Lipid regulation of the ABCB1 and ABCG2 multidrug transporters

Impacts of ABCG2 loss of function variant (p. Gln141Lys, c.421 C > A, rs2231142) on lipid levels and statin efficiency: a systematic review and meta-analysis

BACKGROUND

The latest evidence indicates that ATP-binding cassette superfamily G member 2 (ABCG2) is critical in regulating lipid metabolism and mediating statin or cholesterol efflux. This study investigates whether the function variant loss within ABCG2 (rs2231142) impacts lipid levels and statin efficiency.

METHODS

PubMed, Cochrane Library, Central, CINAHL, and ClinicalTrials.gov were searched until November 18, 2023.

RESULTS

Fifteen studies (34,150 individuals) were included in the analysis. The A allele [Glu141Lys amino acid substitution was formed by a transversion from cytosine (C) to adenine (A)] of rs2231142 was linked to lower levels of high-density lipoprotein cholesterol (HDL-C), and higher levels of low-density lipoprotein cholesterol (LDL-C) and total cholesterol (TC). In addition, the A allele of rs2231142 substantially increased the lipid-lowering efficiency of rosuvastatin in Asian individuals with dyslipidemia. Subgroup analysis indicated that the impacts of rs2231142 on lipid levels and statin response were primarily in Asian individuals.

CONCLUSIONS

The ABCG2 rs2231142 loss of function variant significantly impacts lipid levels and statin efficiency. Preventive use of rosuvastatin may prevent the onset of coronary artery disease (CAD) in Asian individuals with dyslipidemia.

Lipid metabolism as a target for cancer drug resistance: progress and prospects

Cancer is the world's leading cause of human death today, and the treatment process of cancer is highly complex. Chemotherapy and targeted therapy are commonly used in cancer treatment, and the emergence of drug resistance is a significant problem in cancer treatment. Therefore, the mechanism of drug resistance during cancer treatment has become a hot issue in current research. A series of studies have found that lipid metabolism is closely related to cancer drug resistance. This paper details the changes of lipid metabolism in drug resistance and how lipid metabolism affects drug resistance. More importantly, most studies have reported that combination therapy may lead to changes in lipid-related metabolic pathways, which may reverse the development of cancer drug resistance and enhance or rescue the sensitivity to therapeutic drugs. This paper summarizes the progress of drug design targeting lipid metabolism in improving drug resistance, and providing new ideas and strategies for future tumor treatment. Therefore, this paper reviews the issues of combining medications with lipid metabolism and drug resistance.

Identification of clinical and pharmacogenetic factors influencing metformin response in Type 2 diabetes mellitus

Metformin, a hypoglycemic drug for Type 2 diabetes mellitus, shows variability in pharmacokinetics and response due to membrane transporters. This study followed 34 Type 2 diabetes mellitus patients on metformin treatment. Genetic variants in 11 metformin transport-related genes were analyzed, revealing associations. Specifically, SLC47A1 rs2289669 A/A and SLC22A4 rs1050152 T/T genotypes correlated with glycated hemoglobin values at 6 months. SLC47A1 rs2289669 G/A genotype influenced glucose levels at 6 months, while SLC29A4 rs3889348 A/A, SLC47A1 rs2289669 A/A, SLC22A4 rs1050152 C/T and SLC47A2 rs12943590 A/A genotypes were linked to glucose levels at 12 months. Additionally, ABCB1 rs2032582 C/A and ABCG2 rs2231137 C/T genotypes impacted cholesterol levels at 12 months. These findings shed light on metformin response determinants, offering insights for further research.

ABCG2 plays a central role in the dysregulation of 25-hydrovitamin D in Crohn's disease

Vitamin D (VD) deficiency, as indicated by the main circulating form of VD metabolite 25-hydrovitamin D₃ (25(OH)D₃), in patients with Crohn's disease (CD) has been well documented, but the reasons for this remain unclear. In this study, 367 patients with CD and 57 healthy individuals who were enrolled, and the association between 25(OH)D₃ level and clinical biochemical characteristics including hepatic and renal functions, inflammatory response was analyzed with binary logistic regression models. VD metabolic enzymes and transporters were screened with bioinformatical analysis and identified with qRT-PCR and western blot. Compared to the healthy controls, serum 25(OH)D₃ was significantly reduced in patients with CD, but the protein level of adenosine triphosphate (ATP)-binding cassette efflux transporter G2 (ABCG2) was evidently increased in the ileum and colon. Meanwhile, in lipopolysaccharide (LPS)-treated CaCO2 cells, the mRNA and protein levels of ABCG2 were significantly increased, and the overexpression of ABCG2 obviously promoted 25(OH)D₃ efflux, but, Ko143, an ABCG2 inhibitors, substantially prevented the efflux of 25(OH)D₃. In addition, in 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced CD model mice, the ABCG2 protein levels were significantly increased in the ileum, colon, kidney and liver, and administration of Ko143 significantly inhibited the efflux of 25 (OH) D₃in vivo. All of these findings suggest that VD deficiency in patients with CD may be associated with an abnormal increase in ABCG2 expression, but not directly implicated in hepatic and renal function, and inflammatory response in patients with CD.

Regulation of P-Glycoprotein in the Brain

Maintenance of the tightly regulated homeostatic environment of the brain is facilitated by the blood-brain barrier (BBB). P-glycoprotein (P-gp), an ATP-binding cassette transporter, is expressed on the luminal surface of the endothelial cells in the BBB, and actively exports a wide variety of substrates to limit exposure of the vulnerable brain environment to waste buildup and neurotoxic compounds. Downregulation of P-gp expression and activity at the BBB have been reported with ageing and in neurodegenerative diseases. Upregulation of P-gp at the BBB contributes to poor therapeutic outcomes due to altered pharmacokinetics of CNS-acting drugs. The regulation of P-gp is highly complex, but unravelling the mechanisms involved may help the development of novel and nuanced strategies to modulate P-gp expression for therapeutic benefit. This review summarises the current understanding of P-gp regulation in the brain, encompassing the transcriptional, post-transcriptional and post-translational mechanisms that have been identified to affect P-gp expression and transport activity.

Mechanistic Insights Delineating the Role of Cholesterol in Epithelial Mesenchymal Transition and Drug Resistance in Cancer

Despite the significant advancements made in targeted anti-cancer therapy, drug resistance constitutes a multifaceted phenomenon leading to therapy failure and ultimately mortality. Emerging experimental evidence highlight a role of cholesterol metabolism in facilitating drug resistance in cancer. This review aims to describe the role of cholesterol in facilitating multi-drug resistance in cancer. We focus on specific signaling pathways that contribute to drug resistance and the link between these pathways and cholesterol. Additionally, we briefly discuss the molecular mechanisms related to the epithelial-mesenchymal transition (EMT), and the documented link between EMT, metastasis and drug resistance. We illustrate this by specifically focusing on hypoxia and the role it plays in influencing cellular cholesterol content following EMT induction. Finally, we provide a proposed model delineating the crucial role of cholesterol in EMT and discuss whether targeting cholesterol could serve as a novel means of combatting drug resistance in cancer progression and metastasis.

Effect of statins on lipid metabolism-related microRNA expression in HepG2 cells

BACKGROUND

Statins are potent cholesterol-lowering drugs that prevent cardiovascular events. microRNAs (miRNAs) modulate the expression of genes involved in metabolic pathways and cardiovascular functions post-transcriptionally. This study explored the effects of statins on the expression of miRNAs and their target genes involved in lipid metabolism in HepG2 cells.

METHODS

HepG2 cells were treated with atorvastatin or simvastatin (0.1-10 µM) for 24 h. The expression of 84 miRNAs and nine target genes, selected by in silico studies, was measured by qPCR Array and TaqMan-qPCR, respectively.

RESULTS

Five miRNAs were upregulated (miR-129, miR-143, miR-205, miR-381 and miR-495) and two downregulated (miR-29b and miR-33a) in atorvastatin-treated HepG2 cells. Simvastatin also downregulated miR-33a expression. Both statins upregulated LDLR, HMGCR, LRP1, and ABCG1, and downregulated FDFT1 and ABCB1, whereas only atorvastatin increased SCAP mRNA levels. In silico analysis of miRNA-mRNA interactions revealed a single network with six miRNAs modulating genes involved in lipogenesis and lipid metabolism. The statin-dysregulated miRNAs were predicted to target genes involved in cellular development and differentiation, regulation of metabolic process and expression of genes involved in inflammation, and lipid metabolism disorders contributing to metabolic and liver diseases.

CONCLUSIONS

Atorvastatin-mediated miR-129, miR-143, miR-205, miR-381, and miR-495 upregulation, and miR-29b, and miR-33a downregulation, modulate the expression of target genes involved in lipogenesis and lipid metabolism. Thus, statins may prevent hepatic lipid accumulation and ameliorate dyslipidemia.

Human drug efflux transporter ABCC5 confers acquired resistance to pemetrexed in breast cancer

Aim

Pemetrexed, a new generation antifolate drug, has been approved for the treatment of locally advanced or metastatic breast cancer. However, factors affecting its efficacy and resistance have not been fully elucidated yet. ATP-binding cassette (ABC) transporters are predictors of prognosis as well as of adverse effects of several xenobiotics. This study was designed to explore whether ABC transporters affect pemetrexed resistance and can contribute to the optimization of breast cancer treatment regimen.

Methods

First, we measured the expression levels of ABC transporter family members in cell lines. Subsequently, we assessed the potential role of ABC transporters in conferring resistance to pemetrexed in primary breast cancer cells isolated from 34 breast cancer patients and the role of ABCC5 in mediating pemetrexed transport and apoptotic pathways in MCF-7 cells. Finally, the influence of ABCC5 expression on the therapeutic effect of pemetrexed was evaluated in an in vivo xenograft mouse model of breast cancer.

Results

The expression levels of ABCC2, ABCC4, ABCC5, and ABCG2 significantly increased in the pan-resistant cell line, and the ABCC5 level in the MCF-7-ADR cell line was 5.21 times higher than that in the control group. ABCC5 expression was inversely correlated with pemetrexed sensitivity (IC₅₀, r = 0.741; p < 0.001) in breast cancer cells derived from 34 patients. Furthermore, we found that the expression level of ABCC5 influenced the efflux and cytotoxicity of pemetrexed in MCF-7 cells, with IC₅₀ values of 0.06 and 0.20 μg/mL in ABCC5 knockout and over-expression cells, respectively. In the in vivo study, we observed that ABCC5 affected the sensitivity of pemetrexed in breast tumor-bearing mice, and the tumor volume was much larger in the ABCC5-overexpressing group than in the control group when compared with their own initial volumes (2.7-fold vs. 1.3-fold).

Conclusions

Our results indicated that ABCC5 expression was associated with pemetrexed resistance in vitro and in vivo, and it may serve as a target or biomarker for the optimization of pemetrexed regimen in breast cancer treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-01842-x.

The transport pathway in the ABCG2 protein and its regulation revealed by molecular dynamics simulations

Atomic-level structural insight on the human ABCG2 membrane protein, a pharmacologically important transporter, has been recently revealed by several key papers. In spite of the wealth of structural data, the pathway of transmembrane movement for the large variety of structurally different ABCG2 substrates and the physiological lipid regulation of the transporter has not been elucidated. The complex molecular dynamics simulations presented here may provide a breakthrough in understanding the steps of the substrate transport process and its regulation by cholesterol. Our analysis revealed drug binding cavities other than the central binding site and delineated a putative dynamic transport pathway for substrates with variable structures. We found that membrane cholesterol accelerated drug transport by promoting the closure of cytoplasmic protein regions. Since ABCG2 is present in all major biological barriers and drug-metabolizing organs, influences the pharmacokinetics of numerous clinically applied drugs, and plays a key role in uric acid extrusion, this information may significantly promote a reliable prediction of clinically important substrate characteristics and drug-drug interactions.

ACLY: A biomarker of recurrence in breast cancer

OBJECTIVE

ACLY is a cytoplasmic metabolic enzyme involved in lipid synthesis. It also affects proliferation and metastasis of breast cancer. However, the correlation of ACLY expression with breast cancer recurrence is unclear.

METHODS

The Oncomine and TCGA databases were used to investigate the mRNA expression of ACLY in breast cancer. Immunohistochemistry (IHC) was used to evaluate ACLY expression level in tumor tissues and normal tissues from 127 breast cancer patients. Next, the prognostic role of ACLY was explored by analyzing the clinicopathological features and prognosis during follow-up. The role of ACLY in breast cancer cells drug resistance was further detected by CCK-8 assays and quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

ACLY mRNA and protein expression was significantly increased in the breast cancer tissues compared to normal tissues. Clinically, high ACLY levels were associated with ER status, PR status, tumor size, TNM stage, and lymph node invasion. Upregulated ACLY predicted worse tumor relapse-free survival (RFS) of breast cancer patients in univariate analyses and in multivariate models. In subgroup analysis, patients with high ACLY expression showed worse RFS in the TNM III or ER positive subgroups. Moreover, ACLY over-expression induced the resistance of breast cancer cells to docetaxel and promoted the expression of multi-drug resistant protein ABCB1/ABCG2.

CONCLUSIONS

Our study highlights the possibility of ACLY as a potential and independent biomarker for the recurrence prediction in breast cancer patients. It may be related to ACLY promoting drug resistance in breast cancer cells.

Fatty acid receptor GPR120 promotes breast cancer chemoresistance by upregulating ABC transporters expression and fatty acid synthesis

BACKGROUND

Chemoresistance is the major cause of neoadjuvant treatment failure in breast cancer patients. Despite recent progress, the mechanism underlying chemoresistance remains to be further defined.

METHODS

Expression of G protein-coupled receptor 120 (GPR120) was analyzed by immunohistochemistry in the biopsies of primary breast cancer who subsequently underwent preoperative neoadjuvant chemotherapy. In vitro and in vivo loss- and gain-of -function studies were performed to reveal the effects and related mechanism of GPR120 signaling pathway in the chemoresistance of breast cancer cells.

FINDINGS

We identified that GPR120, a receptor for long-chain fatty acids, was important for the acquisition of chemoresistance in breast cancer cells. We showed that GPR120 expression was positively associated with clinical response to neoadjuvant chemotherapy in patients. In breast cancer cells, GPR120 enhanced the de novo synthesis of fatty acids that served as GPR120 ligands to activate GPR120 signaling via a feedback mechanism. Upregulated GPR120 signaling rendered cells resistant to epirubicin-induced cell death by upregulating ABC transporters expression and thus decreasing the intracellular accumulation of epirubicin. Akt/NF-κB pathway was responsible for the GPR120-mediated expression of ABC transporters leading to modulation of the concentration of chemotherapeutic drugs in cells. The functional importance of GPR120 in chemoresistance was further validated using epirubicin-treated tumor xenografts, in which we showed that blockade of GPR120 signaling with AH7614 or GPR120-siRNA significantly compromised chemoresistance.

INTERPRETATION

Our results highlight that GPR120 might be a promising therapeutic target for breast cancer chemoresistance. FUND: National Natural Science Foundation of China, Ministry of Science and Technology of China, Program of Science and Technology Commission of Shanghai Municipality.

Activation of LXRɑ/β by cholesterol in malignant ascites promotes chemoresistance in ovarian cancer

Background

The purpose of this study was to investigate the role of malignant ascites tumor microenvironment in ovarian cancer progression and chemoresistance.

Methods

A total of 45 patients with ovarian cancer and three benign ascites were collected at the time of clinical intervention. Ascites cholesterol levels were quantitated using cholesterol quantitation kit and recurrence free survival (RFS) of ovarian cancer patients were collected. The sensitivity of ovarian cancer cells to cisplatin (CDDP) and paclitaxel (PAC) were assessed by viability assay, flow cytometry and protein expression. Receiver operating characteristics (ROC) curve and Youden index analysis were applied to calculate the optimal cut-off values for ascites cholesterol. Kaplan-Meier curve were applied to compare RFS between high and low ascites cholesterol levels in ovarian cancer patients.

Results

Here we show that cholesterol is elevated in malignant ascites and modulates the sensitivity of ovarian cancer cells to CDDP and PAC by upregulating the expression of drug efflux pump proteins, ABCG2 and MDR1, together with upregulation of LXRɑ/β, the cholesterol receptor. Transfection of LXRɑ/β siRNA inhibited cholesterol-induced chemoresistance and upregulation of MDR1. In addition, the cholesterol level in malignant ascites was negatively correlated with number of CDDP-induced apoptotic cell death, but not with that of PAC-induced apoptotic cell death. Cholesterol depletion by methyl beta cyclodextrin (MβCD) inhibited malignant ascites-induced chemoresistance to CDDP and upregulation of MDR1 and LXRɑ/β. For patients with ovarian cancer, high cholesterol level in malignant ascites correlated with short RFS.

Conclusions

High cholesterol in malignant ascites contributes to poor prognosis in ovarian cancer patients, partly by contributing to multidrug resistance through upregulation of MDR1 via activation of LXRɑ/β.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-5152-5) contains supplementary material, which is available to authorized users.

Interactions Between ABCB1 Genotype and Preoperative Statin Use Impact Clinical Outcomes Among Breast Cancer Patients

Multiple clinical trials investigate statins' effects in breast cancer. The ABCB1 genotype appears to influence statin response and toxicity in the cardiovascular setting. This exploratory study aimed to investigate the interplay between preoperative statin use, ABCB1 genotype, and tumor-specific expression of the statin target 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) in breast cancer. Preoperative statin use, ABCB1 C3435T genotype, and HMGCR expression in relation to outcome were analyzed in 985 primary breast cancer patients from a population-based prospective cohort in Sweden from 2002 to 2012. Preoperative statin use (n = 80) was not associated with ABCB1 C3435T genotype (n = 576), HMGCR expression (n = 848), or clinical outcomes. ABCB1 C3435T TT-carriers had lower risk of breast cancer events than any C-carriers (adjusted hazard ratio (HR_adj) 0.74; 95%CI 0.49, 1.12), but only in non-statin users (P_interaction = 0.042). Statin users with TT genotype had higher risk of distant metastasis (HR_adj 4.37; 95%CI 1.20, 15.91; P_interaction = 0.009) and shorter overall survival than other patients (HR_adj 3.77; 95%CI 1.37, 10.39; P_interaction = 0.019). In conclusion, there were nominally significant interactions between ABCB1 genotype and preoperative statin use on clinical outcomes, while preoperative statin use was not associated with outcomes. Since this is an exploratory study of the impact of the ABCB1 genotype in relation to statin use and clinical outcomes in the breast cancer setting, the results should be interpreted with caution and warrant replication in an independent cohort, preferably in a randomized setting. Since statin use is common in breast cancer patients, it would be of interest to further elucidate the clinical impact of the ABCB1 genotype in breast cancer.

Cancer stem cells (CSCs), cervical CSCs and targeted therapies

Accumulating evidence has shown that cancer stem cells (CSCs) have a tumour-initiating capacity and play crucial roles in tumour metastasis, relapse and chemo/radio-resistance. As tumour propagation initiators, CSCs are considered to be promising targets for obtaining a better therapeutic outcome. Cervical carcinoma is the most common gynaecological malignancy and has a high cancer mortality rate among females. As a result, the investigation of cervical cancer stem cells (CCSCs) is of great value. However, the numbers of cancer cells and corresponding CSCs in malignancy are dynamically balanced, and CSCs may reside in the CSC niche, about which little is known to date. Therefore, due to their complicated molecular phenotypes and biological behaviours, it remains challenging to obtain “purified” CSCs and continuously culture CSCs for further in vitro studies without the cells losing their stem properties. At present, CSC-related markers and functional assays are used to purify, identify and therapeutically target CSCs both in vitro and in vivo. Nevertheless, CSC-related markers are not universal to all tumour types, although some markers may be valid in multiple tumour types. Additionally, functional identifications based on CSC-specific properties are usually limited in in vivo studies. Furthermore, an optimal method for identifying potential CCSCs in CCSC studies has not been previously published, and these techniques are currently of great importance. This article updates our knowledge on CSCs and CCSCs, reviews potential stem cell markers and functional assays for identifying CCSCs, and describes the potential of targeting CCSCs in the treatment of cervical carcinoma.

Suppression of Lipogenesis via Reactive Oxygen Species-AMPK Signaling for Treating Malignant and Proliferative Diseases

AIMS

Systemic diseases often have common characteristics. The aim of this study was to investigate the feasibility of targeting common pathological metabolism to inhibit the progression of malignant and proliferative diseases.

RESULTS

Gefitinib-resistant (G-R) nonsmall-cell lung cancer (NSCLC) and rheumatoid arthritis (RA) were studied as conditions representative of malignant and proliferative diseases, respectively. Strong lipogenic activity and high expression of sterol regulatory element-binding protein 1 (SREBP1) were found in both G-R NSCLC cells and synovial fibroblasts from RA patients (RASFs). Berberine (BBR), an effective suppressor of SREBP1 and lipogenesis regulated through reactive oxygen species (ROS)/AMPK pathway, selectively inhibited the growth of G-R NSCLC cells and RASFs but not that of normal cells. It effectively caused mitochondrial dysfunction, activated ROS/AMPK pathway, and finally suppressed cellular lipogenesis and cell proliferation. Addition of ROS blocker, AMPK inhibitor, and palmitic acid significantly reduced the effect of BBR. In an in vivo study, treatment of BBR led to significant inhibition of mouse tumor xenograft growth and remarkably slowed down the development of adjuvant-induced arthritis in rats. Innovation and Conclusion: Targeting ROS/AMPK/lipogenesis signaling pathway selectively inhibited the growth of G-R NSCLC cells and the progress of RASFs in vitro and in vivo, which provides a new avenue for treating malignancies and proliferative diseases. Antioxid. Redox Signal. 28, 339-357.

Structure-function relationships in ABCG2: insights from molecular dynamics simulations and molecular docking studies

Efflux pumps of the ATP-binding cassette transporters superfamily (ABC transporters) are frequently involved in the multidrug-resistance (MDR) phenomenon in cancer cells. Herein, we describe a new atomistic model for the MDR-related ABCG2 efflux pump, also named breast cancer resistance protein (BCRP), based on the recently published crystallographic structure of the ABCG5/G8 heterodimer sterol transporter, a member of the ABCG family involved in cholesterol homeostasis. By means of molecular dynamics simulations and molecular docking, a far-reaching characterization of the ABCG2 homodimer was obtained. The role of important residues and motifs in the structural stability of the transporter was comprehensively studied and was found to be in good agreement with the available experimental data published in literature. Moreover, structural motifs potentially involved in signal transmission were identified, along with two symmetrical drug-binding sites that are herein described for the first time, in a rational attempt to better understand how drug binding and recognition occurs in ABCG2 homodimeric transporters.

Sphingolipids facilitate age asymmetry of membrane proteins in dividing yeast cells

One proposed mechanism of cellular aging is the gradual loss of certain cellular components that are insufficiently renewed. In an earlier study, multidrug resistance transporters (MDRs) were postulated to be such aging determinants during the yeast replicative life span (RLS). Aged MDR proteins were asymmetrically retained by the aging mother cell and did not diffuse freely into the bud, whereas newly synthesized MDR proteins were thought to be deposited mostly in the bud before cytokinesis. In this study, we further demonstrate the proposed age asymmetry of MDR proteins in dividing yeast cells and investigate the mechanism that controls diffusive properties of MDR proteins to maintain this asymmetry. We found that long-chain sphingolipids, but not the septin/endoplasmic reticulum-based membrane diffusion barrier, are important for restricting MDR diffusion. Depletion of sphingolipids or shortening of their long acyl chains resulted in an increase in the lateral mobility of MDR proteins, causing aged MDR protein in the mother cell to enter the bud. We used a mathematical model to understand the effect of diminished MDR age asymmetry on yeast cell aging, the result of which was qualitatively consistent with the observed RLS shortening in sphingolipid mutants.

Localization of the placental BCRP/ABCG2 transporter to lipid rafts: Role for cholesterol in mediating efflux activity

INTRODUCTION

The breast cancer resistance protein (BCRP/ABCG2) is an efflux transporter in the placental barrier. By transporting chemicals from the fetal to the maternal circulation, BCRP limits fetal exposure to a range of drugs, toxicants, and endobiotics such as bile acids and hormones. The purpose of the present studies was to 1) determine whether BCRP localizes to highly-ordered, cholesterol-rich lipid raft microdomains in placenta microvillous membranes, and 2) determine the impact of cholesterol on BCRP-mediated placental transport in vitro.

METHODS

BCRP expression was analyzed in lipid rafts isolated from placentas from healthy, term pregnancies and BeWo trophoblasts by density gradient ultracentrifugation. BeWo cells were also tested for their ability to efflux BCRP substrates after treatment with the cholesterol sequestrant methyl-β-cyclodextrin (MβCD, 5 mM, 1 h) or the cholesterol synthesis inhibitor pravastatin (200 μM, 48 h).

RESULTS AND DISCUSSION

BCRP was found to co-localize with lipid raft proteins in detergent-resistant, lipid raft-containing fractions from placental microvillous membranes and BeWo cells. Treatment of BeWo cells with MβCD redistributed BCRP protein into higher density non-lipid raft fractions. Repletion of the cells with cholesterol restored BCRP localization to lipid raft-containing fractions. Treatment of BeWo cells with MβCD or pravastatin increased cellular retention of two BCRP substrates, the fluorescent dye Hoechst 33342 and the mycotoxin zearalenone. Repletion with cholesterol restored BCRP transporter activity. Taken together, these data demonstrate that cholesterol may play a critical role in the post-translational regulation of BCRP in placental lipid rafts.

Effects of Catechins and Their Related Compounds on Cellular Accumulation and Efflux Transport of Mitoxantrone in Caco-2 Cell Monolayers

The ability of catechins and their related compounds to inhibit breast cancer resistance protein (BCRP) function in Caco-2 cell monolayers was investigated with mitoxantrone as a BCRP substrate. The gallate or pyrogallol moiety on the catechin structure seemed to promote increased cellular accumulation and inhibit efflux transport of mitoxantrone. The ability of gallate catechins such as (-)-epigallocatechin gallate (EGCG) and (-)-epicatechin gallate (ECG) to increase cellular accumulation and inhibit efflux transport of mitoxantrone was greater than that of nongallate catechins. Gallic acid octyl ester (GAO) also increased intracellular mitoxantrone accumulation. Experiments using GAO derivatives indicated that the gallate moiety required the presence of a long carbon chain for BCRP inhibition. Cellular accumulation and reduced efflux transport of mitoxantrone were greater with epigallocatechin 3-(3″-O-butyl) gallate than with EGCG. EGCG inhibition of BCRP seemed to be restricted by hydrophobicity. The co-administration of catechins, particularly EGCG and related compounds, with greater hydrophobicity may increase the therapeutic activities of BCRP substrates such as mitoxantrone.

A mechanism for overcoming P-glycoprotein-mediated drug resistance: novel combination therapy that releases stored doxorubicin from lysosomes via lysosomal permeabilization using Dp44mT or DpC

The intracellular distribution of a drug can cause significant variability in both activity and selectivity. Herein, we investigate the mechanism by which the anti-cancer agents, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) and the clinically trialed, di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), re-instate the efficacy of doxorubicin (DOX), in drug-resistant P-glycoprotein (Pgp)-expressing cells. Both Dp44mT and DpC potently target and kill Pgp-expressing tumors, while DOX effectively kills non-Pgp-expressing cancers. Thus, the combination of these agents should be considered as an effective rationalized therapy for potently treating advanced and resistant tumors that are often heterogeneous in terms of Pgp-expression. These studies demonstrate that both Dp44mT and DpC are transported into lysosomes via Pgp transport activity, where they induce lysosomal-membrane permeabilization to release DOX trapped within lysosomes. This novel strategy of loading lysosomes with DOX, followed by permeabilization with Dp44mT or DpC, results in the relocalization of stored DOX from its lysosomal 'safe house' to its nuclear targets, markedly enhancing cellular toxicity against resistant tumor cells. Notably, the combination of Dp44mT or DpC with DOX showed a very high level of synergism in multiple Pgp-expressing cell types, for example, cervical, breast and colorectal cancer cells. These studies revealed that the level of drug synergy was proportional to Pgp activity. Interestingly, synergism was ablated by inhibiting Pgp using the pharmacological inhibitor, Elacridar, or by inhibiting Pgp-expression using Pgp-silencing, demonstrating the importance of Pgp in the synergistic interaction. Furthermore, lysosomal-membrane stabilization inhibited the relocalization of DOX from lysosomes to the nucleus upon combination with Dp44mT or DpC, preventing synergism. This latter observation demonstrated the importance of lysosomal-membrane permeabilization to the synergistic interaction between these agents. The synergistic and potent anti-tumor efficacy observed between DOX and thiosemicarbazones represents a promising treatment combination for advanced cancers, which are heterogeneous and composed of non-Pgp- and Pgp-expressing tumor cells.

Jump into a New Fold-A Homology Based Model for the ABCG2/BCRP Multidrug Transporter

ABCG2/BCRP is a membrane protein, involved in xenobiotic and endobiotic transport in key pharmacological barriers and drug metabolizing organs, in the protection of stem cells, and in multidrug resistance of cancer. Pharmacogenetic studies implicated the role of ABCG2 in response to widely used medicines and anticancer agents, as well as in gout. Its Q141K variant exhibits decreased functional expression thus increased drug accumulation and decreased urate secretion. Still, there has been no reliable molecular model available for this protein, as the published structures of other ABC transporters could not be properly fitted to the ABCG2 topology and experimental data. The recently published high resolution structure of a close homologue, the ABCG5-ABCG8 heterodimer, revealed a new ABC transporter fold, unique for ABCG proteins. Here we present a structural model of the ABCG2 homodimer based on this fold and detail the experimental results supporting this model. In order to describe the effect of mutations on structure and dynamics, and characterize substrate recognition and cholesterol regulation we performed molecular dynamics simulations using full length ABCG2 protein embedded in a membrane bilayer and in silico docking simulations. Our results show that in the Q141K variant the introduced positive charge diminishes the interaction between the nucleotide binding and transmembrane domains and the R482G variation alters the orientation of transmembrane helices. Moreover, the R482 position, which plays a role the substrate specificity of the transporter, is located in one of the substrate binding pockets identified by the in silico docking calculations. In summary, the ABCG2 model and in silico simulations presented here may have significant impact on understanding drug distribution and toxicity, as well as drug development against cancer chemotherapy resistance or gout.

Impact of Membrane Drug Transporters on Resistance to Small-Molecule Tyrosine Kinase Inhibitors

Small-molecule inhibitors of tyrosine kinases (TKIs) are the mainstay of treatment for many malignancies and represent novel treatment options for other diseases such as idiopathic pulmonary fibrosis. Twenty-five TKIs are currently FDA-approved and >130 are being evaluated in clinical trials. Increasing evidence suggests that drug exposure of TKIs may significantly contribute to drug resistance, independently from somatic variation of TKI target genes. Membrane transport proteins may limit the amount of TKI reaching the target cells. This review highlights current knowledge on the basic and clinical pharmacology of membrane transporters involved in TKI disposition and their contribution to drug efficacy and adverse drug effects. In addition to non-genetic and epigenetic factors, genetic variants, particularly rare ones, in transporter genes are promising novel factors to explain interindividual variability in the response to TKI therapy.

Recent advances in the exploration of the bile salt export pump (BSEP/ABCB11) function

INTRODUCTION

The bile salt export pump (BSEP/ABCB11), residing in the apical membrane of hepatocyte, mediates the secretion of bile salts into the bile. A range of human diseases is associated with the malfunction of BSEP, including fatal hereditary liver disorders and mild cholestatic conditions. Manifestation of these diseases primarily depends on the mutation type; however, other factors such as hormonal changes and drug interactions can also trigger or influence the related diseases.

AREAS COVERED

Here, we summarize the recent knowledge on BSEP by covering its transport properties, cellular localization, regulation and major mutations/polymorphisms, as well as the hereditary and acquired diseases associated with BSEP dysfunction. We discuss the different model expression systems employed to understand the function of the BSEP variants, their drug interactions and the contemporary therapeutic interventions.

EXPERT OPINION

The limitations of the available model expression systems for BSEP result in controversial conclusions, and obstruct our deeper insight into BSEP deficiencies and BSEP-related drug interactions. The knowledge originating from different methodologies, such as clinical studies, molecular genetics, as well as in vitro and in silico modeling, should be integrated and harmonized. Increasing availability of robust molecular biological tools and our better understanding of the mechanism of BSEP deficiencies should make the personalized, mutation-based therapeutic interventions more attainable.