Frizzled receptors (FZDs) in Wnt signaling: potential therapeutic targets for human cancers

Abstract

Frizzled receptors (FZDs) are key contributors intrinsic to the Wnt signaling pathway, activation of FZDs triggering the Wnt signaling cascade is frequently observed in human tumors and intimately associated with an aggressive carcinoma phenotype. It has been shown that the abnormal expression of FZD receptors contributes to the manifestation of malignant characteristics in human tumors such as enhanced cell proliferation, metastasis, chemotherapy resistance as well as the acquisition of cancer stemness. Given the essential roles of FZD receptors in the Wnt signaling in human tumors, this review aims to consolidate the prevailing knowledge on the specific status of FZD receptors (FZD1-10) and elucidate their respective functions in tumor progression. Furthermore, we delineate the structural basis for binding of FZD and its co-receptors to Wnt, and provide a better theoretical foundation for subsequent studies on related mechanisms. Finally, we describe the existing biological classes of small molecule-based FZD inhibitors in detail in the hope that they can provide useful assistance for design and development of novel drug candidates targeted FZDs.

Keywords: FZD inhibitors; Frizzled receptors; Wnt signaling pathway; cancer therapy; structural foundation.

Fig. 1. Sequence analysis and structure of human FZDs and the binding pattern of haXWnt8–mFzd8_CRD–hLRP6_E1E2 (PDB code 8CTG).

a Multiple sequence alignment of FZD receptors (after shearing) in https://espript.ibcp.fr/ESPript/ESPript/. Ten FZDs constitute a highly conserved family of receptors sharing highly repetitive sequences. b Phylogenetic tree of human class Frizzled receptors FZD1–10 created with the MEGA software using multiple sequence alignment. The family of FZD1–10 can be further divided into four subfamilies based on the amino acid sequence identity: FZD1, FZD2, and FZD7; FZD3 and FZD6; FZD5 and FZD8; FZD4, FZD9, and FZD10. c Schematic representation of Fz receptors (SP signal peptide; TM transmembrane domain; CD cytoplasmic domain). Extracellular segments of FZDs include a CRD, a Connector, and a Linker between TMs. d hLRP6_E1E2 interacts with XWnt8 through two sites of binding (site A and site B) at the top of the structure. XWnt8 interacts with mFzd8CRD through “thumb” and “index” fingers (a special structure similar with a human hand). The ribbon model of Wnt Ternary Complex colored in cyan (hLRP6_E1E2), magenta (XWnt8), and green (mFzd8CRD).

Fig. 2. Canonical Wnt/FZD signal transduction.

(Created with BioRender.com). a In the absence of the extracellular Wnt ligands, the signaling switch is not activated. Then β-catenin is continuously phosphorylated and degraded, Casein kinase 1α (CK1α), axis inhibition protein (AXIN), adenomatous polyposis coli (APC), and the serine/threonine kinase glycogen synthase kinase3β (GSK3β) together form the β-catenin degradation complex. Phosphorylation of β-catenin by the degradation complex will further trigger its degradation, thus preventing the abnormal cellular responses triggered by the canonical pathway. b In the presence of Wnt, it binds to two coreceptors on the cell membrane and the degradation complex is inhibited. Free β-catenin is no longer phosphorylated and accumulates in the cytoplasm, then they are translocated to the nucleus to promote transcription of downstream target genes.

Fig. 3. Non-canonical WNT pathways.

(Created with BioRender.com). a In the Wnt/PCP pathway, the binding of Wnt to FZD and the coreceptor tyrosine kinase-like receptor alone 1/2 (ROR1/2)/receptor-associated tyrosinase (RYK) coreceptor causes the activation of DVL recruitment. The binding of DVL to DAMM1 activates Rho-associated protein kinase and ultimately cause actin skeleton alterations and rearrangements. The activation of DVL can also initiate the JNK signaling cascade by triggering RAC. Then the activation of c-Jun further promotes activator protein 1 (AP-1)-dependent gene transcription, which ultimately regulates cytoskeletal remodeling and cell adhesion. b The Wnt/Ca²⁺ cascade activates phospholipase C (PLC) via heterotrimeric G proteins to generate diacylglycerol and inositol 1,4,5-trisphosphate-3 (IP₃), followed by an increase in cytoplasmic Ca²⁺ concentration. The accumulation of Ca²⁺ activates Ca²⁺-sensitive enzymes such as calcium-regulated protein kinase II (CaMKII) and protein kinase C (PKC), and calcium activation of CaMKII and CaN dephosphorylates nuclear factor of transcription factor-activated T cells (NFAT), leading to their nuclear import and subsequent NFAT-mediated gene expression.