St. John's Wort constituents modulate P-glycoprotein transport activity at the blood-brain barrier

The purpose of this study was to investigate the short-term signaling effects of St. John's Wort (SJW) extract and selected SJW constituents on the blood-brain barrier transporter P-glycoprotein and to describe the role of PKC in the signaling.

Cultured porcine brain capillary endothelial cells (PBCEC) and freshly isolated brain capillaries from pig were used as in vitro/ex vivo blood-brain barrier model. SJW modulation of P-glycoprotein function was studied in PBCEC using a calcein-AM uptake assay and in isolated pig brain capillaries using the fluorescent cyclosporine A derivative NBD-CSA and confocal microscopy.

SJW extract and the constituents hyperforin, hypericin, and quercetin decreased P-glycoprotein transport activity in a dose- and time-dependent manner. SJW extract and hyperforin directly inhibited P-glycoprotein activity, whereas hypericin and quercetin modulated transporter function through a mechanism involving protein kinase C. Quercetin at high concentrations decreased P-glycoprotein transport activity, but increased transporter function at low concentrations. This increase in P-glycoprotein activity was likely due to trafficking and membrane insertion of vesicles containing transporter protein.

Our findings provide new insights into the short-term interaction of SJW with P-glycoprotein at the blood-brain barrier. They are of potential relevance given the wide use of SJW as OTC medication and the importance P-glycoprotein has for CNS therapy.