Peptide substrates of dipeptidyl peptidases

What doesn't kill you makes you stranger: Dipeptidyl peptidase-4 (CD26) proteolysis differentially modulates the activity of many peptide hormones and cytokines generating novel cryptic bioactive ligands

Dipeptidyl peptidase 4 (DPP4) is an exopeptidase found either on cell surfaces where it is highly regulated in terms of its expression and surface availability (CD26) or in a free/circulating soluble constitutively available and intrinsically active form. It is responsible for proteolytic cleavage of many peptide substrates. In this review we discuss the idea that DPP4-cleaved peptides are not necessarily inactivated, but rather can possess either a modified receptor selectivity, modified bioactivity, new antagonistic activity, or even a novel activity relative to the intact parent ligand. We examine in detail five different major DPP4 substrates: glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), peptide tyrosine-tyrosine (PYY), and neuropeptide Y (NPY), and stromal derived factor 1 (SDF-1 aka CXCL12). We note that discussion of the cleaved forms of these five peptides are underrepresented in the research literature, and are both poorly investigated and poorly understood, representing a serious research literature gap. We believe they are understudied and misinterpreted as inactive due to several factors. This includes lack of accurate and specific quantification methods, sample collection techniques that are inherently inaccurate and inappropriate, and a general perception that DPP4 cleavage inactivates its ligand substrates. Increasing evidence points towards many DPP4-cleaved ligands having their own bioactivity. For example, GLP-1 can work through a different receptor than GLP-1R, DPP4-cleaved GIP can function as a GIP receptor antagonist at high doses, and DPP4-cleaved PYY, NPY, and CXCL12 can have different receptor selectivity, or can bind novel, previously unrecognized receptors to their intact ligands, resulting in altered signaling and functionality. We believe that more rigorous research in this area could lead to a better understanding of DPP4's role and the biological importance of the generation of novel cryptic ligands. This will also significantly impact our understanding of the clinical effects and side effects of DPP4-inhibitors as a class of anti-diabetic drugs that potentially have an expanding clinical relevance. This will be specifically relevant in targeting DPP4 substrate ligands involved in a variety of other major clinical acute and chronic injury/disease areas including inflammation, immunology, cardiology, stroke, musculoskeletal disease and injury, as well as cancer biology and tissue maintenance in aging.

Unravelling the immunological roles of dipeptidyl peptidase 4 (DPP4) activity and/or structure homologue (DASH) proteins

Dipeptidyl peptidase (DPP) 4 (CD26, DPP4) is a multi-functional protein involved in T cell activation by co-stimulation via its association with adenosine deaminase (ADA), caveolin-1, CARMA-1, CD45, mannose-6-phosphate/insulin growth factor-II receptor (M6P/IGFII-R) and C-X-C motif receptor 4 (CXC-R4). The proline-specific dipeptidyl peptidase also modulates the bioactivity of several chemokines. However, a number of enzymes displaying either DPP4-like activities or representing structural homologues have been discovered in the past two decades and are referred to as DPP4 activity and/or structure homologue (DASH) proteins. Apart from DPP4, DASH proteins include fibroblast activation protein alpha (FAP), DPP8, DPP9, DPP4-like protein 1 (DPL1, DPP6, DPPX L, DPPX S), DPP4-like protein 2 (DPL2, DPP10) from the DPP4-gene family S9b and structurally unrelated enzyme DPP2, displaying DPP4-like activity. In contrast, DPP6 and DPP10 lack enzymatic DPP4-like activity. These DASH proteins play important roles in the immune system involving quiescence (DPP2), proliferation (DPP8/DPP9), antigen-presenting (DPP9), co-stimulation (DPP4), T cell activation (DPP4), signal transduction (DPP4, DPP8 and DPP9), differentiation (DPP4, DPP8) and tissue remodelling (DPP4, FAP). Thus, they are involved in many pathophysiological processes and have therefore been proposed for potential biomarkers or even drug targets in various cancers (DPP4 and FAP) and inflammatory diseases (DPP4, DPP8/DPP9). However, they also pose the challenge of drug selectivity concerning other DASH members for better efficacy and/or avoidance of unwanted side effects. Therefore, this review unravels the complex roles of DASH proteins in immunology.

CD26/dipeptidylpeptidase IV-chemokine interactions: double-edged regulation of inflammation and tumor biology

Review of how chemokine processing by CD26/DPP IV regulates leukocyte trafficking.

Post‐translational modification of chemokines is an essential regulatory mechanism to enhance or dampen the inflammatory response. CD26/dipeptidylpeptidase IV, ubiquitously expressed in tissues and blood, removes NH₂‐terminal dipeptides from proteins with a penultimate Pro or Ala. A large number of human chemokines, including CXCL2, CXCL6, CXCL9, CXCL10, CXCL11, CXCL12, CCL3L1, CCL4, CCL5, CCL11, CCL14, and CCL22, are cleaved by CD26; however, the efficiency is clearly influenced by the amino acids surrounding the cleavage site and although not yet proven, potentially affected by the chemokine concentration and interactions with third molecules. NH₂‐terminal cleavage of chemokines by CD26 has prominent effects on their receptor binding, signaling, and hence, in vitro and in vivo biologic activities. However, rather than having a similar result, the outcome of NH₂‐terminal truncation is highly diverse. Either no difference in activity or drastic alterations in receptor recognition/specificity and hence, chemotactic activity are observed. Analogously, chemokine‐dependent inhibition of HIV infection is enhanced (for CCL3L1 and CCL5) or decreased (for CXCL12) by CD26 cleavage. The occurrence of CD26‐processed chemokine isoforms in plasma underscores the importance of the in vitro‐observed CD26 cleavages. Through modulation of chemokine activity, CD26 regulates leukocyte/tumor cell migration and progenitor cell release from the bone marrow, as shown by use of mice treated with CD26 inhibitors or CD26 knockout mice. As chemokine processing by CD26 has a significant impact on physiologic and pathologic processes, application of CD26 inhibitors to affect chemokine function is currently explored, e.g., as add‐on therapy in viral infection and cancer.

Blood pressure-lowering effects of incretin-based diabetes therapies

Glucagon-like peptide-1 receptor (GLP-1) agonists and dipeptidyl-peptidase-4 (DPP-4) inhibitors are therapies that are used to treat hyperglycemia in patients with type 2 diabetes mellitus. Although both of these medication types primarily lower prandial and fasting blood glucose levels by enhanced GLP-1 receptor signalling, they have distinct mechanisms of action. Whereas DPP-4 inhibitors boost patient levels of endogenously produced GLP-1 (and glucose-dependent insulinotropic peptide) by preventing its metabolism by DPP-4 enzymatic activity, GLP-1 receptor agonists are either synthetic analogues of human GLP-1 or exendin-4 based molecules. They are tailored to resist hydrolysis by DPP-4 activity and to provide longer durability in the circulation compared with native GLP-1. Several roles for incretin-based diabetes therapies beyond the endocrine pancreas and their glycemic-lowering properties have now been described, including attenuation of cardiac myocyte injury and reduction in post-ischemic infarction size after cardiovascular insult. Favourable outcomes have also been observed on systolic blood pressure reduction, postprandial intestinal lipoprotein metabolism, endothelial cell function, modulation of innate immune-mediated inflammation and surrogate markers of renal function. As hypertension is an independent risk factor for premature death in patients with type 2 diabetes, potential favourable extrapancreatic actions, particularly within the heart, blood vessels and kidney, for this drug class are of considerable clinical interest. Herein, we highlight and provide critical appraisal of the clinical data supporting the antihypertensive effects of GLP-1 receptor agonists and DPP-4 inhibitors and link possible mechanisms of action to clinical outcomes reported for this drug class.

Inhibition of circulating dipeptidyl peptidase 4 activity in patients with metastatic prostate cancer

Cancer is responsible for many deaths and is a major source of healthcare expenditures. The identification of new, non-invasive biomarkers might allow improvement of the direct diagnostic or prognostic ability of already available tools. Here, we took the innovative approach of interrogating the activity of exopeptidases in the serum of cancer patients with the aim of establishing a distinction based on enzymatic function, instead of simple protein levels, as a means to biomarker discovery. We first analyzed two well-characterized mouse models of prostate cancer, each with a distinct genetic lesion, and established that broad exopeptidase and targeted aminopeptidase activity tests reveal proteolytic changes associated with tumor development. We also describe new peptide-based freeze-frame reagents uniquely suited to probe the altered balance of selected aminopeptidases, as opposed to the full array of exopeptidases, and/or their modulators in patient serum or plasma. One particular proteolytic activity was impaired in animals with aggressive disease relative to cancer-free littermates. We identified the protease in question as dipeptidyl peptidase 4 (DPP4) by analyzing selected knockout mice and evaluating the effect of specific inhibitors. DPP4 activity was also reduced in the sera of patients with metastatic prostate cancer relative to patients with localized disease or healthy controls. However, no significant differences in DPP4 serum levels were observed, which established the loss of activity as the result of impaired enzymatic function. Biochemical analysis indicated that reduced activity was the result not of post-translational modifications or allosteric changes, but instead of a low-molecular-weight inhibitor. After we adjusted for age and total prostate-specific antigen, reduced DPP4 activity remained a significant predictor of cancer status. The results of this proof-of-principle study suggest that DPP4 activity might be a potential blood-based indicator of the presence of metastatic cancer of prostatic origin, either by itself or, more likely, as a means to improve the sensitivity and specificity of existing markers.

Identification of altered dipeptidyl-peptidase activities as potential biomarkers for unipolar depression

BACKGROUND

Changes in circulatory aminopeptidases [dipeptidyl-peptidase-IV (DPP-IV), Prolyl-oligopeptidase (POP) and Leucine aminopeptidase (LAP)] activities have been found to be associated with psychiatric illnesses and inflammatory diseases.

METHODS

The discriminatory indices of aminopeptidases activities were assessed by enzymatic assays in plasma samples from 240 unipolar depression (UD) patients and 264 matched controls. In addition the relationship between soluble and cellular DPP-IV activity was determined in plasma and blood cells from healthy subjects.

RESULTS

Greater than 95% of the plasma DPP-IV activity could be blocked by inhibitors, demonstrating the specificity of the assay. Also, DPP-IV protein and activity levels were strongly correlated. In contrast, only 50% of the membrane-bound activity in blood cells was inhibited, which suggested that other similar peptidases may be present in these cells. UD patients had decreased plasma levels of DPP-IV and POP activities compared to healthy controls with a concomitant increase in LAP activity. Finally, testing of the LAP/DPP-IV ratio resulted in good discrimination of UD patients from controls with an area under the curve-receiver operating characteristic of 0.70.

LIMITATIONS

Further biological validation studies using different cohorts are warranted.

CONCLUSIONS

The finding that plasma DPP-IV activity was decreased and LAP activity was increased in UD patients suggests the potential value for testing the levels of these enzymes for improved classification of patients. In addition, the changes in these enzymes, suggests that the proteolytic maturation of their proneuropeptide and prohormone subtrates may also be affected in UD, resulting in altered production of the associated bioactive peptides.

Clinical utility of the dipeptidyl peptidase-4 inhibitor linagliptin

INTRODUCTION

Dipeptidyl peptidase-4 (DPP-4) inhibitors have emerged as new options in the management of type 2 diabetes mellitus, demonstrating meaningful antihyperglycemic effects and good tolerability profiles. Glycemic control is improved by preventing the DPP-4-mediated degradation of incretin hormones, with a resulting increase in insulin secretion and inhibition of glucagon secretion.

PURPOSE

This article provides a discussion of the clinical utility of linagliptin.

RESULTS AND CONCLUSION

Linagliptin is a xanthine-based, oral DPP-4 inhibitor that has been approved in the United States and Europe. It has been evaluated extensively in clinical trials, and results in improved glycemic control when used as monotherapy, initial combination therapy with metformin or pioglitazone, add-on therapy to metformin and/or a sulfonylurea, or add-on therapy to basal insulin (with or without oral antidiabetic drugs). Consistent with other members of its class, the benefits of linagliptin also include a low risk of hypoglycemia and weight gain. However, linagliptin is the first DPP-4 inhibitor to be approved as a once-daily, 5-mg dose and, due to its primarily non-renal route of excretion, no dosage adjustment is required for patients with renal or hepatic impairment. The pharmacokinetics and pharmacodynamics of linagliptin are not affected to a clinically meaningful degree by race or ethnicity and linagliptin has very low potential for drug-drug interactions.

The impact of current and novel anti-diabetic therapies on cardiovascular risk

Type 2 diabetes mellitus (T2DM) has become an overwhelming health condition that is no longer just a threat to developed nations, but to undeveloped nations as well. Current therapies for T2DM are relatively effective in controlling hyperglycemia; examples include metformin, thiazolidinediones, sulfonylurea derivatives, α-glucosidase inhibitors, glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors. Despite their efficacy in controlling hyperglycemia, due to recent findings of increased cardiovascular risk following treatment with either rosiglitazone or intensive glucose lowering, new guidelines from the US FDA recommend that new therapies for diabetes not only improve glycemia, but exert no adverse cardiovascular effects. Based on cardiovascular risk profiles, metformin appears to be the superior anti-diabetic therapy, although studies in humans with glucagon-like peptide-1 receptor agonists are encouraging. As patients with T2DM also often have cardiovascular disease, the increased rigor in drug development should ultimately reduce the health burden of both of these conditions.

The dipeptidyl peptidase-4 inhibitor linagliptin attenuates inflammation and accelerates epithelialization in wounds of diabetic ob/ob mice

In recent years, new and effective therapeutic agents for blood glucose control have been added to standard diabetes therapies: dipeptidyl peptidase-4 (DPP-4) inhibitors, which prolong the bioavailability of the endogenously secreted incretin hormone glucagon-like peptide-1 (GLP-1). Full-thickness excisional wounding was performed in wild-type (C57BL/6J) and diabetic [C57BL/6J-obese/obese (ob/ob)] mice. DPP-4 activity was inhibited by oral administration of linagliptin during healing. Wound tissue was analyzed by using histological, molecular, and biochemical techniques. In healthy mice, DPP-4 was constitutively expressed in the keratinocytes of nonwounded skin. After skin injury, DPP-4 expression declined and was lowest during the most active phase of tissue reassembly. In contrast, in ob/ob mice, we observed increasing levels of DPP-4 at late time points, when delayed tissue repair still occurs. Oral administration of the DPP-4 inhibitor linagliptin strongly reduced DPP-4 activity, stabilized active GLP-1 in chronic wounds, and improved healing in ob/ob mice. At day 10 postwounding, linagliptin-treated ob/ob mice showed largely epithelialized wounds characterized by the absence of neutrophils. In addition, DPP-4 inhibition reduced the expression of the proinflammatory markers cyclooxygenase-2 and macrophage inflammatory protein-2, but enhanced the formation of myofibroblasts in healing wounds from ob/ob mice. Our data suggest a potentially beneficial role of DPP-4 inhibition in diabetes-affected wound healing.

Cardiovascular biology of the incretin system

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that enhances glucose-stimulated insulin secretion and exerts direct and indirect actions on the cardiovascular system. GLP-1 and its related incretin hormone, glucose-dependent insulinotropic polypeptide, are rapidly inactivated by the enzyme dipeptidyl peptidase 4 (DPP-4), a key determinant of incretin bioactivity. Two classes of medications that enhance incretin action, GLP-1 receptor (GLP-1R) agonists and DPP-4 inhibitors, are used for the treatment of type 2 diabetes mellitus. We review herein the cardiovascular biology of GLP-1R agonists and DPP-4 inhibitors, including direct and indirect effects on cardiomyocytes, blood vessels, adipocytes, the control of blood pressure, and postprandial lipoprotein secretion. Both GLP-1R activation and DPP-4 inhibition exert multiple cardioprotective actions in preclinical models of cardiovascular dysfunction, and short-term studies in human subjects appear to demonstrate modest yet beneficial actions on cardiac function in subjects with ischemic heart disease. Incretin-based agents control body weight, improve glycemic control with a low risk of hypoglycemia, decrease blood pressure, inhibit the secretion of intestinal chylomicrons, and reduce inflammation in preclinical studies. Nevertheless, there is limited information on the cardiovascular actions of these agents in patients with diabetes and established cardiovascular disease. Hence, a more complete understanding of the cardiovascular risk to benefit ratio of incretin-based therapies will require completion of long-term cardiovascular outcome studies currently underway in patients with type 2 diabetes mellitus.

Building multidimensional biomarker views of type 2 diabetes on the basis of protein microheterogeneity

BACKGROUND

In 2008, the US Food and Drug Administration (FDA) issued a Guidance for Industry statement formally recognizing (during drug development) the conjoined nature of type 2 diabetes (T2D) and cardiovascular disease (CVD), which has precipitated an urgent need for panels of markers (and means of analysis) that are able to differentiate subtypes of CVD in the context of T2D. Here, we explore the possibility of creating such panels using the working hypothesis that proteins, in addition to carrying time-cumulative marks of hyperglycemia (e.g., protein glycation in the form of Hb A(₁c)), may carry analogous information with regard to systemic oxidative stress and aberrant enzymatic signaling related to underlying pathobiologies involved in T2D and/or CVD.

METHODS

We used mass spectrometric immunoassay to quantify, in targeted fashion, relative differences in the glycation, oxidation, and truncation of 11 specific proteins.

RESULTS

Protein oxidation and truncation (owing to modified enzymatic activity) are able to distinguish between subsets of diabetic patients with or without a history of myocardial infarction and/or congestive heart failure where markers of glycation alone cannot.

CONCLUSION

Markers based on protein modifications aligned with the known pathobiologies of T2D represent a reservoir of potential cardiovascular markers that are needed to develop the next generation of antidiabetes medications.

DPP4 inhibitors for diabetes--what next?

With vildagliptin and sitagliptin on the market for the treatment of type 2 diabetes, dipeptidyl peptidase 4 (DPP4, EC 3.4.14.5) research has entered a new era. Scientists aim to uncover the broader pharmacological profile of DPP4 inhibitors and search for therapeutic opportunities outside diabetes. During the pre-clinical and clinical evaluation of vildagliptin and sitagliptin, there has been a growing awareness of the presence of other DPP4-like peptidases in various cells and tissues. This fuelled the development of more inhibitors with defined selectivity for DPP2, 8 and 9 that were used to investigate the expression, distribution and regulation of these peptidases. In turn, these studies increased the insights in the role of DPP4 in the body's response to various insults.

Neuropeptide Y (NPY) cleaving enzymes: structural and functional homologues of dipeptidyl peptidase 4

N-terminal truncation of NPY has important physiological consequences, because the truncated peptides lose their capability to activate the Y1-receptor. The sources of N-terminally truncated NPY and related peptides are unknown and several proline specific peptidases may be involved. First, we therefore provide an overview on the peptidases, belonging to structural and functional homologues of dipeptidyl peptidase 4 (DP4) as well as aminopeptidase P (APP) and thus, represent potential candidates of NPY cleavage in vivo. Second, applying selective inhibitors against DP4, DP8/9 and DP2, respectively, the enzymatic distribution was analyzed in brain extracts from wild type and DP4 deficient F344 rat substrains and human plasma samples in activity studies as well as by matrix assisted laser desorption/ionisation-time of flight (MALDI-TOF)-mass spectrometry. Third, co-transfection of Cos-1 cells with Dpp4 and Npy followed by confocal lasermicroscopy illustrated that hNPY-dsRed1-N1 was transported in large dense core vesicles towards the membrane while rDP4-GFP-C1 was transported primarily in different vesicles thereby providing no clear evidence for co-localization of NPY and DP4. Nevertheless, the review and experimental results of activity and mass spectrometry studies support the notion that at least five peptidases (DP4, DP8, DP9, XPNPEP1, XPNPEP2) are potentially involved in NPY cleavage while the serine protease DP4 (CD26) could be the principal peptidase involved in the N-terminal truncation of NPY. However, DP8 and DP9 are also capable of cleaving NPY, whereas no cleavage could be demonstrated for DP2.

Dipeptidyl peptidase II (DPPII), a review

An increasing number of biological processes appear to be regulated by Pro-specific N-terminal processing. The proline-specific dipeptidyl peptidases (DPPs) like DPPIV, fibroblast activation protein alpha (FAP), DPPII, DPP8 and DPP9, because of their preference for cleavage after X-Pro in vitro, are likely to be involved in many of these processes. These DPPs are emerging as an important protease family with roles in the regulation of signaling by peptide hormones. Dipeptidyl peptidase II (DPPII, E.C. 3.4.14.2) is an intracellular protease that localizes to the vesicular system. It releases, preferably at acidic pH, N-terminal dipeptides from oligopeptides with Pro or Ala in the penultimate position. Despite the fact that the physiological role of DPPII still has not been elucidated, several suggestions were made on possible functions of the enzyme depending on its localization in different cells, body fluids and organs. DPPII was a.o. suggested to be involved in the processes of cell differentiation and in the protection from cell death, and to have a role in the degradation of collagen fragments, myofibrillar proteins and short neuropeptides. Moreover, changes in the level and distribution of the enzyme provided clues indicating additional roles in disease-related processes. Here we review the DPPII literature, aiming to bring more clarity in the disperse data on this subject and give a state of the art on DPPII research.

Dipeptidyl peptidase II and leukocyte cell death

Dipeptidyl peptidase (DPP) II (E.C. 3.4.14.2) is an intracellular protease that releases, preferably at acidic pH, N-terminal dipeptides from oligopeptides with Pro or Ala in the penultimate position. The natural substrates and the physiological role of DPPII remain unclear. The aim of the present study was to investigate the involvement of DPPII activity in different forms of cell death (apoptosis, necrosis and autophagy) in human leukocytes. We determined specific DPP activities in leukocytes. Compared to other subpopulations of peripheral blood mononuclear cells (PBMC), we observed relatively high DPPII specific activity in monocytic cells, opening new perspectives for further investigation of the DPPII functions. A second intriguing finding was that DPPII specific activity increased during necrosis, whereas induction of apoptosis or autophagy did not affect any of the dipeptidyl peptidase activities. Finally, we showed that inhibition of DPPII (>90%) using the in vitro applicable, highly potent (K(i) of 0.082+/-0.048 nM) and selective DPPII inhibitor UAMC00039, did not induce any form of cell death in leukocytes. These data are of importance for a more precise interpretation of the in vitro and in vivo effects of other dipeptidyl peptidase inhibitors.