Antitumor Triptycene Analogs Directly Interact with Isolated Mitochondria to Rapidly Trigger Markers of Permeability Transition

Abstract

Background: Substituted triptycenes (TT code number), which block nucleoside transport, macromolecule syntheses and DNA topoisomerase activities, induce cytochrome c release and apoptotic DNA fragmentation, inhibit the proliferation of drug-sensitive and -resistant tumor cells in the nM range in vitro and rapidly trigger the collapse of mitochondrial transmembrane potential in cell and cell-free systems. Because mitochondrial permeability transition (MPT) requires more than depolarization, antitumor TTs were tested for their ability to directly trigger specific markers of MPT in isolated mitochondria. Materials and Methods: Large amplitude swelling and Ca²⁺ release were assayed in isolated mitochondria to demonstrate TT-induced MPT. Results: Antitumor TTs interact with isolated mitochondria in a concentration- and time-dependent manner to rapidly cause large amplitude swelling and Ca²⁺ release in relation with their antiproliferative activities in L1210, HL-60 and LL/2 tumor cells in vitro. The ability of 4-10 μM TT15, TT16 and TT24 to maximally induce mitochondrial swelling and Ca²⁺ release within 20 min is similar to that of classic MPT inducers, such as 5 μg/ml alamethicin, 200 μM atractyloside, 5 μM phenylarsine oxide, 100 μM arsenic trioxide and a 100 μM Ca²⁺ overload. TT15 requires a priming concentration of 20 μM Ca²⁺ to trigger mitochondrial swelling and Ca²⁺ release and these 0.1 μM ruthenium red-sensitive MPT events are abolished by 1 μM cyclosporin A, 2 mM ADP and 20 μM bongkrekic acid, which block components of the permeability transition pore (PTP), and by 50-100 μM of various ubiquinones, which interact with the quinone binding site of the PTP and raise the Ca²⁺ load required for PTP opening. Conclusion: Antitumor TTs that trigger MPT in isolated mitochondria might interact with components of the PTP to boost its Ca²⁺-sensitive transition from the closed to the open state and might be valuable to develop mitochondriotoxic drugs that directly activate early components of apoptosis.