SELECTION OF A SUBPOPULATION WITH FEWER DNA TOPOISOMERASE II-ALPHA GENE COPIES IN A DOXORUBICIN-RESISTANT CELL-LINE PANEL

A panel of doxorubicin-resistant sublines of the human small-cell ling carcinoma cell line GLC(4) displays decreasing DNA topoisomerase II a lpha (TopoII alpha) mRNA levels with increasing resistance. In the pre sent study we describe how this decrease may be regulated. No signific ant differences in Topoll(a)lpha mRNA stability or gene arrangement we re found, using mRNA slot-blotting and Southern blotting, in the most resistant cell line compared with the parental cell line. To investiga te if TopoII alpha gene copy loss contributed to the mRNA decrease, fl uorescence in situ hybridisation using a TopoII alpha-specific probe w as performed. During doxorubicin resistance development, the compositi on of the population in each cell line shifted with increasing resista nce, from a population in which most cells contain three TopoII alpha gene copies (GLC(4)) to a population in which most cells contain only two copies. A partial revertant of the most resistant cell line displa yed a shift back to the original situation. We conclude that the TopoI I alpha gene copy number decrease per cell line is in good agreement w ith decreased TopoII alpha mRNA and protein levels, and TopoII activit y levels in these cell lines which were described previously.