Thesis (Ph. D. in Molecular Physiology and Biophysics)--Vanderbilt University, May 2009. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (leaves [42]-50).

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 192-252). Also available in print.

Thesis (doctoral)--Universiẗat, Göttingen, 2004.

BACKGROUND: Elevation of glucagon levels and increase in alpha-cell mass are associated with states of hyperglycemia in diabetes. However, little is known about the mechanisms that control glucagon secretion and alpha-cell mass expansion in normal or diabetogenic conditions. Glucagon is secreted during the fasting state, when glucose levels are low, to stimulate glycogenolysis and gluconeogenesis in the liver to increase the blood glucose level. Amino acids, also, stimulate-glucagon secretion and alpha-cell mass. Amino acids increase glucagon secretion via activation of mTORC1 in alpha-cells. A critical step for mTOR activation is the localization of mTORC1 to the lysosome where it meets Rheb for activation. Amino acids are unique in their ability to localize mTORC1 to the lysosomal membrane for activation through their interaction with a variety of amino acid sensors, such as Sestrin2, which modulates mTORC1 activity via its interaction with GATOR2. Integral to mTORC1’s localization is the Ragulator complex, more specifically, p18, which provides the essential scaffolding necessary for lysosomal docking. Amino acids sensors work upstream of mTORC1 and sense amino acid concentrations with different affinities and are specific to certain amino acids and relay this information to mTORC1. OBJECTIVE: To investigate the role that p18, a component of the Ragulator complex, and GATOR2, a component of an amino acid sensor complex, play in amino-acid dependent mTORC1 lysosomal localization and its effect on alpha-cell function and glucagon secretion. METHODS: Generation of Knockout mice for p18 and GATOR2 in alpha-cells were produced by crossing Glu-Cre mice with P18(flox/flox) and GATOR2(flox/flox). Blood glucose, glucagon, and insulin levels were evaluated during fed, fasting, and insulin-induced hypoglycemic conditions to evaluate glucagon secretion. Isolated islets were also exposed to media containing different glucose or nutrient concentrations to evaluate the effect on glucagon secretion. RESULTS: Our data shows that knockdown experiments in alpha-cells for p18 and GATOR2 have demonstrated the role of these proteins in amino acid dependent localization of mTORC1 to the lysosomal membrane. More specifically, our data demonstrate that animals with a knockdown for p18 or GATOR2 demonstrated decreased glucagon secretion during hypoglycemic conditions. Mice with a knockdown for p18 also demonstrate decreased glucagon secretion in the presence of glucagon secretion stimulators such as arginine and also demonstrated decreased insulin secretion. CONCLUSIONS: Loss of essential components of the amino acid signaling and lysosomal localization in the mTORC1 pathway results in impaired function of alpha-cells and glucagon secretion. Loss of p18 in alpha-cells potentially results in an inability of mTORC1 to dock and bind to the lysosomal membrane, whereas loss of GATOR2 potentially results in chronic inhibition of mTORC1 via GATOR1. Loss of each of these components results in the lost or impaired ability for mTORC1 to migrate and bind to the lysosomal membrane. / 2022-06-02T00:00:00Z

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Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

L’homéostasie glucidique est, durant un jeûne normal, maintenue grâce à un réseau de régulation complexe contrôlé principalement par le glucagon, produit par le pancréas. S’opposant aux effets de l’insuline, celui-ci orchestre notamment l'utilisation, le stockage et la synthèse du glucose par le foie, principal organe de production du glucose au cours du jeûne. Cette dernière s’effectue d’abord suite à la dégradation du glycogène ou glycogénolyse puis par la synthèse de novo de glucose ou néoglucogenèse. La néoglucogenèse hépatique est contrôlée par la modulation de l’activité et/ou de l’expression de différentes enzymes-clefs selon des mécanismes allostériques ou transcriptionnels.De multiples facteurs de transcription sont impliqués dans la régulation, au niveau transcriptionnel, de la néoglucogenèse hépatique. Le récepteur nucléaire des acides biliaires FXR est exprimé dans le foie et dans plusieurs organes impliqués dans le maintien de l’homéostasie glucidique. FXR participe à la régulation de nombreuses fonctions hépatiques essentielles, en contrôlant notamment les métabolismes des acides biliaires et lipidique. Le rôle exact de FXR sur la néoglucogenèse reste toujours débattu. L’objectif de cette thèse a donc été d’étudier le rôle de FXR dans le contrôle de la néoglucogenèse hépatique dans des conditions expérimentales reflétant certains aspects du jeûne. Nous avons démontré que FXR, en présence de glucagon, régulait positivement la néoglucogenèse selon deux mécanismes.Le premier mécanisme implique la phosphorylation de FXR par la PKA, une kinase activée par le glucagon. Cette modification post-traductionnelle de FXR permet une induction synergique de l’expression des enzymes-clefs de la néoglucogenèse par FXR et le facteur de transcription CREB. L’identification de ce mécanisme constitue la majeure partie des travaux présentés dans cette thèse. Ceux-ci ont été intégrés à des travaux menés précédemment dans le laboratoire qui nous ont permis d’identifier un mécanisme additionnel de régulation de la gluconéogenèse. L’interaction directe de FXR avec le facteur de transcription FOXA2, lui-même activé par le glucagon, inhibe la capacité de FXR à induire l’expression de SHP, un récepteur nucléaire inhibiteur de la néoglucogenèse.Ce travail a donc permis d’identifier pour la première fois que la néoglucogenèse hépatique est régulée positivement par FXR dans le cadre de la voie de signalisation du glucagon. Pour cela, FXR intègre le signal « glucagon » par deux mécanismes distincts: via une modification post-traductionnelle, sa phosphorylation par la PKA sur les sérines S325 et S357 et via une interaction protéine-protéine avec FOXA2. / Glucose homeostasis is maintained during normal fasting through a complex regulatory network controlled mainly by glucagon, a pancreatic hormone. Opposing the effects of insulin, it orchestrates the glucose use, storage and synthesis by the liver, the main organ that produces glucose during fasting. The latter is carried out first by the degradation of glycogen or glycogenolysis and then by de novo glucose synthesis or gluconeogenesis. Hepatic gluconeogenesis is controlled by modulation of various key enzymes activity and/or expression according to allosteric or transcriptional mechanisms.Multiple transcription factors are involved in the transcriptional regulation of hepatic gluconeogenesis. The nuclear bile acid receptor FXR is expressed in the liver and in several organs involved in glucose homeostasis. FXR regulates many essential liver functions, including controlling bile acid and lipid metabolism. The exact role of FXR on gluconeogenesis is still debated. The objective of this work was therefore to study the role of FXR in the control of hepatic gluconeogenesis under experimental conditions reflecting certain aspects of fasting. We demonstrated that FXR, in the presence of glucagon, positively regulated gluconeogenesis according to two mechanisms.The first mechanism involves phosphorylation of FXR by PKA, a glucagon-activated kinase. This FXR post-translational modification allows synergistic induction of key gluconeogenic enzymes expression by FXR and the CREB transcription factor. This mechanism identification constitutes the major part of the work presented in this thesis. These were integrated with work previously conducted in the laboratory that allowed us to identify an additional mechanism for regulating gluconeogenesis. The FXR direct interaction with the transcription factor FOXA2, itself activated by glucagon, inhibits the ability of FXR to induce the expression of SHP, a gluconeogenesis inhibitory nuclear receptor.This work has therefore identified for the first time that hepatic gluconeogenesis is positively regulated by FXR in the glucagon signalling pathway. For this, FXR integrates the "glucagon" signal by two distinct mechanisms: via post-translational modification, its phosphorylation by PKA on S325 and S357 serines and via protein-protein interaction with FOXA2.

OBJECTIVE: To review the evidence surrounding a potential association between liraglutide and pancreatitis. DATA SOURCES: A literature search was conducted in MEDLINE (1948-July 12, 2012) and EMBASE (1974-week 27, 2012) using the search terms pancreatitis, liraglutide, and glucagon-like peptide 1/adverse effects. Reference citations from identified publications were reviewed. The manufacturer was contacted and regulatory documents from the Food and Drug Administration website were reviewed for unpublished data related to cases of pancreatitis associated with liraglutide use. STUDY SELECTION AND DATA EXTRACTION: All identified sources that were published in English were considered for inclusion. DATA SYNTHESIS: Eleven cases of pancreatitis have been reported in patients taking liraglutide. Seven were from the LEAD (Liraglutide Effect and Action in Diabetes) studies, 1 was reported in the extension of a clinical trial, and 1 was in an unpublished obesity trial. Two were published postmarketing case reports. Nine of the cases reported were diagnosed as acute pancreatitis, while 2 were classified as chronic pancreatitis. The mean age of the patients was 57.5 years and mean body mass index was 33.92 kg/m2. Six of the 11 cases occurred in male patients. Nine of the patients were white and 1 was African American. In 7 of the cases, onset occurred at liraglutide doses at or above 1.8 mg daily. Common comorbidities included history of pancreatitis, cholelithiasis, and diabetes. One case was fatal. CONCLUSIONS: Pancreatitis is a potential complication with liraglutide therapy. Liraglutide should be used cautiously in patients at risk of pancreatitis (eg, alcohol abuse, history of pancreatitis, cholelithiasis).

Indiana University-Purdue University Indianapolis (IUPUI) / Diabetes mellitus is a group of related metabolic diseases that share a common pathological mechanism: insufficient insulin signaling. Insulin is a hormone secreted from pancreatic β cells that promotes energy storage and consequently lowers blood glucose. In contrast, the hormone glucagon, released by pancreatic α cells, plays a critical complementary role in metabolic homeostasis by releasing energy stores and increasing blood glucose. Restoration of β cell mass in diabetic patients via β cell regeneration is a conceptually proven approach to finally curing diabetes. Moreover, in situ regeneration of β cells from endogenous sources would circumvent many of the obstacles encountered by surgical restoration of β cell mass via islet transplantation. Regeneration may occur both by β cell self-duplication and by neogenesis from non-β cell sources. Although the mechanisms regulating the β cell replication pathway have been highly investigated, the signals that regulate β cell neogenesis are relatively unknown. In this dissertation, I have used zebrafish as a genetic model system to investigate the process of β cell neogenesis following insulin signaling depletion by various modes. Specifically, I have found that after their ablation, β cells primarily regenerate from two discrete cellular sources: differentiation from uncommitted pancreatic progenitors and transdifferentiation from α cells. Importantly, I have found that insulin and glucagon play crucial roles in controlling β cell regeneration from both sources. As with metabolic regulation, insulin and glucagon play counter-balancing roles in directing endocrine cell fate specification. These studies have revealed that glucagon signaling promotes β cell formation by increasing differentiation of pancreas progenitors and by destabilizing α cell identity to promote α to β cell transdifferentiation. In contrast, insulin signaling maintains pancreatic progenitors in an undifferentiated state and stabilizes α cell identity. Finally, I have shown that insulin also regulates pancreatic exocrine cell development. Insufficient insulin signaling destabilized acinar cell fate and impairs exocrine pancreas development. By understanding the roles of pancreatic hormones during pancreas development and regeneration can provide new therapeutic targets for in vivo β cell regeneration to remediate the devastating consequences of diabetes.