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.

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) . 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.

IL-6 enhances CD4 cell motility by sustaining mitochondrial Ca through the noncanonical STAT3 pathway.

Interleukin 6 (IL-6) is known to regulate the CD4 T cell function by inducing gene expression of a number of cytokines through activation of Stat3 transcription factor. Here, we reveal that IL-6 strengthens the mechanics of CD4 T cells. The presence of IL-6 during activation of mouse and human CD4 T cells enhances their motility (random walk and exploratory spread), resulting in an increase in travel distance and higher velocity.

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.

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.

Mitochondrial STAT3 regulates antioxidant gene expression through complex I-derived NAD in triple negative breast cancer.

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor with roles in inflammation and tumorigenicity. A fraction of STAT3 localizes in mitochondria, where it augments tumorigenesis via regulation of mitochondrial functions, including modulation of respiration and redox status. We show a novel mechanism for mitochondrial STAT3 regulation of redox homeostasis in triple-negative breast cancer cells.

STAT3 Inhibitor OPB-51602 Is Cytotoxic to Tumor Cells Through Inhibition of Complex I and ROS Induction.

STAT3 is a transcription factor involved in several cellular activities including inflammation, proliferation, and survival, but it also plays a non-transcriptional role in modulating mitochondrial metabolism. Given its diverse functions in human cancers, it is an emerging therapeutic target. Here we show that OPB-51602, a small molecule inhibitor of STAT3, is highly toxic in a STAT3-dependent manner.

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.

Mitochondrial dysfunction induced by a SH2 domain-targeting STAT3 inhibitor leads to metabolic synthetic lethality in cancer cells.

In addition to its canonical role in nuclear transcription, signal transducer and activator of transcription 3 (STAT3) is emerging as an important regulator of mitochondrial function. Here, we demonstrate that a novel inhibitor that binds with high affinity to the STAT3 SH2 domain triggers a complex cascade of events initiated by interference with mitochondrial STAT3 (mSTAT3). The mSTAT3-drug interaction leads to mitochondrial dysfunction, accumulation of proteotoxic STAT3 aggregates, and cell death.

The kinase TBK1 functions in dendritic cells to regulate T cell homeostasis, autoimmunity, and antitumor immunity.

Dendritic cells (DCs) are crucial for mediating immune responses but, when deregulated, also contribute to immunological disorders, such as autoimmunity. The molecular mechanism underlying the function of DCs is incompletely understood. In this study, we have identified TANK-binding kinase 1 (TBK1), a master innate immune kinase, as an important regulator of DC function.

Cytoplasmic, full length and novel cleaved variant, TBLR1 reduces apoptosis in prostate cancer under androgen deprivation.

TBLR1/TBL1XR1, a core component of the nuclear receptor corepressor (NCoR) complex critical for the regulation of multiple nuclear receptors, is a transcriptional coactivator of androgen receptor (AR) and functions as a tumor suppressor when expressed in the nucleus in prostate. Subcellular localization of a protein is critical for its function, and although TBLR1, as a transcriptional cofactor, has been primarily viewed as a nuclear protein, many cells also express variable levels of cytoplasmic TBLR1 and its cytoplasmic specific functions have not been studied. Prostate cancer (PCa) cells express moderately higher level of cytoplasmic TBLR1 compared to benign prostate cells.

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.

Insertional Mutagenesis Identifies a STAT3/Arid1b/β-catenin Pathway Driving Neurofibroma Initiation.

To identify genes and signaling pathways that initiate Neurofibromatosis type 1 (NF1) neurofibromas, we used unbiased insertional mutagenesis screening, mouse models, and molecular analyses. We mapped an Nf1-Stat3-Arid1b/β-catenin pathway that becomes active in the context of Nf1 loss. Genetic deletion of Stat3 in Schwann cell progenitors (SCPs) and Schwann cells (SCs) prevents neurofibroma formation, decreasing SCP self-renewal and β-catenin activity.

A Synthetic Lethal Interaction between Glutathione Synthesis and Mitochondrial Reactive Oxygen Species Provides a Tumor-Specific Vulnerability Dependent on STAT3.

Increased production of mitochondrion-derived reactive oxygen species (ROS) is characteristic of a metabolic shift observed during malignant transformation. While the exact sources and roles of ROS in tumorigenesis remain to be defined, it has become clear that maintaining redox balance is critical for cancer cell proliferation and survival and, as such, may represent a vulnerability that can be exploited therapeutically. STAT3, a latent cytosolic transcription factor activated by diverse cytokines and growth factors, has been shown to exhibit an additional, nontranscriptional function in mitochondria, including modulation of electron transport chain activity.

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.

STAT3 supports experimental K-RasG12D-induced murine myeloproliferative neoplasms dependent on serine phosphorylation.

Juvenile myelomonocytic leukemia, acute myeloid leukemia (AML), and other myeloproliferative neoplasms (MPNs) are genetically heterogeneous but frequently display activating mutations in Ras GTPases and activation of signal transducer and activator of transcription 3 (STAT3). Altered STAT3 activity is observed in up to 50% of AML correlating with poor prognosis. Activated STAT proteins, classically associated with tyrosine phosphorylation, support tumor development as transcription factors, but alternative STAT functions independent of tyrosine phosphorylation have been documented, including roles for serine-phosphorylated STAT3 in mitochondria supporting transformation by oncogenic Ras.

The MEK-ERK pathway is necessary for serine phosphorylation of mitochondrial STAT3 and Ras-mediated transformation.

Activating mutations in the RasGTPases are the most common oncogenic lesions in human cancer. Similarly, elevated STAT3 expression and/or phosphorylation are observed in the majority of human cancers. We recently found that activated Ras requires a mitochondrial rather than a nuclear activity of STAT3 to support cellular transformation.

The human RVB complex is required for efficient transcription of type I interferon-stimulated genes.

Type I interferons (IFNs) stimulate transcription through a latent heterotrimeric transcription factor composed of tyrosine-phosphorylated STAT1 and STAT2 and the DNA binding partner IRF9, with STAT2 contributing a critical transactivation domain. Human RVB1 and RVB2, which are highly conserved AAA(+) ATP binding proteins contained in chromatin-remodeling complexes such as Ino80, SNF2-related CBP activator protein (SRCAP), and Tip60/NuA4, interacted with the transactivation domain of STAT2 in the nuclei of IFN-stimulated cells. RNA interference (RNAi) experiments demonstrated that RVB proteins were required for robust activation of IFN-α-stimulated genes (ISGs).

Chemokine gene silencing in decidual stromal cells limits T cell access to the maternal-fetal interface.

The chemokine-mediated recruitment of effector T cells to sites of inflammation is a central feature of the immune response. The extent to which chemokine expression levels are limited by the intrinsic developmental characteristics of a tissue has remained unexplored. We show in mice that effector T cells cannot accumulate within the decidua, the specialized stromal tissue encapsulating the fetus and placenta.

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.

Constitutive type I interferon modulates homeostatic balance through tonic signaling.

Interferons (IFNs) were discovered as cytokines induced during and protecting from viral infection. They have been documented to play essential roles in numerous physiological processes beyond antiviral and antimicrobial defense, including immunomodulation, cell cycle regulation, cell survival, and cell differentiation. Recent data have also uncovered a potentially darker side to IFN, including roles in inflammatory diseases, such as autoimmunity and diabetes.

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.

Inhibition of pyrimidine synthesis reverses viral virulence factor-mediated block of mRNA nuclear export.

The NS1 protein of influenza virus is a major virulence factor essential for virus replication, as it redirects the host cell to promote viral protein expression. NS1 inhibits cellular messenger ribonucleic acid (mRNA) processing and export, down-regulating host gene expression and enhancing viral gene expression. We report in this paper the identification of a nontoxic quinoline carboxylic acid that reverts the inhibition of mRNA nuclear export by NS1, in the absence or presence of the virus.

STAT1-deficient mice spontaneously develop estrogen receptor α-positive luminal mammary carcinomas.

Introduction: Although breast cancers expressing estrogen receptor-α (ERα) and progesterone receptors (PR) are the most common form of mammary malignancy in humans, it has been difficult to develop a suitable mouse model showing similar steroid hormone responsiveness. STAT transcription factors play critical roles in mammary gland tumorigenesis, but the precise role of STAT1 remains unclear. Herein, we show that a subset of human breast cancers display reduced STAT1 expression and that mice lacking STAT1 surprisingly develop ERα+/PR+ mammary tumors.