RCN3

RCN3 is a calcium-binding reticulocalbin of the ER lumen that functions as a stress-responsive regulator of fibroblast activation, epithelial injury responses, and TGF-β signaling (PMID:37710230, PMID:41664195). In fibroblasts, TGF-β1 induces RCN3, which then sustains the pathway through a positive feedback loop: RCN3 physically binds the methyltransferase EZH2 and sequesters it in the cytoplasm, relieving EZH2/H3K27me3 repression of TGFBR1 and thereby maintaining persistent TGFBR1 expression and TGF-β1 signaling that drives fibroblast activation in bleomycin-induced lung fibrosis (PMID:37710230). RCN3 also acts non-cell-autonomously: it is secreted from pulmonary epithelial cells via an N140-glycosylation–dependent ER–Golgi route, binds TGFβR1 on fibroblasts, and activates canonical Smad2/3 signaling, with intratracheal recombinant RCN3 alone sufficient to trigger pulmonary fibrosis and early RCN3 neutralization attenuating fibrotic remodeling (PMID:41664195). Within type II alveolar epithelial cells, RCN3 supports the injury-repair process and limits POVPC-induced ferroptosis by restraining lipid peroxidation and iron accumulation and preserving the GSH/GPX4/SLC7A11 axis (PMID:33531801, PMID:41881199). Its EF-hand domains mediate distinct partner interactions—domains 3–4 engage GRP78 and domains 5–6 engage MMP10—coupling RCN3 to calcium handling, ER stress, and metalloproteinase secretion (PMID:40849588, PMID:41966439).

1. 2021 Medium

   Whether RCN3 in alveolar epithelium has a functional role in lung injury was unknown; cell-type-specific deletion established RCN3 in AECIIs as a participant in the injury-repair process.

   Evidence AECII-specific conditional Rcn3 knockout in a PPE-induced emphysema mouse model

   PMID:33531801

   Open questions at the time

   • Molecular mechanism by which RCN3 affects AECII injury was not defined
   • No interaction partners identified in this model
2. 2023 Medium

   It was unclear how RCN3 sustains fibroblast activation; the work showed RCN3 sequesters EZH2 in the cytoplasm to derepress TGFBR1, defining a TGF-β1/RCN3 feedback loop.

   Evidence BioID and co-IP interaction assays, epigenetic (H3K27me3) analysis, and fibroblast-selective Rcn3 knockdown in bleomycin lung fibrosis

   PMID:37710230

   Open questions at the time

   • Structural basis of EZH2 binding not resolved
   • Whether cytoplasmic EZH2 sequestration occurs in other cell types untested
3. 2024 Low

   The role of fibroblast RCN3 in the tumor immune microenvironment was unknown; an RCN3/PCSK6/STAT1 axis was proposed linking CAFs to macrophage polarization in gastric cancer.

   Evidence ChIP, expression assays, and macrophage M1/M2 polarization co-culture in gastric cancer

   PMID:39206893

   Open questions at the time

   • Single study without full mechanistic reconstitution
   • Direct molecular link from RCN3 to PCSK6 not established
   • Whether the axis operates in vivo not shown
4. 2025 Low

   Partner interactions and upstream regulation of RCN3 in cancer were uncharacterized; EF-hand 3–4 binding to GRP78 (with ER stress and PI3K-AKT-mTOR activation) and RELB-driven RCN3 induction by TGF-β were reported.

   Evidence Domain-mapping interaction studies and pathway Western blotting in colorectal cancer; multi-omics and co-culture/xenograft for the RELB/RCN3 feedback loop

   PMID:40849588 PMID:41186513

   Open questions at the time

   • EF-hand 3–4/GRP78 interaction lacks explicit mutagenesis confirmation
   • RELB→RCN3 regulation not fully elaborated experimentally
   • Single lab per claim
5. 2026 Medium

   Whether RCN3 acts intracellularly versus as a secreted ligand, and how it controls epithelial fate, were open; studies showed RCN3 limits ferroptosis in AECIIs and that glycosylation-dependent secreted RCN3 binds TGFβR1 to activate Smad2/3 and drive fibrosis, while EF-hand 5–6 binds MMP10 to promote metastasis.

   Evidence AECII-specific KO with AAV6 rescue and ferroptosis readouts; secretomics with N140-glycosylation mutagenesis, recombinant RCN3 and neutralizing antibody in vivo; co-IP domain mapping with lung metastasis model

   PMID:41664195 PMID:41881199 PMID:41966439

   Open questions at the time

   • Mechanism by which RCN3 restrains lipid peroxidation not molecularly defined
   • Receptor-level details of secreted RCN3/TGFβR1 binding not structurally resolved
   • EF-hand 5–6/MMP10 interaction from abstract without confirmed reconstitution

• How RCN3's distinct EF-hand-mediated partner interactions, its calcium-binding activity, and its dual intracellular/secreted functions are integrated within a single mechanistic framework remains unresolved.
• No structural model of RCN3 EF-hand engagement of distinct partners
• Quantitative role of calcium binding in each function not established
• Determinants directing RCN3 to intracellular vs secreted roles unknown