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*
Department of Pharmacology, University of Illinois, Chicago, IL 60612;
Friedrich Miescher Institute, Basel, Switzerland;
Howard Hughes Medical Institute, South Los Angeles, CA 90095;
Division of Signal Transduction, Beth Israel Deaconess Medical Center and Department of Cell Biology, Harvard Medical School, Boston, MA 02215; and
¶ Department of Molecular Microbiology and Immunology, St. Louis University, St. Louis, MO 63104
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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-/- embryonic fibroblasts
to EMCV-induced cell death is specifically corrected by overexpression
of an activated allele of Akt/protein kinase B, but not
activated mitogen-activated protein kinase extracellular kinase.
Conversely, the augmented sensitivity of p85-/- cells
to HSV-induced cell death was compensated for by expression of an
activated form of mitogen-activated protein kinase extracellular
kinase, but not by activated Akt/protein kinase B. We conclude from
these data that PI3K-activated pathways function in parallel with the
Janus kinase/STAT pathway to protect cells from the lethal effects of
viruses. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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, IFN-
, and IFN-
, while IFN-
is the only known type
II IFN (1, 2). Binding of type I and type II IFNs to their
distinct heterodimeric receptors (also referred to as IFN-R and
IFN-R) results in the activation of the Janus kinase
(Jak)3/STAT
pathway. In IFN- and IFN- pathways, tyrosine kinases
(Jak1/Tyk2 and Jak1/Jak2, respectively) become activated and
phosphorylate receptor chains. The phosphorylated tyrosines act as
docking sites for STAT factors via conserved SH2 domains. STATs
1 and 2 are activated by the type I IFN receptor and STAT1 is activated
by the type II IFN receptor. Jaks presumably phosphorylate the STATs,
which allows two STATs to homodimerize or heterodimerize by reciprocal
phosphotyrosine-SH2 domain interactions. Thus, type I IFNs induce
dimerization of STAT1 and STAT2, which, along with the p48 protein,
form the IFN-stimulated gene factor 3 complex, while IFN- induces
the -activated factor complex composed of STAT1 dimers. These
complexes translocate to the nucleus, bind to the IFN-stimulated
response element (ISRE) and IFN- activated sequence (GAS) motifs in
specific genes, and induce transcription of genes required to produce
an antiviral state (for reviews see Refs. 2, 3, 4) (Fig. 1
A).
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PI3K is probably the most intriguing due to its multiple functions.
PI3K is a dimer composed of adaptor and catalytic subunits and plays a
critical role in many cellular events such as cell survival,
proliferation, and adhesion (21, 22, 23, 24). Cytokines, IFNs, and
growth factors activate class 1 PI3K, which contains one of three forms
of the adaptor subunit that are encoded by three different genes:
p85, p85, and p55. The gene encodes adaptor subunits with
molecular masses of 85, 55, and 50 kDa (p85, p55, and
p50) (reviewed in Refs. 21, 22, 23 and 25).
The catalytic subunit also has several forms designated as p110,
p110, p110, and p110
. p85 and p85 can dimerize with
either p110 or p110. PI3K has both lipid and protein kinase
activity. The kinase generates lipid second messengers by
phosphorylating the 3 position on inositol rings to produce
PtdInsP(3),
PtdInsP2(3, 4), and
PtdInsP3 (3, 4, 5) (22, 26).
Several signaling pathways diverge downstream of activated PI3K,
including two critical in promoting cell survival, the Ser/Thr kinase
Akt/protein kinase B (PKB) (reviewed in Refs. 24 and
27), and extracellular regulated kinase (ERK)
(28). ERK is part of the p21ras
oncoprotein (Ras)-RAF-MEK-MAPK signaling pathway and mediates
both cell growth and survival. Akt/PKB is a Ser/Thr kinase with
important antiapoptotic functions. There are three isoforms of Akt/PKB
that are encoded by three different genes:
Akt1/PKB, Akt2/PKB, and
Akt3/PKB. Akt is composed of an amino-terminal pleckstrin
homology domain, a catalytic domain, and a 70-aa regulatory domain at
the C terminus. Akt/PKB is regulated by phosphorylation of conserved
threonine and serine residues. In Akt1/PKB, these residues are found
in the catalytic domain, T308, and regulatory domain, S473. Akt/PKB is
activated by the phospholipids produced by PI3K activity. These lipids
bind the pleckstrin homology domain of Akt/PKB and target the protein
to the cell membrane where 3'-phosphoinositide kinases 1 and 2
phosphorylate residues T308 and S473, thereby activating the kinase
domain. Activated Akt/PKB promotes cell survival in both the cytosol
and the nucleus in several ways. The proapoptotic protein BAD
and the Forkhead family of transcription factors are both targets of
Akt. Phosphorylation of these proteins by Akt impairs their apoptotic
functions. Akt also promotes cell survival through phosphorylation of
I
B kinases and caspase-9, and acts to stabilize the
mitochondria (reviewed in Ref. 27).
In this paper, we investigated the possible involvement of PI3K in the
cell survival activity induced by IFNs and serum against cell death
induced by viruses. Our data show that mouse embryonal fibroblast (MEF)
clones lacking the p85 adaptor subunit are more susceptible to EMCV-
and HSV-induced cell death. This effect is independent of the
activation of the STAT pathway, implying that PI3K and STAT pathways
function in parallel. Interestingly, overexpression of an activated
form of Akt (Aktm/p) in p85-/- cells was
able to rescue the impaired response to the RNA virus EMCV, but not to
HSV type 1 (HSV-1), a DNA virus. Conversely, expression of an activated
form of MEK (MEK*) conferred resistance to HSV-1 but not to EMCV. These
data suggest that HSV and EMCV very likely induce cell death through
different mechanisms that are inhibited by distinct pathways downstream
of PI3K. We propose that PI3K plays a central role in the regulation of
virus-induced cell death through different downstream pathways.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The pan-anti-p85 Ab was previously described
(29). The anti-Akt Ab was purchased from
Upstate Biotechnology (Lake Placid, NY). Anti-hemagglutinating Ag
(HA) and anti-myc were obtained from Boehringer
Mannheim (Indianapolis, IN). Recombinant murine (mu) IFN-4
and recombinant human IFN-AD were kindly provided by D. Gewert (QT
Genetics, U.K.) and M. Brunda (Hoffman-LaRoche, Nutley, NJ).
Recombinant muIFN- was purchased from Boehringer-Mannheim.
Stable transfectants
MEFp85+/+53.9 and
MEFp85-/-92.1 cells were cotransfected with
pCEP4 vector, which carries the hygromycin resistance gene, and the
HA-tagged activated Akt/PKB (Aktm/p), under the control of the CMV
promoter (30), or with myc-tagged MEK* under
the control of an inducible promoter (kindly provided by M. Roussel,
St. Jude Children’s Research Hospital, Memphis, TN)
(31). Clones were selected in 500 µg/ml hygromycin and
screened by Western blotting with the appropriate anti-tag
Ab.
Susceptibility to virus-induced cell death and protection by IFNs
Cells were seeded in 96-well plates at 15,000 cells/well in DMEM
supplemented with 2 (Fig. 1
B), 0.5, or 6% serum. The
low-serum experiments (0.5% FBS) were performed by seeding the cells
in DMEM supplemented with 10% FBS in 96-well plates (15,000
cells/well) and replacing the medium with the indicated concentration
of FBS after the cells were attached to the plates. Two-fold dilutions
were performed starting at 1/1,000 (multiplicity of infection (MOI)
0.5) for EMCV in a final volume of 100 µl. For experiments using
HSV-1, cells were infected at the indicated MOIs in DMEM containing
10% FBS. Cell viability was determined using MTT after 24 h for
EMCV or 3 and 6 days for HSV-1.
The induction of an antiviral response by IFNs was determined using a
standard antiviral assay (32).
MEFp85-/-92.1 and
MEFp85-/-92.3
(p85-/-.1 and
p85-/-.3) and
p85+/+53.9 (p85+/+)
were incubated with the indicated amounts of murine type I or II IFNs
(muIFN) for 18 h in DMEM containing 0.5% FBS. The medium was
removed and replaced with the lowest EMCV dilution that killed 100% of
the cells in 24 h (MOIs of 0.05 and 0.005–0.0025 for
p85+/+ and p85-/- cells,
respectively). Cell viability was also determined using MTT assay.
Similar results were observed with different muIFN preparations
(natural muIFN-, recombinant muIFN-4, and rIFN-AD),
although these preparations have different specific activities.
Viral replication
Cells were seeded in 24-well plates and infected at the indicated MOIs. The initial time point was 1 h to allow virus adsorption to the cells. Virus titer was determined by a standard plaque assay with Vero cells.
Immunoprecipitation, Western blots, and in vitro kinase assays
Cells were treated with 250 U/ml the indicated IFNs for 20 min.
For experiments with LY294002 (LY), cells were preincubated with 25
µM LY for 1 h before IFN treatment. Cells were lysed and
subjected to immunoprecipitation and Western blotting following
procedures described previously (33). For in vitro kinase
assays, immunoprecipitates were incubated with 2 µg of substrate and
2 µCi of [-32P]ATP in a final volume of 30
µl of kinase buffer (25 mM MgCl2, 25 mM
HEPES (pH 7.5), 2 mM DTT, 25 mM -glycerophosphate, and 0.1 mM
sodium orthovanadate). Myelin basic protein (MBP) and histone 2-B (H2B)
were used as substrates for ERK and Akt, respectively. After 20 min,
reactions were stopped by adding 4x SDS-loading buffer and
boiling.
Northern blotting
Cells were treated with the indicated IFNs for 20 h in the presence or absence of 25 µM of LY. Total RNA was extracted using the Tryazol kit, (Life Technologies, Gaithersburg, MD), and 20 µg were used for Northern blotting. [32P]dCTP-labeled STAT1, oligoadenylate synthetase gene (OAS), and actin were sequentially used for hybridization. The same preparation and concentration of IFNs used in these experiments failed to produce an IC50 in a standard antiviral assay.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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-/- cells have increased susceptibility to
EMCV
Two pathways critical in promoting cell survival diverge
downstream of activated PI3K, the Ser/Thr kinases Akt/PKB (reviewed in
Refs. 24 and 27) and ERK (28).
This potentially places PI3K in a central position with regard to
IFN-mediated inhibition of virus-induced cell death (Fig. 1A). If this premise is correct, then PI3K-deficient cells
should be more susceptible to virus-induced cell death. It was not
possible to develop MEF cell lines with a null mutation of the
PI3K-p110 catalytic subunit because
p110-/- mice died very early during
gestation (34). However, MEFs have been generated from
mice carrying a deletion of the forms of the adaptor subunit
(29). Thus, MEFp85-/- cells
were used to test the hypothesis that activation of PI3K by IFNs could
contribute to the antiviral effect by prolonging survival of
virus-infected cells.
We first determined the sensitivity of
MEFp85-/- and
MEFp85+/+ cells to the cytopathic effect of an
RNA virus, EMCV. Both cell lines were plated in 96-well plates in 2%
serum and treated with virus in 2-fold dilutions. Interestingly, the
MOI required to kill the majority of p85-/-
cells was 
100-fold lower than that of
p85+/+ cells (Fig. 1B), suggesting
that the activation of PI3K by growth factors present in serum plays a
role in resistance against EMCV-induced cell death.
We next determined the capacity of these cells to induce an antiviral
state in response to IFN- or IFN- under low-serum conditions
(0.5% FBS) to minimize PI3K activation by serum factors. Fig. 1
(C and D) shows that
p85-/- cells require more muIFN- or
muIFN- than wild type cells for protection against EMCV. This defect
in the antiviral response was always more pronounced for IFN-. These
results indicate that activation of PI3K by IFN- or IFN-
contributes to the antiviral effect.
Because the attenuated response to IFNs could correlate with low
expression of p85 in MEFp85-/- cell
lines, cell lysates were subjected to Western blotting with a pan-p85
Ab that recognizes both p85 and isoforms (Fig. 1C,
inset, and Ref. 29). We found that the p85
isoform, if present, is able to complement the
p85-/- mutation, resulting in a milder
defect or no alteration at all in the antiviral response (e.g.,
p85-/-.8 and data not shown). These data
strongly suggest that activation of PI3K plays a role in determining
sensitivity to EMCV-induced cell death and the induction of an
antiviral state by IFN- and IFN-.
The Jak/STAT pathway is not impaired in p85-/- cells
We next tested whether increased sensitivity of
p85-/- cells to EMCV corresponded to a
defect in the activation of the Jak/STAT pathway. This was conducted
under IFN concentrations that prevented viral-induced cell death in the
p85+/+, but not in
p85-/- cell lines. (The same preparation and
concentration of IFNs used in these experiments failed to produce
an IC50 in p85-/-
cells in a standard antiviral assay.) We initially assessed the
activation of the Jak/STAT pathway by studying the induction of
tyrosine phosphorylation of Jak1, STAT1, and STAT2 after treatment with
IFN- or IFN- for 15 min. Fig. 2
shows that muIFN- and muIFN- induced comparable levels of
tyrosine phosphorylation of Jak1, STAT1, and STAT2 in both
p85-/- and p85+/+
cells at IFN doses that showed marked impairment in the induction of
the antiviral response of p85-/- cells (Fig. 2, A–C, and data not shown). These results indicate
that the defect observed in p85-/- cells is
not due to a defect in the activation of the Jak/STAT pathway.
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-/- and
MEFp85+/+ cells. These experiments were also
conducted in cells pretreated with the PI3K inhibitor LY to determine
whether PI3K plays a role in the induction of specific genes by IFN-
and/or IFN-. Fig. 2D shows that IFN-stimulated gene
factor 3-driven induction of the OAS by IFN- and induction of
transcription of the STAT1 gene through the GAS element by IFN- and
IFN- were identical in p85-/- and
p85+/+ cells. Additionally, the data show that
preincubation with LY does not inhibit IFN-induced transcription. This
eliminates the possibility that PI3K-induced activation of Akt or MEK
influences Jak/STAT signaling. These results demonstrate that PI3K
contributes to the antiviral activity of IFN- and IFN-
independently of the Jak/STAT pathway, and works as a parallel,
STAT-independent mechanism. Akt and ERK are activated by IFNs through PI3K
Because PI3K signaling diverges into several pathways, we next
wanted to determine which pathways mediated an increased resistance to
virus-induced cell death. The primary targets for investigation were
Akt/PKB and ERK, because activation of these survival pathways could
increase resistance to virus-induced cell death. A previous report
indicated that the ERK pathway may be activated by IFN- and IFN-
(12), possibly through PI3K (35). To
determine whether Akt/PKB and ERK were activated by IFNs through PI3K,
MEFp85+/+ and
MEFp8-/- cells were treated with IFNs
and at 250 U/ml, and then in vitro kinase assays were performed
after immunoprecipitation with anti-Akt (Fig. 3A) or anti-ERK (Fig. 3B) Abs. Treatment with IFN- or IFN- induced an
increase in Akt/PKB activity, as seen by H2B phosphorylation, in
p85+/+ cells, but not in
MEFp85-/- cells (Fig. 3A).
Similarly, IFNs and produced an increase in MBP
phosphorylation, which is indicative of ERK activity, in
p85+/+ cells but not in
p85-/- cells (Fig. 3B).
These results confirm that activation of Akt/PKB and ERK by IFNs
requires functional PI3K (35, 36).
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To determine which PI3K downstream effector was responsible for
the increase in cell survival, we stably transfected constitutively
active forms of Akt/PKB (Akt/PKB m/p-membrane targeted) or MEK (MEK*)
into MEFp85+/+ and
MEFp85-/- cells to bypass the requirement of
PI3K for activation. Fig. 3C shows the expression of
HA-Aktm/p- and myc-MEK*-tagged constructs in different
clones. Immunoblotting with an anti-phospho-Akt shows Akt
activity in stably transfected cells, but not in cells transfected with
empty vector. Similarly, good levels of expression of MEK* were
detected in p85-/- and
p85+/+ cells.
We next tested the susceptibility of these cells to EMCV infection.
Surprisingly, overexpression of activated Akt/PKB not only overcame the
sensitivity to EMCV-induced cell death observed in
p85-/- cells, but also made them highly
resistant to this virus even under conditions in which activation of
PI3K by growth factors present in the serum was minimal (Fig. 4A, left panel). A
similar increase in resistance to EMCV was observed in
p85+/+ cells transfected with activated
Akt/PKB. This increase was not observed in cells transfected with
control vector (Fig. 4A, left panel) or activated
MEK. Activation of PI3K by factors present in a higher concentration of
serum (Fig. 4A, right panel) increased the
resistance to EMCV-induced cell death of control
MEFp85+/+ cells, and, to a lesser extent,
p85-/- cells transfected with the control
vector. This effect was also detected in nontransfected cells (Fig. 1B). These results suggest that activation of the
PI3K-Akt/PKB pathway is responsible for the resistance to virus-induced
cell death.
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The increased survival of cells expressing a constitutively active
form of Akt/PKB could be explained by a decrease in viral replication
through activation of the Jak/STAT pathway. We determined the titer of
EMCV in cultures 24 h after infection using two different MOIs
(0.043 and 0.0043 PFU/cell). Cell lines were infected with virus, then
medium was collected and subjected to a plaque assay. Almost identical
EMCV replication was observed in cells expressing activated Akt/PKB or
empty vector (Fig. 4B). Also, there were no differences in
the activation of the STAT pathway in cells expressing activated
Akt/PKB or control vector (data not shown). These data demonstrate that
the activation of the PI3K-Akt/PKB pathway delays or inhibits
EMCV-induced death without affecting viral replication or activation of
the Jak/STAT pathway.
Constitutively active MEK increases resistance to HSV-1
We also studied the sensitivity of wild type and
p85-/- cells to HSV-1 to test whether
resistance to virus-induced cell death is a characteristic specific to
EMCV or a general mechanism that may also apply to other viruses. The
p85+/+ cells were less susceptible to
HSV-1-induced cell death than were p85-/-
cells transfected with empty vector (Fig. 4C, -/-CMV.2).
Surprisingly, the defect in p85-/- cells was
not corrected by activated Akt (-/-Akt.1 and -/-Akt.4), but rather
by transfection of activated MEK (-/-MEK.9*). There were no
significant differences in HSV-1 replication between
p85-/- cells expressing activated MEK or
control vector (data not shown). The finding that a specific
antiapoptotic pathway is required for a distinct virus, i.e., Akt
protects against EMCV but not HSV-1, indicates that these mechanisms
are specific and not due to a general resistance to apoptosis due to
expression of an activated kinase. These data indicate that HSV-1 and
EMCV induce cell death by activation of different apoptotic pathways
that are inhibited by different survival pathways regulated by
PI3K.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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are more susceptible to virus-induced cell death.
Second, during the antiviral response, activation of PI3K by IFN- or
IFN- increases protection against viral agents. This is particularly
evident at lower doses of IFNs in cells lacking p85 (Fig. 1, C and D). Interestingly, the impaired antiviral
response in MEFp85-/- cells occurs in the
presence of normal STAT activation and gene induction (Fig. 2) and is
not inhibited by PI3K inhibitors. This strongly suggests that PI3K
functions in parallel to the STAT pathway, possibly by activating
specific survival pathways (see below). Therefore, we conclude that
there are two different components involved in the IFN-induced
antiviral state: activation of the STAT pathway that controls viral
replication and activation of PI3K that "helps" the antiviral
response by making cells more resistant to virus-induced cell death. In
this respect, the activation of PI3K by IFNs has the same role as the
activation by serum factors.
The STAT-independent contribution of PI3K is further demonstrated by
the finding that expression of activated downstream effectors of PI3K,
i.e., Akt and MEK, rescue the defect observed in
p85-/- cells by promoting cell survival
without affecting viral replication (Fig. 4B and data not
shown). Moreover, the survival role of PI3K in viral infections is
further underscored by two findings: 1) cells lacking p85 are more
susceptible to EMCV (Fig. 1B); and 2) conditions that
decrease the activation of PI3K, such as serum starvation, increase
susceptibility to EMCV-induced cell death (Fig. 4A). Thus,
activation of PI3K by IFNs during the antiviral response or factors
present in serum during viral infection play a role in fending off
virus-induced cell death.
The role of PI3K in antagonizing virus-induced cell death is not unique to a single virus class, as it can protect cells infected with RNA (EMCV) and DNA (HSV) viruses. However, the downstream effectors are different: ERK for HSV and Akt/PKB for EMCV. This suggests that HSV and EMCV probably induce cell death by activating different apoptotic pathways. This would explain why Akt increases cellular resistance to EMCV and MEK increases resistance to HSV-1. For example, it is possible that EMCV up-regulates BAD or FKHRs. The antiapoptotic effect of Akt/PKB could be through inactivation of these proapoptotic proteins (37). Similarly, the antiapoptotic effect of ERK could be mediated by phosphorylation of BAD by ribosomal S6 protein kinase as recently reported for neurons (38). Thus, PI3K is a central player in the control of virus-induced cell death through distinct downstream effectors.
These findings raise the following question: what is the role of PI3K in the normal antiviral response? It is possible that the PI3K pathway is important as a rapid mechanism against virus-induced cell death, giving the cell time to activate transcription of IFN-stimulated genes (ISGs) responsible for the antiviral effect of IFNs. The role of PI3K could be different in the cells initially infected by the virus and any neighboring cells. For instance, the activation of the PI3K pathway by growth factors could delay virus-induced cell death and maintain survival. This would give cells time to express and secrete IFNs to protect neighboring cells. In this scenario, paracrine IFN will induce transcription of ISGs, specific antiviral genes that will stop viral replication. In the noninfected cells, the survival mechanisms induced by IFN-driven activation of Akt/PKB and ERK could be important in fending off apoptotic pathways activated by the virus, even after induction of a full antiviral response. For example, the initially infected cells secrete IFNs that will activate the STAT pathway in noninfected neighboring cells, with the final result of stopping viral replication. However, if a given virus encodes or contains a protein that induces cell death, the activation of the STAT pathway in the healthy neighboring cells may not be able to stop this effect, particularly if high numbers of viral particles are released by the infected cells. It should also be considered that PI3K activity could help to restrict the spread of certain viruses that require cell death for release, because it has been postulated that viruses may induce apoptosis as a way of leaving the cell without eliciting an immune response.
Finally, it should be noted that previous reports indicated that
activation of ERK by IFN was required for induction of ISGs
(12). Contrary to this, the finding that neither cells
treated with the PI3K inhibitor LY nor
MEFp85-/- cells have a defect in the
induction of ISGs through the ISRE or GAS elements strongly suggests
that ERK activity is not necessary for full activation of the STAT
pathway or ISG induction. It is also worth noting that these results
confirm previous reports (35) which indicated that ERK is
activated downstream of PI3K. The exact mechanism of MAPK activation is
not understood. It may occur through a Ras-dependent or Ras-independent
mechanism (36).
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Acknowledgments |
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4,
respectively. We are also grateful to Andrew Larner for providing us
with anti-STAT1 and -STAT2 rabbit sera. |
Footnotes |
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2 Address correspondence and reprint requests to Dr. Oscar R. Colamonici, Department of Pharmacology, University of Illinois, 835 South Wolcott, Room E403 (M/C 868), Chicago, IL 60612. E-mail address: ocolamon{at}uic.edu
3 Abbreviations used in this paper: Jak, Janus kinase; ISRE, IFN-stimulated response element; GAS, IFN--activated sequence; EMCV, encephalomyocarditis virus; PtdIns, phosphatidylinositol; PI3K, PtdIns 3-kinase; PKB, protein kinase B; ERK, extracellular regulated kinase; MEF, mouse embryonal fibroblasts; MAPK, mitogen-activated protein kinase; MEK, MAPK extracellular kinase; MEK*, an activated form of MEK; HSV-1, HSV type 1; MOI, multiplicity of infection; MBP, myelin basic protein; H2B, histone 2-B; Aktm/p, an activated form of Akt; LY, LY294002; Ras, p21ras oncoprotein; OAS, oligoadenylate synthetase gene; ISG, IFN-stimulated genes; mu, murine; SH2, src homology domain 2; SHP, SH2 phosphatase; HA, hemagglutinating Ag.
Received for publication June 8, 2001. Accepted for publication August 16, 2001.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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induces rapid tyrosine phosphorylation of the vav proto-oncogene product in hematopoietic cells. J. Biol. Chem. 269:3143./ interferon-stimulated Jak/Stat pathway by the SH2 domain-containing tyrosine phosphatase SHPTP1. Mol. Cell. Biol. 15:7050.[Abstract]
- and interferon -stimulated gene expression through STAT proteins. Science 269:1721. or insulin stimulation. Blood 90:2574. engages the insulin receptor substrate-1 to associate with the phosphatidylinositol 3'-kinase. J. Biol. Chem. 270:15938./-induced gene expression. J. Biol. Chem. 271:15862. mediated phosphorylation of Stat3 in interferon receptor 1 (IFNAR1) but not for activation of phosphoinositol 3-kinase. J. Biol. Chem. 274:32507.. Science 283:393. subunit of the interferon receptor that is required for interferon signaling. J. Biol. Chem. 270:21606. chain of the type I interferon receptor are essential for signal transducer and activator of transcription (Stat) 2 activation. J. Biol. Chem. 274:4045. subunit of phosphoinositide 3-kinase. J. Biol. Chem. 274:10963.. J. Immunol. 158:2390.[Abstract]
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