U937 cells lacking mitochondrial DNA (ρ° cells) are auxotrophic for uridine and pyruvate, hypersensitive to hypoglycemic conditions, and resistant to antimycin A-induced apoptosis. In spite of their obvious metabolic defects, ρ° cells possess a normal mitochondrial transmembrane potential, as well as a near-normal capacity to generate superoxide anion after menadione treatment. Similarly to ρ+ controls, ρ° cells undergo apoptosis in response to tumor necrosis factor-α plus cycloheximide. Detailed comparison of the apoptotic process in ρ+ and ρ° cells reveals essentially the same sequence of events. In response to tumor necrosis factor/cycloheximide, cells first lose their mitochondrial transmembrane potential (Δψm) and then manifest late apoptotic alterations, such as generation of reactive oxygen species and DNA fragmentation. Experiments involving isolated mitochondria from ρ+ and ρ° cells confirm that ρ° mitochondria can be induced to undergo permeability transition, a process thought to account for the pre-apoptotic Δψm disruption in cells. Like ρ+ mitochondria, ρ° mitochondria contain a pre-formed soluble factor that is capable of inducing chromatin condensation in isolated nuclei in vitro. This factor is released from mitochondria upon induction of permeability transition by calcium or the specific ligand of the adenine nucleotide translocator atractyloside. In conclusion, it appears that all structures involved in the maintenance and pre-apoptotic disruption of the Δψm, as well as a mitochondrial apoptotic factor(s), are present in ρ° cells and thus are controlled by the nuclear rather than by the mitochondrial genome. These findings underline the contribution of mitochondria to the apoptotic process.

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This work was supported by grants from Association pour la Recherche sur le Cancer, Agence Nationale de Recherche sur le SIDA, Centre National de la Recherche Scientifique, Fondation pour la Recherche Médicale. Institut National de la Santé et de la Recherche Médicale, NATO, Leo Foundation, Ministère de la Recherche (France), Sidaction (to G. K.) and Dirección General de Ciencia y Tecnología (PB92-0355 to J. N.). S. A. S. received a fellowship from the Spanish Ministry of Science.

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