Identification of a 70-base-pair cell cycle regulatory unit within the promoter of the human thymidine kinase gene and its interaction with cellular factors

Thymidine kinase 1 through the ages: a comprehensive review

Proliferation markers, such as proliferating cell nuclear antigen (PCNA), Ki-67, and thymidine kinase 1 (TK1), have potential as diagnostic tools and as prognostic factors in assessing cancer treatment and disease progression. TK1 is involved in cellular proliferation through the recovery of the nucleotide thymidine in the DNA salvage pathway. TK1 upregulation has been found to be an early event in cancer development. In addition, serum levels of TK1 have been shown to be tied to cancer stage, so that higher levels of TK1 indicate a more serious prognosis. As a result of these findings and others, TK1 is not only a potentially viable biomarker for cancer recurrence, treatment monitoring, and survival, but is potentially more advantageous than current biomarkers. Compared to other proliferation markers, TK1 levels during S phase more accurately determine the rate of DNA synthesis in actively dividing tumors. Several reviews of TK1 elaborate on various assays that have been developed to measure levels in the serum of cancer patients in clinical settings. In this review, we include a brief history of important TK1 discoveries and findings, a comprehensive overview of TK1 regulation at DNA to protein levels, and recent findings that indicate TK1’s potential role in cancer pathogenesis and its growing potential as a tumor biomarker and therapeutic target.

The role of thymidine kinase in cancer diseases

Thymidine kinase 1 (TK 1-fetal) is a cell cycle-dependent marker that increases dramatically during the S-phase of the cell cycle. In this review, the authors discuss serum levels of thymidine kinase in a variety of neoplasias. Determination of thymidine kinase helps to monitor the follow-up of solid tumours and haematological malignancies as well as indicating the efficacy of adjuvant and palliative chemotherapy. Elevated levels of thymidine kinase must always be interpreted together with a detailed knowledge of the patient's condition because nonspecific elevations of serum levels (inflammatory and autoimmune diseases) must be excluded.

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.

Multiple androgen response elements cooperate in androgen regulated activity of the type 1 neutral endopeptidase promoter

The neutral endopeptidase (NEP) gene is transcriptionally regulated by androgen in prostate cancer cells. We previously identified in the NEP gene an androgen responsive element (NEP-ARE) and an androgen responsive region (NEP-ARR) that together conveyed only moderate androgen-inducibility [Mol. Cell. Endocrinol. 170 (2000) 131]. Therefore, we characterized the entire genomic structure of the NEP gene and identified ARE1 (ACTCAACAttgTGTCCTTT) and ARE2 (CAGGACAtttTGTCCC), which are located in the 3'-untranslated region and in intron 17, respectively. Steroid-dependent enhancement of transcription was assayed by transfecting the pGL-3-luciferase reporter plasmid containing three copies of ARE1 or ARE2 into PC-3 cells. Luciferase activities were increased 3.6-fold (ARE1) and 5-fold (ARE2) by androgen (AR), 4.2-fold (ARE1) and 8.2-fold (ARE2) by dexamethasone, and 3-fold (ARE1) and 4.1-fold (ARE2) by progesterone. Mutation of the ARE1 and ARE2 sequences completely abrogated androgen-inducibility. We next showed that both ARE1 and ARE2 are involved in the transcriptional regulation of the NEP gene, demonstrating in vitro and in vivo binding with AR as determined by electrophoretic mobility gel shift and chromatin immunoprecipitation (ChIP) assays, Furthermore, ARE1 and ARE2 mediate coordinated androgen-inducibility in both an SV40 promoter and the native NEP type 1 promoter. These data indicate the newly identified ARE1 and ARE2 together with the previously identified NEP-ARE function as androgen response elements, and that androgen regulation of the NEP gene is regulated by the coordinated action of multiple AREs in prostate cancer 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.

CDP and AP-2 mediated repression mechanism of the replication-dependent hamster histone H3.2 promoter

The replication-dependent hamster histone H3.2 promoter contains two tandem CCAAT repeats located upstream of the TATA element. It has been shown that the NF-Y/CBF complex binds to a single CCAAT motif with high affinity, whereas the CCAAT displacement protein (CDP) binds to at least two CCAAT motifs in close proximity. Here, we report that the two CCAAT motifs within the H3.2 promoter confer transcriptional repression of the promoter during the cell cycle. While we cannot detect direct association of CDP with Rb in vitro, we discover that CDP can bind AP-2, a ubiquitous factor that interacts with Rb. The interaction domains between CDP and AP-2 are mapped to the highly conserved cut repeats of CDP as well as the basic and dimerization region of AP-2. Further, in transfection assays, CDP and AP-2 act synergistically to suppress the H3.2 promoter. Together, these data support a repression mechanism mediated by CDP and AP-2 that regulates H3.2 gene expression during the mammalian cell cycle.

Cell cycle regulation of the tobacco ribonucleotide reductase small subunit gene is mediated by E2F-like elements

Ribonucleotide reductase (RNR) is a key enzyme involved in the DNA synthesis pathway. The RNR-encoded genes are cell cycle regulated and specifically expressed in S phase. The promoter of the RNR2 gene encoding for the small subunit was isolated from tobacco. Both in vivo and in vitro studies of the DNA-protein interactions in synchronized BY2 tobacco cells showed that two E2F-like motifs were involved in multiple specific complexes, some of which displayed cell cycle-regulated binding activities. Moreover, these two elements could specifically interact with a purified tobacco E2F protein. Involvement of the E2F elements in regulating the RNR2 promoter was checked by functional analyses in synchronized transgenic BY2 cells transformed with various RNR2 promoter constructs fused to the luciferase reporter gene. The two E2F elements were involved in upregulation of the promoter at the G1/S transition and mutation of both elements prevented any significant induction of the RNR promoter. In addition, one of the E2F elements sharing homology with the animal E2F/cell cycle-dependent element motif behaved like a repressor when outside of the S phase. These data provide evidence that E2F elements play a crucial role in cell cycle regulation of gene transcription in plants.

Cell cycle regulation of the tobacco ribonucleotide reductase small subunit gene is mediated by E2F-like elements

Ribonucleotide reductase (RNR) is a key enzyme involved in the DNA synthesis pathway. The RNR-encoded genes are cell cycle regulated and specifically expressed in S phase. The promoter of the RNR2 gene encoding for the small subunit was isolated from tobacco. Both in vivo and in vitro studies of the DNA-protein interactions in synchronized BY2 tobacco cells showed that two E2F-like motifs were involved in multiple specific complexes, some of which displayed cell cycle-regulated binding activities. Moreover, these two elements could specifically interact with a purified tobacco E2F protein. Involvement of the E2F elements in regulating the RNR2 promoter was checked by functional analyses in synchronized transgenic BY2 cells transformed with various RNR2 promoter constructs fused to the luciferase reporter gene. The two E2F elements were involved in upregulation of the promoter at the G1/S transition and mutation of both elements prevented any significant induction of the RNR promoter. In addition, one of the E2F elements sharing homology with the animal E2F/cell cycle-dependent element motif behaved like a repressor when outside of the S phase. These data provide evidence that E2F elements play a crucial role in cell cycle regulation of gene transcription in plants.

Differential regulation of E2F transcription factors by p53 tumor suppressor protein

The cell cycle is under the control of various positive and negative regulators. Two such regulators are the E2F family of transcription factors and the p53 tumor suppressor protein. While E2F proteins are implicated in promoting the S phase of the cell cycle, p53 has the potential to arrest cells in G1 phase and thereby prevent entry into S phase. Because they perform seemingly opposite functions in the control of cell growth, a possibility of functional interactions between E2F and p53 was investigated. It was found that p53 specifically inhibited activated transcription by E2F-5 but not by E2F-1. Investigation into the mechanism of action established that heterodimer formation and the DNA-binding steps were not significantly inhibited by p53. However, the transcriptional activation step of E2F-5 activity, as examined by using a Gal4 DNA-binding domain chimera, was specifically inhibited by p53. Interestingly, p53 could also inhibit transcriptional activation by E2F-4 but not by E2F-2 or E2F-3. The results indicate that p53 differentially regulates the activities of two subclasses (E2F-1/-2/-3 vs. E2F-4/-5) of E2F transcription factors. Detailed analysis using a two-hybrid approach in mammalian cells indicated lack of physical interaction between p53 and E2F-5, DP-1, or E2F-1. Reciprocal analysis revealed that whereas E2F-1 dramatically inhibited p53-activated transcription, E2F-5 or DP-1 did not. Thus, nonreciprocal functional interactions exist between various members of the E2F family of transcription factors and p53 tumor suppressor protein. The complex interplay between various positive and negative regulators of cell growth, such as E2F and p53 proteins, may be crucial in determining the ultimate outcome in terms of cell cycle arrest, cell growth, or apoptosis.

Intratumoral injection of oligonucleotides to the NF kappa B binding site inhibits cachexia in a mouse tumor model

Cancer cachexia, characterized by anorexia, weight loss and progressive tissue wasting, has been postulated to be mediated by various cytokines. However, the precise mechanism of cachexia induction is not fully explained. We have developed synthetic double-stranded oligodeoxynucleotides (ODN) as 'decoy' cis-elements that block the binding of nuclear factors to promoter regions of targeted genes, resulting in the inhibition of gene transactivation in vivo as well as in vitro. This novel molecular strategy could be useful for treating a broad range of human diseases including cancer. In this study, we injected decoy ODN targeting the transcriptional factor, NF-kappa B (NF kappa B) binding cis-elements, which are essential for transactivation of gene expression of cytokines, directly into tumors of adenocarcinoma colon26 in mice, in order to examine whether or not cachexia is alleviated by inhibiting the action of cytokines. Tumor growth was not affected by transfection of NF kappa B decoy ODN as compared with scrambled decoy ODN. Nevertheless, transfection of NF kappa B decoy, but not scrambled decoy, ODN resulted in attenuation of the reductions in body weight, epididymal fat, gastrocnemius muscle mass and food intake, which were induced by the tumor presence. Interleukin 6 mRNA in the tumor was also markedly decreased by the transfection of NF kappa B decoy ODN. It is known that the transcriptional factor E2F plays a pivotal role in the coordinated transactivation of cell cycle regulatory genes. Therefore, we hypothesized that the introduction of synthetic double-stranded DNA with high affinity for E2F in vivo as 'decoy' cis-elements might inhibit the tumor growth of colon26, resulting in turn in inhibition of cachexia induction. However, injection of E2F decoy ODN failed to inhibit tumor growth and cachexia induction, as compared with mismatched decoy ODN. Overall, the present study demonstrated that cachexia induced by adenocarcinoma colon26 was inhibited by blocking of NF kappa B, using a novel molecular decoy strategy, without an effect on tumor growth, and also that tumor growth and cachexia induction in the colon26 model were not affected by E2F decoy ODN. These results suggest that cytokines regulated by NF kappa B may play a pivotal role in the induction of cachexia by colon26, providing a new therapeutic strategy for cancer cachexia.

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.

The molecular and functional characterization of E2F-5 transcription factor

The E2F activity plays a critical role in the control of cell cycle and action of tumor suppressor proteins and is also a target of the transforming proteins of small DNA tumor viruses. We describe here molecular cloning and functional characterization of a fifth member of the E2F family of transcription factors. E2F-5 protein is more homologous to E2F-4 (72% amino acid identity) than to E2F-1, E2F-2, and E2F-3 (35% amino acid identity). Based on structural and functional criteria, the E2F family appears to comprise two distinct sub-families, one composed of E2F-1, E2F-2, and E2F-3 and the other composed of E2F-4 and E2F-5, E2F-5 mRNA is expressed in a wide variety of human tissues. The protein is expressed as multiple species ranging in size from 46 to 54 kDa as a result of differential phosphorylation. The expression of a reporter gene containing E2F binding sites in the promoter is transcriptionally activated by E2F-5 in a cooperative manner with the DP-1 protein. The interaction between E2F-5 and DP-1 is demonstrated using a two-hybrid system in mammalian cells. We have also demonstrated the presence of a strong transactivation domain at the carboxy terminus (273-346 amino acid residues) of E2F-5 protein.

Cell cycle regulation of histone H4 gene transcription requires the oncogenic factor IRF-2

Histone genes display a peak in transcription in early S phase and are ideal models for cell cycle-regulated gene expression. We have previously shown that the transcription factor interferon regulatory factor 2 (IRF-2) can activate histone H4 gene expression. In this report we establish that a mouse histone H4 gene and its human homolog lose stringent cell cycle control in synchronized embryonic fibroblasts in which IRF-2 has been ablated. We also show that there are reduced mRNA levels of this endogenous mouse histone H4 gene in the IRF-2(-/-) cells. Strikingly, the overall mRNA level and cell cycle regulation of histone H4 transcription are restored when IRF-2 is reintroduced to these cells. IRF-2 is a negative regulator of the interferon response and has oncogenic potential, but little is known of the mechanism of these activities. Our results suggest that IRF-2 is an active player in E2F-independent cell cycle-regulated gene expression at the G1/S phase transition. IRF-2 was previously considered a passive antagonist to the tumor suppressor IRF-1 but can now join other oncogenic factors such as c-Myb and E2F1 that are predicted to mediate their transforming capabilities by actively regulating genes necessary for cell cycle progression.

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.

Cell cycle-dependent regulation of the mouse DNA topoisomerase IIalpha gene promoter

Expression of DNA topoisomerase (topo) IIalpha varies through the cell cycle with its peak in G2/M. To investigate the mechanism controlling the topo IIalpha gene expression, we cloned the 5' upstream region of the mouse topo IIalpha gene. Although there was no TATA-like sequence, two GC and seven CCAAT boxes were found in the upstream region 5' distal to the major transcription start sites, which were located 137, 124, and 105 bp upstream from the ATG start codon. Luciferase vectors with the upstream sequences were constructed and transfected into HeLa cells, followed by cell cycle arrest either in G1 by treatment with mimosine, in S with thymidine, or in G2/M with colcemid. We found that the topo IIalpha gene promoter has the cell cycle-dependent activity, which is low in G1, rises in S, and peaks in G2/M. We suggest that the level of topo IIalpha mRNA is determined by the cell cycle-regulated promoter.

Failure to detect mutations in the retinoblastoma protein-binding domain of the transcription factor E2F-1 in human cancers

The functions of the transcription factor E2F-1 are regulated by the RB protein through the RB-binding domain of E2F-1 and this factor is considered to be an important molecule that functions downstream of the RB protein. In order to determine whether E2F-1 that cannot bind to RB might be associated with various human cancers, we searched for mutations in the RB-binding domain of E2F-1 using samples of DNA from various clinical specimens obtained from 406 cancer patients (with lung, pancreatic, stomach, colon, esophageal, and hepatic cancers) by analysis of polymerase chain reaction-mediated single-strand conformational polymorphism. No mutations or deletions were detected in genes for E2F-1 from any of the tumor tissues examined. These results suggest that a mutation or deletion in E2F-1 that might affect binding of the RB protein is not involved in human cancers.

Isolation and characterization of cDNA for DREF, a promoter-activating factor for Drosophila DNA replication-related genes

DREF, a transcription regulatory factor which specifically binds to the promoter-activating element DRE (DNA replication-related element) of DNA replication-related genes, was purified to homogeneity from nuclear extracts of Drosophila Kc cells. cDNA for DREF was isolated with the reverse-transcriptase polymerase chain reaction method using primers synthesized on the basis of partial amino acid sequences and following screening of cDNA libraries. Deduced from the nucleotide sequences of cDNA, DREF is a polypeptide of 701 amino acid residues with a molecular weight of 80,096, which contains three characteristic regions, rich in basic amino acids, proline, and acidic amino acids, respectively. Deletion analysis of bacterially expressed DREF fused with glutathione S-transferase (GST-DREF) indicated that a part of the N-terminal basic amino acid region (16-115 amino acids) is responsible for the specific binding to DRE. A polyclonal and four monoclonal antibodies were raised against the GST-DREF fusion protein. The antibodies inhibited specifically the transcription of DNA polymerase alpha promoter in vitro. Cotransfection experiments using Kc cells demonstrated that overproduction of DREF protein overcomes the repression of the proliferating cell nuclear antigen gene promoter by the zerknüllt gene product. These results confirmed that DREF is a trans-activating factor for DNA replication-related genes. Immunocytochemical analysis demonstrated the presence of DREF polypeptide in nuclei after the eighth nuclear division cycle, suggesting that nuclear accumulation of DREF is important for the coordinate zygotic expression of DNA replication-related genes carrying DRE sequences.

The age-dependent binding of CBP/tk, a CCAAT binding protein, is deregulated in transformed and immortalized mammalian cells but absent in premature aging cells

CBP/tk, CCAAT Binding Protein for thymidine kinase, has been shown to bind to the distal and proximal CCAAT elements in human TK gene at G1/S boundary in normal human IMR-90 cells after serum stimulation (Pang and Chen, 1993). We now show that the serum-induced binding activity of CBP/tk was inversely related to the population doubling level (PDL) of the normal IMR-90 cells. However, little or almost no CBP/tk binding activity was observed in cells derived from patients with premature aging syndromes (e.g., Werner, Hutchinson-Gilford, and Cockayne syndrome). In contrast, CBP/tk binding activity in SV-40 virus-transformed human cells and in HeLa cells was overexpressed at levels 5- to 15-fold higher than that in normal cells and appeared to be deregulated. The half-life of CBP/tk binding activity in SV-40 transformed cells was at least 10 times longer than that in normal IMR-90 cells, suggesting that posttranslational control may contribute to the deregulation. CBP/tk binding activity detected in other mammalian cells such as murine NIH3T3, an immortal cell line, did not reveal any cell cycle dependence either. Further characterization of CBP/tk binding complex indicates that the binding complex may contain NF-YA and NF-YB and that the binding activity was sensitive to oxidizing reagents. Taken together, our data showed that the age- and cell cycle-dependent nature of CBP/tk is a function of cell types and that CBP/tk binding activity may be subjected to posttranslational and redox regulation.

Introduction to the E2F family: protein structure and gene regulation

E2F is a heterodimer composed of two partners, such as E2F1 and DP1. Although E2F1 can bind DNA as a homodimer and increase promoter activity, optimal DNA-binding and transcriptional activity occurs in the heterodimeric form. A model (Fig. 3) for the involvement of E2F activity in cell growth control that incorporates viral oncoproteins, positive regulators of cell growth (cyclins) and negative regulators of cell growth (tumor suppressor proteins) can now be advanced. Each aspect of this model is addressed in subsequent chapters of this book. It is likely that binding of growth-suppressing proteins, such as Rb, can inhibit the transactivation potential of E2F1, either by blocking the interaction of E2F1 with a separate component of the transcription complex or by bringing a repressor domain to the transcription complex (Flemington et al. 1993; Helin et al. 1993; Weintraub et al. 1992; Zamanian and La Thangue 1993; Zhu et al. 1993). Phosphorylation or sequestration of Rb by viral oncoproteins can free E2F. The influence of viral oncoproteins on E2F activity and the regulation of the different E2F complexes is the focus of the contributions by Cobrinik and by Cress and Nevens. The interaction of the free E2F induces a bend in the DNA that may also play a role in transactivation, perhaps by bringing proteins (such as an Sp1 or CCAAT family member) separated by distance on the promoter DNA into contact (Huber et al. 1994). Because E2F target genes encode proteins critical for cell growth, deregulation of E2F activity can have severe consequences, such as apoptosis or uncontrolled proliferation. The effect of altered expression of E2F activity on the cell cycle and on tumorigenicity is the focus of the contribution by Adams and Kaelin. Finally, a comparison of E2F to the genetically well-characterized factors that regulate G1/S phase transcription in yeast is the subject of the chapter by Breeden. This volume concludes with Farnham's summary of the rapid gains in knowledge concerning the E2F gene family that have been made in the past several years and provides a series of questions and lines of investigation that will be the focus of future studies.

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.

Molecular cloning and characterization of the promoter for the Chinese hamster DNA topoisomerase II alpha gene

To investigate the mechanisms governing the expression of DNA topoisomerase II alpha, the Chinese hamster topoisomerase II alpha gene has been cloned and the promoter region analyzed. There are several transcriptional start sites clustered in a region of 30 base pairs, with the major one being 102 nucleotides upstream from the ATG translation initiation site. Sequencing data reveal one GC box and a total of five inverted CCAAT elements (ICEs) within a region of 530 base pairs upstream from the major transcription start site. Sequence comparison between the human and Chinese hamster topoisomerase II alpha gene promoter regions shows a high degree of homology centered at the ICEs and GC box. In vitro DNase I footprinting results indicate protection by binding proteins at and around each ICE on both DNA strands. However, no obvious protection was observed for the GC box. Competition gel mobility shift assays with oligonucleotides containing either the wild-type or mutated ICE sequences suggest that identical or similar proteins specifically bind at each ICE, although with different affinities for individual ICE sequences. Chloramphenicol acetyltransferase assays employing nested 5'-deletions of the 5'-flanking sequence of the gene demonstrate that the sequence between -186 and +102, which contains three proximal ICEs, is sufficient for near wild-type level of promoter activity. When these three ICEs were gradually replaced with sequences which do not interact with the binding proteins, reducing promoter activity of the resulted constructs was observed. In conjunction with results from footprinting and gel mobility shift studies, the transient gene expression finding suggests that the ICEs are functionally important for the transcriptional regulation of the topoisomerase II alpha gene.

Promoter binding factors regulating cyclin B transcription in the sea urchin embryo

Cyclin B is a key regulatory protein of the cell cycle, central to the control of the G2/M transition. In the developing sea urchin embryo, the cyclin B gene is transcriptionally regulated in concert with changing patterns of cell division. In an effort to understand the mechanism controlling cyclin B expression during development, we have conducted an analysis of the Strongylocentrotus purpuratus cyclin B gene promoter. DNase I foot-printing of the cyclin B upstream region revealed eight binding regions within 435 bp of the start of transcription; seven of these sites were within 215 bp. Found within these regions were consensus sequences for two CCAAT boxes, TATA, and E-boxes and sequences with some similarity to E2F and octamer binding motifs. Upstream sequences were functionally defined by generating cyclin B-CAT fusion genes, containing deletions and base specific mutations, and testing for relative levels of expression by gene transfer. Both CCAAT boxes were found to be essential for maximal levels of expression. A third binding site (PR7) with no recognizable consensus sequence was also found to act as a positive element. Our results suggest that protein binding to the E2F-like sequences may act to reduce expression. Protein binding was further characterized by gel mobility-shift and methylation interference. The CCAAT boxes were found to bind similar, if not identical, proteins. Sequence comparisons and methylation interference data indicate that the likely protein binding these CCAAT sequences is the characterized CCAAT-binding protein CP1. A probe containing site PR7 formed multiple gel shift complexes that, by methylation interference, appeared to be interrelated. One major complex was formed with an oligonucleotide containing the two E2F-like sequences. Protein binding to this probe was specific and required bases within the E2F-like sequences. Our results indicate that cyclin B is subject to positive and negative regulation, involving multiple factors that bind between -200 and -90 bp from the start of transcription.

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.

Selectivity of cell cycle regulation of glucocorticoid receptor function

The restricted expression of some genes to distinct stages of the cell cycle is often brought about through alterations in the activity and/or abundance of specific transcription factors. Many cells have been shown to be unresponsive to glucocorticoid hormone action during the G2 phase of the mammalian cell cycle, suggesting that some activities of the glucocorticoid receptor (GR), a ligand-activated transcription factor, are subjected to cell cycle control. We show here that GR insensitivity in G2 is selective, affecting receptor-mediated transactivation from a simple glucocorticoid response element, but not repression from a composite glucocorticoid response element. Since glucocorticoid-dependent down-regulation of GR protein levels is also unaffected in G2, distinct activities of the receptor that participate in this homologous down-regulation must be operating as effectively in G2-synchronized cells as in asynchronous cells. Finally, the phosphorylation state of the GR is altered in G2-synchronized cells reflecting, in part, both site-specific phosphorylation and dephosphorylation events. These results suggest that, while GR may be a target for cell cycle regulated kinases and phosphatases, the resulting changes in receptor phosphorylation have an impact only on selected GR functions.

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.

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.

Post-transcriptional repression of thymidine kinase expression during cell cycle and growth stimulation

In vertebrates, endogenous thymidine kinase (TK) gene expression is strictly growth-dependent. Here we report that in continuously cycling Ltk-mouse fibroblasts, stably transfected with a vector expressing human TK cDNA from a constitutive promoter, enzyme activity rises 8-fold at the G1/S phase transition and declines again in G2. The mechanism did not involve changes in protein stability. When hTK was put under the control of a hormone-inducible promoter, production of high mRNA levels following addition of dexamethasone did not result in any enzyme activity in resting NIH-3T3tk- cells. After growth stimulation with serum, TK activity rose together with the onset of DNA synthesis only in the simultaneous presence of the hormone.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.

Involvement of transcription factor YB-1 in human T-cell lymphotropic virus type I basal gene expression

Sequences which control basal human T-cell lymphotropic virus type I (HTLV-I) transcription likely play an important role in initiation and maintenance of virus replication. We previously identified and analyzed a 45-nucleotide sequence (downstream regulatory element 1 [DRE 1]), +195 to +240, at the boundary of the R/U5 region of the long terminal repeat which is required for HTLV-I basal transcription. We identified a protein, p37, which specifically bound to DRE 1. An affinity column fraction, containing p37, stimulated HTLV-I transcription approximately 12-fold in vitro. We now report the identification of a cDNA clone (15B-7), from a Jurkat expression library, that binds specifically to the DRE 1 regulatory sequence. Binding of the cDNA fusion protein, similarly to the results obtained with purified Jurkat protein, was decreased by introduction of site-specific mutations in the DRE 1 regulatory sequence. In vitro transcription and translation of 15B-7 cDNA produced a fusion protein which bound specifically to the HTLV-I +195 to +240 oligonucleotide. The partial cDNA encodes a protein which is homologous to the C-terminal 196 amino acids of the 36-kDa transcription factor, YB-1. Cotransfection of a YB-1 expression plasmid increases HTLV-I basal transcription approximately 14-fold in Jurkat T lymphocytes. On the basis of the molecular weight, DNA-binding characteristics, and in vivo transactivation activity, we suggest that the previously identified DRE 1-binding protein, p37, is YB-1.

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.

G1 events and the regulation of genes for S-phase enzymes

The genes for S-phase enzymes are expressed at low levels in quiescent mammalian cells but at high levels during DNA replication. Regulation occurs at multiple levels by mechanisms that are different for each gene. Current research is focused on identifying the control elements and trans-acting factors for each gene and establishing relationships between these regulatory mechanisms and the G1 signal transduction pathway.

Localization and DNA sequence of a replication origin in the rhodopsin gene locus of Chinese hamster cells

A chromosomal origin of DNA replication has been localized within the single-copy rhodopsin gene locus in Chinese hamster (line CHO) cells using two methods. In the first method, single-copy segments were identified at 3 to 15 kb intervals within approximately 75 kb (kb = 10(3) bases) of cloned genomic DNA containing the early-replicating rhodopsin gene near its middle. The cloned single-copy segments were then used as hybridization probes to quantify the replication of their corresponding genomic segments as synchronized cells progressed into S phase. In the second method, genomic DNA synthesized in vivo or in permeabilized early S phase cells was hybridized with slot-blots of the cloned single-copy DNA segments to identify the earliest replicating part of the 75 kb mapped region. The first method indicates that the earliest replicating DNA is located within a 10 kb region beginning 4 kb upstream from and extending 1 kb beyond the rhodopsin gene. The second method confirms the location in the vicinity of the rhodopsin gene and indicates that the earliest replicating region is located within or very near the 4.5 kb rhodopsin gene itself. An extended region of 12 kb that encompasses the entire early-replicating region has been sequenced for analysis and comparison with currently characterized origin regions associated with the CHO dihydrofolate reductase (dhfr) and human c-myc genes. There are several sequence similarities between the dhfr rhodopsin origin regions, including common transcription promoter consensus sequences, rodent Alu repeats with their 3'-A+T rich flanking sequences, A+T-rich yeast ARS and Drosophila SAR consensus sequences, and simple (GA)n repeats, but there are no extended regions of direct similarity. The rhodopsin gene locus is the second sequenced CHO origin region.

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.

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.