Le rôle du stress oxydant dans les changements épigénétiques contribuant aux complications du syndrome métabolique

DNA methylation is one of the epigenetic modifications to CpG islands. This covalent epigenetic modification, catalyzed by DNA methyltransferases (DNMTs), consists of localized cytosine methylation in carbon 5’ of the CpG islands resulting in 5-methylcytosine. DNA methylation is clearly implicated in stable gene inactivation and also in the genetic footprint. It is modulated by nutrition, particularly methyl donors and protein restriction. These epigenetic changes persist later in life and lead to the development of numerous pathologies such as metabolic syndrome and type 2 diabetes. In fact, many key genes undergo a modification to their methylation status in the presence of metabolic syndrome components. This shows that DNA methylation is a major process in the etiology of metabolic syndrome. The first work of this Ph.D focused on writing a journal article that examined the central part of the metabolic syndrome and analyze the role of epigenetic changes that may affect the occurrence of oxidative stress and cardiometabolic complications. Furthermore, intestinal cells Caco-2/15, which have the capacity to differentiate and acquire the physiological characteristics of the small intestine, were used and treated with iron-ascorbate to induce oxidative stress. Iron-ascorbate induced a significant increase in inflammation and lipid peroxidation (malondialdehyde) and alterations of the antioxidant defense (GPx and SOD2) accompanied by epigenetic changes. Moreover, pre-incubation of Caco-2/15 cells with 5-Aza-2′-deoxycytidine, a demethylating agent, and/or Trolox antioxidant normalized the activities of SOD2 and GPx, reduced lipid peroxidation and prevented inflammation. This initial work has shown that changes in the redox and inflammation induced by iron-ascorbate may involve epigenetic changes, particularly DNA methylation. To better define the impact of oxidative stress on the nutritional level, Guinea pigs aged three days were divided into three groups : 1) Control: animals enterally fed with regular chow; 2) Total parenteral nutrition (TPN); 3) H2O2: Control+ i.v. infusion with 350 μM H2O2. After four days, for one group, animals were sacrificed for liver sampling. The other group of animals had free access to regular chow and water until the end of study, 8 weeks later and were sacrificed after this period. This work has shown that at one week of age, DNMT activity and 5’-methyl-2’-deoxycytidine levels were lower in TPN and H2O2 groups compared to controls. At nine weeks, DNMT activity remained lower in TPN group whereas 5’-methyl-2’-deoxycytidine levels were lower in TPN and H2O2 groups. This work has demonstrated that administration of TPN or H2O2, early in life in Guinea pigs, induces a sustained hypomethylation of DNA following inhibition of DNMT activity. Finally, mice that have received a high-fat high-sugar diet (HFHS) were used as in vivo model of metabolic syndrome. Mice were fed either a standard chow diet (controls), a high-fat/high-sucrose (HFHS) diet, or the HFHS diet in combination with GFT505 (30 mg/kg), for 12 weeks. The HFHS diet was effective in inducing metabolic syndrome characteristics in view of increases in body weight, visceral adiposity, insulin insensitivity, plasma and hepatic lipids, oxidative stress and inflammation in liver. These derangements were accompanied with deficient in PPARα and PPARγ hepatic gene expressions and hypermethylation of their respective promoters. Addition of GFT505 to the HFHS diet prevented most of the cardiometabolic effects induced by HFHS diet via negative modulation of promoter hypermethylation, resulting in raised PPARα and PPARγ hepatic gene expression. In conclusion, GFT505 exerts positive metabolic effects by improving HFHS diet-induced metabolic syndrome due to epigenetic alterations of PPARs genes.  Together, the work of this thesis has shown that oxidative stress from nutrition can regulate epigenetic that lead to the development of metabolic syndrome. Nutrition appears as a critical factor in the prevention of fetal reprogramming and development of the metabolic syndrome. Since the mechanisms suggest that oxidative stress acts on metabolites of methionine cycle to alter the epigenetic, supplementation of these molecules and antioxidants could help to restore the redox balance and the epigenetic.

Keyphrases: Nutrition, Inflammation, DNA methylation, Oxidative stress, Antioxidant defense, Inflammation, Metabolic syndrome, Fetal programming