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

Abstract

The obesity is now recognized as one of the biggest health problems worldwide. Obesity can be defined as an excessive accumulation of lipids into cells called adipocytes. As obesity is mainly caused by an imbalance between energy intake and energy consumption, actual anti-obesity treatments mainly rely on the decrease in food/energy intake by promoting satiety or by inhibiting nutrient intake by enterocytes. Unfortunately, such treatments present a low efficiency, especially on the long term run.

Therefore, new anti-obesity strategies must be designed. A promising strategy would be to take advantage of a physiological process: the uncoupling of mitochondria. In several animal models, it has now been clearly demonstrated that the induction of a controlled but sustained mitochondrial uncoupling can limit the lipid accumulation in adipocytes, limiting the deleterious effects associated with adipocyte hypertrophy such as organelle dysfunction and alterations in 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, most of the studies performed so far on the systemic and cell impact of mitochondria uncoupling were mainly focused on the lipid metabolism but poorly addressed the endocrine function of adipocytes. Indeed, it is now commonly admitted that adipocytes also behave as key endocrine cells by synthesizing and secreting more than 600 adipokines.

The aims of the present study was 1) to characterize the effect(s) 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 such as FCCP (carbonyl cyanide-4- (trifluoromethoxy)phenylhydrazone) on lipolysis and 2) 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, the macrolipophagy and the microlipophagy. First, we found that HSL and ATGL-
dependent lipolysis does not play a role in the activated lipolysis (assessed by the glycerol release) observed in response to the uncoupling of mitochondria. We also observed an increase in macroautophagy rate in adipocytes exposed to the mitochondrial uncoupler. However, macroautophagy inhibition does not
seem to prevent the FCCP-induced glycerol release. Based on the results obtained, we hypothesized that a form of microautophagy that directly targets lipid droplets could be the responsible for the release of glycerol 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, we found that the expression of leptin, adiponectin, resistin and angiotensinogen are 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.

In conclusion, the study presented here brings new information regarding the consequences of a mild mitochondrial uncoupling on lipid metabolism and endocrine function of white 3T3-L1 adipocytes in vitro. However, despite their interest, the relevance of the data obtained in this study remains to be validated in other models of mitochondrial uncoupling, tested in human adipocytes, and/or analyzed in more physiological systems using animal models developed to test biological responses to the mitochondrial uncoupling in adipocytes.

Date of Award	26 Apr 2016
Language	English
Awarding Institution	University of Namur
Supervisor	Xavier De Bolle (President), Michel Jadot (Co-Supervisor), Thierry Arnould (Supervisor), Martine Raes (Jury), Jaap KEIJER (Jury) & Luc Bertrand (Jury)