Antagonistic Regulation of StAR Gene Expression and Synergistic Regulation of Mc2R Gene Expression by FoxL2 and SF-1.

Abstract

FoxL2, a gene encoding a forkhead transcription factor, is one of the earliest ovarian markers and plays an important role in regulation of cholesterol and steroid metabolism, inflammation, apoptosis, and ovarian development and function. Mutations and deficiencies of FoxL2 have been shown to cause blepharophimosis-ptosis-epicanthus inversus syndrome (BPES) as well as ovarian defects. Previous reports have shown that FoxL2 represses the promoter activity of the steroidogenic acute regulatory (StAR) gene by binding to StAR promoter. SF-1, an orphan nuclear receptor, is expressed throughout adrenal cortex and gonads where it regulates a number of genes involved in steroid hormone biosynthesis, including StAR and Mc2R genes. It is defined that FoxL2 interacts with SF1 in fish and up-regulates aromatase gene transcription. However, this interaction has not been studied in mammals. Therefore, we are interested in the interplay between FoxL2 and SF-1 in regulating StAR and Mc2R gene expression in mammalian systems. First, we find that FoxL2 interacts with SF-1 in mammalian two-hybrid system. While both FoxL2 and SF-1 are expressed in both ovary and adrenal gland tissues, FoxL2 is predominantly expressed in the ovary. Interestingly, we find that FoxL2 and SF-1 are highly expressed in day 12 and day18 of mouse embryonic tissues. As expected, FoxL2 represses and SF-1 enhances StAR promoter activity. Interestingly, we find that SF-1 cannot rescue FoxL2's repressive activity even at very high doses. This indicates that repressive activity of FoxL2 on StAR promoter may be due to competition binding to SF-1's response element. Numerous genes required for adrenocortical steroidogenesis are also activated by SF-1. Interestingly, Mc2R promoter activity is activated by FoxL2 in a dose-dependent manner. Surprisingly, we find that FoxL2 and SF-1 synergistically up-regulate the luciferase activity of Mc2R promoter. Because the -2000-bp mouse Mc2R promoter contains two SF-1 and multiple forkhead-responsive consensus sites, the Mc2R promoter was truncated to determine the minimal region that is important for synergistically transcriptional up-regulation by FoxL2 and SF-1. The results suggest that the first 1320 bp upstream of the Mc2R transcriptional start site are sufficient for synergistically transcriptional up-regulation by FoxL2 and SF-1. In summary, these results indicate that FoxL2 and SF-1 may play important and divergent roles in regulating adrenal and gonadal tissue differentiation and development as well as steroidogenesis. (poster)