8-hour
time course of ischemia/reperfusion injury and analyzed by EMSA.
Nuclear translocation of STAT6 was quantitated by image analysis of
autoradiographs. Activation of STAT6 in livers from saline-treated mice
was not observed at any time (Figure 1)
.
However, treatment with either IL-4 or IL-13 resulted in significant
increases in STAT6 activation within 1 hour of reperfusion. These
effects persisted throughout the 8-hour time course studied. Similar to
our earlier reports,11,12
NF-
B activation was markedly
increased 1, 4, and 8 hours after reperfusion in mice treated with
saline (Figure 2)
. Treatment with IL-4 or
IL-13 had no effect on nuclear translocation or DNA binding of NF-
B.
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Expression by
IL-4 and IL-13
Because both IL-4 and IL-13 reduce TNF
production in
vitro and in vivo,11,12,18
and because we
have recently shown that suppressed TNF
production in hepatic
ischemia/reperfusion injury by IL-4 or IL-13 is associated with
increased STAT6 activation,11,12
we assessed whether STAT6
was required for IL-4- and IL-13-mediated suppression of TNF
production in this model. Liver RNA extracts from wild-type mice and
STAT6 -/- mice were analyzed by reverse transcriptase-polymerase
chain reaction for expression of TNF
mRNA. As shown in Figure 3
, TNF
mRNA expression was greatly
increased in both wild-type and STAT6 -/- mice after hepatic ischemia
and 8 hours of reperfusion (P < 0.001 compared
to sham controls). Treatment with IL-4 or IL-13 significantly reduced
the expression of TNF
mRNA in wild-type mice. However, in STAT6
-/- mice, neither IL-4 nor IL-13 had any effect on hepatic
ischemia/reperfusion-induced TNF
mRNA expression. Similar results
were obtained when serum levels of TNF
were measured by ELISA. In
both wild-type mice and STAT6 -/- mice, hepatic ischemia/reperfusion
caused a significant increase in the circulating levels of TNF
(P < 0.001 compared to sham controls; Figure 4
). In wild-type mice, treatment with
IL-4 or IL-13 caused significant reductions in serum levels of TNF
.
In contrast, neither IL-4 nor IL-13 had any effect on serum TNF
in
STAT6 -/- mice.
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We have recently shown that IL-4 and IL-13 protect against hepatic
ischemia/reperfusion injury in association with STAT6 activation in
liver.11,12
To determine whether STAT6 was required for
the protective effects of IL-4 and IL-13, we assessed the ability of
IL-4 and IL-13 to suppress liver neutrophil recruitment and
hepatocellular injury in wild-type and STAT6 -/- mice. Neutrophil
recruitment into liver was determined by the content of myeloperoxidase
(MPO) in liver homogenates. In both wild-type and STAT6 -/- mice,
ischemia and 8 hours of reperfusion resulted in significant increases
in liver MPO content compared to sham controls
(P < 0.001; Figure 5
). Treatment of wild-type mice with IL-4
or IL-13 greatly reduced the amount of liver neutrophil accumulation.
However, in STAT6 -/- mice, treatment with IL-4 or IL-13 had no
effect on the hepatic recruitment of neutrophils.
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| Discussion |
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B.9,19
Other studies have shown that IL-4 and IL-13
prevent liver inflammatory injury by an NF-
B-independent
mechanism.11,12
The present studies provide evidence that
activation of the transcription factor STAT6 is required for the
regulation of liver inflammation by IL-4 and IL-13.
In wild-type mice, administration of IL-4 or IL-13 rapidly activated
STAT6 in liver and suppressed TNF
mRNA and protein expression. These
effects were completely lost in STAT6 knockout mice. Because the gene
for TNF
lacks a STAT6 binding site, it is unlikely that STAT6
directly represses TNF
gene transcription. However, STAT6 binds to
the transcriptional coactivator p300, which is required for
transcriptional activation by STAT6.20,21
p300 also binds
to the RelA (p65) subunit of NF-
B and has been shown to be necessary
for NF-
B-mediated gene transcription.22,23
p300 is
present in the nucleus in limited quantities,24
and thus
it is possible that STAT6 competes with NF-
B for p300 as a potential
mechanism of transcriptional deactivation of NF-
B. In this scenario
NF-
B, but not STAT6, would translocate to the nucleus of liver cells
shortly after hepatic reperfusion in untreated mice. The NF-
B binds
to p300 and begins transcription of target genes, such as TNF
.
However, when mice are treated with IL-4 or IL-13, there is nuclear
translocation of STAT6, which can compete with NF-
B for p300.
Decreased availability of p300 to NF-
B may result in decreased
transcriptional activation. Our attempts to investigate the competitive
interactions between p300 and NF-
B or STAT6 have been limited by an
inability of commercially available antibodies to immunoprecipitate
murine p300. However, the role of p300 in the regulation of NF-
B
transcriptional activation has been demonstrated in studies involving
the tumor suppressor p53.25,26
These reports showed that
p53 inhibited the transcriptional activation of NF-
B by outcompeting
NF-
B for binding to p300. Our current data suggest, but do not
prove, that a similar relationship may exist between STAT6 and NF-
B.
The fact that STAT6 was not appreciably activated by
ischemia/reperfusion may suggest that STAT6 is not involved in the
endogenous mechanisms that serve to regulate acute liver inflammation.
In addition, STAT6 knockout mice were indistinguishable from wild-type
mice in their response to hepatic ischemia/reperfusion; there were no
differences in TNF
expression, neutrophil accumulation, or
hepatocellular injury. Whether the expression of IL-4 and/or IL-13 is
up-regulated during hepatic ischemia/reperfusion injury is currently
under investigation. Based on the present data demonstrating that STAT6
is not activated after ischemia/reperfusion, it would seem unlikely
that endogenous production of IL-4 or IL-13 is a relevant component of
the hepatic regulatory machinery.
Functional knowledge of STAT6 has primarily been associated with Th2
cell development and immunoglobulin class switching to
IgE.27,28
More recent studies have suggested that
activation of STAT6 may be involved in the suppression of
proinflammatory mediator expression in macrophages.29-30
Our in vivo data demonstrating a requirement for STAT6 for
IL-4- and IL-13-mediated suppression of TNF
expression and
development of liver inflammatory injury identify a novel function for
STAT6 as a regulator of the acute inflammatory response. The precise
mechanism by which STAT6 confers these effects requires further
investigation, but the present findings suggest that development of
pharmacological agents that activate STAT6, or use of IL-4 and/or
IL-13, may represent plausible therapeutic strategies for the treatment
of inflammatory liver disease.
| Footnotes |
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Supported by National Institutes of Health grant DK56029.
| References |
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in the pathophysiologic alternations after hepatic ischemia/reperfusion injury in the rat. J Clin Invest 1990, 85:1936-1943
B and Rel proteins: evolutionary conserved mediators of immune responses. Annu Rev Immunol 1998, 16:225-260[Medline]
B independently of I
B degradation. Hepatology 1998, 28:1022-1030[Medline]
B. Hepatology 1999, 30:203-208[Medline]
B by cyclin-dependent kinases associated with the p300 coactivator. Science 1997, 275:523-527
B requires multiple coactivators. Mol Cell Biol 1999, 19:6367-6378
and IL-12 by STAT6-dependent and -independent mechanisms. J Immunol 1999, 162:5224-5229This article has been cited by other articles:
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