Expression of the rat growth hormone (rGH) gene in the pituitary and in cultured pituitary tumor cells is regulated by glucocorticoid hormones. After co-transfer of cloned DNA containing the rGH gene with the herpes simplex virus (HSV) thymidine kinase (tk) gene into mouse Ltk- cells, rGH gene transcripts were detected in eight of fifteen tk+ cell lines. However, in all eight clones, the predominant rGH gene transcript was only about 0.75 kb, 0.3 kb shorter than pituitary rGH mRNA. The 0.75-kb transcripts, examined from one clone, L-rGH-4, lacked sequences derived from exons 1 and 2 of the rGH gene. Although transcripts larger than 0.75 kb were detected, the normal 2.2-kb rGH gene primary transcript was present only at very low levels. Nuclease mapping studies also failed to reveal transcripts initiated at the normal rGH gene promoter, but instead revealed transcripts with 5' termini arising within intron B of the gene. These data suggest either that transcripts arise from internal promoters within the rGH gene or that a transcript initiated upstream from the normal promoter was processed abnormally. Dexamethasone increased the levels of the 0.75-kb rGH gene transcripts about fourfold in all eight clones expressing rGH mRNA. These data suggest that structural elements important for glucocorticoid-mediated influences on regulation of GH gene expression are contained within the transferred rGH gene fragment and can function even when the normal rGH gene promoter is not used and the pattern of expression is grossly abnormal.
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