Clinical Role of Multidrug Resistance Protein 1 Expression in Chemotherapy Resistance in Early-Stage Breast Cancer: The Austrian Breast and Colorectal Cancer Study Group

CFTR reduces the proliferation of lung adenocarcinoma and is a strong predictor of survival in both smokers and non-smokers

Background

One of the main hurdles of oncological therapy is the development of drug resistance. The ABC transporter gene family contributes majorly to cancer chemoresistance. However, effects of somatic expression of most ABC transporters on cancer outcomes remain largely unclear.

Methods

We systematically analyzed expression signatures of all 48 human ABC transporters in samples from 8562 patients across 14 different cancer types. The association between CFTR (ABCC7) expression and outcomes was analyzed experimentally using knock-downs and pharmacological CFTR stimulation.

Results

Across 720 analyzed clinical associations with patient outcomes, 363 were nominally significant of which 29 remained significant after stringent Bonferroni correction. Among those were various previously known associations, as well as a multitude of novel factors that correlated with poor prognosis or predicted improved outcomes. The association between low CFTR levels and reduced survival in lung adenocarcinoma was confirmed in two independent cohorts of 246 patients with a history of smoking (logrank P = 0.0021, hazard ratio [HR], 0.49) and 143 never-smokers (logrank P = 0.0023, HR 0.31). Further in vitro experiments using naturally CFTR expressing lung adenocarcinoma cells showed that treatment with CFTR potentiators significantly reduced proliferation at therapeutically relevant concentrations.

Conclusions

These results suggest that CFTR acts as a pharmacologically activatable tumor suppressor and constitutes a promising target for adjuvant therapy in lung adenocarcinoma.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00432-022-04106-x.

A Prognostic and Carboplatin Response Predictive Model in Ovarian Cancer: A Mono-Institutional Retrospective Study Based on Clinics and Pharmacogenomics

Carboplatin is the cornerstone of ovarian cancer (OC) treatment, while platinum-response, dependent on interindividual variability, is the major prognostic factor for long-term outcomes. This retrospective study was focused on explorative search of genetic polymorphisms in the Absorption, Distribution, Metabolism, Excretion (ADME) genes for the identification of biomarkers prognostic/predictive of platinum-response in OC patients. Ninety-two advanced OC patients treated with carboplatin-based therapy were enrolled at our institution. Of these, we showed that 72% of patients were platinum-sensitive, with a significant benefit in terms of OS (p = 0.001). We identified an inflammatory-score with a longer OS in patients with lower scores as compared to patients with the maximum score (p = 0.001). Thirty-two patients were genotyped for 1931 single nucleotide polymorphisms (SNPs) and five copy number variations (CNVs) by the DMET Plus array platform. Among prognostic polymorphisms, we found a potential role of UGT2A1 both as a predictor of platinum-response (p = 0.01) and as prognostic of survival (p = 0.05). Finally, we identified 24 SNPs related to OS. UGT2A1 correlates to an "inflammatory-score" and retains a potential prognostic role in advanced OC. These data provide a proof of concept that warrants further validation in follow-up studies for the definition of novel biomarkers in this aggressive disease.

Targeting multidrug resistance-associated protein 1 (MRP1)-expressing cancers: Beyond pharmacological inhibition

Resistance to chemotherapy remains one of the most significant obstacles to successful cancer treatment. While inhibiting drug efflux mediated by ATP-binding cassette (ABC) transporters is a seemingly attractive and logical approach to combat multidrug resistance (MDR), small molecule inhibition of ABC transporters has so far failed to confer clinical benefit, despite considerable efforts by medicinal chemists, biologists, and clinicians. The long-sought treatment to eradicate cancers displaying ABC transporter overexpression may therefore lie within alternative targeting strategies. When aberrantly expressed, the ABC transporter multidrug resistance-associated protein 1 (MRP1, ABCC1) confers MDR, but can also shift cellular redox balance, leaving the cell vulnerable to select agents. Here, we explore the physiological roles of MRP1, the rational for targeting this transporter in cancer, the development of small molecule MRP1 inhibitors, and the most recent developments in alternative therapeutic approaches for targeting cancers with MRP1 overexpression. We discuss approaches that extend beyond simple MRP1 inhibition by exploiting the collateral sensitivity to glutathione depletion and ferroptosis, the rationale for targeting the shared transcriptional regulators of both MRP1 and glutathione biosynthesis, advances in gene silencing, and new molecules that modulate transporter activity to the detriment of the cancer cell. These strategies illustrate promising new approaches to address multidrug resistant disease that extend beyond the simple reversal of MDR and offer exciting routes for further research.

Predictive Value of MRP-1 in Localized High-Risk Soft Tissue Sarcomas: A Translational Research Associated to ISG-STS 1001 Randomized Phase III Trial

MRP-1 is implicated in multidrug resistance and was described as prognostic in high-risk patients with soft-tissue sarcoma (STS) in a previous study. The current research aimed to validate MRP-1 prognostic/predictive value in localized sarcomas treated with anthracyclines plus ifosfamide within the ISG-1001 phase III study. In addition, the inhibitory activity on MRP-1 was investigated in preclinical studies to identify new combinations able to increase the efficacy of standard chemotherapy in STS. MRP-1 expression was assessed by IHC in tissue microarrays from patients with STS and tested for correlation with disease-free survival (DFS) and overall survival (OS). In vitro studies tested the efficacy of MRP-1 inhibitors (nilotinib, ripretinib, selumetinib, and avapritinib) in sarcoma cell lines. The effect of combinations of the most active MRP-1 inhibitors and chemotherapy was measured on the basis of apoptosis. MRP-1 was evaluable in 231 of 264 cases who entered the study. MRP-1 expression (strong intensity) was independently associated with worse DFS [HR, 1.78; 95% confidence interval (CI), 1.11-2.83; P = 0.016], in the multivariate analysis, with a trend for a worse OS (HR, 1.78; 95% CI, 0.97-3.25; P = 0.062). In vitro studies showed that the addition of MRP-1 inhibitors (nilotinib or avapritinib) to doxorubicin plus palifosfamide, significantly increased cell death in SK-UT-1 and CP0024 cell lines. MRP-1 is an adverse predictive factor in localized high-risk patients with STS treated with neoadjuvant anthracyclines plus ifosfamide followed by surgery. In vitro findings support the clinical assessment of the combination of chemotherapy and MRP-1 inhibitors as a promising strategy to overcome the drug ceiling effect for chemotherapy.

MicroRNA-29b-3p promotes 5-fluorouracil resistance <em>via</em> suppressing TRAF5-mediated necroptosis in human colorectal cancer

Drug resistance in colorectal cancer is a great challenge in clinic. Elucidating the deep mechanism underlying drug resistance will bring much benefit to diagnosis, therapy and prognosis in patients with colorectal cancer. In this study, miR-29b-3p was shown to be involved in resistance to 5-fluorouracil (5-FU)-induced necroptosis of colorectal cancer. Further, miR-29b-3p was shown to target a regulatory subunit of necroptosis TRAF5. Rescue of TRAF5 could reverse the effect of miR-29b-3p on 5-FU-induced necroptosis, which was consistent with the role ofnecrostatin-1 (a specific necroptosis inhibitor). Then it was demonstrated that miR-29b-3p was positively correlated with chemo-resistance in colorectal cancer while TRAF5 negatively. In conclusion, it is deduced that miR-29b-3p/TRAF5 signaling axis plays critical role in drug resistance in chemotherapy for colorectal cancer patients by regulating necroptosis. The findings in this study provide us a new target for interfere therapy in colorectal cancer.

Drug Resistance in Metastatic Breast Cancer: Tumor Targeted Nanomedicine to the Rescue

Breast cancer, specifically metastatic breast, is a leading cause of morbidity and mortality in women. This is mainly due to relapse and reoccurrence of tumor. The primary reason for cancer relapse is the development of multidrug resistance (MDR) hampering the treatment and prognosis. MDR can occur due to a multitude of molecular events, including increased expression of efflux transporters such as P-gp, BCRP, or MRP1; epithelial to mesenchymal transition; and resistance development in breast cancer stem cells. Excessive dose dumping in chemotherapy can cause intrinsic anti-cancer MDR to appear prior to chemotherapy and after the treatment. Hence, novel targeted nanomedicines encapsulating chemotherapeutics and gene therapy products may assist to overcome cancer drug resistance. Targeted nanomedicines offer innovative strategies to overcome the limitations of conventional chemotherapy while permitting enhanced selectivity to cancer cells. Targeted nanotheranostics permit targeted drug release, precise breast cancer diagnosis, and importantly, the ability to overcome MDR. The article discusses various nanomedicines designed to selectively target breast cancer, triple negative breast cancer, and breast cancer stem cells. In addition, the review discusses recent approaches, including combination nanoparticles (NPs), theranostic NPs, and stimuli sensitive or “smart” NPs. Recent innovations in microRNA NPs and personalized medicine NPs are also discussed. Future perspective research for complex targeted and multi-stage responsive nanomedicines for metastatic breast cancer is discussed.

Roles of ABCC1 and ABCC4 in Proliferation and Migration of Breast Cancer Cell Lines

ABCC1 and ABCC4 utilize energy from ATP hydrolysis to transport many different molecules, including drugs, out of the cell and, as such, have been implicated in causing drug resistance. However recently, because of their ability to transport signaling molecules and inflammatory mediators, it has been proposed that ABCC1 and ABCC4 may play a role in the hallmarks of cancer development and progression, independent of their drug efflux capabilities. Breast cancer is the most common cancer affecting women. In this study, the aim was to investigate whether ABCC1 or ABCC4 play a role in the proliferation or migration of breast cancer cell lines MCF-7 (luminal-type, receptor-positive) and MDA-MB-231 (basal-type, triple-negative). The effects of small molecule inhibitors or siRNA-mediated knockdown of ABCC1 or ABCCC4 were measured. Colony formation assays were used to assess the clonogenic capacity, MTT assays to measure the proliferation, and scratch assays and Transwell assays to monitor the cellular migration. The results showed a role for ABCC1 in cellular proliferation, whilst ABCC4 appeared to be more important for cellular migration. ELISA studies implicated cAMP and/or sphingosine-1-phosphate efflux in the mechanism by which these transporters mediate their effects. However, this needs to be investigated further, as it is key to understand the mechanisms before they can be considered as targets for treatment.

Superior Pyrimidine Derivatives as Selective ABCG2 Inhibitors and Broad-Spectrum ABCB1, ABCC1, and ABCG2 Antagonists

In the search for highly effective modulators addressing ABCG2-mediated MDR, 23 pyrimidines were synthesized and biologically assessed. Seven derivatives with (a) nitrogen- and/or halogen-containing residue(s) had extraordinary potencies against ABCG2 (IC₅₀ < 150 nM). The compounds competitively inhibited ABCG2-mediated Hoechst 33342 transport but were not substrates of ABCG2. The most potent MDR reverser, compound 19, concentration-dependently increased SN-38-mediated cancer cell death at 11 nM (EC₅₀), time-dependently doubled SN-38 toxicity in a period of 7 days at 10 nM, and half-maximally accelerated cell death combined with SN-38 at 17 nM. No induction of ABCG2 was observed. Furthermore, 11 pyrimidines were revealed as triple ABCB1/ABCC1/ABCG2 inhibitors. Five possessed IC₅₀ values below 10 μM against each transporter, classifying them as some of the 50 most potent multitarget ABC transporter inhibitors. The most promising representative, compound 37, reversed ABCB1-, ABCC1-, and ABCG2-mediated MDR, making it one of the three most potent ABC transporter inhibitors and reversers of ABC transporters-mediated MDR.

Predictive Value of Molecular Subtypes in Premenopausal Women with Hormone Receptor-positive Early Breast Cancer: Results from the ABCSG Trial 5

PURPOSE

To assess the predictive value of molecular breast cancer subtypes in premenopausal patients with hormone receptor-positive early breast cancer who received adjuvant endocrine treatment or chemotherapy.

EXPERIMENTAL DESIGN

Molecular breast cancer subtypes were centrally assessed on whole tumor sections by IHC in patients of the Austrian Breast and Colorectal Cancer Study Group Trial 5 who had received either 5 years of tamoxifen/3 years of goserelin or six cycles of cyclophosphamide, methotrexate, and fluorouracil (CMF). Luminal A disease was defined as Ki67 <20% and luminal B as Ki67 ≥20%. The luminal B/HER2-positive subtype displayed 3+ HER2-IHC or amplification by ISH. Recurrence-free survival (RFS) and overall survival (OS) were analyzed using Cox models adjusted for clinical and pathologic factors.

RESULTS

185 (38%), 244 (50%), and 59 (12%) of 488 tumors were classified as luminal A, luminal B/HER2-negative and luminal B/HER2-positive, respectively. Luminal B subtypes were associated with poor outcome. Patients with luminal B tumors had a significantly shorter RFS [adjusted HR for recurrence: 2.22; 95% confidence interval (CI), 1.41-3.49; P = 0.001] and OS (adjusted HR for death: 3.51; 95% CI, 1.80-6.87; P < 0.001). No interaction between molecular subtypes and treatment was observed (test for interaction: P = 0.84 for RFS; P = 0.69 for OS).

CONCLUSIONS

Determination of molecular subtypes by IHC is an independent prognostic factor for recurrence and death in premenopausal women with early-stage, hormone receptor-positive breast cancer but is not predictive for outcome of adjuvant treatment with tamoxifen/goserelin or CMF.See related commentary by Hunter et al., p. 5543.

Characterization of the kinetic cycle of an ABC transporter by single-molecule and cryo-EM analyses

ATP-binding cassette (ABC) transporters are molecular pumps ubiquitous across all kingdoms of life. While their structures have been widely reported, the kinetics governing their transport cycles remain largely unexplored. Multidrug resistance protein 1 (MRP1) is an ABC exporter that extrudes a variety of chemotherapeutic agents and native substrates. Previously, the structures of MRP1 were determined in an inward-facing (IF) or outward-facing (OF) conformation. Here, we used single-molecule fluorescence spectroscopy to track the conformational changes of bovine MRP1 (bMRP1) in real time. We also determined the structure of bMRP1 under active turnover conditions. Our results show that substrate stimulates ATP hydrolysis by accelerating the IF-to-OF transition. The rate-limiting step of the transport cycle is the dissociation of the nucleotide-binding-domain dimer, while ATP hydrolysis per se does not reset MRP1 to the resting state. The combination of structural and kinetic data illustrates how different conformations of MRP1 are temporally linked and how substrate and ATP alter protein dynamics to achieve active transport.

Germline variant burden in multidrug resistance transporters is a therapy-specific predictor of survival in breast cancer patients

Multidrug resistance due to facilitated drug efflux mediated by ATP-binding cassette (ABC) transporters is a main cause for failure of cancer therapy. Genetic polymorphisms in ABC genes affect the disposition of chemotherapeutics and constitute important biomarkers for therapeutic response and toxicity. Here we correlated germline variability in ABC transporters with disease-specific survival (DSS) in 960 breast cancer (BRCA), 314 clear cell renal cell carcinoma and 325 hepatocellular carcinoma patients. We find that variant burden in ABCC1 is a strong predictor of DSS in BRCA patients, whereas candidate polymorphisms are not associated with DSS. This association is highly drug-specific for subgroups treated with the MRP1 substrates cyclophosphamide (log-rank p = 0.0011) and doxorubicin (log-rank p = 0.0088) independent of age and tumor stage, whereas no association was found in individuals treated with tamoxifen (log-rank p = 0.13). Structural mapping of significant variants revealed multiple variants at residues involved in protein stability, cofactor stabilization or substrate binding. Our results demonstrate that BRCA patients with high variant burden in ABCC1 are less prone to respond appropriately to pharmacological therapy with MRP1 substrates, thus incentivizing the consideration of genomic germline data for precision cancer medicine.

Overcoming transporter-mediated multidrug resistance in cancer: failures and achievements of the last decades

Multidrug resistance (MDR) is a complex phenomenon caused by numerous reasons in cancer chemotherapy. It is related to the abnormal tumor metabolism, precisely increased glycolysis and lactic acid production, extracellular acidification, and drug efflux caused by transport proteins. There are few strategies to increase drug delivery into cancer cells. One of them is the inhibition of carbonic anhydrases or certain proton transporters that increase extracellular acidity by proton extrusion from the cells. This prevents weakly basic chemotherapeutic drugs from ionization and increases their penetration through the cancer cell membrane. Another approach is the inhibition of MDR proteins that pump the anticancer agents into the extracellular milieu and decrease their intracellular concentration. Physical methods, such as ultrasound-mediated sonoporation, are being developed, as well. To increase the efficacy of sonoporation, various microbubbles are used. Ultrasound causes microbubble cavitation, i.e., periodical pulsation of the microbubble, and destruction which results in formation of temporary pores in the cellular membrane and increased permeabilization to drug molecules. This review summarizes the main approaches to reverse MDR related to the drug penetration along with its applications in preclinical and clinical studies.

Vielanin P enhances the cytotoxicity of doxorubicin via the inhibition of PI3K/Nrf2-stimulated MRP1 expression in MCF-7 and K562 DOX-resistant cell lines

BACKGROUND

Cancer cells that are resistant to structurally and mechanically unrelated anticancer drugs are said to have multidrug resistance (MDR). The overexpression of the ATP-binding cassette (ABC) transporter is one of the most important mechanisms of MDR. Vielanin P (VP), a dimeric guaiane from the leaves of Xylopia vielana, has the potential to reverse multidrug resistance.

PURPOSE

To evaluate the meroterpenoid compound VP as a low cytotoxicity MDR regulator and the related mechanisms.

METHODS

Cell viability was determined by CCK-8 and MTT assays. Apoptosis and the accumulation of doxorubicin (DOX) and 5(6)-carboxyfluorescein diacetate (CFDA) were determined by flow cytometry. We determined mRNA levels by quantitative real-time polymerase chain reaction (qRT-PCR). Protein levels were analyzed by Western blotting and immunofluorescence.

RESULTS

In the MCF-7 and K562 DOX-resistant cell lines, VP treatment (10 μM or 20 μM) enhanced the activity of chemotherapeutic agents. We found that VP selectively inhibited MRP1 mRNA but not MDR1 mRNA. VP enhanced DOX-induced apoptosis and reduced colony formation in the presence of DOX in drug-resistant cells. Moreover, VP increased the accumulation of DOX and the MRP1-specific substrate CFDA. In addition, VP reversed MRP1 protein levels and the accumulation of DOX and CFDA in MRP1-overexpressing MCF-7 and K562 cells. Thus, the mechanism of MDR reversal by VP is MRP1-dependent. Furthermore, we found that the inhibitory effect of VP on MRP1 is PI3K/Nrf2-dependent.

CONCLUSION

These results support the potential therapeutic value of VP as an MDR-reversal agent by inhibiting MRP1 via PI3K/Nrf2 signaling.

Synergistic effects of 7-O-geranylquercetin and siRNAs on the treatment of human breast cancer

AIMS

To investigate the antitumor effect of 7-O-geranylquercetin (GQ) combining with survivin siRNA (siSuvi) or IL-10 siRNA (siIL-10) to breast cancer.

MAIN METHODS

Xenograft tumor model was established by subcutaneously inoculating human breast cancer MCF-7 cells in BALB/c nude mice. Transfection efficiency of siRNA mediated by cationic liposome CDO14 in MCF-7 cells and tumor bearing mice was measured by flow cytometer and living imaging sysytem, respectively. Cell viability was detected using CCK-8 assay. Cell apoptosis was determined by Hoechst33342 staining and AV-PI staining. Tumors bearing mice were administered with GQ by gavage, and/or with liposome CDO14 mediated siRNAs via tail intravenous injection. Expression levels of proteins and cytokines were detected by western blot and ELISA, respectively.

KEY FINDINGS

Liposome CDO14 could deliver siRNA to tumor effectively. Combination of GQ and siSuvi promoted the antiproliferation and pro-apoptosis effects of GQ or siSuvi to MCF-7 cells, and reduced the level of survivin and raised the level of caspase-7 in cells. GQ combining with siSuvi inhibited the growth of tumor, down-regulated the expression of survivin and up-regulated the expression of caspase-7 in tumor tissue. Similarly, GQ combining with siIL-10 inhibited the growth of tumor, decreased the level of IL-10 and increased the level of TNF-α. These results revealed that GQ enhanced the pro-apoptosis effect of siSuvi on tumor cells and the modulating effect of siIL-10 on tumor microenvironment.

SIGNIFICANCES

Synergistic anti-tumor effect of GQ and siRNAs against breast cancer proved that chemical drugs combining with siRNAs is a promising antitumor strategy.

Mechanisms of Anticancer Drug Resistance in Hepatoblastoma

The most frequent liver tumor in children is hepatoblastoma (HB), which derives from embryonic parenchymal liver cells or hepatoblasts. Hepatocellular carcinoma (HCC), which rarely affects young people, causes one fourth of deaths due to cancer in adults. In contrast, HB usually has better prognosis, but this is still poor in 20% of cases. Although more responsive to chemotherapy than HCC, the failure of pharmacological treatment used before and/or after surgical resection is an important limitation in the management of patients with HB. To advance in the implementation of personalized medicine it is important to select the best combination among available anti-HB drugs, such as platinum derivatives, anthracyclines, etoposide, tyrosine-kinase inhibitors, Vinca alkaloids, 5-fluorouracil, monoclonal antibodies, irinotecan and nitrogen mustards. This requires predicting the sensitivity to these drugs of each tumor at each time because, it should be kept in mind, that cancer chemoresistance is a dynamic process of Darwinian nature. For this goal it is necessary to improve our understanding of the mechanisms of chemoresistance involved in the refractoriness of HB against the pharmacological challenge and how they evolve during treatment. In this review we have summarized the current knowledge on the multifactorial and complex factors responsible for the lack of response of HB to chemotherapy.

Over-expression of FSIP1 promotes breast cancer progression and confers resistance to docetaxel via MRP1 stabilization

Fibrous sheath-interacting protein 1 (FSIP1) functions centrally in breast carcinogenesis and progression, although its exact role remains to be clarified. Therefore, we sought to establish a correlation between the clinico-pathological features of breast cancer and FSIP1 expression in breast cancer tissues, as well as to validate its role in tumor progression and chemo-resistance. We analyzed FSIP1 expression in the breast cancer and para-tumor tissues by immunohistochemistry. We performed MTT, Caspase-Glo 3/7 Assay, Annexin V staining, wound healing and trans-well assays to evaluate cellular apoptosis, proliferation, migration and invasion in FSIP1 knockout and wild-type breast cancer cell lines. Additionally, we examined the effects of FSIP1 on docetaxel sensitivity in a nude mice model transplanted with control or FSIP1 knockout breast cancer cells, and also evaluate its role in tumor metastasis. FSIP1 and MRP1 interaction was determined by co-immunoprecipitation and mass spectrometry. We found that breast cancer cells and tissues consistently demonstrated elevated FSIP1 expressions, which correlated with poor overall survival. Notably, patients with high FSIP1 expression in their tumors undergoing docetaxel neoadjuvant chemotherapy had shorter disease-free survival. FSIP1 knockout in breast cancer cells significantly increased their sensitivity to docetaxel both in vitro and in vivo. Mechanistically, FSIP1 bound to the multidrug resistance protein 1 (MRP1) and stabilized it, and knocking out FSIP1 decreased MRP1 expression and increased cellular docetaxel accumulation. In sum, FSIP1 promotes breast carcinogenesis and mediates docetaxel resistance, and may serve as a novel target in the development of breast cancer therapies.

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

Calcitriol and Calcipotriol Modulate Transport Activity of ABC Transporters and Exhibit Selective Cytotoxicity in MRP1-overexpressing Cells

Efflux transporters P-glycoprotein (P-gp/ABCB1), multidrug resistance protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) can affect the efficacy and toxicity of a wide variety of drugs and are implicated in multidrug resistance (MDR). Eight test compounds, recently identified from an intramolecular FRET-based high throughput screening, were characterized for their interaction with MRP1. We report that the active metabolite of vitamin D₃, calcitriol, and its analog calcipotriol are selectively cytotoxic to MRP1-overexpressing cells, besides inhibiting transport function of P-gp, MRP1, and BCRP. Calcitriol and calcipotriol consistently displayed a potent inhibitory activity on MRP1-mediated doxorubicin and calcein efflux in MRP1-overexpressing H69AR and HEK293/MRP1 cells. Vesicular transport studies confirmed a strong inhibitory effect of calcitriol and calcipotriol on MRP1-mediated uptake of tritium-labeled estradiol glucuronide and leukotriene C₄. In cytotoxicity assays, MRP1-overexpressing cells exhibited hypersensitivity toward calcitriol and calcipotriol. Such collateral sensitivity, however, was not observed in HEK293/P-gp and HEK293/BCRP cells, although the vitamin D₃ analogs inhibited calcein efflux in P-gp-overexpressing cells, and mitoxantrone efflux in BCRP-overexpressing cells. The selective cytotoxicity of calcitriol and calpotriol toward MRP1 over-expressing cells can be eliminated with MRP1 inhibitor MK571. Our data indicate a potential role of calcitriol and its analogs in targeting malignancies in which MRP1 expression is prominent and contributes to MDR.

miR-145 sensitizes breast cancer to doxorubicin by targeting multidrug resistance-associated protein-1

Multidrug resistance-associated protein 1 (MRP1) is an important efflux transporter and overexpression of MRP1 usually leads to chemoresistance in breast cancer. Here, we found MRP1 overexpressed in human breast cancer tissues and breast cancer cell lines (compared with normal breast tissues and cell line, respectively). And MRP1 level increased in doxorubicin resistant MCF-7 cells compared with parental MCF-7 cells. Increasing evidences suggest microRNAs (miRNAs) influence chemotherapy response. We found miR-145 level decreased in human breast cancer tissues, breast cancer cell lines and doxorubicin resistant MCF-7 cells, and inversely correlated with MRP1 expression level. In the process of constructing MCF-7 doxorubicin resistant cell line, escalating doxorubicin markedly decreased miR-145 level, following by increased MRP1 level. Further study showed, miR-145 suppressed MRP1 expression by directly targeting MRP1 3'-untranslated regions. Overexpression of miR-145 sensitized breast cancer cells to doxorubicin in vitro and enhanced to doxorubicin chemotherapy in vivo through inducing intracellular doxorubicin accumulation via inhibiting MRP1. Taken together, our study revealed miR-145 sensitizes breast cancer to doxorubicin by targeting MRP1 and indicated the potential application in developing MRP1 inhibitor.

ATP Binding Enables Substrate Release from Multidrug Resistance Protein 1

The multidrug resistance protein MRP1 is an ATP-driven pump that confers resistance to chemotherapy. Previously, we have shown that intracellular substrates are recruited to a bipartite binding site when the transporter rests in an inward-facing conformation. A key question remains: how are high-affinity substrates transferred across the membrane and released outside the cell? Using electron cryomicroscopy, we show here that ATP binding opens the transport pathway to the extracellular space and reconfigures the substrate-binding site such that it relinquishes its affinity for substrate. Thus, substrate is released prior to ATP hydrolysis. With this result, we now have a complete description of the conformational cycle that enables substrate transfer in a eukaryotic ABC exporter.

Effective Drug Delivery in Diffuse Intrinsic Pontine Glioma: A Theoretical Model to Identify Potential Candidates

Despite decades of clinical trials for diffuse intrinsic pontine glioma (DIPG), patient survival does not exceed 10% at two years post-diagnosis. Lack of benefit from systemic chemotherapy may be attributed to an intact bloodbrain barrier (BBB). We aim to develop a theoretical model including relevant physicochemical properties in order to review whether applied chemotherapeutics are suitable for passive diffusion through an intact BBB or whether local administration via convection-enhanced delivery (CED) may increase their therapeutic potential. Physicochemical properties (lipophilicity, molecular weight, and charge in physiological environment) of anticancer drugs historically and currently administered to DIPG patients, that affect passive diffusion over the BBB, were included in the model. Subsequently, the likelihood of BBB passage of these drugs was ascertained, as well as their potential for intratumoral administration via CED. As only non-molecularly charged, lipophilic, and relatively small sized drugs are likely to passively diffuse through the BBB, out of 51 drugs modeled, only 8 (15%)-carmustine, lomustine, erlotinib, vismodegib, lenalomide, thalidomide, vorinostat, and mebendazole-are theoretically qualified for systemic administration in DIPG. Local administration via CED might create more therapeutic options, excluding only positively charged drugs and drugs that are either prodrugs and/or only available as oral formulation. A wide variety of drugs have been administered systemically to DIPG patients. Our model shows that only few are likely to penetrate the BBB via passive diffusion, which may partly explain the lack of efficacy. Drug distribution via CED is less dependent on physicochemical properties and may increase the therapeutic options for DIPG.

Selective Fusion of Heterogeneous Classifiers for Predicting Substrates of Membrane Transporters

Membrane transporters play a crucial role in determining fate of administered drugs in a biological system. Early identification of plausible transporters for a drug molecule can provide insights into its therapeutic, pharmacokinetic, and toxicological profiles. In the present study, predictive models for classifying small molecules into substrates and nonsubstrates of various pharmaceutically important membrane transporters were developed using quantitative structure-activity relationship (QSAR) and proteochemometric (PCM) approaches. For this purpose, 4575 substrate interactions for these transporters were collected from the Metabolism and Transport Database (Metrabase) and the literature. The transporters selected for this study include (i) six efflux transporters, viz., breast cancer resistance protein (BCRP/ABCG2), P-glycoprotein (P-gp/MDR1), and multidrug resistance proteins (MRP1, MRP2, MRP3, and MRP4), and (ii) seven influx transporters, viz., organic cation transporter (OCT1/SO22A1), peptide transporter (PEPT1/SO15A1), apical sodium-bile acid transporter (ASBT/NTCP2), and organic anion transporting peptides (OATP1A2/SO1A2, OATP1B/SO1B1, OATP1B3/SO1B3, and OATP2B1/SO2B1). Various types of descriptors and machine learning methods (classifiers) were evaluated for the development of robust predictive models. Additionally, ensemble models were developed by bagging of homogeneous classifiers and selective fusion of heterogeneous classifiers. It was observed that the latter approach improves the accuracy of substrate/nonsubstrate prediction for transporters (average correct classification rate of more than 0.80 for external validation). Moreover, structural fragments important in determining the substrate specificity across the various transporters were identified. To demonstrate these fragments on the query molecule, contour maps were generated. The prediction efficacy of the developed models was illustrated by a good correlation between the reported logBB value of a molecule and its predicted substrate propensity for blood-brain barrier transporters. Conclusively, this comprehensive modeling analysis can be efficiently employed for the prediction of membrane transporters of a drug, thereby providing insights into its pharmacological profile.

High-content screening of clinically tested anticancer drugs identifies novel inhibitors of human MRP1 (ABCC1)

Multidrug resistance protein 1 (MRP1/ABCC1), an integral transmembrane efflux transporter, belongs to the ATP-binding cassette (ABC) protein superfamily. MRP1 governs the absorption and disposition of a wide variety of endogenous and xenobiotic substrates including various drugs across organs and physiological barriers. Additionally, its overexpression has been implicated in multidrug resistance in chemotherapy of multiple cancers. Here, we describe the development of a high content imaging-based screening assay for MRP1 activity. This live cell-based automated microscopy assay is very robust and allows simultaneous detection of cell permeable, non-toxic and potent inhibitors. The validity of the assay was demonstrated by profiling a library of 386 anti-cancer compounds, which are under clinical trials, for interactions with MRP1. The assay identified 12 potent inhibitors including two known MRP1 inhibitors, cyclosporine A and rapamycin. On the other hand, MRP1-inhibitory activity of tipifarnib, AZD1208, deforolimus, everolimus, temsirolimus, HS-173, YM201636, ESI-09, TAK-733, and CX-6258 has not been previously reported. Inhibition of MRP1 activity was further validated using flow cytometry and confocal microscopy for the respective detection of calcein and doxorubicin in MRP1-overexpressing cells. Among the identified compounds, tipifarnib, AZD1208, rapamycin, deforolimus, everolimus, TAK-733, and temsirolimus resensitized MRP1-overexpressing H69AR cells towards vincristine, a cytotoxic chemotherapeutic agent, by 2-6-fold. Using purified HEK293 membrane vesicles overexpressing MRP1, MRP2, MRP3, and MRP4, we also demonstrated that the identified compounds exert differential and selective response on the uptake of estradiol glucuronide, an endogenous MRP substrate. In summary, we demonstrated the effectiveness of the high content imaging-based high-throughput assay for profiling compound interaction with MRP1.

The role of multidrug resistance protein (MRP-1) as an active efflux transporter on blood-brain barrier (BBB) permeability

Drugs acting on central nervous system (CNS) may take longer duration to reach the market as these compounds have a higher attrition rate in clinical trials due to the complexity of the brain, side effects, and poor blood-brain barrier (BBB) permeability compared to non-CNS-acting compounds. The roles of active efflux transporters with BBB are still unclear. The aim of the present work was to develop a predictive model for BBB permeability that includes the MRP-1 transporter, which is considered as an active efflux transporter. A support vector machine model was developed for the classification of MRP-1 substrates and non-substrates, which was validated with an external data set and Y-randomization method. An artificial neural network model has been developed to evaluate the role of MRP-1 on BBB permeation. A total of nine descriptors were selected, which included molecular weight, topological polar surface area, ClogP, number of hydrogen bond donors, number of hydrogen bond acceptors, number of rotatable bonds, P-gp, BCRP, and MRP-1 substrate probabilities for model development. We identified 5 molecules that fulfilled all criteria required for passive permeation of BBB, but they all have a low logBB value, which suggested that the molecules were effluxed by the MRP-1 transporter.

MRP1 and its role in anticancer drug resistance

The phenomenon of multidrug resistance (MDR) in cancer is associated with the overexpression of the ATP-binding cassette (ABC) transporter proteins, including multidrug resistance-associated protein 1 (MRP1) and P-glycoprotein. MRP1 plays an active role in protecting cells by its ability to efflux a vast array of drugs to sub-lethal levels. There has been much effort in elucidating the mechanisms of action, structure and substrates and substrate binding sites of MRP1 in the last decade. In this review, we detail our current understanding of MRP1, its clinical relevance and highlight the current environment in the search for MRP1 inhibitors. We also look at the capacity for the rapid intercellular transfer of MRP1 phenotype from spontaneously shed membrane vesicles known as microparticles and discuss the clinical and therapeutic significance of this in the context of cancer MDR.

Exercise training does not affect anthracycline antitumor efficacy while attenuating cardiac dysfunction

Highly effective anthracyclines, like doxorubicin (DOX), have limited clinical use due to protracted cardiotoxic effects. While exercise is known to be cardioprotective, it is unclear whether exercise compromises chemotherapy treatment efficacy. To determine the effect of exercise training on DOX antitumor efficacy as well as DOX-induced cardiotoxicity, female Fisher 344 rats were randomly assigned to sedentary + saline (SED+SAL), SED+DOX, wheel run exercise training + SAL (WR+SAL), or WR+DOX. On week 11, animals were inoculated with 1×10(6) MatBIII tumor cells. Once tumors reached ∼1 cm in diameter, animals were treated with 12 mg/kg of DOX or SAL. Animals were killed 1, 3, or 5 days following treatment. Tumor growth and cardiac function were measured at each interval. DOX accumulation and multidrug resistance protein (MRP) expression were quantified in tumor and heart tissue. No significant difference (P > 0.05) existed between DOX-treated SED and WR groups for tumor measurements. Exercise preserved cardiac function up to 5 days following DOX treatment. Exercise reduced ventricular DOX accumulation and upregulated ventricular MPR1 and MPR2. In contrast, no differences were observed in DOX accumulation or MRP expression in tumors of SED and WR animals. Endurance exercise had no effect on DOX antitumor efficacy as evidenced by a definitive DOX-induced reduction in tumor growth in both the SED and WR groups. Although exercise did not affect the antitumor efficacy of DOX, it still provided protection against cardiac dysfunction. These effects may be mediated by the degree of DOX tissue accumulation secondary to the regulation of MRP expression.

Chimeric MicroRNA-1291 Biosynthesized Efficiently in Escherichia coli Is Effective to Reduce Target Gene Expression in Human Carcinoma Cells and Improve Chemosensitivity

In contrast to the growing interests in studying noncoding RNAs (ncRNAs) such as microRNA (miRNA or miR) pharmacoepigenetics, there is a lack of efficient means to cost effectively produce large quantities of natural miRNA agents. Our recent efforts led to a successful production of chimeric pre-miR-27b in bacteria using a transfer RNA (tRNA)-based recombinant RNA technology, but at very low expression levels. Herein, we present a high-yield expression of chimeric pre-miR-1291 in common Escherichia coli strains using the same tRNA scaffold. The tRNA fusion pre-miR-1291 (tRNA/mir-1291) was then purified to high homogeneity using affinity chromatography, whose primary sequence and post-transcriptional modifications were directly characterized by mass spectrometric analyses. Chimeric tRNA/mir-1291 was readily processed to mature miR-1291 in human carcinoma MCF-7 and PANC-1 cells. Consequently, recombinant tRNA/mir-1291 reduced the protein levels of miR-1291 target genes, including ABCC1, FOXA2, and MeCP2, as compared with cells transfected with the same doses of control methionyl-tRNA scaffold with a sephadex aptamer (tRNA/MSA). In addition, tRNA-carried pre-miR-1291 suppressed the growth of MCF-7 and PANC-1 cells in a dose-dependent manner, and significantly enhanced the sensitivity of ABCC1-overexpressing PANC-1 cells to doxorubicin. These results indicate that recombinant miR-1291 agent is effective in the modulation of target gene expression and chemosensitivity, which may provide insights into high-yield bioengineering of new ncRNA agents for pharmacoepigenetics research.

xCT, not just an amino-acid transporter: a multi-functional regulator of microbial infection and associated diseases

Expression of xCT, a component of the x_c^– amino-acid transporter, is essential for the uptake of cystine required for intracellular glutathione (GSH) synthesis and maintenance of the intracellular redox balance. Therefore, xCT plays an important role not only in the survival of somatic and immune cells, but also in other aspects of tumorigenesis, including the growth and malignant progression of cancer cells, resistance to anticancer drugs, and protection of normal cells against oxidative damage induced by carcinogens. xCT also functions as a factor required for infection by Kaposi’s sarcoma-associated herpesvirus (KSHV), the causative agent of Kaposi’s sarcoma (KS) and other lymphoproliferative diseases associated with HIV/AIDS. In spite of these advances, our understanding of the role of xCT in the pathogenesis of infectious diseases is still limited. Therefore, this review will summarize recent findings about the functions of xCT in diseases associated with microbial (bacterial or viral) infections, in particular KSHV-associated malignancies. We will also discuss the remaining questions, future directions, as well as evidence that supports the potential benefits of exploring system x_c^– as a target for prevention and clinical management of microbial diseases and cancer.

Role of ABC transporters in fluoropyrimidine-based chemotherapy response

Since over 50 years, 5-fluorouracil (5-FU) is in use as backbone of chemotherapy treatment regimens for a wide range of cancers including colon, breast, and head and neck carcinomas. However, drug resistance and severe toxicities such as mucositis, diarrhea, neutropenia, and vomiting in up to 40% of treated patients often lead to dose limitation or treatment discontinuation. Because the oral bioavailability of 5-FU is unpredictable and highly variable, 5-FU is commonly administered intravenously. To overcome medical complications and inconvenience associated with intravenous administration, the oral prodrugs capecitabine and tegafur have been developed. Both fluoropyrimidines are metabolically converted intracellularly to 5-FU, which then needs metabolic activation to exert its damaging activity on RNA and DNA. The low response rates of 10-15% of 5-FU monotherapy can be improved by combination regimens of infusional 5-FU and leucovorin together with oxaliplatin (FOLFOX) or irinotecan (FOLFIRI), thereby increasing response rates to 30-40%. The impact of metabolizing enzymes in the development of fluoropyrimidine toxicity and resistance has been studied in great detail. In addition, membrane drug transporters, which are critical determinants of intracellular drug concentrations, may play a role in occurrence of toxicity and development of resistance against fluoropyrimidine-based therapy as well. This review therefore summarizes current knowledge on the role of drug transporters with particular focus on ATP-binding cassette transporters in fluoropyrimidine-based chemotherapy response.

Non-coding polymorphisms in nucleotide binding domain 1 in ABCC1 gene associate with transcript level and survival of patients with breast cancer

OBJECTIVES

ATP-Binding Cassette (ABC) transporters may cause treatment failure by transporting of anticancer drugs outside of the tumor cells. Multidrug resistance-associated protein 1 coded by the ABCC1 gene has recently been suggested as a potential prognostic marker in breast cancer patients. This study aimed to explore tagged haplotype covering nucleotide binding domain 1 of ABCC1 in relation with corresponding transcript levels in tissues and clinical phenotype of breast cancer patients.

METHODS

The distribution of twelve ABCC1 polymorphisms was assessed by direct sequencing in peripheral blood DNA (n = 540).

RESULTS

Tumors from carriers of the wild type genotype in rs35623 or rs35628 exhibited significantly lower levels of ABCC1 transcript than those from carriers of the minor allele (p = 0.003 and p = 0.004, respectively). The ABCC1 transcript levels significantly increased in the order CT-GT>CC-GT>CC-GG for the predicted rs35626-rs4148351 diplotype. Chemotherapy-treated patients carrying the T allele in rs4148353 had longer disease-free survival than those with the GG genotype (p = 0.043). On the other hand, hormonal therapy-treated patients with the AA genotype in rs35628 had significantly longer disease-free survival than carriers of the G allele (p = 0.012).

CONCLUSIONS

Taken together, our study shows that genetic variability in the nucleotide binding domain 1 has a significant impact on the ABCC1 transcript level in the target tissue and may modify survival of breast cancer patients.

Targeting xCT, a cystine-glutamate transporter induces apoptosis and tumor regression for KSHV/HIV-associated lymphoma

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the etiological agent of primary effusion lymphoma (PEL), which represents a rapidly progressing malignancy arising in HIV-infected patients. Conventional chemotherapy for PEL treatment induces unwanted toxicity and is ineffective — PEL continues to portend nearly 100% mortality within a period of months, which requires novel therapeutic strategies. The amino acid transporter, xCT, is essential for the uptake of cystine required for intracellular glutathione (GSH) synthesis and for maintaining the intracellular redox balance. Inhibition of xCT induces growth arrest in a variety of cancer cells, although its role in virus-associated malignancies including PEL remains unclear. In the current study, we identify that xCT is expressed on the surface of patient-derived KSHV⁺ PEL cells, and targeting xCT induces caspase-dependent cell apoptosis. Further experiments demonstrate the underlying mechanisms including host and viral factors: reducing intracellular GSH while increasing reactive oxygen species (ROS), repressing cell-proliferation-related signaling, and inducing viral lytic genes. Using an immune-deficient xenograft model, we demonstrate that an xCT selective inhibitor, Sulfasalazine (SASP), prevents PEL tumor progression in vivo. Together, our data provide innovative and mechanistic insights into the role of xCT in PEL pathogenesis, and the framework for xCT-focused therapies for AIDS-related lymphoma in future.

Importance of ABCC1 for cancer therapy and prognosis

Multidrug resistance presents one of the most important causes of cancer treatment failure. Numerous in vitro and in vivo data have made it clear that multidrug resistance is frequently caused by enhanced expression of ATP-binding cassette (ABC) transporters. ABC transporters are membrane-bound proteins involved in cellular defense mechanisms, namely, in outward transport of xenobiotics and physiological substrates. Their function thus prevents toxicity as carcinogenesis on one hand but may contribute to the resistance of tumor cells to a number of drugs including chemotherapeutics on the other. Within 48 members of the human ABC superfamily there are several multidrug resistance-associated transporters. Due to the well documented susceptibility of numerous drugs to efflux via ABC transporters it is highly desirable to assess the status of ABC transporters for individualization of treatment by their substrates. The multidrug resistance associated protein 1 (MRP1) encoded by ABCC1 gene is one of the most studied ABC transporters. Despite the fact that its structure and functions have already been explored in detail, there are significant gaps in knowledge which preclude clinical applications. Tissue-specific patterns of expression and broad genetic variability make ABCC1/MRP1 an optimal candidate for use as a marker or member of multi-marker panel for prediction of chemotherapy resistance. The purpose of this review was to summarize investigations about associations of gene and protein expression and genetic variability with prognosis and therapy outcome of major cancers. Major advances in the knowledge have been identified and future research directions are highlighted.

DualChip®microarray as a new tool in cancer research

Over the last 5 years, the emergence of gene expression profiling using high-density DNA microarrays led to a better understanding of tumor development and identified new prognostic markers. However, high-density microarrays failed to leap from the researcher's bench to the clinical practice due to their cost, data management and lack of standardization. DualChip low-density DNA microarrays were developed as a new flexible tool that is able to reliably quantify the expression of a limited number of genes of clinical relevance. This review will illustrate how DualChip technology can be applied to tumor diagnosis and tumor-acquired drug resistance.

Metabolism and transport of tamoxifen in relation to its effectiveness: new perspectives on an ongoing controversy

Tamoxifen reduces the rate of breast cancer recurrence by approximately a half. Tamoxifen is metabolized to more active metabolites by enzymes encoded by polymorphic genes, including cytochrome P450 2D6 (CYP2D6). Tamoxifen is a substrate for ATP-binding cassette transporter proteins. We review tamoxifen's clinical pharmacology and use meta-analyses to evaluate the clinical epidemiology studies conducted to date on the association between CYP2D6 inhibition and tamoxifen effectiveness. Our findings indicate that the effect of both drug-induced and/or gene-induced inhibition of CYP2D6 activity is likely to be null or small, or at most moderate in subjects carrying two reduced function alleles. Future research should examine the effect of polymorphisms in genes encoding enzymes in tamoxifen's complete metabolic pathway, should comprehensively evaluate other biomarkers that affect tamoxifen effectiveness, such as the transport enzymes, and focus on subgroups of patients, such as premenopausal breast cancer patients, for whom tamoxifen is the only guideline endocrine therapy.

Small nucleolar RNA-derived microRNA hsa-miR-1291 modulates cellular drug disposition through direct targeting of ABC transporter ABCC1

Multidrug resistance-associated protein 1 (MRP1/ABCC1) is an important membrane transporter that contributes to cellular disposition of many endobiotic and xenobiotic agents, and it can also confer multidrug resistance. This study aimed to investigate the role of human noncoding microRNA-1291 (hsa-miR-1291) in regulation of ABCC1 and drug disposition. Bioinformatics analyses indicated that hsa-miR-1291, localized within the small nucleolar RNA H/ACA box 34 (SNORA34), might target ABCC1 3'-untranslated region (3'UTR). Using splinted ligation small RNA detection method, we found that SNORA34 was processed into hsa-miR-1291 in human pancreatic carcinoma PANC-1 cells. Luciferase reporter assays showed that ABCC1 3'-UTR-luciferase activity was decreased by 20% in cells transfected with hsa-miR-1291 expression plasmid, and increased by 40% in cells transfected with hsa-miR-1291 antagomir. Furthermore, immunoblot study revealed that ABCC1 protein expression was sharply reduced in hsa-miR-1291-stably transfected PANC-1 cells, which was attenuated by hsa-miR-1291 antagomir. The change of ABCC1 protein expression was associated with an alternation in mRNA expression. In addition, hsa-miR-1291-directed downregulation of ABCC1 led to a greater intracellular drug accumulation and sensitized the cells to doxorubicin. Together, our results indicate that hsa-miR-1291 is derived from SNORA34 and modulates cellular drug disposition and chemosensitivity through regulation of ABCC1 expression. These findings shall improve the understanding of microRNA-controlled epigenetic regulatory mechanisms underlying multidrug resistance and interindividual variability in pharmacokinetics.

The expression profile of ATP-binding cassette transporter genes in breast carcinoma

AIM

ATP-binding cassette (ABC) transporters contribute to development of resistance to anticancer drugs via ATP-dependent drug efflux. A major goal of our study was to investigate associations between the expression of ABC transporters and outcome of breast carcinoma patients.

PATIENTS & METHODS

Transcript levels of all 49 human ABC transporters were determined in post-treatment tumor and non-neoplastic tissue samples from 68 breast carcinoma patients treated by neoadjuvant chemotherapy. Six ABC transporters were then evaluated in independent series of 100 pretreatment patients.

RESULTS

ABCA5/6/8/9/10, ABCB1/5/11, ABCC6/9, ABCD2/4, ABCG5 and ABCG8 were significantly downregulated and ABCA2/3/7/12, ABCB2/3/8/9/10, ABCC1/4/5/10/11/12, ABCD1/3, ABCE1, ABCF1/2/3 and ABCG1 were upregulated in post-treatment tumors compared with non-neoplastic tissues. Significant associations of intratumoral levels of ABCC1 and ABCC8 with grade and expression of hormonal receptors were found in both sets of patients. ABCA12, ABCA13 and ABCD2 levels were significantly associated with the response to neoadjuvant chemotherapy in post-treatment patients. Protein expression of ABCA12, ABCC8 and ABCD2 in tumor tissues of patients with breast carcinoma was observed by immunoblotting for the first time.

CONCLUSION

ABCA12, ABCA13, ABCC1, ABCC8 and ABCD2 present potential modifiers of progression and response to the chemotherapy of breast carcinoma.

Virtual screening of ABCC1 transporter nucleotidebinding domains as a therapeutic target in multidrug resistant cancer

ABCC1 is a member of the ATP-binding Cassette super family of transporters, actively effluxes xenobiotics from cells. Clinically, ABCC1 expression is linked to cancer multidrug resistance. Substrate efflux is energised by ATP binding and hydrolysis at the nucleotide-binding domains (NBDs) and inhibition of these events may help combat drug resistance. The aim of this study is to identify potential inhibitors of ABCC1 through virtual screening of National Cancer Institute (NCI) compounds. A threedimensional model of ABCC1 NBD2 was generated using MODELLER whilst the X-ray crystal structure of ABCC1 NBD1 was retrieved from the Protein Data Bank. A pharmacophore hypothesis was generated based on flavonoids known to bind at the NBDs using PHASE, and used to screen the NCI database. GLIDE was employed in molecular docking studies for all hit compounds identified by pharmacophore screening. The best potential inhibitors were identified as compounds possessing predicted binding affinities greater than ATP. Approximately 5% (13/265) of the hit compounds possessed lower docking scores than ATP in ABCC1 NBD1 (NSC93033, NSC662377, NSC319661, NSC333748, NSC683893, NSC226639, NSC94231, NSC55979, NSC169121, NSC166574, NSC73380, NSC127738, NSC115534), whereas approximately 7% (7/104) of docked NCI compounds were predicted to possess lower docking scores than ATP in ABCC1 NBD2 (NSC91789, NSC529483, NSC211168, NSC318214, NSC116519, NSC372332, NSC526974). Analyses of docking orientations revealed P-loop residues of each NBD and the aromatic amino acids Trp653 (NBD1) and Tyr1302 (NBD2) were key in interacting with high-affinity compounds. On the basis of docked orientation and docking score the compounds identified may be potential inhibitors of ABCC1 and require further pharmacological analysis.

ABBREVIATIONS

ABC - ATP-binding cassette, DHS - dehydrosilybin, MDR - multidrug resistance, NBD - nucleotide-binding domain, PDB - protein data bank.

Oxazaphosphorine bioactivation and detoxification The role of xenobiotic receptors

Oxazaphosphorines, with the most representative members including cyclophosphamide, ifosfamide, and trofosfamide, constitute a class of alkylating agents that have a broad spectrum of anticancer activity against many malignant ailments including both solid tumors such as breast cancer and hematological malignancies such as leukemia and lymphoma. Most oxazaphosphorines are prodrugs that require hepatic cytochrome P450 enzymes to generate active alkylating moieties before manifesting their chemotherapeutic effects. Meanwhile, oxazaphosphorines can also be transformed into non-therapeutic byproducts by various drug-metabolizing enzymes. Clinically, oxazaphosphorines are often administered in combination with other chemotherapeutics in adjuvant treatments. As such, the therapeutic efficacy, off-target toxicity, and unintentional drug-drug interactions of oxazaphosphorines have been long-lasting clinical concerns and heightened focuses of scientific literatures. Recent evidence suggests that xenobiotic receptors may play important roles in regulating the metabolism and clearance of oxazaphosphorines. Drugs as modulators of xenobiotic receptors can affect the therapeutic efficacy, cytotoxicity, and pharmacokinetics of coadministered oxazaphosphorines, providing a new molecular mechanism of drug-drug interactions. Here, we review current advances regarding the influence of xenobiotic receptors, particularly, the constitutive androstane receptor, the pregnane X receptor and the aryl hydrocarbon receptor, on the bioactivation and detoxification of oxazaphosphorines, with a focus on cyclophosphamide and ifosfamide.

Taxane resistance in breast cancer: mechanisms, predictive biomarkers and circumvention strategies

BACKGROUND

Taxanes are established in the treatment of metastatic breast cancer (MBC) and early breast cancer (EBC) as potent chemotherapy agents. However, their therapeutic usefulness is limited by de-novo refractoriness or acquired resistance, which are common drawbacks to most anti-cancer cytotoxics. Considering that the taxanes will remain principle chemotherapeutic agents for the treatment of breast cancer, we reviewed known mechanisms of resistance in with an outlook of optimizing their clinical use.

METHODS

We searched the PubMed and MEDLINE databases for articles (from inception through to 9th January 2012; last search 10/01/2012) and journals known to publish information relevant to taxane chemotherapy. We imposed no language restrictions. Search terms included: cancer, breast cancer, response, resistance, taxane, paclitaxel, docetaxel, taxol. Due to the possibility of alternative mechanisms of resistance all combination chemotherapy treated data sets were removed from our overview.

RESULTS

Over-expression of the MDR-1 gene product Pgp was extensively studied in vitro in association with taxane resistance, but data are conflicting. Similarly, the target components microtubules, which are thought to mediate refractoriness through alterations of the expression pattern of tubulins or microtubule associated proteins and the expression of alternative tubulin isoforms, failed to confirm such associations. Little consensus has been generated for reported associations between taxane-sensitivity and mutated p53, or taxane-resistance and overexpression of Bcl-2, Bcl-xL or NFkB. In contrary sufficient in vitro data support an association of spindle assembly checkpoint (SAC) defects with resistance. Clinical data have been limited and inconsistent, which relate to the variety of methods used, lack of standardization of cut-offs for quantitation, differences in clinical endpoints measured and in methods of tissue collection preparation and storage, and study/patient heterogeneity. The most prominent finding is that pharmaceutical down-regulation of HER-2 appears to reverse the taxane resistance.

CONCLUSIONS

Currently no valid practical biomarkers exist that can predict resistance to the taxanes in breast cancer supporting the principle of individualized cancer therapy. The incorporation of several biomarker analyses into prospectively designed studies in this setting are needed.

Targeting MDR in breast and lung cancer: discriminating its potential importance from the failure of drug resistance reversal studies

This special issue of Drug Resistance Updates is dedicated to multidrug resistance protein 1 (MDR-1), 35 years after its discovery. While enormous progress has been made and our understanding of drug resistance has become more sophisticated and nuanced, after 35 years the role of MDR-1 in clinical oncology remains a work in progress. Despite clear in vitro evidence that P-glycoprotein (Pgp), encoded by MDR-1, is able to dramatically reduce drug concentrations in cultured cells, and that drug accumulation can be increased by small molecule inhibitors, clinical trials testing this paradigm have mostly failed. Some have argued that it is no longer worthy of study. However, repeated analyses have demonstrated MDR-1 expression in a tumor is a poor prognostic indicator leading some to conclude MDR-1 is a marker of a more aggressive phenotype, rather than a mechanism of drug resistance. In this review we will re-evaluate the MDR-1 story in light of our new understanding of molecular targeted therapy, using breast and lung cancer as examples. In the end we will reconcile the data available and the knowledge gained in support of a thesis that we understand far more than we realize, and that we can use this knowledge to improve future therapies.

The controversial role of ABC transporters in clinical oncology

The phenomenon of multidrug resistance in cancer is often associated with the overexpression of the ABC (ATP-binding cassette) transporters Pgp (P-glycoprotein) (ABCB1), MRP1 (multidrug resistance-associated protein 1) (ABCC1) and ABCG2 [BCRP (breast cancer resistance protein)]. Since the discovery of Pgp over 35 years ago, studies have convincingly linked ABC transporter expression to poor outcome in several cancer types, leading to the development of transporter inhibitors. Three generations of inhibitors later, we are still no closer to validating the 'Pgp hypothesis', the idea that increased chemotherapy efficacy can be achieved by inhibition of transporter-mediated efflux. In this chapter, we highlight the difficulties and past failures encountered in the development of clinical inhibitors of ABC transporters. We discuss the challenges that remain in our effort to exploit decades of work on ABC transporters in oncology. In learning from past mistakes, it is hoped that ABC transporters can be developed as targets for clinical intervention.

Regulation of miR-19 to breast cancer chemoresistance through targeting PTEN

PURPOSE

To explore whether miR-19 is involved in the regulation of multidrug resistance (MDR), one of the main causes of breast cancer mortality, and modulates sensitivity of tumor cells to chemotherapeutic agents.

METHODS

We analyzed miRNA expression levels in three MDR cell lines in comparison with their parent cell line, MCF-7, using a miRNA microarray. We investigated whether inhibitor of miR-19 sensitized MDR cells to chemotherapeutic agents in vitro and in vivo.

RESULTS

MiR-19 was overexpressed in all three MDR cell lines compared to their parental cell line, MCF-7. Expression levels of miR-19 in MDR cells were inversely consistent with those of PTEN. Inhibitor of miR-19a restored sensitivity of MDR cells to cytotoxic agents; administration of LNA-antimiR-19a, a chemo-modified miR-19a inhibitor, sensitized MDR cells to chemotherapeutic agents in vivo.

CONCLUSION

Our findings demonstrate, for the first time, involvement of miR-19 in multidrug resistance through modulation of PTEN and suggest that miR-19 may be a potential target for preventing and reversing MDR in tumor cells.

Multidrug resistance in breast cancer: from in vitro models to clinical studies

The development of multidrug resistance (MDR) and subsequent relapse on therapy is a widespread problem in breast cancer, but our understanding of the underlying molecular mechanisms is incomplete. Numerous studies have aimed to establish the role of drug transporter pumps in MDR and to link their expression to response to chemotherapy. The ATP-binding cassette (ABC) transporters are central to breast cancer MDR, and increases in ABC expression levels have been shown to correlate with decreases in response to various chemotherapy drugs and a reduction in overall survival. But as there is a large degree of redundancy between different ABC transporters, this correlation has not been seen in all studies. This paper provides an introduction to the key molecules associated with breast cancer MDR and summarises evidence of their potential roles reported from model systems and clinical studies. We provide possible explanations for why despite several decades of research, the precise role of ABC transporters in breast cancer MDR remains elusive.

Reversal of multidrug resistance by small interfering RNA (siRNA) in doxorubicin-resistant MCF-7 breast cancer cells

PURPOSE

Resistance to anticancer drugs is a serious obstacle to cancer chemotherapy. A common form of multidrug resistance (MDR) is caused by the overexpression of transmembrane transporter proteins P-glycoprotein (P-gp) and multidrug resistance-associated protein-1 (MRP1), encoded by MDR1 and MRP1 genes, respectively. These proteins lead to reduced intracellular drug concentration and decreased cytotoxicity by means of their ability to pump the drugs out of the cells. Breast cancer tumor resistance is mainly associated with overexpression of P-gp/MDR1. Although some chemical MDR modulators aim to overcome MDR by interfering functioning of P-gp, their toxicities limit their usage in clinics. Consequently, RNA interference mediated sequence specific inhibition of the expression of P-gp/MDR1 mRNA may be an efficient tool to reverse MDR phenotype and increase the success of chemotherapy. Aim of this study was resensitizing doxorubicin-resistant breast cancer cells to anticancer agent doxorubicin by selective downregulation of P-gp/MDR1 mRNA.

METHODS

The effect of the selected MDR1 siRNA, and MRP1 expression after MDR1 silencing was determined by qPCR analysis. Intracellular drug accumulation and localization was investigated by confocal laser scanning microscopy after treatment with MDR1 siRNA. XTT cell proliferation assay was performed to determine the effect of MDR1 silencing on doxorubicin sensitivity.

RESULTS

The results demonstrated that approximately 90% gene silencing occurred by the selected siRNA targeting MDR1 mRNA. However, the level of MRP1 mRNA did not change after MDR1 downregulation. Silencing of P-gp encoding MDR1 gene resulted in almost complete restoration of the intracellular doxorubicin accumulation and relocalization of the drug in the nuclei. Introduction of siRNA resulted in about 70% resensitization to doxorubicin.

CONCLUSIONS

Selected siRNA duplex was shown to effectively inhibit MDR1 gene expression, restore doxorubicin accumulation and localization, and enhance chemosensitivity of resistant cells, which makes it a suitable candidate for therapeutic applications.

Chemotherapy resistance in metastatic breast cancer: the evolving role of ixabepilone

Resistance to chemotherapy is a major obstacle to the effective treatment of many tumor types. Although many anticancer therapies can alter tumor growth, in most cases the effect is not long lasting. Consequently, there is a significant need for new agents with low susceptibility to common drug resistance mechanisms in order to improve response rates and potentially extend survival. Approximately 30% of the women diagnosed with early-stage disease in turn progress to metastatic breast cancer, for which therapeutic options are limited. Current recommendations for first-line chemotherapy include anthracycline-based regimens and taxanes (paclitaxel and docetaxel). They typically give response rates of 30 to 70% but the responses are often not durable, with a time to progression of 6 to 10 months. Patients with progression or resistance may be administered capecitabine, gemcitabine, vinorelbine, albumin-bound paclitaxel, or ixabepilone, while other drugs are being evaluated. Response rates in this setting tend to be low (20 to 30%); the median duration of responses is <6 months and the results do not always translate into improved long-term outcomes. The present article reviews treatment options in taxane-resistant metastatic breast cancer and the role of ixabepilone in this setting.