Differential activation of STATs 3 and 5 during mammary gland development

Methods for investigating STAT3 regulation of lysosomal function in mammary epithelial cells

The transcription factor STAT3 is activated by multiple cytokines and other extrinsic factors. It plays a key role in immune and inflammatory responses and, when dysregulated, in tumourigenesis. STAT3 is also an indispensable mediator of the cell death process that occurs during post-lactational regression of the mammary gland, one of the most dramatic examples of physiological cell death in adult mammals. During this involution of the gland, STAT3 powerfully enhances the lysosomal system to efficiently remove superfluous milk-producing mammary epithelial cells via a lysosomal-mediated programmed cell death pathway. The lysosome is a membrane-enclosed  cytoplasmic organelle that digests and recycles cellular waste, with an important role as a signalling centre that monitors cellular metabolism. Here, we describe key strategies for investigating the role of STAT3 in regulating lysosomal function using a mammary epithelial cell culture model system. These include protocols for lysosome enrichment and enzyme activity assays, in addition to microscopic analyses of the vesicular compartment in cell lines. Collectively, these approaches provide the tools to investigate multiple aspects of lysosome biogenesis and function, and to define both direct and indirect roles for STAT3.

Interferon-Induced Bone Marrow Stromal Antigen 2 (BST2) Is A Functional Tumor-Initiating Cell Marker In Triple-Negative Breast Cancer

Tumor-initiating cells (TIC) are a tumor cell subpopulation thought to be responsible for therapeutic resistance and metastasis. Using a S ignal T ransducer and A ctivator of T ranscription (STAT) reporter, and a STAT-responsive lineage tracing system, we enriched for cells with enhanced mammosphere-forming potential in some, but not all, triple-negative breast cancer xenograft models (TNBC) indicating TIC-related and TIC-independent functions for STAT signaling. Single-cell RNA sequencing (scRNA-seq) of reporter-tagged xenografts identified a common interferon-associated transcriptional state, previously linked to inflammation and macrophage differentiation, in TIC. Similar transcriptional states exist in human breast cancer patient scRNA-seq datasets. Flow cytometric sorting using bone marrow stromal cell antigen 2 (BST2), a marker of this state, enriched for TIC, and BST2 knockdown reduced mammosphere-forming potential. These results suggest TIC may exploit the interferon response pathway to promote their activity in TNBC. Our results lay the groundwork to target interferon-associated pathways in TIC in a subset of TNBC.

ERBB Receptors and Their Ligands in the Developing Mammary Glands of Different Species: Fifteen Characters in Search of an Author

The ERBB tyrosine kinase receptors and their ligands belong to a complex family that has diverse biological effects and expression profiles in the developing mammary glands, where its members play an essential role in translating hormone signals into local effects. While our understanding of these processes stems mostly from mouse models, there is the potential for differences in how this family functions in the mammary glands of other species, particularly in light of their unique histomorphological features. Herein we review the postnatal distribution and function of ERBB receptors and their ligands in the mammary glands of rodents and humans, as well as for livestock and companion animals. Our analysis highlights the diverse biology for this family and its members across species, the regulation of their expression, and how their roles and functions might be modulated by varying stromal composition and hormone interactions. Given that ERBB receptors and their ligands have the potential to influence processes ranging from normal mammary development to diseased states such as cancer and/or mastitis, both in human and veterinary medicine, a more complete understanding of their biological functions should help to direct future research and the identification of new therapeutic targets.

Stat3 Tyrosine 705 and Serine 727 Phosphorylation Associate With Clinicopathological Characteristics and Distinct Tumor Cell Phenotypes in Triple-Negative Breast Cancer

Signal transducer and activator of transcription 3 (Stat3) is responsible for many aspects of normal development and contributes to the development and progression of cancer through regulating epithelial cell identity and cancer stem cells. In breast cancer, Stat3 is associated with triple-negative breast cancers (TNBC) and its function has been related to the activation of p63, itself a marker of basal-like TNBC and a master regulator of stem cell activities. Stat3 activation is controlled by dual phosphorylation at tyrosine 705 (pTyr705) and serine 727 (pSer727), although it is unclear whether these have equivalent effects, and whether they are related or independent events. To address these issues, we investigated Stat3 phosphorylation at the two sites by immunohistochemistry in 173 patients with TNBC. Stat3 phosphorylation was assessed by automated quantitative measurements of digitized scanned images and classified into four categories based on histoscore. The results were analyzed for associations with multiple markers of tumor phenotype, proliferation, BRCA status, and clinicopathological characteristics. We show that the levels of pTyr705- and pSer727-Stat3 were independent in 34% of tumors. High pTyr705-Stat3 levels were associated with the luminal differentiation markers ERβ/AR and MUC1, whereas tumors with high levels of pSer727-Stat3 were more likely to be positive for the basal marker CK5/6, but were independent of p63 and were EGFR negative. Combined high pSer727- and low Tyr705-Stat3 phosphorylation associated with basal-like cancer. Although high Stat3 phosphorylation levels were associated with less aggressive tumor characteristics, they did not associate with improved survival, indicating that Stat3 phosphorylation is an unfavorable indicator for tumors with an otherwise good prognosis according to clinicopathological characteristics. These findings also show that pTyr705-Stat3 and pSer727-Stat3 associate with specific breast tumor phenotypes, implying that they exert distinct functional activities in breast cancer.

Alveolar cells in the mammary gland: lineage commitment and cell death

The mammary gland provides a spectacular example of physiological cell death whereby the cells that produce milk during lactation are removed swiftly, efficiently, and without inducing inflammation upon the cessation of lactation. The milk-producing cells arise primarily during pregnancy and comprise the alveolar lineage that is specified by signalling pathways and factors that are activated in response to pregnancy hormones. There are at least two alveolar sub-lineages, one of which is marked by the presence of binucleate cells that are especially susceptible to programmed cell death during involution. This process of post-lactational regression, or involution, is carefully orchestrated and occurs in two phases, the first results in a rapid switch in cell fate with the secretory epithelial cells becoming phagocytes whereupon they destroy dead and dying cells from milk. This reversible phase is followed by the second phase that is marked by an influx of immune cells and a remodelling of the gland to replace the alveolar cells with re-differentiated adipocytes, resulting in a return to the pre-pregnant state in preparation for any subsequent pregnancies. The mouse mammary gland provides an excellent experimental tool with which to investigate lineage commitment and the mechanisms of programmed cell death that occur in a normal physiological process. Importantly, involution has highlighted a role for lysoptosis, a mechanism of cell death that is mediated by lysosomal cathepsins and their endogenous inhibitors, serpins. In this review, I discuss alveolar lineage commitment during pregnancy and the programmed cell death pathways that destroy these cells during involution.

PTHrP induces STAT5 activation, secretory differentiation and accelerates mammary tumor development

BACKGROUND

Parathyroid hormone-related protein (PTHrP) is required for embryonic breast development and has important functions during lactation, when it is produced by alveolar epithelial cells and secreted into the maternal circulation to mobilize skeletal calcium used for milk production. PTHrP is also produced by breast cancers, and GWAS studies suggest that it influences breast cancer risk. However, the exact functions of PTHrP in breast cancer biology remain unsettled.

METHODS

We developed a tetracycline-regulated, MMTV (mouse mammary tumor virus)-driven model of PTHrP overexpression in mammary epithelial cells (Tet-PTHrP mice) and bred these mice with the MMTV-PyMT (polyoma middle tumor-antigen) breast cancer model to analyze the impact of PTHrP overexpression on normal mammary gland biology and in breast cancer progression.

RESULTS

Overexpression of PTHrP in luminal epithelial cells caused alveolar hyperplasia and secretory differentiation of the mammary epithelium with milk production. This was accompanied by activation of Stat5 and increased expression of E74-like factor-5 (Elf5) as well as a delay in post-lactation involution. In MMTV-PyMT mice, overexpression of PTHrP (Tet-PTHrP;PyMT mice) shortened tumor latency and accelerated tumor growth, ultimately reducing overall survival. Tumors overproducing PTHrP also displayed increased expression of nuclear pSTAT5 and Elf5, increased expression of markers of secretory differentiation and milk constituents, and histologically resembled secretory carcinomas of the breast. Overexpression of PTHrP within cells isolated from tumors, but not PTHrP exogenously added to cell culture media, led to activation of STAT5 and milk protein gene expression. In addition, neither ablating the Type 1 PTH/PTHrP receptor (PTH1R) in epithelial cells nor treating Tet-PTHrP;PyMT mice with an anti-PTH1R antibody prevented secretory differentiation or altered tumor latency. These data suggest that PTHrP acts in a cell-autonomous, intracrine manner. Finally, expression of PTHrP in human breast cancers is associated with expression of genes involved in milk production and STAT5 signaling.

CONCLUSIONS

Our study suggests that PTHrP promotes pathways leading to secretory differentiation and proliferation in both normal mammary epithelial cells and in breast tumor cells.

Identification of Gene Modules and Hub Genes Involved in Mastitis Development Using a Systems Biology Approach

Objective

Mastitis is defined as the inflammation of the mammary gland, which impact directly on the production performance and welfare of dairy cattle. Since, mastitis is a multifactorial complex disease and the molecular pathways underlying this disorder have not been clearly understood yet, a system biology approach was used in this study to a better understanding of the molecular mechanisms behind mastitis.

Methods

Publicly available RNA-Seq data containing samples from milk of five infected and five healthy Holstein cows at five time points were retrieved. Gene Co-expression network analysis (WGCNA) approach and functional enrichment analysis were then applied with the aim to find the non-preserved module of genes that their connectivity were altered under infected condition. Hub genes were identified in the non-preserved modules and were subjected to protein-protein interactions (PPI) network construction.

Results

Among the 25 modules identified, eight modules were non-preserved and were also biologically associated with inflammation, immune response and mastitis development. Interestingly most of the hub genes in the eight modules were also densely connected in the PPI network. Of the hub genes, 250 genes were hubs in both co-expression and PPI networks and most of them were reported to play important roles in immune response or inflammatory pathways. The blue module was highly enriched in inflammatory responses and STAT1 was suggested to play an important role in mastitis development by regulating the immune related genes in this module. Moreover, a set of highly connected genes were identified such as BIRC3, PSMA6, FYN, F11R, NFKBIZ, NFKBIA, GRO1, PHB, CD3E, IL16, GSN, SOCS2, HCK, VAV1 and TLR6, which have been established to be critical for mastitis pathogenesis.

Conclusion

This study improved the understanding of the mechanisms underlying bovine mastitis and suggested eight non-preserved modules along with several most important genes with promising potential in etiology of mastitis.

Autophagy regulates functional differentiation of mammary epithelial cells

Mitochondria operate as a central hub for many metabolic processes by sensing and responding to the cellular environment. Developmental cues from the environment have been implicated in selective autophagy, or mitophagy, of mitochondria during cell differentiation and tissue development. Mitophagy occurring in this context, termed programmed mitophagy, responds to cell state rather than mitochondrial damage and is often accompanied by a metabolic transition. However, little is known about the mechanisms that engage and execute mitophagy under physiological or developmental conditions. As the mammary gland undergoes post-natal development and lactation challenges mitochondrial homeostasis, we investigated the contribution of mitochondria to differentiation of mammary epithelial cells (MECs). Using lactogenic differentiation of the HC11 mouse MEC line, we demonstrated that HC11 cells transition to a highly energetic state during differentiation by engaging both oxidative phosphorylation and glycolysis. Interestingly, this transition was lost when autophagy was inhibited with bafilomycin A₁ or knockdown of Atg7 (autophagy related 7). To evaluate the specific targeting of mitochondria, we traced mitochondrial oxidation and turnover in vitro with the fluorescent probe, pMitoTimer. Indeed, we found that differentiation engaged mitophagy. To further evaluate the requirement of mitophagy during differentiation, we knocked down the expression of Prkn/parkin in HC11 cells. We found that MEC differentiation was impaired in shPrkn cells, implying that PRKN is required for MEC differentiation. These studies suggest a novel regulation of MEC differentiation through programmed mitophagy and provide a foundation for future studies of development and disease associated with mitochondrial function in the mammary gland.Abbreviations: AA: antimycin A; ATG5: autophagy related 5; BAF: bafilomycin A₁; BNIP3: BCL2 interacting protein 3; BNIP3L/NIX: BCL2 interacting protein 3 like; COX8A: cytochrome c oxidase subunit 8A; CQ: chloroquine; CSN2: casein beta; ECAR: extracellular acidification rate; FCCP: trifluoromethoxy carbonylcyanide phenylhydrazone; FUNDC1: FUN14 domain containing 1; HIF1A: hypoxia inducible factor 1 subunit alpha; L1: lactation day 1; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MEC: mammary epithelial cell; mitoQ: mitoquinol; mROS: mitochondrial reactive oxygen species; OCR: oxygen consumption rate; P: priming; P16: pregnancy day 16; PARP1: poly(ADP-ribose) polymerase 1; PINK1: PTEN induced kinase 1; PPARGC1A: PPARG coactivator 1 alpha; PRKN: parkin RBR E3 ubiquitin protein ligase; shNT: short hairpin non-targeting control; SQSTM1: sequestosome 1; STAT3: signal transducer and activator of transcription 3; TEM: transmission electron microscopy; TFAM: transcription factor A, mitochondrial; U: undifferentiated.

TRPS1 acts as a context-dependent regulator of mammary epithelial cell growth/differentiation and breast cancer development

In this study, Cornelissen et al. set out to elucidate the role of the GATA-type zinc finger transcription factor TRPS1 in breast cancer. Using in vitro and in vivo loss-of-function approaches, the authors demonstrate that TRPS1 can function as a context-dependent tumor suppressor in breast cancer, while being essential for growth and differentiation of normal mammary epithelial cells.

The GATA-type zinc finger transcription factor TRPS1 has been implicated in breast cancer. However, its precise role remains unclear, as both amplifications and inactivating mutations in TRPS1 have been reported. Here, we used in vitro and in vivo loss-of-function approaches to dissect the role of TRPS1 in mammary gland development and invasive lobular breast carcinoma, which is hallmarked by functional loss of E-cadherin. We show that TRPS1 is essential in mammary epithelial cells, since TRPS1-mediated suppression of interferon signaling promotes in vitro proliferation and lactogenic differentiation. Similarly, TRPS1 expression is indispensable for proliferation of mammary organoids and in vivo survival of luminal epithelial cells during mammary gland development. However, the consequences of TRPS1 loss are dependent on E-cadherin status, as combined inactivation of E-cadherin and TRPS1 causes persistent proliferation of mammary organoids and accelerated mammary tumor formation in mice. Together, our results demonstrate that TRPS1 can function as a context-dependent tumor suppressor in breast cancer, while being essential for growth and differentiation of normal mammary epithelial cells.

Morphological, hormonal, and molecular changes in different maternal tissues during lactation and post-lactation

Milk supply and quality during lactation are critical for progeny survival. Maternal tissues and metabolism, influenced by hormonal changes, undergo modification during lactation to sustain breastfeeding. Two organs that suffer essential adjustment are the mammary glands and the bone; however, renal calcium conservation and calcium absorption from the intestine are also modified. Lactation leads to a transient loss of bone minerals to provide adequate amounts of minerals, including calcium for milk production. Physiological, metabolic, and molecular changes in different tissues participate in providing nutrients for milk production. After weaning, the histological, metabolic, and hormonal modifications that take place in lactation are reverted, and bone remineralization is a central function at this time. This study focuses on the hormonal, metabolic, molecular, and tissue modifications that occur in mammary glands, bone, intestine, and kidneys in the mother during lactation and post-weaning periods.

MiR-21-3p Centric Regulatory Network in Dairy Cow Mammary Epithelial Cell Proliferation

MicroRNA-mediated gene regulation is important for the development of the mammary gland and the lactating process. A previous study has shown that the expression of microRNA-21 (miR-21) is different in the dry and early lactation period of the dairy cow mammary gland, but the molecular mechanisms underlying the lactation cycle are not fully understood. Here, the function of miR-21-3p on bovine mammary gland epithelial cells (BMECs) was detected by MTT assay and flow cytometry analysis, which showed that miR-21-3p significantly promoted the cell viability and proliferation. Then, the regulating mechanism of miR-21-3p on cell viability and proliferation was elucidated. Dual luciferase assay, RT-qPCR, and Western blot results revealed that IGFBP5 was a target gene of miR-21-3p. It was known that lncRNA could act as a competing endogenous RNA to sequester miRNAs and reduce the regulatory effect of miRNA-targeted genes. Based on our previous lncRNA-seq data and bioinformatics analysis, lncRNA NONBTAT017009.2 was potentially associated with miR-21-3p, and its expression was specifically inhibited with the transfection of miR-21-3p mimic into BMECs. Inversely, the overexpression of NONBTAT017009.2 significantly decreased the expression level of miR-21-3p in BMECs, while the expression of IGFBP5, the target gene of miR-21-3p, was significantly upregulated. In addition, the promoter region of miR-21 contained two STAT3 binding sites, and the dual luciferase reporter assays revealed that the overexpression of STAT3 significantly reduced the promoter activity of miR-21, implying that the transcription factor STAT3 may act as an upstream regulator affecting the regulation process of miR-21-3p. The overexpression of STAT3 significantly inhibited the expression of miR-21-3p, while the mRNA expression of IGFBP5 was significantly increased compared with the control group. Besides, there are no STAT3 binding sites in the promoter region of IGFBP5 as we predicted by gene-regulation and JASPAR software. Therefore, it could infer that STAT3 might regulate the expression of IGFBP5 by miR-21-3p. Taken together, these results established a regulatory network of miR-21-3p to illustrate the regulating mechanism on promoting cow mammary epithelial cell proliferation.

Transcription Factor Sox4 as a Potential Player in Mammary Gland Involution

Mammary gland involution is a regressive process for the gland to return to its prepregnancy state after lactation and comprises an initial reversible and second remodeling stage. Although many genes and the multiple expression profiles of their mRNAs have been found in this process, the mechanisms controlling the profiles are largely unknown. In this study, we identified and analyzed transcription factor Sox4 in mammary gland involution. Elevated expression of Sox4 gene in the first stage (48 h after weaning) was observed at the mRNA and protein levels in the mouse mammary gland. Immunohistochemistry of the involuting gland indicated that Sox4 was located in the nuclei of epithelial cells. Nuclear Sox4 was also detected in the second stage, but unlikely to be involved in cell death, one of the characteristic events of involution. To clarify the functional roles of Sox4 in involution, we introduced a model, including a normal mammary epithelial cell line, for finding candidate target genes of this transcription factor and examined its effect on tenascin C mRNA expression.

STAT3 in Breast Cancer Onset and Progression: A Matter of Time and Context

Signal transducer and activator of transcription 3 (STAT3) is responsible for mediating the transcriptional programs downstream of several cytokine, growth factor, and oncogenic stimuli. Its expression and activity are consistently linked to cellular transformation, as well as tumor initiation and progression. Due to this central role, STAT3 is widely considered a good target for anti-cancer therapy; however, the success of these approaches has been, so far, very limited. Notably, on one side, STAT3 is aberrantly active in many breast cancers, on the other, at the physiological level, it is the main mediator of epithelial cell death during post-lactation mammary-gland involution, thus strongly suggesting that its biological functions are highly context-specific. One of the most peculiar features of STAT3 is that it can act both in cell-autonomous and non-cell-autonomous manners, simultaneously modulating the phenotypes of the tumor cells and their microenvironment. Here, we focus on the role of STAT3 in breast cancer progression, discussing the potential contrasting roles of STAT3 activation in the establishment of locally recurrent and distant metastatic disease. Based on the most recent literature, depending on the tumor cell type, the local microenvironment status, and the stage of the disease, either STAT3 activation or inactivation can support disease progression. Accordingly, cancer cells dynamically exploit STAT3 activity to carry out transcriptional programs somehow contrasting and complementary, such as supporting survival and growth, dormancy and awakening, stem cell-like features, and inflammation, immune response, and immune evasion. As a consequence, to achieve clinical efficacy, the conception and testing of anti-STAT3 targeted therapies will need a very careful evaluation of these opposing roles and of the most appropriate tumor context, disease stage and patient population to treat.

Stat3 accelerates Myc induced tumor formation while reducing growth rate in a mouse model of breast cancer

Elevated Myc expression has been noted in basal breast cancer but therapies targeting Myc directly are lacking. It is therefore critical to characterize the interaction of Myc with other genes and pathways to uncover future potential therapeutic strategies. In this study, we bioinformatically predicted a role for Stat3 in Myc induced mammary tumors and tested it using mouse models. During normal mammary function, loss of Stat3 in Myc transgenic dams resulted in lethality of pups due to lactation deficiencies. We also observed that deletion of Stat3 in the mammary glands of MMTV-Myc mice unexpectedly resulted in increased and earlier hyperplasia and expedited tumorigenesis. However, despite arising earlier, Myc tumors lacking Stat3 grew more slowly with alterations in the resulting histological subtypes, including a dramatic increase in EMT-like tumors. We also observed that these tumors had impaired angiogenesis and a slight decrease in lung metastases. This metastatic finding is distinct from previously published findings in both MMTV-Neu and MMTV-PyMT mouse models. Together, the literature and our current research demonstrate that Stat3 can function as an oncogene or as a tumor repressor depending on the oncogenic driver and developmental context.

The role of STAT3 in glioblastoma progression through dual influences on tumor cells and the immune microenvironment

Glioblastoma multiforme (GBM) is the most aggressive form of cancer that begins within the brain; generally, the patient has a dismal prognosis and limited therapeutic options. Signal transducer and activator of transcription 3 (STAT3) is a critical mediator of tumorigenesis, tumor progression, and suppression of anti-tumor immunity in GBM. In a high percentage of GBM cells and tumor microenvironments, persistent activation of STAT3 induces cell proliferation, anti-apoptosis, glioma stem cell maintenance, tumor invasion, angiogenesis, and immune evasion. This makes STAT3 an attractive therapeutic target and a prognostic indicator in GBM. Targeting STAT3 affords an opportunity to disrupt multiple pro-oncogenic pathways at a single molecular hub. Unfortunately, there are no successful STAT3 inhibitors currently in clinical trials. However, strong clinical evidence implicating STAT3 as a major factor in GBM justifies the identification of safe and effective strategies for inhibiting STAT3.

Crosstalk between STAT5 activation and PI3K/AKT functions in normal and transformed mammary epithelial cells

Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) have been shown to function downstream of several peptide hormones and cytokines that are required for postnatal development and secretory function of the mammary gland. As part of an extended network, these signal transducers can engage in crosstalk with other pathways to facilitate synergistic, and sometimes antagonistic, actions of different growth factors. Specifically, signaling through the JAK2/STAT5 cascade has been demonstrated to be indispensable for the specification, proliferation, differentiation, and survival of secretory mammary epithelial cells. Following a concise description of major cellular programs in mammary gland development and the role of growth factors that rely on JAK/STAT signaling to orchestrate these programs, this review highlights the significance of active STAT5 and its crosstalk with the PI3 kinase and AKT1 for mediating the proliferation of alveolar progenitors and survival of their functionally differentiated descendants in the mammary gland. Based on its ability to provide self-sufficiency in growth signals that are also capable of overriding intrinsic cell death programs, persistently active STAT5 can serve as a potent oncoprotein that contributes to the genesis of breast cancer. Recent experimental evidence demonstrated that, similar to normal developmental programs, oncogenic functions of STAT5 rely on molecular crosstalk with PI3K/AKT signaling for the initiation, and in some instances the progression, of breast cancer. The multitude by which STATs can interact with individual mediators of the PI3K/AKT signaling cascade may provide novel avenues for targeting signaling nodes within molecular networks that are crucial for the survival of cancer cells.

Co-Expression Network and Pathway Analyses Reveal Important Modules of miRNAs Regulating Milk Yield and Component Traits

Co-expression network analyses provide insights into the molecular interactions underlying complex traits and diseases. In this study, co-expression network analysis was performed to detect expression patterns (modules or clusters) of microRNAs (miRNAs) during lactation, and to identify miRNA regulatory mechanisms for milk yield and component traits (fat, protein, somatic cell count (SCC), lactose, and milk urea nitrogen (MUN)) via miRNA target gene enrichment analysis. miRNA expression (713 miRNAs), and milk yield and components (Fat%, Protein%, lactose, SCC, MUN) data of nine cows at each of six different time points (day 30 (D30), D70, D130, D170, D230 and D290) of an entire lactation curve were used. Four modules or clusters (GREEN, BLUE, RED and TURQUOISE) of miRNAs were identified as important for milk yield and component traits. The GREEN and BLUE modules were significantly correlated (|r| > 0.5) with milk yield and lactose, respectively. The RED and TURQUOISE modules were significantly correlated (|r| > 0.5) with both SCC and lactose. In the GREEN module, three abundantly expressed miRNAs (miR-148a, miR-186 and miR-200a) were most significantly correlated to milk yield, and are probably the most important miRNAs for this trait. DDR1 and DDHX1 are hub genes for miRNA regulatory networks controlling milk yield, while HHEX is an important transcription regulator for these networks. miR-18a, miR-221/222 cluster, and transcription factors HOXA7, and NOTCH 3 and 4, are important for the regulation of lactose. miR-142, miR-146a, and miR-EIA17-14144 (a novel miRNA), and transcription factors in the SMAD family and MYB, are important for the regulation of SCC. Important signaling pathways enriched for target genes of miRNAs of significant modules, included protein kinase A and PTEN signaling for milk yield, eNOS and Noth signaling for lactose, and TGF β, HIPPO, Wnt/β-catenin and cell cycle signaling for SCC. Relevant enriched gene ontology (GO)-terms related to milk and mammary gland traits included cell differentiation, G-protein coupled receptor activity, and intracellular signaling transduction. Overall, this study uncovered regulatory networks in which miRNAs interacted with each other to regulate lactation traits.

Nuclear Expression of STAT5 in Intraductal Papillary Mucinous Neoplasms of the Pancreas

Intraductal papillary mucinous neoplasms (IPMNs) are noninvasive lesions of the pancreas and classified as intraductal papillary mucinous adenomas (IPMAs), borderline IPMNs, and intraductal papillary mucinous carcinomas (IPMCs). Expression patterns of the specific genes alter during IPMN progression. Based on the evidence that signal transducers and activators of transcription (STAT) 5 play important roles in tumor development, we tested STAT5 expression in IPMAs, borderline IPMNs, and IPMCs by immunohistochemical method. STAT5 frequently expressed in the nuclei of tumor cells of borderline IPMNs or IPMCs but was not observed in those of IPMAs. Nuclear expression of STAT5 protein correlated to the Ki-67 labeling index of the examined IPMNs. STAT5 protein could contribute to the progression and proliferation of IPMNs.

Influenza Transmission in the Mother-Infant Dyad Leads to Severe Disease, Mammary Gland Infection, and Pathogenesis by Regulating Host Responses

Seasonal influenza viruses are typically restricted to the human upper respiratory tract whereas influenza viruses with greater pathogenic potential often also target extra-pulmonary organs. Infants, pregnant women, and breastfeeding mothers are highly susceptible to severe respiratory disease following influenza virus infection but the mechanisms of disease severity in the mother-infant dyad are poorly understood. Here we investigated 2009 H1N1 influenza virus infection and transmission in breastfeeding mothers and infants utilizing our developed infant-mother ferret influenza model. Infants acquired severe disease and mortality following infection. Transmission of the virus from infants to mother ferrets led to infection in the lungs and mother mortality. Live virus was also found in mammary gland tissue and expressed milk of the mothers which eventually led to milk cessation. Histopathology showed destruction of acini glandular architecture with the absence of milk. The virus was localized in mammary epithelial cells of positive glands. To understand the molecular mechanisms of mammary gland infection, we performed global transcript analysis which showed downregulation of milk production genes such as Prolactin and increased breast involution pathways indicated by a STAT5 to STAT3 signaling shift. Genes associated with cancer development were also significantly increased including JUN, FOS and M2 macrophage markers. Immune responses within the mammary gland were characterized by decreased lymphocyte-associated genes CD3e, IL2Ra, CD4 with IL1β upregulation. Direct inoculation of H1N1 into the mammary gland led to infant respiratory infection and infant mortality suggesting the influenza virus was able to replicate in mammary tissue and transmission is possible through breastfeeding. In vitro infection studies with human breast cells showed susceptibility to H1N1 virus infection. Together, we have shown that the host-pathogen interactions of influenza virus infection in the mother-infant dyad initiate immunological and oncogenic signaling cascades within the mammary gland. These findings suggest the mammary gland may have a greater role in infection and immunity than previously thought.

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.

STAT3 signaling is activated preferentially in tumor-initiating cells in claudin-low models of human breast cancer

In breast cancer, a subset of tumor-initiating cells (TIC) or "cancer stem cells" are thought to be responsible for tumor maintenance, treatment resistance, and disease recurrence. While current breast cancer stem cell markers (e.g., CD44(high) /CD24(low/neg) , ALDH positive) have allowed enrichment for such cells, they are not universally expressed and may actually identify distinct TIC subpopulations in the same tumor. Thus, additional markers of functional stem cells are needed. The STAT3 pathway is a critical regulator of the function of normal stem cells, and evidence is accumulating for its important role in breast cancer stem cells. However, due to the lack of a method for separating live cells based on their level of STAT3 activity, it remains unknown whether STAT3 functions in the cancer stem cells themselves, or in surrounding niche cells, or in both. To approach this question, we constructed a series of lentiviral fluorescent (enhanced green fluorescent protein, EGFP) reporters that enabled flow cytometric enrichment of cells differing in STAT3-mediated transcriptional activity, as well as in vivo/in situ localization of STAT3 responsive cells. Using in vivo claudin-low cell line xenograft models of human breast cancer, we found that STAT3 signaling reporter activity (EGFP(+) ) is associated with a subpopulation of cancer cells enriched for mammosphere-forming efficiency, as well as TIC function in limiting dilution transplantation assays compared to negative or unsorted populations. Our results support STAT3 signaling activity as another functional marker for human breast cancer stem cells thus making it an attractive therapeutic target for stem-cell-directed therapy in some breast cancer subtypes.

The spectrum of STAT functions in mammary gland development

The signal transducer and activator of transcription (STAT) family of transcription factors have a spectrum of functions in mammary gland development. In some cases these roles parallel those of STATs in other organ systems, while in other instances the function of individual STATs in the mammary gland is specific to this tissue. In the immune system, STAT6 is associated with differentiation of T helper cells, while in the mammary gland, it has a fundamental role in the commitment of luminal epithelial cells to the alveolar lineage. STAT5A is required for the production of luminal progenitor cells from mammary stem cells and is essential for the differentiation of milk producing alveolar cells during pregnancy. By contrast, the initiation of regression following weaning heralds a dramatic and specific activation of STAT3, reflecting its pivotal role in the regulation of cell death and tissue remodeling during mammary involution. Although it has been demonstrated that STAT1 is regulated during a mammary developmental cycle, it is not yet determined whether it has a specific, non-redundant function. Thus, the mammary gland constitutes an unusual example of an adult organ in which different STATs are sequentially activated to orchestrate the processes of functional differentiation, cell death and tissue remodeling.

STAT3 Activation in Glioblastoma: Biochemical and Therapeutic Implications

Signal transducer and activator of transcription 3 (STAT3) is a potent regulator of gliomagenesis through its induction of angiogenesis, host immunosuppression, and tumor invasion. Gain of function mutations result in constitutive activation of STAT3 in glioma cells, making STAT3 an attractive target for inhibition in cancer therapy. Nevertheless, some studies show that STAT3 also participates in terminal differentiation and apoptosis of various cell lines and in glioma with phosphatase and tensin homolog (PTEN)-deficient genetic backgrounds. In light of these findings, the utility of STAT3 as a prognostic indicator and as a target of drug therapies will be contingent on a more nuanced understanding of its pro- and anti-tumorigenic effects.

The Stat3 paradox: a killer and an oncogene

Stat proteins regulate many aspects of mammary gland development, including the profound changes that occur during pregnancy, lactation and involution. Stat3 induces transcriptional activation of genes involved in the inflammatory response, and in seemingly contradictory cellular events such as apoptosis, differentiation and stem cell maintenance. While Stat3 signalling during mammary gland involution induces epithelial cell death, aberrant Stat3 activation is widely implicated in breast tumourigenesis. Specific cytokines may initiate either a Stat3-driven proliferative or death response depending on the cell-type and cell-context specific availability of particular combinations of signals and receptors. The paradoxical functions of Stat3 may also be due to the degree and extent of activation in different circumstances, in addition to paracrine signalling between mammary epithelial cells and the surrounding microenvironment. Deciphering the enigmatic nature of Stat3 in the mammary gland may benefit future therapeutic strategies for inducing cell death in breast tumours.

STAT signaling in mammary gland differentiation, cell survival and tumorigenesis

The mammary gland is a unique organ that undergoes extensive and profound changes during puberty, menstruation, pregnancy, lactation and involution. The changes that take place during puberty involve large-scale proliferation and invasion of the fat-pad. During pregnancy and lactation, the mammary cells are exposed to signaling pathways that inhibit apoptosis, induce proliferation and invoke terminal differentiation. Finally, during involution the mammary gland is exposed to milk stasis, programmed cell death and stromal reorganization to clear the differentiated milk-producing cells. Not surprisingly, the signaling pathways responsible for bringing about these changes in breast cells are often subverted during the process of tumorigenesis. The STAT family of proteins is involved in every stage of mammary gland development, and is also frequently implicated in breast tumorigenesis. While the roles of STAT3 and STAT5 during mammary gland development and tumorigenesis are well studied, others members, e.g. STAT1 and STAT6, have only recently been observed to play a role in mammary gland biology. Continued investigation into the STAT protein network in the mammary gland will likely yield new biomarkers and risk factors for breast cancer, and may also lead to novel prophylactic or therapeutic strategies against breast cancer.

Effects of 5-fluorouracil and gemcitabine on a breast cancer cell line (MCF-7) via the JAK/STAT pathway

Aberrant activation of the JAK/STAT pathway may predispose to malignancy as a consequence of the deregulation of cell proliferation, differentiation or apoptosis such as in cancer of the blood, head and neck, and breast. In our study we aimed to investigate the effects of 5-fluorouracil (5-FU) and gemcitabine on a breast cancer cell line (MCF-7 cells) via the JAK/STAT pathway. Distribution of JAK1, JAK2, JAK3 and STAT2, STAT3, STAT4, STAT5 were evaluated on MCF-7 cells following gemcitabine and 5-FU treatment and in the absence of drug treatment by an indirect immunohistochemical method. It was observed that JAK1, JAK3, STAT5 and particularly STAT2 activation were more effective than the other JAK/STATs in breast cancer progression. Following treatment with 5-FU, JAK1 and STAT5 immunoreactivities were decreased in MCF-7 cells in comparison with both gemcitabine-treated and non-treated groups. These results suggest that the JAK/STAT pathway plays an important role in breast cancer pathogenesis and may be more affected after 5-FU treatment rather than gemcitabine. Drugs which block STAT5 may provide a novel therapeutic approach for the treatment of breast cancer.

Mechanisms of HGF/Met signaling to Brk and Sam68 in breast cancer progression

Signal transduction pathways downstream of receptor tyrosine kinases (RTKs) are often deregulated during oncogenesis, tumor progression, and metastasis. In particular, the peptide growth factor hormone, hepatocyte growth factor (HGF), and its specific receptor, Met tyrosine kinase, regulate cancer cell migration, thereby conferring an aggressive phenotype (Nakamura et al., J Clin Invest 106(12):1511-1519, 2000; Huh et al., Proc Natl Acad Sci U S A 101:4477-4482, 2004). Additionally, overexpression of Met is associated with enhanced invasiveness of breast cancer cells (Edakuni et al., Pathol Int 51(3):172-178, 2001; Jin et al., Cancer 79(4):749-760, 1997; Tuck et al., Am J Pathol 148(1):225-232, 1996). Here, we review the regulation of recently identified novel downstream mediators of HGF/Met signaling, Breast tumor kinase (Brk/PTK6), and Src-associated substrate during mitosis of 68 kDa (Sam68), and discuss their relevance to mechanisms of breast cancer progression.

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

Multiparous Stat1^−/− mice spontaneously develop mammary tumors with increased incidence: at an average age of 12 months, 55% of the animals suffer from mammary cancer, although the histopathology is heterogeneous. We consistently observed mosaic expression or down-regulation of STAT1 protein in wild-type mammary cancer evolving in the control group. Transplantation experiments show that tumorigenesis in Stat1^−/− mice is partially influenced by impaired CTL mediated tumor surveillance. Additionally, STAT1 exerts an intrinsic tumor suppressing role by controlling and blocking proliferation of the mammary epithelium. Loss of STAT1 in epithelial cells enhances cell growth in both transformed and primary cells. The increased proliferative capacity leads to the loss of structured acini formation in 3D-cultures. Analogous effects were observed when Irf1^−/− epithelial cells were used. Accordingly, the rate of mammary intraepithelial neoplasias (MINs) is increased in Stat1^−/− animals: MINs represent the first step towards mammary tumors. The experiments characterize STAT1/IRF1 as a key growth inhibitory and tumor suppressive signaling pathway that prevents mammary cancer formation by maintaining growth control. Furthermore, they define the loss of STAT1 as a predisposing event via enhanced MIN formation.

The two faces of Janus kinases and their respective STATs in mammary gland development and cancer

Since its discovery as “just another kinase” more than twenty years ago, the family of JAK tyrosine kinases and their respective Signal Transducers and Activators of Transcription (STATs) has been a center of attention in the areas of signal transduction, development, and cancer. The subsequent designation of JAKs as Janus kinases after the mythical two-faced Roman God of the doorways accurately portrays the analogous and sometimes contrasting molecular and biological characteristics of these tyrosine kinases. The two “faces” of JAKs are their structurally similar kinase and pseudo-kinase domains. As essential parts of various transmembrane receptor complexes, these tyrosine kinases function at cellular gateways and relay signals from growth factors to their respective intracellular targets. The multifaceted nature of JAKs becomes evident from their ability to activate specific STATs during distinct phases of normal mammary gland development. Studies in breast cancer cells and genetically engineered mouse models also show that JAK/STAT signaling possesses a "two-faced" role during breast cancer initiation and progression. This review will highlight recent findings about important biological functions of JAKs and STATs during normal mammogenesis, with particular emphasis on the Jak2/Stat5 pathway as well as Jak1/2/Stat3 signaling complexes. In addition, we will discuss how the importance of these signaling networks changes during carcinogenesis. With JAK inhibitors currently under development to treat myeloproliferative disorders, determining the essential functions of JAKs at particular stages of disease initiation and progression is of critical importance to predict the efficacy of these agents for targeted therapies against breast cancer.

Mammary gland specific expression of Brk/PTK6 promotes delayed involution and tumor formation associated with activation of p38 MAPK

Introduction

Protein tyrosine kinases (PTKs) are frequently overexpressed and/or activated in human malignancies, and regulate cancer cell proliferation, cellular survival, and migration. As such, they have become promising molecular targets for new therapies. The non-receptor PTK termed breast tumor kinase (Brk/PTK6) is overexpressed in approximately 86% of human breast tumors. The role of Brk in breast pathology is unclear.

Methods

We expressed a WAP-driven Brk/PTK6 transgene in FVB/n mice, and analyzed mammary glands from wild-type (wt) and transgenic mice after forced weaning. Western blotting and immunohistochemistry (IHC) studies were conducted to visualize markers of mammary gland involution, cell proliferation and apoptosis, as well as Brk, STAT3, and activated p38 mitogen-activated protein kinase (MAPK) in mammary tissues and tumors from WAP-Brk mice. Human (HMEC) or mouse (HC11) mammary epithelial cells were stably or transiently transfected with Brk cDNA to assay p38 MAPK signaling and cell survival in suspension or in response to chemotherapeutic agents.

Results

Brk-transgenic dams exhibited delayed mammary gland involution and aged mice developed infrequent tumors with reduced latency relative to wt mice. Consistent with delayed involution, mammary glands of transgenic animals displayed decreased STAT3 phosphorylation, a marker of early-stage involution. Notably, p38 MAPK, a pro-survival signaling mediator downstream of Brk, was activated in mammary glands of Brk transgenic relative to wt mice. Brk-dependent signaling to p38 MAPK was recapitulated by Brk overexpression in the HC11 murine mammary epithelial cell (MEC) line and human MEC, while Brk knock-down in breast cancer cells blocked EGF-stimulated p38 signaling. Additionally, human or mouse MECs expressing Brk exhibited increased anchorage-independent survival and resistance to doxorubicin. Finally, breast tumor biopsies were subjected to IHC analysis for co-expression of Brk and phospho-p38 MAPK; ductal and lobular carcinomas expressing Brk were significantly more likely to express elevated phospho-p38 MAPK.

Conclusions

These studies illustrate that forced expression of Brk/PTK6 in non-transformed mammary epithelial cells mediates p38 MAPK phosphorylation and promotes increased cellular survival, delayed involution, and latent tumor formation. Brk expression in human breast tumors may contribute to progression by inducing p38-driven pro-survival signaling pathways.

Tracking the activation of Stat5 through the expression of an inducible reporter gene in a transgenic mouse line

Signal transducer and activator of transcription 5 (Stat5), a latent cytoplasmic transcription factor, becomes activated by phosphorylation upon cytokine, hormone, and growth factor interactions with their appropriate receptors and induces the transcription of target genes. It plays crucial roles in principal cell fate decisions and regulates cell differentiation, development, proliferation, apoptosis, and inflammation. It is active in the mammary gland, the liver, hematopoietic cells, and other organs and has pleiotropic functions, depending on its activation pathway and its site of action. We derived transgenic mice in which the expression of a LacZ reporter gene is directed by Stat5-specific response elements and visualized the activation of Stat5 in cells of mouse organs at different developmental stages. The reporter gene activity reflects the timing and the location of Stat5 activation and was documented in mammary epithelial cells during developmental stages of the gland, cells of the liver, kidney, spleen, thymus, and uterus and in granulocytes and macrophages of the transgenic lines.

Forced involution of the functionally differentiated mammary gland by overexpression of the pro-apoptotic protein bax

The mammary gland is a developmentally dynamic, hormone-responsive organ that undergoes proliferation and differentiation within the secretory epithelial compartment during pregnancy. The epithelia are maintained by pro-survival signals (e.g., Stat5, Akt1) during lactation, but undergo apoptosis during involution through inactivation of cell survival pathways and upregulation of pro-apoptotic proteins. To assess if the survival signals in the functionally differentiated mammary epithelial cells can override a pro-apoptotic signal, we generated transgenic mice that express Bax under the whey acidic protein (WAP) promoter. WAP-Bax females exhibited a lactation defect and were unable to nourish their offspring. Mammary glands demonstrated: (1) a reduction in epithelial content, (2) hallmark signs of mitochondria-mediated cell death, (3) an increase in apoptotic cells by TUNEL assay, and (4) precocious Stat3 activation. This suggests that upregulation of a single pro-apoptotic factor of the Bcl-2 family is sufficient to initiate apoptosis of functionally differentiated mammary epithelial cells in vivo.

Glucocorticoid-induced impairment of mammary gland involution is associated with STAT5 and STAT3 signaling modulation

The mammary epithelium undergoes cyclical periods of cellular proliferation, differentiation, and regression. During lactation, the signal transducer and activator of transcription factor (STAT)-5A and the glucocorticoid receptor (GR) synergize to induce milk protein expression and also act as survival factors. During involution, STAT3 activation mediates epithelial cell apoptosis and mammary gland remodeling. It has been shown that the administration of glucocorticoids at weaning prevents epithelial cell death, probably by extracellular matrix breakdown prevention. Our results show that the synthetic glucocorticoid dexamethasone (DEX) modulates STAT5A and STAT3 signaling and inhibits apoptosis induction in postlactating mouse mammary glands, only when administered within the first 48 h upon cessation of suckling. DEX administration right after weaning delayed STAT5A inactivation and degradation, preserving gene expression of target genes as β-casein (bcas) and prolactin induced protein (pip). Weaning-triggered GR down-regulation is also delayed by the hormone treatment. Moreover, DEX administration delayed STAT3 activation and translocation into epithelial cells nuclei. In particular, DEX treatment impaired the increment in gene expression of signal transducer subunit gp130, normally up-regulated from lactation to involution and responsible for STAT3 activation. Therefore, the data shown herein indicate that glucocorticoids are able to modulate early involution by controlling the strong cross talk that GR, STAT5, and STAT3 pathways maintains in the mammary epithelium.

Dystroglycan controls signaling of multiple hormones through modulation of STAT5 activity

Receptors for basement membrane (BM) proteins, including dystroglycan (DG), coordinate tissue development and function by mechanisms that are only partially defined. To further elucidate these mechanisms, we generated a conditional knockout of DG in the epithelial compartment of the mouse mammary gland. Deletion of DG caused an inhibition of mammary epithelial outgrowth and a failure of lactation. Surprisingly, loss of DG in vivo did not disrupt normal tissue architecture or BM formation, even though cultured Dag1-null epithelial cells failed to assemble laminin-111 at the cell surface. The absence of DG was, however, associated with a marked loss in activity of signal transducer and activator of transcription 5 (STAT5). Loss of DG perturbed STAT5 signaling induced by either prolactin or growth hormone. We found that DG regulates signaling by both hormones in a manner that is dependent on laminin-111 binding, but independent of the DG cytoplasmic domain, suggesting that it acts via a co-receptor mechanism reliant on DG-mediated laminin assembly. These results demonstrate a requirement for DG in the growth and function of a mammalian epithelial tissue in vivo. Moreover, we reveal a selective role for DG in the control of multiple STAT5-dependent hormone signaling pathways, with implications for numerous diseases in which DG function is compromised.

Nuclear localization of signal transducer and activator of transcription 3 in head and neck squamous cell carcinoma is associated with a better prognosis

PURPOSE

A high frequency of head and neck squamous cell cancers (HNSCC) contain constitutively activated signal transducer and activator of transcription 3 (STAT3). To further elucidate the prognostic role of STAT3 in HNSCC, the expression pattern of STAT3 was correlated with outcome in two independent data sets.

EXPERIMENTAL DESIGN

STAT3 protein expression analysis was done on a test cohort of 102 patients with HNSCC recruited between 1992 and 2005. Automated quantitative analysis was used to assess STAT3 protein expression. We evaluated associations with clinicopathologic parameters and survival prognosis. Associations were validated in a second, independent cohort of 58 patients with confirmed HNSCC enrolled in the Early Detection Research Network-sponsored study who underwent surgical resection with curative intent at the University of Pittsburgh Medical Center between 2000 and 2004.

RESULTS

STAT3 displayed mixed nuclear and cytoplasmic staining. Survival analysis showed that high nuclear STAT3 expression (top tertile versus the rest) was associated with longer progression-free survival (n = 70, mean survival of 88.9 versus 46.7 months, P = 0.012 for the first cohort; n = 37, mean survival of 60.3 versus 33.0 months, P = 0.009 for the second cohort). After best model selection in the multivariable analysis context, only STAT3 was significant, revealing a lower risk of progression and death for patients with high nuclear STAT3-expressing tumors (hazard ratio, 0.28; 95% confidence interval, 0.10-0.82; P = 0.019; and hazard ratio, 0.23; 95% confidence interval, 0.07-0.76; P = 0.016, respectively).

CONCLUSIONS

Our results indicate that high nuclear STAT3 expression levels by automated quantitative analysis are associated with favorable outcome in HNSCC.

STAT3 regulation of glioblastoma pathogenesis

Malignant gliomas are the most common primary brain tumors. Despite efforts to find effective treatments, these tumors remain incurable. The failure of malignant gliomas to respond to conventional cancer therapies may reflect the unique biology of these tumors, underscoring the need for new approaches in their investigation. Recently, progress has been made in characterization of the molecular pathogenesis of glioblastoma using a developmental neurobiological perspective, by exploring the role of signaling pathways that control the differentiation of neural stem cells along the glial lineage. The transcription factor STAT3, which has an established function in neural stem cell and astrocyte development, has been found to play dual tumor suppressive and oncogenic roles in glial malignancy depending on the mutational profile of the tumor. These findings establish a novel developmental paradigm in the study of glioblastoma pathogenesis and provide the rationale for patient-tailored therapy in the treatment of this devastating disease.

A dual role for oncostatin M signaling in the differentiation and death of mammary epithelial cells in vivo

Recent studies in breast cancer cell lines have shown that oncostatin M (OSM) not only inhibits proliferation but also promotes cell detachment and enhances cell motility. In this study, we have looked at the role of OSM signaling in nontransformed mouse mammary epithelial cells in vitro using the KIM-2 mammary epithelial cell line and in vivo using OSM receptor (OSMR)-deficient mice. OSM and its receptor were up-regulated approximately 2 d after the onset of postlactational mammary regression, in response to leukemia inhibitory factor (LIF)-induced signal transducer and activator of transcription-3 (STAT3). This resulted in sustained STAT3 activity, increased epithelial apoptosis, and enhanced clearance of epithelial structures during the remodeling phase of mammary involution. Concurrently, OSM signaling precipitated the dephosphorylation of STAT5 and repressed expression of the milk protein genes beta-casein and whey acidic protein (WAP). Similarly, during pregnancy, OSM signaling suppressed beta-casein and WAP gene expression. In vitro, OSM but not LIF persistently down-regulated phosphorylated (p)-STAT5, even in the continued presence of prolactin. OSM also promoted the expression of metalloproteinases MMP3, MMP12, and MMP14, which, in vitro, were responsible for OSM-specific apoptosis. Thus, the sequential activation of IL-6-related cytokines during mammary involution culminates in an OSM-dependent repression of epithelial-specific gene expression and the potentiation of epithelial cell extinction mediated, at least in part, by the reciprocal regulation of p-STAT5 and p-STAT3.

Immune cell regulators in mouse mammary development and involution

Dramatic changes in cell composition and function occur in the mammary gland during a pregnancy-lactation-involution cycle. We investigated the transcriptional changes associated with these biological events by using microarray analysis and identified the critical genes involved by using genetically modified mice. Two surprising findings arose from these studies. First, the microarray data showed that postlactational regression was associated with an acute phase inflammatory response, in addition to cell death. Conditional deletion of signal transducer and activator of transcription (Stat)3 or the nuclear factor-kappaB regulatory kinase inhibitor of kappa B kinase beta resulted in a failure of cell death induction during involution, an indication that these signaling pathways are essential mediators of the involution process. Both Stat3 and nuclear factor-kappaB have been shown to regulate acute phase gene expression in addition to apoptosis regulators. Four distinct transcriptional profiles are present in the first 4 d of involution, whereas there are 3 in lactation. At the peak of lactation (i.e., d 10 in mouse), more than 400 genes reach their maximum expression before declining dramatically in the first 12 h of involution. A reciprocal pattern was observed for more than 500 genes that were specifically upregulated within the first 12 h of forced involution. We are now investigating the role of a subset of these genes in involution. We also uncovered a role for genes normally associated with immune cell signaling in the differentiation of luminal mammary epithelial cells during pregnancy. Genetic deletion of the transcription factor Stat6 resulted in delayed development during pregnancy, and this phenotype was recapitulated in mammary tissue from IL-4 and IL-13 doubly deficient mice. Furthermore, we showed that mammary epithelial cells secrete T-cell regulatory cytokines. T-helper type 1 cytokines, such as interferon-gamma and IL-12a, are secreted by undifferentiated mammary epithelial cells, whereas T-helper type 2 cytokines, including IL-4 and IL-13, are secreted by differentiated cells. This unexpected finding demonstrates a role for immune cell signaling in mammary epithelial cell fate and function.

The Stat family of transcription factors have diverse roles in mammary gland development

The Stat family of transcription factors have diverse roles in mammary gland development. Genetic studies in mice have revealed an essential requirement for Stat5a in development of secretory alveolar cells during pregnancy while Stat6, which is normally associated with differentiation of T helper cells, is important in the commitment of luminal cells to this alveolar lineage. In contrast, Stat3 is specifically activated at the initiation of post-lactational regression when it has an essential function in the regulation of cell death and tissue remodelling. Stat1 and Stat4 have been shown to be regulated during a mammary developmental cycle although whether they have specific, non-redundant roles is not clear. Thus, the adult mammary gland is somewhat unusual in that it is a tissue where different Stats are sequentially activated to orchestrate the processes of functional differentiation, cell death and tissue remodelling.

Progesterone pre-treatment potentiates EGF pathway signaling in the breast cancer cell line ZR-75

Progesterone in hormone replacement therapy (HRT) preparations increases, while hysterectomy greatly reduces, the incidence of breast cancer. Cross-talk between the progesterone and growth factor signaling pathways occurs at multiple levels and this maybe a key factor in breast cancer survival and progression. To test this hypothesis, we characterized the effect of progesterone pre-treatment on the sensitization of the epidermal growth factor (EGF) signaling pathway to EGF in the breast cancer cell line ZR-75. For the first time in ZR-75 cells and in agreement with previous work using synthetic progestins, we demonstrate that pre-treatment with the natural ligand progesterone increases EGF receptor (EGFR) levels and subsequent ligand-dependent phosphorylation. Downstream we demonstrate that progesterone alone increases erk-1 + 2 phosphorylation, potentiates EGF-phosphorylated erk-1 + 2 and maintains these levels elevated for 24 h; over 20 h longer than in vehicle treated cells. Additionally, progesterone increased the levels of STAT5, another component of the EGF signaling cascade. Progesterone increased EGF mediated transcription of a c-fos promoter reporter and the nuclear localization of the native c-fos protein. Furthermore, progesterone and EGF both alone and in combination, significantly increase cell proliferation. Several results presented herein demonstrate the conformity between the action of the natural ligand progesterone with that of synthetic progestins such as MPA and R5020 and allows the postulation that the progestin/progesterone-dependent increase of EGF signaling provides a survival advantage to burgeoning cancer cells and may contribute to the breast cancer risk associated with endogenous progesterone and with progestin-containing HRT.

STAT5 transcriptional activity is impaired by LIF in a mammary epithelial cell line

Gene regulation mediated by STAT factors has been implicated in cellular functions with relevance to a variety of processes. Particularly, STAT5 and STAT3 play a crucial role in mammary epithelium displaying reciprocal activation kinetics during pregnancy, lactation and involution. Here, we show that LIF treatment of mammary epithelial HC11 cells reduces the phosphorylation levels and transcriptional activity of p-STAT5 in correlation with STAT3 phosphorylation. We have also found that STAT5 activity is negatively modulated by this cytokine, both on a gene whose expression is induced, as well as on a promoter repressed by STAT5. Besides, our results show that lactogenic hormones increase LIF effect on gene induction without modifying STAT3 phosphorylation state. Our findings strongly suggest that there is crosstalk between STAT5 and STAT3 pathways that could modulate their ability to regulate gene expression.

The molecular culprits underlying precocious mammary gland involution

Mammary gland involution, characterized by extensive apoptosis and structural remodelling of the gland, is the process by which the gland is returned to the pre-pregnant state. A key advantage of the mammary gland is the ability to synchronize involution through forced weaning, thus allowing the dissection of biochemical pathways involved in the involution process. Over the past few years, significant advances have been made in understanding the signaling pathways and downstream effectors that regulate epithelial cell apoptosis in the first phase of involution, and the importance of matrix metalloproteinases and their inhibitors in both phases of involution. The precise nature of the triggers for apoptosis, however, and the ultimate perpetrators of cell death are not yet clear. This review focuses on genes whose perturbation, either by targeted deletion or overexpression in transgenic mouse models, leads to precocious involution. The accumulating data point to a complex network of signal transduction pathways that synergize to regulate apoptosis in the involuting mammary gland.

Caspase-dependent proteolytic cleavage of STAT3alpha in ES cells, in mammary glands undergoing forced involution and in breast cancer cell lines

Background

The STAT (Signal Transducers and Activators of Transcription) transcription factor family mediates cellular responses to a wide range of cytokines. Activated STATs (particularly STAT3) are found in a range of cancers. Further, STAT3 has anti-apoptotic functions in a range of tumour cell lines. After observing a proteolytic cleavage in STAT3α close to a potential apoptotic caspase protease cleavage site we investigated whether STAT3α might be a caspase substrate.

Methods

STAT3α status was investigated in vitro in several cell systems:- HM-1 murine embryonic stem (ES) cells following various interventions; IOUD2 murine ES cells following induction to differentiate along neural or adipocyte lineages; and in a number of breast cancer cell lines. STAT3α status was also analysed in vivo in wild type murine mammary glands undergoing controlled, forced involution.

Results

Immunoblotting for STAT3α in HM-1 ES cell extracts detected amino and carboxy terminal species of approximately 48 kDa and 43 kDa respectively – which could be diminished dose-dependently by cell treatment with the nitric oxide (NO) donor drug sodium nitroprusside (SNP). UV irradiation of HM-1 ES cells triggered the STAT3α cleavage (close to a potential caspase protease cleavage site). Interestingly, the pan-caspase inhibitor z-Val-Ala-DL-Asp-fluoromethylketone (z-VAD-FMK) and the JAK2 tyrosine kinase inhibitor AG490 both inhibited cleavage dose-dependently, and cleavage was significantly lower in a heterozygous JAK2 knockout ES cell clone. STAT3α cleavage also occurred in vivo in normal murine mammary glands undergoing forced involution, coinciding with a pulse of phosphorylation of residue Y705 on full-length STAT3α. Cleavage also occurred during IOUD2 ES cell differentiation (most strikingly along the neural lineage) and in several human breast cancer cell lines, correlating strongly with Y705 phosphorylation.

Conclusion

This study documents a proteolytic cleavage of STAT3α into 48 kDa amino and 43 kDa carboxyl terminal fragments in a range of cell types. STAT3α cleavage occurs close to a potential caspase site, and can be inhibited dose-dependently by SNP, AG490 and z-VAD-FMK. The cleavage seems to be caspase-dependent and requires the phosphorylation of STAT3α at the Y705 residue. This highly regulated STAT3α cleavage may play an important role in modulating STAT3 transcriptional activity.

c-myc as a mediator of accelerated apoptosis and involution in mammary glands lacking Socs3

Suppressor of cytokine signalling (SOCS) proteins are critical attenuators of cytokine-mediated signalling in diverse tissues. To determine the importance of Socs3 in mammary development, we generated mice in which Socs3 was deleted in mammary epithelial cells. No overt phenotype was evident during pregnancy and lactation, indicating that Socs3 is not a key physiological regulator of prolactin signalling. However, Socs3-deficient mammary glands exhibited a profound increase in epithelial apoptosis and tissue remodelling, resulting in precocious involution. This phenotype was accompanied by augmented Stat3 activation and a marked increase in the level of c-myc. Moreover, induction of c-myc before weaning using an inducible transgenic model recapitulated the Socs3 phenotype, and elevated expression of likely c-myc target genes, E2F-1, Bax and p53, was observed. Our data establish Socs3 as a critical attenuator of pro-apoptotic pathways that act in the developing mammary gland and provide evidence that c-myc regulates apoptosis during involution.

Conditional deletion of Shp2 in the mammary gland leads to impaired lobulo-alveolar outgrowth and attenuated Stat5 activation

Stat5 and Stat3, two members of the Stat (signal transducer and activator of transcription) family, are known to play critical roles in mammopoiesis/lactogenesis and involution, respectively, in the mammary gland. Phosphotyrosine phosphatase Shp2 has been shown to dephosphorylate and thus inactivate both Stat5 and Stat3 in vitro. Paradoxically, cell culture studies also suggest a positive role of Shp2 in promoting prolactin-stimulated Stat5 activation. We have shown here that selective deletion of Shp2 in mouse mammary glands suppresses Stat5 activity during pregnancy and lactation, resulting in significant impairment of lobulo-alveolar outgrowth and lactation. In contrast, Stat3 activity was slightly up-regulated shortly before/at involution, leading to normal epithelial cell apoptosis/involution in Shp2-deficient mammary gland. Thus, Shp2 acts to promote Stat5 activation by the JAK2.prolactin receptor complex, while negatively modulating Stat3 activity before the onset of involution. This is the first demonstration that Shp2 manipulates Stat5 and Stat3 activities reciprocally in mammary epithelial cells, providing novel insight into the complex mechanisms for regulation of various Stat family members by a cytoplasmic tyrosine phosphatase.

Signal transducers and activators of transcription-3 up-regulates tissue inhibitor of metalloproteinase-1 expression and decreases invasiveness of breast cancer

Signal transducers and activators of transcription (STAT)-3 is an oncogenic protein that is constitutively activated in many human cancers, including 30 to 60% of primary breast cancer. The biological significance of STAT3 activation in breast cancer is not fully understood. We have previously shown that STAT3 up-regulates tissue inhibitors of metalloproteinase (TIMP)-1, a cytokine known to block metalloproteinases and decrease invasiveness in certain cancer cell types. We hypothesize that STAT3 activation may modulate tumor invasiveness of breast cancer by regulating TIMP1 expression. Using MCF-7 cells transfected with tetracycline-off STAT3C (constitutively active STAT3), we generated an in vitro system in which STAT3C levels can be tightly controlled in breast cancer cells. Increasing tetracycline levels gradually decreased STAT3C and TIMP1 in a dose-dependent manner, and down-regulation of these proteins led to a reciprocal decrease in invasiveness of these cells in Matrigel. Addition of a neutralizing anti-TIMP1 antibody increased invasiveness in the same experimental system. Using immunohistochemistry and 142 primary breast tumors, we found a significant association between the expression of the phosphorylated/active form of STAT3 (pSTAT3) and that of TIMP1. Importantly, STAT3 activation correlated significantly with a lower frequency of vascular and lymphatic invasion (P = 0.015 and P = 0.0002, respectively). Our data support the concept that STAT3 activation significantly modulates the biological and clinical behavior of breast cancer.

Systemic evaluation of total Stat3 and Stat3 tyrosine phosphorylation in normal human tissues

Stat3 plays important roles in many biological phenomena including cell survival, growth, proliferation, differentiation and cancer malignancies. As Stat3 emerges as a new therapeutic target for treatment of cancers in which the Stat3 is constitutively activated, the overall evaluation of basal expression of Stat3 and phosphorylated Stat3 at tyrosine residue 705 in human tissues would be very important and informative. We took a pilot study to examine the expression patterns of total Stat3 and phosphorylated Stat3 protein (p-Stat3) using immunohistochemistry in 47 different adult normal human tissues of 10 organ systems. Immunohistochemistry showed that total Stat3 protein was almost universally detected in all tissues except peripheral nerve. Interestingly, majorities of tissues showed to have moderate to high expression levels of total Stat3 protein. Several heart tissues displayed a unique perinuclear immunostaining for both Stat3 and p-Stat3, most likely in Golgi complexes. Based on the cell types, the p-Stat3 was also expressed in glandular, secretory, mucosal epithelial, circulatory endothelial, lymphoid, proliferative, and reabsorption-active cells.