Structural determinants of mitochondrial STAT3 targeting and function

Signal transducer and activator of transcription (STAT) 3 has been found within mitochondria in addition to its canonical role of shuttling between cytoplasm and nucleus during cytokine signaling. Mitochondrial STAT3 has been implicated in modulation of cellular metabolism, largely through effects on the respiratory electron transport chain. However, the structural requirements underlying mitochondrial targeting and function have remained unclear. Here, we show that mitochondrial STAT3 partitions between mitochondrial compartments defined by differential detergent solubility, suggesting that mitochondrial STAT3 is membrane associated. The majority of STAT3 was found in an SDS soluble fraction copurifying with respiratory chain proteins, including numerous components of the complex I NADH dehydrogenase, while a minor component was found with proteins of the mitochondrial translation machinery. Mitochondrial targeting of STAT3 required the amino-terminal domain, and an internal linker domain motif also directed mitochondrial translocation. However, neither the phosphorylation of serine 727 nor the presence of mitochondrial DNA was required for the mitochondrial localization of STAT3. Two cysteine residues in the STAT3 SH2 domain, which have been previously suggested to be targets for protein palmitoylation, were also not required for mitochondrial translocation, but were required for its function as an enhancer of complex I activity. These structural determinants of STAT3 mitochondrial targeting and function provide potential therapeutic targets for disrupting the activity of mitochondrial STAT3 in diseases such as cancer.

Donor T cell STAT3 deficiency enables tissue PD-L1-dependent prevention of graft-versus-host disease while preserving graft-versus-leukemia activity

STAT3 deficiency (STAT3-/-) in donor T cells prevents graft-versus-host disease (GVHD), but the impact on graft-versus-leukemia (GVL) activity and mechanisms of GVHD prevention remains unclear. Here, using murine models of GVHD, we show that STAT3-/- donor T cells induced only mild reversible acute GVHD while preserving GVL effects against nonsusceptible acute lymphoblastic leukemia (ALL) cells in a donor T cell dose-dependent manner. GVHD prevention depended on programmed death ligand 1/programmed cell death protein 1 (PD-L1/PD-1) signaling. In GVHD target tissues, STAT3 deficiency amplified PD-L1/PD-1 inhibition of glutathione (GSH)/Myc pathways that regulate metabolic reprogramming in activated T cells, with decreased glycolytic and mitochondrial ATP production and increased mitochondrial ROS production and dysfunction, leading to tissue-specific deletion of host-reactive T cells and prevention of GVHD. Mitochondrial STAT3 deficiency alone did not reduce GSH expression or prevent GVHD. In lymphoid tissues, the lack of host-tissue PD-L1 interaction with PD-1 reduced the inhibition of the GSH/Myc pathway despite reduced GSH production caused by STAT3 deficiency and allowed donor T cell functions that mediate GVL activity. Therefore, STAT3 deficiency in donor T cells augments PD-1 signaling-mediated inhibition of GSH/Myc pathways and augments dysfunction of T cells in GVHD target tissues while sparing T cells in lymphoid tissues, leading to prevention of GVHD while preserving GVL effects.

Assessing the Presence of Hematopoietic Stem and Progenitor Cells in Mouse Spleen

Transplantation of hematopoietic material into recipient mice is an assay routinely used to determine the presence and function of hematopoietic stem and progenitor cells (HSPCs) in vivo . The principle of the method is to transplant donor cells being tested for HSPCs into a recipient mouse following bone marrow ablation and testing for reconstitution of hematopoiesis. Congenic mouse strains where donor and recipient differ by a distinct cell surface antigen (commonly CD45.1 versus CD45.2) are used to distinguish between cells derived from the donor and any residual recipient cells. Typically, the transplantation is performed using bone marrow cells, which are enriched for HSPCs. Here, we describe an analogous procedure using hematopoietic material from spleen, allowing detection of functional progenitors and/or stem cells in the spleen that can occur under certain pathologies. Key to the success of this procedure is the prior removal of mature T cells from the donor sample, to minimize graft versus host reactions. As such, this protocol is highly analogous to standard bone marrow transplant procedures, differing mainly only in the source of stem cells (spleen rather than bone marrow) and the recommendation for T cell depletion to avoid potential immune incompatibilities. Graphical abstract: Schematic overview for assessment of stem cells in spleen by transplantation. Single cell suspensions from spleens are depleted of potentially pathogenic mature T lymphocytes by magnetic bead immunoselection using biotinylated antibodies against CD4 and CD8, followed by streptavidin magnetic beads, which are subsequently removed by using a magnet (MojoSort, Biolegend). Successful T cell depletion is then evaluated by Fluorescence Activated Cell Sorting (FACS). T-cell depleted cell suspension is injected intravenously through the retro-orbital sinus into lethally irradiated recipients. Recipients are analyzed for successful engraftment by FACS analysis for the presence of donor-derived mature hematopoietic lineages in the peripheral blood. A second serial transplantation can be used to document the presence of long-term reconstituting stem cells in the periphery of the original donor mice.

The Cyclin-Dependent Kinase 8 (CDK8) Inhibitor DCA Promotes a Tolerogenic Chemical Immunophenotype in CD4+ T Cells via a Novel CDK8-GATA3-FOXP3 Pathway

Immune health requires innate and adaptive immune cells to engage precisely balanced pro- and anti-inflammatory forces. We employ the concept of chemical immunophenotypes to classify small molecules functionally or mechanistically according to their patterns of effects on primary innate and adaptive immune cells. The high-specificity, low-toxicity cyclin-dependent kinase 8 (CDK8) inhibitor 16-didehydro-cortistatin A (DCA) exerts a distinct tolerogenic profile in both innate and adaptive immune cells. DCA promotes regulatory T cells (Treg) and Th2 differentiation while inhibiting Th1 and Th17 differentiation in both murine and human cells. This unique chemical immunophenotype led to mechanistic studies showing that DCA promotes Treg differentiation in part by regulating a previously undescribed CDK8-GATA3-FOXP3 pathway that regulates early pathways of Foxp3 expression. These results highlight previously unappreciated links between Treg and Th2 differentiation and extend our understanding of the transcription factors that regulate Treg differentiation and their temporal sequencing. These findings have significant implications for future mechanistic and translational studies of CDK8 and CDK8 inhibitors.

Tonic interferon restricts pathogenic IL-17-driven inflammatory disease via balancing the microbiome

Maintenance of immune homeostasis involves a synergistic relationship between the host and the microbiome. Canonical interferon (IFN) signaling controls responses to acute microbial infection, through engagement of the STAT1 transcription factor. However, the contribution of tonic levels of IFN to immune homeostasis in the absence of acute infection remains largely unexplored. We report that STAT1 KO mice spontaneously developed an inflammatory disease marked by myeloid hyperplasia and splenic accumulation of hematopoietic stem cells. Moreover, these animals developed inflammatory bowel disease. Profiling gut bacteria revealed a profound dysbiosis in the absence of tonic IFN signaling, which triggered expansion of T_H17 cells and loss of splenic T_reg cells. Reduction of bacterial load by antibiotic treatment averted the T_H17 bias and blocking IL17 signaling prevented myeloid expansion and splenic stem cell accumulation. Thus, tonic IFNs regulate gut microbial ecology, which is crucial for maintaining physiologic immune homeostasis and preventing inflammation.

HDAC stimulates gene expression through BRD4 availability in response to IFN and in interferonopathies

In contrast to the common role of histone deacetylases (HDACs) for gene repression, HDAC activity provides a required positive function for IFN-stimulated gene (ISG) expression. Here, we show that HDAC1/2 as components of the Sin3A complex are required for ISG transcriptional elongation but not for recruitment of RNA polymerase or transcriptional initiation. Transcriptional arrest by HDAC inhibition coincides with failure to recruit the epigenetic reader Brd4 and elongation factor P-TEFb due to sequestration of Brd4 on hyperacetylated chromatin. Brd4 availability is regulated by an equilibrium cycle between opposed acetyltransferase and deacetylase activities that maintains a steady-state pool of free Brd4 available for recruitment to inducible promoters. An ISG expression signature is a hallmark of interferonopathies and other autoimmune diseases. Combined inhibition of HDAC1/2 and Brd4 resolved the aberrant ISG expression detected in cells derived from patients with two inherited interferonopathies, ISG15 and USP18 deficiencies, defining a novel therapeutic approach to ISG-associated autoimmune diseases.

PKR Transduces MDA5-Dependent Signals for Type I IFN Induction

Sensing invading pathogens early in infection is critical for establishing host defense. Two cytosolic RIG-like RNA helicases, RIG-I and MDA5, are key to type I interferon (IFN) induction in response to viral infection. Mounting evidence suggests that another viral RNA sensor, protein kinase R (PKR), may also be critical for IFN induction during infection, although its exact contribution and mechanism of action are not completely understood. Using PKR-deficient cells, we found that PKR was required for type I IFN induction in response to infection by vaccinia virus lacking the PKR antagonist E3L (VVΔE3L), but not by Sendai virus or influenza A virus lacking the IFN-antagonist NS1 (FluΔNS1). IFN induction required the catalytic activity of PKR, but not the phosphorylation of its principal substrate, eIF2α, or the resulting inhibition of host translation. In the absence of PKR, IRF3 nuclear translocation was impaired in response to MDA5 activators, VVΔE3L and encephalomyocarditis virus, but not during infection with a RIG-I-activating virus. Interestingly, PKR interacted with both RIG-I and MDA5; however, PKR was only required for MDA5-mediated, but not RIG-I-mediated, IFN production. Using an artificially activated form of PKR, we showed that PKR activity alone was sufficient for IFN induction. This effect required MAVS and correlated with IRF3 activation, but no longer required MDA5. Nonetheless, PKR activation during viral infection was enhanced by MDA5, as virus-stimulated catalytic activity was impaired in MDA5-null cells. Taken together, our data describe a critical and non-redundant role for PKR following MDA5, but not RIG-I, activation to mediate MAVS-dependent induction of type I IFN through a kinase-dependent mechanism.

STAT3 regulated ARF expression suppresses prostate cancer metastasis

Prostate cancer (PCa) is the most prevalent cancer in men. Hyperactive STAT3 is thought to be oncogenic in PCa. However, targeting of the IL-6/STAT3 axis in PCa patients has failed to provide therapeutic benefit. Here we show that genetic inactivation of Stat3 or IL-6 signalling in a Pten-deficient PCa mouse model accelerates cancer progression leading to metastasis. Mechanistically, we identify p19^ARF as a direct Stat3 target. Loss of Stat3 signalling disrupts the ARF–Mdm2–p53 tumour suppressor axis bypassing senescence. Strikingly, we also identify STAT3 and CDKN2A mutations in primary human PCa. STAT3 and CDKN2A deletions co-occurred with high frequency in PCa metastases. In accordance, loss of STAT3 and p14^ARF expression in patient tumours correlates with increased risk of disease recurrence and metastatic PCa. Thus, STAT3 and ARF may be prognostic markers to stratify high from low risk PCa patients. Our findings challenge the current discussion on therapeutic benefit or risk of IL-6/STAT3 inhibition.

IL6-STAT3 signaling is activated in prostate cancer, however inhibiting this pathway has not lead to a survival advantage in patients. Here, Pencik et al. show that loss of the IL6-STAT3 axis in mice and humans leads to metastasis due to loss of ARF, unravelling STAT3 and ARF as potential prognostic markers in prostate cancer.

Induction and function of type I and III interferon in response to viral infection

The type I and III interferon (IFN) families consist of cytokines rapidly induced during viral infection that confer antiviral protection on target cells and are critical components of innate immune responses and the transition to effective adaptive immunity. The regulation of their expression involves an intricate and stringently regulated signaling cascade, initiated by recognition most often of viral nucleic acid in cytoplasmic and endosomal compartments and involving a series of protein conformational rearrangements and interactions regulated by helicase action, ubiquitin modification, and protein aggregation, culminating in kinase activation and phosphorylation of critical transcription factors and their regulators. The many IFN subtypes induced by viruses confer amplification, diversification, and cell-type specificity to the host response to infection, providing fertile ground for development of antiviral therapeutics and vaccines.

STATus report on tetramers

STAT proteins bind DNA as dimers to regulate gene expression. Cooperative recruitment of pairs of dimers (tetramers) to adjacent DNA sites has also been documented. In this issue, Lin et al. (2012) examined tetramer function in vivo and showed that STAT5 tetramers function primarily as transcriptional activators.

Regulatory serine residues mediate phosphorylation-dependent and phosphorylation-independent activation of interferon regulatory factor 7

Interferon regulatory factor (IRF)7 is a key transcription factor required for establishment of antiviral resistance. In response to infection, IRF7 is activated by phosphorylation through the action of the non-canonical IkappaB kinases, IkappaB kinase-epsilon and TANK-binding kinase 1. Activation leads to nuclear retention, DNA binding, and derepression of transactivation ability. Clusters of serine residues located in the carboxyl-terminal regulatory domain of IRF7 are putative targets of virus-activated kinases. However, the exact sites of phosphorylation have not yet been established. Here, we report a comprehensive structure-activity examination of potential IRF7 phosphorylation sites through analysis of mutant proteins in which specific serine residues were altered to alanine or aspartate. Phosphorylation patterns of these mutants were analyzed by two-dimensional gel electrophoresis, and their transcriptional activity was monitored by reporter assays. Essential phosphorylation events were mapped to amino acids 437-438 and a redundant set of sites at either amino acids 429-431 or 441. IRF7 recovered from infected cells was heterogeneously phosphorylated at these sites, and greater phosphorylation correlated with increased transactivation. Interestingly, a distinct serine cluster conserved in the related protein IRF3 was also essential for IRF7 activation and distal phosphorylation. However, the essential role of this motif did not appear to be fulfilled by phosphorylation. Rather, these serine residues and an adjacent leucine were required for phosphorylation at distal sites and may determine a conformational element required for function.