Canonical and non-canonical WNT signaling in cancer stem cells and their niches: Cellular heterogeneity, omics reprogramming, targeted therapy and tumor plasticity (Review)

Abstract

Cancer stem cells (CSCs), which have the potential for self-renewal, differentiation and de-differentiation, undergo epigenetic, epithelial-mesenchymal, immunological and metabolic reprogramming to adapt to the tumor microenvironment and survive host defense or therapeutic insults. Intra-tumor heterogeneity and cancer-cell plasticity give rise to therapeutic resistance and recurrence through clonal replacement and reactivation of dormant CSCs, respectively. WNT signaling cascades cross-talk with the FGF, Notch, Hedgehog and TGFβ/BMP signaling cascades and regulate expression of functional CSC markers, such as CD44, CD133 (PROM1), EPCAM and LGR5 (GPR49). Aberrant canonical and non-canonical WNT signaling in human malignancies, including breast, colorectal, gastric, lung, ovary, pancreatic, prostate and uterine cancers, leukemia and melanoma, are involved in CSC survival, bulk-tumor expansion and invasion/metastasis. WNT signaling-targeted therapeutics, such as anti-FZD1/2/5/7/8 monoclonal antibody (mAb) (vantictumab), anti-LGR5 antibody-drug conjugate (ADC) (mAb-mc-vc-PAB-MMAE), anti-PTK7 ADC (PF-06647020), anti-ROR1 mAb (cirmtuzumab), anti-RSPO3 mAb (rosmantuzumab), small-molecule porcupine inhibitors (ETC-159, WNT-C59 and WNT974), tankyrase inhibitors (AZ1366, G007-LK, NVP-TNKS656 and XAV939) and β-catenin inhibitors (BC2059, CWP232228, ICG-001 and PRI-724), are in clinical trials or preclinical studies for the treatment of patients with WNT-driven cancers. WNT signaling-targeted therapeutics are applicable for combination therapy with BCR-ABL, EGFR, FLT3, KIT or RET inhibitors to treat a subset of tyrosine kinase-driven cancers because WNT and tyrosine kinase signaling cascades converge to β-catenin for the maintenance and expansion of CSCs. WNT signaling-targeted therapeutics might also be applicable for combination therapy with immune checkpoint blockers, such as atezolizumab, avelumab, durvalumab, ipilimumab, nivolumab and pembrolizumab, to treat cancers with immune evasion, although the context-dependent effects of WNT signaling on immunity should be carefully assessed. Omics monitoring, such as genome sequencing and transcriptome tests, immunohistochemical analyses on PD-L1 (CD274), PD-1 (PDCD1), ROR1 and nuclear β-catenin and organoid-based drug screening, is necessary to determine the appropriate WNT signaling-targeted therapeutics for cancer patients.

Figure 1

Therapeutic resistance owing to evolution and plasticity of cancer stem cells (CSCs). CSCs with self-renewal, differentiation and de-differentiation potentials undergo omics reprogramming, such as epigenetic reprogramming, immunoediting (immunological reprogramming), two-way shifts between epithelial and mesenchymal states (epithelial-mesenchymal reprogramming) and two-way shifts between aerobic glycolysis and oxidative phosphorylation in the tricarboxylic acid cycle (metabolic reprogramming). Genetic or epigenetic evolution of CSCs gives rise to a repertoire of drug-resistant CSCs, which cause early recurrence through clonal expansion of drug-resistant CSCs replacing drug-sensitive bulk tumors. By contrast, the plasticity of CSCs with omics reprogramming potential gives rise to dormant CSCs to survive host defense or therapeutic insult, which cause late relapse through reactivation of dormant CSCs into cycling CSCs. CSC-targeted therapeutics are necessary to avoid drug resistance or recurrence after anticancer therapy. MDSC, myeloid-derived suppressor cell; NK, natural killer cell; Treg, regulatory T cell.

Figure 2

Overview of WNT signaling cascades and WNT signaling-targeted therapeutics. WNT signals are transduced by multiple downstream signaling cascades in a cell context-dependent manner. Canonical WNT signaling through Frizzled (FZD) and LRP5/6 receptors is transduced by the WNT/β-catenin and WNT/STOP (stabilization of proteins) signaling cascades, whereas non-canonical WNT signaling through FZD and/or ROR1/ROR2/RYK receptors is transduced by the WNT/PCP (planar cell polarity), WNT/RTK (receptor tyrosine kinase) and WNT/Ca²⁺ signaling cascades. Antibody-based drugs, such as anti-LGR5 antibody-drug conjugate (ADC), anti-RSPO3 monoclonal antibody (mAb), anti-ROR1 mAb and anti-PTK7 ADC, ROR1 chimeric antigen receptor-modified T (CAR-T) cells, porcupine (PORCN) inhibitors and β-catenin inhibitors are representative WNT signaling-targeted therapeutics in clinical trials or preclinical studies for the treatment of cancer patients.

Figure 3

Investigational WNT signaling-targeted therapeutics for genome-based precision medicine. WNT signaling-targeted therapeutics are applicable for mono-therapy of WNT-related human cancers: β-catenin inhibitors for WNT-driven cancers with APC or CTNNB1 alterations; porcupine (PORCN) inhibitors, anti-FZD or anti-RSPO3 monoclonal antibody (mAb) for WNT-driven cancers with RNF43, RSPO2, RSPO3 or ZNRF3 alterations; and anti-ROR1 mAb for WNT-driven cancers with ROR1 upregulation. By contrast, WNT signaling-targeted therapeutics are applicable for combination therapies with tyrosine kinase inhibitors (TKI) to treat a subset of tyrosine kinase (TK)-driven cancers. WNT signaling-targeted therapeutics are also applicable for combination therapies with immune checkpoint blockers (ICB) to treat cancers with immune evasion; however, because WNT signals regulate immune evasion and antitumor immunity in a context-dependent manner, monitoring of WNT signaling and immunity is mandatory to select an appropriate class of WNT signaling-targeted therapeutics for combination immunotherapy. Therefore, omics monitoring, including genome sequencing, transcriptomic, immunohistochemical and organoid-based tests, is necessary before and during selection of WNT signaling-targeted therapeutics for cancer patients. Mut, mutation; Fus, fusion.