RAR related orphan receptor alpha

Discovery

The first three human isoforms of RORα were initially cloned and characterized as nuclear receptors in 1994 by Giguère and colleagues, when their structure and function were first studied.

In the early 2000s, various studies demonstrated that RORα displays rhythmic patterns of expression in a circadian cycle in the liver, kidney, retina, and lung. Of interest, it was around this time that RORα abundance was found to be circadian in the mammalian suprachiasmatic nucleus. RORα is necessary for normal circadian rhythms in mice, demonstrating its importance in chronobiology.

Structure

The protein encoded by this gene is a member of the NR1 subfamily of nuclear hormone receptors. In humans, 4 isoforms of RORα have been identified, which are generated via alternative splicing and promoter usage, and exhibit differential tissue-specific expression. The protein structure of RORα consists of four canonical functional groups: an N-terminal (A/B) domain, a DNA-binding domain containing two zinc fingers, a hinge domain, and a C-terminal ligand-binding domain. Within the ROR family, the DNA-binding domain is highly conserved, and the ligand-binding domain is only moderately conserved. Different isoforms of RORα have different binding specificities and strengths of transcriptional activity.

Regulation of circadian rhythm

The core mammalian circadian clock is a negative feedback loop which consists of Per1/Per2, Cry1/Cry2, Bmal1, and Clock. This feedback loop is stabilized through another loop involving the transcriptional regulation of Bmal1. Transactivation of Bmal1 is regulated through the upstream ROR/REV-ERB Response Element (RRE) in the Bmal1 promoter, to which RORα and REV-ERBα bind. This stabilizing regulatory loop itself is induced by the Bmal1/Clock heterodimer, which induces transcription of RORα and REV-ERBα. RORα, which activates transcription of Bmal1, and REV-ERBα, which represses transcription of Bmal1, compete to bind to the RRE. This feedback loop regulating the expression of Bmal1 is thought to stabilize the core clock mechanism, helping to buffer it against changes in the environment.

Mechanism

Specific association with ROR elements (RORE) in regulatory regions is necessary for RORα’s function as a transcriptional activator. RORα achieves this by specific binding to a consensus core motif in RORE, RGGTCA. This interaction is possible through the association of RORα’s first zinc finger with the core motif in the major groove, the P-box, and the association of its C-terminal extension with the AT-rich region in the 5’ region of RORE.

Homology

RORα, RORβ, and RORγ are all transcriptional activators recognizing ROR-response elements. ROR-alpha is expressed in a variety of cell types and is involved in regulating several aspects of development, inflammatory responses, and lymphocyte development. The RORα isoforms (RORα1 through RORα3) arise via alternative RNA processing, with RORα2 and RORα3 sharing an amino-terminal region different from RORα1. In contrast to RORα, RORβ is expressed in Central Nervous System (CNS) tissues involved in processing sensory information and in generating circadian rhythms while RORγ is critical in lymph node organogenesis and thymopoeisis.

The DNA-binding domains of the DHR3 orphan receptor in Drosophila shows especially close homology within amino and carboxy regions adjacent to the second zinc finger region in RORα, suggesting that this group of residues is important for the proteins' functionalities.

PDP1 and VRI in Drosophila regulate circadian rhythm's by competing for the same binding site, the VP box, similarly to how ROR and REV-ERB competitively bind to RRE. PDP1 and VRI constitute a feedback loop and are functional homologs of ROR and REV-ERB in mammals.

Direct orthologs of this gene have been identified in mice and humans.

Human cytochrome c pseudogene HC2 and RORα share overlapping genomic organization with the HC2 pseudogene located within the RORα2 transcription unit. The nucleotide and deduced amino acid sequences of cytochrome c-processed pseudogene are on the sense strand while those of the RORα2 amino-terminal exon are on the antisense strand.

Interactions

As a drug target

Because RORα and REV-ERBα are nuclear receptors that share the same target genes and are involved in processes that regulate metabolism, development, immunity, and circadian rhythm, they show potential as drug targets. Synthetic ligands have a variety of potential therapeutic uses, and can be used to treat diseases such as diabetes, atherosclerosis, autoimmunity, and cancer. T0901317 and SR1001, two synthetic ligands, have been found to be RORα and RORγ inverse agonists that suppress reporter activity and have been shown to delay onset and clinical severity of multiple sclerosis and other Th17 cell-mediated autoimmune diseases. SR1078 has been discovered as a RORα and RORγ agonist that increases the expression of G6PC and FGF21, yielding the therapeutic potential to treat obesity and diabetes as well as cancer of the breast, ovaries, and prostate. SR3335 has also been discovered as a RORα inverse agonist.