Abstract
ATP-driven efflux transporters at the blood-brain barrier both protect against neurotoxicants and limit drug delivery to the brain. In other barrier and excretory tissues, efflux transporter expression is regulated by certain ligand-activated nuclear receptors. Here we identified constitutive androstane receptor (CAR) as a positive regulator of P-glycoprotein, multidrug resistance-associated protein 2 (Mrp2), and breast cancer resistance protein (BCRP) expression in rat and mouse brain capillaries. Exposing rat brain capillaries to the CAR activator, phenobarbital (PB), increased the transport activity and protein expression (Western blots) of P-glycoprotein, Mrp2, and BCRP. Induction of transport was abolished by the protein phosphatase 2A inhibitor, OA. Similar effects on transporter activity and expression were found when mouse brain capillaries were exposed to the mouse-specific CAR ligand, 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). In brain capillaries from CAR-null mice, TCPOBOP did not increase transporter activity. Finally, treating mice with 0.33 mg/kg TCPOBOP or rats with 80 mg/kg PB increased P-glycoprotein-, Mrp2-, and BCRP-mediated transport and protein expression in brain capillaries assayed ex vivo. Thus, CAR activation selectively tightens the blood-brain barrier by increasing transport activity and protein expression of three xenobiotic efflux pumps.