Expression of a multidrug resistance gene in human cancers

A monoclonal antibody detecting cell surface epitope on some drug resistant human tumour cell lines

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Structural analysis of the mouse mdr1a (P-glycoprotein) promoter reveals the basis for differential transcript heterogeneity in multidrug-resistant J774.2 cells

In multidrug-resistant mouse J774.2 cells, the differential overproduction of functionally distinct phosphoglycoprotein isoforms reflects the amplification or transcriptional activation or both of two mdr gene family members, mdr1a and mdr1b. The mdr1a gene is a complex transcriptional unit whose expression is associated with multiple transcript sizes. Independently selected multidrug-resistant J774.2 cell lines differentially overexpress either 4.6- and 5.0-kilobase (kb) or 4.7- and 5.1-kb mdr1a transcripts. However, abundant overproduction of the mdr1a gene product was observed only in cell lines which overexpressed the 4.6- and 5.0-kb mRNAs. In order to determine the basis for mdr1a transcript heterogeneity and the relationship between transcript size and steady-state mdr1a protein levels, genomic and cDNA sequence analyses of the 5' and 3' ends of the mdr1a gene were carried out. Promoter sequence analysis and primer extension mapping indicated that mdr1a transcripts were differentially initiated from two putative promoters to generate either 5.1- and 4.7-kb or 5.0- and 4.6-kb transcripts in four multidrug-resistant J774.2 cell lines. Sequence analysis of 3' cDNA variants and a 3' genomic fragment revealed that the 5.1- and 5.0-kb mRNAs had identical 3'-untranslated regions which differed from those of the 4.7- and 4.6-kb mRNAs as a result of the utilization of a more downstream alternative poly(A) addition signal. Transcript initiation from the putative upstream promoter correlated with a 70 to 85% decrease in steady-state mdr1a protein levels relative to transcript levels. In addition, the identification of putative AP-1 and AP-2 promoter elements suggests a possible role for protein kinase A and protein kinase C in the regulation of mdr1a. The implications of these findings for mdr gene expression and regulation are discussed.

Patterns of cross-resistance in a multidrug-resistant small-cell lung carcinoma cell line

H69AR is a multidrug-resistant human small-cell lung carcinoma cell line that was selected in doxorubicin and has previously been shown to be cross-resistant to a variety of natural-product-type anticancer drugs. H69AR is unlike many other multidrug-resistant cell lines in that it does not overexpress P-glycoprotein. In the present study, the drug sensitivity and cross-resistance patterns of H69AR cells were further characterized. A total of 15 drugs belonging to a number of chemical classes were screened. These compounds included anthracyclines, DNA binders (anthrapyrazoles, benzothiopyranoindazoles, and pyrazoloacridines), and lipophilic antifolates. The alkylating agent melphalan and the antimetabolite cytosine arabinofuranoside (Ara-C) were also tested. In general, the drug sensitivity and cross-resistance profiles of H69AR cells were consistent with those reported by others using other drug-resistant cell lines. However, there were several unexpected instances of cross-resistance. Thus, the H69AR cell line was more resistant than its parent cell line to the potent 3'-deamino-3'-(3-cyano-4-morpholinyl) doxorubicin, bisantrene, the pyrazoloacridine PD 114541, Ara-C, and melphalan. In addition, no cross-resistance to the four lipophilic antifolates tested, including trimetrexate, was found. The absence of a consistent pattern among the various drug-resistant cell lines indicates that assumptions about the efficacy of anticancer drugs in multidrug resistance should be made with caution.

Transfection of a mutant regulatory subunit gene of cAMP-dependent protein kinase causes increased drug sensitivity and decreased expression of P-glycoprotein

Wild-type Chinese hamster ovary (CHO) cells were transfected with a DNA clone (MT-REV, site A) carrying a mouse gene for a dominant mutant regulatory subunit (RI) gene of cAMP-dependent protein kinase (PKA) from S49 cells along with a marker for G418 resistance. G418-resistant transfectant clone R-2D1 was resistant to 8-Br-cAMP-induced growth inhibition and morphological changes. The cells also did not phosphorylate a 50-kDa protein after cAMP stimulation and had decreased PKA activity, both characteristics of PKA mutants. Northern blot analysis indicated that clone R-2D1 was actively transcribing the MT-REV (site A)-specific RNA. We also tested clone R-2D1 for sensitivity to certain natural product hydrophobic drugs and found increased sensitivity to several drugs including adriamycin. Hypersensitivity to these drugs has previously been shown by us to be a characteristic of a CHO PKA mutant cell line. Expression of the mutant RI gene is also associated with a decrease in expression of the multidrug resistance associated P-glycoprotein (gp170) mRNA and protein. These results show that the PKA mutant RI gene from S49 cells acts as a dominant mutation to reduce the total PKA activity in the CHO transfectants as it does in mouse S49 cells. This study also confirms that reduced PKA activity modulates the basal multidrug resistance of these cells, apparently by causing decreased expression of the mdr gene at the protein and mRNA level.

mdr1/P-glycoprotein gene segments analyzed from various human leukemic cell lines exhibiting different multidrug resistance profiles

Three high-level multidrug-resistant sublines of the human T-lymphoblastoid cell line CCRF-CEM were selected independently with either actinomycin D, vincristine or adriamycin. They exhibited distinct quantitative differences of cross-resistance profiles, and showed amplification and marked expression of the mdrl/P-glycoprotein gene. DNA and RNA were prepared from the cell lines, and additionally from three cell samples of patients suffering from acute lymphatic leukemia. Applying the polymerase chain reaction (PCR) for amplification, we cloned and sequenced from these sources segments of the mdrl/P-glycoprotein gene around the codon 185 which codes for an amino acid residue possibly influencing the drug binding function of the P-glycoprotein. Altogether, only 2 single nucleotide differences in an intron were found in 2 out of 40 recombinants each harboring a 209 bp genomic or a 269 bp cDNA fragment of the mdrl/P-glycoprotein gene. Our result does not support the idea of clustered point mutations in this segment of the P-glycoprotein gene as a cause of different multidrug resistance profiles. We additionally examined another segment of the P-glycoprotein gene in its second half, essentially delivering the same negative result, though.

Non-P-glycoprotein-mediated multidrug resistance with reduced EGF receptor expression in a human large cell lung cancer cell line

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Modulation of vincristine sensitivity of human kidney tumor cells by pharmacological agents interfering with intracellular signals. No apparent relationship to changes in cytoplasmic Ca2+ or pH

The effect of substances proposed to modulate intracellular signal systems on growth and sensitivity to vincristine in the human kidney tumor cell line ACHN was investigated and related to changes in cytoplasmic free Ca2+ concentration ([Ca2+]i) and cytoplasmic pH (pHi). Presence during culture of the protein kinase C (PKC) activator 12-O-tetradecanoyl phorbol 13-acetate (TPA) had no effect on cell growth but significantly increased the EC50 concentration for vincristine inhibited cell growth. There was no indication for endogenous PKC activity being responsible for basal vincristine insensitivity since it was not affected by the PKC inhibitor H-7. The Ca2+ ionophore ionomycin tended to increase cell growth and induced vincristine resistance, whereas the calmodulin inhibitor W-7 had opposite effects. Presence during culture of the adenylate cyclase activator forskolin did not affect basal cell growth but dose-dependently made the cells more sensitive to vincristine. The modulators of vincristine sensitivity had no immediate effect on pHi, whereas after 3 days of incubation ionomycin and forskolin tended to increase pHi. Ionomycin and forskolin induced an immediate increase in [Ca2+]i which remained after 3 days only for ionomycin, whereas TPA decreased [Ca2+]i, a change which tended to remain after 3 days of incubation. It is concluded that perturbation of the intracellular signal system may affect both cell growth and cytotoxic drug sensitivity. However, there is no apparent relationship between immediate or late changes in [Ca2+]i and pHi and vincristine sensitivity.

Reversal of multi-drug resistance in human KB cell lines by structural analogs of verapamil

Several structural analogs of verapamil were studied for their ability to reverse multi-drug resistance (MDR) in human KB cell lines. D595, D792 and verapamil completely reversed resistance to colchicine and adriamycin. D595 and D792 had a higher reversing potency than verapamil. Devapamil, gallopamil, emopamil and D528 partially reversed MDR. The reversing potency of a drug did not correlate with its calcium antagonistic activity. No differences in reversing potency between (R)-isomers, (L)-isomers and the racemic forms were observed in the case of both verapamil and emopamil. (R)-verapamil, which has less calcium antagonistic activity and less in vivo toxicity than racemic verapamil, and D792, which has higher reversing potency and less in vivo toxicity than racemic verapamil, should be suitable for clinical applications to overcome drug resistance in cancer patients.

Expression of mdr1 and gst-pi in human breast tumours: comparison to in vitro chemosensitivity

Increased expression of the mdr1 gene, encoding the 175 kDa P-glycoprotein, and the gst-pi gene, encoding the anionic isozyme of glutathione S-transferase (GST), have previously been detected in continuous human breast cancer cell lines selected in vitro for resistance to doxorubicin. In this present study we have measured RNA levels of mdr1 and gst-pi in primary human breast tumour biopsies prior to chemotherapy and from tumours which have different inherent responses to doxorubicin treatment, including colon, head and neck squamous cell carcinomas and myeloid leukaemias. Detectable levels of mdr1 mRNA was observed in 25 out of 49 breast tumours, with up to a 100-fold range in expression. A narrower range of gst-pi expression has also been observed in these tumours. Chemosensitivity of cells grown in short-term culture from some of the breast tumours has been measured by an in vitro colony forming assay in the presence of doxorubicin. Comparison of the dose of doxorubicin causing 50% inhibition of growth (ID50) with RNA levels showed that the tumours with high mdr1 expression had high ID50, while the more sensitive explants had low mdr1 expression. These results support a role for mdr1 gene expression in determining the response of human breast cancer cells to chemotherapy.

Expression of the mdr3 gene in prolymphocytic leukemia: association with cyclosporin-A-induced increase in drug accumulation

Typical multidrug resistance in human and animal cell lines is caused by overactivity of an unidirectional transmembrane drug efflux pump, encoded by the MDR genes, called mdr genes in mice and humans and pgp genes in hamsters. In humans, two mdr genes, mdr1 and mdr3, with approximately 80% nucleotide homology, have been identified. There is increasing evidence that overexpression of the mdr1 gene plays a role in resistance to anticancer agents in specific tumor types. However, currently no data are available on a possible role for mdr3 in drug resistance. Here we report high levels of expression of mdr3 gene sequences in leukemic cells from 6 out of 6 patients with prolymphocytic leukemia (PLL). No mdr1 expression was detected in 5 out of 6 of these samples, whereas a low level of mdr1 expression was found in a sample from one PLL patient in the course of transformation to non-Hodgkin's lymphoma. Except for this patient, all other PLL cases studied had not received prior chemotherapy. In vitro drug uptake studies showed that daunorubicin accumulation in PLL cells was increased by cyclosporin A. Since cyclosporin A is an inhibitor of the mdr1-encoded P-glycoprotein drug pump, these data suggest that in PLL cells mdr3 also codes for a drug efflux pump. Our findings could partly explain the primary refractoriness of PLL to chemotherapy.

Aspergillus species strain M39 produces two naphtho-gamma-pyrones that reverse drug resistance in human KB cells

One thousand fungi and Actinomycetes were investigated to see whether they produced compounds that reverse multi-drug resistance in KB cells. Only one Aspergillus strain M39 produced agents with resistance-reversing activity and these compounds were identified to be rubrofusarin B and dianhydro-aurasperone C. Rubrofusarin B only slightly reversed the resistance of KB-C2 cells to Adriamycin and daunomycin, partially reversed the resistance to chromomycin A3, and almost completely reversed the resistance to vincristine and mitomycin C. Purified dianhydro-aurasperone C and rubrofusarin B had similar effects on drug resistance in KB-8-5 cells. Dianhydro-aurasperone C enhanced the accumulation of vinblastine in KB-8-5 cells and inhibited the efflux of vinblastine from the cells. Dianhydro-aurasperone C and rubrofusarin B at 10 microM completely inhibited 3H-azidopine photolabelling of P-glycoprotein. The two products of Aspergillus strain M39 appear to reverse multi-drug resistance by interacting with P-glycoprotein and inhibiting its role as an active efflux pump.

Chemo-radioresistance of small cell lung cancer cell lines derived from untreated primary tumors obtained by diagnostic bronchofiberscopy

New cell lines of small cell lung cancer (SCLC) were established from specimens of untreated primary tumors biopsied by diagnostic bronchofiberscopy. The advantage of this method was ease of obtaining specimens from lung tumors. Establishment of cell lines was successful with 4 of 13 specimens (30%). Clinical responses of the tumors showed considerable variation, but were well correlated with the in vitro sensitivity of the respective cell lines to chemotherapeutic drugs and irradiation. One of the cell lines was resistant to all drugs tested and irradiation, while another was sensitive to all of them. Although the acquired resistance of SCLC is the biggest problem in treatment, the natural resistance to therapy is another significant problem. Either acquired or natural, resistance mechanisms of SCLC may be elucidated by the use of such cell lines derived from untreated tumors. This method and these SCLC cell lines are expected to be useful for the serial study of biologic and genetic changes of untreated and pre-treated tumors, or primary and secondary tumors.

Geriatric oncology: perspectives from decision analysis. A review

The management of advanced cancer in the older person is variable. In some patients with common malignancies chemotherapy may palliate symptoms and prolong survival, while in other patients chemotherapy is a cause of overwhelming toxicity and supportive care is the preferred form of treatment. We explored the principles of individualized management for the older person with cancer and we used decision analysis as a guide. From a decisional standpoint, geriatric malignancies may be subdivided into two categories: diseases whose management is not affected by age and diseases whose management may be age-conditioned. In the latter group one can distinguish three clinical situations: diseases with short survival when untreated, which are responsive only to highly toxic chemotherapy; diseases with short survival when untreated, which may be responsive to low-toxicity chemotherapy; and diseases with prolonged survival when untreated, whose clinical course may be affected by chemotherapy. From the analysis of these clinical situations, a critical paucity of information has emerged in five areas: prognostic evaluation of the older patients, interaction of comorbid conditions and cancer chemotherapy, availability of low-toxicity alternative treatment regimens, life expectancy and estimate of outcome utilities. Further research in these areas, according to the lines we propose, may fill critical gaps of knowledge and allow optimal management of geriatric cancer.

The activity of verapamil as a resistance modifier in vitro in drug resistant human tumour cell lines is not stereospecific

The L-isomer of verapamil is a more potent calcium antagonist than the D-isomer. We have examined the two stereoisomers of verapamil for their ability to increase the chemosensitivity in vitro of three drug resistant cell lines (2780AD, MCF7/AdrR and H69LX10). Neither racemic verapamil nor its individual isomers had any effect on the drug sensitivity of the parent cell lines (A2780, MCF7 and NCI-H69). Verapamil (6.6 microM) increased the sensitivity of all three resistant cell lines to Adriamycin by 10-12-fold. This activity was concentration dependent and was maximal at 6-7 microM. The increase in sensitivity was only 2-3-fold at 2 microM, the maximum plasma concentration achieved in patients. Both the D- and L-isomers of verapamil alone at 6.6 microM were as effective as racemic verapamil and the D-isomer demonstrated the same concentration dependent activity as racemic verapamil. The total cellular Adriamycin concentration of both 2780AD and MCF7/AdrR was increased by two-fold in the presence of verapamil (6.6 microM). Both D- and L-verapamil alone increased the amount of drug accumulated to the same extent as racemic verapamil. These results indicate that the resistance modification activity of verapamil is not stereospecific. Use of D-verapamil alone in patients could increase the maximum tolerated plasma concentrations of verapamil and thus D-verapamil may be a more effective resistance modifier in vivo than racemic verapamil.

Overexpression of the mdr1 gene in blast cells from patients with acute myelocytic leukemia is associated with decreased anthracycline accumulation that can be restored by cyclosporin-A

Typical multi-drug resistance (MDR) in human and animal cell lines is caused by overactivity of a unidirectional drug efflux pump. This pump is composed of a 170-kDa transmembrane glycoprotein (P-glycoprotein) that is encoded by the so-called mdr1 gene. The functionally relevant characteristic of MDR cells is a defect in drug accumulation that can be restored by agents which inhibit the P-glycoprotein pump. The purpose of our study was to find out whether P-glycoprotein inhibitors could increase the daunorubicin (DNR) accumulation in acute myelocytic leukemia (AML) cells, overexpressing the mdr1 gene. Using dot blot analysis with an mdr1-specific cDNA probe, we identified leukemic cell samples, obtained from chemotherapy-resistant AML patients, that had relatively high levels of mdr1 expression. These leukemic cells showed a reduced ability to accumulate DNR in vitro, as quantitated by flow cytometry. Addition of cyclosporin-A (Cy-A), a drug known to inhibit the P-glycoprotein pump, to the incubation medium resulted in an increase (up to 60%) in steady-state drug uptake by the leukemic cells. The degree of Cy-A-induced increase in drug accumulation in the leukemic cells correlated approximately with the level of overexpression of the mdr1 gene. Our data indicate that Cy-A is a good candidate for combination chemotherapy with cytotoxic drugs in clinical trials, aimed at the treatment of drug resistance in AML.

Use of recombinant P-glycoprotein fragments to produce antibodies to the multidrug transporter

Multidrug-resistance of human cancer cells may result from expression of a 170,000 dalton multidrug efflux pump called P-glycoprotein. To identify this multidrug transporter, and to study its structure and function, we have generated polyclonal rabbit antibodies against the amino-terminal and carboxy-terminal halves of the molecule using recombinant protein fragments produced in Escherichia coli. Two recombinant P-glycoprotein fragments, representing amino acids 140-228 and 919-1280, were overproduced in Escherichia coli by an inducible T7 expression system, gel-purified and injected into rabbits. Both antisera specifically immunoprecipitate 3H-azidopine and 35S-methionine labeled P-glycoprotein from multidrug-resistant cells and detect P-glycoprotein on Western blots with high sensitivity. Because these antisera were raised against epitopes in the amino- and carboxy-terminal halves of P-glycoprotein, they should be useful as research tools to define the function of these two halves of the molecule.

P-glycoproteins in pathology: the multidrug resistance gene family in humans

Many cancers do not respond to chemotherapy on primary exposure to drugs, thus manifesting intrinsic drug resistance. Other cancers that do initially respond subsequently become resistant to the same drugs and simultaneously to other drugs to which the patient has had no previous exposure. This is a form of acquired drug resistance. There is a pressing need to better understand the mechanisms of drug resistance and to use this information to develop strategies for the chemosensitization of drug-resistant tumors. A goal of the pathology laboratory is to offer chemosensitivity tests that identify intrinsic or acquired resistance of tumors to specific drugs or classes of drugs to enable the clinician to tailor therapy to the biology of cancers in individual patients. Multidrug resistance is one type of drug resistance. It can be present in either an intrinsic or acquired form. The human gene that confers human multidrug resistance, the MDR1 gene, has been cloned and classified as a member of the MDR gene family. Its encoded protein, called Mdr1, is an energy-driven membrane efflux transporter that maintains intracellular concentrations of certain chemotherapeutic drugs at nontoxic levels. Useful model systems for studying multidrug resistance have been developed in several research laboratories. Applying selection pressure by exposing cultured cancer cells to escalating doses of natural product anti-cancer drugs allows cross-resistant cell lines to be produced which share patterns of drug resistance with human cancers. A common feature of these drug-resistant lines is the expression of Mdr1. Using techniques of genetic engineering, molecular probes have been developed that can be used to measure MDR1 mRNA and MDR1 gene amplification. Mdr can be measured by immunochemistry methods. Currently, such measurements are being used to stratify patients in clinical trials designed to determine if chemosensitization by inhibition of the pump function of Mdr is a clinically useful therapeutic strategy. If successful, Mdr/MDR1 mRNA laboratory testing might significantly increase the clinical laboratory's role in cancer patient management.

Bepridil enhances adriamycin-induced DNA biosynthesis inhibition in human myeloid leukemia cells

The utilization of drug response modulators, based on their physico-chemical properties to augment the cytotoxic response of anticancer drugs is now gaining importance. We present in this communication, investigations performed to assess the antitumor activity of Adriamycin (ADR), on chronic myeloid leukemia (CML) cells, and the effect of bepridil, a calcium channel blocker on the ADR cytotoxicity. Inhibition of 3H-thymidine incorporation into DNA was used as an index of the cytotoxic effects of drugs when utilised alone or in combination. The combination of bepridil (1 and 5 micrograms/ml) and ADR (5 and 10 micrograms/ml) indicated a significant (P less than 0.001) enhancement in the DNA biosynthesis inhibition in CML cells, as compared to those samples exposed to ADR alone. The observed inhibition of DNA biosynthesis was found to be totally reversible, partially reversible and completely irreversible when the CML cells were exposed to bepridil alone, ADR alone and ADR plus bepridil, respectively. Bepridil was found to be highly lipid soluble at physiological pH, and this property could be responsible for the modulation of the ADR activity observed in this study. Results obtained, though preliminary due to the small sample size, clearly indicate a necessity for a detailed evaluation of bepridil effects, which would lead to higher therapeutic gains in anticancer chemotherapy in the clinic.

Does chemotherapy fulfill its expectations in cancer treatment?

In 1949, Cornelius P. Rhoads, director of the Memorial Sloan-Kettering Cancer Center, made his famous report on the practical value of nitrogen mustard before the Committee on Growth of the National Research Council, a quasi-governmental agency which had accepted the responsibility for the clinical trial of this drug at the end of the war. Doctor Rhoads assigned to David A. Karnofsky the job of receiving reports from individual physicians and analyzing the results in each type of cancer. The conclusions, based on 2500 case reports, indicated that nitrogen mustard did not cure any form of cancer, but it was temporarily useful in chronic leukemias, Hodgkin's disease, malignant lymphomas, and lung cancer. The most valuable outcome of this report was the beginning of massive and direct attack on cancer in humans. Since that time, in fact, there was enthusiasm and interest in studying cancer; physicians have been educated, interested and motivated to treat the disease; investigators became engaged in the search of new growth-inhibiting compounds and research clinicians in the formulation of new treatment strategies. The result was a progressively growing list of tumors responding to drug treatment with increased complete remission, prolonged disease free and even total survival rates particularly in the early stages of given malignancies; patient care, the most important achievement in cancer medicine, has been drastically improved. Clinical chemotherapy of cancer has a long history of controversies. Though never advocated as panacea, this form of treatment has continuously fluctuated between excessive optimism and detraction. Recently, harsh criticism has pointed to the magnitude of the overall results achieved in terms of decreased national mortality statistics for the various malignancies. An impatient group of clinicians, biostatisticians and epidemiologists began to question whether medical oncology has reached a plateau in its control of cancer or whether the entire treatment strategy for the control of cancer is wrong. Since sweeping generalization cannot be supported for the results of treatment in each tumor subset require separate analysis, let us review the biological principles of cancer treatment, the main strategies utilized, and the results achieved in the major groups of malignancies. The paper will also outline the contributions of chemotherapy to the medical sciences and to the care of the patients with cancer during the last 40 years.

Accelerated genetic destabilization and dormancy: two distinct causes of resistance in metastatic cells; clinical magnitude, therapeutic approaches

Several years of clinical chemotherapy have shown that, despite modern refinements, cytotoxic agents are not able to eradicate metastases of most adult solid tumors but only to prolong survival by achieving a cell kill that is not 100 per cent. Among the possible causes of this phenomenon, two are discussed in detail. The first one is cell autonomy. It is shown that the numbers of generations reached by a metastatic clone until clinical detection is largely in excess of 100, which allows for a considerable number of mutations, and that in addition genetic destabilization leading to autonomy proceeds much more rapidly than anticipated by a random mutation process. Adaptative changes by genetic amplification in response to toxic injury add to this acceleration effect, accounting for the fact that most metastatic cells are totally resistant very early in the natural history of a human tumor. On the other hand, it is shown that dormant metastatic cells do exist, due either to lack of autocrine growth factors or to inhibiting agents secreted by other metastases. These cells can survive chemotherapy and then re-enter a proliferative state due to some mechanisms that are analyzed, accounting for semi-late and late failures. These obstacles call for other strategies of metastases management, such as arresting or differentiating agents, some of which have been successfully tested by the author's group, such as antiprostaglandins, antithrombin, somatostatin, hyaluronidase, and retinoic acid. It remains to study their optimal combinations, and the appropriate timing, in order to achieve, if not eradication, growth suppression for very long periods without toxicity.

Expression of the multidrug transporter, P-glycoprotein, in chronic myelogenous leukaemia cells in blast crisis

The overexpression of a cell-surface glycoprotein termed P-glycoprotein (P-gp) is frequently associated with multidrug resistance (MDR) in cell lines in vitro. To evaluate the implications of P-gp expression in clinical drug-resistance, we examined the expression of P-gp in fresh leukaemia cells from chronic myelogenous leukaemia (CML) patients in blast crisis. By using immunoblotting with a monoclonal antibody against P-gp, C219, we showed that leukaemia cells from three CML patients in blast crisis were P-gp negative at the stage when these patients were in complete remission, and that the cells showed high levels of P-gp expression at times when the same patients had relapsed and had not responded to chemotherapy. Six out of 11 patients (nine in the refractory state) were P-gp positive and they rarely responded to chemotherapy. These data suggest that the expression of P-gp is closely associated with drug-resistance in CML.

Detection of P-glycoprotein isoforms by gene-specific monoclonal antibodies

P-glycoprotein is a highly conserved membrane protein shown to be overexpressed in many multidrug-resistant tumor cell lines. P-glycoprotein is encoded by a small gene family in mammalian cells. Class I and II isoforms cause multidrug resistance, whereas class III does not. In this report, we have characterized three P-glycoprotein-specific monoclonal antibodies (mAbs) by high-resolution epitope mapping with a series of hexapeptides. mAb C494 is gene specific, binding to a sequence present only in the class I isoform of hamster and human. The mAb C32 recognizes a sequence conserved in hamster class I and II isoforms but not in class III isoforms. In contrast, the mAb C219 recognizes a highly conserved amino acid sequence found in all P-glycoprotein isoforms characterized to date. These mAbs were used to reveal differential expression and specific localization of the three P-glycoprotein isoforms in hamster tissues by immunohistochemical staining and competition with epitope-specific peptides. Colonic epithelial cells expressed predominantly the class I isoform in a polarized manner, adrenal cortical cells expressed predominantly the class II isoform, whereas a small percentage of skeletal muscle fibers expressed the class III isoform of P-glycoprotein. These findings suggest that the P-glycoprotein isoforms have distinct physiological roles associated with specialized cell functions.

Expression of a human multidrug resistance cDNA (MDR1) in the bone marrow of transgenic mice: resistance to daunomycin-induced leukopenia

The human multidrug resistance gene (MDR1) encodes a drug efflux pump glycoprotein (P-glycoprotein) responsible for resistance to multiple cytotoxic drugs. A plasmid carrying a human MDR1 cDNA under the control of a chicken beta-actin promoter was used to generate transgenic mice in which the transgene was mainly expressed in bone marrow and spleen. Immunofluorescence localization studies showed that P-glycoprotein was present on bone marrow cells. Furthermore, leukocyte counts of the transgenic mice treated with daunomycin did not fall, indicating that their bone marrow was resistant to the cytotoxic effect of the drug. Since bone marrow suppression is a major limitation to chemotherapy, these transgenic mice should serve as a model to determine whether higher doses of drugs can cure previously unresponsive cancers.

P-glycoprotein expression in treated and untreated human breast cancer

The expression of P-glycoprotein in primary and recurrent human breast cancer was investigated by means of immunohistochemistry, using a monoclonal antibody (C219) and the streptavidin-biotin-peroxidase method. Twelve patients received no chemotherapeutic treatment. The other 11 patients were treated with chemotherapy, and all developed clinical resistance to it. No or only minimal reactivity was found in specimens coming from the untreated patients (12 cases) or from patients treated with substances not involved in the multidrug resistance phenomenon (four cases). In contrast, three out of seven tumours from patients treated with multidrug resistance related substances showed clear reactivity (positive staining in more than 20% of the tumour cells). In one of these cases, where specimens of the tumour could be studied before and after treatment, an association between the latter and expression of P-glycoprotein was suggested. Finally, this marked expression of P-glycoprotein only took place in tumours treated over a longer space of time (five courses or more of multidrug resistance related chemotherapy).

Detection of multidrug resistance (MDR1) gene RNA expression in human tumors by a sensitive ribonuclease protection assay

The human MDR1 gene encoding P-glycoprotein, an energy-dependent drug-efflux pump, was initially isolated from a multidrug-resistant KB carcinoma cell. When a 3 kb genomic sequence isolated from normal human tissue including the major downstream promoter and the first and second exons of the MDR1 gene was compared to the equivalent fragment from KB cells, the MDR1 gene from KB carcinoma cells was found to have a point mutation in the first exon. Although this mutation does not affect the downstream promoter sequence or the coding sequence of the MDR1 gene, it creates a single base mismatch between the 5' KB genomic fragment previously used for RNase protection analysis of MDR1 RNA expression in normal tissues and thereby reduces the sensitivity of this assay. Using the DNA fragment from normal tissues rather than KB cells, we have reanalyzed MDR1 mRNA levels in 12 renal carcinomas and 4 colon adenocarcinomas. By this RNase protection assay, MDR1 RNA levels are as high in these tumors as in the multidrug-resistant cell line, KB-8-5. The ribonuclease protection assay indicated that the major downstream promoter was mainly used in these clinical samples including two samples of RNA from metastatic renal cancer. This assay appears to be a very sensitive and specific assay for detecting MDR1 mRNA levels and mRNA initiation sites in clinical samples.

Effects of cyclosporin A and verapamil on the intracellular daunorubicin accumulation in Chinese hamster ovary cells with increasing levels of drug-resistance

Multidrug-resistance (MDR) is characterized by the presence of a 170 kDa glycoprotein (P-glycoprotein) in the plasma membrane. P-glycoprotein is thought to act as an efflux pump, leading to reduced drug accumulation in MDR cells. This defect in drug accumulation can be overcome by membrane transport modulating agents (MTMAs). We determined the concentration of MTMA needed for maximal restoration of daunorubicin content in 4 Chinese hamster ovary cell lines with increasing levels of drug-resistance using flow cytometry. Stimulation of daunorubicin accumulation occurred in a dose-dependent manner. The required level of MTMA needed for maximal drug accumulation increased with the level of drug-resistance. CHrA3 cells, which have a level of resistance comparable to clinical samples, needed relatively low concentrations of MTMA for maximal restoration of drug accumulation. This indicates that, in trial combining drugs and MTMAs, low dosages of MTMAs could be sufficient for optimal potentiation of cytotoxicity.

Expression of a human multidrug resistance cDNA (MDR1) in the bone marrow of transgenic mice: resistance to daunomycin-induced leukopenia

The human multidrug resistance gene (MDR1) encodes a drug efflux pump glycoprotein (P-glycoprotein) responsible for resistance to multiple cytotoxic drugs. A plasmid carrying a human MDR1 cDNA under the control of a chicken beta-actin promoter was used to generate transgenic mice in which the transgene was mainly expressed in bone marrow and spleen. Immunofluorescence localization studies showed that P-glycoprotein was present on bone marrow cells. Furthermore, leukocyte counts of the transgenic mice treated with daunomycin did not fall, indicating that their bone marrow was resistant to the cytotoxic effect of the drug. Since bone marrow suppression is a major limitation to chemotherapy, these transgenic mice should serve as a model to determine whether higher doses of drugs can cure previously unresponsive cancers.

P-glycoprotein gene (MDR1) cDNA from human adrenal: normal P-glycoprotein carries Gly185 with an altered pattern of multidrug resistance

We isolated a full-length MDR1 cDNA from human adrenal where P-glycoprotein is expressed at high level. The deduced amino acid sequence shows two amino acid differences from the sequence of P-glycoprotein obtained from colchicine-selected multidrug resistant cultured cells. The amino acid substitution Gly----Val at codon 185 in P-glycoprotein from colchicine resistant cells occurred during selection of cells in colchicine. As previously reported, cells transfected with the MDR1 cDNA carrying Val185 acquire increased resistance to colchicine compared to other drugs. The other amino acid substitution Ser----Ala at codon 893 probably reflects genetic polymorphism. The MDR1 gene, the major member of the P-glycoprotein gene family expressed in human adrenal, is sufficient to confer multidrug-resistance on culture cells.

Sensitization of multidrug-resistant colon cancer cells to doxorubicin encapsulated in liposomes

The effectiveness of liposome-encapsulated doxorubicin in overcoming multidrug resistance was studied in various human colon cancer cells. Colon-cancer cell lines SW403, HT29, SW620, and SW620/R overexpressed P-glycoprotein as determined by immunoflow cytometry, thereby confirming the presence of the multidrug-resistant phenotype. Important differences were observed in the cytotoxicity of free doxorubicin as represented by IC50 values of 0.168, 0.058, 0.023, and 9.83 microM for SW403, HT29, SW620, and SW620/R, respectively. Liposomally encapsulated doxorubicin provided an IC50 that was 1.4 times lower than that of the free drug in the doxorubicin-resistant SW 620/R cell line, whereas no difference was evident in the sensitive parental SW620 cells. In addition, liposome-encapsulated doxorubicin exhibited 1.31- and 2.33-fold cytotoxicity to HT-29 and SW403 cells, respectively. The intracellular drug accumulation in SW620/R cells was enhanced by liposomally encapsulated doxorubicin, whereas it was reduced in all other cell lines as compared with that of free drug. The colon-cancer cell lines demonstrated different degrees of doxorubicin-induced DNA strand breakage that correlated with their sensitivities to drug-induced cytotoxicity. However, no difference was observed between DNA breakage caused by the free drug and that induced by liposome-encapsulated doxorubicin in any of the cell lines. The results suggest that the enhanced cytotoxicity of liposomal doxorubicin to colon cancer cells was due to some secondary non-DNA target. However, liposomally encapsulated doxorubicin appears to be effective in diminishing the multidrug-resistant phenotype and may have clinical applications.