Orphan nuclear receptor Nur77 participates in human apolipoprotein A5 gene expression

Abstract

The orphan nuclear receptor Nur77 (NR4A1) has been reported to play a crucial role in the modulation of diverse metabolic processes in liver. Here, we reported the identification of human apolipoprotein A5 (ApoA5), which implicated in lowering plasma triglyceride levels, as a novel target gene of Nur77. Nur77 induced the human ApoA5 promoter activity. Using 5′-deletion and mutagenesis of human ApoA5 promoter analysis and chromatin immunoprecipitation assays, it was shown that Nur77 directly regulated human ApoA5 gene expression by binding to a Nur77 response element (AAAGGTCA) located in the proximal human ApoA5 promoter region. In addition, we demonstrated that blocking of Nur77 transcriptional activity via overexpression of dominant negative Nur77 suppressed human ApoA5 promoter activity and mRNA expression in human hepatoma cells, HepG2. Taken together, our results demonstrated that Nur77 is a novel regulator of human ApoA5 gene expression and provide a new insight into the role of this orphan nuclear receptor in lipoprotein metabolism and triglyceride homeostasis.

Introduction

Nuclear receptor Nur77 (NR4A1) belongs to the NR4A family of the nuclear receptors which also includes Nurr1 (NR4A2) and Nor1 (NR4A3) [1]. It has not been identified specific ligands for the NR4A family hitherto and recent studies demonstrated that the ligand-binding domain of these receptors do not contain a ligand-binding pocket suggesting that these receptors are orphan nuclear receptor and their function is regulated by their expression and post-translational modification rather than ligand-binding [1], [2]. NR4A receptor family shares a highly conserved DNA-binding domain and recognizes its cognate DNA consensus sequence known as the Nur77 response element (NBRE; AAAGGTCA) as a monomer [3], [4]. It has been reported that Nur77 is expressed and modulates diverse metabolic processes including apoptosis, gluconeogenesis and lipogenesis in the liver [5], [6], [7], [8]. Although informations regarding Nur77 function have been accumulated, the physiological role of this orphan nuclear receptor largely remains to be elucidated.

Apolipoproteins play an important role in lipid homeostasis and have a major impact on the risk of cardiovascular disease [9], [10]. A recently identified apolipoprotein, apolipoprotein A5 (ApoA5), has been shown to be important in the regulation of plasma triglyceride (TG) levels [11], [12]. ApoA5 was predominantly expressed in the liver and found in plasma fractions containing the high density lipoprotein (HDL) particles [11], [12]. A crucial role of ApoA5 in the regulation of TG was demonstrated using transgenic and knock-out animal models leading to decreased and increased plasma TG concentration, respectively [12], [13]. Moreover, overexpression of mouse ApoA5 gene using adenovirus reduced serum levels of TG and cholesterol in mice [14].

It has been reported that human ApoA5 gene expression was directly up-regulated by several nuclear receptors including peroxisome proliferator-activated receptor (PPARα), farnesoid X-activated receptor (FXR), retinoid acid receptor-related orphan receptor (RORα), hepatocyte nuclear factor 4α (HNF4α), and thyroid receptor β (TRβ) [15], [16], [17], [18], [19]. Interestingly, these studies revealed an important region for the regulation of human ApoA5 gene expression, direct repeat 1 (DR-1) sequence located at the position from −272 to −260 in the human ApoA5 promoter [15], [16], [17], [18]. It is important to identify and characterize transcription factors that bind to the identified DR-1 sequence to understand the regulation mechanism of human ApoA5 gene expression. In contrast, liver X receptor (LXR) down-regulated ApoA5 gene expression via an indirect pathway involving its downstream target gene sterol regulatory element-binding protein 1c (SREBP-1c) which binding to the functional E-box present in human ApoA5 promoter [20].

In this report, we provided evidences that Nur77 directly regulates human ApoA5 gene expression via NBRE sequence in the promoter and highlighted a critical role of Nur77 in the regulation triglyceride homeostasis.