Gonadal Receptors/Mechanisms Of Action Of Hormones

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The LH receptor: The luteinizing hormone receptor (LRH) is a TATAless gene whose transcription is driven by Sp1/Sp3. Functional studies on the mechanism of orphan receptor-mediated silencing of transcription of the luteinizing hormone receptor have established a functional connection between the orphan receptor EAR3/COUP-TFI bound to a DNA direct repeat and the Sp1/Sp3 complex. EAR3 perturbs the communication between Sp1/Sp3 at the Sp1 site and the basal transcription complex through TFIIB, and reduces recruitment of RNA Pol II. This mechanism has been found to be operative in repressive/inductive states of the LHR during the ovarian cycle. Control of testicular function:The gonadotropin-regulated testicular RNA helicase (GRTH/Ddx25) cloned in this Branch is novel member of the DEAD-box family. This testis-specific enzyme is present in Leydig and germ cells (pachytene spermatocytes, round spermatids), and is the first member found to be regulated by hormones (gonadotropin/androgen). Recent studies have identified three translation initiation codons that are utilized for the generation of multiple species. The studies have demonstrated a cell-specific and androgen-dependent alternative usage of ATG codons in the rat testis. The autocrine actions of testosterone in Leydig cells and paracrine actions at tubule sites (round spermatids) cause increases of GRTH at the transcriptional level and promote the utilization of the 2nd ATG codon in both cell types at the translational level. The Prolactin receptor: Acting through its cognate receptor (hPRLR-long form), prolactin activates Jak2/Stat5 pathway, which is essential for prolactin-induced differentiation of mammary epithelium. It was generally accepted that prolactin-induced homodimerization is required for the activation of subsequent signalling pathways. However,current studies demonstrated that homodimerization of the long form of the receptor can be independent of prolactin. They also demonstrated heterodimerization between the long form of the hPRLR (activating) and either of the two short forms discovered in the branch (dominant repressor forms), indicating that homodimers and heterodimers are constitutively present. The hormonal stimulus could act on the preformed homodimer of the long form to induce the signaling by causing changes in conformation of the cytoplasmic domain. Heterodimerization with either short form could abrogate the hormone-induced structural changes that are required for activation of the dimeric long form of the receptor.