REGULATION OF FOXO TRANSCRIPTION FACTORS BY OXIDATIVE STRESS: ROLE OF GLUTATHIONE

Abstract

FoxO transcription factors are downstream mediators of insulin effects that can bind to DNA and regulate transcription of multiple target genes. These target genes are known for their importance in several biological functions including energy homeostasis, longevity, tumour suppression and development. FoxO activity has been reported to be affected by insulin signaling and oxidative stress. Glutathione (GSH), a potent antioxidant and regulator of cellular redox state could therefore play a role in regulating FoxO activity. Herein, modulators of GSH levels were used to investigate the effect of GSH depletion on insulin-induced FoxO1/3a phosphorylation. HepG2 human hepatoma cells were exposed to diethyl maleate (DEM) to deplete thiols including glutathione, or buthionine sulfoximine (BSO) to inhibit the synthesis of glutathione, followed by insulin treatment, western blotting and immunodetection of FoxO1/3a phosphorylation and other insulin signaling targets. Our results show that DEM and BSO significantly attenuate insulininduced FoxO1/3a phosphorylation. In addition, the mechanism of DEM attenuating phosphorylation of FoxO1/3a has been investigated. DEM was found to be associated with a general impairment of insulin signaling and an activation of stress-induced Jun N-terminal kinases, JNK, which had previously been shown to modulate FoxO activity. In conclusion, depletion of thiols, including glutathione modulates FoxO signaling by interfering with FoxO1/3a phosphorylation.