The role of the multidrug resistance-associated protein 1 gene in neuroblastoma biology and clinical outcome

The ATP-binding cassette proteins ABCB1 and ABCC1 as modulators of glucocorticoid action

Responses to hormones that act through nuclear receptors are controlled by modulating hormone concentrations not only in the circulation but also within target tissues. The role of enzymes that amplify or reduce local hormone concentrations is well established for glucocorticoid and other lipophilic hormones; moreover, transmembrane transporters have proven critical in determining tissue responses to thyroid hormones. However, there has been less consideration of the role of transmembrane transport for steroid hormones. ATP-binding cassette (ABC) proteins were first shown to influence the accumulation of glucocorticoids in cells almost three decades ago, but observations over the past 10 years suggest that differential transport propensities of both exogenous and endogenous glucocorticoids by ABCB1 and ABCC1 transporters provide a mechanism whereby different tissues are preferentially sensitive to different steroids. This Review summarizes this evidence and the new insights provided for the physiology and pharmacology of glucocorticoid action, including new approaches to glucocorticoid replacement.

Extrachromosomal circular DNA in cancer drug resistance and its potential clinical implications

Chemotherapy is widely used to treat patients with cancer. However, resistance to chemotherapeutic drugs remains a major clinical concern. The mechanisms of cancer drug resistance are extremely complex and involve such factors such as genomic instability, DNA repair, and chromothripsis. A recently emerging area of interest is extrachromosomal circular DNA (eccDNA), which forms owing to genomic instability and chromothripsis. eccDNA exists widely in physiologically healthy individuals but also arises during tumorigenesis and/or treatment as a drug resistance mechanism. In this review, we summarize the recent progress in research regarding the role of eccDNA in the development of cancer drug resistance as well as the mechanisms thereof. Furthermore, we discuss the clinical applications of eccDNA and propose some novel strategies for characterizing drug-resistant biomarkers and developing potential targeted cancer therapies.

A comprehensive review on acridone based derivatives as future anti-cancer agents and their structure activity relationships

The development of drug resistance and severe side-effects has reduced the clinical efficacy of the existing anti-cancer drugs available in the market. Thus, there is always a constant need to develop newer anti-cancer drugs with minimal adverse effects. Researchers all over the world have been focusing on various alternative strategies to discover novel, potent, and target specific molecules for cancer therapy. In this direction, several heterocyclic compounds are being explored but amongst them one promising heterocycle is acridone which has attracted the attention of medicinal chemists and gained huge biological importance as acridones are found to act on different therapeutically proven molecular targets, overcome ABC transporters mediated drug resistance and DNA intercalation in cancer cells. Some of these acridone derivatives have reached clinical studies as these heterocycles have shown huge potential in cancer therapeutics and imaging. Here, the authors have attempted to compile and make some recommendations of acridone based derivatives concerning their cancer biological targets and in vitro-cytotoxicity based on drug design and novelty to increase their therapeutic potential. This review also provides some important insights on the design, receptor targeting and future directions for the development of acridones as possible clinically effective anti-cancer agents.

P-glycoprotein: new insights into structure, physiological function, regulation and alterations in disease

The multidrug resistance phenomenon presents a major threat to the pharmaceutical industry. This resistance is a common occurrence in several diseases and is mediated by multidrug transporters that actively pump substances out of the cell and away from their target regions. The most well-known multidrug transporter is the P-glycoprotein transporter. The binding sites within P-glycoprotein can accommodate a variety of compounds with diverse structures. Hence, numerous drugs are P-glycoprotein substrates, with new ones being identified every day. For many years, the mechanisms of action of P-glycoprotein have been shrouded in mystery, and scientists have only recently been able to elucidate certain structural and functional aspects of this protein. Although P-glycoprotein is highly implicated in multidrug resistant diseases, this transporter also performs various physiological roles in the human body and is expressed in several tissues, including the brain, kidneys, liver, gastrointestinal tract, testis, and placenta. The expression levels of P-glycoprotein are regulated by different enzymes, inflammatory mediators and transcription factors; alterations in which can result in the generation of a disease phenotype. This review details the discovery, the recently proposed structure and the regulatory functions of P-glycoprotein, as well as the crucial role it plays in health and disease.

Specifically Targeted Transport of Plasma Membrane Transporters: From Potential Mechanisms for Regulating Cell Health or Disease to Applications

Normal substrate transport and signal transmission are the premise to ensure the health of biological somatic cells. Therefore, a comprehensive understanding of the molecular mechanism of intercellular substrate transport is of great significance for clinical treatment. In order to better understand the membrane protein through its interaction with receptors, to help maintain a healthy cell and the molecular mechanisms of disease, in this paper, we seek to clarify, first of all, the recognition mechanism for different types of membrane protein receptors; pathogen invasion using the transport pathway involved in the membrane; and the latest specific target sites of various kinds of membrane transport carriers; to provide an explanation and summary of the system. Secondly, the downstream receptor proteins and specific substrates of different membrane transporters were classified systematically; the functional differences of different subclasses and their relationship with intracellular transport disorders were analyzed to further explore the potential relationship between cell transport disorders and diseases. Finally, the paper summarizes the use of membrane transporter-specific targets for drug design and development from the latest research results; it points out the transporter-related results in disease treatment; the application prospects and the direction for drug development and disease treatment providing a new train of thought; also for disease-specific targeted therapy, it provides a certain reference value.

Prognostic Value of c-MYC Expression in Patients with Peripheral Neuroblastic Tumors

Objective

Neuroblastic tumors are the most common solid tumors in children. The aim of this study was to explore the prognostic value of immunostaining for cellular-myelocytomatosis viral oncogene (c-MYC) expression in patients with peripheral neuroblastic tumors (NTs).

Methods

A retrospective study was conducted to compare the expression of c-MYC detected by immunohistochemistry and v-myc avian myelocytomatosis viral oncogene neuroblastoma derived homolog (MYCN) by fluorescence in situ hybridization among 177 cases of NTs and determine the associations of c-MYC and MYCN with the clinical stages, morphological types, and survival rates of NTs.

Results

The cases positive for c-MYC were mainly the favorable histology type in stage 3 or 4 with a poor NT prognosis, but no morphological changes related to the poor prognosis were observed in their samples under a microscope. The cases with positive c-MYC expression did not overlap those with MYCN amplification.

Conclusion

Positive c-MYC expression portends a poor prognosis in patients with NTs.

Over-expression of FSIP1 promotes breast cancer progression and confers resistance to docetaxel via MRP1 stabilization

Fibrous sheath-interacting protein 1 (FSIP1) functions centrally in breast carcinogenesis and progression, although its exact role remains to be clarified. Therefore, we sought to establish a correlation between the clinico-pathological features of breast cancer and FSIP1 expression in breast cancer tissues, as well as to validate its role in tumor progression and chemo-resistance. We analyzed FSIP1 expression in the breast cancer and para-tumor tissues by immunohistochemistry. We performed MTT, Caspase-Glo 3/7 Assay, Annexin V staining, wound healing and trans-well assays to evaluate cellular apoptosis, proliferation, migration and invasion in FSIP1 knockout and wild-type breast cancer cell lines. Additionally, we examined the effects of FSIP1 on docetaxel sensitivity in a nude mice model transplanted with control or FSIP1 knockout breast cancer cells, and also evaluate its role in tumor metastasis. FSIP1 and MRP1 interaction was determined by co-immunoprecipitation and mass spectrometry. We found that breast cancer cells and tissues consistently demonstrated elevated FSIP1 expressions, which correlated with poor overall survival. Notably, patients with high FSIP1 expression in their tumors undergoing docetaxel neoadjuvant chemotherapy had shorter disease-free survival. FSIP1 knockout in breast cancer cells significantly increased their sensitivity to docetaxel both in vitro and in vivo. Mechanistically, FSIP1 bound to the multidrug resistance protein 1 (MRP1) and stabilized it, and knocking out FSIP1 decreased MRP1 expression and increased cellular docetaxel accumulation. In sum, FSIP1 promotes breast carcinogenesis and mediates docetaxel resistance, and may serve as a novel target in the development of breast cancer therapies.

Surface marker profiling of SH-SY5Y cells enables small molecule screens identifying BMP4 as a modulator of neuroblastoma differentiation

Neuroblastoma is the most common extra-cranial solid tumor in children. Its broad spectrum of clinical outcomes reflects the underlying inherent cellular heterogeneity. As current treatments often do not lead to tumor eradication, there is a need to better define therapy-resistant neuroblastoma and to identify new modulatory molecules. To this end, we performed the first comprehensive flow cytometric characterization of surface molecule expression in neuroblastoma cell lines. Exploiting an established clustering algorithm (SPADE) for unbiased visualization of cellular subsets, we conducted a multiwell screen for small molecule modulators of neuroblastoma phenotype. In addition to SH-SY5Y cells, the SH-EP, BE(2)-M17 and Kelly lines were included in follow-up analysis as in vitro models of neuroblastoma. A combinatorial detection of glycoprotein epitopes (CD15, CD24, CD44, CD57, TrkA) and the chemokine receptor CXCR4 (CD184) enabled the quantitative identification of SPADE-defined clusters differentially responding to small molecules. Exposure to bone morphogenetic protein (BMP)-4 was found to enhance a TrkA^high/CD15⁻/CD184⁻ neuroblastoma cellular subset, accompanied by a reduction in doublecortin-positive neuroblasts and of NMYC protein expression in SH-SY5Y cells. Beyond yielding novel marker candidates for studying neuroblastoma pathology, our approach may provide tools for improved pharmacological screens towards developing novel avenues of neuroblastoma diagnosis and treatment.

Visfatin mediates doxorubicin resistance in human non-small-cell lung cancer via Akt-mediated up-regulation of ABCC1

OBJECTIVES

Non-small-cell lung cancer (NSCLC) is one of the leading causes of cancer deaths worldwide. Increasing levels of visfatin are correlated with worse clinical prognosis of NSCLC. However, the effects of visfatin on drug resistant are still not well illustrated.

MATERIALS AND METHODS

Effects of visfatin on drug resistant cells were checked by CCK-8 kit. Gene and protein variations were measured by real-time PCR and western blot analysis, respectively.

RESULTS

Our present data confirmed that expression of visfatin was significantly increased in NSCLC cells and tissues. In addition, protein and mRNA expression of visfatin were significantly elevated in doxorubicin (Dox) resistance of NSCLC cells when compared with their corresponding sensitivity parental cells. Overexpression of visfatin can down-regulate the Dox sensitivity of NSCLC cells and up-regulate the mRNA and protein expression of ABCC1, while has no effect on ABCB1. Knockdown of visfatin can down-regulate the expression of ABCC1 in Dox-resistant NSCLC cells. Visfatin can increase the phosphorylation and nuclear localization of Akt in NSCLC cells. LY294002 can decrease the expression of multidrug resistance protein-1 (MRP1) in NSCLC Dox-resistant cells. Chromatin immunoprecipitation assays showed that overexpression of visfatin can significantly increase the binding of Akt with the promoter of ABCC1 in both A549 and H1793 cells.

CONCLUSIONS

These data showed that visfatin can decrease Dox sensitivity of NSCLC cells via activation of Akt/MRP1. It indicated that inhibition of visfatin signals might be a promising therapeutic strategy for the management of chemoresistance of NSCLC patients.

Low dose of arsenic trioxide inhibits multidrug resistant-related P-glycoprotein expression in human neuroblastoma cell line

This study investigated arsenic trioxide (As2O3), cisplatin (DDP) and etoposide (Vp16) on the anticancer effects and P-glycoprotein (P-gp) expression in neuroblastoma (NB) SK-N-SH cells. The potential influence of As2O3, DDP and Vp16 currently included in NB routine treatment protocols on cytotoxicity in SK-N-SH cells was measured by flow cytometry and drug half-maximal inhibitory concentration (IC50) was established. Moreover, chemotherapeutic agent-mediated changes of cellular expression levels of resistant-related P-gp, was monitored using western blotting. The data showed that As2O3, DDP and Vp16 significantly inhibited the growth and survival of the SK-N-SH cells at different concentration. Notably, the levels of apoptosis were upregulated in SK-N-SH cells with an acceleration of the exposure time and the concentration of As2O3, DDP and Vp16. As2O3, DDP and Vp16 were observed with their IC50 values on SK-N-SH cells being 3 µM, 8 and 100 µg/ml, respectively. Flow cytometry analysis showed that As2O3 at low concentrations in SK-N-SH cells led to enhanced accumulation of cell populations in G2/M phase with increasing the exposure time, and increased levels of apoptosis. In contrast, we observed that SK-N-SH cell populations arrested in S phase by DDP and Vp16. In vitro examination revealed that following pretreatment of SK-N-SH cells with As2O3, the expression of P-gp was not increased. The expression of P-gp downregulation were noted following the group treated by As2O3 at 2 and 3 µM. Exposed to As2O3 at 3 µM for 72 h, SK-N-SH cells exhibited lower expression of P-gp than 2 µM As2O3 for 72 h. In contrast, the expression of P-gp was upregulated by DDP and VP16. In summary, SK-N-SH cells were responsive to chemotherapeutic agent-induced apoptosis in a dose-dependent and time-dependent manner. In particular, ours findings showed that low dose of As2O3 markedly reduced the P-gp expression and increased apoptotic cell death in human NB cell line.

Probenecid Sensitizes Neuroblastoma Cancer Stem Cells to Cisplatin

We used both in vitro cultures of neuroblastoma cell lines and nude-mice xenotransplants to explore the effects of co-administration of cisplatin and probenecid. Probenecid sensitized neuroblastoma cells, including tumor cells with stem features, to the effects of cisplatin, both in vitro and in vivo. This effect was mediated by an increase in the apoptotic cell death and a concomitant decrease in cell proliferation. This effect is accompanied by modulation of the mRNA and protein of the drug efflux transporters MDR1, MRP2, and BCRP. The co-administration of probenecid with cisplatin should be explored as a possible therapeutic strategy.

Effect of siRNA-Livin on drug resistance to chemotherapy in glioma U251 cells and CD133+ stem cells

The aim of the present study was to observe the effect of siRNA-Livin on the expression of multidrug resistance-associated protein (MRP) genes in a U251 cell line and U251 stem cells. CD133⁺ cancer stem cells were identified and isolated from the U251 glioblastoma cells, and morphological observations were used to detect the cell survival conditions. In addition, quantitative polymerase chain reaction was used to detect the mRNA expression levels of Livin, MRP1 and MRP3. Following transfection with the lentivirus containing the siRNA-Livin, the expression of Livin was significantly inhibited in the U251 cells and stem cells (P<0.01). Following temozolomide intervention, the proliferation of the U251 cells and U251 stem cells was restrained, with a lot of cell debris present and the structure of the cell spheres destroyed. The inhibitory effect was more significant following transfection with siRNA-Livin. Prior to siRNA-Livin transfection, the expression of MRP1 presented an increasing trend in the U251 cells and U251 stem cells with increasing drug concentrations and intervention times (P<0.05). Following siRNA-Livin transfection, the expression of MRP1 decreased in the U251 cells and U251 stem cells under the same drug concentration and intervention time (P<0.05), while the expression of MRP3 increased in the U251 stem cells under the same intervention concentration and time (P<0.05). Therefore, siRNA-Livin was shown to decrease the expression of MRP1 in U251 cells and U251 stem cells, increase the expression of MRP3 in U251 stem cells and decrease the proliferation of U251 cells and U251 stem cells. Thus, Livin may be associated with the high expression of MRP1, and siRNA-Livin may be used to lower the expression of MRP1 in order to reduce the drug resistance to chemotherapy in cases of glioblastoma.

Multidrug resistance and cancer stem cells in neuroblastoma and hepatoblastoma

Chemotherapy is one of the major modalities in treating cancers. However, its effectiveness is limited by the acquisition of multidrug resistance (MDR). Several mechanisms could explain the up-regulation of MDR genes/proteins in cancer after chemotherapy. It is known that cancer stem cells (CSCs) play a role as master regulators. Therefore, understanding the mechanisms that regulate some traits of CSCs may help design efficient strategies to overcome chemoresistance. Different CSC phenotypes have been identified, including those found in some pediatric malignancies. As solid tumors in children significantly differ from those observed in adults, this review aims at providing an overview of the mechanistic relationship between MDR and CSCs in common solid tumors, and, in particular, focuses on clinical as well as experimental evidence of the relations between CSCs and MDR in neuroblastoma and hepatoblastoma. Finally, some novel approaches, such as concomitant targeting of multiple key transcription factors governing the stemness of CSCs, as well as nanoparticle-based approaches will also be briefly addressed.

The molecular and biophysical characterization of the human blood-nerve barrier: current concepts

The internal microenvironment in peripheral nerves is highly regulated in order to maintain normal axonal impulse transmission to or from the central nervous system. In humans, this regulation is facilitated by specialized tight junction (TJ)-forming endoneurial microvascular endothelial cells and perineurial myofibroblasts that form multiple concentric layers around nerve fascicles. The endoneurial endothelial cells come in direct contact with circulating blood and, thus, can be considered the blood-nerve barrier (BNB). Studies on the molecular and biophysical properties of the human BNB in vivo or in situ are limited. Owing to the recent isolation of primary human endoneurial endothelial cells and the development of simian virus 40 large T-antigen immortalized cell lines, data are emerging on the structural and functional characteristics of these cells. These data aim to increase our understanding of how solutes, macromolecules, nutrients and hematogenous leukocytes gain access into or are restricted from the endoneurium of peripheral nerves. These concepts have clinical relevance in understanding normal peripheral nerve homeostasis, the response of peripheral nerves to external insult and stresses such as drugs and toxins and the pathogenesis of peripheral neuropathies. This review discusses current knowledge in this nascent and exciting field of microvascular biology.

NeuPAT: an intranet database supporting translational research in neuroblastic tumors

Translational research in oncology is directed mainly towards establishing a better risk stratification and searching for appropriate therapeutic targets. This research generates a tremendous amount of complex clinical and biological data needing speedy and effective management. The authors describe the design, implementation and early experiences of a computer-aided system for the integration and management of data for neuroblastoma patients. NeuPAT facilitates clinical and translational research, minimizes the workload in consolidating the information, reduces errors and increases correlation of data through extensive coding. This design can also be applied to other tumor types.

A comparative proteomic study identified LRPPRC and MCM7 as putative actors in imatinib mesylate cross-resistance in Lucena cell line

Background

Although chronic myeloid leukemia (CML) treatment has improved since the introduction of imatinib mesylate (IM), cases of resistance have been reported. This resistance has been associated with the emergence of multidrug resistance (MDR) phenotype, as a BCR-ABL independent mechanism. The classic pathway studied in MDR promotion is ATP-binding cassette (ABC) family transporters expression, but other mechanisms that drive drug resistance are largely unknown. To better understand IM therapy relapse due to the rise of MDR, we compared the proteomic profiles of K562 and Lucena (K562/VCR) cells.

Results

The use of 2-DE coupled with a MS approach resulted in the identification of 36 differentially expressed proteins. Differential mRNA levels of leucine-rich PPR motif-containing (LRPPRC) protein, minichromosome maintenance complex component 7 (MCM7) and ATP-binding cassette sub-family B (MDR/TAP) member 1 (ABCB1) were capable of defining samples from CML patients as responsive or resistant to therapy.

Conclusions

Through the data presented in this work, we show the relevance of MDR to IM therapy. In addition, our proteomic approach identified candidate actors involved in resistance, which could lead to additional information on BCR-ABL-independent molecular mechanisms.

A single-arm pilot phase II study of gefitinib and irinotecan in children with newly diagnosed high-risk neuroblastoma

BACKGROUND

Gefitinib potently inhibits neuroblastoma proliferation in vitro, and the gefitinib/irinotecan combination shows greater than additive activity against neuroblastoma xenografts. This Phase II pilot study estimated the rate of response to two courses of intravenous irinotecan plus oral gefitinib in children with untreated high-risk neuroblastoma.

METHODS

Two courses of irinotecan [15 mg/m(2)/day (daily ×5)×2] were combined with 12 daily doses of gefitinib (112.5 mg/m(2)/day). Response was assessed after 6 weeks. A response rate >55% was sought.

RESULTS

Of the 23 children enrolled, 19 were evaluable for response. Median age at diagnosis was 3.1 years (range, 18 days-12.7 years). Most patients were older than 24 months (n = 20; 87%), male (n = 18; 78%), white (n = 16; 70%), had INSS 4 disease (n = 19; 83%), and had adrenal primary tumors (n = 18; 78%); nine patients (39%) had amplified tumor MYCN. The toxicity of gefitinib/irinotecan was mild and reversible (nausea, 5/20; diarrhea, 8/20; vomiting, 7/20). Five patients had partial responses; 9 others had a 23%-60% decrease in primary tumor volume and/or improved MIBG scans or decreased bone or bone marrow tumor burden. Median (range) systemic irinotecan exposure (AUC) was 283 ng/ml*hr (range, 163-890 ng/ml*hr) and 28 ng/ml*hr (3.6-297 ng/ml*hr) for the active metabolite, SN-38. No relation was observed between response and tumor expression of EGFR, MRP2-4, ABCG2, and Pgp.

CONCLUSIONS

Although the gefitinib/irinotecan combination was very tolerable and induced responses, it was not sufficiently active to warrant further investigation. Initial investigational studies of this type can preclude the necessity for larger, longer, and costlier trials.

Influence of Etoposide on anti-apoptotic and multidrug resistance-associated protein genes in CD133 positive U251 glioblastoma stem-like cells

It has been hypothesized that cancer stem cell is responsible for the refractoriness of glioblastoma therapy. This study is to observe the influence of Etoposide on anti-apoptotic and multidrug resistance-associated protein genes in glioblastoma stem-like cells. U251 glioblastoma cells were cultured and CD133 positive cancer stem-like cells were isolated and identified. Cell counting kit-8 assay, cell morphology and flow cytometry were employed for assaying cell survival condition. Real-time quantitative PCR was chosen for detecting mRNA expression of livin, livinalpha, livinbeta, survivin, MRP1 and MRP3. As results, after Etoposide intervention, the U251 stem-like cells showed more resistant property, more intact morphology and lower apoptotic rate than that in U251 cells (p<0.05). It could be found that the expression of livinbeta in U251 stem-like cells was significantly higher (p<0.05). After Etoposide intervention, only livinalpha was suppressed markedly (p<0.05), while livin expression was not notably decreased with livinbeta increased on the contrary (p<0.05). MRP1 and MRP3 in U251 stem-like cells were significantly higher than that in cancer cells, and after chemotherapy, the expression of MRP1 increased notably (p<0.05). But the expression of survivin and MRP3 did not show these features. In conclusion, after Etoposide intervention glioblastoma stem-like cells showed a stronger resistance to apoptosis and death, and the anti-apoptotic gene livinbeta was more related with the high survival rate and MRP1 appeared to be more related with transporting chemotherapeutics out of glioblastoma stem-like cells.

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.

Molecular cytogenetic characterization of doxorubicin-resistant neuroblastoma cell lines: evidence that acquired multidrug resistance results from a unique large amplification of the 7q21 region

Neuroblastoma is a heterogeneous neural crest-derived embryonic childhood neoplasm that is the second most common solid tumor found in children. Despite recent advances in combined therapy, the overall survival of patients with high-stage disease has not improved in the last decades. Treatment failure is in part attributed to multidrug resistance. To address the mechanisms involved in the development of multidrug resistance, we have generated two doxorubicin-resistant neuroblastoma cell lines (IGRN-91R and LAN-1R). These cells were shown to overexpress the MDR1 gene coding for the P-glycoprotein and were resistant to other MDR1- and non-MDR1-substrate drugs. Indeed, the MDR1 inhibitor verapamil only partially restored sensitivity to drugs, confirming that P-glycoprotein-mediated drug efflux was not responsible for 100% resistance. High-resolution and array-based comparative genomic hybridization analyses revealed the presence of an amplicon in the 7q21 region as the unique genomic alteration common to both doxorubicin-resistant cell lines. In addition to the MDR1 locus, this large amplified region is likely to harbor additional genes potentially involved in the development of drug resistance. This study represents the first molecular cytogenetic and genomic approach to identifying genomic regions involved in the multidrug-resistant phenotype of neuroblastoma. These results could lead to the identification of relevant target genes for the development of new therapeutic modalities.

Multidrug Resistance-Associated Proteins: Expression and Function in the Central Nervous System

Drug delivery to the brain is highly restricted, since compounds must cross a series of structural and metabolic barriers to reach their final destination, often a cellular compartment such as neurons, microglia, or astrocytes. The primary barriers to the central nervous system are the blood-brain and blood-cerebrospinal fluid barriers. Through structural modifications, including the presence of tight junctions that greatly limit paracellular transport, the cells that make up these barriers restrict diffusion of many pharmaceutically active compounds. In addition, the cells that comprise the blood-brain and blood-cerebrospinal fluid barriers express multiple ATP-dependent, membrane-bound, efflux transporters, such as members of the multidrug resistance-associated protein (MRP) family, which contribute to lowered drug accumulation. A relatively new concept in brain drug distribution just beginning to be explored is the possibility that cellular components of the brain parenchyma could act as a "second" barrier to brain permeation of pharmacological agents via expression of many of the same transporters. Indeed, efflux transporters expressed in brain parenchyma may facilitate the overall export of xenobiotics from the central nervous system, essentially handing them off to the barrier tissues. We propose that these primary and secondary barriers work in tandem to limit overall accumulation and distribution of xenobiotics in the central nervous system. The present review summarizes recent knowledge in this area and emphasizes the clinical significance of MRP transporter expression in a variety of neurological disorders.