The Amyloid-β 42 Proxy,Amyloid-β 25-35,Induces Normal Human Cerebral Astrocytes to Produce Amyloid-β 42

Astrocytes in amyloid-β (Aβ)₄₂-accumulating human brains afflicted with Alzheimer's disease (AD) upregulate vascular endothelial growth factor (VEGF)-A synthesis and also become loaded with Aβ₄₂. We have already shown that Aβ_25-35 (surrogate of Aβ₄₂)-induced VEGF-A production in ‘normoxic’ cultures of early passage normal human cerebral astrocytes (NAHAs) is mediated by the stabilization of VEGF gene-stimulating hypoxia-inducible factor (HIF)-1α and nuclear translocation of HIF-1α•HIF-1β complexes. We have now found that treating these NAHAs with Aβ_25-35 also stimulates them to make Aβ₄₂ (appearing in immunoblots as several bands with M_r's from 8 kDa upwards), whose levels peak at 48 h (2.8-fold versus 0 h, p < 0.001) and then start falling slowly. This rise of Aβ₄₂ peptide production coincides with a transiently increased flow of HIF-1α (therefore HIF-1α•HIF-1β complexes; at 24 h, 1.5-fold versus 0 h, p < 0.001) into the nucleus and transient surges first of β-secretase (BACE-1/β-S) mRNA expression (1.2-fold versus 0 h, p = 0.013) and activity peaking at 24-h (1.4-fold versus 0 h, p = 0.001), and then of γ-secretase (γ-S) activity cresting at 48 h (1.6-fold versus 0 h, p < 0.001) that cleave the Aβ₄₂ peptides from amyloid-β protein precursor. Since the genes encoding components of these two secretases have the same HIF-1α•HIF-1β-responsive elements in their promoters as the VEGF gene, these observations suggest that the Aβ₄₂ released from neurons in the AD brain can recruit associated astrocytes via HIF-1α•HIF-1β signaling into the pool of Aβ₄₂-producing cells. In other words, Aβ₄₂ begets Aβ₄₂ in NAHAs.