A mathematically‐based method for ordering cofactor function in glucocorticoid receptor transactivation

Many transcriptional cofactors are known to influence not only the maximal transcriptional activity (A max ) but also the potency (EC 50 ) of glucocorticoid receptor (GR) agonist complexes. However, two critical questions remain: 1) the reaction sequence location of the specific step where the cofactor exerts its function and 2) the precise nature of that function. An extension of our recent mathematical model of steroid hormone action now predicts that graphical analyses of the competition of two factors during GR transactivation can yield the desired information. The utility of this analysis was supported by its determination of the coactivator TIF2 acting as an activator at the same step as the corepressor sSMRT (short form of SMRT), which functions as a competitive inhibitor. In this same assay system, full length NCoR unexpectedly behaves as an activator at a further downstream step. This result was obtained upon competing NCoR with either TIF2 or sSMRT. The internal consistency of these results further attests to the accuracy of the graphical method. Studies in different cell lines revealed that NCoR activity as a competitor or an activator is cell‐line specific. In summary, our new methodology appears to provide a method for defining the relative positioning and mode of action of factors involved in GR transactivation. This research was supported by the Intramural Research Program of the NIH, NIDDK.