1. ¹Cold Spring Harbor Laboratory, One Bungtown Rd, Cold Spring Harbor, NY 11724, USA
2. ²Molecular and Cellular Biology Program, Stony Brook University, Stony Brook, NY 11794, USA

Correspondence: Y Lazebnik, Cold Spring Harbor Laboratory, One Bungtown Rd, Cold Spring Harbor, NY 11724, USA. Tel: 516 367 8357; Fax: 516 367 8461; E-mail: lazebnik@cshl.edu

Received 2 November 2005; Revised 28 March 2006; Accepted 10 April 2006; Published online 26 May 2006.

Abstract

During apoptosis, cytochrome c released from mitochondria activates Apaf-1, a cofactor of caspase-9. The evidence that cytochrome c can activate Apaf-1 is abundant, but the proof that cytochrome c is required for apoptosis is limited to two studies that used genetically modified mice. One of these studies concluded that in some tissues apoptosis may require Apaf-1 but not cytochrome c, which indicated the need to analyze the requirement of cytochrome c beyond the mouse models, and in human tumor cells in particular. In this study, we designed tools to silence cytochrome c expression in human cells and tested these tools in an experimental system of oncogenic transformation. We found that cytochrome c was required for apoptosis induced by both DNA damage and, unexpectedly, TNF. Overall, this study established that cytochrome c is required for apoptosis in human cells and provided tools to dissect mechanisms of apoptosis in various experimental models.