The anthracycline resistance-associated (ara) gene, a novel gene associated with multidrug resistance in a human leukaemia cell line

Drug resistance mechanisms in cancers: Execution of pro-survival strategies

One of the quintessential challenges in cancer treatment is drug resistance. Several mechanisms of drug resistance have been described to date, and new modes of drug resistance continue to be discovered. The phenomenon of cancer drug resistance is now widespread, with approximately 90% of cancer-related deaths associated with drug resistance. Despite significant advances in the drug discovery process, the emergence of innate and acquired mechanisms of drug resistance has impeded the progress in cancer therapy. Therefore, understanding the mechanisms of drug resistance and the various pathways involved is integral to treatment modalities. In the present review, I discuss the different mechanisms of drug resistance in cancer cells, including DNA damage repair, epithelial to mesenchymal transition, inhibition of cell death, alteration of drug targets, inactivation of drugs, deregulation of cellular energetics, immune evasion, tumor-promoting inflammation, genome instability, and other contributing epigenetic factors. Furthermore, I highlight available treatment options and conclude with future directions.

PXE, a Mysterious Inborn Error Clarified

Ever since Garrod deduced the existence of inborn errors in 1901, a vast array of metabolic diseases has been identified and characterized in molecular terms. In 2018 it is difficult to imagine that there is any uncharted backyard left in the metabolic disease landscape. Nevertheless, it took until 2013 to identify the cause of a relatively frequent inborn error, pseudoxanthoma elasticum (PXE), a disorder resulting in aberrant calcification. The mechanism found was not only biochemically interesting but also points to possible new treatments for PXE, a disease that has remained untreatable. In this review we sketch the tortuous road that led to the biochemical understanding of PXE and to new ideas for treatment. We also discuss some of the controversies still haunting the field.

Autophagy and doxorubicin resistance in cancer

Doxorubicin (DOX), also known as adriamycin, is a DNA topoisomerase II inhibitor and belongs to the family of anthracycline anticancer drugs. DOX is used for the treatment of a wide variety of cancer types. However, resistance among cancer cells has emerged as a major barrier to effective treatment using DOX. Currently, the role of autophagy in cancer resistance to DOX and the mechanisms involved have become one of the areas of intense investigation. More and more preclinical data are being obtained on reversing DOX resistance through modulation of autophagy as one of the promising therapeutic strategies. This review summarizes the recent advances in autophagy-targeting therapies that overcome DOX resistance from in-vitro studies to animal models for exploration of novel delivery systems. In-depth understanding of the mechanisms of autophagy regulation in relation to DOX resistance and development of molecularly targeted autophagy-modulating agents will provide a promising therapeutic strategy for overcoming DOX resistance in cancer treatment.

High-Copy Overexpression Screening Reveals PDR5 as the Main Doxorubicin Resistance Gene in Yeast

Doxorubicin is one of the most potent anticancer drugs used in the treatment of various cancer types. The efficacy of doxorubicin is influenced by the drug resistance mechanisms and its cytotoxicity. In this study, we performed a high-copy screening analysis to find genes that play a role in doxorubicin resistance and found several genes (CUE5, AKL1, CAN1, YHR177W and PDR5) that provide resistance. Among these genes, overexpression of PDR5 provided a remarkable resistance, and deletion of it significantly rendered the tolerance level for the drug. Q-PCR analyses suggested that transcriptional regulation of these genes was not dependent on doxorubicin treatment. Additionally, we profiled the global expression pattern of cells in response to doxorubicin treatment and highlighted the genes and pathways that are important in doxorubicin tolerance/toxicity. Our results suggest that many efflux pumps and DNA metabolism genes are upregulated by the drug and required for doxorubicin tolerance.

Molecular diagnosis of ATP-binding cassette transporter-related diseases

ATP-binding cassette (ABC) transporters are involved in a variety of physiologic processes such as xenobiotic defense, lipid metabolism, ion homeostasis and immune functions. A large number of ABC proteins have been causatively linked to rare and common human genetic diseases including familial high-density lipoprotein deficiency, retinopathies, cystic fibrosis, diabetes and cardiomyopathies. Furthermore, genetic variations in ABC transporter genes and dysregulated expression patterns of these molecules significantly contribute to drug resistance in human cancer cells and alter the pharmacokinetic properties of a variety of drugs. In order to analyze DNA sequence alterations or define disease-associated mRNA expression patterns of the complete ABC transporter superfamily, novel high-throughput molecular methods such as quantitative real-time PCR and DNA microarray analysis are emerging. The aim of this review is to provide an overview and to present some examples of human ABC transporters involved in monogenic diseases, cancer and pharmacogenetics. Methodologic aspects of molecular diagnostics applied to analyze genetic variations, mRNA and protein expression levels and functional characteristics of ABC transporters are discussed.

Multidrug resistance proteins (MRPs/ABCCs) in cancer chemotherapy and genetic diseases

The ATP-binding cassette (ABC) transporters are a superfamily of membrane proteins that are best known for their ability to transport a wide variety of exogenous and endogenous substances across membranes against a concentration gradient via ATP hydrolysis. There are seven subfamilies of human ABC transporters, one of the largest being the 'C' subfamily (gene symbol ABCC). Nine ABCC subfamily members, the so-called multidrug resistance proteins (MRPs) 1-9, have been implicated in mediating multidrug resistance in tumor cells to varying degrees as the efflux extrude chemotherapeutic compounds (or their metabolites) from malignant cells. Some of the MRPs are also known to either influence drug disposition in normal tissues or modulate the elimination of drugs (or their metabolites) via hepatobiliary or renal excretory pathways. In addition, the cellular efflux of physiologically important organic anions such as leukotriene C(4) and cAMP is mediated by one or more of the MRPs. Finally, mutations in several MRPs are associated with human genetic disorders. In this minireview, the current biochemical and physiological knowledge of MRP1-MRP9 in cancer chemotherapy and human genetic disease is summarized. The mutations in MRP2/ABCC2 leading to conjugated hyperbilirubinemia (Dubin-Johnson syndrome) and in MRP6/ABCC6 leading to the connective tissue disorder Pseudoxanthoma elasticum are also discussed.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Development of a rapid, reliable genetic test for pseudoxanthoma elasticum

Mutations in the human ABCC6 gene cause pseudoxanthoma elasticum (PXE), a hereditary disorder that impacts the skin, eyes, and cardiovascular system. Currently, the diagnosis of PXE is based on physical findings and histological examination of a biopsy of affected skin. We have combined two simple, polymerase chain reaction (PCR)-based methods to develop a rapid, reliable genetic assay for the majority of known PXE mutations. After PCR amplification and heteroduplex formation, mutations in exon 24 and exon 28 of the ABCC6 gene were detected with Surveyor nuclease, which cleaves double-stranded DNA at any mismatch site. Mutations originating from deletion of a segment of the ABCC6 gene between exon 23 and exon 29 (ex23_ex29del) were detected by long-range PCR. Size analysis of digestion fragments and long-range PCR products was performed by agarose gel electrophoresis. The methods accurately identified mutations or the absence thereof in 16 affected individuals as confirmed by DNA sequencing. Fifteen patients had one or two point mutations, and two of these individuals carried the ex23_ex29del in their second allele. This mutation detection and mapping strategy provides a simple and reliable genetic assay to assist in diagnosis of PXE, differential diagnosis of PXE-like conditions, and study of PXE genetics.

Neoadjuvant chemotherapy in women with large and locally advanced breast cancer: chemoresistance and prediction of response to drug therapy

Patients with large and locally advanced breast cancer (LLABC) present with a therapeutic challenge and undergo multimodality treatment. Many such patients receive neoadjuvant chemotherapy (NAC) prior to surgery. However, a number of these patients do not respond well to NAC and only a percentage (usually less than 30%) obtains a complete or optimal response. A range of mechanisms are believed to be involved in this chemoresistance, including ATP binding cassette (ABC) transporter overexpression, dysregulation of apoptosis and possibly increased numbers of cancer stem cells. The chemoresistant processes may be due to more than one mechanism. The ability to predict a response to NAC would be beneficial, targeting expensive and toxic drug treatment to those likely to respond and providing a therapeutic strategy for further post-operative chemotherapy. Currently, many biomarkers have been studied with a view to establishing a predictor of response. However, no single biomarker appears to be effective. Genomics is a novel biotechnological process which is being used to predict response to drug therapy; this work is currently at an early stage of development

Chromosome microdissection identifies genomic amplifications associated with drug resistance in a leukemia cell line: an approach to understanding drug resistance in cancer

A significant problem encountered in the treatment of cancer patients is that cancer cells often evolve resistance to chemotherapeutic agents. One of the mechanisms responsible for drug resistance is gene amplification. The study of the behavior of genes conferring drug resistance is very important to determine future treatments for cancer patients that will minimize the effect of gene amplification. One of the best methods to investigate this phenomenon is to use chromosome microdissection to directly access the amplified gene or genes. In the present study, chromosome microdissection and fluorescent in-situ hybridization (FISH) were applied for the identification of genes residing in a homogeneously staining region (HSR) in drug-resistant cell sublines developed by treatment of the T-cell leukemia cell line CCRF-CEM with increasing levels of the anthracycline, epirubicin. We have demonstrated that the selection by epirubicin actually elevated the level of the multidrug resistance-associated protein (MRP1) gene. We argue that the breakage fusion bridge (B-F-B) cycle offers a plausible explanation for this amplification. The DNA prepared from the amplified regions by chromosome microdissection provides a resource for future investigations looking for the possible presence of novel genes contributing to drug resistance.

Functions of yeast helicase Ssl2p that are essential for viability are also involved in protection from the toxicity of adriamycin

We have found that, in the yeast Saccharomyces cerevisiae, overexpression of the DNA helicase Ssl2p confers resistance to adriamycin. Ssl2p is involved, as a subunit of the basic transcription factor TFIIH, in the initiation of transcription and in nucleotide-excision repair (NER), and this helicase is essential for the survival of yeast cells. An examination of the relationship between the known functions of Ssl2p and adriamycin resistance indicated that overexpression of Ssl2p caused little or no increase in the rate of RNA synthesis and in NER. The absence of any involvement of NER in adriamycin resistance was supported by the finding that yeast cells that overexpressed the mutant form of Ssl2p that lacked the carboxy-terminal region, which is necessary for NER, remained resistant to adriamycin. When we examined the effects of overexpression in yeast of other mutant forms of Ssl2p with various deletions, we found that, of the 843 amino acids of Ssl2p, the entire amino acid sequence from position 81 to position 750 was necessary for adriamycin resistance. This region is identical to the region of Ssl2p that is necessary for the survival of yeast cells. Although this region contains helicase motifs, the overexpression of other yeast helicases, such as Rad3 and Sgs1, had little or no effect on adriamycin resistance, indicating that a mere increase in the intracellular level of helicases does not result in adriamycin resistance. Our results suggest that the functions of Ssl2p that are essential for yeast survival are also required for protection against adriamycin toxicity.

Overexpression of Ssl2p confers resistance to adriamycin and actinomycin D in Saccharomyces cerevisiae

Adriamycin is one of the most active anticancer drugs but the development of resistance to this drug hampers its efficacy. In an effort to identify novel genes that confer resistance to adriamycin, we introduced a yeast genomic library into Saccharomyces cerevisiae and selected transformants that grew in the presence of a normally toxic concentration of adriamycin. Detailed examination of a plasmid recovered from these transformants revealed that overexpression of the gene for Ssl2p rendered yeast cells resistant to adriamycin. Ssl2p is a protein that is involved in the initiation of transcription and in DNA repair. Overexpression of Ssl2p did not confer resistance to aclarubicin, an anthracycline anticancer drug, which, like adriamycin, is intercalated into DNA. Both adriamycin and aclarubicin inhibit topoisomerase II and, thus, topoisomerase II might not be a major factor in the acquired resistance to adriamycin that results from overexpression of Ssl2p. We tested several other compounds but the only one to which Ssl2p-overexpressing cells were cross-resistant was actinomycin D. Mammalian cells that overexpress P-glycoprotein, which is a transmembrane protein that is involved in the efflux of certain drugs, are resistant to both adriamycin and actinomycin D but not to aclarubicin. However, overexpression of Ssl2p had little or no effect on the intracellular accumulation of adriamycin. Our results suggest that a novel mechanism might be involved in the sensitivity of yeast to both adriamycin and actinomycin D.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

Therapeutic and biological importance of getting nucleotides out of cells: a case for the ABC transporters, MRP4 and 5

The energy dependent transport of drugs contributes to cellular resistance and is undoubtedly a prime suspect in chemotherapeutic failure of a variety of disease processes. Early studies focused on a single gene, the multidrug resistance gene, MDR1, as a main contributor to chemotherapeutic failure. However, the multifaceted nature of cellular resistance lead to the discovery of the MRP gene. This pivotal finding and the concurrent rapid development of gene databases lead to the expansion of the MRP gene family. The purpose of this review is to discuss two of the recently described MRP family members that were orphans until their role in drug resistance was discovered. This review will provide an overview of the current state of our understanding of MRP4 and 5.

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

Multidrug resistance-associated protein 1 (MRP1) is a member of the ATP-binding cassette membrane transport superfamily and is responsible for multidrug resistance in cancer cells. Currently, there are nine known human MRPs. Distinct from many other members of the ATP-binding cassette superfamily, human MRP1 and four other MRPs have an additional membrane-spanning domain (MSD) with a putative extracellular amino terminus. The functional significance of this additional MSD (MSD1) is currently unknown. To understand the role of MSD1 in human MRP1 structure and function, we studied the amino-terminal 33 amino acids. We found that the amino terminus of human MRP1 has two cysteine residues (Cys(7) and Cys(32)) that are conserved among the five human MRPs that have MSD1. Mutation analyses of the two cysteines in human MRP1 revealed that the Cys(7) residue is critical for the MRP1-mediated drug resistance and leukotriene C(4) transport activity. On the other hand, mutation of Cys(32) reduced only moderately the MRP1 function. The effect of Cys(7) mutation on MRP1 activity appears to be due to the 5-7-fold decrease in the maximal transport rate V(max). We also found that mutation of Cys(7) changed the amino-terminal conformation of MRP1. This conformational change is likely responsible for the decrease in V(max) of LTC(4) transport mediated by the mutant MRP1. Based on these studies, we conclude that the amino terminus of human MRP1 is important and that the Cys(7) residue plays a critical role in maintaining the proper structure and function of human MRP1.

Cytoplasmic retraction of the amino terminus of human multidrug resistance protein 1

Human multidrug resistance protein 1 (MRP1) is a member of the ATP-binding cassette (ABC) transport superfamily which also includes human multidrug resistance 1 (MDR1) gene product P-glycoprotein (Pgp). Overexpression of MRP1 or Pgp causes multidrug resistance in cancer cells. Different from Pgp, MRP1 contains an extra membrane-spanning domain (MSD1) with a putative extracellular amino terminus in addition to the core structure of two MSDs and two NBDs (nucleotide-binding domains). The structural and functional significance of the additional MSD1 in MRP1 remains elusive. In this study, we generated an IgG1 subclass monoclonal antibody, IU2H10, specific to the amino terminus of human MRP1 and mapped its epitope to 10 amino acids (S8ADGSDPLWD17). It can be used for Western blot, immunoprecipitation, and indirect immunofluorescence studies of human MRP1. However, surprisingly we found that IU2H10 cannot react with MRP1 unless cells are permeabilized. Furthermore, the IU2H10 epitope is exposed extracellularly when the carboxyl-terminal core domain of human MRP1 is deleted. Examination of the amino-terminal sequence of human MRP1 suggests that it consist of mainly coiled structures. These observations provide evidence for a model that is different from the prevailing extracellular location of the amino terminus of human MRP1. It is possible that part of the amino terminus of human MRP1, following exposure to the lumen of the endoplasmic reticulum, is retracted to the cytoplasm.

MRP subfamily transporters and resistance to anticancer agents

The MRP subfamily of ABC transporters from mammals consists of at least seven members, six of which have been implicated in the transport of amphipathic anions. MRP1, MRP2, and MRP3 bear a close structural resemblance, confer resistance to a variety of natural products as well as methotrexate, and have the facility for transporting glutathione and glucuronate conjugates. MRP1 is a ubiquitously expressed efflux pump for the products of phase II of xenobiotic detoxification, while MRP2, whose hereditary deficiency results in Dubin-Johnson syndrome, functions to extrude organic anions into the bile. MRP3 is distinguished by its capacity to transport the monoanionic bile constituent glycocholate, and may function as a basolateral back-up system for the detoxification of hepatocytes when the usual canalicular route is impaired by cholestatic conditions. MRP4 and MRP5 resemble each other more closely than they resemble MRPs 1-3 and confer resistance to purine and nucleotide analogs which are either inherently anionic, as in the case of the anti-AIDS drug PMEA, or are phosphorylated and converted to anionic amphiphiles in the cell, as in the case of 6-MP. Given their capacity for transporting cyclic nucleotides, MRP4 and MRP5 have also been implicated in a broad range of cellular signaling processes. The drug resistance activity and physiological substrates of MRP6 are unknown. However, its hereditary deficiency results in pseudoxanthoma elasticum, a multisystem disorder affecting skin, eyes, and blood vessels. It is hoped that elucidation of the resistance profiles and physiological functions of the different members of the MRP subfamily will provide new insights into the molecular basis of clinical drug resistance and spawn new strategies for combating this phenomenon.

Analysis of the structure and expression pattern of MRP7 (ABCC10), a new member of the MRP subfamily

The MRP subfamily of ABC transporters currently consists of at least six members, several of which have been demonstrated to transport amphipathic anions and to confer in vitro resistance to chemotherapeutic agents. In searching the data bases we identified the product of a cDNA sequencing project that bears significant similarity to MRP subfamily transporters. In this report the predicted coding sequence, protein product and expression pattern of this cDNA, termed MRP7, are analyzed. The MRP7 cDNA sequence encodes a 1492 amino acid ABC transporter whose structural architecture resembles that of MRP1, MRP2, MRP3, and MRP6, in that its transmembrane helices are arranged in three membrane spanning domains. However, in contrast to the latter transporters, a conserved N-linked glycosylation site is not found at the N-terminus of MRP7. Comparisons of the MRP7 amino acid sequence indicated that while it is most closely related to other MRP subfamily members, its degree of relatedness is the lowest of any of the known MRP-related transporters. The integrity of the predicted MRP7 coding sequence was confirmed by the synthesis of an approximately 158 kDa protein in reticulocyte lysates programmed with the MRP7 cDNA. While MRP7 transcript was detected in a variety of tissues by RT/PCR, it was not readily detectable by RNA blot analysis, suggesting that it is expressed at low levels in these tissues. Fluorescence in situ hybridization indicated that MRP7 maps to chromosome 6p12-21, in proximity to several genes associated with glutathione conjugation and synthesis. On the basis of these findings and evolutionary cluster analysis, we conclude that MRP7 is a member of the MRP subfamily of amphipathic anion transporters.

The (patho)physiological functions of the MRP family

The identification of certain members of the large superfamily of ATP binding cassette transport proteins such as MDR1 -P-glycoprotein and the multidrug resistance protein MRP1 as ATP-dependent drug efflux pumps has been a major contribution in our understanding of the multidrug resistance phenotype of cancer cells. Importantly, both transport proteins that exhibit only low structural homology have a very different substrate specificity but confer resistance to a similar spectrum of natural product chemotherapeutic drugs. In contrast to the drug transporter MDR1, MRP1 mainly transports anionic Phase II-conjugates. In addition MRP1-mediated drug resistance is highly dependent on high intracellular glutathione levels which may be linked to the apparent physiological involvement of MRP1 in glutathione-related cellular processes. This review summarizes the current knowledge about functional aspects of MRP1 and its five recently cloned homologues MRP2-MRP6 and discusses their substrate specificities and cellular localization with emphasis on drug resistance. Copyright 2000 Harcourt Publishers Ltd.

A family of drug transporters: the multidrug resistance-associated proteins

The human multidrug resistance-associated protein (MRP) family currently has seven members. The ability of several of these membrane proteins to transport a wide range of anticancer drugs out of cells and their presence in many tumors make them prime suspects in unexplained cases of drug resistance, although proof that they contribute to clinical drug resistance is still lacking. Recent studies have begun to clarify the function of the MRP family members. MRPs are organic anion transporters; i.e., they transport anionic drugs, exemplified by methotrexate, and neutral drugs conjugated to acidic ligands, such as glutathione (GSH), glucuronate, or sulfate. However, MRP1, MRP2, and MRP3 can also cause resistance to neutral organic drugs that are not known to be conjugated to acidic ligands by transporting these drugs together with free GSH. MRP1 can even confer resistance to arsenite and MRP2 to cisplatin, again probably by transporting these compounds in complexes with GSH. MRP4 overexpression is associated with high-level resistance to the nucleoside analogues 9-(2-phosphonylmethoxyethyl) adenine and azidothymidine, both of which are used as anti-human immunodeficiency virus drugs. MRPs may, therefore, also have a role in resistance against nucleoside analogues used in cancer chemotherapy. Mice without Mrp1, a high-affinity leukotriene C(4) transporter, have an altered response to inflammatory stimuli but are otherwise healthy and fertile. MRP2 is the major transporter responsible for the secretion of bilirubin glucuronides into bile, and humans without MRP2 develop a mild liver disease known as the Dubin-Johnson syndrome. The physiologic functions of the other MRPs are not known. Whether long-term inhibition of MRPs in humans can be tolerated (assuming that suitable inhibitors will be found) remains to be determined.

Deletion of the multidrug resistance protein MRP1 gene in acute myeloid leukemia: the impact on MRP activity

Deletion of the multidrug resistance gene MRP1has been demonstrated in acute myeloid leukemia (AML) patients with inversion of chromosome 16 (inv[16]). These AML patients are known to have a relatively favorable prognosis, which suggests thatMRP1 might play an important role in determining clinical outcome. This study analyzed MRP1 deletion by fluorescent in situ hybridization (FISH), with a focus on inv(16) AML patients. Functional activity of multidrug resistance protein (MRP) was studied in a flow cytometric assay with the use of the MRP substrate carboxyfluorescein (CF) and the inhibitor MK-571. MRP1, MRP2, and MRP6 messenger RNA (mRNA) expression was determined with reverse transcriptase–polymerase chain reaction (RT-PCR). The results were compared with normal bone marrow cells. MRP1deletion was detected in 7 AML patients; 2 cases showed no MRP1FISH signals, and 5 cases had 1 MRP1 signal, whereas in 4 AML patients with inv(16) no MRP1 deletions were observed. A variability in MRP activity, expressed as CF efflux–blocking by MK-571, was observed (efflux-blocking factors varied between 1.2 and 3.6); this correlated with the number of MRP1 genes (r = 0.91, P < .01). MRP activity in the AML cases was not different from normal hematopoietic cells. MRP1 mRNA was detected in patients with 1 or 2 MRP1 FISH signals, but not in patients with no MRP1 signals. MRP2 and MRP6 mRNA were expressed predominantly in AML samples with 1 MRP1 signal, whereas in normal bone marrow cells no MRP2 and MRP6 mRNA was observed. In conclusion, this study shows that MRP activity varies among inv(16) AML cases and does not differ from that in normal hematopoietic cells; this might be in part due to the up-regulation of other MRP genes.

Deletion of the multidrug resistance protein MRP1 gene in acute myeloid leukemia: the impact on MRP activity

Deletion of the multidrug resistance gene MRP1has been demonstrated in acute myeloid leukemia (AML) patients with inversion of chromosome 16 (inv[16]). These AML patients are known to have a relatively favorable prognosis, which suggests thatMRP1 might play an important role in determining clinical outcome. This study analyzed MRP1 deletion by fluorescent in situ hybridization (FISH), with a focus on inv(16) AML patients. Functional activity of multidrug resistance protein (MRP) was studied in a flow cytometric assay with the use of the MRP substrate carboxyfluorescein (CF) and the inhibitor MK-571. MRP1, MRP2, and MRP6 messenger RNA (mRNA) expression was determined with reverse transcriptase–polymerase chain reaction (RT-PCR). The results were compared with normal bone marrow cells. MRP1deletion was detected in 7 AML patients; 2 cases showed no MRP1FISH signals, and 5 cases had 1 MRP1 signal, whereas in 4 AML patients with inv(16) no MRP1 deletions were observed. A variability in MRP activity, expressed as CF efflux–blocking by MK-571, was observed (efflux-blocking factors varied between 1.2 and 3.6); this correlated with the number of MRP1 genes (r = 0.91, P < .01). MRP activity in the AML cases was not different from normal hematopoietic cells. MRP1 mRNA was detected in patients with 1 or 2 MRP1 FISH signals, but not in patients with no MRP1 signals. MRP2 and MRP6 mRNA were expressed predominantly in AML samples with 1 MRP1 signal, whereas in normal bone marrow cells no MRP2 and MRP6 mRNA was observed. In conclusion, this study shows that MRP activity varies among inv(16) AML cases and does not differ from that in normal hematopoietic cells; this might be in part due to the up-regulation of other MRP genes.

The multidrug-resistant phenotype associated with overexpression of the new ABC half-transporter, MXR (ABCG2)

Mechanisms of drug resistance other than P-glycoprotein are of increasing interest as the list of newly identified members of the ABC transport family has grown. We sought to characterize the phenotype of the newly discovered ABC transporter encoded by the mitoxantrone resistance gene, MXR, also known as ABCP1 or BCRP. The pharmacodynamics of mitoxantrone and 12 other fluorescent drugs were evaluated by confocal microscopy in four multidrug-resistant human colon (S1) and breast (MCF-7) cancer cell lines. We utilized two sublines, MCF-7 AdVp3000 and S1-M1-80, and detected overexpression of MXR by PCR, immunoblot assay and immunohistochemistry. These MXR overexpressing sublines were compared to cell lines with P-glycoprotein- and MRP-mediated resistance. High levels of cross-resistance were observed for mitoxantrone, the anthracyclines, bisantrene and topotecan. Reduced levels of mitoxantrone, daunorubicin, bisantrene, topotecan, rhodamine 123 and prazosin were observed in the two sublines with high MXR expression. Neither the P-glycoprotein substrates vinblastine, paclitaxel, verapamil and calcein-AM, nor the MRP substrate calcein, were extruded from MCF-7 AdVp3000 and S1-M1-80 cells. Thus, the multidrug-resistant phenotype due to MXR expression is overlapping with, but distinct from, that due to P-glycoprotein. Further, cells that overexpress the MXR protein seem to be more resistant to mitoxantrone and topotecan than cells with P-glycoprotein-mediated multidrug resistance. Our studies suggest that the ABC half-transporter, MXR, is a potent, new mechanism for conferring multiple drug resistance. Definition of its mechanism of transport and its role in clinical oncology is required.

The novel genetic disorder microhydranencephaly maps to chromosome 16p13.3-12.1

We studied a large consanguineous Anatolian family with children who exhibited hydranencephaly associated with microcephaly. The children were severely affected. This novel genetic disorder is autosomal recessive. We used autozygosity mapping to identify a locus at chromosome 16p13.3-12.1; it has a LOD score of 4.11. The gene locus is within a maximal 11-cM interval between markers D16S497 and D16S672 and within a minimal critical region of 8 cM between markers D16S748 and D16S490.

Increased expression of the MRP5 gene is associated with exposure to platinum drugs in lung cancer

To investigate the role of the multidrug resistance-associated protein (MRP1) homologue MRP5 in relation to platinum drug resistance, we examined the steady-state levels of the mRNAs for MRP5 in both lung cancer cell lines and peripheral mononuclear cells (PMN) after exposure to platinum drug and in normal lung and lung cancer tissue specimens. Firstly, we examined MRP5 gene expression levels in 80 autopsy samples (40 primary tumors and 40 corresponding normal lung tissues) from 40 patients who had died from lung cancer. Next, we monitored MRP5 gene expression levels within 24 hr in both lung cancer cell lines incubated with cisplatin and in PMN from 10 previously untreated lung cancer patients after carboplatin administration alone. The MRP5 gene expression levels were assessed by quantitative reverse transcription polymerase chain reaction or RNase protection assay. The MRP5 expression levels in normal lung tissues and in tumors from patients exposed to platinum drugs during their lifetime were significantly higher than those in tissues from non-exposed patients. On the other hand, the MRP5 expression levels were not rapidly induced by platinum drugs either in lung cancer cell lines or in PMN within 24 hr. Our results suggest that increased expression levels of the MRP5 gene are associated with exposure to platinum drugs in lung cancer in vivo and/or the chronic stress response to xenobiotics.

Current Clinical Practice: Application of Resistance Reversal Agents in Hematologic Malignancies

The clinical application of resistance reversal drugs for patients with hematologic malignancies is reviewed. The phenomenon of multidrug resistance versus other mechanisms are discussed. The pump-like mechanisms of P-glycoprotein, multidrug resistance associated protein, lung resistance protein and of other ATP binding cassette transporter proteins are reviewed briefly, as well as the important substrate drugs and pump-blocking compounds. The problems associated with resistance protein assays in clinical samples and the concept of prognostic versus therapeutic clinical relevance are described, within the context of selected hematologic malignancies. Toxicities and treatment outcomes of phase II and III trials of reversal agents in lymphoma, multiple myeloma, myelodysplastic syndromes, acute myeloid leukemia and blast phase of chronic myeloid leukemia are reviewed. Finally, current options for on-study management of relapsed or refractory hematologic malignancy patients are discussed.

Conjugate export pumps of the multidrug resistance protein (MRP) family: localization, substrate specificity, and MRP2-mediated drug resistance

The membrane proteins mediating the ATP-dependent transport of lipophilic substances conjugated to glutathione, glucuronate, or sulfate have been identified as members of the multidrug resistance protein (MRP) family. Several isoforms of these conjugate export pumps with different kinetic properties and domain-specific localization in polarized human cells have been cloned and characterized. Orthologs of the human MRP isoforms have been detected in many different organisms. Studies in mutant rats lacking the apical isoform MRP2 (symbol ABCC2) indicate that anionic conjugates of endogenous and exogenous substances cannot exit from cells at a sufficient rate unless an export pump of the MRP family is present in the plasma membrane. Several mutations in the human MRP2 gene have been identified which lead to the absence of the MRP2 protein from the hepatocyte canalicular membrane and to the conjugated hyperbilirubinemia of Dubin-Johnson syndrome. Overexpression of recombinant MRP2 confers resistance to multiple chemotherapeutic agents. Because of its function in the terminal excretion of cytotoxic and carcinogenic substances, MRP2 as well as other members of the MRP family, play an important role in detoxification and chemoprevention.

An inventory of the human ABC proteins

Currently 30 human ABC proteins are represented by full sequences in various databases, and this paper provides a brief overview of these proteins. ABC proteins are composed of transmembrane domains (TMDs), and nucleotide binding domains (NBDs, or ATP-binding cassettes, ABSs). The arrangement of these domains, together with available membrane topology models of the family members, are presented. Based on their sequence similarity scores, the members of the human ABC protein family can be grouped into eight subfamilies. At present the MDR/TAP, the ALD, the MRP/CFTR, the ABC1, the White, the RNAseL inhibitor, the ANSA, and the GCN20 subfamilies are identified. Mutations of many human ABC proteins are known to be causative in inherited diseases, and a short description of the molecular pathology of these ABC gene-related genetic diseases is also provided.

The multidrug resistance protein family

The human multidrug resistance protein (MRP) family contains at least six members: MRP1, the godfather of the family and well known as the multidrug resistance protein, and five homologs, called MRP2-6. In this review, we summarize what is known about the protein structure, the expression in tissues, the routing in cells, the physiological functions, the substrate specificity, and the role in multidrug resistance of the individual members of the MRP family.

Pseudoxanthoma elasticum maps to an 820-kb region of the p13.1 region of chromosome 16

We have performed linkage analysis on 21 families with pseudoxanthoma elasticum (PXE) using 10 polymorphic markers located on chromosome 16p13.1. The gene responsible for the PXE phenotype was localized to an 8-cM region of 16p13.1 between markers D16S500 and D16S3041 with a maximum lod score of 8.1 at a recombination fraction of 0.04 for marker D16S3017. The lack of any locus heterogeneity suggests that the major predisposing allele for the PXE phenotype is located in this region. Haplotype studies of a total of 36 PXE families identified several recombinations that further confined the PXE gene to a region (< 1 cM) between markers D16S3060 and D16S79. This PXE locus was identified within a single YAC clone and several overlapping BAC recombinants. From sequence analysis of these BAC recombinants, it is clear that the distance between markers D16S3060 and D16S79 is about 820 kb and contains a total of nine genes including three pseudogenes. We predict that mutations in one of the expressed genes in the locus will be responsible for the PXE phenotype in these families.

Absolute levels of MDR-1, MRP, and BCL-2 MRNA and tumor remission in acute leukemia

Mononuclear cells prepared from peripheral blood or bone marrow of 119 AML and 28 ALL patients prior and following therapy were analyzed for absolute transcript levels of the chemoresistance genes mdr-1 and MRP, and the proto-oncogene bcl-2, by validated contamination-protected quantitative RT-PCR. In newly diagnosed AML mainly tumors of the granulocytic lineage (FAB M1-M2) expressed increased mdr-1 mRNA amounts. The MRP gene was expressed in all investigated samples without relation to a particular FAB class. High initial expression of both genes did not confer a poor prognosis even at high number of CD34+ cells. Data compared prior to and after therapy start (paired samples) revealed that AML patients who did not respond to therapy (NR) expressed increased levels of mdr-1 mRNA, as well as MRP and bcl-2 cDNA normalized to GAPDH reference transcripts, when compared to patients achieving complete remission (CR; p = 0.003, 0.008 and 0.0005, respectively). In ALL-NR the mdr-1 and bcl-2 genes were entirely more active after induction chemotherapy. Arbitrary cut-off values were established in order to delimit pathological from non-pathological gene expression. 59% of studied AML and 33% of ALL-NR exceeded the arbitrary values (mdr-1: > 2 amol/microgram RNA, MRP: > 10 zmol/amol GAPDH, bcl-2: > 5 zmol/amol GAPDH) for one and 11% of AML-NR for two parameters. Only 17% of the AML-CR and none of the ALL-CR group were above these limits. The results indicate that high individual activity of usually one, rarely two of the investigated genes might be associated with poor clinical outcome in treated acute leukemia.

Multidrug resistance protein MRP1, glutathione, and related enzymes. Their importance in acute myeloid leukemia

Multidrug resistance (MDR), which is cross-resistance to structurally and functionally unrelated drugs such as anthracyclines, epipodophyllotoxins and vinca alkaloids, is a major cause of treatment failure in malignant disorders. Known mechanisms of MDR are overexpression of the ATP-dependent membrane proteins P-glycoprotein (P-gp) and multidrug resistance protein (MRP1), or an increased detoxification of compounds mediated by glutathione (GSH) or GSH related enzymes. MRP1 appeared to transport drugs conjugated to GSH and also unmodified cytostatic agents in presence of GSH. The relation between MRP1, GSH and enzymes involved in GSH metabolism or GSH dependent detoxification reactions recently has drawn a lot of attention. Coordinated induction of MRP1 and GSH related enzymes is reported in malignant cells after exposure to cytostatic agents. Besides MRP1, a number of MRP1 homologs are identified, named MRP2, MRP3, MRP4, MRP5 and MRP6. The relation between MDR and expression of these MRP1 homologs is currently under research.

Genome duplications and other features in 12 Mb of DNA sequence from human chromosome 16p and 16q

Several publicly funded large-scale sequencing efforts have been initiated with the goal of completing the first reference human genome sequence by the year 2005. Here we present the results of analysis of 11.8 Mb of genomic sequence from chromosome 16. The apparent gene density varies throughout the region, but the number of genes predicted (84) suggests that this is a gene-poor region. This result may also suggest that the total number of human genes is likely to be at the lower end of published estimates. One of the most interesting aspects of this region of the genome is the presence of highly homologous, recently duplicated tracts of sequence distributed throughout the p-arm. Such duplications have implications for mapping and gene analysis as well as the predisposition to recurrent chromosomal structural rearrangements associated with genetic disease.

MOAT-E (ARA) is a full-length MRP/cMOAT subfamily transporter expressed in kidney and liver

Multidrug resistance-associated protein (MRP) and the canalicular multispecific organic anion transporter (cMOAT) are organic anion pumps that have been linked to cytotoxic drug resistance. We previously reported the isolation of three human MRP/cMOAT-related transporters, MOAT-B (MRP4), MOAT-C (MRP5) and MOAT-D (MRP3). In the present study we describe the fourth MRP/cMOAT-related transporter. We analysed ARA, a human cDNA reported to encode a 453 residue MRP-related transporter, and found that it represents a fused transcript composed of MRP sequences and partial sequences of a novel transporter. The complete coding sequence of this novel transporter, which we designated MOAT-E, was isolated. MOAT-E encodes a 1503 residue transporter that is most closely related to MRP (45%), MOAT-D (44%) and cMOAT (39%), both in terms of amino acid identity and sharing a common topology in which approximately 17 transmembrane spanning helices are distributed within three membrane spanning domains. RNA blot analysis indicated that MOAT-E expression is restricted to kidney and liver. These observations suggest that MOAT-E may function as an organic anion transporter involved in cellular detoxification and possibly in the hepatobiliary and renal excretion of xenobiotics and/or endogenous metabolites. Isolation of MOAT-E helps to define the MRP/cMOAT subfamily of transporters.

Biochemical and molecular aspects of genetic disorders of bilirubin metabolism

Bilirubin, the oxidative product of heme in mammals, is excreted into the bile after its esterification with glucuronic acid to polar mono- and diconjugated derivatives. The accumulation of unconjugated and conjugated bilirubin in the serum is caused by several types of hereditary disorder. The Crigler-Najjar syndrome is caused by a defect in the gene which encodes bilirubin UDP-glucuronosyltransferase (UGT), whereas the Dubin-Johnson syndrome is characterized by a defect in the gene which encodes the canalicular bilirubin conjugate export pump of hepatocytes. Animal models such as the unconjugated hyperbilirubinemic Gunn rat, the conjugated hyperbilirubinemic GY/TR-, and the Eisai hyperbilirubinemic rat, have contributed to the understanding of the molecular basis of hyperbilirubinemia in humans. Elucidation of both the structure of the UGT1 gene complex, and the Mrp2 (cMoat) gene which encodes the canalicular conjugate export pump, has led to a greater understanding of the genetic basis of hyperbilirubinemia.

ARA, a novel ABC transporter, is located at 16p13.1, is deleted in inv(16) leukemias, and is shown to be expressed in primitive hematopoietic precursors

ATP-binding cassette (ABC), ATP-dependent transporters are a large superfamily of proteins that include the multidrug resistance proteins, P-glycoprotein and MRP (multidrug resistance protein). The ARA (anthracycline resistance-associated) gene that codes for a putative member of the ABC transporters has recently been cloned and shown to have high sequence homology to the gene for MRP. We have previously shown MRP to be deleted in a subset of inv(16) leukemic patients. The deletion of MRP was associated with an improved patient survival compared with inv(16) patients who did not have such a deletion. In this study, the ARA gene is mapped to 16p13.1, in the same physical interval as the inv(16) short-arm breakpoint. It is shown to be situated proximal to both MYH11, the gene involved in the primary breakpoint on the short arm of the inv(16), and MRP. A YAC clone has been isolated containing both MRP and ARA. FISH analysis of metaphase chromosomes from inv(16) patients has established the gene order as telomere-MYH11-MRP-ARA-centromere and demonstrated that both ARA and MRP are deleted in a subgroup of the inv(16) leukemias. ARA and MRP are both shown to be expressed in normal hematopoietic precursors including CD34(+) cells. The mapping of ARA to this region and its homology to MRP raises questions about its potential role in the biology of the inv(16) leukemias.

Amplification and expression of the ABC transporters ARA and MRP in a series of multidrug-resistant leukaemia cell sublines

E1000, the most drug-resistant subline from the E-series (CCRF-CEM/E16 to E1000), has been previously shown to express high mRNA levels from two ABC transporter genes associated with multidrug resistance, ARA and MRP. The expression and amplification of both genes has now been characterized for each member of the E-series of drug-resistant sublines and is reported here. Both ARA [detected by reverse transcriptase polymerase chain reaction (RT-PCR)] and MRP (detected by Northern blot analysis) were expressed at low levels in the sensitive parental CEM cell line. An equivalent level of MRP mRNA expression was detected throughout the CEM, E16, E25 and E50 sublines, and there was increasing expression in the E100, E200 and E1000 sublines. ARA expression was not detected in the E16, E25, E50 and E100 sublines but was detected by both RT-PCR and Northern blot analysis in the E200 and E1000 sublines. Southern blot analysis indicated the increased levels of MRP and ARA expression resulted from gene amplification and that MRP was first amplified in the E100 subline and ARA in the E200 subline, suggesting that the two genes were not initially co-amplified. Cytogenetic analysis of E1000 cells demonstrated a large addition to chromosome 16p, around the region where the ARA and MRP genes are located. Increased expression of ARA is associated with increased colchicine resistance in the E-series of sublines and combined with MRP may account for their resistance phenotype.

Induction of broad drug resistance in small cell lung cancer cells and its reversal by paclitaxel

The H82 "variant" and the H69 "classic" small cell lung cancer (SCLC) cell lines were treated with low levels of epirubicin (69 and 14 nM) which caused little cell death but produced the H82/E8 and H69/E8 extended-multidrug resistant sublines. Both were resistant to drugs associated with multidrug resistance (MDR), and to chlorambucil (9.5- and 5.6-fold, respectively) and cisplatin (2.3- and 8.5-fold, respectively). There was increased expression of the multidrug resistance-associated protein (MRP1) in the H82/E8 subline while P-glycoprotein expression was not detected in any cells or sublines. Treatment of the H82 cells for 1 hr with 69 nM epirubicin increased MRP1-mRNA expression within 4 hr and this was associated with an increase in the resistance to epirubicin, chlorambucil, cisplatin and paclitaxel. Further, a 1 hr treatment with non-cytotoxic doses of chlorambucil (2.5 microM), cisplatin (1.3 microM) or paclitaxel (13 nM), drugs not normally associated with MRP1-mediated MDR, also increased MRP1-mRNA expression in the H82 cells with paclitaxel causing the highest increase (4.5-fold). For chlorambucil treatment, this increased MRPI-mRNA expression was accompanied by increased drug resistance while paclitaxel treatment had no effect on drug resistance in the H82 cells. For the drug resistant H82/E8 subline, these drug treatments had no effect on the MRP1-mRNA expression and little effect on increasing the subline drug resistance. However, pretreatment with paclitaxel sensitised the H82/E8 subline to chlorambucil and cisplatin returning the subline to the sensitivity of the H82 cell line. We conclude that treatment with low levels of MDR and non-MDR drugs can induce extended-multidrug resistance in SCLC cells, a process that probably involves the co-ordinate upregulation of MRP1 and other resistance mechanisms. The results also suggest paclitaxel may have a role as a response modifier in the treatment of refractory SCLC.

ATP-Dependent human erythrocyte glutathione-conjugate transporter. II. Functional reconstitution of transport activity

Purified dinitrophenyl S-glutathione (DNP-SG) ATPase was reconstituted into artificial liposomes prepared from soybean asolectin. Electron micrography confirmed the formation of unilamellar vesicles with an average radius of 0.25 micron. Intravesicular volume estimated by incorporation of radiolabled inulin into the vesicles was found to be 19.7 +/- 1.3 microL/mL reconstitution solution. Accumulation of the glutathione-conjugate of CDNB, DNP-SG, and of doxorubicin (DOX) in the proteoliposomes was increased in the presence of ATP as compared to equimolar ADP or adenosine 5'-[beta,gamma-methylene]triphosphate tetralithium. ATP-dependent transmembrane movement of DOX and DNP-SG into DNP-SG ATPase-reconstituted vesicles was saturable with respect to time, sensitive to the osmolarity of the assay medium, and temperature dependent. The energy of activation was found to be 12 and 15 kcal/mol for DNP-SG and DOX, respectively. Optimal temperature for transport was 37 degrees C. Saturable transport was demonstrated for DNP-SG (Vmax of 433 +/- 20 nmol/min/mg of protein, KmATP = 2.4 +/- 0. 3 mM and KmDNP-SG = 36 +/- 5 microM) as well as DOX (Vmax = 194 +/- 19 nmol/min/mg of protein, KmATP = 2.5 +/- 0.6 mM and KmDOX = 2.4 +/- 0.7 microM). The kinetic data for both DNP-SG and DOX transport were consistent with a random bi-bi sequential reaction mechanism. DOX was found to be a competitive inhibitor of DNP-SG transport with Kis of 1.2 +/- 0.2 microM and DNP-SG was found to be a competitive inhibitor of DOX transport with Kis of 13.3 +/- 2.6 microM.

Active efflux by multidrug transporters as one of the strategies to evade chemotherapy and novel practical implications of yeast pleiotropic drug resistance

Mankind is faced by the increasing emergence of resistant pathogens, including cancer cells. An overview of the different strategies adopted by a variety of cells to evade chemotherapy is presented, with a focus on the mechanisms of multidrug transport. In particular, we analyze the yeast network for pleiotropic drug resistance and assess the potentiality of this system for further understanding of the mechanism of broad specificity and for development of novel practical applications.

Increased MRP expression is associated with resistance to radiation, anthracyclines and etoposide in cells treated with fractionated gamma-radiation

The failure of chemotherapy is often associated with the failure of radiotherapy in the treatment of cancer. To investigate this relationship, the CCRF-CEM (CEM) human T-cell leukaemia cell line was treated with fractionated gamma-radiation totalling 75 Gy (10 cycles of 1.5 Gy daily for 5 days). This produced the CEMRR subline which was 1.5-fold resistant to radiation compared with the parental CEM cells. The CEMRR subline was also resistant to daunorbicin, idarubicin and etoposide but not to paclitaxel, cis-platinum or chlorambucil. Treatment with 50 microM buthionine sulphoximine, an inhibitor of glutathione synthesis, reversed the daunorubicin resistance in the CEMRR subline. Multidrug resistance-associated protein (MRP) mRNA was 6-fold higher in the CEMRR subline than in the CEM cells, and there was no detectable expression of P-glycoprotein in either the CEM cells or the CEMRR subline. Treatment of the CEM cells with 2 Gy of gamma-radiation caused an increase in MRP-mRNA within 4 hr which, by 24 hr, was greater than 5-fold that of the untreated CEM cells. No change in MRP mRNA was observed in the CEMRR subline with similar treatment. We conclude that MRP is involved in the immediate response to radiation and it may account for the drug resistance that often develops following radiation treatment.