NOXO1

NOXO1 is a cytosolic organizer subunit of the NOX1 NADPH oxidase complex that scaffolds the assembly of NOX1, p22phox, and NOXA1 into an active superoxide-generating complex at membranes. Its PX domain binds phosphoinositide lipids to confer constitutive membrane localization, while an intramolecular SH3–proline-rich region interaction maintains an autoinhibited state that is relieved by stimuli such as arachidonic acid; PKC-mediated phosphorylation at Ser154 and Thr341 enhances binding to both p22phox and NOXA1 and augments ROS production (PMID:14617635, PMID:17126813, PMID:23322165, PMID:23957209). NOXO1 protein stability is controlled by Grb2/Cbl-dependent ubiquitin-proteasomal degradation and by CYLD-mediated ubiquitination, linking its abundance to growth-factor signaling and tumorigenesis (PMID:26781991, PMID:34742871). Beyond ROS production, NOXO1 exerts a NOX1-independent scaffolding function through interaction with Erbin to modulate EGFR signaling kinetics, TFEB-driven lysosomal biogenesis, and endolysosomal cargo trafficking (PMID:39426288, PMID:41406574).

1. 2003 High

   The fundamental question of how NOXO1 reaches membranes and activates NOX1 was answered: its PX domain binds phosphoinositides for constitutive membrane targeting, and unlike p47phox it lacks classical autoinhibitory and phosphorylation-dependent activation mechanisms, explaining agonist-independent NOX1 activity.

   Evidence Co-transfection reconstitution in HEK293 cells with PX-domain mutagenesis, lipid-binding assays, and subcellular fractionation

   PMID:14617635

   Open questions at the time

   • Structural basis of PX-domain lipid selectivity unresolved
   • In vivo relevance of constitutive membrane targeting not tested
2. 2004 High

   NOXO1 was shown to activate NOX3 independently of any activator subunit, establishing it as a versatile organizer for multiple NOX isoforms and demonstrating that absence of an autoinhibitory region underlies stimulus-independent activation.

   Evidence Cell-based superoxide reconstitution with subunit combinations and activator-domain mutants

   PMID:15181005

   Open questions at the time

   • Physiological context of NOXO1–NOX3 interaction (inner ear) not explored
   • Structural basis for NOX3 versus NOX1 selectivity unknown
3. 2005 Medium

   Alternative splicing of the PX-domain-encoding exon was found to generate four NOXO1 isoforms with differing abilities to activate NOX3 despite identical lipid-binding properties, revealing isoform-specific regulation at the protein level.

   Evidence RT-PCR tissue profiling, recombinant PX domain lipid-binding assays, and Nox1/Nox3 superoxide reconstitution

   PMID:15949904

   Open questions at the time

   • Mechanism by which PX-domain splice variants differentially regulate NOX activation unclear
   • Physiological relevance of individual isoforms not established in vivo
4. 2006 Medium

   An intramolecular SH3–proline-rich region interaction was identified that holds NOXO1 in an autoinhibited state, contradicting the initial 2003 view that NOXO1 entirely lacks autoinhibition; arachidonic acid relieves this autoinhibition to expose binding sites for p22phox and NOXA1.

   Evidence Cell-free gp91phox activation assay, GST-pulldown mapping of intramolecular interactions

   PMID:17126813

   Open questions at the time

   • In vivo stimuli that relieve autoinhibition beyond arachidonic acid not defined
   • Structural model of the autoinhibited conformation absent
5. 2010 High

   Quantitative biophysical measurement showed that the NOXO1 tandem SH3 domains bind p22phox with high affinity in a superSH3 conformation, and the C-terminal tail competes for this binding, providing a thermodynamic framework for the autoinhibitory mechanism.

   Evidence Isothermal titration calorimetry with recombinant NOXO1 domains

   PMID:20454568

   Open questions at the time

   • Full-length NOXO1 structure not determined
   • Affinity measurements not performed in a membrane-reconstituted context
6. 2013 High

   PKC-mediated phosphorylation at Ser154 and Thr341 was identified as a key activating switch: phosphorylation dramatically enhances NOXO1 binding to both p22phox and NOXA1, reconciling how agonist stimulation augments an already membrane-localized organizer.

   Evidence In vitro PKC kinase assays, phosphosite mutagenesis (S154A/D, T341A), co-immunoprecipitation, ROS measurement in HEK293 cells

   PMID:23322165 PMID:23957209

   Open questions at the time

   • Kinase(s) responsible in native tissue not confirmed
   • Interplay between Ser154 and Thr341 phosphorylation not dissected
7. 2012 High

   A physiological role for the NOXO1–NOX1 axis was established in diabetic endothelial dysfunction: NOXO1-dependent NOX1 activation mediates eNOS uncoupling and superoxide production, and genetic ablation of either component is protective.

   Evidence Streptozotocin diabetic mouse model, Nox1-knockout, NOXO1 siRNA, ESR superoxide detection, vasorelaxation assays

   PMID:22549734

   Open questions at the time

   • Direct phosphorylation or activation mechanism of NOXO1 in diabetic endothelium not characterized
   • Contribution of individual NOXO1 phosphosites in vivo unknown
8. 2016 High

   NOXO1 protein stability was shown to be controlled by Grb2-recruited Cbl E3 ligase-mediated ubiquitin-proteasomal degradation, and EGF-induced phosphorylation releases NOXO1 from Grb2 to stimulate ROS and promote tumorigenesis; NOXO1 CRISPR knockout abolishes anchorage-independent growth and xenograft tumors.

   Evidence Co-IP, CRISPR/Cas9 knockout, proteasome inhibitor experiments, soft-agar and xenograft assays in colorectal cancer cells

   PMID:26781991

   Open questions at the time

   • Identity of the kinase phosphorylating NOXO1 to release Grb2 not defined
   • Relevance to non-colorectal cancers untested
9. 2016 Medium

   Distinct hemodynamic roles were defined: NOXO1–NOX1 generates superoxide under oscillatory shear stress causing eNOS uncoupling, while p47phox–NOX2 produces NO under laminar flow, establishing pathway-specific mechanotransduction.

   Evidence RNAi knockdown, immunocytochemistry co-localization, parallel-flow chamber system, superoxide and NO measurements in endothelial cells

   PMID:26826128

   Open questions at the time

   • Signaling cascade linking oscillatory shear to NOXO1–NOX1 activation unresolved
   • In vivo shear-stress validation lacking
10. 2016 Medium

    NoxO1 was linked to angiogenic regulation: NoxO1-dependent ROS activates ADAM17, which cleaves Notch to maintain a stalk-cell phenotype; NoxO1 knockout shifts endothelial cells toward a tip-cell phenotype and increases angiogenesis.

    Evidence NoxO1-knockout mouse, ADAM17 activity assays, Notch signaling readouts, retinal and femoral artery ligation models

    PMID:27283741

    Open questions at the time

    • Direct molecular link between NoxO1-derived ROS and ADAM17 oxidation sites not mapped
    • Whether NOXO1 scaffold function contributes independently of ROS not addressed
11. 2021 Medium

    CYLD was identified as a negative regulator of NOXO1 stability: paradoxically, CYLD (a deubiquitinase) promotes NOXO1 ubiquitination and degradation, adding a second ubiquitin-dependent regulatory layer beyond Cbl.

    Evidence Genome-wide CRISPR/Cas9 DUB-knockout library screen, Co-IP, ubiquitination assays, protein stability measurements, xenograft assays

    PMID:34742871

    Open questions at the time

    • Mechanism by which a DUB promotes ubiquitination of NOXO1 is unexplained
    • Ubiquitin chain type and specific lysine residues on NOXO1 not identified
12. 2024 Medium

    A ROS-independent scaffolding function was discovered: NOXO1 interacts with Erbin via proximity labeling, co-localizes with EGFR, and modulates EGF-stimulated kinase activation and wound closure independently of NOX1 catalytic activity.

    Evidence BioID proximity labeling, proximity ligation assay, co-localization imaging, EGF signaling assays, wound closure, NoxO1 KO cells

    PMID:39426288

    Open questions at the time

    • Direct binding interface between NOXO1 and Erbin not mapped
    • Generalizability beyond colon epithelial cells not tested
13. 2025 Medium

    The NOXO1–Erbin interaction was extended to endolysosomal trafficking: NOXO1 activates TFEB in an Erbin-dependent, ROS-independent manner, expanding early endosomes and lysosomes and slowing cargo degradation while enhancing internalization.

    Evidence Proximity ligation assay, fluorescent cargo tracking (EGF/BSA), EEA1/LAMP1 quantification, TFEB reporter, Erbin genetic ablation, ODE modeling

    PMID:41406574

    Open questions at the time

    • Signal transduction pathway from NOXO1–Erbin to TFEB nuclear translocation not delineated
    • Whether TFEB activation contributes to tumorigenesis unknown

• Key unresolved questions include the structural basis of full-length NOXO1 autoinhibition and activation, the ubiquitin chain types and lysine residues governing NOXO1 turnover, the mechanism by which CYLD promotes rather than reverses NOXO1 ubiquitination, and the physiological integration of ROS-dependent and ROS-independent scaffolding functions in vivo.
• No full-length NOXO1 structure available
• In vivo contribution of individual phosphosites not tested by knock-in
• ROS-independent vs. ROS-dependent functions not genetically separated in animal models