Increased Aβ1-42 Production Sensitizes Neuroblastoma Cells for ER Stress Toxicity

Alzheimers disease (AD) is characterized by the aggregation and subsequent deposition of misfolded β- amyloid (Aβ) peptide. The unfolded protein response (UPR) is activated by misfolded protein stress in the endoplasmic reticulum (ER). In previous studies we demonstrated mild activation of the UPR by extracellularly applied oligomeric but not fibrillar Aβ1-42. In addition, we showed that oligomeric Aβ1-42 is internalized by cells, whereas fibrillar Aβ1-42 remains on the outside of the cell. Inhibition of Aβ uptake specifically inhibits toxicity of Aβ1-42 oligomers, underscoring the toxic potential of intracellular Aβ. Therefore, in the present study, we investigated the connection between intracellularly produced Aβ and the ER stress response, using human neuroblastoma cells overexpressing either wild type APP695 (APPwt) or APP695V717F (APPmut). Both cell lines secrete higher levels of Aβ1-40 and Aβ1-42 compared to the parental line. In addition, APPmut produces more Aβ1-42 than APPwt. Whereas the basal levels of UPR markers are not different, we find augmented UPR induction in response to ER stress in both APP overproducing cell lines compared to the parental cell line, with the strongest UPR activation in APPmut cells. In addition, ER stress toxicity was highest in APPmut cells, strongly suggesting a connection with the production of Aβ1-42. The difference in ER stress mediated toxicity between the APPwt and APPmut cell lines is alleviated by pretreatment with γ-secretase inhibitor, indicating that it is dependent on Aβ production and in particular on Aβ1-42. Our data indicate that increased Aβ1-42 production sensitizes neuroblastoma cells for ER stress toxicity. Keywords: Alzheimer's disease, β-Amyloid, intracellular, endoplasmic reticulum, unfolded protein response