The C-terminal activation domain of the STAT-1 transcription factor is necessary and sufficient for stress-induced apoptosis

iTRAQ protein profile analysis of neuroblastoma (NA) cells infected with the rabies viruses rHep-Flury and Hep-dG

The rabies virus (RABV) glycoprotein (G) is the principal contributor to the pathogenicity and protective immunity of RABV. In a previous work, we reported that recombinant rabies virus Hep-dG, which was generated by reverse genetics to carry two copies of the G-gene, showed lower virulence than the parental virus rHep-Flury in suckling mice with a better immune protection effect. To better understand the mechanisms underlying rabies virus attenuation and the role of glycoprotein G, isobaric tags for relative and absolute quantitation (iTRAQ) was performed to identify and quantify distinct proteins. 10 and 111 differentially expressed proteins were obtained in rHep-Flury and Hep-dG infection groups, respectively. Selected data were validated by western blot and qRT-PCR. Bioinformatics analysis of the distinct protein suggested that glycoprotein over-expression in the attenuated RABV strain can induce activation of the interferon signaling. Furthermore, it may promote the antiviral response, MHC-I mediated antigen-specific T cell immune response, apoptosis and autophagy in an IFN-dependent manner. These findings might not only improve the understanding of the dynamics of RABV and host interaction, but also help understand the mechanisms underlying innate and adaptive immunity during RABV infection.

CDK8 as the STAT1 serine 727 kinase?

Whereas cytokine-induced tyrosine phosphorylation of STAT (signal transducer and activator of transcription) proteins by JAK kinases has been well studied, much less is known about STAT-specific serine kinases and their signal-dependent regulation. The paper by Joanna Bancerek and colleagues published recently in Immunity reports that upon interferon-γ (IFNγ) stimulation of cells the chromatin-associated cyclin-dependent kinase 8 (CDK8) phosphorylates the regulatory serine residue 727 in the transactivation domain of STAT1. The authors state that the CDK8 module of the Mediator complex is a key component in the STAT1 signal pathway, linking serine phosphorylation to gene-specific transcriptional events.

Signal transducer and activator of transcription-1 localizes to the mitochondria and modulates mitophagy

The signal transducer and activator of transcription (STAT) proteins are latent transcription factors that have been shown to be involved in cell proliferation, development, apoptosis, and autophagy. STAT proteins undergo activation by phosphorylation at tyrosine 701 and serine 727 where they translocate to the nucleus to regulate gene expression. STAT1 has been shown to be involved in promoting apoptotic cell death in response to cardiac ischemia/reperfusion and has recently been shown by our laboratory to be involved in negatively regulating autophagy. These processes are thought to promote cell death and restrict cell survival leading to the generation of an infarct. Here we present data that shows STAT1 localizes to the mitochondria and co-immunoprecipitates with LC3. Furthermore, electron microscopy studies also reveal mitochondria from ex vivo I/R treated hearts of STAT1KO mice contained within a double membrane autophagosome indicating that STAT1 may be involved in negatively regulating mitophagy. This is the first description of STAT1 being localized to the mitochondria and also having a role in mitophagy.

IFN Regulatory Factor-2 Regulates Macrophage Apoptosis through a STAT1/3- and Caspase-1-Dependent Mechanism

IFN regulatory factor (IRF)-2(-/-) mice are significantly more resistant to LPS challenge than wild-type littermates, and this was correlated with increased numbers of apoptotic Kupffer cells. To assess the generality of this observation, and to understand the role of IRF-2 in apoptosis, responses of peritoneal macrophages from IRF-2(+/+) and IRF-2(-/-) mice to apoptotic stimuli, including the fungal metabolite, gliotoxin, were compared. IRF-2(-/-) macrophages exhibited a consistently higher incidence of apoptosis that failed to correlate with caspase-3/7 activity. Using microarray gene expression profiling of liver RNA samples derived from IRF-2(+/+) and IRF-2(-/-) mice treated with saline or LPS, we identified >40 genes that were significantly down-regulated in IRF-2(-/-) mice, including Stat3, which has been reported to regulate apoptosis. Compared with IRF-2(+/+) macrophages, STAT3alpha mRNA was up-regulated constitutively or after gliotoxin treatment of IRF-2(-/-) macrophages, whereas STAT3beta mRNA was down-regulated. Phospho-Y705-STAT3, phospho-S727-STAT1, and phospho-p38 protein levels were also significantly higher in IRF-2(-/-) than control macrophages. Activation of the STAT signaling pathway has been shown to elicit expression of CASP1 and apoptosis. IRF-2(-/-) macrophages exhibited increased basal and gliotoxin-induced caspase-1 mRNA expression and enhanced caspase-1 activity. Pharmacologic inhibition of STAT3 and caspase-1 abolished gliotoxin-induced apoptosis in IRF-2(-/-) macrophages. A novel IFN-stimulated response element, identified within the murine promoter of Casp1, was determined to be functional by EMSA and supershift analysis. Collectively, these data support the hypothesis that IRF-2 acts as a transcriptional repressor of Casp1, and that the absence of IRF-2 renders macrophages more sensitive to apoptotic stimuli in a caspase-1-dependent process.

STAT1 as a key modulator of cell death

Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors that mediate various biological responses, including cell proliferation, survival, apoptosis, and differentiation. Among the members of the STAT family, accumulating evidence now indicates an important role for STAT1 in various forms of cell death. Depending upon stimuli or cell types, STAT1 can modulate a broad spectrum of cell death, comprising both apoptotic and non-apoptotic pathways. STAT1-dependent regulation of cell death is largely dependent on a transcriptional mechanism such as the activation of death-promoting genes. However, non-transcriptional mechanisms such as STAT1 interaction with TRADD, p53, or HDAC have been implicated in the regulation of cell death by STAT1. Furthermore, STAT1 itself is also subject to complex forms of regulation such as post-translational protein modification, which can critically affect STAT1 signaling and STAT1-dependent cell death. Given the reports showing that dysregulation of STAT1 signaling is associated with various pathological conditions, including the development of cancer, a better understanding of the mechanism underlying STAT1 regulation of cell death may lead to successful strategies for targeting STAT1 in such pathological settings.

STAT1 regulates p73-mediated Bax gene expression

Although signal transducer and activator of transcription 1 (STAT1) mediated regulation of p53 transcription and apoptosis has been previously reported, modulation of other members of the p53 family of transcription factors remains poorly understood. In this study, we found that STAT1 and TA-p73 can interact directly and that p73-mediated Bax promoter activity was observed to be reduced by STAT1 expression in a p53-independent manner for which STAT1 Tyrosine-701 and Serine-727 are key residues. This study presents the first report physically linking STAT1 and TA-p73 signalling and highlights the modulation of the Bax promoter in the context of IFN-gamma stimulation.

Aggravation of ischemic brain injury by prion protein deficiency: Role of ERK-1/-2 and STAT-1

The cellular isoform of prion protein, PrPc, may confer neuroprotection in the brain, according to recent studies. To elucidate the role of PrPc in stroke pathology, we subjected PrPc-knockout (Prnp(0/0)), wild-type and PrPc-transgenic (tga20) mice to 30 min of intraluminal middle cerebral artery occlusion, followed by 3, 24 or 72 h reperfusion, and examined how PrPc levels influence brain injury and cell signaling. In immunohistochemical experiments and Western blots, we show that PrPc expression is absent in the brains of Prnp(0/0) mice, detectable in wild-type controls and approximately 4.0-fold elevated in tga20 mice. We provide evidence that PrPc deficiency increases infarct size by approximately 200%, while transgenic PrPc restores tissue viability, albeit not above levels in wild-type animals. To elucidate the mechanisms underlying Prnp(0/0)-induced injury, we performed Western blots, which revealed increased activities of ERK-1/-2, STAT-1 and caspase-3 in ischemic brains of Prnp(0/0)mice. Our data suggest a role of cytosolic signaling pathways in Prnp(0/0)-induced cell death.

Essential role of STAT1 in caspase-independent cell death of activated macrophages through the p38 mitogen-activated protein kinase/STAT1/reactive oxygen species pathway

Unlike other immune cells, activation of macrophages by stimulating agents, such as lipopolysaccharide (LPS), confers significant resistance to many apoptotic stimuli, but the underlying mechanism of this phenomenon remains largely unknown. Here, we demonstrate that LPS-induced early caspase activation is essential for macrophage survival because blocking caspase activation with a pancaspase inhibitor (zVAD [benzyloxycarbonyl-Val-Ala-Asp]) rapidly induced death of activated macrophages. This type of death process by zVAD/LPS was principally mediated by intracellular generation of superoxide. STAT1 knockout macrophages demonstrated profoundly decreased superoxide production and were resistant to treatment with zVAD/LPS, indicating the crucial involvement of STAT1 in macrophage death by zVAD/LPS. STAT1 level and activity were reciprocally regulated by caspase activation and were associated with cell death. Activation of STAT1 was critically dependent upon serine phosphorylation induced by p38 mitogen-activated protein kinase (MAPK) because a p38 MAPK inhibitor nullified STAT1 serine phosphorylation, reactive oxygen species (ROS) production, and macrophage death by zVAD/LPS. Conversely, p38 MAPK activation was dependent upon superoxide and was also nullified in STAT1 knockout macrophages, probably due to impaired generation of superoxide. Our findings collectively indicate that STAT1 signaling modulates intracellular oxidative stress in activated macrophages through a positive-feedback mechanism involving the p38 MAPK/STAT1/ROS pathway, which is interrupted by caspase activation. Furthermore, our study may provide significant insights in regards to the unanticipated critical role of STAT1 in the caspase-independent death pathway.

Opposing actions of STAT-1 and STAT-3

The signal transducers and activators of transcription (STATs) are a family of transcription factors, which were originally identified on the basis of their ability to transduce a signal from a cellular receptor into the nucleus and modulate the transcription of specific genes. Interestingly, recent studies have demonstrated that STAT-1 plays a key role in promoting apoptosis in a variety of cell types, whereas STAT-3 has an anti-apoptotic effect. Moreover, whilst STAT-3 promotes cellular proliferation and is activated in a variety of tumour cells, STAT-1 appears to have an anti-proliferative effect. Although the initially characterised signal transduction events mediated by STAT-1 and STAT-3 involve the DNA binding and transcriptional activation domains of the factor, some of their other effects appear not to require DNA binding. Therefore, STAT-1 and STAT-3 can mediate the regulation of gene transcription both by direct DNA binding and via a co-activator mechanism and despite their very similar structures, have antagonistic effects on cellular proliferation and apoptosis.

STAT-1 facilitates the ATM activated checkpoint pathway following DNA damage

STAT-1 plays a role in mediating stress responses to various stimuli and has also been implied to be a tumour suppressor. Here, we report that STAT-1-deficient cells have defects both in intra-S-phase and G2-M checkpoints in response to DNA damage. Interestingly, STAT-1-deficient cells showed reduced Chk2 phosphorylation on threonine 68 (Chk2-T68) following DNA damage, suggesting that STAT-1 might function in the ATM-Chk2 pathway. Moreover, the defects in Chk2-T68 phosphorylation in STAT-1-deficient cells also correlated with reduced degradation of Cdc25A compared with STAT-1-expressing cells after DNA damage. We also show that STAT-1 is required for ATM-dependent phosphorylation of NBS1 and p53 but not for BRCA1 or H2AX phosphorylation following DNA damage. Expression levels of BRCT mediator/adaptor proteins MDC1 and 53BP1, which are required for ATM-mediated pathways, are reduced in cells lacking STAT-1. Enforced expression of MDC1 into STAT-1-deficient cells restored ATM-mediated phosphorylation of downstream substrates. These results imply that STAT-1 plays a crucial role in the DNA-damage-response by regulating the expression of 53BP1 and MDC1, factors known to be important for mediating ATM-dependent checkpoint pathways.

STAT1: a modulator of chemotherapy-induced apoptosis

The anthracyclines, such as doxorubicin, are widely used in the treatment of breast cancer. Previously, we showed that these drugs could activate the transcription factor, nuclear factor kappaB, in a DNA damage-dependent manner. We now show that these drugs can potentiate the activation of signal transducer and activator of transcription 1 (STAT1) in MDA-MB 435 breast cancer cells treated with IFN-gamma. We observed that key markers of STAT1 activation, including tyrosine 701 and serine 727 phosphorylation, were enhanced in the presence of doxorubicin. This potentiation resulted in enhanced nuclear localization of activated STAT1 and led to an increase in the nuclear binding of activated STAT complexes. The observed potentiation was specific for STAT1 and IFN-gamma, as no effects were observed with either STAT3 or STAT5. Furthermore, the type I IFNs (alpha and beta) had little or no effect. The observed effects on STAT1 phosphorylation have previously been linked with maximal transcriptional activation and apoptosis. Cell viability was assessed by crystal violet staining followed by analysis with CalcuSyn to determine combination index values, a measure of synergy. We confirmed that significant synergy existed between IFN-gamma and doxorubicin (combination index = 0.34) at doses lower than IC(50) values for this drug (0.67 micromol/L). In support of this, we observed that apoptotic cell death was also enhanced by measuring poly(ADP-ribose) polymerase and caspase-3 cleavage. Finally, suppression of STAT1 expression by small-interfering RNA resulted in a loss of synergistic apoptotic cell death compared with cells, where no suppression of STAT1 expression was attained with scrambled small-interfering RNA control. We conclude that doxorubicin potentiates STAT1 activation in response to IFN-gamma, and that this combination results in enhanced apoptosis in breast cancer cells.

Gene expression profiling after treatment with the histone deacetylase inhibitor trichostatin A reveals altered expression of both pro- and anti-apoptotic genes in pancreatic adenocarcinoma cells

The histone deacetylase inhibitor trichostatin A (TSA) has been previously shown to block cellular growth in G2 and induce apoptosis in human pancreatic cancer cell lines. In order to better understand this phenomenon, we have analyzed the gene expression profiles in PaCa44 cells after treatment with TSA using microarrays containing 22,283 probesets. TSA was found to cause both the induction and repression of a large number of genes, although the number whose expression was up-regulated was greater than the number of genes that were down-regulated. When a threshold value of 3 was used as a cutoff level, a total of 306 (3.4%) of the detectable genes had altered expression. When categorized according to cellular function, the differentially expressed genes were found to be involved in a wide variety of cellular processes, including cell proliferation, signaling, regulation of transcription, and apoptosis. Moreover, Sp1/Sp3 transcription factor binding sites were significantly more abundant among TSA-induced genes. One prominent feature was the increased ratio between the levels of expression of pro-apoptotic (BIM) and anti-apoptotic (Bcl-XL and Bcl-W) genes. This result was confirmed in eight additional pancreatic cancer cell lines after treatment with TSA, suggesting that this event may be a strong determinant for the induction of apoptosis by TSA.

Protein Kinase Cδ Regulates Apoptosis via Activation of STAT1

Protein kinase Cdelta (PKCdelta) is required for mitochondria-dependent apoptosis; however, little is known about downstream effectors of PKCdelta in apoptotic cells. Here we show that activation of STAT1 is an early response to DNA damage and that STAT1 activation requires PKCdelta. Treatment of HeLa cells with etoposide results in phosphorylation of STAT1 on Ser(727) and the association of STAT1 with PKCdelta. Etoposide increases transcription from STAT1-dependent reporter constructs. Increased transcription, as well as STAT1 Ser(727) phosphorylation, can be blocked by inhibition or depletion of PKCdelta. To ask if STAT1 is required for PKCdelta-mediated apoptosis, we utilized U3A STAT1-deficient cells. Induction of apoptosis by PKCdelta is suppressed in U3A cells but can be rescued by co-transfection with STAT1alpha but not STAT1 mutated at Ser(727). Nuclear accumulation of STAT1, phospho-Ser(727) STAT1, and PKCdelta are detectable 30-60 min after treatment with etoposide. Nuclear localization is necessary for apoptosis, since a nuclear localization mutant of PKCdelta does not induce apoptosis in U3A cells reconstituted with STAT1alpha, and a nuclear localization mutant of STAT1 does not support PKCdelta-induced apoptosis in U3A cells. Our data identify STAT1 as a downstream target of PKCdelta and suggest that PKCdelta may regulate apoptosis by activation of STAT1 target genes.

Soluble receptor (DcR3) and cellular inhibitor of apoptosis-2 (cIAP-2) protect human cytotrophoblast cells against LIGHT-mediated apoptosis

LIGHT (tumor necrosis factor superfamily 14) is among the powerful apoptosis-inducing cytokines synthesized in human placentas. Here, we investigated mechanisms protecting cytotrophoblast (CTB) cells from LIGHT-mediated apoptosis. Viability assays and caspase-3 immunoblots using recombinant LIGHT were done to establish that CTB cells purified from term placentas resist LIGHT-induced apoptosis. Although the cells were also resistant to killing by another placental cytokine, interferon-gamma (IFN-gamma), a combination of the two induced apoptosis. Killing was prevented by DcR3-Fc fragment but not control human-Fc fragment, showing that apoptosis occurs via the LIGHT pathway and that soluble receptors provide protection. Next, two cellular inhibitors of apoptosis expressed in CTB cells, cellular inhibitor of apoptosis (cIAP)-1 and cIAP-2, were investigated for protection. Cellular IAP-1 was unchanged after stimulation with LIGHT whereas cIAP-2 mRNA and protein were elevated. The increase was abrogated by treating CTB cells with LIGHT + IFN-gamma, implying a central role for cIAP-2 in preventing LIGHT-mediated apoptosis and an ability of IFN-gamma to overcome cIAP-2 protection. Definitive evidence was provided in experiments that showed that cIAP-2 anti-sense morpholinos permit LIGHT to induce apoptosis in HT-29 cells. In summary, the data are consistent with the postulate that placental CTB cells are protected from LIGHT-mediated apoptosis by both soluble receptor, DcR3, and cIAP-2.

Analysis of Stat3 (signal transducer and activator of transcription 3) dimerization by fluorescence resonance energy transfer in living cells

Signal transducer and activator of transcription 3 (Stat3) dimerization is commonly thought to be triggered by its tyrosine phosphorylation in response to interleukin-6 (IL-6) or other cytokines. Accumulating evidence from in vitro studies, however, suggests that cytoplasmic Stat3 may be associated with high-molecular-mass protein complexes and/or dimerize prior to its activation. To directly study Stat3 dimerization and subcellular localization upon cytokine stimulation, we used live-cell fluorescence spectroscopy and imaging microscopy combined with fluorescence resonance energy transfer (FRET). Stat3 fusion proteins with spectral variants of green fluorescent protein (GFP), cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) were constructed and expressed in human hepatoma cells (HepG2) and human embryonic kidney cells (HEK-293). Like wild-type Stat3, the fusion proteins redistributed from a preferentially cytoplasmic to nuclear localization upon IL-6 stimulation and supported IL-6-dependent target gene expression. FRET studies in cells co-expressing Stat3-CFP and Stat3-YFP demonstrated that Stat3 dimers exist in the absence of tyrosine phosphorylation. IL-6 induced a 2-fold increase of this basal FRET signal, indicating that tyrosine phosphorylation either increases the dimer/monomer ratio of Stat3 or induces a conformational change of the dimer yielding a higher FRET efficiency. Studies using a mutated Stat3 with a non-functional src-homology 2 (SH2) domain showed that the SH2 domain is essential for dimer formation of phosphorylated as well as non-phosphorylated Stat3. Furthermore, our data show that visualization of normalized FRET signals allow insights into the spatiotemporal dynamics of Stat3 signal transduction.

MR image-guided investigation of regional signal transducers and activators of transcription-1 activation in a rat model of focal cerebral ischemia

BACKGROUND AND PURPOSE

STAT-1 is a member of a family of proteins called signal transducers and activators of transcription (STATs), and recent studies have shown its involvement in the induction of apoptosis. There is limited information on the role of STAT-1 following stroke. In this study we use MRI measurements of cerebral perfusion and bioenergetic status to target measurements of regional STAT-1 activity.

METHODS

Rats were subjected to 60 or 90 min of middle cerebral artery occlusion with and without reperfusion. MRI maps of the apparent diffusion coefficient of water and cerebral blood flow were acquired throughout the study. After the ischemia or reperfusion period, the brain was excised and samples were analyzed by Western blots using anti-phospho-STAT1 and anti-Fas antibodies. Regions were selected for analysis according to their MRI characteristics.

RESULTS

Transcriptional factor STAT-1 was enhanced in the lesion core and, to a lesser extent, in the lesion periphery, following ischemia and reperfusion. This level of activity was greater than for ischemia alone. Western blots demonstrated STAT-1 phosphorylation on tyrosine 701 and not serine 727 after ischemia and 3 h of reperfusion. Enhanced expression of the apoptotic death receptor Fas was confirmed after ischemia followed by reperfusion.

CONCLUSIONS

This study demonstrates that focal ischemia of the rat brain can induce STAT-1 activation, particularly following a period of reperfusion. The activation occurs not only in the lesion core, but also in the lesion periphery, as identified using MRI. STAT-1 may play an important role in the induction of cell death following stroke.

STAT-1: a novel regulator of apoptosis

Extracellular signalling molecules binding to their specific receptors are able to modulate gene expression, leading to changes in development, cell growth and homeostasis. The signal transducers and activators of transcription (STAT) protein family members are among the best studied of the latent cytoplasmic signal-dependent transcription factors. The STAT factors are activated via phosphorylation on the C-terminal domain following cytokine signalling or by various stress-induced stimuli. Recently, STAT-1 has been implicated in modulating pro- and anti-apoptotic genes following several stress-induced responses. These effects are dependent on STAT-1 phosphorylation on serine-727 and require the C-terminal transactivation domain of STAT-1 to enhance its pro-apoptotic effect or inhibit its anti-apoptotic effects. The STAT-1 C-terminal domain has been demonstrated to be important for protein-protein interaction with other transcriptional activators. The reports that STAT-1-deficient mice develop spontaneous and chemically induced tumours more rapidly compared to wild-type mice and that STAT-1-deficient cells are more resistant to agents that induce apoptosis strongly support the argument that STAT-1 acts as a tumour suppressor.

STAT-1 interacts with p53 to enhance DNA damage-induced apoptosis

The STAT-1 transcription factor has been implicated as a tumor suppressor by virtue of its ability to inhibit cell growth and promoting apoptosis. However, the mechanisms by which STAT-1 mediates these effects remain unclear. Using human and mouse STAT-1-deficient cells, we show here that STAT-1 is required for optimal DNA damage-induced apoptosis. The basal level of the p53 inhibitor Mdm2 is increased in STAT-1(-/-) cells, suggesting that STAT-1 is a negative regulator of Mdm2 expression. Correspondingly, both basal p53 levels, and those induced by DNA damage were lower in STAT-1(-/-) cells. In agreement with this lower p53 response to DNA damage in cells lacking STAT-1, the induction of p53 responsive genes, such as Bax, Noxa, and Fas, was reduced in STAT-1-deficient cells. Conversely, STAT-1 overexpression enhances transcription of these genes, an effect that is abolished if the p53 response element in their promoters is mutated. Moreover, STAT-1 interacts directly with p53, an association, which is enhanced following DNA damage. Therefore, in addition to negatively regulating Mdm2, STAT-1 also acts as a coactivator for p53. Hence STAT-1 is another member of a growing family of protein partners able to modulate the p53-activated apoptotic pathway.