Resistance to inhibitors of DNA topoisomerases

Human multiple myeloma cells are sensitized to topoisomerase II inhibitors by CRM1 inhibition

Topoisomerase IIalpha (topo IIalpha) is exported from the nucleus of human myeloma cells by a CRM1-dependent mechanism at cellular densities similar to those found in patient bone marrow. When topo IIalpha is trafficked to the cytoplasm, it is not in contact with the DNA; thus, topo IIalpha inhibitors are unable to induce DNA-cleavable complexes and cell death. Using a CRM1 inhibitor or a CRM1-specific small interfering RNA (siRNA), we were able to block nuclear export of topo IIalpha as shown by immunofluorescence microscopy. Human myeloma cell lines and patient myeloma cells isolated from bone marrow were treated with a CRM1 inhibitor or CRM1-specific siRNA and exposed to doxorubicin or etoposide at high cell densities. CRM1-treated cell lines or myeloma patient cells were 4-fold more sensitive to topo II poisons as determined by an activated caspase assay. Normal cells were not significantly affected by CRM1-topo II inhibitor combination treatment. Cell death was correlated with increased DNA double-strand breaks as shown by the comet assay. Band depletion assays of CRM1 inhibitor-exposed myeloma cells showed increased topo IIalpha covalently bound to DNA. Topo IIalpha knockdown by a topo IIalpha-specific siRNA abrogated the CRM1-topo II therapy synergistic effect. These results suggest that blocking topo IIalpha nuclear export sensitizes myeloma cells to topo II inhibitors. This method of sensitizing myeloma cells suggests a new therapeutic approach to multiple myeloma.

Impact of Topoisomerase II alpha and spermine on the clinical outcome of children with acute lymphoblastic leukemia

It has been reported in the literature that a leukemic cell may be (or become) resistant to anti-cancer treatment because many mechanisms, such as efflux membrane pump (multi-drug resistance, MDR-P170), intracellular transport (LRP, MRP), or different detoxification systems (glutathione transferases, methallothioneines) may be implicated. Topoisomerase II alpha (TopoII) are also reported as responsible for resistance since their main action is to repair DNA breakage. Polyamines are described as having a protective DNA action by stabilizing the double stranded DNA helix. For these reasons we investigated 65 children with acute lymphoblastic leukemia using an immunocytochemical method to elucidate the potential role of Topoisomerase and polyamines in drug resistance. Most children (60/65) were treated with the French (acute lymphoblastic leukemia, ALL) protocol (FRALLE-93) in which B and C arms include (at least) VP16. Children with cytoplasmic TopoII positivity (18 cases) were more resistant since their overall survival was 34 months compared to more than 110 months for negative cases ( P = 0.0003). Polyamines may be associated with drug resistance since the overall survivals were 51 months and 92 months for positive and negative patients, respectively, but the P-value is only 0.13. We conclude that Topoisomerase and polyamines must be tested at diagnosis as new possible markers for chemo-resistance. Larger series are needed to confirm these preliminary results and to verify if the use of anti epipodophillotoxin agents (as it is the case for FRALLE B or C) should be excluded for positive cases.

Inconstant association between 27-kDa heat-shock protein (Hsp27) content and doxorubicin resistance in human colon cancer cells. The doxorubicin-protecting effect of Hsp27

To investigate the role of the small 27-kDa heat-shock protein (Hsp27) in the intrinsic resistance of colon cancer cells to doxorubicin, we modified Hsp27 expression either genetically by transfection or pharmacologically by cisplatin treatment. HT-29 cells were transfected with a full-length Hsp27 construct in the sense or antisense orientation. We found a good correlation between cell survival after doxorubicin treatment and Hsp27 content. A similar correlation was found for the thermoresistance of the Hsp27-transfected cells. In contrast, the sensitivity of the different transfected cells to 5-fluorouracil was not modified. cis-Platinum(II)diammine dichloride (cisplatin) treatment of HT-29 or Caco2 cells dramatically increased their Hsp27 mRNA and protein content. Accordingly, the cells became thermoresistant. Contrary to what has been previously assumed, however, cell resistance to doxorubicin was reduced. Our data suggest that the decreased resistance of the cells to doxorubicin may be due to a concomitant increase of topoisomerase II expression, the main target of anthracyclines. In conclusion, although Hsp27 seems to participate in the natural resistance of colon cancer cells to anthracyclines, its increase after cisplatin treatment is not associated with a decreased cytotoxicity to doxorubicin.

Circumvention of confluence-dependent resistance in a human multi-drug-resistant colon-cancer cell line

Colorectal adenocarcinomas are inherently resistant to anthracyclines and other topoisomerase-II inhibitors. Resistance to doxorubicin of colon cancer cells (Caco2) depends on 2 main mechanisms. The first is typical multi-drug resistance, characterized by the mdr1 gene and its product the P170 membrane glycoprotein. P170 effluxes anthracyclines out of cancer cells and is antagonized in vitro by verapamil. The second mechanism, which develops when cell-culture density increases, we have designated confluence-dependent resistance. Confluence-dependent resistance depends on the reduced topoisomerase II content of the G0/G1-phase cells which accumulate in the confluent population. We show here that short treatments of confluent Caco2 cells with slightly toxic concentrations of DNA-damaging agents (cisplatin, melphalan or mitomycin C) produced a transient accumulation of cells in S- and G2/M-phases of the cell cycle. Concomitantly with the increase in the S-phase population, the topoisomerase II cellular level and the sensitivity of cells to doxorubicin were greatly enhanced. Overcoming confluence-dependent resistance through S-phase accumulation and inhibition of multi-drug resistance by verapamil were fully additive, and a nearly complete reversal of confluent Caco2 cells' resistance to doxorubicin was obtained when both strategies were combined.

Multiple mechanisms of resistance in a series of human testicular teratoma cell lines selected for increasing resistance to etoposide

Mechanisms of resistance to VP-16 were monitored in a series of sublines of the human testicular teratoma cell line (SuSa) derived following exposure either to fractionated X-irradiation (DXR-10) or to VP-16 using pulsed 24-hr exposures (VP10) or continuous exposure conditions (VPC2, VPC3 and VPC4). Orders of resistance expressed (ranging from 3- to 33-fold based on IC50 values derived from colony forming assays) were comparable with those likely to be encountered clinically. All of these resistant sublines showed some cross-resistance to VCR, and the 3 drug-selected sublines tested also proved cross-resistant to ADR. Resistance was not associated with modified 3H-VP-16 accumulation. However, decreased VP-16-induced SSBs were detectable in all the resistant sublines and a strong positive correlation was noted between the extent of SSB formation and VP-16 resistance by linear regression analysis. Topo II alpha protein content, as judged by Western blotting, was significantly decreased only in the sublines derived by continuous exposure to VP-16, but this was not progressive with increasing levels of resistance expressed. RNase protection assays also showed no significant differences in Topo II alpha expression in the low-level resistant DXR-10 and VP10 sublines, contrasting with the 2-fold decreases identified in the VPC2, VPC3 and VPC4 sublines. Significantly, however, mRNA levels of two alternately spliced Topo II beta mRNAs were markedly decreased (2- to 9-fold) in all the drug-selected resistant sublines. No mutations in consensus ATP-binding sequences or in the DNA-binding region of Topo II alpha were detected by single strand conformational polymorphism analysis. Significant Pgp overexpression was only identified in the most highly resistant sublines VPC3 and VPC4, which both showed 4-fold cross-resistance to VCR. Decreased 3H-VCR accumulation and partial reversal of resistance by VPM (6.6 microM) addition was also identified, consistent with a functional Pgp being overexpressed in these sublines. Modifications of Topo II expression therefore appear to precede Pgp overexpression in this series of sequentially derived VP-16 resistant sublines and to represent the predominant mechanism underlying low level (< 10-fold) resistance.

Reduced levels of topoisomerase II alpha and II beta in a multidrug-resistant lung-cancer cell line

We have previously shown that the doxorubicin-selected multidrug-resistant small-cell lung-cancer cell line H69AR is resistant to VP-16-induced single-strand DNA breaks as compared with its parental H69 cell line. Levels of immunoreactive topoisomerase II alpha are also reduced in H69AR cells. In the present study, we found that cleaved complex formation in the presence of VP-16 was decreased in H69AR cells as compared with H69 cells. In addition, the resistant cells contained lower levels of both topoisomerase II alpha and topoisomerase II beta protein and mRNA. However, these changes were not accompanied by a decrease in the P4-unknotting (strand-passing) activity of 0.67 M NaCl nuclear extracts of H69AR cells, nor was there any difference in VP-16 inhibition of unknotting activity in the H69 and H69AR nuclear extracts. These data suggest that reduced levels of topoisomerase II alpha and II beta may contribute to the resistance of H69AR cells to VP-16 and other drugs that target these isoenzymes.

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.

A drug-resistant variant of topoisomerase II alpha in human HL-60 cells exhibits alterations in catalytic pH optimum, DNA binding and sub-nuclear distribution

Anion-exchange chromatography of partially purified human HL-60 topoisomerase II resolves the known alpha (170 kDa) and beta (180 kDa) isoenzymes at 150 mM NaCl and 230 mM NaCl, respectively. An additional topoisomerase II fraction was eluted by > 300 mM NaCl. It could be identified by Western blotting as a late-eluting variant of topoisomerase II alpha, which is functionally altered as compared to the early-eluting form, having the following properties: a shift in the catalytic optimum to pH 9; increased stability in DNA complex formation; approximately 100-fold resistance to orthovanadate; approximately 1000-fold resistance to the cytostatic substances N-[4-(9-acridinylamino)-3-methoxyphenyl]-methanesulphonamide (amsacrine) and the podophyllotoxin etoposide (VP 16). 80% of the late-eluting topoisomerase II alpha could be captured by SDS on calf thymus DNA without further enhancement by drugs. In contrast, the early-eluting topoisomerase II alpha exhibits 10% complex formation with SDS alone, and an increase to 90% complex formation in the presence of drugs. A HL-60 subline (HL-60/R), approximately 1000-fold resistant to etoposide and amsacrine, has equivalent proportions of topoisomerase II alpha and topoisomerase II beta and similar levels of both isoenzymes, as compared to the drug-sensitive HL-60/WT cells. However, determination of the cellular levels of the early-eluting and late-eluting forms of topoisomerase II alpha revealed that the HL-60/R cell line contains approximately 80% of the late-eluting topoisomerase II alpha, whereas the sensitive HL-60/WT cell line contains only 15-20% of this form. The nuclear distribution of the two forms also differs. Sensitive HL-60/WT cells show a diffuse nuclear distribution but in resistant cells the distribution is localized in the nucleoli. Apparently two functionally distinct subforms of topoisomerase II alpha coexist in drug-sensitive and drug-resistant HL-60 cells and changes in their relative levels affect the cellular sensitivity to topoisomerase-II-targeting drugs.

Topoisomerase II in multiple drug resistance

Topoisomerase II is a target of alkaloid, anthracycline and related antitumor agents. Two types of multiple drug resistance are associated with these enzymes. In classical (typical) multidrug resistance, inhibitors are actively effluxed from cells by P-glycoprotein. In atypical multidrug resistance, topoisomerase II is either reduced in cellular content or mutated to a form that does not interact with inhibitors. Because cytotoxicity of most antineoplastic topoisomerase II inhibitors is directly related to the number of active topoisomerase II molecules, a reduction in this number leads to resistance. In the topoisomerase II mechanism, through which the DNA linking number is altered, DNA double strands are cleaved, and the termini transiently bound covalently (5') or noncovalently (3') to the enzyme while a second double strand is passed through the break in the first. This transition state complex then decays to enzyme and DNA of altered linking number. Most cytotoxic topoisomerase II inhibitors stabilize these reaction intermediates as ternary complexes, which are converted to lethal lesions when cells attempt to utilize the damaged DNA as templates. Toxicity is related to topoisomerase II content as well as to drug concentration. Thus, multidrug resistance results from either 1) decreasing cellular content of the inhibitor by P-glycoprotein (typical) or 2) decreasing cellular content and/or activity of the target, topoisomerase II, as, for example, when its content or activity is modulated downward by decreased expression, deactivation, or by mutations to the TopII gene, producing an enzyme that reacts poorly with inhibitors (atypical). Mixed types, i.e., both typical and atypical, are known. Attempts to abrogate or prevent both typical and atypical multidrug resistance to topoisomerase II inhibitors have been described.