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Journal of Virology, July 2000, p. 6203-6206, Vol. 74, No. 13
Department of Surgery1
and Department of Dermatology,3
University of Vienna Medical School, 1090 Vienna, Austria, and
Department of Microbiology, Mount Sinai Medical Center, New
York, New York 100292
Received 29 December 1999/Accepted 7 April 2000
The availability of an influenza virus NS1 gene knockout virus
(delNS1 virus) allowed us to establish the significance of the
biological relationship between the influenza virus NS1 protein and
double-stranded-RNA-activated protein kinase (PKR) in the life cycle
and pathogenicity of influenza virus. Our results show that the lack of
functional PKR permits the delNS1 virus to replicate in otherwise
nonpermissive hosts, suggesting that the major function of the
influenza virus NS1 protein is to counteract or prevent the
PKR-mediated antiviral response.
The alpha/beta interferon
(IFN- We recently generated a viable influenza A/PR/8/34 transfectant virus
that lacks the entire NS1 gene (termed the delNS1 virus). We showed
that the delNS1 virus only replicated efficiently in host systems
defective in IFN production or signaling but not in IFN-competent hosts
(6, 12). This result demonstrated that the NS1 protein of
influenza A virus is dispensable for viral growth in
interferon-deficient systems. It also suggested that NS1 protein is
a virulence factor that counteracts the interferon-mediated antiviral
response. In this study, we have investigated the role of PKR in the
IFN-mediated antiviral response in delNS1 virus-infected cells and
mice. Analysis of infected cell lysates revealed that PKR
phosphorylation was higher in delNS1 virus than in wild-type (wt)
influenza virus A/PR/8/34 (PR8)-infected cells, suggesting that the NS1
protein prevents activation of PKR. Cells not permissive for
delNS1 virus replication produced infectious particles when the
infected cells were incubated with 2-aminopurine (2-AP)
(15), a chemical inhibitor of PKR. To analyze the relevance
of these observations at an organismic level, we determined the
replication properties of delNS1 virus in mice devoid of PKR.
While the delNS1 virus failed to replicate in the lungs of wt
mice, it grew as efficiently as the PR8 wt virus in PKR knockout mice.
Previously, it was suggested that wt influenza virus is capable of
repressing PKR phosphorylation in infected cells (17). If
NS1 inhibits PKR activation, delNS1 virus-infected cells
should contain higher levels of the activated autophosphorylated
form of PKR than cells infected with the wt PR8 virus. Western blot analysis of cell extracts with a PKR-specific antibody allowed us to
differentiate between the activated and the nonactivated forms of PKR.
A 1:1 ratio of the two activation states was found between wt and
mock-infected W138 cell extracts. In contrast, the infection with the
delNS1 virus clearly shifted this balance to the activated form of PKR.
This shift, initiated by delNS1 virus infection, was almost as
pronounced as transfection of W138 cells with dsRNA, the established
activator of PKR (Fig. 1A). We further
demonstrated this increase of the activated form of PKR in
delNS1-infected cells by immunoprecipitation of virus-infected and
32P-labeled cell extracts. This assay selectively
detects the activated form of PKR. Again, infection with the
delNS1 virus correlated with a higher amount of activated PKR than did
mock or wt virus infection. The increase in the autophosphorylated form
of PKR by the delNS1 virus was approximately two- to threefold higher than the baseline level (Fig. 1B). It should be noted that we could not
do these assays at a high multiplicity of infection (MOI), since the
delNS1 virus has a slightly attenuated phenotype. Thus, it is likely
that the observed difference in PKR activation between delNS1 virus and
PKR is a low estimate. However, the data support the hypothesis that
the lack of the NS1 protein prevents the delNS1 virus from inhibiting
PKR activation. In turn, this suggests that activation of PKR is at
least partly responsible for the inability of the delNS1 virus to form
infectious particles in IFN-competent systems.
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Influenza Virus NS1 Protein Counteracts PKR-Mediated
Inhibition of Replication
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)-induced cellular antiviral response is the first line of
defense against a viral infection by the host (31). Major
antiviral effectors induced by IFN include Mx (29, 30, 32),
the 2'-5' oligoadenylate synthetase (2, 8), and the
double-stranded-RNA (dsRNA)-activated protein kinase (PKR)
(25). PKR is a serine/threonine protein kinase, which dimerizes and autophosphorylates upon activation (for reviews, see
references 3, 14, 19, and 35). The
activated form of PKR is capable of blocking protein synthesis through
its ability to phosphorylate the
subunit of eukaryotic translation
initiation factor 2 (eIF-2
). This mechanism inhibits viral
replication. To counteract the antiviral effects of IFN induction and
PKR activation, many eukaryotic viruses have developed strategies to
block the activity of PKR (for a review, see reference
11). In the case of influenza A virus, it is assumed
that the virus can repress PKR activity by two mechanisms. One of these
pathways is characterized by the virus recruiting P58IPK. This cellular
protein was suggested to inhibit PKR by binding directly to the kinase
(10, 23, 24). The second mechanism of PKR blockage during
influenza virus infection involves the viral nonstructural protein NS1
(henceforth NS1). This protein effectively blocks the dsRNA-mediated
activation of purified PKR and eIF-2
in vitro. Correspondingly, NS1
blocks the PKR-induced inhibition of translation in reticulocyte
lysates. It was therefore postulated that NS1 sequesters dsRNA from
activating PKR by binding to dsRNA (21). Other studies
suggested that PKR inhibition is also mediated by an RNA-independent
mechanism through the formation of complexes between the NS1 protein
and PKR (34). However, the direct interaction of NS1 and PKR
remains controversial (7). Interestingly, a
temperature-sensitive influenza A virus mutant with mutations in the
NS1 gene exhibited a defect in protein synthesis at the nonpermissive
temperature that correlated with an increased level of phosphorylated
PKR and eIF-2
(13).

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FIG. 1.
(A) Western blot of PKR in infected W138 cells. Cells
were mock treated, transfected with dsRNA, or infected with delNS1 or
PR8 virus at an MOI of 2. For dsRNA transfection, 50 µg of
poly(I)-poly(C) RNA was transfected using 30 µl of DOTAP transfection
reagent according to the manufacturer's protocol (Boehringer,
Mannheim, Germany). Twenty-four hours postinfection or
posttransfection, respectively, cells were lysed, and equivalent
amounts of cell extracts were subjected to sodium dodecyl
sulfate-polyacrylamide gel electrophoresis. PKR-specific bands were
detected by the PKR-specific antibody K-17 (Santa Cruz Biotechnology
catalogue no. sc 707; Santa Cruz, Calif.). The upper band corresponds
to phosphorylated (active) PKR. The lower band corresponds to
unphosphorylated (inactive) PKR (18). The two PKR bands are
indicated at the right. Lane 1, mock; lane 2, dsRNA; lane 3, delNS1
virus; lane 4, PR8 virus. (B) Immunoprecipitation of phosphorylated PKR
of infected HeLa cells. A total of 106 HeLa cells were mock
treated or infected with influenza delNS1 or PR8 virus at an MOI of
0.5. At 5 h postinfection, the cells were washed with a
phosphate-free buffer and incubated for 2 h in Dulbecco modified
Eagle medium lacking both phosphate and pyruvate (Sigma), containing
500 µCi of [32P]orthophosphate (Amersham). After being
labeled, the cells were washed twice with cold phosphate-buffered
saline and 10 mM EDTA (without Ca2+ and Mg2+)
and lysed for 10 min on ice in lysis buffer. One quarter of the extract
was used for immunoprecipitation carried out with 2 µg of PKR
antibody B-10 (Santa Cruz Biotechnology catalog no. sc 1215), per ml
followed by the addition of 30 µl of protein G-agarose (in a 50/50
ratio) at 4°C. The beads were washed according to the manufacturer's
protocol with wash buffer containing PBSTDS (Oncogene, Cambridge,
Mass.), heated for 2 min at 95°C and analyzed on a sodium dodecyl
sulfate-10% polyacrylamide gel electrophoresis gel. The size of the
bands was determined by a size marker (Benchmark, GIBCO-BRL). The bands
of phosphorylated PKR were visualized by autoradiography for 7 days and
quantified by laser densitometry. Lane 1, mock; lane 2, delNS1 virus;
lane 3, PR8 virus. The size marker is indicated at the left. The PKR
band is indicated at the right.
To further study this hypothesis, we attempted to rescue replication of delNS1 virus in nonpermissive cells by incubating the infected cells in the presence of 2-AP, an inhibitor of PKR and other serine/threonine protein kinases (15). In this experiment, we had to use a cell line that could withstand the required high doses of 2-AP. We therefore used the human melanoma cell line 518A2 (16), in which the PR8 virus grew to high titers of 5.3 log10 and which was not permissive for the delNS1 virus. Incubation of the infected cells in the presence of 5 mM 2-AP allowed the replication of the delNS1 virus to a titer of 2.6 log10 PFU/ml (data not shown). Although 2-AP is a nonspecific protein kinase inhibitor, this experiment supports the hypothesis that PKR is involved in blocking replication of the delNS1 virus in vivo.
To address the relevance of the antiviral effects of PKR for influenza
virus pathogenicity, we took advantage of the availability of PKR
knockout (PKR
/
) mice. These mice were derived from
C57BL/6 mice by the targeted deletion of PKR (36). C57BL/6
wt (PKR+/+) mice were obtained from Bomholtgard (Ry,
Denmark). It should be noted that in untreated PKR-defective mice, the
induction of IFN-
/
genes by virus is unimpaired and that
antiviral responses appear to be normal (1, 36).
Specifically, we analyzed the replication properties of delNS1 and PR8
viruses in mouse lungs. Figure 2 compares
the replication properties of delNS1 virus in wt and PKR knockout mice
to those of PR8 virus. The mean lung virus titers in wt mice after PR8
infection were 2.8 log10 PFU/ml on day 2, 4.5 log10 PFU/ml on day 4, and 4.1 log10 PFU/ml on
day 6 postinfection. After delNS1 infection, the virus titers in lung tissue were less than 50 PFU/ml at each time point analyzed. The lack
of detectable replication of delNS1 in wt mice supports the role of the
NS1 gene in influenza virus pathogenicity. The mean virus titers in
lung tissue in PKR
/
mice after delNS1 infection were
4.8 log10 PFU/ml on day 2, 5.1 log10 PFU/ml on
day 4, and 4.4 log10 PFU/ml on day 6 postinfection. These
titers were comparable to titers achieved in PKR
/
mice
after PR8 virus infection. The latter were 5.2 log10 at day
2, 5.0 log10 at day 4, and 4.3 log10 at day 6 postinfection. The fact that infectious delNS1 virus can be recovered
from the lungs of PKR
/
mice establishes the relevance
of the PKR protein in mediating an antiviral response against the
delNS1 virus. Since the lung virus titers in PKR
/
mice
were generally similar for PR8 and delNS1 viruses, this result also
suggests that one major function of NS1 protein in the influenza virus
life cycle is to counteract the PKR-mediated antiviral response.
|
Infection of wt mice by delNS1 virus did not cause weight loss or
symptoms of disease, such as ruffled fur. In contrast, approximately 30% weight loss was observed in delNS1 virus-infected
PKR
/
mice 6 days after infection (Fig.
3). Correspondingly, all the PKR
/
mice died due to delNS1 virus infection, whereas
the PKR+/+ mice survived challenge with this virus. These
results reflect the replication data in the mouse lungs and confirm the
importance of PKR in viral pathogenicity.
|
In order to gain information on the role of the PKR-mediated pathway in
the IFN-induced antiviral response against influenza virus, we compared
the virus titers in lung tissue of the delNS1 virus and PR8 wt virus
achieved in the PKR knockout mice to those obtained in STAT1 knockout
mice (5). STAT1 is necessary for any IFN-mediated signals,
including the transcriptional activation of the PKR gene. In the STAT1
knockout model, the mean virus titer on day 3 postinfection was
4.9 ± 0.3 log10 PFU/ml for the delNS1 virus
(n = 3) and 6.3 ± 0.3 log10 PFU/ml
for the PR8 virus (n = 3). In wt mice, the PR8 titers
were 5.0 ± 0.1 log10, and no infectious particles
were recovered after delNS1 virus infection (data not shown). The
data indicate that delNS1 virus replication reaches levels similar
to those obtained in the PKR
/
knockout mice. This
suggests that PKR is the major antiviral effector against influenza
virus in the IFN pathway in our system. However, the wt PR8 virus
levels are about 1 log unit higher in the STAT1
/
mice than in the PKR
/
mice, suggesting that
other IFN-activated genes may also play a role in the antiviral
host response. Since the IFN-inducible 2'-5' oligoadenylate synthetase
mRNA was shown to be induced in influenza virus-infected lung cells
(30), RNase L might be a likely candidate. Triply deficient
RNase L, PKR, and Mx knockout mice and cells which have been
recently described (37) should be helpful in
addressing this question. It should also be noted that C57BL/6 mice do
not express functional Mx, another IFN-induced antiviral gene
(32), which was shown to be a very potent antiviral defense. Thus, despite our data demonstrating the potency of PKR to
inhibit viral replication, an exclusive role of PKR in the IFN-mediated
pathway can certainly not be postulated.
Another important question is the role of the cellular PKR inhibitor
P58IPK in influenza virus pathogenicity. Although the ability of P58IPK
to inhibit PKR has been clearly shown in vitro and by overexpression in
cultured cells, its role in an animal model has never been tested. One
interpretation of our observations in the PKR knockout model in
connection with P58IPK is that either recruitment of P58IPK by
influenza virus plays a minor role in influenza virus pathogenicity in
vivo or the NS1 protein is responsible for recruiting P58IPK.
The replication properties of the delNS1 virus in the PKR
knockout model allow us also to speculate about the in vivo relevance
of other NS1 protein-associated functions established by in vitro
studies, such as inhibition of host mRNA polyadenylation, mRNA nuclear
export, and mRNA splicing (9, 20, 27, 28, 22). Since in the
PKR
/
mice the absence of NS1-associated functions did
not affect replication capacity in the mouse lungs, it appears that
these functions are unlikely to play a major role in influenza virus
replication in vivo. Alternatively, the functions ascribed to the NS1
may be redundant and could be taken over by other viral or cellular proteins.
A central role of PKR in the cellular antiviral defense strategies was also demonstrated during reovirus infection. Inhibition of PKR by 2-AP resulted in the growth of the virus in cell lines otherwise not permissive for reovirus replication. Interestingly, the growth of reovirus could also be observed when proteins of the ras signaling pathway, such as EGFR, v-erbB, sos, and ras were overexpressed in cell lines (33). This finding could be explained by the fact that activated ras induces an inhibitor of PKR (26). The potential of reovirus as an oncolytic virus specifically replicating in oncogenic ras-positive tumor cells could be confirmed in a mouse tumor model (4). Since the delNS1 virus has a phenotype similar to that of reovirus with respect to PKR-dependent growth, the delNS1 virus might also be an oncolytic virus that specifically eradicates tumors expressing oncogenic ras. Experiments to test this hypothesis are in progress.
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ACKNOWLEDGMENTS |
|---|
This work was supported by Austrian Science Fund grant MOB-12548 (T.M.), the Niarchos Foundation (K.W.), and grants from the National Institutes of Health to A.G.-S. and P.P.
PKR knockout mice were kindly provided by Charles Weissmann from the University of Zürich. We thank Reinhard Fleischhacker and Ingrid Romirer for technical assistance.
| |
FOOTNOTES |
|---|
* Corresponding author. Mailing address: Department of Dermatology, University of Vienna Medical School, 1090 Vienna, Austria. Phone: 43-1-40400-5441. Fax: 43-1-40400-7790. E-mail: Thomas.Muster{at}Akh-Wien.ac.at.
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