Hypothalamic hormonal regulation of human growth hormone gene expression in somatotroph adenoma cell cultures

Pituitary growth hormone (GH) secretion is stimulated by hypothalamic growth hormonereleasing hormone (GRH) and inhibited by somatotropin release-inhibiting hormone (so-matostatin, SRIH). The effect of GRH and SRIH was therefore tested on human GH gene expression in nine somatotroph tumor cell cultures. SMS 201-995, a long-acting somatostatin analog, was also tested in these long-term pituitary cultures. Cells were grown in serum-free defined medium and treated with GRH, SMS 201-995, or SRIH for up to 72 hours. After 72 hours, total cellular RNA was extracted and subjected to an RNAse protection assay, using a radiolabeled 210 base human GH cRNA probe. This sensitive technique allowed detection of small amounts of GH messenger RNA (mRNA). Nonstimulated somatotroph tumor cells expressed human GH mRNA detected by protection of double-stranded hybrid bands. SMS 201-995 had no effect on GH mRNA levels. GRH induced GH mRNA transcripts four- to sixfold. These results indicate that GRH stimulates both long-term in vitro GH secretion and GH mRNA levels in human GH cell cultures. SMS 201-995 has a potent and sustained long-term inhibitory effect on GH secretion but does not appear to alter human GH mRNA levels. Although 15 nM SMS 201-995 attenuated GH secretion to 30% of controls during 72-hour incubation, the peptide did not alter GH mRNA levels. SMS 201 -995 may therefore act as an inhibitor of GH secretion without altering expression of the human GH gene in somatotroph adenomas.