The carboxylic ionophore monensin inhibits active drug efflux and modulates in vitro resistance in daunorubicin resistant Ehrlich ascites tumor cells

Interaction of crown ethers with the ABCG2 transporter and their implication for multidrug resistance reversal

Overexpression of ABC transporters, such as ABCB1 and ABCG2, plays an important role in mediating multidrug resistance (MDR) in cancer. This feature is also attributed to a subpopulation of cancer stem cells (CSCs), having enhanced tumourigenic potential. ABCG2 is specifically associated with the CSC phenotype, making it a valuable target for eliminating aggressive and resistant cells. Several natural and synthetic ionophores have been discovered as CSC-selective drugs that may also have MDR-reversing ability, whereas their interaction with ABCG2 has not yet been explored. We previously reported the biological activities, including ABCB1 inhibition, of a group of adamantane-substituted diaza-18-crown-6 (DAC) compounds that possess ionophore capabilities. In this study, we investigated the mechanism of ABCG2-inhibitory activity of DAC compounds and the natural ionophores salinomycin, monensin and nigericin. We used a series of functional assays, including real-time microscopic analysis of ABCG2-mediated fluorescent substrate transport in cells, and docking studies to provide comparative aspects for the transporter-compound interactions and their role in restoring chemosensitivity. We found that natural ionophores did not inhibit ABCG2, suggesting that their CSC selectivity is likely mediated by other mechanisms. In contrast, DACs with amide linkage in the side arms demonstrated noteworthy ABCG2-inhibitory activity, with DAC-3Amide proving to be the most potent. This compound induced conformational changes of the transporter and likely binds to both Cavity 1 and the NBD-TMD interface. DAC-3Amide reversed ABCG2-mediated MDR in model cells, without affecting ABCG2 expression or localization. These results pave the way for the development of new crown ether compounds with improved ABCG2-inhibitory properties.

Ketamine promotes the amyloidogenic pathway by regulating endosomal pH

Ketamine is an anesthetic and addictive drug that can cause cognitive dysfunction and neuroinflammation. Studies have shown that carboxy-terminal fragment derived from β-secretase (CTF-β) and amyloid beta (Aβ), the amyloidogenic products of amyloid precursor protein (APP), can also induce neuroinflammation and impair cognitive function. However, it remains unclear whether ketamine regulates the amyloidogenic pathway. In the endosome, APP is cleaved by beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), whose activity is influenced by pH. Endosomal acidification is mainly regulated by sodium hydrogen exchanger 6 (NHE6), which leaks protons out of endosomes, and vacuolar proton translocating ATPases (V-ATPase), which pump protons into endosomes. Therefore, we hypothesized that ketamine lowers the endosomal pH by reducing the endosomal NHE6 protein level, and this hyperacidification promotes the amyloidogenic pathway. We set up C57BL/6 J mouse models using 10, 20, 40, 80, and 100 mg/kg ketamine administration and SH-SY5Y cell models using 1, 10, 100, and 1000 μM ketamine administration to investigate its effects on the amyloidogenic pathway at different doses. Western blotting results showed that 100 mg/kg ketamine treatment in vivo and 1000 μM ketamine treatment in vitro increased endosomal BACE1 and CTF-β protein levels and reduced endosomal NHE6 and APP protein levels. The endosomal accumulation of BACE1 caused by ketamine administration was also observed using confocal imaging. Moreover, flow cytometry indicated that ketamine treatment lowered the endosomal pH value of SH-SY5Y cells. Later, cells were pretreated with monensin to restore the endosomal pH. Monensin did not affect amyloidogenic-related proteins or NHE6 directly; therefore, ketamine-promoted endosomal amyloidogenic processing and BACE1 accumulation were depleted by restoring endosomal acidity through monensin pretreatment. Finally, knockdown of NHE6 promoted the amyloidogenic pathway similarly and prevented further enhancement by ketamine. These results indicated that the effects of ketamine on the amyloidogenic pathway were dependent on the reduction of NHE6 and endosomal pH.

Evidence of nigericin as a potential therapeutic candidate for cancers: A review

Emerging studies have shown that nigericin, an H⁺, K⁺ and Pb²⁺ ionophore, has exhibited a promising anti-cancer activity in various cancers. However, its anti-cancer mechanisms have not been fully elucidated. In this review, the recent progresses on the use of nigericin in human cancers have been summarized. By exchanging H⁺ and K⁺ across cell membranes, nigericin shows promising anti-cancer activities in in vitro and in vivo as a single agent or in combination with other anti-cancer drugs through decreasing intracellular pH (pHi). The underlying mechanisms of nigericin also include the inactivation of Wnt/β-catenin signals, blockade of Androgen Receptor (AR) signaling, and activation of Stress-Activated Protein Kinase/c-Jun N-terminal Kinase (SAPK/JNK) signaling pathways. In many cancers, nigericin is proved to specifically target putative Cancer Stem Cells (CSCs), and its synergistic effects on photodynamic therapy are also reported. Other mechanisms of nigericin including influencing the mitochondrial membrane potentials, inducing an increase in drug accumulation and autophagy, controlling insulin accumulation in nuclei, and increasing the cytotoxic activity of liposome-entrapped drugs, are also discussed. Notably, the potential adverse effects such as teratogenic effects, insulin resistance and eryptosis shall not be ignored. Taken together, these reports suggest that treatment of cancer cells with nigericin may offer a novel therapeutic strategy and future potential of translation to clinics.

Lysosome Inhibitors Enhance the Chemotherapeutic Activity of Doxorubicin in HepG2 Cells

The lysosome inhibitors bafilomycin A1 and chloroquine have both lysosomotropic properties and autophagy inhibition ability, and are promising clinical agents to be used in combination with anticancer drugs. In order to investigate this combination effect, HepG2 cells were treated with bafilomycin A1, chloroquine, or/and doxorubicin, and their proliferative ability, induction of apoptosis, and the changes of lysosomal membrane permeabilization and mitochondrial membrane potential were studied. The results demonstrate that treatment with bafilomycin A1 or chloroquine alone at a relatively low concentration promotes the inhibitory effect of doxorubicin on cell growth and apoptosis. Further studies reveal that bafilomycin A1 and chloroquine promote lysosomal membrane permeabilization and the reduction of mitochondrial membrane potential induced by doxorubicin. Our findings suggest that bafilomycin A1 and chloroquine potentiate the anticancer effect of doxorubicin in hepatic cancer cells and that supplementation of conventional chemotherapy with lysosome inhibitors may provide a more efficient anticancer therapy.

Cytostatic and cytotoxic properties of monensic acid and its biometal(II) complexes against human tumor / non-tumor cell lines

The anticancer activity of monensic acid (MonH) and its biometal(II) complexes [M(Mon)2(H2O)2](M = Mg, Ca, Mn, Co, Ni, Zn) was evaluated against cultured human permanent cell lines established from glioblastoma multiforme (8MGBA) and cancers of the lung (A549), breast (MCF-7), uterine cervix (HeLa) and liver (HepG2). The viability and proliferation of the non-tumor human embryonic cell line Lep3 was also tested. The investigations were carried out using a thiazolyl blue tetrazolium bromide test, neutral red uptake cytotoxicity assay, crystal violet staining, colony forming method and double staining with acridin orange and propidium iodide. The results obtained reveal that the compounds applied at concentrations of 0.5–25 µg mL−1 for 24–72 h decrease the viability and proliferation of the treated cells in a time- and concentration-dependent manner. The metal(II) complexes studied (especially those of Co(II), Ni(II) and Zn(II)) have been found to express stronger cytotoxic and cytostatic activities than the non-coordinated monensic acid. The non-tumor human cell line showed strong chemosensitivity towards compounds tested comparable to that of cultured human tumor cell lines.

Long-circulating monensin nanoparticles for the potentiation of immunotoxin and anticancer drugs

The carboxylic ionophore monensin was formulated into long-circulating nanoparticles with the help of polyethylene glycol/poly (dl-lactide-co-glycolide) diblock copolymers, in an attempt to enhance the cytotoxicity of a ricin-based immunotoxin, anti-My9, and anticancer drugs like adriamycin and tamoxifen. This study looked into various aspects involving the preparation (using a homogenizer and an EmulsiFlex homogenizer-extrusion device) and lyophilization of long-circulating monensin nanoparticles (LMNP) of particle size < 200 nm in diameter. The particle size of LMNP was reduced from 194 nm to 160 nm by passing the nanoparticles through an EmulsiFlex, before freeze-drying. There was a 4.8–83.7% increase in the particle size of LMNP after freeze-drying, which was dependent upon the manufacturing conditions such as use of the EmulsiFlex for size reduction before freeze-drying, the freezing method (rapid/slow) and the concentration of lyoprotectant (mannitol or trehalose) employed for freeze-drying. LMNP freeze-dried with 2.4% of trehalose showed minimal size change (< 9%) after freeze-drying. Further, the freezing method was found to have negligible effect on the particle size of LMNP freeze-dried with trehalose in comparison with mannitol. The entrapment efficiency of monensin in LMNP was found to be 14.2 ± 0.3%. The LMNP were found to be spherical in shape and smooth in surface texture as observed by atomic force microscopy. In-vitro release of monensin from LMNP in phosphate buffered saline (PBS) pH 7.4 or PBS supplemented with 10% human serum indicated that there was an initial rapid release of about 40% in the first 8 h followed by a fairly slow release (about 20%) in the next 88 h. In-vivo studies conducted with Sprague-Dawley rats showed that 20% of monensin remained in circulation 4–8 h after the intravenous administration of LMNP. An in-vitro dye-based cytotoxicity assay (MTS/PMS method) showed that there was 500 times and 5 times potentiation of the cytotoxicity of anti-My9 immunotoxin by LMNP (5 times 10−8  m of monensin) in HL-60 sensitive and resistant human tumour cell lines, respectively. Further, LMNP (5 times 10−8  m of monensin) potentiated the cytotoxicity of adriamycin in MCF 7 and SW 620 cell lines by 100 fold and 10 fold, respectively, and that of tamoxifen by 44 fold in MCF 7 cell line as assessed by crystal violet dye uptake assay. Our results suggest that it is possibleto prepare LMNP possessing appropriate particlesize (< 200 nm), monensin content and in-vitro and in-vivo release characteristics with the help of a homogenizer and an EmulsiFlex homogenizer-extrusion device. LMNP can be freeze-dried with minimal increase in particle size by using a suitable concentration of a lyoprotectant like trehalose. Furthermore, LMNP could potentiate the cytotoxicity of immunotoxin, adriamycin and tamoxifen by 5–500 fold in-vitro, which will be further investigated in-vivo in a suitable animal model.

Effects of monensin liposomes on the cytotoxicity, apoptosis and expression of multidrug resistance genes in doxorubicin-resistant human breast tumour (MCF-7/dox) cell-line

We have evaluated the effects of monensin liposomes on drug resistance reversal, induction of apoptosis and expression of multidrug resistance (MDR) genes in a doxorubicin-resistant human breast tumour (MCF-7/dox) cell line. Monensin liposomes were prepared by the pH-gradient method. MCF-7/dox cells were treated with various anticancer drugs (doxorubicin, paclitaxel and etoposide) alone and in combination with monensin liposomes. The cytotoxicity was assessed using the crystal violet dye uptake method. The induction of apoptosis in MCF-7/dox cells was assessed by established techniques such as TUNEL (terminal deoxynucleotidyl transferase-mediated nick end labelling) staining and caspase-3 assay. The effect of monensin liposomes on doxorubicin accumulation in MCF-7/dox cells was monitored by fluorescent microscopy. Finally, the expression of MDR genes (MDR1 and MRP1) in MCF-7/dox cells following the exposure to doxorubicin alone and in combination with monensin liposomes was evaluated by semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). Our results indicated that monensin liposomes overcame drug resistance in MCF-7/dox cells to doxorubicin, etoposide and paclitaxel by 16.5-, 5.6- and 2.8-times, respectively. The combination of doxorubicin (2.5 μg mL−1) with monensin liposomes (20 times 10−8M) induced apoptosis in approximately 40% cells, whereas doxorubicin (2.5 μg mL−1) or monensin liposomes (20 times 10−8M) alone produced minimal apoptosis (<10%) in MCF-7/dox cells. Fluorescent microscopy revealed that monensin liposomes increased the accumulation of doxorubicin in MCF-7/dox cells. RT-PCR studies demonstrated that the expression of MDR1 and MRP1 was increased by 33 and 57%, respectively, in MCF-7/dox cells following treatment with doxorubicin (2.5 μg mL−1) for 72 h as compared with control MCF-7/dox cells. Furthermore, the levels of MDR1 and MRP1 in MCF-7/dox cells exposed to both doxorubicin and monensin liposomes showed a modest decrease as compared with MCF-7/dox cells treated with doxorubicin alone. In conclusion, the delivery of monensin via liposomes provided an opportunity to overcome drug resistance.

Increased lysosomal uptake of methotrexate-polyglutamates in two methotrexate-resistant cell lines with distinct mechanisms of resistance

Methotrexate (MTX) resistance in mitoxantrone-selected MCF7/MX cells and in MTX-selected CEM/MTX cells is associated with reduced drug accumulation, albeit caused by different mechanisms. In addition, in both resistant cell lines the proportion of active long-chain MTX-polyglutamate (MTX-PG) metabolites is reduced relative to that in the respective parental cell line. Previous studies by others have implied that increased lysosomal uptake could affect the rate of MTX-PG hydrolysis, and hence the length distribution of the polyglutamate chains. However, in the two cell line pairs studied, the number of lysosomes per cell was not different between the corresponding parental and resistant cells. Instead, we observed a two- to three-fold increased facilitative uptake of MTX-Glu4 by the lysosomes from these two independently derived MTX-resistant cell lines, compared to uptake by lysosomes from their corresponding parental cells. Enhanced lysosomal uptake of MTX-Glu4 was reflected in an increased maximal uptake velocity, without a change in the apparent substrate affinity. In addition, the rate of MTX efflux from lysosomes from CEM/MTX cells was two-fold faster than from lysosomes from CEM cells. Consistent with this observation, the relative amount of short-chain MTX-Glu(1+2) species, as a fraction of the total amount of all MTX-Glu(1-4) species combined, was only half as large in lysosomes from CEM/MTX cells as in lysosomes from CEM cells. Together, these results suggest the possibility that increased lysosomal uptake, and hence enhanced sequestration of MTX-PGs in resistant cells, contributes to the development of high-level MTX resistance by decreasing the cytosolic levels of MTX-PGs.

In Vitro Cytotoxicity of Stealth Liposomes Co-encapsulating Doxorubicin and Verapamil on Doxorubicin-Resistant Tumor Cells

Multidrug resistance (MDR) is a major obstacle to successful clinical cancer chemotherapy. A novel doxorubicin anti-resistant Stealth liposomes (DARSLs), prepared by co-encapsulating doxorubicin (DOX) and verapamil (VER) into stealth liposomes, has been developed. The average particle size of DARSLs was 118.1+/-22.3 nm. Encapsulation efficiencies of DOX and VER in DARSLs were greater than 95% and 70%, respectively. The IC(50) of DARSLs as measured by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay in multidrug resistant rat prostate cancer Mat-LyLu-B2 (MLLB2) cells was 0.079+/-0.017 microM, 13 fold less than that for liposomal DOX with free VER (LDFV 0.96+/-0.46 microM) but only about 2 times less than FDFV. The IC(50) cytotoxicity on MLLB2 cells of the various formulations was as follows: DARSLs approximately LDLV<FDFV<FDLV<LDFV<LD<FD, (LD: liposomal DOX; LV: liposomal VER; FD: free DOX; FV: free VER). Similar cytotoxicities were shown between DARSLs and FDFV in DOX-resistant human uterus sarcoma MES-SA/DX5 cells, reversing DOX-resistance to that shown by FD on DOX-sensitive MES-SA cells. For MLLB2 cells, DARSLs was the most cytotoxic, but its intracellular concentration of DOX, measured as mean cellular fluorescence with flow cytometry was lower (p<0.01) than that observed with the FDFV formulation. In conclusion, DARSLs was an effective DOX formulation which could overcome drug resistance in DOX-resistant tumor cells, but its mechanisms of action may be complex.

Efflux Kinetics and Intracellular Distribution of Daunorubicin Are Not Affected by Major Vault Protein/Lung Resistance-Related Protein (Vault) Expression

Vaults may contribute to multidrug resistance by transporting drugs away from their subcellular targets. To study the involvement of vaults in the extrusion of anthracyclines from the nucleus, we investigated the handling of daunorubicin by drug-sensitive and drug-resistant non-small lung cancer cells, including a green fluorescent protein (GFP)-tagged major vault protein (MVP)-overexpressing transfectant (SW1573/MVP-GFP). Cells were exposed to 1 microm daunorubicin for 60 min, after which the cells were allowed to efflux the accumulated drug. No significant differences in daunorubicin efflux kinetics were observed between the sensitive SW1573 and SW1573/MVP-GFP transfectant, whereas the drug-resistant SW1573/2R120 cells clearly demonstrated an increased efflux rate. It was noted that the redistribution of daunorubicin from the nucleus into distinct vesicular structures in the cytoplasm was not accompanied by changes in the intracellular localization of vaults. Similar experiments were performed using mouse embryonic fibroblasts derived from wild-type and MVP knockout mice, which were previously shown to be devoid of vault particles. Both cell lines showed comparable drug efflux rates, and the intracellular distribution of daunorubicin in time was identical. Reintroduction of a human MVP tagged with GFP in the MVP(-/-) cells results in the formation of vault particles but did not give rise an altered daunorubicin handling compared with MVP(-/-) cells expressing GFP. Our results indicate that vaults are not directly involved in the sequestration of anthracyclines in vesicles nor in their efflux from the nucleus.

Stealth monensin immunoliposomes as potentiator of immunotoxins in vitro

Stealth monensin liposomes (SML) were prepared using dipalmitoyl phosphatidylcholine, cholesterol, distearoyl glycerophosphoethanolamine coupled to polyethylene glycol, stearylamine, and N-succinimidyl pyridodithiopropionate linked to stearyl amine, in the molar ratio of 10:5:1.4:1.4:1.5. SML was conjugated to the anti-MY9 antibody by a disulfide linkage to form stealth monensin immunoliposomes (SMIL) by an already established procedure. The encapsulation concentrations of monensin in SML and SMIL were 10(-7) and 4.9x10(-8) M, respectively. More than 20% of monensin remained in circulation after 24 h in BALB/c mice. The ability of SML and SMIL to potentiate the effect of anti-MY9 immunotoxin (anti-MY9-IT) was tested against human leukemia HL-60 sensitive and resistant tumor cells in vitro. SML and SMIL potentiated the activity of anti-MY9-IT by 10-20 times against HL-60 sensitive tumor cell lines. However, greater potentiation of anti-MY9-IT was observed in combination with SML and SMIL against HL-60 resistant tumor cells, found to be 200 and 500 times, respectively. The potentiation of anti-MY9-IT by SMIL was more than two-fold compared with SML against both HL-60 sensitive and resistant tumor cells. Transmission electron microscopy studies conducted with HL-60 resistant cells incubated with anti-MY9-IT and monensin liposomes showed significant dilation of the golgi, which was reversible after re-incubation in fresh medium. Our studies show that SML and SMIL can be successfully used to potentiate the activity of ricin based anti-MY9-IT in vitro, and further in vivo studies will demonstrate the usefulness of this approach.

pH and drug resistance in tumors

Uptake of weakly ionizing drugs by tumours is greatly influenced by the interstitial and intracellular pH, and the ionization properties of the drug. Extracellular pH in tumors is acidic, while the intracellular pH is in the neutral-to-alkaline range. Tumors of the bladder, kidney and gastrointestinal system in particular are exposed to extremes of pH. Strategies for exhancing and exploiting acid-outside plasmalemmal pH gradients to drive the uptake of weak acid drugs into tumors are discussed, as are techniques for alkalinizing tissues to improve response to weak base drugs. The participation of acidic intracellular vesicles in non-specific drug resistance is explored. Copyright 2000 Harcourt Publishers Ltd.

pH and drug resistance. II. Turnover of acidic vesicles and resistance to weakly basic chemotherapeutic drugs

Resistance to chemotherapeutic agents is a major cause of treatment failure in patients with cancer. The primary mechanism leading to a multidrug-resistant phenotype is assumed to be plasma-membrane localized overexpression of drug efflux transporters, such as P-glycoprotein (P-gp). However, acidic intracellular organelles can also participate in resistance to chemotherapeutic drugs. In this study, we investigated, both experimentally and theoretically, the effect of acidic vesicle turnover on drug resistance. We have developed a general model to account for multiple mechanisms of resistance to weakly basic organic cations, e.g. anthracyclines and Vinca alkaloids. The model predicts that lower cytosolic concentrations of drugs can be achieved through a combination of high endosomal turnover rates, a low endosomal pH, and an alkaline-inside pH gradient between cytosol and the extracellular fluid. Measured values for these parameters have been inserted into the model. Computations using conservative values of all parameters indicate that turnover of acidic vesicles can be an important contributor to the drug-resistant phenotype, especially if vesicles contain an active uptake system, such as H+/cation exchange. Even conservative estimates of organic cation-proton antiport activity would be sufficient to make endosomal drug extrusion a potent mechanism of resistance to weakly basic drugs. The effectiveness of such a drug export mechanism would be comparable to drug extrusion via drug pumps such as P-gp. Thus, turnover of acidic vesicles can be an important factor in chemoresistance, especially in cells that do not overexpress plasma membrane-bound drug pumps like P-glycoprotein.

Stealth monensin liposomes as a potentiator of adriamycin in cancer treatment

Small unilamellar stealth monensin liposomes (SMLs) were prepared from multilamellar liposomes (MLVs). The MLVs were prepared by using dipalmitoyl phosphatidylcholine (DPPC), cholesterol, distearoyl glycerophosphoethanolamine coupled to poly(ethylene glycol) (DSPE-PEG) and stearylamine in the molar ratio of 10:5:1.4:1.4 (32.8 mM total lipid). The encapsulation efficiencies of monensin in MLVs and small unilamellar vesicles (SUVs) was 6x10++(-6) and 10(-7) M, respectively. The stability of SMLs was studied at 4 degrees C. The amount of leakage of monensin from SMLs was less than 20% after four weeks of storage. The in vitro release of monensin from SMLs in human serum was determined, and t1/2 was found to be 10 h. Pharmacokinetic studies on SMLs were carried out in BALB/c mice. More than 20% of SMLs remained in blood circulation after 24 h. SMLs increased the uptake of adriamycin (AM) in HL-60-resistant cells by more than two fold, compared to monensin in solution. SMLs potentiated the effect of AM against both sensitive and resistant HL-60 cells (six- and tenfold potentiation, respectively) and human LOVO tumor cells (four- and 200-fold potentiation, respectively). However, the highest potentiation was observed against resistant human breast tumor MCF7 cells, and was found to be 2400 times in comparison to AM alone. Transmission electron microscopic (TEM) studies carried out with HL-60-resistant tumor cells incubated with SMLs showed that SMLs caused dilation of the golgi of tumor cells within 10 min. The dilation of golgi was reversible after reincubation of the cells in fresh medium. SMLs showed considerable potential as a potentiator in combination with AM in overcoming drug resistance.

Characterization of acidic vesicles in multidrug-resistant and sensitive cancer cells by acridine orange staining and confocal microspectrofluorometry

To study the pH gradient status through membranes of acidic vesicles, either in sensitive or in multidrug-resistant living cancer cells, we monitored the fluorescence-emission spectra of acridine orange. Successive stainings with a pH-sensitive dye and AO showed that low-pH organelles were stained red by AO. In these compartments, high AO concentrations are driven by the pH gradient through membrane vesicles. The resulting rise in the dye's oligomeric/monomeric ratio induced an increase in the red/green (655-nm/530-nm) emission intensity ratio. Therefore, the accumulation of AO in acidic organelles was appraised by determination of the contribution of the red emission intensity (R%) in each emission spectrum, using laser scanning confocal microspectrofluorometry. In vesicles of multidrug-resistant K562-R cells, R% is significantly higher (72 +/- 10%) than the value (48 +/- 8%) from K562-sensitive cells (p < 0.001). This result is interpreted as a more important accumulation of AO in acidic cytoplasmic structures of resistant cells, which induces a shift from AO monomers (green emission) to self-associated structures (red emission). Equilibration of the pH gradient through acidic organelles was performed by addition of weak bases and carboxylic ionophores. Ammonium chloride (0.1 mM), methylamine (0.1 mM), monensine (10 microM), or nigericine (0.3 microM) all suppressed the initial difference of local AO accumulation between both cell lines. These agents decreased the red emission intensity for the resistant cell line but not for the sensitive one. The same effects were induced by 50 microM verapamil, a pleiotropic drug-resistance modulator. Our data allow the hypothesis of a higher pH gradient through membranes of acidic organelles, which would be a potential mechanism of multidrug resistance via the sequestration of weak bases inside these organelles.

The multidrug-resistant human lung tumour cell line, DLKP-A10, expresses novel drug accumulation and sequestration systems

Drug accumulation studies with the anticancer agents adriamycin and vincristine were carried out on the MDR variant of the human lung cell lines DLKP, DLKP-A10 which overexpresses the MDR associated P-glycoprotein efflux pump. Reduced cellular accumulation of both agents was observed in the resistant variant. The subsequent addition of verapamil and cyclosporin A resulted in partial restoration of cellular accumulation of both drugs in the DLKP-A10 resistant variant while complete restoration of cellular drug levels was observed in the SKMES-1/ADR cell line. These results suggested that the accumulation defect observed in the SKMES-1/ADR cell line was P-glycoprotein mediated and that accordingly, the cells exhibited characteristics consistent with the classical MDR phenotype. In contrast, while P-glycoprotein also appears to mediate a reduction in cellular drug accumulation in the DLKP-A10 cells, an alternative transport mechanism may also be present. No significant increase in the expression of either the MRP or LRP transport proteins was observed in the resistant cells. Metabolic inhibition by antimycin A (but not sodium azide or 2-deoxy-D-glucose) resulted in complete restoration of drug accumulation suggesting the presence of an alternative energy dependent transport mechanism. Fluorescent microscopy studies indicated different cellular localisation of the drug within the parental and resistant cells despite equivalent intracellular concentrations. These studies also revealed the presence of an ATP-dependent, vesicular sequestration mechanism which may be involved in the reduction of nuclear adriamycin accumulation in the DLKP-A10 cell line. This was indicated by observation of the disruption of cytoplasmic vesicles by antimycin A and also inhibition of cytoplasmic drug sequestration by the carboxylic ionophores, monensin and nigericin, accompanied by increased adriamycin accumulation and redistribution of the drug from the cytoplasm to the nucleus.

Monensin and verapamil do not alter intracellular localisation of daunorubicin in multidrug resistant human KB cells

The effects of monensin, verapamil and several inhibitors of membrane transport processes on the accumulation of [3H] daunorubicin by human KB-A1 cells have been investigated to determine the role of subcellular vesicular transport in the multidrug resistance phenotype. The Golgi inhibitor, brefeldin A, had no effect on drug accumulation, which suggests that vesicular transport is not a significant factor in drug resistance in these cells. KB-A1 cells were collaterally sensitive to both monensin and verapamil. Both of these compounds reduced drug efflux but did not alter subcellular distribution of daunorubicin, consistent with the view that monensin, like verapamil, acts directly on P-glycoprotein.

Defective pH regulation of acidic compartments in human breast cancer cells (MCF-7) is normalized in adriamycin-resistant cells (MCF-7adr)

Alkalinization of normally acidic intracellular compartments or acidification of a mildly alkaline cytoplasm by biochemical or genetic manipulation has been demonstrated to inhibit both endocytosis and secretion (Tartakoff, 1983a; Cosson et al., 1989; Mellman et al., 1986; Davoust et al., 1987; Cosson et al., 1989; van Deurs et al., 1989; Maxfield & Yamashiro, 1991; Hansen et al., 1993). These results provide the basis for the conclusion that the maintenance of pH gradients between acidic vesicular compartments and a mildly alkaline cytoplasm is an essential biochemical requirement for the correct functioning of the endocytotic and secretory machinery. Tumor cells have been shown to have an abnormally acidic cytoplasmic pH (Warburg, 1956; Simon & Schindler, 1994). Here we report that the intracellular vesicular compartments in tumor cells (MCF-7) derived from a human breast cancer fail to acidify. This failure results in a significant decrease in the pH gradient (0.9 pH unit) between the vesicular luminal compartments and the cytoplasm. These defects are correlated with a disruption in the organization and function of the trans-Golgi network (TGN) and the pericentriolar recycling compartment (PRC). In marked distinction, drug-resistant tumor cells (MCF-7adr) derived from the MCF-7 line that are resistant to the most widely employed chemotherapeutic drug, adriamycin, appear normal in both acidification and organization of the PRC and TGN. Treatment of drug-resistant MCF-7adr cells with nigericin and monensin, ionophores demonstrated to disrupt vesicular acidification (Tartakoff, 1983b), leads to a resensitization of these cells to adriamycin. Drug sensitivity is proposed to result from an acidification defect within vesicles of the recycling and secretory pathways. A functional consequence of this defect is the diminished capacity of cells to remove cytotoxic drugs from the cytoplasm by sequestration of protonated drugs within the vesicles, followed by drug secretion through the activity of the secretory and recycling pathways.

The effect of crown ethers, tetraalkylammonium salts, and polyoxyethylene amphiphiles on pirarubicin incorporation in K562 resistant cells

The basic distinguishing feature of all cells expressing functional P-glycoprotein-multidrug resistance (P-gp-MDR) is a decrease in steady-state accumulation drug levels as compared to drug-sensitive controls. In an attempt to identify mechanism(s) by which MDR can be circumvented, we examined the cellular accumulation, in resistant cells, of 4'-O-tetrahydropyranyl-doxorubicin (pirarubicin) alone and in conjunction with various molecules belonging to three different classes: the crown ethers, the tetraalkylammonium salts, and the polyoxethylene amphiphiles. The present study was performed using a spectrofluorometric method which enabled us to follow the uptake and release of fluorescent molecules by living cells while the cells were being incubated with the drug. Erythroleukemia K562 cell lines were used. Our data show that the compounds of these three completely different classes were able to increase the incorporation of pirarubicin provided they had a minimum degree of lipophilicity. Study of the growth inhibitory activity of these compounds revealed that cross-resistance to the tetraalkyl ammonium salt increased with the lipophilicity and was equal to 58 for tetraoctylammonium salt, the most lipophilic compound of this series. This demonstrates that neither the presence of a positive charge nor an aromatic moiety is required for MDR recognition.

The structure of P-glycoprotein and the secretion of lysosomal enzymes in multidrug-resistant cells

We have previously demonstrated that multidrug-resistant cells have a lower content of lysosomal enzymes, a consequence of an increased rate of secretion. The question was therefore to know whether an intact functional P-glycoprotein was necessary for expression of this property. Control NIH3T3 and mdr1-gene-transfected cells (pHaMDR1) were used together with 2 variants either lacking 23 amino acids at the carboxyl terminal (pHaMDRC 23) or in which 4 extra amino acids are inserted (pHaMDRBL2). Transfected and variant cells exhibited reduced uptake of [3H]-vinblastine and [3H]-daunomycin, a finding consistent with their drug resistance. By contrast, only pHaMDR1 cells had a reduced level of N-acetyl glucosaminidase that paralleled an increased rate of secretion of the same enzyme. The mutant cells secreted lysosomal enzyme at the same rate and had the same intracellular lysosomal enzyme content as NIH3T3 cells. Abnormal behavior of lysosomal enzymes in multidrug-resistant cells therefore seemed to require an intact P-glycoprotein molecule. Although sequestration in lysosomes and then secretion of drugs may possibly contribute to protection, it would not be an essential component of multidrug resistance.

Modulation of rhodamine 123 uptake by nigericin in sensitive and multidrug resistant leukemic cells

We have investigated the effect of the ionophore nigericin (NIG) in multidrug resistant (MDR) cells, using intracellular accumulation of the fluorescent dye rhodamine 123 (R123). NIG increased the accumulation of R123 in half of the murine MDR RFLC3 population but not in the human MDR CEM/VLB 100 cells. Co-treatment of RFLC3 with NIG plus verapamil showed additive effect on the accumulation of R123. The increase in R123 accumulation observed in RFLC3 was not the consequence of a direct effect of NIG on P-glycoprotein and was accompanied by a redistribution of the dye throughout the cell and a high cytotoxicity, which prevents the use of NIG as a resistance modulating agent.

Mobile ionophores are a novel class of P-glycoprotein inhibitors. The effects of ionophores on 4'-O-tetrahydropyranyl-adriamycin incorporation in K562 drug-resistant cells

The decrease of the intracellular concentration of drug in resistant cells compared to sensitive cells is, in most cases, correlated with the presence, in the membrane of resistant cells, of a 170-kDa P-glycoprotein responsible for an active efflux of the drug. In an attempt to identify mechanism(s) by which multidrug resistance can be circumvented, we have examined the cellular accumulation of 4'-O-tetrahydropyranyl-adriamycin, alone and in conjunction with various ionophores on the one hand and with cyclosporin A on the other hand. The present study was performed using a spectrofluorometric method with which it is possible to follow continuously the uptake and release of fluorescent molecules by living cells, as the incubation of the cells with the drug proceeds. Erythroleukemia K562 cell lines were used. Using experimental conditions in which these ionophores were unable to modify either the intracellular pH, or the transmembrane potential, or to induce an intracellular ATP depletion, we have shown that mobile ionophores as well as cyclosporin inhibit the P-glycoprotein-mediated efflux of 4'-O-tetrahydropyranyl-adriamycin in K562 resistant cells, whereas gramicidin, a channel-forming ionophore, does not. The concentration that must be used to inhibit 50% of the efflux was 0.7 microM for valinomycin, 0.4 microM for nonactin, 0.2 microM for nigericin, 1.1 microM for monensin, 0.4 microM for lasalocid, 1.2 microM for calcimycin and 0.4 microM for cyclosporin. Due to the high toxicity of the ionophores, the observation that they increased 4'-O-tetrahydropyranyl-adriamycin accumulation in the multidrug-resistant cells is not correlated with an effect of these compounds on drug resistance. However, the correlation exists in the case of cyclosporin. From our data showing that lipophilic neutral complexes, formed between carboxylic ionophores and metal ions, are both able to inhibit the P-glycoprotein-mediated efflux of anthracycline we can infer that the lipophilicity but not the cationic charge is an important physical property.

Reversal of doxorubicin resistance and catalytic neutralization of lysosomes by a lipophilic imidazole

A number of lipophilic nitrogenous bases, designed to act as membrane-active, catalytic proton transfer agents, were tested for their ability to neutralize the acidity of lysosomes, a model for other acidic intracellular vesicles involved in drug sorting. The most successful of these, an imidazole 1, caused a 1.7 unit rise in lysosomal pH of RAW cells at 100 microM, compared to a 0.2 and 1.4 unit rise for ammonium chloride at 100 microM and 10 mM, respectively. Compound 1 also exhibited potent reversal of doxorubicin (DOX) resistance in the HCT116-VM46 cell line by a factor of 14 over the sensitive strain, and superior to that of widely used verapamil (VRP) by a factor of 1.75 at 20 microM. It also has antiviral properties, and potential applications in other lysosome-related areas such as immunotoxin potentiation and the control of bacterial toxins, immune response, prion replication, malaria and intralysosomal microorganisms.

A comparison of rhodamine 123 accumulation and efflux in cells with P-glycoprotein-mediated and MRP-associated multidrug resistance phenotypes

Rhodamine 123 (Rh123) is a fluorescent dye which locates in the mitochondria of cells. It is a substrate for P-glycoprotein (Pgp) and can, therefore, be used as a molecular probe in studies of the multidrug resistance (MDR) phenotype. However, not all MDR cells overexpress Pgp. In some, the MDR phenotype is associated with expression of an alternative transporter molecule, the multidrug resistance-associated protein (MRP). We have studied the accumulation and efflux of Rh123 in MDR cells having both Pgp-mediated and MRP-associated phenotypes. In the mouse tumour parental cell line, EMT6/P, Rh123 accumulates rapidly to reach plateau levels by 90 min. Confocal microscopy confirms a localisation to the mitochondria. In the MDR subline, EMT6/AR1.0, which overexpresses Pgp and which is 10-fold resistant to Rh123 cytotoxicity, accumulation is dramatically reduced. Efflux of Rh123 from both resistant and parental lines is rapid but can be inhibited by reduced temperature or by the presence of cyclosporin A (5 micrograms/ml). Efflux from the parental line is probably due to the presence of very low, but detectable, levels of Pgp but the existence of other mechanisms cannot be ruled out. In contrast, the human lung cancer parental cell line COR-L23/P, and its MRP-associated (but Pgp-negative) MDR subline, COR-L23/R (which is 23-fold resistant to Rh123 cytotoxicity), accumulate Rh123 at similar rates for the first 30 min. The curves then diverge so that, at 180 min, the resistant cells contain only 70% of the Rh123 of parental cells. Confocal microscopy demonstrates a similar distribution of fluorescence in resistant and parental cells. Essentially no efflux of Rh123 occurs from parental cells, whereas 70% of the content is lost from resistant cells over a period of 150 min. Such efflux may again be inhibited by reduced temperature but cyclosporin A (5 micrograms/ml) has little effect. These observations should be borne in mind when interpreting Rh123 efflux data in terms of MDR mechanisms.

The intriguing link between modulation of both multidrug resistance and ligand-toxin conjugate cytotoxicity

Pharmacological agents which possess a chemosensitizing activity (i.e. the ability to modulate the multidrug resistance phenotype) can equally enhance ligand-toxin conjugate cytotoxicity. By confronting results obtained in both fields of research it appears that quite a number of agents, which are structurally unrelated, possess this bilateral effect. We have therefore attempted to provide a brief review of the literature and to discuss a hypothesis by which a common mechanism such as modifications in intracellular vesicle sorting and/or lipid metabolism may be implicated. We believe that these observations may provide clues for future research.

Non-P-glycoprotein multidrug resistance in cell lines which are defective in the cellular accumulation of drug

Non-Pgp mdr related to a defect in drug accumulation has now been documented in a number of different cell lines exposed to certain cytotoxic agents. In studies conducted thus far most isolates have been obtained after selection in either adriamycin or mitoxantrone. The work in this area is in its early stages and very little is known about the molecular events which contribute to this mode of drug resistance. At the present time no protein with drug binding properties comparable to Pgp has been identified in non-Pgp mdr isolates. Evidence based on the finding that all isolates do not respond in the same way to reversal agents such as verapamil suggests the possibility that more than one mechanism may exist for non-Pgp mdr. Future studies may thus reveal that cells contain a multiplicity of genes which upon transcriptional activation can function to alter drug transport processes and thus contribute to the development of mdr. Identifying and characterizing these genes will be important since they may function in transport systems of normal cells. The exact identify of proteins which contribute to non-Pgp mdr remains to be determined. One protein designated P190 has been found to be overexpressed in cell lines of human promyelocytic leukemia, lung and adenocarcinoma treated with adriamycin. The protein also is increased in some clinical samples from patients undergoing chemotherapy. P190 which has a minor sequence homology with Pgp can bind ATP and may thus contribute to the energy dependent drug efflux systems found in cells containing this protein. Transfection studies with a P190 cDNA should determine whether this protein actually contributes to drug resistance. Many other protein changes have been detected in non-Pgp mdr cells but the importance of these in resistance also remains to be determined. In some systems a particular protein change can be identified in multiple independent isolates suggesting a correlation between the development of resistance and the presence of this cellular alteration. Experiments conducted thus far on the mechanism of non-Pgp mdr are intriguing. Studies utilizing fluorescence microscopy to follow the fate of daunomycin suggests that the drug passes to the interior of the cell and eventually localizes in the Golgi apparatus. Drug located at this site may move directly into an efflux pathway for rapid extrusion from the cell. Evidence also indicates that as drug leaves the Golgi some may be sequestered into other organelles such as lysosomes or mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of latent recombinant TGF-beta 2 produced by Chinese hamster ovary cells

Latent recombinant transforming growth factor-beta 2 (LrTGF-beta 2) complex has been purified from serum-free media conditioned by Chinese hamster ovary cells transfected with a plasmid encoding the TGF-beta 2 (414) precursor. Under neutral conditions, LrTGF-beta 2 had an apparent molecular weight of 130 kDa. The complex contained both mature and pro-region sequences. Acidification of LrTGF-beta 2 resulted in the release of mature 24 kDa TGF-beta 2 from the high molecular weight pro-region-containing complex, suggesting that TGF-beta 2 was non-covalently associated with this complex. These results were confirmed by crosslinking experiments performed on partially purified LrTGF-beta 2. Protein sequence analysis of the purified TGF-beta 2 pro-region indicated that signal peptide cleavage occurred between ser(20) and leu(21). The pro-region, which previously was found to contain mannose-6-phosphate, bound to the mannose-6-phosphate receptor. Proteolytic cleavage of mature TGF-beta 2 from pro-TGF-beta 2 was inhibited by monensin and chloroquine suggesting that binding to this receptor and subsequent transport to acidic vesicles may be involved in the processing of rTGF-beta 2 precursor.

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.

Investigation of the relationship between altered intracellular pH and multidrug resistance in mammalian cells

The intracellular pH of a number of multidrug resistant cell lines was compared with that of their parental lines using the fluorescent probe bis-carboxyethylcarboxyfluorescein. In four different cases, cells having 5-fold resistance or more exhibited an intracellular pH which was 0.10-0.17 units higher than that of the parental cell line. A CHO cell line, AB1, and its 180-fold resistant counterpart, CHRC5, were further investigated with regard to the role of Na+/H+ antiport. The Na+/H+ antiport activity was greater at any intracellular pH for the CHRC5 cells than the AB1 cells. To investigate the possible role of higher intracellular pH in multidrug resistance, the effect of several agents which are either known to reverse multidrug resistance or inhibit Na+/H+ antiport activity were examined. Verapamil was found to reverse multidrug resistance but had no effect on intracellular pH while amiloride, which acidifies the cytoxol by blocking Na+/H+ antiport activity, did not cause reversal of drug resistance. In contrast to verapamil, treatment of CHRC5 cells with cyclosporin A had a parallel effect on reversal of their drug resistant phenotype and a lowering of their intracellular pH to that of the sensitive cell level. However, cyclosporin was ineffective in either lowering the intracellular pH or reversing drug resistance in DC3F/ADX cells. Therefore, except for the effect of cyclosporin A on the CHRC5 line, the effects of other agents on reversal of multidrug resistance and intracellular pH did not correlate with each other.