Nerve Growth factor IB

The Nerve Growth factor IB (NGFIB, also known as Nur77) protein is a member of the Nur nuclear receptor family of intracellular transcription factors and is encoded by the gene (nuclear receptor subfamily 4, group A, member 1). NGFIB is involved in cell cycle mediation, inflammation and apoptosis.

The NGFIB protein plays a key role in mediating inflammatory responses in macrophages. In addition, subcellular localization of the NGFIB protein appears to play a key role in the survival and death of cells.

Expression is induced by phytohemagglutinin in human lymphocytes and by serum stimulation of arrested fibroblasts. Translocation of the protein from the nucleus to mitochondria induces apoptosis. Multiple alternatively spliced variants, encoding the same protein, have been identified.

Structure
The NR4A1 gene contains seven exons. An amino terminal transactivation domain is encoded in exon 2, a DNA-binding domain in exons 3 and 4, and dimerisation and ligand-binding domains is exons 5 to 7.

The protein has an atypical ligand-binding domain that is unlike the classical ligand-binding domain in most nuclear receptors. The classical domain contains a ligand-receiving pocket and co-activator site, both of which are lacking in the NR4A family. Where most nuclear receptors have a hydrophobic surface that results in a cleft, NGFI-B has a hydrophilic surface.

Cofactors interact with NGFI-B at a hydrophobic region between helices 11 and 12 to modulate transcription.

Function
Along with the two other Nur family members, NGFIB is expressed in macrophages following inflammatory stimuli. This process is mediated by the NF-κB (nuclear factor-kappa B) complex, a ubiquitous transcription factor involved in cellular response to stress.

NGFIB can be induced by many physiological and physical stimuli. These include physiological stimuli such as "fatty acids, stress, prostaglandins, growth factors, calcium, inflammatory cytokines, peptide hormones, phorbol esters, and neurotransmitters" and physical stimuli including "magnetic fields, mechanical agitation (causing fluid shear stress), and membrane depolarization". Ligands do not bind to NGFIB, so modulation occurs at the level of protein expression and posttranslational modification.

Biochemistry
Nerve Growth factor IB binds as a monomer or homodimer to response element NBRE and as a homodimer to NurRE. It is also capable of heterodimerising with COUP-TF (an orphan nuclear receptor) and retinoid X receptor (RXR) in mediating transcription in response to retinoids.

The binding sites on the response elements for NGFI-B, which are common to the two other members of the Nur family, are :
 * NBRE - 5’-A/TAAAGGTCA,
 * NurRE - a AAAT(G/A)(C/T)CA repeat,
 * RXR - DX, a motif.

Evolution and homology
Nerve Growth factor IB has the systematic HUGO gene symbol NR4A1. It belongs to a group of three closely-related orphan receptors, the Nur family, which has the symbol NR4A. The other two members are nuclear receptor related 1 protein (denoted by symbol NR4A2) and neuron-derived orphan receptor 1 (NR4A3).

NGFIB has a high degree of structural similarity with other family members at the DNA-binding domain with 91-95% sequence conservation. The C-terminal ligand-binding domain is conserved to a lesser extent at 60% and the N-terminal AB region is not conserved, differing in each member.

The three members are similar in biochemistry and function. They are immediate early genes activated in a ligand-independent manner that bind at the same sites on response elements.

NGFIB and the rest of the Nur family are structurally similar to other nuclear receptor superfamily members, but contain an extra intron. The DNA-binding domain at exons 3 and 4 of the NR4A1 gene is conserved among all members of the nuclear receptor.

NR4A1 has homologous genes in a range of species including neuronal growth factor-induced clone B in rats, Nur77 in mice and TR3 in humans.

Pathology
Along with 16 other genes, Nerve Growth factor IB is a signature gene in the metastasis of some primary solid tumours. It is downregulated in this process.