LOXL4

LOXL4 is a copper-dependent amine oxidase that functions as the principal enzyme for pathological collagen cross-linking in vivo and as a context-dependent signaling mediator in cancer biology. Its conserved C-terminal catalytic domain catalyzes oxidative deamination of lysine residues in collagen and elastin; genetic ablation in mice demonstrates that LOXL4, rather than LOXL2, is the dominant LOX family member required for collagen cross-link formation and fibrosis in the lung (PMID:37235663). Intracellularly, LOXL4 activates the FAK/Src pathway through hydrogen peroxide generated by its amine oxidase activity to promote cell migration (PMID:30704479), directly binds the basic domain of p53 to reactivate it and trigger cell death (PMID:30728460), and is transferred via tumor-derived extracellular vesicles to macrophages where it activates STAT1-dependent PD-L1 expression to suppress CD8⁺ T cell cytotoxicity (PMID:38047403). Transcription of LOXL4 is induced by TGF-β1 through cooperative Smad and JunB/Fra2(AP-1) binding at its promoter (PMID:23572561), upregulated by zinc-dependent ZEB1 (PMID:36910659), and repressed by EZH2-dependent silencing of miR-29b and miR-30d that directly target LOXL4 mRNA (PMID:32754259).

1. 2001 High

   Identification of LOXL4 as a novel LOX family member established its domain architecture—four N-terminal SRCR domains plus a conserved C-terminal copper-dependent amine oxidase domain—and defined its tissue expression profile, providing the molecular framework for all subsequent functional studies.

   Evidence cDNA cloning, sequence/domain analysis, and Northern blotting across human tissues and cell lines

   PMID:11691588

   Open questions at the time

   • No enzymatic activity assay was performed
   • Function of the SRCR domains remained uncharacterized
   • No loss-of-function data
2. 2007 Medium

   Demonstration that LOXL4 re-expression suppresses colony formation and antagonizes Ras/ERK signaling in bladder cancer revealed its first intracellular signaling role and identified promoter hypermethylation as a silencing mechanism, establishing LOXL4 as a context-dependent tumor suppressor.

   Evidence Gene reintroduction, colony formation, ERK western blotting, and methylation analysis in bladder cancer cells

   PMID:17456585

   Open questions at the time

   • Mechanism linking LOXL4 to ERK inhibition not defined
   • Tumor-suppressive function not validated in vivo
   • Epigenetic silencing not confirmed in independent cohorts
3. 2008 Medium

   Mapping the LOXL4 promoter in HNSCC cells identified TATA and SP1 elements as drivers of its tumor-specific transcription, and immunocytochemistry placed the protein pericellularly (perinuclear and cell-surface), setting the stage for understanding both its transcriptional regulation and its dual intra-/extracellular activity.

   Evidence Promoter deletion/reporter mapping, EMSA, and immunocytochemistry in carcinoma versus normal cells

   PMID:18499440 PMID:18949373

   Open questions at the time

   • Specific transcription factors binding SP1/TATA sites not identified
   • Localization studied in a single cell line
   • Mechanism of secretion not addressed
4. 2013 High

   Elucidation of TGF-β1-driven LOXL4 transcription through cooperative Smad and JunB/Fra2(AP-1) binding—with ERK-dependent Fra2 phosphorylation required for this cooperation—provided the first complete signaling-to-transcription circuit for LOXL4 induction and linked it to ECM deposition.

   Evidence Promoter mutagenesis, ChIP, reporter assays, ERK inhibition, and ECM deposition assays in aortic endothelial cells

   PMID:23572561

   Open questions at the time

   • Whether this mechanism operates in non-endothelial contexts was not tested
   • Relative contribution of LOXL4 versus other LOX enzymes to TGF-β1-induced ECM remodeling was unknown
5. 2014 Medium

   Multiple studies demonstrated that LOXL4 promotes gastric cancer cell migration/invasion through FAK/Src pathway activation and is a direct target of miR-193a-3p, which when lost drives oxidative stress-mediated multidrug resistance—establishing LOXL4's pro-oncogenic intracellular signaling axis and its post-transcriptional regulation.

   Evidence Overexpression, knockdown, recombinant protein, FAK/Src western blotting in gastric cancer; miRNA luciferase reporter, xenograft in bladder cancer

   PMID:25216702 PMID:25311867

   Open questions at the time

   • Catalytic activity dependence of FAK/Src activation not tested in these systems
   • Oxidative stress pathway mechanism downstream of LOXL4 undefined
6. 2017 Medium

   Loss-of-function studies in triple-negative breast cancer showed that LOXL4 knockdown increases collagen I/IV levels and thickened collagen bundles while promoting tumor growth and lung metastasis, revealing its role in restraining pathological collagen remodeling in the tumor microenvironment.

   Evidence shRNA knockdown, mouse xenograft, second harmonic generation imaging, western blotting

   PMID:28060764

   Open questions at the time

   • Biochemical mechanism of collagen regulation (cross-linking vs. transcriptional) not distinguished
   • Contradicts pro-oncogenic roles observed in other cancers—context dependence not resolved
7. 2019 High

   Two key mechanistic advances established that (1) intracellular LOXL4 activates FAK/Src specifically through H₂O₂ generated by its amine oxidase catalytic activity and is transferred between cells via exosomes to promote angiogenesis, and (2) LOXL4 directly binds the basic domain of p53 to reactivate it and induce tumor cell death, revealing dual pro- and anti-tumorigenic intracellular functions dependent on distinct protein–protein interactions.

   Evidence Domain deletion mutants, catalytic-dead constructs, exosome isolation/transfer, and angiogenesis assays in HCC; co-immunoprecipitation with domain mapping and xenograft for p53 interaction

   PMID:30704479 PMID:30728460

   Open questions at the time

   • Identity of intracellular amine oxidase substrates that generate H₂O₂ unknown
   • p53 interaction confirmed by co-IP but lacks structural or biophysical validation
   • Whether exosomal and p53-activating functions occur in the same cell context is unclear
8. 2020 High

   Discovery that EZH2 epigenetically silences miR-29b and miR-30d via H3K27me3 at their promoters, and that these miRNAs directly target LOXL4 mRNA, defined a complete upstream epigenetic circuit controlling LOXL4 abundance and linked it to macrophage infiltration in breast cancer.

   Evidence ChIP for H3K27me3, dual-luciferase reporter for miRNA–LOXL4 3′UTR, in vivo xenograft with flow cytometry for immune infiltrates

   PMID:32754259

   Open questions at the time

   • Whether EZH2–miR-29b/30d–LOXL4 axis operates outside breast cancer not tested
   • Mechanism by which LOXL4 promotes macrophage infiltration not defined at this point
9. 2023 High

   Three studies collectively resolved critical questions: (1) genetic knockout proved LOXL4—not LOXL2—is the dominant enzyme for pathological collagen cross-linking and lung fibrosis in vivo; (2) tumor EV-delivered LOXL4 activates STAT1/PD-L1 in macrophages to suppress CD8⁺ T cell killing; and (3) zinc-finger-dependent ZEB1 transcriptionally activates LOXL4 to promote cancer invasion.

   Evidence Loxl4⁻/⁻ versus Loxl2⁻/⁻ mice with collagen cross-link biochemistry; EV transfer, macrophage co-culture with CD8⁺ T cells, orthotopic HCC model; ZEB1 zinc-finger deletion mutant with RNA-Seq in TNBC

   PMID:36910659 PMID:37235663 PMID:38047403

   Open questions at the time

   • Specific collagen cross-link types catalyzed by LOXL4 in vivo not resolved
   • Mechanism of STAT1 activation by LOXL4 protein not delineated
   • Direct ZEB1 binding to LOXL4 promoter not confirmed by ChIP
10. 2025 Medium

    TGF-β1 was shown to reduce LOXL4 protein (but not mRNA) through MEK/ERK-dependent proteasomal degradation in breast cancer, and N-glycosylation was identified as required for LOXL4 secretion, while a separate study linked LOXL4 to Wnt/β-catenin activation in osteosarcoma—expanding the repertoire of downstream pathways and revealing post-translational control mechanisms.

    Evidence Proteasome/MEK inhibitor studies, N-glycosylation perturbation in MDA-MB-231; OE/KD with XAV-939 Wnt inhibitor rescue in osteosarcoma cells

    PMID:39862152 PMID:41118012

    Open questions at the time

    • E3 ubiquitin ligase mediating LOXL4 proteasomal degradation not identified
    • Whether Wnt/β-catenin activation is direct or indirect downstream of LOXL4 unknown
    • Glycosylation sites required for secretion not mapped

• Key unresolved questions include the identity of LOXL4's intracellular amine oxidase substrates that generate H₂O₂, the structural basis for LOXL4–p53 and LOXL4–STAT1 interactions, whether the apparently contradictory tumor-suppressive and oncogenic roles reflect cell-type-specific cofactors or splice variants, and whether LOXL4 inhibition can attenuate fibrosis and immune suppression therapeutically.
• No crystal or cryo-EM structure of LOXL4 available
• No LOXL4-selective small-molecule inhibitor characterized
• Intracellular substrates and cofactors remain unidentified