Exploiting multidrug resistance to treat cancer

Suppression of ABCG2 mediated MDR in vitro and in vivo by a novel inhibitor of ABCG2 drug transport

Cancer is a disease whose treatment is often limited due to the development of a phenomenon known as multidrug resistance (MDR). There is an immense demand for development of novel agents that can overcome the MDR in cancer. A group of transmembrane proteins called ATP-binding cassette transporters, present ubiquitously in the human body possesses a modular architecture, contributing immensely towards the development of MDR. An analysis of structural congeners among a group of compounds led to the discovery of CCTA-1523 that could selectively reverse ABCG2-mediated MDR in cancer cells in vitro and in vivo. CCTA-1523 (5μM) sensitized the ABCG2 overexpressing cancer cells and ABCG2 transfected cells to the substrate chemotherapeutic drugs. The reversal ability of CCTA-1523 was primarily due to the inhibition of the efflux function of ABCG2; also there was no change in the protein expression or the localization of the ABCG2 in the presence of CCTA-1523. The reversal effect of CCTA-1523 was reversible. Importantly, co-administration of CCTA-1523 restored the in vivo antitumor activity of doxorubicin in ABCG2 overexpressing tumor xenografts. Taken together, our findings indicate that CCTA-1523 is a potent, selective and reversible modulator of ABCG2 that may offer therapeutic promise for multidrug- resistant malignancies.

Natural lignans from Arctium lappa modulate P-glycoprotein efflux function in multidrug resistant cancer cells

Arctium lappa is a well-known traditional medicinal plant in China (TCM) and Europe that has been used for thousands of years to treat arthritis, baldness or cancer. The plant produces lignans as secondary metabolites which have a wide range of bioactivities. Yet, their ability to reverse multidrug resistance (MDR) in cancer cells has not been explored. In this study, we isolated six lignans from A. lappa seeds, namely arctigenin, matairesinol, arctiin, (iso)lappaol A, lappaol C, and lappaol F. The MDR reversal potential of the isolated lignans and the underlying mechanism of action were studied using two MDR cancer cell lines, CaCo2 and CEM/ADR 5000 which overexpress P-gp and other ABC transporters. In two-drug combinations of lignans with the cytotoxic doxorubicin, all lignans exhibited synergistic effects in CaCo2 cells and matairesinol, arctiin, lappaol C and lappaol F display synergistic activity in CEM/ADR 5000 cells. Additionally, in three-drug combinations of lignans with the saponin digitonin and doxorubicin MDR reversal activity was even stronger enhanced. The lignans can increase the retention of the P-gp substrate rhodamine 123 in CEM/ADR 5000 cells, indicating that lignans can inhibit the activity of P-gp. Our study provides a first insight into the potential chemosensitizing activity of a series of natural lignans, which might be candidates for developing novel adjuvant anticancer agents.

Different roles of TM5, TM6, and ECL3 in the oligomerization and function of human ABCG2

ABCG2 is a member of the ATP-binding cassette transporter superfamily, and its overexpression causes multidrug resistance (MDR) in cancer chemotherapy. ABCG2 may also protect cancer stem cells by extruding cytotoxic materials. ABCG2 has previously been shown to exist as a high-order homo-oligomer consisting of possibly 8-12 subunits, and the oligomerization domain was mapped to the C-terminal domain, including TM5, ECL3, and TM6. In this study, we further investigate this domain in detail for the role of each segment in the oligomerization and drug transport function of ABCG2 using domain swapping and site-directed mutagenesis. We found that none of the three segments (TM5, TM6, and ECL3) is essential for the oligomerization activity of ABCG2 and that any one of these three segments in the full-length context is sufficient to support ABCG2 oligomerization. While TM5 plays an important role in the drug transport function of ABCG2, TM6 and ECL3 are replaceable. Thus, each segment in the TM5-ECL3-TM6 domain plays a distinctive role in the oligomerization and function of ABCG2.

Farnesyl diphosphate synthase is involved in the resistance to zoledronic acid of osteosarcoma cells

We recently demonstrated original anti-tumor effects of zoledronic acid (Zol) on osteosarcoma cell lines independently of their p53 and Rb status. The present study investigated the potential Zol-resistance acquired by osteosarcoma cells after prolonged treatment. After 12 weeks of culture in the presence of 1 μm Zol, the effects of high doses of Zol (10–100 μm) were compared between the untreated rat (OSRGA, ROS) and human (MG63, SAOS2) osteosarcoma cells and Zol-pretreated cells in terms of cell proliferation, cell cycle analysis, migration assay and cytoskeleton organization. Long-term treatment with 1 μm Zol reduced the sensitivity of osteosarcoma cells to high concentrations of Zol. Furthermore, the Zol-resistant cells were sensitive to conventional anti-cancer agents demonstrating that this resistance process is independent of the multidrug resistance phenotype. However, as similar experiments performed in the presence of clodronate and pamidronate evidenced that this drug resistance was restricted to the nitrogen-containing bisphosphonates, we then hypothesized that this resistance could be associated with a differential expression of farnesyl diphos-phate synthase (FPPS) also observed in human osteosarcoma samples. The transfection of Zol-resistant cells with FPPS siRNA strongly increased their sensitivity to Zol. This study demonstrates for the first time the induction of metabolic resistance after prolonged Zol treatment of osteosarcoma cells confirming the therapeutic potential of Zol for the treatment of bone malignant pathologies, but points out the importance of the treatment regimen may be important in terms of duration and dose to avoid the development of drug metabolic resistance.

Drug-selected co-expression of P-glycoprotein and gp91 in vivo from an MDR1-bicistronic retrovirus vector Ha-MDR-IRES-gp91

BACKGROUND

Retroviral transduction of human hematopoietic stem cells is an attractive strategy in gene therapy; however, transduction efficiency and duration of transgene expression may not be satisfactory in current protocols. Co-expression of a human multidrug resistance gene (MDR1) with a therapeutic gene affords selectable growth advantage to genetically modified cells.

METHODS

A bicistronic retrovirus vector, Ha-MDR-IRES-gp91, was constructed for the co-expression of MDR1 and gp91, a gene responsible for X-linked chronic granulomatous disease (X-CGD). Drug-selected co-expression of P-glycoprotein and gp91 was evaluated in transduced cells.

RESULTS

Epstein-Barr virus-transformed B cells from X-CGD patients transduced with Ha-MDR-IRES-gp91 co-expressed human P-glycoprotein and gp91, and acquired superoxide-generating activity. Human CD34-positive cells from an X-CGD patient were transduced with Ha-MDR-IRES-gp91 and subsequently treated with 2 ng/ml vincristine. After 13 days, 20% of Ha-MDR-IRES-gp91-transduced cells were P-glycoprotein- and gp91-positive by FACS analysis. The superoxide-generating activity of the transduced population was 27% of that of normal cells. Mice transplanted with Ha-MDR-IRES-gp91-transduced bone marrow cells showed co-expression of P-glycoprotein and gp91 in peripheral blood mononuclear cells. By administering paclitaxel, the proportions of P-glycoprotein- and gp91-positive cells were increased in all the four mice examined. When mice transplanted with Ha-MDR-IRES-gp91-transduced cells were repeatedly administered paclitaxel, the ratios of P-glycoprotein- and gp91-positive cells were maintained for over 1 year.

CONCLUSIONS

These results suggest that MDR1-bicistronic vectors may be useful to select the transduced hematopoietic cells in vivo. This may lead to the sustained expression of transgenes in the blood cells of patients treated with stem cell gene therapy.

Reversal of drug resistance of hepatocellular carcinoma cells by adenoviral delivery of anti-MDR1 ribozymes

Human cancers, including hepatocellular carcinoma (HCC), are characterized by a high degree of drug resistance. The multidrug resistance (MDR) transporters MDR1-P-glycoprotein and MRP2 (multidrug-associated protein 2) are expressed in almost 50% of human cancers, including HCCs. In this study, we analyzed the effect of anti-MDR1 ribozymes, especially AFP promoter-driven anti-MDR1 ribozymes, to specifically chemosensitize HCC cells. Epirubicin-selected HB8065/R cells were used as MDR1-P-glycoprotein-overexpressing cells. Adenoviral vectors were constructed to allow an efficient gene transfer of anti-MDR1 ribozyme constructs. AFP promoter-driven anti-MDR1 ribozymes reduced the IC(50) 30-fold for epirubicin in HCC cells, whereas human colorectal cancer cells were unaffected. Target sequences were either the translational start site or codon 196 of the human MDR1 gene. Adenoviral delivery of CMV promoter-driven anti-MDR1 ribozymes resulted in a reduced IC(50) for epirubicin and doxorubicin (60- and 20-fold, respectively). They completely restored chemosensitivity in stably transfected anti-MDR1 ribozyme-expressing HCC cells as well as in HCC cells transduced with adenoviruses expressing wild-type anti-MDR1 ribozymes. Adenoviral delivery of ribozymes was so efficient that chemosensitization of HCC cells could be demonstrated in cell cultures without further selection of transduced cells for single anti-MDR1 ribozyme-expressing HCC cell clones. Northern blots showed a decreased MDR1 mRNA expression, and fluorescence-activated cell sorting (FACS) analysis revealed a significantly reduced expression of MDR1-P-glycoprotein on the cell surface of HB8065/R cells after transduction with the anti-MDR1 ribozymes. In conclusion, our data demonstrate that adenoviral delivery of ribozymes can chemosensitize HCC cells and that chemosensitization can be specifically achieved by ribozymes driven by an AFP promoter directed against human MDR1.

Mechanisms of cancer drug resistance

The design of cancer chemotherapy has become increasingly sophisticated, yet there is no cancer treatment that is 100% effective against disseminated cancer. Resistance to treatment with anticancer drugs results from a variety of factors including individual variations in patients and somatic cell genetic differences in tumors, even those from the same tissue of origin. Frequently resistance is intrinsic to the cancer, but as therapy becomes more and more effective, acquired resistance has also become common. The most common reason for acquisition of resistance to a broad range of anticancer drugs is expression of one or more energy-dependent transporters that detect and eject anticancer drugs from cells, but other mechanisms of resistance including insensitivity to drug-induced apoptosis and induction of drug-detoxifying mechanisms probably play an important role in acquired anticancer drug resistance. Studies on mechanisms of cancer drug resistance have yielded important information about how to circumvent this resistance to improve cancer chemotherapy and have implications for pharmacokinetics of many commonly used drugs.

The mechanism of action of multidrug-resistance-linked P-glycoprotein

P-glycoprotein (Pgp), the ATP-binding cassette (ABC) transporter, confers multidrug resistance to cancer cells by extruding cytotoxic natural product amphipathic drugs using the energy of ATP hydrolysis. Our studies are directed toward understanding the mechanism of action of Pgp and recent work deals with the assessment of interaction between substrate and ATP sites and elucidation of the catalytic cycle of ATP hydrolysis. The kinetic analyses of ATP hydrolysis by reconstituted purified Pgp suggest that ADP release is the rate-limiting step in the catalytic cycle and the substrates exert their effect by modulating ADP release. In addition, we provide evidence for two distinct roles for ATP hydrolysis in a single turnover of Pgp, one in the transport of drug and the other in effecting conformational changes so as to reset the transporter for the next catalytic cycle. Detailed kinetic measurements determined that both nucleotide-binding domains behave symmetrically and during individual hydrolysis events the ATP sites are recruited in a random manner. Furthermore, only one nucleotide site hydrolyzes ATP at any given time, causing (in this site) a conformational change that drastically decreases (>30-fold) the affinity of the second site for ATP-binding. Thus, the blocking of ATP-binding to the second site while the first one is in catalytic conformation appears to be the basis for the alternate catalytic cycle of ATP hydrolysis by Pgp, and this may be applicable as well to other ABC transporters linked with the development of multidrug resistance.

Results of retroviral and adenoviral approaches to cancer gene therapy

Genetic modification for cancer treatment has involved the introduction of chemotherapy protection and sensitization genes into normal and tumor cells, respectively, for the purpose of improving the outcome of conventional approaches to the treatment of solid tumor neoplasms. This paper will review the use of multidrug resistance-1 retroviral vectors and cytosine deaminase adenoviral prodrug activation vectors for this purpose.

Phorbol ester-induced P-glycoprotein phosphorylation and functionality in the HTB-123 human breast cancer cell line

The discordance between P-glycoprotein (P-gp) expression and functionality [as measured by the efflux of doxorubicin (DOX)] was analyzed in a DOX-sensitive human breast cancer cell line (HTB-123) with high reactivity against four P-gp specific monoclonal antibodies (C219, MRK-16, UIC2, and 4E3). Reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting analyses confirmed the overexpression of MDR1 mRNA and P-gp in this cell line. However, incubation of cells with efflux blockers, verapamil (VPL) or dipyridamole (DPD), did not enhance cellular (DOX) accumulation or cytotoxicity. Upon incubation with 12-O-tetradecanoylphorbol-13-acetate (TPA), HTB-123 cells retained less DOX than control cells and were sensitive to the efflux blockers verapamil or dipyridamole. These observations suggest that 12-O-tetradecanoylphorbol-13-acetate-induced P-gp phosphorylation may be associated with induction of P-gp-mediated drug efflux in the HTB-123 cell line.