DRASTIC REDUCTION OF TOPOISOMERASE II-ALPHA ASSOCIATED WITH MAJOR ACQUIRED-RESISTANCE TO TOPOISOMERASE-II ACTIVE AGENTS BUT MINOR PERTURBATIONS OF CELL-GROWTH

V511 and V513 cell lines, derived from Chinese hamster V79 cells follo wing alkylating agent mutagenesis and subsequent selection with VP-16, showed resistance to cytotoxicity and DNA strand breaks induced by to poisomerase (topo) II inhibitors and were resistant to VP-16-induced s ister chromatid exchanges. They showed no amplification of the multidr ug-resistant p-glycoprotein. In a kinetoplast-DNA decatenation assay, V511 and V513 showed 51% and 49% topo II activity relative to parental V79 cells, respectively. By western-blot analysis all three logarithm ically growing cell lines showed similar levels of topo II beta (M(r) 180,000), which increased as cells progressed to quiescence. In contra st, immunoreactive levels of topo II alpha (M(r) 170,000) were 6.8% in V511 and 62.4 % in V513 relative to V79. V511 showed drastically decr eased topo II alpha in both log growth and quiescence. In a second app roach, immunoreactive topo II was analyzed in different phases of the cell cycle in logarithmically growing cells fractionated by fluorescen ce-activated cell sorting. All cell lines demonstrated relatively stab le topo II beta throughout the cell cycle. Topo II alpha showed little cell cycle variation in V79 or V513. However, in V511, it was only de tectable at low levels in G2/M phase. When cell growth parameters were measured, V511 and V513 showed a 17% increase in cell doubling time r elative to V79. These studies indicate that cells with a drastic reduc tion in topo II alpha (V511) or mutant topo II alpha (V513) but with n ormal levels of topo II beta show only minor perturbations of cell gro wth.