Abstract
Increased specificity in steroid-regulated gene expression is a long-sought goal of endocrinologists. Considerable progress has resulted from the discovery of coactivators, corepressors, and comodulators that adjust the total activity (A(max)) of gene induction. Two less frequently quantitated, but equally potent, means of improving specificity are the concentration of agonist steroid required for half-maximal activity (EC(50)) and the residual or partial agonist activity displayed by most antisteroids (PAA). It is usually assumed that the modulatory activity of transcriptional cofactors coordinately regulates A(max), EC(50), and PAA. Here we examine the hypothesis that these three parameters can be independently modified by separate protein domains. The test system involves three differently sized fragments of each of three factors (glucocorticoid receptor [GR], coactivator TIF2, and comodulator STAMP), which are shown to form a ternary complex and similarly affect the induction properties of transfected and endogenous genes. Twenty-five different fragment combinations of the ternary complex are examined for their ability to modulate the A(max), EC(50), and PAA of a transiently transfected synthetic reporter gene. Different combinations selectively alter one, two, or all three parameters. These results clearly demonstrate that A(max), EC(50), and PAA can be independently regulated under some conditions by different pathways or molecular interactions. This new mechanistic insight suggests that selected activities of individual transcription factors are attractive targets for small molecules, which would have obvious clinical applications for increasing the specificity of steroids during endocrine therapies.