High cholesterol levels promotes TNF‐α‐dependent necroptosis in Alzheimer’s disease

Abstract

AbstractBackgroundNecroptosis has been reported in post‐mortem Alzheimer’s disease (AD) brains, but still, the events that trigger or regulate this type of inflammatory cell death in AD are not fully known. Oxidative stress may be determinant as to whether a cell initiates necroptosis; Therefore, it is, likely that mitochondrial dysfunction, observed in the early stages of AD, might favor cells to undergo necroptosis. Previous studies from our group have shown that high intracellular cholesterol levels deplete the mitochondrial pool of GSH, thus sensitizing neurons against amyloid‐beta (Ab)‐induced mitochondrial oxidative stress. The present study is aimed to evaluate whether cholesterol can regulate the necroptotic pathway using both APP‐PSEN1 mice that overexpress the cholesterol‐related transcription factor SREBF2 and cholesterol‐enriched SH‐SY5Y cells.MethodCholesterol enrichment was assessed by incubating the cells with soluble cholesterol:methyl‐cyclodextrin complex (50 mg/ml) for 1 h followed by 4 h recovery. To induce necroptosis, cells were treated with TLQ [TNFa (10 ng/ml) plus the SMAC mimetic (LCL‐161, 10 mM) and the pan‐caspase inhibitor qVD‐OPH (10 mM)] for 24 h.ResultsBrains from APP‐PSEN1‐SREBF2 mice show up‐regulated expression of the necroptosis‐related proteins RIPK3 and MLKL, which together with RIPK1 accumulated in the urea‐soluble protein fraction, indicative of necrosome assembly. Moreover, western blot analyses of brain extracts from triple transgenic mice also showed high levels of c‐FLIP, the natural inhibitor of caspase‐8, further pointing to a cholesterol‐regulated engagement of the necroptotic pathway. In SH‐SY5Y cells, cholesterol enrichment resulted in enhanced cell death after exposure to TLQ, which was prevented by RIPK1 and RIPK3 inhibitors (Necrostatin and GSK’872, respectively). Similar protection against TLQ‐induced necroptosis was achieved when mitochondrial GSH levels were recovered by GSH ethyl ester. Intriguingly, cholesterol also regulated the intracellular localization of RIPK3 and MLKL. Confocal microscopy showed an increased presence of nuclear RIPK3 and MLKL in cholesterol‐enriched cells, recently described as a requirement for the subsequent necrosome formation in the cytosol.ConclusionOverall, these findings indicate that high intracellular cholesterol levels compromise neuronal viability, promoting oxidative stress and subsequent necroptosis, a pro‐inflammatory type of cell death, which ultimately may contribute to chronic neuroinflammation in AD.