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

Mechanisms underlying the rapid degradation and elimination of the incretin hormones GLP-1 and GIP

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP, gastric inhibitory peptide) are secreted from intestinal L and K cells and stimulate insulin secretion from pancreatic beta cells. However, they are immediately inactivated mainly via N-terminal degradation by dipeptidyl peptidase IV (DPP IV, CD26), a specialised enzyme located on the cell surface enzyme of endothelial, epithelial and some other cell types. Cleavage by neprilysin (neutral endopeptidase) is a minor degradation route, and renal clearance eliminates incretin/fragments, but appears of less importance for regulating incretin bioactivities. Based on these observations two novel types of drugs for the treatment of type 2 diabetes have been developed: DPP IV inhibitors and DPP IV-resistant incretin analogues. Both have distinct advantages and disadvantages. Potential side effects of DPP IV inhibitors may result from affecting the bioactivity of other hormones, neuropeptides or chemokines and also by their cross-reactivity with DPP IV-related enzymes.

Dipeptidyl peptidase-IV inhibitor use associated with increased risk of ACE inhibitor-associated angioedema

Dipeptidyl peptidase-IV (DPP-IV) inhibitors decrease degradation of the incretins. DPP-IV inhibitors also decrease degradation of peptides, such as substance P, that may be involved in the pathogenesis of angiotensin-converting enzyme (ACE) inhibitor-associated angioedema. This study tested the hypothesis that DPP-IV inhibition affects risk of clinical angioedema, by comparing the incidence of angioedema in patients treated with the DPP-IV inhibitor vildagliptin versus those treated with comparator in Phase III randomized clinical trials. Prospectively defined angioedema-related events were adjudicated in a blinded fashion by an internal medicine adjudication committee and expert reviewer. Concurrent ACE inhibitor or angiotensin receptor blocker exposure was ascertained from case report forms. Study drug exposure was ascertained from unblinded data from phase III studies. Odds ratios and 95% confidence intervals comparing angioedema risk in vildagliptin-treated and comparator-treated patients were calculated for the overall population and for patients taking ACE inhibitors or angiotensin receptor blockers, using both an analysis of pooled data and a meta-analysis (Peto method). Overall, there was no association between vildagliptin use and angioedema. Among individuals taking an ACE inhibitor, however, vildagliptin use was associated with an increased risk of angioedema (14 confirmed cases among 2754 vildagliptin users versus 1 case among 1819 comparator users: odds ratio 4.57 [95% confidence interval 1.57 to 13.28]) in the meta-analysis. Vildagliptin use may be associated with increased risk of angioedema among patients taking ACE inhibitors, although absolute risk is small. Physicians confronted with angioedema in a patient taking an ACE inhibitor and DPP-IV inhibitor should consider this possible drug-drug interaction.

3D-QSAR studies on triazolopiperazine amide inhibitors of dipeptidyl peptidase-IV as anti-diabetic agents

Three-dimensional quantitative structure-activity relationship (3D-QSAR) analyses were carried out on 45 triazolopiperazine amide derivatives as dipeptidyl peptidase IV (DPP-IV) inhibitors in order to elucidate their antidiabetic activities. The studies include Comparative Molecular Field Analysis (CoMFA) and Comparative Molecular Similarity Indices Analysis (CoMSIA). Models with good predictive abilities were generated with the cross-validated r(2) (r(2)(cv)) and conventional r(2) values of 0.589 and 0.868 for CoMFA and 0.586 and 0.868 for CoMSIA, respectively. Both models were validated by a test set of nine compounds and gave satisfactory predictive r(2) (r(2)(pred)) values of 0.816 and 0.863, respectively. CoMFA and CoMSIA contour maps were then used to analyse the structural features of the ligands to account for the activity in terms of positively contributing physicochemical properties: steric, electrostatic, hydrophobic and hydrogen bond acceptor fields. The information obtained from CoMFA and CoMSIA three-dimensional contour maps can be used for further design of triazolopiperazine amide-based analogues as anti-diabetic agents.

Enzyme activity and immunohistochemical localization of dipeptidyl peptidase 8 and 9 in male reproductive tissues

The mRNA expression pattern of dipeptidyl peptidase (DPP) 8 and DPP9, two DPP4 homologs, was studied previously and showed a broad tissue distribution. In this study, protein expression and activity of DPP8 and DPP9 were investigated in male reproductive tissues of different mammals. Based on specific DPP activities and inhibition profiles, the proline-selective DPP activity in the bovine and rat testis could predominantly be attributed to DPP8/9 and not to DPP4. This is in contrast to the epididymis, where most of the activity was caused by DPP4. Bovine sperm preparations had very low or undetectable DPP8/9 activity. After characterization of polyclonal antibodies specific for DPP8 or DPP9, we could localize both enzymes in seminiferous tubules of the testis. A specific staining for DPP9 was found associated with spermatozoids embedded in the epithelium, just before their release into the lumen, and in spermatids. DPP8 was localized in spermatozoids in an earlier stage of maturation. These findings help to provide insight into the physiological role of DPP4-like enzymes in the male reproductive system. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Chronic administration of voglibose, an alpha-glucosidase inhibitor, increases active glucagon-like peptide-1 levels by increasing its secretion and decreasing dipeptidyl peptidase-4 activity in ob/ob mice

Administration of an alpha-glucosidase inhibitor, voglibose, increases the secretion of glucagon-like peptide (GLP)-1, a key modulator of pancreatic islet hormone secretion and glucose homeostasis. In the present study, novel mechanisms by which voglibose increases active GLP-1 circulation were evaluated. Voglibose (0.001 and 0.005%) was administered in the diet to ob/ob mice for 1 day or 3 to 4 weeks to determine effects on incretin profiles and plasma activity of dipeptidyl peptidase-4 (DPP-4), an enzyme responsible for GLP-1 degradation. Voglibose showed no direct inhibitory effect against DPP-4 in vitro (DPP-4 inhibitor alogliptin, IC(50) < 10 nM). Likewise, 1-day treatment with voglibose did not change plasma DPP-4 activity; however, it increased plasma active GLP-1 by 1.6- to 3.4-fold. After chronic treatment, voglibose stimulated GLP-1 secretion, as evidenced by the 1.3- to 1.5-fold increase in plasma active plus inactive amidated GLP-1 levels. Plasma DPP-4 activity was decreased unexpectedly by 40 to 51%, resulting from reduced plasma DPP-4 concentrations in voglibose-treated mice. Voglibose increased GLP-1 content by 1.5- to 1.6-fold and 1.4- to 1.6-fold in the lower intestine and colon, respectively. The increased GLP-1 content in the colon was associated with elevated expression of gut glucagon gene. Chronic treatment with voglibose resulted in 1.9- to 4.1-fold increase in active GLP-1 circulation, which was higher than 1-day treatment. A similar treatment with pioglitazone (0.03%), an insulin sensitizer, did not affect plasma DPP-4 activity or GLP-1 levels. These results suggest that increased GLP-1 secretion, decreased DPP-4 activity, and increased gut GLP-1 content may have contributed to increased active GLP-1 circulation after chronic treatment with voglibose in a glucose control-independent manner in ob/ob mice.

Combining a dipeptidyl peptidase-4 inhibitor, alogliptin, with pioglitazone improves glycaemic control, lipid profiles and beta-cell function in db/db mice

BACKGROUND AND PURPOSE

Alogliptin, a highly selective dipeptidyl peptidase-4 (DPP-4) inhibitor, enhances incretin action and pioglitazone enhances hepatic and peripheral insulin actions. Here, we have evaluated the effects of combining these agents in diabetic mice.

EXPERIMENTAL APPROACH

Effects of short-term treatment with alogliptin alone (0.01%-0.1% in diet), and chronic combination treatment with alogliptin (0.03% in diet) and pioglitazone (0.0075% in diet) were evaluated in db/db mice exhibiting early stages of diabetes.

KEY RESULTS

Alogliptin inhibited plasma DPP-4 activity up to 84% and increased plasma active glucagon-like peptide-1 by 4.4- to 4.9-fold. Unexpectedly, alogliptin alone lacked clear efficacy for improving glucose levels. However, alogliptin in combination with pioglitazone clearly enhanced the effects of pioglitazone alone. After 3-4 weeks of treatment, combination treatment increased plasma insulin by 3.8-fold, decreased plasma glucagon by 41%, both of which were greater than each drug alone, and increased plasma adiponectin by 2.4-fold. In addition, combination treatment decreased glycosylated haemoglobin by 2.2%, plasma glucose by 52%, plasma triglycerides by 77% and non-esterified fatty acids by 48%, all of which were greater than each drug alone. Combination treatment also increased expression of insulin and pancreatic and duodenal homeobox 1 (PDX1), maintained normal beta-cell/alpha-cell distribution in islets and restored pancreatic insulin content to levels comparable to non-diabetic mice.

CONCLUSIONS AND IMPLICATIONS

These results indicate that combination treatment with alogliptin and pioglitazone at an early stage of diabetes improved metabolic profiles and indices that measure beta-cell function, and maintained islet structure in db/db mice, compared with either alogliptin or pioglitazone monotherapy.

New treatments in type 2 diabetes: a focus on the incretin-based therapies

The demonstration that the incretin hormone glucagon-like peptide 1 can improve glycaemic control in patients with type 2 diabetes has led to the rapid development during the last decade of promising new classes of agent for the management of type 2 diabetes. These agents possess a range of physiological effects that are associated with improved glycaemic control in diabetes including stimulation of glucose-dependent insulin secretion, suppression of glucagon secretion, slowing of gastric emptying, and reduction of food intake. In addition, preclinical studies suggest that incretin-based therapies may improve beta-cell function via enhancement of beta-cell mass and induction of genes important for differentiated beta-cell function. Exenatide, and the dipeptidyl peptidase-4 inhibitors, sitagliptin and vildagliptin are already approved, and liraglutide is currently completing Phase 3 trials. As these agents and standard oral therapies for type 2 diabetes lower glucose levels through different, but potentially complementary mechanisms, their use in combination should provide effective, potentially additive, glycaemic control. The incretin-based therapies also offer other advantages such as weight loss with exenatide and liraglutide, a reduced risk of hypoglycaemia, and as suggested by preclinical studies, a potential beta-cell preserving effect. Long-term outcome and safety data are not available for these agents, but they appear generally well-tolerated in comparison with existing therapies for type 2 diabetes. The multiple underlying glucose-lowering actions of the incretin-based therapies, as well as a lack of weight gain or even weight loss, make these important new additions to available antidiabetic agents expanding the treatment options available for patients.

Prodrugs of thyrotropin-releasing hormone and related peptides as central nervous system agents

Prodrug design for brain delivery of small- and medium-sized neuropeptides was reviewed, focusing on thyrotropin-releasing hormone and structurally related peptides as examples. We have summarized our most important advances in methodology, as well as assessed the benefits and limitations of bioreversible chemical manipulation techniques to achieve targeting of the parent molecules into the central nervous system. The value of prodrug-amenable analogues as potential drug-like central nervous systems agents was highlighted.

Drug evaluation: vildagliptin-metformin single-tablet combination

The single-tablet combination of vildagliptin and metformin addresses key defects of type 2 diabetes for improved glycemic control. By inhibiting the dipeptidyl peptidase-4 (DPP-4) enzyme, vildagliptin raises the levels of the active incretin hormones, glucagon-like peptide 1 and glucose-dependent insulinotropic peptide. This leads to increased synthesis and release of insulin from the pancreatic beta cells and decreased release of glucagon from the pancreatic alpha cells. The combination tablet also contains metformin, which addresses insulin resistance. The complementary mechanisms of action of the two agents in combination have been shown to provide additive and sustained reductions in hemoglobin A(1c) compared with metformin monotherapy. In active-controlled trials, the vildagliptin-metformin combination has been shown to produce equivalent reductions in hemoglobin A(1c) to pioglitazone-metformin and glimepiride-metformin combinations, without significant risk of hypoglycemia and without causing weight gain. In clinical trials, the overall incidence of any adverse event was similar in patients randomized to vildagliptin plus metformin and placebo plus metformin. Available data support the use of vildagliptin in combination with metformin as a promising second-line treatment for the management of type 2 diabetes and this is reflected in the latest UK National Institute for Health and Clinical Excellence draft guideline for consultation on new agents for blood glucose control in type 2 diabetes.

Treating type 2 diabetes: how safe are current therapeutic agents?

Sulphonylureas (SUs) and biguanides (metformin) are the current mainstays in the treatment of type 2 diabetes (T2DM) and represent the most commonly used oral hypoglycaemic agents (OHAs). In recent years, a variety of new OHAs have become available, including thiazolidinediones, glinides, alpha-glucosidase inhibitors, glucagon-like peptide-1 agonists, amylin analogues and dipeptidyl peptidase-IV inhibitors, providing physicians with a larger therapeutic catalogue than ever before. The traditional drugs metformin and SUs have an established safety profile through long-term use. However, long-term clinical trials and routine use are lacking for many of the new agents, and some potentially serious side effects have been reported with several of these compounds. Until adequate data is obtained, it is difficult to assess the risk-benefit ratio of these agents in relation to the traditional drugs. Until that becomes fully documented, it may be wise to start pharmacologic treatment of patients on an individual basis, weighing the benefits and costs of each medication. Thus, there remains a place for well-established drugs that have a proven safety record and are supported by years of clinical use for the treatment of T2DM.

Emerging role of dipeptidyl peptidase-4 inhibitors in the management of type 2 diabetes

BACKGROUND

In type 2 diabetes mellitus (T2DM) there is a progressive loss of beta-cell function. One new approach yielding promising results is the use of the orally active dipeptidyl peptidase-4 (DPP-4) inhibitors. However, every new compound for T2DM has to prove long-term safety especially on cardiovascular outcomes.

OBJECTIVES

Systematic review and meta-analysis of the effects of sitagliptin and vildagliptin therapy on main efficacy parameters and safety. SELECTION CRITERIA, DATA COLLECTION, AND ANALYSIS: Randomized controlled clinical studies of at least 12 weeks' duration in T2DM.

RESULTS

DPP-4 inhibitors versus placebo showed glycosylated hemoglobin A1c (A1c) improvements of 0.7% versus placebo but not compared to monotherapy with other hypoglycemic agents (0.3% in favor of controls). The overall risk profile of DPP-4 inhibitors was low, however a 34% relative risk increase (95% confidence interval 10% to 64%, P = 0.004) was noted for all-cause infection associated with sitagliptin use. No data on immune function, health-related quality of life and diabetic complications could be extracted.

CONCLUSIONS

DPP-4 inhibitors have some theoretical advantages over existing therapies with oral antidiabetic compounds but should currently be restricted to individual patients. Long-term data on cardiovascular outcomes and safety are needed before widespread use of these new agents.

Dipeptidyl-peptidase IV and B-type natriuretic peptide. From bench to bedside

B-type natriuretic peptide (BNP) has emerged as a reliable biomarker in patients with congestive heart failure. The mature, biologically active B-type natriuretic peptide, BNP

Clin Chem Lab Med 2009;47:248–52.

Peptidomic analysis of blood plasma after in vivo treatment with protease inhibitors--a proof of concept study

Native peptides can be regarded as surrogate markers for protease activity in biological samples. Analysis of peptides by peptidomics allows to monitor protease activity in vivo and to describe the influence of protease inhibition. To elucidate the potential of peptides as markers for in vivo protease inhibition we analyzed plasma samples from animals treated with either the indirect FXa inhibitor FONDAPARINUX or the dipeptidylpeptidase IV inhibitor AB192. Signals correlating with the treatment were subsequently identified and assessed with respect to protease-dependent consensus cleavage motifs and occurrence of downstream targets. It could be shown that regulated peptides were either substrates, products or downstream targets of the inhibited protease. The results from the present study demonstrate that the in vivo analysis of peptides by peptidomics has the potential to broaden the knowledge of inhibitor related effects in vivo and that this method may pave the way to develop predictive biomarkers.

The dipeptidyl peptidase-4 inhibitor alogliptin in combination with pioglitazone improves glycemic control, lipid profiles, and increases pancreatic insulin content in ob/ob mice

The combination of two agents with different but complementary mechanisms of action is a logical approach for treating patients with type 2 diabetes. Thus, we evaluated chronic combination therapy with alogliptin, a highly selective dipeptidyl peptidase-4 inhibitor that enhances the action of incretins, and pioglitazone, a thiazolidinedione that improves peripheral and hepatic insulin sensitivity. Studies were designed to investigate the chronic metabolic and pancreatic effects of alogliptin (0.03%) plus pioglitazone (0.003%) combination treatment in obese ob/ob mice. After 4-5 weeks of treatment, alogliptin significantly increased plasma active glucagon-like peptide-1 levels up to 4.1-fold and decreased plasma glucagon up to 25%, whereas pioglitazone significantly increased plasma adiponectin up to 1.3-fold. Combination treatment exhibited a complementary effect, increasing plasma insulin levels by 3.2-fold (alogliptin alone, 1.6-fold; pioglitazone alone, 1.5-fold) and decreasing glycosylated hemoglobin by 2.3% (alogliptin alone, 1.0%; pioglitazone alone, 1.5%), and non-fasting and fasting plasma glucose by 37% and 62% (alogliptin alone, 17% and 24%; pioglitazone alone, 30% and 45%), respectively. Combination treatment also decreased plasma triglycerides by 67% and non-esterified fatty acids by 25% (alogliptin alone, 24% and 11%; pioglitazone alone, 54% and 8%). Moreover, combination treatment increased pancreatic insulin content by 2.2-fold (alogliptin alone, 1.3-fold; pioglitazone alone, 1.6-fold), with no significant changes in body weight. These results indicate that combination treatment with alogliptin and pioglitazone improved glycemic control, lipid profiles and increased pancreatic insulin content in ob/ob mice by preventing incretin inactivation and improving insulin resistance. These results provide a strong argument for using alogliptin in combination with pioglitazone.

DPP6 Localization in Brain Supports Function as a Kv4 Channel Associated Protein

The gene encoding the dipeptidyl peptidase-like protein DPP6 (also known as DPPX) has been associated with human neural disease. However, until recently no function had been found for this protein. It has been proposed that DPP6 is an auxiliary subunit of neuronal Kv4 K(+) channels, the ion channels responsible for the somato-dendritic A-type K(+) current, an ionic current with crucial roles in the regulation of firing frequency, dendritic integration and synaptic plasticity. This view has been supported mainly by studies showing that DPP6 is necessary to generate channels with biophysical properties resembling the native channels in some neurons. However, independent evidence that DPP6 is a component of neuronal Kv4 channels in the brain, and whether this protein has other functions in the CNS is still lacking. We generated antibodies to DPP6 proteins to compare their distribution in brain with that of the Kv4 pore-forming subunits. DPP6 proteins were prominently expressed in neuronal populations expressing Kv4.2 proteins and both types of protein were enriched in the dendrites of these cells, strongly supporting the hypothesis that DPP6 is an associated protein of Kv4 channels in brain neurons. The observed similarity in the cellular and subcellular patterns of expression of both proteins suggests that this is the main function of DPP6 in brain. However, we also found that DPP6 antibodies intensely labeled the hippocampal mossy fiber axons, which lack Kv4 proteins, suggesting that DPP6 proteins may have additional, Kv4-unrelated functions.

Modulation of cytokine production and silica-induced lung fibrosis by inhibitors of aminopeptidase N and of dipeptidyl peptidase-IV-related proteases

AIMS

Dipeptidyl peptidase IV (DP IV)-related proteases and aminopeptidase N (APN) are drug targets in various diseases. Here we investigated for the first time the effects of DP-IV-related protease inhibitors and APN inhibitors on chronic inflammatory lung diseases.

MAIN METHODS

A murine model of silica (SiO2)-induced lung fibrosis and in vitro cultures of human lung epithelial cells and monocytes have been used and the influence of silica-treatment and inhibitors on inflammation and fibrosis has been measured.

KEY FINDINGS

We found increased inflammation and secretion of the chemokines IL-6, MCP-1 and MIP-alpha 2 weeks after SiO2 application, and increased lung fibrosis after 3 months. Treatment with the APN inhibitor actinonin reduced chemokine secretion in the lung and bronchoalveolar lavage fluid, and in cell culture, and decreased the level of fibrosis after 3 months. Treatment with inhibitors of DP-IV-related proteases, or a combination of DP IV inhibitors and APN inhibitors, had no significant effect. We found no obvious side effects of long-term treatment with inhibitors of APN and DP IV.

SIGNIFICANCE

Overall, our findings show that actinonin, an inhibitor of aminopeptidase N, might modulate chemokine secretion in the lung and thus attenuate the development of lung fibrosis. Additional targeting of DP-IV-related proteases had no significant effect on these processes.

Weighing the evidence for a ternary protein complex mediating A-type K+ currents in neurons

The subthreshold-operating A-type K(+) current in neurons (I(SA)) has important roles in the regulation of neuronal excitability, the timing of action potential firing and synaptic integration and plasticity. The channels mediating this current (Kv4 channels) have been implicated in epilepsy, the control of dopamine release, and the regulation of pain plasticity. It has been proposed that Kv4 channels in neurons are ternary complexes of three types of protein: pore forming subunits of the Kv4 subfamily and two types of auxiliary subunits, the Ca(2+) binding proteins KChIPs and the dipeptidyl peptidase-like proteins (DPPLs) DPP6 (also known as DPPX) and DPP10 (4 molecules of each per channel for a total of 12 proteins in the complex). Here we consider the evidence supporting this hypothesis. Kv4 channels in many neurons are likely to be ternary complexes of these three types of protein. KChIPs and DPPLs are required to efficiently traffic Kv4 channels to the plasma membrane and regulate the functional properties of the channels. These proteins may also be important in determining the localization of the channels to specific neuronal compartments, their dynamics, and their response to neuromodulators. A surprisingly large number of additional proteins have been shown to modify Kv4 channels in heterologous expression systems, but their association with native Kv4 channels in neurons has not been properly validated. A critical consideration of the evidence suggests that it is unlikely that association of Kv4 channels with these additional proteins is widespread in the CNS. However, we cannot exclude that some of these proteins may associate with the channels transiently or in specific neurons or neuronal compartments, or that they may associate with the channels in other tissues.

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.

DPP4 inhibitors: a new approach in diabetes treatment

The role of dipeptidyl peptidase-IV (DPP4) as both a regulatory enzyme and a signalling factor has been evaluated and described in many studies. DPP4 inhibition results in increased blood concentration of the incretin hormones glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP). This causes an increase in glucose-dependent stimulation of insulin secretion, resulting in a lowering of blood glucose levels. Recent studies have shown that DPP4 inhibitors can induce a significant reduction in glycosylated haemoglobin (HbA(1c)) levels, either as monotherapy or as a combination with other antidiabetic agents. Research has also demonstrated that DPP4 inhibitors portray a very low risk of hypoglycaemia development. This review article focuses on the two leading agents of this category (sitagliptin and vildagliptin), providing an overview of their function along with the latest data regarding their clinical efficacy as antidiabetic agents.

Exosome-like vesicles with dipeptidyl peptidase IV in human saliva

Saliva contains a large number of proteins that participate in the protection of oral tissue. We found, for the first time, small vesicles (30-130 nm in diameter) in human whole saliva. Vesicles from saliva were identified by electron microscopy after isolation by gel-filtration on Sepharose CL-4B. They resemble exosomes, which are vesicles with an endosome-derived limiting membrane that are secreted by a diverse range of cell types. We performed a biochemical characterization of these vesicles by amino acid sequence analysis and Western blot analysis. We found that they contain dipeptidyl peptidase IV (DPP IV), galectin-3 and immunoglobulin A, which have potential to influence immune response. The DPP IV in the vesicles was metabolically active in cleaving substance P and glucose-dependent insulinotropic polypeptide to release N-terminal dipeptides. Our results demonstrate that human whole saliva contains exosome-like vesicles; they might participate in the catabolism of bioactive peptides and play a regulatory role in local immune defense in the oral cavity.

Inhibition of dipeptidyl peptidase IV with sitagliptin (MK0431) prolongs islet graft survival in streptozotocin-induced diabetic mice

OBJECTIVE

Dipeptidyl peptidase-IV (DPP-IV) inhibitors have been introduced as therapeutics for type 2 diabetes. They partially act by blocking degradation of the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), thus increasing circulating levels of active hormones. In addition to their insulinotropic actions, GLP-1 and GIP also promote beta-cell proliferation and survival, and DPP-IV inhibitors exert similar effects in rodent type 2 diabetes models. The study objective was to establish whether DPP-IV inhibitor treatment prolonged survival of transplanted islets and to determine whether positron emission tomography (PET) was appropriate for quantifying the effect of inhibition on islet mass.

RESEARCH DESIGN & METHODS

Effects of the DPP-IV inhibitor MK0431 (sitagliptin) on glycemic control and functional islet mass in a streptozotocin (STZ)-induced type 1 diabetes mouse model were determined with metabolic studies and microPET imaging.

RESULTS

The type 1 diabetes mouse model exhibited elevated plasma DPP-IV levels that were substantially inhibited in mice on an MK0431 diet. Residual beta-cell mass was extremely low in STZ-induced diabetic mice, and although active GLP-1 levels were increased by the MK0431 diet, there were no significant effects on glycemic control. After islet transplantation, mice fed normal diet rapidly lost their ability to regulate blood glucose, reflecting the suboptimal islet transplant. By contrast, the MK0431 group fully regulated blood glucose throughout the study, and PET imaging demonstrated a profound protective effect of MK0431 on islet graft size.

CONCLUSIONS

Treatment with a DPP-IV inhibitor can prolong islet graft retention in an animal model of type 1 diabetes.

Synthesis and use of 4-peptidylhydrazido-N-hexyl-1,8-naphthalimides as fluorogenic histochemical substrates for dipeptidyl peptidase IV and tripeptidyl peptidase I

Gly-Pro-, Gly-Pro-Met- and Ala-Ala-Phe-N'-(2-hexyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)-hydrazides are synthesized by guanidinium/uronium type condensing reagent and used as fluorogenic substrates to localize dipeptidyl peptidase IV and tripeptidyl peptidase I activities in mammalian tissue sections. Enzyme hydrolysis releases 2-hexyl-6-hydrazino-1H-benzo[de]isoquinoline-1,3(2H)-dione, which couples with piperonal to form insoluble fluorescent hydrazone, precipitating on the enzyme locations and marking them. The fluorescent technique reveals precisely the enzymes locations at the lack of background noise in a single incubation step. It avoids most of the drawbacks of the previously proposed fluorescent histochemical techniques and can be valuable for the in situ studies of these enzymes in norm and pathology.

Glucagon-like peptide-1, glucose homeostasis and diabetes

Incretins, enhancers of insulin secretion, are essential for glucose tolerance, and a reduction in their function might contribute to poor beta-cell function in patients with type-2 diabetes mellitus. However, at supraphysiological doses, the incretin glucagon-like peptide-1 (GLP-1) protects pancreatic beta cells, and inhibits glucagon secretion, gastric emptying and food intake, leading to weight loss. GLP-1 mimetics, which are stable-peptide-based activators of the GLP-1 receptor, and incretin enhancers, which inhibit the incretin-degrading enzyme dipeptidyl peptidase-4, have emerged as therapies for type-2 diabetes and have recently reached the market. The pathophysiological basis the clinical use of these therapeutics is reviewed here.

Dipeptidyl peptidase IV inhibition downregulates Na+-H+exchanger NHE3 in rat renal proximal tubule

In the microvillar microdomain of the kidney brush border, sodium hydrogen exchanger type 3 (NHE3) exists in physical complexes with the serine protease dipeptidyl peptidase IV (DPPIV). The purpose of this study was to explore the functional relationship between NHE3 and DPPIV in the intact proximal tubule in vivo. To this end, male Wistar rats were treated with an injection of the reversible DPPIV inhibitor Lys [Z(NO2)]-pyrrolidide (I40; 60 mg·kg−1·day−1ip) for 7 days. Rats injected with equal amounts of the noninhibitory compound Lys[Z(NO2)]-OH served as controls. Na+-H+exchange activity in isolated microvillar membrane vesicles was 45 ± 5% decreased in rats treated with I40. Membrane fractionation studies using isopycnic centrifugation revealed that I40 provoked redistribution of NHE3 along with a small fraction of DPPIV from the apical enriched microvillar membranes to the intermicrovillar microdomain of the brush border. I40 significantly increased urine output (67 ± 9%; P < 0.01), fractional sodium excretion (63 ± 7%; P < 0.01), as well as lithium clearance (81 ± 9%; P < 0.01), an index of end-proximal tubule delivery. Although not significant, a tendency toward decreased blood pressure and plasma pH/HCO3−was noted in I40-treated rats. These findings indicate that inhibition of DPPIV catalytic activity is associated with inhibition of NHE3-mediated NaHCO3reabsorption in rat renal proximal tubule. Inhibition of apical Na+-H+exchange is due to reduced abundance of NHE3 protein in the microvillar microdomain of the kidney brush border. Moreover, this study demonstrates a physiologically significant interaction between NHE3 and DPPIV in the intact proximal tubule in vivo.

Functional ontogeny of the proglucagon-derived peptide axis in the premature human neonate

BACKGROUND

The regulation of intestinal growth and development in human neonates is incompletely understood, which hinders the provision of nutrients enterally. The "hindgut" hormones glucagon-like peptides 1 and 2 have been shown to play an important role in the regulation of nutrient assimilation, intestinal growth, and function.

OBJECTIVE

Our goal was to investigate the production of glucagon-like peptides 1 and 2 in premature human infants and examine the effects of prematurity and feeding on hormone release.

PATIENTS AND METHODS

With informed consent, premature infants who were admitted to a tertiary neonatal intensive care nursery (gestational age: 28-32 weeks) were monitored with weekly determinations of postprandial glucagon-like peptide 1 and 2 levels. Comparison studies with groups of normal infants and adults were performed. Hormone levels were obtained by using specific radioimmunoassay for glucagon-like peptide 1 (1-36) and glucagon-like peptide 2 (1-33), modified for small sample volumes; accurate monitoring of enteral intake was performed at all of the sampling time points.

RESULTS

Forty-five infants with a mean gestational age of 29.6 +/- 1.9 weeks were studied; fasting levels of both glucagon-like peptides 1 and 2 were elevated. There was no correlation between gestational age and glucagon-like peptide 2 output. However, both glucagon-like peptide 1 and 2 levels were correlated with the caloric value of feeds.

CONCLUSIONS

The premature human neonate has significantly higher fasting levels of glucagon-like peptides 1 and 2 compared with adults; feeding increases these levels further. These findings suggest that the proglucagon-derived peptides may have a role in normal intestinal development and nutrient handling.

Initial combination therapy with sitagliptin, a dipeptidyl peptidase-4 inhibitor, and metformin for patients with Type 2 diabetes mellitus

The two incretin hormones, glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide potentiate nutrient-dependent insulin secretion following meal ingestion. Metabolic control can be improved markedly by administration of exogenous GLP-1, but the native peptide is almost immediately degraded by the enzyme dipeptidyl peptidase (DPP)-4 and, therefore, has little clinical value. Oral formulations that inhibit DPP-4, thereby prolonging the duration of endogenous incretin action, have, therefore, been developed. Sitagliptin, a once-daily, orally active, competitive and fully reversible inhibitor of DPP-4, was, as first in its class, introduced to the market as Januvia™. Recently, the US FDA approved initial combination therapy with sitagliptin and metformin (Janumet™) in order to help more patients with Type 2 diabetes mellitus get closer to accepted glycemic control targets, as recommended by standard guidelines. This article reviews initial treatment with Janumet as an alternative to monotherapy.

Effect of bradykinin metabolism inhibitors on evoked hypotension in rats: rank efficacy of enzymes associated with bradykinin-mediated angioedema

BACKGROUND AND PURPOSE

Inhibition of bradykinin metabolizing enzymes (BMEs) can cause acute angioedema, as demonstrated in a recent clinical trial in patients administered the antihypertensive, omapatrilat. However, the relative contribution of specific BMEs to this effect is unclear and confounded by the lack of a predictive pre-clinical model of angioedema.

EXPERIMENTAL APPROACH

Rats were instrumented to record blood pressure and heart rate; inhibitors were infused for 35 min and bradykinin was infused during the last 5 min to elicit hypotension, as a functional marker of circulating bradykinin and relative angioedema risk.

KEY RESULTS

In the presence of omapatrilat bradykinin produced dose-dependent hypotension, an effect abolished by B(2) blockade. In the presence of lisinopril (ACE inhibitor), but not candoxatril (NEP inhibitor) or apstatin (APP inhibitor), bradykinin also elicited hypotension. Lisinopril-mediated hypotension was unchanged with concomitant blockade of NEP or NEP/DPPIV (candoxatril+A-899301). However, hypotension was enhanced upon concomitant blockade of APP and further intensified in the presence of NEP inhibition to values not different from omapatrilat alone.

CONCLUSIONS AND IMPLICATIONS

We demonstrated that bradykinin is degraded in vivo with an enzyme rank-efficacy of ACE>APP>>NEP or DPPIV. These results suggest the effects of omapatrilat are mediated by inhibition of three BMEs, ACE/APP/NEP. However, dual inhibition of ACE/NEP or ACE/NEP/DPPIV elicits no increased risk of angioedema compared to ACE inhibition alone. Thus, novel BME inhibitors must display no activity against APP to avoid angioedema risk due to high prevalence of ACE inhibitor therapy in patients with diabetes and cardiovascular disease.

Selection of anti-cancer antibodies from combinatorial libraries by whole-cell panning and stringent subtraction with human blood cells

Traditional strategies for the identification of cell-surface cancer targets often fall short of their objective. For example, whole-cell panning of antibody libraries to isolate a diverse panel of antibodies directed against targets on cancer cells often identifies all immunogenic and/or abundant cell-surface antigens, not simply tumor-specific or tumor-associated antigens. Here we describe the use of stringent negative selection in combination with positive panning to increase tumor specificity and clinical relevance of selected antibodies. Sera from cancer cell-immunized mice showed strong binding to immunizing cancer cell lines but also cross-reacted strongly with human blood cells. Antisera blood cell binding was considerably decreased after stringent subtraction with human red blood cells (RBCs) and white blood cells (WBCs), yet cancer cell specificity was retained. In order to select for a higher percentage of clinically relevant antibodies for potential therapeutic use, stringent negative selection by RBC subtraction was employed in whole-cell panning of a disease-specific phage displayed antibody library on the prostate cancer cell line, PC-3. Isolated antibodies were found to bind to target antigens implicated in tumorigenicity and cancer cell migration and/or invasion, and included CD26, CDCP1, and the integrin complexes alpha2/beta1, alpha3/beta1, alpha5/beta1, and alpha6/beta4. Compared with traditional cell panning, this method considerably increased the selectivity of antibodies to tumor-associated antigens.

Dual inhibition of dipeptidyl peptidase IV and aminopeptidase N suppresses inflammatory immune responses

The ectopeptidases dipeptidyl peptidase IV (DP IV, CD26) and aminopeptidase N (APN, CD13) are known to regulate T cell activation. Since selective inhibitors of DP IV and APN suppress DNA synthesis and cytokine production of stimulated T cells in a TGF-beta1-dependent manner, we tested whether combined application of DP IV and APN inhibitors enhances this immunomodulatory effect. The results show that simultaneous application of DP IV and APN inhibitors significantly suppressed DNA synthesis in mitogen- or anti-CD3-stimulated human T cells in vitro when compared to the use of a single DP IV or APN inhibitor. Moreover, the combined action of DP IV and APN inhibitors markedly increased TGF-beta1 production associated with the observed immunosuppressive effects. In vivo, targeting both DP IV and APN led to a potent treatment of experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis (MS). This review summarizes the evidence for the role of both enzymes in T cell activation in vitro and in vivo and provides a rationale for using combined and dual peptidase inhibitors to treat autoimmune diseases like MS.

8-(3-(R)-aminopiperidin-1-yl)-7-but-2-ynyl-3-methyl-1-(4-methyl-quinazolin-2-ylmethyl)-3,7-dihydropurine-2,6-dione (BI 1356), a highly potent, selective, long-acting, and orally bioavailable DPP-4 inhibitor for the treatment of type 2 diabetes

A new chemical class of potent DPP-4 inhibitors structurally derived from the xanthine scaffold for the treatment of type 2 diabetes has been discovered and evaluated. Systematic structural variations have led to 1 (BI 1356), a highly potent, selective, long-acting, and orally active DPP-4 inhibitor that shows considerable blood glucose lowering in different animal species. 1 is currently undergoing clinical phase IIb trials and holds the potential for once-daily treatment of type 2 diabetics.

DPP-4 inhibitors

Inhibition of dipeptidyl peptidase 4 (DPP-4) is a novel treatment for type-2 diabetes. DPP-4 inhibition prevents the inactivation of glucagon-like peptide 1 (GLP-1), which increases levels of active GLP-1. This increases insulin secretion and reduces glucagon secretion, thereby lowering glucose levels. Several DPP-4 inhibitors are in clinical development. Most experience so far has been with sitagliptin (Merck; approved by the FDA) and vildagliptin (Novartis; filed). These are orally active compounds with a long duration, allowing once-daily administration. Both sitagliptin and vildagliptin improve metabolic control in type-2 diabetes, both in monotherapy and in combination with metformin and thiazolidinediones. A reduction in HbA(1c) of approximately 1% is seen in studies of DPP-4 inhibition of up to 52 weeks' duration. DPP-4 inhibition is safe and well tolerated, the risk of hypoglycaemia is minimal, and DPP-4 inhibition is body-weight neutral. DPP-4 inhibition is suggested to be a first-line treatment of type-2 diabetes, particularly in its early stages in combination with metformin. However, the durability and long-term safety of DPP-4 inhibition remain to be established.

Dipeptidyl peptidase IV in angiotensin-converting enzyme inhibitor associated angioedema

Angioedema is a potentially life-threatening adverse effect of angiotensin-converting enzyme inhibitors. Bradykinin and substance P, substrates of angiotensin-converting enzyme, increase vascular permeability and cause tissue edema in animals. Studies indicate that amino-terminal degradation of these peptides, by aminopeptidase P and dipeptidyl peptidase IV, may be impaired in individuals with angiotensin-converting enzyme inhibitor-associated angioedema. This case-control study tested the hypothesis that dipeptidyl peptidase IV activity and antigen are decreased in sera of patients with a history of angiotensin-converting enzyme inhibitor-associated angioedema. Fifty subjects with a history of angiotensin-converting enzyme inhibitor-associated angioedema and 176 angiotensin-converting enzyme inhibitor-exposed control subjects were ascertained. Sera were assayed for angiotensin-converting enzyme activity, aminopeptidase P activity, aminopeptidase N activity, dipeptidyl peptidase IV activity, and antigen and the ex vivo degradation half-lives of bradykinin, des-Arg(9)-bradykinin, and substance P in a subset. The prevalence of smoking was increased and of diabetes decreased in case versus control subjects. Overall, dipeptidyl peptidase IV activity (26.6+/-7.8 versus 29.6+/-7.3 nmol/mL per minute; P=0.026) and antigen (465.8+/-260.8 versus 563.1+/-208.6 ng/mL; P=0.017) were decreased in sera from individuals with angiotensin-converting enzyme inhibitor-associated angioedema compared with angiotensin-converting enzyme inhibitor-exposed control subjects without angioedema. Dipeptidyl peptidase IV activity (21.5+/-4.9 versus 29.8+/-6.7 nmol/mL per minute; P=0.001) and antigen (354.4+/-124.7 versus 559.8+/-163.2 ng/mL; P=0.003) were decreased in sera from cases collected during angiotensin-converting enzyme inhibition but not in the absence of angiotensin-converting enzyme inhibition. The degradation half-life of substance P correlated inversely with dipeptidyl peptidase IV antigen during angiotensin-converting enzyme inhibition. Environmental or genetic factors that reduce dipeptidyl peptidase IV activity may predispose individuals to angioedema.

Inhibitors of dipeptidyl peptidase IV-like activity mediate antifibrotic effects in normal and keloid-derived skin fibroblasts

Suppression of collagen and matrix synthesis and inhibition of the fibrogenic cytokine transforming growth factor-beta(1) (TGF-beta(1)) is a major therapeutic goal in the treatment of fibrosis and keloids. Inhibitors of dipeptidyl peptidase IV (DP IV)-like activity affect cell growth and cytokine production and are currently under investigation for the treatment of metabolic, autoimmune and inflammatory diseases. We show here that the inhibitors of DP IV-like activity, Lys[Z(NO(2))]-thiazolidide and Lys[Z(NO(2))]-pyrrolidide, suppress proliferation in human skin fibroblasts and keloid-derived skin fibroblasts in vitro. They significantly decrease TGF-beta(1) expression and secretion of procollagen type I C-terminal peptide in supernatants of both cell types. Furthermore, they abrogate the TGF-beta(1)-induced stimulation of collagen synthesis, matrix deposition, and TGF-beta(1) and fibronectin expression. Both inhibitors lead to dephosphorylation of mitogen-activated protein kinases pp38 and pERK1/2, which are activated upon TGF-beta1 stimulation and have been implicated in fibrogenesis. In a mouse model of dermal fibrosis, induced by repetitive intracutaneous injections of TGF-beta(1), the profibrotic effect of TGF-beta(1) detected by dermal thickening, collagen I, and alpha-smooth muscle actin expression, is significantly suppressed in the presence of inhibitors. Inhibition of DP IV-like enzymatic activity may therefore represent a promising therapeutic approach for the treatment of fibrotic skin disorders and keloids.