Glucagon-like peptide 1 induces differentiation of islet duodenal homeobox-1-positive pancreatic ductal cells into insulin-secreting cells

The long-acting glucagon-like peptide-1 analogue, liraglutide, inhibits beta-cell apoptosis in vitro

We here show that GLP-1 and the long-acting GLP-1 analogue, liraglutide, interfere with diabetes-associated apoptotic processes in the beta-cell. Studies using primary neonatal rat islets showed that native GLP-1 and liraglutide inhibited both cytokine- and free fatty acid-induced apoptosis in a dose-dependent manner. The anti-apoptotic effect of liraglutide was mediated by the GLP-1 receptor as the specific GLP-1 receptor antagonist, exendin(9-39), blocked the effects. The adenylate cyclase activator, forskolin, had an anti-apoptotic effect similar to those of GLP-1 and liraglutide indicating that the effect was cAMP-mediated. Blocking the PI3 kinase pathway using wortmannin but not the MAP kinase pathways by PD98059 inhibited the effects of liraglutide. In conclusion, GLP-1 receptor activation has anti-apoptotic effect on both cytokine, and free fatty acid-induced apoptosis in primary islet-cells, thus suggesting that the long-acting GLP-1 analogue, liraglutide, may be useful for retaining beta-cell mass in both type 1 and type 2 diabetic patients.

Glucagon-like peptide 1 and its derivatives in the treatment of diabetes

Glucagon-like peptide 1 (GLP-1) was discovered as an insulinotropic gut hormone, suggesting a physiological role as an incretin hormone, i.e., being responsible, in part, for the higher insulin secretory response after oral as compared to intravenous glucose administration. This difference, the incretin effect, is partially lost in patients with Type 2 diabetes. The actions of GLP-1 include (a) a stimulation of insulin secretion in a glucose-dependent manner, (b) a suppression of glucagon, (c) a reduction in appetite and food intake, (d) a deceleration of gastric emptying, (e) a stimulation of beta-cell neogenesis, growth and differentiation in animal and tissue culture experiments, and (f) an in vitro inhibition of beta-cell apoptosis induced by different toxins. Intravenous GLP-1 can normalize and subcutaneous GLP-1 can significantly lower plasma glucose in the majority of patients with Type 2 diabetes. GLP-1 itself, however, is inactivated rapidly in vivo and thus does not appear to be useful as a therapeutic agent in the long-term treatment of Type 2 diabetes. Other agents acting on GLP-1 receptors have been found (like exendin-4) or developed as GLP-1 derivatives (like liraglutide or GLP-1/CJC-1131). Clinical trials with exenatide (two injections per day) and liraglutide (one injection per day) have shown reductions in glucose concentrations and HbA1c by more than 1%, associated with moderate weight loss (2-3 kg), but also some nausea and, rarely, vomiting. It is hoped that this new class of drugs interacting with the GLP-1 or other incretin receptors, the so-called "incretin mimetics", will broaden our armamentarium of antidiabetic medications in the nearest future.

Adenoviral-mediated transduction of human pancreatic islets: importance of adenoviral genome for cell viability and association with a deficient antiviral response

As adenoviral vectors are extensively used for genetic manipulation of insulin-producing cells in vitro, there is an increasing need to evaluate their effects on the function, morphology, and viability of transduced pancreatic islets. In the present study we observed that specific adenoviral genotypes, carrying E4 and E1/E3 deletions, correlate with differential induction of necrosis in pancreatic islet cells. In particular, the adenovirus death protein encoded from the E3 region of the adenoviral genome was able to modulate the changes induced in the morphology and viability of the transduced cells. We also propose a putative role for the transcriptional regulator pIX. Although human islet cells showed an increased resistance in terms of viral concentrations required for the induction of cell toxicity, our results showed that they were unable to build up an efficient antiviral response after transduction and that their survival was dependent on the exogenous addition of alpha-interferon. An intact and fully functional beta-cell is crucial for the successful application of gene therapy approaches in type 1 diabetes, and therefore, the implications of our findings need to be considered when designing vectors for gene transfer into pancreatic beta-cells.

Morphogenetic plasticity of adult human pancreatic islets of Langerhans

The aim of this study was to investigate the phenotypic plasticity of pancreatic islets of Langerhans. Quiescent adult human islets were induced to undergo a phenotypic switch to highly proliferative duct-like structures in a process characterized by a loss of expression of islet-specific hormones and transcription factors as well as a temporally related rise in the expression of markers of both duct epithelial and progenitor cells. Short-term treatment of these primitive duct-like structures with the neogenic factor islet neogenesis-associated protein (INGAP104-118) induced their reconversion back to islet-like structures in a PI3-kinase-dependent manner. These neoislets resembled freshly isolated human islets with respect to the presence and topological arrangement of the four endocrine cell types, islet gene expression and hormone production, insulin content and glucose-responsive insulin secretion. Our results suggest that adult human islets possess a remarkable degree of morphogenetic plasticity. This novel observation may have important implications for understanding pancreatic carcinogenesis and islet neogenesis.

Harnessing the therapeutic potential of glucagon-like peptide-1: a critical review

Glucagon-like peptide-1 (GLP-1) is synthesized from proglucagon in enteroendocrine cells and regulates glucose homeostasis via multiple complementary actions on appetite, gastrointestinal motility and islet hormone secretion. GLP-1 is secreted from the distal gut in response to food ingestion, and levels of circulating GLP-1 may be diminished in patients with type 2 diabetes mellitus. GLP-1 administration stimulates glucose-dependent insulin secretion, inhibits glucagon secretion, and lowers blood glucose in normal and diabetic rodents and in humans. GLP-1 exerts additional glucose-lowering actions in patients with diabetes mellitus already treated with metformin or sulfonylurea therapy. GLP-1 inhibits gastric emptying in healthy individuals and those with diabetes mellitus, and excess GLP-1 administration may cause nausea or vomiting in susceptible individuals. Chronic GLP-1 treatment of normal or diabetic rodents is associated with bodyweight loss and GLP-1 agonists transiently inhibit food intake and may prevent bodyweight gain in humans. The potential for GLP-1 therapy to prevent deterioration of beta-cell function is exemplified by studies demonstrating that GLP-1 analogs stimulate proliferation and neogenesis of beta-cells, leading to expansion of beta-cell mass in diabetic rodents. The rapid N-terminal inactivation of bioactive GLP-1 by dipeptidyl peptidase-IV (DPP-IV) limits the utility of the native peptide for the treatment of patients with diabetes mellitus, and has fostered the development of more potent and stable protease-resistant GLP-1 analogs which exhibit longer durations of action. The importance of DPP-IV for glucose control is illustrated by the phenotype of rodents with genetic inactivation of DPP-IV which exhibit reduced glycemic excursion and increased levels of circulating GLP-1 in vivo. Inhibitors of DPP-IV potentiate incretin action by preventing degradation of GLP-1 and glucose-dependent insulinotropic peptide, and lower blood glucose in normal rodents and in experimental models of diabetes mellitus. Hence, orally available DPP-IV inhibitors also represent a new class of therapeutic agents that enhance incretin action for the treatment of patients with type 2 diabetes mellitus.

Glucagon-like peptide 1(GLP-1) in biology and pathology

Post-translational proteolytic processing of the preproglucagon gene in the gut results in the formation of glucagon-like peptide 1 (GLP-1). Owing to its glucose-dependent insulinotropic effect, this hormone was postulated to primarily act as an incretin, i.e. to augment insulin secretion after oral glucose or meal ingestion. In addition, GLP-1 decelerates gastric emptying and suppresses glucagon secretion. Under physiological conditions, GLP-1 acts as a part of the 'ileal brake', meaning that is slows the transition of nutrients into the distal gut. Animal studies suggest a role for GLP-1 in the development and growth of the endocrine pancreas. In light of its multiple actions throughout the body, different therapeutic applications of GLP-1 are possible. Promising results have been obtained with GLP-1 in the treatment of type 2 diabetes, but its potential to reduce appetite and food intake may also allow its use for the treatment of obesity. While rapid in vivo degradation of GLP-1 has yet prevented its broad clinical use, different pharmacological approaches aiming to extend the in vivo half-life of GLP-1 or to inhibit its inactivation are currently being evaluated. Therefore, antidiabetic treatment based on GLP-1 may become available within the next years. This review will summarize the biological effects of GLP-1, characterize its role in human biology and pathology, and discuss potential clinical applications as well as current clinical studies.

Targeting beta-cell cyclic 3'5' adenosine monophosphate for the development of novel drugs for treating type 2 diabetes mellitus. A review

Cyclic 3'5'AMP is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclase, especially in response to the incretin hormones GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic peptide). These hormones are secreted from the small intestine during and following a meal, and are important in producing a full insulin secretory response to nutrient stimuli. Cyclic AMP influences many steps involved in glucose-induced insulin secretion and may be important in regulating pancreatic islet beta-cell differentiation, growth and survival. Cyclic AMP (cAMP) itself is rapidly degraded in the pancreatic islet beta-cell by cyclic nucleotide phosphodiesterase (PDE) enzymes. This review discusses the possibility of targeting cAMP mechanisms in the treatment of type 2 diabetes mellitus, in which insulin release in response to glucose is impaired. This could be achieved by the use of GLP-1 or GIP to elevate cAMP in the pancreatic islet beta-cell. However, these peptides are normally rapidly degraded by dipeptidyl peptidase IV (DPP IV). Thus longer-acting analogues of GLP-1 and GIP, resistant to enzymic degradation, and orally active inhibitors of DPP IV have also been developed, and these agents were found to improve metabolic control in experimentally diabetic animals and in patients with type 2 diabetes. The use of selective inhibitors of type 3 phosphodiesterase (PDE3B), which is probably the important pancreatic islet beta-cell PDE isoform, would require their targeting to the islet beta-cell, because inhibition of PDE3B in adipocytes and hepatocytes would induce insulin resistance.

Human embryonic stem cells: possibilities for human cell transplantation

Human embryonic stem (ES) cells serve as a potentially unlimited renewable source for cell transplantation targeted to treat several diseases. One advantage of embryonic stem (ES) cells over other stem cells under research is their apparently indefinite self-renewal capacity if cultured appropriately, and their ready differentiation into various cell phenotypes of all three germ layers. To date, a number of studies have reported the derivation of specific functional derivatives from human ES cells in vitro. While there have been clinical trials of human embryonal carcinoma (EC) cell-derived neurons in humans there has been no attempt as yet using human ES cell derivatives. However, the latter have been transplanted into recipient animals. In some cases ES-derived cells were shown to undergo further maturation, displayed integration with host tissue and even ameliorated the disease condition in the animal model. Recently, it has been reported that human ES cells can be genetically manipulated. Such procedures could be used to direct differentiation to a specific cell type or to reduce graft rejections by the modification of immune responses. This review highlights some of the recent advances in the field and the challenges that lie ahead before clinical trials using ES-derived cells can be contemplated.

Clinical endocrinology and metabolism. Glucagon-like peptide-1 and glucagon-like peptide-2

The glucagon-like peptides (glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2)) are released from enteroendocrine cells in response to nutrient ingestion. GLP-1 enhances glucose-stimulated insulin secretion and inhibits glucagon secretion, gastric emptying and feeding. GLP-1 also has proliferative, neogenic and antiapoptotic effects on pancreatic beta-cells. More recent studies illustrate a potential protective role for GLP-1 in the cardiovascular and central nervous systems. GLP-2 is an intestinal trophic peptide that stimulates cell proliferation and inhibits apoptosis in the intestinal crypt compartment. GLP-2 also regulates intestinal glucose transport, food intake and gastric acid secretion and emptying, and improves intestinal barrier function. Thus, GLP-1 and GLP-2 exhibit a diverse array of metabolic, proliferative and cytoprotective actions with important clinical implications for the treatment of diabetes and gastrointestinal disease, respectively. This review will highlight our current understanding of the biology of GLP-1 and GLP-2, with an emphasis on both well-characterized and more novel therapeutic applications of these peptides.

Redox-mediated enrichment of self-renewing adult human pancreatic cells that possess endocrine differentiation potential

OBJECTIVES

The limited availability of transplantable human islets has stimulated the development of methods needed to isolate adult pancreatic stem/progenitor cells capable of self-renewal and endocrine differentiation. The objective of this study was to determine whether modulation of intracellular redox state with N-acetyl-L-cysteine (NAC) would allow for the propagation of pancreatic stem/progenitor cells from adult human pancreatic tissue.

METHODS

Cells were propagated from human pancreatic tissue using a serum-free, low-calcium medium supplemented with NAC and tested for their ability to differentiate when cultured under different growth conditions.

RESULTS

Human pancreatic cell (HPC) cultures coexpressed alpha-amylase, albumin, vimentin, and nestin. The HPC cultures, however, did not express other genes associated with differentiated pancreatic exocrine, duct, or endocrine cells. A number of transcription factors involved in endocrine cell development including Beta 2, Islet-1, Nkx6.1, Pax4, and Pax6 were expressed at variable levels in HPC cultures. In contrast, pancreatic duodenal homeobox factor 1 (Pdx-1) expression was extremely low and at times undetectable. Overexpression of Pdx-1 in HPC cultures stimulated somatostatin, glucagon, and carbonic anhydrase expression but had no effect on insulin gene expression. HPC cultures could form 3-dimensional islet-like cell aggregates, and this was associated with expression of somatostatin and glucagon but not insulin. Cultivation of HPCs in a differentiation medium supplemented with nicotinamide, exendin-4, and/or LY294002, an inhibitor of phosphatidylinositol-3 kinase, stimulated expression of insulin mRNA and protein.

CONCLUSION

These data support the use of intracellular redox modulation for the enrichment of pancreatic stem/progenitor cells capable of self-renewal and endocrine differentiation.

Sources of β-cells for human cell-based therapies for diabetes

Recent progress in islet transplantation coupled with the extremely limited supply of primary human islets has spurred the search for alternative sources of beta-cells for transplantation therapy in treating diabetes. Many potential sources of cells are being explored, including embryonic and adult stem cells, identification of intrapancreatic precursor cells, and human beta-cell lines. Here, we review the promise and problems with those cell sources, focusing on our studies in developing functional human beta-cell lines. Those efforts involve a two-step process in which the first is to introduce growth stimulatory genes that induce human beta-cells to enter the cell cycle. Immortalization can then be achieved by expressing the hTERT telomerase subunit. The second step is to induce differentiation. This involves a complex set of manipulations, including the expression of the important beta-cell transcription factor PDX-1. Although PDX-1 is critical for promoting beta-cell differentiation, we do not find increased expression of the glucagon-like peptide-1 receptor, a gene that has been reported to be induced by PDX-1. Further understanding of the factors governing beta-cell development are likely to be required before a robust cell-based therapy is available for the treatment of diabetes.

Metabolic stability, receptor binding, cAMP generation, insulin secretion and antihyperglycaemic activity of novel N-terminal Glu9-substituted analogues of glucagon-like peptide-1

Glucagon-like peptide-1(7-36)amide (GLP-1) is an incretin hormone with therapeutic potential for type 2 diabetes. Rapid removal of the N-terminal dipeptide, His7-Ala8, by the ubiquitous enzyme dipeptidyl peptidase IV (DPP IV) curtails the biological activity of GLP-1. Chemical modifications or substitutions of GLP-1 at His7 or Ala8 improve resistance to DPP-IV action, but this often reduces potency. Little attention has focused on the metabolic stability and functional activity of GLP-1 analogues with amino acid substitution at Glu9, adjacent to the DPP IV cleavage site. We generated three novel Glu9-substituted GLP-1 analogues, (Pro9)GLP-1, (Phe9)GLP-1 and (Tyr9)GLP-1 and show for the first time that Glu9 of GLP-1 is important in DPP IV degradation, since replacing this amino acid, particularly with proline, substantially reduced susceptibility to degradation. All three novel GLP-1 analogues showed similar or slightly enhanced insulinotropic activity compared with native GLP-1 despite a moderate 4-10-fold reduction in receptor binding and cAMP generation. In addition, (Pro9)GLP-1 showed significant ability to moderate the plasma glucose excursion and increase circulating insulin concentrations in severely insulin resistant obese diabetic (ob/ob) mice. These observations indicate the importance of Glu9 for the biological activity of GLP-1 and susceptibility to DPP IV-mediated degradation.

In vivo and in vitro characterization of insulin-producing cells obtained from murine bone marrow

Efforts toward routine islet cell transplantation as a means for reversing type 1 diabetes have been hampered by islet availability as well as allograft rejection. In vitro transdifferentiation of mouse bone marrow (BM)-derived stem (mBMDS) cells into insulin-producing cells could provide an abundant source of autologous cells for this procedure. For this study, we isolated and characterized single cell-derived stem cell lines obtained from mouse BM. In vitro differentiation of these mBMDS cells resulted in populations meeting a number of criteria set forth to define functional insulin-producing cells. Specifically, the mBMDS cells expressed multiple genes related to pancreatic beta-cell development and function (insulin I and II, Glut2, glucose kinase, islet amyloid polypeptide, nestin, pancreatic duodenal homeobox-1 [PDX-1], and Pax6). Insulin and C-peptide production was identified by immunocytochemistry and confirmed by electron microscopy. In vitro studies involving glucose stimulation identified glucose-stimulated insulin release. Finally, these mBMDS cells transplanted into streptozotocin-induced diabetic mice imparted reversal of hyperglycemia and improved metabolic profiles in response to intraperitoneal glucose tolerance testing. These results indicate that mouse BM harbors cells capable of in vitro transdifferentiating into functional insulin-producing cells and support efforts to derive such cells in humans as a means to alleviate limitations surrounding islet cell transplantation.

Cure of overt diabetes in NOD mice by transient treatment with anti-lymphocyte serum and exendin-4

Treatment of overtly diabetic NOD mice with anti-lymphocyte serum (ALS), a polyclonal anti-T-cell antibody, abrogates autoimmunity and achieves partial clinical remission. Here we investigated whether the addition of exendin-4, a hormone that stimulates insulin secretion and beta-cell replication and differentiation, improves induction of remission by ALS. Transient treatment of overtly diabetic NOD mice with ALS and exendin-4 achieved complete remission in 23 of 26 mice (88%) within 75 days, accompanied by progressive normalization of glucose tolerance, improved islet histology, increased insulin content in the pancreas, and insulin release in response to a glucose challenge. Syngeneic islets transplanted into mice cured by treatment with ALS plus exendin-4 remained intact, and cotransfer of lymphocytes from cured mice delayed diabetes induction by adoptive transfer, suggesting the long-lasting presence of autoimmune regulatory cells. Although ALS alone also achieved reversal of diabetes, the frequency of remission was low (40%). No treatment or exendin-4 alone failed to produce remission. These results show that exendin-4 synergistically augments the remission-inducing effect of ALS. The addition of beta-cell growth factors, such as exendin-4, to immunotherapy protocols with anti-T-cell antibodies presents a potential novel approach to the cure of patients with new-onset type 1 diabetes.

Rapid purification of human ductal cells from human pancreatic fractions with surface antibody CA19-9

Generating human insulin-secreting cells for cell therapy of diabetes represents a highly competitive world challenge. Human ductal cells can give rise to islets in vivo and in vitro. The goal of this study was to devise a rapid sorting method to highly purify human ductal cells from pancreatic tissue using a pan-ductal membrane antibody carbohydrate antigen 19-9 (CA19-9). Human pancreatic sections confirmed antibody specificity. The human exocrine fraction (30% ductal cells) was sorted with magnetic bead technology or by FACS. Immunocytochemistry post-sorting determined ductal cell content. The manual magnetic bead technique resulted in 74%+/-2 (n = 4) CA19 positive cells. Whereas the automated AutoMACS technique (n = 5) yielded 92.6%+/-0.5 CA19-9 positive cells with only a minor beta cell contamination (0.2%+/-0.03); cell yield post-sorting was 12.9%+/-2.5 (1.69+/-0.41 x 10(6) cells) with 51.7%+6.5 (n = 5) viability post-sorting. The FACS (n = 6) resulted in 97.1%+/-0.82 CA19-9 positive cells, a cell yield of 25.5%+/-5.6 (5.03+/-1.0 x 10(6)), with 72.1%+/-6.1 viability post-sorting.

Glucagon-like peptide 1 agonists and the development and growth of pancreatic beta-cells

Glucagon-like peptide 1 (GLP-1) is an intestine-derived insulinotropic hormone that stimulates glucose-dependent insulin production and secretion from pancreatic beta-cells. Other recognized actions of GLP-1 are to suppress glucagon secretion and hepatic glucose output, delay gastric emptying, reduce food intake, and promote glucose disposal in peripheral tissues. All of these actions are potentially beneficial for the treatment of type 2 diabetes mellitus. Several GLP-1 agonists are in clinical trials for the treatment of diabetes. More recently, GLP-1 agonists have been shown to stimulate the growth and differentiation of pancreatic beta-cells, as well as to exert cytoprotective, antiapoptotic effects on beta-cells. Recent evidence indicates that GLP-1 agonists act on receptors on pancreas-derived stem/progenitor cells to prompt their differentiation into beta-cells. These new findings suggest an approach to create beta-cells in vitro by expanding stem/progenitor cells and then to convert them into beta-cells by treatment with GLP-1. Thus GLP-1 may be a means by which to create beta-cells ex vivo for transplantation into patients with insulinopenic type 1 diabetes and severe forms of type 2 diabetes.

Expression of pancreatic duodenal hoemobox-1 in pancreatic islet neogenesis after surgical wrapping in rats

BACKGROUND

Surgical wrapping (SW) of the pancreas causes islet neogenesis in rodents. Pancreatic duodenal hoemobox-1 (PDX-1) is one of the transcriptional factors needed by pancreatic stem cells to develop a mature pancreas. The purpose of this study was to determine whether islet neogenesis arises from ductal cells and whether PDX-1 is involved in this process.

METHODS

SW consisted of nonocclusive wrapping of the pancreas in rats. The wrapped pancreas was then harvested, insulin content was measured, and immunohistochemical analysis for insulin, cytokeratin, and PDX-1 was performed.

RESULTS

The endocrine area of the wrapped pancreas significantly increased after SW. Double immunostaining identified cells positive for both insulin and cytokeratin in or along the epithelial cell lining of the ductal structures and in the centroacinar cells. PDX-1-positive cells were detected in both control islets and islets examined after SW, but these cells were observed in the exocrine area only after SW. Double staining also showed that cells positive for PDX-1 but negative for insulin were present in the exocrine area 1 day after SW and that cells positive for both PDX-1 and insulin had developed 3 days after SW.

CONCLUSIONS

In the process of adult islet neogenesis after SW, cells in the acini and ductal structures developed into PDX-1-expressing cells, supposedly progenitor cells, which in turn became insulin-producing cells and thus might be the origin of small islets.

Novel strategies for the pharmacological management of type 2 diabetes

Type 2 diabetes is characterized by high concentrations of glucose in the blood, which is caused by decreased secretion of insulin from the pancreas and decreased insulin action. This condition is prevalent worldwide and is associated with morbidity and mortality secondary to complications such as myocardial infarction, stroke and end-stage renal disease. The importance of tight control of blood glucose in either preventing or delaying the progression of complications is recognized. Currently, there are many therapeutic options to treat hyperglycemia in type 2 diabetes. However, tight control is difficult to achieve and is often associated with side-effects. Recent advances in understanding insulin secretion, action and signaling have led to the development of new pharmacological agents. In this article, we review new molecules that are promising candidates for the future management of diabetes, focusing on their mechanism of action, efficacy, safety profile and potential benefits compared with pharmacological agents that are available currently.

Lys9 for Glu9 substitution in glucagon-like peptide-1(7-36)amide confers dipeptidylpeptidase IV resistance with cellular and metabolic actions similar to those of established antagonists glucagon-like peptide-1(9-36)amide and exendin (9-39)

The incretin hormone glucagon-like peptide-1(7-36)amide (GLP-1) has been deemed of considerable importance in the regulation of blood glucose. Its effects, mediated through the regulation of insulin, glucagon, and somatostatin, are glucose-dependent and contribute to the tight control of glucose levels. Much enthusiasm has been assigned to a possible role of GLP-1 in the treatment of type 2 diabetes. GLP-1's action unfortunately is limited through enzymatic inactivation caused by dipeptidylpeptidase IV (DPP IV). It is now well established that modifying GLP-1 at the N-terminal amino acids, His(7) and Ala(8), can greatly improve resistance to this enzyme. Little research has assessed what effect Glu(9)-substitution has on GLP-1 activity and its degradation by DPP IV. Here, we report that the replacement of Glu(9) of GLP-1 with Lys dramatically increased resistance to DPP IV. This analogue, (Lys(9))GLP-1, exhibited a preserved GLP-1 receptor affinity, but the usual stimulatory effects of GLP-1 were completely eliminated, a trait duplicated by the other established GLP-1-antagonists, exendin (9-39) and GLP-1(9-36)amide. We investigated the in vivo antagonistic actions of (Lys(9))GLP-1 in comparison with GLP-1(9-36)amide and exendin (9-39) and revealed that this novel analogue may serve as a functional antagonist of the GLP-1 receptor.

Glucagon-like peptide 1 and gastric inhibitory polypeptide: potential applications in type 2 diabetes mellitus

Although the insulinotropic actions of gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) have been known for almost 2 decades, the incretin hormones have not yet become available for clinical application. This can be explained by their unfavourable pharmacological properties. Both hormones are rapidly inactivated by the enzyme dipeptidyl peptidase IV (DPP IV), yielding biologically inactive fragments. There have been several attempts to make use of the antidiabetogenic potential of the incretin hormones. Various analogues of GLP-1 and GIP have been generated in order to achieve resistance to DPP IV degradation. The natural GLP-1 receptor agonist exendin-4, found in the saliva of the Gila monster, has a longer biological half-life after subcutaneous injection than GLP-1, and inhibition of DPP IV using, for example, pyrrolidine derivatives provides elevated concentrations of intact, biologically active GIP and GLP-1 endogenously released from the gut. A continuous intravenous infusion of native GLP-1 for a limited time may be suitable in certain clinical situations. Numerous clinical studies are currently underway to evaluate these approaches. Therefore, an antidiabetic treatment based on incretin hormones may become available within the next 5 years.

Glucagon-like peptide 1 receptor agonist ZP10A increases insulin mRNA expression and prevents diabetic progression in db/db mice

We characterized the novel, rationally designed peptide glucagon-like peptide 1 (GLP-1) receptor agonist H-HGEGTFTSDLSKQMEEEAVRLFIEWLKNGGPSSGAPPSK KKKKK-NH2 (ZP10A). Receptor binding studies demonstrated that the affinity of ZP10A for the human GLP-1 receptor was 4-fold greater than the affinity of GLP-1 (7-36) amide. ZP10A demonstrated dose-dependent improvement of glucose tolerance with an ED50 value of 0.02 nmol/kg i.p. in an oral glucose tolerance test (OGTT) in diabetic db/db mice. After 42 days of treatment, ZP10A dose-dependently (0, 1, 10, or 100 nmol/kg b.i.d.; n = 10/group), decreased glycosylated hemoglobin (HbA1C) from 8.4 +/- 0.4% (vehicle) to a minimum of 6.2 +/- 0.3% (100 nmol/kg b.i.d.; p < 0.05 versus vehicle) in db/db mice. Fasting blood glucose (FBG), glucose tolerance after an OGTT, and HbA1C levels were significantly improved in mice treated with ZP10A for 90 days compared with vehicle-treated controls. Interestingly, these effects were preserved 40 days after drug cessation in db/db mice treated with ZP10A only during the first 50 days of the study. Real-time polymerase chain reaction measurements demonstrated that the antidiabetic effect of early therapy with ZP10A was associated with an increased pancreatic insulin mRNA expression relative to vehicle-treated mice. In conclusion, long-term treatment of diabetic db/db mice with ZP10A resulted in a dose-dependent improvement of FBG, glucose tolerance, and blood glucose control. Our data suggest that ZP10A preserves beta-cell function. ZP10A is considered one of the most promising new drug candidates for preventive and therapeutic intervention in type 2 diabetes.

Glucose-dependent insulinotropic polypeptide promotes beta-(INS-1) cell survival via cyclic adenosine monophosphate-mediated caspase-3 inhibition and regulation of p38 mitogen-activated protein kinase

The incretin glucose-dependent insulinotropic polypeptide (GIP) is a major regulator of postprandial insulin secretion in mammals. Recent studies in our laboratory, and others have suggested that GIP is a potent stimulus for protein kinase activation, including the MAPK (ERK1/2) module. Based on these studies, we hypothesized that GIP could regulate cell fate and sought to examine the underlying mechanisms involved in GIP stimulation of cell survival. GIP potentiated glucose-induced beta-(INS-1)-cell growth to levels comparable with GH and GLP-1 while promoting cell survival in the face of serum and glucose-deprivation or treatment with wortmannin or streptozotocin. In the absence of GIP, 50% of cells died after 48 h of serum and glucose withdrawal, whereas 91 +/- 10% of cells remained viable in the presence of GIP [n = 3, P < 0.05; EC50 of 1.24 +/- 0.48 nm GIP (n = 4)]. Effects of GIP on cell survival and inhibition of caspase-3 were mimicked by forskolin, but pharmacological experiments excluded roles for MAPK kinase (Mek)1/2, phosphatidylinositol 3-kinase, protein kinase A, Epac, and Rap 1. Survival effects of GIP were ablated by the inhibitor SB202190, indicating a role for p38 MAPK. Furthermore, caspase-3 activity was also regulated by p38 MAPK, with a lesser role for Mek1/2, based on RNA interference studies. We propose that GIP is able to reverse caspase-3 activation via inhibition of long-term p38 MAPK phosphorylation in response to glucose deprivation (+/-wortmannin). Intriguingly, these findings contrasted with short-term phosphorylation of MKK3/6-->p38 MAPK-->ATF-2 by GIP. Thus, these data suggest that GIP is able to regulate INS-1 cell survival by dynamic control of p38 MAPK phosphorylation via cAMP signaling and lend further support to the notion that GIP regulation of MAPK signaling is critical for its regulation of cell fate.

Metabolism and pharmacokinetics of a dipeptidyl peptidase IV inhibitor in rats, dogs, and monkeys with selective carbamoyl glucuronidation of the primary amine in dogs

The pharmacokinetics and metabolism of the l-threo isoleucine thiazolidide dipeptidyl peptidase IV inhibitor, di-[2S,3S]-2-amino-3-methyl-pentanoic-1,3-thiazolidine fumarate (ILT-threo) and its allo stereoisomer (ILT-allo) were evaluated in rats, dogs, and monkeys. Both compounds were well absorbed (>80%) in all species, and most of the dose (>60%) was recovered in urine. Metabolites identified in all species included a sulfoxide (M1), a sulfone (M2), and a carbamoyl glucuronide (M3). For both compounds, parent drug had moderate systemic clearance in rats and dogs ( approximately 20-35 ml/min/kg in both species) and lower clearance in monkeys ( approximately 6-9 ml/min/kg). In rats, M1 was present in systemic circulation in concentrations similar to that of parent drug, whereas in dogs and monkeys, exposures to M1 were higher than for parent drug. In dogs, exposures to the sulfoxide metabolite were approximately 2 to 3 times higher after administration of ILT-allo than after administration of ILT-threo. Carbamoyl glucuronidation was an important biotransformation pathway in dogs. Circulating levels of M3 were significant in the dog, and present only in trace levels in rats and monkeys. M3 could be produced in in vitro systems in a NaHCO3 buffer under a CO2-saturated atmosphere and in the presence of UDP-glucuronic acid and alamethicin.

Characterization and isolation of promoter-defined nestin-positive cells from the human fetal pancreas

Studies using adult human islets and mouse embryonic stem cells have suggested that the neurepithelial precursor cell marker nestin also identifies and can be used to purify beta-cell precursors. To determine whether nestin can be used to identify beta-cell progenitors in the developing human pancreas, we characterized nestin expression from 12 to 24 gestational weeks, purified nestin+ cells using an enhancer/promoter-driven selection plasmid, and determined whether nestin+ cells can differentiate into beta-cells. Nestin was visualized in the platelet endothelial cell adhesion molecule and alpha smooth muscle actin-positive blood vessels and colocalized with vimentin in the interstitium. Nestin was not observed in pan cytokeratin (pCK)-positive ductal epithelium or insulin cells. Purified nestin+ cells also coexpressed vimentin and lacked pCK immunoreactivity. Purified adult and fetal pancreatic fibroblasts also expressed nestin. The nestin enhancer/promoter used in the selection plasmid was sufficient to drive reporter gene expression, green fluorescent protein, in human fetal pancreatic tissue. Exposure of selected nestin+ cells to nicotinamide, hepatocyte growth factor/scatter factor, betacellulin, activin A, or exendin-4 failed to induce pancreatic and duodenal homeobox gene-1 or insulin message as determined by RT-PCR. Transplantation of nestin+ cells and fetal pancreatic fibroblasts into athymic mice also failed to result in the development of beta-cells, whereas nestin- fetal pancreatic epithelial cells gave rise to functional insulin-secreting beta-cells. We conclude that nestin is not a specific marker of beta-cell precursors in the developing human pancreas.

GLP-1 stimulates glucose-derived de novo fatty acid synthesis and chain elongation during cell differentiation and insulin release

Glucagon-like peptide-1 (GLP-1, 7-36) is capable of restoring normal glucose tolerance in aging, glucose-intolerant Wistar rats and is a potent causal factor in differentiation of human islet duodenal homeobox-1-expressing cells into insulin-releasing beta cells. Here we report stable isotope-based dynamic metabolic profiles of rat pancreatic epithelial (ARIP) and human ductal tumor (PANC-1) cells responding to 10 nM GLP-1 treatment in 48 h cultures. Macromolecule synthesis patterns and substrate flow measurements using gas chromatography/mass spectrometry (MS) and the stable [1,2-13C2]glucose isotope as the tracer showed that GLP-1 induced a significant 20% and 60% increase in de novo fatty acid palmitate synthesis in ARIP and PANC-1 cells, respectively, and it also induced a significant increase in palmitate chain elongation into stearate utilizing glucose as the primary substrate. Distribution of 13C in other metabolites indicated no changes in the rates of nucleic acid ribose synthesis, glutamate oxidation, or lactate production. Tandem high-performance liquid chromatography-ion trap MS analysis of the culture media demonstrated mass insulin secretion by GLP-1-treated tumor cells. Metabolic profile changes in response to GLP-1-induced cell differentiation include selective increases in de novo fatty acid synthesis from glucose and consequent chain elongation, allowing increased membrane formation and greater insulin availability and release.

Enteroinsular signaling: perspectives on the role of the gastrointestinal hormones glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide in normal and abnormal glucose metabolism

PURPOSE OF REVIEW

The gastrointestinal hormones glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide are emerging as essential regulators of insulin secretion and glucose homeostasis. These peptides, termed incretins, are the key intermediaries in a system that links the absorption of nutrients in the gut with important metabolic processes in substrate assimilation. New findings indicate that the enteroinsular system mediated by the incretins is relevant to both the pathophysiology and treatment of diabetes.

RECENT FINDINGS

Important advances have been made in the understanding of mechanisms fundamental to incretin function such as their release from the intestine during meals, their actions on beta-cell secretion, and extrapancreatic effects. In addition, the regulation of islet growth by glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide is a novel area with considerable support from recent studies. Abnormalities of incretin function are present in patients with diabetes and current research has implicated specific defects of both glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide action in diabetes. Finally, several pharmacological applications of the incretin signaling pathways are under active investigation for the treatment of diabetes.

SUMMARY

With the intensified research of the last several years the physiologic importance of the incretins has been clarified. Enteroinsular signaling is an essential component of the metabolic processes that govern carbohydrate, and likely other nutrient metabolism. As a pathophysiology of the incretins emerges, glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide will have increasing clinical relevance. This is currently exemplified by the development of therapeutics for diabetes that work through the incretin signaling pathways.

Covalent histone modifications underlie the developmental regulation of insulin gene transcription in pancreatic beta cells

Histone modifying enzymes contribute to the activation or inactivation of transcription by ultimately catalyzing the unfolding or further compaction, respectively, of chromatin structure. Actively transcribed genes are typically hyperacetylated at Lys residues of histones H3 and H4 and hypermethylated at Lys-4 of histone H3 (H3-K4). To determine whether covalent histone modifications play a role in the beta cell-specific expression of the insulin gene, we performed chromatin immunoprecipitation assays using anti-histone antibodies and extracts from beta cell lines, non-beta cell lines, and ES cells, and quantitated specific histone modifications at the insulin promoter by real-time PCR. Our studies reveal that the proximal insulin promoter is hyperacetylated at histone H3 only in beta cells. This hyperacetylation is highly correlated to recruitment of the histone acetyltransferase p300 to the proximal promoter in beta cells, and is consistent with the role of hyperacetylation in promoting euchromatin formation. We also observed that the proximal insulin promoter of beta cells is hypermethylated at H3-K4, and that this modification is correlated to the recruitment of the histone methyltransferase SET7/9 to the promoter. ES cells demonstrate a histone modification pattern intermediate between that of beta cells and non-beta cells, and is consistent with their potential to express the insulin gene. We therefore propose a model in which insulin transcription in the beta cell is facilitated by a unique combination of transcription factors that acts in the setting of an open, euchromatic structure of the insulin gene.

Human pancreatic precursor cells secrete FGF2 to stimulate clustering into hormone-expressing islet-like cell aggregates

Development of the endocrine pancreas includes a series of early events wherein precursor cells cluster, that is migrate to form cell aggregates, which subsequently differentiate into islets of Langerhans. We show that PANC-1 cells, a human pancreatic cell line, differentiates into hormone-producing islet-like cell aggregates after exposure to a defined serum-free medium. These cells were used to provide the following evidence that fibroblast growth factor (FGF)2 is a paracrine chemoattractant during PANC-1 cell clustering: (i) FGF2 is secreted and remains bound to the extracellular matrix from where it may diffuse to form chemoattractive gradients; (ii) a subset of cells expresses FGF receptors (FGFRs) -1, -2, -3, and -4; (iii) inhibition of FGFR tyrosine kinase inhibits cell clustering; and (iv) FGF2 neutralizing antibody inhibits clustering. In addition, adult human islet-derived precursor cells, which cluster and differentiate in a manner similar to PANC-1 cells, also secrete FGF2 and express FGFRs. We conclude that FGF2, acting as a paracrine chemoattractant, stimulates clustering of precursor cells, an early step leading to islet-like cell aggregate formation. Similar processes may occur during development of the islet of Langerhans in humans.

Transcription factors direct the development and function of pancreatic beta cells

Transcription factors orchestrate intricate pathways of cellular growth and differentiation by regulating the rate of transcription of an array of genes. Genetic and biochemical studies have begun to unravel the complex cascade of factors that controls the proliferation and differentiation of cells in the developing pancreas. The specific pathway leading to the development of the insulin-secreting beta cell has been a focus of many of these studies because an understanding of the transcription factors governing this pathway will be crucial to the engineering of new beta cells to cure diabetes. In recent years, the number of transcription factors that has been implicated in beta-cell differentiation and function has grown considerably. Here, we outline the known role of transcription factors in beta-cell development, and describe how these factors form a network of gene activation signals that mediates insulin transcription.

GLP-1 receptor agonists are growth and differentiation factors for pancreatic islet beta cells

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that, when given exogenously, is capable of normalizing blood glucose in individuals with type 2 diabetes. Until recently most of the research on this compound had been related to its insulinotropic properties. However, GLP-1 also regulates insulin synthesis and proinsulin gene expression, as well as the components of the glucose-sensing machinery. In addition to regulating insulin release, it is involved in regulating the secretion of at least two other islet hormones--glucagon and somatostatin. Extraislet effects of GLP-1 include a role in the central nervous system stress response, hypothalamic-pituitary function, and the suppression of gastric emptying. Recent studies from our own and other laboratories show that GLP-1 can regulate islet growth and is a differentiation factor of the endocrine pancreas. This leads us to propose that GLP-1 and GLP-1 receptor agonists, in the context of long-term treatment of type 2 diabetes, will have broader biological action on the endocrine pancreas than was earlier anticipated. We propose that GLP-1 is a growth factor for pancreatic endocrine cells and can increase islet cell mass. Here we review those reports that have highlighted its role as a factor for islet cell growth and differentiation.

Dipeptidyl peptidase IV inhibitor treatment stimulates beta-cell survival and islet neogenesis in streptozotocin-induced diabetic rats

Recent studies into the physiology of the incretins glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) have added stimulation of beta-cell growth, differentiation, and cell survival to well-documented, potent insulinotropic effects. Unfortunately, the therapeutic potential of these hormones is limited by their rapid enzymatic inactivation in vivo by dipeptidyl peptidase IV (DP IV). Inhibition of DP IV, so as to enhance circulating incretin levels, has proved effective in the treatment of type 2 diabetes both in humans and in animal models, stimulating improvements in glucose tolerance, insulin sensitivity, and beta-cell function. We hypothesized that enhancement of the cytoprotective and beta-cell regenerative effects of GIP and GLP-1 might extend the therapeutic potential of DP IV inhibitors to include type 1 diabetes. For testing this hypothesis, male Wistar rats, exposed to a single dose of streptozotocin (STZ; 50 mg/kg), were treated twice daily with the DP IV inhibitor P32/98 for 7 weeks. Relative to STZ-injected controls, P32/98-treated animals displayed increased weight gain (230%) and nutrient intake, decreased fed blood glucose ( approximately 26 vs. approximately 20 mmol/l, respectively), and a return of plasma insulin values toward normal (0.07 vs. 0.12 nmol/l, respectively). Marked improvements in oral glucose tolerance, suggesting enhanced insulin secretory capacity, were corroborated by pancreas perfusion and insulin content measurements that revealed two- to eightfold increases in both secretory function and insulin content after 7 weeks of treatment. Immunohistochemical analyses of pancreatic sections showed marked increases in the number of small islets (+35%) and total beta-cells (+120%) and in the islet beta-cell fraction (12% control vs. 24% treated) in the treated animals, suggesting that DP IV inhibitor treatment enhanced islet neogenesis, beta-cell survival, and insulin biosynthesis. In vitro studies using a beta-(INS-1) cell line showed a dose-dependent prevention of STZ-induced apoptotic cell-death by both GIP and GLP-1, supporting a role for the incretins in eliciting the in vivo results. These novel findings provide evidence to support the potential utility of DP IV inhibitors in the treatment of type 1 and possibly late-stage type 2 diabetes.

Glucagon-like peptides: regulators of cell proliferation, differentiation, and apoptosis

Peptide hormones are secreted from endocrine cells and neurons and exert their actions through activation of G protein-coupled receptors to regulate a diverse number of physiological systems including control of energy homeostasis, gastrointestinal motility, neuroendocrine circuits, and hormone secretion. The glucagon-like peptides, GLP-1 and GLP-2 are prototype peptide hormones released from gut endocrine cells in response to nutrient ingestion that regulate not only energy absorption and disposal, but also cell proliferation and survival. GLP-1 expands islet mass by stimulating pancreatic beta-cell proliferation and induction of islet neogenesis. GLP-1 also promotes cell differentiation, from exocrine cells or immature islet progenitors, toward a more differentiated beta-cell phenotype. GLP-2 stimulates cell proliferation in the gastrointestinal mucosa, leading to expansion of the normal mucosal epithelium, or attenuation of intestinal injury in experimental models of intestinal disease. Both GLP-1 and GLP-2 exert antiapoptotic actions in vivo, resulting in preservation of beta-cell mass and gut epithelium, respectively. Furthermore, GLP-1 and GLP-2 promote direct resistance to apoptosis in cells expressing GLP-1 or GLP-2 receptors. Moreover, an increasing number of structurally related peptide hormones and neuropeptides exert cytoprotective effects through G protein-coupled receptor activation in diverse cell types. Hence, peptide hormones, as exemplified by GLP-1 and GLP-2, may prove to be useful adjunctive tools for enhancement of cell differentiation, tissue regeneration, and cytoprotection for the treatment of human disease.

Stem/progenitor cells derived from adult tissues: potential for the treatment of diabetes mellitus

In view of the recent success in pancreatic islet transplantation, interest in treating diabetes by the delivery of insulin-producing beta-cells has been renewed. Because differentiated pancreatic beta-cells cannot be expanded significantly in vitro, beta-cell stem or progenitor cells are seen as a potential source for the preparation of transplantable insulin-producing tissue. In addition to embryonic stem (ES) cells, several potential adult islet/beta-cell progenitors, derived from pancreas, liver, and bone marrow, are being studied. To date, none of the candidate cells has been fully characterized or is clinically applicable, but pancreatic physiology makes the existence of one or more types of adult islet stem cells very likely. It also seems possible that pluripotential stem cells, derived from the bone marrow, contribute to adult islet neogenesis. In future studies, more stringent criteria should be met to clonally define adult islet/beta-cell progenitor cells. If this can be achieved, the utilization of these cells for the generation of insulin-producing beta-cells in vitro seems to be feasible in the near future.

Role of endogenous glucagon-like peptide-1 in islet regeneration after partial pancreatectomy

A reduction in beta-cell mass is an important causative factor in type 1 and type 2 diabetes. Glucagon-like peptide-1 (GLP-1) and the long-acting agonist exendin 4 (Ex-4) expand beta-cell mass by stimulating neogenesis and proliferation. In the partial pancreatectomy (Ppx) model, exogenous Ex-4 promotes islet regeneration, leading to sustained improvement in glucose tolerance. In this study, we investigate the potential role of endogenous GLP-1 in islet growth. We examined beta-cell mass regeneration after 70% Ppx in mice receiving the GLP-1 antagonist Ex9-39 and in GLP-1R(-/-) mice. In Ex9-39-treated sham-operated mice, persistent fasting hyperglycemia was observed, but beta-cell mass was not diminished. In pancreatectomized mice, persistent glucose intolerance was noted, but this was not further exacerbated by Ex9-39. Accordingly, beta-cell mass recovery of Ppx mice was not impaired by Ex9-39. In contrast, GLP-1R(-/-) CD1 mice showed worse glucose intolerance after Ppx compared with wild-type CD1 Ppx mice, and this correlated with a significant defect in beta-cell mass regeneration. The recovery of beta-cell mass differed markedly in the BALB/c and CD1 control mice, indicating a significant role of genetic background in the regulation of beta-cell mass. These studies point to a role for endogenous GLP-1 in beta-cell regeneration after Ppx in mice.

Glucagon-like peptide-1 receptor signaling modulates beta cell apoptosis

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion and augments beta cell mass via activation of beta cell proliferation and islet neogenesis. We examined whether GLP-1 receptor signaling modifies the cellular susceptibility to apoptosis. Mice administered streptozotocin (STZ), an agent known to induce beta cell apoptosis, exhibit sustained improvement in glycemic control and increased levels of plasma insulin with concomitant administration of the GLP-1 agonist exendin-4 (Ex-4). Blood glucose remained significantly lower for weeks after cessation of exendin-4. STZ induced beta cell apoptosis, which was significantly reduced by co-administration of Ex-4. Conversely, mice with a targeted disruption of the GLP-1 receptor gene exhibited increased beta cell apoptosis after STZ administration. Exendin-4 directly reduced cytokine-induced apoptosis in purified rat beta cells exposed to interleukin 1beta, tumor necrosis fator alpha, and interferon gamma in vitro. Furthermore, Ex-4-treated BHK-GLP-1R cells exhibited significantly increased cell viability, reduced caspase activity, and decreased cleavage of beta-catenin after treatment with cycloheximide in vitro. These findings demonstrate that GLP-1 receptor signaling directly modifies the susceptibility to apoptotic injury, and provides a new potential mechanism linking GLP-1 receptor activation to preservation or enhancement of beta cell mass in vivo.

Role for VPAC2 receptor-mediated signals in pancreas development

Mature pancreatic cells develop from progenitors that proliferate and differentiate into endocrine and exocrine cells. This development is thought to be controlled by secreted soluble factors acting on their target cells after binding to membrane receptors. Here, we analyzed the impact on embryonic pancreatic development of ligands that bind to protein G-coupled receptors and increase cAMP accumulation. We found that embryonic pancreatic epithelial cells were sensitive to vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide. These factors generate signals after binding to the VPAC2 receptor, which is expressed by immature pancreatic epithelial cells between embryonic days 12 and 16. Finally, in vitro, VIP exposure increased the survival and proliferation of immature pancreatic cells, leading to an increase in the number of endocrine cells that will develop.

Therapeutic potential of dipeptidyl peptidase IV inhibitors for the treatment of type 2 diabetes

Incretins are peptide hormones, exemplified by glucose-dependent insulinotropic peptide and glucagon-like peptide 1 that are released from the gut in response to nutrient ingestion and enhance glucose-stimulated insulin secretion. Incretin action is terminated due to N-terminal cleavage of the peptides by the aminopeptidase dipeptidyl peptidase IV (DPP-IV). Hence, inhibition of glucose-dependent insulinotropic peptide and glucagon-like peptide 1 degradation via reduction of DPP-IV activity represents an innovative strategy for enhancing incretin action in vivo. This review summarises the biology of incretin action, the structure, expression and pleiotropic biological activities of DPP-IV and provides an overview of the rationale, potential merits and theoretical pitfalls in the development of DPP-IV inhibitors for the treatment of type 2 diabetes.

The multiple actions of GLP-1 on the process of glucose-stimulated insulin secretion

The physiological effects of glucagon-like peptide-1 (GLP-1) are of immense interest because of the potential clinical relevance of this peptide. Produced in intestinal L-cells through posttranslational processing of the proglucagon gene, GLP-1 is released from the gut in response to nutrient ingestion. Peripherally, GLP-1 is known to affect gut motility, inhibit gastric acid secretion, and inhibit glucagon secretion. In the central nervous system, GLP-1 induces satiety, leading to reduced weight gain. In the pancreas, GLP-1 is now known to induce expansion of insulin-secreting beta-cell mass, in addition to its most well-characterized effect: the augmentation of glucose-stimulated insulin secretion. GLP-1 is believed to enhance insulin secretion through mechanisms involving the regulation of ion channels (including ATP-sensitive K(+) channels, voltage-dependent Ca(2+) channels, voltage-dependent K(+) channels, and nonselective cation channels) and by the regulation of intracellular energy homeostasis and exocytosis. The present article will focus principally on the mechanisms proposed to underlie the glucose dependence of GLP-1's insulinotropic effect.

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 beta-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 beta-cell lines (betaTC-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 beta-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 Gbetagamma subunits nor Src tyrosine kinase, and we propose that cAMP-based regulation occurs via B-Raf in both CHO-K1 and beta-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 beta-cells.

Functional maturation of fetal porcine beta-cells by glucagon-like peptide 1 and cholecystokinin

Fetal beta-cells are immature in their responsiveness to glucose, and maturation occurs after oral feeding commences at birth. The incretin hormones glucagon-like peptide 1 (GLP-1) and cholecystokinin (CCK) are known to be released from the gut in response to oral feeding and enhance insulin secretion from pancreatic beta-cells. We hypothesized that these fetal beta-cells would mature in their glucose responsiveness if they were previously exposed to incretins. We exposed fetal pig islet-like cell clusters (ICCs) to 100 nM GLP-1, 5 micro M CCK, or 10 mM nicotinamide (NIC; a positive control) for 6 h and demonstrated 3- and 1.7-fold increases in glucose-induced insulin secretion for GLP-1 and CCK, respectively. This effect did not reach statistical significance if the ICCs were exposed to the incretins for 3 d. However, exposure for 4 d enhanced formation of beta-cells from undifferentiated cells, from 8 +/- 1% (controls) to 17 +/- 3% for GLP-1, 20 +/- 4% for CCK, and 15 +/- 1 for NIC (P < 0.001). ICCs exposed to GLP-1 for 3 d also showed a 1.9-fold increase in the intensity of PDX-1(+) cells, as assessed by semiquantitative fluorescent immunocytochemistry. Exposure of ICCs to incretins for 3 d did not show any increase in size of the islet clusters. ICCs exposed to either incretin as well as controls were transplanted into severe combined immunodeficient mice and examined at 1 and 2 months. We found a significant increase in the number of beta-cells in the GLP-1- and NIC-treated groups compared with the untreated controls or CCK. Perfusion of these grafts at 2 months showed that ICCs previously exposed to GLP-1, CCK, and NIC (but not controls), were functional and mature. In conclusion, GLP-1 and CCK have a dual effect on fetal pig ICCs, causing maturation of glucose-induced insulin secretion from beta-cells as well as enhancement of differentiation from undifferentiated precursors.

Long-term treatment with dipeptidyl peptidase IV inhibitor improves hepatic and peripheral insulin sensitivity in the VDF Zucker rat: a euglycemic-hyperinsulinemic clamp study

Upon release into circulation, the potent insulin secretagogues glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are rapidly cleaved and inactivated by the enzyme dipeptidyl peptidase IV (DP IV). Long-term administration of specific DP IV inhibitors, so as to enhance circulating active GIP and GLP-1 levels, has been shown to improve glucose tolerance and beta-cell glucose responsiveness and to reduce hyperinsulinemia in the Vancouver diabetic fatty (VDF) rat model of type 2 diabetes. Using the VDF model, the current study was undertaken to examine the effects of long-term DP IV inhibitor treatment on insulin sensitivity. Euglycemic-hyperinsulinemic clamps were performed on two sets of conscious VDF rats treated with or without the DP IV inhibitor P32/98 (20 mg. kg(-1). day(-1) for 12 weeks). The protocol consisted of three sequential 90-min periods with insulin infusion rates of 0, 5, and 15 mU. kg(-1). min(-1) and included a constant infusion of [ (3)H]glucose for measure of hepatic and peripheral insulin sensitivity. Relative to untreated littermates, the treated animals showed a left shift in the sensitivity of hepatic glucose output to insulin (average reduction approximately 6 micro mol. kg(-1). min(-1)) and a marked gain in peripheral responsiveness to insulin, with glucose disposal rates increasing 105 and 216% in response to the two insulin steps (versus 2 and 46% in controls). These results provide the first demonstration of improved hepatic and peripheral insulin sensitivity after DP IV inhibitor therapy, and coupled with apparent improvements in beta-cell function, they offer strong support for the utility of these compounds in the treatment of diabetes.

Insulinotropic hormone glucagon-like peptide-1 differentiation of human pancreatic islet-derived progenitor cells into insulin-producing cells

Glucagon-like peptide-1 (GLP-1) is an intestinal incretin hormone, derived from the processing of proglucagon, that exerts insulinotropic actions on insulin-producing pancreatic islet beta-cells. Recently GLP-1 was shown to stimulate the growth and differentiation (neogenesis) of beta-cells and appears to do so by inducing the expression of the homeodomain protein IDX-1 (islet duodenum homeobox-1; also known as PDX-1, pancreatic and duodenal homeobox gene; and as IPF-1, insulin promoter factor), which is required for pancreas development and the expression of beta-cell-specific genes. Earlier we identified multipotential progenitor cells in the islet and ducts of the pancreas, termed nestin-positive islet-derived progenitor cells (NIPs). Here we report the expression of functional GLP-1 receptors on NIPs and that GLP-1 stimulates the differentiation of NIPs into insulin-producing cells. Furthermore, confluent NIP cultures express the proglucagon gene and secrete GLP-1. These findings suggest a model of islet development in which pancreatic progenitor cells express both GLP-1 receptors and proglucagon with the formation of GLP-1. Locally produced GLP-1 may act as an autocrine/paracrine developmental morphogen on receptors on NIPs, resulting in the activation of IDX-1 and the expression of the proinsulin gene conferring a beta-cell phenotype. GLP-1 may be an important morphogen both for the embryonic development of the pancreas and for the neogenesis of beta-cells in the islets of the adult pancreas.

Alternatives to unmodified human islets for transplantation

Islet transplantation as a procedure to induce insulin independence is still a long way from benefitting the population of more than I million type I diabetic patients in the United States. In addition to the problems involved with immune suppression, the most significant obstacle is a scarcity of human organs for transplantation. In 1999, only 5882 donated pancreases were available, of which only 50% could be expected to produce islet yields suitable for clinical purposes. In this article, we review various sources with the potential to provide tissue for transplantation. These sources include islet and nonislet cells derived from both human and nonhuman sources, with an emphasis on human cells.