fur gene expression as a discriminating marker for small cell and nonsmall cell lung carcinomas

The Influenza A Virus Replication Cycle: A Comprehensive Review

Influenza A virus (IAV) is the primary causative agent of influenza, colloquially called the flu. Each year, it infects up to a billion people, resulting in hundreds of thousands of human deaths, and causes devastating avian outbreaks with worldwide losses worth billions of dollars. Always present is the possibility that a highly pathogenic novel subtype capable of direct human-to-human transmission will spill over into humans, causing a pandemic as devastating if not more so than the 1918 influenza pandemic. While antiviral drugs for influenza do exist, they target very few aspects of IAV replication and risk becoming obsolete due to antiviral resistance. Antivirals targeting other areas of IAV replication are needed to overcome this resistance and combat the yearly epidemics, which exact a serious toll worldwide. This review aims to summarise the key steps in the IAV replication cycle, along with highlighting areas of research that need more focus.

Significance of Furin Expression in Thyroid Neoplastic Transformation

Angiotensin-Converting Enzyme 2 (ACE2), Transmembrane Serine Protease 2 (TMPRSS2), and Furin were known to be key players in the SARS-CoV-2 infection, and the thyroid gland was revealed to be one of the relevant targets of the virus. Regardless of the viral infection, the expression of these molecules in the thyroid gland and their putative role in the neoplastic transformation of the thyrocytes has not been thoroughly explored. In this work, we aimed to characterize the mRNA and protein expression pattern of ACE2, TMPRSS2, and Furin in a series of patients with thyroid lesions. Our main results revealed a significantly decreased expression of ACE2 mRNA in the thyroid neoplasms in comparison to normal adjacent tissue. Furin mRNA was significantly increased in thyroid neoplasms when compared to normal adjacent tissue. In addition, a higher Furin mRNA level in thyroid carcinomas was associated with the presence of lymph node metastasis. Furin mRNA expression revealed a high discriminatory power between adjacent tissue and neoplasms. Protein expression of these molecules did not correlate with mRNA expression. Our study shows the mRNA downregulation of ACE2 and overexpression of Furin in thyroid neoplasms. Further studies are required to clarify if Furin expression can be a potential diagnostic indicator in thyroid neoplasia.

Decoding the silent walk of COVID-19: Halting its spread using old bullets

Severe acute respiratory syndrome (SARS) develops within 3-14 days when CoV2 invades epithelial, myeloid cells in the nasopharynx and pneumocytes in the respiratory tract through angiotensin converting enzyme (ACE2). Infection swiftly disseminates to gastrointestinal, cardiovascular, renal organs as well as immune system to deregulate their normal functioning through unique and distinct mechanisms. The health system and economy has been intensely thwarted by the rapid spread and exorbitant mortality caused by COVID-19 disease across the globe. The acute progression of the disease and high infection rate pose an enormous challenge for its therapeutic management and critical care. The viral structure, genome and proteome have been deciphered which yielded cues for targeting already available therapeutic entities. More than 200 compounds have been screened and till date approximately 69 therapeutic agents are undergoing clinical trials across the world. Among these, remedesivir (RMD), chloroquine (CQ), hydroxychloroquine (HCQ), noscapine (NOS) and heparin have demonstrated fairly promising results in preclinical and clinical studies. Recently, RMD has been approved by USFDA for the management of COVID 19. However, intense research is going on to screen and ace the 'magic bullets' for the management of SARS-CoV2 infection worldwide. The current review illustrates the plausible therapeutic targets in SARS-CoV2 important for inhibition of virus cycle. In addition, the role of RMD, CQ, HCQ, NOS and heparin in combating infection has been addressed. The importance of vitamin C and D supplements as adjunct therapies in the prevention of SARS-CoV2 virus infection have also been summarized.

Plasma levels of the proprotein convertase furin and incidence of diabetes and mortality

BACKGROUND

Diabetes mellitus is linked to premature mortality of virtually all causes. Furin is a proprotein convertase broadly involved in the maintenance of cellular homeostasis; however, little is known about its role in the development of diabetes mellitus and risk of premature mortality.

OBJECTIVES

To test if fasting plasma concentration of furin is associated with the development of diabetes mellitus and mortality.

METHODS

Overnight fasted plasma furin levels were measured at baseline examination in 4678 individuals from the population-based prospective Malmö Diet and Cancer Study. We studied the relation of plasma furin levels with metabolic and hemodynamic traits. We used multivariable Cox proportional hazards models to investigate the association between baseline plasma furin levels and incidence of diabetes mellitus and mortality during 21.3-21.7 years follow-up.

RESULTS

An association was observed between quartiles of furin concentration at baseline and body mass index, blood pressure and plasma concentration of glucose, insulin, LDL and HDL cholesterol (|0.11| ≤ β ≤ |0.31|, P < 0.001). Plasma furin (hazard ratio [HR] per one standard deviation increment of furin) was predictive of future diabetes mellitus (727 events; HR = 1.24, CI = 1.14-1.36, P < 0.001) after adjustment for age, sex, body mass index, systolic and diastolic blood pressure, use of antihypertensive treatment, alcohol intake and fasting plasma level of glucose, insulin and lipoproteins cholesterol. Furin was also independently related to the risk of all-cause mortality (1229 events; HR = 1.12, CI = 1.05-1.19, P = 0.001) after full multivariable adjustment.

CONCLUSION

Individuals with high plasma furin concentration have a pronounced dysmetabolic phenotype and elevated risk of diabetes mellitus and premature mortality.

Influenza A Virus Cell Entry, Replication, Virion Assembly and Movement

Influenza viruses replicate within the nucleus of the host cell. This uncommon RNA virus trait provides influenza with the advantage of access to the nuclear machinery during replication. However, it also increases the complexity of the intracellular trafficking that is required for the viral components to establish a productive infection. The segmentation of the influenza genome makes these additional trafficking requirements especially challenging, as each viral RNA (vRNA) gene segment must navigate the network of cellular membrane barriers during the processes of entry and assembly. To accomplish this goal, influenza A viruses (IAVs) utilize a combination of viral and cellular mechanisms to coordinate the transport of their proteins and the eight vRNA gene segments in and out of the cell. The aim of this review is to present the current mechanistic understanding for how IAVs facilitate cell entry, replication, virion assembly, and intercellular movement, in an effort to highlight some of the unanswered questions regarding the coordination of the IAV infection process.

The proprotein convertase furin is required to maintain viability of alveolar rhabdomyosarcoma cells

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Success of current therapies is still limited and outcome is particularly poor for metastatic alveolar rhabdomyosarcoma (aRMS). We previously identified the proprotein convertase furin as potential target for specific drug delivery with RMS-homing peptides. Furin is a protease that converts inactive precursor proteins into bioactive proteins and peptides. In this study, we investigate the biological role of furin in aRMS progression in vitro and in vivo. Furin expression was confirmed in over 86% RMS biopsies in a tissue microarray (n=89). Inducible furin silencing in vitro led to significant impairment of cell viability and proliferation in all investigated aRMS cell lines, but not in MRC5 fibroblasts. Furthermore, the aRMS cell lines Rh3 and Rh4 revealed to be very sensitive to furin silencing, undergoing caspase-dependent cell death. Notably, furin silencing in vivo led to complete remission of established Rh4 tumors and to delayed growth in Rh30 tumors. Taken together, these findings identify furin as an important factor for aRMS progression and survival. Thus, we propose furin as a novel therapeutic target for treatment of aRMS.

The proprotein convertase furin in tumour progression

Proprotein convertases are proteases that have been implicated in the activation of a wide variety of proteins. These proteins are generally synthesised as precursor proteins and require limited proteolysis for conversion into their mature bioactive counterparts. Many of these proteins, including metalloproteases, growth factors and their receptors or adhesion molecules, have been shown to facilitate tumour formation and progression. Hence, this review will focus on the proprotein convertase furin and its role in cancer. The expression of furin has been confirmed in a large spectrum of cancers such as head and neck squamous cell carcinoma, breast cancer and rhabdomyosarcoma. Functional studies modulating furin activity uncovered its importance for the processing of many cancer-related substrates and strongly indicate that high furin activity promotes the malignant phenotype of cancer cells. In this review, we summarise the expression and function of furin in different cancer types, discuss its role in processing cancer-related proproteins and give examples of potential therapeutic approaches that take advantage of the proteolytic activity of furin in cancer cells.

T-cell-expressed proprotein convertase FURIN inhibits DMBA/TPA-induced skin cancer development

Proprotein convertases (PCSK) have a critical role in the body homeostasis as enzymes responsible for processing precursor proteins into their mature forms. FURIN, the first characterized member of the mammalian PCSK family, is overexpressed in multiple malignancies and the inhibition of its activity has been considered potential cancer treatment. FURIN has also an important function in the adaptive immunity, since its deficiency in T cells causes an impaired peripheral immune tolerance and accelerates immune responses. We addressed whether deleting FURIN from the immune cells would strengthen anticancer responses by subjecting mouse strains lacking FURIN from either T cells or macrophages and granulocytes to the DMBA/TPA two-stage skin carcinogenesis protocol. Unexpectedly, deficiency of FURIN in T cells resulted in enhanced and accelerated development of tumors, whereas FURIN deletion in macrophages and granulocytes had no effect. The epidermises of T-cell-specific FURIN deficient mice were significantly thicker with more proliferating Ki67+ cells. In contrast, there were no differences in the numbers of the T cells. The flow cytometric analyses of T-cell populations in skin draining lymph nodes showed that FURIN T-cell KO mice have an inherent upregulation of early activation marker CD69 as well as more CD4⁺CD25⁺Foxp3⁺ positive T regulatory cells. In the early phase of tumor promotion, T cells from the T-cell-specific FURIN knockout animals produced more interferon gamma, whereas at later stage the production of Th2- and Th17-type cytokines was more prominent than in wild-type controls. In conclusion, while PCSK inhibitors are promising therapeutics in cancer treatment, our results show that inhibiting FURIN specifically in T cells may promote squamous skin cancer development.

Targeting proprotein convertases in furin-rich lung cancer cells results in decreased in vitro and in vivo growth

Proprotein convertases (PCs) are serine proteases with an active role in the post-translational processing of numerous inactive proteins to active proteins including many substrates of paramount importance in cancer development and progression. Furin (PCSKC3), a well-studied member of this family, is overexpressed in numerous human and experimental malignancies. In the present communication, we treated two furin-overexpressing non-small cell carcinoma (NSCLC) cell lines (Calu-6 and HOP-62) with the PC inhibitor CMK (Decanoyl-Arg-Val-Lys-Arg-chloromethylketone). This resulted in a diminished IGF-1R processing and a simultaneous decrease in cell proliferation of two NSCLC lines. Similarly, growth of subcutaneous xenografts of both cell lines, were partially inhibited by an in vivo treatment with the same drug. These observations point to a potential role of PC inhibitors in cancer therapy. © 2016 Wiley Periodicals, Inc.

The Proprotein Convertase Furin Contributes to Rhabdomyosarcoma Malignancy by Promoting Vascularization, Migration and Invasion

The proprotein convertase (PC) furin cleaves precursor proteins, an important step in the activation of many cancer-associated proteins. Substrates of furin and furin-like PCs play a role in proliferation, metastasis and invasion. Some of them are involved in the progression of the pediatric soft tissue sarcoma rhabdomyosarcoma (RMS). In this study, we show that PCs, and in particular furin, are expressed in RMS cell lines. To investigate the functional role of furin, we generated RMS cell lines with modulated furin activity. Silencing or stable inhibition of furin delayed tumor growth in Rh30 and RD xenografts in vivo, and was correlated with lower microvessel density. Reduced furin activity also decreased migration and invasion abilities in vitro, and inhibition of furin in RMS cells diminished processing of IGF1R, VEGF-C, PDGF-B and MT1-MMP, leading to lower levels of mature proteins. Furthermore, we found that furin activity is required for proper IGF signaling in RMS cells, as furin silencing resulted in reduced phosphorylation of Akt upon IGF1 stimulation. Taken together, our results suggest that furin plays an important role in the malignant phenotype of RMS cells by activating proteins involved in tumor growth and vascularization, metastasis and invasion.

Protection of the Furin Cleavage Site in Low-Toxicity Immunotoxins Based on Pseudomonas Exotoxin A

Recombinant immunotoxins (RITs) are fusions of an Fv-based targeting moiety and a toxin. Pseudomonas exotoxin A (PE) has been used to make several immunotoxins that have been evaluated in clinical trials. Immunogenicity of the bacterial toxin and off-target toxicity have limited the efficacy of these immunotoxins. To address these issues, we have previously made RITs in which the Fv is connected to domain III (PE24) by a furin cleavage site (FCS), thereby removing unneeded sequences of domain II. However, the PE24 containing RITs do not contain the naturally occurring disulfide bond around the furin cleavage sequence, because it was removed when domain II was deleted. This could potentially allow PE24 containing immunotoxins to be cleaved and inactivated before internalization by cell surface furin or other proteases in the blood stream or tumor microenvironment. Here, we describe five new RITs in which a disulfide bond is engineered to protect the FCS. The most active of these, SS1-Fab-DS3-PE24, shows a longer serum half-life than an RIT without the disulfide bond and has the same anti-tumor activity, despite being less cytotoxic in vitro. These results have significance for the production of de-immunized, low toxicity, PE24-based immunotoxins with a longer serum half-life.

Liver-Specific Inactivation of the Proprotein Convertase FURIN Leads to Increased Hepatocellular Carcinoma Growth

Proprotein convertases are subtilisin-like serine endoproteases that cleave and hence activate a variety of proproteins, including growth factors, receptors, metalloproteases, and extracellular matrix proteins. Therefore, it has been suggested that inhibition of the ubiquitously expressed proprotein convertase FURIN might be a good therapeutic strategy for several tumor types. Whether this is also the case for hepatocellular carcinoma (HCC) is currently not clear. In a mouse model for HCC expression of Furin was not altered in the tumors, while those of PC7, PC5/6, and PACE4 significantly decreased, at least at some time points. To investigate the impact of Furin inhibition on the development and progression of HCC in this model, Furin was genetically ablated in the liver. Furin inactivation resulted in an increased tumor mass after 5 weeks. This was not caused by decreased apoptosis, since no differences in the apoptosis index could be observed. However, it could at least partially be explained by increased hepatocyte proliferation at 5 weeks. The tumors of the Furin knockout mice were histologically similar to those in wild type mice. In conclusion, liver-specific Furin inhibition in HCC enhances the tumor formation and will not be a good therapeutic strategy for this tumor type.

Identification of potent and compartment-selective small molecule furin inhibitors using cell-based assays

The proprotein convertase furin is implicated in a variety of pathogenic processes such as bacterial toxin activation, viral propagation, and cancer. Several groups have identified non-peptide compounds with high inhibitory potency against furin in vitro, although their efficacy in various cell-based assays is largely unknown. In this study we show that certain guanidinylated 2,5-dideoxystreptamine derivatives exhibit interesting ex vivo properties. Compound 1b (1,1'-(4-((2,4-diguanidino-5-(4-guanidinophenoxy)cyclohexyl)oxy)-1,3-phenylene)diguanidine) is a potent and cell-permeable inhibitor of cellular furin, since it was able to retard tumor cell migration, block release of a Golgi reporter, and protect cells against Bacillus anthracis (anthrax) and Pseudomonas aeruginosa intoxication, with no evident cell toxicity. Other compounds based on the 2,5-dideoxystreptamine scaffold, such as compound 1g (1,1'-(4,6-bis(4-guanidinophenoxy)cyclohexane-1,3-diyl)diguanidine) also efficiently protected cells against anthrax, but displayed only moderate protection against Pseudomonas exotoxin A and did not inhibit cell migration, suggesting poor cell permeability. Certain bis-guanidinophenyl ether derivatives such as 2f (1,3-bis(2,4-diguanidinophenoxy) benzene) exhibited micromolar potency against furin in vitro, low cell toxicity, and highly efficient protection against anthrax toxin; this compound only slightly inhibited intracellular furin. Thus, compounds 1g and 2f both represent potent furin inhibitors at the cell surface with low intracellular inhibitory action, and these particular compounds might therefore be of preferred therapeutic interest in the treatment of certain bacterial and viral infections.

Analysis of cathepsin and furin proteolytic enzymes involved in viral fusion protein activation in cells of the bat reservoir host

Bats of different species play a major role in the emergence and transmission of highly pathogenic viruses including Ebola virus, SARS-like coronavirus and the henipaviruses. These viruses require proteolytic activation of surface envelope glycoproteins needed for entry, and cellular cathepsins have been shown to be involved in proteolysis of glycoproteins from these distinct virus families. Very little is currently known about the available proteases in bats. To determine whether the utilization of cathepsins by bat-borne viruses is related to the nature of proteases in their natural hosts, we examined proteolytic processing of several viral fusion proteins in cells derived from two fruit bat species, Pteropus alecto and Rousettus aegyptiacus. Our work shows that fruit bat cells have homologs of cathepsin and furin proteases capable of cleaving and activating both the cathepsin-dependent Hendra virus F and the furin-dependent parainfluenza virus 5 F proteins. Sequence analysis comparing Pteropus alecto furin and cathepsin L to proteases from other mammalian species showed a high degree of conservation; however significant amino acid variation occurs at the C-terminus of Pteropus alecto furin. Further analysis of furin-like proteases from fruit bats revealed that these proteases are catalytically active and resemble other mammalian furins in their response to a potent furin inhibitor. However, kinetic analysis suggests that differences may exist in the cellular localization of furin between different species. Collectively, these results indicate that the unusual role of cathepsin proteases in the life cycle of bat-borne viruses is not due to the lack of active furin-like proteases in these natural reservoir species; however, differences may exist between furin proteases present in fruit bats compared to furins in other mammalian species, and these differences may impact protease usage for viral glycoprotein processing.

The cAMP-responsive element binding protein (CREB) transcription factor regulates furin expression during human trophoblast syncytialization

INTRODUCTION

The multinucleated syncytiotrophoblast is formed and maintained by cytotrophoblast cell fusion and serves multiple functions to ensure a successful pregnancy. We have previously reported that the proprotein convertase furin is required for trophoblast syncytialization by processing type 1 insulin-like growth factor receptor (IGF1R).

METHODS

Utilizing trophoblast cell fusion models including induced fusion of choriocarcinoma BeWo cells and spontaneous fusion of primary cultured term cytotrophoblast cells, the expression of furin was evaluated by quantitative real-time PCR, Western blotting and immunofluorescence. The key transcription factor regulating the FUR gene promoter and critical responsive elements were identified by luciferase reporter assays, truncated mutants analysis, site-directed mutagenesis and ChIP.

RESULTS

We demonstrated that the levels of FUR mRNA were significantly stimulated by cAMP/PKA signaling pathway during spontaneous fusion of cytotrophoblast cells and forskolin-induced fusion of BeWo cells. cAMP-responsive element binding protein (CREB) was proven to be the key transcription factor which regulated the FUR P1 promoter during forskolin-induced BeWo cell fusion, and two critical cAMP-responsive elements (CREs) in the P1 promoter were further identified. Finally, we showed that CREB mediated endogenous furin activation and that CREB siRNA attenuated forskolin-induced furin expression and cell fusion in BeWo cells.

DISCUSSION

This provides the first evidence of the upstream regulator of furin during trophoblast cell fusion.

CONCLUSIONS

The above results suggest that the FUR transcription is activated by CREB-dependent stimulation of the FUR P1 promoter during human trophoblast syncytialization.

Control of spontaneous ovarian tumors by CD8+ T cells through NKG2D-targeted delivery of antigenic peptide

Background

There is an urgent need to develop targeted therapies for the control of advanced stage ovarian cancer because it is the most deadly gynecologic cancer. Antigen-specific immunotherapy is a promising approach because of the potential of the immune system to specifically target tumors without the toxicity associated with traditional chemoradiation. However, one of the major limitations for antigen-specific cancer immunotherapy is the pre-existing immune tolerance against endogenous targeted tumor antigens that frequently evolves during carcinogenesis. Here, we described the creation of a therapeutic agent comprised of a tumor-homing module fused to a functional domain capable of selectively rendering tumor cells sensitive to foreign antigen-specific CD8+ T cell-mediated immune attack, thereby circumventing many aspects of immune tolerance. The tumor-homing module, NKG2D, specifically binds to NKG2D ligand that is commonly overexpressed in ovarian tumors. The functional domain is comprised of the Fc portion of IgG2a protein and foreign immunogenic CD8⁺ T cell epitope flanked by furin cleavage sites (R), which can be recognized and cleaved by furin that is highly expressed in the tumor microenvironment.

Results

We show that this therapeutic chimeric protein specifically loaded antigenic epitope onto the surface of NKG2D ligand-expressing ovarian tumor cells, rendering ovarian tumors susceptible to antigen-specific CTL-mediated killing in vitro. Furthermore, we show that intraperitoneal administration of our therapeutic chimeric protein followed by adoptive transfer of antigen-specific CD8+ T cells generates potent antitumor effects and significant accumulation of antigen-specific CD8+ T cells in the tumor loci.

Conclusions

Our findings have promise for bypassing immune tolerance to enhance cancer immunotherapy.

Activatable oligomerizable imaging agents for photoacoustic imaging of furin-like activity in living subjects

Photoacoustic (PA) imaging is continuing to be applied for physiological imaging and more recently for molecular imaging of living subjects. Owing to its high spatial resolution in deep tissues, PA imaging holds great potential for biomedical applications and molecular diagnostics. There is however a lack of probes for targeted PA imaging, especially in the area of enzyme-activatable probes. Here we introduce a molecular probe, which upon proteolytic processing is retained at the site of enzyme activity and provides PA contrast. The probe oligomerizes via a condensation reaction and accumulates in cells and tumors that express the protease. We demonstrate that this probe reports furin and furin-like activity in cells and tumor models by generating a significantly higher PA signal relative to furin-deficient and nontarget controls. This probe could report enzyme activity in living subjects at depths significantly greater than fluorescence imaging probes and has potential for molecular imaging in deep tumors.

Biological significance of a thyroid hormone-regulated secretome

The thyroid hormone, 3,3,5-triiodo-L-thyronine (T3), modulates several physiological processes, including cellular growth, differentiation, metabolism and proliferation, via interactions with thyroid hormone response elements (TREs) in the regulatory regions of target genes. Several intracellular and extracellular protein candidates are regulated by T3. Moreover, T3-regulated secreted proteins participate in physiological processes or cellular transformation. T3 has been employed as a marker in several disorders, such as cardiovascular disorder in chronic kidney disease, as well as diseases of the liver, immune system, endocrine hormone metabolism and coronary artery. Our group subsequently showed that T3 regulates several tumor-related secretory proteins, leading to cancer progression via alterations in extracellular matrix proteases and tumor-associated signaling pathways in hepatocellular carcinomas. Therefore, elucidation of T3/thyroid hormone receptor-regulated secretory proteins and their underlying mechanisms in cancers should facilitate the identification of novel therapeutic targets. This review provides a detailed summary on the known secretory proteins regulated by T3 and their physiological significance. This article is part of a Special Issue entitled: An Updated Secretome.

Alterations in gene expression of proprotein convertases in human lung cancer have a limited number of scenarios

Proprotein convertases (PCs) is a protein family which includes nine highly specific subtilisin-like serine endopeptidases in mammals. The system of PCs is involved in carcinogenesis and levels of PC mRNAs alter in cancer, which suggests expression status of PCs as a possible marker for cancer typing and prognosis. The goal of this work was to assess the information value of expression profiling of PC genes. Quantitative polymerase chain reaction was used for the first time to analyze mRNA levels of all PC genes as well as matrix metalloproteinase genes MMP2 and MMP14, which are substrates of PCs, in 30 matched pairs of samples of human lung cancer tumor and adjacent tissues without pathology. Significant changes in the expression of PCs have been revealed in tumor tissues: increased FURIN mRNA level (p<0.00005) and decreased mRNA levels of PCSK2 (p<0.007), PCSK5 (p<0.0002), PCSK7 (p<0.002), PCSK9 (p<0.00008), and MBTPS1 (p<0.00004) as well as a tendency to increase in the level of PCSK1 mRNA. Four distinct groups of samples have been identified by cluster analysis of the expression patterns of PC genes in tumor vs. normal tissue. Three of these groups covering 80% of samples feature a strong elevation in the expression of a single gene in cancer: FURIN, PCSK1, or PCSK6. Thus, the changes in the expression of PC genes have a limited number of scenarios, which may reflect different pathways of tumor development and cryptic features of tumors. This finding allows to consider the mRNAs of PC genes as potentially important tumor markers.

Targeted coating with antigenic peptide renders tumor cells susceptible to CD8(+) T cell-mediated killing

The potency of immunotherapies targeting endogenous tumor antigens is hindered by immune tolerance. We created a therapeutic agent comprised of a tumor-homing module fused to a functional domain capable of selectively rendering tumor cells sensitive to foreign antigen-specific CD8(+) T cell-mediated immune attack, and thereby, circumventing concerns for immune tolerance. The tumor-homing module is comprised of a single-chain variable fragment (scFv) that specifically binds to mesothelin (Meso), which is commonly overexpressed in human cancers, including ovarian tumors. The functional domain is comprised of the Fc portion of IgG2a protein and foreign immunogenic CD8(+) T cell epitope flanked by furin cleavage sites (R), which can be recognized and cleaved by furin that is highly expressed in the tumor microenvironment. We show that our therapeutic protein specifically loaded antigenic epitope onto the surface of mesothelin-expressing tumor cells, rendering tumors susceptible to antigen-specific cytotoxic CD8(+) T lymphocytes (CTL)-mediated killing in vitro and in vivo. Our findings have important implications for bypassing immune tolerance to enhance cancer immunotherapy.

Dysregulation of FURIN by prostaglandin-endoperoxide synthase 2 in lung epithelial NCI-H292 cells

Because proprotein convertases (PCSKs) activate growth factors and matrix metalloproteinase, these enzymes have been implicated in non-small cell lung cancer tumor progression and aggressiveness. Previous studies indicate that one PCSK member, FURIN is overexpressed in NSCLC, but little is known regarding the mechanisms driving PCSKs expression during malignant change. We sought to determine whether prostaglandin-endoperoxide synthase 2 (prostaglandin G/H synthase and cyclooxygenase) (PTGS2) (aka COX2), whose expression is also frequently increased in NSCLC, differentially regulates PCSK expression and activity between normal (NHBE) and NSCLC epithelial cells (NCI-H292, NCI-H441, A549). NSCLC cells exhibit significantly greater cell-associated and secreted PCSK activity as compared with NHBE. The heightened activity is consistent with increased FURIN, PCSK4, and PCSK6 protein in the NCSLC cells. Inhibition of PTGS2 activity using NS-398 and siRNA decreased FURIN mRNA, protein, activity along with cell proliferation in NCI-H292 cells but not NHBE cells. NSCLC also expressed elevated levels of the transcription factor E2F1. When NCI-H292 cells were transfected with E2F1 siRNA, both PTGS2 expression and PCSK activity were attenuated, arguing a pivotal role for E2F1 in the differential regulation of PCSKs by PTGS2. Our results highlight a novel role for PTGS2 in NSCLC and may provide a mechanism, whereby PTGS2 inhibitors suppress lung cancer cell growth.

Transgenic overexpression of the proprotein convertase furin enhances skin tumor growth

Furin, one of the members of the family of proprotein convertases (PCs), ubiquitously expressed as a type I membrane-bound proteinase, activates several proteins that contribute to tumor progression. In vitro studies using cancer cell lines and clinical specimens demonstrated that furin processes important substrates such as insulin-like growth factor 1 receptor (IGF-1R) and transforming growth factor β, leading to increased tumor growth and progression. Despite the numerous studies associating furin with tumor development, its effects in preclinical models has not been comprehensively studied. In this study, we sought to determine the protumorigenic role of furin in vivo after a two-stage chemical carcinogenesis protocol in transgenic mice in which furin expression was targeted to the epidermal basal layer. We found that processing of the PC substrate IGF-1R and the proliferation rate of mouse epidermis was enhanced in transgenic mice when compared with their WT counterparts. Histopathologic diagnoses of the tumors demonstrated that furin transgenic mice (line F47) developed twice as many squamous carcinomas as the control, WT mice (P < .002). Similarly, tumors cells from transgenic mice were able to process PC substrates more efficiently than tumor cells from WT mice. Furthermore, furin expression resulted in a higher SCC volume in transgenic mice as well as an increase in the percentage of high-grade SCC, including poorly differentiated and spindle cell carcinomas. In conclusion, expression of furin in the basal layer of the epidermis increased tumor development and enhanced tumor growth, supporting the consideration of furin as a potential target for cancer treatment.

Furin overexpression suppresses tumor growth and predicts a better postoperative disease-free survival in hepatocellular carcinoma

Furin is a member of the pro-protein convertase family. It processes several growth regulatory proteins into their active forms, which are critical to tumor progression, metastasis, and angiogenesis. Furin over-expression could occur in liver cancer and a previous study showed that over-expression of furin promoted HepG2 cell invasion in tail vein xenograft models. However, the clinical relevance of furin expression in hepatocellular carcinoma (HCC) remained unknown. Surprisingly, in a postoperative survival analysis for HCC patients, it was found that the tumor/non-tumor (T/N) ratio of furin expression ≥ 3.5 in HCC tissues predicted a better postoperative disease-free survival (DFS) (P = 0.010; log-rank test). Furthermore, subcutaneous xenograft experiments demonstrated a significant suppression effect of tumor growth in the furin-overexpressed xenografts (Huh7-Furin) compared to the mock control. Administration of a synthetic furin inhibitor for inhibition of the pro-protein convertase activity, decanoyl-Arg-Val-Lys-Arg-chloromethylketone (decRVKR-CMK), to the Huh7-Furin xenograft bearing mice restored the repression effect of tumor growth. In contrast, administration of decRVKR-CMK to the mock Huh7 xenograft bearing mice showed no change in growth rate. In conclusion, furin overexpression inhibited HCC tumor growth in a subcutaneous xenograft model and predicted a better postoperative DFS in clinical analysis.

Proprotein convertases in post-menopausal endometrial cancer: distinctive regulation and non-invasive diagnosis

Proprotein convertases (PCs) play critical roles in cleaving precursor proteins (growth factors, hormones, receptors and adhesion molecules) for activation. PCs are implicated in a number of cellular functions, including oncogenesis. Endometrial cancer is the most common gynecological cancer in the developed world, but the involvement of PCs is unclear. To characterize the role of PCs in endometrial cancer, we assessed expression of seven PCs (PC1/3, PC2, PACE4, PC4, furin, PC5/6 and PC7) by RT-PCR in six well characterized endometrial cancer cell lines. Expression was variable in all lines, with furin being most consistently expressed in all cell lines tested. We next determined the cellular localization and expression levels of four ubiquitously expressed PCs (furin, PACE4, PC5/6 and PC7) in post-menopausal endometrial biopsies from control (n=7) and endometrial cancer patients (n=30) by immunohistochemistry. Furin increased in tumors, whereas PC5/6, PACE4 and PC7 expression was reduced with increasing cancer grades. Uterine lavage is a non-invasive source material for evaluating the endometrium. We thus assessed whether total PC activity was altered in uterine lavage of endometrial cancer patients (n=36) compared to controls (n=10). PC activity was detected in all uterine lavage samples, and significantly elevated in all grades of endometrial cancer. This study demonstrates a complex association between individual PCs and endometrial cancer. Importantly, we show that monitoring the total PC activity in uterine lavage may provide a rapid and non-invasive method for the diagnosis of endometrial cancer in postmenopausal women.

Endoprotease-mediated intracellular protein delivery using nanocapsules

Proteins possess distinct intracellular roles allowing them to have vast therapeutic applications. However, due to poor cellular permeability and fragility of most proteins, intracellular delivery of native, active proteins is challenging. We describe a biomimetic protein delivery vehicle which is degradable upon the digestion by furin, a ubiquitous intracellular protease, to release encapsulated cargos. Proteins were encapsulated in a nanosized matrix prepared with monomers and a bisacrylated peptide cross-linker which can be specifically recognized and cleaved by furin. Release of encapsulated protein was confirmed in a cell-free system upon proteolytic degradation of nanocapsules. In vitro cell culture studies demonstrated successful intracellular delivery of both nuclear and cytosolic proteins and confirmed the importance of furin-degradable construction for native protein release. This endoprotease-mediated intracellular delivery system may be extended to effectively deliver various biological therapeutics.

Furin Functions as a Nonproteolytic Chaperone for Matrix Metalloproteinase-28: MMP-28 Propeptide Sequence Requirement

Although MMP-28 is involved in numerous important physiologic and pathologic conditions, the mechanisms of action of this secreted proteinase is not well understood. We now have demonstrated that furin serves as an intermolecular chaperone for MMP-28 secretion by interacting with the propeptide domain of MMP-28. Employing COS-1 cells transfected with MMP-28 cDNA, protein levels of MMP-28 were quite low in conditioned media as compared to cell lysates. Coexpression of MMP-28 with furin cDNA resulted in markedly enhanced MMP-28 secretion. Contrary to expectation, cleavage of MMP-28 at the furin consensus sequence did not occur and proteolytic inactive furin was equally effective in enhancing MMP-28 secretion. Furin and MMP-28 coimmunoprecipitated and were partially coimmunolocalized in the cytoplasm of transfected cells. Cotransfection with furin cDNA also enhanced MMP-28 induced cell migration. In conclusion, our data provide a novel mechanism for MMP-28 function in cells in which furin serves as an intermolecular chaperone.

Functional consequences of the subdomain organization of the sulfs

Sulf-1 and Sulf-2 are novel extracellular sulfatases that act on internal glucosamine 6-O-sulfate modifications within heparan sulfate proteoglycans and regulate their interactions with various signaling molecules, including Wnt ligands. Although the Sulfs are multidomain proteins, there is limited information available about how the subdomains contribute to their enzymatic and signaling activities. In this study, we found that both human Sulfs were synthesized as prepro-enzymes and cleaved by a furin-type proteinase to form disulfide-bond linked heterodimers of 75- and 50-kDa subunits. The mature Sulfs were secreted into conditioned medium, as well as retained on the cell membrane. Although the catalytic center resides in the N-terminal 75-kDa subunit, the C-terminal 50-kDa subunit was indispensable for both arylsufatase and glucosamine 6-O-sulfate-endosulfatase activity. We found that the hydrophilic regions of the Sulfs were essential for endosulfatase activity but not for arylsulfatase activity. Using Edman sequencing, we identified furin-type proteinase cleavage sites in Sulf-1 and Sulf-2. Deletion of these sequences resulted in uncleavable forms of Sulfs. The uncleavable Sulfs retained enzymatic activity. However, they were unable to potentiate Wnt signaling, which may be due to their defective localization into lipid rafts on the plasma membrane.

Thyroid hormone promotes cell invasion through activation of furin expression in human hepatoma cell lines

The objective of this study was to identify genes regulated by thyroid hormone (T(3)) and associated with tumor invasion. The gene encoding furin, as previously identified by cDNA microarray, is known to be up-regulated by T(3) treatment, and stimulated furin production occurs in thyroidectomized rats after administration of T(3). Presently, by using serial deletion of the promoter and EMSAs, the T(3) response element on the furin promoter was localized to the -6317/-6302 region. T(3)-mediated furin up-regulation was cooperative with TGF-beta because T(3) induction increased after Smad3/4 addition. Furthermore, the invasiveness of HepG2-thyroid hormone receptor (TR) cells was significantly increased by T(3) treatment, perhaps due to furin processing of matrix metalloproteinase-2 and -9. In addition, furin up-regulation either by stable overexpression or T(3) and/or TGF-beta induction was evident in severe-combined immune-deficient mice inoculated with HepG2-TRalpha1 cells. The HepG2-furin mice displayed a higher metastasis index and tumor size than HepG2-neo mice. Notably, the increased liver and lung tumor number or size in the hyperthyroid severe-combined immune-deficient mice as well as TGF-beta mice was attributed specifically to furin overexpression in the HepG2-TRalpha1 cells. Furthermore, this study demonstrated that furin overexpression in some types of hepatocellular carcinomas is TR dependent and might play a crucial role in the development of hepatocellular carcinoma. Thus, T(3) regulates furin gene expression via a novel mechanism or in cooperation with TGF-beta to enhance tumor metastasis in vitro and in vivo.

A small-molecule furin inhibitor inhibits cancer cell motility and invasiveness

Furin, a member the proprotein convertase (PC) family, processes inactive precursor proteins to functional proteins within the Golgi/trans-Golgi network secretory pathway. Furin and other PC family members (furin/PCs) activate proteins vital to proper physiological functioning, including growth factors and hormones, receptors, plasma proteins, and matrix metalloproteases (MMPs). Additionally, the expression and activity of furin/PC are necessary for processing substrates important for cell transformation and tumor progression, metastasis, and angiogenesis. Furin processing of the remodeling protease membrane type-1 matrix metalloproteinase (MT1-MMP) enhances cellular motility and invasiveness, contributing to aggression and metastatic potential cancer cells. Whereas overexpression and activity of furin/PC exacerbate the cancer phenotype, inhibition of its activity decreases or nullifies furin/PC-mediated effects, and thus, inhibition of furin may be a viable route to cancer therapy. Recently, we identified a small-molecule inhibitor of furin, named B3, by high-throughput screening with a K(i) and IC(50) of 12 microM. Here, we show that this cell-permeable, small-molecule compound inhibits furin-mediated cleavage of proMT1-MMP, resulting in decreased MMP-2 activation and cell motility in CHO cells expressing proMT1-MMP. Additionally, this molecule inhibited proMT1-MMP processing, complete MMP-2 maturation, and invasiveness of human fibrosarcoma cells (HT1080).

Inhibition of proprotein convertases: approaches to block squamous carcinoma development and progression

Most proprotein convertase (PC) inhibitors are compounds that act as competitive inhibitors. All of them contain the general cleavage motif RXK/RR that binds to the PC's active site impairing further interactions with their physiological substrates. The first inhibitors synthesized were the acyl-peptidyl-chloromethyl ketones that bind to the PC's active site through its peptidyl group and are able to transverse the plasma membrane due to the acyl moiety. For instance, one of the members of this family that exhibits reduced toxicity and has been widely used as an effective general PCs inhbitor is the derivative decanoyl-RVKR-chloromethylketone (CMK). Another approach to PC inhibition is based on proteins that contain either a natural or a bioengineered PC cleavage consensus site. In this context, the bioengineered serpin, alpha-1-antitrypsin Portland (alpha 1-PDX or PDX), proved to be a potent inhibitor of furin, the most studied of the cancer-related PCs. Both PDX and CMK were able to inhibit invasiveness of squamous cell carcinoma cell lines by blocking activation of cancer-associated PC substrates such as MT-MMPs, IGF-1R, and VEGF-C. A similar effect was produced by inhibiting PC-mediated processing using furin prosegment. PDX and CMK have also been assayed in vivo using skin carcinogenesis models. Newer promising small molecules and RNA interference approaches are also being developed to inhibit PCs.

The structure of the TGF-β latency associated peptide region determines the ability of the proprotein convertase furin to cleave TGF-βs

The TGF-beta family members are generated as latent pre-pro-polypeptides. The active mature peptides are cleaved from the latent forms by cellular proteases. TGF-beta 1, for instance, is predominantly processed by a substilisin-like proprotein convertase, furin. TGF-beta 2 has a consensus cleavage site for furin and therefore has been presumed to be cleaved by furin. However, TGF-beta 2 is often secreted as the latent form, which appears to be inconsistent with its postulated sensitivity to furin. We report here that both the regular (short) form of TGF-beta2 and its spliced variant with an additional exon (long form) are insensitive to furin. NIH 3T3 and CHO cells were transfected with expression vectors containing the short or long form of TGF-beta 2 or a chimeric TGF-beta consisting of the TGF-beta1 LAP region, the TGF-beta 2 cleavage site and the TGF-beta 2 mature peptide. The constructs included a c-myc epitope tag in the N-terminal region of the mature peptide. The TGF-betas produced by the transfected cells were analyzed with Western blots and immunocytochemistry. The intracellular proteins harvested from these cells were incubated with furin. Furin only inefficiently cleaved both the long and short forms of TGF-beta 2, but efficiently processed the chimeric TGF-beta. This indicates that the insensitivity of both forms of TGF-beta 2 to furin is a consequence of the tertiary structure of their LAP regions rather than their cleavage site. This differential processing of TGF-beta1 and -beta 2 may be part of the mechanism that generates isoform-specific functions of the TGF-betas.

Correlation of serpin–protease expression by comparative analysis of real-time PCR profiling data

Imbalanced protease activity has long been recognized in the progression of disease states such as cancer and inflammation. Serpins, the largest family of endogenous protease inhibitors, target a wide variety of serine and cysteine proteases and play a role in a number of physiological and pathological states. The expression profiles of 20 serpins and 105 serine and cysteine proteases were determined across a panel of normal and diseased human tissues. In general, expression of serpins was highly restricted in both normal and diseased tissues, suggesting defined physiological roles for these protease inhibitors. A high correlation in expression for a particular serpin-protease pair in healthy tissues was often predictive of a biological interaction. The most striking finding was the dramatic change observed in the regulation of expression between proteases and their cognate inhibitors in diseased tissues. The loss of regulated serpin-protease matched expression may underlie the imbalanced protease activity observed in pathological states.

Endo/exo-proteolysis in neoplastic progression and metastasis

Biological control of individual cells, organs, and organisms is achieved through interplay of a host of specific interactions that involve various peptidic molecules as modulators or effectors. In tumor cells, these processes may result in uncontrolled growth as a consequence of autocrine and/or paracrine actions. In recent years, growing evidence has accumulated for the important role of proprotein convertases (PCs) and peptide alpha-amidation enzymes in these processes. The widespread belief that these enzymes are involved in the major features of tumor progression, namely, invasiveness and metastasis, has taken place because of their capacity to process and activate many protein precursors involved in the neoplastic progression and metastasis. This includes degrading extracellular matrix proteases, growth promoting factors, and adhesion molecules. Usually, when the processing of these precursor proteins is achieved by one or more of the known PC family members within the general motif (K/R)-(X)n-(K/R) downward arrow, where n=0, 2, 4, or 6, and X, any amino acid except Cys, the accomplishment of the maturation of these molecules is attained by various posttranslational modifications, including the carboxy-terminal alpha-amidation. This review article summarizes recent findings on the role of these enzymatic systems in multiple cellular functions that impact on the invasive/metastatic potential of cancer cells and highlight the potential use of their inhibitors in the treatment of multiple cancers.

Furin Directly Cleaves proMMP-2 in the trans-Golgi Network Resulting in a Nonfunctioning Proteinase

Proprotein convertases play an important role in tumorigenesis and invasiveness. Here, we report that a dibasic amino acid convertase, furin, directly cleaves proMMP-2 within the trans-Golgi network leading to an inactive form of matrix metalloproteinase-2 (MMP-2). Co-transfection of COS-1 cells with both proMMP-2 and furin cDNAs resulted in the cleavage of the N-terminal propeptide of proMMP-2. The molecular mass of cleaved MMP-2 (63 kDa), detected in both cell lysates and conditioned medium, is between the intermediate and fully activated forms of MMP-2 induced by membrane type 1-MMP. Furin-cleaved MMP-2 does not possess proteolytic activity as examined in a cell-free assay. Treatment of transfected cells with a furin inhibitor resulted in a dose-dependent inhibition of proMMP-2 cleavage; recombinant tissue inhibitor of metalloproteinase-2, which binds to the active site of membrane type 1-MMP, had no inhibitory effect. Site-directed mutagenesis of amino acids in the furin consensus recognition motif of proMMP-2(R69KPR72) prevented propeptide cleavage, thereby identifying the scissile bond and characterizing the basic amino acids required for cleavage. Other experimental observations were consistent with intracellular furin cleavage of proMMP-2 in the trans-Golgi network. The furin cleavage site in other proMMPs was examined. MMP-3, which contains the RXXR furin consensus sequence, was cleaved in furin co-transfected cells, whereas MMP-1, which lacks an RXXR consensus sequence, was not cleaved. In conclusion, we report the novel observation that furin can directly cleave the RXXR amino acid sequence in the propeptide domain of proMMP-2 leading to inactivation of the enzyme.

Simultaneous expression of furin and vascular endothelial growth factor in human oral tongue squamous cell carcinoma progression

PURPOSE

Squamous cell carcinoma (SCC) of the tongue is a common malignancy of the oral cavity. Furin convertase activates several precursor matrix metalloproteinases involved in the degradation of the extracellular matrix. The pattern of expression of furin and vascular endothelial growth factor-C (VEGF-C), two key molecules in neoplasm development, was examined during the progression from normal epithelium to invasive SCC.

EXPERIMENTAL DESIGN

We evaluated furin and VEGF-C expression and microvessel density (MVD) by immunohistochemistry in human tongue sections harboring normal epithelium, dysplastic epithelium, and/or SCC. Sections from 46 glossectomy specimens were assessed for furin expression. A selected group of 15 cases, each containing normal epithelium, precursor lesions, and invasive SCC, were further studied for furin and VEGF-C expression and MVD quantification. We also evaluated the pattern of furin expression and VEGF-C processing by Western blot analysis in three SCC cell lines with different degrees of aggressiveness.

RESULTS

Furin and VEGF-C expression was notably higher in most precursor lesions and SCCs than in normal epithelia. Approximately 60% (n = 26) and 100% (n = 15) of the normal epithelia showed low-intensity staining for furin and VEGF-C, respectively. Intense staining for furin and VEGF-C was detected in approximately 80% (n = 34) and 100% (n = 15) of the SCCs, respectively. A significant correlation was seen between the expression of these two markers (Spearman's test, P < 0.00002). We found a statistically significant increase in MVD when either dysplasia (432 +/- 19.06; P < 0.05) or SCC (546 +/- 17.24) was compared with normal epithelium (315 +/- 17.27; P < 0.0001). SCC71, the most aggressive cell line analyzed, was the one with the highest furin expression. This cell line totally processed the VEGF-C proform, whereas the less aggressive line SCC9, exhibiting the least furin expression, did not. SCC15, of intermediate aggressiveness and furin expression, showed intermediate pro-VEGF-C processing.

CONCLUSIONS

These findings suggest that furin is a useful marker of tumor progression and is responsible for VEGF-C processing. This in turn would enhance angiogenesis, leading to increased MVD associated with preinvasive and invasive neoplasia.

BCL-2-induced glioma cell invasiveness depends on furin-like proteases

Migration and invasion are prerequisites for the neoplastic phenotype of malignant glioma. Ectopic expression of BCL-2 enhances migration and invasion of glioma cells and promotes their synthesis of transforming growth factor-beta2 (TGF-beta2). We here report that BCL-2-expressing cells show enhanced expression and activity of the proprotein convertase, furin, which processes metalloproteinases (MMP) and TGF-beta. Consistent with a biological role for a BCL-2-dependent increase in furin-like protease (FLP) activity, BCL-2-expressing cells exhibit enhanced MMP activity. Both a pseudosubstrate furin inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethylketone (dec-RVKR-cmk), or alpha 1-anti-trypsin Portland (PDX), a recombinant furin-inhibitory protein, suppress constitutive and BCL-2-mediated MMP activity and invasion. This inhibition is not overcome by TGF-beta or hepatocyte growth factor (HGF). A neutralizing TGF-beta antibody attenuates, but not abrogates, the invasive properties conferred by exogenous expression of BCL-2, whereas the MMP inhibitor o-phenantroline (o-PA) abolishes the pro-invasive action of BCL-2. Exogenous HGF results in enhanced, and expression of dominant-negative ezrin in reduced, FLP activity, and dec-RVKR-cmk blunts the HGF-induced expression of mature TGF-beta2. Consequently, HGF and BCL-2 family proteins use a furin-dependent pathway to promote invasion via TGF-beta and MMP in human malignant glioma cells and the pro-invasive properties of TGF-beta require furin- dependent MMP activity.

Role of subtilisin-like convertases in cadherin processing or the conundrum to stall cadherin function by convertase inhibitors in cancer therapy

Cadherins are a family of intercellular adhesion receptors. Produced as inactive precursors, they become functional adhesion molecules after proteolytic cleavage by subtilisin-like pro-protein convertases (PCs). Owing to their activation and assembly into multiprotein adhesion complexes at sites of cell contacts, adhesion-competent cadherins are prerequisite for tissue integrity. In recent years evidence has accumulated that intercellular junctions not only provide mechanical linkage, but in addition are potent modulators of signalling cascades. This infers a biological role to intercellular adhesion complexes that is significantly more complex and powerful. Currently, the broad implications of disturbances in somatic tissue adhesion components are only just beginning to emerge. Prominent examples of adhesion defects include autoimmune diseases, or tumour invasion and metastasis and malignant transformation. This review reports on our current knowledge of cadherin function and their maturation by pro-protein convertases, and puts special emphasis on the consequences of pro-protein convertase inhibition for epithelial tissue homeostasis.

Hypoxia-enhanced expression of the proprotein convertase furin is mediated by hypoxia-inducible factor-1: impact on the bioactivation of proproteins

Hypoxia is a common tumorigenesis enhancer, mostly owing to its impact on gene expression of many angiogenic and invasion-related mediators, some of which are natural substrates for the proprotein convertase furin. Analysis of furin promoters revealed the presence of putative binding sites for hypoxia-inducible factor-1 (HIF-1), a transcription complex that plays a pivotal role in cellular adaptation to hypoxia. In fact, we demonstrate herein that the levels of fur mRNA, encoding furin, are remarkably increased upon hypoxic challenge. Cotransfection of a HIF-1alpha dominant negative form in wild-type (WT) cells or transfection of a furin promoter-reporter gene in HIF-1-deficient cells indicated the requirement of HIF-1 for furin promoter activation by hypoxia. Direct HIF-1 action on the furin promoter was identified as a canonical hypoxia-responsive element site with enhancer capability. The hypoxic/HIF-1 regulation of furin correlated with an increased proteolytic activation of the substrates membrane-type 1 matrix metalloproteinase and transforming growth factor-beta1. Our findings unveil a new facet of the physiological consequences of hypoxia/HIF-1, through enhanced furin-induced proteolytic processing/activation of proproteins known to be involved in tumorigenesis.

A novel polymorphism of the gene encoding furin, a TGF-β1 activator, and the influence on cardiac allograft vasculopathy formation

BACKGROUND

Coronary vasculopathy (CV) is an important determinant of survival following cardiac transplantation. We have previously shown that G915C polymorphism of the Transforming Growth Factor-beta1 (TGF-beta1) gene strongly influences CV development. Furin is a proprotein convertase enzyme important in TGF-beta1 activation. We investigated for polymorphism within the promoter region of the gene for furin (fur). Allelic variation of the fur gene, in conjunction with TGF-beta1 polymorphism, was subsequently related to the development of CV.

METHODS AND RESULTS

The fur gene promoter region (position -1199 to +39) was analysed by SSCP and sequencing. A C/T single nucleotide substitution polymorphism at position -231* was identified. Using PCR the fur and TGFB1 genotypes were identified in 115 cardiac transplant recipients. CV was diagnosed at routine surveillance post-transplant coronary angiography. Fur polymorphism had no influence on vasculopathy development; median time to diagnosis, *C/C homozygotes, 2.27 years (2.10-4.32), *C/T heterozygotes 2.97 years (2.09-4.24), *T/T homozygotes 2.65 years (2.33-4.08), (P=0.95). Allelic variation did not influence Kaplan Meier actuarial analysis of disease onset (P=0.54). Ninety-three percent of recipients were high TGF-beta1 producers. We used fur polymorphism to substratify patients with the +915*G/G TGFB1 (high producing) allele. Fur polymorphism did not influence CV development within this TGF-beta1 high producer cohort, when analysed by time to first diagnosis and Kaplan Meier testing.

CONCLUSIONS

We have described a novel polymorphism at position -231* in the gene encoding furin. The fur -231* single nucleotide polymorphism in isolation, or in conjunction with TGFB1 polymorphism, is not useful as a genetic risk marker for cardiac transplant associated coronary vasculopathy.

Furin and prohormone convertase 1/3 are major convertases in the processing of mouse pro-growth hormone-releasing hormone

We investigated the proteolytic processing of mouse pro-GHRH [84 amino acids (aa)] by furin, PC1/3, PC2, and PC5/6A. We created six point mutations in the N- and C-terminal cleavage sites, RXXR decreased and RXRXXR decreased, respectively. The following results were obtained after transient transfection/cotransfection and metabolic pulse-chase labeling studies in several neuroendocrine cells. 1) Furin was the most efficient convertase in cleaving the N-terminal RXXR/RXRR site to generate intermediate I, 12-84aa, whereas PC1/3 was the most potent in processing the C-terminal RXRXXR site to yield mature GHRH, 12-53aa. 2) Both PC1/3 and PC5/6A also processed the N-terminal site but less efficiently than furin. 3) PC2 was much weaker in cleaving the C-terminal site relative to PC1/3 to generate mature GHRH. 4) The Q10R mutant was significantly more susceptible to furin cleavage at the N-terminal site than the wild-type pro-GHRH. And 5) the N- and C-terminal P1 Arg residues, R11 and R54, respectively, were essential for mature GHRH production. We also showed localization of the GHRH immunoreactive peptides in Golgi and secretory granules in neuroendocrine cells by an immunofluorescence assay. We conclude that the efficient production of mature GHRH from pro-GHRH is a stepwise process mediated predominantly by furin at the N-terminal cleavage site followed by PC1/3 at the C terminus.

Curbing activation: proprotein convertases in homeostasis and pathology

The proprotein convertases (PCs) are a seven-member family of endoproteases that activate proproteins by cleavage at basic motifs. Expression patterns for individual PCs vary widely, and all cells express several members. The list of substrates activated by PCs has grown to include neuropeptides, peptide hormones, growth and differentiation factors, receptors, enzymes, adhesion molecules, blood coagulation factors, plasma proteins, viral coat proteins, and bacterial toxins. It has become clear that the PC family plays a crucial role in a variety of physiological processes and is involved in the pathology of diseases such as cancer, viral infection, and Alzheimer's disease. Recent studies using PC inhibitors have demonstrated their potential as therapeutic targets. Despite the avalanche of in vitro data, the physiological role of individual PCs has remained largely elusive. Recently, however, knockout mouse models have been developed for furin, PC1, PC2, PC4, PC6B, LPC, and PACE4, and human patients with PC1 deficiency have been identified. The phenotypes range from undetectable to early embryonic lethality. The major lesson learned from these studies is that specific PC-substrate pairs do exist, but that there is substantial redundancy for the majority of substrates. To some extent, redundancy may be cell type and even species dependent.

Effects of adenovirus-mediated SV5 fusogenic glycoprotein expression on tumor cells

BACKGROUND

The fusogenic (F) membrane glycoprotein of the paramyxovirus SV5 allows virus to enter host cells and mediates fusion between neighboring cells, which leads to cell death. F glycoprotein is synthesized as an inactive precursor (F(0)) that is cleaved by cellular protease furine to form the active heterodimer F(1) + F(2). The active protein can induce syncytium formation in the absence of another integral glycoprotein (HN), a property that appears to be unique among paramyxoviruses.

METHODOLOGY

We constructed a non-replicative adenovirus to express SV5 F protein in tumor cells, and its fusion capacity was analyzed by fluorescent and confocal microscopy. Cell viability and bystander effect were compared with the thymidine kinase/ganciclovir suicide gene therapy. The structure of F-expressing cells was studied using electron microscopy.

RESULTS

F glycoprotein expression induced syncytium formation to a maximum at 72 h, after which syncytia progressively lost viability and detached. The cell membrane was disrupted while nuclear structure was preserved. Over-expression of SV5 F protein in tumor cells led to high cytotoxicity comparable with that associated with the thymidine kinase/ganciclovir. A potent bystander killing effect was detected until the ratio of F-transduced to non-transduced cells was 1 : 100.

CONCLUSIONS

These results indicate that the fusogenic glycoprotein of the paramyxovirus SV5 could be used to eliminate tumor cells and may encourage studies aimed at modifying its selectivity and combining its expression with other cytotoxic strategies to improve their efficacy.

Gene therapy of streptozotocin-induced diabetes by intramuscular delivery of modified preproinsulin genes

BACKGROUND

Despite improvements in insulin preparation and delivery, physiological normoglycemia is not easily achieved in diabetics. Therefore, there has been considerable interest in developing gene therapy approaches to supply insulin. We studied a nonviral muscle-based method of gene therapy and demonstrated that it could prevent hyperglycemia in murine streptozotocin (STZ)-induced diabetes.

METHODS

A plasmid encoding mouse furin-cleavable preproinsulin II cDNA (FI), or its B10-analogue (B10FI), and a plasmid encoding furin were coinjected into muscle of CD-1 mice, who were treated a day later with STZ to induce diabetes. Electroporation was applied to increase gene transfer. Blood glucose was measured in fed and fasting mice, and fasting plasma insulin was measured by radioimmunoassay. The form of insulin produced and the presence of C-peptide were analyzed by gel filtration chromatography.

RESULTS

A B10FI plasmid codelivered with a furin plasmid reduced fed and fasting blood glucose levels in STZ-treated diabetic mice. The (pro)insulin levels in plasma were increased by up to 70-fold versus blank plasmid-treated diabetic mice. The administration of FI with furin was less effective. (Pro)insulin levels were greatly increased by using two plasmids carrying different promoter elements (CMV and SV40). Insulin was identified in muscle cells by immunohistochemistry. In plasma, 40-70% of the (pro)insulin was processed to the mature form and free C-peptide was identified. Insulin gene-treated mice had improved growth rates and appeared healthier. A single injection of B10FI with SV40Furin DNA increased plasma (pro)insulin for at least 8 weeks and reduced fed blood glucose levels for 5 weeks and fasting levels for 8 weeks.

CONCLUSIONS

This is the first report that electroporation-enhanced intramuscular gene therapy with B10FI can prevent hyperglycemia in murine STZ-induced diabetes. Gene therapy using various routes and methods of furin-cleavable insulin gene delivery has been previously explored but, in muscle, results comparable to ours have not been reported.

Furin gene (fur) regulation in differentiating human megakaryoblastic Dami cells: involvement of the proximal GATA recognition motif in the P1 promoter and impact on the maturation of furin substrates

The convertase furin is involved in the maturation of key growth/aggregation mediators synthesized by the platelet producers, megakaryocytes, but the regulation of furin in these cells remains unknown. Computer-assisted search of the furin promoter sequence revealed multiple potential binding motifs for GATA-1, suggesting that furin is expressed and regulated in these cells. Using megakaryoblastic Dami cells, we observed that fur mRNA expression increased gradually on phorbol 12-myristate 13-acetate-induced differentiation, reaching maximum levels (8.3-fold increase) at 10 days. Transient transfections with P1, P1A, or P1B fur-LUC-promoter constructs revealed that in Dami cells, the P1 promoter is the strongest and the most sensitive to forced expression of GATA-1. Coexpression of GATA-1 and its comodulator, Friend of GATA-1 (FOG-1), resulted in a cooperative increase in P1 activity. Deletion analysis indicated that important GATA-1-regulated sequences are located in the most proximal region of the P1 promoter. Further analysis revealed 2 potential GATA-binding motifs at positions -66 and +62. Point mutation of each of the 2 motifs indicated that the intactness of the first GATA site is required for full basal and GATA-1-stimulated promoter activity. Finally, the inhibition of furin activity through gene transfer of the inhibitor alpha1-AT-PDX led to a block in maturation of the furin substrates transforming growth factor-beta1 and platelet-derived growth factor. Taken together, these results indicate that the most proximal GATA element in the P1 promoter is needed for fur gene expression in megakaryoblastic cells. They also suggest that proper regulation of the fur gene in megakaryocytes has an impact on the activation of furin substrates involved in megakaryocyte maturation and platelet functions.

Role of furin in delivery of a CTL epitope of an anthrax toxin-fusion protein

Anthrax toxin lethal factor (LF) in combination with anthrax toxin protective antigen (PA) was endocytosed and translocated to the cytosol of mammalian cells. Residues 1-255 of anthrax toxin lethal factor (LFn) was fused to a cytotoxic T lymphocyte (CTL) epitope of an influenza virus. For processing the toxins, PA must be cleaved into a 63-kDa fragment (PA63) by furin, which is a subtilisin-like processing endo-protease expressed by many eukaryotic cells. To test the ability of cells treated with the LFn fusion protein plus PA to deliver the epitope, CTL assay was performed. Two types of cell lines were identified, one was able to deliver CTL epitope while the other failed to efficiently deliver the epitope. To further elucidate the differences between these cells, the role of furin in these cells was examined. Disruption of the furin gene reduced its ability to deliver the CTL epitope. Furin expression in cells capable of efficiently delivering CTL epitope was quantitatively higher than in cells unable to deliver the epitope. The results suggest that furin plays a critical role in delivery of the CTL epitope of LFn fusion protein.