Rbpj conditional knockout reveals distinct functions of Notch4/Int3 in mammary gland development and tumorigenesis

Notch signaling, hypoxia, and cancer

Notch signaling is involved in cell fate determination and deregulated in human solid tumors. Hypoxia is an important feature in many solid tumors, which activates hypoxia-induced factors (HIFs) and their downstream targets to promote tumorigenesis and cancer development. Recently, HIFs have been shown to trigger the Notch signaling pathway in a variety of organisms and tissues. In this review, we focus on the pro- and anti-tumorigenic functions of Notch signaling and discuss the crosstalk between Notch signaling and cellular hypoxic response in cancer pathogenesis, including epithelia-mesenchymal transition, angiogenesis, and the maintenance of cancer stem cells. The pharmacological strategies targeting Notch signaling and hypoxia in cancer are also discussed in this review.

Unique functions for Notch4 in murine embryonic lymphangiogenesis

In mice, embryonic dermal lymphatic development is well understood and used to study gene functions in lymphangiogenesis. Notch signaling is an evolutionarily conserved pathway that modulates cell fate decisions, which has been shown to both inhibit and promote dermal lymphangiogenesis. Here, we demonstrate distinct roles for Notch4 signaling versus canonical Notch signaling in embryonic dermal lymphangiogenesis. Actively growing embryonic dermal lymphatics expressed NOTCH1, NOTCH4, and DLL4 which correlated with Notch activity. In lymphatic endothelial cells (LECs), DLL4 activation of Notch induced a subset of Notch effectors and lymphatic genes, which were distinctly regulated by Notch1 and Notch4 activation. Treatment of LECs with VEGF-A or VEGF-C upregulated Dll4 transcripts and differentially and temporally regulated the expression of Notch1 and Hes/Hey genes. Mice nullizygous for Notch4 had an increase in the closure of the lymphangiogenic fronts which correlated with reduced vessel caliber in the maturing lymphatic plexus at E14.5 and reduced branching at E16.5. Activation of Notch4 suppressed LEC migration in a wounding assay significantly more than Notch1, suggesting a dominant role for Notch4 in regulating LEC migration. Unlike Notch4 nulls, inhibition of canonical Notch signaling by expressing a dominant negative form of MAML1 (DNMAML) in Prox1+ LECs led to increased lymphatic density consistent with an increase in LEC proliferation, described for the loss of LEC Notch1. Moreover, loss of Notch4 did not affect LEC canonical Notch signaling. Thus, we propose that Notch4 signaling and canonical Notch signaling have distinct functions in the coordination of embryonic dermal lymphangiogenesis.

Mammary Development and Breast Cancer: a Notch Perspective

Mammary gland development primarily occurs postnatally, and this unique process is complex and regulated by systemic hormones and local growth factors. The mammary gland is also a highly dynamic organ that undergoes profound changes at puberty and during the reproductive cycle. These changes are driven by mammary stem cells (MaSCs). Breast cancer is one of the most common causes of cancer-related death in women. Cancer stem cells (CSCs) play prominent roles in tumor initiation, drug resistance, tumor recurrence, and metastasis. The highly conserved Notch signaling pathway functions as a key regulator of the niche mediating mammary organogenesis and breast neoplasia. In this review, we discuss mechanisms by which Notch contributes to breast carcinoma pathology and suggest potentials for therapeutic targeting of Notch in breast cancer. In summary, we provide a comprehensive overview of Notch functions in regulating MaSCs, mammary development, and breast cancer.

Cancer Stem Cells, Quo Vadis? The Notch Signaling Pathway in Tumor Initiation and Progression

The Notch signaling pathway regulates cell proliferation, cytodifferentiation and cell fate decisions in both embryonic and adult life. Several aspects of stem cell maintenance are dependent from the functionality and fine tuning of the Notch pathway. In cancer, Notch is specifically involved in preserving self-renewal and amplification of cancer stem cells, supporting the formation, spread and recurrence of the tumor. As the function of Notch signaling is context dependent, we here provide an overview of its activity in a variety of tumors, focusing mostly on its role in the maintenance of the undifferentiated subset of cancer cells. Finally, we analyze the potential of molecules of the Notch pathway as diagnostic and therapeutic tools against the various cancers.

Notch activation in the mouse mammary luminal lineage leads to ductal hyperplasia and altered partitioning of luminal cell subtypes

Hyperactivated Notch signalling has been implicated in breast cancer, but how elevated levels of Notch signalling contribute to mammary dysplasia and tumorigenesis is not fully understood. In this study, we express an activated form of Notch1 in the mouse mammary luminal lineage and analyse the consequences for tumour formation and the transcriptomic landscape in the luminal lineage. Simultaneous conditional activation of a Notch1 intracellular domain (Notch1 ICD) and EGFP in the luminal lineage was achieved by removal of a stop cassette by CRE-recombinase expression from the whey acidic protein (WAP) promoter. Mice in which Notch1 ICD was activated in the luminal lineage (WAP-CRE;R26-N1ICD mice) exhibit ductal hyperplasia after lactation with an increase in branching frequency and in the number of side-branch ends in the ductal tree. A subset of the mice developed mammary tumours and the majority of the tumour cells expressed EGFP (as a proxy for Notch1 ICD), indicating that the tumours originate from the Notch1 ICD-expressing cells. Single-cell transcriptome analysis of the EGFP-positive mammary cells identified six subtypes of luminal cells. The same six subtypes were found in control mice (WAP-CRE;R26-tdTomato mice expressing the tdTomato reporter from WAP-CRE-mediated activation), but the proportion of cells in the various subtypes differed between the WAP-CRE;R26-N1ICD and control WAP-CRE;R26-tdTomato mice. In conclusion, we show that Notch1 ICD expression in the luminal lineage produces a ductal hyperplasia and branching phenotype accompanied by altered luminal cell subtype partitioning.

Multifactorial Contribution of Notch Signaling in Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC) defines a group of solid tumors originating from the mucosa of the upper aerodigestive tract, pharynx, larynx, mouth, and nasal cavity. It has a metastatic evolution and poor prognosis and is the sixth most common cancer in the world, with 600,000 new cases reported every year. HNSCC heterogeneity and complexity is reflected in a multistep progression, involving crosstalk between several molecular pathways. The Notch pathway is associated with major events supporting cancerogenic evolution: cell proliferation, self-renewal, angiogenesis, and preservation of a pro-oncogenic microenvironment. Additionally, Notch is pivotal in tumor development and plays a dual role acting as both oncogene and tumor suppressor. In this review, we summarize the role of the Notch pathway in HNSCC, with a special focus on its compelling role in major events of tumor initiation and growth.

Sulforaphane Suppresses the Growth of Triple-negative Breast Cancer Stem-like Cells In vitro and In vivo

Triple-negative breast cancer (TNBC) represents the poorest prognosis among all of breast cancer subtypes with no currently available effective therapy. In this study, we hypothesized that sulforaphane, a dietary component abundant in broccoli and its sprouts, can inhibit malignant cell proliferation and tumor sphere formation of cancer stem-like cells (CSC) in TNBC. CSC population was isolated using FACS analysis with the combined stem cell surface markers, CD44⁺/CD24^-/CD49f⁺ The effect of sulforaphane on a stem-related embryonic oncogene CRIPTO-1/TDGF1 (CR1) was evaluated via ELISA. In vivo, BalbC/nude mice were supplemented with sulforaphane before and after TNBC cell inoculation (daily intraperitoneal injection of 50 mg sulforaphane/kg for 5 and 3 weeks, respectively), and the effects of sulforaphane during mammary tumor initiation and growth were accessed with NanoString gene analysis. We found that sulforaphane can inhibit cell proliferation and mammosphere formation of CSCs in TNBC. Further analysis of gene expression in these TNBC tumor cells revealed that sulforaphane significantly decreases the expression of cancer-specific CR1, CRIPTO-3/TDGF1P3 (CR3, a homologue of CR1), and various stem cell markers including Nanog, aldehyde dehydrogenase 1A1 (ALDH1A1), Wnt3, and Notch4. Our results suggest that sulforaphane may control the malignant proliferation of CSCs in TNBC via Cripto-mediated pathway by either suppressing its expression and/or by inhibiting Cripto/Alk4 protein complex formation. Thus, the use of sulforaphane for chemoprevention of TNBC is plausible and warrants further clinical evaluation.

Moving Breast Cancer Therapy up a Notch

Breast cancer is the second most common malignancy, worldwide. Treatment decisions are based on tumor stage, histological subtype, and receptor expression and include combinations of surgery, radiotherapy, and systemic treatment. These, together with earlier diagnosis, have resulted in increased survival. However, initial treatment efficacy cannot be guaranteed upfront, and these treatments may come with (long-term) serious adverse effects, negatively affecting a patient's quality of life. Gene expression-based tests can accurately estimate the risk of recurrence in early stage breast cancers. Disease recurrence correlates with treatment resistance, creating a major need to resensitize tumors to treatment. Notch signaling is frequently deregulated in cancer and is involved in treatment resistance. Preclinical research has already identified many combinatory therapeutic options where Notch involvement enhances the effectiveness of radiotherapy, chemotherapy or targeted therapies for breast cancer. However, the benefit of targeting Notch has remained clinically inconclusive. In this review, we summarize the current knowledge on targeting the Notch pathway to enhance current treatments for breast cancer and to combat treatment resistance. Furthermore, we propose mechanisms to further exploit Notch-based therapeutics in the treatment of breast cancer.

Cancer Cells Exploit Notch Signaling to Redefine a Supportive Cytokine Milieu

Notch signaling is a well-known key player in the communication between adjacent cells during organ development, when it controls several processes involved in cell differentiation. Notch-mediated communication may occur through the interaction of Notch receptors with ligands on adjacent cells or by a paracrine/endocrine fashion, through soluble molecules that can mediate the communication between cells at distant sites. Dysregulation of Notch pathway causes a number of disorders, including cancer. Notch hyperactivation may be caused by mutations of Notch-related genes, dysregulated upstream pathways, or microenvironment signals. Cancer cells may exploit this aberrant signaling to "educate" the surrounding microenvironment cells toward a pro-tumoral behavior. This may occur because of key cytokines secreted by tumor cells or it may involve the microenvironment through the activation of Notch signaling in stromal cells, an event mediated by a direct cell-to-cell contact and resulting in the increased secretion of several pro-tumorigenic cytokines. Up to now, review articles were mainly focused on Notch contribution in a specific tumor context or immune cell populations. Here, we provide a comprehensive overview on the outcomes of Notch-mediated pathological interactions in different tumor settings and on the molecular and cellular mediators involved in this process. We describe how Notch dysregulation in cancer may alter the cytokine network and its outcomes on tumor progression and antitumor immune response.

Canonical Notch Signaling Directs the Fate of Differentiating Neurocompetent Progenitors in the Mammalian Olfactory Epithelium

The adult olfactory epithelium (OE) has the remarkable capacity to regenerate fully both neurosensory and non-neuronal cell types after severe epithelial injury. Lifelong persistence of two stem cell populations supports OE regeneration when damaged: the horizontal basal cells (HBCs), dormant and held in reserve; and globose basal cells, a heterogeneous population most of which are actively dividing. Both populations regenerate all cell types of the OE after injury, but the mechanisms underlying neuronal versus non-neuronal lineage commitment after recruitment of the stem cell pools remains unknown. We used both retroviral transduction and mouse lines that permit conditional cell-specific genetic manipulation as well as the tracing of progeny to study the role of canonical Notch signaling in the determination of neuronal versus non-neuronal lineages in the regenerating adult OE. Excision of either Notch1 or Notch2 genes alone in HBCs did not alter progenitor fate during recovery from epithelial injury, whereas conditional knock-out of both Notch1 and Notch2 together, retroviral transduction of progenitors with a dominant-negative form of MAML (mastermind-like), or excision of the downstream cofactor RBPJ caused progeny to adopt a neuronal fate exclusively. Conversely, we show that overexpressing the Notch1-intracellular domain (N1ICD) either genetically or by transduction blocks neuronal differentiation completely. However, N1ICD overexpression requires both alleles of the canonical cofactor RBPJ to specify downstream lineage. Together, our results suggest that canonical RBPJ-dependent Notch signaling through redundant Notch1 and Notch2 receptors is both necessary and sufficient for determining neuronal versus non-neuronal differentiation in the regenerating adult OE.SIGNIFICANCE STATEMENT Despite the substantial reconstitution of the olfactory epithelium and its population of sensory neurons after injury, disruption and exhaustion of neurogenesis is a consequence of aging and a cause of olfactory dysfunction. Understanding the mechanisms underlying the generation of replacement neurons and non-neuronal cells is critical to any therapeutic strategy aimed at rebuilding a functional neuroepithelium. The results shown here demonstrate that canonical Notch signaling determines the balance between neurons and non-neuronal cells during restoration of the epithelium after injury. Moreover, the complexities of the multiple Notch pathways impinging on that decision are dissected in detail. Finally, RBPJ, the canonical Notch transcriptional cofactor, exhibits a heretofore unreported haploinsufficiency in setting the balance among the regenerating populations.

Targeting signal transduction pathways of cancer stem cells for therapeutic opportunities of metastasis

Tumor comprises of heterogeneous population of cells where not all the disseminated cancer cells have the prerogative and "in-build genetic cues" to form secondary tumors. Cells with stem like properties complemented by key signaling molecules clearly have shown to exhibit selective growth advantage to form tumors at distant metastatic sites. Thus, defining the role of cancer stem cells (CSC) in tumorigenesis and metastasis is emerging as a major thrust area for therapeutic intervention. Precise relationship and regulatory mechanisms operating in various signal transduction pathways during cancer dissemination, extravasation and angiogenesis still remain largely enigmatic. How the crosstalk amongst circulating tumor cells (CTC), epithelial mesenchymal transition (EMT) process and CSC is coordinated for initiating the metastasis at secondary tissues, and during cancer relapse could be of great therapeutic interest. The signal transduction mechanisms facilitating the dissemination, infiltration of CSC into blood stream, extravasations, progression of metastasis phenotype and angiogenesis, at distant organs, are the key pathologically important vulnerabilities being elucidated. Therefore, current new drug discovery focus has shifted towards finding "key driver genes" operating in parallel signaling pathways, during quiescence, survival and maintenance of stemness in CSC. Understanding these mechanisms could open new horizons for tackling the issue of cancer recurrence and metastasis-the cause of ~90% cancer associated mortality. To design futuristic & targeted therapies, we propose a multi-pronged strategy involving small molecules, RNA interference, vaccines, antibodies and other biotechnological modalities against CSC and the metastatic signal transduction cascade.

Histological subtypes of mouse mammary tumors reveal conserved relationships to human cancers

Human breast cancer has been characterized by extensive transcriptional heterogeneity, with dominant patterns reflected in the intrinsic subtypes. Mouse models of breast cancer also have heterogeneous transcriptomes and we noted that specific histological subtypes were associated with particular subsets. We hypothesized that unique sets of genes define each tumor histological type across mouse models of breast cancer. Using mouse models that contained both gene expression data and expert pathologist classification of tumor histology on a sample by sample basis, we predicted and validated gene expression signatures for Papillary, EMT, Microacinar and other histological subtypes. These signatures predict known histological events across murine breast cancer models and identify counterparts of mouse mammary tumor types in subtypes of human breast cancer. Importantly, the EMT, Adenomyoepithelial, and Solid signatures were predictive of clinical events in human breast cancer. In addition, a pan-cancer comparison revealed that the histological signatures were active in a variety of human cancers such as lung, oral, and esophageal squamous tumors. Finally, the differentiation status and transcriptional activity implicit within these signatures was identified. These data reveal that within tumor histology groups are unique gene expression profiles of differentiation and pathway activity that stretch well beyond the transgenic initiating events and that have clear applicability to human cancers. As a result, our work provides a predictive resource and insights into possible mechanisms that govern tumor heterogeneity.

The ANK repeats of Notch-4/Int3 activate NF-κB canonical pathway in the absence of Rbpj and causes mammary tumorigenesis

Transgenic mice expressing the Notch-4 intracellular domain (designated Int3) in the mammary gland have two phenotypes exhibited with 100% penetrance: arrest of mammary alveolar/lobular development and mammary tumorigenesis. Notch-4 signaling is mediated primarily through the interaction of Int3 with the transcription repressor/activator Rbpj. Interestingly, WAP-Int3/Rbpj knockout mice have normal mammary gland development but still developed mammary tumors with a slightly longer latency than the WAP-Int3 mice. Thus, Notch-induced mammary tumor development is Rbpj-independent. Here, we show that Int3 activates NF-κB in HC11 cells in absence of Rbpj through an association with the IKK signalosome. Int3 induced the canonical NF-κB activity and P50 phosphorylation in HC11 cells without altering the NF-κB2 pathway. The minimal domain within the Int3 protein required to activate NF-κB consists of the CDC10/Ankyrin (ANK) repeats domain. Treatment of WAP-Int3 tumor bearing mice with an IKK inhibitor resulted in tumor regression. In a soft agar assay, treatment of HC11-Int3 cells with P50-siRNA caused a significant decrease in colony formation. In addition, Wap-Int3/P50 knockout mice did not develop mammary tumors. This data indicates that the activation of NF-κB canonical signaling by Notch-4/Int3 is ANK repeats dependent, Rbpj-independent, and is mediated by IKK activation and P50 phosphorylation causing mammary tumorigenesis.

Notch Signaling in Development, Tissue Homeostasis, and Disease

Notch signaling is an evolutionarily highly conserved signaling mechanism, but in contrast to signaling pathways such as Wnt, Sonic Hedgehog, and BMP/TGF-β, Notch signaling occurs via cell-cell communication, where transmembrane ligands on one cell activate transmembrane receptors on a juxtaposed cell. Originally discovered through mutations in Drosophila more than 100 yr ago, and with the first Notch gene cloned more than 30 yr ago, we are still gaining new insights into the broad effects of Notch signaling in organisms across the metazoan spectrum and its requirement for normal development of most organs in the body. In this review, we provide an overview of the Notch signaling mechanism at the molecular level and discuss how the pathway, which is architecturally quite simple, is able to engage in the control of cell fates in a broad variety of cell types. We discuss the current understanding of how Notch signaling can become derailed, either by direct mutations or by aberrant regulation, and the expanding spectrum of diseases and cancers that is a consequence of Notch dysregulation. Finally, we explore the emerging field of Notch in the control of tissue homeostasis, with examples from skin, liver, lung, intestine, and the vasculature.

Original Research: Featured Article: Imatinib mesylate (Gleevec) inhibits Notch and c-Myc signaling: Five-day treatment permanently rescues mammary development

Wap-Int3 transgenic females expressing the Notch4 intracellular domain (designated Int3) from the whey acidic protein promoter exhibit two phenotypes in the mammary gland: blockage of lobuloalveolar development and lactation, and tumor development with 100% penetrance. Previously, we have shown that treatment of Wap-Int3 tumor bearing mice with Imatinib mesylate (Gleevec) is associated with complete regression of the tumor. In the present study, we show that treatment of Wap-Int3 mice during day 1 through day 6 of pregnancy with Gleevec leads to the restoration of their lobuloalveolar development and ability to lactate in subsequent pregnancies in absence of Gleevec treatment. In addition, these mice do not develop mammary tumors. We investigated the mechanism for Gleevec regulation of Notch signaling and found that Gleevec treatment results in a loss of Int3 protein but not of Int3 mRNA in HC11 mouse mammary epithelial cells expressing Int3. The addition of MG-132, a proteasome inhibitor, shows increased ubiquitination of Int3 in the presence of Gleevec. Thus, Gleevec affects the stability of Int3 by promoting the degradation of Int3 via E3 ubiquitin ligases targeting it for the proteasome degradation. Gleevec is a tyrosine kinase inhibitor that acts on c-Kit and PDGFR. Therefore, we investigated the downstream substrate kinase GSK3β to ascertain the possible role that this kinase might play in the stability of Int3. Data show that Gleevec degradation of Int3 is GSK3β dependent. We have expanded our study of the effects Gleevec has on tumorigenesis of other oncogenes. We have found that anchorage-independent growth of HC11-c-Myc cells as well as tumor growth in nude mice is inhibited by Gleevec treatment. As with Int3, Gleevec treatment appears to destabilize the c-Myc protein but not mRNA. These results indicate that Gleevec could be a potential therapeutic drug for patients bearing Notch4 and/or c-Myc positive breast carcinomas.

Notch Signaling in Neuroendocrine Tumors

Carcinoids and neuroendocrine tumors (NETs) are a heterogeneous group of tumors that arise from the neuroendocrine cells of the GI tract, endocrine pancreas, and the respiratory system. NETs remain significantly understudied with respect to molecular mechanisms of pathogenesis, particularly the role of cell fate signaling systems such as Notch. The abundance of literature on the Notch pathway is a testament to its complexity in different cellular environments. Notch receptors can function as oncogenes in some contexts and tumor suppressors in others. The genetic heterogeneity of NETs suggests that to fully understand the roles and the potential therapeutic implications of Notch signaling in NETs, a comprehensive analysis of Notch expression patterns and potential roles across all NET subtypes is required.

Cripto-1 ablation disrupts alveolar development in the mouse mammary gland through a progesterone receptor-mediated pathway

Cripto-1, a member of the epidermal growth factor-Cripto-1/FRL-1/Cryptic family, is critical for early embryonic development. Together with its ligand Nodal, Cripto-1 has been found to be associated with the undifferentiated status of mouse and human embryonic stem cells. Several studies have clearly shown that Cripto-1 is involved in regulating branching morphogenesis and epithelial-mesenchymal transition of the mammary gland both in vitro and in vivo and together with the cofactor GRP78 is critical for the maintenance of mammary stem cells ex vivo. Our previous studies showed that mammary-specific overexpression of human Cripto-1 exhibited dramatic morphological alterations in nulliparous mice mammary glands. The present study shows a novel mechanism for Cripto-1 regulation of mammary gland development through direct effects on progesterone receptor expression and pathways regulated by progesterone in the mammary gland. We demonstrate a strict temporal regulation of mouse Cripto-1 (mCripto-1) expression that occurs during mammary gland development and a stage-specific function of mCripto-1 signaling during mammary gland development. Our data suggest that Cripto-1, like the progesterone receptor, is not required for the initial ductal growth but is essential for subsequent side branching and alveologenesis during the initial stages of pregnancy. Dissection of the mechanism by which this occurs indicates that mCripto-1 activates receptor activator NF-κB/receptor activator NF-κB ligand, and NF-κB signaling pathways.

Targeting Notch, Hedgehog, and Wnt pathways in cancer stem cells: clinical update

During the past decade, cancer stem cells (CSCs) have been increasingly identified in many malignancies. Although the origin and plasticity of these cells remain controversial, tumour heterogeneity and the presence of small populations of cells with stem-like characteristics is established in most malignancies. CSCs display many features of embryonic or tissue stem cells, and typically demonstrate persistent activation of one or more highly conserved signal transduction pathways involved in development and tissue homeostasis, including the Notch, Hedgehog (HH), and Wnt pathways. CSCs generally have slow growth rates and are resistant to chemotherapy and/or radiotherapy. Thus, new treatment strategies targeting these pathways to control stem-cell replication, survival and differentiation are under development. Herein, we provide an update on the latest advances in the clinical development of such approaches, and discuss strategies for overcoming CSC-associated primary or acquired resistance to cancer treatment. Given the crosstalk between the different embryonic developmental signalling pathways, as well as other pathways, designing clinical trials that target CSCs with rational combinations of agents to inhibit possible compensatory escape mechanisms could be of particular importance. We also share our views on the future directions for targeting CSCs to advance the clinical development of these classes of agents.

Non-small-cell lung carcinoma: role of the Notch signaling pathway

Notch signaling plays a pivotal role during embryogenesis. It regulates three fundamental processes: lateral inhibition, boundary formation, and lineage specification. During post-natal life, Notch receptors and ligands control critical cell fate decisions both in compartments that are undergoing differentiation and in pluripotent progenitor cells. First recognized as a potent oncogene in certain lymphoblastic leukemias and mesothelium-derived tissue, the role of Notch signaling in epithelial, solid tumors has been far more controversial. The overall consequence of Notch signaling and which form of the Notch receptor drives malignancy in humans is deeply debated. Most likely, this is due to the high degree of context-dependent effects of Notch signaling. More recently, it has been discovered that Notch (especially Notch-1) can exert different, even opposite effects in the same tissue under differing microenvironmental conditions. Further complicating the understanding of Notch receptors is the recently discovered role for non-canonical Notch signaling. Additionally, the most frequent Notch signaling antagonists used in biological systems have been inhibitors of the transmembrane protease complex γ-secretase, which itself processes a plethora of class one transmembrane proteins and thus cannot be considered a Notch-specific upstream regulator. Here we review the available empirical evidence gathered in recent years concerning Notch receptors and ligands in non-small-cell lung carcinoma (NSCLC). Although an overview of the field reveals seemingly contradicting results, we propose that Notch signaling can be exploited as a therapeutic target in NSCLC and represents a promising complement to the current arsenal utilized to combat this malignancy, particularly in targeting NSCLC tissues under specific environmental conditions, such as hypoxia.

AKT and 14-3-3 regulate Notch4 nuclear localization

Members of the Notch family of transmembrane receptors, Notch1-4 in mammals, are involved in the regulation of cell fate decisions and cell proliferation in various organisms. The Notch4 isoform, which is specific to mammals, was originally identified as a viral oncogene in mice, Int3, able to initiate mammary tumors. In humans, Notch4 expression appears to be associated with breast cancer stem cells and endocrine resistance. Following ligand binding, the Notch4 receptor undergoes cleavage at the membrane and the Notch4-intracellular domain (ICD), translocates to the nucleus and regulates gene transcription. Little is known on the mechanisms regulating Notch4-ICD and its nuclear localization. Here, we describe the identification of four distinct AKT phosphorylation sites in human Notch4-ICD and demonstrate that AKT binds Notch4-ICD and phosphorylates all four sites in vitro and in vivo. The phosphorylation in cells is regulated by growth factors and is sensitive to phosphatidyl inositol-3 kinase (PI3K) inhibitors. This phosphorylation generates binding sites to the 14-3-3 regulatory proteins, which are involved in the regulation of nucleocytoplasmic shuttling of target proteins, restricting phosphorylated Notch4-ICD to the cytoplasm. Our findings provide a novel mechanism for Notch4-ICD regulation, suggesting a negative regulatory role for the PI3K-AKT pathway in Notch4 nuclear signaling.

Loss of the Notch effector RBPJ promotes tumorigenesis

Kulic et al. show that RBPJ, a transcriptional repressor of Notch, is frequently deleted in human cancers and can function as a tumor suppressor. Loss of RBPJ acts to derepress target gene promoters, allowing Notch-independent activation by alternate transcription factors that promote tumor growth.

Aberrant Notch activity is oncogenic in several malignancies, but it is unclear how expression or function of downstream elements in the Notch pathway affects tumor growth. Transcriptional regulation by Notch is dependent on interaction with the DNA-binding transcriptional repressor, RBPJ, and consequent derepression or activation of associated gene promoters. We show here that RBPJ is frequently depleted in human tumors. Depletion of RBPJ in human cancer cell lines xenografted into immunodeficient mice resulted in activation of canonical Notch target genes, and accelerated tumor growth secondary to reduced cell death. Global analysis of activated regions of the genome, as defined by differential acetylation of histone H4 (H4ac), revealed that the cell death pathway was significantly dysregulated in RBPJ-depleted tumors. Analysis of transcription factor binding data identified several transcriptional activators that bind promoters with differential H4ac in RBPJ-depleted cells. Functional studies demonstrated that NF-κB and MYC were essential for survival of RBPJ-depleted cells. Thus, loss of RBPJ derepresses target gene promoters, allowing Notch-independent activation by alternate transcription factors that promote tumorigenesis.

Non-canonical notch signaling in cancer and immunity

Canonical Notch signaling is initiated by γ-secretase-mediated cleavage of the Notch receptor, leading to the release of the active intra-cellular domain of Notch that migrates to the nucleus and interacts with RBP-Jκ, resulting in the activation of downstream target genes. While canonical Notch signaling is well known to play an active role in several steps during development as well in multiple cell fate decisions, recent evidence from both invertebrate and vertebrate systems indicates that non-canonical, RBP-Jκ-independent signaling is important in several cellular processes including oncogenesis and activation of T lymphocytes. These observations raise the possibility that, through an understanding of non-canonical Notch signaling, novel strategies for inhibiting Notch signaling may prove useful in the design of therapies targeted to block aberrant Notch activity. In this mini-review, we will examine the current data demonstrating a non-canonical role for Notch signaling in both cancer and the immune system and suggest a better understanding of non-canonical signaling may reveal novel strategies to block Notch signaling in disease.

Inhibition of Notch signaling alters the phenotype of orthotopic tumors formed from glioblastoma multiforme neurosphere cells but does not hamper intracranial tumor growth regardless of endogene Notch pathway signature

BACKGROUND

Brain cancer stem-like cells (bCSC) are cancer cells with neural stem cell (NSC)-like properties found in the devastating brain tumor glioblastoma multiforme (GBM). bCSC are proposed a central role in tumor initiation, progression, treatment resistance and relapse and as such present a promising target in GBM research. The Notch signaling pathway is often deregulated in GBM and we have previously characterized GBM-derived bCSC cultures based on their expression of the Notch-1 receptor and found that it could be used as predictive marker for the effect of Notch inhibition. The aim of the present project was therefore to further elucidate the significance of Notch pathway activity for the tumorigenic properties of GBM-derived bCSC.

METHODS

Human-derived GBM xenograft cells previously established as NSC-like neurosphere cultures were used. Notch inhibition was accomplished by exposing the cells to the gamma-secretase inhibitor DAPT prior to gene expression analysis and intracranial injection into immunocompromised mice.

RESULTS

By analyzing the expression of several Notch pathway components, we found that the cultures indeed displayed different Notch pathway signatures. However, when DAPT-treated neurosphere cells were injected into the brain of immunocompromised mice, no increase in survival was obtained regardless of Notch pathway signature and Notch inhibition. We did however observe a decrease in the expression of the stem cell marker Nestin, an increase in the proliferative marker Ki-67 and an increased number of abnormal vessels in tumors formed from DAPT-treated, high Notch-1 expressing cultures, when compared with the control.

CONCLUSION

Based on the presented results we propose that Notch inhibition partly induces differentiation of bCSC, and selects for a cell type that more strongly induces angiogenesis if the treatment is not sustained. However, this more differentiated cell type might prove to be more sensitive to conventional therapies.

Crosstalk between PKCα and Notch-4 in endocrine-resistant breast cancer cells

The Notch pathway is functionally important in breast cancer. Notch-1 has been reported to maintain an estrogen-independent phenotype in estrogen receptor α (ERα)+ breast cancer cells. Notch-4 expression correlates with Ki67. Notch-4 also plays a key role in breast cancer stem-like cells. Estrogen-independent breast cancer cell lines have higher Notch activity than estrogen-dependent lines. Protein kinase Cα (PKCα) overexpression is common in endocrine-resistant breast cancers and promotes tamoxifen (TAM)-resistant growth in breast cancer cell lines. We tested whether PKCα overexpression affects Notch activity and whether Notch signaling contributes to endocrine resistance in PKCα-overexpressing breast cancer cells.Analysis of published microarray data from ERα+ breast carcinomas shows that PKCα expression correlates strongly with Notch-4. Real-time reverse transcription PCR and immunohistochemistry on archival specimens confirmed this finding. In a PKCα-overexpressing, TAM-resistant T47D model, PKCα selectively increases Notch-4, but not Notch-1, expression in vitro and in vivo. This effect is mediated by activator protein-1 (AP-1) occupancy of the Notch-4 promoter. Notch-4 knockdown inhibits estrogen-independent growth of PKCα-overexpressing T47D cells, whereas Notch-4IC expression stimulates it. Gene expression profiling shows that multiple genes and pathways associated with endocrine resistance are induced in Notch-4IC- and PKCα-expressing T47D cells. In PKCα-overexpressing T47D xenografts, an orally active γ-secretase inhibitor at clinically relevant doses significantly decreased estrogen-independent tumor growth, alone and in combination with TAM. In conclusion, PKCα overexpression induces Notch-4 through AP-1. Notch-4 promotes estrogen-independent, TAM-resistant growth and activates multiple pathways connected with endocrine resistance and chemoresistance. Notch inhibitors should be clinically evaluated in PKCα- and Notch-4-overexpressing, endocrine-resistant breast cancers.

Notch inhibitors for cancer treatment

Notch signaling is an evolutionarily conserved cell signaling pathway involved in cell fate during development, stem cell renewal and differentiation in postnatal tissues. Roles for Notch in carcinogenesis, in the biology of cancer stem cells and tumor angiogenesis have been reported. These features identify Notch as a potential therapeutic target in oncology. Based on the molecular structure of Notch receptor, Notch ligands and Notch activators, a set of Notch pathway inhibitors have been developed. Most of these inhibitors had shown anti-tumor effects in preclinical studies. At the same time, the combinatorial effect of these inhibitors with current chemotherapeutical drugs is still under study in different clinical trials. In this review, we describe the basics of Notch signaling and the role of Notch in normal and cancer stem cells as a logic way to develop different Notch inhibitors and their current stage of progress for cancer patient's treatment.

Differential gene expression in nuclear label-retaining cells in the developing mouse mammary gland

The immortal strand theory postulates stem cells protect themselves from DNA replication-associated mutations and subsequent cancer risk through selective segregation of template DNA strands. Stem cells self-renew by asymmetric cellular division. During asymmetric division, stem cells maintain their template DNA strands, while the newly synthesized DNA strands segregate to newly formed daughter cells. Previous studies have demonstrated that self-renewing mammary stem cells originate in the expanding mammary ducts during puberty-associated allometric growth. In this study, we labeled newly forming mammary stem cells with the thymidine analog 5-ethynl-2'-deoxyuridine for 2 weeks during allometric ductal expansion. Cells that incorporate and retain the nuclear label following extended chase periods are termed label-retaining cells (LRCs). A second nuclear label, 5-bromodeoxyuridine, was administered before euthanasia to identify cells traversing the cell cycle. Mammary cells collected following euthanasia were sorted based on nuclear label retention. Members of the Notch and Wnt signaling pathways were found differentially expressed by mammary LRCs. These pathways are involved in the regulation of stem cells in the mouse mammary gland. Upon further analysis, we found that in contrast to non-LRCs, Notch1 and Notch2 are expressed and localized in the nuclei of the LRCs. Expression of Notch-inducible genes, Hes1 and Hey2, was elevated in LRCs. Inhibition of Notch1 by shRNA reduced colony forming potential and label retention by mammary epithelial cells in vitro. These results indicate that genes are differentially regulated in the LRC population of mammary glands and Notch1 mediates asymmetric cell division of mammary progenitor cells.

Non-canonical Notch signaling activates IL-6/JAK/STAT signaling in breast tumor cells and is controlled by p53 and IKKα/IKKβ

Notch signaling is frequently hyperactivated in breast cancer, but how the enhanced signaling contributes to the tumor process is less well understood. In this report, we identify the proinflammatory cytokine interleukin-6 (IL-6) as a novel Notch target in breast tumor cells. Enhanced Notch signaling upregulated IL-6 expression, leading to activation of autocrine and paracrine Janus kinase/signal transducers and activators of transcription signaling. IL-6 upregulation was mediated by non-canonical Notch signaling, as it could be effectuated by a cytoplasmically localized Notch intracellular domain and was independent of the DNA-binding protein CSL. Instead, Notch-mediated IL-6 upregulation was controlled by two proteins in the nuclear factor (NF)-κB signaling cascade, IKKα and IKKβ (inhibitor of nuclear factor kappa-B kinase subunit alpha and beta, respectively), as well as by p53. Activation of IL-6 by Notch required IKKα/IKKβ function, but interestingly, did not engage canonical NF-κB signaling, in contrast to IL-6 activation by inflammatory agents such as lipopolysaccharide. With regard to p53 status, IL-6 expression was upregulated by Notch when p53 was mutated or lost, and restoring wild-type p53 into p53-mutated or -deficient cells abrogated the IL-6 upregulation. Furthermore, Notch-induced transcriptomes from p53 wild-type and -mutated breast tumor cell lines differed extensively, and for a subset of genes upregulated by Notch in a p53-mutant cell line, this upregulation was reduced by wild-type p53. In conclusion, we identify IL-6 as a novel non-canonical Notch target gene, and reveal roles for p53 and IKKα/IKKβ in non-canonical Notch signaling in breast cancer and in the generation of cell context-dependent diversity in the Notch signaling output.

The biology of human breast epithelial progenitors

Current evidence suggests that similar to other tissues in the human body mammary epithelia cells are being maintained by the unique properties of stem cells, undifferentiated as well as lineage-restricted progenitors. Because of their longevity, proliferation and differentiation potentials these primitive breast epithelial cells are likely targets of transforming mutations that can cause them to act as cancer initiating cells. In this context, understanding the molecular mechanisms that regulate the normal functions of the human breast epithelial stem cells and progenitors and how alterations to these same mechanisms can confer a cancer stem cell phenotype on these rare cell populations is crucial to the development of new and more effective therapies again breast cancer. This review article will examine the current state of knowledge about the isolation and characterization of human breast epithelial progenitors and their relevance to breast cancer research.

Canonical and non-canonical Notch signaling in CD4⁺ T cells

For T cells to become fully activated, they must integrate a myriad of signals, both extrinsic and intrinsic. External stimuli accrued through various cell surface receptors are transduced and amplified through a coordinated circuitry of signaling cascades that ultimately result in the transcription of new genes. Along the way, extracellular and intracellular signaling components function to impart a fully activated state. Evidence is accumulating to show that the Notch family of cell surface receptors, long known to function as transcriptional regulators through their interactions with the canonical nuclear binding protein CSL/RBP-J, may also be playing an as-yet-unappreciated role in T cell activation by virtue of its signaling via non-canonical as well as nonnuclear mechanisms. In this review we will discuss these and other better-known means by which Notch signaling influences T cell responses.

Hypo- and hyperactivated Notch signaling induce a glycolytic switch through distinct mechanisms

A switch from oxidative phosphorylation to glycolysis is frequently observed in cancer cells and is linked to tumor growth and invasion, but the underpinning molecular mechanisms controlling the switch are poorly understood. In this report we show that Notch signaling is a key regulator of cellular metabolism. Both hyper- and hypoactivated Notch induce a glycolytic phenotype in breast tumor cells, although by distinct mechanisms: hyperactivated Notch signaling leads to increased glycolysis through activation of the phosphatidylinositol 3-kinase/AKT serine/threonine kinase pathway, whereas hypoactivated Notch signaling attenuates mitochondrial activity and induces glycolysis in a p53-dependent manner. Despite the fact that cells with both hyper- and hypoactivated Notch signaling showed enhanced glycolysis, only cells with hyperactivated Notch promoted aggressive tumor growth in a xenograft mouse model. This phenomenon may be explained by that only Notch-hyperactivated, but not -hypoactivated, cells retained the capacity to switch back to oxidative phosphorylation. In conclusion, our data reveal a role for Notch in cellular energy homeostasis, and show that Notch signaling is required for metabolic flexibility.

Trp53 regulates Notch 4 signaling through Mdm2

Notch receptors and their ligands have crucial roles in development and tumorigenesis. We present evidence demonstrating the existence of an antagonistic relationship between Notch 4 and Trp53, which is controlled by the Mdm2-dependent ubiquitylation and degradation of the Notch receptor. We show that this signal-controlling mechanism is mediated by physical interactions between Mdm2 and Notch 4 and suggest the existence of a trimeric complex between Trp53, Notch 4 and Mdm2, which ultimately regulates Notch activity. Functional studies indicate that Trp53 can suppress NICD4-induced anchorage-independent growth in mammary epithelial cells and present evidence showing that Trp53 has a pivotal role in the suppression of Notch-associated tumorigenesis in the mammary gland.

Expression of truncated eukaryotic initiation factor 3e (eIF3e) resulting from integration of mouse mammary tumor virus (MMTV) causes a shift from cap-dependent to cap-independent translation

Integration of mouse mammary tumor virus (MMTV) at the common integration site Int6 occurs in the gene encoding eIF3e, the p48 subunit of translation initiation factor eIF3. Integration is at any of several introns of the Eif3e gene and causes the expression of truncated Eif3e mRNAs. Ectopic expression of the truncated eIF3e protein resulting from integration at intron 5 (3e5) induces malignant transformation, but by an unknown mechanism. Because eIF3e makes up at least part of the binding site for eIF4G, we examined the effects of 3e5 expression on protein synthesis. We developed an NIH3T3 cell line that contains a single copy of the 3e5 sequence at a predetermined genomic site. Co-immunoprecipitation indicated diminished binding of eIF3 to eIF4G, signifying a reduction in recruitment of the mRNA-unwinding machinery to the 43 S preinitiation complex. Cell growth and overall protein synthesis were decreased. Translation driven by the eIF4G-independent hepatitis C virus internal ribosome entry sequence (HCV IRES) in a bicistronic mRNA was increased relative to cap-dependent translation. Endogenous mRNAs encoding XIAP, c-Myc, CYR61, and Pim-1, which are translated in a cap-independent manner, were shifted to heavier polysomes whereas mRNAs encoding GAPDH, actin, L32, and L34, which are translated in a cap-dependent manner, were shifted to lighter polysomes. We propose that expression of 3e5 diminishes eIF4G interaction with eIF3 and causes abnormal gene expression at the translational level. The correlation between up-regulation of cap-independent translation and MMTV-induced tumorigenesis contrasts with the well established model for malignant transformation involving up-regulation of highly cap-dependent translation.

Expression of Notch receptors, ligands, and target genes during development of the mouse mammary gland

Notch genes play a critical role in mammary gland growth, development and tumorigenesis. In the present study, we have quantitatively determined the levels and mRNA expression patterns of the Notch receptor genes, their ligands and target genes in the postnatal mouse mammary gland. The steady state levels of Notch3 mRNA are the highest among receptor genes, Jagged1 and Dll3 mRNA levels are the highest among ligand genes and Hey2 mRNA levels are highest among expressed Hes/Hey target genes analyzed during different stages of postnatal mammary gland development. Using an immunohistochemical approach with antibodies specific for each Notch receptor, we show that Notch proteins are temporally regulated in mammary epithelial cells during normal mammary gland development in the FVB/N mouse. The loss of ovarian hormones is associated with changes in the levels of Notch receptor mRNAs (Notch2 higher and Notch3 lower) and ligand mRNAs (Dll1 and Dll4 are higher, whereas Dll3 and Jagged1 are lower) in the mammary gland of ovariectomized mice compared to intact mice. These data define expression of the Notch ligand/receptor system throughout development of the mouse mammary gland and help set the stage for genetic analysis of Notch in this context.

Notch-induced mammary tumorigenesis does not involve the lobule-limited epithelial progenitor

The mouse mammary epithelial cell hierarchy contains both multipotent stem cell as well as lineage-limited duct and lobular progenitor cell functions. The latter-also termed parity-identified mammary epithelial cells (PI-MECs)-are marked by beta-galactosidase (β Gal) expression following pregnancy and involution in whey acidic protein promoter (WAP)-Cre/Rosa26-flox-stop-flox-lacZ (WC/R26) mice, and are the targets of tumorigenic transformation in mouse mammary tumor virus-erbB2 transgenic mice. In this study, we demonstrate that an epithelial population distinct from PI-MECs is transformed during WAP-Int3 tumorigenesis. As expected, WAP-Int3/WC/R26 triple-transgenic mice failed to undergo secretory alveolar development, failed to lactate and developed mammary tumors. Following pregnancy and involution, β Gal+ mammary epithelial cells were found in the normal mammary tissue, but the resulting mammary tumors were all β Gal-. WAP-Int3/WC/R26 mammary glands contained ample estrogen receptor alpha (ERα)+ MECs, but only rare (<1%) progesterone receptor (PR)+ and RANKL+ cells. In addition, dissociated MECs from WAP-Int3/WC/R26 glands failed to regenerate a mammary tree upon transplantation into a cleared fat-pad of a nu/nu recipient mouse. However, when mixed with normal MECs, PI-MECs from WAP-Int3/WC/R26 mice contributed progeny to the resulting functional outgrowth. The WAP-Int3/WC/R26-derived PI-MECs displayed all of the properties of fully functional lobular progenitors including giving rise to ERα+, PR+, smooth muscle actin+ and RANKL+ epithelial progeny. These results demonstrate that WAP-Int3 has no oncogenic effect upon PI-MECs and that the expansion of functional lobular progenitors is required for secretory alveolar development and lactation. Furthermore, lobular progenitor function is ultimately controlled by signals within its microenvironment.

Notch signaling as a therapeutic target for breast cancer treatment?

Aberrant Notch signaling can induce mammary gland carcinoma in transgenic mice, and high expressions of Notch receptors and ligands have been linked to poor clinical outcomes in human patients with breast cancer. This suggests that inhibition of Notch signaling may be beneficial for breast cancer treatment. In this review, we critically evaluate the evidence that supports or challenges the hypothesis that inhibition of Notch signaling would be advantageous in breast cancer management. We find that there are many remaining uncertainties that must be addressed experimentally if we are to exploit inhibition of Notch signaling as a treatment approach in breast cancer. Nonetheless, Notch inhibition, in combination with other therapies, is a promising avenue for future management of breast cancer. Furthermore, since aberrant Notch4 activity can induce mammary gland carcinoma in the absence of RBPjκ, a better understanding of the components of RBPjκ-independent oncogenic Notch signaling pathways and their contribution to Notch-induced tumorigenesis would facilitate the deployment of Notch inhibition strategies for effective treatment of breast cancer.

Characterization and comparison of protein complexes initiated by the intracellular domain of individual Notch paralogs

The Notch signaling pathway is essential for embryonic development, organogenesis, and tissue homeostasis. Aberrant Notch signaling is associated with several types of cancers. The active form of Notch receptor is its intracellular domain (NICD), which is released from the cell membrane by serial proteolytic cleavages following ligand binding. Dose-dependent effects of NICD on cellular phenotypes have been observed under several conditions although the underlying mechanisms have not been well studied. Moreover, there are four mammalian Notch paralogs that have redundant as well as unique functions. The molecular basis for this variability is also not well understood. In this study, we used size exclusion chromatography to examine the overall distribution of NICD among NICD-containing protein complexes under conditions of increasing NICD abundance. We found that the assembly of NICD protein complexes was dose-dependent and that the abundance of the canonical complex was limited by, MAML, one of the proteins involved in the formation of canonical NICD transactivation complex, which became saturated with increasing NICD abundance. In addition, N4ICD showed a unique elution profile among the four NICDs. These results help to explain the dose-dependent and paralog-specific activities of NICD. These results are informative for the development of new reagents to block Notch signaling for therapeutic benefit.

Complex interplay between β-catenin signalling and Notch effectors in intestinal tumorigenesis

AIMS

The activation of β-catenin signalling is a key step in intestinal tumorigenesis. Interplay between the β-catenin and Notch pathways during tumorigenesis has been reported, but the mechanisms involved and the role of Notch remain unclear.

METHODS

Notch status was analysed by studying expression of the Notch effector Hes1 and Notch ligands/receptors in human colorectal cancer (CRC) and mouse models of Apc mutation. A genetic approach was used, deleting the Apc and RBP-J or Atoh1 genes in murine intestine. CRC cell lines were used to analyse the control of Hes1 and Atoh1 by β-catenin signalling.

RESULTS

Notch signalling was found to be activated downstream from β-catenin. It was rapidly induced and maintained throughout tumorigenesis. Hes1 induction was mediated by β-catenin and resulted from both the induction of the Notch ligand/receptor and Notch-independent control of the Hes1 promoter by β-catenin. Surprisingly, the strong phenotype of unrestricted proliferation and impaired differentiation induced by acute Apc deletion in the intestine was not rescued by conditional Notch inactivation. Hyperactivation of β-catenin signalling overrode the forced differention induced by Notch inhibition, through the downregulation of Atoh1, a key secretory determinant factor downstream of Notch. This process involves glycogen synthase kinase 3 β (GSK3β) and proteasome-mediated degradation. The restoration of Atoh1 expression in CRC cell lines displaying β-catenin activation was sufficient to increase goblet cell differentiation, whereas genetic ablation of Atoh1 greatly increased tumour formation in Apc mutant mice.

CONCLUSION

Notch signalling is a downstream target of β-catenin hyperactivation in intestinal tumorigenesis. However, its inhibition had no tumour suppressor effect in the context of acute β-catenin activation probably due to the downregulation of Atoh1. This finding calls into question the use of γ-secretase inhibitors for the treatment of CRC and suggests that the restoration of Atoh1 expression in CRC should be considered as a therapeutic approach.

Breast cancer stem cells: something out of notching?

We and others have established that the developmental Notch receptor signaling pathway is active in breast cancer cell lines, as well as in preinvasive and invasive primary samples. Recently, a role for Notch in regulating the hierarchy of stem and progenitor cells in both normal and cancer epithelium has been elucidated. Because inhibiting the Notch receptor signaling pathway is a possible future breast cancer therapy, here, we review the expression and activity of the different ligands and receptors and summarize the various ways in which the pathway's activity can be inhibited, and the likely effects of inhibition on different tumor cell subpopulations.

Non-canonical activation of Notch signaling/target genes in vertebrates

Evolutionarily conserved Notch signaling orchestrates diverse physiological mechanisms during metazoan development and homeostasis. Classically, ligand-activated Notch receptors transduce the signaling cascade through the interaction of DNA-bound CBF1-co-repressor complex. However, recent reports have demonstrated execution of a CBF1-independent Notch pathway through signaling cross-talks in various cells/tissues. Here, we have tried to congregate the reports that describe the non-canonical/CBF1-independent Notch signaling and target gene activation in vertebrates with specific emphasis on their functional relevance.

Epithelial transformation by KLF4 requires Notch1 but not canonical Notch1 signaling

The transcription factors Notch1 and KLF4 specify epithelial cell fates and confer stem cell properties. Suggesting a functional relationship, each gene can act to promote or suppress tumorigenesis in a context-dependent manner, and alteration of KLF4 or Notch pathway genes in mice gives rise to similar phenotypes. Activation of a conditional allele of KLF4 in RK3E epithelial cells rapidly induces expression of Notch1 mRNA and the active, intracellular form of Notch1. KLF4-induced transformation was suppressed by knockdown of endogenous Notch1 using siRNA or an inhibitor of gamma-secretase. Chromatin immunoprecipitation assay shows that KLF4 binds to the proximal Notch1 promoter in human mammary epithelial cells, and siRNA-mediated suppression of KLF4 in human mammary cancer cells results in reduced expression of Notch1. Furthermore, KLF4 and Notch1 expression are correlated in primary human breast tumors (N = 89; Pearson analysis, r > 0.5, p < 0.0001). Like KLF4, Notch1 was previously shown to induce transformation of rat cells immortalized with adenovirus E1A, similar to RK3E cells. We therefore compared the signaling requirements for Notch1- or KLF4-induced malignant transformation of RK3E. As expected, transformation by Notch1 was suppressed by dominant-negative CSL or MAML1, inhibitors of canonical Notch1 signaling. However, these inhibitors did not suppress transformation by KLF4. Therefore, while KLF4-induced transformation requires Notch1, canonical Notch1 signaling is not required, and Notch1 may signal through a distinct pathway in cells with increased KLF4 activity. These results suggest that KLF4 could contribute to breast tumor progression by activating synthesis of Notch1 and by promoting signaling through a non-canonical Notch1 pathway.

Generation and initial characterization of the prolactin-inducible protein (PIP) null mouse: accompanying global changes in gene expression in the submandibular gland

The human prolactin-inducible protein/gross cystic disease fluid protein-15 (hPIP/GCDFP-15) is a secretory glycoprotein found primarily in apocrine tissues including the breast and salivary glands. With largely unknown functions, PIP has been implicated in breast cancer and metastasis, host defense processes and T lymphocyte apoptosis. To begin to address PIP function in vivo, we generated the PIP null mouse (Pip-/-). Additionally, to determine the effect of the loss of PIP on gene expression and to gain insight into some of the molecular mechanisms underlying PIP function, microarray analysis of the submandibular gland was also undertaken. Pip-/- mice developed normally with no overt differences in behaviour or gross morphology and were fertile. However, histological examination of 3-month-old Pip-/- mice sometimes showed enlarged submandibular lymph nodes, lymphocytic aggregations within the prostate lobes, and enlarged medulla in the thymus. Functional analysis of gene expression revealed sets of multiple differentially expressed genes associated with cell death and survival, lipid metabolism, inflammation, immune disease, and cancer, as a consequence of mPIP abrogation. Taken together, these studies lend support to an immunomodulatory role for PIP in vivo and provide further insights into potentially novel signaling pathways and regulatory networks for PIP.

Notch signaling in solid tumors

In recent years a substantial body of evidence derived from not only preclinical but also clinical studies has accumulated in support of Notch signaling playing important oncogenic roles in several types of cancer. The finding that activating Notch mutations are frequently found in patients suffering from acute lymphoblastic leukemia is one of the best examples for a critical role of Notch signaling in cancer, a fact that motivated many researchers and clinicians to study the role of Notch also in solid tumors. Hence Notch signaling has gained increasing attention as a potential therapeutic target. In this book chapter we would like to discuss our current knowledge of Notch signaling within different types of solid cancers as well as advantages and disadvantages of potential new therapies that try to target the oncogenic properties of Notch signaling.