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

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.

Maize Thymidine Kinase Activity Is Present throughout Plant Development and Its Heterologous Expression Confers Tolerance to an Organellar DNA-Damaging Agent

Thymidine kinase 1 (TK1) phosphorylates thymidine nucleosides to generate thymidine monophosphate. This reaction belongs to the pyrimidine salvage route that is phylogenetically conserved. In the model plant Arabidopsis thaliana, TK activity contributes to maintain nuclear and organellar genome integrity by providing deoxythymidine-triphosphate (dTTP) for DNA synthesis. Arabidopsis has two TK1 genes (TK1a and TK1b) and double mutants show an albino phenotype and develop poorly. In contrast, maize (Zea mays L.) has a single TK1 (ZmTK1) gene and mutant plants are albino and display reduced genome copy number in chloroplasts. We studied the role of ZmTK1 during development and genotoxic stress response by assessing its activity at different developmental stages and by complementing Arabidopsis tk1 mutants. We found that ZmTK1 transcripts and activity are present during germination and throughout maize development. We show that ZmTK1 translocation to chloroplasts depends on a 72-amino-acid N-signal and its plastid localization is consistent with its ability to complement Arabidopsis tk1b mutants which are hypersensitive to ciprofloxacin (CIP), a genotoxic agent to organellar DNA. Also, ZmTK1 partly complemented the Arabidopsis double mutant plants during development. Our results contribute to the understanding of TK1 function in monocot species as an organellar enzyme for genome replication and repair.

Role of pyrimidine salvage pathway in the maintenance of organellar and nuclear genome integrity

Nucleotide biosynthesis proceeds through a de novo pathway and a salvage route. In the salvage route, free bases and/or nucleosides are recycled to generate the corresponding nucleotides. Thymidine kinase (TK) is the first enzyme in the salvage pathway to recycle thymidine nucleosides as it phosphorylates thymidine to yield thymidine monophosphate. The Arabidopsis genome contains two TK genes -TK1a and TK1b- that show similar expression patterns during development. In this work, we studied the respective roles of the two genes during early development and in response to genotoxic agents targeting the organellar or the nuclear genome. We found that the pyrimidine salvage pathway is crucial for chloroplast development and genome replication, as well as for the maintenance of its integrity, and is thus likely to play a crucial role during the transition from heterotrophy to autotrophy after germination. Interestingly, defects in TK activity could be partially compensated by supplementation of the medium with sugar, and this effect resulted from both the availability of a carbon source and the activation of the nucleotide de novo synthesis pathway, providing evidence for a compensation mechanism between two routes of nucleotide biosynthesis that depend on nutrient availability. Finally, we found differential roles of the TK1a and TK1b genes during the plant response to genotoxic stress, suggesting that different pools of nucleotides exist within the cells and are required to respond to different types of DNA damage. Altogether, our results highlight the importance of the pyrimidine salvage pathway, both during plant development and in response to genotoxic stress.

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

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

A method for quantification of nucleotides and nucleotide analogues in thymidine kinase assays using lanthanum phosphate coprecipitation

Current methodologies for quantifying radiolabeled nucleoside monophosphates and nucleoside analogues result in high retention of unphosphorylated guanosine nucleosides in the case of lanthanum chloride precipitation or inconsistent retention of nucleotides in the case of DEAE cellulose filter papers. This study describes an innovative method for quantifying thymidine kinase (TK) activity that is compatible with both purine and pyrimidine nucleoside analogues by using lanthanum phosphate coprecipitation at pH 4.0. This methodology maintains quantitative precipitation of nucleoside monophosphates and yields minimal background binding from a variety of nucleoside analogues. In addition, use of PCR thermocyclers enhances the temporal precision of TK assays. This method was shown to be useful for assaying TK activity in a broad range of biochemically relevant systems, including purified enzymes, stable cell lines, and virally infected cells. Use of this methodology should aid researchers in the evaluation of novel nucleoside analogues and TK enzymes while decreasing radioactive waste, minimizing assay time, increasing accuracy, and enhancing dynamic range.

A continuous spectrophotometric assay for thymidine and deoxycytidine kinases

We have developed a continuous spectrophotometric assay for thymidine and deoxycytidine kinase activities by coupling nucleoside 5'-monophosphate formation to a methylation reaction which generates a product absorbing at 340 nm. With thymidine kinase, we used the alternate substrate deoxyuridine and coupled the reaction to thymidylate synthase. For deoxycytidine kinase, we coupled the reaction to a thymidylate synthase mutant which converts the product 2'-deoxycytidine-5'-monophosphate (dCMP) to m5dCMP. In both cases, the methylation reactions are accompanied by conversion of 5,10-methylene-5,6,7,8-tedrahydrofolate to 7,8-dihydrofolate and can be continuously monitored by the increase of absorbance at 340 nm. The assay should be particularly useful for kinetic studies, and for the purification of these enzymes from various sources.

Fialuridine is phosphorylated and inhibits DNA synthesis in isolated rat hepatic mitochondria

Fialuridine (FIAU) is a thymidine analog effective against hepatitis B virus. Toxicity associated with FIAU treatment included clinical signs consistent with mitochondrial dysfunction, including severe lactic acidosis. To understand further the mechanism of FIAU toxicity, we examined the effect of FIAU on DNA synthesis in mitochondria. Mitochondria isolated from livers of naive rats were treated in vitro with concentrations of FIAU or FIAU triphosphate (FIAU-TP) ranging from 0.1 to 200 microM. A 14 or 32% decrease in mitochondrial DNA synthesis compared to controls was observed when isolated mitochondria were treated with 25 microM FIAU or FIAU-TP, respectively. Since it is thought that nucleosides must be phosphorylated to inhibit DNA polymerase, studies were conducted to determine whether isolated rat mitochondria could phosphorylate FIAU. Results using lanthanum chloride precipitation and HPLC analysis showed that enzymes present in a mitochondrial lysate were capable of phosphorylating FIAU to form FIAU monophosphate.

Equine herpesvirus 1 glycoprotein D: mapping of the transcript and a neutralization epitope

Studies with molecular and immunological techniques identified and mapped the transcript encoding glycoprotein D (gD) of equine herpesvirus 1 KyA, as well as two continuous gD antigenic determinants. Three mRNA species of 5.5, 3.8, and 1.7 kb overlap the gD open reading frame and are transcribed from the DNA strand encoding gD. Northern (RNA) blot hybridization with both DNA clones and riboprobes, as well as S1 nuclease analyses, showed the 3.8-kb mRNA to encode gD and to be synthesized as a late (beta-gamma) transcript. The 3.8-kb gD mRNA initiates within the US segment 91 and 34 nucleotides downstream of the CCAAT and TATA elements, respectively, and encodes a potential polypeptide of 392 amino acids. The termination site of this transcript maps within the terminal repeat at a site also used by the 5.5-kb mRNA and the IR6-encoded 1.2-kb mRNA, such that these three transcripts form a 3'-coterminal nested set. The extended size (2,250 nucleotides) of the 3' untranslated region of the gD transcript and its termination within the terminal repeat may result from the deletion of 3,859 bp, which eliminates two consensus polyadenylation signals downstream of the gD open reading frame of EHV-1 KyA. Use of antisera to synthetic peptides of 19 amino acids (residues 4 to 22) and 20 amino acids (residues 267 to 285) in Western immunoblot analyses revealed that gD is present in EHV-1 virions as a 55-kDa polypeptide. In addition, these antisera detected the 55-kDa protein as well as 58- and 47-kDa polypeptides in infected-cell extracts at late times of infection. Residues 4 to 22 make up a continuous neutralizing epitope of gD, since incubation of equine herpesvirus 1 with the anti-19-mer serum prior to infection results in reduced numbers of plaques and reduced levels of virus-encoded thymidine kinase. Complement is not required for neutralization mediated by the anti-19-mer serum.

Cytopathogenicity, drug susceptibility, and thymidine kinase activity of a retinovirulent herpes simplex virus type 2

We investigated some of the biological and biochemical characteristics of a neuroinvasive, retinovirulent herpes simplex virus type 2 strain SL (HSV-2[SL]) and compared them with those of a neurovirulent, nonretinovirulent HSV-2 (186). HSV-2(SL) was shown to spread rapidly and produce large syncytium in vitro. HSV-2(SL) and HSV-2(186) were equally susceptible to acyclovir (ACV) and thymine arabinoside (Ara-T). However, HSV-2(SL) was fourfold and 44-fold more susceptible than HSV-2(186) to iododeoxyuridine (IUdR) and bromovinyldeoxyuridine (BVDU), respectively. In addition, cytosolic TK from HSV-2(SL)-infected cells phosphorylated 4, 20, and 23,000 times more IUdR, iododeoxycytidine (IdCyD), and Ara-T than the TK of HSV-2(186), respectively. Further, HSV-2(186) TK did not phosphorylate Ara-T, but HSV-2(186) replication was inhibited by Ara-T. These studies indicate that the retinovirulent HSV-2(SL) has a syn phenotype and a TK with broad substrate activity.