The growth hormone cascade and its role in mammalian growth

The myriad actions of growth hormone (GH) are still incompletely understood, despite decades of research. Although it is a major regulator of post-natal growth in mammals, much of its effects on skeletal growth are recognized to be mediated indirectly, through the stimulation of production of insulin-like growth factor (IGF)-I, as well as some of the major serum carrier proteins for IGF-I and -II, such as IGF-binding protein-3 (IGFBP-3) and the acid-labile subunit (ALS). The regulation of IGF-I production by GH appears to be mediated entirely by signalling through the Janus kinase (JAK) 2 pathway, via the phosphorylation of the transcription factor, signal transducer and activator of transcription (STAT) 5b. GH also signals, however, through additional pathways that are likely to be critical to the metabolic actions of GH.