SARS-CoV-2 infection of human lung epithelial cells induces TMPRSS-mediated acute fibrin deposition

Severe COVID-associated lung injury is a major confounding factor of hospitalizations and death with no effective treatments. Here, we describe a non-classical fibrin clotting mechanism mediated by SARS-CoV-2 infected primary lung but not other susceptible epithelial cells. This infection-induced fibrin formation is observed in all variants of SARS-CoV-2 infections, and requires thrombin but is independent of tissue factor and other classical plasma coagulation factors. While prothrombin and fibrinogen levels are elevated in acute COVID BALF samples, fibrin clotting occurs only with the presence of viral infected but not uninfected lung epithelial cells. We suggest a viral-induced coagulation mechanism, in which prothrombin is activated by infection-induced transmembrane serine proteases, such as ST14 and TMPRSS11D, on NHBE cells. Our finding reveals the inefficiency of current plasma targeted anticoagulation therapy and suggests the need to develop a viral-induced ARDS animal model for treating respiratory airways with thrombin inhibitors.

Exosome-Mediated Activation of the Prostasin-Matriptase Serine Protease Cascade in B Lymphoma Cells

Prostasin and matriptase are extracellular membrane serine proteases with opposing effects in solid epithelial tumors. Matriptase is an oncoprotein that promotes tumor initiation and progression, and prostasin is a tumor suppressor that reduces tumor invasion and metastasis. Previous studies have shown that a subgroup of Burkitt lymphoma have high levels of ectopic matriptase expression but no prostasin. Reducing the matriptase level via small interfering RNAs in B lymphoma cells impeded tumor xenograft growth in mice. Here, we report a novel approach to matriptase regulation in B cancer cells by prostasin via exosomes to initiate a prostasin-matriptase protease activation cascade. The activation and shedding of matriptase were monitored by measuring its quantity and trypsin-like serine protease activity in conditioned media. Sustained activation of the protease cascade in the cells was achieved by the stable expression of prostasin. The B cancer cells with prostasin expression presented phenotypes consistent with its tumor suppressor role, such as reduced growth and increased apoptosis. Prostasin exosomes could be developed as an agent to initiate the prostasin-matriptase cascade for treating B lymphoma with further studies in animal models.

A critical review on marine serine protease and its inhibitors: A new wave of drugs?

Marine organisms are rich sources of enzymes and their inhibitors having enormous therapeutic potential. Among different proteolytic enzymes, serine proteases, which can be obtained from various marine organisms show a potential to biomedical application as thrombolytic agents. Although this type of proteases plays a crucial role in almost all biological processes, their uncontrolled activity often leads to several diseases. Accordingly, the actions of these types of proteases are regulated by serine protease inhibitors (SPIs). Marine SPIs control complement activation and various other physiological functions, such as inflammation, immune function, fibrinolysis, blood clotting, and cancer metastasis. This review highlights the potential use of serine proteases and their inhibitors as the new wave of promising drugs.

Mild acidity likely accelerates the physiological matriptase autoactivation process: a comparative study between spontaneous and acid-induced matriptase zymogen activation

The pathophysiological functions of matriptase, a type 2 transmembrane serine protease, rely primarily on its enzymatic activity, which is under tight control through multiple mechanisms. Among those regulatory mechanisms, the control of zymogen activation is arguably the most important. Matriptase zymogen activation not only generates the mature active enzyme but also initiates suppressive mechanisms, such as rapid inhibition by HAI-1, and matriptase shedding. These tightly coupled events allow the potent matriptase tryptic activity to fulfill its biological functions at the same time as limiting undesired hazards. Matriptase is converted to the active enzyme via a process of autoactivation, in which the activational cleavage is thought to rely on the interactions of matriptase zymogen molecules and other as yet identified proteins. Matriptase autoactivation can occur spontaneously and is rapidly followed by the formation and then shedding of matriptase-HAI-1 complexes, resulting in the presence of relatively low levels of the complex on cells. Activation can also be induced by several non-protease factors, such as the exposure of cells to a mildly acidic buffer, which rapidly causes high-level matriptase zymogen activation in almost all cell lines tested. In the current study, the structural requirements for this acid-induced zymogen activation are compared with those required for spontaneous activation through a systematic analysis of the impact of 18 different mutations in various structural domains and motifs on matriptase zymogen activation. Our study reveals that both acid-induced matriptase activation and spontaneous activation depend on the maintenance of the structural integrity of the serine protease domain, non-catalytic domains, and posttranslational modifications. The common requirements of both modes of activation suggest that acid-induced matriptase activation may function as a physiological mechanism to induce pericellular proteolysis by accelerating matriptase autoactivation.

Matriptase and prostasin proteolytic activities are differentially regulated in normal and wounded skin

Orchestrated control of multiple overlapping and sequential processes is required for the maintenance of epidermal homeostasis and the response to and recovery from a variety of skin insults. Previous studies indicate that membrane-associated serine protease matriptase and prostasin play essential roles in epidermal development, differentiation, and barrier formation. The control of proteolysis is a highly regulated process, which depends not only on gene expression but also on zymogen activation and the balance between protease and protease inhibitor. Subcellular localization can affect the accessibility of protease inhibitors to proteases and, thus, also represents an integral component of the control of proteolysis. To understand how membrane-associated proteolysis is regulated in human skin, these key aspects of matriptase and prostasin were determined in normal and injured human skin by immunohistochemistry. This staining shows that matriptase is expressed predominantly in the zymogen form at the periphery of basal and spinous keratinocytes, and prostasin appears to be constitutively activated at high levels in polarized organelle-like structures of the granular keratinocytes in the adjacent quiescent skin. The membrane-associated proteolysis appears to be elevated via an increase in matriptase zymogen activation and prostasin protein expression in areas of skin recovering from epidermal insults. There was no noticeable change observed in other regulatory aspects, including the expression and tissue distribution of their cognate inhibitors HAI-1 and HAI-2. This study reveals that the membrane-associated proteolysis may be a critical epidermal mechanism involved in responding to, and recovering from, damage to human skin.

Intramembrane proteolysis of an extracellular serine protease, epithin/PRSS14, enables its intracellular nuclear function

BACKGROUND

Epithin/PRSS14, a type II transmembrane serine protease, is an emerging target of cancer therapy because of its critical roles in tumor progression and metastasis. In many circumstances, the protease, through its ectodomain shedding, exists as a soluble form and performs its proteolytic functions in extracellular environments increasing cellular invasiveness. The seemingly functional integrity of the soluble form raises the question of why the protease is initially made as a membrane-associated protein.

RESULTS

In this report, we show that the epithin/PRSS14 intracellular domain (EICD) can be released from the membrane by the action of signal peptide peptidase-like 2b (SPPL2b) after ectodomain shedding. The EICD preferentially localizes in the nucleus and can enhance migration, invasion, and metastasis of epithelial cancer when heterologously expressed. Unbiased RNA-seq analysis and subsequent antibody arrays showed that EICD could control the gene expression of chemokines involved in cell motility, by increasing their promoter activities. Finally, bioinformatics analysis provided evidence for the clinical significance of the intramembrane proteolysis of epithin/PRSS14 by revealing that the poor survival of estrogen receptor (ER)-negative breast cancer patients with high epithin/PRSS14 expression is further worsened by high levels of SPPL2b.

CONCLUSIONS

These results show that ectodomain shedding of epithin/PRSS14 can initiate a unique and synchronized bidirectional signal for cancer metastasis: extracellularly broadening proteolytic modification of the surrounding environment and intracellularly reprogramming the transcriptome for metastatic conversion. Clinically, this study also suggests that the intracellular function of epithin/PRSS14 should be considered for targeting this protease for anti-cancer treatment.

Cell surface-anchored serine proteases in cancer progression and metastasis

Over the last two decades, a novel subgroup of serine proteases, the cell surface-anchored serine proteases, has emerged as an important component of the human degradome, and several members have garnered significant attention for their roles in cancer progression and metastasis. A large body of literature describes that cell surface-anchored serine proteases are deregulated in cancer and that they contribute to both tumor formation and metastasis through diverse molecular mechanisms. The loss of precise regulation of cell surface-anchored serine protease expression and/or catalytic activity may be contributing to the etiology of several cancer types. There is therefore a strong impetus to understand the events that lead to deregulation at the gene and protein levels, how these precipitate in various stages of tumorigenesis, and whether targeting of selected proteases can lead to novel cancer intervention strategies. This review summarizes current knowledge about cell surface-anchored serine proteases and their role in cancer based on biochemical characterization, cell culture-based studies, expression studies, and in vivo experiments. Efforts to develop inhibitors to target cell surface-anchored serine proteases in cancer therapy will also be summarized.

The intracellular seven amino acid motif EEGEVFL is required for matriptase vesicle sorting and translocation to the basolateral plasma membrane

Matriptase plays important roles in epithelial integrity and function, which depend on its sorting to the basolateral surface of cells, where matriptase zymogen is converted to an active enzyme in order to act on its substrates. After activation, matriptase undergoes HAI-1-mediated inhibition, internalization, transcytosis, and secretion from the apical surface into the lumen. Matriptase is a mosaic protein with several distinct protein domains and motifs, which are a reflection of matriptase’s complex cellular itinerary, life cycle, and the tight control of its enzymatic activity. While the molecular determinants for various matriptase regulatory events have been identified, the motif(s) required for translocation of human matriptase to the basolateral plasma membrane is unknown. The motif previously identified in rat matriptase is not conserved between the rodent and the primate. We, here, revisit the question for human matriptase through the use of a fusion protein containing a green fluorescent protein linked to the matriptase N-terminal fragment ending at Gly-149. A conserved seven amino acid motif EEGEVFL, which is similar to the monoleucine C-terminal to an acidic cluster motif involved in the basolateral targeting for some growth factors, has been shown to be required for matriptase translocation to the basolateral plasma membrane of polarized MDCK cells. Furthermore, time-lapse video microscopy showed that the motif appears to be required for entry into the correct transport vesicles, by which matriptase can undergo rapid trafficking and translocate to the plasma membrane. Our study reveals that the EEGEVFL motif is necessary, but may not be sufficient, for matriptase basolateral membrane targeting and serves as the basis for further research on its pathophysiological roles.

Aberrant regulation favours matriptase proteolysis in neoplastic B-cells that co-express HAI-2

Matriptase is ectopically expressed in neoplastic B-cells, in which matriptase activity is enhanced by negligible expression of its endogenous inhibitor, hepatocyte growth factor activator inhibitor (HAI)-1. HAI-1, however, is also involved in matriptase synthesis and intracellular trafficking. The lack of HAI-1 indicates that other related inhibitor, such as HAI-2, might be expressed. Here, we show that HAI-2 is commonly co-expressed in matriptase-expressing neoplastic B-cells. The level of active matriptase shed after induction of matriptase zymogen activation in 7 different neoplastic B-cells was next determined and characterised. Our data reveal that active matriptase can only be generated and shed by those cells able to activate matriptase and in a rough correlation with the levels of matriptase protein. While HAI-2 can potently inhibit matriptase, the levels of active matriptase are not proportionally suppressed in those cells with high HAI-2. Our survey suggests that matriptase proteolysis might aberrantly remain high in neoplastic B-cells regardless of the levels of HAI-2.

Membrane-Anchored Serine Proteases and Protease-Activated Receptor-2-Mediated Signaling: Co-Conspirators in Cancer Progression

Pericellular proteolysis provides a significant advantage to developing tumors through the ability to remodel the extracellular matrix, promote cell invasion and migration, and facilitate angiogenesis. Recent advances demonstrate that pericellular proteases can also communicate directly to cells by activation of a unique group of transmembrane G-protein-coupled receptors (GPCR) known as protease-activated receptors (PAR). In this review, we discuss the specific roles of one of four mammalian PARs, namely PAR-2, which is overexpressed in advanced stage tumors and is activated by trypsin-like serine proteases that are highly expressed or otherwise dysregulated in many cancers. We highlight recent insights into the ability of different protease agonists to bias PAR-2 signaling and the newly emerging evidence for an interplay between PAR-2 and membrane-anchored serine proteases, which may co-conspire to promote tumor progression and metastasis. Interfering with these pathways might provide unique opportunities for the development of new mechanism-based strategies for the treatment of advanced and metastatic cancers.

Matriptase-Induced Phosphorylation of MET is Significantly Associated with Poor Prognosis in Invasive Bladder Cancer; an Immunohistochemical Analysis

Hepatocyte growth factor (HGF) plays an important role in cancer progression via phosphorylation of MET (c-met proto-oncogene product, receptor of HGF). HGF-zymogen (pro-HGF) must be processed for activation by HGF activators including matriptase, which is a type II transmembrane serine protease and the most efficient activator. The enzymatic activity is tightly regulated by HGF activator inhibitors (HAIs). Dysregulated pro-HGF activation (with upregulated MET phosphorylation) is reported to promote cancer progression in various cancers. We retrospectively analyzed the expression of matriptase, phosphorylated-MET (phospho-MET) and HAI-1 in tumor specimens obtained from patients with invasive bladder cancer by immunohistochemistry. High expression of phospho-MET and increased expression of matriptase were significantly associated with poor prognosis, and high matriptase/low HAI-1 expression showed poorer prognosis. Furthermore, high expression of matriptase tended to correlate with phosphorylation of MET. Increased expression of matriptase may induce the ligand-dependent activation of MET, which leads to poor prognosis in patients with invasive bladder cancer.

Tissue distribution and subcellular localizations determine in vivo functional relationship among prostasin, matriptase, HAI-1, and HAI-2 in human skin

The membrane-bound serine proteases prostasin and matriptase and the Kunitz-type protease inhibitors HAI-1 and HAI-2 are all expressed in human skin and may form a tightly regulated proteolysis network, contributing to skin pathophysiology. Evidence from other systems, however, suggests that the relationship between matriptase and prostasin and between the proteases and the inhibitors can be context-dependent. In this study the in vivo zymogen activation and protease inhibition status of matriptase and prostasin were investigated in the human skin. Immunohistochemistry detected high levels of activated prostasin in the granular layer, but only low levels of activated matriptase restricted to the basal layer. Immunoblot analysis of foreskin lysates confirmed this in vivo zymogen activation status and further revealed that HAI-1 but not HAI-2 is the prominent inhibitor for prostasin and matriptase in skin. The zymogen activation status and location of the proteases does not support a close functional relation between matriptase and prostasin in the human skin. The limited role for HAI-2 in the inhibition of matriptase and prostasin is the result of its primarily intracellular localization in basal and spinous layer keratinocytes, which probably prevents the Kunitz inhibitor from interacting with active prostasin or matriptase. In contrast, the cell surface expression of HAI-1 in all viable epidermal layers renders it an effective regulator for matriptase and prostasin. Collectively, our study suggests the importance of tissue distribution and subcellular localization in the functional relationship between proteases and protease inhibitors.

HAI-2 stabilizes, inhibits and regulates SEA-cleavage-dependent secretory transport of matriptase

It has recently been shown that hepatocyte growth factor activator inhibitor-2 (HAI-2) is able to suppress carcinogenesis induced by overexpression of matriptase, as well as cause regression of individual established tumors in a mouse model system. However, the role of HAI-2 is poorly understood. In this study, we describe 3 mutations in the binding loop of the HAI-2 Kunitz domain 1 (K42N, C47F and R48L) that cause a delay in the SEA domain cleavage of matriptase, leading to accumulation of non-SEA domain cleaved matriptase in the endoplasmic reticulum (ER). We suggest that, like other known SEA domains, the matriptase SEA domain auto-cleaves and reflects that correct oligomerization, maturation, and/or folding has been obtained. Our results suggest that the HAI-2 Kunitz domain 1 mutants influence the flux of matriptase to the plasma membrane by affecting the oligomerization, maturation and/or folding of matriptase, and as a result the SEA domain cleavage of matriptase. Two of the HAI-2 Kunitz domain 1 mutants investigated (C47F, R48L and C47F/R48L) also displayed a reduced ability to proteolytically silence matriptase. Hence, HAI-2 separately stabilizes matriptase, regulates the secretory transport, possibly via maturation/oligomerization and inhibits the proteolytic activity of matriptase in the ER, and possible throughout the secretory pathway.

Selective Inhibition of Prostasin in Human Enterocytes by the Integral Membrane Kunitz-Type Serine Protease Inhibitor HAI-2

Mutations of hepatocyte growth factor activator inhibitor (HAI)-2 in humans cause sodium loss in the gastrointestinal (GI) tract in patients with syndromic congenital sodium diarrhea (SCSD). Aberrant regulation of HAI-2 target protease(s) was proposed as the cause of the disease. Here functional linkage of HAI-2 with two membrane-associated serine proteases, matriptase and prostasin was analyzed in Caco-2 cells and the human GI tract. Immunodepletion-immunoblot analysis showed that significant proportion of HAI-2 is in complex with activated prostasin but not matriptase. Unexpectedly, prostasin is expressed predominantly in activated forms and was also detected in complex with HAI-1, a Kunitz inhibitor highly related to HAI-2. Immunohistochemistry showed a similar tissue distribution of prostasin and HAI-2 immunoreactivity with the most intense labeling near the brush borders of villus epithelial cells. In contrast, matriptase was detected primarily at the lateral plasma membrane, where HAI-1 was also detected. The tissue distribution profiles of immunoreactivity against these proteins, when paired with the species detected suggests that prostasin is under tight control by both HAI-1 and HAI-2 and matriptase by HAI-1 in human enterocytes. Furthermore, HAI-1 is a general inhibitor of prostasin in a variety of epithelial cells. In contrast, HAI-2 was not found to be a significant inhibitor for prostasin in mammary epithelial cells or keratinocytes. The high levels of constitutive prostasin zymogen activation and the selective prostasin inhibition by HAI-2 in enterocytes suggest that dysregulated prostasin proteolysis may be particularly important in the GI tract when HAI-2 function is lost and/or dysregulated.

The role of type II transmembrane serine protease-mediated signaling in cancer

Pericellular proteases have long been implicated in carcinogenesis. Previous research focused on these proteins, primarily as extracellular matrix (ECM) protein-degrading enzymes which allowed cancer cells to breach the basement membrane and invade surrounding tissue. However, recently, there has been a shift in the view of cell surface proteases, including serine proteases, as proteolytic modifiers of particular targets, including growth factors and protease-activated receptors, which are critical for the activation of oncogenic signaling pathways. Of the 176 human serine proteases currently identified, a subset of 17, known as type II transmembrane serine proteases (TTSPs). Many have been shown to be relevant to cancer progression since they were first identified as a family around the turn of the century. To this end, altered expression of TTSPs appeared as a trademark of several tumor types. However, the substrates and underlying signaling pathways remained unclear. Localization of these proteins to the cell surface places them in the unique position to mediate signal transduction between the cell and its surrounding environment. Many of the TTSPs have already been shown to play key roles in processes such as postnatal development, tissue homeostasis, and tumor progression, which share overlapping molecular mechanisms. In this review, we summarize the current knowledge regarding the role of the TTSP family in pro-oncogenic signaling.

Natural Endogenous Human Matriptase and Prostasin Undergo Zymogen Activation via Independent Mechanisms in an Uncoupled Manner

The membrane-associated serine proteases matriptase and prostasin are believed to function in close partnership. Their zymogen activation has been reported to be tightly coupled, either as a matriptase-initiated proteolytic cascade or through a mutually dependent mechanism involving the formation of a reciprocal zymogen activation complex. Here we show that this putative relationship may not apply in the context of human matriptase and prostasin. First, the tightly coupled proteolytic cascade between matriptase and prostasin might not occur when modest matriptase activation is induced by sphingosine 1-phospahte in human mammary epithelial cells. Second, prostasin is not required and/or involved in matriptase autoactivation because matriptase can undergo zymogen activation in cells that do not endogenously express prostasin. Third, matriptase is not required for and/or involved in prostasin activation, since activated prostasin can be detected in cells expressing no endogenous matriptase. Finally, matriptase and prostasin both undergo zymogen activation through an apparently un-coupled mechanism in cells endogenously expressing both proteases, such as in Caco-2 cells. In these human enterocytes, matriptase is detected primarily in the zymogen form and prostasin predominantly as the activated form, either in complexes with protease inhibitors or as the free active form. The negligible levels of prostasin zymogen with high levels of matriptase zymogen suggests that the reciprocal zymogen activation complex is likely not the mechanism for matriptase zymogen activation. Furthermore, high level prostasin activation still occurs in Caco-2 variants with reduced or absent matriptase expression, indicating that matriptase is not required and/or involved in prostasin zymogen activation. Collectively, these data suggest that any functional relationship between natural endogenous human matriptase and prostasin does not occur at the level of zymogen activation.

Matriptase and prostasin are expressed in human skin in an inverse trend over the course of differentiation and are targeted to different regions of the plasma membrane

Matriptase and prostasin, acting as a tightly coupled proteolytic cascade, were reported to be required for epidermal barrier formation in mouse skin. Here we show that, in human skin, matriptase and prostasin are expressed with an inverse pattern over the course of differentiation. Matriptase was detected primarily in epidermal basal keratinocytes and the basaloid cells in the outer root sheath of hair follicles and the sebaceous gland, where prostasin was not detected. In contrast, prostasin was detected primarily in differentiated cells in the epidermal granular layer, the inner root sheath of hair follicles, and the sebaceous gland, where matriptase expression is negligible. While co-expressed in the middle stage of differentiation, prostasin was detected as polarized patches, and matriptase at intercellular junctions. Targeting to different subcellular localizations is also observed in HaCaT human keratinocytes, in which matriptase was detected primarily at intercellular junctions, and prostasin primarily on membrane protrusion. Furthermore, upon induction of zymogen activation, free active prostasin remains cell-associated and free active matriptase is rapidly shed into the extracellular milieu. Our data suggest that matriptase and prostasin likely function as independent entities in human skin rather than as a tightly coupled proteolytic cascade as observed in mouse skin.

Matriptase-1 expression is lost in psoriatic skin lesions and is downregulated by TNFα in vitro

BACKGROUND AND OBJECTIVES

Matriptase-1 participates in terminal keratinocyte (KC) differentiation. Knockdown of matriptase-1 in skin equivalent cultures leads to impaired KC differentiation and retention of nuclei in the stratum corneum. Here, we investigated the expression and regulation of matriptase-1 in psoriatic skin and in KC in vitro.

PATIENTS AND METHODS

Matriptase-1 expression in healthy and psoriatic skin and its regulation in skin equivalents were analyzed by Western blotting, immunofluorescence staining, qRT-PCR, and activity assays. Involvement of the nuclear factor kappa B (NFκB) signaling pathway was investigated by adenoviral overexpression of a dominant-negative form of IKK2.

RESULTS

Matriptase-1 expression was detected in the stratum granulosum of healthy human skin and in skin equivalent cultures. Its expression and activity was strongly reduced in lesional skin of patients with psoriasis. Addition of TNFα to skin equivalent cultures resulted in complete loss of matriptase-1 expression accompanied by disturbed KC differentiation. Mechanistically, we were able to show that TNFα-induced downregulation of matriptase-1 was inhibited by blocking the IKK2/NFκB signaling pathway.

CONCLUSIONS

Given that matriptase-1 participates in terminal KC differentiation, its absence in psoriatic skin lesions indicates that this contributes to the barrier disturbances in this disease. Our data suggests that blocking the IKK2/NFκB-pathway represents a potential target for the treatment of psoriasis.

Expression of Serine Protease Matriptase in Renal Cell Carcinoma: Correlation of Tissue Microarray Immunohistochemical Expression Analysis Results with Clinicopathological Parameters

Serine protease matriptase (matriptase) cleaves and activates proteins implicated in the progression of cancer and represents a potential therapeutic target. Immunohistochemical analysis of matriptase was performed in tissue microarrays of 168 renal cell carcinomas (RCCs). All subtypes of RCC showed significant immunohistochemical expression of matriptase. In contrast, no expression occurred in areas of RCC with sarcomatous differentiation (SRCC) and in normal collecting tubules. The matriptase scores were significantly higher in papillary RCC (341) and clear cell RCC with granular cell differentiation (GRCC; 324) than in other histologic subtypes of RCC. In GRCC, matriptase scores were correlated with TNM staging and nuclear grading. Matriptase was overexpressed in all subtypes of RCC, and matriptase scores could distinguish between conventional clear cell RCC, GRCC, and SRCC.

The basal chorionic trophoblast cell layer: An emerging coordinator of placenta development

During gestation, fetomaternal exchange occurs in the villous tree (labyrinth) of the placenta. Development of this structure depends on tightly coordinated cellular processes of branching morphogenesis and differentiation of specialized trophoblast cells. The basal chorionic trophoblast (BCT) cell layer that localizes next to the chorioallantoic interface is of critical importance for labyrinth morphogenesis in rodents. Gcm1-positive cell clusters within this layer initiate branching morphogenesis thereby guiding allantoic fetal blood vessels towards maternal blood sinuses. Later these cells differentiate and contribute to the syncytiotrophoblast of the fetomaternal barrier. Additional cells within the BCT layer sustain continued morphogenesis, possibly through a repopulating progenitor population. Several mouse mutants highlight the importance of a structurally intact BCT epithelium, and a growing number of studies addresses its patterning and epithelial architecture. Here, we review and discuss emerging concepts in labyrinth development focussing on the biology of the BCT cell layer.

Targeting matriptase in breast cancer abrogates tumour progression via impairment of stromal-epithelial growth factor signalling

Matriptase is an epithelia-specific membrane-anchored serine protease that has received considerable attention in recent years due to its consistent dysregulation in human epithelial tumors, including breast cancer. Mice with reduced levels of matriptase display a significant delay in oncogene-induced mammary tumor formation and blunted tumor growth. The abated tumor growth is associated with a decrease in cancer cell proliferation. Here we demonstrate by genetic deletion and silencing that the proliferation impairment in matriptase deficient breast cancer cells is caused by their inability to initiate activation of the c-Met signaling pathway in response to fibroblast-secreted pro-HGF. Similarly, inhibition of matriptase catalytic activity using a selective small-molecule inhibitor abrogates the activation of c-Met, Gab1 and AKT, in response to pro-HGF, which functionally leads to attenuated proliferation in breast carcinoma cells. We conclude that matriptase is critically involved in breast cancer progression and represents a potential therapeutic target in breast cancer.

A pan-cancer analysis of alternative splicing events reveals novel tumor-associated splice variants of matriptase

High-throughput transcriptome sequencing allows identification of cancer-related changes that occur at the stages of transcription, pre-messenger RNA (mRNA), and splicing. In the current study, we devised a pipeline to predict novel alternative splicing (AS) variants from high-throughput transcriptome sequencing data and applied it to large sets of tumor transcriptomes from The Cancer Genome Atlas (TCGA). We identified two novel tumor-associated splice variants of matriptase, a known cancer-associated gene, in the transcriptome data from epithelial-derived tumors but not normal tissue. Most notably, these variants were found in 69% of lung squamous cell carcinoma (LUSC) samples studied. We confirmed the expression of matriptase AS transcripts using quantitative reverse transcription PCR (qRT-PCR) in an orthogonal panel of tumor tissues and cell lines. Furthermore, flow cytometric analysis confirmed surface expression of matriptase splice variants in chinese hamster ovary (CHO) cells transiently transfected with cDNA encoding the novel transcripts. Our findings further implicate matriptase in contributing to oncogenic processes and suggest potential novel therapeutic uses for matriptase splice variants.

Mechanisms of hepatocyte growth factor activation in cancer tissues

Hepatocyte growth factor/scatter factor (HGF/SF) plays critical roles in cancer progression through its specific receptor, MET. HGF/SF is usually synthesized and secreted as an inactive proform (pro-HGF/SF) by stromal cells, such as fibroblasts. Several serine proteases are reported to convert pro-HGF/SF to mature HGF/SF and among these, HGF activator (HGFA) and matriptase are the most potent activators. Increased activities of both proteases have been observed in various cancers. HGFA is synthesized mainly by the liver and secreted as an inactive pro-form. In cancer tissues, pro-HGFA is likely activated by thrombin and/or human kallikrein 1-related peptidase (KLK)-4 and KLK-5. Matriptase is a type II transmembrane serine protease that is expressed by most epithelial cells and is also synthesized as an inactive zymogen. Matriptase activation is likely to be mediated by autoactivation or by other trypsin-like proteases. Recent studies revealed that matriptase autoactivation is promoted by an acidic environment. Given the mildly acidic extracellular environment of solid tumors, matriptase activation may, thus, be accelerated in the tumor microenvironment. HGFA and matriptase activities are regulated by HGFA inhibitor (HAI)-1 (HAI-1) and/or HAI-2 in the pericellular microenvironment. HAIs may have an important role in cancer cell biology by regulating HGF/SF-activating proteases.

The protease inhibitor HAI-2, but not HAI-1, regulates matriptase activation and shedding through prostasin

The membrane-anchored serine proteases, matriptase and prostasin, and the membrane-anchored serine protease inhibitors, hepatocyte growth factor activator inhibitor (HAI)-1 and HAI-2, are critical effectors of epithelial development and postnatal epithelial homeostasis. Matriptase and prostasin form a reciprocal zymogen activation complex that results in the formation of active matriptase and prostasin that are targets for inhibition by HAI-1 and HAI-2. Conflicting data, however, have accumulated as to the existence of auxiliary functions for both HAI-1 and HAI-2 in regulating the intracellular trafficking and activation of matriptase. In this study, we, therefore, used genetically engineered mice to determine the effect of ablation of endogenous HAI-1 and endogenous HAI-2 on endogenous matriptase expression, subcellular localization, and activation in polarized intestinal epithelial cells. Whereas ablation of HAI-1 did not affect matriptase in epithelial cells of the small or large intestine, ablation of HAI-2 resulted in the loss of matriptase from both tissues. Gene silencing studies in intestinal Caco-2 cell monolayers revealed that this loss of cell-associated matriptase was mechanistically linked to accelerated activation and shedding of the protease caused by loss of prostasin regulation by HAI-2. Taken together, these data indicate that HAI-1 regulates the activity of activated matriptase, whereas HAI-2 has an essential role in regulating prostasin-dependent matriptase zymogen activation.

Imbalanced matriptase pericellular proteolysis contributes to the pathogenesis of malignant B-cell lymphomas

Membrane-associated serine protease matriptase is widely expressed by epithelial/carcinoma cells in which its proteolytic activity is tightly controlled by the Kunitz-type protease inhibitor, hepatocyte growth factor activator inhibitor (HAI-1). We demonstrate that, although matriptase is not expressed in lymphoid hyperplasia, roughly half of the non-Hodgkin B-cell lymphomas analyzed express significant amounts of matriptase. Furthermore, a significant proportion of these tumors express matriptase in the absence of HAI-1. Aggressive Burkitt lymphoma was more likely than indolent follicular lymphoma to express matriptase alone (86% versus 36%). In the absence of significant HAI-1 expression, the lymphoma cells activate and shed active matriptase when the cells are stimulated with mildly acidic buffer or the hypoxia-mimicking agent, CoCl2. The shed active matriptase can initiate pericellular proteolytic cascades by activating urokinase-type plasminogen activator on the cell surface of monocytes, and it can activate prohepatocyte growth factor. In addition, matriptase knockdown suppressed proliferation and colony-forming ability of neoplastic B cells in culture and growth as tumor xenografts in mice. Furthermore, exogenous expression of HAI-1 significantly suppressed proliferation of neoplastic B cells. These studies suggest that dysregulated pericellular proteolysis as a result of unregulated matriptase expression with limited HAI-1 may contribute to the pathological characteristics of several human B-cell lymphomas through modulation of the tumor microenvironment and enhanced tumor growth.

Matriptase autoactivation is tightly regulated by the cellular chemical environments

The ability of cells to rapidly detect and react to alterations in their chemical environment, such as pH, ionic strength and redox potential, is essential for cell function and survival. We present here evidence that cells can respond to such environmental alterations by rapid induction of matriptase autoactivation. Specifically, we show that matriptase autoactivation can occur spontaneously at physiological pH, and is significantly enhanced by acidic pH, both in a cell-free system and in living cells. The acid-accelerated autoactivation can be attenuated by chloride, a property that may be part of a safety mechanism to prevent unregulated matriptase autoactivation. Additionally, the thio-redox balance of the environment also modulates matriptase autoactivation. Using the cell-free system, we show that matriptase autoactivation is suppressed by cytosolic reductive factors, with this cytosolic suppression being reverted by the addition of oxidizing agents. In living cells, we observed rapid induction of matriptase autoactivation upon exposure to toxic metal ions known to induce oxidative stress, including CoCl₂ and CdCl₂. The metal-induced matriptase autoactivation is suppressed by N-acetylcysteine, supporting the putative role of altered cellular redox state in metal induced matriptase autoactivation. Furthermore, matriptase knockdown rendered cells more susceptible to CdCl₂-induced cell death compared to control cells. This observation implies that the metal-induced matriptase autoactivation confers cells with the ability to survive exposure to toxic metals and/or oxidative stress. Our results suggest that matriptase can act as a cellular sensor of the chemical environment of the cell that allows the cell to respond to and protect itself from changes in the chemical milieu.

Human cancer cells retain modest levels of enzymatically active matriptase only in extracellular milieu following induction of zymogen activation

The type 2 transmembrane serine protease matriptase is broadly expressed in human carcinomas and hematological cancers. The proteolytic activity of matriptase is a potential target of drugs and imaging probes. We assessed the fate of active matriptase following the induction of matriptase zymogen activation. Exposing eight human carcinoma cells to pH 6.0 buffer induced robust matriptase zymogen activation followed by rapid inhibition of the nascent active matriptase by hepatocyte growth factor activator inhibitor (HAI)-1. Consequently, no enzymatically active matriptase was detected in these cells. Some active matriptase is, however, rapidly shed to the extracellular milieu by these carcinoma cells. The lack of cell-associated active matriptase and the shedding of active matriptase were also observed in two hematological cancer lines. Matriptase shedding is correlated closely with the induction of matriptase activation, suggesting that matriptase activation and shedding are kinetically coupled. The coupling allows a proportion of active matriptase to survive HAI-1 inhibition by rapid shedding from cell surface. Our study suggests that cellular free, active matriptase is scarce and might not be an effective target for in vivo imaging and drug development.

Overexpression of matriptase correlates with poor prognosis in esophageal squamous cell carcinoma

Matriptase is one of the type II transmembrane serine proteases and is known to be involved in cancer progression. Increased matriptase expression has been reported in a variety of human cancers, and its association with poor prognosis has been highlighted in some cancer types. However, its exact role in cancer progression and its effect on patient survival in esophageal squamous cell carcinoma (ESCC) are still unclear. We performed immunohistochemical staining of matriptase in 171 ESCC samples after antibody validation and evaluated the association of its expression with clinicopathological parameters and prognosis. High matriptase expression was observed in 38.6 % (66/171) of ESCC samples and more frequently in N3 stage and in poorly differentiated tumors. Both overall survival (OS) and disease-free survival (DFS) were significantly lower for patients with high expression of matriptase than for patients with low expression (5-year OS rate, 38.6 vs 55.3 %; p=0.034 and 5-year DFS rate, 30.5 vs 49.4 %; p=0.007). High matriptase expression was an independent prognostic factor for OS [hazard ratio (HR), 1.65 (95 % confidence interval (CI), 1.01-2.68); p=0.045] and for DFS [HR, 1.79 (95 % CI, 1.14-2.81); p=0.012]. In conclusion, higher expression of matriptase is an independent prognostic factor involved in the progression of ESCC, which suggests that matriptase is a factor in ESCC tumor progression and also a potential molecular therapeutic target.

Matriptase expression and zymogen activation in human pilosebaceous unit

Studies of human genetic disorders and mouse models reveal the important roles of matriptase in hair growth. Here, we investigate matriptase expression and zymogen activation in hair follicles. We show: 1) layer-dependent distribution patterns, with much higher matriptase expression in cells of the outer root sheath and matrix cells of the hair bulb than in cells of the inner root sheath; 2) cycle-dependent expression patterns, with matriptase expressed in the anagen and catagen phases of the hair lifecycle, but not in the telogen phase; 3) reduced expression of the matriptase inhibitor, HAI-1, in the catagen phase, suggesting increased proteolytic activity in this phase; and 4) definitive matriptase zymogen activation patterns, with the highest matriptase activation observed in matrix cells and outer root sheath cells in the isthmus/bulge region. In sebaceous glands, matriptase is highly expressed in basal and ductal cells, with much lower expression in the differentiated, lipid-filled cells of the interior. We also show that matriptase potently activates hepatocyte growth factor (HGF) in vitro, and that the HGF receptor, c-Met, is co-expressed in those cells that express activated matriptase. Our observations suggest that the matriptase-HGF-c-MET pathway has the potential to be engaged, primarily in proliferative cells rather than terminally differentiated epithelial cells of the human pilosebaceous unit.

A matriptase-prostasin reciprocal zymogen activation complex with unique features: prostasin as a non-enzymatic co-factor for matriptase activation

Matriptase and prostasin are part of a cell surface proteolytic pathway critical for epithelial development and homeostasis. Here we have used a reconstituted cell-based system and transgenic mice to investigate the mechanistic interrelationship between the two proteases. We show that matriptase and prostasin form a reciprocal zymogen activation complex with unique features. Prostasin serves as a critical co-factor for matriptase activation. Unexpectedly, however, prostasin-induced matriptase activation requires neither prostasin zymogen conversion nor prostasin catalytic activity. Prostasin zymogen conversion to active prostasin is dependent on matriptase but does not require matriptase zymogen conversion. Consistent with these findings, wild type prostasin, activation cleavage site-mutated prostasin, and catalytically inactive prostasin all were biologically active in vivo when overexpressed in the epidermis of transgenic mice, giving rise to a severe skin phenotype. Our finding of non-enzymatic stimulation of matriptase activation by prostasin and activation of prostasin by the matriptase zymogen provides a tentative mechanistic explanation for several hitherto unaccounted for genetic and biochemical observations regarding these two membrane-anchored serine proteases and their downstream targets.

Antithrombin regulates matriptase activity involved in plasmin generation, syndecan shedding, and HGF activation in keratinocytes

Matriptase, a membrane-associated serine protease, plays an essential role in epidermal barrier function through activation of the glycosylphosphatidylinositol (GPI)-anchored serine protease prostasin. The matriptase-prostasin proteolytic cascade is tightly regulated by hepatocyte growth factor activator inhibitor (HAI)-1 such that matriptase autoactivation and prostasin activation occur simultaneously and are followed immediately by the inhibition of both enzymes by HAI-1. However, the mechanisms whereby matriptase acts on extracellular substrates remain elusive. Here we report that some active matriptase can escape HAI-1 inhibition by being rapidly shed from the cell surface. In the pericellular environment, shed active matriptase is able to activate hepatocyte growth factor (HGF), accelerate plasminogen activation, and shed syndecan 1. The amount of active matriptase shed is inversely correlated with the amount of antithrombin (AT) bound to the surface of the keratinocytes. Binding of AT to the surface of keratinocytes is dependent on a functional heparin binding site, Lys-125, and that the N-glycosylation site Asn-135 be unglycosylated. This suggests that β-AT, and not α-AT, is responsible for regulation of pericellular matriptase activity in keratinocytes. Keratinocytes appear to rely on AT to regulate the level of pericellular active matriptase much more than breast and prostate epithelial cells in which AT regulation of matriptase activity occurs at much lower levels than keratinocytes. These results suggest that keratinocytes employ two distinct serine protease inhibitors to control the activation and processing of two different sets of matriptase substrates leading to different biological events: 1) HAI-1 for prostasin activation/inhibition, and 2) AT for the pericellular proteolysis involved in HGF activation, accelerating plasminogen activation, and shedding of syndecans.

Hepatocyte growth factor activator inhibitor-2 prevents shedding of matriptase

Hepatocyte growth factor activator inhibitor-2 (HAI-2) is an inhibitor of many proteases in vitro, including the membrane-bound serine protease, matriptase. Studies of knock-out mice have shown that HAI-2 is essential for placental development only in mice expressing matriptase, suggesting that HAI-2 is important for regulation of matriptase. Previous studies have shown that recombinant expression of matriptase was unsuccessful unless co-expressed with another HAI, HAI-1. In the present study we show that when human matriptase is recombinantly expressed alone in the canine cell line MDCK, then human matriptase mRNA can be detected and the human matriptase ectodomain is shed to the media, suggesting that matriptase expressed alone is rapidly transported through the secretory pathway and shed. Whereas matriptase expressed together with HAI-1 or HAI-2 accumulates on the plasma membrane where it is activated, as judged by cleavage at Arg614 and increased peptidolytic activity of the cell extracts. Mutagenesis of Kunitz domain 1 but not Kunitz domain 2 abolished this function of HAI-2. HAI-2 seems to carry out its function intracellularly as this is where the vast majority of HAI-2 is located and since HAI-2 could not be detected on the basolateral plasma membrane where matriptase resides. However, minor amounts of HAI-2 not undergoing endocytosis could be detected on the apical plasma membrane. Our results suggest that Kunitz domain 1 of HAI-2 cause matriptase to accumulate in a membrane-bound form on the basolateral plasma membrane.

Mechanisms for the control of matriptase activity in the absence of sufficient HAI-1

Matriptase proteolytic activity must be tightly controlled for normal placental development, epidermal function, and epithelial integrity. Although hepatocyte growth factor activator inhibitor-1 (HAI-1) represents the predominant endogenous inhibitor for matriptase and the protein molar ratio of HAI-1 to matriptase is determined to be >10 in epithelial cells and the majority of carcinoma cells, an inverse HAI-1-to-matriptase ratio is seen in some ovarian and hematopoietic cancer cells. In the current study, cells with insufficient HAI-1 are investigated for the mechanisms through which the activity of matriptase is regulated. When matriptase activation is robustly induced in these cells, activated matriptase rapidly forms two complexes of 100- and 140-kDa in addition to the canonical 120-kDa matriptase-HAI-1 complex already described. Both 100- and 140-kDa complexes contain two-chain, cleaved matriptase but are devoid of gelatinolytic activity. Further biochemical characterization shows that the 140-kDa complex is a matriptase homodimer and that the 100-kDa complexes appear to contain reversible, tight binding serine protease inhibitor(s). The formation of the 140-kDa matriptase dimer is strongly associated with matriptase activation, and its levels are inversely correlated with the ratio of HAI-1 to matriptase. Given these observations and the likelihood that autoactivation requires the interaction of two matriptase molecules, it seems plausible that this activated matriptase homodimer may represent a matriptase autoactivation intermediate and that its accumulation may serve as a mechanism to control matriptase activity when protease inhibitor levels are limiting. These data suggest that matriptase activity can be rapidly inhibited by HAI-1 and other HAI-1-like protease inhibitors and "locked" in an inactive autoactivation intermediate, all of which places matriptase under very tight control.

Matriptase is inhibited by extravascular antithrombin in epithelial cells but not in most carcinoma cells

Antithrombin, a major anticoagulant, is robustly transported into extravascular compartments where its target proteases are largely unknown. This serpin was previously detected in human milk as complexes with matriptase, a membrane-bound serine protease broadly expressed in epithelial and carcinoma cells, and under tight regulation by hepatocyte growth factor activator inhibitor (HAI)-1, a transmembrane Kunitz-type serine protease inhibitor that forms heat-sensitive complexes with active matriptase. In the current study, we detect, in addition to matriptase-HAI-1 complexes, heat-resistant matriptase complexes generated by nontransformed mammary, prostate, and epidermal epithelial cells that we show to be matriptase-antithrombin complexes. These findings suggest that in addition to HAI-1, interstitial antithrombin participates in the regulation of matriptase activity in epithelial cells. This physiological mechanism appears, however, to largely be lost in cancer cells since matriptase-antithrombin complexes were not detected in all but two of a panel of seven breast, prostate, and ovarian cancer cell lines. Using purified active matriptase, we further characterize the formation of matriptase-antithrombin complex and show that heparin can significantly potentiate the inhibitory potency of antithrombin against matriptase. Second-order rate constants for the inhibition were determined to be 3.9 × 10(3) M(-1)s(-1) in the absence of heparin and 1.2 × 10(5) M(-1)s(-1) in the presence of heparin, a 30-fold increase, consistent with the established role of heparin in activating antithrombin function. Taken together these data suggest that normal epithelial cells employ a dual mechanism involving HAI-1 and antithrombin to control matriptase and that the antithrombin-based mechanism appears lost in the majority of carcinoma cells.

Membrane-anchored serine proteases in health and disease

Serine proteases of the trypsin-like family have long been recognized to be critical effectors of biological processes as diverse as digestion, blood coagulation, fibrinolysis, and immunity. In recent years, a subgroup of these enzymes has been identified that are anchored directly to plasma membranes, either by a carboxy-terminal transmembrane domain (Type I), an amino-terminal transmembrane domain with a cytoplasmic extension (Type II or TTSP), or through a glycosylphosphatidylinositol (GPI) linkage. Recent biochemical, cellular, and in vivo analyses have now established that membrane-anchored serine proteases are key pericellular contributors to processes vital for development and the maintenance of homeostasis. This chapter reviews our current knowledge of the biological and physiological functions of these proteases, their molecular substrates, and their contributions to disease.

Transport via the transcytotic pathway makes prostasin available as a substrate for matriptase

The matriptase-prostasin proteolytic cascade is essential for epidermal tight junction formation and terminal epidermal differentiation. This proteolytic pathway may also be operative in a variety of other epithelia, as both matriptase and prostasin are involved in tight junction formation in epithelial monolayers. However, in polarized epithelial cells matriptase is mainly located on the basolateral plasma membrane whereas prostasin is mainly located on the apical plasma membrane. To determine how matriptase and prostasin interact, we mapped the subcellular itinerary of matriptase and prostasin in polarized colonic epithelial cells. We show that zymogen matriptase is activated on the basolateral plasma membrane where it is able to cleave relevant substrates. After activation, matriptase forms a complex with the cognate matriptase inhibitor, hepatocyte growth factor activator inhibitor (HAI)-1 and is efficiently endocytosed. The majority of prostasin is located on the apical plasma membrane albeit a minor fraction of prostasin is present on the basolateral plasma membrane. Basolateral prostasin is endocytosed and transcytosed to the apical plasma membrane where a long retention time causes an accumulation of prostasin. Furthermore, we show that prostasin on the basolateral membrane is activated before it is transcytosed. This study shows that matriptase and prostasin co-localize for a brief period of time at the basolateral plasma membrane after which prostasin is transported to the apical membrane as an active protease. This study suggests a possible explanation for how matriptase or other basolateral serine proteases activate prostasin on its way to its apical destination.

A novel signaling axis of matriptase/PDGF-D/ß-PDGFR in human prostate cancer

Increasing evidence indicates the significance of platelet-derived growth factor receptor-β (β-PDGFR) signaling in prostate cancer (PCa). Accordingly, preclinical studies suggest the potential of β-PDGFR as a therapeutic target in metastatic PCa. However, a ligand responsible for β-PDGFR activation in PCa was unknown, and recent clinical trials with imatinib mesylate showed limited success due to normal tissue toxicity. Similarly, in spite of mounting evidence indicating the significance of matriptase in PCa, little is known about its substrates or molecular actions during PCa progression. Here, we identified PDGF-D as a ligand for β-PDGFR in PCa and discovered matriptase as its regulator. Matriptase activates PDGF-D by proteolytic removal of the CUB domain in a 2-step process, creating a hemidimer, followed by growth factor domain dimer (GFD-D) generation. Matriptase can deactivate PDGF-D by further proteolytic cleavage within the GFD, revealing its biphasic regulation. Importantly, PDGF-D/matriptase colocalization is accompanied with β-PDGFR phosphorylation in human PCa tissues. This study unveiled a novel signaling axis of matriptase/PDGF-D/β-PDGFR in PCa, providing new insights into functional interplay between serine protease and growth factor signaling networks.

Matriptase is involved in ErbB-2-induced prostate cancer cell invasion

Deregulation of both ErbB-2 signaling and matriptase activity has been associated with human prostate cancer (PCa) progression. In this communication, we investigated the roles of both ErbB-2 signaling in matriptase zymogen activation and matriptase in ErbB-2-induced PCa malignancy. In a human PCa cell progression model, we observed that advanced PCa C-81 LNCaP cells exhibited an aggressive phenotype with increased cell migration and invasion capacity; these cells concurrently showed both enhanced ErbB-2 phosphorylation and increased matriptase zymogen activation compared with parental C-33 LNCaP cells. Moreover, ErbB2 activation, both ligand-dependent (eg, epidermal growth factor treatment) and ligand-independent (eg, overexpression), was able to induce matriptase zymogen activation in this cell line. Inhibition of ErbB-2 activity by either the specific inhibitor, AG825, in epidermal growth factor-treated C-33 LNCaP cells or ErbB-2 knockdown in C-81 LNCaP cells, reduced matriptase activation. These observations were confirmed by similar studies using both DU145 and PC3 cells. Together, these data suggest that ErbB-2 signaling plays an important role in matriptase zymogen activation. ErbB-2-enhanced matriptase activation was suppressed by a phosphatidylinositol 3-kinase inhibitor (ie, LY294002) but not by a MEK inhibitor (ie, PD98059). Suppression of matriptase expression by small hairpin RNA knockdown in ErbB-2-overexpressing LNCaP cells dramatically suppressed cancer cell invasion. In summary, our data indicate that ErbB-2 signaling via the phosphatidylinositol 3-kinase pathway results in up-regulated matriptase zymogen activity, which contributes to PCa cell invasion.

ST14 gene variant and decreased matriptase protein expression predict poor breast cancer survival

BACKGROUND

Matriptase plays a role in carcinogenesis, but the role of its genetic variation or that of the hepatocyte growth factor activator inhibitor-1 (HAI-1) has not been evaluated. This study aimed to examine the genetic variation of matriptase (ST14 gene) and HAI-1 (SPINT1 gene) in breast cancer risk and prognosis, to assess matriptase and HAI-1 gene and protein expression in breast tumors, and to identify their clinicopathologic correlations and prognostic significance.

METHODS

Five single nucleotide polymorphisms in ST14 and three in SPINT1 were genotyped in 470 invasive breast cancer cases and 446 healthy controls. Gene expression analysis was done for 40 breast cancer samples. Protein expression was assessed by immunohistochemical analyses in 377 invasive breast tumors. The statistical significance of the associations among genotypes, clinicopathologic variables, and prognosis was assessed.

RESULTS

The ST14 single nucleotide polymorphism rs704624 independently predicted breast cancer survival, a poor outcome associated with the minor allele (P = 0.001; risk ratio, 2.221; 95% confidence interval, 1.382-3.568). Moreover, ST14 gene expression levels were lower among the minor allele carriers (P = 0.009), and negative/low matriptase protein expression was independently predictive of poorer survival (P = 0.046; risk ratio, 1.554; 95% confidence interval, 1.008-2.396).

CONCLUSIONS

The ST14 variant rs704624 and protein expression of matriptase have prognostic significance in breast cancer. This study adds to the evidence for the role of matriptase in breast cancer and has found new evidence for the genotypes having an impact in breast cancer.

IMPACT

This is the first study showing that genetic variation in matriptase has clinical importance. The results encourage further study on the genetic variation affecting protein levels and function in type II transmembrane serine proteases.

Regulation of the matriptase-prostasin cell surface proteolytic cascade by hepatocyte growth factor activator inhibitor-1 during epidermal differentiation

Matriptase, a membrane-tethered serine protease, plays essential roles in epidermal differentiation and barrier function, largely mediated via its activation of prostasin, a glycosylphosphatidylinositol-anchored serine protease. Matriptase activity is tightly regulated by its inhibitor hepatocyte growth factor activator inhibitor-1 (HAI-1) such that free active matriptase is only briefly available to act on its substrates. In the current study we provide evidence for how matriptase activates prostasin under this tight control by HAI-1. When primary human keratinocytes are induced to differentiate in a skin organotypic culture model, both matriptase and prostasin are constitutively activated and then inhibited by HAI-1. These processes also occur in HaCaT human keratinocytes when matriptase activation is induced by exposure of the cells to a pH 6.0 buffer. Using this acid-inducible activation system we demonstrate that prostatin activation is suppressed by matriptase knockdown and by blocking matriptase activation with sodium chloride, suggesting that prostatin activation is dependent on matriptase in this system. Kinetics studies further reveal that the timing of autoactivation of matriptase, prostasin activation, and inhibition of both enzymes by HAI-1 binding are closely correlated. These data suggest that, during epidermal differentiation, the matriptase-prostasin proteolytic cascade is tightly regulated by two mechanisms: 1) prostasin activation temporally coupled to matriptase autoactivation and 2) HAI-1 rapidly inhibiting not only active matriptase but also active prostasin, resulting in an extremely brief window of opportunity for both active matriptase and active prostasin to act on their substrates.

TMPRSS2, a serine protease expressed in the prostate on the apical surface of luminal epithelial cells and released into semen in prostasomes, is misregulated in prostate cancer cells

TMPRSS2, a type II transmembrane serine protease, is highly expressed by the epithelium of the human prostate gland. To explore the regulation and function of TMPRSS2 in the prostate, a panel of monoclonal antibodies with high sensitivity and specificity were generated. Immunodetection showed TMPRSS2 on the apical plasma membrane of the prostate luminal cells and demonstrated its release into semen as a component of prostasomes, organelle-like vesicles that may facilitate sperm function and enhance male reproduction. In prostate cancer cells, TMPRSS2 expression was increased and the protein mislocalized over the entire tumor cell membrane. In both LNCaP prostate cancer cells and human semen, TMPRSS2 protein was detected predominantly as inactive zymogen forms as part of an array of multiple noncovalent and disulfide-linked complexes, suggesting that TMPRSS2 activity may be regulated by unconventional mechanisms. Our data suggested that TMPRSS2, an apical surface serine protease, may have a normal role in male reproduction as a component of prostasomes. The aberrant cellular localization, and increased expression of the protease seen in cancer, may contribute to prostate tumorigenesis by providing access of the enzyme to nonphysiological substrates and binding-proteins.

Tumor detection by imaging proteolytic activity

The cell surface protease membrane-type serine protease-1 (MT-SP1), also known as matriptase, is often upregulated in epithelial cancers. We hypothesized that dysregulation of MT-SP1 with regard to its cognate inhibitor hepatocyte growth factor activator inhibitor-1 (HAI-1), a situation that increases proteolytic activity, might be exploited for imaging purposes to differentiate malignant from normal tissue. In this study, we show that MT-SP1 is active on cancer cells and that its activity may be targeted in vivo for tumor detection. A proteolytic activity assay with several MT-SP1-positive human cancer cell lines showed that MT-SP1 antibodies that inhibit recombinant enzyme activity in vitro also bind and inhibit the full-length enzyme expressed on cells. In contrast, in the same assay, MT-SP1-negative cancer cell lines were inactive. Fluorescence microscopy confirmed the cell surface localization of labeled antibodies bound to MT-SP1-positive cells. To evaluate in vivo targeting capability, 0.7 to 2 nmoles of fluorescently labeled antibodies were administered to mice bearing tumors that were positive or negative for MT-SP1. Antibodies localized to MT-SP1-positive tumors (n = 3), permitting visualization of MT-SP1 activity, whereas MT-SP1-negative tumors (n = 2) were not visualized. Our findings define MT-SP1 activity as a useful biomarker to visualize epithelial cancers using a noninvasive antibody-based method.

Matriptase activation, an early cellular response to acidosis

Extracellular acidosis often rapidly causes intracellular acidification, alters ion channel activities, and activates G protein-coupled receptors. In this report, we demonstrated a novel cellular response to acidosis: induction of the zymogen activation of matriptase. Acid-induced matriptase activation is ubiquitous among epithelial and carcinoma cells and is characterized by rapid onset, fast kinetics, and the magnitude of activation seen. Trace amounts of activated matriptase can be detected 1 min after cells are exposed to pH 6.0 buffer, and the vast majority of latent matriptase within the cells is converted to activated matriptase within 20 min. Matriptase activation may be a direct response to proton exposure because acid-induced matriptase activation also occurs in an in vitro, cell-free setting in which intracellular signaling molecules and ion channel activities are largely absent. Acid-induced matriptase activation takes place both on the cell surface and inside the cells, likely due to the parallel intracellular acidification that activates intracellular matriptase. Following matriptase activation, the active enzyme is immediately inhibited by binding to hepatocyte growth factor activator inhibitor 1, resulting in stable matriptase-hepatocyte growth factor activator inhibitor 1 complexes that are rapidly secreted. As an early response to acidosis, matriptase activation can also be induced by perturbation of intracellular pH homeostasis by 5-(N-methyl-N-isobutyl)-amiloride and 5-(N-ethyl-N-isopropyl)-amiloride, both of which inhibit Na(+)/H(+) exchangers, and diisothiocyanostilbene-2,2'-disulfonic acid, which can inhibit other acid-base ion channels. This study uncovers a novel mechanism regulating proteolysis in epithelial and carcinoma cells, and also demonstrates that a likely function of matriptase is as an early response to acidosis.

Polarized epithelial cells secrete matriptase as a consequence of zymogen activation and HAI-1-mediated inhibition

Matriptase, a transmembrane serine protease, is broadly expressed by, and crucial for the integrity of, the epithelium. Matriptase is synthesized as a zymogen and undergoes autoactivation to become an active protease that is immediately inhibited by, and forms complexes with, hepatocyte growth factor activator inhibitor (HAI-1). To investigate where matriptase is activated and how it is secreted in vivo, we determined the expression and activation status of matriptase in seminal fluid and urine and the distribution and subcellular localization of the protease in the prostate and kidney. The in vivo studies revealed that while the latent matriptase is localized at the basolateral surface of the ductal epithelial cells of both organs, only matriptase-HAI-1 complexes and not latent matriptase are detected in the body fluids, suggesting that activation, inhibition, and transcytosis of matriptase would have to occur for the secretion of matriptase. These complicated processes involved in the in vivo secretion were also observed in polarized Caco-2 intestinal epithelial cells. The cells target latent matriptase to the basolateral plasma membrane where activation, inhibition, and secretion of matriptase appear to take place. However, a proportion of matriptase-HAI-1 complexes, but not the latent matriptase, appears to undergo transcytosis to the apical plasma membrane for secretion. When epithelial cells lose their polarity, they secrete both latent and activated matriptase. Although most epithelial cells retain very low levels of matriptase-HAI-1 complex by rapidly secreting the complex, gastric chief cells may activate matriptase and store matriptase-HAI-1 complexes in the pepsinogen-secretory granules, suggesting an intracellular activation and regulated secretion in these cells. Taken together, while zymogen activation and closely coupled HAI-1-mediated inhibition are common features for matriptase regulation, the cellular location of matriptase activation and inhibition, and the secretory route for matriptase-HAI-1 complex may vary along with the functional divergence of different epithelial cells.

Protein expression of matriptase and its cognate inhibitor HAI-1 in human prostate cancer: a tissue microarray and automated quantitative analysis

Recent studies have suggested that matriptase, a transmembrane serine protease and its cognate inhibitor hepatocyte growth factor activator inhibitor-1 (HAI-1) are important in the progression of many cancers. Limited quantitative data are available on these proteins in prostate cancer. To validate the roles of matriptase and HAI-1 in prostate cancer and its progression, a prostate cancer tissue microarray was constructed. The tissue microarray includes 41 localized prostate cancers (Pca_local), 18 aggressive prostate cancers, 18 metastatic prostate cancers, 24 benign prostate hyperplasias, 18 high-grade intraepithelial neoplasias (HGPIN), and 41 benign prostate tissues. The cellular expression levels of matriptase and HAI-1 were quantified using automated quantitative analysis. We found that matriptase expression levels were significantly higher in Pca_local (P<0.0001) and HGPIN (P<0.05) compared with benign prostate tissue. Matriptase levels were significantly decreased in metastatic cancer when compared with all other tissue types (P<0.05). Compared with benign prostate tissue, HAI-1 expression levels were significantly higher in all proliferative prostate diseases (benign prostate hyperplasia, HGPIN, localized and aggressive cancers, and metastases) (P<0.001); yet, no significant differences were found in HAI-1 expression levels among the diseased tissue types. These results suggest that an increase of matriptase may be useful as a marker for detection of Pca_local, whereas a decrease of matriptase expression may signal prostate cancer progression. HAI-1 seems to be a marker of prostate epithelial cell proliferation.

Purification from human milk of matriptase complexes with secreted serpins: mechanism for inhibition of matriptase other than HAI-1

Matriptase, a type 2 transmembrane serine protease, is predominately expressed by epithelial and carcinoma cells in which hepatocyte growth factor activator inhibitor 1 (HAI-1), a membrane-bound, Kunitz-type serine protease inhibitor, is also expressed. HAI-1 plays dual roles in the regulation of matriptase, as a conventional protease inhibitor and as a factor required for zymogen activation of matriptase. As a consequence, activation of matriptase is immediately followed by HAI-1-mediated inhibition, with the activated matriptase being sequestered into HAI-1 complexes. Matriptase is also expressed by peripheral blood leukocytes, such as monocytes and macrophages; however, in contrast to epithelial cells, monocytes and macrophages were reported not to express HAI-1, suggesting that these leukocytes possess alternate, HAI-1-independent mechanisms regulating the zymogen activation and protease inhibition of matriptase. In the present study, we characterized matriptase complexes of 110 kDa in human milk, which contained no HAI-1 and resisted dissociation in boiling SDS in the absence of reducing agents. These complexes were further purified and dissociated into 80-kDa and 45-kDa fragments by treatment with reducing agents. Proteomic and immunological methods identified the 45-kDa fragment as the noncatalytic domains of matriptase and the 80-kDa fragment as the matriptase serine protease domain covalently linked to one of three different secreted serpin inhibitors: antithrombin III, alpha1-antitrypsin, and alpha2-antiplasmin. Identification of matriptase-serpin inhibitor complexes provides evidence for the first time that the proteolytic activity of matriptase, from those cells that express no or low levels of HAI-1, may be controlled by secreted serpins.

Autoactivation of matriptase in vitro: requirement for biomembrane and LDL receptor domain

In live cells, autoactivation of matriptase, a membrane-bound serine protease, can be induced by lysophospholipids, androgens, and the polyanionic compound suramin. These structurally distinct chemicals induce different signaling pathways and cellular events that somehow, in a cell type-specific manner, lead to activation of matriptase immediately followed by inhibition of matriptase by hepatocyte growth factor activator inhibitor 1 (HAI-1). In the current study, we established an analogous matriptase autoactivation system in an in vitro cell-free setting and showed that a burst of matriptase activation and HAI-1-mediated inhibition spontaneously occurred in the insoluble fractions of cell homogenates and that this in vitro activation could be attenuated by a soluble suppressive factor(s) in cytosolic fractions. Immunofluorescence staining and subcellular fractionation studies revealed that matriptase activation occurred in the perinuclear regions. Solubilization of matriptase from cell homogenates by Triton X-100 or sonication of cell homogenates completely inhibited the effect, suggesting that matriptase activation requires proper lipid bilayer microenvironments, potentially allowing appropriate interactions of matriptase zymogens with HAI-1 and other components. Matriptase activation occurred in a narrow pH range (from pH 5.2 to 7.2), with a sharp increase in activation at the transition from pH 5.2 to 5.4, and could be completely suppressed by moderately increased ionic strength. Protease inhibitors only modestly affected activation, whereas 30 nM (5 microg/ml) of anti-matriptase LDL receptor domain 3 monoclonal antibodies completely blocked activation. These atypical biochemical features are consistent with a mechanism for autoactivation of matriptase that requires protein-protein interactions but not active proteases.