The CTNNB1 gene encodes the intracellular scaffold protein β-catenin (1), which interacts with adhesion molecules (E-cadherin/CDH1, N-cadherin/CDH2, VE-cadherin/CDH5 and α-catenins) (2,3), transmembrane mucins (MUC1/CD227 and MUC16/CA125) (4,5), cytoplasmic signaling regulators (APC, AXIN1, AXIN2, BTRC/βTRCP1, BTRC2/βTRCP2 and NHERF1/EBP50) (6,7), and nuclear transcriptional regulators (BCL9, BCL9L, CREBBP/CBP, EP300/p300, FOXM1, LEF1/TCF7L3, MED12, SMARCA4/BRG1, SPDEF, TCF7/TCF-1, TCF7L1/TCF-3 and TCF7L2/TCF-4) (8-10). Based on protein-protein interactions (PPIs), β-catenin is involved in cell-cell adhesion, cellular signaling and transcriptional regulation (Fig. 1A).

β-catenin undergoes post-translational modifications, including acetylation, glycosylation, methylation, phosphorylation and ubiquitylation. Oncogenic tyrosine kinases phosphorylate β-catenin at Y654 to release β-catenin from cadherin complexes, whereas canonical WNT signals prevent the phosphorylation of β-catenin at S33, S37, T41 and S45 to release β-catenin from ubiquitylation-mediated degradation (1). The β-TRCP complex is involved in the poly-ubiquitylation of S33/S37/T41/S45-phosphorylated β-catenin and subsequent proteasome-mediated degradation (9,11), whereas USP7 is involved in the de-ubiquitylation and stabilization of β-catenin (12). Stabilized β-catenin is translocated into the nucleus to activate the transcription of TCF/LEF target genes (Fig. 1B). The acetylation of β-catenin at K49 leads to transcriptional activation, whereas the methylation of β-catenin at K49 leads to transcriptional repression (13,14). The functions of β-catenin are regulated by its localization, PPIs and stability based on post-translational modifications.

Representative β-catenin target genes (Fig. 1C) include ABCB1, ALDH1A1, ASCL2, ATF3, AXIN2, BAMBI, CCND1 (Cyclin D1), CD44, Claudin-1 (CLDN1), CTLA4, DKK1, EDN1, EOMES, FGF18, FGF20, FZD7, GBX1, IL10 (IL-10), Jagged-1 (JAG1), LEF1, LGR5, MITF, MSX1, MYC (c-Myc), NEUROD1, NKD1, NODAL, NOTCH2, NOTUM, NRCAM, osteopontin (OPN), PAX3, PPARD, PTGS2 (COX2), RNF43, SNAI1 (Snail), SP5, TCF7, TERT, TNFRSF19 (Troy), VEGFA (VEGF) and ZNRF3 (1,15-28). CTTTGATAT is the consensus DNA-binding motif of TCF-3, whereas CTCGCGAGA is the major DNA-binding motif of TCF-1 (29). By contrast, lysine acetyltransferases CBP and p300 are reported to differentially regulate β-catenin-dependent transcriptional programs in stem cells to propel them towards self-renewal and differentiation, respectively (30). β-catenin target genes are upregulated in a cell context-dependent manner based on the epigenetic status of their regulatory regions and availability of transcriptional regulators (1).

Inflammation is an immune response to repair tissue damage caused by infectious agents, environmental stimuli and endogenous irritants. The failure to resolve acute inflammation leads to chronic inflammation characterized by the continuous activation of macrophages and lymphocytes in the inflamed tissue microenvironment and elevated levels of the pro-inflammatory cytokines IL-1β, IL-6, IL-17 and TNF-α (31,32). Chronic persistent inflammation then leads to the collapse of homeostatic interactions among epithelial cells, stromal cells and immune cells in the tissue microenvironment, which causes organ fibrosis through the myofibroblast-like transition of tissue-resident fibroblasts, stellate cells or bone marrow-derived fibrocytes, and the subsequent deposition of extracellular matrix (ECM) components, including collagen, fibronectin and hyaluronan (Fig. 2A).

β-catenin signaling dysregulation is involved in chronic inflammation, organ fibrosis, and various types of human cancer (1,33). However, β-catenin-targeted therapy is not yet approved for the treatment of patients with β-catenin-related diseases. As β-catenin is an intracellular protein without intrinsic enzymatic activity, it is difficult to target β-catenin for drug development. In this review, gain- and loss-of-function β-catenin alterations in human cancer types are summarized, and the pathophysiology of β-catenin-related chronic inflammation and/or organ fibrosis are presented with emphases on carcinogenesis in the stomach, liver and lungs. Finally, the development of β-catenin inhibitors targeting its upstream regulators, PPIs and downstream effectors are reviewed.

β-catenin-dependent transcription is aberrantly activated in human cancer due to gain-of-function mutations in the CTNNB1 gene itself, in addition to genetic alterations in the APC, AXIN2, RNF43 and RSPO3 genes involved in the canonical WNT/β-catenin signaling cascade, upregulation of canonical WNT ligands in the tumor microenvironment, or Y654 phosphorylation of β-catenin by oncogenic tyrosine kinases, including BCR-ABL1 fusion, FLT3-ITD mutation and overexpression of MET (34).

Missense mutations at or in-frame deletions around S33, S37, T41 and S45 in β-catenin give rise to gain-of-function β-catenin mutants that are resistant to ubiquitylation-mediated proteasomal degradation and induce the upregulation of oncogenic target genes, including CCND1 and MYC, in adrenocortical tumors, bladder cancer, colorectal cancer, gastric cancer, liver cancer, lung cancer, pancreatic cancer, prostate cancer and uterine cancer (35-40). Aberrant β-catenin-dependent transcriptional activation drives human carcinogenesis through the induction of cancer stem cell (CSC) features, bulk tumor proliferation and the epithelial-to-mesenchymal transition (EMT) in the solid tumors mentioned above.

By contrast, nonsense or frame-shift mutations in β-catenin, including R95*, K335fs, R449fs, E458fs, R474*, R535*, E571* and E642fs, also occur in human cancer, including gastric cancer and head and neck squamous cell carcinoma (37,41-43). In melanoma, decreased β-catenin not only promotes invasion and metastasis through disrupted cell-cell adhesion but also resistance to targeted therapy through MITF/APE1 axis repression (44,45). Ctnnb1 haploinsufficiency has been shown to promote aggressiveness and metastasis in a mouse model of HER2-positive basal breast cancer (46). β-catenin exerts not only oncogenic but also tumor-suppressor functions in a context-dependent manner.

β-catenin mutations are classified as i) gain-of-function mutations clustered at or around S33, S37, T41 and S45, ii) loss-of-function mutations due to nonsense or frame-shift mutations and iii) other mutations to be further characterized. In addition to these coding mutations, copy number gain (47) and regulatory mutations in the proximal promoter region (48) of the CTNNB1 gene encoding β-catenin have been reported in prostate cancer and breast cancer, respectively. Owing to the pro- and anti-oncogenic roles of β-catenin, integrative omics analyses, including whole-genome sequencing, transcriptome and immunohistochemical analyses, are necessary to precisely prescribe β-catenin-targeted therapeutics in personalized or precision medicine in the future.

Helicobacter pylori (H. pylori) is a Gram-negative, helical rod-shaped microaerophilic bacterium that specifically colonizes the human stomach through interactions between bacterial BabA and HopQ adhesins to host ABO blood antigens and CEACAM, respectively (49,50). H. pylori secretes urease for ammonia production from urea to survive in an microenvironment of high acidity, and they deliver CagA, VacA, γ-glutamyl transpeptidase and other virulence factors to the gastric microenvironment to elicit epithelial polarity dysregulation and mucosal damage, leading to pro-inflammatory cytokine secretion and immune tolerance (51,52). Cytokine-mediated T helper cell (Th)1 and Th17 responses induce anti-H. pylori acquired immunity within H. pylori-colonized microenvironments, whereas the dendritic cell (DC)-mediated expansion of regulatory T (Treg) cells leads to immune evasion within the gastric microenvironment and cross-tolerance to allergens in extra-gastric microenvironments (51,53).

In human organoid culture systems infected with H. pylori, CagA is injected into epithelial cells through the type IV secretion system and associates with the HGF receptor (MET) and hyaluronan receptor (CD44) to promote epithelial proliferation, in part through β-catenin signaling activation (54-56). By contrast, in a mouse model with artificial H. pylori infection, Rspo3 is upregulated to expand Lgr5⁺ gastric stem/progenitor cells (57). As WNT2B (WNT-13) was originally cloned and characterized as a canonical WNT ligand derived from the human stomach (58,59), RSPO3 can enhance canonical WNT/β-catenin signaling activation through the release of Frizzled family WNT receptors from RNF43/ZNRN3-mediated repression (1,60) and thus promote epithelial proliferation and gastric hyperplasia. The WNT/β-catenin signaling cascade is aberrantly activated in human gastric cancer due to gain-of-function mutations in the CTNNB1 gene, as mentioned above, and to loss-of-function mutations in the APC and RNF43 genes (37). Together, these findings indicate that β-catenin serves key roles in H. pylori-related chronic active gastritis and gastric cancer.

Decades of persistent H. pylori infection lead to field cancerization in the stomach (Fig. 2B) through the sequential progression of chronic active gastritis, fundic gland atrophy, intestinal metaplasia and gastric cancer (61-63). Although H. pylori eradication in relatively older patients with irreversible gastric atrophy and intestinal metaplasia may not contribute to the prevention of gastric carcinogenesis due to preexisting genetic and epigenetic alterations in the premalignant lesions, H. pylori eradication in relatively younger patients with chronic active gastritis and/or reversible atrophy is optimal for the prevention of gastric carcinogenesis (Fig. 2C). A triple or quadruple regimen (proton-pump inhibitor, amoxicillin and clarithromycin with or without metronidazole) is prescribed for the eradication of H. pylori (64-67). To avoid the prevalence of drug-resistant H. pylori, the quadruple regimen is recommended for H. pylori eradication in Canada, Europe and the US.

By contrast, atezolizumab, avelumab, durvalumab, nivolumab and pembrolizumab are representative immune checkpoint inhibitors that are approved for the treatment of patients with certain types of cancer (34,68). As H. pylori eradication eliminates H. pylori-related chronic inflammation and H. pylori-specific immune evasion, and reduces non-specific immune evasion caused by the DC-mediated Treg expansion and IL-10 elevation in the gastric microenvironment, H. pylori eradication may synergize with immune checkpoint inhibitors for the treatment of advanced gastric cancer with chronic active gastritis.

Chronic liver inflammation associated with hepatitis C virus (HCV) infection, hepatitis B virus (HBV) infection, alcohol abuse, non-alcoholic fatty liver disease and other etiologies leads to liver fibrosis due to the myofibroblast-like transition of hepatic stellate cells or other mesenchymal cells and subsequent accumulation of excessive ECM (31,33). Liver cirrhosis is the most advanced stage of liver fibrosis, which is characterized by impaired liver functions and complications, including ascites, hepatic encephalopathy and upper gastrointestinal bleeding. Persistent liver inflammation leads to field cancerization in the liver through the sequential progression of chronic hepatitis, liver fibrosis and hepatocellular carcinoma (HCC) (69), similar to H. pylori-related field cancerization in the stomach (Fig. 2C).

The canonical WNT/β-catenin signaling cascade is involved in the development and homeostasis of the liver (70,71). Canonical WNT/β-catenin signals promote the proliferation of LGR5⁺ hepatocyte progenitors in the peri-venous zone of hepatic lobules, whereas non-canonical WNT and other signals promote the proliferation of cholangiocyte progenitors in the peri-portal zone of hepatic lobules (71-73). Canonical Wnt-dependent Lgr5⁺ liver stem/progenitor cells in an organoid culture have successfully been applied in transplantation therapy for liver failure in a rat model (74).

HCV is a single-stranded RNA virus that infects hepatocytes to produce Core, NS3/4A, NS5A, NS5B and other viral proteins, and HCV upregulates the expression of β-catenin and MYC in hepatocytes (69). HBV is a partially double-stranded DNA virus that infects hepatocytes to produce HBx, pre-S, S and other viral proteins, and HBx upregulates the expression of EPCAM, β-catenin and MYC and activates NF-κB signaling in hepatocytes (69). The RSPO-dependent activation of the WNT/β-catenin signaling cascade is involved in the activation of hepatic stellate cells to promote liver fibrosis (75), whereas a β-catenin inhibitor (PRI-724) has been shown to prevent HCV-related liver fibrosis in a mouse model (76). By contrast, the WNT/β-catenin signaling cascade is aberrantly activated in human HCC due to gain-of-function mutations in the CTNNB1 gene and loss-of-function mutations in the APC and AXIN1 genes (35). Additionally, gain-of-function β-catenin mutations (S33Y or S45Y) and the overexpression of Met have been shown to synergistically promote liver tumorigenesis in a mouse model (77), whereas an oncolytic adenovirus Ad.wnt-E1A(Δ24 bp)-TSLC1 has been shown to effectively target liver cancer cells with aberrant β-catenin-TCF/LEF signaling activation and repress in vivo tumorigenesis and metastasis (78). β-catenin plays key roles in multiple processes of chronic inflammation-related liver pathophysiology, including hepatocyte proliferation, stellate-cell activation, liver fibrosis and liver tumorigenesis.

The eradication of HCV is predicted to be an optimal approach to prevent HCC; as such, the eradication of H. pylori may be used to prevent gastric cancer. Direct-acting antivirals, including HCV NS3/4A protease inhibitors (glecaprevir and paritaprevir), HCV NS5A inhibitors (ledipasvir, ombitasvir, pibrentasvir and velpatasvir) and HCV NS5B RNA polymerase inhibitors (dasabuvir and sofosbuvir), have been developed for the eradication of HCV. For example, glecaprevir/pibrentasvir and velpatasvir/sofosbuvir are combination drugs that are approved for the treatment of HCV genotypes 1, 2, 3, 4, 5 and 6 (79,80). Studies in Italy and Spain revealed that the occurrence and recurrence of HCC were not prevented by HCV eradication (81,82), whereas a study in Japan revealed that the occurrence of HCC was successfully prevented by HCV eradication (83). As complications of non-viral etiologies of chronic liver inflammation, ethnic or genetic backgrounds, stages of liver fibrosis, and genomic or epigenetic alterations in premalignant lesions may affect the outcomes of HCV eradication, further investigations in larger cohorts are necessary to address the controversy regarding the rate of HCC development following HCV eradication.

Fibrosis is a common pathology of chronic inflammation in the liver and other organs, including the lungs, heart and kidneys (31,32). Irreversible pulmonary fibrosis, cardiac fibrosis or renal fibrosis leads to organ destruction and subsequent decompensation, which is the final serious condition in patients with non-cancerous diseases. For example, cardiac fibrosis is caused by the transformation of cardiac fibroblasts into myofibroblasts and leads to myocardial stiffness and ventricular dysfunction (32), whereas pulmonary fibrosis is caused by chronic inflammation associated with cancer therapy, cigarette smoking, connective tissue diseases, environmental pollution, infection, pulmonary hypertension and idiopathic pulmonary fibrosis (84,85).

Airway damage-induced canonical WNT/β-catenin signaling activation in alveolar epithelial type II cells promotes the activation and remodeling of interstitial fibroblasts; transient remodeling leads to resolution, whereas persistent remodeling leads to pulmonary fibrosis (31,85). Canonical WNT/β-catenin signaling activation in pulmonary endothelial cells activates perivascular fibroblasts to undergo a myofibroblast-like transition, which also leads to ECM accumulation and increased tissue stiffness, further promoting pulmonary fibrosis (86,87). Pulmonary injuries also induce the chemokine-dependent recruitment of monocytes and their subsequent transition into monocyte-derived alveolar macrophages that express higher levels of pro-inflammatory and pro-fibrotic genes than tissue-resident alveolar macrophages (88). However, β-catenin inhibitors, including ICG-001 and XAV939, ameliorate chronic lung injury and prevent the progression to severe pulmonary fibrosis (89,90). The canonical WNT/β-catenin signaling cascade is involved in the pathogenesis of pulmonary fibrosis.

Genetic alterations in the canonical WNT/β-catenin signaling regulators are relatively rare in human lung cancer (91). However, lung CSCs differentiate into WNT-producing supporting cells to maintain the stemness of CSCs and promote the expansion of bulk tumor cells (92). The upregulation of RSPO2 or RSPO3 leads to canonical WNT/β-catenin signaling activation in patient-derived xenograft (PDX) models of human lung cancer (93), and nuclear β-catenin staining is associated with poor prognosis in patients with lung cancer (94,95). Despite relatively infrequent genetic alterations, the canonical WNT/β-catenin signaling cascade is involved in multi-step tumorigenesis in the human lungs through WNT- and RSPO-dependent paracrine signaling.

Receptor tyrosine kinases (RTKs), including ALK, DDR2, EGFR, FGFR1, FGFR2, HER2, MET, NTRK1, RET and ROS1, are aberrantly activated in human lung cancer due to gene amplification, gene fusions or point mutations (96-99). Although ALK inhibitors (alectinib and ceritinib), an ALK/ROS1 inhibitor (crizotinib) and EGFR inhibitors (afatinib, erlotinib, gefitinib and osimertinib) are approved for the treatment of patients with lung cancer, drug resistance and recurrence are difficult to avoid due to acquired mutations in the targeted RTKs, the by-passed activation of other RTKs and β-catenin signaling activation (34). Therapy-related chronic inflammation, in addition to cancer-cell plasticity and intra-tumor heterogeneity, lead to resistance to RTK-targeted therapeutics, in part through canonical WNT signaling activation.

β-catenin is involved in chronic inflammation, organ fibrosis and carcinogenesis; however, β-catenin lacking intrinsic enzymatic activity is a difficult target for drug development. Antibody-based or decoy-receptor drugs targeting ligands or receptors involved in canonical WNT signaling and small-molecule compounds targeting porcupine (PORCN), tankyrase (TNKS) and β-catenin PPIs have been developed as β-catenin inhibitors for preclinical studies and/or clinical trials (34). In addition to these investigational drugs, small-molecule compounds targeting epigenetic/transcriptional regulators involved in β-catenin-dependent transcription and antibody- or peptide-based drugs binding to β-catenin-target gene products have also been suggested as β-catenin inhibitors. β-catenin-targeted therapeutics are functionally classified as follows: i) β-catenin inhibitors targeting upstream regulators, ii) β-catenin inhibitors targeting PPIs, iii) β-catenin inhibitors targeting epigenetic regulators, iv) β-catenin inhibitors targeting mediator complexes, and v) β-catenin inhibitors targeting transcriptional outputs (Fig. 3). Follows is a discussion of the pros and cons of each β-catenin inhibitor class.

Clinical medicine, particularly clinical oncology, is moving toward genomics-based personalized medicine due to the development of nucleotide sequence technologies. Such personalized medicine is expected to further evolve into omics- and clinical record-based precision medicine with relatively inexpensive costs due to increasing medical expenses in aging societies (162). Genomics-based testing platforms, including MSK-IMPACT (43), FoundationOne (163) and Oncomine Comprehensive Panel (164), and organoid- or PDX-based drug screening (165-167) are useful tools for the prescription of targeted therapeutics; however, there remain unmet medical needs for patients with refractory cancer driven by gain-of-function mutations in non-enzymatic oncogenes, including CTNNB1 and KRAS, or loss-of-function mutations in tumor-suppressor genes, including APC and TP53.

β-catenin signaling is involved in myofibroblast activation and subsequent organ fibrosis (Fig. 2A), whereas the activation of β-catenin and NF-κB signaling is involved in field cancerization in the liver associated with HBV, HCV and other etiologies, and field cancerization in the stomach associated with H. pylori infection (Fig. 2B). Vaccines against HBV and H. pylori are available for the prevention of cancer-associated infections (168,169). The eradication of HCV and H. pylori are optimal choices for the first-line prevention of HCC and gastric cancer, respectively (Fig. 2C); however, pathogen eradication is not always successful and may occur too late to reverse the process of field cancerization. β-catenin inhibitors are expected to be applicable for organ fibrosis prevention, second-line HCC prevention and treating β-catenin-driven cancer (Fig. 2C). Although β-catenin without intrinsic enzymatic activity is difficult to target in drug development, several classes of investigational β-catenin inhibitors (Fig. 3) are in preclinical stages or clinical trials for treating patients with β-catenin-related diseases. The multi-layered prevention and treatment strategy of β-catenin-related human diseases is realistic at present and necessary for the implementation of precision medicine in the future.