Characterization by somatic cell genetics of a monoclonal antibody to the MDR1 gene product (P-glycoprotein): determination of P-glycoprotein expression in multi-drug-resistant KB and CEM cell variants

Peritoneal macrophages attenuate retinal ganglion cell survival and neurite outgrowth

Inflammation is a critical pathophysiological process that modulates neuronal survival in the central nervous system after disease or injury. However, the effects and mechanisms of macrophage activation on neuronal survival remain unclear. In the present study, we co-cultured adult Fischer rat retinas with primary peritoneal macrophages or zymosan-treated peritoneal macrophages for 7 days. Immunofluorescence analysis revealed that peritoneal macrophages reduced retinal ganglion cell survival and neurite outgrowth in the retinal explant compared with the control group. The addition of zymosan to peritoneal macrophages attenuated the survival and neurite outgrowth of retinal ganglion cells. Conditioned media from peritoneal macrophages also reduced retinal ganglion cell survival and neurite outgrowth. This result suggests that secretions from peritoneal macrophages mediate the inhibitory effects of these macrophages. In addition, increased inflammation- and oxidation-related gene expression may be related to the enhanced retinal ganglion cell degeneration caused by zymosan activation. In summary, this study revealed that primary rat peritoneal macrophages attenuated retinal ganglion cell survival and neurite outgrowth, and that macrophage activation further aggravated retinal ganglion cell degeneration. This study was approved by the Animal Ethics Committee of the Joint Shantou International Eye Center of Shantou University and the Chinese University of Hong Kong, Shantou, Guangdong Province, China, on March 11, 2014 (approval no. EC20140311(2)-P01).

Ins and outs of the ABCG2 multidrug transporter: an update on in vitro functional assays

The major aim of this chapter is to provide a critical overview of the in vitro methods available for studying the function of the ABCG2 multidrug transporter protein. When describing the most applicable assay systems, in each case we present a short overview relevant to ABC multidrug transporters in general, and then we concentrate on the tools applicable to analysis of substrate-drug interactions, the effects of potential activators and inhibitors, and the role of polymorphisms of the ABCG2 transporter. Throughout this chapter we focus on recently developed assay systems, which may provide new possibilities for analyzing the pharmacological aspects of this medically important protein.

HIV-1 integrase inhibitors are substrates for the multidrug transporter MDR1-P-glycoprotein

Background

The discovery of diketoacid-containing derivatives as inhibitors of HIV-1 Integrase (IN) (IN inhibitors, IINs) has played a major role in validating this enzyme as an important target for antiretroviral therapy. Since the in vivo efficacy depends on access of these drugs to intracellular sites where HIV-1 replicates, we determined whether the IINs are recognized by the multidrug transporter MDR1-P-glycoprotein (P-gp) thereby reducing their intracellular accumulation. To address the effect of IINs on drug transport, nine quinolonyl diketo acid (DKA) derivatives active on the HIV-1 IN strand transfer (ST) step and with EC50 ranging from 1.83 to >50 μm in cell-based assays were tested for their in vitro interaction with P-gp in the CEM-MDR cell system. IINs were investigated for the inhibition and induction of the P-gp function and expression as well as for multidrug resistance (MDR) reversing ability.

Results

The HIV-1 IINs act as genuine P-gp substrates by inhibiting doxorubicin efflux and inducing P-gp functional conformation changes as evaluated by the modulation of UIC2 mAb epitope. Further, IINs chemosensitize MDR cells to vinblastine and induce P-gp expression in drug sensitive revertants of CEM-MDR cells.

Conclusion

To our knowledge, this is the first demonstration that HIV-1 IINs are P-gp substrates. This biological property may influence the absorption, distribution and elimination of these novels anti HIV-1 compounds.

A Strong Glutathione S-Transferase Inhibitor Overcomes the P-glycoprotein-mediated Resistance in Tumor Cells

The new glutathione S-transferase inhibitor 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol (NBDHEX) is cytotoxic toward P-glycoprotein-overexpressing tumor cell lines, i.e. CEM-VBL10, CEM-VBL100, and U-2 OS/DX580. The mechanism of cell death triggered by NBDHEX has been deeply investigated in leukemia cell lines. Kinetic data indicate a similar NBDHEX membrane permeability between multidrug resistance cells and their sensitive counterpart revealing that NBDHEX is not a substrate of the P-glycoprotein export pump. Unexpectedly, this molecule promotes a caspase-dependent apoptosis that is unusual in the P-glycoprotein-overexpressing cells. The primary event of the apoptotic pathway is the dissociation of glutathione S-transferase P1-1 from the complex with c-Jun N-terminal kinase. Interestingly, leukemia MDR1-expressing cells show lower LC50 values and a higher degree of apoptosis and caspase-3 activity than their drug-sensitive counterparts. The increased susceptibility of the multidrug resistance cells toward the NBDHEX action may be related to a lower content of glutathione S-transferase P1-1. Given the low toxicity of NBDHEX in vivo, this compound may represent an attractive basis for the selective treatment of MDR1 P-glycoprotein-positive tumors.

Detection of human P-glycoprotein-like molecule in azole-resistant Candida albicans from HIV+ patients

Azole resistance in Candida albicans may be due to several mechanisms. It has been demonstrated that C. albicans possesses sequences with a high degree of homology with the human MDR-1 gene coding for P-glycoprotein (P-gp), belonging to the ATP-binding cassette transporter (ABC) superfamily and responsible for the multidrug resistance (MDR) in tumor cells. On this basis, the expression and intracellular localization of human P-gp-like molecule in C. albicans strains showing different sensitivity to fluconazole were investigated by flow cytometry and immunoelectron microscopy. Post-embedding immunolabeling revealed that monoclonal antibody (mAb) MM4.17, which recognizes an external epitope of human P-gp, reacted with both fluconazole-sensitive (3153 and CO 23-1) and fluconazole-resistant (AIDS 68 and CO 23-2, isolated from AIDS patient and in vitro drug-selected, respectively) strains of C. albicans. However, the resistant strains displayed a number of MM4.17-reactive epitopes much higher than the drug-sensitive ones. The C. krusei ATCC 6458 strain, whose resistance is not mediated by the presence of ABC transporters, was not reactive at all with mAb MM4.17. The specificity of the immunolabeling was confirmed by a competitive inhibition assay performed by using phage clone particles capable of mimicking the MM4.17-reactive epitope. The flow cytometric analysis confirmed a higher level of intracytoplasmic P-gp expression in azole-resistant strains of C. albicans. Both cyclosporin A and verapamil, which are well-known MDR inhibitors, strongly reduced the MICs for fluconazole and itraconazole of the tested azole-resistant AIDS 68 strain, while they did not influence the MICs of either the sensitive 3153 strain of C. albicans or the ATCC 6458 strain of C. krusei. Overall, our data suggest the existence of a P-gp-like drug efflux pump in C. albicans that may participate in the mechanisms of azole-resistance of this fungus.

Detection of P-glycoprotein in the nuclear envelope of multidrug resistant cells

P-glycoprotein is a plasma membrane efflux pump which is responsible for multidrug resistance of many cancer cell lines. A number of studies have demonstrated the presence of P-glycoprotein molecules, besides on the plasma membrane, also in intracellular sites, such as the Golgi apparatus and the nucleus. In this study, the presence and function of P-glycoprotein in the nuclear membranes of human breast cancer cells (MCF-7 WT) and their multidrug resistant variants (MCF-7 DX) were investigated. Electron and confocal microscopy immunolabelling experiments demonstrated the presence of P-glycoprotein molecules in the nuclear membranes of MCF-7 DX cells. Moreover, the labelling pattern was strongly dependent on pH values of the incubation buffer. At physiological pH (7.2), a strong labelling was detected in the cytoplasm and the nuclear matrix in both sensitive and resistant MCF-7 cells. By raising the pH to 8.0, the P-glycoprotein molecules were easily detected in the cytoplasm (transport vesicles and Golgi apparatus), plasma and nuclear membranes exclusively in MCF-7 DX cells. Furthermore, drug uptake and efflux studies, performed by flow cytometry on isolated nuclei in the presence of the P-glycoprotein inhibitor cyclosporin A, suggested the presence of a functional P-glycoprotein in the nuclear membrane, but not in the nuclear matrix, of drug resistant cells. Therefore, P-glycoprotein in the nuclear envelope seems to represent a further defense mechanism developed by resistant cells against antineoplastic agents.

Epitope mapping of the monoclonal antibody MM12.10 to external MDR1 P-glycoprotein domain by synthetic peptide scanning and phage display technologies

Epitope mapping of MDR1-P-glycoprotein using specific monoclonal antibodies (mAbs) may help in delineating P-glycoprotein topology and hence in elucidating the relationship between its structural organization and drug-efflux pump function. In this work, by using synthetic peptide scanning and phage display technologies, the binding sites of the mAb MM12.10, a novel antibody to intact human multidrug resistant (MDR) cells, were studied. The results we obtained confirm that two regions localized on the predicted fourth and sixth loops are indeed external and that MDR1 peptides covering the inner domain of the current 12 transmembrane segment (TMs) model of P-glycoprotein could form part of the MM12.10 epitope.

P-glycoprotein expression in de novo acute myeloid leukemia

Detection of the multidrug resistance P-glycoprotein (PGP) phenotype was performed at the time of diagnosis in 223 patients with acute myeloid leukemia (AML) by flow cytometry using C219 Monoclonal Antibody (MoAb). On the other hand, JSB1 MoAb was tested in 173 of these samples. At onset, PGP was detected in 57.4% of cases with C219 and 75.9% of cases with JSB1. There was no correlation between PGP expression and sex, age, marrow blast percentage or extramedullary disease. On the contrary, strict correlations were noted either between C219 negativity and FAB M3 subtype or between C219 positivity and FAB M5 group (P = 0.003). Significant correlation was found between PGP phenotype and CD7, as 143 of 223 samples had similar patterns of staining with C219 (P < 0.0001). Finally, there was a close relationship between C219 and JSB1 positivity: all the C219+ cases were positive for JSB1 (P < 0.0001). Concerning the karyotype, most patients with monosomy or del (7) were MDR positive; on the other hand, most patients with t(8;21) or t(15;17) were MDR negative. Rh123 accumulation studies showed a significant decrease of mean fluorescence intensities both in C219 and in JSB1 positive cases in comparison with PGP negative ones (P < 0.001). A significant decrease of remission induction rates (CR) was highlighted both between C219+ and C219- and between JSB1+ and JSB1- cases (32.1% v 62.1% and 32.6% v 73.8%, respectively, with P < 0.0001). The overall survival and the remission duration (CCR) were significantly shorter both in C219+ and in JSB1+ patients with no relationship to age. Furthermore, a higher rate of early relapses was noted among MDR+ when compared with MDR- patients both for C219+ and JSB1+ cases. The combination (C219- JSB1+) identified a subset of patients with an intermediate prognosis. On multivariate analysis, C219 and JSB1 were confirmed to be independent prognostic factors for achievement of CR, overall survival and CCR. In conclusion, the assessment of MDR phenotype by flow cytometry is a crucial prognostic factor of treatment outcome in AML.

Tumor necrosis factor alpha is a powerful apoptotic inducer in lymphoid leukemic cells expressing the P-170 glycoprotein

Multidrug resistance (MDR) is a phenomenon by which tumor cells exposed to a single anti-proliferative agent acquire resistance to other structurally and functionally unrelated drugs. The classical form of MDR is caused by a plasma-membrane protein currently named P-glycoprotein or P-170 encoded by the human mdr-1 gene in its functional isoform. In vitro cell lines expressing P-170 usually also present phenotypic and functional alterations. In the present study we report that the cytotoxicity mediated by tumor necrosis factor alpha (TNF alpha) in MDR variants of the human T-lymphoblastoid CEM cell line is associated with apoptosis (programmed cell death). Susceptibility of MDR cells to apoptosis was increased upon cycloheximide + TNF alpha sequential treatment, whereby the impairment of protein synthesis due to the former agent was followed by the effect of cytokine exposure. Massive apoptosis of P-170-positive cells, but not of controls, was also obtained by depletion of nutrients (i.e., serum starvation). In contrast, TNF-alpha exerted a similar apoptotic effect in epithelial (MCF-7) or myeloma (S8226) drug-sensitive/ -resistant cell pairs. However, the MDR variant of myeloma S8226 was more sensitive to the cytostatic effect of TNF alpha than the parental drug-sensitive cell line. These results suggest that the presence of the MDR phenotype may be associated with increased histotype-dependent cell susceptibility to specific, protein-synthesis-independent, apoptotic pathways.

Topology of MDR1-P-glycoprotein as indicated by epitope mapping of monoclonal antibodies to human MDR cells

The MDR1-P-glycoprotein binding sites of three different murine monoclonal antibodies (MM4.17, MM6.15 and MC57), directed towards living, intact human multidrug-resistant cells were investigated in order to study P-glycoprotein topology. By using synthetic peptide scanning, we demonstrated that well-defined regions localized on the predicted first, fourth and sixth extracellular loops are external. On the basis of the structure of MM6.15 epitope, which is distributed on the above three different extracellular loops (and thus is discontinuous), P-glycoprotein molecules result to be differently organized in the lipid bilayer. Moreover, the outcome of the MC57 and MM4.17 epitopes localization experiments, obtained through the use of phage-displayed peptide libraries, represent an additional challenge to the classical 12-transmembrane domain model of P-glycoprotein, since they agree with the novel topography of the molecule (10-transmembrane domain), which was recently proposed on the basis of biochemical and expression studies.

Isolation of antigenic mimics of MDR1-P-glycoprotein by phage-displayed peptide libraries

To identify an MC57 epitope which is more efficiently expressed on inactivated forms of P-glycoprotein we utilized peptide libraries displayed on filamentous phage. Using this technology, we selected specific phage clones blocking the binding of the murine monoclonal (MAb) MC57 with live human multi-drug-resistant (MDR) cells, and sequenced and analyzed their DNA. The results we obtained indicate that MAb MC57 epitope could be formed by 2 regions localized on the predicted fourth and sixth extracellular loops of the current 12-transmembrane-domain model predicted for MDR1-P-glycoprotein. Surprisingly, a third region, defined by residues 800-807 of the MDR1-P-glycoprotein sequence and postulated to be intracellular, was also identified as a putative part of the MC57 epitope. This finding adds weight to the interesting hypothesis that a P-glycoprotein structure different from the current model may exist.

P-glycoprotein epitope mapping. II. The murine monoclonal antibody MM6.15 to human multidrug-resistant cells binds with three distinct loops in the MDR1-P-glycoprotein extracellular domain

A new murine monoclonal antibody (MAb), MM6.15, to human MDR1 P-glycoprotein was found to be reactive in ELISA with synthetic peptides selected from the predicted sequences of the first, fourth and sixth extracellular loop of MDR1-P-glycoprotein. In order to precisely define the MM6.15-binding site, a peptide library of overlapping 5- to 9-mer residues covering the entire sixth extracellular loop of both human and rodent class-1 P-glycoproteins was synthesized on polyethylene pins and tested for MAb binding. The results of this ELISA demonstrated that the MAb MM6.15 reacts only with human synthetic peptides and that the critical component of the MAb recognition is made up of the amino-acid sequence LVAHKL (residues 963-968 of the MDR1-P-glycoprotein) with histidine (H), lysine (K) and possibly leucine (L), key residues of this immunogenic domain.

P-glycoprotein expression in acute myeloblastic leukemia analyzed by immunocytochemistry and flow cytometry

Using the APAAP technique, we assessed the reactivity of five monoclonal antibodies, JSB1, C219, Mab 57, 2F8 and MRK16, to gp 170 in 60 cases of de novo acute myeloid leukemia (AML) and 13 relapses. Reactivity, varied between the five antibodies, and positivity was obtained with 2F8 > JSB1 > MRK16 > Mab57 > C219. Sixteen of the 60 cases were also studied by flow cytometry. In 10 cases, the results with the two techniques corresponded, in the other 6 cases, flow cytometry proved more sensitive than APAAP in detecting small amounts of gp170. In the flow cytometry analysis, the cells fixed in methanol and paraformaldehyde were more fluorescent than unfixed samples or those fixed in paraformaldehyde or methanol alone. Our results thus reveal that positivity for gp 170 depends on various factors, including the specificity of the monoclonal antibodies, the techniques used and the preservation of the samples. This suggests the need for a clear standardization of the methods to detect gp170.

Zidovudine induces the expression of cellular resistance affecting its antiviral activity

We have previously shown that multidrug-resistant cells expressing the multidrug transporter P-glycoprotein are less sensitive to the antiviral activity of AZT. Subsequently, we addressed the question whether AZT itself is able to induce cellular resistance to the drug. Indeed, CEM cells propagated in the presence of increasing concentrations of AZT become resistant to the antigrowth and antiviral activity of AZT but do not express detectable level of P-glycoprotein. Sensitivity of these cells to other compounds, such as vinblastine, vincristine, ddI, and ddC remained unchanged, indicating that, in contrast to P-glycoprotein-positive cells, AZT-induced resistance is specific for AZT. Interestingly, in AZT-induced resistant cells the intracellular accumulation of AZT and exogenous deoxythymidine, as well as thymidine kinase activity, are significantly reduced when compared with the parental cell line. Our findings show that AZT itself may directly induce the expression of cellular mechanisms leading to the acquisition of specific cellular resistance that can affect its antiviral activity.

Immunohistochemical evaluation of P-glycoprotein in human malignancies by monoclonal antibody MC57

P-glycoprotein expression was analyzed on 137 formalin-fixed, paraffin-embedded human tumours by monoclonal antibody (MAb) MC57. This MAb recognizes an extracellular human specific P-glycoprotein epitope and defines their multidrug resistance (MDR) phenotype by its binding on cells. Immunohistochemistry indicated that this MAb reacted in human cells and tissues in the same pattern as that found with other MAbs to P-glycoprotein. However, the present extensive study demonstrated that MAb MC57 is a highly specific reagent for the evaluation of an extracellular P-glycoprotein epitope preserved after fixation procedures and that this MAb is available to assess P-glycoprotein expression in routinely processed human tumour specimens.

P-glycoprotein epitope mapping. I. Identification of a linear human-specific epitope in the fourth loop of the P-glycoprotein extracellular domain by MM4.17 murine monoclonal antibody to human multi-drug-resistant cells

A new murine monoclonal antibody (MAb), MM4.17, to human multi-drug-resistant (MDR) cells was found to be reactive in an ELISA with a synthetic 16-amino acid peptide selected from the fourth loop of the P-glycoprotein extracellular domain. Immunohistochemistry indicated that this MAb reacted in human tissues in the same pattern as that previously found with other human-specific MAbs to P-glycoprotein. For a precise definition of the MM4.17 epitope, a peptide library consisting of overlapping 4- to 10-mer residues covering the entire P-glycoprotein-fragment was synthesized on polyethylene pins and tested for MAb binding. The results of this ELISA demonstrated that the MM4.17 epitope is constituted by the continuous-linear TRIDDPET amino-acid sequence (residues 750-757 of the human MDRI-P-glycoprotein). The MAb MM4.17 recognizes only the human MDRI-P-glycoprotein isoform, and excess TRIDDPET peptide blocks the binding of the MAb to MDR variants of CEM cells. These results demonstrate that the amino-acid sequence TRIDDPET from the human MDRI gene represents the first continuous-linear epitope identified in the P-glycoprotein extracellular domain.

Idarubicin is active on MDR cells: evaluation of DNA synthesis inhibition on P388 cell lines

Multidrug resistance is frequently found in patients affected by hematological malignancies and has been related to a poor prognosis of acute leukemia. In the present paper we report results concerning the activity of idarubicin, an anthracycline derivative, on the leukemic P388 and P388 doxorubicin-resistant cell lines. The results clearly show that idarubicin inhibits DNA synthesis in the resistant cell line more actively than doxorubicin.

Flunarizine as a modulator of doxorubicin resistance in human colon-adenocarcinoma cells

The potential of the calcium-entry blocker flunarizine in modulating the cytotoxicity of doxorubicin was investigated in human colon-adenocarcinoma cell lines sensitive to (LoVo) or with experimentally induced resistance (LoVo/DX) to doxorubicin. Exposure to 1 to 2 micrograms/ml flunarizine for intervals of up to 24 hr did not affect cell survival in either line. Simultaneous exposure to flunarizine and doxorubicin for 1 hr selectively enhanced doxorubicin activity in the resistant cell line and not in the sensitive cell line. In particular, the doxorubicin concentration able to reduce cell survival by 50% dropped to one third. Moreover, simultaneous exposure to flunarizine significantly increased intracellular doxorubicin accumulation, as evaluated by fluorescence spectrophotometry. Again, flow-cytometric analysis showed hyperpolarization of the membrane in resistant cells, starting from 15 min of exposure to 2 micrograms/ml flunarizine. Finally, in LoVo/DX cells, which normally express gp170, a 24-hr treatment with flunarizine markedly reduced the immunoreactivity of cells with 2 monoclonal antibodies (MAb57 and MRK16) directed against different external epitopes of the glycoprotein. The results from our study indicate the ability of flunarizine to positively modulate doxorubicin-resistance in human colon-adenocarcinoma cells expressing the multidrug-resistance phenotype.

Antibodies in the study of multiple drug resistance

Multidrug resistance (MDR) is a major problem in cancer chemotherapy. As P-glycoprotein is the key molecule in MDR, many investigators have constructed anti-P-glycoprotein monoclonal antibodies (MAbs). Those antibodies, including MRK16 and C219, were used for elucidation of the mechanism of MDR and for overcoming of MDR. This article describes the characterization of the antibodies against the P-glycoprotein and other proteins of multidrug-resistant tumor cells, and discusses the therapeutic implication of the antibodies.

Resistance of HIV-1 to AZT might also involve the cellular expression of multidrug resistance P-glycoprotein

Resistance of tumor cells to the antigrowth activity of several cytotoxic compounds has been associated with the expression of the so-called multidrug resistance protein or P-glycoprotein. This article addresses the question whether the expression of such protein could also affect the sensitivity of HIV to AZT. Our data indicate that this possibility does exist. In fact, multidrug-resistant CEM VBL100 cells, which express high levels of P-glycoprotein, are less sensitive to both the antiproliferative activity and the antiviral action of AZT. Additionally, our data suggest that this phenomenon is specifically mediated by P-glycoprotein since trifluoroperazine, which is known to circumvent multidrug resistance due to the action on P-glycoprotein, increases the intracellular accumulation of AZT and affects the sensitivity of HIV to AZT. Although the biological and clinical significance of these observations has still to be established, this study suggests that cellular factors, other than virus itself, should be taken into account to address the phenomenon of drug resistance of HIV.

Efficient inhibition of P-glycoprotein-mediated multidrug resistance with a monoclonal antibody

P-glycoprotein (Pgp), encoded by the MDR1 gene, is an active efflux pump for many structurally diverse lipophilic compounds. Cellular expression of Pgp results in multidrug resistance (MDR) in vitro and is believed to be a clinically relevant mechanism for tumor resistance to chemotherapy. We have developed a mouse monoclonal antibody, UIC2, that recognizes an extracellular epitope of human Pgp. UIC2 inhibited the efflux of Pgp substrates from MDR cells and significantly increased the cytotoxicity of Pgp-transported drugs, under the conditions where no effect was detectable with other anti-Pgp antibodies. Potentiation of cytotoxicity by UIC2 was observed with all the tested drugs associated with MDR (vinblastine, vincristine, colchicine, taxol, doxorubicin, etoposide, actinomycin D, puromycin, and gramicidin D) but not with any of the drugs to which MDR cells are not cross-resistant (methotrexate, 5-fluorouracil, cis-platinum, G418, and gentamicin). The inhibitory effect of UIC2 in vitro was as strong as that of verapamil (a widely used Pgp inhibitor) at its highest clinically achievable concentrations. Our results suggest that UIC2 or its derivatives provide an alternative or supplement to chemical agents for the reversal of MDR in clinical cancer.

Drug resistance in oncology: from concepts to applications

The complex problem of drug resistance is discussed with respect to host toxicity, to tumor characteristics (kinetic resistance, heterogeneity of cell subpopulations, hypoxia, mutation and gene amplification), and to the medication itself (pharmacokinetic and pharmacodynamic resistance: cell membrane, intracellular metabolism, intracellular target). After detailing each type of resistance, the possibilities of fighting against drug resistance are explored (dealing with host toxicity, tumor characteristics and drugs--intensifying therapy, multiple drug therapy, biochemical modulation, particular modalities of drug administration). Finally, perspectives of research and development of new drugs are summarized.

Expression of lymphocyte homing receptor gene is lost in multi-drug-resistant variants of human T-lymphoblastoid CCRF-CEM cells

The 2.2-kb human cDNA clone PBL32, encoding for the lymphocyte homing receptor (LHR) was used to study the expression of this determinant in multi-drug-resistant (MDR) variants of human T-lymphoblastoid CCRF-CEM (CEM) cells. LHR is significantly associated with the drug-sensitive phenotype, its expression being progressively and quantitatively reduced in MDR variants of CEM cells according to the extent of drug resistance.