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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...

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The retinoid X receptor (RXR) is a member of the nuclear hormone receptor superfamily and heterodimerizes with a variety of other family members such as the thyroid hormone receptor (TR), 1 retinoic acid receptor, vitamin D receptor, and peroxisome proliferator-activated receptor. Therefore, RXR is supposed to play a key role in a ligand-dependent regulation of gene transcription by nuclear receptors. In this study, we have identified the octamer-binding transcription factor-1 (Oct-1) as a novel interaction factor of RXR. In vitro pull-down assays using RXR deletion mutants showed that the interaction surfaces were located in the region encompassing the DNA binding domain (C domain) and the hinge domain (D domain) of RXR. We also showed that RXR interacted with the POU homeodomain but not with the POU-specific domain of Oct-1. Gel shift analysis revealed that Oct-1 reduced the binding of TR/RXR heterodimers to the thyroid hormone response element (TRE). In transient transfection assays using COS1 cells, Oct-1 repressed the T3-dependent transcriptional activity of TR/RXR heterodimers, consistent with in vitro DNA binding data; however, transcriptional activation by Gal4-TR(LBD) (LBD, ligand binding domain), which lacks its own DNA binding domain but retains responsiveness to T3, was not influenced by Oct-1. These results suggest that Oct-1 functionally interacts with RXR and negatively regulates the nuclear receptor signaling pathway by altering the DNA binding ability of the receptors.The steroid/thyroid hormone receptor superfamily is a large group of related transcriptional factors that control cellular differentiation, development, and homeostasis by direct interaction with distinct cis-elements in target genes (1, 2). This superfamily includes receptors for steroids, thyroid, vitamin D3, retinoids, and a large number of orphan receptors whose cognate ligands are still unknown (3). Members of the superfamily are characterized by a highly conserved cysteine-rich DNA binding domain containing two zinc finger structures necessary for sequence-specific DNA interaction (4). The complex carboxyl-terminal region of the receptors contains ligand binding, receptor dimerization, and putative transcriptional activation function (5). Members of the superfamily regulate transcription by binding to response elements containing two or more copies (often degenerate) of the consensus motif AGGTCA (6, 7). Recently it was shown that retinoic acid receptor, TR, VDR, and peroxisome proliferator-activated receptor form heterodimers with the RXR on bipartite hormone response elements composed of non-symmetrical head-to-tail tandem AGGTCA "half-sites" (8 -10). More than half of the orphan receptors have been shown to heterodimerize with RXR (3). Thus, RXRs are supposed to play a key role in ligand-dependent transcriptional activity of nuclear receptors.The mechanisms by which nuclear hormone receptors regulate target gene transcription are currently under intensive investigation. The ligand-activated nuclear receptors may promote fo...

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