Complete prevention of the killing of L929 fibroblasts by tumor necrosis factor alpha (TNF) in the presence of 0.5 microg/ml actinomycin D (ActD) was obtained with cyclosporin A (CyA), an inhibitor of the mitochondrial permeability transition (MPT), and aristolochic acid (ArA), a phospholipase A2 inhibitor. Peripheral benzodiazepine receptor (PBzR) agonists (PK11195, FGIN 1-27, or chlorodiazepam), agents known to potentiate induction of the MPT, potentiated the cytotoxicity of TNF in the absence of ActD, an effect prevented by CyA plus ArA. The MPT was demonstrated independently of its effect on viability as the CyA-sensitive loss of rhodamine 123 fluorescence from cells preloaded with the dye. Treatment with TNF and ActD resulted in the loss of 80% of rhodamine fluorescence within 6 h, a time prior to any loss of viability. CyA plus ArA completely prevented this effect of TNF. Potentiation of the cytotoxicity of TNF by PBzR agonists was associated with induction of the MPT, as assessed by the loss of rhodamine fluorescence. CyA plus ArA completely prevented the loss of rhodamine 123. Ceramide replaced TNF in killing L929 fibroblasts, an effect also prevented by CyA plus ArA. Ceramide in the presence of ActD resulted in the loss of rhodamine fluorescence, an effect that was again prevented by CyA plus ArA. In addition, CyA plus ArA prevented the ability of PBzR agonists to potentiate the cytotoxicity of ceramide. In the presence of each PBzR agonist, ceramide caused the loss of rhodamine fluorescence, an effect completely prevented by CyA plus ArA. D609, an inhibitor of phosphatidylcholine-specific phospholipase C, completely prevented the killing by TNF, but not by ceramide, in the presence of ActD. D609 prevented induction of the MPT occurring with TNF, but not with ceramide. Inhibitors of endocytosis, as well as lysosomotropic amines, prevented the cytotoxicity of TNF, but not that of ceramide. It is concluded that the MPT is causally linked to the genesis of irreversible cell injury with TNF. In the face of an inhibition of protein synthesis, the MPT occurs as a consequence of the formation of ceramide.

The present and a previous study [J. W. Snyder, J. G. Pastorino, A. M. Attie, and J. L. Farber, Am. J. Physiol. 264 (Cell Physiol. 33): C709-C714, 1993] define two mechanisms whereby ATP depletion promotes liver cell death. ATP depletion and cell death are linked by the mitochondrial permeability transition (MPT). Mitochondrial deenergization promotes the MPT, and ATP maintains a membrane potential by reversal of ATP synthase. With an increased influx of Ca2+ induced by the ionophore A-23187, oligomycin depleted the cells of ATP without loss of the mitochondrial membrane potential and further elevated the intracellular Ca2+ concentration. Cyclosporin A (CyA) prevented the accompanying cell killing. Fructose also preserved the viability of the cells. With the increased cytosolic Ca2+ imposed by A-23187, viability is maintained by ATP-dependent processes. Upon depletion of ATP, Ca2+ homeostasis cannot be maintained, and the MPT is induced. Rotenone also depleted the cells of ATP, and A-23187 accelerated the loss of the mitochondrial membrane potential occurring with rotenone alone. CyA and fructose prevented the cell killing with rotenone and A-23187. Oligomycin did not prevent this action of fructose. We conclude that ATP is needed to maintain Ca2+ homeostasis to prevent the MPT and the resultant liver cell death. ATP is also needed to maintain mitochondrial energization when electron transport is inhibited.