Long-term treatment with the dipeptidyl peptidase IV inhibitor P32/98 causes sustained improvements in glucose tolerance, insulin sensitivity, hyperinsulinemia, and beta-cell glucose responsiveness in VDF (fa/fa) Zucker rats.

The incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are responsible for >50% of nutrient-stimulated insulin secretion. After being released into the circulation, GIP and GLP-1 are rapidly inactivated by the circulating enzyme dipeptidyl peptidase IV (DP IV). The use of DP IV inhibitors to enhance these insulinotropic hormonal axes has proven effective on an acute scale in both animals and humans; however, the long-term effects of these compounds have yet to be determined. Therefore, we carried out the following study: two groups of fa/fa Zucker rats (n = 6 each) were treated twice daily for 3 months with the DP IV inhibitor P32/98 (20 mg.kg(-1).day(-1), p.o.). Monthly oral glucose tolerance tests (OGTTs), performed after drug washout, revealed a progressive and sustained improvement in glucose tolerance in the treated animals. After 12 weeks of treatment, peak OGTT blood glucose values in the treated animals averaged 8.5 mmol/l less than in the controls (12.0 +/- 0.7 vs. 20.5 +/- 1.3 mmol/l, respectively). Concomitant insulin determinations showed an increased early-phase insulin response in the treated group (43% increase). Furthermore, in response to an 8.8 mmol/l glucose perfusion, pancreata from controls showed no increase in insulin secretion, whereas pancreata from treated animals exhibited a 3.2-fold rise in insulin secretion, indicating enhanced beta-cell glucose responsiveness. Also, both basal and insulin-stimulated glucose uptake were increased in soleus muscle strips from the treated group (by 20 and 50%, respectively), providing direct evidence for an improvement in peripheral insulin sensitivity. In summary, long-term DP IV inhibitor treatment was shown to cause sustained improvements in glucose tolerance, insulinemia, beta-cell glucose responsiveness, and peripheral insulin sensitivity, novel effects that provide further support for the use of DP IV inhibitors in the treatment of diabetes.

Glucagon-like peptide-1 (7-37) augments insulin-mediated glucose uptake in elderly patients with diabetes.

BACKGROUND: Glucagon-like peptide-1 (GLP-1) is an intestinal insulinotropic hormone that augments glucose-induced insulin secretion in patients with type 2 diabetes. It has also been proposed that a substantial component of the glucose-lowering effects of GLP-1 occurs because this hormone enhances insulin-mediated glucose disposal. However, interpretations of the studies have been controversial. This study determines the effect of GLP-1 on insulin-mediated glucose disposal in elderly patients with type 2 diabetes. METHODS: Studies were conducted on 8 elderly patients with type 2 diabetes (age range, 76 +/- 1 years; body mass index, 28 +/- 1 kg/m(2)). Each subject underwent two 180-minute euglycemic (insulin infusion rate, 40 mU/m(2)/min) insulin clamps in random order. Glucose production (Ra) and disposal (Rd) rates were measured using tritiated glucose methodology. In one study, glucose and insulin alone were infused. In the other study, a primed-continuous infusion of GLP-1 was administered at a final rate of 1.5 pmol x kg(-1) x min(-1) from 30 to 180 minutes. RESULTS: Glucose values were similar between the control and GLP-1 infusion studies. 120- to 180-minute insulin values appeared to be higher during the GLP-1 infusion study (control, 795 +/- 63 pmol/l; GLP-1, 1140 +/- 275 pmol/l; p = not significant [NS]). The higher insulin values were largely due to 2 subjects who had substantial insulin responses to GLP-1 despite euglycemia and hyperinsulinemia. The 120- to 180-minute insulin values were similar in the other 6 subjects (control, 746 +/- 35 pmol/l; GLP-1, 781 +/- 41 pmol/l; p = NS). Basal (control, 2.08 +/- 0.05 mg/kg/min; GLP-1, 2.13 +/- 0.04 mg/kg/min; p = NS) and 120- to 180-minute (control, 0.50 +/- 0.18 mg/kg/min; GLP-1, 0.45 +/- 0.14 mg/kg/min; p = NS) Ra was similar between studies. The 120- to 180-minute Rd values were higher during the GLP-1 infusion studies (control, 4.73 +/- 0.39 mg/kg/min; GLP-1, 5.52 +/- 0.43 mg/kg/min; p <.01). When the 2 subjects who had significant insulin responses to GLP-1 during the euglycemic clamp were excluded, the 120- to 180-minute Rd values were still higher in the GLP-1 infusion study (control, 5.22 +/- 0.32 mg/kg/min; GLP-1, 6.05 +/- 0.37 mg/kg/min; p <.05). CONCLUSIONS: We conclude that GLP-1 may enhance insulin sensitivity in elderly patients with diabetes.

Effect of glucagon-like peptide 1 on non-insulin-mediated glucose uptake in the elderly patient with diabetes.

An important cause of elevated glucose levels in elderly patients with diabetes is an alteration in non-insulin-mediated glucose uptake (NIMGU). Glucagon-like peptide 1 (GLP-1) is an intestinal insulinotropic hormone. It has been proposed that this hormone also lowers glucose levels by enhancing NIMGU. This study was conducted to determine whether GLP-1 augments NIMGU in elderly patients with diabetes, a group in which NIMGU is known to be impaired. Studies were conducted on 10 elderly patients with type 2 diabetes (aged 75 +/- 2 years, BMI 27 +/- 1 kg/m(2)) who underwent paired 240-min glucose clamp studies. In each study, octreotide was infused to suppress endogenous insulin release, and tritiated glucose methodology was used to measure glucose production and disposal rates. For the first 180 min, no glucose was infused. From 180 to 240 min, glucose was increased to 11 mmol/l using the glucose clamp protocol. In the GLP-1 study, GLP-1 was infused from 30 to 240 min. In a subsequent control study, insulin was infused using the glucose clamp protocol from 30 to 240 min to match the insulin levels that occurred during the GLP-1 infusion study. During hyperglycemia, GLP-1 enhanced glucose disposal (control study: 2.52 +/- 0.19 mg x kg(-1) x min(-1); GLP-1 study: 2.90 +/- 0.17 mg x kg(-1) x min(-1); P < 0.0001). Hepatic glucose output was not different between studies. We conclude that GLP-1 may partially reverse the defect in NIMGU that occurs in elderly patients with diabetes.

Effect of aging and diabetes on the enteroinsular axis.

BACKGROUND: The current studies were designed to examine the effect of aging and diabetes on the enteroinsular axis. METHODS: Healthy young control subjects (n = 10 young; age 23 +/- 1 years; body mass index [BMI] 24 +/- 1 kg/m(2)), healthy elderly subjects (n = 10; age 80 +/- 2 years; BMI 26 +/- 1 kg/m(2)), and elderly patients with type 2 diabetes (n = 10; age 76 +/- 2 years; BMI 26 +/- 2 kg/m(2)) underwent a 3-hour oral glucose tolerance test (glucose dose 40 gm/m(2)). RESULTS: Insulin responses were not different between young controls and elderly patients with diabetes but were significantly lower in elderly patients with diabetes and young controls than in elderly controls (young control: 178 +/- 27 pM; elderly control: 355 +/- 57 pM; elderly diabetes: 177 +/- 30 pM; p <.05 elderly control vs young control and elderly diabetes). Total glucagon-like peptide 1 (GLP-1) responses were not significantly different between young and elderly controls and patients with diabetes (young control: 15 +/- 2 pM; old control: 8 +/- 2 pM; elderly diabetes: 12 +/- 3 pM; p = ns). Active GLP-1 responses were also not different between young and elderly controls and patients with diabetes (young control: 5 +/- 1 pM; old control: 6 +/- 1 pM; elderly diabetes: 7 +/- 1 pM; p = ns). However, the difference between total and active GLP levels was significantly greater in the young controls (young control: 10 +/- 2 pM; old control: 2 +/- 2 pM; elderly diabetes: 4 +/- 2 pM; p <.05, young vs elderly). Glucose-dependent insulinotropic polypeptide responses were not different between young and elderly controls and between elderly controls and patients with diabetes but were significantly higher in elderly patients with diabetes than in young controls (young control: 97 +/- 12 pM; elderly control: 121 +/- 16 pM; elderly diabetes: 173 +/- 27 pM; p <.05, young vs elderly diabetes). Glucagon responses were reduced in elderly controls but were similar in young controls and elderly patients with diabetes (young control: 15 +/- 1 pM; elderly control: 9 +/- 1 pM; elderly diabetes: 16 +/- 1 pM; p <.01 elderly control vs young control and elderly diabetes). Dipeptidyl peptidase IV levels were lower in both elderly controls and patients with diabetes when compared with young controls (young control: 0.17 +/- 0.01; elderly control: 0.15 +/- 0.01; elderly diabetes: 0.15 +/- 0.01 DeltaOD/20 minutes; p <.05, elderly vs young). CONCLUSIONS: We conclude that normal aging and diabetes are associated with multiple changes in the enteroinsular axis.

Identification of a bioactive domain in the amino-terminus of glucose-dependent insulinotropic polypeptide (GIP).

The incretins are a class of hormones released from the small bowel that act on the endocrine pancreas to potentiate insulin secretion in a glucose-dependent manner. Due to the requirement for an elevated glucose concentration for activity, the incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1, have potential in the treatment of non-insulin-dependent diabetes mellitus. A series of synthetic peptide GIP fragments was generated for the purpose of elucidating the bioactive domain of the molecule. Peptides were screened for stimulation of cyclic AMP (cAMP) accumulation in Chinese hamster ovary cells transfected with the rat islet GIP receptor. Of the GIP fragments tested, GIP(1-14) and GIP(19-30) demonstrated the greatest cAMP-stimulating ability over the range of concentrations tested (up to 20 microM). In contrast, GIP fragments corresponding to amino acids 15-42, 15-30, 16-30 and 17-30 all demonstrated weak antagonism of GIP(1-42) activity. Competitive-binding displacement studies indicated that these peptides were low-affinity ligands for the GIP receptor. To examine biological activity in vivo, a bioassay was developed in the anesthetized rat. Intravenous infusion of GIP(1-42) (1 pmol/min/100 g) with a concurrent intraperitoneal glucose load (1 g/kg) significantly reduced circulating blood glucose excursions through stimulation of insulin release. Higher doses of GIP(1-14) and GIP(19-30) (100 pmol/min/100 g) also reduced blood glucose excursions.

Defective glucose-dependent insulinotropic polypeptide receptor expression in diabetic fatty Zucker rats.

Glucose-dependent insulinotropic polypeptide (GIP) is a peptide hormone that is released postprandially from the small intestine and acts in concert with glucagon-like peptide (GLP)-1 to potentiate glucose-induced insulin secretion from the pancreatic beta-cell. In type 2 diabetes, there is a decreased responsiveness of the pancreas to GIP; however, the insulin response to GLP-1 remains intact. The literature suggests that the ineffectiveness of GIP in type 2 diabetes may be a result of chronic homologous desensitization of the GIP receptor. Yet, there has been no conclusive evidence suggesting that GIP levels are elevated in diabetes. The hypothesis of the present study is that one cause of decreased responsiveness to GIP in type 2 diabetes is an inappropriate expression of the GIP receptor in the pancreatic islet. This hypothesis was tested using a strain of diabetic fatty Zucker rats. The obese rats displayed basal GIP levels similar to the control animals; however, they were unresponsive to a GIP infusion (4 pmol.min(-1). kg(-1)), whereas the lean animals displayed a significant reduction in blood glucose (GIP levels, 50% control after 60 min, P < 0.05) as well as a significant increase in circulating insulin. GIP also potently stimulated first-phase insulin secretion from isolated perifused islets (10.3 +/- 3.0 x basal), and GIP and GLP-1 potentiated insulin secretion from the perfused pancreas (6 x control area under the curve [AUC]) from lean animals. GIP yielded no significant effect in the Vancouver diabetic fatty Zucker (VDF) rat pancreases, whereas GLP-1 elicited an eightfold increase of insulin secretion from the perfused VDF pancreas. Islets from lean animals subjected to static incubations with GIP showed a 2.2-fold increase in cAMP, whereas GIP failed to increase islet cAMP in the VDF islets. Finally, the expression of both GIP receptor mRNA and protein was decreased in islets from VDF rats. These data suggest that the decreased effectiveness of GIP in the VDF rat and in type 2 diabetes may be a result of a decreased receptor expression in the islet.

A new pathway for glucose-dependent insulinotropic polypeptide (GIP) receptor signaling: evidence for the involvement of phospholipase A2 in GIP-stimulated insulin secretion.

The hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of insulin secretion. GIP has been shown to increase adenylyl cyclase activity, elevate intracellular Ca(2+) levels, and stimulate a mitogen-activated protein kinase pathway in the pancreatic beta-cell. In the current study we demonstrate a role for arachidonic acid in GIP-mediated signal transduction. Static incubations revealed that both GIP (100 nm) and ATP (5 microm) significantly increased [(3)H]arachidonic acid ([(3)H]AA) efflux from transfected Chinese hamster ovary K1 cells expressing the GIP receptor (basal, 128 +/- 11 cpm/well; GIP, 212 +/- 32 cpm/well; ATP, 263 +/- 35 cpm/well; n = 4; p < 0.05). In addition, GIP receptors were shown for the first time to be capable of functionally coupling to AA production through Gbetagamma dimers in Chinese hamster ovary K1 cells. In a beta-cell model (betaTC-3), GIP was found to elicit [(3)H]AA release, independent of glucose, in a concentration-dependent manner (EC(50) value of 1.4 +/- 0.62 nm; n = 3). Although GIP did not potentiate insulin release under extracellular Ca(2+)-free conditions, it was still capable of elevating intracellular cAMP and stimulating [(3)H]AA release. Our data suggest that cAMP is the proximal signaling intermediate responsible for GIP-stimulated AA release. Finally, stimulation of GIP-mediated AA production was shown to be mediated via a Ca(2+)-independent phospholipase A(2). Arachidonic acid is therefore a new component of GIP-mediated signal transduction in the beta-cell.

Metabolism of glucagon by dipeptidyl peptidase IV (CD26).

Glucagon is a 29-amino acid polypeptide released from pancreatic islet alpha-cells that acts to maintain euglycemia by stimulating hepatic glycogenolysis and gluconeogenesis. Despite its importance, there remains controversy about the mechanisms responsible for glucagon clearance in the body. In the current study, enzymatic metabolism of glucagon was assessed using sensitive mass spectrometric techniques to identify the molecular products. Incubation of glucagon with purified porcine dipeptidyl peptidase IV (DP IV) yielded sequential production of glucagon(3-29) and glucagon(5-29). In human serum, degradation to glucagon(3-29) was rapidly followed by N-terminal cyclization of glucagon, preventing further DP IV-mediated hydrolysis. Bioassay of glucagon, following incubation with purified DP IV or normal rat serum demonstrated a significant loss of hyperglycemic activity, while a similar incubation in DP IV-deficient rat serum did not show any loss of glucagon bioactivity. Degradation, monitored by mass spectrometry and bioassay, was blocked by the specific DP IV inhibitor, isoleucyl thiazolidine. These results identify DP IV as a primary enzyme involved in the degradation and inactivation of glucagon. These findings have important implications for the determination of glucagon levels in human plasma.

Effect of ageing and diabetes on glucose-dependent insulinotropic polypeptide and dipeptidyl peptidase IV responses to oral glucose.

AIMS: Glucose-dependent insulinotropic polypeptide (GIP) acts on the pancreas to potentiate glucose-induced insulin secretion (enteroinsular axis). GIP is rapidly inactivated in vivo by the enzyme dipeptidyl dipeptidase IV (DPP-IV). The current studies were designed to examine the effect of ageing, obesity and diabetes on GIP and DPP-IV responses to oral glucose. METHODS: Healthy controls (nine middle-aged, age 42 +/- 2 years, body mass index (BMI) 33 +/- 1 kg/m2; nine elderly, age 71 +/- 1 years, BMI 30 +/- 1 kg/m2) and patients with Type 2 diabetes (12 middle-aged, age 44 +/- 2 years, BMI 34 +/- 2 kg/m2; 19 elderly, age 74 +/- 1 years, BMI 31 +/- 1 kg/m2) underwent a 3-h oral glucose tolerance test (OGTT) (glucose dose 40 g/m2). RESULTS: Insulin responses were similar in elderly controls and patients with diabetes, but were lower in middle-aged patients with diabetes than in controls (308 +/- 65 vs. 640 +/- 109 pM, P < 0.05). GIP responses were similar in controls and patients with diabetes in each age group, but were higher in elderly controls (middle-aged 45 +/- 13; elderly 112 +/- 13 pM, P < 0.01) and patients with diabetes (middle-aged 55 +/- 10; elderly 99 +/- 10 pM, P < 0.01). DPP-IV levels were lower in patients with diabetes in both middle-aged (control 0.241 +/- 0.015; diabetes 0.179 +/- 0.017 delta OD/20 min, P < 0.05) and elderly groups (control 0.223 +/- 0.019; diabetes 0.173 +/- 0.010 delta OD/20 min, P < 0.05). CONCLUSIONS: It was concluded that ageing in obese subjects is associated with enhanced GIP responses to oral glucose. In addition, DPP-IV activity is reduced in middle-aged and elderly obese patients with diabetes.

Role of glucose in chronic desensitization of isolated rat islets and mouse insulinoma (betaTC-3) cells to glucose-dependent insulinotropic polypeptide.

It is well documented that the release of insulin from isolated perifused islets attenuates over time, despite a continued glucose stimulation. In the current study we have shown that potentiation of insulin release by the intestinal hormone glucose-dependent insulinotropic polypeptide (GIP) is also attenuated after its continuous application. In less than 20 h of maintained stimulus with either hyperglycaemia (11.0 mM glucose) or GIP (10 nM) under hyperglycaemic conditions, insulin release returned to basal values. This was not due to loss of islet viability or reduction in the releasable pool of insulin granules, as 1 mM isobutylmethylxanthine was able to stimulate equivalent insulin release under both conditions. Further examination of chronic GIP desensitization was examined in cultured mouse insulinoma (betaTC-3) cells. GIP-stimulated cAMP production was not greatly affected by the prevailing glucose conditions, suggesting that the glucose dependence of GIP-stimulated insulin release occurs distally to the increase in intracellular cAMP in betaTC-3 cells. The GIP-stimulated cAMP response curve after desensitization was of similar magnitude at all glucose concentrations, but GIP pretreatment did not affect forskolin-stimulated cAMP production. Desensitization of the cAMP response in betaTC-3 cells was shown not to involve induction of dipeptidyl peptidase IV or pertussis toxin-sensitive G-proteins, activation of protein kinase C or protein kinase A, or modulation of phosphodiesterase activity. Homologous desensitization of the insulin-potentiating activity of GIP was found to affect both GIP-stimulated and forskolin-stimulated insulin release, indicating desensitization of distal steps in the stimulus-exocytosis cascade.

Dipeptidyl peptidase IV (DPIV/CD26) degradation of glucagon. Characterization of glucagon degradation products and DPIV-resistant analogs.

Over the past decade, numerous studies have been targeted at defining structure-activity relationships of glucagon. Recently, we have found that glucagon(1-29) is hydrolyzed by dipeptidyl peptidase IV (DPIV) to produce glucagon(3-29) and glucagon(5-29); in human serum, [pyroglutamyl (pGlu)(3)]glucagon(3-29) is formed from glucagon(3-29), and this prevents further hydrolysis of glucagon by DPIV (H.-U. Demuth, K. Glund, U. Heiser, J. Pospisilik, S. Hinke, T. Hoffmann, F. Rosche, D. Schlenzig, M. Wermann, C. McIntosh, and R. Pederson, manuscript in preparation). In the current study, the biological activity of these peptides was examined in vitro. The amino-terminally truncated peptides all behaved as partial agonists in cyclic AMP stimulation assays, with Chinese hamster ovary K1 cells overexpressing the human glucagon receptor (potency: glucagon(1-29) > [pGlu(3)]glu- cagon(3-29) > glucagon(3-29) > glucagon(5-29) > [Glu(9)]glu- cagon(2-29)). In competition binding experiments, [pGlu(3)]glucagon(3-29) and glucagon(5-29) both demonstrated 5-fold lower affinity for the receptor than glucagon(1-29), whereas glucagon(3-29) exhibited 18-fold lower affinity. Of the peptides tested, only glucagon(5-29) showed antagonist activity, and this was weak compared with the classical glucagon antagonist, [Glu(9)]glucagon(2-29). Hence, DPIV hydrolysis of glucagon yields low affinity agonists of the glucagon receptor. As a corollary to evidence indicating that DPIV degrades glucagon (Demuth, et al., manuscript in preparation), DPIV-resistant analogs were synthesized. Matrix-assisted laser desorption/ionization-time of flight mass spectrometry was used to assess DPIV resistance, and it allowed kinetic analysis of degradation. Of several analogs generated, only [D-Ser(2)] and [Gly(2)]glucagon retained high affinity binding and biological potency, similar to native glucagon in vitro. [D-Ser(2)]Glucagon exhibited enhanced hyperglycemic activity in a bioassay, whereas [Gly(2)]glucagon was not completely resistant to DPIV degradation.

Characterization of the carboxyl-terminal domain of the rat glucose-dependent insulinotropic polypeptide (GIP) receptor. A role for serines 426 and 427 in regulating the rate of internalization.

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone involved in the regulation of insulin secretion. In non-insulin-dependent diabetes mellitus insulin responses to GIP are blunted, possibly due to altered signal transduction or reduced receptor number. Site-directed mutagenesis was used to construct truncated GIP receptors to study the importance of the carboxyl-terminal tail (CT) in binding, signaling, and receptor internalization. Receptors truncated at amino acids 425, 418, and 405, expressed in COS-7 or CHO-K1 cells, exhibited similar binding to wild type receptors. GIP-dependent cAMP production with the 405 mutant was decreased in COS-7 cells. Maximal cAMP production in CHO-K1 cells was reduced with all truncated forms. Binding was undetectable with a receptor truncated at amino acid 400; increasing tail length by adding 5 alanines restored binding and signaling. Mutants produced by alanine scanning of residues 394-401, adjacent to transmembrane domain 7, were all functional. CT truncation by 30 or more amino acids, mutation of serines 426/427, singly or combined, or complete CT serine knockout all reduced receptor internalization rate. The majority of the GIP receptor CT is therefore not required for signaling, a minimum chain length of approximately 405 amino acids is needed for receptor expression, and serines 426 and 427 are important for regulating rate of receptor internalization.

Improved glucose tolerance in rats treated with the dipeptidyl peptidase IV (CD26) inhibitor Ile-thiazolidide.

The incretins glucose-dependent insulinotropic polypeptide (GIP1-42) and truncated forms of glucagon-like peptide-1 (GLP-1) are hormones released from the gut in response to ingested nutrients, which act on the pancreas to potentiate glucose-induced insulin secretion. These hormones are rapidly inactivated by the circulating enzyme dipeptidyl peptidase IV ([DPIV] CD26). This study describes the effect on glucose tolerance and insulin secretion of inhibiting endogenous DPIV in the rat using Ile-thiazolidide, a specific DPIV inhibitor. High-performance liquid chromatography (HPLC) analysis of plasma following in vivo administration of 125I-labeled peptides showed that inhibition of DPIV by about 70% prevented the degradation of 90.0% of injected 125I-GLP-17-36 after 5 minutes, while only 13.4% remained unhydrolyzed in rats not treated with the DPIV-inhibiting agent after only 2 minutes. Ile-thiazolidide treatment also increased the circulating half-life of intact GLP-17-36 released in response to intraduodenal (ID) glucose (as measured by N-terminal specific radioimmunoassay [RIA]). In addition, inhibition of DPIV in vivo resulted in an earlier increase and peak of plasma insulin and a more rapid clearance of blood glucose in response to ID glucose challenge. When considered with the HPLC data, these results suggest that the altered insulin profile is an incretin-mediated response. DPIV inhibition resulting in improved glucose tolerance may have therapeutic potential for the management of type 2 diabetes mellitus.

Glucose-dependent insulinotropic polypeptide stimulation of lipolysis in differentiated 3T3-L1 cells: wortmannin-sensitive inhibition by insulin.

GIP is an important insulinotropic hormone (incretin) that has also been implicated in fat metabolism. There is controversy regarding the actions of GIP on adipocytes. In the current study, the existence of GIP receptors and effects of GIP on lipolysis were studied in differentiated 3T3-L1 cells. GIP receptor messenger RNA was detected by RT-PCR and RNase protection assay. Receptors were detected in binding studies (IC50 26.7 +/- 0.7 nM). GIP stimulated glycerol release with an EC50 of 3.28 +/- 0.63 nM. GIP (10(-9)-10(-7) M) +/- IBMX increased cAMP production by 1180-2246%. The adenylyl cyclase inhibitor MDL 12330A (10(-4) M) inhibited GIP-induced glycerol production by >90%, and reduced cAMP responses to basal. Preincubation of 3T3-L1 cells with insulin inhibited glycerol responses to GIP, and the inhibitory effect of insulin was blocked by the phosphatidylinositol 3'-kinase inhibitor, wortmannin. It is concluded that GIP stimulates glycerol release in 3T3-L1 cells primarily via stimulation of cAMP production, and that insulin antagonizes GIP-induced lipolysis in a wortmannin-sensitive fashion. It is suggested that effects of GIP on fat metabolism in vivo may depend upon the circulating insulin level, and that meal-released GIP may elevate circulating fatty acids, thus optimizing pancreatic beta-cell responsiveness to stimulation by glucose and GIP.

Improved glucose tolerance in Zucker fatty rats by oral administration of the dipeptidyl peptidase IV inhibitor isoleucine thiazolidide.

The hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (GLP)-1 act on the pancreas to potentiate glucose-induced insulin secretion (enteroinsular axis). These hormones (incretins) are rapidly hydrolyzed by the circulating enzyme dipeptidyl peptidase IV (DP IV) into biologically inactive NH2-terminally truncated fragments. This study describes the effect of inhibiting endogenous DP IV with a specific DP IV inhibitor, isoleucine thiazolidide (Ile-thiazolidide), on glucose tolerance and insulin secretion in the obese Zucker rat. In initial studies, the specificity of Ile-thiazolidide as an inhibitor of incretin degradation was determined using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. These results showed that inhibiting DP IV activity with Ile-thiazolidide blocked the formation of NH2-terminally truncated GIP and GLP-1. Oral administration of Ile-thiazolidide resulted in rapid inhibition of circulating DP IV levels by 65% in obese and lean Zucker rats. Suppression of DP IV levels enhanced insulin secretion in both phenotypes with the most dramatic effect occurring in obese animals (150% increase in integrated insulin response vs. 27% increase in lean animals). Ile-thiazolidide treatment improved glucose tolerance in both phenotypes and restored glucose tolerance to near-normal levels in obese animals. This was attributed to the glucose-lowering actions of increasing the circulating half-lives of the endogenously released incretins GIP and, particularly, GLP-1. This study suggests that drug manipulation of plasma incretin activity by inhibiting the enzyme DP IV is a valid therapeutic approach for lowering glucose levels in NIDDM and other disorders involving glucose intolerance.

Enhanced glucose-dependent insulinotropic polypeptide secretion and insulinotropic action in glucagon-like peptide 1 receptor -/- mice.

Incretins are gastrointestinal hormones that act on the pancreas to potentiate glucose-stimulated insulin secretion. Despite the physiological importance of the enteroinsular axis, disruption of glucagon-like peptide (GLP)-1 action is associated with only modest glucose intolerance in GLP-1 receptor -/- (GLP-1R -/-) mice. We show here that GLP-1R -/- mice exhibit compensatory changes in the enteroinsular axis via increased glucose-dependent insulinotropic polypeptide (GIP) secretion and enhanced GIP action. Serum GIP levels in GLP-1R -/- mice were significantly elevated versus those in +/+ control mice after an oral glucose tolerance test (369 +/- 40 vs. 236 +/- 28 pmol/l; P < or = 0.02). Furthermore, GIP perfusion of mice pancreas and isolated islets in the presence of elevated glucose concentrations elicited a significantly greater insulin response in GLP-1R -/- than in +/+ mice (P < or = 0.02-0.05). In contrast, no significant perturbation in the insulin response to perfused glucagon was detected under conditions of low (4.4 mmol/l) or high (16.6 mmol/l) glucose in GLP-1R -/- mice. Total pancreatic insulin but not glucagon content was significantly reduced in GLP-1R -/- compared with in +/+ mice (77 +/- 9 vs. 121 +/- 10 pmol/mg protein; P < or = 0.005). These observations suggest that upregulation of the GIP component of the enteroinsular axis, at the levels of GIP secretion and action, modifies the phenotype resulting from interruption of the insulinotropic activity of GLP-1 in vivo.

The Xenopus proglucagon gene encodes novel GLP-1-like peptides with insulinotropic properties.

The proglucagon gene encodes several hormones that have key roles in the regulation of metabolism. In particular, glucagon-like peptide (GLP-1), a potent stimulus of insulin secretion, is being developed as a therapy for the treatment of non-insulin-dependent diabetes mellitus. To define structural moieties of the molecule that convey its insulinotropic activity, we have cloned and characterized the proglucagon gene from the amphibian, Xenopus laevis. Unexpectedly, these cDNAs were found to encode three unique glucagon-like-1 peptides, termed xenGLP-1A, xenGLP-1B, and xenGLP-1C in addition to the typical proglucagon-derived hormones glucagon and GLP-2. xenGLP-1A, -1B, and -1C were synthesized and tested for their ability to bind and activate the human GLP-1 receptor (hGLP-1R), and to stimulate insulin release from rat pancreas. All three Xenopus GLP-1-like peptides bind effectively to the hGLP-1R and stimulate cAMP production. Surprisingly, xenGLP-1B(1-30) demonstrated higher affinity for the hGLP-1R than hGLP-1 (IC50 of 1.1 +/- 0.4 nM vs. 4.4 +/- 1.0 nM, respectively, P < 0.02) and was equipotent to hGLP-1 in stimulating cAMP production (EC50 of 0.17 +/- 0.02 nM vs. 0.6 +/- 0. 2 nM, respectively, P > 0.05). Further studies demonstrated that hGLP-1, xenGLP-1A, -1B, and -1C stimulate comparable insulin release from the pancreas. These results demonstrate that despite an average of nine amino acid differences between the predicted Xenopus GLPs and hGLP-1, all act as hGLP-1R agonists.