Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

Le récepteur LSR est un acteur important du métabolisme hépatique, puisqu'il joue un rôle dans la clairance des lipoprotéines à ApoB/ApoE riches en triglycérides durant la période postprandiale. Dans cette étude, nous avons montré qu'un traitement in vitro par DHA peut augmenter les niveaux de protéine et d'activité LSR dans les cellules d'hépatome de souris Hepa 1-6. En toute cohérence, un régime supplémenté en DHA a conduit à élever les niveaux de protéine LSR hépatique chez la souris. Mais aucune de ces deux études n'a montré de changement au niveau des ARNm. Ceci suggère que l'enrichissement en DHA influe positivement sur le microenvironnement de LSR et son ancrage à la surface de la cellule. Nous avons ensuite étudié le rôle du récepteur PPAR[alpha] dans la régulation du gène lsr. Une analyse in silico nous a permis d'identifier des éléments PPRE dans la région 5' régulatrice du gène humain et de ses homologues de souris et de rat. Des traitements pharmacologiques par des agoniste et antagoniste spécifiques de PPAR[alpha] ont montré que ce récepteur est impliqué dans la régulation transcriptionnelle de l'expression du LSR dans les cellules Hepa 1 6. Enfin, une analyse transcriptomique a révélé une diminution de l'expression de PPAR[alpha] et d'autres gènes impliqués dans le métabolisme lipidique hépatique chez la souris LSR+/- sous régime standard ou riche en graisses. En conclusion, toutes ces études indiquent que l'activité LSR hépatique est sous le contrôle de facteurs nutritionnels capables d'activer divers mécanismes de régulation, faisant du LSR une cible d'intérêt potentiel pour des stratégies nutritionnelles ou thérapeutiques destinées à prévenir ou traiter les dyslipidémies / Lipolysis stimulated lipoprotein receptor (LSR) plays an important role in the clearance of ApoB/ApoE containing triglyceride-rich lipoproteins during postprandial phase. In this study, we demonstrated that in vitro treatment of mouse hepatoma cells, Hepa 1-6, with docosahexaenoic acid (DHA) led to an increase in LSR protein levels as well as its activity. Furthermore, the mice placed on the diet supplemented with DHA showed an increase in hepatic LSR protein. However, the mRNA levels remained unchanged in both in vitro and in vivo studies, suggesting that DHA enrichment may result in changes in LSR microenvironment that could affect its anchorage at the surface of cell membrane. Specific peroxisome proliferator response elements were identified in the upstream region of human, mouse and rat lsr gene by in silico analysis. We therefore sought to determine the role of the transcription factor, peroxisome proliferator-activated receptor (PPAR[alpha]), in LSR regulation. In vitro pharmacological studies using PPAR[alpha]-selective agonist and antagonist agents demonstrated that PPAR[alpha] is indeed involved in the transcriptional regulation of LSR expression. Furthermore, qPCR array analysis revealed the downregulation of PPAR[alpha] and various genes involved in hepatic lipid metabolism in LSR+/- mice on standard and high-fat diets. In conclusion, these studies show that the hepatic LSR activity is controlled by dietary factors that can activate various pathways involved in regulating lipid homeostasis, therefore representing LSR as a potential target for either nutritional or therapeutic strategies towards the prevention or treatment of dyslipidemia

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

The alarming rate of increase in incidence and prevalence of the type 2 diabetes mellitus has prompted intense research on understanding the pathogenesis of the type 2 diabetes. It is observed that the development of type 2 diabetes is preceded by a state of insulin resistance and obesity. Previous studies have suggested that the obesity induced insulin resistance may be mediated by elevated levels of circulating free fatty acids (FFAs). The increase in circulating levels of FFAs may be contributed by the release of FFAs from stored triglycerides (TG) in adipocytes via lipolysis. It is hypothesized that the decrease in levels of circulating FFAs by sequestration and storage of FFAs in adipocytes may prevent deleterious effects of FFAs on insulin sensitivity. Recently our lab and others have shown that the storage of TG in adipocytes is promoted by a novel protein, Fat Specific Protein 27 (FSP27). Although, these studies also revealed FSP27 to be a lipid droplet associated protein that suppresses lipolysis to enhance TG accumulation in adipocytes, the role of FSP27 in lipolysis remains largely undetermined. Therefore, this study investigates the role and regulation of FSP27 in adipocytes in both the basal state, as well as during lipolysis. The studies presented here show FSP27 to be a remarkably short-lived protein (half-life=15 min) due to its rapid ubiquitination and proteasomal degradation. Thus, I tested the hypothesis that lipolytic agents like the cytokine, TNF-α and the catecholamine isoproterenol modulate FSP27 protein levels to regulate FFA release. Consistent with this concept, TNF-α markedly decreased FSP27 mRNA and protein along with lipid droplet size as it increased lipolysis in cultured adipocytes. Similarly, FSP27 depletion using siRNA mimicked the effect of TNF-α to enhance lipolysis, while maintaining stable FSP27 protein levels by expression of HA epitope-tagged FSP27 blocked TNF-α mediated lipolysis. In contrast, the robust lipolytic action of isoproterenol is paradoxically associated with increases in FSP27 protein and a delayed degradation rate that corresponds to decreased ubiquitination. This catecholamine-mediated increase in FSP27 abundance, probably a feedback mechanism to restrain excessive lipolysis by catecholamines, is mimicked by forskolin or 8-Bromo-cAMP treatment, and prevented by Protein Kinase A (PKA) inhibitor KT5720 or PKA depletion using siRNA. These results show that isoproterenol stabililizes FSP27 via the canonical PKA pathway and increased cAMP levels. However, the work presented here also suggests that FSP27 does not get phosphorylated in response to isoproterenol treatment, and the stabilization of FSP27 is independent of isoproterenol mediated lipolysis. The data presented in this thesis not only identifies the regulation of FSP27 as an important intermediate in mechanism of lipolysis in adipocytes in response to TNF-α and isoproterenol, but also suggests that FSP27 may be a possible therapeutic target to modulate lipolysis in adipocytes.