Targeted transgenic overexpression of mitochondrial thymidine kinase (TK2) alters mitochondrial DNA (mtDNA) and mitochondrial polypeptide abundance: transgenic TK2, mtDNA, and antiretrovirals

Mitochondrial toxicity limits nucleoside reverse transcriptase inhibitors (NRTIs) for acquired immune deficiency syndrome. NRTI triphosphates, the active moieties, inhibit human immunodeficiency virus reverse transcriptase and eukaryotic mitochondrial DNA polymerase pol-gamma. NRTI phosphorylation seems to correlate with mitochondrial toxicity, but experimental evidence is lacking. Transgenic mice (TGs) with cardiac overexpression of thymidine kinase isoforms (mitochondrial TK2 and cytoplasmic TK1) were used to study NRTI mitochondrial toxicity. Echocardiography and nuclear magnetic resonance imaging defined cardiac performance and structure. TK gene copy and enzyme activity, mitochondrial (mt) DNA and polypeptide abundance, succinate dehydrogenase and cytochrome oxidase histochemistry, and electron microscopy correlated with transgenesis, mitochondrial structure, and biogenesis. Antiretroviral combinations simulated therapy. Untreated hTK1 or TK2 TGs exhibited normal left ventricle mass. In TK2 TGs, cardiac TK2 gene copy doubled, activity increased 300-fold, and mtDNA abundance doubled. Abundance of the 17-kd subunit of complex I, succinate dehydrogenase histochemical activity, and cristae density increased. NRTIs increased left ventricle mass 20% in TK2 TGs. TK activity increased 3 logs in hTK1 TGs, but no cardiac phenotype resulted. NRTIs abrogated functional effects of transgenically increased TK2 activity but had no effect on TK2 mtDNA abundance. Thus, NRTI mitochondrial phosphorylation by TK2 is integral to clinical NRTI mitochondrial toxicity.

Exogenous wt-p53 enhances the antitumor effect of HSV-TK/GCV on C6 glioma cells

OBJECTIVE

To study on the antitumor effect of combining wt-p53 gene with suicide gene therapy (HSV-tk+GCV) for malignant gliomas.

METHODS

AdCMV-p53 was transfected into C6 glioma cells at MOI of (Multiplicity of infection) 0(G100), 10(TPG1), 100(TPG2), then AdCMV-tk was transducted to C6 glioma cells of G100, TPG1 and TPG2, respectively, at MOI of 100. The C6 glioma cells tranfected with both AdCMV-p53 and AdCMV-tk were exposed to various concentration of GCV. The cell survival rate was measured by MTT assay in vitro. Rat glioma model was established by injecting 5 x 10(5) C6 glioma cells into right caudate nucleus of SD rats. AdCMV-p53 and AdCMV-tk were injected into glioma on day 5 and 6, respectively. On day 7, ganciclovir (GCV) was administrated intraperitoneally at 15 mg/kg/day for 14 days. The survival time of all rats was observed. The growth of intracerebral tumors was monitored dynamically by enhanced MRI. Cell apoptosis was evaluated by TUNEL method. Expression of HSV-tk gene was identified by in situ hybridization and expression of exogenous p53 gene was detected with Western blotting.

RESULTS

In vitro, wt-p53 significantly enhanced antitumor effect of HSV-tk/GCV. The concentration of GCV for ID50 of TPG2 cells (0.001 microg/ml GCV) was 10 times lower than that for the cells of tk-GCV group (MOI = 100), while the concentration of GCV for ID100 of TPG2 (0.01 microg/ml GCV) and TPG1(0.1 microg/ml GCV) was 100 and 10 times lower than that for the cells of tk-GCV group (MOI = 100), respectively. Apoptosis of C6 glioma cells also could be induced by transfection with wt-p53 gene slightly. For in vivo study, the survival time of tumor-bearing rats treated with HSV-TK/GCV or wt-p53 combined with HSV-TK/GCV was significantly prolonged and the intracerebral tumors were regressed and disappeared earlier in the combined gene therapy group than those in the HSV-TK/GCV therapy group as shown in enhanced MRI. However, only half dose of GCV for the rats treated with both wt-p53 and HSV-TK/GCV was needed to obtain the same efficacy as those rats treated with HSV-TK/GCV alone. These results indicate that the transfection of wt-p53 potentiates the effect of HSV-TK/GCV therapy.

CONCLUSIONS

The combination of HSV-tk/GCV system with wt-p53 gene transduction is optimal for clinical therapeutic trials of suicide gene therapy for malignant gliomas.

Synthesis and Evaluation of [18F] Labeled Pyrimidine Nucleosides for Positron Emission Tomography Imaging of Herpes Simplex Virus 1 Thymidine Kinase Gene Expression

Synthesis of three novel 2'-deoxy-2'-[18F]fluoro-1-beta-D-arabinofuranosyluracil derivatives [18F]FPAU, [18F]FBrVAU, and [18F]FTMAU is reported. The compounds were synthesized by coupling of 1-bromo-2-deoxy-2-fluoro sugars with corresponding silylated uracil derivatives. In vitro cell uptake indicated that all three compounds are taken up selectively in RG2TK+ cells with negligible uptake in RG2 cells. The results indicate that [18F]FBrVAU and [18F]FTMAU have better uptake profiles in comparison to [18F]FPAU and have potential as PET probes for imaging HSV1-tk gene expression.

Effect of serotonin receptor 2 blockage on liver regeneration after partial hepatectomy in the rat liver

The effect of serotonin receptor 2 blockade (5-HT(2)) on liver regeneration after 30-34% and 60-70% partial hepatectomy in the rat liver was investigated.

MATERIALS AND METHODS

Male Wistar rats were subjected to 60-70% (group I) and 30-34% (group II) partial hepatectomy. Serotonin receptor 2 blockade was exerted by intraperitoneal administration of ketanserin at different doses and time points after partial hepatectomy. The rats of all groups were killed at different time points until 96 h after partial hepatectomy. The rate of liver regeneration was evaluated by the mitotic index in hematoxylin and eosin sections, the immunochemical detection of Ki67 and proliferating cell nuclear antigens, the rate of [(3)H]-thymidine incorporation into hepatic DNA and liver thymidine kinase enzymatic activity.

RESULTS

Liver regeneration peaked at 24 and 32 h after partial hepatectomy in 60-70% hepatectomized rats. In 30-34% hepatectomized rats liver regeneration peaked at 60 h, whereas low rates of regenerative activity were observed between 24 and 72 h after partial hepatectomy. Ketanserin administration arrested liver regeneration only when administered at 16 h after 60-70% partial hepatectomy. Ketanserin also abrogated the observed peak of regenerative activity at 60 h in 30-34% hepatectomized rats when administered at 52 h after partial hepatectomy. All indices of liver regeneration were affected by ketanserin administration.

CONCLUSIONS

Serotonin receptor 2 blockade can arrest liver regeneration only when administered close to G1/S transition point, and that while serotonin may be a cofactor for DNA synthesis, it does not play a role in initiation of liver regeneration.

Platelet-activating factor (PAF) involvement in acetaminophen-induced liver toxicity and regeneration

Acetaminophen-induced toxicity has been attributed to cytochrome P-450-generated metabolites, which covalently modify target proteins. However, the mechanism of liver injury pathogenesis needs to be further elucidated. Platelet-activating factor (PAF) is one of the mediators involved in inflammatory tissue alterations associated with acute liver failure. In this study, alterations in blood PAF levels and the serum activity of PAF-acetylhydrolase (PAF-AH) were investigated over the time course of liver injury and regeneration induced by acetaminophen treatment in rats. The administration of a toxic dose of acetaminophen (3.5 g/kg) in rats caused acute hepatic injury, as evident by alterations of biochemical (serum enzymes: ALT, AST and ALP) and liver histopathological (degree of inflammation and apoptosis) indices between 20 and 40 h post-treatment. The hepatic damage was followed by liver regeneration, made evident by three independent indices ([3H]thymidine incorporation into hepatic DNA, liver thymidine kinase activity and hepatocyte mitotic index), presenting a peak at 72 h. The PAF levels were elevated at 24 and 28 h, presenting a remarkable peak at 32 h post-treatment. PAF-AH activity presented different kinetics to that of PAF. The enzyme activity was relatively low at all time points examined before the rise in PAF activity, peaking later, at 72, 84 and 96 h. Our data demonstrate that PAF is involved in the pathogenesis of acute liver failure and in augmented compensatory liver tissue repair post-acetaminophen treatment. However, the putative role of PAF during liver toxicity and regeneration remains to be established.

The role of hepatic stimulator substance (HSS) on liver regeneration arrest induced by cadmium

BACKGROUND

The mechanism of cadmium-induced liver regeneration arrest in relation to hepatic stimulator substance (HSS) biological activity was investigated.

MATERIALS AND METHODS

In Wistar rats subjected to 65 - 70% partial hepatectomy, saline, cadmium, cadmium and HSS were administered. The rats were also subjected to 30 - 34% partial hepatectomy. Mitotic index, immunochemistry for PCNA, 3[H]-thymidine incorporation into DNA and thymidine kinase activity were used as indices of liver regeneration. HSS biological activity was evaluated in all groups of rats using a bioassay.

RESULTS

Liver regeneration and HSS activity were arrested by cadmium during the first 24 h after partial hepatectomy. Both in normal and in cadmium-treated rats, the HSS activity was increased and liver regeneration coincided. HSS activity was stable in 30 - 34% hepatectomized rats. HSS administration was able to restore liver regeneration arrest induced by cadmium.

CONCLUSION

The biological activity of HSS increased at the time of G1/S transition of hepatocytes in the cell cycle and no increase was observed with asynchronous G1/S transition (30 - 34% partial hepatectomy). The suppression of HSS biological activity by cadmium seems to represent an important factor for liver regeneration arrest induced by the metal and HSS administration is able to restore liver regeneration.

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

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

Lack of reduction of thymidine kinase activity in stavudine-treated HIV-infected patients

It has been demonstrated that prolonged in vivo or in vitro treatment with some nucleosides analogs may favor the selection of cells with a reduced activity of enzymes involved in the phosphorylation of these drugs leading to a reduced sensitivity to their antiretroviral action. The aim of this study was to evaluate the effect, in vivo, of zidovudine and stavudine treatment on thymidine kinase (TK) activity. The results obtained showed that TK levels in PBMC from naive patients and stavudine-treated patients did not significantly differ (naive TK = 4.16 +/- 1.19 U/mg protein; stavudine TK = 3.65 +/- 1.73 U/mg protein; p = 0.42), suggesting that the treatment with this nucleoside analog is not associated to a defect of TK activity. On the contrary, PBMC from zidovudine-treated patients showed a significant reduction in TK activity compared to naive patients (naive TK = 4.16 +/- 1.19 U/mg protein; zidovudine TK = 2.70 +/- 1.54; p = 0.014. Although the clinical significance of these results has to be established, we can speculate that stavudine and zidovudine, which are presumably phosphorylated by the same cellular kinases, might display a different ability to in vivo select cells with a resistant phenotype.

Suppression of DMBA-induced mouse skin tumor development by inositol hexaphosphate and its mode of action

Inositol hexaphosphate (IP6) or phytic acid, contained in most mammalian cells, has been shown to have anticancer and anti-cell-proliferative effects in several experimental models of carcinogenesis. We investigated the effect of topical application of IP6 on 7,12-dimethylbenzanthracene (DMBA)-induced complete carcinogenesis and on selective critical events of proliferation, differentiation, or apoptosis after DMBA exposure. IP6 inhibited skin tumor development significantly in a dose-dependent manner. IP6 induced the DMBA-inhibited transglutaminase activity. DNA synthesis, as determined by [3H]thymidine incorporation, was suppressed by IP6 in a dose-dependent manner. IP6 also inhibited thymidine kinase enzyme, which is responsible for [3H]thymidine incorporation into DNA. Our results show that topical application of IP6 inhibits DMBA-induced mouse skin tumor development and that IP6 exerts its tumor inhibitory effect probably by modulating proliferation, differentiation, or apoptosis. It seems that IP6 is an effective and potential chemopreventive agent for management of skin tumorigenesis.

P53-dependent cell-killing by selective repression of thymidine kinase and reduced prodrug activation

Selective killing of tumor cells is an important goal for cancer therapeutics. The tumor suppressor transcription factor p53 is absent or mutated in more than 50% of human tumors. Thus, determining approaches that use p53 status to regulate therapy may be an important strategy for attaining cancer selectivity. We have shown previously that a designed transcriptional repressor, K2-5F, strongly and selectively reduces the expression of its target gene MDR1. In this study, we exploited p53 status and the strong repressor activity of K2-5F to establish a system for preferential killing of p53-negative cells. In this system, the expression of K2-5F is induced by p53 in normal cells, and the K2-5F repressor then inhibits the expression of herpes simplex virus thymidine kinase (HSV-TK) driven by an MDR1 minipromoter. In p53-deficient cells, little K2-5F is expressed, and thus HSV-TK is expressed, allowing the cells to be killed by ganciclovir (GCV). K2-5F induced by exogenous p53 dramatically reduced the expression of HSV-TK in human embryonic kidney 293 cells, and it subsequently increased cell survival in response to GCV. To further evaluate this approach in a uniform genetic background, we developed Saos-2 cells stably expressing physiological levels of p53 and paired them with wild-type p53-negative Saos-2 cells. Stable expression of moderate levels of p53 in Saos-2 cells was able to induce the expression of K2-5F and reduce HSV-TK expression and resulted in a modest but distinct protection from GCV toxicity. Thus, this system may be suitable for further development as an approach to selective cancer therapy.

Genotoxicity and toxicity of the potential cancer-preventive agent polyphenon E

The potential health benefits of green tea continue to attract public and scientific interests and are attributed in part to polyphenolic catechin constituents. Polyphenon E (Poly E) is a decaffeinated green tea catechin mixture containing about 50% epigallocatechin gallate and 30% other catechins. We evaluated the toxicity and genotoxicity of Poly E by using two in vitro assays: bacterial mutagenesis in a Salmonella typhimurium-E. coli assay and the L5178Y mouse lymphoma cell thymidine kinase (Tk) gene mutation assay. In addition, we used two in vivo genotoxicity assays: the mouse micronucleus assay and the Big Blue cII transgenic mouse mutation assay. Repeat-dose toxicity evaluations were performed in mice in parallel with the Big Blue transgenic mutation assays. No significant increases in the revertant colonies were found in the bacterial mutagenesis assay, but a significant increase in the mutant frequency (MF) at the Tk locus was observed in the mouse lymphoma test system. We observed toxicity in mice when Poly E was administered at doses of 2,000 mg/kg/day. Lower doses produced no significant increases in micronucleated erythrocytes in the bone marrow of Swiss-Webster mice and no significant increases in cII transgene MF in the liver, lung, or spleen compared with controls. These results indicate that Poly E, although toxic at high doses (2,000 mg/kg/day), poses minimal genotoxic concern. In addition, these studies highlight the importance of using both in vitro and in vivo systems in genetic toxicity screening of pharmaceuticals before they are administered to humans.

Systemic activation of the immune system during ganciclovir treatment following intratumoral herpes simplex virus type 1 thymidine kinase gene transfer in an adolescent ependymoma patient

During ganciclovir treatment of an adolescent ependymoma patient two weeks after intracranial implantation of HSVtk retroviral vector producer cells, increasing numbers of peripheral T- and B-cells were found as well as enhanced T-cell activation and elevated serum levels of interleukin 12 and soluble Fas ligand. These findings suggest the systemic activation of the immune system during ganciclovir treatment in our patient. The induction of an immune response by HSVtk/ganciclovir supports the concept of an anti-tumor vaccination effect by prodrug activating gene therapy systems and may open new promising perspectives for enhancing therapeutic efficiency by combined prodrug activating and immunological gene therapy strategies.

Resistance to 2-chloro-2'-deoxyadenosine of the human B-cell leukemia cell line EHEB

The effects of 2-chloro-2'-deoxyadenosine (CdA, cladribine), an adenosine deaminase-resistant analogue toxic for both proliferating and resting lymphoid cells, were investigated in the human leukemia cell line EHEB, which was derived from a patient with B-cell chronic lymphocytic leukemia. These cells were found to be less sensitive to CdA than B-cell chronic lymphocytic leukemia lymphocytes (approximately 25-fold) and other human lymphoblastic cell lines (10-1000-fold). Phosphorylation of CdA by deoxycytidine kinase and intracellular accumulation of 2-chloro-2'-deoxyadenosine triphosphate (CdATP) were similar in EHEB cells and in other CdA-sensitive cell lines. In contrast, the inhibitory effect of CdA on ribonucleotide reductase activity, which was investigated in situ by the conversion of cytidine into deoxyribonucleotides and its incorporation into DNA, was much less pronounced in EHEB cells than in other human lymphoblastic cells. Accordingly, concentrations of deoxynucleoside triphosphates did not decrease and even tended to rise. Unexpectedly, incorporation of thymidine and deoxycytidine into DNA was increased severalfold after a 24-h incubation with CdA. CdA also increased the activities of deoxycytidine kinase and thymidine kinase approximately 4-fold. Analysis of the cell cycle by flow cytometry showed that after 24 h, CdA provoked an increase in the proportion of cells in S phase, synthesizing DNA. We conclude that the EHEB cell line is resistant to the cytotoxic action of CdA not only because of a lack of inhibition of ribonucleotide reduction but also because CdA, in contrast with its known effects, provokes in this cell line an increase in the proportion of cells replicating their DNA. Unraveling of the mechanism of this effect may shed light on clinical resistance to CdA.

Prostate cancer gene therapy

Cancer-specific gene therapy is still in its infancy. Although the first gene therapy trials were initiated in the late 1980s, it was only more recently that the first successful treatment of a genetic disease was reported.3 The current problems with low efficiency of gene transfer coupled with the immunologic difficulties with certain vectors indicate that more effort needs to be directed at the basic science of gene transfer. Ultimately, successful cancer-specific gene therapy will require combinations of the lessons learned from the ex vivo and in vivo paradigms. The next generation of gene therapy trials likely will focus on combination therapy with conventional chemotherapeutic agents, differentiating agents, or radiation therapy. The obstacles to the development of gene-based human therapeutics (i.e., molecular medicine) are formidable, but the benefits are so great that eventually the technical issues of gene transfer methodology will be worked out, and ultimately this will become the standard of care, not only for inborn errors of metabolism, but also for cancer.

Viral and cellular factors for resistance against antiretroviral agents

Long-term treatment of HIV-1-infected patients with antiretroviral agents may result in failure of therapy, due to the rapid emergence of resistant virus mutants with decreased susceptibility to therapeutic agents. However, in addition to viral resistance other factors, i.e. cellular factors, may contribute to the waning efficiency of chemotherapy. It has been shown in vitro that continuous treatment of cell lines with nucleoside reverse transcriptase inhibitors, such as 3'-azido-2',3'-dideoxythymidine (zidovudine, AZT), may induce decreased activity of cellular thymidine kinase (TK). Measurements of TK activity in ex vivo stimulated peripheral blood mononuclear cells of HIV-1-infected patients who undergo AZT long-term monotherapy as well as combination therapy provide evidence that diminished cellular TK activity may develop. This leads to the assumption that due to long-term treatment with nucleoside analogs, altered drug metabolism in host cells may contribute to inefficient activation of chemotherapeutic agents in HIV-1 patients. Thus, intracellular subtherapeutic levels of the active compounds may develop. In this intracellular environment, selection of resistant virus populations may be promoted. Due to the expanding number of antiretroviral compounds and the requirement for lifelong treatment of HIV-1-infected persons with antiretroviral agents, both viral and cellular resistance mechanisms must be considered in the context of failing chemotherapy.

Administration of herpes simplex-thymidine kinase-expressing donor T cells with a T-cell-depleted allogeneic marrow graft

Administration of donor T cells expressing the herpes simplex-thymidine kinase (HS-tk) with a hematopoietic stem cell (HSC) transplantation could allow, if graft-versus-host disease (GVHD) was to occur, a selective in vivo depletion of these T cells by the use of ganciclovir (GCV). The study evaluates the feasibility of such an approach. Escalating numbers of donor HS-tk-expressing CD3(+) gene-modified cells (GMCs) are infused with a T-cell-depleted bone marrow transplantation (BMT). Twelve patients with hematological malignancies received 2 x 10(5) (n = 5), 6 x 10(5) (n = 5), or 20 x 10(5) (n = 2) donor CD3(+) GMCs/kg with a BMT from a human leukocyte antigen (HLA)-identical sibling. No acute toxicity was associated with GMC administration. An early increase of circulating GMCs followed by a progressive decrease and long-lasting circulation of GMCs was documented. GCV treatment resulted in significant rapid decrease in circulating GMCs. Three patients developed acute GVHD, with a grade of at least II, while one patient developed chronic GVHD. Treatment with GCV alone was associated with a complete remission (CR) in 2 patients with acute GVHD, while the addition of glucocorticoids was necessary to achieve a CR in the last case. Long-lasting CR occurred with GCV treatment in the patient with chronic GVHD. Unfortunately, Epstein-Barr virus-lymphoproliferative disease occurred in 3 patients. Overall, the administration of low numbers of HS-tk-expressing T cells early following an HLA-identical BMT is associated with no acute toxicity, persistent circulation of the GMCs, and GCV-sensitive GVHD. Such findings open the way to the infusion of higher numbers of gene-modified donor T cells to enhance post-BMT immune competence while preserving GCV-sensitive alloreactivity.

A rapid phenotypic assay for detection of acyclovir-resistant varicella-zoster virus with mutations in the thymidine kinase open reading frame

Susceptibility assays by cell culture methods are time-consuming and are particularly difficult to perform with varicella-zoster virus (VZV). To overcome this limitation, we have adapted a functional test of the viral thymidine kinase (TK) in TK-deficient (tdk mutant) bacteria to detect ACV-resistant VZV in clinical samples. After PCR amplification, the complete viral TK open reading frame (ORF) is purified from PCR primers, digested with two restriction enzymes, and ligated in an oriented fashion into a bacterial expression vector. The ligation products are then used to transform tdk mutant bacteria. After transformation, an aliquot of the bacteria is plated onto a plate with minimal medium containing (i) ampicillin to select for plasmids carrying the viral TK ORF and (ii) isopropyl beta-D-thiogalactopyranoside (IPTG) to induce its expression. An identical aliquot of bacteria is also plated onto a medium containing, in addition to the components described above, 5-fluorodeoxyuridine (FUdR). Compared to the number of transformants on FUdR-free medium, the number of colonies carrying TK derived from susceptible strains was reduced by 86%, on average, in the presence of FUdR. In contrast, the number of transformants carrying TK from resistant strains with a mutant TK were reduced by only 4%, on average, on FUdR-containing plates. We have assessed the validity of this assay with cell culture isolates and several clinical samples including two cerebrospinal fluid samples from which no virus could be isolated. This colony reduction assay allowed the correct identification of the TK phenotype of each VZV isolate tested and can be completed within 3 days of receipt of the sample.

The multifunctional deoxynucleoside kinase of insect cells is a target for the development of new insecticides

The antiherpetic agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) was found to be an efficient substrate for recombinant Drosophila melanogaster-deoxyribonucleoside kinase with a K(m) of 4.5 microM and a V(max) of 400 nmol/microg protein/h compared with 1.3 microM and 62.5 nmol/microg protein/h, respectively, for the natural substrate thymidine. Mammalian cytosolic thymidine kinase-1 does not recognize BVDU as a substrate. In sharp contrast to mammalian cells, the insect D. melanogaster and Spodoptera frugiperda (Sf) embryonic cells proved highly sensitive to the cytostatic action of BVDU. BVDU was efficiently metabolized to its 5'-mono-, 5'-di- and 5'-triphosphate derivatives in the insect cell cultures and abundantly incorporated into the insect cell DNA. BVDU prevented the D. melanogaster cells to initiate the S phase of their cell cycle, and exposure of S. frugiperda cells to BVDU led to a dose-dependent retardation of the insect cells in the S phase of their cell cycle. Both inhibition of nucleic acid synthesis (through the 5'-triphosphate of BVDU) and inhibition of thymidylate synthase (through the 5'-monophosphate of BVDU) would account for the cytostatic activity of BVDU against the insect cells. Because of the virtual lack of cytotoxicity of BVDU against mammalian cells, the drug should be considered highly selective in its cytostatic action against the insect cells. When added to the food of S. frugiperda larvae, BVDU caused a remarkable decrease in the weight gain of the larvae and heavily compromised the transformation of the larvae to the pupae and their subsequent adult (moth) phase. Our data indicate that insect multifunctional deoxyribonucleoside kinase should be considered an entirely novel and attractive target in the development of new nucleoside types of highly selective insecticidal drugs.

A novel approach for nasopharyngeal carcinoma treatment uses phenylbutyrate as a protein kinase C modulator: implications for radiosensitization and EBV-targeted therapy

Sodium phenylbutyrate (NaPB) represent a new non-toxic class of compounds with antiproliferative activities to different tumors and has been shown to modulate many gene expressions by inhibiting histone deacetylation and DNA methylation as the major mechanism. Butyrate and other protein kinase C (PKC) activators have been reported to be able to activate virus enzymes. The present work investigates whether NaPB has an antiproliferative effect or modulatory effects on EBV-associated nasopharyngeal carcinoma (NPC) and whether EBV thymidine kinase gene can be activated to make cells susceptible to ganciclovir (GCV) therapy. NaPB treatment displayed a dose- and time-dependent antiproliferative effect on the NPC cell line CNE2. Cell cycle analysis revealed an inhibitory effect of NaPB on G1-S-phase progression. Shortly after NaPB treatment, we found that PKC activity was activated rapidly but also decreased rapidly. Down-regulation of PKC-alpha and translocation of PKC-alpha from the cytosol to membrane were seen by Western blot. The decrease in PKC activity by NaPB corresponds to an enhanced response to radiation on CEN2 cells. Moreover, NaPB up-regulated EBV thymidine kinase activity to render EBV-associated Daudi cells susceptible to killing by GCV. Based on the observations of NaPB as a PKC modulator, the combination of NaPB, GCV, and radiation may provide a potential novel approach for treatment of EBV-associated NPC.

Regulation of the activity and polymerization status of recombinant human cytosolic thymidine kinase by thiols and ATP

The cDNA clone encoding human thymidine kinase (hTK), was expressed in E. coli using a prokaryotic expression vector, pKK 223-3. The kinetics of the recombinant hTK (rhTK) were similar to those of cytosolic TK but not of mitochondrial TK. rhTK was highly purified in the presence of either ATP or dithiothreitol (DTT). The specific activity of rhTK purified in the presence of ATP [rhTK(ATP)] was lower than that of rhTK purified in the presence of DTT [rhTK(DTT)]. Activity of the purified rhTK(ATP) was enhanced by addition of thiols including DTT, cysteine, homocysteine and beta-mercaptoethanol but inhibited by various sulfhydryl reagents such as 5,5'-dithio-bis(2-nitrobenzoic acid). Hence, it was suggested that rhTK is a thiol-type enzyme. Apparent Mr of purified rhTK(ATP) was 100 kDa, which corresponds to the size of a tetramer (25 kDa subunit), while that of purified rhTK(DTT) was 50 kDa, the size of a dimer. The tetramer form of rhTK(ATP) was converted to the dimer by replacement of ATP by DTT. On the other hand, the dimer form of rhTK(DTT) was converted to the tetramer by addition of ATP. Thus, the catalytic activity of human cytosolic TK might be regulated by thiols as well as ATP via its polymerization status.

Immunological defects after suicide gene therapy of experimental graft-versus-host disease

Donor T cells are beneficial for engraftment, immune reconstitution, and antileukemic effects after allogeneic marrow transplantation, but they also cause graft-versus-host disease. Treatment with ganciclovir can control graft-versus-host disease if donor T cells are genetically engineered to express viral thymidine kinase. Clinical protocols with thymidine kinase-expressing T cells currently prescribe the curative use of ganciclovir for genetic immunosuppression only after clinical manifestations of graft-versus-host disease have appeared. The aim of this work was to compare early/preventive versus delayed/curative treatment of GVHD. Here, we found that ganciclovir administered early after experimental marrow transplantation was highly effective in preventing graft-versus-host disease caused by thymidine kinase-expressing T cells, and surviving recipient mice were able to mount a T cell-dependent B cell response. In contrast, curative ganciclovir administration later after transplantation was much less effective in treating graft-versus-host disease and surviving recipients had markedly impaired immune function. These findings should be considered in the development of future clinical trials using thymidine kinase-expressing T cells; to date, such trials have envisaged the use of GCV to treat only declared graft-versus-host disease. The use of thymidine kinase-expressing T cells for conditional elimination of activated T cells after allogeneic marrow transplantation offers a promising new approach for the control of graft-versus-host disease. The versatility of the thymidine kinase/ganciclovir system offers clinical options depending on whether thymidine kinase-expressing T cells are infused at the time of bone marrow transplantation or in a delayed manner, and depending on whether GCV is administered in an early/preventive or delayed/curative manner. The rationale underlying these options is more complex than it may appear and is likely to have a profound impact on the efficacy of such treatments. In the present work, we analyze the immunological impact when GCV is administered in an early/preventive or delayed/curative manner. Our results demonstrate that the delayed/curative strategy is clearly associated with severe immunological defects. To our knowledge, this is the first report of immunodeficiency subsequent to suicide gene therapy for GVHD.

Ultrastructural analysis of bone marrow hematopoiesis in mice transgenic for the thymidine kinase gene driven by the alpha IIb promoter

Transgenic mice have been generated with expression of the herpes virus thymidine kinase gene directed by a 2.7-kb fragment of the alphaIIb murine promoter of the gene encoding the alphaIIb-subunit of the platelet integrin alphaIIbbeta3 (Tropel et al, Blood 90:2995, 1997). Administration of ganciclovir (GCV) to these mice resulted not only in an acute cessation of platelet production due to the depletion of the megakaryocytic lineage, but also a decrease in erythrocyte and leukocyte numbers. Immunogold staining on ultrathin frozen sections and electron microscopy has now shown that the remaining population of immature hematopoietic cells contain a high proportion of Sca-1(+) and CD34(+) cells, with CD45R+ cells of the lymphopoietic lineage being maintained. Stromal cells were also preserved. Blood thrombopoietin levels were high. At 4 days of the recovery phase, Sca-1 and CD34 antigen expression decreased with intense proliferation of cells of the three lineages, with megakaryocyte (MK) progenitors being identified by their positivity for glycoprotein IIb-IIIa. These results suggest that transcriptional activity for the alphaIIb gene promoter was present on pluripotent hematopoietic stem cells. At 6 to 8 days after cessation of GCV, numerous mature MK were observed, some of them with deformed shapes crossing the endothelial barrier through thin apertures. Proplatelet production was visualized in the vascular sinus. After 15 days, circulating platelet levels had increased to approximately 65% of normal. Transgenic alphaIIb-tk mice constitute a valuable model to study in vivo megakaryocytopoiesis.

Ethanol represses thymidylate synthase and thymidine kinase at mRNA level in regenerating rat liver after partial hepatectomy

The effects of ethanol on liver regeneration after partial hepatectomy were investigated. The injection of ethanol inhibited the increases in the activities of thymidylate synthase and thymidine kinase in regenerating rat liver at 24 h after partial hepatectomy in a dose-dependent manner, with a concomitant reduction in DNA content. Northern blot analysis showed that the inhibition of thymidylate synthase and thymidine kinase activities was caused by comparable decreases in their mRNA levels. The immunoblotting assay confirmed the protein levels of thymidylate synthase and thymidine kinase as proportional to the activity and mRNA levels. These findings suggest that ethanol inhibits DNA synthesis by the repression of mRNA levels of dTMP-synthesizing enzymes during liver regeneration.

Structure-activity relationship of the affinity of 5-substituted uracil nucleoside analogues for varicella-zoster virus thymidine kinase and their activity against varicella-zoster virus

We investigated structure-activity relationships of 5-substituted uracil nucleoside analogues for their selective antiviral activity against varicella-zoster virus (VZV) and affinity for VZV thymidine kinase (TK). Anti-proliferative activity of the compounds was measured using human lymphoblastoid cells. Most 2'-deoxyribofuranosyluracil, arabinofuranosyluracil (araU) and 2'-deoxy-2'-fluoro-arabinofuranosyluracil derivatives showed selective anti-VZV activity as well as activity against herpes simplex virus types 1 and 2. 2'-Deoxyuridine derivatives showed higher affinity than the corresponding araU analogues. A correlation was seen between the 50% effective doses for VZV and the Ki values for VZV TK, except for 5-ethyl-2'-deoxyuridine and 5-ethyl araU that showed relatively high affinity for VZV TK without showing any activity against VZV. 5-Halogenovinyluracil nucleosides showed the highest affinity and the most potent and selective anti-VZV activity. 2'-Deoxy-2'-fluoro-arabinofuranosyluracil derivatives exhibited high anti-VZV potency though they showed relatively low affinity for VZV TK. Some 3'-deoxythymidine analogues having anti-human immunodeficiency virus activity were inactive against herpesviruses.

Effects of olomoucine, a selective inhibitor of cyclin-dependent kinases, on cell cycle progression in human cancer cell lines

We have studied the effects of olomoucine, a selective inhibitor of cdk2, cdc2 and MAP kinase, on the rate of proliferation and the cell cycle progression in human cancer cells in culture. Olomoucine inhibited the growth of the KB 3-1, MDA-MB-231 and Evsa-T cell lines in a concentration-dependent manner, with EC50 values of 45, 75 and 85 microM, respectively. Incubation of exponentially growing KB 3-1 cells in the presence of olomoucine led to an increased proportion of cells in G1 phase after 24 h or more of incubation. Olomoucine failed to rapidly affect the phosphorylation of the Rb tumor-supressor gene product. However, [3H]thymidine incorporation into the cell DNA was rapidly inhibited. We show that this inhibition is due, at least in part, to the diminution of thymidine entry into the cells. Surprisingly, all these cell lines, when synchronized at the G1/S interface and relaxed in the presence of olomoucine, progressed unhindered through the S phase. Under these conditions, the G2 phase transit was markedly retarded but not prevented. Insufficient permeability of the cell membrane to olomoucine may explain the low activity of the drug.

Purified herpes simplex thymidine kinase Retrovector particles. I. In vitro characterization, in situ transduction efficiency, and histopathological analyses of gene therapy-treated brain tumors

Replication-defective, highly purified retroviral vectors (Retrovector), at titers of 10(8) colony forming units/mL, were prepared that conferred either beta-galactosidase or herpes simplex thymidine kinase (HSV-TK) activity. 9L gliosarcoma cells, transduced efficiently in vitro, were highly sensitive to ganciclovir (GCV). The mean frequency of in situ transduction, measured by flow cytometry of single-cell tumor suspensions isolated from rat brains, was 3.2 +/- 0.6%; similar assessments were made by staining of beta-galactosidase or by immunohistochemistry with anti-HSV-TK. In vitro HSV-TK-transduced and G418-selected 9L-TK gliosarcoma tumors treated with GCV were eradicated in approximately 53% of the animals (10/19) at day 26, however, 89% (17/19) histologically showed < 1% tumor volume. Histologic evaluation at day 26 of animals with established 9L tumors treated with intralesional injection of HSV-TK vector followed by GCV treatment showed that 29% (4/14) had no tumor; 50% (7/14) had < 1% tumor volume. Regression of tumors proceeded over the time since the complete rate was increased at day 60. Neither HSV-TK vector particles nor GCV alone altered the histological profile of 9L tumors, but substantial numbers of CD4+ and CD8+ lymphocytes infiltrated the tumors of animals treated with both. In cured animals, the former tumor bed contained cell debris, immune cells, and fibroblasts and was without damage to adjacent brain. The efficacy of suicide gene therapy for rat gliosarcoma using highly purified virion vectors approaches that of packaging cell lines.

Human recombinant interferon alpha-2a plus 3'-azido-3'-deoxythymidine. Synergistic growth inhibition with evidence of impaired DNA repair in human colon adenocarcinoma cells

We reported that 3'-azidothymidine-3'-deoxythymidine (AZT) plus 5-fluorouracil or methotrexate produces additive cytotoxicity in HCT-8 cells: a reflection of increased AZT metabolism when de novo thymidylate (dTMP) synthesis was inhibited. We now report that AZT plus human recombinant interferon alpha-2a (rIFN-alpha 2a) produces synergistic growth inhibition in these cells. Evaluation of the effect of rIFN-alpha 2a on dTMP metabolism revealed that exposure to rIFN-alpha 2a (+/-AZT) did not affect dTMP synthase activity significantly but increased thymidine (dThd) kinase activity significantly. Consequently, AZT nucleotide production and incorporation into DNA were increased by coexposure to rIFN-alpha 2a. This alone, however, cannot explain the observed synergism. Therefore, the effect of these agents on DNA excision/repair processes was assessed. Isotope clearance studies demonstrated that rIFN-alpha 2a did not alter the rate of [3H]AZT excision from DNA. In contrast, filter-elution studies revealed that rIFN-alpha 2a (+/-AZT) produced more DNA damage and delayed repair compared with the effects produced by AZT alone. Since DNA polymerases alpha and beta are directly involved in gap-filling repair synthesis, experiments next assessed the effect of rIFN-alpha 2a and/or 3'- azido-3'-deoxythymidine-5'-triphosphate (AZTTP) on their activities. Polymerase alpha was inhibited slightly by AZTTP but not by rIFN-alpha 2a. Polymerase beta activity, however, was inhibited dramatically by rIFN-alpha 2a + AZTTP. Finally, western analysis revealed that a 24-hr exposure to 5000 IU/mL rIFN-alpha 2a (+/-20 microM AZT) significantly reduced wild-type p53 expression compared with AZT-exposed cells. We conclude that rIFN-alpha 2a enhances AZT-induced tumor cell growth inhibition by (i) increasing AZT metabolism, and (ii) inhibiting DNA repair and p53-mediated cell cycle control processes.

Glucocorticoid regulation of a transcription factor that binds an initiator-like element in the murine thymidine kinase (Tk-1) promoter

Glucocorticoids inhibit transcription of the murine cytoplasmic thymidine kinase gene (Tk-1). Glucocorticoid regulation of Tk-1 transcription can be demonstrated in cells that are arrested in late G1. This observation indicates that inhibition of Tk-1 expression is not dependent upon redistribution within the cell cycle but is due to glucocorticoid regulation of this gene. Transfection studies have been carried out using chimeric genes in which restriction fragments of the Tk-1 promoter were fused to chloramphenicol acetyltransferase or neomycin phosphotransferase. These chimeric reporters were assayed for stable expression and glucocorticoid regulation in P1798 lymphoma cells. A 140-bp fragment, extending from -143 to -3 bp with respect to the thymidine kinase translational start site, was capable of both basal and glucocorticoid-regulated transcription of reporter genes. The extent of inhibition by glucocorticoids was similar to that observed for the endogenous gene, and no increase in basal expression or the extent of inhibition was observed with constructs containing additional 5'-flanking DNA. The 140-bp Tk-1 core promoter fragment binds to transcription factors in extracts from P1798 cells. Control cell extracts contain factors that bind to and protect (from deoxyribonuclease I) a distal promoter element from -106 to -87 bp, relative to the translational start site. A second, proximal element was protected at -43 to -36 bp. The proximal element of the Tk-1 promoter resembles an RNA polymerase II initiator element. No other elements were protected. Glucocorticoids inhibit the amount or activity of the transcription factor that binds to this initiator-like element within the Tk-1 promoter. This element, when fused to upstream activation sequences from the herpes simplex virus thymidine kinase promoter, conveys glucocorticoid sensitivity in cis.

Characterization of mutations induced by 300 and 320 nm UV radiation in a rat fibroblast cell line

The cytotoxic and mutagenic activities of monochromatic ultraviolet light (UV) at four wavelengths (254, 290, 300 and 320 nm) were determined using a rat fibroblast cell line CREF stably infected with a retroviral vector carrying the neo and HSV-tk markers. In this system, mutations can be positively detected as acyclovir-resistant colonies. Although the action spectra for these activities closely fit some of the previously reported spectra for photochemical DNA modifications, erythema, cell killing and mouse skin carcinogenesis, they diverge at 320 nm from the absorption spectrum for DNA and the action spectrum for bacterial inactivation and mutagenesis. Structural comparison of the HSV-tk mutants detected after irradiation with 300 and 320 nm UV revealed (1) CC dimers and C oligomers as predominant targets at both wavelengths; (2) increased incidence of relatively large deletions at 300 nm; and (3) greatly increased frequency of tandem double mutations at both wavelengths and of clustered multiple mutations at 320 nm. These results suggest the involvement of distinct mechanisms specifically operating, or becoming evident, in UV-mediated mutagenesis at these different wavelengths in mammalian cells.

Inhibition of thymidylate synthase and thymidine kinase by okadaic acid in regenerating rat liver after partial hepatectomy

The effects of okadaic acid, a potent and specific inhibitor of protein phosphatases 1 and 2A, on liver regeneration after partial (70%) hepatectomy were investigated. The injection of okadaic acid (25 micrograms/kg body weight) inhibited the increases in the activities of thymidylate synthase and thymidine kinase in regenerating rat liver at 24 hr after partial hepatectomy, with a concomitant reduction in DNA content. Northern blot analysis showed that the suppression of thymidylate synthase and thymidine kinase activities was caused by comparable decreases in their mRNA levels. The protein levels of thymidylate synthase and thymidine kinase were confirmed by immunoblotting assay to be proportional to the activity and mRNA levels. These findings suggest that okadaic acid-sensitive protein phosphatases are involved in transcriptional control of the dTMP-synthesizing enzymes during liver regeneration.

HSV-tk gene therapy in head and neck squamous cell carcinoma. Enhancement by the local and distant bystander effect

OBJECTIVES

To determine whether the bystander effect demonstrated in vitro for ganciclovir-mediated killing of a herpes simplex virus thymidine kinase (HSV-tk) gene-infected human squamous cell carcinoma is operative in vivo in a nude mouse model.

DESIGN

Prospective study in a murine model.

INTERVENTION

Human head and neck squamous cell carcinoma tumors were grown as xenografts on the flanks of 20 nude mice. The tumors in the left flank were then infected with the HSV-tk gene. Then, after 48 hours, the animals were treated with intraperitoneal ganciclovir twice daily. Assessment of the tumors on both flanks was performed over a 31-day period.

MAIN OUTCOME MEASURES

Resolution of tumors infected with HSV-tk gene in animals treated with ganciclovir; resolution of tumors uninfected with HSV-tk gene on the contralateral flank in animals treated with ganciclovir.

RESULTS

Following HSV-tk gene therapy in nude mice, complete resolution of HSV-tk-gene-infected human head and neck squamous cell carcinoma tumors was observed following ganciclovir treatment. Uninfected tumors were also noted to regress, but not completely resolve, in response to intraperitoneal ganciclovir (distant bystander effect).

CONCLUSIONS

This study confirms that the local and distant bystander effects exist in this murine model, enhancing the possibility of its role for treatment of human squamous cell carcinoma of the head and neck.

Long-term exposure to zidovudine affects in vitro and in vivo the efficiency of phosphorylation of thymidine kinase

The purpose of this study was to investigate the mechanism of acquired cellular resistance to AZT, a mechanism that has been described as a potential source of drug resistance in addition to viral mutations. To study this phenomenon the kinetics parameters of thymidine kinase (TK) activity have been defined in CEMazt, a cell line previously selected for resistance to AZT, in comparison with the parental AZT-sensitive CEM cells. The results revealed that the value of the maximum velocity (Vmax) of TK activity for deoxythymidine (dThd) phosphorylation is decreased in CEMazt as compared to the wild-type cell line (Vmax: CEM = 105.3 +/- 17.6 nmol/hr/mg of protein; CEMazt = 0.3 +/- 0.02 nmol/hr/mg of protein; p < 0.001). Furthermore, the enzyme affinity versus dThd is lower in CEMazt as compared to CEM (Km: CEM = 0.9 +/- 0.2 microM; CEMazt = 1.6 +/- 0.2 microM; p < 0.01). Consequently phosphorylation efficiency, expressed as the ratio between Vmax and Km, is also reduced in CEMazt (p < 0.001). To evaluate whether such a phenomenon may also occur in patients, ex vivo experiments were carried out by using PBMCs from HIV-infected patients, treated or not treated with AZT. The results (mean values from 10 patients for each group) indicate that a prolonged treatment (> 6 months) with AZT may modify the enzymatic kinetics of TK, leading to a significant reduction in the phosphorylation efficiency of the enzyme (4.07 +/- 1.7 in treated patients versus 13.5 +/- 1.7 in untreated patients; p < 0.001). These results indicate that AZT treatment can also induce a defect in TK activity in patients.

1-(2-Tetrahydrofuryl)-5-fluorouracil in combination with uracil suppresses mammary carcinogenesis and growth of tumors induced with 7,12-dimethylbenz[a]anthracene in rats

The effects of 1-(2-tetrahydrofuryl)-5-fluorouracil in combination with uracil (UFT) on mammary carcinogenesis and growth of tumors induced with 7,12-dimethylbenz[a]anthracene (DMBA) were investigated in rats. Daily oral administration of UFT reduced the incidence and number of mammary tumors compared with those of the DMBA control group, resulting in lower activities in DNA synthesizing enzymes, thymidylate synthetase and thymidine kinase, and a reduction of bromodeoxyuridine-immunoreactive (S-phase) cells in mammary tumors of UFT-treated rats.

Development of a novel mouse tk+/- embryonic stem cell line for use in mutagenicity studies

A tk+/- mouse embryonic stem (ES) cell line, designated 1G2, has been created in which one allele of the thymidine kinase (tk) gene was inactivated by targeted homologous recombination. This line is an analog of the mouse lymphoma tk+/- L5178Y cell line, which is used widely to assess the mutagenicity of chemical agents. Treatment of 1G2 cells with the alkylating agent N-ethyl-N-nitrosourea (ENU) resulted in a dose-related increase in trifluorothymidine-resistant colonies. Mutant frequencies of 152 and 296 per 10(6) cells were determined for 0.1 and 0.3 mg/ml doses of ENU, compared with a spontaneous mutant frequency of 15 per 10(6) cells. The data indicate that tk+/- 1G2 ES cells may be useful for the creation of a transgenic mouse model for assessing in vivo mutation using an endogenous autosomal gene.

[Generation of a trans-complementable defective recombinant provirus and loading a transgene]

This work was aimed at generating a novel system for gene transfer to tumor cell, combining the advantages of non-viral gene transfer methods with those of transfer by recombinant retroviruses. We replaced the env gene of an infectious Moloney murine leukemia provirus with the gene coding for the thymidine kinase of Herpes Simplex Virus 1 (HSV1-TK). The sole transfection of this construction allows the production of viral particles, and the encapsidation of a viral genome carrying transgene. We show that this gene is expressed at a level sufficient for conferring sensitivity to ganciclovir, a nucleoside analog that is metabolised in a toxic compound by HSV1-TK. We also show that the complementation of this recombinant defective provirus with a gene coding for a retroviral envelope, either expressed constitutively by the transduced cell, or by co-transfection, leads to the formation of infectious viral particles capable of transducing HSV1-TK into tumor cells.

Low dose exposure of diethylnitrosamine affects mice liver thymidine kinase

Swiss albino mice exposed to 5 and 10 mg diethylnitrosamine kg-1 body weight by intravenous route up to four weeks showed cyto- and genotoxic effects. Distortion of cell and nucleus shapes and extensive necrosis were observed. Thymidine kinase activity in the liver declined in diethylnitrosamine dose and duration dependent manners. The adult-form of thymidine kinase isozyme declined continuously during this period. Simultaneously, two isozymic forms of thymidine kinase, with small anodic migrations in an electrophoretic field, were gradually induced. Significance of theses changes in diethylnitrosamine induced precarcinogenic toxicity has been discussed.

Suppressive effect of TNF-alpha on increased production of FdUMP by IFN-alpha

Interferon-alpha (IFN-alpha) exhibits synergistic antitumor activity when combined with tumor necrosis factor-alpha (TNF-alpha) in vitro and in vivo and increases the cytotoxicity of 5-fluorouracil (5-FU) in vitro. Using a colon cancer cell line transplanted into nude mice, we examined the effects of pretreatment for 3 days with IFN-alpha (10(6) IU) and/or TNF-alpha (2.9 x 10(3) JRU) on 5-FU metabolism. 5-FU 30 mg/kg was administered after the pretreatment. IFN-alpha increased the tumor level of 5-fluorodeoxuridine monophosphate (FdUMP), and decreased the free level of thymidylate synthetase. Pretreatment with TNT-alpha alone decreased HUMP whereas TNF-alpha plus IFN-alpha abolished the enhancement of FdUMP production by IFN-alpha. TNF-alpha also suppressed thymidine kinase activity. Neither IFN-alpha nor TNF-alpha altered the incorporation of 5-FU into RNA.

Localization of a cycloheximide-sensitivity response element in the human thymidine kinase gene promoter

In order to localize the segment of the human thymidine kinase (TK) gene promoter that mediates sensitivity of TK mRNA expression to the presence of cycloheximide (CX), a series of promoter truncation mutants was prepared between the 460-base pair (bp) promoter that was demonstrated previously to be sensitive to CX and the 83-bp promoter that was demonstrated previously to be insensitive to CX. TK promoters containing 370, 300, 160, or 130 bp of 5'-flanking sequence were all sensitive to inhibition by CX. Further truncation to 100 bp of 5'-flanking sequence eliminated CX sensitivity. Electrophoretic mobility shift assays using a probe containing most of this region (but omitting the SP1 binding site at the 5'-end of the 130-bp promoter) identified some complexes whose formation was sensitive to the presence of CX. Comparison of the sequences of oligonucleotides that were able to compete for formation of mobility shift complexes identified the sequence GCGGCC as a putative CX-sensitivity response element. Two such sequences are found between 83 and 130 bp 5' of the TK capsite. Mutation of the distal sequence attenuated sensitivity of TK mRNA expression to CX, while mutation of the proximal sequence had minimal effect on CX sensitivity. Thus, these data have localized a CX-sensitivity response element to a segment of TK promoter about 120 bp 5' of the capsite that includes the hexamer GCGGCC.

Effects of trifluoromethylaniline isomers on enzyme activities in lymphatic organs and hematology of the rat

Three isomers of trifluoromethylaniline (TFMA) were investigated for their possible different toxic effects on the hematopoietic system in male Wistar rats. The effects of isomeric 2-, 3- and 4-TFMA were compared with those of aniline, the prototypic drug. Strong leukocytosis manifested by considerable increase in the number of all respective white blood elements was observed in the peripheral blood 1 day after the administration of 4-TFMA. In contrast, erythropoiesis, as ascertained by erythrocyte count and hemoglobin concentration, was inhibited by 4-TFMA. The determination of the ED50 revealed lymphocytes to be the most responsive elements towards 4-TFMA administration. Besides hyperemic and proliferative splenomegaly the histological changes in maturation of immunocompetent cells following the 4-TFMA administration were found also in thymus. In accord with an enhanced incorporation of [3H]thymidine, the specific activity of thymidine kinase (TdK) in spleen was increased after a single dose of 4-TFMA. Activities of the catabolic enzymes adenosine deaminase (ADA) and inosine phosphorylase (IP) decreased in both organs with the exception of IP activity in thymus. The effects evoked by the 3-TFMA isomer were regularly less pronounced, and 2-TFMA was nearly inactive.

Certain phosphoramidate derivatives of dideoxy uridine (ddU) are active against HIV and successfully by-pass thymidine kinase

As part of our effort to deliver masked phosphates inside living cells we have discovered that certain phosphate triester derivatives of the inactive nucleoside analogue, dideoxy uridine (ddU) are inhibitors of HIV replication at microM levels. Moreover, we note that certain phosphoramidate derivatives retain their activity in thymidine kinase-deficient cells, which indicates that they do indeed act by intracellular release of the free nucleotide, and that they successfully by-pass the nucleoside kinase. The increased structural freedom in drug design which this allows may have implications for dealing with the emergence of resistance and may stimulate the discovery of improved therapeutic agents.

Stereoelectronic properties of five anti-HSV-1 2'-deoxynucleosides analogues with heterocyclic substituents in the 5-position: a comparison with BVDU

Structural and electronic characteristics of 5-(5-chlorothien-2-yl)-2'-deoxyuridine (I), 5-(furan-2-yl)-2'-deoxyuridine (II), 5-(5-bromofuran-2-yl)-2'-deoxyuridine (III), 5-(3-bromoisoxazol-5-yl)-2'-deoxyuridine (V) and 5-(isoxazol-5-yl)-2'-deoxyuridine (IV) have been determined and compared to the BVDU (VI) characteristics in order to explain their respective affinity for the herpes simplex virus type 1 thymidine kinase (TK). Molecular structure of 5-(5-chlorothien-2-yl)-2'-deoxyuridine has been obtained using single crystal X-ray crystallography. Electrostatic potential maps, energy and topology of frontier orbitals were computed at the ab initio MO STO-3G and STO-3G level. These studies reveal that the electrostatic potential energy maps are clearly dependent on the affinity of the compound for the enzyme.

The effects of lithium and potassium on macromolecular synthesis in herpes simplex virus-infected cells

All herpes simplex virus (HSV) infected cell-specific polypeptides (ICSPs) were synthesized in the presence of lithium at a concentration (60 mM) inhibitory to the production of infectious virus. Yields of certain ICSPs were increased and others, in particular glycoprotein C, decreased. HSV DNA synthesis was completely inhibited; synthesis and in vitro activities of HSV DNA polymerase and thymidine kinase were decreased but to a degree insufficient to account for the complete inhibition of HSV DNA synthesis. HSV DNA synthesis was inhibited to an equivalent degree by either incubation with 60 mM-lithium or by potassium starvation; both procedures decreased intracellular potassium by an equivalent amount as adjudged by X-ray microanalysis. We conclude that lithium inhibits HSV DNA synthesis by displacement of potassium from a potassium-dependent biochemical reaction or by other physiological changes brought about by the loss of cellular potassium. The possibility that lithium also directly inhibits a virus replicative event cannot be excluded.

Inhibition of herpes simplex virus replication and protein synthesis by non-smoked tobacco, tobacco alkaloids and nitrosamines

Inhibitory effects of snuff extract and the tobacco chemicals nicotine, anabasine, diethyl-N-nitrosamine (DEN), and the tobacco-specific nitrosamines (TSNA), N-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) on herpes simplex virus type 1 (HSV-1) replication in vitro and on HSV-1 protein synthesis in infected cells were analysed. Snuff extract and nicotine caused a significant reduction of HSV-1 attachment to cell membranes whereas anabasine, DEN, NNN and NNK did not affect adsorption of HSV-1. Virus production assays in the presence of snuff added after virus adsorption resulted in a significantly reduced production of virus at low multiplicities of infection (MOI), but at high MOI the inhibitory effect of snuff extract was less pronounced. DEN, NNN and NNK only affected virus production at toxic concentrations. Nicotine and anabasine reduced virus production in non-toxic doses but not at the concentrations present in snuff extract. In HSV-infected cells exposed to snuff extract, the immediate early (alpha-) infected cell proteins (ICPs) 4 and 27 (as well as the early (beta-) ICPs 6 and 8) were markedly increased, whereas the late (gamma-) ICPs 5, 11 and 29 were reduced. Nicotine had a less pronounced stimulating effect on the production of alpha-proteins but no detectable effect on production of beta- or gamma-proteins. Anabasine, DEN, NNN and NNK did not affect HSV protein synthesis at non-toxic concentrations. Synthesis of thymidine kinase and DNA polymerase was significantly reduced by snuff extract. Also nicotine and anabasine affected thymidine kinase and DNA polymerase but only at toxic concentrations. The production of the cellular protein actin, which almost disappears a few hours after HSV-1 infection, remained at a significant level in HSV-infected cells exposed to snuff. Thus snuff extract blocks the replicative cycle of HSV at an early stage, which results in an increased production of alpha-proteins in the infected cells and in prolonged maintenance of cellular functions. This may be of importance for HSV-induced transformation and the development of HSV-associated tumours.

Activities of thymidylate synthetase and thymidine kinase in gastric cancer

In a previous clinical study, sequential methotrexate and 5-fluorouracil has shown improved efficacy for treating advanced gastric cancer of the poorly differentiated type. In this study, we investigated whether difference in the levels of thymidylate synthetase (TS) and thymidine kinase (TK) activities in gastric cancer tissue account for selectivity of the treatment. Activity of TS was higher in 19 cases of the poorly differentiated type than in 16 cases of the well differentiated type (P < 0.02), whereas TK activity was lower in the poorly differentiated type than in the well differentiated type (P < 0.01). Thus, the TS/TK ratio was significantly higher in poorly differentiated gastric cancers than in well differentiated cancers (P < 0.001). These results suggest that a greater dependence upon the de novo pathway of pyrimidine synthesis in poorly differentiated gastric carcinomas may enhance the efficacy of sequential methotrexate and 5-fluorouracil treatment.

Deoxypyrimidine-induced inhibition of the cytokinetic effects of 1-beta-D-arabinofuranosyluracil

Ara-U-induced S-phase accumulation and the interaction between high concentrations of ara-U (HiCAU) and ara-C were investigated in L1210 leukemia cells in vitro. Treatment of exponentially growing L1210 murine leukemia cells with ara-U (200-1000 microM) for 48 h caused a dose-dependent accumulation of cells in the S-phase. The extent of this ara-U-induced S-phase accumulation correlated with ara-U incorporation into DNA and with increases of up to 172% and 464% in the specific activities of deoxycytidine kinase and thymidine kinase, respectively, over control values. Metabolism of 1 microM ara-C following the exposure of cells to ara-U (1 mM) resulted in 4.5 pmol araC DNA/mg protein vs 2.1 pmol/mg protein in control cells. Although 48-h exposure of cells to 200 and 400 microM ara-U is not cytotoxic, it enhances the cytotoxicity of ara-C (10-100 microM) 4- to 10-fold. Ara-U-induced S-phase accumulation is inhibited by deoxypyrimidine nucleosides but not by pyrimidine or deoxypurine nucleosides. Some of the ara-U and ara-C concentrations used in this study are achievable in clinical practice, and ara-U/ara-C interactions may explain in part the unique therapeutic utility of high-dose ara-C.

Molecular mechanisms of spontaneous and induced loss of heterozygosity in human cells in vitro

The human TK6 lymphoblast cell line is heteroallelic at the thymidine kinase (TK) locus, with one functional and one nonfunctional allele. Cells that have undergone loss of heterozygosity (LOH) at TK can be selected and cloned in an in vitro assay. In order to study the extent of LOH, we have analyzed a total of 166 thymidine kinase-deficient mutants that arose either spontaneously or following induction by X-ray or ethyl methane sulfonate (EMS) using DNA probes in and around the TK gene on chromosome 17. Two distinct groups of mutants with different doubling times were identified. Among slow-growth mutants, the predominant change for both spontaneous and induced mutants was LOH that generally extended through the entire TK gene to both proximal and distal markers on 17q. While the majority of both spontaneous and X-ray-induced normal-growth mutants showed LOH, this was considerably more localized in scale for X-ray-induced mutants, which rarely involved the distal marker. LOH was rare among EMS-induced normal-growth mutants. LOH was never observed with a 17p marker, indicating that nondisjunctional events were not involved in any of the mutant clones examined. Densitometric analysis of the LOH mutants indicated mitotic recombination was a likely mechanism in more than half the spontaneous LOH mutants in both groups, whereas most induced mutants appeared to arise from simple deletions.