Cytotoxic signalling by inhibitors of DNA topoisomerase II

DNA topoisomerase (topo) II inhibitors either stabilize DNA-topo II complex es by blocking DNA religation (e.g. etoposide) or block the enzyme's cataly tic activity (e.g. dexrazoxane). The former class of drugs causes direct DN A damage through topo II, while the latter class does not, but both classes cause apoptosis. We cloned the Fas ligand (FasL) promoter and coupled it t o the luciferase gene. Treatment of cells transfected with this construct r evealed that complex-stabilizing (DNA-damaging) agents induce FasL expressi on, but the catalytic inhibitors do not, suggesting that the FasL pathway m ay not be involved in all cases of topoisomerase-mediated apoptosis. Some t opo II inhibitors activate a pathway involving stress-activated protein kin ases, which include c-Jun N-terminal kinase-1 (JNK-1). We will discuss the effects of these agents on components of this pathway. Our earlier work rev ealed that topo II alpha interacts with the cell cycle regulatory protein, retinoblastoma protein (Rb). This interaction and the subcellular distribut ion of these proteins are altered by topo II inhibitory drugs and lead to a poptosis. In addition, agents that affect Rb, such as E1A and E2F1/DP-1, wh en transfected into cells, also alter topo II alpha -Rb localization, activ ate jun kinase pathways and cause apoptosis. This paper discusses current s tudies that are designed to determine the contributions of these signalling events to the alterations in subcellular protein distribution and apoptosi s. We suggest that protein-protein interactions are important for mediation of cytotoxic signalling by anticancer drugs.