Les acides biliaires sont depuis longtemps connus pour leur propriété d’agents solubilisant des graisses et des vitamines liposolubles, permettant ainsi une absorption efficace de ces nutriments lors de la digestion. Depuis les années 2000, plusieurs équipes ont montré que ces molécules étaient également dotées de propriétés de signalisation, en particulier via l’activation de deux récepteurs : le récepteur nucléaire FXR, et le récepteur membranaire TGR5. Le récepteur TGR5 est exprimé dans de très nombreux tissus, dont les muscles lisses et squelettiques, le tissu adipeux brun, la vésicule biliaire, mais aussi certaines cellules immunitaires et certaines populations cellulaires intestinales telles que les cellules entéroendocrine L. Selon le tissu étudié, l’activation de TGR5 peut être suivie de nombreux effets biologiques. En particulier, au niveau intestinal, l’activation de ce récepteur stimule la sécrétion de l’hormone incrétine GLP-1.Les hormones incrétines sont impliquées dans la régulation de la glycémie, en particulier dans la phase postprandiale, où elles concourent à potentialiser l’action de l’insuline, hormone hypoglycémiante majeure. Or, dans un contexte de diabète, et en particulier de diabète de type 2, l’organisme est devenu moins sensible à l’insuline, ce qui se traduit par un défaut de gestion de la glycémie, pouvant entrainer à terme des complications graves, telles que des amputations, une cécité, ou encore des problèmes cardiovasculaires. La prévalence et l’incidence de cette pathologie ont conduit l’OMS à la considérer comme la première épidémie d’origine non-infectieuse, ce qui illustre son impact sur la santé publique et le besoin médical constant qu’elle génère.Dans ce contexte, TGR5 apparait comme cible thérapeutique potentielle attrayante, de par l’effet GLP-1 sécrétagogue consécutif à son activation. En effet, parmi les thérapies antidiabétiques, deux classes de molécules basent leur efficacité sur une augmentation de la signalisation de la voie incrétine : les incrétinopotentiateurs (inhibiteurs de la DDP4, enzyme responsable de la faible demi-vie du GLP-1) et les incrétinomimétiques (agonistes synthétiques du récepteur au GLP-1). Récemment, cette dernière classe a également fait son apparition dans l’arsenal thérapeutique de l’obésité, confirmant l’intérêt de cette voie de signalisation dans les pathologies issues d’un désordre métabolique. L’obtention de composés GLP-1 sécrétagogues s’avère ainsi prometteuse et représente une approche complémentaire aux deux autres classes.L’objectif de ce travail était donc d’obtenir des agonistes puissants, sélectifs et originaux du récepteur TGR5. Afin de diminuer les risques d’effets indésirables, on-target ou off-target, nous avons choisi de profiter de la localisation intestinale de notre cible d’intérêt en concevant nos composés de manière à limiter l’exposition au seul tractus gastro-intestinal, en limitant leur absorption. Ainsi, nous avons cherché à obtenir des composés non systémiques GLP-1 sécrétagogues.La stratégie employée pour aboutir à ces composés a été le développement de molécules chimériques constituées d’une partie agoniste de TGR5, le pharmacophore, liée à un groupement permettant de limiter la perméabilité membranaire et donc l’absorption intestinale, le kinétophore. Après avoir optimisé la partie pharmacophore et identifié une position permettant l’ajout de différents types de kinétophores sans impact majeur sur l’activité du composé, nous avons pu obtenir plusieurs agonistes de TGR5 puissants, originaux, et dotés d’une très faible perméabilité membranaire. L’étude in vivo de ces molécules a ensuite permis de valider d’une part leur activité GLP-1 sécrétagogue, et d’autre part leur faible exposition systémique. Enfin, l’évaluation du potentiel thérapeutique d’un des meilleurs composés dans des modèles murins de diabète a récemment pu être initiée. / Bile acids have long been known as lipid solubilizing agents, enabling efficient absorption of nutrients and vitamins during digestion. Since 2000, several teams have demonstrated the signaling properties of these molecules, especially through the activation of two receptors : the nuclear receptor FXR and the membrane receptor TGR5.The TGR5 receptor is expressed in various tissues, such as smooth and skeletal muscles, brown adipose tissue, gallbladder, but also on some immune or intestinal cell lines such as the enteroendocrine L cells. Depending on the studied tissue, TGR5 activation can trigger various biological effects. In the intestine, its activation can stimulate the secretion of an incretin hormone, the GLP-1.Incretin hormones play a role in glycaemia regulation, especially during the postprandial phase during which they potentiate the action of the insulin, the main hypoglycemic hormone. Diabetes mellitus correspond to a decreased response of the organism to insulin signaling. This leads to a default in the glycaemia handling that can lead to serious complications, such as amputation, blindness, or cardiovascular problems. Prevalence and incidence of this disease have lead the WHO to define diabetes as the first non-infectious epidemic, illustrating its impact on public health and the constant need for new therapeutic opportunities.In this context, TGR5 appears as an appealing potential therapeutic target, especially because of the GLP-1 secretagogue effect triggered by its activation. Indeed, among the antidiabetic therapeutic options, two classes of drugs work by increasing the incretin signaling: the incretinopotentiators (inhibitors of the DPP4, which is the enzyme responsible for the very short half-life of GLP-1), and the incretinomimetics (synthetic agonists of the GLP-1 receptor). Recently, this last class has also been approved to treat obesity. This demonstrates the interest of this signaling pathway in the treatment of metabolic disorders. Hence, GLP-1 secretagogue compounds may prove to be an interesting approach, and could complement the two other classes.The aim of this work was then to obtain potent, selective and original agonists of the TGR5 receptor. In order to decrease the risk of on-target and off-target effects, we decided to take advantage of the intestinal localization of our target by designing compounds that would only expose the gastro-intestinal tract, by limiting their absorption. Thus, we wanted to obtain non systemic GLP-1 secretagogue compounds.Our strategy was to develop chimeric compounds consisting of a pharmacophore part, which would be a potent and selective agonist of TGR5, linked to a kinetophore part, which would decrease membrane permeability. After having optimized the pharmacophore part and having identified a position where we could link various kinetophore moieties with only weak impact on the activity, we obtained several potent TGR5 agonists with very low membrane permeability. In vivo evaluations of these compounds have validated both their GLP-1 secretagogue activity and their low systemic exposure. In the end, evaluation of our lead compound on mouse model of diabetes was recently started.

<p><strong><p>Abstract</p><em><p>Background and aim:</p>Diabetes mellitus (DM) is a chronic and progressive illness that affects all type of populations and ages. According to World health organization (WHO) by 2030 it will be 366 million people effected world wild. Many new drugs are Glucagon-like peptide-1 (GLP-1) based therapy for treatment of type 2diabetes. GLP-1 is released from the intestinal L-cells, and is a potent stimulator of glucose-dependent insulin secretion. The aim of this study was to investigate the effect of GLP-1 and its stable analogs on cell proliferation of GLP-1 secreting GLUTag cells. <em><p>Material and methods:</p>GluTag cells were incubated for 48h in DMEM medium containing (0.5% fetal bovine serum and 100 IU/ml penicillin and 100 μg/ml streptomycin and 3mM glucose concentration) in the present or absence of the agents. DNA synthesis was measured using 3H- thymidine incorporation and Ki67 antigen staining. Western blot were performed to investigate the present of GLP-1 receptor in GLUTag cells. <em><p>Results/conclusions:</p><p>These results suggest that GLP-1 regulates proliferation of the GLP-1-secreting cell through a feedback mechanism via its receptor. Since serum GLP-1 levels are decreased in type 2 diabetic patients, the effect of GLP-1 on the GLP-1-secreting cell proliferation suggested here provides a novel beneficial long-term effect of the incretin-based drugs in clinical practice i.e. through increase of the GLP-1-secreting cell mass, augmenting the incretin effect. In addition, the feedback mechanism action of GLP-1 reveals a new insight in regulation manner of the L-cell proliferation.</p>GLP-1(7-36) increased cell proliferation in GLUTag cells, an effect which was blocked by the GLP-1 receptor antagonist exendin(9-39). The GLP-1 receptor was expressed in GluTag cells. <em><p>Keywords:</p>Incretin hormone<em>, GLP-1, GLP-1 receptor, Exendin-4, Diabetes </em></em></em></em></em></strong></p>

Abstract Background and aim: Diabetes mellitus (DM) is a chronic and progressive illness that affects all type of populations and ages. According to World health organization (WHO) by 2030 it will be 366 million people effected world wild. Many new drugs are Glucagon-like peptide-1 (GLP-1) based therapy for treatment of type 2diabetes. GLP-1 is released from the intestinal L-cells, and is a potent stimulator of glucose-dependent insulin secretion. The aim of this study was to investigate the effect of GLP-1 and its stable analogs on cell proliferation of GLP-1 secreting GLUTag cells. Material and methods: GluTag cells were incubated for 48h in DMEM medium containing (0.5% fetal bovine serum and 100 IU/ml penicillin and 100 μg/ml streptomycin and 3mM glucose concentration) in the present or absence of the agents. DNA synthesis was measured using 3H- thymidine incorporation and Ki67 antigen staining. Western blot were performed to investigate the present of GLP-1 receptor in GLUTag cells. Results/conclusions: These results suggest that GLP-1 regulates proliferation of the GLP-1-secreting cell through a feedback mechanism via its receptor. Since serum GLP-1 levels are decreased in type 2 diabetic patients, the effect of GLP-1 on the GLP-1-secreting cell proliferation suggested here provides a novel beneficial long-term effect of the incretin-based drugs in clinical practice i.e. through increase of the GLP-1-secreting cell mass, augmenting the incretin effect. In addition, the feedback mechanism action of GLP-1 reveals a new insight in regulation manner of the L-cell proliferation. GLP-1(7-36) increased cell proliferation in GLUTag cells, an effect which was blocked by the GLP-1 receptor antagonist exendin(9-39). The GLP-1 receptor was expressed in GluTag cells. Keywords: Incretin hormone, GLP-1, GLP-1 receptor, Exendin-4, Diabetes

Le récepteur membranaire TGR5 (Takeda G Protein-coupled Receptor 5), est un récepteur ubiquitaire sensible aux acides biliaires. Il est exprimé dans de nombreux tissus et organes dont l’intestin (dans les cellules entéroendocrines L), la vésicule biliaire, les muscles lisses et squelettiques, le tissu adipeux brun et dans certaines cellules immunitaires. Des études menées in vitro et in vivo chez l’animal ont montré des effets bénéfiques de l’activation de TGR5 sur l’homéostasie énergétique et glucidique. Il est maintenant communément admis que les effets bénéfiques de TGR5 sur l'homéostasie du glucose sont, au moins en partie, médiés par sa capacité à promouvoir la sécrétion de l'incrétine intestinale glucagon-like peptide-1 (GLP-1) au niveau des cellules entéroendocrines L.Cependant, de récentes expériences ont montré que l’activation de TGR5 par des agonistes systémiques dans des modèles animaux peut induire des effets non souhaités tels qu’une augmentation du volume de la vésicule biliaire, des démangeaisons et des effets cardiovasculaires. Afin de s’affranchir des effets non désirés d’agonistes systémiques de TGR5, le projet s’est orienté vers le développement d’agonistes de TGR5 présentant une distribution tissulaire ciblée et limitée à l’intestin et dont la biodisponibilité orale serait très faible, voire nulle. Nous avons alors émis l’hypothèse qu’une activation de TGR5 limitée à l’épithélium intestinal sans exposition systémique permettrait d’obtenir des effets bénéfiques sur l’homéostasie du glucose via l’effet GLP-1 sécrétagogue, tout en minimisant les effets non souhaités sur d’autres tissus ou organes exprimant TGR5.A partir des études de relations structure-activités obtenues au laboratoire sur une série d’agonistes de TGR5, nous avons conçu des composés chimériques de la façon suivante : le pharmacophore responsable de l’activité sur le récepteur TGR5 est lié via un bras espaceur à des éléments structuraux appelés kinétophores qui ajustent les propriétés physicochimiques et pharmacocinétiques de nos agonistes pour limiter leur absorption intestinale. Ainsi, l’objectif de ce travail était d’obtenir des agonistes non systémiques de TGR5, puissants et originaux, exerçant leur action dans l’intestin afin de générer la preuve de concept in vivo de l’intérêt d’utiliser de tels agonistes dans le traitement du diabète de type 2.Une étude systématique de l’effet de kinétophores variés a été réalisée. Une trentaine de composés ont été synthétisés en 8 à 12 étapes permettant l’identification d’agonistes puissants et présentant des propriétés pharmacocinétiques en accord avec notre stratégie de composés topiques intestinaux. Des études in vivo ont ensuite permis de valider l’effet GLP-1 sécrétagogue de tels composés. Enfin, l’évaluation d’un des meilleurs composés dans un modèle murin de diabète nous a permis de valider l’hypothèse qu’un agoniste topique intestinal de TGR5 peut avoir un effet bénéfique sur l’homéostasie énergétique et glucidique. / The membrane receptor TGR5 (Takeda G Protein-coupled Receptor 5) is an ubiquitous receptor sensitive to bile acids. It is expressed in many tissues and organs including the intestine (in enteroendocrine L cells), the gallbladder, smooth and skeletal muscles, brown adipose tissue and in some immune cells. In vitro and in vivo studies in animals have shown beneficial effects of TGR5 activation on energy and glucose homeostasis. It is now commonly accepted that the beneficial effects of TGR5 on glucose homeostasis are, at least in part, mediated by its ability to promote the secretion of the intestinal incretin glucagon-like peptide-1 (GLP-1) in enteroendocrine L cells.However, recent experiments have shown that the activation of TGR5 by systemic agonists in animal models can induce unwanted effects such as increased gallbladder volume, itching and cardiovascular issues. In order to avoid the undesired effects of systemic agonists of TGR5, the project focused on the development of TGR5 agonists with an intestine targeted distribution and a very low oral bioavailability. Then, we hypothesized that the activation of TGR5 limited to the intestinal epithelium without systemic exposure would promote the beneficial effects on glucose homeostasis via the GLP-1 secretagogue effect, while minimizing systemic effects on other tissues or organs expressing TGR5.On the basis of structure-activity relationships on a series of TGR5 agonists developed in the laboratory, we have designed chimeric compounds as follows: the pharmacophore responsible for activity on the TGR5 receptor is bound, via a linker, at structural elements called kinetophores that fine-tune the physicochemical and pharmacokinetic properties of our agonists to limit their intestinal absorption. Thus, the aim of this work was to obtain powerful and original non-systemic TGR5 agonists acting in the intestine to generate the in vivo proof of concept of the therapeutic potential of such agonists in the treatment of type 2 diabetes.A systematic study of the effect of various kinetophores was performed. About thirty compounds have been synthesized in 8 to 12 steps allowing the identification of powerful agonists with pharmacokinetic properties in accordance with our strategy of topical intestinal compounds. In vivo studies were then used to validate the GLP-1 secretagogue effect of such compounds. Finally, evaluation of one of the best compounds in a murine model of diabetes allowed us to validate the hypothesis that a topical intestinal agonist of TGR5 can have a beneficial effect on energy and glucose homeostasis.

Le TD2 est caractérisé par un état inflammatoire de bas grade qui compromet la sécrétion et l‘action de l‘insuline. L‘installation de cette inflammation semble être la conséquence de l‘intoxication aux polluants environnementaux tels que les POP (Polluants Organiques Persistants) et/ou le changement de la flore intestinale, induit par un régime riche en graisse (HFD). L‘intestin est l‘un des premiers tissus exposés aux bactéries pathogènes et à certains POP. Cet organe est aussi le premier impliqué dans la régulation de l‘homéostasie glucidique. Il est donc raisonnable de supposer qu‘une inflammation chronique de l‘intestin puisse avoir des conséquences sur le développement du TD2. Nos objectifs généraux étaient donc : d‘évaluer le rôle de l‘inflammation entérocytaire dans le développement du diabète TD2 et de clarifier l‘effet du Resvératrol (RSV) sur cette maladie. Nos résultats suggèrent que l'inflammation intestinale est impliquée dans l‘établissement d‘un TD2. Cette inflammation pourrait intervenir en libérant des cytokines inflammatoires, en diminuant le taux de GLP-1 et en favorisant le passage des facteurs diabétogènes, de l‘intestin vers les tissus cibles de l‘insuline. L‘induction ou la diminution de l‘inflammation intestinale par des facteurs inflammatogènes (BaP) ou anti-inflammatoires (RSV) sont respectivement associées à l‘aggravation ou à l‘amélioration de l‘état diabétique.Nous avons également montré que le RSV pourrait améliorer l‘état diabétique en normalisant la flore intestinale modifiée par un régime HFD et en augmentant la concentration de GLP-1.. De plus, une co-administration du RSV avec le sitagliptine, un inhibiteur de l‘enzyme DPP4, dégradant le GLP-1, montre une potentialisation de l‘effet du RSV par le sitagliptine: Ces résultats ouvrent la porte pour l‘utilisation de RSV dans la prévention et le traitement du TD2. / The TD2 is characterized by a low-grade inflammatory state that impairs the secretion and action of insulin. Environmental pollutants such as POP (Persistent Organic Pollutants) and the change in gut flora induced by a high fat diet (HFD), are suspected to play an important role in the installation of this inflammation.The intestine is one of the first tissues exposed to pathogenic bacteria and some POP. This organ is also the first take part in the regulation of glucose homeostasis. So, it is reasonable to assume that chronic inflammation of the bowel may affect the development of TD2. Our objectives were: to assess the role of inflammation in the enterocyte development of diabetes TD2 and clarify the effect of Resveratrol (RSV) on the disease.Our results suggest that intestinal inflammation is involved in the establishment of a TD2. This inflammation could affect TD2 through releasing inflammatory cytokines, reducing the concentration of GLP-1 and promoting the passage of diabetogenic factors from the intestine to target tissues of insulin. The induction or reduction of intestinal inflammation by factor inflammatory (BaP) or anti-inflammatory (RSV) are respectively associated with worsening or improvement of the diabetic state.We also showed that the RSV could improve the diabetic state by normalizing the intestinal flora modified by diet and HFD, by increasing the concentration of GLP-1. In addition, co-administration of sitagliptin with RSV, an inhibitor of the enzyme DPP4,which degrade GLP-1, shows a potentiation of the effect of RSV in the sitagliptin: These results open the door for the use of RSV in the prevention and treatment of TD2.Keywords: Resveratrol, type 2 diabetes

Enteroendocrine hormones released from the intestine following food intake have several roles in the control of metabolism, some of which are exploited therapeutically for the treatment of type 2 diabetes. Within this thesis, focus has been on the receptors of the gut hormones glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2). In recent years there has been a surge of interest in the enteroendocrine hormones particularly due to the success of GLP-1 mimetics in the treatment of type 2 diabetes. GLP-1 is an incretin hormone, which enhances glucose induced insulin secretion by binding GLP-1 receptors (GLP1R) on pancreatic β-cells. Despite the therapeutic success, several extra-pancreatic clinical effects of GLP-1 remain unexplained. Here, a GLP1R monoclonal antagonistic antibody that can block GLP1R signalling in vivo has been developed and characterised, providing a new tool for the study of GLP1R physiology. GIP is the second incretin hormone, initially referred to as the 'ugly duckling' incretin hormone due to it's ineffectiveness in inducing insulin secretion in type 2 diabetic patients. Aside from the incretin actions, GIP is thought to be involved in the regulation of high-fat diet (HFD) induced obesity. A new transgenic mouse model expressing a fluorescent reporter under the control of the Gipr promoter has been used here to identify GIPR expressing cells. This model showed GIPR expression in the pancreas, adipose tissue, duodenum and nodose ganglia. Surprisingly GIPR expressing cells were found centrally, in areas of the hypothalamus involved in the regulation of food intake and energy expenditure. We consequently sought to investigate the function of GIPR expressing hypothalamic cells. GLP-2, unlike GLP-1 and GIP, is not an incretin hormone. Rather, GLP-2 has been implicated in the regulation of epithelial cell proliferation and apoptosis within the intestine. Therapeutically, an analogue of GLP-2 is used for the treatment of short bowel syndrome. A common missense mutation in the GLP-2 receptor (GLP2R), D470N, has been found to be associated with type 2 diabetes, and here we sought to understand the mechanism underlying this association. The D470N mutant has decreased β-arrestin recruitment, though the significance of this finding will need further research. Overall; the new monoclonal antagonistic GLP1R antibody will help to further understand GLP1R physiology, the new transgenic GIPR mouse model has contributed to the understanding of GIPR localisation, and cell based assays have identified functional implications of a polymorphism in the GLP2R associated with an increased risk of diabetes. It is hoped that further understanding of the physiology of these gut hormone receptors will be critical in the development of new therapeutics for diabetes and obesity.

Glucagon-like peptide-1 (GLP-1) synthetic analog therapies are prescribed for type 2 diabetes due to their effects on insulin and glucagon secretion, and glycemic control. Recent studies also suggest that they may have cardiovascular benefits; however, the mechanism responsible for this is unknown. To examine this question, we evaluated the effects of GLP-1 and the GLP-1 synthetic analog, liraglutide on cell signaling and function in human aortic endothelial cells (HAECs). The results indicate that both agents inhibit TNFα and LPS induced cellular adhesion molecule expression and monocyte adhesion. They also show that incubation with 30pM GLP-1 and 100nM liraglutide stimulates an immediate increase in intracellular calcium, which activates calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ). This in turn led to a 2.5 fold increase in the phosphorylation of both AMP-activated protein kinase (AMPK) and calcium/calmodulin-dependent protein kinase 1 (CaMK1) within 5 minutes. In addition both GLP-1 and liraglutide caused a 2-fold increase in the phosphorylation of the downstream AMPK/CaMK1 targets: endothelial nitric oxide synthase (eNOS) and cAMP response element-binding protein (CREB). Inhibition of CaMKKβ with STO-609 (0.5ug/mL) blocked the phosphorylation of both AMPK and CaMK1, confirming its pivotal role. Incubation of the HAECs for three hours with lipopolysaccharide (LPS, 2ug/mL) and TNFα (10ng/mL) increased the expression of vascular cell adhesion molecule-1 (VCAM-1) and E-selectin by 5 and 2 fold, respectively. Comparable increases in THP-1 monocyte adhesion to the HAECs, a putative initiating event in atherogenesis, also occurred. Pre-incubation for one hour with either GLP-1 or liraglutide inhibited these events. Likewise, pre-incubation with the CaMKK inhibitor STO-609, or use of lentivirus shRNA to knock down AMPK, blocked the inhibitory effects of both GLP-1 and liraglutide on monocyte adhesion. These results suggest that the recently observed cardiovascular benefits of GLP-1 and liraglutide could be mediated by their effects on CaMKKβ, AMPK and CaMK1 activation, which lead to decreased adhesion molecule expression and monocyte adhesion in endothelial cells. The finding that these effects occur at concentrations of GLP-1 (30pM) and liraglutide (100nM) observed in vivo also suggests they are physiologically relevant.

Mechanisms to augment the cellular function and mass of beta-cells may be effective means of treating type 2 diabetes. Important in the physiological control of beta-cell function and nutrient disposal are factors released from gut enteroendocrine cells during nutrient digestion. In enteroendocrine L-cells, post-translational processing of proglucagon gives rise to a number of proglucagon-derived peptides. One such peptide, glucagon-like peptide-1 (GLP-1), acts via its own receptor (GLP-1R) to stimulate beta-cell insulin secretion, proliferation and survival. Another, oxyntomodulin (OXM), weakly activates the GLP-1R and inhibits food intake in a GLP-1R-dependent manner in rodents, which led us to hypothesize that OXM modulates GLP-1R-dependent glucoregulation. While OXM did not mimic the inhibitory effect of GLP-1 on gastric emptying in mice, OXM stimulated insulin secretion, beta-cell survival and improved glucose tolerance in a GLP-1R-dependent manner. In a similar manner to GLP-1, glucose-dependent insulinotropic polypeptide (GIP), secreted from enteroendocrine K-cells, physiologically stimulates insulin secretion via a distinct GIP receptor (GIPR) in beta-cells. Beyond the beta-cell, GIP and GLP-1 appear to exert divergent actions for the control of glucose homeostasis. Moreover, I illustrate that physiological and pharmacological GLP-1R signalling may be comparatively more important for the preservation of beta-cell mass and glucose homeostasis in murine streptozotocin-induced diabetes. Lastly, studies in rodents and humans have showed that metformin increases circulating levels of GLP-1, leading us to hypothesize that GIP and GLP-1 may be involved in the glucoregulatory effects of metformin. Interestingly, transcripts for the Glp1r and Gipr were significantly increased within islets of metformin-treated mice, and metformin treatment enhanced the sensitivity of cultured beta-cells to GIP and GLP-1. In summary, these studies illustrate mechanisms by which enteroendocrine peptides compare and contrast with respect to beta-cell survival and function and the control of glucose homeostasis.

Mechanisms to augment the cellular function and mass of beta-cells may be effective means of treating type 2 diabetes. Important in the physiological control of beta-cell function and nutrient disposal are factors released from gut enteroendocrine cells during nutrient digestion. In enteroendocrine L-cells, post-translational processing of proglucagon gives rise to a number of proglucagon-derived peptides. One such peptide, glucagon-like peptide-1 (GLP-1), acts via its own receptor (GLP-1R) to stimulate beta-cell insulin secretion, proliferation and survival. Another, oxyntomodulin (OXM), weakly activates the GLP-1R and inhibits food intake in a GLP-1R-dependent manner in rodents, which led us to hypothesize that OXM modulates GLP-1R-dependent glucoregulation. While OXM did not mimic the inhibitory effect of GLP-1 on gastric emptying in mice, OXM stimulated insulin secretion, beta-cell survival and improved glucose tolerance in a GLP-1R-dependent manner. In a similar manner to GLP-1, glucose-dependent insulinotropic polypeptide (GIP), secreted from enteroendocrine K-cells, physiologically stimulates insulin secretion via a distinct GIP receptor (GIPR) in beta-cells. Beyond the beta-cell, GIP and GLP-1 appear to exert divergent actions for the control of glucose homeostasis. Moreover, I illustrate that physiological and pharmacological GLP-1R signalling may be comparatively more important for the preservation of beta-cell mass and glucose homeostasis in murine streptozotocin-induced diabetes. Lastly, studies in rodents and humans have showed that metformin increases circulating levels of GLP-1, leading us to hypothesize that GIP and GLP-1 may be involved in the glucoregulatory effects of metformin. Interestingly, transcripts for the Glp1r and Gipr were significantly increased within islets of metformin-treated mice, and metformin treatment enhanced the sensitivity of cultured beta-cells to GIP and GLP-1. In summary, these studies illustrate mechanisms by which enteroendocrine peptides compare and contrast with respect to beta-cell survival and function and the control of glucose homeostasis.

The incidence of obesity is on the rise globally and is associated with many comorbidities, especially type 2 diabetes mellitus (T2DM). Bariatric surgery is the most effective intervention for weight loss and reducing obesity-associated morbidity. The most common bariatric surgeries are roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG). RYGB and SG are equally efficacious at long-term reduction of weight in obese individuals and amelioriation of T2DM. Interestingly, the improvement of glucose regulation is noted before weight loss is observed. The most likely mechanism underlying glucose homeostasis after bariatric surgery is hormonal changes in the intestine. Enteroendrocrine changes favorable of an anti-diabetic profile are noted after only a few days of receiving either RYGB or SG surgery. Most consistently, elevated postprandial GLP-1, a potent regulator of appetite and glucose control, is observed in post-bariatric surgery patients. However, data is limited regarding post-prandial GLP-1 levels beyond two years after surgery. This study will address the gap in literature by assessing postprandial elevations of GLP-1 following RYGB or SG for up to five years. We will recruit obese type-2 diabetics from an outpatient bariatric surgery clinic at Boston Medical Center scheduled to receive RYGB or SG and periodically assess postprandial GLP-1 levels to determine if they remain elevated after 5 years. Additionally, we will provide evidence if there is a correlation among changes in postprandial GLP-1, weight loss, and hemoglobin A1c at five years. Our proposed study will help direct researchers to develop safer and more efficacious interventions for obesity and T2DM.