Modulation of Notch Signaling by Small-Molecular Compounds and Its Potential in Anticancer Studies

Abstract

Notch signaling is responsible for conveying messages between cells through direct contact, playing a pivotal role in tissue development and homeostasis. The modulation of Notch-related processes, such as cell growth, differentiation, viability, and cell fate, offer opportunities to better understand and prevent disease progression, including cancer. Currently, research efforts are mainly focused on attempts to inhibit Notch signaling in tumors with strong oncogenic, gain-of-function (GoF) or hyperactivation of Notch signaling. The goal is to reduce the growth and proliferation of cancer cells, interfere with neo-angiogenesis, increase chemosensitivity, potentially target cancer stem cells, tumor dormancy, and invasion, and induce apoptosis. Attempts to pharmacologically enhance or restore disturbed Notch signaling for anticancer therapies are less frequent. However, in some cancer types, such as squamous cell carcinomas, preferentially, loss-of-function (LoF) mutations have been confirmed, and restoring but not blocking Notch functions may be beneficial for therapy. The modulation of Notch signaling can be performed at several key levels related to NOTCH receptor expression, translation, posttranslational (proteolytic) processing, glycosylation, transport, and activation. This further includes blocking the interaction with Notch-related nuclear DNA transcription. Examples of small-molecular chemical compounds, that modulate individual elements of Notch signaling at the mentioned levels, have been described in the recent literature.

Keywords: cancer; notch signaling; small-molecular compounds.

Figure 1

Canonical and non-canonical Notch signaling pathway. A schematic figure of the structure of the receptors (NOTCH1-4) and their ligands (DLL-1/3/4 and JAG1/2). After synthesis, the NOTCH receptors mature in a stepwise process and are simultaneously transported through the endoplasmic reticulum and Golgi structures to the plasma membrane. Receptor–ligand interactions generate a mechanical pulling force that allows ADAM proteases and the γ-secretase complex to cleave the receptor (at the S2 and S3 cleave site, respectively), followed by the release of the intracellular domain (NICD) to the cytoplasm of the signal-receiving cell. Simultaneously, the extracellular part of the NOTCH (NECD) and the ligand are internalized by the signal-sending cell. Intracellular release of NICD can also occur without ligand activation via NOTCH internalization and γ-secretase activity. The NICD is translocated into the nucleus where it participates in the formation of the Notch ternary complex (NTC), activating the expression of target genes. In non-canonical Notch signaling, the NICD interacts with and modulates other signaling pathways to express target genes without the mediation of the NTC.

Figure 2

Modulation of Notch signaling by interfering with membrane trafficking of NOTCH receptors. Inhibition of the membrane ion transporters SLC39A7 (NVS-ZP7-4) and SERCA (Thapsigargin, CAD204520, Casearin J) disrupts Zn²⁺ and Ca²⁺ ion efflux between endoplasmic reticulum/Golgi structures and the cytoplasm, resulting in impaired protein folding, arrested maturation, and a block of intracellular NOTCH protein transport. Attenuation of receptor exposure to the membrane can also be achieved by compounds blocking NOTCH secretion prior to the exit from the endoplasmic reticulum (FLI-06), or leading to the accumulation of NOTCH receptors in autophagosomes (Chloroquine, more details in the text). Red: attenuation of Notch signaling (NVS-ZP7-4, Thapsigargin, CAD204520, Casearin J, Bepridil, FLI-06, Chloroquine).

Figure 3

Modulation of NICD release. ADAM protease inhibitors (Lomitapide, Aderbasib, Marimastat, ZLDI-8) block NOTCH S2 cleavage and, consequently, further activation of the NOTCH receptor. The use of γ-secretase inhibitors (γSI; GSI) prevents S3 cleavage and subsequent NICD release. The NICD can be translocated to the nucleus where it is involved in canonical Notch signaling or interacts with other signaling pathways as a component of non-canonical Notch signaling. These events are influenced by proteasome activity (inhibited by Bortezomib) and a number of broad-spectrum substances collectively named as Pan-Assay INterference compoundS (PAINS) such as Resveratrol and Curcumin. Red: attenuation of Notch signaling; Blue: pleiotropic effect on Notch signaling.

Figure 4

Modulation of Notch signaling by targeting the NICD-dependent transcription complex. Attenuation of Notch signaling can be achieved by blocking ternary complex formation by inhibiting RBP-J activity (Fidaxamicin) or preventing the binding of MAML to NICD/RPB-J (IMR-1A, SAHM1) or NICD to RBP-J (CB-103). RIN-1 blocks the binding of the SHARP co-repressor to RBP-j, preventing the function of the repressor complex. HDAC inhibitors (Givinostat, Trichostatin, Tubacin, Valproic acid) block the ability of HDACs to deacetylate histones, contributing to the maintenance of an open DNA structure and expression of genes. Finally, some compounds, such as PTC-124, can re-express functional NOTCH1 and induce Notch-regulated gene expression levels. Red: attenuation of Notch signaling (IMR-1, SAHM1, CB-103, Tubacin), Blue: pleiotropic effect on Notch signaling (RIN-1, VPA), Green: activation of Notch signaling.