Hepsin

Hepsin was originally discovered in 1988 by Stephen P. Leytus, Kenneth R. Loeb, Frederic S. Hagen, Koiti Kurachi, and Earl W. Davie, who identified it from a human liver cDNA clone. The gene was isolated using sequences targeting the conserved catalytic domain of trypsin-like serine proteases. == Structure ==

File:PDB 1p57 EBI.jpg|thumb|Cartoon representation of the human hepsin (HPN) protein (PDB ID: 1P57). Image courtesy of Jawahar Swaminathan and MSD staff, European Bioinformatics Institute Hepsin is a type II transmembrane serine protease made up of a short cytoplasmic region, a single transmembrane helix, and an extracellular region. The N-terminus is located in the cytoplasm, and the C-terminus extends into the extracellular space. The cytoplasmic region is approximately 17 amino acids long. The transmembrane helix extends through the lipid bilayer, securing hepsin at the cell surface and allowing the extracellular region to maintain a stable orientation for substrate interaction and proteolytic activity. The extracellular region contains the enzyme's functional domains and represents the largest structural region of the protein. The portion consists of two domains: a catalytic serine protease domain and a smaller non catalytic domain, known as a scavenger receptor cysteine rich (SRCR) domain. Within the serine protease domain, two β-barrel subdomains form the catalytic site, which include the catalytic triad (His, Asp, Ser) that is essential for protease activity. This domain also features a substrate binding pocket, which influences the enzyme's specificity for substrate selectivity. The SRCR domain, located near the membrane, is linked to the protease domain through a disulfide bond and other non-covalent interactions. While the domain does not participate directly in proteolysis, the SRCR domain is believed to help position the protease domain and facilitate protein to protein interactions. The SRCR domain is stabilized by three internal disulfide bonds and a compact hydrophobic core, which help maintain its structure. The monomeric protein has a length of 417 amino acids and a molecular weight of approximately 45,011 Daltons. == Function ==

Hepsin is a type II transmembrane serine protease anchored at the cell surface. The enzyme modifies specific protein substrates, including zymogens, by cleaving them through proteolytic processing. This proteolytic activity promotes cellular interaction with the ECM, supporting tissue organization, structural stability, and cell adhesion. While hepsin is expressed at highest levels in the liver, it is also found in other epithelial tissues, where it contributes to maintaining normal tissue organization. Hepsin is also highly expressed in the kidney, where it plays a role in uromodulin processing. It may additionally activate pro-matrix metalloproteinases, which facilitates the break down of extracellular matrix proteins and release of growth factors that are important for tissue repair and cell movement. Hepsin's activity at the cell surface depends on correct protein folding and localization of the membrane. A sugar molecule attached at Asn112 within the SRCR domain helps hepsin fold properly and ensures that its transport to the cell membrane is successful. Without this modification, hepsin remains in the endoplasmic reticulum and cannot perform its enzymatic functions. Hepsin is also regulated by natural protein inhibitors like HAI-1 and HAI-2, which limit its proteolytic activity, and calpain-1, which can reduce hepsin's functional levels on the cell surface by cleaving the enzyme directly from the cell membrane. In the liver, hepsin's conversion of pro-HGF to HGF stimulates Met receptor signaling. As a result, downstream targets such as AKT, mTOR, and GSK3 are activated. These signaling events increase lipid, glycogen, and protein synthesis within hepatocytes. Beyond the liver, hepsin contributes to multiple processes in other tissues, including protein processing in the kidney, organization of the extracellular matrix, development of the inner ear structure and fat cell activity. Within the kidney, hepsin processes uromodulin at a specific site, allowing it to polymerize and be secreted into urine. This process is important for maintaining electrolyte balance and proper function of the urinary tract. Hepsin deficiency disrupts uromodulin processing and reduces polymer levels in urine, which affects kidney function. In adipose tissue, hepsin inhibits the development and activity of brown cells by increasing HGF, which activates Met, and AKT signaling. Through this mechanism, hepsin limits energy use of fat cells and reduces the production of thermogenic proteins, including Ucp1 and Cidea. In the inner ear, hepsin driven HGF activation and Met signaling supports the formation of cochlear structures. This development is critical for normal auditory function and sound perception in mammals. == Expression and regulation ==

Hepsin is expressed in the liver, where it plays a central role in glucose, lipid, and protein metabolism. The enzyme is also found in multiple non-hepatic tissues, such as adipose tissue, kidney, inner ear, lung, prostate, thyroid, stomach, and breast. In adipose tissue, hepsin affects the development of fat cells and their energy use through specific signaling pathways that control both cell growth and metabolic activity. In the kidney, the enzyme contributes to the proteolytic processing and polymerization of uromodulin in epithelial cells of the thick ascending limb. In the inner ear, hepsin is required for normal auditory function. Research in mice lacking hepsin have shown lower blood glucose and lipid levels, hearing loss, and problems with renal protein processing. At the cellular level, hepsin is a type II transmembrane serine protease, whose active form is localized on the cell surface. The enzyme's function depends on post-translational changes, including zymogen activation and N-glycosylation in the SRCR domain. Disruption of N-glycosylation causes hepsin to remain the endoplasmic reticulum, not allowing it to become active, showing how important intracellular processing is for its function. Hepsin activity at the cell surface is also controlled by ectodomain shedding. Calpain-1, a calcium dependent enzyme protease, helps remove hepsin from the membrane, lowering the amount of active enzyme levels. Additional protein inhibitors, such as hepatocyte growth factor inhibitors HAI-1 and HAI-2 and SerpinB12, can also limit hepsin's enzymatic activity. Together, these mechanisms regulate when and where hepsin is active, allowing it to carry out specific functions in different tissues while preventing unwanted protein cleavage. However, the mechanisms that control hepsin expression in different tissues are not fully understood. == Clinical significance ==

Hepsin has been studied mainly for its role in cancer, where changes in its expression are linked to tumor behavior in multiple tissues. Changes in hepsin activity can affect cellular interactions within their environment, which can contribute to tumor growth or spread. This activity influences proliferation, tissue organization, and the local signaling environment within breast tumors. While normal ovarian tissue shows low levels of hepsin, both low malignant potential tumors and carcinomas show significantly elevated expression of the enzyme. Mice lacking the hepsin gene show hearing impairments detectable early in life, primarily caused due to structural defects in the tectorial membrane (TM) of the cochlea. Hepsin absence disrupts the organization of the tectorial membrane, producing swelling, gaps, and partial separation from the spiral limbus, which disrupts normal cochlear function. As a result, hepsin knockout mice show elevated auditory thresholds and reduced outer hair cell amplification at high frequencies as revealed by ABR and DPOAE tests. These findings suggest that hepsin contributes to the development and structural integrity of the TM, which is essential for proper cochlear mechanics. Although these findings come from animal models, they indicate that proper hepsin activity during development is important for hearing and inner ear physiology. == See also ==