Temporal ordering of SMRT and TIF2 function in steroid‐regulated gene transcription

The Nuclear Receptor Corepressor (NCoR2/SMRT) and the Transcriptional Intermediary Factor 2 (TIF2) are thought to serve as a transcriptional corepressor and coactivator, respectively, for several nuclear receptors including the glucocorticoid receptor (GR) by competitively binding to a common site in the GR ligand binding domain. Numerous studies have shown the potential for these factors to modulate transcription in response to activation of a nuclear receptor. However, few studies have examined the temporal kinetics of cofactor association with steroid receptors in the course of receptor‐mediated transactivation. No studies have been able to identify where, temporally, these cofactors act to elicit transcriptional modulation. Here we show that a theoretical framework for steroid hormone action can make predictions regarding the relative positioning and functioning of cofactors during GR signaling. Using this model system, the data for GR signaling in U2OS cells for both reporter constructs and an endogenous gene indicates that SMRTα acts as a transcriptional activator while a shorter form of SMRT (sSMRT) functions as a competitive inhibitor and TIF2 is a transcriptional activator. Importantly, these data also indicate that sSMRT acts at the same step as TIF2, while preliminary data suggests that SMRTα acts at a different step from TIF2 and sSMRT. Supported by the NIH/NIDDK Intramural Research Program.