Classical Wnt signaling, known as β-catenin-dependent signaling, has been extensively studied (4-7). This pathway comprises three primary components: Cell membrane proteins, degradation complexes and β-catenin. Cell membrane proteins include Wnt ligands (Wnt1, Wnt2, Wnt3a and Wnt8), seven transmembrane receptors (FZD), auxiliary receptors and low-density lipoprotein receptor-related proteins 5/6 (LRP5/6). The degradation complex is primarily composed of glycogen synthase kinase 3β (GSK-3β), adenomatous polyposis coli (APC), casein kinase 1α (CK1α) and scaffolding protein axin (4). GSK-3β is a serine/threonine protein kinase that phosphorylates residues Thr41, Ser33 and Ser37 on β-catenin. APC increases the affinity of other components of the complex to β-catenin, whereas CK1α is a tyrosine kinase that phosphorylates Thr45 on β-catenin. Furthermore, axin serves as a scaffolding protein that keeps the degradation complex tightly bound and stable. β-catenin is a member of the connexin family. Activation of the Wnt/β-catenin signaling pathway also involves transduction of Wnt signaling in the cell membrane, maintenance of β-catenin stability in the cytoplasm, and activation of Wnt-associated target genes in the nucleus (5). When Wnt ligands are absent from the cell surface, most β-catenin located at the cell membrane junctions forms a complex with epithelial-type calcium adhesion protein (E-cadherin) and α-catenin to regulate the cytoskeleton and maintain intercellular adhesion. A small amount of unbound β-catenin in the free state is ubiquitinated by the degradation complex via amino-terminal phosphorylation and recognized by the E3 ubiquitin ligase β-transducin repeat-containing protein, which eventually leads to its degradation by the proteasome. The cytoplasm contains low levels of β-catenin in the free state. Therefore, it cannot enter the nucleus to initiate transcription of T cell factor/lymphoid enhancer factor (TCF/LEF), blocking the expression of downstream target genes (6). This blockage inactivates the Wnt pathway. In the presence of extracellular Wnt ligands, Wnt proteins bind to FZD and LRP5/6 to activate disheveled (DVL) proteins in the cytoplasm. Activated DVL inhibits GSK-3β in the degradation complex. Inactive GSK-3β cannot phosphorylate β-catenin, which gradually accumulates in the cytoplasm. When β-catenin reaches a certain level, it is transferred to the nucleus and initiates the transcription of c-Myc, cyclin D1, Dickkopf-associated protein 1, matrix metalloproteinase (MMP)-7, axin 2 and other downstream target genes by binding to TCF/LEF in the nucleus, leading to abnormal cell proliferation and resistance to apoptosis, thereby inducing tumor formation (7). Tumors are induced during this process.

By contrast with activation of the classical Wnt/β-catenin pathway, activation of non-classical Wnt signaling is not dependent on β-catenin. Activation of the Wnt/PCP pathway is initiated by binding of cell-secreted Wnt ligand proteins to the cell membrane receptor FZD and co-receptors receptor-like tyrosine kinase and receptor tyrosine-kinase-like orphan receptor. These co-receptors control activity of small GTPases and regulate cytoskeletal remodeling (8). The binding of Wnt proteins to FZD recruits DVL to the cell membrane for activation (8). DVL activates DVL-associated activator of morphogenesis 1. This process is followed by activation of ρGTPase, which further activates myosin and ρ-associated kinase, thereby altering actin and cytoskeletal rearrangement in the presence of activated Rac GTPase. Activated Rac stimulates c-Jun amino-terminal kinase activation, leading to downstream target gene expression (9). Moreover, Wnt/Ca²⁺ signaling is activated when Wnt binds to FZD, recruiting DVL to the cell membrane via guanine nucleotide-binding proteins. This activates phospholipase C and calmodulin-dependent kinase II, causing increased intracellular calcium ion release and further regulating downstream signaling pathways (10).

SOX2 participates in various stages of embryonic development by activating Wnt signaling and maintaining CSC stemness. Colon cancer-associated transcript 1 (CCAT1) elevates the expression of SOX2 and activates Wnt signaling in A549 and H460 lung cancer cells. However, the self-renewal capacity of lung CSCs is lost when microRNA (miR)-Let-7c binds CCAT1(40). In NSCLC cell lines, nuclear-enriched abundant transcript 1 may activate the Wnt pathway and promote the CSC phenotype by inhibiting epigallocatechin gallate-upregulated copper transporter 1(41). Octamer binding transcription factor 4 (OCT-4) is a lung cancer surface marker of SCs whose expression is regulated by Wnt signaling. When cisplatin-resistant human lung adenocarcinoma A549/DDP cells are stimulated with lithium chloride, an inhibitor of GSK-3β, expression of Wnt signaling target genes Cyclin D1 and OCT-4 is upregulated. Moreover, the proliferation, clonogenic ability, migration and drug resistance of A549/DDP cells is enhanced (42).

Wnt signaling is also involved in the maintenance of gastric cancer stemness. Stable overexpression of Wnt1 increases proliferation and tumor sphere formation in the human gastric adenocarcinoma cell line AGS. Additionally, AGS cells express the CSC surface markers OCT-4 and CD44. Activation of Wnt1 accelerates gastric CSC proliferation, suggesting that Wnt signaling contributes to self-renewal of gastric CSCs (37). Human epidermal growth factor 2 (HER2)-overexpressing gastric cancer cells induce increased stemness by regulating Wnt/β-catenin signaling (43). Placental growth factor (PlGF) is associated with gastric carcinogenesis. Thus, knockdown of PlGF expression induces apoptosis through Wnt signaling in gastric CSCs (44). Ring finger protein 43 is a member of the E3 ubiquitin ligase family and was originally identified in SCs. It attenuates the stemness of gastric CSC-like cells via Wnt/β-catenin signaling (45). The expression of bromodomain and extra-terminal domain protein is frequently upregulated in gastric cancer tissue and also promotes the stemness of gastric cancer cells by activating Wnt/β-catenin signaling (46).

Colorectal carcinogenesis and disease progression are caused by progressive accumulation of genetic mutations. APC or β-catenin mutations activate Wnt/β-catenin signaling and initiate tumor formation. This suggests that Wnt signaling serve a central role in the regulation of colorectal CSCs (47-49). Markers on the surface of colon CSCs include CD44, CD133, CD24, CD29, CD26, CD166, leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5) and aldehyde dehydrogenase 1 (ALDH1) (50). ALDH1B1 is a member of the ALDH1 family that is highly expressed in colon cancer cells. It can activate Wnt/β-catenin signaling and may be involved in tumorigenesis of colon CSCs (51). Higher β-catenin expression levels induce the expansion of Lgr5(+) cells in colonic crypts and the formation of crypts (52). The transcription factor GATA6 is a key regulator of Wnt signaling in colorectal cancer. It directly drives Lgr5 expression in adenoma SCs. Moreover, GATA6 achieves CSC self-renewal by competing with β-catenin/TCF4 to bind the distal regulatory region of the bone morphogenetic protein locus (38). Homeobox A5 abrogates the self-renewal properties of CSC and blocks tumor growth and metastasis by inhibiting Wnt signaling in colon cancer (53).

Wnt/β-catenin signaling contributes to the maintenance of breast CSC stemness. B cell lymphoma factor 11A (BCL11A) is overexpressed in TNBC cells and participates in tumorigenesis and invasion (54). The high expression of this transcription factor causes SC-like characteristics and maintains stemness in breast CSCs by activating Wnt/β-catenin signaling (39). Similarly, Lgr4 is frequently overexpressed in BC and is associated with poor prognosis. Lgr4 regulates Wnt/β-catenin signaling by mediating breast CSC maintenance (55). The expression of calmodulin 11 (CDH11), a type II calmodulin and mesenchymal protein marker, is positively correlated with β-catenin and Wnt2 in breast cancer (56). When CDH11 is inhibited, it may suppress the mammary CSC-like phenotype by regulating the Wnt/β-catenin pathway (56).

The surface markers of ovarian CSCs include CD24, CD44, CD117, CD133, ALDH, SOX2, OCT-4, NANOG and epithelial cell adhesion molecule, also known as CD326, a single channel type I membrane glycoprotein. Increased expression of these markers enables ovarian CSCs to become sphere-forming in vitro and tumorigenic in vivo, promoting development of epithelial ovarian cancer (EOC). This makes these cells more resistant to drugs and produces tumor progenitor cells that lead to tumor progression, metastasis and recurrence (57). Mounting evidence demonstrates Wnt/β-catenin signaling involvement in the acquisition of stemness in ovarian cancer cells (57-59). In one study, ALDH1A1 was overexpressed in cultured ovarian cancer spheres in vitro and was directly associated with key components of β-catenin signaling. This suggests that β-catenin-regulated ALDH1A1 maintains the sphere-forming ability of ovarian cancer cells (58). Another study confirmed that miR-1207 overexpression increases ovarian CSC-like properties in vitro and in vivo. The effects of miR-1207 are caused by Wnt/β-catenin signaling activation via inhibition of negative regulators of this pathway, such as secreted Frizzled-related protein 1 (SFRP1), axin 2, β-catenin inhibitor and TCF4(59).

Cancer cells expressing Wnt1 resist drug-induced apoptosis. Moreover, Wnt/β-catenin signaling induces transcription of drug resistance factors such as multidrug resistance 1 (MDR-1) that is a membrane glycoprotein encoded by the MDR gene, survivin and livin (76). Platinum-based chemotherapy is the first-line treatment option for advanced NSCLC. However, acquired cisplatin resistance is prevalent in patients with NSCLC (78). One study reported that cytoplasmic inhibition of GSK-3β activates Wnt/β-catenin signaling and upregulates survivin expression, leading to cisplatin resistance in NSCLC (79). In another study, c-Myc, a downstream target gene of β-catenin, regulated A549/DDP resistance to cisplatin (80). Examination of β-catenin expression in NSCLC cell line PC9 and gefitinib-resistant cell line PC9/AB (2) revealed increased nuclear translocation of β-catenin in PC9/AB (2) compared with PC9. In addition, expression of certain components of β-catenin signaling (phosphorylated-GSK-3β, DVL1, c-Myc, c-JUN) increases (81). GDK-100017, a 2,3,6-trisubstituted quinoxaline derivative, inhibits Wnt/β-catenin signaling, blocks β-catenin-TCF/LEF interactions and increases sensitivity of A549/Wnt2 cells to radiotherapy (82). FZD8 is a member of the frizzled Wnt ligand-receptor family. Disruption of FZD8 increases the sensitivity of lung cancer cells to the chemotherapeutic drug paclitaxel (83).

Several molecules are involved in resistance to chemoradiotherapy in gastric cancer. Caveolin-1 (Cav-1) increases cisplatin resistance in gastric cancer cells by activating Wnt signaling (84). Similarly, DOCK6, a guanine nucleotide exchange factor, promotes radiotherapy resistance in gastric cancer by regulating Wnt signaling (85). ICG-001, an inhibitor of β-catenin, reduces the chemoresistance of gastric cancer cells by binding to CREB-binding protein (CBP) and interfering with its interaction with β-catenin, thereby inhibiting Wnt signaling (86). Cheng et al (87) investigated the mechanisms underlying regulation of cisplatin resistance by homologous cassette gene transcript antisense RNA (HOTAIR) in gastric cancer cells. Low HOTAIR expression attenuates cisplatin resistance in gastric cancer cells by inhibiting Wnt signaling. The long noncoding RNA FAM83H-antisense RNA 1 silencing also increases the chemosensitivity of gastric cancer cells via Wnt signaling (88). Similarly, basic leucine zipper ATF-like transcription factor 2, a member of the type I activator protein-1 family, reverses multidrug resistance in gastric cancer cells by inactivating Wnt signaling (89).

Wnt signaling plays a key role in chemoradiotherapy resistance in BC. The MDR1 gene encodes permeability glycoprotein, a transmembrane transporter glycoprotein that is a member of the ATP-binding cassette (ABC) transporter protein superfamily. This protein superfamily mediates drug efflux and is associated with tumor drug resistance. Pygo2 expression is upregulated in drug-resistant BC cells and activates MDR1 via Wnt signaling, thereby mediating chemoresistance in BC (90). The expression of the membrane transporter protein Cav-1 is upregulated in BC chemoresistance. Cav-1 promotes drug resistance in breast CSCs via β-catenin/ABCG2 signaling (91). Activation of classical and non-classical Wnt signaling pathways is detected in the tamoxifen-resistant estrogen receptor (ER)(+) breast cancer cell line MCF7. Furthermore, Wnt3a increases tamoxifen resistance in MCF7 cells (92). Follistatin like protein 1, an extracellular matrix glycoprotein, is associated with regulation of cellular signaling pathways. Its expression is considerably upregulated in drug-resistant BC cells. Moreover, this gene can act through integrin β3-induced activation of Wnt signaling (93). Similarly, lncAFAP1-AS1 can induce radiotherapy resistance in TNBC via Wnt signaling (94).

In addition to its involvement in BC resistance, abnormal ABCG2 expression is associated with drug resistance in ovarian cancer. The SC-associated receptor tyrosine kinase c-kit promotes ovarian cancer drug resistance via the Wnt/β-catenin/ABCG2 signaling axis. Low c-kit expression increases ovarian cancer cell sensitivity to chemotherapeutic agents such as cisplatin and paclitaxel (95). One study showed that chemoresistance in high-grade plasma ovarian cancer is associated with Wnt signaling activation. In addition, the sensitization of ovarian cancer-initiating cells to cisplatin is restored by a Wnt signaling inhibitor (96). Human copper transporter 1 is a transmembrane transporter that allows copper and cisplatin to enter cells through the membrane barrier. Wnt/β-catenin signaling inhibits expression of this protein in cisplatin-resistant EOC cells (97). MMP-10 is highly expressed in cancer stem-like/carcinoma-initiating cells in EOC and is associated with platinum resistance. It acts by inhibiting Wnt5a activation during Wnt signaling (98).

Several studies have shown that Wnt signaling is associated with chemoradiotherapy resistance in cervical cancer (99-102). Therefore, β-catenin nuclear expression can be used as a predictive marker of chemoradiotherapy resistance in cervical squamous carcinoma (99). Fat mass and obesity-associated protein, an N6-methyladenine demethylase with upregulated mRNA expression in cervical squamous carcinoma tissue, enhances radiotherapy resistance by regulating β-catenin (100). One study showed that chemotherapeutic drugs activate Wnt/β-catenin signaling in a eukaryotic translation initiation factor 4 E (eIF4E)-dependent manner. This suggests that eIF4E/β-catenin signaling serves a positive regulatory role in chemoresistance in cervical cancer (101). Similarly, LGR5 acts as a cancer-promoting factor by activating Wnt signaling in cervical cancer. Thus, high LGR5 expression in cervical cancer cells promotes cisplatin resistance (102).

Different tumors express specific Wnt ligands. Therefore, blocking specific Wnt ligand-receptor interactions can inhibit tumor cell proliferation (120,121). In one study, addition of anti-Wnt1 monoclonal antibodies to human NSCLC, BC, mesothelioma and sarcoma cell lines led to apoptosis. In addition, the antibodies inhibited tumor growth in vivo (122). Another study showed that Wnt2 inhibitors decrease clone formation and transplanted tumor volume in NSCLC cell lines (123). After transferring interfering RNA of Wnt5a into the human lung squamous carcinoma cell line H157 and human lung adenocarcinoma cell line A549, the proliferative capacity of both cell lines was decreased (124). The recombinant fusion protein ipafricept (known as OMP-54F28) is formed by fusing the cysteine-rich structural domain of FZD8 with the structural domain of immunoglobulin Fc, which blocks Wnt signaling by binding to Wnt ligands. Preclinical studies have shown that OMP-54F28 slows tumor growth and has a synergistic effect when combined with chemotherapeutic agents (125,126). This human monoclonal antibody interacts with five FZD receptors to block classical Wnt signaling and clinical trials have shown that it has good tolerability (125,127).

PORCN is a membrane-bound O-acetyltransferase that modifies Wnt proteins via palmitoylation; only such modified Wnt proteins can be secreted outside the cellular membrane to activate Wnt signaling by interacting with its co-receptors LRP5/6 and FZD (128). LGK974 is a small-molecule PORCN inhibitor that blocks Wnt signaling and induces tumor regression in MMTV-Wnt1 mice. In addition, LGK974 considerably attenuates clone formation in human head and neck cancer cell line HN30(129). ETC-159 is another PORCN inhibitor that blocks secretion and activation of Wnt proteins. Preclinical studies have shown that ETC-159 is highly effective in treating mouse-transplanted tumors with R-spondin translocations in patients with colon cancer (127,130). In another preclinical study, combination of the PORCN inhibitor RX004 and anti-PD-1 enhanced antitumor immune effects (131). PORCN inhibitors have shown therapeutic potential in colorectal, pancreatic, hepatocellular and head and neck tumors. To date, no PORCN inhibitors have entered clinical use; only LGK974, ETC159, CGX1321 and RXC004 have been investigated in phase I clinical trials (132-135).

End-anchored polymerase (TNKS) is a member of the poly ADP-ribose polymerase (PARP) family, which includes two isoforms, TNKS1 (PARP5a) and TNKS2 (PARP5b). These isoforms regulate classical Wnt signaling via poly ADP-ribosylated axin proteins. TNKS inhibitors promote β-catenin degradation by increasing axin levels (120). Treatment of the NSCLC cell line A549 with XAV939 inhibits cell proliferation and migratory capacity. Furthermore, it decreases TNKS, β-catenin and c-Myc protein levels (136). This TNKS inhibitor also decreases proliferative capacity of the SCLC cell line H446 by inhibiting Wnt signaling (137). The combination of XAV939 and chemotherapeutic agent paclitaxel induces apoptosis and inhibits Wnt signaling in BC cells. In addition, this treatment suppresses EMT and angiogenesis. Similarly, combined XAV939 and low-dose (20 nM) paclitaxel results in comparable therapeutic effects in BC cell lines compared with high-dose (200 nM) paclitaxel alone (138). NVP-TNKS656, another TNKS inhibitor, decreases β-catenin protein expression in the nucleus of colorectal cancer cells when combined with PI3K or AKT inhibitors, thereby reversing resistance to PI3K or AKT inhibitors and inhibiting tumor growth (139). Moreover, the TNKS inhibitor G007-LK has a sensitizing effect on anti-PD-1 antitumor therapy (140).

An effective way of targeting the classical Wnt signaling pathway is to block the interaction of β-catenin with downstream transcription factors (120,121). TCF4 is a member of the TCF/LEF family and binds to β-catenin to initiate target gene transcription when Wnt signaling is activated. Inhibitors of β-catenin-TCF4 interactions include PKF115-584, CGP049090, PKF222-815, PKF118-744, PKF118-310, ZTM000990, iCRT3/5/14, NC043, LF3 and UU-T02/03(141). PKF115-584 inhibits β-catenin transcription and proliferation in the adrenocortical tumor cell line H295R in a dose-dependent manner (142). Similarly, CGP049090 and PKF115-584 effectively kill chronic lymphocytic leukemia cells (143). Three inhibitors, PKF118-310, PKF115-584 and CGP049090, downregulate the expression of TCF4/β-catenin target genes c-Myc, cyclin D1 and survivin in hepatocellular carcinoma. These inhibitors also induce apoptosis and cell cycle arrest and inhibit the growth of transplanted tumors in mice (144).

In addition, β-catenin can also interact with p300/CBP and BCL9(141). Thus, pharmacological blockade of Wnt signaling seems promising in preclinical models (141-144).