Abstract
Dax1 (Nr0b1) is an atypical orphan nuclear receptor that has recently been shown to play a role in mouse embryonic stem (mES) cell pluripotency. Here we describe a mechanism by which Dax1 maintains pluripotency. In steroidogenic cells, Dax1 protein interacts with the NR5A nuclear receptor steroidogenic factor 1 (Nr5a1) to inhibit transcription of target genes. In mES cells, liver receptor homolog 1 (LRH-1, Nr5a2), the other NR5A family member, is expressed, and LRH-1 has been shown to interact with Dax1. We demonstrate by coimmunoprecipitation that Dax1 is, indeed, able to form a complex with LRH-1 in mES cells. Because Dax1 was historically characterized as an inhibitor of steroidogenic factor 1-mediated transcriptional activation, we hypothesized that Dax1 would inhibit LRH-1 action in mES cells. Therefore, we examined the effect of Dax1 on the LRH-1-mediated activation of the critical ES cell factor Oct4 (Pou5f1). Chromatin immunoprecipitation localized Dax1 to the Oct4 promoter at the LRH-1 binding site, and luciferase assays together with Dax1 overexpression and knockdown experiments revealed that, rather than repress, Dax1 accentuated LRH-1-mediated activation of the Oct4 gene. Similar to our previously published studies that defined the RNA coactivator steroid receptor RNA activator as the critical mediator of Dax1 coactivation function, Dax1 augmentation of LRH-1-mediated Oct4 activation is dependent upon steroid receptor RNA activator. Finally, utilizing published chromatin immunoprecipitation data of whole-genome binding sites of LRH-1 and Dax1, we show that LRH-1 and Dax1 commonly colocalize at 288 genes (43% of LRH-1 target genes), many of which are involved in mES cell pluripotency. Thus, our results indicate that Dax1 plays an important role in the maintenance of pluripotency in mES cells through interaction with LRH-1 and transcriptional activation of Oct4 and other genes.
Challenges Predicting Ligand-Receptor Interactions of Promiscuous Proteins: The Nuclear Receptor PXR