The Apoptotic Bile Acid DCA has Preference for Association to Liquid Disordered Lipid Domains and Inhibits the Rigidifying Effect of Cholesterol in Membranes

Abstract

Hydrophobic bile acids (BAs) such as deoxycholic acid (DCA) induce cell death via multiple pathways, including both the intrinsic and extrinsic pathways of apoptosis. On the other hand, it has been shown that hydrophilic BAs, particularly ursodeoxycholic (UDCA) and tauroursodeoxycholic (TUDCA) acids prevent apoptosis. Still, the mechanisms by which these very similar BAs trigger opposite signaling effects remain unclear. We hypothesize that cellular membranes may constitute a primary target for the modulation of BAs during apoptosis.Here, fluorescent derivatives of DCA and UDCA were studied. Both BAs partitioned with higher affinity to liquid disordered than to the cholesterol-enriched liquid ordered domains. From studies with membrane fluorescent probes, unlabeled BAs are shown to have a superficial location in the lipid membrane upon binding. Additionally, the interaction of DCA with lipid membranes resulted in a significant expansion in membrane surface area, as observed by a FRET experiment. This effect might be related to changes in membrane permeability observed after incubation of both mitochondria and model membranes with hydrophobic BAs. Importantly, it is shown that only DCA is able to significantly disrupt the ordering of the membrane by cholesterol, as seen by the fluorescence anisotropy of membrane probes. In this way, it is possible that the apoptotic effects of BAs are achieved through modulation of lipid raft formation. Alternatively, since it has been reported that the oligomerization of the apoptotic protein Bax is dependent on membrane fluidity, DCA-induced apoptosis could be the result of an increase in mitochondrial membrane fluidity, which promotes Bax oligomerization and triggers apoptosis.Acknowledgements: FCT Portugal is acknowledged for financial support (PTDC/QUI-BIQ/119494/2010; PTDC/QUI-BIQ/112067/2009; SFRH/BPD/64320/2009).