Rev-ErbA alpha

Rev-ErbA alpha also known as NR1D1 (nuclear receptor subfamily 1, group D, member 1), is a member of the Rev-ErbA family of nuclear receptors and is a transcriptional repressor. In mammals, Rev-erbα is highly expressed in the liver, skeletal muscle, adipose tissue, and the brain, participating in the development and circadian regulation of these tissues.

Gene and protein structure
Rev-erbα is transcribed from the opposite strand of the thyroid receptor α (c-erbAα) gene on chromosome 17, with a 269-nucleotide overlap between the two transcripts. The other mammalian isoform of the receptor, Rev-erbβ is encoded by another gene on chromosome 3. In addition, there is one non-mammalian homolog, the ecdysone-regulated gene E75, which is present in Drosophila and C. elegans. The Rev-erbα gene itself has multiple transcripts. Two promoters govern the expression of the Rev-erbα gene in human and rat, generating two mRNA isoforms. The full-length isoform encodes a 614-amino acid protein, while a second isoform is generated from an internal promoter and produces a protein that is shorter by 106 amino acids. Both Rev-erbα mRNA isoforms contain E-boxes as well as Rev-erbα response elements, which means that they can be regulated in a circadian manner by the BMAL and Rev-erba proteins. In fact, both transcripts exhibit rhythmic expression in serum-synchronized fibroblasts.

The Rev-erbα protein is structurally unique from other nuclear receptors, in that it lacks Helix 12 in its ligand-binding domain, which is usually responsible for forming the ligand binding pocket in other nuclear receptors. In place of the missing H12, Rev-erbα displays a hydrophobic interface that binds the corepressor N-CoR, making it a potent transcriptional repressor. Interestingly, all members of the Rev-erb family bind heme, which may act as a ligand to regulate their transcriptional activity.

Physiologic function
Rev-erbα regulates gene transcription by directly binding to target response elements (RevREs), comprised of an A/T-rich flank followed by AGGTCA. Rev-erbα mediates repression by recruiting the corepressor N-CoR, which then activates the histone deacetylase (HDAC) 3. A number of target genes has been identified for Rev-erbα, including the lipoproteins ApoA1 and ApoCIII, hydratase dehydrogenase, the circadian factor BMAL, and the anti-fibrinolytic factor PAI-1. Many of these genes are coordinately regulated by Rev-erbα and the RAR-related orphan receptor RORα, which share the same response elements but exert opposite effects on gene transcription. Crosstalk between Rev-erbα and RORα likely acts to fine-tune of their target physiologic networks, such as circadian rhythms, metabolic homeostasis, and inflammation.

Rev-erbα mRNA is induced during adipogenesis and is highly expressed in adipose tissue. One study reported that overexpression of Rev-erbα may enhance adipogenesis in cultured mouse adipocytes, but the mechanism of this effect remains to be elucidated. Rev-erbα expression is also regulated at the post-translational level: it is phosphorylated on the amino terminus by Glycogen Synthase Kinase (GSK) 3β, which contributes to its protein stability. It has been shown that lithium, which inhibits GSK3β, can de-stabilize Rev-erbα protein and affect its function in the circadian clock. This may partly explain lithium’s therapeutic effect on circadian diseases such as Bipolar Disorder.