Défense de thèse de doctorat en sciences biologiques : "mitochondrial uncoupling"

« Effects of a mitochondrial uncoupling on lipolysis and adipokine expression in 3T3-L1 murine adipocytes»

Jury

Jaap KEIJER (Univ. Wageningen), Luc BERTRAND (UCL), Xavier DE BOLLE, président (UNamur), Martine RAES (UNamur), Michel JADOT, co-promoteur (UNamur), Thierry ARNOULD, promoteur (UNamur)

Résumé

Obesity can be defined as an excessive accumulation of lipids into cells called adipocytes. Anti-obesity treatments usually rely on the decrease in energy intake by promoting satiety or by inhibiting nutrient intake by enterocytes. Unfortunately, they present a low efficiency. Therefore, new anti-obesity strategies must be designed. A promising strategy would be to take advantage of mitochondrial uncoupling. In mice, a controlled mitochondrial uncoupling decreases the adipocyte lipid content and limits the deleterious effects associated with adipocyte hypertrophy such as organelle dysfunction and alterations of the expression of genes encoding adipokines. The decrease in lipid content in adipose tissues can be explained by a decrease in lipid synthesis but also by an increase in lipid mobilization from lipid droplets by the lipolysis. However, the precise molecular mechanisms leading to fatty acid mobilization in response to mitochondrial uncoupling are still largely ignored. In addition, the endocrine function of adipocytes facing mitochondrial uncoupling is also still poorly described.

The first aim of the present study was to characterize the effects of a mild mitochondrial uncoupling in 3T3-L1 adipocytes induced by a prolonged incubation (3 days) with low concentration (0.5 µM) of a chemical mitochondrial uncoupler (FCCP) on lipolysis. The second objective was to analyze the expression of several genes encoding adipokines in adipocytes exposed to the uncoupler.

In the first part of this study, we analyzed the different forms of lipolysis identified so far in FCCP-treated adipocytes: HSL (Hormone Sensitive Lipase) and ATGL (Adipose Triglyceride Lipase)-dependent lipolysis, macrolipophagy and microlipophagy. First, we found that HSL and ATGL-dependent lipolysis does not play a role in lipolysis (assessed by the glycerol release) activated in response to uncoupling of mitochondria. Despite an increase in macroautophagy rate in adipocytes exposed to the mitochondrial uncoupler, inhibition of this process does not seem to prevent the FCCP-induced glycerol release. Finally, we found that a form of microautophagy that directly targets lipid droplets could be involved in lipid disposal in these conditions.

In the second part of this work, we identified several adipokines that are differentially expressed at the protein and/or at the mRNA level(s) in adipocytes exposed to FCCP. Indeed, the expression of genes coding for leptin, adiponectin, resistin and angiotensinogen is decreased in this condition. We also observed that the activity of DNMTs (DNA Methyl Transferases), enzymes involved in epigenetic modifications is increased in FCCP-treated adipocytes. In accordance with these results, we found that a fragment of the leptin promoter sequence is hypermethylated in adipocytes incubated with FCCP, which negatively correlates with the fact that leptin expression is strongly repressed in adipocytes incubated with the mitochondrial uncoupler.

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