Functional analysis of the TAN-1 gene, a human homolog of Drosophila notch

Notch Signaling in Acute Inflammation and Sepsis

Notch signaling, a highly conserved pathway in mammals, is crucial for differentiation and homeostasis of immune cells. Besides, this pathway is also directly involved in the transmission of immune signals. Notch signaling per se does not have a clear pro- or anti-inflammatory effect, but rather its impact is highly dependent on the immune cell type and the cellular environment, modulating several inflammatory conditions including sepsis, and therefore significantly impacts the course of disease. In this review, we will discuss the contribution of Notch signaling on the clinical picture of systemic inflammatory diseases, especially sepsis. Specifically, we will review its role during immune cell development and its contribution to the modulation of organ-specific immune responses. Finally, we will evaluate to what extent manipulation of the Notch signaling pathway could be a future therapeutic strategy.

Field cancerization: Definition, epidemiology, risk factors, and outcomes

Field cancerization was first described in 1953 when pathologic atypia was identified in clinically normal tissue surrounding oropharyngeal carcinomas. The discovery of mutated fields surrounding primary tumors raised the question of whether the development of subsequent tumors within the field represented recurrences or additional primary tumors. Since this initial study, field cancerization has been applied to numerous other epithelial tissues, including the skin. Cutaneous field cancerization occurs in areas exposed to chronic ultraviolet radiation, which leads to clonal proliferations of p53-mutated fields and is characterized by multifocal actinic keratoses, squamous cell carcinomas in situ, and cutaneous squamous cell carcinomas. In the first article in this continuing medical education series, we define field cancerization, review the available grading systems, and discuss the epidemiology, risk factors, and outcomes associated with this disease.

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.

Loss-of-function mutations in Notch receptors in cutaneous and lung squamous cell carcinoma

Squamous cell carcinomas (SCCs) are one of the most frequent forms of human malignancy, but, other than TP53 mutations, few causative somatic aberrations have been identified. We identified NOTCH1 or NOTCH2 mutations in ~75% of cutaneous SCCs and in a lesser fraction of lung SCCs, defining a spectrum for the most prevalent tumor suppressor specific to these epithelial malignancies. Notch receptors normally transduce signals in response to ligands on neighboring cells, regulating metazoan lineage selection and developmental patterning. Our findings therefore illustrate a central role for disruption of microenvironmental communication in cancer progression. NOTCH aberrations include frameshift and nonsense mutations leading to receptor truncations as well as point substitutions in key functional domains that abrogate signaling in cell-based assays. Oncogenic gain-of-function mutations in NOTCH1 commonly occur in human T-cell lymphoblastic leukemia/lymphoma and B-cell chronic lymphocytic leukemia. The bifunctional role of Notch in human cancer thus emphasizes the context dependency of signaling outcomes and suggests that targeted inhibition of the Notch pathway may induce squamous epithelial malignancies.

Proteolytic cleavage of Notch: "HIT and RUN"

The Notch pathway is a highly conserved signaling pathway in multicellular eukaryotes essential in controlling spatial patterning, morphogenesis and homeostasis in embryonic and adult tissues. Notch proteins coordinate cell-cell communication through receptor-ligand interactions between adjacent cells. Notch signaling is frequently deregulated by oncogenic mutation or overexpression in many cancer types. Notch activity is controlled by three sequential cleavage steps leading to ectodomain shedding and transcriptional activation. Here we review the key regulatory steps in the activation of Notch, from receptor maturation to receptor activation (HIT) via a rate-limiting proteolytic cascade (RUN) in the context of species-specific differences.

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 signalling in T-cell lymphoblastic leukaemia/lymphoma and other haematological malignancies

Notch receptors participate in a highly conserved signalling pathway that regulates normal development and tissue homeostasis in a context- and dose-dependent manner. Deregulated Notch signalling has been implicated in many diseases, but the clearest example of a pathogenic role is found in T-cell lymphoblastic leukaemia/lymphoma (T-LL), in which the majority of human and murine tumours have acquired mutations that lead to aberrant increases in Notch1 signalling. Remarkably, it appears that the selective pressure for Notch mutations is virtually unique among cancers to T-LL, presumably reflecting a special context-dependent role for Notch in normal T-cell progenitors. Nevertheless, there are some recent reports suggesting that Notch signalling has subtle, yet important roles in other forms of haematological malignancy as well. Here, we review the role of Notch signalling in various blood cancers, focusing on T-LL with an eye towards targeted therapeutics.

Therapeutic targeting of NOTCH1 signaling in T-cell acute lymphoblastic leukemia

The recent identification of activating mutations in NOTCH1 in the majority of T-cell acute lymphoblastic leukemias (T-ALLs) has brought major interest toward targeting the NOTCH signaling pathway in this disease. Small-molecule gamma-secretase inhibitors (GSIs), which block a critical proteolytic step required for NOTCH1 activation, can effectively block the activity of NOTCH1 mutant alleles. However, the clinical development of GSIs has been hampered by their low cytotoxicity against human T-ALL and the development of significant gastrointestinal toxicity derived from the inhibition of NOTCH signaling in the gut. Improved understanding of the oncogenic mechanisms of NOTCH1 and the effects of NOTCH inhibition in leukemic cells and the intestinal epithelium are required for the design of effective anti-NOTCH1 therapies in T-ALL.

Notch signaling in leukemia

Recent discoveries indicate that gain-of-function mutations in the Notch1 receptor are very common in human T cell acute lymphoblastic leukemia/lymphoma. This review discusses what these mutations have taught us about normal and pathophysiologic Notch1 signaling, and how these insights may lead to new targeted therapies for patients with this aggressive form of cancer.

Notch 1 activation in the molecular pathogenesis of T-cell acute lymphoblastic leukaemia

The chromosomal translocation t(7;9) in human T-cell acute lymphoblastic leukaemia (T-ALL) results in deregulated expression of a truncated, activated form of Notch 1 (TAN1) under the control of the T-cell receptor-beta (TCRB) locus. Although TAN1 efficiently induces T-ALL in mouse models, t(7;9) is present in less than 1% of human T-ALL cases. The recent discovery of novel activating mutations in NOTCH1 in more than 50% of human T-ALL samples has made it clear that Notch 1 is far more important in human T-ALL pathogenesis than previously suspected.

Notch1 oncoprotein antagonizes TGF-beta/Smad-mediated cell growth suppression via sequestration of coactivator p300

The Notch proteins constitute a family of transmembrane receptors that play a pivotal role in cellular differentiation, proliferation and apoptosis. Although it has been recognized that excess Notch signaling is potentially tumorigenic, little is known about precise mechanisms through which dysregulated Notch signaling induces neoplastic transformation. Here we demonstrate that Notch signaling has a transcriptional cross-talk with transforming growth factor-beta (TGF-beta) signaling, which is well characterized by its antiproliferative effects. TGF-beta-mediated transcriptional responses are suppressed by constitutively active Notch1, and this inhibitory effect is canceled by introduction of transcriptional coactivator p300. We further show that this blockade of TGF-beta signaling is executed by the sequestration of p300 from Smad3. Moreover, in a human cervical carcinoma cell line, CaSki, in which Notch1 is spontaneously activated, suppression of Notch1 expression with small interfering RNA significantly restores the responsiveness to TGF-beta. Taken together, we propose that Notch oncoproteins promote cell growth and cancer development partly by suppressing the growth inhibitory effects of TGF-beta through sequestrating p300 from Smad3.

T cell acute lymphoblastic leukemia/lymphoma: a human cancer commonly associated with aberrant NOTCH1 signaling

PURPOSE OF REVIEW

Although constitutively activated forms of the NOTCH1 receptor are potent inducers of T cell acute lymphoblastic leukemia/lymphoma when expressed in the bone marrow stem cells of mice, the known involvement of NOTCH1 in human T cell acute lymphoblastic leukemia/lymphoma has been restricted to very rare tumors associated with a (7;9) chromosomal translocation involving the NOTCH1 gene. This picture has changed dramatically in the past year with the discovery of frequent mutations involving NOTCH1 in human T cell acute lymphoblastic leukemia/lymphoma.

RECENT FINDINGS

NOTCH1 point mutations, insertions, and deletions producing aberrant increases in NOTCH1 signaling are frequently present in both childhood and adult T cell acute lymphoblastic leukemia/lymphoma and are detected in tumors from all major molecular subtypes. These observations are particularly important in the light of experiments using human and murine T cell acute lymphoblastic leukemia/lymphoma cell lines indicating that NOTCH1 signals are required for sustained growth and, in a subset of lines, survival. This inference is based in part on experiments conducted with small molecule inhibitors of gamma-secretase, a protease required for normal NOTCH signal transduction and the activity of the mutated forms of NOTCH1 found commonly in human T cell acute lymphoblastic leukemia/lymphoma.

SUMMARY

These findings support a central role for aberrant NOTCH signaling in the pathogenesis of human T cell acute lymphoblastic leukemia/lymphoma, and they provide a rationale for trials of NOTCH inhibitors, such as gamma-secretase antagonists, in this aggressive human malignancy.

Notch subunit heterodimerization and prevention of ligand-independent proteolytic activation depend, respectively, on a novel domain and the LNR repeats

Notch proteins are transmembrane receptors that participate in a highly conserved signaling pathway that regulates morphogenesis in metazoans. Newly synthesized Notch receptors are proteolytically cleaved during transit to the cell surface, creating heterodimeric mature receptors comprising noncovalently associated extracellular (N(EC)) and transmembrane (N) subunits. Ligand binding activates Notch by inducing two successive proteolytic cleavages, catalyzed by metalloproteases and gamma-secretase, respectively, that permit the intracellular portion of N to translocate to the nucleus and activate transcription of target genes. Prior work has shown that the presence of N(EC) prevents ligand-independent activation of N, but the mechanisms involved are poorly understood. Here, we define the roles of two regions at the C-terminal end of N(EC) that participate in maintaining the integrity of resting Notch receptors through distinct mechanisms. The first region, a hydrophobic, previously uncharacterized portion of N(EC), is sufficient to form stable complexes with the extracellular portion of N. The second region, consisting of the three Lin12/Notch repeats, is not needed for heterodimerization but acts to protect N from ligand-independent cleavage by metalloproteases. Together, these two contiguous regions of N(EC) impose crucial restraints that prevent premature Notch receptor activation.

Modulation of Notch signaling by mastermind-like (MAML) transcriptional co-activators and their involvement in tumorigenesis

Notch signaling is mediated by cell-cell interactions and is critical for cell fate determination in many species. Recently, a family of mastermind-like (MAML) transcriptional co-activator genes was identified that encode proteins that cooperate with Notch and CSL to activate transcription. Here, we review our current understanding of the roles of the MAML proteins in Notch signaling, and their involvement in certain human cancers. The mounting biochemical and functional evidence indicate that the MAML genes are critical components of the Notch signaling pathway, likely regulating cellular events involved in both normal development and oncogenesis.

Epidemiology and molecular pathology at crossroads to establish causation: molecular mechanisms of malignant transformation

Epidemiology is a very reliable science for the identification of carcinogens. Epidemiological studies require that the effect, cancer in this case, has already occurred, when of course it would be more desirable to identify potential carcinogenic substances at an earlier stage before they have caused a large number of malignancies and thus become identifiable by epidemiological studies. In the past 30 years, molecular pathology (which includes chemistry, biochemistry, molecular biology, molecular virology, molecular genetics, epigenetics, genomics, proteomics, and other molecular-based approaches) has identified some key alterations that are required for cellular transformation and malignancy. Agents that specifically interfere with some of these mechanisms are suspected human carcinogens. It can be stated that tumor formation requires the following steps: (1) inactivation of Rb and p53 cellular pathways; (2) activation of Ras and/or other growth promoting pathways; (3) inactivation of phosphatase 2A that causes changes in the phosphorylation and activity of several cellular proteins; (4) evasion of apoptosis; (5) telomerase activation or alternative mechanisms of cellular immortalization; (6) angiogenic activity; and (7) the ability to invade surrounding tissues and to metastasize. Here, we review the molecular mechanisms of cellular transformation. The integration of this knowledge with classical epidemiology and animal studies should permit a more rapid and accurate identification of human carcinogens.

Functional diversity ofnotchfamily genes in fetal lung development

In Drosophila, developmental signaling via the transmembrane Notch receptor modulates branching morphogenesis and neuronal differentiation. To determine whether the notch gene family can regulate mammalian organogenesis, including neuroendocrine cell differentiation, we evaluated developing murine lung. After demonstrating gene expression for notch-1, notch-2, notch-3, and the Notch ligands jagged-1 and jagged-2 in embryonic mouse lung, we tested whether altering expression of these genes can modulate branching morphogenesis. Branching of embryonic day (E) 11.5 lung buds increased when they were treated with notch-1 antisense oligodeoxynucleotides in culture compared with the corresponding sense controls, whereas notch-2, notch-3, jagged-1, or jagged-2 antisense oligos had no significant effect. To assess cell differentiation, we immunostained lung bud cultures for the neural/neuroendocrine marker PGP9.5. Antisense to notch-1 or jagged-1 markedly increased numbers of PGP9.5-positive neuroendocrine cells alone without affecting neural tissue, whereas only neural tissue was promoted by notch-3 antisense in culture. There was no significant effect on cell proliferation or apoptosis in these antisense experiments. Cumulatively, these observations suggest that interactions between distinct Notch family members can have diverse tissue-specific regulatory functions during development, arguing against simple functional redundancy.

Surface expression of Notch1 on thymocytes: correlation with the double-negative to double-positive transition

Notch1 plays a critical role in regulating T lineage commitment during the differentiation of lymphoid precursors. The physiological relevance of Notch1 signaling during subsequent stages of T cell differentiation has been more controversial. This is due in part to conflicting data from studies examining the overexpression or targeted deletion of Notch1 and to difficulties in distinguishing between the activities of multiple Notch family members and their ligands, which are expressed in the thymus. We employed a polyclonal antiserum against the extracellular domain of Notch1 to study surface expression during thymopoiesis. We found high levels of Notch1 on the cell surface only on double negative (DN) stage 2 through the immature single-positive stage of thymocyte development, before the double-positive (DP) stage. The Notch signaling pathway, as read out by Deltex1 expression levels, is highly active in DN thymocytes. When an active Notch1 transgene, Notch1IC, is exogenously introduced into thymocytes of recombinase-activating gene 2-deficient mice, it promotes proliferation and development to the DP stage following anti-CD3 treatment without apparently affecting the intensity of pre-TCR signaling. In addition, a stromal cell line expressing the Notch ligand, Delta-like-1, promotes the in vitro expansion of wild-type DN3 thymocytes in vitro. Consistent with other recent reports, these data suggest a role for Notch1 during the DN to DP stage of thymocyte maturation and suggest a cellular mechanism by which Notch1IC oncogenes could contribute to thymoma development and maintenance.

Differential response of primitive human CD34- and CD34+ hematopoietic cells to the Notch ligand Jagged-1

Recent reports indicate that activation of the Notch signaling pathway delays the differentiation of hematopoietic progenitors, suggesting that Notch may be used to develop novel ex vivo culture conditions for the expansion of primitive cells to be used in clinical transplantation. Here, we compare Notch expression and the effects of Jagged-1 treatment on highly purified subfractions of primitive CD34+ and CD34- human hematopoietic cells. Unlike response of cultured CD34+ cells, Jagged-1 treatment did not enhance the proliferation of CD34- cells, or promote differentiation of CD34- cells into CD34+ cells. While CD34+ and AC133-CD34- cells were shown to express all known forms of Notch receptors, Notch-3 and Notch-4 were not detected in AC133+CD34- cells. Similarly, CD34+ progeny of differentiated CD34- cells did not upregulate Notch-3 or Notch-4 upon differentiation, although transcripts for these genes were expressed in CD34+ arising from CD34+ CD38- parents, suggesting that the Notch receptor expression is tightly and differentially controlled. Fringe, known to inhibit Notch signaling in response to specific Notch ligands, was expressed in parent CD34- and CD34+ cells as well as their CD34+ progeny. We suggest that the inability of primitive CD34- cells to positively respond to Jagged-1 may be due in part to the absence of Notch-3 and Notch-4. Taken together, our study illustrates functional distinctiveness of the primitive CD34- subsets to CD34+ counterparts in relation to Jagged-1 response, and represents the first demonstration of a molecular difference among de novo isolated CD34+ compared to in vitro generated CD34+ cells arising from primitive CD34- or CD34+ parents.

Radiation-induced deletions in the 5' end region of Notch1 lead to the formation of truncated proteins and are involved in the development of mouse thymic lymphomas

Notch1 protein is a transmembrane receptor that directs various cell fate decisions. Active forms of Notch1 consisting of a transmembrane domain and an intracellular domain (Notch1TM) or only an intracellular domain (Notch1IC) function as oncoproteins. To elucidate the effect of Notch1 abnormalities in radiation-induced lymphomagenesis, we determined the structure of the Notch1 gene and examined the frequency and the sites of Notch1 rearrangements in radiation-induced mouse thymic lymphomas. The Notch1 gene consists of 37 exons, including three exons upstream of the previously reported exon 1. The transcript starting from exon 1 was the major transcript whereas the transcripts read upstream from exon 1a, in which amino acid sequences in the N-terminal region were changed, were minor. More than 50% of radiation-induced thymic lymphomas exhibited Notch1 rearrangements, suggesting that Notch1 acts as a major oncogene in radiation-induced lymphomagenesis. We identified three rearranged sites: novel sites in the 5' end region encompassing exons 1 and 2, the previously identified juxtamembrane extracellular region, and the 3' end region. The 5' deletion and the insertion of murine leukemia virus in the juxtamembrane region led to the production of abnormal transcripts starting from cryptic transcription start sites located halfway through the Notch1 gene and resulted in transcripts lacking most of the extracellular domain. As a result of these rearrangements, truncated Notch1 polypeptides resembling Notch1TM or Notch1IC were formed. In contrast, the 3' deletion led to the production of a C-terminal PEST motif-deleted transcript. The downstream target gene Hes1 was transcribed in a lymphoma with insertion of murine leukemia virus, but not in a lymphoma with a 5' deletion. These results indicate that in addition to Hes1 expression, other Notch1 pathway(s) have a role in thymic lymphomagenesis and suggest the presence of a novel mechanism for oncogenic activation of Notch1 by 5' deletion.

Structural requirements for assembly of the CSL.intracellular Notch1.Mastermind-like 1 transcriptional activation complex

Ligand binding by Notch receptors triggers a series of proteolytic cleavages that liberate the intracellular portion of Notch (ICN) from the cell membrane, permitting it to translocate to the nucleus. Nuclear ICN binds to a highly conserved DNA-binding transcription factor called CSL (also known as RBP-Jkappa, CBF1, Suppressor of Hairless, and Lag-1) and recruits Mastermind-like transcriptional co-activators to form a transcriptional activation complex. Using bioinformatics tools, we identified a Rel homology region (RHR) within CSL that was used as a guide to determine the minimal protein requirements for ternary complex formation. The RHR of CSL contains both the N- and C-terminal beta-sheet domains (RHR-n and RHR-c) of typical Rel transcription factors, as judged by circular dichroism spectra. Binding of monomeric CSL to DNA requires the entire RHR of CSL and an additional 125-residue N-terminal sequence, whereas binding to ICN requires only the RHR-n domain. Although the RAM (RBP-Jkappa (recombination-signal-sequence-binding protein for Jkappa genes)-associated molecule) domain of ICN is flexible and relatively unstructured as an isolated polypeptide in solution, it associates stably with CSL on DNA. Recruitment of Mastermind-like 1 (MAML1) to CSL.ICN complexes on DNA requires inclusion of the ankyrin repeat domain of ICN, and N- and C-terminal sequences of CSL extending beyond the DNA-binding region. The requirement for cooperative assembly of the MAML1.ICN.CSL.DNA complex suggests that a primary function of ICN is to render CSL competent for MAML loading. On the basis of our results, we present a working structural model for the organization of the MAML1.ICN.CSL.DNA complex.

Perturbation of Ikaros isoform selection by MLV integration is a cooperative event in Notch(IC)-induced T cell leukemogenesis

The chromosomal translocation t(7;9)(q34;q34.3) in human T cell acute lymphoblastic leukemia (T-ALL) results in the aberrant expression of the intracellular domain of Notch (N(ic)). Consistent with the current multistep model for tumorigenesis, mice that express N(ic) in T cell progenitors develop a T-ALL-like disease with a lengthened latency. Proviral insertional mutagenesis greatly accelerated the onset of leukemia in N(ic) transgenic mice. We demonstrate that the Ikaros (Ik) locus is a common target of proviral integration in N(ic) transgenic mice, which results in the loss of Ik DNA binding activity through altered isoform expression. We propose that cooperative leukemogenesis occurs in cells that have constitutive N(ic) and altered Ik isoform expression because genes normally repressed by Ik become activated by N(ic)/CSL.

Growth suppression of pre-T acute lymphoblastic leukemia cells by inhibition of notch signaling

Constitutive NOTCH signaling in lymphoid progenitors promotes the development of immature T-cell lymphoblastic neoplasms (T-ALLs). Although it is clear that Notch signaling can initiate leukemogenesis, it has not previously been established whether continued NOTCH signaling is required to maintain T-ALL growth. We demonstrate here that the blockade of Notch signaling at two independent steps suppresses the growth and survival of NOTCH1-transformed T-ALL cells. First, inhibitors of presenilin specifically induce growth suppression and apoptosis of a murine T-ALL cell line that requires presenilin-dependent proteolysis of the Notch receptor in order for its intracellular domain to translocate to the nucleus. Second, a 62-amino-acid peptide derived from a NOTCH coactivator, Mastermind-like-1 (MAML1), forms a transcriptionally inert nuclear complex with NOTCH1 and CSL and specifically inhibits the growth of both murine and human NOTCH1-transformed T-ALLs. These studies show that continued growth and survival of NOTCH1-transformed lymphoid cell lines require nuclear access and transcriptional coactivator recruitment by NOTCH1 and identify at least two steps in the Notch signaling pathway as potential targets for chemotherapeutic intervention.

Identification of a family of mastermind-like transcriptional coactivators for mammalian notch receptors

The molecular mechanisms by which Notch receptors induce diverse biological responses are not fully understood. We recently cloned a mammalian homologue of the Mastermind gene of Drosophila melanogaster, MAML1 (Mastermind-like-1 molecule) and determined that it functions as a transcriptional coactivator for Notch receptors. In this report, we characterize two additional genes in this Mastermind-like gene family: MAML2 and MAML3. The three MAML genes are widely expressed in adult tissues but exhibit distinct expression patterns in mouse early spinal cord development. All MAML proteins localize to nuclear bodies, share a conserved basic domain in their N termini that binds to the ankyrin repeat domain of Notch, and contain a transcriptional activation domain in their C termini. Moreover, as determined by using coimmunoprecipitation assays, each MAML protein was found to be capable of forming a multiprotein complex with the intracellular domain of each Notch receptor (ICN1 to -4) and CSL in vivo. However, MAML3 bound less efficiently to the ankyrin repeat domain of Notch1. Also, in U20S cells, whereas MAML1 and MAML2 functioned efficiently as coactivators with each of the Notch receptors to transactivate a Notch target HES1 promoter construct, MAML3 functioned more efficiently with ICN4 than with other forms of ICN. Similarly, MAML1 and MAML2 amplified Notch ligand (both Jagged2 and Delta1)-induced transcription of the HES-1 gene, whereas MAML3 displayed little effect. Thus, MAML proteins may modify Notch signaling in different cell types based on their own expression levels and differential activities and thereby contribute to the diversity of the biological effects resulting from Notch activation.

Characterization of a high-molecular-weight Notch complex in the nucleus of Notch(ic)-transformed RKE cells and in a human T-cell leukemia cell line

Notch genes encode a family of transmembrane proteins that are involved in many cellular processes, such as differentiation, proliferation, and apoptosis. It is well established that all four Notch genes can act as oncogenes; however, the mechanism by which Notch proteins transform cells remains unknown. Previously, we reported that both nuclear localization and transcriptional activation are required for neoplastic transformation of RKE cells. Furthermore, we identified cyclin D1 as a direct transcriptional target of constitutively active Notch molecules. In an effort to understand the mechanism by which Notch functions in the nucleus, we sought to determine if Notch formed stable complexes using size exclusion chromatography. Herein, we report that the Notch intracellular domain (N(ic)) forms distinct high-molecular-weight complexes in the nuclei of transformed RKE cells. The largest complex is approximately 1.5 MDa and contains both endogenous CSL (for CBF1, Suppressor of Hairless, and Lag-1) and Mastermind-Like-1 (Maml). N(ic) molecules that do not have the high-affinity binding site for CSL (RAM) retain the ability to associate with CSL in a stable complex through interactions involving Maml. However, Maml does not directly bind to CSL. Furthermore, Maml can rescue Delta RAM transcriptional activity on a CSL-dependent promoter. These results indicate that deletion of the RAM domain does not equate to CSL-independent signaling. Moreover, in SUP-T1 cells, N(ic) exists exclusively in the largest N(ic)-containing complex. SUP-T1 cells are derived from a T-cell leukemia that harbors the t(7;9)(q34;q34.3) translocation and constitutively express N(ic). Taken together, our data indicate that complex formation is likely required for neoplastic transformation by Notch(ic).

The p53 family member genes are involved in the Notch signal pathway

The p53 tumor suppressor is a transcription factor that regulates cell growth and death in response to environmental stimuli such as DNA damage. p63/p51 and p73 were recently identified as members of the p53 gene family. In contrast to p53 however, p63 and p73 are rarely mutated in human cancers. Mice that lack p53 are developmentally normal, while p63 and p73 appear to play critical roles in normal development. To determine how p63 and p73 are involved in normal development, we attempted to identify target genes that are specifically regulated by p63 and/or p73 but not by p53. We found that the Jagged1 (JAG1) and Jagged2 (JAG2) genes, encoding ligands for the Notch receptors, are up-regulated by p63 and p73. Furthermore, we identified a p63-binding site in the second intron of the JAG1 gene, which can directly interact with the p63 protein in vivo, as assessed by a chromatin immunoprecipitation assay. A heterologous reporter assay revealed that this p63-binding site is a functional response element and is specific for p63. We also found a target of Notch signaling, HES-1 was up-regulated in Jurkat cells, in which Notch1 is highly expressed, when co-cultured with p63-transfected cells, suggesting that p63 can trigger the Notch signal pathway in neighboring cells. Our findings show an association between the p53 family genes and Notch signaling and suggest a potential molecular mechanism for the involvement of the p53 family genes in normal development.

Induction of cyclin D1 transcription and CDK2 activity by Notch(ic): implication for cell cycle disruption in transformation by Notch(ic)

Notch genes encode a family of transmembrane proteins that are involved in many cellular processes such as differentiation, proliferation, and apoptosis. Although it is well established that all four Notch genes can act as oncogenes, the mechanism by which Notch proteins transform cells remains unknown. Previously, we have shown that transformation of RKE cells can be conditionally induced by hormone activation of Notch(ic)-estrogen receptor (ER) chimeras. Using this inducible system, we show that Notch(ic) activates transcription of the cyclin D1 gene with rapid kinetics. Transcriptional activation of cyclin D1 is independent from serum-derived growth factors and de novo synthesis of secondary transcriptional activators. Moreover, hormone activation of Notch(ic)-ER proteins induces CDK2 activity in the absence of serum. Upregulation of cyclin D1 and activation of CDK2 by Notch(ic) result in the promotion of S-phase entry. These data demonstrate the first evidence that Notch(ic) proteins can directly regulate factors involved in cell cycle control and affect cellular proliferation. Furthermore, nontransforming Notch(ic) proteins do not induce cyclin D1 expression, indicating that the mechanism of transformation involves cell cycle deregulation through constitutive expression of cyclin D1. Finally, we have identified a CSL [stands for CBF1, Su(H), and Lag-1] binding site within the human and rat cyclin D1 promoters, suggesting that Notch(ic) proteins activate cyclin D1 transcription through a CSL-dependent pathway.

Human homologues of Delta-1 and Delta-4 function as mitogenic regulators of primitive human hematopoietic cells

Delta-mediated Notch signaling controls cell fate decisions during invertebrate and murine development. However, in the human, functional roles for Delta have yet to be described. This study reports the characterization of Delta-1 and Delta-4 in the human. Human Delta-4 was found to be expressed in a wide range of adult and fetal tissues, including sites of hematopoiesis. Subsets of immature hematopoietic cells, along with stromal and endothelial cells that support hematopoiesis, were shown to express Notch and both Delta-1 and Delta-4. Soluble forms of human Delta-1 (hDelta-1) and hDelta-4 proteins were able to augment the proliferation of primitive human hematopoietic progenitors in vitro. Intravenous transplantation of treated cultures into immune-deficient mice revealed that hDelta-1 is capable of expanding pluripotent human hematopoietic repopulating cells detected in vivo. This study provides the first evidence for a role of Delta ligands as a mitogenic regulator of primitive hematopoietic cells in the human.

Human homologues of Delta-1 and Delta-4 function as mitogenic regulators of primitive human hematopoietic cells

Delta-mediated Notch signaling controls cell fate decisions during invertebrate and murine development. However, in the human, functional roles for Delta have yet to be described. This study reports the characterization of Delta-1 and Delta-4 in the human. Human Delta-4 was found to be expressed in a wide range of adult and fetal tissues, including sites of hematopoiesis. Subsets of immature hematopoietic cells, along with stromal and endothelial cells that support hematopoiesis, were shown to express Notch and both Delta-1 and Delta-4. Soluble forms of human Delta-1 (h Delta-1) and h Delta-4 proteins were able to augment the proliferation of primitive human hematopoietic progenitors in vitro. Intravenous transplantation of treated cultures into immune-deficient mice revealed that h Delta-1 is capable of expanding pluripotent human hematopoietic repopulating cells detected in vivo. This study provides the first evidence for a role of Delta ligands as a mitogenic regulator of primitive hematopoietic cells in the human. (Blood. 2001;97:1960-1967)

Ligand-induced signaling in the absence of furin processing of Notch1

Notch is a conserved cell surface receptor that is activated through direct contact with neighboring ligand-expressing cells. The primary 300-kDa translation product of the Notch1 gene (p300) is cleaved by a furin-like convertase to generate a heterodimeric, cell-surface receptor composed of 180- (p180) and 120- (p120) kDa polypeptides. Heterodimeric Notch is thought to be the only form of the receptor which is both present on the cell surface and able to generate an intracellular signal in response to ligand. Consistent with previous reports, we found that disruption of furin processing of Notch1, either by coexpression of a furin inhibitor or by mutation of furin target sequences within Notch1 itself, perturbed ligand-dependent signaling through the well-characterized mediator of Notch signal transduction, CSL (CBF1, Su(H), and LAG-1). Yet contrary to these reports, we could detect the full-length p300 Notch1 product on the cell surface. Moreover, this uncleaved form of Notch1 could suppress the differentiation of C2C12 myoblasts in response to ligand. Taken together, these data support our previous studies characterizing a CSL-independent Notch signaling pathway and identify this uncleaved isoform of Notch as a potential mediator of this pathway. Our results suggest a novel paradigm in signal transduction, one in which two isoforms of the same cell-surface receptor could mediate two distinct signaling pathways in response to ligand.

Notch-1 and Notch-2 exhibit unique patterns of expression in human B-lineage cells

The Notch genes encode a conserved family of receptors that influence developmental fate in many species. Prior studies have indicated that Notch-1 and Notch-2 signaling influence the development of hematopoietic stems cells and thymocytes, but little is known regarding Notch expression and function in B-lineage cells. We analyzed the expression of Notch receptors and Notch ligands in human B-lineage cells and bone marrow (BM) stromal cells. Notch-1 mRNA and protein is expressed throughout normal B cell development and in leukemic B-lineage cells. In contrast, Notch-2 expression is limited to pre-B cells expressing low levels of surface mu. The Notch ligand Delta is expressed in BM B-lineage cells. The Notch ligand Jagged-1 is not expressed in B-lineage cells, but is expressed in BM stromal cells. These results suggest a model wherein lateral signaling between Notch and Delta on B-lineage cells and/or Notch/Jagged-1 interactions between B-lineage cells and BM stromal cells may regulate human B cell development.

MAML1, a human homologue of Drosophila mastermind, is a transcriptional co-activator for NOTCH receptors

Notch receptors are involved in cell-fate determination in organisms as diverse as flies, frogs and humans. In Drosophila melanogaster , loss-of-function mutations of Notch produce a 'neurogenic' phenotype in which cells destined to become epidermis switch fate and differentiate to neural cells. Upon ligand activation, the intracellular domain of Notch (ICN) translocates to the nucleus, and interacts directly with the DNA-binding protein Suppressor of hairless (Su(H)) in flies, or recombination signal binding protein Jkappa (RBP-Jkappa) in mammals, to activate gene transcription. But the precise mechanisms of Notch-induced gene expression are not completely understood. The gene mastermind has been identified in multiple genetic screens for modifiers of Notch mutations in Drosophila. Here we clone MAML1, a human homologue of the Drosophila gene Mastermind, and show that it encodes a protein of 130 kD localizing to nuclear bodies. MAML1 binds to the ankyrin repeat domain of all four mammalian NOTCH receptors, forms a DNA-binding complex with ICN and RBP-Jkappa, and amplifies NOTCH-induced transcription of HES1. These studies provide a molecular mechanism to explain the genetic links between mastermind and Notch in Drosophila and indicate that MAML1 functions as a transcriptional co-activator for NOTCH signalling.

Essential roles for ankyrin repeat and transactivation domains in induction of T-cell leukemia by notch1

Notch receptors participate in a conserved signaling pathway that controls the development of diverse tissues and cell types, including lymphoid cells. Signaling is normally initiated through one or more ligand-mediated proteolytic cleavages that permit nuclear translocation of the intracellular portion of the Notch receptor (ICN), which then binds and activates transcription factors of the Su(H)/CBF1 family. Several mammalian Notch receptors are oncogenic when constitutively active, including Notch1, a gene initially identified based on its involvement in a (7;9) chromosomal translocation found in sporadic T-cell lymphoblastic leukemias and lymphomas (T-ALL). To investigate which portions of ICN1 contribute to transformation, we performed a structure-transformation analysis using a robust murine bone marrow reconstitution assay. Both the ankyrin repeat and C-terminal transactivation domains were required for T-cell leukemogenesis, whereas the N-terminal RAM domain and a C-terminal domain that includes a PEST sequence were nonessential. Induction of T-ALL correlated with the transactivation activity of each Notch1 polypeptide when fused to the DNA-binding domain of GAL4, with the exception of polypeptides deleted of the ankyrin repeats, which lacked transforming activity while retaining strong transactivation activity. Transforming polypeptides also demonstrated moderate to strong activation of the Su(H)/CBF1-sensitive HES-1 promoter, while polypeptides with weak or absent activity on this promoter failed to cause leukemia. These experiments define a minimal transforming region for Notch1 in T-cell progenitors and suggest that leukemogenic signaling involves recruitment of transcriptional coactivators to ICN1 nuclear complexes.

A carboxy-terminal deletion mutant of Notch1accelerates lymphoid oncogenesis in E2A-PBX1transgenic mice

PBX1 is a proto-oncogene that plays important roles in pattern formation during development. It was discovered as a fusion with the E2A gene after chromosomal translocations in a subset of acute leukemias. The resulting E2a-Pbx1 chimeric proteins display potent oncogenic properties that appear to require dimerization with Hox DNA binding partners. To define molecular pathways that may be impacted by E2a-Pbx1, a genetic screen consisting of neonatal retroviral infection was used to identify genes that accelerate development of T-cell tumors in E2A-PBX1 transgenic mice. Retroviral insertions in the Notch1 gene were observed in 88% of tumors arising with a shortened latency. Among these, approximately half created a NotchIC allele, encoding the intracellular, signaling portion of Notch1, suggesting a synergistic interaction between the Notch and E2a-Pbx1 pathways in oncogenesis. The remaining proviral insertions involvingNotch1 occurred in a more 3′ exon, resulting in truncating mutations that deleted the carboxy-terminal region ofNotch1 containing negative regulatory sequences (Notch1ΔC). In contrast toNotchIC, forced expression ofNotch1ΔC in transgenic mice did not perturb thymocyte growth or differentiation. However, mice transgenic for both the E2A-PBX1 and Notch1ΔC genes displayed a substantially shortened latency for tumor development compared with E2A-PBX1 single transgenic mice. These studies reveal a novel mechanism for oncogenic activation ofNotch1 and demonstrate a collaborative relationship between 2 cellular oncogenes that also contribute to cell fate determination during embryonic development.

A carboxy-terminal deletion mutant of Notch1accelerates lymphoid oncogenesis in E2A-PBX1transgenic mice

PBX1 is a proto-oncogene that plays important roles in pattern formation during development. It was discovered as a fusion with the E2A gene after chromosomal translocations in a subset of acute leukemias. The resulting E2a-Pbx1 chimeric proteins display potent oncogenic properties that appear to require dimerization with Hox DNA binding partners. To define molecular pathways that may be impacted by E2a-Pbx1, a genetic screen consisting of neonatal retroviral infection was used to identify genes that accelerate development of T-cell tumors in E2A-PBX1 transgenic mice. Retroviral insertions in the Notch1 gene were observed in 88% of tumors arising with a shortened latency. Among these, approximately half created a NotchIC allele, encoding the intracellular, signaling portion of Notch1, suggesting a synergistic interaction between the Notch and E2a-Pbx1 pathways in oncogenesis. The remaining proviral insertions involvingNotch1 occurred in a more 3′ exon, resulting in truncating mutations that deleted the carboxy-terminal region ofNotch1 containing negative regulatory sequences (Notch1ΔC). In contrast toNotchIC, forced expression ofNotch1ΔC in transgenic mice did not perturb thymocyte growth or differentiation. However, mice transgenic for both the E2A-PBX1 and Notch1ΔC genes displayed a substantially shortened latency for tumor development compared with E2A-PBX1 single transgenic mice. These studies reveal a novel mechanism for oncogenic activation ofNotch1 and demonstrate a collaborative relationship between 2 cellular oncogenes that also contribute to cell fate determination during embryonic development.

Notch(ic)-ER chimeras display hormone-dependent transformation, nuclear accumulation, phosphorylation and CBF1 activation

Notch genes encode a family of evolutionarily conserved transmembrane receptors that are involved in many distinct cellular processes such as differentiation, proliferation and apoptosis. Notch function has been shown to be required both during development and in adult life. Moreover, several studies on spontaneous human tumors and in experimental models demonstrate that three of the four mammalian Notch genes can act as oncogenes. The mechanism by which Notch proteins induce neoplastic transformation is not known. In order to determine the early signaling events mediated by Notch during cellular transformation we constructed several inducible alleles of Notch(ic) by fusing portions of Nic to the hormone-binding domain of the estrogen receptor. Here we show that Notch(ic)-ER chimeras are conditionally activated by 4-Hydroxytamoxifen (OHT) in a dose-dependent manner. Clonal RKE cell lines expressing Notch(ic)-ER chimeras display hormone-dependent transformation in vitro. Transformation mediated by Notch(ic)-ER is reversible and chronic stimulation is necessary for the maintenance of the transformed phenotype. In response to hormone activation Notch(ic)-ER chimeras become hyperphosphorylated and accumulate in the nucleus of the cell; indicating that both phosphorylation and nuclear localization are required for Notch transforming activity.

Neoplastic transformation by Notch requires nuclear localization

Notch proteins are plasma membrane-spanning receptors that mediate important cell fate decisions such as differentiation, proliferation, and apoptosis. The mechanism of Notch signaling remains poorly understood. However, it is clear that the Notch signaling pathway mediates its effects through intercellular contact between neighboring cells. The prevailing model for Notch signaling suggests that ligand, presented on a neighboring cell, triggers proteolytic processing of Notch. Following proteolysis, it is thought that the intracellular portion of Notch (N(ic)) translocates to the nucleus, where it is involved in regulating gene expression. There is considerable debate concerning where in the cell Notch functions and what proteins serve as effectors of the Notch signal. Several Notch genes have clearly been shown to be proto-oncogenes in mammalian cells. Activation of Notch proto-oncogenes has been associated with tumorigenesis in several human and other mammalian cancers. Transforming alleles of Notch direct the expression of truncated proteins that primarily consist of N(ic) and are not tethered to the plasma membrane. However, the mechanism by which Notch oncoproteins (generically termed here as N(ic)) induce neoplastic transformation is not known. Previously we demonstrated that N1(ic) and N2(ic) could transform E1A immortalized baby rat kidney cells (RKE) in vitro. We now report direct evidence that N1(ic) must accumulate in the nucleus to induce transformation of RKE cells. In addition, we define the minimal domain of N1(ic) required to induce transformation and present evidence that transformation of RKE cells by N1(ic) is likely to be through a CBF1-independent pathway.

Two distinct Notch1 mutant alleles are involved in the induction of T-cell leukemia in c-myc transgenic mice

We have previously characterized a large panel of provirus insertion Notch1 mutant alleles and their products arising in thymomas of MMTV(D)/myc transgenic mice. Here, we show that these Notch1 mutations represent two clearly distinct classes. In the first class (type I), proviral integrations were clustered just upstream of sequences encoding the transmembrane domain. Type I Notch1 alleles produced two types of mutant Notch1 RNA, one of which encoded the entire Notch1 cytoplasmic domain [N(IC)] and the other of which encoded a soluble ectodomain [N(EC)(Mut)] which, in contrast to the processed wild-type ectodomain [N(EC)(WT)], did not reside at the cell surface and became secreted in a temperature-dependent manner. A second, novel class of mutant Notch1 allele (type II) encoded a Notch1 receptor with the C-terminal PEST motif deleted (DeltaCT). The type II Notch1(DeltaCT) protein was expressed as a normally processed receptor [N(EC)(WT) and N(IC)(DeltaCT)] at the cell surface, and its ectodomain was found to be shed into the extracellular medium in a temperature- and calcium-dependent manner. These data suggest that both type I and type II mutations generate two structurally distinct Notch1 N(EC) and N(IC) proteins that may participate in tumor formation, in collaboration with the c-myc oncogene, through distinct mechanisms. Constitutive type I N(IC) and type II N(IC)(DeltaCT) expression may enhance Notch1 intracellular signaling, while secreted or shed type I N(EC)(Mut) and type II N(EC) proteins may differentially interact in an autocrine or paracrine fashion with ligands of Notch1 and affect their signaling.

Notch signaling in T cell development

Notch signaling regulates cell fate decisions during development. Recent experiments suggest that Notch signaling is essential for initial commitment to the T cell lineage and may function together with signals from the pre-TCR and the TCR to regulate subsequent steps of T cell development.

Calcium depletion dissociates and activates heterodimeric notch receptors

Notch receptors participate in a highly conserved signaling pathway that regulates morphogenesis in multicellular animals. Maturation of Notch receptors requires the proteolytic cleavage of a single precursor polypeptide to produce a heterodimer composed of a ligand-binding extracellular domain (N(EC)) and a single-pass transmembrane signaling domain (N(TM)). Notch signaling has been correlated with additional ligand-induced proteolytic cleavages, as well as with nuclear translocation of the intracellular portion of N(TM) (N(ICD)). In the current work, we show that the N(EC) and N(TM) subunits of Drosophila Notch and human Notch1 (hN1) interact noncovalently. N(EC)-N(TM) interaction was disrupted by 0.1% sodium dodecyl sulfate or divalent cation chelators such as EDTA, and stabilized by millimolar Ca(2+). Deletion of the Ca(2+)-binding Lin12-Notch (LN) repeats from the N(EC) subunit resulted in spontaneous shedding of N(EC) into conditioned medium, implying that the LN repeats are important in maintaining the interaction of N(EC) and N(TM). The functional consequences of EDTA-induced N(EC) dissociation were studied by using hN1-expressing NIH 3T3 cells. Treatment of these cells for 10 to 15 min with 0.5 to 10 mM EDTA resulted in the rapid shedding of N(EC), the transient appearance of a polypeptide of the expected size of N(ICD), increased intranuclear anti-Notch1 staining, and the transient activation of an Notch-sensitive reporter gene. EDTA treatment of HeLa cells expressing endogenous Notch1 also stimulated reporter gene activity to a degree equivalent to that resulting from exposure of the cells to the ligand Delta1. These findings indicate that receptor activation can occur as a consequence of N(EC) dissociation, which relieves inhibition of the intrinsically active N(TM) subunit.

Notch signaling in the nervous system. Pieces still missing from the puzzle

Notch has been known for many years as a receptor for inhibitory signals that shapes the pattern of the nervous system during its development. Genes in the Notch pathway function to prevent neural determination so that only a subset of the available ectodermal cells become neural precursors. The localization of Notch signaling is crucial for determining where neural precursor cells arise on a cell-by-cell basis. The unresolved problem is that studies of the expression of Notch protein and its ligands are inconsistent with the pattern of neurogenesis. During neural cell fate specification, distributions of Notch protein and of its ligand Delta appear uniform. Under the reigning paradigm, such widespread expression should lead to N signal transduction in all cells and thereby prevent any neural specification. Yet, contrary to this expectation, neural elements still form, in characteristic patterns, hence, Notch signal transduction must have been inactive in the precursor cells. The mechanism preventing Notch signaling in certain cells must be posttranslational but it has not yet been identified. This review will outline the experimental evidence supporting this view of Notch signaling, and briefly evaluate some of the possible mechanisms that have been suggested.

A ligand-induced extracellular cleavage regulates gamma-secretase-like proteolytic activation of Notch1

Gamma-secretase-like proteolysis at site 3 (S3), within the transmembrane domain, releases the Notch intracellular domain (NICD) and activates CSL-mediated Notch signaling. S3 processing occurs only in response to ligand binding; however, the molecular basis of this regulation is unknown. Here we demonstrate that ligand binding facilitates cleavage at a novel site (S2), within the extracellular juxtamembrane region, which serves to release ectodomain repression of NICD production. Cleavage at S2 generates a transient intermediate peptide termed NEXT (Notch extracellular truncation). NEXT accumulates when NICD production is blocked by point mutations or gamma-secretase inhibitors or by loss of presenilin 1, and inhibition of NEXT eliminates NICD production. Our data demonstrate that S2 cleavage is a ligand-regulated step in the proteolytic cascade leading to Notch activation.

Intracellular forms of human NOTCH1 functionally activate essential Epstein-Barr virus major latent promoters in the Burkitt's lymphoma BJAB cell line but repress these promoters in Jurkat cells

We have demonstrated that intracellular forms of NOTCH1 transactivate two major Epstein-Barr virus (EBV) latent promoters, the LMP1 and Cp1 promoters in an EBV-negative B-cell line, BJAB. Truncated intracellular NOTCH1 associated with the nuclear membrane (DeltaE) transactivates the LMP1 promoter fivefold; however, the intranucleus localized form of NOTCH1 (ICN) transactivates this promoter approximately twofold in chloroamphenicol acetyltransferase (CAT) reporter assays in BJAB cells. Additionally, DeltaE activated the major Cp1 promoter 12-fold, whereas the ICN form of NOTCH1 activates at only about half that level when compared to that of DeltaE membrane-bound NOTCH1. This result differs from previously observed data, where intracellular NOTCH1 bound to the nuclear membrane, DeltaE, and nucleus-localized NOTCH1, ICN, all had similar levels of activation in 293 cells. This suggests distinct transcriptional activities in different cell types. Moreover, in Jurkat cells, a T-cell line, intranucleus localized NOTCH1 molecules demonstrated a repressive activity against the two EBV major latent promoters. Only DeltaE activated the Cp1 and LMP1 promoters at a level slightly above background, whereas intranucleus localized NOTCH1 ICN, or the form of NOTCH1 lacking the ankyrin repeats, DeltaE(TAR), surprisingly resulted in the repression of these promoters in Jurkat cells. Similarly, another truncated form of NOTCH1, referred to as ICNW, which contains the tryptophan residue W(1767) within one of the RBP-Jkappa interacting domains, repressed the LMP1 promoter approximately twofold. Further analysis of the truncated NOTCH1 molecules on the LMP1 promoter element, lacking the two RBP-Jkappa binding sites, suggests that repression in Jurkat cells may be affected by the presence of the two RBP-Jkappa binding sites. These studies indicate that intracellular NOTCH1 can activate the EBV major latent promoters in BJAB cells. However, in Jurkat cells, intracellular truncated forms of NOTCH1 lacking the RBP-Jkappa binding sites repress these EBV latent promoters. Only the membrane-bound form of NOTCH1, DeltaE, activated the EBV major latent promoters in Jurkat cells, albeit at a lower level than that seen in BJAB cells. Our data suggest that EBNA2 and truncated intracellular nuclear localized forms of NOTCH1 may be functionally similar in their interactions with RBP-Jkappa; however, these molecules may have distinctly different transcriptional partners in BJAB and Jurkat cells. Moreover, these truncated NOTCH1 molecules may not represent the normal processed forms of NOTCH1 in cells and may exhibit dominant negative phenotypes in the absence of the required posttranslational modifications. Further investigations are necessary to determine the similarity and differences occurring with intracellular NOTCH1 in other B- and T-cell lines.

Identification of a novel NOTCH-4/INT-3 RNA species encoding an activated gene product in certain human tumor cell lines

Ectopic expression of the intracellular domain of NOTCH-4/INT-3 leads to tumorigenesis in the mouse mammary gland. This results from a gain-of-function mutation. To evaluate gain-of-function NOTCH-4/INT-3 activity in human cancers we have surveyed human breast, lung, and colon carcinoma tissue culture cell lines for evidence of increased NOTCH-4/INT-3 RNA expression. High levels of a 1.8 Kb NOTCH-4/INT-3 RNA species are detected in normal human testis but not in other tissues where a 6.5 Kb species is prevalent. Transformed human cancer cell lines express the 1.8 Kb NOTCH-4/INT-3 RNA species. We show that this RNA species encodes a truncated form of the NOTCH-4/INT-3 intracellular domain (ICD). This novel NOTCH-4/INT-3 protein includes the CDC10 repeats and amino acid residues C-terminal to them, but is missing the CBF-1 binding region of the NOTCH-4/INT-3 ICD. This suggests that it has a different mode of action. Furthermore, we show that a transgene which expresses the 1.8 Kb NOTCH-4/INT-3 RNA species in the 'normal' human mammary epithelial cell line MCF-10A enables these cells to grow in soft agar.

Intracellular forms of human NOTCH1 interact at distinctly different levels with RBP-jkappa in human B and T cells

The cellular transcriptional repressor RBP-Jkappa associates with the Epstein-Barr virus nuclear antigens (EBNAs) determined to be essential for transformation of human primary B lymphocytes. It was demonstrated through genetic analysis that interaction between the viral transactivator EBNA2 and RBP-Jkappa is essential for EBV immortalization of primary B lymphocytes. We have shown that the association of RBP-Jkappa with intracellular NOTCH1 differs significantly in B and T cells. Immunoprecipitation analyses with antibodies to both the intracellular forms of NOTCH1 and to RBP-Jkappa demonstrated that little or no RBP-Jkappa is associated with NOTCH1 in B cell lines compared to the RBP-Jkappa associated with NOTCH1 in T cell lines and was further demonstrated in human primary lymphocytes. Additionally, EBNA2 can compete with intracellular NOTCH1 for binding to GST-RBP-Jkappa in vitro. Northern blot for the cellular gene hairy enhancer of split (HES1) demonstrated that HES1 is upregulated in the EBV transformed lymphoblastoid cells expressing high levels of EBNA2 and in a T cell line SupT1 overexpressing intracellular activated NOTCH1. Hence, EBNA2 may be able to compete for the available pool of RBP-Jkappa more effectively in human B cells than in T cells and provides a possible explanation for the ability of EBV to potently and efficiently infect and immortalize human B cells. Leukemia (2000) 14, 84-92.

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.