Biosynthesis and degradation of altered immature forms of intestinal dipeptidyl peptidase IV in a rat strain lacking the enzyme

Glucagon-like peptide-1 response to whey protein is less diminished by dipeptidyl peptidase-4 in comparison with responses to dextrin, a lipid and casein in rats

Although glucose is the best-known nutrient to stimulate glucagon-like peptide-1 (GLP-1) secretion, dietary peptides also potently stimulate GLP-1 secretion. Certain peptide fragments derived from dietary proteins possess dipeptidyl peptidase-4 (DPP-4) inhibitory activity in vitro. Hence, we hypothesised that dietary peptides protect GLP-1 from degradation through attenuating DPP-4 activity in vivo. Here, we compared GLP-1 responses with dietary proteins, a carbohydrate and a lipid (Intralipos) in rats having or not having plasma DPP-4 activity. Plasma GLP-1 concentrations clearly increased by oral administration of whey protein (2-4 g/kg), but not by that of dextrin (2-4 g/kg), in control rats (untreated Sprague-Dawley rats and F344/Jcl rats), having DPP-4 activity. In contrast, dextrin administration increased the plasma GLP-1 concentrations as the whey protein administration did, in rats having reduced or no DPP-4 activity (a DPP-4 inhibitor, sitagliptin-treated Sprague-Dawley rats or DPP-4-deficient F344/DuCrl/Crlj rats). DPP-4 inhibition by sitagliptin treatment also enhanced GLP-1 response to Intralipos, and casein, but the treatment did not further enhance GLP-1 response to whey protein. Intestinal GLP-1 content and gastric emptying rate were not associated with differences in GLP-1 responses to test nutrients. The luminal contents from rats administered whey protein decreased DPP-4 activity in vitro. These results suggest that GLP-1 released by dextrin, Intralipos and casein was immediately degraded by DPP-4, while GLP-1 released by whey protein was less degraded. Our study provides novel in vivo evidence supporting the hypothesis that dietary peptides not only stimulate GLP-1 secretion but also inhibit DPP-4 activity to potentiate GLP-1 response.

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

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

Cut to the chase: a review of CD26/dipeptidyl peptidase-4's (DPP4) entanglement in the immune system

CD26/DPP4 (dipeptidyl peptidase 4/DP4/DPPIV) is a surface T cell activation antigen and has been shown to have DPP4 enzymatic activity, cleaving-off amino-terminal dipeptides with either L-proline or L-alanine at the penultimate position. It plays a major role in glucose metabolism by N-terminal truncation and inactivation of the incretins glucagon-like peptide-1 (GLP) and gastric inhibitory protein (GIP). In 2006, DPP4 inhibitors have been introduced to clinics and have been demonstrated to efficiently enhance the endogenous insulin secretion via prolongation of the half-life of GLP-1 and GIP in patients. However, a large number of studies demonstrate clearly that CD26/DPP4 also plays an integral role in the immune system, particularly in T cell activation. Therefore, inhibition of DPP4 might represent a double-edged sword. Apart from the metabolic benefit, the associated immunological effects of long term DPP4 inhibition on regulatory processes such as T cell homeostasis, maturation and activation are not understood fully at this stage. The current data point to an important role for CD26/DPP4 in maintaining lymphocyte composition and function, T cell activation and co-stimulation, memory T cell generation and thymic emigration patterns during immune-senescence. In rodents, critical immune changes occur at baseline levels as well as after in-vitro and in-vivo challenge. In patients receiving DPP4 inhibitors, evidence of immunological side effects also became apparent. The scope of this review is to recapitulate the role of CD26/DPP4 in the immune system regarding its pharmacological inhibition and T cell-dependent immune regulation.

A novel DPP-4 inhibitor teneligliptin scavenges hydroxyl radicals: In vitro study evaluated by electron spin resonance spectroscopy and in vivo study using DPP-4 deficient rats

AIMS

Recently various dipeptidyl peptidase-4 (DPP-4) inhibitors have emerged because of their high effectiveness and safety. In spite of their common effect of DPP-4 inhibition, the chemical structures are diverse. Here we show that the structure of teneligliptin, a novel DPP-4 inhibitor, has a scavenging activity on hydroxyl radical (·OH).

METHODS

·OH and superoxide (O2(-)) were detected by electron spin resonance (ESR) spectroscopy. ·OH and O2(-) were generated in vitro by the Fenton reaction and a hypoxanthine-xanthine oxidase system, respectively. The level of free radicals was estimated from the ESR signal intensity. The product via teneligliptin and ·OH reaction was identified by thin layer chromatography and mass spectrometry analysis. In vivo effect was also evaluated using DPP-4 deficient rats with streptozotocin-induced diabetes.

RESULTS

ESR spectroscopy analysis showed that teneligliptin did not scavenge O2(-), but scavenged ·OH in a dose dependent manner. Its activity was greater than that of glutathione. The reaction product appeared to have an oxygen-atom added structure to that of teneligliptin, which was identical to the most abundant metabolite of teneligliptin in human plasma. Furthermore, using DPP-4 deficient rat, teneligliptin did not affect plasma glucose levels or body weight, but normalized increased levels of 8-hydroxy-2'-deoxyguanosine in urine, kidney and aorta of diabetic rats, supporting that teneligliptin may have a ·OH scavenging activity in vivo independently of DPP-4 inhibition.

CONCLUSIONS

Teneligliptin is not only effective as DPP-4 inhibitor, but may also be beneficial as ·OH scavenger, which may be useful in the prevention of diabetic complications.

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

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

The increased dipeptidyl peptidase-4 activity is not counteracted by optimized glucose control in type 2 diabetes, but is lower in metformin-treated patients

AIM

Dipeptidyl peptidase (DPP)-4 in responsible for incretin degradation and some observations suggest that DPP-4 activity is increased in type 2 diabetes (T2D). We aimed to assess the effect of T2D and glucose control on DPP-4 activity.

METHODS

In the first set (SET1) of patients, we compared plasma DPP-4 activity between 30 T2D and 20 age- and sex-matched non-diabetic subjects. In the second set (SET2), we measured serum DPP-4 activity in 42 T2D patients before and after a trial of glucose control achieved by add-on basal insulin therapy (NCT00699686). Serum/plasma DPP-4 activity was determined using chromogenic and fluorigenic substrates, as well as several positive and negative controls.

RESULTS

In SET1, plasma DPP-4 activity was significantly higher in T2D vs. controls (32.2 ± 1.2 U/l vs. 21.2 ± 1.1 U/l, p < 10(-6)). From a meta-analysis of the literature, we found that T2D is associated with a 33% increase in DPP-4 activity compared to controls. In SET2, serum DPP-4 activity was not lowered by intensified glucose control, despite an average haemoglobin A1c (HbA1c) reduction of 1.5%. In both sets of diabetic patients, the use of metformin was associated with a significantly lower DPP-4 activity, independently of age, sex, body mass index and HbA1c.

CONCLUSION

DPP-4 activity is increased in T2D, but is not lowered by glucose control, suggesting that hyperglycaemia is not a direct determinant of DPP-4 activity. However, metformin may indirectly reduce DPP-4 activity.

Biochemical and histological changes in the small intestine of mice with dextran sulfate sodium colitis

The dextran sulfate sodium (DSS) model of colitis has been commonly utilized in mice to assess novel treatments for ulcerative colitis. Recent studies have indicated that morphological and biochemical changes extend to the small intestine (SI). This study aimed to characterize histological and biochemical changes in the SI during DSS colitis in wild-type (WT) and DPIV knock-out (DPIV(-/-) ) mice treated with saline or the DPIV inhibitors, Ile-Pyrr-(2-CN)*TFA or Ile-Thia. Groups (n = 10) of DPIV(-/-) and WT mice were orally gavaged twice daily with saline, Ile-Pyrr-(2-CN)*TFA or Ile-Thia. Mice consumed 2% DSS in drinking water for 6 days to induce colitis. Small intestinal tissue was assessed for histological changes, sucrase, and DPIV activity and neutrophil infiltration. Jejunal villus length was increased in all groups after 6 days DSS consumption (P < 0.05). Jejunal DPIV activity was significantly lower by 35% in WT mice receiving Ile-Pyrr-(2-CN)*TFA compared to saline controls. Jejunal MPO activity was significantly increased in the WT + saline and DPIV(-/-)  + saline groups following DSS consumption, compared to WT and DPIV(-/-) controls at day 0. Increased sucrase activity was apparent at day 0 in DPIV(-/-) compared to WT mice (P < 0.05). We conclude that DSS-induced damage is not restricted to the colon, but also extends to the small intestine. Furthermore, reduced or absent DPIV activity resulted in functional adaptations to brush border enzyme activity. DPIV inhibitors are now a recognized therapy for type-II diabetes. The work presented here highlights the need to delineate any long-term effects of DPIV inhibitors on SI function, to further validate their safety and tolerability.

Concentration-dependent plasma protein binding of the novel dipeptidyl peptidase 4 inhibitor BI 1356 due to saturable binding to its target in plasma of mice, rats and humans

Objectives

The purpose of this study was to characterise the plasma protein binding of BI 1356.

Methods

BI 1356 (proposed trade name ONDERO) is a novel dipeptidyl peptidase 4 (DPP-4) inhibitor, which is under clinical development for the treatment of type 2 diabetes. DPP-4 is expressed in various tissues but soluble DPP-4 is also present in plasma. Therefore, binding to soluble DPP-4 may influence the pharmacokinetics of BI 1356. Plasma protein binding of BI 1356 was determined in vitro for wild type mice and rats and the results compared with those for DPP-4 knockout mice and DPP-4 deficient Fischer rats. In addition, protein binding of BI 1356 was examined in plasma from healthy human volunteers and renal excretion of the compound in the DPP-4 knockout mice was compared with that occurring in wild type mice.

Key findings

The results showed that BI 1356 exhibited a prominent concentration-dependent plasma protein binding due to a saturable high affinity binding to the DPP-4 target in plasma. Differences in renal excretion of BI 1356 between DPP-4 knockout mice and wild type mice suggested that saturable binding of BI 1356 to DPP-4 in the body also influenced elimination.

Conclusions

High affinity, but readily saturable binding of BI 1356 to its target DPP-4 accounted primarily for the concentration-dependent plasma protein binding at therapeutic plasma concentrations of BI 1356.

Tissue distribution of the novel DPP-4 inhibitor BI 1356 is dominated by saturable binding to its target in rats

BI 1356 (INN: linagliptin) is an inhibitor of dipeptidyl peptidase-4 (DPP-4). This study investigated whether saturable binding of BI 1356 to its target DPP-4 occurs in tissues and whether drug accumulation occurs at these sites in vivo. In order to test these hypotheses, the tissue distribution of BI 1356 was determined in wild-type and DPP-4 deficient rats at different dose levels by means of whole body autoradiography and measurement of tissue radioactivity concentrations after single i.v. dosing of [(14)C]-radio labeled BI 1356. The accumulation behavior of drug-related radioactivity in tissues was further explored in an oral repeat dose study. Tissue levels of [(14)C]BI 1356 related radioactivity were markedly lower in all investigated tissues of the DPP-4 deficient rats and the difference of the dose-dependent increase of radioactivity tissue levels between both rat strains indicates that tissue distribution at low doses of BI 1356 is dominated by binding of BI 1356 to DPP-4 in tissues. As the binding to DPP-4 is strong but reversible, the tissue binding results in a long terminal half-life in several tissues including plasma. The binding capacity to DPP-4 is, however, limited. In the rat, saturation of DPP-4 binding is suggested at an intravenous dose above 0.01-0.1 mg/kg [(14)C]BI 1356. As the DPP-4 binding capacity is saturated already at low doses, accumulation of BI 1356 in tissues is unlikely, despite the long persistence of low amounts in the body.

Phenotyping of congenic dipeptidyl peptidase 4 (DP4) deficient Dark Agouti (DA) rats suggests involvement of DP4 in neuro-, endocrine, and immune functions

BACKGROUND

Treatment of diabetes type 2 using chronic pharmacological inhibition of dipeptidyl peptidase 4 (DP4) still requires an in-depth analysis of models for chronic DP4 deficiency, because adverse reactions induced by some DP4 inhibitors have been described.

METHODS

In the present study, a novel congenic rat model of DP4 deficiency on a "DP4-high" DA rat genetic background was generated (DA.F344-Dpp4(m)/ SvH rats) and comprehensively phenotyped.

RESULTS

Similar to chronic pharmacological inhibition of DP4, DP4 deficient rats exhibited a phenotype involving reduced diet-induced body weight gain and improved glucose tolerance associated with increased levels of glucagon-like peptide-1 (GLP-1) and bound leptin as well as decreased aminotransferases and triglycerides. Additionally, DA.F344-Dpp4(m)/SvH rats showed anxiolytic-like and reduced stress-like responses, a phenomenon presently not targeted by DP4 inhibitors. However, several immune alterations, such as differential leukocyte subset composition at baseline, blunted natural killer cell and T-cell functions, and altered cytokine levels were observed.

CONCLUSIONS

While this animal model confirms a critical role of DP4 in GLP-1-dependent glucose regulation, genetically induced chronic DP4 deficiency apparently also affects stress-regulatory and immuneregulatory systems, indicating that the use of chronic DP4 inhibitors might have the potential to interfere with central nervous system and immune functions in vivo.

Interrelationship of dipeptidyl peptidase IV (DPP4) with the development of diabetes, dyslipidaemia and nephropathy: a streptozotocin-induced model using wild-type and DPP4-deficient rats

We examined the role of dipeptidyl peptidase IV (DPP4) in the development of diabetes, dyslipidaemia and renal dysfunction induced by streptozotocin (STZ). F344/DuCrlCrlj rats, which lack DPP4 activity, and wild-type rats were treated with STZ. Plasma DPP4 activity and biochemical parameters were measured until 42 days after STZ treatment. At the end of the experiment, renal function and DPP4 expressions of the kidney, liver, pancreas and adipose tissues were determined. Increases in blood glucose, cholesterol and triglycerides were evoked by STZ in both rat strains; however, the onset of hyperglycaemia was delayed in DPP4-deficient rats as compared with wild-type rats. By contrast, more severe dyslipidaemia was observed in DPP4-deficient rats than in wild-type rats after STZ treatment. Plasma DPP4 activity increased progressively with time after STZ treatment in wild-type rats. The kidney of wild-type rats showed decreased DPP4 activity with increased Dpp4 mRNA after STZ treatment. In addition, kidney weight, serum creatinine and excreted amounts of urinary protein, glucose and DPP4 enzyme were enhanced by STZ. DPP4-deficient rats showed increased serum creatinine in accordance with decreased creatinine clearance as compared with wild-type rats after STZ treatment. In conclusion, plasma DPP4 activity increased after STZ treatment, positively correlating to blood glucose. DPP4-deficient rats were resistant to developing diabetes, while susceptible to dyslipidaemia and reduction of glomerular filtration rate by STZ. DPP4 activation may be responsible for hyperglycaemia, lipid metabolism and preservation of renal function.

Improvement of high fat-diet-induced insulin resistance in dipeptidyl peptidase IV-deficient Fischer rats

F344/DuCrj rats are genetically deficient in dipeptidyl peptidase IV (DPPIV). This enzyme degrades glucagon-like peptide-1 (GLP-1), which induces glucose-dependent insulin secretion. Glucose tolerance of F344/DuCrj rats is improved as a result of enhanced insulin release induced by high levels of plasma GLP-1. In this study, we fed F344/DuCrj rats and DPPIV-positive F344/Jcl rats, aged five weeks, on a high-fat (HF) diet to examine the effect of DPPIV deficiency on food intake and insulin resistance. F344/Jcl rats gained significantly more body weight and consumed significantly more food than F344/DuCrj rats from Week 4 on either control or HF diet. Glucose excursion in the oral glucose tolerance test (OGTT) was improved in F344/DuCrj rats fed on the control or HF diet at all times examined, compared with F344/Jcl rats. Homeostasis model assessment (HOMA) insulin resistance values of F344/DuCrj and F344/Jcl rats fed on HF diet were higher than those of animals fed on control diet up to Week 6. However, HOMA insulin resistance values of F344/DuCrj rats fed on HF diet became significantly lower than those of F344/Jcl rats on HF diet during Weeks 8-10. The area under the insulin curve in the OGTT at Week 10 showed that the insulin resistance of HF-diet-fed F344/DuCrj rats was greatly ameliorated. Plasma active GLP-1 concentrations of F344/DuCrj rats in the fed state were significantly higher than those of F344/Jcl rats. These observations suggest that DPPIV deficiency results in improved glucose tolerance and ameliorated insulin resistance owing to enhanced insulin release and inhibition of food intake as a result of high active GLP-1 levels.

Dipeptidyl peptidase IV inhibition improves impaired glucose tolerance in high-fat diet-fed rats: study using a Fischer 344 rat substrain deficient in its enzyme activity

This study was performed to determine the effects of a high-fat diet on glucose metabolism after an oral glucose challenge in high-fat diet-fed dipeptidyl peptidase IV (DPP-IV) positive (+) and deficient (-) Fischer 344 (F344) rats and the effects of novel DPP-IV inhibitor NVP-DPP728 (1-[2-[(5-cyanopyridin-2-yl)amino]ethylamino]acetyl-2-cyano-(S)-pyrrolidine monohydrochloride salt) on glucose tolerance in high-fat diet-fed F344 rats. In DPP-IV(+) rats, a high-fat diet load caused impaired glucose tolerance, such as increases of plasma insulin and blood glucose concentrations after oral glucose challenge, compared with a standard chow-fed group. In contrast, no marked change in glucose tolerance was induced by the high-fat diet in DPP-IV(-) rats. Blood glucose concentrations in DPP-IV(-) rats after glucose challenge were significantly lower than in DPP-IV(+) rats under high-fat diet load conditions. In standard chow and high-fat diet-fed DPP-IV(+) rats, NVP-DPP728 significantly suppressed glucose excursions after glucose challenge by inhibiting the plasma DPP-IV activity, associated with the stimulation of early insulin secretion. NVP-DPP728 did not affect glucose tolerance in DPP-IV(-) rats under both conditions. These results indicate that the amelioration of glucose tolerance by NVP-DPP728 in DPP-IV(+) rats was directly due to the inhibition of plasma DPP-IV activity, which might be via the subsequent increase in endogenous incretin action.

Dipeptidyl peptidase IV inhibitor NVP-DPP728 ameliorates early insulin response and glucose tolerance in aged rats but not in aged Fischer 344 rats lacking its enzyme activity

The aim of this study was to investigate the effects of aging on glucose metabolism after oral glucose challenge in aged dipeptidyl peptidase IV (DPP-IV) positive (+) Fischer 344 (F344), DPP-IV deficient (-) F344 and DPP-IV(+) Wistar rats and to determine the effect of a DPP-IV inhibitor NVP-DPP728 (1-[2-[(5-cyanopyridin-2-yl)amino]ethylamino]acetyl-2-cyano-(S)-pyrrolidine monohydrochloride salt) on glucose tolerance in aged rats. Aging caused a decrease in early insulin response after an oral glucose challenge in aged Wistar or DPP-IV(+) F344 rats, but not in aged DPP-IV(-) F344 rats, compared with young control groups. Glucose tolerance after an oral glucose challenge in aged DPP-IV(-) F344 rats was better than in aged DPP-IV(+) F344 and Wistar rats associated with the preservation of the early insulin response. NVP-DPP728 improved the glucose tolerance after an oral glucose challenge by potentiating the early insulin response throughout the inhibition of plasma DPP-IV activity in aged DPP-IV(+) Wistar and F344 rats. In contrast, NVP-DPP728 did not affect the glucose tolerance after an oral glucose challenge in aged DPP-IV(-) F344 rats. These results indicate that treatment with NVP-DPP728 ameliorated glucose tolerance in aged rats by the direct inhibition of plasma DPP-IV activity and presumably the subsequent increase in endogenous incretin action.

Dietary induction of angiotensin-converting enzyme in proximal and distal rat small intestine

Induction of angiotensin-converting enzyme was examined in proximal and distal intestinal segments of rats fed a low-protein (4%) diet and then switched to a high-protein (gelatin) diet. Animals were killed at varying time points, and brush-border membranes and total RNA were prepared from the segments. In the proximal intestine, there was a fivefold increase in angiotensin-converting enzyme levels after 14 days but only a twofold change in mRNA. In the distal intestine, there was no increase in enzyme activity but mRNA increased 2.4-fold. Organ culture was used to measure changes in enzyme biosynthesis. There was a 5- to 6-fold increase in the biosynthesis of angiotensin-converting enzyme in the proximal intestine 24 h after the switch to the gelatin diet and a 1.6-fold increase in mRNA levels. No change in biosynthesis was observed in the distal small intestine despite an increase in mRNA. These results support the conclusion that rapid dietary induction of intestinal angiotensin-converting enzyme is differentially regulated in proximal and distal segments of the small intestine.

Improved glucose tolerance via enhanced glucose-dependent insulin secretion in dipeptidyl peptidase IV-deficient Fischer rats

Glucagon-like peptide-1 (GLP-1) is an incretin, which induces glucose-dependent insulin secretion. GLP-1 is rapidly degraded by dipeptidyl peptidase IV (DPPIV) after its release. We investigated whether DPPIV-deficient F344/DuCrj rats show improved glucose tolerance when compared with DPPIV-positive F344/Jcl rats. Oral glucose tolerance test indicated improved glucose tolerance in F344/DuCrj rats, but blood glucose levels of the two strains were almost the same 120 min after the glucose bolus. Valine-pyrrolidide, a DPPIV inhibitor, had no effect on the glucose tolerance of F344/DuCrj rats, but improved that of F344/Jcl rats. Enhanced insulin secretion and high plasma active GLP-1 levels were detected in an intraduodenal glucose tolerance test. Glucose tolerance is improved in DPPIV-deficient F344/DuCrj rats via enhanced insulin release mediated by high active GLP-1 levels. Our results suggest that DPPIV inhibition is a rational strategy to treat diabetic patients by improving glucose tolerance with low risk of hypoglycemia.

Identification of upstream stimulatory factor as an activator of the human dipeptidyl peptidase IV gene in Caco-2 cells

The 5' upstream region (-448/-443) of the human dipeptidyl peptidase IV gene promoter containing a consensus E-box (CACGTG) was shown to bind upstream stimulatory factor using nuclear extracts from mouse (3T3) fibroblasts and the human intestinal and hepatic epithelial cell lines Caco-2 and HepG2. Supershift analysis with specific antibodies to USF-1 and USF-2 indicates that USF-1 is the primary isoform binding to the E-box in nuclear extracts from these cells. Using cell culture, transient cotransfection of USF expression vectors with dipeptidyl peptidase IV promoter constructs revealed that both USF-1 and USF-2 caused an approximately tenfold increase in reporter gene expression in Caco-2 cells. Mutant forms of USF-1 and -2 lacking the DNA binding or transcriptional activation domains were unable to stimulate reporter gene expression. Mutation of the E-box prevented binding of USF, although stimulation of reporter gene expression by cotransfection with USF was reduced by only 50%. By using a series of deletion constructs in cotransfection experiments, a second possible site of USF interaction with the dipeptidyl peptidase IV promoter was localized to the -119/-88 region.

The binding site of human adenosine deaminase for CD26/Dipeptidyl peptidase IV: the Arg142Gln mutation impairs binding to cd26 but does not cause immune deficiency

Human, but not murine, adenosine deaminase (ADA) forms a complex with the cell membrane protein CD26/dipeptidyl peptidase IV. CD26-bound ADA has been postulated to regulate extracellular adenosine levels and to modulate the costimulatory function of CD26 on T lymphocytes. Absence of ADA-CD26 binding has been implicated in causing severe combined immunodeficiency due to ADA deficiency. Using human-mouse ADA hybrids and ADA point mutants, we have localized the amino acids critical for CD26 binding to the helical segment 126-143. Arg142 in human ADA and Gln142 in mouse ADA largely determine the capacity to bind CD26. Recombinant human ADA bearing the R142Q mutation had normal catalytic activity per molecule, but markedly impaired binding to a CD26(+) ADA-deficient human T cell line. Reduced CD26 binding was also found with ADA from red cells and T cells of a healthy individual whose only expressed ADA has the R142Q mutation. Conversely, ADA with the E217K active site mutation, the only ADA expressed by a severely immunodeficient patient, showed normal CD26 binding. These findings argue that ADA binding to CD26 is not essential for immune function in humans.

Analysis of dipeptidyl peptidase IV gene regulation in transgenic mice: DNA elements sufficient for promoter activity in the kidney, but not the intestine, reside on the proximal portion of the gene 5'-flanking region

The dipeptidyl peptidase IV (DPPIV) gene encodes a brush border membrane exopeptidase that is expressed in a tissue-restricted fashion. To examine the regulation of DPPIV transcription in various tissues in vivo, we examined the expression of DPPIV 5'-flanking region (promoter)-human growth hormone reporter constructs in transgenic mice. These mice exhibited cell-type specific reporter expression in kidney. Surprisingly, however, only very low to non-detectable levels of reporter were found in small intestine. These results indicate that DNA elements sufficient for DPPIV expression in kidney, but not intestine, reside in the 5'-flanking region of the gene.

Role of hepatocyte nuclear factor 1alpha and 1beta in the transcriptional regulation of human dipeptidyl peptidase IV during differentiation of Caco-2 cells

Caco-2 cells undergo differentiation to an enterocytic-like cell when maintained in a post-confluent state for 1-2 weeks. During this period Caco-2 cells begin to express high levels brush border membrane associated enzymes such as dipeptidyl peptidase IV. Using the dipeptidyl peptidase IV gene promoter in electrophoretic mobility shift assays, we have shown for the first time that levels of hepatocyte nuclear factor 1alpha increase three- to fourfold during Caco-2 cell differentiation. Transient cotransfection experiments with 3T3 cells using dipeptidyl peptidase IV promoter constructs and expression vectors containing hepatocyte nuclear factor 1alpha and beta show that the ratio of alpha and beta modulates reporter gene expression. These results suggest that the increase in levels of hepatocyte nuclear factor 1alpha that occur during intestinal cell differentiation, are important for expression of dipeptidyl peptidase IV and other intestinal proteins.

The significance of hypersialylation of dipeptidyl peptidase IV (CD26) in the inhibition of its activity by Tat and other cationic peptides. CD26: a subverted adhesion molecule for HIV peptide binding

The functionality of DPP-IV, purified from human placenta and isolated from CD4+/CD26+ T cells of noninfected and HIV-1-infected individuals, was investigated as to its ability to bind certain specific peptides. Using isoelectric focusing and the specificity of substrate-impregnated overlay membranes, we found that DPP-IV from term placenta and from T cells of HIV-infected individuals was significantly more sialylated compared with enzyme isozyme patterns of other tissues. We report here that (1) the number of isoforms of DPP-IV and extent of sialylation are critical to function and peptide binding; (2) the number of sialylated isoforms isolated from PBMCs increases significantly with age greater than 40 years; (3) hypersialylation by extreme anionic isoforms is highly associated with HIV infection and pathognomonic to remaining CD4+ cells in overt AIDS; and (4) highly sialylated DPP-IV is more significantly inhibited by Tat and cationic peptides. We conclude that hypersialylation of DPP-IV modifies surface charge of the CD26 antigen, promoting binding of HIV peptides through their cationic domains to the sialic acid residues of DPP-IV, and that certain HIV moieties are likely to engage this phenomenon as an auxiliary adhesion mechanism to fuse with cells. Furthermore, as a consequence of this occurrence, DPP-IV enzymatic activity can be significantly reduced, competitively.

An aminopeptidase N deficiency in dog small intestine

This study has identified a naturally occurring, specific deficiency of a brush border aminopeptidase N (ApN) in the small intestines of five clinically healthy dogs. ApN activity in mucosal homogenates of dog small intestine was reduced significantly in deficient animals (13.4 (1.1) nmol min-1 mg-1 protein, n = 5, P < 0.002) compared to healthy control dogs (95.1 (6.7), n = 22). Alkaline phosphatase, gamma-glutamyl transferase, zinc-resistant alpha-glucosidase, maltase, sucrase and lactase in the ApN deficient dogs exhibited comparable activities to those in the control dogs. Microvillar membranes were analysed by one- and two-dimensional electrophoresis. ApN was represented by a single 145kDa band in all control dogs, identified by immunoblotting and immunoprecipitation. Protein maps from deficient dogs were normal apart from the virtual absence of an ApN spot and there were no apparent abnormalities in the glycosylation of microvillar proteins. The findings suggest that intestinal ApN deficiency in these dogs is a primary lesion involving diminished expression of an otherwise normal enzyme protein.

Regulation of the biological activity of glucagon-like peptide 2 in vivo by dipeptidyl peptidase IV

Species-specific differences in the enzymatic inactivation of peptides is an important consideration in the evaluation of therapeutic efficacy. We demonstrate that glucagon-like peptide 2 (GLP-2), shown to be highly intestinotrophic in mice, promotes an increase in intestinal villus height but has no trophic effect on small bowel weight in rats. The reduced intestinotrophic activity of GLP-2 in rats is attributable to inactivation by the enzyme dipeptidyl peptidase IV (DPP-IV). GLP-2(1-33) was degraded to GLP-2(3-33) following incubation with human placental DPP-IV or rat serum but not by serum from DPP-IV-deficient rats. Administration of rat GLP-2 to DPP-IV-deficient rats was associated with markedly increased bioactivity of rat GLP-2 resulting in a significant increase in small bowel weight. A synthetic GLP-2 analog, r[Gly2]GLP-2, with an alanine to glycine substitution at position 2, was resistant to cleavage by both DPP-IV and rat serum in vitro. Treatment of wild-type rats with r[Gly2]GLP-2 produced a statistically significant increase in small bowel mass. DPP-IV-mediated inactivation of GLP-2 is a critical determinant of the growth factor-like properties of GLP-2.