Induction of thymidylate synthase as a 5-fluorouracil resistance mechanism

Translational up-regulation of antifolate drug targets in the human malaria parasite Plasmodium falciparum upon challenge with inhibitors

The thymidylate cycle in Plasmodium falciparum is essential for cell growth and replication, and dihydrofolate reductase (DHFR), a key enzyme in this cycle, is the target of important antimalarial drugs such as pyrimethamine and cycloguanil. Following previous work, where we found no evidence of upregulation of the dhfr-ts gene upon challenge with pyrimethamine, we investigated the expression at the protein level of the bifunctional gene product, which also carries thymidylate synthase (TS) activity. Challenge of parasite cultures with fluoro-substituted bases that are specific TS inhibitors at levels close to the IC(50) resulted in five to seven-fold increases in enzyme level, as monitored by both DHFR and TS activities, while pyrimethamine and another DHFR-binding inhibitor, WR99210, induced smaller but still significant increases of approximately three-fold. However, when parasites were challenged with tetracycline, an antimalarial not directed at the folate pathway, although an increase was consistently seen above untreated controls, this was at a level of approximately 1.8-fold. These increases reflect enhanced synthesis of the DHFR-TS enzyme, rather than liberation of a latent activity, as they were completely abolished if cultures were pre-incubated with cycloheximide to block de novo protein synthesis. Moreover, none of the above antimalarial drugs was found to significantly alter absolute levels of the dhfr-ts mRNA under the conditions of challenge used. We conclude that, in common with mammalian systems, where a similar phenomenon has been reported, malaria parasites are able to significantly relieve translational constraint when faced with antifolate drug challenge. The data indicate that there is a specific component in addition to a low-level non-specific increment, and that binding to the TS domain of the DHFR-TS protein appears to be better able to relieve this constraint than binding to the DHFR domain.

Discrete signaling pathways participate in RB-dependent responses to chemotherapeutic agents

The retinoblastoma (RB) tumor suppressor has been proposed to function as a key mediator of cell cycle checkpoints induced by chemotherapeutic agents. However, these prior studies have relied on embryonic fibroblasts harboring chronic loss of RB, a condition under which compensation of RB functions is known to occur. Here we utilized primary adult fibroblasts derived from mice harboring loxP sites flanking exon 3 of the Rb gene to delineate the action of RB in the chemotherapeutic response. In this system we find that targeted disruption of Rb leads to little overt change in cell cycle distribution. However, these cells exhibited deregulation of RB/E2F target genes and became aneuploid following culture in the absence of RB. When challenged with both DNA damaging and antimetabolite chemotherapeutics, RB was required for primary adult cells to undergo DNA damage checkpoint responses and loss of RB resulted in enhanced aneuploidy following challenge. In contrast, following spontaneous immortalization and the loss of functional p53 signaling, the antimetabolite 5-fluorouracil (5-FU) failed to induce arrest despite the presence of RB. In these immortal cultures RB/E2F targets were deregulated in a complex, gene-specific manner and RB was required for the checkpoint response to camptothecin (CPT). Mechanistic analyses of the checkpoint responses in primary cells indicated that loss of RB leads to increased p53 signaling and decreased viability following both CPT and 5-FU treatment. However, the mechanism through which these agents act to facilitate cell cycle inhibition through RB were distinct. These studies underscore the critical role of RB in DNA-damage checkpoint signaling and demonstrate that RB mediates chemotherapeutic-induced cell cycle inhibition in adult fibroblasts by distinct mechanisms.

Comparison of gene expression in HCT116 treatment derivatives generated by two different 5-fluorouracil exposure protocols

Background

Established colorectal cancer cell lines subjected to different 5-fluorouracil (5-FU) treatment protocols are often used as in vitro model systems for investigations of downstream cellular responses to 5-FU and to generate 5-FU-resistant derivatives for the investigation of biological mechanisms involved in drug resistance. We subjected HCT116 colon cancer cells to two different 5-FU treatment protocols in an attempt to generate resistant derivatives: one that simulated the clinical bolus regimens using clinically-achievable 5-FU levels, the other that utilized serial passage in the presence of increasing 5-FU concentrations (continuous exposure). HCT116 Bolus3, ContinB, and ContinD, corresponding to independently-derived cell lines generated either by bolus exposure or continuous exposure, respectively, were characterized for growth- and apoptosis-associated phenotypes, and gene expression using 8.5 K oligonucleotide microarrays. Comparative gene expression analyses were done in order to determine if transcriptional profiles for the respective treatment derivatives were similar or substantially different, and to identify the signaling and regulatory pathways involved in mediating the downstream response to 5-FU exposure and possibly involved in development of resistance.

Results

HCT116 ContinB and ContinD cells were respectively 27-fold and >100-fold more resistant to 5-FU and had reduced apoptotic fractions in response to transient 5-FU challenge compared to the parental cell line, whereas HCT116 Bolus3 cells were not resistant to 5-FU after 3 cycles of bolus 5-FU treatment and had the same apoptotic response to transient 5-FU challenge as the parental cell line. However, gene expression levels and expression level changes for all detected genes in Bolus3 cells were similar to those seen in both the ContinB (strongest correlation) and ContinD derivatives, as demonstrated by correlation and cluster analyses. Regulatory pathways having to do with 5-FU metabolism, apoptosis, and DNA repair were among those that were affected by 5-FU treatment.

Conclusion

All HCT116 derivative cell lines demonstrated similar transcriptional profiles, despite the facts that they were generated by two different 5-FU exposure protocols and that the bolus exposure derivative had not become resistant to 5-FU. Selection pressures on HCT116 cells as a result of 5-FU challenge are thus similar for both treatment protocols.

Thymidylate synthetase mRNA levels are increased in liver metastases of colorectal cancer patients resistant to fluoropyrimidine-based chemotherapy

Background

Fluoropyrimidines such as 5-fluorouracil (5-FU) and 5-fluoro-2'deoxyuridine (FUDR) are among the most effective chemotherapeutic agents for treatment of metastatic colorectal cancer (CRC). Increased expression of thymidylate synthetase (TS) in CRC metastases has been proposed to be an important mechanism of resistance to fluoropyrimidine-based chemotherapy.

Methods

The present study investigated whether TS mRNA levels in liver metastases of 20 CRC patients before treatment with FUDR by hepatic arterial infusion (HAI) correlated with frequency of clinical response or survival duration.

Results

Median survival duration of patients with TS mRNA levels above and below the median was 15 and 18 months, respectively (p > 0.05). Clinical response was achieved in 40% of patients with low TS mRNA levels, but in only 20% of patients with high TS mRNA levels (p = 0.01). TS mRNA levels were also measured for liver metastases of 7 of the patients that did not achieve a clinical response. A statistically significant increase in expression of TS mRNA was observed for liver metastases resistant to chemotherapy (21 ± 14) in comparison to liver metastases of the same patients before chemotherapy (8 ± 4) (p = 0.03).

Conclusion

This is the first report to demonstrate increased TS expression in liver metastases from CRC patients resistant to fluoropyrimidine based chemotherapy. These findings are consistent with previous studies indicating that increased TS expression is associated with resistance to fluoropyrimidine-based chemotherapy.

Apoptosis in cancer--implications for therapy

Resistance towards apoptosis is a key factor for the survival of a malignant cell. Cancer results if there is too little apoptosis and cells grow faster and live longer than normal cells. In addition, defects in apoptosis signaling contribute to drug resistance of tumor cells. Thus, one of the main goals for oncologic treatment is to overcome resistance of tumor cells towards apoptosis. The exciting challenge in oncology is to translate the growing knowledge of apoptotic pathways into clinical applications. In this review we address the role of apoptosis signaling in tumorigenesis and drug resistance of tumor cells and discuss therapeutic approaches interfering with apoptosis pathways.

Thymidylate synthase and methylenetetrahydrofolate reductase gene polymorphisms: relationships with 5-fluorouracil sensitivity

The relationship of thymidylate synthase (TS) and methylenetetrahydrofolate reductase (MTHFR) gene polymorphisms on 5-fluorouracil (FU) sensitivity was tested on 19 human cancer cell lines (head and neck, breast, digestive tract) in the absence and presence of folinic acid (FA) supplementation. Thymidylate synthase polymorphisms in the 5′ promoter region (double or triple tandem repeats) and 3′ untranslated region (6-bp deletion) were analysed by PCR. The C677T and A1298C MTHFR polymorphisms were determined by melting curve analyses (LightCycler). Thymidylate synthase activity and intracellular concentration of the reduced folate 5-10 methylenetetrahydrofolate (CH₂FH₄) were measured (biochemical assays). Thymidylate synthase activity was significantly different according to 5′ TS genotype, heterozygous cell lines (2R/3R) exhibiting higher TS activities than homozygous ones (P=0.05). However, whether in the absence or presence of FA, FU sensitivity was not statistically associated with either 5′ or 3′ TS polymorphism. Basal CH₂FH₄ cellular concentrations were lowest in C677T homozygous wild-type (wt) (C/C) cell lines. FU sensitivity was not linked to C677T polymorphism. In contrast, there was a marked trend for a greater FU efficacy in mutated A1298C variants (C/C+A/C) as compared to wt homozygous cell lines (A/A) (P=0.055 and 0.085 without and with FA supplementation, respectively). These results suggest for the first time a potential role of A1298C MTHFR polymorphism on fluoropyrimidine sensitivity.

Digital karyotyping identifies thymidylate synthase amplification as a mechanism of resistance to 5-fluorouracil in metastatic colorectal cancer patients

Resistance to chemotherapy is a major cause of mortality in advanced cancer patients. In this study, digital karyotyping was used to search for genomic alterations in liver metastases that were clinically resistant to 5-fluorouracil (5-FU). In two of four patients, we identified amplification of an approximately 100-kb region on 18p11.32 that was of particular interest because it contained the gene encoding thymidylate synthase (TYMS), a molecular target of 5-FU. Analysis of TYMS by fluorescence in situ hybridization identified TYMS gene amplification in 23% of 31 5-FU-treated cancers, whereas no amplification was observed in metastases of patients that had not been treated with 5-FU. Patients with metastases containing TYMS amplification had a substantially shorter median survival (329 days) than those without amplification (1,021 days, P <0.01). These data suggest that genetic amplification of TYMS is a major mechanism of 5-FU resistance in vivo and have important implications for the management of colorectal cancer patients with recurrent disease.

Methionine dependency and cancer treatment

Conventional chemotherapies have showed their limits, notably for patients with advanced cancer. New therapeutic strategies must be identified, and the metabolic abnormalities of cancer cells offer such opportunities. Many human cancer cell lines and primary tumors have absolute requirements for methionine, an essential amino acid. In contrast, normal cells are relatively resistant to exogenous methionine restriction. The biochemical mechanism for methionine dependency has been studied extensively, but the fundamental mechanism remains unclear. A number of investigators have attempted to exploit the methionine dependence of tumors for therapeutic effects in vivo. To reduce in vivo methionine in plasma and tumours, dietary and pharmacological treatments have been used. Methionine-free diet or methionine-deprived total parenteral nutrition causes regression of a variety of animal tumours. Alternatively, methionine depletion was achieved by the use of methioninase. This enzyme specifically degrades methionine and inhibits tumour growth in preclinical models. Because of potential toxicity and quality of life problems, prolonged methionine restriction with diet or with methioninase is not suitable for clinical use. Methionine restriction may find greater application in association with various chemotherapeutic agents. Several preclinical studies have demonstrated synergy between methionine restriction and various cytotoxic chemotherapy drugs. The experimental results accumulated during the last three decades suggest that methionine restriction can become an additional cancer therapeutic strategy, notably in association with chemotherapy.

Correlation between 5-fluorouracil metabolism and treatment response in two variants of C26 murine colon carcinoma

Following an i.p. dose of 150 mg x kg(-1) 5-fluorouracil (5-FU), drug uptake and metabolism over a 2-h period were studied by in vivo (19)F magnetic resonance spectroscopy (MRS) for the murine colon carcinoma lines C26-B (5-FU-insensitive; n=11) and C26-10 (5-FU-sensitive; n=15) implanted s.c. in Balb/C mice. Time courses for tumour growth, intracellular levels of FdUMP, thymidylate synthase (TS) activity, and 5-FU in RNA were also determined, and the effects of a 9.5-min period of carbogen breathing, starting 1 min before drug administration, on MRS-detected 5-FU metabolism and tumour growth curves were examined. Both tumour variants generated MRS-detectable 5-FU nucleotides and showed similar initial growth inhibition after treatment. However, the growth rate of C26-B tumours returned to normal, while the sensitive C26-10 tumours, which produced larger fluoronucleotide pools, still showed moderate growth inhibition. Carbogen breathing did not significantly influence 5-FU uptake or fluoronucleotide production but did significantly enhance growth inhibition in C26-10 tumours. While both tumour variants exhibited incorporation of 5-FU into RNA and inhibition of TS via FdUMP, clearance of 5-FU from RNA and recovery of TS activity were greater for the insensitive C26-B line, indicating that these processes, in addition to 5-FU uptake and metabolism, may be important determinants of drug sensitivity and treatment response.

Pharmacokinetics of S-1, an oral formulation of ftorafur, oxonic acid and 5-chloro-2,4-dihydroxypyridine (molar ratio 1:0.4:1) in patients with solid tumors

S-1 is an oral formulation of ftorafur (FT), oxonic acid and 5-chloro-2,4-dihydroxypyridine (CDHP) at a molar ratio of 1:0.4:1. FT is a 5-fluorouracil (5-FU) prodrug, CDHP is a dihydropyrimidine dehydrogenase (DPD) inhibitor and oxonic acid is an inhibitor of 5-FU phosphoribosylation in the gastrointestinal mucosa and was included to prevent gastrointestinal toxicity. We determined the pharmacokinetics of S-1 in 28 patients at doses of 25, 35, 40 and 45 mg/m(2). The plasma C(max) values of FT, 5-FU, oxonic acid and CDHP increased dose-dependently and after 1-2 h were in the ranges 5.8-13 microM, 0.4-2.4 microM, 0.026-1.337 microM, and 1.1-3.6 microM, respectively. Uracil levels, indicative of DPD inhibition, also increased dose-dependently from basal levels of 0.03-0.25 microM to 3.6-9.4 microM after 2-4 h, and 0.09-0.9 microM was still present after 24 h. The pharmacokinetics of CDHP and uracil were linear over the dose range. The areas under the plasma concentration curves (AUC) for CDHP and uracil were in the ranges 418-1735 and 2281-8627 micromol x min/l, respectively. The t(1/2) values were in the ranges 213-692 and 216-354 min, respectively. Cumulative urinary excretion of FT was predominantly as 5-FU and was 2.2-11.9%; the urinary excretion of both fluoro-beta-alanine and uracil was generally maximal between 6 and 18 h. During 28-day courses with twice-daily S-1 administration, 5-FU and uracil generally increased. Before each intake of S-1, 5-FU varied between 0.5 and 1 microM and uracil was in the micromolar range (up to 7 microM), indicating that effective DPD inhibition was maintained during the course. In a biopsy of an esophageal adenocarcinoma metastasis that had regressed, thymidylate synthase, the target of 5-FU, was inhibited 50%, but increased four- to tenfold after relapse in subsequent biopsies. In conclusion, oral S-1 administration resulted in prolonged exposure to micromolar 5-FU concentrations due to DPD inhibition, and the decrease in uracil levels after 6 h followed the pattern of CDHP and indicates reversible DPD inhibition.

Thymidylate synthase inhibition triggers apoptosis via caspases-8 and -9 in both wild-type and mutant p53 colon cancer cell lines

Thymidylate synthase (TS) is an important target for chemotherapy and increased levels are associated with resistance to colorectal cancer chemotherapy. TS can be inhibited by 5-fluorouracil (5-FU) and antifolates, ultimately resulting in apoptosis. We aimed to clarify whether activation of caspases and Fas signalling are crucial for the onset of apoptosis after specific inhibition of TS and whether p53 plays a role in activation of these downstream processes. For this purpose, wild-type (wt) and mutant (mt) p53 colon cancer cell lines, Lovo and WiDr, respectively, transfected with mt- and wt-p53, were treated with the specific TS inhibitor, AG337. Treatment with 10xIC(50) values of AG337 for 48 h resulted in S phase arrest in all Lovo and WiDr cells (up to 50% of cells being in S phase), irrespective of their p53 status. After 72 h, the induction of apoptosis was most pronounced in the AG337-sensitive cells. Approximately 30% apoptosis was detected in all of the WiDr cells, 20% in Lovo li (non-functional p53), 12-14% in Lovo 92 and B2 (wt p53) and only 7% in Lovo 175x2 cells (mt p53 transfected). The induction of apoptosis in Lovo cells, as determined using the classical sub-G1 peak after propidium iodide (PI) staining, was associated with an increase in the expression of Fas receptor. In addition, synergistic increases in apoptosis from approximately 10 to 35% after 48 h could be detected after simultaneous treatment of AG337 and the Fas activator antibody, CH11. Only additive effects were measurable in WiDr cells, without an increase in Fas receptor expression. Surprisingly, the Fas inhibitor, ZB4, could not decrease the amount of cell death in both cell lines after AG337 treatment. In contrast, simultaneous exposure of Lovo and WiDr cells to AG337 and inhibitors of caspases 8, 9 and 3 caused a decrease in the number of apoptotic cells compared with AG337 exposure alone. Inhibition of apoptosis by approximately 10-80% in Lovo and approximately 70-80% in WiDr cells could be detected. In conclusion, these results indicate that apoptosis induced after specific inhibition of TS is mediated via the caspases, but without clear involvement of Fas signalling. The status of p53 did not affect the onset of apoptosis by these caspases.

Predictive markers for colorectal cancer: current status and future prospects

Colorectal cancer (CRC) is the second leading cause of cancer death in the United States. Although there is clear evidence of the benefit of chemotherapy in adjuvant and metastatic settings, its use continues to be suboptimal because of intrinsic or acquired drug resistance. 5-Fluorouracil continues to be the mainstay of CRC therapy, and combinations with newer chemotherapeutic agents such as irinotecan and oxaliplatin have resulted in improved response rates and survival. The role of other agents including cyclooxygenase-2 inhibitors, epidermal growth factor receptor, and farnsyl transferase inhibitors remains to be elucidated. Despite these improvements, many patients undergo chemotherapy without benefit. Increased understanding of the biology of CRC has led to the identification of prognostic markers that may help identify patients who will benefit from chemotherapy. Furthermore, studies have also begun to identify markers that predict whether a tumor will respond to a particular chemotherapy. The ultimate goal of this research is to prospectively identify patients who should receive chemotherapy and, thus, to tailor treatment to the molecular profile of the tumor and patient. Such an approach has the potential to dramatically improve response rates. This review highlights potentially important prognostic and predictive factors in CRC and discusses the potential for their use in the treatment of this disease.

Mechanism of 5-fluorouracil required resistance in human hepatocellular carcinoma cell line Bel7402

AIM: To investigate the resistance mechanism of 5-fluorouracil (5-FU) in Bel₇₄₀₂/5-FU cells which was established in our lab by in vitro continuous stepwise exposure of human hepatocellular carcinoma (HCC) cell line Bel₇₄₀₂ to 5-FU.

METHODS: The expression of multidrug resistance-associated protein (MRP) and thymidylate synthase (TS) in Bel₇₄₀₂ cells was detected by immonocytochemistry. The fluorescein (FLU) accumulation, an index of MRP functional activity, was determined by flow cytometry. The distribution of FLU was observed by confocal laser scanning microscope. The spectrofluorometry was used to show the intracelluar content of glutathione (GSH). Cell growth inhibition was determined by MTT assay. The activity of glutathione S-transferases (GSTs) was determined by spectrophotometry.

RESULTS: A higher expression of MRP in the Bel₇₄₀₂/5-FU cells was observed by using monoclonal mouse anti-MRP antibody, MRPr-1, in comparison with Bel₇₄₀₂ cells. Bel₇₄₀₂/5-FU cells also showed a significant decrease of FLU accumulation. FLU mainly accumulated in the nucleus with a high nuclear/cytoplasmic ratio in Bel₇₄₀₂ cells, whereas there was no difference of FLU accumulation between the nucleus and cytoplasm in Bel₇₄₀₂/5-FU cells. The intracellular GSH content in Bel₇₄₀₂/5-FU cells was almost 3 folds higher than that in Bel₇₄₀₂ cells. Addition of D, L-buthione-S, R-sulfoximine (BSO) dose-dependently reduced the GSH content in Bel₇₄₀₂/ 5-FU cells, however, only a weak enhancement on the cytotoxicity of 5-FU and doxorubicin (Dox) to Bel₇₄₀₂/5-FU cells was observed. Bel₇₄₀₂/5-FU cells also exhibited 29.1% higher total GSTs activity than Bel₇₄₀₂ cells. Immunocytochemical staining by using anti-TS monoclonal antibody TS 106 showed that the level of TS in Bel₇₄₀₂/5-FU cells elevated markedly as compared with Bel₇₄₀₂ cells.

CONCLUSION: The continuous exposure of Bel₇₄₀₂ cells to 5-FU led to overexpression of TS and MRP, as well as increased intracellular GSH content and total GST activity.

Future potential of thymidylate synthase inhibitors in cancer therapy

Thymidylate synthase (TS) catalyses the de novo synthesis of deoxythymidylate and is a key rate-limiting enzyme of DNA synthesis. The primary site of action of the classic antifolate methotrexate is direct inhibition of dihydrofolate reductase, but it also inhibits TS indirectly by diminishing levels of the TS cosubstrate 5,10-methylenetetrahydrofolate. Polyglutamated metabolites of methotrexate also directly bind and inhibit TS. The prototype fluoropyrimidine fluorouracil is metabolised to an irreversible inhibitor of TS and is the standard chemotherapy for gastrointestinal carcinomas. It is also frequently used in combination with other anticancer drugs against breast cancer and head and neck cancers. The clinical efficacy of fluorouracil is routinely increased by concomitant administration of the biomodulating compound leucovorin (folinic acid). Both the success and limitations of these early drugs led to a search for new, more efficacious TS inhibitors active against a broader range of neoplasms. Raltitrexed (ZD1694, Tomudex) is an antifolate TS inhibitor developed over the last decade that is similarly effective, yet better tolerated, than fluorouracil against colorectal cancer. Additional antifolate and fluoropyrimidine-based TS inhibitors continue to be developed. Many of these experimental drugs have been designed to exploit or thwart selective metabolism in neoplasms, including specific mechanisms of resistance. As the curative potential of relatively non-selective antiproliferative drugs like TS inhibitors is limited against most neoplasms, the future role of TS inhibitors will likely continue to be adjunctive in surgically resectable tumours and palliative in combination with other agents for non-resectable disease. Although TS inhibitors will eventually be supplanted by yet to be discovered agents targeting more tumour-specific cellular signalling pathways, they will probably remain important for the above uses for some time. Future advances in the effective use of TS inhibitors may be forthcoming in the form of improved dosing, fewer untoward effects and increased tumour selectivity with novel fluorouracil prodrug formulations. Furthermore, there is emerging evidence that some novel antifolate TS inhibitors are active against a broader range of neoplams, including lung carcinomas and mesothelioma, compared to classical TS inhibitors. Other possible advances to come include effective biomodulation of antifolate TS inhibitors with nucleoside transport inhibitors and individualised patient therapy based on tumour gene expression and resistance patterns (pharmacogenetics).

Magnifying endoscopic observation of the gastric mucosa, particularly in patients with atrophic gastritis

The gastric mucosal surface was observed using the magnifying fibergastroscope (FGS-ML), and the fine gastric mucosal patterns, which were even smaller than one unit of gastric area, were examined at a magnification of about 30. For simplicification, we classified these patterns by magnifying endoscopy in the following ways; FP, FIP, FSP, SP and MP, modifying Yoshii's classification under the dissecting microscope. The FIP, which was found to have round and long elliptical gastric pits, is a new addition to our endoscopic classification. The relationship between the FIP and the intermediate zone was evaluated by superficial and histological studies of surgical and biopsy specimens. The width of the band of FIP seems to be related to the severity of atrophic gastritis. Also, the transformation of FP to FIP was assessed by comparing specimens taken from the resected and residual parts of the stomach, respectively. Moreover, it appears that severe gastritis occurs in the gastric mucosa which shows a FIP. Therefore, we consider that the FIP indicates the position of the atrophic border.