FZD1

FZD1 is a seven-transmembrane Wnt receptor that activates the canonical β-catenin signaling pathway and contributes to multidrug resistance, epithelial-mesenchymal transition, and female reproductive function. FZD1 binds Wnt ligands such as Wnt7b through its extracellular cysteine-rich domain and cooperates with LRP5/6 co-receptors to trigger β-catenin nuclear translocation and target gene transactivation, while LRP1 antagonizes this signaling by sequestering the FZD1 CRD and disrupting FZD1–LRP6 complex formation (PMID:15923619, PMID:14739301). In drug-resistant cancer cells, FZD1 drives MDR1/P-gp expression through both canonical Wnt/β-catenin and PKCδ/AP-1 signaling branches, and its silencing restores chemosensitivity in neuroblastoma, breast cancer, and NSCLC models (PMID:19421142, PMID:22484497, PMID:24814288). FZD1 transcription and mRNA stability are regulated by epigenetic mechanisms including SIRT2-mediated H3K27 deacetylation, MBD2a promoter binding, cis-regulatory promoter variants affecting Egr1/E2F1 occupancy, m6A-dependent mRNA stabilization by IGF2BP3, and miRNA targeting, while FZD1 knockout in mice causes subfertility due to impaired cumulus-oocyte complex gene expression (PMID:41485500, PMID:33402389, PMID:18715140, PMID:40706743, PMID:22954793).

1. 2004 High

   Establishing how FZD1 signaling is negatively regulated: LRP1 was shown to sequester FZD1 via its CRD and disrupt FZD1–LRP6 complex formation, providing the first mechanism for non-endocytic attenuation of canonical Wnt signaling at the receptor level.

   Evidence Co-immunoprecipitation, endocytosis-defective LRP1 mutants, canonical Wnt reporter assays in transfected cells

   PMID:14739301

   Open questions at the time

   • Physiological tissues where LRP1-mediated FZD1 sequestration operates remain undefined
   • Whether LRP1 similarly regulates other Frizzled family members through CRD interaction is untested
   • Structural basis of the LRP1–FZD1 CRD interaction is unknown
2. 2005 High

   Defining ligand specificity: Wnt7b was identified as a direct FZD1 ligand that activates canonical but not noncanonical Wnt signaling in cooperation with LRP5, establishing FZD1 as a pathway-selective Wnt receptor.

   Evidence Cell surface binding assay and pathway reporter transfection with biochemical co-receptor analysis

   PMID:15923619

   Open questions at the time

   • Full Wnt ligand selectivity profile for FZD1 is incomplete
   • Whether FZD1 activates noncanonical pathways in other cellular contexts is unresolved
3. 2009 Medium

   Linking FZD1 to drug resistance: FZD1 overexpression was found to sustain canonical Wnt/β-catenin activation in drug-resistant neuroblastoma cells, with FZD1 knockdown reducing MDR1 expression and restoring doxorubicin sensitivity, establishing the FZD1–β-catenin–MDR1 axis in chemoresistance.

   Evidence shRNA knockdown, nuclear β-catenin localization, MDR1 expression, and drug sensitivity assays in neuroblastoma cells

   PMID:19421142

   Open questions at the time

   • Whether FZD1 is a cause versus consequence of the drug-resistant state is unclear
   • Not confirmed in in vivo drug resistance models at this stage
4. 2009 Medium

   Revealing cis-regulatory control of FZD1 transcription: promoter SNPs rs2232158 and rs2232157 were shown to create Egr1 and E2F1 binding sites, respectively, producing allele-specific increases in FZD1 promoter activity in bone cells, linking genetic variation to FZD1 expression levels.

   Evidence EMSA for transcription factor binding and luciferase promoter reporter assays with haplotype-specific constructs in osteoblast-like cells

   PMID:18715140 PMID:20051274

   Open questions at the time

   • Downstream effects of these promoter variants on Wnt pathway output in bone are not demonstrated
   • Association with bone phenotypes in human cohorts is not established
5. 2012 High

   Extending the chemoresistance role to breast cancer and demonstrating in vivo reproductive function: FZD1 silencing in MCF-7/ADM cells reduced MDR1/P-gp and β-catenin, while FZD1 knockout mice showed subfertility with blunted cumulus-oocyte gene expression, establishing FZD1 as required for ovarian function independently of WNT4.

   Evidence siRNA knockdown with drug sensitivity assays in breast cancer cells; gene-targeted knockout mice with microarray, qRT-PCR, and fertility assays

   PMID:22484497 PMID:22954793

   Open questions at the time

   • Which Wnt ligand signals through FZD1 in the ovary is unknown
   • The FZD1–MDR1 connection lacks in vivo validation in tumor models
6. 2014 Medium

   Identifying a noncanonical signaling branch downstream of FZD1: PKCδ and AP-1 were placed downstream of FZD1 in multidrug-resistant cells, with PKCδ inhibition phenocopying FZD1 knockdown for ABCB1 repression, demonstrating FZD1 controls drug resistance through both β-catenin and PKCδ/AP-1 pathways.

   Evidence Epistasis with FZD1 shRNA, PKCδ inhibitor (Rottlerin), PKCδ shRNA, ABCB1 expression, drug efflux, and AP-1 activity assays in MES-SA/Dx5 cells

   PMID:24814288

   Open questions at the time

   • Rottlerin has known off-target effects, so PKCδ specificity requires confirmation
   • Whether canonical and noncanonical FZD1 branches are activated simultaneously or context-dependently is unclear
7. 2016 Medium

   Establishing post-transcriptional repression of FZD1 by miRNA: miR-135b was shown to directly target the FZD1 3′-UTR, and its overexpression phenocopied FZD1 knockdown in sensitizing cisplatin-resistant NSCLC cells, linking miRNA regulation to FZD1-mediated chemoresistance.

   Evidence Dual-luciferase 3′-UTR reporter assay, miRNA mimic transfection, drug sensitivity assay in NSCLC cells

   PMID:27643554

   Open questions at the time

   • In vivo relevance of miR-135b–FZD1 axis in drug resistance is not tested
   • Whether miR-135b regulation of FZD1 is context-specific or generalizable is unknown
8. 2021 Medium

   Uncovering epigenetic and epitranscriptomic layers of FZD1 regulation: MBD2a was shown to bind FZD1 promoter CpG islands under hypoxia to activate transcription promoting EMT, while RBM38 was found to stabilize FZD1 mRNA in leukemia cells, and IGF2BP3 was later shown to stabilize FZD1 mRNA in an m6A-dependent manner promoting FZD1/FZD7 heterodimerization and cancer stemness.

   Evidence ChIP at FZD1 promoter, HIF1/SRSF2 splicing manipulation, EMT assays; RIP-qPCR and actinomycin D stability assays; m6A-RIP, IGF2BP3 KD, FZD1 mRNA stability, β-catenin fractionation, and Fz7-21 inhibitor experiments

   PMID:33402389 PMID:34893109 PMID:40706743

   Open questions at the time

   • Whether m6A modification and RBM38 binding act on the same or distinct FZD1 mRNA pools is unknown
   • The structural basis and stoichiometry of FZD1/FZD7 heterodimerization are undefined
   • Single-lab findings for each mechanism; independent confirmation is lacking
9. 2026 Medium

   Identifying SIRT2-mediated histone deacetylation as an epigenetic brake on FZD1: SIRT2 deacetylates H3K27 at the FZD1 promoter to repress FZD1 transcription and Wnt/β-catenin activation, with FZD1 overexpression rescuing SIRT2's tumor-suppressive effects, establishing a direct epigenetic–transcriptional mechanism.

   Evidence ChIP for H3K27ac at FZD1 promoter, SIRT2 overexpression/knockdown, FZD1 rescue, in vivo xenograft, Western blot for β-catenin in tongue cancer cells

   PMID:41485500

   Open questions at the time

   • Whether SIRT2 regulation of FZD1 is specific or reflects broader chromatin remodeling is untested
   • Single lab; awaits independent confirmation

• Key unresolved questions include the structural basis of Wnt–FZD1 and FZD1–co-receptor interactions, how FZD1 selectively activates canonical versus noncanonical pathways in different tissues, and whether therapeutic targeting of FZD1 can overcome chemoresistance in vivo.
• No crystal or cryo-EM structure of FZD1 in complex with any ligand or co-receptor
• Systematic in vivo studies of FZD1 in tumor drug resistance are absent
• The full repertoire of Wnt ligands and co-receptors partnering with FZD1 is incompletely mapped