Formation of novel N-acetylcysteine-hemin adducts abrogates hemin-induced cytotoxicity and suppresses the NRF2-driven stress response in human pro-erythroid K562 cells

Heme (iron protoporphyrin IX), as the prosthetic group in hemoproteins, regulates vital cellular functions in human tissues. However, free heme released during hemolysis events promotes severe complications to millions of people worldwide. Over the years, thiols like glutathione (GSH) were known to antagonize heme toxicity. In this study, we have uncovered the underlying molecular mechanism by which N-acetylcysteine (NAC), a well-known thiol prevents hemin-induced cytotoxicity (HIC). Hemin-responsive human pro-erythroid K562 cells were employed to assess hemin intracellular accumulation and cytotoxicity at concentrations ≥50 μΜ, in cultures exposed only to hemin and/or both hemin and NAC. NAC inhibited the intracellular accumulation of hemin and prevented hemin-induced cell growth inhibition, cell death, oxidative stress, and accumulation of ubiquitinated proteins. Meanwhile, the activation of the NF-E2-related factor-2 (NRF2)-driven stress gene activation, a key element involved in HIC, was suppressed by NAC. A refined mechanism of the chemical reaction between NAC and hemin leading to adduct formation via a nucleophilic attack on hemin was uncovered for the first time by tandem mass spectrometry analysis (LC-MS/MS). Such thiol-hemin adducts acted as intermediates to mitigate HIC and to suppress hemin-induced NRF2-driven gene activation. Our findings support the concept that NAC-hemin adduct formation is the major novel molecular mechanism rather than the reactive oxygen species-scavenging capacity of thiols to protect cells from HIC. Our results imply that thiols and their derivatives can be of potential therapeutic value in hemolytic disorders.