GnRH signaling and LHbeta gene transcription: Modulation of promoter activity by signaling pathways and transcription factor occupancy

The hypothalamic peptide gonadotropin-releasing hormone (GnRH) is absolutely required for reproduction. It regulates both the synthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Each of these gonadotropin hormones is composed of a common alpha subunit (alphaGSU) and a unique beta subunit (LHbeta and FSHbeta respectively). The pulsatile rate of GnRH secretion differentially controls the transcription of each of these subunits and influences which hormone is ultimately secreted (1;2). It is unknown how GnRH is able to elicit this array of events in the same cell type simply by altering pulse frequency.; In order to understand the downstream effects of GnRH receptor activation, I analyzed LHbeta promoter activity in response to the signaling molecules calcium/calmodulin kinase II (Ca/CaMK II) and cyclic adenosine monophosphate (cAMP). GnRH stimulates Ca/CaMK II by increasing intracellular calcium through L-type calcium channels and intracellular stores. This increase in Ca/CaMK II activity results in an increase in alphaGSU and LHbeta promoter activity. Only the proximal GnRH response element of the LHbeta promoter is required for Ca/CaMK II-mediated stimulation of promoter activity.; The signaling molecule cAMP is also able to stimulate LHbeta promoter activity, both by augmenting GnRH stimulation of LHbeta promoter activity and by independently stimulating the LHbeta promoter. These effects have been demonstrated in transient transfections of primary rat pituitary cultures and a clonal gonadotrope cell line, and in LHbeta-luciferase expressing transgenic mouse pituitary cultures. This cAMP-mediated stimulation of LHbeta promoter activity is regulated by PKA and Ca/CaMK II and requires multiple sites in the proximal GnRH response element of the LHbeta promoter.; The proximal LHbeta GnRH responsive region contains binding sites for the transcription factors Egr-1 and SF-1. Expression levels of these proteins demonstrated that Egr-1 is rapidly induced by GnRH while SF-1 is only slightly induced. Stimulation of cAMP produces a small increase in Egr-1 protein levels but has no effect on SF-1 expression. Both of these factors interact with the native LHbeta promoter and are temporally associated with transcription of the LHbeta gene. The coactivator, SNURF, is able to stimulate basal LHbeta promoter activity by associating with the LHbeta promoter.