Targeting Wnt Signaling via Notch in Intestinal Carcinogenesis

Abstract

Proliferation and differentiation of intestinal epithelial cells is assisted by highly specialized and well-regulated signaling cascades. The Wnt pathway, which is one of the fundamental pathways in the intestine, contributes to the organization of proliferative intestinal crypts by positioning and cycling of intestinal stem cells and their derivatives. The Wnt pathway promotes differentiation of intestinal secretory cell types along the crypt-plateau and crypt-villus axis. In contrast to the Wnt pathway, the intestinal Notch cascade participates in cellular differentiation and directs progenitor cells towards an absorptive fate with diminished numbers of Paneth and goblet cells. Opposing activities of Notch and Wnt signaling in the regulation of intestinal stem cells and the enterocytic cell fate have been elucidated recently. In fact, targeting Notch was able to overcome tumorigenesis of intestinal adenomas, prevented carcinogenesis, and counteracted Paneth cell death in the absence of caspase 8. At present, pharmacological Notch inhibition is considered as an interesting tool targeting the intrinsic Wnt pathway activities in intestinal non-neoplastic disease and carcinogenesis.

Keywords: Notch; Wnt; cancer; caspase 8; intestine; lncRNA; targeting.

Figure 1

Canonical Wnt signaling pathway. Left: In the absence of Wnt ligands, the multi-protein complex with APC, GSK3, and CK1 cause phosphorylation and subsequent proteosomal degradation of β-catenin. Right: Upon binding of a Wnt protein ligand, the phosphorylation activity of the protein complex is inhibited and β-catenin accumulates in the cytoplasm and translocates to the nucleus with activation of gene transcription. APC: adenomatous polyposis coli; β-cat: β-catenin; CBP: cAMP-response element-binding protein; CK1: casein kinase 1; Dsh: Disheveled; GSK3: glycogen synthase kinase 3; LRP: low-density lipoprotein-related protein.

Figure 2

Canonical Notch signaling pathway. After (S1) cleavage by furin or furin-like convertases the heterodimeric Notch receptor translocates to the cell membrane. Right: Following ligand binding, proteolytic cleavage mediated by ADAM/TACE results in the release of NEC (S2) and additional cleavage at the transmembrane region by the γ-secretase complex creates the NICD (S3), which is able to translocate into the nucleus with subsequent activation of gene transcription. Left: Alternatively, after ubiquitination by E3-ligases, internalization of Notch to endosomes is possible. From endosomes, Notch can be recycled to the plasma membrane, pooled by cis-inhibition by DSL or degraded in the lysosome. ADAM/TACE: metalloproteinase containing protein; CSL: C-promoter binding factor-1 (CBF1 in humans, also known as CSL); DSL: Delta/Serrate Ligand; E3: E3-ligases; MAML: mastermind-like; NEC: Notch extracellular; NICD: Notch intracellular domain; SKIP: Ski-interacting protein.

Figure 3

Variants of the Notch-Wnt crosstalk. (a) Co-operative and synergistic regulation of gene transcription when both pathways are active. (b) Sequential signaling is initiated when one active pathway promotes transcription of the ligand of the other pathway (top: active Notch signaling promotes expression of Wnt ligands; below: active Wnt signaling promotes expression of Notch ligands). (c) Examples of direct molecular crosstalk (top: both pathways are activated, but β-catenin is inhibited by NICD (inhibitory crosstalk); below: the Notch pathway is activated and co-activates intracellularly the Wnt cascade (activating crosstalk).

Figure 4

Morphological features of adenomatous polyps in the human large intestine. (a) “Bottom-up” morphology characterized by ascending cells with nuclear atypia (arrows indicate atypical cells) and normal plateau. (b) “Top-down” morphology with accumulation of atypical cells at the mucosal plateau (arrows indicate atypical cells), but normal cell composition, including goblet cells at the crypt basis. Original magnification: 200×.

Figure 5

Targeting Wnt via Notch in a model of Casp8 deletion. (a) Severe cell death in the large intestinal mucosa of Casp8^f/f villinCre+ mice with deletion in Casp8 in intestinal epithelial cells (Casp8^Δint). Arrows indicate examples of apoptotic bodies. (b) Typical features of normal large intestinal mucosa in Casp8^f/f villinCre- control littermates (Casp8^f/f) with a high number of goblet cells. (c) Restoration of goblet cells in the large intestinal mucosa of Casp8^Δint animals treated with the Notch inhibitor dibenzazepine DBZ. (d) Large intestinal mucosa of Casp8^f/f mice after DBZ treatment with mucostatic features at crypts (arrows). Original magnification: 200×.