Letter to the Editor: Does c‐Jun N‐Terminal Kinase Regulate Acetaminophen Hepatotoxicity by Modulating Nuclear Factor Erythroid 2–Related Factor 2–Dependent Genes or Mitochondrial Oxidant Stress?

Potential conflict of interest: Nothing to report. TO THE EDITOR: We read with interest the recent paper by Chen and coworkers, who provided evidence for nuclear factor erythroid 2–related factor 2 (Nrf2) as a target for c‐jun N‐terminal kinase (JNK).(1) Although we agree that JNK activation is a critical part of the signaling mechanism of acetaminophen (APAP) hepatotoxicity,(2) we disagree with the conclusion that phosphorylated JNK phosphorylation of Nrf2, causing down‐regulation of the Nrf2‐dependent antioxidant response element (ARE)–driven genes for reduced nicotinamide adenine dinucleotide phosphate quinone dehydrogenase 1 and glutathione S‐transferases alpha 3, mu 1, and mu 5, is the critical mechanism by which JNK promotes APAP‐induced cell death.(1) First, although JNK appears to phosphorylate Nrf2 in vivo as early as 3‐6 hours after APAP administration, the effects of the JNK inhibitor SP600125 on expression of these proteins were not observed during the main injury phase (6 hours) but only at 24 hours, i.e., at the end of the injury phase.(1) The authors did not provide any injury data at 6 hours.(1) However, similar experiments demonstrated clearly that even delayed treatment with a potent JNK inhibitor effectively protected against APAP‐induced liver injury at 6 hours.(3) Thus, the effects of Nrf2‐dependent protein expression changes occur too late to affect the mechanisms of cell death.(2) This is reiterated by hepatic glutathione levels, which are a functional readout of ARE activation. Glutathione depletion after 300 mg/kg APAP typically occurs within 30 minutes, and levels start recovering by 6 hours,(2) again indicating that the changes in protein expression described may not be relevant to the injury. Second, the authors show extensive Nrf2 phosphorylation at 6 hours, which, in contrast to the centrilobular necrosis, does not seem to have a zone specificity.(1) This is not consistent with the assumed JNK‐dependent Nrf2 phosphorylation being important for JNK‐dependent cell death. Third, and most importantly, selective inhibition of the mitochondrial oxidant stress by the superoxide dismutase–mimetic Mito‐Tempo(4) or pyruvate dehydrogenase kinase 4 deficiency(5) effectively protected against APAP‐induced liver at 6 hours without preventing JNK activation or phosphorylated JNK translocation to the mitochondria. These data strongly suggest that regulation of the mitochondrial oxidant stress by JNK is the critical downstream event in the pathophysiology,(2) not the delayed modulation of Nrf2‐dependent genes.(1)