CD26/dipeptidyl peptidase IV does not work as an adenosine deaminase-binding protein in rat cells

Lymphocytes are a major source of circulating soluble dipeptidyl peptidase 4

Dipeptidyl peptidase 4 (DPP4, CD26) is a serine protease that is expressed constitutively by many haematopoietic and non-haematopoietic tissues. It exists as a membrane-associated protein, as well as in an active, soluble form (herein called sDPP4), present at high concentrations in bodily fluids. Despite the proposed use of sDPP4 as a biomarker for multiple diseases, its cellular sources are not well defined. Here, we report that individuals with congenital lymphocyte immunodeficiency had markedly lower serum concentrations of sDPP4, which were restored upon successful treatment and restoration of lymphocyte haematopoiesis. Using irradiated lymphopenic mice and wild-type to Dpp4-/- reciprocal bone marrow chimeric animals, we found that haematopoietic cells were a major source of circulating sDPP4. Furthermore, activation of human and mouse T lymphocytes resulted in increased sDPP4, providing a mechanistic link between immune system activation and sDPP4 concentration. Finally, we observed that acute viral infection induced a transient increase in sDPP4, which correlated with the expansion of antigen-specific CD8+ T cell responses. Our study demonstrates that sDPP4 concentrations are determined by the frequency and activation state of lymphocyte populations. Insights from these studies will support the use of sDPP4 concentration as a biomarker for inflammatory and infectious diseases.

Adenosine plasma level correlates with homocysteine and uric acid concentrations in patients with coronary artery disease

The role of hyperhomocysteinemia in coronary artery disease (CAD) patients remains unclear. The present study evaluated the relationship between homocysteine (HCys), adenosine plasma concentration (APC), plasma uric acid, and CAD severity evaluated using the SYNTAX score. We also evaluated in vitro the influence of adenosine on HCys production by hepatoma cultured cells (HuH7). Seventy-eight patients (mean age ± SD: 66.3 ± 11.3; mean SYNTAX score: 19.9 ± 12.3) and 30 healthy subjects (mean age: 61 ± 13) were included. We incubated HuH7 cells with increasing concentrations of adenosine and addressed the effect on HCys level in cell culture supernatant. Patients vs. controls had higher APC (0.82 ± 0.5 μmol/L vs 0.53 ± 0.14 μmol/L; p < 0.01), HCys (15 ± 7.6 μmol/L vs 6.8 ± 3 μmol/L, p < 0.0001), and uric acid (242.6 ± 97 vs 202 ± 59, p < 0.05) levels. APC was correlated with HCys and uric acid concentrations in patients (Pearson's R = 0.65 and 0.52; p < 0.0001, respectively). The SYNTAX score was correlated with HCys concentration. Adenosine induced a time- and dose-dependent increase in HCys in cell culture. Our data suggest that high APC is associated with HCys and uric acid concentrations in CAD patients. Whether the increased APC participates in atherosclerosis or, conversely, is part of a protective regulation process needs further investigations.

Efficacy of parenteral administration of bee venom in experimental arthritis in the rat: a comparison with methotrexate

The use of bee venom (BV) to treat inflammation and pain in arthritis has become increasingly common. This study aimed to compare the effects of BV and methotrexate (MTX), the most used disease-modifying anti-rheumatic drug, in arthritic rats. Edema, erythema, cyanosis, hyperalgesia, reduction of the body mass gain, high circulating tumor necrosis factor alpha (TNF-α) and anti-type II collagen antibodies (AACII), and altered activity of basic (APB) and neutral (APN) aminopeptidases and dipeptidyl peptidase IV (DPPIV) are present in arthritic rats. MTX and/or BV do not affect AACII in healthy or arthritic individuals, but restores TNF-α to normal levels in arthritic rats. BV restores body mass gain to normal levels and MTX ameliorates body mass gain. BV contains DPPIV. BV restores APN in synovial fluid (SF) and in soluble fraction (S) from synovial tissue (ST), and DPPIV in solubilized membrane-bound fraction (M) from peripheral blood mononuclear cells (PBMCs). MTX restores APN of SF, as well as ameliorates APB of S-PBMCs, APN of S-ST and DPPIV of M-PBMCs. The combination therapy does not overcome the effects of BV or MTX alone on the peptidase activities. Edema is ameliorated by MTX or BV alone. MTX, but not BV, is effective in reducing hyperalgesia. Data show that anti-arthritic effects of BV at non-acupoints are not negligible when compared with MTX.

Dipeptidyl peptidase in autoimmune pathophysiology

CD26 is a 110-kDa surface glycoprotein with intrinsic dipeptidyl peptidase IV (DPPIV) activity that is expressed on various cell types and has many biological functions. An important aspect of CD26 biology is its peptidase activity and its functional and physical association with molecules with key roles in human immunological programs. CD26 role in immune regulation has been extensively characterized, with recent findings elucidating its link age with signaling pathways and structures involved in T cell activation a well as antigen-presenting cell-T cell interaction, being a marker of diseas behavior clinically as well as playing an important role in autoimmune pathogenesis and development. Through the use of various experimental approaches and agents to influence CD26/DPPIV expression and activity, such as anti-CD26 antibodies, CD26/DPPIV chemical inhibitors, siRNAs to inhibit CD26 expression, overexpressing CD26 transfectants, soluble CD26 molecules and proteomic approach, we have shown that CD26 interacts with structures with essential cellular functions in T cell responses. We will review emerging data that suggest CD26 may be an appropriate therapeutic target for the treatment of selected immune disorders.

Regulation of adenosine receptor engagement by ecto-adenosine deaminase

Adenosine deaminase (ADA) can localize to the cell surface through its interaction with CD26. Using CD26-transfected cells, we demonstrate that cell surface ADA (ecto-ADA) can regulate adenosine receptor engagement by degrading extracellular adenosine (Ado) to inosine. This ability was dependent upon CD26 expression, the extent of CD26 saturation with ecto-ADA, and the kinetics of the cAMP response. Thus, the cAMP response was markedly decreased when CD26-transfected cells were incubated with an exogenous source of ADA to increase ecto-ADA expression. The ability of ecto-ADA to inhibit the cAMP response was demonstrated by treatment with the specific ADA inhibitor 2'-deoxycoformycin. This inhibited the ability of ecto-ADA to degrade Ado and increased the cAMP response. Although CD26 expression on human thymocytes was low compared with that of CD26-transfected cells, it was saturated with ecto-ADA. When thymocytes incubated at high densities (to mimic the situation in tissues) were exposed to exogenous adenosine, the cAMP response was dramatically decreased by ecto-ADA. We conclude that ecto-ADA has the potential to regulate adenosine receptor-mediated cAMP responses in vivo in tissues with CD26+ cells and sufficient cell death caused by apoptosis or inflammation to provide a source of ADA to bind to CD26.

Structural basis of proline-specific exopeptidase activity as observed in human dipeptidyl peptidase-IV

Inhibition of dipeptidyl peptidase IV (DPP-IV), the main glucagon-like peptide 1 (GLP1)-degrading enzyme, has been proposed for the treatment of type II diabetes. We expressed and purified the ectodomain of human DPP-IV in Pichia pastoris and determined the X-ray structure at 2.1 A resolution. The enzyme consists of two domains, the catalytic domain, with an alpha/beta hydrolase fold, and a beta propeller domain with an 8-fold repeat of a four-strand beta sheet motif. The beta propeller domain contributes two important functions to the molecule that have not been reported for such structures, an extra beta sheet motif that forms part of the dimerization interface and an additional short helix with a double Glu sequence motif. The Glu motif provides recognition and a binding site for the N terminus of the substrates, as revealed by the complex structure with diprotin A, a substrate with low turnover that is trapped in the tetrahedral intermediate of the reaction in the crystal.

Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV

Dipeptidyl-peptidase IV/CD26 (DPP IV) is a cell-surface protease belonging to the prolyloligopeptidase family. It selectively removes the N-terminal dipeptide from peptides with proline or alanine in the second position. Apart from its catalytic activity, it interacts with several proteins, for instance, adenosine deaminase, the HIV gp120 protein, fibronectin, collagen, the chemokine receptor CXCR4, and the tyrosine phosphatase CD45. DPP IV is expressed on a specific set of T lymphocytes, where it is up-regulated after activation. It is also expressed in a variety of tissues, primarily on endothelial and epithelial cells. A soluble form is present in plasma and other body fluids. DPP IV has been proposed as a diagnostic or prognostic marker for various tumors, hematological malignancies, immunological, inflammatory, psychoneuroendocrine disorders, and viral infections. DPP IV truncates many bioactive peptides of medical importance. It plays a role in glucose homeostasis through proteolytic inactivation of the incretins. DPP IV inhibitors improve glucose tolerance and pancreatic islet cell function in animal models of type 2 diabetes and in diabetic patients. The role of DPP IV/ CD26 within the immune system is a combination of its exopeptidase activity and its interactions with different molecules. This enables DPP IV/CD26 to serve as a co-stimulatory molecule to influence T cell activity and to modulate chemotaxis. DPP IV is also implicated in HIV-1 entry, malignant transformation, and tumor invasion.

The crystal structure of dipeptidyl peptidase IV (CD26) reveals its functional regulation and enzymatic mechanism

The membrane-bound glycoprotein dipeptidyl peptidase IV (DP IV, CD26) is a unique multifunctional protein, acting as receptor, binding and proteolytic molecule. We have determined the sequence and 1.8 A crystal structure of native DP IV prepared from porcine kidney. The crystal structure reveals a 2-2-2 symmetric tetrameric assembly which depends on the natively glycosylated beta-propeller blade IV. The crystal structure indicates that tetramerization of DP IV is a key mechanism to regulate its interaction with other components. Each subunit comprises two structural domains, the N-terminal eight-bladed beta-propeller with open Velcro topology and the C-terminal alpha/beta-hydrolase domain. Analogy with the structurally related POP and tricorn protease suggests that substrates access the buried active site through the beta-propeller tunnel while products leave the active site through a separate side exit. A dipeptide mimicking inhibitor complexed to the active site discloses key determinants for substrate recognition, including a Glu-Glu motif that distinguishes DP IV as an aminopeptidase and an oxyanion trap that binds and activates the P(2)-carbonyl oxygen necessary for efficient postproline cleavage. We discuss active and nonactive site-directed inhibition strategies of this pharmaceutical target protein.

Characterization and purification of adenosine deaminase 1 from human and chicken liver

Adenosine deaminase 1 (ADA1) was purified from human and chicken liver. The purified enzyme had a molecular weight of approximately 42,000 Da on SDS-PAGE. In humans, ADA1 was mainly purified concomitant with ADA-binding protein, dipeptidyl peptidase IV (DPP IV)/CD26; however, in chickens, only ADA1 without DPP IV was purified. Both human and chicken ADA1s showed similar properties on substrate specificities, sensitivities on inhibitors, and pH profile. However, they had different affinities with adenosine-Sepharose and IgG anti-ADA1-Sepharose. Human ADA1 was not adsorbed in adenosine-Sepharose column, but chicken ADA1 was adsorbed. As for IgG anti-ADA1-Sepharose column, the results were converse. Furthermore, human ADA1 could bind to DPP IV whereas chicken ADA1 could not.

CD26: a multifunctional integral membrane and secreted protein of activated lymphocytes

CD26 has proved interesting in the fields of immunology, endocrinology, cancer biology and nutrition owing to its ubiquitous and unusual enzyme activity. This dipeptidyl aminopeptidase (DPP IV) activity generally inactivates but sometimes alters or enhances the biological activities of its peptide substrates, which include several chemokines. CD26 costimulates both the CD3 and the CD2 dependent T-cell activation and tyrosine phosphorylation of TCR/CD3 signal transduction pathway proteins. CD26 in vivo has integral membrane protein and soluble forms. Soluble CD26 is at significant levels in serum, these levels alter in many diseases and soluble CD26 can modulate in vitro T-cell proliferation. CD26, being an adenosine deaminase binding protein (ADAbp), functions as a receptor for ADA on lymphocytes. The focus of this review is the structure and function of CD26 and the influence of its ligand binding activity on T-cell proliferation and the T cell costimulatory activity of CD26.

Binding to human dipeptidyl peptidase IV by adenosine deaminase and antibodies that inhibit ligand binding involves overlapping, discontinuous sites on a predicted beta propeller domain

Dipeptidyl peptidase IV (DPPIV) is an atypical serine protease that modifies the biological activities of certain chemokines and neuropeptides. In addition, human DPPIV, also known as the T-cell activation antigen CD26, binds adenosine deaminase (ADA) to the T-cell surface, thus protecting the T-cell from adenosine-mediated inhibition of proliferation. Mutations were engineered into DPPIV (five point, 16 single point and six deletion mutations) to examine the binding of ADA and 19 monoclonal antibodies. Deletions of C-terminal residues from the 738-residue extracellular portion of DPPIV showed that the 214 residues C-terminal to Ser552 were not required for ADA binding and that peptidase activity could be ablated by deletion of 20 residues from the C-terminus. Point mutations at either of two locations, Leu294 and Val341, ablated ADA binding. Binding by six anti-DPPIV antibodies that inhibited ADA binding was found to require Leu340 to Arg343 and Thr440/Lys441 but not the 214 residues C-terminal to Ser552. The 13 other antibodies studied bound to a truncated DPPIV consisting of amino acids 1-356. Therefore, the binding sites on DPPIV of ADA and antibodies that inhibit ADA binding are discontinuous and overlapping. Moreover, the 47 and 97 residue spacing of amino acids in these binding sites concords with their location on a beta propeller fold consisting of repeated beta sheets of about 50 amino acids.

CD26, let it cut or cut it down

The costimulatory properties of CD26 have been studied extensively and significant progress has been made in unravelling the complex nature of this molecule. Here, we summarize recent findings on molecular and functional characteristics of CD26. We argue that a multidisciplinary approach might reveal the molecular events underlying the role of CD26 in HIV infection and immune, inflammatory and endocrine responses.

Lung endothelial dipeptidyl peptidase IV promotes adhesion and metastasis of rat breast cancer cells via tumor cell surface-associated fibronectin

Endothelial cell adhesion molecules are partly responsible for the distinct organ distribution of cancer metastases. Dipeptidyl peptidase IV (DPP IV) expressed on rat lung capillary endothelia is shown here to be an adhesion receptor for rat breast cancer cells and to mediate lung colonization by these tumor cells. Fibronectin (FN) assembled on breast cancer cell surfaces into multiple, randomly dispersed globules from cellular and plasma FN is identified as the principal ligand for DPP IV. Ligand expression correlates quantitatively with the tumor cells' capabilities to bind to DPP IV and to metastasize to the lungs. DPP IV/FN-mediated adhesion and metastasis are blocked when tumor cells are incubated with soluble DPP IV prior to conducting adhesion and lung colony assays. Adhesion is also blocked by anti-DPP IV monoclonal antibody 6A3 and anti-FN antiserum. However, adhesion to immobilized FN is unaffected by soluble plasma FN and, thus, can happen during hematogenous spread of cancer cells at high plasma FN concentrations. The ability of many cancer cells to capture FN molecules on their surface and to augment such deposits by FN self-association during passage in the blood suggests that DPP IV/FN binding may be a relatively common mechanism for lung metastasis.

Dipeptidyl-peptidase IV/CD26 on T cells: analysis of an alternative T-cell activation pathway

CD26 is a proteolytic enzyme (dipeptidyl-peptidase IV) with a wide tissue distribution and a unique specificity that was already described 27 years ago. CD26 is expressed on a fraction of resting T cells at low density but is strongly upregulated following T-cell activation. Recent results indicate that CD26 is a multifunctional molecule that may have important functions on T cells and in the immune system. It is associated with molecules of immunological importance such as the protein tyrosine phosphatase CD45 and adenosine deaminase (ADA) on the cell surface. Synthetic inhibitors of the enzymatic activity of CD26 have been shown to suppress certain immune reactions in vitro and in vivo. An interesting feature of CD26 is its ability to transmit a transmembrane signal to trigger functional programs in T cells. This triggering requires crosslinking of CD26 on a cell membrane. The enzymatic activity of CD26 is not obligatory for the activation of T cells via CD26. Since CD26 is a type II membrane protein with only six intracellular amino acids, it must deliver its signal via a signal-transducing molecule. Signaling is dependent on the expression of the T-cell receptor (TCR) complex with a special need for a functional zeta-chain. In this context the zeta-chain of the TCR complex is required for CD26-mediated signaling but, in contrast to other co-stimulatory molecules such as the CD2 molecule, is not sufficient for triggering the T cell.