Isoliquiritigenin Confers Neuroprotection and Alleviates Amyloid-β42-Induced Neuroinflammation in Microglia by Regulating the Nrf2/NF-κB Signaling.

Yue Fu,Jianping Jia

doi:10.3389/fnins.2021.638772

Abstract

BackgroundNeuroinflammation and oxidative stress are two major pathological characteristics of Alzheimer’s disease (AD). Amyloid-β oligomers (AβO), a toxic form of Aβ, promote the neuroinflammation and oxidative stress in the development of AD. Isoliquiritigenin (ISL), a natural flavonoid isolated from the root of liquorice, has been shown to exert inhibitory effects on inflammatory response and oxidative stress.ObjectivesThe main purpose of this study is to assess the influence of ISL on inflammatory response and oxidative stress in BV2 cells stimulated with AβO, and to explore the underlying molecular mechanisms.Methods3-(4,5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2-H- tetrazolium bromide (MTT) and lactate dehydrogenase (LDH) cytotoxicity assays were used to assess the toxic or protective effects of ISL. The expression levels of interleukin-1β, interleukin-6, and tumor necrosis factor-α were assessed by quantitative real-time polymerase chain reaction (qRT-PCR) and enzyme-linked immunosorbent assays. Morphological changes in BV2 cells were assessed by immunofluorescence method. Nitric oxide (NO) assay kit was used to determinate the NO production. Western blot, qRT-PCR and immunofluorescence were used to explore the underlying molecular mechanisms.ResultsISL treatment reduced the production of inflammatory cytokines and NO, and alleviated the morphological changes in BV2 cells induced by AβO. ISL treatment further protected N2a cells from the toxic medium of AβO-stimulated BV2 cells. ISL activated nuclear factor erythroid-2 related factor 2 (Nrf2) signaling and suppressed nuclear factor-κB (NF-κB) signaling in BV2 cells.ConclusionISL suppresses AβO-induced inflammation and oxidative stress in BV2 cells via the regulation of Nrf2/NF-κB signaling. Therefore, ISL indirectly protects neurons from the damage of toxic conditioned media.

Highlights

Alzheimer’s disease (AD), a chronic neurodegenerative disorder and the leading cause of dementia, leads to severe cognitive impairment (Holtzman et al, 2011)
No statistically significant differences were observed between the ISL treated group and the control group
The results further showed that the amyloid-β oligomers (AβO)-treated BV2 cells had significantly lower total nuclear factor erythroid-2 related factor 2 (Nrf2) protein expression (Figures 6D,E; p < 0.01) and significantly lower nuclear translocation of Nrf2, which is validated by western blot or immunofluorescence staining (vs. control group, FIGURE 5 | Neuroprotective effects of ISL on N2a cells cultured with conditioned medium from AβO stimulated BV2 cells. (A) A diagram of the toxic effect of conditioned media from AβO stimulated BV2 cells on N2a cells with or without the ISL pre-treatment. (B,C) N2a cells were treated with conditioned medium for 24 h and cell viability was assessed using MTT and lactate dehydrogenase (LDH) cytotoxicity assays. ∗p < 0.05; ∗∗p < 0.01, ∗ ∗ ∗p < 0.001. n = 6 per group

Summary

Introduction

Alzheimer’s disease (AD), a chronic neurodegenerative disorder and the leading cause of dementia, leads to severe cognitive impairment (Holtzman et al, 2011). Senile plaques, which are formed by the deposition of amyloid-β (Aβ), are considered to be the most characteristic pathological changes in Alzheimer’s disease (Selkoe and Hardy, 2016). Extensive evidence has shown that amyloid-β oligomers (AβO), a toxic form of Aβ, promote a series of pathological changes in AD, including synaptic dysfunction, neuroinflammation, mitochondrial dysfunction, and neuronal death (Hu et al, 2017; Caruso et al, 2019). Microglia surround amyloid plaques and respond to Aβ with a pro-inflammatory phenotype (Hansen et al, 2018; Simpson and Oliver, 2020) This microglial phenotype is characterized by excessive cytokine expression, including tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin1β (IL-1β), resulting in neurotoxicity (Hansen et al, 2018; Simpson and Oliver, 2020). Amyloid-β oligomers (AβO), a toxic form of Aβ, promote the neuroinflammation and oxidative stress in the development of AD. Isoliquiritigenin (ISL), a natural flavonoid isolated from the root of liquorice, has been shown to exert inhibitory effects on inflammatory response and oxidative stress

Objectives

Methods

Results

Conclusion