Circadian regulation of hepatic cytochrome P450 2a5 by peroxisome proliferator-activated receptor γ

Abstract

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 proliferatoractivated receptor g (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-hydroxycoumarinwas 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 21418-to 21396-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.