Genomic Structure and Transcriptional Regulation of the Human Growth Hormone Secretagogue Receptor

Abstract

Synthetic GH secretagogues stimulate GH release through binding to a recently cloned specific GH secretagogue receptor (GHS-R). The endogenous ligand of this receptor may be part of a new endocrine pathway controlling GH secretion. Two different receptor variants, type 1a and 1b, have been described that differ in their 3′-terminal amino acids. We investigated the genomic structure and transcriptional regulation of the human GHS-R. An 18-kb genomic clone including sequences encoding for the two GHS-R variants was isolated. Sequencing revealed that the two variants originate from specific RNA processing of a single gene that spans approximately 4.1 kb. The transcription start site was defined by 5′-inverse PCR analysis at position −227. RT-PCR analysis points to differential transcriptional initiation and processing. Type 1a is encoded by two exons; 2152 bp of intronic sequence are removed by splicing at position 796/797 relative to the translation start site. Type 1b is encoded by a single exon. A putative polyadenylation signal consensus motif was identified at position+ 4118; 2.7 kb of the 5′-flanking region were sequenced, and putative transcription factor binding sites were identified. Transcriptional regulation was investigated by transient transfections using promoter fragments ranging in size from 168-1745 bp; 1745 bp of the GHS-R promoter directed significant levels of luciferase expression in GH4 rat pituitary cells, whereas no activity was detected in monkey kidney COS-7 cells, human endometrium Skut-1B cells, mouse hypothalamic LHRH neuronal GT1–7 cells, or mouse corticotroph pituitary AtT20 cells. A minimal 309-bp promoter allowed pituitary-specific expression. Its activity in COS-7 cells was enhanced by cotransfection of the pituitary-specific transcription factor Pit-1. We did not find any regulation of the GHS-R promoter by forskolin, somatostatin, insulin-like growth factor I, or 12-O-tetraphorbol 12-myristate 13-acetate. Thyroid hormone and estrogen lead to a significant stimulation; glucocorticoids lead to a significant inhibition. Further mapping suggests a thyroid hormone-responsive element, an estrogen-responsive element, and a glucocorticoid-responsive element located between −309 and the translation start codon. These studies demonstrate the nature of the human GHS-R gene and identify its 5′-flanking region. Furthermore, pituitary-specific activity of the promoter and regulation by various hormones are demonstrated.