The presence of bombesin (gastrin-releasing peptide, GRP)-like immunoreactivity in mucosal endocrine cells of human fetal lung is well established. In this study we have investigated the localisation of pro-GRP mRNA and GRP gene products and compared the distribution and levels of extractable GRP-and C-terminal flanking peptide of human pro-GRP-like immunoreactivity in order to verify synthesis and to investigate their coexistence and molecular forms. Human fetal lungs (14 to 23 weeks gestation) were immunostained, and extracts were assayed using regionspecific antisera to pro-GRP. Additional antisera to chromogranin and protein gene product 9.5 (PGP 9.5) were used for immunostaining by the peroxidase anti-peroxidase technique and for double immunofluorescence staining using antisera raised in two species. Immunoreactivity for both bombesin (GRP) and flanking peptide was seen mainly in the same endocrine cells, but more cells were stained with antisera to flanking peptide than with antiserum to bombesin (GRP). In situ hybridisation showed that pro-GRP mRNA was present and thus synthesis of the peptides was taking place. Endocrine cells and nerve fibres were PGP 9.5-immunoreactive, and a subset of cells was immunoreactive for bombesin gene products. Radioimmunoassay and chromatography show that pro-GRP is present in both the uncleaved and cleaved forms, and, in agreement with immunocytochemistry results, that an excess of C-terminal peptide of pro-GRP is detectable. It is therefore concluded that GRP-like peptides and flanking peptide are co-local-ised in human pulmonary endocrine cells, but the latter is found in larger concentrations than free GRP. Thus GRP-like peptides may be secreted separately from the flanking peptide(s) of pro-GRP. Furthermore PGP 9.5 appears to be a useful marker for endocrine cells in the respiratory epithelium of human fetal lung.
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