Matriptase (ST14, Suppressor of Tumorigenicity 14 Protein)
Matriptase is a type II transmembrane serine protease expressed in most types of epithelia (Oberst et al. 2003a). Matriptase was discovered in 1993 as a novel matrix degrading enzyme purified from human breast cancer cells (Shi et al. 1993). Since then, five independent groups have cloned the matriptase cDNA, and the protease has been published under the names membrane-type serine protease 1 (MT-SP1), tumor-associated differentially expressed gene-15 (TADG-15), epithin (in mouse), and SNC19 (Kim et al. 1999; Takeuchi et al. 1999; Cao et al. 2001; Tanimoto et al. 2001). The matriptase gene was given the name suppression of tumorgenecity 14 (ST14) (Zhang et al. 1998).
Matriptase expression studies have been performed in adult human tissue by mRNA, and protein analysis and matriptase expression has been found in all types of epithelium, including columnar, pseudostratified, cuboidal, and squamous (Oberst et al. 2003a). Matriptase expression has also been mapped in mice by enzymatic gene trapping using beta-galactosidase as a reporter, showing that matriptase displays the same overall epithelial expression pattern in mice as in humans (List et al. 2007b). The similar epithelial expression pattern between species suggests that matriptase has a phylogenetically conserved function in mammals.
Matriptase Structure and Activation
Matriptase is a type II transmembrane protease characterized by an N-terminal cytoplasmic domain and an extracellular C-terminal. Matriptase does not contain a typical signal peptide, but contains a 26 AA hydrophobic residue (residue 55–81) that is flanked by charged residues on each side, functioning as a membrane signal anchor sequence similar to that of other membrane anchored serine proteases (Takeuchi et al. 1999; Szabo and Bugge 2011). The extracellular region of matriptase contains a single Sea urchin sperm protein Enteropeptidase, Agrin (SEA) domain (residue 85–193), and two Complement factor 1r–Urchin embryonic growth factor-Bone morphogenetic protein (CUB) domains (residues 214–334 and 340–447) (Oberst et al. 2003b). CUB domains have conserved characteristics, which include four cysteine residues and various conserved hydrophobic and aromatic positions. Following the CUB domains are four low-density lipoprotein receptor (LDLR) repeats (residues 452–486, 487–523, 524–561, and 566–604). These repeats have a highly conserved pattern and spacing of six cysteine residues that together form three intramolecular disulfide bonds. The final domain is the serine protease domain (residue 614–855) that contains a substrate specificity pocket (S1 pocket) composed of aspartic acid 627 at its bottom and glycine 655 and glycine 665 at its neck, indicating that matriptase is a trypsin-like serine protease. The residues serine 805, aspartic acid 711, and histidine 656 constitute the catalytic triad and are all crucial for the catalytic activity of matriptase (Perona and Craik 1997). Matriptase has the preferential substrate cleavage site for amino acid residues with positively charged side chains such as arginine and lysine in position P1 (Beliveau et al. 2009). Human matriptase contains four potential sites for N-glycosylation (asparagine in positions 109, 302, 485, and 772) where the glycosylation site in the catalytic domain has been shown to be important for activation of the protease (Oberst et al. 2003b). Matriptase has an intricate multistep activation mechanism. Matriptase is synthesized as a 95 kDa pro-enzyme and full activation of the protease requires two sequential endoproteolytic cleavages (Fig. 1b).
The single chain pro-enzyme contains a consensus cleavage site in the SEA domain (GSVI) that is a target for the first proteolytic processing cleavage. This cleavage converts matriptase to a two-chain form cleaved at glycine 149, yet the processed 70 kDa extracellular domain remains attached to the transmembrane domain. The activation of matriptase zymogen follows a classical scheme by which a canonical activation motif is cleaved to convert a single-chain zymogen into a disulfide-linked two-chain active enzyme. Activated matriptase consists of the 40 kDa stem domain and the serine protease domain of 30 kDa, which is held together by a disulfide bridge. This second proteolytic cleavage takes place at arginine 614 and proteolytic processing of the SEA domain is a prerequisite for the activation site cleavage (Oberst et al. 2003b). Although matriptase is a membrane-associated protease, it was originally purified from the conditioned media from cells and human milk. This is a result of ectodomain shedding, the final proteolytic processing of matriptase. Matriptase can be shed by proteolytic cleavage at lysine 189 and lysine 204, which yields slightly smaller matriptase fragments in the extracellular space than the membrane associated form.
While the activation cleavage for most serine proteases depends on other upstream active proteases, activation of matriptase zymogen appears to be initiated by the weak intrinsic proteolytic activity of the zymogen form, referred to as autoactivation. Mutations in any of the catalytic triad residues (serine 805, aspartic acid 711, and histidine 656) results in a form of matriptase that is cleaved in the SEA domain but is unable to undergo the final activation site cleavage. Furthermore, the noncatalytic domains of matriptase and the posttranslational modifications are required for the activation of matriptase zymogen.
Matriptase in Epithelial Homeostasis
Prostasin is a GPI-anchored trypsin like serine protease that is involved in the regulation of epithelial sodium channels and is coexpressed with matriptase in most epithelial tissues (List et al. 2007b). Matriptase-deficient mice and prostasin-deficient mice have nearly identical epidermal phenotypes with impaired processing of the epidermis-specific poly-protein pro-filaggrin, compromised epidermal tight junctions, and a lack of terminal epidermal differentiation (Leyvraz et al. 2005). It has been shown in vitro that matriptase is able to cleave the zymogen form of prostasin to generate proteolytically active prostasin. Moreover, only the inactive form of prostasin is found in matriptase-deficient mice and loss of matriptase in primary human keratinocytes is correlated with loss of proteolytic activation of prostasin (Netzel-Arnett et al. 2006). It has also been shown that prostasin is a potent activator of matriptase. However, the ability of prostasin to activate matriptase is independent of its own activity, as both a zymogen-locked and a catalytically inactive form of prostasin can activate matriptase (Friis et al. 2013; Peters et al. 2014; Friis et al. 2016). This indicates that prostasin works as a chaperone, not a protease, for matriptase activation which is consistent with the observation that small molecule protease inhibitors fail to inhibit the matriptase zymogen activation, indicating that no active protease is involved in the process (Oberst et al. 2003b).
Furthermore, the zymogen of matriptase has weak intrinsic activity that is sufficient for activation of prostasin (Friis et al. 2013). This suggests that matriptase and prostasin form a reciprocal activation complex at the pinnacle of the protease cascade, and initiation of the cascade is facilitated by the interaction of the pro-forms of matriptase and prostasin which activates each other.
Matriptase in Epithelial Pathology and Cancer
A rare mutation in the ST14 gene causes a form of the skin disease ichthyosis with hypotrichosis syndrome in humans characterized by scaly skin and abnormal hair (Basel-Vanagaite et al. 2007). This is caused by a mutation in the serine protease domain of matriptase at glycine 827, which results in strongly reduced matriptase activity and consequently a lack of prostasin activation (Desilets et al. 2008). The ichthyosis symptoms are phenocopied in matriptase hypomorphic mice with a 100-fold reduction in epidermal matriptase mRNA levels showing impaired desquamation and brittle thin hair (List et al. 2007a). Recently matriptase has been coupled to another form of ichthyosis seen in the skin disease Netherton Syndrome, where matriptase is capable of initiating premature activation of a pro-kallikrein-related cascade and in the protease inhibitor LEKTI-deficient mice. Additionally, increased matriptase expression has been observed in keratinocytes at the site of inflammation in 16 different skin diseases (Chen et al. 2011).
Matriptase is a type II transmembrane serine protease expressed in most types of epithelia. Matriptase has been shown to play a crucial role in epithelial homeostasis as matriptase-deficient mice and humans have severe epithelial barrier defects. Matriptase is situated at the pinnacle of a protease cascade that is involved in epidermal differentiation and pro-filaggrin processing. The GPI-anchored serine protease prostasin acts as a chaperone for matriptase activation, as prostasin can activate matriptase, however independent of the proteolytic activity of prostasin. Matriptase has been shown to be a potent oncogene when unopposed by its cognate inhibitor HAI-1. Matriptase exerts its oncogenic functions through the cMet-akt pathway and through the G-protein coupled receptor Par-2. The need for tight regulation of matriptase activity by inhibitors, as well as the vital role of the protease, underlines that both the absence and the excess of matriptase can cause serious epithelial defects.
- Basel-Vanagaite L, Attia R, Ishida-Yamamoto A, Rainshtein L, Ben Amitai D, Lurie R, et al. Autosomal recessive ichthyosis with hypotrichosis caused by a mutation in ST14, encoding type II transmembrane serine protease matriptase. Am J Hum Genet. 2007;80:467–77. doi: 10.1086/512487.PubMedPubMedCentralCrossRefGoogle Scholar
- Cao J, Zheng S, Zheng L, Cai X, Zhang Y, Geng L, et al. A novel serine protease SNC19 associated with human colorectal cancer. Chin Med J (Engl). 2001;114:726–30.Google Scholar
- Friis S, Uzzun Sales K, Godiksen S, Peters DE, Lin CY, Vogel LK, et al. A matriptase-prostasin reciprocal zymogen activation complex with unique features: prostasin as a non-enzymatic co-factor for matriptase activation. J Biol Chem. 2013;288:19028–39. doi: 10.1074/jbc.M113.469932.PubMedPubMedCentralCrossRefGoogle Scholar
- Kim MG, Chen C, Lyu MS, Cho EG, Park D, Kozak C, et al. Cloning and chromosomal mapping of a gene isolated from thymic stromal cells encoding a new mouse type II membrane serine protease, epithin, containing four LDL receptor modules and two CUB domains. Immunogenetics. 1999;49:420–8.PubMedCrossRefGoogle Scholar
- List K, Currie B, Scharschmidt TC, Szabo R, Shireman J, Molinolo A, et al. Autosomal ichthyosis with hypotrichosis syndrome displays low matriptase proteolytic activity and is phenocopied in ST14 hypomorphic mice. J Biol Chem. 2007a;282:36714–23. doi: 10.1074/jbc.M705521200.PubMedCrossRefGoogle Scholar
- Peters DE, Szabo R, Friis S, Shylo NA, Uzzun Sales K, Holmbeck K, et al. The membrane-anchored serine protease prostasin (CAP1/PRSS8) supports epidermal development and postnatal homeostasis independent of its enzymatic activity. J Biol Chem. 2014;289:14740–9. doi: 10.1074/jbc.M113.541318.PubMedPubMedCentralCrossRefGoogle Scholar
- Sales KU, Friis S, Konkel JE, Godiksen S, Hatakeyama M, Hansen KK, et al. Non-hematopoietic PAR-2 is essential for matriptase-driven pre-malignant progression and potentiation of ras-mediated squamous cell carcinogenesis. Oncogene. 2015;34:346–56. doi: 10.1038/onc.2013.563.PubMedCrossRefGoogle Scholar
- Szabo R, Hobson JP, Christoph K, Kosa P, List K, Bugge TH. Regulation of cell surface protease matriptase by HAI2 is essential for placental development, neural tube closure and embryonic survival in mice. Development. 2009;136:2653–63. doi: 10.1242/dev.038430.PubMedPubMedCentralCrossRefGoogle Scholar
- Takeuchi T, Shuman MA, Craik CS. Reverse biochemistry: use of macromolecular protease inhibitors to dissect complex biological processes and identify a membrane-type serine protease in epithelial cancer and normal tissue. Proc Natl Acad Sci U S A. 1999;96:11054–61.PubMedPubMedCentralCrossRefGoogle Scholar