Schedule-dependent response of neuroblastoma cell lines to combinations of etoposide and cisplatin.

The growth inhibitory effects of cisplatin and etoposide on neuroblastoma cell lines were investigated in several scheduled combinations. Results were analyzed using median effect and combination index analyses. In all schedules in which cisplatin was administered prior to etoposide a synergistic effect was observed. Conversely, an antagonistic effect was seen in all schedules where etoposide was administered before cisplatin.

Mutagenesis of E477 or K505 in the B' domain of human topoisomerase II beta increases the requirement for magnesium ions during strand passage.

A type II topoisomerase is essential for decatenating DNA replication products, and it accomplishes this task by passing one DNA duplex through a transient break in a second duplex. The B' domain of topoisomerase II contains three highly conserved motifs, EGDSA, PL(R/K)GK(I/L/M)LNVR, and IMTD(Q/A)DXD. We have investigated these motifs in topoisomerase II beta by mutagenesis, and report that they play a critical role in establishing the DNA cleavage-religation equilibrium. In addition, the mutations E477Q (EGDSA) and K505E (PLRGKILNVR) increase the optimal magnesium ion concentration for strand passage, without affecting the Mg(2+) dependence of ATP hydrolysis. It is likely that the binding affinity of the magnesium ion(s) specifically required for DNA cleavage has been reduced by these mutations. The crystal structure of yeast topo II indicates that residues E477 and K505 may help to position the three aspartate residues of the IMTD(Q/A)DXD motif for magnesium ion coordination, and we propose two possible locations for the magnesium ion binding site(s). These observations are consistent with a previous model in which the B' domain is positioned such that these acidic residues lie next to the active site tyrosine residue. A magnesium ion bound by these aspartate residues could therefore mediate the DNA cleavage-religation reaction.

Carbamate analogues of amsacrine active against non-cycling cells: relative activity against topoisomerases IIalpha and beta.

PURPOSE: Methyl N-(4'-(9-acridinylamino)-phenyl)carbamate hydrochloride (AMCA) and methyl N-(4'-(9-acridinylamino)-2-methoxyphenyl)carbamate hydrochloride (mAMCA) are analogues of the topoisomerase II (topo II) poison amsacrine, and are distinguished from amsacrine by their high cytotoxicity towards non-cycling cells. Since mammalian cells contain two forms (alpha and beta) of topo II and the alpha isoform is down-regulated in non-cycling cells, we have considered whether these carbamate analogues target topo IIbeta selectively. METHODS: A drug permeable yeast strain (JN394 top2-4) was transformed using a shuttle vector containing either human top2alpha, human top2alpha or yeast top2 under the control of a GAL1 promoter. The strain was analysed at a non-permissive temperature, where only the plasmid-borne topo II was active. RESULTS: AMCA and mAMCA produced comparable levels of cell killing with human DNA topo IIalpha, human DNA topo IIbeta and yeast DNA topo II. Two other acridine derivatives N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (DACA) and its 7-chloro derivative, which like AMCA and mAMCA are able to overcome multidrug resistance mechanisms, were much more active against human DNA topo IIalpha than against human DNA topo IIbeta and yeast DNA topo II. A series of mutant Chinese hamster and human lines with defined topo lesions, including the HL60/MX2 line that lacks topo IIbeta expression, was also used to compare resistance to amsacrine, AMCA and etoposide. Loss of topo IIbeta activity had a greater effect on amsacrine and AMCA than on etoposide. Resistance of murine Lewis lung cultures in exponential and plateau phase was also measured. Loss of topo IIalpha activity, as measured in both mutant cells expressing lower amounts of enzyme and in cells in plateau phase, resulted in concomitant acquisition of resistance that was greatest for etoposide and least for AMCA. CONCLUSION: We conclude that the carbamate analogues of amsacrine recognize both topo IIalpha and beta in cells.

Complementation of temperature-sensitive topoisomerase II mutations in Saccharomyces cerevisiae by a human TOP2 beta construct allows the study of topoisomerase II beta inhibitors in yeast.

We show herein that human DNA topoisomerase II beta is functional in yeast. It can complement a yeast temperature-sensitive mutation in topoisomerase II. The effect on human topoisomerase II beta of a number of topoisomerase II inhibitors was analysed in a yeast in vivo system and compared with that of human topoisomerase II alpha and wild-type yeast topoisomerase II. A drug permeable yeast strain (JN394 top2-4) was used to analyse the in vivo effects of known anti-topoisomerase II agents on human topoisomerase II beta transformants. A parallel analysis on human topoisomerase II alpha transformants provides the first in vivo analysis of the responses of yeast bearing the individual isoforms to these drugs. The strain was analysed at 35 degrees C, a non-permissive temperature at which only plasmid-borne topoisomerase II is active. A shuttle vector with either human topoisomerase II beta, human topoisomerase II alpha or yeast topoisomerase II under the control of a GAL1 promoter was used. The key findings were that amsacrine produced comparable levels of cell killing with both alpha and beta, whilst etoposide, doxorubicin and mitoxantrone produced higher degrees of cell killing with alpha than with beta or yeast topoisomerase II. Merbarone had the greatest effect on the yeast strain bearing plasmid-borne yeast topoisomerase II. Suramin, quercetin and genistein showed little cell killing in this system. This yeast in vivo system provides a powerful way to analyse the effects of anti-topoisomerase II agents on transformants bearing the individual human isoforms. This system also provides a means of analysing putative drug-resistance mutations in human topoisomerase II beta or to select for drug-resistance mutations in human topoisomerase II beta.

Amsacrine-promoted DNA cleavage site determinants for the two human DNA topoisomerase II isoforms alpha and beta.

Site-specific DNA cleavage by topoisomerase II (EC 5.99.1.3) is induced by many antitumour drugs. Although human cells express two genetically distinct topoisomerase II isoforms, thus far the role and determinants of drug-induced DNA cleavage have been examined only for alpha. Here we report the first high-resolution study of amsacrine (mAMSA) induced DNA breakage by human topoisomerase II beta (overexpressed and purified from yeast) and a direct comparison with the recombinant alpha isoform. DNA cleavage in plasmid pBR322 and SV40 DNA was induced by alpha or beta in the absence or presence of the antitumour agent mAMSA, and sites were mapped using sequencing gel methodology. Low-resolution studies indicated that recombinant human alpha promoted DNA breakage at sites akin to those of beta, although some sites were only cleaved by one enzyme and different intensities were observed at some sites. However, statistical analysis of 70 drug-induced sites for beta and 70 sites for alpha revealed that both isoforms share the same base preferences at 13 positions relative to the enzyme cleavage site, including a very strong preference for A at +1. The result for recombinant alpha isoform is in agreement with previous studies using alpha purified from human cell lines. Thus, alpha and beta proteins apparently form similar ternary complexes with mAMSA and DNA. Previous studies have emphasized the importance of DNA topoisomerase II alpha; the results presented here demonstrate that beta is an in vitro target with similar site determinants, strongly suggesting that beta should also be considered a target of mAMSA in vivo.