Notch tumor suppressor function

Novel function of keratins 5 and 14 in proliferation and differentiation of stratified epithelial cells

Keratin expression in stratified epithelia is tightly regulated during squamous cell differentiation. Keratins 5 and 14 are expressed in mitotically active basal layer cells, but their function is not well defined. Reported here is the possible role of K14 in regulation of cell proliferation/differentiation in stratified epithelial cells.

Keratins are cytoplasmic intermediate filament proteins preferentially expressed by epithelial tissues in a site-specific and differentiation-dependent manner. The complex network of keratin filaments in stratified epithelia is tightly regulated during squamous cell differentiation. Keratin 14 (K14) is expressed in mitotically active basal layer cells, along with its partner keratin 5 (K5), and their expression is down-regulated as cells differentiate. Apart from the cytoprotective functions of K14, very little is known about K14 regulatory functions, since the K14 knockout mice show postnatal lethality. In this study, K14 expression was inhibited using RNA interference in cell lines derived from stratified epithelia to study the K14 functions in epithelial homeostasis. The K14 knockdown clones demonstrated substantial decreases in the levels of the K14 partner K5. These cells showed reduction in cell proliferation and delay in cell cycle progression, along with decreased phosphorylated Akt levels. K14 knockdown cells also exhibited enhanced levels of activated Notch1, involucrin, and K1. In addition, K14 knockdown AW13516 cells showed significant reduction in tumorigenicity. Our results suggest that K5 and K14 may have a role in maintenance of cell proliferation potential in the basal layer of stratified epithelia, modulating phosphatidylinositol 3-kinase/Akt–mediated cell proliferation and/or Notch1-dependent cell differentiation.

Loss of transcription factor KLF5 in the context of p53 ablation drives invasive progression of human squamous cell cancer

Squamous cell cancers account for more than half of all human cancers, and esophageal cancer is the sixth leading cause of cancer death worldwide. The majority of esophageal squamous cell carcinomas have identifiable p53 mutations, yet the same p53 mutations are found at comparable frequencies in precancerous dysplasia, indicating that transformation requires additional somatic changes yet to be defined. Here, we show that the zinc finger transcription factor Krüppel-like factor 5 (KLF5) transactivates NOTCH1 in the context of p53 mutation or loss. KLF5 loss limited NOTCH1 activity and was sufficient on its own to transform primary human keratinocytes harboring mutant p53, leading to the formation of invasive tumors. Restoration of NOTCH1 blocked transformation of KLF5-deficient and p53-mutant keratinocytes. Although human dysplastic epithelia accumulated KLF5, KLF5 expression was lost concurrently with NOTCH1 in squamous cell cancers. Taken together, these results define KLF5 loss as a critical event in squamous cell transformation and invasion. Our findings suggest that KLF5 may be a useful diagnostic and therapeutic target in esophageal squamous carcinomas and possibly more generally in other cancers associated with p53 loss of function.

Notch signalling is off and is uncoupled from HES1 expression in Ewing's sarcoma

Notch can act as an oncogene or as a tumour suppressor and thus can either promote or inhibit tumour cell growth. To establish Notch status in Ewing's sarcoma family of tumours (ESFT), we investigated the Notch pathway by gene expression profiling meta-analysis or immunohistochemistry in samples obtained from 96 and 24 ESFT patients, respectively. We found that although Notch receptors were highly expressed, Notch did not appear to be active, as evidenced by the absence of Notch receptors in cell nuclei. In contrast, we show that Notch receptors known to be active in colon adenocarcinoma, hepatocarcinoma, and pancreatic carcinoma stain cell nuclei in these tumours. High expression of the Notch effector HES1 transcription factor, usually used as a surrogate marker for active Notch, was also restricted to outside of the nucleus in the majority of ESFT, and analysis of HES1 gene targets indicated HES1 to be transcriptionally inactive. Neither forced activation nor pharmacological or genetic blocking of Notch affected HES1 expression in ESFT cells, indicating HES1 expression to be uncoupled from the Notch pathway. Additional functional studies in ESFT cell lines confirmed Notch to be switched off. Finally, unlike experiments in which HES1 expression was modulated, experimental activation of Notch in ESFT cell lines via several means blocked cell proliferation and reduced their clonogenic potential in soft agar. These indicate that HES1 is uncoupled from Notch in ESFT, that EWS-FLI1-mediated inhibition of Notch contributes to ESFT aggressive cell growth, and support a role for Notch in ESFT tumour suppression, at least partly through the Notch effector HEY1.

A novel oncogenic role for the miRNA-506-514 cluster in initiating melanocyte transformation and promoting melanoma growth

Malignant melanoma is the most aggressive form of skin cancer and its incidence has doubled in the last two decades. It represents only 4% of skin cancer cases per year, but causes as many as 74% of skin cancer deaths. Early detection of malignant melanoma is associated with survival rates of up to 90%, but later detection (stage III to stage IV) is associated with survival rates of only 10%. Dysregulation of microRNA (miRNA) expression has been linked to tumor development and progression by functioning either as a tumor suppressor, an oncogene or a metastasis regulator in multiple cancer types. To understand the role of miRNA in the pathogenesis of malignant melanoma and identify biomarkers of metastasis, miRNA expression profiles in skin punches from 33 metastatic melanoma patients and 14 normal healthy donors were compared. We identified a cluster of 14 miRNAs on the X chromosome, termed the miR-506-514 cluster, which was consistently overexpressed in nearly all melanomas tested (30-60 fold, P<0.001), regardless of mutations in N-ras or B-raf. Inhibition of the expression of this cluster as a whole, or one of its sub-clusters (Sub-cluster A) consisting of six mature miRNAs, led to significant inhibition of cell growth, induction of apoptosis, decreased invasiveness and decreased colony formation in soft agar across multiple melanoma cell lines. Sub-cluster A of the miR-506-514 cluster was critical for maintaining the cancer phenotype, but the overexpression of the full cluster was necessary for melanocyte transformation. Our results provide new insights into the functional role of this miRNA cluster in melanoma, and suggest new approaches to treat or diagnose this disease.

Notch signaling and Notch signaling modifiers

Originally discovered nearly a century ago, the Notch signaling pathway is critical for virtually all developmental programs and modulates an astounding variety of pathogenic processes. The DSL (Delta, Serrate, LAG-2 family) proteins have long been considered canonical activators of the core Notch pathway. More recently, a wide and expanding network of non-canonical extracellular factors has also been shown to modulate Notch signaling, conferring newly appreciated complexity to this evolutionarily conserved signal transduction system. Here, I review current concepts in Notch signaling, with a focus on work from the last decade elucidating novel extracellular proteins that up- or down-regulate signal potency.

IKK1 control of epidermal differentiation is modulated by notch signaling

The molecular mechanism by which IκB kinase 1 (IKK1) regulates epidermal differentiation and tumor suppression in the skin is not well understood. As two major regulatory signaling pathways that regulate epidermal homeostasis and differentiation, the p63 and Notch pathways were examined in Ikk1 mutant epidermis and keratinocytes. Ikk1 inactivation in keratinocytes resulted in increased p63 expression and repression of Notch signaling. The impaired differentiation of Ikk1(-/-) keratinocytes was partially rescued by overexpression of the active form of the Notch1 receptor, the Notch intracellular domain (NICD). In contrast, knockdown of p63 expression by RNA interference was unable to rescue the defect. These results suggest that, in the mammalian skin, IKK1 functions as a differentiation regulator and tumor suppressor through the Notch signaling pathway.

Hepatitis C virus NS3 protein can activate the Notch-signaling pathway through binding to a transcription factor, SRCAP

Persistent infections of hepatitis C virus (HCV) are known to be a major risk factor for causing hepatocellular carcinomas. Nonstructural protein 3 (NS3) of HCV has serine protease and RNA helicase domains, and is essential for the viral replication. Further, NS3 is also considered to be involved in the development of HCV-induced hepatocellular carcinomas. In this report, we focus on the function of NS3 protein, and propose a novel possible molecular mechanism which is thought to be related to the tumorigenesis caused by the persistent infection of HCV. We identified SRCAP (Snf2-related CBP activator protein) as a NS3 binding protein using yeast two-hybrid screening, and a co-immunoprecipitation assay demonstrated that NS3 can bind to SRCAP in mammalian cells. The results of a reporter gene assay using Hes-1 promoter which is known to be a target gene activated by Notch, indicate that NS3 and SRCAP cooperatively activate the Hes-1 promoter in Hep3B cells. In addition, we show in this report that also p400, which is known as a protein closely resembling SRCAP, would be targeted by NS3. NS3 exhibited binding activity also to the 1449–1808 region of p400 by a co-immunoprecipitation assay, and further the activation of the Notch-mediated transcription of Hes-1 promoter by NS3 decreased significantly by the combined silencing of SRCAP and p400 mRNA using short hairpin RNA. These results suggest that the HCV NS3 protein is involved in the activation of the Notch-signaling pathway through the targeting to both SRCAP and p400.

A SIRT1-LSD1 corepressor complex regulates Notch target gene expression and development

Epigenetic regulation of gene expression by histone-modifying corepressor complexes is central to normal animal development. The NAD(+)-dependent deacetylase and gene repressor SIRT1 removes histone H4K16 acetylation marks and facilitates heterochromatin formation. However, the mechanistic contribution of SIRT1 to epigenetic regulation at euchromatic loci and whether it acts in concert with other chromatin-modifying activities to control developmental gene expression programs remain unclear. We describe here a SIRT1 corepressor complex containing the histone H3K4 demethylase LSD1/KDM1A and several other LSD1-associated proteins. SIRT1 and LSD1 interact directly and play conserved and concerted roles in H4K16 deacetylation and H3K4 demethylation to repress genes regulated by the Notch signaling pathway. Mutations in Drosophila SIRT1 and LSD1 orthologs result in similar developmental phenotypes and genetically interact with the Notch pathway in Drosophila. These findings offer new insights into conserved mechanisms of epigenetic gene repression and regulation of development by SIRT1 in metazoans.

CD248 facilitates tumor growth via its cytoplasmic domain

Background

Stromal fibroblasts participate in the development of a permissive environment for tumor growth, yet molecular pathways to therapeutically target fibroblasts are poorly defined. CD248, also known as endosialin or tumor endothelial marker 1 (TEM1), is a transmembrane glycoprotein expressed on activated fibroblasts. We recently showed that the cytoplasmic domain of CD248 is important in facilitating an inflammatory response in a mouse model of arthritis. Others have reported that CD248 gene inactivation in mice results in dampened tumor growth. We hypothesized that the conserved cytoplasmic domain of CD248 is important in regulating tumor growth.

Methods

Mice lacking the cytoplasmic domain of CD248 (CD248^CyD/CyD) were generated and evaluated in tumor models, comparing the findings with wild-type mice (CD248^WT/WT).

Results

As compared to the response in CD248^WT/WT mice, growth of T241 fibrosarcomas and Lewis lung carcinomas was significantly reduced in CD248^CyD/CyD mice. Tumor size was similar to that seen with CD248-deficient mice. Conditioned media from CD248^CyD/CyD fibroblasts were less effective at supporting T241 fibrosarcoma cell survival. In addition to our previous observation of reduced release of activated matrix metalloproteinase (MMP)-9, CD248^CyD/CyD fibroblasts also had impaired PDGF-BB-induced migration and expressed higher transcripts of tumor suppressor factors, transgelin (SM22α), Hes and Hey1.

Conclusions

The multiple pathways regulated by the cytoplasmic domain of CD248 highlight its potential as a therapeutic target to treat cancer.

ETOILE regulates developmental patterning in the filamentous brown alga Ectocarpus siliculosus

Brown algae are multicellular marine organisms evolutionarily distant from both metazoans and land plants. The molecular or cellular mechanisms that govern the developmental patterning in brown algae are poorly characterized. Here, we report the first morphogenetic mutant, étoile (etl), produced in the brown algal model Ectocarpus siliculosus. Genetic, cellular, and morphometric analyses showed that a single recessive locus, ETL, regulates cell differentiation: etl cells display thickening of the extracellular matrix (ECM), and the elongated, apical, and actively dividing E cells are underrepresented. As a result of this defect, the overrepresentation of round, branch-initiating R cells in the etl mutant leads to the rapid induction of the branching process at the expense of the uniaxial growth in the primary filament. Computational modeling allowed the simulation of the etl mutant phenotype by including a modified response to the neighborhood information in the division rules used to specify wild-type development. Microarray experiments supported the hypothesis of a defect in cell-cell communication, as primarily Lin-Notch-domain transmembrane proteins, which share similarities with metazoan Notch proteins involved in binary cell differentiation were repressed in etl. Thus, our study highlights the role of the ECM and of novel transmembrane proteins in cell-cell communication during the establishment of the developmental pattern in this brown alga.

DDR1 receptor tyrosine kinase promotes prosurvival pathway through Notch1 activation

DDR1 (discoidin domain receptor tyrosine kinase 1) kinase s highly expressed in a variety of human cancers and occasionally mutated in lung cancer and leukemia. It is now clear that aberrant signaling through the DDR1 receptor is closely associated with various steps of tumorigenesis, although little is known about the molecular mechanism(s) underlying the role of DDR1 in cancer. Besides the role of DDR1 in tumorigenesis, we previously identified DDR1 kinase as a transcriptional target of tumor suppressor p53. DDR1 is functionally activated as determined by its tyrosine phosphorylation, in response to p53-dependent DNA damage. In this study, we report the characterization of the Notch1 protein as an interacting partner of DDR1 receptor, as determined by tandem affinity protein purification. Upon ligand-mediated DDR1 kinase activation, Notch1 was activated, bound to DDR1, and activated canonical Notch1 targets, including Hes1 and Hey2. Moreover, DDR1 ligand (collagen I) treatment significantly increased the active form of Notch1 receptor in the nuclear fraction, whereas DDR1 knockdown cells show little or no increase of the active form of Notch1 in the nuclear fraction, suggesting a novel intracellular mechanism underlying autocrine activation of wild-type Notch signaling through DDR1. DDR1 activation suppressed genotoxic-mediated cell death, whereas Notch1 inhibition by a γ-secretase inhibitor, DAPT, enhanced cell death in response to stress. Moreover, the DDR1 knockdown cancer cells showed the reduced transformed phenotypes in vitro and in vivo xenograft studies. The results suggest that DDR1 exerts prosurvival effect, at least in part, through the functional interaction with Notch1.

Notch signaling proteins: legitimate targets for cancer therapy

Proteins and small peptides (growth factors and hormones) are key molecules in maintaining cellular homeostasis. To that end, Notch signaling pathway proteins are known to play critical roles in maintaining the balance between cell proliferation, differentiation and apoptosis, and thus it has been suggested that Notch may be responsible for the development and progression of human malignancies. Therefore, the Notch signaling pathway proteins may present novel therapeutic targets, which could have promising therapeutic impact on eradicating human malignancies. This review describes the role of Notch signaling pathway proteins in cancer and how its deregulation is involved in tumor development and progression leading to metastasis and the ultimate demise of patients diagnosed with cancer. Further, we summarize the role of several Notch inhibitors especially "natural agents" that could represent novel therapeutic strategies targeting Notch signaling toward better treatment outcome of patients diagnosed with cancer.

Uncovering the role of hypoxia inducible factor-1α in skin carcinogenesis

The hypoxia inducible factor-1α (HIF-1α) is a pleiotropic transcription factor typically activated in response to low oxygen tension as well as other stress factors in normoxic conditions. Upon activation HIF-1α mediates the transcriptional activation of target genes involved in a variety of processes comprising stress adaptation, metabolism, growth and invasion, but also apoptotic cell death. The molecular mechanisms, signaling pathways and downstream targets evoked by the activation of HIF-1α in epidermal cells are becoming increasingly understood and underscore the participation of HIF-1α in crucial processes including malignant transformation and cancer progression. Recent studies have implicated HIF-1α as an integral part of the multifaceted signal transduction initiated by the exposure of keratinocytes to ultraviolet radiation B (UVB), which represents the most ubiquitous hazard for human skin and the principal risk factor for skin cancer. HIF-1α activation by UVB exposure contributes to either repair or the removal of UVB-damaged keratinocytes by inducing apoptosis, thus revealing a tumor suppressor role for HIF-1α in these cells. On the other hand, the constitutive expression of HIF-1α evoked by the mild hypoxic state of the skin has been implicated as a positive factor in the transformation of normal melanocytes into malignant melanoma, one of the most aggressive types of human cancers. Here we review the uncovered and complex role of HIF-1α in skin carcinogenesis.

NOTCH1 and NOTCH3 coordinate esophageal squamous differentiation through a CSL-dependent transcriptional network

BACKGROUND & AIMS

The Notch receptor family regulates cell fate through cell-cell communication. CSL (CBF-1/RBP-jκ, Su(H), Lag-1) drives canonical Notch-mediated gene transcription during cell lineage specification, differentiation, and proliferation in the hematopoietic system, the intestine, the pancreas, and the skin. However, the functional roles of Notch in esophageal squamous epithelial biology are unknown.

METHODS

Normal esophageal keratinocytes were stimulated with calcium chloride to induce terminal differentiation. The squamous epithelia were reconstituted in organotypic 3-dimensional culture, a form of human tissue engineering. Notch was inhibited in culture with a γ-secretase inhibitor or dominant negative mastermind-like 1 (DNMAML1). The roles of Notch receptors were evaluated by in vitro gain-of-function and loss-of-function experiments. Additionally, DNMAML1 was targeted to the mouse esophagus by cytokeratin K14 promoter-driven Cre (K14Cre) recombination of Lox-STOP-Lox-DNMAML1. Notch-regulated gene expression was determined by reporter transfection, chromatin immunoprecipitation assays, quantitative reverse-transcription polymerase chain reaction, Western blotting, immunofluorescence, and immunohistochemistry.

RESULTS

NOTCH1 (N1) was activated at the onset of squamous differentiation in the esophagus. Intracellular domain of N1 (ICN1) directly activated NOTCH3 (N3) transcription, inducing HES5 and early differentiation markers such as involucrin (IVL) and cytokeratin CK13 in a CSL-dependent fashion. N3 enhanced ICN1 activity and was required for squamous differentiation. Loss of Notch signaling in K14Cre;DNMAML1 mice perturbed esophageal squamous differentiation and resulted in N3 loss and basal cell hyperplasia.

CONCLUSIONS

Notch signaling is important for esophageal epithelial homeostasis. In particular, the cross talk of N3 with N1 during differentiation provides novel, mechanistic insights into Notch signaling and squamous epithelial biology.

Transcription factor E4F1 is essential for epidermal stem cell maintenance and skin homeostasis

A growing body of evidence suggests that the multifunctional protein E4F1 is involved in signaling pathways that play essential roles during normal development and tumorigenesis. We generated E4F1 conditional knockout mice to address E4F1 functions in vivo in newborn and adult skin. E4F1 inactivation in the entire skin or in the basal compartment of the epidermis induces skin homeostasis defects, as evidenced by transient hyperplasia in the interfollicular epithelium and alteration of keratinocyte differentiation, followed by loss of cellularity in the epidermis and severe skin ulcerations. E4F1 depletion alters clonogenic activity of epidermal stem cells (ESCs) ex vivo and ends in exhaustion of the ESC pool in vivo, indicating that the lesions observed in the E4F1 mutant skin result, at least in part, from cell-autonomous alterations in ESC maintenance. The clonogenic potential of E4F1 KO ESCs is rescued by Bmi1 overexpression or by Ink4a/Arf or p53 depletion. Skin phenotype of E4F1 KO mice is also delayed in animals with Ink4a/Arf and E4F1 compound gene deficiencies. Our data identify a regulatory axis essential for ESC-dependent skin homeostasis implicating E4F1 and the Bmi1-Arf-p53 pathway.

Identification of epidermal Pdx1 expression discloses different roles of Notch1 and Notch2 in murine Kras(G12D)-induced skin carcinogenesis in vivo

BACKGROUND

The Ras and Notch signaling pathways are frequently activated during development to control many diverse cellular processes and are often dysregulated during tumorigenesis. To study the role of Notch and oncogenic Kras signaling in a progenitor cell population, Pdx1-Cre mice were utilized to generate conditional oncogenic Kras(G12D) mice with ablation of Notch1 and/or Notch2.

METHODOLOGY/PRINCIPAL FINDINGS

Surprisingly, mice with activated Kras(G12D) and Notch1 but not Notch2 ablation developed skin papillomas progressing to squamous cell carcinoma providing evidence for Pdx1 expression in the skin. Immunostaining and lineage tracing experiments indicate that PDX1 is present predominantly in the suprabasal layers of the epidermis and rarely in the basal layer. Further analysis of keratinocytes in vitro revealed differentiation-dependent expression of PDX1 in terminally differentiated keratinocytes. PDX1 expression was also increased during wound healing. Further analysis revealed that loss of Notch1 but not Notch2 is critical for skin tumor development. Reasons for this include distinct Notch expression with Notch1 in all layers and Notch2 in the suprabasal layer as well as distinctive p21 and β-catenin signaling inhibition capabilities.

CONCLUSIONS/SIGNIFICANCE

Our results provide strong evidence for epidermal expression of Pdx1 as of yet not identified function. In addition, this finding may be relevant for research using Pdx1-Cre transgenic strains. Additionally, our study confirms distinctive expression and functions of Notch1 and Notch2 in the skin supporting the importance of careful dissection of the contribution of individual Notch receptors.

The role of Notch signaling pathway in epithelial-mesenchymal transition (EMT) during development and tumor aggressiveness

The Notch signaling pathway maintains a balance between cell proliferation and apoptosis, and thus it is believed that Notch signaling pathways may play an important role in the development and progression of several malignancies. However, the functions of Notch signaling in EMT are largely unknown. This mini review describes the role of Notch signaling pathway in EMT, and cataloging how its deregulation is involved in EMT and tumor aggressiveness. Further attempts have been made to summarize the role of several chemopreventive agents that could be useful for targeted inactivation of Notch signaling, and thus it may cause reversal of EMT, which could become a novel approach for cancer prevention and treatment.

Control of hair follicle cell fate by underlying mesenchyme through a CSL-Wnt5a-FoxN1 regulatory axis

Epithelial-mesenchymal interactions are key to skin morphogenesis and homeostasis. We report that maintenance of the hair follicle keratinocyte cell fate is defective in mice with mesenchymal deletion of the CSL/RBP-Jkappa gene, the effector of "canonical" Notch signaling. Hair follicle reconstitution assays demonstrate that this can be attributed to an intrinsic defect of dermal papilla cells. Similar consequences on hair follicle differentiation result from deletion of Wnt5a, a specific dermal papilla signature gene that we found to be under direct Notch/CSL control in these cells. Functional rescue experiments establish Wnt5a as an essential downstream mediator of Notch-CSL signaling, impinging on expression in the keratinocyte compartment of FoxN1, a gene with a key hair follicle regulatory function. Thus, Notch/CSL signaling plays a unique function in control of hair follicle differentiation by the underlying mesenchyme, with Wnt5a signaling and FoxN1 as mediators.

sel-11 and cdc-42, two negative modulators of LIN-12/Notch activity in C. elegans

Background

LIN-12/Notch signaling is important for cell-cell interactions during development, and mutations resulting in constitutive LIN-12/Notch signaling can cause cancer. Loss of negative regulators of lin-12/Notch activity has the potential for influencing cell fate decisions during development and the genesis or aggressiveness of cancer.

Methodology/Principal Findings

We describe two negative modulators of lin-12 activity in C. elegans. One gene, sel-11, was initially defined as a suppressor of a lin-12 hypomorphic allele; the other gene, cdc-42, is a well-studied Rho GTPase. Here, we show that SEL-11 corresponds to yeast Hrd1p and mammalian Synoviolin. We also show that cdc-42 has the genetic properties consistent with negative regulation of lin-12 activity during vulval precursor cell fate specification.

Conclusions/Significance

Our results underscore the multiplicity of negative regulatory mechanisms that impact on lin-12/Notch activity and suggest novel mechanisms by which constitutive lin-12/Notch activity might be exacerbated in cancer.

Targeting Notch signaling pathway to overcome drug resistance for cancer therapy

Chemotherapy is an important therapeutic strategy for cancer treatment and remains the mainstay for the management of human malignancies; however, chemotherapy fails to eliminate all tumor cells because of intrinsic or acquired drug resistance, which is the most common cause of tumor recurrence. Recently, emerging evidences suggest that Notch signaling pathway is one of the most important signaling pathways in drug-resistant tumor cells. Moreover, down-regulation of Notch pathway could induce drug sensitivity, leading to increased inhibition of cancer cell growth, invasion, and metastasis. This article will provide a brief overview of the published evidences in support of the roles of Notch in drug resistance and will further summarize how targeting Notch by "natural agents" could become a novel and safer approach for the improvement of tumor treatment by overcoming drug resistance.

Cell polarity in motion: redefining mammary tissue organization through EMT and cell polarity transitions

Epithelial to mesenchymal transition (EMT) and its reversion via mesenchymal to epithelial transition (MET), represent a stepwise cycle of epithelial plasticity that allows for normal tissue remodelling and diversification during development. In particular, epithelial-mesenchymal plasticity is central to many aspects of mammary development and has been proposed to be a key process in breast cancer progression. Such epithelial-mesenchymal plasticity requires complex cellular reprogramming to orchestrate a change in cell shape to an alternate morphology more conducive to migration. During this process, epithelial characteristics, including apical-basal polarity and specialised cell-cell junctions are lost and mesenchymal properties, such as a front-rear polarity associated with weak cell-cell contacts, increased motility, resistance to apoptosis and invasiveness are gained. The ability of epithelial cells to undergo transitions through cell polarity states is a central feature of epithelial-mesenchymal plasticity. These cell polarity states comprise a set of distinct asymmetric distributions of cellular constituents that are fashioned to allow specialized cellular functions, such as the regulated homeostasis of molecules across epithelial barriers, cell migration or cell diversification via asymmetric cell divisions. Each polarity state is engineered using a molecular toolbox that is highly conserved between organisms and cell types which can direct the initiation, establishment and continued maintenance of each asymmetry. Here we discuss how EMT pathways target cell polarity mediators, and how this EMT-dependent change in polarity states impact on the various stages of breast cancer. Emerging evidence places cell polarity at the interface of proliferation and morphology control and as such the changing dynamics within polarity networks play a critical role in normal mammary gland development and breast cancer progression.

Notch1 functions as a tumor suppressor in a model of K-ras-induced pancreatic ductal adenocarcinoma

K-ras is the most commonly mutated oncogene in pancreatic cancer and its activation in murine models is sufficient to recapitulate the spectrum of lesions seen in human pancreatic ductal adenocarcinoma (PDAC). Recent studies suggest that Notch receptor signaling becomes reactivated in a subset of PDACs, leading to the hypothesis that Notch1 functions as an oncogene in this setting. To determine whether Notch1 is required for K-ras-induced tumorigenesis, we used a mouse model in which an oncogenic allele of K-ras is activated and Notch1 is deleted simultaneously in the pancreas. Unexpectedly, the loss of Notch1 in this model resulted in increased tumor incidence and progression, implying that Notch1 can function as a tumor suppressor gene in PDAC.

DeltaNp63alpha repression of the Notch1 gene supports the proliferative capacity of normal human keratinocytes and cervical cancer cells

The p53 family member p63 is a master regulator of epithelial development. One of its isoforms, DeltaNp63alpha, is predominantly expressed in the basal cells of stratified epithelia and plays a fundamental role in control of regenerative potential and epithelial integrity. In contrast to p53, p63 is rarely mutated in human cancers, but it is frequently overexpressed in squamous cell carcinomas (SCC). However, its functional relevance to tumorigenesis remains largely unclear. We previously identified the Notch1 gene as a novel transcriptional target of p53. Here, we show that DeltaNp63alpha functions as a transcriptional repressor of the Notch1 gene through the p53-responsive element. Knockdown of p63 caused upregulation of Notch1 expression and marked reduction in proliferation and clonogenicity of both normal human keratinocytes and cervical cancer cell lines overexpressing DeltaNp63alpha. Concomitant silencing of Notch1 significantly rescued this phenotype, indicating the growth defect induced by p63 deficiency to be, at least in part, attributable to Notch1 function. Conversely, overexpression of DeltaNp63alpha decreased basal levels of Notch1, increased proliferative potential of normal human keratinocytes, and inhibited both p53-dependent and p53-independent induction of Notch1 and differentiation markers upon genotoxic stress and serum exposure, respectively. These results suggest that DeltaNp63alpha maintains the self-renewing capacity of normal human keratinocytes and cervical cancer cells partly through transcriptional repression of the Notch1 gene and imply a novel pathogenetical significance of frequently observed overexpression of DeltaNp63alpha together with p53 inactivation in SCCs.

Differential control of Notch1 gene transcription by Klf4 and Sp3 transcription factors in normal versus cancer-derived keratinocytes

In specific cell types like keratinocytes, Notch signaling plays an important pro-differentiation and tumor suppressing function, with down-modulation of the Notch1 gene being associated with cancer development. Besides being controlled by p53, little else is known on regulation of Notch1 gene expression in this context. We report here that transcription of this gene is driven by a TATA-less “sharp peak” promoter and that the minimal functional region of this promoter, which extends from the −342 bp position to the initiation codon, is differentially active in normal versus cancer cells. This GC rich region lacks p53 binding sites, but binds Klf4 and Sp3. This finding is likely to be of biological significance, as Klf4 and, to a lesser extent, Sp3 are up-regulated in a number of cancer cells where Notch1 expression is down-modulated, and Klf4 over-expression in normal cells is sufficient to down-modulate Notch1 gene transcription. The combined knock-down of Klf4 and Sp3 was necessary for the reverse effect of increasing Notch1 transcription, consistent with the two factors exerting an overlapping repressor function through their binding to the Notch1 promoter.

Notch promotes radioresistance of glioma stem cells

Radiotherapy represents the most effective nonsurgical treatments for gliomas. However, gliomas are highly radioresistant and recurrence is nearly universal. Results from our laboratory and other groups suggest that cancer stem cells contribute to radioresistance in gliomas and breast cancers. The Notch pathway is critically implicated in stem cell fate determination and cancer. In this study, we show that inhibition of Notch pathway with gamma-secretase inhibitors (GSIs) renders the glioma stem cells more sensitive to radiation at clinically relevant doses. GSIs enhance radiation-induced cell death and impair clonogenic survival of glioma stem cells but not non-stem glioma cells. Expression of the constitutively active intracellular domains of Notch1 or Notch2 protect glioma stem cells against radiation. Notch inhibition with GSIs does not alter the DNA damage response of glioma stem cells after radiation but rather reduces Akt activity and Mcl-1 levels. Finally, knockdown of Notch1 or Notch2 sensitizes glioma stem cells to radiation and impairs xenograft tumor formation. Taken together, our results suggest a critical role of Notch signaling to regulate radioresistance of glioma stem cells. Inhibition of Notch signaling holds promise to improve the efficiency of current radiotherapy in glioma treatment.

Notch signaling in the regulation of stem cell self-renewal and differentiation

Stem cells are rare and unique precursor cells that participate in the building and rebuilding of tissues and organs during embryogenesis, postnatal growth, and injury repair. Stem cells are distinctively endowed with the ability to both self-renew and differentiate, such that they can replenish the stem cell pool while continuing to produce the differentiated daughter cells that are essential for tissue function. Stem cell self-renewal/differentiation decisions must be carefully controlled during organogenesis, tissue homeostasis, and regeneration, as failure in stem cell maintenance or activation can lead to progressive tissue wasting, while unchecked self-renewal is a hallmark of many cancers. Here, we review evidence implicating the Notch signaling pathway, an evolutionarily conserved cell fate determinant with widespread roles in a variety of tissues and organisms, as a crucial regulator of stem cell behavior. As discussed below, this pathway plays varied and critical roles at multiple stages of organismal development, in lineage-specific differentiation of pluripotent embryonic stem cells, and in controlling stem cell numbers and activity in the context of age-related tissue degeneration, injury-induced tissue repair, and malignancy.

Stem cells in normal development and cancer

In this chapter we provide an overview of stem cells in normal tissues as well as in many different types of cancers. All tissues in the body are derived from organ-specific stem cells that retain the ability to self-renew and differentiate into specific cell types. The cancer stem cell hypothesis suggests that tumors arise from cell populations with dysregulated self-renewal. This may be tissue stem cells or more differentiated cells that acquire self-renewal capabilities. In addition, we outline some useful assays for purification and isolation of cancer stem cells including the dye exclusion side population assay, flow cytometry sorting techniques for identification of putative cancer stem cell markers, tumorspheres assay, aldehyde dehydrogenase activity assay, PKH, and other membrane staining used to label the cancer stem cells, as well as in vivo xenograft transplantation assays. We also examine some of the cell signaling pathways that regulate stem cell self-renewal including the Notch, Hedgehog, HER2/PI3K/Akt/PTEN, and p53 pathways. We also review information demonstrating the involvement of the microenvironment or stem cell niche and its effects on the growth and maintenance of cancer stem cells. Finally, we highlight the therapeutic implications of targeting stem cells by inhibiting these pathways for the treatment and prevention of cancer.

Mammalian target of rapamycin regulates murine and human cell differentiation through STAT3/p63/Jagged/Notch cascade

The receptor tyrosine kinase/PI3K/AKT/mammalian target of rapamycin (RTK/PI3K/AKT/mTOR) pathway is frequently altered in cancer, but the underlying mechanism leading to tumorigenesis by activated mTOR remains less clear. Here we show that mTOR is a positive regulator of Notch signaling in mouse and human cells, acting through induction of the STAT3/p63/Jagged signaling cascade. Furthermore, in response to differential cues from mTOR, we found that Notch served as a molecular switch to shift the balance between cell proliferation and differentiation. We determined that hyperactive mTOR signaling impaired cell differentiation of murine embryonic fibroblasts via potentiation of Notch signaling. Elevated mTOR signaling strongly correlated with enhanced Notch signaling in poorly differentiated but not in well-differentiated human breast cancers. Both human lung lymphangioleiomyomatosis (LAM) and mouse kidney tumors with hyperactive mTOR due to tumor suppressor TSC1 or TSC2 deficiency exhibited enhanced STAT3/p63/Notch signaling. Furthermore, tumorigenic potential of cells with uncontrolled mTOR signaling was suppressed by Notch inhibition. Our data therefore suggest that perturbation of cell differentiation by augmented Notch signaling might be responsible for the underdifferentiated phenotype displayed by certain tumors with an aberrantly activated RTK/PI3K/AKT/mTOR pathway. Additionally, the STAT3/p63/Notch axis may be a useful target for the treatment of cancers exhibiting hyperactive mTOR signaling.

Cross-talk between miRNA and Notch signaling pathways in tumor development and progression

Notch signaling pathways are known to regulate many cellular processes, including cell proliferation, apoptosis, migration, invasion, and angiogenesis, and is one of the most important signaling pathway during normal development. Recently, emerging evidences suggest that microRNAs (miRNAs) can function as key regulators of various biological and pathologic processes during tumor development and progression. Notch signaling has also been reported to be regulated through cross-talk with many pathways and factors where miRNAs appears to play a major role. This article will provide a brief overview of the published evidences for the cross-talks between Notch and miRNAs. Further, we summarize how targeting miRNAs by natural agents could become a novel and safer approach for the prevention of tumor progression and treatment.

LARG at chromosome 11q23 has functional characteristics of a tumor suppressor in human breast and colorectal cancer

Deletion of 11q23-q24 is frequent in a diverse variety of malignancies, including breast and colorectal carcinoma, implicating the presence of a tumor suppressor gene at that chromosomal region. We examined a 6-Mb region on 11q23 by high-resolution deletion mapping, using both loss of heterozygosity analysis and customized microarray comparative genomic hybridization. LARG (leukemia-associated Rho guanine-nucleotide exchange factor) (also called ARHGEF12), identified from the analysed region, is frequently underexpressed in breast and colorectal carcinomas with a reduced expression observed in all breast cancer cell lines (n=11), in 12 of 38 (32%) primary breast cancers, 5 of 10 (50%) colorectal cell lines and in 20 of 37 (54%) primary colorectal cancers. Underexpression of the LARG transcript was significantly associated with genomic loss (P=0.00334). Hypermethylation of the LARG promoter was not detected in either breast or colorectal cancer, and treatment of four breast and four colorectal cancer cell lines with 5-aza-2'-deoxycytidine and/or trichostatin A did not result in a reactivation of LARG. Enforced expression of LARG in breast and colorectal cancer cells by stable transfection resulted in reduced cell proliferation and colony formation, as well as in a markedly slower cell migration rate in colorectal cancer cells, providing functional evidence for LARG as a candidate tumor suppressor gene.

Inhibition of Notch3 enhances sensitivity to gemcitabine in pancreatic cancer through an inactivation of PI3K/Akt-dependent pathway

Notch3 is one of the four Notch receptors identified in mammal, but its role in human pancreatic cancer remains poorly characterized. In this study, we sought to determine the effect of suppressing Notch3 expression on the chemosensitivity to gemcitabine in human pancreatic cancer cell lines BxPC-3 and PANC-1. RNA interference was used to suppress Notch3 expression. Gemcitabine-induced cytotoxicity was determined by MTT. Cell apoptosis was measured by flow cytometry. Caspase 3 activity was assayed using a Caspase Fluorescent Assay Kit. The effect of Notch3-specific siRNA on PI3K/Akt activity was also quantified. Notch3-specific siRNA suppressed Notch3 expression, and furthermore increased gemcitabine-induced, caspase-mediated apoptosis. The suppression of Notch3 expression decreased the average IC(50) in BxPC-3 and PANC-1 cells treated with gemcitabine. PI3K/Akt activity was decreased by the suppression of Notch3 expression. Taken together, these data demonstrated that Notch3 is a potential therapeutic target for pancreatic cancer, and PI3K/Akt is a key signaling component by which activation of the Notch3 signal transduction pathway protects pancreatic cancer cells from chemotherapy-induced cell death.

Mind bomb 1 regulation of cFLIP interactions

Mind bomb 1 (Mib1) is a multidomain E3 ligase that directs ubiquitination of the Notch ligands Delta and Jagged to promote their endocytosis. Here we examine Notch-independent functions of Mib1 and find that its activities are linked to the initiation of the extrinsic cell death pathway. Expression of Mib1 induces a spontaneous, caspase-dependent cell death. Consistent with this, depletion of endogenous Mib1 decreases tumor-necrosis factor (TNF)-induced cell death. Mib1 was found to bind to cellular Fas-associated death domain (FADD)-like IL-1b converting enzyme (FLICE)-like inhibitory proteins (cFLIP-L and cFLIP-S), whereas only cFLIP-s can inhibit Mib1-induced cell death. The interaction between Mib1 and cFLIP decreases the association of caspase-8 with cFLIP, which activates caspase-8 and induces cell death. Collectively, these results suggest that in addition to a central role in Notch signaling, Mib1 has an important role in regulating the extrinsic cell death pathway.

Crosstalk of Notch with p53 and p63 in cancer growth control

Understanding the complexity of cancer depends on an elucidation of the underlying regulatory networks, at the cellular and intercellular levels and in their temporal dimension. This Opinion article focuses on the multilevel crosstalk between the Notch pathway and the p53 and p63 pathways. These two coordinated signalling modules are at the interface of external damaging signals and control of stem cell potential and differentiation. Positive or negative reciprocal regulation of the two pathways can vary with cell type and cancer stage. Therefore, selective or combined targeting of the two pathways could improve the efficacy and reduce the toxicity of cancer therapies.

Numb regulates cell-cell adhesion and polarity in response to tyrosine kinase signalling

Epithelial-mesenchymal transition (EMT), which can be caused by aberrant tyrosine kinase signalling, marks epithelial tumour progression and metastasis, yet the underlying molecular mechanism is not fully understood. Here, we report that Numb interacts with E-cadherin (E-cad) through its phosphotyrosine-binding domain (PTB) and thereby regulates the localization of E-cad to the lateral domain of epithelial cell–cell junction. Moreover, Numb engages the polarity complex Par3–aPKC–Par6 by binding to Par3 in polarized Madin-Darby canine kidney cells. Intriguingly, after Src activation or hepatocyte growth factor (HGF) treatment, Numb decouples from E-cad and Par3 and associates preferably with aPKC–Par6. Binding of Numb to aPKC is necessary for sequestering the latter in the cytosol during HGF-induced EMT. Knockdown of Numb by small hairpin RNA caused a basolateral-to-apicolateral translocation of E-cad and β-catenin accompanied by elevated actin polymerization, accumulation of Par3 and aPKC in the nucleus, an enhanced sensitivity to HGF-induced cell scattering, a decrease in cell–cell adhesion, and an increase in cell migration. Our work identifies Numb as an important regulator of epithelial polarity and cell–cell adhesion and a sensor of HGF signalling or Src activity during EMT.

The role of ADAMs in disease pathophysiology

The ADAMs are a family of multidomain transmembrane and secreted proteins involved in both proteolysis and cell adhesion. Altered expression of specific ADAMs is implicated in the pathophysiology of several diseases including rheumatoid arthritis, Alzheimer's disease, cardiac hypertrophy, asthma and cancer. Of these different diseases, it is in cancer where most research has been carried out. Multiple ADAMs, including ADAM-9, ADAM-10, ADAM-12, ADAM-15 and ADAM-17, have been shown to play a role in either cancer formation or progression. Consistent with these findings, increased expression of specific ADAMs in several cancer types was found to correlate with features of aggressive disease and poor prognosis. Currently, selective ADAM inhibitors against ADAM-10 and ADAM-17 are undergoing clinical trials for the treatment of cancer. Further work is required in order to establish a causative role for ADAMs in rheumatoid arthritis, Alzheimer's disease, cardiac hypertrophy and asthma.

Molecular mechanisms of cervical carcinogenesis by high-risk human papillomaviruses: novel functions of E6 and E7 oncoproteins

Over the last two decades, since the initial discovery of human papillomavirus (HPV) type 16 and 18 DNAs in cervical cancers by Dr. Harald zur Hausen (winner of the Nobel Prize in Physiology or Medicine, 2008), the HPVs have been well characterised as causative agents for cervical cancer. Viral DNA from a specific group of HPVs can be detected in at least 90% of all cervical cancers and two viral genes, E6 and E7, are invariably expressed in HPV-positive cervical cancer cells. Their gene products are known to inactivate the major tumour suppressors, p53 and retinoblastoma protein (pRB), respectively. In addition, one function of E6 is to activate telomerase, and E6 and E7 cooperate to effectively immortalise human primary epithelial cells. Though expression of E6 and E7 is itself not sufficient for cancer development, it seems to be either directly or indirectly involved in every stage of multi-step carcinogenesis. Epidemiological and biological studies suggest the potential efficacy of prophylactic vaccines to prevent genital HPV infection as an anti-cancer strategy. However, given the widespread nature of HPV infection and unresolved issues about the duration and type specificity of the currently available HPV vaccines, it is crucial that molecular details of the natural history of HPV infection as well as the biological activities of the viral oncoproteins be elucidated in order to provide the basis for development of new therapeutic strategies against HPV-associated malignancies. This review highlights novel functions of E6 and E7 as well as the molecular mechanisms of HPV-induced carcinogenesis.

Differential expression of Notch family members in astrocytomas and medulloblastomas

Notch signaling pathway plays an integral role in determining cell fates in development. Growing evidence demonstrates that Notch signaling pathway has versatile effects in tumorigenesis depending on the tumor type, grade and stage. Notch signaling pathway is deregulated in some brain tumors. To examine the differential expression of Notch family members (Notch1, 2, 3, 4) in human astrocytomas and medulloblastomas, and to evaluate their roles in the development of both tumor types. Immunohistochemical staining and Western blot analysis were used to detect Notch1, 2, 3, 4 expression in tissue microarray and freshly resected tissue samples of normal brain, astrocytomas and medulloblastomas. Notch family members were not expressed or barely detectable in normal brain tissues. Notch1, 3, 4 were highly expressed but Notch2 was not expressed in astrocytomas. The percentage of immunopositive tumor cells and level of Notch1 expression was increased with tumor grade. In addition, overexpression of Notch2 was detected in medulloblastomas in contrast to low or no expression of Notch1, 3, 4. Differential expression of Notch1, 2, 3, 4 is detected in astrocytomas and medulloblastomas, that may be related to their different roles playing in the development of brain tumors.

Identification of PDE4D as a proliferation promoting factor in prostate cancer using a Sleeping Beauty transposon-based somatic mutagenesis screen

Retroviral and transposon-based mutagenesis screens in mice have been useful for identifying candidate cancer genes for some tumor types. However, many of the organs that exhibit the highest cancer rates in humans, including the prostate, have not previously been amenable to these approaches. This study shows for the first time that the Sleeping Beauty transposon system can be used to identify candidate prostate cancer genes in mice. Somatic mobilization of a mutagenic transposon resulted in focal epithelial proliferation and hyperplasia in the prostate. Efficient methods were established to identify transposon insertion sites in these lesions, and analysis of transposon insertions identified candidate prostate cancer genes at common insertion sites, including Pde4d. PDE4D was also overexpressed in human prostate cancer patient samples and cell lines, and changes in PDE4D mRNA isoform expression were observed in human prostate cancers. Furthermore, knockdown of PDE4D reduced the growth and migration of prostate cancer cells in vitro, and knockdown of PDE4D reduced the growth and proliferation rate of prostate cancer xenografts in vivo. These data indicate that PDE4D functions as a proliferation promoting factor in prostate cancer, and the Sleeping Beauty transposon system is a useful tool for identifying candidate prostate cancer genes.

Using naturally occurring tumours in dogs and cats to study telomerase and cancer stem cell biology

The recently described cancer stem cell theory opens up many new challenges and opportunities to identify targets for therapeutic intervention. However, the majority of cancer related therapeutic studies rely upon rodent models of human cancer that rarely translate into clinical success in human patients. Naturally occurring cancers in dogs, cats and humans share biological features, including molecular targets, telomerase biology and tumour genetics. Studying cancer stem cell biology and telomere/telomerase dynamics in the cancer bearing pet population may offer the opportunity to develop a greater understanding of cancer biology in the natural setting and evaluate the development of novel therapies targeted at these systems.