Growth inhibition, cytokinesis failure and apoptosis of multidrug-resistant leukemia cells after treatment with P-glycoprotein inhibitory agents

ATP-binding cassette (ABC) transporters in cancer: A review of recent updates

The ATP-binding cassette (ABC) transporter superfamily is one of the largest membrane protein families existing in wide spectrum of organisms from prokaryotes to human. ABC transporters are also known as efflux pumps because they mediate the cross-membrane transportation of various endo- and xenobiotic molecules energized by ATP hydrolysis. Therefore, ABC transporters have been considered closely to multidrug resistance (MDR) in cancer, where the efflux of structurally distinct chemotherapeutic drugs causes reduced itherapeutic efficacy. Besides, ABC transporters also play other critical biological roles in cancer such as signal transduction. During the past decades, extensive efforts have been made in understanding the structure-function relationship, transportation profile of ABC transporters, as well as the possibility to overcome MDR via targeting these transporters. In this review, we discuss the most recent knowledge regarding ABC transporters and cancer drug resistance in order to provide insights for the development of more effective therapies.

In vivo gene delivery mediated by non-viral vectors for cancer therapy

Gene therapy by expression constructs or down-regulation of certain genes has shown great potential for the treatment of various diseases. The wide clinical application of nucleic acid materials dependents on the development of biocompatible gene carriers. There are enormous various compounds widely investigated to be used as non-viral gene carriers including lipids, polymers, carbon materials, and inorganic structures. In this review, we will discuss the recent discoveries on non-viral gene delivery systems. We will also highlight the in vivo gene delivery mediated by non-viral vectors to treat cancer in different tissue and organs including brain, breast, lung, liver, stomach, and prostate. Finally, we will delineate the state-of-the-art and promising perspective of in vivo gene editing using non-viral nano-vectors.

Delivery of siRNA in vitro and in vivo using PEI-capped porous silicon nanoparticles to silence MRP1 and inhibit proliferation in glioblastoma

BACKGROUND

Multidrug resistance-associated protein 1 (MRP1) overexpression plays a major role in chemoresistance in glioblastoma multiforme (GBM) contributing to its notorious deadly nature. Although MRP1-siRNA transfection to GBM in vitro has been shown to sensitise the cells to drug, MRP1 silencing in vivo and the phenotypic influence on the tumour and normal tissues upon MRP1 down-regulation have not been established. Here, porous silicon nanoparticles (pSiNPs) that enable high-capacity loading and delivery of siRNA are applied in vitro and in vivo.

RESULT

We established pSiNPs with polyethyleneimine (PEI) capping that enables high-capacity loading of siRNA (92 µg of siRNA/mg PEI-pSiNPs), and optimised release profile (70% released between 24 and 48 h). These pSiNPs are biocompatible, and demonstrate cellular uptake and effective knockdown of MRP1 expression in GBM by 30%. Also, siRNA delivery was found to significantly reduce GBM proliferation as an associated effect. This effect is likely mediated by the attenuation of MRP1 transmembrane transport, followed by cell cycle arrest. MRP1 silencing in GBM tumour using MRP1-siRNA loaded pSiNPs was demonstrated in mice (82% reduction at the protein level 48 h post-injection), and it also produced antiproliferative effect in GBM by reducing the population of proliferative cells. These results indicate that in vitro observations are translatable in vivo. No histopathological signs of acute damage were observed in other MRP1-expressing organs despite collateral downregulations.

CONCLUSIONS

This study proposes the potential of efficient MRP1-siRNA delivery by using PEI-capped pSiNPs in achieving a dual therapeutic role of directly attenuating the growth of GBM while sensitising residual tumour cells to the effects of chemotherapy post-resection.

ABC transporters in cancer: more than just drug efflux pumps

Multidrug transporter proteins are best known for their contributions to chemoresistance through the efflux of anticancer drugs from cancer cells. However, a considerable body of evidence also points to their importance in cancer extending beyond drug transport to fundamental roles in tumour biology. Currently, much of the evidence for these additional roles is correlative and definitive studies are needed to confirm causality. We propose that delineating the precise roles of these transporters in tumorigenesis and treatment response will be important for the development of more effective targeted therapies.

Identification of compounds selectively killing multidrug-resistant cancer cells

There is a great need for the development of novel chemotherapeutic agents that overcome the emergence of multidrug resistance (MDR) in cancer. We catalogued the National Cancer Institute's DTP drug repository in search of compounds showing increased toxicity in MDR cells. By comparing the sensitivity of parental cell lines with MDR derivatives, we identified 22 compounds possessing MDR-selective activity. Analysis of structural congeners led to the identification of 15 additional drugs showing increased toxicity in Pgp-expressing cells. Analysis of MDR-selective compounds led to the formulation of structure activity relationships and pharmacophore models. This data mining coupled with experimental data points to a possible mechanism of action linked to metal chelation. Taken together, the discovery of the MDR-selective compound set shows the robustness of the developing field of MDR-targeting therapy as a new strategy for resolving Pgp-mediated MDR.

Imatinib resistance in multidrug-resistant K562 human leukemic cells

The multidrug resistance phenotype (MDR) is one of the major causes of failure in cancer chemotherapy and it is associated with the over-expression of P-glycoprotein (P-gp or MDR1) in tumor cell membranes. A constitutive NF-kappaB activity has been observed in several haematological malignancies and this is associated with its anti-apoptotic role. In the present work, the relationship between NF-kappaB and MDR phenotype was evaluated in wild type K562 human leukemic cells (K562-WT) and in its vincristine-resistant counterpart, K562-Vinc cells. These data showed that K562-Vinc cells, which express an active P-gp, exhibited MDR phenotype. The resistant indexes (IC(50)(K562-Vinc)/IC(50)(K562-WT)) for structurally unrelated drugs like imatinib, doxorubicin and colchicine were 8.0+/-0.3, 2.8+/-0.4 and 44.8+/-8.8, respectively. The imatinib resistance was reversed by P-gp blockade suggesting the involvement of P-gp in imatinib transport. We observed that NF-kappaB was constitutively activated in both cell lines but in a lesser extent in K562-Vinc. The inhibition of NF-kappaB with BAY 11-7082 increased the cytotoxicity of imatinib in K562-Vinc cells but not in K562-WT. Further, the co-administration of imatinib and BAY 11-7082 sensitized multidrug-resistant K562 cells to cell death as detected by increased percentage of annexin V positive cells. The induced cell death in K562-Vinc cells was associated with activation of caspases 9 and 3. Finally, we provide data showing that BAY 11-7082 down-regulates the expression of P-gp suggesting that the activity of NF-kappaB could be functionally associated to this protein in K562 cells. Our results indicate that the vincristine-resistant K562 cells which developed MDR phenotype, exhibited resistance to imatinib associated with a functional P-gp over-expression. This resistance could be partially overcome by the inhibition of NF-kappaB pathway.

Hyaluronan oligosaccharides sensitize lymphoma resistant cell lines to vincristine by modulating P-glycoprotein activity and PI3K/Akt pathway

Multidrug resistance (MDR) is one of the main reasons for failure of cancer therapy. It may be mediated by overexpression of ATP-dependent efflux pumps or by alterations in survival or apoptotic pathways. Fragments generated by enzymatic degradation of hyaluronan (oHA) were able to modulate growth and cell survival and sensitize MDR breast cancer cells to cytotoxic drugs. In this work the relationship between oHA and MDR in lymphoid malignancies was analyzed using murine lymphoma cell lines resistant to doxorubicin (LBR-D160) or vincristine (LBR-V160) and a sensitive line (LBR-). After oHA treatment, higher apoptosis levels were observed in the resistant cell lines than in the sensitive one. Besides, oHA sensitized LBR-D160 and LBR-V160 to vincristine showing increased apoptosis induction when used in combination with vincristine. Native hyaluronan failed to increase apoptosis levels. As different survival factors could be modulated by hyaluronan, we investigated the PI3K/Akt pathway through PIP3 production and phosphorylated Akt (p-Akt) and survivin expression was also evaluated. Our results showed that oHA decreased p-Akt in the 3 cell lines while anti-CD44 treatment abolished this effect. Besides, survivin was downregulated only in LBR-V160 by oHA. When Pgp function was evaluated, we observed that oHA were able to inhibit Pgp efflux in murine and human resistant cell lines in a CD44-dependent way. In summary, we report for the first time that oHA per se modulate MDR in lymphoma cells by decreasing p-Akt as well as Pgp activity, thus suggesting that oHA could be useful in combination with classical chemotherapy in MDR hematological malignancies.

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.

Ionic currents in multidrug resistant K562 human leukemic cells

In this study, the expression and functional characterization of currents through the CFTR (cystic fibrosis transmembrane regulator) and ORCC (outwardly rectifying chloride channels) were determined in wild-type K562 chronic human leukemia cells (K562-WT) and in its resistant counterpart, the vincristine resistant cell line (K562-Vinc). Expression of the CFTR and MDR1 (multidrug resistant) gene products was determined by a semi-quantitative RT-PCR protocol. The amplified products in K562-WT and K562-Vinc showed two bands corresponding to CFTR and MDR1. MDR1 mRNA increased by 20-fold in K562-Vinc whereas no change in CFTR mRNA levels was observed. CFTR and ORCC channel activity were measured with a whole cell configuration of the patch clamp technique. Forskolin (40 microM n activator of adenylate cyclase, added to the extracellular side increased the current in both cell lines. A fraction of the activated whole cell currents was inhibited by 500 microM 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS) and subsequent addition of 500 microM diphenylamine-2-carboxylate (DPC plus DIDS) further inhibited the remaining currents. The levels of forskolin-activated currents and subsequent blockade were similar in both cell lines. The effect of forskolin was prevented in cells previously exposed to 500 microM DPC. The effects of DIDS and DPC on the forskolin-activated whole cell currents support the idea that both CFTR and ORCC are generating a significant fraction of these currents with DIDS inhibiting ORCC currents and DPC inhibiting CFTR currents when the blockers are added one after another to the extracellular side. Finally, we show that exposure of K562 cells to vincristine which results in the over expression of MDR1 is not accompanied by a significant down regulation of CFTR as in other cells.

Verapamil and its derivative trigger apoptosis through glutathione extrusion by multidrug resistance protein MRP1

This study demonstrates that verapamil and a newly synthesized verapamil derivative, NMeOHI(2), behave as apoptogens in multidrug resistance protein 1 (MRP1)-expressing cells. When treated with either verapamil or NMeOHI(2), surprisingly, baby hamster kidney-21 (BHK) cells transfected with human MRP1 were killed. Because parental BHK cells were not, as well as cells expressing an inactive (K1333L) MRP1 mutant, this indicated that cell death involved functional MRP1 transporter. Cell death was identified as apoptosis by using annexin V-fluorescein labeling and was no longer observed in the presence of the caspase inhibitor Z-Val-Ala-Asp(OMe)-CH(2)F (Z-VAD-FMK). In vitro, both verapamil and its derivative inhibited leukotriene C4 transport by MRP1-enriched membrane vesicles in a competitive manner, with a K(i) of 48.6 microm for verapamil and 5.5 microm for NMeOHI(2,) and stimulated reduced glutathione (GSH) transport 3-fold and 9-fold, respectively. Treatment of MRP1-expressing cells with either verapamil or the derivative quickly depleted intracellular GSH content with a strong decrease occurring in the first hour of treatment, which preceded cell death beginning at 8-16 h. Furthermore, addition of GSH to the media efficiently prevented cell death. Therefore, verapamil and its derivative trigger apoptosis through stimulation of GSH extrusion mediated by MRP1. This new information on the mechanism of induced apoptosis of MDR cells may represent a novel approach in the selective treatment of MRP1-positive tumors.

Novel plant triterpenoid drug amooranin overcomes multidrug resistance in human leukemia and colon carcinoma cell lines

Amooranin (AMR), a plant terpenoid, isolated from Amoora rohituka, was investigated for its ability to overcome multidrug resistance in human leukemia and colon carcinoma cell lines. AMR IC(50) values of multidrug-resistant leukemia (CEM/VLB) and colon carcinoma (SW620/Ad-300) cell lines were higher (1.9- and 6-fold) than parental sensitive cell lines (CEM and SW620). AMR induced G(2)+M phase-arrest during cell cycle traverse in leukemia and colon carcinoma cell lines and the percentage of cells in G(2)+M phase increased in a dose-dependent manner. Coincubation of tumor cells with both DOX and AMR reversed DOX resistance in 104-fold DOX-resistant CEM/VLB and 111-fold DOX-resistant SW620/Ad-300 cell lines with a dose modification factor of 50.9 and 99.6, respectively. Flow cytometric assay showed that AMR causes enhanced cellular DOX accumulation in a dose-dependent manner. AMR inhibits photolabeling of P-glycoprotein (P-gp) with [(3)H]-azidopine and the blocking effect enhanced with increasing concentrations of AMR. Our results show that AMR competitively inhibits P-gp-mediated DOX efflux, suggestive of a mechanism underlying the enhanced DOX accumulation and reversal of multidrug resistance by AMR.

MDR1 gene overexpression confers resistance to imatinib mesylate in leukemia cell line models

Inappropriate expression of the multidrug resistance (MDR1) gene encoding the P-glycoprotein (Pgp) has been frequently implicated in resistance to different chemotherapeutic drugs. We have previously generated chronic myeloid leukemia (CML) cell lines resistant to the tyrosine kinase inhibitor imatinib mesylate (STI571), and one line (LAMA84-r) showed overexpression not only of the Bcr-Abl protein but also of Pgp. In the present study, we investigated this phenomenon in other cell lines overexpressing exclusively Pgp. Thus, cells from the K562/DOX line, described as resistant to doxorubicin due to MDR1 gene overexpression, grew continuously in the presence of 1 microM imatinib, but died in 4 to 5 days if the Pgp pump modulators verapamil or PSC833 were added to the imatinib-treated culture. Analysis of cell proliferation by the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) assay confirmed the differential sensitivity of K562/DOX to imatinib, which was also reversed by verapamil or PSC833. Flow cytometric analysis of the total phosphotyrosine content by intracytoplasmic staining after a 2-hour incubation with escalating doses of imatinib showed that the inhibitory concentrations of 50% (IC(50)) for inhibition of cellular protein tyrosine phosphorylation were 15, 10, and 5 microM for K562/DOX, K562/DOX plus verapamil, and K562, respectively. Retroviral-mediated transfection of the BCR-ABL(+) AR230 cell line with the MDR1 gene decreased its sensitivity to imatinib, an effect that was also reversed by verapamil. The possible role of MDR overexpression in clinical resistance to imatinib remains to be defined. We therefore confirm that imatinib should be added to the extensive list of drugs that can be affected by the MDR phenomenon.

Apoptosis-resistant phenotype in HL-60-derived cells HCW-2 is related to changes in expression of stress-induced proteins that impact on redox status and mitochondrial metabolism

The onset of resistance to drug-induced apoptosis of tumour cells is a major problem in cancer therapy. We studied a drug-selected clone of promyelocytic HL-60 cells, called HCW-2, which display a complex resistance to a wide variety of apoptosis-inducing agents and we found that these cells show a dramatic increase in the expression of heat shock proteins (Hsps) 70 and 27, while the parental cell line does not. It is known that stress proteins such as Hsps can confer resistance to a variety of damaging agents other than heat shock, such as TNF-alpha, monocyte-induced cytotoxicity, and also play a role in resistance to chemotherapy. This elevated expression of Hsps is paralleled by an increased activity of mitochondrial metabolism and pentose phosphate pathway, this latter leading to high levels of glucose-6-phosphate dehydrogenase and, consequently, of glutathione. Thus, the apoptotic-deficient phenotype is likely because of the presence of high levels of stress response proteins and GSH, which may confer resistance to apoptotic agents, including chemotherapy drugs. Moreover, the fact that in HCW-2 cells Hsp70 are mainly localised in mitochondria may account for the increased performances of mitochondrial metabolism. These observations could have some implications for the therapy of cancer, and for the design of combined strategies that act on antioxidant defences of the neoplastic cell.

The cyclosporin PSC 833 increases survival and delays engraftment of human multidrug-resistant leukemia cells in xenotransplanted NOD-SCID mice

Circumvention of chemoresistance in cancer may involve several modulator drugs with high affinity for the multidrug transporter P-glycoprotein (Pgp), which is expressed in a number of multi-resistant malignancies. Pgp acts as a membrane efflux pump with broad substrate specificity including antineoplastic drugs and endogenous substances such as certain cytokines and sphingolipids. Therefore, the consequence of Pgp blockade could be far more complex than intracellular drug retention. In the present study exposure of the Pgp inhibitor, PSC 833 (1200 ng/ml), to Pgp expressing KG1a/200 human leukemia cells provoked cell cycle arrest and apoptosis in vitro. This finding was put to test in vivo using a xenotransplant model of KG1a/200 human cells intravenously inoculated into non-obese diabetic severe combined immunodeficient (NOD-SCID) mice. The animals were randomly allocated to receive treatment with PSC 833 (n = 32) or placebo (n = 24). PSC 833 (30 mg/kg) was subcutaneously injected six or 12 times separated by 48-96 h. The overall mean whole blood concentration of PSC 833 was 1191 +/- 60 ng/ml (s.e.m.) at 20 h after administration. Tumor engraftment was significantly reduced in the treatment group (P = 0.037), which also had prolonged survival compared to control animals (P = 0.0016). This is the first study that demonstrates antileukemic effects of a Pgp inhibitor as single agent therapy in vivo, and the present data raise the possibility of alternative exploitation of modulators in cancer chemotherapy.

Effects of isoprenoid analogues of SDB-ethylenediamine on multidrug resistant tumor cells alone and in combination with chemotherapeutic drugs

Multidrug resistance (MDR) mediated by P-glycoprotein (Pgp) remains the major obstacle for successful treatment of cancer. Inhibition of Pgp transport is important for higher efficacy of anticancer drugs. Lipophilic cationogenic amines with at least one tertiary N atom, such as verapamil, are classical PgP-blocking agents. In a search for novel accessible compounds potent against MDR tumor cells, we synthesized a series of arylalkylamines that contain isoprenoid side chains of different length. Two out of seven new analogues of the known N,N'-bis(3,4-dimethoxybenzyl)-N-solanesylethylenediamine (SDB-ethylenediamine), namely, compounds with C10 and C15 side chains, at low micromolar concentrations were preferentially toxic for several mammalian tumor cell lines that acquired MDR during prolonged drug selection. Moreover, at noncytotoxic concentrations, these compounds potently sensitized MDR cells to Pgp substrates vinblastine and adriamycin. We conclude that these analogues of SDB-ethylenediamine may have dual therapeutic advantage because (i) they are preferentially toxic for MDR cells when administered alone and (ii) they potentiate the cytotoxicity of Pgp-transported anticancer drugs.

P-glycoprotein targeting: a unique strategy to selectively eliminate immunoreactive T cells

T lymphocytes have been found to harbor P-glycoprotein (Pgp) and to demonstrate modulation of its ion channel transporter function according to the state of activation of T lymphocytes. We hypothesized that cytotoxic chemicals that are extruded by Pgp could be used to specifically eliminate immunoreactive T-cell populations. In this study, we evaluated the capacity of 4,5-dibromorhodamine methyl ester (TH9402), a photosensitizer structurally similar to rhodamine, a dye transported by Pgp, and which becomes highly cytotoxic on activation with visible light to selectively deplete alloreactive T lymphocytes. Stimulation of T cells with mitogens or allogeneic major histocompatibility complex-mismatched cells resulted in the preferential retention of the TH9402 rhodamine-derivative in activated T cells, both CD4+ and CD8+. Photodynamic cell therapy of TH9402-exposed T cells led to the selective elimination of immunoreactive T-cell populations. In addition, this treatment preserved resting T cells and their capacity to respond to third-party cells. Inhibition of Pgp enhanced cellular trapping of the dye in nonactivated T cells and resulted in their depletion after exposure to light. Targeting of Pgp-deficient cells may therefore represent an appealing strategy for the prevention and treatment of graft-versus-host disease and other alloimmune or autoimmune disorders.

Correlation between decreased apoptosis and multidrug resistance (MDR) in murine leukemic T cell lines

Cancer cells may frequently develop cross-resistance to structurally dissimilar chemotherapeutic agents. However, the molecular mechanisms for sensitivity and resistance of tumor cells towards chemotherapy are still partially understood. Antineoplasic drugs have been shown to induce apoptosis in chemosensitive leukemias and solid tumors. In this work, cross-resistance among vincristine (VCR), doxorubicin (DOX) and other antineoplasic agents commonly used in the treatment of leukemia such as etoposide (VP-16), methotrexate (MTX), cyclophosphamide (CTX), dexamethasone (DEX), cytarabine (Ara-C) and L-asparaginase on vincristine resistant (LBR-V160), doxorubicin resistant (LBR-D160) and sensitive (LBR-) murine leukemic T cell lines, was determined. The effect of antineoplasic agents was assayed by tritiated thymidine incorporation. Our results showed that VCR exhibited cross-resistance with DOX, VP-16, DEX and MTX, while DOX demonstrated cross-resistance with VCR, VP-16 and MTX. Ara-C failed to present cross-resistance with any cell line. Apoptosis induced by the above drugs on the same cell lines was analyzed by acridine orange and ethidium bromide staining, DNA hypoploidy (flow cytometry) and oligonucleosomal fragmentation of nuclear DNA showing that therapeutic concentrations of these chemotherapeutic agents induced apoptosis in the LBR- cell line. Our results demonstrated that, except for DEX, none of the drugs presenting cross-resistance were able to induce cell death on LBR-V 160 or LBR-D 160 cell lines.

Phase I/II trial of the multidrug-resistance modulator valspodar combined with cisplatin and doxorubicin in refractory ovarian cancer

PURPOSE

To determine the maximum-tolerated dose (MTD) of doxorubicin when given in combination with cisplatin and the multidrug-resistance (MDR) modulator valspodar and the remission rate induced by this combination in patients with platinum- and anthracycline-resistant ovarian cancer.

PATIENTS AND METHODS

Fifty-nine patients who had failed prior platinum- and anthracycline-based chemotherapy were enrolled. During the dose-finding phase, patients received a loading dose of valspodar (1.5 or 2 mg/kg) via 2-hour intravenous (IV) infusion on day 1 and continuous IV infusion (CIVI) of valspodar (2, 4, or 10 mg/kg/d) over 3 days. Doxorubicin (starting from 20 up to 50 mg/m(2)) and cisplatin (50 mg/m(2)) were administered via 15- to 20-minute IV infusions on day 3. During the efficacy phase, patients received at least two treatment cycles unless toxicity was unacceptable, and responding patients and those with stable disease received four to six cycles.

RESULTS

All patients completed at least one cycle of combined treatment. The MTD of doxorubicin was determined to be 35 mg/m(2) when administered with valspodar at 2 mg/kg loading dose and 10 mg/kg/d CIVI plus 50 mg/m(2) cisplatin. At these doses, valspodar blood concentrations known to reverse MDR in vitro were reached in all patients. Valspodar was well tolerated at all dose levels. Dose-limiting toxicities of the combination were primarily hematologic and included febrile neutropenia and prolonged leucopenia. The addition of valspodar to the treatment did not worsen cisplatin-related toxicity. Among 33 patients treated at the MTD for doxorubicin, one (3%) had a complete response, and four (12%) had a partial response. An additional seven patients experienced a stabilization of their previously progressive disease. The survival rates at 6 and 12 months were 59% and 19%, respectively.

CONCLUSION

Valspodar can be safely coadministered with doxorubicin and cisplatin. Although the regimen used in this trial produced renewed responses in patients with heavily pretreated, refractory ovarian cancer, the value of valspodar in reversing resistance mediated by P-glycoprotein remains to be determined.

The sulphonylurea glibenclamide inhibits multidrug resistance protein (MRP1) activity in human lung cancer cells

1. Glibenclamide, a sulphonylurea widely used for the treatment of non-insulin-dependent diabetes mellitus, has been shown to inhibit the activities of various ATP-binding cassette (ABC) transporters. In the present study, its effects towards multidrug resistance protein 1 (MRP1), an ABC efflux pump conferring multidrug resistance and handling organic anions, were investigated. 2. Intracellular accumulation of calcein, an anionic dye substrate for MRP1, was strongly increased by glibenclamide in a dose-dependent manner in MRP1-overexpressing lung tumour GLC4/Sb30 cells through inhibition of MRP1-related calcein efflux. By contrast, glibenclamide did not alter calcein levels in parental control GLC4 cells. Another sulphonylurea, tolbutamide, was however without effect on calcein accumulation in both GLC4/Sb30 and GLC4 cells. 3. Glibenclamide used at 12.5 microM was, moreover, found to strongly enhance the sensitivity of GLC4/Sb30 cells towards vincristine, an anticancer drug handled by MRP1. 4. Efflux of carboxy-2',7'-dichlorofluorescein, an anionic dye handled by the ABC transporter MRP2 sharing numerous substrates with MRP1 and expressed at high levels in liver, was also strongly inhibited by glibenclamide in isolated rat hepatocytes. 5. In summary, glibenclamide reversed MRP1-mediated drug resistance likely through inhibiting MRP1 activity and blocked organic anion efflux from MRP2-expressing hepatocytes. Such effects associated with the known inhibitory properties of glibenclamide towards various others ABC proteins suggest that this sulphonylurea is a general inhibitor of ABC transporters.

Potentiation of chemosensitivity in multidrug-resistant human leukemia CEM cells by inhibition of DNA-dependent protein kinase using wortmannin

DNA-dependent protein kinase (DNA-PK) is activated by DNA strand breaks and participates in DNA repair. Its regulatory subunit, Ku autoantigen, binds to DNA and recruits the catalytic subunit (DNA-PKcs). We show here a new role of DNA-PK in the development of multidrug resistance (MDR). The Ku-DNA binding activity, the levels of Ku70/Ku80 and DNA-PKcs in MDR variants, CEM/VLB(10-2), CEM/VLB(55-8) and CEM/VLB100 were higher than those in their parental drug-sensitive CEM cells in a drug resistance-dependent fashion. Also, CEM/VLB100 cells showed about 3-fold increase of DNA-PK enzyme activity as compared with CEM cells. Similar results were observed in another MDR cell line, FM3A/M mouse mammary carcinoma cells. Moreover, we observed that CEM/VLB100 cells were about 11-fold sensitive to wortmannin, which inhibits DNA-PK, compared with the CEM cells, and sensitized the MDR cells when combined with either bleomycin or vincristine, but have a little effect on CEM cells. Wortmannin was shown to inhibit DNA-PK and Ku-DNA binding activity in CEM/VLB100 cells dose dependently but had a little or no effect on their parental cells. Our results suggested that enhanced expression of DNA-PK participates in the development of MDR, and the use of DNA-PK inhibitors such as wortmannin is likely to improve the effectiveness of anticancer drugs and thus could partially overcome drug resistance in MDR cells, through its ability to inhibit Ku/DNA-PK activity.