Cell cycle regulation of thymidylate synthetase gene expression in cultured mouse fibroblasts

Phosphorylation of Thymidylate Synthase and Dihydrofolate Reductase in Cancer Cells and the Effect of CK2α Silencing

Our previous research suggests an important regulatory role of CK2-mediated phosphorylation of enzymes involved in the thymidylate biosynthesis cycle, i.e., thymidylate synthase (TS), dihydrofolate reductase (DHFR), and serine hydroxymethyltransferase (SHMT). The aim of this study was to show whether silencing of the CK2α gene affects TS and DHFR expression in A-549 cells. Additionally, we attempted to identify the endogenous kinases that phosphorylate TS and DHFR in CCRF-CEM and A-549 cells. We used immunodetection, immunofluorescence/confocal analyses, reverse transcription-quantitative polymerase chain reaction (RT-qPCR), in-gel kinase assay, and mass spectrometry analysis. Our results demonstrate that silencing of the CK2α gene in lung adenocarcinoma cells significantly increases both TS and DHFR expression and affects their cellular distribution. Additionally, we show for the first time that both TS and DHFR are very likely phosphorylated by endogenous CK2 in two types of cancer cells, i.e., acute lymphoblastic leukaemia and lung adenocarcinoma. Moreover, our studies indicate that DHFR is phosphorylated intracellularly by CK2 to a greater extent in leukaemia cells than in lung adenocarcinoma cells. Interestingly, in-gel kinase assay results indicate that the CK2α' isoform was more active than the CK2α subunit. Our results confirm the previous studies concerning the physiological relevance of CK2-mediated phosphorylation of TS and DHFR.

Inhibition of Protein Kinase CK2 Affects Thymidylate Synthesis Cycle Enzyme Level and Distribution in Human Cancer Cells

Thymidylate synthase (TS), dihydrofolate reductase (DHFR), and serine hydroxymethyltransferase (SHMT) constitute the thymidylate synthesis cycle providing thymidylate for DNA synthesis and repair. Our previous studies indicated that TS and DHFR are the substrates of protein kinase CK2. This work has been aimed at the elucidation of the effect of CK2 activity on cell cycle progression, thymidylate synthesis enzyme expression and localization, and the role of CK2-mediated TS phosphorylation in in vitro di- and trimolecular complex formation. The results were obtained by means of western blot, confocal microscopy, flow cytometry, quantitative polymerase chain reaction (QPCR), quartz crystal microbalance with dissipation monitoring (QCM-D), and microthermophoresis (MST). Our research indicates that CK2 inhibition does not change the levels of the transcripts; however, it affects the protein levels of DHFR and TS in both tested cell lines, i.e., A549 and CCRF-CEM, and the level of SHMT1 in CCRF-CEM cells. Moreover, we show that CK2-mediated phosphorylation of TS enables the protein (pTS) interaction with SHMT1 and leads to the stability of the tri-complex containing SHMT1, DHFR, and pTS. Our results suggest an important regulatory role of CK2-mediated phosphorylation for inter- and intracellular protein level of enzymes involved in the thymidylate biosynthesis cycle.

Diagnostic and prognostic value of thymidylate synthase expression in breast cancer

Nucleotide metabolism is the driving force of cell proliferation, and thymidylate synthase (TYMS) catalyzes a rate-limiting step in the initial synthesis of nucleotides. Previous studies reported that TYMS activity significantly affected the proliferation of tumour cells. However, the diagnostic and prognostic significance of TYMS expression in breast cancer remains unclear. Here, we used the Breast Cancer Integrative Platform (BCIP) to investigate the relationship between progression and prognosis of breast cancer with TYMS expression, and then verified the database analysis using immunohistochemical staining. Our results indicated TYMS expression was greater in breast cancer than adjacent normal tissues and greater in triple-negative breast cancer (TNBC) than non-TNBC tissues. TYMS expression also had significant positive correlations with histological grade, tumour size, and ER negativity, and PR negativity. The increased copy number of the TYMS gene appears to be the reason for its upregulation in breast cancer. Breast cancer patients with higher TYMS expression had poorer prognosis. Our data suggest that TYMS has potential use as a diagnostic and prognostic marker for breast cancer patients.

Design, synthesis and in vitro antiproliferative activity of new thiazolidinedione-1,3,4-oxadiazole hybrids as thymidylate synthase inhibitors

Thymidylate synthase (TS) has been an attention-grabbing area of research for the treatment of cancers due to their role in DNA biosynthesis. In the present study, we have synthesised a library of thiazolidinedione-1,3,4-oxadiazole hybrids as TS inhibitors. All the synthesised hybrids followed Lipinski and Veber rules which indicated good drug likeness properties upon oral administration. Among the synthesised hybrids, compound 9 and 10 displayed 4.5 and 4.4 folds activity of 5-Fluorouracil, respectively against MCF-7 cell line whereas 3.1 and 2.5 folds cytotoxicity against HCT-116 cell line. Furthermore, compound 9 and 10 also inhibited TS enzyme with IC50 = 1.67 and 2.21 µM, respectively. Finally, the docking studies of 9 and 10 were found to be consistent with in vitro TS results. From these studies, compound 9 and 10 has the potential to be developed as TS inhibitors.

A Novel Nomenclature for Repeat Motifs in the Thymidylate Synthase Enhancer Region and Its Relevance for Pharmacogenetic Studies

Inhibition of thymidylate synthase (TS) is the primary mode of action for 5-fluorouracil (5FU) chemotherapy. TS expression is modulated by a variable number of tandem repeats in the TS enhancer region (TSER) located upstream of the TS gene (TYMS). Variability in the TSER has been suggested to contribute to 5FU-induced adverse events. However, the precise genetic associations remain largely undefined due to high polymorphism and ambiguity in defining genotypes. To assess toxicity associations, we sequenced the TSER in 629 cancer patients treated with 5FU. Of the 13 alleles identified, few could be unambiguously named using current TSER-nomenclature. We devised a concise and unambiguous systematic naming approach for TSER-alleles that encompasses all known variants. After applying this comprehensive naming system to our data, we demonstrated that the number of upstream stimulatory factor (USF1-)binding sites in the TSER was significantly associated with gastrointestinal toxicity in 5FU treatment.

Targeting nuclear thymidylate biosynthesis

Thymidylate (dTMP) biosynthesis plays an essential and exclusive function in DNA synthesis and proper cell division, and therefore has been an attractive therapeutic target. Folate analogs, known as antifolates, and nucleotide analogs that inhibit the enzymatic action of the de novo thymidylate biosynthesis pathway and are commonly used in cancer treatment. In this review, we examine the mechanisms by which the antifolate 5-fluorouracil, as well as other dTMP synthesis inhibitors, function in cancer treatment in light of emerging evidence that dTMP synthesis occurs in the nucleus. Nuclear localization of the de novo dTMP synthesis pathway requires modification of the pathway enzymes by the small ubiquitin-like modifier (SUMO) protein. SUMOylation is required for nuclear localization of the de novo dTMP biosynthesis pathway, and disruption in the SUMO pathway inhibits cell proliferation in several cancer models. We summarize evidence that the nuclear localization of the dTMP biosynthesis pathway is a critical factor in the efficacy of antifolate-based therapies that target dTMP synthesis.

Analysis of the interplay between all-trans retinoic acid and histone deacetylase inhibitors in leukemic cells

The treatment of acute promyelocytic leukemia (APL) with all-trans retinoic acid (ATRA) induces granulocytic differentiation. This process renders APL cells resistant to cytotoxic chemotherapies. Epigenetic regulators of the histone deacetylases (HDACs) family, which comprise four classes (I-IV), critically control the development and progression of APL. We set out to clarify the parameters that determine the interaction between ATRA and histone deacetylase inhibitors (HDACi). Our assays included drugs against class I HDACs (MS-275, VPA, and FK228), pan-HDACi (LBH589, SAHA), and the novel HDAC6-selective compound Marbostat-100. We demonstrate that ATRA protects APL cells from cytotoxic effects of SAHA, MS-275, and Marbostat-100. However, LBH589 and FK228, which have a superior substrate-inhibitor dissociation constant (Ki) for the class I deacetylases HDAC1, 2, 3, are resistant against ATRA-dependent cytoprotective effects. We further show that HDACi evoke DNA damage, measured as induction of phosphorylated histone H2AX and by the comet assay. The ability of ATRA to protect APL cells from the induction of p-H2AX by HDACi is a readout for the cytoprotective effects of ATRA. Moreover, ATRA increases the fraction of cells in the G1 phase, together with an accumulation of the cyclin-dependent kinase inhibitor p21 and a reduced expression of thymidylate synthase (TdS). In contrast, the ATRA-dependent activation of the transcription factors STAT1, NF-κB, and C/EBP hardly influences the responses of APL cells to HDACi. We conclude that the affinity of HDACi for class I HDACs determines whether such drugs can kill naïve and maturated APL cells.

Cell cycle regulation and novel structural features of thymidine kinase, an essential enzyme in Trypanosoma brucei

Thymidine kinase (TK) is a key enzyme in the pyrimidine salvage pathway which catalyzes the transfer of the γ-phosphate of ATP to 2'-deoxythymidine (dThd) forming thymidine monophosphate (dTMP). Unlike other type II TKs, the Trypanosoma brucei enzyme (TbTK) is a tandem protein with two TK homolog domains of which only the C-terminal one is active. In this study, we establish that TbTK is essential for parasite viability and cell cycle progression, independently of extracellular pyrimidine concentrations. We show that expression of TbTK is cell cycle regulated and that depletion of TbTK leads to strongly diminished dTTP pools and DNA damage indicating intracellular dThd to be an essential intermediate metabolite for the synthesis of thymine-derived nucleotides. In addition, we report the X-ray structure of the catalytically active domain of TbTK in complex with dThd and dTMP at resolutions up to 2.2 Å. In spite of the high conservation of the active site residues, the structures reveal a widened active site cavity near the nucleobase moiety compared to the human enzyme. Our findings strongly support TbTK as a crucial enzyme in dTTP homeostasis and identify structural differences within the active site that could be exploited in the process of rational drug design.

Overexpression of thymidylate synthase (TYMS) is associated with aggressive tumor features and early PSA recurrence in prostate cancer

Thymidylate synthase (TYMS) plays a role in DNA synthesis and is a target for 5-fluorouracil. In this study TYMS was analyzed by immunohistochemistry on a tissue microarray containing 11,152 prostate cancers. TYMS expression was higher in neoplastic than in normal prostate epithelium and was detectable in 72.9% of 10,223 interpretable cancers. It was considered strong in 21.9%, moderate in 33.4% and weak in 17.6% of tumors. TYMS overexpression was associated with deletions at 5q21 (p < 0.0001), 6q15 (p < 0.0001) and 3p13 (p = 0.0083) and gradually increased with the total number of these deletions present in the respective cancer sample (p < 0.0001). TYMS expression was unrelated to PTEN deletions (p = 0.9535) but tightly linked to high Gleason grade, advanced pathological tumor stage and early PSA recurrence (p < 0.0001). The prognostic value of TYMS was independent from the ERG status and deletions at 3p13, 5q21, and 6q15. In multivariate analyses the prognostic role of TYMS expression was independent of Gleason grade, pT stage, preoperative PSA, pN stage, or resection margins. TYMS expression analysis might result in clinically useful information in prostate cancer. The striking link to some but not all chromosomal aberrations might suggest a mechanistical link with specific types of DNA damage.

Expression of multidrug resistance proteins in invasive ductal carcinoma of the breast

Chemotherapy is commonly used for the treatment of breast cancer. However, the resistance to chemotherapeutic agents, often mediated by multidrug resistance (MDR) mechanisms, is a common occurrence. The present study examined the expression of several MDR-related proteins (MRPs) in invasive ductal carcinoma (IDC) of the breast, and assessed their association with clinicopathological variables and their prognostic significance. In addition, immunohistochemistry was used to measure the expression of MRP, p-glycoprotein (P-gp), topoisomerase 2α (Topo2α), thymidylate synthase (TS) and glutathione-S-transferase π (GST-π) in 156 resected IDCs of the breast. Pearson's χ² test and Spearman's correlation coefficient were used to analyze the association between MDR protein expression and several clinicopathological variables. The association between each of the five MDR proteins was also examined. Furthermore, Kaplan-Meier analysis and Cox regression modeling were used to assess overall survival. The expression of MRP, P-gp, Topo2α, TS and GST-π was detected in 20.5% (32/156), 25.0% (39/156), 84.0% (131/156), 41.7% (65/156) and 41.0% (64/156) of cases examined, respectively. No correlation was identified between MRP and Topo-2α and the clinicopathological variables examined. By contrast, P-gp (χ²=20.226; P<0.0001) and GST-π (χ²=35.032; P<0.0001) were found to positively correlate with tumor grade. In addition, staining for TS was associated with axillary lymph node metastasis (χ²=42.281; P<0.0001). The expression levels of P-gp and GST-π were found to be significantly correlated (r= 0.319; P<0.0001). Furthermore, GST-π expression was elevated in estrogen receptor-negative breast cancer (χ²=17.407; P<0.0001). Tumor histological grade, in addition to TS and GST-π expression, were significant predictors of a poor survival outcome. TS and GST-π are consequently useful prognostic biomarkers in IDC, therefore, when establishing a personalized chemotherapeutic plan, the expression of MDR proteins must be considered.

Analysis of 320 gastroenteropancreatic neuroendocrine tumors identifies TS expression as independent biomarker for survival

Thymidylate synthase (TS), a critical enzyme for DNA synthesis and repair, is both a potential tumor prognostic biomarker as well as a tumorigenic oncogene in animal models. We have now studied the clinical implications of TS expression in gastroenteropancreatic (GEP) neuroendocrine tumors (NETs) and compared these results to other cell cycle biomarker genes. Protein tissue arrays were used to study TS, Ki-67, Rb, pRb, E2F1, p18, p21, p27 and menin expression in 320 human GEP-NETs samples. Immunohistochemical expression was correlated with univariate and multivariate predictors of survival utilizing Kaplan Meier and Cox proportional hazards models. Real time RT-PCR was used to validate these findings. We found that 78 of 320 GEP-NETs (24.4%) expressed TS. NETs arising in the colon, stomach and pancreas showed the highest expression of TS (47.4%, 42.6% and 37.3%, respectively), whereas NETs of the appendix, rectum and duodenum displayed low TS expression (3.3%, 12.9% and 15.4%, respectively). TS expression in GEP-NETs was associated with poorly differentiated endocrine carcinoma, angiolymphatic invasion, lymph node metastasis and distant metastasis (p < 0.05). Patients with TS-positive NETs had markedly worse outcomes than TS-negative NETs as shown by univariate (p < 0.001) and multivariate (p = 0.01) survival analyses. Expression of p18 predicted survival in TS-positive patients that received chemotherapy (p = 0.015). In conclusion, TS protein expression was an independent prognostic biomarker for GEP-NETs. The strong association of increased TS expression with aggressive disease and early death supports the role of TS as a cancer promoting agent in these tumors.

Depletion of deoxyribonucleotide pools is an endogenous source of DNA damage in cells undergoing oncogene-induced senescence

In normal human cells, oncogene-induced senescence (OIS) depends on induction of DNA damage response. Oxidative stress and hyperreplication of genomic DNA have been proposed as major causes of DNA damage in OIS cells. Here, we report that down-regulation of deoxyribonucleoside pools is another endogenous source of DNA damage in normal human fibroblasts (NHFs) undergoing HRAS(G12V)-induced senescence. NHF-HRAS(G12V) cells underexpressed thymidylate synthase (TS) and ribonucleotide reductase (RR), two enzymes required for the entire de novo deoxyribonucleotide biosynthesis, and possessed low dNTP levels. Chromatin at the promoters of the genes encoding TS and RR was enriched with retinoblastoma tumor suppressor protein and histone H3 tri-methylated at lysine 9. Importantly, ectopic coexpression of TS and RR or addition of deoxyribonucleosides substantially suppressed DNA damage, senescence-associated phenotypes, and proliferation arrest in two types of NHF-expressing HRAS(G12V). Reciprocally, short hairpin RNA-mediated suppression of TS and RR caused DNA damage and senescence in NHFs, although less efficiently than HRAS(G12V). However, overexpression of TS and RR in quiescent NHFs did not overcome proliferation arrest, suggesting that unlike quiescence, OIS requires depletion of dNTP pools and activated DNA replication. Our data identify a previously unknown role of deoxyribonucleotides in regulation of OIS.

Dose- and time-dependent expression patterns of zebrafish orthologs of selected E2F target genes in response to serum starvation/replenishment

Targets of E2F transcription factors effectively regulate the cell cycle from worms to humans. Furthermore, the dysregulation of E2F transcription modules plays a highly conserved role in cancers of human and zebrafish. Studying E2F target expression under a given cellular state, such as quiescence, might lead to a better understanding of the conserved patterns of expression in different taxa. In the present study, we used literature searches and phylogeny to identify several targets of E2F transcription factors that are known to be serum-responsive; namely, PCNA, MYBL2, MCM7, TYMS, and CTGF. The transcriptional serum response of zebrafish orthologs of these genes were quantified under different doses (i.e., 0, 0.1, 1, 3, and 10% FBS) and time points (i.e., 6, 24 and 48 hours, h) using quantitative RT-PCR (qRT-PCR) in the zebrafish fibroblast cells (ZF4). Our results indicated that mRNA expression of zebrafish pcna, mybl2, mcm7 and tyms drastically decreased while that of ctgf increased with decreasing serum levels as observed in mammals. These genes responded to serum starvation at 24 and 48 h and to the mitogenic stimuli as early as 6 h except for ctgf whose expression was significantly altered at 24 h. The zebrafish Mcm7 protein levels also were modulated by serum starvation/replenishment. The present study provides a foundation for the comparative analysis of quantitative expression patterns for genes involved in regulation of cell cycle using a zebrafish serum response model.

miR-192/miR-215 influence 5-fluorouracil resistance through cell cycle-mediated mechanisms complementary to its post-transcriptional thymidilate synthase regulation

Thymidylate synthase (TYMS) is a target of the most widely used chemotherapeutic agents against gastrointestinal malignancies, the fluoropyrimidine-based therapy. TYMS expression levels have been identified as predictive biomarkers for 5-fluoruracil (FU) response in colorectal cancer, but their clinical utility remains controversial. The complexity of fluoropyrimidine response must require more mechanisms that currently have not been completely elucidated. In this context, microRNAs (miRNA) may play a role in modulating chemosensitivity. By carrying out an in silico analysis coupled to experimental validation, we detected that miR-192 and miR-215 target TYMS expression in colorectal cancer cell lines. However, downregulation of TYMS by these miRNAs does not sensitize colorectal cancer cell lines to FU treatment. The overexpression of miR-192/215 significantly reduces cell proliferation by targeting cell cycle progression. This effect was partially associated with p53 status, because reduction of cell proliferation and cell cycle arrest was associated with p21 and p27 induction. The decrease of S-phase cells by these miRNAs mitigates the effects of S phase-specific drugs and suggests that other mechanisms different from TYMS overexpression are essential to direct FU resistance. Finally, ectopic expression of miR-192/215 might have stronger impact to predict FU response than TYMS inhibition. Prospective studies to elucidate the role of these miRNAs as predictive biomarkers to FU are necessary.

Regulation and functional contribution of thymidine kinase 1 in repair of DNA damage

Cellular supply of dNTPs is essential in the DNA replication and repair processes. Here we investigated the regulation of thymidine kinase 1 (TK1) in response to DNA damage and found that genotoxic insults in tumor cells cause up-regulation and nuclear localization of TK1. During recovery from DNA damage, TK1 accumulates in p53-null cells due to a lack of mitotic proteolysis as these cells are arrested in the G(2) phase by checkpoint activation. We show that in p53-proficient cells, p21 expression in response to DNA damage prohibits G(1)/S progression, resulting in a smaller G(2) fraction and less TK1 accumulation. Thus, the p53 status of tumor cells affects the level of TK1 after DNA damage through differential cell cycle control. Furthermore, it was shown that in HCT-116 p53(-/-) cells, TK1 is dispensable for cell proliferation but crucial for dTTP supply during recovery from DNA damage, leading to better survival. Depletion of TK1 decreases the efficiency of DNA repair during recovery from DNA damage and generates more cell death. Altogether, our data suggest that more dTTP synthesis via TK1 take place after genotoxic insults in tumor cells, improving DNA repair during G(2) arrest.

Thymidylate synthase expression in gastroenteropancreatic and pulmonary neuroendocrine tumors

PURPOSE

The predictive role of the quantification of thymidylate synthase (TS) in tumors treated with antifolate drugs, such as 5-fluorouracil (5-FU), has been extensively reported in a variety of human tumors. Neuroendocrine tumors (NET) represent potential targets of antifolate agents, but no data on TS expression level in these tumors are currently available.

EXPERIMENTAL DESIGN

A series of 116 NETs were collected, including 58 gastroenteropancreatic (GEP) and 58 lung NETs. In 24 well-differentiated GEP neuroendocrine carcinomas (WD-NEC), a 5-FU-based treatment was given. Total RNA was extracted from microdissected paraffin blocks. TS mRNA quantification was done by real-time PCR, whereas protein expression was evaluated by immunohistochemistry.

RESULTS

By means of both quantification by real-time PCR and immunohistochemistry, a higher TS expression in pulmonary small cell lung cancer and large cell NEC compared with typical and atypical carcinoids was observed (P < 0.01). Similarly, in GEP tumors, a higher TS expression in poorly differentiated carcinomas than both WD-NEC and benign tumors (P < 0.01) was found. In patients with WD-NEC treated with 5-FU, high TS mRNA levels were associated with shorter time to progression (P = 0.002) and overall survival (P = 0.04). This negative prognostic role was confirmed in multivariate analysis adjusting for major prognostic variables (P = 0.01). No association between TS mRNA and survival was observed in WD-NEC patients not receiving 5-FU.

CONCLUSIONS

This study, for the first time, (a) reports the differential TS expression in the spectrum of NETs and (b) indicates TS as a possible predictive marker of treatment efficacy in WD-NEC patients treated with 5-FU.

E2F1 overexpression correlates with thymidylate synthase and survivin gene expressions and tumor proliferation in non small-cell lung cancer

PURPOSE

We investigated the clinical significance of E2F1 gene expression in relation to its target genes, thymidylate synthase (TS) and Survivin, in case of non-small-cell lung cancer (NSCLC).

EXPERIMENTAL DESIGN

One hundred twenty-seven cases of resected NSCLC were analyzed. Quantitative reverse transcription-PCR was done to evaluate the gene expression of E2F1, TS, and Survivin. Immunohistochemistry was done to investigate the protein expression of E2F1, TS, and Survivin. The Ki-67 proliferation index and the apoptotic index using the terminal deoxyribonucleotidyl transferase-mediated dUTP nick-end labeling method were also evaluated.

RESULTS

E2F1 gene expression significantly correlated with the Ki-67 proliferation index (r = 0.487; P < 0.0001), although no correlation was observed between E2F1 gene expression and the apoptotic index. With regard to E2F1 target genes, E2F1 gene expression significantly correlated with TS gene expression (r = 0.709; P < 0.0001) and Survivin gene expression (r = 0.403; P < 0.0001). The overall survival rate was significantly lower in patients with high-E2F1 tumors than in those with low-E2F1 tumors (P = 0.0027), especially among patients with stage II to III NSCLCs (P = 0.0188). A Cox regression analysis showed that the E2F1 status was a significant prognostic factor for NSCLC patients (hazard ratio, 2.052; P = 0.0261).

CONCLUSIONS

The present study revealed that E2F1 gene expression correlates with TS and Survivin gene expressions and tumor proliferation. During the progression of NSCLC, E2F1 overexpression could produce more aggressive tumors with a high proliferation rate and chemoresistance.

Folate-mediated one-carbon metabolism

Tetrahydrofolate (THF) polyglutamates are a family of cofactors that carry and chemically activate one-carbon units for biosynthesis. THF-mediated one-carbon metabolism is a metabolic network of interdependent biosynthetic pathways that is compartmentalized in the cytoplasm, mitochondria, and nucleus. One-carbon metabolism in the cytoplasm is required for the synthesis of purines and thymidylate and the remethylation of homocysteine to methionine. One-carbon metabolism in the mitochondria is required for the synthesis of formylated methionyl-tRNA; the catabolism of choline, purines, and histidine; and the interconversion of serine and glycine. Mitochondria are also the primary source of one-carbon units for cytoplasmic metabolism. Increasing evidence indicates that folate-dependent de novo thymidylate biosynthesis occurs in the nucleus of certain cell types. Disruption of folate-mediated one-carbon metabolism is associated with many pathologies and developmental anomalies, yet the biochemical mechanisms and causal metabolic pathways responsible for the initiation and/or progression of folate-associated pathologies have yet to be established. This chapter focuses on our current understanding of mammalian folate-mediated one-carbon metabolism, its cellular compartmentation, and knowledge gaps that limit our understanding of one-carbon metabolism and its regulation.

Expression of thymidylate synthase in human cells is an early G(1) event regulated by CDK4 and p16INK4A but not E2F

Thymidylate synthase (TS) is the enzyme that catalyses the last step in de novo thymidylate synthesis. It is of interest clinically because it is an effective target for drugs such as 5-fluorouracil, often used in combination therapy. Despite a number of earlier reports indicating that TS is a cell cycle-dependent enzyme, this remains equivocal. Here, we show that in HCT116 cells synchronised by serum starvation, there is a clear dissociation between the expression of cyclin E (a well-characterised cell-cycle protein) and TS. Although both cyclin E and TS mRNA and protein increased during G₁, TS upregulation was delayed. Moreover, TS levels did not decrease following S-phase completion while cyclin E decreased sharply. Similarly, clear differences were seen between cyclin E and TS as asynchronously growing HCT116 cells were growth-inhibited by low-serum treatment. In contrast to previous reports using rodent cells, adenovirus-mediated over-expression of E2F1 and cyclin E in three human cell lines had no effect on TS. Cell-cycle progression was blocked by treatment of cells with pharmacological inhibitors of CDK2 and CDK4 and by ectopic expression of p16INK4A. Whereas CDK2 inhibition had no effect on TS levels, inhibition of CDK4 was associated with decreased TS protein levels. These results provide the first evidence that drugs targeting CDK4 may be useful with anti-TS drugs as combination therapy for cancer.

Neuro-endocrine tumours of the lung. A review of relevant pathological and molecular data

Neuroendocrine (NE) tumours of the lung include pure and mixed forms. In the former group, a continuum of lesions is recognised ranging from benign typical carcinoids to atypical carcinoids (having a low-grade behaviour, although often associated with regional and distant metastases), to the highly aggressive poorly differentiated carcinomas of the small and large cell types. In the mixed tumour group, the NE component is extensively represented in association with any of the non-small cell carcinoma subtypes (so-called combined carcinomas), or the NE component is restricted to a cell population scattered among adenocarcinoma cells (or more rarely within squamous or large cell carcinomas). The molecular profile of NE tumours has been widely investigated to identify features helpful for the diagnosis, prognosis and even therapy for this special lung tumour category. Specific chromosomal alterations, oncogene mutations and cell cycle molecule disregulation has been documented in NE tumours of the lung, as well as the expression of specific receptors or enzymes implicated in the response to biotherapies or to chemotherapeutic agents. The "molecular classification" of NE tumours should be integrated to morphology, for a better definition of the different histological types and a more appropriate selection of the therapeutic strategy.

Evidence for small ubiquitin-like modifier-dependent nuclear import of the thymidylate biosynthesis pathway

Perturbations in folate-mediated one-carbon metabolism increase rates of uracil misincorporation into DNA during replication, impair cellular methylation reactions, and increase risk for neural tube defects and cancer. One-carbon metabolism is compromised by folate deficiency and common genetic polymorphisms. In this study, the mechanism for the preferential partitioning of cytoplasmic serine hydroxymethyltransferase (cSHMT)-derived methylenetetrahydrofolate to de novo thymidylate biosynthesis was investigated. The cSHMT enzyme was shown to interact with UBC9 and was a substrate for UBC9-catalyzed small ubiquitin-like modifier (SUMO) modification in vitro. SUMOylated cSHMT was detected in extracts from S phase MCF-7 cells, and cSHMT was shown to localize to the nucleus and nuclear periphery during the S and G(2)/M phases of the cell cycle. A common single nucleotide polymorphism (L474F-cSHMT) impaired the UBC9-cSHMT interaction and inhibited cSHMT SUMOylation in vitro. The three folate-dependent enzymes that constitute the de novo thymidylate biosynthesis pathway, cSHMT, thymidylate synthase, and dihydrofolate reductase, all contain SUMO modification consensus sequences. Compartmentation of the folate-dependent de novo thymidylate biosynthesis pathway in the nucleus accounts for the preferential partitioning of cSHMT-derived folate-activated one-carbon units into thymidylate biosynthesis; the efficiency of nuclear folate metabolism is likely to be modified by the cSHMT L474F polymorphism.

Transgenic expression of human thymidylate synthase accelerates the development of hyperplasia and tumors in the endocrine pancreas

Thymidylate synthase (TS) is an essential enzyme for DNA synthesis and repair and elevated levels of TS have been identified as an important prognostic biomarker for colorectal cancer and several other common human malignancies. In addition, TS gene expression has been linked with cell-cycle regulation and cell proliferation through the ability of retinoblastoma protein to repress the transcriptional activation of E2F target genes such as TS. Therefore, overproduction of TS could participate in the progression to a neoplastic phenotype. Consistent with this model, a recent study has suggested that ectopic TS expression can induce a transformed phenotype in mammalian cells. To investigate the role of deregulated TS activity in tumor development, we generated transgenic mice that express high levels of catalytically active human TS (hTS) exclusively in the pancreas and low levels of hTS in multiple other tissues. Analyses of pancreatic tissue in TS transgenic mice revealed abnormalities within the endocrine pancreas, ranging from pancreatic islet hyperplasia to the detection of islet cell tumors. Overexpression of hTS in murine islets provides a new model to study genetic alterations associated with the progression from normal cells to hyperplasia to islet cell tumors, and suggests that this mouse model may be useful for regulating TS activity in vivo for development of cancer prevention and new therapies.

The Ets transcription factor GABP is required for cell-cycle progression

The transition from cellular quiescence (G0) into S phase is regulated by the mitogenic-activation of D-type cyclins and cyclin-dependent kinases (Cdks), the sequestration of the Cdk inhibitors (CDKIs), p21 and p27, and the hyperphosphorylation of Rb with release of E2F transcription factors. However, fibroblasts that lack all D-type cyclins can still undergo serum-induced proliferation and key E2F targets are expressed at stable levels despite cyclical Rb-E2F activity. Here, we show that serum induces expression of the Ets transcription factor, Gabpalpha, and that its ectopic expression induces quiescent cells to re-enter the cell cycle. Genetic disruption of Gabpalpha prevents entry into S phase, and selectively reduces expression of genes that are required for DNA synthesis and degradation of CDKIs, yet does not alter expression of D-type cyclins, Cdks, Rb or E2Fs. Thus, GABP is necessary and sufficient for re-entry into the cell cycle and it regulates a pathway that is distinct from that of D-type cyclins and CDKs.

Evaluations of biomarkers associated with 5-FU sensitivity for non-small-cell lung cancer patients postoperatively treated with UFT

The sensitivity to 5-fluorouracil (5-FU) has been reported to be associated with target molecule thymidylate synthase (TS), fluoropyrimidine-metabolising enzymes such as orotate phosphoribosyltransferase (OPRT), and dihydropyrimidine dehydrogenase (DPD). We performed an immunohistochemical study on the clinical significance of TS, OPRT, and DPD expression using 151 resected non-small-cell lung cancer (NSCLC) patients postoperatively treated with a combination of tegafur and uracil (UFT). Eighty-two carcinomas were TS-positive, 105 carcinomas were OPRT-positive, 68 carcinomas were DPD-positive. No correlation was observed in the HSCORE between the TS and OPRT expression (r=0.203), between the TS and DPD expression (r=0.098), or between the OPRT and DPD expression (r=0.074). Regarding the survival of NSCLC patients treated with UFT, the 5-year survival rate of patients with TS-negative tumours was significantly higher than that with TS-positive tumours (P=0.0133). The 5-year survival rate of patients with OPRT-positive stage II to III tumours was significantly higher than that with OPRT-negative stage II to III tumours (P=0.0145). In addition, the 5-year survival rate of patients with DPD-negative tumours was also significantly higher than that with DPD-positive tumours (P=0.0004). A Cox multivariate regression analysis revealed the TS status (hazard ratio 2.663; P=0.0003), OPRT status (hazard ratio 2.543; P=0.0005), and DPD status (hazard ratio 2.840; P<0.0001) to all be significant prognostic factors for the survival of resected NSCLC patients postoperatively treated with UFT.

Multi-level gene expression profiles affected by thymidylate synthase and 5-fluorouracil in colon cancer

Background

Thymidylate synthase (TS) is a critical target for cancer chemotherapy and is one of the most extensively studied biomarkers for fluoropyrimidine-based chemotherapy. In addition to its critical role in enzyme catalysis, TS functions as an RNA binding protein to regulate the expression of its own mRNA translation and other cellular mRNAs, such as p53, at the translational level. In this study, a comprehensive gene expression analysis at the levels of both transcriptional and post-transcriptional regulation was conducted to identify response markers using human genome array with TS-depleted human colon cancer HCT-C18 (TS-) cells and HCT-C18 (TS+) cells stably transfected with the human TS cDNA expression plasmid.

Results

A total of 38 genes were found to be significantly affected by TS based on the expression profiles of steady state mRNA transcripts. However, based on the expression profiles of polysome associated mRNA transcripts, over 149 genes were affected by TS overexpression. This indicates that additional post-transcriptionally controlled genes can be captured with profiling polysome associated mRNA population. This unique approach provides a comprehensive overview of genes affected by TS. Additional novel post-transcriptionally regulated genes affected by 5-fluorouracil (5-FU) treatment were also discovered via similar approach.

Conclusion

To our knowledge, this is the first time that a comprehensive gene expression profile regulated by TS and 5-FU was analyzed at the multiple steps of gene regulation. This study will provide candidate markers that can be potentially used for predicting therapeutic outcomes for fluoropyrimidine-based cancer chemotherapy.

Significance of thymidylate synthase gene expression level in patients with adenocarcinoma of the lung

BACKGROUND

Thymidylate synthase (TS) catalyzes the methylation of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) and is a key enzyme for DNA synthesis. High expression of TS is thought to be associated with poor prognosis in some kinds of cancers. However, this association has not been clarified for nonsmall cell lung carcinoma. In the current study, the authors investigated the clinicopathologic significance of TS mRNA levels and the correlation with cellular proliferation in patients with lung adenocarcinoma.

METHODS

The expression levels of TS mRNA were measured in 47 lung adenocarcinoma tissues using the Taq-Man real-time reverse transcriptase-polymerase chain reaction (RT-PCR) method and examined the clinicopathologic significance of TS expression. To clarify the correlation between TS expression level and cell proliferation, the Ki-67 labeling index was also examined using immunohistochemical staining.

RESULTS

A positive correlation was observed between the expression levels of TS mRNA and stage of disease, lymph node metastasis, or tumor differentiation (P = .015). The higher expression group of TS mRNA showed a significantly poorer prognosis than the group with lower expression (P = .042). Moreover, there was a strong correlation noted between the expression levels of TS mRNA and the Ki-67 labeling index (P = .009).

CONCLUSIONS

The results of the current study demonstrated that TS may be associated with stage of disease, lymph node metastasis, tumor differentiation, prognosis, and tumor cell proliferation. These results suggest that the expression levels of TS mRNA may be useful for predicting the malignant potential in patients with lung adenocarcinoma.

Elevated levels of the 64-kDa cleavage stimulatory factor (CstF-64) in lipopolysaccharide-stimulated macrophages influence gene expression and induce alternative poly(A) site selection

Lipopolysaccharide (LPS) activation of murine RAW 264.7 macrophages influences the expression of multiple genes through transcriptional and post-transcriptional mechanisms. We observed a 5-fold increase in CstF-64 expression following LPS treatment of RAW macrophages. The increase in CstF-64 protein was specific in that several other factors involved in 3'-end processing were not affected by LPS stimulation. Activation of RAW macrophages with LPS caused an increase in proximal poly(A) site selection within a reporter mini-gene containing two linked poly(A) sites that occurred concomitant with the increase in CstF-64 expression. Furthermore, forced overexpression of the CstF-64 protein also induced alternative poly(A) site selection on the reporter minigene. Microarray analysis performed on CstF-64 overexpressing RAW macrophages revealed that elevated levels of CstF-64 altered the expression of 51 genes, 14 of which showed similar changes in gene expression with LPS stimulation. Sequence analysis of the 3'-untranslated regions of these 51 genes revealed that over 45% possess multiple putative poly(A) sites. Two of these 51 genes demonstrated alternative polyadenylation under both LPS-stimulating and CstF-64-overexpressing conditions. We concluded that the physiologically increased levels of CstF-64 observed in LPS-stimulated RAW macrophages contribute to the changes in expression and alternative polyadenylation of a number of genes, thus identifying another level of gene regulation that occurs in macrophages activated with LPS.

Control of dTTP pool size by anaphase promoting complex/cyclosome is essential for the maintenance of genetic stability

Anaphase promoting complex/cyclosome (APC/C)-mediated proteolysis is essential for chromosome segregation, mitotic exit, and G1 entry. Here, we show the importance of APC/C in the control of dTTP pool size in mammalian cells. Two enzymes, thymidine kinase 1 (TK1) and thymidylate kinase (TMPK), involved in dTTP formation are the targets of the APC/C pathway. We demonstrate that TMPK is recognized and degraded by APC/C-Cdc20/Cdh1-mediated pathways from mitosis to the early G1 phase, whereas TK1 is targeted for degradation by APC/C-Cdh1 after mitotic exit. Overexpression of wild-type TK1 and TMPK induces a four- to fivefold increase in the cellular dTTP pool without promoting spontaneous mutations in the hprt (hypoxanthine-guanine phosphoribosyl transferase) gene. In contrast, coexpression of nondegradable TK1 and TMPK expands the dTTP pool size 10-fold accompanied by a drastic dNTP pool imbalance. Most interestingly, disruption of APC/C proteolysis of TK1 and TMPK leads to growth retardation and a striking increase in gene mutation rate. We conclude that down-regulation of dTTP pool size by the APC/C pathway during mitosis and the G1 phase is an essential means to maintain a balanced dNTP pool and to avoid genetic instability.

Clinical application of biological markers for treatments of resectable non-small-cell lung cancers

We performed a clinical study to identify biological markers useful for the treatment of resectable non-small-cell lung cancers (NSCLCs). In all, 173 patients were studied. By immunohistochemistry, we evaluated the Ki-67 proliferation index, tumour vascularity, thymidylate synthase (TS), vascular endothelial growth factor (VEGF)-A, VEGF-C, and E (epithelial)-cadherin. Concerning the survival of NSCLC patients, tumour vascularity (P<0.01), VEGF-A status (P=0.03), VEGF-C status (P=0.03), and E-cadherin status (P=0.03) were significant prognostic factors in patients with stage I NSCLCs. The Ki-67 proliferation index (P=0.02) and TS status (P<0.01) were significant prognostic factors in patients with stage II–III NSCLCs. In patients with stage II–III NSCLCs, furthermore, the survival of UFT (a combination of tegafur and uracil)-treated patients with TS-negative tumours was significantly better than those of any other patients. Biological markers associated with tumour angiogenesis or metastasis are useful for the detection of aggressive tumours among early-stage NSCLCs. Postoperative chemotherapy might be necessary in such tumours even in stage I. In contrast, tumour proliferation rate and TS status are useful markers for identifying less aggressive tumours in locally advanced NSCLCs. Thymidylate synthase expression is also a useful marker to evaluate responsiveness of UFT-based chemotherapy for these tumours.

Thymidylate synthase and dihydropyrimidine dehydrogenase mRNA levels in tumor tissues and the efficacy of 5-fluorouracil in patients with non-small-cell lung cancer

We examined 116 stage I-IIIA non-small-cell lung cancer (NSCLC) patients for intra-tumoral expression of thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD) using TaqMan reverse transcription polymerase chain reaction (RT-PCR) assay to clarify the correlation between gene expression and the efficacy of 5-fluorouracil (5-FU) in patients with NSCLC. Patients who were administered 5-FU alone after surgery comprised the 5-FU group (n = 30), and those who underwent only surgery comprised the control group (n = 86). When dichotomized at the mean TS and DPD mRNA level, patients with low-DPD tumors who were administered 5-FU had a significantly better prognosis than those who did not receive adjuvant treatment (p = 0.041). In addition, in the 5-FU group, 10 patients with both low-TS and low-DPD tumors have not had any relapse, whereas 8 of the 20 patients with either high-TS or high-DPD tumors developed distant metastasis after surgery. Based on these results, the quantitation of TS and DPD mRNA levels may predict the efficacy of 5-FU after surgery for patient with NSCLC.

Thymidylate synthase as an oncogene: a novel role for an essential DNA synthesis enzyme

Thymidylate synthase (TS) is an E2F1-regulated enzyme that is essential for DNA synthesis and repair. TS protein and mRNA levels are elevated in many human cancers, and high TS levels have been correlated with poor prognosis in patients with colorectal, breast, cervical, bladder, kidney, and non-small cell lung cancers. In this study, we show that ectopic expression of catalytically active TS is sufficient to induce a transformed phenotype in mammalian cells as manifested by foci formation, anchorage independent growth, and tumor formation in nude mice. In contrast, comparable levels of two TS mutants carrying single point mutations within the catalytic domain had no transforming activity. In addition, we show that overexpression of TS results in apoptotic cell death following serum removal. These data demonstrate that TS exhibits oncogene-like activity and suggest a link between TS-regulated DNA synthesis and the induction of a neoplastic phenotype.

Prognostic significance of orotate phosphoribosyltransferase activity in bladder carcinoma

BACKGROUND

5-Fluorouracil (5-FU), an antitumor agent, is used clinically against a variety of malignancies, including bladder carcinoma. 5-FU is a prodrug, and orotate phosphoribosyltransferase (OPRT) is the principal enzyme that converts 5-FU directly into an active antitumor metabolite, 5-fluoro-2'-deoxyuridine 5'-monophosphate. In addition, OPRT is the key enzyme in the de novo DNA and RNA synthetic process. To the authors' knowledge, little is known regarding the significance of OPRT in various malignancies, including bladder carcinoma. The authors analyzed the activity levels of OPRT in 60 bladder carcinomas and evaluated the association between the level of OPRT activity and the stage and grade status of bladder carcinoma. They also examined the prognostic significance of OPRT activity in patients with bladder carcinoma and the correlation between OPRT activity levels in bladder carcinoma cells and the sensitivity of those cells to 5-FU.

METHODS

OPRT activity levels in nonfixed, fresh-frozen specimens of bladder carcinoma and normal bladder were determined enzymatically using a 5-FU phosphorylation assay. The sensitivity of bladder cells to 5-FU was assessed using a microculture tetrazolium dye assay.

RESULTS

The activity levels of OPRT were approximately 7.5-fold higher in bladder carcinoma specimens compared with the activity levels in normal bladder specimens. OPRT activity in muscle-invasive bladder carcinoma was 2-fold higher compared with the activity in superficial bladder carcinoma (classified as Ta and T1). In addition, the activity of OPRT in T1 bladder carcinoma was 2-fold higher compared with the activity in Ta bladder carcinoma. The level of OPRT activity in Grade 3 bladder carcinoma was 6-fold and 2-fold higher compared with the activity in Grade 1 and Grade 2 bladder carcinoma, respectively. Patients who had Ta and T1 bladder carcinoma with low OPRT activity had a longer postoperative tumor free period compared with patients who had bladder carcinoma with high OPRT activity in the 3-year follow-up. There was a positive association between the activity levels of OPRT and thymidylate synthase/thymidine kinase, which are the key enzymes in the de novo/salvage DNA synthetic process. OPRT activity in bladder carcinoma cells was correlated positively with their sensitivity to 5-FU.

CONCLUSIONS

to the authors' knowledge, the current study is the first to demonstrate that OPRT activity levels in bladder carcinoma were higher compared with its activity in the normal bladder tissues and that OPRT activity levels were correlated positively with the stage and grade of bladder carcinoma. In addition, high OPRT activity levels in patients with superficial bladder carcinoma predicted early recurrence and high sensitivity to 5-FU. These results suggest that the level of OPRT activity may be used both as a prognostic parameter and as a predictive indicator for 5-FU efficacy in patients with bladder carcinoma and that OPRT may be a molecular therapeutic target in bladder carcinoma.

Significance of Orotate Phosphoribosyltransferase Activity in Renal Cell Carcinoma

PURPOSE

The anticancer agent 5-fluorouracil (5-FU) is clinically used against various cancers, including renal cell carcinoma (RCC). It is a prodrug and orotate phosphoribosyltransferase (OPRT) is the principal enzyme that directly converts 5-FU to the active anticancer metabolite 5-fluoro-2'-deoxyuridine 5'-monophosphate. In addition, OPRT is the key enzyme in the de novo DNA and RNA synthetic process. Little is known about the significance of OPRT in various cancers, including RCC. We investigated the activity of OPRT in 83 RCCs and evaluated the association between the level of OPRT activity and the stage/grade of RCC. The relationship between OPRT activity in RCC cells and their sensitivity to 5-FU was also examined.

MATERIALS AND METHODS

OPRT activity in nonfixed, fresh frozen RCC and normal kidney was determined enzymatically by the 5-FU phosphorylation assay. The sensitivity of RCC cells to 5-FU was assessed by the microculture tetrazolium dye assay.

RESULTS

The activity of OPRT was approximately 8.5-fold higher in RCC than in normal kidney. OPRT activity in stage III/IV RCC was 3-fold higher than in stage I/II RCC. The level of OPRT activity in grade 3 RCC was 3-fold higher than that in grade 1/2 cancer. Patients with RCC with low OPRT activity had longer postoperative disease specific survival than those with high activity at 5-year followup. OPRT activity in RCC cells positively correlated with their sensitivity to 5-FU.

CONCLUSIONS

To our knowledge this is the first study to demonstrate that OPRT activity in RCC was higher than that in normal kidney and OPRT activity positively correlated with RCC stage/grade. In addition, higher OPRT activity in RCC predicted worse prognosis and higher sensitivity to 5-FU. These results suggest that the level of OPRT activity may be used as a prognostic parameter and predictive indicator for 5-FU efficacy in RCC and OPRT may be a molecular therapeutic target in RCC.

Expression of thymidylate synthase is correlated with proliferative activity in non-small cell lung cancer (NSCLC)

Many experimental studies have revealed that enhanced thymidylate synthase (TS) expression is significantly correlated with higher proliferative activity of tumor cells, which may account for a poor prognosis of high-TS patients. However, only a few clinical studies have focused on the correlation between TS status and cell proliferation. Therefore, we assessed the correlation between TS expression and proliferative index (PI) as a marker of cell proliferation or p53 status in a total of 109 patients with completely resected pathologic stage I, non-small cell lung cancer (NSCLC). PI was defined as the percentage of tumor cells with positive staining against proliferative cell nuclear antigen (PCNA). The mean PIs of TS-high and TS-low tumors were 48.2% and 34.4% respectively, showing a significantly higher proliferative activity of TS-high tumor (P=0.020); when stratified according to histological type, the difference was significant in adenocarcinoma (P=0.038), but not in squamous cell carcinoma. There was no significant correlation between TS expression and p53 status. In conclusion, tumor cells with higher TS expression have higher proliferative activity in NSCLC, especially in adenocarcinoma.

Differential Expression of Thymidylate Synthase in Colorectal Tumors and Matched Lymph Nodes: Impact on Adjuvant Treatment

Although the expression of thymidylate synthase (TS) in metastatic colorectal cancer (CRC) may be a better predictor of response to 5-fluorouracil chemotherapy than TS expression in primary CRC, this enzyme has not been well studied in tumor-draining regional lymph nodes. We retrospectively examined TS expression in 12 primary CRC lesions (pT3) and matched sentinel lymph nodes. Of the 8 primary tumors that were TS-positive, 50 per cent (4/8) had tumor-positive lymph nodes and 50 per cent (4/8) had tumor-negative nodes. Of the 4 primary tumors that were TS-negative, 75 per cent (3/4) had tumor-positive nodes and 25 per cent (1/4) had tumor-negative nodes [κ = -0.1386, 95 per cent confidence interval: (-0.4820, 0.2048), P = 0.4284]. Of the 8 TS-positive primaries, 25 per cent (2/8) had TS-positive nodes and 75 per cent (6/8) had TS-negative nodes. Of the 4 TS-negative primaries, 50 per cent (2/4) had TS-positive nodes and 50 per cent (2/4) had TS-negative nodes [κ = -0.0131, 95 per cent confidence interval: (-0.2958, 0.2696), P = 0.9274]. Two of the three TS-negative primaries that had metastasized to regional lymph nodes were associated with TS-positive lymph nodes. Our findings indicate that expression of TS by a primary CRC does not correlate with nodal metastases or nodal TS expression. Nodal expression of TS may be important in predicting response to 5-fluorouracil when a primary CRC is TS-negative.

New prognostic indicator for non-small-cell lung cancer, quantitation of thymidylate synthase by real-time reverse transcription polymerase chain reaction

Thymidylate synthase (TS) is an enzyme that catalyzes an important DNA biosynthesis process. The gene expression of TS has not been reported in non-small-cell lung cancer (NSCLC) patients. To clarify the correlation between TS mRNA levels and clinicopathological features of NSCLC, we examined 70 Stage I and II NSCLC patients for intra-tumoral expression of TS using TaqMan reverse transcription polymerase chain reaction (RT-PCR) assay and immunohistochemistry methods. We also investigated the TS promoter 28 bp polymorphism in 48 cancer tissues using PCR amplification of genomic DNA. Lung cancer tissue showed higher TS mRNA levels than normal lung tissue (Mann-Whitney U-tests; p = 0.0020). Further, TS mRNA expression was correlated with immunohistochemical TS expression (p = 0.029). We obtained 2 different DNA fragments, which indicated triple-repeat (3R) and double-repeat (2R) type alleles. Cancer tissues with the 3R/3R genotype showed significantly higher TS mRNA levels as compared to those with other genotypes (p = 0.0019). The TS genotype was also correlated with immunohistochemical TS expression (chi(2) test; p = 0.0079). The disease-free survival of the low TS mRNA level group was significantly better than those with high TS mRNA levels (log-rank test; p = 0.010), however, there were no significant differences found by immunohistochemical evaluation (p = 0.34) or TS genotype analysis (p = 0.11). A multivariate analysis revealed that high TS mRNA levels independently contributed to disease-free survival. The quantitation of TS mRNA levels is clinically more sensitive and useful for determining the prognosis of Stage I and II NSCLC patients than an immunohistochemical evaluation.

Mice deficient for cytosolic thymidine kinase gene develop fatal kidney disease

The thymidine kinase (Tk) gene codes for a cytosolic protein involved in the pyrimidine nucleotide salvage pathway. A functional Tk gene is not necessary for cells in culture, and a naturally occurring Tk deficient phenotype has not been described in humans or animal models. In order to determine the biological significance of the Tk gene, we created Tk(-/-) knockout (KO) mice through homologous recombination in mouse embryonic stem cells. Tk KO mice have shortened life spans compared with their wild-type or Tk heterozygous (HET) siblings. All Tk KO mice develop sclerosis of kidney glomeruli and die before one year of age of kidney failure. Among other changes in KO animals, the most consistent is a switch from exclusively mucous secretion to predominantly serous secretion in the sublingual salivary gland. HET parents can produce KO mice at a frequency approaching Mendelian inheritance. Other observations in KO animals include an elevated level of serum thymidine, a significant decrease in the cloning efficiency of splenic lymphocytes, an increase in the frequency of hypoxanthine guanine phosphoribosyl transferase gene mutant lymphocytes, and histological alteration in the lymphoid structure of the spleen. In addition, KO animals sporadically exhibit inflammation of the arteries, which taken together with the lymphocyte and spleen abnormalities, suggest an abnormal immune system. Alterations in Tk KO mice indicate that the pyrimidine nucleotide salvage pathway is indispensable in vivo.

Evaluation of thymidylate synthase protein expression by Western blotting and immunohistochemistry on human colon carcinoma xenografts in nude mice

In this study we investigated the relationship between thymidylate synthase (TS) protein expression, evaluated by Western blotting analysis and by immunohistochemistry (IHC), and growth rate in human colon xenograft tumors in nude mice. Human colon cancer cell lines were used to induce xenograft tumors and the tumor mass growth rate was calculated by measuring tumor size variations over time. TS 106 monoclonal antibody was used for both Western blotting and IHC TS detection. Tumor cell growth fraction was measured by Ki67/MIB1 immunolabeling and tumor cell growth rate by evaluating the mean nucleolar size in silver-stained sections. TS Western blotting values were related to tumor mass growth rate (p<0.001) and cell growth rate (p=0.002) but not to cell growth fraction (p=0.676). The degree of the IHC staining showed only a trend to be associated with TS protein expression measured on Western blotting, and was not related either to tumor mass growth or cell proliferation rate. Tumor xenografts were also characterized for TS promoter tandem repeat and p53 status. No relationship was observed between these variables and TS expression evaluated by both Western blotting and IHC analysis. Our results demonstrate that TS expression evaluated by Western blotting analysis is directly related to the tumor mass growth rate and question the use of the IHC approach to obtain precise quantitative information on TS expression in tumor samples.

Combination of 5-fluorouracil and irinotecan on modulation of thymidylate synthase and topoisomerase I expression and cell cycle regulation in human colon cancer LoVo cells: clinical relevance

This study was designed to explore the possible interaction of 5-fluorouracil (5-FU) and 7-ethyl-10-hydroxycamptothecin (SN-38) in vitro. Human colon cancer LoVo cells were treated in both a dose- and time-dependent manner using clinically relevant concentrations of and exposure to 5-FU and/or SN-38. The expression of thymidylate synthase (TS), topoisomerase I, and cell cycle kinetics were evaluated by Western blot analysis and flow cytometry, respectively. Cytotoxicity was evaluated by MTT (3-[4,5-dimethylthiazol-2yl]-2,5-diphenyl tetrazolium bromide) assay. The cytotoxic effects of combination treatment were determined by median effect analysis. Topoisomerase I expression was downregulated following 12 hours of exposure to treatment, and topoisomerase I expression recovered 8 hours after SN-38 was removed. The TS expression was decreased following 24 hours of 5-FU and it remained at reduced levels for > 24 hours after removal of 5-FU. SN-38 induced an arrest at S/G2/M phase, reaching its maximum effect at 12 hours. This cell cycle arrest was reversed 24 hours after SN-38 was removed. 5-FU induced an arrest at the S phase, and maximum arrest occurred at 12 hours and lasted for > 48 hours. After 12 hours of sequential SN-38, LoVo cells were arrested in S phase, thereby potentiating the effect of 5-FU. Cytotoxicity studies confirmed the synergistic interaction between 5-FU and irinotecan. These findings suggest that the proper sequencing of 5-FU/irinotecan depends on regulation of topoisomerase I, and cell cycle kinetics

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.

Thymidylate synthase as a translational regulator of cellular gene expression

Studies from our laboratory have shown that the folate-dependent enzyme, thymidylate synthase (TS), functions as an RNA binding protein. There is evidence that TS, in addition to interacting with its own TS mRNA, forms a ribonucleoprotein complex with a number of other cellular mRNAs, including those corresponding to the p53 tumor suppressor gene and the myc family of transcription factors. Using both in vitro and in vivo model systems, we have demonstrated that the functional consequence of binding of TS protein to its own cognate mRNA, as well as binding of TS to the p53 mRNA, is translational repression. Herein, we review current work on the translational autoregulatory control of TS expression and discuss the molecular elements that are required for the TS protein-TS mRNA interaction. TS may play a critical role in regulating the cell cycle and the process of apoptosis through its regulatory effects on expression of p53 and perhaps other cell cycle related proteins. Finally, the ability of TS to function as a translational regulator may have important consequences with regard to the development of cellular resistance to various anticancer drugs.