COMMITMENT AND EFFECTOR PHASES OF THE PHYSIOLOGICAL CELL-DEATH PATHWAY ELUCIDATED WITH RESPECT TO BCL-2, CASPASE, AND CYCLIN-DEPENDENT KINASE-ACTIVITIES

Physiological cell deaths occur ubiquitously throughout biology and ha ve common attributes, including apoptotic morphology with mitosis-like chromatin condensation and prelytic genome digestion. The fundamental question is whether a common mechanism of dying underlies these commo n hallmarks of death. Here we describe evidence of such a conserved me chanism in different cells induced by distinct stimuli to undergo phys iological cell death. Our genetic and quantitative biochemical analyse s of T- and B-cell deaths reveal a conserved pattern of requisite comp onents. We have dissected the role of cysteine proteases (caspases) in cell death to reflect two obligate classes of cytoplasmic activities functioning in an amplifying cascade, with upstream interleukin-1 beta -converting enzyme-like proteases activating downstream caspase 3-like caspases. Bcl-2 spares cells from death by punctuating this cascade, preventing the activation of downstream caspases while leading upstrea m activity undisturbed. This observation permits an operational defini tion of the stages of the cell death process. Upstream steps, which ar e necessary but not themselves lethal, are modulators of the death pro cess. Downstream steps are effecters of, and not dissociable from, act ual death; the irreversible commitment to cell death reflects the init iation of this downstream phase. In addition to caspase 3-like proteas es, the effector phase of death involves the activation in the nucleus of cell cycle kinases of the cyclin-dependent kinase (Cdk) family. Nu clear recruitment and activation of Cdk components is dependent on the caspase cascade, suggesting that catastrophic Cdk activity may be the actual effector of cell death. The conservation of the cell death mec hanism is not reflected in the molecular identity of its individual co mponents, however. For example, we have detected different cyclin-Cdk pairs in different instances of cell death. The ordered course of even ts that we have observed in distinct cases reflects essential thematic elements of a conserved sequence of modulatory and effector activitie s comprising a common pathway of physiological cell death.