The interaction of GHRH with membrane-bound receptors on somatotroph cells of the anterior pituitary is an important step in the regulation of GH synthesis and secretion. The identification of a
G protein-coupled receptor for GHRH has made it possible to investigate the pathway by which GHRH regulates pituitary somatotroph cell function. To initiate an analysis of the mechanisms regulating expression and function of the
GHRH receptor, the structure of the gene and its promoter region were analyzed. The coding sequence of the rat
GHRH receptor gene is contained within 14 exons spanning approximately 15 kb of genomic
DNA. Four transcription start sites are located within 286 bp upstream of the
initiation codon. The 5' flanking region of the
GHRH receptor gene acts as a functional promoter in rat
pituitary tumor GH3 cells, and basal promoter activity is enhanced in GH3 and COS7 cells by cotransfection of an expression construct encoding the
pituitary-specific transcription factor Pit-1. The rat
GHRH receptor gene is subject to at least 1 alternative RNA processing event that generates 2 receptor
isoforms differing by 41
amino acids within the third intracellular loop (IL) of the
protein. The short
isoform of the
GHRH receptor is predominant in pituitary cells. The MtT/S
pituitary tumor cell line was found to express the
GHRH receptor, and different populations of these cells produce predominantly the long or short
isoforms of the receptor
messenger RNA, suggesting that the alternative splicing can be regulated. Functional analysis of the two
GHRH receptor isoforms demonstrates that both bind GHRH, but only the short
isoform signals through a cAMP-mediated pathway. Neither receptor
isoform is able to stimulate
calcium mobilization from internal stores after GHRH treatment. Our findings indicate that the
pituitary-specific transcription factor Pit-1 is involved in the somatotroph-specific expression of the
GHRH receptor gene and that functionally distinct receptor
proteins are generated by an alternative RNA processing mechanism.