ABCB1 and ABCG2 Regulation at the Blood-Brain Barrier: Potential New Targets to Improve Brain Drug Delivery

Abstract

The drug efflux transporters ABCB1 and ABCG2 at the blood-brain barrier limit the delivery of drugs into the brain. Strategies to overcome ABCB1/ABCG2 have been largely unsuccessful, which poses a tremendous clinical problem to successfully treat central nervous system (CNS) diseases. Understanding basic transporter biology, including intracellular regulation mechanisms that control these transporters, is critical to solving this clinical problem.In this comprehensive review, we summarize current knowledge on signaling pathways that regulate ABCB1/ABCG2 at the blood-brain barrier. In Section I, we give a historical overview on blood-brain barrier research and introduce the role that ABCB1 and ABCG2 play in this context. In Section II, we summarize the most important strategies that have been tested to overcome the ABCB1/ABCG2 efflux system at the blood-brain barrier. In Section III, the main component of this review, we provide detailed information on the signaling pathways that have been identified to control ABCB1/ABCG2 at the blood-brain barrier and their potential clinical relevance. This is followed by Section IV, where we explain the clinical implications of ABCB1/ABCG2 regulation in the context of CNS disease. Lastly, in Section V, we conclude by highlighting examples of how transporter regulation could be targeted for therapeutic purposes in the clinic. SIGNIFICANCE STATEMENT: The ABCB1/ABCG2 drug efflux system at the blood-brain barrier poses a significant problem to successful drug delivery to the brain. The article reviews signaling pathways that regulate blood-brain barrier ABCB1/ABCG2 and could potentially be targeted for therapeutic purposes.

Fig. 1

(A) The neurovascular unit. The neurovascular unit consists of endothelial cells surrounded by a basement membrane, astrocytes, pericytes, and neurons. This four-cell structure also known as the “neurovascular unit” is responsible for the regulation of blood-brain barrier function. (B) History of the blood-brain barrier. Timeline of fundamental discoveries made in the blood-brain barrier field. Created with BioRender.com.

Fig. 2

(A) History of ABCB1. From the discovery of the “permeability glycoprotein” by Juliano and Ling in 1976 to structural insights into substrate and inhibitor discrimination by human ABCB1 revealed by Alam and Locher in 2019. (B) ABCB1 structure. ABCB1 consists of two transmembrane domains TMD1 and TMD2, each of which has six transmembrane spanning α-helices and a nucleotide binding domain (NBD1 and NBD2). ABCB1 is N-glycosylated at the first extracellular loop. Created with BioRender.com.

Fig. 3

(A) History of ABCG2. From the discovery of the “breast cancer resistance protein” ABCG2 in 1998 to its cryo-EM structure and function. (B) ABCG2 structure. ABCG2 consists of one transmembrane domain that has six transmembrane spanning α-helices and one nucleotide binding domain (NBD1). ABCG2 is a half transporter that needs to homodimerize to fully function. (C) ABCB1 and ABCG2 at the blood-brain barrier. ABCB1 and ABCG2 are both located at the luminal membrane of endothelial cells comprising the blood-brain barrier. They act as a “first line of defense” by limiting xenobiotics including a large number of therapeutic drugs from entering into the brain. Created with BioRender.com.

Fig. 4

Transporter-dependent strategies to overcome ABCB1 and ABCG2 drug efflux. Transporter-dependent strategies focus on inhibiting and overcoming ABCB1- and ABCG2-mediated drug efflux by using (1) siRNA, (2) antibodies, (3) nontransporter substrates, or (4) transporter inhibitors. Created with BioRender.com.

Fig. 5

Regulation of ABCB1 and ABCG2 via corticoid receptors RAR/RXR, PXR, and CAR. (A) Upon ligand binding, the corticoid receptor dimer binds to the direct repeat and inverted repeat region of the target gene to increase ABCB1 and ABCG2 mRNA expression levels. (B) Upon ligand binding, RAR and RXR form a heterodimer that binds and activates the RAR response element (RARE), which increases ABCB1 expression. (C) A PXR ligand binds to inactivated PXR in the cytoplasm. Ligand binding then triggers conformational change of PXR during which the corepressor dissociates. Activated PXR translocates into the nucleus and heterodimerizes with retinoic X receptor α (RXRα). The complex PXR-RXRα together with its coactivators binds to the xenobiotic response element in the promotor region on ABCB1. This results in increased transcription of the gene and protein expression. (D) CAR forms a heterodimer with RXR that binds to RARE, which leads to an increase in ABCB1 and ABCG2 levels. Created with BioRender.com.

Fig. 6

Regulation of ABCB1 and ABCG2 via the nuclear receptors PPAR, ER, AhR, and thyroid hormone receptor. (A) PPAR forms a heterodimer with RXR that binds to and activates the PPAR response element, which leads to increased ABCB1 and ABCG2 levels. (B) Genomic regulation of ABCG2 is driven by the estrogen receptor that binds to the estrogen response element in the ABCG2 promotor region. In addition, ABCG2 is also regulated via rapid, nongenomic ER signaling involving PTEN/PI3K/Akt/GSK3. (C) AhR translocates into the nucleus and dimerizes with the aryl hydrocarbon receptor nuclear translocator resulting in the regulation of its target genes, including ABCB1 and ABCG2. (D) The thyroid receptor forms a complex with RXR and coactivators. This complex binds to the thyroid hormone response element and activates transcription of ABCB1. Created with BioRender.com.

Fig. 7

Inflammatory and oxidative stress signaling. (A) NFkB, a primary transcription factor, is activated by infectious and inflammatory stimuli. NF-κB binds to the promoters of its target genes and stimulates transcription. Both ABCB1 and ABCG2 are regulated via NFkB signaling at the blood-brain barrier. (B) Upon activation, Wnt binds to the Frizzled receptor, which recruits axin and inhibits GSK3Β. Consequently, the destruction complex cannot assemble, and β-catenin accumulates in the cytosol. After translocation into the nucleus, β-catenin acts as transcription factor and induces transcription of both ABCB1 and ABCG2. (C) In isolated brain capillaries, TNFα signals through TNF receptor 1 activating the endothelin converting enzyme, which, in turn, leads to the production of endothelin 1, which signals through the endothelin receptor B to activate the inducible nitric oxide synthase. NO stimulates protein kinase C, which leads to the activation of NF-κB, which upregulates ABCB1 protein expression and transport activity. (D) Seizure-induced glutamate release activates NMDAR- cytosolic phospholipase A2-COX-2 signaling that leads to the generation of prostaglandin E2 (PGE2) by the microsomal prostaglandin synthase. PGE2 activates the prostaglandin EP1 receptor, which via NF-κB activation ultimately leads to increased ABC transporter expression and activity levels at the blood-brain barrier. Created with BioRender.com.

Fig. 8

Regulation of ABCB1 by JAK-STAT3. Cytokines activate the JAK-STAT3 cascade, which leads to phosphorylation of STAT3, which leads to activation of the c-Jun NH2 terminal kinase that in turn deactivates c-Jun and reduces Abcb1 mRNA expression levels. Regulation by VEGF. VEGF signals through VEGFR2 to activate the nonreceptor tyrosine kinase Src. Activation of Src then induces phosphorylation of caveolin-1, which is followed by Abcb1 internalization and lysosomal degradation of the transporter. Regulation by the PI3K/Akt pathway. E2 signaling through ERβ inhibits the PTEN/PI3K/Akt/GSK3 pathway, which in turn leads to proteasomal degradation of Abcg2. Created with BioRender.com.

Fig. 9

Other signaling pathways. Diagram showing several other signaling pathways identified to regulate ABCB1 and/or ABCG2 at the blood-brain barrier: (A) adenosine, (B) circadian rhythm, (C) epigenic changes, and (D) P53. Created with BioRender.com.

Fig. 10

Clinical implications. Overview of diseases where ABCB1 and/or ABCG2 are changed and affect the progression and treatment of the respective disease. Created with BioRender.com.