The gastrin-releasing peptide, its protein precursors and encoding gene

GRP is a 27-amino acid peptide initially isolated from porcine stomach? It is the mammalian homologue of the amphibian skin tetradecapeptide bombesin. The two peptides share an identical C-terminal hectapeptide that binds with high affinity to specific cell-surface receptors. Bombesin/ GRP receptors are expressed on a variety of cells, namely secretory cells, smooth muscle cells, fibroblasts, epithelial cells, neuroendocrine cells, peripheral neurons and central neurons. Binding of the ligand to its receptor activates the phosphatidylinositol phosphate pathway, which produces mainly secretion, contraction, mitosis or changes in membrane potential, according to the nature of the target cell. When bombesin and GRP are injected into a variety of mammalian species, they exert complex pharmacological effects. The complexity of these effects is linked to the diversity of the target cells, as well as to the induced release of several hormones, paracrine factors and neurotransmitters. Exogenous bombesin and GRP have major effects on digestive functions. They stimulate most exocrine and endocrine secretions associated with the digestive tract, modulate gastrointestinal transit, modify glucose regulation and reduce food intake; they may also exert trophic effects on gastrin cells and on acinar pancreatic cells. The peptides can mediate these effects by acting upon secretory cells or smooth muscle, or by acting upon intramural neurons that control these effector cells; more importantly, the peptides also act upon central neurons in discrete areas of thje brain to modify gastric and pancreatic secretions, gastrointestinal motility, glucose metabolism and food intake. The physiological relevance of these pharmacological effects is suggested by colonization of GRP-like peptides and GRP-like receptors in the digestive tract, pancreas and relevant brain areas, and by the induced release of endogenous peptides by electrical and chemical stimuli. So far, compelling evidence indicates that endogenous GRP-like peptides are involved in the neural non-cholinergic regulation of antral gastrin release, and the physiological relevance of other effects on digestive functions is being intensively investigated. Another are of great interest concern the trophic effects of bombesin and GRP on damaged cells in adults, and on developing cells during fetal and neonatal life. The mitogenic effect of the peptides is firmly established on mouse embryonic fibroblast cell lines that provide an excellent model system; however, the physiological significance of these effects remains elusive. More interesting but less compelling are the growth-promoting effects on normal bronchial epithelial cells. The mitogenic effect of the peptides on primary culture of bronchial epithelial cells and the increased numbers of immunoreactive neuroendocrine cells in a variety of conditions associated with chronic lung injury suggest that endogenous GRP-like peptides could be involved in bronchial epithelial cell repair. Similarly, the enhanced expression of GRP in pulmonary and thyroid neuroendocrine cells during fetal and neonatal lung development suggests that the peptide could exert trophic effects on normal developing cells in these organs. However, bombesin/GRP receptors have so far not been characterized on bronchial epithelial cells or thyroid cells, and more in vitro work is required to document the mitogenic effect. Finally, a direct or indirect trophic effect of the peptides is observed in the gastrointestinal tract and pancreas of adult and suckling animals; the high concentrations of GRP-like peptides in milk and the mild trophic activity of orally-given bombesin suggest that maternally-derived GRP could exert trophic effects in the suckling animal. The growth-promoting effects of bombesin and GRP on small cell lung carcinoma cell lines has fostered a great deal of speculation concerning the involvement of these peptides as autocrine growth factors in the progression of malignancy. However, the key observations on growth-promoting effects are difficult to reproduce and hence controversial; the same restriction applies to the characterization of bombesin/GRP receptors on small cell lung carcinoma cell lines by binding studies. This last point has recently been clarified by the demonstration that bombesin-related peptides stimulated the catabolism of inositol lipids in at least one such cell line; however this activation is not necesseraly correlated with growth and could equally well reflect the stimulation of secretion in a neuroendocrine cell type. Whether, the peptides stimulate growth and/or secretion in small cell lung carcinoma cell lines remains to be firmly established, but the speculation has provided a strong incentive for the research of competitive antagonists of the bombesin/GRP receptor. Two recently developed analogues, namely (Leu13-ψ-CH2NH-Leu14)-bombesin and N-acetyl GRP20-26-OCH2CH3 are specific and relatively high-affinity antagonists of the bombesin/GRP receptor. It is not excluded that these antagonists could be used ion the future as anti-cancer drug, but they surely provide essential tools for further analysis of the pharmacological and physiological effects of GRP. GRP is derived from a large protein precursor that has been defined by molecular cloning in both man an rat. The availability of molecular tools in these two species is allowing a definitive mapping of GRP-expressing cells which has already reinforce the presumptive trophic role of peptide during human pulmonary and thyroid development. These studies have also suggested novel functional implications for GRP as a neuropeptide in rat brain, namely in the cortex, which should help to understand this important question. However, a comprehensive molecular analysis of GRP-induced cellular responses requires the cloning of the GRP receptor, which should be achieved in the near future by, at least one of the groups working on this subjectThèse d'agrégation de l'enseignement supérieur (Faculté de médecine) -- UCL, 198