Brain capillary endothelial cells express multiple ATP-binding cassette transport proteins on the luminal, blood-facing, plasma membrane. There these transporters function as ATP-driven efflux pumps for xenobiotics and endogenous metabolites, providing an important element of the barrier. When the transporters limit neurotoxicant entry into the central nervous system (CNS), they are neuroprotective; when they limit therapeutic drug entry, they are obstacles to drug delivery to treat CNS diseases. Certainly, changes in the transporter expression and transport activity can have a profound effect on CNS pharmacotherapy, with increased transport activity reducing drug access to the brain and vice versa. Here, I review the signals that alter transporter expression and transport function with an emphasis on P-glycoprotein, MRP2, and breast cancer resistance protein (ABCG2) (BCRP), the efflux transporters for which we have the most detailed picture of regulation. Recent work shows that transporter protein expression can be upregulated in response to inflammatory and oxidative stress, therapeutic drugs, diet, and persistent environmental pollutants; as a consequence, drug delivery to the brain is reduced. For many of these stimuli, the transcription factor, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), appears to be involved. However, NF-κB activation and nuclear translocation is often initiated by upstream signaling. In contrast, basal transport activity of P-glycoprotein and BCRP can be reduced through complex signaling pathways. Targeting such signals provides opportunities to rapidly and reversibly increase brain accumulation of drugs that are transporter substrates. The extent to which such signaling-based strategies can be utilized in the clinic remains to be seen.
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