Growth hormone (GH) action in early embryogenesis: expression of a GH-response gene in sites of GH production and action

Growth hormone (GH) may act as a local growth factor in early embryonic development, since GH- and GH-receptor (GHR) immunoreactivity is present in all tissues and most cells of embryonic chicks during organogenesis. However, as GHR-immunoreactivity could, alternatively, reflect the presence of GH-binding proteins (GHBPs) rather than authentic receptors linked to signal transduction mechanisms. GHR immunoreactivity may not be indicative of GH target sites. The possibility that GH may act as an autocrine or paracrine factor during embryogenesis was therefore assessed in the present study by determining the presence and cellular localization of mRNA for a GH-responsive gene. The mechanism of GH action involves the induction of a number of specific GH-response genes. In chickens a novel GH-responsive gene (GHRG-1) has been identified as a marker of GH action. In situ hybridization, using a 860 by probe for GHRG-1 mRNA, demonstrated widespread expression of the GHRG-1 gene in embryonic tissues known to contain GH- and GHR-immunoreactivity (e.g. in the spinal cord, skin. heart. liver, muscle, bone and lung). GHRG-1 mRNA was not, however, present in all cells of each tissue. It was, furthermore, not present in subepithelial cells of the esophagus and bronchus and was lacking in many spinal cord ependyma. which are also known to lack GH immunoreactivity. These results therefore support the possibility that GH acts as an autocrine/paracrine factor during early chick embryogenesis, which was hitherto thought to be a growth-without-GH syndrome.