FURTHER CHARACTERIZATION OF AN AMSACRINE-RESISTANT LINE OF HL-60 HUMAN LEUKEMIA-CELLS AND ITS TOPOISOMERASE-II - EFFECTS OF ATP CONCENTRATION, ANION CONCENTRATION, AND THE 3-DIMENSIONAL STRUCTURE OF THE DNA TARGET

The characterization of type II topoisomerases from amsacrine-sensitiv e (HL-60) and amsacrine-resistant (HL-60/AMSA) human leukemia cells wa s extended. The intercalator resistance and etoposide sensitivity of t he HL-60/AMSA cells themselves were confirmed, and the stability of th is pharmacologic phenotype over many hundreds of cell generations was demonstrated. Prolonging exposure of HL-60/AMSA cells to amsacrine did not alter their sensitivity relative to that of HL-60 cells. Improved methods of immunoblotting allowed clear demonstration that the topois omerase II within these cells exhibited sensitivity and resistance cha racteristics that mirrored those of the cells and the isolated enzymes themselves. Additional biochemical characterization of the type II to poisomerases indicated that both enzymes relaxed supercoiled DNA in a distributive fashion and that the ATP concentrations at which optimal catalytic activity of the two enzymes was exhibited were identical. Th e enzymes differed, however, in their activity optima in buffers of va rious type and ionic strength. Furthermore, the inability of the HL-60 /AMSA enzyme to exhibit enhanced DNA cleavage in the presence of amsac rine could be overcome if the DNA target molecule contained a bend clo ned into its polylinker region. By contrast. a bend in a DNA plasmid c ontaining no polylinker was resistant to amsacrine-enhanced cleavage i n the presence of HL-60/AMSA topoisomerase II. as was a plasmid contai ning a polylinker with no bend. This suggests that an unusual DNA conf ormation (a bend) in a specific DNA context (a polylinker) may be a fa vored site for topoisomerase II action. It also suggests a mechanism b y which the sites and extent of topoisomerase II activity can be contr olled in cells.