Differential phosphorylation of human thymidine kinase in proliferating and M phase-arrested human cells

The association between the amino acid transporter LAT1, tumor immunometabolic and proliferative features and menopausal status in breast cancer

L-type Amino Acid Transporter 1 (LAT1) facilitates the uptake of specific essential amino acids, and due to this quality, it has been correlated to worse patient outcomes in various cancer types. However, the relationship between LAT1 and various clinical factors, including menopausal status, in mediating LAT1's prognostic effects remains incompletely understood. This is particularly true in the unique subset of tumors that are both obesity-associated and responsive to immunotherapy, including breast cancer. To close this gap, we employed 6 sets of transcriptomic data using the Kaplan-Meier model in the Xena Functional Genomics Explorer, demonstrating that higher LAT1 expression diminishes breast cancer patients' survival probability. Additionally, we analyzed 3'-Deoxy-3'-18F-Fluorothymidine positron emission tomography-computed tomography (18F-FLT PET-CT) images found on The Cancer Imaging Archive (TCIA). After separating all patients based on menopausal status, we correlated the measured 18F-FLT uptake with various clinical parameters quantifying body composition, tumor proliferation, and immune cell infiltration. By analyzing a wealth of deidentified, open-access data, the current study investigates the impact of LAT1 expression on breast cancer prognosis, along with the menopausal status-dependent associations between tumor proliferation, immunometabolism, and systemic metabolism.

Quantitation of deoxynucleoside triphosphates by click reactions

The levels of the four deoxynucleoside triphosphates (dNTPs) are under strict control in the cell, as improper or imbalanced dNTP pools may lead to growth defects and oncogenesis. Upon treatment of cancer cells with therapeutic agents, changes in the canonical dNTPs levels may provide critical information for evaluating drug response and mode of action. The radioisotope-labeling enzymatic assay has been commonly used for quantitation of cellular dNTP levels. However, the disadvantage of this method is the handling of biohazard materials. Here, we described the use of click chemistry to replace radioisotope-labeling in template-dependent DNA polymerization for quantitation of the four canonical dNTPs. Specific oligomers were designed for dCTP, dTTP, dATP and dGTP measurement, and the incorporation of 5-ethynyl-dUTP or C8-alkyne-dCTP during the polymerization reaction allowed for fluorophore conjugation on immobilized oligonucleotides. The four reactions gave a linear correlation coefficient >0.99 in the range of the concentration of dNTPs present in 10⁶ cells, with little interference of cellular rNTPs. We present evidence indicating that data generated by this methodology is comparable to radioisotope-labeling data. Furthermore, the design and utilization of a robust microplate assay based on this technology evidenced the modulation of dNTPs in response to different chemotherapeutic agents in cancer cells.

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.

The Impact of dUTPase on Ribonucleotide Reductase-Induced Genome Instability in Cancer Cells

The appropriate supply of dNTPs is critical for cell growth and genome integrity. Here, we investigated the interrelationship between dUTP pyrophosphatase (dUTPase) and ribonucleotide reductase (RNR) in the regulation of genome stability. Our results demonstrate that reducing the expression of dUTPase increases genome stress in cancer. Analysis of clinical samples reveals a significant correlation between the combination of low dUTPase and high R2, a subunit of RNR, and a poor prognosis in colorectal and breast cancer patients. Furthermore, overexpression of R2 in non-tumorigenic cells progressively increases genome stress, promoting transformation. These cells display alterations in replication fork progression, elevated genomic uracil, and breaks at AT-rich common fragile sites. Consistently, overexpression of dUTPase abolishes R2-induced genome instability. Thus, the expression level of dUTPase determines the role of high R2 in driving genome instability in cancer cells.

PET Radiotracers for Imaging the Proliferative Status of Solid Tumors

Two different strategies have been developed for imaging the proliferative status of solid tumors with the functional imaging technique, Positron Emission Tomography (PET). The first strategy uses carbon-11 labeled thymidine and/or, more recently, fluorine-18 labeled thymidine analogs. These agents are a substrate for the enzyme thymidine kinase-1 (TK-1) and provide a pulse label of the number of cells in S phase. The second method for imaging the proliferative status of a tumor uses radiolabeled ligands that bind to the sigma-2 receptor which has a 10-fold higher density in proliferating (P) tumor cells versus quiescent (Q) tumor cells. This article compares and contrasts the two different strategies for imaging the proliferative status of solid tumors, and describes the strengths and weaknesses of each approach.

A Phase II Study of 3'-Deoxy-3'-18F-Fluorothymidine PET in the Assessment of Early Response of Breast Cancer to Neoadjuvant Chemotherapy: Results from ACRIN 6688

Our objective was to determine whether early change in standardized uptake values (SUVs) of 3'deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) using PET with CT could predict pathologic complete response (pCR) of primary breast cancer to neoadjuvant chemotherapy (NAC). The key secondary objective was to correlate SUV with the proliferation marker Ki-67 at baseline and after NAC.

METHODS

This prospective, multicenter phase II study did not specify the therapeutic regimen, thus, NAC varied among centers. All evaluable patients underwent (18)F-FLT PET/CT at baseline (FLT1) and after 1 cycle of NAC (FLT2); 43 patients were imaged at FLT1, FLT2, and after NAC completion (FLT3). The percentage change in maximum SUV (%ΔSUVmax) between FLT1 and FLT2 and FLT3 was calculated for the primary tumors. The predictive value of ΔSUVmax for pCR was determined using receiver-operating-characteristic curve analysis. The correlation between SUVmax and Ki-67 was also assessed.

RESULTS

Fifty-one of 90 recruited patients (median age, 54 y; stage IIA-IIIC) met the eligibility criteria for the primary objective analysis, with an additional 22 patients totaling 73 patients for secondary analyses. A pCR in the primary breast cancer was achieved in 9 of 51 patients. NAC resulted in a significant reduction in %SUVmax (mean Δ, 39%; 95% confidence interval, 31-46). There was a marginal difference in %ΔSUVmax_FLT1-FLT2 between pCR and no-pCR patient groups (Wilcoxon 1-sided P = 0.050). The area under the curve for ΔSUVmax in the prediction of pCR was 0.68 (90% confidence interval, 0.50-0.83; Delong 1-sided P = 0.05), with slightly better predictive value for percentage mean SUV (P = 0.02) and similar prediction for peak SUV (P = 0.04). There was a weak correlation with pretherapy SUVmax and Ki-67 (r = 0.29, P = 0.04), but the correlation between SUVmax and Ki-67 after completion of NAC was stronger (r = 0.68, P < 0.0001).

CONCLUSION

(18)F-FLT PET imaging of breast cancer after 1 cycle of NAC weakly predicted pCR in the setting of variable NAC regimens. Posttherapy (18)F-FLT uptake correlated with Ki-67 on surgical specimens. These results suggest some efficacy of (18)F-FLT as an indicator of early therapeutic response of breast cancer to NAC and support future multicenter studies to test (18)F-FLT PET in a more uniformly treated patient population.

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

Background

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

Methods

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

Results

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

Conclusion

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

The contribution of mitochondrial thymidylate synthesis in preventing the nuclear genome stress

In quiescent fibroblasts, the expression levels of cytosolic enzymes for thymidine triphosphate (dTTP) synthesis are down-regulated, causing a marked reduction in the dTTP pool. In this study, we provide evidence that mitochondrial thymidylate synthesis via thymidine kinase 2 (TK2) is a limiting factor for the repair of ultraviolet (UV) damage in the nuclear compartment in quiescent fibroblasts. We found that TK2 deficiency causes secondary DNA double-strand breaks formation in the nuclear genome of quiescent cells at the late stage of recovery from UV damage. Despite slower repair of quiescent fibroblast deficient in TK2, DNA damage signals eventually disappeared, and these cells were capable of re-entering the S phase after serum stimulation. However, these cells displayed severe genome stress as revealed by the dramatic increase in 53BP1 nuclear body in the G1 phase of the successive cell cycle. Here, we conclude that mitochondrial thymidylate synthesis via TK2 plays a role in facilitating the quality repair of UV damage for the maintenance of genome integrity in the cells that are temporarily arrested in the quiescent state.

[18F]FLT PET for non-invasive assessment of tumor sensitivity to chemotherapy: studies with experimental chemotherapy TP202377 in human cancer xenografts in mice

Aim

3′-deoxy-3′-[¹⁸F]fluorothymidine ([18F]FLT) is a tracer used to assess cell proliferation in vivo. The aim of the study was to use [18F]FLT positron emission tomography (PET) to study non-invasively early anti-proliferative effects of the experimental chemotherapeutic agent TP202377 in both sensitive and resistant tumors.

Methods

Xenografts in mice from 3 human cancer cell lines were used: the TP202377 sensitive A2780 ovary cancer cell line (n = 8–16 tumors/group), the induced resistant A2780/Top216 cell line (n = 8–12 tumors/group) and the natural resistant SW620 colon cancer cell line (n = 10 tumors/group). In vivo uptake of [18F]FLT was studied at baseline and repeated 6 hours, Day 1, and Day 6 after TP202377 treatment (40 mg/kg i.v.) was initiated. Tracer uptake was quantified using small animal PET/CT.

Results

TP202377 (40 mg/kg at 0 hours) caused growth inhibition at Day 6 in the sensitive A2780 tumor model compared to the control group (P<0.001). In the A2780 tumor model TP202377 treatment caused significant decrease in uptake of [18F]FLT at 6 hours (-46%; P<0.001) and Day 1 (-44%; P<0.001) after treatment start compared to baseline uptake. At Day 6 uptake was comparable to baseline. Treatment with TP202377 did not influence tumor growth or [18F]FLT uptake in the resistant A2780/Top216 and SW620 tumor models. In all control groups uptake of [18F]FLT did not change. Ki67 gene expression paralleled [18F]FLT uptake.

Conclusion

Treatment of A2780 xenografts in mice with TP202377 (single dose i.v.) caused a significant decrease in cell proliferation assessed by [18F]FLT PET after 6 hours. Inhibition persisted at Day 1; however, cell proliferation had returned to baseline at Day 6. In the resistant A2780/Top216 and SW620 tumor models uptake of [18F]FLT did not change after treatment. With [18F]FLT PET it was possible to distinguish non-invasively between sensitive and resistant tumors already 6 hours after treatment initiation.

Staurosporine increases toxicity of gemcitabine in non-small cell lung cancer cells: role of protein kinase C, deoxycytidine kinase and ribonucleotide reductase

Gemcitabine, a deoxycytidine analog, active against non-small cell lung cancer, is phosphorylated by deoxycytidine kinase (dCK) to active nucleotides. Earlier, we found increased sensitivity to gemcitabine in P-glycoprotein (SW-2R160) and multidrug resistance-associated protein (SW-2R120), overexpressing variants of the human SW1573 non-small cell lung cancer cells. This was related to increased dCK activity. As protein kinase C (PKC) is higher in 2R120 and 2R160 cells and may control the dCK activity, we investigated whether gemcitabine sensitivity was affected by the protein kinase C inhibitor, staurosporine, which also modulates the cell cycle. Ten nmol/l staurosporine enhanced the sensitivity of SW1573, 2R120 and 2R160 cells 10-fold, 50-fold and 270-fold, respectively. Staurosporine increased dCK activity about two-fold and the activity of thymidine kinase 2, which may also activate gemcitabine. Staurosporine also directly increased dCK in cell free extracts. Staurosporine decreased expression of the free transcription factor E2F and of ribonucleotide reductase (RNR), a target for gemcitabine inhibition. In conclusion, staurosporine may potentiate gemcitabine by increasing dCK and decreasing E2F and RNR, which will lead to a more pronounced RNR inhibition.

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

Cellular supply of dNTPs is essential in the DNA replication and repair processes. Here we investigated the regulation of thymidine kinase 1 (TK1) in response to DNA damage and found that genotoxic insults in tumor cells cause up-regulation and nuclear localization of TK1. During recovery from DNA damage, TK1 accumulates in p53-null cells due to a lack of mitotic proteolysis as these cells are arrested in the G(2) phase by checkpoint activation. We show that in p53-proficient cells, p21 expression in response to DNA damage prohibits G(1)/S progression, resulting in a smaller G(2) fraction and less TK1 accumulation. Thus, the p53 status of tumor cells affects the level of TK1 after DNA damage through differential cell cycle control. Furthermore, it was shown that in HCT-116 p53(-/-) cells, TK1 is dispensable for cell proliferation but crucial for dTTP supply during recovery from DNA damage, leading to better survival. Depletion of TK1 decreases the efficiency of DNA repair during recovery from DNA damage and generates more cell death. Altogether, our data suggest that more dTTP synthesis via TK1 take place after genotoxic insults in tumor cells, improving DNA repair during G(2) arrest.

[18F]FDG and [18F]FLT uptake in human breast cancer cells in relation to the effects of chemotherapy: an in vitro study

Increased 2′-deoxy-2′-[¹⁸F]fluoro-D-glucose (FDG) uptake is the most commonly used marker for positron emission tomography in oncology. However, a proliferation tracer such as 3′-deoxy-3′-[¹⁸F]fluorothymidine (FLT) might be more specific for cancer. 3′-deoxy-3′-[¹⁸F]fluorothymidine uptake is dependent on thymidine kinase 1 (TK) activity, but the effects of chemotherapeutic agents are unknown. The aim of this study was to characterise FDG and FLT uptake mechanisms in vitro before and after exposure to chemotherapeutic agents. The effects of 5-fluorouracil (5-FU), doxorubicin and paclitaxel on FDG and FLT uptake were measured in MDA MB231 human breast cancer cells in relation to cell cycle distribution, expression and enzyme activity of TK-1. At IC₅₀ concentrations, 5-FU resulted in accumulation in the G1 phase, but doxorubicin and paclitaxel induced a G2/M accumulation. Compared with untreated cells, 5-FU and doxorubicin increased TK-1 levels by >300. At 72 h, 5-FU decreased FDG uptake by 50% and FLT uptake by 54%, whereas doxorubicin increased FDG and FLT uptake by 71 and 173%, respectively. Paclitaxel increased FDG uptake with >100% after 48 h, whereas FLT uptake hardly changed. In conclusion, various chemotherapeutic agents, commonly used in the treatment of breast cancer, have different effects on the time course of uptake of both FDG and FLT in vitro. This might have implications for interpretation of clinical findings.

Mitotic control of dTTP pool: a necessity or coincidence?

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

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

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

In vivoBiological Activity of the Histone Deacetylase Inhibitor LAQ824 Is detectable with 3′-Deoxy-3′-[18F]Fluorothymidine Positron Emission Tomography

Histone deacetylase inhibitors (HDACI) are emerging as growth inhibitory compounds that modulate gene expression and inhibit tumor cell proliferation. We assessed whether 3'-deoxy-3'-[(18)F]fluorothymidine-positron emission tomography ([18F]FLT-PET) could be used to noninvasively measure the biological activity of a novel HDACI LAQ824 in vivo. We initially showed that thymidine kinase 1 (TK1; EC2.7.1.21), the enzyme responsible for [18F]FLT retention in cells, was regulated by LAQ824 in a drug concentration-dependent manner in vitro. In HCT116 colon carcinoma xenograft-bearing mice, LAQ824 significantly decreased tumor [18F]FLT uptake in a dose-dependent manner. At day 4 of treatment, [18F]FLT tumor-to-heart ratios at 60 minutes (NUV60) were 2.16 +/- 0.15, 1.86 +/- 0.13, and 1.45 +/- 0.20 in vehicle, and 5 and 25 mg/kg LAQ824 treatment groups, respectively (P < or = 0.05). LAQ825 at 5 mg/kg also significantly reduced both TK1 levels and [18F]FLT uptake at day 10 but not at day 2 (P < or = 0.05). [18F]FLT NUV60 correlated significantly with cellular proliferation (r = 0.68; P = 0.0019) and was associated with drug-induced histone H4 hyperacetylation. Of interest to [18F]FLT-PET imaging, both TK1 mRNA copy numbers and protein levels decreased in the order vehicle >5 mg/kg LAQ824 > 25 mg/kg LAQ824, providing a rationale for the use of [18F]FLT-PET in this setting. We also observed increases in Rb hypophosphorylation and p21 levels, factors that could have contributed to the alteration in TK1 transcription in vivo. In conclusion, we have shown the utility of [18F]FLT-PET for monitoring the biological activity of the HDACI, LAQ824. Drug-induced changes in tumor [18F]FLT uptake were due, at least in part, to reductions in TK1 transcription and translation.

Quantification of Cellular Proliferation in Tumor and Normal Tissues of Patients with Breast Cancer by [18F]Fluorothymidine-Positron Emission Tomography Imaging: Evaluation of Analytical Methods

There is an unmet need to develop imaging methods for the early and objective assessment of breast tumors to therapy. 3'-Deoxy-3'-[18F]fluorothymidine ([18F]FLT)-positron emission tomography represents a new approach to imaging thymidine kinase activity, and hence, cellular proliferation. We compared graphical, spectral, and semiquantitative analytic methodologies for quantifying [18F]FLT kinetics in tumor and normal tissue of patients with locally advanced and metastatic breast cancer. The resultant kinetic parameters were correlated with the Ki-67 labeling index from tumor biopsies. [18F]FLT accumulation was detected in primary tumor, nodal disease, and lung metastasis. In large tumors, there was substantial heterogeneity in regional radiotracer uptake, reflecting heterogeneity in cellular proliferation; radiotracer uptake in primary tumors also differed from that of metastases. [18F]FLT was metabolized in patients to a single metabolite [18F]FLT-glucuronide. Unmetabolized [18F]FLT accounted for 71.54 +/- 1.50% of plasma radioactivity by 90 minutes. The rate constant for the metabolite-corrected net irreversible uptake of [18F]FLT (Ki) ranged from 0.6 to 10.4 x 10(-4) and from 0 to 0.6 x 10(-4) mL plasma cleared/s/mL tissue in tumor (29 regions, 15 patients) and normal tissues, respectively. Tumor Ki and fractional retention of radiotracer determined by spectral analysis correlated with Ki-67 labeling index (r = 0.92, P < 0.0001 and r = 0.92, P < 0.0001, respectively). These correlations were superior to those determined by semiquantitative methods. We conclude that [18F]FLT-positron emission tomography is a promising clinical tool for imaging cellular proliferation in breast cancer, and is most predictive when analyzed by graphical and spectral methods.

Control of dTTP pool size by anaphase promoting complex/cyclosome is essential for the maintenance of genetic stability

Anaphase promoting complex/cyclosome (APC/C)-mediated proteolysis is essential for chromosome segregation, mitotic exit, and G1 entry. Here, we show the importance of APC/C in the control of dTTP pool size in mammalian cells. Two enzymes, thymidine kinase 1 (TK1) and thymidylate kinase (TMPK), involved in dTTP formation are the targets of the APC/C pathway. We demonstrate that TMPK is recognized and degraded by APC/C-Cdc20/Cdh1-mediated pathways from mitosis to the early G1 phase, whereas TK1 is targeted for degradation by APC/C-Cdh1 after mitotic exit. Overexpression of wild-type TK1 and TMPK induces a four- to fivefold increase in the cellular dTTP pool without promoting spontaneous mutations in the hprt (hypoxanthine-guanine phosphoribosyl transferase) gene. In contrast, coexpression of nondegradable TK1 and TMPK expands the dTTP pool size 10-fold accompanied by a drastic dNTP pool imbalance. Most interestingly, disruption of APC/C proteolysis of TK1 and TMPK leads to growth retardation and a striking increase in gene mutation rate. We conclude that down-regulation of dTTP pool size by the APC/C pathway during mitosis and the G1 phase is an essential means to maintain a balanced dNTP pool and to avoid genetic instability.

Early detection of tumor response to chemotherapy by 3'-deoxy-3'-[18F]fluorothymidine positron emission tomography: the effect of cisplatin on a fibrosarcoma tumor model in vivo

We have assessed the potential of [18F]fluorothymidine positron emission tomography ([18F]FLT-PET) to measure early cytostasis and cytotoxicity induced by cisplatin treatment of radiation-induced fibrosarcoma 1 (RIF-1) tumor-bearing mice. Cisplatin-mediated arrest of tumor cell growth and induction of tumor shrinkage at 24 and 48 hours, respectively, were detectable by [18F]FLT-PET. At 24 and 48 hours, the normalized uptake at 60 minutes (tumor/liver radioactivity ratio at 60 minutes after radiotracer injection; NUV60) for [18F]FLT was 0.76 +/- 0.08 (P = 0.03) and 0.51 +/- 0.08 (P = 0.03), respectively, compared with controls (1.02 +/- 0.12). The decrease in [18F]FLT uptake at 24 hours was associated with a decrease in cell proliferation assessed immunohistochemically (a decrease in proliferating cell nuclear antigen labeling index, LI(PCNA), from 14.0 +/- 2.0% to 6.2 +/- 1.0%; P = 0.001), despite the lack of a change in tumor size. There were G1-S and G2-M phase arrests after cisplatin treatment, as determined by cell cycle analysis. For the quantitative measurement of tumor cell proliferation, [18F]FLT-PET was found to be superior to [18F]fluorodeoxyglucose-PET (NUV60 versus LIPCNA: r = 0.89, P = 0.001 and r = 0.55, P = 0.06, respectively). At the biochemical level, we found that the changes in [18F]FLT and [18F]fluorodeoxyglucose uptake were due to changes in levels of thymidine kinase 1 protein, hexokinase, and ATP. This work supports the further development of [18F]FLT-PET as a generic pharmacodynamic readout for early quantitative imaging of drug-induced changes in cell proliferation in vivo.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Effect of valine 106 on structure-function relation of cytosolic human thymidine kinase

Information on the regulation and structure-function relation of enzymes involved in DNA precursor synthesis is pivotal, as defects in several of these enzymes have been found to cause depletion or deletion of mitochondrial DNA resulting in severe diseases. Here, the effect of amino acid 106 on the enzymatic properties of the cell-cycle-regulated human cytosolic thymidine kinase 1 (TK1) is investigated. On the basis of the previously observed profound differences between recombinant TK1 with Val106 (V106WT) and Met106 (V106M) in catalytic activity and oligomerization pattern, we designed and characterized nine mutants of amino acid 106 differing in size, conformation and polarity. According to their oligomerization pattern and thymidine kinetics, the TK1 mutants can be divided into two groups. Group I (V106A, V106I and V106T) behaves like V106WT, in that pre-assay exposure to ATP induces reversible transition from a dimer with low catalytic activity to a tetramer with high catalytic activity. Group II (V106G, V106H, V106K, V106L and V106Q) behaves like V106M in that they are permanently high activity tetramers, irrespective of ATP exposure. We conclude that size and conformation of amino acid 106 are more important than polarity for the catalytic activity and oligomerization of TK1. The role of amino acid 106 and the sequence surrounding it for dimer-tetramer transition was confirmed by cloning the putative interface fragment of human TK1 and investigating its oligomerization pattern.

Mitotic degradation of human thymidine kinase 1 is dependent on the anaphase-promoting complex/cyclosome-CDH1-mediated pathway

The expression of human thymidine kinase 1 (hTK1) is highly dependent on the growth states and cell cycle stages in mammalian cells. The amount of hTK1 is significantly increased in the cells during progression to the S and M phases, and becomes barely detectable in the early G(1) phase by a proteolytic control during mitotic exit. This tight regulation is important for providing the correct pool of dTTP for DNA synthesis at the right time in the cell cycle. Here, we investigated the mechanism responsible for mitotic degradation of hTK1. We show that hTK1 is degraded via a ubiquitin-proteasome pathway in mammalian cells and that anaphase-promoting complex/cyclosome (APC/C) activator Cdh1 is not only a necessary but also a rate-limiting factor for mitotic degradation of hTK1. Furthermore, a KEN box sequence located in the C-terminal region of hTK1 is required for its mitotic degradation and interaction capability with Cdh1. By in vitro ubiquitinylation assays, we demonstrated that hTK1 is targeted for degradation by the APC/C-Cdh1 ubiquitin ligase dependent on this KEN box motif. Taken together, we concluded that activation of the APC/C-Cdh1 complex during mitotic exit controls timing of hTK1 destruction, thus effectively minimizing dTTP formation from the salvage pathway in the early G(1) phase of the cell cycle in mammalian cells.

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

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

Cytosolic retention of phosphorylated extracellular signal-regulated kinase and a Rho-associated kinase-mediated signal impair expression of p21(Cip1/Waf1) in phorbol 12-myristate-13- acetate-induced apoptotic cells

In response to treatment with phorbol-12-myristate-13-acetate (PMA), the half-population of erythromyeloblast D2 cells, a cytokine-independent variant of TF-1 cells, displayed adhesion and differentiated into a monocyte/macrophage-like morphology, while the other half-population remained in suspension and underwent apoptosis. Expression of the cell cycle inhibitor p21(Cip1/Waf1) was induced after PMA treatment in the adherent cells but not in the proapoptotic cells. We investigated the mechanism responsible for the impairment of p21(Cip1/Waf1) induction in PMA-induced proapoptotic cells. We demonstrated that in PMA-induced adherent cells, upregulation of p21(Cip1/Waf1) requires the activation and nuclear translocation of phosphorylated extracellular signal-regulated kinase (phospho-ERK). Although ERK was phosphorylated to comparable levels in PMA-induced proapoptotic and adherent cells, nuclear distribution of phospho-ERK was seen only in the adherent, not in the proapoptotic cells. We also found that only PMA-induced proapoptotic cells contained the phosphorylated form of myosin light chain, which is dependent on Rho-associated kinase (ROCK) activation, and that expression of a dominant-active form of ROCK suppressed activation of the p21(Cip1/Waf1) promoter during PMA induction. Finally, we demonstrated that inhibition of ROCK restores nuclear distribution of phospho-ERK and activation of p21(Cip1/Waf1) expression. Based on these findings, we propose that a ROCK-mediated signal is involved in interfering with the process of ERK-mediated p21(Cip1/Waf1) induction in PMA-induced proapoptotic TF-1 and D2 cells.

Cap-independent translation conferred by the 5'-untranslated region of human thymidine kinase mRNA

Translational control is one of the mechanisms that regulate thymidine kinase (TK) expression in the cell cycle. Evidence for the TK mRNA sequence that is involved in its own translation has been lacking. In this report, we show that TK-deficient mouse fibroblasts transfected with pFLAG-TK express a TK mRNA containing the 5'-untranslated region (5'UTR) and produce two polypeptides, FLAG-TK and TK, resulting from an alternative initiation of translation. Most interestingly, the 5'UTR of TK allowed the translation of FLAG-TK mRNA to become cap-independent in an in vitro translation system. Furthermore, this 5'UTR sequence decreased significantly the efficiency of translation from the AUG codon of FLAG when the concentration of FLAG-TK RNA was low. Here, we also show that in normal human IMR-90 fibroblasts the induction of TK polypeptide by serum stimulation is insensitive to rapamycin treatment, which is known to inhibit the translations of transcripts of some growth-controlled genes by affecting the cap-binding efficiency. Taken together, we propose that the 5'UTR in TK mRNA might actually confer a secondary structure to regulate ribosome binding during translation in a cap-independent manner.

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

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

DNA-synthesizing enzymes in breast cancer (thymidine kinase, thymidylate synthase and thymidylate kinase): association with flow cytometric S-phase fraction and relative prognostic importance in node-negative premenopausal patients

S-phase fraction (SPF) is a reference for cell-kinetic analysis. In this study, the links between SPF and the essential enzymes participating in the pyrimidine synthesis were investigated in breast cancer and their relationships with the natural history of the disease were compared. We measured thymidine kinase (TK) for salvage synthesis, thymidylate synthase (TS) for de novo synthesis and thymidylate kinase (TMK), which is required for both pathways. Our study population consisted of 211 premenopausal women with node-negative tumors. SPF was assessed prospectively by flow cytometry, whereas enzyme activities were measured retrospectively in cytosols using radioenzymatic methods. Among the enzymes analyzed, only TK demonstrated a strong correlation with SPF (r(s) = 0.59). In univariate analysis, high SPF and high levels of TK were associated with increased risk of developing distant recurrences (p < 0.001). Correlations with other prognostic factors (histological grade, steroid receptors, DNA ploidy status, urokinase plasminogen activator and plasminogen activator inhibitor type 1) confirmed a parallel association of SPF and TK with the most aggressive tumors. In contrast, TS and TMK were not associated with prognosis. After adjustment for SPF, the risk of relapse increased significantly with TK values. Subgroup analysis showed that additional information was provided by TK in the tumors with low SPF. When urokinase plasminogen activator (uPA) was a candidate variable in multivariate analysis, TK remained significant. Combined with SPF and uPA, TK could be useful to define premenopausal node-negative patients with rapidly proliferating tumors at a high risk of metastatic disease.

PKB/Akt interacts with inosine-5' monophosphate dehydrogenase through its pleckstrin homology domain

The pleckstrin homology (PH) domain of the protooncogenic serine/threonine protein kinase PKB/Akt can bind phosphoinositides. A yeast-based two-hybrid system was employed which identified inosine-5' monophosphate dehydrogenase (IMPDH) type II as specifically interacting with PKB/Akts PH domain. IMPDH catalyzes the rate-limiting step of de novo guanosine-triphosphate (GTP) biosynthesis. Using purified fusion proteins, PKB/Akts PH domain and IMPDH associated in vitro and this association moderately activated IMPDH. Purified PKB/Akt also associated with IMPDH in vitro. We could specifically pull-down PKB/Akt or IMPDH from mammalian cell lysates using glutathione-S-transferase (GST)-IMPDH or GST-PH domain fusion proteins, respectively. Additionally, PKB/Akt and IMPDH could be co-immunoprecipitated from COS cell lysates and active PKB/Akt could phosphorylate IMPDH in vitro. These results implicate PKB/Akt in the regulation of GTP biosynthesis through its interaction with IMPDH, which is involved in providing the GTP pool used by signal transducing G-proteins.

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

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

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.

Thymidine inhibits the growth-arrest-specific degradation of thymidine kinase protein in transfected L fibroblasts

The expression of murine thymidine kinase (TK) is strictly dependent on the growth state of the cell. Expressing epitope-tagged TK in LTK cells, we have previously shown that low TK enzyme levels in G0 cells are in part due to a dramatic decrease in TK protein stability. Here we report that thymidine, one of the substrates of TK, is able to counteract the growth-arrest-specific decrease of TK expression. While TK mRNA levels and TK translation rate are almost unaffected by thymidine, the TK protein half-life rose more than sixfold after addition of the nucleoside to resting cells. The effect of thymidine is reversible and is independent of its presence during the protein synthesis of TK. Dideoxythymidine, a specific inhibitor of the TK enzyme activity, also has the capacity to increase TK protein levels in G0 cells, indicating that the substrate itself exerts the stabilising effect on the TK protein.

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.

Overexpression of thymidine kinase mRNA eliminates cell cycle regulation of thymidine kinase enzyme activity

Expression of thymidine kinase (TK) enzyme activity and mRNA is strictly S phase-specific in primary cells. In contrast, DNA tumor virus-transformed cells have enhanced and constitutive levels of TK mRNA during the whole cell cycle. Their TK protein abundance, however, still increases at the G1-S transition and stays high throughout G2 until mitosis. Therefore, post-transcriptional control must account for the decoupling of TK mRNA from protein synthesis in G1. To characterize the underlying mechanism, we studied the consequences of TK mRNA abundance on the cell cycle-dependent regulation of TK activity in nontransformed cells. Constitutive as well as conditional human and mouse TK cDNA vectors were stably transfected into mouse fibroblasts, which were subsequently synchronized by centrifugal elutriation. Low constitutive TK mRNA expression still resulted in a fluctuation of TK activity with a pronounced maximum in S phase. This pattern of cell cycle-dependent TK activity variation reflected the one in primary cell but is caused by post-transcriptional control. Increasing overexpression of TK transcripts after hormonal induction compromised this regulation. At the highest constant mRNA levels, regulation of enzyme activity was totally abolished in each phase of the cell cycle. These data indicate that post-transcriptional regulation of TK is tightly coupled to the amount of mRNA; high concentrations apparently titrate a factor(s) required for repressing TK production during G1 and presumably also G2.

The role of cyclic AMP in the lipopolysaccharide-induced suppression of thymidine kinase activity in macrophage

We observed that lipopolysaccharide (LPS, 1 micrograms/ml) can suppress [3H]thymidine incorporation into acid-insoluble fraction in a mouse macrophage cell line J774 (over 70% at 6 h) without affecting the uptake of [3H]thymidine or DNA polymerase activity. Paralleling this suppression, a decrease in the thymidine kinase (TK) activity, but not of thymidine monophosphate (TMP) kinase and thymidine diphosphate (TDP) kinase, was observed. LPS dose-dependently increased intracellular cAMP levels to about 3.5-times basal at 6 h, proportionally to the decrease of the TK activity. Elevation of intracellular cAMP by several reagents also decreased TK activity. Apparently LPS treatment elevates cAMP concentration by decreasing the low Km cAMP phosphodiesterase activity (58% at 6 h). The time course of cAMP-dependent protein kinase (PK-A) activity during the first 6 h after LPS treatment correlated with that of cAMP concentration. Treatment with a PK-A inhibitor restored about 63% of LPS-induced reduction of TK activity at 6 h. At longer times, however, there was a discrepancy between the change of cAMP concentration or PK-A activity and the reduction of TK activity. Therefore, protein kinase activation caused by the accumulation of intracellular cAMP probably triggers some mechanism responsible for the reduction of the TK activity.

Mammalian deoxyribonucleoside kinases

The mammalian deoxyribonucleoside kinases are deoxycytidine kinase, thymidine kinase 1 and 2 and deoxyguanosine kinase. These enzymes phosphorylate deoxyribonucleosides and thereby provide an alternative to de novo synthesis of DNA precursors. Their activities are essential for the activation of several chemotherapeutically important nucleoside analogues. In recent years, these enzymes have been thoroughly characterised with regard to structure, substrate specificity and patterns of expression. In this review, these results are reviewed and furthermore, the physiologic metabolic role of the anabolic enzymes is discussed in relation to catabolic pathways. The significance of this information for the development of therapeutic protocols and choice of animal model systems is discussed. Finally, alternative pathways for nucleoside analogue phosphorylation are surveyed, such as the phosphotransfer capacity of 5'-nucleotidase.