Cleavage of E2F-1-regulating proteins and activation of E2F-1 during CD95-induced death of thymocytes

The CD95 death receptor activates caspases that cleave a variety of intracellular substrates, including cell cycle control proteins. However, the significance of this cleavage for the induction of apoptosis is unclear. In this study, CD95-induced cleavage of the G1/S checkpoint regulator proteins, retinoblastoma protein (pRb) and murine-double-minute-2 (mdm-2), was associated with an increased protein concentration of a key transcription factor, E2F-1, which is regulated by both of them. Furthermore, DNA-binding activity to E2F sites is increased. In thymocytes, CD95-induced apoptosis was associated with increased E2F-1 DNA-binding activity, while thymocytes that lacked E2F-1 were less susceptible to CD95-induced apoptosis. We conclude that the G1/S checkpoint is an important target of CD95 signalling. CD95-activated caspases cleave regulator proteins to increase E2F-1 activity, and inappropriate activation of E2F-1 is part of the mechanism of CD95-induced apoptosis.