Pregnane X receptor upregulates ABC-transporter Abcg2 and Abcb1 at the blood-brain barrier

ATP-driven efflux transporters are important, selective elements of the blood-brain barrier. Abcg2 (also brain multidrug resistance protein) and Abcb1 (P-glycoprotein) belong to the best known ABC-transporters which limit the access of therapeutic drugs to the brain and impair pharmacotherapy of central nervous system (CNS) disorders. To investigate the question how ATP-binding cassette (ABC)-transporters are regulated, we analyzed the influence of the nuclear receptor, pregnane X receptor (PXR) on transporter expression. PXR is a xenobiotic-activated transcription factor that is highly expressed in barrier tissue. Xenobiotics like rifampicin and hyperforin activate PXR and induce Abcb1 expression. ABC-transporter up-regulation could have potential effects on pharmacokinetics of different drugs. To study the influence of PXR on the two most prominent efflux transporters we used a primary culture of porcine brain capillary endothelial cells (PBCEC) due to higher homologies to the human form of PXR. For activation of the pregnane X receptor the ligands hyperforin and rifampicin were used. We investigated the effects on the transporters on RNA level (quantitative real time PCR), protein level (Western blotting) and transport level (uptake assay, active transport). The stimulation of the PBCEC with rifampicin or hyperforin showed a significant up-regulation of both ABC-transporters on RNA level after 6h, whereas an increased protein expression was strongest after 12h. Also the transport activity intensified after a period of 12h for Abcg2 and Abcb1. In conclusion our data prove PXR activation by rifampicin and hyperforin lead to an increased ABC-transporter expression and transport activity.