WNT Signaling in Tumors: The Way to Evade Drugs and Immunity

Abstract

WNT/β-catenin signaling is involved in many physiological processes. Its implication in embryonic development, cell migration, and polarization has been shown. Nevertheless, alterations in this signaling have also been related with pathological events such as sustaining and proliferating the cancer stem cell (CSC) subset present in the tumor bulk. Related with this, WNT signaling has been associated with the maintenance, expansion, and epithelial-mesenchymal transition of stem cells, and furthermore with two distinctive features of this tumor population: therapeutic resistance (MDR, multidrug resistance) and immune escape. These mechanisms are developed and maintained by WNT activation through the transcriptional control of the genes involved in such processes. This review focuses on the description of the best known WNT pathways and the molecules involved in them. Special attention is given to the WNT cascade proteins deregulated in tumors, which have a decisive role in tumor survival. Some of these proteins function as extrusion pumps that, in the course of chemotherapy, expel the drugs from the cells; others help the tumoral cells hide from the immune effector mechanisms. Among the WNT targets involved in drug resistance, the drug extrusion pump MDR-1 (P-GP, ABCB1) and the cell adhesion molecules from the CD44 family are highlighted. The chemokine CCL4 and the immune checkpoint proteins CD47 and PD-L1 are included in the list of WNT target molecules with a role in immunity escape. This pathway should be a main target in cancer therapy as WNT signaling activation is essential for tumor progression and survival, even in the presence of the anti-tumoral immune response and/or antineoplastic drugs. The appropriate design and combination of anti-tumoral strategies, based on the modulation of WNT mediators and/or protein targets, could negatively affect the growth of tumoral cells, improving the efficacy of these types of therapies.

Keywords: ABCB1; CD47; PD-L1; WNT; cancer; immunity escape; multidrug resistance (MDR); β-catenin.

Figure 1

A schematic illustration representing different WNT signaling pathways. (A) Canonical WNT signaling. Left panel shows inactive pathway. In the absence of WNT ligands, β-catenin is phosphorylated by the destruction complex, constituted by the scaffolding proteins APC and AXIN and the kinases GSK3β and CK1α. Then, β-catenin is ubiquitinated and targeted for proteasomal degradation by the complex containing β-TrCP, FBXW7, NEDDL4, and WTX proteins. Thus, β-catenin degradation prevents its presence in the nucleus where a complex formed by TCF/LEF and TLE/Groucho binds HDACs to inhibit transcription of target genes. Right panel shows canonical WNT signaling active. The binding of WNT ligands to FZD receptors and LRP co-receptors activates WNT signaling. LRP receptors are phosphorylated by CK1α and GSK3β. Then, DVL proteins polymerize and are activated at the plasma membrane inhibiting the destruction complex. This results in stabilization and accumulation of β-catenin in the cytosol and its subsequent translocation into the nucleus where it displaces TLE/Groucho repressors forming an active complex with TCF/LEF proteins that bind co-activators such as CBP/p300, BRG1, BCL9, and PYGO. An alternative way of β-catenin signaling includes the disruption of epithelial E-cadherin interactions, which breaks the binding of β-catenin to the cytoplasmic domain of cadherin and leads to the accumulation of β-catenin first in the cytosol, and later in the nucleus. (B) Schematic illustration representing the main non-canonical WNT pathways. Left panel shows the WNT/PCP pathway. WNT ligands bind to the FZD receptor and the co-receptors ROR 1/2 (or RYK). Then, DVL is recruited and activated followed by VANGL activation. Then DVL binds to the small GTPase RHO A with the collaboration of the cytoplasmic protein DAAM1. The small GTPases RAC1 and RHO activate ROCK and JNK. This leads to rearrangements of the cytoskeleton and/or transcriptional responses via for example, ATF2 and/or NFAT. Right panel shows the WNT/Ca2+ pathway. The signaling is initiated when WNT ligands bind to the FZD receptor and the co-receptor ROR 1/2 (or RYK). Then, DVL is recruited and activated and binds to the small GTPase which activates phospholipase C leading to intracellular calcium fluxes and downstream calcium dependent cytoskeletal and/or transcriptional responses. APC, adenomatous polyposis coli; BCL9, B-cell CLL/lymphoma 9 protein; β-TrCP, β-Transducin repeat-containing protein; BRG1, Brahma related gene 1; CAMKII, calmodulin-dependent protein kinase II; CBP, CREB-binding protein; CDC42, cell division control protein 42; CELSR, cadherin EGF LAG seven-pass G-type receptor; CK1α,ε,δ, casein kinase 1α,ε,δ; DAAM1, DVL associated activator of morphogenesis; DAG, diacylglycerol; DVL, disheveled; FBXW7, F box/WD repeat-containing protein 7; FZD, Frizzled; GSK3β, glycogen synthase kinase 3β; IP3, inositol 1,4,5 triphosphate; JNK, JUN kinase; LGR5, Leucine-rich repeat-containing G-protein-coupled receptor 5; LRP5/6, low-density lipoprotein receptor-related protein 5/6; NEDD4L, neural precursor cell expressed, developmentally downregulated 4-like; NFAT, nuclear factor of activated T cells; NF-κB, nuclear factor kappa B; PK, Prickle; PKC, protein kinase C; PLC, Phospholipase C; p300, E1A Binding Protein p300; RAC, Ras-related C3 botulinum toxin substrate; RHOA, Ras homolog gene family member A; ROCK, Rho kinase; ROR1/2, bind tyrosine kinase-like orphan receptor 1 or 2; RYK, receptor-like tyrosine kinase; TBP, TATA-binding protein; PRCN, Porcupine; PYGO, Pygopus; RNF43, Ring finger protein 43; RSPO, R-spondin; TCF/LEF, T-cell factor/lymphoid enhancer factor; TLE, Transducin-Like Enhancer of Split proteins; VANGL, Van Gogh-like; WTX, Wilms tumor suppressor protein complex; YAP/TAZ, Yes-associated protein/Transcriptional co-activator with a PDZ-binding domain; ZNRF3, Zinc and Ring Finger 3. Created with BioRender.com.

Figure 2

WNT signaling activation in leukocytes. Canonical WNT signaling promotes T cell lymphopoiesis and regulates peripheral T cell activation and differentiation. Thus, canonical WNT activity promotes differentiation into the TH2 and TFH subsets. Regulatory T (Tr) cells survival is also promoted by the canonical WNT pathway. Nevertheless, TCF1 limits the suppressive activity exerted by Tr in the effector T cell population. Finally, WNT/β-catenin activation is essential for the development of innate lymphoid cells (ILCs) natural killer (NK) cells and DC differentiation. Conversely, non-canonical WNT signaling induces the secretion of IL-12 by dendritic cells favoring TH1 responses. CLP, Common lymphoid progenitor; CMP, Common myeloid progenitor; DC, Dendritic cell; ILC, Innate lymphoid cell; iTr, induced regulatory T cell; HSC, hematopoietic stem cell; NK, Natural killer; TFH, T follicular helper cell; TH, T helper. Created with BioRender.com.

Figure 3

Mechanisms of drug resistance induced by WNT/β-catenin signaling in cancer stem cells. (A) Nuclear β-catenin, by means of MYC, TCF/LEF, NF-κB, SNAI1, HIF-1, and other transcription factors, promotes the transcription of genes involved in mechanisms supporting tumor cells survival such as deregulation of molecules involved in drug resistance. (B) ABCB-1 and (C) CD44. Overexpression of ABCB1 and/or CD44 favor the extrusion of chemotherapeutic drugs out of the cell. ABCB1, ATP-binding cassette B1; ERM, Ezrin, Radixin, Moesin; HA, Hyaluronan; HIF-1, Hypoxia inducible factor 1; LRP5/6, Low-density lipoprotein receptor-related protein 5/6; NF-κB, nuclear factor kappa B; MYC, Myelocytomatosis oncogene cellular homolog; SNAIL, Zinc finger protein SNAI1; TCF/LEF, T-cell factor/lymphoid enhancer factor. Created with BioRender.com.

Figure 4

Mechanisms of immune evasion induced by WNT/β-catenin signaling in cancer stem cells. (A) Nuclear β-catenin, by means of MYC, TCF/LEF, NF-κB, SNAI1, and other transcription factors, promotes the transcription of genes involved in mechanisms supporting tumor cells survival such as the deregulation of molecules involved in immune evasion (CCL4, CD47, and PD-L1). (B) The binding of CD47 to SIRPα might prevent phagocytosis of tumor cells (and/or fragments derived from them) by macrophages and dendritic cells with the subsequent absence of T cell activation. (C) Expression of PD-L1 might directly impair effector T cells function within the tumor microenvironment. (D) Inhibition of CCL4 secretion by tumor cells avoids Dendritic cells recruitment to the tumor microenvironment and the subsequent T cell priming and activation. CCL4, CC-chemokine ligand 4; LRP5/6, Low-density lipoprotein receptor-related protein 5/6; NF-κB, Nuclear factor kappa B; MYC, Myelocytomatosis oncogene cellular homolog; PD-1, Programmed cell death 1; PD-L1, programmed death ligand 1; SIRPα, Signal regulatory protein Alpha; SNAIL, Zinc finger protein SNAI1; TCF/LEF, T-cell factor/lymphoid enhancer factor. Created with BioRender.com.

Figure 5

Inhibitors used for antitumoral therapy. (A) Agents targeting molecules of the WNT canonical pathway: (1) Targeting WNT ligands or agonists; (2) Porcupine specific inhibitors; (3) Blocking antibody targeting RSPO3; (4) Frizzled blocking agents; (5) Agents targeting LGR5; (6) GSK3β inhibitors; (7) Transcription complex inhibitors; (8) Agents targeting β-catenin expression. (B) Agents targeting molecules involved in drug resistance: (1) Inhibitors of the extrusion pump ABCB1; (2) Blocking antibodies targeting CD44. (C) Agents targeting molecules involved in immune evasion: (1) CD47 blocking agents; (2) Agents targeting SIRPα; (3) Agents targeting PD-1; (4) Agents targeting PD-L1. Created with BioRender.com.