The role of the Wnt signaling pathway in cancer stem cells: prospects for drug development

Figures & data

Figure 1 Canonical Wnt signaling.

Notes: (A) Wnt off. In the absence of Wnt (Wingless) ligands, a multi-protein destruction complex in the cytoplasm consists of Axin-1 and its interacting partners tumor suppressor adenomtous polyposis coli (APC), glycogen synthase kinase 3β (GSK3β), and casein kinase 1α (CK1). The destruction complex degrades β-catenin by proteasomal degradation via phosphorylation and, thus, maintains low levels of β-catenin. (B) Wnt on. In the presence of Wnt ligand binding to the frizzled/Lrp5/6 (low density lipoprotein receptor-related proteins 5 or 6) receptors a negative regulator of the destruction complex, dishevelled (Dsh) is recruited leading to degradation of Axin and inactivation of GSK3β (glycogen synthase kinase 3β) and CK1α (casein kinase 1α), thereby inhibiting their interaction with other components of the destruction complex. As β-catenin accumulates in the cytoplasm, it translocates to the nucleus, where it forms a transcriptionally active complex with transcription factors of the T-cell factor (TCF)/lymphoid enhancer factor (LEF) family and coactivators, such as CBP (cyclic AMP response element-binding protein) and p300, driving the expression of Wnt target genes. Arrows indicate activation/induction; blunt ended lines indicate inhibition/blockade.
Abbreviation: APC, adenomtous polyposis coli.

Figure 2 Noncanonical Wnt signaling.

Notes: (A) Noncanonical Wnt/calcium pathway. Wnt ligand binding to frizzled receptors activates Dsh and trimeric G-proteins (Gα,β,γ), leading to the generation of IP3 (inositol 1,4,5-triphosphate) and DAG2 (diacylglycerol) by PLC (phospholipase C)-mediated conversion of PIP2 (phosphatidylinositol biphosphate) and triggering release of calcium ions (Ca²⁺). Subsequent calcium release activates CAMKII (calcium/calmodulin-dependent kinase II), TAK-1 (TGF-β activated kinase 1) and NLK (nemo-like kinase). Calcium release also activates PKC (protein kinase C) and Cdc42 (cell division control protein 42), and, thereafter, actin cytoskeleton is rearranged. In addition, calcium release results in activation of calmodulin, calcineurin, and NFAT (nuclear factor of activated T-cell), which is critical for control of tissue polarity. Arrows indicate activation, blunt ended lines indicate inhibition/blockade. (B) Noncanonical Wnt/PCP (planar cell polarity) pathway. Wnt ligand binding to frizzled receptors leads to activation of dishevelled (Dsh), triggering stimulation of GTPases RAC (Ras-related C3 botulinum toxin substrate), Profilin and RHOA (Ras homolog gene family member A) through activation of DAAM1 (dishevelled associated activator of morphogenesis 1). Subsequently, actin cytoskeleton is rearranged. In addition, Dsh activates RAC and finally activates JNK (c-Jun-N-terminal-kinase) and AP1, which has been implicated in cell migration.
Abbreviations: RYK, receptor tyrosine kinase; ROR, receptor tyrosine kinase-like orphan receptor; ROCK, Rho-associated protein kinase; MAPKKK, mitogen-activated protein kinases; MAPKK, mitogen-activated protein kinases and AP1, activator protein 1.

Table 1 Clinically approved nonspecific Wnt antagonist

Clinical antogonist	Disease	Mechanism	Reference
NSAID		PGE2 generated	139–146
(aspirin, sulindac, celecoxib)		via COX suppresses
		β-catenin degradation
Retinoids	APML	Unclear	147
Vitamin D	Colorectal cancer, breast cancer	Unclear	148

Abbreviations: NSAID, nonsteroidal anti-inflammatory drug; PGE2, prostaglandin E2; COX, cyclooxygenase; APML, acute promyelocytic leukemia.

Table 2 Clinical trials of Wnt inhibitors/modulators

Clinical trials	Disease	Mechanism	Reference
OMP18R5, Vantictumab	Solid tumors	Humanized Ab against multiple Fzd receptors	163
OMP-54F28, Fzd8-Fc	Pancreatic, ovarian, hepatocellular, colorectal, and breast	Fc fusion protein with Fzd8, which binds all Wnt ligands	164
PR1-724	Solid tumors, colon and pancreatic cancer, CML, and AML	Small molecule inihibitor of CBP/catenin binding	167
LGK974, Porcupine inhibitor	Melanoma, breast, and pancreatic adenocarcinoma	Wnt posttranslational acylation	168

Abbreviations: Ab, antibody; Fzd, frizzled; CML, chronic myeloid leukemia; CBP, CREB-binding protein; AML, acute myeloid leukemia.

Figure 3 Schematic of Wnt inhibitors currently in clinical trials.

Notes: LGK974 is a porcupine inhibitor that inhibits Wnt posttranslational acylation. OMP-18R5 is a fully humanized monoclonal antibody that binds to multiple frizzled (Fzd) receptors. OMP-54F28, is an Fc fusion protein with Fzd8, which binds all Wnt ligands. PRI-724 binds specifically to the coactivator CBP blocking its interaction with β-catenin. Arrows indicate activation/induction, blunt ended lines indicate inhibition/blockade.
Abbreviations: Dsh, dishevelled; CK1α, casein kinase 1α; GSK3β, glycogen synthase kinase 3β; APC, adenomatous polyposis coli; TCF/LEF, T-cell factor/lymphoid enhancer factor; CBP, cyclic AMP response element-binding protein.