Thyroid hormone regulation of the transfected rat growth hormone promoter

A region of the rat growth hormone gene and 5' flanking DNA has been identified which promotes accurate, thyroid hormone-regulated transcriptional initiation. GC rat pituitary tumor cells were transfected with chimaeric plasmids containing various lengths of rat growth hormone gene and 5' flanking DNA fused to the coding region of the dominant selectable marker gene neo. Thyroid hormone induction of rGH-neo RNA was observed by Northern and dot blot analysis of cells transfected with rGH-neo chimaeric genes sharing the rat growth hormone gene and upstream regions from -235 to +11. Initiation of rGH-neo transcription was mapped by S1 nuclease protection to the in vivo initiation site of the natural growth hormone gene. Transcription of the most deleted thyroid hormone responsive construct involved an induction-attenuation cycle qualitatively similar to the response of the natural gene. However, the 3,5,3'-triiodo-L-thyronine responsiveness of this deleted construct was approximately 2- to 3-fold less than that of less deleted rGH-neo genes tested. These results suggest that, at a minimum, the sequences required for the cyclic 3,5,3'-triiodo-L-thyronine transcriptional response are located within the region of the gene from -235 to +11. Other sequences essential for full responsiveness appear to be located elsewhere in the 5'-flanking DNA. Rat growth hormone promoter utilization appears to be strongly cell-type dependent. We obtained stable transfectants with rGH-neo constructs only in GC cells.