Oxidative Bax dimerization promotes its translocation to mitochondria independently of apoptosis

Abstract

Bax is a cytosolic protein, which in response to stressing apoptotic stimuli, is activated and translocates to mitochondria, thus initiating the intrinsic apoptotic pathway. In spite of many studies and the importance of the issue, the molecular mechanisms that trigger Bax translocation are still obscure. We show by computer simulation that the two cysteine residues of Bax may form disulfide bridges, producing conformational changes that favor Bax translocation. Oxidative, nonapoptogenic treatments produce an up-shift of Bax migration compatible with homodimerization, which is reverted by reducing agents; this is accompanied by translocation to mitochondria. Dimers also appear in pure cytosolic fractions of cell lysates treated with H2O2, showing that Bax dimerization may take place in the cytosol. Bax dimer-enriched lysates support Bax translocation to isolated mitochondria much more efficiently than untreated lysates, indicating that dimerization may promote Bax translocation. The absence of apoptosis in our system allows the demonstration that Bax moves because of oxidations, even in the absence of apoptosis. This provides the first evidence that Bax dimerization and translocation respond to oxidative stimuli, suggesting a novel role for Bax as a sensor of redox imbalance.

Bax activation and mitochondrial insertion during apoptosis

“…Then, data suggest that Bax C-terminal Hα9 could participate to the pore formation [40], maybe through its polar residues. Finally, a study reported the possible formation of intermolecular disulfide bonds involving Bax C62 and C126 residues [56]. Bax mitochondrial localisation may create a favourable environment to the formation of such bonds.…”

Melatonin: A pleiotropic molecule regulating inflammation

“…Besides altering their abundance, melatonin could alter Bax and Bcl-2 activity in other ways. Oxidative alterations cause Bax disulfide dimerization, and the resulting conformational changes are sufficient for its translocation to mitochondria independently of apoptosis [116]. However, melatonin maintains Bax in a monomeric state within mitochondria [108].…”

Multistep and multitask Bax activation

“…Both cysteines are exposed and potentially reactive to form disulfide bridges for either homo-or hetero-dimerization (D'Alessio et al, 2005); in silico models propose that homodimers via disulfide bonds between cysteine 62 and cysteine 126 expose the hydrophobic alpha helix 9 promoting membrane insertion (D'Alessio et al, 2005).…”

“…It was shown that after treatment with H 2 O 2 at low concentrations, or after glutathione depletion in U937 and HepG2 cells, cytochrome c is released in the absence of apoptosis (Ghibelli et al, 1999); at the same time, Bax translocates to mitochondria, and undergo dimerization, as detected in non-reducing but denaturing electrophoresis (SDS-PAGE) (D'Alessio et al, 2005). Cell-free oxidation of cytosolic extracts with H 2 O 2 leads to disulfide dimerization.…”