Affinage

CST6

Cystatin-M · UniProt Q15828

Round 2 corrected
Length
149 aa
Mass
16.5 kDa
Annotated
2026-04-28
47 papers in source corpus 11 papers cited in narrative 11 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

CST6 (cystatin M/E) is a secreted cysteine protease inhibitor that targets cathepsins B, K, V/L, and the asparaginyl endopeptidase legumain, functioning as a key regulator of epidermal differentiation, osteoclast biology, and tumor suppression (PMID:22688893, PMID:36371786, PMID:24742492). In osteoclast precursors, endocytosed CST6 inhibits cathepsin B to stabilize SPHK1, suppressing RANKL-induced p38 signaling, while it also blocks cathepsin-mediated cleavage of NF-κB/p100 and TRAF3 to prevent osteoclast maturation and function (PMID:34815788, PMID:35881476). Loss-of-function mutation in CST6 (p.Gly84Asp) abolishes cathepsin V/L and legumain inhibition and causes a Mendelian skin disorder with dry skin, desquamation, and abnormal keratosis, consistent with the ichq mouse phenotype (PMID:36371786, PMID:12839564). CST6 is frequently silenced in breast cancer and glioma by promoter CpG island hypermethylation, and its transcription is repressed by TBX2/EGR1 and SRC/EGR1 signaling axes in cancer cells (PMID:17043665, PMID:18607344, PMID:24742492, PMID:39695463).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 2003 Medium

    The first genetic evidence established that CST6 is essential for terminal epidermal differentiation, resolving whether it has a physiological role beyond protease inhibition in vitro.

    Evidence Mouse Cst6 loss-of-function (ichq phenotype) with abnormal cornification; human gene sequencing and skin IHC

    PMID:12839564

    Open questions at the time
    • Specific cathepsin substrates mediating cornification defects were not identified
    • No human loss-of-function mutation was reported in this study
    • Mechanism linking CST6 to transglutaminase cross-linking remained uncharacterized
  2. 2006 Medium

    Promoter CpG island hypermethylation was identified as the primary mechanism of CST6 silencing in breast cancer, explaining how tumor cells lose this protease inhibitor.

    Evidence Bisulfite sequencing and 5-aza-2'-deoxycytidine demethylation restoring CST6 expression across breast cancer cell lines

    PMID:17043665

    Open questions at the time
    • Upstream signals driving de novo methylation at the CST6 locus were not identified
    • Functional consequence of CST6 re-expression on tumor phenotype was not tested in this study
  3. 2008 Medium

    CST6 silencing by promoter methylation was extended to gliomas, and ectopic CST6 expression was shown to reduce glioma cell motility and invasion, broadening its tumor-suppressive role beyond breast cancer.

    Evidence Methylation-specific PCR, bisulfite sequencing, and motility/invasion assays after ectopic CST6 expression in glioma lines

    PMID:18607344

    Open questions at the time
    • Direct demonstration that cathepsin B inhibition mediates the invasion-suppressive effect was not provided
    • In vivo glioma model validation was lacking
  4. 2012 High

    CST6 was established as a bona fide secreted suppressor of breast cancer bone metastasis, demonstrating that its extracellular protease-inhibitory activity is sufficient to block metastatic colonization.

    Evidence Secretome proteomics, ectopic overexpression/knockdown in MDA-MB-231 cells, in vitro and in vivo bone metastasis models

    PMID:22688893

    Open questions at the time
    • The specific cathepsin target(s) responsible for the bone metastasis phenotype were not dissected
    • Whether CST6 acts on tumor cells, osteoclasts, or both was unresolved
  5. 2014 High

    Domain mutagenesis revealed that CST6's anti-tumor activity in TBX2-expressing breast cancer operates through legumain (LGMN) inhibition rather than cathepsin inhibition, and identified TBX2/EGR1 as transcriptional repressors of CST6.

    Evidence Legumain- vs cathepsin-inhibitory domain mutants, LGMN activity assay, siRNA epistasis, luciferase reporter assays for TBX2/EGR1 repression

    PMID:24742492

    Open questions at the time
    • LGMN substrates whose cleavage drives proliferation were not identified
    • Intracellular vs secreted CST6 trafficking and localization were not fully mapped
    • Whether the TBX2/EGR1 axis operates across cancer subtypes was untested
  6. 2021 High

    A full signaling axis was delineated: endocytosed CST6 inhibits cathepsin B in osteoclast precursors, stabilizing SPHK1 and suppressing RANKL-induced p38 activation, explaining how tumor-derived CST6 blocks osteoclastogenesis.

    Evidence Endocytosis tracking, CTSB activity assay, SPHK1 overexpression/knockdown, p38 phosphorylation, in vivo bone metastasis model, recombinant CST6 treatment

    PMID:34815788

    Open questions at the time
    • How CTSB cleavage of SPHK1 is regulated and whether other cathepsins contribute was not resolved
    • Whether the CST6–CTSB–SPHK1 axis operates in non-cancer osteoclast biology is unknown
  7. 2022 High

    CST6 was shown to inhibit cathepsin K and block RANKL-dependent cleavage of NF-κB/p100 and TRAF3, defining a second anti-osteoclastogenic mechanism distinct from the SPHK1–p38 axis.

    Evidence Cathepsin K activity assay, NF-κB/p100 and TRAF3 cleavage assays, single-cell RNA-seq of monocyte polarization, ex vivo and in vivo myeloma osteolysis models

    PMID:35881476

    Open questions at the time
    • Relative contributions of cathepsin B vs K inhibition in osteoclast suppression are not quantified
    • Whether CST6 affects osteoblast activity or only osteoclasts is unaddressed
  8. 2022 High

    A human CST6 loss-of-function mutation (p.Gly84Asp) was shown to abolish cathepsin V/L and legumain inhibition and cause a Mendelian skin disorder, providing the first human genetic proof that CST6 is required for normal cornification.

    Evidence Whole-exome sequencing, fluorimetric enzyme inhibition assays, mouse knock-in model with histological and barrier function analysis

    PMID:36371786

    Open questions at the time
    • Full spectrum of human CST6-associated skin disease phenotypes remains to be defined
    • Whether therapeutic protease inhibitors can rescue the skin phenotype was not tested
  9. 2024 Medium

    A SRC–EGR1–CST6 transcriptional axis was mapped, revealing that SRC kinase suppresses EGR1 transcription, which in turn controls CST6 expression, connecting oncogenic kinase signaling to CST6 epigenetic/transcriptional silencing.

    Evidence ChIP/IP for SRC–EGR1 promoter binding, luciferase reporter assay, lentiviral overexpression/knockdown, in vivo bone metastasis model

    PMID:39695463

    Open questions at the time
    • Whether SRC-mediated CST6 repression cooperates with promoter methylation is unresolved
    • Single-study finding requiring independent validation

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key unresolved questions include the structural basis for CST6's dual cathepsin/legumain inhibition, whether CST6 regulates non-canonical (non-cathepsin) pathways in epithelial differentiation, and the therapeutic potential of recombinant CST6 delivery in bone-destructive diseases.
  • No crystal structure of CST6 in complex with any target protease is available
  • In vivo pharmacokinetics and delivery strategies for recombinant CST6 are undeveloped
  • Whether CST6 loss contributes to cancer types beyond breast, glioma, and myeloma is not systematically assessed

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 5
Localization
GO:0005576 extracellular region 4
Pathway
R-HSA-1643685 Disease 3 R-HSA-1266738 Developmental Biology 2 R-HSA-162582 Signal Transduction 2
Partners
CTSBCTSKEGR1LGMNSPHK1TBX2

Evidence

Reading pass · 11 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2012 Secreted CST6 (cystatin E/M) suppresses breast cancer cell proliferation, colony formation, migration, and invasion, and its soluble form inhibits cancer cell motility. Ectopic expression of CST6 in bone-tropic MDA-MB-231 cells rescued mice from osteolytic metastasis, establishing CST6 as a bona fide secreted suppressor of breast cancer bone metastasis. Secretome proteomic profiling (label-free), ectopic overexpression and knockdown in cancer cell lines, in vitro proliferation/migration/invasion assays, in vivo mouse bone metastasis model Cell research High 22688893
2021 Cancer cell-derived CST6 enters osteoclast precursors by endocytosis and inhibits the cysteine protease CTSB (cathepsin B), leading to upregulation of the CTSB hydrolytic substrate SPHK1. SPHK1 in turn suppresses osteoclast maturation by inhibiting RANKL-induced p38 activation, defining a CST6–CTSB–SPHK1–p38 signaling axis in osteoclast differentiation. In vitro osteoclastogenesis assay, endocytosis tracking, CTSB activity assay, SPHK1 overexpression/knockdown, p38 phosphorylation analysis, in vivo bone metastasis mouse model, recombinant CST6 protein treatment Theranostics High 34815788
2008 Ectopic expression of CST6/cystatin E/M in glioma cell lines reduced cell motility and invasion, consistent with its role as an invasion suppressor via inhibition of cathepsin B. CST6 is highly expressed in normal oligodendrocytes and astrocytes but not neurons, and its expression is silenced by CpG island hypermethylation in the majority of primary adult and pediatric gliomas, including in tumor-initiating cells. Cell motility/invasion assays after ectopic CST6 expression, methylation-specific PCR, bisulfite genomic sequencing, pyrosequencing, tissue microarray immunohistochemistry Laboratory investigation Medium 18607344
2014 TBX2 (oncogenic transcription factor) represses CST6 transcription via a co-repression mechanism involving EGR1. CST6-induced apoptosis in TBX2-expressing breast cancer cells requires its legumain (LGMN)-inhibitory domain but not its cathepsin-inhibitory domain, identifying LGMN as the key downstream effector. CST6 does not require secretion or glycosylation for this cell-killing function, indicating an intracellular mode of action. siRNA knockdown confirmed LGMN (not GPI8) maintains breast cancer cell proliferation downstream of CST6 loss. Luciferase reporter assay for TBX2/EGR1-mediated CST6 repression, CST6 domain-specific mutagenesis (LGMN-inhibitory vs cathepsin-inhibitory domains), LGMN activity assay, siRNA knockdown of LGMN and GPI8, apoptosis assay, pan-cathepsin inhibitor treatment Oncotarget High 24742492
2022 Recombinant CST6 protein inhibits cathepsin K activity and blocks osteoclast differentiation and function in multiple myeloma bone disease. CST6 attenuates monocyte-to-osteoclast-precursor polarization (shown by single-cell RNA-seq) and blocks osteoclast differentiation by suppressing cathepsin-mediated cleavage of NF-κB/p100 and TRAF3 following RANKL stimulation. Recombinant protein ELISA, cathepsin K activity assay, ex vivo and in vivo myeloma osteolysis models, single-cell RNA-seq, NF-κB/p100 and TRAF3 cleavage assays The Journal of clinical investigation High 35881476
2022 Homozygous loss-of-function mutation (p.Gly84Asp) in CST6 causes loss of inhibitory activity against cathepsins V and L and legumain, resulting in dry skin, desquamation, and abnormal keratosis. In mouse knock-in models, the mutation causes excessive cornification, desquamation, impaired skin barrier function, and abnormal keratinocyte proliferation and differentiation. Whole-exome sequencing, fluorimetric enzyme inhibition assay with cathepsins V/L and legumain, mouse knock-in model with histological and barrier function analysis Clinical genetics High 36371786
2024 BCMA-CST6 CAR-T cells release CST6 protein upon lysing myeloma cells, and the secreted CST6 suppresses osteoclast differentiation (TRAP+ osteoclast formation). In xenografted MM mice, BCMA-CST6-CAR-T cells (but not BCMA-CAR-T alone) prevented MM-induced bone damage and decreased osteoclast numbers, confirming the functional requirement for CST6 delivery in suppressing osteolysis. CAR-T cell engineering and in vitro co-culture osteoclastogenesis assay (TRAP staining), bioluminescence imaging of xenograft MM mice, micro-computed tomography of bone The Journal of clinical investigation Medium 37883186
2024 Ketamine suppresses SRC kinase activity in breast cancer cells; SRC normally binds the EGR1 promoter to suppress EGR1 transcription. EGR1 in turn activates CST6 transcription. Overexpression of EGR1 or CST6 counteracts SRC-mediated effects, placing CST6 downstream of a SRC–EGR1 transcriptional axis that regulates breast cancer cell viability and osteoclastogenesis. Immunoprecipitation (SRC-EGR1 promoter binding), luciferase reporter assay (EGR1-CST6 transcriptional regulation), lentiviral overexpression/knockdown, in vitro osteoclastogenesis co-culture, in vivo intracardiac bone metastasis mouse model BMC cancer Medium 39695463
2003 The human CST6 gene encodes cystatin M/E, a cysteine proteinase inhibitor expressed in sweat glands, hair follicles, and the stratum granulosum of the epidermis, where it is a putative transglutaminase substrate. Loss-of-function mutations in the mouse Cst6 gene cause the ichq phenotype with abnormal cornification and desquamation, establishing an essential role for CST6 in the final stages of epidermal differentiation. No CST6 mutations were found in 11 harlequin ichthyosis patients, excluding it as a major cause of that condition. Complete human CST6 gene sequencing, mutation screening of patient cohort by sequencing coding regions and intron-exon boundaries, immunohistochemistry for cystatin M/E protein in patient skin The Journal of investigative dermatology Medium 12839564
2006 CST6 expression is silenced in breast cancer cell lines through DNA methylation-dependent epigenetic mechanisms. Treatment with 5-aza-2'-deoxycytidine (a DNA methylation inhibitor) restores CST6 expression in CST6-negative lines. Bisulfite sequencing confirmed that hypermethylation of the CST6 proximal promoter CpG island is strongly associated with gene silencing, establishing promoter methylation as the primary mechanism for CST6 loss in breast cancer. 5-aza-2'-deoxycytidine demethylation treatment, bisulfite sequencing, expression analysis by RT-PCR across a panel of breast cancer cell lines Laboratory investigation Medium 17043665
2026 In vitro co-culture experiments and secretome analysis demonstrated that interaction between CST6-expressing epithelial cells and SPP1+ macrophages in lung adenocarcinoma triggers secretion of pro-metastatic factors TGF-β and MMP9, significantly enhancing tumor cell invasion and migration. Spatial transcriptomics confirmed physical proximity of these two cell subsets at metastatic sites. In vitro co-culture, secretome analysis (TGF-β, MMP9 measurement), spatial transcriptomics, multiplex immunohistochemistry, single-cell RNA-seq International journal of biological macromolecules Low 42002169

Source papers

Stage 0 corpus · 47 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2012 Insights into RNA biology from an atlas of mammalian mRNA-binding proteins. Cell 1718 22658674
2002 Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences. Proceedings of the National Academy of Sciences of the United States of America 1479 12477932
2000 The DNA sequence of human chromosome 21. Nature 808 10830953
2007 Large-scale mapping of human protein-protein interactions by mass spectrometry. Molecular systems biology 733 17353931
2021 Dual proteome-scale networks reveal cell-specific remodeling of the human interactome. Cell 705 33961781
2012 A census of human soluble protein complexes. Cell 689 22939629
1991 The cystatins: protein inhibitors of cysteine proteinases. FEBS letters 670 1855589
2011 Phylogenetic-based propagation of functional annotations within the Gene Ontology consortium. Briefings in bioinformatics 656 21873635
2008 Large-scale proteomics and phosphoproteomics of urinary exosomes. Journal of the American Society of Nephrology : JASN 607 19056867
2006 Hsp90 cochaperone Aha1 downregulation rescues misfolding of CFTR in cystic fibrosis. Cell 517 17110338
1990 The refined 2.4 A X-ray crystal structure of recombinant human stefin B in complex with the cysteine proteinase papain: a novel type of proteinase inhibitor interaction. The EMBO journal 475 2347312
1996 Mutations in the gene encoding cystatin B in progressive myoclonus epilepsy (EPM1). Science (New York, N.Y.) 456 8596935
2004 The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC). Genome research 438 15489334
1986 Isolation of six cysteine proteinase inhibitors from human urine. Their physicochemical and enzyme kinetic properties and concentrations in biological fluids. The Journal of biological chemistry 434 3488317
2015 Panorama of ancient metazoan macromolecular complexes. Nature 407 26344197
2010 Dynamics of cullin-RING ubiquitin ligase network revealed by systematic quantitative proteomics. Cell 318 21145461
1984 The place of human gamma-trace (cystatin C) amongst the cysteine proteinase inhibitors. Biochemical and biophysical research communications 282 6203523
2012 A high-throughput approach for measuring temporal changes in the interactome. Nature methods 273 22863883
2011 A directed protein interaction network for investigating intracellular signal transduction. Science signaling 258 21900206
2009 Proteomic analysis of human parotid gland exosomes by multidimensional protein identification technology (MudPIT). Journal of proteome research 237 19199708
1998 Cysteine proteinases and their endogenous inhibitors: target proteins for prognosis, diagnosis and therapy in cancer (review). Oncology reports 232 9769367
2014 Extracellular matrix signatures of human primary metastatic colon cancers and their metastases to liver. BMC cancer 203 25037231
2020 Systems analysis of RhoGEF and RhoGAP regulatory proteins reveals spatially organized RAC1 signalling from integrin adhesions. Nature cell biology 194 32203420
2017 Characterization of the Extracellular Matrix of Normal and Diseased Tissues Using Proteomics. Journal of proteome research 185 28675934
2020 UFMylation maintains tumour suppressor p53 stability by antagonizing its ubiquitination. Nature cell biology 168 32807901
1991 Human cystatin C. role of the N-terminal segment in the inhibition of human cysteine proteinases and in its inactivation by leucocyte elastase. The Biochemical journal 167 1996959
2009 Gene-centric association signals for lipids and apolipoproteins identified via the HumanCVD BeadChip. American journal of human genetics 164 19913121
2020 AMPK, a Regulator of Metabolism and Autophagy, Is Activated by Lysosomal Damage via a Novel Galectin-Directed Ubiquitin Signal Transduction System. Molecular cell 152 31995728
1997 Unstable minisatellite expansion causing recessively inherited myoclonus epilepsy, EPM1. Nature genetics 152 9090386
1997 Unstable insertion in the 5' flanking region of the cystatin B gene is the most common mutation in progressive myoclonus epilepsy type 1, EPM1. Nature genetics 147 9054946
2012 CST6 promoter methylation in circulating cell-free DNA of breast cancer patients. Clinical biochemistry 65 23006792
2021 CST6 protein and peptides inhibit breast cancer bone metastasis by suppressing CTSB activity and osteoclastogenesis. Theranostics 57 34815788
2012 Differential secretome analysis reveals CST6 as a suppressor of breast cancer bone metastasis. Cell research 53 22688893
2008 Invasion suppressor cystatin E/M (CST6): high-level cell type-specific expression in normal brain and epigenetic silencing in gliomas. Laboratory investigation; a journal of technical methods and pathology 48 18607344
2006 DNA methylation-dependent silencing of CST6 in human breast cancer cell lines. Laboratory investigation; a journal of technical methods and pathology 40 17043665
2014 TBX2 represses CST6 resulting in uncontrolled legumain activity to sustain breast cancer proliferation: a novel cancer-selective target pathway with therapeutic opportunities. Oncotarget 38 24742492
2007 Methylation-dependent silencing of CST6 in primary human breast tumors and metastatic lesions. Experimental and molecular pathology 37 17540367
2022 CST6 suppresses osteolytic bone disease in multiple myeloma by blocking osteoclast differentiation. The Journal of clinical investigation 22 35881476
2003 The human cystatin M/E gene (CST6): exclusion candidate gene for harlequin ichthyosis. The Journal of investigative dermatology 17 12839564
2012 A closed-tube methylation-sensitive high resolution melting assay (MS-HRMA) for the semi-quantitative determination of CST6 promoter methylation in clinical samples. BMC cancer 14 23088560
2024 BCMA- and CST6-specific CAR T cells lyse multiple myeloma cells and suppress murine osteolytic lesions. The Journal of clinical investigation 5 37883186
2022 Loss-of-function mutations in CST6 cause dry skin, desquamation and abnormal keratosis without hypotrichosis. Clinical genetics 4 36371786
2024 Alleviating role of ketamine in breast cancer cell-induced osteoclastogenesis and tumor bone metastasis-induced bone cancer pain through an SRC/EGR1/CST6 axis. BMC cancer 3 39695463
2025 Cystatin 6 (CST6) and Legumain (LGMN) are potential mediators in the pathogenesis of preeclampsia. Scientific reports 2 40234537
2024 Predictive Values of Homeobox Gene A-Antisense Transcript 3 (HOXA-AS3), Cystatin 6 (CST6), and Chromobox Homolog 4 (CBX4) Expressions in Cancer Tissues for Recurrence of Early Colon Cancer After Surgery. International journal of general medicine 2 38196563
2024 Genetic interaction analysis of Candida glabrata transcription factors CST6 and UPC2A in the regulation of respiration and fluconazole susceptibility. Antimicrobial agents and chemotherapy 1 39714155
2026 Single-cell and spatial transcriptomic profiling reveal CST6 + epithelial-SPP1+ macrophage crosstalk driving lung adenocarcinoma metastasis. International journal of biological macromolecules 0 42002169