CST1

CST1 (Cystatin SN) is a secreted cysteine protease inhibitor predominantly expressed in exocrine glands and mucosal epithelia that has been co-opted in multiple cancer types to suppress ferroptosis and promote tumor progression (PMID:11879580, PMID:36369321). CST1 physically interacts with GPX4 and recruits the deubiquitinase OTUB1 to stabilize GPX4 protein, thereby reducing lipid peroxidation and blocking ferroptotic cell death; this axis is transcriptionally activated by TFAP2C, TFAP2A, NFATC2, and BRD9/FOXP1 complexes in a tissue-dependent manner (PMID:36369321, PMID:38243003, PMID:39692484, PMID:41881961, PMID:39306629). Beyond ferroptosis suppression, CST1 activates Wnt/β-catenin, PI3K/AKT, and MEK/ERK/CREB signaling to drive migration and invasion, and in airway epithelial cells it participates in a TSLP/IL-33 positive feedback loop that amplifies eosinophilic inflammation via AKT–SERPINB2 signaling (PMID:33658852, PMID:36354198, PMID:36848349, PMID:29845224, PMID:37075800). CST1 also accumulates in intracellular vesicular structures during cellular senescence and suppresses senescence in cholangiocarcinoma by stabilizing SOX4 (PMID:21636832, PMID:41881961).

1. 2002 Medium

   Mapping CST1 tissue expression established that it is a gland-restricted secretory cysteine protease inhibitor rather than a ubiquitously expressed housekeeping gene, framing its later study in mucosal biology and cancer.

   Evidence RNase protection assay and immunohistochemistry across 23 human tissues

   PMID:11879580

   Open questions at the time

   • No direct demonstration of protease-inhibitory activity in the studied tissues
   • Protein-level quantitation limited to IHC
2. 2011 Medium

   Discovery that CST1 accumulates in intracellular vesicular compartments during senescence broadened its biology beyond secretory protease inhibition and suggested an intracellular role linked to lysosomal proteostasis.

   Evidence Immunofluorescence and immunoblotting in human fibroblasts across multiple senescence triggers

   PMID:21636832

   Open questions at the time

   • No functional test of whether vesicular CST1 is required for senescence phenotype
   • Identity of the vesicular compartment not resolved (lysosomal vs. other)
3. 2018 Medium

   Identification of a CST1–TSLP/IL-33 positive feedback loop in nasal epithelial cells and fibroblasts revealed how CST1 amplifies type-2 airway inflammation, establishing its first defined signaling circuit outside of cancer.

   Evidence Cytokine stimulation of primary nasal epithelial cells and fibroblasts with RT-qPCR and protein measurement

   PMID:29845224

   Open questions at the time

   • Mechanism by which CST1 induces TSLP not defined at the receptor level
   • In vivo airway model not included in this study
4. 2021 Medium

   Demonstration that CST1 activates Wnt/β-catenin signaling to promote gastric cancer migration and invasion provided the first defined oncogenic pathway downstream of CST1, while ENO1 was placed upstream of CST1 in thyroid carcinoma, indicating CST1 acts as a pathway relay node.

   Evidence Luciferase reporter, β-catenin nuclear translocation, Wnt inhibitor rescue (gastric cancer); ENO1 OE/KD with CST1 rescue epistasis (thyroid carcinoma)

   PMID:33658852 PMID:33968941

   Open questions at the time

   • No direct binding partner identified for Wnt pathway activation by CST1
   • ENO1–CST1 regulatory mechanism not resolved at the promoter/protein level
   • Single-lab findings in each cancer type
5. 2022 High

   The critical mechanistic advance was the identification of a CST1–GPX4–OTUB1 ternary complex: CST1 physically binds GPX4 and recruits OTUB1 to deubiquitinate and stabilize GPX4, directly suppressing ferroptosis and promoting metastasis — the first molecular mechanism explaining how a secreted cystatin inhibits cell death.

   Evidence Reciprocal co-IP with mass spectrometry, ubiquitination assays, overexpression/knockdown in vitro and nude mouse metastasis model

   PMID:36369321

   Open questions at the time

   • Structural basis of CST1–GPX4 interaction unknown
   • Whether catalytic cystatin activity is required for GPX4 stabilization not tested
6. 2022 Medium

   CST1 was shown to activate PI3K/AKT in airway smooth muscle cells to promote proliferation/migration, paralleling its oncogenic PI3K/AKT activation and extending its signaling repertoire to non-malignant disease.

   Evidence siRNA/overexpression with PI3K/AKT inhibitor rescue, EdU/CCK-8, Transwell assays in PDGF-BB-treated ASMCs

   PMID:36354198

   Open questions at the time

   • Receptor or cell-surface partner through which secreted CST1 engages PI3K/AKT not identified
7. 2023 Medium

   Identification of MEK/ERK/CREB activation downstream of CST1 in ESCC and AKT–SERPINB2 axis in asthmatic epithelial cells expanded the signaling portfolio of CST1 and placed it upstream of multiple MAPK and AKT effector branches.

   Evidence Western blot for phospho-MEK/ERK/CREB, dual-luciferase for miR-942-5p targeting CST1 (ESCC); CST1 OE/KD with RNA-seq and p-AKT/SERPINB2 readout plus OVA mouse asthma model (airway)

   PMID:36848349 PMID:37075800

   Open questions at the time

   • Direct target/receptor mediating CST1-to-MAPK signal remains unknown
   • SERPINB2 functional role downstream of AKT not fully dissected
8. 2024 Medium

   Upstream transcriptional control of CST1 was resolved: TFAP2C in breast cancer and BRD9/FOXP1 in gallbladder cancer each directly activate the CST1 promoter, linking epigenetic/transcription factor programs to the CST1–GPX4 ferroptosis suppression axis.

   Evidence ChIP + dual-luciferase reporter + siRNA rescue + xenograft models (TFAP2C); ChIP-qPCR + I-BRD9 inhibitor + mRNA-seq + in vivo GBC model (BRD9/FOXP1)

   PMID:38243003 PMID:39306629

   Open questions at the time

   • Whether TFAP2C and BRD9/FOXP1 act in the same or distinct cancer contexts is unexplored
   • Promoter elements mediating each TF's binding not mapped at nucleotide resolution
9. 2025 Medium

   TFAP2A was validated as a second AP-2 family transcriptional activator of CST1 (in lung adenocarcinoma), and UBE2D2 was found to stabilize CST1 protein via suppression of autophagy-dependent degradation, together revealing both transcriptional and post-translational regulatory layers controlling CST1 abundance.

   Evidence ChIP + dual-luciferase + ferroptosis markers + xenograft (TFAP2A); proteomic screening + autophagy/ferroptosis assays + in vivo functional studies (UBE2D2)

   PMID:39692484 PMID:40912429

   Open questions at the time

   • Autophagy receptor or E3 ligase targeting CST1 for autophagic degradation not identified
   • Whether UBE2D2 ubiquitinates CST1 directly or acts indirectly is unresolved
10. 2026 Medium

    NFATC2 was identified as a transcriptional activator of CST1 in cholangiocarcinoma, and CST1 was shown to suppress cellular senescence by stabilizing SOX4, revealing a new anti-senescence function beyond ferroptosis suppression.

    Evidence Multi-omics profiling, gain/loss-of-function, SOX4 rescue, SA-β-gal assay, orthotopic liver and pulmonary metastasis models

    PMID:41881961

    Open questions at the time

    • Mechanism by which CST1 stabilizes SOX4 protein not defined (direct binding vs. intermediate)
    • Relationship between senescence suppression and ferroptosis suppression by CST1 not tested

• The cell-surface receptor or binding partner through which extracellular CST1 activates intracellular signaling cascades (Wnt, PI3K/AKT, MEK/ERK) remains unidentified, and whether its classical cysteine protease inhibitory activity is required for any of its oncogenic or anti-ferroptotic functions has not been tested.
• No receptor identified for CST1 signal transduction
• Protease-inhibitory activity not dissected from signaling functions using catalytic-dead mutants
• Structural basis of CST1–GPX4 interaction unknown