LGMN

LGMN encodes legumain (also called asparaginyl endopeptidase, AEP), a lysosomal cysteine endopeptidase that processes cathepsins L, V, B, and D from single-chain to two-chain mature forms in an activity-dependent manner and regulates nuclear cathepsin L levels, thereby influencing downstream proteolytic cascades involved in cell proliferation, cell cycle control, and inflammation [PMID:bio_10.1101_2025.08.17.670765]. LGMN promotes tumor cell invasion and metastasis by upregulating MMP2 and MMP9 expression, and its loss impairs lysosomal/autophagic degradation, reducing cancer cell migration and experimental lung metastasis (PMID:27656894, PMID:38582932). LGMN expression is post-transcriptionally regulated by multiple competing endogenous RNA axes—including the LGMNP1 pseudogene/miR-495-3p, circLGMN/miR-127-3p, and PCGEM1/miR-642a-5p circuits—and at the translational level by METTL3-mediated m6A modification read by YTHDF1 (PMID:32711096, PMID:34582975, PMID:33121976, PMID:41506595). In macrophages, LGMN drives M2-like polarization and oxidized-LDL-induced ferroptosis contributing to atherosclerotic plaque formation (PMID:34893848, PMID:41506595).

1. 1996 Medium

   Molecular cloning of human LGMN established it as a cysteine protease homologous to plant vacuolar processing enzymes and hemoglobinases, mapping to chromosome 14q32.1 and expressed most abundantly in kidney, providing the foundational gene identity.

   Evidence cDNA cloning, Northern blot, and FISH in human tissues

   PMID:8893817

   Open questions at the time

   • Enzymatic activity and substrate specificity not yet characterized
   • Subcellular localization not experimentally verified
   • No functional studies performed
2. 2016 Medium

   Loss-of-function experiments first linked LGMN to tumor invasiveness by showing that LGMN knockdown reduced MMP2/MMP9 expression and suppressed breast cancer cell migration, invasion, and colony formation.

   Evidence shRNA knockdown in breast cancer cell lines with Western blot for MMP2/MMP9 and functional invasion assays

   PMID:27656894

   Open questions at the time

   • Mechanism connecting LGMN protease activity to MMP2/MMP9 upregulation unclear
   • No in vivo validation in this study
3. 2020 Medium

   Two independent ceRNA axes were identified that post-transcriptionally control LGMN abundance: the LGMNP1 pseudogene sponges miR-495-3p, and lncRNA PCGEM1 sponges miR-642a-5p, each de-repressing LGMN mRNA to promote cancer cell proliferation and invasion.

   Evidence Dual-luciferase reporter assays, miRNA mimic rescue experiments, and in vivo xenograft models in GBM and cervical carcinoma

   PMID:32711096 PMID:33121976

   Open questions at the time

   • Relative contribution of each ceRNA axis in physiological versus tumor contexts unknown
   • No direct measurement of LGMN protein activity changes
4. 2021 Medium

   A third ceRNA circuit was defined: circLGMN sponges miR-127-3p to upregulate LGMN, reinforcing the principle that LGMN is regulated by multiple noncoding RNA sponge mechanisms in GBM.

   Evidence circRNA overexpression, miR-127-3p mimic rescue, dual-luciferase reporter, and xenograft in GBM cells

   PMID:34582975

   Open questions at the time

   • Interplay among LGMNP1, circLGMN, and PCGEM1 ceRNA axes not addressed
   • Tissue specificity of circLGMN regulation unknown
5. 2022 Medium

   LGMN was shown to drive macrophage M2 polarization via extracellular vesicle-mediated transfer of the LGMNP1 pseudogene from ectopic endometrial stromal cells, establishing an immunomodulatory role beyond tumor cell-autonomous functions.

   Evidence EV isolation and characterization (NTA, TEM), EV internalization assay, qRT-PCR/Western blot for M1/M2 markers in THP-1 macrophages

   PMID:34893848

   Open questions at the time

   • Whether LGMN protease activity is required for M2 polarization not tested
   • Limited patient samples; in vivo validation lacking
6. 2022 Medium

   Metformin was found to suppress LGMN expression and induce autophagy through the AKT/mTOR/LC3II axis in a LGMN-dependent manner, positioning LGMN as a functional node linking autophagy regulation to cell proliferation.

   Evidence Dose-response Western blot, LGMN overexpression rescue, autophagy inhibitor/inducer experiments, and xenograft in choriocarcinoma cells

   PMID:36464174

   Open questions at the time

   • Mechanism by which LGMN modulates AKT/mTOR signaling not defined
   • Whether this pathway operates in non-cancer cells unknown
7. 2024 Medium

   CRISPR/Cas9-mediated disruption of LGMN confirmed its requirement for lysosomal/autophagic degradation and demonstrated that LGMN loss reduces breast cancer migration, invasion, and experimental lung metastasis in vivo.

   Evidence CRISPR/Cas9 editing via lipid nanoparticle delivery, in vitro migration/invasion assays, in vivo lung metastasis model

   PMID:38582932

   Open questions at the time

   • Specific lysosomal substrates affected by LGMN loss not identified in this study
   • Off-target CRISPR effects not fully ruled out
8. 2025 High

   Biochemical and proteomic analyses demonstrated that legumain directly processes cathepsins L, V, B, and D to their mature two-chain forms and controls nuclear cathepsin L levels, with N-terminomics revealing putative nuclear substrates involved in proliferation, cell cycle, and ribosomal biogenesis.

   Evidence (preprint) Activity-based probes, immunoblots in LGMN-knockout cells, recombinant in vitro cleavage assays, and NICE N-terminomics pipeline

   PMID:bio_10.1101_2025.08.17.670765

   Open questions at the time

   • Preprint; awaits peer review
   • Functional consequences of individual cathepsin processing events not dissected
   • Nuclear substrates identified by proteomics require individual validation
9. 2025 Medium

   Co-immunoprecipitation revealed that LGMN forms a complex with ITGA5 and FAPα in osteosarcoma cells, suggesting a cell-surface or pericellular signaling role beyond its lysosomal protease function.

   Evidence Co-IP, immunofluorescence colocalization, and in vivo tumor growth assay with inhibitors in osteosarcoma cells

   PMID:41086694

   Open questions at the time

   • Reciprocal Co-IP and stoichiometry not fully established
   • Functional consequence of LGMN–ITGA5–FAPα complex on protease activity unknown
   • Context-dependence on polyethylene microplastic exposure limits generalizability
10. 2026 High

    METTL3-deposited m6A modification on LGMN mRNA, read by YTHDF1, enhances LGMN translation in macrophages, and elevated LGMN promotes oxidized-LDL-induced ferroptosis and atherosclerotic plaque formation, establishing an epitranscriptomic layer of LGMN regulation with cardiovascular disease relevance.

    Evidence m6A mapping, YTHDF1 binding assays, macrophage-specific METTL3 and LGMN knockdown in atherosclerosis mouse model, LGMN overexpression rescue

    PMID:41506595

    Open questions at the time

    • Mechanism by which LGMN induces ferroptosis not fully elucidated
    • Whether m6A regulation of LGMN operates in non-macrophage cell types unknown

• The direct substrates of legumain beyond cathepsins remain poorly validated, the structural basis for its substrate specificity is unresolved, and the mechanism connecting LGMN protease activity to downstream signaling events (MMP upregulation, AKT/mTOR modulation, ferroptosis induction) has not been defined.
• No crystal structure of human legumain in complex with a natural substrate
• Causal chain from LGMN activity to MMP2/MMP9 transcription not delineated
• In vivo contribution of nuclear versus lysosomal LGMN pools unknown