Cell cycle related studies on thymidine kinase and its isoenzymes in Ehrlich ascites tumours

S, M. Z, J. O, P. V, S. E. Analytical and clinical characterization of an optimized dual monoclonal sandwich ELISA for the quantification of thymidine kinase 1 (TK1) protein in human blood samples

Thymidine Kinase 1 (TK1) plays an important role in DNA precursor synthesis and serum TK1 activity has been used as a biomarker for prognosis and therapy monitoring of different malignancies. AroCell has developed a dual monoclonal antibody ELISA for determination of TK1 protein in clinical samples. The purpose of the study is to validate the ELISA analytically in relation to the gold standard, [3H]-deoxythymidine (dThd) phosphorylation assay for TK1 activity using sera from patients with different malignancies. The colorimetric TK 210 ELISA was validated analytically by assessment of precision, linearity, interfering substances, and stability. For the clinical validation, serum samples from patients with hematological malignancies (n = 100), breast cancer (n = 56), prostate cancer (n = 70) and blood donors (n = 159) were analyzed using TK 210 ELISA and TK1 activity by [3H]-deoxythymidine (dThd) phosphorylation assay. The sandwich TK 210 ELISA was highly specific for TK1 protein having a detection limit of 0.12 ng/mL, with a functional sensitivity of 0.25 ng/mL. Within-run CVs ranged from 5.5% to 10% and between-run CVs ranged from 5% to 15%. The ratio of observed to expected dilutional parallelism of 5 serum samples was in the range of 80-120%. Samples exhibited stability through four freeze/thaw cycles and 5 days at 4°C. Further, the ROC curve analysis showed that TK 210 ELISA and [3H]-dThd phosphorylation assay had similar sensitivity (62% vs 59%) in hematological malignancies. However, in the case of breast and prostate cancer sera, TK 210 ELISA had higher sensitivity (59% and 44%) compared to [3H]-dThd phosphorylation assay (47% and 25%) at a specificity of 98%. These data demonstrate that the dual monoclonal antibody based AroCell TK 210 ELISA is a robust, accurate and precise tool for measuring TK1 protein in different malignancies that can improve the clinical applications of TK1 as a biomarker in cancer management.

Concentration of human thymidine kinase 1 discover invisible malignant human tumours

Early discover of risk progression of invisible carcinomas is important for a prerequisite successful treatment. Here, we investigated whether concentration of human thymidine kinase 1 (HTK1) discover invisible malignant human tumours. The HTK1 concentration of tumour cellular based on HTK1 IgY-polyclonal-antibody (HTK1-IgY-pAb) was determined by using a novel automatic chemiluminescence analyser with sandwich biotin-streptavidin (SBSA) platform. Minimum number of cells able to be detected by this technology used cells with low and high concentration of HTK1. The limit visibility by tumour imaging is approximately 1 mm in diameter, corresponding to approximately 10⁹ cells with a cell diameter of 1 µm. Based on a HTK1 standard curve and a molecular weight of HTK1 of 96 kD, the HTK1protein (HTK1p) concentration per cell was calculated to be 0.021 pg. Assuming 200 pg in total protein/cell, approximately 50 × 10⁶ growing malignant cells in the body were calculated to releases HTK1 into 5-liter blood. A HTK1 values of 3.914, 0.435 and 0.009 pmol/L corresponds to 10 × 10⁵, 2 × 10⁵ and 1 × 10⁵ growing malignant cells, respectively. The lowest detectable sensitivity of HTK1 is 0.009 pmol/L in 1 × 10⁵ growing malignant cells and 0.01 pmol/L in blood serum, detectable in health screening. Comparing the novel automatic chemiluminescence analyser with the original ECL dot-blot assay using serum HTK1p (health screening, n = 265) showed high correlation (r = 0.8743, P < .000). In conclusion, the novel automatic chemiluminescence analyser with SBSA platform is a reliable method with high accuracy to determine carcinoma invisible.

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.

The Half-Life of Serum Thymidine Kinase 1 Concentration Is an Important Tool for Monitoring Surgical Response in Patients with Lung Cancer: A Meta-Analysis

AIMS

In this meta-analysis, we evaluated the usefulness of serum thymidine kinase 1 concentration (STK1c) for monitoring the outcome of extensive open surgery in patients with lung cancer. We also compared STK1c between a healthy population and patients with benign and malignant lung tumors to assess its potential value for early detection of lung cancer and for distinguishing between benign lung disease and malignant cancer.

MATERIALS AND METHODS

Related studies were retrieved from publications in PubMed, Cochrane, China National Knowledge Infrastructure, Wanfang databases, and Internet searches. Correlation was evaluated using weighted mean difference. Fixed or random effect models were selected for data analyses based on heterogeneity tested with the chi-square test. Publication bias was assessed using a funnel plot and Egger's test.

RESULTS

Twenty studies were selected for analysis, which showed that STK1c was significantly (p < 0.00001) reduced by 41.7% 1 month after extensive open surgery, approximately corresponding to an STK1c half-life of 1 month. STK1c levels were significantly higher in lung cancer patients than in healthy persons (p < 0.00001) or in patients with benign lung disease (p < 0.00001). There was also a significant difference in STK1c between patients with benign and malignant lung disease (p < 0.0001).

CONCLUSIONS

The half-life of STK1c may be an important tool in the clinical evaluation of surgical response in patients with lung cancer. STK1c may also be beneficial in the early detection of lung cancer.

Expression of cell cycle-regulated genes and prostate cancer prognosis in a population-based cohort

BACKGROUND

Prostate cancer (PCa) is clinically and biologically heterogeneous, making it difficult to predict at detection whether it will take an indolent or aggressive disease course. Cell cycle-regulated genes may be more highly expressed in actively dividing cells, with transcript levels reflecting tumor growth rate. Here, we evaluated expression of cell cycle genes in relation to PCa outcomes in a population-based cohort.

METHODS

Gene expression data were generated from tumor tissues obtained at radical prostatectomy for 383 population-based patients (12.3-years average follow-up). The overall mean and individual transcript levels of 30 selected cell cycle genes was compared between patients with no evidence of recurrence (73%) and those who recurred (27%) or died (7%) from PCa.

RESULTS

The multivariate adjusted hazard ratio (HR) for a change from the 25th to 75th percentile of mean gene expression level (range 8.02-10.05) was 1.25 (95%CI 0.96-1.63; P = 0.10) for PCa recurrence risk, and did not vary substantially by Gleason score, TMPRSS2-ERG fusion status, or family history of PCa. For lethal PCa, the HR for a change (25th to 75th percentile) in mean gene expression level was 2.04 (95%CI 1.26-3.31; P = 0.004), adjusted for clinicopathological variables. The ROC curve for mean gene expression level alone (AUC = 0.740) did not perform as well as clinicopathological variables alone (AUC = 0.803) for predicting lethal PCa, and the addition of mean gene expression to clinicopathological variables did not substantially improve prediction (AUC = 0.827; P = 0.18). Higher TK1 expression was strongly associated with both recurrent (P = 6.7 × 10(-5)) and lethal (P = 6.4 × 10(-6)) PCa.

CONCLUSIONS

Mean expression level for 30 selected cell cycle-regulated genes was unrelated to recurrence risk, but was associated with a twofold increase in risk of lethal PCa. However, gene expression had less discriminatory accuracy than clinical variables alone for predicting lethal events. Transcript levels for several genes in the panel were significantly overexpressed in lethal versus non-recurrent PCa.

Breast and prostate cancer patients differ significantly in their serum Thymidine kinase 1 (TK1) specific activities compared with those hematological malignancies and blood donors: implications of using serum TK1 as a biomarker

Background

Thymidine kinase 1 (TK1) is a cellular enzyme involved in DNA precursor synthesis, and its activity has been used as a proliferation marker for monitoring malignant diseases. Here, for the first time, we evaluated both TK1 activity and protein levels in sera from patients with different malignancies.

Methods

Serum samples from patients with myelodysplastic syndrome (MDS, n = 22), breast cancer (n = 42), prostate cancer (n = 47) and blood donors (n = 30) were analyzed for TK1 protein and activity levels, using a serum TK1 (STK1) protein assay based on antibodies and an activity assay that measured [³H]-deoxythymidine (dThd) phosphorylation. The molecular forms of TK1 in sera from some of these patients were analyzed using size-exclusion chromatography.

Results

Mean STK1 activities in sera from MDS, breast and prostate cancer were 11 ± 17.5, 6.7 ± 19 and 1.8 ± 1.4 pmol/min/mL, differing significantly from blood donors (mean ± standard deviation (SD) = 1.1 ± 0.9 pmol/min/mL). Serum TK1 protein (25 kDa polypeptide) levels were also significantly higher in MDS, breast, prostate cancer compared to blood donors (mean ± SD = 19 ± 9, 22 ± 11, 20 ± 12, and 5 ± 3.5 ng/mL, respectively). The STK1 specific activities of sera from patients with MDS and blood donors were significantly higher when compared with activities in sera from breast and prostate cancer patients. Size-exclusion analysis of sera from breast and prostate cancer showed that the detected active TK1 was primarily a high molecular weight complex, similar to the forms found in sera from MDS patients and blood donors. However, Western blotting demonstrated high TK1 25 kDa protein levels in fractions lacking TK1 activity in sera from cases with breast and prostate cancer.

Conclusions

These results demonstrate that there are differences in the specific activities and the subunit compositions of STK1 in hematological malignancies compared with breast and prostate cancer. This fact has several important implications for the use of STK1 as a tumor biomarker. One is that STK1 protein assays may differentiate early-stage tumor development in breast and prostate cancer more effectively than STK1 activity assays.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1073-8) contains supplementary material, which is available to authorized users.

Quaternary structures of recombinant, cellular, and serum forms of thymidine kinase 1 from dogs and humans

Background

Thymidine kinase 1 (TK1) is a salvage enzyme involved in DNA precursor synthesis, and its expression is proliferation dependent. A serum form of TK1 has been used as a biomarker in human medicine for many years and more recently to monitor canine lymphoma. Canine TK1 has not been cloned and studied. Therefore, dog and human TK1 cDNA were cloned and expressed, and the recombinant enzymes characterized. The serum and cellular forms of canine and human TK1 were studied by size-exclusion chromatography and the level of TK1 protein was determined using polyclonal and monoclonal anti-TK1 antibodies.

Results

Canine TK1 phosphorylated the thymidine (dThd) analog 3'-azido-thymidine (AZT) as efficiently as it did dThd, whereas AZT phosphorylation by human TK1 was less efficient than that of dThd. Dog TK1 was also more thermostable and pH tolerant than the human enzyme. Oligomeric forms were observed with both enzymes in addition to the tetrameric and dimeric forms. Cellular TK1 was predominantly seen in dimeric and tetrameric forms, in the case of both dog TK1 from MDCK cells and human TK1 from CEM cells. Active serum TK1 was found mainly in a high molecular weight form, and treatment with a reducing agent shifted the high molecular weight complex to lower molecular weight forms with reduced total activity. Western blot analysis demonstrated a polypeptide of 26 kDa (dog) and 25 kDa (human) for cellular and serum TK1. There was no direct correlation between serum TK1 activity and protein level. It appears that a substantial fraction of serum TK1 is not enzymatically active.

Conclusions

These results suggest that the serum TK1 protein differs from cellular or recombinant forms, is more active in high molecular weight complexes, and is sensitive to reducing agents. The results presented here provide important information for the future development and use of serum TK1 as a diagnostic biomarker in human and veterinary medicine.

A sensitive and kinetically defined radiochemical assay for canine and human serum thymidine kinase 1 (TK1) to monitor canine malignant lymphoma

Thymidine kinase 1 (TK1) is a cell cycle regulated enzyme with maximum expression during the S phase. Serum TK1 (S-TK1) is a unique biomarker for cell proliferation. Here, an optimized [(3)H]-thymidine (dThd) phosphorylation assay is described, which is as sensitive as the commercially available TK-REA and TK-Liaison assays for measurement of S-TK1 activity in dogs and humans. Serum samples from dogs (35 healthy, 32 with lymphoma, 2 with leukemia, and 35 with solid tumors) and humans (18 healthy, 9 with chronic lymphocytic leukemia, 10 with myelodysplastic syndrome) were analyzed using the [(3)H]-dThd assay. Mean S-TK1 activities were 1.11 ± 0.46 pmol/min/mL in healthy dogs and 1.15 ± 0.32 pmol/min/mL in healthy humans. S-TK1 activities in dogs with hematological malignancies were 24.2 ± 47.9 pmol/min/mL, and the receiver operating characteristic curve showed an area under the curve of 0.88. With a cut-off value of 1.9 pmol/min/mL (mean value ± 2 SD), the sensitivity was 0.94 and the specificity was 0.68. Very similar results were obtained with human samples (healthy and lymphoma cases). S-TK1 activities measured during chemotherapy of six dogs with lymphoma were drastically reduced. In one case, S-TK1 activity increased prior to relapse. S-TK1 levels in dogs with solid tumors did not differ from the healthy group. S-TK1 activities correlated with those determined with the TK-REA and TK-Liaison assays (r=0.92 and r=0.96, respectively). In conclusion, this optimized [(3)H]-dThd assay is fast, sensitive and economical for measuring S-TK1 activity and should increase its clinical use as biomarker.

Serological thymidine kinase 1 is a prognostic factor in oesophageal, cardial and lung carcinomas

In this study we examine the use of the concentration of thymidine kinase 1 in serum (STK1) as a prognostic factor in routine clinical settings. For this purpose we used sera from patients with oesophageal (n=101) and cardial (n=39) carcinomas and nonsmall-cell lung carcinoma (NSCLC) (n=157). Sera from healthy individuals (n=95) were used as controls. STK1 was analysed by a chemiluminiscence dot blot assay. The mean STK1 concentrations and the STK1 positive rates of the patients with oesophageal and cardial carcinomas and with NSCLC were significantly higher as compared with healthy controls (P=0.01). The mean STK1 value of oesophageal carcinoma patients correlated with T-values (P=0.021) and with stage (P<0.005), but not with grade. The mean STK1 value of cardial carcinoma patients did not correlate with grade. No data on stage and T-values were available for these patients, due to advanced disease. The mean STK1 value of NSCLC patients with squamous cell carcinoma was significantly higher as compared with adenocarcinoma type (P=0.024). The mean STK1 value of the NSCLC patients correlated with clinical grade (P=0.006), T-values (P=0.001), stage (P=0.035) and to size of the tumour (P=0.030). The mean STK1 value and the number of STK1 positive patients were also higher in recurrent NSCLC patients. There was a tendency that stage I-II NSCLC patients with an STK1 level above 2 pmol/l showed a higher frequency of recurrence/death than patients below 1 pmol/l. Our results show that STK1 is a useful marker for prognosis in patients with oesophageal, cardial and lung carcinomas.

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.

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.

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.

A comparative study: immunohistochemical detection of cytosolic thymidine kinase and proliferating cell nuclear antigen in breast cancer

To explore the expression of cytosolic thymidine kinase 1 (TK1) as a cell proliferative marker in human breast cancers, immunohistochemistry was used to detect the expression of TK1 in 52 malignant breast lesions, 20 benign breast lesions, and 16 normal breast tissues. The results were compared to the expression of proliferating cell nuclear antigen (PCNA) in the same specimens. The TK1-labelling index (TK1-LI) and PCNA-labeling index (PCNA-LI) were significantly higher in malignant lesions than in nonmalignant lesions (p < 0.0001 and p < 0.0013, respectively). The TK1-LI (78.9%) in malignant lesions was higher compared to PCNA-LI (64.5%). No significant difference was found for TK1-LI and PCNA-LI between benign lesions and normal tissues. Concerning the tumor stages and the tumor grades, TK1-LI showed a significant correlation with the increased tumor stages (p = 0.023) and tumor grades (p = 0.009). However, PCNA-LI was neither significantly different in tumor stages (p = 0.062) nor in tumor grades (p = 0.073). We conclude that TK1 might be a more accurate marker than PCNA for estimation of cell proliferation and malignant potentials in breast carcinomas.

X-irradiation effects on thymidine kinase (TK): I. TK1 and 2 in normal and malignant cells

The effect of radiation on TK is more complicated than would be expected from earlier results on bone marrow cells (Feinendegen et al. 1984, Int. J. Radiat. Biol. 45, 205). TK activity increased at 0.01 Gy and then decreased up to 1 Gy in mouse spleen. In contrast to the results for the spleen, an increase in activity at 0.1 Gy was seen in mouse thymus. The activity of dephosphorylated TK1 (TK1a) in both spleen and thymus was reduced to 50% after irradiation at 0.5-1 Gy. The degree of phosphorylation (TK1b/TK1a ratio) changed in spleen, but not in thymus. The activity of TK2 in mouse liver increased at 3 h after 5 Gy by about 60%. In mouse ascites tumour, a dose-independent (1-5 Gy) oscillating TK1 activity was found up to 24 h, especially for TK1a and TK1b. The amount of TK1 was unchanged up to 12 h, but decreased at 24 h. This suggests that the differences in the changes in the degree of phosphorylation of TK1 after irradiation among spleen, thymus and ascites tumour further underline the complexity of the response of TK1 activity to irradiation. The dramatic change in the activities of TK1a and TK1b may illustrate that both of them are more radiosensitive than TK-h, a variant with mixed TK1 and TK2 properties.

Existence of phosphorylated and dephosphorylated forms of cytosolic thymidine kinase (TK1)

In this study we examine whether different TK1 variants of pI 6.9 and 8.3 found by isoelectric focusing gel electrophoresis (IFE) reflect just a phenotype difference due to phosphorylation modifications or have a real phenotypic background. The phosphorylation degree of purified TK1 variants was analyzed by determining the changes in the pI values after treatment with alkaline phosphatase, using IFE. The genetic origin of the two TK1 variants was studied by determining their mol wt. by means of SDS-gelelectrophoresis. Furthermore, the subcellular distribution of the two TK1 variants was also studied. Alkaline phosphatase treatment changed the pI value of purified TK1 from 6.9 to 8.3. No change in the pI value was found when purified TK1 corresponding to pI 8.3 was treated in the same way. Similar results were obtained when treated a cytosolic fraction with alkaline phosphatase. Antibody raised against the C-terminal part of human TK1 only recognized the dephosphorylated TK1 variant corresponding to pI 8.3. There was no difference in the molecular weight between the two TK1 variants. Thus, we concluded that the TK1 variants corresponding to pI 6.9 and 8.3 are of the same genetic origin, but consist of phosphorylated and dephosphorylated forms.

Effect of hyperthermia on thymidine salvage as related to DNA synthesis

The effect of increasing temperature up to 43 degrees C on thymidine (TdR) salvage, i.e. uptake of TdR, thymidine kinase (TK) activity, intracellular amount of thymidinetriphosphate (dTTP) and specific activity of 3H-dTTP and incorporation of dTTP into DNA, was studied in ascites tumour cells in vitro. While the uptake of [3H]TdR was almost unchanged up to 40.5 degrees C and 2.4 h followed by a decrease at later times and higher temperatures, TK activity declined already after 30 min in a temperature-dependent way, leading to a decrease in cellular amount of dTTP reaching almost undetectable values at 43 degrees C and 6.0 h. The specific activity of [3H]dTTP in the cell was elevated at 39 degrees C and 43 degrees C, but somewhat reduced at 40.5 degrees C. Incorporation into DNA of [3H]TdR decreased in a temperature-dependent way with increasing incubation time. After correction for specific activity of [3H]dTTP, however, the incorporation of [3H]TdR into DNA at 39 degrees C and 40.5 degrees C was unchanged up to 2.4 h and for 40.5 degrees C up to 6.0 h, while incorporation into DNA at 43 degrees C was still reduced. The overall protein synthesis was also reduced in a temperature-dependent way. We concluded that phosphorylation of TdR to dTMP was more sensitive to moderately elevated temperatures than uptake of TdR and DNA synthesis per se, caused by a reduced TK activity. The decline in TK activity was probably due to a decrease in TK polypeptide synthesis.

Lack of correlation between thymidine kinase activity and changes of DNA synthesis with tumour age: an in vivo study in Ehrlich ascites tumour

Thymidine kinase (TK) and its isoenzymes were studied in relation to age of Ehrlich ascites tumour cells growing in vivo. Various steps of the pathway of thymidine through deoxynucleotide metabolism were studied: [3H]-thymidine cellular uptake and incorporation into DNA; the cellular nucleotide pools; and the concentration of thymidine in ascites. In addition, the proportion of cells in the various parts of the cell cycle and the bromodeoxyuridine labelling index were determined. Four isoenzymes at pI 4.1, 5.3, 6.9 and 8.3 were identified using isoelectric focusing. The TK activity declined with age of the tumour by about 90%, mostly due to a decrease of the isoenzyme at pI 8.3. However, this decline was neither related to the changes in DNA synthesis rate of the cells with tumour age, nor to the proportion of cells in S-phase or the bromodeoxyuridine (BrdU) labelling index. In contrast, the contribution of DNA synthesis via the thymidine salvage pathway relative to the total DNA synthesis increased from less than 1% at exponential growth to about 15% at plateau phase of growth. Blocking of DNA synthesis by aphidicolin did not change the TK activity. We therefore conclude that changes in TK activity and changes in cell growth are epiphenomena rather than causally related to each other. All nucleotide pools decreased with tumour age. The inhibition of TK by an increase in the deoxythymidine triphosphate pool could therefore be excluded. With a decrease of the TK activity during tumour growth, increasing amounts of TdR were excreted by the cells and accumulated in the ascites fluid. To explain our results on TK activity we propose a substrate cycle in which thymidine monophosphate supplied by de novo synthesis is dephosphorylated and is then either phosphorylated by TK to thymidine monophosphate or excreted by the cell.