Expression of the type I DNA topoisomerase gene in adenovirus-5 infected human cells

The feasibility of gene therapy in the treatment of head and neck cancer

Standard approach to the treatment of head and neck cancer include surgery, chemotherapy, and radiation. More recently, dramatic increases in our knowledge of the molecular and genetic basis of cancer combined with advances in technology have resulted in novel molecular therapies for this disease. In particular, gene therapy, which involves the transfer of genetic material to cells to produce a therapeutic effect, has become a promising approach. Clinical trials concerning gene therapy strategies in head and neck cancer as well as combination of these strategies with chemotherapy and radiation therapy will be discussed.

Replication-deficient adenovirus induces host topoisomerase I activity: implications for adenovirus-mediated gene expression

Replication-deficient adenoviruses are useful vectors for the transfer of therapeutic transgenes to malignant and non-malignant tissues. Yet their clinical application is limited by the potential toxicity of viral infection and the transient nature of transgene expression. Although transgene expression from adenovirus vectors is initially higher than expression of transgenes transduced by other viral or non-viral vectors, it is often insufficient to generate a significant therapeutic effect. We addressed this issue by searching for DNA-targeted viral-induced host responses potentially restricting transgene expression. Nuclear protein extracts from livers of rats systemically infected with replication-deficient adenovirus exhibited enhanced topoisomerase I activity compared with extracts from uninfected animals. Consequently, the inhibition of topoisomerase I by the anti-cancer drug topotecan greatly enhanced transgene expression in adenovirus-infected hepatic cells, colon cancer and prostate cancer cell cultures, mouse liver, human ex vivo tumor specimens, and mouse tumor in vivo. The enhancement could not be ascribed to non-specific genotoxic stress, cell death, or cell-cycle perturbation. These findings are significant for gene therapy as they reveal novel aspects of the host anti-adenovirus response and set the stage for the development of a rational molecular-pharmacological approach to increase the effectiveness, and safety, of adenovirus-mediated cancer therapeutics.

Delta-24 Increases the Expression and Activity of Topoisomerase I and Enhances the Antiglioma Effect of Irinotecan

PURPOSE

In this study, we sought to determine whether Delta-24 could sensitize glioma cells to the topoisomerase I inhibitor irinotecan (CPT-11) and to identify the mechanisms underlying this enhanced anticancer effect.

EXPERIMENTAL DESIGN

We used human glioblastoma cell lines for the in vitro studies. The expression of topoisomerase I was determined in Western blot analyses, and topoisomerase I activity was determined by measuring the relaxation of a supercoiled DNA. The cell cycle distribution of cells was determined by flow cytometry analysis of the cellular DNA content. Cell viability was quantified by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Tissue culture infection dose assays were used to quantitate adenovirus replication. For the in vivo studies, athymic mice received intracranial/intratumoral injections of Delta-24 in combination with CPT-11, after which animal survival was monitored.

RESULTS

Delta-24 infection caused human glioma cells to accumulate in the S phase and induced the expression and activity of topoisomerase I as shown by Western blot and in vitro enzymatic activity assays. Further, we showed that the sequential administration of Delta-24 and CPT-11 to human glioma cell cultures potentiated the CPT-11-mediated anticancer effect in vitro without modifying the replicative phenotype of the oncolytic adenovirus. In vivo experiments showed that the single intratumoral administration of Delta-24 to intracranially implanted human glioma xenografts followed by the systemic administration of CPT-11 resulted in significantly prolonged animal survival.

CONCLUSIONS

The combination of Delta-24 treatment with CPT-11 showed an enhanced anticancer effect, which suggests that the interaction between adenoviral and human proteins can be exploited in rational anticancer therapies comprising replication-competent adenoviruses and conventional chemotherapeutic agents.

Direct antiviral effect of cycloferon (10-carboxymethyl-9-acridanone) against adenovirus type 6 in vitro

Adenoviruses represent a broad group of human pathogens that currently have no specific and safe drugs for treatment. We demonstrated direct (non IFN-mediated) antiviral activity of cycloferon (10-carboxymethyl-9-acridanone, CMA), a potent interferon inducer, against adenovirus type 6 (Ad6) in Hep-2 cells. Virus production and details of morphogenesis were studied by ELISA with antibodies to the Ad6 hexon protein, and transmission electron microscopy, respectively. Immunoenzyme assay revealed that CMA does not inhibit viral protein synthesis but instead strongly reduces the ability of the virus to generate infectious progeny virus in a dose dependent manner. Ultrastructural study shows that CMA alters the structure of intranuclear virus-specific inclusions. We suggest that CMA suppresses the late stages of viral cycle in the infected cell.

Intraperitoneal adenovirus-mediated suicide gene therapy in combination with either topotecan or paclitaxel in nude mice with human ovarian cancer

A mouse model of human ovarian cancer was used to investigate the effect of adenovirus-mediated thymidine kinase gene therapy (gt) in combination with chemotherapy. One hundred sixty female CD-1 nu/nu mice were injected intraperitoneally with Ov-ca-2774 cells. Onset of intraperitoneal treatment with either topotecan (6 or 12 mg/kg) or paclitaxel (18 or 36 mg/kg) was on day 4 or 8 and was repeated once after 4 days. Animals scheduled for gt received intraperitoneal application of adv/rsv-tk 1 day prior to chemotherapy and were subsequently treated with ganciclovir (gcv; 10 mg/kg, every 12 hours for 6 days). Survival was chosen as study endpoint. Whereas tumor burden had hardly any effect on survival, the lower dose of either cytotoxic agent was seen to be more effective than the higher one. In the topotecan group, an interaction between topotecan and gt was present. Survival was best for animals treated with low dose of topotecan only, the addition of gt reduced survival time significantly. With the higher dose, gt did not affect survival time. With paclitaxel, only slight effects of gt on the survival times were seen. Due to treatment toxicity, this animal model may be problematic for the evaluation of gt and chemotherapy combinations. The effect of dose varied strongly with time. Mice treated with high-dose chemotherapy had a substantially increased risk of dying in the time period following application, whereas this advantage of the lower dose disappeared later.

CPT-11 may provide therapeutic efficacy for esophageal squamous cell cancer and the effects correlate with the level of DNA topoisomerase I protein

CPT-11 is a potent anti-cancer drug and a specific inhibitor of DNA topoisomerase I (Topo I). In this study, we aim to evaluate the effects of CPT-11 on esophageal squamous cell cancers (ESCC) and to determine the correlation between the effects and the levels of Topo I expression. We examined the growth-inhibitory effect caused by SN-38, an active metabolite of CPT-11, in 14 human ESCC cell lines established from 10 primary and 4 metastatic lesions. CPT-11 was considered effective against 5 cell lines from primary lesions and one from metastatic lesions, and thus may show therapeutic efficacy against both primary and metastatic ESCC tumors. Although Topo I mRNA levels in these 14 ESCC cell lines, as quantitated by northern blot analysis, showed no correlation with the IC(50) values, Topo I protein levels, as quantitated by western blot analysis, showed an inverse correlation with the IC(50) values. Topo I protein levels could be an indicator of sensitivity to CPT-11. We also determined Topo I protein levels in 40 ESCC tumors and matched normal mucosae. Thirty-four tumors showed 1.2 - 22.3-fold increases in Topo I levels. Two patients receiving pre-operative chemotherapy and one receiving radiotherapy exhibited increased Topo I protein levels in their tumor lesions. It appeared that CPT-11 could provide selective therapeutic efficacy against ESCC tumors. CPT-11 may be effective for the treatment of metastatic ESCC tumors and as a second-line anti-cancer drug for ESCC.

Regulation of gene expression, cellular localization, and in vivo function of Caenorhabditis elegans DNA topoisomerase I

BACKGROUND

DNA topoisomerase I is dispensable in yeast, but is essential during the embryogenesis of Drosophila and mouse. In order to determine functions of the enzyme in the development of Caenorhabditis elegans, phenotypes resulting from the deficiency were observed and correlated with the expression of the gene.

RESULTS

The transcriptional regulation of the C. elegans DNA topoisomerase I gene was investigated by mRNA localization and reporter gene expression in C. elegans. The mRNA was expressed in the gonad and in the early embryos, followed by a rapid decrease in its level during the late embryonic stage. A reporter gene expression induced by the 5'-upstream DNA sequence appeared at the comma stage of embryos, continued through the L1 larval stage, and began to decrease gradually afterwards. The DNA topoisomerase I protein was immuno-localized in the nuclei of meiotic gonad cells and interphase embryonic cells, and unexpectedly in centrosomes of mitotic embryonic cells. Double-stranded RNA interference of DNA topoisomerase I gene expression resulted in pleiotropic phenotypes showing abnormal gonadogenesis, oocyte development and embryogenesis.

CONCLUSION

These phenotypes, along with expressional regulations, demonstrate that DNA topoisomerase I plays important roles in rapidly growing germ cells and embryonic cells.

Irinotecan in the treatment of colorectal cancer: clinical overview

PURPOSE AND METHODS

For more than three decades, the therapeutic options for patients with advanced colorectal cancer have almost exclusively been based on fluoropyrimidines. With the recognition that topoisomerase-I (TOP-I) is an important therapeutic target in cancer therapy, irinotecan, a semisynthetic TOP-I-interactive camptothecin derivative, has been clinically established in the treatment of colorectal cancer.

RESULTS

Irinotecan was investigated as second-line chemotherapy after prior treatment with fluorouracil (FU)-based regimens in two large randomized phase III trials comparing irinotecan with either best supportive care or an infusional FU/leucovorin (LV) regimen. The outcomes of these trials established irinotecan as the standard therapy in the second-line treatment of colorectal cancer. The therapeutic value of irinotecan in the first-line treatment of metastatic colorectal cancer was investigated in two large randomized phase III trials comparing the combination of irinotecan and FU/LV with FU/LV alone. Both trials demonstrated significant superior efficacy for the combination of irinotecan and FU/LV in terms of response rate, median time to disease progression, and median survival time. Consequently, the combination of irinotecan and FU/LV has been approved as first-line chemotherapy for patients with metastatic colorectal cancer and constitutes the reference therapy against which other treatment options must be tested in the future.

CONCLUSION

In this review, the clinical rationale and update of the present clinical status of irinotecan in the treatment of colorectal cancer and future prospects of irinotecan-based combinations are discussed.

Synergy between the herpes simplex virus tk/ganciclovir prodrug suicide system and the topoisomerase I inhibitor topotecan

An established principle of antineoplastic chemotherapy is that multidrug regimens are generally superior to single-agent therapy. This prompted us to elucidate whether the topoisomerase inhibitor topotecan (TPT) could enhance the efficacy of the herpes simplex virus thymidine kinase gene/ganciclovir (HSV-tk/GCV) system for the treatment of cancer. We assessed the interaction between these two treatments in murine MC38 and human HT-29 colon carcinoma cell lines that were genetically modified to constitutively express HSV-tk, sensitizing them to GCV. Synergistic cell killing was observed in a clonogenic assay over most of the cytotoxic dose range by the median-effect principle of Chou and Talalay (Adv. Enzyme Regul. 1984; 22:27-55). Subcutaneous tumor models, using the same cell lines in C57BL/6 and athymic nude mice, respectively, demonstrated that the combination of GCV and TPT resulted in statistically significant enhanced survival relative to single-agent treatment. In addition, nude mice bearing HT-29 tumor xenografts were treated with an Ad5 E1b Mr 55,000 attenuated replication-competent adenovirus expressing HSV-tk (Ad.TK(RC)) either alone or in combination with GCV and/or TPT. These experiments demonstrated that Ad.TK(RC) followed by GCV and TPT was more efficacious than any other treatment tested. Our results suggest that for antineoplastic therapy, molecular chemotherapy based on the HSV-tk/GCV system combined with traditional chemotherapy is a logical and practical future direction to pursue. Suicide gene therapy is the approach whereby genetically altering a cell makes it susceptible to an otherwise relatively nontoxic prodrug. By this approach it is possible to achieve relatively high concentrations of the toxic metabolites in the transduced cells while maintaining low systemic levels of the active drug. The most often used metabolic suicide gene transfer system is the HSV-tk/GCV paradigm, which is currently being used in cancer therapy or as a safety modality. The low response rate observed in the early clinical HSV-tk cancer trials may be due to failure in achieving adequate transduction efficiency and/or prodrug concentration within the tumor. The combination of such suicide gene prodrug systems with adjunctive drugs resulting in synergistic cytotoxicity might improve the clinical utility of this approach.

Mechanism of action of eukaryotic DNA topoisomerase I and drugs targeted to the enzyme

DNA topoisomerase I is essential for cellular metabolism and survival. It is also the target of a novel class of anticancer drugs active against previously refractory solid tumors, the camptothecins. The present review describes the topoisomerase I catalytic mechanisms with particular emphasis on the cleavage complex that represents the enzyme's catalytic intermediate and the site of action for camptothecins. Roles of topoisomerase I in DNA replication, transcription and recombination are also reviewed. Because of the importance of topoisomerase I as a chemotherapeutic target, we review the mechanisms of action of camptothecins and the other topoisomerase I inhibitors identified to date.

Alternative splicing in the Caenorhabditis elegans DNA topoisomerase I gene

5'-end cDNA fragments of the Caenorhabditis elegans DNA topoisomerase I gene were obtained by rapid amplification of the cDNA ends from C. elegans mRNAs. The presence of a SL1 sequence at the 5'-terminus of the cDNA sequence suggested trans-splicing of the pre-mRNA. By comparing the complete cDNA sequence with the genomic lambda DNA clones, the gene structure composed of five exons was established. Alternative splicing deleting the second exon was observed in the cDNA fragments obtained by a gene-specific reverse transcription followed by polymerase chain reactions. The shorter mRNA missing the second exon was expressed at all the developmental stages, while the full-length mRNA was present only in embryos.

Cloning of a cDNA encoding DNA topoisomerase I in Daucus carota and expression analysis in relation to cell proliferation

DNA topoisomerase I is an enzyme involved in several processes related to DNA metabolism. Despite the physiological importance, the regulation of top1 gene expression has not yet been investigated in plants. In order to monitor the possible correlation between levels of top1 transcripts and the proliferative state of the cell, two partially overlapping cDNAs encoding DNA topoisomerase I from Daucus carota have been isolated from a poly(A)(+)-primed library, using an Arabidopsis thaliana probe, and from a cDNA library spanning the 5' region of the top1 transcript, which was constructed using an antisense specific oligonucleotide. The top1 nucleotide sequence encoded an open reading frame of 2370 bp, predicting a protein of 90 kDa. The deduced amino acid sequence showed a similarity of 51% with A. thaliana, 41% with S. cerevisiae, 40% with S. pombe and 31% with H. sapiens, respectively. Southern blot analysis, performed under moderate stringency conditions, showed the presence of a single-copy gene. Evaluation of the top1 mRNA steady-state level revealed, besides a constitutive expression in vegetative carrot tissues, an induced expression related to cell proliferation.

The human topoisomerase I gene promoter is regulated by NF-IL6

We investigated the expression of the human DNA topoisomerase I (hTOP1) gene in HeLa cells and in adenovirus-transformed 293 cells. A highly conserved proximal promoter element is essential for hTOP1 promoter activity in HeLa cells but not in 293 cells. This correlates with the presence of specific promoter-binding proteins in HeLa cells and their absence in 293 cells. We identified the HeLa binding protein by screening a cDNA expression library with the specific promoter site as a probe and demonstrate now that the activating protein is identical to the nuclear factor for interleukin-6 expression (NF-IL6), a member of the C/EBP family of transcription factors. Overexpression of NF-IL6 strongly stimulates hTOP1 promoter activity in HeLa cells, suggesting that NF-IL6 is a major hTOP1-regulating protein. Because of the presence of adenovirus protein E1A, 293 cells express the hTOP1 gene more efficiently than HeLa cells but do not contain NF-IL6 activity. E1A activation of the hTOP1 promoter is suppressed by NF-IL6 overexpression. This result supports previous observations concerning a functional interaction between viral protein E1A and NF-IL6. Finally, we show that hTOP1 gene expression in differentiating macrophages is correlated with the synthesis of NF-IL6-specific mRNA.

PKA controls a level of topoisomerase I mRNA in mouse L5178Y lymphoma cells treated with db-cAMP

The level of topoisomerase I mRNA was measured in cells of two mouse lymphoma (LY) sublines treated with db-cAMP. A transient increase of the level was observed to be of about 60% of the basic level and to have maximum after the 3 h treatment of LY-S cells. The increase in LY-R subline was two-fold lower. The activity of PKA in a cytosol fraction of LY-S cells was 1.75 times higher than that in LY-R cells. The activity of PKA in membranes and nuclear fraction did not differ significantly in both cell types. When the activity of PKA in LY-S cells was inhibited with H8, no increase of the level of topoisomerase I mRNA was observed upon db-cAMP treatment of cells. We suggest that the activity of PKA in the cytosol controls the expression of topoisomerase I gene in LY cells at high concentration of cAMP.

hnRNP-U/SAF-A is encoded by two differentially polyadenylated mRNAs in human cells

Scaffold attachment factor A (SAF-A; Romig et al. (1992) EMBO J. 11, 3431-3440) is a human nuclear protein with high affinity for SAR DNA (scaffold attached region; Laemmli et al. (1992) Curr. Opin. Cell. Biol. 2, 275-285). We isolated and sequenced two classes of cDNA clones coding for SAF-A and found that their coding sequences are identical with the coding sequence of hnRNP-U (Kiledjian and Dreyfuss (1992) EMBO J. 11, 2655-2664). The two classes of cDNA clones differ, however, in the length of their 3' untranslated region and result from the use of two different polyadenylation signals. This is consistent with Northern blotting experiments which show that two mRNAs with a length of 3.9 kb and 3.1 kb, respectively, exist in human cells. Using selective cDNA fragments as probes we show that the shorter mRNA lacks 800 nucleotides of the untranslated 3' end.

Determinants of cellular sensitivity to topoisomerase-targeting antitumor drugs

It is now clear that topoisomerase activity level is an important determinant of sensitivity to topo drugs. The regulation of topoisomerases is no doubt complex and multifaceted and is probably accomplished through redundancy at many control levels. The mechanism(s) of altered topo I expression in certain tumor types is unknown, but may be related to the central importance of topoisomerases in proliferating cell functions (transcription, replication, etc.), and the aberrant and chronic activation of these functions as a result of specific tumorigenic alterations. Small differences in sensitivity to chemotherapy can have a dramatic effect on cure rates, and therefore subtle cell type-specific differences may be important determinants of drug sensitivity. Whether abnormal topoisomerase quantity and specific activity are associated with resistance or sensitivity to topoisomerase-targeted chemotherapy in the clinic is now being studied. Determinants downstream of cleavable complex formation that affect the sensitivity of tumor versus normal cells to topo drugs in particular and DNA-damaging agents in general are little known. The goal of enhancing selective tumor cell killing relative to the normal cells that are dose limiting may be achieved either by overcoming tumor cell resistance or by protecting normal cells. Both of these strategies will become more feasible as specific molecular differences between tumor and normal cells are being rapidly identified and new combination therapies that take advantage of these differences are being designed and tested.

The promoter region of the human type-I-DNA-topoisomerase gene. Protein-binding sites and sequences involved in transcriptional regulation

We examined the promoter of the human type-I-DNA topoisomerase gene (hTOP1) for regions protected against DNase I digestion by nuclear proteins from HeLa or from adenovirus-transformed 293 cells. We identified ten protected DNA sequences within 580 bp of DNA upstream of the transcriptional-start sites and one additional site, which is located between the two clusters of transcriptional-start sites. Several of these protein-binding sites have significant similarities to recognition sequences of known transcription factors including factors Sp1, octamer transcription factor, cAMP-responsive-element-binding protein (CREB/ATF), NF-kappa B and members of the Myc-related family of basic/helix-loop-helix/leucine-zipper proteins. Other protein-binding sites show less or no similarities to known consensus sequences. We investigated the physiological significance of these protein-binding sites using a set of deletion and nucleotide-exchange mutants. We conclude that the expression of the hTOP1 gene is regulated by a complex network of negatively and positively acting transcription factors.

DNA topoisomerase I is essential in Drosophila melanogaster

Both biochemical and genetic experiments suggest that the type I DNA topoisomerase may participate in DNA replication, recombination, transcription, and other aspects of DNA metabolism. Despite its apparent importance, genetic studies in unicellular organisms including eubacteria and yeasts indicate that topoisomerase I is not essential for viability. We have previously isolated the cDNA clone encoding DNA topoisomerase I from Drosophila melanogaster. We report here the cytogenetic mapping of top1 to the X chromosome at 13C1 and isolation of top1 genomic DNA. Using P-element mutagenesis, we have isolated a mutant deficient in Drosophila topoisomerase I functions. Genetic studies of this mutant show that topoisomerase I is essential for the growth and development of the fruit fly, a multicellular organism. The biological functions of topoisomerase I are inferred from our analysis of the regulation of topoisomerase I expression during Drosophila development.

Structural characterization of the human DNA topoisomerase I gene promoter

We have isolated a genomic DNA fragment from HeLa cells containing the promoter region and the first two exons of the human gene encoding DNA topoisomerase I (hTOP1). Transcription of hTOP1 mRNA initiates at multiple sites which are clustered 247 nucleotides and 210 nucleotides upstream of the translation-initiation site of the protein coding region. The nucleotide sequence of the region preceding the transcription-initiation sites is G/C rich and contains sequence motifs which are known binding sites of the transcription factors Oct1 (octameric transcription factor 1), Sp1 and AP2 (activator protein 2). Furthermore, one cAMP-responsive element is present 50 nucleotides upstream of the transcription-initiation site nearest the 5' end. Neither TATA nor CAAT boxes were found in the promoter region of the hTOP1 gene. A 918-bp fragment containing the sequence elements described above drives the transient expression of a chloramphenicol acetyl transferase (CAT) gene sequence in transfected HeLa and 293 cells. In addition we analyzed a 10-kb fragment containing the promoter and exons 1 and 2 for regions of DNase I hypersensitivity. We detected one prominent DNase-I-hypersensitive region in the promoter close to the putative transcription-factor-binding sites and several weaker regions in intron 2.