Multidrug efflux transporter ABCG2: expression and regulation

Abstract

The adenosine triphosphate (ATP)-binding cassette efflux transporter G2 (ABCG2) was originally discovered in a multidrug-resistant breast cancer cell line. Studies in the past have expanded the understanding of its role in physiology, disease pathology and drug resistance. With a widely distributed expression across different cell types, ABCG2 plays a central role in ATP-dependent efflux of a vast range of endogenous and exogenous molecules, thereby maintaining cellular homeostasis and providing tissue protection against xenobiotic insults. However, ABCG2 expression is subjected to alterations under various pathophysiological conditions such as inflammation, infection, tissue injury, disease pathology and in response to xenobiotics and endobiotics. These changes may interfere with the bioavailability of therapeutic substrate drugs conferring drug resistance and in certain cases worsen the pathophysiological state aggravating its severity. Considering the crucial role of ABCG2 in normal physiology, therapeutic interventions directly targeting the transporter function may produce serious side effects. Therefore, modulation of transporter regulation instead of inhibiting the transporter itself will allow subtle changes in ABCG2 activity. This requires a thorough comprehension of diverse factors and complex signaling pathways (Kinases, Wnt/β-catenin, Sonic hedgehog) operating at multiple regulatory levels dictating ABCG2 expression and activity. This review features a background on the physiological role of transporter, factors that modulate ABCG2 levels and highlights various signaling pathways, molecular mechanisms and genetic polymorphisms in ABCG2 regulation. This understanding will aid in identifying potential molecular targets for therapeutic interventions to overcome ABCG2-mediated multidrug resistance (MDR) and to manage ABCG2-related pathophysiology.

Keywords: ABCG2; ATP-binding cassette transporter; Drug resistance; Efflux transporter; Regulation; Signaling pathways.

Fig. 1

Network of signaling pathways in ABCG2 regulation. The different signaling pathways regulating ABCG2 are represented with different colors/shades. The signaling proteins (colored ellipses), associated transcription factors (down block arrow) and co-regulators (placed above respective transcription factor) of each cascade are shown as per their representative colors of receptors. The interactions between different molecules are shown by arrow (→ , activation) or solid vertical line (–| inhibition). These signal transduction events regulate ABCG2 expression and/or activity at different levels-transcriptional, post-transcriptional or membrane trafficking. Post-transcriptional regulation is indicated by an oval arrow. Thick line indicates regulation (activation/inhibition) of ABCG2 membrane trafficking. Please refer to the article for a detailed mechanism on each regulatory route. Dotted arrows indicate the unconfirmed routes in regard to ABCG2 regulation. Black-colored ball depicts the stimulating ligand which either binds to the corresponding receptors at the cell membrane to initiate signaling or enters passively across membrane to activate the nuclear receptor-mediated regulation. Additionally, each receptor is activated by a specific/group of ligands. Ligand: receptor interactions regulating ABCG2 are TGFβ: TGFβR, PRL: PRLR, GH: GHR, SDF-1: CXCR4, SHH: PTCH1, EGF: EGFR, HGF: c-met, PTH: PTHR, anandamide: CB2, Wnt: Fzd, uric acid: TLR4. Although major crosstalk between signaling molecules regulating ABCG2 are shown in this network, yet additional interactions remain to be discovered and characterized. TGFβR transforming growth factor-β (TGFβ) receptor, Smad2/3 mothers against decapentaplegic homolog 2/3, PRLR prolactin (PRL) receptor, GHR growth hormone (GH) receptor, JAK2 janus kinase 2, STAT5 signal transducer and activator of transcription 5, SDF-1 stromal cell-derived factor-1, CXCR4 C-X-C chemokine receptor type 4, SHH sonic hedgehog, PTCH1 patched-1, Smo smoothened homolog, SUFU suppressor of fused homolog, GLI glioma-associated oncogene, EGFR epidermal growth factor (EGF) receptor, HGFR/c-met hepatocyte growth factor (HGF) receptor, MEKK mitogen-activated protein kinase (MAPK) kinase kinase, MKK/MEK MAPK kinase, JNK c-Jun N-terminal kinase, ERK extracellular-signal-regulated kinase, PI3K phosphatidylinositol-3-kinase, PKB/Akt protein kinase B, PIP2 phosphatidylinositol-4, 5-bisphosphate, PIP3 phosphatidylinositol-3, 4, 5-triphosphate, PTEN phosphatase and tensin homolog, PDK1 3-phosphoinositide-dependent kinase 1, mTOR mammalian target of rapamycin complex, 4E-BP1 translation initiation factor 4E-binding protein 1, eIF4E translation initiation factor 4E, S6K S6 kinase, HIF-1α hypoxia-inducible factor 1-alpha, IKK inhibitory κB kinase, IκBα inhibitor of nuclear factor kappa B, NF-κB nuclear factor kappa B, ATM ataxia telangiectasia mutated, IL-6 interleukin-6, Cav-1 caveolin-1, PP1 protein phosphatase 1, PP2A protein phosphatase 2A, PTHR parathyroid hormone (PTH) receptor, CB2 cannabinoid receptor 2, AC adenylate cyclase, cAMP cyclic adenosine monophosphate, PKA protein kinase A, CREB cAMP responsive element-binding protein, CRTC2 CREB-regulated transcription coactivator 2, Fzd frizzled receptor, DVL disheveled, GSK3β glycogen synthase kinase 3 beta, β-cat β-catenin, TCF T-cell factor, TLR4 toll-like receptor 4, NLRP3 Nod-like receptor protein 3, PDZK1 postsynaptic density 95/disc large/zona occludens (PDZ) domain-containing 1, SIRT1 silent information regulator T1, PGC-1α PPARγ coactivator 1-alpha, PPARG peroxisome proliferator-activated receptor gamma, AhR aryl hydrocarbon receptor, Hsp90 heat shock protein 90, AR androgen receptor, ARNT aryl hydrocarbon receptor nuclear translocator

Fig. 2

Transcription factor binding sites in the human ABCG2 gene region. Blue arrow represents the spanning DNA. Each of the colored bands show the binding site for respective transcription factors as labelled in the figure. Their positions are mentioned in the parenthesis. These positions are relative to the transcriptional start site marked as + 1. Transcription factors binding upstream of the TSS: NF-κB nuclear factor kappa B, E2F1, Sp1 specificity protein 1, HIF-1α hypoxia-inducible factor-1 alpha, ER_C estrogen receptor, PR progesterone receptor, AhR aryl hydrocarbon receptor, BMI-1 B Lymphoma Mo-MLV Insertion Region 1, Smad2/3 mothers against decapentaplegic homolog 2/3, CREB cAMP responsive element-binding protein, GLI glioma-associated oncogene, NRF2 nuclear factor erythroid 2-related factor 2, STAT5 signal transducer and activator of transcription 5, HIF-2α hypoxia inducible factor-2 alpha, YAP1 Yes-associated protein 1, AR androgen receptor, EGFR epidermal growth factor receptor, cJUN, FOXM1 forkhead box protein M1, PPARα peroxisome proliferator-activated receptor alpha, PPARγ peroxisome proliferator-activated receptor gamma, CAR constitutive androstane receptor, IRF6 interferon regulatory factor 6. Transcription factors binding downstream of the TSS: SOX4, SRY (sex-determining region Y)-box 4 is predicted to bind to the region in the first intron

Fig. 3

Epigenetic regulation of ABCG2. A ABCG2 gene silencing involves hypermethylation of CpG island of ABCG2 promoter by DNMT1 and DNMT3a, recruitment of MBDs (MBD2 and MeCP2) along with co-repressor, mSin3A and HDAC1 to the methylated sites. This is accompanied by deacetylation at H3K9 by HDAC1 and the addition of the repressive histone mark H3K9me3 by HMTs. EZH2, an enzymatic component of PRC2, catalyzes trimethylation at H3K27 and represses transcriptional activity. B ABCG2 gene transcriptional activation involves the replacement of repressive histone mark H3K9Me3 with permissive changes which includes H3K4me3 and H3S10p, release of class 1 HDACs and subsequent recruitment of chromatin remodeller Brg-1 and RNA polymerase II to the promoter. Higher expression of chromatin organizer SATB1 also induces ABCG2 expression. C Upregulated PRMT3 cause methylation of R31 residue within the RRM of hnRNPA1 bound to ABCG2 mRNA and increases mRNA stability, thus post-transcriptionally activating ABCG2. Boxes with dotted outline indicates that the exact involvement of these genes in ABCG2 regulation is not yet identified. H3K9me3 H3K9 trimethylation, HMT histone methyltransferase, EZH2 enhancer of zeste homolog 2, PRC2 polycomb repressive complex 2, H3K27 histone 3 lysine 27, H3K4me3 histone H3 lysine 4 trimethylation, H3S10p histone H3 serine 10 phosphorylation, Brg-1 brahma-related gene-1, SATB1 special AT-rich sequence-binding protein 1, PRMT3 protein arginine methyltransferase 3, R31 arginine 31, RRM RNA recognition motif, hnRNPA1 heterogeneous nuclear ribonucleoprotein A1

Fig. 4

Regulation of ABCG2 transporter by protein degradation pathways. ABCG2 normal trafficking pathway is shown by grey arrows. Synthesized ABCG2 peptide after entering into the ER undergoes folding and N-linked glycosylation at amino-acid residue N596 to form WT, S-G protein. This protein is transported to Golgi complex where it undergoes C-G to form a mature protein which is translocated to plasma membrane. ABCG2 peptide is degraded by three pathways proteasomal, lysosomal and calpain mediated as represented by brown, blue and purple arrows, respectively. A Proteasomal degradation. Misfolded and/or non-glycosylated WT ABCG2 or N-linked glycosylation-deficient mutant, N596Q ABCG2 triggers the process of ERAD. Protein is retro-translocated from ER back to cytosol, ubiquitinated by E3 ubiquitin ligase and is subjected to degradation by proteasomes. Derlin-1 (membrane-spanning protein and a cofactor for E3) interacts with E3 to facilitate the retrotranslocation. Over-expressed Derlin-1 (pink) also prevents ABCG2 translocation from ER to Golgi complex and negatively regulates protein maturation of WT S-G ABCG2. B Lysosomal degradation. Mature ABCG2 after performing its role, undergoes internalization into the endosome and undergoes degradation in lysosomes. Xanthine compounds are reported to downregulate ABCG2 protein levels via routing ABCG2 to this pathway. C Calpain-mediated degradation. SAD, a mycotoxin activates a calcium-dependent non-lysosomal protease, calpain and promotes degradation of mature ABCG2. Plasma membrane-localized mature ABCG2 may also be degraded by the proteasomes, though exact mechanism is not known (brown dotted arrow). ER endoplasmic reticulum, N596 asparagine 596, S-G simply-glycosylated, WT wild-type, C-G complex-glycosylation, N596Q asparagine 596 to glutamine, ERAD endoplasmic-reticulum-associated protein degradation, Derlin-1 degradation in endoplasmic reticulum protein 1, SAD secalonic acid D