Role of the CLOCK protein in liver detoxification

Whether and how circadian clock proteins regulate drug detoxification are not known. Here, we have assessed the effects of CLOCK (a core circadian clock protein) on drug metabolism and detoxification.

Regulation by CLOCK protein of drug-metabolizing enzymes was assessed using Clock knockout (Clock^-/- ) mice and Hepa-1c1c7/AML-12 cells. The relative mRNA and protein levels were determined by qPCR and Western blotting respectively. Toxicity and pharmacokinetic experiments were performed with Clock^-/- and wild-type mice after intraperitoneal injection of coumarin or cyclophosphamide. Transcriptional gene regulation was investigated using luciferase reporter, mobility shift, and chromatin immunoprecipitation (ChIP) assays.

Clock deletion disrupted hepatic diurnal expressions of a number of drug-metabolizing enzymes in mice. In particular, CYP2A4/5 expressions were markedly down-regulated, whereas CYP2B10 was up-regulated. Positive regulation of Cyp2a4/5 and negative regulation of Cyp2b10 by CLOCK were confirmed in Hepa-1c1c7 and AML-12 cells. Based on a combination of luciferase reporter, mobility shift, and ChIP assays, we found that CLOCK activated Cyp2a4/5 transcription via specific binding to E-box elements in promoter region and repressed Cyp2b10 transcription through REV-ERBα/β (two target genes of CLOCK and transcriptional repressors of Cyp2b10). Furthermore, Clock ablation sensitized mice to coumarin toxicity by down-regulating CYP2A4/5-mediated metabolism (a detoxification pathway) and to cyclophosphamide toxicity by up-regulating CYP2B10-mediated metabolism (generating the toxic metabolite 4-hydroxycyclophosphamide).

CLOCK protein regulates metabolism by the cytochrome P450 family and drug detoxification. The findings improve our understanding of the crosstalk between circadian clock and drug detoxification.