Carcinogen-induced mdr overexpression is associated with xenobiotic resistance in rat preneoplastic liver nodules and hepatocellular carcinomas

Altered expression of the xenobiotic transporter P-glycoprotein in liver and liver tumours of mummichog Fundulus heteroclitus from a creosote-contaminated environment

P-glycoproteins (Pgps) are involved in efflux of xenobiotics from drug-resistant cell lines and tumours, and in excretion of toxicants from normal tissues. Recently, investigators have proposed that Pgp activity contributes to resistance or tolerance of certain aquatic species to pollutants. In the present study using immunoblot and immunohistochemical techniques, we found elevation of Pgp in liver and liver tumours of creosote-resistant mummichog from a contaminated site in the Elizabeth River, Virginia. Immunoblots of mummichog liver extracts showed an immunoreactive band at 170 kDa and indicated two- to three-fold elevation of Pgp in livers of resistant fish relative to those from a reference site. Laboratory exposures of reference site fish to a model PAH (3-methylcholanthrene), however, produced no increase in liver Pgp levels as measured by immunoblot. Normal mummichog liver sections showed specific immunohistochemical staining for Pgp on the canalicular surface of hepatocytes. In the majority of hepatic neoplasms we observed a high level of over-expression and altered patterns of Pgp expression. However we did not observe Pgp over-expression in early proliferative lesions. Elevation of Pgp in livers and liver tumoursof these resistant mummichog may contribute to their survival in a heavily contaminated environment.

Implication of cancer stem cells in cancer drug development and drug delivery

Conventional therapy against cancer by chemotherapy and radiation therapy has proven successful at decreasing bulk tumor size when cancer is diagnosed early enough. These therapeutic approaches, however, often result in recurrence years later. A number of studies have begun to identify a subpopulation of tumor-initiating cells that may be critical to early tumorigenesis and subsequent recurrence. These cells have been termed cancer stem cells (CSCs) and have been demonstrated to have properties of self-renewal, differentiation into other tumor cell types, and enhanced drug resistance. Given the growing body of evidence that CSCs play a key role in tumor biology in many cancer types, it is important to take into account the properties of these cells when developing novel cancer drugs and designing enhanced methods of drug delivery. This review covers some of the mechanisms by which CSCs escape conventional therapy as well as the potential approaches to targeting CSCs that may be used during cancer drug development.

P-glycoprotein and its inhibition in tumors by phytochemicals derived from Chinese herbs

P-glycoprotein belongs to the family of ATP-binding cassette (ABC) transporters. It functions in cellular detoxification, pumping a wide range of xenobiotic compounds, including anticancer drugs out of the cell. In cancerous cells, P-glycoprotein confers resistance to a broad spectrum of anticancer agents, a phenomenon termed multidrug resistance. An attractive strategy for overcoming multidrug resistance is to block the transport function of P-glycoprotein and thus increase intracellular concentrations of anticancer drugs to lethal levels. Efforts to identify P-glycoprotein inhibitors have led to numerous candidates, none of which have passed clinical trials with cancer patients due to their high toxicity. The search for naturally inhibitory products from traditional Chinese medicine may be more promising because natural products are frequently less toxic than chemically synthesized substances. In this review, we give an overview of molecular and clinical aspects of P-glycoprotein and multidrug resistance in the context of cancer as well as Chinese herbs and phytochemicals showing inhibitory activity towards P-glycoprotein.

CD13 is a therapeutic target in human liver cancer stem cells

Cancer stem cells (CSCs) are generally dormant or slowly cycling tumor cells that have the ability to reconstitute tumors. They are thought to be involved in tumor resistance to chemo/radiation therapy and tumor relapse and progression. However, neither their existence nor their identity within many cancers has been well defined. Here, we have demonstrated that CD13 is a marker for semiquiescent CSCs in human liver cancer cell lines and clinical samples and that targeting these cells might provide a way to treat this disease. CD13+ cells predominated in the G0 phase of the cell cycle and typically formed cellular clusters in cancer foci. Following treatment, these cells survived and were enriched along the fibrous capsule where liver cancers usually relapse. Mechanistically, CD13 reduced ROS-induced DNA damage after genotoxic chemo/radiation stress and protected cells from apoptosis. In mouse xenograft models, combination of a CD13 inhibitor and the genotoxic chemotherapeutic fluorouracil (5-FU) drastically reduced tumor volume compared with either agent alone. 5-FU inhibited CD90+ proliferating CSCs, some of which produce CD13+ semiquiescent CSCs, while CD13 inhibition suppressed the self-renewing and tumor-initiating ability of dormant CSCs. Therefore, combining a CD13 inhibitor with a ROS-inducing chemo/radiation therapy may improve the treatment of liver cancer.

Membrane microparticles mediate transfer of P-glycoprotein to drug sensitive cancer cells

Multidrug resistance (MDR), a significant impediment to the successful treatment of cancer clinically, has been attributed to the overexpression of P-glycoprotein (P-gp), a plasma membrane multidrug efflux transporter. P-gp maintains sublethal intracellular drug concentrations by virtue of its drug efflux capacity. The cellular regulation of P-gp expression is currently known to occur at either pre- or post-transcriptional levels. In this study, we identify a 'non-genetic' mechanism whereby microparticles (MPs) serve as vectors in the acquisition and spread of MDR. MPs isolated from drug-resistant cancer cells (VLB(100)) were co-cultured with drug sensitive cells (CCRF-CEM) over a 4 h period to allow for MP binding and P-gp transfer. Presence of P-gp on MPs was established using flow cytometry (FCM) and western blotting. Whole-cell drug accumulation assays using rhodamine 123 and daunorubicin (DNR) were carried out to validate the transfer of functional P-gp after co-culture. We establish that MPs shed in vitro from drug-resistant cancer cells incorporate cell surface P-gp from their donor cells, effectively bind to drug-sensitive recipient cells and transfer functional P-gp to the latter. These findings serve to substantially advance our understanding of the molecular basis for the emergence of MDR in cancer clinically and lead to new treatment strategies which target and inhibit MP mediated transfer of P-gp during the course of treatment.

Toward a mechanical control of drug delivery. On the relationship between Lipinski's 2nd rule and cytosolic pH changes in doxorubicin resistance levels in cancer cells: a comparison to published data

Based on molecular and physiological resemblance, the mechanism that controls drug bioavailability and toxicity also shares strong similarities to the one that controls drug resistance. In both cases, this mechanism relies on the expression of drug transporters and the physico-chemical properties of drugs, which together alter the intracellular accumulation of chemicals in cells or tissues. However, a parameter that is central and has received great attention in the field of bioavailability, but almost none in the field of drug resistance, is the molecular weight of drugs. In the former area, it is well known that to achieve a reasonable bioavailability, drugs must have-among other properties-a molecular weight less than 500, known as Lipinski's 2nd rule. Accordingly, it is worth questioning whether a similar rule exists in the field of drug resistance and what subsequent mechanism would control the membrane permeability to drugs as a function of their molecular weight. I demonstrate here that cytosolic pH fixes the molecular weight of drugs entering cells, by altering the cell membrane mechanical properties and that, both cytosolic pH and membrane mechanical properties are needed and sufficient to explain doxorubicin resistance levels in different cancerous cell lines. Finally, I discuss the efficiency of a drug handling activity by transporters in MDR and suggest ways to control drug delivery mechanically. In addition, and for the first time, the literal expression of a Law similar to Lipinski's 2nd rule will be described as a function of cytosolic pH and lipid number asymmetry.

The Multixenobiotic Resistance Mechanism in Aquatic Organisms

Many aquatic organisms thrive and reproduce in polluted waters. This fact indicates that they are well equipped with a defense system(s) against several toxic xenobiotics simultaneously because water pollution is typically caused by a mixture of a number of pollutants. We have found that the biochemical mechanism underlying such "multixenobiotic" resistance in freshwater and marine mussel, in several marine sponges, and in freshwater fish is similar to the mechanism of multidrug resistance (MDR) found in tumor cells that became refractory to treatment with a variety of chemotherapeutic agents. All these organisms possess a verapamil-sensitive potential to bind 2-acetylaminofluorene and vincristine onto membrane vesicles. They all express mRNA for mdr1 gene, and mdr1 protein product, the glycoprotein P170. Finally, in in vivo experiments, the accumulation of xenobiotics is enhanced in all investigated organisms in the presence of verapamil, the inhibitor of the P170 extrusion pump. The knowledge that the presence of one xenobiotic may block the pumping out, and hence accelerating accumulation, of others, may help us to understand and interpret our present and past data on different environmental parameters obtained using indicator organisms.

C-Phycocyanin ameliorates 2-acetylaminofluorene induced oxidative stress and MDR1 expression in the liver of albino mice

AIM

To study the effect of C-Phycocyanin (C-PC), a biliprotein isolated from Spirulina platensis, on 2-acetylaminofluorene (2-AAF) induced oxidative stress and MDR1 expression in the liver of albino mice.

METHODS

In the present study, albino mice aged 40-60 days were used. The mice were randomly assigned to four groups of six animals each. The first group was treated with the vehicle (absolute alcohol), the second group was treated with C-PC (50 mg/kg body weight), the third group was treated with 2-AAF (25 mg/kg body weight) and the fourth group was treated with C-PC (50 mg/kg body weight) and 2-AAF, daily for 3 days. The mice were sacrificed and the tissues were collected and stored for histology and biochemical studies.

RESULTS

2-AAF induced liver tissue damage in albino mice. 2-AAF treatment resulted in upregulation of MDR1 expression and enhanced the generation of reactive oxygen species (ROS). It also induced phosphorylation of Akt and nuclear translocation of NF-kappaB. Co-administration of C-PC and 2-AAF inhibited the expression of MDR1 by preventing ROS generation, Akt phosphorylation and NF-kappaB nuclear translocation.

CONCLUSION

2-AAF-induced oxidative stress is reduced by C-PC treatment. C-PC inhibited the 2-AAF induced expression of MDR1 by interfering at the level of ROS generation, Akt phosphorylation and NF-kappaB translocation. This study reveals the usefulness of C-PC in preventing oxidative stress and downregulation of MDR1 induced by xenobiotics like 2-AAF.

Clonal xenobiotic resistance during pollution-induced toxic injury and hepatocellular carcinogenesis in liver of female flounder (Platichthys flesus (L.))

Juvenile and adult female flounder (Platichthys flesus (L.)) were caught either in the estuary of the most polluted European river, the Elbe, or as controls in a reference site to study pollution-induced xenobiotic resistance in their livers in relation to pathological alterations. In juvenile fish, livers displayed reversible and irreversible degenerative toxipathic lesion types but never showed (pre)neoplastic changes. Tumour frequencies up to 70% were found macroscopically in livers of adult female flounder which had progressed to adenomas and carcinomas in the most polluted site. Because male adult flounder show only up to 50% of livers containing early preneoplastic foci but never malignancies, we focussed our study on female individuals. (Pre)neoplastic changes ranged from early eosinophilic foci to basophilic foci, adenomas and hepatocellular carcinomas. Adenomas were generally eosinophilic whereas carcinomas were mainly basophilic. These phenotypical sequential changes strongly resemble those found in chemically-induced liver carcinogenesis in mammals. Characteristic mutations known from mammalian cancers have not been found so far in these flounder livers. Therefore, we investigated whether epigenetic events had induced a metabolic "resistant phenotype" of (pre)malignant cancer cells during hepatocellular carcinogenesis. With a quantitative immunohistochemical approach, we studied expression of P-glycoprotein (P-gp)-mediated multixenobiotic resistance (MXR), cytochrome P4501A1, glutathione-S-transferase-A which are key proteins in xenobiotic metabolism and elimination. Glucose-6-phosphate dehydrogenase (G6PDH) activity, the major source of the reducing power NADPH which is needed for biotransformation, oxyradical scavenging and biosynthesis, was detected as well. We observed upregulation of G6PDH activity already in early preneoplastic eosinophilic foci and subsequent further upregulation in basophilic foci and carcinomas. P-gp started to become overexpressed in basophilic foci and was overexpressed even more strongly in carcinomas and their invasively-growing protrusions (satellites). In carcinomas, P-gp protein was predominantly present in membranes of lysosomes which are the intracellular sites of deposition of xenobiotics. CYP450 was reduced whereas GST-A was increased in these carcinomas. Progression towards malignancy was positively correlated with levels of mitogenic organochlorines in these livers which are "fingerprint contaminants" of the river Elbe. We conclude that (pre)neoplastic hepatocytes in female flounder acquire growth advantages over normal hepatocytes by epigenetic metabolic adaptations during liver carcinogenesis as a result of chronic exposure to (pro)carcinogens in the polluted habitat.

The rise of DNA methylation and the importance of chromatin on multidrug resistance in cancer

In recent years, the different classes of drugs and regimens used clinically have provided an improvement in tumour management. However, treatment is often palliative for the majority of cancer patients. Transformed cells respond poorly to chemotherapy mainly due to the development of the multidrug resistance (MDR) phenotype. Response to treatment does not generally result in complete remission and disease cure is uncommon for patients presenting with advanced stage cancer. Successful treatment of cancer requires a clearer understanding of chemotherapeutic resistance. Here, we examine what is known of one of the most extensively studied mechanisms of cellular drug resistance. The human multidrug resistance gene 1 (MDR1) is associated with expression of p-glycoprotein (Pgp). A transmembrane protein, Pgp acts as an efflux pump and reduces intracellular drug levels and thus its effectiveness as an antitumor agent. The precise mechanism of transcriptional regulation has been unclear due to the complex regulatory nature of the gene. It has become increasingly apparent that trans-activation or genetic amplification is by no means the only mechanism of activation. Consequently, alternative pathways have received more attention in the area of epigenetics to help explain transcriptional competence at a higher level of organization. The goal of this article is to highlight important findings in the field of methylation and explain how they impinge on MDR1 gene regulation. In this review, we cover the current information and postulate that epigenetic modification of MDR1 chromatin influences gene transcription in leukaemia. Finally, we explore transcriptional regulation and highlight recent progress with engineered ZFP's (zinc finger proteins).

Expression of the lung resistance-related protein in human and rat hepatocarcinogenesis

Lung resistance-related protein (LRP) plays an important role in chemoresistance of tumor cells probably by altering nuclear-cytoplasmic transport processes. We analyzed the association between LRP expression and hepatocarcinogenesis in humans and rats by RT-PCR, immunoblotting, and immunohistochemistry. LRP was found in hepatocytes and bile epithelia of normal human and rat liver showing distinct interindividual variations. In human tissues, the LRP expression levels of dysplastic liver nodules, hepatocellular adenomas, and carcinomas were highly variable, including decreased but also distinctly increased staining intensities. Mean expression levels, however, were comparable to the surrounding tissue. Considerable levels of LRP mRNA and protein were also found in human hepatoma cell lines. To study LRP expression from the beginning of hepatocarcinogenesis onward, rats were subjected to a tumor initiation/promotion protocol leading to preneoplastic hepatocytes present as single cells or multicellular clones, followed by adenoma and carcinoma. All of the (pre)neoplastic rat liver lesions expressed, comparable to the surrounding tissue, considerable amounts of LRP. We conclude that LRP might be one mechanism involved in the intrinsically high but variable chemoresistance of normal and (pre)neoplastic hepatocytes.

Expression of P-glycoprotein in killifish (Fundulus heteroclitus) exposed to environmental xenobiotics

P-glycoproteins (P-gp) are transmembrane efflux flippases that prevent the cellular accumulation of moderately hydrophobic compounds and are responsible for certain multidrug resistance phenotypes in tumor cell lines and human patients. We investigated whether P-gps could be involved in a contaminant resistant phenotype observed in a population of fish exposed over generations to high levels of planar halogenated aromatic hydrocarbons (PHAHs). Hepatic and intestinal epithelial P-gp expression was examined by immunoblot and immunohistochemistry in killifish (Fundulus heteroclitus) from New Bedford Harbor, MA (NBH), a Superfund site highly contaminated with PHAHs, and from Scorton Creek on Cape Cod, MA (SC), a relatively unpolluted site. The NBH population has developed resistance to the toxicity of PHAHs. Hepatic P-gp levels were more than 40% greater in fish freshly collected from SC than in fish freshly collected from NBH. When killifish from either site were maintained in clean water for up to 78 days to permit depuration of bioaccumulated contaminants, hepatic P-gp levels decreased approximately 50% by day 8. P-glycoprotein expression was detected in the intestinal epithelium in 55% of freshly collected NBH fish. However, depurated NBH fish and freshly caught and depurated SC fish rarely expressed P-gp in the intestine. In an effort to determine whether environmental chemicals at the two sites might contribute to altered P-gp expression, depurated fish were exposed either to sediment collected from SC or 2,3,7,8-tetrachlorodibenzofuran, a contaminant found at the NBH site and a model aryl hydrocarbon receptor agonist. Neither exposure affected hepatic P-gp levels in killifish. Elevated intestinal P-gp in NBH fish might counter the absorption of P-gp substrates/inducers and thus limit the amount of these compounds reaching the liver, which might account for the lower hepatic P-gp levels in NBH fish compared to SC fish. The differences in hepatic P-gp levels (SC>NBH) and intestinal P-gp (NBH>SC) in freshly collected fish also might reflect environmental exposure to different anthropogenic contaminants or microbial, algal, plant or other natural products via the water column, sediment, or diet at each site.

Induction of P-glycoprotein, glutathione-S-transferase and cytochrome P450 in rat liver by atrazine

We studied the effects of intraperitoneally administered atrazine on two hepatic neoplastic markers, P-glycoprotein (P-gp), and glutathione-S-transferase (GST), and several phase I drug-metabolizing enzyme cytochrome P450 (CYP) subfamilies in hepatic microsomes and cytosol of Fischer rats. The P-gp content was increased after 24 h of atrazine administration at 50 mg/kg, and maximum P-gp induction was observed at 300 mg/kg for 3 days. GST-P was induced at a lower dose than P-gp, from 10 mg/kg, but no other form of GST, such as GST1A1, was induced by the same dose. Among the CYP families, CYP1A2 was highly and CYP2B was slightly induced by atrazine while the CYP3A content remained unchanged. The liver plasma membrane marker alkaline phosphatase (AP) was not induced by the same doses. The inductions of P-gp, GST-P and CYP1A2 observed may explain some of the reported tumor-promoting properties and toxicity of atrazine in vivo.

Induction of human MDR1 gene expression by 2-acetylaminofluorene is mediated by effectors of the phosphoinositide 3-kinase pathway that activate NF-kappaB signaling

The expression of P-glycoprotein encoded by the multidrug resistance (MDR1) gene is associated with the emergence of the MDR phenotype in cancer cells. Human MDR1 and its rodent homolog mdr1a and mdr1b are frequently overexpressed in liver cancers. However, the underlying mechanisms are largely unknown. The hepatocarcinogen 2-acetylaminofluorene (2-AAF) efficiently activates rat mdr1b expression in cultured cells and in Fisher 344 rats. We recently reported that activation of rat mdr1b in cultured cells by 2-AAF involves a cis-activating element containing a NF-kappaB binding site located -167 to -158 of the rat mdr1b promoter. 2-AAF activates IkappaB kinase (IKK), resulting in degradation of IkappaBbeta and activation of NF-kappaB. In this study, we report that 2-AAF could also activate the human MDR1 gene in human hepatoma and embryonic fibroblast 293 cells. Induction of MDR1 by AAF was mediated by DNA sequence located at -6092 which contains a NF-kappaB binding site. Treating hepatoma cells with 2-AAF activated phosphoinositide 3-kinase (PI3K) and its downstream effectors Rac1, and NAD(P)H oxidase. Transient transfection assays demonstrated that constitutively activated PI3K and Rac1 enhanced the activation of the MDR1 promoter by 2-AAF. Treatment of hepatoma cells with 2-AAF also activated another PI3K downstream effector Akt. Transfection of recombinant encoding a dominant activated Akt also enhanced the activation of MDR1 promoter activation by 2-AAF. These results demonstrated that 2-AAF up-regulates MDR1 expression is mediated by the multiple effectors of the PI3K signaling pathway.

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

The MDR1 gene, considered to be involved in multidrug resistance of cancer cells, is expressed in liver, kidney, small intestine and the blood-brain barrier. We investigated MDR1 gene expression in the well-differentiated hepatoma cell line HepG2 after exposure to several stresses and found that sodium arsenite treatment increased MDR1 gene expression 2.6-fold. Deletion analysis of the MDR1 promoter indicated that the transcriptional activation after exposure to arsenite depends on a 60-bp region containing two heat-shock responsive elements.

Multidrug resistance gene (MDR-1) expression as a useful prognostic factor in patients with human hepatocellular carcinoma after surgical resection

BACKGROUND

Multidrug resistance gene (MDR-1) overexpression has been correlated with tumor aggressiveness and worse prognosis in some human neoplasms. The aim of this study is to evaluate the clinical value of MDR-1 mRNA expression as a prognostic factor after surgical resection in human hepatocellular carcinoma (HCC).

METHODS

MDR-1 mRNA levels in tissue samples from 34 patients with HCC, who underwent surgical resection, were measured by quantitative northern blot analysis. We stratified these patients into two groups according to a ratio of MDR-1 mRNA levels of HCC to nontumorous tissue; MDR-1 mRNA ratio > or = 1.0 and < 1.0. The overall and disease-free survival rates were analyzed using multivariate regression analysis.

RESULTS

The median survival periods were 10.3 and 35.8 months for patients with the MDR-1 mRNA ratio > or = 1.0 and < 1.0, respectively, and the corresponding 5-year survival rates were 33 and 54%, respectively, P < 0.05. The multivariate analysis revealed that TNM stage and MDR-1 mRNA ratio were independent factors for predicting overall survival after surgical resection.

CONCLUSION

This study suggested that the measurement of the MDR-1 mRNA levels in HCC and nontumorous liver tissue might be a useful prognostic factor after surgical resection in patients with HCC.

Expression of P-glycoprotein in the gills of oysters, Crassostrea virginica: seasonal and pollutant related effects

The expression of p-glycoprotein (p-gp) in aquatic organisms has been proposed as a biomarker of pollution exposure. Previous research has provided evidence that p-gp is inducible by organic xenobiotics and that p-gp is overexpressed in mussels from degraded areas. However, seasonal changes in expression at polluted sites has not been described previously. The purposes of these studies were to evaluate the expression of p-gp in polluted and unpolluted sites in oysters (Crassostrea virginica) and to determine if there were seasonal differences. P-gp expression and total protein concentrations were measured seasonally in the gills of southeastern oysters, Crassostrea virginica, at degraded and undegraded sites in Charleston Harbor, South Carolina, USA. At all sites, p-gp expression was generally higher during the warmer months and lower in the colder months. Polluted sites tended to show a decrease in p-gp expression in June and September, suggesting that p-gp inhibition may have occurred. P-gp expression was not significantly related to specific classes of sediment contaminants or to overall sediment contaminant loading. Total gill protein concentrations at all sites were lower during the warmer months and higher during the colder months. In general, all sites tended to show decreased total gill protein concentrations when compared to the control site. Total protein concentrations were significantly related to water temperature, specific classes of sediment contaminants and overall sediment contaminant loads, but there was no relationship with salinity.

Modulation of glucose-6-phosphate dehydrogenase activity and expression is associated with aryl hydrocarbon resistance in vitro

The mutagenic effect of environmental carcinogens has been well documented in animal models and in human studies but the mechanisms involved in preventing carcinogen insult have not been fully elucidated. In this study we examined the molecular and biochemical changes associated with carcinogen resistance in a series of aryl hydrocarbon-resistant MCF-7 cell lines developed by exposure to benzo[a]pyrene (BP). The cell lines were designated as AH(R40), AH(R100), and AH(R200) to denote their increasing fold resistance to BP compared with wild type cells. These cell lines were also resistant to another aryl hydrocarbon (AH), dimethylbenz[a]anthracene, but not to pleiotropic drugs (doxorubicin, vinblastine, and taxol). The resistant cell lines showed an increase in the level of the primary intracellular antioxidant, reduced glutathione, corresponding to increasing AH resistance. However, there was no change in glutathione reductase activity. The generation of reduced glutathione requires NADPH, and we therefore examined the activity and expression of the rate-limiting enzyme in NADPH production, glucose-6-phosphate dehydrogenase (G6PD). An increase in G6PD specific activity was associated with increasing aryl hydrocarbon resistance. This was due to an increased expression of G6PD in resistant cells, which was demonstrated by increases in both protein and mRNA levels. However, there was no increase in the transcription rate of G6PD in the resistant cell lines, indicating that the increase G6PD expression is due to a post-transcriptional modulation, which was confirmed by actinomycin D chase experiments. These results demonstrate that modulation of G6PD expression and activity is an important mechanism in AH resistance.

Anticancer drug-mediated induction of multidrug resistance-associated genes and protein kinase C isozymes in the T-lymphoblastoid cell line CCRF-CEM and in blasts from patients with acute lymphoblastic leukemias

The major determinants mediating drug resistance in acute lymphoblastic leukemias (ALL) unresponsive to chemotherapy, are still unclear. For example, it is still unknown whether selection or induction processes are responsible for drug resistance here or whether protein kinase C (PKC) isozymes contribute to the resistant phenotype. Therefore, inducibility of resistance factors or PKC isozymes genes was examined in CCRF-CEM cells treated with diverse anticancer drugs--adriamycin, camptothecin, etoposide or vincristine--at sublethal concentrations for 24 h. MDR1, MRP1, LRP and PKC isozyme alpha, beta(1), beta(2), epsilon, iota, eta, theta, zeta gene expression was determined by cDNA-PCR. We found significant dose-dependent, mostly combined, induction of the MDR1, MRP1 and LRP genes. Significantly enhanced gene expression of the majority of PKC isozyme genes was found after treatment with camptothecin. PKCzeta was upregulated throughout by each anticancer drug applied in this setting. A series of selected CCRF-CEM-derived multidrug resistance (MDR) sublines also showed enhanced expression of the PKC isozymes compared to the parental cell line. MDR1 and PKCeta gene expression levels were correlated highly significantly. Blasts from two patients with ALL during the first week of monotherapy with steroids revealed combined induction of the MDR1, multidrug resistance-associated protein 1 (MRP1), lung cancer resistance-related protein (LRP) and most PKC isozymes, predominantly PKCzeta. Another patient with T-ALL, who failed to respond to four months of intensive chemotherapy, showed an enhanced MRP1 gene expression combined with markedly overexpression of PKCeta and PKCtheta. Furthermore, the camptothecin and etoposide-mediated induction of resistance factors in the CCRF-CEM cell line could be suppressed by staurosporine, a rather unspecific inhibitor of protein kinases. However, selective inhibitors of PKC isozymes (bisindolylmaleimide GO 6850, indolocarbazole GO 6976) produced no significant effects here. Therefore, the PKC isozymes eta, theta and zeta are of interest as potential targets to overcome drug resistance in ALL.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Elevated expression of hepatic proliferative markers during early hepatocarcinogenesis in hepatitis-B virus transgenic mice lacking mdr1a-encoded P-glycoprotein

Recent studies have shown that expression levels of the multidrug resistance gene MDR1, which encodes the drug transporter P-glycoprotein, correlate with prognostic outcomes of certain tumor types. These findings suggest that expression of MDR1 may affect tumor behaviors. To address this issue further, we investigated the expression of mdr1a, a human MDR1 homolog, on the development of hepatocellular carcinoma in a transgenic mouse model carrying the liver-targeted expression of human hepatitis-B virus (HBV) surface antigen. The pathogenetic program was compared in HBV mice carrying either mdr1a(+/+) or mdr1a(-/-). We found that the expressions of proliferative activity markers, Ki67 nuclear antigen, and proliferating cell nuclear antigen were elevated in mdr1a(-/-) mice younger than 10 wk in comparison with those in the same age group of wild-type animals. Replication in the hepatic population as determined by bromodeoxyuridine incorporation tended to support observation that mdr1a(-/-) mice exhibited elevated labeling indices in this age group. Moreover, histologic staining and flow-cytometric analysis showed that the mdr1a(-/-) animals exhibited a higher cell population with polyploidy than did the mdr1a(+/+) counterparts of the same age. However, no significant differences in the expression of the liver-injury markers serum alanine transaminase and aspartate transaminase were observed. Although our results showed that absence of mdr1a expression is correlated with modest enhanced proliferative characteristics in the livers at stage before the development of hepatocellular carcinoma, the overall life spans between these two strains of mice were not significantly different. The implication of these findings to the role of P-glycoprotein in tumor development and cancer chemotherapy is discussed.

Association of tamoxifen biliary excretion rate with prior tamoxifen exposure and increased mdr1b expression

ATPase transporter proteins are commonly found in the hepatocyte canalicular membrane. Some of these, in particular the multidrug resistance (mdr1b) gene, have been previously demonstrated to be inducible genes. In this study, we found that tamoxifen induced expression of the mdr1b gene in the liver up to 40-fold after 14 days' exposure to tamoxifen in the diet at a concentration of 420 ppm. As tamoxifen and its metabolites are primarily excreted into the bile, we investigated if the increased expression of mdr1b in the liver following tamoxifen exposure had any effect on its excretion in rats. We found that the excretion of tamoxifen and its metabolites into bile was increased from 8 +/- 1% to 51 +/- 18% (mean +/- SD) of an administered dose of 180 nmol/kg over a collection period of 3 hr in rats that had received tamoxifen (35 mg/kg) orally for 12 days (plus a 3-day rest) prior to the experiment. These data suggest that prolonged treatment with tamoxifen may result in lower serum and tumour concentrations, due to a self-mediated enhancement of excretion via mdr1b gene-encoded P-glycoprotein. This may have implications for other drugs sharing the same route of excretion and co-administered with tamoxifen.

Osteoblastic differentiation and P-glycoprotein multidrug resistance in a murine osteosarcoma model

A recent study of multidrug resistance (MDR) 1 gene transfected osteosarcoma cells found a cause-effect relationship between increased expression of P-glycoprotein (P-gp) and a low aggressive phenotype. However, several experimental and clinical studies have observed contradictory findings in that P-gp expression has been associated with tumour progression. In the present study, we characterized P-gp-positive and P-gp-negative single-cell clones of a murine osteosarcoma, to further investigate the relationship between P-gp expression and changes in cell phenotype. Although these clones were all selected by doxorubicin (DOX) exposure, they were heterogeneous with respect to MDR1 gene expression. The P-gp-positive clones revealed MDR phenotype, whereas the P-gp-negative clones showed no resistance to drugs. Morphological and functional analysis showed that both the P-gp-positive and P-gp-negative clones were more differentiated than the parent cells in terms of enhanced activity of cellular alkaline phosphatase, an increase in well-organized actin stress fibres and enhanced osteogenic activity. Moreover, these subclones all displayed a decrease in malignant potential such as oncogenic activity, tumour growth rate and metastatic ability, regardless of their P-gp status. These results indicate that the observed osteoblastic differentiation and less aggressive phenotype in DOX-selected osteosarcoma cells may not only be explained by the direct effect of P-gp, and accordingly, consideration of the effect of DOX, as well as P-gp, appears to be important.

Multixenobiotic resistance as a cellular defense mechanism in aquatic organisms

Multixenobiotic resistance in aquatic organisms exposed to natural toxins or anthropogenic contaminants is a phenomenon analogous to multidrug resistance in mammalian tumor cell lines tolerant of anti-cancer drugs. Multidrug resistance is commonly due to the elevated expression of transmembrane P-glycoproteins (P-gp) which actively transport a wide variety of structurally and functionally diverse compounds. The purpose of this review is to place aquatic ecotoxicological data in context of the larger multidrug resistance field of study. Information on P-glycoproteins structure, mechanism of transport, and substrate specificity gained through traditional mammalian and cell culture models is examined in conjunction with recent work on aquatic species exposed to xenobiotics both in the field and in the laboratory. The physiological function of P-glycoproteins is explored through studies of gene knockout models and expression patterns in normal tissues and tumors. The effect of xenobiotic exposures on P-gp activity and protein titer is examined in wild and captive populations of aquatic invertebrates and vertebrates. Substrate overlap and evidence of co-expression of phase I detoxification enzymes (e.g. cytochromes P450) and P-gp are presented. The role of P-gp chemosensitizers as environmental pollutants and the ecotoxicological consequences of P-gp inhibition are highlighted. The overwhelming evidence suggests that P-glycoproteins provide aquatic organisms with resistance to a wide range of natural and anthropogenic toxins.

Expression of drug resistance-related genes in head and neck squamous cell carcinomas and normal mucosa

We examined the expression levels of mRNA for multidrug resistance 1 (MDR1), multidrug resistance-associated protein (MRP), human canalicular multispecific organic anion transporter (cMOAT), lung resistance-related protein (LRP), topoisomerase IIalpha, beta (Topo IIalpha, beta) and topoisomerase I (Topo I) genes in human head and neck squamous cell carcinoma (HNSCC) specimens and mucosa (HNM) specimens, to elucidate their roles in relation to the biological characteristics and drug resistance in vivo. Fifty-eight samples (45 head and neck carcinomas and 13 head and neck mucosa) obtained during surgical resection or biopsy from 38 patients were analyzed using the quantitative reverse transcription-polymerase chain reaction (RT-PCR) method. MDR1, MRP, LRP, Topo IIalpha, Topo IIbeta, and Topo I gene transcripts were detected in all the samples tested, but cMOAT mRNA was not detected in them. Comparisons of the expression levels in HNSCC with those in HNM showed that the Topo IIalpha gene expression level was higher in HNSCC than in HNM (P=0.0298). Moreover, the Topo IIalpha mRNA level was significantly higher in metastatic lymph node samples of HNSCC than in HNM samples (P=0.0205). There were no significant differences in the six genes' expression levels between samples exposed to platinum drugs and those not exposed to platinum drugs. These results suggest that it may be effective in anticancer therapy to use topoisomerase-targetting drugs against HNSCC, especially metastatic neck tumors, and that the expression of these genes in HNSCC is not associated with platinum drug exposure.

Tc-99m MIBI SPECT is useful for noninvasively predicting the presence of MDR1 gene-encoded P-glycoprotein in patients with hepatocellular carcinoma

PURPOSE

Resistance to chemotherapeutic drugs continues to be one of the major unsolved problems in the treatment of cancer. Multidrug resistance is defined as the ability of cells exposed to a single drug to develop resistance to a broad range of structurally and functionally unrelated drugs as a result of enhanced outward transport of drugs mediated by P-glycoprotein that is encoded by multidrug resistance genes. Recent evidence has shown that Tc-99m MIBI is a suitable transport substrate for P-glycoprotein. A potential advantage of Tc-99m MIBI SPECT is its superiority to diagnose noninvasively the presence of P-glycoprotein overexpression in vivo. In this study, the authors determined the association of enhanced MIBI efflux in Tc-99m MIBI SPECT with overexpression of P-glycoprotein in hepatocellular carcinoma.

MATERIALS AND METHODS

Thirty-five patients with hepatocellular carcinoma were enrolled in the study. Tc-99m MIBI SPECT was performed 10 minutes after intravenous injection of 20 mCi Tc-99m MIBI. All patients had liver biopsy or surgery within 1 week of MIBI imaging. Immunohistochemical study of the biopsy or resected hepatocellular carcinoma specimens was performed using the avidin-biotin-peroxidase technique with monoclonal antibody JSB-1 directed against P-glycoprotein.

RESULTS

On Tc-99m MIBI SPECT, 30 of 35 (85.7%) patients with hepatocellular carcinoma had no Tc-99m MIBI uptake in tumor lesions, whereas five patients with hepatocellular carcinoma had Tc-99m MIBI uptake in tumor lesions. P-glycoprotein expression was observed in tumor tissues of all the patients without Tc-99m MIBI uptake, whereas among the five patients with Tc-99m MIBI uptake, no P-glycoprotein expression was seen in tumor lesions (P < 0.015).

CONCLUSION

Tc-99m MIBI SPECT is useful for noninvasively predicting the presence of MDR1 gene-encoded P-glycoprotein in patients with hepatocellular carcinoma.

Effect of partial hepatectomy on the expression of seven rat sulphotransferase mRNAs

1. The effect of partial hepatectomy on the expression of sulphotransferase (SULT) mRNA was studied. SULTs fall into two distinct classes based on substrate preference: phenol SULT1 family (SULT1A1, SULT1B1, SULT1C1 and SULT1E2) and hydroxysteroid SULT2 family (SULT20/21, SULT40/41 and SULT60). 2. Hepatic expression of SULT mRNA was analysed in the sham-hepatectomised rat (sham) and in the partially hepatectomised (PH) rat at various times after PH. Northern-blot analysis with [alpha-32P]dATP-labelled oligonucleotide probes specific for individual SULT mRNAs was used to monitor hepatic SULT mRNA expression. In general, SULT mRNAs underwent a decrease in expression after PH and the magnitude of decrease was dependent on the SULT isoform. 3. The decrease in SULT mRNA expression was resolved and even induced (SULT40/41 in the female rat) by 10-30 days after PH. Of the phenol SULT isoforms, both SULT1C1 and SULT1E2 mRNAs were significantly decreased by 18-24 h after PH in the male rat. The other phenol SULTs (SULT1A1 and SULT1B1) tended to decrease in the male rat after PH, but the decreases were not statistically significant. Expression of SULT20/21 mRNA was decreased in the female rat (80% at 24 h) and fully recovered by 10 days. SULT40/41 mRNA tended to decrease after PH; however, the decrease was not statistically significant. SULT 60 mRNA was decreased from 24 to 96 h after PH. 4. Thus, during the period of rapid liver growth that occurs after partial hepatectomy, SULT mRNA expression is decreased. The phenomenon of decreased SULT mRNA expression is similar to other states of rapid liver growth (e.g. cancer tissue and young animals) in which expression of SULT enzymes is characteristically low.

Divergent mechanisms for loss of Ah-responsiveness in benzo[a]pyrene- and adriamycinR-resistant MCF-7 cells

The intracellular aryl hydrocarbon receptor (AhR) mediates signal transduction by environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene by functioning as a ligand-activated transcription factor. We have investigated AhR signaling in sublines of the human breast cancer cell line MCF-7 selected for resistance to AdriamycinR (AdrR) and benzo[a]pyrene (BP(R)). Previously we reported that AdrR cells have a loss of estrogen receptor (ER) expression and are Ah-nonresponsive. Here we show that AhR mRNA and protein are expressed at normal levels in AdrR cells, and the activated AhR complex is functionally capable of binding a xenobiotic responsive element. In MCF-7 cells AhR was depleted to 15% of normal levels after 4 hr TCDD treatment; however, 45% of AhR remained in AdrR cells during this time course. In BP(R) cells AhR mRNA levels were found to be decreased relative to wild-type cells, which led to decreased AhR protein levels and DNA-binding activity. Cellular ER content has been shown to correlate with Ah-responsiveness in human breast cancer cell lines. BP(R) cells were found to be ER-positive, although chronic (BP(R) cells) and acute (24 hr) exposure to benzo[a]pyrene led to significantly lower ER protein levels in MCF-7 cells. We conclude that loss of Ah-responsiveness occurs by different mechanisms in xenobiotic-resistant MCF-7 sublines: AhR mRNA is down-regulated in BP(R) cells, whereas AdrR cells are deficient in AhR signaling by a mechanism unrelated to AhR expression and activity.

Reactive oxygen species participate in mdr1b mRNA and P-glycoprotein overexpression in primary rat hepatocyte cultures

P-glycoproteins encoded by multidrug resistance type 1 (mdr1) genes mediate ATP-dependent efflux of numerous lipophilic xenobiotics, including several anticancer drugs, from cells. Overexpression of mdr1-type transporters in tumour cells contributes to a multidrug resistance phenotype. Several factors shown to induce mdr1 overexpression (UV irradiation, epidermal growth factor, tumour necrosis factor alpha, doxorubicin) have been associated with the generation of reactive oxygen species (ROS). In the present study, primary rat hepatocyte cultures that exhibit time-dependent overexpression of the mdr1b gene were used as a model system to investigate whether ROS might participate in the regulation of intrinsic mdr1b overexpression. Addition of H2O2 to the culture medium resulted in a significant increase in mdrlb mRNA and P-glycoprotein after 3 days of culture, with maximal (approximately 2-fold) induction being observed with 0.5-1 mM H2O2. Furthermore, H2O2 led to activation of poly(ADP-ribose) polymerase, a nuclear enzyme activated by DNA strand breaks, indicating that ROS reached the nuclear compartment. Thus, extracellularly applied H2O2 elicited intracellular effects. Treatment of rat hepatocytes with the catalase inhibitor 3-amino-1,2,4-triazole (2-4 mM for 72 h or 10 mM for 1 h following the hepatocyte attachment period) also led to an up-regulation of mdrlb mRNA and P-glycoprotein expression. Conversely, antioxidants (1 mM ascorbate, 10 mM mannitol, 2% dimethyl sulphoxide, 10 mM N-acetylcysteine) markedly suppressed intrinsic mdr1b mRNA and P-glycoprotein overexpression. Intracellular steady-state levels of the mdrl substrate rhodamine 123, determined as parameter of mdr1-type transport activity, indicated that mdr1-dependent efflux was increased in hepatocytes pretreated with H2O2 or aminotriazole and decreased in antioxidant-treated cells. The induction of mdr1b mRNA and of functionally active mdr1-type P-glycoproteins by elevation in intracellular ROS levels and the repression of intrinsic mdrlb mRNA and P-glycoprotein overexpression by antioxidant compounds support the conclusion that the expression of the mdr1b P-glycoprotein is regulated in a redox-sensitive manner.

Expression of MDR1 mRNA and encoding P-glycoprotein in archival formalin-fixed paraffin-embedded gall bladder cancer tissues

The aim of this study was to examine the expression of P-glycoprotein (Pgp) and MDR1 mRNA, in gall bladder carcinoma, a chemo-resistant tumour. 26 cases of gall bladder cancer and nine samples of normal gall bladder archival paraffin blocks were investigated for the presence of Pgp protein with immunohistochemistry (IHC) and MDR1 RNA by reverse transcription-polymerase chain reaction (RT-PCR). Monoclonal antibodies JSB-1 and UIC-2, recognising separate epitopes of Pgp, were used for IHC. For RT-PCR, total RNA was extracted from paraffin-embedded tissue. After RT, the samples were subjected to nested PCR (NPCR) using primers specific for the MDR1 gene, and evaluated by electrophoresis. In gall bladder carcinoma, the percentage of positive cases expressing Pgp (77% for JSB-1, 69% for UIC-2) and MDR1 mRNA (52%) was significantly higher than those in normal gall bladder. In earlier TNM stages Pgp and MDR1 mRNA were more frequently expressed (non-significant) than in advanced stages. The results of this study suggested that overexpression of MDR1 mRNA and Pgp in gall bladder carcinoma tissue probably is a very important reason why gall bladder cancer is generally not responsive to chemotherapy.

Wild-type p53-mediated induction of rat mdr1b expression by the anticancer drug daunorubicin

The expression of P-glycoproteins encoded by the mdr gene family is associated with the emergence of the multidrug resistance phenotype in animal cells. mdr expression can be induced by many extracellular stimulants including cytotoxic drugs and chemical carcinogens. However, little is known about the mechanisms involved. Here, we report that the expression of the rat mdr1b can be induced by anticancer drug daunorubicin. Further analysis identified a bona fide p53-binding site spanning from base pairs -199 to -180 (5'-GAACATGTAGAGACATGTCT-3') in the rat mdr1b promoter that is essential for basal and daunorubicin-inducible promoter activities. In addition, our results show that wild-type p53 can up-regulate not only the promoter function but also endogenous expression of the rat mdr1b. To the best of our knowledge, this is the first report showing that a specific p53-binding site is involved in the transcriptional regulation of mdr gene by wild-type p53. Since p53 is a sensor for a wide variety of genotoxic stresses, our finding has broad implications for understanding the mechanisms involved in the inducible expression of mdr gene by anticancer drugs, chemical carcinogens, UV light, and other DNA-damaging agents.

Differential expression of the polyspecific drug transporter OCT1 in rat hepatocarcinoma cells

The polyspecific drug transporter OCT1 is a plasma transmembrane protein involved in the uptake of cationic drugs into hepatocytes. In order to determine whether hepatic OCT1 levels, like those of the other cationic drug transporter P-glycoprotein, may be altered during hepatocarcinogenesis, we have investigated OCT1 expression and activity in rat liver carcinoma cells. Similar levels of OCT1 mRNAs were evident in both normal liver and diethylnitrosamine-induced hepatocarcinomas by Northern blot analysis. In contrast, five hepatoma cell lines (Fao, Faza, H5, HTC and RHC1) showed either a decrease or an absence of OCT1 expression compared to normal hepatocytes; these hepatoma cells also displayed lower intracellular accumulation of tetraethylammonium (TEA), a well-known substrate for OCT1. However, among the hepatoma cell lines, the well-differentiated Fao cell line was found to retain substantial levels of OCT1 expression and of intracellular TEA uptake. Therefore, these data provide the first evidence that OCT1 expression is well-preserved in chemically-induced rat malignant neoplastic liver lesions, whereas it is either decreased or undetectable in hepatoma cell lines, which may be related to the loss of various liver functions usually occurring in these cell lines.

Chemosensitivity of advanced larynx carcinoma cells in vitro and significance of multidrug resistance markers in these tumors

Thirty cases of previously untreated advanced larynx carcinoma were checked for in vitro chemosensitivity and presence of the resistance markers viz. P-glycoprotein (P-gp) glutathione-S-transferase-pi (GST-pi) and protein kinase C (PKC) overexpression. The cytotoxicity testing was done using MTT assay and the resistance markers were checked by immunohistochemical methods using monoclonal antibodies. The drug combinations employed in MIT assay were 5FU* + MTX*, 5FU + cisPt*, 5FU + Mito*, cisPt + Mito and MTX + Mito (*5FU = 5Fluorouracil, MTX-methotrexate, cisPt-cisplatin and Mito = mitomycin C). No statistically significant correlation was observed between resistance to the above drug combinations and presence of the resistance markers under consideration. A statistically significant correlation was observed between node positivity and expression of resistance markers which indicates that presence of one or more of these markers in these tumors may be considered as a negative prognosis marker. CisPt-Mito was found to be the most effective drug combination in vitro, in the cases studied.

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.

p53-dependent regulation of MDR1 gene expression causes selective resistance to chemotherapeutic agents

Loss of functional p53 paradoxically results in either increased or decreased resistance to chemotherapeutic drugs. The inconsistent relationship between p53 status and drug sensitivity may reflect p53's selective regulation of genes important to cytotoxic response of chemotherapeutic agents. We reasoned that the discrepant effects of p53 on chemotherapeutic cytotoxicity is due to p53-dependent regulation of the multidrug resistance gene (MDR1) expression in tumors that normally express MDR1. To test the hypothesis that wild-type p53 regulates the endogenous mdr1 gene we stably introduced a trans-dominant negative (TDN) p53 into rodent H35 hepatoma cells that express P-glycoprotein (Pgp) and have wild-type p53. Levels of Pgp and mdr1a mRNA were markedly elevated in cells expressing TDN p53 and were linked to impaired p53 function (both transactivation and transrepression) in these cells. Enhanced mdr1a gene expression in the TDN p53 cells was not secondary to mdr1 gene amplification and Pgp was functional as demonstrated by the decreased uptake of vinblastine. Cytotoxicity assays revealed that the TDN p53 cell lines were selectively insensitive to Pgp substrates. Sensitivity was restored by the Pgp inhibitor reserpine, demonstrating that only drug retention was the basis for loss of drug sensitivity. Similar findings were evident in human LS180 colon carcinoma cells engineered to overexpress TDN p53. Therefore, the p53 inactivation seen in cancers likely leads to selective resistance to chemotherapeutic agents because of up-regulation of MDR1 expression.

Maintenance of hypomethylation status and preferential expression of exogenous human MDR1/PGY1 gene in mouse L cells by YAC mediated transfer

Selection of cells for resistance to vincristine or doxorubicin often induces overexpression of the multidrug resistance (MDR) genes, which encode the cell surface P-glycoproteins, as a result of gene amplification, transcriptional activation, or mRNA stabilization. The LMD1 and LMD4 cell lines were established after the transfer into mouse L cells of two independent yeast artificial chromosome clones containing 300 and 850 kb, respectively, of the human MDR locus. The human MDR1/PGY1 gene, but not the endogenous mouse mdr1a and mdr1b genes, was overexpressed as a result of gene amplification and transcriptional activation in various sublines of LMD1 and LMD4 cells selected for resistance to vincristine. Then we asked why human MDR1/PGY1 gene, but not mouse relevant gene, was expressed. Determination of the methylation status of cytosine residues at Msp I/Hap II cleavage sites (CCGG) in the promoter regions of human MDR1/PGY1 and mouse mdr1a revealed hypomethylation and hypermethylation of the human and mouse genes, respectively in LMD1, LMD4, and their vincristine-resistant derivatives. Various vincristine-resistant sublines were also established after exposure of LMD1 cells for 48 h to 5-aza-2'-deoxycytidine, an inhibitor of DNA methyltransferase. These sublines exhibited overexpression of mouse mdr1a and mdr1b, but not of human MDR1/PGY1, as well as hypomethylation of the mouse mdr1a promoter region. Thus, the selective expression of human or mouse MDR genes in this cell system appears to be related to the methylation status of the respective gene promoter regions.

A new type of hazardous chemical: the chemosensitizers of multixenobiotic resistance

The purpose of this overview is to introduce the property of a new class of hazardous chemicals-the inhibitors of multixenobiotic resistance (MXR) in aquatic organisms, referred to as chemosensitizers. Aquatic organisms possess MXR, a mechanism similar to the well-known P-glycoprotein extrusion pump in multidrug resistant (MDR) tumor cells. MXR in aquatic organism moves from cells and organisms both endogenous chemicals and xenobiotics, including also some man-made chemicals. MXR in aquatic organisms represents a general biological first-line defense mechanism for protection against environmental toxins. Many chemical agents, the chemosensitizers, may after the function of this fragile mechanism. It is this new, MXR-inhibiting property, unrecognized as yet, that classifies these chemicals among top-rank hazardous water pollutants. The knowledge that the presence of one xenobiotic may block the pumping out of other xenobiotic(s), and hence accelerate their accumulation, may have important implications on environmental parameters like exposure, uptake, bioaccumulation, and toxicity. In this overview we present the evidence for the expression of MXR-phenotype in aquatic organisms, the demonstration of toxic consequences caused by MXR inhibitors, and the description of methods for measurement of concentration of MXR inhibitors in environmental samples.

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.