Effect of transfection of a Drosophila topoisomerase II gene into a human brain tumour cell line intrinsically resistant to etoposide

Adenovirus-mediated human topoisomerase IIalpha gene transfer increases the sensitivity of etoposide-resistant human and mouse breast cancer cells

Cellular resistance to chemotherapeutic agents is attributable to several mechanisms, including alteration of topoisomerase IIa gene expression. Our previous studies have shown that transient transfection with a vector containing either Drosophila or human topoisomerase IIalpha gene into drug-resistant tumor cells enhanced their drug sensitivity. Furthermore, we constructed a recombinant adenovirus, Ad-hTopoIIalpha, containing the human topoisomerase IIa gene that was able to selectively increase etoposide sensitivity in drug-resistant tumor cells. We also examined Ad-hTopoIIalpha for therapeutic efficacy in vitro using additional etoposide-resistant cell lines, including a mouse breast cancer cell line and a human leukemia cell line. The etoposide-resistant mouse breast cancer cell line FvP, which is derived from FM3A, and etoposide-resistant human breast cancer cell line, MDA-VP, which derived from MDA-P cells showed increased sensitivity to etoposide as well as increased expression of human Topoisomerase IIa mRNA, but this was not seen in FM3A and MDA-P cells. On the other hand, the etoposide-resistant human leukemia cell line K562/MX2 and the parental cell line K562/P did not show enhanced sensitivity against etoposide or an increase in human Topoisomerase IIa mRNA. Using a recombinant adenovirus containing beta-galactosidase gene (Ad-beta-gal), K562 cells were not transducted by the recombinant adenovirus, while both etoposide-sensitive FM3A cells and etoposide resistant FvP cells were transducted by recombinant adenovirus. Ad-hTOP2alpha and etopside treatment showed reduced inoculated tumor weight in the mice. We concluded that a recombinant adenovirus containing the human Topoisomerase IIalpha gene might be a powerful tool for overcoming drug resistance in breast cancer cells, but not in leukemia cells.

The 17q12-q21 amplicon: Her2 and topoisomerase-IIalpha and their importance to the biology of solid tumours

Her2 and topoisomerase-IIalpha (T2A) gene amplification are separate events, although the latter is more frequently seen in Her2 amplified (34-90%) than in Her2 non-amplified (5-10%) tumours. There is a better correlation between Her2 amplification and protein overexpression in breast cancer (BC) than in other tumour types. This marker is also considered a powerful prognostic factor in BC, with similar data emerging in other solid tumours such as bladder, ovarian, endometrial, gastro-oesophageal and non-small cell lung cancer. Her2 amplification and/or overexpression are highly predictive of response to HER2-targeted compounds such as trastuzumab and lapatinib but have been inconsistent predictors of response to cytotoxic chemotherapy. There is also evidence that these tumours are relatively resistant to anti-oestrogen therapy (tamoxifen) but not to oestrogen deprivation (e.g. with aromatase inhibitors). T2A aberrations are uncommon events in solid tumours, with an overall prevalence of approximately 10%. T2A amplification has shown inconsistent correlation with T2A protein expression in preclinical and clinical studies, mainly because non-genetic events such as proliferation rate can also affect protein expression. Expression of T2A protein has not been shown to reliably predict response to T2A inhibitors, despite the fact that this enzyme is the direct target for these compounds. In BC, T2A amplification appears to be a good predictor of response to anthracyclines, but these data are still in the process of validation. The significance of T2A deletions is currently under investigation, but contrary to what was previously thought, it may also predict benefit from treatment with T2A inhibitors. The prognostic significance of T2A aberrations is currently unknown.

Altered expression of topoisomerase IIalpha contributes to cross-resistant to etoposide K562/MX2 cell line by aberrant methylation

KRN 8602 (MX2) is a novel morpholino anthracycline derivative having the chemical structure 3′-deamino-3′-morpholino-13-deoxo-10-hydroxycarminomycin hydrochloride. To investigate the mechanisms of resistance to MX2, we established an MX2-resistant phenotype (K562/MX2) of the human myelogeneous leukaemia cell line (K562/P), by continuously exposing a suspension culture to increasing concentrations of MX2. K562/MX2 cells were more resistant to MX2 than the parent cells, and also showed cross-resistance to etoposide and doxorubicin. Topoisomerase (Topo) IIα protein levels in K562/MX2 cells were lower of those in K562/P cells on immunoblot analysis and decreased expression of Topo IIα mRNA was seen in K562/MX2 cells. Topoisomerase II catalytic activity was also reduced in the nuclear extracts from K562/MX2 cells when compared with K562/P cells. Aberrant methylated CpG of Topo IIα gene was observed in K562/MX2 cells when compared with the parent line on methylation-specific restriction enzyme analysis. To overcome the drug resistance to MX2 and etoposide, we investigated treatment with 5-Aza-2′-deoxycytidine (5AZ), which is a demethylating agent, in K562/MX2 cells. 5-Aza-2′-deoxycytidine treatment increased Topo IIα mRNA expression in K562/MX2 cells, but not in K562/P cells, and increased the cytotoxicity of MX2 and etoposide. Methylated CpG was decreased in K562/MX2 cells after 5AZ treatment. We concluded that the mechanism of drug resistance to MX2 and etoposide in K562/MX2 cells might be the combination of decreased expression of Topo IIα gene and increased methylation, and that 5AZ could prove to be a novel treatment for etoposide-resistant cell lines, such as K562/MX2.

HER-2/neu and topoisomerase IIalpha in breast cancer

In breast cancer, the predominant genetic mechanism for oncogene activation is through an amplification of a gene. The HER-2 (also known as ErbB2/c-erbB2/HER-2/neu) oncogene is the most frequently amplified oncogene in breast cancer, and its overexpression is associated with poor clinical outcome. In addition to its important role in breast cancer growth and progression, HER-2 is also a target for a new form of chemotherapy. Breast cancer patients have been treated with considerable success since 1998 with trastuzumab, a recombinant antibody designed to block signaling through HER-2 receptor. HER-2 has also been implicated in altering the chemosensitivity of breast cancer cells to different forms of conventional cytotoxic chemotherapy, particularly of topoII-inhibitors (e.g., anthracyclines). Topoisomerase IIalpha gene is located just by the HER-2 oncogene at the chromosome 17q12-q21 and is amplified or deleted in almost 90% of the HER-2 amplified primary breast tumors. Recent data suggests that amplification and deletion of topoisomerase IIalpha may account for both relative chemosensitivity and resistance to anthracycline therapy, depending on the specific genetic defect at the topoIIalpha locus. Expanding our understanding of HER-2 amplification also changes its role in the pathogenesis of breast cancer. HER-2 is an oncogene that clearly can drive tumor induction and growth and is also a target for a new kind of chemotherapy, but its function as a marker for chemoselection may be due to associated genetic changes, of which topoisomerase IIalpha is a good example. Moreover, despite potential evidence that genes other than HER-2, such as topoisomerase IIalpha, may be more important predictors of therapeutic response in breast cancer, HER-2 status still has a very significant role in therapeutic selection, mainly as the major criterion for administering trastuzumab in treating breast cancer. Thus, the clinical and therapeutic importance of the HER-2 and topoisomerase IIalpha status to breast cancer management should only increase in the next few years.

HER2/neu expression in malignant lung tumors

Despite intensive treatment efforts, the prognosis for lung cancer is very poor; less than 15% of patients survive 5 years. Trastuzumab, a monoclonal antibody targeting the HER2/neu protein receptor, is effective in the treatment of metastatic breast cancer and may be useful in the treatment of non-small cell lung cancer (NSCLC). Using the HercepTest (Dako; Carpenteria, CA), 25% of NSCLC show 2+ or greater HER2/neu expression, but only 6% to 8% of NSCLC tumors have 3+ overexpression. Positive HER2/neu expression is most often seen in adenocarcinomas compared with squamous cell carcinomas or large cell carcinomas, and is rarely seen in small cell lung cancer. As determined by fluorescence in situ hybridization analysis, the high degree of HER2/neu gene expression and gene amplification seen in breast cancer is lower in NSCLC. Polysomy is the cause of increased HER2/neu expression in most NSCLC. Prospective clinical studies with trastuzumab in lung cancer are ongoing. Future studies in NSCLC need to include immunohistochemistry and fluorescence in situ hybridization analysis to determine the method of choice for evaluating clinically relevant HER2/neu-positive tumors.

Effects of HER-2/neu on chemosensitivity of tumor cells

In solid tumors, such as breast and ovarian cancer, the predominant genetic mechanism for oncogene activation is through gene amplification. The HER-2 (also known as ErbB2/c-erbB2/HER-2 / neu) oncogene is the most frequently amplified oncogene in breast cancer and its overexpression is associated with poor clinical outcome. In addition to its role in tumor progression, HER-2 has been implicated in altering tumor cell chemosensitivity to cytotoxic chemotherapy, particularly to anthracyclines. However, sophisticated in vitro studies have recently indicated that HER-2 may not have anything to do with the sensitivity of the cancer cells to cytotoxic drugs. Topoisomerase IIalpha gene is a target gene for many cytotoxic drugs and is located just by the HER-2 at the 17q12-q21. TopoIIalpha amplification and deletion may account for both relative chemosensitivity and resistance to anthracycline-therapy depending on the specific genetic defect at the topoIIalpha locus. Whereas HER-2 is an oncogene that clearly can drive tumor induction and growth, its function as a marker for chemoselection may be due to associated genetic changes in the topoIIalpha gene. Copyright 2000 Harcourt Publishers Ltd.

Amplification and deletion of topoisomerase IIalpha associate with ErbB-2 amplification and affect sensitivity to topoisomerase II inhibitor doxorubicin in breast cancer

Topoisomerase IIalpha (topoIIalpha) is a key enzyme in DNA replication and a molecular target for many anti-cancer drugs called topoII inhibitors. The topoIIalpha gene is located at chromosome band 17q12-q21, close to the ErbB-2 oncogene (HER-2/neu), which is the most commonly amplified oncogene in breast cancer. Because of the physical proximity to ErbB-2, copy number aberrations may also occur in the topoIIalpha gene. These topoIIalpha gene copy number aberrations may be related to the altered chemosensitivity to topoII inhibitors that breast cancers with ErbB-2 amplification are known to have. We used fluorescence in situ hybridization to study copy number aberrations of both topoIIalpha and ErbB-2 in nine breast cancer cell lines and in 97 clinical breast tumors, which were selected for the study according to their ErbB-2 status by Southern blotting. TopoIIalpha-protein expression was studied with Western blot and sensitivity to doxorubicin (a topoII inhibitor) with a 96-well clonogenic in vitro assay. Two of the five cell lines with ErbB-2 gene amplification (SK-BR-3 and UACC-812) showed amplification of topoIIalpha. In MDA-361 cells, ErbB-2 amplification (14 copies/cell) was associated with a physical deletion of topoIIalpha (four copies of chromosome 17 centromere and two copies of topoIIalpha). The topoIIalpha amplification in UACC-812 cells was associated with 5.9-fold-increased topoIIalpha protein expression and 2.5-fold-increased sensitivity to the topoII inhibitor, doxorubicin, whereas the deletion in MDA-361 leads to decreased protein expression (45% of control) and a 2.4-fold-increased chemoresistance in vitro. Of 57 ErbB-2-amplified primary breast carcinomas, 25 (44%) showed ErbB-2-topoIIalpha coamplification and 24 (42%) showed a physical deletion of the topoIIalpha gene. No topoIIalpha copy number aberrations were found in 40 primary tumors without ErbB-2 amplification. TopoIIalpha gene amplification and deletion are common in ErbB-2-amplified breast cancer and are associated with increased or decreased sensitivity to topoII inhibitors in vitro, respectively. These findings may explain the altered chemosensitivity to topoII inhibitors reported in ErbB-2-amplified breast cancers.

Predictive value of topoisomerase IIalpha and other prognostic factors for epirubicin chemotherapy in advanced breast cancer

Although cytotoxic chemotherapy is widely used in advanced breast cancer, there are no powerful predictors for the therapy response. Because topoisomerase IIalpha (Topo IIalpha) is the molecular target for the anthracycline class of anti-cancer drugs, we compared the immunocytochemical assay of Topo IIalpha with other biomarkers in the prediction of clinical response to Topo II inhibitor chemotherapy. Fifty-five patients with advanced breast cancer were treated with a single cytotoxic drug, Topo II-inhibitor, epirubicin (30 mg m(-2) weekly up to 1000 mg m(-2)), as first line cytotoxic chemotherapy. Objective response to treatment was analysed according to UICC criteria. The predictive value of Topo IIalpha expression, c-erbB2 oncoprotein, p53 tumour-suppressor protein, oestrogen (ER) and progesterone receptor (PR), S-phase fraction and DNA ploidy were analysed from representative formalin-fixed paraffin-embedded primary tumour samples. The proportion of Topo IIalpha-positive cells (Topo IIalpha index) failed to predict response to epirubicin therapy. Mean Topo IIalpha scores in 29 responding patients were similar when compared with those with no change in disease progression (n = 13) and those with progressive disease (n = 13) (14.9% +/- 11.4% vs 15.5% +/- 7.6% vs 17.3% +/- 13.2%, not significant). Among the other biomarkers tested, overexpression of c-erbB2 oncoprotein and hormone receptor negativity were significantly associated with poor response. Response rate in patients with c-erbB2-overexpressing tumours was 32% compared with 65% in patients with no c-erbB2 overexpression (P = 0.0058). Accordingly, the response rate for ER-positive patients was 67% compared with 26% in ER-negative patients (P = 0.0021). Although both negative ER status and c-erbB2 overexpression are associated with high Topo IIalpha expression in breast cancer, step-wise logistic regression analysis showed that ER and c-erbB2 were associated with therapy response independent of Topo IIalpha expression. Histological grade, p53, DNA-ploidy, tumour proliferation rate (S-phase fraction), stage of the disease at diagnosis, age of the patient, previous anti-oestrogen therapy or site of metastasis did not predict the response to epirubicin therapy. In conclusion, despite extensive in vitro evidence, expression of Topo IIalpha is unlikely to predict the response to Topo II inhibitor chemotherapy in advanced breast cancer. Among the prognostic biomarkers, overexpression of c-erbB2 oncogene and negative ER may have predictive value in epirubicin therapy in patients with advanced breast cancer.

Transfection of human topoisomerase II alpha into etoposide-resistant cells: transient increase in sensitivity followed by down-regulation of the endogenous gene

We have investigated the possibility of overcoming the resistance of human brain tumour cells (HBT20) to etoposide by transferring the normal human topoisomerase II alpha (H-topo II) gene into these cells. H-topo II in a mammalian expression vector containing a glucocorticoid-inducible mouse mammary tumour virus (MMTV) promoter was transfected into etoposide-resistant HBT20 cells (HBT20-hTOP2MAM). HBT20 cells transfected with pMAMneo vector alone served as control cells (HBT20-MAM). These were stable transfections. Following a 2 h dexamethasone treatment, H-topo II mRNA expression, protein production, etoposide-induced DNA-protein complex formation and sensitivity to etoposide were increased in HBT20-hTOP2MAM cells compared with control HBT20-MAM cells and with HBT20-hTOP2MAM cells not treated with dexamethasone. However, mRNA and protein levels and cell sensitivity returned to baseline when incubation with dexamethasone was continued for 24 h. This decrease from the 2 h values could not be explained by a loss of the MMTV promoter response to dexamethasone. (H-topo II alpha promoter)-(chloramphenicol acetyltransferase) constructs containing regions -559-0 and -2400-0 were significantly down-regulated in HBT20-hTOP2MAM cells treated for 24 h with dexamethasone compared with dexamethasone-treated control cells. H-topo II mRNA stability after 24 h of dexamethasone treatment was not altered compared with that in control cells. Our data indicate that the exogenously produced H-topo II may have a negative-feedback effect on the endogenous topoisomerase II promoter, causing down-regulation of the endogenous gene.