DNA elements recognizing NF-Y and Sp1 regulate the human multidrug-resistance gene promoter

A new strategy for overcoming drug resistance in liver cancer: Epigenetic regulation

Drug resistance in hepatocellular carcinoma has posed significant obstacles to effective treatment. Recent evidence indicates that, in addition to traditional gene mutations, epigenetic recoding plays a crucial role in HCC drug resistance. Unlike irreversible gene mutations, epigenetic changes are reversible, offering a promising avenue for preventing and overcoming drug resistance in liver cancer. This review focuses on various epigenetic modifications relevant to drug resistance in HCC and their underlying mechanisms. Additionally, we introduce current clinical epigenetic drugs and clinical trials of these drugs as regulators of drug resistance in other solid tumors. Although there is no clinical study to prevent the occurrence of drug resistance in liver cancer, the development of liquid biopsy and other technologies has provided a bridge to achieve this goal.

Epigenetic Regulation of Multidrug Resistance Protein 1 and Breast Cancer Resistance Protein Transporters by Histone Deacetylase Inhibition

Multidrug resistance protein 1 (MDR1, ABCB1, P-glycoprotein) and breast cancer resistance protein (BCRP, ABCG2) are key efflux transporters that mediate the extrusion of drugs and toxicants in cancer cells and healthy tissues, including the liver, kidneys, and the brain. Altering the expression and activity of MDR1 and BCRP influences the disposition, pharmacodynamics, and toxicity of chemicals, including a number of commonly prescribed medications. Histone acetylation is an epigenetic modification that can regulate gene expression by changing the accessibility of the genome to transcriptional regulators and transcriptional machinery. Recently, studies have suggested that pharmacological inhibition of histone deacetylases (HDACs) modulates the expression and function of MDR1 and BCRP transporters as a result of enhanced histone acetylation. This review addresses the ability of HDAC inhibitors to modulate the expression and the function of MDR1 and BCRP transporters and explores the molecular mechanisms by which HDAC inhibition regulates these transporters. While the majority of studies have focused on histone regulation of MDR1 and BCRP in drug-resistant and drug-sensitive cancer cells, emerging data point to similar responses in nonmalignant cells and tissues. Elucidating epigenetic mechanisms regulating MDR1 and BCRP is important to expand our understanding of the basic biology of these two key transporters and subsequent consequences on chemoresistance as well as tissue exposure and responses to drugs and toxicants. SIGNIFICANCE STATEMENT: Histone deacetylase inhibitors alter the expression of key efflux transporters multidrug resistance protein 1 and breast cancer resistance protein in healthy and malignant cells.

Epigenetic dysregulation of Mdr1b in the blood-testis barrier contributes to dyszoospermia in mice exposed to cadmium

Cadmium (Cd) has been reported to induce reproductive toxicity. Recent study indicated that aberrant epigenetic regulation of Multidrug resistance 1b (Mdr1b) causes xenobiotic efflux failure at the blood-testis barrier (BTB). However, whether Mdr1b dysregulation is involved in Cd-mediated dyszoospermia and the underlying mechanism remain unknown. In this study, mice were intragastrically administered 0 or 2.5 mg/kg CdCl₂ every other day for 2 months to investigate changes in spermatogenesis and epigenetic regulation of Mdr1b. Mouse Leydig cells TM3 were cultured to detect Mdr1b expression localization. We found that the Cd group revealed BTB disruption concomitant with obvious sperm abnormity and dynamic impairment. Hypermethylation and decreased nuclear factor Ya (Nfya) recruitment to the Mdr1b promoter were correlated with low sperm motility in response to Cd. In conclusion, these findings provide in vivo evidence that epigenetic dysregulation of Mdr1b in the BTB is a potential cause of dyszoospermia upon Cd exposure.

Increased MDR1 Transporter Expression in Human Brain Endothelial Cells Through Enhanced Histone Acetylation and Activation of Aryl Hydrocarbon Receptor Signaling

Multidrug resistance protein 1 (MDR1, ABCB1, P-glycoprotein) is a critical efflux transporter that extrudes chemicals from the blood-brain barrier (BBB) and limits neuronal exposure to xenobiotics. Prior studies in malignant cells demonstrated that MDR1 expression can be altered by inhibition of histone deacetylases (HDAC), enzymes that modify histone structure and influence transcription factor binding to DNA. Here, we sought to identify the mechanisms responsible for the up-regulation of MDR1 by HDAC inhibitors in human BBB cells. Immortalized human brain capillary endothelial (hCMEC/D3) cells were treated with HDAC inhibitors and assessed for MDR1 expression and function. Of the HDAC inhibitors profiled, valproic acid (VPA), apicidin, and suberoylanilide hydroxamic acid (SAHA) increased MDR1 mRNA and protein levels by 30-200%, which corresponded with reduced intracellular accumulation of the MDR1 substrate rhodamine 123. Interestingly, induction of MDR1 mRNA by HDAC inhibitors mirrored increases in the expression of the aryl hydrocarbon receptor (AHR) and its target gene cytochrome P450 1A1. To explore the role of AHR in HDAC inhibitor-mediated regulation of MDR1, a pharmacological activator (β-naphthoflavone, βNF) and inhibitor (CH-223191, CH) of AHR were tested. The induction of MDR1 in cells treated with SAHA was amplified by βNF and attenuated by CH. Furthermore, SAHA increased the binding of acetylated histone H3K9/K14 and AHR proteins to regions of the MDR1 promoter that contain AHR response elements. In conclusion, HDAC inhibitors up-regulate the expression and activity of the MDR1 transporter in human brain endothelial cells by increasing histone acetylation and facilitating AHR binding at the MDR1 promoter.

Cloning and Transcriptional Activity Analysis of the Porcine Abcb1 Gene Promoter: Transcription Factor Sp1 Regulates the Expression of Porcine Abcb1

P-Glycoprotein (P-gp, Abcb1) plays a crucial role in drug disposition and functions by hydrolyzing ATP. However, little is known about the regulatory elements governing the transcription of the porcine Abcb1 gene. In this study, the transcription start site of the pig Abcb1 gene was identified by 5'-RACE. A 1.9-kb fragment of the 5'-flanking region of the Abcb1 gene was cloned from pig genomic DNA and sequenced. The region critical for its promoter activity was investigated via progressive deletions. Further, using mutation assays, two proximal Sp1 binding sites within the 5'-flanking region of Abcb1 were proven to be important cis-regulatory elements involved in regulating the constitutive expression of porcine Abcb1. RNA interference experiments showed that Sp1 regulated the expression of the porcine P-gp at both mRNA and protein levels. Hence, the current work provides valuable information on the regulatory mechanisms of pig Abcb1.

MiR-138 indirectly regulates the MDR1 promoter by NF-κB/p65 silencing

MicroRNAs (miRNAs) are known to mediate post-transcriptional gene silencing in the cytoplasm and recent evidence indicates that may also possess nuclear roles in regulating gene expression. A previous study showed that miR-138 is involved in the multidrug resistance of leukemia cells through down-regulation of the drug efflux pump P-glycoprotein (P-gp), the protein encoded by the human multidrug-resistant ABCB1/MDR1 gene. However, the transcriptional regulatory mechanisms responsible remain to be elucidated. To deepen the description of the mechanism of transcriptional gene silencing on the MDR1 promoter, we initially performed a bioinformatics search for potential miR-138 binding sites in the MDR1 gene promoter sequence. Interestingly, we did not find miR-138 binding sites in this region, suggesting an indirect regulation. From six representative transcriptional factors involved in MDR1 gene regulation, an in silico analysis revealed that NF-κB/p65 has a specific binding site for miR-138. The results of luciferase reporter assay, western blot and flow cytometry shown here suggest that miR-138 might modulate the human MDR1 expression by inhibiting NF-κB/p65 as an indirect mechanism of MDR1 regulation. Furthermore, employing the human macrophage-like cell line U937 we observed comparable results with NF-κB/p65 down-regulation and we also observed a significant reduction in the IL-6 and TNF-α mRNA, as well as in their secreted pro-inflammatory cytokines following miR-138 expression, suggesting that canonical NF-κB target genes might also be potential targets for miR-138 in leukemia cells.

ZHX2 enhances the cytotoxicity of chemotherapeutic drugs in liver tumor cells by repressing MDR1 via interfering with NF-YA

We previously reported the tumor suppressor function of Zinc-fingers and homeoboxes 2 (ZHX2) in hepatocellular carcinoma (HCC). Other studies indicate the association of increased ZHX2 expression with improved response to high dose chemotherapy in multiple myeloma. Here, we aim to test whether increased ZHX2 levels in HCC cells repress multidrug resistance 1(MDR1) expression resulting in increased sensitivity to chemotherapeutic drugs. We showed evidence that increased ZHX2 levels correlated with reduced MDR1 expression and enhanced the cytotoxicity of CDDP and ADM in different HCC cell lines. Consistently, elevated ZHX2 significantly reduced ADM efflux in HepG2 cells and greatly increased the CDDP-mediated suppression of liver tumor growth in vivo. Furthermore, immunohistochemical staining demonstrated the inverse correlation of ZHX2 and MDR1 expression in HCC tissues. Luciferase report assay showed that ZHX2 repressed the MDR1 promoter activity, while knockdown of NF-YA or mutating the NF-Y binding site eliminated this ZHX2-mediated repression of MDR1 transcription. Co-IP and ChIP assay further suggested that ZHX2 interacted with NF-YA and reduced NF-Y binding to the MDR1 promoter. Taken together, we clarify that ZHX2 represses NF-Y-mediated activation of MDR1 transcription and, in doing so, enhances the effects of chemotherapeutics in HCC cells both in vitro and in vivo.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

ERα directly activated the MDR1 transcription to increase paclitaxel-resistance of ERα-positive breast cancer cells in vitro and in vivo

Chemotherapy is commonly used to treat early-stage invasive and advanced-stage breast cancer either before or after surgery. Increasing evidence from clinical analysis and in vitro studies has shown that ER-positive breast cancer cells are insensitive to chemotherapy. Complete understanding of how ERα mediates drug resistance is prerequisite to improvement of the chemotherapeutic efficacy. Over-expression of P-glycoprotein (P-gp) encoded by MDR1 gene is one of the major causes of drug resistance. The association between ERα and MDR1 in breast cancer is still unclear and the limited reports are conflict. This study systematically explored intrinsic link between ERα and the P-gp over-expression in paclitaxel-resistant ERα(+) breast cancer cell lines and mouse model in molecular details. Our data showed that ERα activated the MDR1 transcription in MCF-7/PTX breast cancer cells by binding to ERE1/2 and interacting with Sp1 that bridged to the downstream CG-rich element within the MDR1 promoter. Knockdown of MDR1 restrained the effect of ERα in MCF-7 cells and sensitized the cells to paclitaxel. Treatment of ICI 182,780 that selectively suppressed ERα significantly decreased the MDR1 expression and increased the sensitivity of drug resistant breast cancer cells and xenograft tumors to paclitaxel. Our data strongly demonstrated that ERα was able to increase drug resistance of breast cancer cells through activating MDR1 transcription. This novel mechanism provides new insight to how the ERα signaling regulates response of ERα(+) breast tumors to chemotherapy, which may be exploited for developing novel therapeutic strategies for breast cancer in the future.

Targeting the Y/CCAAT box in cancer: YB-1 (YBX1) or NF-Y?

The Y box is an important sequence motif found in promoters and enhancers containing a CCAAT box - one of the few elements enriched in promoters of large sets of genes overexpressed in cancer. The search for the transcription factor(s) acting on it led to the biochemical purification of the nuclear factor Y (NF-Y) heterotrimer, and to the cloning - through the screening of expression libraries - of Y box-binding protein 1 (YB-1), an oncogene, overexpressed in aggressive tumors and associated with drug resistance. These two factors have been associated with Y/CCAAT-dependent activation of numerous growth-related genes, notably multidrug resistance protein 1. We review two decades of data indicating that NF-Y ultimately acts on Y/CCAAT in cancer cells, a notion recently confirmed by genome-wide data. Other features of YB-1, such as post-transcriptional control of mRNA biology, render it important in cancer biology.

Transcriptional control of the multi-drug transporter ABCB1 by transcription factor Sp3 in different human tissues

The ATP-binding cassette (ABC) transporter ABCB1, encoded by the multidrug resistance gene MDR1, is expressed on brain microvascular endothelium and several types of epithelium, but not on endothelia outside the CNS. It is an essential component of the blood-brain barrier. The aim of this study was to identify cell-specific controls on the transcription of MDR1 in human brain endothelium. Reporter assays identified a region of 500 bp around the transcription start site that was optimally active in brain endothelium. Chromatin immunoprecipitation identified Sp3 and TFIID associated with this region and EMSA (electrophoretic mobility shift assays) confirmed that Sp3 binds preferentially to an Sp-target site (GC-box) on the MDR1 promoter in brain endothelium. This result contrasts with findings in other cell types and with the colon carcinoma line Caco-2, in which Sp1 preferentially associates with the MDR1 promoter. Differences in MDR1 transcriptional control between brain endothelium and Caco-2 could not be explained by the relative abundance of Sp1:Sp3 nor by the ratio of Sp3 variants, because activating variants of Sp3 were present in both cell types. However differential binding of other transcription factors was also detected in two additional upstream regions of the MDR1 promoter. Identification of cell-specific controls on the transcription of MDR1 indicates that it may be possible to modulate multi-drug resistance on tumours, while leaving the blood brain barrier intact.

Y-box-binding protein 1 (YB-1) and its functions

This review describes the structure and functions of Y-box binding protein 1 (YB-1) and its homologs. Interactions of YB-1 with DNA, mRNAs, and proteins are considered. Data on the participation of YB-1 in DNA reparation and transcription, mRNA splicing and translation are systematized. Results on interactions of YB-1 with cytoskeleton components and its possible role in mRNA localization are discussed. Data on intracellular distribution of YB-1, its redistribution between the nucleus and the cytoplasm, and its secretion and extracellular functions are summarized. The effect of YB-1 on cell differentiation, its involvement in extra- and intracellular signaling pathways, and its role in early embryogenesis are described. The mechanisms of regulation of YB-1 expression in the cell are presented. Special attention is paid to the involvement of YB-1 in oncogenic cell transformation, multiple drug resistance, and dissemination of tumors. Both the oncogenic and antioncogenic activities of YB-1 are reviewed. The potential use of YB-1 in diagnostics and therapy as an early cancer marker and a molecular target is discussed.

Expression and induction of p-glycoprotein-1 on cultured human brain endothelium

The ABC-transporter, p-glycoprotein-1 (pgp-1), is expressed on brain endothelium and is reported to be induced by several cytotoxic drugs, which are themselves substrates of pgp-1. Pgp-1 was increased on a human brain endothelial cell line (hCMEC/D3) after treatment with puromycin or verapamil. However, flow cytometry showed that the apparent upregulation caused by puromycin was not because of a global increase in expression levels, but selective cell death of a subpopulation of endothelium expressing the lowest levels of pgp-1. If a cytotoxic substrate of pgp-1 increases pgp-1 expression in vitro, it can easily be misinterpreted as a transcriptional activator of pgp-1.

ABCB1 and ABCC1 expression in peripheral mononuclear cells is influenced by gene polymorphisms and atorvastatin treatment

This study investigated the effects of atorvastatin on ABCB1 and ABCC1 mRNA expression on peripheral blood mononuclear cells (PBMC) and their relationship with gene polymorphisms and lowering-cholesterol response. One hundred and thirty-six individuals with hypercholesterolemia were selected and treated with atorvastatin (10 mg/day/4 weeks). Blood samples were collected for serum lipids and apolipoproteins measurements and DNA and RNA extraction. ABCB1 (C3435T and G2677T/A) and ABCC1 (G2012T) gene polymorphisms were identified by polymerase chain reaction-restriction (PCR)-RFLP and mRNA expression was measured in peripheral blood mononuclear cells by singleplex real-time PCR. ABCB1 polymorphisms were associated with risk for coronary artery disease (CAD) (p<0.05). After atorvastatin treatment, both ABCB1 and ABCC1 genes showed 50% reduction of the mRNA expression (p<0.05). Reduction of ABCB1 expression was associated with ABCB1 G2677T/A polymorphism (p=0.039). Basal ABCB1 mRNA in the lower quartile (<0.024) was associated with lower reduction rate of serum low-density lipoprotein (LDL) cholesterol (33.4+/-12.4%) and apolipoprotein B (apoB) (17.0+/-31.3%) when compared with the higher quartile (>0.085: LDL-c=40.3+/-14.3%; apoB=32.5+/-10.7%; p<0.05). ABCB1 substrates or inhibitors did not affect the baseline expression, while ABCB1 inhibitors reversed the effects of atorvastatin on both ABCB1 and ABCC1 transporters. In conclusion, ABCB1 and ABCC1 mRNA levels in PBMC are modulated by atorvastatin and ABCB1 G2677T/A polymorphism and ABCB1 baseline expression is related to differences in serum LDL cholesterol and apoB in response to atorvastatin.

Sulfinosine enhances doxorubicin efficacy through synergism and by reversing multidrug resistance in the human non-small cell lung carcinoma cell line (NCI-H460/R)

A resistant non-small cell lung carcinoma cell line-NSCLC (NCI-H460/R) was established in order to investigate the potential of sulfinosine (SF) to overcome multidrug resistance (MDR). The cytotoxicity of doxorubicin (DOX) in NCI-H460/R cells was enhanced by interaction with SF. SF improved the sensitivity of resistant cells to DOX when NCI-H460/R cells were pretreated with SF. Synergism was accompanied by the accumulation of cells in S and G(2)/M phases. Pretreatment with SF was more potent in improving the sensitivity to DOX than verapamil (VER). The decrease of mdr1 and topo II alpha expression (assessed by RT-PCR), was consistent with the DOX accumulation assay and cell cycle analysis. Also, SF significantly decreased intracellular glutathione (GSH) concentration. These results point to SF as a potential agent of MDR reversal and a valuable drug for improving chemotherapy of NSCLC.

Positive regulation of promoter activity of human 3-phosphoglycerate dehydrogenase (PHGDH) gene is mediated by transcription factors Sp1 and NF-Y

The PHGDH gene encodes the 3-phosphoglycerate dehydrogenase that catalyzes the transition of 3-phosphoglycerate into 3-phosphohydroxy pyruvate for the phosphorylated pathway of serine biosynthesis. To understand transcriptional regulation of the human PHGDH promoter, a genomic clone containing the 5'-flanking region of the PHGDH gene was isolated from a human genomic library. The 1192-bp PHGDH promoter region was cloned by PCR using the genomic DNA isolated from the PHGDH genomic clone. Sequence analysis of the promoter region exhibited several putative transcription factor binding sites for NF-Y, Sp1, GATA-1, p53, AP2, and AP1, with no TATA-box motif at an appropriate position. Transfection of a series of deletion constructs of the promoter region into HeLa cells revealed that the core positive promoter activity resided in the -276 to +1, which contains two GC-motifs for binding Sp1 and one CCAAT-motif for NF-Y. Mutational analysis and electrophoretic mobility shift assay indicated that both the proximal GC-motif and CCAAT-motif were crucial for full induction of the promoter activity. Chromatin immunoprecipitation analysis confirmed the recruitment of Sp1 and NF-Y to the promoter region in vivo. These results demonstrated that the promoter activity of the human PHGDH gene was positively regulated by the action of transcription factors Sp1 and NF-Y.

Curcumin as chemosensitizer

This overview presents curcumin as a significant chemosensitizer in cancer chemotherapy. Although the review focuses on curcumin and its analogues on multidrug resistance (MDR) reversal, the relevance of curcumin as a nuclear factor (NF)-KB blocker and sensitizer of many chemoresistant cancer cell lines to chemotherapeutic agents will also be discussed. One of the major mechanisms of MDR is the enhanced ability of tumor cells to actively efflux drugs, leading to a decrease in cellular drug accumulation below toxic levels. Active drug efflux is mediated by several members of the ATP-binding cassette (ABC) superfamily of membrane transporters, which have now been subdivided into seven families designated A through G. Among these ABC families, the classical MDR is attributed to the elevated expression of ABCB1 (Pgp), ABCC1 (MRP1), and ABCG2 (MXR). The clinical importance of Pgp, MRP1, and MXR for MDR and cancer treatment has led to the investigation of the inhibiting properties of several compounds on these transporters. At present, due in part to the disappointing results associated with the many side effects of synthetic modulators that have been used in clinical trials, current research efforts are directed toward the identification of novel compounds, with attention to dietary natural products. The advantage is that they exhibit little or virtually no side effects and do not further increase the patient's medication burden.

Construction of double suicide genes system controlled by MDR1 promoter with targeted expression in drug-resistant glioma cells

The multiple drug resistance protein (MDR1) is frequently overexpressed in human glioma. The aim of this study is to clone the MDR1 promoter from C6/ADR, construct the double suicide genes expressive vector controlled by MDR1 promoter, and explore its targeted expression in C6/ADR cells. MDR1 promoter from C6/ADR genomic DNA, which was linked with T vector, was amplified by using Polymerase chain reaction (PCR). After cut by NdeI and HindIII, MDR1 promoter was cloned into pcDNA3-TK (thymidine kinase) plasmid. The cytosine deaminase (CD) gene from pcDNA3-CD-TK plasmid was directly cloned into the above vector to construct pcDNA3-MDR1-promoter-CD-TK vector. Then this vector was transfected into C6 and C6/ADR cells respectively by liposome. After selection by G418, the tumor cell lines were stably established. Then these cell lines were examined through PCR and RT-PCR to respectively detect the integration and expression of TK and CD genes. The results showed the length and sequence of MDR1 promoter amplified by PCR were confirmed by DNA sequencing. The pcDNA3-MDR1-promoter-CD-TK expression vectors were constructed successfully. PCR indicated the double suicide genes were integrated into C6 and C6/ADR cells. RT-PCR revealed that CD and TK genes expressed in C6/ADR/CD-TK cells, whereas not in C6/CD-TK cells. In conclusions, construction of expressive vector containing double suicide genes controlled by MDR1 promoter with targeted expression in C6/ADR will provide a sound basis for targeted gene therapy for multidrug resistance (MDR) glioma.

Bioavailability and tissular distribution of docetaxel, a P-glycoprotein substrate, are modified by interferon-alpha in rats

Interferon-alpha (IFN-alpha) inhibits intestinal P-glycoprotein (P-gp) expression in rats. In the present study, the effects of repeated pre-treatment with recombinant human INF-alpha (rhIFN-alpha) on oral and intravenous pharmacokinetics of a P-gp substrate, docetaxel (DTX; Taxotere) were investigated in a rat model. The bioavailability and distribution in different organs were also studied. Sprague-Dawley rats were subcutaneously pre-treated with either rhIFN-alpha for 8 days (4MIU kg(-1), once daily) or with pegylated-IFN-alpha (ViraferonPeg; 60 microg kg(-1), Days 1, 4 and 7). The rats were then distributed into sub-groups (n = 5-6) according to the pre-treatment type, and received one dose of [(14)C]DTX (20 mgkg(-1)) either orally or intravenously. Pharmacokinetics studies were then performed over 240 min, at the end of which tissues (intestine, liver, kidneys, lung, heart and brain) were immediately removed for radioactivity quantitation. Non-pegylated and pegylated IFN-alpha both increased DTX oral bioavailability parameters: C(max) (17.0+/-4.0 microg L(-1) (P < 0.02) and 18+/-5.5 microg L(-1) (P < 0.05), respectively, vs 7.4+/-2.5 microg L(-1) for the control) and AUC (0.036+/-0.010 microg h mL(-1) (P < 0.01) and 0.033+/-0.009 microg h mL(-1) (P < 0.01), respectively, versus 0.012+/-0.004 microg h mL(-1) for the control). IFN-alpha also delayed DTX absorption from 60 min in controls to about 95 min and 80 min in non-pegylated and pegylated treated animals, respectively. However, IFN-alpha did not affect intravenous DTX pharmacokinetics and it had a limited effect on tissue distribution at 240 min. [(14)C]DTX was decreased in intestine and enhanced in brain in both pre-treated groups. rhIFN-alpha modified the P-gp-dependent pharmacokinetics of DTX, limited its intestinal efflux and markedly enhanced its oral bioavailability.

Design of a hairpin polyamide, ZT65B, for targeting the inverted CCAAT box (ICB) site in the multidrug resistant (MDR1) gene

A novel hairpin polyamide, ZT65B, containing a 3-methylpicolinate moiety was designed to target the inverted CCAAT box (ICB) of the human multidrug resistance 1 gene (MDR1) promoter. Binding of nuclear factor-Y (NF-Y) to the ICB site upregulates MDR1 gene expression and is, therefore, a good target for anticancer therapeutic agents. However, it is important to distinguish amongst different promoter ICB sites so that only specific genes will be affected. All ICB sites have the same sequence but they differ in the sequence of the flanking base pairs, which can be exploited in the design of sequence-specific polyamides. To test this hypothesis, ten ICB-containing DNA hairpins were designed with different flanking base pairs; the sequences ICBa and ICBb were similar to the 3'-ICB site of MDR1 (TGGCT). Thermal-denaturation studies showed that ZT65B effectively targeted ICBa and ICBb (DeltaTM=6.5 and 7.0 degrees C) in preference to the other DNA hairpins (<3.5 degrees C), with the exception of ICBc (5.0 degrees C). DNase I-footprinting assays were carried out with the topoisomerase IIalpha-promoter sequence, which contains five ICB sites; of these, ICB1 and ICB5 are similar to the ICB site of MDR1. ZT65B was found to selectively bind ICB1 and ICB5; footprints were not observed with ICB2, ICB3, or ICB4. A strong, positive induced ligand band at 325 nm in CD studies confirmed that ZT65B binds in the DNA minor groove. The selectivity of ZT65B binding to hairpins that contained the MDR1 ICB site compared to one that did not (ICBd) was confirmed by surface-plasmon studies, and equilibrium constants of 5x10(6)-1x10(7) and 4.6x10(5) M-1 were obtained with ICB1, ICB5,and ICB2 respectively. ZT65B and the previously published JH37 (J. A. Henry, et al. Biochemistry 2004, 43, 12 249-12 257) serve as prototypes for the design of novel polyamides. These can be used to specifically target the subset of ubiquitous gene elements known as ICBs, and thereby affect the expression of one or a few proteins.

Action of transcription factors in the control of transferrin receptor expression in human brain endothelium

Brain endothelium has a distinctive phenotype, including high expression of transferrin receptor, p-glycoprotein, claudin-5 and occludin. Dermal endothelium expresses lower levels of the transferrin receptor and it is absent from lung endothelium. All three endothelia were screened for transcription factors that bind the transferrin receptor promoter and show different patterns of binding between the endothelia. The transcription factor YY1 has distinct DNA-binding activities in brain endothelium and non-brain endothelium. The target-sites on the transferrin receptor promotor for YY1 lie in close proximity to those of the transcription initiation complex containing TFIID, so the two transcription factors potentially compete or interfere. Notably, the DNA-binding activity of TFIID was the converse of YY1, in different endothelia. YY1 knockdown reduced transferrin receptor expression in brain endothelium, but not in dermal endothelium, implying that YY1 is involved in tissue-specific regulation of the transferrin receptor. Moreover a distinct YY1 variant is present in brain endothelium and it associates with Sp3. A model is presented, in which expression from the transferrin receptor gene in endothelium requires the activity of both TFIID and Sp3, but whether the gene is transcribed in different endothelia, is related to the balance between activating and suppressive forms of YY1.

MDR1 genotype-related pharmacokinetics: fact or fiction?

Multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. A number of various types of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics. The first investigation of the effects of MDR1 genotypes on pharmacotherapy was reported in 2000; a silent single nucleotide polymorphism (SNP), C3435T in exon 26, was found to be associated with the duodenal expression of MDR1, and thereby the plasma concentration of digoxin after oral administration. In the last 5 years, clinical studies have been conducted around the world on the association of MDR1 genotype with MDR1 expression and function in tissues, and with the pharmacokinetics and pharmacodynamics of drugs; however, there are still discrepancies in the results on C3435T. In 1995, a novel concept to predict in vivo oral pharmacokinetic performance from data on in vivo permeability and in vitro solubility has been proposed, and this Biopharmaceutical Classification System strongly suggested that the effects of intestinal MDR1 on the intestinal absorption of substrates is minimal in the case of commercially available oral drugs, and therefore MDR1 genotypes are little associated with the pharmacokinetics after oral administration. This review summarizes the latest reports for the future individualization of pharmacotherapy based on MDR1 genotyping, and attempts to explain discrepancies.

Up-regulation of MDR1 and induction of doxorubicin resistance by histone deacetylase inhibitor depsipeptide (FK228) and ATRA in acute promyelocytic leukemia cells

The multidrug resistance 1 (MDR1) gene product P-glycoprotein (P-gp) is frequently implicated in cross-resistance of tumors to chemotherapeutic drugs. In contrast, acute promyelocytic leukemia (APL) cells do not express MDR1 and are highly sensitive to anthracyclines. The combination of ATRA and the novel histone deacetylase inhibitor (HDACI) depsipeptide (FK228) induced P-gp expression and prevented growth inhibition and apoptosis in NB4 APL cells subsequently exposed to doxorubicin (DOX). ATRA/FK228 treatment after exposure to DOX, however, enhanced apoptosis. Both agents, ATRA or FK228, induced MDR1 mRNA. This effect was significantly enhanced by ATRA/FK228 administered in combination, due in part to increased H4 and H3-Lys9 acetylation of the MDR1 promoter and recruitment of the nuclear transcription factor Y alpha (NFYA) transcription activator to the CCAAT box. Cotreatment with specific P-gp inhibitor PSC833 reversed cytoprotective effects of ATRA/FK228. G1 cell-cycle arrest and p21 mRNA induction were also observed in response to ATRA/FK228, which may restrict DOX-induced apoptosis of cells in G2 phase. These results indicate that epigenetic mechanisms involving NF-YA transcription factor recruitment and histone acetylation are activated by ATRA and HDACI, induce MDR1 in APL cells, and point to the critical importance of mechanism-based sequential therapy in future clinical trials that combine HDAC inhibitors, ATRA, and anthracyclines.

The role of nuclear Y-box binding protein 1 as a global marker in drug resistance

Gene expression can be regulated by nuclear factors at the transcriptional level. Many such factors regulate MDR1 gene expression, but what are the sequence elements and transcription factors that control the basal and inducible expression of this gene? The general principles through which transcription factors participate in drug resistance are now beginning to be understood. Here, we review the factors involved in the transcriptional regulation of the MDR1 gene. In particular, we focus on the transcription factor Y-box binding protein 1 and discuss the possible links between Y-box binding protein 1 expression and drug resistance in cancer, which are mediated by the transmembrane P-glycoprotein or non–P-glycoprotein.

Haplotype-Oriented Genetic Analysis and Functional Assessment of Promoter Variants in theMDR1(ABCB1) Gene

Recently, a number of nucleotide variants have been described in the multidrug resistance 1 (MDR1/ABCB1) gene; however, most studies have focused on the coding region. In the present study, we identified promoter variants of the MDR1 gene and evaluated their phenotypic consequences using a reporter gene assay and the real-time polymerase chain reaction method. Ten allelic variants were detected in the promoter region (approximately 2 kilobases), seven of which were newly identified. Certain mutations occurred simultaneously, and a total of 10 haplotypes were observed. These promoter polymorphisms were found more frequently in Japanese than Caucasians. Some haplotypes were associated with changes in luciferase activity and placental and hepatic mRNA levels. We also determined DNA methylation status in the proximal promoter region of the MDR1 gene. The promoter region around potential binding sites for transcription factors was found to be hypomethylated and thus likely to be independent of the gene expression. Nucleotide and/or haplotype variants not only in the coding region but also in the promoter region of the MDR1 gene may be important for interindividual differences of P-glycoprotein expression.

CCAAT/Enhancer-Binding Protein β (Nuclear Factor for Interleukin 6) Transactivates the HumanMDR1 Gene by Interaction with an Inverted CCAAT Box in Human Cancer Cells

We investigated the mechanisms of MDR1 gene activation by CCAAT/enhancer binding protein beta (C/EBPbeta, or nuclear factor for interleukin 6) in human cancer cells. Transfection of the breast cancer cell line MCF-7 and its doxorubicin-selected variant MCF-7/ADR by either C/EBPbeta or C/EBPbeta-LIP (a dominant-negative form of C/EBPbeta) confirmed their roles in the activation or repression of the endogenous, chromosomally embedded MDR1 gene. Cotransfection experiments with promoter constructs revealed a C/EBPbeta interaction on the MDR1 promoter via the region within -128 to -75. Deletions within the putative AP-1 box (-123 to -111) increased MDR1 promoter activity when stimulated by C/EBPbeta, suggesting that the AP-1 site negatively regulates MDR1 activation by C/EBPbeta. Mutations within the inverted CCAAT box (Y box) (-82 to -73) abolished the C/EBPbeta-stimulated MDR1 promoter activity, indicating that the Y box is required for MDR1 activation by C/EBPbeta. Chromatin immunoprecipitation (ChIP) revealed that C/EBPbeta precipitates a transcription complex containing C/EBPbeta, the MDR1 promoter sequences (-250 to +54), and the hBrm protein. In conclusion, alteration of expression or function of C/EBPbeta plays an important role in MDR1 gene regulation. C/EBPbeta activates the endogenous MDR1 gene of MCF-7 cells, and this activation was associated with a novel C/EBPbeta interaction region within the proximal MDR1 promoter (-128 to -75). The mechanisms of MDR1 activation by C/EBPbeta include C/EBPbeta binding of the chromatin of the MDR1 gene and interactions of C/EBPbeta with the Y box and Y box-associated proteins.

Spontaneous Reversal of P-Glycoprotein Expression in Multidrug Resistant Cell Lines*

Increased expression of P-glycoprotein encoded by the mdr-1 gene is a well-characterised mechanism for resistance to cancer chemotherapeutic drugs in cell lines. However, the P-glycoprotein expression after removal of the selection pressure has not fully been elucidated. The stability of P-glycoprotein expression in the presence (+) and absence (-) of vincristine (30 or 150 nM) was studied in multidrug resistant K562 cell lines (VCR30+, VCR150+, VCR30- and VCR150-) for 11 months. The P-glycoprotein protein and mdr-1 mRNA levels were determined at regular intervals using flow cytometry and real-time PCR, respectively. Chemosensitivity to a panel of antineoplastic drugs was measured using an MTT assay. The presence of vincristine (VCR30+ and VCR150+) resulted in high and stable levels of P-glycoprotein and mdr-1 mRNA during the whole period compared to wild type. As for the VCR30- and VCR150- subcultures, the expressions of P-glycoprotein and mdr-1 mRNA were stable for five months, and then the levels decreased rapidly. Concomitantly, the sensitivity to drugs known as P-glycoprotein substrates was restored. In conclusion, resistant cells growing in the presence of the inducing drug have a stable P-glycoprotein expression and resistance level, but removing the inducing drug may result in a sudden and rapid lowering of P-glycoprotein and mdr-1 mRNA levels as long as five months after drug withdrawal.

Transcriptional regulation of ABC drug transporters

P-glycoprotein, the founding member of the ATP-binding cassette (ABC) family of drug transporters, was first identified almost three decades ago and shown to confer resistance to multiple chemotherapeutic agents when overexpressed in human tumors. Subsequent years have witnessed a tremendous effort to characterize the function and regulation of P-glycoprotein, initially spurred by the hope that its inhibition was the key to overcoming clinical resistance to multiple anticancer agents. However, the identification of MRP1, another member of the ABC drug transporter family, led to the realization that the multidrug resistance (MDR) phenotype is considerably more complex than initially believed. Indeed, at the present time at least 10 members of the ABC transporter family have been implicated in an MDR phenotype, and it is likely that more will be added to this list as studies progress. With this complexity comes the imperative to improve our understanding of the function of individual transporters, as well as to delineate the mechanisms underlying their expression in normal and tumor cells, particularly those that may be amenable to therapeutic intervention. Several articles within this volume address the structure and function of drug transporters. This review will focus on our current understanding of the regulation of ABC drug transporters at the level of transcription.

Constitutive rat multidrug-resistance protein 2 gene transcription is down-regulated by Y-box protein 1

BACKGROUND/AIMS

Molecular mechanisms underlying transcriptional rat multidrug-resistance protein 2 (Mrp2, Abcc2) gene regulation are mostly unclear. Given the presence of putative binding sites for the Y-box binding protein YB-1 in the regulatory sequence, its trans-regulatory influence was analyzed.

METHODS

Reporter assays in HepG2 cells with various Mrp2 deletion constructs in the absence and presence of co-transfected YB-1 were performed. DNA binding studies with recombinant YB-1 protein and nuclear extracts obtained from HepG2 cells and rat liver tissue were carried out.

RESULTS

The minimal promoter sequence was confined to the proximal 186 bp. A YB-1 responsive element, Mrp2 YRE-1, was mapped at -186/-157, which exhibits specific YB-1 binding. YB-1 acts as a potent repressor of Mrp2 promoter activity in vitro.

CONCLUSIONS

Constitutive Mrp2 gene expression is conferred through the proximal -186 bp. YB-1 acts as a repressor in vitro by specific binding to a defined element in the proximal promoter sequence.

Transactivation of human cdc2 promoter by adenovirus E1A

Expression of the adenovirus oncoprotein E1A 12S induces the heterotrimeric transcription factor, NF-Y. NF-Y binds to the two CCAAT motifs upstream of the transcriptional start site of the human cdc2 promoter and is required for activation of the promoter by E1A 12S in cycling cells. The observations that a number of eukaryotic cell cycle regulatory genes also contain the CCAAT motifs and NF-Y binds to them support the notion that E1A 12S could play an important role in deregulated expression of these genes through activation of NF-Y gene in cycling cells.

Assessment by molecular dynamics simulations of the structural determinants of DNA-binding specificity for transcription factor Sp1

The DNA-binding domain (DBD) of the ubiquituous transcription factor Sp1 consists of three consecutive zinc fingers that recognize a number of nucleotide sequences different from, but related to and sometimes overlapping, those recognized by the structurally better characterized early growth response protein 1 (EGR1, also known as Zif268, Krox-24, and NGFI-A). The accepted consensus binding sequence for Sp1 is usually defined by the asymmetric hexanucleotide core GGGCGG but this sequence does not include, among others, the GAG (=CTC) repeat that constitutes a high-affinity site for Sp1 binding to the wt1 promoter. Since no 3D structure of the whole DBD of Sp1 is available, either alone or in complex with DNA, a homology-based model was built and its interaction with two DNA 14-mers was studied using nanosecond molecular dynamics simulations in the presence of explicit water molecules. These oligonucleotides represent Sp1 target sites that are present in the promoters of the mdr1 and wt1 genes. For comparative purposes and validation of the protocol, the complex between the DBD of EGR1 and its DNA target site within the proximal mdr1 promoter was simulated under the same conditions. Some water molecules were seen to play an important role in recognition and stabilization of the protein-DNA complexes. Our results, which are supported by the available experimental evidence, suggest that the accuracy in the prediction of putative Sp1-binding sites can be improved by interpreting a set of rules, which are a blend of both stringency and tolerance, for the juxtaposed triplet subsites to which each zinc finger binds. Our approach can be extrapolated to WT1 and other related natural or artificial zinc-finger-containing DNA-binding proteins and may aid in the assignment of particular DNA stretches as allowed or disallowed-binding sites.

Identification of YB-1 as a regulator of PTP1B expression: implications for regulation of insulin and cytokine signaling

Changes in expression of PTP1B, the prototypic protein tyrosine phosphatase, have been associated with various human diseases; however, the mechanisms by which PTP1B expression is regulated have not been defined. We have identified an enhancer sequence within the PTP1B promoter which serves as a binding site for the transcription factor Y box-binding protein-1 (YB-1). Overexpression of YB-1 resulted in increased levels of PTP1B. Furthermore, depletion of YB-1 protein, by expression of a specific antisense construct, led to an approximately 70% decrease in expression of PTP1B, but no change in the level of its closest relative, TC-PTP. Expression of antisense YB-1 resulted in increased sensitivity to insulin and enhanced signaling through the cytokine receptor gp130, which was suppressed by re-expression of PTP1B. Finally, we observed a correlation between the expression of PTP1B and that of YB-1 in cancer cell lines and an animal model of type II diabetes. Our data reveal an important role for YB-1 as a regulator of PTP1B expression, and further highlight PTP1B as a critical regulator of insulin- and cytokine-mediated signal transduction.

NF-kappaB transcription factor induces drug resistance through MDR1 expression in cancer cells

The ubiquitous NF-kappaB transcription factor has been reported to inhibit apoptosis and to induce drug resistance in cancer cells. Drug resistance is the major reason for cancer therapy failure and neoplastic cells often develop multiple mechanisms of drug resistance during tumor progression. We observed that NF-kappaB or P-glycoprotein inhibition in the HCT15 colon cancer cells led to increased apoptotic cell death in response to daunomycin treatment. Interestingly, NF-kappaB inhibition through transfection of a plasmid coding for a mutated IkappaB-alpha inhibitor increased daunomycin cell uptake. Indeed, the inhibition of NF-kappaB reduced mdr1 mRNA and P-glycoprotein expression in HCT15 cells. We identified a consensus NF-kappaB binding site in the first intron of the human mdr1 gene and demonstrated that NF-kappaB complexes could bind with this intronic site. Moreover, NF-kappaB transactivates an mdr1 promoter luciferase construct. Our data thus demonstrate a role for NF-kappaB in the regulation of the mdr1 gene expression in cancer cells and in drug resistance.

MDR1 genotype-related pharmacokinetics and pharmacodynamics

The multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. MDR1 acts as an energy-dependent efflux pump that exports its substrates out of cells. MDR1 was originally isolated from resistant tumor cells as part of the mechanism of multidrug resistance, but over the last decade, it has been elucidated that human MDR1 is also expressed throughout the body to confer intrinsic resistance to the tissues by exporting unnecessary or toxic exogeneous substances or metabolites. A number of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics and pharmacodynamics. In 2000, Hoffmeyer et al. performed a systemic screening for MDR1 polymorphisms and detected 15 single nucleotide polymorphisms (SNPs). They also indicated that a polymorphism in exon 26 at position 3435 (C3435T), a silent mutation, affected the expression level of MDR1 protein in duodenum, and thereby the intestinal absorption of digoxin. To date, the genotype frequencies of C3435T have been investigated extensively using a larger population and interethnic difference has been elucidated, and a total of 28 SNPs have been found at 27 positions on the MDR1 gene. Clinical studies on MDR1 genotype-related MDR1 expression and pharmacokinetics have also been performed around the world; however, results were not always consistent with Hoffmeyer's report. In this review, published reports are summarized for the future individualization of pharmacotherapy based on MDR1 genotyping. In addition, recent investigations have raised the possibility that MDR1 and related transporters play a fundamental role in regulating apoptosis and immunology, and in fact, there are reports of MDR1-related susceptibility to inflammatory bowel disease, HIV infection and renal cell carcinoma. Herein, these issues are also summarized, and the current status of the knowledge in the area of pharmacogenomics of other transporters is briefly introduced.

Transcriptional regulators of the human multidrug resistance 1 gene: recent views

The multidrug resistance (MDR) phenotype is the major cause of failure of cancer chemotherapy. This phenotype is mainly due to the overexpression of the human MDR1 (hMDR1) gene. Several studies have shown that transcriptional regulation of this gene is unexpectedly complex and is far from being completely understood. Current work is aimed mainly at defining unclear and new control regions in the hMDR1 gene promoter as well as clarifying corresponding signaling pathways. Such studies provide new insights into the mechanisms by which xenobiotic molecules might modify the physiological hMDR1 expression as well as the possible role of oncogenes in the pathological dysregulation of the gene. Here we report recent findings on the regulation of hMDR1 which may help define specific targets aimed at modulating its transcription.

Sp1 is a key regulator of the PDGF beta-receptor transcription

The mouse PDGF beta-receptor promoter is tightly controlled by NF-Y that binds to a CCAAT box located upstream of the initiation site [1, 2]. In this report, we show that Sp1 plays an essential role in the PDGF beta-receptor transcription. Within the upstream GC rich area there are two Sp1 binding sites located in close proximity to the CCAAT box. Deletion of the GC rich region resulted in a 50% decrease of the transcriptional activity of the promoter, and a complete loss of its responsiveness to over-expression of Sp1. There was an additive effect between NF-Y and Sp I in reporter activity when they were co-transfected together with the promoter-reporter construct. Furthermore, transfection of NF-Y failed to enhance transcriptional activity when the Sp1 binding sites were deleted from the promoter, suggesting an important role for Sp1 in this NF-Y controlled transcription. We have recently reported that c-Myc represses PDGF beta-receptor transcription through its interference with the transactivation activity of NF-Y [3]. In the case of p21(wafl/cip1) transcription, c-Myc was shown to repress its transcription by sequestering Sp1 [4]. However, we could not find any effect of Sp1 in the c-Myc-mediated repression on the PFDGF beta-receptor promoter, since the deletion of SpI binding sites could not attenuate the repression by c-Myc on the promoter activity.

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.

Hyperthermia-induced nuclear translocation of transcription factor YB-1 leads to enhanced expression of multidrug resistance-related ABC transporters

Genotoxic stress leads to nuclear translocation of the Y-box transcription factor YB-1 and enhanced expression of the multidrug resistance gene MDR1. Because hyperthermia is used for the treatment of colon cancer in combination with chemoradiotherapy, we investigated the influence of hyperthermia on YB-1 activity and the expression of multidrug resistance-related genes. Here we report that hyperthermia causes YB-1 translocation from the cytoplasm into the nucleus of human colon carcinoma cells HCT15 and HCT116. Nuclear translocation of YB-1 was associated with increased MDR1 and MRP1 gene activity, which is reflected in strong efflux pump activity. However, a combination of hyperthermia and drug treatment effectively reduced cell survival of the HCT15 and HCT116 cells. These results demonstrate that activation of MDR1 and MRP1 gene expression and increased efflux pump activity after hyperthermia were insufficient to cause an increase in drug resistance in colon cancer cell lines. The ability of hyperthermia to abrogate drug resistance in the presence of an increase in functional MDR proteins may provide an explanation for the efficacious results seen in the clinic in colon cancer patients treated with a combination of hyperthermia and chemotherapy.

Transcriptional repression by p53 through direct binding to a novel DNA element

The tumor suppressor protein p53 has been well documented as a transcriptional activator involved in the regulation of a number of critical genes involved in the cell cycle, response to DNA damage, and apoptosis. Activation by p53 requires the interaction of the protein with a consensus binding site consisting of two half-sites, each comprising two copies of the sequence PuPuPuC(A/T) arranged head-to-head and separated by 0-13 base pairs. In addition to activation, p53 has been shown to be a potent repressor of transcription. However, the basis for p53-mediated repression is not well understood and has been proposed to occur indirectly through interactions with other promoter-bound transcription factors. In the present study, we show that p53 can repress transcription directly by binding to a novel head-to-tail (HT) site within the MDR1 promoter. A mutation that disrupted p53 binding to the MDR1 HT site blocked p53-mediated repression of the MDR1 promoter in transfection assays. Replacement of the HT site with a head-to-head (HH) site converted the activity of p53 from repression to activation, indicating that simple recruitment of p53 to the promoter is not sufficient for repression and that the orientation of the binding element determines the fate of p53-regulated promoters.

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.

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.

Isolation and characterization of the human A-myb promoter: regulation by NF-Y and Sp1

The A-myb transcription factor shows a restricted tissue distribution and is cell cycle regulated. Furthermore its deregulation has profound effects on the growth and/or differentiation of the cells in which it is normally expressed. We have therefore characterized its promoter. A 12 kb genomic clone was isolated that comprises the first exon, part of the first intron as well as upstream regulatory sequences. Multiple transcription start sites have been identified which operate in both B lymphocytes and epithelial cells and the upsteam region was shown to have promoter, activity. The boundaries of the minimal promoter region (-183-14), of a positive upstream (-538-183) and a negative downstream regulatory region (NRE) (+83+374) have been defined. The NRE is promoter- and orientation-independent but position specific. The A-myb minimal promoter is GC-rich, does not contain any TATA box but has a functional CCAAT box. The CCAAT box and minimal promoter is highly conserved in the corresponding murine sequence. The CCAAT box efficiently binds the NF-Y complex and its mutation decreases basal promoter activity by 50%. Two Sp1 binding sites are present upstream from the CCAAT box which can bind Spl and contribute to A-myb promoter activity by 70 and 30%, respectively. The two Sp1 sites and CCAAT box together contribute to over 80% of A-myb basal promoter activity and are therefore the major regulatory elements. Finally, we show that the promoter is cell cycle regulated and that the SP1 and CCAAT elements are required for S phase induction.

Regulation of MDR1 gene expression: emerging concepts

Drug resistance genes, such as MDR1, involved in drug efflux, and their regulation have been the subject of intense research efforts in the past 10 years. Many factors and cellular signalling pathways play a role in the regulation of MDR1 gene expression. Commonly used chemotherapeutic agents activate in vitro and in vivo general stress response pathways, potential targets of which include MDR1 and other drug resistance genes. The contribution of these agents to the emergence of drug-resistant tumour cells is of concern. Recent evidence points to a role for the epigenetic regulation of MDR1 gene expression. The identification of key components in the DNA methylation/chromatin system of gene regulation may in time lead to more informed and targeted approaches to treating drug-resistant tumours. Copyright 2000 Harcourt Publishers Ltd.

Transcriptional activation of the MDR1 gene by UV irradiation. Role of NF-Y and Sp1

The MDR1 promoter is subject to control by various internal and external stimuli. We have previously shown that the CCAAT box-binding protein, NF-Y, mediates MDR1 activation by the histone deacetylase inhibitors, trichostatin A and sodium butyrate, through the recruitment of the co-activator, P/CAF. We have now extended our investigation to the activation of MDR1 by genotoxic stress. We show that activation of the MDR1 promoter by UV irradiation is also dependent on the CCAAT box (-82 to -73) as well as on a proximal GC element (-56 to -42). Gel shift and supershift analyses with nuclear extracts prepared from human KB-3-1 cells identified NF-Y as the transcription factor interacting with the CCAAT box, while Sp1 was the predominant factor binding to the GC element. Mutations that abrogated binding of either of these factors reduced or abolished activation by ultraviolet irradiation; moreover, co-expression of a dominant-negative NF-Y protein (NF-YA29) reduced UV-activated transcription. Interestingly, YB-1, a transcription factor that also recognizes the CCAAT motif and had been reported to mediate induction of the MDR1 promoter by ultraviolet light, was incapable of interacting with the double-stranded MDR1 CCAAT box oligonucleotide in nuclear extracts, although it did interact with a single-stranded oligonucleotide. Furthermore, a mutation that abolished activation of MDR1 by UV-irradiation had no effect on YB-1 binding and co-transfection of a YB-1 expression plasmid had a repressive effect on UV-inducible transcription. Taken together, these results indicate a role for both NF-Y and Sp1 in the transcriptional activation of the MDR1 gene by genotoxic stress, and indicate that YB-1, if involved, is not sufficient to mediate this activation.