Notch proteins are important in binary cell-fate decisions and inhibiting differentiation in many developmental systems, and aberrant Notch signaling is associated with tumorigenesis. The role of Notch signaling in mammalian skin is less well characterized and is mainly based on in vitro studies, which suggest that Notch signaling induces differentiation in mammalian skin. Conventional gene targeting is not applicable to establishing the role of Notch receptors or ligands in the skin because Notch1-/- embryos die during gestation. Therefore, we used a tissue-specific inducible gene-targeting approach to study the physiological role of the Notch1 receptor in the mouse epidermis and the corneal epithelium of adult mice. Unexpectedly, ablation of Notch1 results in epidermal and corneal hyperplasia followed by the development of skin tumors and facilitated chemical-induced skin carcinogenesis. Notch1 deficiency in skin and in primary keratinocytes results in increased and sustained expression of Gli2, causing the development of basal-cell carcinoma-like tumors. Furthermore, Notch1 inactivation in the epidermis results in derepressed beta-catenin signaling in cells that should normally undergo differentiation. Enhanced beta-catenin signaling can be reversed by re-introduction of a dominant active form of the Notch1 receptor. This leads to a reduction in the signaling-competent pool of beta-catenin, indicating that Notch1 can inhibit beta-catenin-mediated signaling. Our results indicate that Notch1 functions as a tumor-suppressor gene in mammalian skin.

Notch proteins influence cell fate decisions in many developmental systems. During lymphoid development, Notch1 signaling is essential to direct a bipotent T/B precursor toward the T cell fate, but the role of Notch1 at later stages of T cell development remains controversial. We have recently reported that tissue-specific inactivation of Notch1 in immature (CD44(-) CD25(+)) thymocytes does not affect subsequent T cell development. Here, we demonstrate that loss of Notch1 signaling at an earlier (CD44(+)CD25(+)) developmental stage results in severe perturbation of alpha beta but not gamma delta lineage development. Immature Notch1(-/-) thymocytes show impaired VDJ beta rearrangement and aberrant pre-TCR-independent survival. Collectively, our data demonstrate that Notch1 controls several nonredundant functions necessary for alpha beta lineage development.

Notch proteins influence cell-fate decisions in many developing systems. Several gain-of-function studies have suggested a critical role for Notch 1 signaling in CD4-CD8 lineage commitment, maturation and survival in the thymus. However, we show here that tissue-specific inactivation of the gene encoding Notch 1 in immature (CD25+CD44-)T cell precursors does not affect subsequent thymocyte development. Neither steady-state numbers nor the rate of production of CD4+ and CD8+ mature thymocytes is perturbed in the absence of Notch 1. In addition, Notch 1-deficient thymocytes are normally sensitive to spontaneous or glucocorticoid-induced apoptosis. In contrast to earlier reports, these data formally exclude an essential role for Notch 1 in CD4-CD8 lineage commitment, maturation or survival.

Human tumors often contain slowly proliferating cancer cells that resist treatment, but we do not know precisely how these cells arise. We show that rapidly proliferating cancer cells can divide asymmetrically to produce slowly proliferating "G0-like" progeny that are enriched following chemotherapy in breast cancer patients. Asymmetric cancer cell division results from asymmetric suppression of AKT/PKB kinase signaling in one daughter cell during telophase of mitosis. Moreover, inhibition of AKT signaling with smallmolecule drugs can induce asymmetric cancer cell division and the production of slow proliferators. Cancer cells therefore appear to continuously flux between symmetric and asymmetric division depending on the precise state of their AKT signaling network. This model may have significant implications for understanding how tumors grow, evade treatment, and recur.quiescence | epigenetics | cell signaling | drug resistance T umors generally evolve through years of mutation and clonal selection (1). This favors the outgrowth of rapidly proliferating cancer cells over time. However, even advanced tumors contain many cancer cells that appear to be proliferating slowly (2). This proliferative heterogeneity correlates closely with time to clinical detection, growth, metastasis, and treatment response across all tumor types, but we still do not understand clearly how it arises. The rate of mammalian cell proliferation is generally determined by the time spent in G1 of the cell cycle. Critical genetic and epigenetic changes within cancer cells accelerate G1 transit, whereas a suboptimal microenvironment with imbalance of growth factors, nutrients, or oxygen can slow G1 progression (3). Therefore, individual cancer cells within a tumor are thought to vary significantly in their proliferative rate depending on the precise balance of these intrinsic and extrinsic factors. Interestingly, however, many tumor-derived cancer cell lines also produce slowly proliferating cells. These established lines have many acquired mutations that drive cell proliferation. They have also been grown ex vivo for years in a stable microenvironment to promote unbridled proliferation. These factors ought to favor a strong purifying selection against slow proliferators. We worked to understand how slowly proliferating cells seem to arise paradoxically in cancer cell lines.Results G0-Like Cancer Cells in Vitro. We began by studying MCF7, a highly proliferative, aneuploid, ER + /HER2 − human breast cancer cell line. This line displays significant proliferative heterogeneity despite mutations in CDKN2A and PIK3CA that cooperatively drive cell-cycle progression (4). We first hypothesized that slowly proliferating MCF7 cells might produce low levels of reactive oxygen species (ROS). This hypothesis was based on previous observations that slowly cycling hematopoietic, neural, and breast adult stem cells and cancer stem cells produce low levels of ROS (5-7). We stained MCF7 cells with 5-(and-6)chloromethyl 1-2′,7′-dichlorohydrofluorescein diacet...