Chicken growth hormone: further characterization and ontogenic changes of an N-glycosylated isoform in the anterior pituitary gland

Abstract

Glycosylation is one of the post-translational modifications that growth hormone (GH) can undergo. This has been reported for human, rat, mouse, pig, chicken and buffalo GH. The nature and significance of GH glycosylation remains to be elucidated. This present study further characterizes glycosylated chicken GH (G-cGH) and examines changes in the pituitary concentration of G-cGH during embryonic development and post hatching growth. G-cGH was purified from chicken pituitaries by affinity chromatography (Concanavalin A–Sepharose and monoclonal antibody bound to Sepharose). Immunoreactive G-cGH has a MW of 26 kDa or 29 kDa as determined by SDS–PAGE, respectively, under non-reducing and reducing conditions. Evidence that it is N-glycosylated comes from its susceptibility to peptide N-glycosidase F, and its resistance to O-glycosidase. Based on the ability of G-cGH to bind Concanavalin A or wheat germ agglutinin but not other lectins and its susceptibility to peptide N-glycosidase F, a hybrid or biantennary type glycopeptide (GlcNac2, Man) structure is proposed. Some G-cGH can be observed in the pituitary at most ages examined (from 15-day embryo to adult). Moreover, electron microscopy revealed the presence of both immuno-reactive GH and Concanavalin A-reactive sites in the same secretory granules in the somatotrope. There were marked changes in the level and relative proportion of G-cGH in the pituitary gland during development and growth, the proportion of G-cGH rising during late embryonic development (e.g., between 15 and 18 days of development) and with further increases between 9 weeks and 15 weeks old. G-cGH was able to bind to chicken liver membrane preparations with less affinity than non-glycosylated monomer; on the other hand, however, G-cGH stimulated cell proliferation on Nb2 lymphoma bioassay whereas the non-glycosylated monomer was uncapable to do it.