EXPRESSION OF TOPOISOMERASE-II, BCL-2, AND P53 IN 3 HUMAN BRAIN-TUMORCELL-LINES AND THEIR POSSIBLE RELATIONSHIP TO INTRINSIC RESISTANCE TOETOPOSIDE

We characterized three human brain tumor cell lines (D54, HBT-20, and HBT-28) with respect to resistance to etoposide (VP-16), a topoisomera se II-reactive drug, All three cell lines were inherently resistant to VP-16 when compared to other human cell lines, with D54 showing the g reatest resistance using colony formation assays, Resistance to VP-16 has been attributed to decreased drug uptake and changes in topoisomer ase II; however, drug uptake and topoisomerase II protein levels (immu noblot) were no lower in D54 than in HBT-20 and HBT-28, cell lines rel atively more sensitive to VP-16, More to the point, measurement of top oisomerase II-mediated DNA cleavage of cellular DNA after treatment wi th VP-16 showed that the topoisomerase II in these cells was active, T hese data indicate mechanisms other than those attributable to decreas ed drug uptake or altered topoisomerase II exist for clinical resistan ce to VP-16. VP-16-induced DNA cleavage has been associated with apopt osis in some cell lines; however, neither DNA laddering nor morphologi cal changes characteristic of apoptosis were detected in these cell li nes after treatment with VP-16. Bcl-2 and mutant p53 were present in t hese cells, Either of these conditions can prevent apoptosis and could explain a dissociation between the proximal mediator of VP-16-induced cytotoxicity (topoisomerase II-DNA complex formation) and cellular de ath.