PIK3IP1

PIK3IP1 (TrIP) is a transmembrane inhibitor of PI3K/AKT signaling that directly binds the p110 catalytic subunit of PI3K through a p85-homologous intracellular domain to suppress PI3K enzymatic activity (PMID:17475214, PMID:25826393). Its extracellular kringle domain mediates oligomerization, and both the kringle and intracellular domains are required for full inhibition of PI3K (PMID:30429249). By dampening PI3K signaling, PIK3IP1 restrains proliferation, motility, and survival, and its overexpression suppresses spontaneous liver tumorigenesis (PMID:18632611). In T cells PIK3IP1 limits TCR-driven activation — inhibiting NFAT/AP-1 transcription, Akt phosphorylation, and IL-2 production (PMID:22706993) — in part by promoting degradation of the adaptor SLP76 (PMID:31350312), and its loss shifts T cell metabolism from oxidative phosphorylation toward Hif1α-driven aerobic glycolysis, causing overactivation and exacerbated autoimmunity (PMID:36179018). Surface PIK3IP1 is rapidly shed after TCR engagement through TCR-signal-strength- and PKC-dependent recruitment of ADAM family metalloproteases that cleave the extracellular stalk (PMID:39454954), and IL-21 likewise drives downregulation via p38 MAPK and ADAM17 (PMID:36179018). In B cells PIK3IP1 promotes extrafollicular class switching and early IgG production by limiting PI3K signaling during T-dependent responses (PMID:32887751), and in cardiomyocytes it binds p110α and the ETA receptor/PI3Kγ to restrain hypertrophy and apoptosis (PMID:25826393, PMID:35883611). PIK3IP1 abundance is controlled at multiple levels: the Ras–LSD1 axis epigenetically silences its promoter/enhancer (PMID:31900384), miR-let-7g targets its mRNA through the RISC complex (PMID:26655604), and USP14-mediated de-ubiquitination at K198 stabilizes the protein (PMID:38658954).

1. 2007 High

   Established the core biochemical activity of PIK3IP1 — that it physically engages PI3K and acts as a direct enzymatic inhibitor, rather than merely correlating with reduced signaling.

   Evidence In vitro binding and PI3K activity assays, with sequence homology to the p85 regulatory subunit

   PMID:17475214

   Open questions at the time

   • Did not define which protein domain mediates p110 binding
   • No cellular phenotype tested at this stage
2. 2008 High

   Demonstrated that PIK3IP1's biochemical inhibition translates into tumor suppression in vivo, linking PI3K dampening to control of proliferation, motility, survival, and hepatic tumorigenesis.

   Evidence Transgenic mouse hepatocyte overexpression with PI3K activity and phenotypic readouts

   PMID:18632611

   Open questions at the time

   • Overexpression rather than physiological-level study
   • Did not address loss-of-function consequences
3. 2008 Medium

   Defined PIK3IP1 as a membrane protein with a signal peptide and transmembrane domain and identified a kringle-lacking splice isoform, both pro-apoptotic on overexpression.

   Evidence Fluorescence microscopy, flow cytometry, RT-PCR and structural bioinformatics

   PMID:19088825

   Open questions at the time

   • Functional role of the splice isoform vs full-length not resolved
   • Apoptosis assayed only under overexpression
4. 2012 Medium

   Extended PIK3IP1's inhibitory role to T cells, showing it negatively regulates TCR-driven Akt activation, transcription factor activity, and IL-2 output.

   Evidence Ectopic expression and siRNA knockdown with reporter, pAkt Western, and IL-2 ELISA in T cells

   PMID:22706993

   Open questions at the time

   • Single lab, cell-line based
   • Did not address surface regulation or in vivo immunity
5. 2013 Medium

   Placed PIK3IP1 downstream of cell cycle control, showing it is induced by CDK4/6 inhibition-driven G1 arrest and required for sensitization to PI3Kδ inhibition in lymphoma.

   Evidence Transcriptome sequencing and functional knockdown in primary mantle cell lymphoma cells

   PMID:23676220

   Open questions at the time

   • Transcriptional inducer of PIK3IP1 upon arrest not identified
   • Mechanism linking arrest to PIK3IP1 induction unresolved
6. 2015 High

   Confirmed direct p110α binding by reciprocal Co-IP and established PIK3IP1 as a brake on PI3K/AKT/mTOR-driven cardiac hypertrophy.

   Evidence Co-IP, bidirectional knockdown/overexpression and cell size measurement in cardiomyocytes

   PMID:25826393

   Open questions at the time

   • Did not map binding interface
   • Tissue-specific in vivo cardiac knockout not done
7. 2015 Medium

   Identified post-transcriptional control of PIK3IP1 by a Lin28/let-7 axis, connecting hypoxia signaling to PI3K pathway tuning and apoptosis.

   Evidence RISC crosslinking immunoprecipitation, lentiviral overexpression, caspase and Akt assays in cardiac myocytes

   PMID:26655604

   Open questions at the time

   • Single lab
   • Relative contribution of let-7g vs other miRNAs unquantified
8. 2018 High

   Resolved the structure-function logic of PIK3IP1: both kringle and intracellular p85-like domains are needed for inhibition, the kringle mediates oligomerization, and surface downmodulation occurs on T cell activation with loss enhancing immunity in vivo.

   Evidence Domain mutagenesis, inducible knockout mouse, T cell activation and Listeria infection models

   PMID:30429249

   Open questions at the time

   • Mechanism of activation-induced downmodulation not yet defined here
   • Oligomer stoichiometry unresolved
9. 2019 Medium

   Identified SLP76 degradation, driven by kringle-mediated oligomerization, as a downstream mechanism of TCR inhibition and showed loss confers antitumor immunity.

   Evidence Immunoblotting, confocal microscopy, Pik3ip1-/- mice, MC38/B16 tumor models, fusion protein experiments

   PMID:31350312

   Open questions at the time

   • Mechanism linking PIK3IP1 oligomerization to SLP76 degradation not detailed
   • Single lab
10. 2020 Medium

    Showed oncogenic Ras epigenetically silences PIK3IP1 via LSD1-mediated erasure of active histone marks, explaining its loss in transformed cells.

    Evidence ChIP for LSD1 at PIK3IP1 promoter/enhancer with doxycycline-inducible rescue of anchorage-independent growth

    PMID:31900384

    Open questions at the time

    • Direct LSD1 recruitment factors not identified
    • Single lab
11. 2020 Medium

    Defined a B cell role: PIK3IP1 limits PI3K signaling to promote extrafollicular class switching and early IgG production during T-dependent responses.

    Evidence B cell-specific conditional knockout, flow cytometry, PI3K-pathway Western, immunization

    PMID:32887751

    Open questions at the time

    • Mechanism of PI3K-dependent downregulation in B cells unresolved
    • Single lab
12. 2022 Medium

    Mapped a cardiomyocyte ETA–PI3Kγ–AKT anti-apoptotic axis, showing PIK3IP1 binds both ETA and PI3Kγ to restrain oxidative-stress-induced apoptosis.

    Evidence Co-IP, TUNEL/MTT, overexpression/knockdown and receptor antagonists in H9c2 cells

    PMID:35883611

    Open questions at the time

    • Direct vs indirect ETA binding not resolved
    • In vivo relevance untested
13. 2022 High

    Connected PIK3IP1 loss to a Hif1α-driven metabolic shift toward glycolysis causing T cell overactivation and autoimmunity, and identified IL-21/p38/ADAM17 as a downregulation pathway.

    Evidence T cell-specific knockout, EAE model, metabolic flux assays, pharmacological Hif1α/glycolysis inhibition

    PMID:36179018

    Open questions at the time

    • Link between PI3K inhibition and Hif1α metabolic switch mechanistically incomplete
14. 2024 High

    Dissected the surface-shedding mechanism: TCR signal strength and classical PKC isoforms drive ADAM-protease cleavage at the extracellular stalk, controlling constitutive and induced TrIP shedding.

    Evidence Monoclonal antibody staining, truncation mutants, PKC and ADAM inhibitors, flow cytometry

    PMID:39454954

    Open questions at the time

    • Specific ADAM family member(s) not pinned down
    • Fate of cleaved intracellular fragment unknown
15. 2024 Medium

    Established post-translational stabilization of PIK3IP1, showing PDE5A-500aa recruits USP14 to remove K48-linked polyubiquitin at K198, raising PIK3IP1 levels to attenuate PI3K/AKT in cancer.

    Evidence LC-MS/MS, Co-IP, ubiquitination assays, K198 site-directed mutagenesis in esophageal squamous cell carcinoma

    PMID:38658954

    Open questions at the time

    • E3 ligase opposing USP14 at K198 not identified
    • Single lab
16. 2025 Medium

    Tested PIK3IP1 as a CD8 T cell checkpoint, showing deletion enhances tumor control, increases infiltration, delays exhaustion, and broadens TCR clonotype diversity.

    Evidence CD8-specific conditional knockout, B16/MC38 tumor models, flow cytometry, transcriptomics, TCR sequencing (preprint)

    PMID:40791507

    Open questions at the time

    • Preprint, not yet peer-reviewed
    • Therapeutic targeting strategy not established

• How PIK3IP1 oligomerization mechanistically couples to both PI3K inhibition and SLP76 degradation, and whether its multiple tissue roles share a unified molecular logic, remains unresolved.
• No structural model of the PIK3IP1-p110 complex
• Mechanism connecting oligomerization to downstream degradation unknown
• Opposing E3 ligase and specific ADAM protease unidentified