FIBP

FIBP is an intracellular, predominantly nuclear protein originally identified as a selective binding partner of mitogenic acidic fibroblast growth factor (FGF1) that functions in growth regulation, cancer stemness, immune evasion, and DNA damage repair through multiple signaling axes. FIBP forms a stable trimeric complex with CDK5 and KIAA0528 to promote cell proliferation and migration, binds and inhibits GSK3β Tyr216 phosphorylation to activate β-catenin/cyclin D1 signaling and modulate DNA methylation in colorectal cancer stem cells, and interacts with STAT3 to drive EME1-dependent transcription and radioresistance in lung adenocarcinoma (PMID:25096995, PMID:30275459, PMID:37564211). In CD8+ T cells, FIBP positively regulates cholesterol metabolism to suppress anti-tumor effector function, and its deletion enhances T cell-mediated tumor killing in vivo (PMID:35501486). Biallelic loss-of-function mutations in FIBP cause an autosomal recessive overgrowth syndrome in humans, consistent with its role as a negative regulator of cell proliferation (PMID:26660953, PMID:27183861).

1. 1998 High

   The identity of intracellular FGF1-binding partners was unknown; discovery of FIBP as a 42 kDa nuclear protein that selectively binds mitogenic aFGF but not the non-mitogenic K132E mutant established a candidate effector linking internalized FGF1 to intracellular signaling.

   Evidence Yeast two-hybrid screen followed by in vitro MBP-aFGF binding assay, subcellular fractionation, and fluorescence microscopy in mammalian cells

   PMID:9806903

   Open questions at the time

   • No downstream signaling pathway identified
   • Binding interface and stoichiometry uncharacterized
   • No loss-of-function phenotype demonstrated
2. 2000 Medium

   The genomic organization of FIBP was undefined; mapping to 11q13.1 and identification of a CpG-island promoter with two tissue-specific splice variants established FIBP as a broadly expressed, evolutionarily conserved gene with regulated transcription.

   Evidence Gene mapping, promoter-luciferase reporter assay, comparative genomics across human, mouse, Drosophila

   PMID:11104667

   Open questions at the time

   • Transcriptional regulators of the FIBP promoter not identified
   • Functional significance of splice variants unknown
3. 2003 Medium

   Whether FIBP function is conserved beyond mammals was unclear; demonstration that Drosophila FIBP is expressed in tracheal system and ventral midline cells — known FGF signaling sites — supported a conserved role in FGF-mediated processes.

   Evidence Cloning, RT-PCR, and whole-mount embryo immunostaining of DrFIBP in Drosophila

   PMID:12801646

   Open questions at the time

   • No Drosophila loss-of-function phenotype reported
   • Direct binding of DrFIBP to Drosophila FGFs not tested
4. 2014 High

   The protein complexes through which FIBP acts were unknown; identification of a stable CDK5–KIAA0528–FIBP trimeric complex required for breast cancer cell proliferation and migration revealed FIBP as a structural component of a CDK5 signaling module in non-neuronal cells.

   Evidence MS-based interactome with SAINT analysis, reciprocal co-IP, siRNA knockdown with proliferation/migration assays in breast cancer cell lines

   PMID:25096995

   Open questions at the time

   • CDK5 substrates phosphorylated in a FIBP-dependent manner not identified
   • How FIBP stabilizes the trimeric complex is structurally unresolved
5. 2016 Medium

   Whether FIBP loss has consequences in human development was unknown; identification of biallelic FIBP mutations causing autosomal recessive overgrowth with increased fibroblast proliferation established FIBP as a negative regulator of cell growth in vivo.

   Evidence Exome/whole-genome sequencing of affected individuals, RT-PCR showing NMD, and in vitro proliferation assays on patient fibroblasts

   PMID:26660953 PMID:27183861

   Open questions at the time

   • Molecular pathway mediating overgrowth not determined
   • Limited number of families studied
   • No animal model of FIBP-null developmental phenotype
6. 2018 High

   The signaling mechanism by which FIBP promotes cancer cell stemness was unresolved; demonstration that FIBP binds GSK3β and inhibits its Tyr216 phosphorylation, thereby activating β-catenin/cyclin D1 and modulating DNA methylation, defined a specific oncogenic signaling axis for FIBP.

   Evidence Reciprocal co-IP of FIBP–GSK3β, phosphorylation assays, RNA-seq/GSEA, genetic epistasis by double knockdown, xenograft tumor assays, DNA methylation profiling in colorectal cancer stem cells

   PMID:30275459

   Open questions at the time

   • Whether FIBP directly inhibits a Tyr216 kinase or blocks GSK3β autophosphorylation is unclear
   • Structural basis of FIBP–GSK3β interaction unknown
7. 2022 High

   Whether FIBP has immune-regulatory functions was unexplored; showing that FIBP knockout in CD8+ T cells reduces cholesterol metabolism and enhances anti-tumor killing in vitro and in vivo revealed FIBP as a T cell-intrinsic immunosuppressive factor.

   Evidence CRISPR knockout and siRNA knockdown in murine and human CD8+ T cells, co-culture killing assays, in vivo adoptive cell transfer in B16 melanoma model, cholesterol metabolic profiling

   PMID:35501486

   Open questions at the time

   • Specific cholesterol biosynthetic enzymes regulated by FIBP not identified
   • Whether the T cell phenotype depends on CDK5 or GSK3β axes is unknown
8. 2023 Medium

   How FIBP contributes to therapy resistance was undefined; identification of FIBP–STAT3 interaction driving EME1 transcription and radioresistance established a FIBP-dependent DNA repair axis in lung adenocarcinoma.

   Evidence Co-IP of FIBP–STAT3, STAT3 luciferase reporter assay, siRNA knockdown, EME1 rescue experiments, xenograft validation

   PMID:37564211

   Open questions at the time

   • Whether FIBP acts as a transcriptional co-activator or affects STAT3 post-translational modification is unknown
   • Not independently replicated outside a single lab
   • Relationship of STAT3 axis to GSK3β and CDK5 pathways unclear

• A unifying structural and mechanistic model explaining how FIBP simultaneously engages CDK5, GSK3β, and STAT3 — and whether these represent context-dependent or concurrent functions — remains to be established.
• No crystal or cryo-EM structure of FIBP or its complexes
• How FGF1 binding relates to downstream GSK3β/CDK5/STAT3 effector functions is unresolved
• Whether the overgrowth syndrome phenotype maps to a specific FIBP signaling axis is unknown