Effects of Flavonoid Compounds on β-amyloid-peptide-induced Neuronal Death in Cultured Mouse Cortical Neurons

Abstract

Excessive accumulation of β-amyloid peptide (Aβ) is one of the major mechanisms responsible for neuronal death in Alzheimer's disease. Flavonoids, primarily antioxidants, are a group of polyphenolic compounds synthesized in plant cells. The present study aimed to identify flavonoid compounds that could inhibit Aβ-induced neuronal death by examining the effects of various flavonoids on the neurotoxicity of Aβ fragment 25-35 (Aβ25-35) in mouse cortical cultures. Aβ25-35 induced concentration- and exposure-time-dependent neuronal death. Neuronal death induced by 20 µM Aβ25-35 was significantly inhibited by treatment with either Trolox or ascorbic acid. Among 10 flavonoid compounds tested [apigenin, baicalein, catechin, epicatechin, epigallocatechin gallate (EGCG), kaempferol, luteolin, myricetin, quercetin, and rutin], all except apigenin showed strong 1,1-diphenyl-2-pycrylhydrazyl (DPPH) scavenging activity under cell-free conditions. The flavonoid compounds except apigenin at a concentration of 30 µM also significantly inhibited neuronal death induced by 20 µM Aβ25-35 at the end of 24 hours of exposure. Epicatechin, EGCG, luteolin, and myricetin showed more potent and persistent neuroprotective action than did the other compounds. These results demonstrated that oxidative stress was involved in Aβ-induced neuronal death, and antioxidative flavonoid compounds, especially epicatechin, EGCG, luteolin, and myricetin, could inhibit neuronal death. These findings suggest that these four compounds may be developed as neuroprotective agents against Alzheimer's disease.

Keywords: Alzheimer's disease; Flavonoids; β-Amyloid peptide.

FIG. 1

Aβ_25-35-induced neuronal death in mouse cortical cultures. Phase-contrast photomicrographs from typical representative fields (200×field) of cells were taken after a 24-hour exposure to (A) sham wash or (B) 20 µM Aβ_25-35 showing neurotoxic action of 20 µM Aβ_25-35 in mouse cortical cultures (200×field). Treatment with Aβ_25-35 induced concentration- and exposure-time-dependent neuronal death in mixed cortical cultures (C). Each point and bars are the mean±SEM from 8-20 cultured wells (C).

FIG. 2

Effect of co-treatment with Trolox (100 µM) or ascorbic acid (AA, 100 µM) on 20 µM Aβ_25-35-induced neuronal death at the end of 24 and 48 hours of exposure. Each bar is the mean±SEM from 8-16 cultured wells. ^*Significantly different from 24-hour-treated control group (p<0.05). ^#Significantly different from 48-hour-treated control group (p<0.05).

FIG. 3

(A) Effects of epigallocatechin gallate (EGCG, 3, 10, 30 µM) on 20 µM Aβ_25-35-induced neuronal death at the end of 24 or 48 hours of exposure. (B) Effects of luteolin (LTL, 3, 10, 30 µM) on 20 µM Aβ_25-35-induced neuronal death at the end of 24 or 48 hours of exposure. (C) Effects of myricetin (MYR, 3, 10, 30 µM) on 20 µM Aβ_25-35-induced neuronal death at the end of 24 or 48 hours of exposure. Each bar is the mean±SEM from 8-12 cultured wells. Other legends are the same as in Fig. 2.