Selective inhibition of P-glycoprotein expression in multidrug-resistant tumor cells by a designed transcriptional regulator

Overexpression of P-glycoprotein and resistance to Imatinib in chronic myeloid leukemia patients

BACKGROUND

The P-glycoprotein (P-gp) is one of the mechanisms of Imatinib (IM) resistance in chronic myeloid leukemia (CML). P-gp has been identified as an efflux pump involved in releasing of IM outside CML cells. To date, the P-gp involvement in the IM resistance development was not completely understood. Therefore, the present study aimed at measuring the P-gp expression level on lymphocytes from Tunisian patients with CML and correlating this level with a molecular response to IM.

METHOD

The expression of P-gp on peripheral blood lymphocytes from 59 Tunisian patients with CML (27 IM responder patients vs 32 IM non-responder patients) was evaluated by flow cytometry.

RESULT

Our finding showed significantly positive expression of P-gp in the lymphocytes from the IM non-responder group when compared to the IM-responder group (P = .001). In IM non-responder CML patients, the comparison between CCyR achievers and non-achievers showed a high mean fluorescence intensity (MFI) of P-gp expression in patients who did not achieve their CCyR (P = .001). The comparison between patients with primary and secondary resistance to IM showed an increasing MFI value in patients with primary resistance to IM (P = .001). Besides, the comparison between nilotinib-treated and dasatinib-treated patients proved a high value of MFI in nilotinib-treated patients (P = .001).

CONCLUSION

The overexpression of P-gp on lymphocytes has significantly correlated with the failed molecular response to IM in patients with CML.

Mechanisms of Drug Resistance in Veterinary Oncology- A Review with an Emphasis on Canine Lymphoma

Drug resistance (DR) is the major limiting factor in the successful treatment of systemic neoplasia with cytotoxic chemotherapy. DR can be either intrinsic or acquired, and although the development and clinical implications are different, the underlying mechanisms are likely to be similar. Most causes for DR are pharmacodynamic in nature, result from adaptations within the tumor cell and include reduced drug uptake, increased drug efflux, changes in drug metabolism or drug target, increased capacity to repair drug-induced DNA damage or increased resistance to apoptosis. The role of active drug efflux transporters, and those of the ABC-transporter family in particular, have been studied extensively in human oncology and to a lesser extent in veterinary medicine. Methods reported to assess ABC-transporter status include detection of the actual protein (Western blot, immunohistochemistry), mRNA or ABC-transporter function. The three major ABC-transporters associated with DR in human oncology are ABCB1 or P-gp, ABCC1 or MRP1, and ABCG2 or BCRP, and have been demonstrated in canine cell lines, healthy dogs and dogs with cancer. Although this supports a causative role for these ABC-transporters in DR cytotoxic agents in the dog, the relative contribution to the clinical phenotype of DR in canine cancer remains an area of debate and requires further prospective studies.

Reversal of adriamycin resistance in human mammary cancer cells by small interfering RNA of MDR1 and MDR3 genes

The purpose of this paper is to investigate the reversal effect of small interfering RNA (siRNA) targeting MDR1 and MDR3 genes on the resistance of MCF-7/ADR cells to adriamycin. siRNA plasmid vector targeting MDR1 and MDR3 genes was transfected into MCF-7/ADR cells, and then was stained with Annexin-V FITC (fluorescein isothiocyanate conjugated) to detect the early stage cell apoptosis by flow cytometry (FCM). 50% inhibition concentration (IC50) of adriamycin for MCF-7/ADR cells was determined by MTT method. MDR1 and MDR3 mRNA was assessed by RT-PCR. Treatment of MCF-7/ADR cells with the two kinds of siRNAs resulted in a reversal of adriamycin resistance of MDR to different extents. 1) The apoptosis efficiency of MDR1 and MDR3 siRNA vector after transfection was (18.21+/-1.65) % and (9.07+/-2.16) % respectively (P<0.05), and there was significant differences in the apoptosis efficiency between p Suppressor Neo vector and the MDR1 siRNA or MDR3 siRNA vector (P<0.01); 2) The reversal effect of MDR1 siRNA is higher than that of MDR3 siRNA (P<0.05); 3) The expression of MDRI and MDR3 mRNA can be restrained by p Suppressor Neo MDR1 and MDR3 siRNA respectively, and the reduction in the mRNA level was in a time-dependent manner (P<0.01). MDR1 and MDR3 gene silencing can enhance intracellular adriamycin accumulation in MCF-7/ADR cells, improve sensitivity of MCF-7/ADR cells to adriamycin, and induce cell apoptosis. The reversal effect of adriamycin resistance by siRNA of MDR1 was more effective than that of MDR3.

Drugging the undruggable: transcription therapy for cancer

Transcriptional regulation is often the convergence point of oncogenic signaling. It is not surprising, therefore, that aberrant gene expression is a hallmark of cancer. Transformed cells often develop a dependency on such a reprogramming highlighting the therapeutic potential of rectifying cancer-associated transcriptional abnormalities in malignant cells. Although transcription is traditionally considered as undruggable, agents have been developed that target various levels of transcriptional regulation including DNA binding by transcription factors, protein-protein interactions, and epigenetic alterations. Some of these agents have been approved for clinical use or entered clinical trials. While artificial transcription factors have been developed that can theoretically modulate expression of any given gene, the emergence of reliable reporter assays greatly facilitates the search for transcription-targeted agents. This review provides a comprehensive overview of these developments, and discusses various strategies applicable for developing transcription-targeted therapeutic agents.

Increased expression of P-glycoprotein and doxorubicin chemoresistance of metastatic breast cancer is regulated by miR-298

MicroRNAs (miRNAs) are short, noncoding RNA molecules that regulate the expression of a number of genes involved in cancer; therefore, they offer great diagnostic and therapeutic targets. We have developed doxorubicin-resistant and -sensitive metastatic human breast cancer cell lines (MDA-MB-231) to study the chemoresistant mechanisms regulated by miRNAs. We found that doxorubicin localized exclusively to the cytoplasm and was unable to reach the nuclei of resistant tumor cells because of the increased nuclear expression of MDR1/P-glycoprotein (P-gp). An miRNA array between doxorubicin-sensitive and -resistant breast cancer cells showed that reduced expression of miR-298 in doxorubicin-resistant human breast cancer cells was associated with increased expression of P-gp. In a transient transfection experiment, miR-298 directly bound to the MDR1 3' untranslated region and regulated the expression of firefly luciferase reporter in a dose-dependent manner. Overexpression of miR-298 down-regulated P-gp expression, increasing nuclear accumulation of doxorubicin and cytotoxicity in doxorubicin-resistant breast cancer cells. Furthermore, down-regulation of miR-298 increased P-gp expression and induced doxorubicin resistance in sensitive breast cancer cells. In summary, these results suggest that miR-298 directly modulates P-gp expression and is associated with the chemoresistant mechanisms of metastatic human breast cancer. Therefore, miR-298 has diagnostic and therapeutic potential for predicting doxorubicin chemoresistance in human breast cancer.

Synthesis of a dual functional anti-MDR tumor agent PH II-7 with elucidations of anti-tumor effects and mechanisms

Multidrug resistance mediated by P-glycoprotein in cancer cells has been a major issue that cripples the efficacy of chemotherapy agents. Aimed for improved efficacy against resistant cancer cells, we designed and synthesized 25 oxindole derivatives based on indirubin by structure-activity relationship analysis. The most potent one was named PH II-7, which was effective against 18 cancer cell lines and 5 resistant cell lines in MTT assay. It also significantly inhibited the resistant xenograft tumor growth in mouse model. In cell cycle assay and apoptosis assay conducted with flow cytometry, PH II-7 induced S phase cell cycle arrest and apoptosis even in resistant cells. Consistently revealed by real-time PCR, it modulates the expression of genes related to the cell cycle and apoptosis in these cells, which may contributes to its efficacy against them. By side-chain modification and FITC-labeling of PH II-7, we were able to show with confocal microscopy that not only it was not pumped by P-glycoprotein, it also attenuated the efflux of Adriamycin by P-glycoprotein in MDR tumor cells. Real-time PCR and western blot analysis showed that PH II-7 down-regulated MDR1 gene via protein kinase C alpha (PKCA) pathway, with c-FOS and c-JUN as possible mediators. Taken together, PH II-7 is a dual-functional compound that features both the cytotoxicity against cancer cells and the inhibitory effect on P-gp mediated drug efflux.

The Study of Apoptosis Induced by Allicin in HT-9/HL-60 and its Transfection Cell

To detect the effect of reverse in transfected cells by allicin treated. Two different short hairpin RNAs were designed and constructed in a pSilencer 3.1-H1 neo plasmid. The shRNA recombinant plasmids were transfected into HT9 leukemia cells. Flow cytometry was used to select the stably transfected sells. The cell viability of the cells was assessed using MTT assay, flow cytometry was used to monitor cell cycle and apoptosis. Inverted fluorescence microscope and trasmission electron microscope were used to examine the cytomorphology and ultrastructural changes of cells. The results indicated that compared with HT9 cells, HT9-3.1-2 cells, HT9 cells transfected with shRNA recombinant plasmids, had lower cell viability and higher levels of apoptosis after treated with allicin. The characteristic features of apoptosis were more significantly. The results indicate that allicin can induce cell apoptosis of HT9 and HT9-3.1-2 cells, and after stable transfection with the shRNA vectors, the multidrug resistance in HT9 cells was partly reversed.

Modular system for the construction of zinc-finger libraries and proteins

Engineered zinc-finger transcription factors (ZF-TF) are powerful tools to modulate the expression of specific genes. Complex libraries of ZF-TF can be delivered into cells to scan the genome for genes responsible for a particular phenotype or to select the most effective ZF-TF to regulate an individual gene. In both cases, the construction of highly representative and unbiased libraries is critical. In this protocol, we describe a user-friendly ZF technology suitable for the creation of complex libraries and the construction of customized ZF-TFs. The new technology described here simplifies the building of ZF libraries, avoids PCR-introduced bias and ensures equal representation of every module. We also describe the construction of a customized ZF-TF that can be transferred to a number of expression vectors. This protocol can be completed in 9-11 d.

Reversing agents for ATP-binding cassette drug transporters

The multidrug resistance (MDR) phenotype exhibited by cancer cells is believed to be the major barriers to successful chemotherapy in cancer patients. The major form of MDR phenotype is contributed by a group of ATP-binding cassette (ABC) drug transporters which include P-glycoprotein, multidrug resistance-associated protein 1, and breast cancer resistance protein. There has been intense search for compounds which can act to reverse MDR phenotype in cultured cells, in animal models, and ultimately in patients. The ongoing search for MDR modulators, compounds that act directly on the ABC transporter proteins to block their activity, has led to three generations of drugs. Some of the third-generation MDR modulators have demonstrated encouraging results compared to earlier generation MDR modulators in clinical trials. These modulators are less toxic and they do not affect the pharmacokinetics of anti-cancer drugs. Significant numbers of natural products have also been identified for their effectiveness in reversing MDR in a manner similar to the MDR modulators. Other MDR reversing strategies that have been studied quite extensively are also reviewed and discussed in this chapter. These include strategies aimed at destroying mRNAs for ABC drug transporters, approaches in inhibiting transcription of ABC transporter genes, and blocking of ABC transporter activity using antibodies. This review summarizes the development of reversing agents for ABC drug transporters up to the end of 2008, and provides an optimistic view of what we have achieved and where we could go from here.

The use of nanoparticle-mediated targeted gene silencing and drug delivery to overcome tumor drug resistance

Overexpression of drug efflux transporters such as P-glycoprotein (P-gp) enables cancer cells to develop resistance to multiple anticancer drugs. Functional inhibitors of P-gp have shown promising efficacy in early clinical trials, but their long-term safety is yet to be established. A novel approach to overcome drug resistance is to use siRNA-mediated RNA interference to silence the expression of the efflux transporter. Because P-gp plays an important role in the physiological regulation of endogenous and xenobiotic compounds in the body, it is important to deliver P-gp targeted siRNA and anticancer drug specifically to tumor cells. Further, for optimal synergy, both the drug and siRNA may need to be temporally colocalized in the tumor cells. In the current study, we investigated the effectiveness of simultaneous and targeted delivery of anticancer drug, paclitaxel, along with P-gp targeted siRNA, using poly(D,L-lactide-co-glycolide) nanoparticles to overcome tumor drug resistance. Nanoparticles were surface functionalized with biotin for active tumor targeting. Dual agent nanoparticles encapsulating the combination of paclitaxel and P-gp targeted siRNA showed significantly higher cytotoxicity in vitro than nanoparticles loaded with paclitaxel alone. Enhanced therapeutic efficacy of dual agent nanoparticles could be correlated with effective silencing of the MDR1 gene that encodes for P-gp and with increased accumulation of paclitaxel in drug-resistant tumor cells. In vivo studies in a mouse model of drug-resistant tumor demonstrated significantly greater inhibition of tumor growth following treatment with biotin-functionalized nanoparticles encapsulating both paclitaxel and P-gp targeted siRNA at a paclitaxel dose that was ineffective in the absence of gene silencing. These results suggest that that the combination of P-gp gene silencing and cytotoxic drug delivery using targeted nanoparticles can overcome tumor drug resistance.

Drug-inducible and simultaneous regulation of endogenous genes by single-chain nuclear receptor-based zinc-finger transcription factor gene switches

Chemically inducible gene switches that regulate expression of endogenous genes have multiple applications for basic gene expression research and gene therapy. Single-chain zinc-finger transcription factors that utilize either estrogen receptor homodimers or retinoid X receptor-alpha/ecdysone receptor heterodimers are shown here to be effective regulators of ICAM-1 and ErbB-2 transcription. Using activator (VP64) and repressor (Krüppel-associated box) domains to impart regulatory directionality, ICAM-1 was activated by 4.8-fold and repressed by 81% with the estrogen receptor-inducible transcription factors. ErbB-2 was activated by up to threefold and repressed by 84% with the retinoid X receptor-alpha/ecdysone receptor-inducible transcription factors. The dynamic range of these proteins was similar to the constitutive system and showed negligible basal regulation when ligand was not present. We have also demonstrated that the regulation imposed by these inducible transcription factors is dose dependent, sustainable for at least 11 days and reversible upon cessation of drug treatment. Importantly, these proteins can be used in conjunction with each other with no detectable overlap of activity enabling concurrent and temporal regulation of multiple genes within the same cell. Thus, these chemically inducible transcription factors are valuable tools for spatiotemporal control of gene expression that should prove valuable for research and gene therapy applications.

Selective killing of Smad4-negative tumor cells via a designed repressor strategy

Smad4 is a key tumor suppressor that is frequently deleted or inactive in pancreatic and colon tumors. In this report, we describe an approach for attaining selective killing of Smad4-deficient tumor cells. Using a vector system involving a designed repressor with zinc finger binding domains and the herpes simplex virus thymidine kinase (HSV-TK) "suicide gene," we demonstrate Smad4-responsive regulation of HSV-TK expression and consequent altered susceptibility to the prodrug ganciclovir (GCV). In pancreatic tumor cell lines stably transfected with the vector system, a robust differential of HSV-TK expression and GCV toxicity was attained depending on the presence or absence of cotransfected Smad4. In matched colon tumor cell lines lacking Smad4 or expressing physiological levels of Smad4, an adenoviral version of the vector system attained a significant degree of preferential killing of Smad4-negative tumor cells in response to GCV. These findings demonstrate the possibility of achieving selective killing of pancreatic and colon cells depending on their Smad4 status.

Evolution of programmable zinc finger-recombinases with activity in human cells

Site-specific recombinases are important tools for genomic engineering in many living systems. Applications of recombinases are, however, constrained by the DNA targeting endemic of the recombinase used. A tremendous range of recombinase applications can be envisioned if the targeting of recombinase specificity can be made readily programmable. To address this problem we sought to generate zinc finger-recombinase fusion proteins (Rec(ZF)s) capable of site-specific function in a diversity of genetic contexts. Our first Rec(ZF), Tn3Ch15(X2), recombined substrates derived from the native Tn3 resolvase recombination site. Substrate Linked Protein Evolution (SLiPE) was used to optimize the catalytic domains of the enzymes Hin, Gin, and Tn3 for resolution between non-homologous sites. One of the evolved clones, GinL7C7, catalyzed efficient, site-specific recombination in a variety of sequence contexts. When introduced into human cells by retroviral transduction, GinL7C7 excised a 1.4 kb EGFP cassette out of the genome, diminishing fluorescence in approximately 17% of transduced cells. Following this template of rational design and directed evolution, Rec(ZF)s may eventually mediate gene therapies, facilitate the genetic manipulation of model organisms and cells, and mature into powerful new tools for molecular biology and medicine.

Role of drug efflux carriers in the healthy and diseased brain

The blood-brain barrier is a natural diffusion barrier, which expresses active carriers extruding drugs on their way to the brain back into the blood against concentration gradients. Whereas these so-called adenosine triphosphate-binding cassette (ABC) transporters prevent the brain entry of toxic compounds under physiological conditions, they complicate pharmacotherapies in neurological disease. Recent observations in animal models of ischemic stroke, drug-resistant epilepsy, and brain cancer showed that the prototype of ABC transporters, ABCB1, is upregulated on brain injury, deactivation of this carrier considerably enhancing the accumulation of neuroprotective, antiepileptic, and chemotherapeutic compounds. These studies provide the proof of concept that the efficacy of brain-targeting drugs may significantly be improved when drug efflux is blocked. Under clinical conditions, efforts currently are made to enhance drug accumulation by selecting new compounds that do not bind to efflux carriers or deactivating ABC transporters by targeted downregulation or pharmacological inhibition. We predict that strategies aiming at circumventing drug efflux may greatly facilitate progress in neurological therapies.

Targeting multidrug resistance in cancer

Effective treatment of metastatic cancers usually requires the use of toxic chemotherapy. In most cases, multiple drugs are used, as resistance to single agents occurs almost universally. For this reason, elucidation of mechanisms that confer simultaneous resistance to different drugs with different targets and chemical structures - multidrug resistance - has been a major goal of cancer biologists during the past 35 years. Here, we review the most common of these mechanisms, one that relies on drug efflux from cancer cells mediated by ATP-binding cassette (ABC) transporters. We describe various approaches to combating multidrug-resistant cancer, including the development of drugs that engage, evade or exploit efflux by ABC transporters.

Reversal of MDR1 gene-dependent multidrug resistance using low concentration of endonuclease-prepared small interference RNA

Multidrug resistance following initial chemotherapy is commonly associated with MDR1 gene encoding for P-glycoprotein (P-gp). RNA interference of MDR1 gene expression was used as a strategy to reverse MDR1-mediated multidrug resistance phenotypes. Here we report that endonuclease-prepared small interfering RNA (esiRNA) at concentrations as low as 10 ng/ml (about 0.7 nM) can decrease MDR1 expression and increase chemosensitivity in the Adriamycin-induced resistant MCF-7/R cells. When MCF-7/R cells were transiently transfected with esiRNA of MDR1 (esiMDR1), the MDR1 mRNA was reduced by about 50%, drug accumulation increased by about 30%, and the IC50 for daunorubicin was reduced from 4.5 to 1.2 microM. These results provide evidence that esiRNA of MDR1 could be an alternative to P-gp inhibitors with the advantage of avoiding non-specific suppression with a lower effective dosage than using a single siRNA duplex, offering a potential therapeutic application of siRNA.

Gene therapy of the typical multidrug resistance phenotype of cancers: a new hope?

The multidrug resistance (MDR) phenotype of cancers has generated a large amount of research, owing to its constant fatal clinical outcome. Many studies have focused on the discovery of chemomodulators; however, in spite of this huge effort, the side effects that these products induce, and their additive toxicity when used in the presence of anticancer drugs, have led to the disaffection of the pharmaceutical industry and possibly slowed down research in pharmacological modulation. New tools developed using molecular biology techniques have opened the way for gene therapy and given birth to new therapeutic hopes. However, these discoveries and especially their clinical applications have slowed due to a lack of knowledge of the systems that finely regulate the MDR genes. This weakness explains why, to date, no general review has focused on the possibilities of gene therapy of MDR derived form the strategic options now available. Based on molecular foundations and recent fundamental discoveries, we seek to inform clinicians of the therapeutic hopes for chemoresistant tumors brought about by potent and specific new tools such as transcriptional decoys, interfering RNAs, etc. After describing the causes and mechanisms of MDR, we critically review these new strategies and their corresponding clinical trials.

Step into the Groove: Engineered Transcription Factors as Modulators of Gene Expression

Increasing knowledge about the influence of dysregulated gene expression in causing numerous diseases opens up new possibilities for the development of innovative therapeutics. In this chapter, we first describe different mechanisms of misregulated gene expression resulting in various pathophysiological conditions. Then, an overview is given of different technologies developed to readjust expression levels of genes. One of the most promising upcoming approaches in this respect is the development of engineered zinc-finger transcription factors. Results obtained from modulating endogenous gene expression using such engineered transcription factors are reviewed in depth. Finally, we address possible pitfalls of using such transcriptional targeting approaches at the "chromatin level." We describe aspects of studies at this level that influence successful DNA binding of engineered transcription factors, thereby affecting gene activity. Engineered transcription factors have great promise as potent therapeutics. Moreover, this technology is expected to yield fundamental knowledge about the organization and function of the genome.

Role of efflux pumps and metabolising enzymes in drug delivery

The impact of efflux pumps and metabolic enzymes on the therapeutic activity of various drugs has been well established. The presence of efflux pumps on various tissues and tumours has been shown to regulate the intracellular concentration needed to achieve therapeutic activity. The notable members of efflux proteins include P-glycoprotein, multi-drug resistance protein and breast cancer resistance protein. These efflux pumps play a pivotal role not only in extruding xenobiotics but also in maintaining the body's homeostasis by their ubiquitous presence and ability to coordinate among themselves. In this review, the role of efflux pumps in drug delivery and the importance of their tissue distribution is discussed in detail. To improve pharmacokinetic parameters of substrates, various strategies that modulate the activity of efflux proteins are also described. Drug metabolising enzymes mainly include the cytochrome P450 family of enzymes. Extensive drug metabolism due to the this family of enzymes is the leading cause of therapeutic inactivity. Therefore, the role of metabolising enzymes in drug delivery and disposition is extensively discussed in this review. The synergistic relationship between metabolising enzymes and efflux proteins is also described in detail. In summary, this review emphasises the urgent need to make changes in drug discovery and drug delivery as efflux pumps and metabolising enzymes play an important role in drug delivery and disposition.

Engineering Zinc Finger Protein Transcription Factors: The Therapeutic Relevance of Switching Endogenous Gene Expression On or Off at Command

Modulating gene expression directly at the DNA level represents a novel approach to control cellular processes. In this respect, zinc finger protein DNA-binding domains can be engineered to target virtually any gene. Coupling of a transcription activation or repression domain to these zinc fingers permits regulating gene expression at will, providing a platform of unlimited therapeutic applications. In this review, steps involved in the engineering of zinc finger protein transcription factors are described. In addition, an overview of endogenous genes successfully targeted for modulating expression by engineered zinc finger protein transcription factors is given. So far, research has mainly focused on targeting genes involved in cancer and angiogenesis, with encouraging evaluation in vivo and progression into a clinical trial. Altogether, engineered zinc finger proteins offer a new and exciting direction in the field of medical research with promising prospects.

Epigenetic manipulation of gene expression: a toolkit for cell biologists

Cell biologists have been afforded extraordinary new opportunities for experimentation by the emergence of powerful technologies that allow the selective manipulation of gene expression. Currently, RNA interference is very much in the limelight; however, significant progress has also been made with two other approaches. Thus, antisense oligonucleotide technology is undergoing a resurgence as a result of improvements in the chemistry of these molecules, whereas designed transcription factors offer a powerful and increasingly convenient strategy for either up- or down-regulation of targeted genes. This mini-review will highlight some of the key features of these three approaches to gene regulation, as well as provide pragmatic guidance concerning their use in cell biological experimentation based on our direct experience with each of these technologies. The approaches discussed here are being intensely pursued in terms of possible therapeutic applications. However, we will restrict our comments primarily to the cell culture situation, only briefly alluding to fundamental differences between utilization in animals versus cells.

Delivery of MDR1 small interfering RNA by self-complementary recombinant adeno-associated virus vector

Small interfering RNAs (siRNAs) are potentially powerful tools for therapeutic gene regulation. DNA cassettes encoding RNA polymerase III promoter-driven hairpin siRNAs allow long-term expression of siRNA in targeted cells. A variety of viral vectors have been used to deliver such cassettes to cells. Here we report on the development and use of a self-complementary recombinant adeno-associated virus (scAAV) vector for siRNA delivery into mammalian cells. We demonstrate that this modified vector efficiently delivers siRNA into multidrug-resistant human breast and oral cancer cells and suppresses MDR1 gene expression. This results in rapid, profound, and durable reduction in the expression of the P-glycoprotein multidrug transporter and a substantial reversion of the drug-resistant phenotype. This research suggests that scAAV-based vectors can be very effective agents for efficient delivery of therapeutic siRNA.

Selective transcription of p53 target genes by zinc finger-p53 DNA binding domain chimeras

Active p53 stimulates the transcription of a number of key genes, including the pro-apoptotic gene bax, as well as p21, a cell cycle regulator. In this study we constructed novel chimeric zinc finger-p53 DNA binding domain (DBD) transcription factors designed to bind to the promoters of specific p53 regulated genes. In order to selectively increase the expression of Bax, we coupled a pre-selected three-zinc finger (Zif) peptide targeted to a sequence in the bax promoter to a minimal p53 DBD. This chimeric protein could increase reporter gene transcription from a minimal bax promoter (up to 10-fold) but not from a minimal p21 promoter in p53-deficient Saos-2 cells. However, fusion proteins carrying longer p53 DBDs displayed entirely different selectivity and potency. Thus, Zif-p53 DBD chimeras containing N- and C-terminal extensions of the minimal DBD could increase transcription driven by a minimal p21 promoter up to 800-fold. These chimeras preferred the minimal p21 promoter up to 500-fold over the minimal bax promoter. Additionally, endogenous p21 message and protein levels were increased in cells expressing the p21 selective Zif-p53 DBD chimera and expression of the chimeric proteins resulted in partial cell cycle arrest. Cell fractionation experiments indicated that the Zifs enhanced nuclear localization of the Zif-p53 DBD chimera. These studies suggest that it is possible to create chimeric transcription factors able to strongly and selectively activate genes downstream of p53.

Exploring strategies for the design of artificial transcription factors: targeting sites proximal to known regulatory regions for the induction of gamma-globin expression and the treatment of sickle cell disease

Artificial transcription factors can be engineered to interact with specific DNA sequences to modulate endogenous gene expression within cells. A significant hurdle to implementation of this approach is the selection of the appropriate DNA sequence for targeting. We reasoned that a good target site should be located in chromatin, where it is accessible to DNA-binding proteins, and it should be in the close vicinity of known transcriptional regulators of the gene. Here we have explored the efficacy of these criteria to guide our selection of potential regulators of gamma-globin expression. Several zinc finger-based transcriptional activators were designed to target the sites proximal to the -117-position of the gamma-globin promoter. This region is proximal to the binding sites of known and potential natural transcription factors. Design and study of three transcription factors identified the potent transcriptional activator, gg1-VP64-HA. This transcription factor was able to interact directly with the gamma-globin promoter and up-regulate expression of reporter gene constructs as well as the endogenous gene in a selective manner. Transfection of a gg1-VP64-HA expression vector or retroviral delivery of this transcription factor into the erythroleukemia cell line K562 resulted in an increase of fetal hemoglobin. The gamma-globin content of cells expressing gg1-vp64-HA showed up to 16-fold higher levels of fetal hemoglobin than the native K562 cell line. These transcriptional activators constitute a novel class of regulators of the globin locus that may be suitable for treatment of diseases arising from mutations in this locus such as sickle cell disease and thalassemic diseases.

Designing transcription factor architectures for drug discovery

Recent advances in the design, selection, and engineering of DNA binding proteins have led to the emerging field of designer transcription factors (TFs). Modular DNA-binding protein domains can be assembled to recognize a given sequence of a DNA in a regulatory region of a targeted gene. TFs can be readily prepared by linking the DNA-binding protein to a variety of effector domains that mediate transcriptional activation or repression. Furthermore, the interaction between the TF and the genomic DNA can be regulated by several approaches, including chemical regulation by a variety of small molecules. Genome-wide single target specificity has been demonstrated using arrays of sequence-specific zinc finger (ZF) domains, polydactyl proteins. Any laboratory today can easily construct polydactyl ZF proteins by linkage of predefined ZF units that recognize specific triplets of DNA. The potential of this technology to alter the transcription of specific genes, to discover new genes, and to induce phenotypes in cells and organisms is now being applied in the areas of molecular therapeutics, pharmacology, biotechnology, and functional genomics.

Strategies for inhibition of MDR1 gene expression

Several distinct strategies have been used to modulate the expression of cancer-associated genes, including antisense oligonucleotides, small interfering RNAs (siRNAs), and artificial transcriptional factors. One major cause for chemotherapeutic treatment failure in cancer is the overexpression of P-glycoprotein, the product of the multidrug resistance gene MDR1. In this study, we tested the ability of siRNAs to inhibit MDR1 gene expression. We evaluated the efficiency of chemically synthesized dsRNAs as well as vector-based hairpin siRNAs and investigated the behavior of clones of multidrug-resistant NCI/ADR-RES breast carcinoma cells stably transfected with hairpin siRNA vectors. The effects of siRNA on the MDR phenotype were compared with those elicited by antisense oligonucleotides or by designed transcription factors targeting the MDR1 promoter. These studies suggest that there are several comparably effective strategies for inhibiting MDR1 expression.

Promoter-targeted phage display selections with preassembled synthetic zinc finger libraries for endogenous gene regulation

Regulation of endogenous gene expression has been achieved using synthetic zinc finger proteins fused to activation or repression domains, zinc finger transcription factors (TFZFs). Two key aspects of selective gene regulation using TFZFs are the accessibility of a zinc finger protein to its target DNA sequence and the interaction of the fused activation or repression domain with endogenous proteins. Previous work has shown that predicting a biologically active binding site at which a TF(ZF) can control gene expression is not always straightforward. Here, we used a library of preassembled three-finger zinc finger proteins (ZFPs) displayed on filamentous phage, and selected for ZFPs that bound along a 1.4 kb promoter fragment of the human ErbB-2 gene. Following affinity selection by phage display, 13 ZFPs were isolated and sequenced. Transcription factors were prepared by fusion of the zinc finger proteins with a VP64 activation domain or a KRAB repression domain and the transcriptional control imposed by these TFZFs was evaluated using luciferase reporter assays. Endogenous gene regulation activity was studied following retroviral delivery into A431 cells. Additional ZFP characterization included DNaseI footprinting to evaluate the integrity of each predicted protein:DNA interaction. The most promising TFZFs able to both up-regulate and down-regulate ErbB-2 expression were extended to six-finger proteins. The increased affinity and refined specificity demonstrated by the six-finger proteins provided reliable transcriptional control. As a result of studies with the six-finger proteins, the specific region of the promoter most accessible to transcriptional control by VP64-ZFP and KRAB-ZFP fusion proteins was elucidated and confirmed by DNaseI footprinting, flow cytometric analysis and immunofluorescence. The ZFP phage display library strategy disclosed here, coupled with the growing availability of genome sequencing information, provides a route to identifying gene-regulating TFZFs without the prerequisite of well-defined promoter elements.

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

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

P53-dependent cell-killing by selective repression of thymidine kinase and reduced prodrug activation

Selective killing of tumor cells is an important goal for cancer therapeutics. The tumor suppressor transcription factor p53 is absent or mutated in more than 50% of human tumors. Thus, determining approaches that use p53 status to regulate therapy may be an important strategy for attaining cancer selectivity. We have shown previously that a designed transcriptional repressor, K2-5F, strongly and selectively reduces the expression of its target gene MDR1. In this study, we exploited p53 status and the strong repressor activity of K2-5F to establish a system for preferential killing of p53-negative cells. In this system, the expression of K2-5F is induced by p53 in normal cells, and the K2-5F repressor then inhibits the expression of herpes simplex virus thymidine kinase (HSV-TK) driven by an MDR1 minipromoter. In p53-deficient cells, little K2-5F is expressed, and thus HSV-TK is expressed, allowing the cells to be killed by ganciclovir (GCV). K2-5F induced by exogenous p53 dramatically reduced the expression of HSV-TK in human embryonic kidney 293 cells, and it subsequently increased cell survival in response to GCV. To further evaluate this approach in a uniform genetic background, we developed Saos-2 cells stably expressing physiological levels of p53 and paired them with wild-type p53-negative Saos-2 cells. Stable expression of moderate levels of p53 in Saos-2 cells was able to induce the expression of K2-5F and reduce HSV-TK expression and resulted in a modest but distinct protection from GCV toxicity. Thus, this system may be suitable for further development as an approach to selective cancer therapy.

Zinc fingers and a green thumb: manipulating gene expression in plants

Artificial transcription factors can be rapidly constructed from predefined zinc-finger modules to regulate virtually any gene. Stable, heritable up- and downregulation of endogenous genes has been demonstrated in transgenic plants. These advances promise new approaches for creating functional knockouts and conditional overexpression, and for other gene discovery and manipulation applications in plants.

Targeted regulation of imprinted genes by synthetic zinc-finger transcription factors

Epigenetic control of transcription is essential for mammalian development and its deregulation causes human disease. For example, loss of proper imprinting control at the IGF2-H19 domain is a hallmark of cancer and Beckwith-Wiedemann syndrome, with no targeted therapeutic approaches available. To address this deficiency, we engineered zinc-finger transcription proteins (ZFPs) that specifically activate or repress the IGF2 and H19 genes in a domain-dependent manner. Importantly, we used these ZFPs successfully to reactivate the transcriptionally silent IGF2 and H19 alleles, thus overriding the natural mechanism of imprinting and validating an entirely novel avenue for 'transcription therapy' of human disease.

Design of artificial transcription factors to selectively regulate the pro-apoptotic bax gene

The tumor suppressor p53 is the most commonly mutated gene in human cancers. Active p53 is able to stimulate the transcription of a variety of genes including the pro-apoptotic gene bax, as well as p21, a cell cycle regulator. In this study we produced novel zinc finger transcription factors that would selectively increase the expression of bax, but not of other p53 targets. Reporter gene assays in p53-negative Saos-2 cells showed that the novel zinc finger proteins stimulated transcription driven by a minimal bax promoter, but not that driven by a minimal p21 promoter. Moreover, electromobility shift assays demonstrated that the novel transcription factors could bind the bax promoter sequence with high affinity and selectivity. Expression of a five zinc finger protein (5ZFAV) in COS-7 cells resulted in an increase in Bax protein levels, indicating that this novel transcription factor could act on endogenous gene expression. Expression of 5ZFAV also drastically reduced Saos-2 cell survival; this effect could be reversed by the general caspase inhibitor B-D-FMK. These data suggest that 5ZFAV is able to induce apoptosis through increased Bax expression. Further, while expression of 5ZFAV in p53-deficient Saos-2 cells reduced cell survival, there was little effect on U-2 OS cells which have wild-type p53. Thus the selective induction of the pro-apoptotic bax gene may be a valuable adjunct to cancer chemotherapy by diminishing survival of p53-deficient tumor cells.