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

DTYMK is an essential gene in mice and heterozygosity does not cause neural tube defects

Regulation of nucleotide biosynthesis is necessary for maintaining cellular processes including DNA replication and repair. A key enzyme in this process is deoxythymidylate kinase (dTYMK), which catalyzes the initial step in the production of dTTP from dTMP. This gene constitutes the first merged step of dTTP synthesis from the de novo and salvage pathways which regulate dTMP biosynthesis. Decreased de novo dTMP biosynthesis causes dysregulated dTTP:dUTP pools, and leads to increased uracil in DNA and neural tube closure defect (NTD) development in mice. The goal of this research was to investigate if dTYMK, the downstream enzyme in dTTP production, is an essential gene in mice and if impairments in dTYMK play a causal role in development including NTD pathology in mice. Dtymk+/- C57BL/6J females were weaned onto either a control, excess folic acid, or folic acid deficient diet and timed breeding was performed after 8 weeks on diet. The offspring were analyzed for NTDs and other reproductive outcomes at embryonic day 12.5 (E12.5). Dtymk-/- mice were confirmed to be embryonic lethal before E12.5, and Dtymk+/- mice on all three experimental diets did not show the presence of open neural tube defects, spina bifida or exencephaly. However, the expression of dTYMK in Dtymk+/- mouse embryos was confirmed to be decreased by approximately 3-fold compared to Dtymk+/+ embryos. Although dTYMK was demonstrated to be an essential gene in mice and is required for the regulation of nucleotide pools in vitro, there was no evidence of increased risk of NTDs because of a reduction in expression of this enzyme during embryonic development. It is possible that a further reduction in expression may be required to see developmental anomalies in C57BL/6J mice.

Integrated spectroscopic and MD simulation approach to decipher the effect of pH on the structure function of Staphylococcus aureus thymidine kinase

Staphylococcus aureus is a major human pathogen responsible for a variety of clinical infections, becoming increasingly resistant to antibiotics. To address this challenge, there is a need to identify new cellular targets and innovative approaches to expand treatment options. One such target is thymidine kinase (TK), a crucial enzyme in the pyrimidine salvage pathway, which plays a key role in the phosphorylation of thymidine, an essential component in DNA synthesis and repair. In this study, we have successfully cloned, expressed, and purified the TK protein. A comprehensive investigation into how different pH levels affect the structure and functional activity of TK, using a combination of spectroscopy, classical molecular dynamics simulations, and enzyme activity assays was conducted. Our study revealed that variation in pH disrupts secondary and tertiary structures of TK with noticeable aggregate formation at pH 5.0. Enzyme activity studies demonstrated that TK exhibited its maximum kinase activity within the physiological pH range. These findings strongly suggest a connection between structural changes and enzymatic activity, which was further supported by the agreement between the spectroscopic features we measured and the results of our MD simulations. Our study provides a deeper insight into the structural features of TK, which could potentially be harnessed for the development of therapeutic strategies aimed at combatting infectious diseases. Conformational dynamics plays an essential role in the design and development of effective inhibitors. Considering the effects of pH on the conformational dynamics of TK, our findings may be implicated in the development of potent and selective inhibitors.Communicated by Ramaswamy H. Sarma.

TK1 expression influences pathogenicity by cell cycle progression, cellular migration, and cellular survival in HCC 1806 breast cancer cells

Breast cancer is the most common cancer diagnosis worldwide accounting for 1 out of every 8 cancer diagnoses. The elevated expression of Thymidine Kinase 1 (TK1) is associated with more aggressive tumor grades, including breast cancer. Recent studies indicate that TK1 may be involved in cancer pathogenesis; however, its direct involvement in breast cancer has not been identified. Here, we evaluate potential pathogenic effects of elevated TK1 expression by comparing HCC 1806 to HCC 1806 TK1-knockdown cancer cells (L133). Transcriptomic profiles of HCC 1806 and L133 cells showed cell cycle progression, apoptosis, and invasion as potential pathogenic pathways affected by TK1 expression. Subsequent in-vitro studies confirmed differences between HCC 1806 and L133 cells in cell cycle phase progression, cell survival, and cell migration. Expression comparison of several factors involved in these pathogenic pathways between HCC 1806 and L133 cells identified p21 and AKT3 transcripts were significantly affected by TK1 expression. Creation of a protein-protein interaction map of TK1 and the pathogenic factors we evaluated predict that the majority of factors evaluated either directly or indirectly interact with TK1. Our findings argue that TK1 elevation directly increases HCC 1806 cell pathogenicity and is likely occurring by p21- and AKT3-mediated mechanisms to promote cell cycle arrest, cellular migration, and cellular survival.

Thymidine kinase 1 appears to be a marker for the prognosis of hepatocellular carcinoma based on a large-scale, multicenter study

INTRODUCTION

The aim of this study was to investigate the role of thymidine kinase 1 (TK1) levels in hepatocellular carcinoma (HCC) prognosis and to develop a nomogram for predicting HCC prognosis.

METHOD

In this study, 1066 HCC patients were enrolled between August 2018 and April 2022. TK1 levels were measured within one week before enrollment, and the relationship with HCC prognosis was evaluated. Next, all patients were randomly assigned to the training set (70%, n = 746) and the validation set (30%, n = 320). We used multivariate Cox analysis to find independent prognostic factors in the training set to construct a nomogram. The predictive power of the nomogram was assessed using receiver operating characteristic (ROC) curves, calibration curves, and decision curve analysis (DCA). The optimal critical value of TK1 was determined as 2.35 U/L using X-tile software.

RESULT

Before and after propensity score matching (PSM), the median overall survival (mOS) of the low-TK1 group (< 2.35 U/L) remained significantly longer than that of the high-TK1 group (≥ 2.35 U/L) (48.1 vs 16.5 months, p < 0.001; 75.7 vs 19.8 months, p = 0.001). Moreover, multivariate Cox analysis showed that the low TK1 level was an independent positive prognostic indicator. Additionally, the area under the ROC curve for predicting the 1-year, 2-year, and 3-year survival rates was 0.770, 0.758, and 0.805, respectively.

CONCLUSIONS

TK1 could serve as a prognostic marker for HCC. In addition, the nomogram showed good predictive capability for HCC prognosis.

Prediction of Overall Survival by Thymidine Kinase 1 Combined with Prostate-Specific Antigen in Men with Prostate Cancer

Thymidine kinase 1 (TK1) is an intracellular enzyme involved in DNA-precursor synthesis. Increased serum TK1 levels are used as a biomarker in various malignancies. We combined serum TK1 with PSA and evaluated its capacity to predict overall survival (OS) in 175 men with prostate cancer (PCa), detected by screening in 1988-1989 (n = 52) and during follow-up (median 22.6 years) (n = 123). TK1 was measured in frozen serum, age was stratified into four groups, and dates of PCa diagnosis and dates of death were obtained from Swedish population-based registries. The median concentration of TK1 and PSA was 0.25 and 3.8 ng/ml. TK1 was an independent variable of OS. In the multivariate analysis, PSA was not statistically significant in combination with age whereas the significance remained for TK1 + PSA. Measured once, TK1 + PSA predicted a difference of up to 10 years (depending on patient subgroup) in OS at a median of 9 years before PCa diagnosis. The TK1 concentration in 193 controls without malignancies did not differ from that of the PCa patients, hence TK1 was likely not released from incidental PCa. Thus, TK1 in the blood circulation may indicate the release of TK1 from sources other than cancers, nonetheless associated with OS.

Methylation status of TK1 correlated with immune infiltrates in prostate cancer

TK1 is overexpressed in numerous cancers and is associated with to a poor prognosis. However, the relationship between methylation status of TK1 and Immune Infiltrates in Prostate Cancer (PCa) is unknown. The goal of this study was to use comprehensive bioinformatic analyses to elucidate the involvement relationship between methylation status of TK1 and Immune Infiltrates in PCa. TK1 mRNA expression and methylation data in PCa were investigated via GEPIA, TIMER, and UALCAN coupled with MEXPRESS data resources. We employed the LinkedOmics data resource to determine the signaling cascades linked to TK1 expression. Single-cell analysis was performed using the CancerSEA data resource. GeneMANIA and CancerSEA were used to analyze the correlation between TK1 and TK1 coexpressed genes. In addition, TIMER and TISIDB were adopted to assess tumor-invading immune cells and immunomodulators. CTD was utilized to detect the drugs acting on TK1. This study found that TK1 was overexpressed in PCa, and its contents were linked to tumor stage and prognosis. Genes co-expressed with TK1 were enriched in cascades involved in the ribosome, cell cycle, oxidative phosphorylation, DNA replication, oocyte meiosis, and the proteasome. The expression of TK1 along with its methylation status was found to be linked to tumor-invading immune cells, as well as PCa immunomodulators. We also examined the prospect of employing TK1 as a possible target for PCa therapy. This work provides the clinical value of TK1 hypermethylation in PCa and highlights new insights into its novel immunomodulatory functions.

Assessing the Potential Prognostic and Immunological Role of TK1 in Prostate Cancer

Background: It has been reported that thymidine kinase 1 (TK1) was up-regulated in multiple malignancies and participated in the regulation of tumor malignant behavior. However, its specific role in prostate cancer (PCa) remains unclear. Methods: TK1 expression in PCa patients and cell lines was identified via crossover analysis of the public datasets. A series of in vitro experiments and in vivo models was applied to investigate the function of TK1 in PCa. Functional enrichment analyses were further conducted to explore the underlying mechanism. Additionally, TISIDB was applied to explore the correlation between TK1 expression and tumor-infiltrating lymphocytes, immune subtypes, and immune regulatory factors. Results: TK1 expression was significantly up-regulated in PCa patients and cell lines. TK1 ablation inhibited tumor cell proliferation and migration potential, and in vivo experiments showed that TK1 inactivation can significantly restrain tumor growth. Functional enrichment analysis revealed TK1-related hub genes (AURKB, CCNB2, CDC20, CDCA5, CDK1, CENPA, CENPM, KIF2C, NDC80, NUF2, PLK1, SKA1, SPC25, ZWINT), and found that TK1 was closely involved in the regulation of cell cycle. Moreover, elevated mRNA expression of TK1 was related with higher Gleason score, higher clinical stage, higher pathological stage, higher lymph node stage, shorter overall survival, and DFS in PCa patients. Particularly, TK1 represented attenuated expression in C3 PCa and was related with infiltration of CD4+, CD8+ T cells, and dendritic cells as well as immunomodulator expression. Conclusion: Our study indicates that TK1 is a prognostic predictor correlated with poor outcomes of PCa patients, and for the first time represented that TK1 can promote the progression of PCa. Therefore, TK1 may be a potential diagnostic and prognostic biomarker, as well as a therapeutic target for PCa.

Differential expression of enzymes in thymidylate biosynthesis in zebrafish at different developmental stages: implications for dtymk mutation-caused neurodegenerative disorders

Background

Deoxythymidine triphosphate (dTTP) is an essential building block of DNA, and defects in enzymes involved in dTTP synthesis cause neurodegenerative disorders. For instance, mutations in DTYMK, the gene coding for thymidylate kinase (TMPK), cause severe microcephaly in human. However, the mechanism behind this is not well-understood. Here we used the zebrafish model and studied (i) TMPK, an enzyme required for both the de novo and the salvage pathways of dTTP synthesis, and (ii) thymidine kinases (TK) of the salvage pathway in order to understand their role in neuropathology.

Results

Our findings reveal that maternal-stored dNTPs are only sufficient for 6 cell division cycles, and the levels of dNTPs are inversely correlated to cell cycle length during early embryogenesis. TMPK and TK activities are prominent in the cytosol of embryos, larvae and adult fish and brain contains the highest TMPK activity. During early development, TMPK activity increased gradually from 6 hpf and a profound increase was observed at 72 hpf, and TMPK activity reached its maximal level at 96 hpf, and remained at high level until 144 hpf. The expression of dtymk encoded Dtymk protein correlated to its mRNA expression and neuronal development but not to the TMPK activity detected. However, despite the high TMPK activity detected at later stages of development, the Dtymk protein was undetectable. Furthermore, the TMPK enzyme detected at later stages showed similar biochemical properties as the Dtymk enzyme but was not recognized by the Dtymk specific antibody.

Conclusions

Our results suggest that active dNTP synthesis in early embryogenesis is vital and that Dtymk is essential for neurodevelopment, which is supported by a recent study of dtymk knockout zebrafish with neurological disorder and lethal outcomes. Furthermore, there is a novel TMPK-like enzyme expressed at later stages of development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12868-022-00704-0.

A combined strategy of TK1, HE4 and CA125 shows better diagnostic performance than risk of ovarian malignancy algorithm (ROMA) in ovarian carcinoma

BACKGROUND

The dual marker algorithm Risk of Ovarian Malignancy Algorithm (ROMA) has been widely used in the clinic for the identification of equivocal pelvic masses in ovarian carcinoma. To obtain higher diagnostic efficiency, we created a new diagnostic index, Risk of Ovarian Malignancy Index (ROMI), by combing thymidine kinase 1 (TK1), HE4 and CA125.

METHODS

335 patients with pelvic masses on imaging and 46 healthy controls were enrolled. Serum TK1 was analyzed before further study. ROMI and ROMA were evaluated for diagnostic efficiency.

RESULTS

The level of TK1 was elevated in malignant ovarian tumors compared to benign masses (p < 0.001) and healthy controls (p < 0.001). TK1 expression was positively correlated with stage, intrapelvic metastasis, lymphatic metastasis and distant metastasis (all p values < 0.001). The area under the receiver operating characteristic curve (AUC) of ROMI was higher than that of ROMA for both pre- and postmenopausal women. ROMI had better sensitivity, specificity, accuracy, and positive and negative predictive values than ROMA in diagnosis of all-stage or stage I + II ovarian carcinoma for both pre- and postmenopausal women.

CONCLUSIONS

TK1 is a potential biomarker in detection of ovarian carcinoma. ROMI shows better diagnostic performance than ROMA in distinguishing malignant ovarian tumors from benign masses.

Immunohistochemistry for Thymidine Kinase-1 (TK1): A Potential Tool for the Prognostic Stratification of Breast Cancer Patients

Breast cancer (BC) is the most frequent non-cutaneous malignancy in women. Histological grade, expression of estrogen and progesterone receptors (ER and PgR), overexpression/amplification of the human epidermal growth factor receptor 2 (HER2) oncogene, and proliferative activity measured with ki-67 provide important information on the biological features of BC and guide treatment choices. However, a biomarker that allows a more accurate prognostic stratification is still lacking. Thymidine kinase-1 (TK1), a ubiquitous enzyme involved in the pyrimidine nucleotide recovery pathway, is a cell-proliferation marker with potential prognostic and predictive impacts in BC. Eighty (80) cases of invasive BC with a long-term follow-up were retrospectively selected, and clinicopathological data were collected for each patient. TK1 tissue expression was evaluated immunohistochemically. Data suggested that TK1 expression levels are positively correlated with ER and PgR expression, and negatively correlated with HER2 status and the impact on patients’ distant recurrence-free survival (DRFS): in detail, among patients undergoing adjuvant chemotherapy, lower TK1 levels are correlated with better DRFS. Therefore, these results contribute to furthering the knowledge of TK1, suggesting a possible and important role of this enzyme as a biomarker in the stratification of BC patients.

Dynamics of Serum Thymidine Kinase 1 at the First Cycle of Neoadjuvant Chemotherapy Predicts Outcome of Disease in Estrogen-Receptor-Positive Breast Cancer

Simple Summary

Chemotherapy before surgery (NAC) is an option for high-risk breast cancer (BC) patients. Pathologic complete response (pCR) predicts long-term outcome and has become a surrogate biomarker for survival. pCR is, however, reached in only <10% of hormone-receptor-positive (ER+) patients and is of limited prognostic value. Biomarkers able to predict outcome early during NAC would facilitate individualized therapy with the possibility to adjust or interrupt an ineffective therapy. Here, it is shown that differential response of the serum concentration of thymidine kinase 1, an enzyme involved in the DNA synthesis and released from the tumor into the blood, 48 h after the first cycle of NAC, predicts long-term outcome in localized advanced ER+/HER2-BC. The different reactions to chemotherapy could be used to guide this process early during NAC and utilized to identify mechanisms of tumor sensitivity that could provide a prediction of long-term outcome prior to chemotherapy.

Abstract

Pathologic complete response (pCR) predicts the long-term outcome of neoadjuvantly treated (NAC) breast cancer (BC) but is reached in <10% of hormone-receptor-positive patients. Biomarkers enabling adjustment or interruption of an ineffective therapy are desired. Here, we evaluated whether changes in the serum concentration of thymidine kinase 1 (sTK1) during NAC could be utilized as a biomarker. In the PROMIX trial, women with localized HER2- BC received neoadjuvant epirubicin/docetaxel in six cycles. sTK1 was measured with an ELISA in 54 patients at cycles 1–4 and in an additional 77 patients before and 48 h after treatment 1. Treatment resulted in a 2-fold increase of sTK1 before and a 3-fold increase 48 h after the cycles, except for the first cycle, where half of the patients reacted with a significant decrease and the other half with an increase of sTK1. In Kaplan–Meier estimates of ER+ patients divided by the median of the post/pre-treatment sTK1 ratio at the first treatment cycle, OS was 97.7% and 78% (p = 0.005), and DFS was 90.7% and 68% (p = 0.006), respectively. Thus, the response of sTK1 at the first cycle of chemotherapy could be used both as an early biomarker for the guidance of chemotherapy and for the study of inherent tumor chemo-sensitivity, which could predict long-term outcome prior to therapy.

Relationship between thymidine kinase 1 before radiotherapy and prognosis in breast cancer patients with diabetes

In a retrospective study design, we explored the relationship between serum thymidine kinase 1 (TK1) concentration before radiotherapy and clinical parameters and evaluated the prognostic value of serum TK1 concentration before radiotherapy in breast cancer patients with type 2 diabetes mellitus. The present study finally consisted of 428 breast cancer patients with a mean age of 53.0 years. Compared with low TK1 group, the high TK1 group tended to have larger tumor size (P=0.011) and had more lymph node number (P=0.021). Significant differences were also observed in clinical stages I, II and III (P=0.000). There was no significant difference between TK1 and other clinical parameters. For disease-free survival (DFS), the univariate analysis indicated that the high TK1 increased the risk of poor prognosis (HR = 2.38, 95% CI: 1.64–4.23, P=0.000). The Kaplan–Meier curve indicated the high TK1 group was poorer than that in the low TK1 group (P=0.002). For the overall survival (OS), similar results were found that the high TK1 was related to poor OS (HR = 1.89, 95% CI: 1.34–3.67, P=0.000). The multivariate Cox regression indicated that the TK1 was still associated with DFS (HR = 1.83, 95% CI: 1.22–3.17, P=0.001) and OS (HR = 1.63, 95% CI: 1.19–2.08, P=0.006). The high pretreatment serum TK1 levels in breast cancer patients were associated with poor OS and DFS. TK1 could be a potential predictive factor in differential diagnosis of poor prognosis from all patients.

Thymidine kinase 1 through the ages: a comprehensive review

Proliferation markers, such as proliferating cell nuclear antigen (PCNA), Ki-67, and thymidine kinase 1 (TK1), have potential as diagnostic tools and as prognostic factors in assessing cancer treatment and disease progression. TK1 is involved in cellular proliferation through the recovery of the nucleotide thymidine in the DNA salvage pathway. TK1 upregulation has been found to be an early event in cancer development. In addition, serum levels of TK1 have been shown to be tied to cancer stage, so that higher levels of TK1 indicate a more serious prognosis. As a result of these findings and others, TK1 is not only a potentially viable biomarker for cancer recurrence, treatment monitoring, and survival, but is potentially more advantageous than current biomarkers. Compared to other proliferation markers, TK1 levels during S phase more accurately determine the rate of DNA synthesis in actively dividing tumors. Several reviews of TK1 elaborate on various assays that have been developed to measure levels in the serum of cancer patients in clinical settings. In this review, we include a brief history of important TK1 discoveries and findings, a comprehensive overview of TK1 regulation at DNA to protein levels, and recent findings that indicate TK1’s potential role in cancer pathogenesis and its growing potential as a tumor biomarker and therapeutic target.

Polyomavirus Small T Antigen Induces Apoptosis in Mammalian Cells through the UNC5B Pathway in a PP2A-Dependent Manner

UNC5B is a dependence receptor that promotes survival in the presence of its ligand, netrin-1, while inducing cell death in its absence. The receptor has an important role in the development of the nervous and vascular systems. It is also involved in the normal turnover of intestinal epithelium. Netrin-1 and UNC5B are deregulated in multiple cancers, including colorectal, neuroblastoma, and breast tumors. However, the detailed mechanism of UNC5B function is not fully understood. We have utilized the murine polyomavirus small T antigen (PyST) as a tool to study UNC5B-mediated apoptosis. PyST is known to induce mitotic arrest followed by extensive cell death in mammalian cells. Our results show that the expression of PyST increases mRNA levels of UNC5B by approximately 3-fold in osteosarcoma cells (U2OS) and also stabilizes UNC5B at the posttranslational level. Furthermore, UNC5B is upregulated predominantly in those cells that undergo mitotic arrest upon PyST expression. Interestingly, although its expression was previously reported to be regulated by p53, our data show that the increase in UNC5B levels by PyST is p53 independent. The posttranslational stabilization of UNC5B by PyST is regulated by the interaction of PyST with PP2A. We also show that netrin-1 expression, which is known to inhibit UNC5B apoptotic activity, promotes survival of PyST-expressing cells. Our results thus suggest an important role of UNC5B in small-T antigen-induced mitotic catastrophe that also requires PP2A.IMPORTANCE UNC5B, PP2A, and netrin-1 are deregulated in a variety of cancers. UNC5B and PP2A are regarded as tumor suppressors, as they promote apoptosis and are deleted or mutated in many cancers. In contrast, netrin-1 promotes survival by inhibiting dependence receptors, including UNC5B, and is upregulated in many cancers. Here, we show that UNC5B-mediated apoptosis can occur independently of p53 but in a PP2A-dependent manner. A substantial percentage of cancers arise due to p53 mutations and are insensitive to chemotherapeutic treatments that activate p53. Unexpectedly, treatment of cancers having functional p53 with many conventional drugs leads to the upregulation of netrin-1 through activated p53, which is counterintuitive. Therefore, understanding the p53-independent mechanisms of the netrin-UNC5B axis, such as those involving PP2A, assumes greater clinical significance. Anticancer strategies utilizing anti-netrin-1 antibody treatment are already in clinical trials.

The expression and activity of thymidine kinase 1 and deoxycytidine kinase are modulated by hydrogen peroxide and nucleoside analogs

Thymidine kinase 1 (TK1) and deoxycytidine kinase (dCK) are required for the activation of thymidine and deoxycytidine analogs used in antiviral and anticancer therapies. Many anticancer drugs cause oxidative stress, and the rise of GSSG and other reactive oxygen species may lead to alteration in gene expression, protein, nucleic acids and lipid modifications. Here, we investigated the effects of oxidative stress and nucleoside analog on the expression and activity of TK1 and dCK. Treatment with GSSG resulted in glutathionylation of dCK and dGK but not TK1 and Dm-dNK, and glutathionylation led to increased dCK activity but decreased dGK activity. Treatment with hydrogen peroxide resulted in induction of TK1, however, the TK1 activity did not correlate with TK1 protein levels, indicating that TK1 protein was inactive. The cellular expression of dCK, however, was reduced but dCK activity was not affected at concentration ≤ 4 mM. Treatment with TFT or 5FdU resulted in downregulation of both TK1 and dCK. However, araC and dFdC treatment led to increased dCK protein but decreased dCK activity. In contrast, both TK1 protein and activity were upregulated after araC and dFdC treatment. Doxorubicin treatment led to upregulation of the TK1 but downregulation of dCK. In conclusion TK1 and dCK expression and activity are apparently affected by oxidative stress and treatment by nucleoside analogs. These results demonstrate the pharmacokinetic importance of characterizing the expression and activity of TK1 and dCK during chemotherapy with thymidine and deoxycytidine analogs in order to optimize their efficacy.

Loss of thymidine kinase 1 inhibits lung cancer growth and metastatic attributes by reducing GDF15 expression

Metabolic alterations that are critical for cancer cell growth and metastasis are one of the key hallmarks of cancer. Here, we show that thymidine kinase 1 (TK1) is significantly overexpressed in tumor samples from lung adenocarcinoma (LUAD) patients relative to normal controls, and this TK1 overexpression is associated with significantly reduced overall survival and cancer recurrence. Genetic knockdown of TK1 with short hairpin RNAs (shRNAs) inhibits both the growth and metastatic attributes of LUAD cells in culture and in mice. We further show that transcriptional overexpression of TK1 in LUAD cells is driven, in part, by MAP kinase pathway in a transcription factor MAZ dependent manner. Using targeted and gene expression profiling-based approaches, we then show that loss of TK1 in LUAD cells results in reduced Rho GTPase activity and reduced expression of growth and differentiation factor 15 (GDF15). Furthermore, ectopic expression of GDF15 can partially rescue TK1 knockdown-induced LUAD growth and metastasis inhibition, confirming its important role as a downstream mediator of TK1 function in LUAD. Collectively, our findings demonstrate that TK1 facilitates LUAD tumor and metastatic growth and represents a target for LUAD therapy.

Thymidine kinase 1 (TK1) is overexpressed and associated with poor prognosis in a number of different cancers. However, despite these data suggesting an important role for TK1 in cancer pathogenesis, no study thus far has analyzed the functional effect of TK1 inhibition on tumor growth and metastasis. In this study, we performed TK1 knockdown and found that this protein is necessary for lung adenocarcinoma (LUAD) tumor growth and metastasis. Notably, inhibition of another nucleotide kinase, deoxycytidine kinase (DCK), had no effect on LUAD tumor growth and metastatic attributes. We therefore performed experiments to determine if the TK1 mechanism of action in cancer is distinct from its previously reported role in DNA damage, DNA replication, and DNA repair. We found that TK1 can promote LUAD tumor growth and metastasis in a non-canonical manner by activating Rho GTPase activity and growth and differentiation factor 15 (GDF15) expression. Taken together, our data suggest that TK1 may represent a potential target for development of LUAD therapy, due to its critical role in maintaining lung tumor growth and metastasis.

Deoxyuracil in DNA and disease: Genomic signal or managed situation?

Genomic instability is implicated in the etiology of several deleterious health outcomes including megaloblastic anemia, neural tube defects, and neurodegeneration. Uracil misincorporation and its repair are known to cause genomic instability by inducing DNA strand breaks leading to apoptosis, but there is emerging evidence that uracil incorporation may also result in broader modifications of gene expression, including: changes in transcriptional stalling, strand break-mediated transcriptional upregulation, and direct promoter inhibition. The factors that influence uracil levels in DNA are cytosine deamination, de novo thymidylate (dTMP) biosynthesis, salvage dTMP biosynthesis, dUTPase, and DNA repair. There is evidence that the nuclear localization of the enzymes in these pathways in mammalian cells may modify and/or control the levels of uracil accumulation into nuclear DNA. Uracil sequencing technologies demonstrate that uracil in DNA is not distributed stochastically across the genome, but instead shows patterns of enrichment. Nuclear localization of the enzymes that modify uracil in DNA may serve to change these patterns of enrichment in a tissue-specific manner, and thereby signal the genome in response to metabolic and/or nutritional state of the cell.

Doxorubicin effects on leukemia and breast cancer cells in culture on the TK1 protein levels using AroCell TK 210 ELISA: a tool for drug development

In this study changes in thymidine kinase 1 (TK1) levels after 24 hours of Doxorubicin (Dox) exposure of CEM and MDA MB-231 cells were determined using the commercially available AroCell TK 210 ELISA test. In cell extracts, TK1 levels increased twofold with 1 µM Dox in both cell lines, while the TK1 levels in the culture media increased with 5 and 10 µM of Dox only in case of CEM cells. In conclusion, this study reveals that the TK 210 ELISA can measure changes in intra- and extracellular TK1 levels apparently related to the mechanism of cytotoxicity of anti-cancer agents.

Thymidine kinase 1 as a tumor biomarker: technical advances offer new potential to an old biomarker

Thymidine kinase 1 (TK1) is a key enzyme in DNA precursor synthesis. It is upregulated during the S phase of the cell cycle and its presence in cells is an indicator of active cell proliferation. In studies since the 1980s, TK1 has been shown as a clinically valuable biomarker for the management of hematological malignancies. However, TK1 activity assays may underestimate serum TK1 in subjects with solid tumors limiting its sensitivity. The development of TK1 immunoassays has made the assay of TK1 more widely available and increased its applicability to solid tumor diseases. This paper will review TK1 as a tumor biomarker with emphasis on recent studies and technologies plus highlight its potential in drug discovery and as a therapeutic target.

Nm23-H1 is involved in the repair of ionizing radiation-induced DNA double-strand breaks in the A549 lung cancer cell line

BACKGROUND

Although originally identified as a putative metastasis suppressor, increasing studies have confirmed a possible role for Nm23-H1 in DNA repair, through the base excision repair and nucleotide excision repair pathways. In this study, we explored whether Nm23-H1 was also involved in double-strand break repair (DSBR).

METHODS AND RESULTS

We constructed a stable A549-shNm23-H1 cell line with doxycycline-regulated expression of Nm23-H1, and a A549-nNm23-H1 cell line that over expressed a nucleus-localized version of Nm23-H1. Results from both lines confirmed that Nm23-H1 participated in the repair of double-strand breaks induced by X-rays, using Comet and γ-H2AX foci assays. Subsequent studies showed that Nm23-H1 activated the phosphorylation of checkpoint-related proteins including ATM serine/threonine kinase (on S1981), tumor protein p53 (on S15), and checkpoint kinase 2 (Chk2) (on T68). We also detected interactions between Nm23-H1 and the MRE11-RAD50-NBS1 (MRN) complex, as well as Ku80. Moreover, NBS1 and Ku80 levels were comparably higher in Nm23-H1 overexpressing cells than in control cells (t = 14.462, p < 0.001 and t = 5.347, p = 0.006, respectively). As Ku80 is the keystone of the non-homologous end joining (NHEJ) pathway, we speculate that Nm23-H1 promotes DSBR through NHEJ.

CONCLUSIONS

The results indicate that Nm23-H1 participates in multiple steps of DSBR.

Validation of a network-based strategy for the optimization of combinatorial target selection in breast cancer therapy: siRNA knockdown of network targets in MDA-MB-231 cells as an in vitro model for inhibition of tumor development

Network-based strategies provided by systems biology are attractive tools for cancer therapy. Modulation of cancer networks by anticancer drugs may alter the response of malignant cells and/or drive network re-organization into the inhibition of cancer progression. Previously, using systems biology approach and cancer signaling networks, we identified top-5 highly expressed and connected proteins (HSP90AB1, CSNK2B, TK1, YWHAB and VIM) in the invasive MDA-MB-231 breast cancer cell line. Here, we have knocked down the expression of these proteins, individually or together using siRNAs. The transfected cell lines were assessed for in vitro cell growth, colony formation, migration and invasion relative to control transfected MDA-MB-231, the non-invasive MCF-7 breast carcinoma cell line and the non-tumoral mammary epithelial cell line MCF-10A. The knockdown of the top-5 upregulated connectivity hubs successfully inhibited the in vitro proliferation, colony formation, anchorage independence, migration and invasion in MDA-MB-231 cells; with minimal effects in the control transfected MDA-MB-231 cells or MCF-7 and MCF-10A cells. The in vitro validation of bioinformatics predictions regarding optimized multi-target selection for therapy suggests that protein expression levels together with protein-protein interaction network analysis may provide an optimized combinatorial target selection for a highly effective anti-metastatic precision therapy in triple-negative breast cancer. This approach increases the ability to identify not only druggable hubs as essential targets for cancer survival, but also interactions most susceptible to synergistic drug action. The data provided in this report constitute a preliminary step toward the personalized clinical application of our strategy to optimize the therapeutic use of anti-cancer drugs.

TK1 overexpression is associated with the poor outcomes of lung cancer patients: a systematic review and meta-analysis

AIM

The study aimed to unveil the effect of TK1 expression on the clinicopathological significance and prognosis in patients with lung cancer. Results & methodology: Studies for meta-analysis were selected according to a thorough literature search in databases (PubMed, EMBASE and Chinese databases). Ten studies containing 1393 lung cancer patients were investigated in our analysis. The TK1 overexpression was associated with poorer overall survival(OS) in lung cancer patients (hazard ratio = 1.881; 95% CI:1.318-2.684, Z = 3.48; p = 0.001). Furthermore, The TK1 expression is associated with the clinicopathological features of lung cancer patients (tumor type, age, lymph node metastasis, distant metastasis and tumor, node, metastasis [TNM] stages).  Discussion & conclusion: The TK1 expression might have a supportive implication in assessing biological behavior and prognosis of lung cancer patients.

Integrative omics analysis of p53-dependent regulation of metabolism

Accumulated evidence in the last decade implies that regulation of metabolism by p53 represents a reviving mechanism vital to prevent tumorigenesis. To gain a more in-depth understanding of metabolic regulation by baseline levels of p53, we employed both metabolomics and transcriptomics analysis with human colon cancer cell-line HCT116 depleted of p53. Metabolomics analyses with UPLC/quadrupole time-of-flight mass spectrometry identified 283 significantly changed metabolites including 138 important metabolites. Transcriptomics analysis with microarray revealed 1317 differentially expressed genes. By integrated analysis of both omics data, we found nucleotides metabolism and sulfur-related metabolism are of great importance. Our study provided a pilot comprehensive view of the metabolism regulated by p53 and suggests several potential p53 targets in metabolism for further study.

Clinical significance and diagnostic capacity of serum TK1, CEA, CA 19-9 and CA 72-4 levels in gastric and colorectal cancer patients

Despite extensive progress in treatment for cancer in recent decades, the early diagnosis for gastric cancer (GC) and colorectal cancer (CRC) remains poor. In this study, we explore the diagnostic value of joint detection of thymidine kinase 1 (TK1), carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA 19-9) and carbohydrate antigen 72-4 (CA 72-4) in the diagnosis of GC and CRC, and to evaluated the relationship between TK1 expression and clinical pathological characteristics in the patients. Serum TK1, CA 19-9, CA 72-4 and CEA levels were measured in 169 patients with GC, 344 patients with CRC and 75 healthy controls using electro-chemiluminescence. The TK1 concentration was significantly higher in patients with cancer than in healthy controls and patients with clinical stage Ⅲ+Ⅳ had higher TK1 levels than clinical stage Ⅰ+Ⅱ (P<0.05). The levels of TK1 is significantly associated with tumor stage, lymph node metastasis, distant metastasis, tumor differentiation and age (P<0.05). When the tumor markers (TK1, CA 19-9 and CA 72-4) were detected respectively, the area under receiver operating characteristics curve (AUC) of TK1 for three cancers was the highest (0.823-0.895). However, the combination of AUC was higher than that for each tumor marker detected respectively (0.934-0.953), and the Hosmer-Lemeshow test showed an adequate model of calibration (P>0.05). Moreover, the AUCs varied significantly between the combination tests and single biomarker tests (Z test, P<0.01). In conclusion, serum TK1 may be an independent tumor marker for GC and CRC patients, and the combination of TK1, CA 19-9 and CA 72-4 and CEA performed even better. This study suggests that combination detection of four tumor markers may prove to be useful for the diagnosis of GC and CRC.

Targeting Nucleotide Biosynthesis: A Strategy for Improving the Oncolytic Potential of DNA Viruses

The rapid growth of tumors depends upon elevated levels of dNTPs, and while dNTP concentrations are tightly regulated in normal cells, this control is often lost in transformed cells. This feature of cancer cells has been used to advantage to develop oncolytic DNA viruses. DNA viruses employ many different mechanisms to increase dNTP levels in infected cells, because the low concentration of dNTPs found in non-cycling cells can inhibit virus replication. By disrupting the virus-encoded gene(s) that normally promote dNTP biosynthesis, one can assemble oncolytic versions of these agents that replicate selectively in cancer cells. This review covers the pathways involved in dNTP production, how they are dysregulated in cancer cells, and the various approaches that have been used to exploit this biology to improve the tumor specificity of oncolytic viruses. In particular, we compare and contrast the ways that the different types of oncolytic virus candidates can directly modulate these processes. We limit our review to the large DNA viruses that naturally encode homologs of the cellular enzymes that catalyze dNTP biogenesis. Lastly, we consider how this knowledge might guide future development of oncolytic viruses.

Biomarker analysis and clinical relevance of TK1 on the cell membrane of Burkitt's lymphoma and acute lymphoblastic leukemia

TK1 is an enzyme involved in DNA synthesis and repair. TK1 is usually found elevated in cancer patients’ serum, which makes it a useful tumor proliferation biomarker that strongly correlates with cancer stage, metastatic capabilities, and recurrence risk. In this study, we show that TK1 is upregulated and localizes on the plasma membrane of Burkitt’s lymphoma, acute promyelocytic leukemia, T cell leukemia, and acute lymphoblastic leukemia (ALL). Using flow cytometry, we confirmed that TK1 localizes on the surface of Raji, HL60, and Jurkat cell lines and on ALL clinical samples. Using fluorescent microscopy, we found a strong association of TK1 with the plasma membrane in Raji, HL60, and Jurkat cell lines. These findings were also confirmed by scanning electron microscopy. Our study also shows that this phenomenon does not occur on normal resting or proliferating lymphocytes. In addition, we show that membrane TK1 is found in all oligomeric forms ranging from monomer to tetramer and exhibits enzymatic activity. These findings suggest TK1 as a possible target for immunotherapy with the potential to be utilized in the treatment of hematological cancers.

Metformin-induced alterations in nucleotide metabolism cause 5-fluorouracil resistance but gemcitabine susceptibility in oesophageal squamous cell carcinoma

5-Fluorouracil (5-FU) is a chemotherapeutic agent used to treat a variety of gastric cancers including oesophageal squamous cell carcinoma (OSCC), for which the 5-year mortality rate exceeds 85%. Our study investigated the effects of metformin, an antidiabetic drug with established anti-cancer activity, in combination with 5-FU as a novel chemotherapy strategy, using the OSCC cell lines, WHCO1 and WHCO5. Our results indicate that metformin treatment induces significant resistance to 5-FU in WHCO1 and WHCO5 cells, by more than five- and sixfolds, respectively, as assessed by MTT assay. We show that this is due to global alterations in nucleotide metabolism, including elevated expression of thymidylate synthase and thymidine kinase 1 (established 5-FU resistance mechanisms), which likely result in an increase in intracellular dTTP pools and a "dilution" of 5-FU anabolites. Metformin treatment also increases deoxycytidine kinase (dCK) expression and, as the chemotherapeutic agent gemcitabine relies on dCK for its efficient activity, we speculated that metformin would enhance the sensitivity of OSCC cells to gemcitabine. Indeed we show that metformin pre-treatment greatly increases gemcitabine toxicity and DNA fragmentation in comparison to gemcitabine alone. Taken together, our findings show that metformin alters nucleotide metabolism in OSCC cells and while responsible for inducing resistance to 5-FU, it conversely increases sensitivity to gemcitabine, thereby highlighting metformin and gemcitabine as a potentially novel combination therapy for OSCC.

Function of the Plant DNA Polymerase Epsilon in Replicative Stress Sensing, a Genetic Analysis

Faithful transmission of the genetic information is essential in all living organisms. DNA replication is therefore a critical step of cell proliferation, because of the potential occurrence of replication errors or DNA damage when progression of a replication fork is hampered causing replicative stress. Like other types of DNA damage, replicative stress activates the DNA damage response, a signaling cascade allowing cell cycle arrest and repair of lesions. The replicative DNA polymerase ε (Pol ε) was shown to activate the S-phase checkpoint in yeast in response to replicative stress, but whether this mechanism functions in multicellular eukaryotes remains unclear. Here, we explored the genetic interaction between Pol ε and the main elements of the DNA damage response in Arabidopsis (Arabidopsis thaliana). We found that mutations affecting the polymerase domain of Pol ε trigger ATR-dependent signaling leading to SOG1 activation, WEE1-dependent cell cycle inhibition, and tolerance to replicative stress induced by hydroxyurea, but result in enhanced sensitivity to a wide range of DNA damaging agents. Using knock-down lines, we also provide evidence for the direct role of Pol ε in replicative stress sensing. Together, our results demonstrate that the role of Pol ε in replicative stress sensing is conserved in plants, and provide, to our knowledge, the first genetic dissection of the downstream signaling events in a multicellular eukaryote.

Nuclear-targeted siRNA delivery for long-term gene silencing

Developing effective nonviral siRNA delivery systems for long-term gene silencing remains a great challenge. Here we present a nuclear-targeted siRNA delivery system that can induce long-term gene silencing in cancer cells. The nanocarrier consists of gold nanoparticles modified with a dense shell of synthetic siRNAs and nuclear localization signal (NLS) peptides. The NLS peptide could translocate the nanocarrier into the nucleus and the siRNA was designed to target the promoter of thymidine kinase 1 and trigger the RNA-directed DNA methylation, thereby enabling the nuclear-targeted gene silencing. Compared with traditional gene silencing in cytoplasm, long-lasting gene knockdown could be achieved for the nuclear-targeted nanocarrier, which lasts for more than 30 days. The long-term gene silencing induced by nuclear-targeted siRNA delivery could effectively inhibit the proliferation of cancer cells and prevent the formation of a tumor in a mouse model.

Serum TK1 is a more reliable marker than CEA and AFP for cancer screening in a study of 56,286 people

BACKGROUND

The World Health Organization (WHO) has estimated that the number of cancer patients will increase by about 70% during the next 25 years world-wide. To deal with this problem, WHO has suggested a focus on prevention of tumor incidence and health screening for early detection of people with tumors.

OBJECTIVE

To investigate the use of thymidine kinase 1 (TK1), CEA and AFP in serum to discover people with malignant tumors through health cancer screening.

METHODS

Of a cohort in 486,085 people of a routine health screening at the Health Centre, Fujun 180 Hospital, Quanzhou city, China, 56,286 people were investigated according to the presence of cancer during 2009-2014. The concentration of CEA and AFP were determined by an electrochemiluminescence immunoassay from Roche Diagnostics e601GmbH and STK1 by a commercial kit based on an enhanced chemiluminescent dot blot assay.

RESULTS

The cancer incident rate increased from 0.048/100,000 to 0.220/100,000. The most common types of tumors were those of the liver, cervix and lung. STK1 correlated to tumor growth rate, was more sensitive than CEA and AFP for discovering people with malignant tumors and more sensitive among people who had diagnosis of malignant tumor. STK1 was also a prognostic biomarker for death at 10-40 months follow-up, while CEA and AFP were not. A combination of these markers increased the sensitivity by about 30%.

CONCLUSION

STK1 is a reliable biomarker for discovering people with malignant tumors in cancer screening.

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.

Pyrimidine Salvage Enzymes Are Essential for De Novo Biosynthesis of Deoxypyrimidine Nucleotides in Trypanosoma brucei

The human pathogenic parasite Trypanosoma brucei possess both de novo and salvage routes for the biosynthesis of pyrimidine nucleotides. Consequently, they do not require salvageable pyrimidines for growth. Thymidine kinase (TK) catalyzes the formation of dTMP and dUMP and is one of several salvage enzymes that appear redundant to the de novo pathway. Surprisingly, we show through analysis of TK conditional null and RNAi cells that TK is essential for growth and for infectivity in a mouse model, and that a catalytically active enzyme is required for its function. Unlike humans, T. brucei and all other kinetoplastids lack dCMP deaminase (DCTD), which provides an alternative route to dUMP formation. Ectopic expression of human DCTD resulted in full rescue of the RNAi growth phenotype and allowed for selection of viable TK null cells. Metabolite profiling by LC-MS/MS revealed a buildup of deoxypyrimidine nucleosides in TK depleted cells. Knockout of cytidine deaminase (CDA), which converts deoxycytidine to deoxyuridine led to thymidine/deoxyuridine auxotrophy. These unexpected results suggested that T. brucei encodes an unidentified 5'-nucleotidase that converts deoxypyrimidine nucleotides to their corresponding nucleosides, leading to their dead-end buildup in TK depleted cells at the expense of dTTP pools. Bioinformatics analysis identified several potential candidate genes that could encode 5'-nucleotidase activity including an HD-domain protein that we show catalyzes dephosphorylation of deoxyribonucleotide 5'-monophosphates. We conclude that TK is essential for synthesis of thymine nucleotides regardless of whether the nucleoside precursors originate from the de novo pathway or through salvage. Reliance on TK in the absence of DCTD may be a shared vulnerability among trypanosomatids and may provide a unique opportunity to selectively target a diverse group of pathogenic single-celled eukaryotes with a single drug.

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.

The nucleotidohydrolases DCTPP1 and dUTPase are involved in the cellular response to decitabine

Decitabine (5-aza-2'-deoxycytidine, aza-dCyd) is an anti-cancer drug used clinically for the treatment of myelodysplastic syndromes and acute myeloid leukaemia that can act as a DNA-demethylating or genotoxic agent in a dose-dependent manner. On the other hand, DCTPP1 (dCTP pyrophosphatase 1) and dUTPase are two 'house-cleaning' nucleotidohydrolases involved in the elimination of non-canonical nucleotides. In the present study, we show that exposure of HeLa cells to decitabine up-regulates the expression of several pyrimidine metabolic enzymes including DCTPP1, dUTPase, dCMP deaminase and thymidylate synthase, thus suggesting their contribution to the cellular response to this anti-cancer nucleoside. We present several lines of evidence supporting that, in addition to the formation of aza-dCTP (5-aza-2'-deoxycytidine-5'-triphosphate), an alternative cytotoxic mechanism for decitabine may involve the formation of aza-dUMP, a potential thymidylate synthase inhibitor. Indeed, dUTPase or DCTPP1 down-regulation enhanced the cytotoxic effect of decitabine producing an accumulation of nucleoside triphosphates containing uracil as well as uracil misincorporation and double-strand breaks in genomic DNA. Moreover, DCTPP1 hydrolyses the triphosphate form of decitabine with similar kinetic efficiency to its natural substrate dCTP and prevents decitabine-induced global DNA demethylation. The data suggest that the nucleotidohydrolases DCTPP1 and dUTPase are factors involved in the mode of action of decitabine with potential value as enzymatic targets to improve decitabine-based chemotherapy.

The direct interaction of NME3 with Tip60 in DNA repair

Cellular supply of dNTPs via RNR (ribonucleotide reductase) is crucial for DNA replication and repair. It has been shown that DNA-damage-site-specific recruitment of RNR is critical for DNA repair efficiency in quiescent cells. The catalytic function of RNR produces dNDPs. The subsequent step of dNTP formation requires the function of NDP kinase. There are ten isoforms of NDP kinase in human cells. In the present study, we identified NME3 as one specific NDP kinase that interacts directly with Tip60, a histone acetyltransferase, to form a complex with RNR. Our data reveal that NME3 recruitment to DNA damage sites depends on this interaction. Disruption of interaction of NME3 with Tip60 suppressed DNA repair in serum-deprived cells. Thus Tip60 interacts with RNR and NME3 to provide site-specific synthesis of dNTP for facilitating DNA repair in serum-deprived cells which contain low levels of dNTPs.

Thymidine Kinase: A Biomarker for Recently Diagnosed Acute Leukemia in Pediatric Patients According to the Cell Line Involved

BACKGROUND AND AIMS

Acute leukemia (AL) is a heterogeneous group of diseases characterized by a disorganized clone proliferation of hematopoietic cells. Thymidine kinase (TK) is a cell enzyme involved in DNA synthesis and is considered a cellular proliferation marker in some solid tumors.

METHODS

A cross-sectional prospective and comparative study was performed in the Federico Gomez Children's Hospital in Mexico (HIMFG, in Spanish) in 125 samples of patients of the HIMFG with AL and 138 samples of children without leukemia. Serum TK levels were determined for both groups.

RESULTS

Of the children with AL, 90 presented B-cell acute lymphoblastic leukemia (B-ALL); 13, T-cell acute lymphoblastic leukemia (T-ALL); and 22, acute myeloid leukemia (AML). A median (m) TK level of 23.7 IU (IQR 17-35.7) was observed in the group without AL and 91 IU (IQR 98-392) in the AL group. This difference was statistically significant (p <0.0001). When analyzing TK levels according to the type of leukemia, the m was as follows: 68 IU (IQR 35-118) for B-ALL, 470 IU (IQR 88-750) for AML, and 1678 IU (IQR 288-2108) for T- ALL.

CONCLUSION

TK is an enzyme showing heterogeneous levels in B-ALL although it is significantly increased in 90% of patients with T-ALL and AML.

Muscle-invasive bladder cancer is characterized by overexpression of thymidine kinase 1

OBJECTIVE

Thymidine kinases have an important role in the synthesis of DNA and exhibit high activity in rapidly proliferating cells. Thymidine kinase 1 (TK1) activity has been shown to be increased in various cancer types and proposed as a prognostic parameter. Aim of the present study was to investigate TK1 in muscle-invasive urothelial carcinoma (UC).

METHODS

Corresponding UC and benign samples from paraffin embedded tissue of 111 patients treated with cystectomy for invasive UC from 1996 to 2006 were immunohistochemically (IHC) assessed for TK1. IHC expression patterns were evaluated in a semiquantitative fashion by 2 independent reviewers. Localization of staining was categorized into pure nuclear and additional cytoplasmic localization. Uni- and multivariate analyses were performed to assess differential expression in normal and UC tissue and to evaluate the diagnostic and predictive capability of TK1 by correlation to clinical data. To correlate TK1 expression with molecular subtypes of UC, analysis of TK1 RNA expression levels of the Cancer Genome Atlas UC cohort was performed.

RESULTS

TK1 was significantly overexpressed in invasive UC, compared to benign urothelium (P<0.0001), and cytoplasmic expression was more often found in cancer tissue than in benign tissue (P = 0.0001). No correlations of TK1 protein expression patterns to standard histopathological determinants were detected. In univariate analysis, TK1 nuclear and cytoplasmic expression was associated with improved cancer-specific survival (P = 0.0119). However, only metastasis status and histologic grade were identified as independent predictors of cancer-specific survival in multivariate analysis. TK1 expression was merely found in the basal layers of benign urothelium. RNA overexpression of TK1 could be correlated to the biologically more aggressive basal UC subtype.

CONCLUSIONS

TK1 expression is significantly different in invasive UC and benign urothelium, which underlines its potential as a diagnostic marker. Although TK1 is considered to be a marker of proliferation, and TK1 RNA overexpression is associated with an aggressive UC subtype, its capability as a predictive IHC biomarker for invasive UC remains limited.

Synthesis and evaluation of thymidine kinase 1-targeting carboranyl pyrimidine nucleoside analogs for boron neutron capture therapy of cancer

A library of sixteen 2nd generation amino- and amido-substituted carboranyl pyrimidine nucleoside analogs, designed as substrates and inhibitors of thymidine kinase 1 (TK1) for potential use in boron neutron capture therapy (BNCT) of cancer, was synthesized and evaluated in enzyme kinetic-, enzyme inhibition-, metabolomic-, and biodistribution studies. One of these 2nd generation carboranyl pyrimidine nucleoside analogs (YB18A [3]), having an amino group directly attached to a meta-carborane cage tethered via ethylene spacer to the 3-position of thymidine, was approximately 3-4 times superior as a substrate and inhibitor of hTK1 than N5-2OH (2), a 1st generation carboranyl pyrimidine nucleoside analog. Both 2 and 3 appeared to be 5'-monophosphorylated in TK1(+) RG2 cells, both in vitro and in vivo. Biodistribution studies in rats bearing intracerebral RG2 glioma resulted in selective tumor uptake of 3 with an intratumoral concentration that was approximately 4 times higher than that of 2. The obtained results significantly advance the understanding of the binding interactions between TK1 and carboranyl pyrimidine nucleoside analogs and will profoundly impact future design strategies for these agents.

Arabidopsis thaliana thymidine kinase 1a is ubiquitously expressed during development and contributes to confer tolerance to genotoxic stress

Thymidine kinase catalyzes the first step in the nucleotide salvage pathway by transferring a phosphate group to a thymidine molecule. In mammals thymidine kinase supplies deoxyribonucleotides for DNA replication and DNA repair, and the expression of the gene is tightly regulated during the cell cycle. Although this gene is phylogenetically conserved in many taxa, its physiological function in plants remains unknown. The genome of the model plant Arabidopsis thaliana has two thymidine kinase genes (AtTK1a and AtTK1b) and microarray data suggest they might have redundant roles. In this study we analyzed the TK1a function by evaluating its expression pattern during development and in response to genotoxic stress. We also studied its role in DNA repair by the characterization of a mutant that contained the T-DNA insertion in the promoter region of the TK1a gene. We found that TK1a is expressed in most tissues during plant development and it was differentially induced by ultraviolet-C radiation because TK1b expression was unaffected. In the mutant, the T-DNA insertion caused a 40 % rise in transcript levels and enzyme activity in Arabidopsis seedlings compared to wild-type plants. This elevation was enough to confer tolerance to ultraviolet-C irradiation in dark conditions, as determined by root growth, and meristem length and structure. TK1a overexpression also provided tolerance to genotoxins that induce double-strand break. Our results suggest that thymidine kinase contributes to several DNA repair pathways by providing deoxythymidine triphosphate that serve as precursors for DNA repair and to balance deoxyribonucleotides pools.

Deoxycytidine kinase expression underpins response to gemcitabine in bladder cancer

PURPOSE

In a recent phase II clinical trial, low-dose (100 mg/m(2)) gemcitabine showed promise as a radiosensitizer in bladder cancer, but underlying mechanisms lack elucidation. Here, we investigated the mechanism of radiosensitization by low-dose gemcitabine in bladder cancer cell lines.

EXPERIMENTAL DESIGN

Four bladder cancer cell lines were screened for radiosensitization by low-dose gemcitabine using clonogenic assay, and gemcitabine-resistant RT112gem and CALgem cells created by exposure to increasing gemcitabine doses. Four key gemcitabine-regulatory genes were knocked down by transient siRNA. Nude mice carrying CALgem subcutaneous xenografts were exposed to 100 mg/kg gemcitabine ± ionizing radiation (IR) and response assessed by tumor growth delay.

RESULTS

Gemcitabine was cytotoxic in the low nanomolar range (10-40 nmol/L) in four bladder cancer cell lines and radiosensitized all four lines. Sensitizer enhancement ratios at 10% survival were: RT112 1.42, CAL29 1.55, T24 1.63, and VMCUB1 1.47. Transient siRNA knockdown of deoxycytidine kinase (dCK) significantly reduced radiosensitization by gemcitabine (P = 0.02). RT112gem and CALgem cells displayed robust decreases of dCK mRNA and protein levels; reexpression of dCK restored gemcitabine sensitivity. However, CALgem xenografts responded better to combination gemcitabine/IR than either treatment alone (P < 0.001) with dCK strongly expressed in the tumor vasculature and stroma.

CONCLUSIONS

Gemcitabine resistance in bladder cancer cell lines was associated with decreased dCK expression, but gemcitabine-resistant xenografts were responsive to combination low-dose gemcitabine/IR. We propose that dCK activity in tumor vasculature renders it gemcitabine sensitive, which is sufficient to invoke a tumor response and permit tumor cell kill in gemcitabine-resistant tumors.

FISH comets show that the salvage enzyme TK1 contributes to gene-specific DNA repair

Thymidine kinase 1 (TK1) is a salvage enzyme that phosphorylates thymidine, imported from surrounding fluids, to create dTMP, which is further phosphorylated to the DNA precursor dTTP. TK1 deficiency has for a long time been known to cause increased cellular sensitivity to DNA damage. We have examined preferential strand break repair of DNA domains in TK1(+) and TK1(-) clones of the Raji cell line, by the Comet-FISH technique, in bulk DNA and in the actively transcribed tumor suppressor (TP53) and human telomerase reverse transcriptase (hTERT) gene regions, over 1 h after 5Gy γ-irradiation. Results showed that repair of the TP53 and hTERT gene regions was more efficient in TK1(+) compared to TK1(-) cells, a trend also reflected to a lesser degree in genomic DNA repair between the cell-lines. The targeted gene-specific repair in TK(+) cells occurred rapidly, mainly over the first 15 min repair-period. Therefore, TK1 is needed for preferential repair of actively transcribed regions, through a previously unsuspected mechanism. In principle, TK1 could exert its protective effects through supply of a supplementary dTTP pool for accurate repair of damaged genes; but Raji TK1(+) cells in thymidine free media still show preferential repair of transcribed regions. TK1 therefore does not exert its protective effects through dTTP pools, but through another unidentified mechanism, which affects sensitivity to and mutagenicity by DNA damaging agents.

Monitoring tumor response with radiolabeled nucleoside analogs in a hepatoma-bearing mouse model early after doxisome(®) treatment

PURPOSE

This study aims to demonstrate that 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) positron emission tomography (PET) is a promising modality for noninvasively monitoring the therapeutic efficacy of Doxisome(®) in a subcutaneous hepatoma mouse model.

PROCEDURES

Male BALB/c nu/nu mice were inoculated with HepG2 hepatoma xenograft in the right flank. Doxisome(®) (5 mg/kg, three times a week for 2 weeks) was intravenously administrated for treatment. (18)F-FLT-microPET, biodistribution studies, and immunohistochemistry of Ki-67 were performed.

RESULTS

A significant difference (p < 0.05) in tumor volume was observed on day 5 between treated and control groups. The tumor-to-muscle ratio derived from (18)F-FLT-PET and (123)I-ICdR-microSPECT images of Doxisome(®)-treated mice dropped from 12.55 ± 0.76 to 3.81 ± 0.31 and from 2.48 ± 0.42 to 1.59 ± 0.08 after a three-dose treatment, respectively, while that of the control group remained steady. The retarded proliferation rate of treated xenograft was confirmed by Ki-67 immunohistochemistry staining.

CONCLUSIONS

This study clearly demonstrated that Doxisome(®) is an effective anti-cancer drug against the growth of HepG2 hepatoma and that (18)F-FLT-PET could provide early information of tumor response during treatment.

Early assessment of the efficacy of temozolomide chemotherapy in experimental glioblastoma using [18F]FLT-PET imaging

Addition of temozolomide (TMZ) to radiation therapy is the standard treatment for patients with glioblastoma (GBM). However, there is uncertainty regarding the effectiveness of TMZ. Considering the rapid evolution of the disease, methods to assess TMZ efficacy early during treatment would be of great benefit. Our aim was to monitor early effects of TMZ in a mouse model of GBM using positron emission tomography (PET) with 3′-deoxy-3′-[¹⁸F]fluorothymidine ([¹⁸F]FLT).

Methods

Human glioma cells sensitive to TMZ (Gli36dEGFR-1) were treated with sub-lethal doses of TMZ to obtain cells with lower sensitivity to TMZ (Gli36dEGFR-2), as measured by growth and clonogenic assays. Gli36dEGFR-1 and Gli36dEGFR-2 cells were subcutaneously (s.c.) or intracranially (i.c.) xenografted into nude mice. Mice were treated for 7 days with daily injection of 25 or 50 mg/kg TMZ. Treatment efficacy was measured using [¹⁸F]FLT-PET before treatment and after 2 days. Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) were used to determine tumor volumes before treatment and after 7 days.

Results

A significant difference was observed between TMZ and DMSO treated tumors in terms of variations of [¹⁸F]FLT T/B ratio as soon as day 2 in the i.c. as well as in the s.c. mouse model. Variations of [¹⁸F]FLT T/B uptake ratio between days 0 and 2 correlated with variations of tumor size between days 0 and 7 (s.c. model: n_tumor = 17 in n_mice = 11, P<0.01; i.c. model: n_tumor/mice = 9, P<0.01).

Conclusions

Our results indicate that [¹⁸F]FLT-PET may be useful for an early evaluation of the response of GBM to TMZ chemotherapy in patients with glioma.

Nuclear TK1 expression is an independent prognostic factor for survival in pre-malignant and malignant lesions of the cervix

Background

Thymidine kinase 1 (TK1) is a proliferation biomarker that has been found useful for prognostication in cancer patients. Here we investigate for the first time the use of TK1 expression as a prognostic factor for patients with premalignant and malignant lesions of the uterine cervix.

Methods

TK1 expression was determined by immunohistochemistry in cervical lesions (cervical intraepithelial neoplasia (CIN), n = 216; invasive cervical carcinoma, n = 84). TK1 and Ki-67 expressions and pathological/FIGO stages and age were correlated with 5-year survival by Kaplan-Meier, log rank and COX hazard uni- and multivariate analyses.

Results

TK1 labeling index (LI) was significantly correlated with CIN grades and invasive cervical carcinoma stages, while TK1 labeling intensity was only correlated to CIN grades. TK1 LI was significantly higher compared with Ki-67 LI. TK1 LI correlated significantly to 5-year survival in patients with invasive cervical carcinoma, particularly nuclear TK1 LI. In a multivariate analysis, nuclear TK1 expression was independent prognostic factor in patients with in situ/invasive cervical carcinoma or in invasive cervical carcinoma alone. Interestingly, in invasive cervical carcinoma patients with advanced tumors, nuclear TK1 expression could identify patients with significantly better survival rates (80%), while Ki-67 could not.

Conclusions

Nuclear TK1 expression in early grade CIN predicts risk for progression to malignancy. Nuclear TK1 expression is also a prognostic factor for treatment outcome, particularly in patients with advanced cervical carcinomas. Nuclear TK1 expression is more useful than Ki-67 and pathological/FIGO stages.

Pharmacodynamic evaluation of irinotecan therapy by FDG and FLT PET/CT imaging in a colorectal cancer xenograft model

PURPOSE

Longitudinal changes of 3'-[(18) F]fluoro-3'-deoxythymidine (FLT) and 2-deoxy-2-[(18) F]fluoro-D-glucose (FDG) in response to irinotecan therapy in an animal model of colorectal cancer were compared.

PROCEDURES

SCID/CB-17 mice with HCT116 tumors were treated with 50 mg/kg irinotecan by intraperitoneal injection weekly for 3 weeks. FLT and FDG-positron emission tomography (PET) were performed at baseline, the day after each treatment, and 5 days after the first treatment. Proliferation and apoptosis were evaluated by immunohistochemistry (IHC) after day 15 of imaging.

RESULTS

Irinotecan treatment resulted in a suppression of tumor growth. Tumor FLT uptake was decreased the day after each treatment but to a lesser extent 5 days after the first treatment. FDG uptake increased the day after each treatment with a continuous increase throughout the experiment. IHC analysis of phospho-H3 and Ki67 confirmed FLT-PET results, indicating a decrease in proliferation the day after the final irinotecan treatment. Increased apoptosis monitored by caspase-3 was observed after day 15 with irinotecan treatment.

CONCLUSIONS

FLT-PET may be a better method than FDG-PET for assessing treatment response to irinotecan. Changes in imaging occur before changes in tumor volume.

The proliferation marker thymidine kinase 1 in clinical use

Tumor-related biomarkers are used for the diagnosis, prognosis and monitoring of treatments and follow-up of cancer patients, although only a few are fully accepted for the detection of invisible/visible tumors in health screening. Thymidine kinase 1 (TK1), a cell cycle-dependent and thus a proliferation-related marker, has been extensively studied during the last decades, using both biochemical and immunological techniques. Therefore, TK1 is an emerging potential proliferating biomarker in oncology that may be used for the prognosis and monitoring of tumor therapy, relapse and survival. In addition, TK1 concentration in serum (STK1p) is a useful biomarker in healthy screening for the detection of potential malignancy development as well as the identification of early-stage tumors, with a few false-positive cases (ROC value, 0.96; tumor proliferation sensitivity, 0.80; specificity, 0.99). In this review, we examine results regarding the expression of STK1p and TK1 in relation to cancer patients and STK1p in health screening published between 2000 and 2012. The use of tumor-related markers recommended by international cancer organizations is also discussed. This review provides valuable information for applications in tumor patients, in health screening and for cancer research.

Design, synthesis, and in vitro and in vivo biological studies of a 3'-deoxythymidine conjugate that potentially kills cancer cells selectively

Thymidine kinases (TKs) have been considered one of the potential targets for anticancer therapeutic because of their elevated expressions in cancer cells. However, nucleobase analogs targeting TKs have shown poor selective cytotoxicity in cancer cells despite effective antiviral activity. 3′-Deoxythymidine phenylquinoxaline conjugate (dT-QX) was designed as a novel nucleobase analog to target TKs in cancer cells and block cell replication via conjugated DNA intercalating quinoxaline moiety. In vitro cell screening showed that dT-QX selectively kills a variety of cancer cells including liver carcinoma, breast adenocarcinoma and brain glioma cells; whereas it had a low cytotoxicity in normal cells such as normal human liver cells. The anticancer activity of dT-QX was attributed to its selective inhibition of DNA synthesis resulting in extensive mitochondrial superoxide stress in cancer cells. We demonstrate that covalent linkage with 3′-deoxythymidine uniquely directed cytotoxic phenylquinoxaline moiety more toward cancer cells than normal cells. Preliminary mouse study with subcutaneous liver tumor model showed that dT-QX effectively inhibited the growth of tumors. dT-QX is the first molecule of its kind with highly amendable constituents that exhibits this selective cytotoxicity in cancer cells.

Nucleoside salvage pathway kinases regulate hematopoiesis by linking nucleotide metabolism with replication stress

Nucleotide deficiency causes replication stress (RS) and DNA damage in dividing cells. How nucleotide metabolism is regulated in vivo to prevent these deleterious effects remains unknown. In this study, we investigate a functional link between nucleotide deficiency, RS, and the nucleoside salvage pathway (NSP) enzymes deoxycytidine kinase (dCK) and thymidine kinase (TK1). We show that inactivation of dCK in mice depletes deoxycytidine triphosphate (dCTP) pools and induces RS, early S-phase arrest, and DNA damage in erythroid, B lymphoid, and T lymphoid lineages. TK1(-/-) erythroid and B lymphoid lineages also experience nucleotide deficiency but, unlike their dCK(-/-) counterparts, they still sustain DNA replication. Intriguingly, dCTP pool depletion, RS, and hematopoietic defects induced by dCK inactivation are almost completely reversed in a newly generated dCK/TK1 double-knockout (DKO) mouse model. Using NSP-deficient DKO hematopoietic cells, we identify a previously unrecognized biological activity of endogenous thymidine as a strong inducer of RS in vivo through TK1-mediated dCTP pool depletion. We propose a model that explains how TK1 and dCK "tune" dCTP pools to both trigger and resolve RS in vivo. This new model may be exploited therapeutically to induce synthetic sickness/lethality in hematological malignancies, and possibly in other cancers.

Homogeneous assay for real-time and simultaneous detection of thymidine kinase 1 and deoxycytidine kinase activities

Measurement of thymidine kinase-1 (TK1) and deoxycytidine kinase (dCK) activity may be useful in cancer disease management. Therefore, a one-step homogeneous assay for real-time determination of TK1 and dCK was developed by combining enzyme complementation with fluorescent signal generation using primer extension and a quenched probe oligodeoxyribonucleotide system at 37 °C. Complementation, for producing dCTP and TTP from nucleoside substrates, was carried out by dTMP kinase and/or UMP/CMP kinase and nucleoside diphosphate kinase. dNTP was continuously incorporated into a fixed oligodeoxyribonucleotide primer, template, and probe system, and the fluorescent signal was generated by using the combined actions of primer extension and 5' exonuclease activity of Thermophilus aquaticus (Taq) DNA polymerase for specific relief of fluorescent quenching. Fluorescence was captured at 1-min intervals using a real-time polymerase chain reaction (PCR) instrument. A horizontal threshold line, crossing all sample relative fluorescent units (RFU) values at the level of the RFU of the blank sample at the end of the assay (i.e., 90 min), was drawn, obtaining RFU measurement data in minutes for each sample. Duplex proof of principle was demonstrated by the independent determination of different amounts of dCK and TK1 in combination. R(2) values of 0.90 were demonstrated with Prolifigen TK-REA U/L reference values obtained from pathological canine and human serum samples.