Aucubin, a naturally occurring iridoid glycoside inhibits TNF-α-induced inflammatory responses through suppression of NF-κB activation in 3T3-L1 adipocytes

doi:10.1016/j.cyto.2013.04.005

Cytokine

Volume 62, Issue 3, June 2013, Pages 407-412

https://doi.org/10.1016/j.cyto.2013.04.005 Get rights and content

Highlights

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Aucubin inhibits TNF-α-induced production of atherogenic adipokines in adipocytes.
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These effects of aucubin result from suppression of NF-κB activation pathway.
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The mechanism explains effects of aucubin on obesity-related inflammatory responses.

Abstract

Obesity is closely associated with a state of chronic, low-grade inflammation characterized by abnormal cytokine production and activation of inflammatory signaling pathways in adipose tissue. Tumor necrosis factor (TNF)-α is chronically elevated in adipose tissues of obese rodents and humans. Increased levels of TNF-α are implicated in the induction of atherogenic adipokines, such as plasminogen activator inhibitor (PAI)-1, adipose-tissue-derived monocyte chemoattractant protein (MCP)-1, and interleukin (IL)-6. Aucubin, an iridoid glycoside existing in medicinal plants, has been reported to show an anti-inflammatory activity by suppression of TNF-α production in murine macrophages. The present study is aimed to investigate the effects of aucubin on TNF-α-induced atherogenic changes of the adipokines in differentiated 3T3-L1 cells. Aucubin significantly inhibited TNF-α-induced secretion and mRNA synthesis of the atherogenic adipokines including PAI-1, MCP-1, and IL-6. Further investigation of the molecular mechanism revealed that pretreatment with aucubin suppressed extracellular signal-regulated kinase (ERK) activation, inhibitory kappa Bα (IκBα) degradation, and subsequent nuclear factor kappa B (NF-κB) activation. These findings suggest that aucubin may improve obesity-induced atherosclerosis by attenuating TNF-α-induced inflammatory responses.

Introduction

Obesity, which is characterized by excessive accumulation of abdominal fat, is casually associated with the premature development of atherosclerosis, increased risk of stroke, and development of congestive heart failure [1]. Recent studies have indicated that the adipocyte secretes a variety of adipokines involved in energy metabolism, inflammation, and cardiovascular functions [2]. The cellular mechanisms linking obesity and atherosclerosis are complex and have not been fully elucidated. However, increasing evidences suggest that the changes of adipokines including PAI-1, MCP-1, and IL-6 due to excess adipose tissue may be a cause of atherosclerosis [3].

Plasminogen activator inhibitor (PAI)-1 is the primary inhibitor of plasminogen activation. Plasma levels of PAI-1 are markedly elevated in obese individuals as well as in patients with insulin resistance, type 2 diabetes, and cardiovascular disease (CVD) [4], [5]. PAI-1 is thought to be the link between obesity and increased risk for CVDs [6]. Although several tissues are known to produce PAI-1, adipose tissue appears to be the major contributor to elevated PAI-1 levels observed in cases of obesity [7], [8]. Monocyte chemoattractant protein (MCP)-1 chemotactically recruits monocytes to sites of inflammation. Although this protein is traditionally thought to be expressed mainly endothelial cells and macrophages, it has been shown to be primarily expressed by adipose tissues [9], [10]. Adipocyte-derived MCP-1 induces macrophage infiltration into adipose tissues and thus secretes inflammatory cytokines including tumor necrosis factor (TNF)-α, which in turn leads to the dysfunction of adipocytes [11]. In addition, MCP-1 inhibits insulin-dependent glucose uptake and the expression of adipogenic genes [10].

Aucubin is a natural constituent with a monoterpene cyclic ring system found in a wide range of some insects and higher plants such as Aucuba japonica and Plantago asiatica [12], [13]. In recent years, a variety of bio-activities of aucubin have been reported; liver-protective activities against hepato-toxicants, stimulation of bile acid excretion, anti-microbial activities, anti-tumor activities, antidotal activities for noxious amanita mushroom poisoning, anti-viral activities against hepatitis B virus, and anti-inflammatory activities [14], [15], [16], [17], [18], [19], [20], [21], [22]. Interestingly, a number of plants containing aucubin have been used long as medicinal herbs for anti-inflammatics and anti-rheumatics in Oriental and Occidental hemispheres. As for the molecular mechanism for anti-inflammatory activities of aucubin, it was reported that aucubin inhibits the production of TNF-α resulting from the both IκBα degradation and the nuclear translocation of NF-κB in RAW 264.7 cells [21]. However, the inhibitory effects of aucubin on TNF-α-induced inflammatory responses in adipocytes have not been reported previously.

The present study was designed to determine whether it attenuates TNF-α-induced secretion and mRNA production of the atherogenic adipokines including PAI-1, MCP-1, and IL-6 in differentiated 3T3-L1 adipocytes. In addition, the possible mechanisms for inhibitory effects of aucubin on obesity-related inflammatory responses were examined.

Section snippets

Reagents

Aucubin with 99.5% purity was purchased from Wako Pure Chemical Industries Ltd., (Osaka, Japan). Recombinant murine tumor necrosis factor (TNF)-α was from R&D Systems (Minneapolis, MN, USA). Insulin, dexamethasone, and 3-isobutyl-1-methylxanthine were from Sigma (St. Louis, MO, USA). All tissue culture materials were from Gibco-BRL (Rockville, MD, USA). Primary anti-mouse antibodies for extracellular signal-regulated kinases (ERK), phospho-ERK, IκBα, phospho-IκBα, and β-actin were obtained from

Effects of aucubin on the TNF-α-induced secretion of adipokines in 3T3-L1 adipocytes

To examine the effects of aucubin on TNF-α-induced secretion of adipokines, 3T3-L1 adipocytes were pretreated with various concentrations of aucubin for 6 h and then incubated with 10 ng/mL TNF-α for 24 h. After incubation, conditioned medium was collected for ELISA assay. Treatment with aucubin inhibited TNF-α-induced increase in the secretion of MCP-1, PAI-1, and IL-6 in dose-dependent manner with IC50 of 8.20, 9.94, and 6.85 μM, respectively (Fig. 1).

Effects of aucubin on the TNF-α-induced expression of adipokine genes in 3T3-L1 adipocytes

To investigate whether aucubin inhibited

Discussions

Aucubin [1,4a,5,7a-tetra-5-hydroxy-7-(hydroxymethyl)cyclopenta(c)pyran-1-yl-β-D-glucopyranoside] is a common iridoid glycoside, found in a wide range of plants, which are used in folk medicine and traditional Chinese medicine. Those medicinal plants containing aucubin show a variety of biological activities as aforementioned. Of pharmaceutical products as well as dietary supplements containing such medicinal plant materials, agents for treating inflammatory ailments including rheumatic

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Aucubin, a naturally occurring iridoid glycoside inhibits TNF-α-induced inflammatory responses through suppression of NF-κB activation in 3T3-L1 adipocytes

Highlights

Abstract

Introduction

Section snippets

Reagents

Effects of aucubin on the TNF-α-induced secretion of adipokines in 3T3-L1 adipocytes

Effects of aucubin on the TNF-α-induced expression of adipokine genes in 3T3-L1 adipocytes

Discussions

J Thromb Haemost

Phytochemistry

Phytochemistry

Cancer Lett

Brain Res Bull

Obesity and cardiovascular disease: pathogenic mechanisms and potential benefits of weight reduction

Semin Vasc Med

Adipokines: inflammation and the pleiotropic role of white adipose tissue

Br J Nutr

Obesity, metabolic syndrome, and cardiovascular diseases

J Clin Endocrinol Metab

Effect of plasminogen activator inhibitor-1 in diabetes mellitus and cardiovascular disease

Am J Med

Obesity and impaired fibrinolysis: role of adipose production of plasminogen activator inhibitor-1