HETEROGENEOUS EXPRESSION OF DNA TOPOISOMERASE II-ALPHA ISOFORMS IN TUMOR-CELL LINES

DNA topoisomerase II alpha is a nuclear enzyme essential for DNA metab olism and cell cycle progression. Previous studies have shown that hum an tumor cell lines can express more than one topoisomerase II alpha i soform through alternative splicing. A 160-kDa isoform of topoisomeras e II alpha has been described in several cell lines selected for resis tance to inhibitors of DNA topoisomerase, but its physiological functi on has not been defined. In the present study, we have identified two major (160 and 140 kDa) and two minor (150 and 145 kDa) isoforms of to poisomerase II alpha in drug-sensitive human leukemic CEM cells, all o f which have lost C-terminal regions that produce epitopes recognized by specific antibodies. Reverse transcription-polymerase chain reactio n and molecular doning identified four alternatively spliced transcrip ts of topoisomerase II alpha from CEM cells. Furthermore, nucleotide s equencing indicated that the 160-kDa isoform is encoded by two transcr ipts derived from alternative splicing at a different C-terminal site and that the other two transcripts likely code for the 150-kDa isoform . Although the full-length topoisomerase II alpha resided in the cell nucleus, all altered isoforms, except the 160 kDa that was located in both cytoplasmic and nuclear extracts in about equal amount, were show n to be present predominantly in the cytosol. In contrast to the obser vations of other groups, we have not found an association of the topoi somerase II alpha isoforms with drug resistance. Rather, our results s uggest that expression of topoisomerase II alpha isoforms is cell type specific or might be associated with the neoplastic phenotype of the cells. Thus, although T-lineage tumor cell lines examined (CEM, Jurkat , and H9) displayed altered topoisomerase II alpha isoforms, normal T cells expressed only a full-length copy of the gene. Together, these r esults suggest that expression of altered topoisomerase II alpha isofo rms is not limited to drug resistance, but might be a feature of neopl astic cells.