Keynote (green)Advances in targeting the WNT/β-catenin signaling pathway in cancer
Introduction
The WNT/β-catenin signaling is an evolutionarily conserved pathway with a crucial role in embryonic development and regulates adult stem cell homeostasis and tissue regeneration.1, 2, 3, 4, 5 Clinical and experimental data suggest that activation of the WNT pathway is linked to human disease, including cancer. WNT ligands, comprising 19 glycoproteins, are secreted morphogens that exhibit differential spatiotemporal expression and act in a paracrine or autocrine manner. Wnt proteins, post synthesis, are destined for the lumen of the endoplasmic reticulum (ER). Upon palmitoylation by porcupine (PORCN) in the ER, the WNT ligand interacts with Wntless (WLS). The WLS complex is a part of the WNT secretory pathway, which facilitates the release of WNTs.4 Cell–cell transfer of WNT ligands under different physiological conditions can be mediated by passive diffusion or secreted in membrane-enclosed vesicles, such as exosomes, or travel on signaling filopodia (called cytonemes).5, 6
WNT signaling pathways can be further classified as canonical (β-catenin dependent) and non-canonical (β-catenin independent). In canonical signaling, the WNT family of secreted glycolipoproteins acts as a ligand and activates the receptor frizzled (FZD) on the recipient cell.7 In the absence of WNT, β-catenin is bound and regulated by a large protein complex comprising AXIN (scaffold protein), adenomatous polyposis coli (APC; tumor suppressor), casein kinase 1α (CK1α), glycogen synthase kinase 3β (GSK3β), beta-transducin repeats-containing E3 ubiquitin ligase (β-TrCP), and disheveled (DVL). This complex is called the destruction complex because it leads to the phosphorylation and ubiquitination of β-catenin, followed by proteasomal degradation of β-catenin. CK1 mediates phosphorylation, whereas GSK3β carries out subsequent phosphorylation of β-catenin at Ser33, Ser37, and Thr41 at the N-terminal region. Phosphorylated β-catenin is then ubiquitinated by β-TrCP, leading to proteasomal breakdown of β-catenin. The absence of WNT ligands, considered as the inactivated state, results in lower levels of β-catenin in the cell, preventing its nuclear accumulation, and is considered the inactivated state (Fig. 1). In the activated state, the extracellular WNT ligand secreted by another cell activates the FZD receptor, leading to the phosphorylation of a membrane protein called low-density lipoprotein receptor-related protein 5,6 (LRP5/6), resulting in the formation of the WNT-FZD-LRP5/6 complex.5 Once LRP5/6 is phosphorylated, it induces translocation of the destruction complex to the membrane-bound WNT-FZD-LRP5/6 complex. The binding of the scaffolding protein DVL to FZD leads to the phosphorylation of LRP6 and AXIN1 recruitment followed by inhibition of β-catenin phosphorylation. Binding of AXIN1 to the WNT-FZD-LRP5/6 complex results in inhibition of the ubiquitination of β-catenin by β-TrCP.7 In the absence of an active destruction complex, β-catenin levels elevate along with a concomitant increase in its nuclear translocation. Nuclear β-catenin acts as a co-activator for T cell factor/lymphocyte enhancer factor (TCF/LEF) to activate WNT-responsive gene transcription, including c-Myc and Cyclin D1, thus leading to growth and proliferation. In an inactivated state, the WNT-targeted genes remain repressed, because Groucho and histone deacetylase (HDAC) bind to TCF/LEF. An overview of the canonical WNT signaling pathway is provided in Fig. 1. An additional degree of Wnt pathway regulation occurs via the E3 ubiquitin ligases ring finger protein 43 (RNF43) and zinc and ring finger 3 (ZNRF3), which degrade FZD and prevent Wnt-dependent phosphorylation of LRP5/6 and the DVL complex, thereby inhibiting the stabilization of β-catenin and attenuation of the Wnt pathway; thus, they are negative Wnt regulators. In the presence the secreted protein R-spondin (Rspo), ZNRF3/RNF43 is internalized, allowing the interaction of WNT ligand with the WNT receptor complex and activation of the pathway. The WNT pathway is discussed elsewhere in details.4, 5, 8
Several non-canonical WNT pathways are described, but the WNT/PCP and the WNT/Ca2+ pathways are better investigated. Non-canonical WNT/PCP signaling regulates planar cell polarity and cell migration via the FZD-ROR1/ROR2/RYK receptors. The WNT/Ca2+ signaling cascade starts with the WNT–FZD interaction followed by activation of Phospholipase C (PLC) and intracellular Ca2+ release and activation of Ca2+-dependent kinases. The non-canonical pathway is not discussed in this review and is covered in detail elsewhere.9, 10 WNT signaling interacts with several other signaling pathways and is tightly regulated at multiple levels; however, its deregulation is often associated with different cancers.
Section snippets
WNT/β-catenin signaling in cancer
Mutations in the canonical WNT signaling pathway are central to the development of several cancers, including ovarian, pancreatic, colon, lung, and skin.7 Sanchez-Vega et al. performed an extensive landscape analysis of the mechanisms and patterns of somatic alterations in key oncogenic signaling pathways in 33 cancer types (on 9125 tumors) in The Cancer Genome Atlas (TCGA) study. Their data suggest that WNT pathway alterations are the most variable across different cancer types, with the most
WNT/β-catenin pathway inhibitors in clinical development as cancer therapeutics
WNT/β-catenin signaling has an instrumental role in several developmental pathways and regulates stem cell homeostasis in multiple tissue epithelium. By contrast, aberrant WNT/β-catenin signaling is implicated in several cancers and the regulation of CSCs.12 Therefore, various approaches have been used to therapeutically target the pathway and several clinical trials are underway for cancer treatment (Table 1).
Novel approaches toward targeting the WNT/β-catenin pathway
Given the expression of WNT/β-catenin in stem cells and its association with developmental signaling, targeting the WNT pathway might lead to multiple adverse effects and vulnerabilities.12 Therefore, attractive novel druggable-molecular targets that can specifically inhibit the aberrant WNT pathway in tumors with no or minimal adverse effects are desirable. Here, we describe prospective routes to target Wnt signaling in the future.
Challenges to targeting the WNT/β-catenin signaling pathway
A strong association exists between the WNT/β-catenin pathway and different aspects of cancer, including tumor initiation, proliferation, metastasis, drug resistance, dormancy, maintenance of CSCs, and antitumor immunity. Several mutations have been reported that cause aberrant and constitutive activation of the WNT/β-catenin pathway in solid and hematological malignancies. Thus, targeting the WNT/β-catenin signaling pathway presents an exciting opportunity to target cancer. However, several
Artificial intelligence and targeting WNT/β-catenin pathway
The complexity of the molecular interactions of the WNT/β-catenin pathway (19 ligands and more than 15 receptors and co-receptors) and its involvement in different physiological functions make it challenging for drug discovery. PPI studies following the traditional route are time consuming and difficult, especially when targeting novel targets. Accurate prediction of PPI using techniques such as support vector machines and random forest classifiers can help researchers make informed drug design
Concluding remarks and future direction
The WNT pathway has emerged as a promising target for several diseases, especially cancer. The discovery of WNT inhibitors can also aid in understanding the fundamental aspects of signaling under different conditions and can provide clinically relevant information. Targeting the WNT pathway is challenging because normal tissues and cells are also WNT dependent. Research advances have enhanced our knowledge of WNT signaling. Currently, cancer mutational data sets supported by bioinformatics
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Manash K. Paul is a principal investigator and scientist at the University of California Los Angeles. He has contributed significantly to the field of stem cell biology, regenerative medicine, and lung cancer. His research focuses on various signaling process involved in maintaining stem cell homeostasis during injury and repair. Recently his group published the role of Wnt in proximal airway stem cell niche regulation and also identified a potent Wnt signaling inhibitor.
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2023, Experimental NeurologyCitation Excerpt :The Wnt signaling pathway plays a key role in the regulation of neurogenesis and tissue homeostasis in the nervous system (Gao et al., 2019). The central effector of the canonical Wnt signaling cascade is β-catenin, which is phosphorylated by casein kinase 1α (CK1α) and glycogen synthase kinase 3β (GSK3β) when the Wnt signaling pathway is inactive, followed by further ubiquitination and degradation (Chatterjee et al., 2022). When the pathway is activated, β-catenin phosphorylation is blocked, and β-catenin accumulates in the cytoplasm and translocates to the nucleus, where it affects downstream target molecules and further exerts its function.
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Manash K. Paul is a principal investigator and scientist at the University of California Los Angeles. He has contributed significantly to the field of stem cell biology, regenerative medicine, and lung cancer. His research focuses on various signaling process involved in maintaining stem cell homeostasis during injury and repair. Recently his group published the role of Wnt in proximal airway stem cell niche regulation and also identified a potent Wnt signaling inhibitor.
Avradip Chatterjee has a PhD in biochemistry from University College London, UK and performed postdoctoral research at Memorial Sloan Kettering Cancer Center, New York, USA. His work on sister chromatid cohesion using structural and biophysical techniques revealed key early insights into cohesin regulation. Dr Avradip is currently a scientist at Cedars-Sinai Medical Center, Los Angeles, USA, where he leads several drug discovery projects involving computational screening and characterization of protein–small molecule complexes through structural, biophysical and biochemical techniques. A major focus of his work is to develop novel small-molecule inhibitors against cancer-relevant transcription factors.
Shelley Bhattacharya is an internationally recognized professor associated with the Visva Bharati (A Central University), India. Her research work has had significant impacts in the fields of xenobiotics and nanoparticle-based cancer cell targeting.