Beyond Competitive Inhibition: Regulation of ABC Transporters by Kinases and Protein-Protein Interactions as Potential Mechanisms of Drug-Drug Interactions

Abstract

ATP-binding cassette (ABC) transporters are transmembrane efflux transporters mediating the extrusion of an array of substrates ranging from amino acids and lipids to xenobiotics, and many therapeutic compounds, including anticancer drugs. The ABC transporters are also recognized as important contributors to pharmacokinetics, especially in drug-drug interactions and adverse drug effects. Drugs and xenobiotics, as well as pathologic conditions, can influence the transcription of ABC transporters, or modify their activity or intracellular localization. Kinases can affect the aforementioned processes for ABC transporters as do protein interactions. In this review, we focus on the ABC transporters ABCB1, ABCB11, ABCC1, ABCC4, and ABCG2 and illustrate how kinases and protein-protein interactions affect these transporters. The clinical relevance of these factors is currently unknown; however, these examples suggest that our understanding of drug-drug interactions will benefit from further knowledge of how kinases and protein-protein interactions affect ABC transporters.

Fig. 1.

Indirect mechanisms of drug interactions with ABC transporters. Drugs influencing the activity of ABC transporters by factors distinct from direct binding to the transporter. These mechanisms include regulation of mRNA and protein expression through influences on transcription, translation, and/or proteasomal degradation (A); regulation of protein trafficking leading to changes in stabilization of the transporter on the membrane and/or internalization (B); or regulation of transporter activity through protein interactions or phosphorylation by kinases (C).

Fig. 2.

EGF receptor (EGFR) effects on ABC transporters. The EGFR signals through both MAPK and PI3K pathways to affect ABC transporters, differentially, at the transcriptional, translational, and post-translational levels. Following ligand binding, the EGFR dimerizes and signals through its tyrosine kinase domain to lead to the phosphorylation and activation of MEK, which in turn phosphorylates and activates ERK. ERK signaling increases the mRNA and protein expression of ABCG2, decreases the mRNA and protein expression of ABCC4, and decreases the mRNA expression of ABCB1 while preventing the degradation of ABCB1 protein by the proteasome. PI3K can also be activated downstream from EGFR tyrosine kinase activity leading to the activation of Akt. Akt signaling then increases the mRNA expression of ABCG2, ABCB1, and ABCC4, and also increases the protein expression and membrane localization of ABCG2. PI3K signaling can be inhibited by PTEN; PTEN is activated by PPARγ signaling. Pharmacological inhibitors used to elucidate this pathway have been noted at the level of their inhibition; mRNA changes induced by the ERK pathway are noted in blue, while changes induced by the Akt pathway are noted in orange.

Fig. 3.

Predicted phosphorylation sites on ABCB1, ABCC1, ABCG2, and ABCC4. Potential post-translational modifications of the indicated proteins were performed using the default parameters at the following website: www.phosphosite.org.

Fig. 4.

ABCC4 PDZ motif binds to a variety of PDZ-domain containing proteins. (Upper panel) Overview of the workflow for PDZ arrays. (A–C) The purified PDZ domains from the indicated PDZ-domain proteins were immobilized, in duplicate, on the array membrane (Panomics). Biotin-containing ABCC4 purified peptide harboring the ABCC4 PDZ motif was incubated with the array membrane. Unbound ABCC4 was then washed away and bound ABCC4 detected using streptavidin-horseradish peroxidase. The ABCC4 was peptide: biotin-KSGSG-STLTIFETAL_COOH (blue = linker, orange = spacer; green = PDZ motif). (Lower panel) Results of three different PDZ arrays. (D–F) Proteins that have been immobilized on the membrane are noted, with positive binding indicated by the bold font.