Cytochrome c depletion upon expression of Bcl-XS.
We have shown previously that Bcl-XS causes acute cell death in 3T3 cells without activating caspases (Fridman, J. S., Benedict, M. A., and Maybaum, J. (1999) Cancer Res. 59, 5999-6004). In this study, we determined that the explanation for lack of caspase activation is the cellular depletion of cytochrome c. Electron microscopy revealed gross structural changes in the mitochondria of Bcl-XS-expressing cells; however, cytochrome c was not detected in cytosolic fractions from these cells. Surprisingly, it was determined that cellular cytochrome c levels decreased as Bcl-XS expression levels increased. Experiments performed to eliminate other possible explanations for the lack of caspase activation showed that these 3T3 cells have a functional cytoplasmic apoptosome, a complex of proteins that form a functional trigger capable of activating the proximal caspase in an apoptotic pathway Chinnaiyan, A. M. (1999) Neoplasia 1, 5-15, as cytosolic extracts from these cells were capable of cleaving pro-caspase-9. These cells were also able to release cytochrome c from their mitochondria after appropriate stimulation, other than Bcl-XS expression (i.e. withdrawal from serum for 24 h), and initiate a cell death that is inhibited by a dominant negative caspase-9. We conclude that lack of caspase activation is due to a Bcl-XS-induced depletion of active cytochrome c, a phenomenon that represents an alternative cell death effector pathway and/or a novel mechanism for regulating caspase activation.
Fridman
J S
JS
Departments of Pharmacology and Radiation Oncology, University of Michigan Medical School, Ann Arbor, Michigan 48109, USA.
Parsels
J
J
Rehemtulla
A
A
Maybaum
J
J
D016475:3T3 Cells; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003574:Cytochrome c Group / Q000378:metabolism*; D051379:Mice; D008854:Microscopy, Electron; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D051020:bcl-X Protein
C494812:Bcl2l1 protein, mouse; D003574:Cytochrome c Group; D019253:Proto-Oncogene Proteins c-bcl-2; D051020:bcl-X Protein