WNT Signaling in Melanoma

Abstract

WNT-signaling controls important cellular processes throughout embryonic development and adult life, so any deregulation of this signaling can result in a wide range of pathologies, including cancer. WNT-signaling is classified into two categories: β-catenin-dependent signaling (canonical pathway) and β-catenin-independent signaling (non-canonical pathway), the latter can be further divided into WNT/planar cell polarity (PCP) and calcium pathways. WNT ligands are considered as unique directional growth factors that contribute to both cell proliferation and polarity. Origin of cancer can be diverse and therefore tissue-specific differences can be found in WNT-signaling between cancers, including specific mutations contributing to cancer development. This review focuses on the role of the WNT-signaling pathway in melanoma. The current view on the role of WNT-signaling in cancer immunity as well as a short summary of WNT pathway-related drugs under investigation are also provided.

Keywords: WNT; WNT5A; immune evasion; melanoma; signal transduction crosstalk; β-catenin.

Figure 1

Simplified scheme of canonical WNT -signaling pathway. (A) In the absence of WNT ligands (WNT OFF state), β-catenin is phosphorylated by a destruction complex consisting of AXIN, APC, GSK3β and CK1α to be further ubiquitinated for proteasomal degradation. In the absence of R-spondins, E3 ubiquitin ligases RNF43/ZNRF3 target FZD for lysosomal degradation; (B) binding of WNT ligands to FZD receptors and LRP co-receptors activates WNT-signaling (WNT ON state). AXIN is associated with LRP5/6, whereas DVL is recruited to FZD, which results in dissociation of the destructive complex. β-catenin is accumulated and stabilized in the cytosol, and then unphosphorylated β-catenin is translocated to the nucleus to activate the expression of WNT target genes. APC—adenomatosis polyposis coli; AXIN—axis inhibition protein; BCL—B-cell CLL/lymphoma protein; BRG-1—brahma-related gene-1; CBP—(CREB)-binding protein; CK1α—casein kinase 1α; CK1γ—casein kinase 1γ; CK1ε—casein kinase 1ε; DKK1—Dickkopf-1; DVL—disheveled; FZD—frizzled receptor; GSK3β—glycogen synthase kinase 3β; LEF—lymphoid enhancer-binding factor 1; LGR—leucine-rich repeat-containing G-protein coupled receptor; LRP—low-density lipoprotein receptor related protein; MAK—metastasis associated kinase; PAR1—protease-activated receptor 1; PKC—protein kinase C; PYGO—pygopus; RNF43—ring finger protein 43; sFRP—secreted frizzled-related proteins; TCF—T cell factor; β-TrCP—beta-transducin repeats-containing proteins; WIF1—WNT inhibitory factor 1; WISE—WNT modulator in surface ectoderm; Ub; ubiquitin; ZNRF3—zinc and ring finger protein 3.

Figure 2

An overview of non-canonical WNT-signaling pathways: (A) WNT/planar cell polarity-signaling pathway (PCP) is initiated by WNT binding to FZD and ROR, then DVL is recruited and DVL-Daam-1 complex is activated, followed by JNK and ROCK activation and cytoskeletal rearrangement; (B) WNT/Ca²⁺-signaling pathway is initiated by WNT binding to FZD and ROR, with further G-protein triggered phospholipase C activation leading to phospholipase C intracellular calcium fluxes and downstream calcium dependent responses. AP-1—activator protein 1; CaMKII—Ca2+/calmodulin dependent kinase II; CaN—calcineurin; CDC42—cell division cycle 42; DAG—diacylglycerol; DAAM1—DVL associated activator of morphogenesis; DVL—disheveled; FZD—frizzled; JNK—c-Jun N-terminal kinases; NLK—nemo like kinase; NFAT—nuclear factor of activated T-cells; PIP2—phosphatidylinositol (4,5)-biphosphates; PKC—protein kinase C; PLC—phospholipase C; RAC—Ras-related C3 botulinum toxin substrate; RHO—Ras homolog gene family; ROCK—Rho-associated kinase; ROR—RAR-related orphan receptor; TAK1—transforming growth factor beta-activated kinase 1.

Figure 3

Proposed model of the crosstalk between canonical and non-canonical WNT-signaling in melanoma. In the canonical WNT pathway, WNT–FZD/LRP5/6 interaction initiates β-catenin dependent signaling. β-catenin translocates to the nucleus to drive the transcription of target genes. This is critical for early steps of transformation when melanocytes bypass senescence and start to proliferate, thus promoting first the radial then vertical growth of melanoma. An increase of WNT5A that activates non-canonical WNT-signaling inhibits β-catenin-signaling and enhances the invasiveness of melanoma cells crucial for metastatic spreading of melanoma. Green and red arrows indicate increase and decrease, respectively.

Figure 4

Main signal transduction pathways and transcriptional regulators that interact with the WNT-signaling pathways in melanoma. Figure shows the possible crosstalk between different pathways, however, some of the interactions are genetic context-, disease stage- or treatment-dependent. See the text for more details. APC—adenomatosis polyposis coli; AXIN—axis inhibition protein; BCL—B-cell CLL/lymphoma protein; BRG-1—brahma-related gene-1; CBP—(CREB)-binding protein; CK1α—casein kinase 1α; COX-2—cyclooxygenase 2; DVL—disheveled; FZD—frizzled receptor; GSK3β—glycogen synthase kinase 3β; iASPP—inhibitor of apoptosis-stimulating protein of p53; LEF—lymphoid enhancer-binding factor 1; LRP—low-density lipoprotein receptor related protein; MITF—microphthalmia-associated transcription factor; PTEN—phosphatase and tensin homolog deleted on chromosome ten; PYGO—pygopus; mTOR—mammalian target of rapamycin; PI3K—phosphatidylinositol-3-kinase; RAS—Rat sarcoma.; ROR—RAR-related orphan receptor; RTK—receptor tyrosine kinase—SIAH2—seven in absentia homolog 2.

Figure 5

Mechanisms of immune exclusion in melanoma through WNT/β-catenin-signaling [172]. β-catenin induces expression of ATF3 and ATF3 represses transcription of CCL4. CD103⁺ dendritic cells and cytotoxic CD8⁺ T-cells are not recruited to melanoma, which leads to non-inflamed tumor. When β-catenin-signaling is inactive in melanoma cells, ATF3 is not expressed, which restores CCL4 production and secretion. This stimulates recruitment of immune cells, including CD103⁺ dendritic cells that activate CD8⁺ T cells. Recruitment of tumor specific CD8⁺ T cells in the tumor microenvironment results in immune inflamed melanoma. ATF3—activating transcription factor 3; β- cat—β- catenin; CCL4—CC-motif chemokine ligand 4.

Figure 6

Compounds and antibodies affecting WNT/β-catenin-signaling pathway that were investigated in melanoma (preclinical studies, except for LGK974, which is tested in the clinical trial NCT01351103). APC—adenomatosis polyposis coli; AXIN—axis inhibition protein; BRG-1—brahma-related gene-1; BCL-9—B-cell CLL/lymphoma 9 protein; CBP—cAMP response element-binding protein; CK1α—casein kinase 1 α; CK1γ—casein kinase 1 γ; DKK1—Dickkopf-1; DVL—disheveled; ER—endoplasmic reticulum; FZD—frizzled; GSK3β—glycogen synthase kinase 3β; LRP5/6—lipoprotein receptor related protein 5/6; PYGO—pygopus; TKNS—tankyrase; β-TrCP—beta-transducin repeat–containing protein; Ub—ubiquitin.