Stephen Harvey (PhD, DSc, University of Leeds)
Professor & Associate Chair (Education)
7-41B1 Medical Sciences Building
University of Alberta
Canada T6G 2H7
Tel: 780 492-2809
Growth Hormone: A Paracrine / Autocrine?
Growth hormone (GH), as its name suggests, is obligatory for postnatal growth. It does, however, have numerous other effects and it regulates the metabolism of most cells, even in adults and the elderly. The secretion of GH is primarily by the pituitary gland, from where it is distributed throughout the body in the systemic circulation. The actions of pituitary GH are, therefore, endocrine, regulating the activity of distant target sites. The pituitary is not, however, the only site of GH synthesis and release.
In recent years, this laboratory has demonstrated that GH is produced within neural, immune, respiratory and reproductive tissues. The production of GH within these extrapituitary tissues is much less than in the pituitary gland and does not contribute to the GH pool in systemic circulation. These extrapituitary tissues also have GH receptors and are target sites for GH action. It is, therefore, possible that the local production of GH has paracrine (actions on neighboring cells) or autocrine (actions on the GH-secreting cell) actions that may augment or complement the endocrine actions of pituitary GH. The local production of GH may thus reflect an emergency mechanism to rapidly regulate cellular functions that are normally regulated in a strategic way by pituitary GH. This hypothesis is the focus of research in this laboratory, which involves determination of GH action and the influence of GH or GH receptor (GHR) blockade.
Growth Hormone: An Embryonic Growth Factor
Although GH is essential for postnatal growth, embryonic or fetal growth is, paradoxically, thought to be a 'growth-without-GH syndrome'. It occurs prior to the ontogenic appearance of the pituitary gland and the appearance of plasma GH (usually in mid-late gestation). It also occurs in acephalic states and is independent of exogenous GH therapy. The GH gene is, however, expressed in numerous extrapituitary cells during early embryonic development. It is, therefore, possible that GH acts locally during early embrogenesis to regulate growth and development. This possibility is being assessed in chick embryos, which provide an established model of embryogenesis.
The in vivo and in vitro approaches utilized in this laboratory include immunological (radioimmunoassay, immunocytochemistry, Western blotting) and molecular biological (Northern blotting, Southern blotting, in situhybridization, PCR, gene cloning) techniques.