Expression of functional growth hormone secretagogue receptors in human pituitary adenomas: polymerase chain reaction, triple in-situ hybridization and cell culture studies.

We examined the expression of functional growth hormone secretagogue receptors (GHS-R) in a series of 30 human pituitary adenomas-six secreting GH, three GH-PRL, six prolactin (PRL), five adrenocorticotrophic hormone (ACTH), one thyroid stimulating hormone (TSH), four gonadotroph and five non-secreting adenomas. By reverse transcriptase polymerase chain reaction (RT-PCR), the coexpression of the two GHS-R isoforms (Ia and Ib) was found in all the GH-, GH-PRL- and PRL-secreting adenomas, and only in two out of three corticotroph, two out of four gonadotroph and one out of five non-secreting tumours. They were absent in the TSH-secreting adenoma. The PCR products of GHS-R Ia and Ib were identical in size to those from two normal pituitaries. PCR cloning and sequencing of isoforms performed in two somatotroph adenomas revealed only two single, silent base mutations. Triple in-situ hybridization showed colocalization of GHS-R mRNA with messengers of GH and PRL, conjointly or separately, in individual cells of somatotroph, mammosomatotroph, and lactotroph adenomas. The presence of GHS-R mRNA in cells expressing PRL mRNA is emphasized. In cultured cells from six somatotroph and two mammosomatotroph adenomas, the powerful GHS MK-0677 stimulated GH release in a dose-dependent manner, with maximal effect at 6 h. Contrarily, when GHRH was applied, only three somatotrophs and two mamosomatotrophs were stimulated. In the two mammosomatotrophs, the PRL response to MK-0677 and to GHRH was similar to the GH response. An homologous desensitization of the GHS-R and the GHRH receptor was observed 24 h after a first stimulation by a single dose of the corresponding agonist. Heterologous desensitization was not observed. Interestingly, MK-0677 also stimulated, in a dose-dependent way, the hormone release of cells from all tested lactotroph and corticotroph adenomas. The existence of a functional expression of GHS-R in somatotroph, mammosomatotroph, lactotroph and corticotroph adenomas rises the question of the role played by GHS-R in pituitary adenomas, particularly those not engaged in GH secretion.