Notch Signaling Activation as a Hallmark for Triple-Negative Breast Cancer Subtype

Abstract

Triple-negative breast cancer (TNBC) is a subgroup of 15%-20% of diagnosed breast cancer patients. It is generally considered to be the most difficult breast cancer subtype to deal with, due to the lack of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), which usually direct targeted therapies. In this scenario, the current treatments of TNBC-affected patients rely on tumor excision and conventional chemotherapy. As a result, the prognosis is overall poor. Thus, the identification and characterization of targets for novel therapies are urgently required. The Notch signaling pathway has emerged to act in the pathogenesis and tumor progression of TNBCs. Firstly, Notch receptors are associated with the regulation of tumor-initiating cells (TICs) behavior, as well as with the aetiology of TNBCs. Secondly, there is a strong evidence that Notch pathway is a relevant player in mammary cancer stem cells maintenance and expansion. Finally, Notch receptors expression and activation strongly correlate with the aggressive clinicopathological and biological phenotypes of breast cancer (e.g., invasiveness and chemoresistance), which are relevant characteristics of TNBC subtype. The purpose of this up-to-date review is to provide a detailed overview of the specific role of all four Notch receptors (Notch1, Notch2, Notch3, and Notch4) in TNBCs, thus identifying the Notch signaling pathway deregulation/activation as a pathognomonic feature of this breast cancer subtype. Furthermore, this review will also discuss recent information associated with different therapeutic options related to the four Notch receptors, which may be useful to evaluate prognostic or predictive indicators as well as to develop new therapies aimed at improving the clinical outcome of TNBC patients.

Figure 1

Role of Notch signaling in TNBC. (a) Schematic representation of the Notch receptors structure. Abbreviations. NECD: Notch extracellular domain; NTM: Notch transmembrane; NICD: Notch intracellular domain; EGF: epidermal growth factor-like repeats; NRR: negative regulatory region; LNR: Lin12/Notch repeats; HD: heterodimerization domain; PM: plasmatic membrane; RAM: RBP-j associated molecule; NLS: nuclear localization signal; ANK: ankyrin repeats; PEST: proline (P), glutamic acid (E), serine (S), and threonine (T). (b) The cartoon schematically depicts the involvement of each Notch receptor on TNBC initiation and progression.

Figure 2

Notch-targeting therapeutic approaches in TNBCs. (a) The canonical Notch signaling pathway: ligand binding promotes sequential cleavages of the Notch receptors (Notch1-4) by ADAM enzyme and γ-secretase complex, resulting in the release of NICD, which translocates in the nucleus, interacts with transcriptional regulators to transcriptionally activate the canonical Notch target genes (ON), thus leading to the regulation of TNBC growth and progression. (b) Notch inhibitors with lower or absent selectivity, respectively, include mAbs targeting the Notch ligands and GSIs. (1) mAbs against Notch ligands prevent ligand-receptor interaction and the subsequent Notch cleavages, preventing Notch signaling triggering. Little is known about the specific Notch-ligand relationship in TNBC; thus further studies are needed to consider ligand blocking as a potential alternative selective approach in TNBC treatment. (2) GSIs act as pan-Notch inhibitors since they prevent the cleavage of all Notch receptors, thus avoiding the release of any NICD. This unselective mechanism of action is strongly correlated with a high intestinal toxicity in patients, which significantly impairs their clinical use. (3) Lower doses of GSIs used in combination with chemotherapeutic drugs result in improved clinical outcome and less toxicity, which however must be overcome. (c) A higher selectivity can be obtained by using monoclonal antibodies directed against the extracellular domain of a specific Notch receptor (1): mAbs mask the cleavage domain of ADAM, thus preventing the binding of this enzyme and the subsequent γ-secretase cleavage. The final effect will depend on the specific block of the single Notch receptor, also used in combination with chemotherapeutic drugs (2). Several studies detailed in the text have suggested that a greater selectivity in the Notch inhibition approach for TNBCs treatment is strongly correlated with a higher probability of success in favoring tumor regression, associated with less toxicity and therefore with a potential better prognosis of TNBC-bearing patients. Abbreviations. ADAM: a disintegrin and metalloproteinase; CSL: CBF1/Su(H)/Lag-1; CoA: coactivator; CoR: corepressor; GSIs: γ-secretase inhibitors; mAb: monoclonal antibody; MAML1: mastermind-like 1; NECD: Notch extracellular domain; NICD: Notch intracellular domain; NTM: Notch transmembrane; PM: plasmatic membrane.