Métabolisme du glucose et du glycérol dans la cellule pancréatique β et les hépatocytes et identification des voies de détoxification du glucose

Excessive intake of calorigenic nutrients and low levels of physical activity contribute to the rapid increase in obesity and related metabolic diseases such as metabolic syndrome and type 2 diabetes (T2D). Insulin secreted by the β cells of the islets of Langerhans, is a key hormone in energy regulation of the organism. Plasma glucose is the main stimulator of insulin secretion via the production of metabolic coupling factors (MCF), which are key molecules mediating the link between glucose metabolism by β cell and the exocytosis of insulin. Other calorigenic nutrients such as fatty acids and certain amino acids amplify glucose-induced insulin secretion (GSIS), partly via their intracellular metabolism. Nevertheless, the contribution of MCFs in GSIS and the related mechanisms remain partly unknown. Our laboratory has highlighted the contribution of the glycerolipids/free fatty acids (GL/FFA) cycle in insulin secretion induced by glucose and amplified by fatty acids, and this cycle, which encompasses lipogenesis and lipolysis, plays an important role in the overall regulation of glucose and lipid metabolism. In addition, it takes part in regulating the GSIS via MCF production such as monoacylglycerol and in the intracellular availability of nutrients and their derivatives. Disturbed GL/FFA cycle contributes to the pathogenesis of T2D because of its role in lipid signaling and calorigenic nutrients detoxification. Indeed, chronic exposition to elevated glucose and FFA cause pancreatic β-cell dysfunction and apoptosis as well. Nevertheless, β cells must continually take-up glucose for promoting insulin secretion. To cope with the cellular toxicity caused by excessive intake of calorigenic nutrients, cells use metabolic detoxification pathways to convert calorigenic nutrients into more inert molecules and eventually export them extracellularly. Cells have various mechanisms and strategies to eliminate "foreign" molecules such as toxins, drugs and other xenobiotics. However, the mechanisms involved in the detoxification of excess nutrients and their derivatives are mostly unknown whatever the cell type. In the first part of the thesis, we hypothesized the existence of metabolic pathways for calorigenic nutrients detoxification. To highlight these pathways, we evaluated the destination of glucose carbon towards different metabolite candidates. More specifically, one way to identify pathways possibly involved in glucose excess detoxification is to measure metabolic pathway activities and metabolite levels at various concentrations of glucose, and identify those that increase further beyond the maximum glucose concentrations for GSIS. Furthermore, in contrast to most GSIS studies, we used a nutrient enriched incubation medium with a basal level of 4 mmol/l glucose supplemented with glutamine (2 mmol/l) and carnitine (50 μmol/l) when using isolated Langerhans islets. In fact, we realized that the basal level of 2-3 mmol/l glucose alone used in most studies, results in cellular energy depletion. Overall results indicate that detoxification processes could involve different forms of energy storage (TG, glycogen and cholesterol esters) and the extracellular release of metabolites derived from glucose (glycerol, cholesterol and FFA). Furthermore, data indicate that a major fate of glucose carbons is their transformation and released into the medium in the form of glycerol.