Induction of cellular thymidine kinase occurs at the mRNA level

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.

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.

Multi-tracer small animal PET imaging of the tumour response to the novel pan-Erb-B inhibitor CI-1033

PURPOSE

This study was designed as "proof of concept" for a drug development model utilising multi-tracer serial small animal PET imaging to characterise tumour responses to molecularly targeted therapy.

METHODS

Mice bearing subcutaneous A431 human squamous carcinoma xenografts (n=6-8) were treated with the pan-Erb-B inhibitor CI-1033 or vehicle and imaged serially (days 0, 3 and 6 or 7) with [(18)F]fluorodeoxyglucose, [(18)F]fluoro-L: -thymidine, [(18)F]fluoro-azoazomycinarabinoside or [(18)F]fluoromisonidazole. Separate cohorts (n=3) were treated identically and tumours were assessed ex vivo for markers of glucose metabolism, proliferation and hypoxia.

RESULTS

During the study period, mean uptake of all PET tracers generally increased for control tumours compared to baseline. In contrast, tracer uptake into CI-1033-treated tumours decreased by 20-60% during treatment. Expression of the glucose transporter Glut-1 and cell cycle markers was unchanged or increased in control tumours and generally decreased with CI-1033 treatment, compared to baseline. Thymidine kinase activity was reduced in all tumours compared to baseline at day 3 but was sevenfold higher in control versus CI-1033-treated tumours by day 6 of treatment. Uptake of the hypoxia marker pimonidazole was stable in control tumours but was severely reduced following 7 days of CI-1033 treatment.

CONCLUSION

CI-1033 treatment significantly affects tumour metabolism, proliferation and hypoxia as determined by PET. The PET findings correlated well with ex vivo biomarkers for each of the cellular processes studied. These results confirm the utility of small animal PET for evaluation of the effectiveness of molecularly targeted therapies and simultaneously definition of specific cellular processes involved in the therapeutic response.

Isolation and characterization of Dictyostelium thymidine kinase 1 as a calmodulin-binding protein

Probing of a cDNA expression library from multicellular development of Dictyostelium discoideum using a recombinant radiolabelled calmodulin probe (35S-VU1-CaM) led to the isolation of a cDNA encoding a putative CaM-binding protein (CaMBP). The cDNA contained an open reading frame of 951 bp encoding a 227aa polypeptide (25.5 kDa). Sequence comparisons led to highly significant matches with cytosolic thymidine kinases (TK1; EC 2.7.1.21) from a diverse number of species including humans (7e-56; 59% Identities; 75% Positives) indicating that the encoded protein is D. discoideum TK1 (DdTK1; ThyB). DdTK1 has not been previously characterized in this organism. In keeping with its sequence similarity with DdTK1, antibodies against humanTK1 recognize DdTK1, which is expressed during growth but decreases in amount after starvation. A CaM-binding domain (CaMBD; 20GKTTELIRRIKRFNFANKKC30) was identified and wild type DdTK1 plus two constructs (DdTK deltaC36, DdTK deltaC75) possessing the domain were shown to bind CaM in vitro but only in the presence of calcium while a construct (DdTK deltaN72) lacking the region failed to bind to CaM. Thus, DdTK1 is a Ca2+-dependent CaMBP. Sequence alignments against TK1 from vertebrates to viruses show that CaM-binding region is highly conserved. The identified CaMBD overlaps the ATP-binding (P-loop) domain suggesting CaM might affect the activity of this kinase. Recombinant DdTK is enzymatically active and showed stimulation by CaM (113+/-0.5%) an in vitro enhancement that was prevented by co-addition of the CaM antagonists W7 (91.2+/-0.8%) and W13 (96.6+/-0.6%). The discovery that TK1 from D. discoideum, and possibly other species including humans and a large number of human viruses, is a Ca2+-dependent CaMBP opens up new avenues for research on this medically relevant protein.

The modulation of radiation-induced cell death by genistein in K562 cells: activation of thymidine kinase 1

Ionizing radiation is one of the most effective tools in cancer therapy. In a previous study, we reported that protein tyrosine kinase (PTK) inhibitors modulate the radiation responses in the human chronic myelogenous leukemia (CML) cell line K562. The receptor tyrosine kinase inhibitor, genistein, delayed radiation-induced cell death, while non-recepter tyrosine kinase inhibitor, herbimycin A (HMA) enhances radiation-induced apoptosis. In this study, we focused on the modulation of radiation-induced cell death by genistein and performed PCR-select suppression subtractive hybridization (SSH) to understand its molecular mechanism. We identified human thymidine kinase 1 (TK1), which is cell cycle regulatory gene and confirmed expression of TK1 mRNA by Northern blot analysis. Expression of TK1 mRNA and TK1 enzymatic activity were parallel in their increase and decrease. TK1 is involved in G1-S phase transition of cell cycle progression. In cell cycle analysis, we showed that radiation induced G2 arrest in K562 cells but it was not able to sustain. However, the addition of genistein to irradiated cells sustained a prolonged G2 arrest up to 120 h. In addition, the expression of cell cycle-related proteins, cyclin A and cyclin B1, provided the evidences of G1/S progression and G2-arrest, and their relationship with TK1 in cells treated with radiation and genistein. These results suggest that the activation of TK1 may be critical to modulate the radiation-induced cell death and cell cycle progression in irradiated K562 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.

Activation of deoxycytidine kinase by gamma-irradiation and inactivation by hyperosmotic shock in human lymphocytes

Deoxycytidine kinase (dCK) is a key enzyme in the intracellular metabolism of deoxynucleosides and their analogues, phosphorylating a wide range of drugs used in the chemotherapy of leukaemia and solid tumours. Previously, we found that activity of dCK can be enhanced by incubating primary cultures of lymphocytes with substrate analogues of the enzyme, as well as with various genotoxic agents. Here we present evidence that exposure of human lymphocytes to 0.5-2 Gy dosage of gamma-radiation as well as incubation of cells with calyculin A, a potent inhibitor of protein phosphatase 1 and 2A, both elevate dCK activity without changing the level of dCK protein. When cells were gamma-irradiated in the presence of calyculin A, a more pronounced activation of dCK was observed. In contrast, both basal and stimulated dCK activities were reduced by hyperosmotic treatment of the cells. DNA repair determined by the Comet assay and by thymidine incorporation was induced by irradiation. Complete repair of gamma-irradiated DNA was detected within 1 hr following the irradiation along with dCK activation, but the rate of repair was not accelerated by calyculin A. These data provide evidence for the activation of dCK upon DNA damage and repair that seems to be mediated by phosphorylation of the enzyme, suggesting the role of dCK in DNA repair processes.

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

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

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.

Constitutive protection of E2F recognition sequences in the human thymidine kinase promoter during cell cycle progression

The sequences responsible for S phase-specific induction of the human thymidine kinase (TK) gene have been mapped to a small region that contains putative E2F binding sites. We have analyzed protein-DNA interactions at the TK promoter during cell cycle progression in human fibroblasts using an in vivo footprinting approach. We found 14 protein binding sites that were occupied in vivo. All of the sites (among them two inverted CCAAT boxes and several Sp1 sites) bound transcription factors constitutively throughout the cell cycle, i.e. none of the factor binding was cell cycle-dependent. An E2F-like site located between nucleotides -97 and -89 relative to the major transcription start site was protected in G0, G1, S, and G2 phases. This cell cycle-independent protection of E2F sequences in the TK promoter differs from the G0/G1-restricted binding of E2F complexes observed for genes in which the E2F sites function as repressor elements (Tommasi, S., and Pfeifer, G. P. (1995) Mol. Cell. Biol. 15, 6901-6913; Zwicker, J., Liu, N., Engeland, K., Lucibello, F. C., and Müller, R. (1996) Science 271, 1595-1597). A comparison of several genes containing E2F motifs indicates that E2F sites located in proximity to the transcription initiation site (-50 to +20) in TATA-less promoters predominantly function as repressor elements, while in other genes constitutively bound E2F complexes located further upstream mediate activation presumably in conjunction with a functional TATA box.

Mouse thymidine kinase stability in vivo and after in vitro translation

Using a combination of centrifugal elutriation and recultivation of synchronised cell populations we could show that murine thymidine kinase (TK) is rapidly degraded during mitosis in polyoma virus-transformed mouse fibroblasts, in parallel to the time-course for loss of cyclin A. Transformation is no prerequisite for the instability phenotype since artificial overexpression of TK under the control of a constitutive promoter in normal mouse fibroblasts also resulted in rapid turnover of TK during mitosis. The decay of TK protein could be partially mimicked in vitro with enzymatically active protein translated in a rabbit reticulocyte lysate: full length polypeptide was lost slightly more rapidly in the presence of G2/M cytosolic extracts than with G1/S preparations. In addition, an enzymatically active C-terminal truncation of 37 amino acids at Gln-196 was completely stable under the conditions tested, confining the instability domain between residues 196 to 233. These experiments also indicated the border for intact TK since translation products up to Tyr-189 or less were completely inactive. This was also confirmed by a mutant TK protein from mouse F9tk- teratocarcinoma cells which harboured a similar deletion.

Zinc deficiency affects cell cycle and deoxythymidine kinase gene expression in HUT-78 cells

Although zinc is known to be involved in cell proliferation and DNA synthesis, the mechanism by which zinc may regulate these processes is not understood. We have studied the role of zinc on cell proliferation and gene expression of a DNA synthesizing enzyme, deoxythymidine kinase (TK), in a T helper human malignant lymphoblastoid cell line (HUT-78). In zinc-deficient and zinc-sufficient media, the cell doubling time (mean +/- SD) of HUT-78 was 59 +/- 8 hours and 32.6 +/- 6 hours, respectively. The effect of zinc was T cell specific, inasmuch as the cell growth of another T malignant lymphoblastoid cell line, MOLT-3 (immature T cells), was not affected by zinc deficiency. Iron, copper, or manganese did not completely correct the cell growth of zinc-deficient HUT-78 cells. TK activity and the relative accumulation of TK-mRNA were significantly decreased in zinc-deficient cells during the G1 phase of cell cycle in comparison with zinc-sufficient cells. Nuclear run-on experiments and actinomycin-D studies showed that the transcription of TK-mRNA was affected adversely by zinc deficiency. Cell cycle studies showed that more zinc-deficient cells remained in S phase and did not undergo mitosis in comparison with zinc-sufficient cells. In conclusion, our data show that zinc is a T cell-specific growth factor and that a decreased gene expression of DNA-synthesizing enzyme TK in zinc-deficient HUT-78 cells in G1 phase affected adversely the DNA synthesis in S phase and delayed cell cycle.

Differential expression of DNA polymerase alpha in normal and transformed human fibroblasts

The expression of DNA polymerase alpha (pol alpha) was studied in human fibroblast lines W138 (fetal lung) and GM3529 (skin, established from a 66 yr old donor), and their Simian virus 40 (SV40) large tumor antigen (TAg)-transformed corollaries, 2RA and 2-1 respectively. Both SV40-transformed and pSV3.neo (SV40-derived plasmid)-transformed cells express TAg, a virally encoded protein not expressed by the normal parent cell lines. Northern blot hybridization studies showed increased recovery of pol alpha mRNA from transformed cells compared with normal cells. This increase was correlated with increased pol alpha mRNA transcription as determined by nuclear run-on assays. Northern blot analyses also showed an increase in the instability of translationally active pol alpha mRNA in transformed cells. The results suggest that TAg, in addition to its dsDNA binding, pol alpha binding, retinoblastoma protein binding and helicase activities, may be involved either directly or indirectly in regulation of the steady state mRNA levels of pol alpha at the transcriptional level in both fetal and aged donor-derived transformed fibroblasts.

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.

Different regulation of the human thymidine kinase promoter in normal human diploid IMR-90 fibroblasts and HeLa cells

Transcriptional activation of the human thymidine kinase (hTK) promoter plays an important role in the cell cycle control of thymidine kinase expression. Using the luciferase reporter cotransfection assay, we found that the activity of the hTK promoter in IMR-90 normal human diploid fibroblasts was increased by the constitutively over-expressed cyclin A or cyclin E but not by cyclin D, suggesting that the former two cyclins may act as positive regulators for the hTK promoter. The sequence responsible for the transcriptional activation by cyclin E was identified to be located between -133 and -92 of the hTK promoter. Regulation of the hTK promoter in HeLa cells appeared to be different from that in IMR-90 fibroblasts. Firstly, the hTK promoter in HeLa was already highly activated and could not be further activated by ectopically expressed cyclin A or E. Secondly, the -133 to -92 region of the hTK promoter was important for the promoter strength in HeLa cells but not in IMR-90 cells. The steady-state levels of cyclins A and E were readily detected in HeLa cells but not in normal IMR-90 fibroblasts. Based on these results, we propose that the cellular environment of the HeLa cell allows the hTK promoter to stay fully activated for transcription regardless of ectopically expressed cyclin A or E and that transcriptional activation of thymidine kinase gene is deregulated in these tumor cells.

CCAAT-box contributions to human thymidine kinase mRNA expression

In order to examine the role of two inverted CCAAT boxes near the start of transcription of the human thymidine kinase (TK) gene, a series of constructs were prepared in which one or both CCAAT boxes were deleted or mutated. These altered promoters (1.2 kb of 5'-flanking sequence) were used to express a TK minigene containing the first two exons and introns followed by the remainder of the cDNA. RNA blots were prepared from stable cell lines of ts13 cells containing these constructs under three conditions: 1) serum deprived cells, 2) serum stimulated cells, and 3) cells that had been stimulated with serum, but were arrested in the G1 phase of the cell cycle by the temperature sensitive mutation carried by these cells. TK mRNA expression from each construct was suppressed by the temperature sensitive block to cell cycle progression. Measurement of protein expression from the various altered TK promoters indicated that both CCAAT boxes contribute to promoter strength. These experiments also suggested that the two CCAAT boxes were not equivalent and that the distal CCAAT could substitute for the proximal CCAAT, but the converse was not true.

Protein that binds to the distal, but not to the proximal, CCAAT of the human thymidine kinase gene promoter

Mobility shift assays were used to examine protein binding to the human TK gene CCAAT boxes. Similar protein binding patterns were observed with probes containing either the proximal or distal CCAAT. However, probes containing both CCAAT boxes in which one of the CCAAT boxes was inactivated by mutation did not demonstrate identical binding patterns. One of the complexes formed with the longer probes was only observed when the distal CCAAT was intact. This species was not formed with probes that only contained an intact proximal CCAAT, and its formation could only be competed by oligonucleotides containing the distal CCAAT motif. This observation reveals the existence of a protein that can bind to the distal, but not to the proximal, CCAAT of the human TK promoter. This protein may account for the previous observation that the two CCAAT motifs are not functionally equivalent. The protein that binds to the distal, but not to the proximal, CCAAT (DTK-CBP) was also present in two human cell lines. Significantly more DTK-CBP was present in nuclear extracts of HepG2 and WI38 cells than in TK-ts13 cells. However, this protein was not observed in three different murine cell lines and one primary culture. Its abundance in some human cell lines suggests it might modulate the expression of human TK mRNA in cells that express this protein.

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.

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.

Enhanced expression of thymidine kinase in human cells following ionizing radiation

PURPOSE

We investigated the induction of thymidine kinase transcription and enzymatic activity, and the activation of transcription factors binding to the thymidine kinase promoter, in human normal compared to tumor cells in culture before and after ionizing radiation.

METHODS AND MATERIALS

Northern blot, dot-blot, and thymidine kinase enzyme assays were used to observe thymidine kinase transcript and enzymatic changes before and after radiation. Temporal expression of thymidine kinase transcripts following an optimal induction dose of radiation was also studied. Gel mobility shift assays were performed using a 95-base pair fragment of the thymidine kinase promoter (containing the CCAAT box) to analyze transcription factor binding.

RESULTS

Thymidine kinase transcript and enzymatic levels were higher in human tumor compared to normal cells. In contrast, levels of x-ray-activated thymidine kinase transcription factors were not significantly different in human neoplastic compared to normal cells.

CONCLUSIONS

Elevated x-ray-induced thymidine kinase transcripts, enzymatic levels, and transcription factors are consistent with the loss of stringent cell growth regulation associated with neoplastic cells. The induction of thymidine kinase following ionizing radiation may be exploited in chemotherapeutic strategies which use halogenated pyrimidines and/or in various gene therapy strategies.

Methylation-sensitive protein-DNA interaction at the cell cycle regulatory domain of human thymidine kinase promoter

We have investigated the DNA-protein complex formation in nuclear extracts of human cells using the sequence of cell-cycle regulatory unit (CCRU) of human thymidine kinase (TK) promoter. It appeared that a distinct DNA-protein complex was present in three human tumor cell lines and that the CCAAT box within the sequence of CCRU was a necessary element for complex formation. Upon 4 days of serum deprivation, this DNA-protein complex remained unchanged in HeLa cells, but the expression of TK mRNA was decreased. Furthermore, DNA methylation of the Hhal site of the CCRU sequence of the TK promoter greatly reduced the binding activity of nuclear proteins from different human tumor cell lines. On the basis of these data, we proposed a possible role for DNA methylation in the regulation of TK transcription during late G1/S phase progression of the cell cycle.

Cell cycle dependent gene expression in quiescent stimulated and asynchronously cycling arterial smooth muscle cells in culture

The expression of a set of cell cycle dependent (CCD) genes (c-fos, c-myc, ornithine decarboxylase (ODC), and thymidine kinase (TK)) was comparatively studied in cultured arterial smooth muscle cells (SMC) during exit from quiescence and exponential proliferation. These genes, which were not expressed in quiescent SMC, were chronologically induced after serum stimulation. c-fos mRNA were rapidly and transiently expressed very early in the G1 phase; c-myc and ODC peaked a few hours after serum stimulation and then remained at an intermediary level throughout the first cell cycle; TK mRNA and activity then appeared at the G1/S boundary and peak in G2/M phases. Except for c-fos, the other genes were also expressed in asynchronously cycling SMC (ACSMC); their expression was studied in elutriated subpopulations representative of cell cycle progression. c-fos mRNA were undetectable in any sorted subpopulations, even in the pure early G1 population. Despite a slight increase as the cell cycle advanced, c-myc and ODC genes were expressed throughout the ACSMC cell cycle. A faint TK activity was found in G1 subpopulations and increased in populations enriched in other phases; in contrast, TK mRNA remained highly expressed in all elutriated subpopulations. This study demonstrates significant modulations in CCD gene expression between quiescent stimulated and asynchronously cycling SMC in culture. This suggests that the events occurring during the emergence of SMC from quiescence are probably different from those in the G1 phase of ACSMC.

Phenotypic conversion of TK-deficient cells following electroporation of functional TK enzyme

The ability to phenotypically rescue a mutant (Rat-3, thymidine kinase-deficient) cell line by electroporation of functional TK enzyme has been investigated. Extracts of electroporated cells showed a 35-fold increase in TK enzyme levels under conditions where greater than 90% of the cells remained viable. The electroporated enzyme was intracellular, as demonstrated by the fact that cells were able to utilize exogenous [3H]thymidine for DNA synthesis. By in situ autoradiography, 82% of electroporated cells contained functional enzyme and incorporated [3H]thymidine into DNA. Thus, this technique can efficiently provide a missing metabolic function to cultured mammalian cells.

Regulation of O6-methylguanine-DNA methyltransferase expression in the Burkitt's lymphoma cell line Raji

We have investigated the expression of the DNA-repair enzyme O6-methylguanine-DNA methyltransferase in the Burkitt's lymphoma cell line Raji. An existing mutant Raji cell line which lacks thymidine kinase activity had previously been shown to be Mex- and to no longer express O6-methylguanine-DNA methyltransferase. We report here that in addition to the methyltransferase and thymidine kinase, a third enzyme with an unrelated function, galactokinase, is also not expressed in Raji cells. The control of thymidine kinase expression is post-transcriptional and it is possible that galactokinase and methyltransferase can share a common post-transcriptional regulation with thymidine kinase.

A site in the T4 bacteriophage major head protein gene that can promote the inhibition of all translation in Escherichia coli

The cryptic DNA element, e14, synthesizes a protein, Lit, which can inhibit gene expression late in T4 bacteriophage development. This inhibition is due to the interaction between the Lit protein and a short region, the gol region, within gene 23, the major head protein gene of phage T4. We have constructed plasmids in which the gol region is transcribed from the lac promoter and fused translationally and transcriptionally to lacZ and cat (chloramphenicol acetyltransferase). These fusion plasmids were used to demonstrate that, in the presence of Lit protein, the gol region inhibits the expression of genes downstream in the same transcription unit. This local inhibition does not require the gene 23 polypeptide from the gol region. In addition, inducing the transcription and translation of the gol region in the presence of Lit protein causes an immediate global inhibition of all translation in Escherichia coli. This global inhibition does require the gene 23 polypeptide. No more than 75 base-pairs of DNA from the gol region are required for both the local and global inhibitions. The gol region sequence contains a short dyad symmetry. However, it is the sequence of bases in the region of dyad symmetry and not the ability to form a hairpin in the RNA that is required for gol region activity.

Sequence analysis of the human thymidine kinase gene promoter: comparison with the human PCNA promoter

We report the sequence of 4264 nucleotides of 5' flanking sequence of the human thymidine kinase gene, a gene that is maximally expressed at the G1/S boundary of the cell cycle. The position of nucleotide sequences which can act as binding sites for trans-acting factors, Sp-1, AP-1/jun, AP-2, OTF-1 and CAAT box factors as well as other potential cis-acting sequences have been mapped. The organization of these cis-acting sequences in the promoter of the human PCNA gene (another gene that is maximally expressed at the G1/S boundary) are shown for comparison. The potential role that these sequences may play in the transcriptional regulation of these genes is discussed.

Steady-state and nuclear run-on analyses of transcription in a temperature-sensitive Chinese hamster cell mutant with a defect in RNA metabolism

We have further characterized a temperature-sensitive mutant of Chinese hamster lung fibroblasts in tissue culture with a defect in RNA metabolism. The mutant phenotype is reflected in transcription in crude extracts or in isolated nuclei, when these are made from cells shifted to the nonpermissive temperature; however, differential heat inactivation between mutant and wild-type extracts cannot be demonstrated with cell-free systems. We tentatively conclude that the mutation may affect initiation of transcription which cannot be observed in our in vitro systems. Partially purified RNA polymerase I, II, and III fractions are indistinguishable from wild type. A temperature shift does not affect transcription by RNA polymerase III measured with intact cells or by nuclear run-on experiments. The nuclear run-on and other experiments suggest that RNA polymerase II-dependent transcription is inhibited before RNA polymerase I-dependent transcription. This conclusion is also supported by Northern analyses of selected mRNAs in nonsynchronized and synchronized cells after a shift to the nonpermissive temperature.

The lit gene product which blocks bacteriophage T4 late gene expression is a membrane protein encoded by a cryptic DNA element, e14

Escherichia coli lit(Con) mutations cause a severe inhibition of gene expression late in infection by bacteriophage T4 owing to the overproduction of one, and possibly two, proteins (C. Kao, E. Gumbs, and L. Snyder, J. Bacteriol. 169:1232-1238, 1987). One or both of these proteins interact, either directly or indirectly, with a short sequence about one-quarter of the way into the major capsid protein gene of T4, and the inhibition occurs when this late gene of the virus is expressed. In this report we show that lit(Con) mutations are up-promoter mutations in the cryptic DNA element e14 and that only one of the proteins, gplit, of about 34 kilodaltons, is required for the inhibition. We have sequenced the lit gene and the surrounding regions. From the sequence, and from cell fractionation studies, we conclude that gplit is an inner membrane protein. Since the assembly of T4 heads is thought to occur on the inner face of the inner membrane, we propose that gplit interferes with a normal regulation which coordinates the synthesis of proteins and the assembly of T4 heads.

Regulation of protein synthesis by polyamines

Present experimental data show that the synthesis of ribosomal protein S1 and PI protein was stimulated greatly by polyamines at the early stage after addition of putrescine in polyamine-requiring mutants of E. coli. No macromolecular synthesis was stimulated at this stage. Polyamine stimulation of the synthesis of these proteins probably plays an important role for cell growth. In polyamine-deficient bovine lymphocytes, protein synthesis became perturbed before RNA and DNA synthesis. Among enzymes concerned with DNA replication, thymidine kinase activity was most strongly influenced by polyamines. The activity in polyamine-deficient cells was only 7% of the level in normal cells. Judging from the amount of thymidine kinase mRNA and its distribution in polysomes, it was concluded that polyamines mainly regulate the synthesis of thymidine kinase at the level of initiation of protein synthesis. A polyamine-free protein synthetic system, established from components of rabbit reticulocytes, consisted of globin mRNA, salt-washed ribosomes, partially purified initiation factors, and pH 5 enzymes. Spermidine added to this system not only lowered the optimal magnesium concentration required for globin synthesis, but it also stimulated the globin synthesis 8- to 10-fold. The optimal spermidine concentration was 0.4 to 0.6 mM, a concentration similar to that in intact rabbit reticulocytes. The ratio of alpha to beta globin chains synthesized in the presence of spermidine and Mg2+ was approximately 1.0, while the ratio in the presence of only Mg2+ was approximately 1.5. The results strongly suggest that polyamines play an important role in rabbit reticulocyte protein synthesis.

Carbohydrate-binding protein 35: molecular cloning and expression of a recombinant polypeptide with lectin activity in Escherichia coli

Affinity-purified antibodies directed against carbohydrate-binding protein 35 (CBP35), a galactose-specific lectin, were used to screen a lambda gt 11 expression library derived from mRNA of 3T3 fibroblasts. This screening yielded several putative clones containing cDNA for CBP35, one of which was characterized in terms of its expression of a fusion protein containing beta-galactosidase and CBP35 sequences. Limited proteolysis of lysates containing the fusion protein, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting with anti-CBP35, yielded a peptide mapping pattern comparable to that obtained from parallel treatment of authentic CBP35. Such a limited proteolysis followed by affinity chromatography on a Sepharose column coupled with galactose also yielded a 30-kDa polypeptide that exhibited carbohydrate-binding activity. This polypeptide can be immunoblotted with anti-CBP35, but not with antibodies directed against beta-galactosidase. These results indicate that we have identified a cDNA clone for CBP35 that yields a recombinant polypeptide with lectin activity produced in Escherichia coli. Using this cDNA clone as a probe, Northern-blot analysis showed an increased expression of the CBP35 gene when quiescent 3T3 cells were activated by the addition of serum growth factors.

Growth-rate-dependent regulation of the expression and inactivation of thymidylate synthase in Saccharomyces cerevisiae

Thymidylate synthase activity fluctuated dramatically as cultures of Saccharomyces cerevisiae progressed through the different stages of batch culture growth. During logarithmic growth these yeast cultures each contained about 40 microU (1 microU is 1 pmol of 3H released per min) of thymidylate synthase activity per 10(8) haploid cells, but as cultures entered the stationary phase and during the stationary phase, activity dropped dramatically, eventually reaching undetectable levels. Stimulation of stationary-phase cells with fresh medium resulted in rapid reestablishment of log phase levels. Two mechanisms, the regulation of thymidylate synthase-specific transcripts and the irreversible inactivation of thymidylate synthase activity, acted in concert to regulate activity levels. These results suggested that thymidylate synthase represents a special subset of yeast proteins whose levels per cell vary quickly and dramatically in response to changes in proliferation rates.

Transcription of genes encoding enzymes involved in DNA synthesis during the cell cycle of Saccharomyces cerevisiae

We have examined the pattern of transcription exhibited by four genes in the dTTP biosynthetic pathway of Saccharomyces cerevisiae. Consistent with the results reported previously by Storms et al. (1984), the TMP1 (or CDC21) gene encoding thymidylate synthase was found to be transcribed in a periodic manner during the cell cycle with maximal mRNA levels occurring just prior to the onset of DNA replication. Three other genes in this pathway DCD1, DUT1 and DFR1 encoding dCMP deaminase, dUTP pyrophosphatase and dihydrofolate reductase, respectively, exhibited relatively constant levels of transcription throughout the cell cycle. These results, particularly for DFR1, are in marked contrast with those obtained in other eukaryotic systems which have suggested that, in general, genes encoding enzymes involved in DNA precursor synthesis are subject to cell cycle regulation. Thus, periodic transcription is not a property common to all genes involved in DNA replication in this eukaryote.

Expression of the c-myb proto-oncogene during cellular proliferation

In several cell types, messenger RNA levels of the nuclear proto-oncogene c-myb vary as a function of cellular proliferation; a transient increase in c-myb steady-state mRNA, mediated by post-transcriptional mechanisms, occurs during cell-cycle progression. In contrast, both quiescent and proliferating immature thymocytes contain exceptionally high levels of c-myb mRNA as a consequence of increased c-myb transcription.