Mitophagy in Intestinal Epithelial Cells Triggers Adaptive Immunity during Tumorigenesis.

In colorectal cancer patients, a high density of cytotoxic CD8+ T cells in tumors is associated with better prognosis. Using a Stat3 loss-of-function approach in two wnt/ß-catenin-dependent autochthonous models of sporadic intestinal tumorigenesis, we unravel a complex intracellular process in intestinal epithelial cells (IECs) that controls the induction of a CD8+ T cell based adaptive immune response. Elevated mitophagy in IECs causes iron(II)-accumulation in epithelial lysosomes, in turn, triggering lysosomal membrane permeabilization. Subsequent release of proteases into the cytoplasm augments MHC class I presentation and activation of CD8+ T cells via cross-dressing of dendritic cells. Thus, our findings highlight a so-far-unrecognized link between mitochondrial function, lysosomal integrity, and MHC class I presentation in IECs and suggest that therapies triggering mitophagy or inducing LMP in IECs may prove successful in shifting the balance toward anti-tumor immunity in colorectal cancer.

STAT3 suppresses Wnt/ß-catenin signaling during the induction phase of primary Myf5+ brown adipogenesis.

Thermogenic fat is a promising target for new therapies in diabetes and obesity. Understanding how thermogenic fat develops is important to develop rational strategies to treat obesity. Previously, we have shown that Tyk2 and STAT3, part of the JAK-STAT pathway, are necessary for proper development of classical brown fat. Using primary preadipocytes isolated from newborn mice we demonstrate that STAT3 is required for differentiation and robust expression of Uncoupling Protein 1 (UCP1). We also confirm that STAT3 is necessary during the early induction stage of differentiation and is dispensable during the later terminal differentiation stage. The inability of STAT3-/- preadipocytes to differentiate can be rescued using Wnt ligand secretion inhibitors when applied during the induction stage. Through chemical inhibition and RNAi, we show that it is the canonical ß-catenin pathway that is responsible for the block in differentiation; inhibition or knockdown of ß-catenin can fully rescue adipogenesis and UCP1 expression in the STAT3-/- adipocytes. During the induction stage, Wnts 1, 3a, and 10b have increased expression in the STAT3-/- adipocytes, potentially explaining the increased levels and activity of ß-catenin. Our results for the first time point towards an interaction between the JAK/STAT pathway and the Wnt/ß-catenin pathway during the early stages of in-vitro adipogenesis.

Toward a new STATe: the role of STATs in mitochondrial function.

Signal Transducers and Activators of Transcription (STATs) have been studied extensively and have been associated with virtually every biochemical pathway. Until recently, however, they were thought to exert these effects solely as a nuclear transcription factor. The finding that STAT3 localizes to the mitochondria and modulates respiration has opened up a new avenue through which STATs may regulate the cell. Recently, other members of the STAT family (STAT1, STAT2, STAT5, and STAT6) have also been shown to be present in the mitochondria. Coordinate regulation at the nucleus and mitochondria by these proteins places them in a unique position to drive cellular processes to achieve a specific response. This review summarizes recent findings that have led to our current understanding of how STATs influence mitochondrial function in health and disease.

Cardioprotective function of mitochondrial-targeted and transcriptionally inactive STAT3 against ischemia and reperfusion injury.

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that contributes a crucial role in protection against ischemia (ISC)-reperfusion (REP) injury by driving expression of anti-apoptotic and anti-oxidant genes. STAT3 is also present in the mitochondria, where it modulates the activity of the electron transport chain (ETC) and the permeability transition pore. Transgenic mice that overexpress a mitochondrial-targeted, transcriptionally inactive STAT3 in cardiomyocytes (MLS-STAT3E mice) exhibit a persistent, partial blockade of electron transfer through complex I that uniquely did not lead to tissue dysfunction at baseline, yet increased mitochondrial ischemic tolerance. The direct contribution of non-transcriptional, mitochondria-localized STAT3 to protection during ISC-REP remains to be established. We hypothesized that the enhanced mitochondrial tolerance to ischemia present in MLS-STAT3E mice would decrease cardiac injury during ISC-REP. In the isolated buffer-perfused heart model, MLS-STAT3E hearts exhibit a decreased infarct size compared to non-transgenic littermate hearts. Contractile recovery, expressed as a percent of LV developed pressure before ISC, is improved in MLS-STAT3E mice. Mitochondria isolated at the end of 60 min. of REP from MLS-STAT3E hearts show attenuated ROS release. The partial and persistent blockade of complex I present in MLS-STAT3E mice decreases cardiac injury during REP, in part via a persistent decrease in ROS production and attenuation of mitochondrial permeability transition pore opening at the onset of REP. In vivo, MLS-STAT3E hearts exhibit substantially higher postoperative survival rate and a substantial decrease in myocardial infarct size. STAT3 mediates cardioprotection not only via canonical action as a transcription factor, but also as a modulator of ETC activity directly in the mitochondria.

The import of the transcription factor STAT3 into mitochondria depends on GRIM-19, a component of the electron transport chain.

The signal transducer and activator of transcription 3 (STAT3), a nuclear transcription factor, is also present in mitochondria and regulates cellular respiration in a transcriptional-independent manner. The mechanism of STAT3 import into mitochondria remains obscure. In this report we show that mitochondrial-localized STAT3 resides in the inner mitochondrial membrane. In vitro import studies show that the gene associated with retinoid interferon induced cell mortality 19 (GRIM-19), a complex I subunit that acts as a chaperone to recruit STAT3 into mitochondria. In addition, GRIM-19 enhances the integration of STAT3 into complex I. A S727A mutation in STAT3 reduces its import and assembly even in the presence of GRIM-19. Together, our studies unveil a novel chaperone function for GRIM-19 in the recruitment of STAT3 into mitochondria.

Mitochondrial localized Stat3 promotes breast cancer growth via phosphorylation of serine 727.

Signal transducer and activator of transcription 3 (Stat3) is a key mediator in the development of many cancers. For 20 years, it has been assumed that Stat3 mediates its biological activities as a nuclear localized transcription factor activated by many cytokines. However, recent studies from this laboratory and others indicate that Stat3 has an independent function in the mitochondria (mitoStat3) where it controls the activity of the electron transport chain (ETC) and mediates Ras-induced transformation of mouse embryo fibroblasts. The actions of mitoStat3 in controlling respiration and Ras transformation are mediated by the phosphorylation state of serine 727. To address the role of mitoStat3 in the pathogenesis of cells that are transformed, we used 4T1 breast cancer cells, which form tumors that metastasize in immunocompetent mice. Substitution of Ser-727 for an alanine or aspartate in Stat3 that has a mitochondrial localization sequence, MLS-Stat3, has profound effects on tumor growth, complex I activity of the ETC, and accumulation of reactive oxygen species (ROS). Cells expressing MLS-Stat3(S727A) display slower tumor growth, decreased complex I activity of the ETC, and increased ROS accumulation under hypoxia compared with cells expressing MLS-Stat3. In contrast, cells expressing MLS-Stat3(S727D) show enhanced tumor growth and complex I activity and decreased production of ROS. These results highlight the importance of serine 727 of mitoStat3 in breast cancer and suggest a novel role for mitoStat3 in regulation of ROS concentrations through its action on the ETC.

Mitochondrial-targeted Signal transducer and activator of transcription 3 (STAT3) protects against ischemia-induced changes in the electron transport chain and the generation of reactive oxygen species.

Expression of the STAT3 transcription factor in the heart is cardioprotective and decreases the levels of reactive oxygen species. Recent studies indicate that a pool of STAT3 resides in the mitochondria where it is necessary for the maximal activity of complexes I and II of the electron transport chain. However, it has not been explored whether mitochondrial STAT3 modulates cardiac function under conditions of stress. Transgenic mice with cardiomyocyte-specific overexpression of mitochondria-targeted STAT3 with a mutation in the DNA-binding domain (MLS-STAT3E) were generated. We evaluated the role of mitochondrial STAT3 in the preservation of mitochondrial function during ischemia. Under conditions of ischemia heart mitochondria expressing MLS-STAT3E exhibited modest decreases in basal activities of complexes I and II of the electron transport chain. In contrast to WT hearts, complex I-dependent respiratory rates were protected against ischemic damage in MLS-STAT3E hearts. MLS-STAT3E prevented the release of cytochrome c into the cytosol during ischemia. In contrast to WT mitochondria, ischemia did not augment reactive oxygen species production in MLS-STAT3E mitochondria likely due to an MLS-STAT3E-mediated partial blockade of electron transport through complex I. Given the caveat of STAT3 overexpression, these results suggest a novel protective mechanism mediated by mitochondrial STAT3 that is independent of its canonical activity as a nuclear transcription factor.

Stat3 oxidation-dependent regulation of gene expression impacts on developmental processes and involves cooperation with Hif-1α.

Reactive oxygen species are bona fide intracellular second messengers that influence cell metabolism and aging by mechanisms that are incompletely resolved. Mitochondria generate superoxide that is dis-mutated to hydrogen peroxide, which in turn oxidises cysteine-based enzymes such as phosphatases, peroxiredoxins and redox-sensitive transcription factors to modulate their activity. Signal Transducer and Activator of Transcription 3 (Stat3) has been shown to participate in an oxidative relay with peroxiredoxin II but the impact of Stat3 oxidation on target gene expression and its biological consequences remain to be established. Thus, we created murine embryonic fibroblasts (MEFs) that express either WT-Stat3 or a redox-insensitive mutant of Stat3 (Stat3-C3S). The Stat3-C3S cells differed from WT-Stat3 cells in morphology, proliferation and resistance to oxidative stress; in response to cytokine stimulation, they displayed elevated Stat3 tyrosine phosphorylation and Socs3 expression, implying that Stat3-C3S is insensitive to oxidative inhibition. Comparative analysis of global gene expression in WT-Stat3 and Stat3-C3S cells revealed differential expression (DE) of genes both under basal conditions and during oxidative stress. Using differential gene regulation pattern analysis, we identified 199 genes clustered into 10 distinct patterns that were selectively responsive to Stat3 oxidation. GO term analysis identified down-regulated genes to be enriched for tissue/organ development and morphogenesis and up-regulated genes to be enriched for cell-cell adhesion, immune responses and transport related processes. Although most DE gene promoters contain consensus Stat3 inducible elements (SIEs), our chromatin immunoprecipitation (ChIP) and ChIP-seq analyses did not detect Stat3 binding at these sites in control or oxidant-stimulated cells, suggesting that oxidised Stat3 regulates these genes indirectly. Our further computational analysis revealed enrichment of hypoxia response elements (HREs) within DE gene promoters, implying a role for Hif-1. Experimental validation revealed that efficient stabilisation of Hif-1α in response to oxidative stress or hypoxia required an oxidation-competent Stat3 and that depletion of Hif-1α suppressed the inducible expression of Kcnb1, a representative DE gene. Our data suggest that Stat3 and Hif-1α cooperate to regulate genes involved in immune functions and developmental processes in response to oxidative stress.

Tyk2 tyrosine kinase expression is required for the maintenance of mitochondrial respiration in primary pro-B lymphocytes.

Tyk2, a member of the Jak family of protein tyrosine kinases, is critical for the biological actions of alpha/beta interferon (IFN-alpha/beta). Although Tyk2(-/-) mice are phenotypically normal, they exhibit abnormal responses to inflammatory challenges in a variety of cells isolated from Tyk2(-/-) mice. The reported phenotypic alterations in both Tyk2-null cells and mice are consistent with the possibility that the expression of this tyrosine kinase may regulate mitochondrial function. We report here that Tyk2-null pro-B cells are markedly deficient in basal oxygen consumption and exhibit a significant decrease in steady-state cellular ATP levels compared to wild-type cells. Tyk2-null cells also exhibit impaired complex I, III, and IV function of the mitochondrial electron transport chain. Reconstitution of Tyk2-null pro-B cells with either the wild type or a kinase-inactive mutant of Tyk2 restores basal mitochondrial respiration. By contrast, the kinase activity of Tyk2 is required for maintenance of both complex I-dependent mitochondrial respiration as well as induction of apoptosis in cells incubated with IFN-beta. Consistent with the role of Tyk2 in the regulation of tyrosine phosphorylation of Stat3, expression of a constitutively active Stat3 can restore the mitochondrial respiration in Tyk2-null cells treated with IFN-beta. Finally, Tyk2(-/-) mice show decreased exercise tolerance compared to wild-type littermates. Our results implicate a novel role for Tyk2 kinase and Stat3 phosphorylation in mitochondrial respiration.

Type I interferons activate apoptosis in a Jurkat cell variant by caspase-dependent and independent mechanisms.

Although the antiviral actions of interferons (IFNs) are observed in most types of cells, the antiproliferative effects of IFNalpha/beta are variable as are the mechanisms of growth inhibition that may or may not be due to the induction of apoptosis. To understand more about the mechanisms that are responsible for IFNalpha/beta-stimulated apoptosis, we have characterized a new human Jurkat T cell variant named H123 where IFNalpha activates programmed cell death (PCD). No differences in IFNalpha-stimulated, Stat-dependent gene expression were detected between H123 cells and the parental Jurkat cells, which are growth inhibited, but do not undergo apoptosis with IFNalpha. Although IFNalpha stimulates the activity of both caspase 3 and 9 in H123 cells, the general caspase inhibitor Z-VAD only partially reverses the apoptotic actions of IFNalpha. Induction of apoptosis by IFNalpha occurs through a mitochondrial-dependent pathway in H123 cells, as demonstrated by the release of cytochrome C from the mitochondria. Furthermore, IFNalpha treatment of H123 cells stimulates the release of the serine protease HtrA2/Omi from the mitochondria, suggesting that it plays a role in the apoptotic actions of this cytokine. These results provide evidence for a novel type 1 IFN-mediated pathway that regulates apoptosis of T cells through a mitochondrial-dependent and caspase-dependent and independent pathway.

Kinase Inactive Tyrosine Kinase (Tyk2) Supports Differentiation of Brown Fat Cells.

It has been known for decades that brown adipose tissue (BAT) plays a central role in maintaining body temperature in hibernating animals and human infants. Recently, it has become evident that there are also depots of brown fat in adult humans, and the mass of brown fat is inversely correlated with body weight. There are a variety of transcription factors implicated in the differentiation of classical Myf5+ brown preadipocytes, one of the most important of which is PRDM16. We have recently identified that in addition to PRDM16, the tyrosine kinase Tyk2 and the STAT3 transcription factor are required for the differentiation of Myf5 positive brown preadipocytes both in cell culture and in mice. Tyk2 is a member of the Jak family of tyrosine kinases, which are activated by exposure of cells to different cytokines and growth factors. In this study we report the surprising observation that a mutated form of Tyk2, which lacks tyrosine kinase activity (Tyk2KD) restores differentiation of brown preadipocytes in vitro as well as in Tyk2-/- mice. Furthermore, expression of the Tyk2KD transgene in brown fat reverses the obese phenotype of Tyk2-/- animals. Treatment of cells with Jak-selective inhibitors suggests that the mechanism by which Tyk2KD functions to restore BAT differentiation is by dimerizing with kinase active Jak1 or Jak2. These results indicate that there are redundant mechanisms by which members of the Jak family can contribute to differentiation of BAT.

Stress-induced dynamic regulation of mitochondrial STAT3 and its association with cyclophilin D reduce mitochondrial ROS production.

Signal transducer and activator of transcription 3 (STAT3) is associated with various physiological and pathological functions, mainly as a transcription factor that translocates to the nucleus upon tyrosine phosphorylation induced by cytokine stimulation. In addition, a small pool of STAT3 resides in the mitochondria, where it serves as a sensor for various metabolic stressors including reactive oxygen species (ROS). Mitochondrially localized STAT3 largely exerts its effects through direct or indirect regulation of the activity of the electron transport chain (ETC). It has been assumed that the amounts of STAT3 in the mitochondria are static. We showed that various stimuli, including oxidative stress and cytokines, triggered a signaling cascade that resulted in a rapid loss of mitochondrially localized STAT3. Recovery of the mitochondrial pool of STAT3 over time depended on phosphorylation of Ser727 in STAT3 and new protein synthesis. Under these conditions, mitochondrially localized STAT3 also became competent to bind to cyclophilin D (CypD). Binding of STAT3 to CypD was mediated by the amino terminus of STAT3, which was also important for reducing mitochondrial ROS production after oxidative stress. These results outline a role for mitochondrially localized STAT3 in sensing and responding to external stimuli.

A PI-3 kinase-dependent, Stat1-independent signaling pathway regulates interferon-stimulated monocyte adhesion.

Type I interferon (IFN)-alpha/beta and type II IFN-gamma induce the expression of early response genes through activation of the Janus tyrosine kinase/signal transducer and activator of transcription (Stat) pathway. Although IFNs regulate a variety of other signaling cascades, little is known about how they contribute to the biological activities of these cytokines. In this study, we demonstrate that IFN-beta or IFN-gamma induces the phosphorylation of the serine/threonine kinase Akt in primary human peripheral blood monocytes. Abrogation of the IFN-stimulated Akt activation by phosphatidylinositol-3 kinase (PI-3K) inhibitors prevents IFN-induced adhesion in these cells, and IFN activation of the Stat1-dependent guanylate-binding protein (GBP) gene is not affected. Importantly, Stat1-deficient bone marrow macrophages displayed a similar level of IFN-gamma-stimulated adhesion compared with macrophages derived from wild-type littermates. These findings demonstrate for the first time that IFN stimulation of a PI-3K signaling cascade modulates the ability of these cytokines to regulate monocyte adhesion, and this process does not require the expression of Stat1, a primary mediator of IFN-gamma signaling.

The Signal Transducer and Activator of Transcription 1 (STAT1) Inhibits Mitochondrial Biogenesis in Liver and Fatty Acid Oxidation in Adipocytes.

The transcription factor STAT1 plays a central role in orchestrating responses to various pathogens by activating the transcription of nuclear-encoded genes that mediate the antiviral, the antigrowth, and immune surveillance effects of interferons and other cytokines. In addition to regulating gene expression, we report that STAT1-/- mice display increased energy expenditure and paradoxically decreased release of triglycerides from white adipose tissue (WAT). Liver mitochondria from STAT1-/- mice show both defects in coupling of the electron transport chain (ETC) and increased numbers of mitochondria. Consistent with elevated numbers of mitochondria, STAT1-/- mice expressed increased amounts of PGC1α, a master regulator of mitochondrial biogenesis. STAT1 binds to the PGC1α promoter in fed mice but not in fasted animals, suggesting that STAT1 inhibited transcription of PGC1α. Since STAT1-/- mice utilized more lipids we examined white adipose tissue (WAT) stores. Contrary to expectations, fasted STAT1-/- mice did not lose lipid from WAT. ß-adrenergic stimulation of glycerol release from isolated STAT1-/- WAT was decreased, while activation of hormone sensitive lipase was not changed. These findings suggest that STAT1-/- adipose tissue does not release glycerol and that free fatty acids (FFA) re-esterify back to triglycerides, thus maintaining fat mass in fasted STAT1-/- mice.

Thrombomodulin RNA is destabilized through its 3'-untranslated element in cells exposed to IFN-gamma.

Interferon-gamma (IFN-gamma) is a potent activator of mononuclear phagocytes, allowing them to play a prominent role in acute and chronic inflammatory responses. IFN-gamma binding to its cell surface receptor initiates changes in the steady-state levels of cellular RNAs, permitting the proteins encoded by these RNAs to exert its biologic actions. Hundreds of cellular RNAs have been identified whose rates of transcription are altered by incubation of cells with IFNs. The rates of transcription of many of the genes encoding these RNAs are enhanced by IFN-gamma-mediated activation of the Stat1 transcription factor that is tyrosine phosphorylated and translocates to the nucleus, where it binds enhancers present in IFN-stimulated genes (ISGs). IFN-gamma can also modify the concentrations of some RNAs by posttranscriptional mechanisms. However, very little is understood about the molecular mechanisms regulating this phenomenon. We have identified the RNA encoding thrombomodulin (TM), a physiologic receptor for thrombin, that is downregulated in primary human monocytes incubated with IFN-gamma. Using actinomycin D as a transcriptional inhibitor, we show that the mRNA half-life is rapidly shortened by IFN-gamma. The TM transcript contains a large 3'-untranslated region (UTR), with several AU-rich elements (AREs), elements that have been implicated in the regulation of mRNA decay. Using a tetracycline-regulatory promoter system, we analyzed RNA levels in the absence of transcription of TM. Results from these experiments indicate that incubation of cells with IFN-gamma accelerates the decay of TM RNA through its 3'-UTR. This is the first report describing a clear posttranscriptional downregulation of an mRNA by IFN-gamma that identifies the 3'-UTR as a target of IFN-gamma-stimulated destabilization.

Multi-tasking: nuclear transcription factors with novel roles in the mitochondria.

Coordinated responses between the nucleus and mitochondria are essential for the maintenance of homeostasis. For over 15 years, pools of nuclear transcription factors (TFs), such as p53 and nuclear hormone receptors, have been observed in the mitochondria. The contribution of the mitochondrial pool of these TFs to their well-defined biological actions is in some cases clear and in others not well understood. Recently, a small mitochondrial pool of the TF signal transducer and activator of transcription factor 3 (STAT3) was shown to modulate the activity of the electron transport chain (ETC). The mitochondrial function of STAT3 encompasses both its biological actions in the heart as well as its oncogenic effects. This review highlights advances in our understanding of how mitochondrial pools of nuclear TFs may influence the function of this organelle.

Cytoprotection by the modulation of mitochondrial electron transport chain: the emerging role of mitochondrial STAT3.

The down regulation of mitochondrial electron transport is an emerging mechanism of cytoprotective intervention that is effective in pathologic settings such as myocardial ischemia and reperfusion when the continuation of mitochondrial respiration produces reactive oxygen species, mitochondrial calcium overload, and the release of cytochrome c to activate cell death programs. The initial target of deranged electron transport is the mitochondria themselves. In the first part of this review, we describe this concept and summarize different approaches used to regulate mitochondrial respiration by targeting complex I as a proximal site in the electron transport chain (ETC) in order to favor the cytoprotection. The second part of the review highlights the emerging role of signal transducer and activator of transcription 3 (STAT3) in the direct, non-transcriptional regulation of ETC, as an example of a genetic approach to modulate respiration. Recent studies indicate that a pool of STAT3 resides in the mitochondria where it is necessary for the maximal activity of complexes I and II of the electron transport chain (ETC). The overexpression of mitochondrial-targeted STAT3 results in a partial blockade of electron transport at complexes I and II that does not impair mitochondrial membrane potential nor enhance the production of reactive oxygen species (ROS). The targeting of transcriptionally-inactive STAT3 to mitochondria attenuates damage to mitochondria during cell stress, resulting in decreased production of ROS and retention of cytochrome c by mitochondria. The overexpression of STAT3 targeted to mitochondria unveils a novel protective approach mediated by modulation of mitochondrial respiration that is independent of STAT3 transcriptional activity. The limitation of mitochondrial respiration under pathologic circumstances can be approached by activation and overexpression of endogenous signaling mechanisms in addition to pharmacologic means. The regulation of mitochondrial respiration comprises a cardioprotective paradigm to decrease cellular injury during ischemia and reperfusion.

Tyk2 and Stat3 regulate brown adipose tissue differentiation and obesity.

Mice lacking the Jak tyrosine kinase member Tyk2 become progressively obese due to aberrant development of Myf5+ brown adipose tissue (BAT). Tyk2 RNA levels in BAT and skeletal muscle, which shares a common progenitor with BAT, are dramatically decreased in mice placed on a high-fat diet and in obese humans. Expression of Tyk2 or the constitutively active form of the transcription factor Stat3 (CAStat3) restores differentiation in Tyk2(-/-) brown preadipocytes. Furthermore, Tyk2(-/-) mice expressing CAStat3 transgene in BAT also show improved BAT development, normal levels of insulin, and significantly lower body weights. Stat3 binds to PRDM16, a master regulator of BAT differentiation, and enhances the stability of PRDM16 protein. These results define Tyk2 and Stat3 as critical determinants of brown fat lineage and suggest that altered levels of Tyk2 are associated with obesity in both rodents and humans.

The role of Tyk2 in regulation of breast cancer growth.

The antigrowth and immunomodulatory actions of interferons (IFNs) have enabled these cytokines to be used therapeutically for the treatment of a variety of hematologic and solid malignancies. IFNs exert their effects by activation of the Jak/Stat signaling pathway. IFNγ stimulates the tyrosine kinases Jak1 and Jak2, resulting in activation of the Stat1 transcription factor, whereas type 1 IFNs (IFNα/ß) activate Jak1 and Tyk2, which mediate their effects through Stat1 and Stat2. Disruption in the expression of IFNγ, IFNα receptors, or Stat1 inhibits antitumor responses and blunt cancer immunosurveillance in mice. Mutations in Jak2 or constitutive activation of Jak1 or Jak2 also promote the development of a variety of malignancies. Although there are data indicating that Tyk2 plays a role in the pathogenesis of lymphomas, the effects of Tyk2 expression on tumorigenesis are unknown. We report here that Tyk2(-/-) mice inoculated with 4T1 breast cancer cells show enhanced tumor growth and metastasis compared to Tyk2(+/+) animals. Accelerated growth of 4T1 cells in Tyk2(-/-) animals does not appear to be due to decreased function of CD4(+), CD8(+) T cells, or NK cells. Rather, the tumor suppresive effects of Tyk2 are mediated at least in part by myeloid-derived suppressor cells, which appear to be more effective in inhibiting T cell responses in Tyk2(-/-) mice. Our results provide the first evidence for a role of Tyk2 in suppressing the growth and metastasis of breast cancer.

Mitochondrial STAT3 supports Ras-dependent oncogenic transformation.

Signal transducer and activator of transcription 3 (STAT3) is a latent cytoplasmic transcription factor responsive to cytokine signaling and tyrosine kinase oncoproteins by nuclear translocation when it is tyrosine-phosphorylated. We report that malignant transformation by activated Ras is impaired without STAT3, in spite of the inability of Ras to drive STAT3 tyrosine phosphorylation or nuclear translocation. Moreover, STAT3 mutants that cannot be tyrosine-phosphorylated, that are retained in the cytoplasm, or that cannot bind DNA nonetheless supported Ras-mediated transformation. Unexpectedly, STAT3 was detected within mitochondria, and exclusive targeting of STAT3 to mitochondria without nuclear accumulation facilitated Ras transformation. Mitochondrial STAT3 sustained altered glycolytic and oxidative phosphorylation activities characteristic of cancer cells. Thus, in addition to its nuclear transcriptional role, STAT3 regulates a metabolic function in mitochondria, supporting Ras-dependent malignant transformation.