266: Neuroprotective Effects of Korean Citrus Peels Against Oxidative Stress-Mediated Cytotoxicity and Tau Hyperphosphorylation

Introduction

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive cognitive decline. Although the pathogenesis of AD is unclear, oxidative stress and tau hyperphosphorylation have been recognized as key contributors to its initiation and progression. Citrus peel, a significant byproduct of citrus fruit consumption, is a rich source of flavonoids such as flavones, flavonols, and polymethoxyflavones. This study aimed to characterize the flavonoid composition of various Korean citrus peels (Unshiu, Hallbong, Cheonhyehyang, and Hwaggeumhyang) and investigate the neuroprotective effect of citrus peel extracts (CPEs) against H2O2-induced oxidative stress in PC12 cells and tau hyperphosphorylation in HEK293 cells

Methods

The flavonoid composition of citrus peels, including nobiletin, naringin, naringenin, sinensetin, tangeretin, narirutin, rutin, auraptene, hesperidin, hesperetin, quercetin, and didymin, was analyzed by HPLC-PDA. The protective effect of CPEs against H2O2-induced oxidative stress was evaluated by assessing reactive oxygen species (ROS) and glutathione (GSH) levels, and the upregulation of antioxidant phase II enzymes such as HO-1 and NQO1 via the Nrf2-ARE pathways in PC12 cells. The effect of CPEs on tau hyperphosphorylation was measured in HEK293 cells expressing tau with the P301L mutation.

Results

Unshiu peel exhibited the highest total flavonoid content (22,158±1,608.70 μg/g), primarily composed of hesperidin (15,321.88±1,288.9 μg/g). Cheonhyehyang peel was characterized by the highest narirutin content (7,264.56±70.28 μg/g) and significant levels of polymethoxyflavones, including sinensetin (637.28±10.77 μg/g) and nobiletin (688.28±16.99 μg/g). Treatment with CPEs significantly attenuated H₂O₂-induced cytotoxicity in PC12 cells, with Cheonhyehyang peel extract exhibiting the most potent cytoprotective effects. This protective effect was mediated by the antioxidant properties of CPEs, which involved the activation of the Nrf2-ARE signaling pathway, leading to the inhibition of cellular ROS generation, the restoration of depleted GSH levels, and the induction of phase II enzymes such as HO-1 and NQO-1 expression in H₂O₂-induced PC12 cells. Additionally, the treatment of CPEs decreased the level of tau hyperphosphorylation in HEK 293 cells expressing tau.

Significance

The potent neuroprotective effects observed in Cheonhyehyang peel appear to be associated with its higher concentrations of narirutin, sinensetin, and nobiletin. These findings provide that citrus peels could be valuable neuroprotective agents in the fields of functional foods and pharmaceuticals.

Authors: Samuel Park, Jeehye Sung