Abstract
1. α-Lipoic acid (α-LA) activates antioxidant pathways and exerts insulin-like actions via phosphatidylinositol 3-kinase (PI3K), and previous studies from the authors’ laboratory support the essential role of PI3K-dependent CCAAT/enhancer binding protein (C/EBP) activation in antioxidant defence responses.
2. The study investigated whether α-LA treatment promoted phase II antioxidant gene induction through C/EBP activation and, if so, whether combined treatment of α-LA and insulin synergistically increased target gene expression.
3. α-LA treatment induced GSTA2 in H4IIE cells in a concentration- and time-dependent manner. In cells transfected with the regulatory region of the GSTA2 gene, α-LA treatment increased luciferase reporter-gene activity. Immunoblot, immunocytochemistry, and gel shift assays identified the nuclear translocation and DNA binding of C/EBPα and C/EBPβ, but not C/EBPδ, in α-LA-treated cells. Deletion of the C/EBP binding site abolished the ability of α-LA to promote the luciferase gene activity.
4. α-LA, when combined with low concentrations of insulin, transactivated the GSTA2 gene to a greater extent compared with α-LA or a higher concentration of insulin treatment alone. Combined treatment of α-LA with insulin not only potentiated insulin signalling, but also enhanced PI3K-dependent activation of C/EBPα and C/EBPβ forms. α-LA in combination with insulin substantially increased haemoxygenase-1, microsomal epoxide hydrolase and β-nicotinamide adenine dinucleotide phosphate (NADPH) quinone oxidoreductase expression, verifying the enhanced induction of other phase II enzymes. A dominant negative C/EBP transfection experiment indicated that GSTA2 gene induction by simultaneous treatment of α-LA and insulin was also dependent on C/EBPs.
5. The results demonstrate that α-LA induces phase II enzymes via PI3K-dependent C/EBPα and C/EBPβ activation, and enhances the ability of insulin to promote target gene induction.
Correction Statement
This article was originally published with errors, which have now been corrected in the online version. Please see Correction (http://dx.doi.org/10.1080/00498254.2019.1639934)
Acknowledgements
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (Grant Number R11-2007-107-01001-0).