Background: The transcription factor Nrf2 plays an important role in the cellular defensive machinery against oxidative stress and inflammation and is a potential therapeutic target in human disease. The synthetic triterpenoids including CDDO and its methyl (CDDO-Me, bardoxolone methyl) and imidazolide (CDDO-Im) derivatives have been shown to enhance Nrf2-mediated antioxidant activity in a number of diseases. However, how triterpenoids activate Nrf2 is poorly understood. It has been suggested that triterpenoids are capable of reacting with thiols on Keap1, leading to disruption of the Keap1-Nrf2 association and upregulation of an Nrf2 response. Unlike CDDO-Me, CDDO-Im has an extra reactive site which could covalently bind to other amino acid residues than cysteine. This study aims to investigate how CDDO-Im covalently binds to proteins and the potential intracellular protein targets that may be vital for the molecular pharmacological actions of CDDO-Im.
Methods & results: Here, model proteins such as HSA and GSTP were used to determine the potential chemical reactivity of CDDO-Im towards proteins. Mass spectrometric analysis revealed that CDDO-Im, similar to CDDO-Me, formed covalent adduct with cysteine residues on HSA and GSTP through Michael addition. More importantly, for the first time, we have shown that CDDO-Im can covalently modify lysine, arginine, serine, and tyrosine residues on HSA via transacylation mechanisms. Cross-linking adducts derived from acylation and Michael addition to an adjacent cysteine residue and two different tryptic peptides were also detected.
Conclusion: This study provided new insights into the chemical mechanisms of action of CDDO-Im and paved the way for exploring potential novel targets for CDDO-Im.