A novel G11-protein-coupled receptor specific for synthetic GH-releasing
peptides (GHRPs) has recently been cloned and sequenced. Two forms exist, types 1a and 1b, the latter of which is biologically inactive. Using RT-PCR, we looked for the presence in tumorous pituitary cells of messenger
ribonucleic acid (
mRNA) for this novel GH
secretagogue receptor (GHS-R). Both subtypes of GHS-R
mRNA were detected in all six human pituitary somatotropinomas removed from patients with
acromegaly. In culture, four of the
tumors exhibited strong responses to
GHRP-2 in terms of both
phosphatidylinositol (PI) hydrolysis and GH secretion, but two were resistant. There was no apparent difference in the type 1a and type 1b expression pattern, as judged by RT-PCR, between responsive and nonresponsive
tumors. Similarly, the rat
pituitary tumor cell line, GH3, was found to express GHS-R
mRNA, although these cells also did not respond to GHRPs. RT-PCR failed to detect GHS-R
mRNA in eight functionless human
pituitary tumors. In contrast,
prolactinomas were found to express the receptor and, in culture, significant stimulation of PRL secretion and PI hydrolysis occurred in two of three
tumors tested. These results demonstrate that tumorous somatotrophs express the GHS-R gene and that the occasionally observed nonresponsiveness of somatotropinomas to GHRPs is not due to the absence of the biologically active type 1a receptor. Additionally, human pituitary
prolactinomas also express GHS-R and are able to respond to GHRPs in terms of PI hydrolysis and PRL secretion. In contrast, GHS-R gene expression does not appear to be associated with human functionless
pituitary tumors.