An anti-CD19 antibody inhibits the interaction between P-glycoprotein (P-gp) and CD19, causes P-gp to translocate out of lipid rafts, and chemosensitizes a multidrug-resistant (MDR) lymphoma cell line

MAPK/ERK and PI3K/AKT signaling pathways are activated in adolescent and adult acute lymphoblastic leukemia

BACKGROUND

The mitogen-activated protein kinase (MAPK)/ERK signaling cascade and the phosphoinosytol-3 phosphate/Akt (PI3K/Akt) pathways are involved in proliferation and differentiation of hematopoietic cells. The frequency of PI3K/Akt and MAPK pathway activation in adult acute lymphoblastic leukemia (ALL) still need to be elucidated.

AIMS

To assess the activity and prognostic implications of MAPK/ERK and PI3K/Akt pathways in adult (ALL).

METHODS

We examined 28 precursor-B-cell ALL and 6 T-cell primary ALL samples. Flow cytometry was employed to analyze the expression levels of phosphorylated ERK and phosphorylated Akt.

RESULTS

Ten out of 15 (67%) ALL fresh samples (7 B-cell, 3 T-cell) showed constitutive p-ERK expression. The p-ERK mean fluorescent index ratio (MFI (R)) showed a tendency to be higher in ALL than in normal T lymphocytes (1.26 [0.74-3.10] vs. 1.08 [1.02-1.21], respectively [p = .069]) and was significantly lower than in leukemic cell lines (median MFI (R) 3.83 [3.71-5.97] [p < .001]). Expression of p-Akt was found in 35% (12/34) (10 B-cell, 2 T-cell). The median MFI (R) expression for p-Akt in primary blast cell was 1.13 (0.48-9.90) compared to 1.01 (1.00-1.20) in normal T lymphocytes (p = ns) and lower than in leukemic cell lines (median MFI (R) 2.10 [1.77-3.40] [p = .037]). Moreover, expression of p-ERK was negatively associated with the expression of CD34 (1.22 [0.74-1.33] vs. 1.52 [1.15-3.10] for CD34(+) and CD34(-) group, respectively, p = .009).

CONCLUSION

Our findings suggest that both MAPK/ERK and PI3K/Akt are constitutively activated in adult ALL, indicating a targeted therapy potential for ALL by using inhibitors of these pathways.

Cholesterol-Depletion-Induced Membrane Repair Carries a Raft Conformer of P-Glycoprotein to the Cell Surface, Indicating Enhanced Cholesterol Trafficking in MDR Cells, Which Makes Them Resistant to Cholesterol Modifications

The human P-glycoprotein (P-gp), a transporter responsible for multidrug resistance, is present in the plasma membrane's raft and non-raft domains. One specific conformation of P-gp that binds to the monoclonal antibody UIC2 is primarily associated with raft domains and displays heightened internalization in cells overexpressing P-gp, such as in NIH-3T3 MDR1 cells. Our primary objective was to investigate whether the trafficking of this particular P-gp conformer is dependent on cholesterol levels. Surprisingly, depleting cholesterol using cyclodextrin resulted in an unexpected increase in the proportion of raft-associated P-gp within the cell membrane, as determined by UIC2-reactive P-gp. This increase appears to be a compensatory response to cholesterol loss from the plasma membrane, whereby cholesterol-rich raft micro-domains are delivered to the cell surface through an augmented exocytosis process. Furthermore, this exocytotic event is found to be part of a complex trafficking mechanism involving lysosomal exocytosis, which contributes to membrane repair after cholesterol reduction induced by cyclodextrin treatment. Notably, cells overexpressing P-gp demonstrated higher total cellular cholesterol levels, an increased abundance of stable lysosomes, and more effective membrane repair following cholesterol modifications. These modifications encompassed exocytotic events that involved the transport of P-gp-carrying rafts. Importantly, the enhanced membrane repair capability resulted in a durable phenotype for MDR1 expressing cells, as evidenced by significantly improved viabilities of multidrug-resistant Pgp-overexpressing immortal NIH-3T3 MDR1 and MDCK-MDR1 cells compared to their parents when subjected to cholesterol alterations.

Recent Progress of Novel Nanotechnology Challenging the Multidrug Resistance of Cancer

Multidrug resistance (MDR) of tumors is one of the clinical direct reasons for chemotherapy failure. MDR directly leads to tumor recurrence and metastasis, with extremely grievous mortality. Engineering a novel nano-delivery system for the treatment of MDR tumors has become an important part of nanotechnology. Herein, this review will take those different mechanisms of MDR as the classification standards and systematically summarize the advances in nanotechnology targeting different mechanisms of MDR in recent years. However, it still needs to be seriously considered that there are still some thorny problems in the application of the nano-delivery system against MDR tumors, including the excessive utilization of carrier materials, low drug-loading capacity, relatively narrow targeting mechanism, and so on. It is hoped that through the continuous development of nanotechnology, nano-delivery systems with more universal uses and a simpler preparation process can be obtained, for achieving the goal of defeating cancer MDR and accelerating clinical transformation.

Molecular Regulation of Canalicular ABC Transporters

The ATP-binding cassette (ABC) transporters expressed at the canalicular membrane of hepatocytes mediate the secretion of several compounds into the bile canaliculi and therefore play a key role in bile secretion. Among these transporters, ABCB11 secretes bile acids, ABCB4 translocates phosphatidylcholine and ABCG5/G8 is responsible for cholesterol secretion, while ABCB1 and ABCC2 transport a variety of drugs and other compounds. The dysfunction of these transporters leads to severe, rare, evolutionary biliary diseases. The development of new therapies for patients with these diseases requires a deep understanding of the biology of these transporters. In this review, we report the current knowledge regarding the regulation of canalicular ABC transporters' folding, trafficking, membrane stability and function, and we highlight the role of molecular partners in these regulating mechanisms.

Potential of B-cell-targeting therapy in overcoming multidrug resistance and tissue invasiveness associated with P-glycoprotein expressing-B cell compartments

Rheumatoid arthritis (RA) is a systemic autoimmune mediated inflammatory disease characterized by progressive joint damage and extra-articular organ manifestations. Among the effector pathways and cells involved in the development of RA, activated B cells play a pivotal role in the pathological process of RA. P-glycoprotein (P-gp), a member of ATP-binding cassette transporters, is induced on the cell membrane by certain stimuli. P-gp transports various drugs from the cytoplasm to the cell exterior, resulting in the development of drug resistance. P-gp expression on B cells appears in patients with RA as the disease activity increases, and treatment of these patients' results in modification of over-expression of P-gp on activated B cells. Evidence suggests that P-gp expressing-activated B cells play important roles in the pathogenesis and treatment resistance in RA through the efflux of intracellular drugs and progression of infiltration in inflammatory lesions. Therapies designed to target activated B cells might overcome refractory RA. Identification of the subsets of peripheral activated B cells that express P-gp in RA patients might help the selection of suitable treatment strategy.

Metabolomics analysis of multidrug resistance in colorectal cancer cell and multidrug resistance reversal effect of verapamil

Multidrug resistance remains a huge challenge in the chemotherapy of cancer and numerous studies have reported that P-glycoprotein is the most common mechanism of multidrug resistance. Verapamil has been shown to be able to reverse development of multidrug resistance mediated by P-glycoprotein. However, the mechanism of action for verapamil in reversing multidrug resistance at the metabolic level has been rarely reported. In this research, we report the reversal effect of verapamil on multidrug resistance and its mechanisms of action using metabolomics. The results show that the P-glycoprotein-mediated chemotherapy drug resistance was significantly reversed by verapamil in resistant SW620/Ad300 cells. In-depth studies demonstrated that verapamil at reversal concentration had no effect on the P-glycoprotein expression level, but increased intramolecular accumulation of paclitaxel in SW620/Ad300 cells. Metabolomics revealed that the multidrug resistance of SW620/Ad300 cells was related to changes in glycerophospholipid metabolism, sphingolipid metabolism and citric acid cycle, and verapamil could antagonize the multidrug resistance by reversing the above-mentioned glycerophospholipid metabolism and sphingolipid metabolism. This research shows the multidrug resistance reversal mechanism of verapamil at the metabolic level, which helps in understanding the exact multidrug resistance mechanism of verapamil and might be potentially useful to find new multidrug resistance reversal agents. The combination of verapamil (VRP) and paclitaxel (PTX) yielded synergistic effects. VRP had no effect on the expression of P-gp, but increased intramolecular accumulation of PTX. VRP antagonized the MDR by regulating glycerophospholipid metabolism and sphingolipid metabolism.

Hsp90 facilitates acquired drug resistance of tumor cells through cholesterol modulation however independent of tumor progression

Acquired multidrug resistance of cancer cells challenges the chemotherapeutic interventions. To understand the role of molecular chaperone, Hsp90 in drug adapted tumor cells, we have used in vitro drug adapted epidermoid tumor cells as a model system. We found that chemotherapeutic drug adaptation of tumor cells is mediated by induced activities of both Hsp90 and P-glycoprotein (P-gp). Although the high-affinity conformation of Hsp90 has correlated with the enhanced drug efflux activity, we did not observe a direct interaction between P-gp and Hsp90. The enrichment of P-gp and Hsp90 at the cholesterol-rich membrane microdomains is found obligatory for enhanced drug efflux activity. Since inhibition of cholesterol biosynthesis is not interfering with the drug efflux activity, it is presumed that the net cholesterol redistribution mediated by Hsp90 regulates the enhanced drug efflux activity. Our in vitro cholesterol and Hsp90 interaction studies have furthered our presumption that Hsp90 facilitates cholesterol redistribution. The drug adapted cells though exhibited anti-proliferative and anti-tumor effects in response to 17AAG treatment, drug treatment has also enhanced the drug efflux activity. Our findings suggest that drug efflux activity and metastatic potential of tumor cells are independently regulated by Hsp90 by distinct mechanisms. We expose the limitations imposed by Hsp90 inhibitors against multidrug resistant tumor cells.

Dasatinib reverses drug resistance by downregulating MDR1 and Survivin in Burkitt lymphoma cells

Background

Current chemotherapies for Burkitt lymphoma (BL) have dramatically improved its clinical outcome. However, chemoresistance can lead to chemotherapy failure and very poor prognosis; thus, novel strategies are urgently required for patients with drug-resistant BL. To investigate the mechanisms underlying drug resistance in BL, we established drug-resistant BL cell lines: HS-Sultan/ADM (adriamycin-resistant), HS-Sultan/VCR (vincristine-resistant), HS-Sultan/DEX (dexamethasone-resistant), and HS-Sultan/L-PAM (melphalan-resistant).

Methods

Drug transporter and survival factor expression were investigated the using western blotting and real time polymerase chain reaction. Cell survival was analyzed by trypan blue dye exclusion method.

Results

The established cell lines acquired cross-resistance to adriamycin, vincristine, dexamethasone, and melphalan and exhibited 50% inhibitory concentration values 106-, 40-, 81-, and 45-fold higher than the parental cell lines, respectively. We found that protein and mRNA expression of MDR1 and Survivin were higher in drug-resistant BL cells than in the parent cells. Treatment with verapamil, an MDR1 inhibitor, or Survivin siRNA alongside each anti-cancer drug suppressed the proliferation of all drug-resistant BL cells. Src kinase activity was higher in all resistant cell lines than the parental cells; suppressing Src with dasatinib restored drug sensitivity by reducing MDR1 and Survivin expression.

Conclusions

MDR1 and Survivin upregulation are responsible for resistance to conventional drugs and dasatinib can restore drug sensitivity by reducing MDR1 and Survivin expression in drug-resistant BL cells. Src inhibitors could therefore be a novel treatment strategy for patients with drug resistant BL.

Indocyanine green/doxorubicin-encapsulated functionalized nanoparticles for effective combination therapy against human MDR breast cancer

To overcome low therapeutic efficacy of chemotherapy against multidrug resistance (MDR) breast cancer, a combination therapy system based upon functionalized polymer nanoparticles comprising poly(γ-glutamic acid)-g-poly(lactic-co-glycolic acid) (γ-PGA-g-PLGA) as the major component was developed. The NPs were loaded with doxorubicin (DOX) and indocyanine green (ICG) for dual modality cancer treatment and coated with cholesterol-PEG (C-PEG) for MDR abrogation in treatment of human MDR breast cancer. The in vitro cellular uptake of the DOX/ICG loaded nanoparticles (DI-NPs) by MDR cancer cells was significantly enhanced owing to effective inhibition of the P-gp activity by C-PEG and γ-PGA receptor-mediated endocytosis. DOX localization in cytoplasm and nucleus was observed particularly with the photo-thermal effect that facilitated intracellular drug release. As a result, the C-PEG coated DI-NPs after photo-irradiation exhibited a synergistic effect of combination (chemo/thermal) therapy to depress the proliferation of MDR cancer calls. The ex vivo biodistribution study revealed an enhanced tumor accumulation of C-PEG (2000) coated DI-NPs in MCF-7/MDR tumor-bearing nude mice due to the excellent EPR effects by the NP surface PEGylation. The MDR tumor growth was almost entirely inhibited in the group receiving combination therapy from CP2k-DI-NPs and photo-irradiation along with substantial cell apoptosis of tumor tissues examined by immunohistochemical staining. The results demonstrate a promising dual modality therapy system, CP2k-DI-NPs, developed in this work for effective combination therapy of human MDR breast cancer.

MDR1 in immunity: friend or foe?

MDR1 is an ATP-dependent transmembrane transporter primarily studied for its role in the detoxification of tissues and for its implication in resistance of tumor cells to chemotherapy treatment. Several studies also report on its expression on immune cells where it plays a protective role from xenobiotics and toxins. This review provides an overview of what is known on MDR1 expression in immune cells in human, and its implications in different pathologies and their treatment options.

Evaluation of Multidrug Resistance of Leukemia Using Surface-Enhanced Raman Scattering Method for Clinical Applications

P-glycoprotein (P-gp) is an important multidrug resistance (MDR) regulator for leukemia to mediate its development and thus can be considered as a powerful reference for the diagnosis of MDR. The detection of P-gp is of vital significance and has attracted considerable concerns. In this study, we proposed a surface-enhanced Raman scattering (SERS) method for the evaluation of P-gp expression levels in leukemia cell lines. Basically, we utilized an aqueous phase sandwich-type immunoassay to analyze the expression of P-gp. First, anti-CD45-decorated magnetic beads (MBs) and P-gp antibody-decorated SERS probes were fabricated. CD45 is a common protein expressed in all leukemia cells. As a result, a sandwich immunocomplex can be formed by the MBs, P-gp-overexpressed leukemia cells, and SERS probes. The expression level of P-gp determines the amount of SERS probes that can be captured. Consequently, the SERS intensity of the immunocomplex can be used to evaluate the expression level of P-gp. In a typical procedure, we measured the P-gp expression of an MDR leukemia cell line (K562/ADM) as well as unprocessed whole-blood samples. The SERS intensity of K562/ADM cells was highly correlated with the extent of MDR or the incubation time of adriamycin (which is an MDR inducing drug). In addition, the SERS intensity of the refractory/relapsing group was about sixfolds of that of the control group ( P < 0.01). These results demonstrated that the proposed method holds excellent sensitivity, specificity, reliability, and application potential in assessing both cultured cells and clinical samples. With these outstanding features, we anticipated that such a SERS-based method could be very helpful for the clinical diagnosis of early-stage MDR in leukemia.

Crystal structure of B-cell co-receptor CD19 in complex with antibody B43 reveals an unexpected fold

CD19 is a transmembrane protein expressed on malignant B cells, but not in other lineages or other tissues, which makes it an attractive target for monoclonal antibody-mediated immunotherapy. Anti-CD19 antibody B43 was utilized in a bispecific T-cell engager (BiTE) blinatumomab that demonstrated potency for the treatment of relapsed acute lymphoblastic leukemia. To gain insight into the mechanism of action of the antibody, the crystal structure of B43 Fab was determined in complex with CD19 and in the unbound form. The structure revealed the binding epitope, explained the lack of cross-reactivity toward non-human species, and suggested the key-and-lock mechanism of antigen recognition. Most unexpectedly, the structure revealed a unique molecular topology of CD19. Rather than a tandem of c-type immunoglobulin folds predicted from the amino acid sequence, the extracellular domain of CD19 exhibits an elongated β-sandwich formed by two immunoglobulin folds by swapping their C-terminal halves. This is the first structure of CD19, which has no sequence homologs.

The UDP-glucose ceramide glycosyltransferase (UGCG) and the link to multidrug resistance protein 1 (MDR1)

The UDP-glucose ceramide glycosyltransferase (UGCG) is a key enzyme in the sphingolipid metabolism by generating glucosylceramide (GlcCer), the precursor for all glycosphingolipids (GSL), which are essential for proper cell function. Interestingly, the UGCG is also overexpressed in several cancer types and correlates with multidrug resistance protein 1 (MDR1) gene expression. This membrane protein is responsible for efflux of toxic substances and protects cancer cells from cell damage through chemotherapeutic agents. Studies showed a connection between UGCG and MDR1 overexpression and multidrug resistance development, but the precise underlying mechanisms are unknown. Here, we give an overview about the UGCG and its connection to MDR1 in multidrug resistant cells. Furthermore, we focus on UGCG transcriptional regulation, the impact of UGCG on cellular signaling pathways and the effect of UGCG and MDR1 on the lipid composition of membranes and how this could influence multidrug resistance development. To our knowledge, this is the first review presenting an overview about UGCG with focus on the relationship to MDR1 in the process of multidrug resistance development.

Feasibility Study of a Novel Experimental Induction Protocol Combining B43-PAP (Anti-CD19) Immunotoxin With Standard Induction Chemotherapy in Children and Adolescents With Relapsed B-Lineage ALL: A Report From the Children's Oncology Group

BACKGROUND

B43-pokeweed antiviral protein (B43-PAP) is a high-affinity anti-CD19 immunotoxin that is capable of causing apoptotic death in B-lineage leukemic cells with a drug-resistant phenotype. B43-PAP exhibited in vivo antileukemic activity in preclinical studies as well as on a single-agent phase I clinical trial. This pediatric phase I/II study evaluated the toxicity profile and efficacy of B43-PAP immunotoxin in combination with standard induction chemotherapy in children and adolescents with relapsed CD19-positive B-lineage acute lymphoblastic leukemia (B-ALL). Pharmacokinetic profile and immunogenicity of B43-PAP were assessed.

EXPERIMENTAL DESIGN

B43-PAP in combination with standard 3 and 4-drug induction chemotherapy was administered on days 9-13 and 21-25 of a 28-day treatment course with vincristine, prednisone, L-asparaginase, daunomycin, and intrathecal methotrexate. Thirty patients with relapsed B-ALL were enrolled on study CCG-0957.

RESULTS

Grade III/IV nonhematologic dose-limiting toxicities were encountered in 4 patients evaluable for toxicity and included myalgias, motor dysfunction, pulmonary toxicity, and elevated liver transaminase. Dose-limiting toxicities occurred only with the 4-drug regimen. Fourteen patients achieved a complete remission at the end of induction among the 20 patients evaluable for response.

CONCLUSIONS

B43-PAP in combination with standard induction chemotherapy can be safely administered and exhibits clinical antileukemic activity against relapsed B-ALL.

Sustained Epigenetic Drug Delivery Depletes Cholesterol-Sphingomyelin Rafts from Resistant Breast Cancer Cells, Influencing Biophysical Characteristics of Membrane Lipids

Cell-membrane lipid composition can greatly influence biophysical properties of cell membranes, affecting various cellular functions. We previously showed that lipid synthesis becomes altered in the membranes of resistant breast cancer cells (MCF-7/ADR); they form a more rigid, hydrophobic lipid monolayer than do sensitive cell membranes (MCF-7). These changes in membrane lipids of resistant cells, attributed to epigenetic aberration, significantly affected drug transport and endocytic function, thus impacting the efficacy of anticancer drugs. The present study's objective was to determine the effects of the epigenetic drug, 5-aza-2'-deoxycytidine (DAC), delivered in sustained-release nanogels (DAC-NGs), on the composition and biophysical properties of membrane lipids of resistant cells. Resistant and sensitive cells were treated with DAC in solution (DAC-sol) or DAC-NGs, and cell-membrane lipids were isolated and analyzed for lipid composition and biophysical properties. In resistant cells, we found increased formation of cholesterol-sphingomyelin (CHOL-SM) rafts with culturing time, whereas DAC treatment reduced their formation. In general, the effect of DAC-NGs was greater in changing the lipid composition than with DAC-sol. DAC treatment also caused a rise in levels of certain phospholipids and neutral lipids known to increase membrane fluidity, while reducing the levels of certain lipids known to increase membrane rigidity. Isotherm data showed increased lipid membrane fluidity following DAC treatment, attributed to decrease levels of CHOL-SM rafts (lamellar beta [Lβ] structures or ordered gel) and a corresponding increase in lipids that form lamellar alpha-structures (Lα, liquid crystalline phase). Sensitive cells showed marginal or insignificant changes in lipid profile following DAC-treatment, suggesting that epigenetic changes affecting lipid biosynthesis are more specific to resistant cells. Since membrane fluidity plays a major role in drug transport and endocytic function, treatment of resistant cells with epigenetic drugs with altered lipid profile could facilitate anticancer drug transport to overcome acquired drug resistance in a combination therapy.

A double blinded, placebo-controlled pilot study to examine reduction of CD34 +/CD117 +/CD133 + lymphoma progenitor cells and duration of remission induced by neoadjuvant valspodar in dogs with large B-cell lymphoma

We previously described a population of lymphoid progenitor cells (LPCs) in canine B-cell lymphoma defined by retention of the early progenitor markers CD34 and CD117 and “slow proliferation” molecular signatures that persist in the xenotransplantation setting. We examined whether valspodar, a selective inhibitor of the ATP binding cassette B1 transporter (ABCB1, a.k.a., p-glycoprotein/multidrug resistance protein-1) used in the neoadjuvant setting would sensitize LPCs to doxorubicin and extend the length of remission in dogs with therapy naïve large B-cell lymphoma. Twenty dogs were enrolled into a double-blinded, placebo controlled study where experimental and control groups received oral valspodar (7.5 mg/kg) or placebo, respectively, twice daily for five days followed by five treatments with doxorubicin 21 days apart with a reduction in the first dose to mitigate the potential side effects of ABCB1 inhibition. Lymph node and blood LPCs were quantified at diagnosis, on the fourth day of neoadjuvant period, and 1-week after the first chemotherapy dose. Valspodar therapy was well tolerated. There were no differences between groups in total LPCs in lymph nodes or peripheral blood, nor in event-free survival or overall survival. Overall, we conclude that valspodar can be administered safely in the neoadjuvant setting for canine B-cell lymphoma; however, its use to attenuate ABCB1 ⁺ cells does not alter the composition of lymph node or blood LPCs, and it does not appear to be sufficient to prolong doxorubicin-dependent remissions in this setting.

A double blinded, placebo-controlled pilot study to examine reduction of CD34 +/CD117 +/CD133 + lymphoma progenitor cells and duration of remission induced by neoadjuvant valspodar in dogs with large B-cell lymphoma

We previously described a population of lymphoid progenitor cells (LPCs) in canine B-cell lymphoma defined by retention of the early progenitor markers CD34 and CD117 and "slow proliferation" molecular signatures that persist in the xenotransplantation setting. We examined whether valspodar, a selective inhibitor of the ATP binding cassette B1 transporter (ABCB1, a.k.a., p-glycoprotein/multidrug resistance protein-1) used in the neoadjuvant setting would sensitize LPCs to doxorubicin and extend the length of remission in dogs with therapy naïve large B-cell lymphoma. Twenty dogs were enrolled into a double-blinded, placebo controlled study where experimental and control groups received oral valspodar (7.5 mg/kg) or placebo, respectively, twice daily for five days followed by five treatments with doxorubicin 21 days apart with a reduction in the first dose to mitigate the potential side effects of ABCB1 inhibition. Lymph node and blood LPCs were quantified at diagnosis, on the fourth day of neoadjuvant period, and 1-week after the first chemotherapy dose. Valspodar therapy was well tolerated. There were no differences between groups in total LPCs in lymph nodes or peripheral blood, nor in event-free survival or overall survival. Overall, we conclude that valspodar can be administered safely in the neoadjuvant setting for canine B-cell lymphoma; however, its use to attenuate ABCB1 ⁺ cells does not alter the composition of lymph node or blood LPCs, and it does not appear to be sufficient to prolong doxorubicin-dependent remissions in this setting.

Pluronics and MDR reversal: an update

Multidrug resistance (MDR) remains one of the biggest obstacles for effective cancer therapy. Currently there are only few methods that are available clinically that are used to bypass MDR with very limited success. In this review we describe how MDR can be overcome by a simple yet effective approach of using amphiphilic block copolymers. Triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), arranged in a triblock structure PEO-PPO-PEO, Pluronics or “poloxamers”, raised a considerable interest in the drug delivery field. Previous studies demonstrated that Pluronics sensitize MDR cancer cells resulting in increased cytotoxic activity of Dox, paclitaxel, and other drugs by 2–3 orders of magnitude. Pluronics can also prevent the development of MDR in vitro and in vivo. Additionally, promising results of clinical studies of Dox/Pluronic formulation reinforced the need to ascertain a thorough understanding of Pluronic effects in tumors. These effects are extremely comprehensive and appear on the level of plasma membranes, mitochondria, and regulation of gene expression selectively in MDR cancer cells. Moreover, it has been demonstrated recently that Pluronics can effectively deplete tumorigenic intrinsically drug-resistant cancer stem cells (CSC). Interestingly, sensitization of MDR and inhibition of drug efflux transporters is not specific or selective to Pluronics. Other amphiphilic polymers have shown similar activities in various experimental models. This review summarizes recent advances of understanding the Pluronic effects in sensitization and prevention of MDR.

Pharmacogenetic considerations for non-Hodgkin's lymphoma therapy

INTRODUCTION

Chemotherapy is the current standard treatment for hematological malignancies for both curative and palliative purposes. Unfortunately, in the current treatment scenario chemotherapy resistance is an issue that is know to lead to a relapse in cancer. The multidrug resistance 1 (MDR1) gene is often involved in drug resistance and, so far, the best studied mechanism of resistance relates to the level of P-glycoprotein (P-gp) expression on cancer cells; however, correlation with single nucleotide polymorphism (SNP) in the MDR1 gene has also been observed via a number of different mechanisms that interfere with function and expression of P-gp.

AREAS COVERED

This article describes the influence of P-gp expression and SNP on the MDR1 gene in non-Hodgkin's lymphoma (NHL) and their effect on both its risk and outcome. The authors also provide a brief summary of the more important therapeutic options, which aim to overcome this drug resistance mechanism, and discuss their known mechanisms of action.

EXPERT OPINION

There is evidence pertaining to an association between the outcome of NHL and P-gp expression. However, the authors emphasize the need for more studies to reinforce this evidence. Furthermore, there is a definite need for the therapeutic targets, which provide tumor cellular lines of interest, to be tested in humans, in order to better evaluate their toxicity and overall effect on the outcome. The ultimate aim of this research is to develop specifically designed therapies that are tailored to the intrinsic characteristics of specific patients.

Breast cancer-derived microparticles display tissue selectivity in the transfer of resistance proteins to cells

Microparticles (MPs) play a vital role in cell communication by facilitating the horizontal transfer of cargo between cells. Recently, we described a novel "non-genetic" mechanism for the acquisition of multidrug resistance (MDR) in cancer cells by intercellular transfer of functional P-gp, via MPs. MDR is caused by the overexpression of the efflux transporters P-glycoprotein (P-gp) and Multidrug Resistance-Associated Protein 1 (MRP1). These transporters efflux anticancer drugs from resistant cancer cells and maintain sublethal intracellular drug concentrations. By conducting MP transfer experiments, we show that MPs derived from DX breast cancer cells selectively transfer P-gp to malignant MCF-7 breast cells only, in contrast to VLB100 leukaemic cell-derived MPs that transfer P-gp and MRP1 to both malignant and non-malignant cells. The observed transfer selectivity is not the result of membrane restrictions for intercellular exchange, limitations in MP binding to recipient cells or the differential expression of the cytoskeletal protein, Ezrin. CD44 (isoform 10) was found to be selectively present on the breast cancer-derived MPs and not on leukaemic MPs and may contribute to the observed selective transfer of P-gp to malignant breast cells observed. Using the MCF-7 murine tumour xenograft model we demonstrated the stable transfer of P-gp by MPs in vivo, which was found to localize to the tumour core as early as 24 hours post MP exposure and to remain stable for at least 2 weeks. These findings demonstrate a remarkable capacity by MPs to disseminate a stable resistant trait in the absence of any selective pressure.

Lipid raft modulation by Rp1 reverses multidrug resistance via inactivating MDR-1 and Src inhibition

Multidrug resistance (MDR) is a major obstacle to effective cancer therapy. The membrane transporter MDR-1 (P-gp, ABCB1), a member of the ATP-binding cassette (ABC) transporter family, effluxes anti-cancer drugs from cancer cells. Increased activity of MDR-1 is known to be the main mechanism for multidrug resistance. MDR-1 is known to be localized in the cholesterol- and sphingolipid-enriched plasma membrane microdomains, known as lipid rafts. Disruption of lipid rafts by cholesterol depletion alters lipid raft functions, indicating that cholesterol is critical for raft function. Because ginsenosides are structurally similar to cholesterol, in this study, we investigated the effect of Rp1, a novel ginsenoside derivative, on drug resistance using drug-sensitive OVCAR-8 and drug-resistant NCI/ADR-RES and DXR cells. Rp1 treatment resulted in an accumulation of doxorubicin or rhodamine 123 by decreasing MDR-1 activity in doxorubicin-resistant cells. Rp1 synergistically induced cell death with actinomycin D in DXR cells. Rp1 appeared to redistribute lipid rafts and MDR-1 protein. Moreover, Rp1 reversed resistance to actinomycin D by decreasing MDR-1 protein levels and Src phosphorylation with modulation of lipid rafts. Addition of cholesterol attenuated Rp1-induced raft aggregation and MDR-1 redistribution. Rp1 and actinomycin D reduced Src activity, and overexpression of active Src decreased the synergistic effect of Rp1 with actinomycin D. Rp1-induced drug sensitization was also observed with several anti-cancer drugs, including doxorubicin. These data suggest that lipid raft-modulating agents can be used to inhibit MDR-1 activity and thus overcome drug resistance.

Single molecule tracking of P-glycoprotein in live cells reveals dynamic heterogeneity

P-glycoprotein transports chemotherapy drugs from the plasma membrane and allows cancer cells to survive treatment. We transiently transfected PGP labeled with enhanced green fluorescent protein (PGP-EGFP) into MES-SA cells and used single molecule tracking techniques to characterize the dynamics on the surface of live cells. PGP exhibits freely diffusive behavior at short times and is confined at long times with a transition to anomalous diffusion at 0.7 s.

Epigenetic modulation of the biophysical properties of drug-resistant cell lipids to restore drug transport and endocytic functions

In our recent studies exploring the biophysical characteristics of resistant cell lipids, and the role they play in drug transport, we demonstrated the difference of drug-resistant breast cancer cells from drug-sensitive cells in lipid composition and biophysical properties, suggesting that cancer cells acquire a drug-resistant phenotype through the alteration of lipid synthesis to inhibit intracellular drug transport to protect from cytotoxic effect. In cancer cells, epigenetic changes (e.g., DNA hypermethylation) are essential to maintain this drug-resistant phenotype. Thus, altered lipid synthesis may be linked to epigenetic mechanisms of drug resistance. We hypothesize that reversing DNA hypermethylation in resistant cells with an epigenetic drug could alter lipid synthesis, changing the cell membrane's biophysical properties to facilitate drug delivery to overcome drug resistance. Herein we show that treating drug-resistant breast cancer cells (MCF-7/ADR) with the epigenetic drug 5-aza-2'-deoxycytidine (decitabine) significantly alters cell lipid composition and biophysical properties, causing the resistant cells to acquire biophysical characteristics similar to those of sensitive cell (MCF-7) lipids. Following decitabine treatment, resistant cells demonstrated increased sphingomyelinase activity, resulting in a decreased sphingomyelin level that influenced lipid domain structures, increased membrane fluidity, and reduced P-glycoprotein expression. Changes in the biophysical characteristics of resistant cell lipids facilitated doxorubicin transport and restored endocytic function for drug delivery with a lipid-encapsulated form of doxorubicin, enhancing the drug efficacy. In conclusion, we have established a new mechanism for efficacy of an epigenetic drug, mediated through changes in lipid composition and biophysical properties, in reversing cancer drug resistance.

The frequency and clinical relevance of multidrug resistance protein expression in patients with lymphoma

Objective: Multidrug resistance is a cause of treatment failure in patients with malignant lymphoma; however, the frequency and clinical relevance of multidrug resistance protein expression are unclear. The present study aimed to investigate expression of the most common multidrug resistance proteins in a group of lymphoma patients.

Material and Methods: The study included 44 previously untreated lymphoma patients (non-Hodgkin’s lymphoma [n = 21], non-malignant lymphadenopathy [n = 13], and Hodgkin’s lymphoma [n = 10]). MDR1, MRP, and LRP expression was assessed via quantitative PCR of lymph node biopsy specimens.

Results: In the non-Hodgkin’s lymphoma group MDR1 was positive in 23.8% (5/21) of the patients, MRP was positive in 57.14% (12/21), and LRP was positive in 90.47% (19/21). In the non-malignant lymphadenopathy group, MDR1 was positive in 46.15% (6/13) of the patients, MRP was positive in 84.61% (11/13), and LRP was positive in 100% (13/13). In the Hodgkin’s lymphoma group MDR1 was positive in 50% (5/10) of the patients, MRP was positive in 50% (5/10), and LRP was positive in 80% (8/10). MDR1, MRP, and LRP expression did not differ between the 3 groups. Furthermore, MDR1, MRP, and LRP expression wasn’t associated with tumor stage, response to first-line therapy, the erythrocyte sedimentation rate, or C reactive protein, beta 2 microglobulin, serum lactate dehydrogenase, and albumin levels. Additionally, survival time in the MDR1- and MRP-positive, and MDR1- and MRP-negative patients did not differ (comparison of LRP was not possible due to the small number of LRP-negative patients).

Conclusion: According to the present findings, future studies should investigate alternative pathways of multidrug resistance in order to arrive at a better understanding of treatment failure in lymphoma patients.

Multi-modal strategies for overcoming tumor drug resistance: hypoxia, the Warburg effect, stem cells, and multifunctional nanotechnology

Inefficiencies in systemic drug delivery and tumor residence as well as micro-environmental selection pressures contribute to the development of multidrug resistance (MDR) in cancer. Characteristics of MDR include abnormal vasculature, regions of hypoxia, up-regulation of ABC-transporters, aerobic glycolysis, and an elevated apoptotic threshold. Nano-sized delivery vehicles are ideal for treating MDR cancer as they can improve the therapeutic index of drugs and they can be engineered to achieve multifunctional parameters. The multifunctional ability of nanocarriers makes them more adept at treating heterogeneous tumor mass than traditional chemotherapy. Nanocarriers also have preferential tumor accumulation via the EPR effect; this accumulation can be further enhanced by actively targeting the biological profile of MDR cells. Perhaps the most significant benefit of using nanocarrier drug delivery to treat MDR cancer is that nanocarrier delivery diverts the effects of ABC-transporter mediated drug efflux; which is the primary mechanism of MDR. This review discusses the capabilities, applications, and examples of multifunctional nanocarriers for the treatment of MDR. This review emphasizes multifunctional nanocarriers that enhance drug delivery efficiency, the application of RNAi, modulation of the tumor apoptotic threshold, and physical approaches to overcome MDR.

Chimeric, divalent and tetravalent anti-CD19 monoclonal antibodies with potent in vitro and in vivo antitumor activity against human B-cell lymphoma and pre-B acute lymphoblastic leukemia cell lines

CD19 is an attractive therapeutic target for treating human B-cell tumors. In our study, chimeric (c) divalent (cHD37) and tetravalent (cHD37-DcVV) anti-CD19 monoclonal antibodies (MAbs) were constructed, expressed and evaluated for their binding to human 19-positive (CD19(+)) tumor cell lines. They were also tested for proapoptotic activity and the ability to mediate effector functions. The antitumor activity of these MAbs was further tested in mice xenografted with the CD19(+) Burkitt's lymphoma cell line, Daudi or the pre-B acute lymphoblastic leukemia (ALL) cell line, NALM-6. The cHD37 and cHD37-DcVV MAbs exhibited specific binding and comparable proapoptotic activity on CD19(+) tumor cell lines in vitro. In addition, the cHD37 and cHD37-DcVV MAbs were similar in their ability to mediate antibody-dependent cell-mediated phagocytosis (ADCP). However, the tetravalent cHD37-DcVV MAb bound more avidly, had a slower dissociation rate, and did not internalize as well. It also had enhanced antibody-dependent cellular cytotoxicity (ADCC) with human but not murine effector cells. The cHD37 and cHD37-DcVV MAbs exhibited comparable affinity for the human neonatal Fc receptor (FcRn) and similar pharmacokinetics (PKs) in mice. Moreover, all the HD37 constructs were similar in extending the survival of mice xenografted with Daudi or NALM-6 tumor cells. Therefore, the cHD37 and cHD37-DcVV MAbs have potent antitumor activity and should be further developed for use in humans. Although not evident in mice, due to its increased ability to mediate ADCC with human but not mouse effector cells, the cHD37-DcVV MAb should have superior therapeutic efficacy in humans.

Inhibition of multidrug resistance by adamantylgb3, a globotriaosylceramide analog

Multidrug resistance (MDR) via the ABC drug transporter (ABCB1), P-glycoprotein (P-gp/MDR1) overexpression, is a major obstacle in cancer chemotherapy. Many inhibitors reverse MDR but, like cyclosporin A (CsA), have significant toxicities. MDR1 is also a translocase that flips glucosylceramide inside the Golgi to enhance neutral glycosphingolipid (GSL) synthesis. We observed partial MDR1/globotriaosylceramide (Gb3) cell surface co-localization, and GSL removal depleted cell surface MDR1. MDR1 may therefore interact with GSLs. AdamantylGb3, a water-soluble Gb3 mimic, but not other GSL analogs, reversed MDR1-MDCK cell drug resistance. Cell surface MDR1 was up-regulated 1 h after treatment with CsA or adaGb3, but at 72 h, cell surface expression was lost. Intracellular MDR1 accumulated throughout, suggesting long term defects in plasma membrane MDR1 trafficking. AdaGb3 or CsA rapidly reduced rhodamine 123 cellular efflux. MDR1 also mediates gastrointestinal epithelial drug efflux, restricting oral bioavailability. Vinblastine apical-to-basal transport in polarized human intestinal C2BBe1 cells was significantly increased when adaGb3 was added to both sides, or to the apical side only, comparable with verapamil, a standard MDR1 inhibitor. Disulfide cross-linking of mutant MDR1s showed no binding of adaGb3 to the MDR1 verapamil/cyclosporin-binding site between surface proximal helices of transmembrane segments (TM) 6 and TM7, but rather to an adjacent site nearer the center of TM6 and the TM7 extracellular face, i.e. close to the bilayer leaflet interface. Verotoxin-mediated Gb3 endocytosis also up-regulated total MDR1 and inhibited drug efflux. Thus, a functional interplay between membrane Gb3 and MDR1 provides a more physiologically based approach to MDR1 regulation to increase the bioavailability of chemotherapeutic drugs.

Localization of the human breast cancer resistance protein (BCRP/ABCG2) in lipid rafts/caveolae and modulation of its activity by cholesterol in vitro

Breast cancer resistance protein (BCRP/ABCG2) is an active efflux pump that belongs to the ATP-binding cassette (ABC) transporter family. It is located in various tissues involved in drug absorption, distribution, and elimination and plays an important role in multidrug resistance. For P-glycoprotein, another member of the ABC transporter family, it is well established that it is at least partly located in cholesterol and sphingolipid-enriched domains of the plasma membrane called "lipid rafts" and that the composition of the membrane lipids may modulate its efflux activity. This study addressed the compartmentalization of BCRP in the plasma membrane and the influence of membrane cholesterol on the efflux activity of BCRP. As a cell model, we used the canine kidney epithelial cell line MDCKII-BCRP transfected with the cDNA encoding human BCRP and the corresponding parental cell line MDCKII. Cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD) provoked a 40% decrease in BCRP activity (p < 0.01) assessed with flow cytometry (pheophorbide A efflux assay). Cholesterol repletion with MbetaCD/cholesterol-inclusion complexes restored BCRP function, and cholesterol saturation of native cells did not further enhance BCRP activity. Coimmunoprecipitation experiments indicated a physical interaction between BCRP and caveolin-1, and Western blot analysis after density gradient ultracentrifugation demonstrated that BCRP is located in detergent-resistant membranes that also contain caveolin-1. In conclusion, our results demonstrate for the first time that BCRP is located in membrane rafts and that cholesterol has impact on its efflux activity.

Lipid rafts: dream or reality for cholesterol transporters?

As a key constituent of the cell membranes, cholesterol is an endogenous component of mammalian cells of primary importance, and is thus subjected to highly regulated homeostasis at the cellular level as well as at the level of the whole body. This regulation requires adapted mechanisms favoring the handling of cholesterol in aqueous compartments, as well as its transfer into or out of membranes, involving membrane proteins. A membrane exhibits functional properties largely depending on its lipid composition and on its structural organization, which very often involves cholesterol-rich microdomains. Then there is the appealing possibility that cholesterol may regulate its own transmembrane transport at a purely functional level, independently of any transcriptional regulation based on cholesterol-sensitive nuclear factors controling the expression level of lipid transport proteins. Indeed, the main cholesterol "transporters" presently believed to mediate for instance the intestinal absorption of cholesterol, that are SR-BI, NPC1L1, ABCA1, ABCG1, ABCG5/G8 and even P-glycoprotein, all present privileged functional relationships with membrane cholesterol-containing microdomains. In particular, they all more or less clearly induce membrane disorganization, supposed to facilitate cholesterol exchanges with the close aqueous medium. The actual lipid substrates handled by these transporters are not yet unambiguously determined, but they likely concern the components of membrane microdomains. Conversely, raft alterations may provide specific modulations of the transporter activities, as well as they can induce indirect effects via local perturbations of the membrane. Finally, these cholesterol transporters undergo regulated intracellular trafficking, with presumably some relationships to rafts which remain to be clarified.

Anti-Cripto Mab inhibit tumour growth and overcome MDR in a human leukaemia MDR cell line by inhibition of Akt and activation of JNK/SAPK and bad death pathways

Doxorubicin (DOX) selection of CCRF-CEM leukaemia cell line resulted in multidrug resistance (MDR) CEM/A7R cell line, which overexpresses MDR, 1 coded P-glycoprotein (Pgp). Here, we report for the first time that oncoprotein Cripto, a founding member of epidermal growth factor-Cripto-FRL, 1-Criptic family is overexpressed in the CEM/A7R cells, and anti-Cripto monoclonal antibodies (Mab) inhibited CEM/A7R cell growth both in vitro and in an established xenograft tumour in severe combined immunodeficiency mice. Cripto Mab synergistically enhanced sensitivity of the MDR cells to Pgp substrates epirubicin (EPI), daunorubicin (DAU) and non-Pgp substrates nucleoside analogue cytosine arabinoside (AraC). In particular, the combination of anti-Cripto Mab at less than 50% of inhibition concentrations with noncytotoxic concentrations of EPI or DAU inhibited more than 90% of CEM/A7R cell growth. Cripto Mab slightly inhibited Pgp expression, and had little effect on Pgp function, indicating that a mechanism independent of Pgp was involved in overcoming MDR. We demonstrated that anti-Cripto Mab-induced CEM/A7R cell apoptosis, which was associated with an enhanced activity of the c-Jun N-terminal kinase/stress-activated protein kinase and inhibition of Akt phosphorylation, resulting in an activation of mitochondrial apoptosis pathway as evidenced by dephosphorylation of Bad at Ser136, Bcl-2 at Ser70 and a cleaved caspase-9.

Relationships between sterol/phospholipid composition and xenobiotic transport in nematodes

Therapeutic failure limits prophylaxis of nematode diseases and has been mainly attributed to mutations in cellular targets of anthelmintics. Besides these specific mechanisms, alterations of drug transport also occur in parasites resistant to anthelmintics and depend on both the presence of membrane pumps such as P-glycoprotein (Pgp) and on the lipid composition of membranes. We recently showed in the nematode Haemonchus contortus, using eggs as a model, that the total cholesterol (TC) concentration alters the transport of lipophilic molecules due to membrane pumps such as P-glycoprotein and the resistance to anthelmintics. The effect of TC may depend on the presence of other lipids interacting with TC. Therefore, we analysed the lipid composition and its relationship with Pgp and resistance to anthelmintics. Better correlations were found between Pgp and free cholesterol (FC) than with TC. We also showed that the relationships between lipid composition and resistance to anthelmintics or Pgp depended on the equilibrium between FC and phospholipids (PLs), mainly PLs known to be present primarily in either the external leaflets of cell membranes or the internal leaflets. The PLs phosphatidylcholine and phosphatidylethanolamine played the most significant role, but phosphatidic acid also influenced drug resistance.

Rituximab but not Other anti-CD20 Antibodies Reverses Multidrug Resistance in 2 B lymphoma Cell Lines, Blocks the Activity of P-glycoprotein (P-gp), and Induces P-gp to Translocate out of Lipid Rafts

The objective of this study was to investigate the ability of the anti-CD20 antibody, Rituximab (RTX), to inhibit the activity of P-glycoprotein (P-gp), and reverse multidrug resistance (MDR) in 2 P-gp/CD20 lymphoma cell lines. We determined whether RTX would chemosensitize the 2 P-gp cell lines in vitro, and inhibit the ability of the cells to efflux Rhodamine 123. One cell line was infected with an MDR1 vector and the other was generated by drug selection. We also determined whether RTX induced P-gp to translocate out of lipid rafts. RTX chemosensitized 2 different MDR cell lines, inhibited the activity of P-gp in both, and induced P-gp to translocate out of lipid rafts in the 1 cell line that was studied in greater detail. In contrast, 3 other anti-CD20 antibodies did not chemosensitize, inhibit the activity of P-gp, or induce it to translocate out of rafts, despite the fact that 1 antibody recognized the same epitope on CD20. Our results suggest that RTX can chemosensitize 2 CD20/P-gp cell lines in vitro by inhibiting the activity of the P-gp pump. The inhibition of P-gp activity correlated with the ability of RTX to induce P-gp to translocate out of lipid rafts. Although the mechanisms by which RTX effects P-gp translocation and activity are not yet known, they are not associated with acid-sphingomyelinase activation in raft microdomains, as described for the antiproliferative activity of RTX.

Alternative donor transplants for adult acute lymphoblastic leukaemia: a comparison of the three major options

Myeloablative sibling-matched allogeneic transplantation for adult acute lymphoblastic leukaemia provides the best outcome, but most patients lack a suitable, related histocompatible donor. We reviewed three haematopoietic stem cell donor sources used for alternative donor transplantation pointing out drawbacks of these approaches including inherent selection bias. Matched unrelated donor allografts most often are performed in Philadelphia chromosome-positive disease and in second complete remission (CR2); a nearly 30% event-free survival (EFS) can be anticipated in select patients. Transplants using haploidentical donors are most successful if undertaken in CR1 and CR2 and appear to produce EFS rates of about 25%. Limited umbilical cord blood transplant data suggest efficacy, but marked patient and treatment heterogeneity hamper conclusions. Each of these three strategies has unique potential benefits and disadvantages. The growing use of minimal residual disease detection may identify subgroups of patients unlikely to be cured by chemotherapy alone; these patients are candidates for upfront high-dose chemoradiotherapy and cellular immunotherapy. These three approaches are plagued by treatment-related mortality and relapse rates as high as 40%, but advances in technology and supportive care may make each stem cell source more feasible and efficacious.

ABCB1 transporter discriminates human resting naive B cells from cycling transitional and memory B cells

The exact identification of B cell subsets is instrumental to understand their dynamics under physiological and pathological conditions. Human memory B cells are currently identified according to the expression of CD27, which is absent on naive B cells. We report here that the ATP-binding cassette (ABC)B1 transporter is exclusively present on mature CD27- naive B cells, while it is absent in CD27+ memory B cells and in a heterogeneous subset of CD27- cells that comprise both switch memory and transitional B cells. Thus, ABCB1 activity precisely discriminates naive from transitional and all memory B cells. Using this improved method to discriminate human B cell subsets, and Ki67 staining to identify recently divided cells, we show that in both cord blood and adult peripheral blood, mature naive B cells are quiescent while transitional B cells and memory B cells have a high in vivo turnover.

Autologous stem cell transplant in ALL: who should we be transplanting in first remission?

Long-term disease-free survival (DFS) has been reported after autologous stem cell transplantation for acute lymphoblastic leukemia. Phase II studies have evaluated its role in first and subsequent complete remission (CR) with DFS rates of up to 50%. It has been under-utilized in 1st CR in part, due to a concern that patients who relapse after autologous stem cell transplantation (ASCT) have fewer options for salvage treatment of relapsed disease. Unfortunately, survival rates of <5% are reported in patients who relapse, regardless of initial therapy. Few prospective, randomized trials have analyzed large enough numbers of patients to allow us to determine the appropriate patient population for autologous transplantation. Although variability in the available studies makes it difficult to draw a definite conclusion, and many issues remain unresolved, available data suggests that there may be a group of patients for whom ASCT in first remission is a reasonable and perhaps superior treatment choice. Factors such as risk features at diagnosis, and minimal residual disease following induction therapy greatly affect outcome following ASCT. The available data as well as the questions that remain to be answered will be discussed and reviewed.

An update on antifungal targets and mechanisms of resistance in Candida albicans

Much progress has been made in the last decade in identifying genes responsible for antifungal resistance in Candida albicans. Attention has focused on five major C. albicans genes: ABC transporter genes CDR1 and CDR2, major facilitator efflux gene MDR1, and ergosterol biosynthesis genes ERG11 and ERG3. Resistance involves mutations in 14C-lanosterol demethylase, targeted by fluconazole (FLZ) and encoded by ERG11, and mutations that up-regulate efflux genes that probably efflux the antifungals. Mutations that affect ERG3 mutations have been understudied as mechanism resistance among clinical isolates. In vitro resistance in clinical isolates typically involves step-wise mutations affecting more than one of these genes, and often unidentified genes. Different approaches are needed to identify these other genes. Very little is understood about reversible adaptive resistance of C. albicans despite its potential clinical significance; most clinical failures to control infections other than oropharyngeal candidiasis (OPC) occur with in vitro susceptible strains. Tolerance of C. albicans to azoles has been attributed to the calcineurin stress-response pathway, offering new potential targets for next generation antifungals. Recent studies have identified genes that regulate CDR1 or ERG genes. The focus of this review is C. albicans, although information on Saccharomyces cerevisiae or Candida glabrata is provided in areas in where Candida research is underdeveloped. With the completion of the C. albicans genomic sequence, and new methods for high throughput gene overexpression and disruption, rapid progress towards understanding the regulation of resistance, novel resistance mechanisms, and adaptive resistance is expected in the near future.

The implications of P-glycoprotein in HIV: friend or foe?

P-glycoprotein (P-gp), coded by the ABCB1 gene, has a wide tissue distribution. The drug transporter is known to limit the bioavailability of a plethora of drugs and xenobiotics including the human immunodeficiency virus (HIV) protease inhibitors. There remains a considerable degree of debate in the literature with respect to the role of ABCB1 polymorphisms in HIV-treatment outcome and some studies have also implicated antiretroviral drugs as inducers of P-gp. Recent evidence indicates a role for P-gp in the inhibition of viral infectivity and/or release and cellular relationships with other infection-related proteins (and cholesterol). It is becoming increasingly clear that future studies on P-gp in HIV should consider both pharmacological and virological issues.

Isolated Rafts from Adriamycin-Resistant P388 Cells Contain Functional ATPases and Provide an Easy Test System for P-glycoprotein?Related Activities

PURPOSE

P-glycoprotein (P-gp), a membrane ATPase expelling many structurally unrelated compounds out of cells, is one of the major contributors to multidrug resistance. It is enriched in cold TritonX-100 insoluble membrane domains (i.e., rafts). The purpose of this work was to characterize the ATPase activities of raft preparations from P388 cells overexpressing P-gp (P388/ADR) or devoid of P-gp (P388) and to establish a P-gp-enriched screening system for P-gp-interfering compounds.

METHODS

Rafts were extracted with cold TritonX-100. The ATPase activity was characterized in 96-well plates using a fluorescence assay.

RESULTS

The ATPase activity per mg protein was about five times higher in P388/ADR rafts than in crude membranes. The anti-P-gp antibody C219 inhibited 20% of the activity in P388/ADR rafts but only about 10% of the activity in P388/ADR crude membranes and had no effect on the activity of P388 rafts. The known P-gp-activating compounds verapamil, progesterone, and valinomycin revealed the typical bell-shaped activity/concentration profiles in P388/ADR rafts, indicative for activation at low compound concentrations and inhibition at concentrations >10 to 100 microM. The inhibitory effect was also observed in P388 rafts.

CONCLUSIONS

Extracted rafts are rich in functional ATPases. Rafts from P-gp-overexpressing cells display P-gp-typical ATPase activity and provide an easy, P-gp-enriched screening system.

Modulation of cellular cholesterol alters P-glycoprotein activity in multidrug-resistant cells

The drug transporter P-glycoprotein (ABCB1) plays an important role in drug distribution and elimination, and when overexpressed it may confer multidrug resistance (MDR). P-glycoprotein is localized in the plasma membrane, especially within rafts and caveolae, characterized as detergent-resistant membranes (DRMs). This study investigated the effect of cholesterol depletion and repletion as well as saturation on subcellular localization and function of P-glycoprotein to determine the effect of DRM localization on P-glycoprotein-mediated drug efflux. In L-MDR1 overexpressing human P-glycoprotein, cholesterol depletion removed P-glycoprotein from the raft membranes into non-DRM fractions, whereas repletion fully reconstituted raft localization. P-glycoprotein function was assessed by realtime monitoring with confocal laser scanning microscopy using BODIPY-verapamil as substrate. Cholesterol depletion reduced P-glycoprotein function in L-MDR1 cells resulting in intracellular substrate accumulation (159% +/- 43, p < 0.001; control = 100%). Cholesterol repletion reduced intracellular substrate fluorescence (120% +/- 36, p < 0.001) and restored the transporter activity. Addition of surplus cholesterol (saturation) even enhanced drug efflux in L-MDR1 cells, leading to reduced intracellular accumulation of BODIPY-verapamil (69% +/- 10, p < 0.001). Transport of BODIPY-verapamil in cells not expressing human P-glycoprotein (LLC-PK1) was not susceptible to cholesterol alterations. These results demonstrate that cholesterol alterations influence P-glycoprotein localization and function, which might contribute to the large interindividual variability of P-glycoprotein activity known from in vivo studies.