Transcriptional Regulation of Apolipoprotein A-I Gene Expression by the Nuclear Receptor RORα

Abstract

Since elevated concentrations of plasma high density lipoprotein (HDL) and its major apolipoprotein (apo), apoA-I, confer protection against atherosclerosis, considerable research efforts have focussed on the identification of factors regulating apoA-I gene expression in an attempt to increase its production. Nuclear receptors are interesting candidates because they are transcription factors whose activity is ligand-dependent. In the present study we identified the orphan receptor ROR␣1 as an activator of apoA-I gene transcription. In apoA-Iexpressing intestinal Caco-2 cells, overexpression of the ROR␣1, but not the ROR␣2 or ROR␣3 isoforms, increased rat apoA-I gene transcription. Deletion and sitedirected mutagenesis experiments identified a functional ROR-responsive element (RORE) in the rat and mouse apoA-I gene promoters, which overlaps with the TATA box. Gel shift experiments indicated that this RORE binds the ROR␣1 isoform, but not the ROR␣2 or ROR␣3 isoforms. Furthermore, compared with wild type mice, apoA-I mRNA levels were significantly lower in small intestines of staggerer mice homozygous for a deletion in the ROR␣ gene. In addition, reverse transcriptase-polymerase chain reaction analysis revealed the expression of ROR␣ in small intestinal epithelium and in Caco-2 cells. These data indicate a novel, physiological role for ROR␣1 in the regulation of genes involved in lipid and lipoprotein metabolism and possibly in the development of metabolic diseases, such as atherosclerosis.Results from several epidemiological studies have demonstrated that plasma concentrations of high density lipoprotein (HDL) 1 and its major protein component, apolipoprotein (apo) A-I, are inversely correlated to the development of coronary artery disease (1-4). In addition, studies in transgenic mice and rabbits have shown that overexpression of the human apoA-I gene results in increased plasma HDL and apoA-I concentrations and confers protection against atherogenesis (5, 6), whereas elimination of the apoA-I gene by homologous recombination leads to a profound hypo-␣-lipoproteinemia (7). Therefore, a thorough knowledge of the factors controlling apoA-I production and metabolism is essential to understand the causes of hypo-␣-lipoproteinemia, the most common lipoprotein abnormality in patients with coronary artery disease (8).To identify factors regulating apoA-I gene expression, we focussed our attention on various members of the superfamily of nuclear receptors. Nuclear receptors are transcription factors, which upon activation by specific ligands bind to response elements located in the regulatory regions of target genes and thereby modulate their transcriptional activity (9). As such nuclear receptors translate signals coming from the environment into changes in gene expression and are therefore interesting targets for pharmacological intervention. The largest class of nuclear receptors is constituted by the orphan receptors, for which no ligands have yet been identified (10). Furthermore, the physiological functions of mos...

RAR-related orphan receptor A isoform 1 (RORa1) is disrupted by a balanced translocation t(4;15)(q22.3;q21.3) associated with severe obesity

“…The mice also show an aberrant blood plasma lipid profile with lowered levels of circulating HDL and triglycerides. 23 The REV-ERB-alpha binds to an element in the proximal promoter of the apolipoprotein C-III (APOC3 [MIM 107720]) gene that is also a RORalpha1-binding element. Recent data have provided evidence for a crosstalk between REV-ERB-alpha and RORalpha1 in the modulation of the APOC3 promoter.…”

The “CholesteROR” Protective Pathway in the Vascular System

“…ROR␣ has been shown to bind to a RORE in the promoter of the murine apoA-I and apoC-III genes, and to directly regulate gene transcription, 3,4 suggesting that ROR␣ is a regulator of triglyceride and lipoprotein metabolism. ApoA-I is the major constituent of HDL, and apoC-III is a component of both triglyceride-rich lipoproteins and HDL.…”

“…1,2 ROR␣ has been shown to be implicated in the development and/or differentiation of many tissues, and provides protection against age-related degenerative processes, including atherosclerosis. In addition, ROR␣ activates the apolipoprotein A-I (apoA-I) and apoC-III gene transcription, 3,4 modulates the vasomotor tone of small resistance arteries, and participates in the regulation of postischemic angiogenesis. 5,6 This review focuses on the link between ROR␣, vascular biology, and cholesterol homeostasis.…”

Human IL10 Gene Repression by Rev-erbα Ameliorates Mycobacterium tuberculosis Clearance

“…Rev-erb␣ competes with ROR␣, 2 liver X receptor ␣ (LXR␣), and peroxisome proliferator-activated receptor ␣ (PPAR␣) for these response elements and has opposing effects on transcription (11)(12)(13). In several instances, Rev-erb␣ DR2 overlaps the TATA box (14,15), which suggests the possibility that Rev-erb␣ is a component of a basal repressive transcription complex.…”