Silencing Mediator for Retinoid and Thyroid Hormone Receptors Interacts with Octamer Transcription Factor-1 and Acts as a Transcriptional Repressor

Abstract

Octamer transcription factor-1 (Oct-1) is a member of the POU (Pit-1, Oct-1, unc-86) family of transcription factors and is involved in the transcriptional regulation of a variety of gene expressions related to cell cycle regulation, development, and hormonal signals. It has been shown that Oct-1 acts not only as a transcriptional activator but also as a transcriptional repressor for certain genes. The mechanism of the repressive function of Oct-1 has not been well understood. Here we demonstrate by using the glutathione S-transferase pull-down assays and coimmunoprecipitation assays that the POU domain of Oct-1 directly interacts with a silencing mediator for retinoid and thyroid hormone receptors (SMRT). The interaction surfaces are located in the Cterminal region of SMRT, which are different from previously described silencing domains I and II or receptor interacting domains I and II. In transient transfection assays in COS1 cells, overexpression of SMRT attenuated the augmentation of Oct-1 transcriptional activity by OBF-1/OCA-B, activator for Oct-1. In pull-down assays, increasing amounts of SMRT could compete the binding of OCA-B to Oct-1 POU domain. The activity of Oct-1 could be determined by a regulated balance between SMRT and OCA-B. Furthermore, cotransfected unliganded thyroid hormone receptor enhanced the transactivation by Oct-1, and addition of 3,3,5-tri-iodo-L-thyronine obliterated the stimulatory effects. Consequently, in the presence of cotransfected thyroid hormone receptor, the octamer response element acts as an element negatively regulated by 3,3,5-tri-iodo-L-thyronine. The results suggest that the transcriptional activity of Oct-1 can be modulated by interaction through its POU domain by a silencing mediator SMRT resulting in the cross-talk between Oct-1 and nuclear receptors.Octamer transcription factor-1 (Oct-1) 1 activates the octamer motif containing gene promoters that are ubiquitously as well as tissue-specifically expressed genes such as histone H2B, the small nuclear RNA, and Ig (1-3). Oct-1 is a member of a family of transcription factors characterized by the presence of a bipartite DNA-binding domain (POU domain). The POU domain consists of two conserved regions, a POU-specific domain and a POU homeodomain (4, 5). The both subdomains have a helixturn-helix motif, acting not only as a DNA-binding domain but also as a protein-protein interaction domain. A number of transcription factors have been identified to interact with the POU domains of Oct-1 such as TBP, TFIIB, HMG2, and Oct-binding factor 1 (OBF-1) also referred to as Oct-1-associated coactivator (OCA-B) (6 -11). It has been shown that Oct-1 interacts with nuclear hormone receptors such as retinoid X receptor, thyroid hormone receptor (TR), and glucocorticoid receptor and influences their transcriptional activity (12-14). Oct-1 possesses not only transactivation function but also repression function; von Willebrand factor promoter (15), prolactin gene promoter (16), or rGH promoter (14) was shown to be down-regulated by Oct...

The histone deacetylase inhibitor trichostatin A induces GADD45γ expression via Oct and NF-Y binding sites

“…Role of co-repressors and co-activators in the regulation of the murine GADD45g promoter NF-Y and Oct-1 can interact with HAT-containing coactivator complexes and HDAC-containing co-repressor complexes (Nagy et al, 1997;Mantovani, 1999;Kakizawa et al, 2001;Park et al, 2002;Peng and Jahroudi, 2003;Huang et al, 2005). We therefore assessed the contribution of these factors to GADD45g promoter activity regulation.…”

“…We therefore assessed the contribution of these factors to GADD45g promoter activity regulation. Silencing mediator for retinoid and thyroid hormone receptors (SMRT) interact with Oct-1 and acts as a transcriptional co-repressor (Kakizawa et al, 2001). To analyse the possible role of Oct-1-associated SMRT in the regulation of the GADD45g promoter, a SMRT expression plasmid was co-transfected with the À818 to þ 15 promoter reporter plasmid.…”

“…Since Oct-1 and NF-Y can interact with HATcontaining co-activator complexes and HDAC-containing co-repressor complexes (Nagy et al, 1997;Mantovani, 1999;Kakizawa et al, 2001;Park et al, 2002;Peng and Jahroudi, 2003;Huang et al, 2005), we studied the possible role of SMRT, HDAC1, and PCAF in the regulation of the murine GADD45g promoter. SMRT or HDAC1, but not HDAC3, ectopic expression reduced GADD45g promoter activity, while forced expression of PCAF increased it.…”

Regulation of NF-κB-dependent Gene Expression by the POU Domain Transcription Factor Oct-1

“…Oct-1 can also repress expression of certain genes, including the von Willebrand factor promoter (18), prolactin gene promoter (19), and growth hormone promoter (16). Although the mechanism of repression by Oct is not well established, recent studies suggest that it can interact with SMRT (silencing mediator for retinoid and thyroid hormone receptor) (17).…”

“…Oct-1 is a prototypical member of the POU domain family, and one of its functions is to activate S phase-specific expression of the histone H2b gene (15). Additionally, Oct-1 can interact with nuclear hormone receptors, such as retinoid X receptor, thyroid hormone receptor, and glucocorticoid receptor, and influence their transcriptional activity (16,17). Oct-1 can also repress expression of certain genes, including the von Willebrand factor promoter (18), prolactin gene promoter (19), and growth hormone promoter (16).…”