PI3

PI3 Full Name

peptidase inhibitor 3, skin-derived

PI3 Introduction

PI3 encodes elafin, a small secreted protein that protects epithelial surfaces from the destructive activity of proteolytic enzymes and that is also known as skin-derived antileukoproteinase (SKALP) or trappin-2. Elafin belongs to the whey acidic protein (WAP) family of protease inhibitors, named for the conserved WAP domain that contains the inhibitory activity, and it specifically inhibits neutrophil elastase and proteinase 3, enzymes released by neutrophils that are essential for killing microbes but that can damage host tissues when they escape control. Elafin is produced by epithelial cells of the skin, lung, gut, and other surfaces, and it is strongly induced by inflammation and injury, where it functions as part of the local defense against excessive proteolysis. A distinctive feature of elafin is its ability to be cross-linked into the extracellular matrix by transglutaminases, allowing the inhibitor to remain anchored at the site of production and to protect the matrix from degradation. In addition to its antiprotease function, elafin has direct antimicrobial activity and modulates inflammation, and it has been studied for its protective role in diseases ranging from emphysema and cystic fibrosis to inflammatory bowel disease. In cancer, elafin expression is altered in several tumor types, where it can influence tumor invasion, angiogenesis, and the immune response, making it a molecule of considerable interest in both inflammatory and malignant disease.

Figure 1. The structure of PI3.

WAP Domain Architecture and Protease Inhibition
Elafin is synthesized as a larger precursor protein called trappin-2, which contains an N-terminal cementoin domain and a C-terminal WAP domain.
The cementoin domain is rich in glutamine and lysine residues and serves as the substrate for transglutaminase-mediated cross-linking of elafin to extracellular matrix proteins.
The WAP domain, a conserved module of approximately 50 amino acids stabilized by eight cysteine residues forming four disulfide bonds, carries the protease-inhibitory activity.
Within the WAP domain, a specific reactive-site loop binds the active site of neutrophil elastase and proteinase 3, blocking their proteolytic activity with high affinity.
The PI3 gene is located on human chromosome 20 within a cluster of genes encoding related WAP-domain proteins, and its expression is controlled by transcription factors that respond to inflammation and epithelial injury.
In its secreted form, elafin exists as both the full-length trappin-2 and the proteolytically processed mature elafin, both of which retain protease-inhibitory activity.
The precursor and mature forms of the protein can be detected in biological fluids, where their levels reflect the state of epithelial inflammation.

Epithelial Protection, Antimicrobial Defense, and Roles in Disease
By inhibiting neutrophil elastase and proteinase 3, elafin protects the skin, lung, and mucosal surfaces from the tissue destruction that occurs when neutrophils are activated during infection or inflammation.
Elafin is markedly induced in inflammatory skin diseases such as psoriasis, where it is deposited in the stratum corneum, and its levels are also elevated in the airways of patients with cystic fibrosis and chronic obstructive pulmonary disease.
The cross-linking of elafin into the extracellular matrix by transglutaminases anchors the inhibitor at sites of inflammation, where it can neutralize proteases released into the tissue.
Beyond its antiprotease activity, elafin exhibits direct antimicrobial properties and can modulate the innate immune response, contributing to the barrier function of epithelial surfaces.
In animal models, overexpression or exogenous delivery of elafin protects against emphysema, ventilator-induced lung injury, and other protease-driven tissue damage, supporting its potential as a therapeutic protein.
In cancer, elafin expression is dysregulated in tumors of the breast, lung, ovary, and other organs, and depending on the context it has been reported to influence tumor cell invasion, angiogenesis, and the antitumor immune response, making PI3 a subject of active investigation as a biomarker and therapeutic target.

Alternate Names for PI3

PI3; peptidase inhibitor 3, skin-derived; ESI; WAP3; SKALP; WFDC14; cementoin; elafin; PI-3; trappin-2