Volume 10, Issue 1

No abstract available.

For several decades, the regulation of casein gene expression by the lactogenic hormones, prolactin and glucocorticoids, has provided an excellent model system in which to study how steroid and peptide hormones regulate gene expression. Early studies of casein gene regulation defined conserved sequence elements in the 5′ flanking region of these genes, including one of which was identified as a γ-interferon activation sequence (GAS). Although this site was thought to interact with a mammary gland-specific factor, purification and cloning of this factor by Bernd Groner and his colleagues revealed it was instead a new member of the signal transducers and activators of transcription family, Stat5, which was expressed in many tissues. The exquisite tissue-specific expression of the casein genes was subsequently shown to depend not on a single transcription factor but on composite response elements that interacted with a number of ubiquitous transcription factors in response to the combinatorial effects of peptide and steroid hormone signaling. More recent studies have defined cooperative effects of prolactin and glucocorticoids as well as antagonistic effects of progesterone on the chromatin structure of both the casein gene proximal promoter region as well as a distal enhancer. Local chromatin modifications as well as long-range interactions facilitated by DNA looping are required for the hormonal regulation of β- casein gene expression. The casein genes are part of a large gene cluster, and the chromatin landscape of the entire cluster is regulated in a tissue-specific and developmental manner. Finally, newly discovered large non coding RNAs, such as the pregnancy-induced non coding RNA ( PINC ) may play an important role in the epigenetic regulation of mammary gland differentiation.

The signal transducer and activator of transcription (STAT) family is activated by cytokines and conveys biochemical signals to the genome through binding to specific regulatory sequences, called IFN-γ-activated sequence (GAS) motifs. As common GAS motifs (TTCnnnGAA) contain only six conserved nucleotides, the mammalian genome harbors hundreds of thousands of copies of this sequence. However, it is not possible to predict which specific GAS motifs bind to STATs and are of functional significance. Here, we apply several layers of statistical, bioinformatics and experimental analyses to narrow down the number of GAS sites that might be of biological relevance. In particular, we determined the number of bona fide GAS motifs by utilizing publically available genome-wide STAT5 ChIP-seq data sets. Less than 10% of GAS motifs within the mouse genome are recognized by STAT5 in vivo and only a small portion of them are shared across different cell types. However, even bona fide STAT5 binding did not predict that the respective gene was under cytokine-STAT control. Therefore, additional bioinformatics, genomic and epigenetic parameters, such as patterns of histone modifications, are required to more reliably predict the behavior of cytokine-STAT regulatory networks.

Mammary gland regression post-weaning (involution) is a highly regulated, complicated process in which the transcription factor Stat3 is a key player. Over the last decade, microarray analyses have had a profound impact on our understanding of this role. Studies using mammary epithelial cells in which Stat3 was activated in a ligand-independent manner have allowed direct transcriptional targets of Stat3 to be identified. Additionally, global gene expression changes during involution have been profiled by microarray analyses, which allowed characterization of clusters of genes with distinct expression profiles during the first 4 days of involution. Such expression profiling led to the observation that one of the most strikingly upregulated genes in the absence of Stat3 is the serpin Spi2a. This led to the discovery that mammary epithelial cell lysosomes undergo lysosomal membrane permeablisation and leak cathepsins during involution. Stat3 upregulates the expression of cathepsins B and L within 24 h of weaning and is thus the critical inducer of lysosomal-mediated cell death during this process. In addition to its pivotal role in the control of cell death during involution, microarray-based studies have demonstrated that the expression of acute phase and inflammatory genes is regulated by Stat3 and that mammary epithelial expression of this transcription factor modulates the phenotype of macrophages present in the gland during second phase remodelling. Thus, Stat3 signalling may have effects that are not cell-autonomous, in addition to its cell-autonomous role in involution.

Background: The pro-oncogenic transcription factor STAT3 is constitutively active in tumours of many different origins, which often become addicted to its activity. STAT3 is believed to contribute to the initial survival of pre-cancerous cells as well as to hyper-proliferation and, later, metastasis. Materials and methods: To evaluate the contribution of enhanced STAT3 activation in a controlled model system, we generated knock-in mice in which a mutant constitutively active Stat3C allele replaces the endogenous wild-type allele and analysed its contribution to breast tumorigenesis. Moreover, we generated Stat3 C/C MEF cells and analysed their gene expression and metabolic profiles. Results: Constitutively active STAT3 could enhance the tumorigenic power of the rat Neu oncogene in MMTV-Neu transgenic mice and trigger the production of earlier onset and more invasive mammary tumours. Tumour-derived cell lines displayed higher migrating, invading and metastatic ability and showed disrupted distribution of cell-cell junction markers. These features were mediated by STAT3-dependent over-expression of the C-terminal tensin-like (Cten) focal adhesion protein. Moreover, STAT3C alone was able to induce aerobic glycolysis and down-regulate mitochondrial activity, both in primary fibroblasts and in STAT3-dependent tumour cell lines, acting via both HIF-1α-dependent and independent mechanisms. Conclusions: STAT3 can induce a metabolic switch that predisposes cells to aberrant survival, enhanced proliferation and, finally, tumour transformation. Later, enhanced Cten expression contributes to tissue infiltration and metastasis. While not excluding the contribution of many other tumour-specific STAT3 target genes, our data provide a unifying explanation of several pro-oncogenic STAT3 activities.

Breast cancer patients with disseminated metastatic disease still have a very unfavorable prognosis. Investigations into the molecular mechanisms that underlie metastasis formation have a high priority and can possibly result in improved therapeutic interventions. The process of oncogenic epithelial to mesenchymal transition (EMT) has recently become a focus in cancer research because it encompasses many of the phenotypic traits characteristic of metastatic cells, e.g., increased motility, invasion, anoikis resistance, immunosuppression, and cancer stem cell potential. A number of central cellular signaling pathways and transcription factors have been implied in the control of EMT and metastasis formation, among them signal originating from the activation of the transforming growth factor β (TGFβ), epithelial growth factor, Wnt, Notch, and Hedgehog pathways. We have investigated the contribution of TGFβ signaling to metastasis-related cellular properties. TGFβ signaling can have tumor-suppressive and -promoting effects depending on the tumor type and the stage of tumor progression. TGFβ can inhibit the proliferation of mammary epithelial cells (MECs), but it can also induce EMT, invasion, and metastasis, possibly through Smad-independent signaling events. We investigated the effects of TGFβ pathway inhibition on the proliferation, differentiation, and invasion of both normal and malignant MECs. shRNA-mediated downregulation of the Smad4 protein in non-tumorigenic HC11 and tumorigenic 4T1 cells promotes the invasiveness of both cell lines. Mammary gland reconstitution studies, with primary MECs expressing shSmad4, resulted only in subtle effects on the glandular morphogenesis. Orthotopic transplantation of shSmad4-transduced 4T1 tumor cells caused the accelerated growth of mammary tumors and enhanced colonization and macroscopic lung metastases when compared to control cells. Surprisingly, the expression of Smad4 was restored, and a strong activation of Stat3 was found in the metastatic lesions present in the lungs. These lesions express metastatic factors, such as angiopoietin-like-4 and the inhibitor of DNA binding/differentiation 1. We suggest that the downregulation of Smad4 inhibits the tumor-suppressive effects of TGFβ signaling and enhances tumor growth. The downregulation, however, was only transient, and the reactivation of Smad4 expression caused the reversal of EMT, mesenchymal to epithelial transition, and thereby promoted metastasis formation in the lungs.