Pregnane × Receptor (PXR) expression in colorectal cancer cells restricts irinotecan chemosensitivity through enhanced SN-38 glucuronidation
© Raynal et al; licensee BioMed Central Ltd. 2010
Received: 22 September 2009
Accepted: 2 March 2010
Published: 2 March 2010
Clinical efficacy of chemotherapy in colorectal cancer is subjected to broad inter-individual variations leading to the inability to predict outcome and toxicity. The topoisomerase I inhibitor irinotecan (CPT-11) is worldwide approved for the treatment of metastatic colorectal cancer and undergoes extensive peripheral and tumoral metabolism. PXR is a xenoreceptor activated by many drugs and environmental compounds regulating the expression of drug metabolism and transport genes in detoxification organs such as liver and gastrointestinal tract. Considering the metabolic pathway of irinotecan and the tissue distribution of Pregnane × Receptor (PXR), we hypothesized that PXR could play a key role in colon cancer cell response to irinotecan.
PXR mRNA expression was quantified by RT-quantitative PCR in a panel of 14 colon tumor samples and their matched normal tissues. PXR expression was modulated in human colorectal cancer cells LS174T, SW480 and SW620 by transfection and siRNA strategies. Cellular response to irinotecan and its active metabolic SN38 was assessed by cell viability assays, HPLC metabolic profiles and mRNA quantification of PXR target genes. We showed that PXR was strongly expressed in colon tumor samples and displayed a great variability of expression. Expression of hPXR in human colorectal cancer cells led to a marked chemoresistance to the active metabolite SN38 correlated with PXR expression level. Metabolic profiles of SN38 showed a strong enhancement of SN38 glucuronidation to the inactive SN38G metabolite in PXR-expressing cells, correlated with an increase of UDPglucuronosyl transferases UGT1A1, UGT1A9 and UGT1A10 mRNAs. Inhibition of PXR expression by lentivirus-mediated shRNA, led to SN38 chemoresistance reversion concomitantly to a decrease of UGT1A1 expression and SN38 glucuronidation. Similarly, PXR mRNA expression levels correlated to UGT1A subfamily expression in human colon tumor biopsies.
Our results demonstrate that tumoral metabolism of SN38 is affected by PXR and point to potential therapeutic significance of PXR quantification in the prediction of irinotecan response. Furthermore, our observations are pharmacologically relevant since many patients suffering from cancer diseases are often exposed to co-medications, food additives or herbal supplements able to activate PXR. A substantial part of the variability observed among patients might be caused by such interactions
Colorectal cancer is the fourth most common cancer in men and the third in women worldwide, and is currently undergoing a rapid increase in incidence . Approximately two-thirds of patients present potentially curable disease but 30-40% will relapse with metastatic disease. Despite the emergence of targeted therapies, chemotherapy based on conventional fluoropyrimidine associated either with the platinum salt oxaliplatin or with the topoisomerase inhibitor irinotecan remains the first-line treatment . Yet, clinical efficacy of these drugs is limited by the inability to predict chemotherapy outcome and toxicity. Notably, broad inter-individual variations in terms of response as well as of the occurrence of severe toxic side-effects like diarrhea and neutropenia are detected following treatment with compounds such as irinotecan . In this context, identification of biological markers allowing the prediction of both therapeutic and toxic response is a priority issue.
Irinotecan (or CPT-11) is a water-soluble derivative of camptothecin acting as a topoisomerase I inhibitor and currently registered for use in patients with metastatic colorectal cancer. Irinotecan itself has weak, if any, pharmacological activity in vitro. It is thought to exert its antitumor activity in vivo after enzymatic cleavage by carboxylesterases 1 and 2 (predominantly in the liver but also partly at the tumor site) that generate the active metabolite SN38. Irinotecan and SN38 are then subjected to extensive intracellular catabolism yielding inactive metabolites. Irinotecan undergoes phase I oxidation by cytochromes P450 3A4 and 3A5 leading to oxidized inactive metabolites whereas SN38 is metabolised to SN38G through phase II glucuronidation by the UDP-glucuronosyl transferases 1A1, 1A6, 1A9 and 1A10 [4, 5]. In addition, irinotecan and its metabolites are subjected to extracellular efflux through transporters, including P-glycoprotein (MDR1), multidrug resistance-related protein-2 (MRP2) and breast cancer resistance protein (BCRP) [6, 7]. Numerous studies have focused on peripheral irinotecan metabolism, and genetic polymorphisms within genes coding for enzyme implicated in the irinotecan metabolic pathway have been extensively described. Notably, detection of the UGT1A1*28 genotype, found to be predictive for SN38 peripheral glucuronidation and irinotecan toxicity , is now recommended by the US Food and Drug Administration. However, conflicting results on UGT1A1*28 and the plethora of studies on others sequence variations in UGT1A1, but also in ABCB1, ABCC1 or HNF1A genes, suggests that reliable predictions of SN38 exposures cannot be based on the detection of a single polymorphism . Inter-individual variation may be due to a combination of many genetic and non-genetic factors (diet, co-medications, etc.). Indeed, irinotecan pharmacokinetics and disposition is affected by various compounds now identified as ligands of the xenosensor PXR (Pregnane × Receptor, NR1I2) such as rifampicin  or St. John's wort .
PXR is a nuclear receptor acting as a "molecular sentinel" able to bind to a large variety of structurally diverse compounds included drugs, food additive or environmental toxics . It coordinates the detoxification of many lipophilic xenobiotics via transcriptional regulation of a large number of metabolizing enzymes and transporters . Targets genes of PXR are CYP3A4 , MDR1 , CYP2B6 , members of UGTs superfamily  and transporters like the multidrug resistance-related protein-3 (MRP3)  or the organic anion transporting polypeptide-2 (OATP2) . PXR is predominantly expressed in liver and in intestinal tract, but little is known about its expression in tumors. Because PXR controls the expression of key genes involved in anticancer drugs disposition, recent works have focused on its potential role in drug resistance . For instance, PXR is suspected to play a role in both all-trans retinoic acid  and etoposide  resistances through an enhancement of their CYP3A4-mediated metabolism. In addition, it has been shown that PXR induces cell proliferation and inhibits apoptosis in human colon cancer cells . Considering the metabolic profile of irinotecan and the tissue distribution of PXR, we aimed to assess to what extent PXR could affect metabolism and colon cancer cell response to irinotecan. We show that expression of PXR in human colorectal cancer cells led to irinotecan and SN38 chemoresistance through enhancement of its glucuronidation.
Materials and methods
Cell lines, plasmids and transfections
The human colorectal cancer cells LS174T were kindly provided by Dr. Pierre Martineau (IRCM, Montpellier, France). SW480, SW620, HCT116, HT29, HepG2 and HuH7 were from the cells collection of the Macromolecular Biochemistry Research Center (Montpellier, France). All cell lines were grown in RPMI 1640 supplemented with 10% fetal calf serum (FCS), 2 mmol/l glutamine, 100 units/ml penicillin and streptomycin. Selection mediums for transfected cells were supplemented with 250 μg/ml (SW480 and SW620) or 500 μg/ml (LS174T) geneticin. Cells were maintained routinely at 37°C in 5% CO2 humidified atmosphere.
PXR expression vector was built by cloning hPXR-1 cDNA (NM_003889)  in a pcDNA3 vector (Invitrogen). Stable clones overexpressing PXR were obtained by transfecting cells with the pcDNA3-hPXR vector using lipofectamine LTX transfection reagent (Invitrogen), according to manufacturer's instructions. Parent LS174T cells were transfected with empty pcDNA3 vector to yield control mock-transfectant. The shRNA-expressing vectors were constructed by cloning shRNA expression cassettes into FG12 lentiviral vector  (additional file 1). Cells were transduced with lentiviral vectors and GFP positive cells were isolated using a BD FACSAria™ cell sorter as previously reported .
Human specimen samples
Specimens of liver and colon biopsies were obtained from the pathologist after resection according to French government regulations and with approval of the ethical committee (Montpellier and Nîmes Hospitals). Informed consent was obtained from all patients. Tissue samples were stored in liquid nitrogen until further use.
Irinotecan, 5-fluorouracil (5-FU), oxaliplatin and verapamil chlorhydrate solutions were provided by the department of Pharmacy of the Nimes university hospital. SN38 was a kind gift from Dr E. Chatelut (Claudius Regaud Institute, Toulouse, France). Dimethysulfoxide (DMSO), rifampicin, ketoconazole, fumitremorgin C and L-Sulforaphane (SFN) were purchased from Sigma-Aldrich.
RNA extraction and reverse transcription
Total RNA were extracted using RNAeasy kit (Qiagen), according to the manufacturer's instructions. RNA quantity and quality of samples were determined by the 260:280 nm absorbance ratios using a NanoDrop spectrophotometer (Thermo Fisher Scientific). One μg of total RNA from each sample was added to 8.4 μl of reverse transcription mix containing 4 μl of first strand buffer 5×, 0.4 μl of dNTP mix 25 mM, 2 μl of dithiothreitol 10×, 1 μl of oligodT primer solution and of MLV-RT enzyme 200 U/μl. Solution volumes were adjusted to 20 μl by adding RNase free water. Samples were placed at 37°C for 1 hour and at 65°C for 5 minutes. cDNA solution volumes were adjusted to 100 μl by adding 80 μl of PCR grade water and stored at -20°C for further analysis.
Real-time quantitative PCR
mRNAs expression was evaluated by RT-quantitative PCR, using a LightCycler 480 real-time PCR system and SYBRGreen PCR master mix 2× (Roche Diagnostics) in 96-well plates. Quantitative PCR was done using gene-specific primers and β-actin was used as reference gene (additional file 2). Standard curves were generated for all genes by serial dilution of cDNAs. After normalization of threshold cycle values with the amount of β-actin, gene expression levels were expressed as ratios compared with that of vehicle-treated cells. Each sample was run three times in duplicates, and data were analyzed using the 1.5 version of LightCycler 480 software (Roche Diagnostics). Standard curves were generated for all genes by serial dilution of cDNAs from LS174T control for relative quantification in cultured cells and from a pool of human liver biopsies for relative quantification in tumors.
Protein extracts were prepared from cells by using M-PER® mammalian protein extraction reagent (Thermo Scientific) in presence of a protease inhibitor cocktail (Roche), according to the manufacturer's protocol. Proteins (40 μg/lane) were separated by 12% SDS-polyacrylamide gel electrophoresis and transferred to a PROTRAN® nitrocellulose membrane (Schleicher and Schuell). Membranes were sequentially incubated with anti-hPXR (G-11, Santa Cruz Biotechnology) or anti-βactin (Santa Cruz Biotechnology) primary antibody, and with peroxidase-conjugated anti-mouse IgG (Santa Cruz Biotechnology). Signals were detected by chemoluminescence using ECL Western Blotting Detection reagents (GE Healthcare).
Tissues were embedded in paraffin and sections (5 μm) were dewaxed in a xylene bath and rehydrated in graded alcohols. Endogenous peroxidase activity was quenched with 1.5% H2O2 in methanol for 20 min. and washed in PBS. Antigen retrieval was performed by boiling slides in 10 mM sodium citrate buffer, pH 6.0. Nonspecific binding sites were blocked with 1% BSA, 3% normal goat serum, and 0.2% Triton X-100 in PBS for 1 h at RT. Slides were incubated with the primary anti-human polyclonal PXR antibody (Lifespan Biosiences) overnight at 4°C in 50 times-diluted blocking buffer. Universal immuno-peroxydase polymer anti-mouse Histofine® (Nichirei Biosciences, Japan) was used as a secondary reagent, stainings were developed with DAB (brown precipitate, SIGMA) and hematoxylin counterstain was used. After dehydration, sections were mounted in Pertex (Histolab). Then, slides were scanned with high resolution Nanozoomer (Hamamatsu).
Neutral red chemotherapeutic sensitivity assays
Experimental conditions for neutral red assays were adapted from a previously described protocol . Briefly, 20.000 cells were seeded in 96-well microtiter plates. After 24 h incubation, cells were treated for 72 h with increasing concentrations of cytotoxics. After a neutral red incubation at 37°C for four hours, cells were washed with PBS and destained with 150 μl of 1% glacial acetic acid/50% ethanol (vol:vol). The absorbance at 540 nm was measured using a microplate reader (iEMS, Labsystems). The effect of the drugs on cell survival was expressed as the percentage of cell viability compared to untreated cells.
Irinotecan metabolites detection assay
Cells were seeded in six-well plates at 106 cells/well, incubated for 24 hours, and then treated with 0.1% DMSO (solvent) or 10 μM SN38 for 24 h. Cell pellets and supernatants were stored at -80°C for further analysis. Cell pellets were dissolved in 500 μl of a mixture of methanol-acetonitrile (50:50 vol:vol). 400 μl of culture supernatants were added to 800 μl of the mixture of methanol-acetonitrile (50:50 vol:vol). After proteins denaturation by full-speed vortex mixing, samples were then centrifuged at 13000 rpm for 3 minutes. 550 μl of clear supernatants were mixed to 250 μl of 1 M HCl and used for HPLC injection. Irinotecan and its metabolites were detected and quantified by a HPLC method as previously described .
The Mann and Whitney test was used to analyze the difference between two groups of quantitative variables. Alpha value was set at 5%. For comparisons among three groups of quantitative variables, the Kruskal Wallis test was used. In cases where there was a significant difference between the groups, a pairwise comparison was carried out by adjusting the alpha risk by the method of Bonferroni. Student's t-tests were performed when indicated in figures legends. All statistical analyses were carried out by the Department of biostatistics, epidemiology, public Health and medical information of the Nîmes University Hospital using the SAS software (SAS Institute Inc.).
Expression of hPXR in colon tissues and colon cancer cells
Functional characterization of PXR transfected LS174T colorectal cancer cells
PXR induces colorectal cancer cells resistance to irinotecan and SN38
As expected, cells were more sensitive to SN38 than to irinotecan, the latter undergoing minimal conversion into its active metabolite due to very low expression level of carboxylesterases in these cells (additional file 4). In addition, cell viability assays performed in SW480 and SW620 cell lines stably transfected with hPXR, showed similar PXR-dependent enhancement of SN38 resistance (additional file 5). On the other hand, we found no effect of PXR expression on cell sensitivity to both 5-fluorouracil and oxaliplatin sensitivities (additional file 6).
Inhibition of PXR expression reverses chemoresistance to SN38
PXR increases SN38 glucuronidation
Since PXR expression does not affect colon cancer cells proliferation, we hypothesized that SN38 resistance observed in PXR-expressing cells is likely mediated through transcriptional regulation of genes involved in drug metabolism. We first explored several steps of irinotecan metabolism by using pharmacological inhibitors of CYP3A4 (ketoconazole), MDR1 (verapamil) and BCRP (fumitremorgin C). None of these compounds was able to reverse the PXR-dependent chemoresistance (data not shown). We next assessed the metabolic profile of SN38 using a previously described chromatographic detection . Raw data of peak area for intracellular and extracellular SN38 and SN38G allowed us to calculate the metabolic ratios SN38G/SN38.
In this work, we address whether expression of PXR in human colorectal cancer cells could interfere with their sensitivity and metabolism of drugs used in treatment of advanced colorectal cancer. First we showed that hPXR is expressed in both normal and neoplastic human colon tissues with a strong variability in cancer colon tissues. This variability may prove clinically relevant, since a major finding of this study is that expression of PXR in human colorectal cancer cells leads to chemoresistance to the active metabolite of irinotecan, SN38, whereas it did not affect their sensitivity to both 5-fluorouracil and oxaliplatin sensitivities. The opposite effect obtained with pharmacological inactivation of PXR or shRNA-mediated PXR down regulation confirmed the direct involvement of PXR in SN38 chemoresistance. However, in contrast to previous studies showing that PXR affects intrinsic cell survival through the p53 signaling pathway  and cell growth , we found that PXR induced SN38 chemoresistance in LS174T (p53 wt ) as well as in SW480 and SW620 (p53 mut ) without affecting their intrinsic proliferation rates. Instead, we observed that PXR expression lowered cellular SN38 concentration while increasing SN38 metabolism to its glucuronide conjugate. Accordingly, we found that several UGT1A isoenzymes were up-regulated in PXR-expressing cells, most notably UGT1A1 which is the key enzyme responsible of the inactivation of SN38 to SN38G.
Considering its role as master xenobiotics responsive receptor linking DME genes expression to environment stimuli, we think that differences in PXR expression contribute to the well known intra- and inter-subject variability in irinotecan response, and that they participate in the difficulty to clearly identify factors responsible for pharmacogenetics of irinotecan, the so-called "irinogenetics" [43–45]. Indeed, environmental compounds, nutrition and diet affecting PXR expression and/or activation may mask or attenuate pharmacogenetic associations. Moreover, PXR itself display strong genetic polymorphism with more than 300 reported SNPs in the dbSNPs database, some of which well characterized and inducing differences in both gene expression and ligand recognition [43, 44]. PXR expression levels within tumors could be also affected by non-genetic factors such as intra-tumor inflammatory cytokines , microRNA 148a  and methylation status of its exon 3 . In this context, discriminating the roles of genetic influences from environmental effects in drug response, recently coined "pharmacoecology" , will be even harder as expected. Thus, it will be of interest to evaluate the relative importance of these genetic and non-genetic factors in patient response toward irinotecan-based chemotherapy.
In view of the present findings, clinical studies are now needed to evaluate the potential interest of PXR in personalized medicine. Indeed, PXR expression and/or activation level could help physicians in the choice of appropriate chemotherapy regimen for colorectal cancer patients, since therapeutic alternatives to irinotecan already exist (i.e. platinum salt or targeted therapy). Finally, PXR down-regulation could be considered as a novel therapeutic approach to circumvent chemoresistance to chemotherapy.
This work was funded by La Ligue contre le Cancer, Université Montpellier I and CHU Nîmes. We thank Jean-François Bourgaux, Christine Pignodel, Julie Pannequin for providing us colon tissues, Célia Basurko for statistical analysis and Françoise Malosse for helpful technical assistance for HPLC analysis.
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