Glucocorticoids Facilitate Astrocytic Amyloid-β Peptide Deposition by Increasing the Expression of APP and BACE1 and Decreasing the Expression of Amyloid-β-Degrading Proteases

Abstract

In most cases, the molecular mechanism underlying the pathogenesis of sporadic Alzheimer's disease (AD) is unknown. Elevated basal cortisol levels in AD patients suggest that glucocorticoids (GC) may contribute to the development and/or maintenance of AD. Amyloid plaques are the hallmark of AD, and they are considered to play an early role in the AD process. However, little is known about how their formation is regulated by stress and GC. Astrocyte accumulation is one of the earliest neuropathological changes in AD. Here, we report that GC elevated amyloid-β (Aβ) production in primary cultures of astrocytes by increasing amyloid precursor protein (APP) and β-site APP-cleaving enzyme 1 gene expression. Notably, GC administered to normal, middle-aged mice promoted the expression of APP and β-site APP-cleaving enzyme 1 in astrocytes, as determined by double immunofluorescence. Additionally, confocal microscopy and ELISA revealed that GC markedly reduced Aβ degradation and clearance by astrocytes in vitro, indicating a decreased neuroprotective capacity of the astrocytes. This may have been due to the decrease of several Aβ-degrading proteases, such as insulin-degrading enzyme and matrix metalloproteinase-9. These effects occurred through the activation of GC receptors. Taken together, our results demonstrate that GC can enhance the production of Aβ, reduce its degradation in astrocytes, and provide a molecular mechanism linking stress factors to AD. Our study suggests that GC can facilitate AD pathogenesis and that reducing GC in the elderly and early AD patients would be beneficial.