Inhibitory effect of 9-hydroxy-6,7-dimethoxydalbergiquinol from Dalbergia odorifera on the NF-κB-related neuroinflammatory response in lipopolysaccharide-stimulated mouse BV2 microglial cells is mediated by heme oxygenase-1

Highlights

• HDDQ was isolated from Dalbergia odorifera.

• HDDQ suppressed NF-κB-related neuroinflammatory enzymes and cytokines.

• HDDQ increased HO-1 expression via Nrf2 pathway.

• HDDQ showed potent anti-neuroinflammatory effects through HO-1 expression.

• HDDQ might be a beneficial potent therapeutic agent for neuroinflammatory diseases.

Abstract

The heartwood of Dalbergia odorifera T. Chen (Leguminosae) is an important source of traditional Korean and Chinese medicines. 9-Hydroxy-6,7-dimethoxydalbergiquinol (HDDQ), a compound isolated from D. odorifera, has various biological activities. The aim of this study was to determine the efficacy of HDDQ in modulating the regulation of anti-inflammatory activity through the upregulation of heme oxygenase (HO)-1 in BV2 microglia. HDDQ inhibited the protein expression of inducible nitric oxide synthase (iNOS), iNOS-derived nitric oxide (NO), and the production of cyclooxygenase (COX)-2 and COX-2-derived prostaglandin E2 (PGE₂) in lipopolysaccharide (LPS)-stimulated mouse BV2 microglia. HDDQ also reduced tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) production, and suppressed the phosphorylation and degradation of IκB-α and the nuclear translocation of p65 in mouse BV2 microglia in response to LPS. Furthermore, HDDQ upregulated HO-1 expression via nuclear translocation of nuclear factor E2-related factor 2 (Nrf2) in mouse BV2 microglia. Using tin protoporphyrin (SnPP), an HO activity inhibitor, we verified that the inhibitory effects of HDDQ on the proinflammatory mediators NO, PGE₂, TNF-α, and IL-1β, and nuclear factor kappa B (NF-κB) DNA-binding activity are associated with the induction of HO-1 expression. Our data suggest that HDDQ has therapeutic potential against neurodegenerative diseases caused by neuroinflammation.

Introduction

Inflammation is a crucial immune response; however, it also disturbs tissue functions and can lead to disease pathogenesis [1]. Augmentation of inflammation in the central nervous system (CNS) has been implicated in normal aging [2] as well as age-related neurodegenerative diseases [3] such as Alzheimer's disease (AD), Parkinson's disease (PD), and Amyotrophic Lateral Sclerosis (ALS) [4]. Neuroinflammation is induced by activated microglia, which are CNS-stationed immune cells that constitute up to 10% of the total glial cell population in the brain [5]. Microglia activation occurs in response to inducers (e.g., bacterial pathogens) that in turn enhance the signal transduction, which induces inflammatory enzymes (e.g., inducible nitric oxide synthase [iNOS] and cyclooxygenase-2 [COX-2]), cytokines (e.g., interleukin-1β [IL-1β] and tumor necrosis factor-α [TNF-α]), and transcription factors (e.g., nuclear factor-κB [NF-κB]) [6]. Activated microglia also generate high levels of reactive oxygen species (ROS), especially nitric oxide (NO), along with the proinflammatory prostaglandin, PGE₂ [7]. These can lead to oxidative stress-induced neurodegenerative disorders [8].

Heme oxygenase (HO)-1, which catabolizes heme into carbon monoxide (CO), biliverdin/bilirubin, and ferrous iron, has protective effects in multiple tissues and organs [9]. HO-1 is induced as a phase 2 enzyme by electrophilic xenobiotics, oxidative stress, and cellular damage, which trigger cellular protection mechanisms under antioxidant, anti-proliferative, anti-apoptotic, and anti-inflammatory actions. Several lines of evidence have indicated that phytochemicals also induce nuclear factor E2-related factor 2 (Nrf2) binding to Kelch-like ECH-associated protein 1 (Keap1), ultimately upregulating HO-1 [10]. In addition to controlling genes that encode antioxidant proteins, HO-1 generates CO and ferrous iron and attenuates the expression of proinflammatory mediators such as TNF-α, IL-1β, iNOS, COX-2, NO, and PGE₂ [11], [12].

Dalbergia odorifera T. Chen (Leguminosae) (“Jiang xiang” in Chinese, “Kangjinhyang” in Korean, and “Koshinko” in Japanese) grows largely in Central and South America, Africa, Madagascar, and Southern Asia [13]. The heartwood of D. odorifera is traditional medicine used to treat blood disorders, ischemia, swelling, and epigastric pain in Korea and China, and D. odorifera has been in general use in various Chinese herbal preparations including Qi-Shen-Yi-Qi decoction, Guanxin-Danshen pills, and Danshen injection [14]. Previous studies have reported that D. odorifera possesses a variety of beneficial properties, including anti-oxidant [15], anti-microbial [16], and anti-inflammatory activities [17] in diverse cell types. In our previous study, 9-hydroxy-6,7-dimethoxydalbergiquinol (HDDQ), a biologically active chalcone isolated from the heartwood of D. odorifera, exhibited protective effects against glutamate-induced oxidative injury in HT22 cells [18]. However, studies on its biological and pharmacological activity of HDDQ are limited, and its anti-neuroinflammatory effect has not yet been reported. As a part of our ongoing effort for the identification of phytochemicals with anti-neuroinflammatory effects in vitro, we found that treatment of mouse BV2 microglia cells with HDDQ down-regulated the expression of proinflammatory mediators. We provided evidence for the vital role of HO-1 expression in mediating the anti-neuroinflammatory effects of HDDQ.