DPP8 and DPP9 expression in cynomolgus monkey and Sprague Dawley rat tissues

The amino-dipeptidyl peptidases DPP8 and DPP9: Purification and enzymatic assays

Proline residues highly impact protein stability when present either in the first or second N-terminal position. While the human genome encodes for more than 500 proteases, only few proteases are capable of hydrolyzing a proline-containing peptide bond. The two intra-cellular amino-dipeptidyl peptidases DPP8 and DPP9 are exceptional as they possess the rare ability to cleave post-proline. By removing N-terminal Xaa-Pro dipeptides, DPP8 and DPP9 expose a neo N-terminus of their substates, which can consequently alter inter- or intra-molecular interactions of the modified protein. Both DPP8 and DPP9 play key roles in the immune response and are linked to cancer progression, emerging as attractive drug targets. DPP9 is more abundant than DPP8 and is rate limiting for cleavage of cytosolic proline-containing peptides. Only few DPP9 substrates have been characterized; these include Syk, a central kinase for B-cell receptor mediated signaling; Adenylate Kinase 2 (AK2) which is important for cellular energy homeostasis; and the tumor suppressor Breast cancer type 2 susceptibility protein (BRCA2) that is critical for repair of DNA double strand breaks. N-terminal processing of these proteins by DPP9 triggers their rapid turn-over by the proteasome, highlighting a role for DPP9 as upstream components of the N-degron pathway. Whether N-terminal processing by DPP9 leads to substrate-degradation in all cases, or whether additional outcomes are possible, remains to be tested. In this chapter we will describe methods for purification of DPP8 and DPP9 as well as protocols for biochemical and enzymatic characterization of these proteases.

New insights into the role of dipeptidyl peptidase 8 and dipeptidyl peptidase 9 and their inhibitors

Dipeptidyl peptidase 8 (DPP8) and 9 (DPP9) are widely expressed in mammals including humans, mainly locate in the cytoplasm. The DPP8 and DPP9 (DPP8/9) belong to serine proteolytic enzymes, they can recognize and cleave N-terminal dipeptides of specific substrates if proline is at the penultimate position. Because the localization of DPP8/9 is different from that of DPP4 and the substrates for DPP8/9 are not yet completely clear, their physiological and pathological roles are still being further explored. In this article, we will review the recent research advances focusing on the expression, regulation, and functions of DPP8/9 in physiology and pathology status. Emerging research results have shown that DPP8/9 is involved in various biological processes such as cell behavior, energy metabolism, and immune regulation, which plays an essential role in maintaining normal development and physiological functions of the body. DPP8/9 is also involved in pathological processes such as tumorigenesis, inflammation, and organ fibrosis. In recent years, related research on immune cell pyroptosis has made DPP8/9 a new potential target for the treatment of hematological diseases. In addition, DPP8/9 inhibitors also have great potential in the treatment of tumors and chronic kidney disease.

CD26/DPP4 - a potential biomarker and target for cancer therapy

CD26/dipeptidyl peptidase (DPP)4 is a membrane-bound protein found in many cell types of the body, and a soluble form is present in body fluids. There is longstanding evidence that various primary tumors and also metastases express CD26/DPP4 to a variable extent. By cleaving dipeptides from peptides with a proline or alanine in the penultimate position at the N-terminus, it regulates the activity of incretin hormones, chemokines and many other peptides. Due to these effects and interactions with other molecules, a tumor promoting or suppressing role can be attributed to CD26/DPP4. In this review, we discuss the existing evidence on the expression of soluble or membrane-bound CD26/DPP4 in malignant diseases, along with the most recent findings on CD26/DPP4 as a therapeutic target in specific malignancies. The expression and possible involvement of the related DPP8 and DPP9 in cancer are also reviewed. A higher expression of CD26/DPP4 is found in a wide variety of tumor entities, however more research on CD26/DPP4 in the tumor microenvironment is needed to fully explore its use as a tumor biomarker. Circulating soluble CD26/DPP4 has also been studied as a cancer biomarker, however, the observed decrease in most cancer patients does not seem to be cancer specific. Encouraging results from experimental work and a recently reported first phase clinical trial targeting CD26/DPP4 in mesothelioma, renal and urological tumors pave the way for follow-up clinical studies, also in other tumor entities, possibly leading to the development of more effective complementary therapies against cancer.

Hydrolytic Metabolism of Cyanopyrrolidine DPP-4 Inhibitors Mediated by Dipeptidyl Peptidases

Nitrile group biotransformation is an unusual or minor metabolic pathway for most nitrile-containing drugs. However, for some cyanopyrrolidine dipeptidyl peptidase 4 (DPP-4) inhibitors (vildagliptin, anagliptin, and besigliptin, but not saxagliptin), the conversion of nitrile group into carboxylic acid is their major metabolic pathway in vivo. DPP-4 was reported to be partly involved in the metabolism. In our pilot study, it was also observed that saxagliptin, a DPP-4 specific inhibitor, decreased the plasma exposures of besigliptin carboxylic acid in rats by only 20%. Therefore, it is speculated that some other enzymes may participate in nitrile group hydrolysis. After incubating gliptins with the cytosol, microsomes, and mitochondria of liver and kidney, carboxylic acid metabolites could all be formed. In recombinant DPP family such as DPP-4, DPP-2, DPP-8, DPP-9, and fibroblast activation protein-α, more hydrolytic metabolites were found. Among them, DPP-2 had the highest hydrolytic capacity besides DPP-4, and the DPP-4 inhibitor saxagliptin and DPP-2 inhibitor AX8819 can both inhibit the hydrolysis of gliptins. Western blot results showed that DPP-2 and DPP-4 existed in the aforementioned subcellular organelles at varying amounts. In rats, AX8819 decreased the plasma exposures of besigliptin carboxylic acid by 40%. The amide intermediates of gliptins were detected in vivo and in vitro. When the amide derivatives of gliptins were incubated with DPP-4, they were completely hydrolyzed at a rate far more than that from the parent drug, including saxagliptin-amide. Therefore, it was proposed that gliptins, except saxagliptin, were initially hydrolyzed to their amides by DPPs, which was the rate-limiting step in generating the carboxylic end product.

The expression of proline-specific enzymes in the human lung

The pathophysiology of lung diseases is very complex and proteolytic enzymes may play a role or could be used as biomarkers. In this review, the literature was searched to make an overview of what is known on the expression of the proline-specific peptidases dipeptidyl peptidase (DPP) 4, 8, 9, prolyl oligopeptidase (PREP) and fibroblast activation protein α (FAP) in the healthy and diseased lung. Search terms included asthma, chronic obstructive pulmonary disease (COPD), lung cancer, fibrosis, ischemia reperfusion injury and pneumonia. Knowledge on the loss or gain of protein expression and activity during disease might tie these enzymes to certain cell types, substrates or interaction partners that are involved in the pathophysiology of the disease, ultimately leading to the elucidation of their functional roles and a potential therapeutic target. Most data could be found on DPP4, while the other enzymes are less explored. Published data however often appear to be conflicting, the applied methods divers and the specificity of the assays used questionable. In conclusion, information on the expression of the proline-specific peptidases in the healthy and diseased lung is lacking, begging for further well-designed research.

Hepatocellular carcinoma: Mouse models and the potential roles of proteases

Primary liver cancer is the second most common cause of mortality from cancer. The most common models of hepatocellular carcinoma, which use a chemical and/or metabolic insult, xenograft, or genetic manipulation, are discussed in this review. In the tumour microenvironment lymphocytes, fibroblasts, endothelial cells and antigen presenting cells are important determinants of cell fate. These cells make a range of proteases that modify the biological activity of other proteins, particularly extracellular matrix proteins that alter cell migration of tumour cells, fibroblasts and leucocytes, and chemokines that alter leucocyte migration. The DPP4 family of post-proline peptidase enzymes modifies cell movement and the activities of many bioactive molecules including growth factors and chemokines.

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.

Gliptins and their target dipeptidyl peptidase 4: implications for the treatment of vascular disease

Gliptins are accepted as a standard therapy for diabetes mellitus today. By inhibition of the enzyme dipeptidyl peptidase 4 (DPP4), gliptins prolong the GLP1-dependent insulin secretion in the pancreatic β-cells and thus support physiological blood glucose control. Various studies have now raised hope for an additional protective effect of pharmacological DPP4 inhibition in vascular diseases. Besides GLP1, especially, the inhibition of SDF1 cleavage has been shown to depict a relevant mechanism to enhance endothelial regeneration and reduce atherosclerosis progression via the SDF1-CXCR4 axis. Furthermore, several clinical trials have now shown an excellent safety profile of gliptin therapy in cardiovascular risk patients. In this review, we give a comprehensive overview on DPP4-dependent vascular functions and pathophysiological mechanisms with a detailed discussion of the underlying molecular mechanisms. We further analyse the role of pharmacological DPP4 inhibitors and their potential therapeutic impact on endothelial function and regeneration besides their effect during atherosclerosis development. Finally, we discuss presently available data from in vitro and in vivo studies with respect to the results of the recent clinical trials in diabetic and non-diabetic patients.

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.

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-β1 and talin. DPP9 gene silencing and treatment with a DPP8/DPP9 specific inhibitor both reduced cell adhesion and migration. Expression of integrin-β1 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.

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.

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A novel synthetic fluorogenic substrate is proven to be FAP-specific.

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Mice have higher levels of circulating FAP activity compared to baboons or humans.

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No FAP activity was detected in urine or bile but bile contained high DPP4 activity.

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FAP activity is greatest in pancreas, uterus, salivary gland, skin and lymph node.

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FAP activity and protein is elevated in both serum and liver in human liver disease.

Targeted inactivation of dipeptidyl peptidase 9 enzymatic activity causes mouse neonate lethality

Dipeptidyl Peptidase (DPP) 4 and related dipeptidyl peptidases are emerging as current and potential therapeutic targets. DPP9 is an intracellular protease that is regulated by redox status and by SUMO1. DPP9 can influence antigen processing, epidermal growth factor (EGF)-mediated signaling and tumor biology. We made the first gene knock-in (gki) mouse with a serine to alanine point mutation at the DPP9 active site (S729A). Weaned heterozygote DPP9 (wt/S729A) pups from 110 intercrosses were indistinguishable from wild-type littermates. No homozygote DPP9 (S729A/S729A) weaned mice were detected. DPP9 (S729A/S729A) homozygote embryos, which were morphologically indistinguishable from their wild-type littermate embryos at embryonic day (ED) 12.5 to ED 17.5, were born live but these neonates died within 8 to 24 hours of birth. All neonates suckled and contained milk spots and were of similar body weight. No gender differences were seen. No histological or DPP9 immunostaining pattern differences were seen between genotypes in embryos and neonates. Mouse embryonic fibroblasts (MEFs) from DPP9 (S729A/S729A) ED13.5 embryos and neonate DPP9 (S729A/S729A) mouse livers collected within 6 hours after birth had levels of DPP9 protein and DPP9-related proteases that were similar to wild-type but had less DPP9/DPP8-derived activity. These data confirmed the absence of DPP9 enzymatic activity due to the presence of the serine to alanine mutation and no compensation from related proteases. These novel findings suggest that DPP9 enzymatic activity is essential for early neonatal survival in mice.

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.