Doxorubicin resistance in human melanoma cells: MDR-1 and glutathione S-transferase pi gene expression

Discovery of (quinazolin-6-yl)benzamide derivatives containing a 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety as potent reversal agents against P-glycoprotein-mediated multidrug resistance

P-glycoprotein (P-gp) is an important factor leading to multidrug resistance (MDR) in cancer treatment. The co-administration of anticancer drugs and P-gp inhibitors has been a treatment strategy to overcome MDR. In recent years, tyrosine kinase inhibitor Lapatinib has been reported to reverse MDR through directly interacting with ABC transporters. In this work, a series of P-gp inhibitors (1-26) was designed and synthesized by integrating the quinazoline core of Lapatinib into the molecule framework of the third-generation P-gp inhibitor Tariquidar. Among them, compound 14 exhibited better MDR reversal activity than Tariquidar. The docking results showed compound 14 displayed the L-shaped molecular conformation. Importantly, compound 14 increased the accumulation of Adriamycin (ADM) and rhodamine 123 (Rh123) in MCF7/ADM cells. Besides, compound 14 significantly increased ADM-induced apoptosis and inhibited the proliferation, migration and invasion of MCF7/ADM cells. It was also demonstrated that compound 14 significantly inhibited the growth of MCF7/ADM xenograft tumors by increasing the sensitivity of ADM. In summary, compound 14 has the potential to overcome MDR caused by P-gp.

Discovery of 2,5-disubstituted furan derivatives featuring a benzamide motif for overcoming P-glycoprotein mediated multidrug resistance in MCF-7/ADR cell

P-glycoprotein (P-gp) is one of the drug efflux transporters that triggers multidrug resistance (MDR) in cells. Herein, by utilizing the strategies of active skeleton splicing and structural optimization on the lead compound 5 m, a total of 50 novel 2,5-disubstituted furan derivatives were designed, synthesized, and screened for P-gp inhibitory activity. The structure-activity relationship analysis enabled the identification of an important pharmacophore N-phenylbenzamide, which resulted in the discovery of a promising drug lead compound Ⅲ-8. Ⅲ-8 possesses broad-spectrum reversal activity and low toxicity in MCF-7/ADR cells. Western blot and Rh123 accumulation assay demonstrated that Ⅲ-8 displayed the reversal activity by inhibiting P-gp efflux. Molecular docking analysis indicated a potent affinity of Ⅲ-8 to P-gp by forming H-bond interactions with residues Asn 721 and Met 986. Ⅲ-8 was determined to be a highly effective and safe P-gp inhibitor in an MCF-7/ADR xenograft mouse model.

Design, synthesis and biological evaluation of novel phenylfuran-bisamide derivatives as P-glycoprotein inhibitors against multidrug resistance in MCF-7/ADR cell

The co-administration of anticancer drugs and P-glycoprotein (P-gp) inhibitors was a treatment strategy to surmount multidrug resistance (MDR) in anticancer chemotherapy. In this study, novel phenylfuran-bisamide derivatives were designed as P-gp inhibitors based on target-based drug design, and 31 novel compounds were synthesized and screened on MCF-7/ADR cells. The result of bioassay revealed that compound y12d exhibited low cytotoxicity and promising MDR reversal activity (IC₅₀ = 0.0320 μM, reversal fold = 1163.0), 3.64-fold better than third-generation P-gp inhibitor tariquidar (IC₅₀ = 0.1165 μM, reversal fold = 319.3). The results of Western blot and rhodamine 123 accumulation verified that compound y12d exhibited excellent MDR reversal activity by inhibiting the efflux function of P-gp but not expression. Furthermore, molecular docking showed that compound y12d bound to target P-gp by forming the double H-bond interactions with residue Gln 725. These results suggest that compound y12d might be a potential MDR reveal agent acting as a P-gp inhibitor in clinical therapeutics, and provide insight into design strategy and skeleton optimization for the development of P-gp inhibitors.

Targeting the mercapturic acid pathway for the treatment of melanoma

The treatment of metastatic melanoma is greatly hampered by the simultaneous dysregulation of several major signaling pathways that suppress apoptosis and promote its growth and invasion. The global resistance of melanomas to therapeutics is also supported by a highly active mercapturic acid pathway (MAP), which is responsible for the metabolism and excretion of numerous chemotherapy agents. The relative importance of the MAP in melanoma survival was not recognized until demonstrated that B16 melanoma undergoes dramatic apoptosis and regression upon the depletion or inhibition of the MAP transporter protein RLIP. RLIP is a multi-functional protein that couples ATP hydrolysis with the movement of substances. As the rate-limiting step of the MAP, the primary function of RLIP in the plasma membrane is to catalyze the ATP-dependent efflux of unmetabolized drugs and toxins, including glutathione (GSH) conjugates of electrophilic toxins (GS-Es), which are the precursors of mercapturic acids. Clathrin-dependent endocytosis (CDE) is an essential mechanism for internalizing ligand-receptor complexes that promote tumor cell proliferation through autocrine stimulation (Wnt5a, PDGF, βFGF, TNFα) or paracrine stimulation by hormones produced by fibroblasts (IGF1, HGF) or inflammatory cells (IL8). Aberrant functioning of these pathways appears critical for melanoma cell invasion, metastasis, and evasion of apoptosis. This review focuses on the selective depletion or inhibition of RLIP as a highly effective targeted therapy for melanoma that could cause the simultaneous disruption of the MAP and critical peptide hormone signaling that relies on CDE.

Optical clearing agent increases effectiveness of photodynamic therapy in a mouse model of cutaneous melanoma: an analysis by Raman microspectroscopy

Melanoma is the most aggressive type of skin cancer and a relevant health problem due to its poor treatment response with high morbidity and mortality rates. This study, aimed to investigate the tissue changes of an improved photodynamic therapy (PDT) response when combined with optical clearing agent (OCA) in the treatment of cutaneous melanoma in mice. Photodithazine (PDZ) was administered intraperitoneally and a solution of OCA was topically applied before PDT irradiation. Due to a resultant refractive index matching, OCA-treated tumors are more optically homogenous, improving the PDT response. Raman analysis revealed, when combined with OCA, the PDT response was more homogenous down to 725 µm-depth in thickness.

Glutathione, an Antioxidant Tripeptide: Dual Roles in Carcinogenesis and Chemoprevention

Glutathione (GSH or reduced glutathione) is a tripeptide of gamma-Glutamyl-cysteinylglycine and the predominant intracellular antioxidant in many organisms including humans. GSH and associated enzymes are controlled by a transcription factor-nuclear factor-2 related erythroid factor-2 (Nrf2). In cellular milieu, GSH protects the cells essentially against a wide variety of free radicals including reactive oxygen species, lipid hydroperoxides, xenobiotic toxicants, and heavy metals. It has two forms, the reduced form or reduced glutathione (GSH) and oxidized form (GSSG), where two GSH moieties combine by sulfhydryl bonds. Glutathione peroxidase (GPx) and glutathione-s-transferase (GST) essentially perform the detoxification reactions using GSH, converting it into GSSG. Glutathione reductase (GR) operates the salvage pathway by converting GSSG to GSH with the expense of NADPH and restores the cellular GSH pool. Hence, GSH and GSH-dependent enzymes are necessary for maintaining the normal redox balance in the body and help in cell survival under stress conditions. In addition, GST removes various carcinogenic compounds offering a chemopreventive property, whereas the GSH system plays a significant role in regulating the cellular survival by offering redox stability in a variety of cancers including prostate, lung, breast, and colon cancer. Studies have also indicated that GSH inhibitors, such as buthionine sulfoximine, improve the chemo-sensitivity in cancer cells. In addition, GSH and dependent enzymes provide a survival advantage for cancer cells against chemotherapeutic drugs and radiotherapy.

Carbonyl Reductases and Pluripotent Hydroxysteroid Dehydrogenases of the Short-chain Dehydrogenase/reductase Superfamily

Carbonyl reduction of aldehydes, ketones, and quinones to their corresponding hydroxy derivatives plays an important role in the phase I metabolism of many endogenous (biogenic aldehydes, steroids, prostaglandins, reactive lipid peroxidation products) and xenobiotic (pharmacologic drugs, carcinogens, toxicants) compounds. Carbonyl-reducing enzymes are grouped into two large protein superfamilies: the aldo-keto reductases (AKR) and the short-chain dehydrogenases/reductases (SDR). Whereas aldehyde reductase and aldose reductase are AKRs, several forms of carbonyl reductase belong to the SDRs. In addition, there exist a variety of pluripotent hydroxysteroid dehydrogenases (HSDs) of both superfamilies that specifically catalyze the oxidoreduction at different positions of the steroid nucleus and also catalyze, rather nonspecifically, the reductive metabolism of a great number of nonsteroidal carbonyl compounds. The present review summarizes recent findings on carbonyl reductases and pluripotent HSDs of the SDR protein superfamily.

Increased resistance of tumor cells to daunorubicin after transfection of cDNAs coding for anthracycline inactivating enzymes

Carbonyl reduction is a main but undesired metabolic pathway of the anti-cancer drug daunorubicin (DRC). The resulting alcohol metabolite daunorubicinol has a far less anti-tumor potency and, in addition, is responsible for the life-threatening cardiac toxicity that limits the clinical use of DRC. Elevated levels of carbonyl-reducing enzymes in cancer cells may therefore contribute to the development of DRC chemoresistance and affect the clinical outcome. In the present investigation, human pancreas carcinoma cells were transfected with three important DRC reductases, namely carbonyl reductase (CBR1), aldehyde reductase (AKR1A1) and aldose reductase (AKR1B1), and levels of resistance towards DCR determined. Overexpression of all three reductases lead to a higher DRC inactivation and to an elevation of chemoresistance (7-fold for CBR1, 4.5-fold for AKR1A1 and 3.7-fold for AKR1B1), when IC(50)-values were considered. Coadministration of DRC reductase inhibitors in DRC chemotherapy may be desirable since this would reduce the formation of the cardiotoxic alcohol metabolite and prevent drug resistance.

Suppression of P-glycoprotein gene expression in Hs578T/Dox by the overexpression of caveolin-1

Caveolin-1, the principal component of caveolae, is a 21-24 kDa integral membrane protein. The interaction of the caveolin-1 scaffolding domain with signaling molecules can functionally inhibit the activity of these signaling proteins. Little is known about how caveolin-1 influences the expression of P-glycoprotein (P-gp), an ABC transporter encoded by multi-drug resistance (MDR1) gene. To elucidate the possible mechanism between caveolin-1 and P-gp expression, in the present study, we overexpressed caveolin-1 in the Hs578T/Dox breast adenocarcinoma cells, a multidrug resistant line, and then selected single clone cells highly expressing caveolin-1 level. Both Western blot and confocal microscopy analyses showed that caveolin-1 was markedly overexpressed in the transfectants, while P-gp protein was almost abolished. Reverse transcription polymerase chain reaction also showed that the expression of P-gp mRNA was significantly suppressed in the transfectants. It was confirmed further by Northern blot analysis. Moreover, through measuring the changes of drug resistance and P-gp transport activity in the transfectants, we found that overexpression of caveolin-1 reversed drug resistance of transfectants and lowered their P-gp transport activity to the level of Hs578T/S. Taken together, our results indicate that such suppression of P-gp in the transfectants overexpressing caveolin-1 may occur at the transcriptional level.

Mechanisms of anticancer drug resistance

Chemotherapy agents are extremely important in the treatment of liquid malignancies, such as lymphoma, myeloma, and chronic lymphocytic leukemia. In addition, chemotherapy agents have proven effective in the adjuvant treatment of solid tumors, such as osteosarcoma, hemangiosarcoma, transitional cell carcinoma, and others. Unfortunately, chemotherapy resistance in these situations is the most significant cause of treatment failure. Therefore, the ability to predict, treat, or circumvent resistance is extremely likely to improve clinical outcomes. This article has reviewed the most widely investigated forms of chemotherapy resistance, such as reduced drug accumulation, increased DNA damage repair, decreased apoptosis, and others; however, new mechanisms are being found at an alarming pace. In addition, investigations to date have routinely centered on single-cell mechanisms of drug resistance, and cancer is truly a three dimensional disease. The elucidation of mechanisms surrounding (1) how tumors interact with their normal microenvironment, (2) how tumors interact in a three-dimensional environment, and (3) a better understanding of basic tumor physiology and biology may supersede in importance those previously elucidated single-cell mechanisms of chemoresistance.

Suppression of GST-P by treatment with glutathione-doxorubicin conjugate induces potent apoptosis in rat hepatoma cells

A conjugate of doxorubicin and glutathione via glutaraldehyde (GSH-DXR) inhibited glutathione S-transferase (GST) activity of rat hepatoma AH66 cells, and treatment of the cells with GSH-DXR induced caspase-3 activation and DNA fragmentation. After treatment of AH66 cells with 0.1 microM GSH-DXR, GST-P (placental type of rat GST isozymes) mRNA and its protein increased transiently and then decreased thereafter compared with the levels in nontreated cells. Caspase-3 activation and DNA fragmentation were induced following the suppression of GST-P expression by treatment with GSH-DXR. When the cells were treated with 100 microM ethacrynic acid (ECA), an inhibitor of GST, DNA fragmentation and caspase-3 activation were observed. In contrast, treatment of AH66 cells with a low concentration of ECA (1 microM) that showed little inhibition of GST activity induced slight, but significantly enhanced expression and activity of GST-P, and consequent prevention of DXR- and GSH-DXR-induced DNA fragmentation. Overexpression of GST-pi (placental type of human GST isozymes) by transfection of GST-pi sense cDNA into AH66 cells decreased sensitivities to DXR and GSH-DXR, and the suppression of GST-P by transfection of the antisense cDNA into the cells increased drug sensitivity. On the other hand, there was little change in drug sensitivity caused by overexpression of site-directedly mutated GST-P in which the active-site residue Tyr39 was replaced with His (W39H) or the substrate-binding site residue Cys48 was replaced with Ser (C48S) by transfection of those cDNAs into AH66 cells. These results suggested that the suppression of GST-P in AH66 cells treated with GSH-DXR must play an important role in the induction of apoptosis.

Glutathione-S-transferase-pi expression regulates sensitivity to glutathione-doxorubicin conjugate

We have reported that glutathione-doxorubicin conjugate (GSH-DXR) exhibited potent cytotoxicity against tumor cells and inhibited glutathione-S-transferase (GST) enzyme activity. In order to determine whether or not the expression of GST-pi lowered the cytotoxicity of GSH-DXR, cytocidal activity of the conjugate was examined using tumor cells in which the level of GST-pi expression was regulated by transfecting GST-pi cDNA in the correct or reverse direction and comparing with that of DXR. Enhancement of GST-pi expression by transfecting GST-pi sense cDNA into human hepatoblastoma HepG2 cells in which GST-pi expression was extremely low caused an increase in GST activity from 0.26 to 55.0 nmol/mg/min and a marked reduction in transfectant sensitivity to GSH-DXR to 1/120 (0.15-18 nM IC50) although the sensitivity to DXR was slightly decreased to 1/2.6 (380-990 nM IC50). By contrast, a high GST-pi-expressing human colon cancer cell line, HT29, showed a decrease in GST enzyme activity from 72.0 to 45.9 nmol/mg/min after transfecting GST-pi antisense cDNA and a marked improvement in transfectant sensitivity to GSH-DXR was observed (28-2.9 nM IC50) compared with the transfectant sensitivity to DXR (1020-320 nM IC50). Additionally, the expression of GST-pi in HepG2 cells caused a decrease in GSH-DXR-induced activation of caspase-3, which was an apoptotic marker, whereas the suppression of GST-pi in HT29 cells showed an increase in caspase-3 activation. These results suggested that the cytocidal efficacy of GSH-DXR, but not that of DXR, was controlled by the level of GST-pi expression in the cells.

Chemotherapy induces or increases expression of multidrug resistance-associated protein in malignant melanoma cells

BACKGROUND

Human malignant melanoma is notoriously resistant to chemotherapeutic agents. Melanoma-derived cell lines are often markedly chemoresistant, suggesting that cellular mechanisms mediate generation of the multidrug resistance (MDR) phenotype. This phenotype is often due to P-glycoprotein (Pgp) and the MDR-associated protein (MRP), which are drug transporter proteins associated with resistance to a broad spectrum of lipophilic drugs.

OBJECTIVES

To determine the relationships between the expression of the MDR gene MDR-1 (the product of which is Pgp) or the MRP gene, and clinical chemoresistance of malignant melanoma.

METHODS

We examined changes in the expression of MDR-1 and MRP genes at the mRNA level before and after chemotherapy by reverse transcription-polymerase chain reaction (RT-PCR) analysis using formalin-fixed, paraffin-embedded sections of 18 specimens taken from eight melanoma patients. mRNA expression of the MDR-1 and MRP gene-specific PCR products was quantitatively determined by densitometry and compared with that of an internal standard (beta-actin).

RESULTS

Five of seven primary melanomas were found to express the MRP gene to a certain extent even before chemotherapy. After first and second courses of chemotherapy, six patients had an increased ratio of MRP mRNA to beta-actin mRNA compared with the prechemotherapy levels in the same patients. None of the cases of melanoma expressed MDR-1.

CONCLUSIONS

These results suggest that a significant mRNA level of MRP gene was intrinsically present in malignant melanoma even before exposure to chemotherapeutic drugs and increased in its expression after chemotherapy, suggesting that MRP plays a part in increasing the chemoresistance of malignant melanoma during chemotherapy.

Development of daunorubicin resistance in tumour cells by induction of carbonyl reduction

A resistant descendant of the human stomach carcinoma cell line EPG85-257 was selected in the presence of increasing concentrations of daunorubicin (DRC). To avoid the expression and activity of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP), cells were cultured in the presence of verapamil. The resulting cells were used to evaluate an induced carbonyl reduction as a new determinant in DRC resistance. The MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide) toxicity assay was performed to estimate sensitivity to DRC in both cell lines. IC50 values of DRC increased almost 8-fold in the resistant descendants compared to the parental cell line. P-gp transcripts were detectable in both cell lines at only very low levels, and no significant alterations between sensitive and resistant cells were observed. MRP mRNA expression was markedly higher compared to P-gp mRNA, but, as was the case with P-gp, MRP mRNA levels in sensitive and resistant cells showed no alteration. This was probably due to the effect of the presence of verapamil during cell selection. Another known drug resistance factor, the lung resistance-related protein (LRP), was not at all detectable. Interestingly, resistant cells possessed 6-fold higher levels of DRC carbonyl-reducing activity, leading to the less toxic 13-hydroxy metabolite daunorubicinol (DRCOL). The 6-fold higher DRCOL formation roughly parallels the 8-fold increase in DRC IC50 values during cell selection, and therefore may account for DRC resistance in these cells. The determination of specific carbonyl reducing enzymes, known to be involved in DRC detoxification, revealed that mRNA expression of carbonyl reductase (EC 1.1.1.184), aldose reductase (EC 1.1.1.21), and dihydrodiol dehydrogenase 2 (EC 1.3.1.20) increased in the resistant descendant. In contrast, the phase II-conjugating enzyme activities of glutathione S-transferases were significantly lower in resistant than in sensitive cells, whereas those of glucuronosyl transferase were not detectable in either cell line. Apparently, conjugating enzymes are not involved in DRC resistance in human stomach carcinoma cells. These studies indicate that DRC resistance in human stomach carcinoma cells may appear as a result of an induction of metabolic DRC inactivation via carbonyl reduction to the less active 13-hydroxy metabolite DRCOL.

Differential sensitivity of human mammary epithelial and breast carcinoma cell lines to curcumin

Curcumin has anti-inflammatory, antiproliferative, and antitumor effects. To understand the chemopreventive mechanism of curcumin against human malignancies, the cellular and molecular changes induced by this agent in human mammary epithelial (MCF-10A) and breast carcinoma (MCF- 7/TH) cell lines were investigated. The human multidrug- resistant breast cancer cell line was 3.5 fold more sensitive to curcumin than the mammary epithelial cell line. Even though both cell lines accumulated a similar amount of curcumin, a significantly higher percentage of apoptotic cells was induced in breast cancer cells compared to a very low percentage of apoptosis in mammary epithelial cells. Incubation of breast cancer cells with 20 and 40 microM curcumin for 24 h induced G2 block and sub-G0/G1 cell population, respectively. Curcumin treatment caused a reduction in the expression of Ki67, PCNA, and p53 mRNAs in breast cancer cells. The human mammary epithelial cell line showed a down-regulation of p21 mRNA and an up-regulation of Bax mRNA expression with curcumin treatment. The results suggest that apoptosis is involved in the curcumin-induced inhibition of tumor cell growth, and genes associated with cell proliferation and apoptosis may be playing a role in the chemopreventive action of curcumin.

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.

Inhibition of glutathione S-transferase activity in human melanoma cells by alpha,beta-unsaturated carbonyl derivatives. Effects of acrolein, cinnamaldehyde, citral, crotonaldehyde, curcumin, ethacrynic acid, and trans-2-hexenal

The glutathione S-transferase (GST) activity towards 1-chloro-2,4-dinitrobenzene in intact human IGR-39 melanoma cells was determined by the quantification by HPLC-analysis of the excreted glutathione (GSH) conjugate (S-(2,4-dinitrophenyl)glutathione; DNPSG). The major GST subunit expressed in these melanoma cells is the pi-class GST subunit P1. Using this system, the effect of exposure for 1 h to a series of alpha, beta-unsaturated carbonyl compounds at non-toxic concentrations was studied. Curcumin was the most potent inhibitor (96% inhibition at 25 microM), while 67 and 61% inhibition at 25 microM was observed for ethacrynic acid and trans-2-hexenal, respectively. Moderate inhibition was observed for cinnamaldehyde and crotonaldehyde, while no inhibition was found for citral. The reactive acrolein did not inhibit the DNPSG-excretion at 2.5 microM, the highest non-toxic concentration. Up to about 50% GSH-depletion was found after treatment with crotonaldehyde, curcumin and ethacrynic acid, however the consequences for GST conjugation are presumably small. Reversible inhibition of GST was the major mechanism of inhibition of DNPSG-excretion in melanoma cells, except in the cases of curcumin and ethacrynic acid, which compounds also inactivated GSTP1-1 by covalent modification. This was clear from the fact that depending on the dose between 30 and 80% inhibition was still observed after lysis of the cells, under which conditions reversible inhibition was is absent. Intracellular levels of DNPSG remained relatively high in the case of ethacrynic acid. It is possible that ethacrynic acid also inhibits the transport of DNPSG by inhibition of the multidrug resistance-associated protein gene encoding glutathione conjugate export pump (MRP/GS-X pump) in some way.

Induction of daunorubicin carbonyl reducing enzymes by daunorubicin in sensitive and resistant pancreas carcinoma cells

Daunorubicin (DRC) and other anthracyclines are valuable cytotoxic agents in the clinical treatment of certain malignancies. However, as is the case with virtually all anticancer drugs, tumor cell resistance to these agents is one of the major obstacles to successful chemotherapy. In addition to an increased energy-dependent efflux of chemotherapeutic agents, enzymatic drug-inactivating mechanisms also contribute to multidrug resistance of tumor cells. In the case of DRC, carbonyl reduction leads to 13-hydroxydaunorubicinol (DRCOL), the major metabolite of DRC with a significantly lower antineoplastic potency compared to the parent drug. In the present study, we compared two pancreas carcinoma cell lines (a DRC-sensitive parental line and its DRC-resistant subline) with respect to their capacity of DRC inactivation via carbonyl reduction. In addition, we cultured the two cell lines in the presence of increasing sublethal concentrations of DRC. Evidence is presented that DRC treatment itself leads to a concentration-dependent induction of DRC carbonyl reduction in subcellular fractions of both the sensitive and resistant pancreas carcinoma cells, resulting, surprisingly, in different susceptibilities to DRC. The principal difference between the two cell lines becomes most apparent at high-dose DRC supplementation (1 microgram/mL), at which DRC resistant cells exhibited higher inducibility of DRC-inactivating enzymes, whereas respective sensitive cells already showed an impairment of cellular viability. The use of the diagnostic model substrates metyrapone and p-nitrobenzaldehyde reveals that this adaptive enhancement of DRC inactivation can be attributed to the induction of DRC carbonyl reductases different from known aldehyde and carbonyl reductases. In conclusion, these findings suggest that inactivation of anthracyclines by carbonyl reduction is inducible by the substrate itself, a fact that might be considered as one of the enzymatic mechanisms that contribute to the acquired resistance to these drugs.

Tyrosine and phenylalanine restriction sensitizes adriamycin-resistant P388 leukemia cells to adriamycin

Cancer chemotherapy frequently fails, because tumors develop multiple drug resistance (MDR). Pharmacological efforts to reverse this MDR phenotype and sensitize resistant tumor cells have utilized verapamil (VER) to inhibit p-glycoprotein function and buthionine sulfoximine (BSO) to inhibit glutathione synthesis. Our previous results indicate that restriction of two amino acids, tyrosine (Tyr) and phenylalanine (Phe), may potentially suppress the MDR phenotype. These results show that in vivo Tyr and Phe restriction improves the therapeutic response of a metastatic variant of B16-BL6 (BL6) murine melanoma to adriamycin (ADR) and B16 melanoma to levodopa methyl ester. We examine whether in vitro limitation of Tyr and Phe suppresses ADR resistance of BL6 cells and whether Tyr-Phe modulation of the MDR phenotype is applicable to other tumor types, particularly P388 murine leukemia. Mechanisms underlying Tyr-Phe modulation of ADR resistance are examined in the presence of VER and BSO, singly and in combination. Our results indicate that in vitro Tyr and Phe restriction has no effect on BL6 resistance to ADR. However, Tyr and Phe restriction does increase the sensitivity of ADR-resistant P388 cells to ADR without affecting drug efflux, ADR uptake, or glutathione levels. In addition, this enhanced ADR sensitivity of P388 cells is even more pronounced in the presence of BSO. Suppression of ADR resistance in P388-resistant cells by Tyr and Phe restriction indicates that Tyr- and Phe-mediated modulation of the MDR phenotype is possible and that Tyr and Phe restriction may be useful as a potential adjuvant to effective cancer chemotherapy.

Qualitatively different mechanisms of resistance to doxorubicin, both involving altered glutathione pools, in two myeloid cell lines in vitro

Subclones of the two well-characterized myeloid cell lines HL-60 and KG1a were selected for doxorubicin resistance by systematic exposure to increased concentrations of the drug in vitro. Both subclones demonstrated a threefold increased resistance to the drug as evident from cell growth in liquid culture and clonogenicity in a semisolid matrix. Both resistant subclones displayed a similar degree of reduced total and nuclear doxorubicin levels. The HL-60 and the KG1a cells differed qualitatively and quantitatively with respect to glutathione (GSH) levels during culture, with markedly elevated concentrations in the resistant HL-60 subclone during 1 week of culture. Total GSH pools in resistant and sensitive KG1a cells were similar, but maximum GSH levels were reached earlier in the resistant KG1a clones than in the parental cells. Northern blot analysis suggests that resistance was accompanied by increased mdr1 expression in the KG1a but not in the HL-60 cells, whereas alterations in the glutathione S-transferase P1-1 and topoisomerase II message was evident in the latter. The results demonstrate the complex, multifactorial mechanisms behind the in vitro induction of even moderate resistance in anthracyclines.

Expression of drug resistance-associated mdr-1, GST pi, and topoisomerase II genes during cell cycle traverse

The expression of drug resistance-associated mdr-1, GST pi, and topoisomerase II genes was analyzed in cell cycle phase enriched populations of doxorubicin-resistant murine leukemic P388/R-84 cells. Flow cytometric analysis of bromodeoxyuridine (BrdU) incorporation and staining with anti-BrdU antibodies was used to confirm the purity of cell cycle phase enriched populations obtained by centrifugal elutriation. Doxorubicin (DOX) and daunorubicin (DNR) accumulation was significantly lower in S-phase cells, and coincubation with verapamil (VPL) or chlorpromazine (CPZ) enhanced DOX and DNR accumulation more in S-phase than in G1- and G2/M-phase cells. While the cellular content of mdr-1 and topoisomerase II mRNAs changed, GST pi mRNA content remained constant during the cell cycle. S-phase cells had about 3-fold higher mdr-1 mRNA content than G1- and G2/M-phase cells. In G1 cells, P-glycoprotein expression, as determined by C219 monoclonal antibody, was 12% less than that of S and G2/M cells. Topoisomerase II mRNA content increased with the progression of cell cycle and peaked in G2/M cells. These observations suggest that cell cycle stage related changes in expression of drug resistance markers may have a major bearing on chemosensitivity of drug-resistant cells.

Intrinsic MDR-1 gene and P-glycoprotein expression in human melanoma cell lines

Metastatic malignant melanoma is considered a chemotherapy-refractory malignancy. A few previous studies have delivered contradictory results regarding the presence and functionality of P-glycoprotein (P-gp), a transmembranous protein associated with the classical multidrug resistance (cMDR), in malignant melanoma. Therefore we have investigated this issue on 33 cell lines established from primary and metastatic lesions of human malignant melanoma, comparing different cMDR detection methods. Immunocytochemically 33% of the cell lines stained positive for P-gp. The data correlated with those of a P-gp-radioimmunometric (antibody-binding) assay. When RT-PCR was used for MDR-1 mRNA determination, 76% of the melanoma cell lines scored positive. Slot-blot analysis was seen to be less sensitive than RT-PCR. Results from the functional P-gp assays, using daunomycin (DM) as MDR-substrate, showed no influence of P-gp expression on drug accumulation and cytotoxicity. However, the cMDR-modifier verapamil (VP) significantly increased both parameters in those melanoma cells with the highest P-gp levels. We conclude that cMDR is apparently not the decisive but probably a complementary protective mechanism against toxic agents in malignant melanoma.

Doxorubicin retention and chemoresistance in human mesothelioma cell lines

Eight cell lines were established from the pleural effusion of 4 patients with malignant mesothelioma. The most sensitive (FCCMES-4) and the most resistant (FCCMES-2) mesothelioma cell lines had IC50 of 0.66 and 1.85 microM for doxorubicin in clonogenic assays, respectively. In comparison with murine leukemic P388 cells, mesothelioma cell lines were 7.5- to 21-fold more resistant to doxorubicin. Co-incubation with verapamil significantly increased doxorubicin retention in one of the cell lines (FCCMES-2) expressing P-glycoprotein in 16.8% of the cells. These results indicate that doxorubicin resistance may be intrinsic in refractory mesothelioma patients and P-glycoprotein-mediated drug efflux may be involved in resistance of some of the mesotheliomas.

Inhibition of human glutathione S-transferases by dopamine, alpha-methyldopa and their 5-S-glutathionyl conjugates

The reversible and irreversible inhibition of human glutathione S-transferases (GST) by dopamine, alpha-methyldopa and their 5-S-glutathionyl conjugates (termed 5-GSDA and 5-GSMDOPA, respectively) was studied using purified isoenzymes. The reversible inhibition, using CDNB as substrate and expressed as I50, ranged from 0.18-0.24 (GST M1a-1a), 0.19-0.24 (GST M1b-1b) to 0.5-0.54 mM (GST A1-1) for 5-GSDA and 5-GSMDOPA, respectively. About 20% inhibition was observed for GST A2-2 and P1-1, using 0.5 mM of both 5-GSDA and 5-GSMDOPA. No significant reversible inhibition was observed with dopamine and alpha-methyldopa. Tyrosinase was used to generate ortho-quinones from dopamine and alpha-methyldopa which may bind covalently to GST and thereupon irreversibly inhibit GST. In this respect, GST P1-1 was by far the most sensitive enzyme. The inhibition (expressed as a % of control) after incubating 0.5 microM GST in the presence of 100 units/ml tyrosinase with 5 microM of the catecholamines for 10 min at 25 degrees, was 99% and 67% for dopamine and alpha-methyldopa, respectively. Moderate irreversible inhibition of GST A1-1 by both dopamine and alpha-methyldopa (33% and 25%, respectively), and of GST M1b-1b by dopamine (45%) was also observed. GST P1-1 is also the only isoenzyme susceptible to irreversible inhibition by 5-GSDA (33% inhibition), while no significant inhibition was observed with 5-GSMDOPA. A minor part of the inhibition by dopamine (23%), and the complete inhibition by 5-GSDA was restored by reduction with dithiotreitol. This suggests that GST P1-1 is inhibited by disulfide formation in the case of 5-GSDA, while this oxidative pathway also substantially contributes to the inactivation by dopamine. This was supported by the HPLC-profile of the GST P1-1 subunit which was strongly affected by dopamine, while for 5-GSDA after reduction with dithiotreitol the original elution profile of the subunit returned.

Human cell lines as models for multidrug resistance in solid tumours

In spite of our expanding knowledge on the molecular biology of cancer, relatively little progress has been made in improving therapy for the solid tumours which are major killers, e.g., lung, colon, breast. Significant advances over the past 10-15 years in chemotherapy of some tumours such as testicular cancer and some leukaemias indicates that, in spite of the undesirable side-effects, chemotherapy has the potential to effect cure in the majority of patients with certain types of cancer. Multidrug resistance, inherent or acquired, is one important limiting factor in extending this success to most solid tumours. In vitro studies described in this review are now uncovering a diversity of possible mechanisms of cross-resistance to different types of drug. Sensitive methods such as immunocytochemistry, RT-PCR or in situ RNA hybridisation may be necessary to identify corresponding changes in clinical material. Only by classifying individual tumours according to their specific resistance mechanisms will it be possible to define the multidrug resistance problem properly. Such rigorous definition is a prerequisite to design (and choice on an individual basis) of specific therapies suited to individual patients. Since a much larger proportion of cancer biopsies should be susceptible to accurate analysis by the immunochemical and molecular biological techniques described above than to direct assessment of drug response, it seems reasonable to hope that this approach will succeed in improving results for cancer chemotherapy of solid tumours where other approaches such as individualised in vitro chemosensitivity testing have essentially failed. Results from clinical trials using cyclosporin A or verapamil are encouraging, but these agents are far from ideal, and reverse resistance in only a subset of resistant tumours. Proper definition of the other mechanisms of MDR, and how to antagonize them, is an urgent research priority.