A polymorphism in the growth hormone (GH)-releasing hormone (GHRH) receptor gene is associated with elevated response to GHRH by human pituitary somatotrophinomas in vitro

A previous study has suggested that a G to A base change at position 169 of the GHRH-receptor gene in human somatotrophinomas is a mutation and confers hypersensitivity to GHRH. The alternative base converts codon 57 from GCG to AGC, resulting in replacement of alanine (Ala) with threonine (Thr). In the present study, two of five human GH-secreting somatotrophinomas were found to possess the codon 57 AGC sequence. The GCG allele was also detected, indicating heterozygosity. However, the patients' normal blood-derived DNA also yielded the same sequence pattern, indicating that the Ala=> Thr amino acid change is a normal polymorphism, and not a somatic mutation. Nevertheless, in vitro, the tumors possessing the Ala=> Thr amino acid change responded very strongly to GHRH in terms of cAMP formation, being increased 40- and 200-fold, in comparison to the 2-fold increases by tumors without the alternative GHRH-receptor sequence. Likewise, the in vitro response of GH secretion to GHRH was elevated. One of the two tumors with the alternative Thr residue, and the highest responder to GHRH, possessed a gsp muration, despite the fact that these defects are thought to reduce responsiveness to GHRH. These results fail to confirm that the GCG => AGC at codon 57 of the GHRH-receptor gene is a mutation, but do support the concept that the alternative form with Thr confers increased sensitivity to GHRH. (C) 2000 Academic Press.

Cholecystokinin (CCK) receptor and CCK gene expression in human pituitary adenomas and in vitro effects of CCK peptides on GH and gonadotrophin secretion

There is growing evidence that cholecystokinin (CCK) affects growth and differentiation of anterior pituitary cells, via the CCK-B receptor. The possibility of an autocrine / paracrine role for CCK to modulate hormone secretion in human pituitary tumour cells is demonstrated here by RT-PCR and direct sequencing. In support of this conclusion, a neutralising antibody against the CCK peptide exhibited a dose dependent inhibition of hormone secretion by functionless pituitary adenomas. Total RNA was extracted from human pituitary adenomas, reverse transcribed into cDNA and subjected to PCR using primers specific for the gene for CCK, CCK-A and CCK-B receptors. PCR bands of the predicted length were observed in all tumours using human CCK gene and CCK-B receptor primers. Restriction digestion and direct sequence analysis provided further evidence that they represented both the human CCK peptide along with the CCK-A and/B receptor mRNA. CCK-33 and CCK octapeptide sulphate (CCK-8s) both powerfully stimulated phosphatidylinositol hydrolysis, providing evidence for functional activity of the CCK-A and/B receptors. A direct stimulatory effect of CCK peptides on both LH and FSH secretion is reported for the first time, whereas stimulatory effects on GH were blocked by antagonists to CCK. These results may indicate an autocrine role for CCK in the functioning and perhaps development of human pituitary tumours. © J. A. Barth Verlag in Georg Thieme Verlag KG Stuttgart.