From the Immunobiology Center, The Mount Sinai School of Medicine, New York, New York 10029
Received for publication, December 19, 2002, and in revised form, February 4, 2003
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ABSTRACT |
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Type I interferon (IFN) signaling induces
the heterotrimeric transcription complex, IFN-stimulated gene factor
(ISGF) 3, which contains STAT1, STAT2, and the DNA binding subunit,
interferon regulatory factor (IRF) 9. Because IRF9 is targeted to the
nucleus in the absence of IFN stimulation, the potential of IRF9
protein for gene regulation was examined using a GAL4 DNA binding
domain fusion system. GAL4-IRF9 was transcriptionally active in
reporter gene assays but not in the absence of cellular STAT1 and
STAT2. However, the inert IRF9 protein was readily converted to a
constitutively active ISGF3-like activator by fusion with the
C-terminal transcriptional activation domain of STAT2 or the acidic
activation domain of herpesvirus VP16. The IRF9 hybrids are targeted to
endogenous ISGF3 target loci and can activate their transcription.
Moreover, expression of the IRF9-STAT2 fusion can recapitulate the type I IFN biological response, producing a cellular antiviral state that
protects cells from virus-induced cytopathic effects and inhibits virus
replication. The antiviral state generated by regulated IRF9-STAT2 hybrid protein expression is independent of autocrine IFN
signaling and inhibits both RNA and DNA viruses.
Type I interferons (IFN Most of the immediate actions of IFN can be attributed to the activity
of an IFN-inducible transcriptional regulator, the IFN-stimulated gene
factor 3 (ISGF3), and its IFN-stimulated gene (ISG) targets. As a
trimeric complex, the ISGF3 transcription factor is mechanistically
unique compared with other STAT-dependent transcription
pathways. The cytoplasmic tyrosine kinase-based signaling events
leading to STAT protein activation and STAT1:STAT2 dimerization are
dissociable from the nuclear transcription functions of ISGF3 that
require obligatory interactions between STATs and interferon regulatory
factor 9 (IRF9). IRF9 is essential as the DNA binding component of
ISGF3 that binds to consensus IFN-stimulated response elements (ISRE)
and provides specific protein interaction sites for the IFN-activated
STAT1 and STAT2 proteins (9-11). In the nucleus, the modular STAT2
C-terminal transcriptional activation domain (TAD) provides the
essential signals for the induction of target gene transcription
(12).
Although it is generally accepted that IRF9 functions as an adaptor
molecule for tethering a STAT1:STAT2 dimer to the ISRE DNA element, an
emerging body of evidence suggests non-ISGF3 roles for IRF9 in the
regulation of gene expression. For instance, the human IP-10 gene
requires an IRF9·STAT1 complex for its activation by IFN Cell Culture and Transfection--
Human fibrosarcoma 2fTGH,
2fTGH-derived cell lines U3A (STAT1-deficient) and U6A
(STAT2-deficient), 293T, CV-1, Vero, and the 293 Tet-On (purchased from
Clontech) were adapted to and maintained in
Dulbecco's modified Eagle's medium supplemented with 10%
Cosmic calf serum (Hyclone). Transfection of cells with cDNAs was
carried out by standard CaPO4 procedures (16).
Plasmids--
GAL4-IRF9 and GAL4-S1C were constructed by
insertion of PCR-amplified ORF into the GAL4-DBD plasmid pSG424 (17).
Inserts for the IRF9-S1C, IRF9-S2C, and IRF9-VP16 hybrid cDNAs were
created by standard four-primer PCR technique (18). Briefly, the first step involves generation of two PCR fragments encompassing full-length IRF9 and the STAT2 104 C-terminal amino acids, the 38 C-terminal amino
acids of STAT1, or the 80 C-terminal amino acids of the VP16 TAD (17).
These products are then combined for use as templates for a second PCR
reaction, with external 5' and 3' primers to create IRF9-S1C, IRF9-S2C,
or IRF9-VP16 hybrid inserts with BamHI and NotI
restriction sites. Primers used were as follows: T IRF9, 5'-CCCGGATCCCCGCCATGGCATCAGGCAGGGCACGC-3'; S2 REV 3',
5'-GGGGCGGCCGCCTAGAAGTCAGAAGGCATC-3'; IRF9-S2C T,
5'-GCCATTCTGTCCCTGGTGGGGCCAGAGCTAGAGTCT-3'; IRF9-S2C B,
5'-AGACTCTAGCTCTGGCCCCACCAGGGACAGAATGGC-3'; IRF9-VP16 T,
5'-GCCATTCTGTCCCTGGTGTCGACGGCCCCCCCCA-3'; IRF9-VP16 B,
5'-TGGGGGGGGCCGTCGACACCAGGGACAGAATGGC-3'; IRF9-S1C T,
5'-GCCATTCTGTCCCTGGTGGAAGTTCACCCTTCTAGA-3'; IRF9-S1C B,
5'-TCTAGAAGGGTGAACTTCCACCAGGGACAGAATGGC-3'; S1C-REV,
5'-GGGGCGGCCGCTATACTGTGTTCATCATACT-3'. The IRF9-TAD hybrid insert
was subcloned into pCDNA3 (Invitrogen). Inserts for IRF9 and
IRF9-STAT TADs were also subcloned into the tetracycline-regulated plasmid pBI (Clontech). All constructs were
verified by DNA sequencing.
Reporter Gene Assays--
For detection of GAL4 constructs in a
reporter gene assay, 293T, U3A, and U6A cells were transiently
transfected with either vector alone or GAL4-IRF9 fusion constructs,
along with a reporter gene containing five GAL4 upstream activation
sequence elements fused to the luciferase gene (17). For
detection of IRF9-STAT hybrid protein activity, 293T cells were
transiently co-transfected with expression vectors encoding hybrid
proteins and a reporter gene plasmid containing five copies of the
ISG54 ISRE element upstream of a TATA box and firefly luciferase
ORF. Luciferase assays were carried out following the manufacturer's
instructions (Promega) and were normalized to Protein Immunoblot Assays--
Antibodies against IRF9 (C-20)
were obtained from Santa Cruz Biotechnology, Inc. and used according to
the manufacturer's instructions. Total protein extracts were prepared
in whole cell extract buffer (50 mM Tris, pH 8.0, 280 mM NaCl, 0.5% IGEPAL, 0.2 mM EDTA, 2 mM EGTA, 10% glycerol, 1 mM dithiothreitol)
supplemented with a protease inhibitor mixture (Complete; Roche
Molecular Biochemicals). Proteins were separated by SDS-PAGE and
transferred to nitrocellulose filters, immunoblotted by standard
procedures, and prepared for chemiluminescent detection according to
the manufacturer's protocol (Renaissance; PerkinElmer Life Sciences).
RNA Analysis--
Total RNA was prepared by using Trizol reagent
(Invitrogen), digested with DNase I, and subjected to reverse
transcriptase with SuperScript II RNase H Antiviral Assays--
Antiviral responses measured by cytopathic
effect (CPE) assay were conducted as reported previously (19, 20), with
some modifications. First, to create stably transfected cell lines, 293 Tet-On cells (Clontech) were transfected with
tetracycline-regulated pBI plasmids (Clontech)
encoding the IRF9, IRF9-S1C, or IRF9-S2C ORFs. Individual clones were
then selected and then probed by anti-IRF9 immunoblotting analyses for
regulated expression. To determine the extent of protection from
viral-induced CPE, cells were first treated with 1 µg/ml doxycycline
(Dox) for 24 h, challenged with virus infection for 18 h, and
then photographed. Antiviral assays comparing the biologic activity of
IRF9-S2C and IFN with simian virus 5 (SV5; W3A strain), type II human
parainfluenza virus (HPIV2; Greer strain), and VSV infection were
performed as follows: 293T cells carrying the Tet-induced IRF9-S2C
transgene were treated with Dox or IFN for 24 h. Cells were then
washed with serum-free medium, infected with virus at a
multiplicity of 1.0 and/or 0.1 pfu/cell for 2 h, washed with
serum-free medium, and cultured for an additional 48 h (18 h for VSV) in the presence or absence of Dox or IFN in Dulbecco's
modified Eagle's medium with 2% serum. For SV5 antiviral assays
performed in the presence of IFN neutralizing antibodies, cells were
pre-treated with Dox in the presence or absence of 400 neutralizing
units of anti-IFN Transcriptional Activity of IRF9--
A heterologous DNA binding
domain fusion system was used to test the intrinsic transcriptional
capacity of IRF9. Hybrid cDNAs were constructed that fuse the yeast
GAL4-DBD to IRF9, and the resulting fusion proteins were assayed
for the ability to activate transcription of a GAL4-responsive
luciferase reporter gene (17, 21). No reporter gene activity was
observed when the GAL4- DBD alone was expressed, but fusion of the
full-length IRF9 to GAL4 resulted in a fusion protein that was capable
of stimulating transcription 5-10-fold greater than the GAL4-DBD alone
(Fig. 1A). As a positive control, the activity of GAL4-S1C, a fusion with the well characterized 38-amino acid TAD of STAT1 (21-23), was assayed in parallel. GAL4-S1C activated transcription over 100-fold in this system, indicating that
the intrinsic activity of IRF9 was relatively weak (Fig. 1A). To determine the specific IRF9 amino acids responsible
for this modest transcriptional activity, smaller GAL4-IRF9 fusions were engineered and expressed (Fig. 1B). The N-terminal
portion that encompasses the DNA binding and nuclear localization
domain of IRF9 (amino acids 1-200) was transcriptionally inactive as a
GAL4 fusion, and the central region of the molecule (amino acids 100-300) produced only a minor stimulation of the reporter gene above
basal levels. The C-terminal IRF9 domain (amino acids 200-393) was the
only region to exhibit significant transcriptional activity, giving
rise to reporter gene activity that was even greater than the
full-length IRF9 fusion (Fig. 1B). These data indicate that the IRF9 transcriptional activity localizes to its C-terminal half,
previously determined to be the STAT1 and STAT2 binding region
(10).
Role of STAT1 and STAT2 in IRF9 Transcriptional
Activity--
A current model of ISGF3 signaling assumes that
IRF9 is a STAT and DNA binding adaptor protein linking transcriptional
activation to ISRE DNA elements, but this hypothesis has not been
tested rigorously. Because the transcriptional activity of IRF9
localized to the STAT protein interaction region, the role of STAT1 and STAT2 proteins in activation by GAL4-IRF9 was tested in sister cell
lines deficient in one of the IFN-activated STATs and thereby unresponsive to IFN (24, 25). In the absence of STAT1 (U3A cells),
GAL4-IRF9 has a low constitutive level of transcriptional activity, as
observed in the STAT-intact cells, that does not change upon IFN IRF9-TAD Fusions Activate ISRE Transcription--
The primary
transcription activation function of ISGF3 has been attributed to
contributions from the STATs, especially the STAT2 C terminus.
Deletions from this region of STAT2 abrogate the transcriptional
activity of ISGF3 (26), and fusion of the STAT2 C terminus to STAT1 or
GAL4-DBD is sufficient to generate fusion proteins that activate
transcription (23, 27, 28). The constitutive nuclear import of IRF9,
coupled with its lack of transcriptional activity in the absence of
STATs, suggests that one role of IRF9 might be to target STAT
activation domains to the IFN-responsive promoter, ensuring rapid and
specific activation. To test this concept, the ability of IRF9 to
target STAT transcriptional activation domains to ISGF3-responsive
promoters was examined. A hybrid cDNA expression vector was created
that expressed IRF9 fused to the ISGF3 transcriptional activation
domain that resides in the STAT2 C terminus. The C-terminal 104 amino
acids of STAT2 were fused to the IRF9 ORF to create IRF9-S2C (Fig.
2A). The hybrid protein was
expressed in 293T cells and subjected to ISRE-dependent luciferase assays. Expression of IRF9 protein alone resulted in weak
reporter gene activity, consistent with the results observed in the
GAL4-IRF9 system (Fig. 1). The IRF9-S2C fusion, however, was found to
strongly activate ISRE-dependent reporter gene
transcription in the absence of IFN stimulation, resulting in
~50-fold stimulation of reporter gene activity (Fig. 2A,
right panel). The high level of activity of IRF9-S2C was not
altered by stimulation with IFN
To determine whether the activity of the IRF9-S2C chimeric activator
requires endogenous STAT1 or STAT2, similar ISRE-dependent luciferase reporter gene assays were performed in STAT1-deficient U3A
cells, STAT2-deficient U6A cells, or parental 2fTGH cells (Fig.
2B). As a control, a second hybrid protein fusing the IRF9 ORF to the STAT1
To determine the activity of IRF9-S2C at endogenous target loci, the
ability of the IRF9-S2C hybrid to regulate the expression of endogenous
ISGs was determined by RT-PCR analysis. Cells were treated with IFN
The finding that IRF9-S2C exhibited intrinsic ISGF3-like
transcriptional activity but IRF9-S1C did not (Fig. 2B) was
unexpected in view of the established role for the STAT1 C terminus in
IFN IRF9-S2C Recapitulates IFN Biological Responses--
By the tested
criteria, IRF9-TAD fusion proteins duplicate molecular aspects of ISGF3
function. To test the biological activity of this hybrid factor in IFN
response assays, vectors for inducible expression of the IRF9 fusions
were constructed. Tetracycline-regulated vectors with IRF9 and IRF9-S2C
open reading frames were used to create mammalian cell lines that
inducibly express the transgene following stimulation with the
tetracycline analog, Dox. Several independent clones were isolated in
293 Tet-On cells that demonstrated similar levels of Dox-induced
transgene expression (Fig. 3A,
right panel). The inducible cell lines were tested for
resistance to virus-induced cytopathic effects and the ability to
suppress virus replication. Cells were assayed for resistance to VSV by
standard methodology (19, 20). The cell lines were susceptible to
VSV-induced CPE, causing death of infected cell monolayers, and no
nonspecific antiviral effects were observed in Dox-treated control
cells or IRF9 cells (Fig. 3A, left panels).
Induction of transgene expression with Dox did not alter the
susceptibility of the cells to VSV-induced CPE when IRF9 was the
transgene, but cells expressing IRF9-S2C were greatly protected from
viral CPE (Fig. 3A).
As VSV is well known to be susceptible to the IFN-induced antiviral
state, the ability of the IRF9-S2C transgene to block the replication
of an IFN-resistant virus was also tested. SV5, a member of the
Rubulavirus genus of the Paramyxoviridae, evades the
IFN signaling response by targeting STAT1 for ubiquitylation and
proteasomal degradation (32-35). Significantly, SV5 replication is
also inhibited by IRF9-S2C induction (see Fig. 3B and Table I). The antiviral response to RNA virus
infection relies in part on autocrine and paracrine signaling through
newly synthesized IFN. Virus infection can activate the synthesis and
secretion of IFN
To directly and quantitatively compare the activity of IRF9-S2C to
exogenous IFN treatment, assays were carried out in which Dox and IFN
treatments were tested for antiviral activity. The induction of
ISRE-luciferase activity was used to calibrate Dox stimulation of
IRF9-S2C with the ISGF3 transcriptional response to several IFN doses
(Table I). Cells were transfected with the reporter gene and treated
with 10 µg/ml Dox or 10, 100, or 1000 units/ml IFN prior to reporter
gene assays. Results indicate that expression of IRF9-S2C induced by
Dox carries a transcriptional capacity resulting in 87% of the
reporter activity induced by stimulation of cells with 1000 units/ml
IFN (Table I). As 100 units/ml of IFN only induced 56% of the
luciferase activity, the capacity of IRF9-S2C in this particular
inducible system is determined to lie between the ISGF3 activity
induced by 100 and 1000 units/ml IFN. The antiviral responses induced
by these same treatments were evaluated for the test viruses, VSV, SV5,
and HPIV2. For VSV, 6.0 × 107 pfu/ml were recovered
from control, unstimulated cells, but only 5.0 × 106
pfu/ml were collected following Dox induction of IRF9-S2C (Table I).
For SV5, 2.0 × 107 pfu/ml were recovered from control
cells, but only 1.1 × 105 pfu/ml were recovered from
Dox pre-treated cells. Similarly, HPIV2 titers decreased from 7.3 × 105 to 9.2 × 102 pfu/ml after Dox
treatment. For all the viruses tested, Dox treatment resulted in a
replication inhibition level similar to that obtained following
treatment with between 100 and 1000 units/ml IFN. Thus, both
transcriptional response assays and biological antiviral response
assays indicate that in this expression system, IRF9-S2C can induce
ISGF3-like responses similar to native IFN treatment and can afford
antiviral protection against several RNA viruses.
The pleiotropic IFN response is effective against a broad range of
viruses. To expand our observations beyond RNA viruses, the ability of
IRF9-S2C to block a DNA virus, the herpesvirus, HSV-1, was also tested.
Control and Dox-treated IRF9-S2C Tet-On cells were infected with HSV-1.
Virus-containing supernatants were harvested an additional 24 h
later and titered by plaque assay in Vero cells. A dramatic inhibition
of HSV-1 replication was observed in cells upon IRF9-S2C hybrid protein
expression, reducing recovered viral titers from 2 × 108 to 2.6 × 105 pfu/ml (Table I). These
results indicate that augmenting the cellular IFN response by IRF9-S2C
expression can inhibit the replication of diverse virus species.
Because HSV-1 has several strategies for evading IFN responses
(38-42), the ability to inhibit its replication is a powerful
demonstration that the IRF9-TAD fusion strategy may overcome intrinsic
anti-IFN strategies encoded by viruses.
We have investigated the transcriptional capacity of the IRF9
component of the IFN-activated ISGF3 transcription factor. Results from
studying GAL4-IRF9 fusion proteins demonstrate that IRF9 lacks the
intrinsic ability to induce transcriptional responses but can
participate in both basal and IFN-activated transcription through its
C-terminal domain through association with either STAT1 or STAT2. Basal
transcription by the GAL4-IRF9 fusion is dependent on its ability to
associate with cellular STAT2 protein, reflecting the strong
association between STAT2 and IRF9 demonstrated in vitro and
in vivo (10, 11). In cells lacking STAT2, GAL4-IRF9 no
longer induces basal activity but can still activate transcription in
response to IFN In contrast to the weak transcriptional activity of IRF9, a hybrid
IRF9-STAT2 TAD fusion protein was a strong constitutive stimulator of
ISRE-dependent transcription, inducing potent responses both from transfected reporter genes and from endogenous IFN-stimulated loci. The IRF9-S2C hybrid protein can induce potent and specific ISRE-directed transcriptional activity even in the absence of IFN
signaling molecules such as STAT1 and STAT2. The unanticipated inactivity of the STAT1 TAD in this system may reflect the known minor
biological role of the STAT1 TAD in native ISGF3 transcriptional responses (24), because either the transcriptionally active STAT1 Biological response assays in a regulated expression system revealed
that the IRF9-S2C fusion protein can imitate the ability of IFN to
induce an antiviral state that inhibits the replication of VSV, SV5,
HPIV2, and HSV-1. Direct comparisons demonstrate that both the
transcriptional responses and antiviral activity induced by the fusion
protein are equivalent to the level of IFN signaling responses induced
by 100 to 1000 units/ml of IFN. Currently, IFN has been clinically
approved in the treatment of a limited number of virus infections (1,
2), in part because of direct or indirect IFN inhibition mechanisms
evolved by the viruses. The results presented here demonstrated that
the IRF9-S2C fusion protein can counteract both RNA and DNA viruses
that are known to be IFN-resistant and can also achieve high levels of
ISRE-directed transcription in the absence of IFN signaling proteins.
Experiments testing the efficacy of IRF9-S2C against individual viruses
will be needed on a case-by-case basis, but given the observed general imitation of ISGF3 activity and biological responses, it is likely to
be an effective inhibitor of a wide variety of viruses and may be of
use for the future development of gene therapy approaches for viral diseases.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
and IFN
, referred to here
collectively as IFN)1 provide
the first wave of innate immune defense against virus infection and can
also modulate the adaptive immune response (1, 2). Several studies have
examined the use of IFN as a therapeutic agent, either by direct
administration or by gene therapy-based IFN gene expression approaches
(3-8). Despite the high potential as a pharmacologic agent for both
infectious diseases and cancer, a variety of shortcomings have limited
the widespread use of IFN, including the inability to obtain clinically
relevant concentrations in vivo, the short half-life of IFN
in circulation, limited access of exogenous IFN to the site of
infection, and the evolution of viruses that can antagonize enzymes
involved in the antiviral response. Novel strategies for activation of
IFN biological responses by targeting transcriptional regulators to
specific cells might circumvent these problems and ultimately prove to
be an effective therapeutic option.
(13). IRF9 is efficiently targeted to the nucleus via an intrinsic
bipartite localization signal that functions in the absence of IFN
signaling (14), and ectopic expression of IRF9 confers resistance to
anti-microtubule agents in breast cancer cell lines independent of IFN
signals (15). Interestingly, IRF9 mRNA levels are increased in half
of the breast and uterine cancer samples tested (15), suggesting a
potential role for this factor in cancer biology. Therefore, the
potential of IRF9 to act as a direct transcriptional regulator
independent of its role as the ISGF3 DNA binding adaptor was examined.
Results demonstrate that whereas IRF9 lacks intrinsic transcriptional
capacity, IRF9-TAD fusion proteins effectively mimic the activity of
the trimeric ISGF3 complex. The IRF9 hybrids function in the absence of
endogenous IFN signaling and provide a novel strategy for enhancing
endogenous antiviral responses.
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
-galactosidase
activity derived from 1 µg of co-transfected CMV-lacZ. Unless
otherwise noted, all graphs represent average values for a triplicate
experiment ± S.D.
Reverse
Transcriptase (Invitrogen) and PCR analysis as described elsewhere
(10). A mock reaction was carried out with no reverse transcriptase
added (
RT). One-tenth of the resulting cDNA product was used as
template for 25 cycles of PCR in the presence of
[
-32P]dATP (PerkinElmer Life Sciences) using specific
primers for ISG15, ISG54, or glyceraldehyde-3-phosphate dehydrogenase.
As a control for genomic DNA contamination, PCR was carried out with glyceraldehyde-3-phosphate dehydrogenase primers using the mock (
RT)
reaction products as templates. Following gel electrophoresis, products
were detected by autoradiography. For RT-PCR analysis of ISGs induced
by IRF9-VP16, RNA isolation and cDNA synthesis were performed as
above, but PCR was performed using a 32-cycle reaction, and the product
was run on an agarose gel and visualized using ethidium bromide
staining. Primer sequences were as follows: ISG54A,
5'-AATGCCATTTCACCTGGAACTTG-3'; ISG54B,
5'-GTGATAGTAGACCCAGGCATAGT-3'; ISG15A,
5'-CAACGAATTCCAAAGGTCGGAGTCAAC-3'; ISG15B, 5'-CCCTTGTTATTCCTCACC-3'; glyceraldehyde-3-phosphate dehydrogenase A, 5'-GTGAAGGTCGGAGTCAAC-3'; glyceraldehyde-3-phosphate dehydrogenase B,
5'-TGGAATTTGCCATGGGTG-3'.
and anti-IFN
antibodies (PBL Biomedical
Laboratories), a concentration sufficient to prevent the antiviral
activity of exogenous IFN (data not shown). Cells were infected as
above and then cultured for 48 h in the continued presence of
either Dox or IFN and the anti-IFN antibodies as indicated.
Supernatants were then titered in plaque assays using simian CV-1 cells
with an overlay containing 0.5% agar with Dulbecco's modified
Eagle's medium and 10 mM HEPES, pH 7.2. The monolayer was
fixed in 3.7% formaldehyde and stained with 0.1% crystal violet
(Sigma) dissolved in 20% EtOH. Anti-viral assay using herpes simplex
virus (HSV-1; gift from John Blaho) was performed similarly, except
cells were infected at a multiplicity of 5 pfu/cell. Supernatants were
harvested from infected Dox-inducible IRF9-S2C cell lines 24 h
after infection and titered on Vero cells. No antiviral effects or
virus replication inhibition were observed in Dox-treated control cells
or IRF9 cells.
RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
Analysis of IRF9 transcriptional
capacity. A, transcriptional activity was determined
for GAL4-DBD (V), GAL4-IRF9 (IRF9), and GAL4-S1C
(S1C) in reporter gene assays in 293T cells. B,
IRF9 residues 1-200, 100-300, and 200-393 fused to GAL4 were
examined for reporter gene activation. C, control GAL4-DBD
and GAL4-IRF9 were assayed in U3A cells or U6A cells, either without
treatment (UNT) or following 6 h of treatment with 5 ng/ml IFN . D, GAL4-IRF9 is transcriptionally inert in the
absence of STAT1 and STAT2. Reporter gene assays were carried out in
U6A cells with co-expression of vector control, wild-type STAT1, or two
dominant negative forms of STAT1 (STAT1
and STAT1 Y701F). Luciferase
values were read from untreated (
) cells or cells treated with IFN
or IFN
(1000 units/ml) for 6 h.
stimulation (Fig. 1C). In contrast, in the absence of STAT2
(U6A cells), no basal activity was observed, but IFN
stimulation
activated the reporter gene (Fig. 1C). This result suggests
that endogenous STAT2 may be responsible for the basal activity of
GAL4-IRF9, even in the absence of STAT1, and that a STAT2·GAL4-IRF9
complex can shuttle between the cytoplasm and the nucleus. The action
of IFN
on IRF9-dependent transcription is not the result
of direct stimulatory effects, as no additional IFN
induction was
observed in the absence of STAT1 (U3A cells). As STAT1 dimers form in
response to IFN
in STAT2-deficient cells, this result suggests that
STAT1 is responsible for the activity of GAL4-IRF9 in the
IFN
-treated cells. To test this hypothesis, the reporter gene assay
was carried out in the absence of the both STAT1 and STAT2 by using
STAT2-deficient cells (U6A) and dominant negative STAT1 mutants to
block the activity of the remaining endogenous STAT1 (Fig.
1D). Expression in STAT2-deficient U6A cells of dominant
negative forms of STAT1 that either fail to activate and dimerize
(STAT1 Y701F) or lack transcriptional activity (STAT1
) results in a
situation where neither STAT1 nor STAT2 is functional. GAL4-IRF9 failed
to activate the reporter gene in this virtual null cellular
environment, in support of the conclusion that association of GAL4-IRF9
with endogenous STAT2 is required for basal transcription, but
association with STAT1 is responsible for the observed IFN
-inducible
transcription. Together, the results with the GAL4-IRF9 system indicate
that IRF9 lacks intrinsic transcriptional activity, providing
experimental evidence that supports a model where IRF9 functions to
integrate STAT1 or STAT2 transcriptional activation signals to the ISRE
element on target promoters.
, but a small increase was observed
following IFN
stimulation, consistent with contributions by
endogenous IFN
-activated STAT1 dimers interacting with IRF9 to
heighten transcriptional responses (29, 30). These results indicate
that recruitment of the STAT2 TAD to the ISRE DNA element is sufficient
for transcriptional activity.
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Fig. 2.
IRF9-TAD hybrid proteins are constitutive
activators of ISRE-dependent transcription and targeted to
endogenous ISG loci. A, left panel,
anti-IRF9 Western blot of protein extracts shows the apparent molecular
mass of the IRF9-S2C fusion protein to be ~70-kDa. The hybrid
protein is depicted schematically below the autoradiogram.
Numbers on top indicate the amino acid positions
of IRF9, and numbers on the bottom indicate the
amino acids of STAT2. Right panel, luciferase activity of
empty vector (V), IRF9, or IRF9-S2C expression vectors in
293T cells using an ISRE reporter gene. Luciferase activity was also
measured for IRF9-S2C transfected cells that were treated with IFN
or IFN
for 6 h. B, endogenous STAT proteins are not
needed for IRF9-STAT hybrid protein transcriptional activity. 2fTGH,
U3A, and U6A cells were transfected with vector control (V),
IRF9, IRF9-S1C, and IRF9-S2C expression vectors, and luciferase
activity was determined using the co-transfected ISRE reporter gene.
C, activity of IRF9-S2C in endogenous ISG regulation. 293T
cells were transfected with vector (V) or IRF9 and IRF9-S2C
constructs as indicated. At 48 h post-transfection, total RNA was
prepared, reverse-transcribed, and subjected to PCR with ISG54- and
ISG15-specific or control primers. Untransfected cells were treated
with IFN
or IFN
for 6 h prior to RNA preparation.
D, activity of IRF9-VP16. Left panel, 293T cells
were transfected with the ISRE-luciferase reporter and IRF9, IRF9-S1C,
IRF9-S2C, or IRF9-VP16 vectors and analyzed as above. Right
panel, endogenous mRNA analysis from 293T cells transiently
transfected with IRF9-VP16 vector or with empty vector
(V). Total RNA was isolated and processed for RT-PCR
analysis at 12 and 24 h post-transfection.
C-terminal 38 amino acids was also constructed (to
create IRF9-S1C) and included in these assays. Although the cell-specific transfection efficiency and protein accumulation levels
varied between the tested cell lines, resulting in distinct absolute
activation values, all of the IRF9 expression vectors behaved
similarly, regardless of endogenous STAT1 or STAT2 deficiency; IRF9 and
the IRF9-S1C TAD fusion protein only weakly stimulated reporter gene
activity, but the IRF9-S2C TAD fusion was highly active in all cell
lines (Fig. 2B).
or IFN
for 6 h or transfected with IRF9 or IRF9-S2C expression
vectors. RNA was prepared and processed for RT-PCR assays to examine
the accumulation of ISG15 and ISG54, two genes known to be regulated
transcriptionally by ISGF3. For both ISGs, treatment with IFN
, but
not IFN
, resulted in a strong mRNA induction (Fig.
2C). In the absence of IFN stimulation, transfection of the
IRF9-S2C fusion protein resulted in a similar increase in mRNA
abundance for these two ISGs. A small stimulation of the ISGs was
observed for expression of IRF9 that is consistent with earlier
conclusions regarding the transcriptional activity of IRF9 and may
represent contributions from endogenous STAT proteins. These data
indicate that the IRF9-STAT fusion is active on chromatinized endogenous loci that are normally targets of ISGF3. Based on these and
previous studies (26-28), we conclude that a primary and essential TAD
for ISGF3 resides within the C-terminal 104 amino acids of STAT2.
Furthermore, targeting this minimal STAT2 domain to endogenous promoters is sufficient to mediate ISG transcription.
-dependent transcription (reviewed in Ref. 12). To
determine whether the STAT2 TAD is unique in its ability to activate
IRF9, a distinct heterologous TAD was tested in this system. The well
characterized acidic activation domain of herpesvirus VP16 protein was
fused to IRF9 to create IRF9-VP16. Expression of this construct in 293T cells resulted in strong ISRE-directed reporter gene activity (Fig.
2D), ~3-fold higher than observed with IRF9-S2C, in
agreement with results published for a similar comparison using a GAL4
system (23, 31). The activity of IRF9-VP16 was also tested for ability to induce endogenous ISG transcription. Consistent with the reporter gene assays, expression of IRF9-VP16 was able to induce transcription of both ISG15 and ISG54 (Fig. 2D).
View larger version (41K):
[in a new window]
Fig. 3.
IRF9-S2C expression induces an antiviral
state independent of autocrine IFN. A, left
panel, cytopathic effect in Tet-regulated IRF9 and IRF9-S2C cell
lines upon challenge with VSV infection (multiplicity = 5 pfu/cell). Infected cells were untreated ( Dox) or
pre-treated (+Dox) to regulate transgene expression.
Right panel, comparison of transgene expression in parallel
samples by anti-IRF9 immunoblot. ns, nonspecific protein.
B, antiviral state induced by IRF9-S2C is independent of
autocrine IFN. Dox-regulated IRF9-S2C cells were untreated
(
Dox) or treated with Dox (+Dox) for 24 h
before infection in the presence (+) or absence (
) of IFN
neutralizing antibodies (IFN Abs). Cells were infected with
0.1 pfu of SV5 per cell and cultured in the continued presence or
absence of antibodies for 48 h prior to titration of supernatants
on CV-1 cells.
and IFN
, which, in turn, can activate ISGF3
signaling in the infected cell, as well as adjacent cells, to amplify
antiviral responses (36, 37). To determine whether the mechanism of protection provided by the IRF9-S2C expression relies on IFN
production, antiviral assays for SV5 were performed in the continued
presence of IFN-neutralizing antibodies (Fig. 3B).
Pretreatment with Dox to express the IRF9-S2C transgene for 24 h
before infection produced a cellular antiviral state, and addition of
IFN-neutralizing antibodies beginning at the time of Dox treatment did
not significantly alter the IRF9-S2C-induced antiviral state. These
results indicate that the antiviral effects are because of IRF9-S2C
transcriptional activity rather than autocrine/paracrine signaling
downstream of induced IFN synthesis.
Comparison of IRF9-S2C and interferon stimulation
DISCUSSION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
stimulation, by association with IFN
-activated STAT1 dimers. Independent STAT1-IRF9 contacts are in general
agreement with earlier studies of STAT1-IRF9 interaction
carried out in vitro and in vivo (10, 29, 30).
These results collectively support a model wherein IRF9 serves as a DNA
binding adapter for IFN-activated ISGF3 that must recruit the STAT
protein transcriptional activation domains for functionality.
or
its splice variant STAT1
(lacking the C-terminal 38-amino acid TAD)
can be used interchangeably for ISGF3 signaling (24, 25). The
expression of IRF9-VP16 also efficiently activated ISRE-dependent transcription, and the observed hierarchy
for IRF9-TAD constructs (IRF9-VP16 > IRF9-S2C > IRF9-S1C)
indicates that any number of other heterologous activation domains may
also serve to activate IRF9, enabling the generation of IFN mimetics
with varying degrees of transcriptional potency. This arrangement is in
general agreement with a previous analysis of STAT TADs, where VP16 TAD
was more active than STAT2 TAD, which was found to be a more potent
activator than STAT1 (23). Despite its increased transcriptional
potency, IRF9-VP16 is equally effective as IRF9-S2C in SV5 antiviral
assays.2 Gene expression
profiling comparing IFN stimulation with transgene expression will
better determine the specificity of these IRF9-TAD fusions for
activating individual antiviral genes.
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ACKNOWLEDGEMENTS |
---|
We are grateful to many colleagues for providing reagents and expertise for these studies. Initial experiments with IRF9 fusions were conducted in part by Adam L. Stone as part of the Mount Sinai Summer Undergraduate Research Program. VP16 TAD constructs and good advice were provided by Aseem Ansari (University of Wisconsin), and GAL4-Stat1C construct was provided by Jillian Zhang (Weill Cornell Medical School). We thank Griffith Parks (Wake Forest University) for providing HPIV2 and SV5, John Blaho (Mount Sinai) for providing HSV-1, and George Stark (Lerner Institute) for 2fTGH and daughter cell lines. We also thank members of the Horvath laboratory, Marcus Kretzschmar, Lloyd Mayer, and Tom Moran, for helpful discussions and reading this manuscript.
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FOOTNOTES |
---|
* This work was supported in part by American Cancer Society Research Scholar Award RSG-02-049-01 and National Institutes of Health Grant AI48722 (to C. M. H.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Contributed equally to this work.
§ Supported by Fellowship PDF0100731 from the Susan G. Komen Breast Cancer Foundation.
¶ To whom correspondence should be addressed: Box 1630, East Bldg., Rm. 12-20D, One Gustave L. Levy Pl., New York, NY 10029. Tel.: 212-659-9406; Fax: 212-849-2525; E-mail: curt.horvath@mssm.edu.
Published, JBC Papers in Press, February 5, 2003, DOI 10.1074/jbc.M212972200
2 T. A. Kraus and C. M. Horvath, unpublished observations.
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ABBREVIATIONS |
---|
The abbreviations used are: IFN, interferon; ISG, IFN-stimulated gene; ISGF, ISG factor; STAT, signal transducers and activators of transcription; IRF, interferon regulatory factor; ISRE, IFN-stimulated response elements; TAD, transcriptional activation domain; ORF, open reading frame; DBD, DNA binding domain; RT, reverse transcriptase; CPE, cytopathic effect; Dox, doxycycline; SV, simian virus; HPIV, human parainfluenza virus; VSV, vesicular stomatitis virus; pfu, plaque-forming unit; HSV, herpes simplex virus.
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