Combination treatment of db/db mice with exendin‐4 and gastrin preserves β‐cell mass by stimulating β‐cell growth and differentiation

Preservation of β‐cell mass is crucial for maintaining long‐term glucose homeostasis. Therapies based on incretin and its mimetics are expected to achieve this goal through various biological functions, particularly the restoration of β‐cell mass. Here we tested the effects of gastrin and exendin‐4 in type 2 diabetic animals.

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title: Role of apoptosis in failure of beta-cell mass compensation for insulin resistance and beta-cell defects in the male Zucker diabetic fatty rat.
abstract: To define the mechanisms involved in the evolution of diabetes in the Zucker diabetic fatty (ZDF) rat, beta-cell mass and replication rates were determined by immunochemistry, point-counting morphometry, and 6-h 5-bromo-2'-deoxyuridine (BrdU) incorporation. The beta-cell mass in 5- to 7-week-old prediabetic ZDF rats (4.3 +/- 0.06 mg) was similar to age-matched insulin-resistant Zucker fatty (ZF) rats (3.7 +/- 0.05 mg) and greater than that in Zucker lean control (ZLC) rats (1.9 +/- 0.3, P < 0.05). At 12 weeks (after diabetes onset), beta-cell mass in the ZDF rats (8.1 +/- 1.7 mg) was significantly lower than the ZF rats (15.7 +/- 1.8 mg). The mass in the ZF rats was significantly greater than in the ZLC rats (4.3 +/- 0.8 mg, P < 0.05). The beta-cell proliferation rate (mean of both time points) was significantly greater in the ZDF rats (0.88 +/- 0.1%) compared with the ZF and ZLC rats (0.53 +/- 0.07%, 0.62 +/- 0.07%, respectively, P < 0.05), yet ZDF rats have a lower beta-cell mass than the ZF rats despite a higher proliferative rate. Morphological evidence of neogenesis and apoptosis is evident in the ZF and ZDF rats. In addition, even at 5-7 weeks a modest defect in insulin secretion per beta-cell unit was found by pancreas perfusion. These studies provide evidence that the expansion of beta-cell mass in response to insulin resistance and insulin secretory defects in diabetic ZDF rats is inadequate. This failure of beta-cell mass expansion in the ZDF rat does not appear to be from a reduction in the rate of beta-cell proliferation or neogenesis, suggesting an increased rate of cell death by apoptosis.

title: Insulin resistance and insulin secretory dysfunction as precursors of non-insulin-dependent diabetes mellitus. Prospective studies of Pima Indians.
abstract: The relative roles of obesity, insulin resistance, insulin secretory dysfunction, and excess hepatic glucose production in the development of non-insulin-dependent diabetes mellitus (NIDDM) are controversial. We conducted a prospective study to determine which of these factors predicted the development of the disease in a group of Pima Indians.



A body-composition assessment, oral and intravenous glucose-tolerance tests, and a hyperinsulinemic--euglycemic clamp study were performed in 200 non-diabetic Pima Indians (87 women and 113 men; mean [+/- SD] age, 26 +/- 6 years). The subjects were followed yearly thereafter for an average of 5.3 years.



Diabetes developed in 38 subjects during follow-up. Obesity, insulin resistance (independent of obesity), and low acute plasma insulin response to intravenous glucose (with the degree of obesity and insulin resistance taken into account) were predictors of NIDDM: The six-year cumulative incidence of NIDDM was 39 percent in persons with values below the median for both insulin action and acute insulin response, 27 percent in those with values below the median for insulin action but above that for acute insulin response, 13 percent in those with values above the median for insulin action and below that for acute insulin response, and 0 in those with values originally above the median for both characteristics.



Insulin resistance is a major risk factor for the development of NIDDM: A low acute insulin response to glucose is an additional but weaker risk factor.

title: Beta-cell deficit and increased beta-cell apoptosis in humans with type 2 diabetes.
abstract: Type 2 diabetes is characterized by impaired insulin secretion. Some but not all studies suggest that a decrease in beta-cell mass contributes to this. We examined pancreatic tissue from 124 autopsies: 91 obese cases (BMI >27 kg/m(2); 41 with type 2 diabetes, 15 with impaired fasting glucose [IFG], and 35 nondiabetic subjects) and 33 lean cases (BMI <25 kg/m(2); 16 type 2 diabetic and 17 nondiabetic subjects). We measured relative beta-cell volume, frequency of beta-cell apoptosis and replication, and new islet formation from exocrine ducts (neogenesis). Relative beta-cell volume was increased in obese versus lean nondiabetic cases (P = 0.05) through the mechanism of increased neogenesis (P < 0.05). Obese humans with IFG and type 2 diabetes had a 40% (P < 0.05) and 63% (P < 0.01) deficit and lean cases of type 2 diabetes had a 41% deficit (P < 0.05) in relative beta-cell volume compared with nondiabetic obese and lean cases, respectively. The frequency of beta-cell replication was very low in all cases and no different among groups. Neogenesis, while increased with obesity, was comparable in obese type 2 diabetic, IFG, or nondiabetic subjects and in lean type 2 diabetic or nondiabetic subjects. However, the frequency of beta-cell apoptosis was increased 10-fold in lean and 3-fold in obese cases of type 2 diabetes compared with their respective nondiabetic control group (P < 0.05). We conclude that beta-cell mass is decreased in type 2 diabetes and that the mechanism underlying this is increased beta-cell apoptosis. Since the major defect leading to a decrease in beta-cell mass in type 2 diabetes is increased apoptosis, while new islet formation and beta-cell replication are normal, therapeutic approaches designed to arrest apoptosis could be a significant new development in the management of type 2 diabetes, because this approach might actually reverse the disease to a degree rather than just palliate glycemia.

title: Decreased beta-cell mass in diabetes: significance, mechanisms and therapeutic implications
abstract: Increasing evidence indicates that decreased functional beta-cell mass is the hallmark of both Type 1 and Type 2 diabetes. This underlies the absolute or relative insulin insufficiency in both conditions. In this For Debate, we consider the possible mechanisms responsible for beta-cell death and impaired function and their relative contribution to insulin insufficiency in diabetes. Beta-cell apoptosis and impaired proliferation consequent to hyperglycaemia is one pathway that could be operating in all forms of diabetes. Autoimmunity and other routes to beta-cell death are also considered. Recognition of decreased functional beta-cell mass and its overlapping multifactorial aetiology in diabetic states, leads us to propose a unifying classification of diabetes.

title: Brain glucagon-like peptide-1 increases insulin secretion and muscle insulin resistance to favor hepatic glycogen storage.
abstract: Intestinal glucagon-like peptide-1 (GLP-1) is a hormone released into the hepatoportal circulation that stimulates pancreatic insulin secretion. GLP-1 also acts as a neuropeptide to control food intake and cardiovascular functions, but its neural role in glucose homeostasis is unknown. We show that brain GLP-1 controlled whole-body glucose fate during hyperglycemic conditions. In mice undergoing a hyperglycemic hyperinsulinemic clamp, icv administration of the specific GLP-1 receptor antagonist exendin 9-39 (Ex9) increased muscle glucose utilization and glycogen content. This effect did not require muscle insulin action, as it also occurred in muscle insulin receptor KO mice. Conversely, icv infusion of the GLP-1 receptor agonist exendin 4 (Ex4) reduced insulin-stimulated muscle glucose utilization. In hyperglycemia achieved by i.v. infusion of glucose, icv Ex4, but not Ex9, caused a 4-fold increase in insulin secretion and enhanced liver glycogen storage. However, when glucose was infused intragastrically, icv Ex9 infusion lowered insulin secretion and hepatic glycogen levels, whereas no effects of icv Ex4 were observed. In diabetic mice fed a high-fat diet, a 1-month chronic i.p. Ex9 treatment improved glucose tolerance and fasting glycemia. Our data show that during hyperglycemia, brain GLP-1 inhibited muscle glucose utilization and increased insulin secretion to favor hepatic glycogen stores, preparing efficiently for the next fasting state.

title: Glucose competence of the hepatoportal vein sensor requires the presence of an activated glucagon-like peptide-1 receptor.
abstract: Activation of the hepatoportal glucose sensors by portal glucose infusion leads to increased glucose clearance and induction of hypoglycemia. Here, we investigated whether glucagon-like peptide-1 (GLP-1) could modulate the activity of these sensors. Mice were therefore infused with saline (S-mice) or glucose (P-mice) through the portal vein at a rate of 25 mg/kg. min. In P-mice, glucose clearance increased to 67.5 +/- 3.7 mg/kg. min as compared with 24.1 +/- 1.5 mg/kg. min in S-mice, and glycemia decreased from 5.0 +/- 0.1 to 3.3 +/- 0.1 mmol/l at the end of the 3-h infusion period. Coinfusion of GLP-1 with glucose into the portal vein at a rate of 5 pmol/kg. min (P-GLP-1 mice) did not increase the glucose clearance rate (57.4 +/- 5.0 ml/kg. min) and hypoglycemia (3.8 +/- 0.1 mmol/l) observed in P-mice. In contrast, coinfusion of glucose and the GLP-1 receptor antagonist exendin-(9-39) into the portal vein at a rate of 0.5 pmol/kg. min (P-Ex mice) reduced glucose clearance to 36.1 +/- 2.6 ml/kg. min and transiently increased glycemia to 9.2 +/- 0.3 mmol/l at 60 min of infusion before it returned to the fasting level (5.6 +/- 0.3 mmol/l) at 3 h. When glucose and exendin-(9-39) were infused through the portal and femoral veins, respectively, glucose clearance increased to 70.0 +/- 4.6 ml/kg. min and glycemia decreased to 3.1 +/- 0.1 mmol/l, indicating that exendin-(9-39) has an effect only when infused into the portal vein. Finally, portal vein infusion of glucose in GLP-1 receptor(-/-) mice failed to increase the glucose clearance rate (26.7 +/- 2.9 ml/kg. min). Glycemia increased to 8.5 +/- 0.5 mmol/l at 60 min and remained elevated until the end of the glucose infusion (8.2 +/- 0.4 mmol/l). Together, our data show that the GLP-1 receptor is part of the hepatoportal glucose sensor and that basal fasting levels of GLP-1 sufficiently activate the receptor to confer maximum glucose competence to the sensor. These data demonstrate an important extrapancreatic effect of GLP-1 in the control of glucose homeostasis.

title: Effects of exenatide (exendin-4) on glycemic control and weight over 30 weeks in metformin-treated patients with type 2 diabetes.
abstract: This study evaluates the ability of the incretin mimetic exenatide (exendin-4) to improve glycemic control in patients with type 2 diabetes failing to achieve glycemic control with maximally effective metformin doses.



A triple-blind, placebo-controlled, 30-week study at 82 U.S. sites was performed with 336 randomized patients. In all, 272 patients completed the study. The intent-to-treat population baseline was 53 +/- 10 years with BMI of 34.2 +/- 5.9 kg/m(2) and HbA(1c) of 8.2 +/- 1.1%. After 4 weeks of placebo, subjects self-administered 5 microg exenatide or placebo subcutaneously twice daily for 4 weeks followed by 5 or 10 microg exenatide, or placebo subcutaneously twice daily for 26 weeks. All subjects continued metformin therapy.



At week 30, HbA(1c) changes from baseline +/- SE for each group were -0.78 +/- 0.10% (10 microg), -0.40 +/- 0.11% (5 microg), and +0.08 +/- 0.10% (placebo; intent to treat; adjusted P < 0.002). Of evaluable subjects, 46% (10 microg), 32% (5 microg), and 13% (placebo) achieved HbA(1c) < or =7% (P < 0.01 vs. placebo). Exenatide-treated subjects displayed progressive dose-dependent weight loss (-2.8 +/- 0.5 kg [10 microg], -1.6 +/- 0.4 kg [5 microg]; P < 0.001 vs. placebo). The most frequent adverse events were gastrointestinal in nature and generally mild to moderate. Incidence of mild to moderate hypoglycemia was low and similar across treatment arms, with no severe hypoglycemia.



Exenatide was generally well tolerated and reduced HbA(1c) with no weight gain and no increased incidence of hypoglycemia in patients with type 2 diabetes failing to achieve glycemic control with metformin.

title: Alpha cell function in health and disease: influence of glucagon-like peptide-1
abstract: Although there is abundant evidence that hyperglucagonaemia plays a key role in the development of hyperglycaemia in type 2 diabetes, efforts to understand and correct this abnormality have been overshadowed by the emphasis on insulin secretion and action. However, recognition that the incretin hormone glucagon-like peptide-1 (GLP-1) exerts opposing effects on glucagon and insulin secretion has revived interest in glucagon, the neglected partner of insulin, in the bihormonal hypothesis. In healthy subjects, glucagon secretion is regulated by a variety of nutrient, neural and hormonal factors, the most important of which is glucose. The defect in alpha cell function that occurs in type 2 diabetes reflects impaired glucose sensing. GLP-1 inhibits glucagon secretion in vitro and in vivo in experimental animals, and suppresses glucagon release in a glucose-dependent manner in healthy subjects. This effect is also evident in diabetic patients, but GLP-1 does not inhibit glucagon release in response to hypoglycaemia, and may even enhance it. Early clinical studies with agents acting through GLP-1 signalling mechanisms (e.g. exenatide, liraglutide and vildagliptin) suggest that GLP-1 can improve alpha cell glucose sensing in patients with type 2 diabetes. Therapeutic approaches based around GLP-1 have the potential to improve both alpha cell and beta cell function, and could be of benefit in patients with a broad range of metabolic disorders.

title: Effects of intravenous glucagon-like peptide-1 on gastric emptying and intragastric distribution in healthy subjects: relationships with postprandial glycemic and insulinemic responses.
abstract: The inhibitory action of glucagon-like peptide-1 (GLP-1) on gastric emptying (GE) is likely to be important in mediating its effects on postprandial glycemia, appetite, and gastrointestinal symptoms.



The objective of the study was to evaluate the effects of "low" and "high" doses of iv GLP-1 on GE, intragastric meal distribution, glycemia, insulinemia, and appetite.



Ten healthy males were studied on 3 d. GE of a solid (ground beef)/liquid (glucose) meal, blood glucose, plasma insulin, glucagon and glucose-dependent insulinotropic peptide, appetite perceptions, and gastrointestinal symptoms were evaluated during iv infusion of: 1) GLP-1 at 0.3 pmol x kg(-1) x min(-1) (GLP-1 0.3); 2) GLP-1 at 0.9 pmol x kg(-1) x min(-1) (GLP-1 0.9); and 3) 0.9% saline.



GLP-1 0.3 and 0.9 slowed GE of solid (intragastric retention at t = 100 min; saline: 28 +/- 5%; GLP-1 0.3: 53 +/- 6%; GLP-1 0.9: 58 +/- 7%; P < 0.001) and liquid (time for 50% of the liquid to empty, saline: 28 +/- 2 min; GLP-1 0.3: 42 +/- 7 min; GLP-1 0.9: 50 +/- 9 min; P < 0.001). Both doses of GLP-1 induced gastroparesis in about half the cohort and increased meal retention in the distal stomach (P < 0.05). GLP-1 attenuated the rises in glucose, insulin, and glucose-dependent insulinotropic peptide (P < 0.05). There was an inverse relationship between blood glucose at t = 15 min and the time for 50% of the liquid to empty (r = -0.70, P < 0.001).



In healthy subjects exogenous GLP-1 increases meal retention in the distal stomach and, even when administered in a "low" dose, frequently induces "gastroparesis," and the effects of GLP-1 on postprandial glycemia are predictable on the basis of its effect on GE, supporting the concept that GE is a major target mechanism for the clinical use of incretin mimetics.

title: Glucagon-like peptide-1 mediates the therapeutic actions of DPP-IV inhibitors
abstract: Our opponents in this debate, Nauck and El-Ouaghlidi, have challenged the standard view that the therapeutic actions of the dipeptidyl peptidase-IV (DPP-IV) inhibitors are mediated by glucagon-like peptide-1 (GLP-1) . Based on the findings that exogenously administered GLP-1 is rapidly and extensively degraded by DPP-IV , and that this degradation appears to be mediated by DPP-IV , it was proposed that DPP-IV inhibitors could enhance the survival of intact, biologically active GLP-1 and thus be of use in the treatment of type 2 diabetes mellitus [2, 4]. Although the kinetic studies performed by Mentlein et al. clearly demonstrated that DPP-IVmay have substrates in addition toGLP1 [5, 6], the effect of this enzyme on GLP-1 was considered to be of particular importance because: (1) GLP-1 is extremely susceptible to rapid degradation by DPP-IV, as compared with the majority of other substrates; (2) DPP-IV degradation is the primary route of GLP-1 inactivation; (3) GLP-1 is perhaps the most potent and efficacious insulinotropic hormone in the body [7], whereas gastric inhibitory peptide (GIP), the other major incretin hormone, has no insulinotropic effects in patients with type 2 diabetes [8]; and (4) DPP-IV inhibitors have no metabolic effect in mice with a targeted deletion of the GLP-1 and GIP receptor genes . The authors of the previous article have listed five arguments that have caused them to doubt that GLP-1 is the only, or at least the major, mediator of the remarkable hypoglycaemic effects of DPP-IV inhibitors observed in clinical studies [10–12].Wewould certainly agree that there are other potential substrates for DPP-IV whose extended survival might contribute to the therapeutic effects of these inhibitors; nonetheless, we believe that the protection of GLP-1 against degradation is the major mechanism involved. In the remainder of this paper, we shall respond to each of the five arguments in turn.

title: Glucagon-like peptide-1 receptor signaling modulates beta cell apoptosis.
abstract: 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.

title: Glucagon-like peptide-1 treatment delays the onset of diabetes in 8 week-old db/db mice
abstract: AbstractAims/hypothesis. Glucagon-like peptide-1 ameliorates the symptoms of diabetes through stimulation of insulin secretion and enhancement of beta-cell mass. We have therefore investigated the effects of glucagon-like peptide-1 on the development of diabetes, using db/db mice as a model of Type II diabetes.
 Methods. The potent glucagon-like peptide-1 analogue Exendin-4 or vehicle (control) was administered (i.p.; 1 nmol/kg) to obese 6-week old db/db mice daily for 14 days (n=10).
 Results. By 8 weeks of age, control db/db mice developed hyperglycaemia (fasting: 10.4±0.5 mmol/l), hyperinsulinaemia and impaired glucose tolerance. However, Exendin-4 treatment prevented hyperglycaemia (fasting: 6.1±1.0 mmol/l, p<0.01), with reduced plasma insulin concentrations (p<0.001) and improved glucose tolerance (p<0.05). Peripheral insulin sensitivity was not affected. However, insulin release in vivo and in vitro from the perfused pancreas was improved by Exendin-4, as were pancreatic insulin concentrations (0.54±0.02 vs 0.32±0.01 µg/mg protein, p<0.05). These changes occurred in conjunction with increased beta-cell mass (3.01±0.31 vs 2.22±0.22 mg, p<0.05) and proliferation (BrdU+ beta-cells: 1.08±0.20 vs 0.47±0.11%, p<0.05), as well as decreased apoptosis (Tunel+ beta-cells: 0.37±0.06 vs 1.20±0.21%). Western blot demonstrated increased expression of Akt1 (by fivefold, p<0.01) and p44 MAP kinase (by sixfold, p<0.01), and decreased activation of caspase-3 (by 30%, p<0.05).
  Our results suggest that Ex4 treatment delays the onset of diabetes in 6–8 week old db/db mice, through a mechanism involving Akt1 and expansion of the functional beta-cell mass.

title: Exendin-4 stimulates both beta-cell replication and neogenesis, resulting in increased beta-cell mass and improved glucose tolerance in diabetic rats.
abstract: Diabetes is a disease of increasing prevalence in the general population and of unknown cause. Diabetes is manifested as hyperglycemia due to a relative deficiency of the production of insulin by the pancreatic beta-cells. One determinant in the development of diabetes is an inadequate mass of beta-cells, either absolute (type 1, juvenile diabetes) or relative (type 2, maturity-onset diabetes). Earlier, we reported that the intestinal hormone glucagon-like peptide I (GLP-I) effectively augments glucose-stimulated insulin secretion. Here we report that exendin-4, a long-acting GLP-I agonist, stimulates both the differentiation of beta-cells from ductal progenitor cells (neogenesis) and proliferation of beta-cells when administered to rats. In a partial pancreatectomy rat model of type 2 diabetes, the daily administration of exendin-4 for 10 days post-pancreatectomy attenuates the development of diabetes. We show that exendin-4 stimulates the regeneration of the pancreas and expansion of beta-cell mass by processes of both neogenesis and proliferation of beta-cells. Thus, GLP-I and analogs thereof hold promise as a novel therapy to stimulate beta-cell growth and differentiation when administered to diabetic individuals with reduced beta-cell mass.

title: Glucagon-like peptide 1 and exendin-4 convert pancreatic AR42J cells into glucagon- and insulin-producing cells.
abstract: In this article, we show that glucagon-like peptide 1 (GLP-1) can induce AR42J cells to differentiate into insulin, pancreatic polypeptide, and glucagon-positive cells. In their natural state, these cells, which are derived from a chemically induced pancreatic tumor, possess exocrine and neuroendocrine properties but are negative for islet hormones and their mRNAs. We found that when these cells were exposed to GLP-1 (1 or 10 nmol), a peptide normally released from the gut in response to food and a modulator of insulin release, intracellular cAMP levels were increased, and proliferation of cells was increased for the first 24 h, followed by inhibition. Up to 50% of the cells became positive for islet hormones. The mRNAs for glucose transporter 2 and glucokinase were detected in the GLP-1-treated cells. Insulin was detected by radioimmunoassay (RIA) in the medium of GLP-1-treated cells, and the cells were capable of releasing insulin in a glucose-mediated fashion. Exendin-4, an analog of GLP-1, in some critical experiments performed in a similar manner to GLP-1, with the exception of it being 10-fold more potent. We therefore propose that GLP-1 and exendin-4 are capable of causing pancreatic precursor cells to differentiate into islet cells.

title: Interplay of glucagon-like peptide-1 and transforming growth factor-beta signaling in insulin-positive differentiation of AR42J cells.
abstract: The differentiation of pancreatic exocrine AR42J cells into insulin-expressing endocrine cells has served as an important model for both endogenous in vivo beta-cell differentiation as well as potential application to beta-cell engineering of progenitor cells. Exogenous activin, possibly working through intracellular smad 2 and/or smad 3, as well as exogenous exendin-4 (a long-acting glucagon-like peptide-1 agonist) have both been shown to induce insulin-positive/endocrine differentiation in AR42J cells. In this study, we present evidence of significant interplay and interdependence of these two pathways as well as potential synergy between the pathways. In particular, insulin-positive differentiation seems to entail an exendin-4-induced drop in smad 2 and elevation in smad 3 in RNA levels. The latter appears to be dependent on endogenous transforming growth factor (TGF)-beta isoform release by the AR42J cells and may serve as a mechanism to promote beta-cell maturation. The drop in smad 2 may mediate early endocrine commitment. The coapplication of exogenous exendin-4 and, specifically, low-dose exogenous TGF-beta1 led to a dramatic 20-fold increase in insulin mRNA levels, supporting a novel synergistic and codependent relationship between exendin-4 signaling and TGF-beta isoform signaling.

title: Expansion of Pdx1-expressing pancreatic epithelium and islet neogenesis in transgenic mice overexpressing transforming growth factor alpha.
abstract: The progenitor cells responsible for transforming growth factor (TGF)-alpha-induced pancreatic ductal metaplasia and neoplasia remain uncharacterized. During pancreatic development, differentiated cell types arise from ductal progenitor cells expressing the Pdx1 homeodomain transcription factor. The aims of this study were, first, to evaluate the role of Pdx1-expressing stem cells in MT-TGFalpha transgenic mice, and second, to further characterize cell proliferation and differentiation in this model.



To assess Pdx1 gene expression in normal and metaplastic epithelium, we performed in vivo reporter gene analysis using heterozygous Pdx1(lacZ/+) and bigenic Pdx1(lacZ/+)/MT-TGFalpha mice.



Pdx1(lacZ/+)/MT-TGFalpha bigenics showed up-regulated Pdx1 expression in premalignant metaplastic ductal epithelium. In addition to Pdx1 gene activation, TGF-alpha-induced metaplastic epithelium demonstrated a pluripotent differentiation capacity, as evidenced by focal expression of Pax6 and initiation of islet cell neogenesis. The majority of Pdx1-positive epithelial cells showed no expression of insulin, similar to the pattern observed during embryonic development.



Overexpression of TGF-alpha induces expansion of a Pdx1-expressing epithelium characterized by focal expression of Pax6 and initiation of islet neogenesis. These findings suggest that premalignant events induced by TGF-alpha in mouse pancreas may recapitulate a developmental program active during embryogenesis.

title: Epidermal growth factor increases undifferentiated pancreatic embryonic cells in vitro: a balance between proliferation and differentiation.
abstract: During embryonic life, the development of a proper mass of mature pancreatic tissue is thought to require the proliferation of precursor cells, followed by their differentiation into endocrine or acinar cells. We investigated whether perturbing the proliferation of precursor cells in vitro could modify the final mass of endocrine tissue that develops. For that purpose, we used activators or inhibitors of signals mediated by receptor tyrosine kinases. We demonstrated that when embryonic day 13.5 rat pancreatic epithelium is cultured in the presence of PD98059, an inhibitor of the mitogen-activated protein (MAP) kinase, epithelial cell proliferation is decreased, whereas endocrine cell differentiation is activated. On the other hand, in the presence of epidermal growth factor (EGF), an activator of the MAP kinase pathway, the mass of tissue that develops is increased, whereas the absolute number of endocrine cells that develops is decreased. Under this last condition, a large number of epithelial cells proliferate but remain undifferentiated. In a second step, when EGF is removed from the pool of immature pancreatic epithelial cells, the cells differentiate en masse into insulin-expressing cells. The total number of insulin-expressing cells that develop can thus be increased by first activating the proliferation of immature epithelial cells with growth factors, thus allowing an increase in the pool of precursor cells, and next allowing their differentiation into endocrine cells by removing the growth factor. This strategy suggests a possible tissue engineering approach to expanding beta-cells.

title: Growth factor-mediated proliferation and differentiation of insulin-producing INS-1 and RINm5F cells: identification of betacellulin as a novel beta-cell mitogen.
abstract: It is not clear which growth factors are crucial for the survival, proliferation, and differentiation of pancreatic beta-cells. We used the relatively differentiated rat insulinoma cell line INS-1 to elucidate this issue. Responsiveness of the DNA synthesis of serum-starved cells was studied to a wide variety of growth factors. The most potent stimulators were PRL, GH, and betacellulin, a member of the epidermal growth factor (EGF) family that has not previously been shown to be mitogenic for beta-cells. In addition to these, only vascular endothelial growth factor, insulin-like growth factor-1 and -2, had significant mitogenic activity, whereas hepatocyte growth factor, nerve growth factor-beta, platelet-derived growth factors, basic fibroblast growth factor, EGF, transforming growth factor-alpha (TGF-alpha), neu differentiation factor, and TGF-beta were inactive. None of these factors affected the insulin content of INS-1 cells. In contrast, certain differentiation factors, including nicotinamide, sodium butyrate, activin A, and 1,25-dihydroxyvitamin D3 inhibited the DNA synthesis and increased the insulin content. Also all-trans-retinoic acid had an inhibitory effect on cell DNA synthesis but no effect on insulin content. From these findings betacellulin emerges as a novel growth factor for the beta-cell. Half-maximal stimulation of INS-1 DNA synthesis was obtained with 25 pM betacellulin. Interestingly, betacellulin had no effect on RINm5F cells, whereas both EGF and TGF-alpha were slightly mitogenic. These effects may possibly be explained by differential expression of the erbB receptor tyrosine kinases. In RINm5F cells a spectrum of erbB gene expression was detected (EGF receptor/erbB-1, erbB-2/neu, and erbB-3), whereas INS-1 cells showed only expression of EGF receptor. Expression of the erbB-4 gene was undetectable in these cell lines. In summary, our results suggest that the INS-1 cell line is a suitable model for the study of beta-cell growth and differentiation because the responses to previously identified beta-cell mitogens were essentially similar to those reported in primary cells. In addition, we have identified betacellulin as a possible modulator of beta-cell growth.

title: Recombinant human betacellulin promotes the neogenesis of beta-cells and ameliorates glucose intolerance in mice with diabetes induced by selective alloxan perfusion.
abstract: Betacellulin (BTC), a member of the epidermal growth factor family, is expressed predominantly in the human pancreas and induces the differentiation of a pancreatic acinar cell line (AR42J) into insulin-secreting cells, suggesting that BTC has a physiologically important role in the endocrine pancreas. In this study, we examined the in vivo effect of recombinant human BTC (rhBTC) on glucose intolerance and pancreatic morphology using a new mouse model with glucose intolerance induced by selective alloxan perfusion. RhBTC (1 microg/g body wt) or saline was injected subcutaneously every day from the day after alloxan treatment. The intraperitoneal glucose tolerance test revealed no difference between rhBTC-treated and rhBTC-untreated glucose-intolerant mice at 2-4 weeks. However, glucose tolerance was significantly improved and body weight was significantly increased in rhBTC-treated mice compared with untreated mice at 8 weeks. Islet-like cell clusters, consisting mainly of beta-cells, were increased in the pancreas and were localized in contact with the ductal lining cells and sometimes with acinar cells. In conclusion, administration of rhBTC improved glucose tolerance in this mouse model by increasing beta-cell volume, primarily through accelerated neogenesis from ductal lining cells.

title: Betacellulin and activin A coordinately convert amylase-secreting pancreatic AR42J cells into insulin-secreting cells.
abstract: Rat pancreatic AR42J cells possess exocrine and neuroendocrine properties. Activin A induces morphological changes and converts them into neuron-like cells. In activin-treated cells, mRNA for pancreatic polypeptide (PP) but not that for either insulin or glucagon was detected by reverse transcription-PCR. About 25% of the cells were stained by anti-PP antibody. When AR42J cells were incubated with betacellulin, a small portion of the cells were stained positively with antiinsulin and anti-PP antibodies. The effect of betacellulin was dose dependent, being maximal at 2 nM. Approximately 4% of the cells became insulin positive at this concentration, and mRNAs for insulin and PP were detected. When AR42J cells were incubated with a combination of betacellulin and activin A, approximately 10% of the cells became insulin positive. Morphologically, the insulin-positive cells were composed of two types of cells: neuron-like and round-shaped cells. Immunoreactive PP was found in the latter type of cells. The mRNAs for insulin, PP, glucose transporter 2, and glucokinase, but not glucagon, were detected. Depolarizing concentration of potassium, tolbutamide, carbachol, and glucagon-like peptide-1 stimulated the release of immunoreactive insulin. These results indicate that betacellulin and activin A convert amylase-secreting AR42J cells into cells secreting insulin. AR42J cells provide a model system to study the formation of pancreatic endocrine cells.

title: p38 MAPK Is Involved in Activin A- and Hepatocyte Growth Factor-mediated Expression of Pro-endocrine Gene Neurogenin 3 in AR42J-B13 Cells*
abstract: Neurogenin3 (ngn3) is a transcription factor that is essential for the differentiation of pancreatic endocrine cells. To investigate the signaling pathway that regulates ngn3 expression, we used AR42J-B13 cells as a model of the differentiation of pancreatic islets. In these cells, treatment with activin A and hepatocyte growth factor (HGF) induced the expression of ngn3. Reporter gene analysis using human ngn3 gene (NEUROG3) promoter fragments of various lengths identified the region between –402 and –327 bp of NEUROG3 as an activin A- and HGF-responsive DNA sequence. This DNA sequence normally functions as a repressor in AR42J-B13 cells, but treatment with activin A and HGF negates the repressor activity. Interestingly, function of the activin A- and HGF-responsive sequence was not influenced by the overexpression of the Smad inhibitory factor, Smad7. Instead, activin A and HGF activation was inhibited by overexpression of a dominant-negative mutant of transforming growth factor-β-activated kinase 1 (TAK1), or mitogen-activated protein kinase kinase 3 (MKK3), or by treatment with a p38 MAPK-specific inhibitor, SB203580. Activin A and HGF function through the TAK1-MKK3-p38 MAPK pathway to relieve transcription repressors located between –402 and –326 bp on the NEUROG3 promoter, and consequently activate ngn3 expression and endocrine differentiation of AR42J-B13 cells.

title: Combination therapy with epidermal growth factor and gastrin increases beta-cell mass and reverses hyperglycemia in diabetic NOD mice.
abstract: Combination therapy with epidermal growth factor (EGF) and gastrin induces beta-cell regeneration in rodents with chemically induced diabetes. We investigated whether EGF plus gastrin could correct hyperglycemia in NOD mice with autoimmune diabetes. Combined treatment with EGF (1 mug/kg) and gastrin (3 mug/kg) for 2 weeks restored normoglycemia after diabetes onset in NOD mice, whereas EGF or gastrin alone did not. Fasting blood glucose remained normal (3.5-6.5 mmol/l) or mildly elevated (<11 mmol/l) in five of six mice (83%) for 10 weeks after EGF plus gastrin treatment was stopped, whereas all mice treated with vehicle or EGF or gastrin alone became severely hyperglycemic (12-35 mmol/l). Pancreatic beta-cell mass was increased threefold and insulin content was increased eightfold in mice treated with EGF plus gastrin compared with pretreatment values. The correction of hyperglycemia correlated significantly with increases in pancreatic beta-cell mass and insulin content. In addition, splenic cells from mice treated with EGF plus gastrin delayed diabetes induction by adoptive transfer of diabetogenic cells into immunodeficient NOD-scid mice, suggesting the induction of immunoregulatory cells in NOD mice treated with EGF plus gastrin. We conclude that a short course of combined EGF and gastrin therapy increases pancreatic beta-cell mass and reverses hyperglycemia in acutely diabetic NOD mice; the impact of this combined therapy may result from the effects of EGF and gastrin on beta-cells, immune cells, or both.

title: Combined gastrin and epidermal growth factor treatment induces islet regeneration and restores normoglycaemia in C57Bl6/J mice treated with alloxan
abstract: beta-cell mass and/or function could restore glucose homeostasis in diabetes mellitus. Hitherto, trophic factors for beta-cell regeneration after toxic events have been difficult to identify. We evaluated the application of gastrin and epidermal growth factor after alloxan-induced pancreatic beta-cell damage.MethodsAfter alloxan treatment (70 mg/kg), mice were implanted with Alzet osmotic minipumps releasing gastrin and epidermal growth factor for one week. We monitored glycaemia, did histological analyses of the pancreata and quantified pancreatic beta-cell mass and insulin content.ResultsAlloxan treatment alone resulted in a persisting hyperglycaemic state. Combined gastrin and epidermal growth factor treatment restored normoglycaemia in 3 days, an effect which seemed permanent. Glucose tolerance tests showed normal glucose responsiveness. Gastrin on its own and epidermal growth factor on its own did not alleviate hyperglycaemia. Islet mass, islet density and pancreatic insulin content were higher in mice treated with gastrin and epidermal growth factor than in untreated mice with persisting hyperglycaemia. In normoglycaemic control mice treatment with gastrin and epidermal growth factor did not affect these parameters. We detected transitional cytokeratin-positive ductal to endocrine insulin-expressing cells and noted increased ductal but not beta-cell proliferation.interpretationOur results show that combined treatment with gastrin and epidermal growth factor can induce sufficient regeneration of a functional islet mass to restore glucose homeostasis.

title: Pharmacological treatment of chronic diabetes by stimulating pancreatic beta-cell regeneration with systemic co-administration of EGF and gastrin.
abstract: Transgenic expression of gastrin and EGF receptor ligands stimulates islet neogenesis in adult mice, significantly increasing islet mass. The present study aimed to determine whether pharmacological treatment with gastrin and EGF can significantly stimulate beta-cell regeneration in chronic, severe insulin-dependent diabetes. Diabetes was induced by intravenous streptozotocin, resulting in >95% beta cell destruction. Four weeks later, blood glucose levels were restored to normal range by exogenous insulin therapy and rats were treated with EGF/gastrin in combination, gastrin alone, or EGF alone given subcutaneously. After 14 days treatment blood glucose was significantly lower in the EGF/gastrin group compared to the untreated diabetic controls. Along with improved glucose tolerance, EGF/gastrin treatment significantly increased plasma C peptide and pancreatic insulin content compared to diabetic controls. Histological analysis showed that EGF/gastrin treatment significantly increased beta-cell mass as determined by point counting morphometrics. The EGF/gastrin group had a significantly greater number of BrdU labelled beta-cells/section consistent with stimulation of beta-cell replication or neogenesis. An increased number of gastrin receptor positive cells were observed in the EGF/gastrin-treated groups. In contrast to the effectiveness of the EGF/gastrin combination, neither gastrin nor EGF alone improved glucose tolerance in severely streptozotocin-diabetic rats. These studies indicate that physiologically significant improvement in glucose tolerance can be achieved through stimulating beta-cell regeneration with gastrin/EGF administered systemically as conventional pharmacological therapy.

title: Combination therapy with epidermal growth factor and gastrin induces neogenesis of human islet {beta}-cells from pancreatic duct cells and an increase in functional {beta}-cell mass.
abstract: Pancreatic islet transplantation is a viable treatment for type 1 diabetes, but is limited by human donor tissue availability. The combination of epidermal growth factor (EGF) and gastrin induces islet beta-cell neogenesis from pancreatic exocrine duct cells in rodents. In this study we investigated whether EGF and gastrin could expand the beta-cell mass in adult human isolated islets that contain duct as well as endocrine cells. Human islet cells were cultured for 4 wk in serum-free medium (control) or in medium with EGF (0.3 mug/ml), gastrin (1.0 mug/ml), or the combination of EGF and gastrin. beta-Cell numbers were increased in cultures with EGF plus gastrin (+118%) and with EGF (+81%), but not in cultures with gastrin (-3%) or control medium (-62%). After withdrawal of EGF and gastrin and an additional 4 wk in control medium, beta-cell numbers continued to increase only in cultures previously incubated with both EGF and gastrin (+232%). EGF plus gastrin also significantly increased cytokeratin 19-positive duct cells (+678%) in the cultures. Gastrin, alone or in combination with EGF, but not EGF alone, increased the expression of pancreatic and duodenal homeobox factor-1 as well as insulin and C peptide in the cytokeratin 19-positive duct cells. Also, EGF plus gastrin significantly increased beta-cells and insulin content in human islets implanted in immunodeficient nonobese diabetic-severe combined immune deficiency mice as well as insulin secretory responses of the human islet grafts to glucose challenge. In conclusion, combination therapy with EGF and gastrin increases beta-cell mass in adult human pancreatic islets in vitro and in vivo, and this appears to result from the induction of beta-cell neogenesis from pancreatic exocrine duct cells.

title: Combination therapy with glucagon-like peptide-1 and gastrin induces beta-cell neogenesis from pancreatic duct cells in human islets transplanted in immunodeficient diabetic mice.
abstract: Pancreatic islet transplantation as a treatment for type 1 diabetes is limited by human donor tissue availability. We investigated whether the beta-cell mass in human isolated islets could be expanded by treatments with glucagon-like peptide-1 (GLP-1) and gastrin, peptides reported to stimulate beta-cell growth in mice and rats with deficits in beta-cell mass. Human islets with low endocrine cell purity (7% beta-cells, 4% alpha-cells) and abundant exocrine cells (29% duct cells and 25% acinar cells) were implanted under the renal capsule of nonobese diabetic-severe combined immune deficiency (NOD-scid) mice made diabetic with streptozotocin. The mice were treated with GLP-1 and gastrin, separately and together, daily for 5 weeks. Blood glucose was significantly reduced only in mice implanted with human pancreatic cells and treated with GLP-1 plus gastrin. Correction of hyperglycemia was accompanied by increased insulin content in the human pancreatic cell grafts as well as by increased plasma levels of human C-peptide in the mice. Immunocytochemical examination revealed a fourfold increase in insulin-positive cells in the human pancreatic cell grafts in GLP-1 plus gastrin-treated mice, and most of this increase was accounted for by the appearance of cytokeratin 19-positive pancreatic duct cells expressing insulin. We conclude that combination therapy with GLP-1 and gastrin expands the beta-cell mass in human islets implanted in immunodeficient diabetic mice, largely from pancreatic duct cells associated with the islets, and this is sufficient to ameliorate hyperglycemia in the mice.

title: Glucagon-like peptide 1 induces pancreatic beta-cell proliferation via transactivation of the epidermal growth factor receptor.
abstract: We previously provided evidence that glucagon-like peptide 1 (GLP-1) induces pancreatic beta-cell growth nonadditively with glucose in a phosphatidylinositol (PI) 3-kinase- and protein kinase C zeta-dependent manner. However, the exact mechanism by which the GLP-1 receptor (GLP-1R), a member of the G protein-coupled receptor (GPCR) superfamily, activates the PI 3-kinase signaling pathway to promote beta-cell growth remains unknown. We hypothesized that the GLP-1R could activate PI 3-kinase and promote beta-cell proliferation through transactivation of the epidermal growth factor (EGF) receptor (EGFR), an event possibly linked to GPCRs via activation of c-Src and the production of putative endogenous EGF-like ligands. Both the c-Src inhibitor PP1 and the EGFR-specific inhibitor AG1478 blocked GLP-1-induced [(3)H]thymidine incorporation in INS(832/13) cells as well as in isolated rat islets, while only AG1478 inhibited the proliferative action of betacellulin (BTC), an EGFR agonist. Both compounds also suppressed GLP-1-induced PI 3-kinase activation. A time-dependent increase in tyrosine phosphorylation of the EGFR in response to GLP-1 was observed in INS(832/13) cells. This transactivation of the EGFR was sensitive to both the pharmacological agents PP1 and AG1478. The action of GLP-1 and BTC on INS cell proliferation was found to be not additive. Overexpression of a dominant-negative EGFR in INS cells with a retroviral expression vector curtailed GLP-1-induced beta-cell proliferation. GLP-1 treatment of INS cells caused a decrease in cell surface-associated BTC, as shown by FACS analysis. Also, the metalloproteinase inhibitor GM6001 and an anti-BTC neutralizing antibody suppressed the GLP-1 proliferative effect. Finally, coculturing the prostatic cancer cell line LNCaP that lacks GLP-1 responsiveness with INS cells increased LNCaP cell proliferation in the presence of GLP-1, thus revealing that INS cells secrete a growth factor in response to GLP-1. GM6001 and an anti-BTC neutralizing antibody suppressed increased LNCaP cell proliferation in the presence of GLP-1 in the coculture experiments. The results are consistent with a model in which GLP-1 increases PI 3-kinase activity and enhances beta-cell proliferation via transactivation of the EGFR that would require the proteolytic processing of membrane-anchored BTC or other EGF-like ligands.

title: Protein kinase Cdelta plays a non-redundant role in insulin secretion in pancreatic beta cells.
abstract: Protein kinase C (PKC) is considered to modulate glucose-stimulated insulin secretion. Pancreatic beta cells express multiple isoforms of PKCs; however, the role of each isoform in glucose-stimulated insulin secretion remains controversial. In this study we investigated the role of PKCdelta, a major isoform expressed in pancreatic beta cells on beta cell function. Here, we showed that PKCdelta null mice manifested glucose intolerance with impaired insulin secretion. Insulin tolerance test showed no decrease in insulin sensitivity in PKCdelta null mice. Studies using islets isolated from these mice demonstrated decreased glucose- and KCl-stimulated insulin secretion. Perifusion studies indicated that mainly the second phase of insulin secretion was decreased. On the other hand, glucose-induced influx of Ca2+ into beta cells was not altered. Immunohistochemistry using total internal reflection fluorescence microscopy and electron microscopic analysis showed an increased number of insulin granules close to the plasma membrane in beta cells of PKCdelta null mice. Although PKC is thought to phosphorylate Munc18-1 and facilitate soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors complex formation, the phosphorylation of Munc18-1 by glucose stimulation was decreased in islets of PKCdelta null mice. We conclude that PKCdelta plays a non-redundant role in glucose-stimulated insulin secretion. The impaired insulin secretion in PKCdelta null mice is associated with reduced phosphorylation of Munc18-1.

title: Beneficial effects of candesartan, an angiotensin II type 1 receptor blocker, on beta-cell function and morphology in db/db mice.
abstract: Several epidemiological studies suggested that treatment with angiotensin II type 1 receptor blocker (ARB) provided a risk reduction of developing type 2 diabetes. In this study, we investigated whether and how ARB treatment can improve abnormalities of pancreatic islets in diabetes state. We randomized db/db mice, a model of type 2 diabetes with obesity, at the age of 8 weeks to receive candesartan, an ARB, for 6 weeks. We also studied age-matched db/misty mice as control. Glucose tolerance test revealed that candesartan treatment improved glucose tolerance with the modest increase in serum insulin level in db/db mice. Concurrently, candesartan increased beta-cell mass, increased staining intensity of insulin, and decreased staining intensity of components of NAD(P)H oxidase, p22phox and gp91phox, and those of oxidative stress markers in beta-cells. These changes were accompanied by reduction of mitochondrial volume. Treatment with candesartan also reduced fibrosis in and around the islets and prevented the loss of endothelial cells in islets. Our results showed that candesartan partially prevented deterioration of glucose tolerance by providing protection against progressive beta-cell damage in diabetes.

title: Mitogenic effect of gastrin and expression of gastrin receptors in duct-like cells of rat pancreas.
abstract: Ductular metaplastic cells are observed during pancreas injury. Growth control by gastrin and expression of gastrin/cholecystokinin (CCK) B receptors were evaluated in these cells.



Acinoductal transdifferentiation was induced in vitro by culturing of acinar cells, and ductular metaplasia was obtained in vivo by ligation of the pancreatic ducts. Mitogenic effects of gastrin I on ductal complexes in vivo and of tetragastrin, pentagastrin, and gastrin I and II, with or without the CCK-B receptor antagonist L-365,260, on duct-like cells in vitro were analyzed by 5-bromo-2'-deoxyuridine labeling. Immunocytochemistry, Western blotting, and reverse-transcription polymerase chain reaction were applied for detection of the CCK-B receptor.



Gastrin analogues induced a mitogenic stimulus in the duct-like cells in vitro and in ductal complexes in duct-ligated rat pancreas. Immunocytochemistry showed expression of CCK-B receptors in these models and in fetal but not normal adult exocrine pancreas. Additionally, up-regulation of CCK-B receptors during ductular metaplasia was shown by Western blotting and reverse-transcription polymerase chain reaction.



Duct-like pancreatic epithelial cells in vitro and ductal complexes in vivo express gastrin/CCK-B receptors and proliferate in response to gastrin.

title: Exendin-4 is a high potency agonist and truncated exendin-(9-39)-amide an antagonist at the glucagon-like peptide 1-(7-36)-amide receptor of insulin-secreting beta-cells.
abstract: Exendin-4 purified from Heloderma suspectum venom shows structural relationship to the important incretin hormone glucagon-like peptide 1-(7-36)-amide (GLP-1). We demonstrate that exendin-4 and truncated exendin-(9-39)-amide specifically interact with the GLP-1 receptor on insulinoma-derived cells and on lung membranes. Exendin-4 displaced 125I-GLP-1, and unlabeled GLP-1 displaced 125I-exendin-4 from the binding site at rat insulinoma-derived RINm5F cells. Exendin-4 had, like GLP-1, a pronounced effect on intracellular cAMP generation, which was reduced by exendin-(9-39)-amide. When combined, GLP-1 and exendin-4 showed additive action on cAMP. They each competed with the radio-labeled version of the other peptide in cross-linking experiments. The apparent molecular mass of the respective ligand-binding protein complex was 63,000 Da. Exendin-(9-39)-amide abolished the cross-linking of both peptides. Exendin-4, like GLP-1, stimulated dose dependently the glucose-induced insulin secretion in isolated rat islets, and, in mouse insulinoma beta TC-1 cells, both peptides stimulated the proinsulin gene expression at the level of transcription. Exendin-(9-39)-amide reduced these effects. In conclusion, exendin-4 is an agonist and exendin-(9-39)-amide is a specific GLP-1 receptor antagonist.

title: Enhancing incretin action for the treatment of type 2 diabetes.
abstract: To examine the mechanisms of action, therapeutic potential, and challenges inherent in the use of incretin peptides and dipeptidyl peptidase-IV (DPP-IV) inhibitors for the treatment of type 2 diabetes.



The scientific literature describing the biological importance of incretin peptides and DPP-IV inhibitors in the control of glucose homeostasis has been reviewed, with an emphasis on mechanisms of action, experimental diabetes, human physiological experiments, and short-term clinical studies in normal and diabetic human subjects.



Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) exert important effects on beta-cells to stimulate glucose-dependent insulin secretion. Both peptides also regulate beta-cell proliferation and cytoprotection. GLP-1, but not GIP, inhibits gastric emptying, glucagon secretion, and food intake. The glucose-lowering actions of GLP-1, but not GIP, are preserved in subjects with type 2 diabetes. However, native GLP-1 is rapidly degraded by DPP-IV after parenteral administration; hence, degradation-resistant, long-acting GLP-1 receptor (GLP-1R) agonists are preferable agents for the chronic treatment of human diabetes. Alternatively, inhibition of DPP-IV-mediated incretin degradation represents a complementary therapeutic approach, as orally available DPP-IV inhibitors have been shown to lower glucose in experimental diabetic models and human subjects with type 2 diabetes.



GLP-1R agonists and DPP-IV inhibitors have shown promising results in clinical trials for the treatment of type 2 diabetes. The need for daily injections of potentially immunogenic GLP-1-derived peptides and the potential for unanticipated side effects with chronic use of DPP-IV inhibitors will require ongoing scrutiny of the risk-benefit ratio for these new therapies as they are evaluated in the clinic.

title: Exendin-4 uses Irs2 signaling to mediate pancreatic beta cell growth and function.
abstract: The insulin receptor substrate 2 (Irs2) branch of the insulin/insulin-like growth factor-signaling cascade prevents diabetes in mice because it promotes beta cell replication, function, and survival, especially during metabolic stress. Because exendin-4 (Ex4), a long acting glucagon-like peptide 1 receptor agonist, has similar effects upon beta cells in rodents and humans, we investigated whether Irs2 signaling was required for Ex4 action in isolated beta cells and in Irs2(-/-) mice. Ex4 increased cAMP levels in human islets and Min6 cells, which promoted Irs2 expression and stimulated Akt phosphorylation. In wild type mice Ex4 administered continuously for 28 days increased beta cell mass 2-fold. By contrast, Ex4 failed to arrest the progressive beta cell loss in Irs2(-/-) mice, which culminated in fatal diabetes; however, Ex4 delayed the progression of diabetes by 3 weeks by promoting insulin secretion from the remaining islets. We conclude that some short term therapeutic effects of glucagon-like peptide 1 receptor agonists can be independent of Irs2, but its long term effects upon beta cell growth and survival are mediated by the Irs2 branch of the insulin/insulin-like growth factor signaling cascade.

title: Changes in the expression of transcription factors in pancreatic AR42J cells during differentiation into insulin-producing cells.
abstract: Pancreatic AR42J cells possess both exocrine and neuroendocrine properties and convert to insulin-producing cells upon treatment with activin A and hepatocyte growth factor (HGF). We studied changes in the mRNA expression of various transcription factors during the course of differentiation. Among the transcription factors studied, expression levels of Pax4 and neurogenin3 changed significantly. These two factors were not detected in naive cells, whereas their mRNA levels were markedly increased after treatment with activin A and HGF. Thus, these two factors were induced by activin A. Transfection of Pax4 did not induce any changes in morphology or expression of pancreatic polypeptide (PP). Furthermore, introduction of antisense Pax4 did not affect the conversion into insulin-producing cells induced by activin A and HGF. In contrast, transfection of neurogenin3 induced morphological changes similar to those induced by activin A. In addition, transfection of neurogenin3 induced the expression of PP. Conversely, introduction of antisense neurogenin3 blocked the differentiation of AR42J cells induced by activin A and HGF. These results indicate that activin A regulates the expression of neurogenin3, which is critical for the differentiation of AR42J into endocrine cells.

title: Enhanced expression of PDX-1 and Ngn3 by exendin-4 during beta cell regeneration in STZ-treated mice.
abstract: Progenitor cells exist in the adult pancreas and transform to endocrine cells in pathological conditions. To address the mechanism of beta cell regeneration, mice were treated with streptozotocin (STZ group) or streptozotocin and exendin-4 (STZ + Ex-4 group), and the expression of PDX-1, Ngn3, insulin, IRS-2, and Foxo1 was investigated. PDX-1 mRNA was upregulated biphasically and induction of Ngn3 mRNA occurred shortly after the first increase of PDX-1 expression, a pattern similar to that observed during embryogenesis. PDX-1-positive cells appeared only in islet-like cell clusters (ICCs) in STZ group, but they appeared both in ducts and ICCs in STZ + Ex-4 group. Ngn3-positive cells emerged in ICCs but not in ducts. Therefore, regeneration seemed to occur mainly from intra-islet stem/progenitor cells. Exendin-4 upregulated PDX-1 expression which paralleled increased IRS-2 expression and translocation of Foxo1 from nucleus to cytoplasm. Further analysis of beta cell regeneration should help in the design of novel therapy for diabetes.

title: Disruption of IRS-2 causes type 2 diabetes in mice
abstract: Human type 2 diabetes is characterized by defects in both insulin action and insulin secretion. It has been difficult to identify a single molecular abnormality underlying these features. Insulin-receptor substrates (IRS proteins) may be involved in type 2 diabetes: they mediate pleiotropic signals initiated by receptors for insulin and other cytokines. Disruption of IRS-1 in mice retards growth, but diabetes does not develop because insulin secretion increases to compensate for the mild resistance to insulin,. Here we show that disruption of IRS-2 impairs both peripheral insulin signalling and pancreatic β-cell function. IRS-2-deficient mice show progressive deterioration of glucose homeostasis because of insulin resistance in the liver and skeletal muscle and a lack of β-cell compensation for this insulin resistance. Our results indicate that dysfunction of IRS-2 may contribute to the pathophysiology of human type 2 diabetes.

title: Glucokinase and IRS-2 are required for compensatory beta cell hyperplasia in response to high-fat diet-induced insulin resistance.
abstract: Glucokinase (Gck) functions as a glucose sensor for insulin secretion, and in mice fed standard chow, haploinsufficiency of beta cell-specific Gck (Gck(+/-)) causes impaired insulin secretion to glucose, although the animals have a normal beta cell mass. When fed a high-fat (HF) diet, wild-type mice showed marked beta cell hyperplasia, whereas Gck(+/-) mice demonstrated decreased beta cell replication and insufficient beta cell hyperplasia despite showing a similar degree of insulin resistance. DNA chip analysis revealed decreased insulin receptor substrate 2 (Irs2) expression in HF diet-fed Gck(+/-) mouse islets compared with wild-type islets. Western blot analyses confirmed upregulated Irs2 expression in the islets of HF diet-fed wild-type mice compared with those fed standard chow and reduced expression in HF diet-fed Gck(+/-) mice compared with those of HF diet-fed wild-type mice. HF diet-fed Irs2(+/-) mice failed to show a sufficient increase in beta cell mass, and overexpression of Irs2 in beta cells of HF diet-fed Gck(+/-) mice partially prevented diabetes by increasing beta cell mass. These results suggest that Gck and Irs2 are critical requirements for beta cell hyperplasia to occur in response to HF diet-induced insulin resistance.

title: Gastrin stimulates beta-cell neogenesis and increases islet mass from transdifferentiated but not from normal exocrine pancreas tissue.
abstract: It is still unclear which factors regulate pancreatic regeneration and beta-cell neogenesis and which precursor cells are involved. We evaluated the role of intravenously infused gastrin in regenerating pancreas of duct-ligated rats. The ligation of exocrine ducts draining the splenic half of the pancreas resulted in acinoductal transdifferentiation within the ligated part but not in the unligated part. We found that infusion of gastrin from day 7 to 10 postligation resulted in a doubling of the beta-cell mass in the ligated part as measured by morphometry. This increase in insulin-expressing cells was not associated with increased proliferation, hypertrophy, or reduced cell death of the beta-cells. Furthermore, we found an increased percentage of single, extra-insular beta-cells and small beta-cell clusters induced by gastrin infusion. These changes occurred only in the ligated part of the pancreas, where transdifferentiation of the exocrine acinar cells to ductlike cells (metaplasia) had occurred, and was not found in the normal unaffected pancreatic tissue. In conclusion, we demonstrate that administration of gastrin stimulates beta-cell neogenesis and expansion of the beta-cell mass from transdifferentiated exocrine pancreas.

title: Glucagon-like peptide (GLP)-2 reduces chemotherapy-associated mortality and enhances cell survival in cells expressing a transfected GLP-2 receptor.
abstract: Chemotherapeutic agents produce cytotoxicity via induction of apoptosis and cell cycle arrest. Rapidly proliferating cells in the bone marrow and intestinal crypts are highly susceptible to chemotherapy, and damage to these cellular compartments may preclude maximally effective chemotherapy administration. Glucagon-like peptide (GLP)-2 is an enteroendocrine-derived regulatory peptide that inhibits crypt cell apoptosis after administration of agents that damage the intestinal epithelium. We report here that a human degradation-resistant GLP-2 analogue, h[Gly2]-GLP-2 significantly improves survival, reduces bacteremia, attenuates epithelial injury, and inhibits crypt apoptosis in the murine gastrointestinal tract after administration of topoisomerase I inhibitor irinotecan hydrochloride or the antimetabolite 5-fluorouracil.  significantly improved survival and reduced weight loss but did not impair chemotherapy effectiveness in tumor-bearing mice treated with cyclical irinotecan. Furthermore, h[Gly2]-GLP-2 reduced chemotherapy-induced apoptosis, decreased activation of caspase-8 and -3, and inhibited poly(ADP-ribose) polymerase cleavage in heterologous cells transfected with the GLP-2 receptor. These observations demonstrate that the antiapoptotic effects of GLP-2 on intestinal crypt cells may be useful for the attenuation of chemotherapy-induced intestinal mucositis.

title: Regulation of rat pancreatic CCKB receptor and somatostatin expression by insulin.
abstract: The cholecystokinin B receptor (CCK B R) is localized on pancreatic endocrine somatostatin δ-cells. Pancreatic somatostatin content was increased in diabetic rats. The mechanisms involved in this phenomenon are unknown, and we believe insulin is involved. In this study, four groups of rats were used: controls, streptozotocin-induced diabetic, streptozotocin-induced diabetic with insulin, and streptozotocin-induced diabetic with insulin and its cessation. Rats were killed after 7–28 days of treatment for diabetes, and somatostatin mRNA expression and pancreatic somatostatin content, CCK B R mRNA and protein expression evaluation in total pancreas and purified islets, and the cellular localization of somatostatin and CCK B R in islets was measured. Data indicate that diabetes is established after 7 days, is controlled by insulin, and reappears after treatment cessation. Pancreatic somatostatin mRNA expression and somatostatin content were increased during diabetes, normalized during insulin treatment, and reaugmented after treatment cessation. Gland and islet CCK B R mRNA and protein almost disappeared during diabetes; CCK B mRNA reappeared in response to insulin, but the protein did not. Confocal microscopy confirmed data obtained on somatostatin and CCK B R as established biochemically in the course of the treatments. In conclusion, these data strongly suggest that insulin can negatively control pancreatic somatostatin mRNA and hormone content and positively control CCK B R mRNA; the CCK B R protein appears to be delayed.

title: Molecular mechanisms for the antiapoptotic action of gastrin.
abstract: Gastrin (G17) has a CCK-B receptor-mediated growth-promoting effect on the AR42J rat acinar cell line. We examined whether G17 inhibits apoptosis induced by serum withdrawal of AR42J cells and CHO-K1 cells stably expressing CCK-B receptors (CHO-K1/CCK-B cells). Cellular apoptosis was measured by flow cytometry and the terminal deoxynucleotidyltransferase-mediated dUTP-FITC nick end-labeling method. Serum withdrawal induced AR42J and CHO-K1/CCK-B cell apoptosis. Addition of 10 nM G17 reversed these effects. We examined the action of G17 (10 nM) on phosphorylation and activation of protein kinase B/Akt, a kinase known to promote cell survival. Akt phosphorylation and activation were measured by kinase assays and Western blots with an anti-phospho-Akt antibody. G17 stimulated Akt phosphorylation and activation. G17 induction of Akt phosphorylation was inhibited by the phosphoinositide 3-kinase (PI 3-kinase) inhibitors LY-294002 (10 microM) and wortmannin (200 nM) but not by the mitogen-activated protein kinase kinase 1 inhibitor PD-98059 (50 microM). To study the role of p38 kinase in G17 signaling to Akt, we examined the effect of G17 on p38 kinase activation and phosphorylation using kinase assays and Western blots with an anti-phospho-p38 kinase antibody. G17 induced p38 kinase activity at doses and with kinetics similar to those observed for Akt induction. The p38 kinase inhibitor SB-203580 inhibited G17 induction of Akt phosphorylation and activation at a concentration (10 microM) 10-fold higher than necessary to block p38 kinase (1 microM), suggesting the possible involvement of kinase activities other than p38 kinase. Transduction of AR42J cells with the adenoviral vector Adeno-dn Akt, which overexpresses an inhibitor of Akt, reversed the antiapoptotic action of G17. In conclusion, G17 promotes AR42J cell survival through the induction of Akt via PI 3-kinase and SB-203580-sensitive kinase activities.

title: Pancreatic gastrin stimulates islet differentiation of transforming growth factor alpha-induced ductular precursor cells.
abstract: Gastrin is transiently expressed in fetal islets during a critical period of their development from protodifferentiated islet precursors in fetal pancreatic ducts. To examine the possible role of gastrin as an islet cell growth factor, postnatal islet growth was studied in transgenic mice which overexpress gastrin and TGF alpha in their pancreas. Overexpression of a TGF alpha transgene causes metaplastic ductules containing numerous insulin expressing cells that resemble protodifferentiated precursors of the fetal pancreas. However, islet mass of the TGF alpha transgenic mice was not increased. Pancreatic overexpression of gastrin from a chimeric insulin/gastrin transgene transcribed from the insulin promoter markedly decreased the TGF alpha-stimulated increase in pancreatic duct mass. Furthermore, pancreatic coexpression of both gastrin and TGF alpha significantly increased islet mass in mice expressing both transgenes. These findings indicate that TGF alpha and gastrin can act synergistically to stimulate islet growth, although neither peptide alone is sufficient. Islet growth may possibly be stimulated through gastrin promoting the differentiation of insulin-positive cells in the TGF alpha-induced metaplastic ducts. This transgenic study suggests that islet neogenesis can be reactivated in the ductular epithelium of the adult pancreas by local expression of two growth factors, gastrin and TGF alpha.