Heat-shock responsive elements in the induction of the multidrug resistance gene (MDR1)

HIF1, HSF1, and NRF2: Oxidant-Responsive Trio Raising Cellular Defenses and Engaging Immune System

Cellular homeostasis is continuously challenged by damage from reactive oxygen species (ROS) and numerous reactive electrophiles. Human cells contain various protective systems that are upregulated in response to protein damage by electrophilic or oxidative stress. In addition to the NRF2-mediated antioxidant response, ROS and reactive electrophiles also activate HSF1 and HIF1 that control heat shock response and hypoxia response, respectively. Here, we review chemical and biological mechanisms of activation of these three transcription factors by ROS/reactive toxicants and the roles of their gene expression programs in antioxidant protection. We also discuss how NRF2, HSF1, and HIF1 responses establish multilayered cellular defenses consisting of largely nonoverlapping programs, which mitigates limitations of each response. Some innate immunity links in these stress responses help eliminate damaged cells, whereas others suppress deleterious inflammation in normal tissues but inhibit immunosurveillance of cancer cells in tumors.

Heat Shock Proteins and HSF1 in Cancer

Fitness of cells is dependent on protein homeostasis which is maintained by cooperative activities of protein chaperones and proteolytic machinery. Upon encountering protein-damaging conditions, cells activate the heat-shock response (HSR) which involves HSF1-mediated transcriptional upregulation of a group of chaperones - the heat shock proteins (HSPs). Cancer cells experience high levels of proteotoxic stress due to the production of mutated proteins, aneuploidy-induced excess of components of multiprotein complexes, increased translation rates, and dysregulated metabolism. To cope with this chronic state of proteotoxic stress, cancers almost invariably upregulate major components of HSR, including HSF1 and individual HSPs. Some oncogenic programs show dependence or coupling with a particular HSR factor (such as frequent coamplification of HSF1 and MYC genes). Elevated levels of HSPs and HSF1 are typically associated with drug resistance and poor clinical outcomes in various malignancies. The non-oncogene dependence ("addiction") on protein quality controls represents a pancancer target in treating human malignancies, offering a potential to enhance efficacy of standard and targeted chemotherapy and immune checkpoint inhibitors. In cancers with specific dependencies, HSR components can serve as alternative targets to poorly druggable oncogenic drivers.

Decreased expression of FBXW7 by ERK1/2 activation in drug-resistant cancer cells confers transcriptional activation of MDR1 by suppression of ubiquitin degradation of HSF1

The acquisition of MDR1-mediated chemoresistance poses a major obstacle to the success of conventional chemotherapeutic agents. HSF1 is also involved in chemoresistance, and several studies have demonstrated the relationship between HSF1 and MDR1 but without any consistent results. Paclitaxel- and doxorubicin-resistant cancer cells showed higher expression of MDR1 and HSF1. Depletion of HSF1 decreased mdr1 expression at mRNA level, and HSF1 directly interacted with the promoter site of mdr1, suggesting its role as a transcriptional regulator of MDR1. Phosphorylation of Ser303/307, which was involved in protein stability of HSF1 by FBXW7-mediated degradation, was found to be important for transcriptional activation of mdr1. Drug-resistant cells showed decreased expression of FBXW7, which was mediated by the activation of ERK1/2, thus indicating that over-activation of ERK1/2 in drug-resistant cells decreased FBXW7 protein stability, which finally inhibited protein degradation of pHSF1 at Ser303/307. There was a positive correlation between immunofluorescence data of pHSF1 at Ser303/307 and MDR1 in carcinogen-induced rat mammary tumors and human lung cancers. These findings identified the post-translational mechanisms of HSF1 transcription in MDR1 regulation of drug resistance development.

Vitexin induces apoptosis by suppressing autophagy in multi-drug resistant colorectal cancer cells

Cancer treatment is limited due to the diverse multidrug resistance acquired by cancer cells and the collateral damage caused to adjacent normal cells by chemotherapy. The flavonoid compound vitexin exhibits anti-oxidative, anti-inflammatory and anti-tumor activity. This study elucidated the antitumor effects of vitexin and its underlying mechanisms in a multi-drug resistant human colon cancer cell line (HCT-116^DR), which exhibits higher levels of multidrug-resistant protein 1 (MDR1) expression as compared with its parental cell line (HCT-116). Here, we observed that vitexin suppressed MDR-1 expression and activity in HCT-116^DR cells and showed cytotoxic effect in HCT-116^DR cells by inhibiting autophagy and inducing apoptosis in a concentration-dependent manner. Additionally, vitexin treatment caused cleavage of caspase-9 and caspase-3, and upregulated the expression of the pro-apoptotic proteins, BID and Bax. Moreover, the expression of autophagy-related proteins, such as ATG5, Beclin-1 and LC3-II, was markedly reduced by vitexin treatment. Furthermore, in vivo experiments showed that vitexin induced apoptosis and suppressed tumor growth in HCT-116^DR xenograft model. These results revealed that vitexin induced apoptosis through suppression of autophagy in vitro and in vivo and provide insight into the therapeutic potential of vitexin for the treatment of chemo-resistant colorectal cancer.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Overexpression of Heat Shock Transcription Factor 1 enhances the resistance of melanoma cells to doxorubicin and paclitaxel

Background

Heat Shock Transcription Factor 1 (HSF1) is activated under stress conditions. In turn, it induces expression of Heat Shock Proteins (HSPs), which are well-known regulators of protein homeostasis. Elevated levels of HSF1 and HSPs were observed in many types of tumors. The aim of the present study was to determine whether HSF1 could have an effect on the survival of cancer cells treated with chemotherapeutic cytotoxic agents.

Methods

We constructed mouse (B16F10) and human (1205Lu, WM793B) melanoma cells overexpressing full or mutant form of human HSF1: a constitutively active one with a deletion in regulatory domain or a dominant negative one with a deletion in the activation domain. The impact of different forms of HSF1 on the expression of HSP and ABC genes was studied by RT-PCR and Western blotting. Cell cultures were treated with increasing amounts of doxorubicin, paclitaxel, cisplatin, vinblastine or bortezomib. Cell viability was determined by MTT, and IC₅₀ was calculated. Cellular accumulation of fluorescent dyes and side population cells were studied using flow cytometry.

Results

Cells overexpressing HSF1 and characterized by increased HSPs accumulation were more resistant to doxorubicin or paclitaxel, but not to cisplatin, vinblastine or bortezomib. This resistance correlated with the enhanced efflux of fluorescent dyes and the increased number of side population cells. The expression of constitutively active mutant HSF1, also resulting in HSPs overproduction, did not reduce the sensitivity of melanoma cells to drugs, unlike in the case of dominant negative form expression. Cells overexpressing a full or dominant negative form of HSF1, but not a constitutively active one, had higher transcription levels of ABC genes when compared to control cells.

Conclusions

HSF1 overexpression facilitates the survival of melanoma cells treated with doxorubicin or paclitaxel. However, HSF1-mediated chemoresistance is not dependent on HSPs accumulation but on an increased potential for drug efflux by ABC transporters. Direct transcriptional activity of HSF1 is not necessary for increased expression of ABC genes, which is probably mediated by HSF1 regulatory domain.

Inhibition of P-Glycoprotein Function and Expression by Kaempferol and Quercetin

The 170 kDa plasma membrane P-glycoprotein (Pgp) causes the efflux of chemotherapeutic drugs from cells and is believed to be an important mechanism in multidrug resistance (MDR) in human cancer. This study demonstrates that some putative flavonoids, i.e., flavonols (quercetin and kaempferol) and isoflavones (genistein and daidzein) markedly increase the sensitivity of the multidrug-resistant human cervical carcinoma KB-V1 cells (high Pgp expression) to vinblastine and paclitaxel dose-dependently, and also decrease the relative resistance of these anti-cancer-drugs in KB-V1 cells. None of the flavonoids had a significant effect on vinblastine and paclitaxel cytotoxicity in wildtype drug-sensitive KB-3-1 cells (lacking Pgp). These flavonoids also caused an increase in intracellular accumulation, and reduced the efflux of Rh123 and 3[H]vinblastine in KB-V1 cells, but not in KB-3-1 cells. The flavonols increased the inhibitory effectiveness of Pgp activity in MDR KB-V1 cells more than isoflavones. Only treatment with flavonols up to 48 h was able to significantly decrease the Pgp expression in a dose-dependent manner in KB-V1 cells. These findings provide evidence that flavonols reduced Pgp expression and function resulting in the inhibition of Pgp activity, but isoflavones modulated intracellular drug levels by inhibiting Pgp function with no effect on Pgp expression. Among the flavonoids tested, flavonols, particularly kaempferol, exhibit the most potent MDR reversing property in KB-V1 cells.

P-glycoprotein (P-gp) in the monogonont rotifer, Brachionus koreanus: molecular characterization and expression in response to pharmaceuticals

P-glycoprotein is involved in the efflux of diverse chemicals, including hydrophobic compounds and pharmaceuticals as a first line of defense. Here, we firstly identified and characterized the P-gp (Bk-P-gp) gene in the rotifer, Brachionus koreanus. Bk-P-gp was highly conserved in genomic organization compared to the human P-gp gene. Messenger RNA expression of Bk-P-gp revealed that it would be regulated by temperature change via 14 heat shock response elements in its promoter region. Bk-P-gp showed a high similarity of motifs/domains compared to those of vertebrates in its amino acid sequences. To check whether Bk-P-gp would be inducible, we exposed B. koreanus to six pharmaceuticals including antibiotics for use in aquaculture and observed dose- and time-dependency on transcripts of Bk-P-gp for 24h over a wide range of concentration. Efflux assay and membrane topology supported its conserved function for transportation of a number of chemicals upon cellular damage. To reveal the effect of pharmaceuticals on the rotifer, we measured survival rate and population growth rate after exposure to six pharmaceuticals. In an acute toxicity test, both NOEC and LC₅₀ values for all the pharmaceuticals were high for 24 h. ATP, CBZ, SMX, and TMP markedly inhibited the population growth of B. koreanus after exposure up to 100 mg/L for 10 days. In this paper, we demonstrated that various pharmaceuticals can retard growth rate with up-regulation of the P-gp gene as a cellular defense system. This finding provides a better understanding of molecular mechanisms involved in pharmaceutical-mediated cellular damage in B. koreanus.

Expression of chemoresistance-related genes and heat shock protein 72 in hyperthermic isolated limb perfusion of malignant melanoma: an experimental study

Hyperthermic isolated limb perfusion (HILP) is considered an established treatment for multiple locoregional intransit metastases in malignant melanoma of the extremities. Various mechanisms such as the expression of chemoresistance genes and heat shock proteins by the tumor may be responsible for varying response rates and locoregional recurrences of the treatment. The aim of the experimental animal study was to investigate the direct impact of HILP on such mechanisms of resistance. Tissue temperature, administration of the cytostatic drug, and duration of perfusion were varied. Expression of the chemoresistance genes mdr1, mrp1, mrp2, and lrp and of heat shock protein 72 (HSP72) in the tumor tissue was analysed using RT-PCR and western blot analysis. The untreated SK-MEL-3 tumor expressed mdr1, mrp1, and lrp, but not mrp2. Neither variation of temperature, administration of the cytostatic drug, nor duration of perfusion changed the expression of this “resistance pattern”. In contrast to the cytostatic drug, hyperthermia causes a persistent induction of HSP72. Both observations could offer a potential explanation for failure of HILP in malignant melanoma.

Heat-responsive gene expression for gene therapy

Therapy-inducible vectors are useful for conditional expression of therapeutic genes in gene therapy, which is based on the control of gene expression by conventional treatment modalities. By this approach, combination of chemotherapy, radiation or hyperthermia with gene therapy can result in considerable, additive or synergistic improvement of therapeutic efficacy. This concept has been successfully tested in particular for gene therapy of cancer. The identification of efficient heat-responsive gene promoters provided the rationale for heat-regulated gene therapy. The objective of this review is to provide insights into the cellular mechanisms of heat-shock response, as prerequisite for therapeutic actions of hyperthermia and into the field of heat-responsive gene therapy. Furthermore, the major strategies of heat-responsive gene therapy systems in particular for cancer treatment are summarized. The developments for heat-responsive vector systems for in vitro and in vivo approaches are discussed. This review will provide an overview for this gene therapy strategy and its potential for multimodal therapeutic concepts in the clinic.

P-Glycoprotein-mediated resistance to Hsp90-directed therapy is eclipsed by the heat shock response

Despite studies that show the antitumor activity of Hsp90 inhibitors, such as geldanamycin (GA) and its derivative 17-allylamino-demethoxygeldanamycin (17-AAG), recent reports indicate that these inhibitors lack significant single-agent clinical activity. Resistance to Hsp90 inhibitors has been previously linked to expression of P-glycoprotein (P-gp) and the multidrug resistant (MDR) phenotype. However, the stress response induced by GA treatment can also cause resistance to Hsp90-targeted therapy. Therefore, we chose to further investigate the relative importance of P-gp and the stress response in 17-AAG resistance. Colony-forming assays revealed that high expression of P-gp could increase the 17-AAG IC(50) 6-fold in cells transfected with P-gp compared with parent cells. A549 cells selected for resistance to GA overexpressed P-gp, but verapamil did not reverse the resistance. These cells also overexpressed Hsp27, and Hsp70 was induced with 17-AAG treatment. When the GA and 17-AAG resistant cells were transfected with Hsp27 and/or Hsp70 small interfering RNA (siRNA), the 17-AAG IC(50) decreased 10-fold compared with control transfected cells. Transfection with siRNA directed against Hsp27, Hsp70, or Hsp27 and Hsp70 also increased sensitivity to EC78, a purine scaffold-based Hsp90 inhibitor that is not a P-gp substrate. We conclude that P-gp may contribute, in part, to resistance to 17-AAG, but induction of stress response proteins, such as Hsp27 and Hsp70, by Hsp90-targeted therapy plays a larger role. Taken together, our results indicate that targeting of Hsp27 and Hsp70 should be exploited to increase the clinical efficacy of Hsp90-directed therapy.

The Effects of Crinum asiaticum on the Apoptosis Induction and the Reversal of Multidrug Resistance in HL-60/MX2

The present study investigated the anti-proliferative and chemosensitizing effects of Crinum asiaticum var. japonicum against multi-drug resistant (MDR) cancer cells. The 80% methanol extract, chloroform (CHCI3) fraction and butanol (BuOH) fraction of C asiaticum inhibited the growth of mitoxantrone (MX) resistant HL-60 (HL-60/MX2) cells. When HL-60/MX2 cells were treated with the CHCI3 and BuOH fractions, DNA ladder and sub-G1 hypodiploid cells were observed. Furthermore, the fractions reduced Bcl-2 mRNA levels, whereas Bax mRNA levels were increased. These results suggest that the inhibitory effect of C. asiaticum on the growth of the HL-60/MX2 cells might arise from the induction of apoptosis. Treatment of HL-60/MX2 cells with the fractions markedly decreased the mRNA levels of the multi-drug resistance protein-1 and breast cancer resistance protein. The CHCI3 fraction and hexane fraction increased MX accumulation in HL-60/MX2 cells. These results imply that the CHCI3 fraction of C asiaticum plays a pivotal role as a chemosensitizer. We suggest that components of C asiaticum might have a therapeutic potential for the treatment of MDR leukemia.

Genomic variation at the MDR1 promoter and P-glycoprotein expression and activity in AML patients

Sequence variation at the proximal MDR1 promoter of 72 patients with acute myeloid leukemia (AML) was investigated and its association with P-glycoprotein (Pgp) expression and activity using flow cytometry were analyzed. Two variants were found: -129T/C and a non-described A/T substitution at position +68 of intron 1 in one patient. Three different genotypes were identified for single nucleotide polymorphism (SNP) -129T/C: 60 patients TT, 11 individuals TC, and 1 CC. No significant association was found between SNP variants and Pgp activity and expression, at protein level. Our data also suggested that an evaluation of MDR1 promoter polymorphisms is of uncertain prognostic value.

Curcumin as chemosensitizer

This overview presents curcumin as a significant chemosensitizer in cancer chemotherapy. Although the review focuses on curcumin and its analogues on multidrug resistance (MDR) reversal, the relevance of curcumin as a nuclear factor (NF)-KB blocker and sensitizer of many chemoresistant cancer cell lines to chemotherapeutic agents will also be discussed. One of the major mechanisms of MDR is the enhanced ability of tumor cells to actively efflux drugs, leading to a decrease in cellular drug accumulation below toxic levels. Active drug efflux is mediated by several members of the ATP-binding cassette (ABC) superfamily of membrane transporters, which have now been subdivided into seven families designated A through G. Among these ABC families, the classical MDR is attributed to the elevated expression of ABCB1 (Pgp), ABCC1 (MRP1), and ABCG2 (MXR). The clinical importance of Pgp, MRP1, and MXR for MDR and cancer treatment has led to the investigation of the inhibiting properties of several compounds on these transporters. At present, due in part to the disappointing results associated with the many side effects of synthetic modulators that have been used in clinical trials, current research efforts are directed toward the identification of novel compounds, with attention to dietary natural products. The advantage is that they exhibit little or virtually no side effects and do not further increase the patient's medication burden.

Heat-inducible in vivo gene therapy of colon carcinoma by human mdr1 promoter-regulated tumor necrosis factor-alpha expression

The promoter of the human multidrug resistance gene (mdr1) harbors defined heat-responsive elements, which could be exploited for construction of heat-inducible expression vectors. To analyze the hyperthermia inducibility of the mdr1 promoter in vitro and in vivo, we used the pcDNA3-mdrp-hTNF vector construct for heat-induced tumor necrosis factor alpha (TNF-alpha) expression in transfected HCT116 human colon carcinoma cells at mRNA level by quantitative real-time reverse transcription-PCR and at protein level by TNF-alpha ELISA. For the in vitro studies, the pcDNA3-mdrp-hTNF-transfected tumor cells were treated with hyperthermia at 43 degrees C for 2 h. In the animal studies, stably transfected or in vivo jet-injected tumor-bearing Ncr:nu/nu mice were treated for 60 min at 42 degrees C to induce TNF-alpha expression. Both the in vitro and in vivo experiments show that hyperthermia activates the mdr1 promoter in a temperature- and time-dependent manner, leading to an up to 4-fold increase in mdr1 promoter-driven TNF-alpha expression at mRNA and an up to 3-fold increase at protein level. The in vivo heat-induced TNF-alpha expression combined with Adriamycin (8 mg/kg) treatment leads to the inhibition of tumor growth in the animals. These experiments support the idea that heat-induced mdr1 promoter-driven expression of therapeutic genes is efficient and feasible for combined cancer gene therapy approaches.

Expression of the breast cancer resistance protein (Bcrp1/Abcg2) in tissues from pregnant mice: effects of pregnancy and correlations with nuclear receptors

The breast cancer resistance protein (BCRP) plays an important role in drug disposition, including limiting drug penetration across the placental barrier. Our goal was to investigate the effects of pregnancy on Bcrp1 expression in pregnant mice. We examined Bcrp1 expression in placenta, kidney, liver, and small intestine at various gestational ages. Bcrp1 protein levels peaked at gestation day (gd) 15 in placenta, at gd 10 and 15 in kidney, and at gd 15 in liver; however, Bcrp1 protein levels in small intestine did not change significantly with gestational ages. Immunohistochemistry analysis revealed that the cellular localization of Bcrp1 in placenta, kidney, liver, and small intestine was not influenced by pregnancy. Bcrp1 mRNA levels were analyzed by quantitative real-time RT-PCR. In general, the effects of pregnancy on Bcrp1 protein somewhat lagged behind the effects on Bcrp1 mRNA. To further investigate the possible roles of nuclear receptors in the regulation of the Bcrp1 gene during pregnancy, we examined mRNA levels of aryl hydrocarbon receptor (AhR), hypoxia-inducible factor 1alpha (HIF1alpha), estrogen receptor alpha (ERalpha), estrogen receptor beta (ERbeta), or progesterone receptor and compared them with those of Bcrp1. Bcrp1 mRNA was significantly correlated with mRNA of AhR, HIF1alpha, and ERbeta in placenta, with mRNA of HIF1alpha in kidney, and with mRNA of AhR and ERalpha in liver. These data suggest that Bcrp1 expression in mouse tissues can be altered by pregnancy in a gestational age-dependent manner. Such effects are likely mediated by certain nuclear receptors through a transcriptional mechanism.

Heat shock-independent induction of multidrug resistance by heat shock factor 1

The screening of two different retroviral cDNA expression libraries to select genes that confer constitutive doxorubicin resistance has in both cases resulted in the isolation of the heat shock factor 1 (HSF1) transcription factor. We show that HSF1 induces a multidrug resistance phenotype that occurs in the absence of heat shock or cellular stress and is mediated at least in part through the constitutive activation of the multidrug resistance gene 1 (MDR-1). This drug resistance phenotype does not correlate with an increased expression of heat shock-responsive genes (heat shock protein genes, or HSPs). In addition, HSF1 mutants lacking HSP gene activation are also capable of conferring multidrug resistance, and only hypophosphorylated HSF1 complexes accumulate in transduced cells. Our results indicate that HSF1 can activate MDR-1 expression in a stress-independent manner that differs from the canonical heat shock-activated mechanism involved in HSP induction. We further provide evidence that the induction of MDR-1 expression occurs at a posttranscriptional level, revealing a novel undocumented role for hypophosphorylated HSF1 in posttranscriptional gene regulation.

EXPRESSION AND FUNCTIONAL ACTIVITY OF BREAST CANCER RESISTANCE PROTEIN (BCRP, ABCG2) TRANSPORTER IN THE HUMAN CHORIOCARCINOMA CELL LINE BEWO

1. Breast cancer resistance protein (BCRP, ABCG2) is a drug efflux transporter that is believed to affect the drug disposition of several drugs and xenobiotics. In the present study, we evaluated the localization and functional expression of BCRP in the human choriocarcinoma cell line BeWo, an in vitro model of the human trophoblast, and compared it with the expression of P-glycoprotein (MDR1, ABCB1) as the most widely studied placental transporter. In addition, the expression of BCRP at the mRNA level was compared with that of MDR1 in the human term placenta. 2. Western blotting analysis revealed high endogenous expression of BCRP protein in BeWo cells. Using indirect immunofluorescence microscopy, we found that the BCRP transporter appears to be localized predominantly at the apical plasma membrane. Functional studies showed a significant effect of the BCRP inhibitors GF120918 (5 micromol/L) and Ko143 (1 micromol/L) on mitoxantrone accumulation and, thus, confirmed efflux activity of BCRP in BeWo cells. 3. Using absolute mRNA quantification with real-time reverse transcription-polymerase chain reaction, we found high expression of BCRP in BeWo cells, whereas no transcript of MDR1 (P-glycoprotein), the most extensively studied drug transporter, was detected. 4. In the human placenta, BCRP was localized predominantly in the syncytiotrophoblast layer; however, immunopositivity for the BXP-21 antibody was also observed in fetal vessels of the chorionic villi. The number of BCRP transcripts in the human term placenta was found to be more than 10-fold higher compared with the expression of MDR1. 5. In conclusion, we suggest that BeWo cells could serve as a suitable in vitro model to study trans-trophoblast transport of BCRP substrates and that placental BCRP can play an important role in preventing the accumulation of potentially toxic xenobiotics in the trophoblast cells.

Increased functional expression of P-glycoprotein in Caco-2 TC7 cells exposed long-term to cadmium

The objective of this study was to investigate whether P-glycoprotein (P-gp) functional expression in intestinal cells is modified after long-term exposure to the food contaminant cadmium (Cd). The Caco-2 cell line, clone TC7, was first validated as a cellular model for long-term exposure to cadmium. Cytotoxicity tests after acute exposure of 24 h showed a significant concentration-dependent decrease in cellular viability at cadmium levels higher than 10 microM and led us to select the cadmium ranges for long-term exposure: 1, 5, and 10 microM. Intestinal cells were exposed to these cadmium concentrations for four consecutive weeks without inducing DNA condensation or fragmentation. In the second part of this work, we studied the functional expression of the drug efflux pump multidrug resistance P-glycoprotein after long-term exposure to cadmium by immunoblotting with the monoclonal antibody F4 and measurement of calcein-AM+/-the P-gp inhibitor verapamil. Western blot analysis with the F4 antibody detected a single band of 170 to 180 kDa which is the size previously reported for P-gp. Calcein-AM assay showed that four weeks exposure of intestinal cells to 1, 5, and 10 microM Cd increased P-gp functional expression in proportion to the Cd concentration.

Identification of BCRP as transporter of benzo[ a ]pyrene conjugates metabolically formed in Caco-2 cells and its induction by Ah-receptor agonists

Breast cancer resistance protein (BCRP/ABCG2) is known to actively transport various anticancer drugs and to restrict the uptake of the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine from the gut lumen. The present study reveals that BCRP is involved in the transport of phase-2 metabolites of the carcinogen benzo[a]pyrene (BP) in the human intestinal cell line Caco-2. Treatment with the selective BCRP inhibitor Ko 143 (5 microM) inhibited the apical transport of BP-3-sulfate (BP3S) to 83% of control levels in TC7 cells and to 64% of control levels in Caco-2 cells. The apical transport of BP-3-glucuronide was inhibited by Ko 143 to 76% of control levels in TC7 cells. Furthermore, the expression of BCRP is most likely aryl hydrocarbon receptor (AhR) dependent, as treatment of Caco-2 cells with known AhR agonists including 2,3,7,8-tetrachlorodibenzo-p-dioxin, BP, indolo[3,2-b]carbazole and benzo[k]fluoranthene increased both mRNA and protein levels of BCRP. Induced BCRP protein was found to be functionally active, since pre-treatment of TC7 cells with oltipraz, indolo[3,2-b]carbazole or benzo[k]fluoranthene increased the amount of apically transported BP3S to as much as 180% of that in the controls. The induction of BCRP (mRNA and protein expression) by indolo[3,2-b]carbazole was inhibited in Caco-2 cells by co-incubation with the AhR antagonist PD98059 (2'-amino-3'-methoxyflavone). In summary, this study provides strong evidence that BCRP is an important part of the intestinal barrier protecting the body from food-associated contaminants such as the carcinogen BP.

Expression of mdr49 and mdr65 multidrug resistance genes in larval tissues of Drosophila melanogaster under normal and stress conditions

In situ expression of 2 multidrug resistance genes, mdr49 and mdr65, of Drosophila melanogaster was examined in wild-type third instar larval tissues under physiological conditions and after heat shock or colchicine feeding. Expression of these 2 genes was also examined in tumorous tissues of lethal (2) giant larvae I(2)gl4 mutant larvae. These 2 mdr genes show similar constitutive expression in different larval tissues under physiological conditions. However, they are induced differentially by endogenous (tumorous growth) and exogenous stresses (colchcine feeding or heat shock): whereas heat shock and colchicine feeding induce mdr49, tumorous condition is accompanied by enhanced expression of mdr49 and mdr65 genes.

The role of nuclear Y-box binding protein 1 as a global marker in drug resistance

Gene expression can be regulated by nuclear factors at the transcriptional level. Many such factors regulate MDR1 gene expression, but what are the sequence elements and transcription factors that control the basal and inducible expression of this gene? The general principles through which transcription factors participate in drug resistance are now beginning to be understood. Here, we review the factors involved in the transcriptional regulation of the MDR1 gene. In particular, we focus on the transcription factor Y-box binding protein 1 and discuss the possible links between Y-box binding protein 1 expression and drug resistance in cancer, which are mediated by the transmembrane P-glycoprotein or non–P-glycoprotein.

Transcriptional regulation of ABC drug transporters

P-glycoprotein, the founding member of the ATP-binding cassette (ABC) family of drug transporters, was first identified almost three decades ago and shown to confer resistance to multiple chemotherapeutic agents when overexpressed in human tumors. Subsequent years have witnessed a tremendous effort to characterize the function and regulation of P-glycoprotein, initially spurred by the hope that its inhibition was the key to overcoming clinical resistance to multiple anticancer agents. However, the identification of MRP1, another member of the ABC drug transporter family, led to the realization that the multidrug resistance (MDR) phenotype is considerably more complex than initially believed. Indeed, at the present time at least 10 members of the ABC transporter family have been implicated in an MDR phenotype, and it is likely that more will be added to this list as studies progress. With this complexity comes the imperative to improve our understanding of the function of individual transporters, as well as to delineate the mechanisms underlying their expression in normal and tumor cells, particularly those that may be amenable to therapeutic intervention. Several articles within this volume address the structure and function of drug transporters. This review will focus on our current understanding of the regulation of ABC drug transporters at the level of transcription.

Transcriptional regulators of the human multidrug resistance 1 gene: recent views

The multidrug resistance (MDR) phenotype is the major cause of failure of cancer chemotherapy. This phenotype is mainly due to the overexpression of the human MDR1 (hMDR1) gene. Several studies have shown that transcriptional regulation of this gene is unexpectedly complex and is far from being completely understood. Current work is aimed mainly at defining unclear and new control regions in the hMDR1 gene promoter as well as clarifying corresponding signaling pathways. Such studies provide new insights into the mechanisms by which xenobiotic molecules might modify the physiological hMDR1 expression as well as the possible role of oncogenes in the pathological dysregulation of the gene. Here we report recent findings on the regulation of hMDR1 which may help define specific targets aimed at modulating its transcription.

Expression of multidrug resistance genes MVP, MDR1, and MRP1 determined sequentially before, during, and after hyperthermic isolated limb perfusion of soft tissue sarcoma and melanoma patients

PURPOSE

Isolated, hyperthermic limb perfusion (ILP) with recombinant human tumor necrosis factor alpha and melphalan is a highly effective treatment for advanced soft tissue sarcoma (STS) and locoregional metastatic malignant melanoma. Multidrug resistance (MDR)-associated genes are known to be inducible by heat and drugs; expression levels of the major vault protein (MVP), MDR1, and MDR-associated protein 1 (MRP1) were determined sequentially before, during, and after ILP of patients.

PATIENTS AND METHODS

Twenty-one STS or malignant melanoma patients were treated by ILP. Tumor tissue temperatures were recorded continuously and ranged from 33.4 degrees C initially to peak values of 40.4 degrees C during ILP. Serial true-cut biopsy specimens from tumor tissues were routinely microdissected. Expression analyses for MDR genes were performed by real-time reverse transcriptase polymerase chain reaction and immunohistochemistry.

RESULTS

In 83% of the patients, MVP expression was induced during hyperthermic ILP. MVP-mRNA inductions often paralleled the increase in temperature during ILP. Increased MVP protein expressions either were observed simultaneously with the MVP-mRNA induction or were delayed until after the induction at the transcriptional level. Inductions of MDR1 and MRP1 were observed in only 13% and 27% of the specimens analyzed. Temperatures and drugs applied preferentially led to an induction of MVP and were not sufficient to induce MDR1 and MRP1 in the majority of tumors.

CONCLUSION

This study is the first to analyze the expression of MDR-associated genes sequentially during ILP of patients and demonstrates that treatment might lead to increased levels of MVP, whereas enhanced levels of MDR1 and MRP1 remain rare events.

Use of the human MDR1 promoter for heat-inducible expression of therapeutic genes

The promoter of the human multidrug resistance gene (mdr1) harbors stress-responsive elements, which can be induced e.g., by heat or cytostatic drugs. In previous studies the drug-responsiveness of the mdr1 promoter was successfully used for the drug-inducible expression of the human TNF-alpha gene in vitro and in vivo. Beside the drug-responsive elements of the mdr1 promoter, heat-shock responsive elements have also been identified, which could be exploited for construction of heat-inducible expression vectors. To analyze the hyperthermia-inducibility of the mdr1 promoter we used the pmdr-p-CAT and pM3mdr-p-hTNF vector constructs. Both constructs carry the mdr1 promoter fragment spanning from -207 to +153 to drive expression of the CAT-reporter or TNF-alpha gene. We tested the heat-induced CAT-reporter and TNF-alpha expression in vitro in transduced HCT15 and HCT116 human colon carcinoma cells. For the studies the transduced tumor cells were treated with hyperthermia at 41.5 degrees C or 43 degrees C for 2 hr to induce CAT or TNF-alpha expression. Cells and supernatants were harvested before hyperthermia and at certain time points (0-120 hr) after heat shock. The heat-induced CAT-reporter expression or TNF-alpha secretion was determined by specific ELISA. The experiments indicate that hyperthermia activates the mdr1 promoter in a temperature and time dependent manner. This induction leads to an 2- to 4-fold increase in CAT-reporter or 2- to 7-fold increase in TNF alpha expression in the tumor cell lines. These experiments reveal that the mdr1 promoter driven expression of therapeutic genes can be employed for combined cancer gene therapy approaches.

Impact of BCRP/MXR, MRP1 and MDR1/P-Glycoprotein on thermoresistant variants of atypical and classical multidrug resistant cancer cells

The impact of the ABC transporters breast cancer resistance protein/mitoxantrone resistance associated transporter (BCRP/MXR), multidrug resistance-associated protein 1 (MRP1) and multidrug resistance gene-1/P-glycoprotein (MDR1/PGP) on the multidrug resistance (MDR) phenotype in chemoresistance and thermoresistance was investigated in the parental human gastric carcinoma cell line EPG85-257P, the atypical MDR subline EPG85-257RNOV, the classical MDR subline EPG85-257RDB and their thermoresistant counterparts EPG85-257P-TR, EPG85-257RNOV-TR and EPG85-257RDB-TR. Within the atypical MDR subline EPG85-257RNOV expression of BCRP/MXR and of MRP1 were clearly enhanced (vs. parental and classical MDR lines). MDR1/PGP expression was distinctly elevated in the classical MDR subline EPG85-257RDB (vs. parental and atypical MDR sublines). In all thermoresistant counterparts basal expression of BCRP/MXR, MRP1 and MDR1/PGP was increased relative to thermosensitive sublines. Although it could be shown that the overexpressed ABC transporters were functionally active, however, no decreased drug accumulations of doxorubicin, mitoxantrone and rhodamine 123 were observed. Thus, expression of BCRP/MXR, MRP1 and MDR1/PGP was found to be dependent on the appropriate type of chemoresistance; correlating with a classical or atypical MDR phenotype. Within the thermoresistant variants, however, the increase in ABC transporter expression did obviously not influence the MDR phenotype.

P-glycoprotein in the catfish intestine: inducibility by xenobiotics and functional properties

The p-glycoprotein (pgp)-mediated multixenobiotic resistance (MXR) mechanism of aquatic animals has been associated with protection against pollution. Recent studies in mammals suggest that intestinal pgp may modulate intestinal bioavailability of dietary xenobiotics. In order to further delineate this mechanism in the catfish, these studies: (1) examined the pgp-related distribution in the intestine and liver of catfish, (2) evaluated the MXR response following exposure to various dietary xenobiotics and a prototypic pgp inducer and (3) evaluated pgp functional activity in membrane vesicles, using prototypic substrates and inhibitors. For this purpose, catfish were exposed in vivo to the pgp inducer vincristine (VIN), and the xenobiotics beta-naphthoflavone (BNF), benzo[a]pyrene (BaP), and 3,4,3',4'-tetrachlorobiphenyl (TCB). Membrane vesicles, prepared from liver and intestine (proximal and distal sections) of control and exposed catfish, were subjected to SDS PAGE, Western Blot, and detection with the pgp C219 monoclonal antibody. Transport activity was evaluated in vitro using the pgp substrate [3H]vinblastine (VBL), and the pgp inhibitor verapamil (VP). Immunoblot studies demonstrated a pgp-related protein of approximately 170 kDa in the intestine and liver of catfish. This protein appears to be very susceptible to degradation, and was present in higher levels in the liver, in comparison to the intestine, where regional differences were not observed. Dietary exposure to the pgp substrate VIN, or the xenobiotics BNF, BaP, and TCB, did not appear to affect pgp-related reactivity. Transport studies with VBL indicate that the pgp-related protein of the catfish intestine displays classic pgp-mediated multidrug resistance (MDR) characteristics, such as energy-dependency, and sensitivity to VP. These studies suggest that the pgp-related protein in the catfish intestine and liver is not only immunochemically, but also functionally related to the mammalian MDR. Moreover, the results presented indicate that pgp-related reactivity and transport in intestinal vesicles of catfish may be influenced by factors including method sensitivity, sample collection, sample preparation, and immunoblot conditions.

Hyperthermia-induced nuclear translocation of transcription factor YB-1 leads to enhanced expression of multidrug resistance-related ABC transporters

Genotoxic stress leads to nuclear translocation of the Y-box transcription factor YB-1 and enhanced expression of the multidrug resistance gene MDR1. Because hyperthermia is used for the treatment of colon cancer in combination with chemoradiotherapy, we investigated the influence of hyperthermia on YB-1 activity and the expression of multidrug resistance-related genes. Here we report that hyperthermia causes YB-1 translocation from the cytoplasm into the nucleus of human colon carcinoma cells HCT15 and HCT116. Nuclear translocation of YB-1 was associated with increased MDR1 and MRP1 gene activity, which is reflected in strong efflux pump activity. However, a combination of hyperthermia and drug treatment effectively reduced cell survival of the HCT15 and HCT116 cells. These results demonstrate that activation of MDR1 and MRP1 gene expression and increased efflux pump activity after hyperthermia were insufficient to cause an increase in drug resistance in colon cancer cell lines. The ability of hyperthermia to abrogate drug resistance in the presence of an increase in functional MDR proteins may provide an explanation for the efficacious results seen in the clinic in colon cancer patients treated with a combination of hyperthermia and chemotherapy.

Regulation of multidrug resistance 1 (MDR1)/P-glycoprotein gene expression and activity by heat-shock transcription factor 1 (HSF1)

Infection of HeLa cells with adenovirus-carrying HSF1(+) cDNA, which encodes a mutated form of HSF1 with constitutive transactivation capacity, increased multidrug resistance 1 (MDR1) mRNA level and P-glycoprotein (P-gp) cell surface content and stimulated rhodamine 123 accumulation and vinblastine efflux activity. On the other hand, infection with adenovirus-carrying HSP70 and HSP27 cDNAs did not increase MDR1/P-gp expression. HSF1 regulates MDR1/P-gp expression at the transcriptional level, since HSF1(+) bound the heat-shock consensus elements (HSEs) in the MDR1 gene promoter and also activated the expression of an MDR1 promoter-driven reporter plasmid (pMDR1(-1202)). In addition, heat-shock increased pMDR1(-1202) promoter activity but not the activity of a similar reporter plasmid with point mutations at specific HSEs, and the heat-induced increase was totally inhibited by co-transfection with an expression plasmid carrying HSF1(-), a dominant negative mutant of HSF1. The stress inducers arsenite, butyrate, and etoposide also increased pMDR1(-1202) promoter activity, but the increase was not inhibited (in the case of butyrate) or was only partially inhibited (in the case of arsenite and etoposide) by HSF1(-). These results demonstrate that HSF1 regulates MDR1 expression, and that the HSEs present in the -315 to -285 region mediate the heat-induced activation of the MDR1 promoter. However, other factors may also participate in MDR1 induction by stressing agents.

Thermosensitivity of multidrug-resistant human gastric and pancreatic carcinoma cells

Often, tumour cells acquire drug resistance phenotypes, which include the classical multidrug resistance (MDR) phenomenon accompanied by the synthesis of the P-glycoprotein (Pgp) and atypical MDR phenotypes mediated by different, in part unknown, mechanisms. To investigate the susceptibility of tumour cells exhibiting different kinds of MDR to treatment with heat, the hyperthermic survival of established human gastric and pancreatic carcinoma cell lines were studied and sublines exhibiting a classical and an atypical MDR phenotype were derived, respectively. Arrhenius analysis of this panel of gastrointestinal tumour cells revealed that both the classical and the atypical MDR variants exhibited no breaking points (T*) in contrast to the parent tumour cells. The activation enthalpies E(A) were about 40% lower at T > T* in comparison to the E(A) at lower temperatures. Classical MDR variants of both gastrointestinal tumour cell types exhibited a similar E(A) value, whereas the E(A) of atypical MDR gastric carcinoma cells was 1.6-fold higher than the E(A) of corresponding pancreatic carcinoma cells. In comparison to the parent lines, the drug resistant variants exhibited a 2.1-fold (gastric carcinoma, classical MDR), 2.7-fold (gastric carcinoma, atypical MDR) and 1.4-fold (pancreatic carcinoma, classical MDR) increase of activation enthalpies and a nearby unchanged E(A) in pancreatic carcinoma cells exhibiting an atypical MDR.

Induction of the multixenobiotic defense mechanism (MXR), P-glycoprotein, in the mussel Mytilus californianus as a general cellular response to environmental stresses

A multixenobiotic resistance mechanism (MXR) related to the P-glycoprotein multidrug transporter protein (p-gp) has been identified and characterized in several marine invertebrates. p-gp activity and protein titer is induced by exposure to toxins, supporting the suggestion that the role for this transporter is protection from xenobiotics by reducing accumulation of toxins in cells. In this study, we report on the specificity of the induction of the transporter by various chemical and physical stressors. p-gp substrates (including the pesticides pentachlorophenol and chlorthal) as well as non-substrates (including DDE and sodium arsenite) induced p-gp activity and protein titer in the gill tissues of the mussel Mytilus californianus. Similarly, mussels exposed to heat shock of 20 degrees C or 25 degrees C exhibited increased p-gp titer and activity compared to mussels held at ambient (12 degrees C) temperature seawater. Some of the same treatments that induced an increase in p-gp caused a concomitant increase in hsp70, but hsp induction was not always associated with induction of the p-gp protein. These findings suggest that p-gp induction in mussels may be part of a general cellular stress response. This response, however, does not appear to be always coupled with the hsp70 response in mussels.

Regulation of MDR1 gene expression: emerging concepts

Drug resistance genes, such as MDR1, involved in drug efflux, and their regulation have been the subject of intense research efforts in the past 10 years. Many factors and cellular signalling pathways play a role in the regulation of MDR1 gene expression. Commonly used chemotherapeutic agents activate in vitro and in vivo general stress response pathways, potential targets of which include MDR1 and other drug resistance genes. The contribution of these agents to the emergence of drug-resistant tumour cells is of concern. Recent evidence points to a role for the epigenetic regulation of MDR1 gene expression. The identification of key components in the DNA methylation/chromatin system of gene regulation may in time lead to more informed and targeted approaches to treating drug-resistant tumours. Copyright 2000 Harcourt Publishers Ltd.

A novel association between the human heat shock transcription factor 1 (HSF1) and prostate adenocarcinoma

A search for differentially expressed genes in a pair of nonmetastatic (PC-3) versus metastatic variant (PC-3M) human prostate carcinoma cell lines led to identification of the human heat shock factor (HSF1) as an overexpressed gene product in PC-3M cells. Analysis of primary prostate cancer specimens indicated that HSF1 is generally up-regulated in most of the malignant prostate epithelial cells relative to the normal prostate cells. Among the known effectors of HSF1 action, constitutive levels of HSP70 and HSP90 are not significantly altered by the naturally elevated expression of HSF1 as in PC-3M cells or by transduced overexpression of HSF1 in PC-3 cells. The basal levels of HSP27 in both cases are, however, consistently increased by two- to threefold. With respect to response to heat shock, high basal concentration of HSP90 is not further enhanced in these cells, and HSP70 is up-regulated irrespective of HSF1 level. Heat shock, however, causes an increase in HSP27 when HSF1 is up-regulated, except when the expression of HSF1 is already too high. These results document for the first time that HSF1 is overexpressed in human prostate cancer cells, at least one consequence of which in the prostate cancer cell lines tested is stimulation of both basal and stress-induced expression of HSP27, an important factor in cell growth, differentiation, or apoptosis.

Up-regulation of multidrug resistance P-glycoprotein via nuclear factor-kappaB activation protects kidney proximal tubule cells from cadmium- and reactive oxygen species-induced apoptosis

Cadmium-mediated toxicity of cultured proximal tubule (PT) cells is associated with increased production of reactive oxygen species (ROS) and apoptosis. We found that cadmium-dependent apoptosis (Hoechst 33342 and annexin V assays) decreased with prolonged CdCl(2) (10 microM) application (controls: 2.4 +/- 1.6%; 5 h: +5.1 +/- 2.3%, 20 h: +5.7 +/- 2.5%, 48 h: +3.3 +/- 1.0% and 72 h: +2.1 +/- 0.4% above controls), while cell proliferation was not affected. Reduction of apoptosis correlated with a time-dependent up-regulation of the drug efflux pump multidrug resistance P-glycoprotein (mdr1) in cadmium-treated cells ( approximately 4-fold after 72 h), as determined by immunoblotting with the monoclonal antibody C219 and measurement of intracellular accumulation of the fluorescent probe calcein +/- the mdr1 inhibitor PSC833 (0.5 microM). When mdr1 inhibitors (PSC833, cyclosporine A, verapamil) were transiently added to cells with mdr1 up-regulation by pretreatment for 72 h with cadmium, cadmium-induced apoptosis increased significantly and to a percentage similar to that obtained in cells with no mdr1 up-regulation (72-h cadmium: 5.2 +/- 0.9% versus 72-h cadmium + 1-h PSC833: 7.2 +/- 1.4%; p < or = 0.001). Cadmium-induced apoptosis and mdr1 up-regulation depended on ROS, since co-incubation with the ROS scavengers N-acetylcysteine (15 mM) or pyrrolidine dithiocarbamate (0.1 mM) abolished both responses. Moreover, cadmium- and ROS-associated mdr1 up-regulation was linked to activation of the transcription factor NF-kappaB; N-acetylcysteine, pyrrolidine dithiocarbamate, and the IkappaB-alpha kinase inhibitor Bay 11-7082 (20 microM) prevented both, mdr1 overexpression and degradation of the inhibitory NF-kappaB subunit, IkappaB-alpha, induced by cadmium. The data show that 1) cadmium-mediated apoptosis in PT cells is associated with ROS production, 2) ROS increase mdr1 expression by a process involving NF-kappaB activation, and 3) mdr1 overexpression protects PT cells against cadmium-mediated apoptosis. These data suggest that mdr1 up-regulation, at least in part, provides anti-apoptotic protection for PT cells against cadmium-mediated stress.

Hyperthermia for treatment of rectal cancer: evaluation for induction of multidrug resistance gene (mdr1) expression

Environmental stress factors, such as heat, may induce multidrug resistance gene (mdr1) expression, which could result in the disadvantageous multidrug resistance (MDR) phenotype. To evaluate this possibility in a clinical situation, we investigated mdr1 gene expression in patients with locally advanced rectal cancer who underwent preoperative radio-chemo-thermo-therapy (RCTT). Patients were classified into groups according to the treatment schedule of RCTT vs. radio-chemo-therapy (RCT) without hyperthermia (control group). Expression of the mdr1 gene was analyzed in tumors and normal rectal tissues prior to and post-treatment (RCTT or RCT, respectively) by means of semi-quantitative and quantitative reverse transcription-polymerase chain reaction (RT-PCR). The data were correlated with therapeutic response and survival parameters. Based on our evaluation criteria, in 2 of 19 tumors of the RCTT group, mdr1 gene expression was increased more than 2-fold; in 3 of 19 tumors of this group, however, mdr1 expression was decreased more than 2-fold. In the patient control group, levels of mdr1 gene expression were reduced in 2 of 8 tumors. Thus, hyperthermia combined with RCT (RCTT) in comparison with RCT alone does not lead to an increase in mdr1 gene expression in patients with locally advanced rectal cancer within the preoperative treatment schedule. The risk of inducing the classical multidrug resistance phenotype by hyperthermia was thus minimal in this clinical setting. Subsequent adjuvant chemotherapy should thus not be hindered.

Transcriptional regulation of MDR genes

The emergence of resistance in a tumor population is most often associated with a disregulation of gene expression, usually at the level of transcription. A major goal in the field of cancer chemotherapy is to define the mechanisms underlying transcriptional regulation of drug resistance genes in an effort to identify targets for therapeutic intervention. Recently, considerable progress has been made in identifying the molecular mechanisms involved in the transcriptional regulation of the P-glycoprotein (Pgp) gene. When overexpressed in tumor cells, Pgp confers resistance to a variety of chemotherapeutic agents; this resistance has been termed MDR (multidrug resistance). Moreover, Pgp is a normal component of a variety of highly differentiated cell types and, as such, is regulated by both internal and external environmental stimuli. In this review, we will discuss the current knowledge regarding the DNA elements and protein factors involved in both constitutive and inducible regulation of Pgp transcription in normal and tumor cells.

The chemosensitizers of multixenobiotic resistance mechanism in aquatic invertebrates: a new class of pollutants

Mechanism of multixenobiotic resistance (MXR), identical to multidrug resistance (MDR) in tumor cells, has been found in aquatic invertebrates. The presence of this ATP-dependent membrane P-glycoprotein (Pgp) pump was confirmed by biochemical ('binding'), molecular (immunohistochemical, Western, Northern), physiological (verapamil-sensitivity) and toxicological (modulation of toxicity) methods. The inducibility of MXR in the presence of xenobiotics and its wide taxonomic distribution suggests its role as a general biological defense mechanism that rescues organisms by pumping potentially toxic xenobiotics out of the cells. Some xenobiotics, the chemosensitizers, can inhibit this defense mechanism. The presence of these MXR-inhibitors has important implications on environmental parameters like exposure, uptake, internal dose, bioaccumulation, response, synergism and toxicity. Such MXR-inhibitors, for example, enhance the accumulation of carcinogenic aromatic amines in mussel, with subsequent enhancement in production of their mutagenic metabolites, in induction of single strand breaks in DNA, and in induction of DNA-adducts. The property to inhibit defense mechanism of organisms classifies MXR-inhibitors among top-hazardous environmental chemicals. Therefore, we measured the concentration of chemosensitizers in water concentrates or sediment extracts as their potential to modulate the accumulation of fluorescent dyes in a cell-culture of NIH 3T3 mouse fibroblasts stable transfected with human MDR1 gene, or as the potential of native waters to decrease the efflux-rate of Rhodamine B from gills of mussels. We found significantly higher concentrations of MXR-inhibitors in samples from polluted marine sites or from polluted rivers than in samples from corresponding unpolluted sites. These concentrations were able to enhance the accumulation of fluorescent dyes or carcinogenic aromatic amines in clams, mussels, snails and sponges exposed to these xenobiotics, demonstrating the ecotoxicological relevance of MXR-inhibitors present in polluted waters.

Regulation of expression of the human heme oxygenase-1 gene in transfected chick embryo liver cell cultures

Induction of heme oxygenase (HO) has been proposed as a protective cellular mechanism against oxidative damage. In previous work (Tyrrell et al., Carcinogenesis [1993] 14, 761-765), portions of the 5' promoter region of the human HO-1 gene linked to the reporter gene chloramphenicol acetyl transferase (CAT), had been transiently expressed in HeLa cells. To extend the study of human HO gene expression into primary liver cells, these reporter gene fusion constructs, containing 121 or 1416 base pairs of the untranscribed 5'-upstream sequences of the human HO-1 gene, were used along with pSV beta-Gal plasmid to dually transfect primary cultures of chick embryo liver cells (CELC). The transfected cells were treated with selected metals, heme, phorbol ester, and chemical agents that produce oxidative stress (H2O2 or sodium arsenite). Reporter gene activities were measured 18-20 h later. Our major findings are: (1) these HO-CAT constructs were expressed in CELC; (2) unlike HeLa cells, the expression of CAT was detected in CELC without the need for the SV40 enhancer; (3) sodium arsenite and cobalt chloride induced the expression of the HO-CAT constructs whereas heme had no effect on or decreased CAT expression for all of the transfected constructs; (4) study of endogenous chick HO-1 gene expression in CELC showed that HO-1 responded to sodium arsenite treatment in a dose-dependent fashion, and the response was rapid and transient. We conclude that, in chick liver cell cultures, induction of the HO-1 gene by heme is fundamentally different from that produced by transition metals or sodium arsenite. Furthermore, the results suggest that expression of the HO-1 gene is highly conserved across species.

DNA elements recognizing NF-Y and Sp1 regulate the human multidrug-resistance gene promoter

Regulation of the human multidrug resistance gene (hMDR1) was studied by mapping DNA elements in the proximal promoter necessary for efficient transcription. Transient transfection analysis in tumor cell lines (HCT116, HepG2, and Saos2) of promoter deletions identified several regulatory domains. These cell lines expressed hMDR1 mRNA. Removal of an element between +25 and +158 reduced promoter activity by 2-3-fold, whereas deletion of sequences from approximately -5000 to -138 base pairs gave a approximately 2-fold increase. The activity of the hMDR1 promoter (-137 to +25) was comparable in activity to the SV40 early promoter and enhancer combination. Deletion of the hMDR1 promoter between -86 and -44 reduced activity by 5-10-fold, identifying an important regulatory region. This minimal region (-88 to -37) activated transcription when inserted upstream of a synthetic promoter, suggesting that it acts independently of other regulatory sequences. Two DNA elements within 85 base pairs of the transcriptional start site were required to confer efficient gene expression. A double-point mutation in the Y box (inverted CCAAT box) between -70 and -80 reduced activity of the promoter by 5-10-fold, and a single-point mutation at -52 within a GC-rich element reduced activity by 3-fold. Thus, both the Y-box and GC elements must each remain intact for optimal promoter activity. DNA-binding analyses suggest that the transcription factor NF-Y, but not YB-1 or c/EBP, is most likely responsible for controlling the activity of the Y-box element in these tumor cell lines. DNA-binding analyses also suggest that Sp1, alone or in combination with other nuclear factors, likely controls the activity of the GC element.

The P-glycoprotein multidrug transporter

1. P-glycoprotein (P-gp) is a transmembrane protein involved in ATP-dependent efflux of various structurally unrelated anticancer drugs. Its overexpression in cancer cells decreases intracellular drug concentrations and, thus, confers a multidrug resistance phenotype. 2. P-gp is encoded by MDR genes, which constitute a small gene family comprising two genes in humans and three genes in rodents. Only the MDR1 gene in humans and mdr1 and mdr3 genes in rodents have been demonstrated to be involved in drug resistance. 3. P-gp encoded by the human MDR1 gene is a phosphorylated and glycosylated protein 1289 amino acids long, and consists of 2 halves that share a high degree of similarity. 4. A wide variety of cancers have been shown to express P-gp, including solid tumors and hematological malignancies. This P-gp positivity can be evidenced at the time of diagnosis prior to chemotherapy or at relapse after treatment, and has been correlated with treatment failure and poor prognosis in several types of cancer. In addition, P-gp is also expressed by some normal tissues, such as liver and kidney. 5. P-gp expression is regulated by various factors, including xenobiotics and hormones. 6. P-gp-mediated multidrug resistance can be reversed by various unrelated compounds called chemosensitizers or reversing agents. These drugs act through inhibition of P-gp function and have entered clinical trials.

Vincristine induction of mutant and wild-type human multidrug-resistance promoters is cell-type-specific and dose-dependent

To investigate multidrug-resistance gene (MDR1) promoter efficacy and drug inducibility in cells with different multidrug-resistance phenotypes, multidrug-resistant HCT15 and drug-sensitive KM12 human colon carcinoma cell lines were transfected with constructs incorporating the chloramphenicol acetyltransferase (CAT) reporter gene, driven by wild-type and point-mutated MDR1 promoter regions. The basal CAT expression level in HCT15 cells was markedly elevated compared to KM12 cells. CAT induction by vincristine was dose-dependent over a broad concentration range (40-500 ng/ml) in both lines. The induction levels were related to the cells' MDR phenotype, with the multidrug-resistant HCT15 cells showing the greater effect. In both cell types, basal and drug-induced CAT expression were significantly enhanced by the point-mutated promoter regions. The findings support the possible exploitation of the MDR1 promoter for construction of drug-inducible and MDR-cell-targeted expression vectors for use in gene therapy.

Regulation of metallothionein gene expression. Studies in transfected primary cultures of chick embryo liver cells

To study the regulation of expression of the metallothionein gene in normal liver cells, we transfected chick embryo liver cells in primary cultures with constructs containing luciferase or chloramphenicol acetyl transferase (as reporter genes) under the control of differing lengths of the 5'-promoter region of the chick metallothionein gene (containing 30, 122, 190, or 623 base pairs upstream of the transcriptional start site). We controlled for efficiency of transfection by co-transfections with a plasmid containing a bacterial beta-galactosidase gene under the control of the SV 40 promoter and enhancer. Treatment of the transfected cells with transition metallic ions (cadmium, cobalt, and zinc) or sodium arsenite produced increases in activities of luciferase or chloramphenicol acetyl transferase, relative to beta-galactosidase, and this activity mapped to the first 122 base pairs of the promoter. Although heme has recently been reported to induce the endogenous metallothionein gene in chick embryo liver cells, 10-50 microM heme did not increase reporter gene activities in transfected cells. Nevertheless, the heme-dependent induction of endogenous heme oxygenase-1 in these cells was normal. We conclude that the heme-dependent induction of the liver metallothionein gene depends upon DNA region(s) outside the regulatory region of the chick metallothionein gene studied here and that elements within the first 122 base pairs of the metallothionein promoter are sufficient to confer responsiveness to transition metals or sodium arsenite.

Enhanced secretion of glycocholic acid in a specially adapted cell line is associated with overexpression of apparently novel ATP-binding cassette proteins

Secretion of anionic endo- and xenobiotics is essential for the survival of animal and plant cells; however, the underlying molecular mechanisms remain uncertain. To better understand one such model system--i.e., secretion of bile acids by the liver--we utilized a strategy analogous to that employed to identify the multidrug resistance (mdr) genes. We synthesized the methyl ester of glycocholic acid (GCE), which readily enters cells, where it is hydrolyzed to yield glycocholic acid, a naturally occurring bile acid. The rat hepatoma-derived HTC cell line gradually acquired resistance to GCE concentrations 20-fold higher than those which inhibited growth of naive cells, yet intracellular accumulation of radiolabel in resistant cells exposed to [14C]GCE averaged approximately 25% of that in nonresistant cells. As compared with nonresistant cells, resistant cells also exhibited (i) cross-resistance to colchicine, a known mdr substrate, but not to other noxious substances transported by hepatocytes; (ii) increased abundance on Northern blot of mRNA species up to 7-10 kb recognized by a probe for highly conserved nucleotide-binding domain (NBD) sequences of ATP-binding cassette (ABC) proteins; (iii) increased abundance, as measured by RNase protection assay, of mRNA fragments homologous to a NBD cRNA probe; and (iv) dramatic overexpression, as measured by Western blotting and immunofluorescence, of a group of 150- to 200-kDa plasma membrane proteins recognized by a monoclonal antibody against a region flanking the highly conserved NBD of mdr/P-glycoproteins. Finally, Xenopus laevis oocytes injected with mRNA from resistant cells and incubated with [14C]GCE secreted radiolabel more rapidly than did control oocytes. Enhanced secretion of glycocholic acid in this cell line is associated with overexpression of ABC/mdr-related proteins, some of which are apparently novel and are likely to include a bile acid transport protein.

Characterization of the basal and carcinogen regulatory elements of the rat mdr1b promoter

In this report we characterized the transcriptional regulation of the rat mdr1b gene by xenobiotics. The expression of this gene was increased in primary rat hepatocytes and in the H4-II-E hepatoma cell line by exposure to carcinogens such as aflatoxin B1, N-acetoxy-2-acetylaminofluorene, and methyl methanesulfonate. Nuclear run-on experiments indicated that the higher steady-state levels of mdr1b mRNA were due to an increase in transcription. The 5'-flanking region of the mdr1b gene was isolated, sequenced, and functionally characterized in transient and stable transfection assays. A single transcription start site was identified for this gene; no alternate start sites were used after induction with aflatoxin B1. Deletion analysis of this promoter demonstrated that the sequence between nt -214 and -178 was critical for basal promoter activity. This region did not contain any consensus-binding sites for previously identified transcription factors. A negative regulatory region was also identified between nt -940 and -250. No specific carcinogen-responsive element was identified; the xenobiotic response required a large part of the promoter. These data suggest that the carcinogen induction of mdr1b expression is mediated through sequences that overlap or that are identical to the basal promoter element.

Rapid up-regulation of mdr1 expression by anthracyclines in a classical multidrug-resistant cell line

Studies were carried out in a variant human multidrug-resistant (MDR) cell line CEM/A7R, which expresses very low levels of mdr1 mRNA and P-glycoprotein (P-gp). The induction of mdr1 RNA expression by three anthracyclines, (doxorubicin, daunorubicin, epirubicin), VP-16 and two vinca alkaloids (vincristine, vinblastine) was semiquantitatively assessed by scanning Northern blots on a phosphorimager. The relative level of mdr1 expression was expressed as ratio of mdr1 to the internal RNA (actin). A significant increase (P < 0.02) in expression of mdr1 was noted within 4 hrs of exposure to 1.5 micrograms ml-1 daunorubicin or epirubicin. Neither vinblastine nor vincristine had any effect on mdr1 levels after an 8 h exposure. With increasing concentrations of daunorubicin or epirubicin in a fixed 24 h time period, mdr1 expression increased, although a biphasic response was seen. Based on MRK 16 binding, an increase in P-gp levels was seen in the CEM/A7R line after a 24 h exposure to 1 microgram ml-1 daunorubicin or epirubicin. The rapid increase in mdr1 expression after a short period of exposure to doxorubicin, daunorubicin or epirubicin suggests that induction of mdr1 expression may have an important role in the development of drug-resistant tumours.