Molecular analysis of transcriptional transactivation by the androgen receptor.

Abstract

The physiological effects of steroid hormones are mediated through intracellular receptors that regulate transcription of hormone response element (HRE)-containing genes. Although the androgen receptor (AR) and the glucocorticoid receptor (GR) are coexpressed in many tissues and bind identical HREs, biological actions of each hormone are distinct. Elucidating the mechanisms by which AR and GR regulate transcription of their target genes is vital to understanding cell- and receptor-specific effects of steroid hormones. These receptor-specific effects were investigated using a system in which AR and GR differ in their abilities to activate transcription from the same reporter genes. To determine if the differential activity was due to inherent differences between AR and GR functional domains, the activities of AR/GR chimeric receptors were examined. Functional differences in the N-terminal modulatory domains and, to a lesser degree, the DNA binding domains, contributed to the differential transactivation. A panel of AR derivatives was constructed to examine the function of the N-terminal domain of this receptor. The AR modulatory domain contains a tract of glutamine residues encoded by the trinucleotide CAG. Expansion of this trinucleotide repeat is correlated with the incidence and severity of the degenerative neuromuscular syndrome Kennedy's disease. To investigate the relationship of this repeat to AR function, receptors that varied in the presence, position or size of the polyglutamine tract were constructed. Elimination of the tract resulted in elevated transactivation. Progressive expansion of the repeat caused a linear decrease in transcriptional activation. These results indicate the polyglutamine tract is inhibitory to AR transactivation function. Further analysis of the AR modulatory domain revealed two regions are necessary for maximal transactivation. Secondary structure prediction and site-directed mutagenesis of one region suggest a ten residue acidic amphipathic α-helix is critical for activity. The second region may be a member of the proline-rich class of activation domains. Together, these two regions may form an interaction surface that contacts a limiting factor(s) required for activated transcription. Receptor-selective interactions with promoter- or cell-specific auxiliary factors could control the specificity of steroid-regulated gene networks.