A fourth subtype of retinoic acid receptor-related orphan receptors is activated by oxidized all- retinoic acid in medaka ().

Background: The three known subtypes of the retinoic acid receptor-related orphan receptor (ROR) have been implicated in the control of immunity, brain function, and circadian rhythm in mammals. Here, we demonstrate by phylogenetic analysis that there were originally four subtypes of RORs in vertebrates. One of the novel paralogs, ( in the Ensembl database), is conserved among teleosts, but absent in mammals. Using medaka () as a model teleost, we evaluated the expression pattern of this gene, its transactivational properties for endogenic chemicals, and its ability to activate the promoters of putative target genes.

Results: In eyes, the transcript of was expressed at higher levels during the day than at night. Interestingly, cholesterol derivatives, which are well-known ligands for mammalian RORs, did not efficiently promote transcriptional activity via RORd1. Thus we sought to identify the ligands that regulate the transcriptional activity of RORd1 using a luciferase reporter cell-based screening system. Using this system, we identified two metabolites of all- retinoic acid (ATRA), 4OH-ATRA and 4-keto ATRA, as potential ligands of RORd1. Moreover, RORd1 activated the promoter of in a 4OH-ATRA -dependent manner.

Conclusions: A novel subtype, rord has two paralogs, and , in teleost. Rord1 mRNA is highly abundant in the eyes of medaka during light periods, suggesting that expression is involved in the regulation of circadian rhythm. We identified two ATRA metabolites, 4OH-ATRA and 4 K-ATRA, as endogenous candidate ligands of RORd1. We also show that 4-oxygenated ATRA metabolites have the potential to activate cyp26a1, the metabolic enzyme of ATRA. Our results support the notion that RORd1 is involved in the metabolism of ATRA in medaka.