Lucio Miele

Notch signaling is a direct determinant of keratinocyte growth arrest and entry into differentiation

The role of Notch signaling in growth/differentiation control of mammalian epithelial cells is still poorly defined. We show that keratinocyte‐specific deletion of the Notch1 gene results in marked epidermal hyperplasia and deregulated expression of multiple differentiation markers. In differentiating primary keratinocytes in vitro endogenous Notch1 is required for induction of p21WAF1/Cip1 expression, and activated Notch1 causes growth suppression by inducing p21WAF1/Cip1 expression. Activated Notch1 also induces expression of ‘early’ differentiation markers, while suppressing the late markers. Induction of p21WAF1/Cip1 expression and early differentiation markers occur through two different mechanisms. The RBP‐Jκ protein binds directly to the endogenous p21 promoter and p21 expression is induced specifically by activated Notch1 through RBP‐Jκ‐dependent transcription. Expression of early differentiation markers is RBP‐Jκ‐independent and can be induced by both activated Notch1 and Notch2, as well as the highly conserved ankyrin repeat domain of the Notch1 cytoplasmic region. Thus, Notch signaling triggers two distinct pathways leading to keratinocyte growth arrest and differentiation.

Activation of Notch-1 signaling maintains the neoplastic phenotype in human Ras-transformed cells

Truncated Notch receptors have transforming activity in vitro and in vivo. However, the role of wild-type Notch signaling in neoplastic transformation remains unclear. Ras signaling is deregulated in a large fraction of human malignancies and is a major target for the development of novel cancer treatments. We show that oncogenic Ras activates Notch signaling and that wild-type Notch-1 is necessary to maintain the neoplastic phenotype in Ras-transformed human cells in vitro and in vivo. Oncogenic Ras increases levels and activity of the intracellular form of wild-type Notch-1, and upregulates Notch ligand Delta-1 and also presenilin-1, a protein involved in Notch processing, through a p38-mediated pathway. These observations place Notch signaling among key downstream effectors of oncogenic Ras and suggest that it might be a novel therapeutic target.

The nervous system and innate immunity: the neuropeptide connection

Notch receptors are processed by γ-secretase acting in synergy with T cell receptor signaling to sustain peripheral T cell activation. Activated CD4+ T cells differentiate into T helper type 1 (TH1) or TH2 subsets. Molecular cues directing TH1 differentiation include expression of the TH1-specific transcription factor T-bet, encoded by Tbx21. However, the regulation of Tbx21 remains incompletely defined. Here we report that Notch1 can directly regulate Tbx21 through complexes formed on the Tbx21 promoter. In vitro, γ-secretase inhibitors extinguished expression of Notch, interferon-γ and Tbx21 in TH1-polarized CD4+ cells, whereas ectopic expression of activated Notch1 restored Tbx21 transcription. In vivo, administration of γ-secretase inhibitors substantially impeded TH1-mediated disease progression in the mouse experimental autoimmune encephalomyelitis model of multiple sclerosis. Thus, using γ-secretase inhibitors to modulate Notch signaling may prove beneficial in treating TH1-mediated autoimmunity.

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.

Arbiter of differentiation and death: Notch signaling meets apoptosis

Notch‐ligand interactions are a highly conserved mechanism that regulates cell fate decisions. Over the past few years, numerous observations have shown that this mechanism operates to regulate cell differentiation in an enormous variety of developmental and cell maturation processes. Recent studies indicate that in addition to cell differentiation, Notch signaling has direct effects on proliferation and programmed cell death. The picture emerging from these findings suggests that, depending on cellular and developmental context, Notch signaling may function as a general “arbiter” of cell fate, regulating differentiation potential, rate of proliferation, and apoptotic cell death. In this review, we briefly summarize the current knowledge of the structure and function of Notch receptors and discuss the recent evidence that Notch signaling regulates apoptotic cell death. The possible mechanisms of this effect and its potential implications for developmental biology, immunobiology, neuropathology, and tumor biology are discussed. J. Cell. Physiol. 181:393–409, 1999.

Cross-talk between Notch and the Estrogen Receptor in Breast Cancer Suggests Novel Therapeutic Approaches

High expression of Notch-1 and Jagged-1 mRNA correlates with poor prognosis in breast cancer. Elucidating the cross-talk between Notch and other major breast cancer pathways is necessary to determine which patients may benefit from Notch inhibitors, which agents should be combined with them, and which biomarkers indicate Notch activity in vivo. We explored expression of Notch receptors and ligands in clinical specimens, as well as activity, regulation, and effectors of Notch signaling using cell lines and xenografts. Ductal and lobular carcinomas commonly expressed Notch-1, Notch-4, and Jagged-1 at variable levels. However, in breast cancer cell lines, Notch-induced transcriptional activity did not correlate with Notch receptor levels and was highest in estrogen receptor alpha-negative (ERalpha(-)), Her2/Neu nonoverexpressing cells. In ERalpha(+) cells, estradiol inhibited Notch activity and Notch-1(IC) nuclear levels and affected Notch-1 cellular distribution. Tamoxifen and raloxifene blocked this effect, reactivating Notch. Notch-1 induced Notch-4. Notch-4 expression in clinical specimens correlated with proliferation (Ki67). In MDA-MB231 (ERalpha(-)) cells, Notch-1 knockdown or gamma-secretase inhibition decreased cyclins A and B1, causing G(2) arrest, p53-independent induction of NOXA, and death. In T47D:A18 (ERalpha(+)) cells, the same targets were affected, and Notch inhibition potentiated the effects of tamoxifen. In vivo, gamma-secretase inhibitor treatment arrested the growth of MDA-MB231 tumors and, in combination with tamoxifen, caused regression of T47D:A18 tumors. Our data indicate that combinations of antiestrogens and Notch inhibitors may be effective in ERalpha(+) breast cancers and that Notch signaling is a potential therapeutic target in ERalpha(-) breast cancers.

Targeting the NF-κB signaling pathway in Notch1-induced T-cell leukemia

T-cell acute lymphoblastic leukemia (T-ALL), unlike other ALL types, is only infrequently associated with chromosomal aberrations, but it was recently shown that most individuals with T-ALL carry activating mutations in the NOTCH1 gene. However, the signaling pathways and target genes responsible for Notch1-induced neoplastic transformation remain undefined. We report here that constitutively active Notch1 activates the NF-κB pathway transcriptionally and via the IκB kinase (IKK) complex, thereby causing increased expression of several well characterized target genes of NF-κB in bone marrow hematopoietic stem cells and progenitors. Our observations demonstrate that the NF-κB pathway is highly active in established human T-ALL and that inhibition of the pathway can efficiently restrict tumor growth both in vitro and in vivo. These findings identify NF-κB as one of the major mediators of Notch1-induced transformation and suggest that the NF-κB pathway is a potential target of future therapies of T-ALL.

Proteasome inhibitors trigger NOXA-mediated apoptosis in melanoma and myeloma cells

Patients with metastatic melanoma or multiple myeloma have a dismal prognosis because these aggressive malignancies resist conventional treatment. A promising new oncologic approach uses molecularly targeted therapeutics that overcomes apoptotic resistance and, at the same time, achieves tumor selectivity. The unexpected selectivity of proteasome inhibition for inducing apoptosis in cancer cells, but not in normal cells, prompted us to define the mechanism of action for this class of drugs, including Food and Drug Administration-approved bortezomib. In this report, five melanoma cell lines and a myeloma cell line are treated with three different proteasome inhibitors (MG-132, lactacystin, and bortezomib), and the mechanism underlying the apoptotic pathway is defined. Following exposure to proteasome inhibitors, effective killing of human melanoma and myeloma cells, but not of normal proliferating melanocytes, was shown to involve p53-independent induction of the BH3-only protein NOXA. Induction of NOXA at the protein level was preceded by enhanced transcription of NOXA mRNA. Engagement of mitochondrial-based apoptotic pathway involved release of cytochrome c, second mitochondria-derived activator of caspases, and apoptosis-inducing factor, accompanied by a proteolytic cascade with processing of caspases 9, 3, and 8 and poly(ADP)-ribose polymerase. Blocking NOXA induction using an antisense (but not control) oligonucleotide reduced the apoptotic response by 30% to 50%, indicating a NOXA-dependent component in the overall killing of melanoma cells. These results provide a novel mechanism for overcoming the apoptotic resistance of tumor cells, and validate agents triggering NOXA induction as potential selective cancer therapeutics for life-threatening malignancies such as melanoma and multiple myeloma.

Notch-1 Regulates NF-κB Activity in Hemopoietic Progenitor Cells

We investigated the interaction between two elements critical for differentiation of hemopoietic cells, the Notch-1 receptor and the transcription factor NF-κB. These factors were studied in hemopoietic progenitor cells (HPC) using Notch-1 antisense transgenic (Notch-AS-Tg) mice. DNA binding of NF-κB as well as its ability to activate transcription was strongly decreased in HPC from Notch-AS-Tg mice. NF-κB-driven transcriptional activity was completely restored after transduction of the cells with retroviral constructs containing activated Notch-1 gene. HPC from Notch-AS-Tg mice have decreased levels of several members of the NF-κB family, p65, p50, RelB, and c-Rel and this is due to down-regulation of the gene expression. To investigate functional consequences of decreased NF-κB activity in transgenic mice, we studied LPS-induced proliferation of B cells and GM-CSF-dependent differentiation of dendritic cells from HPC. These two processes are known to be closely dependent on NF-κB. B cells from Notch-AS-Tg mice had almost 3-fold lower response to LPS than B cells isolated from control mice. Differentiation of dendritic cells was significantly affected in Notch-AS-Tg mice. However, it was restored by transduction of activated Notch-1 into HPC. Taken together, these data indicate that in HPC NF-κB activity is regulated by Notch-1 via transcriptional control of NF-κB.

Gamma secretase inhibitor blocks Notch activation and induces apoptosis in Kaposi's sarcoma tumor cells

Kaposis sarcoma (KS) is a common neoplasm in HIV-1-infected individuals causing significant morbidity and mortality. Despite recent advances, the pathogenesis of this potentially life-threatening neoplasm remains unclear, and there is currently no cure for KS. Notch proteins are known to play a fundamental role in cell fate decisions including proliferation, differentiation, and apoptosis. It is, therefore, not surprising that Notch proteins have been implicated in tumorigenesis and appear to function as either oncogenes or tumor suppressor proteins depending on cellular context. In this report, we demonstrate elevated levels of activated Notch-1, -2, and -4 in KS tumor cells in vivo and in vitro compared to endothelial cells, the precursor of the KS cell. Notch activation was confirmed through luciferase reporter assays and localization of Hes (Hairy/Enhancer of Split)-1 and Hey (Hairy/Enhancer of Split related with YRPW)1 (primary targets of the Notch pathway) in KS cell nuclei. Studies using γ-secretase inhibitors (GSI and LY-411,575), which block Notch activation, resulted in apoptosis in primary and immortalized KS cells. Similar studies injecting GSI into established KS cell tumors on mice demonstrated growth inhibition or tumor regression that was characterized by apoptosis in treated, but not control tumors. The results indicate that KS cells overexpress activated Notch and interruption of Notch signaling inhibits KS cell growth. Thus, targeting Notch signaling may be of therapeutic value in KS patients.