Elucidating the mechanisms for circadian expression of drug-metabolizing enzymes is essential for a better understanding of dosing time-dependent drug metabolism and pharmacokinetics. CYP2B6 (Cyp2b10 in mice) is an important enzyme responsible for metabolism and detoxification of approximately 10% of drugs. Here, we aimed to investigate a potential role of nuclear receptor co-repressor RIP140 in circadian regulation of Cyp2b10 in mice.

We first uncovered diurnal rhythmicity in hepatic RIP140 mRNA and protein with peak values at ZT10 (ZT, zeitgeber time). RIP140 ablation up-regulated Cyp2b10 expression and blunted its rhythm in mice and in AML-12 cells. Consistent with a negative regulatory effect, overexpression of RIP140 inhibited Cyp2b10 promoter activity and reduced cellular Cyp2b10 expression.

Furthermore, RIP140 suppressed Car- and Pxr-mediated transactivation of Cyp2b10, and the suppressive effects were attenuated when the RIP140 gene was silenced. Chromatin immunoprecipitation assays revealed that recruitment of RIP140 protein to the Cyp2b10 promoter was circadian time-dependent in wild-type mice. More extensive recruitment was observed at ZT10 than at ZT2 consistent with the rhythmic pattern of RIP140 protein. However, the time-dependency of RIP140 recruitment was lost in RIP140^−/− mice.

Additionally, we identified a D-box and a RORE cis-element in RIP140 promoter. D-box- and RORE-acting clock components such as Dbp, E4bp4, Rev-erbα/β and Rorα transcriptionally regulated RIP140, potentially accounting for its rhythmic expression.

In conclusion, RIP140 regulates diurnal expression of Cyp2b10 in mouse liver through periodical repression of Car- and Pxr-mediated transactivation. This co-regulator-driven mechanism represents a novel source of diurnal rhythmicity in drug-metabolizing enzymes.

阐明药物代谢酶的昼夜节律表达机制对于更好地了解剂量依赖性药物代谢和药代动力学至关重要。CYP2B6（在小鼠中为Cyp2b10）是一种重要的酶，负责大约10％的药物的代谢和排毒。在这里，我们旨在研究核受体共抑制子RIP140在小鼠Cyp2b10的昼夜节律调节中的潜在作用。

我们首先发现了肝脏RIP140 mRNA和蛋白质的昼夜节律性，其峰值位于ZT10（ZT，时代时间）。RIP140消融上调了Cyp2b10的表达，并减弱了小鼠和AML-12细胞的节律。与负调节作用一致，RIP140的过表达抑制Cyp2b10启动子活性并降低细胞Cyp2b10表达。

此外，RIP140抑制了Car和Pxr介导的Cyp2b10的反式激活，当RIP140基因沉默时，抑制作用减弱。染色质免疫沉淀试验表明，在野生型小鼠中，RIP140蛋白募集到Cyp2b10启动子是昼夜节律的。与RIP140蛋白的节奏模式相一致，在ZT10观察到的征募要比在ZT2征募的征兵更广泛。然而，在RIP140 ^-/-小鼠中RIP140募集的时间依赖性丧失。

此外，我们在RIP140启动子中鉴定了一个D-box和一个RORE顺式元件。D-box和RORE起作用的时钟成分，例如Dbp，E4bp4，Rev-erbα/β和Rorα转录调节RIP140，可能解释了其有节奏的表达。

总之，RIP140通过周期性抑制Car和Pxr介导的反式激活来调节Cyp2b10在小鼠肝脏中的昼夜表达。这种共同调节因子驱动的机制代表了药物代谢酶昼夜节律的新来源。