Lycopene inhibits ICAM-1 expression and NF-κB activation by Nrf2-regulated cell redox state in human retinal pigment epithelial cells

Abstract

Aims

Age-related macular degeneration (AMD) is one of the most common diseases leading to blindness in elderly people. The progression of AMD may be prevented through anti-inflammation and antioxidation in retinal pigment epithelium (RPE) cells. Lycopene, a carotenoid, has been shown to possess both antioxidative and anti-inflammatory properties. This research was conducted to detail the mechanisms of these effects of lycopene-treated RPE cells.

Main methods

We exposed ARPE-19 cells to TNFα after pretreatment with lycopene, and measured monocyte adhesion, ICAM-1 expression, NF-κB nuclear translocation, and transcriptional activity. Cell viability was assayed with Alamar Blue. The cell redox state was tested by glutathione (GSH) and reactive oxygen species (ROS) levels. The importance of the Nrf2 pathway was tested in nuclear translocation, promoter reporter assay, and siRNA.

Key findings

Lycopene could reduce TNF-α-induced monocyte adhesion and H₂O₂– induced cell damage in RPE cells. Furthermore, lycopene inhibits ICAM-1 expression and abolishes NF-κB activation for up to 12 h in TNFα-treated RPE cells. Lycopene upregulates Nrf2 levels in nuclear extracts and increases the transactivity of antioxidant response elements. The use of Nrf2 siRNA blocks the inhibitory effect of lycopene in TNF-α-induced ICAM-1 expression and NF-κB activation. Glutamate-cysteine ligase (GCL) is the rate-limiting enzyme in the de novo synthesis of GSH. We found that lycopene increases intracellular GSH levels and GCL expression. Following lycopene treatment, TNF-α-induced ROS production was abolished.

Significance

The Nrf2-regulated antioxidant property plays a pivotal role in the anti-inflammatory mechanism underlying the inhibition of NF-κB activation in lycopene-treated ARPE-19 cells.

Introduction

Age-related macular degeneration (AMD) is characterized by a progressive loss of central vision attributable to degeneration at the ocular interface between the neural retina and the underlying choroid [1]. Previous studies have indicated that the pathogenesis of AMD is associated with oxidative stress and inflammation, which ultimately lead to degeneration of the retinal pigment epithelium (RPE) [2].

During ocular inflammation, lymphocytes and macrophages migrate to the posterior compartment of the eye and secrete proinflammatory cytokines [3]. RPE cells are a component of the outer blood–retinal barrier and are sensitive to proinflammatory stimuli, particularly TNFα, which is secreted by macrophages and vascular endothelial cells [4]. The RPE is the site for leukocyte recruitment and adhesion to the retina upon stimulation with TNFα. The processes are mediated by cell adhesion molecules. Intercellular adhesion molecule-1 (ICAM-1), a transmembrane glycoprotein, is upregulated for leukocyte recruitment and adhesion to the retina [4]. TNFα can activate nuclear factor-κB (NF-κB), which plays a key role in ICAM-1 expression [5]. After stimulation, a kinase cascade is activated, which induces subsequent IκB phosphorylation. Phosphorylated IκB rapidly degrades, resulting in the release of NF-κB, which enters the nucleus and regulates gene transcription [6].

Evidence from in vitro and in vivo studies has shown that Nrf2 is a major transcription factor that regulates the antioxidant response element (ARE) driving the expression of phase II detoxifying enzymes and antioxidant enzymes such as heme oxygenease-1 (HO-1), quinone oxidoreductase, and glutamate-cysteine ligase (GCL) [7], [8], [9]. GCL is the rate-limiting enzyme regulating the synthesis of glutathione (GSH), which is one of the most critical antioxidants in vivo. GCL consists of a catalytic heavy subunit (GCLC) and a modulatory light subunit (GCLM) [9]. Nrf2 can influence GSH synthesis through regulating the expression of GCLC and GCLM subunits. Reduced GSH in cells serves to maintain a reduced cellular environment and may also act as a crucial posttranslational modification in numerous cellular proteins [10]. A previous study discussed Nrf2-related GSH levels involved in the anti-inflammatory mechanisms associated with the induction of NF-κB/p65 protein glutathionylation [11]. An imbalance in cellular thiol redox state has been implicated in the progression of AMD [12]. Therefore, protection of RPE cells from oxidative stress through Nrf2 activation may provide a therapeutic target for treating AMD.

Lycopene is a natural carotenoid present in tomatoes and tomato-based products. Other dietary sources of lycopene include dried apricots, guava, watermelon, papaya, and pink grapefruit [13]. Dietary intake of tomatoes containing lycopene can reduce the risk of chronic diseases and various cancers [14]. Lycopene exerts an antioxidant effect that reduces proinflammatory cytokine and chemokine expression in macrophages [15], [16]. Lycopene has been found to reduce the expression of several genes by modulating the NF-κB signaling pathway [17]. These results may relate lycopene's antioxidant activity to its anti-inflammatory effects. Lycopene was shown to attenuate adhesion molecule expression and interactions between monocytes and endothelial cells [18]. A recent study found that lycopene contributed by tomato ketchup extracts reduced release of the proinflammatory cytokines in endothelial cells [19]. The anti-inflammatory mechanism of lycopene in RPE cells has not yet been described thoroughly. The present study investigated the molecular mechanisms underlying the anti-inflammatory properties of lycopene in RPE cells. We investigated whether lycopene's antioxidant properties contribute to lycopene-regulated anti-inflammatory responses. We found that lycopene-induced Nrf2 activation is closely associated with GSH upregulation, which leads to anti-inflammatory effects in RPE cells.