Stimulatory effect of thyroid hormone on growth hormone gene expression in a human pituitary cell line

A cell line (GX) derived from a pituitary tumor of a boy with gigantism, which exhibits morphological characteristics of epithelial cells, has been established and maintained in culture for over 2 yr. Initially, GX cells secreted GH which was stimulated by T3. However, after 2 yr in culture, GH production in GX cells is no longer detectable, though GH messenger RNA (mRNA) synthesis has been maintained. The GX cell line is a somatomammotroph line as indicated by the presence of both GH and PRL mRNAs. GX cells were used to study the effect of T3 on GH gene expression. A nearly 3-fold increase in GH mRNA accumulation was observed in GX cells in response to the addition of T3 to the culture medium. This increase was also observed in serum-free medium, indicating that the effect of T3 does not require the presence of other hormones or serum factors. Neither PRL nor beta-actin mRNA accumulation was stimulated by T3 in GX cells. Nuclear runoff experiments demonstrated that the stimulatory effect of T3 on GH mRNA accumulation is related to transcriptional activation of the human GH gene.