Auraptene increases the production of amyloid-ß via c-Jun N-terminal kinase-dependent activation of γ-secretase.

Amyloid-ß (Aß) peptide plays a major role in the pathogenesis of Alzheimer's disease (AD), and is generated by ß- and γ-secretase-mediated proteolytic processing of amyloid-ß protein precursor (AßPP). In the present study, we investigated the effect of 118 natural compounds on Aß production in the medium of HEK293 cells stably expressing human AßPP695 (HEK293-AßPP) using Aß42 sandwich ELISA to find natural compounds that can modulate Aß production. We found that a coumarin derivative of citrus fruits, auraptene, increased Aß production. Treatment of HEK293-AßPP cells and rat primary cortical neurons with auraptene significantly increased the secretion of Aß40, Aß42, and the Aß42/40 ratio. However, auraptene did not change the protein levels of the AßPP processing enzymes, a disintegrin and metalloproteinases 10 (ADAM10, α-secretase), ß-site AßPP cleaving enzyme-1 (BACE-1, ß-secretase), and presenilin 1 (PS1, γ-secretase component). Auraptene increased the activity of γ-secretase but not that of α- and ß-secretase. Furthermore, auraptene enhanced γ-secretase-mediated production of Aß from AßPP or AßPP-C99, but not through α- and ß-secretase. Auraptene also phosphorylated c-Jun N-terminal kinase (JNK), and pretreatment with the JNK inhibitor, SP600125, reduced auraptene-induced γ-secretase activity. Overall, our results suggest that auraptene-mediated activation of JNK may contribute to the production of Aß by promoting γ-secretase activity.

Beta-amyloid increases the expression level of ATBF1 responsible for death in cultured cortical neurons.

BACKGROUND: Recently, several lines of evidence have shown the aberrant expression of cell-cycle-related proteins and tumor suppressor proteins in vulnerable neurons of the Alzheimer's disease (AD) brain and transgenic mouse models of AD; these proteins are associated with various paradigms of neuronal death. It has been reported that ATBF1 induces cell cycle arrest associated with neuronal differentiation in the developing rat brain, and that gene is one of the candidate tumor suppressor genes for prostate and breast cancers in whose cells overexpressed ATBF1 induces cell cycle arrest. However, the involvement of ATBF1 in AD pathogenesis is as yet unknown. RESULTS: We found that ATBF1 was up-regulated in the brains of 17-month-old Tg2576 mice compared with those of age-matched wild-type mice. Moreover, our in vitro studies showed that Aß1-42 and DNA-damaging drugs, namely, etoposide and homocysteine, increased the expression ATBF1 level in primary rat cortical neurons, whereas the knockdown of ATBF1 in these neurons protected against neuronal death induced by Aß1-42, etoposide, and homocysteine, indicating that ATBF1 mediates neuronal death in response to these substances. In addition, we found that ATBF1-mediated neuronal death is dependent on ataxia-telangiectasia mutated (ATM) because the blockage of ATM activity by treatment with ATM inhibitors, caffeine and KU55933, abolished ATBF1 function in neuronal death. Furthermore, Aß1-42 phosphorylates ATM, and ATBF1 interacts with phosphorylated ATM. CONCLUSIONS: To the best of our knowledge, this is the first report that Aß1-42 and DNA-damaging drugs increased the ATBF1 expression level in primary rat cortical neurons; this increase, in turn, may activate ATM signaling responsible for neuronal death through the binding of ATBF1 to phosphorylated ATM. ATBF1 may therefore be a suitable target for therapeutic intervention of AD.

Honokiol increases ABCA1 expression level by activating retinoid X receptor beta.

ABCA1, a member of the ATP-binding cassette transporter family, regulates high-density lipoprotein (HDL) metabolism and reverses cholesterol transport. Its expression is upregulated mainly by the activation of the liver X receptor (LXR), retinoid X receptor (RXR), and peroxisome proliferator-activated receptors (PPARs). To identify natural compounds that can upregulate ABCA1 expression, we developed a reporter assay using U251-MG (human glioma cell line) cells that stably express a human ABCA1 promoter-luciferase and performed a cell-based high-throughput screening of 118 natural compounds. Using this system, we identified honokiol, a compound extracted from Magnolia officinalis, as an activator of the ABCA1 promoter. We found that honokiol also increased ABCA1 mRNA and protein expression levels in a dose-dependent manner in U251-MG cells without significant cell death and also increased ABCA1, ABCG1 and apolipoprotein E (apoE) expression levels in THP-1 macrophages. PPAR antagonists did not diminish the induction of ABCA1 expression by honokiol in U251-MG cells. Cotreatment of the cells with honokiol and T0901317 (synthetic LXR ligand) further increased the ABCA1 expression level, whereas cotreatment with 9-cis retinoic acid had no additive effect compared with treatment with honokiol alone. We also found that honokiol has binding affinity to RXRbeta. In this study, we identified for the first time honokiol as an upregulator of ABCA1 expression, which is mediated by the binding of honokiol to RXRbeta as a ligand.