PTPRH

PTPRH (SAP-1) is a receptor-type protein tyrosine phosphatase that localizes to intestinal epithelial microvilli and functions as a negative regulator of integrin signaling, growth factor receptor signaling, and T cell activation through dephosphorylation of key signaling intermediates. Its identified substrates include the focal adhesion proteins p130cas and FAK, the adaptor p62(dok), the kinases Lck and c-Src, the microvillar transmembrane protein CEACAM20, and EGFR at tyrosine 1197, through which it suppresses downstream PI3K/AKT/mTOR and NF-κB signaling, cell motility, and proliferation (PMID:11278335, PMID:12837766, PMID:26195794, PMID:36054194). PTPRH forms redox-sensitive homodimers via its extracellular and transmembrane domains, and dimerization inhibits catalytic activity, providing a mechanism for regulation by the cellular redox environment (PMID:15850787). In vivo, PTPRH deficiency increases intestinal paracellular permeability and modulates intestinal tumorigenesis in an APC-mutant background, while loss of PTPRH in the IL-10-deficient setting worsens colitis, establishing roles in epithelial barrier integrity and mucosal immune homeostasis (PMID:19170756, PMID:26195794, PMID:28431964).

1. 2001 High

   Identifying the first substrates of PTPRH established it as a phosphatase that targets focal adhesion signaling components and mediates contact inhibition of cell growth.

   Evidence Substrate-trapping mutant approach in cultured cells identified p130cas, FAK, and p62(dok) as substrates; functional assays showed inhibited cell spreading and colony formation

   PMID:11278335

   Open questions at the time

   • No in vivo validation of substrate dephosphorylation
   • Mechanism by which cell-cell adhesion increases SAP-1 activity not defined
   • Structural basis for substrate recognition unknown
2. 2003 High

   Demonstrating that PTPRH directly binds and dephosphorylates Lck extended its functional scope beyond focal adhesion signaling to T cell receptor signaling, showing it suppresses TCR-triggered MAPK activation and T cell migration.

   Evidence Direct binding assay, in vitro phosphatase assay with catalytic-dead control, and overexpression in Jurkat T cells measuring ZAP-70/LAT phosphorylation and CD69 upregulation

   PMID:12837766

   Open questions at the time

   • Endogenous relevance in primary T cells not demonstrated
   • Whether PTPRH is physiologically expressed in T cells in vivo unclear
3. 2003 Medium

   Showing that PTPRH is downregulated in hepatocellular carcinoma and that its re-expression suppresses cell motility and growth suggested a tumor-suppressive function in liver.

   Evidence Immunohistochemistry and immunoblot of HCC tissue, recombinant SAP-1 re-expression with migration and growth assays

   PMID:12879010

   Open questions at the time

   • No substrate identified in HCC context
   • Lack of in vivo tumor model confirmation
4. 2005 High

   Revealing that PTPRH forms redox-sensitive homodimers through its extracellular/transmembrane domains, with dimerization inhibiting catalytic activity, provided a regulatory mechanism and identified c-Src as an additional substrate.

   Evidence Chemical cross-linking, co-immunoprecipitation, reducing agent treatment, and catalytic activity assays

   PMID:15850787

   Open questions at the time

   • Structural details of dimer interface unresolved
   • Physiological redox conditions that regulate dimerization not defined
   • Whether dimerization is regulated by ligand binding unknown
5. 2009 High

   Localizing PTPRH specifically to intestinal epithelial microvilli and showing that its knockout inhibits APC-driven intestinal tumorigenesis established an unexpected pro-tumorigenic role in the gut, contrasting with its anti-proliferative effects in vitro.

   Evidence Immunofluorescence in gastrointestinal tissue and genetic cross of SAP-1 knockout with APC heterozygous mice with tumor quantification

   PMID:19170756

   Open questions at the time

   • Molecular mechanism by which PTPRH promotes intestinal tumorigenesis not identified
   • Apparent contradiction with tumor-suppressive in vitro data not resolved
6. 2015 High

   Identifying CEACAM20 as a microvillar substrate of PTPRH and linking its dephosphorylation to suppression of NF-κB/IL-8 signaling connected PTPRH to intestinal innate immune regulation, with loss-of-function worsening colitis in IL-10-deficient mice.

   Evidence Substrate identification in SAP-1 knockout mice, co-IP of SAP-1/CEACAM20 complex, in vitro c-Src phosphorylation, NF-κB reporter assay, colitis phenotype in IL-10 KO mice

   PMID:26195794

   Open questions at the time

   • Whether CEACAM20 dephosphorylation fully accounts for colitis phenotype not tested
   • PTPRH regulation of other NF-κB pathway components not explored
7. 2017 Medium

   Two studies filled distinct gaps: one showed PTPRH expression in colorectal tumors is silenced by promoter DNA methylation and H3K27me3, and the other demonstrated that PTPRH deficiency increases intestinal paracellular permeability, broadening its role to epithelial barrier function.

   Evidence Pyrosequencing, ChIP for H3K27me3/Pol II, and 5-aza-dC rescue (epigenetic); SAP-1 knockout mice with everted ileal sac and colonic loop transport assays (permeability)

   PMID:28431964 PMID:28713969

   Open questions at the time

   • Tight junction substrate(s) of PTPRH not identified
   • Whether epigenetic silencing causally drives tumor progression not tested in vivo
8. 2022 High

   Pinpointing EGFR Y1197 as a direct dephosphorylation target of PTPRH in NSCLC, using knockout and catalytic-dead rescue, established a specific phosphosite-level mechanism and revealed mutual exclusivity of PTPRH and EGFR mutations with therapeutic implications for TKI sensitivity.

   Evidence PTPRH knockout cells, wild-type vs. catalytic-dead rescue, phospho-EGFR Y1197 immunoblot, osimertinib dose-response, xenograft model

   PMID:36054194

   Open questions at the time

   • Whether PTPRH dephosphorylates other EGFR tyrosine sites not addressed
   • Physical interaction between PTPRH and EGFR not confirmed by co-IP
9. 2023 Medium

   Placing PTPRH upstream of PI3K/AKT/mTOR in NSCLC and showing it promotes glycolysis and proliferation via this pathway addressed the downstream signaling cascade but did not identify the direct substrate.

   Evidence Colony, EdU, Transwell, wound healing assays, 18F-FDG uptake, PI3K inhibitor epistasis, xenograft model

   PMID:37974250

   Open questions at the time

   • Direct phosphatase substrate in PI3K/AKT/mTOR axis not identified
   • Context-dependent tumor-promoting vs. tumor-suppressive roles unresolved

• Key unresolved questions include the structural basis for PTPRH substrate selectivity, the identity of any extracellular ligand, the mechanistic explanation for context-dependent tumor-promoting versus tumor-suppressive activities, and the direct substrates mediating its effects on tight junction permeability and PI3K/AKT/mTOR signaling.
• No extracellular ligand identified
• No crystal structure of full-length PTPRH or its dimer
• Conflicting tumor-promoting (intestinal) versus tumor-suppressive (HCC, NSCLC) roles not mechanistically reconciled