C4BPA

C4BPA encodes the alpha-chain of C4b-binding protein, a complement regulator transcribed from a hepatic, TATA-less promoter that absolutely requires HNF1 binding at -38 bp and is induced by acute-phase cytokines (IL-6, IL-1β, IFN-γ) while being repressed by TNF-α, with IFN-γ acting differentially on the alpha versus beta chains (PMID:7772049, PMID:7561114). The gene lies in the regulators of complement activation (RCA) cluster at chromosome 1q32 alongside C4BPB (PMID:2237642). Beyond its canonical extracellular role, C4BPA is S-palmitoylated—at Cys15 in mouse and Cys13/Cys23 in pig, the latter by the palmitoyl transferase ZDHHC8—a modification required for its loading into epididymosomes and transfer onto the sperm surface, where it protects sperm from complement C4-mediated damage and preserves motility (PMID:39370067, PMID:41619901). C4BPA also acts intracellularly and in non-canonical signaling contexts: it binds the NF-κB subunit RelA to stabilize inhibitory IκBα-RelA complexes and sensitize cancer cells to oxaliplatin-induced apoptosis (PMID:33205012), and its loss activates Pink1/Parkin-mediated mitophagy with suppression of NF-κB and pro-inflammatory cytokines (PMID:41745945). In disease settings, fetal C4BPA in cord-blood exosomes binds trophoblast CD40 to drive non-canonical NF-κB (p100→p52) processing and pro-labor gene induction (PMID:30940885), tumor-derived C4BPA activates JAK2/STAT3 and a C5a→C5aR1→STAT3 axis that polarizes macrophages toward an M2 pro-tumoral phenotype (PMID:41237697), and C4BPA secreted by dystrophic fibroadipogenic precursors impairs myoblast differentiation and contractile function (PMID:41851078).

1. 1990 High

   Establishing the genomic context resolved whether the C4BP alpha and beta chains are co-regulated members of a complement gene cluster.

   Evidence In situ hybridization and Southern blotting of somatic cell hybrids mapping C4BPA/C4BPB to human 1q32 and rat chromosome 13

   PMID:2237642

   Open questions at the time

   • Does not address transcriptional co-regulation mechanism
   • No functional consequence of clustering tested
2. 1995 Medium

   Defining the C4BPA promoter answered how its expression is restricted to liver and identified the transcription factor controlling it.

   Evidence Promoter deletion constructs and gel-shift/footprinting in hepatic cells identifying an absolute HNF1 requirement at -38 bp in a TATA-less promoter

   PMID:7772049

   Open questions at the time

   • Other regulatory elements beyond -369 bp not characterized
   • No in vivo confirmation of HNF1 dependence
3. 1995 Medium

   Cytokine-response profiling explained how C4BPA is regulated as an acute-phase protein and how alpha/beta stoichiometry is maintained.

   Evidence Cytokine treatment of Hep3B hepatoma cells with mRNA quantification plus acute-phase patient serum isoform analysis

   PMID:7561114

   Open questions at the time

   • Signaling pathways linking cytokines to differential chain regulation not mapped
   • Protein-level induction inferred from mRNA
4. 2019 Medium

   Identifying fetal C4BPA as a CD40 ligand revealed a signaling, non-complement role in placental biology and parturition.

   Evidence Cord-blood exosome proteomics, computational docking, crystal structural analysis, and functional assays in trophoblast cells showing CD40-driven p100→p52 processing

   PMID:30940885

   Open questions at the time

   • In-cell rescue detail limited
   • Physiological contribution to labor timing not established in vivo
5. 2020 Medium

   Discovery of intracellular C4BPA binding RelA established a cell-autonomous role in apoptosis regulation independent of complement.

   Evidence Co-IP of C4BPA with RelA, patient-mutation cell lines, and in vitro/in vivo oxaliplatin apoptosis and IκBα-RelA stability assays

   PMID:33205012

   Open questions at the time

   • No reciprocal Co-IP or structural basis for RelA interaction
   • Mechanism of intracellular C4BPA retention not defined
6. 2024 Medium

   Mapping S-palmitoylation at Cys15 explained how C4BPA is trafficked into epididymosomes and onto sperm to confer complement protection.

   Evidence Palmitoylation assays, C15S mutagenesis, epididymosome isolation, and sperm motility/complement C4 challenge in mouse

   PMID:39370067

   Open questions at the time

   • Enzyme catalyzing mouse palmitoylation not identified
   • Mechanism of epididymosome sorting not detailed
7. 2025 Medium

   Characterizing tumor-derived C4BPA defined a JAK2/STAT3 and C5a→C5aR1→STAT3 axis that remodels the tumor immune microenvironment.

   Evidence C4BPA knockdown/overexpression in gastric cancer lines, organoids, xenografts, and THP-1 macrophage co-culture with recombinant C5a rescue

   PMID:41237697

   Open questions at the time

   • Direct receptor for C4BPA on tumor cells not identified
   • Single tumor type tested
8. 2026 Medium

   Identifying ZDHHC8 and the Cys13/Cys23 sites in pig extended the palmitoylation-trafficking mechanism and named the responsible enzyme.

   Evidence Palmitoylation site mapping, ZDHHC8 identification, and epididymosome/sperm complement-protection assays in porcine system

   PMID:41619901

   Open questions at the time

   • Whether ZDHHC8 acts on C4BPA in other species not tested
   • Subcellular site of palmitoylation not localized
9. 2026 Medium

   C4BPA knockout in mammary epithelial cells linked the protein to suppression of mitophagy and NF-κB-driven inflammation.

   Evidence C4BPA knockout in bovine mammary epithelial cells with EM, LC3-mitochondria immunofluorescence, NF-κB pathway Western blots, ROS, and cytokine readouts

   PMID:41745945

   Open questions at the time

   • Molecular link between C4BPA and Pink1/Parkin not defined
   • Whether intracellular RelA interaction underlies this effect untested
10. 2026 Medium

    Secretome analysis of dystrophic FAPs identified C4BPA as a paracrine inhibitor of myogenic differentiation in muscular dystrophy.

    Evidence FAP secretome mass spectrometry, recombinant C4BPA treatment of myoblasts, 3D engineered muscle, and siRNA knockdown in DMD FAPs

    PMID:41851078

    Open questions at the time

    • Receptor mediating myoblast effects unidentified
    • Signaling pathway downstream of C4BPA in myoblasts not mapped

• The unifying receptor logic and structural basis that allow a secreted complement regulator to also act as an intracellular RelA partner and a surface signaling ligand across reproductive, tumor, and muscle contexts remain unresolved.
• No single mechanism reconciles intracellular vs extracellular C4BPA functions
• Receptors for tumor and myoblast effects unknown
• Trigger determining secreted vs retained C4BPA fate undefined