Over-expression of Bcl-2 does not protect cells from hypericin photo-induced mitochondrial membrane depolarization, but delays subsequent events in the apoptotic pathway

Hypericin (HY) is a powerful photo-inducer of apoptosis in Jurkat cells as measured by caspase-3 activation, cell shrinkage, phosphatidylserine (PS) e xposure and the appearance of hypoploid DNA, These processes are preceded b y rapid Bcl-2-independent mitochondrial transmembrane depolarization and a drop in cytoplasmic pH, Pre-incubation of cells with inhibitors of the mito chondrial permeability transition pore, such as cyclosporin A or bongkrekic acid, does not protect cells from mitochondrial membrane potential (Delta psi(m)) decrease. However, monitoring of mitochondrial entrapped calcein by confocal fluorescence imaging gives clear evidence of HY photo-induced mit ochondrial permeability, This should be considered as the result of a non-s pecific alteration of mitochondrial membrane integrity brought about by lip id peroxidation, Nevertheless, synthesis of the anti-apoptotic protein Bcl- 2 appears to delay the subsequent time course of PS exposure and to reduce caspase-3 activation and the fraction of cells which become hypoploid, We i nterpret this partially protective effect as the consequence of a direct in teraction of Bcl-2 with cytosolic cytochrome c previously released from mit ochondria upon Delta psi(m), decrease and/or of Bcl-2 inhibition of the del eterious retro-effect of caspase-3 on the mitochondrial permeability transi tion pore and/or the mitochondrial membrane components, (C) 1999 Federation of European Biochemical Societies.