Circadian Clock Is Involved in Regulation of Hepatobiliary Transport Mediated by Multidrug Resistance-Associated Protein 2

There has been a growing interest in circadian regulation of the expression and function of drug transporters. In this study, we investigated circadian rhythm in the expression and function of multidrug resistance-associated protein 2 (Mrp2) in mouse liver and involvement of circadian clock in their regulations by using the circadian clock genes (period 1 and period 2) knockout mice. The mRNA and protein expression of Mrp2, P-glycoprotein, and breast cancer resistance protein was measured in the mouse liver at different times of the day. Circadian variation of hepatobiliary excretion of phenolsulfonphthalein, a model substrate of Mrp2, was also investigated in mice. Circadian oscillation of Mrp2 protein expression was clearly observed in the mouse liver with levels down at the light phase and up at the dark phase. The cumulative biliary excretion and biliary clearance of phenolsulfonphthalein from the liver to the bile was 2.37- and 1.74-fold greater in mice administered during the dark phase than in those administered during the light phase, respectively. The circadian oscillation in mRNA expression of Mrp2 disappeared in period 1 and period 2 double knockout mice. These results suggest that the expression and function of Mrp2 show the circadian rhythm, controlled by circadian clock genes.