Identification of Novel Natural Substrates of Fibroblast Activation Protein-alpha by Differential Degradomics and Proteomics.

Fibroblast activation protein-alpha (FAP) is a cell-surface transmembrane-anchored dimeric protease. This unique, constitutively active serine protease has both dipeptidyl aminopeptidase and endopeptidase activities and can hydrolyze the post-proline bond. FAP expression is very low in adult organs but is upregulated by activated fibroblasts in sites of tissue remodeling, including fibrosis, atherosclerosis, arthritis and tumors. To identify the endogenous substrates of FAP, we immortalized primary mouse embryonic fibroblasts (MEFs) from FAP gene knockout embryos and then stably transduced them to express either enzymatically active or inactive FAP. The MEF secretomes were then analyzed using degradomic and proteomic techniques. Terminal amine isotopic labeling of substrates (TAILS)-based degradomics identified cleavage sites in collagens, many other extracellular matrix (ECM) and associated proteins, and lysyl oxidase-like-1, CXCL-5, CSF-1, and C1qT6, that were confirmed in vitro In addition, differential metabolic labeling coupled with quantitative proteomic analysis also implicated FAP in ECM-cell interactions, as well as with coagulation, metabolism and wound healing associated proteins. Plasma from FAP-deficient mice exhibited slower than wild-type clotting times. This study provides a significant expansion of the substrate repertoire of FAP and provides insight into the physiological and potential pathological roles of this enigmatic protease.

Circulating fibroblast activation protein activity and antigen levels correlate strongly when measured in liver disease and coronary heart disease.

BACKGROUND AND AIM: Circulating fibroblast activation protein (cFAP) is a constitutively active enzyme expressed by activated fibroblasts that has both dipeptidyl peptidase and endopeptidase activities. We aimed to assess the correlation between cFAP activity and antigen levels and to compare variations in levels. METHODS: In plasma of 465 control individuals, 368 patients with coronary heart disease (CHD) and 102 hepatitis C virus (HCV) infected patients with severe liver disease before and after liver transplant, cFAP activity levels were measured with a newly developed cFAP activity assay. In the same samples, cFAP antigen levels were measured using a commercially available cFAP ELISA. Correlation analyses between activity and antigen levels were performed by calculating Pearson's correlation coefficient (ρ). Additionally, normal ranges, determinants and differences between cohorts and between anticoagulants were investigated. RESULTS: cFAP activity and antigen levels significantly correlated in controls (ρ: 0.660, p<0.001) and in CHD patients (ρ: 0.709, p<0.001). cFAP activity and antigen levels in the HCV cohort were significantly lower in the samples taken after liver transplantation (p<0.001) and normalized toward levels of healthy individuals. Furthermore, cFAP activity and antigen levels were higher in men and significantly associated with body mass index. Also, cFAP activity and antigen levels were higher in EDTA plasma as compared to the levels in citrated plasma from the same healthy individuals. CONCLUSIONS: For analyzing cFAP levels, either activity levels or antigen levels can be measured to investigate differences between individuals. However, it is of importance that blood samples are collected in the same anticoagulant.

Dipeptidyl peptidase 9 enzymatic activity influences the expression of neonatal metabolic genes.

The success of dipeptidyl peptidase 4 (DPP4) inhibition as a type 2 diabetes therapy has encouraged deeper examination of the post-proline DPP enzymes. DPP9 has been implicated in immunoregulation, disease pathogenesis and metabolism. The DPP9 enzyme-inactive (Dpp9 gene knock-in; Dpp9 gki) mouse displays neonatal lethality, suggesting that DPP9 enzyme activity is essential in neonatal development. Here we present gene expression patterns in these Dpp9 gki neonatal mice. Taqman PCR arrays and sequential qPCR assays on neonatal liver and gut revealed differential expression of genes involved in cell growth, innate immunity and metabolic pathways including long-chain-fatty-acid uptake and esterification, long-chain fatty acyl-CoA binding, trafficking and transport into mitochondria, lipoprotein metabolism, adipokine transport and gluconeogenesis in the Dpp9 gki mice compared to wild type. In a liver cell line, Dpp9 knockdown increased AMP-activated protein kinase phosphorylation, which suggests a potential mechanism. DPP9 protein levels in liver cells were altered by treatment with EGF, HGF, insulin or palmitate, suggesting potential natural DPP9 regulators. These gene expression analyses of a mouse strain deficient in DPP9 enzyme activity show, for the first time, that DPP9 enzyme activity regulates metabolic pathways in neonatal liver and gut.

Neuropeptide Y is a physiological substrate of fibroblast activation protein: Enzyme kinetics in blood plasma and expression of Y2R and Y5R in human liver cirrhosis and hepatocellular carcinoma.

Fibroblast activation protein (FAP) is a dipeptidyl peptidase (DPP) and endopeptidase that is weakly expressed in normal adult human tissues but is greatly up-regulated in activated mesenchymal cells of tumors and chronically injured tissue. The identities and locations of target substrates of FAP are poorly defined, in contrast to the related protease DPP4. This study is the first to characterize the physiological substrate repertoire of the DPP activity of endogenous FAP present in plasma. Four substrates, neuropeptide Y (NPY), peptide YY, B-type natriuretic peptide and substance P, were analyzed by mass spectrometry following proteolysis in human or mouse plasma, and by in vivo localization in human liver tissues with cirrhosis and hepatocellular carcinoma (HCC). NPY was the most efficiently cleaved substrate of both human and mouse FAP, whereas all four peptides were efficiently cleaved by endogenous DPP4, indicating that the in vivo degradomes of FAP and DPP4 differ. All detectable DPP-specific proteolysis and C-terminal processing of these neuropeptides was attributable to FAP and DPP4, and plasma kallikrein, respectively, highlighting their combined physiological significance in the regulation of these neuropeptides. In cirrhotic liver and HCC, NPY and its receptor Y2R, but not Y5R, were increased in hepatocytes near the parenchymal-stromal interface where there is an opportunity to interact with FAP expressed on nearby activated mesenchymal cells in the stroma. These novel findings provide insights into the substrate specificity of FAP, which differs greatly from DPP4, and reveal a potential function for FAP in neuropeptide regulation within liver and cancer biology.

Identification of novel dipeptidyl peptidase 9 substrates by two-dimensional differential in-gel electrophoresis.

Dipeptidyl peptidase 9 (DPP9) is a member of the S9B/DPPIV (DPP4) serine protease family, which cleaves N-terminal dipeptides at an Xaa-Pro consensus motif. Cytoplasmic DPP9 has roles in epidermal growth factor signalling and in antigen processing, whilst the role of the recently discovered nuclear form of DPP9 is unknown. Mice lacking DPP9 proteolytic activity die as neonates. We applied a modified 2D differential in-gel electrophoresis approach to identify novel DPP9 substrates, using mouse embryonic fibroblasts lacking endogenous DPP9 activity. A total of 111 potential new DPP9 substrates were identified, with nine proteins/peptides confirmed as DPP9 substrates by MALDI-TOF or immunoblotting. Moreover, we also identified the dipeptide Val-Ala as a consensus site for DPP9 cleavage that was not recognized by DPP8, suggesting different in vivo roles for these closely related enzymes. The relative kinetics for the cleavage of these nine candidate substrates by DPP9, DPP8 and DPP4 were determined. This is the first identification of DPP9 substrates from cells lacking endogenous DPP9 activity. These data greatly expand the potential roles of DPP9 and suggest different in vivo roles for DPP9 and DPP8.

Circulating dipeptidyl peptidase-4 activity correlates with measures of hepatocyte apoptosis and fibrosis in non-alcoholic fatty liver disease in type 2 diabetes mellitus and obesity: A dual cohort cross-sectional study.

BACKGROUND: Intrahepatic expression of dipeptidyl peptidase-4 (DPP4), and circulating DPP4 (cDPP4) levels and its enzymatic activity, are increased in non-alcoholic fatty liver disease (NAFLD) and in type 2 diabetes mellitus and/or obesity. DPP4 has been implicated as a causative factor in NAFLD progression but few studies have examined associations between cDPP4 activity and NAFLD severity in humans. This study aimed to examine the relationship of cDPP4 activity with measures of liver disease severity in NAFLD in subjects with diabetes and/or obesity. METHODS: cDPP4 was measured in 106 individuals with type 2 diabetes who had transient elastography (Cohort 1) and 145 individuals with morbid obesity who had liver biopsy (Cohort 2). Both cohorts had caspase-cleaved keratin-18 (ccK18) measured as a marker of apoptosis. RESULTS: Natural log increases in cDPP4 activity were associated with increasing quartiles of ccK18 (Cohorts 1 and 2) and with median liver stiffness ≥10.3 kPa (Cohort 1) and significant fibrosis (F ≥ 2) on liver biopsy (Cohort 2). CONCLUSIONS: In diabetes and/or obesity, cDPP4 activity is associated with current apoptosis and liver fibrosis. Given the pathogenic mechanisms by which DPP4 may progress NAFLD, measurement of cDPP4 activity may have utility to predict disease progression and DPP4 inhibition may improve liver histology over time.

Dipeptidyl peptidase 9 subcellular localization and a role in cell adhesion involving focal adhesion kinase and paxillin.

Dipeptidyl peptidase 9 (DPP9) is a ubiquitously expressed member of the DPP4 gene and protease family. Deciphering the biological functions of DPP9 and its roles in pathogenesis has implicated DPP9 in tumor biology, the immune response, apoptosis, intracellular epidermal growth factor-dependent signaling and cell adhesion and migration. We investigated the intracellular distribution of DPP9 chimeric fluorescent proteins and consequent functions of DPP9. We showed that while some DPP9 is associated with mitochondria, the strongest co-localization was with microtubules. Under steady state conditions, DPP9 was not seen at the plasma membrane, but upon stimulation with either phorbol 12-myristate 13-acetate or epidermal growth factor, some DPP9 re-distributed towards the ruffling membrane. DPP9 was seen at the leading edge of the migrating cell and co-localized with the focal adhesion proteins, integrin-ß1 and talin. DPP9 gene silencing and treatment with a DPP8/DPP9 specific inhibitor both reduced cell adhesion and migration. Expression of integrin-ß1 and talin was decreased in DPP9-deficient and DPP9-enzyme-inactive cells. There was a concomitant decrease in the phosphorylation of focal adhesion kinase and paxillin, indicating that DPP9 knockdown or enzyme inhibition suppressed the associated adhesion signaling pathway, causing impaired cell movement. These novel findings provide mechanistic insights into the regulatory role of DPP9 in cell movement, and may thus implicate DPP9 in tissue and tumor growth and metastasis.

Understanding fibroblast activation protein (FAP): substrates, activities, expression and targeting for cancer therapy.

Fibroblast activation protein (FAP) is best known for its heightened expression in tumour stroma. This atypical serine protease has both dipeptidyl peptidase and endopeptidase activities, cleaving substrates at a post-proline bond. FAP expression is difficult to detect in non-diseased adult organs, but is greatly upregulated in sites of tissue remodelling, which include liver fibrosis, lung fibrosis, atherosclerosis, arthritis, tumours and embryonic tissues. Due to its restricted expression pattern and dual enzymatic activities, FAP is emerging as a unique therapeutic target. However, methods to exploit and target this protease are advancing more rapidly than knowledge of the fundamental biology of FAP. This review highlights this imbalance, emphasising the need to better define the substrate repertoire and expression patterns of FAP to elucidate its role in biological and pathological processes.

Quantitation of fibroblast activation protein (FAP)-specific protease activity in mouse, baboon and human fluids and organs.

The protease fibroblast activation protein (FAP) is a specific marker of activated mesenchymal cells in tumour stroma and fibrotic liver. A specific, reliable FAP enzyme assay has been lacking. FAP's unique and restricted cleavage of the post proline bond was exploited to generate a new specific substrate to quantify FAP enzyme activity. This sensitive assay detected no FAP activity in any tissue or fluid of FAP gene knockout mice, thus confirming assay specificity. Circulating FAP activity was ∼20- and 1.3-fold less in baboon than in mouse and human plasma, respectively. Serum and plasma contained comparable FAP activity. In mice, the highest levels of FAP activity were in uterus, pancreas, submaxillary gland and skin, whereas the lowest levels were in brain, prostate, leukocytes and testis. Baboon organs high in FAP activity included skin, epididymis, bladder, colon, adipose tissue, nerve and tongue. FAP activity was greatly elevated in tumours and associated lymph nodes and in fungal-infected skin of unhealthy baboons. FAP activity was 14- to 18-fold greater in cirrhotic than in non-diseased human liver, and circulating FAP activity was almost doubled in alcoholic cirrhosis. Parallel DPP4 measurements concorded with the literature, except for the novel finding of high DPP4 activity in bile. The new FAP enzyme assay is the first to be thoroughly characterised and shows that FAP activity is measurable in most organs and at high levels in some. This new assay is a robust tool for specific quantitation of FAP enzyme activity in both preclinical and clinical samples, particularly liver fibrosis.

Advances in understanding the expression and function of dipeptidyl peptidase 8 and 9.

DPP8 and DPP9 are recently identified members of the dipeptidyl peptidase IV (DPPIV) enzyme family, which is characterized by the rare ability to cleave a post-proline bond two residues from the N-terminus of a substrate. DPP8 and DPP9 have unique cellular localization patterns, are ubiquitously expressed in tissues and cell lines, and evidence suggests important contributions to various biological processes including: cell behavior, cancer biology, disease pathogenesis, and immune responses. Importantly, functional differences between these two proteins have emerged, such as DPP8 may be more associated with gut inflammation whereas DPP9 is involved in antigen presentation and intracellular signaling. Similarly, the DPP9 connections with H-Ras and SUMO1, and its role in AKT1 pathway downregulation provide essential insights into the molecular mechanisms of DPP9 action. The recent discovery of novel natural substrates of DPP8 and DPP9 highlights the potential role of these proteases in energy metabolism and homeostasis. This review focuses on the recent progress made with these post-proline dipeptidyl peptidases and underscores their emerging importance.

Neuropeptide Y, B-type natriuretic peptide, substance P and peptide YY are novel substrates of fibroblast activation protein-α.

Fibroblast activation protein-α (FAP) is a cell surface-expressed and soluble enzyme of the prolyl oligopeptidase family, which includes dipeptidyl peptidase 4 (DPP4). FAP is not generally expressed in normal adult tissues, but is found at high levels in activated myofibroblasts and hepatic stellate cells in fibrosis and in stromal fibroblasts of epithelial tumours. FAP possesses a rare catalytic activity, hydrolysis of the post-proline bond two or more residues from the N-terminus of target substrates. α(2)-antiplasmin is an important physiological substrate of FAP endopeptidase activity. This study reports the first natural substrates of FAP dipeptidyl peptidase activity. Neuropeptide Y, B-type natriuretic peptide, substance P and peptide YY were the most efficiently hydrolysed substrates and the first hormone substrates of FAP to be identified. In addition, FAP slowly hydrolysed other hormone peptides, such as the incretins glucagon-like peptide-1 and glucose-dependent insulinotropic peptide, which are efficient DPP4 substrates. FAP showed negligible or no hydrolysis of eight chemokines that are readily hydrolysed by DPP4. This novel identification of FAP substrates furthers our understanding of this unique protease by indicating potential roles in cardiac function and neurobiology.

Sequence diversity in the A domain of Staphylococcus aureus fibronectin-binding protein A.

BACKGROUND: Fibronectin-binding protein A (FnBPA) mediates adhesion of Staphylococcus aureus to fibronectin, fibrinogen and elastin. We previously reported that S. aureus strain P1 encodes an FnBPA protein where the fibrinogen/elastin-binding domain (A domain) is substantially divergent in amino acid sequence from the archetypal FnBPA of S. aureus NCTC8325, and that these variations created differences in antigenicity. In this study strains from multilocus sequence types (MLST) that spanned the genetic diversity of S.aureus were examined to determine the extent of FnBPA A domain variation within the S. aureus population and its effect on ligand binding and immuno-crossreactivity. RESULTS: Seven different isotype forms (I - VII) of the FnBPA A domain were identified which were between 66 to 76% identical in amino acid sequence in any pair-wise alignment. The fnbA allelic variants in strains of different multilocus sequence type were identified by DNA hybridization using probes specific for sequences encoding the highly divergent N3 sub-domain of different isotypes. Several isotypes were not restricted to specific clones or clonal complexes but were more widely distributed. It is highly likely that certain fnbA genes have been transferred horizontally. Residues lining the putative ligand-binding trench were conserved, which is consistent with the ability of each A domain isotype to bind immobilized fibrinogen and elastin by the dock-latch-lock mechanism. Variant amino acid residues were mapped on a three-dimensional model of the FnBPA A domain and were predicted to be surface-exposed. Polyclonal antibodies raised against the recombinant isotype I A domain bound that protein with a 4 - 7 fold higher apparent affinity compared to the A domains of isotypes II - VII, while some monoclonal antibodies generated against the isotype I A domain showed reduced or no binding to the other isotypes. CONCLUSION: The FnBPA A domain occurs in at least 7 different isotypes which differ antigenically and exhibit limited immuno-crossreactivity, yet retain their ligand-binding functions. Antigenic variation of the FnBPA A domain may aid S. aureus to evade the host's immune responses. These findings have implications for the development of vaccines or immunotherapeutics that target FnBPA.

The N-terminal A domain of Staphylococcus aureus fibronectin-binding protein A binds to tropoelastin.

Staphylococcus aureus is an important human pathogen. Its virulence factors include a variety of MSCRAMMs (microbial surface component recognizing adhesive matrix molecules), each capable of binding specifically to the host extracellular matrix. The fibronectin-binding protein, FnBPA, has been shown previously to bind immobilized fibronectin, fibrinogen, and alpha-elastin peptides. Here we show that region A of FnBPA (rAFnBPA) binds to recombinant human tropoelastin. Binding occurs to three separate truncates of tropoelastin, encompassing domains 2-18, 17-27, and 27-36, signifying that the interaction occurs at multiple sites. The greatest affinity was for the N-terminal truncate. We observed a pH dependency for the rAFnBPA-tropoelastin interaction with strong, nonsaturable binding at low pH. The interaction ceased at higher pH. These data support a model of surface-surface interactions between the negative charges present on rAFnBPA and the positive lysines of tropoelastin. A protein lacking the negatively charged C-terminal fibronectin-binding motif of the A domain of FnBPA and another construct lacking subdomain N1 were both capable of binding immobilized tropoelastin with a lower affinity. The binding properties of five site-directed mutants of rAFnBPA were compared with wild-type rAFnBPA. There was no decreased affinity for immobilized tropoelastin, in contrast to the defective binding of these mutants to alpha-elastin and fibrinogen. The data indicate novel interactions between tropoelastin and FnBPA that include the use of surface charges. These results demonstrate that FnBPA is capable of directly binding tropoelastin prior to its incorporation into elastin.

Fibrinogen and elastin bind to the same region within the A domain of fibronectin binding protein A, an MSCRAMM of Staphylococcus aureus.

The fibronectin binding protein, FnBPA, is a multifunctional microbial surface component recognizing adhesive matrix molecule (MSCRAMM) that promotes bacterial adherence to immobilized fibrinogen and elastin via the N-terminal A domain. The binding site for fibrinogen and elastin was localized to subdomains N2N3. A three-dimensional structural model of FnBPA was created based on the known crystal structure of the domains N2N3 of clumping factor A (ClfA). The role of individual residues in the putative ligand binding trench was examined by testing the affinity of mutants for fibrinogen and elastin. Two residues (N304 and F306) were crucial for binding both ligands and are in the equivalent positions to residues known to be important for fibrinogen binding by ClfA. A peptide comprising the C-terminus of the gamma-chain of fibrinogen and a monoclonal anti-rAFnBPA antibody were potent inhibitors of the FnBPA-elastin interaction. This suggests that FnBPA binds to fibrinogen and elastin in a similar manner. Amino acid sequence divergence of 26.5% occurred between the A domains of FnBPA from strains 8325-4 and P1. Most variant residues were predicted to be located on the surface of domains N2N3 while few occurred in the putative ligand binding trench and the latching peptide explaining limited immunocross reactivity while ligand binding activity is conserved.