PPP4R1

PPP4R1 (PP4R1) is the regulatory subunit of the protein phosphatase 4 (PP4c) holoenzyme that targets the phosphatase to substrates within the NF-κB signaling axis and acts as a negative regulator of inflammatory and immune responses (PMID:25134449, PMID:38424148). It engages TRAF2 and TRAF6 in a RING finger domain-dependent manner, dephosphorylating TRAF2 at Ser11 and suppressing TRAF6 polyubiquitination to dampen NF-κB activation downstream of TNF and the EBV oncoprotein LMP1 (PMID:25134449). As part of a PP4R1/PP4c complex, it dephosphorylates the IKK complex; this activity is antagonized by AMBRA1, which competitively binds PP4R1 through its N-terminal F1 domain to disrupt the complex and sustain IKK phosphorylation and NF-κB-driven intestinal inflammation (PMID:38424148). This regulatory module is co-opted by Merkel cell polyomavirus small T antigen, which forms a complex with PP4R1/PP4c that bridges the viral protein to NEMO and deactivates NF-κB to suppress pro-inflammatory cytokine responses (PMID:28445980). Beyond immune control, PP4R1 functions in hepatic metabolism as a target of miR-338-3p, where its derepression impairs AKT/GSK3β signaling and drives FOXO1-dependent gluconeogenesis and insulin resistance (PMID:28467989, PMID:30132533), and it promotes tumor progression by interacting with HMGA2 to activate MAPK/ERK-driven EMT in lung cancer (PMID:32077156), by binding PKM2 to enhance ERK1/2-mediated PKM2 nuclear translocation and glycolysis in gallbladder cancer (PMID:38301910), and by upregulating HSPA6 to activate ER stress in NSCLC (PMID:37739270).

1. 2014 High

   Established PP4R1 as a negative regulator of NF-κB by defining its direct substrates within TRAF signaling, answering how PP4 is targeted to this pathway.

   Evidence Yeast two-hybrid, reciprocal Co-IP, RNAi, NF-κB reporter, phospho-TRAF2 Ser11 and ubiquitination assays

   PMID:25134449

   Open questions at the time

   • Whether PP4c catalytic activity is required for the observed TRAF2 dephosphorylation in cells was not isolated
   • Mechanism of TRAF6 ubiquitination suppression (direct vs indirect) not resolved
2. 2017 High

   Showed that a viral oncoprotein hijacks the PP4R1/PP4c complex to silence antiviral NF-κB signaling, revealing the complex as a target of viral immune evasion.

   Evidence Reciprocal Co-IP, siRNA depletion, tAg mutagenesis, NF-κB reporter and cytokine measurements

   PMID:28445980

   Open questions at the time

   • Identity of the NEMO-proximal phospho-substrate dephosphorylated in this context not pinned down
   • Structural basis of the tAg–PP4R1–NEMO bridge unresolved
3. 2017 Medium

   Placed PP4R1 under post-transcriptional control of miR-338-3p in hepatocytes, linking its expression to insulin signaling and glycogen synthesis.

   Evidence Luciferase 3′UTR reporter, miRNA mimic/inhibitor, western blot, ChIP, db/db and HFD mouse models

   PMID:28467989

   Open questions at the time

   • Whether PP4R1 acts on AKT/GSK3β components directly via PP4c not demonstrated
   • Single lab
4. 2018 Medium

   Established PP4R1 as the downstream effector of miR-338-3p in driving hepatic gluconeogenesis through FOXO1, defining a metabolic epistatic relationship.

   Evidence Double knockdown epistasis, qPCR of gluconeogenic genes, pyruvate tolerance testing in mice

   PMID:30132533

   Open questions at the time

   • Direct phosphatase action on FOXO1 not shown
   • Single lab
5. 2020 Medium

   Extended PP4R1 function to cancer, linking it to HMGA2 and MAPK/ERK-driven EMT in lung cancer.

   Evidence Co-IP, migration/invasion assays, EMT marker and phospho-ERK western blot, overexpression/knockdown

   PMID:32077156

   Open questions at the time

   • Single Co-IP without reciprocal validation of the HMGA2 interaction
   • Whether the effect depends on PP4c phosphatase activity unknown
6. 2023 Medium

   Identified HSPA6 induction and ER stress as a downstream mechanism for PP4R1-driven NSCLC progression.

   Evidence RNA-seq, HSPA6 rescue/epistasis, ER stress markers, in vivo tumor and metastasis models

   PMID:37739270

   Open questions at the time

   • Mechanism connecting PP4R1 to HSPA6 transcription not defined
   • Single lab
7. 2024 High

   Defined the AMBRA1–PP4R1 competition as a switch controlling IKK dephosphorylation, explaining how PP4R1's anti-inflammatory activity is reciprocally regulated.

   Evidence Reciprocal Co-IP, F1 domain mapping, AMBRA1 S1043 mutagenesis, ubiquitination assay, colitis mouse model

   PMID:38424148

   Open questions at the time

   • Direct IKK phospho-site dephosphorylated by PP4R1/PP4c not mapped
   • Stoichiometry of AMBRA1 vs PP4c competition for PP4R1 unresolved
8. 2024 Medium

   Showed PP4R1 promotes glycolysis in gallbladder cancer by strengthening ERK1/2–PKM2 interaction and driving PKM2 nuclear translocation.

   Evidence Co-IP, direct interaction pulldown, subcellular fractionation/imaging, ERK inhibitor, in vitro and in vivo tumor assays

   PMID:38301910

   Open questions at the time

   • Role of PP4c phosphatase activity in PKM2 regulation not addressed
   • Single lab

• How a single PP4 regulatory subunit reconciles its phosphatase-targeting role in NF-κB suppression with its pro-tumorigenic, phosphatase-independent scaffolding of kinase complexes remains unresolved.
• No structural model of PP4R1 substrate-targeting determinants
• Whether catalytic PP4c activity is required across the diverse reported functions is unclear
• Tissue-specific determinants of pro- vs anti-tumor roles undefined