Silencing mediator for retinoid and thyroid hormone receptors interacts with octamer transcription factor-1 and acts as a transcriptional repressor

Octamer transcription factor-1 (Oct-l) is a member of the POU (Pit-l, Oct-1 , unc-86) family of transcription factors and is involved in the transcript ional regulation of a variety of gene expressions related to cell cycle reg ulation, development, and hormonal signals. It has been shown that Oct-1 ac ts not only as a transcriptional activator but also as a transcriptional re pressor for certain genes. The mechanism of the repressive function of Oct- 1 has not been well understood. Here we demonstrate by using the glutathion e S-transferase pull-down assays and coimmunoprecipitation assays that the POU domain of Oct-1 directly interacts with a silencing mediator for retino id and thyroid hormone receptors (SMRT). The interaction surfaces are locat ed in the C-terminal region of SMRT, which are different from previously de scribed silencing domains I and II or receptor interacting domains I and II . In transient transfection assays in COS1 cells, overexpression of SMRT at tenuated 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 co uld be determined by a regulated balance between SMRT and OCA-B, Furthermor e, cotransfected unliganded thyroid hormone receptor enhanced the transacti vation by Oct-1, and addition of 3,3',5-tri-iodo-L-thyronine obliterated th e stimulatory effects. Consequently, in the presence of cotransfected thyro id hormone receptor, the octamer response element acts as an element negati vely 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 bet ween Oct-1 and nuclear receptors.