Expression of the MDR1 gene in human cancers

Crizotinib and Doxorubicin Cooperatively Reduces Drug Resistance by Mitigating MDR1 to Increase Hepatocellular Carcinoma Cells Death

As the sixth most lethal cancers worldwide, hepatocellular carcinoma (HCC) has been treated with doxorubicin (Dox) for decades. However, chemotherapy resistance, especially for Dox is an even more prominent problem due to its high cardiotoxicity. To find a regimen to reduce Dox resistance, and identify the mechanisms behind it, we tried to identify combination of drugs that can overcome drug resistance by screening tyrosine kinase inhibitor(s) with Dox with various HCC cell lines in vitro and in vivo. We report here that combination of Crizo and Dox has a synergistic effect on inducing HCC cell death. Accordingly, Crizo plus Dox increases Dox accumulation in nucleus 3-16 times compared to Dox only; HCC cell death enhanced at least 50% in vitro and tumor weights reduced ranging from 35 to 65%. Combining these two drugs reduces multiple drug resistance 1 (MDR1) protein as a result of activation of protein kinase RNA-like endoplasmic reticulum kinase (PERK), which phosphorylates eIF2α, leading to protein translational repression. Additionally, PERK stimulation activates C-Jun terminal kinase (JNK), resulting in accumulation of unfused autophagosome to enhance autophagic cell death via Poly-ADP-ribosyltransferase (PARP-1) cleavage. When the activity of PERK or JNK is blocked, unfused autophagosome is diminished, cleaved PARP-1 is reduced, and cell death is abated. Therefore, Crizo plus Dox sensitize HCC drug resistance by engaging PERK-p- eIF2α-MDR1, and kill HCC cells by engaging PERK-JNK- autophagic cell death pathways. These newly discovered mechanisms of Crizo plus Dox not only provide a potential treatment for HCC but also point to an approach to overcome MDR1 related drug resistance in other cancers.

β-Elemene, a compound derived from Rhizoma zedoariae, reverses multidrug resistance mediated by the ABCB1 transporter

In the present in vitro study, we examined the effect of the compound β-elemene on the response of KB-C2 cells overexpressing the ABCB1 transporter to specific antineoplastic compounds. The MTT assay was used to determine the effects of β-elemene in combination with other anticancer drugs on ABCB1-overexpressing cancer cell lines. Furthermore, we used [3H]-paclitaxel accumulation, efflux assay, immunofluorescence experiments, western blot assays and docking analysis to ascertain the mechanism of action of β-elemene. The incubation of KB-C2 cells overexpressing ABCB1 transporter with β-elemene (100 µM) significantly augmented the antineoplastic efficacy of colchicine, vinblastine and paclitaxel when compared to KB-C2 cells incubated with these drugs alone. In HEK293 cells overexpressing the ABCB1 transporter, β-elemene significantly increased the cytotoxicity of paclitaxel. In addition, 100 µM of β-elemene significantly increased the accumulation of [3H]-paclitaxel and this was due to a decrease in [3H]-paclitaxel efflux when compared to controls. The incubation of KB-C2 cells with β-elemene (100 µM) for 72 h did not significantly alter the expression of ABCB1 protein levels. Immunofluorescence experiments indicated that β-elemene did not significantly alter the subcellular localization of the ABCB1 transporter. Docking analysis indicated that β-elemene binds to the drug-binding site of ABCB1 transporter. Finally, β-elemene at 100 µM partially (~50%) increased the sensitivity of the BCRP-overexpressing cell line, NCI-H460/MX20, to mitoxantrone, but β-elemene did not significantly alter the resistance of MRP1-transfected HEK293/MRP1 cells to vincristine. Overall, our in vitro findings indicated that β-elemene potentiates the cytotoxic effects of various antineoplastic drugs in cell lines overexpressing the ABCB1 transporter and that this is due to the inhibition of the efflux component of the ABCB1 transporter.

GW583340 and GW2974, human EGFR and HER-2 inhibitors, reverse ABCG2- and ABCB1-mediated drug resistance

The overexpression of ATP binding cassette (ABC) transporters often leads to the development of multidrug resistance (MDR) and results in a suboptimal response to chemotherapy. Previously, we reported that lapatinib (GW572016), a human epidermal growth factor receptor (EGFR) and HER-2 tyrosine kinase inhibitor (TKI), significantly reverses MDR in cancer cells by blocking the efflux function of ABC subfamily B member 1 (ABCB1) and ABC subfamily G member 2 (ABCG2). In the present study, we conducted in vitro experiments to evaluate if GW583340 and GW2974, structural analogues of lapatinib, could reverse ABCB1- and ABCG2-mediated MDR. Our results showed that GW583340 and GW2974 significantly sensitized ABCB1 and ABCG2 overexpressing MDR cells to their anticancer substrates. GW583340 and GW2974 significantly increased the intracellular accumulation of [(3)H]-paclitaxel in ABCB1 overexpressing cells and [(3)H]-mitoxantrone in ABCG2 overexpressing cells respectively. In addition, GW583340 and GW2974 significantly inhibited ABCG2-mediated transport of methotrexate in ABCG2 overexpressing membrane vesicles. There was no significant change in the expression levels of ABCB1 and ABCG2 in the cell lines exposed to 5μM of either GW583340 or GW2974 for 3 days. In addition, a docking model predicted the binding conformation of GW583340 and GW2974 to be within the transmembrane region of homology modeled human ABCB1 and ABCG2. We conclude that GW583340 and GW2974, at clinically achievable plasma concentrations, reverse ABCB1- and ABCG2-mediated MDR by blocking the drug efflux function of these transporters. These findings may be useful in developing combination therapy for cancer treatment with EGFR TKIs.

Expression and function of p-glycoprotein in normal tissues: effect on pharmacokinetics

ATP-binding cassette (ABC) drug efflux transporters limit intracellular concentration of their substrates by pumping them out of cell through an active, energy dependent mechanism. Several of these proteins have been originally associated with the phenomenon of multidrug resistance; however, later on, they have also been shown to control body disposition of their substrates. P-glycoprotein (Pgp) is the first detected and the best characterized of ABC drug efflux transporters. Apart from tumor cells, its constitutive expression has been reported in a variety of other tissues, such as the intestine, brain, liver, placenta, kidney, and others. Being located on the apical site of the plasma membrane, Pgp can remove a variety of structurally unrelated compounds, including clinically relevant drugs, their metabolites, and conjugates from cells. Driven by energy from ATP, it affects many pharmacokinetic events such as intestinal absorption, brain penetration, transplacental passage, and hepatobiliary excretion of drugs and their metabolites. It is widely believed that Pgp, together with other ABC drug efflux transporters, plays a crucial role in the host detoxication and protection against xenobiotic substances. On the other hand, the presence of these transporters in normal tissues may prevent pharmacotherapeutic agents from reaching their site of action, thus limiting their therapeutic potential. This chapter focuses on P-glycoprotein, its expression, localization, and function in nontumor tissues and the pharmacological consequences hereof.

Future perspectives for the development of P-glycoprotein modulators

Resistance to chemotherapeutic agents constitutes one of the major obstacles to the successful treatment of cancer. While several mechanisms underlying drug resistance have been elucidated, the most widely studied mechanism involves the efflux of antineoplastic drugs from cancer cells by P-glycoprotein, the 170 kD glycoprotein product of the MDR-I gene. The observation that several compounds are able to inhibit P-glycoprotein in vitro created optimism that the problem of multidrug resistance in cancer could be quickly resolved by moving these compounds into the clinic. However, despite a large number of clinical trials with several different putative Pgp modulators, the value of Pgp modulation in clinical oncologic practice remains unresolved. While these initial trials have not answered the question of whether Pgp is an important mechanism of resistance in human cancers, or whether modulation of Pgp is likely to positively impact on the treatment of cancer, they have provided insights regarding the problems inherent in conducting trials of this nature. These clinical insights, along with knowledge gained from continued basic research on drug resistance mediated by Pgp and related transporters, will form a strong foundation for future research into the role of Pgp and Pgp modulation in the treatment of cancer. The ubiquitous nature of transporters and the high prevalence of transporter substrates among antineoplastic drugs, compel the development of modulators that can be used to prevent or reverse drug resistance.

P-glycoprotein in the placenta: Expression, localization, regulation and function

Detailed understanding of the mechanisms employed in transfer of drugs across the placenta is essential for optimization of pharmacotherapy during pregnancy. Disclosure of drug efflux transporters as an "active component" of the placental barrier has brought new important insights into the field of transplacental pharmacokinetics. P-glycoprotein (P-gp, MDR1) is the first discovered and so far the best characterized of drug efflux transporters, whose role in the regulation of drug disposition to the fetus has been extensively studied. Expression of P-gp in the placental trophoblast layer was confirmed at the mRNA and protein levels in all phases of pregnancy, and several in vitro and in vivo studies demonstrated functional activity of the transporter in materno-fetal drug transport. P-gp is able to actively pump drugs and other xenobiotics from trophoblast cells back to the maternal circulation, providing thus protection to the fetus. This review summarizes the current knowledge on the expression, localization and function of P-gp in the placenta. In addition, we include the latest data concerning transcriptional regulation of placental P-gp expression and polymorphisms of the MDR1 gene. Clinical significance of placental P-gp and its future perspectives for pharmacotherapy during pregnancy are also discussed.

Block copolymeric biotransport carriers as versatile vehicles for drug delivery

This review describes block co-polymer-based systems that are used in drug delivery. The main focus is on amphiphilic block co-polymers, the application of which modifies the pharmacological performance of various classes of drugs and is attracting more and more attention. The two main reasons for this are the high tendency of block co-polymer-based drug formulations to self-assemble and the flexibility of block co-polymer chemistry, which allows precise tailoring of the carrier to virtually any chemical entity. The combination of these and some other features makes it possible to adjust block co-polymer-based drug formulations to achieve the most beneficial balance in their biological interactions (biotransport), with systems that control drug removal from the body and those that are responsible for drug therapeutic activity. The following major aspects are considered: The role of physical properties of formulations in their pharmacological performance. The chemistry and physico-chemistry of block co-polymers and structure-function relationships in these systems. Examples describing the effects of biotransport systems on drug transport and activity in cells and some results on their in vivo applications with various drugs.

Block copolymeric biotransport carriers as versatile vehicles for drug delivery

This review describes block copolymer-based systems that are used in drug formulation development. The use of amphiphilic block copolymers to modify pharmacological performance of various classes of drugs attracts more and more attention. This is largely attributable to the high tendency of block copolymer-based drug formulations to self-assemble, as well as flexibility of block copolymer chemistry, which allows precise tailoring of the carrier to virtually any chemical entity. Combination of these features allows adjustment of block copolymer-based drug formulations to achieve the most beneficial balance in drug biological interactions with the systems that control its circulation in and removal from the body and its therapeutic activity. The following major aspects are considered: 1) physical properties of formulations and the methods used to adjust these properties towards the highest pharmacological performance of the product; 2) combinatorial methods for optimisation of block copolymer-based formulations; 3) biological response modifying properties of block copolymer-based formulations.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

ATP-dependent uptake of natural product cytotoxic drugs by membrane vesicles establishes MRP as a broad specificity transporter

MRP is a recently isolated ATP-binding cassette family transporter. We previously reported transfection studies that established that MRP confers multidrug resistance [Kruh, G. D., Chan, A., Myers, K., Gaughan, K., Miki, T. & Aaronson, S. A. (1994) Cancer Res. 54, 1649-1652] and that expression of MRP is associated with enhanced cellular efflux of lipophilic cytotoxic agents [Breuninger, L. M., Paul, S., Gaughan, K., Miki, T., Chan, A., Aaronson, S. A. & Kruh, G. D. (1995) Cancer Res. 55, 5342-5347]. To examine the biochemical mechanism by which MRP confers multidrug resistance, drug uptake experiments were performed using inside-out membrane vesicles prepared from NIH 3T3 cells transfected with an MRP expression vector. ATP-dependent transport was observed for several lipophilic cytotoxic agents including daunorubicin, etoposide, and vincristine, as well as for the glutathione conjugate leukotriene C4 (LTC4). However, only marginally increased uptake was observed for vinblastine and Taxol. Drug uptake was osmotically sensitive and saturable with regard to substrate concentration, with Km values of 6.3 microM, 4.4 microM, 4.2 microM, 35 nM, and 38 microM, for daunorubicin, etoposide, vincristine, LTC4, and ATP, respectively. The broad substrate specificity of MRP was confirmed by the observation that daunorubicin transport was competitively inhibited by reduced and oxidized glutathione, the glutathione conjugates S-(p-azidophenacyl)-glutathione (APA-SG) and S-(2,4-dinitrophenyl)glutathione (DNP-SG), arsenate, and the LTD4 antagonist MK571. This study establishes that MRP pumps unaltered lipophilic cytotoxic drugs, and suggests that this activity is an important mechanism by which the transporter confers multidrug resistance. The present study also indicates that the substrate specificity of MRP is overlapping but distinct from that of P-glycoprotein, and includes both the neutral or mildly cationic natural product cytotoxic drugs and the anionic products of glutathione conjugation. The widespread expression of MRP in tissues, combined with its ability to transport both lipophilic xenobiotics and the products of phase II detoxification, indicates that the transporter represents a widespread and remarkably versatile cellular defense mechanism.

Identification of a distinctive P-glycoprotein-mediated resistance phenotype in human ovarian carcinoma cells after their in vitro exposure to fractionated X-irradiation

BACKGROUND

Clinical drug resistance is recognized in patients previously treated with radiotherapy and after chemotherapy. In vitro exposure of mammalian tumor cells to fractionated X-irradiation also resulted in the expression of drug resistance. Analysis of the resistance phenotype of irradiated Chinese hamster ovary sublines revealed P-glycoprotein overexpression, without any concomitant increase in P-glycoprotein messenger RNA, under posttranslational regulation. This study aimed to determine whether this distinctive resistance phenotype could also be identified in irradiated human tumor cells.

METHODS

Irradiated sublines established from two human ovarian tumor cell lines, SK-OV-3 and JA-T, which showed resistance to vincristine and to etoposide, were studied. Protein and RNA expression were quantitated by Western and Northern blotting or RNase protection assays. P-glycoprotein turnover was measured after immunoprecipitation of metabolically labelled cells.

RESULTS

Significant P-glycoprotein overexpression was detected using the C219 and C494 monoclonal antibodies in the two irradiated human ovarian tumor sublines. No concomitant increase in P-glycoprotein messenger RNA was detectable in the SK-OV-3/DXR10 subline, contrasting with the increased message characteristic of vincristine-selected SKVCR sublines. In addition, turnover of P-glycoprotein was significantly reduced in these DXR10 cells when compared with that measured in a vincristine-selected subline. These irradiated sublines showed reduced levels of epidermal growth factor receptors and unchanged levels of topoisomerase II, but they overexpressed c-erbB2 marginally and heat shock protein 27 significantly. These latter elevations in protein levels, however, were associated with concomitant increases in their respective messenger RNAs, implicating regulation at the transcriptional level.

CONCLUSIONS

Exposure of human ovarian tumor cells to fractionated X-irradiation in vitro resulted in the expression of a distinctive multiple drug resistance phenotype unusually involving posttranslational regulation of P-glycoprotein. Monitoring tumor biopsies for P-glycoprotein-associated drug resistance in patients treated with radiotherapy should evaluate protein levels rather than, or as well as, MDR1 mRNA expression.

Kinetic resistance to anticancer agents

Adherent epithelial cancer cells, such as colon cancer cells, are much more resistant to anthracyclines and to many other major anticancer agents when the cell population reaches confluence. Our purpose is to analyze the mechanisms of this confluence dependent resistance (CDR) that is probably the major cause of the natural resistance of solid tumors to chemotherapy. Some drugs (anthracyclines, etoposide and vincristine) but not others (cisplatin, melphalan and 5-fluorouracil) accumulate less in confluent than in nonconfluent cells. A decrease of the passive transmembrane drug transport in confluent cells is associated to a reduced membrane fluidity. However, the predominant mechanism of CDR is an increase in the intrinsic resistance of the DNA to the drug-induced damage. This mechanism is now relatively well understood for anthracyclines and etoposide that act mainly through an inhibition of the topoisomerase II: as the enzyme level is low in slowly proliferating confluent cells, the number of drug-induced DNA strand breaks is lower than in rapidly growing nonconfluent cells which highly express the topoisomerase II gene. Mechanisms of CDR for the other drugs are less clear and could involve an increase in the ability to repair damaged DNA. Attempts to circumvent CDR could consist in the stimulation of the cell proliferation by hormones or growth factors, or in the recruitment of quiescent cells into the S and G2 phases by previous treatment of confluent cells with infratoxic concentration of DNA-damaging agents.

Drug resistance in oncology: from concepts to applications

The complex problem of drug resistance is discussed with respect to host toxicity, to tumor characteristics (kinetic resistance, heterogeneity of cell subpopulations, hypoxia, mutation and gene amplification), and to the medication itself (pharmacokinetic and pharmacodynamic resistance: cell membrane, intracellular metabolism, intracellular target). After detailing each type of resistance, the possibilities of fighting against drug resistance are explored (dealing with host toxicity, tumor characteristics and drugs--intensifying therapy, multiple drug therapy, biochemical modulation, particular modalities of drug administration). Finally, perspectives of research and development of new drugs are summarized.