Cytochrome c is a key molecule in mitochondria-mediated apoptosis. It also plays a pivotal role in cell respiration. The switch between these two functions occurs at the moment of its release from mitochondria. This process is therefore extremely relevant for the fate of the cell. Since cytochrome c mediates respiration, we studied the changes in respiratory chain activity during the early stages of apoptosis in order to contribute to unravel the mechanisms of cytochrome c release. We found that, during staurosporine (STS)- induced apoptosis in PC12 cells, respiration is affected before the release of cytochrome c, as shown by a decrease in the endogenous uncoupled respiration and an uncoupling event, both occurring independently of cytochrome c release. The decline in the uncoupled respiration occurs also upon Bcl-2 overexpression (which inhibits cytochrome c release), while the uncoupling event is inhibited by Bcl-2. We also observed that the first stage of nuclear condensation during STS-induced apoptosis does not depend on the release of cytochrome c into the cytosol and is a reversibile event. These findings may contribute to understand the mechanisms affecting mitochondria during the early stages of apoptosis and priming them for the release of apoptogenic factors. 1. Introduction Mitochondria play a key role in apoptosis triggered by a wide variety of stimuli since they release important proapoptotic factors from their intermembrane space. The first mitochondrial apoptogenic molecule discovered was the hemoprotein cytochrome [1]. Cytochrome is involved in two critical cell processes. Normally, it acts as mobile electron carrier shuttling electrons between ubiquinol cytochrome oxidoreductase (complex III) and cytochrome oxidase (complex IV) of the respiratory chain allowing cell life. On the other hand, upon apoptotic induction, cytochrome is released from mitochondria in the cytosol where it carries out a proapoptotic function by binding the adapter protein apoptosis protease-activating factor-1 (Apaf1). Consequently, it promotes, in presence of ATP/dATP, the assembly of the multiproteic complex apoptosome, which binds and activates the caspase-9, thereby initiating the activation of a caspase cascade which leads to apoptotic cell death [2, 3]. Mitochondrial outer membrane permeabilization (MOMP) and cytochrome release from mitochondria during apoptosis are tightly regulated by the proteins of the Bcl-2 family. This family of proteins includes both antiapoptotic members (e.g., Bcl-2 and Bcl-XL) repressing MOMP and release of apoptogenic factors
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