A switch from prohormone convertase (PC)-2 to PC1/3 expression in transplanted alpha-cells is accompanied by differential processing of proglucagon and improved glucose homeostasis in mice

Pancreas-derived DPP4 is not essential for glucose homeostasis under metabolic stress

Mice systemically lacking dipeptidyl peptidase-4 (DPP4) have improved islet health, glucoregulation, and reduced obesity with high-fat diet (HFD) feeding compared to wild-type mice. Some, but not all, of this improvement can be linked to the loss of DPP4 in endothelial cells (ECs), pointing to the contribution of non-EC types. The importance of intra-islet signaling mediated by α to β cell communication is becoming increasingly clear; thus, our objective was to determine if β cell DPP4 regulates insulin secretion and glucose tolerance in HFD-fed mice by regulating the local concentrations of insulinotropic peptides. Using β cell double incretin receptor knockout mice, β cell- and pancreas-specific Dpp4-/- mice, we reveal that β cell incretin receptors are necessary for DPP4 inhibitor effects. However, although β cell DPP4 modestly contributes to high glucose (16.7 mM)-stimulated insulin secretion in isolated islets, it does not regulate whole-body glucose homeostasis.

Acute Inhibition of Intestinal Neprilysin Enhances Insulin Secretion via GLP-1 Receptor Signaling in Male Mice

The peptidase neprilysin modulates glucose homeostasis by cleaving and inactivating insulinotropic peptides, including some produced in the intestine such as glucagon-like peptide-1 (GLP-1). Under diabetic conditions, systemic or islet-selective inhibition of neprilysin enhances beta-cell function through GLP-1 receptor (GLP-1R) signaling. While neprilysin is expressed in intestine, its local contribution to modulation of beta-cell function remains unknown. We sought to determine whether acute selective pharmacological inhibition of intestinal neprilysin enhanced glucose-stimulated insulin secretion under physiological conditions, and whether this effect was mediated through GLP-1R. Lean chow-fed Glp1r+/+ and Glp1r-/- mice received a single oral low dose of the neprilysin inhibitor thiorphan or vehicle. To confirm selective intestinal neprilysin inhibition, neprilysin activity in plasma and intestine (ileum and colon) was assessed 40 minutes after thiorphan or vehicle administration. In a separate cohort of mice, an oral glucose tolerance test was performed 30 minutes after thiorphan or vehicle administration to assess glucose-stimulated insulin secretion. Systemic active GLP-1 levels were measured in plasma collected 10 minutes after glucose administration. In both Glp1r+/+ and Glp1r-/- mice, thiorphan inhibited neprilysin activity in ileum and colon without altering plasma neprilysin activity or active GLP-1 levels. Further, thiorphan significantly increased insulin secretion in Glp1r+/+ mice, whereas it did not change insulin secretion in Glp1r-/- mice. In conclusion, under physiological conditions, acute pharmacological inhibition of intestinal neprilysin increases glucose-stimulated insulin secretion in a GLP-1R-dependent manner. Since intestinal neprilysin modulates beta-cell function, strategies to inhibit its activity specifically in the intestine may improve beta-cell dysfunction in type 2 diabetes.

The Essential Role of Pancreatic α-Cells in Maternal Metabolic Adaptation to Pregnancy

Pancreatic α-cells are important in maintaining metabolic homeostasis, but their role in regulating maternal metabolic adaptations to pregnancy has not been studied. The objective of this study was to determine whether pancreatic α-cells respond to pregnancy and their contribution to maternal metabolic adaptation. With use of C57BL/6 mice, the findings of our study showed that pregnancy induced a significant increase of α-cell mass by promoting α-cell proliferation that was associated with a transitory increase of maternal serum glucagon concentration in early pregnancy. Maternal pancreatic GLP-1 content also was significantly increased during pregnancy. Using the inducible Cre/loxp technique, we ablated the α-cells (α-null) before and during pregnancy while maintaining enteroendocrine L-cells and serum GLP-1 in the normal range. In contrast to an improved glucose tolerance test (GTT) before pregnancy, significantly impaired GTT and remarkably higher serum glucose concentrations in the fed state were observed in α-null dams. Glucagon receptor antagonism treatment, however, did not affect measures of maternal glucose metabolism, indicating a dispensable role of glucagon receptor signaling in maternal glucose homeostasis. However, the GLP-1 receptor agonist improved insulin production and glucose metabolism of α-null dams. Furthermore, GLP-1 receptor antagonist Exendin (9-39) attenuated pregnancy-enhanced insulin secretion and GLP-1 restored glucose-induced insulin secretion of cultured islets from α-null dams. Together, these results demonstrate that α-cells play an essential role in controlling maternal metabolic adaptation to pregnancy by enhancing insulin secretion.

Neprilysin inhibition improves intravenous but not oral glucose-mediated insulin secretion via GLP-1R signaling in mice with -cell dysfunction

Type 2 diabetes is associated with the upregulation of neprilysin, a peptidase capable of cleaving glucoregulatory peptides such as glucagon-like peptide-1 (GLP-1). In humans, use of the neprilysin inhibitor sacubitril in combination with an angiotensin II receptor blocker was associated with increased plasma GLP-1 levels and improved glycemic control. Whether neprilysin inhibition per se is mediating these effects remains unknown. We sought to determine whether pharmacological neprilysin inhibition on its own confers beneficial effects on glycemic status and β-cell function in a mouse model of reduced insulin secretion, and whether any such effects are dependent on GLP-1 receptor (GLP-1R) signaling. High-fat-fed male wild-type (Glp1r^+/+) and GLP-1R knockout (Glp1r^-/-) mice were treated with low-dose streptozotocin (STZ) to recapitulate type 2 diabetes-associated β-cell dysfunction, or vehicle as control. Mice were continued on high-fat diet alone or supplemented with the neprilysin inhibitor sacubitril for 8 wk. At the end of the study period, β-cell function was assessed by oral or intravenous glucose-tolerance test. Fasting and fed glucose were significantly lower in wild-type mice treated with sacubitril, although active GLP-1 levels and insulin secretion during oral glucose challenge were unchanged. In contrast, insulin secretion in response to intravenous glucose was significantly enhanced in sacubitril-treated wild-type mice, and this effect was blunted in Glp1r^-/- mice. Similarly, sacubitril enhanced insulin secretion in vitro in islets from STZ-treated Glp1r^+/+ but not Glp1r^-/- mice. Together, our data suggest the insulinotropic effects of pharmacological neprilysin inhibition in a mouse model of β-cell dysfunction are mediated via intra-islet GLP-1R signaling.NEW & NOTEWORTHY The neprilysin inhibitor, sacubitril, improves glycemic status in a mouse model of reduced insulin secretion. Sacubitril enhances intravenous but not oral glucose-mediated insulin secretion. The increased glucose-mediated insulin secretion is GLP-1 receptor-dependent. Neprilysin inhibition does not raise postprandial circulating active GLP-1 levels.

Intra-islet glucagon confers β-cell glucose competence for first-phase insulin secretion and favors GLP-1R stimulation by exogenous glucagon

We report that intra-islet glucagon secreted from α-cells signals through β-cell glucagon and GLP-1 receptors (GcgR and GLP-1R), thereby conferring to rat islets their competence to exhibit first-phase glucose-stimulated insulin secretion (GSIS). Thus, in islets not treated with exogenous glucagon or GLP-1, first-phase GSIS is abolished by a GcgR antagonist (LY2786890) or a GLP-1R antagonist (Ex[9-39]). Mechanistically, glucose competence in response to intra-islet glucagon is conditional on β-cell cAMP signaling because it is blocked by the cAMP antagonist prodrug Rp-8-Br-cAMPS-pAB. In its role as a paracrine hormone, intra-islet glucagon binds with high affinity to the GcgR, while also exerting a "spillover" effect to bind with low affinity to the GLP-1R. This produces a right shift of the concentration-response relationship for the potentiation of GSIS by exogenous glucagon. Thus, 0.3 nM glucagon fails to potentiate GSIS, as expected if similar concentrations of intra-islet glucagon already occupy the GcgR. However, 10 to 30 nM glucagon effectively engages the β-cell GLP-1R to potentiate GSIS, an action blocked by Ex[9-39] but not LY2786890. Finally, we report that the action of intra-islet glucagon to support insulin secretion requires a step-wise increase of glucose concentration to trigger first-phase GSIS. It is not measurable when GSIS is stimulated by a gradient of increasing glucose concentrations, as occurs during an oral glucose tolerance test in vivo. Collectively, such findings are understandable if defective intra-islet glucagon action contributes to the characteristic loss of first-phase GSIS in an intravenous glucose tolerance test that is diagnostic of type 2 diabetes in the clinical setting.

Alpha-cell paracrine signaling in the regulation of beta-cell insulin secretion

As an incretin hormone, glucagon-like peptide 1 (GLP-1) lowers blood glucose levels by enhancing glucose-stimulated insulin secretion from pancreatic beta-cells. Therapies targeting the GLP-1 receptor (GLP-1R) use the classical incretin model as a physiological framework in which GLP-1 secreted from enteroendocrine L-cells acts on the beta-cell GLP-1R. However, this model has come into question, as evidence demonstrating local, intra-islet GLP-1 production has advanced the competing hypothesis that the incretin activity of GLP-1 may reflect paracrine signaling of GLP-1 from alpha-cells on GLP-1Rs on beta-cells. Additionally, recent studies suggest that alpha-cell-derived glucagon can serve as an additional, albeit less potent, ligand for the beta-cell GLP-1R, thereby expanding the role of alpha-cells beyond that of a counterregulatory cell type. Efforts to understand the role of the alpha-cell in the regulation of islet function have revealed both transcriptional and functional heterogeneity within the alpha-cell population. Further analysis of this heterogeneity suggests that functionally distinct alpha-cell subpopulations display alterations in islet hormone profile. Thus, the role of the alpha-cell in glucose homeostasis has evolved in recent years, such that alpha-cell to beta-cell communication now presents a critical axis regulating the functional capacity of beta-cells. Herein, we describe and integrate recent advances in our understanding of the impact of alpha-cell paracrine signaling on insulin secretory dynamics and how this intra-islet crosstalk more broadly contributes to whole-body glucose regulation in health and under metabolic stress. Moreover, we explore how these conceptual changes in our understanding of intra-islet GLP-1 biology may impact our understanding of the mechanisms of incretin-based therapeutics.

Mof acetyltransferase inhibition ameliorates glucose intolerance and islet dysfunction of type 2 diabetes via targeting pancreatic α-cells

BACKGROUND

Previously, we reported that Mof was highly expressed in α-cells, and its knockdown led to ameliorated fasting blood glucose (FBG) and glucose tolerance in non-diabetic mice, attributed by reduced total α-cell but enhanced prohormone convertase (PC)1/3-positive α-cell mass. However, how Mof and histone 4 lysine 16 acetylation (H4K16ac) control α-cell and whether Mof inhibition improves glucose handling in type 2 diabetes (T2DM) mice remain unknown.

METHODS

Mof overexpression and chromatin immunoprecipitation sequence (ChIP-seq) based on H4K16ac were applied to determine the effect of Mof on α-cell transcriptional factors and underlying mechanism. Then we administrated mg149 to α-TC1-6 cell line, wild type, db/db and diet-induced obesity (DIO) mice to observe the impact of Mof inhibition in vitro and in vivo. In vitro, western blotting and TUNEL staining were used to examine α-cell apoptosis and function. In vivo, glucose tolerance, hormone levels, islet population, α-cell ratio and the co-staining of glucagon and PC1/3 or PC2 were examined.

RESULTS

Mof activated α-cell-specific transcriptional network. ChIP-seq results indicated that H4K16ac targeted essential genes regulating α-cell differentiation and function. Mof activity inhibition in vitro caused impaired α-cell function and enhanced apoptosis. In vivo, it contributed to ameliorated glucose intolerance and islet dysfunction, characterized by decreased fasting glucagon and elevated post-challenge insulin levels in T2DM mice.

CONCLUSION

Mof regulates α-cell differentiation and function via acetylating H4K16ac and H4K16ac binding to Pax6 and Foxa2 promoters. Mof inhibition may be a potential interventional target for T2DM, which led to decreased α-cell ratio but increased PC1/3-positive α-cells.

Pathways of Glucagon Secretion and Trafficking in the Pancreatic Alpha Cell: Novel Pathways, Proteins, and Targets for Hyperglucagonemia

Patients with diabetes mellitus exhibit hyperglucagonemia, or excess glucagon secretion, which may be the underlying cause of the hyperglycemia of diabetes. Defective alpha cell secretory responses to glucose and paracrine effectors in both Type 1 and Type 2 diabetes may drive the development of hyperglucagonemia. Therefore, uncovering the mechanisms that regulate glucagon secretion from the pancreatic alpha cell is critical for developing improved treatments for diabetes. In this review, we focus on aspects of alpha cell biology for possible mechanisms for alpha cell dysfunction in diabetes: proglucagon processing, intrinsic and paracrine control of glucagon secretion, secretory granule dynamics, and alterations in intracellular trafficking. We explore possible clues gleaned from these studies in how inhibition of glucagon secretion can be targeted as a treatment for diabetes mellitus.

Altered islet prohormone processing: A cause or consequence of diabetes?

Peptide hormones are first produced as larger precursor prohormones that require endoproteolytic cleavage to liberate the mature hormones. A structurally conserved but functionally distinct family of nine prohormone convertase enzymes (PCs) are responsible for cleavage of protein precursors of which PC1/3 and PC2 are known to be exclusive to neuroendocrine cells and responsible for prohormone cleavage. Differential expression of PCs within tissues define prohormone processing; whereas glucagon is the major product liberated from proglucagon via PC2 in pancreatic α-cells, proglucagon is preferentially processed by PC1/3 in intestinal L cells to produce glucagon-like peptides 1 and 2 (GLP-1, GLP-2). Beyond our understanding of processing of islet prohormones in healthy islets, there is convincing evidence that proinsulin, proIAPP, and proglucagon processing is altered during prediabetes and diabetes. There is predictive value of elevated circulating proinsulin or proinsulin : C-peptide ratio for progression to type 2 diabetes and elevated proinsulin or proinsulin : C-peptide is predictive for development of type 1 diabetes in at risk groups. After onset of diabetes, patients have elevated circulating proinsulin and proIAPP and proinsulin may be an autoantigen in type 1 diabetes. Further, preclinical studies reveal that α-cells have altered proglucagon processing during diabetes leading to increased GLP-1 production. We conclude that despite strong associative data, current evidence is inconclusive on the potential causal role of impaired prohormone processing in diabetes, and suggest that future work should focus on resolving the question of whether altered prohormone processing is a causal driver or merely a consequence of diabetes pathology.

Evidence for the existence and potential roles of intra-islet glucagon-like peptide-1

Glucagon-Like Peptide-1 (GLP-1) is an important peptide hormone secreted by L-cells in the gastrointestinal tract in response to nutrients. It is produced by the differential cleavage of the proglucagon peptide. GLP-1 elicits a wide variety of physiological responses in many tissues that contribute to metabolic homeostasis. For these reasons, therapies designed to either increase endogenous GLP-1 levels or introduce exogenous peptide mimetics are now widely used in the management of diabetes. In addition to GLP-1 production from L-cells, recent reports suggest that pancreatic islet alpha cells may also synthesize and secrete GLP-1. Intra-islet GLP-1 may therefore play an unappreciated role in islet health and glucose regulation, suggesting a potential functional paracrine role for islet-derived GLP-1. In this review, we assess the current literature from an islet-centric point-of-view to better understand the production, degradation, and actions of GLP-1 within the endocrine pancreas in rodents and humans. The relevance of intra-islet GLP-1 in human physiology is discussed regarding the potential role of intra-islet GLP-1 in islet health and dysfunction.

GLP-1 receptor signaling increases PCSK1 and β cell features in human α cells

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that potentiates glucose-stimulated insulin secretion. GLP-1 is classically produced by gut L cells; however, under certain circumstances α cells can express the prohormone convertase required for proglucagon processing to GLP-1, prohormone convertase 1/3 (PC1/3), and can produce GLP-1. However, the mechanisms through which this occurs are poorly defined. Understanding the mechanisms by which α cell PC1/3 expression can be activated may reveal new targets for diabetes treatment. Here, we demonstrate that the GLP-1 receptor (GLP-1R) agonist, liraglutide, increased α cell GLP-1 expression in a β cell GLP-1R-dependent manner. We demonstrate that this effect of liraglutide was translationally relevant in human islets through application of a new scRNA-seq technology, DART-Seq. We found that the effect of liraglutide to increase α cell PC1/3 mRNA expression occurred in a subcluster of α cells and was associated with increased expression of other β cell-like genes, which we confirmed by IHC. Finally, we found that the effect of liraglutide to increase bihormonal insulin+ glucagon+ cells was mediated by the β cell GLP-1R in mice. Together, our data validate a high-sensitivity method for scRNA-seq in human islets and identify a potentially novel GLP-1-mediated pathway regulating human α cell function.

Protein restriction during pregnancy impairs intra-islet GLP-1 and the expansion of β-cell mass

We evaluated whether protein restriction during pregnancy alters the morphometry of pancreatic islets, the intra-islet glucagon-like peptide-1 (GLP-1) production, and the anti-apoptotic signalling pathway modulated by GLP-1. Control non-pregnant (CNP) and control pregnant (CP) rats were fed a 17% protein diet, and low-protein non-pregnant (LPNP) and low-protein pregnant (LPP) groups were fed a 6% protein diet. The masses of islets and β-cells were similar in the LPNP group and the CNP group but were higher in the CP group than in the CNP group and were equal in the LPP group and the LPNP group. Both variables were lower in the LPP group than in the CP group. Prohormone convertase 2 and GLP-1 fluorescence in α-cells was lower in the low-protein groups than in the control groups. The least PC2/glucagon colocalization was observed in the LPP group, and the most was observed in the CP group. There was less prohormone convertase 1/3/glucagon colocalization in the LPP group than in the CP group. GLP-1/glucagon colocalization was similar in the LPP, CP and CNP groups, which showed less GLP-1/glucagon colocalization than the LPNP group. The mRNA Pka, Creb and Pdx-1 contents were higher in islets from pregnant rats than in islets from non-pregnant rats. Protein restriction during pregnancy impaired the mass of β-cells and the intra-islet GLP-1 production but did not interfere with the transcription of genes of the anti-apoptotic signalling pathway modulated by GLP-1.

Revisiting Proinsulin Processing: Evidence That Human β-Cells Process Proinsulin With Prohormone Convertase (PC) 1/3 but Not PC2

Insulin is first produced in pancreatic β-cells as the precursor prohormone proinsulin. Defective proinsulin processing has been implicated in the pathogenesis of both type 1 and type 2 diabetes. Though there is substantial evidence that mouse β-cells process proinsulin using prohormone convertase 1/3 (PC1/3) and then prohormone convertase 2 (PC2), this finding has not been verified in human β-cells. Immunofluorescence with validated antibodies revealed that there was no detectable PC2 immunoreactivity in human β-cells and little PCSK2 mRNA by in situ hybridization. Similarly, rat β-cells were not immunoreactive for PC2. In all histological experiments, PC2 immunoreactivity in neighboring α-cells acted as a positive control. In donors with type 2 diabetes, β-cells had elevated PC2 immunoreactivity, suggesting that aberrant PC2 expression may contribute to impaired proinsulin processing in β-cells of patients with diabetes. To support histological findings using a biochemical approach, human islets were used for pulse-chase experiments. Despite inhibition of PC2 function by temperature blockade, brefeldin A, chloroquine, and multiple inhibitors that blocked production of mature glucagon from proglucagon, β-cells retained the ability to produce mature insulin. Conversely, suppression of PC1/3 blocked processing of proinsulin but not proglucagon. By demonstrating that healthy human β-cells process proinsulin by PC1/3 but not PC2, we suggest that there is a need to revise the long-standing theory of proinsulin processing.

Glucagon Receptor Signaling and Glucagon Resistance

Hundred years after the discovery of glucagon, its biology remains enigmatic. Accurate measurement of glucagon has been essential for uncovering its pathological hypersecretion that underlies various metabolic diseases including not only diabetes and liver diseases but also cancers (glucagonomas). The suggested key role of glucagon in the development of diabetes has been termed the bihormonal hypothesis. However, studying tissue-specific knockout of the glucagon receptor has revealed that the physiological role of glucagon may extend beyond blood-glucose regulation. Decades ago, animal and human studies reported an important role of glucagon in amino acid metabolism through ureagenesis. Using modern technologies such as metabolomic profiling, knowledge about the effects of glucagon on amino acid metabolism has been expanded and the mechanisms involved further delineated. Glucagon receptor antagonists have indirectly put focus on glucagon's potential role in lipid metabolism, as individuals treated with these antagonists showed dyslipidemia and increased hepatic fat. One emerging field in glucagon biology now seems to include the concept of hepatic glucagon resistance. Here, we discuss the roles of glucagon in glucose homeostasis, amino acid metabolism, and lipid metabolism and present speculations on the molecular pathways causing and associating with postulated hepatic glucagon resistance.

Physiology and Pharmacology of DPP-4 in Glucose Homeostasis and the Treatment of Type 2 Diabetes

Dipeptidyl peptidase-4 (DPP-4), also known as the T-cell antigen CD26, is a multi-functional protein which, besides its catalytic activity, also functions as a binding protein and a ligand for a variety of extracellular molecules. It is an integral membrane protein expressed on cells throughout the body, but is also shed from the membrane and circulates as a soluble protein in the plasma. A large number of bioactive molecules can be cleaved by DPP-4 in vitro, but only a few of these have been demonstrated to be physiological substrates. One of these is the incretin hormone, glucagon-like peptide-1 (GLP-1), which plays an important role in the maintenance of normal glucose homeostasis, and DPP-4 has been shown to be the key enzyme regulating its biological activity. This pathway has been targeted pharmacologically through the development of DPP-4 inhibitors, and these are now a successful class of anti-hyperglycaemic agents used to treat type 2 diabetes (T2DM). DPP-4 may additionally influence metabolic control via its proteolytic effect on other regulatory peptides, but it has also been reported to affect insulin sensitivity, potentially mediated through its non-enzymatic interactions with other membrane proteins. Given that altered expression and activity of DPP-4 are associated with increasing body mass index and hyperglycaemia, DPP-4 has been proposed to play a role in linking obesity and the pathogenesis of T2DM by functioning as a local mediator of inflammation and insulin resistance in adipose and hepatic tissue. As well as these broader systemic effects, it has also been suggested that DPP-4 may be able to modulate β-cell function as part of a paracrine system involving GLP-1 produced locally within the pancreatic islets. However, while it is evident that DPP-4 has the potential to influence glycaemic control, its overall significance for the normal physiological regulation of glucose homeostasis in humans and its role in the pathogenesis of metabolic disease remain to be established.

Chronic Exposure to Palmitate Impairs Insulin Signaling in an Intestinal L-cell Line: A Possible Shift from GLP-1 to Glucagon Production

Obesity and type 2 diabetes mellitus (T2DM) are characterized by insulin resistance and impaired glucagon-like peptide-1 (GLP-1) secretion/function. Lipotoxicity, a chronic elevation of free fatty acids in the blood, could affect insulin-signaling in many peripheral tissues. To date, the effects of lipotoxicity on the insulin receptor and insulin resistance in the intestinal L-cells need to be elucidated. Moreover, recent observations indicate that L-cells may be able to process not only GLP-1 but also glucagon from proglucagon. The aim of this study was to investigate the effects of chronic palmitate exposure on insulin pathways, GLP-1 secretion and glucagon synthesis in the GLUTag L-cell line. Cells were cultured in the presence/absence of palmitate (0.5 mM) for 24 h to mimic lipotoxicity. Palmitate treatment affected insulin-stimulated GLP-1 secretion, insulin receptor phosphorylation and IRS-1-AKT pathway signaling. In our model lipotoxicity induced extracellular signal-regulated kinase (ERK 44/42) activation both in insulin stimulated and basal conditions and also up-regulated paired box 6 (PAX6) and proglucagon expression (Gcg). Interestingly, palmitate treatment caused an increased glucagon secretion through the up-regulation of prohormone convertase 2. These results indicate that a state of insulin resistance could be responsible for secretory alterations in L-cells through the impairment of insulin-signaling pathways. Our data support the hypothesis that lipotoxicity might contribute to L-cell deregulation.

COMPARISON OF THE LEVELS OF C-REACTIVE PROTEIN, GLP-1 AND GLP-2 AMONG INDIVIDUALS WITH DIABETES, MORBID OBESITY AND HEALTHY CONTROLS: AN EXPLORATORY STUDY

BACKGROUND

The glucagon-like peptides 1 and 2 (GLP-1/GLP-2) are gut hormones that may directly affect the glucose homeostasis and their activity seems to be significantly affected by chronic inflammation.

OBJECTIVE

To evaluate the postprandial levels of glucagon-like peptides 1 and 2 (GLP-1/GLP-2), C-reactive protein (CRP), and the postprandial glucose and insulin levels among individuals with obesity, type 2 diabetes, and healthy controls.

METHODS

An exploratory cross-sectional study, which involved individuals awaiting for bariatric/metabolic surgery and healthy controls. Postprandial levels of GLP-1, GLP-2, glucose, and insulin were obtained after a standard meal tolerance test. Inflammation was assessed by means of CRP.

RESULTS

There were 30 individuals enrolled in the study, divided into three groups: non-diabetic with morbid obesity (NDO; n=11 individuals), diabetic with mild obesity (T2D; n=12 individuals), and healthy controls (C; n=7 individuals). The mean CRP levels were significantly higher in the NDO group (6.6±4.7 mg/dL) than in the T2D (3.3±2.2 mg/dL) and C groups (2.5±3.2 mg/dL) (P=0.038). The GLP-1 levels following standard meal tolerance test and the area under the curve of GLP-1 did not differ among the three groups. The GLP-2 levels were significantly lower in the NDO and T2D than in the C group following standard meal tolerance test at all the times evaluated. The area under the curve of the GLP-2 was significantly lower in the NDO and T2D groups than in the C group (P=0.05 and P=0.01, respectively).

CONCLUSION

GLP-2 levels were impaired in the individuals with obesity and diabetes. This mechanism seems to be enrolled in preventing the worsening of the glucose homeostasis in these individuals.

17-β Estradiol regulates proglucagon-derived peptide secretion in mouse and human α- and L cells

Clinical and experimental data indicate a beneficial effect of estrogens on energy and glucose homeostasis associated with improved insulin sensitivity and positive effects on insulin secretion. The aim of the study was to investigate the impact of estrogens on proglucagon-producing cells, pancreatic α cells, and enteroendocrine L cells. The consequences of sexual hormone deprivation were evaluated in ovariectomized mice (ovx). Ovx mice exhibited impaired glucose tolerance during oral glucose tolerance tests (OGTT), which was associated with decreased GLP-1 intestinal and pancreatic secretion and content, an effect that was reversed by estradiol (E2) treatment. Indeed, E2 increased oral glucose-induced GLP-1 secretion in vivo and GLP-1 secretion from primary culture of mouse and human α cells through the activation of all 3 estrogen receptors (ERs), whereas E2-induced GLP-1 secretion from mouse and human intestinal explants occurred only by ERβ activation. Underlying the implication of ERβ, its selective agonist WAY20070 was able to restore glucose tolerance in ovx mice at least partly through plasma GLP-1 increase. We conclude that E2 directly controls both α- and L cells to increase GLP-1 secretion, in addition to its effects on insulin and glucagon secretion, highlighting the potential beneficial role of the estrogenic pathway and, more particularly, of ERβ agonists to prevent type 2 diabetes.

Abnormal CFTR Affects Glucagon Production by Islet α Cells in Cystic Fibrosis and Polycystic Ovarian Syndrome

Glucagon, produced by islet α cells, functions to increase blood glucose. Abnormal glucose levels are often seen in cystic fibrosis (CF), a systematic disease caused by mutations of the CF transmembrane conductance regulator (CFTR), and in polycystic ovarian syndrome (PCOS), an endocrine disorder featured with hyperandrogenism affecting 5-10% women of reproductive age. Here, we explored the role of CFTR in glucagon production in α cells and its possible contribution to glucagon disturbance in CF and PCOS. We found elevated fasting glucagon levels in CFTR mutant (DF508) mice compared to the wildtypes. Glucagon and prohormone convertase 2 (PC2) were also upregulated in CFTR inhibitor-treated or DF508 islets, as compared to the controls or wildtypes, respectively. Dihydrotestosterone (DHT)-induced PCOS rats exhibited significantly lower fasting glucagon levels with higher CFTR expression in α cells compared to that of controls. Treatment of mouse islets or αTC1-9 cells with DHT enhanced CFTR expression and reduced the levels of glucagon and PC2. The inhibitory effect of DHT on glucagon production was blocked by CFTR inhibitors in mouse islets, and mimicked by overexpressing CFTR in αTC1-9 cells with reduced phosphorylation of the cAMP/Ca2+ response element binding protein (p-CREB), a key transcription factor for glucagon and PC2. These results revealed a previously undefined role of CFTR in suppressing glucagon production in α-cells, defects in which may contribute to glucose metabolic disorder seen in CF and PCOS.

Intra-islet glucagon-like peptide 1

PURPOSE

Glucagon-like peptide-1 (GLP-1) is originally identified in the gut as an incretin hormone, and it is potent in stimulating insulin secretion in the pancreas. However, increasing evidence suggests that GLP-1 is also produced locally within pancreatic islets. This review focuses on the past and current discoveries regarding intra-islet GLP-1 production and its functions.

MAIN FINDINGS

There has been a long-standing debate with regard to whether GLP-1 is produced in the pancreatic α cells. Early controversies lead to the widely accepted conclusion that the vast majority of proglucagon is processed to form glucagon in the pancreas, whereas an insignificant amount is cleaved to produce GLP-1. With technological advancements, recent studies have shown that bioactive GLP-1 is produced locally in the pancreas, and the expression and secretion of GLP-1 within islets are regulated by various factors such as cytokines, hyperglycemia, and β cell injury.

CONCLUSIONS

GLP-1 is produced by the pancreatic α cells, and it is fully functional as an incretin. Therefore, intra-islet GLP-1 may exert insulinotropic and glucagonostatic effects locally via paracrine and/or autocrine actions, under both normal and diabetic conditions.

GLP2: an underestimated signal for improving glycaemic control and insulin sensitivity

Glucagon-like peptide 2 (GLP2) is a proglucagon-derived peptide produced by intestinal enteroendocrine L-cells and by a discrete population of neurons in the brainstem, which projects mainly to the hypothalamus. The main biological actions of GLP2 are related to the regulation of energy absorption and maintenance of mucosal morphology, function and integrity of the intestine; however, recent experimental data suggest that GLP2 exerts beneficial effects on glucose metabolism, especially in conditions related to increased uptake of energy, such as obesity, at least in the animal model. Indeed, mice lacking GLP2 receptor selectively in hypothalamic neurons that express proopiomelanocortin show impaired postprandial glucose tolerance and hepatic insulin resistance (by increased gluconeogenesis). Moreover, GLP2 acts as a beneficial factor for glucose metabolism in mice with high-fat diet-induced obesity. Thus, the aim of this review is to update and summarize current knowledge about the role of GLP2 in the control of glucose homeostasis and to discuss how this molecule could exert protective effects against the onset of related obesity type 2 diabetes.

Activation of Transmembrane Bile Acid Receptor TGR5 Modulates Pancreatic Islet α Cells to Promote Glucose Homeostasis

The physiological role of the TGR5 receptor in the pancreas is not fully understood. We previously showed that activation of TGR5 in pancreatic β cells by bile acids induces insulin secretion. Glucagon released from pancreatic α cells and glucagon-like peptide 1 (GLP-1) released from intestinal L cells regulate insulin secretion. Both glucagon and GLP-1 are derived from alternate splicing of a common precursor, proglucagon by PC2 and PC1, respectively. We investigated whether TGR5 activation in pancreatic α cells enhances hyperglycemia-induced PC1 expression thereby releasing GLP-1, which in turn increases β cell mass and function in a paracrine manner. TGR5 activation augmented a hyperglycemia-induced switch from glucagon to GLP-1 synthesis in human and mouse islet α cells by GS/cAMP/PKA/cAMP-response element-binding protein-dependent activation of PC1. Furthermore, TGR5-induced GLP-1 release from α cells was via an Epac-mediated PKA-independent mechanism. Administration of the TGR5 agonist, INT-777, to db/db mice attenuated the increase in body weight and improved glucose tolerance and insulin sensitivity. INT-777 augmented PC1 expression in α cells and stimulated GLP-1 release from islets of db/db mice compared with control. INT-777 also increased pancreatic β cell proliferation and insulin synthesis. The effect of TGR5-mediated GLP-1 from α cells on insulin release from islets could be blocked by GLP-1 receptor antagonist. These results suggest that TGR5 activation mediates cross-talk between α and β cells by switching from glucagon to GLP-1 to restore β cell mass and function under hyperglycemic conditions. Thus, INT-777-mediated TGR5 activation could be leveraged as a novel way to treat type 2 diabetes mellitus.

Neurotransmitters and Neuropeptides: New Players in the Control of Islet of Langerhans' Cell Mass and Function

Islets of Langerhans control whole body glucose homeostasis, as they respond, releasing hormones, to changes in nutrient concentrations in the blood stream. The regulation of hormone secretion has been the focus of attention for a long time because it is related to many metabolic disorders, including diabetes mellitus. Endocrine cells of the islet use a sophisticate system of endocrine, paracrine and autocrine signals to synchronize their activities. These signals provide a fast and accurate control not only for hormone release but also for cell differentiation and survival, key aspects in islet physiology and pathology. Among the different categories of paracrine/autocrine signals, this review highlights the role of neurotransmitters and neuropeptides. In a manner similar to neurons, endocrine cells synthesize, accumulate, release neurotransmitters in the islet milieu, and possess receptors able to decode these signals. In this review, we provide a comprehensive description of neurotransmitter/neuropetide signaling pathways present within the islet. Then, we focus on evidence supporting the concept that neurotransmitters/neuropeptides and their receptors are interesting new targets to preserve β-cell function and mass. A greater understanding of how this network of signals works in physiological and pathological conditions would advance our knowledge of islet biology and physiology and uncover potentially new areas of pharmacological intervention. J. Cell. Physiol. 231: 756-767, 2016. © 2015 Wiley Periodicals, Inc.

Sustained treatment with a stable long-acting oxyntomodulin analogue improves metabolic control and islet morphology in an experimental model of type 1 diabetes

AIM

To assess the therapeutic benefits of regulatory peptides other than insulin, which have to date received limited consideration in the context of type 1 diabetes.

METHODS

We assessed the effects of subchronic administration of the stable, oxyntomodulin (Oxm) analogue, (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL], for 28 days in streptozotocin (STZ)-induced insulin-deficient diabetic mice.

RESULTS

Twice-daily injection with (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL] significantly countered the excessive food and fluid intake in STZ-induced diabetic mice, and maintained normal body weight. Lean body mass was normalized, whilst fat mass was significantly increased compared with control STZ-induced diabetic mice. In addition, circulating glucose was significantly reduced by the Oxm analogue, whilst plasma and pancreatic insulin concentrations were increased and glucagon decreased by day 28. Plasma lipid profile was normalized by (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL] administration and circulating amylase was not significantly altered by induction of diabetes or Oxm analogue therapy. This was associated with significantly improved glucose tolerance and insulin secretion. Peripheral insulin sensitivity was also significantly improved by Oxm analogue treatment. Histological examination of pancreata showed beneficial elevations of total islet and β-cell area, associated with an increase in the number of smaller-sized islets. Further analysis revealed enhanced islet cell proliferation relative to apoptosis in Oxm analogue-treated mice.

CONCLUSION

These studies emphasize the potential of stable Oxm-based peptides, such as (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL], as therapeutic agents for insulin-deficient type 1 diabetes.

Functional GIP receptors play a major role in islet compensatory response to high fat feeding in mice

BACKGROUND

Consumption of high fat diet and insulin resistance induce significant changes in pancreatic islet morphology and function essential for maintenance of normal glucose homeostasis. We have used incretin receptor null mice to evaluate the role of gastric inhibitory polypeptide (GIP) in this adaptive response.

METHODS

C57BL/6 and GIPRKO mice were fed high fat diet for 45 weeks from weaning. Changes of pancreatic islet morphology were assessed by immunohistochemistry. Body fat, glucose, insulin, glucagon, glucagon-like peptide 1 (GLP-1) and GIP were assessed by routine assays.

RESULTS

Compared with normal diet controls, high fat fed C57BL/6 mice exhibited increased body fat, hyperinsulinaemia and insulin resistance, associated with decreased pancreatic glucagon, unchanged pancreatic GLP-1 and marked increases of insulin, islet number, islet size and both beta- and alpha-cell areas. Beta cell proliferation and apoptosis were increased under high fat feeding, but the overall effect favoured enhanced beta cell mass. A broadly similar pattern of change was observed in high fat fed GIPRKO mice but islet compensation was severely impaired in every respect. The inability to enhance beta cell proliferation was associated with the depletion of pancreatic GLP-1 and lack of hyperinsulinaemic response, resulting in non-fasting hyperglycaemia. GIP and GLP-1 were expressed in islets of all groups of mice but high fat fed GIPRKO mice displayed decreased numbers of GLP-1 containing alpha cells plus non-functional enhancement of pancreatic GIP content.

GENERAL SIGNIFICANCE

These data suggest that GIP released from islet alpha-cells and intestinal K-cells plays an important role in islet adaptations to high fat feeding.

Islet remodeling in female mice with spontaneous autoimmune and streptozotocin-induced diabetes

Islet alpha- and delta-cells are spared autoimmune destruction directed at beta-cells in type 1 diabetes resulting in an apparent increase of non-beta endocrine cells in the islet core. We determined how islet remodeling in autoimmune diabetes compares to streptozotocin (STZ)-induced diabetes. Islet cell mass, proliferation, and immune cell infiltration in pancreas sections from diabetic NOD mice and mice with STZ-induced diabetes was assessed using quantitative image analysis. Serial sections were stained for various beta-cell markers and Ngn3, typically restricted to embryonic tissue, was only upregulated in diabetic NOD mouse islets. Serum levels of insulin, glucagon and GLP-1 were measured to compare hormone levels with respect to disease state. Total pancreatic alpha-cell mass did not change as autoimmune diabetes developed in NOD mice despite the proportion of islet area comprised of alpha- and delta-cells increased. By contrast, alpha- and delta-cell mass was increased in mice with STZ-induced diabetes. Serum levels of glucagon reflected these changes in alpha-cell mass: glucagon levels remained constant in NOD mice over time but increased significantly in STZ-induced diabetes. Increased serum GLP-1 levels were found in both models of diabetes, likely due to alpha-cell expression of prohormone convertase 1/3. Alpha- or delta-cell mass in STZ-diabetic mice did not normalize by replacement of insulin via osmotic mini-pumps or islet transplantation. Hence, the inflammatory milieu in NOD mouse islets may restrict alpha-cell expansion highlighting important differences between these two diabetes models and raising the possibility that increased alpha-cell mass might contribute to the hyperglycemia observed in the STZ model.

Role of endogenous GLP-1 and GIP in beta cell compensatory responses to insulin resistance and cellular stress

Role of GLP-1 and GIP in beta cell compensatory responses to beta cell attack and insulin resistance were examined in C57BL/6 mice lacking functional receptors for GLP-1 and GIP. Mice were treated with multiple low dose streptozotocin or hydrocortisone. Islet parameters were assessed by immunohistochemistry and hormone measurements were determined by specific enzyme linked immunoassays. Wild-type streptozotocin controls exhibited severe diabetes, irregularly shaped islets with lymphocytic infiltration, decreased Ki67/TUNEL ratio with decreased beta cell and increased alpha cell areas. GLP-1 and GIP were co-expressed with glucagon and numbers of alpha cells mainly expressing GLP-1 were increased. In contrast, hydrocortisone treatment and induction of insulin resistance increased islet numbers and area, with enhanced beta cell replication, elevated mass of beta and alpha cells, together with co-expression of GLP-1 and GIP with glucagon in islets. The metabolic responses to streptozotocin in GLP-1RKO and GIPRKO mice were broadly similar to C57BL/6 controls, although decreases in islet numbers and size were more severe. In contrast, both groups of mice lacking functional incretin receptors displayed substantially impaired islet adaptations to insulin resistance induced by hydrocortisone, including marked curtailment of expansion of islet area, beta cell mass and islet number. Our observations cannot be explained by simple changes in circulating incretin concentrations, suggesting that intra-islet GLP-1 and GIP make a significant contribution to islet adaptation, particularly expansion of beta cell mass and compensatory islet compensation to hydrocortisone and insulin resistance.

Incretin receptor null mice reveal key role of GLP-1 but not GIP in pancreatic beta cell adaptation to pregnancy

Islet adaptations to pregnancy were explored in C57BL6/J mice lacking functional receptors for glucagon-like peptide 1 (GLP-1) and gastric inhibitory polypeptide (GIP). Pregnant wild type mice and GIPRKO mice exhibited marked increases in islet and beta cell area, numbers of medium/large sized islets, with positive effects on Ki67/Tunel ratio favouring beta cell growth and enhanced pancreatic insulin content. Alpha cell area and glucagon content were unchanged but prohormone convertases PC2 and PC1/3 together with significant amounts of GLP-1 and GIP were detected in alpha cells. Knockout of GLP-1R abolished these islet adaptations and paradoxically decreased pancreatic insulin, GLP-1 and GIP. This was associated with abolition of normal pregnancy-induced increases in plasma GIP, L-cell numbers, and intestinal GIP and GLP-1 stores. These data indicate that GLP-1 but not GIP is a key mediator of beta cell mass expansion and related adaptations in pregnancy, triggered in part by generation of intra-islet GLP-1.

Chronic exposure to GLP-1 increases GLP-1 synthesis and release in a pancreatic alpha cell line (α-TC1): evidence of a direct effect of GLP-1 on pancreatic alpha cells

Aims/Hypothesis

Incretin therapies, which are used to treat diabetic patients, cause a chronic supra-physiological increase in GLP-1 circulating levels. It is still unclear how the resulting high hormone concentrations may affect pancreatic alpha cells. The present study was designed to investigate the effects of chronic exposure to high GLP-1 levels on a cultured pancreatic alpha cell line.

Methods

α-TC1-6 cell line was cultured in the presence or absence of GLP-1 (100 nmol/l) for up to 72 h. In our model GLP-1 receptor (GLP-1R) was measured. After the cells were exposed to GLP-1 the levels of glucagon secretion were measured. Because GLP-1 acts on intracellular cAMP production, the function of GLP-1R was studied. We also investigated the effects of chronic GLP-1 exposure on the cAMP/MAPK pathway, Pax6 levels, the expression of prohormone convertases (PCs), glucagon gene (Gcg) and protein expression, glucagon and GLP-1 production.

Results

In our model, we were able to detect GLP-1R. After GLP-1 exposure we found a reduction in glucagon secretion. During further investigation of the function of GLP-1R, we found an activation of the cAMP/MAPK/Pax6 pathway and an increase of Gcg gene and protein expression. Furthermore we observed a significant increase in PC1/3 protein expression, GLP-1 intracellular content and GLP-1 secretion.

Conclusions/Interpretation

Our data indicate that the chronic exposure of pancreatic alpha cells to GLP-1 increases the ability of these cells to produce and release GLP-1. This phenomenon occurs through the stimulation of the transcription factor Pax6 and the increased expression of the protein convertase PC1/3.

Genetic variants in PCSK1 gene are associated with the risk of coronary artery disease in type 2 diabetes in a Chinese Han population: a case control study

Background

Insulin and glucagon-like peptide 1 (GLP-1), converted by proprotein convertase 1 (PC1/3) from proinsulin and proglucagon, are associated with type 2 diabetes (T2DM) and coronary artery disease (CAD). The aim of this study is to investigate the association of PCSK1 gene, which encodes PC1/3, with the risk of CAD in Chinese patients with T2DM.

Methods

We selected and genotyped 5 haplotype-tagging single nucleotide polymorphisms (SNPs) at PCSK1 gene (across 39873bp locus) in a case-control study of Chinese Han population involving 425 diabetic patients (62.1% male, mean age 63.2 years) with CAD as positive cases and 258 diabetic patients (44.2% male, mean age 62.0 years) without CAD as controls.

Results

The allele frequencies at rs3811951 were significantly different between cases and controls (30.7% vs. 37.2%), with the allele G associated with decreased risk for CAD (OR = 0.75, 95% CI = 0.59–0.94, p = 0.013). In recessive inheritance mode, the carriers of GG had a lower risk (OR = 0.50, 95%CI = 0.31–0.82, p = 0.005), even after adjusted for gender, age, BMI and smoking (OR = 0.43, 95%CI = 0.24–0.77, p = 0.004). The carriers of the minor allele A at rs156019 had a higher risk (OR = 1.66, 95%CI = 1.10–2.50, p = 0.016 after adjustment) in dominant inheritance mode. The SNP rs6234 was also significantly associated with CAD risk in women, with the carriers of the minor allele G at rs6234 associated with a reduced CAD risk in recessive inheritance mode (OR = 0.42, 95% CI = 0.18–0.95, p = 0.036 after adjustment).

Conclusions

Our results found that common genetic variants in PCSK1 were associated with CAD in Chinese patients with T2DM.

Anti-diabetic actions of glucagon-like peptide-1 on pancreatic beta-cells

Glucagon-like peptide-1 (GLP-1), an incretin hormone, is released from intestinal L-cells in response to nutrients. GLP-1 lowers blood glucose levels by stimulating insulin secretion from pancreatic beta-cells in a glucose-dependent manner. In addition, GLP-1 slows gastric emptying, suppresses appetite, reduces plasma glucagon, and stimulates glucose disposal, which are beneficial for glucose homeostasis. Therefore, incretin-based therapies such as GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase IV, an enzyme which inactivates GLP-1, have been developed for treatment of diabetes. This review outlines our knowledge of the actions of GLP-1 on insulin secretion and biosynthesis, beta-cell proliferation and regeneration, and protection against beta-cell damage, as well as the involvement of recently discovered signaling pathways of GLP-1 action, mainly focusing on pancreatic beta-cells.

Use of glucagon-like peptide-1 agonists to improve islet graft performance

Human islet transplantation is an effective and promising therapy for type I diabetes. However, long-term insulin independence is both difficult to achieve and inconsistent. De novo or early administration of incretin-based drugs is being explored for improving islet engraftment. In addition to its glucose-dependent insulinotropic effects, incretins also lower postprandial glucose excursion by inhibiting glucagon secretion, delaying gastric emptying, and can protect beta-cell function. Incretin therapy has so far proven clinically safe and tolerable with little hypoglycemic risk. The present review aims to highlight the new frontiers in research involving incretins from both in vitro and in vivo animal studies in the field of islet transplant. It also provides an overview of the current clinical status of incretin usage in islet transplantation in the management of type I diabetes.

desHis¹Glu⁹-glucagon-[mPEG] and desHis¹Glu⁹(Lys³⁰PAL)-glucagon: long-acting peptide-based PEGylated and acylated glucagon receptor antagonists with potential antidiabetic activity

Glucagon is hormone secreted from the pancreatic alpha-cells that is involved in blood glucose regulation. As such, antagonism of glucagon receptor signalling represents an exciting approach for treating diabetes. To harness these beneficial metabolic effects, two novel glucagon analogues, desHis¹Glu⁹-glucagon-[mPEG] and desHis¹Glu⁹(Lys³⁰PAL)-glucagon, has been evaluated for potential glucagon receptor antagonistic properties. Both novel peptides were completely resistant to enzymatic breakdown and significantly (P<0.05 to P<0.001) inhibited glucagon-mediated elevations of cAMP production in glucagon receptor transfected cells. Similarly, desHis¹Glu⁹-glucagon-[mPEG] and desHis¹Glu⁹(Lys³⁰PAL)-glucagon effectively antagonised glucagon-induced increases of insulin secretion from BRIN BD11 cells. When administered acutely to normal, high fat fed or ob/ob mice, both analogues had no significant effects on overall blood glucose or plasma insulin levels when compared to saline treated controls. However, desHis¹Glu⁹-glucagon-[mPEG] significantly (P<0.05) annulled glucagon-induced increases in blood glucose and plasma insulin levels in normal mice and had similar non-significant tendencies in high fat and ob/ob mice. In addition, desHis¹Glu⁹(Lys³⁰PAL)-glucagon effectively (P<0.05 to P<0.001) antagonised glucagon-mediated elevations of blood glucose levels in high fat fed and ob/ob mice, but was less efficacious in normal mice. Further studies confirmed the significant persistent glucagon receptor antagonistic properties of both novel enzyme-resistant analogues 4h post administration in normal mice. These studies emphasise the potential of longer-acting peptide-based glucagon receptor antagonists, and particularly acylated versions, for the treatment of diabetes.

A local glucagon-like peptide 1 (GLP-1) system in human pancreatic islets

AIMS/HYPOTHESIS

Glucagon-like peptide 1 (GLP-1) is a major incretin, mainly produced by the intestinal L cells, with beneficial actions on pancreatic beta cells. However, while in vivo only very small amounts of GLP-1 reach the pancreas in bioactive form, some observations indicate that GLP-1 may also be produced in the islets. We performed comprehensive morphological, functional and molecular studies to evaluate the presence and various features of a local GLP-1 system in human pancreatic islet cells, including those from type 2 diabetic patients.

METHODS

The presence of insulin, glucagon, GLP-1, proconvertase (PC) 1/3 and PC2 was determined in human pancreas by immunohistochemistry with confocal microscopy. Islets were isolated from non-diabetic and type 2 diabetic donors. GLP-1 protein abundance was evaluated by immunoblotting and matrix-assisted laser desorption-ionisation-time of flight (MALDI-TOF) mass spectrometry. Single alpha and beta cell suspensions were obtained by enzymatic dissociation and FACS sorting. Glucagon and GLP-1 release were measured in response to nutrients.

RESULTS

Confocal microscopy showed the presence of GLP-1-like and PC1/3 immunoreactivity in subsets of alpha cells, whereas GLP-1 was not observed in beta cells. The presence of GLP-1 in isolated islets was confirmed by immunoblotting, followed by mass spectrometry. Isolated islets and alpha (but not beta) cell fractions released GLP-1, which was regulated by glucose and arginine. PC1/3 (also known as PCSK1) gene expression was shown in alpha cells. GLP-1 release was significantly higher from type 2 diabetic than from non-diabetic isolated islets.

CONCLUSIONS/INTERPRETATION

We have shown the presence of a functionally competent GLP-1 system in human pancreatic islets, which resides in alpha cells and might be modulated by type 2 diabetes.

α-cell role in β-cell generation and regeneration

This review considers the role of α-cells in β-cell generation and regeneration. We present recent evidence obtained from lineage-tracing studies showing that α-cells can serve as progenitors of β-cells and present a hypothetical model how injured β-cells might activate α-cells in adult islets to promote β-cell regeneration. β-cells appear to arise by way of their trans-differentiation from undifferentiated α progenitor cells, pro-α-cells, both during embryonic development of the islets and in the adult pancreas in response to β-cell injuries. Plasticity of α-cells is endowed by the expression of the gene encoding proglucagon, a prohormone that can give rise to glucagon and glucagon-like peptides (GLPs). The production of glucagon from proglucagon is characteristic of fully-differentiated α-cells whereas GLP-1 is a product of undifferentiated α-cells. GLP-1, a cell growth and survival factor, is proposed to promote the expansion of neurogenin3-expressing, undifferentiated pro-α-cells during development. β-cells arise from pro-α-cells by a change in the relative amounts of the transcription factors Arx and Pax4, master regulators of the α- and β-cell lineages, respectively. A paracrine/autocrine model is proposed whereby injuries of β-cells in adult islets induce the production and release of factors, such as stromal cell-derived factor-1, that cause the de-differentiation of adjacent α-cells into pro-α-cells. Pro-α-cells produce GLP-1 and its receptor that renders them competent to trans-differentiate into β-cells. The trans-differentiation of pro-α-cells into β-cells provides a potentially exploitable mechanism for the regeneration of β-cells in individuals with type 1 diabetes.

Reprogramming gut and pancreas endocrine cells to treat diabetes

Multiple approaches have been investigated with the ultimate goal of providing insulin independence to patients with either type 1 or type 2 diabetes. Approaches to produce insulin-secreting cells in culture, convert non-β-cells into functional β-cells or engineer autologous cells to express and secrete insulin in a meal-responsive manner have all been described. This research has been facilitated by significant improvements in both viral and non-viral gene delivery approaches that have enabled new experimental strategies. Many studies have examined possible avenues to confer islet cytoprotection against immune rejection, inflammation and apoptosis by genetic manipulation of islet cells prior to islet transplantation. Here we review several reports based on the reprogramming of pancreas and gut endocrine cells to treat diabetes.

Stromal cell-derived factor-1 (SDF-1)/chemokine (C-X-C motif) receptor 4 (CXCR4) axis activation induces intra-islet glucagon-like peptide-1 (GLP-1) production and enhances beta cell survival

AIMS/HYPOTHESIS

The endogenous production of stromal cell-derived factor-1 (SDF-1) in beta cells in transgenic mice attenuates the development of diabetes in response to streptozotocin. Here we propose that beta cell injury induces SDF-1 production, and the SDF-1/chemokine (C-X-C motif) receptor 4 (CXCR4) interaction auto-activates Sdf1 expression, resulting in the autocrine production of SDF-1 by beta cells and the paracrine activation of glucagon-like peptide-1 (GLP-1) production by alpha cells.

METHODS

SDF-1 production in adult mouse and human islets and rat INS-1 cells was measured in models of beta cell injury. The paracrine actions of SDF-1 on GLP-1 production in alpha cells were explored. The potential synergism between the growth-promoting actions of GLP-1 and the pro-survival actions of SDF-1 on the preservation of cell mass was evaluated by cell viability assays.

RESULTS

In adult islets and INS-1 cells, Sdf1 expression was re-induced in response to injury. The interaction of SDF-1 with its receptor on alphaTC1 cells activated protein kinase Akt, stimulated cell proliferation and induced the expression of prohormone convertase 1/3 and the consequent production of GLP-1 in alpha cells. The combination of GLP-1 and SDF-1 additively enhanced both the growth and longevity of INS-1 beta cells.

CONCLUSIONS/INTERPRETATION

The results of these studies suggest that in response to beta cell injury and the ensuing induction of SDF-1, the biological function of alpha cells switches from the production of glucagon to the provision of the local growth factor GLP-1 which, in combination with SDF-1, promotes the growth, survival and viability of the beta cells.

Activation of the GLP-1 receptor signalling pathway: a relevant strategy to repair a deficient beta-cell mass

Recent preclinical studies in rodent models of diabetes suggest that exogenous GLP-1R agonists and DPP-4 inhibitors have the ability to increase islet mass and preserve beta-cell function, by immediate reactivation of beta-cell glucose competence, as well as enhanced beta-cell proliferation and neogenesis and promotion of beta-cell survival. These effects have tremendous implication in the treatment of T2D because they directly address one of the basic defects in T2D, that is, beta-cell failure. In human diabetes, however, evidence that the GLP-1-based drugs alter the course of beta-cell function remains to be found. Several questions surrounding the risks and benefits of GLP-1-based therapy for the diabetic beta-cell mass are discussed in this review and require further investigation.

Production of functional glucagon-secreting α-cells from human embryonic stem cells

OBJECTIVE

Differentiation of human embryonic stem (hES) cells to fully developed cell types holds great therapeutic promise. Despite significant progress, the conversion of hES cells to stable, fully differentiated endocrine cells that exhibit physiologically regulated hormone secretion has not yet been achieved. Here we describe an efficient differentiation protocol for the in vitro conversion of hES cells to functional glucagon-producing α- cells.

RESEARCH DESIGN AND METHODS

Using a combination of small molecule screening and empirical testing, we developed a six-stage differentiation protocol for creating functional α-cells. An extensive in vitro and in vivo characterization of the differentiated cells was performed.

RESULTS

A high rate of synaptophysin expression (>75%) and robust expression of glucagon and the α-cell transcription factor ARX was achieved. After a transient polyhormonal state in which cells coexpress glucagon and insulin, maturation in vitro or in vivo resulted in depletion of insulin and other β-cell markers with concomitant enrichment of α-cell markers. After transplantation, these cells secreted fully processed, biologically active glucagon in response to physiologic stimuli including prolonged fasting and amino acid challenge. Moreover, glucagon release from transplanted cells was sufficient to reduce demand for pancreatic glucagon, resulting in a significant decrease in pancreatic α-cell mass.

CONCLUSIONS

These results indicate that fully differentiated pancreatic endocrine cells can be created via stepwise differentiation of hES cells. These cells may serve as a useful screening tool for the identification of compounds that modulate glucagon secretion as well as those that promote the transdifferentiation of α-cells to β-cells.

Treatment of diabetes with glucagon-like peptide-1 gene therapy

IMPORTANCE OF THE FIELD

Glucagon-like peptide (GLP)-1 receptor agonists are in widespread clinical use for the treatment of diabetes. While effective, these peptides require frequent injections to maintain efficacy. Therefore, alternative delivery methods including gene therapy are currently being evaluated.

AREAS COVERED IN THIS REVIEW

Here, we review the biology of GLP-1, evidence supporting the clinical use of the native peptide as well as synthetic GLP-1 receptor agonists, and the rationale for their delivery by gene therapy. We then review progress made in the field of GLP-1 gene therapy for both type 1 and type 2 diabetes.

WHAT THE READER WILL GAIN

Efforts to improve the biological half-life of GLP-1 receptor agonists are discussed. We focus on the development of both viral and non-viral gene delivery methods, highlighting vector designs and the strengths and weaknesses of these approaches. We also discuss the utility of targeting regulated GLP-1 production to tissues including the liver, muscle, islet and gut.

TAKE HOME MESSAGE

GLP-1 is a natural peptide possessing several actions that effectively combat diabetes. Current delivery methods for GLP-1-based drugs are cumbersome and do not recapitulate the normal secretion pattern of the native hormone. Gene therapy offers a useful method for directing long-term production and secretion of the native peptide. Targeted production of GLP-1 using tissue-specific promoters and delivery methods may improve therapeutic efficacy, while also eliminating the burden of frequent injections.

Common PCSK1 haplotypes are associated with obesity in the Chinese population

Prohormone convertase subtilisin/kexin type 1 (PCSK1) genetic polymorphisms have recently been associated with obesity in European populations. This study aimed to examine whether common PCSK1 genetic variation is associated with obesity and related metabolic phenotypes in the Chinese population. We genotyped nine common tag single-nucleotide polymorphisms (tagSNP) of the PCSK1 gene in 1,094 subjects of Chinese origin from the Stanford Asia-Pacific Program for Hypertension and Insulin Resistance (SAPPHIRe) family study. One SNP in the PCSK1 gene (rs155971) were nominally associated with risk of obesity in the SAPPHIRe cohort (P = 0.01). A common protective haplotype was associated with reduced risk of obesity (23.79% vs. 32.89%, P = 0.01) and smaller waist circumference (81.71 +/- 10.22 vs. 84.75 +/- 10.48 cm, P = 0.02). Another common haplotype was significantly associated with increased risk of obesity (37.07% vs. 23.84%, P = 0.005). The global P value for haplotype association with obesity was 0.02. We also identified a suggestive association of another PCSK1 SNP (rs3811951) with fasting glucose, fasting insulin, homeostasis model assessment of insulin resistance (HOMA(IR)), triglycerides, and high-density lipoprotein cholesterol (P = 0.05, 0.003, 0.001, 0.04, and 0.04, respectively). These data indicate common PCSK1 genetic variants are associated with obesity in the Chinese population.

Mesenchymal stem cells derived from bone marrow of diabetic patients portrait unique markers influenced by the diabetic microenvironment

Cellular microenvironment is known to play a critical role in the maintenance of human bone marrow-derived mesenchymal stem cells (BM-MSCs). It was uncertain whether BM-MSCs obtained from a 'diabetic milieu' (dBM-MSCs) offer the same regenerative potential as those obtained from healthy (non-diabetic) individuals (hBM-MSCs). To investigate the effect of diabetic microenvironment on human BM-MSCs, we isolated and characterized these cells from diabetic patients (dBM-MSCs). We found that dBM-MSCs expressed mesenchymal markers such as vimentin, smooth muscle actin, nestin, fibronectin, CD29, CD44, CD73, CD90, and CD105. These cells also exhibited multilineage differentiation potential, as evident from the generation of adipocytes, osteocytes, and chondrocytes when exposed to lineage specific differentiation media. Although the cells were similar to hBM-MSCs, 6% (3/54) of dBM-MSCs expressed proinsulin/C-peptide. Emanating from the diabetic microenvironmental milieu, we analyzed whether in vitro reprogramming could afford the maturation of the islet-like clusters (ICAs) derived from dBM-MSCs. Upon mimicking the diabetic hyperglycemic niche and the supplementation of fetal pancreatic extract, to differentiate dBM-MSCs into pancreatic lineage in vitro, we observed rapid differentiation and maturation of dBM-MSCs into islet-like cell aggregates. Thus, our study demonstrated that diabetic hyperglycemic microenvironmental milieu plays a major role in inducing the differentiation of human BM-MSCs in vivo and in vitro.

New insights into the role of cAMP in the production and function of the incretin hormone glucagon-like peptide-1 (GLP-1)

The proglucagon gene (gcg) encodes both glucagon and glucagon-like peptide-1 (GLP-1), produced in pancreatic alpha cells and intestinal endocrine L cells, respectively. The incretin hormone GLP-1 stimulates insulin secretion and pro-insulin gene transcription. GLP-1 also enhances pancreatic beta-cell proliferation, inhibits cell apoptosis, and has been utilized in the trans-differentiation of insulin producing cells. A long-term effective GLP-1 receptor agonist, Byetta, has now been developed as the drug in treating type II diabetes and potentially other metabolic disorders. The expression of gcg and the production of GLP-1 can be activated by the elevation of the second messenger cyclic AMP (cAMP). Recent studies suggest that in addition to protein kinase A (PKA), exchange protein activated by cAMP (Epac), another effector of cAMP, and the crosstalk between PKA and the Wnt signaling pathway, are involved in cAMP-stimulated gcg transcription and GLP-1 production as well. Finally, functions of GLP-1 in pancreatic beta cells are also mediated by PKA, Epac, as well as the effector of the Wnt signaling pathway. Together, these novel findings bring us a new insight into the role of cAMP in the production and function of the incretin hormone GLP-1.

The avian proglucagon system

Understanding how the proglucagon system functions in maintaining glycemic control and energy balance in birds, as well as defining its specific roles in regulating metabolism, gastrointestinal tract function and food intake requires detailed knowledge of the components that comprise this system. These include proglucagon, a precursor protein from which glucagon and two glucagon-like peptide hormones (GLP-1 and GLP-2) are derived, and the membrane bound G-protein-coupled receptors that specifically bind glucagon, GLP-1 and GLP-2 to mediate their individual physiological actions. Another key feature of the proglucagon system that is important for regulating its activity in different tissues involves post-translational processing of the proglucagon precursor protein and the individual peptide hormones derived from it. Currently, there is limited information about the proglucagon system in birds with the majority of that coming from studies involving chickens. By summarizing what is currently known about the proglucagon system in birds, this review aims to provide useful background information for future investigations that will explore the nature and actions of this important hormonal system in different avian species.

Mining incretin hormone pathways for novel therapies

The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), are produced predominantly by enteroendocrine cells and have multiple blood glucose-lowering effects. Recent years have seen a surge of interest in understanding the basic physiology and pathophysiology of incretins and in applying this knowledge to the treatment of diabetes and obesity. Considerable gains have been made in elucidating the mechanisms controlling incretin secretion, and there is growing evidence to suggest that incretins might be involved in the rapid reversal of diabetes observed in gastric bypass patients. Here, we review these recent advances and outline the multiple strategies being pursued to exploit the potential therapeutic benefits of GIP and GLP-1.