PML depletion disrupts normal mammary gland development and skews the composition of the mammary luminal cell progenitor pool

Promyelocytic leukemia protein regulates angiogenesis and epithelial-mesenchymal transition to limit metastasis in MDA-MB-231 breast cancer cells

Promyelocytic leukemia protein (PML) modulates diverse cell functions that contribute to both tumor suppressor and pro-oncogenic effects, depending on the cellular context. We show here that PML knockdown (KD) in MDA-MB-231, but not MCF7, breast cancer cells, prolonged stem-cell-like survival, and increased cell proliferation and migration, which is in line with gene-enrichment results from their RNA sequencing analysis. Of note, increased migration was accompanied by higher levels of the epithelial-mesenchymal transition (EMT) regulator Twist-related protein 2 (TWIST2). We showed here that PML binds to TWIST2 via its basic helix-loop-helix (bHLH) region and functionally interferes with the suppression of the epithelial target of TWIST2, CD24. In addition, PML ablation in MDA-MB-231 cells led to higher protein levels of hypoxia-inducible factor 1-alpha (HIF1a), resulting in a higher cell hypoxic response. Functionally, PML directly suppressed the induction of the HIF1a target gene vascular endothelial growth factor A (VEGFa). In line with these results, tumor xenografts of MDA-MB-231 PML-KD cells had enhanced aggressive properties, including higher microvessel density, faster local growth, and higher metastatic ability, with a preference for lung. Collectively, PML suppresses the cancer aggressive behavior by multiple mechanisms that impede both the HIF-hypoxia-angiogenic and EMT pathways.

Cancer invasion and metastasis: Insights from murine pubertal mammary gland morphogenesis

Cancer invasion and metastasis accounts for the majority of cancer related mortality. A better understanding of the players that drive the aberrant invasion and migration of tumors cells will provide critical targets to inhibit metastasis. Postnatal pubertal mammary gland morphogenesis is characterized by highly proliferative, invasive, and migratory normal epithelial cells. Identifying the molecular regulators of pubertal gland development is a promising strategy since tumorigenesis and metastasis is postulated to be a consequence of aberrant reactivation of developmental stages. In this review, we summarize the pubertal morphogenesis regulators that are involved in cancer metastasis and revisit pubertal mammary gland transcriptome profiling to uncover both known and unknown metastasis genes. Our updated list of pubertal morphogenesis regulators shows that most are implicated in invasion and metastasis. This review highlights molecular linkages between development and metastasis and provides a guide for exploring novel metastatic drivers.

Genome Assembly and Evolutionary Analysis of the Mandarin Duck Aix galericulata Reveal Strong Genome Conservation among Ducks

The mandarin duck, Aix galericulata, is popular in East Asian cultures and displays exaggerated sexual dimorphism, especially in feather traits during breeding seasons. We generated and annotated the first mandarin duck de novo assembly, which was 1.08 Gb in size and encoded 16,615 proteins. Using a phylogenomic approach calibrated with fossils and molecular divergences, we inferred that the last common ancestor of ducks occurred 13.3-26.7 Ma. The majority of the mandarin duck genome repetitive sequences belonged to the chicken repeat 1 (CR1) retroposon CR1-J2_Pass, which underwent a duck lineage-specific burst. Synteny analyses among ducks revealed infrequent chromosomal rearrangements in which breaks were enriched in LINE retrotransposons and DNA transposons. The calculation of the dN/dS ratio revealed that the majority of duck genes were under strong purifying selection. The expanded gene families in the mandarin duck are primarily involved in olfactory perception as well as the development and morphogenesis of feather and branching structures. This new reference genome will improve our understanding of the morphological and physiological characteristics of ducks and provide a valuable resource for functional genomics studies to investigate the feather traits of the mandarin duck.

Neurodegeneration & imperfect ageing: Technological limitations and challenges?

Cellular homeostasis is regulated by the protein quality control (PQC) machinery, comprising multiple chaperones and enzymes. Studies suggest that the loss of the PQC mechanisms in neurons may lead to the formation of abnormal inclusions that may lead to neurological disorders and defective aging. The questions could be raised how protein aggregate formation precisely engenders multifactorial molecular pathomechanism in neuronal cells and affects different brain regions? Such questions await thorough investigation that may help us understand how aberrant proteinaceous bodies lead to neurodegeneration and imperfect aging. However, these studies face multiple technological challenges in utilizing available tools for detailed characterizations of the protein aggregates or amyloids and developing new techniques to understand the biology and pathology of proteopathies. The lack of detection and analysis methods has decelerated the pace of the research in amyloid biology. Here, we address the significance of aggregation and inclusion formation, followed by exploring the evolutionary contribution of these structures. We also provide a detailed overview of current state-of-the-art techniques and advances in studying amyloids in the diseased brain. A comprehensive understanding of the structural, pathological, and clinical characteristics of different types of aggregates (inclusions, fibrils, plaques, etc.) will aid in developing future therapies.

PML Regulates the Epidermal Differentiation Complex and Skin Morphogenesis during Mouse Embryogenesis

The promyelocytic leukemia (PML) protein is an essential component of nuclear compartments called PML bodies. This protein participates in several cellular processes, including growth control, senescence, apoptosis, and differentiation. Previous studies have suggested that PML regulates gene expression at a subset of loci through a function in chromatin remodeling. Here we have studied global gene expression patterns in mouse embryonic skin derived from Pml depleted and wild type mouse embryos. Differential gene expression analysis at different developmental stages revealed a key role of PML in regulating genes involved in epidermal stratification. In particular, we observed dysregulation of the late cornified envelope gene cluster, which is a sub-region of the epidermal differentiation complex. In agreement with these data, PML body numbers are elevated in basal keratinocytes during embryogenesis, and we observed reduced epidermal thickness and defective hair follicle development in PML depleted mouse embryos.

PML Bodies in Mitosis

Promyelocytic leukemia (PML) bodies are dynamic intracellular structures that recruit and release a variety of different proteins in response to stress, virus infection, DNA damage and cell cycle progression. While PML bodies primarily are regarded as nuclear compartments, they are forced to travel to the cytoplasm each time a cell divides, due to breakdown of the nuclear membrane at entry into mitosis and subsequent nuclear exclusion of nuclear material at exit from mitosis. Here we review the biochemical and biophysical transitions that occur in PML bodies during mitosis and discuss this in light of post-mitotic nuclear import, cell fate decision and acute promyelocytic leukemia therapy.

Promyelocytic leukemia protein (PML) controls breast cancer cell proliferation by modulating Forkhead transcription factors

The multitasking promyelocytic leukemia (PML) protein was originally recognized as a tumor‐suppressive factor, but more recent evidence has implicated PML in tumor cell prosurvival actions and poor patient prognosis in specific cancer settings. Here, we report that inducible PMLIV expression inhibits cell proliferation as well as self‐renewal and impairs cell cycle progression of breast cancer cell lines in a reversible manner. Transcriptomic profiling identified a large number of PML‐deregulated genes associated with various cell processes. Among them, cell cycle‐ and division‐related genes and their cognitive regulators are highly ranked. In this study, we focused on previously unknown PML targets, namely the Forkhead transcription factors. PML suppresses the Forkhead box subclass M1 (FOXM1) transcription factor at both the RNA and protein levels, along with many of its gene targets. We show that FOXM1 interacts with PMLIV primarily via its DNA‐binding domain and dynamically colocalizes in PML nuclear bodies. In parallel, PML modulates the activity of Forkhead box O3 (FOXO3), a factor opposing certain FOXM1 activities, to promote cell survival and stress resistance. Thus, PMLIV affects the balance of FOXO3 and FOXM1 transcriptional programs by acting on discrete gene subsets to favor both growth inhibition and survival. Interestingly, PMLIV‐specific knockdown mimicked ectopic expression vis‐à‐vis loss of proliferative ability and self‐renewal, but also led to loss of survival ability as shown by increased apoptosis. We propose that divergent or similar effects on cell physiology may be elicited by high or low PMLIV levels dictated by other concurrent genetic or epigenetic cancer cell states that may additionally account for its disparate effects in various cancer types.

Isolation and Propagation of Mammary Epithelial Stem and Progenitor Cells

Several methods of mammary gland dissociation have been described that utilize a combined strategy of mechanical and enzymatic dissociation to isolate mammary epithelial cells (MECs) from intact tissue (Smalley et al., J Mammary Gland Biol Neoplasia 17:91-97, 2012). Here we detail a robust method that enables the isolation of all major stem and progenitor MEC populations, which has been successfully used to study stem cell behavior when coupled with transplantation and in vitro assays (Shackleton et al., Nature 439:84-88, 2006; Bouras et al., Cell Stem Cell 3:429-441, 2008; Sheridan et al., BMC Cancer 15:221, 2015; Jamieson et al., Development 144:1065-1071, 2017). Furthermore, we outline two prominent methods for culturing MECs for the purposes of ex vivo manipulation or study: 2D feeder layer cultures and 3D Matrigel colony assays. Importantly, all outlined methods retain stem and progenitor cell behaviors and can be used in combination with downstream in vivo, in vitro, or in silico analyses.

Coordinated collective migration and asymmetric cell division in confluent human keratinocytes without wounding

Epithelial sheet spreading is a fundamental cellular process that must be coordinated with cell division and differentiation to restore tissue integrity. Here we use consecutive serum deprivation and re-stimulation to reconstruct biphasic collective migration and proliferation in cultured sheets of human keratinocytes. In this system, a burst of long-range coordinated locomotion is rapidly generated throughout the cell sheet in the absence of wound edges. Migrating cohorts reach correlation lengths of several millimeters and display dependencies on epidermal growth factor receptor-mediated signaling, self-propelled polarized migration, and a G₁/G₀ cell cycle environment. The migration phase is temporally and spatially aligned with polarized cell divisions characterized by pre-mitotic nuclear migration to the cell front and asymmetric partitioning of nuclear promyelocytic leukemia bodies and lysosomes to opposite daughter cells. This study investigates underlying mechanisms contributing to the stark contrast between cells in a static quiescent state compared to the long-range coordinated collective migration seen in contact with blood serum.

Epithelial sheet migration requires polarized and coordinated cell movement. Here, the authors demonstrate serum-activated collective migration followed by polarized asymmetric cell divisions in otherwise quiescent human keratinocyte monolayers in the absence of wound edges.

PML: Regulation and multifaceted function beyond tumor suppression

Promyelocytic leukemia protein (PML) was originally identified as a fusion partner of retinoic acid receptor alpha in acute promyelocytic leukemia patients with the (15;17) chromosomal translocation, giving rise to PML–RARα and RARα–PML fusion proteins. A body of evidence indicated that PML possesses tumor suppressing activity by regulating apoptosis, cell cycle, senescence and DNA damage responses. PML is enriched in discrete nuclear substructures in mammalian cells with 0.2–1 μm diameter in size, referred to as alternately Kremer bodies, nuclear domain 10, PML oncogenic domains or PML nuclear bodies (NBs). Dysregulation of PML NB formation results in altered transcriptional regulation, protein modification, apoptosis and cellular senescence. In addition to PML NBs, PML is also present in nucleoplasm and cytoplasmic compartments, including the endoplasmic reticulum and mitochondria-associated membranes. The role of PML in tumor suppression has been extensively studied but increasing evidence indicates that PML also plays versatile roles in stem cell renewal, metabolism, inflammatory responses, neural function, mammary development and angiogenesis. In this review, we will briefly describe the known PML regulation and function and include new findings.

Myogenic differentiation triggers PML nuclear body loss and DAXX relocalization to chromocentres

The promyelocytic leukemia protein (PML) is expressed in most normal human tissues and forms nuclear bodies (NBs) that have roles in gene regulation and cellular processes such as DNA repair, cell cycle control, and cell fate decisions. Using murine C2C12 myoblasts, we demonstrate that activation of skeletal muscle differentiation results in loss of PML and PML NBs prior to myotube fusion. Myotube formation was associated with marked chromatin reorganization and the relocalization of DAXX from PML NBs to chromocentres. MyoD expression was sufficient to cause PML NB loss, and silencing of PML induced DAXX relocalization. Fusion of C2C12 cells using the reptilian reovirus p14 fusogenic protein failed to disrupt PML NBs yet still promoted DAXX redistribution and loss; whereas ectopic expression of PML in differentiated cells only partially restored PML NB formation and DAXX localization at NBs. Finally, we determined that the C-terminal SUMO-interacting motif of DAXX is required for its colocalization with ATRX in heterochromatin domains during myotube formation. These data support a model in which activation of myogenic differentiation results in PML NB loss, chromatin reorganization and DAXX relocalization, and provides a paradigm for understanding the consequence of PML loss in other cellular contexts, such as during cancer development and progression.

Promyelocytic Leukemia Protein, a Protein at the Crossroad of Oxidative Stress and Metabolism

SIGNIFICANCE

Cellular metabolic activity impacts the production of reactive oxygen species (ROS), both positively through mitochondrial oxidative processes and negatively by promoting the production of reducing agents (including NADPH and reduced glutathione). A defined metabolic state in cancer cells is critical for cell growth and long-term self-renewal, and such state is intrinsically associated with redox balance. Promyelocytic leukemia protein (PML) regulates several biological processes, at least in part, through its ability to control the assembly of PML nuclear bodies (PML NBs). Recent Advances: PML is oxidation-prone, and oxidative stress promotes NB biogenesis. These nuclear subdomains recruit many nuclear proteins and regulate their SUMOylation and other post-translational modifications. Some of these cargos-such as p53, SIRT1, AKT, and mammalian target of rapamycin (mTOR)-are key regulators of cell fate. PML was also recently shown to regulate oxidation.

CRITICAL ISSUES

While it was long considered primarily as a tumor suppressor protein, PML-regulated metabolic switch uncovered that this protein could promote survival and/or stemness of some normal or cancer cells. In this study, we review the recent findings on this multifunctional protein.

FUTURE DIRECTIONS

Studying PML scaffolding functions as well as its fine role in the activation of p53 or fatty acid oxidation will bring new insights in how PML could bridge oxidative stress, senescence, cell death, and metabolism. Antioxid. Redox Signal. 26, 432-444.

USP11 regulates PML stability to control Notch-induced malignancy in brain tumours

The promyelocytic leukaemia (PML) protein controls multiple tumour suppressive functions and is downregulated in diverse types of human cancers through incompletely characterized post-translational mechanisms. Here we identify USP11 as a PML regulator by RNAi screening. USP11 deubiquitinates and stabilizes PML, thereby counteracting the functions of PML ubiquitin ligases RNF4 and the KLHL20-Cul3 (Cullin 3)-Roc1 complex. We find that USP11 is transcriptionally repressed through a Notch/Hey1-dependent mechanism, leading to PML destabilization. In human glioma, Hey1 upregulation correlates with USP11 and PML downregulation and with high-grade malignancy. The Notch/Hey1-induced downregulation of USP11 and PML not only confers multiple malignant characteristics of aggressive glioma, including proliferation, invasiveness and tumour growth in an orthotopic mouse model, but also potentiates self-renewal, tumour-forming capacity and therapeutic resistance of patient-derived glioma-initiating cells. Our study uncovers a PML degradation mechanism through Notch/Hey1-induced repression of the PML deubiquitinase USP11 and suggests an important role for this pathway in brain tumour pathogenesis.

A Long-Lived Luminal Subpopulation Enriched with Alveolar Progenitors Serves as Cellular Origin of Heterogeneous Mammary Tumors

It has been shown that the mammary luminal lineage could be maintained by luminal stem cells or long-lived progenitors, but their identity and role in breast cancer remain largely elusive. By lineage analysis using Wap-Cre mice, we found that, in nulliparous females, mammary epithelial cells (MECs) genetically marked by Wap-Cre represented a subpopulation of CD61+ luminal progenitors independent of ovarian hormones for their maintenance. Using a pulse-chase lineage-tracing approach based on Wap-Cre adenovirus (Ad-Wap-Cre), we found that Ad-Wap-Cre-marked nulliparous MECs were enriched with CD61+ alveolar progenitors (APs) that gave rise to CD61- alveolar luminal cells during pregnancy/lactation and could maintain themselves long term. When transformed by different oncogenes, they could serve as cells of origin of heterogeneous mammary tumors. Thus, our study revealed a type of long-lived AP within the luminal lineage that may serve as the cellular origin of multiple breast cancer subtypes.

Hormone-sensing mammary epithelial progenitors: emerging identity and hormonal regulation

The hormone-sensing mammary epithelial cell (HS-MEC-expressing oestrogen receptor-alpha (ERα) and progesterone receptor (PGR)) is often represented as being terminally differentiated and lacking significant progenitor activity after puberty. Therefore while able to profoundly influence the proliferation and function of other MEC populations, HS-MECs are purported not to respond to sex hormone signals by engaging in significant cell proliferation during adulthood. This is a convenient and practical simplification that overshadows the sublime, and potentially critical, phenotypic plasticity found within the adult HS-MEC population. This concept is exemplified by the large proportion (~80 %) of human breast cancers expressing PGR and/or ERα, demonstrating that HS-MECs clearly proliferate in the context of breast cancer. Understanding how HS-MEC proliferation and differentiation is driven could be key to unraveling the mechanisms behind uncontrolled HS-MEC proliferation associated with ERα- and/or PGR-positive breast cancers. Herein we review evidence for the existence of a HS-MEC progenitor and the emerging plasticity of the HS-MEC population in general. This is followed by an analysis of hormones other than oestrogen and progesterone that are able to influence HS-MEC proliferation and differentiation: androgens, prolactin and transforming growth factor-beta1.

Evidence for a multipotent mammary progenitor with pregnancy-specific activity

Introduction

The mouse mammary gland provides a powerful model system for studying processes involved in epithelial tissue development. Although markers that enrich for mammary stem cells and progenitors have been identified, our understanding of the mammary developmental hierarchy remains incomplete.

Methods

We used the MMTV promoter linked to the reverse tetracycline transactivator to induce H2BGFP expression in the mouse mammary gland. Mammary epithelial cells (MECs) from virgin mice were sorted by flow cytometry for expression of the mammary stem cell/progenitor markers CD24 and CD29, and H2BGFP. Sorted populations were analyzed for in vivo repopulation ability, expression of mammary lineage markers, and differential gene expression.

Results

The reconstituting activity of CD24⁺/CD29⁺ cells in cleared fat pad transplantation assays was not distinguished in GFP⁺ compared to GFP^- subpopulations. However, within the CD24⁺/CD29^lo luminal progenitor-enriched population, H2BGFP⁺, but not H2BGFP^-, MECs formed mammary structures in transplantation assays; moreover, this activity was dramatically enhanced in pregnant recipients. These outgrowths contained luminal and myoepithelial mammary lineages and produced milk, but lacked the capacity for serial transplantation. Transcriptional microarray analysis revealed that H2BGFP⁺/CD24⁺/CD29^lo MECs are distinct from H2BGFP^-/CD24⁺/CD29^lo MECs and enriched for gene expression signatures with both the stem cell (CD24⁺/CD29⁺) and luminal progenitor (CD24⁺/CD29^lo/CD61⁺) compartments.

Conclusions

We have identified a population of MECs containing pregnancy-activated multipotent progenitors that are present in the virgin mammary gland and contribute to the expansion of the mammary gland during pregnancy.

RUNX1, a transcription factor mutated in breast cancer, controls the fate of ER-positive mammary luminal cells

RUNX1 encodes a RUNX family transcription factor (TF) and was recently identified as a novel mutated gene in human luminal breast cancers. We found that Runx1 is expressed in all subpopulations of murine mammary epithelial cells (MECs) except the secretory alveolar luminal cells. Conditional knockout of Runx1 in MECs by MMTV-Cre led to a decrease in luminal MECs, largely due to a profound reduction in the estrogen receptor (ER)-positive mature luminal subpopulation, a phenotype that could be rescued by the loss of either Trp53 or Rb1. Mechanistically RUNX1 represses Elf5, a master regulatory TF gene for alveolar cells, and regulates mature luminal TF/co-factor genes (e.g., Foxa1 and Cited1) involved in the ER program. Collectively, our data identified a key regulator of the ER⁺ luminal lineage whose disruption may contribute to the development of ER⁺ luminal breast cancer when under the background of either TP53 or RB1 loss.

DOI:http://dx.doi.org/10.7554/eLife.03881.001

Stem cells can develop into the many types of specialized cell found in the body. Several proteins regulate these transformations by switching on and off the expression of genes that are specific to different cell types. Disrupting these proteins can cause the development of cells to go awry and can lead to cancer.

A protein called RUNX1 controls gene expression to direct the development of blood cells. Mutations in the gene encoding this protein have been linked to blood cancers and a particular type of breast cancer, which begins in the cells that line the ducts that carry milk towards the nipple.

Mammary duct-lining cells develop from a pool of stem cells that produces breast tissue cells. Now van Bragt et al. have found that RUNX1 is expressed in the cells lining the ducts of the mammary glands, except those that produce milk. Deleting the gene for RUNX1 in mice reduced the number of duct-lining cells, especially a subgroup of cells that are the sensors for the hormone estrogen. Through experiments on breast cancer cells, van Bragt et al. found that RUNX1 is able to dictate the fate of duct-lining breast cells by controlling other protein regulators. RUNX1 boosts the activity of at least one regulator that encourages the cells to become duct-lining cells and represses another regulatory protein that turns cells into milk-producing cells.

Next, van Bragt et al. found that, in mice lacking the gene for RUNX1, reducing the amounts of certain proteins that normally suppress the formation of tumors restored the populations of estrogen-sensing duct-lining cells. This suggests that mutations in the gene encoding RUNX1, coupled with the loss of a tumor-suppressing protein, may contribute to the development of cancer in the cells that line the breast ducts.

The next challenge is to determine exactly how RUNX1 mutations work together with the loss of the tumor-suppressing protein to drive breast cancer development. This knowledge may translate into new approaches to prevent or treat this type of breast cancer.

DOI:http://dx.doi.org/10.7554/eLife.03881.002

Retinoic acid signaling in cancer: The parable of acute promyelocytic leukemia

Inevitably fatal some 40 years, acute promyelocytic leukemia (APL) can now be cured in more than 95% of cases. This clinical success story is tightly linked to tremendous progress in our understanding of retinoic acid (RA) signaling. The discovery of retinoic acid receptor alpha (RARA) was followed by the cloning of the chromosomal translocations driving APL, all of which involve RARA. Since then, new findings on the biology of nuclear receptors have progressively enlightened the basis for the clinical efficacy of RA in APL. Reciprocally, the disease offered a range of angles to approach the cellular and molecular mechanisms of RA action. This virtuous circle contributed to make APL one of the best-understood cancers from both clinical and biological standpoints. Yet, some important questions remain unanswered including how lessons learnt from RA-triggered APL cure can help design new therapies for other malignancies.

Tip30 controls differentiation of murine mammary luminal progenitor to estrogen receptor-positive luminal cell through regulating FoxA1 expression

Estrogen receptor-alpha positive (ER⁺) breast cancers comprise the majority of human breast cancers, but molecular mechanisms underlying this subtype of breast cancers remain poorly understood. Here, we show that ER⁺ mammary luminal tumors arising in Tip30^−/−MMTV-Neu mice exhibited increased enrichment of luminal progenitor gene signature. Deletion of the Tip30 gene increased proportion of mammary stem and progenitor cell populations, and raised susceptibility to ER⁺ mammary luminal tumors in female Balb/c mice. Moreover, Tip30^−/− luminal progenitors displayed increases in propensity to differentiate to mature ER⁺ luminal cells and FoxA1 expression. Knockdown of FoxA1 expression in Tip30^−/− progenitors by shRNA specific for FoxA1 reduced their differentiation toward ER⁺ mature luminal cells. Taken together, our results suggest that TIP30 is a key regulator for maintaining ER⁺ and ER⁻luminal pools in the mammary luminal lineage, and loss of it promotes expansion of ER⁺ luminal progenitors and mature cells and ER⁺ mammary tumorigenesis.

The role of PML in hematopoietic and leukemic stem cell maintenance

The tumor suppressor promyelocytic leukemia (PML) was first identified as a component of PML-RARα fusion protein, one of the initiating cytogenetic abnormalities in acute promyelocytic leukemia. PML is now known to have diverse functions regulating the DNA-damage response, apoptosis, senescence, and angiogenesis. Recent investigations have identified PML as a regulator of metabolic pathways in stem cell compartments, including the hematopoietic system, and have provided researchers with new strategies for controlling stem cell maintenance and differentiation. Studies of PML in leukemia-initiating cells demonstrate that PML is also an essential component of their maintenance, which has drawn tremendous attention to PML from scientists in various stem cell fields. Here, we review research into PML and its associated pathways, including recent studies of PML as it relates to stem cell biology, as well as our finding that PML regulates fatty acid oxidation, which is essential to the maintenance of normal hematopoietic stem cells. We also discuss the therapeutic potential of controlling PML-associated pathways. In particular, we describe promising evidence for the use of arsenic trioxide in the treatment of chronic myeloid leukemia.

PML-mediated signaling and its role in cancer stem cells

The promyelocytic leukemia (PML) protein, initially discovered as a part of the PML/retinoic acid receptor alpha fusion protein, has been found to be a critical player in oncogenesis and tumor progression. Multiple cellular activities, including DNA repair, alternative lengthening of telomeres, transcriptional control, apoptosis and senescence, are regulated by PML and its featured subcellular structure, the PML nuclear body. In correspondence with its role in many important life processes, PML mediates several complex downstream signaling pathways. The determinant function of PML in tumorigenesis and cancer progression raises the interest in its involvement in cancer stem cells (CSCs), a subpopulation of cancer cells that share properties with stem cells and are critical for tumor propagation. Recently, there are exciting discoveries concerning the requirement of PML in CSC maintenance. Growing evidences strongly suggest a positive role of PML in regulating CSCs in both hematopoietic cancers and solid tumors, whereas the underlying mechanisms may be different and remain elusive. Here we summarize and discuss the PML-mediated signaling pathways in cancers and their potential roles in regulating CSCs.

Tumor suppressive pathways in the control of neurogenesis

The generation of specialized neural cells in the developing and postnatal central nervous system is a highly regulated process, whereby neural stem cells divide to generate committed neuronal progenitors, which then withdraw from the cell cycle and start to differentiate. Cell cycle checkpoints play a major role in regulating the balance between neural stem cell expansion and differentiation. Loss of tumor suppressors involved in checkpoint control can lead to dramatic alterations of neurogenesis, thus contributing to neoplastic transformation. Here we summarize and critically discuss the existing literature on the role of tumor suppressive pathways and their regulatory networks in the control of neurogenesis and transformation.

Global changes in the mammary epigenome are induced by hormonal cues and coordinated by Ezh2

The mammary epithelium is a dynamic, highly hormone-responsive tissue. To explore chromatin modifications underlying its lineage specification and hormone responsiveness, we determined genome-wide histone methylation profiles of mammary epithelial subpopulations in different states. The marked differences in H3K27 trimethylation between subpopulations in the adult gland suggest that epithelial cell-fate decisions are orchestrated by polycomb-complex-mediated repression. Remarkably, the mammary epigenome underwent highly specific changes in different hormonal contexts, with a profound change being observed in the global H3K27me3 map of luminal cells during pregnancy. We therefore examined the role of the key H3K27 methyltransferase Ezh2 in mammary physiology. Its expression and phosphorylation coincided with H3K27me3 modifications and peaked during pregnancy, driven in part by progesterone. Targeted deletion of Ezh2 impaired alveologenesis during pregnancy, preventing lactation, and drastically reduced stem/progenitor cell numbers. Taken together, these findings reveal that Ezh2 couples hormonal stimuli to epigenetic changes that underpin progenitor activity, lineage specificity, and alveolar expansion in the mammary gland.

Atbf1 regulates pubertal mammary gland development likely by inhibiting the pro-proliferative function of estrogen-ER signaling

ATBF1 is a candidate tumor suppressor that interacts with estrogen receptor (ER) to inhibit the function of estrogen-ER signaling in gene regulation and cell proliferation control in human breast cancer cells. We therefore tested whether Atbf1 and its interaction with ER modulate the development of pubertal mammary gland, where estrogen is the predominant steroid hormone. In an in vitro model of cell differentiation, i.e., MCF10A cells cultured in Matrigel, ATBF1 expression was significantly increased, and knockdown of ATBF1 inhibited acinus formation. During mouse mammary gland development, Atbf1 was expressed at varying levels at different stages, with higher levels during puberty, lower during pregnancy, and the highest during lactation. Knockout of Atbf1 at the onset of puberty enhanced ductal elongation and bifurcation and promoted cell proliferation in both ducts and terminal end buds of pubertal mammary glands. Enhanced cell proliferation primarily occurred in ER-positive cells and was accompanied by increased expression of ER target genes. Furthermore, inactivation of Atbf1 reduced the expression of basal cell markers (CK5, CK14 and CD44) but not luminal cell markers. These findings indicate that Atbf1 plays a role in the development of pubertal mammary gland likely by modulating the function of estrogen-ER signaling in luminal cells and by modulating gene expression in basal cells.

The Stat6-regulated KRAB domain zinc finger protein Zfp157 regulates the balance of lineages in mammary glands and compensates for loss of Gata-3

Lineage commitment studies in mammary glands have focused on identifying cell populations that display stem or progenitor properties. However, the mechanisms that control cell fate have been incompletely explored. Herein we show that zinc finger protein 157 (Zfp157) is required to establish the balance between luminal alveolar pStat5- and Gata-3-expressing cells in the murine mammary gland. Using mice in which the zfp157 gene was disrupted, we found that alveologenesis was accelerated concomitantly with a dramatic skewing of the proportion of pStat5-expressing cells relative to Gata-3⁺ cells. This suppression of the Gata-3⁺ lineage was associated with increased expression of the inhibitor of helix-loop-helix protein Id2. Surprisingly, Gata-3 becomes dispensable in the absence of Zfp157, as mice deficient for both Zfp157 and Gata-3 lactate normally, although the glands display a mild epithelial dysplasia. These data suggest that the luminal alveolar compartment of the mammary gland is comprised of a number of distinct cell populations that, although interdependant, exhibit considerable cell fate plasticity.

Paternal RLIM/Rnf12 is a survival factor for milk-producing alveolar cells

In female mouse embryos, somatic cells undergo a random form of X chromosome inactivation (XCI), whereas extraembryonic trophoblast cells in the placenta undergo imprinted XCI, silencing exclusively the paternal X chromosome. Initiation of imprinted XCI requires a functional maternal allele of the X-linked gene Rnf12, which encodes the ubiquitin ligase Rnf12/RLIM. We find that knockout (KO) of Rnf12 in female mammary glands inhibits alveolar differentiation and milk production upon pregnancy, with alveolar cells that lack RLIM undergoing apoptosis as they begin to differentiate. Genetic analyses demonstrate that these functions are mediated primarily by the paternal Rnf12 allele due to nonrandom maternal XCI in mammary epithelial cells. These results identify paternal Rnf12/RLIM as a critical survival factor for milk-producing alveolar cells and, together with population models, reveal implications of transgenerational epigenetic inheritance.

Integrins in mammary development

Integrins are ubiquitously expressed major cell surface receptors for extracellular matrix. Integrin interaction with their extracellular ligands triggers activation of the intracellular signaling pathways that control cell shape, motility, proliferation, survival, cell-type-specific gene expression. In this review, we summarize recent studies analyzing contribution of integrins to the control of the mammary morphogenesis and differentiation, function and maintenance of mammary stem and progenitor cells and resume the data from mouse models revealing the contribution of the integrin-mediated signaling to mammary tumorigenesis.

Conditional deletion of Stat3 in mammary epithelium impairs the acute phase response and modulates immune cell numbers during post-lactational regression

Mammary gland regression following weaning (involution) is associated with extensive cell death and the acquisition of an inflammatory signature. Characterizing the interplay between mammary epithelial cells, the re-emerging stroma and immune cells has implications for the understanding of the pathogenesis of pregnancy-associated breast cancer. Stat3 has a role in orchestrating cell death and involution, and we sought to determine whether expression of Stat3 by the mammary epithelium also influences the innate immune environment and inflammatory cell influx in the gland. We examined mice in which Stat3 is conditionally deleted only in the mammary epithelium. Distinct sets of genes associated with the acute phase response and innate immunity are markedly up-regulated during first phase involution in a Stat3-dependent manner. During second phase involution, chitinase 3-like 1, which has been associated with wound healing and chronic inflammatory conditions, is dramatically up-regulated by Stat3. Also at this time, the number of mammary macrophages and mast cells increases per unit area, and this increase is impaired in the absence of epithelial Stat3. Furthermore, expression of arginase-1 and Ym1, markers of alternatively activated macrophages, is significantly decreased in the absence of Stat3, whilst iNOS, a marker associated with classically activated macrophages, shows significantly increased expression in the Stat3-deleted glands. Thus, Stat3 is a key transcriptional regulator of genes associated with innate immunity and wound healing and influences mammary macrophage and mast cell numbers. The presence of epithelial Stat3 appears to polarize the macrophages and epithelial cells towards an alternatively activated phenotype, since in the absence of Stat3, the gland retains a phenotype associated with classically activated macrophages. These findings have relevance to the study of pregnancy-associated breast cancer and the role of Stat3 signalling in recruitment of alternatively activated tumour-associated macrophages in breast cancer. Copyright © 2012 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Role of the promyelocytic leukaemia protein in cell death regulation

The promyelocytic leukaemia gene PML was originally identified at the t(15;17) translocation of acute promyelocytic leukaemia, which generates the oncogene PML-retinoic acid receptor α. PML epitomises a subnuclear structure called PML nuclear body. Current models propose that PML through its scaffold properties is able to control cell growth and survival at many different levels. Here we discuss the current literature and propose new avenues for investigation.

TRIM proteins and cancer

Emerging clinical evidence shows that the deregulation of ubiquitin-mediated degradation of oncogene products or tumour suppressors is likely to be involved in the aetiology of carcinomas and leukaemias. Recent studies have indicated that some members of the tripartite motif (TRIM) proteins (one of the subfamilies of the RING type E3 ubiquitin ligases) function as important regulators for carcinogenesis. This Review focuses on TRIM proteins that are involved in tumour development and progression.

Molecular regulators of pubertal mammary gland development

The pubertal mammary gland is an ideal model for experimental morphogenesis. The primary glandular branching morphogenesis occurs at this time, integrating epithelial cell proliferation, differentiation, and apoptosis. Between birth and puberty, the mammary gland exists in a relatively quiescent state. At the onset of puberty, rapid expansion of a pre-existing rudimentary mammary epithelium generates an extensive ductal network by a process of branch initiation, elongation, and invasion of the mammary mesenchyme. It is this branching morphogenesis that characterizes pubertal mammary gland growth. Tissue-specific molecular networks interpret signals from local cytokines/growth factors in both the epithelial and stromal microenvironments. This is largely orchestrated by secreted ovarian and pituitary hormones. Here, we review the major molecular regulators of pubertal mammary gland development.

Regulation of E2Fs and senescence by PML nuclear bodies

The tumor suppressor PML (promyelocytic leukemia protein) regulates cellular senescence and terminal differentiation, two processes that implicate a permanent exit from the cell cycle. Here, we show that the mechanism by which PML induces a permanent cell cycle exit and activates p53 and senescence involves a recruitment of E2F transcription factors bound to their promoters and the retinoblastoma (Rb) proteins to PML nuclear bodies enriched in heterochromatin proteins and protein phosphatase 1α. Blocking the functions of the Rb protein family or adding back E2Fs to PML-expressing cells can rescue their defects in E2F-dependent gene expression and cell proliferation, inhibiting the senescent phenotype. In benign prostatic hyperplasia, a neoplastic disease that displays features of senescence, PML was found to be up-regulated and forming nuclear bodies. In contrast, PML bodies were rarely visualized in prostate cancers. The newly defined PML/Rb/E2F pathway may help to distinguish benign tumors from cancers, and suggest E2F target genes as potential targets to induce senescence in human tumors.

Role of promyelocytic leukemia protein in host antiviral defense

Several pathways have been implicated in the establishment of antiviral state in response to interferon (IFN), one of which implicates the promyelocytic leukemia (PML) protein. The PML gene has been discovered 20 years ago and has led to new insights into oncogenesis, apoptosis, cell senescence, and antiviral defense. PML is induced by IFN, leading to a marked increase of expression of PML isoforms and the number of PML nuclear bodies (NBs). PML is the organizer of the NBs that contains at least 2 permanent NB-associated proteins, the IFN-stimulated gene product Speckled protein of 100 kDa (Sp100) and death-associated dead protein (Daxx), as well as numerous other transient proteins recruited in these structures in response to different stimuli. Accumulating reports have implicated PML in host antiviral defense and revealed various strategies developed by viruses to disrupt PML NBs. This review will focus on the regulation of PML and the implication of PML NBs in conferring resistance to DNA and RNA viruses. The role of PML in mediating an IFN-induced antiviral state will also be discussed.

Estrogen and progesterone in normal mammary gland development and in cancer

There is emerging evidence that the mammary epithelium in both mice and humans is arranged as a hierarchy that spans from stem cells to differentiated hormone-sensing, milk-producing and myoepithelial cells. It is well established that estrogen is an important mediator of mammary gland morphogenesis and exposure to this hormone is associated with increased breast cancer risk. Yet surprisingly, the primitive cells of the mammary epithelium do not express the estrogen receptor-α (ERα) or the progesterone receptor. This article will review the mammary epithelial cell hierarchy, possible cells of origin of different types of breast tumors, and the potential mechanisms on how estrogen and progesterone may influence the different subcomponents in normal development and in cancer. Also presented are some hypothetical scenarios on how this underlying biology may be reflected in the behavior of ERα(+) and ERα(-) breast tumors.

The role of PML in the nervous system

The promyeloctic leukemia protein PML is a tumor suppressor that was originally identified due to its involvement in the (15;17) translocation of acute promyelocytic leukemia. While the majority of early research has focused upon the role of PML in the pathogenesis of leukemia, more recent evidence has identified important roles for PML in tissues outside the hemopoietic system, including the central nervous system (CNS). Here, we review recent literature on the role of PML in the CNS, with particular focus on the processes of neurodevelopment and neurodegeneration, and propose new lines of investigation.

Acute promyelocytic leukaemia: novel insights into the mechanisms of cure

The fusion oncogene, promyelocytic leukaemia (PML)-retinoic acid receptor-α (RARA), initiates acute promyelocytic leukaemia (APL) through both a block to differentiation and increased self-renewal of leukaemic progenitor cells. The current standard of care is retinoic acid (RA) and chemotherapy, but arsenic trioxide also cures many patients with APL, and an RA plus arsenic trioxide combination cures most patients. This Review discusses the recent evidence that reveals surprising new insights into how RA and arsenic trioxide cure this leukaemia, by targeting PML-RARα for degradation. Drug-triggered oncoprotein degradation may be a strategy that is applicable to many cancers.

CCAAT/enhancer binding protein beta regulates stem cell activity and specifies luminal cell fate in the mammary gland

The bZIP transcription factor C/EBP beta is important for mammary gland development and its expression is deregulated in human breast cancer. To determine whether C/EBP beta regulates mammary stem cells (MaSCs), we employed two different knockout strategies. Using both a germline and a conditional knockout strategy, we demonstrate that mammosphere formation was significantly decreased in C/EBP beta-deficient mammary epithelial cells (MECs). Functional limiting dilution transplantation assays indicated that the repopulating ability of C/EBP beta-deleted MECs was severely impaired. Serial transplantation experiments demonstrated that C/EBP beta deletion resulted in decreased outgrowth potential and premature MaSC senescence. In accord, fluorescence-activated cell sorting analysis demonstrated that C/EBP beta-null MECs contained fewer MaSCs, the loss of luminal progenitors and an increase in differentiated luminal cells as compared with wild-type. Gene profiling of C/EBP beta-null stem cells revealed an alteration in cell fate specification, exemplified by the expression of basal markers in the luminal compartment. Thus, C/EBP beta is a critical regulator of both MaSC repopulation activity and luminal cell lineage commitment. These findings have critical implications for understanding both stem cell biology and the etiology of different breast cancer subtypes.

PML nuclear bodies

PML nuclear bodies are matrix-associated domains that recruit an astonishing variety of seemingly unrelated proteins. Since their discovery in the early 1960s, PML bodies have fascinated cell biologists because of their beauty and their tight association with cellular disorders. The identification of PML, a gene involved in an oncogenic chromosomal translocation, as the key organizer of these domains drew instant interest onto them. The multiple levels of PML body regulation by a specific posttranslational modification, sumoylation, have raised several unsolved issues. Functionally, PML bodies may sequester, modify or degrade partner proteins, but in many ways, PML bodies still constitute an enigma.

The epigenetic landscape of mammary gland development and functional differentiation

Most of the development and functional differentiation in the mammary gland occur after birth. Epigenetics is defined as the stable alterations in gene expression potential that arise during development and proliferation. Epigenetic changes are mediated at the biochemical level by the chromatin conformation initiated by DNA methylation, histone variants, post-translational modifications of histones, non-histone chromatin proteins, and non-coding RNAs. Epigenetics plays a key role in development. However, very little is known about its role in the developing mammary gland or how it might integrate the many signalling pathways involved in mammary gland development and function that have been discovered during the past few decades. An inverse relationship between marks of closed (DNA methylation) or open chromatin (DnaseI hypersensitivity, certain histone modifications) and milk protein gene expression has been documented. Recent studies have shown that during development and functional differentiation, both global and local chromatin changes occur. Locally, chromatin at distal regulatory elements and promoters of milk protein genes gains a more open conformation. Furthermore, changes occur both in looping between regulatory elements and attachment to nuclear matrix. These changes are induced by developmental signals and environmental conditions. Additionally, distinct epigenetic patterns have been identified in mammary gland stem and progenitor cell sub-populations. Together, these findings suggest that epigenetics plays a role in mammary development and function. With the new tools for epigenomics developed in recent years, we now can begin to establish a framework for the role of epigenetics in mammary gland development and disease.

Keeping abreast of the mammary epithelial hierarchy and breast tumorigenesis

The epithelium of the mammary gland exists in a highly dynamic state, undergoing dramatic morphogenetic changes during puberty, pregnancy, lactation, and regression. The recent identification of stem and progenitor populations in mouse and human mammary tissue has provided evidence that the mammary epithelium is organized in a hierarchical manner. Characterization of these normal epithelial subtypes is an important step toward understanding which cells are predisposed to oncogenesis. This review summarizes progress in the field toward defining constituent cells and key molecular regulators of the mammary epithelial hierarchy. Potential relationships between normal epithelial populations and breast tumor subtypes are discussed, with implications for understanding the cellular etiology underpinning breast tumor heterogeneity.

Targeting promyelocytic leukemia protein: a means to regulating PML nuclear bodies

The promyelocytic leukemia protein (PML) is involved in many cellular processes including cell cycle progression, DNA damage response, transcriptional regulation, viral infection, and apoptosis. These cellular activities often rely on the localization of PML to unique subnuclear structures known as PML nuclear bodies (NBs). More than 50 cellular proteins are known to traffic in and out of PML NBs, either transiently or constitutively. In order to understand the dynamics of these NBs, it is important to delineate the regulation of PML itself. PML is subject to extensive regulation at transcriptional, post-transcriptional, and post-translational levels. Many of these modes of regulation depend on the cellular context and the presence of extracellular signals. This review focuses on the current knowledge of regulation of PML under normal cellular conditions as well as the role for regulation of PML in viral infection and cancer.