Overexpression of bcl-2 results in reduction of cytochrome c content and inhibition of complex I activity

Bcl-2 has been shown to exert its antiapoptotic activity predominantly at t he level of mitochondria by preventing cytochrome c release, Whether Bcl-2 is involved in the regulation of mitochondrial function prior to an apoptot ic stimulus remains elusive. Using functional and spectrophotometric measur ements in an inducible PC12-Tet-on-bcl-2 cell line we demonstrate that indu ction of Bcl-2 overexpression rapidly reduced cytochrome b and c levels as well as complex I activity, To confirm that these changes were specific for Bcl-2 we generated a bcl-2 antisense construct under the control of the te tracycline responsive promotor. Transient transfection with this antisense plasmid prevented both the decrease of cytochrome b and c levels and the lo ss of complex I activity. The decrease of cytochrome b levels was parallele d by a decrease of cytochrome b mRNA levels while Northern blot analysis of cytochrome c mRNA expression did not reveal any overt changes in Bcl-2 cel ls. We propose that the antiapoptotic properties of Bcl-2 are related to th e reduction of mitochondrial complex I activity and lowered mitochondrial c ytochrome b and c levels. (C) 2001 Academic Press.