Human multidrug-resistant cell lines: increased mdr1 expression can precede gene amplification

Evidence of post-translational regulation of P-glycoprotein associated with the expression of a distinctive multiple drug-resistant phenotype in Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells following exposure to fractionated X-irradiation in vitro dominantly expressed a distinctive multiple drug-resistant phenotype, characterised by resistance to vinca alkaloids, epipodophyllotoxins and colchicine, but not to anthracyclines, together with overexpression of P-glycoprotein (Pgp), but without any concomitant elevation in Pgp mRNA (J Natl Cancer Inst 1990, 82, 607-612; 1992, 85, 48-53). To investigate the mechanism of this Pgp overexpression, Pgp stability ws examined in an X-irradiation pretreated subline and compared with that of two colchicine-selected drug-resistant CHO sublines. These studies revealed a slower turnover of Pgp in the X-irradiated cells (T1/2 > or = 40 h) relative to the drug-selected sublines (T1/2 = 17 h), indicating that Pgp overexpression appears to be differently regulated in these independently-derived resistant sublines. These data add support to our proposal that the development of drug resistance following X-irradiation may arise by a mechanism distinct from that operating after drug selection.

MDR-1 gene expression, anthracycline retention and cytotoxicity in human lung-tumor cells from refractory patients

Lung-tumor cells from pleural effusion of four refractory patients and in cell lines established from them were analyzed for anthracycline retention, cytotoxicity, and MDR-1 gene and P-glycoprotein expression. Murine leukemic P388 and doxorubicin-resistant P388/R84 lines were used as controls. The 50% growth-inhibitory concentration (IC50) for doxorubicin among lung-tumor lines varied from 0.16 to 0.31 microM in soft agar. Heterogeneity in doxorubicin or daunorubicin retention and response to the efflux-blocking action of 25 microM prochlorperazine was noted in pleural effusion of FCCL-1, -4, and -8. Among the cell lines established, an efflux-blocking effect in a subpopulation was noticed only in FCCL-1 and -4. Although the MDR-1 gene was present in all cell lines, including P388, its expression was pronounced only in P388/R84 and FCCL-1. In situ hybridization of antisense RNA probe to tumor cells showed high heterogeneity for MDR-1 message in the human lung-tumor cells as compared with the murine cells. Northern and slot blot hybridization confirmed in situ hybridization in lines with high levels of MDR-1 expression. The synthesis of MDR-1 mRNA and P-glycoprotein in tumor lines was correlated. The results suggest that because of extensive tumor-cell heterogeneity in human tumors, monitoring of MDR expression by in situ hybridization, quantitation of P-glycoprotein content by laser flow cytometry (and/or immunohistochemical methods), and drug efflux (by laser flow cytometry) may be the best ways to monitor multidrug resistance in human tumors.

[Multidrug resistance induced by surexpression of the mdrl gene]

Drug resistance of some tumors and hematopoietic diseases is a major problem for clinicians. One of the mechanisms of resistance is called the multidrug resistance. Multidrug resistance is a cross-reactive resistance between molecules different in structure and activities. Multidrug resistance is characterized by an active drug efflux through the cell membrane with a decrease in drug cellular accumulation. This efflux out of the cell is due to a membrane glycoprotein called P-glycoprotein or P-gp 170 encoded by the mdr1 gene. In humans, high levels of mdr1 gene expression are observed in normal tissues as adrenal gland, colon and kidney. Tumors derived from these tissues are usually resistant to chemotherapy. By contrast to this intrinsic drug resistance, acquired drug resistance is defined by the mdr1 gene expression in tumors which relapse and become refractory to chemotherapy as acute lymphoïd and myeloïd leukemia, breast and ovarian cancers, non-Hodgkin lymphomas and multiple myeloma. When the clinical significance of multidrug resistance will be defined, identification of non-responders patients should lead to the use of therapeutic regimens including mdr reversing agents.

Multidrug resistance and mutagenesis

Multidrug resistance, the phenomenon whereby the development of resistance to one drug is sometimes accompanied by the simultaneous development of resistance to a variety of other, often structurally unrelated, drugs, is frequently associated with the presence of an energy-dependent membrane-transport system which reduces the concentration of a drug or other chemical in the cytoplasm. The latter process (termed here MDR) occurs naturally in a number of normal mammalian tissues, including colon, jejunum, liver, kidney and bone marrow, as well as in other species including bacteria. The presence of MDR can reduce the mutagenic potential of a variety of compounds in mammalian and microbiological assays. MDR can be reversed by a diverse collection of compounds, many of which are hydrophobic cations with other physiological effects. An important consequence of these considerations is that MDR-reversing agents are potentially dangerous because, while having no intrinsic mutagenicity, they may significantly increase the mutagenicity of other compounds by poisoning protective MDR mechanisms in the body.

Studies on low-level MDR cells

Acquired or spontaneous resistance is a major clinical problem in the treatment of cancer. Low levels of MDR gene expression or P-glycoprotein have been correlated with a high level of drug resistance in vitro and a poor response to chemotherapy in some tumors. A strong correlation between MDR mRNA, P-glycoprotein levels and degree of drug resistance has not been found in several resistant model tumor cell lines. In some cell lines at low and high level of resistance different mechanisms seem to be involved.

Molecular cytogenetics of multiple drug resistance

The refractory nature of many human cancers to multi-agent chemotherapy is termed multidrug resistance (MDR). In the past several decades, a major focus of clinical and basic research has been to characterize the genetic and biochemical mechanisms mediating this phenomenon. To provide model systems in which to study mechanisms of multidrug resistance, in vitro studies have established MDR cultured cell lines expressing resistance to a broad spectrum of unrelated drugs. In many of these cell lines, the expression of high levels of multidrug resistance developed in parallel to the appearance of cytogenetically-detectable chromosomal anomalies resulting from gene amplification. This review describes cytogenetic and molecular-based studies that have characterized DNA amplification structures in MDR cell lines and describes the important role gene amplification played in the cloning and characterization of the mammalian multidrug resistance genes (mdr). In addition, this review discusses the genetic selection generally used to establish the MDR cell lines, and how drug selections performed in transformed cell lines generally favor the genetic process of gene amplification, which is still exploited to identify drug resistance genes that may play an important role in clinical MDR.

The use of reverse transcriptase-polymerase chain reaction (RT-PCR) to investigate specific gene expression in multidrug-resistant cells

Expression of specific genes at the level of mRNA can be studied using techniques such as Northern blot, slot/dot blot, RNase protection assay, in situ hybridisation and RT-PCR. In this article these methods of analysis are compared; RT-PCR offers higher levels of specificity and sensitivity than traditional methods of RNA analysis and as such has become the method of choice for the study of gene expression. The RT-PCR technique is described in detail with sections dealing with RNA extraction, choice of primers (including the use of cDNA sequence data bases), PCR and RT-PCR protocols in addition to the limitations of the method. The study of one particular mRNA transcript (MDR1) using RT-PCR is discussed in detail. Recently described methods for quantitation of PCR products are discussed. Quantitative PCR would appear to offer a method of studying gene expression in a more extensive way than has been possible to date.

Effect of multidrug-resistant P-glycoprotein gene expression on chloroaluminum tetrasulfonate phthalocyanine concentration

The effect of multidrug-resistant P-glycoprotein gene expression (MDR1) in 3T3 cells on cellular concentrations and cytotoxicity induced by the photodynamic agent chloroaluminum tetrasulfonate phthalocyanine (AlSPc) was evaluated. 3T3 cells transfected with a retroviral vector expressing human MDR1 cDNA were resistant to colchicine. Resistant cells incubated with daunomycin accumulated only 40-50% of the quantity of daunomycin accumulated in control cells. Resistant cells incubated with daunomycin in the presence of verapamil had intracellular daunomycin concentrations approximately equal to control cells without verapamil. When these MDR1 3T3 cells were incubated with AlSPc, cellular concentrations of AlSPc did not differ between cells resistant to colchicine and those that were not. Similarly, there was little difference in cytotoxicity demonstrated by 51Cromium release in the two cell lines exposed to AlSPc and light (675 nm; 6 J/cm2). This study suggests photodynamic therapy using AlSPc may be a useful treatment modality for tumors in which the MDR1 P-glycoprotein confers resistance to cancer chemotherapeutics.

Transfer and expression of the human multiple drug resistance gene into live mice

The human multiple drug resistance (MDR) gene has been used as a selectable marker to increase the proportion of bone marrow cells that contain and express this gene by drug selection. By constructing retroviral vectors containing and expressing the MDR gene and a nonselectable gene such as the beta-globin gene, enrichment for cells containing both of these genes can be achieved. A retroviral construct containing MDR cDNA in a Harvey virus-based vector has been used to transfect our ecotropic 3T3 retroviral packaging line GP+E86. Clones have been isolated by exposure of the retrovirally transfected cells (MDR producer cells) to colchicine (60 ng/ml), a selective agent that kills MDR-negative cells. Flow cytometry analysis (fluorescence-activated cell sorting) with an antibody to MDR demonstrates expression of human MDR protein on the surface of these colchicine-resistant producer clones. Untransfected GP+E86 cells are negative. Colchicine-resistant clones were titered using clone supernatants and the highest titer clone (4 x 10(4) viral particles per ml) was cocultured with 10(6) donor mouse bone marrow cells for 24-48 hr. The donor cells were then injected into congenic irradiated mice, and the presence of the MDR gene was assayed by the polymerase chain reaction (PCR) analysis using MDR-specific primers. In one experiment eight of nine transduced mice were positive for MDR by PCR of peripheral blood 14 and 50 days posttransplantation; after 240 days three of nine transduced mice were positive. Bone marrow obtained from one of these positive animals was stained with the MDR monoclonal antibody and the granulocyte population was analyzed by FACS. Approximately 14% of the total granulocyte pool contain increased levels of MDR protein. In addition, the bone marrow cells of several mice initially positive for MDR gene by PCR, and subsequently negative, were exposed to taxol, a drug whose detoxification depends on MDR gene expression; a positive signal was obtained in all of these mice, indicating drug selection of MDR-positive marrow cells. Cell sorting studies of these mice also show an increased number of high-MDR-expressing marrow cells, selected after exposure to taxol. Thus, in this live animal model MDR transduction is effective in selecting a human MDR-expressing population of marrow cells resistant to taxol chemotherapy. This strategy may, thus, be useful in humans to prevent the marrow toxicity induced by anticancer agents such as taxol and as a selectable marker to enrich for cells simultaneously transduced with a nonselectable gene.

Separation of drug transport and chloride channel functions of the human multidrug resistance P-glycoprotein

The human multidrug resistance P-glycoprotein is an active transporter that pumps cytotoxic drugs out of cells. Expression of P-glycoprotein is also associated with a volume-activated chloride channel. Here we address the relationship between these two functions. Drug transport requires ATP hydrolysis while, in contrast, ATP binding is sufficient to enable activation of the chloride channel. The chloride channel and drug transport activities of P-glycoprotein appear to reflect two distinct functional states of the protein that can be interconverted by changes in tonicity. Transportable drugs prevent channel activation but have no effect on channel activity once it has been preactivated by hypotonicity. The transport and channel functions of P-glycoprotein have been separated by directed mutations in the nucleotide-binding domains of the protein. These data provide further evidence that P-glycoprotein is bifunctional with both transport and channel activities. Implications for the design of chemotherapeutic drugs and for the function of the related cystic fibrosis gene product, CFTR, are discussed.

Kinetics of daunorubicin transport by P-glycoprotein of intact cancer cells

Drug permeation across the plasma membrane of multidrug-resistant cells depends on the kinetics of the P-glycoprotein-mediated pump activity as well as on the passive permeation of the drug. We here demonstrate a method to characterize kinetically the pump in intact cells. To this purpose, we examined the membrane-transport properties of daunorubicin in various sensitive cancer cell lines and in their multidrug resistant (MDR) counterparts. First, we determined the passive permeability coefficient for daunorubicin. Then, using a flow-through system, the drug flux into the cell was measured after inhibition of the P-glycoprotein-mediated efflux pump. Combining the two results allowed us to calculate the intracellular free concentration of the drug. In the steady-state, the pump rate must equal the net rate of passive diffusion of the drug and, therefore, the same experiments gave us the pumping rate of daunorubicin. These experiments were then repeated at various extracellular drug concentrations. By plotting the pumping rate versus the intracellular drug concentration, we then characterized the P-glycoprotein kinetically. Four independent methods were used to measure the passive permeability coefficient for the cell line A2780. Similar values were obtained. Maximal pump rates (Vmax) showed a good correlation with the amount of P-glycoprotein in the cell lines used. We obtained saturation curves for the variation of the pump rates with the intracellular daunorubicin concentrations. These curves were typical for positive cooperativity, which provides evidence that at least two binding sites for daunorubicin are present on the active transport system of daunorubicin. The apparent Km values for P-glycoprotein-mediated transport, the intracellular free cytosolic daunorubicin concentrations at half-maximal velocity for the cell lines used, were approximately 1.5 microM. Except for the cell lines with the highest amount of P-glycoprotein, the passive efflux rate of daunorubicin proved to be a substantial part of the total daunorubicin efflux rate for the cell lines used. In cell lines with relatively low levels of P-glycoprotein, passive daunorubicin efflux was even the main route of daunorubicin transport from the cells, determining the intracellular steady-state concentrations of daunorubicin.

Modulation of expression of multidrug resistance gene (mdr-1) by adriamycin

The acquired resistance to various drugs in cancer is mediated by P-glycoprotein (P-gp) which is encoded by the mdr-1 gene. An increased level of mdr-1/P-gp was demonstrated after chemotherapy administered to treat cancer in humans. To clarify the direct effect of anticancer drugs on mdr-1/P-gp expression, we investigated the change in transport of adriamycin (ADR), and the expression of the mdr-1 gene and P-gp in an ADR-treated, multidrug-resistant leukemic cell line (K562/ADR500). The addition of ADR induced the over-expression of mdr-1/P-gp, which led to a transient decrease in the intracellular accumulation of ADR although the difference was not statistically significant. A maximal effect was observed after 4 h incubation, returning to the baseline level after further incubation for 12-24 h. The phosphorylation of P-gp was inversely correlated with the levels of P-gp. These observations suggest that ADR itself modulates both the expression and function of P-gp. Determination of the optimal schedule for administering adriamycin is essential to achieving the optimal effect in treating cancer.

The genetic basis of multidrug resistance

Cellular multidrug resistance, a common side-effect of anticancer chemotherapy frequently leading to failure of the treatment, has been characterized as an acquired resistance to several antimitotic drugs simultaneously. Multidrug resistance could mainly be attributed to the overexpression of the P-170 glycoprotein, considered as a drug-efflux pump encoded by the mdr 1 gene. Overexpression of this protein can be induced either by an accidental amplification or activation or both of the mdr 1 gene. Recent investigations focused on these mechanisms, aiming at a better understanding of the appearance of multidrug resistance during a chemotherapy. P-glycoprotein mediated drug resistance, however, is only one, albeit quite an important detoxification pathway, and some observations revealed genetic interactions with other systems. On the basis of this new knowledge, the development of novel therapeutic strategies to circumvent this clinical side-effect of cancer treatment has already begun.

Regulation of P-glycoprotein gene expression in hepatocyte cultures and liver cell lines by a trans-acting transcriptional repressor

Previously we have demonstrated that expression of the multidrug resistance (mdr) genes in rat liver and primary rat hepatocyte cultures is induced by exposure to 2-acetylaminofluorene and 3-methylcholanthrene. The mdr expression induced by both of these compounds occurs primarily via increased gene transcription. To determine the nature of possible regulatory proteins involved in mdr gene regulation we inhibited protein synthesis using cycloheximide or emetine in primary rat hepatocyte cultures, mouse (HePa 1), human (Hep G2) and rat (H4-II-E) cell lines. Each cell type responded by strongly increasing its steady state mdr1 mRNA levels. In hepatocytes increased mdr expression was observed after greater than 50% inhibition of protein synthesis, and was first detected after 2h of protein synthesis inhibition with maximal induction occurring by 24h. Nuclear run-on analysis showed that the increased steady state mRNA level was due to increased gene transcription without alteration of the transcription start site. Combined these data indicate that one regulatory mechanism by which mdr gene expression is controlled is via a trans-acting transcriptional repressor.

High cell density-dependent resistance and P-glycoprotein-mediated multidrug resistance in mitoxantrone-selected Chinese hamster cells

Mitoxantrone (MIT) resistance has been studied in a colony selected from the CHO AA8 parental line in one step under a low degree of selective pressure (9 nM). The cells of the clonal isolate AA8/MIT C1(0) were sensitive to 9 nM MIT at low cell density but able to grow at high density. Parental AA8 cells were not able to grow under the latter condition. Decreased MIT accumulation (-20%) was observed at this step (step 0) in the absence of overexpression of mdr RNA coding for the drug efflux pump P-glycoprotein. Furthermore, AA8/MIT C1(0) did not exhibit cross resistance to vincristine, Adriamycin and etoposide at low cell density. During subsequent controlled growth for 2 months at high cell density in the presence of 9 nM drug, an additional selection occurred leading to a 4-fold MIT-resistant subline AA8/MIT C1(+). This subline was characterized at this step (step I) and after an additional 4 months of culture in the presence of 9 nM MIT (step II). Analysis of mdr gene expression and gene copy number showed an increase in mdr RNA and a pattern of mdr gene amplification which changed between step I and II. AA8/MIT C1(+)II exhibited a classical multidrug resistance phenotype with decreased accumulation of [14C]MIT and cross-resistance to vincristine, Adriamycin and etoposide. The ability to form the cleavable complex in the presence of etoposide in DNA topoisomerase II-containing nuclear extracts was identical in AA8/MIT C1(+)II and AA8 cell lines. These results demonstrate a new sequence of events in MIT resistance: low level of drug resistance at high cell density followed by mdr gene amplification.

Double minute chromosomes carrying the human multidrug resistance 1 and 2 genes are generated from the dimerization of submicroscopic circular DNAs in colchicine-selected KB carcinoma cells

This study characterizes amplified structures carrying the human multidrug resistance (MDR) genes in colchicine-selected multidrug resistant KB cell lines and strongly supports a model of gene amplification in which small circular extrachromosomal DNA elements generated from contiguous chromosomal DNA regions multimerize to form cytologically detectable double minute chromosomes (DMs). The human MDR1 gene encodes the 170-kDa P-glycoprotein, which is a plasma membrane pump for many structurally unrelated chemotherapeutic drugs. MDR1 and its homolog, MDR2, undergo amplification when KB cells are subjected to stepwise selection in increasing concentrations of colchicine. The structure of the amplification unit at each step of drug selection was characterized using both high-voltage gel electrophoresis and pulsed-field gel electrophoresis (PFGE) techniques. An 890-kb submicroscopic extrachromosomal circular DNA element carrying the MDR1 and MDR2 genes was detected in cell line KB-ChR-8-5-11, the earliest step in drug selection in which conventional Southern/hybridization analyses detected MDR gene amplification. When KB-ChR-8-5-11 was subjected to stepwise increases in colchicine, this circular DNA element dimerized as detected by PFGE with and without digestion with Not 1, which linearizes the 890-kb amplicon. This dimerization process, which also occurred at the next step of colchicine selection, resulted in the formation of cytologically detectable DMs revealed by analysis of Giemsa-stained metaphase spreads.

From amplification to function: the case of the MDR1 gene

This review describes the features of gene amplification associated with the selection of multidrug-resistant cell lines. Some of these lines carry multiple copies of the MDR1 gene that encodes P-glycoprotein, a broad specificity efflux pump. The MDR1 gene was initially identified as the common component of the amplicons found in multidrug-resistant cell lines selected with different drugs. Subsequent studies have established that increased MDR1 expression is sufficient for the multidrug-resistant phenotype. MDR1-containing amplicons may include a number of additional transcribed genes that do not appear to contribute to multidrug resistance. MDR1 amplification is associated with specific chromosomal changes and apparently non-random recombinational events. Increased expression of the MDR1 gene, however, does not necessarily require gene amplification. Although amplification of the MDR1 gene has not been found in clinical tumor samples, increased expression of this gene is commonly observed in different types of cancer and appears to be a significant marker of clinical drug resistance.

Probing daunorubicin accumulation defects in non-P-glycoprotein expressing multidrug-resistant cell lines using digitonin

Multidrug resistance (MDR) in tumor cells is frequently associated with reduced cellular cytostatic drug accumulation, caused by the drug efflux protein, P-glycoprotein (Pgp). The action of Pgp in tumor cells can be detected by measuring the increase of daunorubicin accumulation upon blocking Pgp with drugs such as verapamil. A number of MDR cell lines have been described, characterized by decreased drug accumulation without Pgp being present. For such non-Pgp MDR cells no gene probes or functional assays are available to study this phenotype in clinical tumor specimens. We have worked out a method which enables the detection of drug-transport-related decreases in cellular daunorubicin accumulations without the need for the use of specific Pgp blockers. The cells used were SW-1573-, GLC4- and HT1080-sensitive cell lines, which accumulated (corrected for DNA content) 272%, 1,288% and 203% more daunorubicin than the non-Pgp MDR sublines SW-1573/2R120, GLC4/ADR and HT1080/DR4. When the plasma membranes of these MDR lines were permeabilized with 20 microM digitonin an increase to 282%, 1,260% and 239% of 14C-daunorubicin control accumulation was measured (at pH = 7.35). The intracellular pH measured with BCECF was the same in parent and corresponding MDR cells, excluding the role of pH differences in the measured effects. This method provides a tool allowing the detection of cellular mechanisms (including Pgp) which are related to active outward transport of daunorubicin.

Development of multidrug resistance in a primitive neuroectodermal tumor cell line

✓ Drug resistance remains a formidable obstacle to the successful treatment of pediatric primitive neuroectodermal tumors. Resistance to chemotherapeutic agents may be related, in part, to expression of the multidrug resistance gene 1 (MDR1). The protein product of this gene, P-glycoprotein, confers resistance to multiple unrelated antineoplastic drugs.

The cell line DAOY, derived from a primitive neuroectodermal tumor, was used as an in vitro model to examine the development of drug resistance. Cell lines resistant to actinomycin D were developed by the growth of DAOY in increasing concentrations of the drug. The IC50 (concentration of drug needed to induce a 50% reduction in cell growth) of the resultant lines to actinomycin D was more than 10 times that of the parental line. The resistant lines were cross-resistant to VP-16 (etoposide), despite lack of previous exposure to this drug. The resistance to actinomycin D was attenuated in the presence of verapamil, a known inhibitor of P-glycoprotein. The MDR1 gene was not expressed by the parental DAOY line at the messenger ribonucleic acid (RNA) and protein level. Expression of the MDR1 gene was documented in the resistant lines by RNA blot and immunoblot techniques. These results suggest that exposure to chemotherapeutic drugs can induce classical multidrug resistance in primitive neuroectodermal tumors.

Volume-regulated chloride channels associated with the human multidrug-resistance P-glycoprotein

Expression of P-glycoprotein, the product of the MDR1 gene, confers multidrug resistance on cell lines and human tumours (reviewed in refs 1,2). P-glycoprotein (relative molecular mass 170,000) is an ATP-dependent, active transporter which pumps hydrophobic drugs out of cells, but its normal physiological role is unknown. It is a member of the ABC (ATP-binding cassette) superfamily of transporters, which includes many bacterial transport systems, the putative peptide transporter from the major histocompatibility locus, and the product of the cystic fibrosis gene (the cystic fibrosis transmembrane regulator, CFTR). CFTR is located in the apical membranes of many secretory epithelia and is associated with a cyclic AMP-regulated chloride channel. At least two other chloride channels are present in epithelial cells, regulated by cell volume and by intracellular Ca2+, respectively. Because of the structural and sequence similarities between P-glycoprotein and CFTR, and because P-glycoprotein is abundant in many secretory epithelia, we examined whether P-glycoprotein might be associated with one or other of these channels. We report here that expression of P-glycoprotein generates volume-regulated, ATP-dependent, chloride-selective channels, with properties similar to channels characterized previously in epithelial cells.

Function and regulation of the human multidrug resistance gene

The discovery of an energy-dependent pump system for natural product anticancer drugs has important implications for the biology of related energy-dependent transport systems as well as for the treatment of human cancer. To fully realize the therapeutic potential associated with manipulation of the multidrug transporter, it will be necessary to understand the mechanisms of action of the transporter and its mode of regulation. This review has summarized recent developments in these areas which suggest that both the activity of the pump and its genetic regulation are potential targets for new anticancer therapies.

Evaluation of the cytotoxic mechanisms mediated by the broad-spectrum antitumor alkaloid acronycine and selected semisynthetic derivatives

Acronycine (I) is a broad-spectrum antitumor agent whose development as a clinically useful agent has been hindered, in part, due to its poor solubility characteristics. With the goal of acquiring information that may prove of value in the development of structurally related compounds of greater clinical utility, mechanistic studies were performed with acronycine (I) and two semisynthetic derivatives, 2-nitroacronycine (II) and acronycine azine (III). These three substances demonstrated cytotoxic activity with several human tumor cell lines (breast, colon, lung, melanoma, KB-3, and drug-resistant KB-V1). Compounds II and III demonstrated greater activity than I, and more detailed studies were performed with cultured human breast cancer cells (UISO-BCA-1). Acronycine azine (III) induced the cells to accumulate in the G0/G1 phase of the cell cycle. It effectively inhibited the in vitro catalytic activities of partially purified DNA and RNA polymerases in a manner that was competitive with respect to DNA substrate. As judged by spectrophotometric titration, compound III interacted with calf thymus DNA, calf liver RNA, and a variety of single- and double-stranded (deoxy)ribonucleotides. Although no nucleic acid base specificity was discernable, this interaction appeared to be related to the cytotoxic mechanism of this dimeric substance. Monomeric compounds I and II did not interact with nucleic acids, but were effective inhibitors of DNA and RNA synthesis as judged by in vitro systems comprised of cultured UISO-BCA-1 cells or homogenates derived from these cells. The relative inhibitory activities of compounds I and II correlated with their cytotoxic activities suggesting a causal relationship. In addition, these two compounds induced cultured cells to accumulate in the phase of the cell cycle wherein the DNA content ranged from 2n-4n (S + G2/M), and inhibited in vitro DNA and RNA synthesis in a manner that was competitive with respect to nucleotide (TTP or UTP) substrate. Compounds I and II demonstrated greater cytotoxic activity with drug-resistant KB-V1 cells as compared with the parent (drug-sensitive) cell line, whereas this was not the case with compound III. Based on these results and previous literature reports, compounds I, II and III are likely to function by multiple mechanisms of action. However, it appears that alteration of nucleic acid metabolism is key to the activity of each of the substances.

Genetic aspects of multidrug resistance

Gene amplification is responsible both for dihydrofolate reductase induced methotrexate resistance, and for the P-glycoprotein encoding multigene family induced multidrug resistance. The 6 pairs of hydrophobic regions of the P-glycoprotein fold up in a snake-like structure through the lipidic layers of the cell membrane. Other detoxification mechanisms include the glutathione S-transferase 'pi', but without gene amplification.

Multidrug resistance gene and P-glycoprotein expression in gastric adenocarcinoma and precursor lesions

Overexpression of the Multiple Drug Resistance gene (MDR1) has been proposed as a major mechanism related to both intrinsic and acquired resistance to chemotherapeutic agents. The gene product is a membrane protein (P-glycoprotein), that acts as an energy-dependent drug efflux pump decreasing drug accumulation in resistant tumor cells. We have characterized MDR1 and P-Glycoprotein expression in human gastric adenocarcinoma and in precursor lesions. MDR1 mRNAs, analyzed by dot-blot technique, were detected in 9 of 10 non-tumoral gastric mucosae and in 8 of 10 gastric adenocarcinomas. Immunohistochemical analysis, using the MRK16 monoclonal antibody, revealed heterogeneous expression of P-Glycoprotein in individual cells. The P-Glycoprotein was found on the surface of cells of gastric areas with intestinal metaplasia subtype III. This type of intestinal metaplasia, also called "colonic metaplasia", has been strongly associated with a high risk for the development of gastric cancer. The fact that the P-Glycoprotein was detected in this precursor lesion is consistent with the intestinal metaplasia-dysplasia and carcinoma sequence proposed in the histogenesis of this tumour. The finding that P-Glycoprotein was heterogeneously expressed in malignant cells of some gastric adenocarcinomas also suggests that this transporter system probably contributes to primary and secondary multidrug resistance in this neoplasm.

Structural and functional analyses of the promoter of the murine multidrug resistance gene mdr3/mdr1a reveal a negative element containing the AP-1 binding site

We have previously shown that the multidrug-resistance/P-glycoprotein gene, mdr3/mdr1a, is activated in mouse hepatocellular carcinomas (HCC). In this study, we show that in a number of HCC-derived cell lines (Hepa1c1c, Hepa1c1c-BprC1, and Hepa1-6) mdr3 is expressed at high levels. To investigate transcriptional regulation of mdr3 in these cells, we have isolated a DNA fragment containing the 5' portion of the mouse mdr3 gene and performed a functional analysis of its promoter. Transient transfection assays using various lengths of the promoter sequence to direct expression of the chloramphenicol acetyltransferase (CAT) reporter gene revealed that the sequence located -94 nucleotides upstream from mouse mdr3 transcription start site functions as a negative element in mouse hepatoma cells. A canonical AP-1 binding sequence TGA-GTCA located at -117 is at least in part responsible for the negative effect from the following observations: (i) Alteration of this AP-1 sequence by site-directed mutagenesis enhanced CAT expression. (ii) Expression of CAT reporter gene was elevated when double-stranded DNA containing the AP-1 sequence, but not mutated sequences, was used as a competitor in cotransfection experiment. (iii) Enhancement of the CAT expression was also seen in cotransfection experiments using recombinant plasmid DNA expressing the c-jun/c-fos proteins, which interact with AP-1 sequences. Interestingly, the proximal region of the hamster pgp1 promoter shares striking sequence similarity with that of the mouse mdr3 gene, including the AP-1 site, but the AP-1 site in the hamster promoter serves as a positive regulator. Although previous studies have demonstrated that positive and negative transcription factors can modulate gene expression through interactions with c-jun/c-fos, this is the first study to show that an AP-1 site functions as a negative cis-element in the regulation of gene expression.

Newly formed chromosome-like structures in independent mouse P388 sublines with developed in vivo mdr1 gene amplification

Mouse leukemia P388 sublines that acquired the resistance to multiple drugs as a result of treatment in vivo with anthracyclines (rubomycin, ruboxyl) and/or vincristine were studied. The mdr gene amplification was found in all tested cell lines: in four of five sublines all three members of the mdr gene family showed increased copy numbers, and in one cell line, developed after treatment with ruboxyl, mdr1a and mdr1b genes were amplified to the same degree, whereas the mdr2 gene was not amplified at all. The levels of amplification of mdr genes varied in different cell lines from 30-fold to 50-fold. Unusual cytological manifestations--relatively large newly formed chromosomelike structures, were revealed in four of five long-term independent sublines. Some of these structures did not contained C blocks; the others, in contrast, were enriched by C-heterochromatin. In situ hybridization showed the presence of mdr genes in newly formed bodies. In the majority of cases, the formation of chromosomelike structures was preceded by the appearance of other, smaller size, structures: the so-called "small chromatin bodies" (minichromosomes) and/or homogeneously G-positive small ring chromosomes.

Structure and expression of the cytosolic aldehyde dehydrogenase gene in cyclophosphamide-resistant murine leukemia L1210 cells

These investigations were performed to clarify the molecular basis for the enhanced expression of cytosolic aldehyde dehydrogenase (ALDH-1) enzymatic activity in the cyclophosphamide-resistant L1210/CPA murine leukemia cell line, as compared to the parental L1210/O strain. Western immunoblot analysis was performed using a 15-fold greater quantity of cytosolic protein from the L1210/O as compared to the L1210/CPA cell line. Nevertheless, ALDH-1 immunoreactive protein could be detected only in the L1210/CPA cells. Northern analyses, performed using total cellular and polyadenylated RNA, again demonstrated ALDH-1-specific transcripts only in the L1210/CPA cell line. This transcript was identical in size to the ALDH-1 message expressed by normal murine hepatocytes. On Southern analysis, no evidence of gene amplification, gene rearrangement, or significant mutations of length was detected. These studies suggest that the ALDH-1 protein produced by the L1210/CPA cell line is structurally normal. Moreover, overexpression of the gene does not appear to have arisen as a result of an incremental process, such as gene amplification. Rather, a qualitative abnormality in the regulation of this gene appears to exist in the L1210/CPA cells, which distinguishes them from L1210/O cells and from normal murine lymphocytes.

Identification and characterization of three DNA-binding proteins on the promoter of the human MDR1 gene in drug-sensitive and -resistant cells

In multidrug-resistant cell lines (K562/ADM), MDR1 was amplified and transcriptionally activated. But the mechanism of MDR1 expression is unknown in K562/ADM cells. A 131 bp DNA fragment upstream from the major cap site of MDR1 contained CAAT-like and GC-box-like motifs and showed promoter activity in a CAT expression assay, resulting in a 3.5-fold enhancement of MDR1 promoter activity in K562/ADM as compared with that in K562 cells. In the CAT assay using deletion mutants we found that sequences containing the CAAT-like motif and the GC-box-like motif were required for MDR1 proximal promoter activity. To understand the details of MDR1 transcription in K562/ADM cells we analyzed the interaction between the proximal promoter and DNA-binding protein(s). We have identified three DNA-binding proteins on the MDR1 promoter from K562/ADM and parental K562 cells. The first binding protein to the MDR1 promoter was NF-R1, which recognized the sequences containing the CAAT-like motif and the GC-box-like motif as revealed by using a gel mobility shift assay. The second protein was NF-R2, which bound to the sequence containing the CAAT-like motif. The third protein, which bound to the sequence containing the GC-box-like motif was designated NF-R3. NF-R2 and NF-R3 in the resistant cells formed different bands in the gel mobility shift assays as compared with those in the sensitive cells, respectively. The observed difference might be related to the transcriptional activation of MDR1 in K562/ADM cells.

Modulation of drug resistance in homoharringtonine-resistant C-1300 neuroblastoma cells with cyclosporine A and dipyridamole

The development of resistance accounts for therapy failure in the majority of advanced cases of neuroblastoma in children. A new transplantable murine C-1300 neuroblastoma cell line was developed in vitro, by repeated exposure of a sensitive cell line to increasing, but sublethal, doses of Homoharringtonine (HHT). The ED50 of the highly resistant cells for HHT, using a standard agar colony assay, is 480 ng/ml, compared with 13 ng/ml for the sensitive parental line. The resistant cells have cross-resistance to a number of other agents, including adriamycin, vinca alkaloids, melphalan, and CCNU. Western blot analysis revealed progressive increases in P-glycoprotein, parallel to the graded development of resistance with a 29-fold elevation in the highest resistant cells. High-performance liquid chromatography (HPLC) indicated that resistant cells have a significantly lower uptake of HHT than parental sensitive cells. cyclosporine A (CsA) and dipyridamole (DPM) could modulate the acquired resistance and completely restore the cytotoxic effects of HHT and adriamycin as determined by the clonogenic assay. The reversal of resistance by CsA and DPM was dose dependent. With the relative low toxicity of dipyridamole and CsA in doses required for modulation of resistance, these agents may be candidates for clinical utilization in chemotherapy of resistant neuroblastoma.

Characterization of an etoposide-resistant human K562 cell line, K/eto

An etoposide-resistant K562 cell line (K/eto) was obtained by stepwise exposure, in culture, to increasing concentrations of etoposide, without the use of mutagens. This cell line was resistant to etoposide, and slightly resistant to adriamycin, but sensitive to anti-cancer drugs such as camptothecin, vincristine, actinomycin D and so on. P-Glycoprotein, the mdr1 gene product, was not detected in this cell line, as assessed by immunocytochemistry, immunoprecipitation and flow cytometry. Overexpression of mdr1 mRNA was also not found. Interestingly, expression of 85 kD protein recognized by MRK 20 monoclonal antibody was noted. The level of DNA topoisomerase II protein, detected by antibody staining, decreased concomitantly with a general decrease in DNA topoisomerase II unknotting activity, while DNA topoisomerase I activity was not affected. Cellular accumulation of [3H]etoposide was reduced by 75% in the resistant line compared with parental K562. Karyotype analysis showed that the number of chromosomes in K/eto was 55 and neither a homogeneous staining region nor double-minute chromosomes were detected. These results indicate that this resistance is not due to an altered interaction between the drug and cellular transport machinery, i.e. MDR1, associated with the "classic" multiple drug resistance phenotype, but rather is due to the existence of other mechanism(s) of resistance, decreased transport of the drug and decreased target enzyme, DNA topoisomerase II.

Multidrug resistance after retroviral transfer of the human MDR1 gene correlates with P-glycoprotein density in the plasma membrane and is not affected by cytotoxic selection

Multidrug resistance (MDR) in mammalian cells is associated with the expression of the MDR1 gene encoding P-glycoprotein (P-gp), an and active efflux pump for various lipophilic compounds. MDR transfectants can be isolated after MDR1 gene transfer and selection with cytotoxic drugs; low levels of drug resistance have also been observed in unselected NIH 3T3 mouse cells after retrovirus-mediated transfer of mouse mdr1 cDNA. MDR cell lines possess multiple phenotypic changes, suggesting that P-gp function could be complemented by some additional mechanisms associated with cytotoxic selection. To determine whether cytotoxic selection contributes to the MDR phenotype of MDR1-expressing cells, NIH 3T3 cells infected with a recombinant retrovirus carrying the human MDR1 gene were selected by two different procedures: (i) noncytotoxic selection for increased P-gp expression on the cell surface by multiple rounds of immunofluorescence labeling and flow sorting or (ii) one or more steps of selection with a cytotoxic drug. The levels of MDR in both types of infectants showed an excellent correlation with the P-gp density in the plasma membrane, expressed as immunoreactivity with a P-gp-specific antibody normalized by reactivity with an antibody against an unrelated antigen. Cytotoxic selection conferred no additional increase in resistance relative to P-gp density. These results indicate that P-gp density in the plasma membrane may be sufficient to determine the level of MDR.

A phase II study of epidoxorubicin in colorectal cancer and the use of cyclosporin-A in an attempt to reverse multidrug resistance

We determined the ability of the multidrug resistance (MDR) reversal agent cyclosporin-A to increase anthracycline drug accumulation in colorectal tumour cells in vitro, using the technique of on-line flow cytometry. Data of four previously untreated patients showed that cyclosporin-A can increase intracellular net-uptake of daunorubicin. A phase II study was initiated in 24 colorectal cancer patients. They received cyclosporin-A at a dose of 3 mg kg-1 over 1 h as i.v. infusion, at 7 h and at 1 h preceding cytotoxic drug administration. At the end of the second cyclosporin-A administration epidoxorubicin 90 mg m-2 was administered as i.v. bolus. Cycles were repeated every 3 weeks. Median cyclosporin-A peak blood levels and levels at 18 h after cytotoxic drug administration appeared to be 6248 ng ml-1 and 1012 ng ml-1 respectively. Only one partial response was observed, despite these high cyclosporin-A levels. Cyclosporin-A did not cause major toxicity, only a 29% incidence of hot flushes was observed. Epidoxorubicin toxicities were as expected but the frequency of severe leucocytopenia was striking. This treatment schedule can not be considered active in colorectal cancer.

Multi-drug resistance genes in the management of neoplastic disease

P-gp can function as an ATP-dependent cytotoxic drug-efflux pump. In normal tissues, protein expression is localized to cell surfaces that face excretory lumina; hence, P-gp may function as a toxic-waste disposal system. Tumors that are derived from these tissues can be high expressors of P-gp, and these tumors tend to display intrinsic chemoresistance. Other non-expressing tumors can become P-gp positive after treatment or at relapse, suggesting that mdr may be involved in acquired resistance. The use of MDR-modifying agents has had some clinical success, and further trials of chemosensitizers are proceeding. P-gp overexpression does not explain how clinical resistance to anthracyclines, alkylating agents, and cis-platinum can arise simultaneously. In these cases, multiple genetic mechanisms of resistance may coexist. Eventually, mdr status can be used to select the most effective chemotherapy protocol for the individual. Currently, conversion of a previously mdr negative tumor to mdr expression, in the face of clinical resistance, justifies changing to a non-MDR drug protocol, or if not feasible, the use of MDR sensitizers.

Reversal of multidrug resistance in HL-60 cells by verapamil and liposome-encapsulated doxorubicin

The means of circumventing multidrug resistance was investigated in HL-60 and HL-60R (a drug resistance variant) cell lines. The HL-60R cell line was developed from the parent line by serial exposure to increasing concentrations of doxorubicin over a 4-month period. This drug resistant cell line expressed P-glycoprotein in its cell surface and is 80-fold more resistant than the parent cell line. Multidrug resistance, as evaluated by a cell cytotoxicity assay using doxorubicin, can be overcome by use of verapamil. Multidrug resistance can also be circumvented when doxorubicin is encapsulated in liposomes. The combination of verapamil and doxorubicin-encapsulated liposomes does enhance circumvention of multidrug resistance beyond the effect of each agent alone, implying a synergistic effect. The lipid composition of the liposomes does affect the rate of drug uptake but not the overall cytotoxic effect of doxorubicin. The synergistic reversal of multidrug resistance by doxorubicin-encapsulated liposomes and verapamil suggests a multifactorial basis for drug resistance in this cell line.

Spontaneous multidrug transport in human glioma cells is regulated by transforming growth factors type beta

The multidrug transporting cell membrane molecule P-glycoprotein can be spontaneously expressed in human glioma cells. Transcripts of mdr genes were detected in glial tumor cells by polymerase chain reaction and Northern blotting, expression of P-glycoprotein was analyzed by immunocytochemistry and functional activity by cytofluorometry of fluorescent probe transport. In vitro treatment of glioma cells with vincristine induced coordinate over-expression of both mdr1 and mdr3 genes associated with very high P-glycoprotein-mediated multidrug transport, resistant to the inhibitory activity of chemosensitizers like verapamil. The physiological modulators of multidrug transport are as yet unknown. We therefore initiated a screening program to analyze the effects of cytokines on multidrug transport. We observed, that transforming growth factors (TGF)-beta 1, -beta 2, and -beta 1.2-but not the related bone morphogenetic protein (BMP) 2--inhibited multidrug transport. Interestingly, BMP 2 antagonized the TGF-beta induced inhibition of multidrug transport.

Amplification and expression of mdr genes and flanking sequences in verapamil hypersensitive hamster cell lines

The properties of several multidrug resistant (MDR) Chinese hamster ovary (CHO) cell lines which are verapamil hypersensitive have been investigated, extending our earlier study of two such cell lines. It was observed that increasing levels of multidrug resistance are associated with increasing verapamil and nicardipine sensitivity, although the cell lines are not hypersensitive to cyclosporin A. Although there is appreciable amplification of the P-glycoprotein gene at higher levels of multidrug resistance/verapamil hypersensitivity, there is only very low or no amplification of five flanking genes, including the sorcin gene. Low levels of resistance (3-10 fold) appear to involve increased P-glycoprotein gene expression at the level of transcription. P-glycoprotein levels of the cell lines have been measured by flow cytometry using the monoclonal antibody C219, and there is a general correlation between P-glycoprotein overexpression, increased levels of the corresponding mRNA and the level of verapamil hypersensitivity.

Glycosylation of P-glycoprotein in a multidrug-resistant KB cell line, and in the human tissues

P-glycoprotein (P-gp) is thought to transport anti-cancer drugs and to be responsible for the multidrug-resistant (MDR) phenotype. Immunohistochemistry reveals that P-gp is also expressed in normal human tissues, such as the adrenal gland, kidney, liver, and the capillary endothelium of the brain and testis. However, little is known about the structural and functional variations of P-gp in these tissues. With immunoblotting and photoaffinity labeling, we found that the molecular mass of P-gp in these tissues varied between 130-140 kDa. To clarify the post-translational modification of P-gp, we studied the biosynthesis of P-gp in a human multidrug-resistant cell line (KB-C2). We found that P-gp was produced in KB-C2 cells as a 125 kDa precursor and was slowly processed (t1/2 = 45-60 min) to the mature form of 140 kDa. In the presence of tunicamycin, a 120 kDa form of P-gp was synthesized and this form was no longer processed. Treating the 125 kDa precursor form with endo-beta-N-acetylglucosaminidase H (Endo H) and the 140 kDa mature form with N-glycanase diminished the molecular size of P-gp to that of the tunicamycin-treated form. N-Glycanase almost completely removed [3H]glucosamine labeling from P-gp. These data indicate that the major modification of P-gp is N-linked glycosylation. P-gps from KB-C2 cells, kidney and adrenal gland had a different lectin-binding capacity. There seems to be a variety of N-linked glycosylations in tissue and tumor P-gps.

Expression of P-glycoprotein in human glioma cell lines and surgical glioma specimens

✓ The expression of P-glycoprotein, a product of multidrug resistance gene 1, was studied by Western blotting and immunohistochemistry in five human glioma cell lines. One glioma cell line was resistant to vincristine, Adriamycin (doxorubicin), and etoposide, and the other four glioma cell lines were sensitive to each drug. The multidrug-resistant cell line showed a high expression of P-glycoprotein in Western blot analysis and a positive immunostaining for P-glycoprotein mainly along the cell membrane, whereas all multidrug-sensitive glioma cell lines demonstrated no expression of P-glycoprotein in Western blotting and no immunostaining for P-glycoprotein, thus showing a good correlation between the expression level of P-glycoprotein and the extent of multidrug resistance.

In 18 human surgical glioma specimens, there was no evidence of complete absence of immunostaining for P-glycoprotein. With a definition of more than 20% of P-glycoprotein-positive cells as positive, from 10% to 20% as intermediate, and less than 10% as negative, immunostaining for P-glycoprotein was positive in one specimen and intermediate in six of 15 specimens taken from virgin gliomas, and positive in two specimens and intermediate in one of three recurrent gliomas treated previously with irradiation, ACNU (1-(4-amino-2-methyl-pyrimidine-5-yl)-methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride), cisplatin, vincristine, and/or procarbazine.

Genetic mechanisms of drug resistance. A review

An overview of our present understanding of mechanisms of resistance against cytotoxic drugs is presented. Most of this understanding has come from studies on tumor cells made resistant in vitro, but there is reason to think that similar mechanisms are responsible for resistance in patients. After a brief overview of biochemical mechanisms of drug resistance, the types of mutations in tumor cells that can alter drug handling are discussed. Three examples of resistance are analysed in more detail: resistance to the folate analogue methotrexate; the multidrug resistance caused by increased levels of P-glycoprotein, which extrudes drugs from the cell; and resistance to alkylating agents.

Regulation of 2-acetylaminofluorene-and 3-methylcholanthrene--mediated induction of multidrug resistance and cytochrome P450IA gene family expression in primary hepatocyte cultures and rat liver

Previous studies by this laboratory have indicated that expression of the multidrug resistance (mdr) gene can be increased in vivo by exposure to a variety of xenobiotics. Because of the nature of these compounds, it was proposed that mdr gene expression might, at least in part, be regulated by the arylhydrocarbon (Ah) receptor. In the present study, we used a primary hepatocyte culture model to examine the relationship between induction of cytochrome P450IA and mdr expression in vitro. Both 3-methylcholanthrene (MC) and 2-acetylaminofluorene (AAF) were efficient inducers of mdr expression in this model. Induction of mdr gene expression by both MC and AAF obeyed a log10 concentration/response relationship. In contrast, 2,3,7,8-tetrachlorodibenzo-P-dioxin did not induce mdr expression at concentrations that yielded maximum induction of cytochrome P450IA expression. These data suggest that mdr induction was not mediated via the Ah receptor. Nuclear run-off analysis indicated that both AAF and MC induced mdr expression by increasing transcription. Primer extension analysis indicated that mdr gene transcription was initiated at one major site 151 bp upstream of the ATG site in both the uninduced and induced state in vivo and in vitro. The sequence of the primer and the site of initiation of gene transcription indicate that the main gene induced was the mdr 1b gene.

A murine plasmacytoma MOPC 104E resistant to cyclophosphamide is resistant to immunotherapy

A murine plasmacytoma MOPC 104E (MOPC) is highly sensitive to chemotherapeutic agents such as cyclophosphamide and mitomycin C as well as to immunotherapy (OK-432-combined adoptive immunotherapy using interleukin-2-cultured killer cells). In the present study, we prepared cyclophosphamide-resistant MOPC cells (MOPC-CPA/R) by serial in vivo passage of tumor cells following cyclophosphamide treatment. The in vivo sensitivity of MOPC-CPA/R to mitomycin C or to immunotherapy (OK-432-combined adoptive immunotherapy) was significantly decreased compared to the parent MOPC. In vitro experiments showed that MOPC-CPA/R were more resistant (five-fold) to lysis by cultured immune spleen cells than MOPC. Inhibition of the lytic activity of cultured immune spleen cells against MOPC was significantly increased (P less than 0.05) by the addition of unlabeled MOPC compared to unlabeled MOPC-CPA/R. These results suggest that MOPC-CPA/R express weaker antigenicity than MOPC. However, the transfer of immune spleen cells cultured with tumor extract derived from MOPC-CPA/R significantly prolonged the survival of MOPC-CPA/R-inoculated mice. Thus, by repeated cyclophosphamide treatment, tumor cells with low-antigenicity were selected. These tumor cells had lower sensitivity to another chemotherapeutic agent and immunotherapy. Such an immunological response may play an important role in cancer therapy.