Characterization of a chlorambucil-resistant human ovarian carcinoma cell line overexpressing glutathione S-transferase mu

Rhodamine B-Conjugated Fluorescent Block Copolymer Micelles for Efficient Chlorambucil Delivery and Intracellular Imaging

The clinical development of the anticancer drug chlorambucil (CHL) is limited by its low solubility in water, poor bioavailability, and off-target toxicity. Besides, another constraint for monitoring intracellular drug delivery is the non-fluorescent nature of CHL. Nanocarriers based on block copolymers of poly(ethylene glycol)/poly(ethylene oxide) (PEG/PEO) and poly(ε-caprolactone) (PCL) are an elegant choice for drug delivery applications due to their high biocompatibility and inherent biodegradability properties. Here, we have designed and prepared block copolymer micelles (BCM) containing CHL (BCM-CHL) from a block copolymer having fluorescent probe rhodamine B (RhB) end-groups to achieve efficient drug delivery and intracellular imaging. For this purpose, the previously reported tetraphenylethylene (TPE)-containing poly(ethylene oxide)-b-poly(ε-caprolactone) [TPE-(PEO-b-PCL)₂] triblock copolymer was conjugated with RhB by a feasible and effective post-polymerization modification method. In addition, the block copolymer was obtained by a facile and efficient synthetic strategy of one-pot block copolymerization. The amphiphilicity of the resulting block copolymer TPE-(PEO-b-PCL-RhB)₂ led to the spontaneous formation of micelles (BCM) in aqueous media and successful encapsulation of the hydrophobic anticancer drug CHL (CHL-BCM). Dynamic light scattering and transmission electron microscopy analyses of BCM and CHL-BCM revealed a favorable size (10-100 nm) for passive targeting of tumor tissues via the enhanced permeability and retention effect. The fluorescence emission spectrum (λ_ex 315 nm) of BCM demonstrated Förster resonance energy transfer between TPE aggregates (donor) and RhB (acceptor). On the other hand, CHL-BCM revealed TPE monomer emission, which may be attributed to the π-π stacking interaction between TPE and CHL molecules. The in vitro drug release profile showed that CHL-BCM exhibits drug release in a sustained manner over 48 h. A cytotoxicity study proved the biocompatibility of BCM, while CHL-BCM revealed significant toxicity to cervical (HeLa) cancer cells. The inherent fluorescence of RhB in the block copolymer offered an opportunity to directly monitor the cellular uptake of the micelles by confocal laser scanning microscopy imaging. These results demonstrate the potential of these block copolymers as drug nanocarriers and as bioimaging probes for theranostic applications.

Rational Design of an Amphiphilic Chlorambucil Prodrug Realizing Self-Assembled Micelles for Efficient Anticancer Therapy

The application of anticancer drug chlorambucil (CLB) in chemotherapy is severely restricted by its insolubility, lability, and toxic side effects; therefore, it is challenging to realize a highly efficient anticancer therapy of chlorambucil. To solve the above drawbacks encountered by chlorambucil, herein we proposed an amphiphilic chlorambucil prodrug-based self-assembled micelle strategy to realize the highly efficient anticancer therapy of chlorambucil. 1,6-Hexanediamine hydrochloride (HDH) serving as the hydrophilic segment was covalently bound to hydrophobic CLB to prepare an amphiphilic prodrug CLB-HDH which could self-assemble into micelles in aqueous solution. These micelles can passively target tumor tissues via the enhanced permeability and retention (EPR) effect, leading to enhanced cellular internalization. Both the cytotoxicity assay in vitro and anticancer study in vivo confirmed the excellent therapeutic activity of CLB-HDH micelles in comparison with free chlorambucil. Moreover, the hemolysis examination and histological analysis demonstrated the designed CLB-HDH micelles are safe in drug delivery. Therefore, our designed amphiphilic prodrug CLB-HDH micelles bring new opportunity for chlorambucil clinical application to combat cancers.

Triclosan Disrupts SKN-1/Nrf2-Mediated Oxidative Stress Response in C. elegans and Human Mesenchymal Stem Cells

Triclosan (TCS), an antimicrobial chemical with potential endocrine-disrupting properties, may pose a risk to early embryonic development and cellular homeostasis during adulthood. Here, we show that TCS induces toxicity in both the nematode C. elegans and human mesenchymal stem cells (hMSCs) by disrupting the SKN-1/Nrf2-mediated oxidative stress response. Specifically, TCS exposure affected C. elegans survival and hMSC proliferation in a dose-dependent manner. Cellular analysis showed that TCS inhibited the nuclear localization of SKN-1/Nrf2 and the expression of its target genes, which were associated with oxidative stress response. Notably, TCS-induced toxicity was significantly reduced by either antioxidant treatment or constitutive SKN-1/Nrf2 activation. As Nrf2 is strongly associated with aging and chemoresistance, these findings will provide a novel approach to the identification of therapeutic targets and disease treatment.

Biotransformation of xenobiotics in the human colon and rectum and its association with colorectal cancer

In humans, the liver is generally considered to be the major organ contributing to drug metabolism, but studies during the last years have suggested an important role of the extra-hepatic drug metabolism. The gastrointestinal tract (GI-tract) is the major path of entry for a wide variety of compounds including food, and orally administered drugs, but also compounds - with neither nutrient nor other functional value - such as carcinogens. These compounds are metabolized by a large number of enzymes, including the cytochrome P450 (CYP), the glutathione S-transferase (GST) family, the uridine 5'-diphospho- glucuronosyltransferase (UDP-glucuronosyltransferase - UGT) superfamily, alcohol-metabolizing enzymes, sulfotransferases, etc. These enzymes can either inactivate carcinogens or, in some cases, generate reactive species with higher reactivity compared to the original compound. Most data in this field of research originate from animal or in vitro studies, wherein human studies are limited. Here, we review the human studies, in particular the studies on the phenotypic expression of these enzymes in the colon and rectum to get an impression of the actual enzyme levels in this primary organ of exposure. The aim of this review is to give a summary of currently available data on the relation between the CYP, the GST and the UGT biotransformation system and colorectal cancer obtained from clinical and epidemiological studies in humans.

Quantitative PCR-based measurement of nuclear and mitochondrial DNA damage and repair in mammalian cells

In this chapter, we describe a gene-specific quantitative PCR (QPCR)-based assay for the measurement of DNA damage, using amplification of long DNA targets. This assay has been used extensively to measure the integrity of both nuclear and mitochondrial genomes exposed to different genotoxins and has proven to be particularly valuable in identifying reactive oxygen species-mediated mitochondrial DNA damage. QPCR can be used to quantify both the formation of DNA damage as well as the kinetics of damage removal. One of the main strengths of the assay is that it permits monitoring the integrity of mtDNA directly from total cellular DNA without the need for isolating mitochondria or a separate step of mitochondrial DNA purification. Here we discuss advantages and limitations of using QPCR to assay DNA damage in mammalian cells. In addition, we give a detailed protocol of the QPCR assay that helps facilitate its successful deployment in any molecular biology laboratory.

Structural modifications of mitochondria-targeted chlorambucil alter cell death mechanism but preserve MDR evasion

Multidrug resistance (MDR) remains one of the major obstacles in chemotherapy, potentially rendering a multitude of drugs ineffective. Previously, we have demonstrated that mitochondrial targeting of DNA damaging agents is a promising tool for evading a number of common resistance factors that are present in the nucleus or cytosol. In particular, mitochondria-targeted chlorambucil (mt-Cbl) has increased potency and activity against resistant cancer cells compared to the parent compound chlorambucil (Cbl). However, it was found that, due to its high reactivity, mt-Cbl induces a necrotic type of cell death via rapid nonspecific alkylation of mitochondrial proteins. Here, we demonstrate that by tuning the alkylating activity of mt-Cbl via chemical modification, the rate of generation of protein adducts can be reduced, resulting in a shift of the cell death mechanism from necrosis to a more controlled apoptotic pathway. Moreover, we demonstrate that all of the modified mt-Cbl compounds effectively evade MDR resulting from cytosolic GST-μ upregulation by rapidly accumulating in mitochondria, inducing cell death directly from within. In this study, we systematically elucidated the advantages and limitations of targeting alkylating agents with varying reactivity to mitochondria.

Inhibition and induction of glutathione S-transferases by flavonoids: possible pharmacological and toxicological consequences

Many studies reviewed herein demonstrated the potency of some flavonoids to modulate the activity and/or expression of glutathione S-transferases (GSTs). Because GSTs play a crucial role in the detoxification of xenobiotics, their inhibition or induction may significantly affect metabolism and biological effects of many drugs, industrials, and environmental contaminants. The effect of flavonoids on GSTs strongly depends on flavonoid structure, concentration, period of administration, as well as on GST isoform and origin. Moreover, the results obtained in vitro are often contrary to the vivo results. Based on these facts, the revelation of important flavonoid-drug or flavonoid-pollutant interaction has been complicated. However, it should be borne in mind that ingestion of certain flavonoids in combination with drugs or pollutants (e.g., acetaminophen, simvastatin, cyclophosphamide, cisplatine, polycyclic aromatic hydrocarbons, chlorpyrifos, acrylamide, and isocyanates), which are GST substrates, could have significant pharmacological and toxicological consequences. Although reasonable consumptions of a flavonoids-rich diet (that may lead to GST induction) are mostly beneficial, the uncontrolled intake of high concentrations of certain flavonoids (e.g., quercetin and catechins) in dietary supplements (that may cause GST inhibition) may threaten human health.

Glutathione transferases and development of new principles to overcome drug resistance

Chemoresistance is a multifactorial phenomenon and many studies clearly show that a coordinated expression of efflux transporter proteins and phase II conjugating enzymes in tumor cells is linked to the development of the multidrug resistance phenotype. In particular, the overexpression of glutathione S-transferases and efflux pumps in tumors may reduce the reactivity of various anticancer drugs. In recent years it has become evident that glutathione S-transferases are also involved in the control of apoptosis through the inhibition of the JNK signaling pathway. As such, the glutathione S-transferase superfamily has become the focus of extensive pharmaceutical research in attempt to generate more efficient anticancer agents. Here we present an overview of the GST inhibitors and the GST-activated pro-drugs utilized to date to overcome drug resistance.

Gene network analysis of oxidative stress-mediated drug sensitivity in resistant ovarian carcinoma cells

Drug resistance in cancer cells involves complex molecular mechanisms and ovarian carcinoma cells become resistant to chlorambucil (Cbl) after continuous treatment. This drug- and ionizing radiation-resistant cells have lower level of endogenous ROS (reactive oxygen species) compared with sensitive cells. Elevation of the cellular ROS level by exogenous ROS generation increases the sensitivity of Cbl to resistant cells. In contrast, antioxidants prevent the sensitization of resistant cells to Cbl by H(2)O(2), COS (chronic oxidative stress) or NOO(-). The molecular mechanism of drug sensitivity with COS has been investigated by microarray gene expressions followed by gene network analysis and it reveals that a cdc42/rac1 guanine exchange factor, ARHGEF6, with p53 and DNA-Pkc (PRKDC) is central to induce apoptosis in Cbl(cos) (Cbl with COS) cells. mRNA and protein levels of major gene network pathway differ significantly in Cbl(cos) cells than in Cbl-treated cells. Moreover, DNA-PKc physically interacts with ARHGEF6 and p53 mostly in the nucleus of Cbl-treated cells, whereas in Cbl(cos)-treated cells, its interactions are mostly in the cytoplasm. These results suggest that low doses of Cbl and very low doses of COS together kill Cbl-resistant ovarian carcinoma cells and ARHGEF6 signaling may have an instrumental role in induction of apoptosis in Cbl(cos) cells.

Structural basis for the binding of the anticancer compound 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol to human glutathione s-transferases

Glutathione S-transferases (GST) constitute a superfamily of enzymes with diversified functions including detoxification from xenobiotics. In many human cancers, Pi class GST (GSTP1-1) is overexpressed and contributes to multidrug resistance by conjugating chemotherapeutics. In addition, GSTP1-1 displays antiapoptotic activity by interacting with c-Jun NH(2)-terminal kinase, a key regulator of apoptosis. Therefore, GSTP1-1 is considered a promising target for pharmaceutical treatment. Recently, a potent inhibitor of GSTs, 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol (NBDHEX), was identified and tested on several tumor cell lines demonstrating high antiproliferative activity. To establish the structural basis of NBDHEX activity, we determined the crystal structure of NBDHEX bound to either GSTP1-1 or GSTM2-2 (mu class). NBDHEX in both cases binds to the H-site but occupies different positions. Furthermore, the compound is covalently attached to the GSH sulfur in the GSTM2-2 crystal, forming a sigma-complex, although it is bound but not conjugated in the GSTP1-1 crystal. Several differences in the H-sites of the two isozymes determine the higher affinity of NBDHEX for GSTM2-2 with respect to GSTP1-1. One such difference is the presence of Ile(104) in GSTP1-1 close to the bound NBDHEX, whereas the corresponding position is occupied by an alanine in GSTM2-2. Mutation of Ile(104) into valine is a frequent GSTP1-1 polymorphism and we show here that the Ile(104)Val and Ile(104)Ala variants display a 4-fold higher affinity for the compound. Remarkably, the GSTP1-1/Ile(104)Ala structure in complex with NBDHEX shows a considerable shift of the compound inside the H-site. These data might be useful for the development of new anticancer compounds.

Regulatory role of human AP-endonuclease (APE1/Ref-1) in YB-1-mediated activation of the multidrug resistance gene MDR1

Human AP-endonuclease (APE1/Ref-1), a central enzyme involved in the repair of oxidative base damage and DNA strand breaks, has a second activity as a transcriptional regulator that binds to several trans-acting factors. APE1 overexpression is often observed in tumor cells and confers resistance to various anticancer drugs; its downregulation sensitizes tumor cells to such agents. Because the involvement of APE1 in repairing the DNA damage induced by many of these drugs is unlikely, drug resistance may be linked to APE1's transcriptional regulatory function. Here, we show that APE1, preferably in the acetylated form, stably interacts with Y-box-binding protein 1 (YB-1) and enhances its binding to the Y-box element, leading to the activation of the multidrug resistance gene MDR1. The enhanced MDR1 level due to the ectopic expression of wild-type APE1 but not of its nonacetylable mutant underscores the importance of APE1's acetylation in its coactivator function. APE1 downregulation sensitizes MDR1-overexpressing tumor cells to cisplatin or doxorubicin, showing APE1's critical role in YB-1-mediated gene expression and, thus, drug resistance in tumor cells. A systematic increase in both APE1 and MDR1 expression was observed in non-small-cell lung cancer tissue samples. Thus, our study has established the novel role of the acetylation-mediated transcriptional regulatory function of APE1, making it a potential target for the drug sensitization of tumor cells.

Defining regional infusion treatment strategies for extremity melanoma: comparative analysis of melphalan and temozolomide as regional chemotherapeutic agents

Five different human melanoma xenografts were used in a xenograft model of extremity melanoma to evaluate the variability of tumor response to regionally administered melphalan or temozolomide and to determine if various components of pertinent drug resistance pathways for melphalan [glutathione S-transferase (GST)/glutathione] and temozolomide [O(6)-alkylguanine DNA alkyltranferase (AGT)/mismatch repair (MMR)] could be predictive of tumor response. Xenograft-bearing rats underwent regional isolated limb infusion with either melphalan (90 mg/kg) or temozolomide (2,000 mg/kg). The levels of AGT activity, GST activity, glutathione level, and GST/AGT expression were examined in this group of xenografts and found to be quite heterogeneous. No correlation was identified between melphalan sensitivity and the GST/glutathione cellular detoxification pathway. In contrast, a strong correlation between the levels of AGT activity and percentage increase in tumor volume on day 30 (r = 0.88) was noted for tumors treated with temozolomide. Regional therapy with temozolomide was more effective when compared with melphalan for the xenograft with the lowest AGT activity, whereas melphalan was more effective than temozolomide in another xenograft that had the highest AGT activity. In three other xenografts, there was no significant difference in response between the two chemotherapy agents. This study shows that AGT activity may be useful in predicting the utility of temozolomide-based regional therapy for advanced extremity melanoma tumors. Our observations also point out the limited ability of analysis of the GST/glutathione pathway to predict response to chemotherapies like melphalan whose resistance is primarily mediated through a complex mechanism of detoxification.

Proteomic Analysis of the Resistance to Aplidin in Human Cancer Cells

Aplidin (plitidepsin) is an antitumoral agent that induces apoptosis via Rac1-JNK activation. A proteomic approach using 2D-DIGE technology found 52 cytosolic and 39 membrane proteins differentially expressed in wild-type and Aplidin-resistant HeLa cells, of which 39 and 27 were identified by MALDI-TOF mass spectrometry and database interrogation. A number of proteins involved in apoptosis pathways were found to be deregulated. Alterations in Rab geranylgeranyltransferase, protein disulfide isomerase (PDI), cystathionine gamma-lyase, ezrin, and cyclophilin A (CypA) were confirmed by immunoblotting. Moreover, the role of PDI and CypA in Aplidin resistance was functionally confirmed by using the inhibitor bacitracin and overexpression, respectively. These deregulated proteins are candidates to mediate, at least partially, Aplidin action and might provide a route to the cells to escape the induction of apoptosis by this drug.

Acridine Orange based platinum(II) complexes inducing cytotoxicity and cell cycle perturbation in spite of GSTP1 up-regulation

A series of new ionic Pt(II) complexes of general formula [Pt(II)(A)n(Cl)(AO)]X (A=en, NH3; n=1, 2; X-=BF4-, NO3-, PF6-, CF3SO3-), 1-5, containing Acridine Orange (AO) bound to the metal atom through the endocyclic N atom, have been tested in human melanoma cells (M14, JR8 and PLF2), human neuroblastoma cell line SH-SY5Y and its cis-platin resistant subline SH-SY5Yres. The Pt(II) compounds, and in particular complexes 1 and 4, exhibit higher cytotoxic activity at lower concentration compared to cis-DDP in melanoma cells, affecting cell growth behavior and causing cell cycle perturbation. Moreover, M14 and JR8 cell lines were not able to rescue the impairment due to the new Pt(II) complexes since perturbation of cell cycle phases and cell proliferation inhibition were found after 72 h of recovery time. In order to evaluate whether GSTP1 may play a role in chemo-resistance of our melanoma model, we investigated the effect of the treatment with these Pt(II) compounds on GSTP1 gene expression. Up-regulation of GSTP1, evaluated by Qreal-time PCR was observed after treatment with complexes 1 and 4, showing that the effect of these Pt(II) compounds is GSTP1 indipendent. The lack of resistance of the new Pt(II)-AO complexes and their cytotoxicity, cell growth and cell cycle recovery in melanoma cells provide the basis for the development of new platinum anticancer compounds, directed to those tumors that over express GSTs enzymes.

Free radical theory of autoimmunity

Background

Despite great advances in clinical oncology, the molecular mechanisms underlying the failure of chemotherapeutic intervention in treating lymphoproliferative and related disorders are not well understood.

Hypothesis

A hypothetical scheme to explain the damage induced by chemotherapy and associated chronic oxidative stress is proposed on the basis of published literature, experimental data and anecdotal observations. Brief accounts of multidrug resistance, lymphoid malignancy, the cellular and molecular basis of autoimmunity and chronic oxidative stress are assembled to form a basis for the hypothesis and to indicate the likelihood that it is valid in vivo.

Conclusion

The argument set forward in this article suggests a possible mechanism for the development of autoimmunity. According to this view, the various sorts of damage induced by chemotherapy have a role in the pattern of drug resistance, which is associated with the initiation of autoimmunity.

Enhanced γ-glutamylcysteine synthetase activity decreases drug-induced oxidative stress levels and cytotoxicity

Multidrug resistance of cancer cells can be intrinsic or acquired and occurs due to various reasons, including increased repair of genotoxic damage, an enhanced ability to remove/detoxify chemical agents, or reactive oxygen species (ROS), and repression of apoptosis. Human A2780/100 ovarian carcinoma cells exhibit resistance to DNA cross-linking agents, chlorambucil (Cbl), cisplatin (Cpl), melphalan (Mel), and ionizing radiation (IR) compared to the parental cell line, A2780. In the present study, we show that when A2780/100 and A2780 cells were treated with Cbl, GSH was extruded via methionine or cystathionine-inhibitable transporters of intact plasma membrane. GSH loss was followed by a rapid increase in ROS levels. The resistant, but not drug-sensitive cells normalized the intracellular GSH concentration along with ROS levels within 4-6 h after Cbl addition, and survived drug treatment. Normalization of GSH and ROS levels in A2780/100 cells correlated well with elevated gamma-glutamylcysteine synthetase (gamma-GCS) activity (10 +/- 1.8-fold over A2780 cells). Ectopic overexpression of the gamma-GCS heavy subunit in drug-sensitive cells nearly restored GSH and ROS to pre-treatment levels consequently increased cellular resistance to genotoxic agents (Cbl, Cpl, and IR), while overexpression of gamma-GCS light subunit had no such effects. Thus, in our model system, drug-resistant cells have the inherent ability to maintain increased gamma-GCS activity, reestablish physiological GSH, and cellular redox state and maintain increased cellular resistance to DNA cross-linking agents and IR.

SYNTHESIS AND CHARACTERIZATION OF SOME NEW PHASE II METABOLITES OF THE ALKYLATOR BENDAMUSTINE AND THEIR IDENTIFICATION IN HUMAN BILE, URINE, AND PLASMA FROM PATIENTS WITH CHOLANGIOCARCINOMA

The alkylating agent bendamustine is currently in phase III clinical trials for the treatment of hematological malignancies and breast, lung, and gastrointestinal tumors. Renal elimination mainly as the parent compound is thought to be the primary route of excretion. Because polar biliary conjugates were expected metabolites of bendamustine, three cysteine S-conjugates were synthesized, purified by quantitative high-performance liquid chromatography (HPLC), and characterized by NMR spectroscopy and mass spectrometry (MS). HPLC assays with MS, as well as fluorescence detection of bile, urine, and plasma after single-dose intravenous infusion of 140 mg/m(2) bendamustine in five subjects with cholangiocarcinoma, indicated the existence of these phase II metabolites, which were identified as cysteine S-conjugates by comparison with the previously characterized synthetic reference standards. The sum of the three cysteine S-conjugates of bendamustine was determined in human bile and urine to be 95.8 and 26.0%, respectively, expressed as mean percentage of the sum of the parent compound and identified metabolites. The percentage of administered dose recovered in urine as cysteine S-conjugates ranged from 0.9 to 4.1%, whereas the total percentage of the administered dose excreted in urine as the parent drug and seven metabolites ranged from 3.8 to 16.3%. The identification of cysteine S-conjugates provide evidence that a major route of bendamustine metabolism in humans involves conjugation with glutathione. Results indicate the importance of phase II conjugation in the elimination of bendamustine, besides phase I metabolism and hydrolytic degradation, and require further investigation.

Metabolism of melphalan by rat liver microsomal glutathione S-transferase

One of the major problems in the treatment of human cancer is the phenomenon of drug resistance. Increased glutathione (gamma-glutamylcysteinylglycine, GSH) conjugation (inactivation) due to elevated level of cytosolic glutathione S-transferase (GST) is believed to be an important mechanism in tumor cell resistance. However, the potential involvement of microsomal GST in the establishment of acquired drug resistance (ADR) remains uncertain. In our experiments, a combination of liquid chromatography/electrospray ionization/mass spectrometry (LC/ESI/MS) was employed for structural characterization of the resulting conjugates between GSH and melphalan, one of the alkylating agents. The spontaneous reaction of 1mM melphalan with 5mM GSH at 37 degrees C in aqueous phosphate buffer for 1h gave primarily the monoglutathionyl and diglutathionyl melphalan derivatives, with small amounts of mono- and dihydroxy melphalan derivatives. We demonstrated that rat liver microsomal GST presented a strong catalytic effect on the reaction as determined by the increase of monoglutathionyl and diglutathionyl melphalan derivatives and the decrease of melphalan. We showed that microsomal GST was activated by melphalan in a concentration- and time-dependent manner. Microsomal GST which was stimulated approximately 1.5-fold with melphalan had a stronger catalytic effect. Thus microsomal GST may play a potential role in the metabolism of melphalan in biological membranes, and in the development of ADR.

Inhibition of glutathione S-transferases by thonningianin A, isolated from the African medicinal herb, Thonningia sanguinea, in vitro

There is evidence that increased expression of glutathione S-transferase (EC: 2.5.1.18, GST) is involved in resistance of tumor cells against chemotherapeutic agents. In this study we investigated the inhibitory effects of thonningianin A (Th A), a novel antioxidant isolated from the medicinal herb, Thonningia sanguinea on uncharacterized rat liver GST and human GST P1-1. Using 1-chloro-2,4-dinitrobenzene (CDNB) as substrate, rat liver cytosolic GST activity was inhibited by Th A in a concentration dependent manner with 50% inhibition concentration (IC50) of 1.1 microM. When Th A was compared with known potent GST inhibitors the order of inhibition was tannic acid>cibacron blue>hematin>Th A>ethacrynic acid with CDNB as substrate. Th A also exhibited non-competitive inhibition towards both CDNB and glutathione. Furthermore, using 1,2-dichloro-4-nitrobenzene, ethacrynic acid and 1,2-epoxy-3-(p-nitrophenoxy) propane as substrates Th A at 1.0 microM inhibited cytosolic GST by 2%, 12% and 36% respectively. Human GST P1-1 was also inhibited by Th A with an IC50 of 3.6 microM. While Th A showed competitive inhibition towards CDNB it exhibited non-competitive inhibition towards GSH of the human GST P1-1. These results suggest that Th A represents a new potent GST in vitro inhibitor.

Metabolism of chlorambucil by rat liver microsomal glutathione S-transferase

Clinical efficacy of alkylating anticancer drugs, such as chlorambucil (4-[p-[bis [2-chloroethyl] amino] phenyl]-butanoic acid; CHB), is often limited by the emergence of drug resistant tumor cells. Increased glutathione (gamma-glutamylcysteinylglycine; GSH) conjugation (inactivation) of alkylating anticancer drugs due to overexpression of cytosolic glutathione S-transferase (GST) is believed to be an important mechanism in tumor cell resistance to alkylating agents. However, the potential involvement of microsomal GST in the establishment of acquired drug resistance (ADR) to CHB remains uncertain. In our experiments, a combination of lipid chromatography/electrospray ionization mass spectrometry (LC/ESI/MS) was employed for structural characterization of the resulting conjugates between CHB and GSH. The spontaneous reaction of 1mM CHB with 5 mM GSH at 37 degrees C in aqueous phosphate buffer for 1 h gave primarily the monoglutathionyl derivative, 4-[p-[N-2-chloroethyl, N-2-S-glutathionylethyl] amino]phenyl]-butanoic acid (CHBSG) and the diglutathionyl derivative, 4-[p-[2-S-glutathionylethyl] amino]phenyl]-butanoic acid (CHBSG2) with small amounts of the hydroxy-derivative, 4-[p-[N-2-S-glutathionylethyl, N-2-hydroxyethyl] amino]phenyl]-butanoic acid (CHBSGOH), 4-[p-[bis[2-hydroxyethyl] amino]phenyl]-butanoic acid (CHBOH2), 4-[p-[N-2-chloroethyl, N-2-S-hydroxyethyl]amino]phenyl]-butanoic acid (CHBOH). We demonstrated that rat liver microsomal GST presented a strong catalytic effect on these reactions as determined by the increase of CHBSG2, CHBSGOH and CHBSG and the decrease of CHB. We showed that microsomal GST was activated by CHB in a concentration and time dependent manner. Microsomal GST which was stimulated approximately two-fold with CHB had a stronger catalytic effect. Thus, microsomal GST may play a potential role in the metabolism of CHB in biological membranes, and in the development of ADR.

GlutathioneS-Transferase M1 and Multidrug Resistance Protein 1 Act in Synergy to Protect Melanoma Cells from Vincristine Effects

Previous studies have shown that glutathione S-transferases (GSTs) can operate in synergy with efflux transporters, multi-drug resistance proteins (MRPs), to confer resistance to several carcinogens, mutagens and anticancer drugs. To address the poorly documented role of the GSTM1 in cancer chemoresistance, we used CAL1 human melanoma cells expressing no endogenous GSTM1 and a high level of MRP1. Cells were transfected with an expression vector containing the GSTM1 cDNA, and different clones were selected expressing different levels of GSTM1 (RT-PCR, Western blot, and enzyme activity). Cells overexpressing GSTM1 displayed a 3- to 4-fold increase in resistance to anticancer drugs vincristine (VCR) and chlorambucil (CHB) in proliferation, cytotoxic, and clonogenic survival assays. Inhibitors of MRP1 (sulfinpyrazone, verapamil) and GST (dicumarol, curcumin) completely reversed the GSTM1-associated resistance to VCR, indicating that a MRP efflux function is necessary to potentiate GSTM1-mediated resistance to VCR. Conversely, MRP1 inhibitors had no effect on the sensitivity to CHB. Using immunofluorescence assay, GSTM1 was also shown to protect microtubule network integrity from VCR-induced inhibition of microtubule polymerization. In conclusion, these results show that GSTM1 alone is involved in melanoma resistance to CHB, whereas it can act in synergy with MRP1 to protect cells from toxic effects of VCR.

Reduced DNA double strand breaks in chlorambucil resistant cells are related to high DNA-PKcs activity and low oxidative stress

Modulation of DNA repair represents a strategy to overcome acquired drug resistance of cells to genotoxic chemotherapeutic agents, including nitrogen mustards (NM). These agents induce DNA inter-strand cross-links, which in turn produce double strand breaks (dsbs). These breaks are primarily repaired via the nonhomologous end-joining (NHEJ) pathway. A DNA-dependent protein kinase (DNA-PK) complex plays an important role in NHEJ, and its increased level/activity is associated with acquired drug resistance of human tumors. We show in this report that the DNA-PK complex has comparable levels and kinase activity of DNA-PK catalytic subunit (DNA-PKcs) in a nearly isogenic pair of drug-sensitive (A2780) and resistant (A2780/100) cells; however, treatment with chlorambucil (Cbl), a NM-type of drug, induced differential effects in these cells. The kinase activity of DNA-PKcs was increased up to 2h after Cbl treatment in both cell types; however, it subsequently decreased only in sensitive cells, which is consistent with increased levels of DNA dsbs. The decreased kinase activity of DNA-PKcs was not due to a change in its amount or the levels of Ku70 and Ku86, their subcellular distribution, cell cycle progression or caspase-mediated degradation of DNA-PK. In addition to DNA cross-links, Cbl treatment of cells causes a 2.2-fold increase in the level of reactive oxygen species (ROS) in both cell types. However, the ROS in A2780/100 cells were reduced to the basal level after 3-4h, while sensitive cells continued to produce ROS and undergo apoptosis. Pre-treatment of A2780 cells with the glutathione (GSH) precursor, N-acetyl-L-cysteine prevented Cbl-induced increase in ROS, augmented the kinase activity of DNA-PKcs, decreased the levels of DNA dsbs and increased cell survival. Depletion in GSH from A2780/100 cells by L-buthionine sulfoximine (BSO) resulted in sustained production of ROS, lowered DNA-PKcs kinase activity, enhanced levels of DNA dsbs, and increased cell killing by Cbl. We propose that oxidative stress decreases repair of DNA dsbs via lowering kinase activity of DNA-PKcs and that induction of ROS could be the basis for adjuvant therapies for sensitizing tumor cells to nitrogen mustards and other DNA cross-linking drugs.

Gene expression patterns in ovarian carcinomas

We used DNA microarrays to characterize the global gene expression patterns in surface epithelial cancers of the ovary. We identified groups of genes that distinguished the clear cell subtype from other ovarian carcinomas, grade I and II from grade III serous papillary carcinomas, and ovarian from breast carcinomas. Six clear cell carcinomas were distinguished from 36 other ovarian carcinomas (predominantly serous papillary) based on their gene expression patterns. The differences may yield insights into the worse prognosis and therapeutic resistance associated with clear cell carcinomas. A comparison of the gene expression patterns in the ovarian cancers to published data of gene expression in breast cancers revealed a large number of differentially expressed genes. We identified a group of 62 genes that correctly classified all 125 breast and ovarian cancer specimens. Among the best discriminators more highly expressed in the ovarian carcinomas were PAX8 (paired box gene 8), mesothelin, and ephrin-B1 (EFNB1). Although estrogen receptor was expressed in both the ovarian and breast cancers, genes that are coregulated with the estrogen receptor in breast cancers, including GATA-3, LIV-1, and X-box binding protein 1, did not show a similar pattern of coexpression in the ovarian cancers.

Conjugation of chlorambucil with GSH by GST purified from human colon adenocarcinoma cells and its inhibition by plant polyphenols

Chlorambucil (CMB) combines with glutathione (GSH) spontaneously in vitro to form monochloromonoglutathionyl CMB (MG-CMB). This was identified and quantified by an HPLC-UV method. Glutathione S-transferase (GST) purified from human colon adenocarcinoma cells increased the formation of the conjugate significantly. The GST-mediated conjugation, represented by the difference between total and spontaneous conjugation showed Michaelis-Menten kinetics with apparent Km and Vmax values of 0.2 mM and 75.8 nmol/min/mg for CMB and 5.2 mM and 127.0 nmol/min/mg for GSH respectively. Unexpectedly, we found in our study that both the spontaneous and the enzymatic conjugation of chlorambucil with GSH were affected markedly by a change in pH from 6.0 to 8.0. The optimum for the enzymatic conjugation was about 7.0, above which the spontaneous conjugation increased rapidly, while the enzymatic conjugation became lower. The plant polyphenols namely tannic acid, butein, quercetin, morin, 2-hydroxychalcone and 2'-hydroxychalcone at 40 microM inhibited the GST-mediated conjugation of CMB with GSH by 38 to 62%. Their action in this respect may contribute to sensitisation of tumour cells to anticancer drugs.

Differential protein expression in the cytosol fraction of an MCF-7 breast cancer cell line selected for resistance toward melphalan

Analysis of differential protein expression in the cytosol of melphalan-resistant and -susceptible MCF-7 cell lines has been carried out using a combination of two-dimensional gel electrophoresis, mass spectrometry, and bioinformatics. Comparison of multiple digitized gel arrays detected several spots as candidates for differentially expressed proteins in melphalan-resistant MCF-7 cells. The up-regulated proteins included retinoic acid binding protein II, an isoform of the macrophage migration inhibition factor, and other unidentified proteins. The down-regulated proteins included calreticulin, cyclophin A, and an isoform of the 27 kD heat shock protein. Correlation of the differential expression of some of the proteins with acquired resistance of MCF7 cells to melphalan is discussed.

Biochemical changes associated with a multidrug-resistant phenotype of a human glioma cell line with temozolomide-acquired resistance

Temozolomide (TMZ) is a newly approved alkylating agent for the treatment of malignant gliomas. To investigate resistance mechanisms in a multidrug therapeutic approach, a TMZ-resistant human glioma cell line, SF188/TR, was established by stepwise exposure of human SF188 parental cells to TMZ for approximately 6 months. SF188/TR showed 6-fold resistance to TMZ and cross-resistance to a broad spectrum of other anticancer agents that included 3-5-fold resistance to melphalan (MEL), gemcitabine (GEM), paclitaxel (PAC), methotrexate (MTX), and doxorubicin (DOX), and 1.6-2-fold resistance to cisplatin (CDDP) and topotecan (TPT). Alkylguanine alkyltransferase (AGT) activity was increased significantly in the resistant cell line compared with the parental cell line (P<0.05), whereas no significant differences occurred in the cellular uptake of TMZ and PAC between resistant and parental cells. Depletion of AGT by O(6)-benzylguanine significantly increased the cytotoxicity of TMZ in both the sensitive and resistant cell lines, but did not influence the cytotoxicity of the other drugs tested. Treatment with TMZ caused SF188 cells to accumulate in S phase, whereas SF188/TR cells were unaffected. Expression of Bcl-2 family members in SF188/TR cells compared with SF188 cells indicated that the pro-apoptotic proteins (i.e. Bad, Bax, Bcl-X(S)) were reduced 2-4-fold in the resistant cell line, whereas the anti-apoptotic proteins Bcl-2 and Bcl-X(L) were expressed at similar levels in both cell lines. In conclusion, the mechanism of resistance of SF188/TR cells to TMZ involved increased activity of AGT, a primary resistance mechanism, whereas the broad cross-resistance pattern to other anticancer drugs was due to a common secondary resistance mechanism related to alterations in the relative expression of the pro-apoptotic and anti-apoptotic proteins.

Inhibition of glutathione S-transferases by antimalarial drugs possible implications for circumventing anticancer drug resistance

A strategy to overcome multidrug resistance in cancer cells involves treatment with a combination of the antineoplastic agent and a chemomodulator that inhibits the activity of the resistance-causing protein. The aim of our study was to investigate the effects of antimalarial drugs on human recombinant glutathione S-transferase (GSTs) activity in the context of searching for effective and clinically acceptable inhibitors of these enzymes. Human recombinant GSTs heterologously expressed in Escherichia coli were used for inhibition studies. GST A1-1 activity was inhibited by artemisinin with an IC(50) of 6 microM, whilst GST M1-1 was inhibited by quinidine and its diastereoisomer quinine with IC(50)s of 12 microM and 17 microM, respectively. GST M3-3 was inhibited by tetracycline only with an IC(50) of 47 microM. GST P1-1 was the most susceptible enzyme to inhibition by antimalarials with IC(50) values of 1, 2, 1, 4, and 13 microM for pyrimethamine, artemisinin, quinidine, quinine and tetracycline, respectively. The IC(50) values obtained for artemisinin, quinine, quinidine and tetracycline are below peak plasma concentrations obtained during therapy of malaria with these drugs. It seems likely, therefore, that GSTs may be inhibited in vivo at doses normally used in clinical practice. Using the substrate ethacrynic acid, a diuretic drug also used as a modulator to overcome drug resistance in tumour cells, GST P1-1 activity was inhibited by tetracycline, quinine, pyrimethamine and quinidine with IC(50) values of 18, 27, 45 and 70 microM, respectively. The ubiquitous expression of GSTs in different malignancies suggests that the addition of nontoxic reversing agents such as antimalarials could enhance the efficacy of a variety of alkylating agents.

hMYH cell cycle-dependent expression, subcellular localization and association with replication foci: evidence suggesting replication-coupled repair of adenine:8-oxoguanine mispairs

The human MutY homolog, hMYH, is an adenine-specific DNA glycosylase that removes adenines or 2-hydroxyadenines mispaired with guanines or 8-oxoguanines. In order to prevent mutations, this activity must be directed to the newly synthesized strand and not the template strand during DNA synthesis. The subcellular localization and expression of hMYH has been studied in serum-stimulated, proliferating MRC5 cells. Using specific antibodies, we demonstrate that endogenous hMYH protein localized both to nuclei and mitochondria. hMYH in the nuclei is distinctly distributed and co-localized with BrdU at replication foci and with proliferating cell nuclear antigen (PCNA). The levels of hMYH in the nucleus increased 3- to 4-fold during progression of the cell cycle and reached maximum levels in S phase compared to early G(1). Similar results were obtained for PCNA, while there were no notable changes in expression of 8-oxoguanine glycosylase or the human MutT homolog, MTH1, throughout the cell cycle. The cell cycle-dependent expression and localization of hMYH at sites of DNA replication suggest a role for this glycosylase in immediate post-replication DNA base excision repair.

Activity of allelic variants of Pi class human glutathione S-transferase toward chlorambucil

Clinical efficacy of alkylating anticancer drugs, such as chlorambucil, is often limited by the emergence of drug resistant tumor cells. Increased glutathione (GSH) conjugation (inactivation) of alkylating anticancer drugs or their activated metabolites due to overexpression of the Pi class GSH S-transferase (hGSTP1-1) is believed to be an important mechanism in tumor cell resistance to alkylating agents. Interestingly, the hGSTP1 locus is polymorphic in human populations and involves amino acid residues in positions 104 (isoleucine or valine) and/or 113 (alanine or valine). Here, we report that the allelic variants of hGSTP1-1 significantly differ in their efficiency in catalyzing the GSH conjugation of chlorambucil. Catalytic efficiency of the hGSTP1-1(I104,A113) isoform toward chlorambucil was approximately 2.5-, 7.5- and 15-fold higher compared with I104,V113, V104,A113 and V104,V113 variants of hGSTP1-1, respectively. The results of the present study suggest that hGSTP1-1 polymorphism may be an important factor in GST-mediated tumor cell resistance to some alkylating agents.

Acquired alkylating drug resistance of a human ovarian carcinoma cell line is unaffected by altered levels of pro- and anti-apoptotic proteins

In a systematic study to elucidate the involvement of pro- and anti-apoptotic proteins in alkylating drug resistance of tumor cells, we utilized the A2780(100) line, that was selected by repeated exposure of A2780 cell line (human ovarian carcinoma line) to chlorambucil (CBL). A2780(100) was 5 - 10-fold more resistant to nitrogen mustards (IC50 of 50 - 60 microM) and other DNA crosslinking agents, e.g., cisplatin, and also to DNA topoisomerase inhibitor etoposide (ETO) than A2780. CBL (125 microM) induced extensive apoptosis in A2780 associated with mitochondrial damage but not in A2780(100). No significant differences were observed between A2780 and A2780(100) cells in the basal levels, or the enhanced levels in some cases after CBL treatment, of DNA repair proteins involved in repair of alkyl base adducts or in repair of DNA crosslinks or double strand break repair. However, the basal levels of anti-apoptotic proteins Bcl-xL and Mcl-1 were 4 - 8-fold higher in A2780(100) than in A2780 neither of which expressed Bcl-2. In contrast, the levels of pro-apoptotic Bax and Bak were 3 - 5-fold higher in the CBL-treated A2780 but not in A2780(100). ETO (5 microM) induced apoptosis in A2780 without altering the levels of Bax and Bak in these cells. At the same time, neither overexpression of Bcl-xL in A2780, nor its antisense expression in A2780(100), and nor overexpression of Bax in A2780(100), significantly affected drug sensitivity of either line. Our results suggest that a change in an early step in DNA damage processing which affects intracellular signaling, such as enhanced DNA double-strand break repair, could be the primary cause for development of resistance in A2780(100) cells to drugs which induce DNA crosslinks or double strand-breaks.