Putting the brakes on mammary tumorigenesis: loss of STAT1 predisposes to intraepithelial neoplasias

STAT family of transcription factors in breast cancer: Pathogenesis and therapeutic opportunities and challenges

Breast cancer is the most commonly diagnosed cancer and second-leading cause of cancer deaths in women. Breast cancer stem cells (BCSCs) promote metastasis and therapeutic resistance contributing to tumor relapse. Through activating genes important for BCSCs, transcription factors contribute to breast cancer metastasis and therapeutic resistance, including the signal transducer and activator of transcription (STAT) family of transcription factors. The STAT family consists of six major isoforms, STAT1, STAT2, STAT3, STAT4, STAT5, and STAT6. Canonical STAT signaling is activated by the binding of an extracellular ligand to a cell-surface receptor followed by STAT phosphorylation, leading to STAT nuclear translocation and transactivation of target genes. It is important to note that STAT transcription factors exhibit diverse effects in breast cancer; some are either pro- or anti-tumorigenic while others maintain dual, context-dependent roles. Among the STAT transcription factors, STAT3 is the most widely studied STAT protein in breast cancer for its critical roles in promoting BCSCs, breast cancer cell proliferation, invasion, angiogenesis, metastasis, and immune evasion. Consequently, there have been substantial efforts in developing cancer therapeutics to target breast cancer with dysregulated STAT3 signaling. In this comprehensive review, we will summarize the diverse roles that each STAT family member plays in breast cancer pathobiology, as well as, the opportunities and challenges in pharmacologically targeting STAT proteins and their upstream activators in the context of breast cancer treatment.

To explore immune synergistic function of Quercetin in inhibiting breast cancer cells

Background

The precancerous disease of breast cancer is an inevitable stage in the tumorigenesis and development of breast neoplasms. Quercetin (Que) has shown great potential in breast cancer treatment by inhibiting cell proliferation and regulating T cell function. γδ T cells are a class of nontraditional T cells that have long attracted attention due to their potential in immunotherapy. In this study, we revealed the immunomodulatory function of Que through regulation of the JAK/STAT1 signaling pathway, which was followed by the synergistic killing of breast cancer cells.

Methods

In the experimental design, we first screened target genes with or without Que treatment, and we intersected the Que target with the disease target by functional enrichment analysis. Second, MCF-10A, MCF-10AT, MCF-7 and MDA-MB-231 breast cancer cell lines were treated with Que for 0 h, 24 h and 48 h. Then, we observed the expression of its subsets by coculturing Que and γδ T cells and coculturing Que and γδ T cells with breast tumor cells to investigate their synergistic killing effect on tumor cells. Finally, Western blotting was used to reveal the changes in proteins related to the JAK/STAT1 signaling pathway after Que treatment in MCF-10AT and MCF-7 cells for 48 h.

Results

The pathway affected by Que treatment was the JAK/STAT1 signaling pathway and was associated with precancerous breast cancer, as shown by network pharmacology analysis. Que induced apoptosis of MCF-10AT, MCF-7 and MDA-MB-231 cells in a time- and concentration-dependent manner (P < 0.05). Most importantly, Que promoted the differentiation of γδ T cells into the Vδ2 T cell subpopulation. The best ratio of effector cells to target cells (E/T) was 10:1, the killing percentages of γδ T cells against MCF-10A, MCF-10AT, MCF-7, and MDA-MB-231 were 61.44 ± 4.70, 55.52 ± 3.10, 53.94 ± 2.74, and 53.28 ± 1.73 (P = 0.114, P = 0.486, and P = 0.343, respectively), and the strongest killing effect on precancerous breast cancer cells and breast cancer cells was found when the Que concentration was 5 μM and the E/T ratio was 10:1 (64.94 ± 3.61, 64.96 ± 5.45, 55.59 ± 5.98, and 59.04 ± 5.67, respectively). In addition, our results showed that Que increased the protein levels of IFNγ-R, p-JAK2 and p-STAT1 while decreasing the protein levels of PD-L1 (P < 0.0001).

Conclusions

In conclusion, Que plays a synergistic role in killing breast cancer cells and promoting apoptosis by regulating the expression of IFNγ-R, p-JAK2, p-STAT1 and PD-L1 in the JAK/STAT1 signaling pathway and promoting the regulation of γδ T cells. Que may be a potential drug for the prevention of precancerous breast cancer and adjuvant treatment of breast cancer.

Type I interferon activation and endothelial dysfunction in caveolin-1 insufficiency-associated pulmonary arterial hypertension

Interferonopathies, interferon (IFN)-α/β therapy, and caveolin-1 (CAV1) loss-of-function have all been associated with pulmonary arterial hypertension (PAH). Here, CAV1-silenced primary human pulmonary artery endothelial cells (PAECs) were proliferative and hypermigratory, with reduced cytoskeletal stress fibers. Signal transducers and activators of transcription (STAT) and phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) were both constitutively activated in these cells, resulting in a type I IFN-biased inflammatory signature. Cav1 ^-/- mice that spontaneously develop pulmonary hypertension were found to have STAT1 and AKT activation in lung homogenates and increased circulating levels of CXCL10, a hallmark of IFN-mediated inflammation. PAH patients with CAV1 mutations also had elevated serum CXCL10 levels and their fibroblasts mirrored phenotypic and molecular features of CAV1-deficient PAECs. Moreover, immunofluorescence staining revealed endothelial CAV1 loss and STAT1 activation in the pulmonary arterioles of patients with idiopathic PAH, suggesting that this paradigm might not be limited to rare CAV1 frameshift mutations. While blocking JAK/STAT or AKT rescued aspects of CAV1 loss, only AKT inhibitors suppressed activation of both signaling pathways simultaneously. Silencing endothelial nitric oxide synthase (NOS3) prevented STAT1 and AKT activation induced by CAV1 loss, implicating CAV1/NOS3 uncoupling and NOS3 dysregulation in the inflammatory phenotype. Exogenous IFN reduced CAV1 expression, activated STAT1 and AKT, and altered the cytoskeleton of PAECs, implicating these mechanisms in PAH associated with autoimmune and autoinflammatory diseases, as well as IFN therapy. CAV1 insufficiency elicits an IFN inflammatory response that results in a dysfunctional endothelial cell phenotype and targeting this pathway may reduce pathologic vascular remodeling in PAH.

The mechanism of how CD95/Fas activates the Type I IFN/STAT1 axis, driving cancer stemness in breast cancer

CD95/Fas is an apoptosis inducing death receptor. However, it also has multiple nonapoptotic activities that are tumorigenic. Chronic stimulation of CD95 on breast cancer cells can increase their cancer initiating capacity through activation of a type I interferon (IFN-I)/STAT1 pathway when caspases are inhibited. We now show that this activity relies on the canonical components of the CD95 death-inducing signaling complex, FADD and caspase-8, and on the activation of NF-κB. We identified caspase-2 as the antagonistic caspase that downregulates IFN-I production. Once produced, IFN-Is bind to their receptors activating both STAT1 and STAT2 resulting in upregulation of the double stranded (ds)RNA sensor proteins RIG-I and MDA5, and a release of a subset of endogenous retroviruses. Thus, CD95 is part of a complex cell autonomous regulatory network that involves activation of innate immune components that drive cancer stemness and contribute to therapy resistance.

Constitutive activation of the EGFR-STAT1 axis increases proliferation of meningioma tumor cells

Background

Meningiomas are the most frequent primary brain tumors of the central nervous system. The standard of treatment is surgery and radiotherapy, but effective pharmacological options are not available yet. The well-characterized genetic background stratifies these tumors in several subgroups, thus increasing diversification. We identified epidermal growth factor receptor–signal transducer and activator of transcription 1 (EGFR–STAT1) overexpression and activation as a common identifier of these tumors.

Methods

We analyzed STAT1 overexpression and phosphorylation in 131 meningiomas of different grades and locations by utilizing several techniques, including Western blots, qPCR, and immunocytochemistry. We also silenced and overexpressed wild-type and mutant forms of the gene to assess its biological function and its network. Results were further validated by drug testing.

Results

STAT1 was found widely overexpressed in meningioma but not in the corresponding healthy controls. The protein showed constitutive phosphorylation not dependent on the JAK–STAT pathway. STAT1 knockdown resulted in a significant reduction of cellular proliferation and deactivation of AKT and ERK1/2. STAT1 is known to be activated by EGFR, so we investigated the tyrosine kinase and found that EGFR was also constitutively phosphorylated in meningioma and was responsible for the aberrant phosphorylation of STAT1. The pharmaceutical inhibition of EGFR caused a significant reduction in cellular proliferation and of overall levels of cyclin D1, pAKT, and pERK1/2.

Conclusions

STAT1–EGFR-dependent constitutive phosphorylation is responsible for a positive feedback loop that causes its own overexpression and consequently an increased proliferation of the tumor cells. These findings provide the rationale for further studies aiming to identify effective therapeutic options in meningioma.

JAK-STAT Signaling: A Double-Edged Sword of Immune Regulation and Cancer Progression

Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling mediates almost all immune regulatory processes, including those that are involved in tumor cell recognition and tumor-driven immune escape. Antitumor immune responses are largely driven by STAT1 and STAT2 induction of type I and II interferons (IFNs) and the downstream programs IFNs potentiate. Conversely, STAT3 has been widely linked to cancer cell survival, immunosuppression, and sustained inflammation in the tumor microenvironment. The discovery of JAK-STAT cross-regulatory mechanisms, post-translational control, and non-canonical signal transduction has added a new level of complexity to JAK-STAT governance over tumor initiation and progression. Endeavors to better understand the vast effects of JAK-STAT signaling on antitumor immunity have unearthed a wide range of targets, including oncogenes, miRNAs, and other co-regulatory factors, which direct specific phenotypical outcomes subsequent to JAK-STAT stimulation. Yet, the rapidly expanding field of therapeutic developments aimed to resolve JAK-STAT aberrations commonly reported in a multitude of cancers has been marred by off-target effects. Here, we discuss JAK-STAT biology in the context of immunity and cancer, the consequences of pathway perturbations and current therapeutic interventions, to provide insight and consideration for future targeting innovations.

Janus Kinase 1 Plays a Critical Role in Mammary Cancer Progression

Janus kinases (JAKs) and their downstream STAT proteins play key roles in cytokine signaling, tissue homeostasis, and cancer development. Using a breast cancer model that conditionally lacks Janus kinase 1, we show here that JAK1 is essential for IL-6-class inflammatory cytokine signaling and plays a critical role in metastatic cancer progression. JAK1 is indispensable for the oncogenic activation of STAT1, STAT3, and STAT6 in ERBB2-expressing cancer cells, suggesting that ERBB2 receptor tyrosine kinase complexes do not directly activate these STAT proteins in vivo. A genome-wide gene expression analysis revealed that JAK1 signaling has pleiotropic effects on several pathways associated with cancer progression. We established that FOS and MAP3K8 are targets of JAK1/STAT3 signaling, which promotes tumorsphere formation and cell migration. The results highlight the significance of JAK1 as a rational therapeutic target to block IL-6-class cytokines, which are master regulators of cancer-associated inflammation.

Interferons α and β in cancer: therapeutic opportunities from new insights

Over the past decade, preclinical and clinical research have confirmed the essential role of interferons for effective host immunological responses to malignant cells. Type I interferons (IFNα and IFNβ) directly regulate transcription of >100 downstream genes, which results in a myriad of direct (on cancer cells) and indirect (through immune effector cells and vasculature) effects on the tumour. New insights into endogenous and exogenous activation of type I interferons in the tumour and its microenvironment have given impetus to drug discovery and patient evaluation of interferon-directed strategies. When combined with prior observations or with other effective modalities for cancer treatment, modulation of the interferon system could contribute to further reductions in cancer morbidity and mortality. This Review discusses new interferon-directed therapeutic opportunities, ranging from cyclic dinucleotides to genome methylation inhibitors, angiogenesis inhibitors, chemoradiation, complexes with neoantigen-targeted monoclonal antibodies, combinations with other emerging therapeutic interventions and associations of interferon-stimulated gene expression with patient prognosis - all of which are strategies that have or will soon enter translational clinical evaluation.

Dichotomal functions of phosphorylated and unphosphorylated STAT1 in hepatocellular carcinoma

Abstract

Interferons (IFNs) with antiviral and immune-stimulatory functions have been widely used in prevention and treatment of hepatocellular carcinoma (HCC). Signal transducer and activator of transcription 1 (STAT1) is a key element of the IFN signaling, and the function of STAT1 is critically determined by its phosphorylation state. This study aims to understand the functions of phosphorylated (p-) and unphosphorylated (u-) STAT1 in HCC. We found that u-STAT1 is significantly elevated in patient HCC tumor tissues and predominantly expressed in cytoplasm; while p-STAT1 is absent. Loss of u-STAT1 potently arrested cell cycle and inhibited cell growth in HCC cells. Induction of p-STAT1 by IFN-α treatment effectively triggers the expression of interferon-stimulated genes (ISGs), but has moderate effect on HCC cell growth. Interestingly, both u-STAT1 and p-STAT1 are induced by IFN-α, through with distinct time-dependent process. Furthermore, the ISG induction patterns mediated by p-STAT1 and u-STAT1 are also distinct. Importantly, artificial blocking of the induction of u-STAT1, but not p-STAT1, sensitizes HCC cells to treatment of IFNs. Therefore, p-STAT1 and u-STAT1 exert dichotomal functions and coordinately regulate the responsiveness to IFN treatment in HCC.

Key Messages

STAT1 is upregulated and predominantly presented as u-STAT1 in HCC, while p-STAT1 is absent.

U-STAT1 sustains but p-STAT1 inhibits HCC growth.

The dynamic change of phosphorylation state of STAT1 control the responsiveness to IFN treatment.

Electronic supplementary material

The online version of this article (10.1007/s00109-018-1717-7) contains supplementary material, which is available to authorized users.

Aging Mouse Models Reveal Complex Tumor-Microenvironment Interactions in Cancer Progression

Mouse models and genetically engineered mouse models (GEMM) are essential experimental tools for the understanding molecular mechanisms within complex biological systems. GEMM are especially useful for inferencing phenocopy information to genetic human diseases such as breast cancer. Human breast cancer modeling in mice most commonly employs mammary epithelial-specific promoters to investigate gene function(s) and, in particular, putative oncogenes. Models are specifically useful in the mammary epithelial cell in the context of the complete mammary gland environment. Gene targeted knockout mice including conditional targeting to specific mammary cells can reveal developmental defects in mammary organogenesis and demonstrate the importance of putative tumor suppressor genes. Some of these models demonstrate a non-traditional type of tumor suppression which involves interplay between the tumor susceptible cell and its host/environment. These GEMM help to reveal the processes of cancer progression beyond those intrinsic to cancer cells. Furthermore, the, analysis of mouse models requires appropriate consideration of mouse strain, background, and environmental factors. In this review, we compare aging-related factors in mouse models for breast cancer. We introduce databases of GEMM attributes and colony functional variations.

STAT1 is a sex-specific tumor suppressor in colitis-associated colorectal cancer

The interferon-inducible transcription factor STAT1 is a tumor suppressor in various malignancies. We investigated sex-specific STAT1 functions in colitis and colitis-associated colorectal cancer (CRC) using mice with specific STAT1 deletion in intestinal epithelial cells (STAT1∆IEC ). Male but not female STAT1∆IEC mice were more resistant to DSS-induced colitis than sex-matched STAT1flox/flox controls and displayed reduced intraepithelial infiltration of CD8+ TCRαβ+ granzyme B+ T cells. Moreover, DSS treatment failed to induce expression of T-cell-attracting chemokines in intestinal epithelial cells of male but not of female STAT1∆IEC mice. Application of the AOM-DSS protocol for induction of colitis-associated CRC resulted in increased intestinal tumor load in male but not in female STAT1∆IEC mice. A sex-specific stratification of human CRC patients corroborated the data obtained in mice and revealed that reduced tumor cell-intrinsic nuclear STAT1 protein expression is a poor prognostic factor in men but not in women. These data demonstrate that epithelial STAT1 is a male-specific tumor suppressor in CRC of mice and humans.

Stimulation of murine cell-mediated immunity by dietary administration of a cell preparation of Enterococcus faecalis strain KH-2 and its possible activity against tumour development in mice

It is well known that dietary lactic acid bacteria (LAB) stimulate cell-mediated immunity such as natural killer (NK) activity in mice. Here, we aimed to assay the immunomodulatory effects of a cell preparation of Enterococcus faecalis strain KH-2 (CPEF). We further evaluated the possibility of antitumour activity caused by CPEF administration, because NK cells actively participate in the prevention of tumour formation. NK cell activity and gene expression of IFN-γ and Perforin 1, which were induced most likely by a synergetic action of their cytotoxic activity, were higher in splenocytes of CPEF-administered mice than they were in control mice. Moreover, unlike those of control mice, the splenocytes of CPEF-administered mice had significantly higher CD28⁺CD69⁺/CD4⁺ and CD28⁺CD69⁺/CD8⁺ ratios that resulted in a survival rate with a tendency toward improvement after 47 days of CPEF administration (p=0.1) in Meth-A fibrosarcoma-bearing mice. In conclusion, we showed that CPEF might be effective in treating Meth-A fibrosarcoma in mice, as it helped increase their survival rate via stimulation of an immune response in splenocytes, which involved systemic cellular immunity processes such as cytotoxic activity, and active T cells.

The relationship between total and phosphorylated STAT1 and STAT3 tumour cell expression, components of tumour microenvironment and survival in patients with invasive ductal breast cancer

The aim of the present study was to examine the relationship between tumour cell expression of total and phosphorylated STAT1 (ph-STAT1) and STAT3 (ph-STAT-3), components of tumour microenvironment and survival in patients with invasive ductal breast cancer.

Immunohistochemical analysis of total and ph-STAT1, and STAT3 were performed on tissue microarray of 384 breast cancer specimens. Tumour cell expression of STAT1 and STAT3 at both cytoplasmic and nuclear locations were combined and identified as STAT1/STAT3 tumour cell expression. These results were related to cancer specific survival (CSS) and phenotypic features of the tumour and the host.

High ph-STAT1 and ph-STAT3 tumour cell expression were associated with increased ER (both P≤0.001) and PR (both P <0.05), reduced tumour grade (P=0.015 and P<0.001 respectively) and necrosis (both P=0.001). Ph-STAT1 was associated with increased general inflammatory infiltrate (P=0.007) and ph-STAT3 was associated with lower CD4+ infiltration (P=0.024). In multivariate survival analysis, only high ph-STAT3 tumour cell expression was a predictor of improved CSS (P=0.010) independent of other tumour and host-based factors.

STAT1 and STAT3 tumour cell expression appeared to be an important determinant of favourable outcome in patients with invasive ductal breast cancer. The present results suggest that STAT1 and STAT3 may affect disease outcome through direct impact on tumour cells, counteracting aggressive tumour features, as well as interaction with the surrounding microenvironment.

CD95/Fas Increases Stemness in Cancer Cells by Inducing a STAT1-Dependent Type I Interferon Response

Stimulation of CD95/Fas drives and maintains cancer stem cells (CSCs). We now report that this involves activation of signal transducer and activator of transcription 1 (STAT1) and induction of STAT1-regulated genes and that this process is inhibited by active caspases. STAT1 is enriched in CSCs in cancer cell lines, patient-derived human breast cancer, and CD95^high-expressing glioblastoma neurospheres. CD95 stimulation of cancer cells induced secretion of type I interferons (IFNs) that bind to type I IFN receptors, resulting in activation of Janus-activated kinases, activation of STAT1, and induction of a number of STAT1-regulated genes that are part of a gene signature recently linked to therapy resistance in five primary human cancers. Consequently, we identified type I IFNs as drivers of cancer stemness. Knockdown or knockout of STAT1 resulted in a strongly reduced ability of CD95L or type I IFN to increase cancer stemness. This identifies STAT1 as a key regulator of the CSC-inducing activity of CD95.

Truncating Prolactin Receptor Mutations Promote Tumor Growth in Murine Estrogen Receptor-Alpha Mammary Carcinomas

Estrogen receptor alpha-positive (ERα+) luminal tumors are the most frequent subtype of breast cancer. Stat1(-/-) mice develop mammary tumors that closely recapitulate the biological characteristics of this cancer subtype. To identify transforming events that contribute to tumorigenesis, we performed whole genome sequencing of Stat1(-/-) primary mammary tumors and matched normal tissues. This investigation identified somatic truncating mutations affecting the prolactin receptor (PRLR) in all tumor and no normal samples. Targeted sequencing confirmed the presence of these mutations in precancerous lesions, indicating that this is an early event in tumorigenesis. Functional evaluation of these heterozygous mutations in Stat1(-/-) mouse embryonic fibroblasts showed that co-expression of truncated and wild-type PRLR led to aberrant STAT3 and STAT5 activation downstream of the receptor, cellular transformation in vitro, and tumor formation in vivo. In conclusion, truncating mutations of PRLR promote tumor growth in a model of human ERα+ breast cancer and warrant further investigation.

Pathobiology of the 129:Stat1 -/- mouse model of human age-related ER-positive breast cancer with an immune infiltrate-excluded phenotype

Background

Stat1 gene-targeted knockout mice (129S6/SvEvTac-Stat1^tm1Rds) develop estrogen receptor-positive (ER⁺), luminal-type mammary carcinomas at an advanced age. There is evidence for both host environment as well as tumor cell-intrinsic mechanisms to initiate tumorigenesis in this model. In this report, we summarize details of the systemic and mammary pathology at preneoplastic and tumor-bearing time points. In addition, we investigate tumor progression in the 129:Stat1^−/− host compared with wild-type 129/SvEv, and we describe the immune cell reaction to the tumors.

Methods

Mice housed and treated according to National Institutes of Health guidelines and Institutional Animal Care and Use Committee-approved methods were evaluated by histopathology, and their tissues were subjected to immunohistochemistry with computer-assisted quantitative image analysis. Tumor cell culture and conditioned media from cell culture were used to perform macrophage (RAW264.7) cell migration assays, including the 129:Stat1^−/−-derived SSM2 cells as well as control Met1 and NDL tumor cells and EpH4 normal cells.

Results

Tumorigenesis in 129:Stat1^−/− originates from a population of FoxA1⁺ large oval pale cells that initially appear and accumulate along the mammary ducts in segments or regions of the gland prior to giving rise to mammary intraepithelial neoplasias. Progression to invasive carcinoma is accompanied by a marked local stromal and immune cell response composed predominantly of T cells and macrophages. In conditioned media experiments, cells derived from 129:Stat1^−/− tumors secrete both chemoattractant and chemoinhibitory factors, with greater attraction in the extracellular vesicular fraction and inhibition in the soluble fraction. The result appears to be recruitment of the immune reaction to the periphery of the tumor, with exclusion of immune cell infiltration into the tumor.

Conclusions

129:Stat1^−/− is a unique model for studying the critical origins and risk reduction strategies in age-related ER⁺ breast cancer. In addition, it can be used in preclinical trials of hormonal and targeted therapies as well as immunotherapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0892-8) contains supplementary material, which is available to authorized users.

STAT1 gene deficient mice develop accelerated breast cancer growth and metastasis which is reduced by IL-17 blockade

Signal transducer and activator of transcription 1 (STAT1) mediates interferon gamma signaling which activates the expression of various genes related to apoptosis, inflammation, cell cycle and angiogenesis. Several experimental and clinical studies have investigated the role of STAT1 in primary tumor growth in breast cancer; however, its role in tumor metastasis remains to be determined. To determine the role of STAT1 in breast cancer metastasis, we analyzed growth and metastasis in WT or STAT1^−/− mice orthotopically implanted with metastatic 4T1.2 cells. Primary tumor development was faster in STAT1^−/− mice and these mice developed significantly bigger primary tumors and displayed more lung metastasis compared with WT counterparts. STAT1^−/− mice showed elevated Ly6G⁺CD11b⁺ granulocytic MDSC infiltration in their primary tumors and spleens with concomitant upregulation of Mmp9 and Cxcl1 expression in tumors compared with WT counterparts. Blockade of IL-17A in primary tumor-bearing STAT1^−/− mice suppressed accumulation of Ly6G⁺CD11b⁺ cells and markedly reduced lung metastasis. These data show that STAT1 is an important suppressor of primary breast tumor growth and metastasis. Importantly, we found anti-IL-17 treatment can rescue STAT1 deficient animals from developing exacerbated metastasis to the lungs which could be important for immunotherapies for immunocompromised breast cancer patients.

The good and the bad faces of STAT1 in solid tumours

Signal transducer and activator of transcription (STAT) 1 is part of the Janus kinase (JAK)/STAT signalling cascade and is best known for its essential role in mediating responses to all types of interferons (IFN). STAT1 regulates a variety of cellular processes, such as antimicrobial activities, cell proliferation and cell death. It exerts important immune modulatory activities both in the innate and the adaptive arm of the immune system. Based on studies in mice and data from human patients, STAT1 is generally considered a tumour suppressor but there is growing evidence that it can also act as a tumour promoter. This review aims at contrasting the two faces of STAT1 in tumourigenesis and providing an overview on the current knowledge of the underlying mechanisms or pathways.

STAT1 inhibits human hepatocellular carcinoma cell growth through induction of p53 and Fbxw7

Background

Aberrant STAT1 signaling is observed in human hepatocellular carcinoma (HCC) and has been associated with the modulation of cell proliferation and survival. However, the role of STAT1 signaling in HCC and its underlying mechanism remain elusive.

Methods

We transiently transfected pcDNA3.1-STAT1 and STAT1 siRNA into SMMC7721 and HepG2 cells. Western blot and qRT-PCR examined the expression of protein and RNA of target genes. Cell viability was assessed using MTT assay, and cell cycle and apoptosis were analyzed by flow cytometry.

Results

We found that STAT1 overexpression increased protein expression of p53 and Fbxw7, and downregulated the expression of cyclin A, cyclin D1, cyclin E, CDK2, Hes-1 and NF-κB p65. These changes led to growth inhibition and induced G0/G1 cell cycle arrest and apoptosis in SMMC7721 and HepG2 cells. Conversely, ablation of STAT1 had the opposite effect on p53, Fbxw7, Hes-1, NF-κB p65, cyclin A, cyclin D1, cyclin E and CDK2, and improved the viability of SMMC7721 and HepG2 cells.

Conclusions

Our data indicate that STAT1 exerts tumor-suppressive effects in hepatocarcinogenesis through induction of G0/G1 cell cycle arrest and apoptosis, and may provide a basis for the design of new therapies for the intervention of HCC in the clinic.

Abnormal Mammary Development in 129:STAT1-Null Mice is Stroma-Dependent

Female 129:Stat1-null mice (129S6/SvEvTac-Stat1^tm1Rds homozygous) uniquely develop estrogen-receptor (ER)-positive mammary tumors. Herein we report that the mammary glands (MG) of these mice have altered growth and development with abnormal terminal end buds alongside defective branching morphogenesis and ductal elongation. We also find that the 129:Stat1-null mammary fat pad (MFP) fails to sustain the growth of 129S6/SvEv wild-type and Stat1-null epithelium. These abnormalities are partially reversed by elevated serum progesterone and prolactin whereas transplantation of wild-type bone marrow into 129:Stat1-null mice does not reverse the MG developmental defects. Medium conditioned by 129:Stat1-null epithelium-cleared MFP does not stimulate epithelial proliferation, whereas it is stimulated by medium conditioned by epithelium-cleared MFP from either wild-type or 129:Stat1-null females having elevated progesterone and prolactin. Microarrays and multiplexed cytokine assays reveal that the MG of 129:Stat1-null mice has lower levels of growth factors that have been implicated in normal MG growth and development. Transplanted 129:Stat1-null tumors and their isolated cells also grow slower in 129:Stat1-null MG compared to wild-type recipient MG. These studies demonstrate that growth of normal and neoplastic 129:Stat1-null epithelium is dependent on the hormonal milieu and on factors from the mammary stroma such as cytokines. While the individual or combined effects of these factors remains to be resolved, our data supports the role of STAT1 in maintaining a tumor-suppressive MG microenvironment.

Stat1 stimulates cap-independent mRNA translation to inhibit cell proliferation and promote survival in response to antitumor drugs

The signal transducer and activator of transcription 1 (Stat1) functions as a tumor suppressor via immune regulatory and cell-autonomous pathways. Herein, we report a previously unidentified cell-autonomous Stat1 function, which is its ability to exhibit both antiproliferative and prosurvival properties by facilitating translation of mRNAs encoding for the cyclin-dependent kinase inhibitor p27(Kip1) and antiapoptotic proteins X-linked inhibitor of apoptosis and B-cell lymphoma xl. Translation of the select mRNAs requires the transcriptional function of Stat1, resulting in the up-regulation of the p110γ subunit of phosphoinositide 3-kinase (PI3K) class IB and increased expression of the translational repressor translation initiation factor 4E (eIF4E)-binding protein 1 (4EBP1). Increased PI3Kγ signaling promotes the degradation of the eIF4A inhibitor programmed cell death protein 4, which favors the cap-independent translation of the select mRNAs under conditions of general inhibition of protein synthesis by up-regulated eIF4E-binding protein 1. As such, Stat1 inhibits cell proliferation but also renders cells increasingly resistant to antiproliferative effects of pharmacological inhibitors of PI3K and/or mammalian target of rapamycin. Stat1 also protects Ras-transformed cells from the genotoxic effects of doxorubicin in culture and immune-deficient mice. Our findings demonstrate an important role of mRNA translation in the cell-autonomous Stat1 functions, with implications in tumor growth and treatment with chemotherapeutic drugs.

SIAH2 antagonizes TYK2-STAT3 signaling in lung carcinoma cells

The Janus tyrosine kinases JAK1-3 and tyrosine kinase-2 (TYK2) are frequently hyperactivated in tumors. In lung cancers JAK1 and JAK2 induce oncogenic signaling through STAT3. A putative role of TYK2 in these tumors has not been reported. Here, we show a previously not recognized TYK2-STAT3 signaling node in lung cancer cells. We reveal that the E3 ubiquitin ligase seven-in-absentia-2 (SIAH2) accelerates the proteasomal degradation of TYK2. This mechanism consequently suppresses the activation of STAT3. In agreement with these data the analysis of primary non-small-cell lung cancer (NSCLC) samples from three patient cohorts revealed that compared to lung adenocarcinoma (ADC), lung squamous cell carcinoma (SCC) show significantly higher levels of SIAH2 and reduced STAT3 phosphorylation levels. Thus, SIAH2 is a novel molecular marker for SCC. We further demonstrate that an activation of the oncologically relevant transcription factor p53 in lung cancer cells induces SIAH2, depletes TYK2, and abrogates the tyrosine phosphorylation of STAT1 and STAT3. This mechanism appears to be different from the inhibition of phosphorylated JAKs through the suppressor of cytokine signaling (SOCS) proteins. Our study may help to identify molecular mechanisms affecting lung carcinogenesis and potential therapeutic targets.

Caspase-3 and caspase-6 cleave STAT1 in leukemic cells

Signal Transducer and Activator of Transcription-1 (STAT1) is phosphorylated upon interferon (IFN) stimulation, which can restrict cell proliferation and survival. Nevertheless, in some cancers STAT1 can act in an anti-apoptotic manner. Moreover, certain malignancies are characterized by the overexpression and constitutive activation of STAT1. Here, we demonstrate that the treatment of transformed hematopoietic cells with epigenetic drugs belonging to the class of histone deacetylase inhibitors (HDACi) leads to the cleavage of STAT1 at multiple sites by caspase-3 and caspase-6. This process does not occur in solid tumor cells, normal hematopoietic cells, and leukemic cells that underwent granulocytic or monocytic differentiation. STAT1 cleavage was studied under cell free conditions with purified STAT1 and a set of candidate caspases as well as with mass spectrometry. These assays indicate that unmodified STAT1 is cleaved at multiple sites by caspase-3 and caspase-6. Our study shows that STAT1 is targeted by caspases in malignant undifferentiated hematopoietic cells. This observation may provide an explanation for the selective toxicity of HDACi against rapidly proliferating leukemic cells.

Mouse models of breast cancer

Breast cancer is the most common cause of cancer death in women worldwide. This malignancy is a complex disease, which is defined by an intrinsic heterogeneity on the histopathological and molecular level as well as response to therapy and outcome. In addition to classical histopathological features, breast cancer can be categorized into at least five major subtypes based on comprehensive gene expression profiling: luminal A, luminal B, basal-like, ERBB2-positive, and normal-like breast cancer. Genetically engineered mouse models can serve as tools to study the molecular underpinnings for this disease. Given the genetic complexity that drives the initiation and progression of individual breast cancer subtypes, it is evident that certain models can reflect only particular aspects of this malignancy. In this book chapter, we will primarily focus on advances in modeling breast cancer at defined stages of carcinogenesis using genetically engineered mice. We will discuss the ability as well as shortcomings of these models to faithfully recapitulate the spectrum of human breast cancer subtypes.

STAT1 and STAT3 in tumorigenesis: A matter of balance

The transcription factors STAT1 and STAT3 appear to play opposite roles in tumorigenesis. While STAT3 promotes cell survival/proliferation, motility and immune tolerance and is considered as an oncogene, STAT1 mostly triggers anti-proliferative and pro-apoptotic responses while enhancing anti-tumor immunity. Despite being activated downstream of common cytokine and growth factor receptors, their activation is reciprocally regulated and perturbation in their balanced expression or phosphorylation levels may re-direct cytokine/growth factor signals from proliferative to apoptotic, or from inflammatory to anti-inflammatory. Here we review the functional canonical and non-canonical effects of STAT1 and STAT3 activation in tumorigenesis and their potential cross-regulation mechanisms.

The tumor suppressor function of STAT1 in breast cancer

The anti-tumor function of STAT1 through its capacity to control the immune system and promote tumor immune surveillance has been well understood. However, little is known about cell autonomous (i.e., tumor cell-specific) functions of STAT1 in tumor formation. Recent studies have provided strong evidence that STAT1 suppresses mouse mammary gland tumorigenesis by both, immune regulatory and tumor cell-specific functions of STAT1. Specifically, STAT1 deficiency in the mouse mammary gland inhibits ErbB2/Neu-mediated tumorigenesis and contributes to spontaneous formation of estrogen receptor α (ER α)-positive as well as ER α-negative tumors closely resembling human disease. Herein, we review the anti-tumor functions of STAT1 revealed from investigations of murine breast cancer models and from characterization of the signaling properties of STAT1 in human breast tumor cells. The significance of STAT1 in breast cancer is underscored by studies proposing a prognostic value for the expression and/or phosphorylation of STAT1 for specific molecular types of breast cancer. Furthermore, STAT1 dependent transcription is proposed to contribute to therapeutic responses by modulating the efficacy of chemotherapeutic drugs and the development of drug resistance.

High STAT1 mRNA levels but not its tyrosine phosphorylation are associated with macrophage infiltration and bad prognosis in breast cancer

Background

STAT1 has been attributed a function as tumor suppressor. However, in breast cancer data from microarray analysis indicated a predictive value of high mRNA expression levels of STAT1 and STAT1 target genes belonging to the interferon-related signature for a poor response to therapy. To clarify this issue we have determined STAT1 expression levels and activation by different methods, and investigated their association with tumor infiltration by immune cells. Additionally, we evaluated the interrelationship of these parameters and their significance for predicting disease outcome.

Methods

Expression of STAT1, its target genes SOCS1, IRF1, CXCL9, CXCL10, CXCL11, IFIT1, IFITM1, MX1 and genes characteristic for immune cell infiltration (CD68, CD163, PD-L1, PD-L2, PD-1, CD45, IFN-γ, FOXP3) was determined by RT-PCR in two independent cohorts comprising 132 breast cancer patients. For a subset of patients, protein levels of total as well as serine and tyrosine-phosphorylated STAT1 were ascertained by immunohistochemistry or immunoblotting and protein levels of CXCL10 by ELISA.

Results

mRNA expression levels of STAT1 and STAT1 target genes, as well as protein levels of total and serine-phosphorylated STAT1 correlated with each other in neoplastic tissue. However, there was no association between tumor levels of STAT1 mRNA and tyrosine-phosphorylated STAT1 and between CXCL10 serum levels and CXCL10 expression in the tumor. Tumors with increased STAT1 mRNA amounts exhibited elevated expression of genes characteristic for tumor-associated macrophages and immunosuppressive T lymphocytes. Survival analysis revealed an association of high STAT1 mRNA levels and bad prognosis in both cohorts. A similar prognostically relevant correlation with unfavorable outcome was evident for CXCL10, MX1, CD68, CD163, IFN-γ, and PD-L2 expression in at least one collective. By contrast, activation of STAT1 as assessed by the level of STAT1-Y701 phosphorylation was linked to positive outcome. In multivariate Cox regression, the predictive power of STAT1 mRNA expression was lost when including expression of CXCL10, MX1 and CD68 as confounders.

Conclusions

Our study confirms distinct prognostic relevance of STAT1 expression levels and STAT1 tyrosine phosphorylation in breast cancer patients and identifies an association of high STAT1 levels with elevated expression of STAT1 target genes and markers for infiltrating immune cells.

Inducible, dose-adjustable and time-restricted reconstitution of STAT1 deficiency in vivo

Signal transducer and activator of transcription (STAT) 1 is a key player in interferon (IFN) signaling, essential in mediating host defense against viruses and other pathogens. STAT1 levels are tightly regulated and loss- or gain-of-function mutations in mice and men lead to severe diseases. We have generated a doxycycline (dox) -inducible, FLAG-tagged Stat1 expression system in mice lacking endogenous STAT1 (i.e. Stat1^ind mice). We show that STAT1 expression depends on the time and dose of dox treatment in primary cells and a variety of organs isolated from Stat1^ind mice. In bone marrow-derived macrophages, a fraction of the amount of STAT1 present in WT cells is sufficient for full expression of IFN-induced genes. Dox-induced STAT1 established protection against virus infections in primary cells and mice. The availability of the Stat1^ind mouse model will enable an examination of the consequences of variable amounts of STAT1. The model will also permit the study of STAT1 dose-dependent and reversible functions as well as of STAT1's contributions to the development, progression and resolution of disease.

Dysregulated STAT1-SOCS1 control of JAK2 promotes mammary luminal progenitor cell survival and drives ERα(+) tumorigenesis

We previously reported that STAT1 expression is frequently abrogated in human estrogen receptor-α-positive (ERα(+)) breast cancers and mice lacking STAT1 spontaneously develop ERα(+) mammary tumors. However, the precise mechanism by which STAT1 suppresses mammary gland tumorigenesis has not been fully elucidated. Here we show that STAT1-deficient mammary epithelial cells (MECs) display persistent prolactin receptor (PrlR) signaling, resulting in activation of JAK2, STAT3 and STAT5A/5B, expansion of CD61(+) luminal progenitor cells and development of ERα(+) mammary tumors. A failure to upregulate SOCS1, a STAT1-induced inhibitor of JAK2, leads to unopposed oncogenic PrlR signaling in STAT1(-/-) MECs. Prophylactic use of a pharmacological JAK2 inhibitor restrains the proportion of luminal progenitors and prevents disease induction. Systemic inhibition of activated JAK2 induces tumor cell death and produces therapeutic regression of pre-existing endocrine-sensitive and refractory mammary tumors. Thus, STAT1 suppresses tumor formation in mammary glands by preventing the natural developmental function of a growth factor signaling pathway from becoming pro-oncogenic. In addition, targeted inhibition of JAK2 may have significant therapeutic potential in controlling ERα(+) breast cancer in humans.

CDK8-mediated STAT1-S727 phosphorylation restrains NK cell cytotoxicity and tumor surveillance

The transcription factor STAT1 is important in natural killer (NK) cells, which provide immediate defense against tumor and virally infected cells. We show that mutation of a single phosphorylation site (Stat1-S727A) enhances NK cell cytotoxicity against a range of tumor cells, accompanied by increased expression of perforin and granzyme B. Stat1-S727A mice display significantly delayed disease onset in NK cell-surveilled tumor models including melanoma, leukemia, and metastasizing breast cancer. Constitutive phosphorylation of S727 depends on cyclin-dependent kinase 8 (CDK8). Inhibition of CDK8-mediated STAT1-S727 phosphorylation may thus represent a therapeutic strategy for stimulating NK cell-mediated tumor surveillance.

Cytokine-induced killer (CIK) cells bound with anti-CD3/anti-CD133 bispecific antibodies target CD133(high) cancer stem cells in vitro and in vivo

CD133 is a common marker of cancer stem cells (CSCs). We generated an anti-CD3/anti-CD133 bispecific antibody (BsAb) and bound it to the cytokine-induced killer (CIK) cells as effector cells (BsAb-CIK) to target CD133(high) CSCs. The killing of CD133(high) pancreatic (SW1990) and hepatic (Hep3B) cancer cells by the BsAb-CIK cells was significantly (p<0.05) higher than the killing by the parental CIK or by CIK cells bound with anti-CD3 (CD3-CIK) without CD133 targeting. In nude mice, the BsAb-CIK cells inhibited CD133(high) tumor growth significantly (p<0.05) more than that by CIK or CD3-CIK cells, or by the BsAb alone. BsAb-CIK cells co-cultured with CD133(high) cells produced significantly (p<0.05) higher amount of IFN-γ. Treatment with the BsAb-CIK cells significantly downregulated the expression of S100P and IL-18bp, but upregulated STAT1. The findings may help with the development of novel immunotherapies for patients with cancer containing CD133(high) CSCs by selectively targeting this cell population.

Acetylation modulates the STAT signaling code

A fascinating question of modern biology is how a limited number of signaling pathways generate biological diversity and crosstalk phenomena in vivo. Well-defined posttranslational modification patterns dictate the functions and interactions of proteins. The signal transducers and activators of transcription (STATs) are physiologically important cytokine-induced transcription factors. They are targeted by a multitude of posttranslational modifications that control and modulate signaling responses and gene expression. Beyond phosphorylation of serine and tyrosine residues, lysine acetylation has recently emerged as a critical modification regulating STAT functions. Interestingly, acetylation can determine STAT signaling codes by various molecular mechanisms, including the modulation of other posttranslational modifications. Here, we provide an overview on the acetylation of STATs and how this protein modification shapes cellular cytokine responses. We summarize recent advances in understanding the impact of STAT acetylation on cell growth, apoptosis, innate immunity, inflammation, and tumorigenesis. Furthermore, we discuss how STAT acetylation can be targeted by small molecules and we consider the possibility that additional molecules controlling STAT signaling are regulated by acetylation. Our review also summarizes evolutionary aspects and we show similarities between the acetylation-dependent control of STATs and other important molecules. We propose the concept that, similar to the 'histone code', distinct posttranslational modifications and their crosstalk orchestrate the functions and interactions of STAT proteins.

Conditional Stat1 ablation reveals the importance of interferon signaling for immunity to Listeria monocytogenes infection

Signal transducer and activator of transcription 1 (Stat1) is a key player in responses to interferons (IFN). Mutations of Stat1 cause severe immune deficiencies in humans and mice. Here we investigate the importance of Stat1 signaling for the innate and secondary immune response to the intracellular bacterial pathogen Listeria monocytogenes (Lm). Cell type-restricted ablation of the Stat1 gene in naïve animals revealed unique roles in three cell types: macrophage Stat1 signaling protected against lethal Lm infection, whereas Stat1 ablation in dendritic cells (DC) did not affect survival. T lymphocyte Stat1 reduced survival. Type I IFN (IFN-I) signaling in T lymphocytes reportedly weakens innate resistance to Lm. Surprisingly, the effect of Stat1 signaling was much more pronounced, indicating a contribution of Stat1 to pathways other than the IFN-I pathway. In stark contrast, Stat1 activity in both DC and T cells contributed positively to secondary immune responses against Lm in immunized animals, while macrophage Stat1 was dispensable. Our findings provide the first genetic evidence that Stat1 signaling in different cell types produces antagonistic effects on innate protection against Lm that are obscured in mice with complete Stat1 deficiency. They further demonstrate a drastic change in the cell type-dependent Stat1 requirement for memory responses to Lm infection.

Signal transducer and activator of transcription 1 (Stat1) is an indispensable component of the cellular response to interferons (IFN) during immune reactions to pathogens. Stat1 deficiency leads to severe immune defects in humans and mice. The sensitivity of animals with complete Stat1 ablation to microbial pathogens prevented determining its contribution to various effector systems of the immune response. By way of tissue-restricted Stat1 ablation we now decipher the impact of Stat1 signaling in different cell populations on the innate and adaptive immune response to the intracellular pathogen Listeria monocytogenes. Our data highlight the importance of and requirement for IFNγ-activated macrophages for clearance of the pathogen during early phases of infection, and show a yet unanticipated detrimental role for T cell Stat1. During secondary responses the picture changes and Stat1 in T cells is crucial for proper clearance of L. monocytogenes. Likewise, Stat1 signaling in dendritic cells plays a fundamental role for adaptive immunity to L. monocytogenes. Exploring the local response to L. monocytogenes infection we reveal a role of Stat1 in shaping the cellular composition of inflammatory infiltrates. Furthermore, Stat1 deficiency in dendritic cells increases the proliferation of regulatory T cells, an effect likely to dampen the antibacterial response.