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 C-terminal 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.
Highlights
Octamer transcription factor-1 (Oct-1) possesses transactivation function and repression function; von Willebrand factor promoter [15], prolactin gene promoter [16], or rGH promoter [14] was shown to be down-regulated by Oct-1
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) referred to as Oct-1-associated coactivator (OCA-B) (6 –11)
We have examined the interaction between SMRT and POU domain of Oct-1
Summary
Oct-1, octamer transcription factor-1; TR, thyroid hormone receptor; SMRT, silencing mediator for retinoid and thyroid hormone receptors; N-CoR, nuclear hormone receptor corepressor; GST, glutathione S-transferase; T3, 3,3Ј,5tri-iodo-L-thyronine; POU, Pit-1, Oct-1, unc-86; TSA, trichostatin A; OBF-1, Oct-binding motif containing gene promoters that are ubiquitously as well as tissue- expressed genes such as histone H2B, the small nuclear RNA, and Ig [1,2,3]. Silencing mediator for retinoid and thyroid hormone receptors (SMRT) and the nuclear hormone receptor corepressor (N-CoR) were identified as interacting proteins for unliganded nuclear receptors that repress the basal transcriptional activity of target genes [17, 18]. These corepressors associate with hinge domain of nuclear receptors. Previous studies demonstrated that several homeodomain proteins such as Pit-1 and Pbx functionally interact with N-CoR/SMRT resulted in a formation of the factor 1; OCA-B, Oct-1-associated coactivator; PBS, phosphate-buffered saline; PAGE, polyacrylamide gel electrophoresis; PVDF, polyvinylidene difluoride; RID, receptor interacting domain; SD, silencing domain. These results indicate that SMRT and N-CoR are involved in a wide array of biological processes and signaling pathways
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