Prevalence of type 2 diabetes mellitus (T2DM) is still rising and even so in the juvenile population. Obesity is highly associated with increased risk for developing T2DM. The development has been related to elevated fasting concentrations of the pancreatic islet hormones insulin and glucagon as well as to an increase in plasma lipids that occurs during obesity. Specifically, research has indicated that chronic exposure to high levels of saturated free fatty acids cause dysfunction in islet alpha- and beta-cells. Fatty acids can affect islet cells by various mechanisms one of which is the G-protein coupled receptor FFAR1/GPR40. The role of the receptor in the effects of fatty acids on pancreatic islet-cell function is not clear. The aim of this thesis was to clarify the role of FFAR1 in how fatty acids, and more specifically the long-chain saturated fatty acid palmitate, affect insulin and glucagon secretion. In children and adolescents with obesity elevated fasting levels of insulin and glucagon were positively correlated with lipid parameters. Specifically, plasma triglycerides and free fatty acids were positively correlated with insulin and glucagon at fasting as well as with visceral adipose tissue volume. Elevated glucagon levels at fasting were associated with worsening of glucose tolerance in the same population. In in vitro studies of isolated human islets palmitate stimulated basal insulin and glucagon secretion as well as mitochondrial respiration at fasting glucose levels. The effect was mediated by FFAR1 and fatty acid beta-oxidation. At higher glucose concentrations the receptor was involved in the potentiation of insulin secretion from isolated human islets and insulin-secreting MIN6 cells. Furthermore, we found that the effects of palmitate on hormone secretion were associated with enhanced mitochondrial respiration mediated by FFAR1 Gαq signaling and PKC activity as well as increased intracellular metabolism induced by the fatty acid. When islets were exposed to palmitate for long time periods and in the presence of FFAR1 antagonist, normalized insulin and glucagon secretion during culture and insulin response to glucose after culture were observed. In MIN6 cells chronic palmitate treatment increased mitochondrial uncoupling irrespective of FFAR1 involvement. However, FFAR1 antagonism during palmitate exposure resulted in elevated respiration and reduced apoptosis. In conclusion, children and adolescents with obesity have elevated fasting concentrations of insulin and glucagon that correlate with free fatty acids and fatty acid sources. High glucagon levels are linked to worsening of glucose tolerance in these subjects. In vitro the combination or synergy of FFAR1 activation and intracellular metabolism caused by palmitate is decisive for both the short-term enhancement effects and the negative chronic effects on insulin and glucagon secretion.

Rainbow trout (Oncorhynchus mykiss) is an important freshwater fish whose glucose intolerance, white muscle lactate retention and high lipolytic inertia, have interested comparative physiologists for decades. Recent advancements in mammalian G-protein coupled receptor deorphanization research have identified many endogenous metabolites as regulators of energy metabolism, including lactate and long-chain fatty acids. In addition to being essential metabolic fuels, lactate and long-chain fatty acids regulate lipolysis and lipogenesis by binding to hydroxycarboxylic acid receptor 1 (HCAR1) and the free fatty acid receptors (FFAR1 and 4), respectively. Therefore, the goal of this thesis was to quantify the effects of exogenous lactate and lipids on glucose and fatty acid mobilization in rainbow trout and identify potential signaling mechanisms by monitoring the expression and activity of key glycolytic, gluconeogenic, lipolytic, lipogenic and β-oxidation targets in the liver, muscle and adipose tissue. In Chapter 2, in vivo measurements of metabolic fuel kinetics show that lactate (i) strongly reduced hepatic glucose production by substituting glucose for lactate and (ii) exhibited no lipolytic suppression suggesting HCAR1 signaling is weak in trout. In Chapter 3, in vivo measurements of energy mobilization show that Intralipid strongly induced lipolysis by saturating circulating lipases while transcriptional induction of gluconeogenesis compensates for the acute reduction in hepatic glucose production. Intralipid infusion increased total fatty acid concentration and altered fatty acid composition while suppressing lipid metabolism of trout liver and adipose tissue. In Chapter 4, I identify the presence (hcar1 and ffar1) and absence (ffar4) of these G-protein coupled receptor genes in the rainbow trout genome and describe their evolutionary origins, using in silico approaches of microsynteny, amino acid sequence similarity and critical residue conservation. However, their importance in fish physiology remains relatively unknown, thus future studies are warranted to further investigate such metabolic signals.

Le diabète de type 2 (DT2) résulte de l’incapacité des cellules β sécrétrices d’insuline à compenser la résistance à l’insuline qui s’installe chez les patients obèses. Un traitement potentiel viserait donc à augmenter la sécrétion d’insuline. Dans ce sens, les récepteurs aux acides gras GPR120 et GPR40 potentialisent la sécrétion d’insuline. Cependant, la signalisation GPR120 dans les îlots est méconnue. L’activation de GPR120 diminue la sécrétion de somatostatine (SST), un inhibiteur de la sécrétion d’insuline, par les cellules δ. Ces deux récepteurs régulent l’homéostasie du glucose et sont donc possiblement complémentaires. Nos objectifs étaient d’étudier la signalisation GPR120 dans les îlots pancréatiques, ainsi que la complémentarité des récepteurs GPR120 et GPR40 dans le contrôle de l’homéostasie glucidique. À l’aide d’îlots isolés de souris n’exprimant pas GPR120, constitutivement ou uniquement dans les cellules δ, nous avons étudié le rôle de GPR120 dans les sécrétions d’insuline, glucagon et de SST. Nous avons ensuite étudié des souris n’exprimant pas GPR40, GPR120 ou les deux, sous une diète riche en gras pendant 12 semaines pour étudier la complémentarité des deux récepteurs. L’activation de GPR120 diminue la sécrétion de SST et stimule les sécrétions d’insuline et de glucagon dans les îlots isolés. Cet effet est aboli par la délétion de GPR120 dans les cellules δ in vitro, et la double délétion de GPR120 et GPR40 ne révèle pas d’action complémentaire dans l’homéostasie glucidique. Ces résultats suggèrent que la signalisation GPR120 dans les cellules δ est responsable de l’amélioration de la fonction des îlots. Une meilleure compréhension du rôle joué par GPR120 dans la fonction des îlots et l’homéostasie du glucose est cruciale et pourrait permettre le développement de nouvelles options thérapeutiques dans le traitement du diabète. / In obese patients, type 2 diabetes stems from the failure of the insulin-secreting beta cells to compensate for insulin resistance. Increasing insulin secretion is therefore a viable treatment strategy. In this regard, G protein-coupled receptors (GPCR) are proven therapeutic targets. Activation of the GPCR for long-chain saturated and unsaturated fatty acid GPR40 and GPR120 increase insulin secretion in response to glucose. However, exactly how GPR120 potentiates insulin secretion is unknown. GPR120 and GPR40 both regulate glucose homeostasis and therefore could act in a complementary manner. We aimed to decipher GPR120 signalling in the pancreatic islets and study the complementary roles of GPR120 and GPR40 in maintaining glucose homeostasis. To this aim, we first measured insulin, glucagon and somatostatin secretion following GPR120 activation in isolated islets from mice with a global or somatostatin-cell-specific knock-out of GPR120. Then we studied glucose metabolism in mice with global deletion of GPR120, GPR40 or both, under a high fat diet for 12 weeks. We observed increased insulin and glucagon secretions mirrored by a decreased in somatostatin release following GPR120 activation in isolated islets, an effect abolished by a global or δ-specific deletion of GPR120. A double deletion of GPR120 and GPR40 did not have more impact on glucose metabolism or beta-cell function compared to a simple deletion of either receptor. A better understanding of the GPR120 role in islet function is crucial and could lead to the discovery of new therapeutic options.