SERPINA5

SERPINA5 (protein C inhibitor/PCI) is a secreted, broad-specificity serpin that inhibits multiple coagulation, fibrinolytic, and kallikrein-family proteases, including activated protein C, plasma and tissue kallikreins, and the sperm protease acrosin (PMID:8796266). Its inhibitory activity and substrate selectivity are tuned by cofactors that engage its heparin-binding site: heparin and surface glycosaminoglycans stimulate most protease reactions while interfering with the tissue kallikrein interaction (PMID:8796266), and anionic phospholipids—oxidized phosphatidylethanolamine and phosphatidylserine—act as heparin-like cofactors that stimulate activated protein C inhibition in a Ca2+-dependent manner, an effect lost in a heparin-binding-site mutant (PMID:17332248). Beyond protease inhibition, the intact N-terminal A+-helix of SERPINA5 functions as a cell-penetrating peptide that drives phosphatidylethanolamine-dependent membrane crossing and nuclear translocation; cleavage near the N-terminus by the GPI-anchored protease testisin removes this element and regulates cellular uptake (PMID:25488662, PMID:27989561). Extracellularly, secreted SERPINA5 suppresses tumor cell migration and invasion by binding fibronectin and disrupting fibronectin–integrin β1/FAK signaling in hepatocellular and endometrial cancers (PMID:24388360, PMID:35951287), whereas in gastric cancer it promotes proliferation via inhibition of CBL and activation of PI3K/AKT/mTOR signaling (PMID:36000536). SERPINA5 additionally acts as an interferon-stimulated gene that enhances antiviral innate immunity by promoting STAT1 phosphorylation and nuclear translocation (PMID:36982532), and elevated SERPINA5 worsens preeclampsia-like features in a rat model through enhanced coagulation cascade activation (PMID:38136662).

1. 1996 Medium

   Established SERPINA5 as a broad-specificity serpin and showed that glycosaminoglycans set both its activity and protease selectivity, framing it as a cofactor-regulated inhibitor rather than a single-target enzyme.

   Evidence In vitro protease inhibition assays, epithelial cell-binding studies, and a mouse IVF model defining substrate scope and heparin/GAG modulation

   PMID:8796266

   Open questions at the time

   • Structural basis of how GAG binding redirects specificity not resolved
   • Physiological cofactor identity in each tissue not defined
   • Relative contribution of each protease target in vivo unknown
2. 2003 Low

   Placed SERPINA5 in prostate epithelium as a candidate local regulator of seminal kallikrein activity, extending its protease targets to hK2 and PSA.

   Evidence Immunohistochemistry, in situ hybridization, and Western blotting across benign, PIN, and tumor prostate tissue and cell lines

   PMID:14518028

   Open questions at the time

   • Inhibitory activity inferred rather than directly demonstrated in this study
   • No functional consequence in seminal fluid established
   • Causal role in prostate pathology untested
3. 2004 Low

   Tested whether SERPINA5 mutations underlie male subfertility and found none, excluding the gene as a common genetic cause of impaired semen parameters.

   Evidence Direct gene sequencing in subfertile men, RFLP in controls, and ELISA of seminal plasma antigen

   PMID:15377716

   Open questions at the time

   • Negative result; does not exclude rare or regulatory variants
   • Small variant-carrier sample
   • Functional role in fertility not directly addressed
4. 2007 High

   Showed that anionic phospholipids act as heparin-like cofactors specific to SERPINA5, defining a phospholipid-dependent, Ca2+-regulated route to enhanced activated protein C inhibition.

   Evidence ELISA phospholipid-binding assays, heparin/annexin V competition, Ca2+-controlled aPC inhibition, heparin-binding-site mutagenesis and peptide competition, plus atherosclerotic plaque colocalization

   PMID:17332248

   Open questions at the time

   • In vivo relevance of phospholipid cofactor at lesion sites not quantified
   • Which physiological membranes present these lipids to SERPINA5 unclear
   • Structural model of the lipid-bound inhibitor absent
5. 2013 Medium

   Identified a non-inhibitory extracellular function: SERPINA5 binds fibronectin to disrupt fibronectin-integrin signaling and suppress HCC migration and metastasis, linking it to tumor suppression.

   Evidence Migration/invasion and in vivo metastasis assays, direct SERPINA5-fibronectin interaction assay, and exogenous secreted protein treatment with DNA dosage/expression correlation in HCC

   PMID:24388360

   Open questions at the time

   • Binding interface on fibronectin not mapped
   • Whether protease-inhibitory activity contributes unknown
   • Mechanism of integrin signaling disruption not fully resolved
6. 2014 High

   Defined the N-terminal A+-helix as a cell-penetrating peptide and testisin as a protease that cleaves SERPINA5 to regulate its internalization, establishing a mechanism for cellular uptake.

   Evidence Recombinant and membrane-anchored testisin cleavage assays, synthetic N-terminal peptide penetration tests, and truncation-mutant internalization in Jurkat T cells

   PMID:25488662

   Open questions at the time

   • Functional consequence of nuclear SERPINA5 not defined here
   • Physiological context where testisin gates uptake unclear
   • Receptor/transport machinery beyond the peptide not identified
7. 2016 Medium

   Extended the uptake model by showing PE-dependent membrane crossing and nuclear translocation across multiple cell types and into pure lipid vesicles, demonstrating receptor-independent bilayer penetration.

   Evidence Cell fractionation, immunofluorescence nuclear translocation, phospholipid vesicle penetration, and N-terminal truncation studies

   PMID:27989561

   Open questions at the time

   • Nuclear function of internalized SERPINA5 unknown
   • Quantitative contribution of PE versus other lipids unresolved
   • Synthesizes single-group data
8. 2022 Medium

   Showed context-dependent roles in cancer: SERPINA5 suppresses endometrial cancer via integrin β1/FAK inhibition but promotes gastric cancer proliferation by inhibiting CBL to activate PI3K/AKT/mTOR.

   Evidence Overexpression/knockdown with phospho-FAK and PI3K/AKT/mTOR Western blots, migration/invasion assays, exosomal delivery, and xenograft models

   PMID:35951287 PMID:36000536

   Open questions at the time

   • SERPINA5-CBL interaction not directly validated by co-IP
   • Basis of opposing tumor roles across tissues unexplained
   • Whether intracellular versus secreted pools drive each phenotype unclear
9. 2023 Medium

   Placed SERPINA5 in innate immunity as an interferon-stimulated gene that amplifies STAT1 signaling, and in vascular pathology as a coagulation-promoting aggravator of preeclampsia-like disease.

   Evidence Gain/loss-of-function with phospho- and nuclear-STAT1 readouts, IFN reporter assays, and viral infection models; plus adenoviral SERPINA5 manipulation in a rat preeclampsia model with placental mRNA-seq

   PMID:36982532 PMID:38136662

   Open questions at the time

   • Molecular link between SERPINA5 and STAT1 phosphorylation not defined
   • Whether antiviral and coagulation roles share a mechanism unknown
   • Direct protease targets driving preeclampsia phenotype not identified

• How a single secreted serpin integrates cofactor-tuned protease inhibition, cell penetration and nuclear localization, fibronectin/integrin signaling, and STAT1-dependent immunity into coherent physiology remains unresolved.
• No unifying mechanism connecting extracellular inhibitory and intracellular signaling functions
• Functional output of nuclear-translocated SERPINA5 unknown
• Structural basis of cofactor-switched specificity not determined