Mitochondrial Permeability Transition Triggers Lymphocyte Apoptosis

In a number of experimental systems, the early stage of the apoptotic process, i.e., the stage that precedes nuclear disintegration, is characterized by the breakdown of the inner mitochondrial transmembrane potential (Δψ_m). This Δψ_m disruption may involve mitochondrial permeability transition (PT). Here, we address the question of whether PT transition would suffice to cause apoptosis or, rather, it would constitute a secondary event not causally involved in the apoptotic cascade. Protoporphyrin IX (PPIX), a ligand of the mitochondrial benzodiazepin receptor that is well known for its PT-triggering capacity, induces Δψ_m disruption, enhanced generation of superoxide anions, as well as signs of nuclear apoptosis in thymocytes and in T cells. The sequence of events triggered by PPIX mimics that observed in natural apoptosis. The PT inhibitory compound bongkrekic acid, a specific ligand of the mitochondrial adenine nucleotide translocator, is an efficient inhibitor of protoporphyrin IX-induced Δψ_m disruption. Bongkrekic acid prevents all PPIX-induced phenomena, including superoxide anion generation, chromatinolysis, and oligonucleosomal DNA fragmentation. In contrast, inhibitors of mRNA or protein synthesis fail to suppress PPIX-induced Δψ_m disruption and apoptosis. Transfection-enforced hyperexpression of the apoptosis-inhibitory proto-oncogene bcl-2 also inhibits PPIX-induced Δψ_m disruption, hyperproduction of reactive oxygen species, and nuclear DNA loss. The Δψ_m-stabilizing effect of Bcl-2 is observed both in cells and in isolated mitochondria. In conclusion, these data are compatible with the hypotheses that mitochondrial PT is self-sufficient to trigger apoptosis and that Bcl-2 may directly regulate PT.