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Regulates Cisplatin Cytotoxicity, Mutagenicity, and The Rate of Development of Cisplatin Resistance
Department of Medicine and the Cancer Center, University of California San Diego, La Jolla, California
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ABSTRACT |
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 Top ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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participates in translesional bypass replication. Here we show that reduced expression of the catalytic subunit hREV3 renders human fibroblasts more sensitive to the cytotoxic effect of cisplatin, reduces their sensitivity to the ability of cisplatin exposure to generate drug resistant variants in the surviving population, and reduces the rate of emergence of resistance to cisplatin at the population level. Reduction of REV3 mRNA did not alter the rate of cisplatin adduct removal but did impair both spontaneous and cisplatin-induced extrachromosomal homologous recombination and attenuated bypass replication as reflected by reduced ability to express luciferase from a platinated plasmid. Cisplatin induced a concentration- and time-dependent increase in hREV3 mRNA. The results indicate that, following formation of cisplatin adducts in DNA, REV3 mRNA levels increase, and polymerase functions to promote both cell survival and the generation of drug-resistant variants in the surviving population. We conclude that when cisplatin adducts are present in the DNA, polymerase is an important contributor to cisplatin-induced genomic instability and the subsequent emergence of resistance to this chemotherapeutic agent. |
INTRODUCTION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, polymerase 
, polymerase 
, polymerase µ, and polymerase 
(1)
. Analysis of null mutants of REV3, the catalytic subunit of polymerase , in Saccharomyces cerevisiae revealed that a large fraction of all of the mutations induced by DNA damaging agents and the majority of spontaneous mutations are attributable to the activity of polymerase (2)
. Inhibition of the expression of the REV3 subunit in cultured human fibroblasts by expression of an antisense REV3 mRNA was shown to reduce UV-induced mutagenesis, indicating that polymerase also plays a crucial role in mutagenesis in mammalian cell (3)
.
The activity of polymerase is of concern with respect to the use of DNA-damaging chemotherapeutic agents such as cisplatin. The most abundant lesions produced in DNA are intrastrand cross-links, and these are believed to be important to both the cytotoxicity and the mutagenicity of the drug. Its ability to function as a mutagen has been documented in bacterial (4
, 5)
and mammalian cells (6, 7, 8, 9, 10, 11)
. Whereas cisplatin produces gross chromosomal changes (12)
and probably gene amplification (13)
, the molecular basis for much of its mutagenicity is believed to be related to bypass replication across cisplatin adducts by the eukaryotic DNA polymerase ß and/or members of the class containing polymerases , , and (14, 15, 16)
. Current evidence suggests that in bacteria and yeast when nonerror-prone mechanisms for repairing DNA damage, such as base excision repair, nucleotide excision repair, and homologous recombination, are disabled or overwhelmed, cells make increased use of specialized low-fidelity error-prone DNA polymerases to bypass DNA lesions that block normal replicative polymerases (17)
. This appears to be an important contributor to the mutagenicity of cisplatin adducts (18)
.
Several investigators reported previously that loss of polymerase function markedly reduces UV and benzo(a)pyrene diol epoxide-induced hypoxanthine guanine phosphoribosyl transferase mutations (19
, 20) . A key question with respect to the clinical use of cisplatin is whether cisplatin-induced mutagenesis contributes to the acquisition of cisplatin resistance and whether suppression of cisplatin-induced mutagenesis can reduce the rate of acquisition of cisplatin resistance, because this remains the major cause of treatment failure. In this study, we used the diploid human fibroblast cell line 9N58 and the 6I subline that was engineered to express high levels of hREV3 antisense RNA (19)
. We report here that reduction in polymerase function renders cells more sensitive to the cytotoxic effect of cisplatin but also markedly decreases its mutagenicity. Most importantly, it significantly reduces the rate at which cells acquire resistance to cisplatin. These results strongly support the hypothesis that polymerase is, in large part, responsible for the ability of cells to replicate their DNA and survive in the face of a large cisplatin adduct load and for the error-prone bypass replication that is important to the mutagenicity of this drug and its ability to generate drug-resistant variants in the surviving population.
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MATERIALS AND METHODS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Measurement of Rate of Generation of Resistant Variants.
The rate at which highly drug-resistant variants spontaneously appeared in the population was measured using the "maximum likelihood estimation" technique (21)
. The frequencies of highly drug-resistant variants in the 9N58 and 6I population were measured (vide infra), and 106 cells were subcultured and allowed to expand exponentially for 4 days. The frequency of resistant variants was then measured again, and the process repeated for a total of four iterations. Total cell numbers were determined at each step, along with the plating efficiencies from the previous selection and the exact number of population doublings determined from the following equation: Population doubling = [Ln(total number of cells) – Ln(number of cells plated x plating efficiency)]/Ln2. The rate of generation of resistant variants was then obtained by plotting the observed resistant variant frequency as a function of population doubling and by calculating the slope by linear regression. The slope of the curve yields the rate of generation of resistant variants (resistant variants/clonogenic cell/generation).
Plasmid Reactivation Assay.
The pRL-CMV vector (Promega, Madison, WI) containing Renilla luciferase cDNA was platinated to 1.5 ± 1.4 pg/µg DNA, which is equivalent to 9.3 adducts per plasmid or 3.2 adducts per Luc coding region as reported previously (22)
. Similar levels of platination have been shown previously not to affect the efficiency of transfection (23)
. Twenty-four hours after transfection of 1 µg of platinated or control unplatinated vector, luciferase activity was measured using the Promega Renilla Luciferase Assay System (Promega).
Clonogenic Assay.
Clonogenic assays were performed by seeding 1,000 cells into 60-mm plastic dishes in 5 mL of complete medium. After 24 hours, appropriate amounts of cisplatin were added to the dishes, and the cells were exposed for 1 hour. Colonies of at least 50 cells were visually scored after 10 to 14 days. Each experiment was performed a minimum of three times using triplicate cultures for each drug concentration. IC50 values were determined by log-linear interpolation, and the relative cytotoxicity was determined using the ratio of the slopes of the survival curves.
Measurement of Cisplatin Mutagenicity.
The sensitivity of cells to the mutagenic effects of cisplatin was measured by determining the frequency of variants highly resistant to either 10 µmol/L 6TG or to 6 µmol/L cisplatin itself in the surviving population 20 days after a 1-hour exposure to increasing concentrations of cisplatin as reported previously (10
, 11)
. Each experiment was performed a minimum of three times, and the data are presented as mean ± SEM. When testing for 6TG-resistant variants, the cells were grown in HAT medium containing 0.4 µmol/L aminopterin, 16 µmol/L thymidine, and 100 µmol/L hypoxanthine for a minimum of 14 days before testing to reduce the number of pre-existing hypoxanthine guanine phosphoribosyl transferase mutants.
Measurement of Platinum in DNA.
Aliquots of the DNA were digested in 70% nitric acid at 65°C for 2 hours and diluted to 5% nitric acid by adding appropriate volume of double distilled deionized water. The picograms of platinum per microgram of DNA in the hydrolysate was quantified by inductively coupled plasma optical emission spectroscopy or inductively coupled plasma mass spectroscopy as described previously (24)
. When assessing the time course of the loss of platinum from DNA, the cells were treated with 200 µmol/L cisplatin for 1 hour to obtain quantifiable levels of platinum over the entire period of the experiment. DNA was isolated at 0, 6, 12, 18, and 24 hours after drug exposure.
Measurement of the Frequency of Extrachromosomal Homologous Recombination.
Homologous recombination was assayed by determining the extent of recombination between 2 green fluorescent protein sequences in plasmid DNA as described previously (25)
. The pBHRF vector contains an intact "blue" variant of green fluorescent protein (enhanced blue fluorescent protein) that includes an 
300 nucleotide sequence with perfect homology to a second truncated nonfunctional copy of green fluorescent protein. In the absence of homologous recombination within the vector only enhanced blue fluorescent protein is expressed; however, homologous recombination between the enhanced blue fluorescent protein and truncated green fluorescent protein sequences creates a functional green fluorescent protein, and if this occurs the cell expresses green fluorescent protein as well as enhanced blue fluorescent protein, which is expressed from other plasmids in the cell that have not undergone recombination. Cells were seeded into six-well plates overnight and then exposed to 0 or 10 µmol/L cisplatin for 1 hour. The untreated or surviving cells were then transfected with pBHRF 24 hours later with siPORT XP-1 transfection agent (Ambion Inc., Austin, TX) in the presence of serum according to the manufacturer’s instruction. Four hours after transfection, BoosterExpress reagent (Gene Therapy Systems, Inc., San Diego, CA) was added, and the cells were then analyzed by two-color flow cytometry 48 hours after transfection. The recombination frequency was calculated as [(enhanced blue fluorescent protein+ and green fluorescent protein+) + (green fluorescent protein+)]/[(enhanced blue fluorescent protein+ and green fluorescent protein+) + (green fluorescent protein+) + (enhanced blue fluorescent protein+)] where enhanced blue fluorescent protein and green fluorescent protein represent the number of blue and green fluorescent cells, respectively, in the sample.
Quantitation of hREV3 mRNA by Reverse Transcription-PCR.
Total RNA was extracted with TRIzol reagent (Invitrogen, Carlsbad, CA). First strand cDNA was synthesized using SuperScript II reverse transcriptase (Invitrogen) and random primers. For REV3 gene expression, forward (5'-TGATGTCTTCAGCTG GTATCATGA-3') and reverse (5'-CCGCCCTTCAGGTT CACTT-3') primers were used for amplification under the following conditions: 5 minutes of predenaturation at 95°C, 30 seconds of denaturation at 94°C, 30 seconds of annealing at 59°C, 1 minute of extension at 72°C, and an additional 10 minutes of extension at 72°C. The fluorochrome-labeled probe that was displaced to yield the fluorescent signal during the reverse transcription-PCR reaction was as follows: 5'-TTACCAAGGATCCATAAAGCTA CCAGCTCCTC-3'.
Relative Rate of Development Resistance to Cisplatin.
The rate at which the cell population became resistant to cisplatin during repeated cycles of 1-hour exposures to the drug was determined by measuring the IC50 for cisplatin using a clonogenic assay after each round of selection. The cisplatin concentration used for selection was the IC90 for the population under study. For each round of selection, 106 cells were exposed to cisplatin for 1 hour. When the cells had recovered to 90% confluence, an aliquot was used to determine cell number and the slope of the cisplatin concentration–survival curve in a clonogenic assay, and another aliquot was again exposed to cisplatin. Total cell number and plating efficiency was determined at each step; this information, along with the exact number cells subcultured, was used to calculate population doubling according to the equation described above. The rate of acquisition of resistance to cisplatin was then calculated by plotting the slope of the cisplatin concentration–survival curve as a function of population doubling. The slope of the latter plot yields the rate of relative resistance development.
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RESULTS |
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Function on the Spontaneous Rate of Generation of Resistant Variants.-proficient 9N58 human fibroblast cell line and the 6I subline that had been molecularly engineered to express an antisense mRNA directed at the hREV3 subunit of polymerase . That the 6I cells express high levels of hREV3 antisense mRNA and have a lower frequency of mutants induced by UV and benzo(a)pyrene diol epoxide in their HPRT gene relative to the parental 9N58 cells has been documented previously (19)
. To provide additional confirmation that the 6I cells had diminished polymerase activity, the spontaneous rate of generation of variants resistant to 6TG was determined by serially measuring the frequency of resistant variants in expanding populations. The results, presented in Fig. 1
, show that the hREV3 antisense-expressing 6I cells exhibited a 2.9-fold decrease in spontaneous rate of generation of variants resistant to 6TG as compared with the parental 9N58 cells (P < 0.01). This result is consistent with the reduction in spontaneous mutation rate observed in other studies of polymerase -deficient cells (26
, 27)
.
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complex in the intact 9N58 and 6I cells, but the overall translesional synthetic capability in cells can be assessed indirectly by determining the ability of the cell to successfully express the Renilla luciferase from an expression vector that has been extensively platinated by treatment with cisplatin before transfection. By comparing luciferase expression in the uninjured 9N58 and 6I cells it is possible to assess what fraction of host cell luciferase expression is due to specifically polymerase -mediated adduct bypass as opposed to translesional synthesis mediated by other polyermases. Fig. 2
shows that luciferase activity was equivalent in the 9N58 and 6I cells when they were transfected with nonplatinated vector. When the platinated vector was transfected into the polymerase -proficient 9N58 cells, there was little impairment in the generation of luciferase activity (1.4-fold reduction, P > 0.05). However, when the same platinated vector was transfected into the polymerase antisense-expressing 6I cells, the luciferase expression was markedly reduced (3.9-fold, P < 0.01). Thus, polymerase activity is important to the ability to express luciferase from a platinated plasmid, and this activity was reduced in the 6I cells. Together with the reduced rate of generation of spontaneous 6TG mutants these results validate this experimental system for the study of the effect of polymerase on cisplatin pharmacodynamics.
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Function on Sensitivity to the Cytotoxic Effect of Cisplatin. on sensitivity to the cytotoxic effect of cisplatin. As shown in Fig. 3
, the IC50 was 1.3 ± 0.04 µmol/L (SEM) for the 6I cells and 2.5 ± 0.01 µmol/L (SEM) for the parental cells (P < 0.05). On the basis of the ratio of the slopes of survival curves, the 6I cells were 1.4-fold (P < 0.05) more sensitive to the cytotoxic effect of cisplatin than the polymerase -replete 9N58 cells. Thus, hREV3 antisense-expressing cells demonstrated hypersensitivity to the cytotoxic effect of cisplatin.
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Function on the Ability of Cisplatin to Generate Resistant Variants. in cisplatin-induced mutagenesis, the 9N58 and 6I cells were exposed to 10 µmol/L cisplatin for 1 hour, and then 20 days later the number of clonogenic cells demonstrating high-level resistance to 6TG or to cisplatin itself was determined. When measured at the end of the 1 hour of cisplatin exposure, the DNA of the 9N58 and 6I cells contained 3.49 ± 0.30 (SEM) and 3.32 ± 0.36 (SEM) pg of platinum/µg, respectively, indicating no difference in the extent of initial adduct formation. As shown in Fig. 4A
, the frequency of 6TG-resistant variants induced by exposure to 10 µmol/L cisplatin was significantly lower (3.6-fold) in 6I cells than in the parental 9N58 cells (P < 0.01). Fig. 4B
shows that after exposure to cisplatin, the polymerase -proficient cells yielded 1.4-fold more colonies that were highly resistant to cisplatin itself than the hREV3 antisense-expressing cells (P < 0.01). When normalized to the extent of clonogenic cell kill produced by a 1-hour exposure to 10 µmol/L, the frequency of 6TG and cisplatin-resistant variants was 6.3- and 3.6-fold lower, respectively, in the 6I cells. Thus, cisplatin was able to generate variants in the surviving population that were highly resistant to 6TG or to cisplatin itself, and this mutagenic effect was reduced in the 6I cells both on an absolute basis and when normalized for the extent of clonogenic cell kill in the absence of any difference in the DNA platinum content between the two cell types. This is consistent with the hypothesis that polymerase plays an important role in generating mutations when cisplatin adducts are present in DNA.
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Function on the Disappearance of Platinum from DNA. impaired DNA adduct removal. The rate of disappearance of platinum from the DNA accurately mirrors the rate of removal of the most common cisplatin adducts (29
, 30)
. The initial picogram of platinum per microgram of DNA and the rate of disappearance of platinum from total cellular DNA was measured in both cell lines after a 1-hour exposure to 200 µmol/L cisplatin. The initial adduct levels were nearly identical, being 32.94 and 32.30 pg platinum/µg DNA, respectively, for the 9N58 and 6I cells. Fig. 5
shows that there was no significant difference in the kinetics of platinum disappearance from DNA in the two cell lines. Because nucleotide excision repair is responsible for removal of the majority of cisplatin adducts, this suggests that the degree of impairment of polymerase function in the 6I cells did not interfere significantly with this pathway of DNA repair.
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Function on Spontaneous and Cisplatin-Induced Homologous Recombination. function on basal and cisplatin-induced rates of homologous recombination we used the pBHRF recombination-sensitive reporter vector described by Slebos and Taylor (25)
. This plasmid constitutively expresses an intact, emission-shifted, "blue" variant of green fluorescent protein (enhanced blue fluorescent protein) and also contains a COOHterminally truncated form of green fluorescent protein in which there exists a 300-bp homologous sequence for recombination with identical nucleotide stretch within the intact enhanced blue fluorescent protein sequence. In the absence of homologous recombination within or between the enhanced blue fluorescent protein and green fluorescent protein sequences in the vector only enhanced blue fluorescent protein is expressed in transfected cells; however, homologous recombination event can create a functional green fluorescent protein, in which case the cell expresses both enhanced blue fluorescent protein and green fluorescent protein, because most cells acquire multiple copies of the vector during transfection, only some of which recombine. The polymerase -proficient 9N58 and hREV3-antisense expressing 6I cells were exposed to 10 µmol/L cisplatin for 1 hour or left untreated, and the pBHRF was transfected into the cells 24 hours later. After an additional 48 hours the fraction of blue fluorescent protein-positive cells that also expressed green fluorescent protein was determined by flow cytometry. As shown in Fig. 6
, the spontaneous homologous recombination frequency was 2.4-fold lower in the 6I cells than the parental cells (P < 0.05). Exposure to10 µmol/L cisplatin for 1 hour increased the recombination frequency in both cell types; however, the frequency was still significantly lower (2.5-fold, P < 0.05) in the 6I than in the 9N58 cells. Thus, polymerase function appears to be important to both spontaneous and cisplatin-induced homologous recombination.
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function during the injury response after drug exposure. At the present time, no antibody is available with sufficient avidity and specificity to permit accurate Western blot analysis of hREV3 protein levels. Thus, real-time PCR was used to measure changes in hREV3 mRNA level using primers that directed amplification from the COOH-terminal region of the coding sequence. Fig. 7A
shows that the basal hREV3 mRNA level was higher in the parental 9N58 cells than in the antisense-expressing 6I cells. Moreover, cisplatin induced a concentration-dependent increase in hREV3 mRNA level in the 9N58 cells when measured at 24 hours after a 1-hour drug exposure. Interestingly, although smaller in magnitude, cisplatin also induced an increase in hREV3 mRNA level in the 6I antisense-expressing cells, suggesting that the ability of the antisense to mediate hREV3 degradation may be overwhelmed by an increase in endogenous hREV3 mRNA production.
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shows the change in hREV3 mRNA level as a function of time after exposure to 10 µmol/L cisplatin for 1 hour. The hREV3 mRNA level continued to increase up to 24 hours in both cell lines but was generally lower in the antisense-expressing 6I cells than in the parental 9N58 cells. These results indicate that cisplatin induced an increase in hREV3 mRNA level that peaked at a time similar to the time of maximum cell cycle arrest produced by cisplatin (11)
. Whether this results in an increase in hREV3 protein level cannot currently be determined.
Effect of Loss of Polymerase Function on the Rate of Development of Cisplatin Resistance.
The emergence of drug resistance in a population during cisplatin-repeated cycles of drug exposure may be due to enrichment for pre-existing resistant clones, cisplatin-induced generation of new resistant variants, or some combination of both. As shown above, loss of polymerase function reduced the ability of cisplatin to generate drug-resistant variants in the surviving population. If this ability of cisplatin is central to the emergence of acquired cisplatin resistance in the whole population, then reduction of polymerase activity would be expected to reduce the rate at which resistance emerges. We measured the rate of development of resistance in the whole population of 9N58 and 6I cells starting with 500,000 cells. The cells were exposed to an IC90 concentration of cisplatin for 1 hour, and the exposure was repeated as soon as log phase growth resumed. After each round of drug treatment, the sensitivity of the whole population to cisplatin was measured by determining survival over 2 logs of cell kill as a function of cisplatin concentration in a clonogenic assay. Fig. 8
shows that cisplatin resistance emerges quite rapidly in both cell lines but that the rate of development of resistance was reduced by an average of 3-fold (P < 0.05) in the 6I cells relative to the 9N58 cells. Thus, polymerase plays a central role in the acquisition of cisplatin resistance. Because loss of polymerase does not appear to alter the extent of adduct formation or the time course of platinum removal from DNA, these results are consistent with the concept that mutagenic translesional synthesis across cisplatin adducts is responsible for generating drug-resistant variants that become enriched in the population by subsequent rounds of cisplatin exposure.
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DISCUSSION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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activity could not be measured directly, the phenotypic differences between the 9N58 and 6I cells are consistent with reduced polymerase activity in response to the expression of antisense hREV3 mRNA. Similar changes in phenotype have been observed in fibroblasts from transgenic mice expressing antisense RNA to mREV3 (20)
. The results of the current studies indicate that such impairment of polymerase function caused a moderate increase in sensitivity to the cytotoxic effect of cisplatin without altering the rate at which total platinum adducts were removed from DNA. This suggests that, as for other types of adducts that block the progression of the replicative polymerases (15
, 33)
, polymerase is involved in a pathway that normally carries out enough translesional synthesis to allow some cells to complete DNA synthesis and survive. The observation that impaired polymerase function also reduced the ability of cisplatin to generate highly drug-resistant clones in the surviving population provides strong evidence that the translesional synthesis pathway in which polymerase functions is error-prone when it bypasses cisplatin adducts. It appears that this pathway normally fosters the development of resistance to cisplatin both by permitting the survival of cells that contain mutagenic adducts in their DNA and by generating new mutations in genes that mediate the resistant phenotype.
The actual extent to which polymerase function was disabled in the 6I cells is not known. Knockout of both alleles in mice causes embryonic lethality (34, 35, 36)
, but knockout of both alleles in chicken B cells only causes slowing of proliferation (37)
. On the basis of the magnitude of the phenotypic effects relative to those observed in knockout cells, it is likely that some polymerase function remains in 6I cells, a presumption that is supported by the persistence of measurable amounts of hREV3 mRNA. However, the extent of loss of polymerase function was sufficient to yield an easily detectable phenotype with respect to the pharmacodynamic effects of cisplatin.
What role the proteins of the polymerase complex might play in DNA repair mechanisms other than translesional bypass is only just becoming defined. The observation that, despite being 1.4-fold hypersensitive to the cytotoxic effect of cisplatin, the time course of removal of platinum from DNA was the same in the polymerase -proficient and antisense-expressing cells suggests that nucleotide excision repair is not highly dependent on at least hREV3. However, the finding that recombination-dependent expression of green fluorescent protein from the pBHRF vector was impaired in the 6I cells suggests a role for hREV3 or the entire polymerase complex in at least extrachromosomal homologous recombination. Caution is needed in interpreting the pBHRF results, because the assay reflects only extrachromosomal recombination; however, this finding is in agreement with the results of studies in knockout yeast and chicken B cells that also suggest that REV3 plays an important role in chromosomal homologous recombination (37
, 38)
. Homologous recombination is important to the survival of cells after cisplatin exposure (39, 40, 41)
, and it may be essential for repair of interstrand cross-links (42, 43, 44)
. The finding that cisplatin exposure enhances pBHRF recombination suggests that cisplatin up-regulates this putatively nonmutagenic repair mechanism, an effect expected to improve the ability of the cell to survive the DNA damage produced by this agent. Polymerase may be particularly important to cisplatin pharmacodynamics because its loss simultaneously impairs both translesional synthesis and homologous recombination, each of which alone perhaps could handle the adduct load if it were intact.
The expression of a large number of genes is altered after cisplatin-induced injury (45)
, and hREV3 appears to be one of these. hREV3 mRNA levels increased in proportion to the extent of injury, and the level peaked at 24 hours after a 1-hour exposure to 10 µmol/L cisplatin. It is not known whether the increase in hREV3 mRNA translates into increased polymerase activity, but it is reasonable to expect that it does. Thus, not only are cisplatin adducts mutagenic when bypassed by polymerase , but the data are consistent with the concept that cisplatin also increases polymerase levels and/or activity and, thus, additionally enhances its own mutagenicity.
The single most important finding of these studies is that reduction of polymerase function can reduce the rate at which the whole population of cells becomes resistant to cisplatin. Reduction in the rate at which cisplatin resistance emerges is a key clinical goal. Whereas we have shown previously (10)
and confirmed in the current studies that treatment with cisplatin produces clones in the surviving population that are highly resistant to cisplatin itself and several other classes of drugs, whether this is really important to acquisition of cisplatin resistance by the entire population remained unknown. The current results establish three important points: (1) acquisition of resistance by the entire population is not just due to enrichment for drug-resistant clones that existed in small numbers before drug exposure; (2) the genes that mediate cisplatin resistance are susceptible to adduction by cisplatin and to mutagenic bypass replication by polymerase ; and (3) the magnitude of the effect of polymerase on the rate of resistance acquisition is quite large. Thus, these studies identify polymerase as a target of which the pharmacological inhibition may stabilize the genome during the cellular injury response triggered by cisplatin and reduce the rate of emergence of resistance in patients treated with this important chemotherapeutic agent.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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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.
Requests for reprints: Stephen B. Howell, Department of Medicine 0058, University of California San Diego, La Jolla, CA 92093. Phone: 858-822-1110; Fax: 858-822-1111; E-mail: showell{at}ucsd.edu
Received 12/16/03. Revised 3/12/04. Accepted 4/12/04.
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, parental 9N58 cells; 
, hREV3 antisense-expressing 6I cells. Each curve shows the change in frequency of resistant variants with increasing numbers of population doublings. The rate of generation of resistant variants is given by the slope of the linear regression line. Each data point is the mean of three experiments. Vertical bars, ± SEM.
