An isoform of Nurr1 functions as a negative inhibitor of the NGFI-B familysignaling

NGFI-B, Nur1 and NOR-1 constitute a distinct subfamily within the nuclear r eceptor superfamily. To clarify the transcriptional regulation by the NGFI- B family, we searched for other components that can bind to the NBRE respon se element, a known target sequence for these transcription factors. By low stringency hybridization using the DNA binding domain of NOR-1 as a probe, a C-terminal truncated Nurr1 isoform, named Nurr2, was isolated from a mou se MC3T3-E1 cell cDNA library. Nurr2 had a novel cryptic exon located upstr eam in the Nurr1 promoter region, and was generated by alternative splicing at exons 1, 2 and 6. The C-terminal region was encoded by frame-shifted ex on 6, and so Nurr2 lacked the C-terminal sequences corresponding to the put ative ligand binding domain or dimerization domain. Quantitative reverse tr anscriptase-PCR experiments confirmed the presence of the Nurr2 isoform in mouse, rat and human. It was, like Nurr1, highly expressed in the pituitary and the cerebral cortex. Nurr2 and Nurr1 were also concomitantly induced b y forskolin in NIH3T3 cells. Functional analysis using a reporter gene, con taining NBRE response elements, indicated that while the isoform was inacti ve by itself, it could inhibit transactivation by the members of the NGFI-B family. These results indicate that the C-terminal truncated isoform, Nurr 2, may act as a negative regulator of the NGFI-B family signaling. (C) 1999 Elsevier Science B.V. All rights reserved.