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Given its similarities with mammalian systems, the zebrafish has emerged as a potential model to study the blood-brain barrier (BBB). Capillary endothelial cells at the human BBB express high levels of P-glycoprotein (P-gp, encoded by the ABCB1 gene) and ABCG2 (encoded by the ABCG2 gene). However, little information has been available about ATP-binding cassette transporters expressed at the zebrafish BBB. In this study, we focus on the characterization and tissue localization of two genes that are similar to human ABCB1, zebrafish abcb4 and abcb5. Cytotoxicity assays with stably-transfected cell lines revealed that zebrafish Abcb5 cannot efficiently transport the substrates doxorubicin and mitoxantrone compared to human P-gp and zebrafish Abcb4. Additionally, zebrafish Abcb5 did not transport the fluorescent probes BODIPY-ethylenediamine or LDS 751, while they were readily transported by Abcb4 and P-gp. A high-throughput screen conducted with 90 human P-gp substrates confirmed that zebrafish Abcb4 has overlapping substrate specificity with P-gp. Basal ATPase activity of zebrafish Abcb4 and Abcb5 was comparable to that of human P-gp. In the brain vasculature, RNAscope probes to detect abcb4 colocalized with staining by the P-gp antibody C219, while abcb5 was not detected. Zebrafish abcb4 also colocalized with claudin-5 expression in brain endothelial cells. Abcb4 and Abcb5 had different tissue localizations in multiple zebrafish tissues, consistent with different functions. The data suggest that zebrafish Abcb4 most closely phenocopies P-gp and that the zebrafish may be a viable model to study the role of the multidrug transporter P-gp at the BBB.

Most patients who die of cancer have disseminated disease that has become resistant to multiple therapeutic modalities. Ample evidence suggests that the expression of ATP-binding cassette (ABC) transporters, especially the multidrug resistance protein 1 (MDR1, also known as P-glycoprotein or P-gp), which is encoded by ABC subfamily B member 1 (ABCB1), can confer resistance to cytotoxic and targeted chemotherapy. However, the development of MDR1 as a therapeutic target has been unsuccessful. At the time of its discovery, appropriate tools for the characterization and clinical development of MDR1 as a therapeutic target were lacking. Thirty years after the initial cloning and characterization of MDR1 and the implication of two additional ABC transporters, the multidrug resistance-associated protein 1 (MRP1; encoded by ABCC1)), and ABCG2, in multidrug resistance, interest in investigating these transporters as therapeutic targets has waned. However, with the emergence of new data and advanced techniques, we propose to re-evaluate whether these transporters play a clinical role in multidrug resistance. With this Opinion article, we present recent evidence indicating that it is time to revisit the investigation into the role of ABC transporters in efficient drug delivery in various cancer types and at the blood–brain barrier.

P-glycoprotein (P-gp), an ATP-dependent efflux pump, is associated with the development of multidrug resistance in cancer cells. Antibody-mediated blockade of human P-gp activity has been shown to overcome drug resistance by re-sensitizing resistant cancer cells to anticancer drugs. Despite the potential clinical application of this finding, the epitopes of the three human P-gp-specific monoclonal antibodies MRK-16, UIC2 and 4E3, which bind to the extracellular loops (ECLs) have not yet been mapped. By generating human-mouse P-gp chimeras, we mapped the epitopes of these antibodies to ECLs 1 and 4. We then identified key amino acids in these regions by replacing mouse residues with homologous human P-gp residues to recover binding of antibodies to the mouse P-gp. We found that changing a total of ten residues, five each in ECL1 and ECL4, was sufficient to recover binding of both MRK-16 and 4E3 antibodies, suggesting a common epitope. However, recovery of the conformation-sensitive UIC2 epitope required replacement of thirteen residues in ECL1 and the same five residues replaced in the ECL4 for MRK-16 and 4E3 binding. These results demonstrate that discontinuous epitopes for MRK-16, UIC2 and 4E3 are located in the same regions of ECL1 and 4 of the multidrug transporter.

The System for Continuous Observation of Rodents in Home-cage Environment (SCORHE) was developed to demonstrate the viability of compact and scalable designs for quantifying activity levels and behavior patterns for mice housed within a commercial ventilated cage rack. The SCORHE in-rack design provides day- and night-time monitoring with the consistency and convenience of the home-cage environment. The dual-video camera custom hardware design makes efficient use of space, does not require home-cage modification, and is animal-facility user-friendly. Given the system's low cost and suitability for use in existing vivariums without modification to the animal husbandry procedures or housing setup, SCORHE opens up the potential for the wider use of automated video monitoring in animal facilities. SCORHE's potential uses include day-to-day health monitoring, as well as advanced behavioral screening and ethology experiments, ranging from the assessment of the short- and long-term effects of experimental cancer treatments to the evaluation of mouse models. When used for phenotyping and animal model studies, SCORHE aims to eliminate the concerns often associated with many mouse-monitoring methods, such as circadian rhythm disruption, acclimation periods, lack of night-time measurements, and short monitoring periods. Custom software integrates two video streams to extract several mouse activity and behavior measures. Studies comparing the activity levels of ABCB5 knockout and HMGN1 overexpresser mice with their respective C57BL parental strains demonstrate SCORHE's efficacy in characterizing the activity profiles for singly- and doubly-housed mice. Another study was conducted to demonstrate the ability of SCORHE to detect a change in activity resulting from administering a sedative.

Physiologic barriers such as the blood placenta barrier (BPB) and the blood brain barrier protect the underlying parenchyma from pathogens and toxins. ATP-binding cassette (ABC) transporters are transmembrane proteins found at these barriers and function to efflux xenobiotics and maintain chemical homeostasis. Despite the plethora of ex vivo and in vitro data showing the function and expression of ABC transporters, no imaging modality exists to study ABC transporter activity in vivo at the BPB. In the present study, we show that in vitro models of the placenta possess ABCG2 activity and can specifically transport D-luciferin, the endogenous substrate of firefly luciferase. To test ABCG2 transport activity at the BPB, we devised a breeding strategy to generate a bioluminescent pregnant mouse model to demonstrate transporter function in vivo. We found that coadministering the ABCG2 inhibitors Ko143 and gefitinib with D-luciferin increased bioluminescent signal from fetuses and placentae, whereas the control P-gp inhibitor DCPQ had no effect. We believe that our bioluminescent pregnant mouse model will facilitate greater understanding of the BPB and ABCG2 activity in health and disease.

To determine whether the small endosomal recycling GTPase, RAB8, plays a role in TMEM205-associated resistance to the chemotherapeutic drug cisplatin.Antibodies were used as markers for both genes; confocal microscopy was used to visualize their localization in cisplatin-resistant cells. Both single and dual-transfections were performed.Expression of RAB8 was markedly elevated in human cisplatin-resistant cells. We found that TMEM205 was co-localized with RAB8. Dual transfectants with over-expression of both TMEM205 and RAB8 were found to be up to 4-fold more resistant to cisplatin, while cells transfected with RAB8 alone were ~2-fold more resistant.The development of cisplatin resistance appears to be a consequence of pleotropic epigenetic alterations. We unravel the role of one gene, the GTPase RAB8, in this process. Because its highest expression was at an early step of cisplatin resistance, it may be involved in early development of resistance. Increased expression of TMEM205 and RAB8 in double-transfected cells and their increased resistance to cisplatin indicate an additive effect of these two genes, mediating cisplatin resistance. These two proteins are potential biomarkers or targets for gene or chemotherapy.

Over the last two decades, small interfering RNA (siRNA)-mediated gene silencing has quickly become one of the most powerful techniques used to study gene function in vitro and a promising area for new therapeutics. Delivery remains a significant impediment to realizing the therapeutic potential of siRNA, a problem that is also tied to immunogenicity and toxicity. Numerous delivery vehicles have been developed, including some that can be categorized as pseudovirions: these are vectors that are directly derived from viruses but whose viral coding sequences have been eliminated, preventing their classification as viral vectors. Characteristics of the pseudovirions discussed in this review, namely phagemids, HSV amplicons, SV40 in vitro-packaged vectors, influenza virosomes, and HVJ-Envelope vectors, make them attractive for the delivery of siRNA-based therapeutics. Pseudovirions were shown to deliver siRNA effector molecules and bring about RNA interference (RNAi) in various cell types in vitro, and in vivo using immune-deficient and immune-competent mouse models. Levels of silencing were not always determined directly, but the duration of siRNA-induced knockdown lasted at least 3 days. We present examples of the use of pseudovirions for the delivery of synthetic siRNA as well as the delivery and expression of DNA-directed siRNA.

At the Eleventh International Conference on Gene Therapy of Cancer (December 12-14, 2002, San Diego, CA) progress on using gene transfer technology to treat cancer was presented. Although there is as yet no cancer gene therapy being marketed, considerable progress has been made in defining likely strategies and likely targets for gene therapy of cancer. These strategies, including viral and non-viral delivery systems, and potential targets in cancer cells linked to our developing knowledge of cancer cell biology, are reviewed in this paper. Use of gene therapy to sensitize tumors to radiation and chemotherapy is one promising area of investigation. Some of the ancillary benefits of research on cancer gene therapy, including the development of public-private partnerships, recruitment of laboratory scientists into clinical research, and credentialing of potential cancer cell targets for therapies other than gene therapy, are noted.

Background Resistance to therapy remains a major cause of the failure of cancer treatment. A major challenge in cancer therapy is to design treatment strategies that circumvent the higher-level homeostatic functions of the robust cellular network that occurs in resistant cells. There is a lack of understanding of mechanisms responsible for the development of cancer and the basis of therapy-resistance mechanisms. Cellular signaling networks have an underlying architecture guided by universal principles. A robust system, such as cancer, has the fundamental ability to survive toxic anticancer drug treatments or a stressful environment mainly due to its mechanisms of redundancy. Consequently, inhibition of a single component/pathway would probably not constitute a successful cancer therapy. Results We developed a computational method to study the mechanisms of redundancy and to predict communications among the various pathways based on network theory, using data from gene expression profiles of hepatocellular carcinoma (HCC) of patients with poor and better prognosis cancers. Our results clearly indicate that immune system pathways tightly regulate most cancer pathways, and when those pathways are targeted by drugs, the network connectivity is dramatically changed. We examined the main HCC targeted treatments that are currently being evaluated in clinical trials. One prediction of our study is that Sorafenib combined with immune system treatments will be a more effective combination strategy than Sorafenib combined with any other targeted drugs. Conclusions We developed a computational framework to analyze gene expression data from HCC tumors with varying degrees of responsiveness and non-tumor samples, based on both Gene and Pathway Co-expression Networks. Our hypothesis is that redundancy is one of the major causes of drug resistance, and can be described as a function of the network structure and its properties. From this perspective, we believe that integration of the redundant variables could lead to the development of promising new methodologies to selectively identify and target the most significant resistance mechanisms of HCC. We describe three mechanisms of redundancy based on their levels of generalization and study the possible impact of those redundancy mechanisms on HCC treatments.

ABC transporters are found in all known organisms, and approximately 1,100 different transporters belonging to this family have been described in the literature. The family is defined by homology within the ATP-binding cassette (ABC) region, which extends outside of the more typical Walker motifs found in all ATP-binding proteins. Most family members also contain transmembrane domains involved in recognition of substrates, which are transported across, into, and out of cell membranes, but some members utilize ABCs as engines to regulate ion channels. There are approximately 50 known ABC transporters in the human, and there are currently 13 genetic diseases associated with defects in 14 of these transporters. The most common genetic disease conditions include cystic fibrosis, Stargardt disease, age-related macular degeneration, adrenoleukodystrophy, Tangier disease, Dubin-Johnson syndrome and progressive familial intrahepatic cholestasis. At least 8 members of this family are involved in the transport of a variety of amphipathic compounds, including anticancer drugs, and some appear to contribute to the resistance of cancer cells to chemotherapy.

Severe combined immunodeficiency (SCID) mice1 are defective in their ability to rearrange their variable (V), diversity (D) and joining (J) genetic elements to generate functional immunoglobulin (Ig) and T-cell receptor (TCR) molecules; as a result, they lack mature B and T cells2. These mice are highly sensitive to ionizing radiation, suggesting that the product of the scid gene plays a critical role in both V(D)J recombination and DMA double-strand break repair3–5. Recent studies suggest that the SCID defect lies in the gene encoding the catalytic subunit of DMA-dependent protein kinase (DNA-PK; refs 6–8), a nuclear protein made up of the Ku 70 and Ku 86 subunits as well as the large catalytic subunit, DNA-PKcs9,10. Other reports have implied that the SCID phenotype correlates with nonsense mutations at the extreme 3′ end of Prkdc, the DNA-PKcs gene11–14. The identity of the gene remains in doubt, however, because the consequences of genetic inactivation of Prkdc have not been determined. This study shows that complete inactivation of Prkdc in a novel insertional mouse mutant recapitulates the SCID phenotype and that Prkdc and scid are allelic. Significantly, DNA-PKcs null mice demonstrate complete penetrance of thymic lymphoblastic lymphomas, strongly suggesting that Prkdc functions in mice as a T-cell tumour suppressor and, by virtue of its association with DNA repair and recombination, belongs to the ‘caretaker’ class of tumour-suppressor genes that includes ATM, BRCA1 and BRCA2(ref.15).

We have established a novel ascites tumour model (MDA435/LCC6) from the oestrogen receptor-negative, invasive and metastatic MDA-MB-435 human breast cancer cell line. MDA435/LCC6 cells grow as both malignant ascites and solid tumours in vivo in nude mice and nude rats, with a tumour incidence of approximately 100%. Untreated mice develop ascites following i.p. inoculation of 1 x 10(6) cells and have a reproducible life span of approximately 30 days, with all animals dying within a 48 h period. The in vivo response of MDA435/LCC6 ascites to several cytotoxic drugs, including doxorubicin, etoposide (VP-16), BCNU and mitomycin C, closely reflects the activity of these single agents in previously untreated breast cancer patients. MDA435/LCC6 cells also retain the anchorage-dependent and anchorage-independent in vitro growth properties of the parental MDA-MB-435 cells, and can be used in standard in vitro drug screening assays. The drug resistance pattern of the MDA435/LCC6 cells suggests that they may have few active endogenous drug resistance mechanisms. To generate a model for the screening of MDR1-reversing agents, MDA435/LCC6 were transduced with a retroviral vector directing the constitutive expression of the MDR1 cDNA, producing a cell line with a classical MDR1 resistance pattern (MDA435/LCC6MDR1). THese ascites models may be a viable alternative to the murine leukaemia ascites (L1210, P388) and, in conjunction with other breast cancer cell lines, facilitate the in vitro and in vivo screening of new cytotoxic drugs and drug combinations. Images Figure 1

Cells expressing elevated levels of the membrane phosphoprotein P-glycoprotein exhibit a multidrug resistance phenotype. Studies involving protein kinase activators and inhibitors have implied that covalent modification of P-glycoprotein by phosphorylation may modulate its biological activity as a multidrug transporter. Most of these reagents, however, have additional mechanisms of action and may alter drug accumulation within multidrug resistant cells independent of, or in addition to, their effects on the state of phosphorylation of P-glycoprotein. The protein kinase(s) responsible for P-glycoprotein phosphorylation has(ve) not been unambiguously identified, although several possible candidates have been suggested. Recent biochemical analyses demonstrate that the major sites of phosphorylation are clustered within the linker region that connects the two homologous halves of P-glycoprotein. Mutational analyses have been initiated to confirm this finding. Preliminary data obtained from phosphorylation- and dephosphorylation-defective mutants suggest that phosphorylation of P-glycoprotein is not essential to confer multidrug resistance.

Multidrug resistance in animal cells is defined as the simultaneous resistance to a variety of compounds which appear to be structurally and mechanistically unrelated. One type of multidrug resistance is characterized by the decreased accumulation of hydrophobic natural product drugs, a phenotype which is mediated by an ATP-dependent integral membrane multidrug transporter termed P-glycoprotein or P170. The gene coding for P170 is called MDR. The nucleotide-binding domain of P-glycoprotein shares sequence homology with a family of bacterial permease ATP-binding components. In addition, P170 as a whole is structurally very similar to a number of prokaryotic and eukaryotic proteins believed to be involved in transport activities. This review summarizes our current knowledge of the molecular biology and clinical significance of MDR expression and P-glycoprotein transport activity, as well as some theories about the function of this protein in normal cells.

Crystal structures of a mammalian multidrug efflux pump bound to peptide inhibitors may reveal drug-binding sites.

Microarray gene-expression profiles are generally validated one gene at a time by real-time RT-PCR. We describe here a different approach based on simultaneous mutual validation of large numbers of genes using two different expression-profiling platforms. The result described here for the NCI-60 cancer cell lines is a consensus set of genes that give similar profiles on spotted cDNA arrays and Affymetrix oligonucleotide chips. Global concordance is parameterized by a 'correlation of correlations' coefficient.

From mutagenized Chinese hamster ovary (CHO) cells we have isolated, in a single step, 11 independent mutants resistant to the growth-inhibitory effects of 8-Br-cyclic AMP, cholera toxin, and methylisobutylxanthine. Two major classes and several subclasses of mutants were obtained. Mutants from all classes have a normal doubling time. None of the mutants respond to cyclic AMP treatment with increased flattening and elongation as do the parental cells. Members of the first class have an altered protein kinase activity which has either an increased Ka for cyclic AMP or an absent response to cyclic AMP. Most of those mutations which result in a protein kinase with increased Ka for cyclic AMP (6/11) are dominant in somatic cell hybrids. Those mutations which result in a protein kinase with little or no response to cyclic AMP (3/11) are recessive. Members of the second major class (2/11) have normal levels of basal and cyclic AMP-dependent protein kinase activity. One is recessive and one is dominant by genetic tests. The basis for the defect in this second class of mutants has not been determined.