The Inhibition Mechanisms Of Curcumin On Aβ Levels

Alzheimer’s disease（AD）,the main cause of dementia,is a progressive neurodegenerative disease caused by multiple factors.Its main physiological and biochemical characteristics are senile plaques composed of amyloid-β（Aβ）and neurofibrillary tangles（NFT）formed of hyperphosphorylated tau protein.Recent years,studies have found that neuroinflammation is also present in the brain of AD patients,which is the third major pathological feature.At present,the specific pathogenesis of AD remains unclear.The amyloid cascade hypothesis dominates among all hypotheses,pointing out that Aβis the initiative cascade of other abnormal events in Alzheimer’s disease.Aβis produced by sequential cleavage of amyloid precursor proteins followed by different secretases.Theβ-site amyloid precursor protein cleaving enzyme 1（BACE1）is the rate-limiting enzyme in the Aβproduction step.Therefore,pharmacological inhibition of BACE1 is currently one of the main targets for the treatment of AD.Curcumin is a natural polyphenolic derived from the rhizome of the plant turmeric（Curcuma longa Linn）,which has anti-oxidant,anti-inflammatory and anti-viral properties.Curcumin has been reported to cross the blood-brain barrier and prevent Aβaggregation in the AD model.However,its neuroprotective mechanism needs to be clarified.In this study,we revealed that curcumin has an anti-amyloidogenic effect.First,we established a stable transgenic cell line that overexpressed APPswedish（K595N/M596L）.We found that curcumin is capable to low the secretion of extracellular Aβ₄₂ by enzyme-linked immunosorbent assay.Next,we examined the associated secretory enzymes and intermediates involved in this pathway and found that curcumin inhibited the expression of BACE1 gene at the transcriptional and translational levels in SH-SY5Y cells.Then,we further explored the mechanism involved in the regulation of BACE1 gene by curcumin.Using siRNA or overexpression of p65 strategies,we found that NF-κB positively regulates the expression of BACE1 and affects the level of Aβ—the final product of APP.By constructing different NF-κB-containing elements plasmids,we reasoned that curcumin can reduce the ability of NF-κB binding to the BACE1 promoter.Further protein experiments have shown that curcumin inhibits the Akt-NF-κB axis and suppresses the activity of the NF-κB signaling pathway.Specifically,it inhibits the upstream signaling molecule p-Akt and the phosphorylation of its downstream molecule IκBα,as well as prevent the NF-κB subunit p65 from entering the nucleus,then reducing the phosphorylation level of p65 ser536.The immunofluorescence results also supported the conclusion that curcumin inhibited p65 into the nucleus.This part of the results indicates that the anti-inflammatory properties of curcumin make it capable of lowering BACE levels.Finally,using in vitro receptor binding assay,dual fluorescent reporter gene and other methods,results show that curcumin is a selective agonist of the estrogen receptorβ.Combining with the treatment of ICI182,780,the modulation of curcumin on BACE1is thought to be accomplished by estrogen receptorβ.Through overexpression and agonist experiments we found that activation of the estrogen receptorβdirectly affects the upstream factors of the NF-κB signaling pathway.These results for the first time demonstrate that the estrogenic properties of curcumin contribute to its neuroprotective effects.In summary,curcumin selectively activates estrogen receptorβand inhibits the NF-κB signaling pathway,thereby regulating the expression of BACE1 at the promoter level,ultimately inhibiting the amyloid cleavage pathway and reducing the amount of Aβsecreted to the extracellular.From different perspectives,our study unravels the mechanism of curcumin’s neuroprotective action,providing a theoretical basis for the basic research and clinical treatment of natural compounds in neurodegenerative diseases.