Critical upstream signals of cytochrome c release induced by a novel Bcl-2 inhibitor

Cytochrome c release is a central step in the apoptosis induced by many death stimuli. Bcl-2 plays a critical role in controlling this step. In this study, we investigated the upstream mechanism of cytochrome c release induced by ethyl 2-amino-6-bromo-4-(1-cyano-2-ethoxy-2- oxoethyl)-4H-chromene-3-carboxylate (HA14-1), a recently discovered small molecule inhibitor of Bcl-2. HA14-1 was found to induce cytochrome c release from the mitochondria of intact cells but not from isolated mitochondria. Cytochrome c release from isolated mitochondria requires the presence of both HA14-1 and exogenous Ca2+. This suggests that both mitochondrial and extramitochondrial signals are important. In intact cells, treatment with HA14-1 caused Ca2+ spike, change in mitochondrial membrane potential (DeltaPsi(m)) transition, Bax translocation, and reactive oxygen species (ROS) generation prior to cytochrome c release. Pretreatment with either EGTA acetoxymethyl ester or vitamin E resulted in a significant decrease in cytochrome c release and cell death induced by HA14-1. Furthermore pretreatment with RU-360, an inhibitor of the mitochondrial Ca2+ uniporter, or with EGTA acetoxymethyl ester, but not with vitamin E, prevented the HA14-1-induced DeltaPsi(m) transition and Bax translocation. This suggests that ROS generation is an event that occurs after the DeltaPsi(m) transition and Bax translocation. Together these data demonstrate that the Ca2+ spike, mitochondrial Bcl-2 presensitization, and subsequent DeltaPsi(m) transition, Bax translocation, and ROS generation are important upstream signals for cytochrome c release upon HA14-1 stimulation. The involvement of endoplasmic reticulum and mitochondrial signals suggests both organelles are crucial for HA14-1-induced apoptosis.