Potentiation of topoisomerase inhibitor-induced DNA strand breakage and cytotoxicity by tumor necrosis factor: enhancement of topoisomerase activity as a mechanism of potentiation

A combination of tumor necrosis factor (TNF) and the topoisomerase I inhibitor, camptothecin, or the topoisomerase II inhibitors, teniposide and amsacrine, produced dose-dependent synergistic cytotoxicity against the murine L929 fibrosarcoma cells. Similar synergy was not observed with a combination of TNF and bleomycin. To define the role of TNF in the augmentation of tumor cell killing by topoisomerase I or II inhibitors, the effect of TNF on the production of enzyme-linked DNA strand breaks induced in cells by topoisomerase inhibitors was investigated. L929 cells incubated for 1 h with the topoisomerase inhibitors contained protein-linked strand breaks. In contrast, TNF alone did not induce DNA strand breakage. However, when cells were incubated simultaneously with TNF and camptothecin, amsacrine, Adriamycin, actinomycin D, teniposide, or etoposide, increased numbers of strand breaks were produced. Preincubation of the cells with TNF for 30 min or 3 h before the addition of camptothecin or etoposide resulted in no more strand breaks than that observed in cells incubated with the drugs alone. TNF treatment of L929 cells produced a rapid and transient increase in specific activity of extractable topoisomerases I and II. These increases were maximum at 2-5 min of TNF treatment and by 30 min the activities of extractable enzymes were equal to or less than those detected in extracts from untreated cell controls. The transient nature of the increase in extractable topoisomerase activity may explain the kinetics and significance of the order of addition of TNF and inhibitors for maximal synergistic activity. These data are consistent also with a role for topoisomerase-linked DNA lesions in the TNF-mediated potentiation of killing of L929 cells by topoisomerase inhibitors.     

Reduced formation of protein-associated DNA strand breaks in Chinese hamster cells resistant to topoisomerase II inhibitors

DNA intercalating drugs and the epipodophyllotoxins etoposide and teniposide interfere with the action of mammalian DNA topoisomerase II by trapping an intermediate complex of the enzyme covalently linked to the 5'-termini of DNA breaks. This effect can be observed in intact cells by alkaline elution measurement of protein-associated DNA strand breaks. To assess the cytotoxic role of this effect, we have studied a subline of DC3F Chinese hamster lung cells selected for resistance to the intercalating agent 9-hydroxyellipticine. This subline (DC3F/9-OHE) was cross-resistant to other intercalators as well as to etoposide. Resistance to Adriamycin was associated with reduced uptake. However, resistance to 4'-(9-acridinylamino)methanesulfon-m-aniside and 2-methyl-9-hydroxyellipticinium was observed in the absence of changes in drug uptake, suggesting a second mode of resistance. DC3F/9-OHE cells formed fewer protein-associated DNA strand breaks in response to 4'-(9-acridinylamino)methanesulfon-m-aniside, 2-methyl-9-hydroxyellipticinium, or etoposide than did the sensitive parental cells. The same was true for isolated nuclei from these cells, which is consistent with a mode of resistance unrelated to drug uptake through the plasma membrane. These data suggest that resistance to DNA topoisomerase II inhibitors exhibited by DC3F/9-OHE cells is due in part to a modification of topoisomerase II activity.     

Potentiation by tumor necrosis factor of mitoxantrone cytotoxicity to human ovarian cancer cell lines.

The cytotoxic activity of human recombinant tumor necrosis factor (rHuTNF) (from 0.01 to 10000 U/ml) was assayed on six human ovarian cancer cell lines and one human cervical carcinoma cell line using a crystal violet assay. rHuTNF was cytotoxic to four cell lines (A2780, A2774, SW626, PA1), while 3 cell lines (IGROV1, SKOV3, Me180) were marginally sensitive to its activity. However, under the same experimental conditions rHuTNF markedly enhanced the cytotoxicity of mitoxantrone, a chemotherapeutic drug targeted at DNA topoisomerase II, in six cell lines. The potentiation of mitoxantrone cytotoxicity was not caused by increased drug accumulation after rHuTNF treatment. No significant increase in cytotoxicity to Me180 cell line was seen when rHuTNF was added to mitoxantrone.     

DNA topoisomerase II immunostaining in human leukemia and rhabdomyosarcoma cell lines and their responses to topoisomerase II inhibitors.

DNA topoisomerase II is an enzyme that affects nuclear structure and function and is the target of a number of anticancer drugs in clinical use, including teniposide (VM-26). We have used our polyclonal antisera that recognize both the M(r) 170,000 and 180,000 forms of topoisomerase II to examine the nuclear distribution of topoisomerase II in cytospin preparations of drug-sensitive (CEM) and VM-26-resistant (CEM/VM-1 and CEM/VM-1-5) human leukemic lymphoblasts. We have also examined the nuclear distribution of topoisomerase II in monolayer cultures of a human rhabdomyosarcoma (Rh30) cell line. In the absence of drug, we observed a focal "patchy" staining of nuclear topoisomerase II in all cell lines, that was especially notable in the lymphoblastic cells. Treatment of CEM and Rh30 cells with VM-26 under conditions that increase the number of covalent topoisomerase II-DNA complexes increased both the intensity and the homogeneity of nuclear topoisomerase II staining in a subpopulation of cells; focal staining was less evident after treatment with drug. These responses were roughly proportional to the concentration of VM-26 used and required only brief (approximately 25-min) incubation with drug. We also found that treatment of CEM cells with 4'-(9-acridinylamino)methanesulfon-m-anisidide similarly increased the intensity and homogeneity of nuclear topoisomerase II immunostaining. In contrast, 4'-(9-acridinylamino)methanesulfon-o-anisidide and 1-beta-D-arabinofuranosylcytosine, agents that do not inhibit topoisomerase II, did not produce this effect. Finally, the VM-26-mediated alteration in topoisomerase II staining intensity and distribution was attenuated in proportion to the degree of VM-26 resistance in the CEM/VM-1 and CEM/VM-1-5 sublines. These results appear to be related to the ability of the drug to stabilize DNA-topoisomerase covalent ("cleavable") complexes in intact cells. Our findings indicate that anti-topoisomerase II drugs, such as VM-26, have profound effects on the ability to detect topoisomerase II in the nucleus and provide a novel way of examining drug-stabilized DNA topoisomerase II complexes in intact single tumor cells.     

Cross-resistance of topoisomerase I and II inhibitors in neuroblastoma cell lines

Purpose: We have previously shown that neuroblastoma cell lines established from patients after intensive chemotherapy show sustained resistance to various drugs and especially high resistance to etoposide (up to 51 times higher than a clinically achievable level). To determine whether topoisomerase I inhibitors (topotecan and CPT-11) are effective against etoposide-resistant neuroblastomas, we studied the response to topotecan and the active metabolite of CPT-11 (SN-38) in 19 cell lines with a spectrum of sensitivities to etoposide. Materials and methods: The panel included cell lines established at diagnosis and after disease progression either during induction chemotherapy or after myeloablative therapy supported with bone marrow transplantation. Cytotoxicities of topotecan, SN-38, and etoposide were determined using a microplate digital image microscopy (DIMSCAN) assay with a 4-log dynamic range. Results: All six etoposide-resistant cell lines were resistant to topotecan and SN-38 (resistance defined as LC₉₀ higher then clinically achievable levels for the drug). Significant cross-resistance by Pearson's correlation analysis (r ≥ 0.6) occurred between topotecan + etoposide, topotecan + SN-38, and etoposide + SN-38. Conclusions: Topotecan and CPT-11 do not have significant activity against most etoposide-resistant neuroblastoma cell lines and this suggests that agents other than topoisomerase inhibitors should be explored for the treatment of recurrent neuroblastomas. Received: 19 January 1999 / Accepted: 24 May 1999     

Mechanisms of resistance to DNA topoisomerase II inhibitors]

Cytotoxicity of anticancer drugs are depended on cellular differences of drug transport processes, drug metabolism or proliferation process including DNA synthesis. Thus, resistance to anticancer drugs is likely due to many cellular changes. Mechanisms of resistance to anticancer drugs have been studied with cells selected for their ability to grow in the presence of drugs. In this paper, the mechanisms of cellular resistance to DNA topoisomerase II inhibitors was described.     

Novel actions of inhibitors of DNA topoisomerase II in drug-resistant tumor cells

We review herein current work on the cytotoxic and cellular actions of two classes of inhibitors of DNA topoisomerase II: one represented by etoposide and teniposide, which stabilize DNA-protein complexes, and another represented by merbarone and aclarubicin, which do not stabilize such complexes. We discuss current concepts of protooncogene activation and cell cycle perturbations by some of these inhibitors and summarize recent findings of novel actions of the latter compounds in tumor cells that express a mutant topoisomerase II.Paper presented at the Topoisomerase Inhibitors Conference, University of Maryland Cancer Center, 27–30 October 1993. Supported in part by Bristol-Myers Oncology Division.     

Synergistic effect of inhibitors of topoisomerase I and II on chromosome damage and cell killing in cultured Chinese hamster ovary cells

Simultaneous treatment of cultured Chinese hamster ovary cells with the topoisomerase I inhibitor camptothecin and the topoisomerase II inhibitor 4-(9-acridinylamino)-methanesulfon-m-anisidide results in a clear synergistic effect on both chromosome damage detected at metaphase and loss of colony-forming ability. In contrast, the effect of combined treatment with these topoisomerase inhibitors on sister chromatid exchanges was not significantly different from that expected if the effects were additive. Taken as a whole, these results seem to support the hypothesis that topoisomerase inhibitors can lead to cell death, presumably when DNA replication forks collide with drug-stabilized cleavable complexes. Nevertheless, no evidence of apoptosis was obtained from DNA fragmentation analysis. The possible clinical implications of our findings are discussed.     

Dependence of etoposide-induced cytotoxicity and topoisomerase II-mediated DNA strand breakage on the intracellular ionic environment

We have found that blockade of the Na+,K+-pump by the cardiac glycoside ouabain protects human A549 and hamster V79 cells from the cytotoxic effects of the topoisomerase II poison etoposide. One thousand-fold higher concentrations of ouabain were required to protect V79 cells compared to A549 cells. Since this difference parallels previously measured differences in pump sensitivity, it suggests that protection is mediated directly through pump blockade. Ouabain affected neither the cellular influx nor efflux of etoposide. However, pump blockade did decrease the formation of etoposide-induced DNA-topoisomerase, II-cleavable complexes, assessed as single and double strand DNA breaks using alkaline and neutral elution. To determine if this decrease were a direct effect of change in ionic environment produced by pump blockade, experiments with isolated nuclei and partially purified topoisomerase II were performed. Etoposide-induced cleavable complex formation and topoisomerase-mediated decatenation were assessed in buffers which mimicked either normal intracellular ionic conditions or those produced by ouabain. Compared to the buffer which resembled the normal intracellular ionic conditions, the buffer that mimicked the conditions produced by pump blockade produced fewer etoposide-mediated cleavable complexes in isolated nuclei and less decatenating activity of partially purified topoisomerase II. These findings demonstrate that inhibition of the Na+,K+-pump causes an alteration in the intracellular ionic environment which decreases the activity of topoisomerase II, thus producing a decrease in etoposide-induced cleavable complex formation and cytotoxicity. Since ionic changes occur inside normal cells during progression through the cell cycle as well as in cells that have undergone transformation, these data suggest that the intracellular ionic environment plays a role in determining the sensitivity of normal and malignant cells to this group of chemotherapeutic agents.     

Enhancement of lysosomal enzyme activity by recombinant human tumor necrosis factor and its role in tumor cell killing in vitro.

We investigated the effect of recombinant human tumor necrosis factor (TNF) on the lysosomal enzyme activity of various established cell lines in vitro. Incubation of 1 x 10(6) TNF-sensitive mouse tumorigenic fibroblasts (L-M cells) in the presence of TNF (100 U/ml) for 48 h increased the total (the sum of the enzyme activities in the lysosomes and the cytoplasm) acid phosphatase and beta-glucuronidase activities by 3.7- and 4.2-fold, respectively. The same increase was observed even when 1 U/ml of TNF was added to some cultures and no further augmentation occurred at 10 or 100 U/ml. Measurement of total and free enzyme activities showed that TNF stimulation not only enhanced the total intracellular enzyme activity but also accelerated the conversion into free (cytoplasmic) enzyme activity. Addition of a lysosomotropic agent (methylamine) suppressed both the enhancement of lysosomal enzyme activity and the cytotoxicity of TNF. A similar enhancement of lysosomal enzyme activities was also detected in various TNF-sensitive tumor cell lines, and a strong correlation (acid phosphatase: r = 0.836, beta-glucuronidase: r = 0.910) was observed between the enhancement of enzyme activity and sensitivity to TNF. No such increase was detected in TNF-resistant human diploid cells. These results show that TNF induces the activation and release of lysosomal enzymes in TNF-sensitive cells, and suggest that such events may play an important role in TNF-mediated cytotoxicity.     

Mechanisms of acquired resistance to DNA topoisomerase I inhibitors]

The mechanisms of acquired resistance to mammalian DNA topoisomerase I inhibitor, camptothecin or its derivative (CPT-11) were described. Tumor cell lines containing altered or reduced topoisomerase I were reviewed. It might be necessary to further study on reduced inhibitor uptake and other unknown mechanism.     

Activation of programmed cell death by recombinant human tumor necrosis factor plus topoisomerase II-targeted drugs in L929 tumor cells.

The mechanism of death induced by recombinant human tumor necrosis factor (rHuTNF) in L929 tumor cells of C3H mice was investigated. Treatment with rHuTNF led to fragmentation of DNA into nucleosomal oligomers and to induction of the expression of TRPM-2, a programmed cell death-associated gene. Both events preceded cell death by several hours. Treatment with DNA topoisomerase II inhibitors accelerated both the rHuTNF-mediated DNA fragmentation and the elevation in TRPM-2 messenger RNA levels. These results suggest that rHuTNF exerts its cytotoxicity on L929 cells by activating programmed cell death, leading to apoptosis, and that topoisomerase II inhibitors enhance rHuTNF-mediated cytotoxicity by accelerating this process.     

Molecular effects of topoisomerase II inhibitors in AML cell lines: correlation of apoptosis with topoisomerase II activity but not with DNA damage.

We examined the cellular effects of topo II inhibitors in two human myeloid cell lines, HL-60 and KG-1 cells, with the purpose of finding molecular markers for the sensitivity of leukemia cells to topo II inhibitors. These cell lines are widely used, well characterized and they differ in their sensitivities to topo II inhibitors. Despite the fact that HL-60 cells are p53-negative, they are much more sensitive than KG-1 cells. Three different topo II inhibitors with distinct molecular ways of action have been used. Daunorubicin and aclarubicin are DNA intercalators that secondarily interact with topo II; etoposide, on the other hand, directly binds to the enzyme. In contrast to daunorubicin, which induces protein-associated DNA double-strand breaks due to the blockage of topo II action, aclarubicin inhibits the access of DNA by topo II. No correlation could be established between the drug-induced DNA damage and apoptosis. In fact, the amount and pattern of DNA damage examined with the 'comet assay' was characteristic for each drug in both cell lines. The DNA binding of daunorubicin was slightly higher in HL-60 cells, but there was no notable variance between the cell lines for aclarubicin. The most striking difference could be found for the nuclear topo II activity, which was about half in KG-1 cells and, additionally, less than 1% of the nuclear topo II activity was bound to the DNA in KG-1 cells when compared to HL-60 cells. This fraction of topo II interacts with the inhibitors; subsequently these findings might well explain the variance in the cellular sensitivity. Additional factors are alterations of the apoptotic pathways, eg loss of p53 in HL-60 cells. Although we found no differences in the quantity of DNA damage between the cell lines after drug treatment, the quality of DNA damage appeared to be distinct for each topo II inhibitor. The morphological appearance of the comet tails after treatment was characteristic for each drug. Further studies are necessary to decide whether these in vitro data are compatible with the clinical situation.     

Induction of DNA double-strand breaks by 8-methoxycaffeine: cell cycle dependence and comparison with topoisomerase II inhibitors

We have studied the ability of 8-methoxycaffeine (8-MOC)—oneof the most effective caffeine derivatives in inducing chromosomalaberrations—to induce DNA double strand breaks (DSB) inpurified human T lymphocytes during the cell cycle. Etoposide-or ellipticine-mediated DNA break frequency was used as a parameterof topoiso merase II activity. DNA-DSB induced by either 8-MOCor VP16 or ellipticine rose co-ordinately with the level ofDNA topoisomerase II and with the onset of DNA replication.At concentrations between 10 and 50 .µM 8-MOC was     

Relationship between expression of topoisomerase II isoforms and intrinsic sensitivity to topoisomerase II inhibitors in breast cancer cell lines.

Topoisomerase II is a key target for many anti-cancer drugs used to treat breast cancer. In human cells there are two closely related, but differentially expressed, topoisomerase II isoforms, designated topoisomerase II alpha and beta. Here, we report the production of a new polyclonal antibody raised against a fragment of the C-terminal domain of the 180 kDa form of topoisomerase II (the beta isoform), which does not cross-react with the 170 kDa form (the alpha isoform). Using this antibody, together with a polyclonal antibody specific for the 170 kDa isoform of topoisomerase II, we have examined the relationship between the sensitivity of a panel of human breast cancer cell lines to different classes of topoisomerase II inhibitors and cellular levels of the topoisomerase II alpha and beta proteins. We found that sensitivity to amsacrine showed a correlation with the level of expression of topoisomerase II alpha protein, and that sensitivity to etoposide showed a similar correlation with the level of expression of topoisomerase II beta protein. There was also a relationship between sensitivity of these cell lines to mitoxantrone and the cellular level of both isoforms of topoisomerase II. No relationship was found between the level of mRNA for topoisomerase II alpha or beta, and either sensitivity of breast cancer cell lines to topoisomerase II inhibitors or the level of topoisomerase II protein expression.     

DNA topoisomerase I and II expression in drug resistant germ cell tumours

A small number of testicular germ cell tumours are refractory to current chemotherapy regimens. DNA topoisomerase I is the target for several new drugs and a potential candidate treatment for chemorefractory germ cell tumours. DNA topoisomerase II alpha is the target for etoposide, which is currently used regularly in germ cell tumour treatment. The expression of DNA topoisomerase I and II alpha were therefore assessed immunohistochemically in a range of testicular tumours, especially those with persistent malignant elements on retroperitoneal lymph node dissection. Pre-chemotherapy orchidectomy specimens were matched with post-chemotherapy retroperitoneal lymph node dissections to examine changes in expression. There was considerable variation in the expression of topoisomerase I in different tumour types. Both yolk sac tumours and teratoma, mature showed universal expression of topoisomerase I, while 38% of seminomas and 30% of embryonal carcinomas were positive. Strong topoisomerase II alpha expression was found in embryonal carcinoma. There was a negative correlation between topoisomerase I and II alpha expression (P=0.004) and downregulation of topoisomerase II alpha after chemotherapy (P=0.02). Topoisomerase I expression appears to increase in those cases with residual teratoma, mature, but is largely unchanged in those cases remaining as embryonal carcinoma. These results suggest that topoisomerase I inhibitors may be useful in chemorefractory germ cell tumours, especially yolk sac tumours and where there are unresectable residual teratoma, mature deposits.     

Determinants of response to the DNA topoisomerase II inhibitors doxorubicin and etoposide in human lung cancer cell lines.

BACKGROUND: Small-cell lung cancer (SCLC) is more sensitive to anticancer agents than non-small-cell lung cancer (NSCLC), but few studies have analyzed the mechanisms of natural drug resistance responsible for this difference. PURPOSE: To elucidate these mechanisms, we determined drug sensitivity and evaluated the biochemical parameters affecting response to the DNA topoisomerase II inhibitors doxorubicin and etoposide in both types of cancer cell lines, in particular the activity and content of DNA topoisomerase II, as well as etoposide uptake and cell doubling time. METHODS: Drug sensitivity and cellular uptake of etoposide were determined by clonogenic assay and accumulation of radiolabeled drug, respectively. The topoisomerase II activity was assayed by decatenation of kinetoplast DNA to minicircle DNA using nuclear protein, and the content was determined by immunoblot analysis of nuclear extracts. We also compared the topoisomerase II content in parent cell lines with that in lines with cisplatin resistance acquired in vitro. RESULTS: Sensitivities to doxorubicin and etoposide were higher in SCLC cell lines than in NSCLC lines, and the difference was statistically significant. Etoposide uptake in SCLC cells was higher than in NSCLC cells; the difference was statistically significant, but this difference may not be sufficient to account for the variation in sensitivities of the cell lines. Topoisomerase II activities of nuclear protein from SCLC cell lines were reproducibly twofold higher than those for NSCLC cell lines. The topoisomerase II content in nuclear protein appeared to be higher in SCLC cell lines than in NSCLC cell lines and corresponded to the sensitivities to doxorubicin and etoposide. In the cisplatin-resistant NSCLC cell lines PC-7/CDDP and PC-14/CDDP, the topoisomerase II content was increased compared with that in the parent lines, but the topoisomerase II content in other cisplatin-sensitive parent lines was similar to that in resistant sublines. CONCLUSIONS: These findings suggest that the topoisomerase II activity and content may be major factors in determining sensitivity to topoisomerase II inhibitors.     

Elevated expression of DNA topoisomerase II in camptothecin-resistant human tumor cell lines

In a previous study, we established camptothecin (CPT)-resistant cell lines, A549/CPT and HT-29/CPT, from human lung cancer A549 and human colon cancer HT-29. A549/CPT was shown to express similar amounts of DNA topoisomerase I (topo I) as the parental line, and HT-29/CPT was shown to express lower amounts of topo I than its parental line. DNA topoisomerases I and II are known to be functionally related. In the present study, the possible alterations in topo II expression were examined in these human CPT-resistant lines. In A549/CPT and HT-29/CPT, the cellular contents of topo II and its mRNA were elevated over that seen in each parental line. Nuclear extracts from A549/CPT and HT-29/CPT showed higher topo II activity than those from the corresponding parental lines when the same amounts of nuclear protein were used. Topo II was partially purified from HT-29 and HT-29/CPT by hydroxylapatite column chromatography, and the enzyme activities were compared. HT-29/CPT showed higher topo II activity in the hydroxylapatite column-eluted fractions than HT-29. These results indicate the possible activation of topo II expression in the CPT-resistant cell lines.