Cyclooxygenase-2, P-glycoprotein-170 and drug resistance; is chemoprevention against multidrug resistance possible?

BACKGROUND: It is generally accepted that P-glycoprotein 170 (MDR1/Pgp170) expression in breast tumors results in poor response to chemotherapy due to its ability to export chemotherapeutic agents. Studies indicate that the use of non-steroidal anti-inflammatory drugs (NSAIDs) may enhance the anti-tumor activity of cancer chemotherapeutic agents and reduce the risk of many cancers. The best known function of NSAIDs is to block the enzyme cyclooxygenase (Cox), the rate limiting enzyme in the conversion of arachidonic acid to prostaglandins. In this study we investigated whether expression of the inducible isoform of Cox (Cox-2) is linked with the multidrug resistance phenotype in breast cancer. METHODS: Expression of Cox-2 and MDR1/Pgp170 was investigated in tumor specimens along with normal epithelium in breast cancer patients using immunohistochemisrty. Expression of Cox-2, MDR1/Pgp170, Protein Kinase C (PKC), and Activator Protein 1 (AP1) were investigated in a series of increasingly resistant human MCF-7 breast cancer cells compared to wild type using immunohistochemistry, Western blots, Northern blots, RT-PCR, and Southern blots. RESULTS: Immunohistochemical analyses of human breast tumor specimens revealed a strong correlation between expression of Cox-2 and MDR1/Pgp170. In drug resistant cell lines that over-express MDR1/Pgp170 there was also significant up-regulation of Cox-2 expression. In addition, PKC and AP1 subunits c-Jun and c-Fos were also upregulated. We hypothesized that increased prostaglandin production by Cox-2 induces PKC and the expression of transcriptional factor c-Jun, which in turn, induces the expression of MDR1/Pgp170. CONCLUSION: We propose that pretreatment with selective Cox-2 inhibitors may be useful in the prevention of multidrug resistance in response to cancer chemotherapy and should be further evaluated.

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.

Increased AP-1 activity in drug resistant human breast cancer MCF-7 cells.

The expression, DNA binding, and transactivating activity of activator protein 1 (AP-1) was examined in a series of multidrug resistant (MDR) MCF-7 human breast cancer cells that have increasing levels of MDR1 gene expression. We observed an increase in the amount of both c-jun and c-fos mRNA in cells with 12-, 65-, or 200-fold higher resistance to adriamycin when compared to drug-sensitive MCF-7 wild type (WT) cells. Electrophoretic mobility shift assays (EMSA) demonstrated an increase in the DNA binding activity of an AP-1 complex in nuclear extracts from MDR MCF-7 cells when compared to extracts from WT cells. We observed a proportional increase in luciferase expression from a reporter vector containing consensus AP-1 binding sites in transiently transfected MDR cells when compared to WT cells, indicating that AP-1 mediated gene expression is increased in drug-resistant MCF-7 cells. Since the MDR1 promoter contains a putative AP-1 binding site, we used EMSA to examine AP-1 binding activity to an oligonucleotide probe that contained the relevant MDR1 promoter sequences (-123 to -108). Nuclear extracts from resistant MCF-7 cells displayed an increased level of DNA binding of Jun/Jun dimers to the probe, indicating that AP-1 was capable of binding to this promoter site. A luciferase reporter construct containing triplicate copies of the MDR1 promoter sequence was expressed at higher levels in transiently transfected MDR cells when compared to expression in WT cells. Co-transfection of WT cells with a c-jun expression vector and either of the AP-1 luciferase constructs demonstrated that c-jun could activate gene expression from both the consensus and the MDR1 AP-1 sites in a dose dependent manner. In addition, RT-PCR and western blot analysis showed that levels of MDR1 mRNA and Pgp were increased in c-jun transfected WT cells. Taken together, these data indicate that increased AP-1 activity may be an important mediator of MDR by regulating the expression of MDR1.

Dietary flavonols quercetin and kaempferol are ligands of the aryl hydrocarbon receptor that affect CYP1A1 transcription differentially.

Transcriptional activation of the human CYP1A1 gene (coding for cytochrome P450 1A1) is mediated by the aryl hydrocarbon receptor (AhR). In the present study we have examined the effect of the common dietary polyphenolic compounds quercetin and kaempferol on the transcription of CYP1A1 and the function of the AhR in MCF-7 human breast cancer cells. Quercetin caused a time- and concentration-dependent increase in the amount of CYP1A1 mRNA and CYP1A1 enzyme activity in MCF-7 cells. The increase in CYP1A1 mRNA caused by quercetin was prevented by the transcription inhibitor actinomycin D. Quercetin also caused an increase in the transcription of a chloramphenicol reporter vector containing the CYP1A1 promoter. Quercetin failed to induce CYP1A1 enzyme activity in AhR-deficient MCF-7 cells. Gel retardation studies demonstrated that quercetin activated the ability of the AhR to bind to an oligonucleotide containing the xenobiotic-responsive element (XRE) of the CYP1A1 promoter. These results indicate that quercetin's effect is mediated by the AhR. Kaempferol did not affect CYP1A1 expression by itself but it inhibited the transcription of CYP1A1 induced by the prototypical AhR ligand 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), as measured by a decrease in TCDD-induced CYP1A1 promoter-driven reporter vector activity, and CYP1A1 mRNA in cells. Kaempferol also abolished TCDD-induced XRE binding in a gel-shift assay. Both compounds were able to compete with TCDD for binding to a cytosolic extract of MCF-7 cells. Known ligands of the AhR are, for the most part, man-made compounds such as halogenated and polycyclic aromatic hydrocarbons. These results demonstrate that the dietary flavonols quercetin and kaempferol are natural, dietary ligands of the AhR that exert different effects on CYP1A1 transcription.

Resveratrol inhibits transcription of CYP1A1 in vitro by preventing activation of the aryl hydrocarbon receptor.

Resveratrol, a compound present in a variety of plants, was recently shown to have potent chemopreventive activity against aryl hydrocarbon-induced tumorigenesis in mice. Therefore, in the present study, we examined the effect of resveratrol on the function of the aryl hydrocarbon receptor (AHR) and the transcription of CYP1A1 in human HepG2 hepatoma cells. Resveratrol inhibited the increase in cytochrome P450 (CYP) 1A1 mRNA caused by the AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a concentration-dependent manner. The induction of transcription of an aryl hydrocarbon-responsive reporter vector containing the CYP1A1 promoter by TCDD was likewise inhibited by resveratrol. Resveratrol also inhibited the constitutive level of CYP1A1 mRNA and reporter vector transcription in HepG2 cells. The increase in CYP1A1 enzyme activity induced by TCDD was inhibited by resveratrol. Resveratrol prevented the TCDD-induced transformation of the cytosolic AHR to its nuclear DNA-binding form. However, resveratrol had no effect on the binding of TCDD to the cytosolic AHR. These data demonstrate that resveratrol inhibits CYP1A1 expression in vitro, and that it does this by preventing the binding of the AHR to promoter sequences that regulate CYP1A1 transcription. This activity may be an important part of the chemopreventive activity of resveratrol.

Effect of curcumin on the aryl hydrocarbon receptor and cytochrome P450 1A1 in MCF-7 human breast carcinoma cells.

We examined the interaction of curcumin, a dietary constituent and chemopreventive compound, with the carcinogen activation pathway mediated by the aryl hydrocarbon receptor (AhR) in MCF-7 mammary epithelial carcinoma cells. Curcumin caused a rapid accumulation of cytochrome P450 1A1 (CYP1A1) mRNA in a time- and concentration-dependent manner, and CYP1A1 monooxygenase activity increased as measured by ethoxyresorufin-O-deethylation. Curcumin activated the DNA-binding capacity of the AhR for the xenobiotic responsive element of CYP1A1 as measured by the electrophoretic-mobility shift assay (EMSA). Curcumin was able to compete with the prototypical AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin for binding to the AhR in isolated MCF-7 cytosol, indicating that it interacts directly with the receptor. Although curcumin could activate the AhR on its own, it partially inhibited the activation of AhR, as measured by EMSA, and partially decreased the accumulation of CYP1A1 mRNA caused by the mammary carcinogen dimethylbenzanthracene (DMBA). Curcumin competitively inhibited CYP1A1 activity in DMBA-treated cells and in microsomes isolated from DMBA-treated cells. Curcumin also inhibited the metabolic activation of DMBA, as measured by the formation of DMBA-DNA adducts, and decreased DMBA-induced cytotoxicity. These results suggest that the chemopreventive effect of curcumin may be due, in part, to its ability to compete with aryl hydrocarbons for both the AhR and CYP1A1. Curcumin may thus be a natural ligand and substrate of the AhR pathway.