Thymidine kinase, DNA synthesis and cancer

Thymidine kinase 1 silencing retards proliferative activity of pancreatic cancer cell via E2F1-TK1-P21 axis

OBJECTIVES

Thymidine kinase 1 (TK1) is one of the salvage enzymes engaged in the synthesis of DNA. Although a pro-carcinogenetic role of TK1 has been reported in various types of cancers, its role in pancreatic ductal adenocarcinoma (PDAC) is still unknown. The study is aimed to elaborate the function of TK1 in PDAC and the potential mechanisms in the following study.

MATERIALS AND METHODS

TK1 expression was analysed by immunohistochemistry, real-time PCR and Western blot, and its relationship with clinicopathological characteristics of PDAC patients was further investigated. To verify the function of TK1 and potential mechanism, TK1 siRNA was used to transfect PDAC cells and performed a series of assays in cell and animal models.

RESULTS

The level of TK1 expression was higher in cancerous tissues compared with matched adjacent tissues. TK1 overexpression was associated with progression of PDAC and poor prognosis. Knockdown of TK1 could suppress cell proliferation via inducing S phase arrest mediated by upregulation of P21. Further mechanism investigation suggested that transcription factor E2F-1 could directly regulate the TK1 and promote tumour proliferation.

CONCLUSIONS

The results suggested that TK1 might be involved in the development and progression of PDAC by regulating cell proliferation and show that TK1 may work as a promising therapeutic target in patients with PDAC.

Role of growth factors in oncogenesis: growth factor-proto-oncogene pathways of mitogenesis

Cellular genes which encode proteins involved in the response of cells to stimulation by growth factors may be potential oncogenes. The factors involved in the signal transmission from growth factor-receptor interaction to DNA synthesis constitute a cascade system which we call the 'growth factor-proto-oncogene pathway(s) of mitogenesis'. For each growth factor, all the responsive cells, regardless of cell types and tissue source, have specific growth factor receptors which are similar, if not identical, in molecular weight and biological activity. Thus, we believe that the growth factor-proto-oncogene pathway(s) functions in the same manner in all responsive cells. Platelet-derived growth factor (PDGF), epidermal growth factor (EGF), and brain-derived growth factor (BDGF) are major growth factors for connective tissue cells and do not share a common pathway in mitogenesis in responsive cells. The gene product of c-myc may be involved in the cellular response of cells stimulated by PDGF or FGF, but not directly in the signal transmission which leads to DNA synthesis.

Developmental arrest in Caenorhabditis elegans dauer larvae causes high expression of enzymes involved in thymidylate biosynthesis, similar to that found in Trichinella muscle larvae

Crude extract specific activities of thymidylate synthase, dUTPase, thymidine kinase and dihydrofolate reductase were high during the development of Caenorhabditis elegans, the dauer larva activities being similar to those previously determined in Trichinella spiralis and T. pseudospiralis muscle larvae (with the exception of thymidine kinase, not detected in Trichinella). High thymidylate synthase expression in developmentally arrested larvae, demonstrated also at the mRNA and protein levels, is in agreement with a global cell cycle arrest of dauer larvae and indicates this unusual cell cycle regulation pattern can be shared by developmentally arrested larvae of C. elegans and the two Trichnella species. Hence, the phenomenon may be characteristic for developmentally arrested larvae of different nematodes, rather than specific for the parasitic Trichinella muscle larvae. Endogenous C. elegans thymidylate synthase was purified and its molecular properties compared with those of the recombinant protein, expression of the latter in E. coli cells confirming the NCBI database sequence identity.

Immunopontentiating and antitumor activities of the purified polysaccharides from Phellodendron chinese SCHNEID

The polysaccharide fractions were isolated and purified from Phellodendron chinese SCHNEID, and antitumor activities were examined at dosages of 2, 5 and 10 mg/100 g. F-7 and F-8 showed the highest tumor inhibitory activities (inhibition ratio 96.4 and 98.2% in 2 mg/100 g), and in dose of 5 mg/100 g, the inhibitory ratios were 95.3 and 97.5%, respectively. Furthermore, 10 mg/100 g of intraperitoneal (i.p.) injection gave 97.3 and 98.7% of inhibition. In oral administration, the inhibitory activities were not markedly observed, indicating that the polysaccharides are directly acting to immune system. Also the polysaccharides increased the number of circulating blood leukocytes and total peritoneal exudate cells. Although implantation of tumor cells greatly decreased the productivity of antibody (antibody-mediated) and T lymphocyte reactivity (delayed-type) as 6.3 from 9.3 and 5.9 from 7.7, represented by the increase of footpad thickness, respectively. The polysaccharides elevated the reactivity of T lymphocyte in tumor-bearing mice, which were rapidly recovered by discontinuance of sample treatments. Especially, F-2, F-5, F-7 and F-8 remarkably recovered the decreased sensitivity. When the effects on thymidylate synthase (TS) and thymidine kinase (TK) activities were determined, TS activities in the F-2 and F-7-treated mice were markedly suppressed to 73.7% and 79.5% of that in the control (p < 0.01), while there was little difference in TK activity with a slight decrease in F-2 only. However, in i.p. injection, TS activities in the F-2, F-5, F-7 and F-8-treated mice were markedly suppressed to 83% to 85% of that in the control (p < 0.01). Furthermore, there were also significant differences in TK activities in F-2, F-5, F-7 and F-8-treated mice (p < 0.05). These results clearly indicated that the i.p. injection is much effective to suppress tumor growth than oral administration.

Suppression of DMBA-induced mouse skin tumor development by inositol hexaphosphate and its mode of action

Inositol hexaphosphate (IP6) or phytic acid, contained in most mammalian cells, has been shown to have anticancer and anti-cell-proliferative effects in several experimental models of carcinogenesis. We investigated the effect of topical application of IP6 on 7,12-dimethylbenzanthracene (DMBA)-induced complete carcinogenesis and on selective critical events of proliferation, differentiation, or apoptosis after DMBA exposure. IP6 inhibited skin tumor development significantly in a dose-dependent manner. IP6 induced the DMBA-inhibited transglutaminase activity. DNA synthesis, as determined by [3H]thymidine incorporation, was suppressed by IP6 in a dose-dependent manner. IP6 also inhibited thymidine kinase enzyme, which is responsible for [3H]thymidine incorporation into DNA. Our results show that topical application of IP6 inhibits DMBA-induced mouse skin tumor development and that IP6 exerts its tumor inhibitory effect probably by modulating proliferation, differentiation, or apoptosis. It seems that IP6 is an effective and potential chemopreventive agent for management of skin tumorigenesis.

Prognostic significance of thymidine kinase activity in bladder carcinoma

BACKGROUND

Thymidine kinase (TK) plays a key role in the complimentary or alternative salvage pathway of pyrimidine synthesis. Little is known about the significance of TK activity in bladder carcinoma. The authors examined the activity of TK in 55 patients with bladder carcinoma to determine the prognostic significance of TK activity.

METHODS

TK activity in nonfixed, fresh-frozen specimens of bladder carcinoma and normal bladder tissue was determined by using the diethylaminoethanol cellulose disc method.

RESULTS

The activity of TK was approximately two-fold higher in bladder carcinoma specimens compared with normal bladder tissues. The TK activity in muscle-invasive bladder carcinoma was two-fold higher compared with the activity in superficial bladder carcinoma (Ta and T1). In addition, the activity of TK in T1 tumors was two-fold higher compared with the TK activity in Ta tumors. The level of TK activity in Grade 3 bladder tumors was two-fold higher compared with the activity in Grade 1 and Grade 2 tumors. Patients with Ta and T1 bladder carcinoma who had low TK activity had a longer postoperative tumor free period compared with the patients who had high TK activity during the 2 years of follow-up. TK activity was correlated with the activity of thymidine synthase, which is a key enzyme for pyrimidine synthesis in the de novo pathway.

CONCLUSIONS

This study is the first to demonstrate that the level of TK activity is correlated with both disease stage and tumor grade in patients with bladder carcinoma and that elevated TK activity predicts early recurrence in patients with Ta and T1 disease. These results suggest that the level of TK activity may be used as a prognostic parameter and that TK may be a molecular therapeutic target in patients with bladder carcinoma.

Ornithine decarboxylase and thymidine kinase activities and polyamine levels from selected organs of adult miniature swine receiving three concentrations of dietary menhaden oil

Mature, female swine were randomly assigned to one of seven dietary groups. Swine in groups 1-3 were fed a cholesterol-rich diet for 55 days while the remaining groups remained on a basal swine diet. At the end of the cholesterol(Chol)-preloading period the swine in groups 1-7 were placed on menhaden oil (MO) and/or corn oil (CO) as follows: groups 1 and 4, 15% CO (control); groups 2 and 5, 0.75% MO+14.25% CO; groups 3 and 7, 15% MO; and group 6, 7.5% MO+7.5% CO. Animals were killed at the end of the approximately 6-month feeding period and portions of liver, pancreas and colon mucosa were analyzed for both ornithine decarboxylase (ODC) and thymidine kinase (TK) activity while polyamine levels were measured in the liver and pancreas. Statistical analyses were carried out by one-way and two-way ANOVA and by trend analysis. In the pancreas, the highest MO group (group 7) had significantly higher ODC levels when compared with the CO control (group 4) and the next to highest MO group (group 6) (one-way ANOVA)-all non-cholesterol preloaded groups. Using a two-way ANOVA (Chol-by-MO), liver ODC was significantly lower in the CO control when compared with the lowest and highest MO groups (groups 5 and 7, respectively), again in the non-cholesterol-preloaded animals. In the colon, the swine in the Chol-low MO group (group 2) had significantly lower TK activity than the Chol/CO control group (group 1) and Chol/Hi MO group (group 3) (one-way ANOVA) and also had significantly lower activity than all groups except the CO control (group 4) (two-way ANOVA). Liver acetylputrescine in the lowest and highest MO groups (groups 5 and 7, respectively) was significantly higher than in the CO group (group 4). Liver spermidine in the Chol-Hi MO group (group 3) was significantly higher than the Chol-Lo MO group (group 2), while the highest MO group (group 7) had a statistically higher level than the other non-cholesterol groups (groups 4-6) (one-way ANOVA). Liver spermine was significantly higher in the Chol-Hi MO group (group 3) when compared to the CO control (group 1) and the Chol-Lo MO group (group 2) (one-way ANOVA). Pancreatic putrescine in the CO control (group 4) was significantly higher than all other groups (two-way ANOVA) while spermine from the 2 Chol-MO groups (groups 2 and 3) was higher than the Chol-CO control (group 1) (one-way ANOVA). Using trend analysis, liver TK, putrescine and spermidine increased in the non-cholesterol preloaded groups with increasing dietary MO, similar to the increase seen in ODC. Thus, of the three organs studied, only liver responded to menhaden oil with changes in both ODC itself or some of its metabolic engendered products and thymidine kinase; at least for one of the parameters, ODC, change associated with dietary MO was dependent on whether the swine were preloaded with cholesterol.

Specific recognition of cytosolic thymidine kinase in the human lung tumor by monoclonal antibodies raised against recombinant human thymidine kinase

Anti-TK monoclonal antibodies (mAbs) were raised against recombinant human cytosolic thymidine kinase (rhTK) and characterized by Western immunoblotting, enzyme-linked immunosorbent assay (ELISA) and immunostaining of tumor cells. Twenty-three clones of TK mAbs were characterized to recognize specifically not only rhTK produced by Escherichia coli but also TK subunit of 25 kDa in human lung cancer. The anti-TK mAbs reacted specifically with cytosolic TK but not with mitochondrial TK. Only one clone of the mAbs inhibited the catalytic activity of TK. By solid phase sandwich enzyme immunoassay using these mAbs, we could quantitate the cytosolic TK content in tissues. Immunohistochemical staining analysis using one of the TK mAbs showed that human lung adenocarcinoma and squamous cell carcinoma exhibited much higher staining intensity than stromal cells. These mAbs are useful for biochemical studies on the regulation of human TK in proliferating cells such as tumor cells and for diagnosis of highly proliferating tumors.

Induction of thymidine kinase in mouse skin exposed to 12-O-tetradecanoylphorbol-13-acetate

Topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA) exhibited an increase in the epidermal thymidine kinase (TK) activity in a dose dependent manner. Maximum induction of TK was observed at a TPA concentration of 2.5 microg per animal. The induction of TK by TPA appeared to be a function of time with the maximum TK induction between 4 and 16 h after TPA application. Repeated applications of TPA every 24 h did not show any cumulative effect rather TK activity appeared to be normal after two applications. However, repeated applications of TPA at an interval of 48 h exhibited increased TK activity even after 16 applications. Cycloheximide and actinomycin D, the inhibitors of protein and RNA synthesis, inhibited the TPA induced activation of TK. Our results demonstrated that TPA induced the TK activity may be, by increasing de novo synthesis of enzyme protein and this induction might lead to increased de novo DNA synthesis after TPA application. DMBA was used as a reference compound. As far as the authors are aware, this is the first report on TK induction by topical application of TPA.

Prediction of radiosensitivity of oral cancers by serial cytological assay of nuclear changes

BACKGROUND AND PURPOSE

To identify the relationship between the radiosensitivity of oral cancers and the induction of micronucleation, nuclear budding and multinucleation (polynucleation) evaluated by serial cytology during fractionated radiotherapy.

MATERIALS AND METHODS

Forty-four patients with epidermoid cancer of the oral cavity receiving radiotherapy (60 Gy in 25 fractions over 5 weeks) were studied. Serial scrape smears were taken from the tumour before and during radiotherapy and stained by Giemsa and the frequency of micronucleated cells (MNC), nuclear budded cells (NBC) and multinucleated cells (PNC) was evaluated by light microscopy. After a minimum follow-up period of 30 months the patients were classified as having resistant or sensitive tumours, depending on whether the primary tumour had recurred or not within that time. Within-group and between-group analysis on the induction of the above individual parameters and two combined parameters, the micro- or multinucleated cell (MPC) count and the abnormally nucleated cell (ANC) count, was done. The counts were expressed per 1000 uni-nucleated cells.

RESULTS

In both groups each parameter showed a statistically significant increase with dose, the increase being higher in the sensitive group. The ANC count showed the greatest increase, the mean counts before treatment and after 28.8 Gy being 24.3 and 157.8 (P < 0.0005), respectively, in the sensitive group and 21.0 and 65.2 (P < 0.0005), respectively, in the resistant group. After 28.8 Gy the sensitive tumours had significantly higher ANC (P = 0.01), MPC (P < 0.05) and PNC (P < 0.05) counts.

CONCLUSION

The study shows that serial cytological assay of nuclear changes (SCANCing) during radiotherapy is a potentially useful test to predict radiosensitivity. The fact that multinucleation showed the greatest relation with radiosensitivity suggests that injury to the cytokinetic apparatus is important in determining tumour radiosensitivity.

Serine 13 is the site of mitotic phosphorylation of human thymidine kinase

It has been reported that the polypeptide of thymidine kinase type 1 (TK1) from human and mouse cells can be modified by phosphorylation. Our laboratory has further shown that the level of human TK phosphorylation increases during mitotic arrest in different cell types (Chang, Z.-F., Huang, D.-Y., and Hsue, N.-C. (1994) J. Biol. Chem. 269:21249-21254). In the present study, we demonstrated that a mutation converting Ser13 to Ala abolished the mitotic phosphorylation of native TK1 expressed in Ltk- cells. Furthermore, we expressed recombinant proteins of wild-type and mutated human TK1 with fused FLAG epitope in HeLa cells, and confirmed the occurrence of mitotic phosphorylation on Ser13 of hTK1. By using an in vitro phosphorylation assay, it was shown that wild-type hTK1, but not mutant TK1(Ala13), could serve as a good substrate for Cdc2 or Cdk2 kinase. Coexpression of p21(waf1/cip1), which is a universal inhibitor of Cdk kinases, in Ltk- fibroblasts also suppressed mitotic phosphorylation of hTK1 expressed in this cell line. Thus, Cdc2 or related kinase(s) is probably involved in mitotic phosphorylation on Ser13 of the hTK1 polypeptide. We also found that mutation on Ser13 did not affect the functional activity of hTK1. As the sequences around Ser13 are highly conserved in vertebrate TK1s, we speculate that phosphorylation of Ser13 may play a role in the regulation of TK1 expression in the cell cycle.

Evidence for dual effect of bile acids on thymidine anabolism and catabolism by the regenerating rat liver

Bile acids have been reported to modify DNA synthesis by rodent livers in regeneration, which may be due in part to their ability to interact with the machinery responsible for deoxyribonucleotide synthesis. The aim of this work was to gain information on the effect of taurocholate (TC) on both anabolic and catabolic pathways accounting for the fate of [methyl-14C]thymidine in the liver of two-third hepatectomized rats. Using high-pressure liquid chromatography, the soluble fraction of liver homogenate was used to measure the ability of TC to modify both the rate of thymidine monophosphate formation from thymidine - i.e., thymidine kinase (TK) activity - and the rate of thymidine release from thymidine, which is the result of at least three different reactions catalyzed by thymidine phosphorylase, nucleosidase and nucleoside deoxyribosyl transferase. TC was found to induce a dose-dependent inhibition of both processes. The nature of this inhibition seems to be in part competitive. Apparent Ki values were 1.5 mM for TK and 4 mM for thymidine release. These inhibitory effects were mimicked by glycocholate but not by taurine. To investigate the relevance of the TC-induced modification of anabolism and catabolism in the whole organ, experiments on regenerating perfused rat livers were carried out. The donors underwent two-third hepatectomy 24 h before liver isolation. They were either fasted during this period (F) or allowed free access to food (NF). DNA synthesis, as measured by [methyl-14C]thymidine incorporation into DNA, was significantly increased in both groups, as compared with control non-hepatectomized animals. However, enhancement in DNA synthesis in group F was only 50% of the value found in the NF group. Intravenous TC administration before and/or during liver perfusions induced a dose-dependent recovery of DNA synthesis in the F group. This effect was accompanied by opposed modifications in the amount of radiolabelled metabolites contained in the non-DNA fraction of liver homogenate, consistent with a marked inhibition of thymidine catabolism. These results suggest that, in addition to the previously reported effects of TC on thymidine anabolism, bile acids are also able to affect the thymidine catabolism. The overall results of this dual effect on the fate of thymidine in the regenerating rat liver depend on the metabolic situation. Under circumstances of no nutrient restriction, the effect of TC is characterized by inhibition of thymidine incorporation into DNA. By contrast, under depressed DNA synthesis due to fasting, the overall effect of TC is a partial recovery of this process.

Overexpression of human thymidine kinase mRNA without corresponding enzymatic activity in patients with chronic lymphatic leukemia

The level of cytosolic thymidine kinase (TK1) mRNA in lymphocytes from six healthy people and in lymphocytes from five patients with untreated chronic lymphatic leukemia (CLL) was determined with competitive polymerase chain reaction (competitive PCR). Using this procedure we have shown that in patients with CLL, there is an overexpression of TK1 mRNA without corresponding enzymatic activity. The TK1 mRNA level is approximately 100-fold higher in lymphocytes from CLL patients than in lymphocytes from healthy persons. A high level of TK1 mRNA without corresponding enzyme activity may indicate a defect in the processing of the enzyme. This may disturb the cells' normal feedback system and thereby influence the development of malignant conditions.

Analysis of thymidine kinase gene expression in preimplantation mouse embryos

Thymidine kinase (TK) activity was examined during the development of preimplantation mouse embryos. TK activity was increased approximately 20-fold from day 2 embryos (2-cell) to day 5 embryos (late blastocyst). TK activity did not change along with the progression into S-phase of the first and the second cell cycles but increased sharply at S-phase of the third cell cycle. Analysis of TK mRNA with a reverse transcription-polymerase chain reaction (RT-PCR) method showed that the level of TK mRNA was low in ovulated eggs and 1-cell embryos and was hardly detectable in day 2 embryos (2-cell), but sharply increased in day 3 embryos (mixture of 5- to 8-cell and morula). The functional role of 5'-flanking sequence of TK gene was also investigated in preimplantation embryos after microinjection with the DNA construct of 5'-flanking sequence of TK (2.4 kb) linked to bacterial lacZ gene (TK2.5lacZ) into the pronucleus of 1-cell and subsequently by histochemical staining with X-gal. beta-Galactosidase activity was first detected in day 3 embryos (8-cell), and 30% of embryos were stained with X-gal in day 4 and day 5 embryos, respectively. These results show that an increase in TK activity occurred after 2-cell stage, and this increase was primarily due to the embryonic activation of TK gene expression. Also, it appears that the 5'-flanking sequence of TK may directly regulate the TK gene expression at the transcriptional level during preimplantation murine development.

Difference in allelic expression of genes probably associated with tumor progression in murine fibrosarcomas and cell lines

Allelic expression was examined by single-strand conformation polymorphism analysis in murine fibrosarcomas from inter-subspecific F1 mice between C57BL/6 and MSM. Ten genes encoding p53, mdm2, E-cadherin, 72 kD metalloproteinase and its inhibitor (Timp2), thymidine kinase and four glucose transporters (Gluts) were examined. These genes were chosen because of their probable association with tumor development and progression. In some of the tumors and cell lines, p53, E-cadherin and Glut3 genes showed remarkable differences in allelic expression, one allele being poorly expressed. The allele-specificity persisted in nine cell lines obtained by repeated transplantations from one tumor. These results suggest that expression of some genes is allele-specific in tumor cells and the pattern of specificity is stable. Such a decrease or a loss of expression in one of the alleles may be functionally equivalent to the loss of heterozygosity of the gene, and therefore this may confer malignant properties on tumor cells. It is also suggested that differential expression of two alleles is a common event in tumor cells.

Why do most primary bladder neoplasms first appear around the ureteric orifices?

The majority of primary bladder neoplasms are known to arise within the mucosa around the ureteric orifices and bladder base. This may be due to the mucosa in this area being more susceptible to carcinogens than other areas of the bladder. Deficiency in the nucleotide salvage pathway enzyme thymidine kinase (TK), and especially its TK1 isozyme, has been shown to predispose cell lines to increased mutagenesis. Total TK and TK1 activities were measured in mucosal samples taken adjacent to the ureteric orifices and dome in 32 normal bladders and both total TK and TK1 were shown to be significantly decreased in the mucosa adjacent to the ureteric orifices. This may explain why primary bladder neoplasms occur more commonly in this site.

Mammalian thymidine kinase 2. Direct photoaffinity labeling with [32P]dTTP of the enzyme from spleen, liver, heart and brain

Thymidine kinase 2 (TK2), also called mitochondrial thymidine kinase, is a pyrimidine deoxyribonucleoside kinase expressed in all cells and tissues. It was recently purified to apparent homogeneity from human leukemic spleen and the active enzyme was shown to be a monomer of a 29-kDa polypeptide. The enzyme is feedback-inhibited by both end products, dCTP and dTTP. Here we show that TK2 purified from several different sources, including purified beef heart mitochondria, could be directly photoaffinity labeled with radioactive dTTP (approximately 18% of all TK2 molecules were cross-linked to dTTP after 20 min of ultraviolet irradiation) or to a lower extent with dCTP. Photo-incorporation was inhibited by the presence of the other effector but also the phosphate donor ATP blocked photolabeling, with dTTP. Addition of nucleoside substrates gave only a marginal inhibition of photo-incorporation. There were no detectable difference in the molecular size of photolabeled TK2 isolated from human spleen, brain or placenta, monkey liver, beef heart and beef heart mitochondria. Nor was there any significant differences in the enzyme kinetic properties of these enzymes. Cleavage of labeled TK2 with cyanogen bromide showed that dTTP was incorporated into a single 3-kDa peptide. TK2 was the only pyrimidine deoxynucleoside kinase expressed in liver, heart and brain. A detailed characterization of the subunit structure and substrate specificity of this enzyme is of importance for the design of new antiviral and cytostatic therapies based on nucleoside analogs.

Evolution of thymidine and thymidylate kinases: the possibility of independent capture of TK genes by different groups of viruses

Phylogenetic analysis of viral and cellular thymidine and thymidylate kinases was performed using computer-assisted methods. Multiple alignments and tentative phylogenetic trees were generated for the two families of these enzymes, which include a) thymidine kinases (TK) of mammals, poxviruses, African swine fever virus, E. coli, and bacteriophage T4; and b) thymidylate kinases (ThyK) of yeast and poxviruses and distantly related herpesvirus proteins with both enzymatic activities. Analysis of the alignment of the TKs of the first family highlighted three strongly conserved segments. Two of these corresponded to the A and B motifs of the purine NTP-binding pattern. The third, C-terminal segment, showing the highest conservation, encompassed a modified Zn finger motif. It is speculated that this motif might be involved in TK oligomerization. Phylogenetic trees constructed by three different methods suggested that cellular TK genes could be captured independently by T4 bacteriophage, African swine fever virus, fowlpox virus, and the other poxviruses. The observed tree topologies appear to contradict the popular virus-host coevolution schemes and to imply that different subdivisions of poxviruses diverged at earlier stages of evolution than their hosts did. It was shown that deoxynucleoside monophosphate kinase of bacteriophage T4 is related to the ThyK family. Phylogenetic analysis suggested that ThyK genes probably have been acquired independently by phage T4, poxviruses, and herpes-viruses.

Deoxynucleoside phosphorylating enzymes in monkey and human tissues show great similarities, while mouse deoxycytidine kinase has a different substrate specificity

Three key enzymes in the anabolic phosphorylation of deoxyribonucleosides and deoxyribonucleoside analogs were purified i.e. cytoplasmic thymidine kinase (TK1), mitochondrial thymidine kinase (TK2) and cytoplasmic deoxycytidine kinase (dCK) from human, mouse and monkey liver and spleen. Their subunit structure and substrate specificities were compared. Extensive purification of TK1 and dCK from mouse spleen and TK2 from mouse and monkey livers revealed major polypeptide bands of 25, 30 and 28 kD, respectively, on sodium dodecyl sulphate-polyacrylamide gel electrophoresis which are very similar to the subunit molecular weights of the corresponding human enzymes. Affinity purified polyclonal antibodies against human dCK also cross-reacted with 30 kD bands in extracts from both mouse and monkey spleen. Thus, the molecular weights of the subunits of these three enzymes appeared to be very similar in all three species. TK1 and TK2 from these different sources appeared to have similar substrate specificities against several deoxyribonucleoside analogs. However, mouse dCK differed significantly from monkey and human dCK in its capacity to phosphorylate dAdo and 2',3'-dideoxycytidine (ddCyd) with a Vmax approximately 10-fold lower than that of the two latter enzymes. The Km and Vmax values for dCyd and arabinocytosine appeared to be very similar with the enzymes from all three species. The fact that mouse dCK shows low activity with dAdo and ddCyd explains differences reported previously in the metabolism of dAdo and ddCyd in mouse compared to that in human lymphocytes. These results argue against the use of mice as model systems for human deoxynucleoside metabolism.

Purification and characterization of thymidine kinase from regenerating rat liver

Thymidine kinase (EC 2.7.1.21) from regenerating rat liver has been purified 70,000-fold to apparent homogeneity by affinity chromatography. Molecular weight of the native enzyme was found to be about 54,000, as determined by gel filtration. Electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate yielded a single band with a molecular weight of 26,000, suggesting that thymidine kinase is a dimer of very similar or identical subunits. The Michaelis constant for thymidine is 2.2 microM. ATP acts as a sigmoidal substrate with a 'Km' of 0.2 mM. Reaction kinetics and product inhibition studies reveal the enzymatic mechanism to be sequential.

Thymidine kinase from Tetrahymena thermophila. Purification and immunological analysis

Thymidine kinase is an enzyme involved in DNA precursor metabolism and DNA replication. The synthesis of this enzyme is highly regulated during the cell cycle and the activity of the enzyme is also regulated by feedback inhibition. Genes encoding thymidine kinase have been extremely useful as selectable markers for introducing DNA into a number of cells. In order to study cell cycle regulation of thymidine kinase, the gene which encodes this enzyme, as well as aspects of DNA replication in the ciliated protozoan Tetrahymena thermophila, we have purified thymidine kinase from Tetrahymena. Two forms of thymidine kinase with native molecular masses of 59 kDa and 80 kDa have been identified and purified 6800- and 4600-fold, respectively. The 59-kDa enzyme, a homodimer of 30-kDa subunits, has been purified to near homogeneity and polyclonal antibodies have been raised against the 30-kDa subunit. Serological studies indicate that the two enzymes are antigenically distinct. The antibody against the Tetrahymena protein cross-reacts with a polypeptide in Chinese hamster ovary (CHO) cell extracts of 26 kDa which corresponds to the reported size of Chinese hamster thymidine kinase protein.

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.

DNA polymerase-primase complex in wild-type and ts A1S9 mouse L-cells, temperature-sensitive for DNA replication during cell cycle progression

ts A1S9 mutant cells, derived from wild type WT-4 mouse L-cells, are temperature-sensitive (ts) for DNA synthesis and cell division. We try to determine the cause of the arrest of DNA replication in ts A1S9 cells at the nonpermissive temperature by comparing the modifications induced by the shift of temperature on the activity and the synthesis of DNA polymerase-alpha and DNA primase as a function of time. Forty-seven hours after temperature upshift DNA polymerase-alpha activity of ts A1S9 cells was inhibited by 90% while primase activity was barely detectable. By contrast, the activities of both enzymes increased to a plateau level in WT-4 cultured at either temperature and in ts A1S9 cells grown at the low permissive temperature. Study of the synthesis of DNA polymerase-alpha primase and of the structure of the enzyme complex during cell cycle progression was approached by immunoprecipitation of [35S]-labelled cells, with a specific monoclonal antibody directed against DNA polymerase-alpha. We have found that, irrespective of temperature of cultivation of WT-4 or ts A1S9 cells, this antibody precipitated polypeptides of 220, 186, 150, 110, 68-70, 60, and 48 kDa from cell extracts. With ts A1S9 cells cultivated at 38.5 degrees C for 48 hr the polypeptides of 220 and 186 kDa, associated with alpha-polymerase activity, were considerably more abundant than in the control cells, with a concomitant decline in the polypeptides of 60 and 48 kDa, implicated in primase activity. Thus the inhibition of DNA polymerase-alpha cannot be due to a decreased synthesis of the 186 kDa subunit but to its temperature inactivation. Consistent with a recent asymmetric dimeric model where polymerase-alpha complex and polymerase delta complex synthesize co-ordinately at the replication fork lagging and leading DNA strands, the observed alterations of polymerase-alpha and primase content explain the inhibition of DNA synthesis and the cell cycle arrest of the ts A1S9 cells at the nonpermissive temperature.

A rapid two-step purification of rat liver fetal thymidine kinase

The fetal isoenzyme of thymidine kinase was purified to apparent homogeneity from cytosols of rat fetuses liver. A two-step purification including anion exchange chromatography and affinity chromatography was developed. The purified enzyme appears as oligomeric with a relative molecular weight of 71 kDa. In denaturing media its molecular weight was 24 kDa, and its pHi 8.3.

Thymidine kinase in human leukemia--expression of three isoenzyme variants in six patients with chronic myelocytic leukemia

In six patients with untreated, chronic myelocytic leukemia (CML), the dominating thymidine kinase (TK) activity was compared with the fetal form, TK 1, from mitogen stimulated and the adult form TK 2 from unstimulated normal human lymphocytes, and with TK-1-onc, TK-3-onc and TK-4-onc. This was done in human acute, myelocytic and monocytic leukemias, using the combined thymidine/dTTP enzyme kinetics for isoenzyme characterization. TK-1-onc was found in one, TK-2-onc in two and TK-3-onc in three CML patients. The suffix -onc indicates the difference in ATP kinetics and molecular weights between the normal and the leukemic thymidine kinases. A possible relation between the isoenzyme forms and the types of leukemias is discussed.

Metabolism of pyrimidine analogues and their nucleosides

The pyrimidine antimetabolite drugs consist of base and nucleoside analogues of the naturally occurring pyrimidines uracil, thymine and cytosine. As is typical of antimetabolites, these drugs have a strong structural similarity to endogenous nucleic acid precursors. The structural differences are usually substitutions at one of the carbons in the pyrimidine ring itself or substitutions at on of the hydrogens attached to the ring of the pyrimidine or sugar (ribose or deoxyribose). Despite the differences noted above, these analogues, can still be taken up into cells and then metabolized via anabolic or catabolic pathways used by endogenous pyrimidines. Cytotoxicity results when the antimetabolite either is incorporated in place of the naturally occurring pyrimidine metabolite into a key molecule (such as RNA or DNA) or competes with the naturally occurring pyrimidine metabolite for a critical enzyme. There are four pyrimidine antimetabolites that are currently used extensively in clinical oncology. These include the fluoropyrimidines fluorouracil and fluorodeoxyuridine, and the cytosine analogues, cytosine arabinoside and azacytidine.

Transition from cytosolic to mitochondrial thymidine kinase during development in human fetal tissues

The transition from cytosolic ("fetal") to mitochondrial ("adult") thymidine kinase, as detected by electrophoresis, was examined in six human fetal tissues of gestational ages 11-40 weeks. In all tissues there was an early period during development in which only the fetal form was detected, followed by a transitional period in which both fetal and adults forms were present, followed by a later period in which only the adult enzyme occurred. Transitional periods were 23-25 wk. gestational age for colon, 13-15 wk. for kidney, 18-20 wk. for liver, 14-18 wk. for lung, 34-36 wk. for serum, and 25-28 wk. for thyroid. In all cases, only the adult form was present by the time of birth and persisted during the first 18 months of extrauterine life. The adult form, but not the fetal form, was inhibited by dCTP.

Transcriptional and posttranscriptional mechanisms regulate murine thymidine kinase gene expression in serum-stimulated cells

We previously isolated and characterized the structure of murine thymidine kinase (tk) genomic and cDNA sequences to begin a study designed to identify regions of the tk gene important for regulated expression during the transition of cells from G0 to a proliferating state. In this report, we describe the stable transfection of the cloned gene into L-M(TK-) cells and show that both thymidine kinase (TK) enzyme activity and DNA synthesis increase in parallel when transfectants in G0 arrest are stimulated by serum. To define promoter and regulatory regions more precisely, we have constructed a series of tk minigenes and have examined their expression in stable transfectants after serum stimulation. We have identified a 291-base-pair DNA fragment at the 5' end of the tk gene that has promoter function, and we have determined its sequence. In addition, we have found that DNA sequences which mediate serum-induced expression of TK are transcribed, since expression of the murine tk cDNA, fused to a promoter from either the murine tk gene, the simian virus 40 early region, or the herpes simplex virus tk gene, is stimulated by serum. Our constructs also reveal that the murine tk polyadenylation signal is not required for regulation, nor is most of the 3' untranslated region. RNA dot blot analysis indicates that murine cytoplasmic tk mRNA levels always parallel TK enzyme activity. Nuclear runon transcription assays show less than a 2-fold increase in transcription from the cloned tk gene in serum-stimulated transfectants, but an 11-fold increase in mouse L929 cells, which are inherently TK+. These results taken together suggest that the murine tk gene is controlled in serum-stimulated cells by a transcriptional mechanism influenced by DNA sequences that flank tk and also by a posttranscriptional system linked to gene sequences that are transcribed.

Detection of thymidine kinase activity using an assay based on the precipitation of nucleoside monophosphates with lanthanum chloride

A new assay method for the measurement of thymidine kinase (TK) is described. Cytosols were prepared from TK- and TK+ cells and evaluated for TK activity using an assay which is based on the phosphorylation of [125I]-iododeoxyuridine, [125I]-iododeoxycytidine, or [3H]thymidine and the precipitation of the monophosphates of these nucleosides by lanthanum chloride. The specificity, reproducibility, sensitivity, and convenience of this assay are demonstrated.

Mouse thymidine kinase: the promoter sequence and the gene and pseudogene structures in normal cells and in thymidine kinase deficient mutants

The mouse genome carries one gene and two pseudogenes for cytoplasmic thymidine kinase. The overall structure of these genes was determined with the help of cosmids and lambda phage clones and the upstream sequence containing the promoter was determined. The data allow an allocation of bands seen in the complex patterns of genomic Southern blots obtained from the DNA of wild type cells and of thymidine kinase deficient mutants to the gene as well as to the two pseudogenes. The much used LTK cell line was found to lack the entire gene but to retain the pseudogenes. Two other TK cell lines had DNA patterns indistinguishable from the wild type. Whereas the LTK line did not produce any TKmRNA, the two other mutants had normal amounts of TKmRNA but no cytoplasmic TK activity.

Transcriptional and posttranscriptional mechanisms regulate murine thymidine kinase gene expression in serum-stimulated cells

We previously isolated and characterized the structure of murine thymidine kinase (tk) genomic and cDNA sequences to begin a study designed to identify regions of the tk gene important for regulated expression during the transition of cells from G0 to a proliferating state. In this report, we describe the stable transfection of the cloned gene into L-M(TK-) cells and show that both thymidine kinase (TK) enzyme activity and DNA synthesis increase in parallel when transfectants in G0 arrest are stimulated by serum. To define promoter and regulatory regions more precisely, we have constructed a series of tk minigenes and have examined their expression in stable transfectants after serum stimulation. We have identified a 291-base-pair DNA fragment at the 5' end of the tk gene that has promoter function, and we have determined its sequence. In addition, we have found that DNA sequences which mediate serum-induced expression of TK are transcribed, since expression of the murine tk cDNA, fused to a promoter from either the murine tk gene, the simian virus 40 early region, or the herpes simplex virus tk gene, is stimulated by serum. Our constructs also reveal that the murine tk polyadenylation signal is not required for regulation, nor is most of the 3' untranslated region. RNA dot blot analysis indicates that murine cytoplasmic tk mRNA levels always parallel TK enzyme activity. Nuclear runon transcription assays show less than a 2-fold increase in transcription from the cloned tk gene in serum-stimulated transfectants, but an 11-fold increase in mouse L929 cells, which are inherently TK+. These results taken together suggest that the murine tk gene is controlled in serum-stimulated cells by a transcriptional mechanism influenced by DNA sequences that flank tk and also by a posttranscriptional system linked to gene sequences that are transcribed.

Estrogen-like effect of 5-androstene-3 beta,17 beta-diol on the induction of fetal thymidine kinase in the rat uterus

The action of 5-androstene-3 beta,17 beta-diol (Adiol) on Thymidine Kinase (TK) was studied in the rat uterus. It appeared that Adiol, like 17 beta-estradiol (E2), could induce the synthesis of the fetal isoenzyme of TK. Its effects compared with those of E2, were delayed by 6 h and higher doses were required to reach the same levels of induction. The action of Adiol on TK was exclusively observed in tissues where E2 receptors were present. Moreover, Adiol could stimulate the activity of uterine DNA-polymerase alpha. These observations are additional evidence of the estrogen-like behaviour of Adiol. They suggest that it could participate in proliferative processes of tissues which are dependent on estrogens.

Nuclear posttranscriptional processing of thymidine kinase mRNA at the onset of DNA synthesis

The posttranscriptional regulatory mechanism(s) underlying thymidine kinase (TK) mRNA accumulation was investigated in BALB/c 3T3 cells during their progression from G0 into S phase of the cell cycle. Very little TK mRNA could be detected in either the nuclear or the cytoplasmic compartment from cells harvested in G0 or G1. At the onset of S phase, however, the level of nuclear TK mRNA precursors and mature TK mRNAs increased dramatically. The high molecular weight TK heterogeneous nuclear RNA species detected in the nuclei of S-phase cells were polyadenylylated and hybridized to intron sequences derived from the TK gene. A series of high molecular weight precursors could be chased to lower molecular weight species in the presence of actinomycin D, suggesting an ordered removal of intron sequences with the kinetics of a precursor-product relationship. These results demonstrate a striking change in the nuclear posttranscriptional processing of TK heterogeneous nuclear RNA at the G1-S boundary and, furthermore, define a model system for the examination of RNA-processing events in vivo.

Thymidine kinase activities in mononuclear leukocytes and serum from breast cancer patients

Levels of the nucleotide pathway enzyme thymidine kinase (TK) were assayed in the mononuclear leukocytes and serum of 70 female patients with breast cancer and 98 male and 77 female non-cancer hospital patients. The total TK levels in both mononuclear leukocytes and serum from patients with breast cancer were significantly higher than in controls. The serum TK levels showed a significant correlation with cancer stage. No such correlation was observed with mononuclear leukocyte TK levels. Serum TK from 20 patients with breast cancer and 19 control patients was further assayed to ascertain the relative contributions of the thymidine kinase isozymes TK1 and TK2 to total TK levels. The increase in serum TK from breast cancer patients appears to be due to an increase in both TK1 and TK2 levels.

Stimulation of liver growth and DNA synthesis by glucosylceramide

The nature of the growth-stimulating effect of glucosylceramide was studied. Mice were injected intraperitoneally with emulsified glucosylceramide and conduritol B epoxide, an inhibitor of cerebroside glucosidase. Within one or two days, the liver grew 18-24%, as reported. Two enzymes involved in DNA synthesis also increased more than the weight. The total liver activity of thymidine kinase increased 46-73%, and the total activity of ornithine decarboxylase increased as much as 101%. It is suggested that elevated liver levels of glucocerebroside stimulate cell proliferation through a relatively direct mechanism.

Thymidine kinase in human leukemia. Expression of the lymphoblastic isoenzyme in three patients with acute myelocytic leukemia

The dominating thymidine kinase activity in mononuclear white blood cells from three patients with untreated acute myelocytic leukemia (AML) was compared with TK 1 from phytohemagglutinin-stimulated and TK 2 from unstimulated, normal lymphocytes. The enzyme activity in the AML cells and the stimulated lymphocytes was found to be in the same range. Regarding the combined thymidine and dTTP kinetics, the enzymes from the three AML patients resembled TK 1, but the ATP kinetics were different and the molecular weights were lower, as previously found for thymidine kinases from other leukemic cells. Therefore, the designation TK-1-onc is suggested for the thymidine kinases from the AML cells.

Cell cycle regulated synthesis of stable mouse thymidine kinase mRNA is mediated by a sequence within the cDNA

The cDNA for mouse thymidine kinase (TK) was isolated from a cDNA library in lambda-gt11 and sequenced. It was used as a probe to follow the time course of TK mRNA expression in growth stimulated mouse fibroblasts. Linked to the HSV-TK promoter the cDNA was able to transform LTK-cells to the TK+ phenotype. The transformed cells expressed the TK mRNA and enzyme activity in a growth dependent fashion suggesting that the regulatory element is localized on the cDNA.

The mutagenicity of 5-azacytidine and other inhibitors of replicative DNA synthesis in the L5178Y mouse lymphoma cell

The mutagenic potential of the cytidine analog, 5-azacytidine (Aza Cyd), was tested at the thymidine kinase (TK) gene locus of L5178Y mouse lymphoma cells. 3-h exposure to as little as 20 ng/ml Aza Cyd yielded a substantial increase in TK-deficient L5178Y cells as measured by drug-induced resistance to trifluorothymidine (TFTres) 48 h later. This mutagenic effect was diminished up to 75% when Aza Cyd was tested in the presence of either enzymatically active or heat-denatured 9000 X g supernatant prepared from rat liver homogenate. The mutagenicity of Aza Cyd was also decreased in the presence of 1-5 X 10(-3) M thymidine and eliminated in the presence of greater than 1 X 10(-5) M cytidine. Two L5178Y TK-deficient cell lines had no selective survival advantage compared to TK-competent L5178Y cell stock when plated in soft-agar medium that contained Aza Cyd. Four other specific inhibitors of scheduled DNA synthesis in mammalian cells, deoxyadenosine, aphidicolin, 1-beta-D-arabinofuranosylcytosine, and hydroxyurea were also L5178Y/TK mutagens. These data along with other published results suggest that chemicals known to disrupt nucleotide biosynthesis, alter deoxyribonucleotide pools, or directly inhibit DNA polymerase can cause stable, heritable increases in TFT resistance through mechanisms dependent upon altered replicative DNA synthesis, yet not necessarily dependent upon DNA incorporation or the binding of these mutagenic agents to nuclear DNA.

Inhibition by Celiptium of the fetal thymidine kinase synthesis induced by estrogens in the rat uterus

Celiptium (Ce) is an antitumor drug used in the therapy of breast carcinomas, which are to a large extent dependent on estrogens. We have studied the effect of Ce on some proteins induced by estradiol (E2) in the rat uterus. It was observed that Ce administered at the same time or before E2, was able to inhibit the induction by E2 of fetal thymidine kinase (TK-F), of creatine kinase of brain-type (CK-BB) and of progesterone receptor (PR). When Ce was given after E2, its inhibitory effects were less evident. Results seemed to indicate that Ce could bind the acceptor sites for E2 receptor and thus inhibit the activity of E2-regulated genes. This assumption was corroborated by the fact that Ce did not modify the activity of enzymes not submitted to E2 regulation.

Changes in thymidine kinase activity during differentiation of HL-60 leukemic cells

Induction of granulocyte maturation in HL-60 leukemic cells by DMSO (1.2%) or RA (1 microM) is accompanied by a 50-60% decrease in cellular thymidine kinase activity. Similarly, the differentiation of HL-60 cells into monocyte-macrophage phenotype by the addition of PMA is paralleled by a 60-80% suppression of thymidine kinase specific activity. Measurement of thymidine kinase kinetic parameters shows that the Vmax decreases from 0.7 pmol/min in control cells to 0.43 pmol/min in PMA-treated cells and to 0.38 pmol/min in RA-treated cells. The Km of the enzyme is not affected by either inducing agent and remains at 2.1 microM. Studies with PMA analogs suggest that thymidine kinase modulation is coupled to HL-60 differentiation.