Of the 600+ known proteases identified to date in mammals, a significant percentage is involved or implicated in pathogenic and cancer processes. The dipeptidyl peptidase IV (DPIV) gene family, comprising four enzyme members [DPIV (EC 3.4.14.5), fibroblast activation protein, DP8 and DP9] and two nonenzyme members [DP6 (DPL1) and DP10 (DPL2)], are interesting in this regard because of their multiple diverse functions, varying patterns of distribution/localization and subtle, but significant, differences in structure/substrate recognition. In addition, their engagement in cell biological processes involves both enzymatic and nonenzymatic capabilities. This article examines, in detail, our current understanding of the biological involvement of this unique enzyme family and their overall potential as therapeutic targets.

Fibroblast activation protein-a (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. a 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. Structured digital abstractl FAP cleaves GLP-1-amide by protease assay (View interaction) l DPP4 cleaves PACAP by protease assay (View interaction) l FAP cleaves PYY by protease assay (View interaction) l FAP cleaves NPY by protease assay (View interaction) l DPP4 cleaves Substance P by protease assay (View interaction) l DPP4 cleaves GIP by protease assay (View interaction) l DPP4 cleaves CCL11-Eotaxin by protease assay (View interaction) l DPP4 cleaves GLP-1-amide by protease assay (View interaction) l DPP4 cleaves CXCL12-SDF1a by protease assay (View interaction)

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.

), human lymphoma cell lines and mouse splenocytes stimulated with pokeweed mitogen (PWM) or lipopolysaccharide (LPS), and in dithiothreitol (DTT) and mitomycin-C treated Raji cells. DPP8 and DPP9 expression were measured in epidermal growth factor (EGF) treated Huh7 hepatoma cells, in fibrotic liver samples from mice treated with carbon tetrachloride (CCl4) and from multidrug resistance gene 2 (Mdr2 /Abcb4 ) gene knockout (gko) mice with biliary fibrosis, and in human end stage primary biliary cirrhosis (PBC). RESULTS:All three lymphocyte subsets expressed DPP8 and DPP9 mRNA. DPP8 and DPP9 expression were upregulated in both PWM and LPS stimulated mouse splenocytes and in both Jurkat T-and Raji B-cell lines. DPP8 and DPP9 were downregulated in DTT treated and upregulated in mitomycin-C treated Raji cells. DPP9-transfected Raji cells exhibited more annexin V + cells and associated apoptosis. DPP8 and DPP9 mRNA were upregulated in CCl4 induced fibrotic livers but not in the lymphocytes isolated from such livers, while DPP9 was upregulated in EGF stimulated Huh7 cells. In contrast, intrahepatic DPP8 and DPP9 mRNA expression levels were low in the Mdr2 gko mouse and in human PBC compared to non-diseased livers. CONCLUSION:These expression patterns point to biological roles for DPP8 and DPP9 in lymphocyte activation and apoptosis and in hepatocytes during liver disease pathogenesis.

Liver fibrosis is a progressive pathological process involving inflammation and extracellular matrix deposition. Dipeptidyl peptidase 4 (DPP4), also known as CD26, is a cell surface glycoprotein and serine protease. DPP4 binds to fibronectin, can inactivate specific chemokines, incretin hormone and neuropeptides, and influences cell adhesion and migration. Such properties suggest a pro-fibrotic role for this peptidase but this hypothesis needs in vivo examination. Experimental liver injury was induced with carbon tetrachloride (CCl) in DPP4 gene knockout (gko) mice. DPP4 gko had less liver fibrosis and inflammation and fewer B cell clusters than wild type mice in the fibrosis model. DPP4 inhibitor-treated mice also developed less liver fibrosis. DNA microarray and PCR showed that many immunoglobulin (Ig) genes and some metabolism-associated transcripts were differentially expressed in the gko strain compared with wild type. CCl-treated DPP4 gko livers had more IgM and IgG intrahepatic lymphocytes, and fewer CD4, IgD and CD21 intrahepatic lymphocytes. These data suggest that DPP4 is pro-fibrotic in CCl-induced liver fibrosis and that the mechanisms of DPP4 pro-fibrotic action include energy metabolism, B cells, NK cells and CD4 cells.

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In recent years, the dipeptidyl peptidase-4 (DPP-4) enzyme family has created intense pharmaceutical interest. DPP-4 inhibitors have proven successful as a therapy for the growing type 2 diabetes epidemic and have potential to treat other diseases. A large number of recently developed DPP-4 inhibitors are in various phases of clinical development, with four gliptin class inhibitors already in clinical use. The unique distribution of Fibroblast Activation Protein (FAP), the closest relative of DPP-4, has led to numerous investigations of it as a target and marker for epithelial cancers. The roles of the newer members, DPP-8 and DPP-9, are yet to be fully characterized, but early evidence suggests possible roles in various aspects of cell biology and disease. There are a number of favorable circumstances that have contributed to the therapeutic approaches of targeting DPP-4 and FAP. Specific targeting of DPP-4 and FAP is made easier by the small size of this enzyme family and several structural differences at their active sites. Additionally, observations over the past decade that the DPP-4 and FAP gene knockout mice are healthy suggests that selective inhibition of each of these proteases would be safe, and this is reflected in the excellent safety profiles of the DPP-4-selective gliptins. These proteins also have interesting extra-enzymatic activities that are expected to be retained in the presence of protease inhibition. This feature also points to a low likelihood of off-target effects. Thus, an overall understanding of DPP-4 and FAP structure–function relationships, distribution, and enzymatic and extra-enzymatic biological roles provides an insight into their therapeutic usefulness as disease targets.

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