Abstract
Pituitary FSH promotes pubertal timing and normal gametogenesis by binding its receptor (FSHR) located on Sertoli and granulosa cells of the testis and ovary, respectively. Studies on Fshr transcription provide substantial evidence that upstream stimulatory factor (USF) 1 and USF2, basic helix-loop-helix leucine zipper proteins, regulate Fshr through an E-box within its promoter. However, despite the strong in vitro support for USF1 and USF2 in Fshr regulation, there is currently no in vivo corroborating evidence. In the present study, chromatin immunoprecipitation demonstrated specific binding of USF1 and USF2 to the Fshr promoter in both Sertoli and granulosa cells, in vivo. Control cells lacking Fshr expression showed no USF-Fshr promoter binding, thus correlating USF-promoter binding to gene activity. Evaluation of Fshr expression in Usf1 and Usf2 null mice further explored USF's role in Fshr transcription. Loss of either gene significantly reduced ovarian Fshr levels, whereas testis levels were unaltered. Chromatin immunoprecipitation analysis of USF-Fshr promoter binding in Usf-null mice indicated differences in the composition of promoter-bound USF dimers in granulosa and Sertoli cells. Promoter-bound USF dimer levels declined in granulosa cells from both null mice, despite increased USF2 levels in Usf1-null ovaries. However, compensatory increases in promoter-bound USF homodimers were evident in Usf-null Sertoli cells. In summary, this study provides the first in vivo evidence that USF1 and USF2 bind the Fshr promoter and revealed differences between Sertoli and granulosa cells in compensatory responses to USF loss and the USF dimeric composition required for Fshr transcription.