7β-Hydroxycholesterol-induced energy stress leads to sequential opposing signaling responses and to death of C6 glioblastoma cells

Abstract

7β-Hydroxycholesterol cytotoxicity has been shown in vivo and in vitro to be dependent on the accumulation of its esters. We show in our study, using a detergent-free raft preparation and LC/MS lipid content analysis, that membrane microdomains isolated from 7β-hydroxycholesterol-treated C6 cells have a reduced cholesterol: cholesterol ester ratio and accumulate 7keto-hydroxycholesterol, 7β-hydroxycholesterol and 7β-hydroxycholesterol esters. These modifications in lipid content are accompanied by a redistribution of flotillin-1 in the lipid rafts. Transient increases of AMPK phosphorylation and mitochondrial activity during the first 12 h of 7β-hydroxycholesterol treatment indicate that C6 cells undergo energy stress and increase oxidative phosphorylation. Even so, ATP levels are maintained during 15 h until glucose uptake decreases. The cell's answers to raft modifications and energy stress are sequential activations of different signaling pathways such as ERK, AMPK and PI3K/Akt. These pathways, known to be activated under energy stress conditions, are transiently activated at 6 h (ERK, AMPK) and 12 h (Akt) of treatment respectively suggesting a shift from cell survival to cell proliferation. The persistence of 7β-hydroxycholesterol-induced stress led after 24 h to P38 activation, loss of GSK3β activation and to cell death. Finally we demonstrate that the observed signaling responses depend on 7β-hydroxycholesterol esterification, confirming that esterification of 7β-hydroxycholesterol is essential for cytotoxicity.