KRT19

KRT19 is a type I intermediate filament keratin that, beyond its structural role, functions as a signaling scaffold and transcriptional corepressor across multiple epithelial cell contexts. KRT19 directly binds β-catenin—alone or in a β-catenin/RAC1 complex depending on tissue type—to promote nuclear β-catenin translocation and modulate Wnt and Notch pathway crosstalk; in breast cancer this suppresses Notch via NUMB upregulation, whereas in colon cancer and hepatocellular carcinoma it activates LEF/TCF-driven transcription and Notch signaling (PMID:27345400, PMID:30650643, PMID:33767587). In bronchial epithelial cells, KRT19 stabilizes CFTR at the apical plasma membrane by blocking Rab7A-mediated lysosomal degradation, thereby sustaining chloride secretion (PMID:31450978). A nuclear pool of KRT19 interacts with HDAC1 and RCOR1 to enhance CoREST deacetylase activity and repress hepatocyte-differentiation genes such as HNF4A, promoting liver dedifferentiation and tumorigenesis (PMID:38557414). KRT19 expression is regulated epigenetically by promoter methylation, post-translationally by Ser35 phosphorylation modulated by the lncRNA Linc-KILH, and post-transcriptionally by miRNAs including miR-26a and miR-374b-5p (PMID:20874491, PMID:33767587, PMID:27833076, PMID:39280598).

1. 1997 Medium

   The question of whether KRT19 plays a passive structural role or an active functional role in epithelial cells was first addressed: K8 co-expression stabilizes endogenous K19 into filaments, and K8/K19-expressing cells retain motile and tumorigenic properties distinct from K8/K18-expressing cells, establishing that KRT19-containing filaments have functional consequences beyond cytoskeletal support.

   Evidence Stable transfection of K8 ± K18 in pancreatic adenocarcinoma cells with soft-agar colony, motility, and syngeneic tumor assays

   PMID:9152022

   Open questions at the time

   • Mechanism by which K19 vs K18 filaments differentially affect motility was not identified
   • No direct signaling interactors of K19 were mapped
2. 2010 Medium

   Whether KRT19 expression is epigenetically silenced in cancer was resolved: KRT19 promoter CpG methylation represses transcription in renal carcinoma, and demethylation plus HDAC inhibition restores expression, establishing epigenetic regulation as a layer controlling KRT19 levels.

   Evidence Methylation-specific PCR, bisulfite sequencing, 5-aza-dC + TSA treatment in renal carcinoma cell lines

   PMID:20874491

   Open questions at the time

   • Upstream signals directing promoter methylation were not identified
   • Whether methylation-based silencing occurs in non-renal cancers was not tested
3. 2014 Medium

   The mechanism by which lncRNAs regulate KRT19 abundance was established: Linc00974 acts as a ceRNA sponging miR-642 to upregulate KRT19, which in turn activates Notch and TGF-β signaling to promote hepatocellular carcinoma progression.

   Evidence ceRNA luciferase assay, KRT19 knockdown, cDNA microarray pathway analysis, xenograft models

   PMID:25476897

   Open questions at the time

   • Direct binding between KRT19 protein and Notch/TGF-β pathway components was not demonstrated
   • ceRNA stoichiometry in physiological conditions was not assessed
4. 2016 High

   The central signaling mechanism of KRT19 was defined: KRT19 directly scaffolds a β-catenin/RAC1 complex, promoting nuclear β-catenin translocation and NUMB-mediated Notch suppression in breast cancer cells, thereby linking an intermediate filament protein to Wnt/Notch crosstalk.

   Evidence Reciprocal Co-IP, nuclear fractionation, reporter assays, knockdown/overexpression with proliferation, migration, invasion, and sphere formation readouts in patient-derived CSLCs

   PMID:27345400

   Open questions at the time

   • Structural basis of KRT19–β-catenin–RAC1 ternary complex was not resolved
   • Whether the interaction occurs outside breast cancer lineages was untested at this point
5. 2016 Medium

   Post-transcriptional regulation of KRT19 by miRNA was established: miR-26a directly binds the KRT19 3′UTR to suppress protein expression, with functional consequences on the side-population fraction and tumor growth in cholangiocarcinoma.

   Evidence 3′UTR luciferase reporter assay, gain/loss-of-function assays, SP flow cytometry, xenograft model

   PMID:27833076

   Open questions at the time

   • Whether miR-26a regulation of KRT19 operates in non-biliary epithelia was not examined
   • Other miRNA targets contributing to the phenotype were not excluded
6. 2018 Medium

   The downstream signaling consequences of KRT19 in cancer stem cells were extended: KRT19 modulates Src and GSK3β phosphorylation and CSC marker expression (ALDH1, CXCR4, CD133) in breast cancer, connecting its scaffold function to kinase-level signaling.

   Evidence KRT19 knockdown/overexpression, sphere formation, drug resistance assay, Western blot for phospho-Src and phospho-GSK3β

   PMID:29747452

   Open questions at the time

   • Direct binding of KRT19 to Src or GSK3β was not tested—kinase effects remain correlative
   • Mechanism linking β-catenin scaffold to Src phosphorylation was not delineated
7. 2019 Medium

   Tissue-context dependence of KRT19 signaling was established: in colon cancer KRT19 interacts with β-catenin alone (not RAC1), enabling LEF/TCF promoter binding and activating—rather than suppressing—Notch, contrasting with the β-catenin/RAC1/NUMB axis in breast cancer.

   Evidence Co-IP, ChIP assay at LEF1/TCF7 promoters, knockdown with proliferation/invasion readouts in colon vs breast cancer cells

   PMID:30650643

   Open questions at the time

   • Determinants specifying whether RAC1 is included in the complex remain unknown
   • Whether context-dependent effects reflect stoichiometric differences or distinct post-translational modifications was not resolved
8. 2019 High

   A non-signaling, membrane-stabilization function of KRT19 was discovered: KRT19 physically interacts with CFTR at the apical membrane and prevents Rab7A-mediated lysosomal trafficking, stabilizing both wild-type and pharmacologically rescued F508del-CFTR and sustaining chloride secretion.

   Evidence Co-IP of CK19–CFTR, surface biotinylation, Rab7A dominant-negative rescue, Ussing chamber Cl⁻ secretion in primary bronchial epithelial cells

   PMID:31450978

   Open questions at the time

   • Binding interface between KRT19 and CFTR was not mapped
   • Whether reduced CK19 in CF epithelia is a cause or consequence of disease pathology was not fully resolved
9. 2019 Medium

   An upstream oncogenic inducer of KRT19 was identified: BRAFV600E drives KRT19 expression in thyroid cancer, and KRT19 promotes EMT-mediated metastasis.

   Evidence BRAFV600E overexpression, KRT19 knockdown with proliferation/migration assays, EMT marker Western blot

   PMID:31289571

   Open questions at the time

   • Direct transcriptional mechanism by which BRAFV600E upregulates KRT19 promoter was not mapped
   • EMT effects were correlative without direct β-catenin interaction data in thyroid cells
10. 2021 Medium

    Post-translational control of KRT19 localization was delineated: the lncRNA Linc-KILH binds KRT19 and inhibits Ser35 phosphorylation, redirecting KRT19 from cytoplasm to plasma membrane where it engages β-catenin (without RAC1) to activate Notch1 in hepatocellular carcinoma.

    Evidence RNA-protein pulldown, phosphorylation assay, subcellular fractionation, immunofluorescence, Co-IP, in vivo metastasis assays

    PMID:33767587

    Open questions at the time

    • Kinase responsible for Ser35 phosphorylation was not identified
    • Structural basis for lncRNA-mediated phosphorylation inhibition is unknown
11. 2024 High

    A nuclear function of KRT19 was discovered: nuclear KRT19 binds HDAC1 and RCOR1, enhances CoREST deacetylase activity, and represses hepatocyte-differentiation genes including HNF4A, establishing KRT19 as a transcriptional corepressor that promotes liver dedifferentiation and tumorigenesis.

    Evidence Tandem affinity purification/mass spectrometry, Co-IP, HDAC activity assay, ChIP-seq at HNF4A locus, in vivo multiplexed genome editing, patient-derived xenograft experiments

    PMID:38557414

    Open questions at the time

    • Whether nuclear KRT19 retains filament structure or exists as soluble monomers/oligomers is unknown
    • Genome-wide target spectrum of KRT19-CoREST beyond HNF4A was not comprehensively defined
12. 2024 Medium

    Additional miRNA regulators of KRT19 were identified: miR-374b-5p directly targets KRT19 to inhibit Wnt/β-catenin and EMT in pancreatic cancer, while HIF1α-driven hypoxia and loss of miR-642a-5p converge to upregulate KRT19.

    Evidence 3′UTR luciferase reporter assay, overexpression/knockdown with proliferation/migration/invasion assays, hypoxia/HIF1α manipulation

    PMID:39280598

    Open questions at the time

    • Relative contributions of miR-374b-5p vs miR-642a-5p to KRT19 regulation under physiological conditions were not quantified
    • Whether HIF1α directly binds the KRT19 promoter was not demonstrated

• Major unresolved questions include: the structural basis of KRT19's interactions with β-catenin, CFTR, and the CoREST complex; the determinants specifying tissue-context-dependent partner selection (RAC1 inclusion vs exclusion); the identity of the Ser35 kinase; and whether the nuclear corepressor function and membrane scaffold function operate simultaneously or are mutually exclusive pools in the same cell.
• No structural or cryo-EM data for any KRT19 signaling complex
• Mechanism sorting KRT19 between cytoplasmic filaments, plasma membrane, and nuclear pools is undefined
• In vivo genetic models (knockout mice) have not clearly separated structural from signaling functions