Phenotypic drug resistance in mammalian cells in vitro

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<ul><li><p>Somatic Cell Genetics, Vol. 8, No. 3, 1982, pp. 307-317 </p><p>Phenotypic Drug Resistance in Mammalian Cells in Vitro </p><p>P.C.E.M. Verschure and J.W.I.M. Simons </p><p>Department of Radiation Genetics and Chemical Mutagenesis, State University of Leiden, Sylvius Laboratories, Wassenaarseweg 72, 2353 AL Leiden, The Netherlands </p><p>Received 22 September 1981--Final 23 December 1981 </p><p>Abstract--When mammalian cells are cultured at low concentrations of toxic drugs, they often become phenotypically resistant. We studied whether this phenotypic resistance is due to selection of preexisting variants. The drugs 8-azaguaine (AG) and 6-thioguanine (TG) were used and, as a parameter for resistance, the incorporation of hypoxanthine was determined. Preexisting variation among clones in the uptake of hypoxanthine was found, and this variation has a hereditary component. This transmission of aberrant incorporation of hypoxanthine does not appear a stable trait, and the aberrant cell lines returned gradually to the original steady state. There are indications that within a cell population cells with altered levels of incorpo- ration of hypoxanthine arise continuously and at a high frequency. Treat- ment with marginally toxic concentrations of AG or TG indicates that, at least for AG, survival is not related to the preexisting variation in hypoxan- thine uptake. The observed phenomena could be of importance for the selection of drugs to be used in cancer chemotherapy. </p><p>INTRODUCTION </p><p>Until some years ago it was uncertain whether the mutants which are obtained in the mutational assay systems with mammalian cells in vitro are due to alterations in the DNA which codes for the genes involved or whether the mutants arise because of stable phenotypic variation (1-3). This question has largely subsided since it became gradually clear that mutants show alterations in their enzyme kinetics if stringent selection conditions are used (4--8). This is supported by the finding that with the BrdU-light methods mutants can be induced in specific parts of the S phase, indicating that these mutants are induced at the time of DNA replication (9). Moreover the genetic </p><p>307 </p><p>0098-0366/82/0500-0307503.00/0 9 1982 Plenum Publishing Corporation </p></li><li><p>308 Verschure and Simons </p><p>nature of the mutants became evident in studies with repair-deficient mutants in which the defective repair of DNA damage results in elevated induced mutant frequencies (10, 11). Over the past years it also gradually became clear that a phenotypic drug resistance can be obtained if the selection conditions used are not so stringent and the nonmutant cells do not die shortly after the application of the selective drug. Unstable phenotypic drug resis- tance can be obtained after low concentrations of 8-azaguanine (12), and recently it has been shown that prolonged application of low concentrations of purine analogs may even lead to stable resistance (13). The same phenomenon also has been observed after prolonged application of dexamethasone (14) or other drugs (15). The nature of this stable phenotypic variation is unknown. </p><p>The aim of this study was to gain more insight in this phenomenon. We considered phenotypic drug resistance to be an important problem as it could occur in vivo in patients which are treated with cytostatics, leading to resistant tumor cells (16) or to very high frequencies of resistant lymphocytes (17). In particular, the question we wanted to study was whether phenotypic drug resistance was due to selection of cells or to adaptation of cells. Selection of cells could occur if there is a preexisting variation among cells which is continuously arising at a high frequency. For resistance to purine analogs this would be reflected in hypoxanthine uptake. The existence of variation in enzyme activities among clonal populations has been shown to occur (18). Therefore a method was developed to measure the uptake of hypoxanthine in a large number of clones. It was found that in mammalian cells a kind of steady state occurs in the uptake of hypoxanthine which can alter spontaneously. It is probable that this type of change is involved in the occurrence of stable phenotypic drug resistance. </p><p>MATERIALS AND METHODS </p><p>Cell Cultures. The experiments were performed with BSC-1 cells (Afri- can green monkey) and with human diploid skin fibroblasts. As standard medium Ham's F- 10 was used, modified by the omission of hypoxanthine and supplemented with 15% newborn calf serum, 100 units/ml penicillin, and 0.1 mg/ml streptomycin sulfate. </p><p>The cells were grown in plastic petri dishes. For subculturing, the petri dishes were rinsed once with Ca-free and Mg-free Hanks' BSS (Gibco Bio-cult) and incubated at 37~ for 5 min with 1 ml 0.5% trypsin and 0.02% EDTA (Gibco Bio-cult) in Hanks' BSS. The tryptic activity was neutralized by the addition of standard medium. For the experiments, petri dishes 90 mm (from Greiner, P90) and 30 mm in diameter (from Limbro, P30) were used. Clones were obtained within 14-21 days after seeding 150-300 cells per P90 </p></li><li><p>Phenotypic Drug Resistance 309 </p><p>and changing the medium twice a week. The clones were grown to at least 5-7 mm in diameter before studying their incorporation of hypoxanthine. </p><p>Measurement of Uptake of FH]Hypoxanthine in Clones. For this assay the medium from the cultures was replaced by F-10 medium without serum containing [3H]hypoxanthine (570 mCi/mmol, Amersham); 4 ml/P90 or 1 ml/P30. The final radioactivity was 7.2 ~Ci/ml. After 24 h the nonincorpo- rated hypoxanthine was removed from the cells by incubation in 5% cold TCA (Baker). Subsequently the cultures were fixed with methanol (Merck). The amount of incorporation was measured in a Philips liquid scintillator after cutting out the clones and dissolving the piece of plastic with the clone in scintillation fluid (toluene containing 0.4% PPO, Baker). </p><p>Determination of Amount of Protein in Clones in Situ. The clones were stained with a saturated solution of naphthol yellow S (NYS, Fluka) in methanol at pH 2.8 for 2 min. The NYS was removed by washing 3 times with methanol. Subsequently the extinction per clone was measured in a flying spot densitometer (Vitatron LTD100) connected with an integrating recorder (Vitatron UR406). </p><p>Hypoxanthine-Guanine-Phosphoribosyltransferase Assay. The cells were lysed in 0.01 M Tris HC1, pH 7.4. The HGPRT activity was assayed as described previously (19). </p><p>RESULTS </p><p>Determination of Variation in Uptake of Hypoxanthine in Clones in Situ. As it was considered important to characterize the hypoxanthine uptake of clones as soon as possible after their origin, the amount of uptake per unit protein was determined in the clones in situ. After the incorporation of labeled hypoxanthine, the clones were stained with NYS and the extinction per clone measured in a densitometer (see Materials and Methods). This measurement is in IU (integrator units). The incorporation per clone was expressed per 100 IU which corresponds with 3.2 #g protein or about 11,000 cells. </p><p>The application of NYS for staining and quantitative determination of proteins has been described (20, 21). This staining proved to be suitable for our purpose. The clones stain within 15 sec, and no further increase in staining was observed c~ver a period of 12 h. Extensive washing of the stained clones with methonol did not reduce the stain. The extinction is linearly related with cell density (measured up to 2.5 x 106 cells/P90). Furthermore the stain does not influence the number Of cpm during liquid scintillation counting. </p><p>The degree of accuracy of this method was tested by determining the uptake of hypoxanthine in colonies of BSC-1 cells. These colonies were obtained with droplets of 20t~l containing 8000 cells. These droplets were </p></li><li><p>310 Verschure and Simons </p><p>placed in P90 dishes, and the cells were allowed to attach for 6 h. After attachment, medium with [3H]hypoxanthine was given for 24 h after which the colonies were fixed and stained with NYS The mean incorporation per 100 IU in 42 colonies measured was 9652 cpm with a standard deviation of 2202 cpm (Fig. 1A). This standard deviation is 23% of the mean. In two other experiments this percentage was 30% and 26%, respectively. The weighed mean value for the standard deviation amounted to 28%, which can be considered to be the accuracy for each individual measurement. Therefore the 95% confidence limits for the determination will be about 12% for 20 measurements, 10% for 30 measurement, and 8% for 50 measurements. </p><p>In order to determine the biological variation among clones in the uptake of hypoxanthine, the incorporation was determined in 53 clones of BSC- 1 cells (Fig. 1B). A mean of 8267 cpm with a standard deviation of 3579 cpm was found. This standard deviation is 43% of the mean. Therefore the standard deviation in this experiment is much larger than the standard deviation obtained for the accuracy of the method (43% instead of 28%). This indicates that there is a biological variation among clones in the uptake of hypoxan- thine. This variation proved not to be correlated with the size of the clones. This uptake of hypoxanthine in clones of BSC-1 cells appeared to be quite reproducible in three experiments (Table 1), which supports the conclusion about the occurrence of a biological variation among clones. </p><p>Experiments on Heredity of Differences in Uptake of Hypoxanthine. To determine whether there is a hereditary component in this biological variation </p><p>1A </p><p>Number of colonies </p><p>15 </p><p>lO </p><p>5 </p><p>0 20 40 60 -tin 80 100 120 </p><p>1B </p><p>Number of clones </p><p>20 40 60 80 100 120 1/.0 '160 </p><p>10 2 CPM/IoOI.U </p><p>180 200 </p><p>Fig. 1. Histogram of [~H] hypoxanthine uptake in cell populations of BSC-1 cells. (A) Uptake of hypoxanthine in 42 colonies of BSC-1 cells. The colonies were obtained by means of droplets of cell suspension (8000 cells in 20 #1). (B) Uptake of hypoxanthine in 53 clones of BSC-1 cells. </p></li><li><p>Phenotypic Drug Resistance 311 </p><p>Table 1. Incorporation of ~H-Labeled Hypoxanthine in Clones of BSC-1 Cells </p><p>Mean incorporation Standard deviation Number of of hypoxanthine expressed as </p><p>Experiment clones (cpm/100 IU/24 h) percentage No. measured 95% confidence limits of the mean </p><p>1 43 8421 _+ 1092 41 2 53 8267 _+ 988 43 3 64 8469 _+ 918 43 </p><p>1 + 2 + 3 160 8389 _+ 562 43 </p><p>among clones in the uptake of hypoxanthine, an experiment was performed analogous to the classic experiment of Johannsen on the heredity of bean size (23). </p><p>About 60 clones obtained from Bsc - i cells were isolated with a micropipet in such a way that half of the clone was removed and the other half left in the dish. This procedure allowed for a subsequent determinat ion of hypoxanthine uptake in the unremoved part of the clone. In this way it was possible to proceed only with those clones which were character ized by a high or a low uptake of hypoxanthine. Therefore, from the 60 clones, only 5 were propagated to cell lines, and the uptake of hypoxanthine in these cell lines was measured in clones derived from them (Table 2). In order to avoid semantic ditticulties, clones obtained from the original population are called pr imary clones and clones derived from pr imary clones are called secondary clones (Tables 3, 4, and 5); tert iary clones are clones derived from secondary clonal populations (table 4). Table 2 shows that three of the five clones have the same mean incorporation of hypoxanthine as the parental cells in Table 1; two clones are character ized by a significantly lower incorporation, which points to a hereditary factor in the variation in uptake of hypoxanthine. When there is such a hereditary factor, one would expect that the clonal populations would have standard deviations which are lower than the standard deviation of the parental population. According to Table 2 this appears to be the case for three of the clones but not for the two others, which could mean that a new variation is present within the clones. Therefore the stabil ity of the reduction in </p><p>Table 2. Incorporation of 3H-Labeled Hypoxanthine in Secondary Clones of BSC- 1 Cells </p><p>Number of Mean incorporation Standard deviation secondary of hypoxanthine expressed as </p><p>Primary clones clones (cpm/100 IU/24 h) percentage of BSC- 1 cells measured 95% confidence limits of the mean </p><p>P-I 42 3669 _+ 470 41 P-2 65 8082 _+ 932 47 P-3 75 8244 _+ 676 36 P-4 37 8626 _+ 978 34 P-5 57 6073 _+ 563 35 </p></li><li><p>312 Verschure and Simons </p><p>Table 3. Incorporation of 3H-Labeled Hypoxanthine in Secondary Clones of BSC-1 Cells Obtained from Primary Clones at Different Passages </p><p>Number of Mean incorporation Standard deviation secondary of hypoxanthine expressed as </p><p>Primary Passage clones (cpm/100 IU/24 h) percentage clone number measured 95% confidence limits of the mean </p><p>P-5 4 57 6073 _+ 563 35 6 77 8602 _+ 961 49 7 57 8880 _+ 1006 43 9 53 8724_+ 731 30 </p><p>P-I 4 42 3669 _+ 470 41 6 42 4415 _+ 1048 76 7 79 5605 _+ 549 43 </p><p>16 72 8700 _+ 1141 42 </p><p>hypoxanthine uptake was determined (Table 3). The latter table shows that the amount of incorporation returns to that demonstrated by the parental line and that therefore the trait is not stable. To check whether this restoration occurs in all the cells concomitantly, some clones derived from passage 4 were isolated, grown to cell lines, and again tested (Table 4). It turned out that, while two of the clones showed reversion to parental values, one clone still demonstrated a reduced incorporation and one other had a significantly higher incorporation. This indicates that within the clonal cell populations new variations are arising continuously and at a high frequency. </p><p>An explanation for this could be chromosomal variation as BSC-1 cells are aneuploid and differences between clones in chromosomal content are bound to occur. This point was investigated by performing experiments with normal human skin fibroblasts, which are character ized by a stable diploid karyotype (Table 5). This table shows that three of the seven clones are character ized by a significantly lower uptake of hypoxanthine. Therefore this phenomenon is also present in diploid cell strains. Also in this case no reduction in standard deviation is apparent, which points to instabil ity in these clones also. </p><p>Table 4. Incorporation of 3H-Labeled Hypoxanthine in Tertiary Clones of BSC-1 Cells Indicates Heterogeneity in Uptake of Hypoxanthine within Clonal Populations </p><p>Number of Mean incorporation Standard deviation tertiary of hypoxanthine expressed as </p><p>Primary Secundary clones (cpm/100 IU/24 h) percentage clone clone measured 95% confidence limits of the mean </p><p>P.1 P-I-i 22 5970 _+ 1056 40 P-l-2 11 8457 _+ 1987 35 </p><p>P-5 P-5-1 3! 8215 2360 79 P-5.2 25 13162 1866 34 </p></li><li><p>Phenotypic Drug Resistance 313 </p><p>Table 5. Incorporation of all-Labeled Hypoxanthine in Primary and Secondary Clones of Human Diploid Skin Fibroblasts ~ </p><p>Mean incorporation Standard deviation Clones Number of of hypoxanthine expressed as derived clones (cpm/100 IU/24 h) percentage from measured 95% confidence limits of the mean...</p></li></ul>