Human leukemic cells resistant to 5-fluoro-2'-deoxyuridine contain a thymidylate synthetase with lower affinity for nucleotides

Implications of a Neuronal Receptor Family, Metabotropic Glutamate Receptors, in Cancer Development and Progression

Cancer is the second leading cause of death, and incidences are increasing globally. Simply defined, cancer is the uncontrolled proliferation of a cell, and depending on the tissue of origin, the cancer etiology, biology, progression, prognosis, and treatment will differ. Carcinogenesis and its progression are associated with genetic factors that can either be inherited and/or acquired and are classified as an oncogene or tumor suppressor. Many of these genetic factors converge on common signaling pathway(s), such as the MAPK and PI3K/AKT pathways. In this review, we will focus on the metabotropic glutamate receptor (mGluR) family, an upstream protein that transmits extracellular signals into the cell and has been shown to regulate many aspects of tumor development and progression. We explore the involvement of members of this receptor family in various cancers that include breast cancer, colorectal cancer, glioma, kidney cancer, melanoma, oral cancer, osteosarcoma, pancreatic cancer, prostate cancer, and T-cell cancers. Intriguingly, depending on the member, mGluRs can either be classified as oncogenes or tumor suppressors, although in general most act as an oncogene. The extensive work done to elucidate the role of mGluRs in various cancers suggests that it might be a viable strategy to therapeutically target glutamatergic signaling.

Phosphorylation of thymidylate synthase affects slow-binding inhibition by 5-fluoro-dUMP and N(4)-hydroxy-dCMP

Endogenous thymidylate synthases, isolated from tissues or cultured cells of the same specific origin, have been reported to show differing slow-binding inhibition patterns. These were reflected by biphasic or linear dependence of the inactivation rate on time and accompanied by differing inhibition parameters. Considering its importance for chemotherapeutic drug resistance, the possible effect of thymidylate synthase inhibition by post-translational modification was tested, e.g. phosphorylation, by comparing sensitivities to inhibition by two slow-binding inhibitors, 5-fluoro-dUMP and N(4)-hydroxy-dCMP, of two fractions of purified recombinant mouse enzyme preparations, phosphorylated and non-phosphorylated, separated by metal oxide/hydroxide affinity chromatography on Al(OH)3 beads. The modification, found to concern histidine residues and influence kinetic properties by lowering Vmax, altered both the pattern of dependence of the inactivation rate on time from linear to biphasic, as well as slow-binding inhibition parameters, with each inhibitor studied. Being present on only one subunit of at least a great majority of phosphorylated enzyme molecules, it probably introduced dimer asymmetry, causing the altered time dependence of the inactivation rate pattern (biphasic with the phosphorylated enzyme) and resulting in asymmetric binding of each inhibitor studied. The latter is reflected by the ternary complexes, stable under denaturing conditions, formed by only the non-phosphorylated subunit of the phosphorylated enzyme with each of the two inhibitors and N(5,10)-methylenetetrahydrofolate. Inhibition of the phosphorylated enzyme by N(4)-hydroxy-dCMP was found to be strongly dependent on [Mg(2+)], cations demonstrated previously to also influence the activity of endogenous mouse TS isolated from tumour cells.

Inside the biochemical pathways of thymidylate synthase perturbed by anticancer drugs: Novel strategies to overcome cancer chemoresistance

Our current understanding of the mechanisms of action of antitumor agents and the precise mechanisms underlying drug resistance is that these two processes are directly linked. Moreover, it is often possible to delineate chemoresistance mechanisms based on the specific mechanism of action of a given anticancer drug. A more holistic approach to the chemoresistance problem suggests that entire metabolic pathways, rather than single enzyme targets may better explain and educate us about the complexity of the cellular responses upon cytotoxic drug administration. Drugs, which target thymidylate synthase and folate-dependent enzymes, represent an important therapeutic arm in the treatment of various human malignancies. However, prolonged patient treatment often provokes drug resistance phenomena that render the chemotherapeutic treatment highly ineffective. Hence, strategies to overcome drug resistance are primarily designed to achieve either enhanced intracellular drug accumulation, to avoid the upregulation of folate-dependent enzymes, and to circumvent the impairment of DNA repair enzymes which are also responsible for cross-resistance to various anticancer drugs. The current clinical practice based on drug combination therapeutic regimens represents the most effective approach to counteract drug resistance. In the current paper, we review the molecular aspects of the activity of TS-targeting drugs and describe how such mechanisms are related to the emergence of clinical drug resistance. We also discuss the current possibilities to overcome drug resistance by using a molecular mechanistic approach based on medicinal chemistry methods focusing on rational structural modifications of novel antitumor agents. This paper also focuses on the importance of the modulation of metabolic pathways upon drug administration, their analysis and the assessment of their putative roles in the networks involved using a meta-analysis approach. The present review describes the main pathways that are modulated by TS-targeting anticancer drugs starting from the description of the normal functioning of the folate metabolic pathway, through the protein modulation occurring upon drug delivery to cultured tumor cells as well as cancer patients, finally describing how the pathways are modulated by drug resistance development. The data collected are then analyzed using network/netwire connecting methods in order to provide a wider view of the pathways involved and of the importance of such information in identifying additional proteins that could serve as novel druggable targets for efficacious cancer therapy.

Analysis of the thymidylate synthase gene structure in colorectal cancer patients and its possible relation with the 5-Fluorouracil drug response

Thymidylate synthase (TS) catalyzes methylation of dUMP to dTMP and it is the target for the 5-Fluorouracil (5-FU) activity. Barbour et al. showed that variant structural forms of TS in tumour cell lines confer resistance to fluoropyrimidines. We planned to perform the whole TS gene structure by means of sequencing techniques in human colorectal cancer (CRC) samples to try to identify the presence of any possible TS variant form that could be responsible of fluoropyrimidines drug resistance and of the worse prognosis. We performed the TS-DNA gene sequence in 68 CRC from patients of A, B, and C Dukes' stages and different histological grade, but we did not find any mutation in the TS-DNA structure. In the future we intend to widen the TS structure analysis to the metastatic CRCs, because due to their higher genomic instability, they could present a TS variant form responsible of the fluoropyrimidines drug resistance and the worse prognosis.

Overview of resistance to systemic therapy in patients with breast cancer

Breast cancer is the most common cancer and the second leading cause of cancer death in American women. It was the second most common cancer in the world in 2002, with more than 1 million new cases. Despite advances in early detection and the understanding of the molecular bases of breast cancer biology, about 30% of patients with early-stage breast cancer have recurrent disease. To offer more effective and less toxic treatment, selecting therapies requires considering the patient and the clinical and molecular characteristics of the tumor. Systemic treatment of breast cancer includes cytotoxic, hormonal, and immunotherapeutic agents. These medications are used in the adjuvant, neoadjuvant, and metastatic settings. In general, systemic agents are active at the beginning of therapy in 90% of primary breast cancers and 50% of metastases. However, after a variable period of time, progression occurs. At that point, resistance to therapy is not only common but expected. Herein we review general mechanisms of drug resistance, including multidrug resistance by P-glycoprotein and the multidrug resistance protein family in association with specific agents and their metabolism, emergence of refractory tumors associated with multiple resistance mechanisms, and resistance factors unique to host-tumor-drug interactions. Important anticancer agents specific to breast cancer are described. Breast cancer is the most common type of cancer and the second leading cause of cancer death in American women. In 2002, 209,995 new cases of breast cancer were registered, and 42,913 patients died of it. In 5 years, the annual prevalence of breast cancer will reach 968,731 cases in the United States. World wide, the problem is just as significant, as breast cancer is the most frequent cancer after nonmelanoma skin cancer, with more than 1 million new cases in 2002 and an expected annual prevalence of more than 4.4 million in 5 years. Breast cancer treatment currently requires the joint efforts of a multidisciplinary team. The alternatives for treatment are constantly expanding. With the use of new effective chemotherapy, hormone therapy, and biological agents and with information regarding more effective ways to integrate systemic therapy, surgery, and radiation therapy, elaborating an appropriate treatment plan is becoming more complex. Developing such a plan should be based on knowledge of the benefits and potential acute and late toxic effects of each of the therapy regimens. Despite advances in early detection and understanding of the molecular bases of breast cancer biology, approximately 30% of all patients with early-stage breast cancer have recurrent disease, which is metastatic in most cases. The rates of local and systemic recurrence vary within different series, but in general, distant recurrences are dominant, strengthening the hypothesis that breast cancer is a systemic disease from presentation. On the other hand, local recurrence may signal a posterior systemic relapse in a considerable number of patients within 2 to 5 years after completion of treatment. To offer better treatment with increased efficacy and low toxicity, selecting therapies based on the patient and the clinical and molecular characteristics of the tumor is necessary. Consideration of these factors should be incorporated in clinical practice after appropriate validation studies are performed to avoid confounding results, making them true prognostic and predictive factors. A prognostic factor is a measurable clinical or biological characteristic associated with a disease-free or overall survival period in the absence of adjuvant therapy, whereas a predictive factor is any measurable characteristic associated with a response or lack of a response to a specific treatment. The main prognostic factors associated with breast cancer are the number of lymph nodes involved, tumor size, histological grade, and hormone receptor status, the first two of which are the basis for the AJCC staging system. The sixth edition of the American Joint Committee on Cancer staging system allows better prediction of prognosis by stage. However, after determining the stage, histological grade, and hormone receptor status, the tumor can behave in an unexpected manner, and the prognosis can vary. Other prognostic and predictive factors have been studied in an effort to explain this phenomenon, some of which are more relevant than others: HER-2/neu gene amplification and protein expression, expression of other members of the epithelial growth factor receptor family, S phase fraction, DNA ploidy, p53 gene mutations, cyclin E, p27 dysregulation, the presence of tumor cells in the circulation or bone marrow, and perineural and lymphovascular space invasion. Systemic treatment of breast cancer includes the use of cytotoxic, hormonal, and immunotherapeutic agents. All of these agents are used in the adjuvant, neoadjuvant, and metastatic setting. Adjuvant systemic therapy is used in patients after they undergo primary surgical resection of their breast tumor and axillary nodes and who have a significant risk of systemic recurrence. Multiple studies have demonstrated that adjuvant therapy for early-stage breast cancer produces a 23% or greater improvement in disease-free survival and a 15% or greater increase in overall survival rates. Recommendations for the use of adjuvant therapy are based on the individual patient's risk and the balance between absolute benefit and toxicity. Anthracycline-based regimens are preferred, and the addition of taxanes increases the survival rate in patients with lymph node-positive disease. Adjuvant hormone therapy accounts for almost two thirds of the benefit of adjuvant therapy overall in patients with hormone-receptor-positive breast cancer. Tamoxifen is considered the standard of care in premenopausal patients. In comparison, the aromatase inhibitor anastrozole has been proven to be superior to tamoxifen in postmenopausal patients with early-stage breast cancer. The adjuvant use of monoclonal antibodies and targeted therapies other than hormone therapy is being studied. Interestingly, some patients have an early recurrence even though they have a tumor with good prognostic features and at a favorable stage. These recurrences have been explained by the existence of certain cellular characteristics at the molecular level that make the tumor cells resistant to therapy. Selection of resistant cell clones of micrometastatic disease has also been proposed as an explanation for these events. Neoadjuvant systemic therapy, which is the standard of care for patients with locally advanced and inflammatory breast cancer, is becoming more popular. It reduces the tumor volume, thus increasing the possibility of breast conservation, and at the same time allows identification of in vivo tumor sensitivity to different agents. The pathological response to neoadj uvant systemic therapy in the breast and lymph nodes correlates with patient survival. Use of this treatment modality produces survival rates identical to those obtained with the standard adjuvant approach. The rates of pathological complete response (pCR) to neoadjuvant systemic therapy vary according to the regimen used, ranging from 6% to 15% with anthracycline-based regimens to almost 30% with the addition of a noncross-resistant agent such as a taxane. In one study, the addition of neoadjuvant trastuzumab in patients with HER-2-positive breast tumors increased the pCR rate to 65%. Primary hormone therapy has also been used in the neoadjuvant systemic setting. Although the pCR rates with this therapy are low, it significantly increases breast conservation. Currently, neoadjuvant systemic therapy is an important tool in not only assessing tumor response to an agent but also studying the mechanisms of action of the agent and its effects at the cellular level. However, no tumor response is observed in some cases despite the use of appropriate therapy. The tumor continues growing during treatment in such cases, a phenomenon called primary resistance to therapy. The use of palliative systemic therapy for metastatic breast cancer is challenging. Five percent of newly diagnosed cases of breast cancer are metastatic, and 30% of treated patients have a systemic recurrence. Once metastatic disease develops, the possibility of a cure is very limited or practically nonexistent. In this heterogeneous group of patients, the 5-year survival rate is 20%, and the median survival duration varies from 12 to 24 months. In this setting, breast cancer has multiple clinical presentations, and the therapy for it should be chosen according to the patient's tumor characteristics, previous treatment, and performance status with the goal of improving survival without compromising quality of life. Treatment resistance is most commonly seen in such patients. They initially may have a response to different agents, but the responses are not sustained, and, in general, the rates of response to subsequent agents are lower. Table 1 summarizes metastatic breast cancer response rates to single-agent systemic therapy.

Down-regulation of mitochondrial F1F0-ATP synthase in human colon cancer cells with induced 5-fluorouracil resistance

5-Fluorouracil (5-FU) is widely used for treatment of advanced colorectal cancer. However, it is common for such patients to develop resistance to 5-FU, and this drug resistance becomes a critical problem for chemotherapy. The mechanisms underlying this resistance are largely unknown. To screen for proteins possibly responsible for 5-FU resistance, cells resistant to 5-FU were derived from human colon cancer cell lines and two-dimensional gel electrophoresis-based comparative proteomics was done. Two-dimensional gel electrophoresis data showed there was lower expression of the alpha subunit of mitochondrial F(1)F(0)-ATP synthase (ATP synthase) in 5-FU-resistant cells compared with parent cells. Western blotting showed that expression of other ATP synthase complex subunits was also lower in 5-FU-resistant cell lines and that these resistant cells also showed decreased ATP synthase activity and reduced intracellular ATP content. The ATP synthase inhibitor, oligomycin A, strongly antagonized 5-FU-induced suppression of cell proliferation. When 5-FU sensitivity was compared with ATP synthase activity in six different human colon cancer cell lines, a positive correlation has been found. Furthermore, suppressed ATP synthase d-subunit expression by siRNA transfection increased cell viability in the presence of 5-FU. Bioenergetic dysfunction of mitochondria has been reported as a hallmark of many types of cancers (i.e., down-regulation of ATP synthase beta-subunit expression in liver, kidney, colon, squamous oesophageal, and lung carcinomas, as well as in breast and gastric adenocarcinomas). Our findings show that ATP synthase down-regulation may not only be a bioenergetic signature of colorectal carcinomas but may also lead to cellular events responsible for 5-FU resistance.

Metabotropic glutamate receptor 4-mediated 5-Fluorouracil resistance in a human colon cancer cell line

PURPOSE

5-Fluorouracil (5-FU) has been the mainstay treatment for colorectal cancer for the past few decades. However, as with other cancers, development of 5-FU resistance has been a major obstacle in colorectal cancer chemotherapy. The purpose of this study was to gain further understanding of the mechanisms underlying 5-FU resistance in colorectal cancer cells.

EXPERIMENTAL DESIGN

A 5-FU-resistant cell line was established from the human colon cancer cell line SNU-769A. Protein extracts from these two cell lines (parent and resistant) were analyzed using comparative proteomics to identify differentially expressed proteins.

RESULTS

5-FU-resistant human colon cancer cells were found to overexpress metabotropic glutamate receptor 4 (mGluR4). Other experiments showed cellular resistance to 5-FU (i.e., cell survival) was altered by the mGluR4 agonist l-2-amino-4-phosphonobutyric acid (L-AP 4), and by the mGluR4 antagonist (S)-amino-2-methyl-4-phosphonobutanoic acid (MAP 4), in that L-AP 4 increased 5-FU resistance in SNU-769A cells, whereas MAP 4 ablated 5-FU resistance in 5-FU-resistant cells. However, there was no significant effect of L-AP 4 or MAP 4 on basal cAMP and thymidylate synthase levels. Interestingly, 5-FU down-regulated mGluR4 expression, and MAP 4 suppressed proliferation in both cell lines.

CONCLUSIONS

We here report mGluR4 expression in human colon cancer cell line, which provides further evidence for extra-central nervous system expression of glutamate receptors. Overexpression of mGluR4 may tentatively be responsible for 5-FU resistance and, although activation by agonist promotes cell survival in the presence of 5-FU, decreased mGluR4 expression or inactivation by antagonist contributes to cell death.

Induction of thymidylate synthase as a 5-fluorouracil resistance mechanism

Thymidylate synthase (TS) is a key enzyme in the de novo synthesis of 2'-deoxythymidine-5'-monophosphate (dTMP) from 2'-deoxyuridine-5'-monophosphate (dUMP), for which 5,10-methylene-tetrahydrofolate (CH(2)-THF) is the methyl donor. TS is an important target for chemotherapy; it is inhibited by folate and nucleotide analogs, such as by 5-fluoro-dUMP (FdUMP), the active metabolite of 5-fluorouracil (5FU). FdUMP forms a relatively stable ternary complex with TS and CH(2)THF, which is further stabilized by leucovorin (LV). 5FU treatment can induce TS expression, which might bypass dTMP depletion. An improved efficacy of 5FU might be achieved by increasing and prolonging TS inhibition, a prevention of dissociation of the ternary complex, and prevention of TS induction. In a panel of 17 colon cancer cells, including several variants with acquired resistance to 5FU, sensitivity was related to TS levels, but exclusion of the resistant variants abolished this relation. For antifolates, polyglutamylation was more important than the intrinsic TS level. Cells with low p53 levels were more sensitive to 5FU and the antifolate raltitrexed (RTX) than cells with high, mutated p53. Free TS protein down-regulates its own translation, but its transcription is regulated by E2F, a cell cycle checkpoint regulator. Together, this results in low TS levels in stationary phase cells. Although cells with a low TS might theoretically be more sensitive to 5FU, the low proliferation rate prevents induction of DNA damage and 5FU toxicity. TS levels were not related to polymorphisms of the TS promoter. Treatment with 5FU or RTX rapidly induced TS levels two- to five-fold. In animal models, 5FU treatment resulted in TS inhibition followed by a two- to three-fold TS induction. Both LV and a high dose of 5FU not only enhanced TS inhibition, but also prevented TS induction and increased the antitumor effect. In patients, TS levels as determined by enzyme activity assays, immunohistochemistry and mRNA expression, were related to a response to 5FU. 5FU treatment initially decreased TS levels, but this was followed by an induction, as seen with an increased ratio of TS protein over TS-mRNA. The clear retrospective relation between TS levels and response now forms the basis for a prospective study, in which TS levels are measured before treatment in order to determine the treatment protocol.

Reduced cellular transport and activation of fluoropyrimidine nucleosides and resistance in human lymphocytic cell lines selected for arabinosylcytosine resistance

Arabinofuranosylcytosine (araC) resistant H9-araC0.05 and H9-araC0.5 sublines were obtained following in vitro exposure of H9 cells to 0. 05 and 0.5 microM araC, respectively. These cell lines were 83.3- and 266.7-fold, 21- and 80-fold, and 2.4- and 4.0-fold more resistant to 5-fluorouridine (FUR), 5-fluoro-2'-deoxyuridine (FdUR), and 5-fluorouracil (FU), respectively. Compared with H9 cells, the cellular accumulation of FUR was 2.2 and 0.2%, FdUR 15.6 and 0.9%, and FU 56.9 and 66.5% in H9-araC0.05 and H9-araC0.5 cells, respectively. An araC resistant HL60 cell line (promyelocytic cell line) was 5.0- and 1.7-fold resistant to FUR and FdUR, respectively, but displayed no resistance to FU. The lower FUR and FdUR nucleotide levels in the resistant cells were a result of reduced cellular transport and uridine kinase (UR kinase) and thymidine kinase (TK) activities. Compared with the parental cell line, the p-nitrobenzyl thioinosine (an inhibitor of nucleoside transport) binding sites also were lower in the araC resistant cells. There was no difference in the expression of multidrug-resistant protein and thymidylate synthase mRNA in the parental and the resistant cell lines. Data presented here suggest that araC exposure of H9 cells, in addition to araC resistance, induced/selected cells that were resistant to FUR and FdUR. These cells had altered cellular drug transport and lower TK and UR kinase activities. Further studies to understand molecular mechanisms of this phenomenon are warranted.

Isolation and characterization of a thymidylate synthase-deficient human colon tumor cell line

Following mutagenesis of the human colorectal tumor cell line HCT C with ethyl methanesulfonate, clonal sublines were isolated that survived on medium toxic to cells expressing thymidylate synthase (TS). The subline exhibiting the lowest TS activity, designated as C18, was characterized. Extracts from C18 cells were mixed with extracts from parental C cells to determine whether the TS-deficient phenotype is trans-acting. No effect was observed on the activity of TS in parental extracts. The levels of functional TS in C18 cells were analyzed by the binding of the mechanism-based inhibitor 5-fluoro-2'-deoxyuridylate (FdUMP) under conditions that allowed for the detection of 10 fmol of TS. Only a low level of FdUMP-TS complexes was detected in C18 extracts. The level of TS expression in C18 cells was similar to that in parental C cells, as indicated by immunoblot and RNA analyses. DNA sequence analysis of TS cDNA from C18 cells revealed the existence of a point mutation (C-->T) at nucleotide 647 that predicts the replacement of Ser216 by a leucine residue. That the C18 cell line was homozygous for this mutation was indicated by restriction fragment-length polymorphism analysis and by primer extension analysis. To provide additional evidence that substitution of Ser216 by a leucine residue created a defective protein, a TS-deficient bacterial strain was transformed with an expression vector containing the mutated human TS cDNA. The transformed strain exhibited thymidine auxotrophy, indicating that the mutant TS (Leu216) is nonfunctional.

Collateral resistance of a dideoxycytidine-resistant cell line to 5-fluoro-2'-deoxyuridine

Exposure of a human lymphocytic cell line, H9 cells, to 0.5 microM and 5.0 microM dideoxycytidine (ddC) resulted in isolation of ddC-resistant H9-ddC0.5w and H9-ddC5.0w cell lines. In addition, these cell lines were also resistant to azidothymidine and had reduced deoxycytidine kinase and thymidine kinase activities. We now show that these cell lines are 4-fold and 2000-fold collaterally resistant to 5-fluoro-2'-deoxyuridine (FdUR), respectively, but not to 5-fluorouracil (FU). Biochemical evaluations show that, compared to the parental cells, the FdUR phosphorylation was reduced to 36.3% and 9.2% and the FdUMP levels were decreased to 48.1% and 1.2% in these cell lines. Taken together, the data suggest that ddC, an antiviral agent, is capable of inducing resistance to FdUR-a drug that is not its analog and which has a different metabolism, target site, and mechanism of action.

Random sequence mutagenesis and resistance to 5-fluorouridine in human thymidylate synthases

Thymidylate synthase (TS) catalyzes the methylation of dUMP to dTMP and is the target for the widely used chemotherapeutic agent 5-fluorouracil. We used random sequence mutagenesis to replace 13 codons within the active site of TS and obtain variants that are resistant to 5-fluorodeoxyuridine (5-FdUR). The resulting random library was selected for its ability to complement a TS-deficient Escherichia coli strain, and sequence analysis of survivors found multiple substitutions to be tolerable within the targeted region. An independent selection of the library was carried out in the presence of 5-FdUR, resulting in a more limited spectrum of mutations. One specific mutation, C199L, was observed in more than 46% of 5-FdUR-resistant clones. A 5-FdUR-resistant triple mutant, A197V/L198I/C199F, was purified to apparent homogeneity. Kinetic studies with the substrate dUMP indicate that this mutant is similar to the wild type in regards to kcat and Km values for dUMP and the cosubstrate CH2H4-folate. In contrast, equilibrium binding studies with the inhibitor, FdUMP, demonstrate that the dissociation constant (Kd) for FdUMP binding into the ternary complex was 20-fold higher than values obtained for the wild-type enzyme. This 5-FdUMP-resistant mutant, or others similarly selected, is a candidate for use in gene therapy to render susceptible normal cells resistant to the toxic effects of systemic 5-fluorouracil.

Mechanisms of acquired resistance to modulation of 5-fluorouracil by leucovorin in HCT-8 human ileocecal carcinoma cells

Repeated (10x) exposure of HCT-8 human ileocecal carcinoma cells to 5-fluorouracil (5-FU) for 2 or 72 hr, both incubations in the continuous presence of 20 microM leucovorin (LV), yielded two stable modulation-resistant sublines, FL2h and FL72h. Although LV potentiated growth inhibition by 5-FU 2-fold in parental HCT-8 cells, it did not potentiate the effect of 5-FU in the FL2h or FL72h sublines. LV modulation of 5-fluorodeoxyuridine (5-FdUrd) was also reduced (FL72h) or eliminated (FL2h). In the FL2h and FL72h sublines, the level of thymidylate synthase (TS) protein and TS activity in cell extracts, TS activity in situ, the rate of cellular uptake and metabolism of LV, and the level of 5-FU incorporation into total cellular RNA were similar to those in parental HCT-8 cells. However, LV significantly (P < 0.01) potentiated the inhibition of TS activity in situ in HCT-8 cells at 24 hr after a 2-hr treatment with either 5-FU or 5-FdUrd, but had no such activity in the FL2h and FL72h sublines (P > 0.1). Resistance to modulation of 5-FU by LV was associated with the inability of LV to increase the formation of intracellular TS-FdUMP-methylenetetrahydrofolate ternary complexes, and these complexes dissociated more rapidly (T1/2 > 1.5- to 3-fold faster) in the presence of different concentrations of 5,10-methylenetetrahydropteroylpentaglutamate. Thus, decreased stability of ternary complexes appears to be the mechanism of acquired resistance to the LV modulation of fluoropyrimidine cytotoxicity, possibly due to mutation(s) of TS in these two modulation-resistant HCT-8 sublines.

Critical factors for optimizing the 5-fluorouracil-folinic acid association in cancer chemotherapy

BACKGROUND

The 5-fluorouracil (FU)-folinic acid (FA) association has demonstrated clinical efficacy in colorectal cancer, both in adjuvant and metastatic situations. However, there is no clear consensus about the optimal FU-FA schedule and dose. In addition, it would be of interest to identify FU-FA-responsive tumors.

DESIGN

Our purpose was to review preclinical and clinical data dealing with prediction of FU-FA sensitivity and optimization of FU-FA schedules.

RESULTS

Preclinical studies have highlighted the importance of thymidylate synthase (TS), the cellular target of the FU-FA mechanism of action, for predicting FU sensitivity. It appears that the more sensitive cell lines express the lowest TS activity. Interestingly, the cell lines sensitive to FA supplementation are those more sensitive to FU. The role of TS in FU-FA responsiveness has been clearly demonstrated in patients with colorectal and gastric cancers. Preliminary in vitro and clinical data have shown that the folylpolyglutamate synthetase (FPGS), the enzyme responsible for folate polyglutamylation, is another promising tool for identifying FU-FA-responsive tumors. So far, results of clinical trials do not form a clear consensus regarding the need to administer high FA doses for improving FU-FA treatment. Experimental studies on human cancer cell lines have demonstrated the wide variability among cell lines, ranging from 0.05 to 200 microns, of 1 FA concentrations required for maximal FU potentiation. In addition, pharmacokinetic studies have reported a significant variability of active folates in plasma after administration of standard-dose FA. Altogether, these observations favour high-dose FA administration to achieve high folate concentrations in plasma and thus to counteract the variability of the 1 FA concentrations required. With respect to the choice of FU-FA schedule, it appears from experimental data that increasing the duration of exposure to FA enhances FU-FA cytotoxicity, probably through an increased formation of reduced folate polyglutamate forms. Considering the S-phase specificity of FU cytotoxicity as well as its rapid elimination from plasma, a schedule of prolonged exposure to both FU and FA should be considered preferable.

CONCLUSIONS

Results of the new FU-FA administration schedules such as the one consisting of a 2-hour FA administration followed by a combination of FU bolus and FU infusion, or the chronomodulated FU-FA infusion, open up promising approaches for improving the therapeutic index of FU-FA chemotherapy. Finally, future clinical studies should investigate tumoral parameters pharmacologically linked to FU-FA sensitivity such as pre-treatment TS and FPGS activities. Such tumoral investigations along with FU and FA pharmacokinetic investigations should provide a better understanding of inter-patient variability in response to FU-FA therapy and an optimal management of this chemotherapy regimen.

Reduced activity of anabolizing enzymes in 5-fluorouracil-resistant human stomach cancer cells

The mechanism of resistance to 5-fluorourcil (5-FU) was studied with NUGC-3/5FU/L, a human stomach cancer cell line which had acquired resistance as a consequence of repeated 5-day exposures to stepwise-increasing concentrations of 5-FU in vitro. NUGC-3/5FU/L was 200-fold and over 16-fold resistant to 96-h and 1-h exposures to 5-FU, respectively. NUGC-3/5FU/L incorporated less 5-FU into RNA, indicating resistance to the RNA-directed action of 5-FU. On the other hand, NUGC-3/5/5FU/L also showed resistance to in situ thymidylate synthase (TS) inhibition by 5-FU. Polymerase chain reaction-single-strand conformation polymorphism analysis of TS cDNA and a FdUMP ligand binding assay showed that quantitative and qualitative alterations of TS are not responsible for this resistance. In contrast, the ability to metabolize 5-FU to its active metabolites, FUTP and FdUMP, was reduced in NUGC-3/5FU/L. We found that not only the activities of uridine phosphorylase/kinase and orotate phosphoribosyl-transferase (OPRT), but also the level of phosphoribosyl pyrophosphate, a cosubstrate for OPRT, were significantly lower in NUGC-3/5FU/L than in the parent NUGC-3. These results indicated that resistance to 5-FU in NUGC-3/5FU/L is due to reduced activities of 5-FU-anabolizing enzymes, but not to an alteration of TS. 2'-Deoxyinosine effectively enhanced TS inhibition by 5-FU in the resistant cells, thus markedly sensitizing them to 5-FU.

Modulation of target enzyme associated with the action of antifolates

The cytotoxicity and molecular effects of antifolate thymidylate synthase inhibitor, ICI-D1694, against human ileocecal carcinoma, were evaluated. The drug concentration for 50% inhibition of cell growth by ICI-D1694 is 73 nM and 3 nM following 2 hr and 72 hr exposure, respectively. The drug induces high level of DNA single strand breaks in a time dependent manner, but subsequent to maximum inhibition of thymidylate synthase. Drug effects can be reversed by thymidine and leucovorin at > 1 microM concentrations. Leucovorin action is primarily at the cell membrane level, competing with the transport and activation of ICI-D1694. Thymidine, however, exerts its competitive effect primarily at the level of thymidylate synthase.

Isolation and characterization of a human ileocecal carcinoma cell line (HCT-8) subclone resistant to fluorodeoxyuridine

A 5-fluoro-2'-deoxyuridine (FdUrd)-resistant subclone (Fd9XR) of HCT-8 (human ileocecal carcinoma) cells was established by two schedules of drug exposure. Initially, cells were exposed to short-term (3 hr) 100 nM FdUrd repeatedly (9 cycles over 8 months), and cells were then exposed to 10 nM FdUrd continuously. During this latter stage, a colony (Fd9XR) with fast growth rate was isolated, expanded, and characterized with respect to mechanisms of resistance to FdUrd and cross-resistance to other chemotherapeutic agents. Fd9XR cells were 1000-fold resistant to FdURD, but 3-fold more sensitive to 5-fluorouracil (FUra) than HCT-8 cells. After a 3-hr treatment with FdUrd, Fd9XR cells accumulated 6630-, 69-, and 3.7-fold less fluorodeoxyuridylate (FdUMP), fluorouridine triphosphate (FUTP) and acid-insoluble materials, respectively, than HCT-8 cells. However, when FUra was substituted for FdUrd, Fd9XR cells accumulated 9.2-, 3.1-, and 2.3-fold more FdUMP, FUTP and acid-insoluble materials, respectively, than HCT-8 cells. Fd9XR and HCT-8 were similar in their growth rates, combined pools of 5,10-methylenetetrahydrofolates (5,10-CH2H4PteGlun) and tetrahydrofolates (H4PTeGlun), thymidine phosphorylase (TP) activity, and level and activity of thymidylate synthase (TS). In contrast, thymidine kinase (TK) activity of Fd9XR was 0.23 and 0.35% of that of HCT-8, for thymidine (dThd) and FdUrd as substrates, respectively. Furthermore, Fd9XR cells exhibited greater sensitivity to the antifolate TS inhibitor ICI D1694 and to methotrexate (MTX) than HCT-8 cells. In addition, dThd alone and in combination with hypoxanthine did not offer any protection against the cytotoxic effect of ICI D1694 in Fd9XR cells. These results indicate that in Fd9XR cells (1) TK deficiency is the primary mechanism of resistance to FdUrd; (2) the greater sensitivity to FUra was associated with higher pools of FdUMP and FUTP with a subsequently higher level of incorporation into cellular RNA; and (3) antifolate compounds, e.g. ICI D1694 and MTX, could be useful agents in the treatment of FdUrd-resistant tumors associated with decreased TK activity and decreased capacity of utilizing dThd.

Genetic variation in thymidylate synthase confers resistance to 5-fluorodeoxyuridine

The human colorectal tumor cell line HCT 116 was resident to the cytotoxic effects of 5-fluorodeoxyuridine (FdUrd). The response to FdUrd was increased only slightly by the presence of 10 microM folinic acid (CF). HCT 116 formed FdUMP and CH2H4PteGlu polyglutamates after exposure to FdUrd and CF. The sensitivity to FdUrd correlated well with the extent of TS inhibition. The role of TS in the resistance of the cells to FdUrd was examined. HCT 116 expresses two TS enzymes, which differ in pI. The more basic TS has been detected in only HCT 116 cells. The other TS is identical in pI to the enzymes detected in other human cells. The variant TS differs from the common by His replacement of Tyr at residue 33. The variant TS exhibited a 3-fold lower affinity for FdUMP than the common TS. The enzymes co-expressed in HCT 116 exhibited an FdUMP binding constant similar to that of the variant TS. TS-deficient cells were transfected with cDNAs encoding the two TS polypeptides. Transfectants expressing the variant TS were more resistant to FdUrd cytotoxicity than cells expressing the common TS. Thus, the structural variation in TS reduced enzyme affinity for FdUMP and conferred resistance to FdUrd.

In vitro enhancement of fluoropyrimidine-induced cytotoxicity by leucovorin in colorectal and gastric carcinoma cell lines but not in non-small-cell lung carcinoma cell lines

Leucovorin (LV) increases the cytotoxic effect of fluorouracil (FUra) and 5-fluoro-2'-deoxyuridine (FdUrd) by enhancing the formation of the fluorodeoxyuridine monophosphate (FdUMP) thymidylate synthase (TS) 5,10-methylenetetrahydrofolate (mTHF) ternary complex. To study the difference in the efficacy of this combination against different tumors, we compared the effect of LV (20 microM) on the cytotoxicity of FUra, FdUrd, and 5-fluorouridine (FUrd) in vitro against cell lines of five colorectal carcinomas (CC), five gastric carcinomas (GC), and four non-small-cell lung carcinomas (NSCLC) using the colony-forming assay. At the concentration used in the experiments, LV alone failed to inhibit colony formation in any of the cell lines tested. The NSCLC cell lines were more resistant to FdUrd than were the CC and GC lines. LV modulated the cytotoxicity of FdUrd in all five CC lines and in three of the five GC lines but failed to do so in any of the NSCLC lines. In addition, following 20 h treatment with 1 microM [3H]-FdUrd, formation of the FdUMP/TS/mTHF ternary complex was enhanced by LV in the LV-sensitized CC and GC cell lines but not in the LV-refractory NSCLC lines. These in vitro data corresponded well to the results of clinical trials. Therefore, the colony-forming assay may be useful for the identification of the sensitivity of tumors according to phenotype.

Clinical relevance of biochemical modulation of 5-fluorouracil

Biochemical modulation is a special type of combination chemotherapy which aims to selectively improve the therapeutic index by increasing the antitumor effect and protecting against toxic side effects. Biochemical modulation seems to be an attractive way to circumvent quantitative and qualitative heterogeneity of tumors. In the past decade a number of biochemical modulation approaches have been tested to improve the activity of 5-fluorouracil (5FU). 5FU itself has only modest anticancer activity but has been shown to be a very attractive target for biochemical modulation. A number of the combinations have been ineffective in the clinic despite extensive testing in a number of schedules. Some other combinations were initially tested in an inappropriate schedule, but were active when applied in another schedule. The latter was made possible by a systematic preclinical development of combinations with a proper translation to the clinic accompanied by pharmacodynamic evaluation. This review describes a number of biochemical modulation combinations, both inactive and active. The main conclusion is that properly applied biochemical modulation schedules may lead to successful use in the clinic.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

Thymidylate synthase from untreated human colorectal cancer and colonic mucosa: enzyme activity and inhibition by 5-fluoro-2'-deoxy-uridine-5'-monophosphate

Inhibition of thymidylate synthase (TS) by the 5-fluorouracil (5-FU) metabolite FdUMP is considered to be the main mechanism of action of 5-FU. TS from colorectal tumours and normal colon mucosa from 10 untreated patients was studied. There was a large variation in the activity of tumour TS both at 1 and 10 mumol/l of its substrate dUMP; in normal mucosa this variation was less. Inhibition by 10 nmol/l FdUMP in tumours varied from 80 to 90% at 1 mumol/l dUMP; in normal mucosa, inhibition varied from 10 to 80%. The number of FdUMP binding sites ranged from 0.1 to 1 in tumours but such binding sites were not detectable in normal mucosa. The ratio between TS activity and FdUMP binding sites varied considerably in tumours but not in normal mucosa. The deviations from normal kinetics may represent a mutant TS form. Alterations in TS may partly account for differences in response to 5-FU.

Thymidylate synthase inhibition as a predictor of tumor sensitivity of fluorinated pyrimidines

The thymidylate synthase (TS) content of tumor tissue was assayed and the levels of 5-fluorouracil (5-Fu) in serum and tumor tissue were determined in 21 patients with cervical cancer who were treated with UFT, a fluorinated pyrimidine. Subrenal capsule assay, a predictive tumor sensitivity test, was conducted concurrently on cervical tumor samples. TS inhibition and serum and tissue 5-Fu levels widely varied between both samples. Investigation of the relationship between the tumor growth inhibition rate determined by subrenal capsule assay and the above parameters (TS inhibition, serum and tissue 5-Fu levels) showed that TS inhibition and tumor growth inhibition were well correlated (r = 0.73). This suggests that a TS inhibition assay in tumor tissue can be a useful predictive test for tumor sensitivity of fluorinated pyrimidines.

Purification and characterization of thymidylate synthetase in the liver of the mouse bearing Ehrlich ascites tumor

The activity of thymidylate synthetase in the liver of the ddY strain male mouse increased transitorily according to the increase in tumor cell number at maximum 7-9 days after ip transplantation of Ehrlich ascites tumor. The enzyme was able to be purified from the tumor host mouse liver or from the normal mouse liver in the same manner as from tumor cells using Affi-Gel blue and methotrexate-Sepharose 4B affinity column chromatography. The three enzyme preparations obtained were purified at 27,000-38,000-, and 8,000-fold, and yielded total activities of 11, 3, and 16% of these homogenates, respectively. These preparations were similar in molecular weight to the whole enzyme (67,000) and its subunit (34,000), optimum pH, and Km values either for deoxyuridine 5'-monophosphate or tetrahydrofolate in the presence of formaldehyde. Furthermore, the amount of 5-fluoro-2'-deoxyuridine 5'-monophosphate forming the ternary complex with the enzyme and tetrahydrofolate paralleled the enzyme activities in the cytosol fractions of the three tissues. The characteristics of the tumor host liver enzyme were similar to those of the proliferating tissues, the Ehrlich ascites tumor.

The role of thymidylate synthase in the response to fluoropyrimidine-folinic acid combinations

A panel of human colorectal tumor cell lines has been examined to determine the role of TS in the response to fluoropyrimidine antimetabolites. Among these cell lines, the response to FdUrd does not correlate with the levels of TS. In cell lines HCT 116 and RCA, which are poorly responsive to FdUrd, structural alterations in TS have been identified. In HCT 116, two TS polypeptides are present: a common form, occurring in all the cell lines and a variant form. The variant TS polypeptide has a reduced affinity for the TS ligands, FdUMP and CH2H4PteGlu, relative to the common TS polypeptide. Clonal populations of HCT 116 that overproduce each form have been isolated. Clones that overproduce the variant polypeptide are 4-fold less responsive to TS-directed cytotoxic agents than those that overproduce the common; thus, the presence of the variant TS is associated with a reduced response to TS-directed cytotoxic agents. The response of cell line RCA to FdUrd is dependent upon the extracellular CF concentration: response increases as CF is increased. RCA contains a TS enzyme with reduced affinity for CH2H4PteGlu, relative to cell line C, which is sensitive to FdUrd at all CF concentrations. Both cells form high chain-length polyglutamates of CH2H4PteGlu at CF concentrations in which the response to FdUrd differs by 4-fold. In RCA, the TS structural gene is variant, relative to the other cell lines. This variation may underlie the altered enzyme affinity for CH2H4PteGlu and the sensitivity to modulation of FdUrd response by CF.

Inhibition of thymidylate synthase after administration of doxifluridine in a transplantable colon carcinoma in the rat

Parameters for inhibition of thymidylate synthase (TS) in a DMH-induced transplantable rat colon carcinoma were studied after intraperitoneal administration of bolus doxifluridine (5'-dFUR) 200 mg/kg. Levels of 5'-dFUR, 5-fluorouracil (5-FU), fluorouridinediphosphate (FUDP), and fluorouridinetriphosphate (FUTP) were determined by use of high-performance liquid chromatography. Micromethods for analysis of 5-fluoro-2'-deoxyuridylate (FdUMP) and TS were used to study the in vivo intracellular pharmacokinetics of TS inhibition. Peak values of 5'-dFUR and 5-FU were found at 30 min and showed exponential declines with values close to zero at 5 hr. Substantial levels of FUDP and FUTP were found throughout the 24 h observation time. Peak FdUMP levels were modest compared to those observed after equimolar administration of 5-FU, but FdUMP persisted in amounts well above available binding sites on TS for the 24 h observation time. Reduction of free TS enzyme to undetectable levels (less than 0.05 pmol/g) lasted for 4 h, and at 24 h, there was still almost 70% enzyme inhibition. The total amount of TS (TStot) defined as free [3H]FdUMP-titrable enzyme (TSf) plus TS bound to FdUMP in a ternary complex (TSb) increased as a result of 5'-dFUR bolus injection from 15 to 50 pmol/g during the 24 hr observation time. We conclude from these data that 5'-dFUR is converted to 5-FU and subsequently to FdUMP, and the results suggest that 5'-dFUR exerts its cytotoxic effects through inhibition of TS and incorporation into RNA.

Relationship between 5-fluoro-2'-deoxyuridylate, 2'-deoxyuridylate, and thymidylate synthase activity subsequent to 5-fluorouracil administration, in xenografts of human colon adenocarcinomas

5-Fluorouracil (FUra) has been administered to mice bearing xenografts of human colon adenocarcinomas. In two tumor lines, HxGC3 and HxVRC5, intrinsically resistant to FUra, 2'-deoxyuridylate (dUMP) accumulated 13.4- and 23.9-fold above basal levels. In HxELC2 xenografts, which demonstrated some sensitivity to FUra, there was a decrease in dUMP concentration after drug administration. Maximal intratumor levels of 5-fluoro-2'-deoxyuridylate (FdUMP) were found at 1 hr, but decreased in all tumor lines by 4 hr after administration of FUra. Data derived in tumor cytosols suggested that FdUMP levels in situ were not rate-limiting for formation of covalent ternary complex, but that accumulation of dUMP would retard the rate of complex formation. Subsequent to administration of FUra, thymidylate synthase activity was reduced greater than 75% in all tumors, but it recovered rapidly in tumors resistant to FUra. In addition, the pretreatment level of activity of thymidylate synthase was 12.7-fold greater in HxVRC5 tumors than in HxELC2 tumors. This elevated activity in HxVRC5 tumors appears not to be a consequence of gene amplification. Formation of FdUMP or the accumulation of dUMP did not correlate with the activity of phosphatases measured at pH 5.8 or pH 9.2 in each tumor line. Further, inhibition of phosphatase activity did not alter, significantly, the net rate of dissociation of the FdUMP-thymidylate synthase-[6R]-CH2-H4PteGlu complex.

Binding of 5-fluorodeoxyuridylate to thymidylate synthase in human colon adenocarcinoma xenografts

The formation and stability of the covalent ternary complex formed between thymidylate synthase (E.C. 2.1.1.45), 5-fluoro 2'-deoxyuridylate (FdUMP) and 5,10-methylenetetrahydrofolate (CH2-H4PteGlu) has been examined in cytosols derived from xenografts of human colon adenocarcinomas. The rate of association (ka) for FdUMP was low being between 3.4 +/- 0.9 and 10.2 +/- 2.6 X 10(6) M-1 min-1, with the lowest ka value being determined in cytosols from a tumor (HxELC2) which has demonstrated some sensitivity to 5-fluoropyrimidines. Relative to reported ka values for human leukemic cells, the rate of association of FdUMP was 20- to 59-fold lower. This difference is not a consequence of FdUMP catabolism, or metabolism of CH2-H4PteGlu. In cytosols the apparent Km values for dUMP (3.6-4.2 microM) and and [6RS]- CH2-H4PteGlu (25-26.7 microM) were similar to reported values for human enzyme. Data derived from cytosols were similar to those derived using affinity purified enzyme from HxVRC5 colon adenocarcinoma xenografts. The net dissociation of [6-3H] FdUMP from the covalent ternary complex was 31-33 min in the absence of added CH2-H4PteGlu, and the rate of dissociation was dependent upon the concentration of cofactor. The concentration of [6RS]-CH2-H4PteGlu required to stabilize ternary complex derived from HxELC2 cytosols was slightly lower than that required for the same degree of stabilization of complex formed in cytosols from resistant tumors (HxGC3,HxVRC5). Addition of 5-CHO-H4PteGlu, 5-CH3-H4PteGlu, H2PteGlu, and PteGlu did not stabilize the covalent complex, but H4PteGlu substituted for CH2-H4PteGlu.

Purification and properties of mouse thymus thymidylate synthase. Comparison of the enzyme from mammalian normal and tumour tissues

Mouse thymus thymidylate synthase has been purified to apparent electrophoretic homogeneity and compared with the enzyme from mouse tumour L1210 and Ehrlich ascites carcinoma cells. The enzyme is a dimer composed of 35,000 mol. wt monomers. Mouse thymus and tumour enzymes exhibit allosteric properties reflected by cooperative binding of both dUMP and 5-fluoro-dUMP. Activation energy for the reaction, catalyzed by thymidylate synthase from mouse tumour but not from mouse thymus, lowers at temperatures above 34 degrees C, reflecting a change of rate-limiting step in dTMP formation. MgATP at millimolar concentrations inhibits mouse thymus enzyme.

Resistance of CCRF-CEM cloned sublines to 5-fluorodeoxyuridine associated with enhanced phosphatase activities

Resistance of human CCRF-CEM leukemic cells in tissue culture to 5-fluoro-2'-deoxyuridine (FdUrd) has been examined following a single drug exposure (FS sublines). In two FS sublines generated by soft agar cloning of FdUrd sensitive cells in the presence of 10 nM FdUrd, the level of drug resistance was maintained at 22- to 30-fold following 1 month growth in the absence of FdUrd. Characteristic of the FS sublines was a decreased accumulation and retention of free intracellular 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) averaging 3% of FdUrd sensitive cells, a more rapid rate of disappearance of free FdUMP and FdUMP-bound thymidylate synthase (EC 2.1.1.45, 5,10-methylenetetrahydrofolate:dUMP C-methyltransferase), and enhanced alkaline and acid phosphatase activities. There was no significant difference in the number of nucleoside transport sites per cell among the FS sublines and FdUrd-sensitive cells, indicating that the decreased accumulation of FdUMP in the resistant sublines was not the result of impaired FdUrd transport across the plasma membrane. The more rapid turnover of FdUMP-bound TMP synthase observed in the FS sublines was neither accompanied by a decreased stability of the TMP synthase-FdUMP-5,10-methylenetetrahydrofolate ternary complex, nor an enhanced rate of degradation of FdUrd to the less potent agent, 5-fluorouracil. In addition, the growth rates of the two FS sublines were similar to that of FdUrd sensitive cells in medium containing hypoxanthine, methotrexate, and thymidine, indicating that there was no depletion of thymidine kinase (EC 2.7.1.21, ATP : thymidine-5'-phosphotransferase) in the FS sublines. Therefore, we propose that enhanced activities of acid and alkaline phosphatases, which influence the intracellular accumulation and retention of FdUMP, are important determinants of stable FdUrd resistance in CCRF-CEM cells.

Mutants of Chinese hamster cells deficient in thymidylate synthetase

Stable mutants of Chinese hamster V79 cells deficient in thymidylate synthetase (TS; E.C. 2.1.1.45) have been selected from cultures grown in medium supplemented with folinic acid, aminopterin, and thymidine (FAT). After chemical mutagenesis, the frequency of colonies resistant to the "FAT" medium increased more than 100-fold over the spontaneous frequency. The optimal expression time of the mutant phenotype was 5-7 days after mutagen treatment. The recovery of FAT-resistant colonies in the selective medium was not affected by the presence of wild-type cells at a density below 9,000 cells per cm2. All 21 mutants tested exhibited thymidine auxotrophy; neither folinic acid nor deoxyuridine could support mutant cell growth. There was no detectable TS activity in all 11 mutants so far examined and only about 50% of wild-type activity in three prototrophic revertants, as measured by whole-cell and cell-free enzyme assays. The apparent Michaelis-Menten constant (Km) for deoxyuridine-5'-monophosphate and inhibition constant (Ki) for 5-fluoro-deoxyuridine-5'-monophosphate, measured by whole-cell enzyme assay, appear to be similar for the wild-type and revertant cell lines. Using 5-fluoro-[6-3H]-2'-deoxyuridine 5'-monophosphate as active site titrant, the relative amounts of TS in crude cell extract from the parental, revertant, and mutant cells were shown to exist in a 1:0.5:0 ratio. Furthermore, the enzymes from two revertants were more heat labile than that of V79 cells. These properties, taken together, suggest that the FAT-resistant, thymidine auxotrophic phenotype may be the result of a structural gene mutation at the TS locus. The availability of such a mutant facilitates studies on thymidylate stress in relation to DNA metabolism, cell growth, and mutagenesis.