Transcriptional regulation of apolipoprotein A-I gene expression by the nuclear receptor ROR alpha

Since elevated concentrations of plasma high density lipoprotein (HDL) and its major apolipoprotein (ape), 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 alpha 1 as an activator of apoA-I gene transcription. In apoA-I-expressing intestinal Caco-2 cells, overexpression of the ROR alpha 1, but not the ROR alpha 2 or ROR alpha 3 isoforms, increased rat apoA-I gene transcription. Deletion and site-directed 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 alpha 1 isoform, but not the ROR alpha 2 or ROR alpha 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 alpha gene. In addition, reverse transcriptase-polymerase chain reaction analysis revealed the expression of ROR alpha in small intestinal epithelium and in Caco-2 cells. These data indicate a novel, physiological role for ROR alpha 1 in the regulation of genes involved in lipid and lipoprotein metabolism and possibly in the development of metabolic diseases, such as atherosclerosis.