Genomic changes identified by comparative genomic hybridisation in docetaxel-resistant breast cancer cell lines

Two-step hypothesis testing to detect gene-environment interactions in a genome-wide scan with a survival endpoint

Defined by their genetic profile, individuals may exhibit differential clinical outcomes due to an environmental exposure. Identifying subgroups based on specific exposure-modifying genes can lead to targeted interventions and focused studies. Genome-wide interaction scans (GWIS) can be performed to identify such genes, but these scans typically suffer from low power due to the large multiple testing burden. We provide a novel framework for powerful two-step hypothesis tests for GWIS with a time-to-event endpoint under the Cox proportional hazards model. In the Cox regression setting, we develop an approach that prioritizes genes for Step-2 G × E testing based on a carefully constructed Step-1 screening procedure. Simulation results demonstrate this two-step approach can lead to substantially higher power for identifying gene-environment ( G × E ) interactions compared to the standard GWIS while preserving the family wise error rate over a range of scenarios. In a taxane-anthracycline chemotherapy study for breast cancer patients, the two-step approach identifies several gene expression by treatment interactions that would not be detected using the standard GWIS.

A Unique Morphological Phenotype in Chemoresistant Triple-Negative Breast Cancer Reveals Metabolic Reprogramming and PLIN4 Expression as a Molecular Vulnerability

The major obstacle in successfully treating triple-negative breast cancer (TNBC) is resistance to cytotoxic chemotherapy, the mainstay of treatment in this disease. Previous preclinical models of chemoresistance in TNBC have suffered from a lack of clinical relevance. Using a single high dose chemotherapy treatment, we developed a novel MDA-MB-436 cell-based model of chemoresistance characterized by a unique and complex morphologic phenotype, which consists of polyploid giant cancer cells giving rise to neuron-like mononuclear daughter cells filled with smaller but functional mitochondria and numerous lipid droplets. This resistant phenotype is associated with metabolic reprogramming with a shift to a greater dependence on fatty acids and oxidative phosphorylation. We validated both the molecular and histologic features of this model in a clinical cohort of primary chemoresistant TNBCs and identified several metabolic vulnerabilities including a dependence on PLIN4, a perilipin coating the observed lipid droplets, expressed both in the TNBC-resistant cells and clinical chemoresistant tumors treated with neoadjuvant doxorubicin-based chemotherapy. These findings thus reveal a novel mechanism of chemotherapy resistance that has therapeutic implications in the treatment of drug-resistant cancer. IMPLICATIONS: These findings underlie the importance of a novel morphologic-metabolic phenotype associated with chemotherapy resistance in TNBC, and bring to light novel therapeutic targets resulting from vulnerabilities in this phenotype, including the expression of PLIN4 essential for stabilizing lipid droplets in resistant cells.

Establishment and characterization of a triple negative basal-like breast cancer cell line with multi-drug resistance

Triple-negative breast carcinoma (TNBC) is one of the most aggressive subtypes of breast cancer and is associated with an unfavorable prognosis. The management of TNBC is currently based on the use of classical cytotoxic drugs, i.e., anthracyclines and/or microtubule-binding agents (TBAs). However, conventional chemotherapy is not always effective in these tumors and a systemic relapse is often observed, potentially due to the development of multi-drug resistance (MDR). Therefore, an improved understanding of MDR mechanisms may improve the therapeutic strategies for TNBC. In the present study, a paclitaxel-resistant (TxR) breast cancer cell subline of HCC1806 TNBC cells was established and characterized. The resistance index of this subline was calculated according to the IC₅₀ of HCC1806-TxR relative to the parental HCC1806 cells (16.86-fold). TxR-cells also exhibited cross-resistance to vinblastin, doxorubicin and etoposide (~14-, ~4- and ~3-fold, respectively). As assessed with reverse transcription-quantitative polymerase chain reaction, TxR-resistant cells exhibited the upregulated expression of a number of multidrug resistance-associated genes, including MDR-1, MRP-1, -5, -6 and YB-1. The TxR cells also exhibited an increased expression of MDR-related proteins including MDR1 and MRP-1, which led to a substantial increase (5.4-fold) of the paclitaxel efflux from TxR-cells. In addition, the pro-apoptotic protein Fas was downregulated, whereas the anti-apoptotic Bcl-2 was upregulated, in TxR-cells. This may explain why a reduced extent of apoptosis was observed when TxR cells were exposed to TBAs and topoisomerase type II inhibitors, relative to the parental HCC1806 cells. Thus, the HCC1806-TxR cell line may serve as an appropriate model for the analysis of chemoresistance mechanisms in TNBCs, and for the investigation of novel anticancer agents for overcoming MDR-mediated mechanisms in TNBC.

Polymeric mixed micelles as nanomedicines: Achievements and perspectives

During the past few decades, polymeric micelles have raised special attention as novel nano-sized drug delivery systems for optimizing the treatment and diagnosis of numerous diseases. These nanocarriers exhibit several in vitro and in vivo advantages as well as increased stability and solubility to hydrophobic drugs. An interesting approach for optimizing these properties and overcoming some of their disadvantages is the combination of two or more polymers in order to assemble polymeric mixed micelles. This review article gives an overview on the current state of the art of several mixed micellar formulations as nanocarriers for drugs and imaging probes, evaluating their ongoing status (preclinical or clinical stage), with special emphasis on type of copolymers, physicochemical properties, in vivo progress achieved so far and toxicity profiles. Besides, the present article presents relevant research outcomes about polymeric mixed micelles as better drug delivery systems, when compared to polymeric pristine micelles. The reported data clearly illustrates the promise of these nanovehicles reaching clinical stages in the near future.

The stepwise evolution of the exome during acquisition of docetaxel resistance in breast cancer cells

BACKGROUND

Resistance to taxane-based therapy in breast cancer patients is a major clinical problem that may be addressed through insight of the genomic alterations leading to taxane resistance in breast cancer cells. In the current study we used whole exome sequencing to discover somatic genomic alterations, evolving across evolutionary stages during the acquisition of docetaxel resistance in breast cancer cell lines.

RESULTS

Two human breast cancer in vitro models (MCF-7 and MDA-MB-231) of the step-wise acquisition of docetaxel resistance were developed by exposing cells to 18 gradually increasing concentrations of docetaxel. Whole exome sequencing performed at five successive stages during this process was used to identify single point mutational events, insertions/deletions and copy number alterations associated with the acquisition of docetaxel resistance. Acquired coding variation undergoing positive selection and harboring characteristics likely to be functional were further prioritized using network-based approaches. A number of genomic changes were found to be undergoing evolutionary selection, some of which were likely to be functional. Of the five stages of progression toward resistance, most resistance relevant genomic variation appeared to arise midway towards fully resistant cells corresponding to passage 31 (5 nM docetaxel) for MDA-MB-231 and passage 16 (1.2 nM docetaxel) for MCF-7, and where the cells also exhibited a period of reduced growth rate or arrest, respectively. MCF-7 cell acquired several copy number gains on chromosome 7, including ABC transporter genes, including ABCB1 and ABCB4, as well as DMTF1, CLDN12, CROT, and SRI. For MDA-MB-231 numerous copy number losses on chromosome X involving more than 30 genes was observed. Of these genes, CASK, POLA1, PRDX4, MED14 and PIGA were highly prioritized by the applied network-based gene ranking approach. At higher docetaxel concentration MCF-7 subclones exhibited a copy number loss in E2F4, and the gene encoding this important transcription factor was down-regulated in MCF-7 resistant cells.

CONCLUSIONS

Our study of the evolution of acquired docetaxel resistance identified several genomic changes that might explain development of docetaxel resistance. Interestingly, the most relevant resistance-associated changes appeared to originate midway through the evolution towards fully resistant cell lines. Our data suggest that no single genomic event sufficiently predicts resistance to docetaxel, but require genomic alterations affecting multiple pathways that in concert establish the final resistance stage.

Identification of Copy Number Aberrations in Breast Cancer Subtypes Using Persistence Topology

DNA copy number aberrations (CNAs) are of biological and medical interest because they help identify regulatory mechanisms underlying tumor initiation and evolution. Identification of tumor-driving CNAs (driver CNAs) however remains a challenging task, because they are frequently hidden by CNAs that are the product of random events that take place during tumor evolution. Experimental detection of CNAs is commonly accomplished through array comparative genomic hybridization (aCGH) assays followed by supervised and/or unsupervised statistical methods that combine the segmented profiles of all patients to identify driver CNAs. Here, we extend a previously-presented supervised algorithm for the identification of CNAs that is based on a topological representation of the data. Our method associates a two-dimensional (2D) point cloud with each aCGH profile and generates a sequence of simplicial complexes, mathematical objects that generalize the concept of a graph. This representation of the data permits segmenting the data at different resolutions and identifying CNAs by interrogating the topological properties of these simplicial complexes. We tested our approach on a published dataset with the goal of identifying specific breast cancer CNAs associated with specific molecular subtypes. Identification of CNAs associated with each subtype was performed by analyzing each subtype separately from the others and by taking the rest of the subtypes as the control. Our results found a new amplification in 11q at the location of the progesterone receptor in the Luminal A subtype. Aberrations in the Luminal B subtype were found only upon removal of the basal-like subtype from the control set. Under those conditions, all regions found in the original publication, except for 17q, were confirmed; all aberrations, except those in chromosome arms 8q and 12q were confirmed in the basal-like subtype. These two chromosome arms, however, were detected only upon removal of three patients with exceedingly large copy number values. More importantly, we detected 10 and 21 additional regions in the Luminal B and basal-like subtypes, respectively. Most of the additional regions were either validated on an independent dataset and/or using GISTIC. Furthermore, we found three new CNAs in the basal-like subtype: a combination of gains and losses in 1p, a gain in 2p and a loss in 14q. Based on these results, we suggest that topological approaches that incorporate multiresolution analyses and that interrogate topological properties of the data can help in the identification of copy number changes in cancer.

Collateral chemoresistance to anti-microtubule agents in a lung cancer cell line with acquired resistance to erlotinib

Various alterations underlying acquired resistance to epidermal growth factor receptor-tyrosine kinase inhibitors (EGFR-TKIs) have been described. Although treatment strategies specific for these mechanisms are under development, cytotoxic agents are currently employed to treat many patients following failure of EGFR-TKIs. However, the effect of TKI resistance on sensitivity to these cytotoxic agents is mostly unclear. This study investigated the sensitivity of erlotinib-resistant tumor cells to five cytotoxic agents using an in vitro EGFR-TKI-resistant model. Four erlotinib-sensitive lung adenocarcinoma cell lines and their resistant derivatives were tested. Of the resistant cell lines, all but one showed a similar sensitivity to the tested drugs as their parental cells. HCC4006ER cells with epithelial mesenchymal transition features acquired resistance to the three microtubule-targeting agents, docetaxel, paclitaxel and vinorelbine, but not to cisplatin and gemcitabine. Gene expression array and immunoblotting demonstrated that ATP-binding cassette subfamily B, member 1 (ABCB1) was up-regulated in HCC4006ER cells. ABCB1 knockdown by siRNA partially restored sensitivity to the anti-microtubule agents but not to erlotinib. Moreover, the histone deacetylase inhibitor entinostat sensitized HCC4006ER cells to anti-microtubule agents through ABCB1 suppression. Our study indicates that sensitivity of tumor cells to cytotoxic agents in general does not change before and after failure of EGFR-TKIs. However, we describe that two different molecular alterations confer acquired resistance to EGFR-TKIs and cytotoxic agents, respectively. This phenomenon should be kept in mind in selection of subsequent therapy after failure of EGFR-TKIs.

Docetaxel-loaded thermosensitive and bioadhesive nanomicelles as a rectal drug delivery system for enhanced chemotherapeutic effect

PURPOSE

To investigate the potential of thermosensitive and biadhesive nanomicelles in improving the bioavailability of docetaxel (DCT) and its chemotherapeutic effect.

METHOD

DCT-loaded nanomicelles were prepared by emulsufication and characterized in terms of physico-chemical and visco-elastic parameters. The optimzed formulation was evaluated for in vivo localization, pharmacokinetic and anti-tumor efficacy.

RESULTS

The hydrodynamic size of DCT-loaded nanomicelles was approximately 13 nm and the nanomicelles exhibited a sufficient gelation strength (9250 mPa·s) and bioadhesive force (2100 dyn/cm²) to be retained in the upper part of rectum. We observed a high rectal bioavailability of 29% DCT compared to that following oral administration in rats, as it successfully evaded the multidrug efflux transporters and hepatic first-pass metabolism. Plasma concentration around ∼50 ng/mL was maintained throughout the study period (12 h) while Taxotere® attained subtherapeutic range within 4 h of drug administration. Results also revealed that the rectally administered DCT-loaded nanomicelles exhibited a significant anti-tumor effect (200 mm³) with a reduced toxicity profile when compared to orally administered DCT (950 mm³). Furthermore, histological study showed that the rectal mucosa was completely intact with no signs of irritation upon treatment with DCT-loaded nanomicelles.

CONCLUSIONS

Taken together, our novel thermosensitive and biadhesive nanomicelles demonstrated the ability to improve the bioavailability and chemotherapeutic potential of DCT in vivo. To the best of our knowledge, this is the first report describing the rectal delivery of DCT-loaded nanomicelles.

Taxane resistance in breast cancer: a closed HER2 circuit?

Microtubule inhibitors, such as the taxanes docetaxel and paclitaxel, are commonly used drugs for the treatment of breast cancer. Although highly active in a large fraction of individuals a considerable number of patients show poor response due to either intrinsic or acquired drug resistance. Extensive research in the past identified several taxane resistance-related mechanisms being activated by pathologically altered single gene function. To date, however, a clinically relevant predictive biomarker for taxanes has not been derived yet from this knowledge, most likely due to the manifold of resistance mechanisms that may combine in one tumor, thereby fostering escape from taxane cytotoxicity. Here, we aimed to comprehensively review the current literature on taxane resistance mechanisms in breast cancer. Interestingly, besides altered microtubule physiology we identified the HER2 signaling cascade as a major dominator influencing several routes of cytotoxicity escape, such as cell survival, apoptosis, drug efflux, and drug metabolism. Furthermore, the transcription factor YBX-1, activated by HER2, facilitates a sustaining HER2 signaling feedback loop contributing to the establishment of cellular survival detours. In conclusion, taxane resistance in breast cancer follows a multiplex establishment of drug cytotoxicity escape routes, which may be most efficiently therapeutically targeted by interference with their mutually governing signaling nodes.

SNPs in genes coding for ROS metabolism and signalling in association with docetaxel clearance

The dose of docetaxel is currently calculated based on body surface area and does not reflect the pharmacokinetic, metabolic potential or genetic background of the patients. The influence of genetic variation on the clearance of docetaxel was analysed in a two-stage analysis. In step one, 583 single-nucleotide polymorphisms (SNPs) in 203 genes were genotyped on samples from 24 patients with locally advanced non-small cell lung cancer. We found that many of the genes harbour several SNPs associated with clearance of docetaxel. Most notably these were four SNPs in EGF, three SNPs in PRDX4 and XPC, and two SNPs in GSTA4, TGFBR2, TNFAIP2, BCL2, DPYD and EGFR. The multiple SNPs per gene suggested the existence of common haplotypes associated with clearance. These were confirmed with detailed haplotype analysis. On the basis of analysis of variance (ANOVA), quantitative mutual information score (QMIS) and Kruskal-Wallis (KW) analysis SNPs significantly associated with clearance of docetaxel were confirmed for GSTA4, PRDX4, TGFBR2 and XPC and additional putative markers were found in CYP2C8, EPHX1, IGF2, IL1R2, MAPK7, NDUFB4, TGFBR3, TPMT (2 SNPs), (P<0.05 or borderline significant for all three methods, 14 SNPs in total). In step two, these 14 SNPs were genotyped in additional 9 samples and the results combined with the genotyping results from the first step. For 7 of the 14 SNPs, the results are still significant/borderline significant by all three methods: ANOVA, QMIS and KW analysis strengthening our hypothesis that they are associated with the clearance of docetaxel.

The effects of mixed MPEG-PLA/Pluronic copolymer micelles on the bioavailability and multidrug resistance of docetaxel

A mixed micelle that comprised of MPEG-PLA (MPP) and Pluronic copolymers was developed for enhanced bioavailability and to overcome multidrug resistance of docetaxel in cancer therapy. The mixed micelles that sufficiently solubilized docetaxel were evaluated for the effect of Pluronic copolymers weight ratio on the mixed micelles with respect to drug loading and drug release. In vitro, cell viability and cytotoxicity studies in KB and KBv cells revealed that the mixed micellar formulations were more potent than the commercial docetaxel formulation (Taxotere). In vivo pharmacokinetics study in rats showed that the mixed micelles significantly enhanced the bioavailability of docetaxel (3.6 fold) than Taxotere. Moreover, antitumor activity assessed in KBv cancer xenograft BALB/C nude mice models showed that the mixed micelles significantly reduced the tumor size than the control (Taxotere). Clear differences in the intracellular uptake of docetaxel between MPP and mixed micelles were observed using confocal laser scanning microscopy. This study presents not only a new micelle structure for a diblock-triblock copolymer system, but also a method for enhanced bioavailability of docetaxel and to overcome some of the limitations on its multidrug resistance in cancer therapy.

Molecular mechanisms of drug resistance in single-step and multi-step drug-selected cancer cells

Multidrug resistance (MDR) remains one of the key determinants in chemotherapeutic success of cancer patients. Often, acquired resistance is mediated by the overexpression of ATP-binding cassette (ABC) drug transporters. To study the mechanisms involved in the MDR phenotype, investigators have generated a variety of in vitro cell culture models using both multi-step and single-step drug selections. Sublines produced from multi-step selections have led to the discovery of several crucial drug transporters including ABCB1, ABCC1, and ABCG2. Additionally, a number of mechanisms causing gene overexpression have been elucidated. To more closely mimic in vivo conditions, investigators have also established MDR sublines with single-step drug selections. Here, we examine some of the multi-step and single-step selected cell lines generated to elucidate the mechanisms involved in the development of MDR in cancer cells.

Development of radioresistance in drug resistant human MCF-7 breast cancer cells

Background and purpose: Radiotherapy is used for the treatment of malignant tumours, and may be used as the primary therapy. It is also common to combine radiotherapy with surgery, chemotherapy, hormone therapy or some combination of them. Even if the tumour is treated intensively, women diagnosed with breast cancer may develop a recurrence. Most recurrences may be in the form of distant metastases, development of multi-drug resistance phenotype or both together. This study demonstrated that some of the multi-drug resistant cancer cells may also become radioresistant.

Materials and Methods: Chemoresistance in paclitaxel (MCF-7/Pac), docetaxel (MCF-7/Doc), vincristine (MCF-7/Vinc), doxorubicin (MCF-7/Dox) and zoledronic acid (MCF-7/Zol) resistant MCF-7 cells were demonstrated by XTT assay. MDR1 gene expression was detected by real-time PCR in human MCF-7 breast cancer cells. Drug resistant and sensitive cells were exposed to γ-radiation and development of radioresistance was investigated.

Results: Results have indicated that paclitaxel, docetaxel, vincristine, doxorubicin and zoledronic acid–selected cells gained varying degrees of resistance to their selective drugs when compared with original MCF-7/S. MCF-7/Pac, MCF-7/Doc, MCF-7/Vinc and MCF-7/Dox cells have all acquired MDR1 expression. Among the resistant sub-lines, MCF-7/Pac and MCF-7/Doc cells were significantly cross-resistant to irradiation compared to the sensitive cells.

Conclusion: MCF-7/Pac and MCF-7/Doc cell lines were found radioresistant to γ-radiation. On the contrary, doxorubicin, vincristine and zoledronic acid resistant cancer cells were still sensitive to radiation.

Cdc7-Dbf4 kinase overexpression in multiple cancers and tumor cell lines is correlated with p53 inactivation

Cdc7 is a conserved serine/threonine kinase essential for the initiation of DNA replication, likely by activating the MCM DNA helicase at the G(1-) to S-phase transition. Cdc7 kinase activity requires association with its regulatory subunit Dbf4/activator of S-phase kinase. Cdc7-Dbf4 is also downstream of the conserved Ataxia telangectasia and RAD3-related kinase that responds to stalled replication forks or DNA damage. In this study, we found that Cdc7 protein was very low or undetectable in normal tissues and cell lines but had increased expression in approximately 50% of the 62 human tumor cell lines we examined. Most cell lines with increased Cdc7 protein levels also had increased Dbf4 abundance, and some tumor cell lines had extra copies of the DBF4 gene. A high expression of Cdc7 protein was also detected in primary breast, colon, and lung tumors but not in the matched normal tissues. We also found a high correlation between p53 loss and increased CDC7 and DBF4 expression in primary breast cancers (P = 3.6 x 10(-9) and 1.8 x 10(-10), respectively) and in the cancer cell lines we studied. Therefore, increased Cdc7-Dbf4 abundance may be a common occurrence in human malignancies.

Genomic copy number alterations as predictive markers of systemic recurrence in breast cancer

We tried to establish models that predict systemic recurrence in breast cancer by selecting marker clones with DNA copy number alterations (CNAs) using an array comparative genomic hybridization (CGH). Array CGH containing 4,044 human bacterial artificial chromosome clones was used to assess CNAs in 62 primary breast cancer tissues from 31 patients with systemic recurrence within 5 years after surgery and clinicopathologically well matched 31 patients who had no evidence of disease for at least 5years. Fourteen significant clones (11 clones showing gain and 3 showing loss) were identified by systemic recurrence-free survival (SRFS) analysis and 23 significant clones (17 clones showing gain and 6 showing loss) identified by chi(2) test and FDR test were selected as predictive markers of systemic breast cancer recurrence. The significant CNAs were found in the chromosomal regions of 5p15.33, 11q13.3, 15q26.3, 17q25.3, 18q23 and 21q22.3 with gain and 9p12, 11q24.1 and 14q32.33 with loss. We devised 2 prediction models for the systemic recurrence of breast cancer based on the 14 clones and the 23 clones, respectively. The survivals of the patients were significantly separated according to the scores from each model at the optimal cut off values in SRFS and overall survival analysis. We found candidate clones and genes of which CNAs were significantly associated with systemic recurrence of breast cancer. The devised prediction models with these clones were effective at differentiating the recurrence and nonrecurrence.

Genome-wide DNA copy number predictors of lapatinib sensitivity in tumor-derived cell lines

A common aim of pharmacogenomic studies that use genome-wide assays on panels of cancers is the unbiased discovery of genomic alterations that are associated with clinical outcome and drug response. Previous studies of lapatinib, a selective dual-kinase inhibitor of epidermal growth factor receptor (EGFR) and HER2 tyrosine kinases, have shown predictable relationships between the activity of these target genes and response. Under the hypothesis that additional genes may play a role in drug sensitivity, a predictive model for lapatinib response was constructed from genome-wide DNA copy number data from 24 cancer cell lines. An optimal predictive model which consists of aberrations at nine distinct genetic loci, includes gains of HER2, EGFR, and loss of CDKN2A. This model achieved an area under the receiver operating characteristic curve of approximately 0.85 (80% confidence interval, 0.70-0.98; P < 0.01), and correctly classified the sensitivity status of 8 of 10 head and neck cancer cell lines. This study shows that biomarkers predictive for lapatinib sensitivity, including the previously described copy number gains of EGFR and HER2, can be discovered using novel genomic assays in an unbiased manner. Furthermore, these results show the utility of DNA copy number profiles in pharmacogenomic studies.

Gene expression profiles as biomarkers for the prediction of chemotherapy drug response in human tumour cells

Genome profiling approaches such as cDNA microarray analysis and quantitative reverse transcription polymerase chain reaction are playing ever-increasing roles in the classification of human cancers and in the discovery of biomarkers for the prediction of prognosis in cancer patients. Increasing research efforts are also being directed at identifying set of genes whose expression can be correlated with response to specific drugs or drug combinations. Such genes hold the prospect of tailoring chemotherapy regimens to the individual patient, based on tumour or host gene expression profiles. This review outlines recent advances and challenges in using genome profiling for the identification of tumour or host genes whose expression correlates with response to chemotherapy drugs both in vitro and in clinical studies. Genetic predictors of response to a variety of anticancer agents are discussed, including the anthracyclines, taxanes, topoisomerase I and II inhibitors, nucleoside analogs, alkylating agents, and vinca alkaloids.

Molecular cytogenetic characterization of doxorubicin-resistant neuroblastoma cell lines: evidence that acquired multidrug resistance results from a unique large amplification of the 7q21 region

Neuroblastoma is a heterogeneous neural crest-derived embryonic childhood neoplasm that is the second most common solid tumor found in children. Despite recent advances in combined therapy, the overall survival of patients with high-stage disease has not improved in the last decades. Treatment failure is in part attributed to multidrug resistance. To address the mechanisms involved in the development of multidrug resistance, we have generated two doxorubicin-resistant neuroblastoma cell lines (IGRN-91R and LAN-1R). These cells were shown to overexpress the MDR1 gene coding for the P-glycoprotein and were resistant to other MDR1- and non-MDR1-substrate drugs. Indeed, the MDR1 inhibitor verapamil only partially restored sensitivity to drugs, confirming that P-glycoprotein-mediated drug efflux was not responsible for 100% resistance. High-resolution and array-based comparative genomic hybridization analyses revealed the presence of an amplicon in the 7q21 region as the unique genomic alteration common to both doxorubicin-resistant cell lines. In addition to the MDR1 locus, this large amplified region is likely to harbor additional genes potentially involved in the development of drug resistance. This study represents the first molecular cytogenetic and genomic approach to identifying genomic regions involved in the multidrug-resistant phenotype of neuroblastoma. These results could lead to the identification of relevant target genes for the development of new therapeutic modalities.

Drug resistance in cancer

Cancer Research UK has recently sponsored a meeting, organized by the UK Medical Research Council, on cancer drug resistance. Several of the molecular mechanisms responsible for this clinical outcome, such as DNA interstrand crosslink repair, apoptosis evasion, cytochrome P450 and P-glycoprotein, were discussed. There was a special focus on leukaemia, breast and ovarian cancer, and the potential use of positron-emission tomography to study anticancer-drug resistance. The progress made in translating these findings to the clinic, like Gefitinib, P-glycoprotein phenotyping, or genome-wide analysis technology, was also discussed.

Alterations of β‐tubulin isotypes in breast cancer cells resistant to docetaxel

Docetaxel is one of the most active drugs used to treat breast cancer. The cellular target of docetaxel is the microtubule, specifically the beta-tubulin subunit, that comprises a series of isotypes and that can modulate function. This study has examined the role of alteration in beta-tubulin isotypes in vitro and has sequenced the beta-tubulin gene to determine if there were mutations, both of which may represent important mechanisms of acquired resistance to docetaxel. Breast cancer cells, MCF-7 (oestrogen-receptor positive) and MDA-MB-231, (oestrogen-receptor negative) were made resistant to docetaxel in vitro. Expression of beta-tubulin isotypes (class I, II, III, IVa, IVb, and VI) was determined at the RNA and protein level using RT-PCR and western analysis, respectively. DNA sequencing evaluated the beta-tubulin gene. At the mRNA level, class I, II, III, and IVa beta-tubulin mRNA isotypes were over-expressed in docetaxel-resistant MCF-7 cells when compared with the docetaxel-sensitive parental cells. However, class VI beta-tubulin mRNA isotype expression was decreased in resistant cells. In MDA-MB-231 cells, there was a decrease in expression of the class I and class IVa beta-tubulin mRNA. However, there were increased expressions in class II, IVb, and VI beta-tubulin mRNA isotypes in resistant cells. Western analysis has confirmed corresponding increases in beta-tubulin protein levels in MCF-7 cells. However, in MDA-MB-231 cells, there were decreased protein levels for class II and class III beta-tubulin. This study demonstrates that altered expression of mRNA beta-tubulin isotypes and modulation of beta-tubulin protein levels are associated with acquired docetaxel resistance in breast cancer cells. This allows further understanding and elucidation of mechanisms involved in resistance to docetaxel.