Patterns of cross-resistance in a multidrug-resistant small-cell lung carcinoma cell line

Total steroidal saponins from black nightshade (Solanum nigrum L.) overcome tumor multidrug resistance by inducing autophagy-mediated cell death in vivo and in vitro

Multiple drug resistance (MDR) often occurs after prolonged chemotherapy, leading to refractory tumors and cancer recurrence. In this study, we demonstrated that the total steroidal saponins from Solanum nigrum L. (SN) had broad-spectrum cytotoxic activity against various human leukemia cancer cell lines, especially in adriamycin (ADR)-sensitive and resistant K562 cell lines. Moreover, SN could effectively inhibit the expression of ABC transporter in K562/ADR cells in vivo and in vitro. In vivo, by establishing K562/ADR xenograft tumor model, we demonstrated that SN might overcome drug resistance and inhibit the proliferation of tumors by regulating autophagy. In vitro, the increased LC3 puncta, the expression of LC3-II and Beclin-1, and the decreased expression of p62/SQSTM1 in SN-treated K562/ADR and K562 cells demonstrated autophagy induced by SN. Moreover, using the autophagy inhibitors or transfecting the ATG5 shRNA, we confirmed that autophagy induced by SN was a key factor in overcoming MDR thereby promoting cell death in K562/ADR cells. More importantly, SN-induced autophagy through the mTOR signaling pathway to overcome drug resistance and ultimately induced autophagy-mediated cell death in K562/ADR cells. Taken together, our findings suggest that SN has the potential to treat multidrug-resistant leukemia.

Plasmodium falciparum Multidrug Resistance Proteins (pfMRPs)

The capacity of the lethal Plasmodium falciparum parasite to develop resistance against anti-malarial drugs represents a central challenge in the global control and elimination of malaria. Historically, the action of drug transporters is known to play a pivotal role in the capacity of the parasite to evade drug action. MRPs (Multidrug Resistance Protein) are known in many phylogenetically diverse groups to be related to drug resistance by being able to handle a large range of substrates, including important endogenous substances as glutathione and its conjugates. P. falciparum MRPs are associated with in vivo and in vitro altered drug response, and might be important factors for the development of multi-drug resistance phenotypes, a latent possibility in the present, and future, combination therapy environment. Information on P. falciparum MRPs is scattered in the literature, with no specialized review available. We herein address this issue by reviewing the present state of knowledge.

Targeting multidrug resistance in cancer

Effective treatment of metastatic cancers usually requires the use of toxic chemotherapy. In most cases, multiple drugs are used, as resistance to single agents occurs almost universally. For this reason, elucidation of mechanisms that confer simultaneous resistance to different drugs with different targets and chemical structures - multidrug resistance - has been a major goal of cancer biologists during the past 35 years. Here, we review the most common of these mechanisms, one that relies on drug efflux from cancer cells mediated by ATP-binding cassette (ABC) transporters. We describe various approaches to combating multidrug-resistant cancer, including the development of drugs that engage, evade or exploit efflux by ABC transporters.

The role of transporters in drug interactions

Transport proteins play an important role in the adsorption, distribution and elimination of a wide variety of drugs. Therefore, it is not surprising that transporter-based drug interactions can occur in the clinic. These interactions can lead to changes in toxicity and/or efficacy of the affected drug. Here, we review such interactions and ask if these interactions could have been predicted from in vitro data. Conducting such in vitro-in vivo correlation is important for predicting future transporter-based drug interactions.

Phase I/II trial of pyrazoloacridine and carboplatin in patients with recurrent glioma: a North Central Cancer Treatment Group trial

PURPOSE

Novel therapeutic agents in the treatment of recurrent gliomas are urgently needed. Pyrazoloacridine (PZA), a rationally synthesized acridine derivative, has shown promising antitumor activity against glioma lines in combination with platinum compounds. This phase I/II trial of the PZA/carboplatin combination in recurrent glioma patients consisted of two phase I studies (studies 1 and 2) and a phase II trial (study 3). The objectives of studies 1 and 2 were to (a) assess the safety and toxicity and to establish the phase II dose of the pyrazoloacridine/carboplatin combination for recurrent glioma patients on P450 inducing anticonvulsants, and (b) to confirm the phase II dose for patients not on P450 inducing anticonvulsants. The primary objectives of study 3 were to determine the efficacy of the pyrazoloacridine/carboplatin combination in patients with recurrent gliomas, to further assess the toxicity of the combination, and to evaluate the impact of enzyme-inducing anticonvulsants on the pyrazoloacridine metabolism.

EXPERIMENTAL DESIGN

Both carboplatin and pyrazoloacridine were administered intravenously every 28 days. Treatment was continued until unacceptable toxicity, tumor progression or patient withdrawal.

RESULTS

14 patients were treated in the two phase I studies and 32 patients in the phase II trial. The phase II dose of the combination was PZA 400 mg/m(2) and carboplatin AUC of 5 every 28 days. Neutropenia (4 patients) and dyspnea (1 patient) was the dose limiting toxicity in the phase I studies. In the phase II trial, the most frequent toxicity was myelosuppression with grade 3 and 4 hematologic adverse events being observed in 22 and 19% of the patients, respectively. The antitumor activity of this regimen was limited; the response rate in the phase II trial was 0%, (95% CI:0-11%) while 12 of the 32 patients (38%) had stable disease with a median duration of 2 months. The percentage of phase II patients who were progression free at three months was 22% and at six months was 16%. Median survival from study entry was 5.0 months for phase I patients and 5.8 months for phase II patients. Pharmacokinetic analysis performed in 8 phase I patients demonstrated no significant impact of the enzyme-inducing anticonvulsants on the pharmacokinetics of pyrazoloacridine.

CONCLUSIONS

The phase II dose of the pyrazoloacridine/carboplatin combination is pyrazoloacridine 400 mg/m(2) in combination with carboplatin AUC of 5. Antitumor activity in patients with recurrent gliomas was limited. Initial disease stabilization occurred in approximately 38% of the patients, with median duration of 2 months. Enzyme-inducing anticonvulsants did not affect the pyrazoloacridine metabolism.

Substrate recognition and transport by multidrug resistance protein 1 (ABCC1)

Multidrug resistance protein (MRP) 1 belongs to the 'C' branch of the ABC transporter superfamily. MRP1 is a high-affinity transporter of the cysteinyl leukotriene C(4) and is responsible for the systemic release of this cytokine in response to an inflammatory stimulus. However, the substrate specificity of MRP1 is extremely broad and includes many organic anion conjugates of structurally unrelated endo- and xenobiotics. In addition, MRP1 transports unmodified hydrophobic compounds, such as natural product type chemotherapeutic agents and mutagens, such as aflatoxin B(1). Transport of several of these compounds has been shown to be dependent on the presence of reduced glutathione (GSH). More recently, GSH has also been shown to stimulate the transport of some conjugated compounds, including sulfates and glucuronides. Here, we summarize current knowledge of the substrate specificity and modes of transport of MRP1 and discuss how the protein may recognize its structurally diverse substrates.

Synergistic Cytotoxicity of Pyrazoloacridine with Doxorubicin, Etoposide, and Topotecan in Drug-Resistant Tumor Cells

Pyrazoloacridine (NSC 366140, PD115934, PZA) is a new class of acridine anticancer agents under investigation in Phase II clinical trials in patients with advanced cancers. Although poor responses in patients to the treatment with PZA alone have been observed, this class of agents remains of interest because of its distinct mechanism of action from other topoisomerase poisons. Therefore, the combination of PZA with conventional anticancer agents presents an attractive approach to treat drug-resistant human tumors. In the present study, the cytotoxic effects of PZA combined with doxorubicin, topotecan, and etoposide were determined using paired parental and doxorubicin-resistant human colon carcinoma (SW-620 and SW620/AD-300) and breast cancer cell lines (MCF-7 and MCF-7/TH). Cytotoxicity was measured by soft agar clonogenic assays. Dose effect and combination effects were analyzed by the method of Chou and Talalay. The combination of PZA with doxorubicin, topotecan, and etoposide in fixed ratios demonstrated synergistic cytotoxicity on both SW-620 and SW620/AD-300 cell lines. The combination of PZA with doxorubicin also exhibited synergistic cytotoxicity against both MCF-7 and MCF-7/TH cell lines. The mechanism of synergism appeared independent of topoisomerase I and II inhibition, and interference with protein-DNA complexes. Strategies to define optimal drug combinations are proving to be of significant value when considering potential clinical applications of new and established agents.

Doxorubicin-resistant, MRP1-expressing U-1285 cells are sensitive to idarubicin

A doxorubicin-resistant subline (U-1285dox(900)) was derived from the human small cell lung carcinoma cell line U-1285. U-1285dox(900) was exposed to a wide range of anticancer agents to determine its resistance profile. In contrast to U-1285 cells, the resistant subline U-1285dox(900) expressed elevated MRP1 mRNA detected by reversed transcriptase-polymerase chain reaction (RT-PCR) and MRP1 protein analyzed with Western blot. Neither MDR1 mRNA nor P-glycoprotein could be detected in the parental cell line or resistant subline. U-1285dox(900) exhibited high resistance to doxorubicin, epirubicin, daunorubicin, and vincristine, an intermediate resistance to mitoxantrone, and a low resistance to etoposide. A collateral sensitivity to cytosine arabinoside, chlorodeoxyadenosine, and melphalan was observed. The resistance could be reversed by buthionine-sulphoximine and verapamil for all tested drugs. Compared with daunorubicin, resistance to idarubicin was very low, 14-fold and 2.6-fold, respectively. This was associated with a higher accumulation due to a slower transport of idarubicin out of U-1285dox(900) cells.

A phase I and pharmacologic study of pyrazoloacridine (NSC 366140) and carboplatin in patients with advanced cancer

BACKGROUND

Pyrazoloacridine (PZA) is the first of a new class of rationally synthesized acridine derivatives to undergo clinical testing as an anticancer agent. We previously demonstrated cytotoxic synergy between combinations of PZA and platinum compounds. Subsequent studies revealed that PZA inhibits removal of platinum-DNA adducts in cultured A549 cells. Based on these results, we undertook a phase I study of the combination of PZA and carboplatin (CBDCA).

PATIENTS AND METHODS

Twenty-eight patients received 76 28-day courses (median 2.5, range 1-6) of CBDCA (30-minute infusion) followed by PZA (3-hour infusion), through six dose levels [PZA/CBDCA] (200/AUC 3, 400/AUC 3, 400/AUC 4, 400/AUC 5, 500/AUC 5, 600/AUC 5 mg/m2/AUC). Pharmacokinetic analyses were performed to evaluate the disposition of PZA. Retention of platinum-DNA adducts in peripheral blood mononuclear cells of patients was also evaluated.

RESULTS

The most common and dose-limiting toxicity was myelosuppression, consisting of neutropenia and leukopenia. Non-hematologic toxicities of anorexia, nausea and stomatitis were mild to moderate. In six patients evaluated at the MTD, CBDCA did not appear to affect the pharmacokinetics of PZA. One patient with malignant melanoma had a partial response. Disease stabilization for greater than 4 courses of treatment occurred in 4 patients.

CONCLUSION

The combination of PZA and CBDCA was well tolerated and may have utility in some tumor types.

Mutation of a single conserved tryptophan in multidrug resistance protein 1 (MRP1/ABCC1) results in loss of drug resistance and selective loss of organic anion transport

Multidrug resistance protein 1 (MRP1/ABCC1) belongs to the ATP-binding cassette transporter superfamily and is capable of conferring resistance to a broad range of chemotherapeutic agents and transporting structurally diverse conjugated organic anions. In this study, we found that substitution of a highly conserved tryptophan at position 1246 with cysteine (W1246C-MRP1) in the putative last transmembrane segment (TM17) of MRP1 eliminated 17beta-estradiol 17-(beta-d-glucuronide) (E(2)17betaG) transport by membrane vesicles prepared from transiently transfected human embryonic kidney cells while leaving the capacity for leukotriene C(4)- and verapamil-stimulated glutathione transport intact. In addition, in contrast to wild-type MRP1, leukotriene C(4) transport by the W1246C-MRP1 protein was no longer inhibitable by E(2)17betaG, indicating that the mutant protein had lost the ability to bind the glucuronide. A similar phenotype was observed when Trp(1246) was replaced with Ala, Phe, and Tyr. Confocal microscopy of cells expressing Trp(1246) mutant MRP1 molecules fused at the C terminus with green fluorescent protein showed that they were correctly routed to the plasma membrane. In addition to the loss of E(2)17betaG transport, HeLa cells stably transfected with W1246C-MRP1 cDNA were not resistant to the Vinca alkaloid vincristine and accumulated levels of [(3)H]vincristine comparable to those in vector control-transfected cells. Cells expressing W1246C-MRP1 were also not resistant to cationic anthracyclines (doxorubicin, daunorubicin) or the electroneutral epipodophyllotoxin VP-16. In contrast, resistance to sodium arsenite was only partially diminished, and resistance to potassium antimony tartrate remained comparable to that of cells expressing wild-type MRP1. This suggests that the structural determinants required for transport of heavy metal oxyanions differ from those for chemotherapeutic agents. Our results provide the first example of a tryptophan residue being so critically important for substrate specificity in a eukaryotic ATP-binding cassette transporter.

Current status of pyrazoloacridine as an anticancer agent

Pyrazoloacridine (PZA) is the first of a new class of rationally synthesized acridine derivatives to undergo clinical testing as an anticancer agent. Recent studies suggest that PZA might be a dual inhibitor of DNA topoisomerase I and DNA topoisomerase II that exerts its effects by diminishing the formation of topoisomerase-DNA adducts. Consistent with this unique mechanism of action, PZA exhibits broad spectrum antitumor activity in preclinical models in vivo. In addition, this agent displays several unique properties including solid tumor selectivity, activity against hypoxic cells, and cytotoxicity in noncycling cells. PZA also retains full activity against cells that are resistant to other agents on the basis of overexpression of P-glycoprotein or the multidrug resistance-associated protein (MRP). PZA has been studied in phase I trials in adults and children, and is currently undergoing broad phase II trials in a number of tumor types. No significant anti-tumor activity has been seen in gastrointestinal malignancies and prostate cancer. Results from ongoing or recently completed trials are awaited before the utility of this agent in our current armamentarium can be defined. Because of its unique properties, combination studies with other antineoplastic agents are warranted.

Localization of a substrate specificity domain in the multidrug resistance protein

Multidrug resistance protein (MRP) confers resistance to a number of natural product chemotherapeutic agents. It is also a high affinity transporter of some physiological conjugated organic anions such as cysteinyl leukotriene C(4) and the cholestatic estrogen, 17beta-estradiol 17(beta-D-glucuronide) (E(2)17betaG). We have shown that the murine orthologue of MRP (mrp), unlike the human protein, does not confer resistance to common anthracyclines and is a relatively poor transporter of E(2)17betaG. We have taken advantage of these functional differences to identify region(s) of MRP involved in mediating anthracycline resistance and E(2)17betaG transport by generating mrp/MRP hybrid proteins. All hybrid proteins conferred resistance to the Vinca alkaloid, vincristine, when transfected into human embryonic kidney cells. However, only those in which the COOH-terminal third of mrp had been replaced with the corresponding region of MRP-conferred resistance to the anthracyclines, doxorubicin, and epirubicin. Exchange of smaller segments of the COOH-terminal third of the mouse protein by replacement of either amino acids 959-1187 or 1188-1531 with those of MRP produced proteins capable of conferring some level of resistance to the anthracyclines tested. All hybrid proteins transported cysteinyl leukotriene C(4) with similar efficiencies. In contrast, only those containing the COOH-terminal third of MRP transported E(2)17betaG with an efficiency comparable with that of the intact human protein. The results demonstrate that differences in primary structure of the highly conserved COOH-terminal third of mrp and MRP are important determinants of the inability of the murine protein to confer anthracycline resistance and its relatively poor ability to transport E(2)17betaG.

ATPase activity of purified multidrug resistance-associated protein

Human multidrug resistance protein (MRP) was expressed at high levels in stably transfected baby hamster kidney (BHK-21) cells. These cells exhibited a pattern of cross-resistance to several different drugs typical of an MRP-mediated phenotype despite the addition of 10 histidine residues at the C terminus to facilitate purification. Consistent with this functional evidence of the presence of MRP at the surface of these transfectants, strong signals were detected by immunoblotting and immunofluorescence using a specific monoclonal antibody to MRP. There was intense uniform staining of the cell surface as well as weaker staining of intracellular membranes. MRP-containing membranes were solubilized in 1% N-dodecyl-beta-D-maltoside in the presence of 0.4% sheep brain phospholipids. Two sequential affinity purification steps on Ni-NTA agarose and wheat germ agglutinin agarose provided substantial enrichment, and contaminating bands were not detected. ATPase activity of the purified protein was assayed in the presence of the phospholipids, which had been maintained throughout all purification steps. ATP was hydrolyzed in proportion to the amount of purified protein assayed, and typical Michaelis-Menten behavior was exhibited, yielding estimations of Km of approximately 3.0 mM and Vmax of 0.46 micromol mg-1 min-1. This activity was moderately stimulated by the drugs that others have shown to be transported by MRP-containing membrane vesicles. This stimulation was enhanced by reduced glutathione as is its drug transport, and oxidized glutathione, itself a substrate for transport, caused a strong stimulation. These data describe the first purification of MRP and provide the first direct evidence that the molecule possesses drug-stimulated ATPase activity.

Pharmacological characterization of the murine and human orthologs of multidrug-resistance protein in transfected human embryonic kidney cells

Overexpression of the human multidrug-resistance protein (MRP) causes a form of multidrug resistance similar to that conferred by P-glycoprotein, although the two proteins are only distantly related. In contrast to P-glycoprotein, human MRP has also been shown to be a primary active transporter of a structurally diverse range of organic anionic conjugates, some of which may be physiological substrates. At present, the mechanism by which MRP transports these compounds and mediates multidrug resistance is not understood. With the objective of developing an animal model for studies on the normal functions of MRP and its ability to confer multidrug resistance in vivo, we recently cloned the murine ortholog of MRP (mrp). To assess the degree of functional conservation between mrp and MRP, we directly compared the drug cross-resistance profiles they confer when transfected into human embryonic kidney cells, as well as their ability to actively transport leukotriene C4, 17beta-Estradiol 17beta-(D-glucuronide), and vincristine; mrp and MRP conferred similar drug resistance profiles, with the exception that only MRP conferred resistance to the anthracyclines tested. Consistent with these findings, accumulation of [3H]vincristine and [3H]VP-16 was decreased, and efflux of [3H]vincristine was increased in both murine and human MRP-transfected cell populations, whereas only human MRP-transfected cells displayed decreased accumulation and increased efflux of [3H]daunorubicin. Membrane vesicles derived from both transfected cell populations transported leukotriene C4 in an ATP-dependent manner with comparable efficiency, although the efficiency of 17beta-estradiol 17beta-(D-glucuronide) transport was somewhat higher with MRP transfectants. ATP-dependent transport of vincristine was also observed with vesicles from mrp and MRP transfectants but only in the presence of glutathione. These studies reveal intrinsic differences between the murine and human MRP orthologs with respect to their ability to confer resistance to a major class of chemotherapeutic drugs.

Standardization of a single-cell assay for sensitive detection of multidrug resistance protein expression in normal and malignant cells in archival clinical samples

Multidrug resistance protein (MRP), like P170, confers multidrug resistance, but its clinical relevance is uncertain, whereas P170 is an accepted cause of chemotherapy failure for which ongoing reversal trials are being conducted. Because such trials have been only modestly successful, we must investigate alternative drug resistance mechanisms such as MRP, which is poorly blocked by P170 inhibitors. The significance of MRP has remained undefined because MRP mRNA is difficult to assay in archival material, does not necessarily reflect MRP levels, and is widely expressed in normal or hematopoietic cells within tumors and bone marrow. Because conventional immunoblot or immunocytochemistry may not be sensitive enough to detect low or heterogeneous MRP expression in clinical samples, we elected to score MRP in single tumor cells by modifying our P170 assays that have proven valuable for correlating P170 expression with the outcome of pediatric cancer chemotherapy. We enhanced the signal-to-noise ratio with several peroxidase-tagged secondary antibody layers and staining refinements, standardizing the assay with MRP-negative and MRP-positive but P170-negative transfected or drug-selected controls in which MRP was quantified by immunoblot. We confirmed sensitivity by staining a very low MRP-expressing revertant line and "mixed" samples containing small numbers of positive cells; we confirmed specificity by applying two antibodies directed against separate MRP epitopes. We examined neuroblastoma, osteosarcoma, rhabdomyosarcoma, and retinoblastoma samples, identifying MRP-positive malignant cells, which were distinguishable from MRP-positive normal cells. This assay may be valuable for early diagnosis of low but potentially important MRP expression, which would allow timely application of alternative therapy, perhaps with MRP-specific blockers.

Multidrug resistance mediated by the multidrug resistance protein (MRP) gene

Inherent or acquired resistance to multiple natural product drugs is a major obstacle to the success of chemotherapy. Two proteins have been shown to cause this type of multidrug resistance in human tumour cells, the 170 kDa P-glycoprotein and the 190 kDa multidrug resistance protein (MRP). Overexpression of these N-glycosylated phosphoproteins in mammalian cells is associated with reduced drug accumulation. Both MRP and p-glycoprotein belong to the ATP-binding cassette superfamily of transmembrane transport proteins, but they share only 15% amino acid identity. Furthermore, their predicted membrane topologies differ considerably, with MRP containing three multispanning transmembrane domains compared with the two that are present in P-glycoprotein. The drug cross-resistance profiles of cells that overexpress MRP or P-glycoprotein are similar but not identical. For example, lower levels of taxol resistance are associated with overexpression of MRP than with overexpression of P-glycoprotein. There also appear to be fundamental differences in the mechanisms by which the two proteins transport chemotherapeutic drugs. P-glycoprotein-enriched membrane vesicles have been shown to directly transport several chemotherapeutic drugs, whereas vincristine transport by MRP-enriched membrane vesicles is demonstrable only in the presence of reduced glutathione. Several potential physiologic substrates of MRP including leukotriene C4 and 17 beta-estradiol-17-(beta-D-glucuronide) have been identified. In contrast, these conjugated organic anions are transported poorly, if at all, by P-glycoprotein. Finally, agents that reverse P-glycoprotein-associated resistance are usually much less effective in MRP-associated resistance. Antisense oligonucleotide-mediated suppression of MRP synthesis offers a highly specific alternative approach to circumventing resistance mediated by this novel drug resistance protein.

Sequential emergence of MRP- and MDR1-gene over-expression as well as MDR1-gene translocation in homoharringtonine-selected K562 human leukemia cell lines

To investigate the mechanism of resistance to an antineoplastic natural product homoharringtonine (HHT) in leukemic cells, we have established 5 sub-lines of human myeloid leukemia K562 cells, designated as K-H30, K-H100, K-H200, K-H300 and K-H400, which showed progressive resistance to different concentrations of HHT. These sub-lines were cross-resistant to daunorubicin, vincristine, etoposide and mitoxantrone, but not to melphalan. Immunofluorescence with monoclonal anti-Pgp antibody MRK16 and Northern-blot analysis demonstrated that resistance to HHT is related to the sequential emergence of MRP- and MDR1-gene over-expression. In the low-level-resistant K-H30 sub-line, the MDR1 gene was not over-expressed, but the MRP gene was over-expressed 2.1-fold. In the intermediate-level-resistant K-H100 and K-H200 sublines, both the MRP and the MDR1 genes were over-expressed. However, in the high-level-resistant K-H300 and K-H400 sublines, MDR1-gene over-expression predominated (20- and 21-fold respectively). On the other hand, GST pi-gene expression was decreased in all 5 sub-lines. Southern-blot analysis revealed no MRP-gene amplification in any of the 5 sub-lines, whereas the MDR1 gene was amplified in the high-level-resistant K-H300 and K-H400 sub-lines. The most interesting observation is a homogeneously staining region (HSR) found in chromosome 2 of the K-H300 and K-H400 sub-lines. Chromosome painting and in situ hybridization demonstrated that this HSR was translocated from chromosome 7 and consisted of the amplified MDR1 gene, suggesting that there is a relationship between MDR1-gene, translocation and MDR1-gene amplification.

Multidrug resistance-associated protein-mediated multidrug resistance modulated by cyclosporin A in a human bladder cancer cell line

A doxorubicin-resistant subline (5637/DR5.5) from human bladder cancer cells (5637) was induced by stepwise increase in the doxorubicin concentration. 5637/DR5.5 cells were cross-resistant to vinblastine and etoposide but not to mitomycin C and cisplatin. We analyzed the mdr1, MRP (multidrug resistance-associated protein), and DNA topoisomerase II gene expression using the reverse transcription polymerase chain reaction assay (RT-PCR) and investigated possible differences in the accumulation and efflux of radiolabeled daunorubicin. 5637/DR5.5 cells do not express the mdr1 gene, but the expression levels of MRP are markedly higher than in drug-sensitive 5637 cells. The intracellular accumulation of radiolabeled daunorubicin was markedly decreased in the 5637/DR5.5 cells in comparison with the parent cells. This reduced drug accumulation was associated with an enhanced drug efflux, but was reversed when cells were incubated with cyclosporin A. Cyclosporin A at the concentration of 5 microM caused 3.4-fold enhancement of daunorubicin-sensitivity in the 5637/DR5.5 cells. On the other hand, there was no difference in DNA-topoisomerase II activity between the parent and resistant cells. The resistance of the 5637/DR5.5 cells is therefore associated with an enhanced drug efflux mediated by the MRP gene overexpression, as distinct from P-glycoprotein, and is modulated by cyclosporin A.

MRP gene overexpression in a human doxorubicin-resistant SCLC cell line: alterations in cellular pharmacokinetics and in pattern of cross-resistance

The development of non-P-glycoprotein-mediated multi-drug resistance is a frequent event among lung-cancer cell lines. In an attempt to understand the underlying mechanisms of this phenotype, we have selected a multi-drug-resistant subline (POGB/DX) in vitro for doxorubicin resistance. The original cell line (POGB) was established in vitro from a non-treated patient with a small-cell lung cancer. POGB/DX cells were cross-resistant to other drugs, associated with MDR phenotype. In contrast, they were not resistant to taxol, camptothecin or melphalan, but were instead hypersensitive to 5-fluorouracil. Although expression of the mdr-1 gene was not detected in POGB/DX cells, cellular pharmacokinetics showed a reduced drug accumulation and altered intracellular localization in the POGB/DX cell line. This defect in drug accumulation was associated with overexpression and amplification of the MRP gene. Interestingly, verapamil, a known modulator of P-glycoprotein function, was able to reverse drug resistance and to increase drug accumulation. In Northern-blot analysis no differences in expression of topoisomerase I and II (alpha and beta), DNA polymerase beta, or HSP70 and HSP60 genes were observed between POGB and POGB/DX. Coupled to lack of changes in expression of known resistance factors, overexpression of MRP and modulation by verapamil strongly support a role for this gene product in the development of drug resistance in this SCLC cell system. This study provides evidence that (a) altered cellular pharmacokinetics is related to MRP expression; (b) MRP-mediated phenotype is characterized by a specific pattern of cross-resistance, which does not involve taxol; and (c) verapamil may be effective in modulating the function of the MRP gene product.

The role of methoxymorpholino anthracycline and cyanomorpholino anthracycline in a sensitive small-cell lung-cancer cell line and its multidrug-resistant but P-glycoprotein-negative and cisplatin-resistant counterparts

The cytotoxic action of two morpholino anthracyclines, methoxymorpholino anthracycline (MRA-MT, FCE 23,762) and cyanomorpholino anthracycline (MRA-CN), was compared with the cytotoxicity of doxorubicin (DOX), the topoisomerase II inhibitor etoposide (VP-16), the topoisomerase I inhibitor camptothecin, methotrexate, and cisplatin in GLC4, a human small-cell lung-cancer cell line, in GLC4-Adr, its P-glycoprotein (Pgp)-negative, multidrug-resistant (MDR; 100-fold DOX-resistant) subline with overexpression of the MDR-associated protein (MRP) and a lowered topoisomerase II activity, and in GLC4-CDDP, its cisplatin-resistant subline. GLC4-Adr was about 2-fold cross-resistant for the morpholino anthracyclines and GLC4-CDDP was, relative to GLC4, more resistant for the morpholino anthracyclines than for DOX. Overall, MRA-CN was about 2.5-fold more cytotoxic than MRA-MT. The cytotoxicity profile of the morpholino anthracyclines in these cell lines mimicked that of camptothecin.

Reduced levels of topoisomerase II alpha and II beta in a multidrug-resistant lung-cancer cell line

We have previously shown that the doxorubicin-selected multidrug-resistant small-cell lung-cancer cell line H69AR is resistant to VP-16-induced single-strand DNA breaks as compared with its parental H69 cell line. Levels of immunoreactive topoisomerase II alpha are also reduced in H69AR cells. In the present study, we found that cleaved complex formation in the presence of VP-16 was decreased in H69AR cells as compared with H69 cells. In addition, the resistant cells contained lower levels of both topoisomerase II alpha and topoisomerase II beta protein and mRNA. However, these changes were not accompanied by a decrease in the P4-unknotting (strand-passing) activity of 0.67 M NaCl nuclear extracts of H69AR cells, nor was there any difference in VP-16 inhibition of unknotting activity in the H69 and H69AR nuclear extracts. These data suggest that reduced levels of topoisomerase II alpha and II beta may contribute to the resistance of H69AR cells to VP-16 and other drugs that target these isoenzymes.

Elevated expression of annexin II (lipocortin II, p36) in a multidrug resistant small cell lung cancer cell line

The doxorubicin-selected multidrug resistant small cell lung cancer cell line, H69AR, is cross-resistant to the Vinca alkaloids and epipodophyllotoxins, but does not overexpress P-glycoprotein, a 170 kDa plasma membrane efflux pump usually associated with this type of resistance. Monoclonal antibodies were raised against the H69AR cell line and one of these, MAb 3.186, recognises a peptide epitope on a 36 kDa phosphorylated protein that is membrane associated, but not presented on the external surface of H69AR cells (Mirski & Cole, 1991). In the present study, in vitro translation and molecular cloning techniques were used to determine the relative levels of mRNA corresponding to the 3.186 antigen. In addition, a cDNA clone containing an insert of approximately 1.4 kb was obtained by screening an H69AR cDNA library with 125I-MAb 3.186. Fragments of this cloned DNA hybridised to a single mRNA species of approximately 1.6 kb that was 5 to 6-fold elevated in H69AR cells. Partial DNA sequencing and restriction endonuclease mapping revealed identity of the cloned DNA with p36, a member of the annexin/lipocortin family of Ca2+ and phospholipid binding proteins.

Differential growth inhibition and enhancement of major histocompatibility complex class I antigen expression by interferons in a small-cell lung cancer cell line and its doxorubicin-selected multidrug-resistant variant

Expression of class I and class II major histocompatibility complex antigens on a human small-cell lung cancer cell line and its multidrug-resistant variant was examined before and after exposure to interferon alpha (IFN alpha) and IFN gamma by flow cytometry. Neither IFN alpha nor IFN gamma induced class II antigen expression on the drug-sensitive or resistant cell line. Induction of class I antigen expression along with an inhibition of proliferation was observed in both cell lines after IFN alpha treatment. On the other hand, IFN gamma treatment resulted in growth inhibition and enhancement of class I antigen expression in the sensitive cell line but not the resistant cell line. The differential response of the two cell lines to IFN gamma cannot be directly attributed to the acquisition of drug resistance but it suggests that further investigation of the possibility that drug-sensitive and resistant small-cell lung tumors may respond differently to immunotherapies that include IFN gamma is warranted.