Flavonoids lower Alzheimer's A beta production via an NF kappa B dependent mechanism

Alzheimer's disease (AD) is characterized by the brain accumulation of A beta peptides and by the presence of neurofibrillary tangles. A beta is believed to play an important role in AD and it has been shown that certain flavonoids can affect A beta production. Recently, it was suggested that the A beta lowering properties of flavonoids are mediated by a direct inhibition the beta-secretase (BACE-1) activity, the rate limiting enzyme responsible for the production of A beta peptides. Western-blots and ELISAs were employed to monitor the impact of flavonoids on amyloid precursor protein processing and A beta production. A cell free chemoluminescent assay using human recombinant BACE-1 was used to assess the effect of flavonoids on BACE-1 activity. The effect of flavonoids on NF kappa B activation was determined by using a stable NF kappa B luciferase reporter cell line. Molecular docking simulations were performed to predict the binding of flavonoids to the BACE-1 catalytic site. Real time quantitative PCR was used to determine the effect of flavonoids on BACE-1 transcription. We show in a cell free assay that flavonoids are only weak inhibitors of BACE-1 activity. Docking simulation studies with different BACE-1 structures also suggest that flavonoids are poor BACE-1 inhibitors as they appear to adopt various docking poses in the active site pocket and have weak docking scores that differ as a function of the BACE-1 structures studied. Moreover, a weak correlation was observed between the effect of flavonoids on A beta production in vitro and their ability to lower BACE-1 activity suggesting that the A beta lowering properties of flavonoids in whole cells are not mediated via direct inhibition of BACE-1 activity. We found however a strong correlation between the inhibition of NF kappa B activation by flavonoids and their A beta lowering properties suggesting that flavonoids inhibit A beta production in whole cells via NF kappa B related mechanisms. As NF kappa B has been shown to regulate BACE-1 expression, we show that NF kappa B lowering flavonoids inhibit BACE-1 transcription in human neuronal SH-SY5Y cells. Altogether, our data suggest that flavonoids inhibit A beta and sAPP beta production by regulating BACE-1 expression and not by directly inhibiting BACE-1 activity.