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* Department of Immunology and Host Defenses, Ehime University School of Medicine, Shigenobu, Ehime, Japan; and
Department of Immunology, Graduate School of Medicine, University of Tokyo, Tokyo, Japan
| Abstract |
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B in the presence
of IFN-
. IFN-
induces IFN regulatory factor-1 (IRF-1), which in
turn induces the transcription of the IL-12 p40 gene. However, the
IRF-1 binding site in the promoter region of the IL-12 p40 gene has not
yet been formally determined. In the present study, we demonstrated
that IRF-1 directly binds to the IL-12 p40 gene promoter and identified
its binding site. The IRF-1 binding site in the promoter region of the
IL-12 p40 gene is shown to be in the -72 to -58 area of the
5'-upstream region. The -63 to -61 position is the critical site
within this region for the binding of IRF-1 to the IL-12 p40 gene
promoter. While IFN-
must be present for IL-12 p40 gene induction,
the p35 gene is strongly induced by LPS, even in the absence of
IFN-
, and therefore the induction of the p35 gene is IRF-1
independent. | Introduction |
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and
IL-12. Bacterial stimuli activate macrophages and subsequently NK cells
in the innate immune response to produce IL-12 and IFN-
,
respectively (8). IL-12 subsequently induces NK cells to
produce IFN-
(9, 10, 11), which in turn activates
macrophages to present Ags to Ag-specific T cells (12).
The involvement of IL-12 in type 1 T cell differentiation is well
established in many systems (8, 13, 14, 15, 16, 17). This type of
innate immune response and its accompanying Ag-specific T cell response
work to eradicate microbial pathogens (1, 4, 5).
The induction of the IL-12 p40 gene in macrophages is regulated by
NF-
B in the presence of IFN-
(3, 18). Signals from
IFN-
receptors are initially transduced by IFN-stimulated gene
factor (ISGF)6-3 and
IFN-
-activated factor (GAF). Transcription factors such as IFN
regulatory factor-1 (IRF-1) are used for further proper regulation of
the broad range of genes induced by IFN, e.g., inducible NO and IL-1
converting enzyme (19, 20, 21, 22). In a previous study, we
demonstrated that IRF-1 gene-disrupted mice are defective in inducing
IL-12 p40 mRNA, suggesting that gene expression is regulated by
transcription factor IRF-1 (23, 24). However, the binding
site of IRF-1 to the promoter region of the IL-12 p40 gene has not yet
been formally determined. We further analyzed within this study the
regulation of IL-12 p40 gene expression and showed that IRF-1 directly
binds to the IL-12 p40 gene promoter and up-regulates gene expression.
Although IL-12 p35 is thought to be constitutively expressed, we show
the IL-12 p35 gene is also inducible by LPS stimulation, even in the
absence of IFN-
, and is regulated differently from the IL-12
p40 gene.
| Materials and Methods |
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Recombinant murine IFN-
was purchased from Genzyme
(Cambridge, MA). Bacterial LPS was purchased from Sigma-Aldrich (Tokyo,
Japan). Ab specific for IRF-1 protein was purchased from Santa Cruz
Biotechnology (Santa Cruz, CA).
Mice
C57BL/6 mice were purchased from Charles River Japan (Yokohama,
Japan). IRF-1-deficient mice have been described earlier
(25) and were maintained by backcrossing to C57BL/6 mice.
ISGF3
-deficient (p48) mice have been described previously
(26). The littermates of each mutant strain were used as
control mice. These mutant mice and their littermates were reared under
specific pathogen-free conditions in the animal facility of either the
University of Tokyo or Ehime University School of Medicine. All mice
were used in accordance with our institutional guidelines for animal
experimentation.
Culture conditions for cells
Cells were cultured at 37°C in a 5% CO2 humidified air atmosphere. The medium used was RPMI 1640 supplemented with 2 mM L-glutamine, 100 U/ml penicillin, 100 µg/ml streptomycin, 1 mM sodium pyruvate, 1-time nonessential amino acid, 50 µM 2-ME, and 10% heat-inactivated FCS.
Isolation and stimulation of peritoneal exudate cells (PECs) for IL-12 induction
PECs taken from mice treated with thioglycolate 3 days earlier
were allowed to adhere to tissue culture plates for 1 h, and
nonadherent cells were removed. Adherent cells were cultured in the
presence of medium alone, LPS (10 µg/ml), or LPS plus recombinant
mouse IFN-
(100 U/ml). Total RNA was extracted 16 h later and
subjected to Northern blotting with probes for IL-12 p40 (a gift from
Dr. H. Yamamoto, Osaka University) and TNF-
(a gift from Dr. T.
Yokota, University of Tokyo).
RNA isolation and Northern blot analysis
Total cellular RNA was isolated by the guanidinium-thiocyanate
method. The procedure for Northern blot analysis is described by Harada
et al. (27). To prepare DNA probes, fragments of TNF-
,
IL-12 p35, IL-12 p40, and
-actin cDNA were labeled by the random
primer method. The specific activity of the IL-12 p35 probe and that of
the IL-12 p40 probe were comparable. The intensity of the band was
measured by BAS2000 (Fuji Film, Tokyo, Japan).
DNA transfection and luciferase assay
Cos7 cells were resuspended in 10% FCS-containing RPMI 1640 medium at 1 x 106 cells/ml. Further, 10 µg of plasmid DNA was mixed with Cos7 cells in a 1-ml electrochamber (PKG/36; Life Technologies, Gaithersburg, MD). DNA was electroporated to Cos7 cells at 750 V/cm and 880 µF using the Electroporation System I (Life Technologies). The cells for luciferase assay were harvested 60 h after transfection. The same amount of cell extract was used in each experiment.
Electrophoretic mobility shift analysis
Electrophoretic mobility shift analysis was done as described by Harada et al. (27). A total of 2 µl of in vitro-translated IRF-1 were incubated with a 32P-labeled DNA probe in a buffer containing 10 mM Tris-HCl (pH 7.6), 50 mM NaCl, 1 mM DTT, 1 mM EDTA, and 5% glycerol at 25°C for 60 min. Protein-DNA complexes were analyzed by 4% polyaclylamide gel electrophoresis.
DNase I footprinting assay
A PCR fragment of -133 to +51 in the IL-12 p40 promoter region was labeled with 32P by incubating with T4 kinase and followed by digestion with SacI and HindIII. The sense strand was mixed with rIRF-1 protein and incubated at 30°C for 1 h. The mixture was digested with DNase I (0.2 U/µl) at 25°C for 1 min, treated with phenol, and precipitated with ethanol. The pellet was dissolved in a loading buffer and analyzed on an 8% polyacrylamide-8 M urea gel (28).
Chromatin immunoprecipitation (ChIP) assay
PECs were stimulated with LPS (10 µg/ml) plus recombinant
mouse IFN-
(100 U/ml). At 0 and 2 h later, formaldehyde
solution was added directly to the culture at a final concentration of
1%. Cross-linking of proteins on chromatin was allowed to occur at
37°C for 10 min, and the cells were lysed by Nonidet P-40 lysis
buffer with protease inhibitors. Chromatin in the lysate was sonicated
to an average length of 200500 bp as determined by agarose gel
electrophoresis. IRF-1 protein binding to chromatin was
immunoprecipitated, washed, and eluted. Cross-links were reversed by
0.5 M NaCl. After proteinase K digestion, DNA in samples was phenol
extracted, ethanol precipitated, and used for PCR amplication to detect
IL-12 p40 promoter segment.
| Results |
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We first examined the requirement of IFN-
for IL-12 p35 and
IL-12 p40 gene expression. Although the IL-12 p40 gene is thought to be
inducible and IL-12 p35 gene is constitutively expressed, both genes
are induced by stimulation with LPS plus IFN-
(Fig. 1, A and B).
However, the requirement for IFN-
is distinctly different in the two
genes. The induction of IL-12 p40 is minimal in the absence of
IFN-
and is enhanced by the presence of IFN-
, confirming previous
findings (2). In contrast, the effect of IFN-
on IL-12
p35 gene induction is minimal. Therefore, IL-12 p70 production is
profoundly influenced by the presence of IFN-
.
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receptor is transduced by IRF-1 and
ISGF3
(p48), IL-12 p40 and p35 gene induction with LPS plus IFN-
was evaluated by Northern blot analysis using thioglycorate-induced
peritoneal adherent cells from IRF-1-/- mice,
p48-/- mice, and their littermates as wild-type
mice (Fig. 1C). Costimulation with LPS and IFN-
induced
the IL-12 p40 gene in wild-type mice, whereas the same stimulation
failed to induce the gene in IRF-1-/- mice.
Gene induction in p48-/- mice was achieved by
costimulation with LPS plus IFN-
, and the induction level was
comparable to their wild-type littermates. This result suggests that
IRF-1 but not p48 is responsible for the regulation of the IL-12 p40
gene. In contrast with IL-12 p40 gene induction, neither IRF-1
nor p48 is required for IL-12 p35 gene induction. The result proves
that induction of the IL-12 p40 and p35 genes is differentially
regulated by IFN-
via IRF-1 gene induction. The former is IRF-1
dependent and the latter is IRF-1 independent. IRF-1 up-regulates the transcription of IL-12 p40 gene
To determine the IRF-1 regulatory region of the IL-12 p40 gene,
the promoter region in the published DNA sequence at -677 to +51 of
the IL-12 p40 gene (18, 29) was obtained by PCR using an
IL-12 p40 genomic clone of C57BL/6 origin as a template. The DNA region
obtained was inserted into the 5'-upstream region of a luciferase gene
lacking the promoter region (basic vector, pLuc) and the reporter
plasmid (p40677-pLuc) was constructed. RAW264.7 cells were
transfected with this plasmid. The transfectant was stimulated with LPS
plus IFN-
and the luciferase activity of the cells was measured. As
shown in Fig. 2A, the
treatment activated the transfectant and resulted in the induction of
luciferase activity. This result proved that the chosen promoter region
of the IL-12 p40 gene is appropriate for further study. Because it has
been shown that the LPS signal required for IL-12 p40 gene activation
was mediated by NF-
B (18), and that the p65 subunit of
the NF-
B complex is a potent transcriptional activator in the
apparent absence of the p50 subunit (30), we used a
NF-
B p65 expression plasmid (CMIN-p65, a gift of Dr. Ruben,
Roche Institute of Molecular Biology, Nutley, NJ) as a substitute for
treatment with LPS. The reporter plasmid containing the IL-12 p40
(p40677-pLuc) promoter region was electrically transfected to Cos7
cells and luciferase activity was measured. Transfection of the plasmid
to Cos7 cells gave baseline enzyme activity. Cotransfection of CMIN-p65
with p40677-pLuc to Cos7 cells did not augment luciferase activity.
On the other hand, cotransfection of IRF-1 expression plasmid (pAct-1,
containing actin-promoter and full length of IRF-1 cDNA) with
p40677-pLuc to Cos7 cells augmented the luciferase activity. This
augmentation was further enhanced by the copresence of CMIN-p65 (Fig. 2B). These results show that the binding region for IRF-1 is
present within the DNA sequence at -677 to +51 of the IL-12 p40
gene.
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B half site) was
obtained by PCR and inserted into the 5'-upstream region of pLuc. The
constructed plasmid (p40133-pLuc) was electrically transfected to
Cos7 cells and luciferase activity was measured (Fig. 2C).
Cotransfection of pAct-1 with p40133-pLuc to Cos7 cells augmented the
luciferase activity. These results show that the IRF-1 binding site in
the IL-12 p40 gene is present in -133 to +51 of the 5'-upstream region
of the gene. The -63 to -61 position of the IL-12 p40 promoter region is critical for the binding of IRF-1 protein
Whether IRF-1 protein binds to the putative IRF-1 binding site of the IL-12 p40 promoter region was directly tested by EMSA. An in vitro-translated IRF-1 protein, which contains a His tag, was incubated with the -133 to -31 region of the IL-12 p40 promoter (which lacks a TATA box), and the resulting solution was applied to the gel. As shown in Fig. 3, the recombinant IRF-1 protein formed a band, and the addition of Ab specific for the His tag of the recombinant IRF-1 altered the electrophoretic mobility of the band. The result shows that IRF-1 binds to the used promoter region of the IL-12 p40 gene.
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The position of the actual binding site was further determined by DNase
I footprinting assay (Fig. 5A). The -72 to -58 region
was protected from DNase I digestion, which demonstrates that this
region is the binding site of IRF-1 protein to the IL-12 p40 promoter.
Actual binding of IRF-1 to IL-12 p40 promoter of PECs in vivo was
demonstrated by ChIP assay. The anti-IRF-1 Ab precipitated the
IL-12 p40 promoter region from LPS plus IFN-
-stimulated PECs (Fig. 5B).
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| Discussion |
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(2, 18, 32). IL-12 p40 was shown to be one of the IFN-
-inducible
genes that are activated by a phosphorylated complex, GAF, which
consists of dimerized STAT1
and possibly another DNA binding protein
(19, 20). IRF-1 is one of the GAF-activated proteins and
has been shown to modulate the cellular response to IFN-
(19). IL-12 is a heterodimer composed of p40 and p35. The
former is induced by microbial infection, whereas the latter is known
to be constitutively expressed (2, 3). Although
IL-12 p40 gene induction requires costimulation with IFN-
, the IL-12
p35 gene is rapidly and strongly induced in PECs during bacterial
infection even in the absence of IFN-
(Y. Asano, unpublished
observation). Because signals from IFN-
receptor are transduced by
transcription factors, IRF-1 and p48, we assessed the requirement of
these transcription factors for IL-12 p40 and p35 genes expression in
the present study. We demonstrated that induction of IL-12 p40 and p35
genes is differentially regulated by IFN-
via IRF-1 but not by p48.
IL-12 p40 is IRF-1-dependent and IL-12 p35 is IRF-1-independent.
We analyzed the promoter region required for the regulation by the
transcription factor IRF-1. Stimulation of RAW cells transfected with
the -677 to +51 region of the IL-12 p40 gene resulted in the induction
of luciferase activity. This finding suggested that the transcription
factors induced by LPS and IFN-
bind to this DNA region, and it was
further confirmed by cotransfection experiments using NF-
B and IRF-1
expression plasmids. The cotransfection of CMIN-p65 and pACt-1 induced
the luciferase activity in a synergistic manner confirming that a LPS
and IFN-
responsive region is present in this promoter segment.
Although IL-12 p40 is barely induced in PECs by stimulation with
IFN-
alone, IRF-1 (pAct-1) augmented the -677 promoter activity of
IL-12 p40 in the absence of CMIN-p65 (Fig. 2B). This finding
suggests the possibility that the negative regulatory element might be
present in the upstream area of the promoter region. Because we found
that IRF-1 binds to the promoter region of the IL-12 p40 gene, we
further determined the binding site of IRF-1. We showed by an EMSA
using the DNA segment at -133 to -31 of the IL-12 p40 promoter region
that the important DNA segment for IRF-1 binding is present between
-72 to -58 of the 5'-upstream region of the IL-12 p40 gene
(Figs. 3 and 5). In addition, we found through the mutation experiment
that the DNA sequence at -63 to -61 of IL-12 p40 gene is the critical
site for binding of IRF-1 (Fig. 4). This is the first direct
demonstration of the binding site of IRF-1 in the promoter region of
the IL-12 p40 gene. The copresence of NF-
B and IRF-1 maximally
up-regulates the gene transcription, consistent with the observation of
in vitro induction of the gene by LPS plus IFN-
.
Although we demonstrated the direct binding of IRF-1 protein to the
promoter of the mouse IL-12 p40 gene in the present study, the
copresence of NF-
B (CMIN-p65) with IRF-1 (pAct-1) resulted in the
maximum augmentation of the promoter activity. It is demonstrated that
IRF-1 forms a complex with other nuclear factors during stimulation
with IFN-
or LPS and the complex up-regulates the promoter activity
of the human IL-12 p40 gene (33). However, the formation
of an IRF-1 complex does not always up-regulate promoter activity. We
determined in a previous study that an IRF-1-containing complex was
found in the nuclear extract of Plasmodium berghei-infected
PECs and the complex bound to the IRF-1-binding motif. The IL-12 p40
transcription and IL-12 p70 production was inhibited in these PECs
(34). This finding also suggests the possibility of the
presence of a negative regulatory component in the IL-12 p40 promoter
region.
| Acknowledgments |
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probe, animal
care, and recombinant mouse IL-12, respectively. We also thank Dr.
Tadatsugu Taniguchi for his warm support. | Footnotes |
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2 Current address: Division of Clinical Immunology and Rheumatology, Department of Medicine, University of Alabama, Birmingham, AL 35294. ![]()
3 Current address: Institute of Medical Sciences, University of Tokyo, Tokyo 108-8639, Japan. ![]()
4 Current address: Second Department of Medical Chemistry, Saitama Medical College, Saitama 350-0495, Japan. ![]()
5 Address correspondence and reprint requests to Dr. Yoshihiro Asano, Department of Immunology and Host Defenses, Ehime University School of Medicine, Shigenobu, Ehime 791-0295, Japan. E-mail address: asanoy{at}m.ehime-u.ac.jp ![]()
6 Abbreviations used in this paper: ISGF, IFN-stimulated gene factor; GAF,
-activated factor; IRF-1, IFN regulatory factor-1; PECs, peritoneal exudate cells; ChIP, chromatin immunoprecipitation. ![]()
Received for publication July 1, 2002. Accepted for publication November 5, 2002.
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