Search

The islet of Langerhans plays a key role in glucose homeostasis through regulated secretion of the hormones insulin and glucagon. Islet research has focused on the insulin-secreting β-cells, even though aberrant glucagon secretion from α-cells also contributes to the aetiology of diabetes. Despite its importance, the mechanisms controlling glucagon secretion remain controversial. Proper α-cell function requires the islet milieu, where β- and δ-cells drive and constrain α-cell dynamics. The response of glucagon to glucose is similar between isolated islets and that measured in vivo, so it appears that the glucose dependence requires only islet-intrinsic factors and not input from blood flow or the nervous system. Elevated intracellular free Ca 2+ is needed for α-cell exocytosis, but interpreting Ca 2+ data is tricky since it is heterogeneous among α-cells at all physiological glucose levels. Total Ca 2+ activity in α-cells increases slightly with glucose, so Ca 2+ may serve a permissive, rather than regulatory, role in glucagon secretion. On the other hand, cAMP is a more promising candidate for controlling glucagon secretion and is itself driven by paracrine signalling from β- and δ-cells. Another pathway, juxtacrine signalling through the α-cell EphA receptors, stimulated by β-cell ephrin ligands, leads to a tonic inhibition of glucagon secretion. We discuss potential combinations of Ca 2+ , cAMP, paracrine and juxtacrine factors in the regulation of glucagon secretion, focusing on recent data in the literature that might unify the field towards a quantitative understanding of α-cell function., (© 2018 John Wiley & Sons Ltd.)

Biosynthesis of peptide hormones by pancreatic islet endocrine cells is a tightly orchestrated process that is critical for metabolic homeostasis. Like neuroendocrine peptides, insulin and other islet hormones are first synthesized as larger precursor molecules that are processed to their mature secreted products through a series of proteolytic cleavages, mediated by the prohormone convertases Pc1/3 and Pc2, and carboxypeptidase E. Additional posttranslational modifications including C-terminal amidation of the β-cell peptide islet amyloid polypeptide (IAPP) by peptidyl-glycine α-amidating monooxygenase (Pam) may also occur. Genome-wide association studies (GWAS) have showed genetic linkage of these processing enzymes to obesity, β-cell dysfunction, and type 2 diabetes (T2D), pointing to their important roles in metabolism and blood glucose regulation. In both type 1 diabetes (T1D) and T2D, and in the face of metabolic or inflammatory stresses, islet prohormone processing may become impaired; indeed elevated proinsulin:insulin (PI:I) ratios are a hallmark of the β-cell dysfunction in T2D. Recent studies suggest that genetic or acquired defects in proIAPP processing may lead to the production and secretion of incompletely processed forms of proIAPP that could contribute to T2D pathogenesis, and additionally that impaired processing of both PI and proIAPP may be characteristic of β-cell dysfunction in T1D. In islet α-cells, the prohormone proglucagon is normally processed to bioactive glucagon by Pc2 but may express Pc1/3 under certain conditions leading to production of GLP-1(7-36 NH2 ). A better understanding of how β-cell processing of PI and proIAPP, as well as α-cell processing of proglucagon, are impacted by genetic susceptibility and in the face of diabetogenic stresses, may lead to new therapeutic approaches for improving islet function in diabetes., (© 2018 John Wiley & Sons Ltd.)

Pancreatic islet hormone secretion is central in the maintenance of blood glucose homeostasis. During development of hyperglycaemia, the β-cell is under pressure to release more insulin to compensate for increased insulin resistance. Failure of the β-cells to secrete enough insulin results in type 2 diabetes (T2D). MicroRNAs (miRNAs) are short non-coding RNA molecules suitable for rapid regulation of the changes in target gene expression needed in β-cell adaptations. Moreover, miRNAs are involved in the maintenance of α-cell and β-cell phenotypic identities via cell-specific, or cell-enriched expression. Although many of the abundant miRNAs are highly expressed in both cell types, recent research has focused on the role of miRNAs in β-cells. It has been shown that highly abundant miRNAs, such as miR-375, are involved in several cellular functions indispensable in maintaining β-cell phenotypic identity, almost acting as "housekeeping genes" in the context of hormone secretion. Despite the abundance and importance of miR-375, it has not been shown to be differentially expressed in T2D islets. On the contrary, the less abundant miRNAs such as miR-212/miR-132, miR-335, miR-130a/b and miR-152 are deregulated in T2D islets, wherein the latter three miRNAs were shown to play key roles in regulating β-cell metabolism. In this review, we focus on β-cell function and describe miRNAs involved in insulin biosynthesis and processing, glucose uptake and metabolism, electrical activity and Ca 2+ -influx and exocytosis of the insulin granules. We present current status on miRNA regulation in α-cells, and finally we discuss the involvement of miRNAs in β-cell dysfunction underlying T2D pathogenesis., (© 2018 John Wiley & Sons Ltd.)

Aims: Maintainance of glucagon response to hypoglycaemia is important as a safeguard against hypoglycaemia during glucose-lowering therapy in type 2 diabetes. During recent years, DPP-4 (dipeptidyl peptidase-4) inhibition has become more commonly used in elderly patients. However, whether DPP-4 inhibition affects the glucagon response to hypoglycaemia in the elderly is not known and was the aim of this study., Methods: In a single-centre, double-blind, randomized, placebo-controlled crossover study, 28 subjects with metformin-treated type 2 diabetes (17 male, 11 female; mean age, 74 years [range 65-86]; mean HbA1c, 51.5 mmol/mol [6.9%]) received sitagliptin (100 mg once daily) as add-on therapy or placebo for 4 weeks with a 4-week washout period in between. After each treatment period, the subjects underwent a standard breakfast test, followed by a 2-step hyperinsulinaemic hypoglycaemic clamp (target 3.5 and 3.0 mmol/L), followed by lunch., Results: Glucagon levels after breakfast and lunch, and the glucagon response at 3.5 mmol/L, were lower after sitagliptin than after placebo. However, the glucagon response to hypoglycaemia at 3.1 mmol/L did not differ significantly between the two. Similarly, the noradrenaline, adrenaline and cortisol responses were lower with sitagliptin than with placebo at 3.5 mmol/L, but not at 3.1 mmol/L glucose. Responses in pancreatic polypeptide did not differ between the two., Conclusions: Elderly subjects with metformin-treated type 2 diabetes have lower glucagon levels at 3.5 mmol/L glucose, but maintain the glucagon response to hypoglycaemia at 3.1 mmol/L during DPP-4 inhibition, which safeguards against hypoglycaemia and may contribute to decreasing the risk of hypoglycaemia by DPP-4 inhibition in this age group., (© 2018 John Wiley & Sons Ltd.)

While it is well recognized that obesity is associated with an increased β-cell mass, the association with α-cell mass is less clear. Type 2 diabetes (T2DM) associated with obesity is a bihormonal disease characterized by inadequate insulin secretion and hyperglucagonaemia. We examined β- and α-cell mass throughout the pancreas in obese and lean subjects. Pancreatic tissue of the head, body and tail region of the pancreas was examined from 15 obese subjects (body mass index [BMI] ≥ 27 kg/m 2 ) and 15 age-matched lean subjects (BMI ≤ 25 kg/m 2 ) without diabetes. In obese subjects both β- and α-cell mass were proportionally higher compared with lean subjects, thereby maintaining the α- to β-cell ratio. The adaptation to obesity occurred preferentially in the head of the pancreas. As data so far have been derived from histological studies of β- and α-cell adaptation, in which the head region of the human pancreas was not included, the adaptive capacity of humans to obesity has previously been underestimated. Obesity is associated with an increased α-cell mass, which could contribute to the hyperglucagonaemia observed in people with T2DM., (© 2017 John Wiley & Sons Ltd.)

The "second messenger" archetype cAMP is one of the most important cellular signalling molecules with central functions including the regulation of insulin and glucagon secretion from the pancreatic β- and α-cells, respectively. cAMP is generally considered as an amplifier of insulin secretion triggered by Ca 2+ elevation in the β-cells. Both messengers are also positive modulators of glucagon release from α-cells, but in this case cAMP may be the important regulator and Ca 2+ have a more permissive role. The actions of cAMP are mediated by protein kinase A (PKA) and the guanine nucleotide exchange factor Epac. The present review focuses on how cAMP is regulated by nutrients, hormones and neural factors in β- and α-cells via adenylyl cyclase-catalysed generation and phosphodiesterase-mediated degradation. We will also discuss how PKA and Epac affect ion fluxes and the secretory machinery to transduce the stimulatory effects on insulin and glucagon secretion. Finally, we will briefly describe disturbances of the cAMP system associated with diabetes and how cAMP signalling can be targeted to normalize hypo- and hypersecretion of insulin and glucagon, respectively, in diabetic patients., (© 2017 John Wiley & Sons Ltd.)

The progressive loss of pancreatic β-cell mass that occurs in both type 1 and type 2 diabetes is a primary factor driving efforts to identify strategies for effectively increasing, enhancing or restoring β-cell mass. While factors that seem to influence β-cell proliferation in specific contexts have been described, reliable stimulation of human β-cell proliferation has remained a challenge. Importantly, β-cells exist in the context of a complex, integrated pancreatic islet microenvironment where they interact with other endocrine cells, vascular endothelial cells, extracellular matrix, neuronal projections and islet macrophages. This review highlights different components of the pancreatic microenvironment, and reviews what is known about how signaling that occurs between β-cells and these other components influences β-cell proliferation. Future efforts to further define the role of the pancreatic islet microenvironment on β-cell proliferation may lead to the development of successful approaches to increase or restore β-cell mass in diabetes., (© 2017 John Wiley & Sons Ltd.)

Diabetes mellitus is characterized by chronically elevated blood glucose levels accelerated by a progressive decline of insulin-producing β-cells in the pancreatic islets. Although medications are available to transiently adjust blood glucose to normal levels, the effects of current drugs are limited when it comes to preservation of a critical mass of functional β-cells to sustainably maintain normoglycemia. In this review, we recapitulate recent evidence on the role of pancreatic N-methyl-D-aspartate receptors (NMDARs) in β-cell physiology, and summarize effects of morphinan-based NMDAR antagonists that are beneficial for insulin secretion, glucose tolerance and islet cell survival. We further discuss NMDAR-mediated molecular pathways relevant for neuronal cell survival, which may also be important for the preservation of β-cell function and mass. Finally, we summarize the literature for evidence on the role of NMDARs in the development of diabetic long-term complications, and highlight beneficial pharmacologic aspects of NMDAR antagonists in diabetic nephropathy, retinopathy as well as neuropathy., (© 2017 John Wiley & Sons Ltd.)

Aims: Incretin-based therapies have been associated with an increased risk of pancreatitis. Recently, various histological abnormalities have been reported in human pancreatic tissue from brain-dead organ donors who had been exposed to incretin-based drugs. In the present study we examined pancreatic tissue collected at surgery., Methods: Human pancreatic tissue from 7 type 2-diabetic patients treated with incretin-based drugs (type 2-I), 6 diabetic patients without incretin treatment (type 2-NI), 11 patients without diabetes (no diabetes group) and 9 brain-dead organ donors (BDOD group) was examined., Results: Fractional beta-cell area was reduced in the type 2-NI group compared to the group without diabetes (P < .05), but there was no difference compared to the type 2-I patients. Alpha-cell area (P = .30), beta-cell replication (P = .17) and alpha-cell replication (P = .91) were not different. There were also no differences in acinar cell (P = .13) and duct cell replication (P = .099). Insulin-positive duct cells were more frequent in the type 2-I and the BDOD groups (P = .034). No co-expression of insulin and glucagon was detected. Pancreatic intraepithelial neoplasia (PanIN) lesions were very rare, all low-grade (PanIN 1a and 1b) and tended to occur more frequently in the type 2-I group (P = .084)., Conclusions: The present results did not reveal marked histological abnormalities in the pancreas of incretin-treated patients with type 2 diabetes. Low numbers of specimens available and a large inter-individual variability of the findings warrant caution regarding the interpretation of histological data concerning drug effects on the human pancreas., (© 2016 John Wiley & Sons Ltd.)

During embryonic development, endocrine cells of the pancreas are specified from multipotent progenitors. The transcription factor Neurogenin 3 (NEUROG3) is critical for this development and it has been shown that all endocrine cells of the pancreas arise from endocrine progenitors expressing NEUROG3. A thorough understanding of the role of NEUROG3 during development, directed differentiation of pluripotent stem cells and in models of cellular reprogramming, will guide future efforts directed at finding novel sources of β-cells for cell replacement therapies. In this article, we review the expression and function of NEUROG3 in both mouse and human and present the further characterization of a monoclonal antibody directed against NEUROG3. This antibody has been previously been used for detection of both mouse and human NEUROG3. However, our results suggest that the epitope recognized by this antibody is specific to mouse NEUROG3. Thus, we have also generated a monoclonal antibody specifically recognizing human NEUROG3 and present the characterization of this antibody here. Together, these antibodies will provide useful tools for future studies of NEUROG3 expression, and the data presented in this article suggest that recently described expression patterns of NEUROG3 in human foetal and adult pancreas should be re-examined., (© 2016 John Wiley & Sons Ltd.)

Several hormones are regulated by circadian rhythms to adjust the metabolism to the light/dark cycles and feeding/activity patterns throughout the day. Circadian rhythms are mainly governed by the central clock located in the suprachiasmatic nucleus but also by clocks present in peripheral organs, like the endocrine pancreas. Plasma glucose levels and the main pancreatic hormones insulin and glucagon also exhibit daily variations. Alterations in circadian rhythms are associated with metabolic disturbances and pathologies such as obesity and diabetes. The molecular components of central and peripheral clocks and their regulatory mechanisms are well established. Among the different clock genes, Rev-erbα is considered one of the key links between circadian rhythms and metabolism. Rev-erbα is a critical part of a negative feedback loop in the core circadian clock and modulates the clock oscillatory properties. In addition, Rev-erbα plays an important role in the regulation of lipid and glucose metabolism, thermogenesis, adipocyte and muscle differentiation as well as mitochondrial function. In the endocrine pancreas, Rev-erbα regulates insulin and glucagon secretion and pancreatic β-cell proliferation. In the present review, we discuss all these subjects and, particularly, the role of the clock gene Rev-erbα in the endocrine pancreas., (© 2015 John Wiley & Sons Ltd.)

Absolute or relative hyperglucagonaemia has been recognized for years in all experimental or clinical forms of diabetes. It has been suggested that excess secretion of glucagon by the islet α cells is a direct consequence of intra-islet insulin secretory defects. Recent studies have shown that knockout of the glucagon receptor or administration of a monoclonal specific glucagon receptor antibody make insulin-deficient type 1 diabetic rodents thrive without insulin. These observations suggest that glucagon plays an essential role in the pathophysiology of diabetes and that targeting the α cell and glucagon are innovative approaches in the management of diabetes. Despite active research and identification of promising compounds, no one selective glucagon antagonist is presently used in the treatment of diabetes. Interestingly, besides insulin, several drugs used today in the management of diabetes appear to exert their effects, in part, by inhibiting glucagon secretion (glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors, α-glucosidase inhibitors and, possibly, sulphonylureas) or glucagon action (metformin). The potential risks associated with total glucagon suppression include α-cell hyperplasia, increased mass of the pancreas, increased susceptibility to hepatosteatosis and hepatocellular injury and increased risk of hypoglycaemia, and these should be considered in the search and development of new compounds reducing glucagon receptor signalling. More than 40 years after its initial description, hyperglucagonaemia in diabetes can no longer be ignored or minimized, and its correction represents an attractive way to improve diabetes management., (© 2015 John Wiley & Sons Ltd.)

Background: Glucagon-like peptide 1 (GLP-1) and glucagon share the same precursor molecule proglucagon, but each arises from a distinct posttranslational process in a tissue-specific manner. Recently, it has been shown that GLP-1 is co-expressed with glucagon in pancreatic islet cells. This study was aimed to investigate the progressive changes of GLP-1 versus glucagon production in pancreatic islets during the course of diabetes development., Methods: Both type 1 (non-obese diabetes mice) and type 2 (db/db mice) diabetes models were employed in this study. The mice were monitored closely for their diabetes progression and were sacrificed at different stages according to their blood glucose levels. GLP-1 and glucagon expression in the pancreatic islets was examined using immunohistochemistry assays. Quantitative analysis was performed to evaluate the significance of the changes., Results: The ratio of GLP-1-expressing cells to glucagon-expressing cells in the islets showed significant, progressive increase with the development of diabetes in db/db mice. The increase of GLP-1 expression was in agreement with the upregulation of PC1/3 expression in these cells. Interestingly, intra-islet GLP-1 expression was not significantly changed during the development of type 1 diabetes in non-obese diabetes mice., Conclusions: The study demonstrated that GLP-1 was progressively upregulated in pancreatic islets during type 2 diabetes development. In addition, the data suggest clear differences in intra-islet GLP-1 production between type 1 and type 2 diabetes developments. These differences may have an effect on the clinical and pathophysiological processes of these diseases and may be a target for therapeutic approaches., (Copyright © 2014 John Wiley & Sons, Ltd.)

Intracellular glucose signalling pathways control the secretion of glucagon and insulin by pancreatic islet α- and β-cells, respectively. However, glucose also indirectly controls the secretion of these hormones through regulation of the autonomic nervous system that richly innervates this endocrine organ. Both parasympathetic and sympathetic nervous systems also impact endocrine pancreas postnatal development and plasticity in adult animals. Defects in these autonomic regulations impair β-cell mass expansion during the weaning period and β-cell mass adaptation in adult life. Both branches of the autonomic nervous system also regulate glucagon secretion. In type 2 diabetes, impaired glucose-dependent autonomic activity causes the loss of cephalic and first phases of insulin secretion, and impaired suppression of glucagon secretion in the postabsorptive phase; in diabetic patients treated with insulin, it causes a progressive failure of hypoglycaemia to trigger the secretion of glucagon and other counterregulatory hormones. Therefore, identification of the glucose-sensing cells that control the autonomic innervation of the endocrine pancreatic and insulin and glucagon secretion is an important goal of research. This is required for a better understanding of the physiological control of glucose homeostasis and its deregulation in diabetes. This review will discuss recent advances in this field of investigation., (© 2014 John Wiley & Sons Ltd.)

Background: Some patients with long-standing type 1 diabetes (T1D) maintain detectable levels of C-peptide. The quantitative and qualitative aspects of insulin secretion in these subjects have not been assessed, but may shed light on the basis for maintained β cell function. Our objective was to characterize insulin secretion in subjects with varying duration of T1D., Methods: Data from mixed-meal tolerance tests were collected in this cross-sectional study. We screened 58 subjects with T1D <1 year and 34 subjects with T1D >2 years, 20 of whom had previously participated in trials of anti-CD3 monoclonal antibody. Data from 38 historical non-diabetic controls were utilized. Insulin secretory rates were calculated from C-peptide levels from mixed-meal tolerance tests. Patterns and rates of insulin secretion were characterized along with relationships between insulin secretion and clinical parameters., Results: C-peptide was detected in 68% of subjects with T1D duration >2 years. Insulin secretion was negatively correlated with HgbA(1c) and insulin use. A decline in total insulin secretion was seen with increasing disease duration (p < 0.0001). More subjects with long duration of T1D had a delayed time to peak secretion compared with those with new onset T1D or non-diabetic subjects. Insulin and glucagon secretory responses appeared unrelated., Conclusions: Meal-stimulated insulin secretory responses are seen in those with long-standing T1D and detectable C-peptide. Delayed insulin secretory responses are more common in individuals with longer disease duration. Residual insulin secretory responses are associated with improved clinical parameters., (Copyright © 2013 John Wiley & Sons, Ltd.)

Aim: To investigate the acute and chronic effects of l-leucine on pancreatic α-cell function in vitro. Furthermore, we wanted to explore if glucagon-like peptide-1 (GLP-1), isosteviol (ISV) and 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR) counteract changes in α-cell function induced by chronic exposure to leucine., Methods: Isolated mice islets were incubated with 10 mM leucine for 2 or 72 h. We investigated glucagon and insulin secretion at 2 mM and 16.7 mM glucose. In addition, we cultured clonal α-TC1-6 cells with 5 mM leucine, 5 mM leucine plus GLP-1 (10(-6) M), or ISV (10(-6) M) or AICAR (10(-5) M) at high glucose for 72 h. We measured the glucagon secretion, cholesterol (CHO) and triglyceride (TG) content, cell proliferation as well as gene expression., Results: Ten millimolar of leucine for 2 h significantly stimulated glucagon and insulin secretion both at 2 and 16.7 mM glucose in mice islets. After 72 h incubation with 10 mM leucine the glucagon secretion was enhanced at both 2 and 16.7 mM glucose, whereas the glucose-stimulated insulin secretion (16.7 mM glucose) was inhibited. Chronic exposure to 5 mM leucine increased glucagon secretion, CHO and TG content, cell proliferation and Pcsk2 (p < 0.001), MafB (p < 0.05), Gcg (p < 0.001), Prkaa1 (p < 0.01), Hmgcr (p < 0.001), Srebf2 (p < 0.001), Acaca (p < 0.001), Mtor (p < 0.05) mRNA expression in clonal α-TC1-6 cells. While GLP-1 was cable of reducing glucagon hypersecretion and Pcsk2 (p < 0.05) mRNA expression. ISV and AICAR had no effect on leucine-induced glucagon hypersecretion., Conclusions: Long-term exposure to leucine induces hypersecretion of glucagon secretion, that is, aminoacidotoxicity and influences some key genes of pancreatic α-cells. Interestingly, GLP-1 counteracts the leucine-induced α-cell dysfunction., (© 2012 Blackwell Publishing Ltd.)

Glucagon is a hormone secreted from the alpha cells of the pancreatic islets. Through its effect on hepatic glucose production (HGP), glucagon plays a central role in the regulation of glucose homeostasis. In patients with type 2 diabetes mellitus (T2DM), abnormal regulation of glucagon secretion has been implicated in the development of fasting and postprandial hyperglycaemia. Therefore, new therapeutic agents based on antagonizing glucagon action, and hence blockade of glucagon-induced HGP, could be effective in lowering both fasting and postprandial hyperglycaemia in patients with T2DM. This review focuses on the mechanism of action, safety and efficacy of glucagon antagonists in the treatment of T2DM and discusses the challenges associated with this new potential antidiabetic treatment modality., (© 2011 Blackwell Publishing Ltd.)

Type 1 diabetes is caused by the loss of insulin-producing β-cells as a result of an autoimmune condition. Despite current therapeutic approaches aimed at restoring the insulin supply, complications caused by variations in glycaemia may still arise with age. There is therefore mounting interest in the establishment of alternative therapies. Most current approaches consist in designing rational protocols for in vitro or in vivo cell differentiation/reprogramming from a number of cell sources, including stem, progenitor or differentiated cells. Towards this ultimate goal, it is clear that we need to gain further insight into the interplay between signalling events and transcriptional networks that act in concert throughout pancreatic morphogenesis. This short review will therefore focus on the main events underlying pancreatic development with particular emphasis on the genetic determinants implicated, as well as on the relatively new concept of endocrine cell reprogramming, that is the conversion of pancreatic α-cells into cells displaying a β-cell phenotype., (© 2011 Blackwell Publishing Ltd.)

The glucagon gene is expressed in α-cells of the pancreas, L cells of the intestine and the hypothalamus. The determinants of the α-cell-specific expression of the glucagon gene are not fully characterized, although Arx, Pax6 and Foxa2 are critical for α-cell differentiation and glucagon gene expression; in addition, the absence of the β-cell-specific transcription factors Pdx1, Pax4 and Nkx6.1 may allow for the glucagon gene to be expressed. Pax6, along with cMaf and MafB, binds to the DNA control element G(1) which confers α-cell specificity to the promoter and to G(3) and potently activates glucagon gene transcription. In addition, to its direct role on the transcription of the glucagon gene, Pax6 controls several transcription factors involved in the activation of the glucagon gene such as cMaf, MafB and NeuroD1/Beta2 as well as different steps of glucagon biosynthesis and secretion. We conclude that Pax6 independently of Arx and Foxa2 is critical for α-cell function by coordinating glucagon gene expression as well as glucagon biosynthesis and secretion., (© 2011 Blackwell Publishing Ltd.)

In type-2 diabetes, hyperglucagonaemia aggravates elevated blood glucose levels. Relative to our knowledge of the β-cell and insulin secretion, there remains a limited understanding of glucagon secretion in α-cells. Regulation of glucagon may be dependent on a combination of factors, which include direct glucose sensing by the α-cell, innervations from the autonomic nervous system and potential 'paracrine' actions by hormones and factors that are released by adjacent endocrine cells within the islets. The list of potential 'paracrine' regulators within the islet includes insulin, somatostatin, γ-aminobutyric acid, glutamate and zinc. Zinc crystallises with insulin in β-cells and is co-secreted with insulin. In the scientific literature, the effect of exogeneous zinc on glucagon secretion has been debated. Here, we confirm that an increase in exogeneous zinc does inhibit glucagon secretion. To determine if there are physiological effects of zinc on glucagon secretion we used a β-cell-specific ZnT8 knockout (Znt8BKO) mouse model. Znt8BKO mice, despite showing lower granular zinc content in β-cells, showed no changes in fasted plasma glucagon levels and glucose regulated glucagon secretion. These findings suggest that zinc secreted from β-cell does not regulate glucagon secretion., (© 2011 Blackwell Publishing Ltd.)

The development of the endocrine pancreas and the differentiation of its five cell types, α, β, δ, ε and pancreatic polypeptide (PP) cells, are a highly complex and tightly regulated process. Proper differentiation and function of α- and β-cells are critical for blood glucose homeostasis. These processes are governed by multiple transcription factors and other signalling systems, and its dysregulation results in diabetes. The differentiation of α-cells and the maintenance of α-cell function can be influenced at several stages during development and in the maturing islet. Many transcription factors, such as neurogenin 3 (Ngn3), pancreatic duodenal homeobox 1 (Pdx1) and regulatory factor x6 (Rfx6), play a crucial role in the determination of the endocrine cell fate, while other transcription factors, such as aristaless-related homeobox (Arx) and forkhead box A2 (Foxa2), are implicated in the initial or terminal differentiation of α-cells. In vivo and in vitro studies have shown that preproglucagon transcription, and therefore the maintenance of α-cell function, is regulated by several factors, including forkhead box A1 (Foxa1), paired box 6 (Pax6), brain4 (Brn4) and islet-1 (Isl-1). Detailed information about the regulation of normal and abnormal α-cell differentiation gives insight into the pathogenesis of diabetes, identifies further targets for diabetes treatment and provides clues for the reprogramming of α- to β-cells for replacement therapy., (© 2011 Blackwell Publishing Ltd.)

Signalling by intraislet β-cells to neighbouring α-cells was recognized almost 40 years ago, leading to the hypothesis that this is an essential mechanism to regulate the glucagon counterregulatory response to hypoglycaemia. The thesis was that during normoglycaemia or hyperglycaemia insulin secretion from β-cells would enter the islet periportal circulation and travel downstream to α-cells to dampen glucagon secretion. As a corollary, during hypoglycaemia β-cells would stop secreting insulin, which would permit α-cells to release glucagon into the hepatic portal circulation so it could travel to the liver to increase glucose production and thereby correct hypoglycaemia. This mini-review briefly mentions the early work that established this hypothesis and more extensively examines more recent work that has provided direct evidence supporting the hypothesis. A new twist has been introduced based on the fact that zinc is bound to insulin within β-cells and co-secreted with insulin. Zinc is released from insulin when it reaches the higher pH of blood, and zinc has recently been shown to negatively regulate α-cell secretion. It is now suggested that a switch-off signal provided by a sudden cessation of zinc secretion from β-cells during hypoglycaemia may play a critical role in stimulating glucagon secretion that is independent of the effect of insulin., (© 2011 Blackwell Publishing Ltd.)

Excessive production of glucose by the liver contributes to fasting and postprandial hyperglycaemia, hallmarks of type 2 diabetes. A central feature of this pathologic response is insufficient hepatic insulin action, due to a combination of insulin resistance and impaired β-cell function. However, a case can be made that glucagon also plays a role in dysregulated hepatic glucose production and abnormal glucose homeostasis. Plasma glucagon concentrations are inappropriately elevated in diabetic individuals, and α-cell suppression by hyperglycaemia is blunted. Experimental evidence suggests that this contributes to greater rates of hepatic glucose production in the fasting state and attenuated reduction after meals. Recent studies in animal models indicate that reduction of glucagon action can have profound effects to mitigate hyperglycaemia even in the face of severe hypoinsulinaemia. While there are no specific treatments for diabetic patients yet available that act specifically on the glucagon signalling pathway, newer agents including glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors reduce plasma glucagon and this is thought to contribute to their action to lower blood glucose. The α-cell and glucagon receptor remain tempting targets for novel diabetes treatments, but it is important to understand the magnitude of benefit new strategies would provide as preclinical models suggest that chronic interference with glucagon action could entail adverse effects as well., (© 2011 Blackwell Publishing Ltd.)

Multiple bioactive peptides are produced from proglucagon encoded by glucagon gene (Gcg). Glucagon is produced in islet α-cells through processing by prohormone convertase 2 (Pcsk2) and exerts its action through the glucagon receptor (Gcgr). Although it is difficult to produce a genetic model that harbours isolated glucagon deficiency without affecting the production of other peptides derived from proglucagon, three different animal models that harbour deficiencies in glucagon signalling have been generated by gene targeting strategy. Although both Pcsk2(-/-) and Gcgr(-/-) mice display lower blood glucose levels, homozygous glucagon-GFP knock-in mice (Gcg(gfp/gfp) ) display normoglycaemia despite complete glucagon deficiency. In Gcg(gfp/gfp) mice, the metabolic impact of glucagon deficiency is probably ameliorated by lower plasma insulin levels and glucagon-independent mechanisms that maintain gluconeogenesis. As both Pcsk2(-/-) and Gcgr(-/-) mice exhibit increased production of glucagon-like peptide-1 (GLP-1), which is absent in Gcg(gfp/gfp), GLP-1 is the likely cause of the difference in metabolic impact of glucagon deficiency in these animal models. Although all the three models display islet 'α'-cell hyperplasia, the mechanisms involved remain to be elucidated. Studies using Pcsk2(-/-), Gcgr(-/-) and Gcg(gfp/gfp) mice, especially in combination with α-cell ablation models such as pancreas-specific aristaless-related homeobox (ARX) knockout mice, should further clarify the physiological and pathological roles of glucagon in the regulation of metabolism and the control of islet cell differentiation and proliferation., (© 2011 Blackwell Publishing Ltd.)

Glucagon secretion is regulated by glucose but the mechanisms involved remain hotly debated. Both intrinsic (within the α-cell itself) and paracrine (mediated by factors released β- and/or δ-cells) have been postulated. Glucagon secretion is maximally suppressed by glucose concentrations that do not affect insulin and somatostatin secretion, a finding that highlights the significance of intrinsic regulation of glucagon secretion. Experiments on islets from mice lacking functional ATP-sensitive potassium channels (K(ATP)-channels) indicate that these channels are critical to the α-cell's capacity to sense changes in extracellular glucose. Here, we review recent data on the intrinsic and paracrine regulation of glucagon secretion in human pancreatic islets. We propose that glucose-induced closure of the K(ATP)-channels, via membrane depolarization, culminates in reduced electrical activity and glucagon secretion by voltage-dependent inactivation of the ion channels involved in action potential firing. We further demonstrate that glucagon secretion measured in islets isolated from donors with type-2 diabetes is reduced at low glucose and that glucose stimulates rather than inhibits secretion in these islets. We finally discuss the relative significance of paracrine and intrinsic regulation in the fed and fasted states and propose a unifying model for the regulation of glucagon secretion that incorporates both modes of control., (© 2011 Blackwell Publishing Ltd.)

Glucagon plays critical roles in regulating glucose homeostasis, mainly by counteracting the effects of insulin. Consequently, the dysregulated glucagon secretion that is evident in type 2 diabetes has significant implications in the pathophysiology of the disease. Glucagon secretion from pancreatic α-cells has been suggested to be modulated by blood glucose, signals from the nervous system and endocrine components. In addition to these regulators, intraislet factors acting in a paracrine manner from neighbouring β-cells are emerging as central modulator(s) of α-cell biology. One of the most important of these paracrine factors, insulin, modulates glucagon secretion. Indeed, the α-cell-specific insulin receptor knockout (αIRKO) mouse manifests hypersecretion of glucagon in the postprandial stage and exhibits defective secretion in fasting-induced hypoglycaemia, together mimicking the α-cell defects observed in type 2 diabetes. Interestingly, αIRKO mice display a progressive increase in β-cell mass and a concomitant decrease in α-cells. Lineage trace analyses reveal that the new β-cells originate, in part, from the insulin receptor-deficient α-cells indicating a critical role for α-cell insulin signalling in determining β-cell origin. Our studies also reveal that glucagon-like peptide-1 (GLP-1) treatment of αIRKO mice suppresses glucagon secretion despite absence of functional insulin receptors precluding a role for insulin in GLP-1 action on α-cells in this model. These findings highlight the significance of insulin signalling in the regulation of α-cell biology., (© 2011 Blackwell Publishing Ltd.)

Pancreatic islet cell regeneration is considered to be important in the onset and progression of diabetes and as a potential cell therapy. Current hypotheses, largely based on rodent studies, indicate continuous turnover and plasticity of α- and β-cells in adults; cell populations in rodents respond to increased secretory demand in obesity (30-fold β-cell increase) and pregnancy. Turnover and plasticity of islet cells decrease in mice within >1 year. In man, morphometric observations on postmortem pancreas have indicated that the cellular expansion is much smaller than the increased insulin secretion that accompanies obesity. Longevity of β-cells in humans >20-30 years has been shown by (14) C measurements and bromo-deoxyuridine (BrdU) incorporation and there is an age-related decline in the expression of proteins associated with cell division and regeneration including cyclin D3 and PDX-1. Quantitative estimation and mathematical modelling of the longevity marker, cellular lipofuscin body content, in islets of subjects aged 1-84 years indicated an age-related increase and that 97% of the human β-cell population was established by the age of 20. New data show that human α-cell lipofuscin content is less than that seen in β-cells, but the age-related accumulation is similar; lipofuscin-positive (aged) cells form ≥ 95% of the population after 20 years. Increased turnover of cellular organelles such as mitochondria and endoplasmic reticulum could contribute to lipofuscin accumulation with age in long-lived cells. Induction of regeneration of human islet cells will require understanding of the mechanisms associated with age-related senescence., (© 2011 Blackwell Publishing Ltd.)

In health, the pancreatic islet cells work as a network with highly co-ordinated signals over time to balance glycaemia within a narrow range. In type 1 diabetes (T1DM), with autoimmune destruction of the β-cells, lack of insulin is considered the primary abnormality and is the primary therapy target. However, replacing insulin alone does not achieve adequate glucose control and recent studies have focused on controlling the endogenous glucagon release as well. In T1DM, glucagon secretion is disordered but not absolutely deficient; it may be excessive postprandially yet it is characteristically insufficient and delayed in response to hypoglycaemia. We review our system-level analysis of the pancreatic endocrine network mechanisms of glucagon counterregulation (GCR) and their dysregulation in T1DM and focus on possible use of α-cell inhibitors (ACIs) to manipulate the glucagon axis to repair the defective GCR. Our results indicate that the GCR abnormalities are of 'network origin'. The lack of β-cell signalling is the primary deficiency that contributes to two separate network abnormalities: (i) absence of a β-cell switch-off trigger and (ii) increased intraislet basal glucagon. A strategy to repair these abnormalities with ACI is proposed, which could achieve better control of glycaemia with reduced hypoglycaemia risk., (© 2011 Blackwell Publishing Ltd.)

Background: Pseudoislet studies have concentrated on single beta-cell lines or a combination of insulin and glucagon-secreting cells, overlooking the potential role of somatostatin in insulin release. This study sought to evaluate a heterotypic pseudoislet model containing insulin- (MIN6), glucagon- (αTC1.9) and somatostatin (TGP52)-secreting cells of mouse origin and to compare these pseudoislets with traditional monolayer preparations., Methods: Cellular viability (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide and lactate dehydrogenase assays), proliferation (5-bromo-2-deoxyuridine ELISA), hormone content and functional insulin release in response to a variety of stimuli were measured. Differential expression of E-cadherin, connexin 36 and connexin 43 was assessed by reverse transcriptase-polymerase chain reaction and Western blot to determine a possible role for adherens in insulin release from these pseudoislets., Results: All pseudoislet cells displayed reduced proliferation coupled with an increase in cell death which may contribute to their static size in culture. While MIN6 and TGP52 cells expressed E-cadherin and showed sustained or improved hormone content when configured as pseudoislets, αTC1.9 lacked E-cadherin and contained less glucagon following pseudoislet formation. MIN6 and αTC1.9 cells expressed connexin 36, but not connexin 43 and TGP52 cells expressed connexin 43 only. In the presence of Alanine, Arginine and glucagon-like peptide-1, heterotypic pseudoislet cultures secreted levels of insulin that were comparable to that of MIN6 pseudoislets. In addition, pseudoislets comprising all three cell lines released more insulin into the surrounding culture medium than MIN6 pseudoislets when studied over a 1-week period., Conclusions: The current model may prove useful in studying the role of islet cell interactions in the release of insulin from pancreatic islets.

Background: While the effect of fatty acids and ectopic triglyceride storage in pancreatic beta cells has been well-defined, only limited information is available on alpha cells. This study evaluates the long-term impact of fatty acids on alpha cell function and proliferation as well as fatty acid oxidation., Methods: Clonal alpha cells were cultured with fatty acids in the presence of high glucose for up to 3 days. The influence of fatty acids on glucagon secretion, glucagon content and triglyceride accumulation from 24 to 72 h was investigated. After a - 72 h culture, cell proliferation, carnitine palmitoyl transferase-1 mRNA level and the effect of etomoxir were also elucidated., Results: Fatty acids stimulated glucagon secretion and increased triglyceride accumulation in a time- and dose-dependent manner, but inhibited alpha cell proliferation. Lower concentrations (0.125-0.25 mM) of fatty acids significantly increased glucagon secretion at 48 and 72 h, but did not affect triglyceride content. However, a marked increment in triglyceride accumulation occurred in the presence of 0.5 mM fatty acids. Fatty acids caused an up-regulation of the expression of carnitine palmitoyl transferase-1 gene. Etomoxir (1 microM) reversed fatty acid-induced glucagon hypersecretion, but did not inhibit carnitine palmitoyl transferase-1 mRNA level., Conclusions: Our data indicates that compared with triglyceride accumulation, glucagon secretion is more sensitive to changes in fatty acid concentration. The effect of fatty acids on the glucagon response is mediated through their oxidation. The high carnitine palmitoyl transferase-1 gene expression and the accumulation of triglyceride may initially be a compensatory oxidation reaction to elevated fatty acids., ((c) 2007 John Wiley & Sons, Ltd.)