Multienzyme-mediated stable and transient multidrug resistance and collateral sensitivity induced by xenobiotics

Synthesis and antitumor activity of selenium-containing quinone-based triazoles possessing two redox centres, and their mechanistic insights

Selenium-containing quinone-based 1,2,3-triazoles were synthesized using click chemistry, the copper catalyzed azide-alkyne 1,3-dipolar cycloaddition, and evaluated against six types of cancer cell lines: HL-60 (human promyelocytic leukemia cells), HCT-116 (human colon carcinoma cells), PC3 (human prostate cells), SF295 (human glioblastoma cells), MDA-MB-435 (melanoma cells) and OVCAR-8 (human ovarian carcinoma cells). Some compounds showed IC50 values < 0.3 μM. The cytotoxic potential of the quinones evaluated was also assayed using non-tumor cells, exemplified by peripheral blood mononuclear (PBMC), V79 and L929 cells. Mechanistic role for

NAD(P)H

Quinone Oxidoreductase 1 (NQO1) was also elucidated. These compounds could provide promising new lead derivatives for more potent anticancer drug development and delivery, and represent one of the most active classes of lapachones reported.

Esterase-activatable β-lapachone prodrug micelles for NQO1-targeted lung cancer therapy

Lung cancer is one of the most lethal forms of cancer and current chemotherapeutic strategies lack broad specificity and efficacy. Recently, β-lapachone (β-lap) was shown to be highly efficacious in killing non-small cell lung cancer (NSCLC) cells regardless of their p53, cell cycle and caspase status. Pre-clinical and clinical use of β-lap (clinical form, ARQ501 or 761) is hampered by poor pharmacokinetics and toxicity due to hemolytic anemia. Here, we report the development and preclinical evaluation of β-lap prodrug nanotherapeutics consisting of diester derivatives of β-lap encapsulated in biocompatible and biodegradable poly(ethylene glycol)-b-poly(D,L-lactic acid) (PEG-b-PLA) micelles. Compared to the parent drug, diester derivatives of β-lap showed higher drug loading densities inside PEG-b-PLA micelles. After esterase treatment, micelle-delivered β-lap-dC3 and -dC6 prodrugs were converted to β-lap. Cytotoxicity assays using A549 and H596 lung cancer cells showed that both micelle formulations maintained

NAD(P)H

quinone oxidoreductase 1 (NQO1)-dependent cytotoxicity. However, antitumor efficacy study of β-lap-dC3 micelles against orthotopic A549 NSCLC xenograft-bearing mice showed significantly greater long-term survival over β-lap-dC6 micelles or β-lap-HPβCD complexes. Improved therapeutic efficacy of β-lap-dC3 micelles correlated with higher area under the concentration-time curves of β-lap in tumors, and enhanced pharmacodynamic endpoints (e.g., PARP1 hyperactivation, γH2AX, and ATP depletion). β-Lap-dC3 prodrug micelles provide a promising strategy for NQO1-targeted therapy of lung cancer with improved safety and antitumor efficacy.

Networking of differentially expressed genes in human cancer cells resistant to methotrexate

Background

The need for an integrated view of data obtained from high-throughput technologies gave rise to network analyses. These are especially useful to rationalize how external perturbations propagate through the expression of genes. To address this issue in the case of drug resistance, we constructed biological association networks of genes differentially expressed in cell lines resistant to methotrexate (MTX).

Methods

Seven cell lines representative of different types of cancer, including colon cancer (HT29 and Caco2), breast cancer (MCF-7 and MDA-MB-468), pancreatic cancer (MIA PaCa-2), erythroblastic leukemia (K562) and osteosarcoma (Saos-2), were used. The differential expression pattern between sensitive and MTX-resistant cells was determined by whole human genome microarrays and analyzed with the GeneSpring GX software package. Genes deregulated in common between the different cancer cell lines served to generate biological association networks using the Pathway Architect software.

Results

Dikkopf homolog-1 (DKK1) is a highly interconnected node in the network generated with genes in common between the two colon cancer cell lines, and functional validations of this target using small interfering RNAs (siRNAs) showed a chemosensitization toward MTX. Members of the UDP-glucuronosyltransferase 1A (UGT1A) family formed a network of genes differentially expressed in the two breast cancer cell lines. siRNA treatment against UGT1A also showed an increase in MTX sensitivity. Eukaryotic translation elongation factor 1 alpha 1 (EEF1A1) was overexpressed among the pancreatic cancer, leukemia and osteosarcoma cell lines, and siRNA treatment against EEF1A1 produced a chemosensitization toward MTX.

Conclusions

Biological association networks identified DKK1, UGT1As and EEF1A1 as important gene nodes in MTX-resistance. Treatments using siRNA technology against these three genes showed chemosensitization toward MTX.

The mechanism of cytotoxicity and DNA adduct formation by the anticancer drug ellipticine in human neuroblastoma cells

Ellipticine is an antineoplastic agent, whose mode of action is based mainly on DNA intercalation, inhibition of topoisomerase II and formation of covalent DNA adducts mediated by cytochromes P450 and peroxidases. Here, the molecular mechanism of DNA-mediated ellipticine action in human neuroblastoma IMR-32, UKF-NB-3 and UKF-NB-4 cancer cell lines was investigated. Treatment of neuroblastoma cells with ellipticine resulted in apoptosis induction, which was verified by the appearance of DNA fragmentation, and in inhibition of cell growth. These effects were associated with formation of two covalent ellipticine-derived DNA adducts, identical to those formed by the cytochrome P450- and peroxidase-mediated ellipticine metabolites, 13-hydroxy- and 12-hydroxyellipticine. The expression of these enzymes at mRNA and protein levels and their ability to generate ellipticine-DNA adducts in neuroblastoma cells were proven, using the real-time polymerase chain reaction, Western blotting analyses and by analyzing ellipticine-DNA adducts in incubations of this drug with neuroblastoma S9 fractions, enzyme cofactors and DNA. The levels of DNA adducts correlated with toxicity of ellipticine to IMR-32 and UKF-NB-4 cells, but not with that to UKF-NB-3 cells. In addition, hypoxic cell culture conditions resulted in a decrease in ellipticine toxicity to IMR-32 and UKF-NB-4 cells and this correlated with lower levels of DNA adducts. Both these cell lines accumulated in S phase, suggesting that ellipticine-DNA adducts interfere with DNA replication. The results demonstrate that among the multiple modes of ellipticine antitumor action, formation of covalent DNA adducts by ellipticine is the predominant mechanism of cytotoxicity to IMR-32 and UKF-NB-4 neuroblastoma cells.

Drug metabolism by tumours: its nature, relevance and therapeutic implications

Drug-metabolising enzymes (DMEs) are present in tumours and are capable of biotransforming a variety of antineoplastics. Tumoural drug metabolism is both a potential mechanism of resistance and a means of achieving optimal therapy. This review addresses the classes of DMEs, their cytotoxic substrates and distribution in specific malignancies. The limitations of preclinical and clinical studies are highlighted. Their role in predicting therapeutic response, the activation of prodrugs and the potential for their modulation for gain is also addressed. The contribution of tumoural DMEs to cancer therapy can only be ascertained through large prospective studies and supported by new technologies. Only then can efforts be concentrated in the design of better prodrugs or combination therapy to optimise individual therapy.

The Anticancer Drug Ellipticine Is a Potent Inducer of Rat Cytochromes P450 1A1 and 1A2, Thereby Modulating Its Own Metabolism

Ellipticine is an antineoplastic agent whose mode of action is based mainly on DNA intercalation, inhibition of topoisomerase II, and formation of covalent DNA adducts mediated by cytochromes P450 (P450s) and peroxidases. Here, this drug was found to induce CYP1A1 and/or 1A2 enzymes and their enzymatic activities in livers, lungs, and kidneys of rats treated (i.p.) with ellipticine. The induction is transient. In the absence of repeated administration of ellipticine, the levels and activities of the induced CYP1A decreased almost to the basal level 2 weeks after treatment. The ellipticine-mediated CYP1A induction increases the DNA adduct formation by the compound. When microsomal fractions from livers, kidneys, and lungs of rats treated with ellipticine were incubated with ellipticine, DNA adduct formation, measured by (32)P-postlabeling analysis, was up to 3.8-fold higher in incubations with microsomes from pretreated rats than with controls. The observed stimulation of DNA adduct formation by ellipticine was attributed to induction of CYP1A1 and/or 1A2-mediated increase in ellipticine oxidative activation to 13-hydroxy- and 12-hydroxyellipticine, the metabolites generating two major DNA adducts in human and rat livers. In addition to these metabolites, increased formation of the excretion products 9-hydroxy- and 7-hydroxyellipticine was also observed in microsomes of rats treated with ellipticine. Taken together, these results demonstrate for the first time that by inducing CYP1A1/2, ellipticine increases its own metabolism, leading both to an activation of this drug to reactive species-forming DNA adducts and to detoxication metabolites, thereby modulating to some extent its pharmacological and/or genotoxic potential.

DNA adduct formation by the anticancer drug ellipticine in human leukemia HL-60 and CCRF-CEM cells

Ellipticine induces formation of two DNA adducts in leukemia HL-60 and CCRF-CEM cells, identical with deoxyguanosine adducts generated by ellipticine metabolites 13-hydroxyellipticine and 12-hydroxyellipticine in vitro and in vivo. The ellipticine cytotoxicity to HL-60 (IC(50)=0.64microM) and CCRF-CEM cells (IC(50)=4.7microM) correlates with levels of DNA adducts. The different expressions of enzymes activating ellipticine in cells explain this finding. While cytochrome P450 1A1 and cyclooxygenase-1 are expressed in both cells, HL-60 cells express also high levels of another activator, myeloperoxidase. The results suggest the adduct formation as a new mode of antitumor action of ellipticine for leukemia.

Molecular mechanisms of antineoplastic action of an anticancer drug ellipticine

Ellipticine is a potent antineoplastic agent exhibiting the multimodal mechanism of its action. This article reviews the mechanisms of predominant pharmacological and cytotoxic effects of ellipticine and shows the results of our laboratories indicating a novel mechanism of its action. The prevalent mechanisms of ellipticine antitumor, mutagenic and cytotoxic activities were suggested to be intercalation into DNA and inhibition of DNA topoisomerase II activity. We demonstrated a new mode of ellipticine action, formation of covalent DNA adducts mediated by its oxidation with cytochromes P450 (CYP) and peroxidases. The article reports the molecular mechanism of ellipticine oxidation by CYPs and identifies human and rat CYPs responsible for ellipticine metabolic activation and detoxication. It also presents a role of peroxidases (i.e. myeloperoxidase, cyclooxygenases, lactoperoxidase) in ellipticine oxidation leading to ellipticine-DNA adducts. The 9-hydroxy- and 7-hydroxyellipticine metabolites formed by CYPs and the major product of ellipticine oxidation by peroxidases, the dimer, in which the two ellipticine skeletons are connected via N(6) of the pyrrole ring of one ellipticine molecule and C9 in the second one, are the detoxication metabolites. On the contrary, 13-hydroxy- and 12-hydroxyellipticine, produced by ellipticine oxidation with CYPs, the latter one formed also spontaneously from another CYP- and peroxidase-mediated metabolite, ellipticine N(2)-oxide, are metabolites responsible for formation of two ellipticine-derived deoxyguanosine adducts in DNA. The results reviewed here allow us to propose species, two carbenium ions, ellipticine-13-ylium and ellipticine-12-ylium, as reactive species generating two major DNA adducts seen in vivo in rats treated with ellipticine. The study forms the basis to further predict the susceptibility of human cancers to ellipticine.

Mammalian peroxidases activate anticancer drug ellipticine to intermediates forming deoxyguanosine adducts in DNA identical to those foundin vivo and generated from 12-hydroxyellipticine and 13-hydroxyellipticine

Ellipticine is a potent antineoplastic agent, whose mode of action is considered to be based mainly on DNA intercalation, inhibition of topoisomerase II and cytochrome P450-mediated formation of covalent DNA adducts. This is the first report on the molecular mechanism of ellipticine oxidation by peroxidases (human myeloperoxidase, human and ovine cyclooxygenases, bovine lactoperoxidase, horseradish peroxidase) to species forming ellipticine-DNA adducts. Using NMR spectroscopy, the structures of 2 ellipticine metabolites were identified; the major product is the ellipticine dimer, in which the 2 ellipticine skeletons are connected via N(6) of the pyrrole ring of one ellipticine molecule and C9 in the second one. The minor metabolite is ellipticine N(2)-oxide. Using (32)P-postlabeling and [(3)H]-labeled ellipticine, we showed that ellipticine binds covalently to DNA after its activation by peroxidases. The DNA adduct pattern induced by ellipticine consisted of a cluster of up to 4 adducts. The 2 adducts are indistinguishable from the 2 major adducts generated between deoxyguanosine in DNA and either 13-hydroxy- or 12-hydroxyellipticine or in rats treated with ellipticine, or if ellipticine was activated with human hepatic and renal microsomes. The results presented here are the first characterization of the peroxidase-mediated oxidative metabolites of ellipticine and we have proposed species, 2 carbenium ions, ellipticine-13-ylium and ellipticine-12-ylium, as reactive species generating 2 major DNA adducts seen in vivo in rats treated with ellipticine. The study forms the basis to further predict the susceptibility of human cancers to ellipticine.

Tumoral drug metabolism: overview and its implications for cancer therapy

Drug-metabolizing enzymes (DME) in tumors are capable of biotransforming a variety of xenobiotics, including antineoplastics, resulting in either their activation or detoxification. Many studies have reported the presence of DME in tumors; however, heterogeneous detection methodology and patient cohorts have not generated consistent, firm data. Nevertheless, various gene therapy approaches and oral prodrugs have been devised, taking advantage of tumoral DME. With the need to target and individualize anticancer therapies, tumoral processes such as drug metabolism must be considered as both a potential mechanism of resistance to therapy and a potential means of achieving optimal therapy. This review discusses cytotoxic drug metabolism by tumors, through addressing the classes of the individual DME, their relevant substrates, and their distribution in specific malignancies. The limitations of preclinical models relative to the clinical setting and lack of data on the changes of DME with disease progression and host response will be discussed. The therapeutic implications of tumoral drug metabolism will be addressed-in particular, the role of DME in predicting therapeutic response, the activation of prodrugs, and the potential for modulation of their activity for gain are considered, with relevant clinical examples. The contribution of tumoral drug metabolism to cancer therapy can only be truly ascertained through large-scale prospective studies and supported by new technologies for tumor sampling and genetic analysis such as microarrays. Only then can efforts be concentrated in the design of better prodrugs or combination therapy to improve drug efficacy and individualize therapy.

The Anticancer Drug Ellipticine Forms Covalent DNA Adducts, Mediated by Human Cytochromes P450, through Metabolism to 13-Hydroxyellipticine and Ellipticine N2-Oxide

Ellipticine is an antineoplastic agent, the mode of action of which is considered to be based on DNA intercalation and inhibition of topoisomerase II. We found that ellipticine also forms the cytochrome P450 (CYP)-mediated covalent DNA adducts. We now identified the ellipticine metabolites formed by human CYPs and elucidated the metabolites responsible for DNA binding. The 7-hydroxyellipticine, 9-hydroxyellipticine, 12-hydroxyellipticine, 13-hydroxyellipticine, and ellipticine N(2)-oxide are generated by hepatic microsomes from eight human donors. The role of specific CYPs in the oxidation of ellipticine and the role of the ellipticine metabolites in the formation of DNA adducts were investigated by correlating the levels of metabolites formed in each microsomal sample with CYP activities and with the levels of the ellipticine-derived deoxyguanosine adducts in DNA. On the basis of this analysis, formation of 9-hydroxyellipticine and 7-hydroxyellipticine was attributable to CYP1A1/2, whereas production of 13-hydroxyellipticine and ellipticine N(2)-oxide, the metabolites responsible for formation of two major DNA adducts, was attributable to CYP3A4. Using recombinant human enzymes, oxidation of ellipticine to 9-hydroxyellipticine and 7-hydroxyellipticine by CYP1A1/2 and to 13-hydroxyellipticine and N(2)-oxide by CYP3A4 was corroborated. Homologue modeling and docking of ellipticine to the CYP3A4 active center was used to explain the predominance of ellipticine oxidation by CYP3A4 to 13-hydroxyellipticine and N(2)-oxide.

Antiepileptic drugs inhibit cell growth in the human breast cancer cell line MCF7

Several antiepileptic drugs (AEDs) are associated with anti-cancer activity. At the same time, many AEDs alter endocrine function with phenytoin (PHT) and phenobarbital (PB) causing-reduced free fractions of sex-steroid hormones, while VPA induces hyperandrogenism. Changes in sex-steroid hormone levels are known to affect apoptosis in endocrine tissue. The aim of the study was to investigate the influence of the antiepileptic drugs PHT, PB, VPA and lamotrigine (LTG) on estrogen-stimulated cell growth of human breast cancer cells (MCF-7), and to evaluate whether this effect could be related to a direct estrogen receptor (ER) binding. VPA reduced cell growth at therapeutically relevant concentrations; half-maximum effect of VPA on cell growth was 230 microM. PHT (100 microM) and PB (10 microM) reduced cell growth by 47 and 21%, respectively. None of the drugs had affinity to isolated estrogen receptors, and excess of estrogen was not able to abolish the growth inhibition provoked by VPA. However, sub-therapeutic concentrations of VPA (100 microM) mimicked estrogen by inducing cell growth (11%) in an estrogen-depleted medium, an effect that was abolished by adding an estrogen receptor antagonist. In conclusion; the estrogen receptor appear to be indirectly activated by sub-therapeutic concentrations of VPA, but therapeutic concentrations of VPA inhibits cell growth by mechanisms that do not seem to involve the estrogen receptor or estrogen stimulation.

DNA Adduct Formation by the Anticancer Drug Ellipticine and Its Hydroxy Derivatives in Human Breast Adenocarcinoma MCF-7 Cells

The cytotoxicity of the antineoplastic agent ellipticine and its 9- and 7-hydroxylated metabolites to human breast adenocarcinoma MCF-7 cells and their ability to generate DNA adducts in these cancer cells were investigated. Ellipticine and its 9-hydroxylated metabolite were found to be toxic to MCF-7 cells with IC50values of 1.25 and 3.25 μmol l-1for ellipticine and 9-hydroxyellipticine, respectively. In contrast, no toxicity to these cancer cells was detectable for 7-hydroxyellipticine. The nuclease P1 version of the32P-postlabeling assay yielded a pattern of ellipticine-DNA adducts with two major and one minor adducts in MCF-7 cells, similar to the pattern of adducts detected in DNA reacted with ellipticine and the reconstituted cytochrome P450 enzyme systemin vitroand in DNAin vivo. The identity of two major adducts formed in DNA of MCF-7 cells with those formed by cytochrome P450-mediated ellipticine activationin vitrowas confirmed by HPLC of the isolated adducts. 9-Hydroxyellipticine was also capable of inducing DNA adducts in MCF-7 cells, but to a lesser extent. In addition, the adducts generated by 9-hydroxyellipticine were different from those generated by ellipticine. Negligible levels of DNA adducts were detectable in DNA of MCF-7 cells exposed to 7-hydroxyellipticine. The results presented here are the first report showing the formation of covalent DNA adducts with ellipticine in human breast cancer cells in culture, and suggest the formation of covalent DNA adducts as a new mode of antitumor action of ellipticine in breast cancer.

Inducibility of the P-glycoprotein transport activity in the marine mussel Mytilus galloprovincialis and the freshwater mussel Dreissena polymorpha

Previous investigations directed to the determination of the P-glycoprotein (Pgp) expression in aquatic organisms have indicated the possibility of the multixenobiotic resistance mechanism (MXR) induction as a response to organic pollution. However, in numerous cases no significant and/or no clear relationship between Pgp contents and pollution level was detected. Concerning these discrepancies the results of an extensive, 3-year study of the Pgp mediated MXR induction in the selected freshwater (Dreissena polymorpha) and marine (Mytilus galloprovincialis) bivalves are presented here. The main goals of the study were to ascertain the rate-dynamic, level, as well as the possible usability of MXR in environmental biomonitoring. Since the primary result of MXR induction should be the decreased intracellular accumulation of xenobiotics, the determination of MXR induction was performed using the measurement of Pgp transport activity. We measured the accumulation or the efflux rate of the model Pgp substrate rhodamine B (RB) in gills of the mussels previously exposed to pollution. The study was performed in several steps: from the exposure experiments in laboratory, using model inducers rhodamine 123 (R123) and water extract of Diesel-2 oil (D2), to the final in situ testing in real environmental conditions. Our results confirmed that Pgp activity is induced/induces according to the level of pollution, and that 4-days period was already long enough for the significant induction and deinduction of MXR activity. However, the inducibility of Pgp transport activity was significantly limited--the maximal level of induction obtained in this study resulted in 50-60% lower RB accumulation in the gills of induced specimens (laboratory or in situ exposed to pollution), when compared to control, non-induced animals. The obtained level of Pgp related MXR induction, resulting in halfway lesser accumulation of a model pollutant (RB), extrapolated to the similar scenario with toxic xenobiotics may have significant environmental relevance. However, our results also suggest that for the use of the MXR as a relevant biomarker the Pgp transport activity should be measured along with the determination of DNA, mRNA or/and protein expression. Based on the data from this study several factors that may have had critical influence on the effectiveness and the level of MXR induction are additionally discussed.

Human aldehyde dehydrogenases: potential pathological, pharmacological, and toxicological impact

Aldehyde dehydrogenases catalyze the pyridine nucleotide-dependent oxidation of aldehydes to acids. Seventeen enzymes are currently viewed as belonging to the human aldehyde dehydrogenase superfamily. Summarized herein, insofar as the information is available, are the structural composition, physical properties, tissue distribution, subcellular location, substrate specificity, and cofactor preference of each member of this superfamily. Also summarized are the chromosomal locations and organization of the genes that encode these enzymes and the biological consequences when enzyme activity is lost or substantially diminished. Broadly, aldehyde dehydrogenases can be categorized as critical for normal development and/or physiological homeostasis (1). even when the organism is in a friendly environment or (2). only when the organism finds itself in a hostile environment. The primary, if not sole, evolved raison d'être of first category aldehyde dehydrogenases appears to be to catalyze the biotransformation of a single endobiotic for which they are relatively specific and of which the resultant metabolite is essential to the organism. Most of the human aldehyde dehydrogenases for which the relevant information is available fall into this category. Second category aldehyde dehydrogenases are relatively substrate nonspecific and their evolved raison d'être seems to be to protect the organism from potentially harmful xenobiotics, specifically aldehydes or xenobiotics that give rise to aldehydes, by catalyzing their detoxification. Thus, the lack of a fully functional first category aldehyde dehydrogenase results in a gross pathological phenotype in the absence of any insult, whereas the lack of a functional second category aldehyde dehydrogenase is ordinarily of no consequence with respect to gross phenotype, but is of consequence in that regard when the organism is subjected to a relevant insult.

Transient induction of increased aldehyde dehydrogenase 3A1 levels in cultured human breast (adeno)carcinoma cell lines via 5'-upstream xenobiotic, and electrophile, responsive elements is, respectively, estrogen receptor-dependent and -independent

Transient up-regulation of ALDH3A1, CYP1A1 and CYP1B1 transcription by transient exposure to aryl hydrocarbon receptor (AhR) ligands, e.g. 3-methylcholanthrene, is via transient transactivation of xenobiotic responsive elements (XRE) present in the 5'-upstream regions of these genes. Others have shown that AhR ligand-mediated induction of increased CYP1A1 levels in cultured human breast (adeno)carcinoma cell lines is apparently estrogen receptor (ER)-dependent, i.e. it was observed in ER(+) cell lines but not in ER(-) cell lines, whereas AhR ligand-mediated induction of increased CYP1B1 levels is ER-independent, i.e. it was observed in both ER(+) and ER(-) cell lines. The present investigation established that transient, AhR ligand/XRE-mediated induction of increased ALDH3A1 levels in human breast (adeno)carcinoma cell lines was, like that of CYP1A1 and unlike that of CYP1B1, apparently ER-dependent. Thus, transient exposure to 3-methylcholanthrene induced increased levels of ALDH3A1 in five cultured human breast (adeno)carcinoma cell lines that were documented as being ER(+), viz., MCF-7/0, MCF-7/OAP, T-47D, ZR-75-1 and MDA-MB-468, but failed to induce increased levels of this enzyme in four cultured human breast (adeno)carcinoma cell lines that have been historically viewed as being ER(-), viz., MDA-MB-231, SK-BR-3, HS-578-T and MDA-MB-435. Somewhat at odds with the foregoing, transient exposure to 3-methylcholanthrene also induced increased levels of ALDH3A1 and CYP1A1 in cultured, essentially ER(-), human breast epithelial MCF-10A cells. These cells, like cultured human breast (adeno)carcinoma cells, are immortal, but unlike the latter, are not tumorigenic. Transient induction of increased ALDH3A1 levels can also be effected by agents that are not AhR ligands, viz., electrophiles such as catechol, and thus, cannot up-regulate ALDH3A1 transcription via transactivation of a 5'-upsteam region XRE. Rather, they are thought to up-regulate ALDH3A1 transcription via transient transactivation of an electrophile responsive element (EpRE) that is putatively also present in the 5'-upstream region of this gene. Electrophile-initiated/EpRE-mediated induction of increased ALDH3A1 levels was found to be ER-independent. Thus, catechol transiently induced increased levels of ALDH3A1 in the five ER(+) human breast (adeno)carcinoma cell lines, the four ER(-) human breast (adeno)carcinoma cell lines, and the ER(-), immortal but not tumorigenic, human breast epithelial cell line.

The anticancer agent ellipticine on activation by cytochrome P450 forms covalent DNA adducts

Ellipticine is a potent antitumor agent whose mechanism of action is considered to be based mainly on DNA intercalation and/or inhibition of topoisomerase II. Using [3H]-labeled ellipticine, we observed substantial microsome (cytochrome P450)-dependent binding of ellipticine to DNA. In rat, rabbit, minipig, and human microsomes, in reconstituted systems with isolated cytochromes P450 and in Supersomes containing recombinantly expressed human cytochromes P450, we could show that ellipticine forms a covalent DNA adduct detected by [32P]-postlabeling. The most potent human enzyme is CYP3A4, followed by CYP1A1, CYP1A2, CYP1B1, and CYP2C9. Another minor adduct is formed independent of enzymatic activation. The [32P]-postlabeling analysis of DNA modified by activated ellipticine confirms the covalent binding to DNA as an important type of DNA modification. The DNA adduct formation we describe is a novel mechanism for the ellipticine action and might in part explain its tumor specificity.

Three different stable human breast adenocarcinoma sublines that overexpress ALDH3A1 and certain other enzymes, apparently as a consequence of constitutively upregulated gene transcription mediated by transactivated EpREs (electrophile responsive elements) present in the 5'-upstream regions of these genes

ALDH3A1 catalyzes the detoxification of cyclophosphamide, mafosfamide, 4-hydroperoxycyclophosphamide and other oxazaphosphorines. Constitutive ALDH3A1 levels, as well as those of certain other drug-metabolizing enzymes, e.g. NQO1 and CYP1A1, are relatively low in cultured, relatively oxazaphosphorine-sensitive, human breast adenocarcinoma MCF-7 cells. However, transient cellular insensitivity to the oxazaphosphorines can be brought about in these cells by transiently elevating ALDH3A1 levels in them as a consequence of transient exposure to: (1) electrophiles such as catechol that induce the transcription of a battery of genes, e.g. ALDH3A1 and NQO1, having in common an electrophile responsive element (EpRE) in their 5'-upstream regions; or (2) Ah-receptor agonists, e.g. indole-3-carbinol and polycyclic aromatic hydrocarbons such as 3-methylcholanthrene, that induce the transcription of a battery of genes, e.g. ALDH3A1, NQO1 and CYP1A1, having in common a xenobiotic responsive element (XRE) in their 5'-upstream regions. Further, MCF-7 sublines that are constitutively, i.e. when grown in the absence of the original selecting pressure, relatively oxazaphosphorine-insensitive as a consequence of constitutively relatively elevated cellular ALDH3A1 levels evolved when MCF-7 cells were: (1) continuously exposed for several months to gradually increasing concentrations of 4-hydroperoxycyclophosphamide or benz(a)pyrene; or (2) briefly exposed (once for 30 min) to a high concentration (1 mM) of mafosfamide. Each of these three stable sublines is constitutively relatively cross-insensitive to benz(a)pyrene and other polycyclic aromatic hydrocarbons. Cellular levels of NQO1, but not of CYP1A1, are also constitutively relatively elevated in each of the three sublines. RT-PCR-based experiments established that ALDH3A1 mRNA levels are constitutively elevated ( approximately 5- to 8-fold) in each of the three sublines. The elevated ALDH3A1 mRNA levels are not the consequence of gene amplification, hypomethylation of a relevant regulatory element, or ALDH3A1 mRNA stabilization. Collectively, these observations suggest that constitutively elevated levels of ALDH3A1 and certain other enzymes in the three stable sublines are probably the consequence of a constitutive change in the cellular concentration of a key component of the EpRE signaling pathway, such that the cellular concentration of the relevant ultimate transactivating factor is constitutively elevated, i.e. gene transcription promoted by transactivated EpREs is constitutively upregulated. Further, constitutively upregulated gene transcription mediated by transactivated EpREs can be relatively easily induced, whereas that mediated by transactivated XREs cannot, at least in MCF-7 cells. Still further, the three sublines may facilitate study of the signaling pathway that leads to transactivation of the EpREs present in the 5'-upstream regions of ALDH3A1, NQO1 and other gene loci.

The multidrug-resistant phenotype associated with overexpression of the new ABC half-transporter, MXR (ABCG2)

Mechanisms of drug resistance other than P-glycoprotein are of increasing interest as the list of newly identified members of the ABC transport family has grown. We sought to characterize the phenotype of the newly discovered ABC transporter encoded by the mitoxantrone resistance gene, MXR, also known as ABCP1 or BCRP. The pharmacodynamics of mitoxantrone and 12 other fluorescent drugs were evaluated by confocal microscopy in four multidrug-resistant human colon (S1) and breast (MCF-7) cancer cell lines. We utilized two sublines, MCF-7 AdVp3000 and S1-M1-80, and detected overexpression of MXR by PCR, immunoblot assay and immunohistochemistry. These MXR overexpressing sublines were compared to cell lines with P-glycoprotein- and MRP-mediated resistance. High levels of cross-resistance were observed for mitoxantrone, the anthracyclines, bisantrene and topotecan. Reduced levels of mitoxantrone, daunorubicin, bisantrene, topotecan, rhodamine 123 and prazosin were observed in the two sublines with high MXR expression. Neither the P-glycoprotein substrates vinblastine, paclitaxel, verapamil and calcein-AM, nor the MRP substrate calcein, were extruded from MCF-7 AdVp3000 and S1-M1-80 cells. Thus, the multidrug-resistant phenotype due to MXR expression is overlapping with, but distinct from, that due to P-glycoprotein. Further, cells that overexpress the MXR protein seem to be more resistant to mitoxantrone and topotecan than cells with P-glycoprotein-mediated multidrug resistance. Our studies suggest that the ABC half-transporter, MXR, is a potent, new mechanism for conferring multiple drug resistance. Definition of its mechanism of transport and its role in clinical oncology is required.