SERPINA5

SERPINA5 (protein C inhibitor/PCI) is a secreted serpin-family serine protease inhibitor with broad specificity that functions in hemostasis, reproduction, cell invasion, and innate immunity. It inhibits activated protein C, urokinase, kallikreins, and acrosin, with its activity and target specificity modulated by heparin, glycosaminoglycans, and phospholipids—oxidized phosphatidylethanolamine and phosphatidylserine stimulate its inhibition of activated protein C through its heparin-binding site in a Ca²⁺-dependent manner (PMID:8796266, PMID:17332248). Extracellularly, SERPINA5 suppresses tumor cell migration and metastasis by binding fibronectin and disrupting integrin β1/FAK signaling (PMID:24388360, PMID:35951287), and it can penetrate cell membranes via an N-terminal cell-penetrating peptide in a phosphatidylethanolamine-dependent manner, translocating to the nucleus (PMID:27989561, PMID:28639251). As an interferon-stimulated gene, intracellular SERPINA5 promotes antiviral responses by enhancing STAT1 phosphorylation and nuclear translocation (PMID:36982532).

1. 1996 High

   Establishing SERPINA5 as a broad-specificity serpin resolved the question of whether PCI acts only on activated protein C or has wider protease targets, and revealed its role in reproduction by showing it inhibits acrosin and blocks sperm-egg binding.

   Evidence In vitro protease inhibition assays against multiple substrates, heparin modulation studies, immunocytochemistry on spermatozoa, and mouse in vitro fertilization model

   PMID:8796266

   Open questions at the time

   • Structural basis for broad specificity not determined
   • Physiological relevance of GAG modulation in vivo not established
   • Whether SERPINA5 is essential for fertility in humans remained unclear
2. 2004 Low

   Genetic screening addressed whether SERPINA5 mutations underlie male subfertility, finding no causal variants and indicating that PCI deficiency is not a common genetic cause of teratozoospermia or azoospermia.

   Evidence Direct sequencing of SERPINA5 gene and seminal plasma PCI antigen ELISA in subfertile men

   PMID:15377716

   Open questions at the time

   • Small cohort limits generalizability
   • Functional consequences of identified rare variants not tested in protease inhibition assays
   • Does not exclude a role for SERPINA5 in other aspects of reproductive biology
3. 2007 High

   Demonstrating that oxidized phospholipids (OxPE, PS) stimulate SERPINA5 inhibition of activated protein C through its heparin-binding site revealed a novel lipid-dependent regulatory mechanism distinct from classical GAG modulation.

   Evidence ELISA-based phospholipid binding, competition assays with heparin and annexin V, in vitro aPC inhibition with heparin-binding site mutant, immunohistochemistry of atherosclerotic plaques

   PMID:17332248

   Open questions at the time

   • In vivo contribution of OxPE/PS-mediated activation in thrombosis or atherosclerosis not tested
   • Structural details of phospholipid interaction with the heparin-binding site unresolved
4. 2013 High

   Discovery that SERPINA5 directly binds fibronectin and disrupts integrin β1 signaling to suppress HCC cell migration established a non-protease-inhibitory anti-metastatic mechanism for this serpin.

   Evidence Co-immunoprecipitation/pulldown for fibronectin interaction, in vitro migration/invasion assays, in vivo metastasis model with overexpression and knockdown

   PMID:24388360

   Open questions at the time

   • Whether anti-metastatic effect requires serpin protease-inhibitory activity or is independent
   • Fibronectin binding site on SERPINA5 not mapped
5. 2016 Medium

   Showing that SERPINA5 is internalized and translocated to the nucleus via an N-terminal cell-penetrating peptide in a phosphatidylethanolamine-dependent manner revealed an unexpected intracellular life for this secreted serpin.

   Evidence Cell fractionation, fluorescence microscopy, phospholipid binding assays, N-terminal deletion/mutation experiments in platelets, granulocytes, HL-60, and Jurkat cells

   PMID:27989561 PMID:28639251

   Open questions at the time

   • Nuclear function of internalized SERPINA5 not identified
   • Findings from a single lab and partly presented in review format
   • Mechanism of membrane translocation at the biophysical level not resolved
6. 2022 Medium

   Replication of the integrin β1/FAK suppression mechanism in endometrial cancer, including exosomal delivery, confirmed that SERPINA5's anti-invasive activity generalizes across tumor types and can be mediated by exosomes.

   Evidence Overexpression/knockdown in EC cells, Western blot for integrin β1/FAK pathway, xenograft mouse model, ELISA for exosomal SERPINA5

   PMID:35951287

   Open questions at the time

   • Whether exosomal SERPINA5 acts identically to free secreted protein not distinguished
   • Receptor or uptake mechanism for exosomal delivery not characterized
7. 2023 Medium

   Identification of SERPINA5 as an interferon-stimulated gene that enhances STAT1 phosphorylation and nuclear translocation linked its intracellular function to innate antiviral immunity, providing a functional consequence for nuclear translocation.

   Evidence Overexpression/knockdown, Western blot for STAT1 phosphorylation, nuclear fractionation, viral infection assays, reporter assays

   PMID:36982532

   Open questions at the time

   • Direct molecular target through which SERPINA5 enhances STAT1 phosphorylation unknown
   • Whether antiviral function requires serpin protease-inhibitory activity or cell-penetrating peptide not tested
   • Findings from single lab
8. 2023 Medium

   In vivo gain- and loss-of-function in a rat preeclampsia model showed SERPINA5 promotes coagulation activation and worsens disease features, connecting its hemostatic function to placental pathology.

   Evidence Adenoviral overexpression/suppression in rats, blood pressure and proteinuria measurement, placental histology, mRNA sequencing

   PMID:38136662

   Open questions at the time

   • Specific protease target(s) mediating placental coagulation activation not identified
   • Relevance to human preeclampsia not validated

• The direct molecular target by which intracellular SERPINA5 activates STAT1 signaling, the nuclear function of translocated SERPINA5, and whether its protease-inhibitory and non-inhibitory (fibronectin-binding, cell-penetrating) activities are functionally separable in vivo remain unresolved.
• No structural model of SERPINA5 in complex with fibronectin or phospholipids
• No separation-of-function mutant distinguishing protease inhibition from anti-metastatic or antiviral activities
• Nuclear targets or gene-regulatory role of internalized SERPINA5 uncharacterized