Cytotoxic bile acids, but not cytoprotective species, inhibit the ordering effect of cholesterol in model membranes at physiologically active concentrations

Abstract

Submillimolar concentrations of cytotoxic bile acids (BAs) induce cell death via apoptosis. On the other hand, several cytoprotective BAs were shown to prevent apoptosis in the same concentration range. Still, the mechanisms by which BAs trigger these opposite signaling effects remain unclear. This study was aimed to determine if cytotoxic and cytoprotective BAs, at physiologically active concentrations, are able to modulate the biophysical properties of lipid membranes, potentially translating into changes in the apoptotic threshold of cells. Binding of BAs to membranes was assessed through the variation of fluorescence parameters of suitable derivatized BAs. These derivatives partitioned with higher affinity to liquid disordered than to the cholesterol-enriched liquid ordered domains. Unlabeled BAs were also shown to have a superficial location upon interaction with the lipid membrane. Additionally, the interaction of cytotoxic BAs with membranes resulted in membrane expansion, as concluded from FRET data. Moreover, it was shown that cytotoxic BAs were able to significantly disrupt the ordering of the membrane by cholesterol at physiologically active concentrations of the BA, an effect not associated with cholesterol removal. On the other hand, cytoprotective bile acids had no effect on membrane properties. It was concluded that, given the observed effects on membrane rigidity, the apoptotic activity of cytotoxic BAs could be potentially associated with changes in plasma membrane organization (e.g. modulation of lipid domains) or with an increase in mitochondrial membrane affinity for apoptotic proteins.

Deoxycholic acid modulates cell death signaling through changes in mitochondrial membrane properties

“…Interestingly, this effect was much less effi cient in the Chol-rich liquid-ordered phase ( l o ), possibly as a consequence of a lower affi nity of the bile acids toward more ordered membranes (14)(15)(16). This was also recently confi rmed in giant plasma membrane vesicles (GPMVs) isolated from rat tumor mast cells ( 17 ).…”

“…In support of the hypothesis that the plasma membrane is the target of cytotoxic bile acids, a recent study showed that cytotoxic bile acids such as DCA or chenodeoxycholic acid have the ability to disrupt to some extent the membrane-ordering effect induced by cholesterol (Chol) ( 14 ). Interestingly, this effect was much less effi cient in the Chol-rich liquid-ordered phase ( l o ), possibly as a consequence of a lower affi nity of the bile acids toward more ordered membranes (14)(15)(16).…”

“…The product of the difference from the mean (PDM) and Pearson's correlation coefficient ( R r ) ( 27 ) parameters were obtained through WCIF ImageJ (http://www.uhnresearch.ca/facilities/ wcif/imagej/). ( 14,17 ) have confi rmed that apoptosis-inducing bile acids are able to modulate lipid membrane fl uidity, suggesting that their physiological activity could be associated with this effect. Different organelles have been anticipated as targets for cytotoxic bile acids, namely, the plasma membrane ( 17 ) and the mitochondria ( 20 ).…”

“…Fluorescence steady-state measurements of di-4-ANEPPDHQ were performed as previously described ( 14 ). The fl uorescence intensities were calculated through integration of the emission spectra obtained, and background fl uorescence intensity was always subtracted.…”

“…5(6)-Carboxyfl uorescein (CF) was obtained from Sigma-Aldrich. Fluorescently labeled bile acid derivatives were obtained as previously described ( 14 ).…”