Silencing Mediator of Retinoid and Thyroid Hormone Receptors and Activating Signal Cointegrator-2 as Transcriptional Coregulators of the Orphan Nuclear Receptor Nur77

Abstract

For the orphan nuclear receptor subfamily that includes Nur77 (NGFI-B), Nurr1, and NOR-1, no transcriptional coregulators have been identified thus far. In this report, we found that Ca 2؉ /calmodulin-dependent protein kinase IV enhances Nur77 transactivation in cotransfections either alone or in synergy with AF2-dependent coactivator ASC-2, whereas corepressor silencing mediator for retinoid and thyroid hormone receptors (SMRT) is repressive. Interestingly, Nur77 interacted with SMRT but did not directly bind ASC-2, and accordingly, the putative AF2 core domain of Nur77 did not affect the Nur77 transactivation. SMRT harbors transferable repression domains that associate with various histone deacetylases. Surprisingly, histone deacetylase inhibitor trichostatin A was unable to block the repressive effect of SMRT while dramatically stimulating the Nur77 transactivation. These results suggest that SMRT and ASC-2 are specific coregulators of Nur77 and that SMRT may dynamically compete with a putative adaptor molecule, which links ASC-2 to Nur77, for the identical binding sites within Nur77 in vivo.The nuclear receptor superfamily is a group of ligand-dependent transcriptional regulatory proteins that function by binding to specific DNA sequences named hormone response elements in the promoters of target genes (for a review, see Ref.1). The superfamily includes receptors for a variety of small hydrophobic ligands, such as steroids, T3, and retinoids, as well as a large number of related proteins that do not have known ligands referred to as orphan nuclear receptors. Functional analysis of nuclear receptors has shown that the ligand-binding domain (LBD) 1 exhibits ligand-dependent transcriptional activation function referred to as activation function-2 (AF2). Notably, this region has been shown to play a critical role in mediating transactivation by serving as a ligand-dependent interaction interface with many different coactivators (for reviews, see Refs. 2 and 3). Transcriptional coactivators either bridge transcription factors and the components of the basal transcriptional apparatus and/or remodel the chromatin structures. In particular, cAMP-response element-binding protein (CREB)-binding protein (CBP) and its functional homologue p300, steroid receptor coactivator-1 (SRC-1) and its family members, and activating signal cointegrator-2 (ASC-2) were shown to be essential for the activation of transcription by a large number of regulated transcription factors, including many members of the nuclear receptor superfamily (2, 3). Interestingly, SRC-1 and its family member activator of thyroid and retinoic acid receptors (ACTR) along with CBP and p300 were recently shown to contain histone acetyltransferase activities and associate with another histone acetyltransferase protein p300/CBP-associated factor (P/CAF) (2, 3). In contrast, nuclear receptor corepressor (N-CoR) and its homologue silencing mediator of retinoid and thyroid hormone receptors (SMRT) harbor transferable repression domains that can associate with va...

Differential recruitment of nuclear receptor coactivators may determine alternative RNA splice site choice in target genes

“…Indeed, like many other transcriptional coregulators, ASC-2 is known to be targeted by various signaling pathways (24,30,31). In other words, posttranslational modifications of ASC-2 by different signaling pathways could allow ASC-2 to preferentially interact with CoAA or CAPER, and therefore in response to steroid hormones, ASC-2 could recruit one or the other coactivator, depending on the signaling context within the cell.…”

Anti-steroidogenic Factor ARR19 Inhibits Testicular Steroidogenesis through the Suppression of Nur77 Transactivation

“…ARR19 Competes with SRC-1 for Binding to Nur77-It has been reported that SRC-1 and SRC-2 can directly bind to the AF-2 and AF-1 domains of Nur77, respectively, and enhance the transcriptional activity of Nur77 (11,23). Because the AF-2 domain of Nur77 is the binding site for both SRC-1 and ARR19 (Fig.…”

“…Although Nur77 has been well characterized as an immediate early response gene (2, 14 -17) and for its post-translational modifications (18 -22), coregulators involved in Nur77 transactivation are not fully characterized. Recently, steroid receptor coactivator (SRC)-1 and silencing mediator for retinoid and thyroid hormone receptors have been shown to regulate Nur77 transactivation through direct protein-protein interactions (11,23).…”

Structural Basis for the Cell-specific Activities of the NGFI-B and the Nurr1 Ligand-binding Domain

“…For the NR4A subfamily, the AF1 appears to play the predominant role in transcriptional activation and co-factor recruitment, and its activity can be regulated by phosphorylation (21)(22)(23)(24)(25)(26). Recent results show that the AF1 directly recruits co-activators such as members of the SRC family, p300, pCAF, and DRIP205 (25,26).…”

“…Importantly, the AF2 does not directly interact with SRCs, p300, pCAF, or DRIP205, and little is known about the regulation of its activity (12,25,27,28). However, the co-repressor SMRT, the atypical orphan receptor DAX-1, and the SUMO ubiquitin ligase PIAS␥ have been reported to interact with the LBD of either NGFI-B or Nurr1, and play a role in AF2 regulation (22,29,30).…”