AR suppresses transcription of the LHbeta subunit by interacting with steroidogenic factor-1.

Abstract

Synthesis of LH is suppressed by feedback from gonadal steroids. Previously, we demonstrated that 779 bp of the bovine LHbeta promoter was sufficient to target expression of a chloramphenicol acetyltransferase reporter gene specifically to the pituitary in transgenic mice, and found that it was appropriately suppressed after administration of T or E2. In this study, we report that ligand-bound AR, but not ligand-bound ER, directly suppressed activity of the bovine LHbeta promoter when examined in a gonadotrope-derived cell line. Additional studies with mutated bovine LHbeta promoter constructs focused on the proximal 5'-flanking region because of the presence of several cis-acting elements that are highly conserved across all mammals. These include regulatory elements that bind steroidogenic factor 1 (SF-1), Egr-1, and Pitx1. When tested by cotransfection with AR, overexpression of Egr-1, Pitx1, and constitutively active steroidogenic factor 1 (SF-1DeltaLBD) each individually rescued androgen-mediated suppression of the bovine LHbeta promoter. This suggested a functional interaction between each of these transcription proteins and AR. In contrast, overexpression of full-length SF-1 was incapable of relieving the bovine LHbeta promoter from the suppressive effect imposed by AR. This suggested that the ligand-binding domain of SF-1 plays an important role in functional interactions that occur between this protein and AR. This notion was further supported by binding assays performed with glutathione-S-transferase-AR: these identified SF-1 as a key interactive partner and localized this interaction to the ligand-binding domain of the protein. Additional binding studies indicated that protein interactions between SF-1, Pitx1, and Egr-1 interfere with formation of a binary complex that contains AR and SF-1. Thus, we conclude that AR suppresses activity of the bovine LHbeta promoter through protein-protein interactions with SF-1 and that the degree of this interaction can be modified by the presence of Egr-1 and Pitx1.