Antitumor triptycene analogs induce a rapid collapse of mitochondrial transmembrane potential in HL-60 cells and isolated mitochondria

Abstract

Since synthetic analogs of triptycene (TT code number), such as bisquinones TT2 and TT13, can trigger cytochrome c release without caspase activation and retain their ability to induce apoptosis in multidrug-resistant (MDR) tumor cells, fluorescent probes of transmembrane potential have been used to determine whether these antitumor compounds might directly target mitochondria in cell and cell-free systems to cause the collapse of mitochondrial membrane potential ( downward arrow Deltapsim) that is linked to permeability transition pore (PTP) opening. Using JC-1 dye, the abilities of various TT analogs to induce the downward arrow Deltapsim in wild-type and MDR HL-60 cells are rapid (within 5-20 min), irreversible after drug removal, concentration dependent in the 0.64-25 microM range, and generally related to their antitumor activities in vitro. The downward arrow Deltapsim caused by TT2 and TT13, which are more potent than mitoxantrone, staurosporine and the reference depolarizing agent, carbonyl cyanide m-chlorophenylhydrazone (CCCP), in HL-60 cells, are not prevented by caspase-2 or -8 inhibitors, suggesting that activation of these apical caspases upstream of mitochondria is not involved in this process. Antitumor TT analogs (0.64-25 microM) also mimic the abilities of the known depolarizing agents, CCCP, alamethicin, gramicidin A and 100 microM CaCl(2), to directly induce within 20 min the downward arrow Deltapsim in isolated mitochondria prepared from mouse liver and loaded with rhodamine 123 dye. The fact that 20 microM Ca(2+), which is insufficient to trigger depolarization on its own, is required to reveal the depolarizing effect of TT2 in isolated mitochondria suggests that antitumor TT analogs might interact with the PTP to alter its conformation and increase its Ca(2+) sensitivity. Indeed, such Ca(2+)-dependent downward arrowDeltapsim of isolated mitochondria treated with 25 microM TT2 or 100 microM Ca(2+) are blocked by ruthenium red. Daunorubicin (DAU) is unable to mimic the rapid downward arrowDeltapsim caused by antitumor TT bisquinones within 5-40 min of treatment in HL-60 cells or isolated mitochondria. Moreover, the downward arrowDeltapsim caused by 25 microM TT2 or 100 microM Ca(2+) in isolated mitochondria are similarly blocked by cyclosporin A (CsA), bongkrekic acid and decylubiquinone, which prevent PTP opening, suggesting that, in contrast to DAU, antitumor TT analogs that directly target mitochondria to trigger the Ca(2+)-dependent and CsA-sensitive downward arrowDeltapsim, might induce PTP opening and the mitochondrial pathway of apoptosis even in the absence of nuclear signals.