Open Collections

Ligand-independent activation of steroid hormone receptors by gonadotropin-releasing hormone

University of British Columbia

Nuclear receptors including estrogen receptors (ERs) and progesterone receptors (PRs) are activated by their ligands as well as by signaling pathways in response to peptide hormones and growth factors. In gonadotrophs, gonadotropin releasing hormones (GnRHs) act via the GnRH receptor (GnRHR). Both GnRH-I and GnRH-II activate an estrogen response element (ERE)-driven luciferase reporter gene in LβT2 mouse pituitary cells, and GnRH-I is more potent in this regard. The ERα is phosphorylated at Ser¹¹⁸ in the nucleus and at Ser¹⁶⁷ in both nucleus and cytoplasm after GnRI-I treatments, and this coincides with increased ERct binding to its co-activator, the P300/CBP-associated factor (PCAF). Most importantly, both GnRH subtypes robustly up-regulate expression of the immediate early response gene, Fosb, while co-treatment with ERα siRNA or PCAF siRNA attenuates this effect. This appears to occur at the transcriptional level because co-recruitment of ERα and PCAF to an ERE within the endogenous Fosb promoter is increased by GnRH treatments, as shown by chromatin immunoprecipitation assays. Furthermore, cross-talk between GnRH-I and PR accentuates gonadotropin production. GnRH-I activates a progesterone response element (PRE)-driven luciferase reporter gene and gonadotropin a subunit (Gsua) gene expression in two mouse gonadotroph cell lines, αT3-1 and LβT2. Up-regulation of the PRE-luciferase reporter gene by GnRH-I is attenuated by pre-treatment with protein kinase A (H89) and protein kinase C (GF109203X) inhibitors, while only GF109203X inhibits GnRH-1-induced Gsua mRNA levels. In both cell lines within the same time-frame, knockdown of PR levels by siRNA reduces GnRH-I activation of Gsua mRNA levels by approximately 40%. Both GnRH-I and GnRH-II also increase mouse Gnrhr-luciferase promoter activity and this is significantly reduced by knockdown of PR in LβT2 cells. We conclude that the effects of GnRH-I on Fosb and Gsua expression, as well as mouse Gnrhr promoter activity in mouse gonadotrophs are mediated by ligand-independent activation of ERα and PR. These ligand-independent effects of GnRHs on steroid hormone receptor function may influence the magnitude of changes in the expression of specific genes in the pituitary during the mouse estrous cycle, which in this context may serve as a model in the human menstrual cycle.

Text

2011-05-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/34980

Reproductive and Developmental Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/