Do current incretin mimetics exploit the full therapeutic potential inherent in GLP-1 receptor stimulation?

Glucagon-like peptide-1 (GLP-1) receptor agonists (RAs) are incretin-derived glucose-lowering agents that have been used for the treatment of type 2 diabetes since 2007. Agents such as exenatide (short-acting and once weekly preparations), liraglutide, taspoglutide, albiglutide and lixisenatide lower fasting glucose and HbA1c upon subcutaneous injection, leading to glycaemic control that is equivalent to, or better than, that observed with other oral glucose-lowering agents or bedtime insulin. However, varying proportions of patients report nausea and vomiting, adverse events that typically narrow the therapeutic dose range. Furthermore, GLP-1 RAs reduce fasting glucose to a clinically meaningful extent, but not into the normal range. In contrast, where GLP-1 is administered as a short-term intravenous infusion, a full normalisation of glucose concentrations (approximately 5 mmol/l) has been observed without any risk of gastrointestinal side effects. Subcutaneous infusions or injections of GLP-1 are much less effective. The present analysis relates the proportion of patients who report nausea following treatment with GLP-1 and GLP-1 RAs to the clinical effectiveness of the treatment (represented by the fasting glucose concentration achieved with treatment). The results suggest that GLP-1 RAs injected into the subcutaneous compartment do not exploit the full potential inherent in GLP-1 receptor activation. Reasons for this may include modifications of the peptide molecules in the subcutaneous environment or high local concentrations triggering side effects through GLP-1 receptors on autonomic nerves in subcutaneous adipose tissue. Elucidation of the mechanisms underlying differential responses to GLP-1/GLP-1 RAs administered intravenously vs subcutaneously may help to develop improved agents or modes of administration that are more effective and have fewer side effects.

Prolonged culture of human islets induces ER stress.

Type 2 diabetes (T2D) is characterized by islet dysfunction and beta-cell deficiency caused by apoptosis. One mechanism underlying induction of beta-cell apoptosis is stress in the endoplasmic reticulum (ER). Isolated human islets are a frequently used model to examine islet pathophysiology in T2D. Therefore it is important to establish how function and beta-cell turnover of human islets change in culture. Islets from four organ donors were cultured over four weeks. At 0, 1, 2, 3 and 4 weeks aliquots of islets were used for analysis of a) islet-cell turnover (replication by Ki-67 and apoptosis by TUNEL staining), b) the ER stress level (CHOP and phospho-eIF2alpha staining), c) fractional beta-cell content (insulin staining) and d) islet function (2 h static incubation). Culture duration positively correlated to replication (p=0.03) and negatively correlated to apoptosis (p=0.003). In comparison to islets in situ islet cell turnover is accelerated (>10-fold). The ER stress level was stable during the first three weeks, but showed a sharp increase (p<0.05) at four weeks. The fractional beta-cell content increased from 29+/-2% to 41+/-2% (p=0.0004). Islet function improved (p<0.0001). In conclusion, isolated human islets may be used for in vitro experiments for up to three weeks. During this time islet function and islet-cell turnover are stable. If islet culture is extended beyond three weeks ER stress may impair islet viability. Studies analyzing the pathophysiology of human T2D at the level of the endocrine pancreas need to confirm results obtained with isolated human islets by analysis of primary human pancreatic tissue.

[Invasive diagnostic procedures for insulinomas of the pancreas]. / Invasive Diagnostik bei Insulinomen des Pankreas.

Insulinomas are the most common cause for hypoglycemia with endogenous hyperinsulinism. Insulinomas are the most frequent endocrine tumor of the pancreas and 10% occur as multiple tumors (e.g. multiple endocrine neoplasia type I) or in rare cases as islet cell hyperplasia. A further 10-15% of insulinomas are malignant. Non-invasive imaging modalities, such as computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (US) and somatoreceptor scintigraphy (SRN) show a lower sensitivity for detection and localization of tumors, because in many cases insulinomas are smaller than 2 cm in size. Invasive pre-operative diagnostic procedures, such as transhepatic peripancreatic venous blood sampling (TPVB) and the intra-arterial calcium stimulation test (ASVS) are much more time-intensive compared to CT, MRI and US with an examination time of 2-3 h but achieve a more exact pre-operative detection and localization with sensitivities mostly greater than 95% and are therefore the diagnostic methods of choice.

Preserved GLP-1 effects in a diabetic patient with Cushing's disease.

CONTEXT: A patient with diabetes mellitus, who participated in a study with intravenous administration of GLP-1, was later found to have Cushing's disease (markedly elevated 24 h urinary cortisol excretion and inadequate suppression of fasting cortisol with 2 mg dexamethasone). His diabetic state disappeared (2 h plasma glucose after 75 g oral glucose 159 mg/dl=IGT) after successful pituitary surgery (normal 24 h urinary cortisol excretion and adequate cortisol suppression with 2 mg dexamethasone). OBJECTIVE: The present analysis was undertaken to compare GLP-1 actions on fasting glycemia in diabetes mellitus due to Cushing's disease with GLP-1 actions in typical type 2 diabetes. DESIGN AND METHODS: GLP-1 (1.2 pmol/kg/min) and placebo had been infused into ten patients with diabetes mellitus over 4 h in the fasting state. The results from the patient with Cushing's disease (C) were compared to the data from the remaining nine patients with type 2 diabetes (D). RESULTS: Within 4 h glucose decreased from basal (C: 12.9; D: 12.9+/-0.7 mmol/l) to normal fasting values (C: 5.0; D: 4.9+/-0.4 mmol/l). The stimulation of insulin secretion and suppression of glucagon secretion was similar in the patient with Cushing's disease compared to those with type 2 diabetes. CONCLUSIONS: The insulinotropic, glucagonostatic and glucose-lowering actions of GLP-1 in a patient with diabetes mellitus due to cortisol excess were similar to actions in typical type 2 diabetes. Therefore incretin mimetics might be a novel therapeutic strategy for the treatment of glucocorticoid-induced diabetes mellitus.

Increased vulnerability of newly forming beta cells to cytokine-induced cell death.

AIMS/HYPOTHESIS: Beta cell destruction in type 1 diabetes is apparently mediated by the release of cytokines. We questioned whether cytokine-induced apoptosis preferentially kills replicating beta cells. MATERIALS AND METHODS: In the first experiment, rat insulinoma (RIN) cells were studied for 36 h by time-lapse video microscopy. Cells were exposed to three doses of a cytokine mixture (maximal concentration: IL-1beta 50 U/ml; TNF-alpha 1,000 U/ml; IFN-gamma 1,000 U/ml) or vehicle and analysed for the total cell number (2-h intervals) and timing of each cell death and division. In the second experiment, isolated human islets were incubated with the same cytokine mixture for 24 h and examined for replication and paired (postmitotic) apoptosis. RESULTS: In the first experiment, after application of cytokines, apoptosis occurred most frequently immediately after the next or subsequent cell mitosis (p<0.05). In the second experiment, cytokines caused increased apoptosis in human islets, with an increase in the proportion of postmitotic apoptotic pairs (p<0.001). CONCLUSIONS/INTERPRETATION: Cytokine-induced beta cell death preferentially affects newly forming beta cells, which implies that replicating beta cells might be more vulnerable to cytokine destruction. Efforts to expand beta cell mass in type 1 diabetes by fostering beta cell replication are likely to fail unless cytokine-induced apoptosis is concurrently suppressed.

The mass, but not the frequency, of insulin secretory bursts in isolated human islets is entrained by oscillatory glucose exposure.

Insulin is secreted in discrete insulin secretory bursts. Regulation of insulin release is accomplished almost exclusively by modulation of insulin pulse mass, whereas the insulin pulse interval remains stable at approximately 4 min. It has been reported that in vivo insulin pulses can be entrained to a pulse interval of approximately 10 min by infused glucose oscillations. If oscillations in glucose concentration play an important role in the regulation of pulsatile insulin secretion, abnormal or absent glucose oscillations, which have been described in type 2 diabetes, might contribute to the defective insulin secretion. Using perifused human islets exposed to oscillatory vs. constant glucose, we questioned 1) whether the interval of insulin pulses released by human islets is entrained to infused glucose oscillations and 2) whether the exposure of islets to oscillating vs. constant glucose confers an increased signal for insulin secretion. We report that oscillatory glucose exposure does not entrain insulin pulse frequency, but it amplifies the mass of insulin secretory bursts that coincide with glucose oscillations (P < 0.001). Dose-response analyses showed that the mode of glucose drive does not influence total insulin secretion (P = not significant). The apparent entrainment of pulsatile insulin to infused glucose oscillations in nondiabetic humans in vivo might reflect the amplification of underlying insulin secretory bursts that are detected as entrained pulses at the peripheral sampling site, but without changes in the underlying pacemaker activity.