Minor role of pregnane-x-receptor for acquired multidrug resistance in head and neck squamous cell carcinoma in vitro

Regulation of PXR Function by Coactivator and Corepressor Proteins: Ligand Binding Is Just the Beginning

The pregnane X receptor (PXR, NR1I2) is a nuclear receptor which exerts its regulatory function by heterodimerization with the retinoid-X-receptor α (RXRα, NR2B1) and binding to the promoter and enhancer regions of diverse target genes. PXR is involved in the regulation of drug metabolism and excretion, metabolic and immunological functions and cancer pathogenesis. PXR activity is strongly regulated by the association with coactivator and corepressor proteins. Coactivator proteins exhibit histone acetyltransferase or histone methyltransferase activity or associate with proteins having one of these activities, thus promoting chromatin decondensation and activation of the gene expression. On the contrary, corepressor proteins promote histone deacetylation and therefore favor chromatin condensation and repression of the gene expression. Several studies pointed to clear cell- and ligand-specific differences in the activation of PXR. In this article, we will review the critical role of coactivator and corepressor proteins as molecular determinants of the specificity of PXR-mediated effects. As already known for other nuclear receptors, understanding the complex mechanism of PXR activation in each cell type and under particular physiological and pathophysiological conditions may lead to the development of selective modulators with therapeutic potential.

Effect of 5-fluorouracil on mRNA expression of drug metabolizing enzyme and transporter genes in human hepatoma cell lines

Fluoropyrimidines such as 5-fluorouracil (5-FU) are well known to have drug-drug interactions with anticoagulant medications such as warfarin. This study investigated the mRNA expression of pharmacokinetic (PK)-related genes in response to 5-FU using the hepatocarcinoma cell lines after examining relevant gene expression via RNA sequencing. We used HepaRG cells for 5-FU treatment analysis because these cells displayed PK-related gene expression. 5-FU exposure significantly reduced cytochrome P450 3A4 (CYP3A4) mRNA expression. Additionally, the mRNA expression of nuclear receptor subfamily 1 group I member 2 (also known as pregnane X receptor), a nuclear receptor transcription factor that promotes the expression of many CYP genes, was also decreased in HepaRG cells following 5-FU treatment. The mRNA expressions of the CYP2B6 and ATP-binding cassette transporter genes were decreased after 5-FU treatment. This study revealed that 5-FU treatment reduced PK-related gene expression in HepaRG cells. These findings should be useful for further drug-drug interaction research.

Acquired ABC-transporter overexpression in cancer cells: transcriptional induction or Darwinian selection?

Acquired multidrug resistance (MDR) in tumor diseases has repeatedly been associated with overexpression of ATP-binding cassette transporters (ABC-transporters) such as P-glycoprotein. Both in vitro and in vivo data suggest that these efflux transporters can cause MDR, albeit its actual relevance for clinical chemotherapy unresponsiveness remains uncertain. The overexpression can experimentally be achieved by exposure of tumor cells to cytotoxic drugs. For simplification, the drug-mediated transporter overexpression can be attributed to two opposite mechanisms: First, increased transcription of ABC-transporter genes mediated by nuclear receptors sensing the respective compound. Second, Darwinian selection of sub-clones intrinsically overexpressing drug transporters being capable of extruding the respective drug. To date, there is no definite data indicating which mechanism truly applies or whether there are circumstances promoting either mode of action. This review summarizes experimental evidence for both theories, suggests an algorithm discriminating between these two modes, and finally points out future experimental approaches of research to answer this basic question in cancer pharmacology.

Time-Resolved Effect of Interferon-Alpha 2a on Activities of Nuclear Factor Kappa B, Pregnane X Receptor and on Drug Disposition Genes

Interferon-alpha (IFN-α) is suggested to cause pharmacokinetic drug interactions by lowering expression of drug disposition genes through affecting the activities of nuclear factor kappa B (NF-ĸB) and pregnane X receptor (PXR). The time-resolved impact of IFN-α 2a (1000 U/mL; 5000 U/mL; 2 h to 30 h) on the activities of NF-ĸB and PXR and mRNA expression (5000 U/mL; 24 h, 48 h) of selected drug disposition genes and on cytochrome P450 (CYP3A4) activity in LS180 cells (5000 U/mL; 24 h, 48 h) was evaluated using luciferase-based reporter gene assays, reverse transcription polymerase chain reaction, and luminescence-based CYP3A4 activity assays. The cross-talk between NF-ĸB activation and PXR suppression was evaluated by NF-ĸB blockage (10 µM parthenolide). IFN-α 2a initially (2 h, 6 h) enhanced NF-ĸB activity 2-fold and suppressed PXR activity by 30%. mRNA of CYP3A4 was halved, whereas UGT1A1 was increased (1.35-fold) after 24 h. After 48 h, ABCB1 expression was increased (1.76-fold). CYP3A4 activity remained unchanged after 24 h, but was enhanced after 48 h (1.35-fold). IFN-α 2a demonstrated short-term suppressive effects on PXR activity and CYP3A4 mRNA expression, likely mediated by activated NF-ĸB. Longer exposure enhanced CYP3A4 activity. Clinical trials should evaluate the relevance by investigating the temporal effects of IFN-α on CYP3A4 using a sensitive marker substrate.

Functional role of miR-148a in oropharyngeal cancer: influence on pregnane X receptor and P-glycoprotein expression

MicroRNAs are short noncoding RNAs of about 19-25 nucleotides that usually target the 3' untranslated regions of mRNAs thus mediating post-transcriptional regulation of gene expression. Previous data indicate a role for miR-148a in the regulation of the pregnane X receptor (PXR/NR1I2), a nuclear receptor that regulates the expression of drug transporters like P-glycoprotein (P-gp/ABCB1). Our study investigated the effect of miR-148a on the post-transcriptional regulation of PXR and its target gene ABCB1 in oropharyngeal cancer cell lines (OPSCC). miR-148a was over-expressed and knocked-down in three OPSCC cell lines (HNO41, HNO206, and HNO413) by transfection with miR-148a mimic and miR-148a antagomir, respectively. Expression of miR-148a, NR1I2, and ABCB1 mRNA was quantified via real-time qPCR, protein expression of PXR was assessed by immunoblotting. Transfection of miR-148a mimic led to increased miR-148a levels in all cell lines and transfection of miR-148a antagomir reduced miR-148a expression in HNO206 and HNO413. Whereas these changes had no significant effect on PXR mRNA expression, protein expression was reduced in HNO41 by transfection with miR-148a and increased in HNO413 by transfection with miR-148a antagomir. Transfection of miR-148a downregulated ABCB1 mRNA in all cell lines, whereas antagonizing miR-148a had no significant effect. Our data demonstrate a modulation of PXR/NR1I2 and ABCB1 expression in OPSCC by miR-148a, however the effect was not uniform in all cell lines and depended on the range of expression of miR-148 and the genotype of rs1054190 SNP in NR1I2 3'UTR. Thus, our findings argue against an unequivocal association between miR-148a and PXR levels in OPSCC.

Human papilloma virus (HPV) 18 proteins E6 and E7 up-regulate ABC transporters in oropharyngeal carcinoma. Involvement of the nonsense-mediated decay (NMD) pathway

Oropharyngeal cancer incidence increased dramatically in the last decades, being infection with human papillomaviruses (HPV) a determinant of this trend. Concerning etiology, treatment response and prognosis, HPV⁺ and HPV^- oropharyngeal cancers constitute different disease entities. The underlying molecular background is not completely understood. ATP-binding cassette (ABC) transporters mediate the efflux of anticancer drugs and are regulated by changes in the intracellular milieu. Furthermore, a role in cancer pathogenesis besides drug transport was reported. We evaluated the effect of transfection with E6 and E7 oncogenes from HPV16 and HPV18 on ABC transporters in oropharyngeal cancer cells. HPV18E6/E7 up-regulated P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1) and MRP2 expression in HNO206 cells and breast cancer resistance protein (BCRP) in HNO206 and HNO413 cells. While P-gp was regulated translationally, MRP1, MRP2 and BCRP up-regulation resulted from mRNA stabilization. For MRP1 and MRP2, the nonsense-mediated decay pathway was involved. In general, resistance to substrates of up-regulated transporters was increased. Transfection with oncogenes individually indicated a major role of HPV18E7. Our findings suggest ABC transporters as molecular players leading to differences in the pathogenesis of HPV⁺ and HPV^- oropharyngeal cancer.

Biphasic modulation of cAMP levels by the contraceptive nomegestrol acetate. Impact on P-glycoprotein expression and activity in hepatic cells

ABC transporters are key players in drug excretion with alterations in their expression and activity by therapeutic agents potentially leading to drug-drug interactions. The interaction potential of nomegestrol acetate (NMGA), a synthetic progestogen increasingly used as oral contraceptive, had never been explored. In this work we evaluated (1) the effect of NMGA on ABC transporters in the human hepatic cell line HepG2 and (2) the underlying molecular mechanism. NMGA (5, 50 and 500 nM) increased P-glycoprotein (P-gp) expression at both protein and mRNA levels and reduced intracellular calcein accumulation, indicating an increase also in transporter activity. This up-regulation of P-gp was corroborated in Huh7 cells and was independent of the classical progesterone receptor. Instead, using a siRNA-mediated silencing approach, we demonstrated the involvement of membrane progesterone receptor α. Moreover, we found that the activation of this receptor by NMGA led to a falling-rising profile in intracellular cAMP levels and protein kinase A activity over time, ultimately leading to transcriptional P-gp up-regulation. Finally, we identified inhibitory G protein and phosphodiesterases as mediators of this novel biphasic modulation. These results demonstrate the ability of NMGA to selectively up-regulate hepatic P-gp expression and activity and constitute the first report of ABC transporter modulation by membrane progesterone receptor α. If a similar regulation took place in vivo, decreased bioavailability and therapeutic efficacy of NMGA-coadministered P-gp substrates could be expected. This holds special importance considering long-term administration of NMGA and broad substrate specificity of P-gp.

The pregnane X receptor (PXR) and the nuclear receptor corepressor 2 (NCoR2) modulate cell growth in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is the sixth most frequent cancer worldwide. The pregnane X receptor (PXR) is a nuclear receptor regulating several target genes associated with cancer malignancy. We here demonstrated a significant effect of PXR on HNSCC cell growth, as evidenced in PXR knock-down experiments. PXR transcriptional activity is more importantly regulated by the presence of coactivators and corepressors than by PXR protein expression. To date, there is scarce information on the regulation of PXR in HNSCC and on its role in the pathogenesis of this disease. Coactivator and corepressor expression was screened through qRT-PCR in 8 HNSCC cell lines and correlated to PXR activity, determined by using a reporter gene assay. All cell lines considerably expressed all the cofactors assessed. PXR activity negatively correlated with nuclear receptor corepressor 2 (NCoR2) expression, indicating a major role of this corepressor in PXR modulation and suggesting its potential as a surrogate for PXR activity in HNSCC. To test the association of NCoR2 with the malignant phenotype, a subset of three cell lines was transfected with an over-expression plasmid for this corepressor. Subsequently, cell growth and chemoresistance assays were performed. To elucidate the mechanisms underlying NCoR2 effects on cell growth, caspase 3/7 activity and protein levels of cleaved caspase 3 and PARP were evaluated. In HNO97 cells, NCoR2 over-expression decreased cell growth, chemoresistance and increased cleaved caspase 3 levels, caspase activity and cleaved PARP levels. On the contrary, in HNO124 and HNO210 cells, NCoR2 over-expression increased cell growth, drug resistance and decreased cleaved caspase 3 levels, caspase activity and cleaved PARP levels. In conclusion, we demonstrated a role of PXR and NCoR2 in the modulation of cell growth in HNSCC. This may contribute to a better understanding of the highly variable HNSCC therapeutic response.

Pregnane X receptor, constitutive androstane receptor and hepatocyte nuclear factors as emerging players in cancer precision medicine

Great research effort has been focused on elucidating the contribution of host genetic variability on pharmacological outcomes in cancer. Nuclear receptors have emerged as mediators between environmental stimuli and drug pharmacokinetics and pharmacodynamics. The pregnane X receptor, constitutive androstane receptor and hepatocyte nuclear factors have been reported to regulate transcription of genes that encode drug metabolizing enzymes and transporters. Altered nuclear receptor expression has been shown to affect the metabolism and pharmacological profile of traditional chemotherapeutics and targeted agents. Accordingly, polymorphic variants in these genes have been studied as pharmacogenetic markers of outcome variability. This review summarizes the state of knowledge about the roles played by pregnane X receptor, constitutive androstane receptor and hepatocyte nuclear factor expression and genetics as predictive markers of anticancer drug toxicity and efficacy, which can improve cancer precision medicine.

Pregnane X Receptor and Cancer: Context-Specificity is Key

Pregnane X receptor (PXR) is an adopted orphan nuclear receptor that is activated by a wide-range of endobiotics and xenobiotics, including chemotherapy drugs. PXR plays a major role in the metabolism and clearance of xenobiotics and endobiotics in liver and intestine via induction of drug-metabolizing enzymes and drug-transporting proteins. However, PXR is expressed in several cancer tissues and the accumulating evidence strongly points to the differential role of PXR in cancer growth and progression as well as in chemotherapy outcome. In cancer cells, besides regulating the gene expression of enzymes and proteins involved in drug metabolism and transport, PXR also regulates other genes involved in proliferation, metastasis, apoptosis, anti-apoptosis, inflammation, and oxidative stress. In this review, we focus on the differential role of PXR in a variety of cancers, including prostate, breast, ovarian, endometrial, and colon. We also discuss the future directions to further understand the differential role of PXR in cancer, and conclude with the need to identify novel selective PXR modulators to target PXR in PXR-expressing cancers.

The phytoestrogen genistein enhances multidrug resistance in breast cancer cell lines by translational regulation of ABC transporters

Breast cancer is the most frequent malignancy in women. Multidrug resistance due to overexpression of ABC drug transporters is a common cause of chemotherapy failure and disease recurrence. Genistein (GNT) is a phytoestrogen present in soybeans and hormone supplements. We investigated the effect of GNT on the expression and function of ABC transporters in MCF-7 and MDA-MB-231 breast cancer cell lines. Results demonstrated an induction at the protein level of ABCC1 and ABCG2 and of ABCC1 in MCF-7 and MDA-MB-231, respectively. MCF-7 cells showed a concomitant increase in doxorubicin and mitoxantrone efflux and resistance, dependent on ABCG2 activity. ABCC1 induction by GNT in MDA-MB-231 cells modified neither drug efflux nor chemoresistance due to simultaneous acute inhibition of the transporter activity by GNT. All inductions took place at the translational level, as no increment in mRNA was observed and protein increase was prevented by cycloheximide. miR-181a, already demonstrated to inhibit ABCG2 translation, was down-regulated by GNT, explaining translational induction. Effects were independent of classical estrogen receptors. Results suggest potential nutrient-drug interactions that could threaten chemotherapy efficacy, especially in ABCG2-expressing tumors treated with substrates of this transporter.

Antiproliferative efficacies but minor drug transporter inducing effects of paclitaxel, cisplatin, or 5-fluorouracil in a murine xenograft model for head and neck squamous cell carcinoma

Drug-induced multidrug resistance (MDR) has been linked to overexpression of drug transporting proteins in head and neck squamous cell carcinoma (HNSCC) in vitro. The aim of this work was to reassess these findings in a murine xenograft model. NOD-SCID mice xenotransplanted with 10 (6) HNO97 cells were treated for four consecutive weeks with weekly paclitaxel, biweekly cisplatin (both intraperitoneal), or 5-fluorouracil (5-FU, administered by osmotic pump). Tumor volume and body weight were weekly documented. Expression of drug transporters and Ki-67 marker were examined using quantitative real-time polymerase chain reaction and/or immunohistochemistry. Both paclitaxel and cisplatin significantly reduced tumor volumes after 2-3 weeks. 5-FU-treated animals had significantly lower body weights after 2 or 4 weeks of chemotherapy. None of the drugs affected expression of drug transporters at the mRNA level. However, P-glycoprotein (Pgp) protein expression was increased by paclitaxel (P<0.01). Ki-67 expression did not change during treatment irrespective of the drug applied. Paclitaxel and cisplatin are effectively tumor volume reducing drugs in a murine xenograft model of HNSCC. Paclitaxel enhanced Pgp expression at the protein level, but not at the mRNA level suggesting transcriptional induction to be of minor relevance. In contrast, posttranscriptional mechanisms or Darwinian selection of intrinsically drug transporter overexpressing MDR cells might lead to iatrogenic chemotherapy resistance in HNSCC.