Method comparison of dipeptidyl peptidase IV activity assays and their application in biological samples containing reversible inhibitors

A sensitive fluorescence assay of serum dipeptidyl peptidase IV activity to predict the suitability of its inhibitors in patients with type 2 diabetes mellitus

DPP-IV inhibitors, which are close to the natural hypoglycemic pathway of human physiology and have few side effects, have been extensively employed in the management of type 2 diabetes mellitus (T2DM). However, there are currently no specific blood indicators that can indicate or predict a patient's suitability for DPP-IV inhibitors. In this study, based on the self-developed high-specificity fluorescent substrate glycyl-prolyl-N-butyl-4-amino-1, 8-naphthimide (GP-BAN), a detection method of human serum DPP-IV activity was established and optimized. The method demonstrates a favorable lower limit of detection (LOD) at 0.32 ng/mL and a satisfactory lower limit of quantification (LOQ) of 1.12 ng/mL, and can be used for the detection of DPP-IV activity in trace serum (2 μL). In addition, Vitalliptin and Sitagliptin showed similar IC50 values when human recombinant DPP-IV and human serum were used as enzyme sources, and the intra-day and inter-day precision obtained by the microplate analyzer were less than 15 %. These results indicate that the microplate reader based detection technique has good accuracy, repeatability and reproducibility. A total of 700 volunteers were recruited, and 646 serum samples were tested for DPP-IV activity. The results showed that serum DPP-IV activity was higher in patients with T2DM than in controls (P < 0.01). However, the statistical data of family history of diabetes, gender and age of diabetic patients showed no statistical significance, and there was no contrast difference. The DPP-IV activity of serum in T2DM patients ranged from 2.4 μmol/min/L to 78.6 μmol/min/L, with a huge difference of up to 32-fold. These results suggest that it is necessary to test DPP-IV activity in patients with T2DM when taking DPP-IV inhibitors to determine the applicability of DPP-IV inhibitors in T2DM patients. These results suggest that it is necessary to detect the activity of DPP-IV in blood before taking DPP-IV inhibitors in patients with T2DM to judge the applicability of DPP-IV inhibitors in patients with T2DM.

High pressure-assisted enzymatic hydrolysis promotes the release of a bi-functional peptide from cowhide gelatin with dipeptidyl peptidase IV (DPP-IV) inhibitory and antioxidant activities

The process of generating functional peptides from cowhide gelatin is challenged by inefficient enzymatic hydrolysis. In this study, the researchers attempted to enhance the hydrolysis and potential functional properties of the peptides by subjecting the cowhide gelatin to high-pressure treatment (200, 300, and 400 MPa) for 20 min, followed by enzymatic hydrolysis. The highest 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, 2,2' azinobis(3 ethylbenzothiazoline 6 sulfonic acid) (ABTS) radical scavenging activity, and DPP-IV inhibitory activity of the hydrolysate were obtained at 200 MPa, accompanied with an increase in the content of hydrophobic, acidic, and basic amino acids (P < 0.05). Correspondingly, the high-pressure pretreatment (200 MPa) reduced the thermal stability, particle size, and morphological integrity of cowhide gelatin, with a corresponding increase in the exposure of hydrophobic regions. Altogether, these results indicated that appropriate high-pressure-assisted enzymatic hydrolysis reinforced the release of bi-functional peptides by modifying the structure of cowhide gelatin.

Natural carboxyterminal truncation of human CXCL10 attenuates glycosaminoglycan binding, CXCR3A signaling and lymphocyte chemotaxis, while retaining angiostatic activity

BACKGROUND

Interferon-γ-inducible protein of 10 kDa (IP-10/CXCL10) is a dual-function CXC chemokine that coordinates chemotaxis of activated T cells and natural killer (NK) cells via interaction with its G protein-coupled receptor (GPCR), CXC chemokine receptor 3 (CXCR3). As a consequence of natural posttranslational modifications, human CXCL10 exhibits a high degree of structural and functional heterogeneity. However, the biological effect of natural posttranslational processing of CXCL10 at the carboxy (C)-terminus has remained partially elusive. We studied CXCL10(1-73), lacking the four endmost C-terminal amino acids, which was previously identified in supernatant of cultured human fibroblasts and keratinocytes.

METHODS

Relative levels of CXCL10(1-73) and intact CXCL10(1-77) were determined in synovial fluids of patients with rheumatoid arthritis (RA) through tandem mass spectrometry. The production of CXCL10(1-73) was optimized through Fmoc-based solid phase peptide synthesis (SPPS) and a strategy to efficiently generate human CXCL10 proteoforms was introduced. CXCL10(1-73) was compared to intact CXCL10(1-77) using surface plasmon resonance for glycosaminoglycan (GAG) binding affinity, assays for cell migration, second messenger signaling downstream of CXCR3, and flow cytometry of CHO cells and primary human T lymphocytes and endothelial cells. Leukocyte recruitment in vivo upon intraperitoneal injection of CXCL10(1-73) was also evaluated.

RESULTS

Natural CXCL10(1-73) was more abundantly present compared to intact CXCL10(1-77) in synovial fluids of patients with RA. CXCL10(1-73) had diminished affinity for GAG including heparin, heparan sulfate and chondroitin sulfate A. Moreover, CXCL10(1-73) exhibited an attenuated capacity to induce CXCR3A-mediated signaling, as evidenced in calcium mobilization assays and through quantification of phosphorylated extracellular signal-regulated kinase-1/2 (ERK1/2) and protein kinase B/Akt. Furthermore, CXCL10(1-73) incited significantly less primary human T lymphocyte chemotaxis in vitro and peritoneal ingress of CXCR3+ T lymphocytes in mice. In contrast, loss of the four endmost C-terminal residues did not affect the inhibitory properties of CXCL10 on migration, proliferation, wound closure, phosphorylation of ERK1/2, and sprouting of human microvascular endothelial cells.

CONCLUSION

Our study shows that the C-terminal residues Lys74-Pro77 of CXCL10 are important for GAG binding, signaling through CXCR3A, T lymphocyte chemotaxis, but dispensable for angiostasis.

In vitro profiling and functional assessments of the anti-diabetic capacity of phenolic-rich extracts of Bulbine natalensis and Bulbine frutescens

AIMS

Bulbine natalensis (BN) and Bulbine frutescens (BF) are recommended in South African traditional medicine to treat diabetes, but their modes of action are unknown. This study assessed the phenolic acid profiles, mineral composition and in vitro functional effects of BN and BF to better understand their glucose-lowering capabilities.

METHODS

Phenolic acid and mineral composition of BN and BF methanolic extracts were determined by HPLC and inductively coupled plasma optical emission spectroscopy respectively. Antioxidant capacity was assessed by potassium ferricyanide reducing power and 2,2-diphenyl-2-picrylhydrazyl radical scavenging assays, and inhibition of alpha-amylase, alpha-glucosidase, pancreatic lipase and DPP4 was evaluated by standard enzyme assays. The effects of BN and BF extracts on insulin secretion were investigated using static incubations of isolated mouse islets and molecular docking analysis was used to identify interactions of BN and BF with partners that could mediate stimulatory effects on insulin secretion.

RESULTS

Methanolic extracts of BN and BF contained high concentrations of protocatechuic and gallic acids, and high levels of Zn, Mn and Cr. The extracts inhibited alpha-glucosidase, alpha-amylase, pancreatic lipase and DPP4 activities, and they also inhibited free radical generation. Both extracts significantly potentiated glucose-stimulated insulin secretion without significantly affecting basal insulin secretion or islet cell viability. Protocatechuic acid, the most abundant phenolic acid in the extracts, showed high affinity for PKA, PKC, DPP4 and CaMK II in the docking analysis.

CONCLUSIONS

BN and BF have multiple beneficial effects on glucoregulatory pathways and they, or their derivatives, could be developed to treat type 2 diabetes.

Proline-specific peptidase activities (DPP4, PRCP, FAP and PREP) in plasma of hospitalized COVID-19 patients

BACKGROUND

COVID-19 patients experience several features of dysregulated immune system observed in sepsis. We previously showed a dysregulation of several proline-selective peptidases such as dipeptidyl peptidase 4 (DPP4), fibroblast activation protein alpha (FAP), prolyl oligopeptidase (PREP) and prolylcarboxypeptidase (PRCP) in sepsis. In this study, we investigated whether these peptidases are similarly dysregulated in hospitalized COVID-19 patients.

METHODS

Fifty-six hospitalized COVID-19 patients and 32 healthy controls were included. Enzymatic activities of DPP4, FAP, PREP and PRCP were measured in samples collected shortly after hospital admission and in longitudinal follow-up samples.

RESULTS

Compared to healthy controls, both DPP4 and FAP activities were significantly lower in COVID-19 patients at hospital admission and FAP activity further decreased significantly in the first week of hospitalization. While PRCP activity remained unchanged, PREP activity was significantly increased in COVID-19 patients at hospitalization and further increased during hospital stay and stayed elevated until the day of discharge.

CONCLUSION

The changes in activities of proline-selective peptidases in plasma are very similar in COVID-19 and septic shock patients. The pronounced decrease in FAP activity deserves further investigation, both from a pathophysiological viewpoint and as its utility as a part of a biomarker panel.

Comparison of biomarker and chromatographic analytical approaches to pharmacokinetic study of sitagliptin

The pharmacokinetic profiling of active compounds is necessary for drug development and application. Approaches to a pharmacokinetic study based on biological markers are alternatives to traditional approaches based on chromatographic methods. The aim of the study was to compare two analytical approaches to pharmacokinetics investigation for an example of sitagliptin in rabbits after one dose oral administration. The method for sitagliptin quantification in rabbit plasma samples based on a correlation between its concentration and dipeptidyl peptidase IV activity was proposed, validated, and applied. The high-performance liquid chromatography (HPLC)-ultraviolet (UV) method was also validated and applied for the same sample analysis. The plasma pharmacokinetics of sitagliptin after oral administration to the rabbits in one dose was characterized after two analytical assays. The close values of the main pharmacokinetic parameters were obtained after two approaches. The nontraditional approach based on correlation of special marker activity and active substance concentration appears to be more sensitive than HPLC-UV. Thus, the sitagliptin concentrations determined by biomarker assay were higher than the lower limit of quantification (LLOQ) for a longer period (more timepoints) than after the HPLC-UV assay. This feature may influence the values of some calculated concentration-dependent (area under the curve [AUC]_0-t , etc.) and time-dependent parameters (mean residence time [MRT], T_1/2 , etc.). The values of T_max obtained by the two approaches were similar and adequate for oral drug administration that confirms the correctness of biomarker selection for pharmacokinetics assessment. The obtained results on the example of sitagliptin confirms that the biomarker approach is adequate and applicable for a pharmacokinetics study. Similar approaches may be effective for individual compounds and complex mixtures when it is difficult or impossible to analyze them traditionally by chromatographic methods.

Advance in dietary polyphenols as dipeptidyl peptidase-IV inhibitors to alleviate type 2 diabetes mellitus: aspects from structure-activity relationship and characterization methods

Dietary polyphenols with great antidiabetic effects are the most abundant components in edible products. Dietary polyphenols have attracted attention as dipeptidyl peptidase-IV (DPP-IV) inhibitors and indirectly improve insulin secretion. The DPP-IV inhibitory activities of dietary polyphenols depend on their structural diversity. Screening methods that can be used to rapidly and accurately identify potential polyphenol DPP-IV inhibitors are urgently needed. This review focuses on the relationship between the structures of dietary polyphenols and their DPP-IV inhibitory effects. Different characterization methods used for polyphenols as DPP-IV inhibitors have been summarized and compared. We conclude that the position and number of hydroxyl groups, methoxy groups, glycosylated groups, and the extent of conjugation influence the efficiency of inhibition of DPP-IV. Various combinations of methods, such as in-vitro enzymatic inhibition, ex-vivo/in-vivo enzymatic inhibition, cell-based in situ, and in-silico virtual screening, are used to evaluate the DPP-IV inhibitory effects of dietary polyphenols. Further investigations of polyphenol DPP-IV inhibitors will improve the bioaccessibility and bioavailability of these bioactive compounds. Exploration of (i) dietary polyphenols derived from multiple targets, that can prevent diabetes, and (ii) actual binding interactions via multispectral analysis, to understand the binding interactions in the complexes, is required.

The C-terminal cleavage of angiotensin II and III is mediated by prolyl carboxypeptidase in human umbilical vein and aortic endothelial cells

The renin-angiotensin system, with the octapeptide angiotensin II as key player, is important in the renal, cardiac and vascular physiology. Prolyl carboxypeptidase (PRCP), prolyl endopeptidase (PREP) and angiotensin converting enzyme 2 (ACE2) are reported to be involved in the conversion of angiotensin II to angiotensin (1-7). Previous investigations showed that the processing of angiotensin II is cell- and species-specific and little is known about its conversion in human endothelial cells. Therefore, we aimed to investigate the C-terminal processing of angiotensin II and III in comparison to the processing of des-Arg⁹-bradykinin in human endothelial cells. To this end, human umbilical vein and aortic endothelial cells (HUVEC and HAoEC) were incubated with the peptides for different time periods. Mass spectrometry analysis was performed on the supernatants to check for cleavage products. Contribution of PRCP, ACE2 and PREP to the peptide cleavage was evaluated by use of the selective inhibitors compound 8o, DX600 and KYP-2047. The use of these selective inhibitors revealed that the C-terminal cleavage of angiotensin II and III was PRCP-dependent in HUVEC and HAoEC. In contrast, the C-terminal cleavage of des-Arg⁹-bradykinin was PRCP-dependent in HUVEC and PRCP- and ACE2-dependent in HAoEC. With this study, we contribute to a better understanding of the processing of peptides involved in the alternative renin-angiotensin system. We conclude that PRCP is the main enzyme for the C-terminal processing of angiotensin peptides in human umbilical vein and aortic endothelial cells. For the first time the contribution of PRCP was investigated by use of a selective PRCP-inhibitor.

Purification, identification, and molecular mechanism of DPP-IV inhibitory peptides from defatted Antarctic krill powder

Dipeptidyl peptidase-IV (DPP-IV) inhibitors can reduce the blood sugar levels of diabetic patients by preventing the rapid decomposition of incretin hormone and prolonging its physiological effects. In this study, DPP-IV inhibitory peptides FAGDDAPR and LAPPRGSL were isolated from defatted Antarctic krill powder (DAKP) protein by the sequential purification of ultrafiltration, gel filtration chromatography, and RP-HPLC, and IC₅₀ values of the two peptides were 349.70 ± 3.66 μM and 461.14 ± 0.87 μM, respectively. The FAGDDAPR and LAPPRGSL were identified by LC-MS/MS method, and the molecular models of DPP-IV and the two peptides were further constructed by AutoDock Vina software, the results revealed that the inhibition activity of FAGDDAPR and LAPPRGSL was mainly attributed to the formation of strong hydrophobic interactions and hydrogen bonds with amino acids of DPP-IV. PRACTICAL APPLICATIONS: DAKP is an economical by-product produced in the production of krill oil and contains high-quality protein, but these products were mainly used as fish feed and had low utility value in the past. DPP-IV inhibitors are an efficacious drug employed in the treatment of hyperglycemia processes. However, these drugs can cause undesirable side effects. Thus, the development of new natural hypoglycemic drugs with low side effects is a valuable strategy to be applied in therapeutic interventions.

A high-throughput screening assay for dipeptidyl peptidase-IV inhibitors using human plasma

Dipeptidyl peptidase-IV (DPP-IV) plays a critical role in glucose metabolism and has become an important target for type 2 diabetes mellitus. We previously reported a two-photon fluorescent probe glycyl-prolyl-N-butyl-4-amino-1,8-naphthalimide (GP-BAN) for DPP-IV detection with high specificity and sensitivity. In this study, a high-throughput screening (HTS) method for DPP-IV inhibitors using human plasma as the enzyme source was established and optimized. Further investigations demonstrate that the IC50 value of sitagliptin (listed as the DPP-IV inhibitor) determined with human recombinant DPP-IV (36.22 nM) is very similar to that in human plasma (39.18 nM), and sitagliptin acts as a competitive inhibitor against human plasma DPP-IV-mediated GP-BAN hydrolysis. These results indicate that expensive human recombinant DPP-IV can be replaced by human plasma in this GP-BAN-based assay. On this basis, GP-AMC (commercial probe) was used as a comparison to verify this method, and the catalytic efficacy (Vmax/Km) for GP-AMC (0.09 min-1) hydrolysis in human plasma is lower than that for GP-BAN (0.21 min-1). Further analysis of inhibition kinetics (sitagliptin) and molecular docking (GP-BAN and GP-AMC) showed that GP-BAN has better specificity and affinity for enzymes than GP-AMC. Finally, the optimized method was used for the HTS of DPP-IV inhibitors in 69 natural alkaloids.

Prolyl Carboxypeptidase Mediates the C-Terminal Cleavage of (Pyr)-Apelin-13 in Human Umbilical Vein and Aortic Endothelial Cells

The aim of this study was to investigate the C-terminal cleavage of (pyr)-apelin-13 in human endothelial cells with respect to the role and subcellular location of prolyl carboxypeptidase (PRCP). Human umbilical vein and aortic endothelial cells, pre-treated with prolyl carboxypeptidase-inhibitor compound 8o and/or angiotensin converting enzyme 2 (ACE2)-inhibitor DX600, were incubated with (pyr)-apelin-13 for different time periods. Cleavage products of (pyr)-apelin-13 in the supernatant were identified by mass spectrometry. The subcellular location of PRCP was examined via immunocytochemistry. In addition, PRCP activity was measured in supernatants and cell lysates of LPS-, TNFα-, and IL-1β-stimulated cells. PRCP cleaved (pyr)-apelin-13 in human umbilical vein and aortic endothelial cells, while ACE2 only contributed to this cleavage in aortic endothelial cells. PRCP was found in endothelial cell lysosomes. Pro-inflammatory stimulation induced the secretion of PRCP in the extracellular environment of endothelial cells, while its intracellular level remained intact. In conclusion, PRCP, observed in endothelial lysosomes, is responsible for the C-terminal cleavage of (pyr)-apelin-13 in human umbilical vein endothelial cells, while in aortic endothelial cells ACE2 also contributes to this cleavage. These results pave the way to further elucidate the relevance of the C-terminal Phe of (pyr)-apelin-13.

Chemometric-enhanced metabolic profiling of five Pinus species using HPLC-MS/MS spectrometry: Correlation to in vitro anti-aging, anti-Alzheimer and antidiabetic activities

Detailed metabolic profiling of needles of five Pinus species was investigated using complementary HPLC-MS/MS techniques together with supervised and unsupervised chemometric tools. This resulted in putative identification of 44 compounds belonging to flavonoids, phenolics, lignans, diterpenes and fatty acids. Unsupervised principal component analysis showed that differences were maintained across the metabolites characteristic of each Pinus species, are mainly related to di-O-p-coumaroyltrifolin, p-coumaroyl quinic acid derivative, arachidonic acid, hydroxypalmitic acid, isopimaric acid and its derivative. A supervised Partial Least Squares regression analysis was performed to correlate HPLC-MS/MS profiles with the variation observed in the in vitro anticholinesterase, antiaging and anti-diabetic potential. All investigated Pinus extracts exerted their antiaging activity via increasing telomerase and TERT levels in normal human melanocytes cells compared to the control (untreated cells). Profound inhibition activities of acetylcholinesterase and dipeptidyl peptidase-4 were also observed with P. pinea and P. canariensis extracts having comparable antidiabetic activities to sitagliptin as a standard antidiabetic drug. Our findings suggested that pine needles are a good source of phenolics and diterpenoids that have possible health promoting activities in management and alleviation of diabetic conditions and Alzheimer disease.

Circulating dipeptidyl peptidase-4 is independently associated with the presence and severity of NAFLD/NASH in individuals with and without obesity and metabolic disease

INTRODUCTION

Dipeptidyl peptidase 4 (DPP4) levels are associated to metabolic and cardiovascular diseases in humans; initial evidence reported a relationship between DPP4 and chronic liver diseases. Aim of this study was to investigate hepatic and systemic DPP4 levels/activity in relation to NAFLD/NASH in individuals with and without metabolic disease.

METHODS

We recruited fifty-two obese individuals undergoing bariatric surgery and intra-operative liver biopsy at Sapienza University, Rome, Italy. The association between DPP4 levels/activity and NAFLD was also evaluated in 126 non-obese individuals recruited in the same setting.

RESULTS

NAFLD patients had significantly higher circulating DPP4 activity than no-NAFLD in both the obese and non-obese cohorts; plasma DPP4 activity and levels linearly correlated with steatosis grade and inflammation at the liver biopsy. Hepatic DPP4 mRNA was not associated to either its circulating levels/activity or NAFLD. In the multivariate logistic regression analysis on all the study participants (n = 178), higher circulating DPP4 activity was associated with NAFLD independently of potential confounders with OR (95% CI): 3.5 (1.2-10.21), p = 0.022.

CONCLUSIONS

This study demonstrates the coexistence of increased plasma DPP4 levels and activity in NAFLD. Circulating DPP4 measurement may represent a novel cost-effective strategy for NAFLD/NASH risk stratification and a potential tool for monitoring disease's progression in established NAFLD.

(-)-Epicatechin and Anthocyanins Modulate GLP-1 Metabolism: Evidence from C57BL/6J Mice and GLUTag Cells

BACKGROUND

Generated in intestinal L cells through cleavage of proglucagon (Gcg), glucagon-like peptide 1 (GLP-1) is secreted and rapidly inactivated by dipeptidyl peptidase IV (DPP-IV). GLP-1 regulates insulin secretion and overall glucose homeostasis. The capacity of dietary bioactives to increase GLP-1 circulating levels, and therefore increase insulin secretion and glucose metabolism, has gained significant interest of late.

OBJECTIVES

We evaluated the effects of (-)-epicatechin (EC) and different anthocyanins (ACs) and AC metabolites on GLP-1 metabolism in mice and on GLUTag cells.

METHODS

We fed 6-week-old C57BL/6J male mice a control diet or a control diet supplemented with either 40 mg AC or 20 mg EC/kg body weight for 14 weeks (AC) or 15 weeks (EC). Intestinal mRNA levels of Gcg and Dpp-iv were measured. In vitro, GLUTag cells were incubated in the presence or absence of different ACs, the AC metabolite protocatechuic acid (PCA), and EC. GLP-1 secretion and the main pathways involved in its release were assessed.

RESULTS

Long-term supplementation with EC or AC increased mouse GLP-1 plasma concentrations (55% and 98%, respectively; P < 0.05). In mice, 1) EC and AC increased Gcg mRNA levels in the ileum (91%) and colon (41%), respectively (P < 0.05); and 2) AC lowered ileum Dpp-iv mRNA levels (35%), while EC decreased plasma DPP-IV activity (15%; P < 0.05). In GLUTag cells, 1) cyanidin, delphinidin, PCA, and EC increased GLP-1 secretion (53%, 33%, 53%, and 68%, respectively; P < 0.05); and 2) cyanidin, delphinidin, EC, and PCA increased cyclin adenosine monophosphate levels (25-50%; P < 0.05) and activated protein kinase A (PKA; 100%, 50%, 80%, and 86%, respectively; P < 0.05).

CONCLUSIONS

In mice, EC and ACs regulated different steps in GLP-1 regulation, leading to increased plasma GLP-1. Cyanidin, delphinidin, PCA, and EC promoted GLP-1 secretion from GLUTag cells by activating the PKA-dependent pathway. These findings support the beneficial actions of these flavonoids in sustaining intestinal and glucose homeostasis through the modulation of the GLP-1 metabolism.

The Emerging Role of Polyphenols in the Management of Type 2 Diabetes

Type 2 diabetes (T2D) is a fast-increasing health problem globally, and it results from insulin resistance and pancreatic β-cell dysfunction. The gastrointestinal (GI) tract is recognized as one of the major regulatory organs of glucose homeostasis that involves multiple gut hormones and microbiota. Notably, the incretin hormone glucagon-like peptide-1 (GLP-1) secreted from enteroendocrine L-cells plays a pivotal role in maintaining glucose homeostasis via eliciting pleiotropic effects, which are largely mediated via its receptor. Thus, targeting the GLP-1 signaling system is a highly attractive therapeutic strategy to treatment T2D. Polyphenols, the secondary metabolites from plants, have drawn considerable attention because of their numerous health benefits, including potential anti-diabetic effects. Although the major targets and locations for the polyphenolic compounds to exert the anti-diabetic action are still unclear, the first organ that is exposed to these compounds is the GI tract in which polyphenols could modulate enzymes and hormones. Indeed, emerging evidence has shown that polyphenols can stimulate GLP-1 secretion, indicating that these natural compounds might exert metabolic action at least partially mediated by GLP-1. This review provides an overview of nutritional regulation of GLP-1 secretion and summarizes recent studies on the roles of polyphenols in GLP-1 secretion and degradation as it relates to metabolic homeostasis. In addition, the effects of polyphenols on microbiota and microbial metabolites that could indirectly modulate GLP-1 secretion are also discussed.

Optimization and validation of a fluorogenic dipeptidyl peptidase 4 enzymatic assay in human plasma

During the development of a specific dipeptidyl peptidase 4 (DPP4) inhibitor to treat type 2 diabetes, a fluorogenic kinetic analysis for DPP4 enzymatic activity using Gly-Pro-Aminomethylcoumarin (AMC) as a substrate was optimized and validated for recombinant DPP4 and human plasma samples. The sensitivity, calibration curve, detection range, accuracy, precision, recovery efficiency, Km constant, short/long-term stability, and stability after freezing-thawing cycles were analyzed. DPP4 enzymatic activity (mU/min) was measured as the initial velocity (Vo) of the enzymatic reaction over time. The sensitivity of the Vo value was 14,488 mU/min for recombinant DPP4 and 17,995 mU/min for human plasma samples. The dynamic ranges of the calibration curve were linear and reliable between 1.11 × 10⁴-1.86 × 10⁶ mU/min of the mean Vo value and in the DPP4 concentration range of 23.4-3,000 ng/mL. The assay's accuracy and precision met acceptance criteria for all samples. Plasma DPP4 was stable under various storage temperatures, even after three freeze-thaw cycles. Our optimized, validated bioanalytic method for measuring DPP4 activity in plasma samples was successfully employed to evaluate the effect of evogliptin (DA-1229) tartrate, which irreversibly and dose-dependently inhibits DPP4 enzymatic activity, without the dilution effect of human plasma samples and irrespective of the co-treated metformin.

Dysregulated activities of proline-specific enzymes in septic shock patients (sepsis-2)

The proline-specific enzymes dipeptidyl peptidase 4 (DPP4), prolylcarboxypeptidase (PRCP), fibroblast activation protein α (FAP) and prolyl oligopeptidase (PREP) are known for their involvement in the immune system and blood pressure regulation. Only very limited information is currently available on their enzymatic activity and possible involvement in patients with sepsis and septic-shock. The activity of the enzymes was measured in EDTA-plasma of patients admitted to the intensive care unit (ICU): 40 septic shock patients (sepsis-2) and 22 ICU control patients after major intracranial surgery. These data were used to generate receiver operating characteristic (ROC) curves. A survival analysis (at 90 days) and an association study with other parameters was performed. PRCP (day 1) and PREP (all days) enzymatic activities were higher in septic shock patients compared to controls. In contrast, FAP and DPP4 were lower in these patients on all studied time points. Since large differences were found, ROC curves were generated and these yielded area under the curve (AUC) values for PREP, FAP and DPP4 of 0.88 (CI: 0.80-0.96), 0.94 (CI: 0.89-0.99) and 0.86 (CI: 0.77-0.95), respectively. PRCP had a lower predicting value with an AUC of 0.71 (CI: 0.58-0.83). A nominally significant association was observed between survival and the DPP4 enzymatic activity at day 1 (p<0.05), with a higher DPP4 activity being associated with an increase in survival. All four enzymes were dysregulated in septic shock patients. DPP4, FAP and PREP are good in discriminating between septic shock patients and ICU controls and should be further explored to see whether they are already dysregulated in earlier stages, opening perspectives for their further investigation as biomarkers in sepsis. DPP4 also shows potential as a prognostic biomarker. Additionally, the associations found warrant further research.

Nitrothiadiazolo[3,2-a]pyrimidines as promising antiglycating agents

Managing the advanced glycation end-products (AGEs) concentration is a reliable approach to achieve control over the pathogenesis of diabetic vascular complications. Inhibition of dipeptidyl peptidase-4 (DPP-4) is also an attractive way to tackle type 2 diabetes mellitus (T2DM). We showed previously that azoloazine heterocycles have the potential to prevent the formation of AGEs and in this work, we conducted docking studies with DPP-4 of 5-alkylamino-6-nitro-1,3,4-thiadiazolo[3,2-a]pyrimidines. Consequently, we have developed a synthetic approach to these structures by chlorodeoxygenation and amination reactions. Antidiabetic properties of obtained compounds were studied by evaluating DPP-4 (ex vivo/in vitro) and AGEs formation inhibition (in vitro). It was shown that the nitrothiadiazolopyrimidines exhibit a higher antiglycation activity than reference compound aminoguanidine, but only moderate inhibition of DPP-4. The most active DPP-4 inhibitor 1l had IC₅₀ of 55.87 μM and showed the ability to inhibit serum DPP-4 activity in rats after 10 mg/kg oral administration but with the less and shorter effect than vildagliptin. At the same time, 1l was the most active antiglycating compound in the series (IC₅₀ 134.4 μM). Copper chelation properties of synthesized compounds were also investigated since the formation of AGEs is catalyzed by the transition metal cations. A noticeable correlation between antiglycation activity and metal chelation was revealed. Both activities (antiglycation and copper chelation) correlated with quantum-chemical properties (calculated with ab initio) of the tested compounds. These findings will allow us to predict both activities in the future, without the need to model multiple steps of glycation reaction.

Greater circulating DPP4 activity is associated with impaired flow-mediated dilatation in adults with type 2 diabetes mellitus

BACKGROUND AND AIM

Dipeptidyl peptidase 4 (DPP4) is a key enzyme involved in the regulation of the incretin system exerted by cleaving the glucagon-like peptide 1 (GLP-1); the blockage of DPP4, exerted by the antidiabetic agents DPP4-inhibitors (DPP4-I), results in greater GLP-1 concentration and improved glycaemic control. DPP4 acts also as a pro-inflammatory molecule and mediates vascular damage in experimental models. The relationship between DPP4 activity and endothelial function in diabetes has not been explored yet. Aim of this study was to investigate systemic plasma DPP4 activity in relation to endothelial function in patients with type 2 diabetes mellitus (T2DM).

METHODS AND RESULTS

Sixty-two T2DM individuals were recruited in our Diabetes outpatient clinics, Sapienza University, Rome, Italy. All participants underwent complete clinical work-up; endothelial function was evaluated by flow-mediated dilatation (FMD) test; plasma DPP4 activity was assessed by measuring the 7-amino-4-methylcoumarin (AMC) cleavage rate from the synthetic substrate H-glycyl-prolyl-AMC and compared with DPP4 activity measured in sixty-two age-, sex-, BMI-matched non-diabetic subjects. Patients with T2DM had significantly higher DPP4 activity than non-diabetic individuals (211,466 ± 87657 vs 158,087 ± 60267 nmol/min/ml, p < 0.001); in T2DM patients, greater DPP4 activity significantly correlated with lower FMD whereas was not associated with BMI and metabolic control. Greater systemic DPP4 activity was an independent predictor of reduced FMD after adjusting for age, gender and other confounders.

CONCLUSIONS

Circulating DPP4 activity is increased in individuals with T2DM and associated with signs of endothelial dysfunction such as impaired FMD. DPP4 may negatively affect endothelial function through mechanisms beyond glucose homeostasis and metabolic control.

The dipeptidyl peptidase-4 inhibitory effect of flavonoids is hindered in protein rich environments

Dipeptidyl peptidase-4 (DPP-4) inhibitors present a unique approach for the management of type 2 diabetes (T2D). In the present study, the inhibition of DPP-4 was evaluated for a large panel of flavonoids, important components of the human diet, using in vitro and ex vivo models. The activity of the isolated enzyme was assayed in vitro. Subsequently, the most active flavonoids were tested ex vivo in human whole blood and plasma. In this study, contrary to the in vitro fluorometric tests, flavonoids did not show inhibitory activity against DPP-4. Due to the discrepancy in the results between the in vitro and ex vivo approaches, plasma protein binding values were determined, presenting values from 43.9 to 100.0%. This work provides a new insight into the inhibitory activity for DPP-4, based on the flavonoid scaffold. Additionally, the obtained results showed that the inhibitory effect of flavonoids against DPP-4 was hindered in protein rich environments, like that occurring in blood, and indicated the need for experimental refinement in drug discovery for blood targets.

Development of a 13C Stable Isotope Assay for Dipeptidyl Peptidase-4 Enzyme Activity A New Breath Test for Dipeptidyl Peptidase Activity

Dipeptidyl peptidase-4 inhibitors (DPP4i) are a class of orally available, small molecule inhibitors for the management of Type-II diabetes. A rapid, real-time, functional breath test for DPP4 enzyme activity could help to define DPP4i efficacy in patients that are refractory to treatment. We aimed to develop a selective, non-invasive, stable-isotope ¹³C-breath test for DPP4. In vitro experiments were performed using high (Caco-2) and low (HeLa) DPP4 expressing cells. DPP gene expression was determined in cell lines by qRT-PCR. A DPP4 selective ¹³C-tripeptide was added to cells in the presence and absence of the DPP4 inhibitor Sitagliptin. Gas samples were collected from the cell headspace and ¹³CO₂ content quantified by isotope ratio mass spectrometry (IRMS). DPP4 was highly expressed in Caco-2 cells compared to HeLa cells and using the ¹³C-tripeptide, we detected a high ¹³CO₂ signal from Caco2 cells. Addition of Sitaglitpin to Caco2 cells significantly inhibited this ¹³CO₂ signal. ¹³C-assay DPP4 activity correlated positively with the enzyme activity detected using a colorimetric substrate. We have developed a selective, non-invasive, ¹³C-assay for DPP4 that could have broad translational applications in diabetes and gastrointestinal disease.

The In Vitro Biotransformation of the Fusion Protein Tetranectin-Apolipoprotein A1

As more and more protein biotherapeutics enter the drug discovery pipelines, there is an increasing interest in tools for mechanistic drug metabolism investigations of biologics in order to identify and prioritize the most promising candidates. Understanding or even predicting the in vivo clearance of biologics and to support translational pharmacokinetic modeling activities is essential, however there is a lack of effective and validated in vitro cellular tools. Although different mechanisms have to be adressed in the context of biologics disposition, the scope is not comparable to the nowadays widely established tools for early characterization of small molecule disposition. Here, we describe a biotransformation study of the fusion protein tetranectin apolipoprotein A1 by cellular systems. The in vivo biotransformation of tetranectin apolipoprotein A1 has been described previously, and the same major biotransformation product could also be detected in vitro, by a targeted and highly sensitive detection method based on chymotrypsin digest. In addition, the protease responsible for the formation of this biotransformation product could be elucidated to be DPP4. To our knowledge, this is one of the first reports of an in vitro biotransformation study by cells of a therapeutic protein.

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.

DPP8/DPP9 inhibition elicits canonical Nlrp1b inflammasome hallmarks in murine macrophages

Activating germline mutations in the human inflammasome sensor NLRP1 causes palmoplantar dyskeratosis and susceptibility to Mendelian autoinflammatory diseases. Recent studies have shown that the cytosolic serine dipeptidyl peptidases DPP8 and DPP9 suppress inflammasome activation upstream of NLRP1 and CARD8 in human keratinocytes and peripheral blood mononuclear cells. Moreover, pharmacological inhibition of DPP8/DPP9 protease activity was shown to induce pyroptosis in murine C57BL/6 macrophages without eliciting other inflammasome hallmark responses. Here, we show that DPP8/DPP9 inhibition in macrophages that express a Bacillus anthracis lethal toxin (LeTx)-sensitive Nlrp1b allele triggered significantly accelerated pyroptosis concomitant with caspase-1 maturation, ASC speck assembly, and secretion of mature IL-1β and IL-18. Genetic ablation of ASC prevented DPP8/DPP9 inhibition-induced caspase-1 maturation and partially hampered pyroptosis and inflammasome-dependent cytokine release, whereas deletion of caspase-1 or gasdermin D triggered apoptosis in the absence of IL-1β and IL-18 secretion. In conclusion, blockade of DPP8/DPP9 protease activity triggers rapid pyroptosis and canonical inflammasome hallmarks in primary macrophages that express a LeTx-responsive Nlrp1b allele.

Dipeptidyl Peptidase 4 Activity Is Related to Body Composition, Measures of Adiposity, and Insulin Resistance in Subjects with Excessive Adiposity and Different Degrees of Glucose Tolerance

BACKGROUND

The enzyme dipeptidyl peptidase 4 (DPP4) has been recently recognized as an adipo-myokine. However, studies that associate its constitutive activity with body composition, anthropometry, and insulin resistance (IR) are very scarce and included only healthy people.

METHODS

First, we investigated the relationships of constitutive DPP4 activity, body composition (assessed by bioelectrical impedance analysis), and measures of adiposity and IR in fifty-two subjects of both sexes, 18-50 years, and BMI ≥25.0 kg/m² who comprised three groups according to glucose tolerance. Additionally, we evaluated associations among DPP4 activity and adipokines, gut peptides, and biochemical variables at fasting and 30 and 60 min after a standardized meal intake.

RESULTS

DPP4 activity was no different among the three groups. At fasting, pooled analysis showed it was positively correlated with measures of central adiposity, such as WC (P = 0.011) and WHR (P = 0.009), and with all measures of IR, but inversely related to indexes of general adiposity, such as fat mass percentage (P = 0.014) and BAI (P = 0.0003). DPP4 activity was also associated with lean mass (r = 0.57, P < 0.0001). After meal intake, DPP4 activity remained significantly associated with insulin, leptin, and resistin. In multiple regression analysis, BAI, WHR, percent lean mass, HOMA-IR, and leptin influenced DPP4 activity and explained approximately 26% of the variance on it.

CONCLUSIONS

Constitutive DPP4 activity is positively associated with lean mass, central adiposity, and IR and negatively to general adiposity. Furthermore, it seems to be influenced by body composition and IR and could also be viewed as an adipo-myokine in subjects with excessive adiposity and different stages of glucose tolerance.

Reduced post-operative DPP4 activity associated with worse patient outcome after cardiac surgery

Cardiac surgery with cardiopulmonary bypass (CPB) triggers myocardial ischemia/reperfusion injury contributing to organ dysfunction. Preclinical studies revealed that dipeptidyl peptidase (DPP4) inhibition is protective during myocardial infarction. Here, we assessed for the first time the relation of peri-operative DPP4-activity in serum of 46 patients undergoing cardiac surgery with patients' post-operative organ dysfunction during intensive care unit (ICU) stay. Whereas a prior myocardial infarction significantly reduced pre-operative DDP4-activity, patients with preserved left ventricular function showed an intra-operative decrease of DPP4-activity. The latter correlated with aortic cross clamping time, indicative for the duration of surgery-induced myocardial ischemia. As underlying mechanism, mass-spectrometry revealed increased DPP4 oxidation by cardiac surgery, with DPP4 oxidation reducing DPP4-activity in vitro. Further, post-operative DPP4-activity was negatively correlated with the extent of post-operative organ injury as measured by SAPS II and SOFA scoring, circulating levels of creatinine and lactate, as well as patients' stay on the ICU. In conclusion, cardiac surgery reduces DPP4-activity through oxidation, with low post-operative DPP4-activity being associated with organ dysfunction and worse outcome of patients during the post-operative ICU stay. This likely reflects the severity of myocardial ischemia/reperfusion injury and may suggest potential beneficial effects of anti-oxidative treatments during cardiac surgery.

Potential disease biomarkers: dipeptidyl peptidase 4 and fibroblast activation protein

The importance of the dipeptidyl peptidase 4 (DPP4) gene family in regulating critical biochemical pathways continues to emerge. The two most well-studied members of the family, DPP4 and fibroblast activation protein (FAP), have been investigated both as therapeutic targets for disease and as diagnostic biomarkers. The interest in DPP4 and FAP as potential disease biomarkers has been driven primarily by observations of altered expression profiles in inflammatory diseases and cancer. Furthermore, the stability and persistence of soluble DPP4 and FAP in the serum make them attractive candidate serology markers. This review summarises investigations into DPP4 and FAP as biomarkers of autoimmune disease, gut inflammation, psychosomatic disorders and malignancy and discusses their potential likelihood as clinically useful tools.

Improved glycaemia in high-fat-fed neprilysin-deficient mice is associated with reduced DPP-4 activity and increased active GLP-1 levels

AIM/HYPOTHESIS

Neprilysin, a widely expressed peptidase, is upregulated in metabolically altered states such as obesity and type 2 diabetes. Like dipeptidyl peptidase-4 (DPP-4), neprilysin can degrade and inactivate the insulinotropic peptide glucagon-like peptide-1 (GLP-1). Thus, we investigated whether neprilysin deficiency enhances active GLP-1 levels and improves glycaemia in a mouse model of high fat feeding.

METHODS

Nep ^+/+ and Nep ^-/- mice were fed a 60% fat diet for 16 weeks, after which active GLP-1 and DPP-4 activity levels were measured, as were glucose, insulin and C-peptide levels during an OGTT. Insulin sensitivity was assessed using an insulin tolerance test.

RESULTS

High-fat-fed Nep ^-/- mice exhibited elevated active GLP-1 levels (5.8 ± 1.1 vs 3.5 ± 0.8 pmol/l, p < 0.05) in association with improved glucose tolerance, insulin sensitivity and beta cell function compared with high-fat-fed Nep ^+/+ mice. In addition, plasma DPP-4 activity was lower in high-fat-fed Nep ^-/- mice (7.4 ± 1.0 vs 10.7 ± 1.3 nmol ml^-1 min^-1, p < 0.05). No difference in insulin:C-peptide ratio was observed between Nep ^-/- and Nep ^+/+ mice, suggesting that improved glycaemia does not result from changes in insulin clearance.

CONCLUSIONS/INTERPRETATION

Under conditions of increased dietary fat, an improved glycaemic status in neprilysin-deficient mice is associated with elevated active GLP-1 levels, reduced plasma DPP-4 activity and improved beta cell function. Thus, neprilysin inhibition may be a novel treatment strategy for type 2 diabetes.

Truncation of CXCL12 by CD26 reduces its CXC chemokine receptor 4- and atypical chemokine receptor 3-dependent activity on endothelial cells and lymphocytes

The chemokine CXCL12 or stromal cell-derived factor 1/SDF-1 attracts hematopoietic progenitor cells and mature leukocytes through the G protein-coupled CXC chemokine receptor 4 (CXCR4). In addition, it interacts with atypical chemokine receptor 3 (ACKR3 or CXCR7) and glycosaminoglycans. CXCL12 activity is regulated through posttranslational cleavage by CD26/dipeptidyl peptidase 4 that removes two NH₂-terminal amino acids. CD26-truncated CXCL12 does not induce calcium signaling or chemotaxis of mononuclear cells. CXCL12(3-68) was chemically synthesized de novo for detailed biological characterization. Compared to unmodified CXCL12, CXCL12(3-68) was no longer able to signal through CXCR4 via inositol trisphosphate (IP₃), Akt or extracellular signal-regulated kinases 1 and 2 (ERK1/2). Interestingly, the recruitment of β-arrestin 2 to the cell membrane via CXCR4 by CXCL12(3-68) was abolished, whereas a weakened but significant β-arrestin recruitment remained via ACKR3. CXCL12-induced endothelial cell migration and signal transduction was completely abrogated by CD26. Intact CXCL12 hardly induced lymphocyte migration upon intra-articular injection in mice. In contrast, oral treatment of mice with the CD26 inhibitor sitagliptin reduced CD26 activity and CXCL12 cleavage in blood plasma. The potential of CXCL12 to induce intra-articular lymphocyte infiltration was significantly increased in sitagliptin-treated mice and CXCL12(3-68) failed to induce migration under both CD26-inhibiting and non-inhibiting conditions. In conclusion, CD26-cleavage skews CXCL12 towards β-arrestin dependent recruitment through ACKR3 and destroys the CXCR4-mediated lymphocyte chemoattractant properties of CXCL12 in vivo. Hence, pharmacological CD26-blockade in tissues may enhance CXCL12-induced inflammation.

Optimization of gold nanoparticle-based real-time colorimetric assay of dipeptidyl peptidase IV activity

Dipeptidyl peptidase IV (DPP-IV also referred to as CD-26) is a serine protease enzyme with remarkable diagnostic and prognostic value in a variety of health and disease conditions. Herein, we describe a simple and real-time colorimetric assay for DPP-IV/CD-26 activity based on the aggregation of gold nanoparticles (AuNPs) functionalized with the peptide substrates: Gly-Pro-Asp-Cys (GPDC) or Val-Pro-ethylene diamine-Asp-Cys (VP-ED-DC). Cleavage of the substrates by DPP-IV resulted in aggregation of the AuNPs with accompanying color change in the solution from red to blue that was monitored using either a UV-visible spectrophotometer or by the naked eye. Factors, such as time course of the reaction, stability of the functionalized AuNPs and the structure of the substrate that influence the cleavage reaction in solution were investigated. The effects of potential interference from serum proteins (lysozyme, thrombin and trypsin) on the analytical response were negligible. The detection limits when GPDC or VP-EN-DC functionalized AuNPs were used for DPP-IV assay were 1.2U/L and 1.5U/L, respectively. The VP-EN-DC method was preferred for the quantitative determination of DPP-IV activity in serum because of its wide linear range 0-30U/L compared to 0-12U/L for the GPDC assay. Recoveries from serum samples spiked with DPP-IV activity, between 5 and 25U/L, and using the VP-EN-DC modified AuNPs method ranged between 83.6% and 114.9%. The two colorimetric biosensors described here are superior to other conventional methods because of their simplicity, stability, selectivity and reliability.

Circulating fibroblast activation protein and dipeptidyl peptidase 4 in rheumatoid arthritis and systemic sclerosis

AIM

To quantify circulating fibroblast activation protein (cFAP) and dipeptidyl peptidase 4 (cDPP4) protease activities in patients with rheumatoid arthritis (RA), systemic sclerosis (SSc), and a control group with mechanical back pain and to correlate plasma levels with disease characteristics.

METHODS

Plasma was collected from patients with RA (n = 73), SSc (n = 37) and control subjects (n = 26). DPP4 and FAP were quantified using specific enzyme activity assays.

RESULTS

Median cDPP4 was significantly lower in the RA group (P = 0.02), and SSc group (P = 0.002) compared with controls. There were no significant differences in median cFAP between the three groups. DPP4 and FAP demonstrated a negative correlation with inflammatory markers and duration of disease. There were no associations with disease subtypes in RA, including seropositive and erosive disease. Decreased cDPP4 was found in SSc patients with myositis. Plasma FAP was lower in RA patients receiving prednisone (P = 0.001) or leflunomide (P = 0.04), but higher with biologic agents (P = 0.01). RA patients receiving leflunomide also had decreased cDPP4 (P = 0.014). SSc patients receiving prednisone (P = 0.02) had lower cDPP4 but there was no association with cFAP.

CONCLUSIONS

No association was found between cFAP and RA or SSc. Plasma DPP4 was decreased in RA and SSc when compared with controls. cDPP4 and cFAP correlated negatively with inflammatory markers and there were no significant correlations with disease characteristics in this RA cohort.

The Dipeptidyl Peptidases 4, 8, and 9 in Mouse Monocytes and Macrophages: DPP8/9 Inhibition Attenuates M1 Macrophage Activation in Mice

Atherosclerosis remains the leading cause of death in Western countries. Dipeptidyl peptidase (DPP) 4 has emerged as a novel target for the prevention and treatment of atherosclerosis. Family members DPP8 and 9 are abundantly present in macrophage-rich regions of atherosclerotic plaques, and DPP9 inhibition attenuates activation of human M1 macrophages in vitro. Studying this family in a mouse model for atherosclerosis would greatly advance our knowledge regarding their potential as therapeutic targets. We found that DPP4 is downregulated during mouse monocyte-to-macrophage differentiation. DPP8 and 9 expression seems relatively low in mouse monocytes and macrophages. Viability of primary mouse macrophages is unaffected by DPP4 or DPP8/9 inhibition. Importantly, DPP8/9 inhibition attenuates macrophage activation as IL-6 secretion is significantly decreased. Mouse macrophages respond similarly to DPP inhibition, compared to human macrophages. This shows that the mouse could become a valid model species for the study of DPPs as therapeutic targets in atherosclerosis.

The serine protease, dipeptidyl peptidase IV as a myokine: dietary protein and exercise mimetics as a stimulus for transcription and release

Dipeptidyl-peptidase IV (DPP-IV) is an enzyme with numerous roles within the body, mostly related to regulating energy metabolism. DPP-IV is also a myokine, but the stimulus for its release is poorly understood. We investigated the transcription and release of DPP-IV from skeletal muscle in a three-part study using C2C12 myotube cultures, an acute rat exercise and postexercise feeding model, and human feeding or human exercise models. When myotubes were presented with leucine only, hydrolyzed whey protein, or chemicals that cause exercise-related signaling to occur in cell culture, all caused an increase in the mRNA expression of DPP-IV (1.63 to 18.56 fold change, P < 0.05), but only whey protein caused a significant increase in DPP-IV activity in the cell culture media. When rats were fed whey protein concentrate immediately following stimulated muscle contractions, DPP-IV mRNA in both the exercised and nonexercised gastrocnemius muscles significantly increased 2.5- to 3.7-fold (P < 0.05) 3-6 h following the exercise/feeding bout; of note exercise alone or postexercise leucine-only feeding had no significant effect. In humans, plasma and serum DPP-IV activities were not altered by the ingestion of whey protein up to 1 h post consumption, after a 10 min bout of vigorous running, or during the completion of three repeated lower body resistance exercise bouts. Our cell culture and rodent data suggest that whey protein increases DPP-IV mRNA expression and secretion from muscle cells. However, our human data suggest that DPP-IV is not elevated in the bloodstream following acute whey protein ingestion or exercise.

Acute Ischemic Stroke Severity, Progression, and Outcome Relate to Changes in Dipeptidyl Peptidase IV and Fibroblast Activation Protein Activity

Dipeptidyl peptidase IV (DPPIV) inhibition may be a promising therapeutic strategy for acute stroke treatment, given its potential to prolong the biological half-life of neuroprotective substrates. A related protease, fibroblast activation protein (FAP), was recently shown to inactivate the same substrates. Therefore, it should also be investigated as a potential target in stroke. The study aimed to investigate whether stroke severity and outcome correlate with DPPIV and FAP activities and their kinetics shortly after acute ischemic stroke. DPPIV and FAP activities were analyzed in the serum of 50 hyperacute stroke patients at admission, 1 day, 3 days, and 7 days after stroke onset and in 50 age-matched healthy controls. This was done as part of the Middelheim's Interdisciplinary Stroke Study. DPPIV activity tended to increase shortly after stroke compared to the control population. DPPIV and FAP activities steadily decreased in the first week after stroke onset. Higher infarct volumes (≥5 ml) and a more severe stroke (NIHSS >7) at admission were correlated with a stronger decrease in the activities of both enzymes. Moreover, these patients more often developed a progressive stroke, were more often institutionalized. Patients with a stronger increase in DPPIV activity at admission and decrease in the activity of both DPPIV and FAP during the first week after stroke onset had a more severe stroke and worse short-term outcomes.

Plasma dipeptidyl peptidase IV activity and measures of body composition in apparently healthy people

AIM

Based on its regulatory action on glucagon-like peptide 1, dipeptidyl peptidase IV (DPP-IV) has increasingly been linked to Type 2 diabetes. However, there is no evidence as to how this normal modulatory enzyme leads to pathology. It is thought that DPP-IV is affected by the development of obesity, which is a common precursor to Type 2 diabetes. Little is known about the relationship between DPP-IV activity in plasma and specific body composition measures.

MAIN METHODS

In the current study, plasma DPP-IV activity and body composition measures were collected from 111 healthy subjects between the ages of 19 and 70 years old for analysis.

KEY FINDINGS

The mean plasma DPP-IV activity was 35.9U/L ± 12.3, falling within normal reference value range presented by Durinx et al. DPP-IV activity was negatively correlated with absolute body fat mass, but absolute lean mass was positively correlated. Consistent with the findings, DPP-IV activity was also negatively correlated with absolute gynoid fat (p = 0.0047). DPP-IV activity did not have a significant correlation with absolute android fat mass, visceral adipose tissue, BMI, and age.

SIGNIFICANCE

From these results, it can be concluded that high activity of DPP-IV is not indicative of pathology, and specific body composition components may influence soluble DPP-IV activity in the blood.

Design, synthesis and biological evaluation of novel pyrazolo-pyrimidinones as DPP-IV inhibitors in diabetes

We report the design, synthesis, biological activity and docking studies of series of novel pyrazolo[3,4-d]pyrimidinones as DPP-IV inhibitors in diabetes. Molecules were synthesized and evaluated for their DPP-IV inhibition activity. Compounds 5e, 5k, 5o and 6a were found to be potent inhibitors of DPP-IV enzyme. Amongst all the synthesized compounds, 6-methyl-5-(4-methylpyridin-2-yl)-1-phenyl-1H-pyrazolo[3,4-d]pyrimidin-4(5H)-one (5k) was found to be the most active based on in vitro DPP-IV studies and also exhibited promising in vivo blood glucose lowering activity in male Wistar rats.

Potential impact of sitagliptin on collagen-derived dipeptides in diabetic osteoporosis

It is known that diabetes coincides with an increased risk of osteoporosis. While a disturbed collagen metabolism is proposed as a possible cause, much remains unknown about the enzymes involved and changes in the collagen-derived dipeptides and amino acids. Therefore, we sought to study this intricate pathway and the effect of dipeptidyl peptidase 4 (DPP4) inhibitors. Control and streptozotocin-nicotinamide-induced diabetic rats were treated for 12 weeks with vehicle or sitagliptin, a DPP4 inhibitor (Con/VH, Con/SG, DM/VH and DM/SG). The activities of four key enzymes involved in collagen breakdown were determined in serum (DPP4, matrix metalloproteinase 2 and 9 and prolidase). Dipeptide (Ala-Pro, Gly-Pro, Pro-Pro and Pro-Hyp) and amino acid (Pro and Hyp) concentrations were measured by liquid chromatography coupled to mass spectrometry. We found three-fold higher MMP9 activities in DM/VH than in controls, while in DM/SG this rise was attenuated. MMP2 and prolidase did not differ in the investigated groups. Furthermore, we are the first to report on two-fold higher Ala-Pro and Pro-Pro levels in diabetes compared to controls. In contrast, Pro-Hyp concentrations were lower in diabetes (DM/VH and DM/SG). DPP4 inhibition does not seem to have a direct influence on the collagen metabolism in streptozotocin-nicotinamide-induced diabetic rats. Instead, it probably acts through its effect on osteoprotective substrates. In diabetes, increased MMP9 activities seem to favour the production of Ala-Pro and Pro-Pro containing collagen fragments. The high Pro-Hyp levels in untreated controls might have a bone-stimulating effect. Nevertheless, the biological significance of these dipeptides is not yet clear and should be further investigated.

Left ventricular diastolic dysfunction and myocardial stiffness in diabetic mice is attenuated by inhibition of dipeptidyl peptidase 4

AIMS

Obesity and Type 2 diabetes mellitus (DM) induce left ventricular (LV) diastolic dysfunction, which contributes to an increasing prevalence of heart failure with a preserved LV ejection fraction. We investigated the effects of sitagliptin (SITA), an inhibitor of dipeptidylpeptidase-4 (DPP-4) and anti-diabetic drug, on LV structure and function of obese mice with Type 2 DM.

METHODS AND RESULTS

Obese Type 2 diabetic mice (Lepr(db/db), BKS.Cg-Dock7(m)+/+ Lepr(db)/J), displaying increased cardiomyocyte and LV stiffness at the age of 16 weeks, were treated with SITA (300 mg/kg/day) or vehicle for 8 weeks. SITA severely impaired serum DPP-4 activity, but had no effect on glycaemia. Invasive haemodynamic recordings showed that SITA reduced LV passive stiffness and increased LV stroke volume; LV end-systolic elastance remained unchanged. In addition, SITA reduced resting tension of isolated single cardiomyocytes and intensified phosphorylation of the sarcomeric protein titin. SITA also increased LV concentrations of cGMP and increased activity of protein kinase G (PKG). In vitro activation of PKG decreased resting tension of cardiomyocytes from vehicle-treated mice, but had no effect on resting tension of cardiomyocytes from SITA-treated mice.

CONCLUSIONS

In obese Type 2 diabetic mice, in the absence of hypoglycaemic effects, inhibition of DPP-4 decreases LV passive stiffness and improves global LV performance. These effects seem at least partially mediated by stimulatory effects on the myocardial cGMP-PKG pathway and, hence, on the phosphorylation status of titin and the hereto coupled cardiomyocyte stiffness modulus.

DPP IV inhibitor treatment attenuates bone loss and improves mechanical bone strength in male diabetic rats

Dipeptidyl peptidase IV (DPP IV) modulates protein activity by removing dipeptides. DPP IV inhibitors are currently used to improve glucose tolerance in type 2 diabetes patients. DPP IV substrates not only increase insulin secretion but also affect bone metabolism. In this study, the effect of DPP IV inhibitor sitagliptin on bone was evaluated in normal and streptozotocin-induced diabetic rats. This study included 64 male Wistar rats divided into four groups (n = 16): two diabetic and two control groups. One diabetic and one control group received sitagliptin through drinking water. Tibiae were scanned every 3 wk using an in vivo μCT scanner. After 6 and 12 wk, rats were euthanized for histomorphometric analysis of bone parameters. The mechanical resistance of femora to fracture was assessed using a three-point bending test, and serum levels of bone metabolic markers were measured. Efficient DPP IV inhibition was achieved in sitagliptin-treated groups. Trabecular bone loss, the decrease in trabecular number, and the increase in trabecular spacing was attenuated through sitagliptin treatment in diabetic rats, as shown by in vivo μCT. Bone histomorphometry was in line with these results. μCT analysis furthermore showed that sitagliptin prevented cortical bone growth stagnation in diabetic rats, resulting in stronger femora during three-point bending. Finally, the serum levels of the resorption marker CTX-I were significantly lower in sitagliptin-treated diabetic animals compared with untreated diabetic animals. In conclusion, sitagliptin treatment attenuates bone loss and increases bone strength in diabetic rats probably through the reduction of bone resorption and independent of glycemic management.