Cell cycle regulation of thymidine kinase: residues near the carboxyl terminus are essential for the specific degradation of the enzyme at mitosis

Nomogram model of serum thymidine kinase 1 combined with ultrasonography for prediction of central lymph node metastasis risk in patients with papillary thyroid carcinoma pre-surgery

Objective

The aim of this study was to develop a nomogram, using serum thymidine kinase 1 protein (STK1p) combined with ultrasonography parameters, to early predict central lymph node metastasis (CLNM) in patients with papillary thyroid carcinoma (PTC) pre-surgery.

Methods

Patients with PTC pre-surgery in January 2021 to February 2023 were divided into three cohorts: the observation cohort (CLNM, n = 140), the control cohort (NCLNM, n = 128), and the external verification cohort (CLNM, n = 50; NCLNM, n = 50). STK1p was detected by an enzyme immunodot-blot chemiluminescence analyzer and clinical parameters were evaluated by ultrasonography.

Results

A suitable risk threshold value for STK1p of 1.7 pmol/L was selected for predicting CLNM risk by receiver operating characteristic (ROC) curve analysis. Multivariate analysis identified the following six independent risk factors for CLNM: maximum tumor size >1 cm [odds ratio (OR) = 2.406, 95% confidence interval (CI) (1.279-4.526), p = 0.006]; capsule invasion [OR = 2.664, 95% CI (1.324-5.360), p = 0.006]; irregular margin [OR = 2.922; 95% CI (1.397-6.111), p = 0.004]; CLN flow signal [OR = 3.618, 95% CI (1.631-8.027), p = 0.002]; tumor-foci number ≥2 [OR = 4.064, 95% CI (2.102-7.859), p < 0.001]; and STK1p ≥1.7 pmol/L [OR = 7.514, 95% CI (3.852-14.660), p < 0.001]. The constructed nomogram showed that the area under the ROC curve for the main dataset was 0.867 and that for the validation dataset was 0.830, exhibiting effectivity, and was recalculated to a total score of approximately 383. Through monitoring the response post-surgery, all patients were assessed as tumor-free at 12 months post-surgery, which was significantly associated with a reduction in STK1p to disease-free levels.

Conclusion

We demonstrate for the first time that a novel nomogram including STK1p combined with ultrasonography can assist in the clinical prevention of CLNM, by facilitating timely, individualized prophylactic CLNM dissection, thereby reducing the risk of secondary surgery and the probability of recurrence.

A Novel Model Using Serum Thymidine Kinase 1 and Low-dose Computed Tomography Parameters to Predict Three-year Lung Cancer Risk in People with Pulmonary Nodules: A Retrospective Study

This study was designed to develop a model of serum thymidine kinase 1 protein (STK1p) concentration in combination with low-dose computed tomography (LDCT) to predict the risk of benign pulmonary nodules progressing into lung cancer within three years in a large screening population. The study included a retrospective cohort of 6,841 individuals aged > 30 years who had LDCT-detected pulmonary nodules, but no cancer history or baseline cancer. The outcome was a lung cancer diagnosis recorded within three years after the first detection of pulmonary nodules. The adaptive least absolute shrinkage and selection operator was used to select candidate predictors and fit a logistic model. The model was validated internally by examining discrimination (area under the receiver operating characteristic curve (AUC), calibration (calibration plot)) and net benefit. A web application was developed based on the model. The results showed that the proportion of incident lung cancer cases was 0.79% (n=52). Predictors selected for the model were STK1p and three LDCT parameters (nodule size, type, and count). The AUC of the model was 0.91 (95% confidence interval (CI): 0.86, 0.96). The model had satisfactory discrimination at internal validation (AUC: 0.90 (0.84, 0.96)) and in subgroups (AUC=0.69-0.93). The high-risk group identified by the model exhibited a significantly higher three-year lung cancer risk than the low-risk group (odds ratio (OR): 66.03 (95% CI: 30.49, 162.98)). We concluded that the novel model of STK1p and LDCT parameters together can be used to accurately predict the three-year risk of lung cancer in people with pulmonary nodules.

Efficient establishment of reactivatable latency by an acyclovir-resistant herpes simplex virus 1 thymidine kinase substitution mutant with reduced neuronal replication

Herpes simplex virus 1 causes recurrent diseases by reactivating from latency, which requires the viral thymidine kinase (TK) gene. An acyclovir-resistant mutation in TK, V204G, was previously repeatedly identified in a patient with recurrent herpetic keratitis. We found that compared with its parental strain KOS, a laboratory-derived V204G mutant virus was impaired in replication in cultured neurons despite little defect in non-neuronal cells. After corneal inoculation of mice, V204G exhibited defects in ocular replication that were modest over the first three days but severe afterward. Acute replication of V204G in trigeminal ganglia was significantly impaired. However, V204G established latency with viral loads as high as KOS and reactivated with high frequency albeit reduced kinetics. Acyclovir treatment that drastically decreased ocular and ganglionic replication of KOS had little effect on V204G. Thus, despite reduced neuronal replication due to impaired TK activity, this clinically relevant drug-resistant mutant can efficiently establish reactivatable latency.

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.

STK1p as a prognostic biomarker for overall survival in non-small-cell lung carcinoma, based on real-world data

Aim:

A prospective investigation of serum thymidine kinase 1 concentration (STK1p) was performed to evaluate its prognostic value in patients with non-small-cell lung carcinoma (NSCLCs).

Patients & methods:

The STK1p values of 127 patients were determined by an enhanced chemiluminescent dot blot assay. The patients were recruited from March 2011 to December 2017.

Results:

Kaplan–Meier plot showed that patients with elevated STK1p values had worse overall survival (OS), especially patients of early/middle stages. Multi-variable COX regression showed that STK1p value and combined treatment surgery + chemotherapy were independent prognostic factors for favorable OS.

Conclusion:

STK1p is helpful in predicting OS of early/middle stages (I–IIIA) NSCLCs patients following a nonrandomized individual adapted treatment, but is may be not recommended in advanced stages (IIIB + IV) of NSCLCs.

Lung cancer is one of the most common types of tumors, with a high mortality rate. We investigate if thymidine kinase 1 in serum (STK1p) is a reliable prognostic marker for survival in non-small-cell lung carcinoma. We recruited 127 patients in this study. STK1p level was determined using a high-sensitive chemiluminescent dot blot assay. Patients with elevated STK1p values had worse overall survival, especially patients in the early/middle cancer stages. Analysis showed that STK1p is an independent prognostic factors for overall survival. We concluded that STK1p is helpful predicting the efficacy of treatment in non-small-cell lung carcinoma for those in the early/middle stages.

Novel monoclonal antibodies against thymidine kinase 1 and their potential use for the immunotargeting of lung, breast and colon cancer cells

BACKGROUND

Thymidine kinase 1 (TK1) is a pyrimidine salvage pathway enzyme that is up-regulated in malignant tissues and elevated in the serum of cancer patients. While TK1 has been well established as a tumor biomarker, little has been done to explore its potential as a tumor target. Recently, we reported the membrane expression of TK1 on malignant cells, but not on normal cells. This study explores the possible use of monoclonal antibodies for the targeting of membrane associated TK1 in lung, breast, colon and prostate cancer cells.

METHODS

We generated and evaluated a panel of monoclonal antibodies against six different epitopes exposed in the tetrameric form of TK1. Antibodies were developed with hybridoma technology and validated with Western blot, siRNA TK1 knockdown, enzyme-linked immunosorbent assay (ELISA) and flow cytometry. The therapeutic potential of the antibodies was evaluated in vitro in antibody-dependent cell-mediated-cytotoxicity (ADCC) experiments.

RESULTS

Binding of the antibodies to TK1 was confirmed by Western blot in purified recombinant protein, cancer serum, and cell lysate. After a TK1 knockdown was performed, a reduction of TK1 expression was observed with five antibodies. Using indirect ELISA, we identified 3B2E11, 9C10, 7H2, 3B4, 8G2 among the most sensitive antibodies (LOD = 10.73-66.9 pg/ml). Surface expression of TK1 on the membrane of various cancer cell lines was analyzed with flow cytometry. Antibodies 8G2, 3B4, 7HD and 5F7G11 detected TK1 on the membrane of various cancer cell lines, including lung, prostate, colon and breast. No significant binding was detected on normal lymphocytes. Increased cytolysis of lung (~ 70%. p = 0.0001), breast (~ 70%, p = 0.0461) and colon (~ 50% p = 0.0216) cancer cells by effector cells was observed when anti-TK1 antibodies were added during ADCC experiments.

CONCLUSIONS

The antibodies developed showed potential to be used to detect and target TK1 on the membrane of various tumor cells. The targeting of TK1 in malignant cells using monoclonal antibodies may be a feasible approach for the elimination of high TK1 expressing tumor cells.

CCNB2, NUSAP1 and TK1 are associated with the prognosis and progression of hepatocellular carcinoma, as revealed by co-expression analysis

The mortality rate associated with hepatocellular carcinoma (HCC) is the third highest among all digestive system tumors. However, the causes of HCC development and the underlying mechanisms have remained to be fully elucidated. In the present bioinformatics study, genetic markers were identified and their association with HCC was determined. The mRNA expression datasets GSE87630, GSE74656 and GSE76427 were downloaded from the Gene Expression Omnibus (GEO) database. A total of 96 differentially expressed genes (DEGs) were screened from the 3 GEO datasets, including 25 upregulated and 71 downregulated genes. DEGs were uploaded to the database for Annotation, Visualization and Integrated Discovery to screen for enriched Gene Ontology terms in various categories and the Search Tool for the Retrieval of Interacting Genes/Proteins was used to identify the interactions and functions of the DEGs. A total of 3 genetic markers were identified in a stepwise pathway and functional analysis in a previous study. The association of the genetic markers with prognosis was analysed using the UALCAN online analysis tool. Regression analysis was also performed to identify the relationship between HCC grade and disease recurrence and the expression of genetic markers using The Cancer Genome Atlas HCC dataset. In addition, the expression of the 3 genetic markers in HCC tissues was determined using reverse transcription-quantitative PCR, the Oncomine database and the Human Protein Atlas database. The expression levels of the 3 genetic markers cyclin B2 (CCNB2), nucleolar and spindle-associated protein 1 (NUSAP1) and thymidine kinase 1 (TK1) were significantly correlated with each other and high mRNA expression of CCNB2 was significantly associated with poor overall survival of patients with HCC. Receiver operating characteristic curve analysis indicated that NUSAP1 and TK1 were capable of distinguishing between recurrent and non-recurrent HCC. Furthermore, CCNB2, NUSAP1 and TK1 were highly correlated with the HCC grade. It was also indicated that the mRNA expression of CCNB2, NUSAPA and TK1 was increased in primary HCC tissues when compared with that in adjacent tissues. The present study identified that the CCNB2, NUSAP1 and TK1 genes may serve as prognostic markers for HCC, and may be of value from the perspectives of basic research and clinical treatment of HCC.

Evaluation of serum thymidine kinase 1 activity as a biomarker for treatment effectiveness and prediction of relapse in dogs with non-Hodgkin lymphoma

BACKGROUND

Serum thymidine kinase 1 (sTK1) activity is closely correlated with DNA synthesis.

OBJECTIVES

Evaluate sTK1 activity as a biomarker for treatment response and early detection of relapse in dogs with lymphoma.

ANIMALS

Ninety-seven client-owned dogs with naive or relapsed lymphoma and 23 healthy dogs.

METHODS

Prospective study. Serum TK1 activity measured by refined ELISA-based method (DiviTum assay, Biovica International) before treatment, at clinical response, and every 4 weeks until relapse or last follow-up.

RESULTS

Serum TK1 activity was ≤20 Du/L in 96% (22/23) of healthy dogs. Pretreatment sTK1 activity was >20 Du/L in 88% (85/97) dogs with lymphoma. At clinical response, sTK1 activity was significantly lower in dogs with complete (CR, n = 36) versus partial (PR, n = 29) response (P < .0001). Sensitivity (Se) and specificity (Sp) of sTK1 activity for detecting nonfully responders were 76% and 100%, respectively, with cutoff of 119.5 Du/L (AUC, 0.90; 95%-CI, 0.81-0.98; P < .0001). In dogs with CR, a 5-fold increase in sTK1 activity at a 4-week interval predicted relapse at the subsequent 4-week assessment with a Se 50% and Sp 94% (AUC, 0.72; 95%-CI, 0.55-0.90; P = .02). An increase of sTK1 activity (>2.7-fold value measured at clinical response) predicted relapse at subsequent 4-week assessment with a Se 61% and Sp 88% (AUC, 0.79; 95%-CI, 0.64-0.95; P = .004).

CONCLUSIONS AND CLINICAL IMPORTANCE

Monitoring sTK1 activity could help to detect complete responders and early disease progression in dogs with lymphoma.

The Clinical Significance of Thymidine Kinase 1 Measurement in Serum of Breast Cancer Patients Using Anti-TK1 Antibody

The activity of total thymidine kinase in serum (S-TK) has been used as a tumor maker for decades. To date such activity has been determined using [125]I-iodo-deoxyuridine as a substrate. The aim of this study was to develop a new, antibody-based technique for the measurement of cytoplasmic thymidine kinase (TK1) in serum. Both mono- and polyclonal antibodies against S-TK1 were used in dot blot assay. S-TK1 was characterized by SDS and IEF techniques. Sixty-five breast cancer patients were studied, including 17 preoperative and 38 postoperative tumor-free patients and 10 patients with metastases to the lymph nodes (N1–2). They were compared to patients with benign tumors (n=21) and healthy volunteers (n=11). S-TK1 was low (0–1.0 pM) in healthy volunteers, while in preoperative patients the level was increased 6–110-fold. Significant differences were observed between preoperative patients and healthy volunteers (p=0.005), preoperative patients and patients with benign tumors (p<0.001), and preoperative patients and postoperative patients without metastases (p<0.001). No significant difference was observed between preoperative patients and postoperative patients with metastases (p=0.191). The S-TK activity in preoperative patients was also high in serum, but no decrease was observed following surgery. In conclusion, the anti-TK1 antibody could be a good marker for monitoring the response of breast cancer patients to therapy.

Whole genome sequencing of matched tumor, adjacent non-tumor tissues and corresponding normal blood samples of hepatocellular carcinoma patients revealed dynamic changes of the mutations profiles during hepatocarcinogenesis

Hepatocellular carcinoma (HCC) has become the third most deadly disease worldwide and HBV is the major factor in Asia and Africa. We conducted 9 WGS (whole genome sequencing) analyses for matched samples of tumor, adjacent non-tumor tissues and normal blood samples of HCC patients from three HBV positive patients. We then validated the mutations identified in a larger cohort of 177 HCC patients. We found that the number of the unique somatic mutations (average of 59,136) in tumor samples is significantly less than that in adjacent non-tumor tissues (average 83, 633). We discovered that the TP53 R249S mutation occurred in 7.7% of the HCC patients, and it was significantly associated with poor diagnosis. In addition, we found that the L104P mutation in the VCX gene (Variable charge, X-linked) was absent in white blood cell samples, but present at 11.1% frequency in the adjacent tissues and increased to 14.6% in HCC tissues, suggesting that this mutation might be a tumor driver gene driving HCC carcinogenesis. Finally, we identified a TK1-RNU7 fusion, which would result in a deletion of 103 amino acids from its C-terminal. The frequencies of this fusion event decreased from the adjacent tissues (29.2%) to the tumors (16.7%), suggesting that a truncated thymidine Kinase1 (TK1) caused by the fusion event might be deleterious and be selected against during tumor progression. The three-way comparisons allow the identification of potential driver mutations of carcinogenesis. Furthermore, our dataset provides the research community a valuable dataset for identifying dynamic changes of mutation profiles and driver mutations for HCC.

The Half-Life of Thymidine Kinase 1 in Serum Measured by ECL Dot Blot: A Potential Marker for Monitoring the Response to Surgery of Patients with Gastric Cancer

Thymidine kinase 1 in serum (STK1) of patients with gastric cancer was determined by two methods: ECL dot blot and radioactivity assay. Both measurements showed significantly different values for preoperative STK1 and healthy STK1 (p=0.012 for ECL dot blot and p=0.003 for the radioactivity assay). The preliminary results of ECL dot blot STK1 measurement showed that in tumor-free subjects the level of the enzyme was significantly reduced to 52.7% 35 days after surgery (n=8, p=0.0106). The decrease in STK1 levels in the tumor-free subjects paralleled the decline of the half-life of the STK1 enzyme. In patients with distant metastases (n=6) the enzyme level had increased to 173% 35 days postoperatively. By contrast, with the radioactivity assay no significant differences in thymidine kinase activity for 0-day-postoperative patients and 35-day-postoperative tumor-free patients was found (p=0.329). The activity decreased to 80% in 35-day-postoperative patients with metastatic disease. We suggest that the value of the half-life of STK1 measured by ECL dot blot can be used as a potential marker for monitoring the response to surgery in patients with gastric or other cancers one month after surgery.

Drug-Free Approach To Study the Unusual Cell Cycle of Giardia intestinalis

Giardia intestinalis is a protozoan parasite that causes giardiasis, a form of severe and infectious diarrhea. Despite the importance of the cell cycle in the control of proliferation and differentiation during a giardia infection, it has been difficult to study this process due to the absence of a synchronization procedure that would not induce cellular damage resulting in artifacts. We utilized counterflow centrifugal elutriation (CCE), a size-based separation technique, to successfully obtain fractions of giardia cultures enriched in G₁, S, and G₂. Unlike drug-induced synchronization of giardia cultures, CCE did not induce double-stranded DNA damage or endoreplication. We observed increases in the appearance and size of the median body in the cells from elutriation fractions corresponding to the progression of the cell cycle from early G₁ to late G₂. Consequently, CCE could be used to examine the dynamics of the median body and other structures and organelles in the giardia cell cycle. For the cell cycle gene expression studies, the actin-related gene was identified by the program geNorm as the most suitable normalizer for reverse transcription-quantitative PCR (RT-qPCR) analysis of the CCE samples. Ten of 11 suspected cell cycle-regulated genes in the CCE fractions have expression profiles in giardia that resemble those of higher eukaryotes. However, the RNA levels of these genes during the cell cycle differ less than 4-fold to 5-fold, which might indicate that large changes in gene expression are not required by giardia to regulate the cell cycle. IMPORTANCE Giardias are among the most commonly reported intestinal protozoa in the world, with infections seen in humans and over 40 species of animals. The life cycle of giardia alternates between the motile trophozoite and the infectious cyst. The regulation of the cell cycle controls the proliferation of giardia trophozoites during an active infection and contains the restriction point for the differentiation of trophozoite to cyst. Here, we developed counterflow centrifugal elutriation as a drug-free method to obtain fractions of giardia cultures enriched in cells from the G₁, S, and G₂ stages of the cell cycle. Analysis of these fractions showed that the cells do not show side effects associated with the drugs used for synchronization of giardia cultures. Therefore, counterflow centrifugal elutriation would advance studies on key regulatory events during the giardia cell cycle and identify potential drug targets to block giardia proliferation and transmission.

Serum Thymidine Kinase 1, Canine-C-Reactive Protein, Haptoglobin, and Vitamin D Concentrations in Dogs with Immune-Mediated Hemolytic Anemia, Thrombocytopenia, and Polyarthropathy

Background

Relapses of immune‐mediated hemolytic anemia (IMHA), thrombocytopenia (ITP), or polyarthropathy (IMPA) occur despite normal hematologic and cytologic parameters. Thymidine kinase 1 (TK1), canine C‐reactive protein (c‐CRP), haptoglobin (HPT), and 25‐Hydroxyvitamin‐D (25(OH)D) might be adjunct to current monitoring strategies.

Hypothesis/Objectives

Compare serum concentrations of TK1, c‐CRP, HPT, and 25(OH)D in dogs with well‐ and poorly controlled primary IMHA, ITP, or IMPA.

Animals

Thirty‐eight client‐owned dogs.

Methods

Prospective descriptive study. Dogs diagnosed with IMHA, ITP, or IMPA had serum biomarker concentrations measured commercially. Disease control was assessed by hematocrit/PCV and reticulocyte count, platelet count, and synovial fluid cytology for IMHA, ITP, and IMPA, respectively. Statistical analysis performed by Mann‐Whitney rank‐sum tests and receiver operating characteristic curves.

Results

TK1 and c‐CRP, but not HPT significantly decreased with well‐ versus poorly controlled IMHA (P = 0.047, P = 0.028, P = 0.37). C‐CRP, but not TK or HPT was significantly lower with well‐ versus poorly controlled IMPA (P = 0.05, P = 0.28, P = 0.84). Sensitivity and specificity of TK and c‐CRP (simultaneously) for detecting dogs with poorly controlled IMHA were 88 and 100%, respectively. Sensitivity and specificity of c‐CRP for detecting poorly controlled dogs with IMPA were 13 and 100%, respectively. 92% of dogs were vitamin D insufficient (<100 ng/mL) regardless of disease control.

Conclusions and Clinical Importance

Combining TK1 and c‐CRP might act markers of disease control in dogs with IMHA. Canine‐CRP cannot be recommended as an independent marker of disease control in IMPA. 25(OH)D insufficiency in immune‐mediated disorders might benefit from further study to determine if supplementation could improve therapeutic response or reduce disease risk.

18F-FLT and 18F-FDG PET-CT imaging in the evaluation of early therapeutic effects of chemotherapy on Walker 256 tumor-bearing rats

The present study aimed to evaluate the early therapeutic effects of chemotherapy on Walker 256 tumor-bearing Wistar rats via F-18-fluoro-3'-deoxy-3'-L-fluorothymidine (¹⁸F-FLT) and F-18-fluoro-deoxyglucose (¹⁸F-FDG) positron emission tomography-computed tomography (PET-CT) imaging. Walker 256 tumor-bearing Wistar rats were subjected to ¹⁸F-FLT and ¹⁸F-FDG PET-CT imaging prior to and 24 and 48 h after epirubicin chemotherapy. ¹⁸F-FLT and ¹⁸F-FDG uptake [tumor/muscle (T/M)], the percentage of injected dose per gram (% ID/g), and the Ki-67 labeling index (LI-Ki-67) were quantitatively determined for each rat prior to and following epirubicin chemotherapy. The correlation between % ID/g and tumor LI-Ki-67 was analyzed. Both ¹⁸F-FLT and ¹⁸F-FDG tumor uptake decreased significantly at 24 and 48 h after chemotherapy (P<0.01 and P<0.05, respectively). LI-Ki-67 also significantly reduced 24 and 48 h after chemotherapy (P<0.001). Furthermore, ¹⁸F-FLT and ¹⁸F-FDG T/M tumor uptake correlated positively with LI-Ki-67 before and after chemotherapy (r=0.842 and 0.813, respectively). During the early post-chemotherapy stage, ¹⁸F-FLT and ¹⁸F-FDG uptake in Walker 256 tumors reduced significantly, which correlated positively with the tumor cell proliferative activity.

Whole genome and normalized mRNA sequencing reveal genetic status of TK6, WTK1, and NH32 human B-lymphoblastoid cell lines

Closely related TK6, WTK1, and NH32 human B-lymphoblastoid cell lines differ in their p53 functional status. These lines are used frequently in genotoxicity studies and in studies aimed at understanding the role of p53 in DNA repair. Despite their routine use, little is known about the genetic status of these cells. To provide insight into their genetic composition, we sequenced and analyzed the entire genome of TK6 cells, as well as the normalized transcriptomes of TK6, WTK1, and NH32 cells. Whole genome sequencing (WGS) identified 21,561 genes and 5.17×10(6) small variants. Within the small variants, 50.54% were naturally occurring single nucleotide polymorphisms (SNPs) and 49.46% were mutations. The mutations were comprised of 92.97% single base-pair substitutions and 7.03% insertions or deletions (indels). The number of predicted genes, SNPs, and small mutations are similar to frequencies observed in the human population in general. Normalized mRNA-seq analysis identified the expression of transcripts bearing SNPs or mutations for TK6, WTK1, and NH32 as 2.88%, 2.04%, and 1.71%, respectively, and several of the variant transcripts identified appear to have important implications in genetic toxicology. These include a single base deletion mutation in the ferritin heavy chain gene (FTH1) resulting in a frame shift and protein truncation in TK6 that impairs iron metabolism. SNPs in the thiopurine S-methyltransferase (TPMT) gene (TPMT*3A SNP), and in the xenobiotic metabolizing enzyme, NADPH quinine oxidoreductase 1 (NQO1) gene (NQO1*2 SNP), are both associated with decreased enzyme activity. The clinically relevant TPMT*3A and NQO1*2 SNPs can make these cell lines useful in pharmacogenetic studies aimed at improving or tailoring drug treatment regimens that minimize toxicity and enhance efficacy.

Serum thymidine kinase 1 is a reliable maker for the assessment of the risk of developing malignancy: A case report

With regard to different types of malignancies, thymidine kinase 1 (TK1) is a useful prognostic marker in clinical oncology, both as a serum proliferation marker and in immunohistochemistry. The present study investigated the use of serum TK1 protein (STK1p) for the identification of multiple proliferating diseases linked to the risk of developing cancer, by following one patient during the period of 2003-2014. The patient presented with adenomatous polyps in the stomach in 2003, follicular cervicitis in 2007 and hyperplasia of the breast/fibrocystic breasts in 2010. The breast cysts increased from 4×5 mm in size in 2010 to 8×7 mm in size in 2013, and were assessed as a suspicious malignancy at the end of this period. In parallel, the STK1p values increased from 2.0 to 7.6 pM. Based on this information, a minimally invasive surgery using the Mammotome® Biopsy System was performed. Immunohistochemistry on the cyst tissue showed strong staining of TK1 in the ductal epithelial cells and thus confirmed the abnormal proliferation in the lesion. One week after the surgery, the STK1p value had decreased to almost normal values (1.6 pM), but then fluctuated above 2.0 pM for the next 7 months. After the surgery, the patient was re-examined and small foci with squamous cell hyperplasia and a suspected ulcerated cervix, as well as flat gastric erosive, were identified, but not treated; this may explain why the STK1 P-values did not return to within normal values. The patient is currently being followed up using STK1p analysis combined with imaging/pathology in order to begin therapeutic intervention as early as possible to avoid the risk of developing cancer. Overall, STK1p is useful in health screening to identify individuals at risk of developing premalignancy/malignancy.

Re-evaluation of the Mutagenic Response to Phosphorothioate Nucleotides in Human Lymphoblastoid TK6 Cells

The degradation of phosphorothioate oligonucleotides (PS-ONDs) and the release of potentially genotoxic modified mononucleotides raise a safety concern for OND-based therapeutics. Deoxyadenosine monophosphorothioate (dAMPαS), a PS nucleotide analog, has been reported to be a potent in vitro mutagen at the thymidine kinase (TK) locus in human TK6 lymphoblastoid cells. This led us to explore the mechanism behind the apparent positive response induced by dAMPαS in the TK gene-mutation assay in TK6 cells. In this work, treatment of TK6 cells with dAMPαS produced a dose-dependent increase in cytotoxicity and mutant frequency at the TK locus. Surprisingly, when the colonies from dAMPαS were re-challenged with the selective agent trifluorothymidine (TFT), the TFT-resistant phenotype was lost. Moreover, dAMPαS-induced colonies displayed distinct growth kinetics and required longer incubation time than 4-nitroquinoline-1-oxide-induced colonies to start growing. Treatment of TK6 cells with dAMPαS induced cell cycle arrest at the G1 phase, enabling cells to grow, and form a colony after the efficacy of TFT in the culture medium was lost. Our findings suggest that a fraction of parental "nonmutant" TK6 cells escaped the toxicity of TFT, possibly via G1 arrest, and resumed growth after the degradation of TFT. We conclude that dAMPαS did not induce real TFT-resistant mutants and caution should be taken with interpretation of mutation data from TK gene-mutation assay in TK6 cells when assessing modified nucleotides.

Phosphorylation status of thymidine kinase 1 following antiproliferative drug treatment mediates 3'-deoxy-3'-[18F]-fluorothymidine cellular retention

Background

3′-Deoxy-3′-[¹⁸F]-fluorothymidine ([¹⁸F]FLT) is being investigated as a Positron Emission Tomography (PET) proliferation biomarker. The mechanism of cellular [¹⁸F]FLT retention has been assigned primarily to alteration of the strict transcriptionally regulated S-phase expression of thymidine kinase 1 (TK1). This, however, does not explain how anticancer agents acting primarily through G2/M arrest affect [¹⁸F]FLT uptake. We investigated alternative mechanisms of [¹⁸F]FLT cellular retention involving post-translational modification of TK1 during mitosis.

Methods

[¹⁸F]FLT cellular retention was assessed in cell lines having different TK1 expression. Drug-induced phosphorylation of TK1 protein was evaluated by MnCl₂-phos-tag gel electrophoresis and correlated with [¹⁸F]FLT cellular retention. We further elaborated the amino acid residues involved in TK1 phosphorylation by transient transfection of FLAG-pCMV2 plasmids encoding wild type or mutant variants of TK1 into TK1 negative cells.

Results

Baseline [¹⁸F]FLT cellular retention and TK1 protein expression were associated. S-phase and G2/M phase arrest caused greater than two-fold reduction in [¹⁸F]FLT cellular retention in colon cancer HCT116 cells (p<0.001). G2/M cell cycle arrest increased TK1 phosphorylation as measured by induction of at least one phosphorylated form of the protein on MnCl₂-phos-tag gels. Changes in [¹⁸F]FLT cellular retention reflected TK1 phosphorylation and not expression of total protein, in keeping with the impact of phosphorylation on enzyme catalytic activity. Both Ser13 and Ser231 were shown to be involved in the TK1 phosphorylation-modulated [¹⁸F]FLT cellular retention; although the data suggested involvement of other amino-acid residues.

Conclusion

We have defined a regulatory role of TK1 phosphorylation in mediating [¹⁸F]FLT cellular retention and hence reporting of antiproliferative activity, with implications especially for drugs that induce a G2/M cell cycle arrest.

N3-substituted thymidine bioconjugates for cancer therapy and imaging

The compound class of 3-carboranyl thymidine analogues (3CTAs) are boron delivery agents for boron neutron capture therapy (BNCT), a binary treatment modality for cancer. Presumably, these compounds accumulate selectively in tumor cells via intracellular trapping, which is mediated by hTK1. Favorable in vivo biodistribution profiles of 3CTAs led to promising results in preclinical BNCT of rats with intracerebral brain tumors. This review presents an overview on the design, synthesis, and biological evaluation of first- and second-generation 3CTAs. Boronated nucleosides developed prior to 3CTAs for BNCT and non-boronated N3-substituted thymidine conjugates for other areas of cancer therapy and imaging are also described. In addition, basic features of carborane clusters, which are used as boron moieties in the design and synthesis of 3CTAs, and the biological and structural features of TK1-like enzymes, which are the molecular targets of 3CTAs, are discussed.

The role of thymidine kinase in cancer diseases

Thymidine kinase 1 (TK 1-fetal) is a cell cycle-dependent marker that increases dramatically during the S-phase of the cell cycle. In this review, the authors discuss serum levels of thymidine kinase in a variety of neoplasias. Determination of thymidine kinase helps to monitor the follow-up of solid tumours and haematological malignancies as well as indicating the efficacy of adjuvant and palliative chemotherapy. Elevated levels of thymidine kinase must always be interpreted together with a detailed knowledge of the patient's condition because nonspecific elevations of serum levels (inflammatory and autoimmune diseases) must be excluded.

PET/CT today and tomorrow in veterinary cancer diagnosis and monitoring: fundamentals, early results and future perspectives

Functional imaging using positron emission tomography (PET) plays an important role in the diagnosis, staging, image-guided treatment planning and monitoring of malignant diseases. PET imaging complements conventional anatomical imaging such as computed tomography (CT) and magnetic resonance imaging (MRI). The strength of CT scanning lies in its high spatial resolution, allowing for anatomical characterization of disease. PET imaging, however, moves beyond anatomy and characterizes tissue based on functions such as metabolic rate. Combined PET/CT scanners were introduced commercially in 2001 and a number of technological advancements have since occurred. Radiolabelled tracers such as (18)F-fluorodeoxyglucose (FDG) and (18)F-fluorothymidine (FLT) allow visualization of various metabolic processes within cancer cells. Many studies in human oncology evaluating the utility of PET/CT have demonstrated clinical benefits. Few veterinary studies have been performed, but initial studies show promise for improved detection of malignancy, more thorough staging of canine cancer and determination of early response and disease recrudescence.

Comparative aspects of the proliferation marker thymidine kinase 1 in human and canine tumour diseases

As cell proliferation is one of the hallmarks of cancer, various types of proliferation markers are used as important tools in diagnosis, prognosis, treatment decision-making and follow-up in clinical oncology. The S phase-specific protein thymidine kinase 1 (TK1) can be used in immunohistochemistry for RNA/protein expression in tissue specimens and for activity or protein/peptide levels in serum from patients. TK1 has been used mainly in haematologic malignancies in humans, but also found beneficial in canine malignancies. As the protein sequence homology is high between humans and dogs, findings in canine models will have a high comparative value in further human research as well. In the present review, we will focus on the recent results concerning TK1's S phase-correlated expression, increased serum levels of TK1 in patients with malignancies and the relevance for veterinary and comparative oncology. Finally, the benefit of recently developed specific anti-TK1 antibodies suitable for immunologic assay is discussed.

The future of imaging: developing the tools for monitoring response to therapy in oncology: the 2009 Sir James MacKenzie Davidson Memorial lecture

Since the days of Sir James MacKenzie Davidson, radiology discoveries have been shaping the way patients are managed. The lecture on which this review is based focused not on anatomical imaging, which already has a great impact on patient management, but on the molecular imaging of cancer targets and pathways. In this post-genomic era, we have several tools at our disposal to enable the discovery of new probes for stratifying patients for therapy and for monitoring response to therapy sooner than is possible using conventional cross-sectional imaging methods. I describe a chemical library approach to discovering new imaging agents, as well as novel methods for improving the metabolic stability of existing probes. Finally, I describe the evaluation of these probes for clinical use in both pre-clinical and clinical validation. The chemical library approach is exemplified by the discovery of isatin sulfonamide probes for imaging apoptosis in tumours. This approach allowed important desirable features of radiopharmaceuticals to be incorporated into the design strategy. A lead compound, [(18)F]ICMT11, is selectively taken up in vitro in cancer cells and in vivo in tumours undergoing apoptosis. Improvement of the metabolic stability of a probe is exemplified by work on [(18)F]fluoro-[1,2-(2)H(2)]choline ("[(18)F]-D4-choline"), a novel probe for imaging choline metabolism. Deuterium substitution significantly reduced the systemic metabolism of this compound relative to that of non-deuteriated analogues, supporting its future clinical use. In order for probes to be useful for monitoring response a number of validation and/or qualification studies need to be performed, including assessments of whether the probe measures the target or pathway of interest in a specific and reproducible manner, whether the probe is stable to metabolism in vivo, what is the best time to assess response with these probes and finally whether changes in radiotracer uptake are associated with clinical outcome. [(18)F]Fluorothymidine, a probe for proliferation imaging has been validated and qualified for use in breast cancer. In summary, the ability to create new molecules that can report on specific targets and pathways provides a strategy for studying response to treatment in cancer earlier than it is currently possible. This could fundamentally change the way medicine is practised in the next 5-10 years.

Serum thymidine kinase 1 correlates to clinical stages and clinical reactions and monitors the outcome of therapy of 1,247 cancer patients in routine clinical settings

BACKGROUND

Thymidine kinase 1 in serum (STK1) has been found to be a reliable proliferation marker in clinical trials. In this study, we examined the significance of STK1 in routine clinical settings.

METHODS

The concentration of STK1 was determined by a sensitive dot blot ECL assay. The STK1 value was correlated to clinical stage and reactions and used for monitoring the outcome of surgery and/or multidrug chemotherapy of 1,247 patients with five different types of carcinomas (lung, esophagus, gastric, head and neck, and thyroid) in routine clinical settings.

RESULTS

The STK1 values correlated with the clinical stage in patients with lung, esophagus, thyroid, and gastric carcinomas. After treatment, STK1 declined in all tumor groups after treatments (P < 0.01). The STK1 was low (<2 pM) or decreasing during treatment in patients with clinical reactions of complete response (CR) or partial response (PR), but high (>2 pM) or increasing in patients with stable disease (SD) or progressive disease (PD), some of them showing metastasis. STK1 also reflected the differences in clinical reactions when surgery and chemotherapy were compared.

CONCLUSION

We concluded that the concentration of TK1 in serum correlates to clinical stages and clinical reactions and monitors the effect of tumor therapies, not only in controlled clinical trials, but also in routine clinical settings.

Serum thymidine kinase 1 concentration as a prognostic factor of chemotherapy-treated non-Hodgkin's lymphoma patients

PURPOSE

This study was performed to examine possible use of thymidine kinase 1 concentration in serum (STK1) for prognosis of non-Hodgkin's lymphoma patients following chemotherapy treatment.

METHODS

The STK1 levels of 37 patients were determined by enhanced chemiluminescent dot-blot assay on the day before chemotherapy, and on day 1 and day 28 after start of the treatment. The specificity and sensitivity was evaluated by Western blot with anti-TK1 IgY antibody and by receiver operating characteristic (ROC) analysis.

RESULTS

Western blot and ROC analysis of TK1 in serum showed high specificity and sensitivity. The mean STK1 level of the non-Hodgkin's lymphoma patients was significantly higher compared to healthy persons (p < 0.001). The mean STK1 level increased significantly (p < 0.001) on day 1 and then declined, reaching on day 28 values corresponding to those of healthy persons. The mean STK1 values before treatment and at 1 and 28 days after start of the treatment also correlated significantly with the clinical response (CR, PR and NR) and five-year survival.

CONCLUSION

Although the number of patients was limited in this study, TK1 in serum might possess an important reference value in the evaluation of treatment and prognosis of non-Hodgkin's lymphoma following chemotherapy.

Thymidine kinase 1 expression defines an activated G1 state of the cell cycle as revealed with site-specific antibodies and ArrayScan assays

Thymidine kinase 1 (TK1) is a DNA salvage enzyme involved in the synthesis of thymidine triphosphate needed during S phase. Although TK1 has been utilized as a cell proliferation marker for many years no well-characterized antibodies are available. The preparation and properties of two types of poly- and monoclonal anti-TK1 peptide antibodies are described and they are used to determine the levels of TK1 in intact cells. Expression of TK1, c-fos, cyclin B1, Ki67, phosphorylated histone H3, phosphorylated ribosomal protein S6, as well as bromodeoxyuridine (BrdU) incorporation in human normal dermal fibroblast cultures were studied with high-content ArrayScan fluorescence microscopy. The levels of TK1 increased 6-7h after serum re-addition to starved cells as they passed through G1, S and G2/M phases, which was earlier than the increase in Ki67 protein levels and before BrdU incorporation was detected. Thus, a population of activated G1 cells with high TK1 and low Ki67 expression could be identified and their role in cell proliferation can now be clarified.

Serum thymidine kinase 1 concentration in Chinese patients with chronic lymphocytic leukemia and its correlation with other prognostic factors

Chronic lymphocytic leukemia (CLL) shows a remarkable heterogeneity, with some patients having an almost normal lifespan, others surviving only several months after diagnosis despite intensive therapy. The aim of this study was to investigate the serum thymidine kinase 1 (TK1) concentration in Chinese patients with CLL and its correlation with well-established other prognostic factors. Enhanced chemiluminescent dot blot assay was performed to measure serum TK1 concentration in 80 CLL patients. The concentration of TK1 was significantly increased in patients with Binet C (P = 0.002), higher levels of serum lactate dehydrogenase (LDH) (P = 0.012) and beta2-microglobulin (beta2-MG) (P = 0.025), unmutated IGHV status (P < 0.001), or higher expression levels of ZAP-70 (P = 0.014) and CD38 (P = 0.018) groups compared to the patients with Binet A, lower levels of serum LDH and beta2-MG, mutated IGHV status, or lower expression levels of ZAP-70 and CD38 groups, respectively. Strong correlation of TK1 level with IGHV mutations (r = 0.412, P < 0.001) or ZAP-70 (r = 0.263, P = 0.024) was observed. According to receiver operating characteristic curve analysis for serum TK1 concentration and IGHV mutational status, area under the curve was 0.757 (P = 0.001) and the optimal cut-off value of serum TK1 concentration level was 1.75 pM, with a 87.8% specificity, a 63.6% sensitivity. It was showed that serum TK1 concentration could be a predictive marker of IGHV mutational status, and might be applied for the assessment of prognosis in patients with CLL.

A simple quantitative assay for the activity of thymidine kinase 1 in solid tumors

INTRODUCTION

The activity of the pyrimidine salvage pathway enzyme thymidine kinase 1 (TK1) is tightly cell cycle regulated and has been investigated as a prognostic indicator of cancer in a variety of tissues. However, using the in vitro assay of TK1 to rank order a series of unique tumor samples by their TK1 activity can be problematic due to the complex nature of TK1 enzyme substrate kinetics. We present a refined TK1 in vitro assay and method of analysis which address these problems.

METHODS

Extracts were prepared of the resected lung lesions from eight patients and assayed for TK1 activity using an in vitro assay modified to account for nonlinearities in extract protein concentration. A separate extract of exponentially growing A549 human lung carcinoma cells was used as a cross-assay control.

RESULTS

In extracts prepared from eight frozen samples of resected human lung lesions, TK1 activity (mean=0.0070+/-0.0077 pmol [(3)H]-TMP/microg protein/minute) was 2 orders of magnitude below that of exponentially growing A549 human lung carcinoma cells (mean=0.1572+/-0.0218 pmol [(3)H]-TMP/microg protein/minute; n=9). TK1 activity was nonlinear with respect to extract protein concentration in both groups, with A549 cell extracts exhibiting evidence of positive cooperativity which could not be explained by the presence of detergents in the cell lysis buffer. Lung tumor extracts demonstrated evidence of negative cooperativity.

CONCLUSIONS

The modified TK1 assay takes into account these nonlinearities by averaging the results of several complete time-course curves measured over a range of extract protein concentrations. An extract prepared from exponentially growing A549 cells is included in each assay for use as a cross-assay control. We demonstrate that these modifications allow for the accurate rank ordering of TK1 activity in solid tumors.

Mitotic control of dTTP pool: a necessity or coincidence?

The fidelity of DNA replication in eukaryotic cells requires a balanced dNTP supply in the S phase. During the cell cycle progression, the production of dTTP is highly regulated to coordinate with DNA replication. Intracellular thymidine is salvaged to dTTP by cytosolic thymidine kinase (TK1) and thymidylate kinase (TMPK), both of which expression increase in the G1/S transition and diminish in the mitotic phase via proteolytic destruction. Anaphase promoting complex/cyclosome (APC/C)-mediated ubiquitination targets TK1 and TMPK to undergo proteasomal degradation in mitosis, by which dTTP pool is minimized in the early G1 phase of the next cell cycle. In this review, we will focus on regulation of TK1 in the post-S phase and the importance of mitotic proteolysis in controlling dNTP balance, replication stress and genomic stability. Finally, we discuss how thymidine pool and oligomeric forms of TK1 can affect mitotic control of dTTP.

p53R2-dependent ribonucleotide reduction provides deoxyribonucleotides in quiescent human fibroblasts in the absence of induced DNA damage

Human fibroblasts in culture obtain deoxynucleotides by de novo ribonucleotide reduction or by salvage of deoxynucleosides. In cycling cells the de novo pathway dominates, but in quiescent cells the salvage pathway becomes important. Two forms of active mammalian ribonucleotide reductases are known. Each form contains the catalytic R1 protein, but the two differ with respect to the second protein (R2 or p53R2). R2 is cell cycle-regulated, degraded during mitosis, and absent from quiescent cells. The recently discovered p53-inducible p53R2 was proposed to be linked to DNA repair processes. The protein is not cell cycle-regulated and can provide deoxynucleotides to quiescent mouse fibroblasts. Here we investigate the in situ activities of the R1-p53R2 complex and two other enzymes of the de novo pathway, dCMP deaminase and thymidylate synthase, in confluent quiescent serum-starved human fibroblasts in experiments with [5-(3)H]cytidine, [6-(3)H]deoxycytidine, and [C(3)H(3)]thymidine. These cells had increased their content of p53R2 2-fold and lacked R2. From isotope incorporation, we conclude that they have a complete de novo pathway for deoxynucleotide synthesis, including thymidylate synthesis. During quiescence, incorporation of deoxynucleotides into DNA was very low. Deoxynucleotides were instead degraded to deoxynucleosides and exported into the medium as deoxycytidine, deoxyuridine, and thymidine. The rate of export was surprisingly high, 25% of that in cycling cells. Total ribonucleotide reduction in quiescent cells amounted to only 2-3% of cycling cells. We suggest that in quiescent cells an important function of p53R2 is to provide deoxynucleotides for mitochondrial DNA replication.

Effect of C-terminal of human cytosolic thymidine kinase (TK1) on in vitro stability and enzymatic properties

Thymidine kinase (TK1) is a key enzyme in the salvage pathway of nucleotide metabolism and catalyzes the first rate-limiting step in the synthesis of dTTP, transfer of a gamma-phosphate group from a nucleoside triphosphate to the 5'-hydroxyl group of thymidine, thus forming dTMP. TK1 is cytosolic and its activity fluctuates during cell cycle coinciding with the DNA synthesis rate and disappears during mitosis. This fluctuation is important for providing a balanced supply of dTTP for DNA replication.The cell cycle specific activity of TK1 is regulated at the transcriptional level, but posttranslational mechanisms seem to play an important role for the level of functional TK1 protein as well. Thus, the C-terminal of TK1 is known to be essential for the specific degradation of the enzyme at the G2/M phase. In this work, we have studied the effect of deletion of the C-terminal 20, 40, and 44 amino acids of TK1 on in vitro stability, oligomerization, and enzyme kinetics. We found that deletion of the C-terminal fold markedly increased the stability as well as the catalytic activity.

Hiding human thymidine kinase 1 from APC/C-mediated destruction by thymidine binding

Thymidine kinase 1 (TK1) is a key cytosolic enzyme in the salvage pathway for dTTP synthesis. In mitotic exit, human TK1 (hTK1) is degraded via the anaphase-promoting complex/cyclosome (APC/C)-Cdh1 pathway to limit dTTP production. In this study, we show that thymidine binding stabilizes hTK1 during growth arrest. By in vitro degradation, ubiquitination, and Cdh1 binding analyses, we provide direct evidence that thymidine binding protects wild-type hTK1 protein from APC/C-Cdh1-mediated destruction. In contrast, mutant-type hTK1 protein defective in thymidine binding ability could still be polyubiquitinated by APC/C-Cdh1 in the presence of thymidine. These results suggest that the status of thymidine binding to hTK1 protein determines its susceptibility to degradation due to APC/C targeting. Our in vivo experimental data also demonstrated that thymidine treatment abolished Cdh1/proteasome-responsive suppression of hTK1 expression. Moreover, exposure of mitotic-arrested K562 cells to thymidine (100 microM) stabilized endogenous TK1, causing nucleotide imbalance in the early G1 phase and an increase of S phase accumulation. In conclusion, thymidine is not only a substrate of TK1 but also acts as its expression regulator by modulating its proteolytic control during mitotic exit, conferring a feed-forward regulation of dTTP formation.

[18F]FLT-PET in oncology: current status and opportunities

In recent years, [18F]-fluoro-3'-deoxy-3'-L: -fluorothymidine ([18F]FLT) has been developed as a proliferation tracer. Imaging and measurement of proliferation with PET could provide us with a non-invasive staging tool and a tool to monitor the response to anticancer treatment. In this review, the basis of [18F]FLT as a proliferation tracer is discussed. Furthermore, an overview of the current status of [18F]FLT-PET research is given. The results of this research show that although [18F]FLT is a tracer that visualises cellular proliferation, it also has certain limitations. In comparison with the most widely used PET tracer, [18F]FDG, [18F]FLT uptake is lower in most cases. Furthermore, [18F]FLT uptake does not always reflect the tumour cell proliferation rate, for example during or shortly after certain chemotherapy regimens. The opportunities provided by, and the limitations of, [18F]FLT as a proliferation tracer are addressed in this review, and directions are given for further research, taking into account the strong and weak points of the new tracer.

The uptake of 3?-deoxy-3?-[18F]fluorothymidine into L5178Y tumours in vivo is dependent on thymidine kinase 1 protein levels

PURPOSE

The aim of this study was to investigate the role of thymidine kinase 1 (TK1) protein in 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) positron emission tomography (PET) studies.

METHODS

We investigated the in vivo kinetics of [18F]FLT in TK1+/- and TK1-/- L5178Y mouse lymphoma tumours that express different levels of TK1 protein.

RESULTS

[18F]FLT-derived radioactivity, measured by a dedicated small animal PET scanner, increased within the tumours over 60 min. The area under the normalised tumour time-activity curve were significantly higher for the TK1+/- compared with the -/- variant (0.89+/-0.02 vs 0.79+/-0.03 MBq ml(-1) min, P=0.043; n=5 for each tumour type). Ex vivo gamma counting of tissues excised at 60 min p.i. (n=8) also revealed significantly higher tumour [18F]FLT uptake for the TK1+/- variant (6.2+/-0.6 vs 4.6+/-0.4%ID g(-1), P=0.018). The observed differences between the cell lines with respect to [18F]FLT uptake were in keeping with a 48% higher TK1 protein in the TK1+/- tumours versus the -/- variant (P=0.043). On average, there were no differences in ATP levels between the two tumour variants (P=1.00). A positive correlation between [18F]FLT accumulation and TK1 protein levels (r=0.68, P=0.046) was seen. Normalisation of the data for ATP content further improved the correlation (r=0.86, P=0.003).

CONCLUSION

This study shows that in vivo [18F]FLT kinetics depend on TK1 protein expression. ATP may be important in realising this effect. Thus, [18F]FLT-PET has the potential to yield specific information on tumour proliferation in diagnostic imaging and therapy monitoring.

Sensitive nonradiometric method for determining thymidine kinase 1 activity

BACKGROUND

Thymidine kinase 1 (TK1) is a cytoplasmic enzyme, produced only in the S-phase of proliferating cells, that has potential as a tumor marker. Specific determination of TK1 in serum is difficult, in part because of differences in the physical properties of serum TK1 compared with cytoplasmic TK1.

METHODS

The first step in the new assay was phosphorylation of 3'-azido-2',3'-deoxythymidine (AZT) to AZT 5'-monophosphate (AZTMP) by TK1 present in patient material. The AZTMP formed was measured in a competitive immunoassay with specific anti-AZTMP antibodies and AZTMP-labeled peroxidase. Results were compared with those of a TK radioenzyme assay (REA) for 78 samples from patients suffering from hematologic diseases.

RESULTS

The detection limit was 78 microIU/L, and within-run CVs <20% were seen for samples with TK1 down to 130 microIU/L. Cross-determination of the mitochondrial isoenzyme TK2 activity was <0.1%. Between-assay imprecision (CV) was 3.5-7.4%, and the within-assay imprecision was 4.1-9.1%. In studies of recovery and linearity on dilution, measured values ranged from 84% to 115% of expected at concentrations of 0.26-10.4 mIU/L. Results of the new assay (mIU/L) = 0.109 x TK REA (U/L) + 0.092. Heterophilic antibodies did not interfere in the assay. The upper 95th percentile, in 100 healthy individuals, was 0.94 mIU/L, and the median value was 0.43 mIU/L.

CONCLUSION

The TK1 enzyme-labeled immunoassay uses a stable substrate, is precise, appears to be accurate, and is resistant to interferences. It may provide a practical tool in the management of hematologic malignancies.

Perturbation of ATP-induced tetramerization of human cytosolic thymidine kinase by substitution of serine-13 with aspartic acid at the mitotic phosphorylation site

Human cytosolic thymidine kinase (TK1) is tightly regulated in the cell cycle by multiple mechanisms. Our laboratory has previously shown that in mitotic-arrested cells human TK1 is phosphorylated at serine-13, accompanied by a decrease in catalytic efficiency. In this study we investigated whether serine-13 phosphorylation regulated TK1 activity and found that substitution of serine-13 with aspartic acid (S13D), which mimics phosphorylation, not only diminished the ATP-activating effect on the enzyme, but also decreased its thymidine substrate affinity. Our experimental results further showed that the S13D mutation perturbed ATP-induced tetramerization of TK1. Given that the dimeric form of TK1 is less active than the tetrameric, we propose that mitotic phosphorylation of serine-13 is of physiological importance, in that it may counteract ATP-dependent activation of TK1 by affecting its quaternary structure, thus attenuating its enzymatic function at the G2/M phase.

Molecular characterization of thymidine kinase from Ureaplasma urealyticum: nucleoside analogues as potent inhibitors of mycoplasma growth

Ureaplasma urealyticum (U. urealyticum), belonging to the class Mollicutes, is a human pathogen colonizing the urogenital tract and causes among other things respiratory diseases in premature infants. We have studied the salvage of pyrimidine deoxynucleosides in U. urealyticum and cloned a key salvage enzyme, thymidine kinase (TK) from U. urealyticum. Recombinant Uu-TK was expressed in E. coli, purified and characterized with regards to substrate specificity and feedback inhibition. Uu-TK efficiently phosphorylated thymidine (dThd) and deoxyuridine (dUrd) as well as a number of pyrimidine nucleoside analogues. All natural ribonucleoside/deoxyribonucleoside triphosphates, except dTTP, served as phosphate donors, while dTTP was a feedback inhibitor. The level of Uu-TK activity in U. urealyticum extracts increased upon addition of dUrd to the growth medium. Fluoropyrimidine nucleosides inhibited U. urealyticum and M. pneumoniae growth and this inhibitory effect could be reversed by addition of dThd, dUrd or deoxytetrahydrouridine to the growth medium. Thus, the mechanism of inhibition was most likely the depletion of dTTP, either via a blocked thymidine kinase reaction and/or thymidylate synthesis step and these metabolic reactions should be suitable targets for antimycoplasma chemotherapy.

Production and characterisation of a novel chicken IgY antibody raised against C-terminal peptide from human thymidine kinase 1

Egg yolk is a good source of highly specific antibodies against mammalian antigens because of the phylogenetic distance between birds and mammals. Chicken egg yolk immunoglobulins (IgY) were generated to a synthetic 31-amino acid peptide from the C-terminal of human HeLa thymidine kinase 1 (TK1) enzyme. The anti-TK1 IgY antibody was purified using affinity chromatography against the 31-amino acid peptide. The purified antibody inhibited the catalytic activity of the TK1 enzyme in the CEM TK1(+) cells and recognized the 25-kDa subunit and tetrameric form of TK1, which has a pI value of 8.3. No immunoreaction was observed in CEM TK1(-) cells. Western blot of the serum TK1 (S-TK1) also showed that only a single band was found in the serum of patients with malignancies. No band was seen in healthy serum. Furthermore, dot blots and enhanced chemiluminescence (ECL) detection of S-TK1 performed on sera of preoperative patients with gastric cancer (GC) (n=31) and healthy controls (n=62) showed that the levels of S-TK1 in the sera of cancer patients were significantly different (P<0.01). Using ECL dot blots, 0.1 pg of TK1 in 3 microl sera could be detected. Immunohistostaining of tissues in the 11 advanced-stage cancer patients (four breast carcinomas, three hepatocarcinomas and four thyroid carcinomas) indicated that a strong staining of TK1 enzyme was found in the cytoplasm of malignant cells. No staining or weak staining was seen in normal tissues. We suggest that screening for TK1 using anti-TK1 IgY may be potentially useful for serological and immunohistochemical detection of TK1 as an early prognosis and for monitoring patients undergoing treatment.

Deoxynucleoside anabolic enzyme levels in acute myelocytic leukemia and chronic lymphocytic leukemia cells

The deoxynucleoside kinase reaction is often rate-limiting in the anabolism of pharmacologically active anti-cancer nucleosides. The levels of thymidine kinase (TK), deoxycytidine kinase, deoxyguanosine kinase (dGK), and thymidylate kinase were determined in leukocyte extracts from patients with chronic lymphocytic leukemia (CLL) and acute myelocytic leukemia (AML). The extracts from AML patients showed significantly higher TK activity than the ones from CLL patients. There were no differences in the levels of the other three kinases. In the case of dGK, the determinations were carried out with both an immunoblotting assay and selective enzyme activity measurements.

Valine, not methionine, is amino acid 106 in human cytosolic thymidine kinase (TK1). Impact on oligomerization, stability, and kinetic properties

Cytosolic thymidine kinase (TK1) cDNA from human lymphocytes was cloned, expressed in Escherichia coli, purified, and characterized with respect to the ATP effect on thymidine affinity and oligomerization. Sequence analysis of this lymphocyte TK1 cDNA and 21 other cDNAs or genomic TK1 DNAs from healthy cells or leukemic or transformed cell lines revealed a valine at amino acid position 106. The TK1 sequence in NCBI GenBank(TM) has methionine at this position. The recombinant lymphocyte TK1(Val-106) (rLy-TK1(Val-106)) has the same enzymatic and oligomerization properties as endogenous human lymphocyte TK1 (Ly-TK1); ATP exposure induces an enzyme concentration-dependent reversible transition from a dimer to a tetramer with 20-30-fold higher thymidine affinity (K(m) about 15 and 0.5 microm, respectively). Substitution of Val-106 with methionine to give rLy-TK1(Met-106) results in a permanent tetramer with the high thymidine affinity (K(m) about 0.5 microm), even without ATP exposure. Furthermore, rLy-TK1(Met-106) is considerably less stable than rLy-TK1(Val-106) (t(12) at 15 degrees C is 41 and 392 min, respectively). Because valine with high probability is the naturally occurring amino acid at position 106 in human TK1 and because this position has high impact on the enzyme properties, the Val-106 form should be used in future investigations of recombinant human TK1.

Substrate binding is a prerequisite for stabilisation of mouse thymidine kinase in proliferating fibroblasts

Thymidine kinase (TK) expression in mammalian cells is strictly growth regulated, with high levels of the enzyme present in proliferating cells and low levels in resting cells. We have shown that mouse TK expressed from a constitutive promoter is still subject to this regulation. The drastic decline in TK enzyme levels in resting cells is largely due to a pronounced reduction in the half-life of the protein. Deletion of the 30 C-terminal amino acid residues from TK abrogates growth regulation, rendering the enzyme very stable. Moreover, the substrate thymidine was sufficient to stabilise the labile TK protein in quiescent cells. Here, we report that the ability of TK to bind substrates is essential for both growth-dependent regulation and stabilisation by the substrate. By mutation or elimination of the binding sites for either of the two substrates, ATP and thymidine, we expressed TK proteins lacking enzymatic activity which abolished growth-regulated expression in both cases. Mutant TK proteins impaired in substrate binding were subject to rapid degradation in exponentially growing cells and thymidine was no longer sufficient to inhibit this rapid decay. A C-terminal truncation known to stabilise the TK wild-type protein in resting cells did not affect the rapid turnover of enzymatically inactive TK proteins. Proteasome inhibitors also failed to stabilise these substrate-binding mutants. By cross-linking experiments, we show that TK proteins with mutated substrate-binding sites exist only as monomers, whereas active TK enzyme forms dimers and tetramers. Our data indicate that, In addition to the C terminus intact substrate-binding sites are required for growth-dependent regulation of TK protein stability.

Human high-Km 5'-nucleotidase effects of overexpression of the cloned cDNA in cultured human cells

5'-Nucleotidases participate, together with nucleoside kinases, in substrate cycles involved in the regulation of deoxyribonucleotide metabolism. Three major classes of nucleotidases are known, one on the plasma membrane and two in the cytosol. The two cytosolic classes have been named high-Km nucleotidases and 5'(3')-nucleotidases. Starting from two plasmids with partial sequences (Oka, J., Matsumoto, A., Hosokawa, Y. & Inoue, S. (1994) Biochem. Biophys. Res. Commun. 205, 917-922) we cloned the complete cDNA of the human high-Km nucleotidase into vectors suitable for transfection of Escherichia coli or mammalian cells. After transfection, E. coli overproduced large amounts of the enzyme. Most of the enzyme was present in inclusion bodies that also contained many partially degraded products of the protein. Part of the enzyme, corresponding to approximately 2% of the soluble proteins, was in a soluble active form. Stably transfected human 293 cells were obtained with a vector where the 3'-end of the nucleotidase coding sequence is linked to the 5'-end of the green fluorescent protein coding sequence. Several green clones overproduced both mRNA and fusion protein. Two clones with 10-fold higher enzyme activity were analyzed further. The nucleotidase activity of cell extracts showed the same substrate specificity and allosteric regulation as the high-Km enzyme. The growth rate of the two clones did not differ from the controls. The cells were not resistant to deoxyguanosine or deoxyadenosine, and did not show an increased ability to phosphorylate dideoxyinosine. Both ribonucleoside and deoxyribonucleoside triphosphate pools were decreased slightly, suggesting participation of the enzyme in their regulation.