Non-genomic effects of the NR4A1/Nur77/TR3/NGFIB orphan nuclear receptor.

The orphan nuclear receptor NR4A1/Nur77/TR3/NGFIB acts primarily as a transcription factor to regulate the expression of multiple genes. However, increasing research attention has recently been given to non-genomic activities of NR4A1. The first description of a non-genomic action of NR4A1 referred to the conversion of anti-apoptotic Bcl-2 into a pro-apoptotic protein by direct interaction with NR4A1. In response to certain apoptotic stimuli, NR4A1 translocates from the nucleus to the mitochondrial outer membrane (MOM) where it associates with Bcl-2 and thereby causes apoptosis. Afterwards, it appeared that NR4A1 could also bind and convert other anti-apoptotic Bcl-2 family members. The latest studies indicate a significant role of NR4A1 in the process of autophagy. For example, a new NR4A1-mediated pathway specific for melanoma cells has been described where NR4A1 interacts with the adenine nucleotide translocase 1 (ANT1) on the mitochondrial inner membrane (MIM) leading to induction of the autophagy pathway. Moreover, NR4A1 interaction with cytoplasmic p53 may also contribute to the induction of autophagy. In addition to mitochondria, NR4A1 could be translocated to the outer membrane of the endoplasmic reticulum (ER) and associate with Bcl-2 or translocon-associated protein subunit γ (TRAPγ) causing ER stress-induced apoptosis. NR4A1 also contributes to the proteasomal degradation of β-catenin in colon cancer cells in vitro and in vivo, as well as to the stabilization of hypoxia-inducible factor-1α (HIF-1α) under non-hypoxic conditions. This review summarizes research findings on non-genomic effects of NR4A1 in normal and cancer cells.