Glucocorticoid signaling is perturbed by the atypical orphan receptor and corepressor SHP

SHP (NROB2) is an atypical orphan nuclear receptor that lacks a DNA-binding domain but contains a putative ligand-binding domain. Previous studies have revealed that SHP interacts with a variety of nuclear receptors and inhibits their transcriptional activity, thereby acting as a corepressor. In this report we identify the glucocorticoid receptor (GR) as a novel down-stream target receptor for SHP inhibition. SHP potently inhibits dexamethasone-induced transcriptional GR activity in mammalian cells, and the inhibition involves a functional second NR-box within SHP. Interestingly, this motif shows a high homology with the NR-box in the glucocorticoid and cAMP-inducible GR coactivator PGC-1, indicating similar binding specificity and shared target receptors. We show that SHP antagonizes PGC-1 coactivation and, in addition, we identify the PGC-1-regulated phospho(enol)pyruvate carboxykinase (PEPCK) promoter as a novel target promoter for SHP inhibition. This implies a physiologically relevant role for SHP in modulating hepatic glucocorticoid action. Furthermore, when coexpressing green fluorescent protein-tagged GR together with SHP, an intranuclear redistribution of GR was observed. As inhibition-deficient SHP mutants were unable to induce this redistribution, intranuclear tethering of target receptors may represent yet another, previously uncovered, aspect of SHP inhibition.