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[Cancer Research 61, 172-177, January 1, 2001]
© 2001 American Association for Cancer Research

Experimental Therapeutics

Simultaneous Treatment with 1-ß-D-Arabinofuranosylcytosine and Daunorubicin Induces Cross-Resistance to Both Drugs due to a Combination-specific Mechanism in HL60 Cells1

Haruyuki Takemura, Yoshimasa Urasaki, Akira Yoshida, Toshihiro Fukushima and Takanori Ueda2

First Department of Internal Medicine, Fukui Medical University, Fukui 910-1193, Japan


   ABSTRACT
Top
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
We have established a human myelogenous leukemia cell line (HL60/AD) that is 10-fold cross-resistant to both 1-ß-D-arabinofuranosylcytosine (ara-C) and daunorubicin; the cell line was isolated from HL60 by simultaneous treatment with these two agents at low drug concentrations attainable in clinical trials. HL60/AD was found to have multiple resistance mechanisms. With regard to ara-C, HL60/AD cells showed decreased deoxycytidine kinase activity but did not show elevation of cytidine deaminase activity or a decrease in ara-C influx. With regard to daunorubicin, a decrease in topoisomerase II activity was found. A decrease in intracellular accumulation of daunorubicin was also found. P-glycoprotein was not detected, but the multidrug resistance-associated protein was expressed. Furthermore, an increase of total cellular glutathione (GSH) content was found. Interestingly, the resistance of HL60/AD cells not only to daunorubicin but also to ara-C was markedly reversed by treatment with L-buthionine-(S,R)-sulfoximine (BSO), a potent inhibitor of GSH synthesis. After exposure of HL60/AD to ara-C, mitochondrial membrane potential and reactive oxygen intermediates showed no significant change, but a considerable loss of mitochondrial membrane potential and an increase in reactive oxygen intermediate generation were caused by preincubation with BSO. Neither elevation of GSH nor reversal of resistance by BSO was found in ara-C-resistant HL60 cells that were selected only with ara-C. These findings suggest that in addition to the summation of the mechanisms of resistance to each agent reported previously, an increased level of GSH plays an important role in the cross-resistance induced in HL60/AD cells by simultaneous exposure to both drugs.


   INTRODUCTION
Top
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Recently, remission induction rates of AML3 have gradually increased, but curative treatment of AML is still a serious problem. One of the reasons for this is the development of multidrug resistance, which is a major obstacle to successful treatment. Ara-C and daunorubicin are the most commonly used drugs for treatment of AML (1 , 2) . These two agents are commonly applied in combination to treat AML patients in clinical trials and have proved effective. However, more than half of the patients treated with both drugs are primary refractory or suffer relapse, suggesting that the leukemic cells of such patients acquire resistance to both drugs. Furthermore, the exact mechanism by which leukemic cells develop cross-resistance to these agents in vivo has not been clearly elucidated.

To date, investigators have reported on the mechanisms of action of ara-C (3) and daunorubicin (4) and elucidated the mechanisms of resistance to each agent, but cross-resistant cells induced by simultaneous treatment with ara-C and daunorubicin have not been studied until now. Furthermore, in many reports, the mechanisms of resistance were studied mainly in cell lines resistant to high drug levels, which do not reflect the conditions in most clinical trials (5) . To elucidate the mechanisms of drug resistance under conditions reflecting those in clinical trials, it would be more relevant to establish cell lines resistant to low drug levels selected with drug concentrations attainable in vivo and study the mechanism of the resistance.

In the present study, we established such a human myelogenous leukemia cell line (HL60/AD) resistant to both ara-C and daunorubicin, which was selected by simultaneous treatment with these drugs at low concentrations, and we examined the mechanisms of cross-resistance to these drugs. In particular, we focused on whether HL60/AD cells would acquire a new mechanism of resistance that differed from the mechanisms seen in HL60 cells selected with each single agent.


   MATERIALS AND METHODS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Drugs and Chemicals.
Daunorubicin was kindly supplied by Meiji Seika Co. (Tokyo, Japan). Ara-C was obtained from Nippon Shinyaku Co. (Kyoto, Japan). [3 H]Ara-C and NCS were purchased from Amersham Japan (Tokyo, Japan). [3 H]Daunorubicin was obtained from Daiichi Pure Chemicals (Tokyo, Japan). 1-Chloro-2,4-dinitrobenzene, 5,5'-dithiobis-(2-nitrobenzoic acid), and BSO were obtained from Nacalai Tesque (Kyoto, Japan). GSH reductase from yeast, and DiOC6(3) were obtained from Sigma Chemical Co. (St. Louis, MO). MRPm6 was obtained from Kamiya Biomedical Co. (Seattle, WA). UIC2 was obtained from Immunotec (Marseille, France). NADPH and GSH (reduced form) were obtained from Kohjin Co. (Tokyo, Japan). kDNA was obtained from Wako Co. (Osaka, Japan). All other chemicals were usually obtained from commercial sources.

Cells and Drug Sensitivity.
HL60/AD cells were isolated from a human myelogenous leukemia cell line (HL60) by a series of stepwise selections with simultaneous treatment of ara-C and daunorubicin. Resistant cells were cloned by the limiting dilution method. Drug sensitivity was tested by the trypan blue dye exclusion method. The IC50 is defined as the drug concentration that resulted in a 50% reduction in cell number at 72 h relative to untreated control. The degree of resistance was calculated by dividing the IC50 value of the resistant cells by that of the parental cells. In some experiments, a colony-forming assay was performed concurrently to compare survival with growth inhibition. All studies were carried out with exponentially growing cells.

Measurement of Ara-C Influx.
Ara-C influx was determined by the method of Wiley et al. (6) . An aliquot of 5 x 106 cells was suspended in medium and preincubated at 20°C for 5 min. The cells were incubated in the presence of 0.2 µM [3 H]ara-C for 0–60 s, after which they were separated by centrifugation at 12,000 x g for 6 s. A 2-ml volume of NCS tissue solubilizer was added to the pellet, and then the mixture was kept at room temperature overnight to allow the cells to lyse. The amount of radioactivity in the mixture was determined in scintillation fluid consisting of toluene and Triton X-100 (2:1, v:v), PPO (4.0 grams/liter), and 2,2'-p-phenylene bis(5-phenyl oxazole) (0.1 gram/liter).

CDD Assay.
The CDD assay was performed by the method of Steuart and Burke (7) , with a slight modification. Crude enzyme was obtained by sonication of cells suspended in 200 µM [3 H]ara-C (specific activity, 25 µCi/µmol) and 50 mM Tris-HCl (pH 8.0) containing 1 mM EDTA and 10 mM 2-mercaptoethanol. The enzyme was incubated in a reaction buffer containing 50 mM Tris-HCl (pH 8.0), 2 mM 2-mercaptoethanol, and 1 mM EDTA. The acid-soluble fraction was subjected to TLC.

DCK Assay.
DCK was assayed by the method of Ives and Durhum (8) . Briefly, crude enzyme was obtained by sonication of cells suspended in 50 mM Tris-HCl (pH 8.0) containing 2 mM DTT and then clarified by centrifugation at 100,000 x g for 60 min at 4°C. The enzyme was incubated in a reaction buffer containing 20 µM [3 H]ara-C (specific activity, 500 µCi/µmol), 40 mM Tris-HCl (pH 8.0), 10 mM MgCl2, 10 mM ATP, 10 mM 2-mercaptoethanol, and 1 mM tetrahydrouridine.

topo II Assay.
The nuclear extract was prepared by the method of Kunkel et al. (9) . topo II activity was assayed by decatenation of kDNA into minicircles. Briefly, an aliquot of 1 x 108 cells was prepared and incubated in TKM buffer containing 0.25 M sucrose and 0.25% Triton X-100 at 4°C for 5 min. Nuclei were pelleted by centrifugation at 1,600 x g for 5 min and washed with TKM buffer containing 0.25 M sucrose. Nuclear protein was extracted at 4°C for 60 min in TKE buffer containing 0.5 M NaCl, 1 mM DTT, and 1 mM phenylmethylsulfonyl fluoride. Nuclear extracts were obtained by centrifugation (100,000 x g, 60 min, 4°C). They were incubated for 30 min at 30°C with kDNA in reaction buffer containing 50 mM Tris-HCl (pH 7.5), 85 mM KCl,10 mM MgCl2, 5 mM DTT, 0.5 mM EDTA, 1 mM ATP, and 0.01% BSA.

Measurement of Intracellular Daunorubicin Accumulation.
Intracellular daunorubicin accumulation was determined by the method of Fukushima et al. (10) . An aliquot of 2.0 x 106 cells was suspended and incubated in the presence of 0.1 mM [3 H]daunorubicin for a designated time at 37°C, and then cells were separated from the medium as described above.

Detection of P-glycoprotein and MRP.
P-glycoprotein was analyzed by the method of Chaudhary et al. (11) . An aliquot of 1 x 106 cells was incubated with the P-glycoprotein Moab UIC2 for 30 min on ice and then washed twice. MRP was analyzed by the method of Flens et al. (12) . Cells (1 x 106) were incubated with MRP Moab m6 for 1 h on ice, washed twice, and analyzed using a FACScan (Becton Dickinson).

Total Cellular GSH Content.
Total cellular GSH content was measured by the method of Tietze (13) , with a minor modification. Briefly, 5 x 106 cells were prepared and washed twice with cold PBS and suspended in 125 mM sodium phosphate buffer containing 6.3 mM EDTA (pH 7.5; sodium phosphate buffer). After sonication, 12% 5-sulfsulicylic acid was added to the lysates, and the mixture was allowed to precipitate for 2 h at 4°C. After centrifugation at 10,000 x g for 15 min, protein-free lysates were obtained. The reaction mixture for determination of GSH content consisted of lysates, 0.3 mM NADPH, 6 mM 5,5'-dithiobis-(2-nitrobenzoic acid), and 0.5 unit of GSH reductase. The absorbance at 412 nm was monitored for 6 min using a plate reader (SPECTRA Max250; Molecular Devices). The content of GSH was calculated from the change in the rate of absorbance on the basis of a standard curve.

GST Activity.
GST activity was measured as described by Habig et al. (14) , with a slight modification. An aliquot of 1 x 107 cells was harvested, washed twice with cold PBS, and resuspended in 10 mM Tris-HCl (pH 7.8) containing 0.2 M NaCl. After sonication, the lysates were centrifuged at 10,000 x g for 30 min at 4°C. The supernatant was assayed for GST activity using 1-chloro-2,4-dinitrobenzene in a spectrophotometric assay. The change of absorbance at 340 nm was monitored for 5 min. The enzymatic activity was expressed in nmol/min/mg protein.

Calcein Efflux Assay.
Calcein efflux assay was performed by using the method of Feller et al. (15) , with a slight modification.

Briefly, 1 x 106 cells were incubated with 0.1 µM calcein-AM with or without 2 mM probenecid for 15 min at 37°C. After centrifugation, cells were resuspended in fresh medium, and the efflux of calcein was allowed for 90 min at 37°C. Intracellular calcein accumulation was measured by collecting the fluorescent calcein through a 530 nm bandpass filter with excitation at 488 nm.

Detection of Mit-MP.
Cells were incubated with various concentrations of ara-C for 6 h. They were resuspended in PBS and immediately incubated with 40 nM DiOC6(3) for 15 min at 37°C in a dark room. Cells were washed twice with PBS and then analyzed by flow cytometry as described above.

ROI Generation.
ROI generation was measured by using a fluorescent probe, DCFH-DA. Cells were exposed to1 µM ara-C for 24 h and then incubated with 20 µM DCFH-DA for 12 min at 37°C. The cells were then washed twice with PBS and analyzed by flow cytometry as described above.

Expression of bcl-2 and bcl-XL.
Cell lysates were separated by 12.5% SDS-PAGE. Gels were transferred to nitrocellulose, blocked with 5% blocking agent, and incubated with mouse anti-bcl-2 or anti-bcl-xL antiserum. Blots were developed with a chemiluminescent substrate.

Analysis of Data.
To ascertain the significance of the differences observed between HL60 and HL60/AD cells subjected to various experimental conditions, statistical analysis of the data was performed using Student’s t test.


   RESULTS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Isolation of the Resistant Cell Line HL60/AD.
After simultaneous stepwise increases of the ara-C and daunorubicin concentrations in the culture medium up to 0.4 µM ara-C and 0.1 µM daunorubicin, respectively, we isolated a resistant cell line by the limiting dilution method and named it HL60/AD. It showed about 10-fold resistance to both ara-C (IC50 = 0.40 ± 0.06 µM in HL60/AD and 0.04 ± 0.007 µM in HL60) and daunorubicin (IC50 = 0.1 ± 0.02 µM in HL60/AD and 0.01 ± 0.002 µM in HL60), as compared with the parental cells in the cell growth inhibition study, respectively. A colony-forming assay also revealed about 8.5-fold resistance to ara-C and 10-fold resistance to daunorubicin.

Ara-C Influx.
Ara-C influx was linear with incubation time up to 60 s and reached a steady state. No significant differences were found between HL60/AD and parental cells in uptake (1.17 and 1.16 pmol/5 x 106 cells/min, respectively) or in the steady-state concentration (24.4 and 23.1 pmol/h, respectively).

DCK and CDD Activities.
DCK and CDD enzymatic activities were determined using ara-C as substrate. The DCK activity was 530.4 ± 52.7 pmol/mg/h in HL60/AD cells and 944.1 ± 76.4 pmol/mg/h in the parental cells (P < 0.01). The DCK activity was about 50% lower in HL60/AD cells than in HL60 cells.

In contrast, there was no significant difference in CDD activities (218.4 ± 36.0 pmol/mg protein/h in HL60/AD and 217.3 ± 22.5 pmol/mg protein/h in HL60).

topo II Activity.
Daunorubicin is known to be a topo II inhibitor (16) . Therefore, we examined the activity of topo II in the cells by means of a kDNA decatenation assay. As shown in Fig. 1Citation , the topo II activity of HL60/AD cells was substantially decreased as compared with that of the parental cells. Furthermore, the expression of topo II{alpha} was decreased as compared with that in the parental cells (data not shown).



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  		Fig. 1. DNA topo II activity from HL60 and HL60/AD cells. Lane N, negative control; Lanes 1 and 2, HL60; Lanes 3 and 4, HL60/AD; 0.30 µg of protein, Lanes 1 and 3; 0.15 µg of protein, Lanes 2 and 4.

 
Daunorubicin Accumulation.
Fig. 2Citation shows that the intracellular accumulation of daunorubicin in HL60/AD cells was substantially slower and lower than that in HL60 cells.



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  		Fig. 2. Intracellular daunorubicin accumulation. HL60 and HL60/AD cells were incubated with 1.0 µM [3H]daunorubicin at 37°C for the designated times. Drug accumulation is expressed as the amount of drug in 2 x 106 cells. Points are the means of duplicate determinations. {circ}, HL60; •, HL60/AD.

 
Expression of P-glycoprotein and MRP.
To determine the cause of the decrease in intracellular daunorubicin accumulation, the expression of P-glycoprotein was examined, but it was not detected in either HL60/AD cells or the parental cells (data not shown). Furthermore, we performed a rhodamine 123 efflux assay, which is a functional assay of P-glycoprotein (17) , but we found no difference between the two cell lines (data not shown). Recently, a second energy-dependent efflux pump, MRP, has been identified in mammalian cells (18) . MRP can confer resistance to a wide spectrum of natural product drugs (19) . As shown in Fig. 3Citation , expression of MRP was found in HL60/AD cells, but not in the parental cells.



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  		Fig. 3. Analysis of MRP phenotype. MRPm6 is an anti-MRP Moab. MS-IgG1 (mouse polyclonal antibody) was used as the negative control. HL60, A; HL60/AD, B. Thin solid line, MS-IgG1; thick solid line, MRPm6.

 
MRP Function in HL60/AD Cells.
To examine the function of MRP, we performed a calcein efflux assay. As shown in Fig. 4ACitation , intracellular calcein accumulation was significantly lower in HL60/AD cells than in HL60 cells. However, this reduction was partially reversed in the presence of probenecid, which is known to be an inhibitor of MRP synthesis, in HL60/AD cells. On the other hand, it has been reported that the function of MRP was associated with GSH and GST (20 , 21) . The GSH content was therefore measured in the HL60/AD cells and parental cells. The level of GSH in the cells was normalized to the amount of total cellular protein. As shown in Table 1Citation , the cellular GSH content was about 1.5-fold higher in HL60/AD cells than in HL60 cells (P < 0.001). In addition, we examined GST activity because GST is the enzyme family catalyzing conjugation with GSH. As for GST activity, HL60/AD cells contained 101.3 ± 2.9 nmol/min/mg protein, as compared with 80.6 ± 3.3 nmol/min/mg protein in the parental cells (P < 0.01). To examine the role of GSH, cells were preincubated with and without a sublethal concentration of BSO (50 µM), a potent inhibitor of GSH synthesis, for 24 h. Then they were incubated with and without ara-C for 72 h at 37°C and subjected to the calcein efflux assay. The accumulation of calcein in HL60/AD cells was substantially increased by pretreatment with BSO (Fig. 4B)Citation .



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  		Fig. 4. Flow cytometric histograms showing functional incorporation of calcein-AM. A, HL60 and HL60/AD with or without probenecid. Black-shaded histogram, control (without calcein-AM); thick black line, HL60 without probenecid; thin black line, HL60/AD without probenecid; dotted gray line, HL60/AD with probenecid. B, HL60/AD treated or not treated with BSO. Thin black line, HL60/AD not treated with BSO; thick black line, HL60/AD treated with BSO.

 

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  		Table 1 Comparison of activity of DCK and CDD and GSH content in HL60, HL60/AD, and HL60/ara-C

 
Reversal of Daunorubicin Resistance by BSO.
In an attempt to clarify whether GSH was actually correlated with the mechanism of daunorubicin resistance, cells were preincubated with or without a sublethal concentration of BSO for 24 h. They were then incubated with or without daunorubicin for 72 h at 37°C. The GSH level was reduced by less than 5% after 16 h of incubation in the presence of BSO. BSO alone did not affect the survival in any cell line. After pretreatment with BSO, there was a 3.3-fold increase in drug sensitivity as compared with the untreated cells. In contrast, the sensitivity to HL60 was not changed (Fig. 5A)Citation . These results in HL60/AD cells were confirmed by a colony-forming assay. The IC50 showed a 3.5-fold increase as compared with the untreated cells.



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  		Fig. 5. Sensitization with BSO in both cell types. After treatment with 50 µM BSO for 24 h, cells were incubated with ara-C (A) or daunorubicin (B) at various concentrations for 72 h. {circ}, HL60; •, HL60 + BSO; {square}, HL60/AD; {blacksquare}, HL60/AD + BSO.

 
MRP-GSH System in Ara-C Resistance Mechanism.
The same experiments shown in Fig. 5ACitation were performed using ara-C instead of daunorubicin. Interestingly, the sensitivity to ara-C was also remarkably recovered (Fig. 5B)Citation . This indicates that inhibition of GSH synthesis with BSO modifies not only daunorubicin cytotoxicity but also ara-C cytotoxicity in HL60/AD cells.

Comparison of HL60/AD Cells with Cells Selected by a Single Agent.
To confirm the possibility that simultaneous treatment with ara-C and daunorubicin induced an elevation of GSH, we established an ara-C-resistant HL60 cell line (10-fold resistance) selected with ara-C alone (HL60/ara-C). HL60/ara-C showed low DCK activity and almost the same CDD activity and ara-C influx as seen in HL60 cells. The activity of topo II was not decreased in HL60/ara-C. GSH did not increase compared with that in the parental cells (Table 1)Citation . In addition, the IC50 was 0.4 µM with BSO and 0.5 µM without BSO, respectively. Thus, reversal of ara-C resistance with BSO was not found in this cell line. As a model for the singly selected daunorubicin-resistant cells, we already established and characterized K562/D1-9, which is 28.0-fold more resistant to daunorubicin than the parental cells (22) . Briefly, compared with the wild type, K562/D1-9 showed decreased intracellular daunorubicin retention, decreased topo II activity, and positive expression of P-glycoprotein but did not express MRP or have an increased level of GSH. Compared with the wild type, the activities of DCK were not decreased (423.4 versus 447.4 pmol/mg/h), and CDD activities were not increased (443.8 versus 456.1 pmol/mg/h, respectively), In addition, the ara-C influx in K562/D1-9 was similar to that in parental cells. The IC50 with and without BSO was 5.6 and 5.2 µM, respectively, showing no reversal effect of BSO.

Mit-MP and ROI Generation.
Using DiOC6(3) as a fluorescent probe to assess Mit-MP, two distinct subpopulations appeared after exposure to ara-C. Cells with low fluorescence intensity have a loss of Mit-MP (23 , 24) . As shown in Fig. 6Citation , the percentage of cells with high Mit-MP decreased after treatment with ara-C in the parental cells but did not obviously decrease in HL60/AD cells. ROI generation in both cell types is shown in Fig. 7Citation . HL60 cells showed an increase in ROI generation after ara-C exposure. In contrast, in HL60/AD cells, ROI generation did not increase after ara-C exposure but increased significantly after preincubation with BSO. There were no substantial changes of Mit-MP and ROI after exposure to ara-C + daunorubicin and daunorubicin alone with BSO pretreatment in HL60/AD cells (data not shown).



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  		Fig. 6. Comparison of the changes of Mit-MP after ara-C exposure in parental and HL60/AD cells with or without BSO. Cells were incubated with various concentrations of ara-C for 6 h. Mit-MP was determined by flow cytometry using the fluorescent probe DiOC6(3). Cells with high Mit-MP constitute a subpopulation whose Mit-MP has not been lost. The loss of Mit-MP is shown as a decrease in the percentage of cells with high Mit-MP. {circ}, HL60; •, HL60 with BSO; {square}, HL60/AD; {blacksquare}, HL60/AD with BSO.

 


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  		Fig. 7. ROI generation after ara-C exposure in parental and HL60/AD cells. One µM ara-C was added to HL60 (A) and HL60/AD cells (B) for 24 h with or without BSO. ROI was determined using 20 µM DCFH-DA. Gray dotted line, control (without ara-C); thin black line, 10 µM ara-C without BSO; thick black line, 10 µM ara-C with BSO.

 
DNA Fragmentation and Expression of bcl-2 and bcl-XL.
After preincubation with BSO, HL60/AD cells were exposed to 10 µM ara-C for 4 h. Ara-C-induced internucleosomal DNA fragmentation was not inhibited (Fig. 8A)Citation . HL60 is known to express bcl-2 protein, and no difference in bcl-2 expression was found between the two cell lines (Fig. 8B)Citation . In addition, bcl-XL, which is reported to inhibit drug-induced apoptosis, could not be detected in either cell line (Fig. 8C)Citation .



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  		Fig. 8. A, DNA fragmentation. Cells were exposed to 10 µM ara-C for 4 h after preincubation with or without BSO. Lane 1, HL60 without BSO; Lane 2, HL60/AD without BSO; Lane 3, HL60/AD with BSO. B, expression of bcl-2. C, detection of bcl-XL. Lane C, positive control (bcl-XL protein); Lane 1, HL60; Lane 2, HL60/AD.

 

   DISCUSSION
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Resistance to ara-C is generally considered to arise from reduced influx of ara-C across cell membranes (6) , decrement of 1-ß-D-arabinofuranosylcytosine 5'-triphosphate, which is an active metabolite of ara-C, caused by decreased activities in DCK (4) , and increased conversion to1-ß-D-arabinofuranosyluracil, which is an inactive metabolite of ara-C, by high level of CDD (7) and so on. On the other hand, the resistance to anthracycline, including daunorubicin and doxorubicin, is known to be correlated with reduced accumulation of drugs by an overexpression of P-glycoprotein (22) , MRP (25) , and other ATP-binding cassette transporter and quantitative or qualitative alterations of topo II, and so forth. It is also well known that resistance to antimetabolites, including ara-C, usually does not cross to resistance to anthracyclines. In the present study, HL60/AD cells selected with simultaneous treatment with both ara-C and daunorubicin presented multiple mechanisms of resistance to these agents. Because they were selected with drug concentrations attainable in vivo, they showed low level resistance as compared with the parental cells.

As for resistance to ara-C, DCK activities were decreased in HL60/AD cell line, but differences of CDD activities were not found between the cell lines. Furthermore, there were no differences of ara-C incorporation into cells between the cell lines. Therefore, the change of DCK activitiy is one of the mechanisms of resistance to ara-C.

With regard to daunorubicin, HL60/AD cells showed a decreased activity of DNA topo II (Ref. 26 ; Fig. 1Citation ). topo II inhibitors, such as anthracyclines, etoposide, or amsacrine, stabilize a cleavable complex between DNA and topo II. This stabilized cleavable complex seems to be the trigger for cell death (27) . Previous studies have reported that cells resistant to these agents displayed quantitative or qualitative alterations of topo II (28 , 29) , resulting in a decrease of activity. Decreased intracellular accumulation was also found in HL60/AD cells. This result suggests the existence of a drug efflux system in the cells.

P-glycoprotein is an energy-dependent efflux pump and related to drug resistance by decreasing intracellular drug accumulation (30) . However, HL60/AD cells showed no expression of P-glycoprotein, but they showed the expression of MRP (Fig. 3)Citation . Recently, it has been suggested that MRP, a member of ATP-binding cassette transporter superfamily, can confer resistance to a broad range of natural product drugs by extruding the drugs from the cells (31 , 32) . Furthermore, it is known that resistant cells overexpressing MRP often show alterations in topo II. Some studies reported that MRP expression had an impact on clinical outcome in AML (33) , and some did not (34) . In this regard, a correlation between MRP and GSH has been reported recently (20 , 21) . Versantvoort et al. (31) indicated that transport of drugs in MRP-overexpressing but not in P-glycoprotein-overexpressing multidrug resistance cells can be regulated by cellular GSH levels. Therefore, MRP function may be dependent on intracellular GSH level. To elucidate the correlation between MRP and GSH, we studied whether the function of MRP was affected by the level of GSH, using a fluorescent probe, calcein-AM. It was reported that the calcein efflux assay could explore MRP activity (35) . In brief, cells exposed to calcein-AM become fluorescent after the cleavage of calcein-AM by cellular esterase that produces a fluorescent derivative calcein. A fluorescent calcein can be actively extruded by MRP. On the other hand, probenecid, which is a specific and effective inhibitor of MRP function, can modulate the efflux of calcein (36) . As shown in Fig. 5Citation , calcein efflux was prevented in the presence of probenecid. Furthermore, it was prevented by preincubation with BSO, which is a potent inhibitor of GSH synthesis. If MRP plays a role in transporting daunorubicin together with GSH, daunorubicin efflux would also be inhibited by lowering GSH levels with BSO. As shown in Fig. 4ACitation , the resistance of HL60/AD cells to daunorubicin was remarkably reversed by BSO pretreatment. Accordingly, these results suggest that MRP in HL60/AD cells may function so as to decrease the intracellular accumulation of drugs, depending on the level of GSH. In addition, we assayed total GST catalytic activity in each cell line. The GSTs constitute an enzyme family catalyzing with GSH, and four classes of GSTs are known. GST{pi} is the form most commonly overexpressed in many organs and is known to be associated with drug resistance (37) . Its mechanism of action remains obscure, but GSH and GSTs are closely related to each other. Although it is still unclear whether daunorubicin itself is transported by MRP (38) , our results suggest that the expression of MRP is correlated with an elevated level of GSH, probably in combination with increased activity of GST.

Interestingly, the resistance to ara-C was also remarkably reversed by BSO pretreatment in HL60/AD cells. This evidence suggests that the elevation in cellular GSH content takes part in the resistance to ara-C in HL60/AD cells. However, GSH is not usually considered to be associated with resistance to antimetabolite drugs, such as ara-C and methotrexate. In this respect, it has been reported that AML blast stem cells can be protected against ara-C lethality by N-acetyl cysteine (39) . N-Acetyl cysteine is a free radical scavenger and is known to protect cells against damage by free radicals. Fig. 6Citation shows the changes of Mit-MP after 6 h of treatment with ara-C. The decrease in Mit-MP observed in parental cells after treatment with ara-C was not remarkable in HL60/AD cells. The generation of ROI was increased after 24 h of treatment with ara-C in parental cells, but not in HL60/AD cells (Fig. 7)Citation . However, HL60/AD cells showed a significant loss of Mit-MP and a high level of ROI generation when they were coincubated with BSO before being exposed to ara-C. These observations suggest that GSH can protect cells against damage by oxidative stress. Whereas the source of ROI is uncertain, the generation of ROI is known to occur after ara-C incorporation into DNA (29) . As a hypothesis, it might be possible that the inner mitochondrial membrane changes and the mitochondrial permeability transition can occur due to changes in the pore of the structure (23) . The permeability transition produces the loss of Mit-MP due to oxidative stress and hence the uncoupling of oxidative phosphorylation. Finally, the loss of a feedback inhibition of respiratory chain activity occurs due to the depletion of cellular ATP, thereby causing a further increase of ROI. Our results suggested that oxidative stress was a critical part of the distal events in AML blasts treated with ara-C (29) and was, by itself, injurious to cells. GSH was thought to act as an antioxidant defense mechanism against oxidative stress (40) . Although this mechanism does not work in typical ara-C-resistant cells selected by exposure to ara-C, it could be induced in particular situations, such as exposure to a combination of other drugs.

However, it is not clear whether GSH plays an independent role in antioxidant defense. In this respect, bcl-2 is considered to have antioxidant activity (41) , and it is reported that bcl-2-expressing cells lose the suppression of apoptosis by depleting cellular thiols (42) . As shown in Fig. 8Citation , DNA fragmentation was not inhibited in HL60/AD cells when they were exposed to ara-C after preincubation with BSO. These pieces of evidence suggested that resistance to ara-C was not caused by the prevention of cell death by a downstream apoptotic pathway. Considering these observations, the role of GSH may be the control of the redox state in HL60/AD cells.

In conclusion, we established HL60/AD cell line resistant to both ara-C and daunorubicin, which was selected with simultaneous exposure to these two agents. The characteristics of HL60/AD cells were as follows: (a) resistance to ara-C arising from decreased DCK activity; and (b) resistance to daunorubicin related to alteration of topo II activity and the expression of MRP.

Interestingly, the elevation of GSH was shown as a common resistant mechanism to both drugs. These findings suggest that the mechanism of cross-resistance to ara-C and anthracycline may not be the summation of each drug but that a new mechanism could be induced in such a case. Because it is unclear whether the MRP-GSH mechanism would be induced in other dual-selected cell lines, it would be necessary to establish and extensively characterize such resistant cell lines including a cell line transfected with MRP. In conclusion, we suggest that a new modality of circumvention of resistance should be established in patients with refractory or relapsed leukemia after combination chemotherapy.


   ACKNOWLEDGMENTS
 
We thank R. Nishi and A. Murai for technical and secretarial assistance. We also thank Meiji Seika Co. for the generous gift of daunorubicin.


   FOOTNOTES
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 Supported in part by a Grant in Aid from the Ministry of Education, Japan. Back

2 To whom requests for reprints should be addressed, at First Department of Internal Medicine, Fukui Medical University, 23-3, Shimoaizuki, Matsuoka, Fukui 910-1193, Japan. Phone: 81-776-61-8343; Fax: 81-776-61-8109. Back

3 The abbreviations used are: AML, acute myelogenous leukemia; ara-C, 1-ß-D-arabinofuranosylcytosine; MRP, multidrug resistance-associated protein; DCK, deoxycytidine kinase; CDD, cytidine deaminase; topo II, topoisomerase II; TKM, 50 mM Tris HCl, 25 mM KCl, 5 mM MgCl2; TKE, 50 mM Tris HCl, 25 mM KCl, 1 mM EDTA; kDNA, kinetoplast DNA; calcein-AM, calcein acetoxymethyl ester; GSH, glutathione; GST, glutathione S-transferase; BSO, L-buthionine-(S,R)-sulfoximine; Moab, monoclonal antibody; Mit-MP, mitochondrial membrane potential; ROI, reactive oxygen intermediate; DiOC6(3), 3,3'-dihexyloxacarbocyanine iodide; DCFH-DA, 6-carboboxy-2',7'-dichlorodihydrofluorescein diacetate di(acetoxymethyl ester). Back

Received 11/15/99. Accepted 11/ 1/00.


   REFERENCES
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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