Transcriptional Regulation of Apolipoprotein C-III Gene Expression by the Orphan Nuclear Receptor RORα

Abstract

Triglyceride-rich remnant lipoproteins are considered as major risk factors contributing to the pathogenesis of atherosclerosis. Because apolipoprotein (apo) C-III is a major determinant of plasma triglyceride and remnant lipoprotein metabolism, it is important to understand how the expression of this gene is regulated. In the present study, we identified the orphan nuclear receptor ROR␣1 as a regulator of human and mouse apo C-III gene expression. Plasma triglyceride and apo C-III protein concentrations in staggerer (sg/sg) mice, homozygous for a deletion in the ROR␣ gene, were significantly lower than in wild type littermates. The lowered plasma apo C-III levels were associated with reduced apo C-III mRNA levels in liver and intestine of sg/sg mice. Transient transfection experiments in human hepatoma HepG2, human colonic CaCO2, and rabbit kidney RK13 cells demonstrated that overexpression of the human ROR␣1 isoform specifically increases human apo C-III promoter activity, indicating that ROR␣1 enhances human apo C-III gene transcription. ROR␣1 response elements were mapped by promoter deletion analysis and gel shift experiments to two AGGTCA halfsites located at positions ؊83/؊78 (within the C3P site) and ؊23/؊18 (downstream of the TATA box) in the human apo C-III promoter, with the ؊23/؊18 site exhibiting the highest binding affinity. Transfection of sitedirected mutated constructs in HepG2 cells indicated that the ROR␣1 effect is predominantly mediated by the ؊23/؊18 site. This site is conserved in the mouse apo C-III gene promoter. Moreover, ROR␣ binds to the equivalent mouse site and activates constructs containing three copies of the mouse site cloned in front of an heterologous promoter. Taken together, our data identify ROR␣ as a transcriptional regulator of apo C-III gene expression, providing a novel, physiological role for ROR␣1 in the regulation of genes controlling triglyceride metabolism.

The “CholesteROR” Protective Pathway in the Vascular System

“…ApoA-I is the major constituent of HDL, and apoC-III is a component of both triglyceride-rich lipoproteins and HDL. Rora sg/sg mutants show significantly reduced plasma triglyceride levels associated with a strong decrease in apoC-III plasma concentrations, 4 suggesting a physiological role of ROR␣ in the regulation of plasma triglyceride metabolism in the mouse. This finding is likely to also apply to humans, since human ROR␣1 overexpression in HepG2 cells activates human apoC-III promoter activity in cotransfection assays.…”

“…This finding is likely to also apply to humans, since human ROR␣1 overexpression in HepG2 cells activates human apoC-III promoter activity in cotransfection assays. 4 Although elevated triglycerides likely affect atherosclerosis in humans, the effect of hypertriglyceridemia in mice is modest. 69 This might explain why, despite their decreased hepatic apoC-III gene expression, 70 Rora sg/sg mice fed an atherogenic diet develop more severe atherosclerosis than wild type mice.…”

“…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.…”

Rorα deficiency and decreased adiposity are associated with induction of thermogenic gene expression in subcutaneous white adipose and brown adipose tissue

“…Homozygous sg/sg mice have a lean and dyslipidemic phenotype. Several studies on Ror␣ (16,19,23,32,33,39) have revealed a role for this nuclear receptor in fatty acid homeostasis. These mice display hypoalphalipoproteinemia, decreased serum triacylglycerol and HDL cholesterol, and vulnerability to atherosclerosis (23).…”