The mammalian circadian pacemaker stays in the paired suprachiasmatic nuclei (SCN). Recent several studies reveal that the circadian rhythms of physiology and behavior are controlled by clock genes. In addition, the effectiveness and toxicity of many drugs vary depending on dosing time associated with 24-h rhythms of biochemical, physiological, and behavioral processes under the control of the circadian clock. Acetaminophen (APAP) is a widely used analgesic drug, and is mainly biotransformed and eliminated as nontoxic conjugates with glucuronic acid and sulfuric acid. Only a small portion of the dose is mainly bioactivated by CYP2E1 to N-acetyl-p-benzoquinone imine (NAPQI), a reactive toxic intermediate. For APAP overdose, glucuronidation and sulfation are saturated and the formation of NAPQI increases. However, the exact mechanisms underlying the chronotoxicity of APAP have not been clarified yet. In the present study, we have clarified that there was a significant dosing time-dependent difference in hepatotoxicity induced by APAP in mice. The mechanism may be related to the rhythmicity of CYP2E1 activity and GSH conjugation. In additon, we investigated whether the liver transcription factor hepatic nuclear factor-1alpha (HNF-1alpha) and clock genes undergoing astriking 24-h rhythm in mouse liver contribute to the 24-h regulation of CYP2E1 activity. A significant 24-h rhythmicity was demonstrated for CYP2E1 activity, protein levels and mRNA levels. HNF-1alpha and clock genes may contribute to produce the 24-h rhythm of CYP2E1 mRNA levels. Metabolism by CYP and GSH conjugation are common metabolic pathways for many drugs such as APAP. These findings support the concept that choosing the most appropriate time of day to administer the drugs associated with metabolic rhythmicity such as CYP and GSH conjugation may reduce hepatotoxicity in experimental and clinical situations. 24-h rhythm of CYP2E1 activity was controlled by HNF-1alpha and clock gene, in a transcriptional level. Identification of rhythmic marker for selecting dosing time will lead improved progress and diffusion of chronopharmacotherapy.
