aphidicolin and Colonic-Neoplasms

The error frequency of uracil-initiated base excision repair (BER) DNA synthesis in human and Escherichia coli cell-free extracts was determined by an M13mp2 lacZ alpha DNA-based reversion assay. Heteroduplex M13mp2 DNA was constructed that contained a site-specific uracil target located opposite the first nucleotide position of opal codon 14 in the lacZ alpha gene. Human glioblastoma U251 and colon adenocarcinoma LoVo whole-cell extracts repaired the uracil residue to produce form I DNA that was resistant to subsequent in vitro cleavage by E. coli uracil-DNA glycosylase (Ung) and endonuclease IV, indicating that complete uracil-initiated BER repair had occurred. Characterization of the BER reactions revealed that (1) the majority of uracil-DNA repair was initiated by a uracil-DNA glycosylase-sensitive to Ugi (uracil-DNA glycosylase inhibitor protein), (2) the addition of aphidicolin did not significantly inhibit BER DNA synthesis, and (3) the BER patch size ranged from 1 to 8 nucleotides. The misincorporation frequency of BER DNA synthesis at the target site was 5.2 x 10(-4) in U251 extracts and 5.4 x 10(-4) in LoVo extracts. The most frequent base substitution errors in the U251 and LoVo mutational spectrum were T to G > T to A >> T to C. Uracil-initiated BER DNA synthesis in extracts of E. coli BH156 (ung) BH157 (dug), and BH158 (ung, dug) was also examined. Efficient BER occurred in extracts of the BH157 strain with a misincorporation frequency of 5.6 x 10(-4). A reduced, but detectable level of BER was observed in extracts of E. coli BH156 cells; however, the mutation frequency of BER DNA synthesis was elevated 6.4-fold.

Topics: Adenocarcinoma; Aphidicolin; Bacteriophage M13; Cell Extracts; Cell-Free System; Colonic Neoplasms; DNA Damage; DNA Glycosylases; DNA Repair; DNA Replication; DNA, Bacterial; DNA, Neoplasm; DNA, Single-Stranded; Escherichia coli; Lac Operon; Mutation; N-Glycosyl Hydrolases; Neoplasm Proteins; Tumor Cells, Cultured; Uracil; Uracil-DNA Glycosidase; Viral Proteins

Cancer gene therapy by retroviral vectors is mainly limited by the level of transduction. Retroviral gene transfer requires target cell division. Cell synchronization, obtained by drugs inducing a reversible inhibition of DNA synthesis, could therefore be proposed to precondition target cells to retroviral gene transfer. We tested whether drug-mediated cell synchronization could enhance the transfer efficiency of a retroviral-mediated gene encoding herpes simplex virus thymidine kinase (HSV-tk) in two colon cancer cell lines, DHDK12 and HT29.. Synchronization was induced by methotrexate (MTX), aracytin (ara-C) or aphidicolin. Gene transfer efficiency was assessed by the level of HSV-TK expression. Transduced cells were driven by ganciclovir (GCV) towards apoptosis that was assessed using annexin V labeling by quantitative flow cytometry.. DHDK12 and HT29 cells were synchronized in S phase with MTX but not ara-C or aphidicolin. In synchronized DHDK12 and HT29 cells, the HSV-TK transduction rates were 2 and 1.5-fold higher than those obtained in control cells, respectively. Furthermore, the rate of apoptosis was increased two-fold in MTX-treated DHDK12 cells after treatment with GCV.. Our findings indicate that MTX-mediated synchronization of target cells allowed a significant improvement of retroviral HSV-tk gene transfer, resulting in an increased cell apoptosis in response to GCV. Pharmacological control of cell cycle may thus be a useful strategy to optimize the efficiency of retroviral-mediated cancer gene therapy.

Topics: Animals; Antimetabolites, Antineoplastic; Antiviral Agents; Aphidicolin; Apoptosis; Cell Culture Techniques; Cell Cycle; Colonic Neoplasms; Cytarabine; Flow Cytometry; Ganciclovir; Gene Transfer Techniques; Genetic Therapy; Genetic Vectors; Humans; Methotrexate; Mice; Retroviridae; Simplexvirus; Thymidine Kinase; Tumor Cells, Cultured

This paper describes the inhibitory activities of diacylglyceride phospholipids, such as phosphatidylcholine (lecithin), phosphatidylethanolamine (cephalin), phosphatidylserine, phosphatidylglycerol, bisphosphatidylglycerol (cardiolipin), phosphatidylinositol, and phosphatidic acid (phosphatidate) (compounds 1 - 7, respectively) against DNA polymerase (pol), DNA topoisomerase (topo), and human cancer cell growth. Among the compounds tested, compounds 3 - 7 were revealed to be potent inhibitors of animal pols: compound 4 was the strongest inhibitor, with IC(50) values for different pols of 1.7 - 15 mM. Compounds 4 - 7 also inhibited the activity of human topo II: compound 7 was the strongest inhibitor, with an IC50 value of 20 mM. The glycerophospholipids had no effect on the activities of plant (cauliflower) pol a, prokaryotic pols, or other DNA metabolic enzymes, such as calf primase of pol a, T7 RNA polymerase, T4 polynucleotide kinase, and bovine deoxyribonuclease I. These results suggest that compounds 3 - 7 are selective inhibitors of animal pols and human topos. Compounds 4 and 7 also suppressed the growth of a human colon carcinoma cell line that lacked p53 (HCT116 p53(-/-)); their LD(50) values were 63.6 and 51.1 mM, respectively, suggesting that cell growth inhibition by these compounds leads to the inhibition of pols and/or topos. From these findings, diacylglyceride phospholipids, which are present in various foods, might be effective nutrients for promoting human anti-cancer health promotion.

Topics: Animals; Antineoplastic Agents, Phytogenic; Aphidicolin; Cattle; Cell Proliferation; Colonic Neoplasms; DNA Topoisomerases, Type I; DNA Topoisomerases, Type II; DNA-Directed DNA Polymerase; Glycerophospholipids; HCT116 Cells; Humans; Inhibitory Concentration 50; Nucleic Acid Synthesis Inhibitors; Topoisomerase Inhibitors; Tumor Suppressor Protein p53

Cyclin E is the Cdk2-regulatory subunit required for the initiation of DNA replication at the G1/S transition. It accumulates in late G1 phase and gets rapidly degraded by the ubiquitin/proteasome pathway during S phase. The degradation of cyclin E is a consequence of its phosphorylation and subsequent isomerization by the peptidyl-prolyl isomerase Pin1. We show that in the colon cancer cells HT-29 the inhibition of the chaperone function of Hsp90 by geldanamycin (GA) enhances the ubiquitinylation of cyclin E and triggers active degradation via the proteasome pathway. As Hsp90 forms multiprotein complexes with and regulates the function and cell contents of numerous signaling proteins, this observation suggests a direct interaction between Hsp90 and cyclin E. However, experiments using cell lysate fractionation did not reveal the presence of complexes containing both Hsp90 and cyclin E. Coupled transcription/translation experiments also failed to detect the formation of complexes between newly synthesized cyclin E and Hsp90. We conclude that Hsp90 can regulate the degradation of cellular proteins without binding to them, by an indirect mechanism. This conclusion postulates a new category of proteins that are affected by the inactivation of Hsp90. Our observations do not support the possible involvement of a PPIase in this indirect mechanism. Besides, we did not observe active geldanamycin-dependent degradation of cyclin E in the prostate cancer-derived cell line DU-145, indicating that the Hsp90-dependent stabilization of cyclin E requires specific regulatory mechanism which may be lost in certain types of cancer cells.

Topics: Antibiotics, Antineoplastic; Aphidicolin; Benzoquinones; Cell Cycle; Cell Line, Tumor; Colonic Neoplasms; Cyclin E; DNA Replication; Homeostasis; HSP90 Heat-Shock Proteins; Humans; Lactams, Macrocyclic; Male; Neoplasm Proteins; Prostatic Neoplasms; Protein Biosynthesis; Transcription, Genetic; Ubiquitin

This study provides evidence for the importance of p21(CDKN1A) for the repair of replication-mediated DNA double-strand breaks (DSBs) induced by topoisomerase I. We report that defects of p21(CDKN1A) and p53 enhance camptothecin-induced histone H2AX phosphorylation (gammaH2AX), a marker for DNA DSBs. In human colon carcinoma HCT116 cells with wild-type (wt) p53, gammaH2AX reverses after camptothecin removal. By contrast, gammaH2AX increases after camptothecin removal in HCT116 cells deficient for p53 (p53-/-) or p21(CDKN1A) (p21-/-) as the cells reach the late-S and G2 phases. Since p21-/- cells exhibit similar S-phase arrest as wt cells in response to camptothecin and aphidicolin does not abrogate the enhanced gammaH2AX formation in p21-/- cells, we conclude that enhanced gammaH2AX formation in p21-/- cells is not due to re-replication. The cell cycle checkpoint abrogator and Chk1/Chk2 inhibitor 7-hydroxystaurosporine (UCN-01) also increases camptothecin-induced gammaH2AX formation and inhibits camptothecin-induced p21(CDKN1A) upregulation in HCT116 wt cells. TUNEL (terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling) assays demonstrate that gammaH2AX formation in late S and G2 cells following CPT treatment corresponds to DNA breaks. However, these breaks are not related to apoptotic DNA fragmentation. We propose that p21(CDKN1A) prevents the collapse of replication forks damaged by stabilized topoisomerase I cleavage complexes.

Topics: Aphidicolin; Apoptosis; Chromosome Breakage; Colonic Neoplasms; Cyclin-Dependent Kinase Inhibitor p21; DNA; DNA Damage; DNA Repair; DNA Replication; DNA Topoisomerases, Type I; HCT116 Cells; Histones; Humans; In Situ Nick-End Labeling; Phosphorylation; Protein Kinase C; Protein Kinase Inhibitors; S Phase; Staurosporine; Tumor Suppressor Protein p53

We studied the cytotoxic effects of various DNA replication inhibitors on MMR-deficient and -proficient colon carcinoma cell lines. DNA polymerase (pol) inhibitors including aphidicolin and gemcitabine, and hydroxyurea were more toxic (1.7 to 2.8-fold) to hMLH1-deficient HCT116 than to hMLH1-proficient HCT116+ch3. Similarly, pol inhibitors were more toxic to hMSH2-deficient LoVo than to hMSH2-proficient LoVo+ch2. In contrast, DNA topoisomerase I inhibitors, such as CPT-11, SN-38, and topotecan, were more toxic to MMR-proficient cells. Our results suggest that MMR-deficient colon carcinoma cells are hypersensitive to inhibitors of the pol reaction.

Topics: Adaptor Proteins, Signal Transducing; Antineoplastic Agents; Aphidicolin; Base Pair Mismatch; Carrier Proteins; Colonic Neoplasms; DNA Repair; HCT116 Cells; Humans; MutL Protein Homolog 1; Neoplasm Proteins; Nuclear Proteins; Nucleic Acid Synthesis Inhibitors; Tumor Cells, Cultured

cDNA microarray technology can be used to establish associations between characteristic gene expression patterns and molecular responses to drug therapy. In this study, we used cDNA microarrays of 1694 cancer-related genes to monitor the gene expression consequences of the treatment of HCT116 colon cancer cells with the topoisomerase I inhibitor camptothecin (CPT). To obtain a more homogeneous cellular response, we synchronized the cells in S-phase using aphidicolin (APH) before CPT treatment. Brief incubation with 20 and 1000 nM CPT caused reversible and irreversible G(2) arrest, respectively, and the patterns of gene expression change (with reference to untreated controls) were strikingly different at the two concentrations. Thirty-three genes, mainly divided into three groups, showed characteristic changes in the first 20 h as a consequence of treatment. Northern blots performed for five of these genes (each under eight experimental conditions) were quite consistent with the microarray results (average correlation coefficient, 0.86). Several p53-activated stress response genes were up-regulated after treatment with 1000 nM CPT or prolonged exposure to APH, but it seemed that the up-regulation did not directly cause cell cycle arrest because the up-regulation induced by prolonged treatment with APH did not prevent cell cycle progression after removal of APH. In contrast, cell cycle-dependent up-regulation of a group of mitosis-related genes was delayed or blocked after CPT treatments. The interrupted up-regulation of this group of genes was directly associated with G(2) arrest. In addition, we observed down-regulation of gene expression in cells that were recovering from cell cycle delay. The observations reported here suggest a fundamental difference at the gene expression level between the molecular mechanism of reversible G(2) delay that follows mild DNA damage and the mechanism of permanent G(2) arrest that follows more extensive DNA damage.

Topics: Aphidicolin; Camptothecin; Cell Cycle; Colonic Neoplasms; DNA Damage; Dose-Response Relationship, Drug; Enzyme Inhibitors; G2 Phase; Gene Expression Profiling; Gene Expression Regulation, Neoplastic; Genes, cdc; Humans; Mitosis; Oligonucleotide Array Sequence Analysis; Topoisomerase I Inhibitors; Transcription, Genetic

We have previously demonstrated with several cell lines in vitro that hydroxyurea (HU) synergistically enhances ganciclovir (GCV)-mediated cytotoxicity in bystander cells. In this study, we evaluated the role of DNA synthesis inhibition on enhanced bystander killing and assessed whether addition of HU would improve the efficacy of the HSV-TK/GCV system in vivo. Compared with GCV treatment alone, addition of HU resulted in increased DNA synthesis inhibition and delayed progression through S phase following removal of drug. In a xenograft tumor model, 1:10 and 1:1 mixtures of HSVtk- and LacZ-expressing SW620 cells were injected s.c. in the flanks of nude mice and treated i.p. (100 mg/kg GCV, 1500 mg/kg HU) daily for 5 days. Tumors from mice treated with GCV alone grew rapidly and increased to 10 times their initial size in 15.7 +/- 1.8 and 16.0 +/- 0.9 days for 1:10 and 1:1 mixtures, respectively. However, when both GCV and HU were administered in combination, a single complete tumor regression was observed in both the 1:10 and 1:1 groups. In the remaining mice treated with GCV/HU, it took 23.2 +/- 2.1 (1:10) and 26.4 +/- 3.8 days (1:1) to obtain a similar 10-fold increase in tumor size.

Topics: Animals; Antimetabolites; Antiviral Agents; Aphidicolin; Bystander Effect; Cell Cycle; Cell Line; Colonic Neoplasms; Drug Synergism; Female; Ganciclovir; Genetic Therapy; Humans; Hydroxyurea; Mice; Mice, Nude; Neoplasm Transplantation; Simplexvirus; Thymidine Kinase

The expression frequency of common fragile sites induced by aphidicolin (Apc), bromodeoxyuridine (BrdU), and caffeine was evaluated on prometaphase chromosomes obtained from the peripheral blood lymphocytes of 32 patients with colon cancer, 30 of their clinically healthy family members and 30 age-matched normal controls. The proportion of damaged cells (P < 0.001), the mean number of chromosomal aberrations and the expression frequencies of fragile sites were significantly higher in the patient and relative groups compared to the control group. Our findings show an increased genetic instability in patients with colon cancer and their first-degree relatives. In addition, common fragile sites can be used as a suitable marker for determining genetic predisposition to cancer.

Topics: Adenocarcinoma; Adolescent; Adult; Aged; Alu Elements; Aphidicolin; Bromodeoxyuridine; Caffeine; Child; Child, Preschool; Chromosome Aberrations; Chromosome Fragile Sites; Chromosome Fragility; Chromosomes, Human; Colonic Neoplasms; Female; Gene Expression Profiling; Genes, Tumor Suppressor; Genetic Predisposition to Disease; Humans; Lymphocytes; Male; Middle Aged; Pedigree

An alkylating camptothecin (CPT) derivative, 7-chloromethyl-10,11-methylenedioxy-camptothecin (7-CM-MDO-CPT) was recently shown to produce irreversible topoisomerase I (top1) cleavage complexes by binding to the +1 base of the scissile strand of a top1 cleavage site. We demonstrate that 7-CM-EDO-CPT (7-chloromethyl-10,11-ethylenedioxy-camptothecin) also induces irreversible top1-DNA complexes. 7-CM-MDO-CPT, 7-CM-EDO-CPT, and the nonalkylating derivative 7-ethyl-10,11-methylenedioxy-camptothecin (7-E-MDO-CPT) also induced reversible top1 cleavable complexes, which were markedly more stable to salt-induced reversal than those induced by 7-ethyl-10-hyroxy-CPT, the active metabolite of CPT-11. This greater stability of the top1 cleavable complexes was contributed by the 7-alkyl and the 10,11-methylene- (or ethylene-) dioxy substitutions. Studies in SW620 cells showed that 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT are more potent inducers of cleavable complexes and more cytotoxic than CPT. The reversal of the cleavable complexes induced by 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT was markedly slower after drug removal than that for CPT, which is consistent with the data with purified top1. By contrast to CPT, 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT were cytotoxic irrespective of the presence of 10 microM aphidicolin. These results suggest that 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT are more potent top1 poisons than CPT and produce long lasting top1 cleavable complexes and greater cytotoxicity than CPT in cells.

Topics: Antineoplastic Agents, Phytogenic; Aphidicolin; Base Sequence; Camptothecin; Colonic Neoplasms; DNA Damage; DNA Topoisomerases, Type I; Humans; Molecular Sequence Data; Structure-Activity Relationship; Tumor Cells, Cultured

Previous cell line comparisons indicated that neither S-phase fraction nor topoisomerase I (top1) levels are sufficient to predict camptothecin (CPT) cytotoxicity (F. Goldwasser el al., Cancer Res., 55: 2116-2121, 1995.). To identify new determinants for CPT activity, two mutant p53 human colon cancer cell lines, SW620 and KM12, that were previously reported to have similar top1 levels and differential sensitivity to CPT were studied. No difference in the kinetics of top1-mediated DNA single-strand breaks or DNA synthesis inhibition were observed after 1 h exposure to 1 microM CPT. Pulse-labeling alkaline elution showed deficiency of damaged replicon elongation in the more sensitive SW620 cells. Consistentiy, flow cytometry analyses showed that KM12 was arrested in G2, whereas SW620 cells were irreversibly blocked in S phase. Aphidicolin protection was minimal in KM12 and more pronounced in the more sensitive SW620 cells. Thus, CPT appears to have two cytotoxic mechanisms, one protectable by aphidicolin and present in SW620 and the other not protectable by aphidicolin and common to both cell lines. SW620 exhibited also a greater capacity to break through the G2 checkpoint after DNA damage. Consistently, SW620 cells failed to down-regulate cyclin B-cdc2 kinase activity, whereas KM12 cells down-regulated cyclin B/cdc2 kinase activity within 30 min to 20 % of control level after CPT treatment. Analysis of the 7 human colon carcinoma cell lines of the NCI Anticancer Drug Screen showed that defects in replicon elongation and G2 breakthrough capability correlate with sensitivity to CPT. Our results suggest that misrepair of damaged replicons and/or alterations in DNA damage checkpoints is critical to defining chemosensitivity to CPT-induced top1-cleavable complexes and that CPT appears to have two cytotoxic mechanisms, one protectable by aphidicolin, and the other not.

Topics: Antineoplastic Agents, Phytogenic; Aphidicolin; Camptothecin; Carcinoma; CDC2 Protein Kinase; Colonic Neoplasms; Cyclins; DNA Damage; DNA Repair; DNA Replication; Enzyme Inhibitors; G2 Phase; Humans; S Phase; Topoisomerase I Inhibitors; Tumor Cells, Cultured

Src and Yes protein-tyrosine kinase activities are elevated in malignant and premalignant tumors of the colon. To determine whether Src activity is elevated throughout the human colon carcinoma cell cycle as it is in polyomavirus middle T antigen- or F527 Src-transformed cells, and whether Yes activity, which is lower than that of Src in the carcinoma cells, is regulated differently, we measured their activities in cycling cells. We observed that the activities of both kinases were higher throughout all phases of the HT-29 colon carcinoma cell cycle than in corresponding phases of the fibroblast cycle. In addition, during mitosis of HT-29 cells, Src specific activity increased two- to threefold more, while Yes activity and abundance decreased threefold. The decreased steady-state protein levels of Yes during mitosis appeared to be due to both decreased synthesis and increased degradation of the protein. Inhibition of tyrosine but not serine/threonine phosphatases abolished the mitotic activation of Src. Mitotic Src was phosphorylated at novel serine and threonine sites and dephosphorylated at Tyr-527. Two cellular proteins (p160 and p180) were phosphorylated on tyrosine only during mitosis. Tyrosine phosphorylation of several other proteins decreased during mitosis. Thus, Src in HT-29 colon carcinoma cells, similar to Src complexed to polyomavirus middle T antigen or activated by mutation at Tyr-527, is highly active in all phases of the cell cycle. Moreover, Src activity further increases during mitosis, whereas Yes activity and abundance decrease. Thus, Src and Yes appear to be regulated differently during mitosis of HT-29 colon carcinoma cells.

Topics: Aphidicolin; Colonic Neoplasms; CSK Tyrosine-Protein Kinase; DNA Polymerase II; G2 Phase; Humans; Mitosis; Nocodazole; Phosphoprotein Phosphatases; Phosphorylation; Protein Tyrosine Phosphatases; Protein-Tyrosine Kinases; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-yes; src-Family Kinases; Tumor Cells, Cultured

The growth-suppressive activity of the retinoblastoma (RB) protein is suggested to be regulated by phosphorylation. In studies on the kinase that phosphorylates the RB proteins, we have previously found that RB proteins can be phosphorylated by purified cdc2 kinase. In this study, we noted that RB proteins immunoprecipitated from human cell lysates are weakly phosphorylated in the absence of purified cdc2 kinase. Immunoblot analysis showed the presence of p34cdc2 in the immunoprecipitates with anti-RB monoclonal antibody. In addition, the coprecipitated kinase was found to have the same substrate specificity as cdc2 kinase. The associated kinase activity was particularly high in cells arrested in G1/S and S phase by aphidicolin. Furthermore, RB proteins were shown to be phosphorylated in nuclear extracts by some endogenous cdc2-like kinase(s). These results suggest that cdc2-like kinase is the main kinase for phosphorylation of RB proteins in vivo.

Topics: Amino Acid Sequence; Animals; Aphidicolin; CDC2 Protein Kinase; Cell Cycle; Cell Line, Transformed; Cell Nucleus; Colonic Neoplasms; HeLa Cells; Humans; Leukemia; Mammary Neoplasms, Experimental; Mice; Molecular Sequence Data; Neuroblastoma; Peptides; Phosphorylation; Retinoblastoma Protein; Substrate Specificity

Previous studies in rapidly proliferating rodent cells have suggested that the lethal effect of the DNA topoisomerase I inhibitor, camptothecin (CPT) is dependent upon the active participation of DNA replication (Holm et al. Cancer Res. 49:6365-6368; 1989). The purpose of the current study was to determine if this relationship applies to more slowly growing human cells. In our present study, we employed the human colon carcinoma cell line, HT-29 (45 hr doubling time). Flow cytometric determination of S-phase cells either by S-phase fit model or rectangle fit model analysis predicted that 21% of exponentially growing HT-29 cells were undergoing DNA replication. These findings were confirmed by immunofluorescence microscopy of bromodeoxyuridine labeled cells. Based on these findings, we would have expected only 20-30% of the cells to be susceptible to brief treatment (30 min) with CPT. Instead, 90-95% of HT-29 cells were killed. This apparent disparity was not due to prolonged cellular retention of drug after treatment because protein-linked DNA strand breaks reversed within 15 min of drug removal. Moreover, the DNA replication inhibitor, aphidicolin, fully protected HT-29 cells against CPT-induced killing but did not affect the production of CPT-induced protein-linked DNA strand breaks. Similar results were also obtained using the CPT-analog, 10,11-methylenedioxy-camptothecin, which was 5- to 10-fold more potent than camptothecin (O'Connor et al. Cancer Commun. 2:395-400; 1990).(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Aphidicolin; Camptothecin; Cell Division; Cell Line; Cell Survival; Colonic Neoplasms; Diterpenes; DNA Polymerase II; DNA Replication; DNA, Neoplasm; Humans; Molecular Structure; S Phase; Topoisomerase I Inhibitors; Tumor Cells, Cultured

Leucovorin augments the growth inhibitory effect of 5-fluoropyrimidines on neoplastic cells. The effect is paralleled by much higher levels of DNA fragmentation than in cells treated with 5-fluoropyrimidines alone at the same concentration. The lesions are induced by a mechanism independent of incorporation of the drug into DNA, in all probability due to reduced repair of DNA lesions induced independently of the drug treatment. Thymidine added after the treatment with fluoropyrimidines partly rescues the cells and reduces the level of DNA fragmentation.

Topics: Adenocarcinoma; Aphidicolin; Cell Line; Cell Survival; Colonic Neoplasms; Diterpenes; DNA; DNA Damage; Drug Synergism; Floxuridine; Fluorouracil; Humans; Leucovorin; Pyrimidines; Thymidine

Prior reports demonstrated more than additive cytotoxic effects of cis-diamminedichloroplatinum(II) (CDDP) and 1-beta-D-arabinofuranosylcytosine (ara-C) in LoVo colon carcinoma cells. We have extended these findings by analyzing mechanisms that may underlie the effect of ara-C on CDDP-induced cytotoxicity. In contrast to a previous study, ara-C neither enhances DNA interstrand cross-link formation by CDDP nor affects the excision of platinum from DNA. Features peculiar to ara-C, such as its misincorporation into DNA, probably contribute since more than additive cytotoxic effects do not occur by combinations of CDDP with inhibitors of DNA synthesis that are not incorporated into DNA. Also, while ara-C does not significantly enhance the degree of inhibition of DNA synthesis caused by CDDP, the recovery of DNA synthesis after drug removal is significantly slowed when cells are exposed to both drugs. These findings contrast with those obtained with CDDP and aphidicolin (the latter agent resembles ara-C in competing with dCTP for binding to DNA polymerase alpha but, unlike ara-C, is not incorporated into DNA). Lastly, ara-C is incorporated into LoVo cell DNA undergoing replicative synthesis as well as into DNA undergoing repair synthesis after CDDP-induced induced DNA damage.

Topics: Aphidicolin; Carcinoma; Cell Line; Cell Survival; Cisplatin; Colonic Neoplasms; Cytarabine; Diterpenes; DNA Damage; DNA Repair; Humans

The present study was undertaken to determine whether cytotoxicity by 1-beta-D-arabinofuranosylcytosine (ara-C) in LoVo colon carcinoma cells, which are resistant to high concentrations of ara-C, would be enhanced by concurrent exposure to hydroxyurea (HU). Since mechanisms underlying the effects of HU on ara-C induced cytotoxicity are unclear, we also evaluated the effect of HU on the incorporation of ara-C into DNA, as well as potential consequences of misincorporation. Our results demonstrate that HU synergistically enhances cytotoxicity by ara-C in these cells. This effect was not present when HU was combined with aphidicolin, an agent that resembles ara-C in competing with dCTP for binding to polymerase alpha but that is not incorporated into DNA. Further, cells exposed to HU and ara-C incorporated up to 5-fold as much ara-C into DNA as cells solely treated with ara-C. While the extent of inhibition of DNA synthesis was comparable with cells exposed to HU and aphidicolin as those treated with HU and ara-C, recovery of DNA synthesis was delayed more significantly by the latter combination. These findings suggest that HU synergistically potentiates ara-C induced cytotoxicity by enhancing incorporation of ara-C in LoVo cell DNA.

Topics: Aphidicolin; Cell Line; Cell Survival; Colonic Neoplasms; Cytarabine; Diterpenes; DNA; Drug Resistance; Drug Synergism; Hydroxyurea

The effect of 5-fluorouracil on the stability of DNA and the synthesis of DNA replication intermediates was analyzed in human colon adenocarcinoma cells. Density gradient analysis showed that some of the drug is incorporated into DNA. Moreover, DNA fragments are released when cells with drug-containing DNA are lysed in dilute alkali. The DNA fragments can be separated from the bulk DNA by agarose gel electrophoresis. The fragmentation of the DNA can be prevented by pretreatment with aphidicolin which inhibits DNA polymerase alpha. In 5-fluorouracil-treated cells, a heterogeneous population of DNA replication intermediates is formed, instead of discrete DNA replication intermediates which are formed in untreated cells. Aphidicolin prevents the formation of the heterogeneous population of DNA fragments. However, replication intermediates formed before the blockade with aphidicolin are ligated to high-molecular-weight DNA. In cells released from aphidicolin inhibition, there is preferential labeling of the heterogeneous population of DNA fragments. This population, therefore, shows the same characteristics as the discrete DNA populations formed in untreated cells.

Topics: Adenocarcinoma; Antibiotics, Antineoplastic; Aphidicolin; Cell Line; Colonic Neoplasms; Diterpenes; DNA Replication; DNA, Neoplasm; Fluorouracil; Humans; Kinetics