Regulation of growth hormone-releasing hormone gene expression and biosynthesis.

Growth hormone-releasing hormone (GRH) was initially isolated, characterized, sequenced, and cloned from human tumors and subsequently from the hypothalamus of humans and other animal species. Extensive structure-function studies have indicated the amino terminus to be most important for its biologic action, and the primary mechanism of its bioinactivation occurs by cleavage of an amino terminal dipeptide. The GRH gene is expressed primarily in the hypothalamic arcuate nucleus but also in the placenta. Expression of the GRH gene is regulated by growth hormone in a classical feedback manner, with hypophysectomy leading to increased expression that is reversed by growth hormone treatment. GRH gene overexpression in transgenic mice leads to a syndrome similar to that of ectopic GRH secretion with massive pituitary hyperplasia and markedly enhanced growth. The transgenic mouse has been used for studies of GRH biosynthesis and provides a suitable model for the study of precursor processing to the mature hormone.