The biochemistry of programmed cell death.

Programmed cell death (PCD) is involved in the removal of superfluous and damaged cells in most organ systems. The induction phase of PCD or apoptosis is characterized by an extreme heterogeneity of potential PCD-triggering signal transduction pathways. During the subsequent effector phase, the numerous PCD-inducing stimuli converge into a few stereotypical pathways and cells pass a point of no return, thus becoming irreversibly committed to death. It is only during the successive degradation phase that vital structures and functions are destroyed, giving rise to the full-blown phenotype of PCD. Evidence is accumulating that cytoplasmic structures, including mitochondria, participate in the critical effector stage and that alterations commonly considered to define PCD (apoptotic morphology of the nucleus and regular, oligonucleosomal chromatin fragmentation) have to be ascribed to the late degradation phase. The decision as to whether a cell will undergo PCD or not may be expected to be regulated by "switches" that, once activated, trigger self-amplificatory metabolic pathways. One of these switches may reside in a perturbation of mitochondrial function. Thus, a decrease in mitochondrial transmembrane potential, followed by mitochondrial uncoupling and generation of reactive oxygen species, precedes nuclear alterations. It appears that molecules that participate in apoptotic decision-making also exert functions that are vital for normal cell proliferation and intermediate metabolism.