Cytokines such as interleukin-6 induce tyrosine and serine phosphorylation of Stat3 that results in activation of Stat3-responsive genes. We provide evidence that Stat3 is present in the mitochondria of cultured cells and primary tissues, including the liver and heart. In Stat3−/− cells, the activities of complexes I and II of the electron transport chain (ETC) were significantly decreased. We identified Stat3 mutants that selectively restored the protein's function as a transcription factor or its functions within the ETC. In mice that do not express Stat3 in the heart, there were also selective defects in the activities of complexes I and II of the ETC. These data indicate that Stat3 is required for optimal function of the ETC, which may allow it to orchestrate responses to cellular homeostasis.

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 was originally identified as an IL-6-induced transcriptional activator of acute phase genes (1). However, other members of the IL-6 family, which utilize gp-130 receptor, as well as leptin, IL-12, IFN␣/␤, IL-10, GM-CSF, several growth factors, oncogenes, and stress such as hypoxia, also activate STAT3 (1). STAT3 is vital to embryonic development and STAT3-null mice are embryonic lethal (2). Analysis of tissuespecific conditional STAT3 knock-out mice has provided strong evidence that transcriptional activity of STAT3 plays a central role in the control of cell growth and host responses to inflammation and cellular stress (1). STAT3 positively regulates expression of anti-apoptotic (Bcl-2 and Bcl-xL) (1) and antioxidative proteins (MnSOD and metallothionein-1 and -2) (3, 4).Expression of STAT3 in the heart is associated with cardiac survival (5). When STAT3 is selectively deleted in cardiomyocytes, mice develop enhanced cardiac inflammation with fibrosis, dilated cardiomyopathy, and die prematurely due to congestive heart failure (5). Female mice, where STAT3 is not expressed in cardiomyocytes, develop post-partum cardiomyopathy, which is also seen in humans with reduced STAT3 expression in the myocardium (6). Ventricles from STAT3-null hearts show elevated levels of reactive oxygen species (ROS) 2 (6). Ischemic and pharmacologic preconditioning protected the viability of wild type but not STAT3 Ϫ/Ϫ cardiomyocytes (5). When STAT3 is overexpressed in cardiomyocytes, mice are less sensitive to the cardiotoxic effects of doxorubicin, which exerts i...

Background: A pool of the nuclear transcription factor Stat3 in the mitochondria (mitoStat3) controls respiration and Ras transformation. Results: Serine phosphorylation of mitoStat3 controls accumulation of reactive oxygen species and growth of breast cancer in mice. Conclusion:These results provide the first evidence for a mechanism by which mitoStat3 contributes to tumorigenesis. Significance: The data suggest new therapeutic approaches to treatment of breast cancer.

Purpose Recent advances in immunotherapy of advanced human cancers underscored the need to address and eliminate tumor immune evasion. The myeloid-derived suppressor cells (MDSCs) are important inhibitors of T cell responses in solid tumors, such as prostate cancers. However, targeting MDSCs proved challenging due to their phenotypic heterogeneity. Experimental Design Myeloid cell populations were evaluated using flow cytometry on blood samples, functional assays and immunohistochemical/immunofluorescent stainings on tumor/tumor-draining lymph node specimens from healthy subjects, localized and metastatic castration-resistant prostate cancer patients. Results Here, we identify a population of Lin−CD15HICD33LO granulocytic MDSCs that accumulate in patients’ circulation during prostate cancer progression from localized to metastatic disease. The prostate cancer-associated MDSCs potently inhibit autologous CD8+ T cells proliferation and production of IFNγ and Granzyme-B. The circulating MDSCs have high levels of activated STAT3, which is a central immune checkpoint regulator. The granulocytic pSTAT3+ cells are also detectable in patients’ prostate tissues. We previously generated an original strategy to silence genes specifically in Toll-like Receptor-9 (TLR9) positive myeloid cells using CpG-siRNA conjugates. We demonstrate that human granulocytic MDSCs express TLR9 and rapidly internalize naked CpG-STAT3siRNA thereby silencing STAT3 expression. STAT3 blocking abrogates immunosuppressive effects of patients-derived MDSCs on effector CD8+ T cells. These effects depended on reduced expression and enzymatic activity of Arginase-1, a downstream STAT3 target gene and a potent T cell inhibitor. Conclusions Overall, we demonstrate the accumulation of granulocytic MDSCs with prostate cancer progression and the feasibility of using TLR9-targeted STAT3siRNA delivery strategy to alleviate MDSC-mediated immunosuppression.

• CpG(A)-siRNA oligonucleotides allow for targeting genes specifically in human TLR9 ϩ immune cells and blood cancer cells.• Tumoricidal and immunostimulatory properties of CpG(A)-STAT3 siRNA provide a novel therapeutic opportunity for hematologic malignancies.STAT3 operates in both cancer cells and tumor-associated immune cells to promote cancer progression. As a transcription factor, it is a highly desirable but difficult target for pharmacologic inhibition. We have recently shown that the TLR9 agonists CpG oligonucleotides can be used for targeted siRNA delivery to mouse immune cells. In the present study, we demonstrate that a similar strategy allows for targeted gene silencing in both normal and malignant human TLR9 ؉ hematopoietic cells in vivo. We have developed new human cell-specific CpG(A) - IntroductionThe proliferation and survival of the majority of hematologic cancers depends on constitutive activity of STAT transcription factors. 1,2 The first evidence linking STAT activity with human blood cancer was derived from studies on multiple myeloma (MM). Permanent activity of STAT3 observed in myeloma cells was critical for their survival because of up-regulation of antiapoptotic BCL-X L protein. 3 Later reports identified constitutive activation of STAT3 not only in myeloma but also in other hematologic malignancies, with the highest frequency in B-cell lymphoma (BCL) and acute myeloid leukemia (AML) patient blasts. 1,4,5 The presence of activated STAT3 in leukemic blasts was associated with decreased disease-free survival of AML patients. 4 As a point of convergence for downstream signaling from cytokine and growth factor receptors, STAT3 plays a critical role in mediating cross-talk within the tumor microenvironment, which promotes tumor immune tolerance, vascularization, and metastasis. 6 Because STAT3 operates in both cancer cells and nonmalignant tumorassociated cells, it represents a highly desirable target for cancer therapy. 6 These important findings instigated numerous attempts to develop STAT3 inhibitors; however, pharmacologic inhibition of a protein lacking enzymatic activity is challenging. 4,7 An additional complication is the close structural similarity between oncogenic STAT3 and functionally distinct STAT1, a transcriptional factor critical for generation of antitumor immunity by IFNs. 8,9 The tyrosine kinase inhibitors upstream from STAT3, such as JAK, SRC, c-KIT, and FLT3 in leukemia, gained attention as promising blood cancer therapeutics. 4 However, the effect of small-molecule drugs, including FLT3 inhibitors, in most clinical trials was short-lived. 10,11 Other conventional treatment regimens for hematologic malignancies are limited by the high toxicity to normal tissues, development of drug resistance, and low disease-free survival rates. For personal use only. on May 11, 2018. by guest www.bloodjournal.org FromThe emergence of therapeutic strategies based on RNA interference (RNAi) created a unique opportunity to silence any diseaserelated target gene. 13,14 The major obstacle ...

Cancer radiotherapy may be immunogenic, but it is unclear why its immunogenic effects are rarely sufficient to prevent tumor recurrence. Here we report a novel Toll receptor-9 (TLR9)-dependent mechanism that initiates tumor regrowth after local radiotherapy. Systemic inhibition of TLR9, but not TLR4, delayed tumor recurrence in mouse models of B16 melanoma, MB49 bladder cancer and CT26 colon cancer after localized high-dose tumor irradiation. Soluble factors in the microenvironment of regressing tumors triggered TLR9 signaling in freshly recruited myeloid cells appearing within four days of radiotherapy. The tumorigenic effects of TLR9 depended on MyD88/NF-κB-mediated upregulation of IL-6 expression, which in turn resulted in downstream activation of Jak/STAT3 signaling in myeloid cells. By comparing global gene expression in wild-type, TLR9- or STAT3-deficient myeloid cells derived from irradiated tumors, we identified a unique set of TLR9/STAT3-regulated genes involved in tumor-promoting inflammation and re-vascularization. Blocking STAT3 function by two myeloid-specific genetic strategies corrected TLR9-mediated cancer recurrence after radiation therapy. Our results suggest that combining localized tumor irradiation with myeloid cell-specific inhibition of TLR9/STAT3 signaling may help eliminate radiation-resistant cancers.

• Blocking STAT3 in acute myeloid leukemia cells stimulates their TLR9-induced immunogenicity and antigenspecific activation of CD8

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.