Structural and functional consequences of mutating cysteine residues in the amino terminus of human multidrug resistance-associated protein 1

Reversible promoter demethylation of PDGFD confers gemcitabine resistance through STAT3 activation and RRM1 upregulation

Drug resistance is a major problem in cancer treatment with traditional or targeted therapeutics. Gemcitabine is approved for several human cancers and the first line treatment for locally advanced or metastatic pancreatic ductal adenocarcinoma (PDAC). However, gemcitabine resistance frequently occurs and is a major problem in successful treatments of these cancers and the mechanism of gemcitabine resistance remains largely unknown. In this study, we identified 65 genes that had reversible methylation changes in their promoters in gemcitabine resistant PDAC cells using whole genome Reduced Representation Bisulfite Sequencing analyses. One of these genes, PDGFD, was further studied in detail for its reversible epigenetic regulation in expression and shown to contribute to gemcitabine resistance in vitro and in vivo via stimulating STAT3 signaling in both autocrine and paracrine manners to upregulate RRM1 expression. Analyses of TCGA datasets showed that PDGFD positively associates with poor outcome of PDAC patients. Together, we conclude that the reversible epigenetic upregulation plays an important role in gemcitabine resistance development and targeting PDGFD signaling alleviates gemcitabine resistance for PDAC treatment.

A Structure-Based View on ABC-Transporter Linked to Multidrug Resistance

The discovery of the first ATP-binding cassette (ABC) transporter, whose overexpression in cancer cells is responsible for exporting anticancer drugs out of tumor cells, initiated enormous efforts to overcome tumor cell multidrug resistance (MDR) by inhibition of ABC-transporter. Because of its many physiological functions, diverse studies have been conducted on the mechanism, function and regulation of this important group of transmembrane transport proteins. In this review, we will focus on the structural aspects of this transporter superfamily. Since the resolution revolution of electron microscope, experimentally solved structures increased rapidly. A summary of the structures available and an overview of recent structure-based studies are provided. More specifically, the artificial intelligence (AI)-based predictions from AlphaFold-2 will be discussed.

Targeting multidrug resistance-associated protein 1 (MRP1)-expressing cancers: Beyond pharmacological inhibition

Resistance to chemotherapy remains one of the most significant obstacles to successful cancer treatment. While inhibiting drug efflux mediated by ATP-binding cassette (ABC) transporters is a seemingly attractive and logical approach to combat multidrug resistance (MDR), small molecule inhibition of ABC transporters has so far failed to confer clinical benefit, despite considerable efforts by medicinal chemists, biologists, and clinicians. The long-sought treatment to eradicate cancers displaying ABC transporter overexpression may therefore lie within alternative targeting strategies. When aberrantly expressed, the ABC transporter multidrug resistance-associated protein 1 (MRP1, ABCC1) confers MDR, but can also shift cellular redox balance, leaving the cell vulnerable to select agents. Here, we explore the physiological roles of MRP1, the rational for targeting this transporter in cancer, the development of small molecule MRP1 inhibitors, and the most recent developments in alternative therapeutic approaches for targeting cancers with MRP1 overexpression. We discuss approaches that extend beyond simple MRP1 inhibition by exploiting the collateral sensitivity to glutathione depletion and ferroptosis, the rationale for targeting the shared transcriptional regulators of both MRP1 and glutathione biosynthesis, advances in gene silencing, and new molecules that modulate transporter activity to the detriment of the cancer cell. These strategies illustrate promising new approaches to address multidrug resistant disease that extend beyond the simple reversal of MDR and offer exciting routes for further research.

Single-nucleotide polymorphisms in a short basic motif in the ABC transporter ABCG2 disable its trafficking out of endoplasmic reticulum and reduce cell resistance to anticancer drugs

ATP-binding cassette (ABC) subfamily G member 2 (ABCG2) belongs to the ABC transporter superfamily and has been implicated in multidrug resistance of cancers. Although the structure and function of ABCG2 have been extensively studied, little is known about its biogenesis and the regulation thereof. In this study, using mutagenesis and several biochemical analyses, we show that the positive charges in the vicinity of the RKR motif downstream of the ABC signature drive trafficking of nascent ABCG2 out of the endoplasmic reticulum (ER) onto plasma membranes. Substitutions of and naturally occurring single-nucleotide polymorphisms within these positively charged residues disabled the trafficking of ABCG2 out of the ER. A representative ABCG2 variant in which the RKR motif had been altered underwent increased ER stress-associated degradation. We also found that unlike WT ABCG2, genetic ABCG2 RKR variants have disrupted normal maturation and do not reduce accumulation of the anticancer drug mitoxantrone and no longer confer resistance to the drug. We conclude that the positive charges downstream of the ABC signature motif critically regulate ABCG2 trafficking and maturation. We propose that single-nucleotide polymorphisms of these residues reduce ABCG2 expression via ER stress-associated degradation pathway and may contribute to reduced cancer drug resistance, improving the success of cancer chemotherapy.

FASN regulates cellular response to genotoxic treatments by increasing PARP-1 expression and DNA repair activity via NF-κB and SP1

Fatty acid synthase (FASN), the sole cytosolic mammalian enzyme for de novo lipid synthesis, is crucial for cancer cell survival and associates with poor prognosis. FASN overexpression has been found to cause resistance to genotoxic insults. Here we tested the hypothesis that FASN regulates DNA repair to facilitate survival against genotoxic insults and found that FASN suppresses NF-κB but increases specificity protein 1 (SP1) expression. NF-κB and SP1 bind to a composite element in the poly(ADP-ribose) polymerase 1 (PARP-1) promoter in a mutually exclusive manner and regulate PARP-1 expression. Up-regulation of PARP-1 by FASN in turn increases Ku protein recruitment and DNA repair. Furthermore, lipid deprivation suppresses SP1 expression, which is able to be rescued by palmitate supplementation. However, lipid deprivation or palmitate supplementation has no effect on NF-κB expression. Thus, FASN may regulate NF-κB and SP1 expression using different mechanisms. Altogether, we conclude that FASN regulates cellular response against genotoxic insults by up-regulating PARP-1 and DNA repair via NF-κB and SP1.

Cancer drug resistance: redox resetting renders a way

Disruption of redox homeostasis is a crucial factor in the development of drug resistance, which is a major problem facing current cancer treatment. Compared with normal cells, tumor cells generally exhibit higher levels of reactive oxygen species (ROS), which can promote tumor progression and development. Upon drug treatment, some tumor cells can undergo a process of 'Redox Resetting' to acquire a new redox balance with higher levels of ROS accumulation and stronger antioxidant systems. Evidence has accumulated showing that the 'Redox Resetting' enables cancer cells to become resistant to anticancer drugs by multiple mechanisms, including increased rates of drug efflux, altered drug metabolism and drug targets, activated prosurvival pathways and inefficient induction of cell death. In this article, we provide insight into the role of 'Redox Resetting' on the emergence of drug resistance that may contribute to pharmacological modulation of resistance.

Anticancer efficacy of a nitric oxide-modified derivative of bifendate against multidrug-resistant cancer cells

The development of multidrug resistance (MDR) not only actively transports a wide range of cytotoxic drugs across drug transporters but is also a complex interaction between a number of important cellular signalling pathways. Nitric oxide donors appear to be a new class of anticancer therapeutics for satisfying all the above conditions. Previously, we reported furoxan‐based nitric oxide‐releasing compounds that exhibited selective antitumour activity in vitro and in vivo. Herein, we demonstrate that bifendate (DDB)‐nitric oxide, a synthetic furoxan‐based nitric oxide‐releasing derivative of bifendate, effectively inhibits the both sensitive and MDR tumour cell viability at a comparatively low concentration. Interestingly, the potency of DDB‐nitric oxide is the independent of inhibition of the functions and expressions of three major ABC transporters. The mechanism of DDB‐nitric oxide appears to be in two modes of actions by inducing mitochondrial tyrosine nitration and apoptosis, as well as by down‐regulating HIF‐1α expression and protein kinase B (AKT), extracellular signal‐regulated kinases (ERK), nuclear factor κB (NF‐κB) activation in MDR cells. Moreover, the addition of a typical nitric oxide scavenger significantly attenuated all the effects of DDB‐nitric oxide, indicating that the cytotoxicity of DDB‐nitric oxide is as a result of higher levels of nitric oxide release in MDR cancer cells. Given that acquired MDR to nitric oxide donors is reportedly difficult to achieve and genetically unstable, compound like DDB‐nitric oxide may be a new type of therapeutic agent for the treatment of MDR tumours.

Reversible epigenetic regulation of 14-3-3σ expression in acquired gemcitabine resistance by uhrf1 and DNA methyltransferase 1

Although gemcitabine is the most commonly used drug for treating pancreatic cancers, acquired gemcitabine resistance in a substantial number of patients appears to hinder its effectiveness in successful treatment of this dreadful disease. To understand acquired gemcitabine resistance, we generated a gemcitabine-resistant pancreatic cancer cell line using stepwise selection and found that, in addition to the known mechanisms of upregulated expression of ribonucleotide reductase, 14-3-3σ expression is dramatically upregulated, and that 14-3-3σ overexpression contributes to the acquired resistance to gemcitabine and cross-resistance to cytarabine. We also found that the increased 14-3-3σ expression in the gemcitabine-resistant cells is due to demethylation of the 14-3-3σ gene during gemcitabine selection, which could be partially reversed with removal of the gemcitabine selection pressure. Most importantly, the reversible methylation/demethylation of the 14-3-3σ gene appears to be carried out by DNA methyltransferase 1 under regulation by Uhrf1. These findings suggest that the epigenetic regulation of gene expression may play an important role in gemcitabine resistance, and that epigenetic modification is reversible in response to gemcitabine treatment.

Determinants of 14-3-3σ protein dimerization and function in drug and radiation resistance

Many proteins exist and function as homodimers. Understanding the detailed mechanism driving the homodimerization is important and will impact future studies targeting the "undruggable" oncogenic protein dimers. In this study, we used 14-3-3σ as a model homodimeric protein and performed a systematic investigation of the potential roles of amino acid residues in the interface for homodimerization. Unlike other members of the conserved 14-3-3 protein family, 14-3-3σ prefers to form a homodimer with two subareas in the dimeric interface that has 180° symmetry. We found that both subareas of the dimeric interface are required to maintain full dimerization activity. Although the interfacial hydrophobic core residues Leu(12) and Tyr(84) play important roles in 14-3-3σ dimerization, the non-core residue Phe(25) appears to be more important in controlling 14-3-3σ dimerization activity. Interestingly, a similar non-core residue (Val(81)) is less important than Phe(25) in contributing to 14-3-3σ dimerization. Furthermore, dissociating dimeric 14-3-3σ into monomers by mutating the Leu(12), Phe(25), or Tyr(84) dimerization residue individually diminished the function of 14-3-3σ in resisting drug-induced apoptosis and in arresting cells at G2/M phase in response to DNA-damaging treatment. Thus, dimerization appears to be required for the function of 14-3-3σ.

Overexpression of asparagine synthetase and matrix metalloproteinase 19 confers cisplatin sensitivity in nasopharyngeal carcinoma cells

Platinum-based concurrent chemoradiotherapy is considered a standard treatment approach for locoregionally advanced nasopharyngeal carcinoma. However, only a minority of patients benefit from this treatment regimen compared with radiotherapy alone. Identification of a set of molecular markers predicting sensitivity of platinum-based chemotherapy may contribute to personalized treatment of patients with nasopharyngeal carcinoma for better clinical outcome with less toxicity. Previously, we generated a cisplatin-sensitive nasopharyngeal carcinoma cell line, S16, by clonal selection from CNE-2 cells and found that eIF3a is upregulated and contributes to cisplatin sensitivity by downregulating the synthesis of nucleotide excision repair proteins. In this study, we conducted a gene expression profiling analysis and found three other genes, asparagine synthetase (ASNS), choriogonadotropin α subunit (CGA), and matrix metalloproteinase 19 (MMP19), that are upregulated in the cisplatin-sensitive S16 cells compared with the CNE-2 cells. However, only ASNS and MMP19, but not CGA, contributes to cisplatin sensitivity by potentiating cisplatin-induced DNA damage and apoptosis. Thus, ASNS and MMP19, along with eIF3a, are the sensitivity factors for cisplatin treatment and may serve as potential candidate molecular markers for predicting cisplatin sensitivity of advanced nasopharyngeal carcinoma.

Multidrug resistance-associated protein 1 (MRP1/ABCC1) polymorphism: from discovery to clinical application

Multidrug resistance-associated protein 1(MRP1/ABCC1) is the first identified member of ABCC subfamily which belongs to ATP-binding cassette (ABC) transporter superfamily. It is ubiquitously expressed in almost all human tissues and transports a wide spectrum of substrates including drugs, heavy metal anions, toxicants, and conjugates of glutathione, glucuronide and sulfate. With the advance of sequence technology, many MRP1/ABCC1 polymorphisms have been identified. Accumulating evidences show that some polymorphisms are significantly associated with drug resistance and disease susceptibility. In vitro reconstitution studies have also unveiled the mechanism for some polymorphisms. In this review, we present recent advances in understanding the role and mechanism of MRP1/ABCC1 polymorphisms in drug resistance, toxicity, disease susceptibility and severity, prognosis prediction, and Methods to select and predict functional polymorphisms.

New use for an old drug: inhibiting ABCG2 with sorafenib

Human ABCG2, a member of the ATP-binding cassette transporter superfamily, represents a promising target for sensitizing MDR in cancer chemotherapy. Although lots of ABCG2 inhibitors were identified, none of them has been tested clinically, maybe because of several problems such as toxicity or safety and pharmacokinetic uncertainty of compounds with novel chemical structures. One efficient solution is to rediscover new uses for existing drugs with known pharmacokinetics and safety profiles. Here, we found the new use for sorafenib, which has a dual-mode action by inducing ABCG2 degradation in lysosome in addition to inhibiting its function. Previously, we reported some novel dual-acting ABCG2 inhibitors that showed closer similarity to degradation-induced mechanism of action. On the basis of these ABCG2 inhibitors with diverse chemical structures, we developed a pharmacophore model for identifying the critical pharmacophore features necessary for dual-acting ABCG2 inhibitors. Sorafenib forms impressive alignment with the pharmacophore hypothesis, supporting the argument that sorafenib is a potential ABCG2 inhibitor. This is the first article that sorafenib may be a good candidate for chemosensitizing agent targeting ABCG2-mediated MDR. This study may facilitate the rediscovery of new functions of structurally diverse old drugs and provide a more effective and safe way of sensitizing MDR in cancer chemotherapy.

Different roles of TM5, TM6, and ECL3 in the oligomerization and function of human ABCG2

ABCG2 is a member of the ATP-binding cassette transporter superfamily, and its overexpression causes multidrug resistance (MDR) in cancer chemotherapy. ABCG2 may also protect cancer stem cells by extruding cytotoxic materials. ABCG2 has previously been shown to exist as a high-order homo-oligomer consisting of possibly 8-12 subunits, and the oligomerization domain was mapped to the C-terminal domain, including TM5, ECL3, and TM6. In this study, we further investigate this domain in detail for the role of each segment in the oligomerization and drug transport function of ABCG2 using domain swapping and site-directed mutagenesis. We found that none of the three segments (TM5, TM6, and ECL3) is essential for the oligomerization activity of ABCG2 and that any one of these three segments in the full-length context is sufficient to support ABCG2 oligomerization. While TM5 plays an important role in the drug transport function of ABCG2, TM6 and ECL3 are replaceable. Thus, each segment in the TM5-ECL3-TM6 domain plays a distinctive role in the oligomerization and function of ABCG2.

Human ABCC1 interacts and colocalizes with ATP synthase α, revealed by interactive proteomics analysis

Human ABCC1 is a member of the ATP-binding cassette (ABC) transporter superfamily, and its overexpression has been shown to cause multidrug resistance by active efflux of a wide variety of anticancer drugs. ABCC1 has been shown to exist and possibly function as a homodimer. However, a possible heterocomplex involving ABCC1 has been indicated. In this study, we performed an interactive proteomics study to examine proteins that bind to and form heterocomplexes with ABCC1 using coimmunoprecipitation and tandem mass spectrometry (MS/MS) analyses. We found that ATP synthase α binds to ABCC1 in plasma membranes with a ratio of 2:1. The ATP synthase α binding site in ABCC1 is located in the linker domain at the carboxyl core of ABCC1, and phosphorylation of the linker domain at the protein kinase A site enhances ATP synthase α binding. The interaction between ABCC1 and ATP synthase α in a heterocomplex may indicate a novel function of ABCC1 in regulating extracellular ATP level and purinergic signaling cascade.

Role of eIF3a in regulating cisplatin sensitivity and in translational control of nucleotide excision repair of nasopharyngeal carcinoma

Translational control at the initiation step has been recognized as a major and important regulatory mechanism of gene expression. eIF3a, a putative subunit of eIF3 complex, has recently been shown to play an important role in regulating translation of a subset of mRNAs and found to correlate with prognosis of cancers. In this study, using nasopharyngeal carcinoma (NPC) cells as a model system we tested the hypothesis that eIF3a negatively regulates synthesis of nucleotide excision repair (NER) proteins and, thus, NER activities and cellular response to treatments with DNA damaging agents such as cisplatin. We found that a cisplatin-sensitive subclone S16 isolated from a NPC cell line CNE2 via limited dilution has increased eIF3a expression. Knocking down its expression in S16 cells increased cellular resistance to cisplatin, NER activity, and synthesis of NER proteins XPA, XPC, RAD23B, and RPA32. Altering eIF3a expression also changed cellular response to cisplatin and UV treatment in other NPC cell lines. Taken together, we conclude that eIF3a plays an important role in cisplatin response and NER activity of nasopharyngeal carcinomas by suppressing synthesis of NER proteins.

Dynamic vs static ABCG2 inhibitors to sensitize drug resistant cancer cells

Human ABCG2, a member of the ATP-binding cassette transporter superfamily, plays a key role in multidrug resistance and protecting cancer stem cells. ABCG2-knockout had no apparent adverse effect on the development, biochemistry, and life of mice. Thus, ABCG2 is an interesting and promising target for development of chemo-sensitizing agents for better treatment of drug resistant cancers and for eliminating cancer stem cells. Previously, we reported a novel two mode-acting ABCG2 inhibitor, PZ-39, that induces ABCG2 degradation in addition to inhibiting its activity. In this manuscript, we report our recent progresses in identifying two different groups of ABCG2 inhibitors with one inhibiting only ABCG2 function (static) and the other induces ABCG2 degradation in lysosome in addition to inhibiting its function (dynamic). Thus, the inhibitor-induced ABCG2 degradation may be more common than we previously anticipated and further investigation of the dynamic inhibitors that induce ABCG2 degradation may provide a more effective way of sensitizing ABCG2-mediated MDR in cancer chemotherapy.

Role of transmembrane segment 5 and extracellular loop 3 in the homodimerization of human ABCC1

Resistance to multiple anticancer agents is a major obstacle in the successful treatment of cancers. Overexpression of some ATP-binding cassette (ABC) membrane transporters such as ABCC1 has been shown to be a major contributor of multidrug resistance (MDR) in both laboratory cell line models and the clinical setting. ABCC1 has been thought to function as a homodimer with a putative dimerization domain located in the first 281 amino acid residues, including MSD0 and L0 domains. In this study, we further mapped in detail the dimerization site and placed it in TM5 and ECL3 in MSD0 using co-expression and co-immunoprecipitation of a series of deletion constructs. TM5 and ECL3 in one subunit appear to interact with TM5 and ECL3 in the opposing subunit in a sequence-independent manner, but their physical location together with the hydrophobicity of TM5 and the length of ECL3 appears to be important contributors to the dimerization ability of ABCC1.

A novel two mode-acting inhibitor of ABCG2-mediated multidrug transport and resistance in cancer chemotherapy

BACKGROUND

Multidrug resistance (MDR) is a major problem in successful treatment of cancers. Human ABCG2, a member of the ATP-binding cassette transporter superfamily, plays a key role in MDR and an important role in protecting cancer stem cells. Knockout of ABCG2 had no apparent adverse effect on the mice. Thus, ABCG2 is an ideal target for development of chemo-sensitizing agents for better treatment of drug resistant cancers and helping eradicate cancer stem cells.

METHODS/PRELIMINARY FINDINGS

Using rational screening of representatives from a chemical compound library, we found a novel inhibitor of ABCG2, PZ-39 (N-(4-chlorophenyl)-2-[(6-{[4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl]amino}-1,3-benzothiazol-2-yl)sulfanyl]acetamide), that has two modes of actions by inhibiting ABCG2 activity and by accelerating its lysosome-dependent degradation. PZ-39 has no effect on ABCB1 and ABCC1-mediated drug efflux, resistance, and their expression, indicating that it may be specific to ABCG2. Analyses of its analogue compounds showed that the pharmacophore of PZ-39 is benzothiazole linked to a triazine ring backbone.

CONCLUSION/SIGNIFICANCE

Unlike any previously known ABCG2 transporter inhibitors, PZ-39 has a novel two-mode action by inhibiting ABCG2 activity, an acute effect, and by accelerating lysosome-dependent degradation, a chronic effect. PZ-39 is potentially a valuable probe for structure-function studies of ABCG2 and a lead compound for developing therapeutics targeting ABCG2-mediated MDR in combinational cancer chemotherapy.

Characterization and analyses of multidrug resistance-associated protein 1 (MRP1/ABCC1) polymorphisms in Chinese population

OBJECTIVE

To explore the distribution frequencies of four common single nucleotide polymorphisms (SNPs) of MRP1/ABCC1 in a mainland Chinese population and investigate whether these SNPs affect the expression and function of the MRP1/ABCC1.

METHODS

The genotype of 208 healthy volunteers was determined using PCR-restriction fragment length polymorphism. The four candidated SNPs were recreated by site-directed mutagenesis and tested for their effect on MRP1/ABCC1 expression and multidrug resistance function in stable transfected HEK293 and CHO-K1 cell lines. Real-time PCR, western blot and confocal microscopy were used to determine the mRNA, protein expression, and protein trafficking. At last, the effect of mutations on MRP1/ABCC1-mediate drug resistance was determined using methyl thiazolyl tetrazolium assay.

RESULTS

The allelic frequencies of Cys43Ser (128G>C), Thr73Ile (218C>T), Arg723Gln (2168G>A), and Arg1058Gln (3173G>A) in mainland Chinese were 0.5, 1.4, 5.8, and 0.5%, respectively. None of these mutations had any effect on MRP1/ABCC1 expression and trafficking, but that Arg723Gln mutation significantly reduced MRP1/ABCC1-mediated resistance to daunorubicin, doxorubicin, etoposide, vinblastine, and vincristine. The Cys43Ser mutation did not affect all tested drug resistance. In contrast, the Thr73Ile mutation reduced resistance to methotrexate and etoposide, whereas the Arg1058Gln mutation increased the response of two anthracycline drugs and etoposide in HEK293 and CHO-K1 cells as well as vinblastine and methotrexate in CHO-K1 cells.

CONCLUSION

The allelic frequency of the Arg723Gln mutation is relatively higher than other SNPs in mainland Chinese population and therefore this mutation significantly reduces MRP1/ABCC1 activity in multidrug resistance.

Redox regulation of multidrug resistance in cancer chemotherapy: molecular mechanisms and therapeutic opportunities

The development of multidrug resistance to cancer chemotherapy is a major obstacle to the effective treatment of human malignancies. It has been established that membrane proteins, notably multidrug resistance (MDR), multidrug resistance protein (MRP), and breast cancer resistance protein (BCRP) of the ATP binding cassette (ABC) transporter family encoding efflux pumps, play important roles in the development of multidrug resistance. Overexpression of these transporters has been observed frequently in many types of human malignancies and correlated with poor responses to chemotherapeutic agents. Evidence has accumulated showing that redox signals are activated in response to drug treatments that affect the expression and activity of these transporters by multiple mechanisms, including (a) conformational changes in the transporters, (b) regulation of the biosynthesis cofactors required for the transporter's function, (c) regulation of the expression of transporters at transcriptional, posttranscriptional, and epigenetic levels, and (d) amplification of the copy number of genes encoding these transporters. This review describes various specific factors and their relevant signaling pathways that are involved in the regulation. Finally, the roles of redox signaling in the maintenance and evolution of cancer stem cells and their implications in the development of intrinsic and acquired multidrug resistance in cancer chemotherapy are discussed.

A new mechanism of drug resistance in breast cancer cells: fatty acid synthase overexpression-mediated palmitate overproduction

Multidrug resistance is a major problem in successful cancer chemotherapy. Various mechanisms of resistance, such as ABC transporter-mediated drug efflux, have been discovered using established model cancer cell lines. While characterizing a drug-resistant breast cancer cell line, MCF7/AdVp3000, we found that fatty acid synthase (FASN) is overexpressed. In this study, we showed that ectopic overexpression of FASN indeed causes drug resistance and that reducing the FASN expression increased the drug sensitivity in breast cancer cell lines MCF7 and MDA-MB-468 but not in the normal mammary epithelial cell line MCF10A1. Use of FASN inhibitor, Orlistat, at low concentrations also sensitized cells with FASN overexpression to anticancer drugs. The FASN-mediated drug resistance appears to be due to a decrease in drug-induced apoptosis from an overproduction of palmitic acid by FASN. Together with previous findings of FASN as a poor prognosis marker for breast cancer patients, our results suggest that FASN overexpression is a new mechanism of drug resistance and may be an ideal target for chemosensitization in breast cancer chemotherapy.

Effect of cysteine mutagenesis on the function and disulfide bond formation of human ABCG2

ABCG2 is a member of the ATP-binding cassette (ABC) transporter superfamily. Its overexpression causes multidrug resistance in cancer chemotherapy. Based on its apparent half size in sequence when compared with other traditional ABC transporters, ABCG2 has been thought to exist and function as a homodimer linked by intermolecular disulfide bonds. However, recent evidence suggests that ABCG2 may exist as a higher form of oligomers due to noncovalent interactions. In this study, we attempted to create a cysless mutant ABCG2 as a tool for further characterization of this molecule. However, we found that the cysless mutant ABCG2 is well expressed but not functional. Mapping of the cysteine residues showed that three cysteine residues (Cys284, Cys374, and Cys438) are required concurrently for the function of ABCG2 and potentially for intramolecular disulfide bond formation. We also found that the cysteine residues (Cys592, Cys603, and Cys608) in the third extracellular loop are involved in forming intermolecular disulfide bonds and that mutation of these residues does not affect the expression or drug transport activity of human ABCG2. Thus, we conclude that Cys284, Cys374, and Cys438, which may be involved in intramolecular disulfide bond formation, are concurrently required for ABCG2 function, whereas Cys592, Cys603, and Cys608, potentially involved in intermolecular disulfide bond formation, are not required.

A novel comonomer-free light-cured glass-ionomer cement for reduced cytotoxicity and enhanced mechanical strength

OBJECTIVE

The objective of this study was to develop a novel comonomer-free light-cured glass-ionomer system based on the 4-arm star-shape poly(acrylic acid). The mechanical strengths and in vitro cytotoxicity of the formed system were evaluated and compared with those of several representative commercial glass-ionomer cements.

MATERIALS AND METHODS

The 4-arm poly(acrylic acid) was synthesized using ATRP and tethered with glycidyl methacrylate (GM). The GM-tethered polymer was formulated with water, photo-initiators, and Fuji II LC filler. Fuji II, Fuji II LC and Vitremer were used for comparison. Compressive strength (CS) and MTT assay were used as tools to evaluate the mechanical strengths and in vitro cytotoxicity of the cements, respectively.

RESULTS

The experimental cement exhibited significantly high compressive, diametral tensile and flexural strengths as compared to commercial glass-ionomer cements, Fuji II, Fuji II LC and Vitremer. The effects of polymer/water (P/W) ratio, GM-grafting ratio, glass powder/polymer liquid (P/L) ratio and aging in water on strengths were significant. Similar to conventional glass-ionomer cement Fuji II, the eluates from the experimental cement showed little in vitro cytotoxicity to Balb/c mouse fibroblast cells, as compared to Fuji II LC and Vitremer that contain HEMA as a comonomer.

CONCLUSIONS

It appears that this novel comonomer-free light-cured glass-ionomer cement will be a promising dental restorative because it demonstrated significantly improved mechanical strengths and almost no in vitro cytotoxicity as compared to current commercial light-cured glass-ionomer cements.

Regulation of function by dimerization through the amino-terminal membrane-spanning domain of human ABCC1/MRP1

Overexpression of some ATP-binding cassette (ABC) membrane transporters such as ABCB1/P-glycoprotein/MDR1 and ABCC1/MRP1 causes multidrug resistance in cancer chemotherapy. It has been thought that half-ABC transporters with one nucleotide-binding domain and one membrane-spanning domain (MSD) likely work as dimers, whereas full-length transporters with two nucleotide-binding domains and two or three MSDs function as monomers. In this study, we examined the oligomeric status of the human full-length ABC transporter ABCC1/MRP1 using several biochemical approaches. We found 1) that it is a homodimer, 2) that the dimerization domain is located in the amino-terminal MSD0L0 (where L0 is loop 0) region, and 3) that MSD0L0 has a dominant-negative function when coexpressed with wild-type ABCC1/MRP1. These findings suggest that ABCC1/MRP1 may exist and function as a dimer and that MSD0L0 likely plays some structural and regulatory functions. It is also tempting to propose that the MSD0L0-mediated dimerization may be targeted for therapeutic development to sensitize ABCC1/MRP1-mediated drug resistance in cancer chemotherapy.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

Sensitizing hormone-refractory prostate cancer cells to drug treatment by targeting 14-3-3sigma

Advanced and hormone-refractory prostate cancer has long been considered as a chemoresistant disease. Recently, it was found that 14-3-3sigma expression increases as prostate tumor progresses, and that 14-3-3sigma contributes significantly to drug resistance in breast cancers. We, thus, hypothesized that advanced and hormone-refractory prostate cancers may have an increased level of 14-3-3sigma, which in turn may contribute to drug resistance in advanced and hormone-refractory prostate cancers. In this study, we tested this hypothesis and found that, indeed, the expression level of 14-3-3sigma in androgen-independent prostate cancer cell lines DU145, PC3, and CWR22RV are much higher than that in the androgen-dependent cell line LNCaP, and that the androgen-independent cells are more resistant to mitoxantrone and Adriamycin than the androgen-dependent cells. Depleting 14-3-3sigma expression in DU145 and CWR22RV by RNA interference significantly sensitized these cells to mitoxantrone and Adriamycin by abrogating G2-M checkpoint and increasing apoptosis, whereas restoring 14-3-3sigma expression in LNCaP cells enhanced drug resistance. We also showed that 14-3-3sigma deficiency caused nuclear localization of Cdc2 and dephosphorylation of the Tyr15 residue upon DNA damage. Based on these studies, we propose that therapeutic intervention targeting 14-3-3sigma may be useful for sensitizing hormone-refractory prostate cancers to chemotherapy by both G2-M checkpoint abrogation and apoptosis enhancement.

The amino terminus of the human multidrug resistance transporter ABCC1 has a U-shaped folding with a gating function

Multidrug resistance is a serious problem in successful cancer chemotherapy. Studies using model cell lines have demonstrated that overexpression of some members of the ATP-binding cassette (ABC) transporter superfamily, such as ABCC1, causes enhanced efflux and, thus, decreased accumulation of multiple anticancer drugs, which leads to increased cell survival. Unlike most other ABC transporters, ABCC1 has an additional membrane-spanning domain (MSD0) with a putative extracellular amino terminus of 32 amino acids. However, the function of MSD0 and the role of the extracellular amino terminus are largely unknown. In this study, we examined the structural folding and the function of the amino terminus. We found that it has a U-shaped folding with the bottom of the U-structure facing cytoplasm and both ends in extracellular space. We also found that this U-shaped amino terminus probably functions as a gate to regulate the drug transport activity of human ABCC1.

Expression profiling of ABC transporters in a drug-resistant breast cancer cell line using AmpArray

ATP-binding cassette (ABC) membrane proteins comprise a superfamily of transporters with a wide variety of substrates. Humans have 49 members in this superfamily. Several human ABC transporters, such as ABCB1 and ABCC1, have been attributed to cause multidrug resistance (MDR) in cancer treatment when over-expressed. In the past, an MDR cancer cell line MCF7/AdVp3000 has been selected, and overexpression of ABCG2 was thought to cause MDR in this cell line. However, ectopic overexpression of ABCG2 in MCF7 cells could not explain the high drug resistance level observed with the selected cell line. In this study, we designed an AmpArray analysis to profile whether other ABC transporters were also selected to contribute to the increased drug resistance in MCF7/AdVp3000 cells. We found that 16 ABC transporters, including ABCG2, had >/=1.5-fold altered expression in MCF7/AdVp3000 compared with the parental MCF7 cells. In particular, the expression of ABCA4 and ABCC3 was increased by 132- and 459-fold, respectively, whereas ABCG2 was increased by approximately 3000-fold. Furthermore, the elevated expression of these three transporters reversed with the reversed drug resistance phenotype, and silencing ABCC3 expression in MCF7/AdVp3000 cells significantly reduced doxorubicin resistance. Thus, other ABC transporters in addition to ABCG2 are likely to contribute to the MDR selected in MCF7/AdVp3000 cells. This study also shows that AmpArray can be used as a quick and easy tool to profile the expression of ABC transporters in resistant cell lines and tumor samples for potential use in individualized design of therapy.

Mutations of Charged Amino Acids in or near the Transmembrane Helices of the Second Membrane Spanning Domain Differentially Affect the Substrate Specificity and Transport Activity of the Multidrug Resistance Protein MRP1 (ABCC1)

Multidrug resistance protein 1 (MRP1) belongs to the ATP-binding cassette superfamily of transport proteins. In addition to drugs, MRP1 mediates the active transport of many conjugated and unconjugated organic anions. MRP1 consists of two membrane-spanning domains (MSD2 and MSD3) each followed by a nucleotide binding domain plus a third NH2-terminal MSD1. MSD2 contains transmembrane (TM) helices 6 through 11, and previously, we identified two charged residues in TM6 as having important but markedly different roles in MRP1 transport activity and substrate specificity by characterizing mutants containing nonconservative substitutions of Lys332 and Asp336. We have now extended these studies and found that the same-charge TM6 mutant K332R, like the nonconservatively substituted Lys332 mutants, exhibits a selective decrease in leukotriene C4 (LTC4) transport, associated with substantial changes in both Km and Vmax and LTC4 binding. The overall organic anion transport activity of the same-charge mutant of Asp336 (D336E) also remained very low, as observed for D336R. In addition, nonconservative substitutions of TM6-associated Lys319 and Lys347 resulted in a selective decrease in GSH transport. Of eight other charged residues in or proximal to TM7 to TM11 that were investigated, nonconservative substitutions of three of them [Lys396 (TM7), Asp436 (TM8), and Arg593 (TM11)] caused a substantial and global reduction in transport activity. However, unlike TM6 Asp336, wild-type transport activity could be reestablished in these MRP1 mutants by conservative substitutions. We conclude that MSD2-charged residues in or proximal to TM6, TM7, TM8, and TM11 play critical but differential roles in MRP1 transport activity and substrate specificity.

Characterization of oligomeric human half-ABC transporter ATP-binding cassette G2

Human ATP-binding cassette G2 (ABCG2, also known as mitoxantrone resistance protein, breast cancer-resistance protein, ABC placenta) is a member of the superfamily of ATP-binding cassette (ABC) transporters that have a wide variety of substrates. Overexpression of human ABCG2 in model cancer cell lines causes multidrug resistance by actively effluxing anticancer drugs. Unlike most of the other ABC transporters which usually have two nucleotide-binding domains and two transmembrane domains, ABCG2 consists of only one nucleotide-binding domain followed by one transmembrane domain. Thus, ABCG2 has been thought to be a half-transporter that may function as a homodimer. In this study, we characterized the oligomeric feature of human ABCG2 using non-denaturing detergent perfluoro-octanoic acid and Triton X-100 in combination with gel filtration, sucrose density gradient sedimentation, and gel electrophoresis. Unexpectedly, we found that human ABCG2 exists mainly as a tetramer, with a possibility of a higher form of oligomerization. Monomeric and dimeric ABCG2 did not appear to be the major form of the protein. Further immunoprecipitation analysis showed that the oligomeric ABCG2 did not contain any other proteins. Taken together, we conclude that human ABCG2 likely exists and functions as a homotetramer.

The MRP family of drug efflux pumps

The MRP family is comprised of nine related ABC transporters that are able to transport structurally diverse lipophilic anions and function as drug efflux pumps. Investigations of this family have provided insights not only into cellular resistance mechanisms associated with natural product chemotherapeutic agents, antifolates and nucleotide analogs, but also into factors that influence drug distribution in the body, membrane systems that are involved in the extrusion of reduced folates, cysteinyl leukotrienes and bile acids, and the molecular basis of two hereditary conditions in humans. The review will describe the biochemical properties, drug resistance activities and potential in vivo functions of these unusual pumps.

Mutation of proline residues in the NH(2)-terminal region of the multidrug resistance protein, MRP1 (ABCC1): effects on protein expression, membrane localization, and transport function

The Multidrug Resistance Protein, MRP1 (ABCC1) confers drug resistance and transports organic anions such as leukotriene C(4) (LTC(4)) and 17beta-estradiol 17-(beta-D-glucuronide) (E(2)17betaG). Previous studies showed that portions of the first membrane spanning domain (MSD1) and the cytoplasmic loop (CL3) connecting it to MSD2 are important for MRP1 transport function. We have replaced 12 prolines in MSD1 and CL3 with alanine and determined the effects of these substitutions on MRP1 expression and transport activity. All singly substituted MRP1-Pro mutants could be expressed in HeLa cells, except MRP1-P104A. The expressed mutants also transported LTC(4) and E(2)17betaG, and their K(m) (LTC(4)) values were similar to wild-type MRP1. Expression of the double mutant MRP1-P42/51A was reduced by >80% although it localized to the plasma membrane and transported organic anions. MRP1 expression was also reduced when the first transmembrane helix (amino acids 37-54) was deleted. In contrast, the phenotypes of the multiply substituted CL3 mutants MRP1-P196/205/207/209A and MRP1-P235/255A were comparable to wild-type MRP1. However, Pro(255)-substituted MRP1 mutants showed reduced immunoreactivity with a monoclonal antibody (MAb) whose epitope is located in CL3. We conclude that certain prolines in MSD1 and CL3 play a role in the expression and structure of MRP1.

Sestamibi is a substrate for MDR1 and MDR2 P-glycoprotein genes

Technetium-99m sestamibi has attracted interest for assessment of the function of P-glycoproteins, which are well expressed in the liver and have roles in biliary transport and the removal of chemotherapeutic drugs. To further examine the cross-reactivity of (99m)Tc-sestamibi for P-glycoprotein family members, we conducted studies in animals. Hepatobiliary secretion of (99m)Tc-sestamibi was determined in normal FVB/N mice, mutant mice with specific P-glycoprotein deficiencies in the FVB/N background, normal Long-Evans Agouti (LEA) rats, and Long-Evans Cinnamon (LEC) rats with abnormal copper transport and liver disease but intact P-glycoprotein expression. After intrasplenic injection, (99m)Tc-sestamibi was rapidly incorporated in the mouse and rat liver, with maximal accumulation after 102+/-31 and 109+/-16 s, respectively ( P=NS). In normal mice and rats, 55%+/-11% and 55%+/-6%, respectively, of the maximal sestamibi activity was retained in the liver after 1 h ( P=NS). In double knockout mice lacking both mdr1a and mdr1b homologs of the human MDR1 ( ABCB1) gene, 88%+/-11% of maximal sestamibi activity was retained in the liver after 1 h ( P<0.001). In knockout mice deficient in either mdr1a gene or mdr2 ( ABCB4) gene, biliary sestamibi excretion was also impaired, although this impairment was relatively less pronounced in ABCB4-deficient mice than in double knockout mice lacking both ABCB1 gene homologs ( P<0.03). Hepatobiliary sestamibi excretion in LEC rats was not different from that in control normal rats, despite the presence of significant liver disease in the former. Hepatobiliary sestamibi excretion requires P-glycoproteins and is unperturbed in chronic liver disease. Sestamibi appears to be a substrate for both ABCB1 and ABCB4 genes, although the former utilizes it far more efficiently. Assessment of P-glycoprotein activity with sestamibi should consider how regulation of ABCB1 and related family members might modulate sestamibi incorporation.

Identification of domains participating in the substrate specificity and subcellular localization of the multidrug resistance proteins MRP1 and MRP2

The human multidrug resistance protein MRP1 and its homolog, MRP2, are both thought to be involved in cancer drug resistance and the transport of a wide variety of organic anions, including the cysteinyl leukotriene C4 (LTC4) (Km = 0.1 and 1 microm). To determine which domain of these proteins is associated with substrate specificity and subcellular localization, we constructed various chimeric MRP1/MRP2 molecules and expressed them in polarized mammalian LLC-PK1 cells. We examined the kinetic properties of each chimeric protein by measuring LTC4 and methotrexate transport in inside-out membrane vesicles, sensitivity to an anticancer agent, etoposide, and subcellular localization by indirect immunofluorescence methods. The following results were determined in these studies: (i) when the NH2-proximal 108 amino acids of MRP2, including transmembrane (TM) helices 1-3, were exchanged with the corresponding region of MRP1, Km(LTC4) values of the chimera decreased approximately 4-fold and Km(methotrexate) values increased approximately 5-fold relative to those of wild-type MRP2 and MRP1, respectively, whereas resistance to etoposide increased approximately 3-fold; (ii) when the NH2-proximal region up to TM9 of MRP2 was exchanged with the corresponding region of MRP1, a further increase in etoposide resistance was observed, and subcellular localization moved from the apical to the lateral membrane; (iii) when two-thirds of MRP2 at the NH2 terminus were exchanged with the corresponding MRP1 region, the chimeric protein transported LTC4 with an efficiency comparable with that achieved by the wild-type MRP1; and (iv) exchange of the COOH-terminal 51 amino acids between MRP1 and MRP2 did not affect the localization of either of the proteins. These results provide a strong framework for further studies aimed at determining the precise domains of MRP1 and MRP2 with affinity for LTC4 and anticancer agents.

Transport of leukotriene C4 by a cysteine-less multidrug resistance protein 1 (MRP1)

Overexpression of multidrug resistance protein 1 (MRP1), an ATP-binding cassette protein, causes multidrug resistance. We developed a functional cysteine-less version of MRP1 that provides a framework for detailed biochemical and biophysical studies. The 18 Cys residues of a truncated MRP1 (tMRP1; lacking the first multispanning transmembrane domain) were replaced with Ala to generate Cys-less tMRP1 (CL tMRP1). CL tMRP1 expressed in Saccharomyces cerevisiae membranes displayed high-affinity ATP-dependent transport of the MRP1 substrate leukotriene C4. Compared with full-length MRP1, the K m for leukotriene C4 transport by CL tMRP1 was increased approximately 3-fold, while V max was not affected. Thus a functional CL tMRP1 can be expressed using a low-cost and rapid-generation yeast expression system. This Cys-less protein can be used for biochemical, spectroscopic and structural studies to elucidate the mechanism of drug transport by MRP1.