Pro-urokinase-type plasminogen activator is a substrate for hepsin

Extracellular proteolysis in cancer: Proteases, substrates, and mechanisms in tumor progression and metastasis

A vast ensemble of extracellular proteins influences the development and progression of cancer, shaped and reshaped by a complex network of extracellular proteases. These proteases, belonging to the distinct classes of metalloproteases, serine proteases, cysteine proteases, and aspartic proteases, play a critical role in cancer. They often become dysregulated in cancer, with increases in pathological protease activity frequently driven by the loss of normal latency controls, diminished regulation by endogenous protease inhibitors, and changes in localization. Dysregulated proteases accelerate tumor progression and metastasis by degrading protein barriers within the extracellular matrix (ECM), stimulating tumor growth, reactivating dormant tumor cells, facilitating tumor cell escape from immune surveillance, and shifting stromal cells toward cancer-promoting behaviors through the precise proteolysis of specific substrates to alter their functions. These crucial substrates include ECM proteins and proteoglycans, soluble proteins secreted by tumor and stromal cells, and extracellular domains of cell surface proteins, including membrane receptors and adhesion proteins. The complexity of the extracellular protease web presents a significant challenge to untangle. Nevertheless, technological strides in proteomics, chemical biology, and the development of new probes and reagents are enabling progress and advancing our understanding of the pivotal importance of extracellular proteolysis in cancer.

Intersection of Coagulation and Fibrinolysis by the Glycosylphosphatidylinositol (GPI)-Anchored Serine Protease Testisin

Hemostasis is a delicate balance between coagulation and fibrinolysis that regulates the formation and removal of fibrin, respectively. Positive and negative feedback loops and crosstalk between coagulation and fibrinolytic serine proteases maintain the hemostatic balance to prevent both excessive bleeding and thrombosis. Here, we identify a novel role for the glycosylphosphatidylinositol (GPI)-anchored serine protease testisin in the regulation of pericellular hemostasis. Using in vitro cell-based fibrin generation assays, we found that the expression of catalytically active testisin on the cell surface accelerates thrombin-dependent fibrin polymerization, and intriguingly, that it subsequently promotes accelerated fibrinolysis. We find that the testisin-dependent fibrin formation is inhibited by rivaroxaban, a specific inhibitor of the central prothrombin-activating serine protease factor Xa (FXa), demonstrating that cell-surface testisin acts upstream of factor X (FX) to promote fibrin formation at the cell surface. Unexpectedly, testisin was also found to accelerate fibrinolysis by stimulating the plasmin-dependent degradation of fibrin and enhancing plasmin-dependent cell invasion through polymerized fibrin. Testisin was not a direct activator of plasminogen, but it is able to induce zymogen cleavage and the activation of pro-urokinase plasminogen activator (pro-uPA), which converts plasminogen to plasmin. These data identify a new proteolytic component that can regulate pericellular hemostatic cascades at the cell surface, which has implications for angiogenesis, cancer biology, and male fertility.

Clinical Significance of Hepsin and Underlying Signaling Pathways in Prostate Cancer

The hepsin gene encodes a type II transmembrane serine protease. Previous studies have shown the overexpression of hepsin in prostate cancer, and the dysregulation of hepsin promotes cancer cell proliferation, migration, and metastasis in vitro and in vivo. The review incorporated with our work showed that hepsin expression levels were specifically increased in prostate cancer, and higher expression in metastatic tumors than in primary tumors was also observed. Moreover, increased expression was associated with poor outcomes for patients with prostate cancer. Using in silico protein–protein interaction prediction, mechanistic analysis showed that hepsin interacted with eight other oncogenic proteins, whose expression was significantly correlated with hepsin expression in prostate cancer. The oncogenic functions of hepsin are mainly linked to proteolytic activities that disrupt epithelial integrity and regulatorily interact with other genes to influence cell-proliferation, EMT/metastasis, inflammatory, and tyrosine-kinase-signaling pathways. Moreover, genomic amplifications of hepsin, not deletions or other alterations, were significantly associated with prostate cancer metastasis. Targeting hepsin using a specific inhibitor or antibodies significantly attenuates its oncogenic behaviors. Therefore, hepsin could be a novel biomarker and therapeutic target for prostate cancer.

Hepsin regulates TGFβ signaling via fibronectin proteolysis

Transforming growth factor‐beta (TGFβ) is a multifunctional cytokine with a well‐established role in mammary gland development and both oncogenic and tumor‐suppressive functions. The extracellular matrix (ECM) indirectly regulates TGFβ activity by acting as a storage compartment of latent‐TGFβ, but how TGFβ is released from the ECM via proteolytic mechanisms remains largely unknown. In this study, we demonstrate that hepsin, a type II transmembrane protease overexpressed in 70% of breast tumors, promotes canonical TGFβ signaling through the release of latent‐TGFβ from the ECM storage compartment. Mammary glands in hepsin CRISPR knockout mice showed reduced TGFβ signaling and increased epithelial branching, accompanied by increased levels of fibronectin and latent‐TGFβ1, while overexpression of hepsin in mammary tumors increased TGFβ signaling. Cell‐free and cell‐based experiments showed that hepsin is capable of direct proteolytic cleavage of fibronectin but not latent‐TGFβ and, importantly, that the ability of hepsin to activate TGFβ signaling is dependent on fibronectin. Altogether, this study demonstrates a role for hepsin as a regulator of the TGFβ pathway in the mammary gland via a novel mechanism involving proteolytic downmodulation of fibronectin.

Hepsin: a multifunctional transmembrane serine protease in pathobiology

Cell membrane-bound serine proteases are important in the maintenance of physiological homeostasis. Hepsin is a type II transmembrane serine protease highly expressed in the liver. Recent studies indicate that hepsin activates prohepatocyte growth factor in the liver to enhance Met signaling, thereby regulating glucose, lipid, and protein metabolism. In addition, hepsin functions in nonhepatic tissues, including the adipose tissue, kidney, and inner ear, to regulate adipocyte differentiation, urinary protein processing, and auditory function, respectively. In mouse models, hepsin deficiency lowers blood glucose, lipid, and protein levels, impairs uromodulin assembly in renal epithelial cells, and causes hearing loss. Elevated hepsin expression has also been found in many cancers. As a type II transmembrane protease, cell surface expression and zymogen activation are essential for hepsin activity. In this review, we discuss the current knowledge regarding hepsin biosynthesis, activation, and functions in pathobiology.

Priming of SARS-CoV-2 S protein by several membrane-bound serine proteinases could explain enhanced viral infectivity and systemic COVID-19 infection

The ongoing COVID-19 pandemic has already caused over a million deaths worldwide, and this death toll will be much higher before effective treatments and vaccines are available. The causative agent of the disease, the coronavirus SARS-CoV-2, shows important similarities with the previously emerged SARS-CoV-1, but also striking differences. First, SARS-CoV-2 possesses a significantly higher transmission rate and infectivity than SARS-CoV-1 and has infected in a few months over 60 million people. Moreover, COVID-19 has a systemic character, as in addition to the lungs, it also affects the heart, liver, and kidneys among other organs of the patients and causes frequent thrombotic and neurological complications. In fact, the term "viral sepsis" has been recently coined to describe the clinical observations. Here I review current structure-function information on the viral spike proteins and the membrane fusion process to provide plausible explanations for these observations. I hypothesize that several membrane-associated serine proteinases (MASPs), in synergy with or in place of TMPRSS2, contribute to activate the SARS-CoV-2 spike protein. Relative concentrations of the attachment receptor, ACE2, MASPs, their endogenous inhibitors (the Kunitz-type transmembrane inhibitors, HAI-1/SPINT1 and HAI-2/SPINT2, as well as major circulating serpins) would determine the infection rate of host cells. The exclusive or predominant expression of major MASPs in specific human organs suggests a direct role of these proteinases in e.g., heart infection and myocardial injury, liver dysfunction, kidney damage, as well as neurological complications. Thorough consideration of these factors could have a positive impact on the control of the current COVID-19 pandemic.

Excess hepsin proteolytic activity limits oncogenic signaling and induces ER stress and autophagy in prostate cancer cells

The serine protease hepsin is frequently overexpressed in human prostate cancer (PCa) and is associated with matrix degradation and PCa progression in mice. Curiously, low expression of hepsin is associated with poor survival in different cancer types, and transgenic overexpression of hepsin leads to loss of viability in various cancer cell lines. Here, by comparing isogenic transfectants of the PCa cell line PC-3 providing inducible overexpression of wild-type hepsin (HPN) vs. the protease-deficient mutant HPN^S353A, we were able to attribute hepsin-mediated tumor-adverse effects to its excess proteolytic activity. A stem-like expression signature of surface markers and adhesion molecules, Notch intracellular domain release, and increased pericellular protease activity were associated with low expression levels of wild-type hepsin, but were partially lost in response to overexpression. Instead, overexpression of wild-type hepsin, but not of HPN^S353A, induced relocalization of the protein to the cytoplasm, and increased autophagic flux in vitro as well as LC3B punctae frequency in tumor xenografts. Confocal microscopy revealed colocalization of wild-type hepsin with both LC3B punctae as well as with the autophagy cargo receptor p62/SQSTM1. Overexpression of wild type, but not protease-deficient hepsin induced expression and nuclear presence of CHOP, indicating activation of the unfolded protein response and ER-associated protein degradation (ERAD). Whereas inhibitors of ER stress and secretory protein trafficking slightly increased viability, combined inhibition of the ubiquitin-proteasome degradation pathway (by bortezomib) with either ER stress (by salubrinal) or autophagy (by bafilomycin A1) revealed a significant decrease of viability during overexpression of wild-type hepsin in PC-3 cells. Our results demonstrate that a precise control of Hepsin proteolytic activity is critical for PCa cell fate and suggest, that the interference with ERAD could be a promising therapeutic option, leading to induction of proteotoxicity in hepsin-overexpressing tumors.

Inhibition of nuclear factor-κB signaling suppresses Spint1-deletion-induced tumor susceptibility in the ApcMin/+ model

Hepatocyte growth factor activator inhibitor type 1 (HAI-1), encoded by the Spint1 gene, is a membrane-bound serine protease inhibitor expressed on the epithelial cell surface. We have previously reported that the intestine-specific Spint1-deleted ApcMin/+ mice showed accelerated formation of intestinal tumors. In this study, we focused on the role of nuclear factor-κB (NF-κB) signaling in the HAI-1 loss-induced tumor susceptibility. In the HAI-1-deficient intestine, inflammatory cytokines, such as tumor necrosis factor-α and interleukin-6, were upregulated in normal mucosa. Furthermore, increased nuclear translocation of NF-κB was observed in both normal mucosa and tumor tissues of HAI-1-deficient ApcMin/+ intestines, and an NF-κB target gene, such as urokinase-type plasminogen activator, was upregulated in the HAI-1-deficient tumor tissues. Thus, we investigated the effect of dehydroxymethylepoxyquinomicin (DHMEQ), a synthetic inhibitor of NF-κB, on intestinal HAI-1-deficient ApcMin/+ mice. Treatment with DHMEQ reduced the formation of intestinal tumors compared with vehicle control in the HAI-1-deficient ApcMin/+ mice. These results suggested that insufficient HAI-1 function promotes intestinal carcinogenesis by activating NF-κB signaling.

The serine proteinase hepsin is an activator of pro-matrix metalloproteinases: molecular mechanisms and implications for extracellular matrix turnover

Increasing evidence implicates serine proteinases in the proteolytic cascades leading to the pathological destruction of extracellular matrices such as cartilage in osteoarthritis (OA). We have previously demonstrated that the type II transmembrane serine proteinase (TTSP) matriptase acts as a novel initiator of cartilage destruction via the induction and activation of matrix metalloproteinases (MMPs). Hepsin is another TTSP expressed in OA cartilage such that we hypothesized this proteinase may also contribute to matrix turnover. Herein, we demonstrate that addition of hepsin to OA cartilage in explant culture induced significant collagen and aggrecan release and activated proMMP-1 and proMMP-3. Furthermore, hepsin directly cleaved the aggrecan core protein at a novel cleavage site within the interglobular domain. Hepsin expression correlated with synovitis as well as tumour necrosis factor α expression, and was induced in cartilage by a pro-inflammatory stimulus. However, a major difference compared to matriptase was that hepsin demonstrated markedly reduced capacity to activate proteinase-activated receptor-2. Overall, our data suggest that hepsin, like matriptase, induces potent destruction of the extracellular matrix whilst displaying distinct efficiencies for the cleavage of specific substrates.

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.

Hepatitis B virus X protein binding to hepsin promotes C3 production by inducing IL-6 secretion from hepatocytes

Hepatitis B virus (HBV) X protein (HBx) is an important effector for HBV-associated pathogenesis. In this study, we identified hepsin as an HBx-interacting protein and investigated the effects of hepsin on HBx-mediated complement component 3 (C3) secretion in hepatocytes. In vivo and in vitro binding between HBx and hepsin was confirmed by co-immunoprecipitation and Glutathione S-transferase pull-down assays. HBx synergized with hepsin to promote C3 production by potentiating interleukin-6 (IL-6) secretion. Knockdown of endogenous hepsin attenuated C3 and IL-6 secretion induced by HBx in hepatic cells. In addition, levels of hepsin protein correlated positively with C3 expression in human non-tumor liver tissues. Further exploration revealed that HBx and hepsin increased C3 promoter activity by up-regulating the expression and phosphorylation of the transcription factor CAAT/enhancer binding protein beta (C/EBP-β), which binds to the IL-6/IL-1 response element in the C3 promoter. HBx and hepsin synergistically enhanced IL-6 mRNA levels and promoter activity by increasing the nuclear translocation of nuclear factor kappaB (NF-κB). Our findings show for the first time that binding between HBx and hepsin promotes C3 production by inducing IL-6 secretion in hepatocytes.

Low expression levels of hepsin and TMPRSS3 are associated with poor breast cancer survival

BACKGROUND

Hepsin, (also called TMPRSS1) and TMPRSS3 are type II transmembrane serine proteases (TTSPs) that are involved in cancer progression. TTSPs can remodel extracellular matrix (ECM) and, when dysregulated, promote tumor progression and metastasis by inducing defects in basement membrane and ECM molecules. This study investigated whether the gene and protein expression levels of these TTSPs were associated with breast cancer characteristics or survival.

METHODS

Immunohistochemical staining was used to evaluate hepsin levels in 372 breast cancer samples and TMPRSS3 levels in 373 samples. TMPRSS1 mRNA expression was determined in 125 invasive and 16 benign breast tumor samples, and TMPRSS3 mRNA expression was determined in 167 invasive and 23 benign breast tumor samples. The gene and protein expression levels were analyzed for associations with breast cancer-specific survival and clinicopathological parameters.

RESULTS

Low TMPRSS1 and TMPRSS3 mRNA expression levels were independent prognostic factors for poor breast cancer survival during the 20-year follow-up (TMPRSS1, P = 0.023; HR, 2.065; 95 % CI, 1.106-3.856; TMPRSS3, P = 0.013; HR, 2.106; 95 % CI, 1.167-3.800). Low expression of the two genes at the mRNA and protein levels associated with poorer survival compared to high levels (log rank P-values 0.015-0.042). Low TMPRSS1 mRNA expression was also an independent marker of poor breast cancer prognosis in patients treated with radiotherapy (P = 0.034; HR, 2.344; 95 % CI, 1.065-5.160). Grade III tumors, large tumor size, and metastasis were associated with low mRNA and protein expression levels.

CONCLUSIONS

The results suggest that the TTSPs hepsin and TMPRSS3 may have similar biological functions in the molecular pathology of breast cancer. Low mRNA and protein expression levels of the studied TTSPs were prognostic markers of poor survival in breast cancer.

Targeted inhibition of cell-surface serine protease Hepsin blocks prostate cancer bone metastasis

The development of effective therapies inhibiting prostate cancer progression and metastasis may substantially impact prostate cancer mortality and potentially reduce the rates of invasive treatments by enhancing the safety of active surveillance strategies. Hepsin (HPN) is a cell surface serine protease amplified in a subset of human sarcomas (7.2%), as well as in ovarian (10%), lung adeno (5.4%), lung squamous cell (4.5%), adenoid cystic (5%), breast (2.6%), uterine (1.7%) and colon (1.4%) carcinomas. While HPN is not amplified in prostate cancer, it is one of the most prominently overexpressed genes in the majority of human prostate tumors and genetic experiments in mice indicate that Hepsin promotes prostate cancer metastasis, particularly metastasis to the bone marrow. We report here the development, analysis and animal trial of the small-molecule Hepsin inhibitor HepIn-13. Long-term exposure to HepIn-13 inhibited bone, liver and lung metastasis in a murine model of metastatic prostate cancer. These findings indicate that inhibition of Hepsin with small-molecule compounds could provide an effective tool for attenuation of prostate cancer progression and metastasis.

Hepsin inhibits CDK11p58 IRES activity by suppressing unr expression and eIF-2α phosphorylation in prostate cancer

Hepsin is a type II transmembrane serine protease frequently overexpressed in prostate cancer (PCa). However, the role of hepsin in PCa remains unclear. In this study, we found that hepsin inhibited the internal ribosome entry site (IRES) activity and expression of CDK11p58, which is associated with cell cycle progression and pro-apoptotic signaling in PCa. Hepsin suppressed CDK11p58 IRES activity in PCa by modulating unr expression and eIF-2α phosphorylation. Further studies revealed that hepsin inhibited the expression of unr by directly binding to unr IRES element and suppressing its activity, and also repressed eIF-2α phosphorylation through down-regulating the expression and phosphorylation of general control non-derepressible-2 (GCN2). Taken together, our data suggest a novel role of hepsin in regulating CDK11p58 IRES activity, and imply that hepsin may act on the machinery of translation to modulate cell cycle progression and survival in PCa cells.

Cleavage specificity analysis of six type II transmembrane serine proteases (TTSPs) using PICS with proteome-derived peptide libraries

Background

Type II transmembrane serine proteases (TTSPs) are a family of cell membrane tethered serine proteases with unclear roles as their cleavage site specificities and substrate degradomes have not been fully elucidated. Indeed just 52 cleavage sites are annotated in MEROPS, the database of proteases, their substrates and inhibitors.

Methodology/Principal Finding

To profile the active site specificities of the TTSPs, we applied Proteomic Identification of protease Cleavage Sites (PICS). Human proteome-derived database searchable peptide libraries were assayed with six human TTSPs (matriptase, matriptase-2, matriptase-3, HAT, DESC and hepsin) to simultaneously determine sequence preferences on the N-terminal non-prime (P) and C-terminal prime (P’) sides of the scissile bond. Prime-side cleavage products were isolated following biotinylation and identified by tandem mass spectrometry. The corresponding non-prime side sequences were derived from human proteome databases using bioinformatics. Sequencing of 2,405 individual cleaved peptides allowed for the development of the family consensus protease cleavage site specificity revealing a strong specificity for arginine in the P1 position and surprisingly a lysine in P1′ position. TTSP cleavage between R↓K was confirmed using synthetic peptides. By parsing through known substrates and known structures of TTSP catalytic domains, and by modeling the remainder, structural explanations for this strong specificity were derived.

Conclusions

Degradomics analysis of 2,405 cleavage sites revealed a similar and characteristic TTSP family specificity at the P1 and P1′ positions for arginine and lysine in unfolded peptides. The prime side is important for cleavage specificity, thus making these proteases unusual within the tryptic-enzyme class that generally has overriding non-prime side specificity.

Potent and specific inhibition of the biological activity of the type-II transmembrane serine protease matriptase by the cyclic microprotein MCoTI-II

Matriptase is a type-II transmembrane serine protease involved in epithelial homeostasis in both health and disease, and is implicated in the development and progression of a variety of cancers. Matriptase mediates its biological effects both via as yet undefined substrates and pathways, and also by proteolytic cleavage of a variety of well-defined protein substrates, several of which it shares with the closely-related protease hepsin. Development of targeted therapeutic strategies will require discrimination between these proteases. Here we have investigated cyclic microproteins of the squash Momordica cochinchinensis trypsin-inhibitor family (generated by total chemical synthesis) and found MCoTI-II to be a high-affinity (Ki 9 nM) and highly selective (> 1,000-fold) inhibitor of matriptase. MCoTI-II efficiently inhibited the proteolytic activation of pro-hepatocyte growth factor (HGF) by matriptase but not by hepsin, in both purified and cell-based systems, and inhibited HGF-dependent cell scattering. MCoTI-II also selectively inhibited the invasion of matriptase-expressing prostate cancer cells. Using a model of epithelial cell tight junction assembly, we also found that MCoTI-II could effectively inhibit the re-establishment of tight junctions and epithelial barrier function in MDCK-I cells after disruption, consistent with the role of matriptase in regulating epithelial integrity. Surprisingly, MCoTI-II was unable to inhibit matriptase-dependent proteolytic activation of prostasin, a GPI-anchored serine protease also implicated in epithelial homeostasis. These observations suggest that the unusually high selectivity afforded by MCoTI-II and its biological effectiveness might represent a useful starting point for the development of therapeutic inhibitors, and further highlight the role of matriptase in epithelial maintenance.

Function and clinical relevance of kallikrein-related peptidases and other serine proteases in gynecological cancers

Gynecological cancers, including malignant tumors of the ovaries, the endometrium and the cervix, account for approximately 10% of tumor-associated deaths in women of the Western world. For screening, diagnosis, prognosis, and therapy response prediction, the group of enzymes known as serine (Ser-)proteases show great promise as biomarkers. In the present review, following a summary of the clinical facts regarding malignant tumors of the ovaries, the endometrium and the cervix, and characterization of the most important Ser-proteases, we thoroughly review the current state of knowledge relating to the use of proteases as biomarkers of the most frequent gynecological cancers. Within the Ser-protease group, the kallikrein-related peptidase (KLK) family, which encompasses a subgroup of 15 members, holds particular promise, with some acting via a tumor-promoting mechanism and others behaving as protective factors. Further, the urokinase-type plasminogen activator (uPA) and its inhibitor PAI-1 (plasminogen activator inhibitor-1) seem to play an unfavorable role in gynecological tumors, while down-regulation of high-temperature requirement proteins A 1, 2 and 3 (HtrA1,2,3) is associated with malignant disease and cancer progression. Expression/activity levels of other Ser-proteases, including the type II transmembrane Ser-proteases (TTSPs) matriptase, hepsin (TMPRSS1), and the hepsin-related protease (TMPRSS3), as well as the glycosyl-phosphatidylinositol (GPI)-anchored Ser-proteases prostasin and testisin, may be of clinical relevance in gynecological cancers. In conclusion, proteases are a rich source of biomarkers of gynecological cancer, though the enzymes' exact roles and functions merit further investigation.

HLA-A2-restricted cytotoxic T lymphocyte epitopes from human hepsin as novel targets for prostate cancer immunotherapy

Hepsin is a type II transmembrane serine protease that is overexpressed in prostate cancer, and it is associated with prostate cancer cellular migration and invasion. Therefore, HPN is a biomarker for prostate cancer. CD8(+) T cells play an important role in tumour immunity. This study predicted and identified HLA-A2-restricted cytotoxic T lymphocyte (CTL) epitopes in human hepsin protein. HLA-A2-restricted CTL epitopes were identified using the following four-step procedure: (1) a computer program generated predicted epitopes from the amino acid sequence of human hepsin; (2) an HLA-A2-binding assay detected the affinity of the predicted epitopes to the HLA-A2 molecule; (3) the primary T cell response against the predicted epitopes was stimulated in vitro; and (4) the induced CTLs towards different types of hepsin- or HLA-A2-expressing prostate cancer cells were detected. Five candidate peptides were identified. The effectors that were induced by human hepsin epitopes containing residues 229 to 237 (Hpn229; GLQLGVQAV), 268 to 276 (Hpn268; PLTEYIQPV) and 191 to 199 (Hpn199; SLLSGDWVL) effectively lysed LNCaP prostate cancer cells that were hepsin-positive and HLA-A2 matched. These peptide-specific CTLs did not lyse normal liver cells with low hepsin levels. Hpn229, Hpn268 and Hpn199 increased the frequency of IFN-γ-producing T cells compared with the negative peptide. These results suggest that the Hpn229, Hpn268 and Hpn199 epitopes are novel HLA-A2-restricted CTL epitopes that are capable of inducing hepsin-specific CTLs in vitro. Hpn229, Hpn268 and Hpn199 peptide-based vaccines may be useful for immunotherapy in patients with prostate cancer.

Serine protease hepsin regulates hepatocyte size and hemodynamic retention of tumor cells by hepatocyte growth factor signaling in mice

The liver architecture plays an important role in maintaining hemodynamic balance, but the mechanisms that underlie this role are not fully understood. Hepsin, a type II transmembrane serine protease, is predominantly expressed in the liver, but has no known physiological functions. Here, we report that hemodynamic balance in the liver is regulated through hepsin. Deletion of hepsin (hepsin(-/-) ) in mice resulted in enlarged hepatocytes and narrowed liver sinusoids. Using fluorescent microbeads and antihepsin treatment, we demonstrated that metastatic cancer cells preferentially colonized the hepsin(-/-) mouse liver as a result of the retention of tumor cells because of narrower sinusoids. The enlarged hepatocytes expressed increased levels of connexin, which resulted from defective prohepatocyte growth factor (pro-HGF) processing and decreased c-Met phosphorylation in the livers of hepsin(-/-) mice. Treatment of hepsin(-/-) mice with recombinant HGF rescued these phenotypes, and treatment of wild-type mice with an HGF antagonist recapitulated the phenotypes observed in hepsin(-/-) mice.

CONCLUSION

Our findings show that the maintenance of hepatic structural homeostasis occurs through HGF/c-Met/connexin signaling by hepsin, and hepsin-mediated changes in liver architecture significantly enhance tumor metastasis to the liver.

Allosteric antibody inhibition of human hepsin protease

Hepsin is a type II transmembrane serine protease that is expressed in several human tissues. Overexpression of hepsin has been found to correlate with tumour progression and metastasis, which is so far best studied for prostate cancer, where more than 90% of such tumours show this characteristic. To enable improved future patient treatment, we have developed a monoclonal humanized antibody that selectively inhibits human hepsin and does not inhibit other related proteases. We found that our antibody, hH35, potently inhibits hepsin enzymatic activity at nanomolar concentrations. Kinetic characterization revealed non-linear, slow, tight-binding inhibition. This correlates with the crystal structure we obtained for the human hepsin-hH35 antibody Fab fragment complex, which showed that the antibody binds hepsin around α3-helix, located far from the active centre. The unique allosteric mode of inhibition of hH35 is distinct from the recently described HGFA (hepatocyte growth factor activator) allosteric antibody inhibition. We further explain how a small change in the antibody design induces dramatic structural rearrangements in the hepsin antigen upon binding, leading to complete enzyme inactivation.

Activation of pro-uPA is critical for initial escape from the primary tumor and hematogenous dissemination of human carcinoma cells

Urokinase-type plasminogen activator (uPA) and plasmin have long been implicated in cancer progression. However, the precise contributions of the uPA/plasmin system to specific steps involved in cancer cell dissemination have not been fully established. Herein, we have used a highly disseminating variant of the human PC-3 prostate carcinoma cell line, PC-hi/diss, as a prototype of aggressive carcinomas to investigate the mechanisms whereby pro-uPA activation and uPA-generated plasmin functionally contribute to specific stages of metastasis. The PC-hi/diss cells secrete and activate significant amounts of pro-uPA, leading to efficient generation of plasmin in solution and at the cell surface. In a mouse orthotopic xenograft model, treatment with the specific pro-uPA activation-blocking antibody mAb-112 significantly inhibited local invasion and distant metastasis of the PC-hi/diss cells. To mechanistically examine the uPA/plasmin-mediated aspects of tumor cell dissemination, the anti-pro-uPA mAb-112 and the potent serine protease inhibitor, aprotinin, were used in parallel in a number of in vivo assays modeling various rate-limiting steps in early metastatic spread. Our findings demonstrate that, by generating plasmin, activated tumor-derived uPA facilitates early stages of PC-hi/diss dissemination, specifically the escape from the primary tumor and tumor cell intravasation. Moreover, through a series of in vitro and in vivo analyses, we suggest that PC-hi/diss-invasive escape and dissemination may be enhanced by cleavage of stromal fibronectin by uPA-generated plasmin. Together, our findings point to inhibition of pro-uPA activation at the apex of the uPA/plasmin cascade as a therapy-valid approach to control onset of tumor escape and ensuing metastatic spread.

An allosteric anti-hepsin antibody derived from a constrained phage display library

The serine protease hepsin is highly upregulated in prostate cancer and is implicated in tumor progression. Therefore, specific inhibition of hepsin enzymatic activity by an antibody constitutes an attractive therapeutic approach. Here, we report the identification of the anti-hepsin antibody Fab25 by screening of a Fab phage display library with a restricted chemical diversity at the complementary determining regions. Hepsin with its S1 pocket occupied by 3,4-dichloro-isocoumarin was used as the 'bait' for library screening. Fab25 was highly specific and it potently inhibited hepsin activity toward a panel of synthetic and macromolecular substrates. Biochemical and enzymatic studies with synthetic substrates of variable length suggested that Fab25 acts as an allosteric inhibitor based on non-competitive inhibition kinetics. Isothermal titration calorimetric experiments showed that the high-affinity (K(D) 6.1 nM) binding of Fab25 with hepsin is enthalpically driven. Despite an unusually long CDR-H3 loop with several potential hepsin cleavage sites (Lys, Arg residues), Fab25 was not processed by hepsin. Antibody-25 should be valuable for investigating hepsin's role in cancer progression and for potential therapeutic applications. Furthermore, the herein presented phage display strategy using an active site-modified protease should be widely applicable for identifying potential allosteric anti-protease antibodies.

Matrix-dependent regulation of AKT in Hepsin-overexpressing PC3 prostate cancer cells

The serine-protease hepsin is one of the most prominently overexpressed genes in human prostate carcinoma. Forced expression of the enzyme in mice prostates is associated with matrix degradation, invasive growth, and prostate cancer progression. Conversely, hepsin overexpression in metastatic prostate cancer cell lines was reported to induce cell cycle arrest and reduction of invasive growth in vitro. We used a system for doxycycline (dox)-inducible target gene expression in metastasis-derived PC3 cells to analyze the effects of hepsin in a quantitative manner. Loss of viability and adhesion correlated with hepsin expression levels during anchorage-dependent but not anchorage-independent growth. Full expression of hepsin led to cell death and detachment and was specifically associated with reduced phosphorylation of AKT at Ser(473), which was restored by growth on matrix derived from RWPE1 normal prostatic epithelial cells. In the chorioallantoic membrane xenograft model, hepsin overexpression in PC3 cells reduced the viability of tumors but did not suppress invasive growth. The data presented here provide evidence that elevated levels of hepsin interfere with cell adhesion and viability in the background of prostate cancer as well as other tissue types, the details of which depend on the microenvironment provided. Our findings suggest that overexpression of the enzyme in prostate carcinogenesis must be spatially and temporally restricted for the efficient development of tumors and metastases.

Targeting the autolysis loop of urokinase-type plasminogen activator with conformation-specific monoclonal antibodies

Tight regulation of serine proteases is essential for their physiological function, and unbalanced states of protease activity have been implicated in a variety of human diseases. One key example is the presence of uPA (urokinase-type plasminogen activator) in different human cancer types, with high levels correlating with a poor prognosis. This observation has stimulated efforts into finding new principles for intervening with uPA's activity. In the present study we characterize the so-called autolysis loop in the catalytic domain of uPA as a potential inhibitory target. This loop was found to harbour the epitopes for three conformation-specific monoclonal antibodies, two with a preference for the zymogen form pro-uPA, and one with a preference for active uPA. All three antibodies were shown to have overlapping epitopes, with three common residues being crucial for all three antibodies, demonstrating a direct link between conformational changes of the autolysis loop and the creation of a catalytically mature active site. All three antibodies are potent inhibitors of uPA activity, the two pro-uPA-specific ones by inhibiting conversion of pro-uPA to active uPA and the active uPA-specific antibody by shielding the access of plasminogen to the active site. Furthermore, using immunofluorescence, the conformation-specific antibodies mAb-112 and mAb-12E6B10 enabled us to selectively stain pro-uPA or active uPA on the surface of cultured cells. Moreover, in various independent model systems, the antibodies inhibited tumour cell invasion and dissemination, providing evidence for the feasibility of pharmaceutical intervention with serine protease activity by targeting surface loops that undergo conformational changes during zymogen activation.

Proteolytic activation of pro-macrophage-stimulating protein by hepsin

Macrophage-stimulating protein (MSP) is a plasminogen-related growth factor and ligand for the receptor tyrosine kinase RON. The MSP/RON system promotes wound healing and invasive tumor growth and suppresses proinflammatory immune response. MSP binding to RON requires proteolytic conversion of the inactive single-chain form (pro-MSP) into the disulfide-linked α/β heterodimer. The pro-MSP cleavage sequence (Ser-Lys-Leu-Arg(483)↓Val(484)) closely matches the substrate recognition sequences of hepsin, a type II transmembrane serine protease, that is overexpressed in several cancers. Here, we show that recombinant hepsin cleaves pro-MSP at the consensus site Arg(483)-Val(484) with superior efficiency compared with the known activators MT-SP1 and hepatocyte growth factor activator (HGFA). At least 50% of pro-MSP was processed within 1 hour at a hepsin concentration of 2.4 nmol/L and at a molar enzyme to substrate ratio of 1:500. An uncleavable single-chain variant of MSP weakly bound to a RON-Fc fusion protein, whereas hepsin-cleaved MSP bound with a K(D) of 10.3 nmol/L, suggesting that the high-affinity binding site in MSP β-chain was properly formed. LNCaP prostate cancer cells overexpressing hepsin on the cell surface efficiently activated pro-MSP, which was blocked by a specific anti-hepsin antibody. Incubation of pro-MSP with hepsin led to robust RON-mediated phosphorylation of mitogen-activated protein kinase, ribosomal S6 protein, and Akt in human A2780 ovarian carcinoma cells stably expressing RON protein. In macrophages, pro-MSP with hepsin induced chemotaxis and attenuated lipopolysaccharide-dependent production of nitric oxide. These findings suggest that the MSP/RON signaling pathway may be regulated by hepsin in tissue homeostasis and in disease pathologies, such as in cancer and immune disorders.

The role of fibrinolytic genes and proteins in the development of allograft vascular disease

BACKGROUND

We have previously shown that lack of plasminogen activator inhibitor-1 (PAI-1) expression in donor tissue greatly increases intimal proliferation (IP) after allogeneic transplantation. We sought to determine the relative role of PAI-1 and other fibrinolytic proteins in the development of IP.

METHODS

We used an abdominal aortic transplant model in mice to investigate IP in 3 groups of 6 recipients. In the isograft group, CBA/J strain mice were donors and recipients, donors for allograft group were C57BL/6J mice, and for the allograft/knockout group, C57BL/6J PAI-1 knockout mice. All groups received weekly injections of anti-CD8/CD4 monoclonal antibodies. IP was calculated at 50 days, and sections were analyzed for fibrinolytic proteins, messenger RNA (mRNA) and PAI-1 activity using immunohistochemistry (IHC), in situ hybridization (ISH), reverse transcription-polymerase chain reaction (RT-PCR), and Western blot analysis.

RESULTS

Significantly more IP developed in the allograft/knockout group vs the isograft (p < 0.001) and the allograft groups (p = 0.003). There was marked intimal expression of tissue plasminogen activator (tPA), urokinase PA (uPA), and uPA receptor (uPAR) proteins and mRNA in the allograft and allograft/knockout groups vs the isograft group. Allografts also showed significant intimal staining for PAI-1 protein and mRNA. RT-PCR demonstrated a stepwise increase in profibrinolytic protein mRNA from isograft to allograft to allograft/knockout groups, particularly uPA (p = 0.02) and uPAR (p = 0.016). Western blot data showed complementary findings. PAI-1 activity was persistently present in isograft and allograft animals, only. Intimas in allograft and allograft/knockout groups were primarily smooth muscle cells.

CONCLUSIONS

PAI-1 reduces IP by limiting smooth muscle cell activity, with little change in matrix composition likely by modulating profibrinolytic protein expression.

Activation of the zymogen to urokinase-type plasminogen activator is associated with increased interdomain flexibility

A key regulatory step for serine proteases of the trypsin clan is activation of the initially secreted zymogens, leading to an increase in activity by orders of magnitude. Zymogen activation occurs by cleavage of a single peptide bond near the N-terminus of the catalytic domain. Besides the catalytic domain, most serine proteases have N-terminal A-chains with independently folded domains. Little is known about how zymogen activation affects the interplay between domains. This question is investigated with urokinase-type plasminogen activator (uPA), which has an epidermal growth factor domain and a kringle domain, connected to the catalytic domain by a 15-residue linker. uPA has been implicated under several pathological conditions, and one possibility for pharmacological control is targeting the conversion of the zymogen pro-uPA to active uPA. Therefore, a small-angle X-ray scattering study of the conformations of pro-uPA and uPA in solution was performed. Structural models for the proteins were derived using available atomic-resolution structures for the various domains. Active uPA was found to be flexible with a random conformation of the amino-terminal fragment domain with respect to the serine protease domain. In contrast, pro-uPA was observed to be rigid, with the amino-terminal fragment domain in a fixed position with respect to the serine protease domain. Analytical ultracentrifugation analysis supported the observed difference between pro-uPA and uPA in overall shape and size seen with small-angle X-ray scattering. Upon association of either of two monoclonal Fab (fragment antigen-binding) fragments that are directed against the catalytic domain of, respectively, pro-uPA and uPA, rigid structures were formed.

Mechanisms of prostate cancer initiation and progression

Prostate cancer is a major health problem as it continues to be the most frequently diagnosed cancer in men in the Western world. While improved early detection significantly decreased mortality, prostate cancer still remains the second leading cause of cancer-related death in Western men. Understanding the mechanisms of prostate cancer initiation and progression should have a significant impact on development of novel therapeutic approaches that can help to combat this disease. The recent explosion of novel high-throughput genetic technologies together with studies in animal models and human tissues allowed a comprehensive analysis and functional validation of the molecular changes. This chapter will summarize and discuss recently identified critical genetic and epigenetic changes that drive prostate cancer initiation and progression. These discoveries should help concentrate the efforts of drug development on key pathways and molecules, and finally translate the knowledge that is gained from mechanistic studies into effective treatments.

Laminin-332 cleavage by matriptase alters motility parameters of prostate cancer cells

BACKGROUND

Matriptase, a type II transmembrane serine protease, has been linked to initiation and promotion of epidermal carcinogenesis in a murine model, suggesting that deregulation of its role in epithelia contributes to transformation. In human prostate cancer, matriptase expression correlates with progression. It is therefore of interest to determine how matriptase may contribute to epithelial neoplastic progression. One approach for studying this is to identify potential matriptase substrates involved in epithelial integrity and/or transformation like the extracellular matrix macromolecule, laminin-332 (Ln-332), which is found in the basement membrane of many epithelia, including prostate. Proteolytic processing of Ln-332 regulates cell motility of both normal and transformed cells, which has implications in cancer progression.

METHODS

In vitro cleavage experiments were performed with purified Ln-332 protein and matriptase. Western blotting, enzyme inhibition assays, and mass spectrometry were used to confirm cleavage events. Matriptase overexpressing LNCaP prostate cancer cells were generated and included in Transwell migration assays and single cell motility assays, along with other prostate cells.

RESULTS

We report that matriptase proteolytically cleaves Ln-332 in the β3 chain. Substrate specificity was confirmed by blocking cleavage with the matriptase inhibitor, Kunitz domain-1. Transwell migration assays showed that DU145 cell motility was significantly enhanced when plated on matriptase-cleaved Ln-332. Similarly, Transwell migration of matriptase-overexpressing LNCaP cells was significantly increased on Ln-332 and, as determined by live single-cell microscopy, two motility parameters of this cell line, speed and directional persistence, were also higher.

CONCLUSIONS

Proteolytic processing of Ln-332 by matriptase enhances speed and directional persistence of prostate cancer cells.

Toxin-based therapeutic approaches

Protein toxins confer a defense against predation/grazing or a superior pathogenic competence upon the producing organism. Such toxins have been perfected through evolution in poisonous animals/plants and pathogenic bacteria. Over the past five decades, a lot of effort has been invested in studying their mechanism of action, the way they contribute to pathogenicity and in the development of antidotes that neutralize their action. In parallel, many research groups turned to explore the pharmaceutical potential of such toxins when they are used to efficiently impair essential cellular processes and/or damage the integrity of their target cells. The following review summarizes major advances in the field of toxin based therapeutics and offers a comprehensive description of the mode of action of each applied toxin.

The role of hepatocyte growth factor activator inhibitor (HAI)-1 and HAI-2 in endometrial cancer

Hepatocyte growth factor activator inhibitors (HAI-1 and HAI-2) are Kunitz-type serine protease inhibitors that have a broad inhibitory spectrum against serine proteases. This is the first study to investigate the role of HAI-1 and HAI-2 in endometrial cancer. We investigated the biological functions of HAI-1 and HAI-2 using KLE and HEC-251 endometrial cancer cell lines, thus HAI-1 and HAI-2 were examined in uterine normal endometrium, endometrial hyperplasia and cancer specimens by immunohistochemistry. HAI-1 and HAI-2 showed potential inhibitory effects on cell proliferation, migration and cellular invasion by reduction of matriptase and hepsin expression. This in turn led to an increase in the levels of E-cadherin and Slug, and a reduction in the levels of Vimentin, SIP1, Snail and Twist, and hence ER and PR signal transduction in endometrial cancer cells. The levels of HAI-1 and HAI-2 expression were significantly decreased in endometrial cancer specimens relative to the corresponding normal endometrium specimens. Low HAI-1 and HAI-2 expression was a significant predictor for a poor prognosis compared with high HAI-1 and HAI-2 expression. These findings indicate that HAI-1 and HAI-2 could be considered as therapeutic targets and used as favorable prognosis markers for endometrial cancer.

Isolation, purification, and study of properties of recombinant hepsin from Escherichia coli

A recombinant hepsin-producing strain of Escherichia coli was obtained and the conditions for hepsin expression in a bacterial system were optimized. To study the physicochemical properties of the enzyme, a procedure for purification of active recombinant hepsin using metal-chelate affinity chromatography and ion-exchange chromatography was developed. The interaction of recombinant hepsin with various peptide substrates is characterized. The dose-dependent inhibition of the recombinant hepsin enzyme activity by anthralin in vitro and an increase in the hepsin enzymatic activity in the presence of resveratrol were revealed.

The cutting edge: membrane-anchored serine protease activities in the pericellular microenvironment

The serine proteases of the trypsin-like (S1) family play critical roles in many key biological processes including digestion, blood coagulation, and immunity. Members of this family contain N- or C-terminal domains that serve to tether the serine protease catalytic domain directly to the plasma membrane. These membrane-anchored serine proteases are proving to be key components of the cell machinery for activation of precursor molecules in the pericellular microenvironment, playing vital functions in the maintenance of homoeostasis. Substrates activated by membrane-anchored serine proteases include peptide hormones, growth and differentiation factors, receptors, enzymes, adhesion molecules and viral coat proteins. In addition, new insights into our understanding of the physiological functions of these proteases and their involvement in human pathology have come from animal models and patient studies. The present review discusses emerging evidence for the diversity of this fascinating group of membrane serine proteases as potent modifiers of the pericellular microenvironment through proteolytic processing of diverse substrates. We also discuss the functional consequences of the activities of these proteases on mammalian physiology and disease.

Association of hepsin gene variants with prostate cancer risk and prognosis

BACKGROUND

Hepsin (HPN) is one of the most consistently overexpressed genes in prostate cancer and there is some evidence supporting an association between HPN gene variants and prostate cancer risk. We report results from a population-based case-control genetic association study for six tagging single nucleotide polymorphisms (tagSNPs) in the HPN gene.

METHODS

Prostate cancer risk was estimated using adjusted unconditional logistic regression in 1,401 incident prostate cancer cases diagnosed in 1993 through 1996 or 2002 through 2005 and 1,351 age-matched controls. Risks of disease recurrence/progression and prostate cancer-specific mortality were estimated using Cox proportional hazards (PH) regression in 437 cases with long-term follow-up.

RESULTS

There were 135 recurrence/progression events and 57 cases who died of prostate cancer. Contrary to some earlier studies, we found no evidence of altered risk of developing prostate cancer overall or when clinical measures of tumor aggressiveness were considered for any of the tagSNPs, assessed either individually or by haplotypes. There was no evidence of altered risks of tumor recurrence/progression or prostate cancer death associated with variants in the HPN gene.

CONCLUSIONS

Germline genetic variation of HPN does not seem to contribute to risk of prostate cancer or prognosis.

Nonproteolytic induction of catalytic activity into the single-chain form of urokinase-type plasminogen activator by dipeptides

Serine proteases are initially synthesized as single-chain proenzymes with activities that are many orders of magnitude lower than those of the mature enzyme. Proteolytic cleavage of an exposed loop liberates a new amino terminus that inserts into a hydrophobic pocket and forms a stabilizing salt bridge with a ubiquitously conserved aspartate residue, resulting in a conformational change organizing the mature oxyanion hole. In a decisive 1976 work, Huber and Bode [Bode, W., and Huber, R. (1976) FEBS Lett. 68, 231-236] demonstrated that peptides sequentially similar to the new amino terminus in combination with a catalytic site inhibitor could specifically induce a trypsin-like conformation in trypsinogen. We now demonstrate that an Ile-Ile or Ile-Val dipeptide can induce limited enzyme activity in the single-chain zymogen form of urokinase-type plasminogen activator (uPA) or its K158A variant, which cannot be activated proteolytically. Furthermore, the slow formation of a covalent serpin-protease complex between single-chain uPA and PAI-1 is significantly accelerated in the presence of specific dipeptide sequences. The technique of using a dipeptide mimic as a surrogate for the liberated amino terminus further provides a novel means by which to covalently label the immature active site of single-chain uPA with a fluorescent probe, permitting fluorescence approaches for direct observations of conformational changes within the protease domain during zymogen activation. These data demonstrate the structural plasticity of the protease domain, reinforce the notion of "molecular sexuality", and provide a novel way of studying conformational changes of zymogens during proteolytic activation.

Pegylated kunitz domain inhibitor suppresses hepsin-mediated invasive tumor growth and metastasis

The transmembrane serine protease hepsin is one of the most highly upregulated genes in prostate cancer. Here, we investigated its tumor-promoting activity by use of a mouse orthotopic prostate cancer model. First, we compared the tumor growth of low hepsin-expressing LnCaP-17 cells with hepsin-overexpressing LnCaP-34 cells. After implantation of cells into the left anterior prostate lobe, LnCaP-34 tumors not only grew faster based on increased serum prostate-specific antigen levels but also metastasized to local lymph nodes and, most remarkably, invaded the contralateral side of the prostate at a rate of 100% compared with only 18% for LnCaP-17 tumors. The increased tumor growth was not due to nonspecific gene expression changes and was not predicted from the unaltered in vitro growth and invasion of LnCaP-34 cells. A likely explanation is that the in vivo effects of hepsin were mediated by specific hepsin substrates present in the tumor stroma. In a second study, mice bearing LnCaP-34 tumors were treated with a PEGylated form of Kunitz domain-1, a potent hepsin active site inhibitor derived from hepatocyte growth factor activator inhibitor-1 (K(i)(app) 0.30 +/- 0.02 nmol/L). Treatment of established tumors with PEGylated Kunitz domain-1 decreased contralateral prostate invasion (46% weight reduction) and lymph node metastasis (50% inhibition). Moreover, serum prostate-specific antigen level remained reduced during the entire treatment period, reaching a maximal reduction of 76% after 5 weeks of dosing. The findings show that hepsin promotes invasive prostate tumor growth and metastasis and suggest that active site-directed hepsin inhibition could be effective in prostate cancer therapy.

Delineating protease functions during cancer development

Much progress has been made in understanding how matrix remodeling proteases, including metalloproteinases, serine proteases, and cysteine cathepsins, functionally contribute to cancer development. In addition to modulating extracellular matrix metabolism, proteases provide a significant protumor advantage to developing neoplasms through their ability to modulate bioavailability of growth and proangiogenic factors, regulation of bioactive chemokines and cytokines, and processing of cell-cell and cell-matrix adhesion molecules. Although some proteases directly regulate these events, it is now evident that some proteases indirectly contribute to cancer development by regulating posttranslational activation of latent zymogens that then directly impart regulatory information. Thus, many proteases act in a cascade-like manner and exert their functionality as part of a proteolytic pathway rather than simply functioning individually. Delineating the cascade of enzymatic activities contributing to overall proteolysis during carcinogenesis may identify rate-limiting steps or pathways that can be targeted with anti-cancer therapeutics. This chapter highlights recent insights into the complexity of roles played by pericellular and intracellular proteases by examining mechanistic studies as well as the roles of individual protease gene functions in various organ-specific mouse models of cancer development, with an emphasis on intersecting proteolytic activities that amplify programming of tissues to foster neoplastic development.

Plasmin-mediated proteolysis is required for hepatocyte growth factor activation during liver repair

The physiological relevance of the activation of hepatocyte growth factor (Hgf) by the plasminogen (Plg) system of proteases and its contribution to tissue repair are largely undefined. Here, we investigated whether the defective liver repair in mice lacking Plg is due to impaired activation of Hgf. Loss of Plg in vivo suppressed Hgf activation and signaling through its Met tyrosine kinase receptor. Without Plg, hepatocytes were unresponsive to Hgf-induced proliferation and migration, with a more pronounced impairment in hepatocyte movement within the hepatic environment. Most notably, circumventing the defect in proteolytic activation of Hgf by the downstream expression of an activated Met receptor corrected the functional deficits and improved liver repair in Plg-deficient mice. These findings support a fibrinolysis-unrelated role for Plg in modulating cell proliferation and migration by activation of Hgf.

Probing the substrate specificities of matriptase, matriptase-2, hepsin and DESC1 with internally quenched fluorescent peptides

Type II transmembrane serine proteases are an emerging class of proteolytic enzymes involved in tissue homeostasis and a number of human disorders such as cancer. To better define the biochemical functions of a subset of these proteases, we compared the enzymatic properties of matriptase, matriptase-2, hepsin and DESC1 using a series of internally quenched fluorogenic peptide substrates containing o-aminobenzoyl and 3-nitro-tyrosine. We based the sequence of the peptides on the P4 to P4' activation sequence of matriptase (RQAR-VVGG). Positions P4, P3, P2 and P1' were substituted with nonpolar (Ala, Leu), aromatic (Tyr), acid (Glu) and basic (Arg) amino acids, whereas P1 was fixed to Arg. Of the four type II transmembrane serine proteases studied, matriptase-2 was the most promiscuous, and matriptase was the most discriminating, with a distinct specificity for Arg residues at P4, P3 and P2. DESC1 had a preference similar to that of matriptase, but with a propensity for small nonpolar amino acids (Ala) at P1'. Hepsin shared similarities with matriptase and DESC1, but was markedly more permissive at P2. Matriptase-2 manifested broader specificities, as well as substrate inhibition, for selective internally quenched fluorescent substrates. Lastly, we found that antithrombin III has robust inhibitory properties toward matriptase, matriptase-2, hepsin and DESC1, whereas plasminogen activator inhibitor-1 and alpha(2)-antiplasmin inhibited matriptase-2, hepsin and DESC1, and to a much lesser extent, matriptase. In summary, our studies revealed that these enzymes have distinct substrate preferences.

New insights into the molecular mechanisms of the fibrinolytic system

Fibrinolysis is regulated by specific molecular interactions between its main components. Activation of plasminogen by tissue-type plasminogen activator (t-PA) is enhanced in the presence of fibrin or at the endothelial cell surface. Urokinase-type plasminogen activator (u-PA) binds to a specific cellular u-PA receptor (u-PAR), resulting in enhanced activation of cell-bound plasminogen. Inhibition of fibrinolysis occurs at the level of plasminogen activation or at the level of plasmin. Assembly of fibrinolytic components at the surface of fibrin results in fibrin degradation. Assembly at the surface of cells provides a mechanism for generation of localized cell-associated proteolytic activity. This review includes novel proteins such a thrombin-activatable fibrinolysis inhibitor (TAFI) and discusses new insights into molecular mechanisms obtained from the rapidly growing knowledge of crystal structures of proteins.

A novel mode of intervention with serine protease activity: targeting zymogen activation

Serine proteases are secreted from cells as single-chain zymogens, typically having activities orders of magnitude lower than those of the mature two-chain enzymes. Activation occurs by a conformational change initiated by cleavage of a specific peptide bond. We have derived a monoclonal antibody (mAb-112) which binds with subnanomolar affinity to pro-uPA, the zymogen form of urokinase-type plasminogen activator (uPA). We mapped the epitope of the antibody to the autolysis loop, one of the structural elements known to change conformation during zymogen activation. A mechanistic evaluation with biophysical methods elucidated a novel bifunctional inhibitory mechanism whereby mAb-112 not only delays the proteolytic conversion of single-chain pro-uPA into the two-chain form but also subsequently averts the conformational transition to a mature protease by sequestering the two-chain form in a zymogen-like, noncatalytic state. Functional studies employing two variants of human HT-1080 cells, exhibiting high and low levels of dissemination in a chorioallantoic membrane assay, demonstrate that mAb-112 is an effective inhibitor of tumor cell intravasation. These findings show that pharmacological interference with zymogen activation is a plausible and robust means to regulate uPA activity and the downstream effects of plasminogen activation in the spread of cancer and other processes of pathological tissue remodeling. A strategy that targets regions related to pro-enzyme activation likely provide a unique inhibitor-protease interaction surface and is, thus, expected to enhance the chances of engineering high inhibitor specificity. Our results provide new information about the structural flexibility underlying the equilibrium between active and inactive forms of serine proteases.