Stimulation of the preprothyrotropin-releasing hormone gene by epidermal growth factor

Regulation of the expression of the prepro-TRH (ppTRH) gene by epidermal growth factor (EGF) was investigated. The i.p. injection of EGF significantly stimulated hypothalamic ppTRH messenger RNA levels in rats. To clarify whether this stimulatory effect of EGF could be exerted at the level of gene transcription, the 5'-flanking region (-1893/+127) of the mouse ppTRH gene fused to a luciferase reporter gene was transiently transfected into pituitary GH4C1 cells, and the effect of EGF on gene transcription was measured by a luciferase assay. EGF stimulated ppTRH gene promoter activity in a time- and dose-dependent manner. Deletion analysis revealed that two different regions of the promoter, between -254 and -218 [EGF response element-1 (EGFRE1)] and between -130 and -84 (EGFRE2) were required for full stimulation by EGF. The two EGFREs possessed putative binding sequences for the transcription factor Sp1, and they functioned cooperatively in heterologous promoters. Nuclear extracts from GH4C1 cells specifically bound those two EGFREs in gel retardation assays. Two protein-DNA complexes were found on EGFRE1, whereas four complexes were observed on EGFRE2. Although the binding of nuclear extracts to EGFRE1 was competed for by the consensus Sp1 binding sequence, the complexes on EGFRE1 were not supershifted by an Sp1 antibody. Formation of the slower migrating protein complex on EGFRE1 was prevented by EDTA, suggesting that one of the EGFRE1-binding proteins might be an Sp1-related zinc finger protein. Competition and supershift experiments demonstrated that the EGFRE2-binding protein showing that the slowest migration possessed a characteristic similar to that of Sp1. Selective mutations of the Sp1-binding site in EGFRE2 markedly diminished the EGF-induced stimulation. These results suggest that EGF may function as a positive regulator of ppTRH gene expression, and that the stimulatory effect may be mediated through a cooperative interaction between Sp1 or Sp1-related proteins and additional factors that bind to two separate DNA regions.