Evidence for transcriptional and post-transcriptional control of the cellular thymidine kinase gene

SAMHD1 … and Viral Ways around It

The SAM and HD domain-containing protein 1 (SAMHD1) is a dNTP triphosphohydrolase that plays a crucial role for a variety of different cellular functions. Besides balancing intracellular dNTP concentrations, facilitating DNA damage repair, and dampening excessive immune responses, SAMHD1 has been shown to act as a major restriction factor against various virus species. In addition to its well-described activity against retroviruses such as HIV-1, SAMHD1 has been identified to reduce the infectivity of different DNA viruses such as the herpesviruses CMV and EBV, the poxvirus VACV, or the hepadnavirus HBV. While some viruses are efficiently restricted by SAMHD1, others have developed evasion mechanisms that antagonize the antiviral activity of SAMHD1. Within this review, we summarize the different cellular functions of SAMHD1 and highlight the countermeasures viruses have evolved to neutralize the restriction factor SAMHD1.

Efficient establishment of reactivatable latency by an acyclovir-resistant herpes simplex virus 1 thymidine kinase substitution mutant with reduced neuronal replication

Herpes simplex virus 1 causes recurrent diseases by reactivating from latency, which requires the viral thymidine kinase (TK) gene. An acyclovir-resistant mutation in TK, V204G, was previously repeatedly identified in a patient with recurrent herpetic keratitis. We found that compared with its parental strain KOS, a laboratory-derived V204G mutant virus was impaired in replication in cultured neurons despite little defect in non-neuronal cells. After corneal inoculation of mice, V204G exhibited defects in ocular replication that were modest over the first three days but severe afterward. Acute replication of V204G in trigeminal ganglia was significantly impaired. However, V204G established latency with viral loads as high as KOS and reactivated with high frequency albeit reduced kinetics. Acyclovir treatment that drastically decreased ocular and ganglionic replication of KOS had little effect on V204G. Thus, despite reduced neuronal replication due to impaired TK activity, this clinically relevant drug-resistant mutant can efficiently establish reactivatable latency.

SAMHD1 Functions and Human Diseases

Deoxynucleoside triphosphate (dNTP) molecules are essential for the replication and maintenance of genomic information in both cells and a variety of viral pathogens. While the process of dNTP biosynthesis by cellular enzymes, such as ribonucleotide reductase (RNR) and thymidine kinase (TK), has been extensively investigated, a negative regulatory mechanism of dNTP pools was recently found to involve sterile alpha motif (SAM) domain and histidine-aspartate (HD) domain-containing protein 1, SAMHD1. When active, dNTP triphosphohydrolase activity of SAMHD1 degrades dNTPs into their 2'-deoxynucleoside (dN) and triphosphate subparts, steadily depleting intercellular dNTP pools. The differential expression levels and activation states of SAMHD1 in various cell types contributes to unique dNTP pools that either aid (i.e., dividing T cells) or restrict (i.e., nondividing macrophages) viral replication that consumes cellular dNTPs. Genetic mutations in SAMHD1 induce a rare inflammatory encephalopathy called Aicardi-Goutières syndrome (AGS), which phenotypically resembles viral infection. Recent publications have identified diverse roles for SAMHD1 in double-stranded break repair, genome stability, and the replication stress response through interferon signaling. Finally, a series of SAMHD1 mutations were also reported in various cancer cell types while why SAMHD1 is mutated in these cancer cells remains to investigated. Here, we reviewed a series of studies that have begun illuminating the highly diverse roles of SAMHD1 in virology, immunology, and cancer biology.

Mechanisms shaping the mutational landscape of the FRA3B/FHIT-deficient cancer genome

Genome instability is an enabling characteristic of cancer that facilitates the acquisition of oncogenic mutations that drive tumorigenesis. Underlying much of the instability in cancer is DNA replication stress, which causes both chromosome structural changes and single base-pair mutations. Common fragile sites are some of the earliest and most frequently altered loci in tumors. Notably, the fragile locus, FRA3B, lies within the fragile histidine triad (FHIT) gene, and consequently deletions within FHIT are common in cancer. We review the evidence in support of FHIT as a DNA caretaker and discuss the mechanism by which FHIT promotes genome stability. FHIT increases thymidine kinase 1 (TK1) translation to balance the deoxyribonucleotide triphosphates (dNTPs) for efficient DNA replication. Consequently, FHIT-loss causes replication stress, DNA breaks, aneuploidy, copy-number changes (CNCs), small insertions and deletions, and point mutations. Moreover, FHIT-loss-induced replication stress and DNA breaks cooperate with APOBEC3B overexpression to catalyze DNA hypermutation in cancer, as APOBEC family enzymes prefer single-stranded DNA (ssDNA) as substrates and ssDNA is enriched at sites of both replication stress and DNA breaks. Consistent with the frequent loss of FHIT across a broad spectrum of cancer types, FHIT-deficiency is highly associated with the ubiquitous, clock-like mutation signature 5 occurring in all cancer types thus far examined. The ongoing destabilization of the genome caused by FHIT loss underlies recurrent inactivation of tumor suppressors and activation of oncogenes. Considering that more than 50% of cancers are FHIT-deficient, we propose that FRA3B/FHIT fragility shapes the mutational landscape of cancer genomes.

Cell cycle regulation of folate-mediated one-carbon metabolism

Folate-mediated one-carbon metabolism (FOCM) comprises a network of interconnected folate-dependent metabolic pathways responsible for serine and glycine interconversion, de novo purine synthesis, de novo thymidylate synthesis and homocysteine remethylation to methionine. These pathways are compartmentalized in the cytosol, nucleus and mitochondria. Individual enzymes within the FOCM network compete for folate cofactors because intracellular folate concentrations are limiting. Although there are feedback mechanisms that regulate the partitioning of folate cofactors among the folate-dependent pathways, less recognized is the impact of cell cycle regulation on FOCM. This review summarizes the evidence for temporal regulation of expression, activity and cellular localization of enzymes and pathways in the FOCM network in mammalian cells through the cell cycle. This article is categorized under: Biological Mechanisms > Metabolism Physiology > Mammalian Physiology in Health and Disease.

Intracellular nucleotide levels and the control of retroviral infections

Retroviruses consume cellular deoxynucleoside triphosphates (dNTPs) to convert their RNA genomes into proviral DNA through reverse transcription. While all retroviruses replicate in dividing cells, lentiviruses uniquely replicate in nondividing cells such as macrophages. Importantly, dNTP levels in nondividing cells are extremely low, compared to dividing cells. Indeed, a recently discovered anti-HIV/SIV restriction factor, SAMHD1, which is a dNTP triphosphohydrolase, is responsible for the limited dNTP pool of nondividing cells. Lentiviral reverse transcriptases (RT) uniquely stay functional even at the low dNTP concentrations in nondividing cells. Interestingly, Vpx of HIV-2/SIVsm proteosomally degrades SAMHD1, which elevates cellular dNTP pools and accelerates lentiviral replication in nondividing cells. These Vpx-encoding lentiviruses rapidly replicate in nondividing cells by encoding both highly functional RTs and Vpx. Here, we discuss a series of mechanistic and virological studies that have contributed to conceptually linking cellular dNTP levels and the adaptation of lentiviral replication in nondividing cells.

Initiation of genome instability and preneoplastic processes through loss of Fhit expression

Genomic instability drives tumorigenesis, but how it is initiated in sporadic neoplasias is unknown. In early preneoplasias, alterations at chromosome fragile sites arise due to DNA replication stress. A frequent, perhaps earliest, genetic alteration in preneoplasias is deletion within the fragile FRA3B/FHIT locus, leading to loss of Fhit protein expression. Because common chromosome fragile sites are exquisitely sensitive to replication stress, it has been proposed that their clonal alterations in cancer cells are due to stress sensitivity rather than to a selective advantage imparted by loss of expression of fragile gene products. Here, we show in normal, transformed, and cancer-derived cell lines that Fhit-depletion causes replication stress-induced DNA double-strand breaks. Using DNA combing, we observed a defect in replication fork progression in Fhit-deficient cells that stemmed primarily from fork stalling and collapse. The likely mechanism for the role of Fhit in replication fork progression is through regulation of Thymidine kinase 1 expression and thymidine triphosphate pool levels; notably, restoration of nucleotide balance rescued DNA replication defects and suppressed DNA breakage in Fhit-deficient cells. Depletion of Fhit did not activate the DNA damage response nor cause cell cycle arrest, allowing continued cell proliferation and ongoing chromosomal instability. This finding was in accord with in vivo studies, as Fhit knockout mouse tissue showed no evidence of cell cycle arrest or senescence yet exhibited numerous somatic DNA copy number aberrations at replication stress-sensitive loci. Furthermore, cells established from Fhit knockout tissue showed rapid immortalization and selection of DNA deletions and amplifications, including amplification of the Mdm2 gene, suggesting that Fhit loss-induced genome instability facilitates transformation. We propose that loss of Fhit expression in precancerous lesions is the first step in the initiation of genomic instability, linking alterations at common fragile sites to the origin of genome instability.

Normal cells have robust mechanisms to maintain the proper sequence of their DNA; in cancer cells these mechanisms are compromised, resulting in complex changes in the DNA of tumors. How this genome instability begins has not been defined, except in cases of familial cancers, which often have mutations in genes called “caretaker” genes, necessary to preserve DNA stability. We have defined a mechanism for genome instability in non-familial tumors that occur sporadically in the population. Certain fragile regions of our DNA are more difficult to duplicate during cell division and are prone to breakage. A fragile region, FRA3B, lies within the FHIT gene, and deletions within FRA3B are common in precancer cells, causing loss of Fhit protein expression. We find that loss of Fhit protein causes defective DNA replication, leading to further DNA breaks. Cells that continue DNA replication in the absence of Fhit develop numerous chromosomal aberrations. Importantly, cells established from tissues of mice that are missing Fhit undergo selection for increasing DNA alterations that can promote immortality, a cancer cell hallmark. Thus, loss of Fhit expression in precancer cells is the first step in the initiation of genomic instability and facilitates cancer development.

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.

2', 3'-Dideoxycytidine represses thymidine kinases 1 and 2 expression in T-lymphoid cells

In vitro culture of H9 human lymphoid cells in the presence of 5.0 microM dideoxycytidine (ddC), for about 40-45 days, selected cells (H9-ddC cells), which were resistant to the drug and cross-resistant to AZT (zidovudine) and 5-fluoro-2'-deoxyuridine (FdUR). The major mechanism of cross-resistance to AZT and FdUR in these cells was low cellular activity of thymidine kinase (TK). To explore molecular mechanisms of the reduced TK activity in H9-ddC cells, the mRNA expression of TK1 and TK2 and western blot analysis of TK1 protein were performed. RT-PCR analysis revealed that in H9-ddC cells the expression of both TK1 and TK2 mRNA was reduced to 27.1% and 79.4%, respectively. The reduced TK1 gene expression was confirmed by an absence of a detectable TK1 protein band in western blot of H9-ddC cells. These results demonstrate that long-term treatment of H9 cells in the presence of ddC down-regulated TK1 and TK2 gene expression and reduced the expression and activity of TK in the resistant cells.

Global analysis of gene expression in neural progenitors reveals specific cell-cycle, signaling, and metabolic networks

The genetic programs underlying neural stem cell (NSC) proliferation and pluripotentiality have only been partially elucidated. We compared the gene expression profile of proliferating neural stem cell cultures (NS) with cultures differentiated for 24 h (DC) to identify functionally coordinated alterations in gene expression associated with neural progenitor proliferation. The majority of differentially expressed genes (65%) were upregulated in NS relative to DC. Microarray analysis of this in vitro system was followed by high throughput screening in situ hybridization to identify genes enriched in the germinal neuroepithelium, so as to distinguish those expressed in neural progenitors from those expressed in more differentiated cells in vivo. NS cultures were characterized by the coordinate upregulation of genes involved in cell cycle progression, DNA synthesis, and metabolism, not simply related to general features of cell proliferation, since many of the genes identified were highly enriched in the CNS ventricular zones and not widely expressed in other proliferating tissues. Components of specific metabolic and signal transduction pathways, and several transcription factors, including Sox3, FoxM1, and PTTG1, were also enriched in neural progenitor cultures. We propose a putative network of gene expression linking cell cycle control to cell fate pathways, providing a framework for further investigations of neural stem cell proliferation and differentiation.

Replicative enzymes and ageing: importance of DNA polymerase alpha function to the events of cellular ageing

A hallmark of cellular ageing is the failure of senescing cells to initiate DNA synthesis and transition from G1 into S phase of the cell cycle. This transition is normally dependent on or concomitant with expression of a set of genes specifying cellular proteins, some of which directly participate in DNA replication. Deregulation of this gene expression may play a pivotal role in the ageing process. The number of known enzymes and co-factors required to maintain integrity of the genome during eukaryotic DNA replication has increased significantly in the past few years, and includes proteins essential for DNA replication and repair, as well as for cell cycle regulation. In eukaryotic cells, ranging from yeast to man, a replicative enzyme essential for initiation of transcription is DNA polymerase alpha (pol alpha), the activity of which is coordinately regulated with the initiation of DNA synthesis. DNA pol alpha, by means of its primase subunit, has the unique ability to initiate de novo DNA synthesis, and as a consequence, is required for the initiation of continuous (leading-strand) DNA synthesis at an origin of replication, as well as for initiation of discontinuous (lagging-strand) DNA synthesis. The dual role of the pol alpha-primase complex makes it a potential interactant with the regulatory mechanisms controlling entry into S phase. The purpose of this review is to address the regulation and/or modulation of DNA pol alpha during ageing that may play a key role in the cascade of events which ultimately leads to the failure of old cells to enter or complete S phase of the cell cycle.

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.

Valine, not methionine, is amino acid 106 in human cytosolic thymidine kinase (TK1). Impact on oligomerization, stability, and kinetic properties

Cytosolic thymidine kinase (TK1) cDNA from human lymphocytes was cloned, expressed in Escherichia coli, purified, and characterized with respect to the ATP effect on thymidine affinity and oligomerization. Sequence analysis of this lymphocyte TK1 cDNA and 21 other cDNAs or genomic TK1 DNAs from healthy cells or leukemic or transformed cell lines revealed a valine at amino acid position 106. The TK1 sequence in NCBI GenBank(TM) has methionine at this position. The recombinant lymphocyte TK1(Val-106) (rLy-TK1(Val-106)) has the same enzymatic and oligomerization properties as endogenous human lymphocyte TK1 (Ly-TK1); ATP exposure induces an enzyme concentration-dependent reversible transition from a dimer to a tetramer with 20-30-fold higher thymidine affinity (K(m) about 15 and 0.5 microm, respectively). Substitution of Val-106 with methionine to give rLy-TK1(Met-106) results in a permanent tetramer with the high thymidine affinity (K(m) about 0.5 microm), even without ATP exposure. Furthermore, rLy-TK1(Met-106) is considerably less stable than rLy-TK1(Val-106) (t(12) at 15 degrees C is 41 and 392 min, respectively). Because valine with high probability is the naturally occurring amino acid at position 106 in human TK1 and because this position has high impact on the enzyme properties, the Val-106 form should be used in future investigations of recombinant human TK1.

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.

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.

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.

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.

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.

Analysis of sequence-specific binding activity of cis-elements in human thymidine kinase gene promoter during G1/S phase transition

Expression of thymidine kinase (TK) gene in normal human diploid, cells is both cell cycle and age dependent and appears to be transcriptionally regulated. Several studies have indicated that the G1/S control sequence may reside within the region of about 130 bp upstream of the transcription initiation site. We have previously shown that a trans-acting factor, CBP/tk (CCAAT binding protein for TK gene), binds to either one of the two inverted CCAAT boxes in a cell cycle- and age-dependent manner (Pang and Chen, 1993, J. Biol. Chem., 268:2909-2916). An upstream 25 bp fragment (-109/-84), containing both Yi-like and E2F-like binding sites, has recently been proposed to be essential for the G1/S regulation of human TK gene. To assess the contribution of various cis-elements in human TK promoter to the G1/S regulation, we have examined the binding activity of these cis-elements in the nuclear extracts derived from human IMR-90 cells at low passage number. Our results indicated that no binding activity could be detected using either the 25 bp fragment (-109/-94) or the authentic Yi sequence. However, Yi binding activity was observed in SV-40 transformed IMR-90 cells. In contrast, the 28 bp fragment (-91/-64) that contains the distal inverted CCAAT box exhibited a strong binding in serum-stimulated young IMR-90 cells. The binding of CBP/tk to the 28 bp fragment was abolished by a single base mutation in the CCAAT box. The CBP/tk binding of the 28 bp fragment could not be displaced by either the 25 bp fragment or the authentic Yi element. A deletion of the 5'-flanking region of the 28 bp fragment up to 5 bases also abolished the binding activity. The CBP/tk binding in IMR-90 cells was supershifted by antiserum against NF-Ya, but not by antiserum made against p107, pRb, cyclin A, p33cdk2, or p34cdc2. Taken together, our results suggest that the G1/S regulatory cis-element in human TK promoter may be confined only to CBP/tk binding sites.

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.

Human thymidine kinase 1. Regulation in normal and malignant cells

In mammalian cells, salvage pathway phosphorylation of thymidine is catalyzed by two thymidine kinases: the cell-cycle regulated cytoplasmic TK1 and the constitutively expressed mitochondrial TK2. Since TK1 is virtually absent in non-dividing cells, TK2 is probably the only thymidine kinase present in these cells. In cellular metabolism, TK1 and TK2 presumably serve to maintain sufficient dTTP for DNA replication and repair. TK1 purified from phytohemagglutinin-stimulated human lymphocytes is a dimer in the absence and a tetramer in the presence of ATP. In addition to the molecular weight transition, incubation with ATP at 4 degrees C or storage with ATP induces a reversible, enzyme concentration-dependent, kinetically slow transition from a low to a high affinity form of TK1, with Km values of 14 microM and 0.5 microM, respectively. This affinity difference implies that at cellular thymidine concentrations, the difference in catalytic activity between the two TK1 forms will be 3-5-fold. Calculations of cellular TK1 concentration suggested that the low affinity dimer form was dominant in G0/G1 cells and the high affinity tetramer form in S-phase cells. Hence, the transition may serve to fine-tune the cell-cycle regulation of thymidine kinase activity on the post-translational level. To study the ATP effect on the molecular level, an IPTG inducible T7 RNA polymerase-dependent expression system for the entire human TK1 polypeptide in E. coli was established. The recombinant TK1 has the same subunit mass and specific activity as the native enzyme. However, the recombinant TK1 solely displayed the kinetics of the high affinity form, with Km values of 0.3-0.4 microM regardless of pre-exposure to ATP, indicating that the ATP effect may be dependent on post-translational modifications absent in E. coli. Surprisingly, we did not observe any effect of ATP on TK1 purified from bone-marrow cells from a patient with acute monocytic leukemia (AMOL). Furthermore, the Km values of TK1 from these cells were 45 microM for the ATP-free enzyme and 65 microM for the ATP-incubated enzyme. With TK1 purified from HL-60 cells, we obtained the same pattern and kinetic values as for TK1 from lymphocytes. In the light of the results with the recombinant TK1, we presume that the lack of ATP effect and very high Km values observed for the AMOL TK1 may be due to changes in post-translational regulatory mechanisms in acute monocytic cells.

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

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

Analysis of thymidine kinase gene expression in preimplantation mouse embryos

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

International Commission for Protection Against Environmental Mutagens and Carcinogens. Deoxyribonucleoside triphosphate levels: a critical factor in the maintenance of genetic stability

DNA precursor pool imbalances can elicit a variety of genetic effects and modulate the genotoxicity of certain DNA-damaging agents. These and other observations indicate that the control of DNA precursor concentrations is essential for the maintenance of genetic stability, and suggest that factors which offset this control may contribute to environmental mutagenesis and carcinogenesis. In this article, we review the biochemical and genetic mechanisms responsible for regulating the production and relative amounts of intracellular DNA precursors, describe the many outcomes of perturbations in DNA precursor levels, and discuss implications of such imbalances for sensitivity to DNA-damaging agents, population monitoring, and human diseases.

Expression and role of c-myc protooncogene in murine preimplantation embryonic development

PURPOSE

The present study was conducted to investigate the expression and possible role of the c-myc protooncogene in preimplantation embryos by using reverse-transcriptase/polymerase chain reaction (RT-PCR) technique and microinjection of synthetic antisense c-myc oligonucleotide probe, respectively. Total RNA was extracted from oocytes and two cell-, four cell-, early morula-, late morula-, early blastocyst-, and late blastocyst-stage embryos, and cDNA was constructed using MMLV reverse transcriptase. Sense primer (P1) and antisense primer (P2) used were based on the c-myc gene sequence bp 1609-1629 and bp 3279-3299, respectively, that span a 1.37-kb intron. PCR-amplified products of cDNA from oocyte-, two cell-, four cell-, early and late morula-, and blastocyst-stage embryos demonstrated the expected 313-bp product in Southern blot hybridization using a c-myc specific DNA probe, with an indication of lower levels in oocytes and early morulae.

RESULTS

Cytoplasmic injection of the antisense c-myc oligonucleotide probe (P2) and not the sense probe (P1) into pronuclear-stage zygotes caused a significant (P = 0.02 to 0.0001) inhibition of development to blastocysts in a concentration-dependent manner, with a maximal inhibition at the first cleavage of zygotes to two cell-stage embryos. There was no effect on the P2 antisense injection on pronucleus formation.

CONCLUSION

These results indicate that the c-myc protooncogene is expressed in preimplantation embryos and may have an essential role in normal embryogenesis in mice.

Reversible shutdown of replicon initiation by transient hypoxia in Ehrlich ascites cells. Dependence of initiation on short-lived protein

The O2-dependent regulation of replication in Ehrlich ascites cells, characterized by a reversible shutdown of replicon initiation during hypoxia, was scrutinized with respect to the involvement of gene expression. Synchronous and asynchronous cells were subjected to transient hypoxia and examined for expression of selected 'late' growth-regulated mRNA and for the influence of inhibitors of transcription and translation on DNA replication. Irrespective of whether replicon initiation was suppressed by hypoxia or retriggered by reoxygenation, the levels of thymidine kinase mRNA and of proliferating cell-nuclear antigen/cyclin mRNA were as high as in untreated replicating cells. The level of histone H3.1 mRNA followed, with a distinct delay, the replicative activity of the cells governed by the imposed changes of pO2. The response of replication to inhibition of transcription and translation was virtually the same as to hypoxia, i.e. a selective suppression of replicon initiation. It was demonstrated that replicon initiation depends on one or several short-lived protein(s) (lifetime about 5 min) which is (are) formed under hypoxic conditions as well. The lifetime of the corresponding RNA message(s) is in the range of several hours. It is suggested that the expression of genes conditioning resting cells for DNA replication remains unaffected by hypoxia or by restoring the normal pO2. Hypoxic cell appear to rest in a state fully prepared for entering DNA replication, but a yet unknown event essential for replicon initiation is blocked. This event depends on a critical oxygen tension as well as on short-lived protein(s).

Transcription initiation and temporal expression of thymidine kinase mRNA in Chinese hamster cells

The induction of thymidine kinase mRNA has proved to be a valuable model for understanding regulatory events at the G1/S boundary of the cell cycle (1, 2, 3). As an initial step toward characterizing the regulation of this gene in Chinese hamster cells, I have mapped the transcription start sites for TK mRNA in CHEF/18 cells. Two closely spaced sites of transcription initiation were detected downstream of a nonconsensus TATAA element in the promoter region. Using primer extension analyses, I demonstrated that the transcription initiation sites remained constant while the absolute levels of TK mRNA varied during the cell cycle in synchronized populations of Chinese hamster cells.

Protein binding sites within the human thymidine kinase promoter

DNase I footprint analysis, using total HeLa cell nuclear extract and purified transcription factor Sp1, was carried out to determine the various protein binding sites within the human thymidine kinase promoter. The promoter has two separate CCAAT elements and multiple Sp1-binding sites, as well as at least one undefined protein binding site. Detailed analysis of protein binding to the two CCAAT elements showed that changing the spacing between the two CCAAT elements altered both protein binding to the distal CCAAT element as well as promoter activity. Both CCAAT elements can act as functional transcription elements, but are not oriented for optimal promoter strength in the human tk promoter. Our studies show that a promoter fragment that has been previously shown to be the minimal region to maintain a serum responsive promoter regulation apparently contains only a single Sp1-binding domain and the more distal of the CCAAT elements.

Marek's disease virus-mediated enhancement of avian leukosis virus gene expression and virus production

Direct interaction between two viruses in coinfected cells may promote replication and pathogenesis of one or both virus types. Synergism between herpesviruses and retroviruses is an important factor in diagnosis, treatment, and prevention of animal and human diseases. In birds, Marek's disease virus (MDV) may be an important cofactor in avian leukosis virus induced disease. Infection of susceptible cells with non-oncogenic serotype 2 MDV, an avian herpesvirus, and Rous-associated virus type 2 (RAV-2 ALV), a leukemogenic avian retrovirus, results in enhanced (greater than 3-fold) transcription of retroviral genes, relative to infection with ALV alone. A direct relationship between concentrations of retroviral gene expression and amount of input MDV suggests that MDV-encoded or -induced factors are responsible for enhanced ALV gene expression, ultimately leading to increased accumulation of ALV-specific RNA (greater than 5-fold) and protein (greater than 10-fold). At lower doses of input MDV, ALV virus production increased over 3-fold, relative to cells infected with ALV alone. Interactive laser cytometry was used to detect accumulation of both MDV and ALV antigens within single cells from coinfected cultures. These results suggest a direct role for MDV-encoded or -induced factors in enhancement of ALV gene expression and demonstrate the importance of herpesviruses as cofactors in retrovirus replication and pathogenesis in coinfected cells.

Temporal events regulating the early phases of the mammalian cell cycle

It is proposed that the regulation of the pathways directing mammalian cell cycle progression involves several oncogenes. A summary of what is known about some of these regulatory oncogenes (fos, jun, myc, and Rb-1) and where they might function in the progression of a cell from G0 to G1 and G1 to S is presented. Data on two replication-dependent genes, those encoding histones and thymidine kinase, respectively, are also presented as models for describing transcriptional and post-transcriptional events at the G1-S border.

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

Thymidine kinase (TK) activity was measured in relation to the cell cycle of in vivo growing ascites tumour cells. The cells were synchronized by means of centrifugal elutriation and the cell cycle composition of the cell fractions was determined by flow cytometry. TK activity was low in G1, increased during S phase and declined in G2. A half-life of TK activity of about 45 min was found throughout the cell cycle. Four isoenzymes at pI values of 4.1, 5.3, 6.9 and 8.3, denoted as isoenzymes 1-4, were identified using isoelectric focusing. Isoenzymes 3 and 4 were responsible for the profound cell cycle related changes in the TK activity. Corresponding isoenzymes were also found in the fetal mouse liver. In the adult mouse liver isoenzyme 2 was the dominating isoenzyme. The half-life of the isoenzymes was in the same range as for the total TK activity. We conclude that the low TK activity in G1 is due to degradation of the enzyme in G2 at a normal rate combined with an arrest in the synthesis of TK. We also conclude that isoenzyme 4 and the intermediate isoenzyme 3, which had earlier been suggested to be a mitochondrial form of TK, in fact represent cytoplasmatic forms of TK. According to cell cycle and pI studies, isoenzyme 2 belongs to the mitochondrial form. Studies with various phosphor donors and specific substrates, however, indicate that it also contains a cytoplasmic component.

A new class of reversible cell cycle inhibitors

The effects of three compounds on the cell cycle of HL-60 promyeloid leukemia cells has been examined. Ciclopirox olamine, an antifungal agent, and the compound Hoechst 768159 reversibly block the cell cycle at a point occurring roughly 1 h before the arrest mediated by aphidicolin, an inhibitor of DNA polymerase alpha activity, which acts in early S phase. Similar results are also obtained with the compound mimosine, a plant amino acid. Based on these data, it is concluded that all three agents inhibit cell cycle traverse at or very near the G1/S phase boundary and identify a previously undefined reversible cell cycle arrest point.

Suppression of programmed death and G1 arrest in B-cell hybridomas by interleukin-6 is not accompanied by altered expression of immediate early response genes

The murine B-cell hybridoma B9 requires interleukin-6 (IL-6) for its survival and proliferation in vitro. We show here that withdrawal of IL-6 from B9 cultures results in programmed death, concomitant with arrest of the cells in the G1 phase of the cell cycle. Unlike several other systems that undergo programmed cell death, no induction of transcripts corresponding to the testosterone-repressed message-2 or transglutaminase genes is observed during this process. Upon readdition of IL-6 to G1-arrested B9 cells, viability is maintained and entry into S phase occurs after a lag period of 10 to 12 hr. Northern blot analysis showed that the immediate-early mRNAs normally induced shortly after growth factor stimulation in quiescent fibroblasts (c-fos, c-jun, Egr-1, c-myc, JE, and KC), and other growth-related genes (2F1, c-Ha-ras, and p53), are either not induced or remain unchanged during G1 to S phase progression. A correlation was found, however, between the temporal pattern of expression of several G1/S phase genes (dihydrofolate reductase, thymidine kinase, transferrin receptor, and histone H3) and DNA synthesis. These results demonstrate that IL-6-induced viability and growth of hybridoma (and, presumably, plasmacytoma) cells is mediated via novel signal transduction pathways.

Bidirectional promoter activity of the 5' flanking region of the mouse thymidine kinase gene

The 5' flanking region of the gene coding for cytoplasmic thymidine kinase (TK) in the mouse (a total of 490 bp upstream of the initiation codon) was tested for promoter activity using the chloramphenicol acetyltransferase gene as reporter. It was found that the region can be divided into two parts, one of which carries promoter activity in the direction of TK, whereas the 5'-half has promoter activity in the opposite direction. A fragment of 140 bp was sufficient for growth-dependent promoter activity in the direction of TK, although about 100 bp further upstream, enhanced the activity. Expression from the divergent promoter was independent of cell growth.