Jean-Marc Guettier

Localization of Insulinomas to Regions of the Pancreas by Intraarterial Calcium Stimulation: The NIH Experience

CONTEXT Selective intraarterial calcium injection of the major pancreatic arteries with hepatic venous sampling [calcium arterial stimulation (CaStim)] has been used as a localizing tool for insulinomas at the National Institutes of Health (NIH) since 1989. The accuracy of this technique for localizing insulinomas was reported for all cases until 1996. OBJECTIVES The aim of the study was to assess the accuracy and track record of the CaStim over time and in the context of evolving technology and to review issues related to result interpretation and procedure complications. CaStim was the only invasive preoperative localization modality used at our center. Endoscopic ultrasound (US) was not studied. DESIGN AND SETTING We conducted a retrospective case review at a referral center. PATIENTS Twenty-nine women and 16 men (mean age, 47 yr; range, 13-78) were diagnosed with an insulinoma from 1996-2008. INTERVENTION A supervised fast was conducted to confirm the diagnosis of insulinoma. US, computed tomography (CT), magnetic resonance imaging (MRI), and CaStim were used as preoperative localization studies. Localization predicted by each preoperative test was compared to surgical localization for accuracy. MAIN OUTCOME We measured the accuracy of US, CT, MRI, and CaStim for localization of insulinomas preoperatively. RESULTS All 45 patients had surgically proven insulinomas. Thirty-eight of 45 (84%) localized to the correct anatomical region by CaStim. In five of 45 (11%) patients, the CaStim was falsely negative. Two of 45 (4%) had false-positive localizations. CONCLUSION The CaStim has remained vastly superior to abdominal US, CT, or MRI over time as a preoperative localizing tool for insulinomas. The utility of the CaStim for this purpose and in this setting is thus validated.

Chronic activation of a designer Gq-coupled receptor improves β cell function

Type 2 diabetes (T2D) has emerged as a major threat to human health in most parts of the world. Therapeutic strategies aimed at improving pancreatic β cell function are predicted to prove beneficial for the treatment of T2D. In the present study, we demonstrate that drug-mediated, chronic, and selective activation of β cell G(q) signaling greatly improve β cell function and glucose homeostasis in mice. These beneficial metabolic effects were accompanied by the enhanced expression of many genes critical for β cell function, maintenance, and differentiation. By employing a combination of in vivo and in vitro approaches, we identified a novel β cell pathway through which receptor-activated G(q) leads to the sequential activation of ERK1/2 and IRS2 signaling, thus triggering a series of events that greatly improve β cell function. Importantly, we found that chronic stimulation of a designer G(q)-coupled receptor selectively expressed in β cells prevented both streptozotocin-induced diabetes and the metabolic deficits associated with the consumption of a high-fat diet in mice. Since β cells are endowed with numerous receptors that mediate their cellular effects via activation of G(q)-type G proteins, our findings provide a rational basis for the development of novel antidiabetic drugs targeting this class of receptors.

Beneficial Metabolic Effects Caused by Persistent Activation of β-Cell M3 Muscarinic Acetylcholine Receptors in Transgenic Mice

Previous studies have shown that β-cell M(3) muscarinic acetylcholine receptors (M3Rs) play a key role in maintaining blood glucose homeostasis by enhancing glucose-dependent insulin release. In this study, we tested the hypothesis that long-term, persistent activation of β-cell M3Rs can improve glucose tolerance and ameliorate the metabolic deficits associated with the consumption of a high-fat diet. To achieve the selective and persistent activation of β-cell M3Rs in vivo, we generated transgenic mice that expressed the Q490L mutant M3R in their pancreatic β-cells (β-M3-Q490L Tg mice). The Q490L point mutation is known to render the M3R constitutively active. The metabolic phenotypes of the transgenic mice were examined in several in vitro and in vivo metabolic tests. In the presence of 15 mm glucose and the absence of M3R ligands, isolated perifused islets prepared from β-M3-Q490L Tg mice released considerably more insulin than wild-type control islets. This effect could be completely blocked by incubation of the transgenic islets with atropine (10 μm), an inverse muscarinic agonist, indicating that the Q490L mutant M3R exhibited ligand-independent signaling (constitutive activity) in mouse β-cells. In vivo studies showed that β-M3-Q490L Tg mice displayed greatly improved glucose tolerance and increased serum insulin levels as well as resistance to diet-induced glucose intolerance and hyperglycemia. These results suggest that chronic activation of β-cell M3Rs may represent a useful approach to boost insulin output in the long-term treatment of type 2 diabetes.

Hepatic Muscarinic Acetylcholine Receptors Are Not Critically Involved in Maintaining Glucose Homeostasis in Mice

OBJECTIVE An increase in the rate of hepatic glucose production is the major determinant of fasting hyperglycemia in type 2 diabetes. A better understanding of the signaling pathways and molecules that regulate hepatic glucose metabolism is therefore of great clinical importance. Recent studies suggest that an increase in vagal outflow to the liver leads to decreased hepatic glucose production and reduced blood glucose levels. Since acetylcholine (ACh) is the major neurotransmitter of the vagus nerve and exerts its parasympathetic actions via activation of muscarinic ACh receptors (mAChRs), we examined the potential metabolic relevance of hepatocyte mAChRs. RESEARCH DESIGN AND METHODS We initially demonstrated that the M3 mAChR is the only mAChR subtype expressed by mouse liver/hepatocytes. To assess the physiological role of this receptor subtype in regulating hepatic glucose fluxes and glucose homeostasis in vivo, we used gene targeting and transgenic techniques to generate mutant mice lacking or overexpressing M3 receptors in hepatocytes only. RESULTS Strikingly, detailed in vivo phenotyping studies failed to reveal any significant metabolic differences between the M3 receptor mutant mice and their control littermates, independent of whether the mice were fed regular or a high-fat diet. Moreover, the expression levels of genes for various key transcription factors, signaling molecules, and enzymes regulating hepatic glucose fluxes were not significantly altered in the M3 receptor mutant mice. CONCLUSIONS This rather surprising finding suggests that the pronounced metabolic effects mediated by activation of hepatic vagal nerves are mediated by noncholinergic signaling pathways.

Leptin therapy for partial lipodystrophy linked to a PPAR‐γ mutation

Aims/hypothesis  Partial lipodystrophy (PL) is most commonly characterized by loss of subcutaneous fat in the extremities with preservation of truncal fat and is associated with insulin resistance, diabetes and hyperlipidaemia. Recombinant human leptin (r‐metHuLeptin) therapy has been shown to be effective in treating metabolic abnormalities associated with congenital or acquired generalized lipodystrophy and PL associated with lamin A/C (LMNA) gene mutations or highly active antiretroviral therapy (HAART). Our aim was to assess the effectiveness of leptin therapy in treating metabolic complications of PL associated with heterozygous peroxisome proliferator activated receptor gamma (PPARG) mutations. This is the first report to detail the clinical response of a patient with PL due to a PPARG mutation treated with r‐metHuLeptin.

Novel insights into the function of β-cell M3 muscarinic acetylcholine receptors: therapeutic implications.

Impaired function of pancreatic β-cells is one of the hallmarks of type 2 diabetes. β-cell function is regulated by the activity of many hormones and neurotransmitters, which bind to specific cell surface receptors. The M(3) muscarinic acetylcholine receptor (M3R) belongs to the superfamily of G protein-coupled receptors and, following ligand dependent activation, selectively activates G proteins of the G(q/11) family. Recent studies with M3R mutant mice strongly suggest that β-cell M3Rs play a central role in promoting insulin release and maintaining correct glucose homeostasis. In this review, we highlight recent studies indicating that β-cell M3Rs and components of downstream signaling pathways might represent promising new targets for the treatment of type 2 diabetes.

Insulin secretion and insulin-producing tumors.

Insulinomas are rare neuroendocrine tumors of pancreatic islet cells that retain the ability to produce and secrete insulin. In contrast to normally differentiated β-cells, insulinoma cells continue to secrete insulin and proinsulin at low blood glucose. This deregulated insulin secretion manifests clinically as fasting hypoglycemia. The molecular pathways that characterize normal insulin secretion and β-cell growth are reviewed and contrasted to the biology of insulinomas. The second half of this review summarizes the clinical approach to the disorder. The diagnosis of insulinoma is established by demonstrating inappropriately high insulin levels with coincident hypoglycemia at the time of a supervised fast. Localization of insulinomas is challenging owing to their small size but should be attempted to maximize the chance for successful surgical resection and avoid risks associated with reoperation. In the majority of cases, successful surgical resection leads to lifelong cure.

Critical metabolic roles of β-cell M3 muscarinic acetylcholine receptors.

Muscarinic acetylcholine (ACh) receptors (mAChRs; M(1)-M(5)) regulate the activity of an extraordinarily large number of important physiological processes. We and others previously demonstrated that pancreatic β-cells are endowed with M(3) mAChRs which are linked to G proteins of the G(q) family. The activation of these receptors by ACh or other muscarinic agonists leads to the augmentation of glucose-induced insulin release via multiple mechanisms. Interestingly, in humans, ACh acting on human β-cell mAChRs is released from adjacent α-cells which express both choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (vAChT), indicative of the presence of a non-neuronal cholinergic system in human pancreatic islets. In order to shed light on the physiological roles of β-cell M(3) receptors, we recently generated and analyzed various mutant mouse models. Specifically, we carried out studies with mice which overexpressed M(3) receptors or mutant M(3) receptors in pancreatic β-cells or which selectively lacked M(3) receptors or M(3)-receptor-associated proteins in pancreatic β-cells. Our findings indicate that β-cell M(3) receptors play a key role in maintaining proper insulin release and whole body glucose homeostasis and that strategies aimed at enhancing signaling through β-cell M(3) receptors may prove useful to improve β-cell function for the treatment of type 2 diabetes (T2D).