Section 3.2.1: Wnt signaling pathway in CSCs (from DOI: 10.1038/s41392-020-0110-5)
From publication: "Targeting cancer stem cell pathways for cancer therapy" published as Signal Transduct Target Ther; 2020 ; 5 8; DOI: https://doi.org/10.1038/s41392-020-0110-5
Section 3.2.1: Wnt signaling pathway in CSCs
Figure 1: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7005297/bin/41392_2020_110_Fig1_HTML.jpg
Figure 1 caption: Wnt/beta-catenin pathway in cancer stem cells. The canonical Wnt/beta-catenin pathway regulates the pluripotency of CSCs and determines the differentiation fate of CSCs. In the absence of Wnt signaling, beta-catenin is bound to the Axin complex, which contains APC and GSK3beta, and is phosphorylated, leading to ubiquitination and proteasomal degradation through the beta-Trcp pathway. However, the complex (TAZ/YAP), the long noncoding RNA TIC1 and proteins (TRAP1 and TIAM1) regulate the beta-Trcp pathway. In the presence of Wnt signaling, the binding of LRP5/6 and Fzd inhibits the activity of the Axin complex and the phosphorylation of beta-catenin, which makes beta-catenin enter the nucleus, and then bind to TEF/TCF to form a complex, which then recruits cofactors to initiate downstream gene expression. Some proteins (DKK2 (Dickkopf-related protein 2), DACT1, CDH11, GECG, PKM2, EZH2, CD44v6, MYC, and TERT), microRNAs (miR-1246, miR-9, miR-92a, miR-544a, and miR-483-5p), and long noncoding RNAs (lncR-beta-catm and lncR-TCF7) regulate the activation of the Wnt/beta-catenin pathway in CSCs
Wnts include large protein ligands that affect diverse processes, such as the generation of cell polarity, and cells fate. The Wnt pathway is highly complex and evolutionarily conserved and includes 19 Wnt ligands and more than 15 receptors. The Wnt signaling pathway can be divided into canonical Wnt signaling (through the FZD-LRP5/6 receptor complex, leading to derepression of beta-catenin) and noncanonical Wnt signaling (through FZD receptors and/or ROR1/ROR2/RYK coreceptors, activating PCP, RTK, or Ca2+ signaling cascades). In canonical Wnt signaling, in the absence of Wnt ligands (inactive Wnt signaling state, Fig. 1, left), beta-catenin is phosphorylated by glycogen synthase kinase 3beta (GSK3beta), which leads to beta-catenin degradation via beta-TrCP200 ubiquitination and inhibits translocation of beta-catenin from the cytoplasm to the nucleus. In contrast, in the presence of Wnt ligands (e.g., Wnt3a and Wnt1), the ligands combine with Fzd receptors and LRP coreceptors (active Wnt signaling, Fig. 1, right). LRP receptors are phosphorylated by GSK3beta and CK1alpha. beta-Catenin is released from the Axin complex to enter the nucleus. In addition, beta-catenin combines with LEF/TCF and enhances the recruitment of histone-modifying coactivators, such as BCL9, Pygo, CBP/p300, and BRG1, to activate transcription. Noncanonical Wnt signaling does not involve beta-catenin. During Wnt/PCP signaling, Dvl is activated through binding of Wnt ligands and the ROR-Frizzled receptor. Dvl inhibits the binding of the small GTPase Rho and the cytoplasmic protein DAAM1. The small GTPases Rac1 and Rho together trigger ROCK (Rho kinase) and JNK (c-Jun N-terminal kinase). This results in cytoskeletal rearrangement and/or transcriptional responses. Wnt/Ca2+ signaling is activated by G protein-triggered phospholipase C activity, which results in intracellular calcium flux and downstream calcium-dependent cytoskeletal and/or transcriptional responses.
Aberrant Wnt signaling is found in many cancers, such as invasive ductal breast carcinomas, colorectal cancer, papillary thyroid cancer, esophageal cancer, and colorectal cancer. The activation of Wnt signaling is different in different tumors. Some Wnt activation is caused by mutations in Wnt components, such as Axin mutation in gastrointestinal cancers, APC mutation in colorectal cancer, and beta-catenin mutation in gastric cancer and liver cancer. GSK3 genes are critical for beta-catenin regulation; therefore, many researchers expect the occurrence of GSK3 mutations, but GSK3 mutations are not correlated with cancer occurrence. In addition, some genes (pyruvate kinase isozyme M2 (PKM2) in breast cancer and telomerase reverse transcriptase (TERT) in prostate cancer) and microRNAs (miR-164a in colorectal cancer and miR-582-3p in non-small-cell lung cancer) inhibit the activity of APC, Axin, and GSK3beta to promote the accumulation of beta-catenin in the cytoplasm.
Stem cell signaling pathways and transcriptional circuits are related to the alteration or reactivation of signaling pathways. Tumor dormancy is a lag phenomenon in tumor growth. Dormancy may occur during primary tumor formation or in the diffusion of some of the constituent tumor cells. However, primary tumor dormancy and metastatic dormancy seem to be different processes. In some cases, cells in the TME produce cytokines, such as Wnt proteins, secreted inhibitors of bone morphogenetic protein (BMP), and Delta, which activate the signaling pathway to maintain the self-renewal ability of CSCs. Activation of Wnt induce the transformation of dormant CSCs into active CSCs to promote cell cycle progression through beta-catenin, increasing the expression of downstream cyclin D1 and MYC, and MYC also promotes the expression of the polycomb repressor complex 1 component Bmi-1 and induces the combination E2F with cyclin E. The extracellular matrix (ECM) protein tenascin C often exists in the gap of stem cells, which supports the cell cycle in breast cancer cells by increasing Wnt signals. In addition, aberrant Wnt signaling has also been observed in the self-renewal of CSCs (Fig. 1). Many reports have proven that numerous proto-oncogenes stimulate this process through the Wnt signaling pathway. PKM2 catalyzes the last step of glycolysis and plays an essential role in the proliferation of breast CSCs by associating with increased beta-catenin levels at regions "-410 to 180 and -2250 to 2000". Enhancer of zeste homolog 2 (EZH2), a key component of the polycomb PRC2 complex, promotes self-renewal of CSCs by activating beta-catenin. Moreover, TERT, an RNA-dependent DNA polymerase, acts as a cofactor and forms a complex with beta-catenin to activate Wnt downstream targets in prostate CSCs. Capillary morphogenesis gene 2 increases the expression of nuclear beta-catenin to regulate the self-renewal and tumorigenicity of gastric CSCs, and SMYD3, which is located downstream of the Wnt pathway, has a similar effect. In addition, long noncoding RNAs and microRNAs also promote self-renewal of CSCs through the Wnt signaling pathway. LncTCF7 recruits the SWI/SNF complex to regulate the expression of the TCF7 promoter in liver CSCs. Lnc-beta-Catm associates with the methyltransferase EZH2 to suppress the ubiquitination of beta-catenin and promote its stability, and LncTIC1 interacts with beta-catenin and maintains its stability, activating Wnt/beta-catenin signaling. MicroRNA-1246, miR-19, and miR-92a suppress the expression of AXIN and GSK3beta in CSCs. MicroRNA-544a downregulates GSK3beta in lung CSCs. MicroRNA-483-5p upregulates the expression of beta-catenin in gastric CSCs. In addition, there are still many genes, microRNAs, and noncoding RNAs in CSCs' self-renewal through the Wnt signaling pathway.
Wnt signaling also plays an important role in the dedifferentiation of CSCs. HOXA5, which is a member of the HOX family, induces the differentiation of colorectal CSCs. However, Wnt indirectly suppresses indirectly via MYC, which is an important direct target of beta-catenin/TCF in the intestine. PMP22, an integral membrane glycoprotein in myelin in the peripheral nervous system, induces the differentiation of gastric CSCs, but its mRNA level declines with activation of the Wnt/beta-catenin pathway. Moreover, TRAP1, a component of the HSP90 (heat-shock protein 90) chaperone family, inhibits the differentiation of colorectal carcinoma stem cells by modulating beta-catenin ubiquitination and phosphorylation. Lgr5, a member of the G protein-coupled receptor (GPCR) family of proteins, is located downstream of the Wnt signaling pathway and restrains the differentiation of esophageal SCC stem cells.
Wnt signaling also plays an important role in regulating CSC apoptosis. Dickkopf-related protein 2 induces G0/G1 arrest and cell apoptosis by suppressing beta-catenin activity in breast CSCs. DACT1, a homolog of Dapper that is located at chromosomal region 14q23.1, promotes apoptosis in breast CSCs by antagonizing the Wnt/beta-catenin signaling pathway. Cadherin-11, a proapoptotic tumor suppressor, reduces the level of active phospho-beta-catenin (ser552) to induce apoptosis in colorectal CSCs. Epigallocatechin-3-gallate increases apoptosis by degrading beta-catenin in lung CSCs. The small-molecule inhibitor CWP232228 antagonizes the binding of beta-catenin to TCF in the nucleus to induce apoptosis in liver CSCs. In addition, temozolomide combined with miR-125b significantly induces apoptosis by targeting the Wnt/beta-catenin signaling pathway in glioma stem cells.
Wnt/beta-catenin signaling has been implicated in CSC-mediated metastasis. In the cytomembrane, Frizzled8 promotes bone metastasis in prostate CSCs. The leucine-rich repeat containing GPCR4 (LGR4, or GPR48), together with its family members LGR5/6, binds to R-spondins 1-4 and leads to Wnt3A potentiation, activating Wnt signaling in breast CSCs. Increased levels of CD44v6 mRNA in human pancreatic CSCs, lung CSCs, and colon CSCs promote migration and metastasis through the activation of beta-catenin. In the cytoplasm, TAZ/YAP interacts directly with beta-catenin and restricts beta-catenin degradation, but TIAM1 antagonizes TAZ/YAP accumulation and translocation from the cytoplasm to the nucleus. Moreover, CDH11 inhibits the migration and invasion of colorectal CSCs by inhibiting Wnt/beta-catenin and AKT/RhoA signaling. Wnt signaling decreases the expression of HOXA5 to promote CSC metastasis. These data suggest that amplified Wnt signaling is important for self-renewal, dedifferentiation, apoptosis inhibition, and metastasis of CSCs.
