科研之友
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摘要
GnRH is the main modulator of LH secretion and transcription of the LH subunit genes in pituitary gonadotropes. The LH beta gene is preferentially transcribed during pulsatile GnRH stimuli of one pulse/30 min and is thus carefully controlled by specific signaling pathways and transcription factors. We now show that GnRH-stimulated LH beta transcription is also influenced by the ubiquitin-proteasome system. GnRH-stimulated activity of an LH beta reporter gene was prevented by proteasome inhibitors MG-132 and lactacystin. Inhibition was not rescued by overexpression of two key transcription factors for LH beta, early growth response-1 (Egr-1) and steroidogenic factor-1 (SF-1). Increased endogenous LH beta transcription after GnRH treatment was also prevented by MG-132, as measured by primary transcript assays. To investigate possible mechanisms of LH beta transcriptional inhibition by proteasome blockade, we employed chromatin immunoprecipitation to measure LH beta promoter occupancy by transcription factors. Without GnRH, binding was low and unorganized. With GnRH, Egr-1 and SF-1 associations were stimulated, cyclic, and coincidental, with a period of approximately 30 min. MG-132 disrupted GnRH-induced Egr-1 and SF-1 binding and prevented phosphorylated RNA polymerase II association with the LH beta promoter. Egr-1, but not SF-1, protein was induced by GnRH and accumulated with MG-132. Egr-1 and SF-1 were ubiquitinated in gonadotropes and ubiquitinated forms of these factors associated with the LH beta promoter, suggesting their degradation may be key for LH beta proteasome-dependent transcription. Together, these results demonstrate that degradation via the proteasome is vital to GnRH-stimulated LH beta expression, and this occurs in part by allowing proper transcription factor associations with the LH beta promoter. (Molecular Endocrinology 23: 237-250, 2009)
- 出版日期2009-2
