Sustained glucagon receptor antagonism in insulin-deficient high-fat-fed mice.

Discerning modification to the amino acid sequence of native glucagon can generate specific glucagon receptor (GCGR) antagonists, that include desHis1Pro4Glu9-glucagon and the acylated form desHis1Pro4Glu9(Lys12PAL)-glucagon. In the current study, we have evaluated the metabolic benefits of once-daily injection of these peptide-based GCGR antagonists for 18 days in insulin-resistant high-fat-fed (HFF) mice with streptozotocin (STZ)-induced insulin deficiency, namely HFF-STZ mice. Administration of desHis1Pro4Glu9-glucagon moderately (P < 0.05) decreased STZ-induced elevations of food intake. Body weight was not different between groups of HFF-STZ mice and both treatment interventions delayed (P < 0.05) the onset of hyperglycaemia. The treatments reduced (P < 0.05-P < 0.001) circulating and pancreatic glucagon, whilst desHis1Pro4Glu9(Lys12PAL)-glucagon also substantially increased (P < 0.001) pancreatic insulin stores. Oral glucose tolerance was appreciably improved (P < 0.05) by both antagonists, despite the lack of augmentation of glucose-stimulated insulin release. Interestingly, positive effects on i.p. glucose tolerance were less obvious suggesting important beneficial effects on gut function. Metabolic benefits were accompanied by decreased (P < 0.05-P < 0.01) locomotor activity and increases (P < 0.001) in energy expenditure and respiratory exchange ratio in both treatment groups. In addition, desHis1Pro4Glu9-glucagon increased (P < 0.01-P < 0.001) O2 consumption and CO2 production. Together, these data provide further evidence that peptidic GCGR antagonists are effective treatment options for obesity-driven forms of diabetes, even when accompanied by insulin deficiency.

Metabolic effects of combined glucagon receptor antagonism and glucagon-like peptide-1 receptor agonism in high fat fed mice.

Ablation of glucagon receptor (GCGR) signalling is a potential treatment option for diabetes, whilst glucagon-like peptide-1 (GLP-1) receptor agonists are clinically approved for both obesity and diabetes. There is a suggestion that GCGR blockade enhances GLP-1 secretion and action, whilst GLP-1 receptor activation is known to inhibit glucagon release, implying potential for positive interactions between both therapeutic avenues. The present study has examined the ability of sustained GCGR antagonism, using desHis1Pro4Glu9-glucagon, to augment the established benefits of the GLP-1 mimetic, exendin-4, in high fat fed (HFF) mice. Twice-daily injection of desHis1Pro4Glu9-glucagon, exendin-4 or a combination of both peptides to groups of HFF mice for 10 days had no impact on body weight or energy intake. Circulating blood glucose and glucagon concentrations were significantly (P < 0.05-0.01) decreased by all treatment regimens, with plasma insulin levels elevated (P < 0.001) when compared to lean control mice. Intraperitoneal and oral glucose tolerance were improved (P < 0.05-0.01) by all treatments, despite lack of enhanced glucose-stimulated insulin secretion. Following exogenous glucagon administration, all HFF treatment groups displayed reduced (P < 0.05-0.001) glucose and insulin levels compared to HFF saline controls, although peripheral insulin sensitivity was largely unchanged across all animals. Interestingly, all treatments had tendency to increase pancreatic insulin content with pancreatic glucagon content significantly elevated (P < 0.05) by all interventions. These studies highlight the capacity of peptide-based GCGR inhibition, or GLP-1 receptor activation, to significantly improve metabolism in HFF mice but suggest no obvious additive benefits of combined therapy.

Modelling pancreatic ß-cell inflammation in zebrafish identifies the natural product wedelolactone for human islet protection.

Islet inflammation and cytokine production are implicated in pancreatic ß-cell dysfunction and diabetes pathogenesis. However, we lack therapeutics to protect the insulin-producing ß-cells from inflammatory damage. Closing this clinical gap requires the establishment of new disease models of islet inflammation to facilitate screening efforts aimed at identifying new protective agents. Here, we have developed a genetic model of Interleukin-1ß (Il-1ß)-driven islet inflammation in zebrafish, a vertebrate that allows for non-invasive imaging of ß-cells and in vivo drug discovery. Live imaging of immune cells and ß-cells in our model revealed dynamic migration, increased visitation and prolonged macrophage retention in the islet, together with robust activation of NF-κB signalling in ß-cells. We find that Il-1ß-mediated inflammation does not cause ß-cell destruction but, rather, it impairs ß-cell function and identity. In vivo, ß-cells exhibit impaired glucose-stimulated calcium influx and reduced expression of genes involved in function and maturity. These defects are accompanied by α-cell expansion, glucose intolerance and hyperglycemia following a glucose challenge. Notably, we show that a medicinal plant derivative (wedelolactone) is capable of reducing the immune-cell infiltration while also ameliorating the hyperglycemic phenotype of our model. Importantly, these anti-diabetic properties in zebrafish are predictive of wedelolactone's efficacy in protecting rodent and human islets from cytokine-induced apoptosis. In summary, this new zebrafish model of diabetes opens a window to study the interactions between immune and ß-cells in vivo, while also allowing the identification of therapeutic agents for protecting ß-cells from inflammation.

Islet neuropeptide Y receptors are functionally conserved and novel targets for the preservation of beta-cell mass.

AIMS: Two unmet therapeutic strategies for diabetes treatment are prevention of beta-cell death and stimulation of beta-cell replication. Our aim was to characterize the role of neuropeptide Y receptors in the control of beta-cell mass. MATERIALS AND METHODS: We used endogenous and selective agonists of the NPY receptor system to explore its role in the prevention of beta-cell apoptosis and proliferation in islets isolated from both mouse and human donors. We further explored the intra-cellular signalling cascades involved, using chemical inhibitors of key signalling pathways. As proof of principle we designed a long-acting analogue of [Leu31 Pro34 ]-NPY, an agonist of the islet-expressed Y receptors, to determine if targeting this system could preserve beta-cell mass in vivo. RESULTS: Our data reveal that NPY Y1, 4 and 5 receptor activation engages a generalized and powerful anti-apoptotic pathway that protects mouse and human islets from damage. These anti-apoptotic effects were dependent on stimulating a Gαi-PLC-PKC signalling cascade, which prevented cytokine-induced NFkB signalling. NPY receptor activation functionally protected islets by restoring glucose responsiveness following chemically induced injury in both species. NPY receptor activation attenuated beta-cell apoptosis, preserved functional beta-cell mass and attenuated the hyperglycaemic phenotype in a low-dose streptozotocin model of diabetes. CONCLUSION: Taken together, our observations identify the islet Y receptors as promising targets for the preservation of beta-cell mass. As such, targeting these receptors could help to maintain beta-cell mass in both type 1 and type 2 diabetes, and may also be useful for improving islet transplantation outcomes.

Inhibitory effect of somatostatin on insulin secretion is not mediated via the CNS.

The inhibitory effect of somatostatin (SST) on insulin secretion in vivo is attributed to a direct effect on pancreatic beta cells, but this is inconsistent with some in vitro results in which exogenous SST is ineffective in inhibiting secretion from isolated islets. We therefore investigated whether insulin secretion from the pancreatic islets may partly be regulated by an indirect effect of SST mediated via the CNS. Islet hormone secretion was assessed in vitro by perifusion and static incubations of isolated islets and in vivo by i.v. or i.c.v. administration of the SST analogue BIM23014C with an i.v. glucose challenge to conscious, chronically catheterised rats. Hormone content of samples was assessed by ELISA or RIA and blood glucose levels using a glucose meter. Exogenous SST14/SST28 or BIM23014C did not inhibit the release of insulin from isolated rodent islets in vitro, whereas peripheral i.v. administration of BIM23014C (7.5 µg) with glucose (1 g/kg) led to decreased plasma insulin content (2.3±0.5 ng insulin/ml versus 4.5±0.5 ng/ml at t=5 min, P<0.001) and elevated blood glucose levels compared with those of the controls (29.19±1.3 mmol/l versus 23.5±1.7 mmol/l, P<0.05). In contrast, central i.c.v. injection of BIM23014C (0.75 µg) had no significant effect on either plasma insulin (3.3±0.4 ng/ml, P>0.05) or blood glucose levels (23.5±1.7 mmol/l, P>0.05) although i.v. administration of this dose increased blood glucose concentrations (32.3±0.7 mmol/l, P<0.01). BIM23014C did not measurably alter plasma glucagon, SST, GLP1 or catecholamine levels whether injected i.v. or i.c.v. These results indicate that SST does not suppress insulin secretion by a centrally mediated effect but acts peripherally on islet cells.

Ablation of glucagon receptor signaling by peptide-based glucagon antagonists improves glucose tolerance in high fat fed mice.

Modification to the structure of glucagon has provided a number of glucagon receptor antagonists with possible therapeutic application for diabetes. These novel peptide analogs include desHis(1)Pro(4)Glu(9)-glucagon and desHis(1)Pro(4)Glu(9)(Lys(30)PAL)-glucagon. This study has evaluated the metabolic benefits of once daily administration of desHis(1)Pro(4)Glu(9)-glucagon and desHis(1)Pro(4)Glu(9)(Lys(30)PAL)-glucagon in high fat (45%) fed mice for 15 days. Administration of desHis(1)Pro(4)Glu(9)-glucagon and desHis(1)Pro(4)Glu(9)(Lys(30)PAL)-glucagon had no significant effect on body weight, food intake or circulating glucose concentrations during the treatment period. However, both peptides significantly (P<0.05 to P<0.01) reduced circulating plasma insulin concentrations from day 6 onwards. Oral glucose tolerance and insulin sensitivity, as assessed by exogenous insulin administration, were significantly (P<0.01 to P<0.001) improved by both desHis(1)Pro(4)Glu(9)-glucagon and desHis(1)Pro(4)Glu(9)(Lys(30)PAL)-glucagon. These metabolic benefits were accompanied by significantly (P<0.01) increased pancreatic insulin stores. No significant differences in blood triacylglycerol or cholesterol levels were noted with desHis(1)Pro(4)Glu(9)-glucagon, however desHis(1)Pro(4)Glu(9)(Lys(30)PAL)-glucagon treatment significantly (P<0.01) increased HDL-cholesterol levels. Glucagon-mediated elevations of glucose and insulin were effectively (P<0.01 to P<0.001) annulled in both treatment groups on day 15. Interestingly, glucose levels during an intraperitoneal glucose tolerance test were not altered by either desHis(1)Pro(4)Glu(9)-glucagon or desHis(1)Pro(4)Glu(9)(Lys(30)PAL)-glucagon treatment. These data provide further evidence that glucagon antagonism could provide an effective means of treating T2DM.

Metabolic phenotyping guidelines: assessing glucose homeostasis in rodent models.

The pathophysiology of diabetes as a disease is characterised by an inability to maintain normal glucose homeostasis. In type 1 diabetes, this is due to autoimmune destruction of the pancreatic ß-cells and subsequent lack of insulin production, and in type 2 diabetes it is due to a combination of both insulin resistance and an inability of the ß-cells to compensate adequately with increased insulin release. Animal models, in particular genetically modified mice, are increasingly being used to elucidate the mechanisms underlying both type 1 and type 2 diabetes, and as such the ability to study glucose homeostasis in vivo has become an essential tool. Several techniques exist for measuring different aspects of glucose tolerance and each of these methods has distinct advantages and disadvantages. Thus the appropriate methodology may vary from study to study depending on the desired end-points, the animal model, and other practical considerations. This review outlines the most commonly used techniques for assessing glucose tolerance in rodents and details the factors that should be taken into account in their use. Representative scenarios illustrating some of the practical considerations of designing in vivo experiments for the measurement of glucose homeostasis are also discussed.

Effects of short-term chemical ablation of glucagon signalling by peptide-based glucagon receptor antagonists on insulin secretion and glucose homeostasis in mice.

Glucagon is a hormone with important effects on blood glucose regulation. This study has utilized the stable glucagon receptor antagonists, desHis¹Pro4Glu9-glucagon and desHis¹Pro4Glu9(Lys¹²PAL)-glucagon, to evaluate the effects of sustained inhibition of glucagon receptor signalling in normal mice. Twice-daily injection of either analogue for 10 days had no effect on food intake, body weight and non-fasting plasma glucose concentrations. However, insulin levels were significantly raised (p<0.05 to p<0.01) from day 3 onwards in desHis¹Pro4Glu9-glucagon mice. After 10 days, glucose tolerance was improved (p<0.05) in desHis¹Pro4Glu9-glucagon treated mice. Glucose-mediated insulin secretion and circulating cholesterol levels were significantly (p<0.05 to p<0.01) decreased in both treatment groups. Importantly, the effects of glucagon to increase blood glucose and insulin concentrations were still annulled on day 10. Insulin sensitivity was almost identical in all groups of mice at the end of the study. In addition, no changes in pancreatic insulin and glucagon content or islet morphology were observed in either treatment group. Finally, acute injection of desHis¹Pro4Glu9-glucagon followed by a 24-h fast in treatment naïve mice was not associated with any hypoglycaemic episodes. These data indicate that peptide-based glucagon receptor antagonists represent safe and effective treatment options for type 2 diabetes.

desHis¹Glu9-glucagon-[mPEG] and desHis¹Glu9(Lys³°PAL)-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(Lys³°PAL)-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(Lys³°PAL)-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(Lys³°PAL)-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.