Circadian Regulation of Hepatic Cytochrome P450 2a5 by Peroxisome Proliferator-Activated Receptor γ
Human CYP2A6 (Cyp2a5 in mice) plays an important role in metabolism and detoxification of various drugs and chemicals. Here, we investigated a potential role of peroxisome proliferator-activated receptor γ (Ppar-γ) in circadian regulation of the Cyp2a5 enzyme. We first showed that Cyp2a5 mRNA and protein in mouse liver displayed robust circadian oscillations. Consistent with a circadian protein pattern, Cyp2a5-mediated 7-hydroxylation of coumarin was circadian time–dependent. Formation of 7-hydroxycoumarin was more extensive at a dosing time of Zeitgeber time 2 (ZT2) than that at ZT14. Interestingly, the nuclear receptor Ppar-γ was also a circadian gene. Circadian Ppar-γ protein level was strongly correlated with the Cyp2a5 mRNA level (r = 0.989). Furthermore, Ppar-γ activation (by a selective agonist, rosiglitazone) upregulated Cyp2a5 expression in Hepa-1c1c7 cells, whereas Ppar-γ knockdown downregulated Cyp2a5 expression. Also, Ppar-γ knockdown blunted the rhythmicity of Cyp2a5 mRNA in serum-shocked Hepa-1c1c7 cells. In addition, a combination of promoter truncation analysis, mobility shift, and chromatin immunoprecipitation assays revealed that Ppar-γ directly bound to a PPAR response element (i.e., the −1418- to −1396-bp region) within Cyp2a5 promoter and activated the gene transcription. Taken together, Ppar-γ was a transcriptional activator of Cyp2a5, and its rhythmic expression contributed to circadian expression of Cyp2a5.
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