The involvement of growth hormone in growth regulation, energy homeostasisand immune function in the gilthead sea bream (Sparus aurata): a short review

The aim of this mini-review is to provide a comprehensive survey of the phy siological role of growth hormone (GH) in the Mediterranean sea bream (Spar us aurata). For this purpose, sea bream GH is now available as a recombinan t and bioactive protein in a practically unlimited amount. In juvenile and adult fish, the liver is the most important target for the direct action of GH. Nevertheless, it must be noted that in sea bream larvae the greater co ncentration of GH-binding sites occurs in the head region, which also exhib its a fast growth. This finding suggests that GH exerts at this early stage of development a direct action on growing tissues rather than a systemic o ne mediated by hepatic IGF-I. However, the GH-liver axis is later a sensiti ve marker of growth performance, and seasonal changes in circulating GH lev els and hepatic GH-binding are well characterized. The effect of age, water temperature and feeding regimes has also been studied. As a characteristic feature, fasting and malnourished fish show a decrease in hepatic GH-bindi ng and circulating IGF-I, which increases pituitary GH release due to a lac k of negative feedback inhibition. Interestingly, the up-regulation of plas ma GH levels has also been described in fish fed to visual satiety. This, t ogether with a decreased feed conversion, is more evident in fish fed high energy diets, and it has been suggested that this metabolic derangement is an adaptive response to protect adipose tissue and perhaps other organs and tissues from the excessive lipid deposition, when abundant energy is avail able. Experimental evidence also indicates that GH is able to exert a direc t effect on sea bream erythroid and immunocytes. It is now recognized that GH receptors are present in erythroid, lymphoid and myeloid cells, and both GH and IGF-I have a mitogenic effect. GH also acts as a phagocytic-activat ing factor, and the presence of GH transcripts in the head kidney has been demonstrated by RT-PCR. All this provides suitable evidence for a pleiotrop ic and crucial role of GH in a lower vertebrate species such as sea bream.