Sequential activation of genetic programs in mouse mammary epithelium during pregnancy depends on STAT5A/B concentration

In-silico QTL mapping of postpubertal mammary ductal development in the mouse uncovers potential human breast cancer risk loci

Genetic background plays a dominant role in mammary gland development and breast cancer (BrCa). Despite this, the role of genetics is only partially understood. This study used strain-dependent variation in an inbred mouse mapping panel, to identify quantitative trait loci (QTL) underlying structural variation in mammary ductal development, and determined if these QTL correlated with genomic intervals conferring BrCa susceptibility in humans. For about half of the traits, developmental variation among the complete set of strains in this study was greater (P < 0.05) than that of previously studied strains, or strains in current common use for mammary gland biology. Correlations were also detected with previously reported variation in mammary tumor latency and metastasis. In-silico genome-wide association identified 20 mammary development QTL (Mdq). Of these, five were syntenic with previously reported human BrCa loci. The most significant (P = 1 × 10(-11)) association of the study was on MMU6 and contained the genes Plxna4, Plxna4os1, and Chchd3. On MMU5, a QTL was detected (P = 8 × 10(-7)) that was syntenic to a human BrCa locus on h12q24.5 containing the genes Tbx3 and Tbx5. Intersection of linked SNP (r(2) > 0.8) with genomic and epigenomic features, and intersection of candidate genes with gene expression and survival data from human BrCa highlighted several for further study. These results support the conclusion that mammary tumorigenesis and normal ductal development are influenced by common genetic factors and that further studies of genetically diverse mice can improve our understanding of BrCa in humans.

Role of STAT5 and epigenetics in lactation-associated upregulation of multidrug transporter ABCG2 in the mammary gland

The multidrug resistance efflux transporter ATP-binding cassette subfamily G member 2 (ABCG2) is not only overexpressed in certain drug-resistant cancers but is also highly expressed in the mammary gland during lactation, carrying xenobiotics and nutrients into milk. We sought to investigate the molecular mechanisms involved in the upregulation of ABCG2 during lactation. Expression profiling of different mouse Abcg2 mRNA isoforms (E1a, E1b, and E1c) revealed that E1b is predominantly expressed and induced in the lactating mouse mammary gland. Despite this induction, analyses of CpG methylation status and published ChIP-seq datasets reveal that E1b promoter sequences in the virgin gland are already hypomethylated and marked with the open chromatin histone mark H3K4me2. Using a forced-weaning model to shut down lactation, we found that within 24 h there was a significant reduction in Abcg2 mRNA expression and a loss of signal transducer and activator of transcription-5 (STAT5) occupancy at the mouse Abcg2 gene. Luciferase reporter assays further showed that some of these STAT5-binding regions that contained interferon-γ-activated sequence (GAS) motifs function as an enhancer after prolactin treatment. We conclude that Abcg2 is already poised for expression in the virgin mammary gland and that STAT5 plays an important role in Abcg2 expression during lactation.

The STAT5-regulated miR-193b locus restrains mammary stem and progenitor cell activity and alveolar differentiation

The transcription factor STAT5 mediates prolactin signaling and controls functional development of mammary tissue during pregnancy. This study has identified the miR-193b locus, also encoding miRNAs 365-1 and 6365, as a STAT5 target in mammary epithelium. While the locus was characterized by active histone marks in mammary tissue, STAT5 binding and expression during pregnancy, it was silent in most non-mammary cells. Inactivation of the miR-193b locus in mice resulted in elevated mammary stem/progenitor cell activity as judged by limiting dilution transplantation experiments of primary mammary epithelial cells. Colonies formed by mutant cells were larger and contained more Ki-67 positive cells. Differentiation of mammary epithelium lacking the miR-193b locus was accelerated during puberty and pregnancy, which coincided with the loss of Cav3 and elevated levels of Elf5. Normal colony development was partially obtained upon ectopically expressing Cav3 or upon siRNA-mediated reduction of Elf5 in miR-193b-null primary mammary epithelial cells. This study reveals a previously unknown link between the mammary-defining transcription factor STAT5 and a microRNA cluster in controlling mammary epithelial differentiation and the activity of mammary stem and progenitor cells.

Tissue-specific changes in molecular clocks during the transition from pregnancy to lactation in mice

Circadian clocks regulate homeostasis and mediate responses to stressors. Lactation is one of the most energetically demanding periods of an adult female's life. Peripartum changes occur in almost every organ so the dam can support neonatal growth through milk production while homeostasis is maintained. How circadian clocks are involved in adaptation to lactation is currently unknown. The abundance and temporal pattern of core clock genes' expression were measured in suprachiasmatic nucleus, liver, and mammary from late pregnant and early lactation mice. Tissue-specific changes in molecular clocks occurred between physiological states. Amplitude and robustness of rhythms increased in suprachiasmatic nucleus and liver. Mammary rhythms of core molecular clock genes were suppressed. Attenuated rhythms appeared to be a physiological adaptation of mammary to lactation, because manipulation of timing of suckling resulting in significant differences in plasma prolactin and corticosterone had no effect on amplitude. Analysis of core clock proteins revealed that the stoichiometric relationship between positive (CLOCK) and negative (PER2) components remained 1:1 in liver but was increased to 4:1 in mammary during physiological transition. Induction of differentiation of mammary epithelial cell line HC11 with dexamethasone, insulin, and prolactin resulted in similar stoichiometric changes among positive and negative clock regulators, and prolactin induced phase shifts in HC11 Arntl expression rhythm. Data support that distinct mechanisms drive periparturient changes in mammary clock. Stoichiometric change in clock regulators occurs with gland differentiation. Suppression of mammary clock gene expression rhythms represents a physiological adaptation to suckling cues. Adaptations in mammary clock are likely needed in part to support suckling demands of neonates.

Trp63 is regulated by STAT5 in mammary tissue and subject to differentiation in cancer

Transformation-related protein 63 (Trp63), the predominant member of the Trp53 family, contributes to epithelial differentiation and is expressed in breast neoplasia. Trp63 features two distinct promoters yielding specific mRNAs encoding two major TRP63 isoforms, a transactivating transcription factor and a dominant negative isoform. Specific TRP63 isoforms are linked to cell cycle arrest, apoptosis, survival, and epithelial mesenchymal transition (EMT). Although TRP63 overexpression in cultured cells is used to elucidate functions, little is known about Trp63 regulation in normal and cancerous mammary tissues. This study used ChIP-seq to interrogate transcription factor binding and histone modifications of the Trp63 locus in mammary tissue and RNA-seq and immunohistochemistry to gauge gene expression. H3K4me2 and H3K4me3 marks coincided only with the proximal promoter, supporting RNA-seq data showing the predominance of the dominant negative isoform. STAT5 bound specifically to the Trp63 proximal promoter and Trp63 mRNA levels were elevated upon deleting Stat5 from mammary tissue, suggesting its role as a negative regulator. The dominant negative TRP63 isoform was localized to nuclei of basal mammary epithelial cells throughout reproductive cycles and retained in a majority of the triple-negative cancers generated from loss of full-length Brca1. Increased expression of dominant negative isoforms was correlated with developmental windows of increased progesterone receptor binding to the proximal Trp63 promoter and decreased expression during lactation was correlated with STAT5 binding to the same region. TRP63 is present in the majority of triple-negative cancers resulting from loss of Brca1 but diminished in less differentiated cancer subtypes and in cancer cells undergoing EMT.

Peroxisomal membrane channel Pxmp2 in the mammary fat pad is essential for stromal lipid homeostasis and for development of mammary gland epithelium in mice

To understand the functional role of the peroxisomal membrane channel Pxmp2, mice with a targeted disruption of the Pxmp2 gene were generated. These mice were viable, grew and bred normally. However, Pxmp2(-/-) female mice were unable to nurse their pups. Lactating mammary gland epithelium displayed secretory lipid droplets and milk proteins, but the size of the ductal system was greatly reduced. Examination of mammary gland development revealed that retarded mammary ductal outgrowth was due to reduced proliferation of epithelial cells during puberty. Transplantation experiments established the Pxmp2(-/-) mammary stroma as a tissue responsible for suppression of epithelial growth. Morphological and biochemical examination confirmed the presence of peroxisomes in the mammary fat pad adipocytes, and functional Pxmp2 was detected in the stroma of wild-type mammary glands. Deletion of Pxmp2 led to an elevation in the expression of peroxisomal proteins in the mammary fat pad but not in liver or kidney of transgenic mice. Lipidomics of Pxmp2(-/-)mammary fat pad showed a decrease in the content of myristic acid (C14), a principal substrate for protein myristoylation and a potential peroxisomal β-oxidation product. Analysis of complex lipids revealed a reduced concentration of a variety of diacylglycerols and phospholipids containing mostly polyunsaturated fatty acids that may be caused by activation of lipid peroxidation. However, an antioxidant-containing diet did not stimulate mammary epithelial proliferation in Pxmp2(-/-) mice. The results point to disturbances of lipid metabolism in the mammary fat pad that in turn may result in abnormal epithelial growth. The work reveals impaired mammary gland development as a new category of peroxisomal disorders.

Coregulation of genetic programs by the transcription factors NFIB and STAT5

Mammary-specific genetic programs are activated during pregnancy by the common transcription factor signal transducer and activator of transcription (STAT) 5. More than one third of these genes carry nuclear factor I/B (NFIB) binding motifs that coincide with STAT5 in vivo binding, suggesting functional synergy between these two transcription factors. The role of NFIB in this governance was investigated in mice from which Nfib had been inactivated in mammary stem cells or in differentiating alveolar epithelium. Although NFIB was not required for alveolar expansion, the combined absence of NFIB and STAT5 prevented the formation of functional alveoli. NFIB controlled the expression of mammary-specific and STAT5-regulated genes and chromatin immunoprecipitation-sequencing established STAT5 and NFIB binding at composite regulatory elements containing histone H3 lysine dimethylation enhancer marks and progesterone receptor binding. By integrating previously published chromatin immunoprecipitation-sequencing data sets, the presence of NFIB-STAT5 modules in other cell types was investigated. Notably, genomic sites bound by NFIB in hair follicle stem cells were also occupied by STAT5 in mammary epithelium and coincided with enhancer marks. Many of these genes were under NFIB control in both hair follicle stem cells and mammary alveolar epithelium. We propose that NFIB-STAT5 modules, possibly in conjunction with other transcription factors, control cell-specific genetic programs.

Miz1 deficiency in the mammary gland causes a lactation defect by attenuated Stat5 expression and phosphorylation

Miz1 is a zinc finger transcription factor with an N-terminal POZ domain. Complexes with Myc, Bcl-6 or Gfi-1 repress expression of genes like Cdkn2b (p15^Ink4) or Cdkn1a (p21^Cip1). The role of Miz1 in normal mammary gland development has not been addressed so far. Conditional knockout of the Miz1 POZ domain in luminal cells during pregnancy caused a lactation defect with a transient reduction of glandular tissue, reduced proliferation and attenuated differentiation. This was recapitulated in vitro using mouse mammary gland derived HC11 cells. Further analysis revealed decreased Stat5 activity in Miz1ΔPOZ mammary glands and an attenuated expression of Stat5 targets. Gene expression of the Prolactin receptor (PrlR) and ErbB4, both critical for Stat5 phosphorylation (pStat5) or pStat5 nuclear translocation, was decreased in Miz1ΔPOZ females. Microarray, ChIP-Seq and gene set enrichment analysis revealed a down-regulation of Miz1 target genes being involved in vesicular transport processes. Our data suggest that deranged intracellular transport and localization of PrlR and ErbB4 disrupt the Stat5 signalling pathway in mutant glands and cause the observed lactation phenotype.

Developmental programming mediated by complementary roles of imprinted Grb10 in mother and pup

Developmental programming links growth in early life with health status in adulthood. Although environmental factors such as maternal diet can influence the growth and adult health status of offspring, the genetic influences on this process are poorly understood. Using the mouse as a model, we identify the imprinted gene Grb10 as a mediator of nutrient supply and demand in the postnatal period. The combined actions of Grb10 expressed in the mother, controlling supply, and Grb10 expressed in the offspring, controlling demand, jointly regulate offspring growth. Furthermore, Grb10 determines the proportions of lean and fat tissue during development, thereby influencing energy homeostasis in the adult. Most strikingly, we show that the development of normal lean/fat proportions depends on the combined effects of Grb10 expressed in the mother, which has the greater effect on offspring adiposity, and Grb10 expressed in the offspring, which influences lean mass. These distinct functions of Grb10 in mother and pup act complementarily, which is consistent with a coadaptation model of imprinting evolution, a model predicted but for which there is limited experimental evidence. In addition, our findings identify Grb10 as a key genetic component of developmental programming, and highlight the need for a better understanding of mother-offspring interactions at the genetic level in predicting adult disease risk.

MiR-21 is under control of STAT5 but is dispensable for mammary development and lactation

Development of mammary alveolar epithelium during pregnancy is controlled by prolactin, through the transcription factors STAT5A/B that activate specific sets of target genes. Here we asked whether some of STAT5's functions are mediated by microRNAs. The miR-21 promoter sequence contains a bona-fide STAT5 binding site and miR-21 levels increased in HC11 mammary cells upon prolactin treatment. In vivo miR-21 was abundantly expressed in mammary epithelium at day 6 of pregnancy. Analysis of mice lacking miR-21 revealed that their mammary tissue developed normally during pregnancy and dams were able to nurse their pups. Our study demonstrated that although expression of miR-21 is under prolactin control through the transcription factors STAT5A/B its presence is dispensable for mammary development and lactation.

Identification of STAT5A and STAT5B target genes in human T cells

Signal transducer and activator of transcription (STAT) comprises a family of universal transcription factors that help cells sense and respond to environmental signals. STAT5 refers to two highly related proteins, STAT5A and STAT5B, with critical function: their complete deficiency is lethal in mice; in humans, STAT5B deficiency alone leads to endocrine and immunological problems, while STAT5A deficiency has not been reported. STAT5A and STAT5B show peptide sequence similarities greater than 90%, but subtle structural differences suggest possible non-redundant roles in gene regulation. However, these roles remain unclear in humans. We applied chromatin immunoprecipitation followed by DNA sequencing using human CD4⁺ T cells to detect candidate genes regulated by STAT5A and/or STAT5B, and quantitative-PCR in STAT5A or STAT5B knock-down (KD) human CD4⁺ T cells to validate the findings. Our data show STAT5A and STAT5B play redundant roles in cell proliferation and apoptosis via SGK1 interaction. Interestingly, we found a novel, unique role for STAT5A in binding to genes involved in neural development and function (NDRG1, DNAJC6, and SSH2), while STAT5B appears to play a distinct role in T cell development and function via DOCK8, SNX9, FOXP3 and IL2RA binding. Our results also suggest that one or more co-activators for STAT5A and/or STAT5B may play important roles in establishing different binding abilities and gene regulation behaviors. The new identification of these genes regulated by STAT5A and/or STAT5B has major implications for understanding the pathophysiology of cancer progression, neural disorders, and immune abnormalities.

Stat5 regulates the phosphatidylinositol 3-kinase/Akt1 pathway during mammary gland development and tumorigenesis

Stat5 (signal transducer and activator of transcription 5) is an essential mediator of cytokine receptor signaling and plays important roles in the proliferation of alveolar progenitors and the survival of functionally differentiated epithelial cells in the mammary gland. A deregulated expression and activation of Stat5 leads to precocious alveolar development in the absence of pregnancy hormones, impaired mammary gland remodeling following the cessation of lactation, and mammary tumor formation. We reported previously that Stat5 induces the transcription of the Akt1 gene from a novel promoter. In this report, we provide experimental evidence that Akt1 is an essential mediator for the biological function of Stat5 as a survival factor. Additionally, Stat5 controls the expression of the regulatory and catalytic subunits of the phosphatidylinositol 3-kinase (PI3K) (p85α and p110α), thereby greatly augmenting signaling through the prosurvival PI3K/Akt pathway. In agreement with this model, we observed that the constitutive activation of Stat5 cooperates with the loss of function of the tumor suppressor PTEN by accelerating the formation of preneoplastic lesions and mammary tumors. The mammary gland-specific ablation of Stat5 is sufficient to prevent mammary carcinogenesis in a genuine mouse model for Cowden syndrome. Therefore, targeting the Jak2/Stat5 pathway might be a suitable strategy to prevent breast cancer in patients that carry a mutant PTEN allele.

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.

Canonical and non-canonical roles of the histone methyltransferase EZH2 in mammary development and cancer

Although hormones and downstream transcription factors are considered main drivers directing mammary gland development and oncogenic transformation, an emerging body of evidence suggests these processes are modulated by dynamic histone methylation landscapes. The methyltransferase EZH2 catalyzes the formation of trimethyl groups on lysine 27 of histone 3 (H3K27me3) and loss- and gain-of-function studies have provided insight into its role in normal mammary development and oncogenic transformation. EZH2 controls the homeostasis of mouse mammary stem cells, and mammary epithelium devoid of EZH2 does not undergo functional development during pregnancy, possibly due to a paucity of stem cells. EZH2 levels are frequently elevated in breast cancer suggesting a link between H3K27me3 and cell proliferation. In addition to its role as epigenetic regulator, recent studies have placed EZH2 into the category of transcriptional co-activators and thus opened the possibility of non-canonical signaling pathways. In contrast to solid tumors, loss of EZH2 from hematopoietic cells has been linked to malignancies (Fig. 1). The challenge will be to understand not only cell-specific functions of EZH2, but also the extent to which it relies on its enzymatic activity versus its ability to serve as a transcriptional co-factor.

Basal cell signaling by p63 controls luminal progenitor function and lactation via NRG1

The mammary epithelium is organized as a bilayer of luminal and basal/myoepithelial cells. During pregnancy, the luminal compartment expands for milk production, while basal cells are thought to provide structural and contractile support. Here, we reveal a pregnancy-specific role of basal epithelia as a central coordinator of lactogenesis. We demonstrate that genetic deletion of the transcription factor p63 (Trp63) gene exclusively within basal cells of the adult gland during pregnancy leads to dramatic defects in luminal cell proliferation and differentiation, resulting in lactation failure. This phenotype is explained by direct transcriptional activation of the epidermal growth factor family ligand gene Nrg1 by p63 selectively in basal cells, which is required for luminal ERBB4/STAT5A activation and consequent luminal progenitor cell maturation. Thus, paracrine basal-to-luminal cell signaling, controlled by p63 via NRG1, orchestrates the entire lactation program. Collectively, these findings redefine the paradigm for cellular interactions specifying the functional maturation of the mammary gland.

Mammary-specific gene activation is defined by progressive recruitment of STAT5 during pregnancy and the establishment of H3K4me3 marks

Differentiation of mammary secretory epithelium during pregnancy is characterized by sequential activation of genes over several orders of magnitude. Although the transcription factor STAT5 is key to alveolar development, it is not clear to what extent it controls temporal activation of genetic programs in secretory epithelium. To uncover molecular mechanisms effecting progressive differentiation, we explored genome-wide STAT5 binding and H3K4me3 (i.e., trimethylated histone H3 at K4) marks in mammary tissues at early and midpregnancy and at parturition. STAT5 binding to genes induced during pregnancy was low in immature mammary tissue but increased with epithelial differentiation. Increased STAT5 binding was associated with the establishment of H3K4me3 marks and transcriptional activation. STAT5 binding preceded the formation of H3K4me3 marks in some mammary-specific genes. De novo STAT5 binding was also found at distal sites, indicating enhancers. Furthermore, we established an exhaustive mammary transcriptome. Through integration of RNA-seq and STAT5 and H3K4me4 ChIP-seq data, we discovered novel mammary-specific alternative promoters and genes, including noncoding RNAs. Our findings suggest that STAT5 is an early step in establishing transcription complexes on genes specifically expressed in mammary epithelium. This is the first study in an organ that links progressive chromatin occupancy of STAT5 to the acquisition of H3K4me3 marks and transcription during hormone-induced differentiation.

Identification of a STAT5 target gene, Dpf3, provides novel insights in chronic lymphocytic leukemia

STAT5 controls essential cellular functions and is encoded by two genes, Stat5a and Stat5b. To provide insight to the mechanisms linking hematologic malignancy to STAT5 activation/regulation of target genes, we identified STAT5 target genes and focused on Dpf3 gene, which encodes for an epigenetic factor. Dpf3 expression was induced upon IL-3 stimulation in Ba/F3 cells, while strong binding of both STAT5a and STAT5b was detected in its promoter. Reduced expression of Dpf3 was detected in Ba/F3 cells with Stat5a and Stat5b knock-down, suggesting that this gene is positively regulated by STAT5, upon IL-3 stimulation. Furthermore, this gene was significantly up-regulated in CLL patients, where DPF3 gene/protein up-regulation and strong STAT5 binding to the DPF3 promoter, correlated with increased STAT5 activation, mainly in non-malignant myeloid cells (granulocytes). Our findings provide insights in the STAT5 dependent transcriptional regulation of Dpf3, and demonstrate for the first time increased STAT5 activation in granulocytes of CLL patients. Novel routes of investigation are opened to facilitate the understanding of the role of STAT5 activation in the communication between non-malignant myeloid and malignant B-cells, and the functions of STAT5 target genes networks in CLL biology.