Molecular mechanisms of bile-acid induced apoptosis and cytoprotection

Tânia Patrícia Marques de Sousa

Key information

Authors:

Tânia Patrícia Marques de Sousa (Tânia Patrícia Marques de Sousa)

Supervisors:

Fábio Monteiro Fernandes (Fábio Monteiro Fernandes); Manuel José Estevez Prieto (Manuel José Estevez Prieto)

Published in

10/17/2018

Abstract

Certain bile acids are known for their ability to modulate the apoptotic process in liver and non-liver cells. Deoxycholic acid (DCA) is a known inducer of apoptosis whereas ursodeoxycholic (UDCA) and tauroursodeoxycholic acids (TUDCA) have cytoprotective properties. Although the opposing effects of these molecules are strongly associated with their hydrophobicity, the exact mechanism by which these effects are triggered is still unknown. In this work, we used a combination of spectroscopic and microscopic techniques to study the possible mechanism of action of both apoptotic and cytoprotective bile acids. These studies were performed on model membrane systems, isolated mitochondria, human cell lines and rat hepatocytes, following a bottom-up approach. Our results demonstrate that the induction of apoptosis by DCA is most likely associated with changes in the properties of mitochondrial membranes. In fact, using membrane-sensitive probes in isolated mitochondria, it was confirmed that changes in the structure of the outer mitochondrial membrane after exposure to apoptotic concentrations of DCA, precede the onset of the mitochondrial permeability transition (MPT) process. However, in model membrane systems, extensive DCA-dependent permeabilization it is not observed, suggesting that other characteristics of mitochondrial membranes, not associated with the lipid environment, are responsible for the induction of apoptosis by DCA, possibly mitochondrial proteins. Considering Bax, one of the most important proteins in the intrinsic pathway of apoptosis, as a possible target for the activity of cytoprotective bile acids, we have shown that these molecules interact with Bax, have the ability to inhibit the interaction of this protein both with an activator peptide and with lipid membranes, and also inhibit its pore forming activity in membranes. However, in Bax-deficient HCT116 cells, it has been observed that DCA-induced apoptosis and UDCA cytoprotection are not strictly dependent on the presence of Bax. While for DCA such observation is not surprising, considering its permeabilization of isolated mitochondria, these results also suggest that for cytoprotective bile acids the molecular target is not exclusively Bax. Since changes in the surface charge of mitochondrial membranes intervene in the recruitment of signaling molecules during apoptosis, the impact of bile acids on the surface charge of the mitochondria and plasma membrane was also evaluated. Measurements of zeta potential in liposomes mimicking the lipid composition of these organelles confirm that DCA does indeed induce an increase in the anionic character of these membranes. The interaction of a charge sensing cationic peptide, Kφ, with mimetic liposomes from the inner leaflet of the plasma membrane and outer mitochondrial membrane, was characterized in the presence of cytoprotective and apoptotic bile acids, as was its distribution between the two organelles in HEK293T cells, also in the presence of bile acids.