CTSH

CTSH (cathepsin H) is a lysosomal cysteine protease that protects pancreatic β-cells from cytokine-induced apoptosis by suppressing JNK and p38 MAPK signaling and reducing expression of the proapoptotic effectors Bim, DP5, and c-Myc, while simultaneously promoting Ins2 transcription and insulin secretion (PMID:24982147). Proinflammatory cytokines downregulate CTSH transcription in human islets through Tet1/3-dependent DNA hypermethylation of CpG sites in an intronic regulatory element (PMID:33992646). In human microglia, CTSH acts as a negative regulator of amyloid-β phagocytosis, linking a protective genetic variant (rs2289702) that reduces CTSH expression to enhanced Aβ clearance (PMID:36739351). CTSH mRNA stability is post-transcriptionally enhanced by the m6A reader IGF2BP2, and elevated CTSH levels promote cancer cell proliferation, migration, and epithelial–mesenchymal transition (PMID:41100504).

1. 2014 High

   Before this work it was unknown whether CTSH had functions beyond general lysosomal proteolysis; overexpression and knockout experiments revealed that CTSH suppresses JNK/p38-mediated apoptosis in β-cells and is required for normal insulin content and secretion, establishing a direct cytoprotective and secretory role in pancreatic islets.

   Evidence Overexpression in insulin-secreting cells with cytokine challenge, Ctsh−/− mouse islet analysis, signaling pathway readouts, insulin secretion assays

   PMID:24982147

   Open questions at the time

   • The direct proteolytic substrate(s) through which CTSH modulates JNK/p38 signaling remain unidentified
   • Whether CTSH acts catalytically or via a non-enzymatic mechanism in β-cell protection is unresolved
   • Relevance to human type 1 or type 2 diabetes in vivo has not been demonstrated
2. 2021 Medium

   How cytokines silence CTSH was unclear; bisulfite sequencing and reporter assays showed that IL-1β/TNF-α/IFN-γ drive DNA hypermethylation of three intronic CpG sites via reduced Tet1/3 demethylase activity, providing an epigenetic mechanism for cytokine-mediated CTSH downregulation in human islets.

   Evidence Luciferase reporter assay with methylated vs. unmethylated CpG constructs in HEK293 cells; bisulfite sequencing of cytokine-treated human islets; Tet1/3 activity measurement

   PMID:33992646

   Open questions at the time

   • Reporter assays were performed in HEK293 cells rather than β-cells, limiting direct cellular relevance
   • Whether pharmacological demethylation can restore CTSH expression and rescue β-cell function has not been tested
   • The upstream signals linking cytokine receptors to Tet1/3 suppression are not defined
3. 2023 Medium

   CTSH had no established role in neuroinflammation; knockout in human microglia demonstrated that CTSH limits Aβ phagocytosis, and a protective genetic variant (rs2289702-T) reduces CTSH expression by disrupting transcription factor binding, connecting CTSH to microglial Aβ clearance.

   Evidence CTSH knockout in human microglia with Aβ phagocytosis assay; transcription factor binding affinity assay for rs2289702

   PMID:36739351

   Open questions at the time

   • The mechanism by which CTSH suppresses phagocytic uptake (e.g., receptor processing, signaling modulation) is unknown
   • In vivo relevance to Alzheimer's disease pathology has not been tested
   • The transcription factor whose binding is disrupted by rs2289702 is not definitively identified
4. 2025 Medium

   Post-transcriptional regulation of CTSH was unexplored; RIP and RNA pull-down experiments showed that the m6A reader IGF2BP2 binds and stabilizes CTSH mRNA, and CTSH overexpression rescued the anti-tumorigenic effects of IGF2BP2 knockdown, placing CTSH as a key downstream effector of epitranscriptomic signaling in cancer.

   Evidence RIP-qPCR, RNA pull-down, CCK8/EdU/Transwell assays, xenograft model, IGF2BP2 knockdown with CTSH rescue in thyroid cancer cells

   PMID:41100504

   Open questions at the time

   • The specific m6A site(s) on CTSH mRNA that mediate IGF2BP2 binding have not been mapped
   • Whether the pro-tumorigenic activity requires CTSH catalytic function is untested
   • Generalizability beyond thyroid cancer is unknown

• The direct proteolytic substrates of CTSH responsible for its signaling effects in β-cells, microglia, and cancer cells remain unidentified, and no structural or biochemical basis connects CTSH catalytic activity to modulation of JNK/p38, phagocytosis, or EMT.
• No substrate has been identified that mechanistically links CTSH proteolysis to JNK/p38 suppression or phagocytic regulation
• Catalytic-dead mutant experiments have not been performed in any of the reported contexts
• No structural model explains CTSH's aminopeptidase specificity in these biological settings