From fly wings to targeted cancer therapies: a centennial for notch signaling

Abstract

Since Notch phenotypes in Drosophila melanogaster were first identified 100 years ago, Notch signaling has been extensively characterized as a regulator of cell-fate decisions in a variety of organisms and tissues. However, in the past 20 years, accumulating evidence has linked alterations in the Notch pathway to tumorigenesis. In this review, we discuss the protumorigenic and tumor-suppressive functions of Notch signaling, and dissect the molecular mechanisms that underlie these functions in hematopoietic cancers and solid tumors. Finally, we link these mechanisms and observations to possible therapeutic strategies targeting the Notch pathway in human cancers.

Figure 1. Protein structure and mutations of a typical Notch receptor

The structure of the NOTCH1 receptor and genetic alterations of the protein in representative types of cancer are depicted. ADAM metalloproteases and the γ-secretase complex cleave the receptor and free the ICN domain. Major mutations are clustered according to their effects on protein activity. Both gain-and loss-of-function mutations are shown. The majority of the T-ALL mutations are clustered in the heterodimerization (HD) and PEST domains controlling processing of the receptors by proteases and the stability of the protein correspondingly. Different characteristic cases of hematopoietic disorders (affecting NOTCH2 as well) are shown. In CLL tumors there is an apparent mutational hotspot at the PEST domain of NOTCH1. In the case of SqCC mutations, they are mainly clustered in the EGF repeat region potentially affecting interaction with the ligands. T-ALL: T-cell acute lymphoblastic leukemia, SMZL: splenic marginal zone lymphoma, CLL: chronic lymphocytic leukemia, DLBCL: diffuse large B cell lymphoma, SqCC: squamous cell carcinoma. Percentages are approximations based on current literature.

Figure 2. Overview of the Notch signaling pathway

A visual description of the signaling cascade is shown for the signal-receiving cell (i.e. the cell expressing the Notch receptor). Pathway inhibitors used include antibodies against NOTCH receptors and DLL ligands, γ-secretase complex inhibitors (GSI), and small peptides inhibiting formation of the transcriptional complex. Antibody-based treatments are shown in purple, GSI compounds in pink, peptide-based drugs in red. Potential epigenetic inhibitors (in green) can include BRD inhibitors like JQ1. HDAC: histone deacetylase, ICN1: intracellular part of NOTCH1, LSD1: lysine specific demethylase 1, SMRT: Silencing-Mediator for Retinoid/Thyroid hormone receptors, GSK3β: glycogen synthase kinase 3 beta, DNMAML1: dominant negative MAML1.

Figure 3. In vivo mapping of Notch pathway activity using a Hes1^GFP reporter

(A) Targeting strategy for the generation of transgenic animals expressing Emerald GFP (emGFP) from the endogenous Hes1 locus. (B) Immuno-fluorescence staining for thymus of the Hes1^GFP mice. DAPI stains DNA (nucleus), VE-cadherin is a vascular endothelial marker and K14 is a marker of thymic medullary cells. (C) Increased levels of Notch pathway help differentiation of thymic T cell progenitors through the DN2/3 CD4⁻8⁻ differentiation stage and the pathway activity is decreased immediately at the DP stage. (D) Activity of the Notch pathway in the mouse bone marrow is detected at the HSC level and is decreased as cells differentiate. Subsequently it is reactivated at the level of a megakaryocytic-erythrocytic progenitor (MEP). HSC: hematopoietic stem cells, MPP: multipotent progenitors, CMP: common myeloid progenitors, CLP: common lymphoid progenitors, MEP: megakaryocyte-erythrocyte progenitor, GMP: granulocyte-macrophage progenitor, DN: double negative (CD4⁻CD8⁻), DP: double positive (CD4⁺CD8⁺), SP: single positive.

Figure 4. Simplified scheme of Notch interactions with other signaling pathways in cancer

The TGFβ, PI3K, NFκB, and WNT pathways are some of the most important pathways interacting with NOTCH. Notably, Jagged 1 is activated by TGFβ pathway and in turn activates NOTCH receptors in neighboring cells. Phosphorylation of NOTCH from the WNT-induced GSK3β leads to ubiquitination through FBXW7 and final degradation. Also, a classical NOTCH target, HES1, represses PTEN, a competitor of another pathway with oncogenic roles, PI3K, which in turn activates NFκB, a pathway important for leukemia progression. Important parameters of the interactions, such as regulation of NFκB pathway by NOTCH through HES1 action, or the interaction of NOTCH with the WNT member DVL (Dishevelled) protein that inhibits both WNT and NOTCH pathways are not shown in this figure. ICJ1: intracellular part of JAG1, ECJ1: extracellular part of JAG1.