Oxidarive stress modulates GSK3β signaling associated with lipolysis in fat cells

Abstract

Oxidative stress (OS) modulates fat metabolism and triggers chronic inflammation, promoting the onset of metabolic diseases such as type 2 diabetes and obesity. We have previously demonstrated that iron-induced OS increases lipolysis in mouse white adipose tissue as well as in in vitro differentiated adipocytes. We found that iron treatment triggered β-catenin upregulation and exacerbated lipolysis by ATGL activation. In addition, adipocytes where β-catenin had been deleted showed no ATGL upregulation by OS. Our aim was to study the role of GSK3β kinase in the modulation of β-catenin cascade in adipocytes exposed to OS. For this purpose, we used differentiated 3T3-L1 adipocytes (0.5 mM IBMX, 1 μM dexamethasone, 6 μg/ml insulin, and 5 µM rosiglitazone) exposed to 500 µM ferric ammonium citrate (FAC) for 24 h. We also analyzed adipose tissue and liver isolated from mice challenged with iron overload. As previously reported in adipose tissue, OS enhanced lipolysis in liver (p<0.05). To unravel the role of GSK3β pathway in OS-activated lipolysis, we performed experiments using LiCl (20 mM), a pharmacological inhibitor of the kinase. FAC and LiCl concentrations used in cell culture experiments had no effect on cell viability. A very well-known mechanism of β-catenin regulation and subcellular localization is its phosphorylation by GSK3β. We performed subcellular fractionation in 3T3-L1 adipocytes exposed to iron overload to study the localization of GSK3β/β-catenin. In nuclear fractions, GSK3β expression showed no significant difference between adipocytes treated with FAC, LiCl, and FAC plus LiCl. However, β-catenin expression was increased in LiCl-exposed adipocytes compared to controls (p<0.001) indicating that GSK3β inhibition favors β-catenin translocation to the nucleus. Our results show that nuclear translocation of β-catenin is promoted by its dephosphorylated form and that this mechanism is involved in the lipolytic response of the adipocyte to iron-triggered OS.