Notch1-induced mammary tumor development is cyclin D1-dependent and correlates with expansion of pre-malignant multipotent duct-limited progenitors

Breast cancer stem cells

Breast cancer stem cells (BCSCs) constitute a subpopulation of tumor cells that express stem cell-associated markers and have a high capacity for tumor generation in vivo. Identification of BCSCs from tumor samples or breast cancer cell lines has been based mainly on CD44(+)/CD24(-/low) or ALDH(+) phenotypes. BCSCs isolation has allowed the analysis of the molecular mechanisms involved in their origin, self-renewal, differentiation into tumor cells, resistance to radiation therapy and chemotherapy, and invasiveness and metastatic ability. Molecular genetic analysis using knockout animals and inducible transgenics has identified NF-κB, c-Jun, p21(CIP1), and Forkhead-like-protein Dach1 involvement in BCSC expansion and fate. Clinical analyses of BCSCs in breast tumors have found a correlation between the proportion of BCSCs and poor prognosis. Therefore, new therapies that specifically target BCSCs are an urgent need. We summarize recent evidence that partially explain the biological characteristics of BCSCs.

Functionally recurrent rearrangements of the MAST kinase and Notch gene families in breast cancer

Breast cancer is a heterogeneous disease, exhibiting a wide range of molecular aberrations and clinical outcomes. Here we employed paired-end transcriptome sequencing to explore the landscape of gene fusions in a panel of breast cancer cell lines and tissues. We observed that individual breast cancers harbor an array of expressed gene fusions. We identified two classes of recurrent gene rearrangements involving microtubule associated serine-threonine kinase (MAST) and Notch family genes. Both MAST and Notch family gene fusions exerted significant phenotypic effects in breast epithelial cells. Breast cancer lines harboring Notch gene rearrangements are uniquely sensitive to inhibition of Notch signaling, and over-expression of MAST1 or MAST2 gene fusions had a proliferative effect both in vitro and in vivo. These findings illustrate that recurrent gene rearrangements play significant roles in subsets of carcinomas and suggest that transcriptome sequencing may serve to identify patients with rare, actionable gene fusions.

Examining the role of cyclin D1 in breast cancer

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration and inhibit mitochondrial metabolism. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the significance of cyclin D1 in breast cancer with emphasis on its role in breast cancer stem cell expansion.

Notch regulation of tumor angiogenesis

The growth of new blood vessels (angiogenesis) is critical for tumor growth and progression. The highly conserved Notch signaling pathway is involved in a variety of cell fate decisions and regulates many cellular biological processes, including angiogenesis. Aberrant Notch signaling has also been implicated in tumorigenesis. Notch ligands and receptors are expressed on many different cell types present within the tumor, including tumor cells and the stromal compartment. This article highlights in particular the various mechanisms by which Notch signaling can mediate tumor angiogenesis. The most studied Notch ligands, Delta-like 4 and Jagged1, competitively regulate tumor angiogenesis. Studies have demonstrated that Delta-like 4 functions as a negative regulator of tumor angiogenesis, whereas Jagged1 promotes angiogenesis. Understanding the implications of Notch signaling in various tumor backgrounds will enable the effects of specific Notch signaling inhibition on tumor angiogenesis and growth to be evaluated as a potential for a novel antiangiogenic therapy in the clinic.

Notch signalling in solid tumours: a little bit of everything but not all the time

The discovery of Notch in Drosophila melanogaster nearly a century ago opened the door to an ever-widening understanding of cellular processes that are controlled or influenced by Notch signalling. As would be expected with such a pleiotropic pathway, the deregulation of Notch signalling leads to several pathological conditions, including cancer. A role for Notch is well established in haematological malignancies, and more recent studies have provided evidence for the importance of Notch activity in solid tumours. As it is thought to act as an oncogene in some cancers but as a tumour suppressor in others, the role of Notch in solid tumours seems to be highly context dependent.

The role of breast cancer stem cells in metastasis and therapeutic implications

Cancer stem cells (CSCs) possess the capacity to self-renew and to generate heterogeneous lineages of cancer cells that comprise tumors. A substantial body of evidence supports a model in which CSCs play a major role in the initiation, maintenance, and clinical outcome of cancers. In contrast, analysis of the role of CSCs in metastasis has been mainly conceptual and speculative. This review summarizes recent data that support the theory of CSCs as the source of metastatic lesions in breast cancer, with a focus on the key role of the microenvironment in the stemness-metastasis link.

Role of Notch and its oncogenic signaling crosstalk in breast cancer

The Notch signaling plays a key role in cell differentiation, survival, and proliferation through diverse mechanisms. Notch signaling is also involved in vasculogenesis and angiogenesis. Moreover, Notch expression is regulated by hypoxia and inflammatory cytokines (IL-1, IL-6 and leptin). Entangled crosstalk between Notch and other developmental signaling (Hedgehog and Wnt), and signaling triggered by growth factors, estrogens and oncogenic kinases, could impact on Notch targeted genes. Thus, alterations of the Notch signaling can lead to a variety of disorders, including human malignancies. Notch signaling is activated by ligand binding, followed by ADAM/tumor necrosis factor-α-converting enzyme (TACE) metalloprotease and γ-secretase cleavages that produce the Notch intracellular domain (NICD). Translocation of NICD into the nucleus induces the transcriptional activation of Notch target genes. The relationships between Notch deregulated signaling, cancer stem cells and the carcinogenesis process reinforced by Notch crosstalk with many oncogenic signaling pathways suggest that Notch signaling may be a critical drug target for breast and other cancers. Since current status of knowledge in this field changes quickly, our insight should be continuously revised. In this review, we will focus on recent advancements in identification of aberrant Notch signaling in breast cancer and the possible underlying mechanisms, including potential role of Notch in breast cancer stem cells, tumor angiogenesis, as well as its crosstalk with other oncogenic signaling pathways in breast cancer. We will also discuss the prognostic value of Notch proteins and therapeutic potential of targeting Notch signaling for cancer treatment.