Structure and Mechanism of Human ABC Transporters

Cryo-EM structures of ABCC1 revealing new conformational dynamics in the resting state

ABCC1/MRP1 in the C branch of Adenosine triphosphate binding cassette (ABC) transporters superfamily, is directly linked to multiple drug resistance in chemotherapy. Here, to further understand the conformational dynamics of ABCC1, we performed single-particle cryo-electron microscopy analysis of purified bovine ABCC1. Two conformational states were found coexisted with nearly equal population. While one state has a wider substrate transporting pathway, akin to the previously reported apo structure, the other is narrower, despite the empty substrate pocket. In addition, multiple lipid-binding interfaces were identified based on the presence of rod-shaped, unmodeled, non-protein densities in the resolved density maps, potentially contributing to the stabilization of TMD0 domain and activity regulation of ABCC1. Further, we found that three asparagine residues in bovine ABCC1 are glycosylated. Together, our study provides fresh insights into the structural features and conformational dynamics of bovine ABCC1, offering a new framework for understanding the function and regulatory mechanisms of ABCC1.

Mechanistic insights into P-glycoprotein ligand transport and inhibition revealed by enhanced molecular dynamics simulations

P-glycoprotein (P-gp) plays a crucial role in cellular detoxification and drug efflux processes, transitioning between inward-facing (IF) open, occluded, and outward-facing (OF) states to facilitate substrate transport. Its role is critical in cancer therapy, where P-gp contributes to the multidrug resistance phenotype. In our study, classical and enhanced molecular dynamics (MD) simulations were conducted to dissect the structural and functional features of the P-gp conformational states. Our advanced MD simulations, including kinetically excited targeted MD (ketMD) and adiabatic biasing MD (ABMD), provided deeper insights into state transition and translocation mechanisms. Our findings suggest that the unkinking of TM4 and TM10 helices is a prerequisite for correctly achieving the outward conformation. Simulations of the IF-occluded conformations, characterized by kinked TM4 and TM10 helices, consistently demonstrated altered communication between the transmembrane domains (TMDs) and nucleotide binding domain 2 (NBD2), suggesting the implication of this interface in inhibiting P-gp's efflux function. A particular emphasis was placed on the unstructured linker segment connecting the NBD1 to TMD2 and its role in the transporter's dynamics. With the linker present, we specifically noticed a potential entrance of cholesterol (CHOL) through the TM4-TM6 portal, shedding light on crucial residues involved in accommodating CHOL. We therefore suggest that this entry mechanism could be employed for some P-gp substrates or inhibitors. Our results provide critical data for understanding P-gp functioning and developing new P-gp inhibitors for establishing more effective strategies against multidrug resistance.

MiRNAs function in the development of resistance against doxorubicin in cancer cells: targeting ABC transporters

Resistance to chemotherapeutic agents poses a significant challenge in cancer treatment, particularly with doxorubicin, a widely used drug for various cancers, including breast cancer, leukaemia, osteosarcoma, and gastrointestinal cancers. This review aims to elucidate the critical role of microRNAs (miRNAs) in the development of doxorubicin resistance, focusing on their interactions with ATP-binding cassette (ABC) transporters. Despite extensive research, the molecular mechanisms governing doxorubicin resistance still need to be completed, particularly regarding the regulatory influence of miRNAs on ABC transporter expression. By analyzing current literature, this review identifies a notable gap: the lack of comprehensive insight into how specific miRNAs modulate the expression and activity of ABC transporters in cancer cells, contributing to doxorubicin resistance. We systematically examine recent findings on the interplay between miRNAs and ABC transporters, providing a detailed assessment of potential therapeutic strategies that leverage miRNA modulation to overcome drug resistance. Ultimately, this review underscores the significance of integrating miRNA research into existing therapeutic frameworks to enhance the efficacy of doxorubicin in cancer treatment.

The Unveiled Novel regulator of Adeno-associated virus production in HEK293 cells

The field of gene therapy using Adeno-associated viral (AAV) vector delivery is rapidly advancing in the biotherapeutics industry. Despite its successes, AAV manufacturing remains a challenge due to limited production yields. The triple plasmid transfection of HEK293 cells represents the most extensively utilized system for AAV production. The regulatory factors and mechanisms underlying viral production in HEK293 cells are largely unknown. In this study, we isolated high-titer AAV production clones from a parental HEK293 population using a single limiting dilution step, and subsequently elucidating their underlying molecular mechanisms through whole transcriptome analysis. LncRNA TCONS_00160397 was upregulated in clones and shown to promoted HEK293 cells proliferation and improved the titer of AAV production. Mechanistically, results from proteomics and metabolomics indicated that TCONS_00160397 regulated the ABC transporters pathway. These findings furnish a rich repository of knowledge and actionable targets for the rational optimization of HEK293-based producer lines, thereby paving the way for tangible improvements in AAV vector output and expediting the broad implementation of gene therapies.

The association of ABC proteins with multidrug resistance in cancer

Multidrug resistance (MDR) poses one of the primary challenges for cancer treatment, especially in cases of metastatic disease. Various mechanisms contribute to MDR, including the overexpression of ATP-binding cassette (ABC) proteins. In this context, we reviewed the literature to establish a correlation between the overexpression of ABC proteins and MDR in cancer, considering both in vitro and clinical studies. Initially, we presented an overview of the seven subfamilies of ABC proteins, along with the subcellular localization of each protein. Subsequently, we identified a panel of 20 ABC proteins (ABCA1-3, ABCA7, ABCB1-2, ABCB4-6, ABCC1-5, ABCC10-11, ABCE1, ABCF2, ABCG1, and ABCG2) associated with MDR. We also emphasize the significance of drug sequestration by certain ABC proteins into intracellular compartments. Among the anticancer drugs linked to MDR, 29 were definitively identified as substrates for at least one of the three most crucial ABC transporters: ABCB1, ABCC1, and ABCG2. We further discussed that the most commonly used drugs in standard regimens for mainly breast cancer, lung cancer, and acute lymphoblastic leukemia could be subject to MDR mediated by ABC transporters. Collectively, these insights will aid in conducting new studies aimed at a deeper understanding of the clinical MDR mediated by ABC proteins and in designing more effective pharmacological treatments to enhance the objective response rate in cancer patients.

Reversal potentials of Tween 20 in ABC transporter-mediated multidrug-resistant cancer and treatment-resistant depression through interacting with both drug-binding and ATP-binding areas on MDR proteins

Drug efflux transporters, especially those belonging to the ATP-binding cassette (ABC) transporter superfamily, play a crucial role in various drug resistance issues, including multidrug resistance (MDR) in cancer and treatment-resistant depression (TRD) in individuals with major depressive disorder. Key transporters in this context include P-glycoprotein (P-gp), multidrug resistance protein 1 (MRP1), and breast cancer resistance protein (BCRP). This study aimed to investigate the modulatory effects of polyoxyethylene (20) sorbitan monolaurate (Tween 20) on these efflux transporters in vitro and to evaluate its potential for overcoming drug resistance in two models: an in vitro cancer MDR model and an in vivo TRD model. The findings indicated that 0.001% Tween 20 significantly inhibited the efflux actions of all three transporters. Additionally, 0.005% Tween 20 effectively reversed resistance to paclitaxel, vincristine, doxorubicin, and mitoxantrone in various cancer MDR cell lines. In the in vivo depression-like behaviour model, 0.01% Tween 20 markedly enhanced the antidepressant and anxiolytic effects of fluoxetine. Given its strong inhibitory effects on P-gp, MRP1, and BCRP, along with its capacity to reverse drug resistance both in vitro and in vivo, Tween 20 is a compelling candidate for tackling transporter-mediated drug resistance.

Structural insights into brassinosteroid export mediated by the Arabidopsis ABC transporter ABCB1

Brassinosteroids (BRs) are steroidal phytohormones indispensable for plant growth, development, and responses to environmental stresses. The export of bioactive BRs to the apoplast is essential for BR signaling initiation, which requires binding of a BR molecule to the extracellular domains of the plasma membrane-localized receptor complex. We have previously shown that the Arabidopsis thaliana ATP-binding cassette (ABC) transporter ABCB19 functions as a BR exporter and, together with its close homolog ABCB1, positively regulates BR signaling. Here, we demonstrate that ABCB1 is another BR transporter. The ATP hydrolysis activity of ABCB1 can be stimulated by bioactive BRs, and its transport activity was confirmed in proteoliposomes and protoplasts. Structures of ABCB1 were determined in substrate-unbound (apo), brassinolide (BL)-bound, and ATP plus BL-bound states. In the BL-bound structure, BL is bound to the hydrophobic cavity formed by the transmembrane domain and triggers local conformational changes. Together, our data provide additional insights into ABC transporter-mediated BR export.

Unraveling the secrets: Evolution of resistance mediated by membrane proteins

Membrane protein-mediated resistance is a multidisciplinary challenge that spans fields such as medicine, agriculture, and environmental science. Understanding its complexity and devising innovative strategies are crucial for treating diseases like cancer and managing resistant pests in agriculture. This paper explores the dual nature of resistance mechanisms across different organisms: On one hand, animals, bacteria, fungi, plants, and insects exhibit convergent evolution, leading to the development of similar resistance mechanisms. On the other hand, influenced by diverse environmental pressures and structural differences among organisms, they also demonstrate divergent resistance characteristics. Membrane protein-mediated resistance mechanisms are prevalent across animals, bacteria, fungi, plants, and insects, reflecting their shared survival strategies evolved through convergent evolution to address similar survival challenges. However, variations in ecological environments and biological characteristics result in differing responses to resistance. Therefore, examining these differences not only enhances our understanding of adaptive resistance mechanisms but also provides crucial theoretical support and insights for addressing drug resistance and advancing pharmaceutical development.

Methionine restriction promotes cisplatin sensitivity of gastric cancer resistant cells by down-regulating circ-CDK13 level

BACKGROUND

Methionine restriction (MR) is a research direction in the treatment of gastric cancer (GC). The aim of this study was to investigate the molecular mechanism of MR on enhancing cisplatin (DDP) sensitivity of drug-resistant GC cells.

METHODS

Twenty pairs of GC tissues and adjacent normal gastric mucosa tissues were collected. DDP-resistant cell lines (KATO/DDP and MKN45/DDP), mouse model of GC and GC patient-derived organoid (PDO) models were established. Lentivirus-mediated METase overexpression was used for MR. Cell viability and apoptosis were detected by MTT assay and flow cytometry. Western blotting was used to detect multi-drug resistance-1 (MDR1), MDR-associated protein 1 (MRP1) eukaryotic initiation factor 4A-Ⅲ (EIF4A3), and METase protein expressions. The levels of circRNAs were detected by qRT-PCR. Tumor volume and weight were measured. The proliferation of tumor cells was detected by immunohistochemical staining.

RESULTS

The differentially expressed circRNAs of GC were screened in Gene Expression Omnibus database. MR in KATO/DDP and MKN45/DDP cells significantly down-regulated circ-CDK13 level. Overexpression of circ-CDK13 significantly inhibited apoptosis of sensitive cells (KATO III and MKN45). Interference with circ-CDK13 significantly promoted apoptosis of drug-resistant cells (KATO/DDP and MKN45/DDP). MR enhanced the DDP sensitivity of GC resistant cells, GC PDO and GC mice by down-regulating circ-CDK13. EIF4A3 binds to the downstream flanking sequence of circ-CDK13, and interference with EIF4A3 reduces circ-CDK13 levels, but does not affect CDK13. The expressions of circ-CDK13 and EIF4A3 in GC clinical samples were increased and positively correlated. Simultaneously overexpression of METase and EIF4A3 in resistant cells inhibited apoptosis, and further interference with circ-CDK13 reversed this effect.

CONCLUSION

MR inhibits circ-CDK13 level by down-regulating EIF4A3, thereby increasing the sensitivity of GC drug-resistant cells to DDP.

lncRNAs: New players of cancer drug resistance via targeting ABC transporters

Cancer drug resistance poses a significant obstacle to successful chemotherapy, primarily driven by the activity of ATP-binding cassette (ABC) transporters, which actively efflux chemotherapeutic agents from cancer cells, reducing their intracellular concentrations and therapeutic efficacy. Recent studies have highlighted the pivotal role of long noncoding RNAs (lncRNAs) in regulating this resistance, positioning them as crucial modulators of ABC transporter function. lncRNAs, once considered transcriptional noise, are now recognized for their complex regulatory capabilities at various cellular levels, including chromatin modification, transcription, and post-transcriptional processing. This review synthesizes current research demonstrating how lncRNAs influence cancer drug resistance by modulating the expression and activity of ABC transporters. lncRNAs can act as molecular sponges, sequestering microRNAs that would otherwise downregulate ABC transporter genes. Additionally, they can alter the epigenetic landscape of these genes, affecting their transcriptional activity. Mechanistic insights reveal that lncRNAs contribute to the activity of ABC transporters, thereby altering the efflux of chemotherapeutic drugs and promoting drug resistance. Understanding these interactions provides a new perspective on the molecular basis of chemoresistance, emphasizing the regulatory network of lncRNAs and ABC transporters. This knowledge not only deepens our understanding of the biological mechanisms underlying drug resistance but also suggests novel therapeutic strategies. In conclusion, the intricate interplay between lncRNAs and ABC transporters is crucial for developing innovative solutions to combat cancer drug resistance, underscoring the importance of continued research in this field.

Age-Dependent Differences in Radiation-Induced DNA Damage Responses in Intestinal Stem Cells

Age at exposure is a critical modifier of the risk of radiation-induced cancer. However, the effects of age on radiation-induced carcinogenesis remain poorly understood. In this study, we focused on tissue stem cells using Lgr5-eGFP-ires-CreERT2 mice to compare radiation-induced DNA damage responses between Lgr5+ and Lgr5- intestinal stem cells. Three-dimensional immunostaining analyses demonstrated that radiation induced apoptosis and the mitotic index more efficiently in adult Lgr5- stem cells than in adult Lgr5+ stem cells but not in infants, regardless of Lgr5 expression. Supporting this evidence, rapid and transient p53 activation occurred after irradiation in adult intestinal crypts but not in infants. RNA sequencing revealed greater variability in gene expression in adult Lgr5+ stem cells than in infant Lgr5+ stem cells after irradiation. Notably, the cell cycle and DNA repair pathways were more enriched in adult stem cells than in infant stem cells after irradiation. Our findings suggest that radiation-induced DNA damage responses in mouse intestinal crypts differ between infants and adults, potentially contributing to the age-dependent susceptibility to radiation carcinogenesis.

Lansoprazole (LPZ) reverses multidrug resistance (MDR) in cancer through impeding ATP-binding cassette (ABC) transporter-mediated chemotherapeutic drug efflux and lysosomal sequestration

AIMS

Lansoprazole is one of the many proton pump inhibitors (PPIs) that acts more strongly with ABCB1 and ABCG2. The present study is to investigate the potential of lansoprazole on reversal of ABCB1/G2-mediated MDR in cancer, in vitro and in vivo.

METHODS

Reversal studies and combination evaluation were conducted to determine the synergistic anti-MDR effects on lansoprazole. Lysosomal staining was used to determination of lansoprazole on ABCB1-mediated lysosomal sequestration. Substrate accumulation and efflux assays, ATPase activity, and molecular docking were conducted to evaluate lansoprazole on ABCB1/G2 functions. Western blot and immunofluorescence were used to detect lansoprazole on ABCB1/G2 expression and subcellular localization. MDR nude mice models were established to evaluate the effects of lansoprazole on MDR in vivo.

RESULTS

Lansoprazole attenuated ABCB1/G2-mediated MDR and exhibited synergistic effects with substrate drugs in MDR cells. In vivo experiments demonstrated that lansoprazole attenuated ABCB1/G2-mediated MDR and exhibited synergistic effects that augmented the sensitivity of substrate anticancer drugs in ABCB1/G2-mediated settings without obvious toxicity. Lansoprazole impeded lysosomal sequestration mediated by ABCB1, leading to a substantial increase in intracellular accumulation of substrate drugs. The effects of lansoprazole were not attributable to downregulation or alterations in subcellular localization of ABCB1/G2. Lansoprazole promoted the ATPase activity of ABCB1/G2 and competitively bound to the substrate-binding region of ABCB1/G2.

CONCLUSIONS

These findings present novel therapeutic avenues whereby the combination of lansoprazole and chemotherapeutic agents mitigates MDR mediated by ABCB1/G2 overexpression.

Characterization of a deazaflavin analog as a potent inhibitor of multidrug resistance-associated protein 1

Selective inhibition of overexpressed ATP binding cassette (ABC) transporters is an attractive approach to enhancing the efficacy of chemotherapeutics in multidrug resistant cancers. Previously, we reported that the cancer sensitizing effect of deazaflavin analogs, an important chemotype for developing combination treatments with topoisomerase II (TOP2) poisons, is associated with increased intracellular drug accumulation. Here we report the characterization of ZW-1226, a deazaflavin analog, as a potent inhibitor of multidrug resistance-associated protein 1 (MRP1). Specifically, ZW-1226 inhibited MRP1 with a 16-fold higher potency than the most widely used positive control MK-571 in vesicular transport assay and displayed excellent selectivity indices exceeding 100 over other major ABC transporters, including P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), MRP2 and MRP3. Mechanistically, we revealed that its MRP1 inhibitory action requires the participation of GSH. In chemo-sensitization test, ZW-1226 fully reversed the MRP1-mediated drug resistance to TOP2 poisons etoposide (ETP) and doxorubicin (DOX) in H69AR cells and conferred CC50s comparable to those in the sensitive parental NCI-H69 cells. The sensitization was associated with boosted intracellular accumulation of ETP and DOX and elevated endogenous GSH. Moreover, ZW-1226 showed potential to occupy the leukotriene C4 binding site in molecular docking with bovine MRP1, presumably with the help of GSH. Lastly, ZW-1226 exhibited high tissue to plasma partitions in mice but did not alter ETP distribution to normal tissues, suggesting it could be a viable lead with desirable pharmacokinetic properties to warrant further investigation.

Structural and functional characterization of the nucleotide-binding domains of ABCA4 and their role in Stargardt disease

ABCA4 is an ATP-binding cassette (ABC) transporter that prevents the buildup of toxic retinoid compounds by facilitating the transport of N-retinylidene-phosphatidylethanolamine across membranes of rod and cone photoreceptor cells. Over 1500 missense mutations in ABCA4, many in the nucleotide-binding domains (NBDs), have been genetically linked to Stargardt disease. Here, we show by cryo-EM that ABCA4 is converted from an open outward conformation to a closed conformation upon the binding of adenylyl-imidodiphosphate. Structural information and biochemical studies were used to further define the role of the NBDs in the functional properties of ABCA4 and the mechanisms by which mutations lead to the loss in activity. We show that ATPase activity in both NBDs is required for the functional activity of ABCA4. Mutations in Walker A asparagine residues cause a severe reduction in substrate-activated ATPase activity due to the loss in polar interactions with residues within the D-loops of the opposing NBD. The structural basis for how disease mutations in other NBD residues, including the R1108C, R2077W, R2107H, and L2027F, affect the structure and function of ABCA4 is described. Collectively, our studies provide insight into the structure and function of ABCA4 and mechanisms underlying Stargardt disease.

Coupling the role of lipids to the conformational dynamics of the ABC transporter P-glycoprotein

The ATP-binding cassette transporter P-glycoprotein (P-gp) is a multidrug efflux pump that is overexpressed in a variety of cancers and associated with the drug-resistance phenomenon. P-gp structures were previously determined in detergent and in nanodiscs, in which different transmembrane helix conformations were found, "straight" and "kinked," respectively, indicating a possible role of the lipid environment on the P-gp structural ensemble. Here, we investigate the dynamic conformational ensembles and protein-lipid interactions of two human P-gp inward-open conformers, straight and kinked, employing all-atom molecular dynamics (MD) simulations in asymmetric multicomponent lipid bilayers that mimic the highly specialized hepatocyte membrane in which P-gp is expressed. The two conformers are found to differ in terms of the accessibility of the substrate cavity. The MD simulations show how cholesterol and different lipid species wedge, snorkel, and partially enter into the cavity of the straight P-gp conformer solved in detergent. However, access to the cavity of the kinked P-gp conformer solved in nanodiscs is restricted. Furthermore, the volume and dynamic fluctuations of the substrate cavity largely differ between the two P-gp conformers and are modulated by the presence (or absence) of cholesterol in the membrane and/or of ATP. From the mechanistic perspective, the findings indicate that the straight conformer likely precedes the kinked conformer in the functional working cycle of P-gp, with the latter conformation representing a post substrate-bound state. The inaccessibility of the main transmembrane cavity in the kinked conformer might be crucial in preventing substrate disengagement and transport withdrawal. Remarkably, in our unbiased MD simulations, one transmembrane helix (TM10) of the straight conformer underwent a spontaneous transition to a kinked conformation, underlining the relevance of both conformations in a native phospholipid environment and revealing structural descriptors defining the transition between the two P-gp conformers.

Alterations in the Intestinal Microbiome and Metabolic Profile of British Shorthair Kittens Fed with Milk Replacer

With the rising popularity of pet cats as companion animals, the survival of newborn kittens is often threatened by factors such as inadequate nursing, maternal behavior and blood incompatibility. These challenges require the use of milk replacers for nurturing. To investigate the effects that feeding kittens with an experimental milk replacer (EMR) have on growth and development, intestinal microbiota, immune response and nutrient metabolism, 12 British shorthair kittens were randomly divided into two groups after nursing for the first week of life. Kittens were fed queen's milk or EMR, whereby kittens fed queen's milk served as the control (CON) group. The findings revealed that the CON group exhibited superoxide dismutase (SOD) activity and total antioxidant capacity (T-AOC) (p < 0.01) on day 7. However, the EMR group had better growth performance during the later stage of the experiment (p < 0.05); the immunocompetence and antioxidant capacity of the EMR group were not significantly different from those of the CON group in the middle and late stages of the experiment, and the mean values of all the indexes were slightly better than those of the control group. Sequencing of the 16S rRNA gene in microbiota demonstrated that EMR increased the colonization of bacterial genera, including Lachnospiraceae, Enterococcus, Rothia and Ligilactobacillus. Compared to the CON group, acetate acid (p < 0.05), propionate acid (p < 0.01) and total SCFAs (p < 0.01) in the EMR group were significantly increased. Moreover, the intake of the EMR resulted in the production of distinct metabolites implicated in the metabolism of lipids and amino acids, among other nutrients, thus invigorating the associated metabolic pathways. These results elucidate the impact of administering a milk replacer on gastrointestinal health and nutrient assimilation in kittens. The study provides insights into the use of milk powder alternatives and sets the stage for future research on the formulation and effectiveness of kitten milk replacers.

Fractionated irradiation induces radioresistant oral carcinoma cells with enhanced malignant phenotypes

OBJECTIVE

The fact that certain oral carcinoma patients experience radiotherapy failure implies that a more radioresistant and aggressive phenotype of surviving cancer cells potentially occurs during treatment. Our study aimed to establish radioresistant oral cancer cells through a fractionated irradiation protocol that mimics clinically relevant radiotherapy dosing strategies and to investigate all-round alterations in the malignant phenotype.

METHODS

Radioresistant oral carcinoma cells were generated by exposing Cal27 and Detroit 562 cells to 60 Gy radiation in 10 dose-escalating fractions and verified by cell immunofluorescence. Specific markers related to the epithelial-mesenchymal transition (EMT) process and the cancer stem cell (CSC) phenotype were assessed by Western blotting. Cell invasion and migration were evaluated using Matrigel-coated transwell and wound healing assays, respectively. Nontargeted metabolomics was used to mechanistically delineate the potential metabolic patterns linked to EMT and CSCs; the CSC phenotype was also examined by sphere formation assays and cell immunofluorescence.

RESULTS

Radioresistant oral carcinoma cell lines were successfully established and validated. These cells exhibited enhanced EMT and increase in both cell invasion and migration. These radioresistant cells further demonstrated a high metabolic profile, notably marked by lipid metabolism reprogramming and functional enrichment of ATP-binding cassette (ABC) transporters. Consistently, enhanced CSC phenotype in radioresistant cells was confirmed by elevated expression of stemness markers and increased sphere-forming capacity.

CONCLUSION

Radioresistant oral carcinoma cells subjected to fractionated radiation exhibit an augmented malignant phenotype. The metabolic characteristics linked to enhanced EMT and CSC phenotypes provide potential targets for improving radiotherapy in oral carcinoma.

The ATP-bound inward-open conformation of ABCC4 reveals asymmetric ATP binding for substrate transport

The multidrug resistance-associated protein (MRP) ABCC4 facilitates substrate transport across the cytoplasmic membrane, crucial for normal physiology and mediating multidrug resistance in tumor cells. Despite intensive studies on MRPs, ABCC4's transport mechanism remains incompletely understood. In this study, we unveiled an inward-open conformation with an ATP bound to degenerate NBD1. Additionally, we captured the structure with both ATP and substrate co-bound in the inward-open state. Our findings uncover the asymmetric ATP binding in ABCC4 and provide insights into substrate binding and transport mechanisms. ATP binding to NBD1 is parallel to substrate binding to ABCC4, and is a prerequisite for ATP-bound NBD2-induced global conformational changes. Our findings shed new light on targeting ABCC4 in combination with anticancer therapy.

Efflux ABC transporters in drug disposition and their posttranscriptional gene regulation by microRNAs

ATP-binding cassette (ABC) transporters are transmembrane proteins expressed commonly in metabolic and excretory organs to control xenobiotic or endobiotic disposition and maintain their homeostasis. Changes in ABC transporter expression may directly affect the pharmacokinetics of relevant drugs involving absorption, distribution, metabolism, and excretion (ADME) processes. Indeed, overexpression of efflux ABC transporters in cancer cells or bacteria limits drug exposure and causes therapeutic failure that is known as multidrug resistance (MDR). With the discovery of functional noncoding microRNAs (miRNAs) produced from the genome, many miRNAs have been revealed to govern posttranscriptional gene regulation of ABC transporters, which shall improve our understanding of complex mechanism behind the overexpression of ABC transporters linked to MDR. In this article, we first overview the expression and localization of important ABC transporters in human tissues and their clinical importance regarding ADME as well as MDR. Further, we summarize miRNA-controlled posttranscriptional gene regulation of ABC transporters and effects on ADME and MDR. Additionally, we discuss the development and utilization of novel bioengineered miRNA agents to modulate ABC transporter gene expression and subsequent influence on cellular drug accumulation and chemosensitivity. Findings on posttranscriptional gene regulation of ABC transporters shall not only improve our understanding of mechanisms behind variable ADME but also provide insight into developing new means towards rational and more effective pharmacotherapies.

The Folding Pathway of ABC Transporter CFTR: Effective and Robust

De novo protein folding into a native three-dimensional structure is indispensable for biological function, is instructed by its amino acid sequence, and occurs along a vectorial trajectory. The human proteome contains thousands of membrane-spanning proteins, whose biosynthesis begins on endoplasmic reticulum-associated ribosomes. Nearly half of all membrane proteins traverse the membrane more than once, including therapeutically important protein families such as solute carriers, G-protein-coupled receptors, and ABC transporters. These mediate a variety of functions like signal transduction and solute transport and are often of vital importance for cell function and tissue homeostasis. Missense mutations in multispan membrane proteins can lead to misfolding and cause disease; an example is the ABC transporter Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). Even though our understanding of multispan membrane-protein folding still is rather rudimental, the cumulative knowledge of 20 years of basic research on CFTR folding has led to development of drugs that modulate the misfolded protein. This has provided the prospect of a life without CF to the vast majority of patients. In this review we describe our understanding of the folding pathway of CFTR in cells, which is modular and tolerates many defects, making it effective and robust. We address how modulator drugs affect folding and function of CFTR, and distinguish protein stability from its folding process. Since the domain architecture of (mammalian) ABC transporters are highly conserved, we anticipate that the insights we discuss here for folding of CFTR may lay the groundwork for understanding the general rules of ABC-transporter folding.

Evidence for the Role of the Mitochondrial ABC Transporter MDL1 in the Uptake of Clozapine and Related Molecules into the Yeast Saccharomyces cerevisiae

Clozapine is an antipsychotic drug whose accumulation in white cells can sometimes prove toxic; understanding the transporters and alleles responsible is thus highly desirable. We used a strategy in which a yeast (Saccharomyces cerevisiae) CRISPR-Cas9 knock-out library was exposed to cytotoxic concentrations of clozapine to determine those transporters whose absence made it more resistant; we also recognised the structural similarity of the fluorescent dye safranin O (also known as safranin T) to clozapine, allowing it to be used as a surrogate marker. Strains lacking the mitochondrial ABC transporter MDL1 (encoded by YLR188W) showed substantial resistance to clozapine. MDL1 overexpression also conferred extra sensitivity to clozapine and admitted a massive increase in the cellular and mitochondrial uptake of safranin O, as determined using flow cytometry and microscopically. Yeast lacking mitochondria showed no such unusual accumulation. Mitochondrial MDL1 is thus the main means of accumulation of clozapine in S. cerevisiae. The closest human homologue of S. cerevisiae MDL1 is ABCB10.

Structural basis for the transport and regulation mechanism of the Multidrug resistance-associated protein 2

Multidrug resistance-associated protein 2 (MRP2) is an ATP-powered exporter important for maintaining liver homeostasis and a potential contributor to chemotherapeutic resistance. Deficiencies in MRP2 function are associated with Dubin-Johnson Syndrome and increased vulnerability to liver injury from cytotoxic drugs. Using cryogenic electron microscopy (cryo-EM), we determined the structures of human MRP2 in three conformational states: an autoinhibited state, a substrate-bound pre-translocation state, and an ATP-bound post-translocation state. These structures show that MRP2 functions through the classic alternating access model, driven by ATP binding and hydrolysis. Its cytosolic regulatory (R) domain serves as a selectivity gauge, wherein only sufficiently high concentrations of substrates can effectively compete with and disengage the R domain to initiate transport. Comparative structural analyses of MRP2 in complex with different substrates reveal how the transporter recognizes a diverse array of compounds, highlighting the transporter's role in multidrug resistance.

Golgi dispersal in cancer stem cells promotes chemoresistance of colorectal cancer via the Golgi stress response

Chemotherapy is a crucial treatment for colorectal tumors. However, its efficacy is restricted by chemoresistance. Recently, Golgi dispersal has been suggested to be a potential response to chemotherapy, particularly to drugs that induce DNA damage. However, the underlying mechanisms by which Golgi dispersal enhances the capacity to resist DNA-damaging agents remain unclear. Here, we demonstrated that DNA-damaging agents triggered Golgi dispersal in colorectal cancer (CRC), and cancer stem cells (CSCs) possessed a greater degree of Golgi dispersal compared with differentiated cancer cells (non-CSCs). We further revealed that Golgi dispersal conferred resistance against the lethal effects of DNA-damaging agents. Momentously, Golgi dispersal activated the Golgi stress response via the PKCα/GSK3α/TFE3 axis, resulting in enhanced protein and vesicle trafficking, which facilitated drug efflux through ABCG2. Identification of Golgi dispersal indicated an unexpected pathway regulating chemoresistance in CRC.

ATP-Binding Cassette Transporter of Clinical Significance: Sideroblastic Anemia

The ATP-binding cassette (ABC) transporters are a vast group of 48 membrane proteins, some of which are of notable physiological and clinical importance. Some ABC transporters are involved in functions such as the transport of chloride ions, bilirubin, reproductive hormones, cholesterol, and iron. Consequently, genetic or physiological disruption in these functions is manifested in various disease processes like cystic fibrosis, Tangier disease, and sideroblastic anemia. Among other etiologies, primary sideroblastic anemia results from a genetic mutation in the ATP-binding cassette-7 (ABCB7), a member of the ABC transporter family. There are not many articles specifically tackling the disease processes caused by ABC transporters in detail. Some testing methodologies previously reported in the available literature for investigating sideroblastic anemia need updating. Here, we expound on the relevance of ABCB7 as a clinically important ABC transporter and a rare participant in the disease process of Sideroblastic anemia. The other genetic and secondary etiologies of sideroblastic anemia, which do not involve mutations in the ABCB7 protein, are also described. We review the pathophysiology, clinical course, symptoms, diagnosis, and treatment of sideroblastic anemia with a focus on modern technologies for laboratory testing.

Cadmium transport by mammalian ATP-binding cassette transporters

Cellular responses to toxic metals depend on metal accessibility to intracellular targets, reaching interaction sites, and the intracellular metal concentration, which is mainly determined by uptake pathways, binding/sequestration and efflux pathways. ATP-binding cassette (ABC) transporters are ubiquitous in the human body-usually in epithelia-and are responsible for the transfer of indispensable physiological substrates (e.g. lipids and heme), protection against potentially toxic substances, maintenance of fluid composition, and excretion of metabolic waste products. Derailed regulation and gene variants of ABC transporters culminate in a wide array of pathophysiological disease states, such as oncogenic multidrug resistance or cystic fibrosis. Cadmium (Cd) has no known physiological role in mammalians and poses a health risk due to its release into the environment as a result of industrial activities, and eventually passes into the food chain. Epithelial cells, especially within the liver, lungs, gastrointestinal tract and kidneys, are particularly susceptible to the multifaceted effects of Cd because of the plethora of uptake pathways available. Pertinent to their broad substrate spectra, ABC transporters represent a major cellular efflux pathway for Cd and Cd complexes. In this review, we summarize current knowledge concerning transport of Cd and its complexes (mainly Cd bound to glutathione) by the ABC transporters ABCB1 (P-glycoprotein, MDR1), ABCB6, ABCC1 (multidrug resistance related protein 1, MRP1), ABCC7 (cystic fibrosis transmembrane regulator, CFTR), and ABCG2 (breast cancer related protein, BCRP). Potential detoxification strategies underlying ABC transporter-mediated efflux of Cd and Cd complexes are discussed.

Understanding the impact of ABCG2 polymorphisms on drug pharmacokinetics: focus on rosuvastatin and allopurinol

INTRODUCTION

In addition to the well-established understanding of the pharmacogenetics of drug-metabolizing enzymes, there is growing data on the effects of genetic variation in drug transporters, particularly ATP-binding cassette (ABC) transporters. However, the evidence that these genetic variants can be used to predict drug effects and to adjust individual dosing to avoid adverse events is still limited.

AREAS COVERED

This review presents a summary of the current literature from the PubMed database as of February 2024 regarding the impact of genetic variants on ABCG2 function and their relevance to the clinical use of the HMG-CoA reductase inhibitor rosuvastatin and the xanthine oxidase inhibitor allopurinol.

EXPERT OPINION

Although there are pharmacogenetic guidelines for the ABCG2 missense variant Q141K, there is still some conflicting data regarding the clinical benefits of these recommendations. Some caution appears to be warranted in homozygous ABCG2 Q141K carriers when rosuvastatin is administered at higher doses and such information is already included in the drug label. The benefit of dose adaption to lower possible side effects needs to be evaluated in prospective clinical studies.

SLC38A3 Promotes the Proliferation and Migration of Tumor Cells and Predicts Poor Prognosis in Colorectal Cancer

Previous studies have revealed that abnormal expressions of membrane transporters were associated with colorectal cancer (CRC). We herein performed a comprehensive bioinformatics analysis to identify the key transporter protein-related genes involved in CRC and potential mechanisms. Differentially expressed transporter protein-related genes (DE-TPRGs) were identified from CRC and normal samples using The Cancer Genome Atlas database. SLC38A3 expression was validated by immunohistochemistry and RT-qPCR, and the potential mechanism was explored. A total of 63 DE-TPRGs (29 up-regulated and 34 down-regulated) were screened. Inside, ABCC2, ABCG2, SLC4A4, SLC9A3, SLC15A1, and SLC38A3 were identified as hub genes. SLC38A3 is indeed upregulated in colorectal cancer patients. Furthermore, we found that knockdown of SLC38A3 inhibited the proliferation and migration of HCT116 cells, and Hsp70 ATPase activator could rescue it. Overall, SLC38A3 is a novel potential biomarker involved in CRC progression and promotes the proliferation and migration of tumor cells by positively regulating the function of Hsp70.

Exploration of Lactiplantibacillus plantarum P101 ameliorated the alcohol-induced testicular dysfunction based on metabolome analysis

The decline in male sperm quality caused by multiple factors has become a widespread concern. Alcohol excessive consumption is one of the factors that induce testicular dysfunction. Testicular dysfunction caused by alcohol abuse is related to oxidative stress and inflammation. Probiotics can ameliorate alcohol-induced testicular dysfunction. However, the specific mechanism is not explicit. This study aimed to elucidate the underlying mechanism by which Lactiplantibacillus plantarum P101 ameliorates the alcohol-induced testicular dysfunction. The model of alcohol-induced testicular dysfunction in C57B/6 male mice was established according to the National Institute on Alcohol Abuse and Alcoholism, and Lactiplantibacillus plantarum P101 supplementation was orally administered to mice during the experiment. The results showed that Lactiplantibacillus plantarum P101 promoted androgen production, reduced testis inflammation, and improved testis antioxidant capacity, thereby improving sperm quality and sperm motility and ultimately ameliorating alcohol-induced testicular disorder. Three key metabolite pathways and six key metabolites were identified by metabolome analysis.

Utilizing non-coding RNA-mediated regulation of ATP binding cassette (ABC) transporters to overcome multidrug resistance to cancer chemotherapy

Multidrug resistance (MDR) is one of the primary factors that produces treatment failure in patients receiving cancer chemotherapy. MDR is a complex multifactorial phenomenon, characterized by a decrease or abrogation of the efficacy of a wide spectrum of anticancer drugs that are structurally and mechanistically distinct. The overexpression of the ATP-binding cassette (ABC) transporters, notably ABCG2 and ABCB1, are one of the primary mediators of MDR in cancer cells, which promotes the efflux of certain chemotherapeutic drugs from cancer cells, thereby decreasing or abolishing their therapeutic efficacy. A number of studies have suggested that non-coding RNAs (ncRNAs), particularly microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), play a pivotal role in mediating the upregulation of ABC transporters in certain MDR cancer cells. This review will provide updated information about the induction of ABC transporters due to the aberrant regulation of ncRNAs in cancer cells. We will also discuss the measurement and biological profile of circulating ncRNAs in various body fluids as potential biomarkers for predicting the response of cancer patients to chemotherapy. Sequence variations, such as alternative polyadenylation of mRNA and single nucleotide polymorphism (SNPs) at miRNA target sites, which may indicate the interaction of miRNA-mediated gene regulation with genetic variations to modulate the MDR phenotype, will be reviewed. Finally, we will highlight novel strategies that could be used to modulate ncRNAs and circumvent ABC transporter-mediated MDR.

Structural View of Cryo-Electron Microscopy-Determined ATP-Binding Cassette Transporters in Human Multidrug Resistance

ATP-binding cassette (ABC) transporters, acting as cellular "pumps," facilitate solute translocation through membranes via ATP hydrolysis. Their overexpression is closely tied to multidrug resistance (MDR), a major obstacle in chemotherapy and neurological disorder treatment, hampering drug accumulation and delivery. Extensive research has delved into the intricate interplay between ABC transporter structure, function, and potential inhibition for MDR reversal. Cryo-electron microscopy has been instrumental in unveiling structural details of various MDR-causing ABC transporters, encompassing ABCB1, ABCC1, and ABCG2, as well as the recently revealed ABCC3 and ABCC4 structures. The newly obtained structural insight has deepened our understanding of substrate and drug binding, translocation mechanisms, and inhibitor interactions. Given the growing body of structural information available for human MDR transporters and their associated mechanisms, we believe it is timely to compile a comprehensive review of these transporters and compare their functional mechanisms in the context of multidrug resistance. Therefore, this review primarily focuses on the structural aspects of clinically significant human ABC transporters linked to MDR, with the aim of providing valuable insights to enhance the effectiveness of MDR reversal strategies in clinical therapies.

Transport mechanism of human bilirubin transporter ABCC2 tuned by the inter-module regulatory domain

Bilirubin is mainly generated from the breakdown of heme when red blood cells reach the end of their lifespan. Accumulation of bilirubin in human body usually leads to various disorders, including jaundice and liver disease. Bilirubin is conjugated in hepatocytes and excreted to bile duct via the ATP-binding cassette transporter ABCC2, dysfunction of which would lead to Dubin-Johnson syndrome. Here we determine the structures of ABCC2 in the apo, substrate-bound and ATP/ADP-bound forms using the cryo-electron microscopy, exhibiting a full transporter with a regulatory (R) domain inserted between the two half modules. Combined with substrate-stimulated ATPase and transport activity assays, structural analysis enables us to figure out transport cycle of ABCC2 with the R domain adopting various conformations. At the rest state, the R domain binding to the translocation cavity functions as an affinity filter that allows the substrates of high affinity to be transported in priority. Upon substrate binding, the R domain is expelled from the cavity and docks to the lateral of transmembrane domain following ATP hydrolysis. Our findings provide structural insights into a transport mechanism of ABC transporters finely tuned by the R domain.

Blood-brain barrier transporters: An overview of function, dysfunction in Alzheimer's disease and strategies for treatment

The blood-brain-barrier (BBB) has a major function in maintaining brain homeostasis by regulating the entry of molecules from the blood to the brain. Key players in BBB function are BBB transporters which are highly expressed in brain endothelial cells (BECs) and critical in mediating the exchange of nutrients and waste products. BBB transporters can also influence drug delivery into the brain by inhibiting or facilitating the entry of brain targeting therapeutics for the treatment of brain disorders, such as Alzheimer's disease (AD). Recent studies have shown that AD is associated with a disrupted BBB and transporter dysfunction, although their roles in the development in AD are not fully understand. Modulation of BBB transporter activity may pose a novel approach to enhance the delivery of drugs to the brain for enhanced treatment of AD. In this review, we will give an overview of key functions of BBB transporters and known changes in AD. In addition, we will discuss current strategies for transporter modulation for enhanced drug delivery into the brain.

Linking ABC transporters to the hallmarks of cancer

Human ATP-binding cassette (ABC) transporters are ubiquitously expressed and transport a broad range of endogenous and xenobiotic substrates across extra- and intracellular membranes. Mutations in ABC genes cause 21 monogenic diseases, and polymorphisms in these genes are associated with susceptibility to complex diseases. ABC transporters also play a major role in drug bioavailability, and they mediate multidrug resistance in cancer. At least 13 ABC transporters were shown to be involved in drug resistance in vitro. In the past decade, efforts have been made to elucidate their roles in tumor biology. Herein, we explore their involvement in tumorigenesis, focusing on the hallmarks of cells as they make their way from normalcy to neoplastic growth states.

Carboranes as Potent Phenyl Mimetics: A Comparative Study on the Reversal of ABCG2-Mediated Drug Resistance by Carboranylquinazolines and Their Organic Isosteres

Multidrug resistance is a major challenge in clinical cancer therapy. In particular, overexpression of certain ATP-binding cassette (ABC) transporter proteins, like the efflux transporter ABCG2, also known as breast cancer resistance protein (BCRP), has been associated with the development of resistance to applied chemotherapeutic agents in cancer therapies, and therefore targeted inhibition of BCRP-mediated transport might lead to reversal of this (multidrug) resistance (MDR). In a previous study, we have described the introduction of a boron-carbon cluster, namely closo-dicarbadodecaborane or carborane, as an inorganic pharmacophore into a polymethoxylated 2-phenylquinazolin-4-amine backbone. In this work, the scope was extended to the corresponding amide derivatives. As most of the amide derivatives suffered from poor solubility, only the amide derivative QCe and the two amine derivatives DMQCc and DMQCd were further investigated. Carboranes are often considered as sterically demanding phenyl mimetics or isosteres. Therefore, the organic phenyl and sterically demanding adamantyl analogues of the most promising carborane derivatives were also investigated. The studies showed that the previously described DMQCd, a penta-methoxylated N-carboranyl-2-phenylquinazolin-4-amine, was by far superior to its organic analogues in terms of cytotoxicity, inhibition of the human ABCG2 transporter, as well as the ability to reverse BCRP-mediated mitoxantrone resistance in MDCKII-hABCG2 and HT29 colon cancer cells. Our results indicate that DMQCd is a promising candidate for further in vitro as well as in vivo studies in combination therapy for ABCG2-overexpressing cancers.

Muropeptides and muropeptide transporters impact on host immune response

In bacteria, the cell envelope is the key element surrounding and protecting the bacterial content from mechanical or osmotic damages. It allows the selective interchanges of solutes, ions, cellular debris, and drugs between the cellular compartments and the external environment, thanks to the presence of transmembrane proteins called transporters. The major component of the cell envelope is the peptidoglycan, consisting of long linear glycan strands cross-linked by short peptide stems. During cell growth or under stress conditions, peptidoglycan fragments, the muropeptides, are released by bacteria and recognized by the host Pattern Recognition Receptor, promoting the activation of their innate defense mechanisms. The review sums up the salient aspects of microbiota-host interaction with a focus on the NOD-dependent immune response to bacterial peptidoglycan and on the accountability of muropeptide transporters in the crosstalk with the host and in antibiotic resistance. Furthermore, it retraces the discoveries and applications of microorganisms-derived components such as vaccines or vaccine adjuvants.

Association of ABCG5 and ABCG8 Transporters with Sitosterolemia

Sitosterolemia is a rare genetic lipid disorder, mainly characterized by the accumulation of dietary xenosterols in plasma and tissues. It is caused by inactivating mutations in either ABCG5 or ABCG8 subunits, a subfamily-G ATP-binding cassette (ABCG) transporters. ABCG5/G8 encodes a pair of ABC half transporters that form a heterodimer (G5G8). This heterodimeric ATP-binding cassette (ABC) sterol transporter, ABCG5/G8, is responsible for the hepatobiliary and transintestinal secretion of cholesterol and dietary plant sterols to the surface of hepatocytes and enterocytes, promoting the secretion of cholesterol and xenosterols into the bile and the intestinal lumen. In this way, ABCG5/G8 function in the reverse cholesterol transport pathway and mediate the efflux of cholesterol and xenosterols to high-density lipoprotein and bile salt micelles, respectively. Here, we review the biological characteristics and function of ABCG5/G8, and how the mutations of ABCG5/G8 can cause sitosterolemia, a loss-of-function disorder characterized by plant sterol accumulation and premature atherosclerosis, among other features.

Evaluation of the FDA-approved kinase inhibitors to uncover the potential repurposing candidates targeting ABC transporters in multidrug-resistant cancer cells: an in silico approach

Multiple drug resistance (MDR) is characterized by the resistance of cancer cells to a broad spectrum of anticancer drugs. The main mechanism underlying the MDR phenotype is the overexpression of ATP-binding cassette (ABC) transporters by promoting active drug efflux from cancer cells. Some small-molecule protein kinase inhibitors have been found to overcome MDR by inhibiting ABC transporters as substrates or modulators. This study investigated the chemical activity of 58 FDA-approved anticancer kinase inhibitors against three multidrug resistance-related proteins. The studied proteins are ATP-Binding Cassette Sub-Family B Member 1 (ABCB1), ATP-Binding Cassette Subfamily C Member 1 (ABCC1), and ATP-binding cassette superfamily G member 2 (ABCG2). The drug-binding domain and ATP binding sites of the proteins were considered the kinase inhibitors' probable target. High-throughput virtual screening and molecular docking were employed to find the hit drugs, and the drugs with the highest binding affinity were further evaluated using the molecular dynamics (MD) simulation. The virtual screening revealed that several kinase inhibitors could be considered potential inhibitors of ABCB1, ABCC1, and ABCG2, among which larotrectinib, entrectinib, and infigratinib showed the highest binding affinity, respectively. Based on the obtained results from MD simulation, these drugs can form strong interactions with the essential residues of the target proteins. In silico investigation revealed that larotrectinib, entrectinib, and infigratinib can target the key residues of the studied proteins. Therefore, these approved kinase inhibitors could be considered potential therapies for MDR cancers by targeting these transporters.Communicated by Ramaswamy H. Sarma.

Asymmetric conformations and lipid interactions shape the ATP-coupled cycle of a heterodimeric ABC transporter

Here we used cryo-electron microscopy (cryo-EM), double electron-electron resonance spectroscopy (DEER), and molecular dynamics (MD) simulations, to capture and characterize ATP- and substrate-bound inward-facing (IF) and occluded (OC) conformational states of the heterodimeric ATP binding cassette (ABC) multidrug exporter BmrCD in lipid nanodiscs. Supported by DEER analysis, the structures reveal that ATP-powered isomerization entails changes in the relative symmetry of the BmrC and BmrD subunits that propagates from the transmembrane domain to the nucleotide binding domain. The structures uncover asymmetric substrate and Mg2+ binding which we hypothesize are required for triggering ATP hydrolysis preferentially in one of the nucleotide-binding sites. MD simulations demonstrate that multiple lipid molecules differentially bind the IF versus the OC conformation thus establishing that lipid interactions modulate BmrCD energy landscape. Our findings are framed in a model that highlights the role of asymmetric conformations in the ATP-coupled transport with general implications to the mechanism of ABC transporters.

The cation-leaky hereditary stomatocytosis syndromes: A tale of six proteins

This review concerns a series of dominantly inherited haemolytic anaemias in which the membrane of the erythrocyte 'leaks' the univalent cations, compromising the osmotic stability of the cell. The majority of the conditions are explained by mutations in one of six genes, coding for multispanning membrane proteins of different structure and function. These are: RhAG, coding for an ammonium carrier; SLC4A1, coding for the band 3 anion exchanger; PIEZO1, coding for a mechanosensitive cation channel; GLUT1, coding for a glucose transporter; KCNN4, coding for an internal-calcium-activated potassium channel; and ABCB6, coding for a porphyrin transporter. This review describes the five clinical syndromes associated with genetic defects in these genes and their variable genotype/phenotype relationships.

Redefine the role of d-α-Tocopheryl polyethylene glycol 1000 succinate on P-glycoprotein, multidrug resistance protein 1, and breast cancer resistance protein mediated cancer multidrug resistance

Cancer drug resistance is an ever-changing problem that most patients need to face in their later stages of treatment, especially the multidrug resistant (MDR) type. The drug efflux transporters, including P-glycoprotein (P-gp), multidrug resistance protein 1 (MRP1), and breast cancer resistance protein (BCRP), play the crucial roles in this sophisticated battle. In recent decades, researchers try to find potential inhibitors to impede the drug efflux function of above transporters. d-α-Tocopheryl polyethylene glycol 1000 succinate (vitamin E TPGS) is a prevalently used excipient in the formulation design. In the present study, the modulatory effects and mechanisms of vitamin E TPGS on the efflux transporters were investigated. And the cancer MDR reversing ability of vitamin E TPGS was evaluated as well. Stable-cloned transporter over-expressed cell lines were used for mechanisms study, while several types of MDR cancer cell lines were adopted as reversing evaluation models. The results exhibited that vitamin E TPGS significantly inhibited the efflux function of P-gp, MRP1, and BCRP under non-cytotoxic concentrations, but not influencing the protein expression levels. Through efflux assay and molecular docking, vitamin E TPGS was found to be an uncompetitive, non-competitive, and competitive inhibitor on chemotherapeutic drug doxorubicin efflux in P-gp, MRP1, and BCRP over-expressing cell lines, respectively. Furthermore, the basal ATPase activity of three transporters were significantly inhibited by vitamin E TPGS at 10 μM. And the cell membrane fluidity of P-gp over-expressing cell line was enhanced by 22.58% with 5 μM vitamin E TPGS treatment, compared to the parental Flp-In™-293 cell line (without P-gp). The resistance reversing ability of vitamin E TPGS was prominent in MCF-7/DOX MDR breast cancer cell line, which over-expressed P-gp, MRP1, and BCRP. These significant results suggested that vitamin E TPGS is a promising modulator on transporters mediated cancer MDR. Vitamin E TPGS is not an inert excipient, but possesses MDR-reversing pharmacological effects, and deserves a re-purposing application on the future combinatorial regimen design for MDR cancer treatment.

Perspectives on drug repurposing to overcome cancer multidrug resistance mediated by ABCB1 and ABCG2

The overexpression of the human ATP-binding cassette (ABC) transporters in cancer cells is a common mechanism involved in developing multidrug resistance (MDR). Unfortunately, there are currently no approved drugs specifically designed to treat multidrug-resistant cancers, making MDR a significant obstacle to successful chemotherapy. Despite over two decades of research, developing transporter-specific inhibitors for clinical use has proven to be a challenging endeavor. As an alternative approach, drug repurposing has gained traction as a more practical method to discover clinically effective modulators of drug transporters. This involves exploring new indications for already-approved drugs, bypassing the lengthy process of developing novel synthetic inhibitors. In this context, we will discuss the mechanisms of ABC drug transporters ABCB1 and ABCG2, their roles in cancer MDR, and the inhibitors that have been evaluated for their potential to reverse MDR mediated by these drug transporters. Our focus will be on providing an up-to-date report on approved drugs tested for their inhibitory activities against these drug efflux pumps. Lastly, we will explore the challenges and prospects of repurposing already approved medications for clinical use to overcome chemoresistance in patients with high tumor expression of ABCB1 and/or ABCG2.

ATP-dependent transporters: emerging players at the crossroads of immunity and metabolism

Nearly 50 ATP-binding cassette (ABC) transporters are encoded by mammalian genomes. These transporters are characterized by conserved nucleotide-binding and hydrolysis (i.e., ATPase) domains, and power directional transport of diverse substrate classes - ions, small molecule metabolites, xenobiotics, hydrophobic drugs, and even polypeptides - into or out of cells or subcellular organelles. Although immunological functions of ABC transporters are only beginning to be unraveled, emerging literature suggests these proteins have under-appreciated roles in the development and function of T lymphocytes, including many of the key effector, memory and regulatory subsets that arise during responses to infection, inflammation or cancers. One transporter in particular, MDR1 (Multidrug resistance-1; encoded by the ABCB1 locus in humans), has taken center stage as a novel player in immune regulation. Although MDR1 remains widely viewed as a simple drug efflux pump in tumor cells, recent evidence suggests that this transporter fills key endogenous roles in enforcing metabolic fitness of activated CD4 and CD8 T cells. Here, we summarize current understanding of the physiological functions of ABC transporters in immune regulation, with a focus on the anti-oxidant functions of MDR1 that may shape both the magnitude and repertoires of antigen-specific effector and memory T cell compartments. While much remains to be learned about the functions of ABC transporters in immunobiology, it is already clear that they represent fertile new ground, both for the definition of novel immunometabolic pathways, and for the discovery of new drug targets that could be leveraged to optimize immune responses to vaccines and cancer immunotherapies.

The β Isoform of Human ATP-Binding Cassette B5 Transporter, ABCB5β, Localizes to the Endoplasmic Reticulum

ABCB5β is a member of the ABC transporter superfamily cloned from melanocytes. It has been reported as a marker of skin progenitor cells and melanoma stem cells. ABCB5β has also been shown to exert an oncogenic activity and promote cancer metastasis. However, this protein remains poorly characterized. To elucidate its subcellular localization, we tested several anti-ABCB5 antibodies and prepared several tagged ABCB5β cDNA constructs. We then used a combination of immunofluorescence and biochemical analyses to investigate the presence of ABCB5β in different subcellular compartments of HeLa and MelJuSo cell lines. Treatment of the cells with the proteasome inhibitor MG132 showed that part of the population of newly synthesized ABCB5β is degraded by the proteasome system. Interestingly, treatment with SAHA, a molecule that promotes chaperone-assisted folding, largely increased the expression of ABCB5β. Nevertheless, the overall protein distribution in the cells remained similar to that of control conditions; the protein extensively colocalized with the endoplasmic reticulum marker calnexin. Taken together with cell surface biotinylation studies demonstrating that the protein does not reach the plasma membrane (even after SAHA treatment), the data indicate that ABCB5β is a microsomal protein predominantly localized to the ER.

The Physiological and Pathological Role of Acyl-CoA Oxidation

Fatty acid metabolism, including β-oxidation (βOX), plays an important role in human physiology and pathology. βOX is an essential process in the energy metabolism of most human cells. Moreover, βOX is also the source of acetyl-CoA, the substrate for (a) ketone bodies synthesis, (b) cholesterol synthesis, (c) phase II detoxication, (d) protein acetylation, and (d) the synthesis of many other compounds, including N-acetylglutamate-an important regulator of urea synthesis. This review describes the current knowledge on the importance of the mitochondrial and peroxisomal βOX in various organs, including the liver, heart, kidney, lung, gastrointestinal tract, peripheral white blood cells, and other cells. In addition, the diseases associated with a disturbance of fatty acid oxidation (FAO) in the liver, heart, kidney, lung, alimentary tract, and other organs or cells are presented. Special attention was paid to abnormalities of FAO in cancer cells and the diseases caused by mutations in gene-encoding enzymes involved in FAO. Finally, issues related to α- and ω- fatty acid oxidation are discussed.