Characterization of adriamycin-resistant human breast cancer cells which display overexpression of a novel resistance-related membrane protein

The Relationship between Trop-2, Chemotherapeutic Drugs, and Chemoresistance

Trop-2 is a highly conserved one-pass transmembrane mammalian glycoprotein that is normally expressed in tissues such as the lung, intestines, and kidney during embryonic development. It is overexpressed in many epithelial cancers but is absent in non-epithelial tumors. Trop-2 is an intracellular calcium signal transducer that participates in the promotion of cell proliferation, migration, invasion, metastasis, and probably stemness. It also has some tumor suppressor effects. The pro-tumoral actions have been thoroughly investigated and reported. However, Trop-2's activity in chemoresistance is less well known. We review a possible relationship between Trop-2, chemotherapy, and chemoresistance. We conclude that there is a clear role for Trop-2 in some specific chemoresistance events. On the other hand, there is no clear evidence for its participation in multidrug resistance through direct drug transport. The development of antibody conjugate drugs (ACD) centered on anti-Trop-2 monoclonal antibodies opened the gates for the treatment of some tumors resistant to classic chemotherapies. Advanced urothelial tumors and breast cancer were among the first malignancies for which these ACDs have been employed. However, there is a wide group of other tumors that may benefit from anti-Trop-2 therapy as soon as clinical trials are completed.

Targeting ABCG2 transporter to enhance 5-aminolevulinic acid for tumor visualization and photodynamic therapy

5-Aminolevulinic acid (ALA) has been approved by the U. S. FDA for fluorescence-guided resection of high-grade glioma and photodynamic therapy (PDT) of superficial skin precancerous and cancerous lesions. As a prodrug, ALA administered orally or topically is metabolized in the heme biosynthesis pathway to produce protoporphyrin IX (PpIX), the active drug with red fluorescence and photosensitizing property. Preferential accumulation of PpIX in tumors after ALA administration enables the use of ALA for PpIX-mediated tumor fluorescence diagnosis and PDT, functioning as a photo-theranostic agent. Extensive research is currently underway to further enhance ALA-mediated PpIX tumor disposition for better tumor visualization and treatment. Particularly, the discovery of PpIX as a specific substrate of ATP binding cassette subfamily G member 2 (ABCG2) opens the door to therapeutic enhancement with ABCG2 inhibitors. Studies with human tumor cell lines and human tumor samples have demonstrated ABCG2 as an important biological determinant of reduced ALA-PpIX tumor accumulation, inhibition of which greatly enhances ALA-PpIX fluorescence and PDT response. These studies strongly support targeting ABCG2 as an effective therapeutic enhancement approach. In this review, we would like to summarize current research of ABCG2 as a drug efflux transporter in multidrug resistance, highlight previous works on targeting ABCG2 for therapeutic enhancement of ALA, and provide future perspectives on how to translate this ABCG2-targeted therapeutic enhancement strategy from bench to bedside.

ABCG2 in Acute Myeloid Leukemia: Old and New Perspectives

Despite recent advances, prognosis of acute myeloid leukemia (AML) remains unsatisfactory due to poor response to therapy or relapse. Among causes of resistance, over-expression of multidrug resistance (MDR) proteins represents a pivotal mechanism. ABCG2 is an efflux transporter responsible for inducing MDR in leukemic cells; through its ability to extrude many antineoplastic drugs, it leads to AML resistance and/or relapse, even if conflicting data have been reported to date. Moreover, ABCG2 may be co-expressed with other MDR-related proteins and is finely regulated by epigenetic mechanisms. Here, we review the main issues regarding ABCG2 activity and regulation in the AML clinical scenario, focusing on its expression and the role of polymorphisms, as well as on the potential ways to inhibit its function to counteract drug resistance to, eventually, improve outcomes in AML patients.

Transport Mechanisms at the Blood-Brain Barrier and in Cellular Compartments of the Neurovascular Unit: Focus on CNS Delivery of Small Molecule Drugs

Ischemic stroke is a primary origin of morbidity and mortality in the United States and around the world. Indeed, several research projects have attempted to discover new drugs or repurpose existing therapeutics to advance stroke pharmacotherapy. Many of these preclinical stroke studies have reported positive results for neuroprotective agents; however, only one compound (3K3A-activated protein C (3K3A-APC)) has advanced to Phase III clinical trial evaluation. One reason for these many failures is the lack of consideration of transport mechanisms at the blood-brain barrier (BBB) and neurovascular unit (NVU). These endogenous transport processes function as a "gateway" that is a primary determinant of efficacious brain concentrations for centrally acting drugs. Despite the knowledge that some neuroprotective agents (i.e., statins and memantine) are substrates for these endogenous BBB transporters, preclinical stroke studies have largely ignored the role of transporters in CNS drug disposition. Here, we review the current knowledge on specific BBB transporters that either limit drug uptake into the brain (i.e., ATP-binding cassette (ABC) transporters) or can be targeted for optimized drug delivery (i.e., solute carrier (SLC) transporters). Additionally, we highlight the current knowledge on transporter expression in astrocytes, microglia, pericytes, and neurons with an emphasis on transport mechanisms in these cell types that can influence drug distribution within the brain.

Multifunctional Role of Astrocyte Elevated Gene-1 (AEG-1) in Cancer: Focus on Drug Resistance

Simple Summary

Chemotherapy is a major mode of treatment for cancers. However, cancer cells adapt to survive in stressful conditions and in many cases, they are inherently resistant to chemotherapy. Additionally, after initial response to chemotherapy, the surviving cancer cells acquire new alterations making them chemoresistant. Genes that help adapt the cancer cells to cope with stress often contribute to chemoresistance and one such gene is Astrocyte elevated gene-1 (AEG-1). AEG-1 levels are increased in all cancers studied to date and AEG-1 contributes to the development of highly aggressive, metastatic cancers. In this review, we provide a comprehensive description of the mechanism by which AEG-1 augments tumor development with special focus on its ability to regulate chemoresistance. We also discuss potential ways to inhibit AEG-1 to overcome chemoresistance.

Abstract

Cancer development results from the acquisition of numerous genetic and epigenetic alterations in cancer cells themselves, as well as continuous changes in their microenvironment. The plasticity of cancer cells allows them to continuously adapt to selective pressures brought forth by exogenous environmental stresses, the internal milieu of the tumor and cancer treatment itself. Resistance to treatment, either inherent or acquired after the commencement of treatment, is a major obstacle an oncologist confronts in an endeavor to efficiently manage the disease. Resistance to chemotherapy, chemoresistance, is an important hallmark of aggressive cancers, and driver oncogene-induced signaling pathways and molecular abnormalities create the platform for chemoresistance. The oncogene Astrocyte elevated gene-1/Metadherin (AEG-1/MTDH) is overexpressed in a diverse array of cancers, and its overexpression promotes all the hallmarks of cancer, such as proliferation, invasion, metastasis, angiogenesis and chemoresistance. The present review provides a comprehensive description of the molecular mechanism by which AEG-1 promotes tumorigenesis, with a special emphasis on its ability to regulate chemoresistance.

Disease-Induced Modulation of Drug Transporters at the Blood-Brain Barrier Level

The blood–brain barrier (BBB) is a highly selective and restrictive semipermeable network of cells and blood vessel constituents. All components of the neurovascular unit give to the BBB its crucial and protective function, i.e., to regulate homeostasis in the central nervous system (CNS) by removing substances from the endothelial compartment and supplying the brain with nutrients and other endogenous compounds. Many transporters have been identified that play a role in maintaining BBB integrity and homeostasis. As such, the restrictive nature of the BBB provides an obstacle for drug delivery to the CNS. Nevertheless, according to their physicochemical or pharmacological properties, drugs may reach the CNS by passive diffusion or be subjected to putative influx and/or efflux through BBB membrane transporters, allowing or limiting their distribution to the CNS. Drug transporters functionally expressed on various compartments of the BBB involve numerous proteins from either the ATP-binding cassette (ABC) or the solute carrier (SLC) superfamilies. Pathophysiological stressors, age, and age-associated disorders may alter the expression level and functionality of transporter protein elements that modulate drug distribution and accumulation into the brain, namely, drug efficacy and toxicity. This review focuses and sheds light on the influence of inflammatory conditions and diseases such as Alzheimer’s disease, epilepsy, and stroke on the expression and functionality of the BBB drug transporters, the consequential modulation of drug distribution to the brain, and their impact on drug efficacy and toxicity.

Tariquidar-related triazoles as potent, selective and stable inhibitors of ABCG2 (BCRP)

Tariquidar derivatives have been described as potent and selective ABCG2 inhibitors. However, their susceptibility to hydrolysis limits their applicability. The current study comprises the synthesis and characterization of novel tariquidar-related inhibitors, obtained by bioisosteric replacement of the labile moieties in our previous tariquidar analog UR-ME22-1 (9). CuAAC ("click" reaction) gave convenient access to a triazole core as a substitute for the labile amide group and the labile ester moiety was replaced by different acyl groups in a Sugasawa reaction. A stability assay proved the enhancement of the stability in blood plasma. Compounds UR-MB108 (57) and UR-MB136 (59) inhibited ABCG2 in a Hoechst 33342 transport assay with an IC₅₀ value of about 80 nM and belong to the most potent ABCG2 inhibitors described so far. Compound 57 was highly selective, whereas its PEGylated analog 59 showed some potency at ABCB1. Both 57 and 59 produced an ABCG2 ATPase-depressing effect which is in agreement with our precedent cryo-EM study identifying 59 as an ATPase inhibitor that exerts its effect via locking the inward-facing conformation. Thermostabilization of ABCG2 by 57 and 59 can be taken as a hint to comparable binding to ABCG2. As reference substances, compounds 57 and 59 allow additional mechanistic studies on ABCG2 inhibition. Due to their stability in blood plasma, they are also applicable in vivo. The highly specific inhibitor 57 is suited for PET labeling, helping to further elucidate the (patho)physiological role of ABCG2, e.g. at the BBB.

Metabolic and non-metabolic pathways that control cancer resistance to anthracyclines

Anthracyclines Doxorubicin, Epirubicin, Daunorubicin and Idarubicin are used to treat a variety of tumor types in the clinics, either alone or, most often, in combination therapies. While their cardiotoxicity is well known, the emergence of chemoresistance is also a major issue accounting for treatment discontinuation. Resistance to anthracyclines is associated to the acquisition of multidrug resistance conferred by overexpression of permeability glycoprotein-1 or other efflux pumps, by altered DNA repair, changes in topoisomerase II activity, cancer stemness and metabolic adaptations. This review further details the metabolic aspects of resistance to anthracyclines, emphasizing the contributions of glycolysis, the pentose phosphate pathway and nucleotide biosynthesis, glutathione, lipid metabolism and autophagy to the chemoresistant phenotype.

Comprehensive Regression Model for Dissociation Equilibria of Cell-Specific Aptamers

A comprehensive nonlinear regression model for dissociation equilibria of cell-specific aptamers is proposed by considering the effect of receptor expression level. Benefiting from the global regression of simultaneous equations, the fitted parameters reach a very significant level, indicating the statistical validity of this updated model. According to the fitting results, we found that dissociation constants fitted using the previous model are obviously larger than the updated values, which can be explained by the effect of receptor number on curve fitting. In addition, equivalent receptor density can be estimated using the updated model, which may lead to some new judgments about reported results of cell-SELEX.

Involvement of α-methylene-γ- and δ-lactones in the suppression of multidrug resistance in MCF-7 cells

BACKGROUND

The development of multidrug resistance to chemotherapy remains a challenge in the treatment of cancer and is a major factor causing failure of many forms of chemotherapy. The ATP binding cassette (ABC) family of proteins are efflux pumps that transport various potentially dangerous substances out of the cells. Several of the ABC transporters are related to chemoresistance, as the rapidly dividing malignant cells use them to protect themselves from medical interventions. Inhibitors of ABC transporters have the potential to enhance the efficacy of anticancer drugs. Two new synthetic compounds, AD-06 and AD-013, were tested as possible multidrug resistance inhibitors in MCF-7 cells.

METHODS

The cytotoxicity of new compounds was tested in MCF-7 and MCF-10A cell lines using the MTT method. Gene expression was measured by real-time PCR and changes in the protein levels were evaluated by flow cytometry and ELISA. A method based on the use of a fluorescent dye, being a marker of the ABC transporter activity, was used for screening the tested compounds as potential multidrug resistance inhibitors.

RESULTS

AD-06 and AD-013 down-regulated NF-κB mRNA levels and decreased the population of cells with activated NF-κB. Both compounds were found to be strong ABCB1 and ABCG2 transporter inhibitors. They showed synergistic effects when incubated with taxol or oxaliplatin.

CONCLUSIONS

α-Methylene-γ- and -δ-lactones AD-06 and AD-013 are promising lead structures for further development as multidrug resistance inhibitors.

Biogenesis of zinc storage granules in Drosophila melanogaster

Membrane transporters and sequestration mechanisms concentrate metal ions differentially into discrete subcellular microenvironments for use in protein cofactors, signalling, storage or excretion. Here we identify zinc storage granules as the insect's major zinc reservoir in principal Malpighian tubule epithelial cells of Drosophila melanogaster The concerted action of Adaptor Protein-3, Rab32, HOPS and BLOC complexes as well as of the white-scarlet (ABCG2-like) and ZnT35C (ZnT2/ZnT3/ZnT8-like) transporters is required for zinc storage granule biogenesis. Due to lysosome-related organelle defects caused by mutations in the homologous human genes, patients with Hermansky-Pudlak syndrome may lack zinc granules in beta pancreatic cells, intestinal paneth cells and presynaptic vesicles of hippocampal mossy fibers.

Resistance mechanisms to drug therapy in breast cancer and other solid tumors: An opinion

Cancer is an important contributor to mortality worldwide. Breast cancer is the most common solid tumor in women. Despite numerous drug combinations and regimens, all patients with advanced breast cancer, similarly to other solid tumors, inevitably develop resistance to treatment. Identified mechanisms of resistance could be classified into intra- and extracellular mechanisms. Intracellular mechanisms include drug metabolism and efflux, target modulations and damage restoration. Extracellular mechanisms might be attributed to the crosstalk between tumor cells and environmental factors. However, current knowledge concerning resistance mechanisms cannot completely explain the phenomenon of multi-drug resistance, which occurs in the vast majority of patients treated with chemotherapy. In this opinion article, we investigate the role of these factors in the development of drug-resistance.

Cytotoxic analysis and chemical characterization of fractions of the hydroalcoholic extract of the Euterpe oleracea Mart. seed in the MCF-7 cell line

OBJECTIVES

To analyse the antineoplastic activity of fractions derived from the hydroalcoholic extract of Euterpe oleracea Mart. seed in the MCF-7 cell line and to identify the compounds responsible for the antineoplastic action.

METHODS

Cells were treated with 10, 20, 40 and 60 μg/ml with the hexane, chloroform and ethyl acetate fraction (EAF) of the hydroalcoholic extract of açaí seed, for 24 and 48 h. After treatment, cell viability was measured using MTT assay and cell death was assessed using the Annexin-Pi assay. The most cytotoxic fraction under study was analysed by mass spectrometry using an electrospray ionization source and a cyclotron analyser coupled to a Fourier transform. Data were analysed statistically by analysis of variance (ANOVA) or by Student's t-test, where appropriate.

KEY FINDINGS

All fractions caused significant reduction in the cell viability, but the EAF was the most cytotoxic (P < 0.001). It was observed the absence of significant annexin staining but increase Pi staining (P < 0.001). The EAF is composed of epicatechin, proanthocyanidin A₂ and trimeric and tetrameric procyanidins.

CONCLUSIONS

In this study, we demonstrated that EAF was the most effective fraction in reducing cell viability and causing necroptosis in the MCF-7 cell.

Leveraging Physiology for Precision Drug Delivery

Physiological characteristics of diseases bring about both challenges and opportunities for targeted drug delivery. Various drug delivery platforms have been devised ranging from macro- to micro- and further into the nanoscopic scale in the past decades. Recently, the favorable physicochemical properties of nanomaterials, including long circulation, robust tissue and cell penetration attract broad interest, leading to extensive studies for therapeutic benefits. Accumulated knowledge about the physiological barriers that affect the in vivo fate of nanomedicine has led to more rational guidelines for tailoring the nanocarriers, such as size, shape, charge, and surface ligands. Meanwhile, progresses in material chemistry and molecular pharmaceutics generate a panel of physiological stimuli-responsive modules that are equipped into the formulations to prepare “smart” drug delivery systems. The capability of harnessing physiological traits of diseased tissues to control the accumulation of or drug release from nanomedicine has further improved the controlled drug release profiles with a precise manner. Successful clinical translation of a few nano-formulations has excited the collaborative efforts from the research community, pharmaceutical industry, and the public towards a promising future of smart drug delivery.

11a-N-Tosyl-5-deoxi-pterocarpan, LQB-223, a novel compound with potent antineoplastic activity toward breast cancer cells with different phenotypes

Multidrug resistance is the major obstacle for successful treatment of breast cancer, prompting the investigation of novel anticancer compounds.

PURPOSE

In this study, we tested whether LQB-223, an 11a-N-Tosyl-5-deoxi-pterocarpan newly synthesized compound, could be effective toward breast cancer cells.

METHODS

Human breast cell lines MCF-7, MDA-MB-231, HB4a and MCF-7 Dox(R) were used as models for this study. Cell culture, MTT and clonogenic assay, flow cytometry and Western blotting were performed.

RESULTS

The LQB-223 decreased cell viability, inhibited colony formation and induced an expressive G2/M arrest in breast cancer cells. There was an induction in p53 and p21(Cip1) protein levels following treatment of wild-type p53 MCF-7 cells, which was not observed in the mutant p53 MDA-MB-231 cell line, providing evidence that the compound might act to modulate the cell cycle regardless of p53 status. In addition, LQB-223 resulted in decreased procaspase levels and increased annexin V staining, suggesting that the apoptotic cascade is also triggered. Importantly, LQB-223 treatment was shown to be less cytotoxic to non-neoplastic breast cells than docetaxel and doxorubicin. Strikingly, exposure of doxorubicin-resistant MCF-7-Dox(R) cells to LQB-223 resulted in suppression of cell viability and proliferation in levels comparable to MCF-7. Of note, MCF-7-Dox(R) cells have an elevated expression of the P-glycoprotein efflux pump when compared to MCF-7.

CONCLUSION

Together, these results show that LQB-223 mediates cytotoxic effects in sensitive and resistant breast cancer cells, while presenting low toxicity to non-neoplastic cells. The new compound might represent a potential strategy to induce toxicity in breast cancer cells, especially chemoresistant cells.

Glycosylation in Cancer: Interplay between Multidrug Resistance and Epithelial-to-Mesenchymal Transition?

The expression of unusual glycan structures is a hallmark of cancer progression, and their functional roles in cancer biology have been extensively investigated in epithelial-to-mesenchymal transition (EMT) models. EMT is a physiological process involved in embryonic development and wound healing. It is characterized by loss of epithelial cell polarity and cell adhesion, permitting cell migration, and thus formation of new epithelia. However, this process is unwanted when occurring outside their physiological limit, resulting in fibrosis of organs and progression of cancer and metastasis. Several studies observed that EMT is related to the acquisition of multidrug resistance (MDR) phenotype, a condition in which cancer cells acquire resistance to multiple different drugs, which has virtually nothing in common. However, although some studies suggested interplay between these two apparently distinct phenomena, almost nothing is known about this possible relationship. A common pathway to them is the need for glycosylation, a post-translational modification that can alter biological function. Thus, this review intends to compile the main facts obtained until now in these two areas, as an effort to unravel the relationship between EMT and MDR.

Altered Expression of Transporters, its Potential Mechanisms and Influences in the Liver of Rodent Models Associated with Diabetes Mellitus and Obesity

Diabetes mellitus is becoming an increasingly prevalent disease that concerns patients and healthcare professionals worldwide. Among many anti-diabetic agents in clinical uses, numerous reports are available on their altered pharmacokinetics because of changes in the expression of drug transporters and metabolic enzymes under diabetic states. These changes may affect the safety and efficacy of therapeutic agents and/or drug-drug interaction with co-administered agents. Therefore, the changes in transporter expression should be identified, and the underlying mechanisms should be clarified. This review summarizes the progress of recent studies on the alterations in important uptake and efflux transporters in liver of diabetic animals and their regulatory pathways.

Blockade of a key region in the extracellular domain inhibits HER2 dimerization and signaling

BACKGROUND

Several treatment strategies target the human epidermal growth factor receptor 2 (HER2) in breast carcinomas, including monoclonal antibodies directed against HER2's extracellular domain (ECD) and small molecule inhibitors of its tyrosine kinase activity. Yet, novel therapies are needed that prevent HER2 dimerization with other HER family members, because current treatments are only partially effective.

METHODS

To test the hypothesis that HER2 activation requires a protein sequence in the HER2-ECD that mediates HER2 homo- and heterodimerization, we introduced a series of deletion mutations in the third subdomain of HER2-ECD. These deletion mutants were retrovirally expressed in breast cancer (BC) cells that naturally overexpress HER2 and in noncancerous, HER2-negative breast epithelial cells. One-factor analysis of variance or Student's t test were used to analyze differences. All statistical tests were two-sided.

RESULTS

The smallest deletion in the ECD domain of HER2, which removed only 16 amino acids (HER2-ECDΔ451-466), completely disrupted the oncogenic potential of HER2. In contrast to wild-type HER2, the mutant-inhibited anchorage-independent growth (mean number of colonies: mutant, 70, 95% confidence interval [CI] = 55 to 85; wild-type, 400, 95% CI = 320 to 480, P < .001) increased sensitivity to paclitaxel treatment in both transformed and nontransformed cells. Overexpression of HER2Δ451-466 efficiently inhibited activation of HER1, HER2, and HER3 in all cell lines tested.

CONCLUSIONS

These findings reveal that an essential "activating" sequence exists in the extracellular domain of HER2. Disruption of this sequence disables the HER2 dimerization loop, blocks subsequent activation of HER2-driven oncogenic signaling, and generates a dominant-negative form of HER2. Reagents specifically against this molecular activation switch may represent a novel targeted approach for the management of HER2-overexpressing carcinomas.

Blood brain barrier: a challenge for effectual therapy of brain tumors

Brain tumors are one of the most formidable diseases of mankind. They have only a fair to poor prognosis and high relapse rate. One of the major causes of extreme difficulty in brain tumor treatment is the presence of blood brain barrier (BBB). BBB comprises different molecular components and transport systems, which in turn create efflux machinery or hindrance for the entry of several drugs in brain. Thus, along with the conventional techniques, successful modification of drug delivery and novel therapeutic strategies are needed to overcome this obstacle for treatment of brain tumors. In this review, we have elucidated some critical insights into the composition and function of BBB and along with it we have discussed the effective methods for delivery of drugs to the brain and therapeutic strategies overcoming the barrier.

ARRY-334543 reverses multidrug resistance by antagonizing the activity of ATP-binding cassette subfamily G member 2

ARRY-334543 is a small molecule inhibitor of ErbB1 and ErbB2 tyrosine kinases. We conducted this study to determine whether ARRY-334543 can enhance the efficacy of conventional anticancer drugs through interaction with ABC transporters. Lung cancer cell line NCI-H460 and its ABCG2-overexpressing NCI-H460/MX20, as well as the ABCG2-, ABCB1-, and ABCC10-overexpressing transfected cell lines were used for the reversal study. Our results demonstrated that ARRY-334543 (1.0 μM) significantly reversed ABCG2-mediated multidrug resistance (MDR) by directly inhibiting the drug efflux function of ABCG2, resulting in the elevated intracellular accumulation of chemotherapeutic drugs in the ABCG2-overexpressing cell lines. In addition, in isolated membranes, ARRY-334543 stimulated ATPase activity and inhibited photolabeling of ABCG2 with [(125)I]-iodoarylazidoprazosin in a concentration-dependent manner indicating that this drug directly interacts at the drug-binding pocket of this transporter. ARRY-334543 (1.0 μM) only slightly reversed ABCB1- and partially reversed ABCC10-mediated MDR suggesting that it exhibits high affinity toward ABCG2. Moreover, homology modeling predicted the binding conformation of ARRY-334543 at Arg482 centroid-based grid of ABCG2. However, ARRY-334543 at reversal concentrations did not affect the expression level of ABCG2, AKT and ERK1/2 and regulate the re-localization of ABCG2. We conclude that ARRY-334543 significantly reverses drug resistance mediated by ABCG2.

Multidrug resistance: Physiological principles and nanomedical solutions

Multidrug resistance (MDR) is a pathophysiological phenomenon employed by cancer cells which limits the prolonged and effective use of chemotherapeutic agents. MDR is primarily based on the over-expression of drug efflux pumps in the cellular membrane. Prominent examples of such efflux pumps, which belong to the ATP-binding cassette (ABC) superfamily of proteins, are Pgp (P-glycoprotein) and MRP (multidrug resistance-associated protein), nowadays officially known as ABCB1 and ABCC1. Over the years, several strategies have been evaluated to overcome MDR, based not only on the use of low-molecular-weight MDR modulators, but also on the implementation of 1-100(0) nm-sized drug delivery systems. In the present manuscript, after introducing the most important physiological principles of MDR, we summarize prototypic nanomedical strategies to overcome multidrug resistance, including the use of carrier materials with intrinsic anti-MDR properties, the use of nanomedicines to modify the mode of cellular uptake, and the co-formulation of chemotherapeutic drugs together with low- and high-molecular-weight MDR inhibitors within a single drug delivery system. While certain challenges still need to be overcome before such constructs and concepts can be widely applied in the clinic, the insights obtained and the progress made strongly suggest that nanomedicine formulations hold significant potential for improving the treatment of multidrug-resistant malignancies.

Use of the proton pump inhibitor pantoprazole to modify the distribution and activity of doxorubicin: a potential strategy to improve the therapy of solid tumors

PURPOSE

Limited drug distribution within solid tumors is an important cause of drug resistance. Basic drugs (e.g., doxorubicin) may be sequestered in acidic organelles, thereby limiting drug distribution to distal cells and diverting drugs from their target DNA. Here we investigate the effects of pantoprazole, a proton pump inhibitor, on doxorubicin uptake, and doxorubicin distribution and activity using in vitro and murine models.

EXPERIMENTAL DESIGN

Murine EMT-6 and human MCF-7 cells were treated with pantoprazole to evaluate changes in endosomal pH using fluorescence spectroscopy, and uptake of doxorubicin using flow cytometry. Effects of pantoprazole on tissue penetration of doxorubicin were evaluated in multilayered cell cultures (MCC), and in solid tumors using immunohistochemistry. Effects of pantoprazole to influence tumor growth delay and toxicity because of doxorubicin were evaluated in mice.

RESULTS

Pantoprazole (>200 μmol/L) increased endosomal pH in cells, and also increased nuclear uptake of doxorubicin. Pretreatment with pantoprazole increased tissue penetration of doxorubicin in MCCs. Pantoprazole improved doxorubicin distribution from blood vessels in solid tumors. Pantoprazole given before doxorubicin led to increased growth delay when given as single or multiple doses to mice bearing MCF7 xenografts.

CONCLUSIONS

Use of pantoprazole to enhance the distribution and cytotoxicity of anticancer drugs in solid tumors might be a novel treatment strategy to improve their therapeutic index.

Sequence mutations of the substrate binding pocket of stem cell factor and multidrug resistance protein ABCG2 in renal cell cancer: a possible link to treatment resistance

ABCG2 is a multidrug cellular transport protein that is associated with resistance to certain treatments in patients, particularly anticancer treatment. The tumor-protective properties of ABCG2 expression are reported to be a feature of a subset of stem cell-like tumor cells. While protection against chemotherapy has been well analyzed, the role of ABCG2 in the treatment with tyrosine kinase inhibitors is only partially understood. Tyrosine kinase inhibitors are currently the main treatment option in irresectable renal cell carcinomas. To investigate possible underlying sequence variations in the ABCG2 gene with relevance to the functional properties of the protein, 36 patient samples were analyzed. Using sequence analysis and single-nucleotide polymorphism databases, sequence variations in the highly conserved domains of the binding pocket of ABCG2 were analyzed. The resulting variations were used for computational protein prediction algorithms to identify conformational alterations. A relevant shift from A to G at position 1376 (resulting in Y→C at 459 aa) was identified and found to be present in 8.3% of the patients. These patients are currently in follow-up after resection, thus, further analysis will reveal whether this mutation has relevance to treatment efficacy.

The novel BCR-ABL and FLT3 inhibitor ponatinib is a potent inhibitor of the MDR-associated ATP-binding cassette transporter ABCG2

Ponatinib is a novel tyrosine kinase inhibitor with potent activity against BCR-ABL with mutations, including T315I, and also against fms-like tyrosine kinase 3. We tested interactions between ponatinib at pharmacologically relevant concentrations of 50 to 200 nmol/L and the MDR-associated ATP-binding cassette (ABC) proteins ABCB1, ABCC1, and ABCG2. Ponatinib enhanced uptake of substrates of ABCG2 and ABCB1, but not ABCC1, in cells overexpressing these proteins, with a greater effect on ABCG2 than on ABCB1. Ponatinib potently inhibited [(125)I]-IAAP binding to ABCG2 and ABCB1, indicating binding to their drug substrate sites, with IC(50) values of 0.04 and 0.63 μmol/L, respectively. Ponatinib stimulated ABCG2 ATPase activity in a concentration-dependent manner and stimulated ABCB1 ATPase activity at low concentrations, consistent with it being a substrate of both proteins at pharmacologically relevant concentrations. The ponatinib IC(50) values of BCR-ABL-expressing K562 cells transfected with ABCB1 and ABCG2 were approximately the same as and 2-fold higher than that of K562, respectively, consistent with ponatinib being a substrate of both proteins, but inhibiting its own transport, and resistance was also attenuated to a small degree by ponatinib-induced downregulation of ABCB1 and ABCG2 cell-surface expression on resistant K562 cells. Ponatinib at pharmacologically relevant concentrations produced synergistic cytotoxicity with ABCB1 and ABCG2 substrate chemotherapy drugs and enhanced apoptosis induced by these drugs, including daunorubicin, mitoxantrone, topotecan, and flavopiridol, in cells overexpressing these transport proteins. Combinations of ponatinib and chemotherapy drugs warrant further testing.

Role of breast cancer resistance protein (BCRP/ABCG2) in cancer drug resistance

Since cloning of the ATP-binding cassette (ABC) family member breast cancer resistance protein (BCRP/ABCG2) and its characterization as a multidrug resistance efflux transporter in 1998, BCRP has been the subject of more than two thousand scholarly articles. In normal tissues, BCRP functions as a defense mechanism against toxins and xenobiotics, with expression in the gut, bile canaliculi, placenta, blood-testis and blood-brain barriers facilitating excretion and limiting absorption of potentially toxic substrate molecules, including many cancer chemotherapeutic drugs. BCRP also plays a key role in heme and folate homeostasis, which may help normal cells survive under conditions of hypoxia. BCRP expression appears to be a characteristic of certain normal tissue stem cells termed "side population cells," which are identified on flow cytometric analysis by their ability to exclude Hoechst 33342, a BCRP substrate fluorescent dye. Hence, BCRP expression may contribute to the natural resistance and longevity of these normal stem cells. Malignant tissues can exploit the properties of BCRP to survive hypoxia and to evade exposure to chemotherapeutic drugs. Evidence is mounting that many cancers display subpopulations of stem cells that are responsible for tumor self-renewal. Such stem cells frequently manifest the "side population" phenotype characterized by expression of BCRP and other ABC transporters. Along with other factors, these transporters may contribute to the inherent resistance of these neoplasms and their failure to be cured.

Effect of bone morphogenetic protein 4 in the human brain glioma cell line U251

The role of bone morphogenetic protein 4 (BMP4) in gliomas is not clear. We hypothesized that BMP4 inhibits proliferation in the brain glioma cell line U251 through a signaling pathway involving BMP4 and the mothers against decapentaplegic homolog 4 (SMAD4) protein. We exposed U251 cells to Adriamycin (1 g) for 48 h; cell proliferation (MTT assay), expression of BMP4 and SMAD4 (mRNA: qPCR; protein: Western blot) were studied. We further altered expression of BMP4 by overexpression or siRNA silencing, and documented cell responses to Adriamycin. Proliferation of U251 cells was significantly inhibited upon exposure to Adriamycin. This inhibition was associated with increased expression of BMP4. Further, proliferation of U251 cells was inhibited when BMP4 was overexpressed. BMP4 expression negatively correlated with expression of SMAD4, such that elevated levels of BMP4 were associated with decreased expression of SMAD4 and vice versa. The Adriamycin-induced inhibition of proliferation of U251 cells was attenuated when BMP4 was knocked down by siRNA. To conclude, BMP4 is associated with inhibition of proliferation of U251 cells; the effects of BMP4 involve the BMP4-Smad signaling pathway. BMP4 has a potential as a target for glioma therapy.

In vitro and in vivo evidence for the importance of breast cancer resistance protein transporters (BCRP/MXR/ABCP/ABCG2)

The breast cancer resistance protein (BCRP/ABCG2) is a member of the G-subfamiliy of the ATP-binding cassette (ABC)-transporter superfamily. This half-transporter is assumed to function as an important mechanism limiting cellular accumulation of various compounds. In context of its tissue distribution with localization in the sinusoidal membrane of hepatocytes, and in the apical membrane of enterocytes ABCG2 is assumed to function as an important mechanism facilitating hepatobiliary excretion and limiting oral bioavailability, respectively. Indeed functional assessment performing mouse studies with genetic deletion or chemical inhibition of the transporter, or performing pharmacogenetic studies in humans support this assumption. Furthermore the efflux function of ABCG2 has been linked to sanctuary blood tissue barriers as described for placenta and the central nervous system. However, in lactating mammary glands ABCG2 increases the transfer of substrates into milk thereby increasing the exposure to potential noxes of a breastfed newborn. With regard to its broad substrate spectrum including various anticancer drugs and environmental carcinogens the function of ABCG2 has been associated with multidrug resistance and tumor development/progression. In terms of cancer biology current research is focusing on the expression and function of ABCG2 in immature stem cells. Recent findings support the notion that the physiological function of ABCG2 is involved in the elimination of uric acid resulting in higher risk for developing gout in male patients harboring genetic variants. Taken together ABCG2 is implicated in various pathophysiological and pharmacological processes.

Molecular mechanism of suppression of MDR1 by puerarin from Pueraria lobata via NF-kappaB pathway and cAMP-responsive element transcriptional activity-dependent up-regulation of AMP-activated protein kinase in breast cancer MCF-7/adr cells

Multidrug resistance (MDR) is a major obstacle in cancer chemotherapy and its inhibition is an effective way to reverse cancer drug resistance. In the present study, we investigated that puerarin, a natural isoflavonoid from Pueraria lobata, down-regulated MDR1 expression in MCF-7/adriamycin (MCF-7/adr), a human breast MDR cancer cell line. Puerarin treatment significantly inhibited MDR1 expression, MDR1 mRNA and MDR1 promoter activity in MCF-7/adr cells. The suppression of MDR1 was accompanied by partial recovery of intracellular drug accumulation, leading to increased toxicity of adriamycin and fluorescence of rhodamine 123, indicating that puerarin reversed the MDR phenotype by inhibiting the drug efflux function of MDR1. Moreover, nuclear factor kappa-B activity and IkappaB degradation were inhibited by puerarin. Puerarin stimulated AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase and glycogen synthase kinase-3beta phosphorylation, but puerarin decreased cAMP-responsive element-binding protein phosphorylation. The puerarin-induced suppression of MDR1 expression was reduced by AMPK inhibitor (compound C). Furthermore, both MDR1 protein expression and the transcriptional activity of cAMP-responsive element (CRE) were inhibited by puerarin and protein kinase A/CRE inhibitor (H89). Taken together, our results suggested that puerarin down-regulated MDR1 expression via nuclear factor kappa-B and CRE transcriptional activity-dependent up-regulation of AMPK in MCF-7/adr cells.

MicroRNA and drug resistance

Chemotherapy is the preferred treatment for malignancies. However, a successful long-term use of chemotherapy is often prevented by the development of drug resistance. Many mechanisms such as gene mutation, DNA methylation and histone modification have important roles in the resistance of cancer cells to chemotherapeutic agents. Climent suggested miR-125b was involved in the development of drug resistance by microRNA (miRNA) dysregulation. miRNAs are endogenously expressed small non-coding RNAs, which are evolutionarily conserved and function as regulators of gene expression. Much effort has been exerted in analyzing the role of miRNAs in the development of drug resistance in a variety of malignancies. Several research groups have shown that the expressions of miRNAs in chemoresistant cancer cells and their parental chemosensitive ones are different. The molecular targets and mechanisms of chemosensitivity and chemoresistance are also elucidated. This article reviews the functions of miRNAs in the development of drug resistance.

ABCG2: the key to chemoresistance in cancer stem cells?

Multi-drug chemoresistance remains one of the most common reasons for chemotherapy failure. The membrane transporter protein ABCG2/BCRP1 has been shown in vitro to effectively reduce the intracellular concentrations of several prominent anticancer chemotherapeutic agents such as mitoxantrone and doxorubicin. Intriguingly, cancer stem cells are known to be characterized by multi-drug chemoresistance. Taking into account that the ABCG2(+) subset of tumor cells are often enriched with cells with cancer stem-like phenotypes, it has been proposed that ABCG2 activity underlies the ability of cancer cells to regenerate post-chemotherapy. Furthermore, we also review evidence suggesting that tyrosine kinase inhibitors, including imatinib and gefitinib, are both direct and downstream inactivators of ABCG2 and, therefore, serve as candidates to reverse cancer stem cell chemoresistance and potentially target cancer stem cells.

Impact of breast cancer resistance protein on cancer treatment outcomes

Breast cancer resistance protein (BCRP/ABCG2) was discovered in multidrug resistant breast cancer cells having an ATP-dependent transport-based resistance phenotype. This ABC transporter functions (at least in part) as a xenobiotic protective mechanism for the organism: in the gut and biliary tract, it prevents absorption and enhances elimination of potentially toxic substances. As a placental barrier, it protects the fetus; similarly, it serves as a component of blood-brain and blood-testis barrier; BCRP is expressed in stem cells and may protect them from potentially harmful agents. Therefore, BCRP could influence cancer outcomes by (a) endogenous BCRP affecting the absorption, distribution, metabolism, and elimination of anticancer drugs; (b) BCRP expression in cancer cells may directly cause resistance by active efflux of anticancer drugs; (c) BCRP expression in cancer cells could be a manifestation of the activity of metabolic and signaling pathways that impart multiple mechanisms of drug resistance, self-renewal (stemness), and invasiveness (aggressiveness)--i.e. impart a poor prognosis--to cancers. This chapter presents a synopsis of translational clinical studies relating BCRP expression in leukemias, lymphomas, and a variety of solid tumors with clinical outcome. Data are emerging that expression of BCRP, like P-glycoprotein/ABCB1, is associated with adverse outcomes in a variety of human cancers. Whether this adverse prognostic effect results from resistance imparted to the cancer cells as the direct result of BCRP efflux of anticancer drugs, or whether BCRP expression (and also Pgp expression - coexpression of these transporters is common among poor risk cancers) serves as indicators of the activity of signaling pathways that enhance cancer cellular proliferation, metastases, genomic instability, enhance drug resistance, and oppose programmed cell death mechanisms is yet unknown.

Mechanisms of chemotherapeutic drug resistance in cancer therapy--a quick review

Chemotherapy is one of the principal modes of treatment for cancer patients. Clinically, many tumors present a satisfactory response when they are first exposed to the chemotherapeutic drugs. However, drug resistance occurs sooner or later in these tumors, and the majority of the patients develop progressive disease. The mechanisms of treatment failure of chemotherapeutic drugs have been well studied. Via a unique protection system, i.e. multidrug resistance (MDR), the cancer cells can escape the toxic effect of most commonly used cancer drugs in spite of their different chemical structures and different mechanisms of intracellular activity. There are two classes of transporter proteins at the cellular surface which are responsible for MDR in tumors. One is the adenosine triphosphate-binding cassette transporter superfamily, which is an energy-requiring efflux pump with the function of extruding toxic chemotherapeutic drugs from the cancer cells. The other is the solute carrier transporter superfamily, which mediates the cellular uptake of anticancer drugs, and drug resistance may result from decreased activity of these transporters. Although transporters of MDR are responsible for the tumor resistance to many chemotherapeutic drugs currently used in cancer therapy, the mechanisms of resistance to platinum-based antitumor agents are through different pathways. In this article, the mechanisms of MDR transporters mediating resistance to the commonly used chemotherapeutic drugs and to platinum-based agents are reviewed. Finally, with the finding of cancer stem cells in more and more solid tumors, it is recognized that the cancer stem cell is spared along with its normal tissue stem cell counterparts with very subtle differences. One characteristic of the normal tissue stem cell is the self-protection ability through innate MDR transporters. Therefore, the essential self-protection property is also present in the cancer stem cells. The quiescent tumor stem cell with constitutive MDR is the main barrier to therapy. Successful cancer therapy will depend on the ability to discern the subtle differences between the tumor and normal stem cells so that approaches can be developed to eliminate the tumor stem cells without excessive toxicity to normal stem cells.

Mechanisms of multidrug resistance in cancer treatment

Advanced breast cancer responds to a range of cytotoxic agents, but resistance always develops. Understanding the mechanisms of resistance may provide new therapeutic options. There are several major groups of resistance mechanisms. 1) The multidrug resistant phenotype. This is due to a membrane pump that can extrude a wide range of anticancer drugs--the P-glycoprotein. It is inhibited by a range of clinically used calcium channel blockers such as nifedipine and verapamil. Several other membrane proteins of 180 KD, 170 KD, 300 KD and 85 KD have been reported and are associated with MDR. 2) Glutathione transferences and detoxification mechanisms. These are a multigene family of enzymes that conjugate glutathione to chemically reactive groups. There are 3 major groups of enzymes--acidic, basic and neutral. They have been implicated in resistance to doxorubicin, melphalan cisplatinum chlorambucil and other alkylating agents. Other protecting systems include metallothionein and selenium dependent glutathione peroxidase. HSP27 confers doxorubicin resistance. 3) Topoisomerase II. DNA topoisomerases are involved in several aspects of DNA metabolism in particular genetic recombination, DNA transcription, chromosome segregation. They are a target for doxorubicin, mitoxantrone, VP16. Low levels of expression are associated with resistance. However, it is oestrogen inducible and this may be of therapeutic value. A novel topo IIb which is more drug resistant has been reported. 4) DNA repair. A score or more of genes are involved in the repair of DNA damage by drugs and radiation. Defective DNA repair may predispose to cancer of the breast and be responsible for adverse radiation reactions. Enhanced repair has been shown to be a mechanism of cisplatinum resistance. Several genes are inducible by DNA damage and may confer resistance e.g. A45. 5) Drug activation. Mitomycin C as well as cyclophosphamide and VP16 require activation for their effects. Low levels of cytochrome p450 reductase are associated with MMC resistance.

Systematic analysis of microRNA involved in resistance of the MCF-7 human breast cancer cell to doxorubicin

Multidrug resistance remains a major clinical obstacle to successful treatment of breast cancer and leads to poor prognosis for the patients. Recently studies have shown that microRNAs play an important role in breast cancer cell resistance to chemotherapeutic agents. In this study, microRNA expression profiles of MCF-7/AdrVp and MCF-7 were analyzed using microarray and the results were confirmed by real-time RT-polymerase chain reaction. Gene Ontology (GO) and pathways mapping tools were employed to analyse systemically the biological processes and signaling pathways affected by differential expression microRNAs. Here, we showed that 181 human microRNAs were differentially expressed between two cell lines. Compared to MCF-7 cells, there were 16 microRNAs down-regulated and 165 microRNAs up-regulated in MCF-7/AdrVp. Western blot confirmed the correlation between specific microRNA expression and corresponding changes in protein levels of their targets, specifically those that have a documented role in cancer drug resistance. Furthermore, we validated that signaling pathway highlighted in the study was involved in drug resistance. These results indicated that breast cancer cell resistant to chemotherapy was associated with a group of microRNAs. GO and pathway mapping are valid and effective approach to analyse the function of microRNAs and the results could be a guideline for further investigation.

Identification of genes that confer tumor cell resistance to the aurora B kinase inhibitor, AZD1152

AZD1152 is a highly selective Aurora B kinase inhibitor currently undergoing Phase I and II clinical evaluation in patients with acute myelogenous leukemia and advanced solid malignancies. We have established two AZD1152-resistant cell lines from SW620 colon and MiaPaCa pancreatic carcinoma lines, which are >100-fold resistant to the active metabolite of AZD1152, AZD1152 HQPA and interestingly, cross-resistant to the pan-Aurora kinase inhibitor, VX-680/MK0457. Using whole-genome microarray analysis and comparative genomic hybridization, we were able to identify MDR1 and BCRP as the causative genes that underlie AZD1152 HQPA-resistance in these models. Furthermore, the upregulation of either of these genes is sufficient to render in vivo tumor growth insensitive to AZD1152. Finally, the upregulation of MDR1 or BCRP is predictive of tumor cell sensitivity to this agent, both in vitro and in vivo. The data provide a genetic basis for resistance to Aurora kinase inhibitors, which could be utilized to predict clinical response to therapy.

ABCG2: a perspective

ABCG2, or breast cancer resistance protein (BCRP), is an ABC transporter that has been the subject of intense study since its discovery a decade ago. With high normal tissue expression in the brain endothelium, gastrointestinal tract, and placenta, ABCG2 is believed to be important in the protection from xenobiotics, regulating oral bioavailability, forming part of the blood-brain barrier, the blood-testis barrier, and the maternal-fetal barrier. Notably, ABCG2 is often expressed in stem cell populations, where it likely plays a role in xenobiotic protection. However, clues to its epigenetic regulation in various cell populations are only beginning to emerge. While ABCG2 overexpression has been demonstrated in cancer cells after in vitro drug treatment, endogenous ABCG2 expression in certain cancers is likely a reflection of the differentiated phenotype of the cell of origin and likely contributes to intrinsic drug resistance. Notably, research into the transporter's role in cancer drug resistance and its development as a therapeutic target in cancer has lagged. Substrates and inhibitors of the transporter have been described, among them chemotherapy drugs, tyrosine kinase inhibitors, antivirals, HMG-CoA reductase inhibitors, carcinogens, and flavonoids. This broad range of substrates complements the efficiency of ABCG2 as a transporter in laboratory studies and suggests that, while there are redundant mechanisms of xenobiotic protection, the protein is important in normal physiology. Indeed, emerging studies in pharmacology and toxicology assessing polymorphic variants in man, in combination with murine knockout models have confirmed its dynamic role. Work in pharmacology may eventually lead us to a greater understanding of the physiologic role of ABCG2.

Sagopilone (ZK-EPO): from a natural product to a fully synthetic clinical development candidate

BACKGROUND

Tubulin is among the most established and clinically validated targets in oncology. The taxanes, paclitaxel and docetaxel, stabilize microtubules and have shown significant clinical activity, but factors such as the development of resistance can limit their clinical use. The epothilones are a novel class of natural microtubule-stabilizing products with potential activity in an expanded spectrum of tumour indications.

OBJECTIVE

In an extensive lead optimization programme, we selected sagopilone from 350 compounds produced by total synthesis because of its combination of potent activity and good tolerability in tumour models. It is the first fully synthetic epothilone in clinical development.

METHODS

Here we review the directed optimization of the natural product epothilone B to produce sagopilone, along with its mechanism of action, preclinical data and emerging clinical results.

RESULTS/CONCLUSIONS

We show how this optimization process translated into superior preclinical activity, coupled with a favourable tolerability profile. Activity has been determined in a number of animal models, including those from tumours resistant to other systemic treatments. The approach used to develop sagopilone may become more common as structure-driven research is increasingly employed to exploit the enormous potential of natural products, in parallel with other targeted approaches, heralding a new era of anticancer therapy.

A new mechanism of drug resistance in breast cancer cells: fatty acid synthase overexpression-mediated palmitate overproduction

Multidrug resistance is a major problem in successful cancer chemotherapy. Various mechanisms of resistance, such as ABC transporter-mediated drug efflux, have been discovered using established model cancer cell lines. While characterizing a drug-resistant breast cancer cell line, MCF7/AdVp3000, we found that fatty acid synthase (FASN) is overexpressed. In this study, we showed that ectopic overexpression of FASN indeed causes drug resistance and that reducing the FASN expression increased the drug sensitivity in breast cancer cell lines MCF7 and MDA-MB-468 but not in the normal mammary epithelial cell line MCF10A1. Use of FASN inhibitor, Orlistat, at low concentrations also sensitized cells with FASN overexpression to anticancer drugs. The FASN-mediated drug resistance appears to be due to a decrease in drug-induced apoptosis from an overproduction of palmitic acid by FASN. Together with previous findings of FASN as a poor prognosis marker for breast cancer patients, our results suggest that FASN overexpression is a new mechanism of drug resistance and may be an ideal target for chemosensitization in breast cancer chemotherapy.