Review
Regulation of P-glycoprotein and other ABC drug transporters at the blood–brain barrier

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ATP-binding cassette (ABC) transporters are important selective elements of the blood–brain barrier. They line the luminal plasma membrane of the brain capillary endothelium, facing the vascular space, and both protect the central nervous system from entry of neurotoxicants and limit the access of therapeutic drugs to the brain parenchyma. Recent studies highlight the multiple signaling pathways through which the expression and activity of P-glycoprotein and other ABC transporters are modulated in response to xenobiotics, stress and disease. The results show that increased transporter expression occurs in response to signals that activate specific transcription factors, including pregnane-X receptor, constitutive androstane receptor, nuclear factor-κB and activator protein-1, and that reduced transporter activity occurs rapidly and reversibly in response to signaling through Src kinase, protein kinase C and estrogen receptors. A detailed understanding of such regulation can provide the basis for improved neuroprotection and enhanced therapeutic drug delivery to the brain.

Introduction

Delivery of drugs to the central nervous system (CNS) is one of the final frontiers of pharmacotherapy. To a large extent this is because of the inability of many candidate drugs to readily cross the blood–brain barrier (BBB) and reach sufficiently high concentrations at sites of action within the brain. This barrier resides within the 5–8-μm-diameter microvessels that comprise the brain capillary endothelium (Box 1). One defining feature of the brain capillary phenotype is the expression of ATP-driven drug efflux pumps [ATP-binding cassette (ABC) transporters] on the luminal blood-facing plasma membrane of the endothelial cells (Figure 1). These are members of the B, C and G families of ABC transporters that collectively restrict the uptake of numerous lipophilic xenobiotics that, on the basis of structure, should readily diffuse across endothelial cell membranes. Of these BBB efflux transporters, we have the most complete picture of function and regulation for P-glycoprotein (ABCB1), which handles a surprisingly large number of therapeutic drugs (polyspecificity) and is expressed at high levels in the brain capillary endothelium 1, 2. P-Glycoprotein knockout mice have been available for over a decade, and for many drugs that are P-glycoprotein substrates, these animals show large increases in brain-to-plasma concentration ratios over wild-type controls. In addition, several animal studies show remarkably increased effectiveness of chemotherapeutics against implanted human tumors when P-glycoprotein inhibitors are co-administered 3, 4, 5. P-Glycoprotein has proved to be a primary obstacle to drug delivery to the brain.

Other ATP-driven xenobiotic efflux pumps expressed at the luminal membrane include multidrug resistance-associated proteins (Mrp1, 2, 4 and 5; ABC C family) [6] and breast cancer-related protein (Bcrp, ABCG2; Figure 1) 7, 8. Experiments with Bcrp- or Mrp4-null mice and specific inhibitors of these transporters show increased brain accumulation of a restricted list of therapeutic drugs, such as imatinib (Bcrp) [8] and topotecan (Bcrp [9] and Mrp4 [10]), whereas no such list is available for Mrp2 in spite of the long availability of two strains of Mrp2-null rats and, more recently, Mrp2-null mice. It is likely, however, that all of these transporters, along with basolateral uptake transporters, such as organic anion transporter 3 (Oat3) and organic-anion-transporting polypeptides (Oatp1a4, Oatp1a5; Figure 1), contribute to brain-to-blood transport of potentially toxic endogenous metabolic wastes, which are largely organic anions. This aspect of BBB function is certainly understudied.

There are four compelling reasons for wanting to understand regulation of these transporters. First, we lack a basic understanding of how environmental factors, including diet, therapy and toxicant exposure, alter barrier properties and affect CNS pharmacotherapy. In certain situations, such as chemotherapy to the periphery, it might be advantageous to upregulate ABC transporter expression to increase CNS protection; we should devise strategies to do this in a safe manner. Second, ABC transporter expression in other barrier and excretory tissues seems to be affected by inflammatory and oxidative stress. Because inflammation and oxidative stress are factors in nearly all CNS disorders, it is important to understand how they affect ABC transporter expression at the BBB. Third, it is critical to know how transporter function is altered in specific CNS diseases, because alterations in BBB transporters will affect the efficacy of CNS-acting drugs. Fourth, the use of ABC transporter-specific inhibitors to improve drug delivery has not translated well to the clinic so far. Identification and targeting of signals that have the potential to rapidly modulate transporter activity could provide an alternative strategy.

Here I review recent findings on signals that regulate BBB ABC transporters in the context of the four reasons discussed above and attempt to integrate findings from in vitro and in vivo studies (Box 1). The material that follows is in three major sections: one dealing with signals that increase transporter protein expression, a second dealing with signals that reduce transporter activity without altering expression, and a third discussing the complexities of BBB signaling and transport.

Section snippets

Xenobiotic exposure

In peripheral barrier and excretory tissues, signals from several ligand-activated intracellular receptors increase the expression of both xenobiotic-metabolizing enzymes and excretory transporters. Prominent among these receptors are the former ‘orphan’ receptors pregnane-X receptor (PXR) and constitutive androstane receptor (CAR), which are considered to be a major part of the first line of defense against both endogenous toxicants and xenobiotics 11, 12. Both receptors are activated by

Altered transport activity

Despite the success of specific transport inhibitors in improving drug delivery to the brain in animal studies, the results have not been translated to the clinic. One problem is that we possess incomplete knowledge about transport function in the human BBB in situ. Indeed, it has been suggested that P-glycoprotein is present in excess in brain capillaries. As a result, more complete inhibition of transport activity would be needed in vivo than would be predicted based on in vitro dose–response

Conclusions

It is now clear from studies with animal models and with patient samples that the expression and activity of P-glycoprotein and other ABC transporters at the BBB can be moving targets, affected by genetics, disease, pharmacotherapy and diet. Indeed, we are rapidly adding to maps of the signals and signaling pathways involved with a view to improving both CNS protection and the delivery of small-molecule drugs to the brain. Thus, an understanding of signaling could provide opportunities to both

Acknowledgements

My work is supported by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences. I thank current and past members of the Miller Laboratory for helpful discussions.

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