Endocrine Abstracts

The gonadotrophin hormones comprise a common alpha subunit and a hormone specific beta subunit and their expression profiles differ during pituitary ontogeny. The transcriptional regulatory cascade that is required to mediate this is largely unknown. However, perturbation of the expression of homeodomain transcription factors, Paired-like homeodomain transcription factor 1 (Pitx1) and Homeobox gene expressed in embryonic stem cells 1 (Hesx1) disparately affects gonadotrophin gene expression in vivo . Pitx1 up-regulates gonadotrophin gene expression, but the transcriptional targets of Hesx1 are unknown. Therefore we have investigated if the transcriptional repressor Hesx1 affects alpha GSU and LH beta gene expression in vitro and tested for interactions with known transactivators of gonadotrophin gene expression.

Hesx1 repressed expression from alpha GSU and LH beta gene promoters. This mapped to the Pitx1 binding site in the alpha GSU promoter, but mapped distally to the binding site required for Pitx1 transactivation in the LH beta promoter. The Pitx1 DNA binding sites on alpha GSU and LH beta are similar, so Pitx1 and Hesx1 were co-transfected to test if they interacted via these sites. Hesx1 did not affect Pitx1 transactivation of either alpha GSU promoter constructs or a minimal reporter construct containing the Pitx1 DNA binding site from alpha GSU. In contrast, Hesx1/Rpx could antagonise Pitx1 activation of LH beta promoter constructs, but not synergy between Pitx1 and steroidogenic factor-1 (SF-1). These interactions were further characterised using minimal reporters containing the LH beta Pitx1 DNA binding site and the distal SF-1 DNA binding site. Hesx1 blocked Pitx1 transactivation via the Pitx1 site. Hesx1 did not block synergy between Pitx1 and SF-1 via the SF-1 DNA binding site.