Raymond A. Pederson

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 NHg-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 desorption7sol;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.

Gastric Inhibitory Polypeptide: Its Physiologic Release and Insulinotropic Action in the Dog

Studies were carried out in conscious dogs in which the immunoreactive gastric inhibitory polypeptide (IR-GIP) response to graded doses of oral fat (triglycerides) and glucose was investigated. The IR-GIP response to the doses of triglycerides used was greater and more prolonged than the response to the glucose loads employed. In addition, the relative insulinotropic potencies of exogenous porcine GIP and IR-GIP released by fat as against those released by oral glucose were assessed. When glucose was administered by the oral route, the immunoreactive insulin (IRI) response was magnified above the IRI response to a comparable intravenous glucose load. The serum IRI response to oral glucose was accompanied by a concomitant rise in serum IR-GIP levels, suggesting a causal relationship. IR-GIP released by oral fat was shown to augment the IRI response to an intravenous glucose load, resulting in an improvement of glucose tolerance. Fat-released IR-GIP augmented IRI levels to a lessor degree than either oral glucose or an infusion of porcine GIP.

Further purification of a polypeptide demonstrating enterogastrone activity

1. The further purification of a polypeptide having potent enterogastrone activity, without CCK—PZ effects, is described.

Dipeptidyl peptidase IV inhibitors: how do they work as new antidiabetic agents?

A number of new approaches to diabetes therapy are currently undergoing clinical trials, including those involving stimulation of the pancreatic beta-cell with the gut-derived insulinotropic hormones (incretins), GIP and GLP-1. The current review focuses on an approach based on the inhibition of dipeptidyl peptidase IV (DP IV), the major enzyme responsible for degrading the incretins in vivo. The rationale for this approach was that blockade of incretin degradation would increase their physiological actions, including the stimulation of insulin secretion and inhibition of gastric emptying. It is now clear that both GIP and GLP-1 also have powerful effects on beta-cell differentation, mitogenesis and survival. By potentiating these pleiotropic actions of the incretins, DP IV inhibition can therefore preserve beta-cell mass and improve secretory function in diabetics.

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.

Long-Term Effects of Vanadyl Treatment on Streptozocin-Induced Diabetes in Rats

The vanadate and vanadyl forms of vanadium have been shown by many investigators to have insulinlike effects on glucose metabolism. Many investigators have shown that vanadium, or its salts, counteracts the hyperglycemie associated with streptozocin-induced diabetes (STZ-D) in the rat, although insulin secretion remains depressed. Studies of the action of vanadate on insulin secretion and glucose metabolism have not addressed the question of possible long-term effects of this compound on glucose metabolism extending beyond the period of oral administration. This study was undertaken to assess the effects of treatment (3 wk) and withdrawal of vanadyl sulfate (13 wk) on glucose metabolism, insulin secretion, and islet insulin content of STZ-D rats. Our results indicate that STZ-D rats that have had blood glucose levels normalized by 3 wk of vanadyl treatment remain normoglycemic after 13 wk of withdrawal from treatment. Normal glucose tolerance was observed in vanadyl-treated diabetic animals despite depressed fasting and glucose-stimulated plasma insulin levels. Insulin secretion from the isolated perfused pancreas was greater after vanadyl treatment than in untreated diabetic rats, although it was only 12% of values from controls. Three weeks of vanadyl treatment of STZ-D rats, followed by 13 wk of withdrawal, yielded islets close in size and insulin content of control islets, even though in vivo and in vitro insulin secretion was impaired. This study has shown that short-term vanadyl treatment of STZ-D rats yields normalization of glucose tolerance and protection of islets from destruction by STZ. The relationship between normal glucose tolerance, normal islet insulin content, and reduced insulin secretion in vanadyl-treated STZ-D rats remains to be investigated.

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 glucagon1–29 is hydrolyzed by dipeptidyl peptidase IV (DPIV) to produce glucagon3–29 and glucagon5–29; in human serum, [pyroglutamyl (pGlu)3]glucagon3–29 is formed from glucagon3–29, 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: glucagon1–29 > [pGlu3]glu- cagon3–29 > glucagon3–29 > glucagon5–29 > [Glu9]glu- cagon2–29). In competition binding experiments, [pGlu3]glucagon3–29 and glucagon5–29 both demonstrated 5-fold lower affinity for the receptor than glucagon1–29, whereas glucagon3–29 exhibited 18-fold lower affinity. Of the peptides tested, only glucagon5–29 showed antagonist activity, and this was weak compared with the classical glucagon antagonist, [Glu9]glucagon2–29. 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-Ser2] and [Gly2]glucagon retained high affinity binding and biological potency, similar to native glucagon in vitro. [d-Ser2]Glucagon exhibited enhanced hyperglycemic activity in a bioassay, whereas [Gly2]glucagon was not completely resistant to DPIV degradation.

Glucose-dependent insulinotropic polypeptide activates the Raf-Mek1/2-ERK1/2 module via a cyclic AMP/cAMP-dependent protein kinase/Rap1-mediated pathway.

The gastrointestinal hormone, glucose-dependent insulinotropic polypeptide (GIP), is one of the most important regulators of insulin secretion following ingestion of a meal. GIP stimulates insulin secretion from the pancreatic β-cell via its G protein-coupled receptor activation of adenylyl cyclase and other signal transduction pathways, but there is little known regarding subsequent protein kinase pathways that are activated. A screening technique was used to determine the relative abundance of 75 protein kinases in CHO-K1 cells expressing the GIP receptor and in two pancreatic β-cell lines (βTC-3 and INS-1 (832/13) cells). This information was used to identify kinases that are potentially regulated following GIP stimulation, with a focus on GIP regulation of the ERK1/2 MAPK pathway. In CHO-K1 cells, GIP induced phosphorylation of Raf-1 (Ser-259), Mek1/2 (Ser-217/Ser-221), ERK1/2 (Thr-202 and Tyr-204), and p90 RSK (Ser-380) in a concentration-dependent manner. Activation of ERK1/2 was maximal at 4 min and was cAMP-dependent protein kinase-dependent and protein kinase C-independent. Studies using a β-cell line (INS-1 clone 832/13) corroborated these findings, and it was also demonstrated that the ERK1/2 module could be activated by GIP in the absence of glucose. Finally, we have shown that GIP regulation of the ERK1/2 module is via Rap1 but does not involve Gβγ subunits nor Src tyrosine kinase, and we propose that cAMP-based regulation occurs via B-Raf in both CHO-K1 and β-cells. These results establish the importance of GIP in the cellular regulation of the ERK1/2 module and identify a role for cAMP in coupling its G protein-coupled receptors to ERK1/2 activity in pancreatic β-cells.

Inhibition of Histamine-, Pentagastrin-, and Insulin-Stimulated Canine Gastric Secretion by Pure “Gastric Inhibitory Polypeptide”

Gastric inhibitory polypeptide isolated from a partially purified preparation of the gastrointestinal hormone cholecystokinin-pancreozymin (Gastrointestinal Hormone Research Laboratory, Karolinska Institutet, Stockholm, Sweden) has been used in doses of 0.25 to 4.0 μg per kg-hr, in studies on the inhibition of H+ secretion from Bickel-type pouches and the innervated gastric remnant in dogs. A dose of 4.0 μg per kg-hr was shown to produce 40% inhibition of H+ secretion stimulated by histamine dihydrochloride from the Bickeltype pouches and 75% inhibition of pepsin output. The polypeptide produced 25% inhibition of H+ secretion to pentagastrin stimulation in a dose as small as 0.25 μg per kg-hr and a dose of 1.0 μg per kg-hr produced 75% inhibition of H+ secretion. Similar degrees of inhibition of pepsin outputs were also observed. H+ secretion from the gastric remnant, in response to insulin hypoglycemia, was inhibited by 45% and pepsin output was inhibited by 35%. The ability of the polypeptide to inhibit histamine-stimulated H+ secretion satisfies one of the necessary criteria for a claim that this polypeptide could be the enterogastrone released when fat is instilled into the duodenum. However such a claim to hormonal status must await identification of the polypeptide in blood.

A novel pathway for regulation of glucose-dependent insulinotropic polypeptide (GIP) receptor expression in beta cells.

Glucose‐dependent insulinotropic polypeptide (GIP) is secreted postprandially and acts in concert with glucose to stimulate insulin secretion from the pancreas. Here, we describe a novel pathway for the regulation of GIP receptor (GIPR) expression within clonal β‐cell lines, pancreatic islets, and in vivo. High (25 mM) glucose was able to significantly reduce GIPR mRNA levels in INS(832/13) cells after only 6 h. In contrast, palmitic acid (2 mM) and WY 14643 (100 μM) stimulated approximate doublings of GIPR expression in INS(832/13) cells under low (5.5 mM), but not high (25 mM), glucose conditons, suggesting that fat can regulate GIPR expression via PPARα in a glucose‐dependent manner. Both MK‐886, an antagonist of PPARα, and a dominant negative form of PPARα transfected into INS(832/13) cells caused a significant reduction in GIPR expression in low, but not high, glucose conditions. Finally, in hyperglycemic clamped rats, there was a 70% reduction in GIPR expression in the islets and a 71% reduction in GIP‐stimulated insulin secretion from the perfused pancreas. Thus, evidence is presented that the GIPR is controlled at normoglycemia by the fatty acid load on the islet; however, when exposed to hyperglycemic conditions, the GIPR is down‐regulated, which may contribute to the decreased responsiveness to GIP that is observed in type 2 diabetes.