SERPINA5

SERPINA5 Full Name

serpin peptidase inhibitor, clade A (alpha-1 antiproteinase, antitrypsin), member 5

SERPINA5 Introduction

SERPINA5, more commonly known in the scientific literature as Protein C Inhibitor (PCI), is a highly versatile member of the serine protease inhibitor (serpin) superfamily. Initially discovered in human blood plasma as a primary inhibitor of activated protein C (APC), this single-chain glycoprotein is synthesized predominantly by the liver. However, unlike many other serpins that are restricted to circulation, SERPINA5 is widely distributed across various tissues and is abundantly secreted into bodily fluids, most notably seminal plasma, urine, and cerebrospinal fluid. A defining structural characteristic of SERPINA5 is its strong dependency on negatively charged glycosaminoglycans, such as heparin. The binding of these cofactors induces a critical conformational change in the protein, which dramatically accelerates its inhibitory activity against specific target proteases.

The physiological functions of SERPINA5 are remarkably broad and promiscuous, allowing it to regulate multiple distinct biological cascades. In the realm of hemostasis, it acts as a complex dual regulator. By neutralizing activated protein C, SERPINA5 dampens the body's natural anticoagulant pathway, thereby exerting a procoagulant effect. Conversely, it can also inhibit powerful procoagulant enzymes like thrombin and factor Xa, as well as essential profibrinolytic factors such as urokinase (uPA) and tissue plasminogen activator (tPA). Beyond blood coagulation and fibrinolysis, SERPINA5 is a crucial player in reproductive biology. In the male reproductive tract, where it exists in exceptionally high concentrations, it regulates the activity of prostate-specific antigen (PSA) and other kallikreins, playing a necessary role in semen liquefaction, sperm motility, and successful fertilization.

Figure 1. The inhibitory points of PCI in the pathway of the events (inflammatory response, microvascular thrombi formation and secondary fibrinolysis) occurring in sepsis-related DIC. (Source: Fujita M, et al. 2004)

Clinically, the dysregulation of SERPINA5 is implicated in a diverse array of human pathologies. Because of its pivotal position in hemostasis, systemic imbalances in SERPINA5 levels can disrupt the delicate equilibrium between clot formation and bleeding, contributing to severe thromboembolic disorders or hemorrhagic tendencies. In reproductive medicine, diminished expression or structural mutations of SERPINA5 within seminal fluid are heavily correlated with male infertility and aberrant spermatogenesis. Furthermore, SERPINA5 has become a molecule of significant interest in oncology. Since it actively inhibits tissue-degrading proteases like uPA, it generally functions as a protective factor against tumor invasion. Consequently, the downregulation or loss of SERPINA5 expression is frequently observed in aggressive malignancies, including prostate, breast, and renal cell carcinomas. In these contexts, its absence facilitates tumor cell migration, extracellular matrix degradation, and metastasis, making it a valuable prognostic biomarker and a potential therapeutic target.

Alternate Names for SERPINA5

SERPINA5; serpin peptidase inhibitor; clade A (alpha-1 antiproteinase; antitrypsin); member 5; PCI; PAI3; PAI-3; PCI-B; PROCI; PLANH3; plasma serine protease inhibitor; protein C inhibitor; plasminogen activator inhibitor-3; acrosomal serine protease inhibitor; plasminogen activator inhibitor III; serine (or cysteine) proteinase inhibitor; clade A (alpha-1 antiproteinase; antitrypsin); member 5; anti-PCI

• 1. Yang H, et al. Cell penetrating SERPINA5 (ProteinC inhibitor, PCI): More questions than answers. Semin Cell Dev Biol. 2017 Feb;62:187-193.
  2. Fujita M, et al. Protein C inhibitor as an anti-disseminated intravascular coagulation agent--mechanism and modification. Curr Med Chem Cardiovasc Hematol Agents. 2004 Jan;2(1):21-7.