Background & Aims: P-glycoprotein, the product of the multidrug resistance (mdr) gene, functions as a xenobiotic transporter contributing to the intestinal barrier. Although intestinal expression of the mdr1a gene and its efflux pump function has been shown to exhibit 24-hour variation, the mechanism of the variations remains poorly understood. Here, we demonstrated that the molecular components of the circadian clock act as regulators to control 24-hour variation in the expression of the mdr1a gene. Methods: Luciferase reporter assay and gel mobility shift assay were used to study the mechanism of transcriptional regulation of the mdr1a gene by clock gene products. The messenger RNA levels and protein abundances in colon 26 cells and mouse intestine were measured by quantitative real-time polymerase chain reaction and Western blotting, respectively. Results: Hepatic leukemia factor (HLF) and E4 promoter binding protein-4 (E4BP4) regulated transcription of the mdr1a gene by competing with each other for the same DNA binding site. Molecular and biochemical analyses of HLF- and E4BP4-down-regulated colon 26 cells and the intestinal tract of Clock mutant mice suggested that these 2 proteins consisted of a reciprocating mechanism in which HLF activated the transcription of the mdr1a gene, whereas E4BP4 periodically suppressed transcription at the time of day when E4BP4 was abundant. Conclusions: The intestinal expression of the mdr1a gene is influenced by the circadian organization of molecular clockwork. Our present findings provide a link between the circadian timekeeping system and xenobiotic detoxification.
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