Murine monoclonal antibody recognizing a 90-kDa cell-surface determinant selectively lost by multi-drug-resistant variants of CEM cells

Potential proton pump inhibitor-related adverse effects

Proton pump inhibitors (PPIs) are one of the most common medications taken by patients worldwide. PPIs are used to treat acid-related disorders, including gastroesophageal reflux disease, peptic ulcer disease, Helicobacter pylori infection, and nonsteroidal anti-inflammatory drug/stress ulceration. For some of these diseases, long-term treatment is necessary. With such prolonged use, concern and investigation into potential adverse effects has increased. In addition, data are available regarding potential anticancer effects of PPIs, especially regarding solid tumors. The aim of this review is to assess the literature on PPIs with regard to common concerns, such as drug-drug interactions, the intestinal microbiome, dementia and central nervous system disease, and osteoporosis, as well as to highlight potential negative and positive impacts of the drug in cancer.

Tumor acidity, chemoresistance and proton pump inhibitors

Tumor microenvironment may play a key role in tumor malignancy. It is hypothesized that hypoxia and acidity may contribute to the progression from benign to malignant growth. In particular, the unfavorable environment may induce the selection of tumor cells able to survive in acidic and hypoxic conditions. In fact, the common components of the cancer phenotype result from active selection, and characteristics of tumor microenvironment may create the best condition for this selection. Acidity, in particular, has been shown to have a role in resistance to chemotherapy, proliferation and metastatic behavior. In fact, a mechanism of resistance to cytotoxic drugs may be the alteration of the tumor microenvironment through changes of the pH gradient between the extracellular environment and cell cytoplasm. The extracellular pH of solid tumors is significantly more acidic than that of normal tissues, thus impairing the uptake of weakly basic chemotherapeutic drugs and reducing their effect on tumors. An important determinant of tumor acidity is the anaerobic metabolism that allows selection of cells able to survive in an hypoxic-anoxic environment with the generation of lactate. However, this is not the major mechanism responsible for the development of an acidic environment within solid tumors. It appears clear that a complex framework of protein-protein, protein-lipid and lipid-lipid interactions underlay the pH homeostasis in mammalian cells. Malignant tumor cells seem to hijack some of these mechanism to protect themselves from the acidic environment and to maintain acidity in an environment unsuitable for normal or more differentiated cells. Recent data suggest that vacuolar-type (V-type) H(+)-ATPases, that pump protons across the plasma membrane, may have a key role in the acidification of the tumor microenvironment. Some human tumor cells are characterized by an increased V-type H(+)-ATPase expression and activity, and pretreatment with proton pump inhibitors -- a class of H(+)-ATPase inhibitors -- sensitized tumor cell lines to the effect of a variety of anticancer drugs. Proton pump inhibitor pretreatment has been associated with inhibition of V-type H(+)-ATPase activity and increase in both extracellular pH and pH of lysosomal organelles. In vivo experiments in human/mouse xenografts have shown that oral pretreatment with proton pump inhibitors is able to sensitize human solid tumors to anticancer drugs. These data suggest that tumor alkalinization may represent a key target of future antitumor strategies.

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.

Proton pump inhibitors may reduce tumour resistance

Resistance to cytotoxic agents is a major problem in treating cancer. The mechanisms underlying this phenomenon appear to take advantage of functions involved in the control of cell homeostasis. A mechanism of resistance may be alteration of the tumour microenvironment via changes in the pH gradient between the extracellular environment and the cell cytoplasm. The extracellular pH of solid tumours is significantly more acidic than that of normal tissues, thus impairing the uptake of weakly basic chemotherapeutic drugs and reducing their effect on tumours. An option to revert multi-drug resistance is the use of agents that disrupt the pH gradient in tumours by inhibiting the function of pumps generating the pH gradient, such as vacuolar H(+)-ATPases (V-H(+)-ATPases). V-H(+)-ATPases pump protons across the plasma membrane and across the membranes of various intracellular compartments. Some human tumour cells, particularly those selected for multi-drug resistance, exhibit enhanced V-H(+)-ATPase activity. A class of V-H(+)-ATPase inhibitors, called proton pump inhibitors (PPIs), have emerged as the drug class of choice for treating patients with peptic diseases. These drugs inhibit gastric acid secretion by targeting the gastric acid pump, but they also directly inhibit V-H(+)-ATPases. PPIs (including omeprazole, esomeprazole, lansoprazole, pantoprazole and rabeprazole) are protonable weak bases which selectively accumulate in acidic spaces. Recent findings from our group have shown that PPI pretreatment sensitised tumour cell lines to the effect of cisplatin, 5-fluoro-uracil and vinblastine. PPI pretreatment was associated with the inhibition of V-H(+)-ATPase activity and an increase of both extracellular pH and the pH of lysosomal organelles, consistent with a cytoplasmic retention of the cytotoxic drugs and targeting to the nucleus in the case of doxorubicin. In vivo experiments showed that oral pretreatment with omeprazole induced a sensitivity of the human solid tumours to anticancer drugs.

Effect of proton pump inhibitor pretreatment on resistance of solid tumors to cytotoxic drugs

BACKGROUND

Resistance to antitumor agents is a major cause of treatment failure in patients with cancer. Some mechanisms of tumor resistance to cytotoxic drugs may involve increased acidification of extracellular compartments. We investigated whether proton pump inhibitors (PPIs), currently used in the anti-acid treatment of peptic disease, could inhibit the acidification of the tumor microenvironment and increase the sensitivity of tumor cells to cytotoxic agents.

METHODS

We pretreated cell lines derived from human melanomas, adenocarcinomas, and lymphomas with the PPIs omeprazole, esomeprazole, or pantoprazole and tested their response to cytotoxic drugs in cell death assays. We also evaluated extracellular and intracellular pH and vacuolar-H+-ATPase (V-H+-ATPase) expression, distribution, and activity in PPI-pretreated cells by using western blot analyses, immunocytochemistry, laser scanning confocal analysis, and bioluminescence assays. Finally, we evaluated human melanoma growth and cisplatin sensitivity with or without omeprazole pretreatment in xenografted SCID/SCID mice.

RESULTS

PPI pretreatment sensitized tumor cell lines to the effects of cisplatin, 5-fluorouracil, and vinblastine, with an IC50 value reduction up to 2 logs. PPI pretreatment was associated with the inhibition of V-H+-ATPase activity and increases in both extracellular pH and the pH of lysosomal organelles. PPI pretreatment induced a marked increase in the cytoplasmic retention of the cytotoxic drugs, with clear targeting to the nucleus in the case of doxorubicin. In in vivo experiments, oral pretreatment with omeprazole was able to induce sensitivity of human solid tumors to cisplatin.

CONCLUSION

Our results open new possibilities for the treatment of drug-resistant tumors through combination strategies based on the use of well-tolerated pH modulators such as PPIs.

Saquinavir induces stable and functional expression of the multidrug transporter P-glycoprotein in human CD4 T-lymphoblastoid CEMrev cells

BACKGROUND

The multidrug transporter P-glycoprotein (P-gp) is expressed in HIV-1 target cells, in a range of pharmacological barriers and in AIDS-associated tumours. P-gp substrates include HIV-1 protease inhibitors (PIs) and anticancer drugs, which are efficiently effluxed from multidrug-resistant (MDR) cells.

OBJECTIVES

The aim of this study was to investigate the effect on human CD4 T-lymphoblastoid CEMrev cells of saquinavir and other PIs in terms of P-gp expression and to characterize the functional and biochemical patterns of PI-induced P-gp molecules.

METHODS

CEMrev cells no longer expressing detectable amounts of P-gp were cultured for a prolonged period in the presence of 10 microg/mL saquinavir (CEMsaq10) and tested for P-gp expression and function. Subsequently, CEMsaq10 cells were transferred into medium containing 15 microg/mL saquinavir (CEMsaq15) and cultured for several months. These cell lines were continuously monitored for P-gp expression, function and immunochemical patterns. A similar strategy was adopted to determine whether other PIs, such as ritonavir and indinavir, were able to induce P-gp expression in CEMrev cells.

RESULTS

Compared with the drug-diluent control, the exposure of CEMrev cells to 10 microg/mL saquinavir induced, in a consistent fraction of cells (45-50%), de novo expression of functioning P-gp molecules. The transfer of CEMsaq10 cells to 15 microg/mL saquinavir was associated with a dramatic increase in P-gp expression and function (85-90% of CEMsaq15 cells expressed P-gp and effluxed P-gp dye substrates). These saquinavir-induced P-gp molecules included 75-kDa molecules as well as the classical 170-kDa form of P-gp, suggesting induction of a particular isoform of P-gp termed mini-P-glycoprotein. Conversely, ritonavir and indinavir induced transient P-gp expression in a small percentage of the CEMrev cells.

CONCLUSIONS

Treatment of human CD4 T-lymphoblastoid CEMrev cells with saquinavir caused over-expression of functioning P-gp molecules. This de novo acquired MDR phenotype, which differed from that induced by other PIs, was stable, as expression and activity of P-gp were observed in CEMsaq10 and CEMsaq15 cells during prolonged in vitro culturing, even in drug-free conditions.

What is the relationship between P-glycoprotein and adhesion molecule expression in melanoma cells?

A number of studies have reported that increased P-glycoprotein expression in drug-resistant tumour cells may be associated with decreased expression of a family of surface glycoproteins. However, despite its potential biological and clinical relevance, this phenomenon has not been extensively studied. In this study the phenotypic alterations that are associated with the acquisition of the multidrug-resistant phenotype in tumour cells, together with drug transporter overexpression, were investigated in human melanoma cells. The expression of cell adhesion molecules was analysed in a panel of multidrug-resistant melanoma cell lines (M14Dx) showing different degrees of resistance to doxorubicin and different levels of the expression of the drug transporter P-glycoprotein. In particular, expression of intercellular adhesion molecule-1 (ICAM-1), CD44, very late activation antigen (VLA)-5 and VLA-2 was determined by flow cytometry in the different resistant cell lines. A progressive downregulation of all the adhesion molecules examined was revealed in M14Dx cells, in parallel with an increasing level of expression of the drug transporter P-glycoprotein. The results obtained raise the question of the role of P-glycoprotein in the invasive and metastatic behaviour of tumour cells.

P-glycoprotein-actin association through ERM family proteins: a role in P-glycoprotein function in human cells of lymphoid origin

P-glycoprotein is a 170-kd glycosylated transmembrane protein, expressed in a variety of human cells and belonging to the adenosine triphosphate-binding cassette transporter family, whose membrane expression is functionally associated with the multidrug resistance phenotype. However, the mechanisms underlying the regulation of P-glycoprotein functions remain unclear. On the basis of some evidence suggesting P-glycoprotein-actin cytoskeleton interaction, this study investigated the association of P-glycoprotein with ezrin, radixin, and moesin, a class of proteins that cross-link actin filaments with plasma membrane in a human cell line of lymphoid origin and that have been shown to link other ion-pump-related proteins. To this purpose, a multidrug-resistant variant of CCRF-CEM cells (CEM-VBL100) was used as a model to investigate the following: (1) the cellular localizations of P-glycoprotein and ezrin, radixin, and moesin and their molecular associations; and (2) the effects of ezrin, radixin, and moesin antisense oligonucleotides on multidrug resistance and P-glycoprotein function. The results showed that: (1) P-glycoprotein colocalized and coimmunoprecipitated with ezrin, radixin, and moesin; and (2) treatment with antisense oligonucleotides for ezrin, radixin, and moesin restored drug susceptibility consistently with inhibition of both drug efflux and actin-P-glycoprotein association and induction of cellular redistribution of P-glycoprotein. These data suggest that P-glycoprotein association with the actin cytoskeleton through ezrin, radixin, and moesin is key in conferring to human lymphoid cells a multidrug resistance phenotype. Strategies aimed at inhibiting P-glycoprotein-actin association may be helpful in increasing the efficiency of both antitumor and antiviral therapies.

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.

Idiotypic vaccination: immunoprotection mediated by anti-idiotypic antibodies with antibiotic activity

Anti-Id antibodies were raised in mice against a monoclonal antibody (MoAb KT4) that neutralized the in vitro activity of a Pichia anomala yeast killer toxin. Monoclonal antibody was administered to BALB/C syngeneic mice with different schedules of immunization before intravenous challenge with increasing amounts of yeast killer toxin-sensitive Candida albicans cells. The course of candidosis was studied in comparison with mice non-immunized and immunized with an isotype-matched unrelated MoAb subdivided into control groups. Protection was reflected by statistically significant increases in survival rate of mice immunized with MoAb KT4 which showed variable serum levels of yeast killer toxin-like anti-Id antibodies. MoAb KT4 affinity chromatography purified mouse anti-Id antibodies were capable of killing in vitro the yeast cells of the Candida albicans strain used for the experimental infection. This is the first report of antimicrobial protection that exploits the role of anti-idiotypic antibodies presumably acting in vivo as antibiotics (idiotypic vaccination).

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.

The gene encoding for MC56 determinant (drug-sensitivity marker) is located on the short arm of human chromosome 11

A panel of mouse x human B- and T-cell hybrids was analyzed for the expression of MC56 determinant which marks the drug-sensitive state of CEM cells. Karyotypic and phenotypic analyses of the tested clones showed that the expression of MC56 determinant correlated to the presence of human chromosome 11 and segregated concordantly to the epitopes recognized by monoclonal antibodies in the CD44 cluster. By using a particular class of interspecific rodent x human-cell hybrids in which the human genome counterpart is represented in the different clones only by human chromosome 11 or its fragments, we showed that the gene encoding for MC56 determinant is located on the region p13-pter of the short arm of chromosome 11. Therefore, the hypothesized homology between the drug-sensitivity marker MC56 and the CD44 determinant is supported also by gene mapping studies.

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.

What's new in cytostatic drug resistance and pathology

A major problem in cytostatic treatment of malignant tumors is the development of chemoresistant cell clones. An increased understanding of chemoresistance related mechanisms, improved methods for the detection and localization of resistant cell populations including predictive conclusions on the effectiveness of cytostatic drugs would contribute to the advancement of anti-tumor strategies. This paper reviews current concepts suggested for the development of cellular resistance to natural product drugs (anthracyclines, Vinca alkaloids, epipodophyllotoxines, antibiotics; so-called multidrug resistance substances), alkylating agents (nitrosureas, busulfan and mitomycin C), heavy metal compounds (cisplatin) and antifolates (5-fluorouracil, methotrexate) and describes the role of drug transporting and binding proteins (P-170-glycoprotein), detoxifying enzymes (glutathion-S-transferase, dihydrofolate reductase), DNA repair enzymes (topoisomerase I and II, polymerase alpha and beta), and genomic alterations (amplification, double minutes and homogeneous staining regions) due to resistance. It is focussed on the employment of morphological methods (light microscopy, immunocytochemistry, electron microscopy, fluorescence analysis, in situ hybridization, computer aided morphometric analysis) which will help to detect resistant cell clones in tumor biopsies. First correlations between histological data and clinical course will be reported. In the future, the morphological determination of chemoresistance may play an important role in applied functional tumor pathology.

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

We isolated an IgG2a murine monoclonal antibody (MAb) termed MAb57, specifically reactive with multi-drug-resistant (MDR) human cells. Its specificity toward the MDRI gene product (P-glycoprotein) has been demonstrated by the concordant segregation of the MAb57 epitope with the MDRI gene in interspecific mouse x human cell hybrids, and the reactivity of several different MDRI gene-expressing cells with MAb57, particularly insect cells acutely infected with a baculovirus encoding the MDRI gene. MAb57 can be used to detect, by flow cytometry, variations in the relative drug-resistance levels of several MDR KB and CEM cell variants. This immunological probe has also proven useful in selectively destroying MDR target cells in an antibody-dependent cell-mediated (ADCC) assay system as well as in detecting P-glycoprotein expression in normal and malignant tissues and cells.