Role of inflammation in the pathogenesis of insulin resistance in obesity : specific role of reactive oxygen and reactive nitrogen species

Authors: Issa, Nahla
Advisor: Marette, André
Abstract: Chronic low-grade inflammation is considered one of the triggers of obesity-associated insulin resistance. Metabolic inflammation goes along with increased oxidative and nitrosative stress, but whether this promotes insulin resistance in obesity remains ill-defined. Thus, the primary objective of this thesis was to study the role of oxidative and nitrosative stress in the development of inflammation mediated insulin resistance and in particular to highlight the role of superoxide anion production, lipid radical generation, and iNOS induction, in mediating tyrosine nitration of insulin signaling proteins and other metabolic dysfunctions associated with obesity. In chapter I, we showed for the first time that treatment of adipocytes with cytokines induced NADPH oxidase-3 (NOX3) expression along with increasing superoxide production. Cytokine treatment also increased lipolysis as indicated by measuring free glycerol release and caused increase in the phosphorylation of hormone sensitive lipase. Interestingly, pharmacological inhibition of NOX activity by Diphenyleneiodonium (DPI) reversed the effect of cytokines on lipolysis and on the phosphorylation of HSL in line with decreasing superoxide production. Specific knockdown of NOX3 gene expression in adipocytes displayed the same effects as those exerted by DPI. These results indicate that NOX3 is the major NOX involved in superoxide production in 3T3L1 adipocytes and a regulator of lipolysis in inflammatory settings. In chapter II, we identified a new role of NOX3 and superoxide production in mediating tyrosine nitration on Akt in FAO hepatic cells. NOX3 expression was increased in primary hepatocytes after cytokine treatment together with an increase in 3-nitrotyrosine. Interestingly, primary hepatocytes isolated from high fat (HF) fed mice displayed more tyrosine nitration when compared to primary hepatocytes isolated from mice on chow diet. Also, we showed for the first time a tendency for high fat feeding to increase tyrosine nitration specifically on Akt. More importantly, two novel tyrosine nitrated sites on Akt1 were identified: tyrosine 152 and tyrosine 38, which seem to play a role in negatively regulating Akt activity when tyrosine nitrated. In chapter III, scavenging lipid radicals by α -(4-Pyridyl-1-oxide)-N-tert-butylnitrone (POBN) reversed the metabolic disorders caused by HF feeding in mice. POBN treated mice exhibited decrease in fat mass when compared to their HF counterparts. This effect was associated with enhanced glucose tolerance and insulin sensitivity. Also, adipose tissue inflammation was alleviated and mitochondrial function was ameliorated, insulin signaling in skeletal muscle was restored and mitochondrial oxidative metabolism was also enhanced. In the liver, POBN treatment prevented fat accumulation and enhanced lipid and glucose metabolism. Together these results highlight the important role of NOX3 generated superoxide in mediating tyrosine nitration in liver and in altering metabolic dysfunction in adipocytes. Also, two important tyrosine nitrated sites on Akt were identified that may possibly be involved in regulating its activity. Finally, the lipid radical scavenger, POBN, displayed anti-obesity effects in HF fed mice and this effect was associated with amelioration of several metabolic parameters.
Document Type: Thèse de doctorat
Issue Date: 2016
Open Access Date: 23 April 2018
Grantor: Université Laval
Collection:Thèses et mémoires

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